Accepted Manuscript

Hydroxyapatite: A review of syntheses, structure and applications in heteroge-

neous catalysis

Aziz Fihri, Christophe Len, Rajender S. Varma, Abderrahim Solhy

PII: S0010-8545(17)30160-1
DOI: http://dx.doi.org/10.1016/j.ccr.2017.06.009
Reference: CCR 112468

To appear in: Coordination Chemistry Reviews

Received Date: 1 April 2017

Accepted Date: 13 June 2017

Please cite this article as: A. Fihri, C. Len, R.S. Varma, A. Solhy, Hydroxyapatite: A review of syntheses, structure
and applications in heterogeneous catalysis, Coordination Chemistry Reviews (2017), doi: http://dx.doi.org/10.1016/
j.ccr.2017.06.009

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 Hydroxyapatite: A review of syntheses, structure and

applications in heterogeneous catalysis

Aziz Fihri a*, Christophe Len b, Rajender S. Varma c, Abderrahim Solhy d*

a
MAScIR Foundation, VARENA Center, Rabat Design, Rue Mohamed El Jazouli, Madinat Al

Irfane 10100 Rabat, Morocco.

b
Sorbonne Universités, Université de Technologie Compiègne, Centre de Recherche Royallieu,

CS 60319, F-60203, Compiègne Cedex, France.

c
Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical

Chemistry, Palacky University, Šlechtitelů 11, 783 71, Olomouc, Czech Republic.

d
Materials Science and Nanoengineering Department, Mohamed VI Polytechnic University,

Lot 660 – Hay Moulay Rachid, 43150 Benguerir, Morocco.

E-mail address: fihri239@yahoo.fr ; Abderrahim.Solhy@um6p.ma

Fax: (+212) 05-30-27-98-27
Tel: (+212) 06-61-98-31-39

1
CONTENTS
1. Introduction
2. Principal methods for the synthesis of HAP
2.1. Dry methods
2.2. Wet methods
2.2.1. Co-precipitation method
2.2.2. Sol–gel method
2.2.3. Emulsionmethod
2.2.4. Hydrolysis method
2.2.5. Hydrothermal methods
2.3. Alternate energy input methods
2.3.1. Microwave (MW)-assisted method
2.3.2. Ball-milling method
2.2.3. Sonochemical method
2.4. Other methods
3. Structure-reactivity studies of hydroxyapatite related to catalysis performances
4. Catalytic applications of hydroxyapatite
4.1. Cross-Coupling reactions
4.2. Condensation reactions
4.2.1 Knoevenagel reaction
4.2.2 Claisen-Schmidt reaction
4.2.3 Michael addition
4.4. Oxidation reactions
4.4.1. Oxidation of alcohols
4.4.2. Oxidations of silanols
4.4.3. Epoxidation reaction
4.4.4 Oxidative dehydrogenation of ethane and propane
4.5 Photocatalytic reactions
4.6. Other miscellaneous reactions
4.6.1 Multicomponent reactions
4.6.2 Alkylation reactions
4.6.3 Hydration of nitriles
4.6.4 Transesterification Reactions
4.6.5 Hydroformylation reactions
4.6.6. Hydrogenolysis reactions
4.6.7. Hydrogenation reactions
Summary and outlook
References

2
ABSTRACT
The synthesis of hydroxyapatite (HAP) and its applications in heterogeneous catalysis are
abridged in view of its demonstrated prowess and utility in chemical, material and industrial
industry. This overview documents the strength sand weaknesses of the synthetic routes
that have been applied to the synthesis of hydroxyapatite with description of salient
structural properties of hydroxyapatite responsible for its reactivity. The significant
applications of hydroxyapatite both, as an inorganic support and as acatalyst are described
with an emphasis on its performance, stability and reusability. Several newer findings as well
as the current challenges pertaining to the use of hydoxyapetite are summarized.

Keyword: Hydroxyapatite, modified hydroxyapatite, structure-activity, catalysis.

3
1. Introduction

Heterogeneous catalysis is the cornerstone of the chemical industry and without it the

existing technological society would not be what it is today [1]; several processes in the

chemical industry involve heterogeneous catalysis. More than 90% of oil and gas molecules

encounter a catalyst in refineries, generating fuels and large number of chemical

intermediates [2]. Additonally, the heterogeneous catalysis is implicated in many other

varied industrial processes, ranging from inorganic chemistry (NH3, H2SO4, HNO3...), to the

synthesis of fine chemicals, and drug molecules, and environmental treatment of emissions

and polluted discharges, etc.[3]. The study of heterogeneous catalysis requires a good

understanding of solid catalysts and their preparation, characterization, shape selectivity

including the kinetics and mechanisms of reaction [4-9]. On the economic front, in the year

2010, the annual market value of solid catalysts was approximately 29 billion USD [10]. This

value is expected to increase by an average of 7% by the year 2015, regardless of the current

financial and economic crisis [11]. Aside from economic considerations, another driving

reason for the development of heterogeneous catalysis is its ecological impact [12]. Present-

day manufacturers have to satisfy increasingly severe legislations to develop manufacturing

processes that release the minimum amount of possible waste. In this context, the use of

cleaner catalytic reactions can lead to a substantial reduction of the amount of toxic

chemical pollution by reducing the formation of by-products [13].

At present, it is well known that a heterogeneous catalyst must have three characteristics:

high activity, high selectivity and high stability. Thus, the development of a new generation

of solids catalysts is a subject of increasing interest in the manufacture offine chemicals,

refinery operations as well as in the domain of energy and environmental protection [14]. In

the past few years, the design of materials with a controlled porosity or possessing

4
hierarchically texture has garnered a great interest due to their wide applications in a

number of fields, especially heterogeneous catalysis [15].

Hydroxyapatite (HAP) is a catalyst that may address the aforementioned needs. Indeed,

HAP is a material of varying properties depending on its mode of preparation. Phosphate

comprising the ideal formula of Ca10(PO4)6(OH)2 belongs to the apatite family; HAP is one of

the most usual forms of calcium phosphate. The special attribute of this structure lies in its

ability to form solid solutions and to accept a large number of anionic and cationic

substituents. The variability of properties in HAP enables the possibility to use the material

in diverse applications. For instance, due to HAP’s similarity in chemical composition to the

mineral phase of bone tissues, it is known for its applications in medicine as synthetic bone

substitutes [16]. In addition to its biological importance, HAP is studied for various

applications such as fluorescent lamps [17], material for fuel cell [18], or an adsorption and

stabilization matrix for radioactive waste and harmful metals [19]; significant catalytic

activities of HAP are subject matter of numerous publications and patents [20,21].

This present review aims to provide insight in the field of synthesis and the catalytic

applications of HAP. The reason for our efforts is due to its wide accessibility to chemical

community, including non-specialists in material science area. The first section of the review

gives an overview of the different synthetic routes that were applied to the synthesis of the

HAP. The second section is devoted to structure-reactivity studies of HAP related to its

catalysis performances. The main section of this review covers the important applications of

the HAP in catalysis.

2. Principal methods for the synthesis of HAP

The synthesis of HAP, with its various structures, morphologies and textures, has

stimulated a great deal of interest in academic and industrial research for numerous
5
heterogeneous catalysis applications. In the past thirty years, a number of synthetic routes

for producing HAP powders have been developed. Production of HAP powders are classified

under four different methods: i) dry methods, ii) wet methods, iii) microwave (MW)-assisted

methods, ball-millingor ultrasound and iv) miscellaneous methods. In each category, there

are several variations depending on the conditions of synthesis and reagents deployed. In

this part of review, the most important synthetic methods used to obtain HAP powder are

documented.

2.1. Dry methods

The dry route preparation of HAP is based on the heat treatment of finely ground mixed

precursors. Extensive research on this method has shown that variation of some factors can

influence the ensuing results. The essential requirement for dry route method is to have a

mixture that is perfectly homogenous for completion of the reaction. The purity of the final

product is dependent upon precise weighing procedures during preparation; the formation

of a very stable intermediate phase can occur. These variable factors can potentially limit the

formation of the final desired compound. Tromel et al. identified the optimum conditions for

the formation of HAP via the calcination of the mixture of reagents at 1050 °C in air (Scheme

1) [22,23].

Scheme 1. Dry synthesis route to HAP [22].

In general, the reactions in the solid state usually yield well-crystallized stoichiometric

products. Adversely, this method requires a relatively high temperature which can penalize

in terms of the porosity of product [24,25].

6
2.2. Wet methods

Wet methods comprising double decomposition or co-precipitation, emulsion, hydrolysis

method, sol-gel method and hydrothermal approach, are widely used duetothe simplicity of

the procedures. These methods allow for perfect control over the structure, texture and

morphology, and leads to a high yield of HAP. The wet methods can be performed in water,

or in organic solvents by several reactions involving diverse reagents and auxiliary additives

and apparatus. They can be performed at ambient temperature or elevated temperatures,

under atmospheric pressure or under high pressures. The major drawback of the wet

methods is that they sometimes give rise to structures that are not crystallographically pure;

other phases of phosphates are present with HAP. In addition, various ions in aqueous

solution can be incorporated into the crystal structure, leading to trace impurities.

2.2.1. Co-precipitation method

Co-precipitation is the most straightforward and frequently used method for the

preparation of HAP. This chemical process consists of a reacting source of PO43- ligand with a

source of calcium in the presence of other additives (e.g., base or acid) [26-28]; several

sources of these two reagents are used. The conditions of the co-precipitation method are

variable, but in general, this process is usually carried out at pH values ranging from 3 to 12

and at temperatures ranging from room temperature to the boiling temperature of water. In

addition, this method is sometimes performed in the presence of templates [29-23].

2.2.2. Sol–gel method

The sol-gel process is a method of mineralization from precursors in a solution, preferably

organometallic compounds or other suitable precursors. This exemplary method for the

synthesis of porous HAP occurs under conditions called soft-chemistry conditions that are

significantly lower than those of conventional synthetic routes in terms of temperature. The

7
sol-gel process has limitations that hinder its expansion into industrial scale production. The

main obstacles are: i) the high cost and scarcity of often used alkoxides-based precursors

and ii) the delicate process control culminating inusually time consuming processes. This

protocol involves hydrolysis of the precursors and the formation of micelles around

templates in either an aqueous or an organic phase followed by the polycondensation of

these species via formation of a 3D inorganic network [34,35]. However, the rate of gel

formation depends strongly on: i) the nature of the solvent; ii) the temperature and pH used

during the process and iii) the chemical nature of the reagents used. In addition, lack of

control of certain parameters during the growth of HAP may cause the appearance of

secondary phases such as CaO, Ca2P2O7, Ca3(PO4)2 and CaCO3 [36,37]. It should be noted that

a non-alkoxide sol-gel process for synthesis of HAP, without need to adjust the pH, has also

been developed; only the conventional sources of calcium and phosphate have been used in

these methods [38-40].

2.2.3. Emulsion methods

Among methods developed for the synthesis of HAP, the emulsion process is considered

to be more efficient, simple and suitable for producing a nano-structured HAP powder [41-

43] allowing precise control of the morphology and distribution of grain size. In general, this

technique was originally developed to create a porous material as well as to overcome the

issue of particle agglomeration. Several sources of calcium and phosphate have been tested,

but most often calcium nitrate and phosphoric acid are used [44] in terms of the reports of

reaction media, more organic solvents are blended with water. Among the surfactants used

to prepare the emulsion, some examples include: dioctyl sodium sulfosuccinate salt, dodecyl

phosphate, polyoxyethylene(5) nonylphenol ether, polyoxyethylene(12) ether nonylphenol,

polyoxyethylene and polysorbate 80, cetyltrimethylammonium bromide and sodium dodecyl

8
sulfate [45]. The main synthesis parameters focus on the type of surfactant, ratio of aqueous

and organic phases, pH and temperature.

2.2.4. Hydrolysis methods

The aqueous hydrolysis of calcium phosphates that forms HAP crystallites usually follows

two stages dissolution and precipitation [46-49]. Many calcium phosphates phases are used

as precursors to prepare HAP especially octacalcium phosphate, dicalcium phosphate

dihydrate and tricalcium phosphate [50,51]. The hydrolysis process applicable to these

precursors depends strongly on pH and temperature. The additions of other calcium and

phosphate sources are sometimes required in the order to obtain the stoichiometric HAP the

porous texture is dependent upon these parameters.

2.2.5. Hydrothermal methods

The hydrothermal process is a rathermature technique for the growth of crystalline HAP

[52-54]. This process is the generic term used to describe a reaction between the calcium

source and the phosphate precursor in the presence of the following: i) water or organic

solvent, or ii) a mixture of water/organic solvent (water: hydrothermal, organic solvent:

solvothermal, water/organic solvent: solvo-hydrothermal). The process occurs in a confined

environment with a higher temperature and pressure greater than autogenously ambient

pressure, inside an autoclave or a pressure vessel; the medium could be subcritical or

supercritical, depending on the pressure and temperature. Through the effect of

condensation and an increase of reactivity, and depending on the value of pressure and

temperature, the hydrothermal method creates chemical bonds and formation of nuclei that

ensurein a relatively stoichiometric and highly crystalline synthesis of HAP [55-58]. It should

be noted that high pressure permits the formatting of HAP in the form of micro or nano-

sized crystallites, with controlled morphology and porosity. The hydrothermal method

9
serves to modulate the interactions between solid/solvent, especially in terms of solubility

and also functions as a conduit in the control of the nucleation and growth processes

[59,60]. Moreover, this technique is often combined with conventional methods such as co-

precipitation or sol-gel protocols.

2.3. Alternate energy input methods

These assisted methods are often combined with other conventional methods to create a

synergy between the different techniques to increase the yield in an expeditious fashion

[61-66].

2.3.1. Microwave (MW)-assisted methods

Due to enormous progress in this field of alternative heating technique, there has been a

continued interest in methods involving MW activation. The MW-assisted preparation of

HAP produces an increased yield of perfectly crystalline powder that is particularly

homogeneous in terms of size, porosity and morphology [67-71]. The activation results from

two contributing factors: i) a purely thermal origin, resulting inmolecular agitation that is

caused by the inversion of dipole with the exteremely rapit alternations of the electric field,

and ii) an electrostatic origin, involving interactions like dipole-dipole between polar

molecules and the electric field. This has a direct effect on the kinetics of the decrease of the

activation energy. Our research group has developed a technique that uses sol-gel followed

by a hydrothermal treatment assisted by MW irradiation [72,73].

2.3.2. Ball-milling method

The ball-milling method, also termed mechanochemical process, has been widely used to

synthesize HAP with the advantages such as simplicity, reproducibility, and large-scale

production of HAP [74,75]. The control of the growth of HAP by this technique closely

depends on the type of reagents used, the milling medium, the diameter of the milling balls,

10
the type of atmosphere, the duration of the milling steps and interval pauses, the powder-

to-ball mass ratio and the rotational speed [76-79].

2.3.3. Sonochemical method

The sonochemical approach is based on the reactions activated by powerful ultrasound

radiation [80-82]. This method invariably delivers nanosized products and elicits perfect

control of morphology, porosity and size [83,84]; enhanced stimulation of the reaction

between the calcium and phosphate precursors accelerate the rate of reaction in a

remarkable manner [85,86].

2.4. Other methods

Bio-inspired chemistry is fields of chemistry that is aimed at creating materials that

mimic efficient biological systems by adopting their functional expressions [87-90].

Developed under the realm of bio-inspired chemistry, these two methods to prepare HAP

are combustion and pyrolysis or spray pyrolysis; the combustion process is a promising

method for preparation of HAP [91-93]. The fundamental principle of this method comes

from the thermochemical concepts used in the field of propellants and explosives chemistry

exothermic and self-sustaining redox reaction occurs in an aqueous phase between

precursors and a suitable organic fuel (e.g., glycerin, urea, sucrose, citric acid, and succinic

acid) [94-97]. The pyrolysis process consists of evaporated liquid reagents (precursors and

calcium phosphate) [98-100] and involves the spraying of the precursor solutions into a

flame or a hot zone of an electric furnace using an ultrasonic generator [101,102].

3. Structure-reactivity studies of hydroxyapatite related to catalysis

performances
HAP belongs to a large family of isomorphic compounds and is one of the most common

forms of calcium phosphate. HAP crystallizes in the hexagonal system (space group P63/m)

11
with the following crystallographic parameters: a = 9.418 Ǻ, c = 6.881 Ǻ, β = 120 ° C (JCPDS

No. 9-432) [103]. The crystalline network of the stoichiometric HAP can be described as a

compact assemblage of tetrahedral PO4 groups, where P5+ ions are in the center of the

tetrahedrons and whose tops are occupied by 4 oxygen atoms. Each PO4 tetrahedron is

shared by a column and delimits two types of unconnected channels (Fig. 1). The first

channel has a diameter of 2.5 Å and is surrounded by Ca2+ ions, denoted Ca(I) (4 per unit

cell). They are in coordination 9 with the oxygen atoms of the PO4 tetrahedrons resulting in

the formation of a polyhedron as shown in Fig. 1a. The second type of channel plays an

important role in the properties of apatites. It has a diameter larger than the previous one

(3~4.5 Å), and contains six other Ca2+ ions, referred to as Ca(II). The latter are located at the

periphery of the channel. These ions are located two dimensions 1/4 and 3/4 of the unit cell

along c axis and form alternate equilateral triangles around the helicoidal senary axis. Their

coordination is 7, and they are surrounded by six atoms of oxygen belonging to [XO4]

tetrahedron and one OH- anion in position 2a (Fig. 1b) [103]. The existence of two different

calcium sites is of special interest because the properties of HAP can be tuned by specific

modification of the site [104]. These channels host OH- groups along the c axis to balance the

positive charge of the matrix. The OH- ions are presentin columns perpendicular to the unit

cell face, at the centerof the large channels type II. The oxygen present in the hydroxyl group

is located at 0.4 Å out of the plane formed by the calcium ion, and the hydrogen of the

hydroxyl group is located at 1 Å, which is almost on the triangle plane of calcium. The

dimension of the tunnel endows certain mobility to these ions and consequently allows their

circulation along the tunnels in the direction of OZ axis (Fig. 1c). The HAP is a highly non-

stoichiometric calcium phosphate compound with a Ca/P molar ratio ranging from 1.50 to

1.67 [105]. The Ca/P molar ratio of a stoichiometric form of hydroxyapatite is 1.67. The

12
preparation of the non-stoichiometric hydroxyapatite can be rationalized by the fact that the

loss of Ca2+ ions and the ensuing electrical imbalance are corrected by the introduction of H+

ions and depletion of OH- ions, denoted by the formula Ca10-Z(HPO4)Z(PO4)6-Z(OH)2-Z; 0<Z≤1

[106]. Moreover, the environment surrounding the OH- sites is very attractive for

substitutions as it enables one to control the ratio between acid-base sites. With a Ca/P ratio

of 1.50, HAP acts as an acid catalyst with the existence of basic sites. In contrast, with a Ca/P

ratio of 1.67, HAP acts as a basic catalyst while acid sites are still present [107].

Fig. 1. (a) Projection of the unit cell of HAP according to plan (001); (b) projection showing

the arrangement of octahedrons [Ca(1)O6] in the HAP structure; (c) projection showing the

sequence of octahedral [Ca(1)O6] and tetrahedral [PO4] in the HAP structure; and (d)

projection showing the sequence of octahedral: [Ca(1)O6] and [Ca(2)O6], and also tetrahedral

[PO4] in the HAP structure.

13
 However, a key element of the apatite structure is that it allows a great number of

substitutions that leave the crystallographic structure unchanged. Table 1 shows some

examples of possible substitutions in consideration of the following general chemical

formula: Me10(XO4)6(Y)2, wherein Me is a monovalent, divalent or trivalent cation (alkaline

earth metal, d-block elements, alkali metal, or lanthanide,...), XO4 a trivalent anion (PO43-,

AsO43-, ...) and Y a monovalent anion (OH-, F-, ...). XO43- groups may also be substituted by

divalent or tetravalent groups, but the existence of apatite containing XO43- deficient sites

has never been reported. The second anionic Y site can also be occupied by monovalent

ions, divalent. The possibility of substitution allows the existence of non-stoichiometric

hydroxyapatites. Non-stoichiometry results in: i) the presence of a vacancy defect in cationic

site and OH-, ii) a state of crystallinity that is especially poor the further hydroxyapatite is

from stoichiometry, and iii) more solubility with increasing distance from stoichiometry.

Table 1

Examples of substitutions in the apatite structure [ref]

Me2+ XO43- Y-
Mg2+ Cd2+ PO43- SiO43- AsO43- OH- F- Cl-
Sr2+ Cu2+
Pb2+ SO43- MnO43- VO43- Br- I-
Ba2+ Zn2+ CrO43- CO32- HPO42- S2- O2- CO32-
Na+ K+ Eu3+

4. Catalytic applications of hydroxyapatite

4.1. Cross-Coupling reactions
Carbon-carbon cross-coupling reactions are among the most important synthetic

transformations developed in the 20th century [108-111]. These reactions are often catalyzed

using homogeneous catalytic systems because of their high reactivity, higher turnover

numbers, and notably the possibility of coupling the widely available and relatively low-cost

aryl chlorides [112-117]. However, these traditional systems continue to pose difficulties

14
related to product purification and toxic waste produced after separation of palladium (Pd)

catalyst. One of the most favorable ways to overcome these issues is the use of

heterogeneous catalysis because it offers a number of advantages namely excellent stability,

easy separation from the reaction mixture by filtration or decantation, recyclability often

with minimal loss of activity, and the wide accessibility of support [118-120]. Relatively few

studies have reported the use of hydroxyapatite as catalysts for cross-coupling reactions. In

this context, Kaneda and colleagues reported the preparation of two types of PdHAP by

selecting appropriate Ca/P molar ratios to give the stoichiometric Ca10(PO4)6(OH)2 (Ca/P =

1.67, HAP-0) and the non-stoichiometric Ca-deficient hydroxyapatite Ca9(HPO4)(PO4)5(OH)

(Ca/P = 1.50, HAP-1) [121]. The treatment of the HAP-0 with PdCl2(PhCN)2 in acetone

afforded the HAP-bound Pd complex PdHAP-0, while the PdHAP-1 complex was obtained

using HAP-1 by the same method. The PdHAP-1 was found to be an outstanding catalyst for

Suzuki-Miyaura cross-coupling reaction of activated and deactivated aryl bromides, with

phenylboronic acid using K2CO3 as the base and o-xylene as the solvent at 120°C (Table 2).

Additionally, the catalyst could be recycled, and there was not any Pd leaching observed.

Table 2

Coupling of aryl bromides with phenylboronic acid catalyzed by PdHAP-1 [121]

Entry R1 Time (h) Yield (%) TON

1 H 4 80 40000

2 OMe 6 91 45500

3 (CO)Me 4 94 47000

15
 In a subsequent study, the same research group expanded the promising initial results of

the application of the PdHAP-1 to Suzuki–Miyaura coupling reactions of aryl chlorides [122].

Under the various conditions explored, the prepared catalysts proved effective in coupling

reactions of aryl chlorides bearing electron-withdrawing substituents with phenyl boronic

acid (Table 3). However, chlorobenzene was not active and only a moderate yield was

obtained, even when the catalyst loading was increased to 0.3 mol %. The PdHAP-1 catalyst

system exhibited higher catalytic activity compared to other conventional heterogeneous

catalysts such as Pd/Carbon, Pd/Al2O3, and Pd/SiO2. It is interesting to note that for the

chloride substrates, the addition of tetrabutylammonium bromide and a small amount of

water was required for better conversion.

Table 3

Coupling of aryl chlorides and phenylboronic acids using PdHAP-1 [122]

Entry R Time (h) Yield (%) TON

1 H 5 30 100

2 CHO 2 70 233

3 (CO)Me 1 92 613

4 NO2 1 > 99 > 660

The same catalytic system was employed to investigate the Mizoroki-Heck reaction of aryl

bromides with styrene and n-butyl acrylate. In the presence of 0.002 mol% of Pd catalyst,

the best results were obtained with K2CO3 as the base in N-methylpyrrolidone (NMP) at

130°C (Table 4). In addition, the recycling of the PdHAP-1 was briefly investigated in the

16
coupling of bromobenzene with styrene. The results showed that the catalyst could be

recycled at least three times without any loss of activity. Unfortunately, the Heck reaction of

aryl chlorides was not reported for this catalytic system under these conditions.

Table 4

Heck reactions of aryl bromides witholefins catalyzed PdHAP-1 [122]

Entry R1 R2 Time (h) Yield (%) TON

1 H Ph 24 94 47000

2 OMe Ph 24 90 45000

3 (CO)Me Ph 20 96 48000

4 H (CO)O-nBu 20 91 45500

5 OMe (CO)O-nBu 24 94 47000

6 (CO)Me (CO)O-nBu 20 98 49000

Paul and colleagues reported the preparation of HAP-supported palladium particles

(average size = 20 nm) by treating hydroxyapatite and Pd(OAc)2 in ethanol, followed by

reduction with hydrazine hydrate. The ensuing material was investigated as a catalyst and its

application in Suzuki-Miyaura coupling reactions [123]. Using 0.33-0.55 mol% of Pd catalyst

and K2CO3 as the base in the presence of TBAB as phase-transfer agent, this catalyst

provided good yields for the synthesis of biaryls, polyaryls and heteroaryls by coupling aryl

bromides with arylboronic acids (Scheme 2). The catalyst was recyclable over at least five

cycles without any apparent deactivation in the coupling of 4-bromoacetophenone with

phenylboronic acid. This result demonstrated the chemical stability of the HAP-supported Pd

17
catalyst. In a scale-up study, 9 g of 4-acetylbiphenyl was isolated in a 92% yield starting from

a mixture of 50 mmol of 4-bromoacetophenone, 75 mmol of phenylboronic acid, 12 g of

catalyst, 50 mmol of TBAB, and 150 mmol of K2CO3,in 250 mL of H2O during 7.5 hours at

100°C. Unfortunately, additional information concerning the Pd leaching and the nature of

active species were not reported for this system. A study on a more diverse array of

substrates would be necessary to determine the scope of this catalyst.

Scheme 2. Selected examples of Suzuki-Miyaura reaction using Pd/HAP as catalyst [123].

In a recent study, Masuyama and colleagues prepared PdHAP by introduction of Pd via

ion-exchange with calcium in HAP matrices [124] where in Pd could be firmly supported by

the chelation of phosphate moieties of HAP, which suppresses the leaching of the Pd(II) or in

situ reduced Pd(0) into the reaction medium. The performance of this heterogeneous

catalyst was evaluated in Suzuki–Miyaura-type cross-coupling reaction of aryl bromides and

potassium aryltrifluoroborates, in the presence of triphenylphosphine and K2CO3 as a base,

at 50°C using methanol as the solvent (Scheme 3). The catalyst was shown to provide the

desired product in good to excellent yields using water as the solvent, but with slower

kinetics compared to methanol as the solvent. The recyclability of this catalyst was examined

in the coupling of ethyl 4-bromobenzoate or 4-bromoanisole with potassium

phenyltrifluoroborates. The catalyst was found to be recyclable ten times with only a slight
18
loss of activity for the reaction of ethyl 4-bromobenzoate; its activity began to drop

appreciably after the sixth cycle for the reaction of 4-bromoanisole.

Scheme 3. PdHAP-catalyzed Suzuki-Miyaura-type cross-coupling reactions of aryl bromides

and potassium phenyltrifluoroborates [124].

Despite being outside of the main scope of the present review, the first example of the

exploration of highly basic fluorapatite as support in Suzuki-Miyaura reactions by Kantam

and colleagues is worth mentioning [125]; they synthesized a fluorapatite-supported Pd

catalyst by treatment of fluorapatite with PdCl2(PhCN)2 in acetone. Under optimized

conditions that involve 0.15 mol% of the palladium catalyst, Na2CO3 as the base, and MeOH

as the solvent, aryl iodides and bromides were successfully coupled with aryl boronic acids at

room temperature. However, the coupling of the widely available and low-cost aryl chlorides

with phenylboronic acid required higher PdFAP loading, elevated temperatures and the use

of TBAB as an additive (Table 5). The heterogeneous Pd catalyst could easily be recovered by

simple filtration; it could also be reused at least three times with consistent activity.

More recently, the synthesis of a new HAP supported Pd catalyst and its use as efficient

catalyst in the Suzuki-Miyaura cross-coupling reactions in water has been reported [126].

This heterogeneous catalyst was obtained by immobilizing dichloro(1,5-

cyclooctadiene)palladium on the HAP. By using this catalyst, electron-rich, electron-neutral,

electron-poor and sterically hindered aryl boronic acids were efficiently coupled with aryl

halides containing electron-donating and electron withdrawing substituents. This included

19
aryl chlorides in the presence of potassium carbonate as the base and tetrabutylammonium

bromide as the phase-transfer agent (Scheme 4). The catalyst could be recycled several

times without any detectable deactivation or leaching of the catalyst. Heterogeneity tests

clearly indicated that the reaction occurred at the surface of the support through a truly

heterogeneous catalysis. Importantly, under similar reaction conditions, this catalyst

exhibited superior catalytic properties compared to catalyst prepared by reduction of HAP

supported Pd(2+) catalyst with sodium borohydride in ethanol.

Table 5

Suzuki–Miyaura coupling of aryl chlorides using PdFAP catalyst [125]

Entry R Time (h) Yield (%) TON

1 NO2 6 94 335

2 CN 6 92 328

3 (CO)Me 6 94 335

4 H 30 45 160

5 OMe 30 32 114

Scheme 4. Suzuki coupling of chloroarenes with arylboronic acids catalysed by Pd(0)HAP [126].

20
4.2. Condensation reactions
4.2.1 Knoevenagel reaction

Recently, solid base materials have attracted increased interest due to their potential

application as environmentally friendly catalysts in a number of important industrial

reactions, especially fine in chemical synthesis [127-129]. In this context, Knoevenagel

condensations of aldehydes with active methylene compound are among the most

important processes in organic chemistry, ranging in use from the synthesis of small

molecules, to the elegant intermediates for anti-hypertensive drugs and calcium antagonists

[130-132]. These reactions allow the synthesis of cinnamic acid, in which its ester and

carboxylic functional derivatives are very important components in flavors, perfumes,

synthetic indigo and pharmaceuticals. These reactions were mostly carried out by

homogeneous catalytic systems using weak bases like primary, secondary, tertiary amine

and ammoniums salts and Lewis acids [133-138]. However, these catalysts have numerous

disadvantages such as waste production, corrosion and lack of catalyst recovery. As a result

of these shortcomings, other heterogeneous catalysts have been developed in recent years

[139-143]. Boulaajaj and colleagues investigated the Knoevenagel reaction by using

hydroxyapatite at room temperature in the absence of any solvents and reported the use of

hydroxyapatite as a heterogeneous catalyst [144]. The HAP catalyst was able to promote the

reaction, although longer reaction times were required. By adding water and

benzyltriethylammonium chloride (BTEAC) separately, there was a significant improvement

in the scope and mildness of this catalyst system; excellent yields were obtained by

simultaneous addition of these components (Tables 6 and 7). It is believed that the addition

of water dissolved the ammonium salt and facilitated its interaction with HAP, which

therefore allowed the activation of catalyst. This heterogeneous catalyst can be readily

21
recovered by filtration and subsequently calcined at 700°C before being reused nine times

without marked loss of catalytic activity.

Table 6

Synthesis of alkenes via Knoevenagel condensation catalyzed by HAP [144].

Entry Solvent Time (h) Yield (%)

1 No solvent 15 24

2 H2O 10 83

3 BTEAC 3 87

4 H2O/BTEAC 3 96

Table 7

Synthesis of alkenes via Knoevenagel condensation catalyzed by HAP [144].

Entry Solvent Time (h) Yield (%)

1 No solvent 30 20

2 H2O 30 22

3 BTEAC 20 37

4 H2O/BTEAC 20 64

22
 Another independent study examined Knoevenagel condensation using HAP prepared

by the co-precipitation method [145] where methanol was found to be the best solvent

among those that were surveyed. The catalyst displayed superior over fluorapatite in the

condensation of benzaldehyde with methylcyanoacetate as a model reaction; doping of HAP

with potassium fluoride was beneficial for the rate and reaction yield (Tables 8 and 9). It was

shown that the reactions took place at the surface of the catalyst in heterogeneous media

and the catalyst could be reused, washed, and dried after filtration; catalyst maintained

good catalytic activity after three runs.

Table 8

Knoevenagel condensation catalyzed by modified HAP [145].

Entry Catalyst Yield (%)

1 KF 5

2 FAP 60

3 HAP 50

4 KF/FAP 80

5 KF/HAP 91

Table 9

Knoevenagel condensation catalyzed by modified HAP [145].

23
 Entry Catalyst Yield (%)

1 KF 17

2 FAP 13

3 HAP 25

4 KF/FAP 79

5 KF/HAP 95

In a similar study, the Knoevenagel condensation of benzaldehyde with ethyl

cyanoacetate at room temperature was reported, in the presence of as-synthesized and

aluminum-enriched HAP as catalysts [146]. The latter exhibited significantly enhanced

catalytic activity which was ascribed to an increased surface area and the presence of HPO42−

species. The presence of HPO42− species can lead to the formation of Al3+–O2− acid–base pair

sites by the interaction between Al of AlPO4 with the OH group of HPO42−.

Ionic liquids recently emerged as a promising media for homogeneous catalysis and as

well as catalysts. These new compounds have unique physico-chemical properties including

negligible vapor pressure, wide liquid range, higher ionic conductivity and excellent solubility

[147-150]. Xia and colleagues took an interesting approach in their study by describing the

synthesis of a series of imidazole-based ionic liquid with various alkyl-chain lengths grafted

on γ-Fe2O3, encapsulated in a hydroxyapatite [151]. These basic magnetic nanopaticles were

used as efficient and recyclable heterogeneous catalysts for Knoevenagel condensation

between various aldehydes and malononitrile, and were put under mild conditions in an

aqueous environment. XRD, XPS, and TEM measurements confirmed the presence of 1-3 nm

24
γ-Fe2O3 particles in the HAP matrix. This heterogeneous catalyst showed higher catalytic

activity than the carrier HAP-γ-Fe2O3 and the homogeneous basic ionic liquid [Bmim]OH

(Bmim = 1-n-butyl-3-methylimidazolium). The high catalytic activity was attributed to the

cooperation between the base sites generated by HAP framework and supported basic ionic

liquids; activity decreased gradually in the sequence of CH3> C4H9> C8H17> C16H33. This

decrease of activity could be ascribed to the low accessibility to the basic centers with an

increase of steric hindrance, as well as a lower loading of basic centers (Scheme 5).

Scheme 5. Knoevenagel reaction catalyzed byionic liquids functionalized HAP-γ-Fe2O3 [151].

The catalyst recovery was quantified by simple magnetic decantation using a permanent

magnet. The catalyst was reused several times and remained consistent even after the

fourth cycle. Similarly, Xia and colleagues also reported the synthesis and catalytic activity of

the HAP-encapsulated γ-Fe2O3 nanoparticles functionalized with diethyl aliphatic amine basic

ionic liquids [152]; good to high yields were obtained in the condensation of various

aldehydes with malononitrile in water at 30°C (Table 10). Importantly, this catalyst was

found to be superior to homogeneous basic ionic liquids and the basic ionic liquid-modified

polystyrene resin. This excellent catalytic activity was attributed to the cooperativity

between the base sites generated by framework HAP and the supported basic ionic liquids.

25
In addition, the results revealed that catalyst could be recoveredand recycled by simple

magnetic decantation, and it could be reused for more than four consecutive trials without

significant loss of catalytic activity.

Table 10

Knoevenagel reaction facilitated by ionic liquids functionalized HAP-γ-Fe2O3 [152].

Entry R1 R2 T (°C) Time (h) Yield (%)

1 CN (CO)OEt 50 3 88

2 (CO)OEt (CO)OEt 50 8 50

3 (CO)Me (CO)Me 80 9 91

4 (CO)Me (CO)OEt 80 10 86

Benhida and colleagues reported the use of porous calcium HAP as recoverable

heterogeneous catalyst [153] using MW irradiation. The catalyst calcined at 300°C provided

good yields in Knoevenagel condensation for a range of aldehydes that incorporated

electron-donating and electron withdrawing substituents, with malonitrile and ethyl

cyanoacetate (Scheme 6). This catalyst exhibited superior catalytic properties compared to

the catalysts calcined at 100, 800°C and KF/Al2O3 a presumably influenced by the specific

26
surface area and the chemical nature of the solid surface; catalyst could be recovered by

simple filtration and reused for at least ten reaction cycles without loss of activity.

Scheme 6. Knoevenagel condensation using HAP under microwave heating [153]

HAP analogues of the formula NaLaCa3(PO4)3OH and NaLaSr3(PO4) have been synthesized

to evaluate catalytic activity in the Knoevenagel condensation (Table 11) [154] which were

found to be stable but less active. The addition of water led to the activation of these

catalysts and the enhanced catalytic activity. An increase of OH- sites ions on the surface

may have been the cause of enhanced catalytic acitvity. Unfortunately, recycling

experiments were not reported in this study and a more diverse array of substrates is

necessary to determine the further scope of these catalysts.

Table 11

Knoevenagel reaction catalyzed by apatite phosphates containing heterovalent cations [154]

Entry Catalyst Solvent Time (h) Yield (%)

1 NaLaCa3(PO4)3OH neat 30 29

2 NaLaCa3(PO4)3OH H2O 30 42

27
 3 LaSr3(PO4)3OH neat 30 22

4 LaSr3(PO4)3OH H2O 15 41

Jonnalagadda and colleagues reported the use of cobalt HAP as catalyst, under solvent-

free conditions (Scheme 7) [155] which provided products in high yields and with facilitated

recovery and reuse. However, deactivation was observed possibly due to the loss of catalyst

during recovery and the poisoning of active sites on the surface of catalyst via interaction

with other products.

Scheme 7. Solvent-free Knoevenagel reaction catalyzed by cobalt HAP [155].

4.2.2 Claisen-Schmidt reaction

The preparation of chalcones is of interest in organic chemistry due to its wide presence

in many bioactive natural products [156-159] as they are important intermediates in the

biosynthesis of flavonoids and flavones, which are present in the soy beans and soy derived

products [160-162]. Claisen-Schmidt condensation is a useful method for the generation of

chalcones from arylaldehydes and acetophenones, using various homogeneous catalysts.

However in recent years, many heterogeneous catalysts have been used to promote this

reaction [163-166]. The research group of Sebti reported an efficient and convenient route

to the heterogeneous synthesis of several chalcones using NaNO3/HAP [167]. The catalyst

was prepared by adding HAP to an aqueous solution of sodium nitrate, followed by stirring

28
and calcination at 900°C. The XRD analysis showed the apparition of news phases; one phase

observed was probably NaCaPO4. The best results were obtained with NaNO3/HAP = ½ and

used methanol as a solvent. The addition of benzyltriethylammonium chloride was beneficial

for the reaction yield (Table 12). It is noteworthy that catalytic activity decreased

progressively when the recovered catalyst was dried at 150°C.

Table 12

Claisen-Schmidt condensation catalyzed bymodified HAP [167].

Entry Catalyst Time (h) Yield (%)

1 HAP 12 7

2 NaNO3/HAP 6 92

3 NaNO3/HAP/BTEAC 2 95

The same group described the synthesis of chalcones derivatives via Claisen-Schmidt by

using the HAP under MW irradiation (Scheme 8) [168]. The solvent had a pronounced effect

in these reactions, of which water proved to be the best solvent; DMF, methanol and

ethanol produced moderate yields. According to the study, the water acted as a co-catalyst

at a specific volume, however, the increase of its quantity led to a decrease of the catalytic

activity of the HAP which may be due to the formation of a water-film on the active surface

of the catalyst, affecting its hydrophobicity. The study did not explain the precise role of the

activation of recovered catalyst via calcination at various temperatures with regard to the

activity of the catalysts.

29
 Scheme 8. Synthesis of chalcones using HAP under microwave irradiation [168].

4.2.3 Michael addition

Michael addition is one of the most useful methods for the formation of carbon–carbon

bonds in organic chemistry under mild conditions [169-171]. Various heterogeneous

catalysts have been developed [172-175] to replace soluble strong bases that are

traditionally used for this reaction, which can then lead to side reactions like auto-oxidation

or retro-Michael type decompositions [175-178]. HAP prepared by the co-precipitation

method was found to be highly efficient catalysts for Michael addition of mercaptans and

chalcone derivatives (Scheme 9) [179] in ethanol and methanol were the best solvents

among those surveyed. Zahouily and colleagues indicated that the reaction occurred at the

surface since the dimensions of the tunnels of the catalyst were not large enough, and

therefore inaccessible to reactants. The acidic character of HAP probably induced the

polarization of the C=O bond, and the basic sites probably helped enhance the

thiolnucleophilicity. Consequently, the S–C bond formation accelerated and the final product

was obtained after protonation of the resulting enolate. This catalyst exhibited a higher

efficiency compared to natural phosphate, synthetic diphosphate, Na2CaP2O7 and

fluoroapatite; it showed slightly lower efficiency than potassium fluoride doped with natural

phosphate. Moreover, the catalyst was quantitatively recovered by simple filtration, calcined

at 500°C and reused for seven cycles with an almost consistent activity.

30
 Scheme 9. Michael reaction using HAP as catalyst [179].

The same research group reported that HAP modified by sodium nitrate, which was found

to be effective for selective hydration of nitriles, can also be applied to Michael-type

addition of aliphatic and aromatic amines with α,β-unsaturated carbonyl compounds at

room temperature [180]. In this study, the NaHAP was more active for the reactions in

methanol than HAP alone although author’s did not explain the cause of the difference in

reactivity (Table 13). The catalyst was recycled, washed with dichloromethane, and calcined

at 900 °C.

Table 13. Formation of carbon-nitrogen bond using of HAP and NaHAP catalysts [180].

Entry R1 R2 Catalyst Time (h) Yield (%)

1 H C6H5 HAP 24 30

2 H C6H5 NaHAP 1.5 99

3 Cl C6H5 HAP 24 80

4 Cl C6H5 NaHAP 6 96

5 OMe C6H5 HAP 24 2

6 OMe C6H5 NaHAP 4 96

31
 7 Me C6H5 HAP 24 78

8 Me C6H5 NaHAP 6 90

9 H 4-MeOC6H5 HAP 24 42

10 H 4-MeOC6H5 NaHAP 1 98

11 Cl 4-MeOC6H5 HAP 24 44

12 Cl 4-MeOC6H5 NaHAP 8 70

13 OMe 4-MeOC6H5 HAP 24 20

14 OMe 4-MeOC6H5 NaHAP 7 92

15 H C6H5CH2 HAP 48 62

16 H C6H5CH2 NaHAP 5 94

17 Cl C6H5CH2 HAP 48 85

18 Cl C6H5CH2 NaHAP 5 92

The β -amino carbonyl compounds synthesized by HAP-modified sodium nitrate were

successfully used as a direct precursor for synthesized β-amino acids (Scheme 20). For

example, the 1,3-diphenyl-3-(phenylamino)propan-1-one was synthesized by a reaction of

aniline with 1,3-diphenyl-2-propenone, which then was converted to oxime in the presence

of NaHAP with an excellent yield. The latter was subjected to the reaction induced by thionyl

chloride and chemoselective Beckmann rearrangement to provide the N-phenylamino-3-

phenyl-3-(phenylamino)propan-1-one. This produced the 3-phenyl-3-(phenylamino)-

propanoic acid after esterification reaction, followed by hydrolysis.

32
 Scheme 10. Synthesis of β -amino acidusing modified HAP as catalyst [180].

A similar study reported the preparation of zinc bromide supported on HAP, and its

application as an efficient heterogeneous catalyst to promote the synthesis of 3-substituted

indoles from Michael addition toindoles to α,β-unsaturated ketones (Table 14) [181]; the

substitution of the indole nucleus occurred exclusively at the 3-position without the

formation of N-alkylation products. Unfortunately, the study did not report any information

regarding leaching or mechanisms.

Gruselle and co-workers reported thatthe calcium hydroxyapatite with different

compositions and various specific surface areas was highly active in the Michael reaction of

ethyl 2-oxocyclopentanecarboxylate with 3-buten-2-one, in the absence of solvents [182];

products were obtained with 90% conversion at 65°C, regardless of its different

stoichiometries and specific surface areas. Using deuterium-labeling experiments to explain

33
the origin of the basicity of this catalyst, the authors proposed a mechanism based on the

basic properties of the calcium HAP surfaces. The NMR studies of the reaction products

showed that part of the deuterium atoms had been transferred from the partially

deuterated apatite to the methylene group, which is adjacent to the ketone function of the

3-oxo-butyl chain of the final product (Scheme 11). These results confirmed that the

hydroxyl ions located on the surface of the apatites were responsible for the basicity and

acted as a basic catalyst that led to a carbanion by abstraction of an acidic proton.

Table 14. ZnHAP-catalyzedMichael addition of indoles to electron-deficient olefins [181]

Entry R1 R2 R3 Time (h) Yield (%)

1 CH3 CH3 H 4 98

2 H CH3 H 4 89

3 C6H5 CH3 H 20 98

4 H C6H5 C6H5 24 70

5 CH3 C6H5 C6H5 24 95

6 C6H5 CH3 C6H5 24 21

34
Scheme 11. Proposed mechanism for the deuterium transfer from the apatite surface to the
final product [182].

The use of the Michael reaction was extended to calcium HAP with Ca/P = 1.61. The reaction

of ethyl 2-oxocyclopentanecarboxylate, methyl 2-oxocyclopentanecarboxylate, ethyl 2-

oxocyclohexanecarboxylate, methyl 1-oxoindan-2-carboxylate and ethyl 3-oxo-3-

phenylpropanoate with 3-buten-2-one produced good to excellent yields (Scheme 12).

Importantly, this heterogeneous catalyst was readily recovered by filtration, subsequently

washed with ethanol or dichloromethane, then was reused several times without marked

loss of catalytic activity.

Scheme 12. Michael carbon-carbon bond formation using HAP as catalyst [182].

35
 Kaneda and colleagues described the use of calcium vanadate apatite (VAP), in which a

[PO4]3- group of HAP was substituted by [VO4]3-; for a Michael reaction of 2-oxo-

cyclopentane carboxylic acid ethyl ester with 2-cyclohexen-1-one as a model reaction [183].

The catalytic activity was attributed to a [VO3OH]2- moiety which ensued from partial

dissolution of the apatite in water and the calcination process was necessary to achieve

catalytic activity. It is interesting to note that orthovanadate and pyrovanadate derivatives

that contained isolated VO4 tetrahedra such as Na3VO4, K3VO4, and Mg2V2O7 were found less

effective compared to calcined calcium vanadate apatite; procedure was extended to

include various sets of donors and acceptors (Scheme 13).

Scheme 13. Aqueous Michael reaction catalyzed by calcium vanadate apatite [183].

The calcium vanadate apatite was also tested as a catalyst in Michael addition of ethyl 2-

oxocyclopentanecarboxylate with 3-buten-2-one where a 99% yield of the targeted ethyl 2-

oxo-1-(3-oxocyclohexyl)cyclopentanecarboxylate compound was consistently obtained

during four consecutive cycles (Table 15) [184]. There was no observable leaching of any

vanadium in the aqueous phase was discerned. To check the heterogeneity of this system,

the catalyst was separated by a simple filtration from the reaction mixture with 55%

36
conversion; the reaction was continued for an additional 1 hour. The conversion remained

almost unchanged, which clearly demonstrated that vanadium did not leach as confirmed by

ICP analysis and the absence of vanadium in the filtrate.

Table 15. Recycling experiments of VAP-catalyzed aqueous Michael reaction [184]

Entry Cycle Conversion (%) Yield (%)

1 First cycle 99 99

2 Second cycle 99 99

3 Third cycle 99 99

4 Fourth cycle 99 99

Electropositive properties and the coordination ability of lanthanide ions have been

exploited by HAP-bound lanthanum catalyst prepared via a cation-exchange method in the

Michael addition reaction of 2-oxocyclopentanecarboxylate with methyl vinyl ketone under

aqueous or solvent-free conditions. The main findings were that the LaHAP catalyst was

more active compared to ScHAP, YHAP, YbHAP and LaFAP (Table 16) [185,186]. Among the

solvents surveyed, neat and aqueous conditions provided higher yields than those using

organic solvents. It is worth noting that this methodology could be extended to other 1,3-

dicarbonyl compounds and enones. Due to its heterogeneous character, the catalyst was

reused during three cycles under solvent-free conditions.

37
Table 16.

Michael catalyzed by various modified HAPs [185].

Entry Catalyst Solvent Yield (%)

1 LaHAP neat >99

2 ScHAP neat 90

3 YHAP neat 67

4 LaFAP neat 41

5 LaHAP H 2O >99

6 LaHAP C6H5CH3 58

7 LaHAP CH3CN 35

8 LaHAP CH2Cl2 30

9 LaHAP C6H12 21

10 LaHAP THF 9

11 LaHAP DMF trace

This approach was extended to an asymmetric version of Michael reaction where in the

heterogenized La3+ species and (R,R)-tartaric acid on HAP were used. The supported catalyst

was successfully tested in the addition of cyclic acetoacetates to methyl vinyl ketone in

toluene, affording the respective product a 99% yield and 30% ee (Table 17). Unfortunately,

the catalytic system proved to be less enantioselective than LaFAP.

38
Table 17

Asymmetric Michael reaction catalyzed by calcium lanthanide apatite [186].

Entry Catalyst Yield (%) ee (%)

1 LaFAP 97 60

2 LaHAP >99 30

4.4. Oxidation reactions

Selective oxidation reactions constitute industrial core technologies for converting bulk

chemicals to useful products of a higher oxidation state and recently many heterogenous

catalyst systems have been developed for selective oxidation catalysis [187,188].

4.4.1. Oxidation of alcohols

The selective oxidation reactions of alcohols are undeniably part of the key reactions of

organic synthesis; they allow the synthesis of carbonyl compounds which are very useful for

the synthesis of important pharmaceutical intermediates [189,190]. A considerable effort

has been devoted to develop efficient heterogeneous catalysts to replace the classical

oxidation methods that employing stoichiometric permanganate, dichromate or nitrogen

oxides in oxidations carried out with nitric acid. Among the developed catalytic materials,

supported ruthenium (Ru) and Pd catalysts are transition metals that offer excellent

opportunities for aerobic oxidation of alcohols.

39
 Despite the remarkable efforts since 1967, and in contrast to Suzuki reactions, there are

relatively few examples of heterogeneous Pd-catalyzed aerobic oxidation of alcohols [191].

Among the catalysts developed, the Pd on activated carbon [192], Pd on hydrotalcite [193],

Pd on simple oxides such as titania [194], and Pd/polymeric materials [195] are most

reliable. However, these systems suffer from a number of drawbacks namely moderate

catalytic activity and a limited scope of substrates. An important contribution in the

development of heterogeneous Pd catalysts for selective oxidation of alcohols was described

by Kaneda and colleagues in 2002 and 2004 [121,196] wherein HAP-supported Pd

nanoclusters PdHAP-0 were shown to be efficient not only for Heck and Suzuki reactions, but

were also useful for aerobic oxidation of alcohols. Using 0.2 to 0.6 mol% Pd catalyst and

molecular oxygen as oxidant, a wide variety of alcohols (including bearing heteroatoms

substrates) were oxidized in trifluorotoluene at 90°C, in water at 110°C or under solvent-free

conditions (Scheme 14). This methodology could be extended to a larger-scale study of the

oxidation of 1-phenylethanol under solvent-free conditions at 160°C. Importantly, TEM

analysis revealed that during reaction Pd(2+) was reduced to Pd(0) by the reactant alcohol

and the active catalytic species were apatite-supported Pd nanoparticles, with an average

diameter of 4 nm. Pd leaching from this heterogeneous catalyst was evaluated in the

oxidation of 1-phenylethanol which clearly demonstrated that no leaching of Pd in active

form occurred. The catalyst was recovered by centrifugation, followed by filtration, and was

reused three times without any loss of catalytic activity when another equivalent of the

1-phenylethanol and trifluorotoluene was added.

40
Scheme 14. Oxidation of various alcohols catalyzed by PdHAP-0 using molecular oxygen [196].

The probable reaction mechanism was initiated by oxidative addition of O-H alcohol bond

by the unsaturated Pd(0) species located at the edge to afford a Pd-alcoholate species. The

latter underwent a β-hydride elimination that produced the carbonyl compounds and a

hydrido-palladium species, which reacted with molecular oxygen to regenerate the Pd(0)

species. This reaction closed the catalytic cycle as well as the formation of O2 and H2O

(Scheme 15).

41
Scheme 15. Proposed reaction pathway for the oxidation of alcohols on the surface of Pd(0)
nanoclusters [196].

Conducting oxidation reactions in aqueous media can be advantageous, particularly for

large-scale industrial applications. Some of the benefits of an aqueous media include the

ease of purification, environmental friendliness and use of water as a reaction medium

[197,198]. Paul and colleagues demonstrated that he Pd(0)HAP which is efficient for Suzuki

couplings, could also perform the selective aerobic oxidation of benzyl alcohols to

corresponding aldehydes, using water as the solvent under atmospheric air (Table 18) [123].

The reusability of this catalyst was examined in the oxidation of benzyl alcohol, and the

results of the study showed that the catalyst could be recycled at least five times with only a

slight loss of activity (from 95 to 86% yield after five cycles); leaching studies were not

reported for this system.

42
Table 18

Oxidation of benzyl alcohols in presence of Pd(0)HAP as catalyst [123].

Entry R1 Yield (%)

1 H 95

2 4-MeO 96

3 4-Me 92

4 4-Cl 91

5 4-F 93

6 4-NO2 90

7 3-NO2 87

Studies have shown that a variety of low valent Ru species supported on solid matrices

such as zeolites [199], alumina [200], silicotungstate [201], and hydrotalcite [202] catalyze

the oxidation of alcohols with negligible Ru leaching. However, these catalysts display strong

shape selectivity due to uniform pore size, but their activity is moderate compared with

some platinum and Pd-based catalysts. Monomeric Ru species were immobilized on the

surface of HAP by an equimolar substitution of Ca(2+) ions with Ru(3+) [203]. This

heterogeneous catalyst gave efficient conversions of primary, secondary and functionalized

alcohols using molecular oxygen but required high catalyst loading (Scheme 16).

43
 Scheme 16. Oxidation of various alcohols over RuHAP [203].

Kaneda and colleagues indicated conclusively that the active species are monomeric Ru

cation surrounded by four oxygen and chlorine which contrasts with the findings of Opre and

colleagues for a similar catalyst [204]. The reusability of RuHAP catalyst was assessed in the

oxidation of benzyl alcohol with up to three consecutive cycles with consistent activity; the

amount of Ru content in the filtrate was below the detection limit of 10 ppb and the

negligible leaching was attributed to the strong coordinations of Ru in the apatite

framework. The oxidation was initiated by a ligand exchange between the alcohol and the

chlorine moiety of RuHAP. In the second step, the Ru-alcoholate underwent

β -hydride elimination that produced the carbonyl compound and a hydrido-ruthenium

species. The latter reoxidized by molecular oxygen to afford the Ru–OOH species, which

underwent a ligand exchange with alcohol to regenerate the Ru-alcoholate species together

with the formation of O2 and H2O (Scheme 17).

44
 Scheme 17. Plausible reaction pathway for the oxidation of alcohols using RuHAP [203].

In an independent study, the same group described a very original approach based on the

synthesis of recoverable RuHAP-encapsulated superparamagnetic nanoparticles catalyst

(RuHAP-γ-Fe2O3) by the simultaneous crystallization of HAP and γ-Fe2O3, under basic

conditions [205]. The ion exchange capabilities of the external HAP surface were utilized to

substitute calcium with Ru. The physicochemical characterization revealed that the γ-Fe2O3

nanoparticles were uniformly dispersed on the HAP matrix and had an average size of ~8 nm.

The catalyst provided a very efficient system for aerobic oxidation of a variety of benzylic,

allylic, aliphatic, aliphatic and heterocyclic alcohols at room temperature in trifluorotoluene

or toluene in the absence of any additives (Scheme 18).

45
 Scheme 18. RuHAP-γ-Fe2O3-catalyzed oxidation of alcohols [205].

Importantly, this system was found equally active in the oxidation of a series of sterically

bulky alcohols. For example, 3,5-dibenzyloxybenzyl alcohol, cholestanol and hexadec-7-yn-1-

ol were successfully converted quantitatively into the corresponding carbonyl compounds,

under relatively mild conditions. The catalyst could be separated from the reaction mixture

by an external magnet, avoiding the usual filtration or centrifugation process and was

recycled several times without any detectable deactivation and nor leaching of the catalyst

to the organic layer. The Ru species on the RuHAP-γ-Fe2O3s urface is a monomeric Ru(OH)22+

surrounded by four phosphate ligands, an observation which contrasts with a previous

report by Opre and colleagues on RuHAP [206]. According to the authors, the lowering of the

Ru content and incorporation of γ-Fe2O3 nanoparticles might play an important role on the

local environment of the active Ru species. 1-phenylethanol was oxidized giving

benzaldehyde in 93% yield; the turnover number based on Ru approached 930 after 24

hours. Baiker and colleagues reported the effect of organic modification on the catalytic

properties of RuHAP in the oxidation of aromatic and aliphatic alcohols using molecular

oxygen as the sole source of oxidant [207]. This new strategy involved the modification of

the host HAP and added polar organic compounds that were able to form strong hydrogen

bonds with the OH and phosphate functions of HAP. The incorporation of Ru was not

46
affected by the ion-exchange but by an adsorption process controlled by the polar organic

compounds; organic modification tripled the catalytic activity of RuHAP due to higher

intrinsic activity of Ru species and coordination in organically modified HAP. Interestingly,

during the oxidation process, the modifiers leach out and thus do not disturb the catalytic

oxidation of alcohols. To check for leaching, the organically modified RuHAP was removed at

60% conversion by simple filtration; the filtrate reacted under the reaction conditions. The

above treatment of the filtrate did not provide any yield. It clearly demonstrated that there

was not any leaching of ruthenium in active form. The reusability of this heterogeneous

catalyst was also tested. After complete conversion of benzyl alcohol, an equimolar amount

of 4-methylbenzyl alcohol was added to the reaction mixture and the reaction was

completed. The same procedure was repeated with the opposite sequence of alcohol

addition. The results indicated that the average reaction rates in the oxidation of benzyl

alcohol were almost identical, independent of the order of the addition of the two

substrates. In a subsequent study, the catalytic activity of RuHAP could be improved by the

incorporation of various promoter ions into the HAP lattice [207]. After incorporation of Ru,

the thermal treatments of the catalysts were critical due to the dehydration and irreversible

restructuring of the active species Ru(OH)2+ and its surroundings. On the basis of various

methods of characterization, it was thought that the active site consisted of Ru(OH)22+

surrounded by four oxygen atoms. The RuCoHAP and RuPbHAP that dried at moderate

temperature gave satisfactory yield and showed selectivity in oxidation of benzyl alcohol in

the presence of molecular oxygen. It is interesting to note that catalysts that were prepared

with a short contact time were more active than those prepared with a longer contact time.

This finding is due to the minimization of restructuring via dissolution-redeposition. The

scope of the RuCoHAP catalyst, prepared by reaction of CoHAP and aqueous RuCl3 solution

47
at room temperature for 10 minutes, was studied in the oxidation of various alcohols

(Scheme 19). Under the optimized conditions that involve 1.7 mol% of the ruthenium

catalyst, toluene as the solvent and molecular oxygen as oxidant, the benzylic and

cycloaliphatic alcohols were successfully transformed selectively to the corresponding

carbonyl compounds. However, the oxidation of primary and secondary aliphatic alcohols

required high catalyst loading compared to aromatic alcohols; no leaching of Ru from the

catalyst occurred under the reaction conditions.

Scheme 19. Oxidation of alcohols over RuCoHAP [207].

The exact mechanism of the RuCoHAP catalyzed alcohol oxidation reaction is unknown

and is generally supposed to take place through the hydridometal pathway (Scheme20)

48
[208]. The first step of the mechanism involves the formation of a Ru-alcoholate species,

which undergoes α-hydride elimination to produce the carbonyl compound, followed by a

hydrido-ruthenium species which are then reoxidized by molecular oxygen to complete the

catalytic cycle (Scheme 20).

Scheme 20. Mechanism of alcohol oxidation over RuCoHAP [208].

Fukahori and colleagues reported the selective oxidation of benzyl alcohol [209] using a

papermaking technique to make a RuHAP catalyst that entailed impregnating sheet

composites with RuHAP powder; this technique showed superior performance compared to

RuHAP beads in oxidation of benzyl alcohol and benzaldehyde in a batch reaction process.

Using this heterogeneous catalyst, the oxidation of benzyl alcohol in a continuous system

and a fixed-bed reactor were also examined; RuHAP sheets showed more oxidative

efficiency compared to the powder catalyst as the porous sheet structure improves the mass

transfer within the sheet. Although not relevant to the present section, the racemization of

chirally secondary alcohols by employing immobilized Ru on HAP has been demonstrated

49
[210]; both α-benzylic and aliphatic alcohols were racemized at 80°C in toluene in a

substrate to catalyst ratio of 33.

4.4.2. Oxidations of silanols

Silanol compounds have attracted worldwide attention in the synthesisof silicon-based

polymeric materials as well as organics [211-214]. These silanol compounds are mostly used

as valuable nucleophilic partners in transition-metal catalyzed carbon–carbon cross-coupling

reactions [215-217], and in organocatalysts for activating carbonyl compounds [218-220].

Typically these compounds are prepared by hydrolysis of the corresponding of chlorosilanes,

or from siloxanes and hydrosilanes via nucleophilic substitution and oxidation [221-225].

However, the catalytic oxidation of hydrosilanes using water as oxidant, and under neutral

conditions is an attractive route for clean and economic synthesis of silanols in comparison

to other preparations [226]. The RuHAP catalyst system, which was found to be effective for

aerobic oxidation of alcohols, can also be applied to oxidation of silanes [227]. Ethyl acetate,

(EtAc) acetonitrile and dimethylformamide were the best solvents among those that were

surveyed, and the use of typical heterogeneous catalystssuch as Ru/carbon, RuO2 and

Ru/Al2O3etc. were less effective than RuHAP. A variety of silanes, including aliphatic silanes,

were successfully converted to corresponding silanols in good yields, by employing 5 mol%

of catalyst in the presence of ethyl acetate as a solvent at 80°C (Scheme 21). Recycling

studies showed that the catalyst could be reused at least five times with consistent activity.

These leach-proof catalysts leave no remnants of metal in the end products, an especially

important feature since metal contamination is highly regulated in the industry.

50
 Scheme 21. RuHAP-catalysed oxidation of silanes [227].

Although RuHAP was an effective catalyst for this process, its cost was a drawback.

Fortunately, AgHAP prepared by similar strategy is significantly less expensive and was found

to be more active thanthe Ru catalyst in water (Table 19) [228]. However, the silver (Ag)

catalyst was effective only for aromatic silanes which could interact strongly with Ag

nanoparticles thus concluding the existence of cooperative action between Ag nanoparticles

and HAP. The increasing concentration of nucleophiles OH− from water activated on the

AgHAP surface promotes formation of silanols by suppressing condensation to disiloxane;

the biphasic nature of the organosilane/H2O system reduces the effective concentration of

water in the organosilane phase [229].

Table 19

Oxidation of silanes by in water with Ag/HAP or Ru/HAP catalysts [228].

51
 Entry Catalyst Solvent Time (h) Yield (%) TON Reused time

1 Ag/HAP (3 mol%) H2O 0.25 99 33 5

2 Ru/HAP (5 mol%) EtAc 3 99 20 5

Kaneda and colleagues reported successful use of gold (Au) nanoparticles embedded in

hydroxyapatite (AuHAP) in the catalytic oxidation of silanes to silanols using water as an

oxidant [230]; catalyst was synthesized by treatment of HAP, with an aqueous solution of

HAuCl4. TEM measurements revealed that the average size of the gold nanoparticles was 3.0

nm. Using 0.83 mol% of catalyst and under organic-solvent-free conditions, various silanes

with different structures were converted selectively into the corresponding silanols in high

yield (Scheme 22) with the exception of triphenylsilane presumably due to strong steric

hindrance (Scheme 23). The catalyst offered a number of advantages, namely excellent

stability, easy separation from the reaction mixture by filtration, use of water as a clean

oxidant, and reusability over several times without loss of activity and selectivity. Another

advantage is that AuHAP catalysts can offer the possibility of the oxidation of a wide range of

silanes, including various aliphatic silanes.

52
 Scheme 22. Oxidation of silanes over AuHAP catalyst in water [230].

Scheme 23. Oxidation of triphenylsilane using AuHAP catalyst [230].

4.4.3. Epoxidation reaction

Epoxidation of olefin is a widely used transformation in the organic synthesis of fine

chemicals [231,232]. It is an oxygen-transfer reaction for which hydrogen peroxide and its

derivatives are particularly well suited. A large number of epoxidation processes have been

developed in the recent years using homogeneous and heterogeneous catalysts [233-237].

An effective and environmentally friendly protocol for the epoxidation of olefins and

α,β-unsaturated ketones using HAP as catalyst, has been developed [238] via a

sonochemical route and was deployed for a wide range of olefins and olefinic ketones using

53
hydrogen peroxide under relatively mild reaction conditions. The acetonitrile and acetone

were found to be the best solvents and no reaction was observed in other solvents studied.

The cyclic and linear olefins such as cyclopentene, cyclohexene, substituted cyclohexenes,

cyclooctene, norbornene, 1-hexene, 2-hexene, 1-octene, and 2-octene afforded the

corresponding epoxides as sole products in good to excellent yields. However, the

epoxidation of cyclooctadiene gave a mixture of mono and diepoxides. It is interesting to

note that the epoxidation of styrene led to styrene oxide in poor yield, with the formation of

a small quantity of phenyl acetaldehyde. The epoxidation of stilbene also gave poor yield,

but its yields were greatly improved by conducting the reaction in a mixture of acetonitrile

and acetone. This could be attributed to the enhancement of solubility of stilbene in

acetonitrile-acetone mixture when compared to that in acetonitrile. A variety of

α,β-unsaturated ketones were efficiently converted to the corresponding epoxides, but it

was found that the presence of a substituent at the olefinic carbon inhibited the reaction,

perhaps, due to the steric hindrance of the substituent. The catalyst could easily be

separated from the reaction mixture by simple filtration and reused at least four times in the

epoxidation of cyclohexene, with no apparent deactivation (Table20).

Table 20

Reusabilityof HAP catalyst in the epoxidation of cyclohexene [238].

Entry Cycle Yield (%)

1 1 65

54
 2 2 66

3 3 64

4 4 64

Parvulescu and colleagues reported the epoxidation of cyclohexene and indene using

peroxotungstic species complexed with phosphate groups at the surface of HAP in the

presence of hydrogen peroxide [239]. In this study, the approach was to neutralize the basic

OH groups of HAP with tungstic acid or a complex of tungstic acid with Aliquat 336; bonding

of WO3 and/or WO(O2)2 units to the surface of phosphates generated the phosphotungstic

species. However, anchoring on HAP was limited by the number of accessible OH groups and

by the strength of the ensuing bond. The epoxidation of cyclohexene and indene occurred

with varying TOFs and selectivities; cyclohexene led to rather high TOF values, but the

maximum selectivity to epoxide was 30%. Leached tungsten resulted in acidic species that

catalyzed the hydrolysis of epoxide to diols. In the case of the recycledcatalysts, for which

the leaching was much reduced, the hydrolysis of epoxides to diols could be associated with

the properties of the remaining free OH groups on HAP. In the second run, after the weakly

bonded tungsten leached out, both the TOF and selectivity to epoxide increased.

A new magnetically recyclable and efficient nanocomposite catalyst for the epoxidation of

olefins was reported using molybdenum oxide nanoparticles supported on HAP

encapsulated γ-Fe2O3 nanocrystallites [240]. The catalytic system comprising tert-butyl

hydroperoxide as oxidant and chloroform as the solvent, in the presence of 20 wt%

MoOx/HAP-γ-Fe2O3 and showed good activity for the epoxidation of cyclohexene; cyclic and

linear alkenes were all converted into the corresponding epoxides with excellent conversions

and epoxide selectivities. The successful recycling tests are an advantage of this

55
methodology, since the recovered catalyst was reused six times with a consistent activity;

ICP analysis of the filtrate confirmed that the metal content was negligible (Mo < 5 μg/L, Fe <

3 μg/L). The epoxidation of styrene catalyzed by Auclusters supported on HAP has been

reported [241]. This heterogeneous catalytic system was prepared by dispersion of

Au25(SG)18 (SG = glutathione ligand), followed by calcination to remove the SG ligand, and

was able to oxidize styrene with 100% conversion and 92% selectivity in the presence of tert-

butylhydroperoxide (TBHP) as an oxidant at 80°C. The morphology-dependant catalyst

prepared from the Au25(SG)18 precursor exhibited higher selectivity to styrene oxide than the

same catalyst prepared from HAuCl4 by adsorption technique, despite a similar conversion

and size of the supported Au nanoparticles. In addition, recyclability and high selectivity

make this catalyst a promising system for the selective epoxidation of alkenes. In another

study, Jin and colleagues showed that the very small colloidal clusters of Au, Au25, Au38, and

Au144, capped with phenylethanethiolate, (e.g., Au 25(SC2H4Ph)18), was supported on HAP and

SiO2 for application in catalysis; oxidation of styrene in the presence of TBHP or oxygen as

the main oxidant, in toluene as the solvent occurred at 80°C. With TBHP as the oxidant, the

catalytic activity was almost independent of the size of the cluster but when O2 was used as

the oxidant, a distinct size effect was observed for (Au) ncatalysts in selective oxidation of

styrene, the order of catalytic activity being: Au25 > Au38 > Au144. After calcination, the Au

nanocatalysts were still active. However, aggregation of gold was observed which might

contrast the expected benefit of decapping the catalytic centers. Using O2 as the sole

oxidant, the uncalcined Au n(SR)m catalysts achieved higher activity than the calcined

Aun(SR)m catalysts. This is due to cluster sintering in calcination and the larger ensuing

particles cannot effectively activate O2. Using Au25/HAP capped catalyst at 80°C in toluene

under similar reaction conditions, TBHP was more effective than O2, while TBHP as initiator

56
and O2 as the main oxidant exhibited an intermediate conversion. More importantly, in the

case of TBHP and (O2 + TBHP), the oxidation of styrene led to benzaldehyde with high

selectivity (100%) and 80% selectivity with sole O2 as oxidant; acetophenone and styrene

oxide were formed only as minor reaction products. The catalytic activity of the thiolate-

capped cluster was surprising, considering the expected poisoning effect of sulfur

compounds [242]. In brief, the selective oxidation of styrene catalyzed by Au25(SCH2CH2Ph)18

nanoclusters, using three different oxidation systems, involves the activation of oxygen on

small Au clusters and the role of peroxidic species;these gold nanocatalysts can be readily

recovered without any precautions and reused several times with only a slight loss of

activity.

4.4.4 Oxidative dehydrogenation of ethane and propane

To date, steam cracking, fluid-catalytic-cracking and catalytic dehydrogenation processes

are the main methods to produce olefins, the oxidative dehydrogenation being an attractive

alternative for the efficient production of olefins with salient advantages that include:

reduced cost, lower green house gas emissions, low energy consumption and the low

deposition of carbon during the process [243,244]. In addition, this reaction is exothermic,

avoiding the thermodynamic constraints of non-oxidative routes by forming water as a

byproduct. Extensive works on this topic have been reported [245-247] and the yield of

alkenes obtained by oxidative dehydrogenation on most catalysts is limited by alkene

combustion to CO and CO2. The effects of the addition of tetrachloromethane (TCM) into the

feed-stream of the oxidative dehydrogenation of propane have been thoroughly

investigated by Sugiyama and colleagues; strontium and barium supported on HAP, with and

without treatment with Cu2+ and Pb2+ have been used [248]. In the absence of TCM, the

system SrHAP was significantly more efficient than BaHAP. For both the SrHAP and BaHAP,

57
the selectivity to propylene decreased with increasing time-on-stream when the TCM was

added. The introduction of lead by ion-exchange, with calcium in the HAP, resulted in the

decrease of the yield of propylene, independently of the addition of TCM in the reaction

mixture. In the presence of TCM, the incorporation of copper in the HAP structures

dramatically improved the conversion of propane and the selectivity to propylene 16 and

80%, respectively; the activities remained constant during 6 hours on-stream. The

understanding of the origin of this improvement still remains an open question with a

popular belief that the TCM contributed to the reduction of Cu2+ to metallic Cu.

HAP partially substituted with vanadate, Ca10(PO4)6-x(VO4)x(OH)2 where V/P = 0, 0.025, 0.05

and 0.10 (atomic ratio), have been used in oxidative dehydrogenation of propane to

propylene [249]; the incorporation of vanadate in a molar ratio of V/P = 0.05 increased the

conversion of propane to 7.6 to 17.2% and the selectivity from 3.5 to 52.4%, values being

close to those obtained by magnesium pyrovanadate, which is one of the most active

catalysts for the oxidative dehydrogenation [250]. It is evident that the lattice oxygen in the

vanadate plays an important role in this improvement.

In order to identify the active site on HAPs for oxidation of alkanes, the oxidative

dehydrogenation of propane using strontium HAP and incorporated cobalt cation and H-D

exchange behaviors of OH groups in those catalysts with D2O have been studied [251].

Interestingly, based on 1H and 31

P MAS NMR studies, the active oxygen species formed by

abstraction of hydrogen of OH groups in the HAP was enhanced on the Co2+ rich catalysts.

These species contributed directly to the hydrogen abstraction from propane, which allowed

improvement of catalytic activity of the catalysts in propane dehydrogenation.

Ziyad and colleagues reported the oxidative dehydrogenation of propane using the

chromium-loaded HAP catalysts Cr(x)/HAP (0.1 ≤x≥ 3.7 wt.% Cr), prepared by cation-

58
exchange method between HAP and chromium(III) nitrate [252]. According to the authors,

several isolated chromium species were identified after calcinations, mainly being Cr2+, Cr+6

and Cr3+. The latter is preponderant at a much higher loading and located on the apatite

surface as isolated Cr3+–O–Ca2+ or Cr3+–O–Cr3+, whereas the Cr6+ is preponderant at low

loading as monochromates; Cr6+ species might be allowed only to initiate the cracking of

propane since they were reduced by the reaction mixture. It has been proposed that the Cr3+

present in Cr3+–O–Ca2+ was the most active species because it favors the release of the

neighboring lattice oxygen, leaving either an oxygen vacancy or a Cr2+ species. The Cr3+–O–

Cr3+ species were considered less active because they were less basic than Cr3+–O–Ca2+. The

basicity of the HAP allowed abstraction of the proton from propane and the oxygen lability

was favored by the presence Cr3+. However, the decrease of the basicity induced by the

fixation of Cr3+ counterbalanced the positive effect of chromium on oxygen reactivity, which

limited its performance.

The same research group reported the synthesis of Fe3+-loaded HAP (FeHAP) and its

catalytic activity toward propane oxidative dehydrogenation [253]; the characterization of

FeHAP revealed the presence of Fe3+ species in distorted octahedral and lower coordination

site forms. The catalyst exhibited good propene selectivity (34-90%) but with low

conversion. Importantly, the maximum propene yield of 6.2% was obtained at low Fe

content with the formation of carbon oxides and ethylene. The Fe3+–O–Ca2+ species

identified at low Fe content led to the maximum propene yield (6.2 %) at 550°C with the

formation of carbon oxides and ethylene. This may be due to low Fe content, which allowed

avoidance of the decrease of the basicity and maintained it for the hydrogen abstraction

from the propane.

59
 On a side note, vanadium pentoxide, with loadings varying from 2.5 to 15 wt%, supported

on HAP via wet impregnation techniques have been used in oxidative dehydrogenation of

n-octane [254]. The XRD and IR characterizations revealed that the vanadium exhibits two

types of phases, the vanadium pentoxide at lower loadings and pyrovanadate phase for

weight loadings in excess of 10%. The electron microscopy measurements revealed that the

vanadium species were highly dispersed on the HAP. Importantly, the temperature

programmed reduction showed that total acidity of the catalysts increased as the vanadium

loading increased. The vanadium pentoxide exhibited high selectivity towards octenes and

low selectivity towards aromatics. The pyrovanadate phase also produced octenes as a

major product, but the selectivity towards aromatics was improved compared to the other

catalysts. Moderate to high selectivity towards carbon oxides were obtained with all the

catalysts since vanadium pentoxide also favors the formation of oxygenates. C2–C7 is the

favorable cyclization mode for the formation of aromatics, and aromatics selectivity

increased with both vanadium content and temperature.

4.5 Photocatalytic reactions

The preparation of HAP-titania composites (HAP/TiO2) has attracted considerable interest

with several methods developed for their preparation to improve the photocatalytic

activities [255]. Anmin and colleagues reported the synthesis of HAP/TiO2 via hydrothermal

treatment of HAP powders with colloidal Ti(OH)4 solutions [256]. TEM characterizations

concluded that TiO2 granules were deposed on HAP as anatase crystals with a particle size

distribution in the range of 3-5 nm. The photocatalytic activity of this hydroxyapatite-titania

composite was evaluated in the decomposition of methylene blue wherein this nano-

composite was more effective than that of HAP and undoped TiO2. The synthesis and the

characterization of such photocatalysts with different titanium concentrations at varying

60
hydrothermal temperatures have been reported [257]. The study indicated that at higher

hydrothermal temperatures synthesis, the increasing titanium substitution led to an increase

in the crystallinity of HAP/TiO2 with adsorption capacities being in the range of UV-vis

spectra. Photocatalytic activity efficiency in the decomposition of methylene blue

significantly increased with the Ti/(Ca + Ti) mole ratio concluding that the photocatalytic

properties of this composite may be related with the particle size, crystalline quality,

morphology, and specific surface area. Hu and Zhong have reported similar observations in

the photocatalytic degradation of formaldehyde and acetaldehyde using Ti4+-substituted

HAP [258].

Colloidal calcium HAP doped with Ti(IV) ions in different atomic ratios, Ti/(Ca + Ti) = XTi,

by a co-precipitation method has been reportedin which H3PO4 was added to the solution

containing Ca(NO3)2 and Ti(SO4)2 [259]. For charge balance in the HAP lattice, the added

Ti(IV) may exist in the HAP crystals as divalent cations such as [Ti(OH)2]2+ and [TiHPO4]2+. The

photocatalytic activities of this modified HAP were examined in the decomposition reactions

of acetaldehyde and albumin; modified HAP particles were active while the HAP particles

were completely inactive towards the decomposition of both the materials.

A study used the Quartz crystal microbalance technique to investigate the adsorption and

degradation behavior of bilirubin on HAP-modified titania coatings [260,261]. Interestingly,

the authors noted that the adsorption and the photocatalytic degradation of bilirubin under

UV irradiation was closely related to the amount of HAP deposited on titania coatings, and

the most degradation efficiency of bilirubin was found when the percentage of HAP was 0.7

wt%; the degradation-rate constant of bilirubin on HAP (0.7 wt%)-titania coatings were (1.18

± 0.12) x 10-3 s-1, which is higher than that on other titania coatings.

61
 Nanometer-sized TiO2 island structures on the lamellate hydroxyapatite nanocrystals

were prepared via a two-step emulsion process [262]. After the preparation of platy HAP

nanocrystals with a particle size distribution in the range of 70-200 nm by heat-treatment at

1078 K, the HAP nanocrystals were then immersed in NaH2PO4 solution to generate the

formation of hydroxyl group on its surface. In the second step, the titanium

tetraisopropoxide reacted with the hydroxyl group of HAP surface to form TiO2 nanoparticles

on the surface of HAP nanocrystals. The ensuing HAP nanocrystals loaded with TiO2

nanoparticles showed the high photocatalytic activity compared to the commercial TiO2

catalyst in the photodegradation of benzaldehyde.

The synthesis of TiO2/hydroxyapatite composite photocatalyst with a novel morphology

was accomplished from commercially available α-tri-calcium phosphate and TiO2 powders,

using a process based on the liquid immiscibility effect, followed by precalcination and

hydrothermal treatment [263]; a blue shift of the absorption edge of 16 nm was observed

upon hybridization of TiO2 and HAP. Microstructure analysis revealed that the granule

contained rod-shaped HAP crystals with a surface adorned by nanosized TiO2 particles.

Importantly, the active anatase surface was retained effectively in the composite granules,

and they operatedas three-dimensional, highly porous, size-controllable small reactors.

Compared with that of bulk TiO2, in the photodegradation of methylene blue, the

photocatalytic reaction rate greatly increased when using this composite granule.

The preparation of modified Ag-TiO2 powders using the wet chemical strategy has been

documented wherein the modified Ag-TiO2 powders photocatalytically decompose gaseous

acetone under visible-light illumination [264]. The modified Ag-TiO2-HAP powders showed

higher catalytic activity and photochemical stability compared to Ag-TiO2, undoped TiO2 and

P25 powders. These improvements could be attributed to strong absorption in the visible-

62
light region, low recombination rate of the electron–hole pair and the large BET specific

surface area of hydroxyapatite modified by Ag-TiO2.

Pd-TiO2-HAP nanoparticles with 3 wt% of Pd and 25 wt% of TiO2 were successfully

synthesized by an impregnation method [265]. The catalytic performance of this catalyst was

evaluated in the photocatalytic oxidation of potassium cyanide into carbon dioxide and

nitrogen gases under visible light irradiation; the catalyst displayed significantly higher

photocatalytic activity as compared to Pd-TiO2, undoped TiO2, and P25 powders. This was due

to the absorption of radiation in the visible light region, the small recombination rate of the

electron-hole pair, and the high surface area. The photocatalytic degradation of cyanide

followed first-order kinetics. Recycling experiments confirmed the relative stability of the

catalyst. Recently, Vaimakis and colleagues reported the synthesis of HAP/TiO2

nanocomposites and itsphotocatalytic applications in the oxidation of nitrogen monoxide

[266]. The morphology was dependent upon the volume ratio between HAP and TiO2 in the

initial solution. At a low HAP/TiO2 ratio, an aggregate of spherical particles were observed,

while the decrease of the TiO2 amount resulted in formation of needle and lath like particles.

The composite with the highest amount of TiO2 in the initial solution (VHA/VTiO2 = 1, volume

ration) demonstrated higher •OH production capability than pure TiO2. The HAP/TiO2

composite exhibited different activities for each part of the NO→NO2→NO3− photocatalytic

process. Even still, the composites exhibited increased NO→NO2 oxidation in comparison to

the pure TiO2; their activity in the NO2→NO3− oxidation was extremely low, especially for the

samples with volume ratio more than 1. The behavior of the HAP/TiO2 composites was

attributed to the presence of residual acidic groups which was due to the HAP precipitation

procedure.

63
 Wei and colleagues reported the synthesis and photocatalytic activity of HAP modified by

nitrogen-doped TiO2 catalyst [267]. This new photocatalyst was evaluated in the

decomposition of gaseous acetone under visible light irradiation; 10%-HAP-N-TiO2 was

superior to that of the commercial Degussa P25 counterpart. The enhanced catalytic activity

was attributed to the synergistic effect of HAP that adsorbed the chemical compound and

the TiO2 photocatalyst that generated oxygen reactive species, which diffused and reacted

with the molecules located on the HAP. The doped nitrogen species played a key role in

narrowing the band gap and made the composite active under visible-light irradiation.

A HAP/TiO2 thin film was deposited on glass using a radio frequency magnetron sputtering

process [268] which showed that the decomposition of formaldehyde gas on such film

showed a higher decomposition rate than either the TiO2 or the HAP film alone.

The recovery and reuse of expensive catalysts after catalytic reactions are important

factors to consider for sustainable processing. Although nanocatalysts are highly active, they

are not easy to isolate from the reaction mixture, which hampers overall process

sustainability. However, due to its magnetic properties, the use of magnetic nanoparticles as

a support seems to be a promising option to overcome this separation problem [269,270]. In

this context, Liu and coworkers prepared Fe3O4/HAP nanoparticles by homogeneous

precipitation method and used it as photocatalyst in the photodegradation of diazinon [271].

XRD and TEM measurements revealed that the obtained nanoparticles were composed of

Fe3O4 and HAP, and these particles were almost spherical with an average particle size of 25

nm in diameter. The effect of calcination temperature on photocatalytic activity of

Fe3O4/HAP nanoparticles was investigated, and the results showed that the catalyst that

calcined at 400°C possessed photocatalytic activity in comparison with that calcined at other

temperatures.

64
 A similar study involved the development of a novel Pd/HAP/Fe3O4 for photocatalytic

degradation of azo dyes [272]. This catalyst showed high catalytic activity and good stability

without any visible light irradiation or application of adulterant; the catalytic activity being

significantly enhanced under acidic conditions. A reaction mechanism involving the reaction

of Pd/HAP/Fe3O4 with dissolved oxygen, with the assistance of acid to form a Pd

hydroperoxide that oxidizes azo dyes under HAP catalysis, was proposed. Interestingly, the

concentrations of dyes changed exponentially with time and high rate constants were

obtained for the degradation of these dyes. The pseudo-first-order equation was shown to

fit degradation kinetics in most cases. Due to the magnetic properties of nano-Fe3O4, this

catalyst was separated from the reaction mixture by an external magnet, thus avoiding the

usual filtration or centrifugation process.

In a report by Nishikawa and Omamiuda, the photocatalytic decomposition of methyl

mercaptane over HAP under UV irradiation was investigated [273]. From the results of

electron spin resonance studies, the researchers concluded that the radical species formed

under UV irradiation was a superoxide anion radical species. It was also reported that the

photocatalytic activity of HAP treated at 200°C was different from that of HAP after

treatment at 1150°C for decomposition of methyl mercaptane. The photocatalytic activity of

HAP was attributed to the production of sufficient amounts of superoxide anion radical

species under UV irradiation. In this study, it was shown that the photocatalytic activity of

the HAP sample used for dimethyl sulfide was not satisfactory under UV irradiation, which

indicated that the oxidation of dimethyl sulfide is more difficult than that of methyl

mercaptane.

The photocatalytic decomposition of dimethyl sulfide under UV irradiation using two

types of HAP with Ca/P molar ratios (Ca/P = 1.67, HAP-1) and (Ca/P = 1.67, HAP-2) [274]

65
showed that the HAP-2 was more active than HAP-1 and β-tricalcium phosphate in the

conversion of dimethyl sulfide, which led to stoichiometrical carbon dioxide, sulfur dioxide

and water. This high photocatalytic activity could not be due to the surface area of HAP

alone, but must be due to the crystallinity toward the a-axis of HAP crystal.

The HAP modified Pt-ZnO nanoparticles were synthesized by using a template-ultrasonic

assisted method [275]. TEM measurements for the ensuing Pt/ZnO/HAP particles were

located on the surface HAP nanorod with an average diameter of 9 nm. The photocatalytic

activities of these nanoparticles were evaluated in the photocatalytic oxidation

decomposition of benzene under a visible-light irradiation; Pt/ZnO/HAP was found to be

significantly higher than that of ZnO, P25 and Pt-ZnO. This was attributed to strong

absorption in the visible-light region, a low recombination rate of the electron-hole pair, and

the large BET specific surface area of Pt/ZnO/HAP.

4.6. Othermiscellaneous reactions

4.6.1. Multicomponent reactions

Multicomponent reactions are generally reactions whereby more than two starting

materials react in a sequential manner to give highly selective products that essentially

retain the atoms of the starting material [276-278]; relatively few examples of HAP-

supported catalysts existin multicomponent reactions. Choudary reported that use of HAP-

supported copper catalyst (CuHAP) could catalyze the formation of propargylamines via the

coupling of aldehydes, amines, and alkynes without any co-catalyst or additive [279],

acetonitrile beingthe best solvent with optimum ratio of aldehyde, amine and alkyne

1:1.2:1.3. Among the catalysts screened, thereactivity order was CuHAP>Cu(OAc)2>RuHAP>

Cu >FeHAP for reactions that accomodated various aldehydes with a variety of terminal

alkynes and amines possessing a wide range of functional groups (Scheme 24); they were

66
found to be more rapid compared to earlier reported case using agin ionic liquids [280]. This

recoverable catalyst could be reused four times without any significant decrease in its

catalytic activity, and there was not any detection of leaching of copper species from the

support upon atomic adsorption spectroscopy of the reaction mixture.

Scheme 24. Three-component coupling over CuHAP catalyst [279].

Mahajan and colleagues used antimony chloride doped on HAP(SbCl3-HAP) as a Lewis

solid acids catalyst for the one-pot stereoselective synthesis of trans-pyrano[3,2-

c]quinolones from anilines, benzaldehydes and 3,4-dihydro-2H-pyran (Scheme 25) [281];

significant and notable feature being the formation of a single trans isomer in contrast to

SbCl3 and SbCl3-Al2O3, which led the formation of both cis and trans.

Scheme 25. SbCl3-HAP-catalyzed stereoselective one-pot synthesis of pyrano[3,2-

c]quinolones [281].

67
 The catalyst was quantitatively recovered from the reaction, and its reusability was

explored with 4-tolualdehyde, o-toluidine and 3,4-dihydro-2H-pyranas a model reaction. The

catalyst was easily recycled up to ten cycles without significant any loss in activity. However,

the leaching study was not reported for this system.

Sun and colleagues reported on one-pot synthesis of α-aminophosphonate via

chlorinated benzyl alcohol and aniline with dimethyl phosphite under solvent-free

conditions, using Au supported on HAP (AuHAP) [282]. The reaction proceeded smoothly at

100°C under atmospheric pressure to furnish product in a good 86% yield (Scheme 26).

Scheme 26. Three components route toα-aminophosphonates using Au/HAP catalyst [282].

Interestingly, a one-pot synthesis of naphthopyrans has been developed by Solhy

research group via a three-component coupling from β-naphthol and aldehydes, with

malononitrile, using HAP and sodium-modified HAP in aqueous media [283]; sodium nitrate

was used to impregnate to create the sodium-modified HAP (SMH) which was superior to

HAP alone (Table 21). The SMH catalyst was readily recovered by filtration, followed by

calcination at 450°C. The recycling of the catalyst had been briefly examined in the reaction

of malononitrile with β-naphthol and benzaldehyde. The results showed that the catalyst

could be recycled at least five times without any loss of activity.

68
Table 21

ZnHAP-catalyzed Michael addition of indoles to electron-deficient olefins [283].

Entry R1 Catalyst Time (h) Yield (%)

1 H HAP 6 74

2 H SMH 3 95

3 4-Me HAP 6 71

4 4-Me SMH 3 79

5 4-OMe HAP 6 61

6 4-OMe SMH 3 72

7 4-Cl HAP 6 75

8 4-Cl SMH 3 96

9 3-NO2 HAP 87 6

10 3-NO2 SMH 93 2

11 4-NO2 HAP 82 6

12 4-NO2 SMH 2 92

4.6.2. Alkylation reactions

Acylation of aromatic compounds is a powerful and widely used method for the

production of fine chemicals [284]. The classic methodology for Friedel-Crafts acylation uses

stoichiometric amounts of Lewis acids or strong mineral acids as catalysts, and results in a

substantial amount of waste and corrosion problems [285]. One of the best ways to

69
overcome these problems is the use of solid acids such as zeolites, clays, Nafion-H, and

heteropoly acids [286-289]. Sebti and colleagues explored these options and successfully

used HAP alone or activated with metal halides in the Friedel-Crafts alkylation of benzene,

toluene and p-xylene by benzyl chloride (Scheme 27) [290]; HAP alone can catalyze the

selective alkylation to monoalkyl-compounds thus confirming the acid character of the

material. The obtained products of monoalkylation mostly consist of: benzene benzylation

(68% monoalkyl, 32% dialkyl); toluene benzylation (88% monoalkyl, 12% dialkyl); p-xylene

benzylation (87% monoalkyl, 13% dialkyl). It is noteworthy that the reactivity of the aromatic

nucleus increases with the number of electron donor groups. To improve the acid catalytic

activity of HAP, authors impregnated the phosphate with metal halides such as ZnCl 2, NiCl2

and CuCl2 and these three catalytic systems showed good catalytic activity and high

selectivity to produce monoalkylation in the Friedel-Crafts alkylation; ZnCl2/HAP being much

more effective than the catalysts NiCl2/HAP and CuCl2/HAP. The selectivity towards

monoalkylation was greater with toluene and p-xylene, with a ratio of monoalkyl/dialkyl that

varied between 95/5 and 87/13; for benzene, the ratio varied between 83/17 and 68/32.

The para-substituted product was the major compound (about 63% para/37% ortho) in the

alkylation of toluene. Unfortunately, recycling experiments were not reported for these

systems.

Scheme 27. Friedel-Crafts alkylationof benzene over different Lewis acid supported on HAP [290].
70
It is worth noting – even if it is out of the main scope of the present review, that the

fluorapatite (FAP) alone or impregnated by transition metal halides easily catalyzed the

Friedel-Crafts alkylation [291]; fluorapatite was found to be less acidic than HAP. Among

many halides such as ZnCl2, ZnBr2, CuCl2, CuBr2 and NiCl2 doped on FAP, the best results

were obtained with ZnCl2/FAP and ZnBr2/FAP; catalysts were recycled through simple

filtration and drying methods, although leaching of metal of the catalyst to the organic layer

caused progressive deactivation.

Masuyama and colleagues developed a Pd-exchanged HAP and explored its catalytic

properties in allylic alkylation [292]. Using 1 mol% of catalyst in aqueous medium, and 0.01

mmol of PPh3 in water at 50°C, this catalyst system gave satisfactory yields of monoallylated

product in the reaction of allyl methyl carbonate with active methylene compounds such as

diethyl malonate, 2-ethoxycarbonylcyclopentanone,2-ethoxycarbonylcyclohexanone and

2,2-dimethyl-1,3-dioxane-4,6-dione under mild reaction conditions (Scheme 28). The

reusability was tested in allylic alkylation of allyl methyl carbonate with diethyl malonate, in

the presence of PPh3 which reduced Pd(II) to active Pd(0) in situ; catalyst could be reused ten

times without losing activity presumably via the coordination of phosphate moieties to Pd.

71
 Scheme 28. Allylic alkylation of allyl methyl carbonate with active methylene compounds
over PdHAP catalyst [292].

Synthesis and the catalytic activity of heterogeneous HAP impregnated with zinc bromide

have been explored for Friedel-Crafts acylation of anisole (Scheme 29) [293]. The high

catalytic efficiency of ZnBr2/HAP displayed high selectivity and could be reused at least five

times without obvious activity loss.

Scheme 29. Friedel-Crafts acylation of anisole catalyzed by ZnBr2/HAP catalyst [293].

4.6.3. Hydration of nitriles

The catalytic hydration of nitriles to the corresponding amides is an efficient and

economical route to useful amides from an industrial point of view. Amides are found in

numerous natural products and are also widely used in the organic synthesis for lubricants,
72
detergent additives, drug stabilizers, and functional polymers [294-297]. This catalytic

hydration is usually performed using a strong acid or a base [298,299]. However, under

these conditions various by-products, notably carboxylic acids which are formed by

overhydrolysis of amides, and generation of salts after neutralization of the catalysts. These

harsh conditions can be circumvented using heterogeneous catalysts instead. Hydration of

nitriles using sodium nitrate impregnated HAP has been reported [300] wherein a variety of

benzonitriles derivatives including heteroaromatic nitriles such as 3-cyanopyridine, 4-

cyanopyridine and prazinecarbonitrile were smoothly hydrated to corresponding amides in

excellent yield (Scheme 30). However, the substituents bearing electron-withdrawing groups

were significantly more reactive than that of the bearing-electron donating groups.

Scheme 30. Selective hydration of nitriles over HAP as catalyst [300].

HAP-supported Ag (AgHAP) nanoparticles have been described as an efficient and

recyclable catalytic system for the selective hydration of nitriles [301]; catalyst was prepared

by treating HAP with AgNO3 followed by reduction with KBH4, the average size of metal

particles in AgHAP being 7.6 nm (Scheme 31). The limitations of the system were found to be

aliphatic nitriles, which were not converted to corresponding amides even at 180°C.

Nonetheless, it is significant that this methodology could extend to various heteroaromatic

nitriles giving useful to excellent isolated yields of expected amides; the hydration of

pyrazinecarbonitrile could be maintained at 99% after four recycling steps, by using 3 mol%

of catalyst.

73
 Scheme 31. Hydration of nitriles to amides using AgHAP as catalyst [301].

4.6.4. Transesterification

Transesterification is an important organic reaction that can be employed to synthesize

esters [302] and this organic transformation reaction is more advantageous than the ester

synthesis from carboxylic acids and alcohols. Transesterification has traditionally been

carried out by the use of strong acids or by soluble bases such as caustic soda [303-305].

However, traditional homogeneous transterification poses significant issues related to

product separation which could be addressed by the use of heterogeneous catalysis namely

HAP supported zinc chloride [306] exemplified in the transesterification reaction between

methyl benzoate and butan-1-ol as a model reaction; supported Lewis acid catalyst exhibited

higher catalytic activity than HAP alone. The general procedure included various aromatic

and aliphatic esters with aliphatic, cyclic and aromatic alcohols; fairly good to excellent yields

of corresponding esters were obtained under reflux conditions (Table 22).

Table 22

Transesterification reactions catalyzed by HAP and ZnCl2/HAP [306].

Entry R1 R2 Catalyst Time (h) Yield (%)

1 C6H5 C6H13 HAP 24 60

74
 2 C6H5 C6H13 ZnCl2/HAP 6 92

3 C6H5 C6H5CH2 HAP 24 49

4 C6H5 C6H5CH2 ZnCl2/HAP 6 93

5 C6H5 C6H11 HAP 24 60

6 C6H5 C6H11 ZnCl2/HAP 3 75

7 C6H11 C8H17 HAP 24 57

8 C6H11 C8H17 ZnCl2/HAP 6 93

9 C6H11 C6H5CH2 HAP 24 50

10 C6H11 C6H5CH2 ZnCl2/HAP 6 90

11 C3H7 C8H17 HAP 24 29

12 C3H7 C8H17 ZnCl2/HAP 6 58

The catalyst recovered after filtration was dried at 150°C, and then successfully reused in

3 runs with only a 3% reduction in yield. There was not any reactivity found in a filtration

test, and no Zinc leaching was detected by atomic absorption spectroscopy.

The transesterification of soybean oil with methanol has been reported using a solid base

catalyst, prepared by calcining the mixture of porous HAP and Sr(NO3)2 at 873K [307]; the

activity of the catalysts showing strong dependency upon the loading amount of Sr(NO3)2

onto porous HAP. The catalyst of 20 wt% Sr(NO3)2/HAP showed the highest catalytic activity

towards the transesterification and the catalytic activity decreased when the loading amount

of Sr(NO3)2 exceeded 20 wt% presumably due to the decrease of specific surface area.

Importantly, no catalytic activity was discerned when the Sr(NO3)2/HAP was non-calcined,

which is due to the lack of strong basic sites.The reaction of the solid phase ion-exchange

75
between Ca5(PO4)3(OH) and SrO was prompted by decomposition of Sr(NO3)2 at 873 K and

led to the formation of Ca5-xSrx(PO4)3(OH) and CaxSr1-xO. The latter was dispersed onto the

surface of HAP, which prevented the decomposition and recrystallization of the porous HAP,

and maintained porous morphology of HAP at 873 K with the active base sites were probably

derived from the CaxSr1-xO phase. The catalytic activity of KF/HAP prepared by a wet-

impregnation method was investigated in the synthesis of glycerol carbonate via

transesterification reaction from glycerol and dimethyl carbonate [308]. SEM

characterizations concluded that KF was well dispersed on the surface of HAP, which was

consistent with XRD results. Using dimethyl carbonate/glycerol in a molar ratio of 2:1,

catalyst/glycerol in a weight ratio of 3% with 50 minutes reaction time at 78°C, the

conversion of glycerol and yield of glycerol carbonate reached 99.3% and 99.0%,

respectively. Comparing conversions under the same reaction methods, the KF/HAP catalyst

was superior to Zr/HAP, Li/HAP, Ce/HAP, Li/HAP and K/HAP (Table 23); the selectivity to the

glycerol carbonate being higher than 98% of the overall modified HAP catalysts. It is

particularly noteworthy that the catalytic performance of KF/HAP was comparable to the

homogenous KF catalyst and better than that of K2CO3 and HAP alone.

Table 23

Synthesis of glycerol carbonate over different modified HAP catalysts [308].

Entry Catalyst Time (h) Conversion (%) Yield (%)

1 HAP 2 5.0 4.5

76
 2 Zr/HAP 2 46.9 46.6

3 Ce/HAP 2 50.5 49.7

4 Li/HAP 2 34.1 33.7

5 La/HAP 2 54.7 54

6 K/HAP 2 64.0 62.9

7 KF/HAP 0.83 99.1 99.0

To establish the reusability of the catalyst for glycerol transesterification, the catalyst was

filtered after the first reaction and then washed with methanol. It was then dried in an oven

at 383 K before being reused for up to four cycles; a slow decrease in conversion of glycerol

and an increase of reaction time occurred. To check the heterogeneity of the catalyst, it was

separated by simple filtration after the second run, then another equivalent of the reactants

were added. The reaction was performed and the conversion of glycerol was only 2.1%,

indicating that only a very small amount of the KF used for the reaction leached into the

product mixture.

4.6.5. Hydroformylation reactions

Hydroformylation, also known as oxo synthesis, is a chemical process that adds a formyl

group and a hydrogen atom to an alkene to form an aldehyde.This reaction was accidentally

discovered by Otto Roelen in 1938 during his investigations into the origin of oxygenated

products formed in cobalt-catalyzed Fischer-Tropsch reactions [309]. The hydroformylation

reaction represents one of the most important synthetic transformations developed in the

20th century [310] and is generally catalyzed by soluble rhodium or cobalt complexes with

various ligands, using homogeneous catalytic systems because of its high reactivity,

selectivity, and turnover numbers, and milder reaction conditions. Rhodium catalysts

77
typically work under mild conditions (100°C, 25 bar), giving good activity and selectivity (80

to 90%) for the desired linear aldehyde. Nonetheless, most commercial plants carrying out

this reaction use cobalt catalysts, which require much harsher conditions (typically 200°C,

100 bar) and give poorer selectivity; this may be due to rhodium catalysts’ decomposition

while attempting to distill the product from them [311]. To overcome these separation

problems, chemists and engineers have investigated two strategies other than distillation for

recycling of the catalyst. In the first strategy, the catalyst is anchored to some kind of soluble

or insoluble support, and the separation is conducted via a filtration technique often

referred to as heterogenizing homogeneous catalysts. The other strategy involves designing

the catalyst so that it is solubilized in a solvent that, under some conditions, is immiscible

with the reaction product. These reactions involve two phases and are often referred to as

biphasic systems. To date, there are relatively few examples of HAP-supported catalysts

used in hydroformylation reactions. One study used the apatitictricalcium phosphate as

support for supported aqueous phase hydroformylation of octene at 80°C in toluene, using a

dinuclear rhodium complex bearing as tris(m-sulfonatophenyl)phosphine hydrophilic ligands.

Kalck and colleagues attributed the high aldehyde yield to the favorable orientation of the

catalytic molecule that arose from the absorption of some of thetris(m-

sulfonatophenyl)phosphine ligands onto the hydrophilic surface of apatite [312].

Wang and colleagues prepared HAP supported rhodium and explored its catalytic

prowess in the hydroformylation of 1-hexene [313]. This catalyst was prepared by treating

the HAP with Rh(CO)2(acetylacetonate) in acetone at room temperature (Scheme 32) and

the supported catalyst was characterized by infrared and solid state NMR. It was proposed

that the proton in the P–OH group of HAP reacted with the acetylacetonate ligand to

generate acetylacetone, which coordinated with Rh dicarbonyl; the latter being anchored on

78
the surface via the resulting P–O− group. This supported catalyst exhibited high catalytic

activity in catalyzing hydroformylation of 1-hexene without the need of any auxiliary ligand

and Importantly, was found to be more active than the unsupported

Rh(CO)2(acetylacetonate) homogeneous system. The catalyst was recycled by simple

filtration and reused, although there was a slight reduction in the yield.

Scheme 32. Hydroformylation of 1-hexene over HAP supported rhodium [313].

4.6.6. Hydrogenolysis reactions

The hydrogenolysis reactions haveattracted increased interest due to their potential

application in many organic processes such as the conversion of glycerol into commodity

chemicals [314]; chemical bond dissociation and simultaneous addition of hydrogen to the

ensuing molecular fragments occured. In this context, Kaneda and co-workers examined the

efficiency of PdHAP as catalyst in hydrogenolysis of C−X bonds of various organic halides

[315]. Pd(0)HAP catalyst showed good catalytic activity in hydrodehalogenation of

chlorobenzene to benzene using 0.05 mol% of catalyst and in molecular hydrogen

atmosphere. Additionally, PdHAP was able to promote the dehalogenation of halophenols

such as 4-chlorophenol in water under an atmospheric pressure of molecular hydrogen

(Scheme 33); hydrogenolysis of bromooarenes namely 4-bromochlorobenzene to

chlorobenzene occurs inhigh yield (95%).

79
 Scheme 33. Hydrogenolysis of 4-chlorphenol over PdHAP [315].

It is worth noting that the PdHAP was highly effective for the deprotection of N-

benzyloxycarbonyl group from amino acids in the presence of molecular hydrogen and in

methanol as solvent (Table 24) [316]. This catalyst exhibited superior catalytic properties

compared to most common catalyst deployed for such reaction such as Pd/C, Pd/SiO2, and

Pd/Al2O3. The PdHAP catalyst was also efficient in the deprotection of N-benzyloxycarbonyl

group poly(amido amine) dendrimer functionalized with n-hexyl groups.

Table 24

Hydrogenolysis of Z-protected amino acids catalyzed by PdHAP [316].

Entry Catalysts (mol%) Yield (%)

1 PdHAP (0.4) 92

2 Pd/C (5.0) 68

3 Pd/SiO2 (0.5) 64

4 Pd/Al2O3 (0.5) 60

In another study, the same research group used this catalyst in the hydrogenolysis reactions

of 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT) and its derivatives 1,1-dichloro-2,2-

bis(4-chlorophenyl)ethene (DDE) and 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane (DDD)

80
using hydrogen under moderate reaction conditions (Scheme 34) [317]. Importantly, this

catalyst was found to be more active compared to other catalytic systems such as Pd/Al2O3

and Pd/C.

Scheme 34. Hydrogenolysis of DDT, DDE and DDT usingPdHAP catalyst [317].

4.6.7. Hydrogenation reactions

Hydrogenation is catalytic process of great importance to the petrochemical and fine

chemical industries [318]. This process is lately investigated to convert carbon dioxide to

methanol and alkenes and aromatics into saturated alkanes and cycloalkanes [319,320].

Several research groups have used the HAP as a support to design heterogeneous catalysts

for hydrogenation reactions [321]. Venugopal and co-workers investigated the catalytic

activity of HAP-supported Ru, Pt, Pd, Ni catalysts also in the hydrogenation of levulinic acid

to γ-valerolactone using water as the sole reaction medium [322]. Among the evaluated

catalysts, the RuHAP exhibited good catalytic activity and selectivity to γ-valerolactone at

70 °C under 0.5 MPa H2 pressure (Scheme 35). Recycling studies have shown that the
81
Ru/HAP catalyst can be readily recovered and reused up to four times with only a slight loss

of activity. The HAP supported Ru was found to be an active catalyst for the hydrogenation

of quinoline at 150 °C and 5 MPa of hydrogen pressure in cyclohexane [323]. The authors

believe that this high performance of this catalyst is attributed to the cooperation between

the hydroxyl groups on support surface and active metal centers. Zahmakıran and co-

workers reported the preparation of Ru(0) and Rh(0) nanoclusters supported on HAP and

their use for the hydrogenation of aromatics to their corresponding cyclohexane derivatives

under mild conditions [324,325]. Bimetallic Ru-Zn catalysts supported on HAP was prepared

via ion-exchange method and used for the partial hydrogenation of benzene to cyclohexene

[326]. The hydrogenation reaction was conducted over RuZnHAP catalyst at the optimized

conditions and the catalyst can be recovered and reused at least four times without any loss

of catalytic activity.

Scheme 35. Hydrogenation of levulinic acid over RuHAP catalyst [322].

Huang and co-workers have shown RhHAP catalyst to be an effective catalyst for the

hydrogenation of nitroarenes and olefins with under mild conditions [327]. A variety of

nitroarenes bearing various halides functional groups such as fluorine, chlorine, bromine

were hydrogenated selectively to the corresponding anilines over the RhHAP catalyst in the

presence of hydrazine as a reducing agent (Scheme 36). Moreover, olefins were also

hydrogenated to the corresponding alkanes in good yields over the RhHAP. This catalyst

could be simply recovered by filtration and reused without a significant loss of catalytic

activity for at least five times without loss of activity.

82
 Scheme 36. RhHAP-catalyzed hydrogenation of nitroarenes with hydrazine [327].

Finally, it is worth noting that Dai and co-workers have prepared CuHAP catalysts by facile

ammonia-assisted one-pot process and investigated their catalytic activity in the selective

hydrogenation of dimethyl oxalate to methyl glycolate and ethylene glycol [328]. The

experimental findings showed that methyl glycolate was the main product of the reaction at

210°C and when the temperature was raised to 240°C, the catalytic behavior for the Cu/HAP

catalysts were similar to the conventional Cu/SiO2 catalysts generating ethylene glycol as the

main product.

83
Summary and outlook

As demonstrated in this review, tremendous progress has been achieved during the last

few years pertaining to the synthesis and use of HAP in heterogeneous catalysis. This solid

material offers numerous advantages such as high thermal stability and weak acid-base

characters that may prevent occurrence of side reactions promoted by the support itself.

The use of HAP also offers the possibility of generating modified chemical composition and

transitioning it from the stoichiometric form to the non-stoichiometric Ca-deficient

composition. In addition, the replacement of Ca2+ sites in CaHAP by divalent or trivalent

cations such as Sr2+, Ba2+, Pd2+, Ru3+ leads to stable material formation with enhancement of

catalytic performances. However, a number of challenges remain untapped. For instance,

the textural properties and the mesoporosity of such material are still difficult to control.

Another weakness of HAP is its poor mechanical properties that are not adequate enough to

prevent its success in long-term load bearing applications. It is also worth noticing that HAP

may decompose in an acid medium, which poses as a significant drawback of the material.

For all aforementioned reasons, the interest on studying the earth-abundant HAP will

continue to be a fast-progressing topic in the coming years.

84
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1. Synthetic methods for obtaining hydroxyapatite are reviewed.

2. The structure-reactivity studies of hydroxyapatite as related to catalysis

performances are discussed for a number of significant reactions.

3. The catalytic applications of hydroxyapatite are introduced and discussed.

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