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 Page 1 of 33 RSC Advances
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DOI: 10.1039/C5RA17623A

Glycerol acetins: Fuel additive synthesis by acetylation and esterification

of glycerol using cesium phosphotungstate catalyst
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Swetha Sandesh, Pandian Manjunathan, Anand B. Halgeri and Ganapati V. Shanbhag*

Materials Science Division,

RSC Advances Accepted Manuscript

Poornaprajna Institute of Scientific Research (PPISR),
Bidalur Post, Devanahalli, Bengaluru-562164, Karnataka State, India

(* corresponding author: shanbhag@poornaprajna.org, Phone: +91-80-27408552)

1
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2
3 Abstract
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4 Glycerol acetylation and esterification reactions with acetic anhydride and acetic acid

5 respectively give acetins, in which di and tri acetins are commercially important products

RSC Advances Accepted Manuscript

6 used as fuel additives. Acetylation and esterification of glycerol were studied over various

7 solid acid catalysts namely, cesium phosphotungstate, amberlyst-15, H-beta, sulfated zirconia

8 and montmorillonite K-10 under mild reaction conditions. The catalysts were characterized

9 by XRD, FTIR, SEM and acidity measurements. Among all the catalysts evaluated in this

10 study, cesium phosphotungstate showed highest activity with > 98% conversion for both the

11 reactions, whereas di and triacetins selectivity was 99.1% for acetylation and 75% for

12 esterification reaction. The catalyst with high Brnsted acidity gave high activity for both the

13 reactions, whereas selectivity for di and tri acetins depends on nature of active sites.

14
15
16
17

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1
2 1. Introduction

3 The growing scarcity of fossil hydrocarbons made the researchers to find alternative
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4 source of energy. Biomass is considered to be a potential raw material for making renewable

5 fuels. Transesterification of vegetable oils or animal fats through catalytic route produces

RSC Advances Accepted Manuscript

6 biodiesel as the main product (90 wt%) and glycerol as a byproduct (10 wt%).1 Increase in

7 the production of biodiesel causes increase in glycerol availability in tons. Glycerol is a

8 propanetriol containing three hydroxyl groups which can be functionalized as well as

9 removed through catalytic processes. Transformation of available glycerol to value-added

10 products like oxygenated fuel additives, chemicals, solvents etc. can add value to it and make

11 the process economically viable and valuable.2-8

12 Glycerol undergoes acetylation with acetic acid or acetic anhydride in presence of

13 acid catalyst to yield acetins namely monoacetin, diacetin and triacetin. Di and triacetins can

14 be used as fuel additive which have been introduced in biodiesel formulation to improve its

15 viscosity property as cold flow improver and it has also been used as an antiknock additive

16 for gasoline. Triacetin is also used in cosmetics, whereas monoacetin and diacetin are used as

17 plasticizer in cigarette filters and as raw materials for the production of biodegradable

18 polyesters.9

19 Homogeneous acids such as H3PW12O40 showed higher catalytic activities towards

20 acetylation reaction10,11 but faced several practical difficulties in separation of catalyst,

21 recyclability and handling. To overcome these practical difficulties, variety of Brnsted solid

22 acid catalysts have been reported for the reaction of glycerol with acetic acid or acetic

23 anhydride. Solid acid catalysts such as amberlyst-15, montmorillonite K-10, beta zeolite and
12,13
24 H-USY were applied as catalysts for this reaction which showed 100% glycerol
14-20
25 conversion using higher catalyst concentrations. Supported sulfonic acid catalysts and

3
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21
1 mixed oxides like MoO3/TiO2ZrO2, Y/SBA-3 22 have been reported to be active catalysts

2 for acetylation of glycerol. Even supported heteropoly acid catalysts like PW on silica, Cs-

3 containing zirconia, carbon, niobic acid with high thermal stability and high surface area have
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4 been used but showed less efficiency for this reaction.23-27 Silver ion exchanged

phosphotungstic acid catalyst is also used for the esterification of glycerol with acetic acid.28

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5

6 Recently, cesium exchanged heteropoly acid has received greater attention as a

7 catalyst in many reactions due to its heterogeneous property, high thermal stability and higher

8 surface area compared to parent heteropoly acid. The catalytic properties of metal ion

9 exchanged heteropoly acid can be tuned by choosing appropriate metal salt and by varying

10 the extent of ion exchange. The studies in the literature show that the cesium

11 phosphotungstate is more active than parent PWA due to its high surface protonic acidity

12 with high acid strength of the proton associated to the polyanion.29-32

13 The aim of this work is to explore a catalyst for the synthesis of glycerol acetins under

14 mild reaction conditions (low temperature, mole ratio and catalyst concentration) and to get

15 higher activity and selectivity for di and triacetins. Different well known solid acid catalysts

16 like cesium phosphotungstate, zeolites, resin, clay and sulfated zirconia were studied for both

17 acetylation and esterification reactions of glycerol. The best catalyst was taken further for

18 detailed studies. The physicochemical properties of the catalysts were correlated with

19 catalytic activity and selectivity for acetins.

20 2. Experimental

21 2.1. Chemicals and catalysts

22 Glycerol and acetic acid were purchased from Merck India Ltd. Cesium carbonate and

23 phosphotungstic acid (PWA) were procured from SD fine chemicals, India. Amberlyst-15

24 (hereafter AB-15) was obtained from Alfa Aesar, USA. The montmorillonite K-10 clay

25 (hereafter K-10) was purchased from Sigma Aldrich, USA. H-beta (SAR-25) was kindly

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1 donated by Sd-Chemie India Pvt Ltd. All the chemicals were of research grade and used

2 without any further purification.

3 The acidic cesium phosphotungstate (hereafter CsPWA) was prepared following a

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4 literature procedure.33 The final composition of the salt was found to be Cs2.5H0.5PW12O40.

Other solid acid catalyst, sulfated zirconia (SZ) was synthesized by literature method.34

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5

6 2.2. Catalyst characterization

7 Powder X-ray diffraction patterns of CsPWA and PWA were recorded with Bruker

8 D2 phaser X-ray diffractometer using CuK radiation ( = 1.542 ) with high resolution

9 Lynxeye detector. All the samples were scanned in the 2 range of 5 80. The specific

10 surface areas of the catalysts were determined by nitrogen sorption measurement using

11 Quantachrome NOVA instrument at 77 K.

12 The nature of acidic sites of catalysts was investigated by pyridine adsorption study

13 using Pyridine-FT-IR (alpha-T, Bruker) and the spectra were obtained in the range of 1400

14 1600 cm1. The catalyst pellets were saturated by pyridine followed by degassing at 150 C

15 for 1 h. The FTIR spectra in absorbance mode for pyridine treated sample were subtracted

16 with pyridine untreated sample to obtain the peaks only due to pyridineacid interaction.34

17 In addition to above method, acidity of the catalysts was determined by

18 potentiometric titration. About 0.05 g of sample was suspended in 5 ml of n-butylamine

19 solution (0.05 N) in acetic acid and sonicated for 5 min to attain uniform dispersion. Then the

20 above solution was suspended in excess of acetic acid (90 mL) and potentiometrically titrated

21 against perchloric acid (0.1 N) in acetic acid. Prior to sample titration, a blank titration of

22 acetic acid and n-butyl amine against perchloric acid was carried out to check the acidity

23 contribution from solutions used. ICP-OES was performed using a Thermo-iCAP 6000 series

24 in order to study the leaching of cesium in the reaction mixture.

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1 Scanning electron microscope (SEM) images of CsPWA catalyst were recorded on

2 Zeiss microscope to investigate the crystallite size and morphology.

3 2.3 Catalytic activity studies

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4 Catalytic activity studies were performed in a liquid phase glass batch reactor. Prior to

the reaction, the catalysts (except Amberlyst-15) were activated at 120 C to remove the

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5

6 moisture.

7 a) Acetylation reaction of glycerol with acetic anhydride: In a typical procedure, the reaction

8 was performed in a 100 ml two-necked glass reactor equipped with a magnetic stirring bar, a

9 Liebig condenser, and a thermometer. The glycerol and acetic anhydride were taken in the

10 ratio of 1: 3 in the glass reactor and 4 wt% of catalyst (with respect to total reactants) were

11 added into it. The reaction was performed under stirring at room temperature.

12 b) Esterification reaction of glycerol with acetic acid: In a typical procedure, the reaction was

13 carried out in a 100 ml two-necked glass reactor equipped with a magnetic stirring bar, a

14 Liebig condenser, and a thermometer. The required amounts of glycerol and acetic acid were

15 taken in the reactor and desired catalyst weight was added into it. The reaction was performed

16 under stirring at desired temperature.

17 For both the reactions, same separation procedure was followed; the reaction mixture

18 was taken out and centrifuged for 10 min to separate the catalyst from liquid phase. The

19 obtained product was analyzed in gas chromatography (Shimadzu, GC-2014) with flame

20 ionization detector (FID) equipped with capillary column (0.25mm I.D and 30 m length,

21 Stabilwax, Restek). All the products were confirmed by gas chromatography with mass

22 spectroscopy (Shimadzu, GCMS QP 2010).

23 3. Results and discussion

24 3.1. Catalyst characterization

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1 The formation of crystalline phase of CsPWA catalyst was confirmed by XRD in

2 comparison with phosphotungstic acid (Fig.1a). XRD pattern of PWA shows the diffraction

3 peaks corresponding to cubic Pn3m crystalline structure. Interestingly, the diffraction peak of
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4 CsPWA became significantly broader with a right shift in 2 value (25) compared to PWA.

The shift of diffraction peak towards higher angle in CsPWA is attributed to the formation of

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5

6 body centered cubic structure.29,35

7 The characteristic presence of Keggin structure of CsPWA and phosphotungstic acid

8 was confirmed by FTIR studies (Fig. 1b). Four bands at 700-1100 cm-1 region corresponding

9 to Keggin unit (PWA) structural vibrations are observed for PWA and CsPWA suggesting

10 that the framework of primary Keggin structure remained unaltered after modification of

11 PWA with cesium salt. The peaks corresponding to Keggin anion vibration are as follows.

12 The stretching frequency of PO in the central PO4 tetrahedron is at 1084 cm-1. The peak at

13 991 cm-1 is due to the terminal W=O vibration in the WO6 octahedron and the peak at 890

14 and 794 cm-1 were assigned to WObW and WOcW bridges respectively. Weaker peak

15 appearing at 595 cm-1 is due to bending vibrations of WOW bonds. 32

16 The physicochemical properties of CsPWA, AB-15, K-10, H-beta and sulfated

17 zirconia are tabulated in Table 1. The specific surface area of as-prepared CsPWA was found

18 to be 110 m2/g. H-beta and K-10 exhibited higher surface area of 450 and 250 m2/g

19 respectively, whereas amberlyst-15 and sulfated zirconia gave lower surface areas < 60 m2/g.

20 The interaction of pyridine nitrogen with acidic sites gave two different frequency of

21 bending vibrations. The bending vibrations around 1445 cm-1 and 1540 cm-1 are assigned as

22 Lewis (L) and Brnsted (B) acid sites respectively and B/L ratio were measured using the

23 peak intensities. Pyridine-FTIR spectra of CsPWA catalyst showed a strong Brnsted acidity

24 due to the presence of protons (peak at 1540 cm-1) and weak Lewis acid sites (peak at 1445

25 cm-1) as depicted in Fig. 2. The CsPWA contained high B/L ratio of 3.86 compared to other

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1 solid acid catalysts used in this study except AB-15 (Table 1). AB-15 is a pure Brnsted acid

2 catalyst with sulfonic acid groups on polystyrene chain. The B/L ratio decreased in the order;

3 AB-15 > CsPWA > K-10 > H-beta > sulfated zirconia.
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4 Potentiometric acid-base titration revealed the total acidity of the catalysts (tabulated

in Table 1). The total acidity of CsPWA, H-beta, K-10 and SZ was found to be 1.87, 1.49,

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6 1.10 and 1.48 mmol/g respectively. H-beta zeolite and sulfated zirconia has same amount of

7 acidic sites. Acidity of AB-15 was 4.7 mmol/g as given by the manufacturer.

8 CsPWA catalyst exhibited the morphology of the spherical shaped particles with size

9 ranging from 70-200 nm as shown in Fig. 3.

10 3.2. Catalyst screening

11 Acetylation and esterification of glycerol was studied over various solid acid catalysts

12 namely, CsPWA, AB-15, H-beta, sulfated zirconia and K-10 using acetic anhydride and

13 acetic acid respectively. The performance of the catalyst is measured by glycerol conversion

14 and the selectivity to di and triacetins.

15 Acetylation of glycerol using acetic anhydride was carried out over different Brnsted

16 solid acid catalysts at room temperature (30 C) (Fig. 4.). Prior to the catalytic reaction, a

17 blank run was carried out without a catalyst, which resulted in negligible glycerol conversion

18 (2.5%) with 100% selectivity to monoacetins. Among the solid acid catalysts screened, the

19 catalyst containing higher amount of acid sites viz. CsPWA (1.87 mmol/g) and AB-15 (4.7

20 mmol/g) resulted in maximum glycerol conversion (100%) with higher glycerol diacetins and

21 glycerol triacetin selectivity of 99.1 and 99.9% respectively. Among the solid acid catalysts

22 screened, the catalyst containing higher amount of acid sites viz. CsPWA (1.87 mmol/g) and

23 AB-15 (4.7 mmol/g) gave maximum glycerol conversion (100%) with higher selectivity

24 towards diacetins (17 and 23%) and triacetin (82 and 77%) respectively. The catalytic

25 activity of CsPWA showed higher triacetin selectivity of 82% at room temperature compared

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1 to all other solid acid catalysts for 2 h of reaction time. This result shows that the utilization

2 of acetic anhydride is maximum for CsPWA and AB-15 with higher selectivity to triacetin

3 compared with other catalysts namely K-10, H-beta and sulfated zirconia. The glycerol
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4 conversion reached to a maximum of 100% at the initial time period, but the triacetin

selectivity was found to increase with time for CsPWA and AB-15 with a decrease in mono

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5

6 and diacetins selectivity (Fig. 4). K-10 containing B/L ratio of 2.3 gave lower triacetin

7 selectivity of 32%, whereas H-beta catalyst (B/L ratio of 1.92) showed 80% diacetin

8 selectivity. Sulfated zirconia, the catalyst with higher Lewis acidic sites showed very low

9 glycerol conversion of 25%. These results clearly show that the catalyst with higher Brnsted

10 acidic sites gives higher glycerol conversion with high di and triacetins selectivity. The

11 glycerol conversion and triacetin selectivity increased for the catalysts in the following order;

12 SZ < H-beta < K10 < AB-15 < Cs/PWA.

13 Since CsPWA and AB-15 showed complete glycerol conversion at 30 min, it was not

14 possible to decide the best catalyst among the two. Therefore, the catalyst concentration was

15 reduced to 1 wt% (w.r.t. total reactants) and as a result, the catalytic performance of AB-15

16 showed lower glycerol conversion of 25% at 30 min. As the time increased, glycerol

17 converted completely with increase in triacetin selectivity. But CsPWA catalyst showed 99.8

18 % glycerol conversion even at less catalyst amount for 30 min with higher triacetin selectivity

19 compared to AB-15 (Fig. S1). The turn over frequency of all the catalysts (Table 1)

20 increased in the following order; SZ < AB-15 < H-beta < K-10 < CsPWA. Highest TOF/h of

21 267 was obtained for CsPWA which clearly proves that CsPWA is highly active catalyst for

22 acetylation reaction of glycerol. The high selectivity towards triacetin using acetic anhydride

23 as acetylating agent compared to acetic acid can be explained on the basis of formation of

24 intermediate acylium ion (Scheme S1).12

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1 In order to study the catalytic behaviour at lower reactant mole ratio, the reaction was

2 studied with glycerol : acetic anhydride of 1:1.5 and 1:2 (Table 2). It showed relatively lower

3 selectivity towards triacetin compared to higher reactant mole ratio of 1:3 suggesting that the
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4 formation of triacetin is greater with higher amount of acetylating agent.

Further, the esterification of glycerol was studied using acetic acid under reaction

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6 conditions; glycerol : acetic acid of 1:8, 85 C and 7 wt% of catalyst referred to total

7 reactants (Fig. 5). As observed in the acetylation reaction, a similar catalytic performance was

8 observed with high performance of CsPWA and AB-15 compared with other catalysts. The

9 glycerol conversion reached to 98% using CsPWA and AB-15 within 2 h with increase in

10 diacetins and triacetin selectivity. CsPWA exhibited higher catalytic performance with

11 triacetin selectivity of 27%, whereas AB-15 gave 22% triacetin selectivity. Among these two

12 catalysts, CsPWA exhibited much higher TOF at 30.5 h-1 compared to AB-15 (12.3 h-1)

13 (Table 1).

14 Among lower active catalysts, large pore H-beta zeolite exhibited comparatively

15 greater catalytic performance than K-10 and sulfated zirconia. Glycerol conversion increased

16 from 28 to 80% with increase in time from 1 to 5 h using H-beta and finally reached to 37%

17 diacetins selectivity (5 h). Triacetin did not form with H-beta catalyst. In contrast, K-10 clay

18 showed lower glycerol conversion (63%) compared to H-beta zeolite but it gave triacetin

19 selectivity of 4% (5 h). Glycerol conversion of 70% with 20% diacetin selectivity was

20 observed for sulfated zirconia catalyst. It exhibited lower activity compared to other acid

21 catalysts which could be due to lower B/L ratio (1.46), since the esterification reactions are

22 predominantly catalyzed by Brnsted acid sites. Thus, the catalytic activity towards

23 esterification of glycerol with acetic acid gives a clear picture with respect to nature of acidic

24 sites (B/L ratio) of the catalyst. The turn over frequency of the screened catalysts increased in

25 the following order; SZ < AB-15 H-Beta < K-10 < CsPWA.

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1 3.3 Influence of reaction conditions on esterification reaction of glycerol with acetic acid

2 The influence of reaction parameters viz. catalyst concentration, reaction temperature,

3 and reactants mole ratio on catalytic activity using CsPWA catalyst were studied for glycerol
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4 esterification with acetic acid.

Effect of catalyst concentration was studied with glycerol to acetic acid mole ratio of

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6 1 : 8 at 85 C for 2 h. The catalyst concentration was varied from 3 to 9 wt% (Table 3). The

7 glycerol conversion was found to increase from 56 to 98% with increase in the catalyst

8 concentration from 3 to 7 wt%. The lesser catalytic activity with catalyst concentration of 3

9 and 5 wt% indicates the requirement of higher active sites for the reaction. Selectivity to

10 diacetins (31 and 34%) was almost the same for 3 and 5 wt% catalysts, but the triacetin was

11 formed with 5 wt% catalyst (5% selectivity), whereas for 3 wt% catalyst, triacetin was not

12 observed. The catalytic activity was found to be almost the same with 7 and 9 wt% catalyst

13 concentrations. The glycerol conversion increased from ~ 84 to 98% as the time increased

14 from 30 min to 2 h. The maximum of 98% glycerol conversion was attained at 1 h using 7

15 wt% catalyst concentration, but the selectivity to diacetin varied from 55 to 59% after 2 h.

16 The maximum triacetin selectivity of 16% was obtained after 2 h. No major variation in

17 catalytic performance was observed with further increase in catalyst concentration to 9 wt%.

18 Moreover, the selectivity to all the acetins remained the same as in the case of 7 wt% catalyst

19 concentration. This indicates that the amount of active acidic sites in 7 wt% catalyst

20 concentration is sufficient to get the maximum activity of glycerol conversion and selectivity

21 to the desired product.

22 The effect of glycerol to acetic acid mole ratio was studied from 1:4 to 1:10 at 85 C

23 for 2 h (Table 4). The conversion of glycerol increased with increase in mole ratio from 1:4 to

24 1:8 due to increase in availability of accessible acetic acid with glycerol. The glycerol

25 conversion and selectivity to acetins remained almost the same with further increase in mole

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1 ratio of reactants from 1:8 to 1:10. The reaction condition with 1:8 mole ratio was found to be

2 the best compared with other mole ratios. A gradual increase in glycerol conversion from 45

3 to 69% with increase in reaction time was observed for 1:4. Formation of triacetin was found
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4 to be nil at this mole ratio. This indicates that at 1:4 mole ratio, the amount of accessible

acetic acid was not sufficient for the maximum conversion of glycerol to yield higher amount

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6 of diacetin and triacetin. For mole ratio 1:6, the glycerol conversion increased from 67 to

7 92% with the increase in time from 30 to 120 min. The catalytic activity with 1:8 and 1:10

8 mole ratio was found to be almost the same. The glycerol reached a maximum conversion of

9 98% with negligible changes in the acetins selectivity (diacetins and triacetin was 59 and

10 16% respectively). Therefore, 1:8 reactants mole ratio was found to be the best mole ratio for

11 further studies.

12 The effect of temperature was studied at four different temperatures ranging from 65

13 to 95 C using glycerol to acetic acid mole ratio of 1:8 for 2 h. From Table 5, it is observed

14 that the glycerol conversions were low and slowly increased with time at temperatures of 65

15 and 75 C, which could be attributed to lesser formation of acylium ion from acetic acid at

16 lower temperatures. At higher temperatures of 85 and 95 C, glycerol conversion reached to a

17 maximum of 98% and remained almost the same, indicating that the formation of acylium ion

18 is faster at these temperatures. It is also observed that di and triacetins increased with increase

19 in reaction time. The catalytic activity at 85 C was found to be best temperature for

20 esterification reaction since the glycerol reached a maximum conversion of 98% with the

21 selectivity to di and triacetin of 59 and 16% respectively.

22

23 3.4. Plausible reaction mechanism for esterification reaction of glycerol with acetic acid

24 The plausible reaction mechanism for esterification of glycerol with acetic acid

25 proceeds by the activation of acetic acid carbonyl group by CsPWA catalyst whereby

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1 electrophilicity of carbonyl carbon increases (Scheme 1). Then the carbonyl carbon is

2 attacked by the oxygen of glycerol. The transfer of proton from the intermediate to the second

3 hydroxyl group of glycerol gives an activated complex with the formation of water molecule.
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4 Later, with the loss of water molecule gives monoacetin. The above-mentioned mechanism

continues sequentially further by the reaction of monoacetin with acetic acid to form diacetin

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6 and triacetin.

8 3.5. Catalyst reusability and leaching studies

9 Catalyst recyclability test was performed for CsPWA catalyst under optimized

10 reaction conditions for both acetylation and esterification reactions. The catalyst showed

11 good recyclability with similar activity after 3 recycles (Table 6). In case of acetylation, the

12 selectivity of triacetin was retained after initial decrease in the 1st recycle. For esterification

13 reaction, with each recycle, triacetin decreased marginally with the increase of monoacetin

14 and diacetin. XRD analysis of the fresh and 3 times recycled catalyst showed no change in

15 the phase purity of the catalyst (Fig. 6). FTIR analysis of the spent CsPWA catalyst was also

16 performed and it showed no change in the characteristic peak of Keggin structure after the

17 recycle (Fig. S2).

18 The leaching test was carried out for acetylation and esterification reactions by

19 investigating the leaching of Cs in the catalyst into the reaction media. The study was

20 performed under the optimized reaction conditions where the reaction was stopped at 2 and 5

21 h for acetylation and esterification reactions respectively and the catalyst was filtered out.

22 Thus obtained filtrate was subjected to ICP-OES analysis of Cs in order to find the leaching

23 of Cs. The analysis confirmed the absence of Cs in the reaction mixture under the detection

24 limit of 0.01ppm which suggests that the catalyst is truly heterogeneous.

25

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2 3.6. Comparison of CsPWA with the reported catalysts for the esterification of glycerol

3 The active CsPWA catalyst was compared with the reported catalysts for the
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4 esterification of glycerol and the data was tabulated in Table S1. Among the reported

catalysts, supported heteropoly acids viz. HSiW/ZrO2 and HPW/ZrO2 catalysts showed

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6 slightly higher selectivity for di and triacetins compared to CsPWA catalyst. However, higher

7 reaction temperature (difference of 35 C) and glycerol to acetic acid mole ratio were used

8 for these catalysts. At lower temperature and reactants mole ratio, CsPWA catalyst showed

9 higher activity and selectivity towards di and triacetins compared to the reported catalysts.

10 4. Conclusions

11 Acetylation and esterification of glycerol were studied with acetic anhydride and

12 acetic acid respectively using different solid acid catalysts. The yields of mono, di and

13 triacetins were differed with the nature of acid catalysts. Among the solid acid catalysts

14 screened, the catalyst containing higher amount of acid sites viz. CsPWA (1.87 mmol/g) and

15 AB-15 (4.7 mmol/g) resulted in maximum glycerol conversion (100%) with higher di and

16 triacetins selectivity of 99.1 and 99.9% respectively for acetylation reaction. CsPWA showed

17 highest triacetin selectivity of 82% at room temperature compared to all other solid acid

18 catalysts. The turn over frequency for acetylation of glycerol increased in the following order;

19 SZ < Amberlyst-15 < H-beta < K-10 < CsPWA with highest TOF/h of 267 for CsPWA

20 catalyst. CsPWA catalyst also gave highest activity and selectivity for di and triacetins for

21 esterification of glycerol with acetic acid. The catalytic activity towards the reaction was

22 correlated with B/L ratio of the catalyst. Higher catalytic activities of CsPWA and AB-15 are

23 due to higher B/L ratio of the catalysts. Among the two catalysts, CsPWA gave highest di and

24 triacetins selectivity which could be due to the nature of active sites present in the catalyst.

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1 The catalyst exhibited good recyclability with marginal decrease in the activity after 3

2 recycles.

3 5. Acknowledgement
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4 Swetha S. acknowledges CSIR, New Delhi for providing Senior Research Fellowship

and also thankful to Manipal University for permitting this research as a part of the Ph. D

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6 programme.

8 6. References

9 1 J. C. S Ruiz, R. Luque, A. S. Escribano, Chem. Soc. Rev., 2011, 40, 5266.

10 2 M. O. Sonnati, S. Amigoni, E. P. T de Givenchy, T. Darmanin, O. Choulet, F. Guittard,

11 Green Chem., 2013, 15, 283-306.

12 3 M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C. D. Pina, Angew. Chem. Int. Ed., 2007,

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16 5 S. Sandesh, G. V. Shanbhag, A. B. Halgeri, RSC Adv., 2014, 4, 974-977.

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21 2015, DOI: 10.1039/c5cy01252j.

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1 12 L. N. Silva, V. L. C. Goncalves, C. J. A. Mota, Catal. Commun., 2010, 11, 1036-1039.

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15 I. D. Rodriguez, C. Adriany, E. M. Gaigneaux, Catal. Today., 2011, 67,5663.

RSC Advances Accepted Manuscript

5

6 16 J. A. Melero, R. V. Grieken, G. Morales, M. Paniagua, Energy. Fuels., 2007, 21, 1782-

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4 30 K. Johnson, B. Viswanathan, T. K. Varadarajan, Stud. Surf. Sci. Catal., 1998, 113, 233-

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9 170176.

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17
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Figures
Fig. 1. XRD patterns (a) and FTIR spectra (b) of CsPWA and PWA.
Fig. 2. Pyridine-FTIR spectra of catalysts.
Fig. 3. SEM images of CsPWA.
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Fig. 4. Catalytic activity of different solid acid catalysts with glycerol and acetic anhydride.
Reaction conditions: Glycerol : Acetic anhydride = 1 : 3, Temperature = 30 C, Catalyst

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weight = 3 wt%.
Fig. 5. Catalytic activity of different solid acid catalysts with glycerol and acetic acid.
Reaction conditions: Glycerol : Acetic acid = 1 : 8, Temperature = 85 C, Time= 5 h, Catalyst
weight = 7 wt%.
Fig. 6. XRD patterns of fresh and recycled catalyst
Scheme 1. Plausible reaction mechanism for the esterification
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Fig. 1. XRD patterns (a) and FTIR spectra (b) of CsPWA and PWA.
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Fig. 2. Pyridine-FTIR spectra of catalysts.

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Page 21 of 33

Fig. 3. SEM images of CsPWA catalyst

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Fig. 4. Catalytic activity of different solid acid catalysts with glycerol and acetic anhydride.
Reaction conditions: Glycerol : Acetic anhydride = 1 : 3, Temperature = 30 C, Time= 120
min, Catalyst weight = 4 wt%.
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Fig. 5. Catalytic activity of different solid acid catalysts with glycerol and acetic acid.
Reaction conditions: Glycerol : Acetic acid = 1: 8, Temperature = 85 C, Time= 5 h, Catalyst
weight = 7 wt%.
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Fig. 6. XRD patterns of fresh and recycled CsPWA catalyst
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Scheme 1.Plausible reaction mechanism for the esterification

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Tables
Table 1. Physicochemical properties and activities of different catalysts for acetylation and
esterification of glycerol.
Table 2. Effect of reactant mole ratio on catalytic performance in acetylation of glycerol with
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acetic anhydride.
Reaction conditions: Temperature = 30 C, Time= 120 min, CsPWA catalyst = 4 wt%.

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Table 3. Effect of catalyst concentration on catalytic activity in esterification of glycerol with
acetic acid.
Reaction conditions: Glycerol : Acetic acid = 1 : 8 , Temperature = 85 C, Catalyst =
CsPWA.
Table 4. Effect of reactant mole ratio on catalyst performance in esterification of glycerol
with acetic acid.
Reaction conditions: Temperature = 85 C, CsPWA catalyst = 7 wt%.
Table 5. Effect of temperature on catalyst performance in esterification of glycerol with
acetic acid.
Reaction conditions: Glycerol : Acetic acid = 1: 8 , CsPWA catalyst = 7 wt%.
Table 6. Reusability test of CsPWA catalyst on acetylation and esterification of glycerol.
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Table 1: Physicochemical properties and activities of different catalysts for acetylation and
esterification of glycerol.
Catalyst SBET Amount Py-FTIR Acetylationa Esterificationc
(m2/g) of acidity B/L ratio TOF TOF
(mmol /g) (h-1) (h-1)
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CsPWA 110 1.87 3.86 69.7 30.5

b

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267
AB -15 39 4.7 - 27.7 12.3
28.3b
H-beta 450 1.49 1.92 55.2 12.4
K-10 250 1.1 2.30 58.6 14.7
Sulfated zirconia 57 1.48 1.44 11.9 9.2
Turn over frequency (TOF) = Moles of glycerol converted per mole of acid site per hour.
a
Reaction conditions as in Fig. 4, b 1wt% catalyst, c Reaction conditions as in Fig. 5
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Table 2. Effect of reactant mole ratio on catalytic performance in acetylation of glycerol with
acetic anhydride.

Time Glycerol : Glycerol Acetin Selectivity (mol%)

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(h) Acetic conversion Mono Di Tri

anhydride (mol%)
1h 1:1.5 92.6 19.3 40.3 40.4

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2h 1:1.5 98 13.3 41.2 45.5
1h 1:2 98.1 12.8 33.3 54.0
2h 1:2 98.3 12.7 31.3 56.1
1h 1:3 100 1.2 21.5 77.3
2h 1:3 100 0.9 17.1 82

Reaction conditions: Temperature = 30 C, Time= 120 min, CsPWA catalyst = 4 wt%.

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Table 3. Effect of catalyst concentration on catalytic activity in esterification of glycerol with

acetic acid.

Catalyst Time Glycerol Acetin Selectivity (mol%)

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wt% (min) conversion Mono Di Tri

(mol%)
3 30 30 95 5 0

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3 60 35.4 80 20 0
3 90 40 72 28 0
3 120 56 69 31 0
5 30 40 86 14 0
5 60 52 75 25 0
5 90 62 68 30 2
5 120 70 61 34 5
7 30 84 37 55 8
7 60 97.5 35 56 9
7 90 98 27 58 15
7 120 98.1 25 59 16
9 30 85 36 54 10
9 60 95.6 30 58 12
9 90 97 28 58 14
9 120 98.2 26 58 16

Reaction conditions: Glycerol : Acetic acid = 1 : 8 , Temperature = 85 C, Catalyst =

CsPWA.
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Table 4. Effect of reactant mole ratio on catalyst performance in esterification of glycerol

with acetic acid.
Mole ratio Time Glycerol Acetin Selectivity (mol%)
Gly : Acetic (min) conversion
acid (mol%) Mono Di Tri
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1:4 30 45.4 81 19 0
1:4 60 58.7 75 25 0
1:4 90 62 73 27 0

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1:4 120 69 65 35 0
1:6 30 67 77 23 0
1:6 60 85.4 74 26 0
1:6 90 90 73 24 3
1:6 120 92 59 35 6
1:8 30 84 37 55 8
1:8 60 97.5 35 56 9
1:8 90 98 27 58 15
1:8 120 98.1 25 59 16
1:10 30 85 36 54 10
1:10 60 90.7 31 57 12
1:10 90 97 28 58 14
1:10 120 98.2 26 58 16

Reaction conditions: Temperature = 85 C, CsPWA catalyst = 7 wt%.

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Table 5. Effect of temperature on catalyst performance in esterification of glycerol with

acetic acid.

Temp Time Glycerol Acetin Selectivity (mol%)

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(C) (min) conversion Mono Di Tri

(mol%)
65 30 35.4 93 7 0
65 60 45.8 75 25 0

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65 90 52 69 31 0
65 120 65.4 68 32 0
75 30 57 90 10 0
75 60 62.5 84 16 0
75 90 88 74 23 3
75 120 92 60 34 6
85 30 84 37 55 8
85 60 97.5 35 56 9
85 90 98 27 58 15
85 120 98.1 25 59 16
95 30 85 35 55 10
95 60 87.3 32 56 12
95 90 97 27 59 14
95 120 98.2 25 59 16

Reaction conditions: Glycerol : Acetic acid = 1: 8 , CsPWA catalyst = 7 wt%.

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Table 6. Reusability test of CsPWA catalyst on acetylation and esterification of glycerol.

Catalyst Temp Glycerol Acetins Selectivity (mol%)
(C) conversion Mono Di Tri
(mol%)
Fresh 30 100 1 17 82
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Recycle-1 30 100 2 23 75
Recycle-2 30 100 6 19 75
Recycle-3 30 100 4 20 76
Reaction conditions: Glycerol : Acetic anhydride = 1:3,

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CsPWA catalyst = 4 wt%, Temp : 30 C, Time = 2 h
Fresh 85 98.1 20 53 27
Recycle-1 85 98.2 22 54 24
Recycle-2 85 98.5 23 57 20
Recycle-3 85 98 25 59 16
Reaction conditions: Glycerol : Acetic acid = 1:8,
CsPWA catalyst = 7 wt%, Temp : 85 C, Time = 5 h
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Graphical abstract
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