Copper catalyzed transformation of ethanol to value added chemicals: Studies on

particle size of Cu vs. Activity and selectivity

P. Gidyonu1*, Syed.Rafi2
*1&2
Department of chemistry, PACE Institute of Technology & Sciences(Autonomous),Ongole-
523272,Andhrapradesh,India.
paletigidyonu@gmail.com*
rafiresearch2@gmail.com

Abstract
The influence of particle size of Cu on the activity and selectivity in conversion of
ethanol to value-added chemicals such as acetaldehyde and diethylacetal (DEE) was
studied. Various Cu catalysts with different particle sizes were prepared by subjecting the
CuO in the calcination range of 350 to 500 °C. The physicochemical characteristics of these
catalysts were achieved by X-ray diffraction, N2O pulse chemisorption, TEM and H2-TPR
techniques. The copper particle sizes were in the range of 23-28 nm. The conversion of
ethanol and selectivity to acetaldehyde and DEE were strongly influenced by the particle
size of copper.

Key Words: Ethanol, Acetaldehyde, Diethylacetal, Copper, Particles Size

Introduction
The selective conversion of alcohols to aldehydes, ketones and acids is an essential
transformation for the synthesis of value-added chemicals [1]. It is much attractive
transformation if value-added chemicals are obtained from biomass-derived feedstock such as
glycerol, ethanol [2-4]. Ethanol is a platform chemical for converting many value-added
chemicals such as acetaldehyde, diethyl acetal, and it is derived from carbohydrate biomass
and donates to the reduction of fossil fuel resources [5]. Acetaldehyde derived from ethanol is
an important molecule and it is used as an intermediate in the synthesis of acetic acid and
many valuable chemicals such as ethyl acetate, crotonaldehyde, butadiene etc. [6]. There are
generally three methods for producing acetaldehyde, rather than ethanol, as the starting
molecule which are partial oxidation of ethane, oxidation of ethylene and hydration of
acetylene [7]. But these methods require high reaction conditions and high toxicity catalysts,
which is not good in terms of economy, environment and fine research. The ethane to
acetaldehyde method requires an expensive catalyst and also uses high reaction temperatures.
Acetaldehyde formed from ethylene produced condensed and polymerization products which
is high price and causes environmental troubles. A toxic catalyst such as the mercury
complex is used in the production of acetaldehyde from acetylene. The conversion of ethanol
to acetaldehyde is another important method. There are two ways to produce acetaldehyde
from ethanol, one is the oxidative dehydrogenation of ethanol and the other is the
dehydrogenation of ethanol. The reaction in the oxidative dehydrogenation pathways requires
air, which affects the cost of the reaction. But dehydrogenation methods do not use air;
therefore the reaction cost is reduced. From an environmental point of view, ethanol
dehydrogenation is good for producing acetaldehyde. There are many reports in the literature
on the conversion of ethanol to acetaldehyde in homogeneous and heterogeneous catalysts [8-
16].
Another important molecule derived from ethanol is diethyl acetyl. Diethyl acetyl is a
solvent, a protective agent for carbonyl compounds, the raw material in the pharmaceutical
industry, used as a diesel blending and reduces the release of nitrates and particles [17]. There
are two methods two prepared diethyl acetal. In the first method ethanol and acetaldehyde
react with each other to produce diethyl acetyl. The second method has no use of aldehyde
and in this method one mole of ethanol undergoes dehydrogenation to produce acetaldehyde,
which then react with the acetaldehyde with the remaining ethanol to produce diethyl acetal
[17]. There are some reports to produce diethyl acetal from ethanol by using different
catalysts in both batch mode and gas phase reactions [17,18].
The conversion of alcohols to value-added chemicals is strongly influenced by the
metal particle size [19]. For example, the activity and/or selectivity of some reactions are
good at large particle sizes of metals and some reactions are at small particle sizes of metals.
This is because; the shape and symmetry of the metal particle size can be strongly influenced
by the site population and geometry [20]. Another two more factors are there to influences
the alcohol conversion such as, support effect and addition of a base [21, 22]. The present
work mainly focus on the effect of metal particle size on the activity of ethanol to value-
added chemicals. In the recent years, the research activity is increased in the area of synthesis
and application of different size and shape of metal nanoparticles. Nanometer sized particles
have different properties such as optical, electronic, magnetic and chemical properties and
can be used in various fields such as biological sensors, nano devices, optoelectronics and
catalysis [23]. Among many available metals like gold, silver, palladium, platinum and
copper and copper based materials are the most important inexpensive materials in the area of
catalysis.

Copper is very important in chemistry because it exhibits different oxidation states

such as 0, +1 and +2 and can have one or two electron mechanism reactions. Similar to
palladium, copper can involve both radical and two electron bond forming mechanisms.
Under mild conditions copper is very active and does not react with other chemical agents.
Copper metal reacts with oxygen in the air to form brown-black copper oxide, which does not
react with water. Due to these special properties of copper, it can have different catalytic
reactions [24]. Herein, we examined how the different particle size of the copper obtained
from different calcination temperature and their catalytic performance on the conversion of
ethanol to acetaldehyde and diethyl acetal.
Experimental section
Preparation of copper catalysts with different particle size
A series of copper particle catalysts of five different sizes were synthesized using different
calcination temperatures. The copper oxide (CuO) chemical was purchased from (M/s Sigma-
Aldrich, USA) and was used as such without any further purification. The copper catalysts
with different particle sizes were obtained by subjecting CuO to calcination under static air in
the temperature range of 350-500 °C for 12 h at ramping 5 °C/min and followed by reduction
in a H2 flow at 280 °C for 3 h. The obtained solid mass was labelled as Cu-X where X
represents the calcination temperature. For example Cu-350 means, the CuO calcined at 350
°C.
Characterization of the catalysts
Ultima IV diffractometer (M/s. Rigaku Corporation, Japan) was used to record
Powder X-ray diffraction (XRD) patterns of all the catalysts with a scanning rate of 0.02
using Ni filtered Cu Ka radiation (l ¼ 1.5406 °A) with a scan speed of 4 o min-1 and a scan
range of 10–80o at 40 kV and 20 mA.
JEM 2000EXII apparatus (M/s. JEOL, Switzerland) operating between 160 and 180
kV was used to record TEM images of catalyst samples. For TEM analysis, the samples were
suspended ultrasonically in ethanol and then dropped onto the carbon coated copper grid, the
solvent was then evaporated in an air oven at 80o C for 6 hours.
Temperature programmed reduction (TPR) studies aimed to find out the reduction
behavior of reducible species present in the catalyst was carried out on a homemade quartz
reactor system. In the present case TPR studies are helpful to find out reducing behavior of
the nickel precursor and also metal support interaction. Prior to TPR analysis, the catalyst
was heated to 413 K at a rate of 20 K min-1 in N 2 flow and held for 30 min to remove the
moisture followed by cooling to 313 K. While heating to 1073 K at a rate of 20 K min-1 in a

flow of gas consisting of 5% H2 balance Argon (50 ml min-1), the TPR patterns were
recorded on thermal conductivity equipped gas chromatograph (M/s. CIC Instruments, India)
which was connected with standard GC software.
Activity test
Catalyst tests were performed in a fixed-bed reactor (14 mm id and 300 mm long) at
atmospheric pressure. During each catalyst run, a mixture of 1 g of catalyst and the same
amount of quartz particles is loaded in the center of the reactor. Above the catalyst bed,
quartz beeds which acts as preheating zone were placed. Prior to the reaction, the catalyst
were reduced at 280 oC for 3 h in a H 2 flow of 30 cm3 min-1. Then the temperature was
brought down to the reaction temperature where the liquid reactant was continuously fed at a
flow rate of 1 cm3 h-1 using a syringe feed pump (M/s. B. Braun, Germany) along with N 2.
The product mixture was collected in an ice cooled trap at hourly intervals and the
components present in the mixture was quantified in a flame ionization detector (FID)
equipped gas chromatograph, GC-17A (M/s. Shimadzu Instruments, Japan) and confirmed by
GC-MS, QP-5050 (M/s. Shimadzu Instruments, Japan).

Results and discussions

XRD Studies
The XRD patterns of reduced Cu catalysts such as un-calcined copper catalyst (reduced
pristine CuO) and calcined copper catalysts are shown in Fig. 1. From XRD results it is
observed that there are three diffraction peaks at 2θ = 43.48°, 50.5° and 74.1° characteristic
of Cu0 (JCPDS 04-0836), which confirm the presence of metallic Cu in all of the five
catalysts. The crystallite size (calculated using Scherrer equation) of metallic copper is
increased with increasing the calcination temperature from (350 to 500 °C). The calculated
crystalline sizes of all catalysts are presented in the Table 1.
Table 1: The crystallite sizes of copper catalysts.
 Catalyst Crystallite size (nm)

UCu 20.74
Cu-350 21.25
Cu400 23.33
Cu-450 25.93
Cu-500 28.52

Fig.1 XRD patterns of the reduced Cu catalysts. (A) UCu (B) Cu-350 (C) Cu-400 (D) Cu-
450 and (E) Cu-500.
H2-TPR studies
To evaluate the reduction behaviour of copper species in CuO catalysts examined by
H2-TPR analysis was conducted and the resultant reduction profiles are shown in Fig. 2. In
the H2-TPR profile it is observed that all the catalysts shows a single symmetric reduction
peak and which indicates the CuO species present in the all catalyst are reduced completely
into Cu0 (CuO + H2 - Cu + H2O) in a single-step. From the H2-TPR profile it is clearly
observed that the calcination temperature has an impact on the reduction behavior of CuO
species because on increase in calcination temperature from 350 to 500 oC, the particle size of
the copper increase from 21.25 nm to 28.52 nm indicating the achievement bulk nature of
catalysts and this bulk nature of catalyst also confirmed from the H 2-TPR profile. In H2-TPR
profile, from catalyst B to E the there is a marginal shift in temperature because bulk nature
of copper catalysts.
Fig.2 H2-TPR profiles of CuO catalysts. (A) UCu (B) Cu-350 (C) Cu-400 (D) Cu-450 and
(E) Cu-500.
N2O pulse chemisorption studies
The estimated surface properties such as copper particle size and copper metal surface area of
copper catalysts are presented in Table 2. The N 2O pulse chemisorption is an established
technique for the determination of metal area and surface properties of copper catalysts. From
the N2O pulse data, it is clearly observed that the copper metal surface area is gradually
decreased from the catalyst UCu to Cu-500 catalyst due to increase in the copper particle
size. The N2O uptake of the catalyst increases, from U Cu catalyst to Cu-500 catalysts
because bulk nature of the catalyst increases.
Table 2 N2O pulse chemisorption data of copper catalysts.
Catalysts Cu PS N2O uptake Cu SA
(nm) (μmol g−1) (m2 g−1)
U CuO 18.75 227.30 35.95
Cu-350 20.09 243.50 33.55
Cu-400 21.50 261.35 31.26
Cu-450 24.36 295.26 27.67
Cu-500 26.20 317.57 25.72

TEM analysis
TEM images of (A) UCu (B) Cu-350 (C) Cu-400 (D) Cu-450 and (E) Cu-500 catalysts are
shown in Fig 3. In all the catalysts dark black spots are observed they indicate the presence of
copper particles. In the TEM images from A to E, the size of the dark black spots increases
because increase in the size of the copper particles i.e. increase in calcination temperature. It
is also evidenced from the XRD and N2O studies.
Fig. 3 TEM images of (A) UCu (B) Cu-350 (C) Cu-400 (D) Cu-450 and (E) Cu-500.

Activity studies
The conversion of ethanol to acetaldehyde and diethyl acetal is shown in equations 1
and 2. Acetaldehyde formation is a single step process in which acetaldehyde is formed by
ethanol dehydrogenation. Diethyl acetal formation is a two step process in which
acetaldehyde is formed in the first step and the formed acetaldehyde is condensed with
remaining ethanol to form diethyl acetal in the second step.
(1)
C2H5OH CH3CHO
(2)
CH3CHO + C2H5OH CH3CH (OC2H5)2

Influence of copper particle size on direct conversion of ethanol to acetaldehyde and

diethylacetal
The catalytic activity of five catalysts with different Cu particle size in the conversion of
ethanol to acetaldehyde and diethylacetal is presented in Fig 5. The dehydrogenation reaction
was performed under N2 atmosphere at 300 °C. The conversion of ethanol increases from 20

to 44% with increases in copper particle size with 20.7 nm to 25.9 nm because change in
particle size and active metal surface area with the calcination temperature. The conversion
of ethanol decreases to 35% on further increasing the copper particle size to 28.5 nm due to
decrease in active metal surface area.
Fig.4 Catalytic activity (A) UCu (B) Cu-350 (C) Cu-400 (D) Cu-450 and (E) Cu-500
catalysts. Reaction conditions: temperature: 300 oC, catalyst weight: 2 g, N 2 flow: 30 ml min-
1, ethanol feed flow: 2 ml h-1.
The variation of catalytic activity and selectivity in the conversion of ethanol to value-
added chemicals with copper particle size is presented in Fig.5. The conversion of ethanol
reaches a maximum at a copper particle size of 25.9 nm i.e. for Cu-450 catalyst and then
decreases over Cu-500 catalyst.

Fig.5 Variation of catalytic activity and selectivity with Cu metal particle size. Reaction
conditions: Reaction conditions: temperature: 300 oC, catalyst weight: 2 g, N 2 flow: 30 ml
min-1, ethanol feed flow: 2 ml h-1.
Influence of reaction temperature
The conversion of ethanol to acetaldehyde and diethyl acetal over the better catalyst, that is
copper catalyst (Cu-450) with 25.9 nm particle size was examined at various reaction
temperatures such as 250 oC, 275 oC, 300 oC and 325oC and results were presented in Fig 6. It
was observed that conversion of ethanol increases from 29% to 44% when the temperature
increases from 250 to 300 oC. The increase in ethanol conversion with increasing temperature
is due to the endothermic nature of the dehydrogenation reactions and has shown that
temperature is an important requirement in this reaction. As the temperature increases from
250 to 300oC, the selectivity to acetaldehyde increases from 56 to 64% and the selectivity to
diethyl acetal is from 30% to 33%.On further increasing temperature from 300 to 325 oC, the
conversion of ethanol decreases to 38% this is because the reaction between ethanol to
acetaldehyde and diethyl acetal is an equilibrium reaction and promotion of reverse reaction
has been found hence conversion of ethanol decreases and the same time the selectivity to
acetaldehyde decrease to 57% and selectivity to diethyl acetal decreases to 32%. Therefore
300oC is the optimum temperature for maximum conversion of ethanol.

Fig.6 Influence of reaction temperature on dehydrogenation of ethanol. (A) 250 oC (B) 275
o
C (C) 300 oC (D) 325 oC. Reactions conditions: Catalyst weight: 2 g, N 2 flow: 30 ml min-1,
ethanol feed flow 2 ml h-1.

Influence of gas hourly space velocity (GHSV)

The effect of GHSV on the conversion of ethanol to acetaldehyde and diethyl acetal was
studied and results were displayed in the Table 4. From the results, it is clearly observed that
the conversion of ethanol decreases with increases the GHSV due to decrease in contact time
and amount of adsorbed reactant. At low GHSV, the reactant molecule spends long time in
catalyst and in this stage reactant molecule undergoes adsorption and diffusion on the surface
of catalyst, therefore the ethanol conversion is more in low GHSV.
Table 3: Influence of GHSV on the conversion of ethanol to acetaldehyde and diethyl acetal
over Cu-450 catalyst.
GHSV (h-1) Conv. Sel. Sel.
C2H5OH (%) CH3CHO (%) CH3CH (OC2H5)2 (%)
5870 44 64 33
7071 38 70 28
8400 31 75 22
Reactions conditions: Catalyst weight: 2 g, Temperature: 300 oC, ethanol feed flow: 2 ml/ h-1.

Effect of ethanol feed flow

The effect of ethanol feed flow (1-4 ml/h) on the conversion of ethanol to acetaldehyde and
diethyl acetal was studied over Cu-450 catalyst at 300oC and the results were presented in
Fig. 7. At 1ml/h and 2 ml/h ethanol feed flows there is no significance change in conversion
of ethanol and selectivity to acetaldehyde and diethyl acetal. On further increasing ethanol
feed flow that is at 3 ml/h and 4 ml/h, the conversion of ethanol decreases due to the
deficient number of Cu active species for high amount of reactant ethanol. Therefore 2 ml/h
of ethanol is optimum for better conversion of ethanol to acetaldehyde and diethyl acetal.
Fig.7: Influence of ethanol feed flow the conversion of ethanol to acetaldehyde and diethyl
acetal over Cu-450 catalyst. (A) 1ml/h (B) 2ml/h (C) 3 ml/h (D) 4 ml/h. Reactions conditions:
catalyst weight: 2 g, temperature: 300 oC, N2 flow: 30 ml min-1.
Time on stream study
Under optimized reaction conditions (catalyst weight: 2 g, N 2 flow: 30 ml min-1, temperature
300 °C, ethanol feed flow 2 ml h -1) the time on stream study was performed on the Cu-450
catalyst and the results are shown in Figure 8. It can be observed from the figure that the
conversion of ethanol (44%) is constant from 1 to 5 h during the reaction time. The
selectivity of acetaldehyde (64%) and diethyl acetal (33%) is also stable up to 5 h. After 5 h
the conversion of ethanol gradually decreases and the selectivity of acetaldehyde and diethyl
acetyl decreases slightly. The activity of the catalyst was lost due to the formation of coke on
the surface of the active sites of Cu-450 and this was confirmed by CHNS analysis.
Moreover, there was no change in the structural features of the Cu-450 catalyst.
Figure 8 Influence of time on stream over Cu-450 catalyst. Reaction conditions: temperature:
300 oC, catalyst weight: 2 g, N2 flow: 30 ml min-1, ethanol feed flow: 2 ml h-1.

Conclusions
In conclusion, the present work offers an efficient and easy way for the conversion of ethanol
to acetaldehyde and diethyl acetal over copper catalysts with different particle sizes. Of the
five different copper particle size catalysts obtained by five different calcination

Temperatures, copper particle size 25.9 nm catalyst calcined at 450 °C is better for converting
ethanol to acetaldehyde and diethyl acetyl with maximum selectivity and activity.

Ethical approvals
This study does not involve experiments on animals or human subjects.

Author contributions
All authors made substantial contributions to conception and design, acquisition of data, or analysis and
interpretation of data; took part in drafting the article or revising it critically for important intellectual
content; agreed to submit to the current journal; gave final approval of the version to be published; and
agree to be accountable for all aspects of the work. All the authors are eligible to be an author as per the
international committee of medical journal editors (ICMJE) requirements/guidelines.

Conflict of interest:
None.
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