Production of acetic acid from ethanol over CuCr catalysts: Via dehydrogenation-(aldehyde-water shift) reaction

Article abstract of DOI:10.1039/c7ra05922a

A series of CuCr catalysts were prepared by co-precipitation method and used to produce acetic acid from ethanol via dehydrogenation-(aldehyde-water shift) reaction. The catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller analysis (BET), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and temperature programmed reduction (TPR). The effects of copper contents and atmosphere on catalytic performance were investigated. The enhanced catalytic performance can be ascribed to the existence of chromium oxide. A possible mechanism for the production of acetic acid from ethanol without oxidant was also proposed.

Full text of DOI:10.1039/c7ra05922a

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Production of acetic acid from ethanol over CuCr
catalysts via dehydrogenation-(aldehyde–water
shift) reaction
Cite this: RSC Adv., 2017, 7, 38586
Ning Xiang,
Peng Xu, Nianbo Ran and Tongqi Ye*
A series of CuCr catalysts were prepared by co-precipitation method and used to produce acetic acid from
ethanol via dehydrogenation-(aldehyde–water shift) reaction. The catalysts were characterized by X-ray
diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller analysis (BET),
inductively coupled plasma atomic emission spectroscopy (ICP-AES), and temperature programmed
reduction (TPR). The effects of copper contents and atmosphere on catalytic performance were
investigated. The enhanced catalytic performance can be ascribed to the existence of chromium oxide.
A possible mechanism for the production of acetic acid from ethanol without oxidant was also proposed.
Received 26th May 2017
Accepted 31st July 2017
DOI: 10.1039/c7ra05922a
rsc.li/rsc-advances
use O as oxidant, whether pure oxygen or air. As Scheme 1
shows, oxidation of ethanol to acetic acid generally proceeds in
2
Introduction
Nowadays, the outlook and motivation towards the conversion two steps. Ethanol is rstly oxidized to acetaldehyde and the
of renewable biomass to fuels and commodity chemicals is acetaldehyde is then oxidized to acetic acid. This reaction is
growing due to the diminishing petroleum resources and global considered as a green catalytic process for the only reduction
1
14
warming problems. Bio-ethanol is one of the most widely dis- product is water. However, safety is one of the major issues
cussed options: it could provide alternative routes to produce concerning O -based oxidation process, in relation to amma-
chemicals, such as acetaldehyde, acetic acid, ethylene oxide and bility limits and explosion hazards.
ethyl acetate, which are currently produced from ethane,
In fact, other than O , water can also inuence or take part in
2
15
2
2
ethene, or methanol. As one of the most promising the process of conversion ethanol to acetic acid. P. R. S.
approaches, catalytic oxidation of ethanol to acetic acid has Medeiros et al. studied the role of water in ethanol oxidation
been widely studied in recent decades.
2
over SnO -supported molybdenum oxides, showing that the
A number of heterogeneous catalysts have been reported for presence of water in the stream decreased ethanol conversion
8
the conversion of ethanol to acetic acid in gas phase, such as but increased selectivity to acetic acid. While M. M. Rahman
3
4
5–7
Mo0.61
Mo–CeO
V
0.31Nb0.08
O
x
/TiO
2
, MoV0.3Nb0.12Te0.23
O
x
, V
2
O
5
/TiO
2
,
et al. reported that the water promoted the secondary acetal-
8
x
/SnO
2
, etc. On the other hand in liquid phase oxida- dehyde oxidation to acetic acid conversion over ZnO catalyst
tion, catalysts mainly consist of noble metal compounds have and inhibited the acetaldehyde aldol-condensation to croto-
9,10
16
also been investigated, such as Au/MgAl
2
O
4
,
Au/Ni0.95Cu0.05
-
naldehyde. When O
O , RuO /CeO , Ru(OH) /CeO , etc. Most of these catalysts ethanol is dehydrogenated to produce acetaldehyde, and then
2
is nonexistence in the reaction system,
11
12
13
x
x
2
x
2
water, acting as the oxidizer, reacts with acetaldehyde to
17–19
produce acetic acid, concomitantly releasing hydrogen gas.
As Scheme 2 shows, the latter one is also named “aldehyde–
water shi” reaction (AWS).
A number of homogeneous catalysts employing noble metal
species have been used to the AWS reaction. Based on the study
Scheme 1 Two-step oxidation of ethanol to acetic acid.
School of Chemistry and Chemical Engineering, Hefei University of Technology,
Xuancheng, Anhui, 242000, PR China. E-mail: yetq@hfut.edu.cn; Tel:
Scheme 2 Ethanol conversion to acetic acid via dehydrogenation-
+
86-563-3831065
AWS reaction.
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ꢀ
1
of the dehydrogenation of alcohols to carboxylic acids, Milstein 1300 W, ow rate: 1.5 mL min . Cu element was measured in
and Gr u¨ tzmacher et al. proposed the AWS was an intermediate the radial mode of the ICP-AES.
reaction step involving dehydrogenation of a hydrated aldehyde
Nitrogen adsorption experiments for pore size distribution,
20,21
(geminal-diol).
Recently, Heinekey et al. reported that a series pore volume, and surface area measurements were conducted
of (p-cymene) ruthenium(II) diamine complexes and p-arene (or on a COULTER SA 3100 analyser. All samples were calcined at
cyclopentadienyl) complexes of iridium, rhodium, and ruthe- 673 K under vacuum before the measurements. The X-ray
nium were shown to be active catalysts for the conversion of diffraction (XRD) was measured on an X'pert Pro Philips
1
7,18
aldehydes and water to carboxylic acids.
However, all these diffractometer with a CuKa radiation (l ¼ 0.154 nm). The
ꢁ
homogeneous reaction systems have the obvious disadvantages measurement conditions were in the range of 2q ¼ 5–80 , step
ꢁ
on the separation of products and the recycling of catalyst. Thus counting time 5 s, and step size 0.017 at 298 K. The surface
a more attractive process for industrial application would be elements and their states were analysed by X-ray photoelectron
heterogeneous reaction system especially over the base metal spectroscopy (XPS). The XPS measurements were performed on
catalysts, because of the convenience of catalyst separation, facile an ESCALAB-250 (Thermo-VG Scientic, USA) spectrometer with
continuous process operation and cost effectiveness. We noticed AlKa (1486.6 eV) irradiation source.
that some patents were related to the production of acetic acid
The reducibility of the calcined catalysts was determined by
22,23
from ethanol via dehydrogenation-AWS reaction,
but none of temperature programmed reduction (TPR) with a heating rate
ꢀ
1
them studied the catalytic mechanism of this process.
of 10 K min to 823 K under a owing atmosphere of 10 vol%
It is well known that the copper based catalysts are active for H /Ar. The copper dispersions and particle sizes were deter-
2
dehydrogenation of alcohols to produce correspondence alde- mined by the dissociative N O adsorption method. The experi-
2
24
hydes. According to Colley, ethanol dissociated on the Cu ments were performed using the same apparatus as for the TPR
component to form ethoxy species, and then ethoxy species measurements. The catalysts were rst reduced at 623 K under
2
5
were dehydrogenated to acetyl species. Work by Jiang on the 10 vol% H
2
/Ar for 2 h. Aer cooling to 333 K in a He ow, the
O ow for 0.5 h at 333 K.
proposed that the Cu species played an important role in Finally, the re-oxidized samples were cooled to room tempera-
ethanol dehydrogenation to acetaldehyde, while the Lewis ture to start another TPR run with 10 vol% H /Ar at a ramping
acidic site on Cr O3 phase might be responsible for the rate of 10 K min to 773 K. The average copper particle size and
dehydrogenation of ethanol to ethyl acetate on CuCr catalyst reduced samples were exposed to a N
2
0
2
ꢀ
1
2
1
9
desorption of product ethyl acetate from CuCr catalyst's surface. dispersion were calculated by assuming 1.4 ꢂ 10 copper
2
26
In this paper, an attempt to develop CuCr catalyst for ethanol atoms per m and a molar stoichiometry N O/Cu ¼ 0.5, where
2
s
dehydrogenation along with AWS reaction to produce acetic acid the symbol Cu
s
means the copper atoms on the surface. The
was given. Catalysts were characterized by various techniques dispersion of copper was calculated by the following equation:
2 2
including BET, ICP-AES, XRD, XPS, H -TPR and N O chemi-
sorption. The relationship between the composition of the CuCr
catalysts and catalytic performance were also discussed.
Cusurface
Cutotal
Dispersion ð%Þ ¼
ꢂ 100
Catalytic test
Experimental
The “free-oxygen” conversion of ethanol to acetic acid was carried
out in the continuous ow systems, using a quartz xed-bed
reactor under atmospheric pressure. In a typical experiment,
Catalyst preparation
The CuCr catalysts with various Cu contents (calculated by CuO
of 100 wt%, 70 wt%, 50 wt%, 30 wt%, 0 wt%) were prepared by co-
precipitation method using respective metal nitrates solutions as
precursors and Na CO solution as precipitator. Sodium
1
g of the catalyst diluted with an equal amount of quartz grains
was introduced into the reactor. The catalyst bed was packed with
quartz grains, which serve as the preheated zone. Prior to the
2
3
carbonate was added dropwise to the stirred aqueous solutions of
reaction, the catalysts were pre-reduced by H stream at 623 K for
2
2
+
3+
mixtures of Cu and/or Cr nitrates at 343 K until pH 7 was
reached. The precipitate was aged at 343 K in the mother liquor
for 2 h, and then it was washed thoroughly with deionized water
and dried in air at 393 K for 12 h. The formed CuCr mixed
hydroxides were thereaer calcined at 623 K for 4 h in air to
obtain the corresponding mixed oxide catalysts. The latter ones
were nally made into granules with 40–60 mesh sizes.
3
h. The liquid feed consisting of 25 wt% of ethanol and water
mixer was fed into the reactor using a syringe pump during the
reaction. High purity Ar (99.999%) was used as carrier gas in the
experiments, keeping constant ow rate by using mass ow
controllers. The products were analysed by gas chromatography
equipped with a ame ionization detector (FID) connected to
a PEG-20M capillary column (for the liquid products) and a TCD
connected to a Porapak Q packed column (for the gas products).
Catalyst characterization
An ICP-AES (Optima 7300 DV, Perkin Elmer, Korea) was used for
the determination of Cu element content in calcined catalysts.
Brief operation conditions and optics of the ICP-AES used in the
Results and discussion
Results of characterization
ꢀ1
present study are as follows: axial mode plasma: 15 L min
,
As shown in Table 1, the Cu contents in the catalysts analysed by
ꢀ
1
ꢀ1
auxiliary: 0.2 L min , nebulizer: 0.65 L min , RF powder: ICP-AES are in correspondence with the feed ratio. Some other
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important properties of fresh catalysts, such as BET surface 323 K. Aer that, H
2
-TPR characterization is performed. Similar
areas, Cu dispersions and particle sizes are listed in Table 1. to the TPR prole of pristine catalyst, the N O treated Cu70
2
Compared to the pure copper and chromium of Cu100 and Cu0, sample also shows two separate reduction peaks that shi to
the hybrid of the two components gives much higher BET lower temperatures of 398 K and 417 K. However, other catalysts
surface areas and Cu dispersion. The Cu70 shows maximal BET show only one peak just like their pristine samples. The results
surface area and pore volume among the mixed metal oxides. indicate the two hydrogen consumption processes for the Cu70
With the increase of Cr content, it decreases from 42.8 to 26.4 sample are corresponding to two kinds of Cu species, rather
2
ꢀ1
3
ꢀ1
m g and 0.237 to 0.120 cm g respectively. However, the than the two steps of CuO reduction (CuO to Cu
Cu50 gives the largest pore size and Cu dispersion. Cu). Compare the peak shape with Cu100 sample; we would like
Fig. 1a shows the XRD patterns of the fresh calcined catalysts attribute the lower reduction peak to the reduction of CuCr
with various Cu contents. Diffraction peaks of CuO and Cr O mixed oxide on the surface, while the other to the reduction of
2 2
O and Cu O to
2
3
are obviously observed in Cu100 and Cu0 respectively. However, CuO.
they are not shown in the mixed oxide catalysts of Cu70, Cu50
The catalytic performance of CuCr catalysts on conversion of
and Cu30, indicating good dispersion in these hybrid samples. ethanol to acetic acid without oxidant is investigated. Before
Aer reduction and 8 hours' catalytic reaction, as Fig. 1b and c reaction, the catalysts are pre-reduced at 623 K for 3 h by
shows, diffraction peaks of Cu are obviously found on the hydrogen. Then the ethanol solution is preheated and injected
samples of Cu100, Cu70, Cu50 and Cu30 with declining inten- under argon atmosphere. For there is no oxidant in the reaction
sity, which is well corresponding to the Cu content of the four system, we speculate the conversion of ethanol to acetic acid
catalysts. However, the Cu0 catalyst keeps the same due to should consists of two steps. The rst is dehydrogenation of
a higher reduction temperature of Cr O . The average particle ethanol to acetaldehyde, and the second is the reaction of
2
3
0
size of Cu on the used catalysts are calculated by Debye– acetaldehyde with water which named “aldehyde–water shi”
17,18
Scherrer equation and shown in the Table 1. Results show the (AWS) reaction to acetic acid.
average particle sizes are similar and in the range of 15.0 to shows, pure Cr exhibits none of catalytic activity which
7.0 nm, except for the pure copper sample shows a much larger indicates at least the rst step of dehydrogenation can only
As the reaction results of Fig. 3
2 3
O
1
particle size of 23.0 nm.
proceed on copper species. As expected, all of the Cu-containing
The reduction behaviour of the catalysts with various Cu catalysts exhibited high activity with the ethanol conversion
contents is studied by H -TPR characterization and the ranging from 62% to 97%.
2
respective proles are shown in Fig. 2a. The reduction peaks in
From the perspective of selectivity, Cu70 catalyst is the best
all the samples are assigned to the reduction of copper species one which shows highest acetic acid selectivity of 48.6%.
for the chromic oxide is hard to be reduced. Considering of the Furthermore, some deep oxidation product of CO
Cu70 sample, the mix with chromium decreases the reduction the Cu50 and Cu30 catalysts.
2
appears on
temperature of copper species for the dispersing effect.
The ethanol dehydrogenation on heterogeneous catalysts is
However, further increase of Cr content is not good for the a well-researched process. Ethanol is rstly adsorbed as ethoxy
reduction of Cu species. With the increase of Cr content, the and the adsorbed ethoxy species could easily dehydrogenated to
25
reduction temperature increases gradually. Furthermore, form acetyl species via aldehyde. Then the acetyl species
different with others, the Cu70 sample shows two separate combine with another ethoxy species to produce adsorbed ethyl
reduction peaks centred at 508 K and 521 K respectively. Some acetate which then desorbed. However, when water is in the
authors reported the two hydrogen consumption processes reaction system, it will compete with ethanol and some other
2
resulted from the two steps of CuO reduction (CuO to Cu O and adsorbed species for the same catalytic sites, thus inhibits
27–29
8
Cu
2
O to Cu),
some others proposed that the two hydrogen bridging carboxylates and in favour of form acetic acid. Orozco
consumption processes revealed the presence of different CuO et al. carefully investigated the role of water in the oxidation of
3
0
species. To conrm that, the reduction behaviour of Cu O is aldehyde to carboxylic acid use ZrO and CeO as model cata-
2
2
2
31–33
studied by H -TPR characterization and the respective proles lysts.
Surface hydroxyl group (from water adsorption and
2
are shown in Fig. 2b. The catalysts are reduced at 623 K for 2 h decomposition) could combine with adsorbed aldehyde to form
and treated with N O for another 0.5 h aer cooling down to aldehyde hydrate, which then is able to transfer a hydride
2
Table 1 Physico-chemical properties of CuCr catalysts
a
2
ꢀ1
3
ꢀ1
b
c
Sample
Cu (wt%)
S
BET (m g
)
V
p
(cm g
)
D
p
(nm)
Dispersion (%)
dCu (nm)
Cu100
Cu70
Cu50
Cu30
Cu0
99.9
68.7
52.1
33.3
0
4.0
0.021
0.237
0.203
0.120
0.012
20.6
22.1
23.0
18.0
14.1
1.6
10.1
17.5
15.7
—
23.0
16.1
17.0
15.0
—
42.8
32.1
26.4
3.5
a
b
c
CuO content (wt%) measured by ICP-AES. Cu dispersion calculated by N
Scherrer equation.
2
O desorption. Cu particle size for used catalysts calculated by Debye–
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Fig. 2 Typical H -TPR profiles of CuCr catalysts: (a) fresh calcined
2
catalysts and (b) reduced catalysts that treated with N O.
reactor under the same conditions with above experiments. As
the results shown in Fig. 4, no acetaldehyde conversion was
detected on the pure Cr O which indicates the step of AWS
2 3
reaction also performed only on surface Cu species. Interest-
ingly, none of the catalysts exhibited more than 50% of acetal-
dehyde conversion. We speculate that the abundant surface
proton (from water adsorption and decomposition) competed
with aldehyde and adsorbed acetyl species for the same catalytic
sites, thus hindered the reaction between aldehyde and
hydroxyl group, led to a rather low aldehyde conversion.
Considering of the decisive role of surface elemental compo-
sitions in catalytic performance, we have taken XPS analysis on
the Cu-containing catalysts to investigate the relationship
between surface compositions and catalytic performance. The
Fig. 1 X-ray diffraction patterns of the CuCr catalysts: (a) fresh
calcined catalysts; (b) reduced catalysts; and (c) used catalysts (reac-
tion conditions: catalyst loading ¼ 1 g, T ¼ 623 K, carrier gas: argon,
ꢀ1
reactant: 25 wt% ethanol solution, total flow rate ¼ 2 mL h , t ¼ 6 h). XPS spectra of used catalysts are shown in Fig. 5. The peak at
0
+
9
32.5 eV can correspond to Cu and/or Cu , because both spectra
are identical within 0.2 eV range. Nonetheless, the spectra of the
species and further form molecular hydrogen. Meanwhile, with Cu LMM Auger peaks shown by Fig. 5b indicate the coexistence of
0
+
34,35
a surface proton derived from the initial water adsorption, Cu and Cu species for all samples.
However, the peak at
2
+
carboxylic acid product can be formed. Different with ZrO
2
and 933.8 eV which assigned to Cu (2p3/2) is observed for used Cu50
CeO catalysts, the further ketonization of carboxylic acids is and Cu30 catalysts. It manifests that the surface copper species
2
suppressed on the CuCr catalysts thus carboxylic acid is the get more stable at a relatively higher valence state with the
main product.
increase of Cr and decrease of Cu contents in the catalysts. The
2
+
To elucidate the active site for the AWS reaction on the CuCr high valence of Cu induces deep oxidation of ethanol and
2
catalysts, the acetaldehyde solution was pumped into the generation of CO , steam reforming reaction may occur.
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Fig. 4 Influence of Cu content on the (a) conversion and (b) selectivity
of acetic acid in the AWS reaction of acetaldehyde (reaction condi-
tions: catalyst loading ¼ 1 g, T ¼ 623 K, carrier gas: argon, reactant:
Fig. 3 Influence of Cu content on the (a) conversion and (b) product
distribution of the dehydrogenation-AWS reaction of bio-ethanol
(
reaction conditions: catalyst loading ¼ 1 g, T ¼ 623 K, carrier gas:
ꢀ1
ꢀ1
25 wt% acetaldehyde solution, total flow rate ¼ 2 mL h , t ¼ 6 h).
argon, reactant: 25 wt% ethanol solution, total flow rate ¼ 2 mL h
,
t ¼ 6 h).
species. On the contrary, the selectivity of acetic acid in air
The surface elemental compositions of fresh and used reaches 79.5%. The main reason for this phenomenon is
catalysts are listed in Table 2. With the decrease of Cu content, ascribed to the consumption of adsorbed hydrogen on catalyst
the surface Cu content decreases as expected. And with the surface by oxygen gas. Therefore, the concentration of surface
increase of Cr content, the surface metal compositions ((Cu + oxygen species is enhanced. However, over-oxidation of ethanol
Cr)%) decreases gradually, but much more carbon is detected is obviously a disadvantage in oxidative atmosphere, which
on the surface, especially for the used samples. However, Cu70 liberates undesirable product: carbon dioxide. Especially on the
catalyst has a comparatively higher surface oxygen content, Cu100 catalyst, the CO
leading to more chance of aldehyde react with hydroxyl group, oxidative atmosphere.
2
selectivity even reached to 31% under
thus the better selectivity of acetic acid (Fig. 3b). As Fig. 6 shows,
there is nearly a linear relationship between the selectivity of tion of best chosen Cu70 catalyst with time on stream was
acetic acid and surface oxygen content.
conducted at 623 K in Ar atmosphere, and the results are shown
To evaluate the stability of the catalysts, performance varia-
The inuence of redox atmosphere is also investigated on in Fig. 8. As can be seen, the catalytic activity did not change
the pure Cu and Cu70 catalysts. Three representative gases have obviously in the earlier stage but declined fast aer 8 h reaction.
been studied: hydrogen (reductive), argon (inert) and air The selectivity of acetic acid was continuous declined along with
(
oxidative). As can be seen from Fig. 7, all catalytic tests show the acetaldehyde selectivity arised gradually. TG analysis
high ethanol conversion over 75%, even under atmosphere of method was used to evaluate the inuence of carbon deposition
. It indicates that the dehydrogenation of ethanol on Cu is on the catalytic performance. However, almost no carbon was
H
2
little affected by atmosphere. The atmosphere mainly inu- detected. Although aldehyde as a product in the reaction
ences the latter AWS reaction and shows rather different system, the high content of water suppressed the aldol
selectivity preference. Take catalytic test of Cu70 as an example, condensation to form carbon deposition. Thus the sintering of
the selectivity of acetic acid is 48% under inert argon, while 24% Cu particles may play an important role in the catalyst deacti-
36,37
under reducing H
2
. It manifests that the hydrogen gas vation.
As the XRD results show above, the Cu particle size
suppresses the bonding of adsorbed aldehyde with oxygen increased from undetectable level to about 16 nm aer 8 h
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Fig. 6 The relationship between the selectivity of acetic acid and
surface oxygen content (reaction conditions: catalyst loading ¼ 1 g,
T ¼ 623 K, carrier gas: argon, reactant: 25 wt% ethanol solution, total
ꢀ
1
flow rate ¼ 2 mL h , t ¼ 6 h).
Fig. 5 XPS spectra of (a) Cu 2p spectra for used catalysts and (b) Cu
LMM Auger electron spectra for used catalysts (reaction conditions:
catalyst loading ¼ 1 g, T ¼ 623 K, carrier gas: argon, reactant: 25 wt%
ꢀ1
ethanol solution, total flow rate ¼ 2 mL h , t ¼ 6 h).
Table 2 Surface element content of different catalysts
a
b
Atomic (%)
Atomic (%)
Catalyst
Cu
Cr
O
Cu
Cr
O
Cu100
Cu70
Cu50
Cu30
44
21
14
7
—
9
11
15
56
70
50
53
45
12
9
—
22
11
3
55
66
45
47
5
a
b
Fresh calcined catalysts. Used catalysts.
reaction. Results show that the catalyst stability needs improve
in our future work.
In summary, a possible mechanism for the production of
acetic acid from ethanol without oxidant is shown in Scheme 3.
Ethanol is rstly adsorbed on the Cu component as ethoxy
species. Subsequently, the latter one is dehydrogenated to
produce adsorbed acetaldehyde. At the same time, water is
adsorbed onto the catalyst surface and dissociates to form same
Fig. 7 Effect of atmosphere on the ethanol conversion and the
product distribution of (a) Cu100 and (b) Cu70 catalysts (reaction
conditions: catalyst loading ¼ 1 g, T ¼ 623 K, reactant: 25 wt% ethanol
ꢀ1
amount of H proton and hydroxyl group. The adsorbed acetal- solution, total flow rate ¼ 2 mL h , t ¼ 6 h).
dehyde either desorbed to gas phase or react with hydroxyl
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Acknowledgements
The project was supported by the National Natural Science
Foundation of China (21406045), the Natural Science Founda-
tion of Anhui Province (1508085QB46) and the Fundamental
Research Funds for the Central Universities (JZ2017HGTB0231).
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Conclusions
The production of acetic acid from ethanol was performed on
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