Regulating Aromatic Alcohols Distributions by Cofeeding Methanol with Ethanol over Cobalt-Hydroxyapatite Catalyst

Article abstract of DOI:10.1002/cctc.202000010

Aromatic alcohols, often used for the synthesis of plasticizers, coatings and pharmaceuticals, are currently produced from the oxidation of petroleum-derived aromatic hydrocarbons. Herein, we report a non-petroleum and environmentally friendly route for the production of aromatic alcohols: cofeeding methanol with ethanol over cobalt-hydroxyapatite catalyst, in which the distribution of aromatic alcohols products can be regulated by varying methanol pressure. Co species on hydroxyapatite can activate both methanol and ethanol to yield their corresponding aldehydes (formaldehyde and acetaldehyde). The followed cross- and self-condensations of aldehydes are catalyzed by hydroxyapatite to produce acrolein and 2-butenal, which are the key intermediates for the formation of aromatic oxygenates. The sequential cross-condensation and dehydrocyclization of acrolein and 2-butenal yield benzaldehyde, which is then hydrogenated to benzyl alcohol. The direct production of benzyl alcohol from methanol and ethanol could be regarded as a cutting-edge example, which ensures a promising route for sustainable aromatic alcohols production.

Full text of DOI:10.1002/cctc.202000010

Accepted Article
Title: Regulating Aromatic Alcohols Distributions by Cofeeding
Methanol with Ethanol over Cobalt-Hydroxyapatite Catalyst
Authors: Bai-Chuan Zhou, Qing-Nan Wang, Xue-Fei Weng, Lei He,
Wen-Cui Prof. Li, and An-Hui Lu
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ChemCatChem
10.1002/cctc.202000010
FULL PAPER
Regulating Aromatic Alcohols Distributions by Cofeeding
Methanol with Ethanol over Cobalt-Hydroxyapatite Catalyst
Bai-Chuan Zhou, Qing-Nan Wang, Xue-Fei Weng, Lei He, Wen-Cui Li and An-Hui Lu*[a]
Abstract: Aromatic alcohols, often used for the synthesis of
plasticizers, coatings and pharmaceuticals, are currently produced
from the oxidation of petroleum-derived aromatic hydrocarbons.
Herein, we report a non-petroleum and environmentally friendly
route for the production of aromatic alcohols: cofeeding methanol
with ethanol over cobalt-hydroxyapatite catalyst, in which the
distribution of aromatic alcohols products can be regulated by
varying methanol pressure. Co species on hydroxyapatite can
activate both methanol and ethanol to yield their corresponding
aldehydes (formaldehyde and acetaldehyde). The followed cross-
and self-condensations of aldehydes are catalyzed by
hydroxyapatite to produce acrolein and 2-butenal, which are the key
intermediates for the formation of aromatic oxygenates. The
sequential cross-condensation and dehydrocyclization of acrolein
and 2-butenal yield benzaldehyde, which is then hydrogenated to
benzyl alcohol. The direct production of benzyl alcohol from
methanol and ethanol could be regarded as a cutting-edge example,
which ensures a promising route for sustainable aromatic alcohols
production.
ethanol) over MgO and hydroxyapatite, respectively.
Nevertheless, the selectivity was unsatisfying, mainly because of
the presence of non-selective intermolecular hydrogen transfer
between enal intermediates and ethanol, leading to the
dominant formation of saturated aliphatic alcohols. Recently, our
group reported a direct production of methylbenzyl alcohols
(MB-OH) from ethanol, using
a
bifunctional cobalt-
2+
hydroxyapatite catalyst.[23] The existent Co promotes ethanol
dehydrogenation and simultaneously suppresses the non-
selective hydrogenation of unsaturated enal intermediates,
thereby allowing the selective production of 2-methylbenzyl
alcohol (2-MB-OH) from ethanol. In this process, 2-butenal, a
key intermediate, undergoes self-condensation and/or coupling
with 2-butenol derived from the selective hydrogenation of 2-
butenal to form the targeted aromatic alcohols. Inspired by the
synthesis route, we attempt to regulate the distribution of
aromatic alcohols by altering either the structure or the carbon
number of enal intermediates.
Benzyl alcohol is also commonly used for fine chemical
manufacturing.[24] Thus, exploring its sustainable synthesis route
from lower alcohols is challenging, which has not been reported
yet. Methanol is known as an abundant C1 platform molecule
Introduction
[25,26]
from syngas chemistry.
Cofeeding of methanol and ethanol
was recently reported to form acrolein,[ iso-butanal,[28] and iso-
27]
Aromatic alcohols with oxygen functional groups are essential
precursors in pharmaceuticals manufacturing.[1,2] In industry,
aromatic alcohols are mainly produced via the oxidation of
butanol[29,30] over metal and/or acid-base catalysts. During these
reactions, formaldehyde as a key intermediate for subsequent
cascade reactions was confirmed as the primary product from
methanol dehydrogenation and its cross-condensation with
acetaldehyde yields acrolein. We therefore propose that the
cofeeding of methanol (which holds an odd carbon number) with
ethanol could be an attractive potential route to regulate the
aromatic alcohols distributions. In this study, catalytic results
petroleum-derived
aromatic
hydrocarbons,
such
as
methylbenzene, xylene, etc.[3] Moreover, shifting refinery targets
towards gasoline production certainly causes a global shortage
of aromatic hydrocarbons.[4] Although the supply of aromatic
hydrocarbons has been explored by other non-oil routes, such
as
lignocellulose
conversion,[5,6]
aromatization of
sugar-derived
alcohols,
trimerization
furans
x
proved that the formaldehyde (CH O) derived from methanol can
cycloaddition,[
7]
[8,9]
syngas
regulate the structure of enal intermediates, and in turn, change
the product distributions of aromatic alcohols. As a result, benzyl
alcohol (BA-OH) is formed over cobalt-hydroxyapatite, and its
selectivity increases with the increment of methanol pressure.
The possible reaction pathways for BA-OH production was
proposed from kinetic experiments and in situ IR studies.
transformation,[10,11,12]
ethylene
and
cycloaddition,[13,14] and methane nonoxidative conversion, the
production of aromatic alcohols by using any sustainable
feedstock rather than aromatic hydrocarbons would be more
environmentally benign.
Ethanol, an available platform molecule, has been
investigated for the production of value-added chemicals, such
[4]
as acetaldehyde, butadiene, and higher aliphatic alcohols (C4-12
)
Results and Discussion
dehydrogenation,[
15,16]
dehydration,[17]
and
CC
by
coupling.[
18,19,20]
However, by far, few studies were reported to
Dependence of product distributions on methanol
concentration
produce aromatic oxygenates from ethanol and its derivates.
Resasco[
21]
and Flaherty
[22]
observed the formation of
methylbenzaldehydes (MB=O) from acetaldehyde (instead of
Figure 1 shows that the distribution of aromatic alcohols over
Co-HAP is greatly switched from methylbenzyl alcohol (C8, MB-
OH) to benzaldehyde (C7, BA=O) and benzyl alcohol (C7, BA-
OH) by cofeeding methanol with ethanol. When ethanol is fed
alone, MB=O/-OH are presented as the only aromatic products,
in accordance with our previous report.[23] Strikingly, after
cofeeding methanol with ethanol, BA=O/-OH are clearly
observed. The distribution of aromatic alcohols depends strongly
on the ratio of methanol to ethanol, that is, the selectivity of
[
a]
B.-C. Zhou, Dr. Q.-N. Wang, Dr. X.-F. Weng, Dr. L. He, Prof. W.-C.
Li, and Prof. A.-H. Lu
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering
Dalian University of Technology
Dalian 116024, P. R. China
E-mail: anhuilu@dlut.edu.cn
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ChemCatChem
10.1002/cctc.202000010
FULL PAPER
MB=O/-OH decreases monotonically, while the selectivity of
BA=O/-OH gradually increases and reaches a maximum of 27%
in the feed of methanol mole fraction of 0.67 at 325 °C. The total
selectivity towards aromatic oxygenates reaches 45% under
these reaction conditions. A combination of kinetic experiments
and in situ FTIR studies demonstrates that ethanol undergoes
dehydrogenation, self-condensation, dehydrocyclization, and
rehydrogenation to MB-OH, in which 2-butenal is the key
intermediate.[23] The carbon number of BA=O/-OH (C
) shown in
7
Figure 1, together with their structure, indicates that cross-
condensation reactions of different enal intermediates derived
from methanol and ethanol take place. Simultaneously, the
product distributions of saturated aliphatic alcohols (C
3 5
and C ,
C
4
and C ) show an opposite trend (Table 1). These findings
6
reveal that high concentration of methanol possibly favors the
formation of enal intermediates with odd carbon number, such
as acrolein,[27] which then undergoes cross-condensation with 2-
butenal and dehydrocyclization to BA=O/-OH, rather than
hydrogenation to n-propanol. As shown in Table 1,
approximately 60% carbon atoms in the consumption of CH
are incorporated into the aromatic alcohols. Notably,
formaldehyde, dimethyl ether, CO (>90 mol%) and no aromatic
oxygenates were observed when methanol was only fed over
Co-HAP (Table 2). CO likely arises from the over oxidation and
3
OH
x
x
decomposition of formaldehyde in the presence of water, which
was coproduced from the C-C coupling under reaction
conditions.[
31]
Nevertheless, in the case of methanol-ethanol
selectivity is below 0.5 mol% among all the
cofeeding, the CO
x
Figure 1. Dependence of product distributions on the partial pressure of
methanol. Reaction activity was measured over Co-HAP (0.8 wt%) at 325 C
in a feed gas of 6 vol% (CH OH+C H OH) balanced with N (30 mLmin ).
3 2 5 2
products, indicating that the cross-condensation of formaldehyde
with acetaldehyde is dominant by overwhelming its
oxidation/decomposition reactions.
-
1
Table 1. Reactivity of methanol and ethanol over Co-HAP catalyst a
Conversion (%)
Temperature
Selectivity (%)
(
C)
Others b
C
2
H
5
OH
CH
3
OH
CH
3
CHO
C
3
H
4
O
C
4
H
6
O
C
3-6OH
BA=O
1.9
BA-OH
20.3
MB=O
1.4
MB-OH
15.2
16.7
11.7
10
3
3
3
3
00
25
50
75
10.9
19.3
36.8
59.8
2.5
5.1
19.4
22
trace.
0.5
2.8
3.4
3.9
2.2
35.5
22.1
16.5
14.1
3.5
6.7
4.1
22.4
2.1
7.8
21.3
19.3
0.9
8.5
24.6
3.2
9.4
12.5
1.4
14.7
20.6
5.2
12.5
a
3 2 5 2 4
Reaction activity was measured over Co-HAP (0.8 wt%) at 325 C with a feed gas of 4vol% CH OH and 2vol% C H OH, N
balanced. b Other products include ethyl acetate and C
olefins, with minor acetone, ethylene, and diethyl ether.
Table 2. Reactivity of methanol over HAP and Co-HAP catalyst a
Conversion (%)
Catalyst
Selectivity (%)
CH
3
OCH
3
CO
x
Others b
3
CH OH
HAP
2.1
100
6.2
-
-
Co-HAP
37.1
93.5
OH. b Other products include methyl formate.
3
0.3
a
Reaction activity was measured over HAP and Co-HAP (0.8 wt%) at 325 C with a feed gas of 4vol% CH
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Table 3. Reactivity of methanol and ethanol over HAP catalyst a
Conversion (%)
Temperature
Selectivity (%)
i-C4-OH
5.3
(
C)
Others b
C
2 5
H OH
CH
3
OH
CH
3
CHO
C
3
H
6
O
C
3
-OH
n-C4-OH
32.5
C
5
-OH
C
6
-OH
Aromatics
0.1
300
325
350
375
14.9
25.9
37.6
54.3
0.3
3.1
2.5
2.8
3.2
4.3
1.0
1.1
1.7
3.4
30.5
28.6
23.0
18.6
10.9
13.3
16.3
14.6
6.3
7.6
7.5
5.8
10.9
12.4
18.8
28.6
26.2
7.8
0.2
5.8
18.6
10.6
0.3
12.2
11.3
13.1
0.3
a
3 2 5 2
OH and 2vol% C H OH, N
balanced. b Other products include ethyl acetate, methyl acetate
Reaction activity was measured over HAP at 325 C with a feed gas of 4vol% CH
and C olefins, with minor acetone, ethylene, and diethyl ether.
4
On the other hand, when bare HAP is used, feeding ethanol or
methanol-ethanol mainly produces saturated aliphatic alcohols
(from 47% to 38%) and aromatic oxygenates selectivity (from
45% to 40%) (Figure 2).
(
>80%, Table 3), with only traces of aromatics (less than 0.3%),
which agrees well with the literature.[32] The condensation
reaction of methanol with ethanol produces a mixture of C
、C and C alcohols. The selectivity of C alcohol decreases
with the increase in temperature, while the selectivity of iso-
butanol and C alcohols shows a slight increase. Moreover, only
dimethyl ether was observed over HAP by feeding methanol
3
、
C
4
5
6
3
5
(
Table 2). The suitable acid/base property endows HAP a good
catalyst for C-C bond coupling.[
22,23,32,33,34,35,36]
The catalysts
acid/base property modified by Co was carefully considered that
.8wt% loading of Co reduced ca. 15% of base sites, which
0
should decrease the C-C coupling rate if the acid/base property
is the only factor,[33,35,36,37,38] however, Co-HAP shows higher
catalytic reactivity as compared to bare HAP towards both
acetaldehyde condensation[23] and methanol conversion (Table
1
-3). These results indicate that the presence of Co is more
Figure 2. Reaction stability test was measured at 325 C over Co-HAP (0.8 wt%)
important than the acid/base property modification in shifting
reaction route of lower alcohols upgrading.
Furthermore, good stability of the Co-HAP catalyst in the
reaction is evidenced by its operation for 50 h at 325 C, where
a slight decline was observed in terms of ethanol conversion
3 2 5 2
catalyst with a feed gas of 6 vol% (CH OH+C H OH)/N . The weight hourly space
velocity of ethanol and methanol is 0.37 gC2H5OHgCat.-1h-1 and 0.51 gCH3OHgCat. h ,
-1
-1
respectively.
Figure 3. Dependence of the product distributions on the residence time, (a) 1/WHSVC2H5OH and (b) 1/WHSVCH3OH. The dash lines are presented only to guide the
eye. Reaction was conducted at 325 C over Co-HAP (0.8 wt%) in a feed gas of 4 vol% CH OH and 2 vol% C OH balanced with N
3
2
H
5
2
.
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Identifying the reaction intermediates
To reveal the possible reaction route of methanol-ethanol over
Co-HAP, the effect of residence time on product distributions
was investigated. As shown in Figure 3, the steep initial rise in
acetaldehyde formation with residence time (1/WHSV) indicated
that acetaldehyde is a primary product obtained directly from
ethanol dehydrogenation. What’s more, formaldehyde was also
a primary product via the dehydrogenation of methanol, which
acts as an active electrophile.[39] The yields of acetaldehyde and
formaldehyde soon reached plateaus as they converted to 2-
butenal and acrolein via secondary self- or cross-condensation.
The yield of 2-butenal or acrolein also reached a plateau, while
the formation rate of BA=O/-OH and MB=O/-OH increased with
the residence time. The low yields and slopes of 2-butenal and
acrolein, compared to aromatic oxygenates, could only be
rationalized by the fast transformation to BA=O/-OH and MB=O/-
OH. 2-butenal has been verified to undergo self-condensation
and dehydrocyclization to form MB=O, which was then
hydrogenated to MB-OH.[22,23] Therefore, BA=O is likely
produced from the cross-condensation of 2-butenal and acrolein,
and the following dehydrocyclization (vide infra). In these
upgrading reactions, cross-condensation of 2-butenal with
acrolein competes with its self-condensation. The data from
Figure 1 display that increasing methanol pressure efficiently
improves BA=O/-OH selectivity, possibly by boosting the
formation of formaldehyde and acrolein. Therefore, the formation
rate of BA=O/-OH is proportional to the concentration of these
aldehydes intermediates under reaction conditions. In addition, a
small amount of 2-propenol and 2-butenol is also observed; thus
Figure 4. Dependence of the product distributions on the residence time,
(1/WHSVC2H5OH). Reaction activity was measured over HAP at 325 C with a
feed gas of 4 vol% CH OH and 2 vol% C OH balanced with N
3
2
H
5
2
.
report,[29] ethanol and methanol proceed direct cross- and self-
condensation reactions and form aliphatic alcohols (C3-6). At all
residence times (1/WHSV
< 0.5 h), acetaldehyde and
formaldehyde are always minor products, and the selectivity of
aromatic oxygenates is below 0.3%. These results demonstrate
that the Co2 sites change the activation behavior of alcohols,
thus shifting the major products from saturated aliphatic alcohols
to aromatic oxygenates, such as BA=O/-OH and MB=O/-OH.
In situ DRIFTS experiments were conducted to better
understand the activation of methanol over Co-HAP and HAP.
As shown in Figure 5, a weak signal at 2956 cm-1 appears after
methanol dosing over both Co-HAP and HAP, corresponding to
the v(C−H) mode of adsorbed methoxy species, which transform
+
their cross-condensation with enals is possibly existing, which
yields aromatic alcohols directly.[23] Neither cyclohexanone nor
phenol is observed, thereby indicating that the self-condensation
and dehydrocyclization of acrolein do not occur. Moreover,
aliphatic C3-6 alcohols were also yielded, via the hydrogenation
of unsaturated intermediate aldehydes, using either alcohols or
hydrogen as the H-donor.[40]
In the case of HAP, saturated aliphatic alcohols (C3-6) are
always the major products (Figure 4), in consistence with the
prior work.[32] Based on our kinetic studies and literature
-1
quickly into formate species (2830, 2710, 1596 and 1353 cm )
over Co-HAP but not over HAP. This finding clearly
demonstrates that methanol dehydrogenation occurs over Co-
x
HAP, and the CH O species (such as formaldehyde) derived
from methanol possibly further participate in subsequent
reactions, which is in line with our previous study.[23]
Figure 5. In situ DRIFTS spectra recorded over (a) Co-HAP (0.8 wt% Co), and (b) HAP, both at 300 C after one dose of methanol.
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Conclusions
We have demonstrated that by cofeeding methanol together
with ethanol, benzyl alcohol can be produced over Co-HAP. The
distribution of aromatic alcohols can be regulated from
methylbenzyl alcohols by feeding ethanol to benzaldehyde (C
7
,
BA=O) and benzyl alcohol (C , BA-OH) by cofeeding methanol
7
and ethanol, and to aromatic alcohols free by feeding methanol
only. This study provides a direct route for the selective
production of aromatic alcohols from methanol and ethanol over
a tailored bifunctional Co-HAP catalyst. The introduction of
methanol results in the formation of acrolein, which undergoes
secondary
cross-condensation
with
2-butenal,
dehydrocyclization, and hydrogenation to benzyl alcohols.
3
Figure 6. Product distributions from acetaldehyde (CH CHO, 1 kPa) alone and
cofed with acrolein (CH =CHCHO, 1 kPa). Catalytic measurement was
2
conducted at 325 C.
Experimental Section
To explore the reaction pathways and identify the key
intermediates that control the selectivity of aromatic products,
the reaction intermediates (acetaldehyde and acrolein) were fed
Catalyst preparation
Hydroxyapatite (HAP) was synthesized by precipitation method
according to the literature.[32] Briefly, an aqueous solution of Ca(NO3)2
directly as reactants onto Co-HAP catalyst. As shown in Figure 6, (0.6 M, Sinopharm) was added dropwise to a solution of (NH
4
)
2
HPO
O (25 wt%, Sinopharm) was added to the
solution to achieve an initial system pH of 10.3. The slurry was stirred for
4 h at 80 C. After filtration, drying, and calcination (600 C for 2 h in
4
(0.4
MB=O is observed to be sole aromatic products by feeding
acetaldehyde. Interestingly, when acrolein was cofed with
acetaldehyde, BA=O is obviously observed and becomes major
aromatics. These results indicate that 2-butenal and acrolein
both act as key intermediates for the formation of aromatic
alcohols with odd carbon number.
3 2
M, Sinopharm). Then, NH ·H
2
static air), white HAP was obtained. The overall Ca/P ratio was ca. 1.67,
as measured by inductively-coupled plasma optical emission
spectrometry (ICP-OES, Optima2000DV).
3 2
HAP was impregnated with an aqueous solution (0.35 M) of Co(NO )
Scheme
1
illustrates the possible reaction network for
(Sinopharm) by incipient wetness impregnation and held at room
temperature for 30 min. After drying at 50 C for 12 h in air and
calcination at 350 C for 2 h in air, the as prepared Co-HAP catalyst was
obtained. The actual Co content was determined to be 0.8 wt% by ICP-
OES.
methanol and ethanol conversion over Co-HAP. According to
our previous study, Co2+ sites are able to catalyze
dehydrogenation of ethanol to acetaldehyde, which rapidly
undergoes HAP-catalyzed condensation and dehydrocyclization
to form MB=O. 2-butenal, the key intermediate, undergoes self-
condensation, dehydrocyclization, and hydrogenation to 2-MB-
OH. More importantly, Co2+ sites could prevent hydrogenation of
the C=C bonds in unsaturated enal intermediates. For methanol,
it proceeds dehydrogenation to form formaldehyde, which is
subjected to nucleophilic attack from acetaldehyde to produce
Characterization
The characterization of HAP and Co-HAP catalysts refers to our
previous work.[23] Briefly, the Co species of cobalt-hydroxyapatite (Co-
HAP) exist as atomically dispersed Co2+ located into the lattice on HAP
(Ca/P= 1.67) surface. In this study, the actual Co content was determined
by ICP-OES to be 0.8wt % and the chemical state of Co was +2, as
acrolein.
dehydrocyclization of 2-butenal with acrolein probably yields
BA=O, which is then hydrogenated to BA-OH by alcohols or H
as H-donor, i.e., C O + C O  BA=O  BA-OH.
The
subsequent
cross-condensation
and
confirmed by XPS.
DRIFTS spectra were collected at a resolution of 4 cm-1 on a Bruker
2
70V spectrometer equipped with an in situ reaction cell (HARRICK) and
MCT detector. 50 mg catalyst powder was pre-treated at 400 C for 1 h in
a flow of 8% H /N . Then, the sample was maintained at 300 C in
2 2
3
H
4
4
H
6
Scheme 1. Proposed Pathways to Form Aromatic Oxygenates from Ethanol and Methanol.
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ChemCatChem
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flowing N
into the chamber by N
collected at different reaction time. The amount of methanol (6×10-8 mol)
delivered to the cell was controlled by a sample loop and the saturated
vapor pressure at a given temperature.
2
to collect the background spectrum. Methanol vapor was fed
bubbling, and the spectra were simultaneously
Keywords: lower alcohols • aromatic alcohols • cobalt-
hydroxyapatite • dehydrogenation • dehydrocyclization
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(Benzaldehyde, Benzyl alcohol, AR;
C
-C
1 6
alcohols, 2- and 4-
methylbenzyl alcohols, Sinopharm; acrolein, Energy chemical;
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This work was supported by a Joint Sino-German Research
Project (21761132011), the State Key Program of the National
Natural Science Foundation of China (21733002), and the
Cheung Kong Scholars Program of China (T2015036).
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ChemCatChem
10.1002/cctc.202000010
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The distribution of aromatic alcohols
on cobalt-hydroxyapatite is greatly
switched from methylbenzyl alcohol
Bai-Chuan Zhou, Qing-Nan Wang, Xue-
Fei Weng, Lei He, Wen-Cui Li and An-
Hui Lu*
(
(
feeding ethanol) to benzaldehyde
, BA=O) and benzyl alcohol (C
C
7
7
,
Page No. – Page No.
BA-OH) by cofeeding of methanol and
ethanol.
Regulating Aromatic Alcohols
Distributions by Cofeeding Methanol
with Ethanol over Cobalt-
Hydroxyapatite Catalyst
This article is protected by copyright. All rights reserved.