Synthesis of Higher Alcohols via Syngas on Cu/Zn/Si Catalysts. Effect of Polyethylene Glycol Content

Article abstract of DOI:10.1134/S0036024418050059

Cu/Zn/Si catalysts with different polyethylene glycol (PEG) content were prepared by a complete liquid-phase method, and characterized by XRD, H₂-TPR, N₂-adsorption, and XPS. The influence of PEG content on the higher alcohols synthesis from syngas was investigated. The results showed that addition of PEG can influence the texture and surface properties of the catalysts, and therefore affect their activity and product distribution. With an increase in PEG content, BET surface area, Cu crystallite size and surface active ingredient content of the catalysts first increased and then decreased, the CO conversion had similar variation tendency. However, the pore volume and pore diameter of the catalyst increased, and the binding energy of the active component and the content of Cu₂O decreased, which resulted in higher catalyst selectivity towards higher alcohols. The highest C²⁺OH selectivity in total alcohols was 60.6 wt %.

Full text of DOI:10.1134/S0036024418050059

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2018, Vol. 92, No. 5, pp. 884–888. © Pleiades Publishing, Ltd., 2018.
CHEMICAL KINETICS
AND CATALYSIS
Synthesis of Higher Alcohols via Syngas on Cu/Zn/Si Catalysts.
Effect of Polyethylene Glycol Content1
Rong-Ji Cuia, Xing Yanb, Jin-Chuan Fanb,*, and Wei Huangc,**
aCollege of Electrical and Power Engineering, Taiyuan University of Technology, Taiyuan, 030024 China
bCollege of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024 China
cKey Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province,
Taiyuan University of Technology, Taiyuan, Shanxi, 030024 China
*e-mail: fanjinchuan@163.com
**e-mail: huangwei@tyut.edu.cn
Received February 9, 2017
Abstract—Cu/Zn/Si catalysts with different polyethylene glycol (PEG) content were prepared by a complete
liquid-phase method, and characterized by XRD, H2-TPR, N2-adsorption, and XPS. The influence of PEG
content on the higher alcohols synthesis from syngas was investigated. The results showed that addition of
PEG can influence the texture and surface properties of the catalysts, and therefore affect their activity and
product distribution. With an increase in PEG content, BET surface area, Cu crystallite size and surface
active ingredient content of the catalysts first increased and then decreased, the CO conversion had similar
variation tendency. However, the pore volume and pore diameter of the catalyst increased, and the binding
energy of the active component and the content of Cu2O decreased, which resulted in higher catalyst selec-
tivity towards higher alcohols. The highest C2+OH selectivity in total alcohols was 60.6 wt %.
Keywords: higher alcohols, syngas, Cu/Zn/Si catalysts, CO hydrogenation, PEG
DOI: 10.1134/S0036024418050059
1. INTRODUCTION
tion [11]. In our previous study, the ethanol selectivity
of Cu based catalysts without alkalis or F–T elements
could reach an unexpected point via complete liquid-
phase technology; this was discovered accidentally,
but it was difficult to repeat this result [12]. Fan et al.
prepared a series of Cu/Zn/Al catalysts via complete
liquid-phase technology with different surfactants
used as the modifiers. The result showed that non-
ionic surfactant can promote performance of the cata-
lyst via adjusting its specific surface area, pore struc-
ture, phase composition and crystallite size [13].
Meanwhile, it was reported that polyethylene glycol
(PEG) used to modify the silica surface, provides
effective stabilization and immobilization of metal
nanoparticles [14]. Therefore, in this paper, the
Cu/Zn/Si catalysts were prepared via complete liquid-
phase method, and the influence of PEG content on
the higher alcohols formation was clarified.
Environmental pollution and growing demand for
energy force researchers to dedicate to the utilization
of fossil fuels in more clean ways [1–4]. Higher alco-
hols synthesis from syngas has been paid much atten-
tion because they can be used as fuel additives and
alternative transportation fuel [5–7]. However, low
selectivity towards higher alcohols and poor stability of
the catalysts make this process difficult to commer-
cialize [8, 9]. Therefore, developing an efficient and
highly selective catalyst is an actual problem.
Nowadays, the catalysts applied in higher alcohol
synthesis from syngas include modified Fischer–
Tropsch (F–T) catalysts, modified methanol synthe-
sis catalysts, Mo and Rh based catalysts. Modified
F‒T catalysts, mainly including Cu–Co and Cu–Fe
based ones, are regarded as the most promising for this
process [10]. Huang suggested a novel preparation
method of slurry catalysts for slurry reactor, named it
complete liquid-phase method. Its main innovative
thought lied in preparing the catalysts directly from
solution to slurry. It is expected that the stability of
catalysts can be improved, because the catalytic mate-
rials can be used in the same environment as its forma-
2. EXPERIMENTAL
2.1. Catalyst Preparation
Cu/Zn/Si catalysts with Cu : Zn : Si molar ratio
2 : 1 : 0.8 with various PEG content were prepared by
a complete liquid-phase method. A certain amount of
tetraethyl orthosilicate (TEOS) was dissolved in anhy-
1
The article is published in the original.
884

SYNTHESIS OF HIGHER ALCOHOLS VIA SYNGAS ON Cu/Zn/Si CATALYSTS
885
Textual properties of the catalysts were studied by a
nitrogen adsorption method using a Micromeritics
Quantachrome instrument at 77 K. Pore size distribu-
tion was calculated by the Barrett–Joyner–Halenda
(BJH) method, and the specific surface areas were
calculated from the isotherms using the Brunauer-
Emmett-Teller (BET) method.
Cu
ZnO
CZS-4CZ
X-ray photoelectron spectra (XPS) were recorded
on an ESCALAB 250 spectrometer (VG Scientific
Ltd., UK) with AlKα radiation (hν = 1486.6 eV) under
CZS-2CZ
CZS-1CZ
ultrahigh vacuum (7 × 10–8 Pa).
H2-TPR tests (10 K/min) were performed to inves-
tigate the reducibility of the catalysts using a
chemisorption instrument; the sweep gas was detected
using an OmniStar instrument. The catalyst (50 mg)
was pre-treated to remove physically adsorbed water
and other impurities at 573 K under a flow of N2
(32 mL/min) for 1 h and then cooled to 323 K; after
cooling, the gas was changed to a H2/N2 = 0.05 mix-
ture (35 mL/min).
CZS-0
10
20
30
40
50
60
70
80
2θ, deg
Fig. 1. XRD patterns of the catalysts with different PEG
content.
drous ethanol, followed by addition of the mixed solu-
tion containing anhydrous ethanol, distilled water and
concentrated nitric acid and stirred for 2 h and then
kept for 12 h to obtain a silica sol. Then, Cu(NO3)2 ·
3H2O, Zn(NO3)2 · 6H2O, and PEG-600, in turn, were
dissolved in anhydrous ethanol and the resulting solu-
tion was slowly added to the silica sol. A homogeneous
gel was obtained after stirring at 313 K for 4 h, and
aging at room temperature for 7 days. The gel was dis-
persed in liquid paraffin and heated from 323 to 533 K
at a heating rate of 5 K/min and then kept for 7 h at
533 K. To remove paraffin, the catalysts were
extracted with petroleum ether in a Soxhlet apparatus
for 24 h, and then dried in air. The catalysts were
named as CZS–xCZ (x = 0, 1, 2, 4), where x =
16n(PEG-600)/n(Cu2+).
2.3. Catalytic Activity Test
The activity of the catalyst was evaluated in a
500 mL slurry reactor. We added the slurry catalyst
(30 g) into the reactor, and added liquid paraffin to
total volume of 300 mL. Prior to reaction, the catalysts
were reduced at 523 K in a flow of 20 vol % H2 + N2
for 4 h. After cooling to room temperature, the syngas
(H2/CO = 2) was introduced into the reactor at a feed
flow rate of 150 mL/min at 523 K and 4 MPa. The
steady-state activity of the catalyst was estimated from
gas chromatographic analysis of the gaseous products
(online) and analyzing the liquid products (offline).
3. RESULTS AND DISCUSSION
3.1. XRD Characterization
2.2. Characterization of the Catalysts
The XRD patterns of the catalysts with different
PEG content are presented in Fig. 1; the crystallite
size of Cu particles in different catalysts are shown in
Table 1. The diffraction peaks of Cu and ZnO could be
found in the CZS-0 catalyst, but the ZnO diffraction
peaks gradually weakened to disappear in XRD pat-
tern with the increasing PEG content in the catalyst,
indicating that ZnO is well dispersed in it.
XRD patterns of the catalysts were recorded on
Rigaku D/MAX-2500 diffractometer with CuKα radi-
ation (40 kV, 100 mA) in 2θ range from 5° to 85°. The
crystallite size of Cu was estimated from selected
Cu(111) reflections using the Scherrer equation.
The ether oxygen atom of PEG can form coordina-
tion bond with Cu2+ and Zn2+ ions, so PEG can pro-
mote the dispersion of species by scattering Cu2+ and
Zn2+ ions around its chain [13]. In addition, no crys-
talline silica species were detected by XRD. As shown
in Table 1, the average crystallite size of Cu first
decreased and then increased with further increase in
PEG content, implying that a suitable amount of PEG
improves the dispersion of copper species, but an
excess of PEG is unfavorable. Firstly, PEG would
intertwine with the excessive amount of PEG added,
Table 1. Textural properties and crystallite size of Cu parti-
cles of different catalysts
BET
surface
area, m2/g cm3/g
Pore
volume,
Pore
Cu
Catalyst
diameter, crystallite
nm
size, nm
CZS-0
98.3
113.9
82.8
0.170
0.235
0.264
0.349
6.9
8.3
12.7
17.5
25.0
23.2
23.9
28.0
CZS-1CZ
CZS-2CZ
CZS-4CZ
79.8
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886
RONG-JI CUI et al.
so that the gel microspheres mutually reunited, lead-
ing to aggregation of the copper. Secondly, the strong
reducibility of PEG can accelerate the reduction of
CuO with the formation of Cu particles with larger
crystallite size during heat treatment [15].
Besides, it was reported [16] that the surface of
large copper particles has higher CHx and CO cover-
age and lower H coverage.
CZS-4CZ
CZS-2CZ
CZS-1CZ
CZS-0
3.2. Textural Properties
The textural properties of the catalysts are listed in
Table 1. The BET surface areas first increased and
then decreased with the increasing PEG content, sug-
gesting that a suitable amount of PEG allows obtain-
ing higher surface areas. The pore diameters and pore
volumes increase with increasing PEG content, indi-
cating the PEG role in pore formation. Fan et al. [17]
suggested that the catalyst pore size affects the cata-
lytic activity as well as the products distribution
because of the differences in the diffusion of both
reactants and products.
100
150
200
250
300
350
Temperature, °C
Fig. 2. H2-TPR spectra of catalysts with different PEG
content.
For all the catalysts, the Cu XPS peaks were around
931.8–932.4 eV (BE); there are no shake-up satellite
peaks in the range of 940–945 eV attributable to Cu+
and Cu0 species [20–22], which is in accordance with
the XRD and H2-TPR results. Besides, with the addi-
tion of PEG, the Cu 2p, Zn 2p, and Si 2p peaks shifted
toward lower BE proving the formation of Cu, Zn, and
Si species with higher electron density. PEG mole-
cules contain hydroxyl groups which can bind with
metal ions, especially transition metal ions. As a strong
electron-donating group, hydroxyl groups can share
its lone pair of electrons with cation resulting in a
higher electron density of Cu/Zn/Si catalysts contain-
ing PEG.
3.3. H2-TPR Characterization
The reduction of the catalysts with different PEG
content was studied using H2-TPR method. As shown
in Fig. 2, all catalysts demonstrated the peak of H2
consumption which can be attributed to the reduction
of Cu2O, which was not detected by XRD [18]. With
the increasing PEG content, the position of the
hydrogen consumption peak shifted towards the lower
temperature showing that Cu2O reduction proceeds
more easily. This may be due to different dispersities of
Cu2O [19]. Moreover, the area of the hydrogen con-
sumption peak decreased with increasing PEG con-
tent, indicating that the amount of reducible of Cu2O
was gradually reduced. Possibly, PEG in the heat
treatment process facilitates the reduction of copper
species, which is consistent with XRD result.
Surface compositions of the studied catalysts are
listed in Table 2. Both the Cu/Zn and (Cu+Zn)/Si
atomic ratios are far less than theoretical ones, indicat-
ing that the surface of the catalysts was enriched with
Zn and Si; enrichment with Si is more significant.
With an increase in PEG content, the content of Si on
the catalyst surface gradually increases and the Zn
content decreases, while the surface copper content
shows an irregular trend. In addition, the (Cu +
Zn)/Si atomic ratio first increased and then decreased
with increasing PEG content. It is shown that the
3.4. XPS Characterization
The surface chemical state of the components of
the catalysts with different PEG content were charac-
terized by XPS. The binding energies (BE) for
Cu 2p3/2, Zn 2p3/2, and Si 2p are presented in Table 2.
Table 2. The binding energies (BE) for Cu 2p3/2, Zn 2p3/2, and Si 2p
Binding energy, eV
Atomic composition, %
Atomic ratio
Catalysts
Cu 2p3/2
Zn 2p3/2
Si 2p
Cu
Zn
Si
Cu/Zn
(Cu + Zn)/Si
CZS-0
932.3
932.4
932.3
931.8
1022.1
1022.2
1021.9
1021.2
103.0
103.1
102.8
102.3
1.21
1.32
1.15
1.21
3.13
2.83
2.81
2.76
5.99
5.61
6.53
7.77
0.39
0.47
0.41
0.46
0.72
0.74
0.61
0.52
CZS-1CZ
CZS-2CZ
CZS-4CZ
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SYNTHESIS OF HIGHER ALCOHOLS VIA SYNGAS ON Cu/Zn/Si CATALYSTS
Table 3. Performance of the catalysts with different PEG content
887
Selectivity, mol %
HC DME
C
2+OH/ROH,
CO conversion,
%
Catalysts
%
CO2
ROH
CZS-0
17.0
20.0
9.9
71.6
71.0
64.5
47.9
1.5
0.8
3.4
14.2
16.2
15.8
0.5
12.7
12.0
16.3
29.6
0.3
1.8
11.2
60.6
CZS-1CZ
CZS-2CZ
CZS-4CZ
3.5
22.0
Reaction conditions: T = 523 K, P = 4 MPa, H /CO = 2, feed flow rate = 150 mL/min, ROH for total alcohols and HC for hydrocarbons.
2
Table 4. Product distributions for catalysts with different PEG content
ROH, wt %
HC, wt %
C3
Catalysts
MeOH
EtOH
PrOH
BuOH
C1
C2
C4
C5+
CZS-0
99.7
98.2
88.8
39.4
0.2
1.0
6.0
0.1
0.3
2.2
3.5
0.0
0.5
3.0
7.5
24.7
15.9
9.4
16.9
11.5
19.0
19.5
13.9
8.3
7.2
18.7
23.0
20.6
11.2
25.8
41.3
43.8
49.6
CZS-1CZ
CZS-2CZ
CZS-4CZ
49.6
8.1
11.6
higher (Cu + Zn)/Si atomic ratio can favor the cata- reported that CO hydrogen-assisted dissociation to
lytic activity.
form CH2 and then CO insertion into CH2 to form C2
oxygenates were two important reactions of the higher
alcohols formation [24]. Therefore, the synthesis of
higher alcohols required both dissociated and non-
dissociated CO molecules. Besides, Chen et al. sug-
gested that the activity of the catalyst depends on the
rate of CO hydrogenation, but excessive reaction rates
at this step tend to promote the formation of metha-
nol. The reaction rate of CO insertion affects the
selectivity towards higher alcohols. So the higher
selectivity towards higher alcohols requires a balance
between CO hydrogenation and CO insertion capaci-
ties of the catalyst [25]. It was generally believed that
Cu2O has a stronger ability to adsorb and dissociate
CO, and metallic Cu is a non-dissociative adsorption
site for CO in Cu-based catalysts. In CZS-2CZ and
CZS-4CZ catalysts, a decrease in the binding energy
of Cu promoted non-dissociative adsorption of CO,
and a decrease in Cu2O concentration weakened the
CO dissociation facilitating the CO insertion capacity
of the catalyst simultaneously impairing its CO hydro-
genation capacity. In addition, larger copper particle
size and larger pore volume also provide an essential
conditions for the carbon chain growth. An increase in
selectivity towards CO2 may be due to the fact that
hydrocarbons and alcohols formation is always
accompanied by the production of water, most of
which appears to be converted to CO2 through the
water-gas-shift reaction [26].
3.5. Catalytic Performance
The catalytic performance of different catalysts in
CO hydrogenation is summarized in Tables 3 and 4.
One can see that higher alcohols formation on the
CZS-0 catalyst without PEG is insignificant and the
main product is methanol.
The catalytic performance of CZS-1CZ catalyst
prepared with a small amount of PEG was not signifi-
cantly different from that of CZS-0 catalyst except
higher CO conversion, which corresponds to larger
specific surface area of this the catalyst, smaller Cu
grain size and higher active component content of the
catalyst surface. With further increase in PEG content
the CO conversion and total alcohols selectivity
decreased, and the selectivity towards hydrocarbons
and carbon dioxide increased. The mass fractions of
higher alcohols in resulting alcohol mixture increased,
as well as the fraction of long-chain hydrocarbons.
When n(PEG-600)/n(Cu2+) = 4/16, C2+ alcohol frac-
tion reached 60.6 wt %, and C2+ hydrocarbon fraction
reached 91.9 wt %. We can suggest that the addition of
a sufficient amount of PEG promotes carbon chain
growth; these results are consistent with the data
reported in [23]. Assuming the results of the catalysts
characterization, we can conclude that a decrease in
CO conversion is attributed to a decrease in the spe-
cific surface area and to an increase in Cu particle size.
The formation capacity of higher alcohols and long-
chain hydrocarbons is related to a decrease in the
binding energy of Cu, the growth of Cu grains size and
CONCLUSIONS
The PEG content had a significant influence on
the increase in pore size of the catalyst. Zhang et al. the surface properties and catalytic performance of the
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 92 No. 5 2018

888
RONG-JI CUI et al.
10. K. Fang, D. Li, M. Lin, M. Xiang, W. Wei, and Y. Sun,
catalysts prepared via complete liquid-phase method.
A suitable amount of PEG favors an increase in BET
surface area and the wide dispersion of the active
ingredient, thus improving the CO conversion. Higher
amounts of PEG provide the reduction of the binding
energy of the active component, the pore volume and
diameter increase, leading to the formation of higher
alcohols and C2+ hydrocarbons.
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ACKNOWLEDGMENTS
15. Z. Li, Z. Zuo, W. Huang, and K. Xie, Appl. Surf. Sci.
This work was supported by Natural Science Foun-
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of Natural Science Foundation of China (grant
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