Partial hydrogenation of dimethyl oxalate on Cu/SiO2 catalyst modified by sodium silicate

Article abstract of DOI:10.1016/j.cattod.2019.08.048

Cu/SiO₂ catalyst modified with Na₂SiO₃ was prepared by the ammonia evaporation impregnation method and applied in the partial hydrogenation of dimethyl oxalate (DMO) to methyl glycolate (MG). The addition of Na₂SiO₃ led to a serious shrinkage of those accumulation pores in the catalyst, but hardly affect the structure of the mesopores bellow 8 nm. Moreover, the trace amount of Na₂SiO₃ enhanced the formation of copper phyllosilicate, which resulted in a minor increment in Cu⁺ species as well as a decrease of Cu⁰ species. An unexpected high MG yield of about 83% and MG selectivity of 99.8% was achieved over the Cu/SiO₂ catalyst modified with 0.5% Na₂SiO₃ in the partial hydrogenation of DMO. The loss of some smaller accumulation pores due to the presence of Na₂SiO₃ dopant could be key reason for the higher selectivity, because the larger pores can ensure the fast transfer of MG to the external surface. Thus, the further hydrogenation of MG can be prevented. Moreover, the decrement of Cu⁰ species induced by doping of Na₂SiO₃ could be another reason for the higher selectivity to MG, since insufficient activated H₂ could be provided for the further hydrogenation of MG.

Full text of DOI:10.1016/j.cattod.2019.08.048

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Partial hydrogenation of dimethyl oxalate on Cu/SiO2 catalyst modified by
sodium silicate
Huijiang Huang, Bo Wang, Yue Wang, Yujun Zhao⁎, Shengping Wang, Xinbin Ma
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cu/SiO2 catalyst modified with Na2SiO3 was prepared by the ammonia evaporation impregnation method and
applied in the partial hydrogenation of dimethyl oxalate (DMO) to methyl glycolate (MG). The addition of
Na2SiO3 led to a serious shrinkage of those accumulation pores in the catalyst, but hardly affect the structure of
the mesopores bellow 8 nm. Moreover, the trace amount of Na2SiO3 enhanced the formation of copper phyl-
losilicate, which resulted in a minor increment in Cu+ species as well as a decrease of Cu0 species. An un-
expected high MG yield of about 83% and MG selectivity of 99.8% was achieved over the Cu/SiO2 catalyst
modified with 0.5% Na2SiO3 in the partial hydrogenation of DMO. The loss of some smaller accumulation pores
due to the presence of Na2SiO3 dopant could be key reason for the higher selectivity, because the larger pores can
ensure the fast transfer of MG to the external surface. Thus, the further hydrogenation of MG can be prevented.
Moreover, the decrement of Cu0 species induced by doping of Na2SiO3 could be another reason for the higher
selectivity to MG, since insufficient activated H2 could be provided for the further hydrogenation of MG.
Hydrogenation
Dimethyl oxalate
Methyl glycolate
Cu/SiO2
Na2SiO3
1. Introduction
the CeC bond[9–11]. Ammonia-evaporation method has been studied
systematically in the preparation of Cu/SiO2 catalysts, including the
Methyl glycolate (MG) is an excellent organic solvent and important
fine chemical intermediate [1]. Due to its unique molecular structure
with both hydroxyl and ester groups, MG can undergo reactions like
carbonylation, oxidation, hydrolyzation etc. It has been widely applied
in the manufacture of glycolic acid, DL-glycine and malonate ester [2].
Traditionally, MG can be synthesized via the coupling reaction of for-
maldehyde and methyl formate over various acid catalysts, however it
is not pratically feasible in large scale production due to the low yield
and environment contamination [3–7]. Recently, as one of the crucial
step in the synthesis technology of ethylene glycol via syngas, hydro-
genation of dimethyl oxalate (DMO) has drawn much attention from
both scientific and industrial fields. The hydrogenation of DMO is a
complex reaction comprising of several consecutive and parallel reac-
tions, among which, the hydrogenation of only one of the carbonyls of
dimethyl oxalate can produce methyl glycolate with higher selectivity.
Therefore, production of MG via hydrogenation of dimethyl oxalate
(DMO) is promising as an emerging C1 chemistry route owing to its
higher yields, environment friendly and atomic economy [8,9].
evaporation temperature [12], Cu loading [13], doping [14–17], mor-
phology [18] and so on. Lin et.al [19] found that the unstable active
centers was caused by methanol, and the CO splitting from methanol
and the oxidation of alcohol over Cu/SiO2 catalysts are the main rea-
sons for sintering of copper, which resulted in deactivation. Yue and co-
works [20] designed a Cu/SiO2 -based monolithic catalyst for hydro-
genation of DMO to EG. The catalyst is low cost, stable, and exhibits
high activity in the reaction of hydrogenation of DMO, achieving a
100% conversion of DMO and more than 95% selectivity to EG. Cu-Au
catalysts and silver-manganese nanocatalysts were also used in for
ethylene glycol (EG) production via DMO hydrogenation [21–24].
However, it is difficult to achieve excellent selectivity to MG on Cu/
SiO2 catalyst at the conditions of high DMO conversion because the
thermodynamic equilibrium constant of the MG hydrogenation to EG is
two orders of magnitude larger than that of DMO hydrogenation to MG
[1]. Therefore, the synthesis of MG via DMO needs a moderate reaction
condition and a catalyst with relatively weak hydrogenation property.
Previous reports showed that silver-based catalysts exhibited high cat-
alytic performance for partial hydrogenation of DMO to MG at low
temperature [25–27]. Besides, several bimetallic catalysts (such as
CuAu, CuZr, AuAg alloy etc.) are also used for the synthesis of MG via
Generally, Cu/SiO2 catalysts are widely used for ethylene glycol
(EG) production via DMO hydrogenation due to its high activity in the
adsorption of CeO/C]O group and the lower activity in dissociating
⁎ Corresponding author.
E-mail address: yujunzhao@tju.edu.cn (Y. Zhao).
https://doi.org/10.1016/j.cattod.2019.08.048
Received 13 March 2019; Received in revised form 14 August 2019; Accepted 27 August 2019
0920-5861/©2019ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:HuijiangHuang,etal.,CatalysisToday,https://doi.org/10.1016/j.cattod.2019.08.048

H. Huang, et al.
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the hydrogenation of DMO [15,28–30].
were carried out on a Perkin-Elmer PHI 1600 ESCA system operated at a
pass energy of 187.85 eV for survey spectra with an Mg Kα X-ray source
(E =1253.6 eV).
Recently, Dai et al. [31] reported a copper-based hydroxyapatite
(HAP) supported (Cu/HAP) catalysts with excellent catalytic perfor-
mance for the synthesis of MG with a yield of 70%. The author pointed
out that the relatively lower hydrogen activation ability was one of the
important reasons for the high selectivity to MG. Sun et al. [32] re-
ported a sputtering (SP) Cu/SiO2 catalysts with a selectivity of more
than 87% via freezing Cu in a zero-valence state. The author pointed
out that the regulation of Cu chemical properties by changing the
electron structure is a feasible strategy to control the hydrogenation
products.
The specific surface area of metallic copper was measured by the
adsorption and decomposition of N2O using the pulse titration method
described in the literature [1,2]. Briefly, 100 mg of catalyst sample was
reduced in 5% H2-Ar at 623 K for 4 h and cooled to 363 K. Then 15%
N2O-Ar was introduced at a rate of 30 ml/min for 2 h, ensuring that
surface Cu atoms were completely oxidized according to the reaction
2Cu(s) + N2O → Cu2O(s) + N2. The quantity of chemisorbed N2O was
measured by a hydrogen pulse chromatographic technique on a Mi-
cromeritics Autochem II 2920 equipped with a TCD. Hydrogen pulse
reduction of surface Cu2O to metallic copper was conducted at 623 K to
ensure that the chemisorbed N2O on the copper surface immediately
reacted with hydrogen gas introduced from the pulse loop. Hydrogen
pulse-dosing was repeated until the pulse area no longer changed. The
consumed amount of hydrogen was the value obtained by subtracting
the small area of the first few pulses from the area of the other pulses.
The specific area of metallic copper was calculated from the total
amount of hydrogen consumption with 1.46 × 1019 copper atoms per
m2 [3,5].
As reported by the previous work, copper phyllosilicates, which is
responsible for the formation of Cu+, can be formed during the pre-
paration of Cu/SiO2 catalysts by ammonia evaporation (AE) method
[33]. Under the ammonia conditions, Cu2+ complex can react with the
≡
silanol groups of the silica surface via hydrolytic adsorption to form
SiOCuII monomer, which is difficult to be reduced to metallic copper
due to the strong interaction between the copper species and SiO2. In
this paper, Na2SiO3 was applied to modify the surface characteristic of
the Cu/SiO2 catalyst to modulate its hydrogen activation ability. The
4−
SiO4
group derived from the hydrolysis of Na2SiO3 was found to be
able to enhance the formation of copper phyllosilicates. Thus, the
amount of exposed metallic copper decreased in the final reduced
catalysts and the hydrogen activation ability of the catalyst can be
modulated. Fortunately, the Cu/SiO2 decorated with sodium silicate in
the present work displays an un-expectative catalytic performance with
a high yield of MG (YMG = 83%). Moreover, the effect of the copper
valence state distribution on the hydrogenation of DMO to MG was
evaluated to clarify the formation mechanism of MG.
Textual properties of the catalyst were determined by the mercury
intrusion porosimetry (MIP) test using a Micromeritics Autopore IV.
2.3. Catalytic activity test
The catalytic performance was carried out in a continuous flow unit
equipped with a stainless-steel tube reactor placed vertically inside a
furnace with a temperature controller. The catalyst (40–60 meshes) was
packed in the center of the tube reactor. The reaction was carried out
after the catalyst was reduced in pure hydrogen atmosphere at 623 K for
4 h. The reactant (20 wt. % DMO (99.9% purity) in methanol (AR
purity) solution) was injected from the top of the reactor through a
high-pressure pump (Lab Alliance Series II pump) with a system pres-
sure of 2.5 MPa. The reaction was performed at 473 K, with the weight
liquid hourly space velocity (WLHSV) of 1.5 gDMO gcat−1 h−1 (for short
2. Experimental
2.1. Catalyst preparation
Cu/SiO2 powder was prepared by the ammonia-evaporation (AE)
method which has been described in detail in our previous reports
[13,20]. Na2SiO3-modified Cu/SiO2 catalyst was prepared by the im-
pregnation method through immersing uncalcined Cu/SiO2 powder in
sodium silicate solution, stirred for 4 h, subsequently evaporated at
353 K using vacuum rotary, and then dried at 393 K for 4 h, calcined at
673 K in flowing air for 4 h. The final calcined sample was designed as
xSS-Cu/SiO2, where SS and x represented Na2SiO3 and its mass content
to the catalyst, respectively.
h
−1). The products collected in a condenser were analyzed on an
Agilent Micro GC 6820 with an HP-INNOWAX capillary column
(Hewlett-Packard Company, 30 m ×0.32 mm ×0.50 μm) equipped
with a flame ionization detector (FID). To ensure repeatability, 4–6
separate GC samples were taken and the results were averaged for each
experimental data point, and uncertainties were typically within 3%.
3. Results and discussion
2.2. Catalyst characterization
3.1. Effect of Na2SiO3 on structure properties of the catalysts
Fourier-transform infrared radiation (FTIR) spectra were recorded
on a Thermo Scientific Nicolet 6700 in the range of 4000–400 cm−1
The samples were finely grounded, dispersed in KBr, and pelletized. The
spectral resolution was 4 cm−1, and 32 scans were recorded for each
spectrum.
Transmission electron microscopy (TEM) images were obtained
using a Philips TECNAI G2 F20 system electron micro-scope at 100 kV
equipped with a field emission gun.
.
The physicochemical properties of the catalysts modified with/
without Na2SiO3 were listed in Table 1. According to the data measured
by N2 physisorption, the specific surface area (SBET) and the average
pore diameter of the catalysts only showed neglect changes with the
increment of Na2SiO3. However, when come to the results obtained by
MIP method, the pore volume declined dramatically from 0.46 to
0.27 cm3/g and the average pore diameter increased nearly doubled
from 16 to 30 nm with increasing of Na2SiO3. It has been reported that
for the Cu/SiO2 catalyst, the pore about 3 nm belonged to the pores
between the layers of the copper phyllosilicate, and the pores about
15 nm were attributed to the accumulation pore [11]. Because we could
only detect the pore structure larger than 10 nm by MIP method, above
results suggested that addition of Na2SiO3 changed the structure of
large pore and had little influence on the mesoporous between 2 nm
and 8 nm. This suggestion can be also well illustrated by their pore size
distribution curves shown in Fig. 1 This could be attributed to the
condensation polymerization of silicate ions (SiO44−) derived from the
hydrolysis of Na2SiO3, which were anchored on the surface of the
catalyst during the preparation procedure. As a result, it led to the
Temperature-programmed reduction (TPR) was carried out on a
Micromeritics Autochem II 2920. Catalyst of 50 mg was loaded into a
quartz tube and dried in an argon stream at 393 K for 1 h before the
reduction. The catalyst was then heated in 30 ml/min of 5%H2-Ar at a
heating rate of 10 K/min up to 973 K. The amount of H2 consumption
was monitored by a thermal conductivity detector (TCD).
Powder X-ray diffraction (XRD) analysis of the catalysts were per-
formed using a Rigaku C/max-2500 diffractometer employing the gra-
phite filtered Cu Kα radiation (λ =1.5406 Å) at room temperature.
Data points were acquired by step scanning with a rate of 12˚/min from
2θ = 10˚ to 2θ = 90˚.
X-ray photoelectron spectroscopy (XPS) analysis of the catalysts
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Table 1
The textural properties and chemical compositions of Cu/SiO2 catalysts with different content of Na2SiO3.
a
a
b
b
c
d
e
f
Catalysts
SBET (m2/
g)
Vpore (cm3/
g)
dpore (nm) Vpore (cm3/
dpore (nm) Cu dispersionc (%) SCu (m2 g−1
)
SCu+ (m2 g−1
)
XCu0
STYMG (gMG
L-1 h-1−1
0
g)
)
Cu/SiO2
0.25 SS-Cu/SiO2 400
0.5 SS-Cu/SiO2
413
0.98
1
1.07
9.5
10
11
0.46
0.34
0.27
16
24
30
8.8
8.6
8.2
11
10.8
10.3
9.4
10.2
10.3
0.539 10.3
0.513 654
0.498 1033
388
a
Pore volume and average pore diameter determined by BET.
Pore volume and average pore diameter are determined by MIP.
b
c
d
e
f
Cu dispersion and the Cu0 surface area are determined by N2O titration.
0
Surface area of Cu+ is calculated by SCu /XCu0×(1- XCu0).
Peaks area ratio between Cu0 and (Cu0+Cu+) by deconvolution of Cu LMM XAES.
STY represents the space time yield, grams of product per liter of catalyst per hour.
Scheme 1. Diagram for the effect of Na2SiO3 dipping on the catalyst and DMO
hydrogenation.
Fig. 1. Pore size distribution of Cu/SiO2 catalysts with different content of
Na2SiO3 determined by mercury intrusion method.
Fig. 2. FTIR spectra of calcined Cu/SiO2 catalysts with different content of
Na2SiO3.
serious shrinkage of those accumulation pores. It suggested that trace
amount of Na2SiO3 could change the catalyst structure by shrinking the
accumulation mesopores (Scheme 1).
Fig. 1 clearly shows that the relative amount of copper phyllosilicate
4-
was improved with increasing Na2SiO3. The reaction between SiO4
and Cu2+ to form copper phyllosilicate could be the reason for the
increment. It could also be illustrated from the TEM images of calcined
samples shown in Fig. 3.
3.2. Effect of Na2SiO3 on the chemical state of copper
Fourier-transform IR (FTIR) spectroscopy was used to investigate
the structural evolution of the catalysts and the effect of Na2SiO3. As
shown in Fig. 2, the formation of copper phyllosilicate in all calcined
samples was confirmed by the appearance of the δOH bands at 1040 and
670 cm−1 [34]. While, the bands at 938 and 690 cm−1 were owed to
the frequencies of the δOH bands for copper hydroxide. Furthermore,
the relative amount of copper phyllosilicate can be evaluated by con-
sidering the integrated intensity of the δOH band at 670 cm−1 normal-
ized to the integrated intensity of the symmetric δSiO band of amor-
phous silica at 800 cm−1, which are named I670/I800 [35,36]. Inset in
It has been reported that the presence of copper phyllosilicate can
enhance the metal–support interactions significantly, which are vital
for the high activity and stability in hydrogenation of DMO [11,33,34].
This could be proved by the H2-TPR profiles. As shown in Fig. 4, the
temperature of main reduction peak rose apparently with the increase
of the Na2SiO3 content. It indicated that trace Na2SiO3 will make the
copper species more difficult to be reduced, which should be ascribed to
the strengthened interaction between the copper species and the surface
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Fig. 3. TEM images of (A) calcined Cu/SiO2 monolithic catalyst and (B) calcined 0.25 SS-Cu/SiO2 monolithic catalyst and (C) calcined 0.5 SS-Cu/SiO2 monolithic
catalyst.
Fig. 4. H2-TPR profiles of Cu/SiO2 catalysts with different content of Na2SiO3.
dopants.
In our previous work, it was found that the cooperative effect be-
tween Cu+ and Cu0 was responsible for the high catalytic activity for
hydrogenation of DMO to ethanol [37,38]. The Cu+ species is formed
upon the reduction of the copper phyllosilicate under the experimental
conditions (e.g. 623 K), since the further reduction of Cu+ to Cu0 re-
quires a temperature higher than 873 K [19]. While the Cu0 species is
formed upon the reduction of the CuO in the calcined catalyst. As
shown in Fig. 5, the change of copper species was exhibited by the XRD
patterns of the calcined and reduced catalysts. The broad peak at 22°
was attributed to amorphous silica. Moreover, the characteristic peaks
of CuO at 35.4° and 38.6° (JCPDS05-0661) cannot be found for all the
samples (Fig. 5A), indicating that the copper species was highly dis-
persed in these catalysts. Strong diffraction peaks of the reduced cata-
Fig. 5. XRD patterns of the calcined (A) and reduced (B) Cu/SiO2 catalysts with
different content of Na2SiO3.
lysts (Fig. 5B) at 36.4° can be ascribed to the characteristic of the Cu2O
(111) plane, which become stronger with the introduction of
into the catalyst. Fig. 6B showed that the Cu 2p satellite peak at about
Na2SiO3.The formation of Si-O-Cu is due to that [Cu(NH3)4]2+ can
944 eV in the XPS patterns disappeared after reduction, suggesting that
exchange ions with Si−OH on surface of silica sol. [39] It implied that
Na2SiO3 hardly affect the reduction of Cu2+ to Cu+ or Cu0 for all
Na2SiO3 could prompt the formation of Cu+ species in Cu/SiO2 cata-
samples. It is well known that Cu0 and Cu+ species cannot be accurately
lysts. Additionally, the characteristic diffraction peaks at 43.3° for Cu0
confirmed since the binding energy of them overlap in Cu 2p XPS
spectra, but they could be distinguished by their different kinetic en-
ergies in the Cu LMM XAES spectra. As shown in Fig. 6C, the broad and
was not detected in all the samples, probably due to its higher disper-
sion.
XPS analysis was conducted to further discriminate the chemical
asymmetric Cu LMM XAES spectra indicated that Cu+ and Cu0 may
states of copper species in the catalysts. As shown in Fig.6A, two in-
coexist in a stable state on the catalyst surface. Deconvolution of the
tensive photoelectron peaks at 933.7 eV (with a satellite peak at
original spectra can generate peaks of 916.8 eV and 913.5 eV, which
944 eV) and 953.3 eV were ascribed to the Cu 2p3/2 and 2p1/2 binding
were attributed to Cu0 and Cu+, respectively (Fig. 5C). As listed in
energy (BE), and no obvious variation was found when doping Na2SiO3
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Fig. 6. Cu2p X-ray photoelectron spectrum of (A) calcined and (B) reduced Cu/SiO2 catalysts with different content of Na2SiO3. (C) Cu LMM XAES spectra of various
catalysts.
Table 1, the Cu0/(Cu0+Cu+) intensity ratio derived by fitting the Cu
LMM peak decreased from 0.539 to 0.498 when increasing the content
of Na2SiO3 from 0 to 0.5 wt.%. The decrease of the ratio could be re-
lated to the increase of the copper phyllosilicate in calcined catalysts.
In order to determine quantitatively the role of Na2SiO3 on tuning
the copper valence state distribution, N2O titration were applied to
measure the surface area of Cu0 in the catalysts. It was found that the
both the Cu0 surface areas and dispersion presented a slightly decline
(Table 1), as a result of the introduction of Na2SiO3 into the catalysts.
DMO is the rate-determining step for the hydrogenation of DMO to EG
upon Cu/SiO2 prepared by ammonia evaporation (AE) method [40].
However, it is worth noting that the conversion of DMO presented a
decrease instead of increase trend although the surface area of Cu+ is
slightly improved by the addition of Na2SiO3. Meanwhile, the activa-
tion of H2 on Cu0 sites should not be the key reason for the lower ac-
tivity of Na2SiO3-modified Cu/SiO2 catalysts, since there is only a minor
decrease in the surface area of Cu0 species. According to the texture
properties of the catalysts, the obvious decrease of the accumulation
pore in the regime of pore diameter at 8–16 nm (Fig. 1b) could be the
reason. It indicates that the active sites in the accumulation pores could
be responsible for DMO hydrogenation. The loss of accumulation pores
could decrease the accessibility of the active sites by DMO molecular,
resulting in lower DMO conversion. As for the product distribution, a
high EG selectivity of 92.2% was obtained over the Cu/SiO2 catalyst,
accompanied by the generation of only trace amount of MG
(SMG = 0.8%). However, barely any other product except MG was
produced over the 0.5 SS-Cu/SiO2 catalyst and the MG selectivity
reached 99.8%. According to the structure properties of the catalyst, the
loss of some smaller accumulation pores could be key reason for the
higher selectivity, because the larger pore can ensure the fast transfer of
MG to the external surface. Thus, the further hydrogenation of MG can
be prevented. Some previous studies [29,31] showed that catalyst de-
activation is obvious when conversion of DMO is much lower. It in-
dicated that the presence of large amount of MG could affect the sta-
bility of copper-based catalyst. In this work, the catalyst activity did not
show a dramatic decline when the conversion of DMO is lower than
90% with the increase of the WLHSV (in Fig.S1). The presence of so-
dium silicate should play an important role in stabilizing the catalyst
activity, which is deserved a further study. In addition, the decrease of
the Cu0 surface area could be another possible reason for the higher
selectivity of MG over Na2SiO3-modified Cu/SiO2 catalysts. The in-
sufficient Cu0 species prohibits the further hydrogenation of MG to EG
due to the lack of enough activated hydrogen. Anyway, Na2SiO3-mod-
ified Cu/SiO2 catalyst gave a high MG yield of above 83% at nearly
100% selectivity at 1.5 h−1. And the space time yields (STY) of MG over
the 0.5 SS-Cu/SiO2 catalyst were 100 times more than the Cu/SiO2
catalyst (Table 1). These results may provide an inspiration on fabri-
cating effective catalyst for the partial hydrogenation of DMO to MG.
On the contrary, the Cu+ surface areas (obtained by SCu = SCu
/
+
0
XCu0×(1- XCu0)) increase from 9.4 to 10.3 m2/g with the increase of
Na2SiO3 content. Evidently, appropriate amount of Na2SiO3 had a po-
sitive influence on the formation of Cu+ species, leading to the decrease
of Cu0/(Cu0+Cu+).
3.3. Catalytic performance in the hydrogenation of DMO
Vapor-phase hydrogenation of DMO is known to involve several
continuous reactions, including DMO hydrogenation to MG, MG hy-
drogenation to ethylene glycol (EG), and deep hydrogenation of EG to
ethanol [29]. Fig. 7 presented the catalytic performance of partial hy-
drogenation of DMO over various catalysts. When 0.5 wt.% Na2SiO3
was introduced into the Cu/SiO2 catalyst, the conversion of DMO de-
crease from 100% to 84%. It has been reported that the Cu0 species
activates H2 and the Cu+ species adsorbs the carbonyl group [6]. Our
previous kinetic study has revealed that the dissociation adsorption of
4. Conclusions
In summary, we have demonstrated a simple method to modify the
Cu/SiO2 catalyst with Na2SiO3 for achieving high yields of MG via
hydrogenation of DMO. According to the structure properties of the
catalysts, the modification of Cu/SiO2 catalyst by Na2SiO3 led to a
serious shrinkage of those smaller accumulation pores in the catalyst,
but hardly affect the structure of the mesopores bellow 8 nm. This could
Fig. 7. Correlation of the catalytic performance of the Cu/SiO2 catalysts mod-
ified by Na2SiO3 with the Cu0 surface areas. Reaction conditions: T =473 K,
P = 2.5 MPa, H2/DMO = 80 (mol/mol), Weight liquid hourly space velocity
(WLHSV) =1.5 h−1
.
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be attributed to the condensation polymerization of silicate ions
(SiO44−) derived from the hydrolysis of Na2SiO3 during the prepara-
tion. Moreover, the trace amount of Na2SiO3 enhanced the formation of
copper phyllosilicate, resulting in a decrease of Cu0 species in the re-
duced catalyst. The disappearance of some smaller accumulation pores
led to an increase of the proportion of larger accumulation mesopores,
which will contribute to the rapid diffusion of MG. Thus, the deep
hydrogenation of MG can be prevented. Moreover, lower amount of Cu0
species could also benefit the chemosynthesis of MG by supplying in-
sufficient activated H2 for its further hydrogenation. An unexpected MG
yield of about 83% and selectivity of 99.8% were achieved on the
0.5SS-Cu/SiO2 catalyst due to the relative higher proportion of larger
mesopores and lower hydrogen activation ability, showing a pro-
spective future of the catalyst in industrial applications.
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