High catalytic activity of CuY catalysts prepared by high temperature anhydrous interaction for the oxidative carbonylation of methanol

Article abstract of DOI:10.1039/c9ra10501h

CuY catalysts were prepared by high temperature anhydrous interaction between NH₄Y zeolite and copper(ii) acetylacetonate Cu(acac)₂ and the activities were measured for the oxidative carbonylation of methanol to dimethyl carbonate under atmospheric pressure. The bulk and surface properties of the as-prepared catalyst were characterized by XRD, H₂-TPR and XPS techniques. The activation atmosphere of the CuY catalyst and the testing temperature of the catalytic activity was systematically studied. During activation, nitrogen promotes the auto-reduction of Cu²⁺ to form the Cu⁺ active center, but deposited carbon on the surface of the CuY catalyst covers the active center, even plugging the channel, resulting in lower catalytic activity. Oxygen eliminates deposited carbon, but is not so good for the auto-reduction of Cu²⁺. Nitrogen doped with a small amount of oxygen not only eliminates the deposited carbon, but also promotes the auto-reduction of Cu²⁺ to form more Cu⁺ active centers. With the testing temperature increasing, the catalytic activity increases first and then decreases. When the testing temperature is 170 °C, the CuY catalyst with satisfactory activity and stability showed an excellent catalytic activity with 525.1 mg g^-1 h^-1 space time yield of DMC (STY_DMC) and 18.9% methanol conversion. Then the longevity was investigated at 170 °C for 150 h. During the initial reaction period of 40 h, the STY_DMC value was constant. In the next 20 h, the catalytic activity slightly decreased. But in the last 90 h, the catalytic performance is very stable and the STY_DMC value remains 480 mg g^-1 h^-1. The main cause of deactivation is the growth of the particles.

Full text of DOI:10.1039/c9ra10501h

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High catalytic activity of CuY catalysts prepared by
high temperature anhydrous interaction for the
oxidative carbonylation of methanol
Cite this: RSC Adv., 2020, 10, 3293
a
a
b
a
c
Yuchun Wang, * Zhaorong Liu, Chao Tan, Hong Sun and Zhong Li
CuY catalysts were prepared by high temperature anhydrous interaction between NH
4
Y zeolite and
copper(II) acetylacetonate Cu(acac)
methanol to dimethyl carbonate under atmospheric pressure. The bulk and surface properties of the as-
prepared catalyst were characterized by XRD, H -TPR and XPS techniques. The activation atmosphere of
the CuY catalyst and the testing temperature of the catalytic activity was systematically studied. During
2
and the activities were measured for the oxidative carbonylation of
2
2+
+
activation, nitrogen promotes the auto-reduction of Cu to form the Cu active center, but deposited
carbon on the surface of the CuY catalyst covers the active center, even plugging the channel, resulting
in lower catalytic activity. Oxygen eliminates deposited carbon, but is not so good for the auto-reduction
2+
of Cu . Nitrogen doped with a small amount of oxygen not only eliminates the deposited carbon, but
2+
+
also promotes the auto-reduction of Cu
to form more Cu active centers. With the testing
temperature increasing, the catalytic activity increases first and then decreases. When the testing
ꢀ
temperature is 170 C, the CuY catalyst with satisfactory activity and stability showed an excellent
ꢁ1
ꢁ1
catalytic activity with 525.1 mg g
h space time yield of DMC (STYDMC) and 18.9% methanol
Received 13th December 2019
Accepted 14th January 2020
4
ꢀ
conversion. Then the longevity was investigated at 170 C for 150 h. During the initial reaction period of
0 h, the STYDMC value was constant. In the next 20 h, the catalytic activity slightly decreased. But in the
DOI: 10.1039/c9ra10501h
ꢁ1 ꢁ1
last 90 h, the catalytic performance is very stable and the STYDMC value remains 480 mg g
main cause of deactivation is the growth of the particles.
h . The
rsc.li/rsc-advances
At present, the oxidative carbonylation of methanol to DMC
has attracted increased attention due to its low-cost and green
Introduction
12,13
Recently, the global annual methanol production capacity has virtues.
EniChem rst commercialized a DMC production
exceeded 100 million tons, but its demand was only around 65– process based on a liquid phase oxidative carbonylation of
1
14
7
0 million tons. Under this environment, to develop green methanol in the presence of cuprous chloride as a catalyst.
technologies for the conversion of methanol to its downstream This process, however, suffers from rapid deactivation of cata-
products is of great importance. The environmentally friendly lyst, corrosion of equipment and the separation of products
dimethyl carbonate (DMC), as the downstream product of from the catalyst. To overcome these drawbacks, vapor phase
2–5
methanol, has been attracting wide attention. DMC is being oxidative carbonylation processes have been proposed and was
considered as a component of reformulated fuels owing to its commercialized on the CuCl /activated carbon (AC) by Dow
2
6
–8
15
good blending properties and high oxygen content of 53%.
Compared with the usual methyltert-butyl ether with 18%
oxygen content, DMC not only signicantly improves the fuel different support and found CuY catalyst is one of the effective
Chemical Company.
Some studies had compared copper-based catalyst with
16,17
oxygenation but also displays a more valuable efficiency due to catalysts. King
showed that the activity and stability of CuY
reducing the exhaust level of carbon monoxide, formaldehyde prepared by solid state ion exchange with helium purge is
18
2
and unburned hydrocarbons, which reduces environmental superior to that of the CuCl /AC catalyst. Zhang et al. showed
9
–11
pollution.
that, compared with Cu-ZSM-5 and Cu-MOR catalysts, the CuY
catalyst had the higher activity and selectivity for the oxidative
carbonylation of methanol to DMC. This is mainly because
a
+
Department of Applied Chemistry, Yuncheng University, Fudan Street 1155, Yuncheng
methanol was more easily absorbed on surface bound-Cu of
0
b
44000, Shanxi, China. E-mail: wyc0104@126.com; Tel: +86 359 2513059
the CuY catalyst to form Cu–OCH and CO inserted into Cu–
3
Key Lab of Process Analysis and Control of Sichuan Universities, Yibin University,
OCH bonds, resulting in the formation of DMC. And many
3
Yibin 644000, Sichuan, China
+
c
documents have recorded the Cu ion on the CuY catalyst is
active center by a density functional theory.
Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi
19,20
Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
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Research shows that the activation condition would deter- Catalyst preparation
+
mine the Cu content on the CuY catalyst, and then the catalytic
activity for oxidative carbonylation of methanol to DMC. Richter
Firstly, 30.002 g NaY zeolite (Si/Al ¼ 2.7) was added to 300.00 mL
ꢁ1
0.50 mol L
4 3
NH NO aqueous solution and stirred to ion
2
1
et al. recently reported that NH Y with copper(II) nitrate solu-
ꢀ
ꢀ
4
exchange for 4 h at 30 C, and dried at 100 C. Then the
ꢀ
tion by incipient wetness impregnation and drying at 120 C has
no activity. Aer calcination at 400 C in static air for 2 h, the
4
procedures were repeated and the nal solid material was NH Y
zeolite.
ꢀ
samples revealed only limited activity. But a dry argon stream
Typical preparation process of the CuY catalyst included two
stages. In the rst stage, a physical mixture with 2.454 g
Cu(acac) and 5.001 g NH Y zeolite was ground with a mortar
ꢀ
activation at 650 C will improve the catalytic activity of CuY
22
catalyst. Nam et al. prepared the Cu/Y-zeolite by precipitation
2
4
ꢀ
sedimentation, aer drying for 12 h at 120 C, the sample
and pestle to form an intimate gray-blue mixture. Then the
exhibited a good catalytic activity with DMC yield of 49.9%. But
mixture was placed in a crucible, and put in the muffle furnace.
ꢀ
aer the sample was calcined at 550 C for 2 h in an air, DMC
ꢀ
And the furnace temperature rstly increased from 25 C to
yield decreased to 39.1%. All this proves that activation condi-
tion is a critical factor of affecting the catalytic activity of the
CuY catalyst. Moreover, it was reported that nitrogen is the
optimal activation atmosphere for the CuY catalyst, which is
ꢀ
ꢀ
ꢁ1
ꢀ
2
4
50 C at 3 C min , and then was held constant at 250 C for
h. The mixture turned brown and the nal solid material was
named as precursor. In the following activation stage, the
precursor was placed in a tube furnace. And the furnace
2
+
+
conducive to the reduction of Cu and the formation of the Cu
ꢀ
ꢁ1
ꢀ
temperature was ramped at rate of 3 C min to 650 C, held
for 4 h and decreased slowly to room temperature. To investi-
gate the effects of activation atmosphere, the CuY catalysts were
23–25
active center.
Our preliminary investigations
2
6–28
+
showed that NH4 of
NH Y was ion-exchanged by Cu of copper(II) acetylacetonate
2
+
4
activated using N
2
, O
2
, mixture of both gas with the ratio of 4 : 1,
Cu(acac)
interaction between NH
exchanged Cu was reduced to Cu active center when the
CuY catalyst was activated in a muffle furnace, and the as-
prepared CuY catalyst has a good catalytic activity for oxida-
tive carbonylation of methanol to DMC. But under an inert
2
under the condition of high temperature anhydrous
Y zeolite and Cu(acac) . The ion-
ꢁ1
10 : 1, 54 : 1 at a ow rate of 120 mL min , the CuY catalysts are
4
2
labeled CuY-1, CuY-2, CuY-3, CuY-4, CuY-5 respectively. The
CuY-1 is black, and the light blue becomes shallow gradually
from CuY-2 to CuY-4, CuY-5 is grey white. The as-prepared
catalysts were stored in a drybox. Before catalytic measure-
ments, the catalyst was pressed and sieved to 40–60 mesh.
2
+
+
2
+
gas atmosphere the auto-reduction of Cu is most likely to
occur and form more Cu active centers. However, for the CuY
+
Characterization techniques
catalysts prepared by high temperature anhydrous interaction
4 2
between NH Y and Cu(acac) , we was surprised to nd that X-ray diffraction (XRD) patters were obtained using CuKa
the surface of the as-prepared CuY catalyst activated under radiation (l ¼ 0.154056 nm) on a Rigaku D/max 2500 diffrac-
nitrogen atmosphere was covered by a black material and the tometer at 40 kV target voltage and 100 mA tube current in the
ꢀ
ꢀ
ꢀ
ꢁ1
catalytic activity is poor. Then activation atmosphere of the 2q range from 5 to 65 at a scanning rate of 8 min
.
CuY catalyst was investigated. Evaluation of catalytic activity
Temperature-programmed reduction (H -TPR) was per-
2
for oxidative carbonylation of methanol to DMC and formed on a Micromeritics Autochem II 2920 chemical
a detailed characterization of CuY catalysts were also per- adsorption instrument. Firstly, about 20 mg sample was loaded
formed. The major objectives are to elucidate the inuence of in a U-shaped quartz tube and degassed at a heating rate of
ꢀ
ꢁ1
ꢀ
activation atmosphere on copper species and catalytic activity 10 C min to 300 C for 30 min at a N2 ow rate of 30
ꢁ1
by the characterization methods of X-ray, H
2
-TPR, XPS and mL min . Aer cooling to room temperature, the gas was
in argon ow, and the sample was
TEM. In addition, the inuence of oxidative carbonylation switched to 10 vol% H
2
ꢀ
ꢁ1
ꢀ
temperature on the selectivity of organic products and the heated at a heating rate of 10 C min to 1000 C. The effluent
space time yield of DMC was investigated. Finally, the catalyst gas was passed through a cold trap to remove water before
ꢀ
life time was studied at 170 C of oxidative carbonylation detection, then the hydrogen consumption was monitored by
temperature.
a thermal conductivity.
X-ray photoelectron spectroscopy (XPS) data were collected
on an AXIS ULTRA DLD electron spectrometer by using a 150 W
monochromatic Al Ka radiation source operating at 1486.6 eV.
The catalyst holder was placed into a fast entry air load-lock
chamber without exposure to air and the pressure was main-
Experimental section
Materials
ꢁ7
NaY zeolite with a Si/Al molar ratio of 2.7 was purchased tained at 3 ꢂ 10 Pa overnight. Then the catalyst holder was
from Nankai Catalyst Factory. Reagents like NH
Tianjin Chemical Reagent No. 3 Factory), Cu(acac)
Sinopharm Chemical Reagent Co. Ltd), CH
Kemiou Chemical Reagent Company) as well as carbon
4
NO
3
(AR, transferred to the analysis chamber for XPS study. The binding
2
(AR, energy and the Auger kinetic energy scales were referenced to
3
OH (AR, Tianjin the C1s line at 284.8 eV from adventitious carbon.
Transmission electron microscope (TEM) photographs were
monoxide (99.99%, Beijing Haipu Beifen Gas Industry Co., carried out using a JEOL JEM-2100F electron microscope oper-
LTD), oxygen, nitrogen and hydrogen (99.99%, TISCO) were ating at 200 kV. Exposition times were conned to about 2 s due
used.
to the beam sensitivity of the samples. The samples were
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prepared by dispersing the powdered products as slurry in
ethanol, and then deposited on a carbon lm coated on
a copper grid.
Catalytic tests
Catalytic measurements were performed in a continuous xed
bed micro reactor (F8 mm ꢂ 500 mm) under atmospheric
pressure with an on-line Agilent 6890 gas chromatograph (GC).
0.6006 g (about 1.50 mL) catalyst sample was packed in the
tubular reactor and the reactor was positioned in the reactor
ꢁ
1
furnace. 0.05 mL min methanol was fed into by a constant-
ux pump (2PB-05) and vaporized in the pre-heater. In the

ꢁ
1
ꢁ1
meantime, 55.0 mL min carbon monoxide and 5.5 mL min
oxygen was introduced into the pre-heater and mixed well with
vaporized methanol, and then the mixed reactants were fed into
the catalyst bed to catalyze oxidative carbonylation of methanol.
The activity testing time is 10 h and testing temperature is
4
Fig. 1 The XRD patterns of NH Y and the CuY catalysts.
29
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Tang reported that some materials were dispersed spontane-
ously onto a highly specic surface of a suitable support aer
the heat treatment, which was identied by the disappearance
of the crystalline phase of the matter from their XRD patterns.
So it is concluded that copper species were well dispersed on the
surface of Y zeolite.
140 C, 150 C, 160 C, 170 C, 180 C respectively.
The thermal gas products were analyzed by an on-line GC
with three valves and four columns every 12 minutes. The
products owed through the HP-PLOT/Q capillary column (30
m ꢂ 0.53 mm ꢂ 40 mm), the organic products were hold back.
And then the excurrent inorganic products owed through the
Porapak-Q packed column: CO rstly owed out and entered
2
into a thermal conductivity detector (TCD) and was detected;
secondly, CO and O
A capillary column (30 m ꢂ 0.53 mm ꢂ 25 mm) and detected by
TCD. The products owed through the HP-INNOWax (30 m ꢂ
2
were separated by the HP-PLOT Molesieve/
5
0
.53 mm ꢂ 1 mm) capillary column and entered into a ame
ionization detector (FID), thus MeOH, methyl formate (MF),
dimethoxymethane (DMM), dimethyl ether (DME) and DMC
were detected.
ꢀ
The catalyst longevity was studied at 170 C of oxidative
carbonylation temperature, and the thermal gas products were
analyzed every 30 minutes.
Results and discussion
Characterization
4
Fig. 1 shows the XRD pattern of NH Y zeolite and the CuY
catalysts with different activation atmosphere. As shown in
Fig. 1, all the patterns show the characteristic diffraction peaks
of Y zeolite, which proves that HAI between Cu(acac) and NH Y
2 4
zeolite does not affect the integrity of the Y zeolite crystal
structure, even under the high temperature activation treat-
ꢀ
ment at 650 C. With the nitrogen content of the activation
atmosphere, the peak strength of (220) and (331) lattice plane
becomes gradually weaken and the ratio of I220/I331 gradual
decreases, the value is 2.5, 2.0, 1.8 from CuY-3, CuY-4 to CuY-5
respectively. It indicates that aer HAI between Cu(acac)
2
and
NH Y zeolite and activation at high temperature, copper species
4
enter into the pore of Y zeolite and the (220) and (331) lattice
plane is mainly lled by disordered copper species. And the

lling effect depends highly on activation atmosphere.
And no obvious diffraction peaks corresponding to crystal-
Fig. 2 Hydrogen consumption of CuY catalysts. (a) TPR, (b) hydrogen
lized copper phase are observed in all the XRD patterns. Xie and consumption of the signal peak.
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2
Fig. 2 presents the H -TPR (a) of the CuY catalyst and
hydrogen consumption of each reduction peak (b). According to
30–32
2+
the literatures,
the reactions of ion-exchanged Cu on
zeolites involved in the reduction process occur by a two-step
mechanism:
2
Cu + 0.5H
+
+
+
2
/ Cu + H
(1)
(2)
+
0
+
Cu + 0.5H / Cu + H
2
2
+
+
The reduction of Cu to Cu on the CuY catalyst is charac-
ꢀ
terized by a H
2
-TPR peak below 500 C, and the difficulty of
reduction depends on the location of the cations, in the order of
hexagonal prism > sodalite cage > supercage. The subsequent
+
0
reduction step of Cu to Cu is observed at the higher temper-
ꢀ
atures of 700 C.
ꢀ
The H
2
consumption peak at about 230 C corresponds to
the reduction of CuO on the CuY catalyst proceeds in one step.
0
CuO + H / Cu + H O
(3)
2
2
As mentioned above, the low temperature reduction peaks
ꢀ
ꢀ
below 600 C (LP) are dened peak I at 195 C, peak III at about
ꢀ ꢀ ꢀ
3
80 C, peak IV at 504 C, peak II at about 260 C, which is
2
+
+
assigned to the reduction of Cu to Cu in the supercage,
sodalite cage, hexagonal prism and the reduction of CuO to Cu
0
respectively. The high temperature reduction peaks above
ꢀ
+
0
6
00 C (HP) is attributed to the reduction of Cu to Cu .
For the CuY-1 catalyst activated under nitrogen atmosphere,
ꢀ
ꢀ
there are weak LPs and broad HPs from 600 C to 1000 C, Fig. 3 XPS spectra of Cu2p (a) and Auger spectrum (b) of Cu in CuY
catalysts.
especially, the peak I and peak II are very weak. This shows that
2
+
+
the reduction reaction of Cu is easier to form Cu during
activation of the CuY-1 catalyst under nitrogen atmosphere. In
addition, it is clear that the total hydrogen consumption of the
CuY-1 catalyst is low. Other four catalysts, with the increase the
nitrogen content in activation atmosphere, the strength of the
peak I increases at rst and decreases aerwards. It illustrates
indicates that the bonding type and the relative content of
different copper species of all catalysts might be different. And
the Auger analysis can complement the data obtained from XPS.
33
Alonso et al. reported on Cu Auger spectrum, the kinetic
2
+
energies at 918.2, 916.0 and 917.6 eV are assigned to Cu, Cu O
that the ion-exchange reaction and the auto-reduction of Cu to
2
+
+
and CuO respectively. In addition, the kinetic energy for Cu in
Cu are more likely to occur in relatively higher nitrogen content
3
4
the CuY catalyst is 3 eV below that of Cu O, at 913.0 eV.
According to literature and the H
of activation atmosphere under the premise of no deposited
carbon on the CuY catalyst. Besides the CuY-1 catalyst, the peak
II becomes broad and the hydrogen consumption decreases
gradually. The peak V shis to low temperature from the CuY-3
to CuY-5 catalyst, and the hydrogen consumption is increased
2
27
2
-TPR characterization
(Fig. 2(a)), we determine that metallic copper is absent in the
CuY catalyst. So the two peaks at 913.0 eV and 916.7 eV are
assigned to monovalent copper species and divalent copper
species respectively.
+
gradually, thus the Cu content also increases. So a small
For XPS spectra of Cu2p, it is generally known that the over-
lapping peak from 930.0 eV to 940.0 eV can be resolved into two
amount of oxygen ensures without detrimental deposited
carbon to catalytic activity on the CuY catalyst. And in the
+
2+
2
+
2
p3/2 peaks at 932.4 eV and 934.9 eV attributed to Cu and Cu
activation atmosphere with a small amount of oxygen the Cu
35–38
+
respectively.
By means of XPS-peak-differentiating analysis,
is auto-reduced to Cu to the greatest extent, the content of the
+
the relative content of copper species was determined and the
results were listed in Table 1. As shown in Table 1, the total
active center Cu reach to the maximum.
In order to study the surface composition of the catalysts and
the nature of the surface copper species, XPS characterization
was carried out. And Fig. 3 shows the XPS spectra for Cu2p and
their curve-tting results (a), Cu Auger spectrum (b) of all CuY
catalysts activated under different activation atmosphere. The
XPS peaks were different in shape as well as intensity. This
2
+
+
copper content and relative contents of Cu and Cu vary with
different activation atmosphere. The black CuY-1 catalyst, the
total copper content is only 3.7 wt% due to deposited carbon on
+
the surface. Therefore, the Cu content is also low, only 2.4 wt%.
The CuY-2 catalyst, the total copper content reaches 7.7 wt%, but
+
the Cu content is only 3.1 wt% due to oxidation of oxygen. The
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Table 1 Quantitative analysis of the XPS curve-fitting of CuY catalysts
Peak area
+
2+
+
+
2+
+
Catalyst
Cu peak
Cu peak
Cu /(Cu + Cu ) (%)
Total Cu (wt%)
Cu (wt%)
CuY-1
CuY-2
CuY-3
CuY-4
CuY-5
1582.5
1882.9
2136.4
2509.4
2491.6
921.4
2784.2
2205.9
1925.0
1520.0
63.2
40.3
49.2
56.6
62.1
3.7
7.7
6.4
5.9
5.6
2.4
3.1
3.1
3.4
3.5
total copper content decreases gradually with the increase of the carbonylation of methanol with a feed containing 62.5 vol% CO,
nitrogen content, when the mixture of nitrogen and oxygen used 5.7 vol% O
2
, 31.8 vol% CH
3
OH, and gaseous hourly space
+
ꢁ1
as activation atmosphere. And the Cu content increases from 3.1 velocity (GHSV) 3520 h , 10 h testing time at temperature
ꢀ
to 3.5 wt%. The above analysis results show that nitrogen 140 C and under atmospheric pressure. Space time yield of
3
3
promotes migration of copper species from the surface to bulk, in DMC (STYDMC), conversion of CH OH (XCH OH) and selectivity of
+
addition, facilitates the formation of the Cu active center. The organic products DMC, DMM, DME, MF are shown in Fig. 4. All
results are accord with that of H -TPR.
CuY catalysts prepared by HAI between NH Y zeolite and
2
4
Cu(acac)2 under different activation atmosphere exhibit
different catalytic activity and no deactivation had taken place
Catalytic performance
The effect of activation atmosphere on the catalytic activity is
demonstrated for all CuY catalysts in the direct vapor oxidative
Fig. 5 Catalytic activities of the CuY-5 catalyst with different testing
temperature in the vapor-phase oxidative carbonylation of methanol
Fig. 4 Catalytic activities of CuY catalysts in the vapor-phase oxidative
carbonylation of methanol to DMC at atmospheric pressure. (a) Time-
on-stream performance of all CuY catalysts. (b) Selectivity of organic
to DMC at atmospheric pressure. (a) STYDMC and XCH OH, (b) selectivity
3
of organic products. Reaction time is 10 h and the testing temperature
ꢀ ꢀ ꢀ ꢀ ꢀ
is 140 C, 150 C, 160 C, 170 C, 180 C respectively.
ꢀ
products and XCH OH. Testing time and temperature is 10 h and 140 C
3
respectively.
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in 10 h testing time (Fig. 4(a)). From Fig. 4(a) and (b), it was organic products are showed in Fig. 5(b). STYDMC and XCH OH
3
found that STYDMC and XCH3OH shows the same pattern. When increase followed by a decrease with the increase of the testing
activated under nitrogen atmosphere, the black CuY-1 catalyst temperature. However, the selectivity of DMC monotonically
ꢁ
1
ꢁ1
shows poor catalytic activity with 163.3 mg g
h
STYDMC
,
decreases with the increase of testing temperature and can hold
ꢀ
5
.1% XCH OH and 59.8% DMC selectivity. The catalytic activity of about 50% at the testing temperature 170 C. This is a decrease
3
the light blue CuY-2 catalyst activated in oxygen is better than mirrored by a corresponding increase in DME and MF selec-
that of the CuY-1 catalyst, although the CuY-2 catalyst has tivity. The reason is the thermal decomposition of DMC can
+
+
39
ꢀ
4
0.3% Cu and the CuY-1 catalyst has 63.2% Cu . It is likely to produce DME. Therefore, at 170 C testing temperature, the
+
be because deposited carbon on the CuY-1 covers the Cu active CuY-5 catalyst has the optimal catalytic performance with
centers, even plugs channel. Hence, for the CuY catalyst STYDMC of 525.1 mg g
ꢁ
1
ꢁ1
h , and XCH3OH reaches to 18.9%, far
prepared by HAI between NH Y zeolite and Cu(acac) , activation higher than the methanol conversion (5–12%) for a catalyst with
4
2
atmosphere shows a signicant inuence on the activities of the 17.4 wt% Cu loading at the elevated pressure reported by
2
1
CuY catalyst in DMC synthesis through oxidative carbonylation Richter.
of methanol. For attaining the CuY catalyst with more Cu active
+
The long-term stability and activity of catalysts are vital for
centers and no deposited carbon on the surface of the CuY oxidative carbonylation of methanol to dimethyl carbonate
catalyst, the gas mixture of nitrogen and oxygen was used as from both academic and industrial viewpoints. The effects on
activation atmosphere to prepare catalysts, the nitrogen content catalyst longevity of vapor-phase oxidative carbonylation of
ꢀ
increasing sequence is as follow: CuY-3 < CuY-4 < CuY-5, and methanol to DMC reactions have been investigated at 170 C
the STYDMC and XCH3OH also increase in the same order, and the and the catalytic results of 150 h were presented in Fig. 6. In
selectivity is not changed obviously (see Fig. 4(b)). So the CuY-5 time on stream of 150 h, the selectivities of DMM and DME have
catalyst activated under the mixture of nitrogen with a small been constant. However, during the initial reaction period of
amount of oxygen (only 1.8 vol%) shows the best catalytic 40 h, the selectivity of MF is slowly falling which monitored by
ꢁ1
ꢁ1
activity with STYDMC of 370.7 mg g
DMC selectivity of 63.5%.
h
, XCH OH of 10.5% and an increase of the CH
3
OH selectivity. But CH
3
OH conversion
3
gradually increases. Finally, STYDMC have remained broadly
In order to determine the optimal temperature of oxidative stable. Aer a reaction period of 40 h, the CuY-5 catalytic activity
carbonylation of methanol to DMC, so the next work to study decreased. In the next 20 h, the selectivities of DMC and MF also
the testing temperature on the CuY-5 catalyst with other remains constant, but compared with the initial value, the
ꢀ
ꢀ
conditions xed. The testing temperature was 140 C, 150 C, deactivation degree of XCH3OH and STYDMC is 7.8% and 7.0%
ꢀ ꢀ ꢀ
1
60 C, 170 C, 180 C respectively and testing time was 10 h. respectively. In the last 90 h, the catalytic activity with 480 mg
STYDMC and XCH OH are showed in Fig. 5(a) and the selectivity of
ꢁ1
ꢁ1
g
h
of STYDMC remains basically unchanged.
3
Fig. 6 The long-term catalytic performance of the CuY-5 catalyst. (a) STYDMC, (b) XCH OH, (c) DMC selectivity, (d) DME selectivity, (e) DMM
3
ꢀ
selectivity, (f) MF selectivity. The testing temperature is 170 C.
3298 | RSC Adv., 2020, 10, 3293–3300
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In order to investigate the reason for catalyst deactivation, 4–9 nm are formed on the used CuY-2 compared with 1–5 nm of
TEM characterization was carried out and the TEM images and the fresh catalyst, which may be the main cause of activity
the size distribution of CuO particles for the fresh CuY-5 and the decrease.
used CuY-5 catalyst showed in Fig. 7. The bigger particles about
Conclusions
For the CuY catalysts prepared by HAI between NH
4
Y zeolite
and Cu(acac) , the activation atmosphere of the CuY catalyst
2
show a signicant inuence on the activities of CuY catalysts in
DMC synthesis through oxidative carbonylation of methanol.
When the CuY catalyst is activated under pure nitrogen ow,
there is deposited carbon on the catalyst. And the catalytic
activity is inferior to other as-prepared catalysts because
+
deposited carbon blocks the pores and covers the Cu active
center, even causes the DMC selectivity decrease. Pure oxygen
used as activation atmosphere, there is no deposited carbon on
+
the as-prepared CuY-2 catalyst, but the formation of the Cu is
limited, which is adverse to the catalytic activity. So nitrogen
doped a small amount of oxygen used as activation atmosphere
is the best choice. It not only assures no deposited carbon on
+
the CuY catalyst, but also promotes the formation of the Cu
active center.
Additionally, the testing temperature of oxidative carbonyl-
ation of methanol to DMC is also a key factor that inuence the
catalytic activity of the CuY catalyst. As the testing temperature
rises, STYDMC and XCH3OH increase at rst and decrease aer-
wards, the DMC selectivity decrease gradually due to thermal
ꢀ
decomposition of DMC to DME. At 170 C testing temperature,
the CuY catalyst exhibits the optimal catalytic activity and an
ꢀ
excellent stability. At 170 C the service life of the CuY-5 catalyst
ꢁ1
ꢁ1
is very long and its catalytic activity with 480 mg g
STYDMC remains basically unchanged for 150 h.
h
of
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors are grateful for the nancial support from the
Doctor Research Fund of Yuncheng University [YQ-2019024];
Applied Basic Research Programs of Foundation for Youths of
Shanxi Province of China [201701D221045]; Subject Research
Project of Foundation of Yuncheng University [XK-2019018] and
the open Foundation of Key Lab of Process Analysis and Control
of Sichuan Universities [Yibin University Grant No. 2015003].
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