Effect of NH4+ exchange on CuY catalyst for oxidative carbonylation of methanol

Article abstract of DOI:10.1016/S1872-2067(16)62490-7

NaY and ion exchanged NaNH₄Y zeolite with NH₄NO₃ were used as the support to prepare CuY catalysts by a high temperature anhydrous interaction between the support and copper (II) acetylacetonate Cu(acac)₂. The catalysts were used for the oxidative carbonylation of methanol to dimethyl carbonate (DMC) at atmospheric pressure. The textural and acidic properties of NaNH₄Y zeolite and the CuY catalysts were investigated by X-ray diffraction, scanning electron microscopy, N₂ adsorption-desorption, temperature programmed reduction of H₂, X-ray photoelectron spectroscopy and temperature programmed desorption of NH₃. With increasing NH₄NO₃ concentration, the NH₄⁺ exchange degree increased while the crystallinity of the zeolite remained intact. Crystalline CuO was formed when the NH₄⁺ exchange degree of NaNH₄Y was low, and the corresponding CuY catalyst showed low catalytic activity. With increasing of the NH₄⁺ exchange degree of NaNH₄Y, the content of surface bound Cu⁺ active centers increased and the catalytic activity of the corresponding CuY catalyst also increased. The surface bound Cu⁺ content reached its maximum when the NH₄⁺ exchange degree of NaNH₄Y reached towards saturation. The CuY exhibited optimal catalytic activity with 267.3 mg/(g·h) space time yield of DMC, 6.9% conversion of methanol, 68.5% selectivity of DMC.

Full text of DOI:10.1016/S1872-2067(16)62490-7

Chinese Journal of Catalysis 37 (2016) 1403–1412
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
+
Effect of NH4 exchange on CuY catalyst for oxidative carbonylation
of methanol
Yuchun Wanga,b, Huayan Zhenga, Zhong Lia,*
a Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024,
Shanxi, China
b Department of Applied Chemistry, Yuncheng University, Yuncheng 044000, Shanxi, China
A R T I C L E I N F O
A B S T R A C T
Article history:
NaY and ion exchanged NaNH4Y zeolite with NH4NO3 were used as the support to prepare CuY cata‐
lysts by a high temperature anhydrous interaction between the support and copper (II) acety‐
lacetonate Cu(acac)2. The catalysts were used for the oxidative carbonylation of methanol to dime‐
thyl carbonate (DMC) at atmospheric pressure. The textural and acidic properties of NaNH4Y zeolite
and the CuY catalysts were investigated by X‐ray diffraction, scanning electron microscopy, N2 ad‐
sorption‐desorption, temperature programmed reduction of H2, X‐ray photoelectron spectroscopy
Received 4 May 2016
Accepted 15 June 2016
Published 5 August 2016
Keywords:
High temperature anhydrous
interaction
NH4+ exchange degree
CuY catalyst
+
and temperature programmed desorption of NH3. With increasing NH4NO3 concentration, the NH4
exchange degree increased while the crystallinity of the zeolite remained intact. Crystalline CuO was
+
formed when the NH4 exchange degree of NaNH4Y was low, and the corresponding CuY catalyst
showed low catalytic activity. With increasing of the NH4+ exchange degree of NaNH4Y, the content
of surface bound Cu+ active centers increased and the catalytic activity of the corresponding CuY
Oxidative carbonylation
Dimethyl carbonate
+
catalyst also increased. The surface bound Cu+ content reached its maximum when the NH4 ex‐
change degree of NaNH4Y reached towards saturation. The CuY exhibited optimal catalytic activity
with 267.3 mg/(g·h) space time yield of DMC, 6.9% conversion of methanol, 68.5% selectivity of
DMC.
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
[7–9].
It is well known that the surface‐bound Cu+ on the CuY cat‐
Owing to its negligible toxicity, low bioaccumulation and
high reactivity, dimethyl carbonate (DMC) has attracted much
attention for the so‐called ‘sustainable society’ and ‘green
chemistry’ chemicals [1–4]. Oxidative carbonylation of metha‐
nol is one of the most promising approaches to produce DMC
[5,6]. In the process, supported Cu‐based catalysts exhibit ex‐
cellent catalytic activity and has attracted extensive interest.
Among these catalysts, copper‐exchanged zeolite Y (CuY) is
interesting for the oxidative carbonylation of methanol to DMC
alyst is the active species for the carbonylation reaction
[10–14]. Richter et al. [15] prepared a CuO/NH4Y zeolite by the
deposition‐precipitation method. After calcination at 400 °C in
static air for 2 h, the zeolite has only marginal activity for the
carbonylation reaction. The high activity achieved by the inert
activation procedure at 700–750 °C for 15 h is associated with
the formation of a favorable active site Cu+. Zhang et al. [8]
proved that on the CuY catalyst, methanol was absorbed on
surface bound Cu+ to form Cu–OCH3 and CO inserted into
* Corresponding author. Tel/Fax: +86‐351‐6018526; E‐mail: lizhong@tyut.edu.cn
This work was supported by the National Natural Science Foundation of China (21276169).
DOI: 10.1016/S1872‐2067(16)62490‐7 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 8, August 2016

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Yuchun Wang et al. / Chinese Journal of Catalysis 37 (2016) 1403–1412
Cu–OCH3 bonds resulting in the formation of DMC, and inser‐
tion of CO is the rate limiting step. However, on Cu‐ZSM‐5 and
Cu‐MOR catalysts, because the formation of Cu–OCH3 on sur‐
face bound Cu+ was limited due to the strong adsorption of CO,
dimethoxy methane (DMM) was the primary product.
grammed reduction of H2 (H2‐TPR), X‐ray photoelectron spec‐
troscopy (XPS) and temperature‐programmed desorption of
NH3 (NH3‐TPD). The major objectives were to establish the
+
relationship between the catalytic performance and NH4 ex‐
change degree of NaNH4Y support.
2. Experimental
The traditional way for the surface bound Cu+ active center
to be introduced into Y zeolite is that the H+ ions (Brönsted
acidic sites) of HY are exchanged by Cu+ of CuCl during the
heating of a physical mixture of CuCl and HY. That is, a solid
state ion exchange (SSIE) formed the effective active sites for
oxidative carbonylation [16]. The distribution of Brönsted
acidic sites in Y zeolite affects the location of the Cu active spe‐
cies, and further affects the catalytic performance of oxidative
carbonylation. Huang et al. [17] established a quantitative rela‐
tionship between the amount of Brönsted acid sites and cata‐
lytic activity for oxidative carbonylation over CuY catalysts with
different Si/Al ratios. Zhang et al. [18] also investigated the
activity of CuY with different Si/Al ratios for the oxidative car‐
bonylation of methanol to DMC systematically by a density
functional theory method. They indicated that the CuY zeolite
with a Si/Al ratio = 6.5 had the highest catalytic activity.
2.1. NaNH4Y preparation
10.003 g commercial NaY zeolite (Si/Al = 2.7) was added to
100.00 mL NH4NO3 aqueous solution with the concentration of
0.10, 0.30, 0.50 mol/L, and stirred to ion exchange for 4 h at 30
°C. Then, the suspension was filtered and the resulting cake
was washed with deionized water several times and dried at
100 °C for 24 h. The procedure was repeated once and twice in
the NH4NO3 aqueous solution (0.50 mol/L). The final solid ma‐
+
terial was designated as NaNH4Y‐X zeolite with the NH4 ex‐
change degree of X.
2.2. Catalyst preparation
However, for zeolite Y with different Si/Al ratios, there are
differences in not only the Brönsted acid sites, but also other
factors in the catalytic activity such as the composition of the
framework, bond angle of Si–O–Al and degree of dispersion of
the Al ions [19]. Thus, there is a need for a more valid way to
evaluate the quantitative relationship between the Brönsted
acid sites of the support and catalytic activity of the CuY. Our
preliminary investigations [20,21] showed that Cu(acac)2 ad‐
sorbed and highly dispersed in Y zeolite support due to its sub‐
limation. Cu2+ of Cu(acac)2 ion exchanged with NH4+ of NH4Y at
a lower temperature of 174 °C, then during activation,
Cu(acac)2 is easily decomposed and the decomposition prod‐
ucts are useful for the Cu+ active center, resulting in the for‐
mation of more active centers. The unreacted and adsorbed
Cu(acac)2 on Y zeolite decomposed to form CuO species which
improved the catalytic activity of the Cu+ active center. More
importantly, NaY zeolite does not have a solid state ion change
with copper (II) acetylacetonate Cu(acac)2 during the anhy‐
drous heating treatment of the physical mixture. However, an
ion exchange reaction between NH4Y zeolite and Cu(acac)2
According to the literature [21], the typical preparation of
the CuY catalyst included two stages. In the first stage, 5.001 g Y
zeolite support and 2.454 g Cu(acac)2 were mixed and ground
well with a mortar and pestle to form an intimate gray‐blue
mixture. Then the mixture was heated in the muffle furnace
from the ambient temperature to 250 °C at 3 °C/min, and then
held at 250 °C for 4 h. The mixture turned brown and was
named as precursor. In the following activation stage, the pre‐
cursor material was activated to form the CuY catalyst by heat‐
ing from the ambient temperature to 650 °C at a heating rate of
3 °C/min and at 650 °C for 4 h. The CuY catalysts were desig‐
nated as CuY‐X. The catalyst support was NaNH4Y‐X zeolite
with the NH4+ exchange degree of X. All the prepared catalysts
were stored in a desiccator and pressed to 40–60 mesh parti‐
cles before the catalytic tests.
2.3. Characterization
The sodium content was determined by a VARIAN AA240FS
atomic absorption spectroscopy (AAS). The test solution was
prepared by the following steps. A mixture of 0.202 g fresh
catalyst and 2.012 g KOH was calcined at 500 °C for 6 h. Be‐
cause the excess base destroyed the crystal structure of Y zeo‐
lite, Cu, Si and Al existed in the form of dissolvable compounds
in hydrochloric acid. Then the calcined mixture was digested in
a hydrochloric acid solution and diluted in a 250.00 mL volu‐
metric flask with deionized water. Then 5.00 mL of this solu‐
tion was transferred to a 100.00 mL volumetric flask, diluted
with deionized water to volume (Cu solution concentration 1–5
mg/L for the AAS experiment). Finally, 50 mL of this solution
was used to test the actual Cu content.
+
does occur. Compared with SSIE, NH4 of NaNH4Y plays the
same role as H+ of HY, which provides the exchangeable sites
for ion exchange of copper ions, and forms surface bound‐Cu2+
on the CuY. Moreover, the released NH3 promotes the reduction
of surface bound‐Cu2+ to form the surface bound Cu+ active
center for oxidation carbonylation of methanol to DMC.
In the present study, we studied the relationship between
+
NH4 of the NaNH4Y support and catalytic activity of CuY pre‐
pared by high temperature anhydrous interaction (HAI) be‐
tween Cu(acac)2 and NaNH4Y. First, the CuY catalysts from the
+
NaNH4Y support with different NH4 ion exchange degrees
were prepared by HAI. Then the activities of the CuY catalysts
for oxidative carbonylation of methanol were evaluated. De‐
tailed characterization of the CuY catalysts were also per‐
formed by X‐ray diffraction (XRD), scanning election micros‐
copy (SEM), N2 adsorption‐desorption, temperature‐pro‐
XRD patterns were obtained using Cu Kα radiation (λ =
0.154056 nm) on a Rigaku D/max 2500 diffractometer at 40 kV
target voltage and 100 mA tube current in the 2θ range from 5°
to 65° at a scanning rate of 8°/min.

Yuchun Wang et al. / Chinese Journal of Catalysis 37 (2016) 1403–1412
1405
BET surface areas and pore volume of the CuY catalyst were
determined from N2 physisorption measurements on a Mi‐
cromeritics ASAP 2010 instrument. The samples were first
outgassed under vacuum at 90 °C for 1 h and at 350 °C for 15 h.
The total surface area was obtained by the BET equation using
the relative pressure range of 0.05–0.16 in the linear part of the
nitrogen adsorption isotherm (molecular cross‐sectional area
for N2 of 0.162 nm2). The micropore and mesopore volumes
were calculated by the t‐plot and BJH method, respectively.
Morphology analysis was performed by a S‐4800 field emis‐
sion scanning electron microscope (SEM). The powder samples
were carefully placed on a double‐sided tape with the alumi‐
num stub as the base. The observations were made at different
magnifications.
NH3 temperature programmed decomposition (NH3‐TPDec)
of Y zeolite was performed on a Micromeritics AutochemⅡ
2920 chemical adsorption instrument with a thermal conduc‐
tivity detector (TCD). 0.3003 g sample was loaded in a
U‐shaped quartz tube and heated from 50 to 600 °C at 10
°C/min in He (30 mL/min). Ammonium gas released from the
decomposition was continuously monitored by a TCD. Before
detection, the gas flow was conducted through a KOH trap to
remove desorbed water.
0.6 g (about 1.5 mL) catalyst sample was packed in the tubular
reactor and the reactor was positioned in the reactor furnace.
0.05 mL/min methanol was fed into by a constant flow pump
(2PB‐05) and vaporized in the pre‐heater. In the meantime,
55.0 mL/min CO and 5.5 mL/min O2 were 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 test time was
10 h and testing temperature was 140 °C.
The gas products were analyzed by an online GC with three
valves and four columns every 12 min. The products flowed
through the HP‐PLOT/Q capillary column (30 m × 0.53 μm × 40
μm). The organic products were held back. Then the inorganic
products flowed through the Poropack‐Q packed column: CO2
first flowed out and entered a TCD and was detected; second,
CO and O2 were separated by the HP‐PLOT Molesieve/5A ca‐
pillary column (30 m × 0.53 μm × 25 μm) and detected by TCD.
The products flowed through the HP‐INNOWax (30 m × 0.53
μm × 1 μm) capillary column and into a flame ionization detec‐
tor (FID), thus MeOH, methyl formate (MF), dimethoxymethane
(DMM), dimethyl ether (DME) and DMC were detected.
3. Results and discussion
NH3‐TPD was carried out on a Micromeritics AutochemⅡ
2920 chemical adsorption instrument. 20 mg sample was
loaded in a U‐shaped quartz tube and first degassed at 300 °C
for 0.5 h in He (30 mL/min). After degassing, the temperature
was dropped to 50 °C, and then 15 vol% NH3/He (30 mL/min)
was introduced into the sample cell to allow the sample to ab‐
sorb ammonia. After saturated adsorption, the temperature of
the sample was raised from 50 to 600 °C at 10 °C/min. The
ammonia gas released by desorption was continuously moni‐
tored by a TCD.
3.1. Characterization
In order to determine the NH4+ exchange degree of NaNH4Y
zeolite, the Na+ and NH4+ contents were determined by AAS and
NH3‐TPD, respectively. Fig.
1 presents the profiles of
NH3‐TPDec from NaNH4Y with different NH4+ exchange degree
(X) with temperature rise. There were three peaks over all
samples and they were enhanced gradually with the increase of
+
the NH4 exchange degree. The released NH3 amount was de‐
H2‐TPR was performed on a Micromeritics AutochemⅡ
2920 chemical adsorption instrument. First, 20 mg sample was
loaded in a U‐shaped quartz tube and was degassed at 300 °C
for 60 min in N2 (30 mL/min), then cooled to room tempera‐
ture. The gas was switched to 10 vol% H2 in argon flow, and the
sample was heated to 1000 °C at 10 °C/min. Hydrogen con‐
sumption was monitored by a TCD. The effluent gas was passed
through a liquid nitrogen cold trap to remove water before
detection.
termined by the peak area. Then combined with the content of
the Na+ determined by AAS, the NH4 exchange degree was
+
(6)
(2)
XPS data were collected on an on Scientific ESCALab220i‐XL
electron spectrometer equipped with a 300 W Al Ka X‐ray
source with a Thermo‐VG. The catalyst holder was placed in a
fast entry air load‐lock chamber without exposure to air and
the pressure was maintained at 3×10−7 Pa overnight. Then the
catalyst holder was transferred to the analysis chamber for XPS
study. The binding energy and the Auger kinetic energy scales
were referenced to the C1s line at 284.6 eV from adventitious
carbon.
(1)
0
100
200
300
400
500
600
Temperature C
2.4. Catalytic tests
Fig. 1. Profiles of NH3 released from the decomposition of NaNH4Y with
different NH4+ exchange degree during temperature programmed inert
heating. Heating rate 10 °C/min, sample weight 0.3 g, He 30 mL/min (1)
NaNH4Y‐0; (2) NaNH4Y‐18; (3) NaNH4Y‐46; (4) NaNH4Y‐63; (5)
NaNH4Y‐68; (6) NaNH4Y‐69.
Catalytic measurements were performed on a continuous
fixed bed microreactor (Φ 8 mm × 500 mm) under atmospheric
pressure with an online Agilent 6890 gas chromatograph (GC).

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Yuchun Wang et al. / Chinese Journal of Catalysis 37 (2016) 1403–1412
prepare the CuY catalysts by HAI between NaNH4Y and
Table 1
Na+ content, NH4+ content and NH4+ exchange degree of NaNH4Y.
Cu(acac)2. The XRD patterns of the CuY catalysts are presented
in Fig. 2 (b). For all the CuY catalysts, the characteristic diffrac‐
tion peaks of Y zeolite were observed. This indicated that the
structure of the Y zeolite remained after the ion exchange of
NH4NO3 and succeeding heat treatment. It is worth noting that
the CuY‐68 catalyst exhibited almost no crystalline copper ox‐
ide, but other catalysts contain crystalline CuO. CuO (tenorite,
synthetic, PDF‐Nr. 48‐1548) has intense diffraction peaks (rela‐
tive intensities in parentheses) at 2θ = 35.418° (37), 35.544°
(100), 38.709° (99), 38.903° (21) and 48.717° (30). The two
most intense diffraction peaks at 2θ = 35.544° and 38.709° can
clearly be distinguished. There was no indication for the pres‐
ence of crystalline Cu2O or metallic Cu. With the increase of the
+
NH4NO3
Ion
Na+
NH4
Exchange
degree (%)
Sample
concentra‐ exchange contenta contentb
tion (mol/L) times (mmol/g) (mmol/g)
NaNH4Y‐0
0
1
1
1
1
2
3
3.80
3.12
2.03
1.41
1.27
1.26
0
0
NaNH4Y‐18
NaNH4Y‐46
NaNH4Y‐63
NaNH4Y‐68
NaNH4Y‐69
0. 1
0.3
0.5
0.5
0.5
0.70
1.76
2.42
2.72
2.77
18.3
46.4
63.2
68.1
68.7
a From AAS; b From NH3‐TPDec.
+
calculated, as listed in Table 1. The initial increase of the NH4
exchange degree was very obvious. Then the increments were
small with further increase of the NH4NO3 solution concentra‐
tion. When the concentration of the NH4NO3 solution was 0.5
mol/L and the NaY zeolite ion exchanged twice, the NH4 ex‐
change degree reached 68.1%, 7.7% higher than that of NaY ion
+
NH4 exchange degree, the XRD peak intensities of crystalline
CuO decrease gradually. The results may indicate that the con‐
tent of crystalline CuO decreased or the dispersion of CuO spe‐
cies became better.
+
The textural properties and porous structure of NaNH4Y
+
exchanged once. The NH4 exchange degree of NaY ion ex‐
+
samples with different NH4 exchange degrees and the CuY
changed three times only increased 0.9% more. So when NaY
zeolite was ion exchanged with 0.5 mol/L NH4NO3 solution
twice, the NH4+ exchange degree was beginning to stabilize and
reached the maximum.
catalysts were characterized by N2 adsorption and the results
were shown in Table 2. The specific surface area of the Y zeolite
decreased slightly from 797.0 to 775.5 m2/g with increase of
the NH4+ exchange degree. This was because the pore structure
of the Y zeolite remained intact, which was proved by XRD.
After loading of Cu, the surface area of the CuY catalyst was less
than that of the support. When the NH4+ exchange degree was
zero, the specific surface area of the CuY‐0 catalyst dropped to
647.9 from 797.0 m2/g for the support and the pore volume
decreased from 0.35 to 0.27 cm3/g. Combined with the H2‐TPR
and SEM results, this was because the pores of the Y zeolite
Fig. 2(a) shows the XRD patterns of the NaNH4Y samples
+
with different NH4 exchange degrees. Compared with
NaNH4Y‐0 zeolite, the diffraction peak intensity of the other
NaNH4Y zeolites decreased slightly. Sato et al. [22] found that
ion exchange of NaY with (NH4)2SO4 resulted in structural
changes and the relative crystallinity of the NaY decreased by
approximately 20% after the first ion exchange. In the second
and third ion exchanges, the structural changes depended on
the Si/Al ratio of the starting NaY zeolite. Y zeolite with a me‐
dium Si/Al ratio of 2.8 retained their high crystallinity. Com‐
pared with the results reported by Sato et al. [22], the reduction
of the crystallinity of Y zeolite was less in this study. This may
be due to the medium Si/Al ratio of 2.7 and the low concentra‐
tion of the NH4NO3 solution. These NaNH4Y samples with dif‐
+
have been partly blocked by CuO. With increase of the NH4
exchange degree, the CuO amount decreased, and there was a
reduced specific surface area. That eventually resulted in the
increase of the specific surface area of the CuY catalyst with
increase of the NH4+ exchange degree.
Fig. 3 shows the SEM images of the CuY catalysts. For all the
CuY catalysts, the octahedron morphology with smooth crystal
faces and sharp edges was clearly visible, and CuO species were
+
ferent NH4 exchange degrees were used as the support to
(b)
(a)
CuO
CuY-68
CuY-63
CuY-46
CuY-18
CuY-0
NaNH4Y-68
NaNH4Y-63
NaNH4Y-46
NaNH4Y-18
NaNH4Y-0
5
10 15 20 25 30 35 40 45 50 55 60 65
5
10 15 20 25 30 35 40 45 50 55 60 65
2 /(o)
2 /(o)
Fig. 2. XRD patterns of NaNH4Y with different NH4+ exchange degrees (a) and the prepared CuY catalysts (b).

Yuchun Wang et al. / Chinese Journal of Catalysis 37 (2016) 1403–1412
1407
Table 2
Specific surface area and pore volume of catalysts with different Cu
loadings.
III
I
IV
CuY-68
CuY-63
II
550
a
b
b
c
d
SBET
Smicro
Smeso
Vmicro
Vmeso
Sample
(m2/g)
797.0
795.4
785.1
784.2
775.5
647.9
653.8
671.9
674.6
720.5
(m2/g)
726.8
737.8
739.2
744.7
732.1
605.0
608.3
629.1
638.0
677.4
(m2/g)
70.2
57.5
45.9
39.5
43.4
42.9
45.5
42.8
36.6
43.1
(cm3/g)
0.35
0.34
0.33
0.33
0.33
0.27
0.27
0.28
0.30
0.30
(cm3/g)
0.17
0.13
0.10
0.10
0.53
0.09
0.10
0.12
0.13
0.32
NH4Y‐0
NH4Y‐18
NH4Y‐46
NH4Y‐63
NH4Y‐68
CuY‐0
CuY‐18
CuY‐46
CuY‐63
CuY-46
CuY-18
CuY‐68
a
BET surface area calculated using the relative pressure range of
0.05‐0.16.
b The specific surface area of micropore and mesoporous calculated by
t‐plot.
c The pore of micropore calculated by HK.
d The pore of mesoporous calculated by BJH.
CuY-0
200 300 400 500 600 700 800 900 1000
Temperature (C)
Fig. 4. H2‐TPR of CuY catalysts.
observed. With increase of the NH4+ exchange degree, the CuO
content decreased. This showed that for all the CuY catalysts,
the replacement of Na+ by NH4+ and the supported copper spe‐
cies did not affect the morphology of the Y zeolite.
In order to investigate the influence of the NH4+ exchanged
degree on the distribution of the reducible copper species on
the CuY catalysts, H2‐TPR was performed. The results are
shown in Fig. 4. The hydrogen consumption peaks from low
temperature to high were labeled as peak I, peak II, peak III,
and peak IV.
2Cu+ + H2 → 2Cu0 + 2H+
(2)
The reduction of Cu2+ to Cu+ is characterized by a H2 ‐TPR
peak below 500 °C. The difficulty of reduction depends on the
location of Cu2+, in the order of hexagonal prism > sodalite cage
> supercage. The second reduction step of Cu+ to Cu0 occurs
above 700 °C
Reduction of CuO on the CuY catalyst proceeded in one step
at 230 °C.
The following facts from the literatures [15,23–25] help to
CuO+ H2 → Cu0 + H2O
(3)
understand the H2‐TPR:
The reduction of ion exchanged Cu2+ on zeolite occurs via a
two‐step mechanism.
So peak I, peak III, and peak II were ascribed to the reduc‐
tion of Cu2+→Cu+ in the supercage and sodalite cage, and
CuO→Cu0, respectively. Peak IV was assigned to the reduction
of Cu+→Cu0. For the CuY‐0 catalyst, there was only peak II of
2Cu2+ + H2 → 2Cu+ + 2H+
(1)
(a)
）
(c)
(b)
(d)
(e)
Fig. 3. SEM images of CuY catalysts. (a) CuY‐0; (b) CuY‐18; (c) CuY‐46; (d) CuY‐63; (e) CuY‐68.

1408
Yuchun Wang et al. / Chinese Journal of Catalysis 37 (2016) 1403–1412
CuO→Cu0. This indicated the absence of other reducible copper
species besides CuO. With increasing the NH4 exchange de‐
Table 3
+
Quantitative analysis of the XPS curve fitting of the CuY catalysts.
gree, peak I, peak III and peak IV increased and peak II de‐
creased gradually. Meanwhile, the reduction temperature of
peak IV moved towards low temperature. All these observa‐
tions suggested that with the increasing NH4+ exchange degree,
the Cu2+ and Cu+ contents increased and the CuO content de‐
creased, which was in agreement with the XRD results. The
Cu2+ and Cu+ contents of CuY‐68 catalyst were much higher
than these of the CuY‐63 catalyst, and the Cu+ reduction tem‐
perature of the CuY‐68 catalyst was also lower than other cata‐
lysts. Moreover, the catalytic activity of the CuY‐68 catalyst was
superior to the other catalysts. This indicated that the Cu+ ac‐
tive center for the oxidative carbonylation of methanol to DMC
was formed by HAI between NaNH4Y and Cu(acac)2, and the
Cu+/Cusum
(%)
Cu+ loading
(wt%)
Peak area
Cu+ peak
Catalyst
Cu2+ peak
5907.8
3229.2
5469.5
5654.4
3545.3
CuY‐0
0
0
0
CuY‐18
CuY‐46
CuY‐63
CuY‐68
2238.0
4211.1
4823.0
6309.9
40.9
43.5
46.0
64.0
4.9
5.2
5.5
7.7
spectra was calculated, i. e., the relative amounts of copper spe‐
cies with different valence states, which are listed in Table 3.
With the increasing of the NH4 exchange degree of NaNH4Y,
the monovalent copper content increased and divalent copper
content decreased gradually.
+
+
more the NH4 exchange degree, the more Cu+ active centers
To investigate the surface acidity of the CuY catalysts from
the NaNH4Y support with the different NH4+ exchange degrees,
NH3‐TPD was performed. The results are presented in Fig. 6. It
was only below 250 °C that a NH3 desorption peak (α) was
observed over NaNH4Y‐0, which was attributed to the physi‐
cally adsorbed and hydrogen‐bound NH3 of Y zeolite [32,33].
There was no desorption of NH3 at high temperature. The
CuY‐0 catalyst from the NaNH4Y‐0 support has a similar
NH3‐TPD profile to the NaNH4Y‐0 zeolite. From the H2‐TPR
result (Fig. 4) for the CuY‐0 catalyst, there existed only CuO
species, and no other copper species. So these provide evidence
that CuO species adsorb NH3 molecule very weakly and the NH3
adsorption are generally negligible. Besides, for the CuY cata‐
were formed and increased the activity of the CuY catalyst.
In order to analyze the copper species distribution on the
CuY catalyst surface, XPS was performed. The XPS spectra of Cu
2p and Auger spectra are presented in Fig. 5. Divalent copper
can be confirmed by the Cu 2p3/2 binding energy in the range of
933.0–936.0 eV and the shake‐up satellite peak. Monovalent
copper can be assigned to the binding energy of the XPS peaks
ranging from 932.0 to 933.0 eV without any shake‐up satellite
peaks [26–29]. Usually, the two peaks from divalent and mon‐
ovalent copper overlapped. From the different peak shape and
relative intensity, the distribution of copper species in the CuY
catalysts were different. From the Auger analysis of the Cu spe‐
cies, the kinetic energies for Cu, Cu2O and CuO are 918.2, 916.0
and 917.6 eV, respectively [30]. In addition, the kinetic energy
for Cu+ in the CuY catalyst was 3 eV below that of Cu2O, at 913.0
eV [31]. Under the HAI preparation conditions, metallic copper
was not possible in the CuY catalyst. Combined with the H2‐TPR
results, the CuY‐0 catalyst only contained CuO species, so the
two peaks at 912.6 and 916.7 eV in the Auger spectra were
assigned to monovalent copper species and divalent copper
species, respectively. To determine exactly the content of the
copper species on the surface of CuY catalyst, the relative pro‐
portion of the peak area of the curve fitting of the Cu 2p3/2
+
lysts from the NaNH4Y support with different NH4 ion ex‐
change degree, the NH3 desorption peak below 200 °C was
splitted into two overlapped peaks and two NH3 desorption
peaks (β and γ) above 200 °C were observed, which was caused
by the introduction of the surface‐bound copper ions, as al‐
ready proposed [25]. That is, the introduction of copper ion
into NaY not only creates a more strongly bonded NH3 adsorp‐
tion center but also increased the amount of weakly bonded
NH3 centers. In order to quantify the NH3 desorption capacity
of the CuY catalysts, all the NH3‐TPD curves were processed by
932.9
(a)
912.6
(b)
CuY-68
CuY-63
916.7
935.0
CuY-68
CuY-63
CuY-46
CuY-18
CuY-0
CuY-46
CuY-18
CuY-0
930
940
950
Binding energy (eV)
Fig. 5. Cu 2p XPS spectra (a) and Cu Auger spectrum (b) of CuY catalysts.
960
970
905
910
915
920
925
930
Kinetic energy (eV)

Yuchun Wang et al. / Chinese Journal of Catalysis 37 (2016) 1403–1412
1409
Table 4


Quantitative analysis of the NH3‐TPD profiles of NaNH4Y‐0 and CuY
catalysts.

CuY-69
Cu+ + Cu2+
loading (wt%)
Desorption capacity of NH3 (mmol/g)
Catalyst
α
3.02
β
0
γ
0
Amount
3.02
CuY-68
CuY-63
CuY-46
CuY-18
NaNH4Y‐0
CuY‐0
—
—
3.12
0
0
3.12
CuY‐18
CuY‐46
CuY‐63
CuY‐68
CuY‐69
1.14 1.45
0.68 2.56
0.42 3.45
0.37 4.29
0.36 4.31
0.89
1.35
1.49
1.83
1.84
0.71
1.15
1.24
1.48
1.50
4.19
5.74
6.60
4.4
6.8
7.4
9.0
9.1
7.97
8.01
CuY-0
α: < 200 °C; β: 200–300 °C; γ: 300–400 °C.
NaNH4Y-0
no the surface‐bound Cu+ active center instead of CuO. When
the NH4 exchange degree was 18.3%, the copper ion loading
100
200
300
400
500
600
Temperature (C)
+
was 4.4 wt%. With the increasing of the NH4+ exchange degree
of NaNH4Y zeolite, the copper ion loading on the CuY catalyst
also increased, and reached 9.0 wt%, when the NH4+ exchange
degree was 68.1%. It was conducive to forming Cu+ active cen‐
ters and also will increase the activity of CuY.
Fig. 6. NH3‐TPD profiles of NaNH4Y‐0 and CuY catalysts.
means of a peak‐differentiating analysis. The results of the
quantitative analysis are listed in Table 4. It can be seen that
the NH3 desorption capacity of the CuY catalyst increased with
the increasing of the NH4+ exchange degree of NaNH4Y zeolite.
Therefore, there was a positive correlation between the
3.2. Activity testing
+
amount of copper ions on the CuY catalyst and the NH4 ex‐
Experiments were undertaken to determine the influence of
the NH4+ exchange degree on the activity for the oxidative car‐
bonylation of methanol to DMC. The results are showed in Fig.
7. In Fig. 7(a), a plot of the DMC synthesis is given in terms of
space time yield (STYDMC) as a function of time on stream. The
STYDMC on all the catalysts increased with time on stream, be‐
fore steadying off within 60 min, then kept constant for 600
min of time on stream. For the CuY‐0 catalyst, there existed
only CuO species, not the surface‐bound Cu+ active center, so it
has no catalytic activity for the oxidative carbonylation of
methanol to DMC. Under the same testing reactions, the activi‐
ties of all the other catalysts from NaNH4Y support exhibited
significant increase. The average value of STYDMC, conversion of
change degree of NaNH4Y zeolite. Kieger et al. [25] reported
that the peak at 327 °C on the NH3‐TPD is attributed to NH3
desorbing from Cu2+ ions on the Cu2+Y. However, after NH3
treatment at 500 °C and desorption in He at 500 °C, most of the
Cu2+ ions on the CuY have been reduced to Cu+ [34]. A partial
reduction of Cu2+ to Cu+ results in the the NH3 desorption peak
above 227 °C shifting toward a low temperature and forming a
broad peak from 77 to 327 °C. So the β and γ peak were at‐
tributed to surface‐bound Cu+ and Cu2+. It was reported that
three NH3 ligands were bound to Cu2+ ions on the CuY catalyst
and only two NH3 ligands were bound to Cu+ ions on the CuY
catalyst [25]. The ion exchange copper ion loading was ob‐
tained and listed in Table 4. For the CuY‐0 catalyst, there was
300
(a)
(b)
14
12
280
250
240
200
160
STYDMC
SDMC
XCH3OH
200
150
100
50
10
8
6
120
80
40
0
CuY-0
CuY-46
CuY-68
CuY-18
CuY-63
CuY-69
4
2
0
0
0
100
200
300
400
500
600
0
10
20
30
40
50
60
70
+
The NH4 exchange degree (%)
Time on stream (min)
Fig. 7. Catalytic activity of CuY catalysts in the vapor phase oxidative carbonylation of methanol to DMC. (a) Time‐on‐stream performance of CuY
catalysts; (b) STYDMC, SDMC and XCH3OH. Testing conditions: feed composition 31.8 vol% CH3OH, 62.5 vol% CO, 5.7 vol% O2, GHSV = 3250 h–1, 10 h, 140
°C, atmospheric pressure.

1410
Yuchun Wang et al. / Chinese Journal of Catalysis 37 (2016) 1403–1412
CH3OH (XCH3OH) and selectivity of DMC (SDMC) are presented Fig.
7 (b). It can be seen that with the increase of the NH4+ exchange
degree, STYDMC and XCH3OH increased and SDMC decreased slight‐
slowly decreased. At the time on stream of 60 h, the deactiva‐
tion degrees for CuY‐18, CuY‐63 and CuY‐68 catalysts were
19.6%, 32.6%, and 33.3%, respectively.
ly. When the NH4+ exchange degree was 18.3%, STYDMC, XCH3OH
,
and SDMC were 49.1 mg/(g·h), 1.2%, and 73.6%, respectively.
When the NH4+ exchange degree reached 63.2%, the STYDMC on
the CuY‐63 catalyst increased to 184.5 mg/(g·h). When the
NH4+ exchange degree reached 68.1%, it was surprised that the
corresponding CuY‐68 catalyst has higher activity than the
CuY‐63 catalyst. Its STYDMC increased about 45% and reached
267.3 mg/(g·h), but SDMC remained unchanged. The catalytic
activity of the CuY‐69 catalyst from the NaNH4Y‐69 support
was a little higher than that of the CuY‐68 catalyst. The experi‐
4. Conclusions
+
CuY catalysts from a NaNH4Y support with different NH4
exchange degrees were prepared by high temperature anhy‐
drous interaction and tested for the oxidative carbonylation of
methanol. The NH4+ of NH4Y was needed to form the Cu+ active
center and the NH4+ exchange degree of NaNH4Y had a signifi‐
cant influence on the activities of the CuY catalysts. With the
+
increase of the NH4 exchange degree of NaNH4Y, the surface
+
mental results can be explained by the following. First, the NH4
exchange degree of 68% has reached the limit. In addition,
bound‐Cu2+ and surface bound Cu+ content of the CuY catalyst
increased and the CuO species decreased. The NH4+ of NaNH4Y
made copper ion exchange occur more easily and was promot‐
ed to form the Cu+ active centers, which increased the activity
of the CuY catalyst for the oxidative carbonylation of methanol
to DMC. The surface bound Cu+ content reached the maximum
+
+
when the NH4 exchange degree was 63.2%, NH4 was com‐
pletely ion exchanged by Cu2+ of Cu(acac)2, and the ion ex‐
changed copper ions have reached the maximum. When the
+
NH4 exchange degree continued to increase from 63.2% to
68.7%, ion exchange between NH4 and Cu2+ does not occur
anymore, resulting in the rest of the NH4 remaining on the
+
+
when the NH4 has almost completely exchanged the Na+ of
+
NaY zeolite, and this CuY exhibited the optimal catalytic activi‐
ty.
catalyst precursor. It has been reported that the released NH3
from NH4Y decomposition facilitates the reduction of copper
species during high temperature treatment [35] and the for‐
mation of more Cu+ active centers [23].
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Yuchun Wang et al. / Chinese Journal of Catalysis 37 (2016) 1403–1412
1411
Graphical Abstract
Chin. J. Catal., 2016, 37: 1403–1412 doi: 10.1016/S1872‐2067(16)62490‐7
Effect of NH4+ exchange on CuY catalyst for oxidative carbonylation of methanol
Yuchun Wang, Huayan Zheng, Zhong Li*
Taiyuan University of Technology; Yuncheng University
280
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With different NH4
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CuY-46
CuY-68
CuY-18
CuY-63
CuY-69
high temperature
anhydrous interaction
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of methanol to DMC
40
NaNH4Y
Cu(acac)2
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