Investigation of the interaction between Cu(acac)2 and NH4Y in the preparation of chlorine-free CuY catalysts for the oxidative carbonylation of methanol to a fuel additive

Article abstract of DOI:10.1039/c5ra19941g

The high temperature anhydrous interaction between copper(ii) acetylacetonate Cu(acac)₂ and NH₄Y was investigated to prepare a chlorine-free CuY catalyst for the oxidative carbonylation of methanol to dimethyl carbonate. When a physical mixture of Cu(acac)₂ and NH₄Y is heated from ambient temperature to 230 °C, Cu(acac)₂ firstly sublimates and then is adsorbed immediately onto the surface of the Y zeolite. Simultaneously the ion exchange between Cu(acac)₂ and NH₄Y occurs at about 174 °C. During the activation process from 230 to 500 °C, the exchanged Cu²⁺ is reduced to a Cu⁺ active center, and the adsorbed and unreacted Cu(acac)₂ on the NH₄Y surface decomposes to nano-CuO. For NaY zeolite, no solid state ion-exchange occurs between Cu(acac)₂ and NaY during the heat treatment and only CuO exists on the Cu/NaY catalyst surface. While for HY zeolite, there is less ion-exchanged Cu⁺ in the supercages. The Cu/NaY catalyst has no catalytic activity and the Cu/HY catalyst exhibits lower activity than the Cu/NH₄Y catalyst. Strong evidence is provided that during heat treatment, a solid state ion-exchange between Cu(acac)₂ and NH₄Y occurs and makes more of the Cu⁺ located in the supercages accessible to reactants.

Full text of DOI:10.1039/c5ra19941g

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Investigation of the interaction between Cu(acac)2
and NH4Y in the preparation of chlorine-free CuY
catalysts for the oxidative carbonylation of
methanol to a fuel additive
Cite this: RSC Adv., 2015, 5, 102323
a
a
Yuchun Wang,ab Huayan Zheng, Zhong Li and Kechang Xiea
*
The high temperature anhydrous interaction between copper(II) acetylacetonate Cu(acac)2 and NH4Y was
investigated to prepare a chlorine-free CuY catalyst for the oxidative carbonylation of methanol to dimethyl
carbonate. When a physical mixture of Cu(acac)2 and NH4Y is heated from ambient temperature to 230 ꢀC,
Cu(acac)2 firstly sublimates and then is adsorbed immediately onto the surface of the Y zeolite.
Simultaneously the ion exchange between Cu(acac)2 and NH4Y occurs at about 174 ꢀC. During the
activation process from 230 to 500 ꢀC, the exchanged Cu2+ is reduced to a Cu+ active center, and the
adsorbed and unreacted Cu(acac)2 on the NH4Y surface decomposes to nano-CuO. For NaY zeolite, no
solid state ion-exchange occurs between Cu(acac)2 and NaY during the heat treatment and only CuO
exists on the Cu/NaY catalyst surface. While for HY zeolite, there is less ion-exchanged Cu+ in the
supercages. The Cu/NaY catalyst has no catalytic activity and the Cu/HY catalyst exhibits lower activity
than the Cu/NH4Y catalyst. Strong evidence is provided that during heat treatment, a solid state ion-
exchange between Cu(acac)2 and NH4Y occurs and makes more of the Cu+ located in the supercages
accessible to reactants.
Received 27th September 2015
Accepted 12th November 2015
DOI: 10.1039/c5ra19941g
www.rsc.org/advances
catalysts were considered to reduce the inuence of chlorine
and have been widely explored.10
Introduction
King11,12 reported that when heating a physical mixture of
In the last few decades, nontoxic and easily biodegraded
dimethyl carbonate (DMC) has been found to have huge
potential applications. Compared with the conventional octane
enhancing additive methyl tert-butyl ether (MTBE), DMC is
a more suitable component of reformulated fuels owing to its
good blending properties, high oxygen content and a signicant
reduction in pollutants in exhaust gases.1–4 DMC is also more
valuable as a green organic intermediate to substitute the toxic
phosgene dimethyl sulfate.5,6 Of course, many innovative strat-
egies for the synthesis of DMC have been developed.7,8 Among
them, the oxidative carbonylation of methanol was considered
as an environmentally friendly synthesis of DMC and has been
widely investigated.8,9 As early as the 1980s, the Enichem
Company (Italy) had demonstrated the oxidative carbonylation
of methanol to DMC using cuprous chloride CuCl catalyst.7 But
during the catalytic process, the catalyst suffers from loss of
chlorine to form HCl, which results in catalytic deactivation and
the equipment seriously corrodes. Then, supported Cu-based
CuCl and HY zeolite at high temperature, the solid state ion
exchange (SSIE) of Cu+ of CuCl with H+ of HY occurred, and the
formed Cu+Y catalyst catalyzed the oxidative carbonylation of
methanol to DMC with a higher than 5% conversion of meth-
anol and 80% DMC selectivity. It means that without chlorine,
Cu+ also has catalytic activity. Aer that, many solid acids, such
as MCM-41,13,14 Hb zeolite,15 HZSM-5 zeolite,16 SiO2–Al2O3,17
SiO2–ZrO2,18 and SiO2–TiO2,19 were employed to prepare Cu+
supported catalysts through SSIE. Among them, HY is of great
interest to many researchers due to its special supercage
microstructure and large surface area.20–25 However, a little
chlorine inevitably remains and then forms HCl which corrodes
the equipment.
To avoid the negative effects of chlorine, intensive efforts
have been made to prepare a chlorine-free CuY catalyst using
non-chloric copper salts, such as Cu2O,12 Cu(NO3)2 (ref. 26–31)
and Cu(CH3COO)2.32 A CuY catalyst prepared through heating
ꢀ
the powder mixture of Cu2O and HY zeolite at 650 C for 70 h
with a helium purge was active for producing DMC, but its
activity was very low and quickly decreased.12 In our previous
work,28–30 it was shown that when a Cu2+Y zeolite material
prepared through liquid solution ion exchange (LSIE) of NaY
zeolite with Cu(NO3)2 and cuprammonia was heated at high
aKey Laboratory of Coal Science and Technology of Ministry of Education and Shanxi
Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China. E-mail:
lizhong@tyut.edu.cn; Fax: +86 351 6018526; Tel: +86 351 6018526
bDepartment of Applied Chemistry, Yuncheng University, Yuncheng 044000, Shanxi,
China
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temperature, Cu2+ in the supercages was auto-reduced to active Sinopharm Chemical Reagent Co. Ltd), CH3OH (AR, Tianjin
Cu+ and the activated materials showed superior activity. Kemiou Chemical Reagent Company) as well as carbon
Furthermore, during high temperature activation of the monoxide (99.99%, Beijing Haipu Beifen Gas Industry Co.,
Cu(NO3)2/Y zeolite prepared through incipient wetness LTD), oxygen, nitrogen and hydrogen (99.99%, TISCO) were
impregnation (IWIM), HY undergoes easier ion-exchange with used.
Cu2+ than NaY and the former exhibited a better catalytic
activity. Richter et al.32 also found that for heat treatment above
700 C of the CuY zeolite prepared through deposition–precip-
Catalyst preparation
ꢀ
itation (DP) of Cu(CH3COO)2 on NH4Y zeolite in aqueous Firstly, 30.0 g of NaY zeolite was added to 300 mL of a 0.5 mol
solution, ion exchange does occur to form Cu2+ on the surface of Lꢁ1 NH4NO3 aqueous solution and stirred to ion exchange for 4
Y zeolite and the released NH3 is favorable to reduce Cu2+ to h at 30 ꢀC, and dried at 100 ꢀC. Then the procedures were
Cu+.
Recently, an organometallic compound Cu(acac)2 has
repeated once and the nal solid material was NH4Y zeolite.
Then, some of the prepared NH4Y zeolite was packed in
demonstrated a unique charm in the eld of copper material a crucible and then put into a muffle furnace in order to calci-
preparation due to its properties of sublimation and decom- nate NH4Y zeolite to HY zeolite. The furnace temperature was
position at low temperatures.33–36 Goel et al.33 reported that, rstly raised from the ambient temperature to 200 C at 3 C
through chemical vapor deposition of a Cu(acac)2 precursor, minꢁ1, then raised from 200 ꢀC to 500 ꢀC at 1 ꢀC minꢁ1 and held
Cu2O and CuO were generated at temperatures of 195 ꢀC to 430 constant at 500 ꢀC for 6 h. Finally, the cooled solid material was
ꢀC respectively, but at 285 ꢀC, both Cu2O and CuO coexisted. HY zeolite.
ꢀ
ꢀ
Nasibulin et al.35 found that when the oxygen concentration of
The Cu/NH4Y catalyst was prepared through heat treatment
the oxygen/nitrogen carrier gas was 0.5%, through Cu(acac)2 of a mixture of Cu(acac)2 and NH4Y according to the following
vapor decomposition, nanoparticles of Cu2O and CuO were two stages. In the rst stage, 2.045 g of Cu(acac)2 and 5.000 g of
prepared at 432 ꢀC and 705 ꢀC respectively; when the oxygen NH4Y were mixed and ground well with a mortar and pestle to
ꢀ
concentration was above 10.0%, above 596 C only CuO nano- form an intimate gray-blue mixture. Then the mixture was
particles were generated; when the oxygen concentration was heated in a muffle furnace from the ambient temperature to 180
ꢁ1
ꢀ
ꢀ
below 0.0003%, only Cu nanoparticles were generated. What’s ꢀC at 3 C min , and then held at 180 C for 4 h to disperse
more, oxygen plays
impurities.
a
very important role in removing Cu(acac)2 very well on the surface of the NH4Y zeolite to form
the precursor. In the second stage, the precursor material was
Therefore, similar to the process of SSIE between CuCl and activated to form the Cu/NH4Y catalyst through heating at
ꢀ
NH4Y zeolite, it is also expected that when a physical mixture of temperatures of 300, 400, or 500 C for 4 h from the ambient
NH4Y zeolite and Cu(acac)2 is heated in a muffle furnace, temperature at a heating rate of 3 C minꢁ1. The activated Cu/
ꢀ
Cu(acac)2 would sublimate and be adsorbed onto the NH4Y NH4Y catalysts were denoted as Cu/NH4Y-300, Cu/NH4Y-400
surface several times and become dispersed uniformly, then the and Cu/NH4Y-500 respectively.
NH4Y zeolite would decompose to HY zeolite and Cu(acac)2
According to the above procedure, NaY and HY were also
would decompose to Cu2O, and simultaneously SSIE between used to substitute NH4Y to prepare Cu/NaY-400 and Cu/HY-400
HY zeolite and Cu2O would form the Cu+Y catalyst. Decompo- catalysts respectively. The activation temperature was 400 ꢀC.
sition gases such as CO, NH3, N2, CO2 form the reducing All catalysts were stored in a dryer, and pressed to 40–60 mesh
atmosphere in the heating process, which is favorable for particles before catalytic tests.
retaining Cu+. To the best of our knowledge, this novel process
for the preparation of a chlorine-free Cu+ catalyst has not been
reported previously.
Characterization techniques
In our recent work,37 chlorine-free CuY catalysts were Thermogravimetric analyses (TG/DSC) were performed on
successfully prepared through high temperature anhydrous a SETSYS Evo 16/18 thermogravimetric analyzer (Setaram)
interaction (HAI) between Cu(acac)2 and NH4Y and were used in under an air ow ꢁr1ate of 100 mL minꢁ1, employing a heating
the oxidation carbonylation of methanol. It was found that the rate of 10 ꢀC min up to a nal temperature of 800 ꢀC. The
catalytic activity depends highly on the heating temperature. In evolved gas analysis of the decomposition reaction was moni-
this study, the interaction process of heating the mixture of tored with an omnistar quadrupole Pierre mass spectrometer
Cu(acac)2 and NH4Y under anhydrous conditions and the (MS) linked to the thermobalance. The MS operated with an
formation of the Cu catalytic active centres were investigated in ionization energy of 40 eV, using a Channeltron detector (1000
more detail using TG/DSC, XRD, H2-TPR, XPS, TEM, etc.
V).
Temperature-programmed reduction (H2-TPR) was per-
formed on Micromeritics Autochem II 2920 chemical
a
Experimental section
Materials
adsorption instrument. 20 mg of sample was loaꢁd1ed in a U-
ꢀ
ꢀ
shaped quartz tube, and then heated at 10 C min to 300 C
NaY zeolite with a Si/Al molar ratio of 2.7 was purchased from with 30 mL minꢁ1 argon for 30 min to remove adsorbed mois-
the Nankai Catalyst Factory. Reagents such as NH4NO3 (AR, ture. Aer cooling to room temperature, the gas was switched to
Tianjin Chemical Reagent no. 3 Factory), Cu(acac)2 (AR, 10 vol% H2 in an argon ow, and the sample was heated to 1000
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ꢀC. The hydrogen consumption was monitored through thermal methyl formate (MF), dimethoxymethane (DMM), dimethyl
conductivity.
ether (DME) and DMC were detected.
X-ray diffraction (XRD) data were collected by using CuKa
radiation (l ¼ 0.154056 nm) on a Rigaku D/max 2500 diffrac-
tometer at a 40 kV target voltage and 100 mA tube curꢁre1nt in the
Results and discussion
Thermal analysis
2q range from 5ꢀ to 80ꢀ at a scanning rate of 8ꢀ min
.
Transmission electron microscopy (TEM) images were It is well known that when NH4Y zeolite is heated at 400 ꢀC, NH3
recorded using a JEOL JEM-2100F electron microscope oper- is dissociated, and the NH4Y zeolite converts to HY zeolite.38
ating at 200 kV. Exposition times were conned to about 2 s due
Then the dehydroxylation of silicol on the surface of the HY
to the beam sensitivity of the samples. Sample preparation was zeolite takes place at heating temperatures above 450 ꢀC.38,39
carried out by dispersing 5 mg of ground powder in 5 mL of The TG/DTG-DSC curves of the as-prepared NH4Y zeolite are
ethanol using ultrasonication, followed by dropping the illustrated in Fig. 1(a). It is clearly shown that when the
suspension on a rounded carbon-coated Cu-Grid mesh and temperature is below 200 ꢀC, the desorption of zeolite-adsorbed
drying at room temperature.
water causes an obvious weight loss and endothermic
X-ray photoelectron spectroscopy (XPS) data were collected phenomenon;40 between 200 and 440 ꢀC, the dissociation of
on an AXIS ULTRA DLD electron spectrometer by using a 150 W NH3 from the NH4Y zeolite causes a slight weight loss with
monochromatic Al Ka radiation source operating at 1486.6 eV. endothermic phenomenon; above 440 ꢀC the dehydroxylation of
The binding energy and the Auger kinetic energy scales were silicol on the surface of the HY zeolite causes another slight
referenced to the C 1s line at 284.8 eV from adventitious carbon. weight loss with endothermic phenomenon.
Atomic absorption spectroscopy (AAS) was used to deter-
For the Cu(acac)2 sample, only one obvious weight loss peak
ꢀ
mine the Cu loading of the prepared catalyst by using VARIAN in the DTG at around 250 C and only one strong exothermic
AA240FS equipment. The testing solution was prepared peak on the DSC at 247 ꢀC are observed in Fig. 1(b). It strongly
through the following steps: 0.2 g of fresh catalyst was calcined indicates that Cu(acac)2 decomposed at 150–350 ꢀC, because
with 2.0 g of potassium hydroxide at 500 ꢀC for 6 h, digested in there is no further weight loss and no heat changes above
a hydrochloric acid solution, and diluted through making the 350 ꢀC. The nal material from Cu(acac)2 decomposition should
volume up to 250 mL in a volumetric ask with deionized be CuO, and the theoretical weight loss is 69.4%. But the actual
water. Then 5.0 mL of this solution was transferred to a 100 mL weight loss of the TG in Fig. 1(b) is as high as 93.7%. If the DSC
volumetric ask, diluted with deionized water to the volume, curve is enlarged as shown in Fig. 1(b), a weak endothermic
and mixed.
peak at 220 ꢀC is clearly observed before the strong exothermic
peak at 247 ꢀC. This must be caused by the sublimation of
Cu(acac)2. Therefore, it is concluded that Cu(acac)2 rstly
sublimates and then decomposes to CuO at temperatures from
Catalytic tests
Catalytic measurements were performed in a continuous xed 150 C to 350 ꢀC.
ꢀ
bed micro reactor (F 6 mm ꢂ 450 mm) under atmospheric
For a well physical mixture of NH4Y with 28.6 wt%
pressure with an on-line Agilent 6890 gas chromatograph (GC). Cu(acac)2, the thermal behavior is so complicated that it is
0.45 g of catalyst (about 1.0 mL) was packed in the tubular not the simple addition of two raw materials, and the TG of
reactor and the reactor was positioned in the furnace. 0.02 mL the mixture and the simple addition are labeled as curve 1
minꢁ1 of methanol was introduced using a constant-ux pump and curve 2 respectively, shown in Fig. 1(c). It is clear that the
(2PB-05) and vaporized in the pre-heater. In the meantime, 28.0 weight loss undergoes four steps according to the TG/DTG
mL minꢁ1 of carbon monoxide and 2.8 mL minꢁ1 of oxygen curves of the mixture. In the rst step, zone I, at tempera-
were introduced into the pre-heater and mixed well with the tures below 120 ꢀC, the weight loss of 8.8% for the peak at
vaporized methanol, and then the mixed reactants were fed into 75 ꢀC must be attributed to the loss of adsorbed water in the
the catalyst bed to catalyze the oxidative carbonylation of NH4Y zeolite. This water desorption peak temperature on
methanolꢀ. The temperature and time of the catalytic reaction curve 1 is obviously lower than that of pure NH4Y and on
were 140 C and 10 h respectively.
curve 2. It indicates that some of the Cu(acac)2 would have
The thermal gas products were analyzed using an on-line GC adsorbed onto the surface of the zeolite and facilitated the
with three valves and four columns every 20 minutes. The release of zeolite-adsorbed water. In the second step, zone II,
products owed through the HP-PLOT/Q capillary column (30 at temperatures between 120 ꢀC and 230 ꢀC, the peak at
m ꢂ 0.53 mm ꢂ 40 mm), the organic products were held back. 182 ꢀC is for a weight loss of 10.1% for curve 1 which is lower
Then the expelled inorganic products owed through a Prop- than the loss of 13.1% for curve 2, which is perhaps because
ack-Q packed column: CO2 rstly owed out and entered into Cu(acac)2 begins to sublimate and was partly absorbed on
a thermal conductivity detector (TCD) and was detected; NH4Y. In the third step, zone III, at temperatures between
secondly, CO and O2 were separated using an HP-PLOT 230 ꢀC and 500 ꢀC, the rate of weight loss for curve 1 is slower
Molesieve/5A capillary column (30 m ꢂ 0.53 mm ꢂ 25 mm) and and lower than that for curve 2, so the weight loss of 13.7%
ꢀ
detected using TCD. The products owed through an HP- with a broad peak at 356 C is assigned to the mild decom-
INNOWax (30 m ꢂ 0.53 mm ꢂ 1 mm) capillary column and position of Cu(acac)2. In the fourth step, zone IV, at
entered into a ame ionization detector (FID), thus MeOH, temperatures above 500 ꢀC, nearly no weight loss is observed.
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Fig. 1 TG/DTG-DSC curves of the samples. Curve 2 is the TG resulting from the addition of the TG of Cu(acac)2 and NH4Y.
Over the total temperature range, the actual weight loss is
To obtain more detailed information during heat treatment
32.6% for curve 1 and 45.4% for curve 2. Moreover, endo- of the mixture of Cu(acac)2 and NH4Y, TG-MS measurements
thermic and exothermic peaks on the DSC curve of the were performed and the results are shown in Fig. 2. The signals
mixture are overꢀlapping and the largest exothermic peak also at m/z ¼ 17, 18 and 44 are assigned to the main volatile products
ꢀ
shis from 247 C to 349 C. This shows that a complicated NH3, H2O and CO2 respectively, and the signals at m/z ¼ 58, 60
interaction exists between Cu(acac)2 and NH4Y during heat and 100 are ascribed to the trace organic products acetone,
treatment.
propyl alcohol and acetylacetone respectively.41 During heat
treatment of the mixture, water is released at 76 C below the
ꢀ
water desorption peak temperature (121 ꢀC) of pure NH4Y,
which has been demonstrated from TG/DTG. With the increase
of the heat treatment temperature, the release of NH3 proceeds
in two steps from the curve of m/z ¼ 17. In the rst step, NH3 is
ꢀ
released at 174 C, simultaneously, a trace amount of organic
products are also observed. Additionally, only above 200 ꢀC, the
NH4Y zeolite decomposes and releases NH3. So it was proven
+
that the ion exchange between the Cu2+ of Cu(acac)2 and NH4
of NH4Y does occur at 174 ꢀC. In the second step, at the
temperature of 417 ꢀC, the release of NH3 comes from the
residual NH4 decomposition of NH4Y or the desorption of
+
adsorbed NH3 from the ion exchange. When the heat treatment
temperature reaches about 360 ꢀC, the organic products
acetone, propyl alcohol and acetylacetone were observed in
addition to CO2 and H2O.
According to the above analysis, during heat treatment of the
mixture, (a) in temperature zone I, the zeolite-adsorbed water of
NH4Y is released; (b) in temperature zone II, Cu(acac)2 subli-
mates and Cu(acac)2 vapor is mostly adsorbed on the Y zeolite,
and in the meantime, ion exchange between Cu(acac)2 and
NH4Y occurs; (c) in temperature zone III, there is
Fig. 2 MS spectra of gaseous products of the physical mixture of
NH4Y and Cu(acac)2.
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+
decomposition of adsorbed Cu(acac)2, and residual NH4
decomposition of NH4Y.
small cages during activation, so the particles of copper species
in the Cu/NH4Y catalysts become smaller and more uniform,
resulting in less impact on the zeolite structures.
Fig. 4 presents the TEM images of the Cu/NH4Y-400 catalyst.
Richter et al.26 reported that the CuY samples suffered major
modications during TEM imaging. In order to suppress the
inuence of the electron beam, imaging was carried out
immediately aer the beam was focused. From Fig. 4(A), the
faujasite structure of zeolite Y is unbroken, nanosized copper
particles are well dispersed on the surface of zeolite Y, and the
particle sizes are in a range of 2–6 nm. From Fig. 4(B), light grey
Cu species of 1–2 nm are observed in the channel of zeolite Y.
Meanwhile, some larger black Cu species also exist, caused by
incident electron beam.
Structural properties of CuY catalysts
The XRD patterns of the samples are compared in Fig. 3. For the
mixture, the diffraction peaks of Cu(acac)2 and NH4Y are
simultaneously observed. However, the diffraction peaks of the
precursor and the Cu/NH4Y catalysts are similar to those of Y
zeolite and no diffraction peaks of the corresponding Cu species
are observed. It proves that Cu species are well dispersed on the
surface of the Y zeolite during the preparation of the CuY
catalyst. In addition, these results also indicate that the prepa-
ration of the CuY catalyst does not have signicant effects on
the framework structure of the zeolite, only the diffraction peak
intensity is slightly weakened.
Analysis of copper valence and state
Owing to the ion-exchanged ions substituting the Na+ of NaY
and undergoing rearrangement, there is alteration of the rela-
tive intensities of the (220) and (311) crystal planes of Y zeolite.42
As can be seen in Fig. 3, compared with NH4Y, I111 of the
precursor decreases obviously, I220 and I311 also decrease
slightly, but I220 decreases more greatly than I311. It illustrates
that Cu2+ substitutes the NH4+ of the NH4Y zeolite through the
To determine the distribution of the reducible copper species
on the CuY catalyst, H2-TPR was carried out and the proles are
shown in Fig. 5. The H2-TPR prole of the Cu/NaY-400 catalyst
only appears as a sharp reduction peak at around 224 ꢀC.
However, the proles of the Cu/NH4Y and Cu/HY-400 catalysts
exhibit more than one reduction peak from ambient tempera-
ture to 1000 ꢀC. In order to investigate the content and location
of the reducible copper species, the H2-TPR proles were
studied by means of peak-differentiating analysis (dotted lines
in Fig. 5).
ion exchange between the Cu2+ of Cu(acac)2 and the NH4 of
+
NH4Y. I111, I220 and I311 also decrease for the Cu/NH4Y catalysts,
yet compared with the precursor, I111 and I220 increase slightly.
This difference indicates that adsorbed Cu(acac)2 decomposes
to copper oxide and copper ions migrate from supercages to
The reduction of the reducible copper species on CuY has
been discussed by many researchers and the following conclu-
sions26,28–30,43–45 are accepted: (i) the reduction of exchanged Cu2+
Fig. 3 XRD patterns of Cu(acac)2 (a), NH4Y (b), the mixture (c), the
precursor (d), Cu/NH4Y-300 (e), Cu/NH4Y-400 (f) and Cu/NH4Y-500
(g).
Fig. 4 TEM images of the Cu/NH4Y-400 catalyst.
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ꢀ
ꢀ
consumption peaks from 300 C to 700 C. This suggests that
the initial temperature of exchange for Cu2+ being reduced to
the Cu+ active center is 280 ꢀC. When the activation temperature
+
ꢀ
is 400 C, the CuY catalyst has the highest Cu content of 4.3
wt% (Table 1). Comparing Cu/NH4Y-400 with Cu/HY-400, it is
found that the content of CuO and Cu2+ of Cu/NH4Y-400 are
both lower than for Cu/HY-400, but the content of Cu+ in the
supercages of Cu/NH4Y-400 is more than double that of Cu/HY-
400.
It’s worth mentioning that many works have shown that the
exchanged copper ions on CuY were populated preferentially in
small cages.46,47 However, Cu(acac)2 cannot enter into small
cages because of steric hindrance, so up to 95% of the Cu2+ ions
on the Cu/NH4Y-400 catalyst located in the supercage are
accessible to reactants. This is an advantage of preparing the
CuY catalyst with high catalytic activity for the oxidation
carbonylation of methanol to DMC.
XPS is a surface analysis technique, however, due to the large
voids of zeolite Y, the sampling depth is higher in zeolites and
the effective depth is about 5–10 zeolite unit cells below the
surface.48 Fig. 6 presents the Cu 2p3/2 XPS spectra and the Cu
LMM Auger spectra of Cu/NH4Y. It was reported that the kinetic
energies of the Cu LMM Auger electrons at 913.7 eV and 916.6
eV are ascribed to Cu+ and Cu2+ 32 and the kinetic energy of Cu0
,
is in the range from 918 to 920 eV.29 As can be seen in Fig. 6, the
Cu LMM Auger spectra strongly supports the presence of Cu+
with a kinetic energy of 912.8 eV and Cu2+ with a kinetic energy
of 915.9 eV, and there is no Cu0 peak in the range from 918 to
920 eV. The characteristics of divalent copper can be further
conrmed by the Cu 2p3/2 binding energy within 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.49,50 The XPS spectra of Cu 2p (Fig. 6) shows an over-
lapping Cu 2p3/2 signal, and from curve-tting of the Cu 2p3/2
spectra, it can be tted into two peaks at 933.4 eV attributed to
Cu+ and 935.7 eV attributed to Cu2+ respectively. From the
proportion of area percentages, the calculated Cu+ content
percentages of the catalysts are 46.9%, 52.5%, and 39.1%
respectively when the activation temperatures are 300 ꢀC, 400 ꢀC
and 500 ꢀC. The change trend coincides with that of the Cu+
content percentage which is shown by the H2-TPR results
(Table 1).
Fig. 5 H2-TPR profiles: Cu/NH4Y-200 (a), Cu/NH4Y-220 (b), Cu/
NH4Y-250 (c), Cu/NH4Y-280 (d), Cu/HY-400 (e), Cu/NH4Y-300 (f),
Cu/NH4Y-400 (g), Cu/NH4Y-500 (h), Cu/NaY-400 (i).
to Cu+ occurs from 200 ꢀC to 300 ꢀC, and the reduction
temperature depends highly on the location and in the order:
supercages < sodalite cages < hexagonal prisms. (ii) The
reduction of dispersed CuO to Cu0 occurs at around 250 ꢀC. (iii)
The exchanged Cu+ is reduced at distinctly higher temperatures.
So for the H2-TPR of the as-prepared CuY catalyst in Fig. 5, the
peaks at 192 ꢀC, 284 ꢀC and 347 ꢀC are attributed to the
reduction of Cu2+ to Cu+ located in supercages, sodalite cages
and hexagonal prisms of the Y zeolite, respectively. The peaks at
224 ꢀC of the Cu/NaY-400 catalyst and at 245 ꢀC of the Cu/NH4Y
and Cu/HY-400 catalysts belong to the reduction of CuO to Cu0
in one step. The reduction peaks above 700 ꢀC are attributed to
the reduction of Cu+ to Cu0. Thus it can be seen that only CuO
exists on Cu/NaY-400, while there is ion-exchanged Cu2+ and
Cu+ besides CuO on Cu/NH4Y and Cu/HY-400. The hydrogen
consumption and the relative content of reducible Cu species
were calculated from the area of the hydrogen consumption
peak and are listed in Table 1.
As indicated above, it is deduced that the main reactions are
as follows: during heat treatment, rstly Cu(acac)2 sublimates
and is adsorbed onto the NH4Y surface, and in the meantime
the ion exchange between the Cu2+ of Cu(acac)2 and the NH4+ of
NH4Y to form Cu2+Y takes place.
ꢀ
ꢀ
CuðacacÞ þ NH Y 150ꢁ350 C CuðacacÞ NH4Y
ƒƒƒƒ!
(1)
(2)
4
2
2
From Fig. 5, for the Cu/NH4Y catalysts, until the activation
ꢀ
ꢀC
ꢀ
temperature reaches 280 C, the hydrogen consumption peaks
CuðacacÞ NH Y 120ꢁ230 Cu2þ
Y
ƒƒƒƒ!
4
of the CuO and Cu+ species appear on the H2-TPR prole of Cu/
2
NH4Y-280. When the activation temperature is lower than
+
ꢀ
280 C, the hydrogen consumption peaks of the CuO and Cu
species are not observed on the corresponding H2-TPR prole of
Cu/NH4Y, and there are some undetermined hydrogen
At the activation stage, the adsorbed unreacted Cu(acac)2
starts to decompose, and at the same time, NH4Y decomposes
to form HY and releases NH3. Meanwhile ion exchanged Cu2+ is
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Table 1 Quantitative analysis of the H2-TPR profiles of the catalysts
H2 consumption (cm3 gꢁ1
)
Cu mass (%)
Cu2+ / Cu+
Cu+ / Cu0
CuO /
Catalyst
H2sum
Sup.
Sod.
Hex.
Cu0
Sup.
Sod.
Hex.
Cu2+
Cu+
CuO
Cu+/Cusum (%)
Cu/NaY-400
Cu/NH4Y-250
Cu/NH4Y-280
Cu/NH4Y-300
Cu/NH4Y-400
Cu/NH4Y-500
Cu/HY-400
28.4
19.8
25.8
26.3
25.5
27.3
26.7
0
0
0
0
0
0
0
0
2.1
28.4
0
0
0
0
0
0
0
0
0
0
4.2
—
—
—
—
8.1
0
0
0
8.9
9.1
8.7
7.4
6.3
3.2
1.1
1.3
0.6
0.4
0.7
3.1
9.8
5.7
3.3
5.2
2.5
2.5
0.7
1.4
2.4
7.4
7.0
9.0
12.3
10.1
10.4
4.6
5.9
5.3
4.5
4.0
4.8
2.5
3.6
4.3
3.4
1.7
0.2
0.4
0.7
2.1
2.0
32.9
38.7
45.3
35.8
20.0
230ꢁ500 ꢀ
Hacac=NH Y þ O ðgÞ
C Y þ CO ðgÞ þ 6H OðgÞ
(7)
ƒƒƒƒ!
4
2
2
2
Catalytic properties of CuY catalysts
Catalytic properties of the as-prepared CuY catalysts in the
direct gas phase oxidative carbonylation of methanol to DMC
are listed in Table 2. For comparison, some CuY catalysts in the
literature24,28,32 are also listed in Table 2 for reference. The as-
prepared Cu/NaY-400 catalyst has no catalytic activity, and the
catalytic activity of the Cu/NH4Y-400 catalyst with 153.0 mg gꢁ1
hꢁ1 of STYDMC is better than that of the Cu/HY-400 catalyst with
65.4 mg gꢁ1 hꢁ1 of STYDMC. The above analysis of the results has
+
shown that the NH4 of NH4Y can introduce Cu2+ into extra-
framework cation locations of zeolite Y through ion exchange
+
between the Cu2+ of Cu(acac)2 and the NH4 of NH4Y, and the
decomposed NH3 from NH4Y facilitates the reduction of Cu2+ to
Cu+ to form more active centres. Compared with the Cu/HY
catalyst, more copper ions on the Cu/NH4Y catalyst located in
the supercages are accessible to the reactants. However, the Na+
of NaY can’t exchange with the Cu2+ of Cu(acac)2 under the
same experimental conditions, and there was no Cu+ active
center on the Cu/NaY-400 catalyst, only CuO species.
Fig. 6 Cu 2p XPS and Cu LMM Auger spectra of Cu/NH4Y-300 (a), Cu/
NH4Y-400 (b), and Cu/NH4Y-500 (c).
As for Cu/NH4Y catalysts with different activation tempera-
tures (300 ꢀC, 400 ꢀC, 500 ꢀC), when the activation temperature
auto-reduced to form the Cu+ active centre for the oxidative
carbonylation of methanol to DMC.
was 300 ꢀC, the conversion of methanol (XCH OH) and the space-
3
ꢀ
ꢀ
time yield of DMC (STYDMC) were 7.5% and 137.2 mg gꢁ1 hꢁ1
,
CuðacacÞ NH Y 230ꢁ500 C CuO=NH Y
ƒƒƒƒ!
(3)
(4)
(5)
4
4
2
respectively, for Cu/NH4Y-300. However, the Cu/NH4Y-400
catalyst activated at 400 ꢀC exhibited an excellent catalytic
ꢀ
NH Y 200ꢁ440 C HY þ NH
ƒƒƒƒ!
4
3
activity with 8.2% XCH OH and 153.0 mg gꢁ1 hꢁ1 STYDMC. When
3
the activation temperature was 500 ꢀC, X
and STYDMC
CH3OH
300ꢁ500 ꢀ
Cu2þ
Y
C CuþY
ƒƒƒƒ!
decreased to 6.2% and 113.3 mg gꢁ1 hꢁ1, respectively, for Cu/
NH4Y-500. The H2-TPR and XPS characterisation have shown
that the Cu/NH4Y-400 catalyst has the highest Cu+ active center
content.
In addition, Lu et al.51 reported that CuY is the most active
catalyst for NH3 oxidation from 200 ꢀC to 300 ꢀC. Also, organic
ligands and their products react with oxygen in the air and
generate CO2 and H2O. So the following reactions are possible
during the preparation of the CuY catalyst through HAI.
In addition, it was also found that the catalytic activity of the
Cu/NH4Y-400 catalyst is better than those of the chloride-free
CuY catalysts prepared through LSIE,28 IWIM28 or DP32 and
the Cu/HY(S)-650 catalyst prepared through SSIE.24 For example,
the STYDMC of the Cu/NH4Y-400 catalyst with a 9.5 wt% Cu
loading reaches 153.0 mg gꢁ1 hꢁ1, which is higher than those of
the CuY catalysts with a 10.0 wt% Cu loading prepared through
200ꢁ300 ꢀ
4NH ðgÞ þ 3O ðgÞ
C 2N ðgÞ þ 6H OðgÞ
(6)
ƒƒƒƒ!
3
2
2
2
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Table 2 Catalytic activity of the catalysts in the oxidative carbonylation of methanola
Support/activation
temperature (ꢀC)
STYDMC
Catalysts
Preparation
Cu (wt%)
(mg gꢁ1 hꢁ1
)
XCH OH (%)
SDMC (%)
Ref.
3
Cu/NaY-400
HAI
HAI
HAI
HAI
HAI
LSIE
IWIM
IWIM
SSIE
DP
NaY/400
NH4Y/300
NH4Y/400
NH4Y/500
HY/400
NaY/600
HY/600
NaY/600
HY/650
8.2c
9.5c
0
0
0
Cu/NH4Y-300
Cu/NH4Y-400
Cu/NH4Y-500
Cu/HY-400
Cu/NaY(L)-600
Cu/HY(I)-600
Cu/NaY(I)-600
Cu/HY(S)-650
Cu/NH4Y(D)-400b
Cu/NH4Y(D)-750b
137.2
153.0
113.3
65.4
131.4
95.6
16.3
97.3
0
7.5
8.2
6.2
3.5
6.5
5.1
1.1
4.4
0
61.2
61.9
60.9
60.7
68.5
62.0
50.7
74.6
0
9.5c
9.5c
8.5c
6.3c
28
28
28
24
32
32
10.0d
10.0d
12.2c
10.0d
10.0d
NH4Y/400
NH4Y/750
DP
84.4
8.1
53.5
a
Reaction conditions: VCH OH : VCO : VO ¼ 4 : 10 : 1, GHSV ¼ 2520 hꢁ1, T ¼ 140 ꢀC, and P ¼ 0.1 MPa. b Reaction conditions: VCH OH : VCO : VO
¼
6 : 8 : 1, 1% Ar (He balance), GHSV ¼ 32000 hꢁ1, T ¼ 140 ꢀC, and P ¼ 0.1 MPa. The Cu/NH4Y(D)-400 catalyst prepared through DP had an NH4Y
3
3
2
c
support and was activated at 400 ꢀC. The naming rules are used to name the other catalysts from the literature. The actual Cu loading was
determined using atomic absorption spectrometry. d The theoretical Cu loading was calculated through copper addition.
IWIM,28 including the Cu/HY(I)-600 catalyst with an STYDMC of amounts of CuOx agglomerates cover the Cu+ active centers,
95.6 mg gꢁ1 hꢁ1 and the Cu/NaY(I)-600 catalyst with an STYDMC resulting in a decrease in the catalytic activity of the CuY
of 16.3 mg gꢁ1 hꢁ1. Even the STYDMC of the Cu/NH4Y-400 catalyst.37 Therefore, we suggest that the Cu+ ions on the CuY
catalyst is higher (57.2%) than that of the Cu/HY(S)-650 cata- catalyst are the active centers for the oxidation carbonylation
lyst with a 12.2 wt% Cu loading prepared through SSIE.24 The of methanol to DMC and a modest amount of CuOx agglom-
Cu/NH4Y(D)-400 catalyst has no catalytic activity, and the Cu/ erates contribute lattice oxygen to the formation of DMC and
NH4Y(D)-750 catalyst has an STYDMC of 84.4 mg gꢁ1 hꢁ1 which is improve the catalytic performance.
also lower (44.8%) than that of the Cu/NH4Y-400 catalyst. For
the Cu/NaY(L)-600 catalyst with an STYDMC of 131.4 mg gꢁ1 hꢁ1
prepared through LSIE,28 the 6.3 wt% Cu loading is the
maximum permissible loading for LSIE, and the STYDMC is
lower (14.4%) than that of the Cu/NH4Y-400 catalyst.
Conclusions
During heat treatment of a physical mixture of Cu(acac)2 and
NH4Y, from 120 ꢀC to 230 ꢀC, Cu(acac)2 sublimates and
Cu(acac)2 vapor is mostly adsorbed and well dispersed on the
surface of Y zeolite. In the meantime, ion exchange between
Cu(acac)2 and NH4Y occurs in the supercages. Above 230 ꢀC,
unreacted Cu(acac)2 decomposes to form copper oxides, and
simultaneously NH4Y decomposes and releases NH3, which
facilitates the reduction of Cu2+ to Cu+. Above 500 ꢀC, no
physical or chemical processes occur.
For the NaY zeolite, no solid state ion-exchange occurs
between Cu(acac)2 and NaY during heat treatment, and the
as-prepared Cu/NaY catalyst has no catalytic activity. For the
HY zeolite, there is less ion-exchanged Cu+ in the supercages
and the as-prepared Cu/HY catalyst exhibits lower activity
than the Cu/NH4Y catalyst. The distribution of Cu2+, Cu+ and
CuO on the Cu/NH4Y catalyst depends highly on the activa-
tion temperature. When the activation temperature is 400 ꢀC,
the content of Cu+ attains the highest value, the corre-
sponding Cu/NH4Y-400 catalyst also shows the best cata-
lytic activity, and the conversion of methanol, selectivity of
DMC and the DMC space-time yields are 8.2%, 61.9% and
153.0 mg gꢁ1 hꢁ1 respectively.
According to previous studies,23 the proposed mechanism
for the oxidative carbonylation of methanol to dimethyl
carbonate can be concluded as follows. Firstly, CH3OH with
oxygen adsorbs on Cu+ Lewis sites through formation of
either mono- or di-methoxide species bound to the Cu+
cations. Then, adsorption and insertion of carbon monoxide
into the methoxide species leads to the formation of DMC
directly or monomethyl carbonate species (MMC) as an
intermediate. Last, the MMC species react with methanol to
form DMC. The reaction of carbon monoxide into methoxide
is the rate-limiting step. The Cu+ of CuY has been demon-
strated to improve the adsorption energy of co-adsorbed CO
in the co-adsorbed CO/CH3O system, and stabilizes the tran-
sition state for the CO insertion reaction to produce mono-
methyl carbonate (MMC) species.52–54 In our experiments, the
content of Cu+ attained the highest value for the Cu/NH4Y-400
catalyst, which exhibits the best catalytic performance.
Besides, Engeldinger et al.27,31 also conclude that the forma-
tion of DMC is initiated by the reaction of molecularly
adsorbed methanol on Cu+ cations. More signicantly, they
found that CuOx agglomerates in the supercages favor the
oxidation and oxocarbonylation reactions of methanol and
enhances the formation of DMC. According to our studies, the
Cu/NaY catalyst, which only contains CuO species, has no
catalytic activity. When the Cu loading is too high, excessive
Acknowledgements
The authors are grateful for the nancial support from the
National Natural Science Foundation of China (21276169).
102330 | RSC Adv., 2015, 5, 102323–102331
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29 L. Zhong, F. Tingjun, W. Ruiyu, N. Yanyan and Z. HuaYan,
Chem. J. Chin. Univ., 2011, 1366–1372.
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