RESEARCH PAPER

CHINESE JOURNAL OF CATALYSIS Volume 29, Issue 7, July 2008 Online English edition of the Chinese language journal

Cite this article as: Chin J Catal, 2008, 29(7): 643–648.

Received date: 29 December 2007. * Corresponding author. Tel: +86-351-6018526; Fax: +86-351-6018453; E-mail: lizhong@tyut.edu.cn Foundation item: Supported by the National Natural Science Foundation of China (20576085, 20606022) and the National Basic Research Program of China (973 Program, 2005CB221204). Copyright © 2008, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.

Surface Structure and Catalytic Performance of CuCl/SiO2-Al2O3 Catalysts for Methanol Oxidative Carbonylation

LI Zhong*, MENG Fanhui, REN Jun, ZHENG Huayan, XIE Kechang Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China

Abstract: Aluminum was doped into amorphous silica gel to modify its surface structure. The obtained SiO2-Al2O3 support was used to prepare the CuCl/SiO2-Al2O3 catalyst by solid-state ion exchange, and the catalyst activity for liquid-phase oxidative carbonylation of methanol to dimethyl carbonate was investigated. The results showed that the prepared SiO2-Al2O3 support kept the amorphous structure of the silica gel. The BET specific surface area of the silica gel was decreased to 200 m2/g, and the surface acid sites (including Brønsted acid sites) were increased. In the CuCl/SiO2-Al2O3 catalyst, CuCl was not only dispersed on surface but also was ion exchanged with surface Brønsted acid sites of the SiO2-Al2O3 support to form Cu+ species, which resulted in a decrease in BET specific surface area to 148 m2/g. These two kinds of Cu+ species on the catalyst surface were both active centers for the oxidative carbonylation of methanol to dimethyl carbonate. When the catalyst was prepared with Si/Al molar ratio of 5 and was calcined at 500 °C, the selectivity and space-time yield of dimethyl carbonate reached 74% and 1.27 g/(g·h), respectively.

Key words: cuprous chloride; alumina-silica mixed oxide; methanol; oxidative carbonylation; dimethyl carbonate

Dimethyl carbonate (DMC) is an environmentally friendly green chemical compound. It can be used to substitute for highly toxic phosgene, chloromethyl formate, and dimethyl sulfate in organic synthesis processes due to the presence of the carbonyl group, methyl group, and methoxyl group in it. It can also be used as a solvent and additive in gasoline and diesel oil [1]. The oxidative carbonylation of methanol to DMC has received more attention than the other processes to make DMC because of its abundant and inexpensive raw materials and its green process. When a promoter and/or ligand is added, the activity and selectivity of the homogeneous catalyst can be enhanced [2–4], but the difficulties in catalyst separation from the product and its recovery for recycling for the system still remain. Research on heterogeneous catalysts is mainly focused on copper [5], cobalt [6], and complexes of copper and palla-dium [7]. Copper heterogeneous catalysts feature low toxicity and low cost and show high activity and selectivity to DMC when CuCl and CuCl2 are used as the active components, but

they have several problems such as the corrosion of the equipment and deactivation of the catalyst due to Cl− loss. In order to stabilize the Cu active centers, Li et al. [8] used CuCl supported on zeolites (ZSM-5, DASY), Anderson et al. [9] used Cu+X and Cu+ZSM-5 catalysts, and Sato et al. [10] used organic ligands and a polymer support for the CuCl2 catalyst.

Compared with a single metal oxide, mixed oxides have larger BET specific surface area, better thermal stability, and stronger surface acidity and basicity [11]. The surface structure of mixed oxides can be controlled by selected preparation parameters to make them catalyst supports with excellent ca-pability. SiO2-Al2O3 mixed oxides prepared by the sol-gel method have been widely investigated [12–14]. Silica gel (SiO2) possesses a large specific surface area and pore volume, and when its surface is modified by Al atoms, Brønsted acid sites can be formed. In this study, Al was doped on silica gel to form Brønsted acid sites, and the obtained SiO2-Al2O3 material was used to prepare a CuCl/SiO2-Al2O3 catalyst by solid-state

LI Zhong et al. / Chinese Journal of Catalysis, 2008, 29(7): 643–648

ion exchange. The catalyst activity was investigated for the oxidative carbonylation of liquid methanol to DMC.

1 Experimental

1.1 Catalyst preparation

Aluminum nitrate (Tianjin Kermel Chemical) was dissolved in distilled water, and ground silica gel (ABET = 300–400 m2/g, pore size 8–10 nm, Qingdao Sea Chemical) was then added into the solution with vigorous stirring. The molar ratio of Si/Al was set at 5. The mixed suspension was stirred continu-ously for 2 h at 80 °C, then dried at 80 °C for 12 h, and calcined at 500 °C for 4 h. The prepared SiO2-Al2O3 solid mixed oxide was used to support CuCl by solid-state ion exchange in a ball mill, and then calcination at 500 °C for 4 h under argon. The amount of CuCl used was 1/4 (wt/wt) of the SiO2-Al2O3 sup-port.

1.2 Catalyst evaluation

Catalytic DMC synthesis by oxidative carbonylation of methanol was performed in a 0.25 L-CJF stainless steel auto-clave (Dalian Tongda, China) with 50 ml methanol and 2 g catalyst, into which CO (1.6 MPa) and O2 (0.8 MPa) were introduced. The reaction was operated at 140 °C for 90 min with stirring at 700 r/min. The products were analyzed using an Agilent GC 6890 gas chromatograph with a HP-Innowax cap-illary column and FID at detector temperature of 250 °C and oven temperature of 80 °C.

1.3 Catalyst characterization

X-ray diffraction (XRD) measurements were performed with a Rigaku D/Max 2500 powder diffractometer using Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 100 mA. The patterns were recorded in steps of 0.01° with the scanning rate at 8º/min from 5° to 85° under atmospheric pressure.

Temperature-programmed desorption (TPD) of NH3 was carried out on an Autochem II 2920 (Micromeritics) Instru-ment to study the acidity of the samples. The sample (40 mg) was pretreated at a heating rate of 10 °C/min to 400 oC and was held for 30 min under a flow of helium. After cooling to 120 oC, a mixture of 15% NH3-85% He was introduced until the acid sites of the catalyst were saturated with NH3. Finally, helium was introduced to remove physically adsorbed NH3, and desorption was carried out from room temperature to 1000 oC at a heating rate of 10 oC/min.

Elemental analysis of the catalysts was conducted with a HITACHI 1800-800 equipment. Cu contents of the catalysts were estimated by atomic absorption spectroscopy using the DZG-93-01 standard.

FT-IR spectra were measured on a Shimadzu FT-IR-8400

infrared spectrometer. The spectra of the catalysts were ac-quired in the 400–4500 cm−1 wavenumber range using a KBr pellet. Pyridine-IR was used to distinguish the nature of the acid sites (Lewis or Brønsted). The samples were pretreated at 300 °C for 1 h, then cooled to room temperature to adsorb pyridine, and then heated at 150 °C for 20 min.

BET specific surface area of the samples was obtained on a Sorptmantic 1990 equipment using nitrogen as adsorbate at −196 °C. Before the measurement, the sample was degassed at 250 °C and 0.01 Pa for 12 h.

2 Results and discussion

2.1 Surface structure and acidity of the SiO2-Al2O3 support

The XRD patterns of silica gel and the SiO2-Al2O3 mixed oxide are shown in Fig. 1. The broad XRD peaks detected around 2θ = 23° are attributed to amorphous silica gel. These suggest that the SiO2-Al2O3 mixed oxide had the same structure as the original SiO2, and the amorphous structure of silica gel was kept.

Fig. 2(a) shows the acidity of silica gel that was calcined at 500 and 1000 °C estimated by TPD. The large and broad de-sorption peaks around 630 oC are attributed to the dehydroxy-lation of silanol groups on the surface of silica gel. This means that the surface of silica gel still had many silanol groups after the catalyst was calcined at 500 oC. When the silica gel was calcined at 1000 oC, all OH groups on the sample surface were desorbed, as deduced from the absence of the desorption peaks in curve (4). Both the TPD curves of silica gel without NH3 (curve (1)) and with NH3 (curve (2)) have big desorption peaks, but there was no desorption peak of NH3 in curve (3), which is the subtraction of curve (2) from curve (1). Therefore, it was concluded that the original silica gel had no acidity at all.

Fig. 2(b) shows the TPD of the SiO2-Al2O3 support prepared by doping Al into the silica gel. There were desorption peaks

Fig. 1. XRD patterns of silica gel and the SiO2-Al2O3 support pre-pared with a Si/Al molar ratio of 5.

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below 500 oC from the SiO2-Al2O3 mixed oxide in both the TPD curves with NH3 and without NH3 adsorption. The larger peak around 202 oC in curve (3) obtained by subtracting curve (2) from curve (1) was attributed to the acid sites of NH3 ad-sorption. Based on the desorption temperature between 150 and 250 °C, the acid site can be classified as weakly acidic. Therefore, it was concluded that the SiO2-Al2O3 mixed oxide possessed a great quantity of acid sites compared with the original silica gel, which had no acid sites at all.

Fig. 3(a) shows FT-IR spectra of the silica gel and SiO2- Al2O3 support. The absorption peak at 960 cm−1 from silica gel calcined at 500 oC was attributed to the characteristic stretching vibrations of Si–OH [15], which was evidence that Si–OH desorbed incompletely at 500 oC. The band at 960 cm−1 was not observed with the SiO2-Al2O3 support because of the formation of the Si–O–Al bond by the interaction of Si–OH and Al–OH groups, which resulted in the decrease of the Si–OH amount. However, because few Si–O–Al bonds were formed on the silica gel surface, and the absorbance of Si–O–Al was near that of Si–O–Si of silica gel [16], it was not easy to identify the Si–O–Al absorbance bands in the IR spectra.

The pyridine-IR spectra of silica gel and the SiO2-Al2O3 support are shown in Fig. 3(b). The IR band at 1450 cm−1 was due to the adsorption of pyridine on Lewis acidic centers, the band at 1490 cm−1 was due to the interaction with both Lewis and Brønsted acid sites, and the bands at 1540 and 1620 cm−1

were due to adsorption of pyridine on Brønsted acid centers [17]. The figure shows that silica gel had no detectable acidic centers, while the SiO2-Al2O3 support exhibited intensive in-frared bands of both Lewis and Brønsted acid sites. This indi-cates that the SiO2-Al2O3 support possessed both Lewis and Brønsted acid sites.

From the above discussion, the surface of silica gel still had many –OH groups even after it was calcined at 500 oC. During the process of doping Al, aluminum nitrate was hydrolyzed to aluminum hydroxide first, then H2O was removed as a result of the reaction of aluminum hydroxide with Si–OH on the surface of silica gel when heated, and Al combined with Si via O to form Si–O–Al. Thus the distribution of the charge was changed, and the Brønsted acid site was generated as below:

2.2 Physicochemical properties of the CuCl/SiO2-Al2O3 catalyst

Fig. 4 shows the XRD patterns of the CuCl/SiO2-Al2O3 catalysts with different Si/Al molar ratios. No CuCl diffraction

Fig. 2. TPD spectra of silica gel (a) and the SiO2-Al2O3 support cal-cined at 500 °C (b). (1) TPD without NH3 adsorption; (2) NH3-TPD; (3) Curve (2) subtracting from curve (1); (4) NH3-TPD of silica gel calcined at 1000 °C.

Fig. 3. FT-IR spectra (a) and pyridine-IR spectra (b) of silica gel and SiO2-Al2O3 support.

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peaks were observed in the XRD patterns when Si/Al < 5. The acid strength of the silica gel can be changed when calcined at high temperature after silica gel was doped with Al [18], and the Cu+ of CuCl can be exchanged with the H+ of the Brønsted acid site on the support when the mixture of CuCl and solid Brønsted acid was calcined in an inert atmosphere [19]. With increasing Si/Al ratio, the amount of exchangeable H+ de-creased, so that less Cu+ was exchanged and loaded, and more Cu+ existed as CuCl crystallites, leading to the appearance of CuCl crystal diffraction peaks in the XRD patterns.

Fig. 5 shows the XRD patterns of catalysts calcined at dif-ferent temperatures. Only CuCl diffractions were observed. It has been reported that CuCl can be dispersed as a monolayer on the surface of the supports when the heating temperature was below the melting point of CuCl (430 oC) [20]. The detected CuCl diffraction peaks in the XRD patterns indicated that the CuCl loading exceeded the monolayer threshold of CuCl on the supports. When the calcination temperature exceeded the melting point of CuCl, such as calcination at 500 oC, CuCl can be highly dispersed on the supports and interacted with Brøn-sted acid sites, and some part of the Cu+ existed as the Cu+–O form, which can efficiently increase the number of active spe-cies on the surface. No diffraction peak of CuCl was observed in the XRD pattern when the catalyst was calcined above 550 oC. It is well known that the sublimation of CuCl is accelerated at a high temperature, which resulted in a rapid loss of CuCl from the CuCl/SiO2-Al2O3 catalyst, with the exception of the Cu+ ion exchanged with Brønsted H+ [21]. No diffraction peak

of CuCl appeared in the XRD patterns when calcined at 600 oC. However, under the same condition, the catalyst prepared at 400 oC showed stronger diffraction peaks of CuCl than the catalyst prepared at 300 oC. This needs further investigation.

N2 adsorption isotherms indicated that the BET surface area of the SiO2-Al2O3 support was 200 m2/g, which decreased to 148 m2/g after loading with CuCl. The pore volume of SiO2-Al2O3 decreased after loading CuCl, which can be ac-counted for by the efficient dispersion of CuCl on the SiO2-Al2O3 support.

The elemental analysis showed that the mass content of Cu and Cl in the CuCl/SiO2-Al2O3 catalyst decreased from 12.83% to 10.87% and 7.17% to 4.98%, respectively. This indicates that during the preparation of the catalyst, CuCl had ion ex-changed with the SiO2-Al2O3 support Brønsted H+ to form Cu+ on the support and lost some Cl as HCl.

2.3 Catalytic performance of the CuCl/SiO2-Al2O3 catalyst

Fig. 6(a) shows the catalytic activity of the catalysts with different Si/Al ratios for DMC synthesis from methanol. The CuCl/SiO2-Al2O3 catalyst had the highest selectivity (74%) and space-time yield of DMC (1.27 g/(g·h)). The catalytic activity decreased when the support Si/Al ratio was higher than 5. With the increase of Si/Al, less Al was doped into silica gel, so that the exchangeable H+ of the Brønsted acid sites decreased, which resulted in decreased exchanged Cu+, and the catalytic activity decreased.

Fig. 6(b) shows the effect of calcination temperature on the

Fig. 4. XRD patterns of the CuCl/SiO2-Al2O3 catalysts with different Si/Al ratios. Si/Al: (1) 0.5; (2) 2; (3) 5; (4) 20; (5) 100; (6) Pure silica gel.

Fig. 5. XRD patterns of the CuCl/SiO2-Al2O3 catalysts calcined at different temperatures.

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catalytic activity of CuCl/SiO2-Al2O3. The catalyst calcined at 500 ºC showed the best catalytic activity. When CuCl/SiO2- Al2O3 was calcined below 500 ºC, more CuCl exchanged with the Brønsted acid sites of the support and less was sublimated. However, when calcined at 600 ºC, less CuCl was exchanged with the H+ of Brønsted acid sites to form Cu+ active centers and more CuCl was sublimated, resulting in the decrease of catalyst activity. It was concluded that both CuCl dispersed on the SiO2-Al2O3 support and Cu+ species were responsible for the catalyst activity in methanol oxidative carbonylation to DMC.

3 Conclusions

A SiO2-Al2O3 mixed oxide support obtained by doping Al into silica gel had the amorphous structure of silica gel. The Brønsted acid centers of the SiO2-Al2O3 support were gener-

ated by the combination of the Al atom and Si atom via Si–O–Al bonds. A CuCl/SiO2-Al2O3 catalyst prepared by the solid-state ion-exchange method had two catalytic active cen-ters for the oxidative carbonylation of methanol to dimethyl carbonate, the Cu+ species formed by ion exchange and the CuCl crystallites dispersed on the surface of the SiO2-Al2O3 support.

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Fig. 6. Effect of Si/Al ratio (a) and calcination temperature (b) on the catalytic activity of the CuCl/SiO2-Al2O3 catalysts.