Journal of Natural Gas Chemistry 21(2012)189–193 Comparative study between gas phase and liquid phase for the production of DMC from methanol and CO 2 Ahmed Aouissi ∗ , Salem S. Al-Deyab Department of Chemistry, King Saud University, P. O. Box 2455, Riyadh 11451, Saudi Arabia [ Manuscript received August 15, 2011; revised October 9, 2011 ] Abstract Direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide over Co 1.5 PW 12 O 40 in liquid and in gas phase is investigated. The synthesized catalyst has been characterized by means of FTIR and XRD. Liquid phase experiment results showed that high pressures are favorable for the synthesis of DMC. However, DMC formation is limited by the reaction with co-produced water. DMC selectivity is more strongly dependent on the temperature than on the pressure of CO 2 . As for the reactions in gas phase, it has been found that both CH 3 OH conversion and DMC selectivity decreased with increasing temperature, owing to the decomposition of DMC at high temperatures. High temperatures and more amount of Co 1.5 PW 12 O 40 catalyst favor the formation of dimethoxymethane (DMM) and methyl formate (MF). Key words heteropolyanion; Keggin structure; methanol; dimethyl carbonate; carbon dioxide 1. Introduction Two routes can be used to perform the natural gas up- grading: direct conversion of methane and indirect conver- sion via methanol. In fact, methanol is manufactured from syngas, which is typically produced from the steam reform- ing of methane. Thus, the conversion of methanol into valu- able chemicals is within the framework of upgrading natural gas. Among the products obtained from methanol, dimethyl carbonate is considered as an important product in chemi- cal syntheses as well as in industry [1-3], which is an en- vironmentally benign chemical product with a wide range of applications. It has been used as a good solvent [4], an alkylation agent [5] and a substitute for highly toxic phos- gene and dimethyl sulfate in many chemical processes [6-9]. It is also used as an intermediate in the synthesis of poly- carbonates and isocyanates [10,11]. In addition, it is ex- pected to replace the gasoline oxygenate methyl tert-butyl ether (MTBE), because of its high oxygen content, low toxi- city and rapid biodegradability [3,4,12-14]. Currently, DMC is produced mainly by oxidative carbonylation of methanol (non-phosgene route) [15]. The synthesis can be carried out in both liquid- and gas-phases [16-19]. However, both routes use poisonous gas of carbon monoxide and there is the possi- bility of an explosion. Recently, the conversion of methanol in the presence of carbon dioxide has drawn much attention, due to the abundance, the cheapness, the non-toxicity and the non-flammability of carbon dioxide, compared with cur- rently used phosgene and carbon monoxide [18,20]. There- fore, the development of efficient heterogeneous catalytic sys- tems has attracted more attention. Bian et al. [21] studied the reaction over Cu-Ni/graphite nanocomposite catalyst in gaseous phase. They obtained 10.13% CH 3 OH conversion and 89.04% DMC selectivity at 105 ◦ C. Wu et al. [22] studied the synthesis of DMC from gaseous methanol and CO 2 over H 3 PO 4 modified V 2 O 5 catalyst with various molar ratios of H 3 PO 4 /V 2 O 5 (P/V). The best conversion (1.95%) and selec- tivity of DMC (92.12%) was obtained at 130 ◦ C over the cata- lyst H 3 PO 4 /V 2 O 5 (P/V = 0.20). The relatively low DMC yield obtained from the direct synthesis from methanol and CO 2 is due to the fact that CO 2 is thermodynamically stable and ki- netically inert and also the deactivation of catalysts induced by water formation during reaction process [23,24]. The inert- ness of carbon dioxide and the reaction of water produced by the reaction bring serious drawbacks for further applications. The main problem is to find an efficient catalyst, as well as optimal operating conditions. Therefore, the direct synthesis of DMC from methanol and carbon dioxide in liquid and gas phase is still a subject of numerous investigations in order to ∗ Corresponding author. Tel: 00 966 1 4675958; Fax: 00 966 1 4675992; E-mail: aouissed@yahoo.fr Copyright©2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(11)60353-8