The Synthesis Of Dimethyl Ether From The Hydrolysis Of Methyl Bromide Over ZnO/HZSM-5Catalyst

Dimethyl ethter (DME) is an important feed stock and intermediate in chemical industry, which is widely used as fuel, aerosols, blowing agents, cooling agents and so on. DME is known as "clean fuel of the21st century". In recent years, the synthesis of DME has gained increasing concern. Traditionally, manufacture of DME maily depends on the synthesis gas process. However, catalytic reforming of natural gas to synthsize syngas is carried out at a temperature higher than800℃, which consumes too much energy and emits large amount of CO2. A new non-syngas process of DME synthesis from methane, with a high methane conversion, less energy consumption and less CO2emissions, was created early in our experimental group. In this paper, new hydrolysis catalysts and process conditions were investigated.In the fixed-bed reactor prepared by ourselves, the influences of carriers, ratio of silicate to aluminium of zeolite, metal oxides as active components and loading weight on the catalyst activity were studied. It was found that activity of supported catalysts was higher than that of carriers, especially HZSM-5. Catalysts supported by ZnO, MnO2, CuO and so on exhibited better catalytic activity, and50wt%-ZnO/HZSM-5(SiO2/Al2O3=60) catalyst showed the best catalytic performance. Then, methods of preparating catalyst and catalyst calcination conditions including calcination temperature and calcination time were investigated. The results showed that catalyst preparated by chemical precipitation and calcinated at550℃for5hours exhibited the best catalytic performance. After that, over ZnO/HZSM-5(SiO2/Al2O3=60) catalyst, the influence of process conditions including reaction temperature, H2O/CH3Br molar ratios and so on on the reaction was studied. The results showed that with increase of reaction temperature, methyl bromide conversion and methanol selectivity increased, while DME selectivity decreased. With the increase of H2O/CH3Br molar ratio, methyl bromide conversion and DME selectivity decreased, while methanol selectivity increased. Under the optimal reaction condition, hydrolysis of methyl bromide was carried out at180℃with H2O/CH3Br molar ratio of1.2/1.0and WHSVCH3Br=1.18h-1over50wt%-ZnO/HZSM-5(SiO2/Al2O3=60) catalyst. The methyl bromide conversion reached56.5%with DME selectivity of90.4%and methanol selectivity of9.6%.In the online reaction, it was found that50wt%-ZnO/HZSM-5catalyst deactivated after6.5h of online reaction. The deactivated catalyst could be regenerated in air at high temperature. In the characterizations of BET, XRD, EDS, and TG-DSC, it was found that the catalyst deactivation was because of ZnBr2formation and carbon deposition on catalyst surface.Finally, the reaction mechanism of hydrolysis of methyl bromide to DME over ZnO/HZSM-5was studied. By CD3OD isotope tracer experiment, comparing hydrolysis experiment with anhydrous experiment, it was demonstrated that methanol was a reaction intermediate, and hydrolysis of methyl bromide to methanol was the fist step, and then generation of DME mainly depended on dehydration of methanol and reaction between methanol and methyl bromide. Moreover, the function of ZnO/HZSM-5catalyst was discussed. By comparing the activity of ZnO/HZSM-5with that of HZSM-5to hydrolysis of methyl bromide to methanol, and measuring their surface acidity by ammonia-temperature programmed desorption, we preliminarily speculated that the active center of hydrolysis of methyl bromide to methanol was on ZnO, and the active center of methanol dehydration to DME was on HZSM-5. ZnO/HZSM-5had a dual function and was a bi-functional catalyst.