(283a) Effect of a Supercritical Isooctane Reaction Medium on the Conversion of Methanol to Hydrocarbons over H-ZSM-5

Optimizing the production of fuels and chemicals from carbon resources other than petroleum has been an area of ongoing investigation for more than eighty years. By steam reforming natural gas or by gasifying coal or biomass, a mixture of carbon monoxide and hydrogen known as syngas is produced, which can be subsequently converted to hydrocarbon fuels, heavy waxes, or oxygenated compounds, including methanol. Methanol synthesis from syngas proceeds with high selectivity, and further upgrading of methanol over solid-acid catalysts can selectively produce dimethyl ether, light olefins or fuel-range hydrocarbon species, depending upon the catalyst type and operating conditions. High yields of high-octane gasoline have been achieved by carrying out the methanol-to-hydrocarbons (MTH) reaction over zeolite catalyst H-ZSM-5. However, catalyst deactivation due to coking as well as heat management challenges due to the exothermic nature of the reaction hinder utilization of MTH technology. Our group, among others, have previously demonstrated the benefits of employing supercritical fluids (SCFs) as reaction media in heterogeneous catalysis in order to facilitate heat removal and heavy product extraction from catalyst active sites. This particular study explores the feasibility of using SCF media to enhance MTH reaction performance relative to traditional, gas-phase operation. Conversion of methanol over H-ZSM-5 was carried out in a fixed-bed reactor in the presence of supercritical isooctane. Isooctane was selected as the SCF medium primarily because of its low reactivity over H-ZSM-5. The performance of SCF-assisted MTH was compared with that of gas-phase MTH at both 300°C and 370°C. Relative to gas phase operation at the same conditions, the use of supercritical isooctane resulted in improved catalyst maintenance and increased hydrocarbon productivity with increasing time on stream.