Conversion of methanol to hydrocarbons over silica-alumina. Selective Formation of Lower Olefins

The catalytic conversion of methanol to hydrocarbons, particularly to the lower olefins, has been studied in a fixed-bed flow reactor over amorphous silica-alumina and over silica-alumina containing small amounts of zeolite. The amorphous silica-alumina catalysts were prepared by the cogelation method such that the alumina content varied from 1.5 to 30 wt.-%. The composite catalysts were prepared by adding crystallites of HAlZSM-5 (SiO₂/Al₂O₃=35) during the cogelation procedure. In each of these composite catalysts the silica-alumina matrix contained 15 wt.-% alumina whereas the ratio of the HAIZSM-5 to the silica-alumina matrix varied from 4 to 19 wt.-%. The conversion of methanol to hydrocarbons other than dimethyl ether at 375°C, 1 atm and 1 h^-1 space velocity, over the synthesized amorphous silica-alumina catalysts increased from 22.5 to 96.5% as the alumina content increased from 5 to 20 wt.-%. The selectivity to C₂-C₄ olefins was essentially constant (62 to 65%) over this range of SiO₂/Al₂O₃ ratios. These amorphous silica-alumina catalysts deactivated significantly over a period of 15 h. When small amounts (5 to 10 wt.-%) of HAlZSM-5 zeolite were added to the silica-alumina matrix having 15 wt.-% Al₂O₃, a more stable and active catalyst for the conversion of methanol to lower olefins was obtained.