PRODUCT SELECTIVITY IN METHANOL TO HYDROCARBON CONVERSION FOR ISOSTRUCTURAL COMPOSITIONS OF AFI AND CHA MOLECULAR-SIEVES

Isostructural molecular sieves based upon AlPO4 and SiO2 chemistry were made for comparison of catalytic selectivity. The AFI and CHA structures were compared with B and Al substitution of SiO2 and Mg and Si substitution in the AlPO4 case. The conversion of methanol to hydrocarbons was studied. Materials were characterized for acidity by NH3 TPD and NH3 microcalorimetry. Methanol conversion was carried out with products analyzed by GC-MS and spent catalysts by C-13 MAS NMR. Borosilicate sieves have acidity too low to carry out this catalytic transformation. Other substituting components were successful but product selectivity seemed to be governed by geometric features of the sieves, rather than by variable acidity due to different types of lattice substitution. Products from small pore molecular sieves SAPO-34 and SSZ-13 were largely olefinic and comprised of C5 and smaller. The large pore sieves, SAPO-5, MAPO-5, and SSZ-24, all produced aromatic-rich products. A considerable quantity of the recovered hydrocarbon was incorporated into penta- and hexamethylbenzene.