HYDROCARBON SYNTHESIS FROM CO HYDROGENATION OVER PD SUPPORTED ON SAPO MOLECULAR-SIEVES

Palladium impregnated on silicoaluminophosphate molecular sieves (SAPO-5, SAPO-11, SAPO-34) and alumina has been studied during CO hydrogenation. Pd/SAPO-5, which exhibited the highest turnover frequencies, initially produced only methane and oxygenates (methanol and dimethyl ether), but catalyzed significant C2+ hydrocarbon formation under the same conditions after temporary increase in gas contact time or exposure to propylene. While high yields of C2-C4 alkanes were observed over Pd/SAPO-34, reaction over Pd/SAPO-11 and Pd/Al2O3 yielded only oxygenates. Methane yields increased significantly with higher temperature, particularly over Pd/SAPO-5. Higher H2 partial pressures increased oxygenate production and decreased C2+ fractions. Many similarities between the results of this work and findings from studies of Pd/ZSM-5 indicate that hydrocarbon synthesis over Pd/SAPO catalysts occurs via a bifunctional pathway, involving methanol synthesis on Pd and autocatalytic conversion of oxygenates responsible for C2+ hydrocarbons on the SAPO molecular sieves. Differences in the product distributions from Pd-impregnated molecular sieves arise mainly from the effect of acidity and shape selectivity in methanol conversion on the SAPO molecular sieves. © 1990.