| The catalytic properties of ZSM-5 and silicoaluminophosphate (SAPO) molecular sieves, studied during CO hydrogenation over Pd/ZSM-5 and Pd/SAPO catalysts and during propylene conversion over ZSM-5 and SAPO catalysts, have been correlated with their physico-chemical characteristics, as determined by infrared spectroscopy, ammonia desorption experiments, and electron microscopy. Reaction rates and product distributions from syngas conversion over Pd/ZSM-5 were greatly affected by the zeolitic cation. While sodium and lanthanum cations enhanced the CO hydrogenation activity of the palladium sites, Pd/NaZSM-5 yielded primarily oxygenates (methanol and dimethyl ether), while Pd/HZSM-5 and Pd/LaZSM-5 catalyzed the formation of light alkanes and polymethylbenzenes. Synthesis of C{dollar}sb2{dollar}+ hydrocarbons from syngas occurred through a bifunctional pathway, which included palladium-catalyzed methanol synthesis followed by methanol conversion on strong acid sites within the ZSM-5 micropores. A similar route accounted for the formation of C{dollar}sb2{dollar}+ alkanes during CO hydrogenation over palladium-impregnated silicoaluminophosphate (SAPO) molecular sieves. The conversion of oxygenates to hydrocarbons over SAPO catalysts followed an autocatalytic pathway. For both Pd/ZSM-5 and Pd/SAPO catalysts, the rate of palladium-catalyzed olefin hydrogenation and the characteristics of the molecular sieves, including acidity, micropore diameter, and particle size, played an important role in determining the product distribution.; The physical and chemical properties of ZSM-5 and SAPO molecular sieves were also compared to their activity for propylene conversion. ZSM-5 catalysts possessing strong acid sites yielded the most products, primarily of C{dollar}sb4{dollar}-C{dollar}sb7{dollar} olefins. Despite attaining propylene conversion levels far below equilibrium predictions, the reaction over HZSM-5 and CoZSM-5 appeared to be thermodynamically controlled. SAPO molecular sieves, showing significantly less activity than the ZSM-5 catalysts, deactivated quickly. Mass transfer phenomena influenced the rate of catalyst deactivation as well as the product distribution over SAPO-5 and SAPO-34. Conversion of propylene over SAPO-11 yielded only hexene products. As with the CO hydrogenation studies, trends observed during propylene conversion over ZSM-5 and SAPO catalysts were directly related to specific structural or chemical properties of the molecular sieves. |
