Syngas conversion to gasoline-range hydrocarbons over Pd/ZnO/Al 2 O 3 and ZSM-5 composite catalyst system

A composite Pd/ZnO/Al2O3–HZSM-5 (Si/Al=40) catalytic system was evaluated for the synthesis of gasoline-range hydrocarbons directly from synthesis gas. A bifunctional catalyst comprising PdZn metal and zeolitic acid sites provides the required catalytically active sites necessary for the methanol synthesis, methanol dehydration, and dimethyl ether-to-gasoline reactions. Using a molar syngas H2/CO feed ratio of 2, the effects of temperature (310–375°C), pressure (300–1000psig), and gas hourly space velocity (740–2970h−1) were investigated. The liquid hydrocarbon product provided by the Pd/ZnO/Al2O3+ZSM-5 composite catalyst is aromatic-rich, and contains a significant amount of methylated benzenes. Catalytic stability was favorable due to the presence of hydrogen in the syngas, thus mitigating coke formation within the zeolite. When ZSM-5 is replaced by zeolite-Y (Si/Al=15), the aromatic content of the hydrocarbon liquid markedly decreased while branched and cyclic hydrocarbons increased. The Pd/Zn/Al2O3 catalyst was found to be highly stable and resistant to sintering under the conditions of the testing, in contrast to the industry standard Cu/ZnO/Al2O3 methanol catalyst. Yield to C5+ liquid hydrocarbon product was limited by alternative syngas conversion pathways (water gas shift, methanation) and by hydrogenation of light olefins that would otherwise convert to a liquid hydrocarbon product.