Selective synthesis of C₃-C₄ hydrocarbons through carbon dioxide hydrogenation on hybrid catalysts composed of a methanol synthesis catalyst and SAPO

Direct synthesis of hydrocarbons through carbon dioxide hydrogenation was investigated over hybrid catalysts composed of methanol synthesis catalysts (Cu/ZnO/ZrO₂ and Cu/ZnO/Al₂O₃) and molecular sieves (H-ZSM-5, SAPO-5 and SAPO-44). It was found that the hybrid catalyst with SAPO-5 or SAPO-44 was effective for the synthesis of C₂₊ hydrocarbons. The high hydrocarbon yield appears to be due to the abundance of weak- and medium-strength acid sites in SAPO, which could be evidenced through temperature-programmed desorption of ammonia. The product distribution of hydrocarbon products was influenced by the acidity as well as the pore size of the molecular sieves. The selectivity to isobutane was the highest on the hybrid catalysts with SAPO-5. Propane was the main product on the hybrid catalyst with SAPO-44. Carbon dioxide conversion increased with reaction temperature, but a maximum yield of C₂₊ hydrocarbon was obtained at 340°C. An increase in contact time lowered the carbon monoxide formation and increased the hydrocarbon formation. Addition of carbon monoxide or ethene to the feed increased the hydrocarbon yield. The reaction pathway to hydrocarbons is thought to be composed of methanol synthesis from carbon dioxide and hydrogen, methanol/dimethyl ether to lower alkene, alkene oligomerization, isomerization and hydrogenation to alkane.