(A) study on the characteristics of hybrid catalysts in $CO_2$ hydrogenation for hydrocarbon synthesis

Abstract

In $CO_2$ hydrogenation over $Cu/ZrO_2$ based catalysts, $ZrO_2$ enhanced the methanol formation activity by increasing Cu surface area. The reaction intermediates of methanol synthesis were carbonate, formate, formaldehyde and/or methoxy, and the rate determining step for methanol synthesis seems to be the conversion of formate into formaldehyde or methoxy. Synthesis of oxygenates from $CO_2$ hydrogenation was investigated over hybrid catalyst composed of methanol synthesis catalyst and zeolite. A hybrid catalyst composed of $Cu/ZnO/ZrO_2$ and H-Ga-Sililcate showed highest oxygenates yield. Reducibility of hybrid catalysts was modified due to a solid-solid interaction and was found to affect the yield of oxygenates. Methanol conversion into hydrocarbons was investigated by using SAPO-34``s, which were modified by various pretreatments. The acidity was controlled by changing the $SiO_2/Al_2O_3$ ratio, by different (hydro) thermal treatments, by preadsorbing $NH_3$ and/or by depositing TEOS. However, all the pretreated catalysts exhibited almost the same activities and selectivities, and the acid strength difference induced by above treatments did not seem to alter hydrocarbon pool over SAPO-34. The reaction parameters such as the methanol partial pressure and the water content in the feed, together with coke formation, had a major impact on the selectivity pattern. Direct syntheses of hydrocarbons from $CO_2$ hydrogenation were investigated over hybrid catalysts consisting of methanol synthesis catalyst $(CuZnOZrO_2)$ and zeolites (MFI or SAPO). The Cu component from methanol synthesis catalyst seemed to migrate onto zeolite``s Broensted acid sites. This kind of solid-solid interaction between methanol synthesis catalyst and zeolite resulted in medium acid sites regarded as active sites for hydrocarbon synthesis. And the deactivation of hybrid catalyst was due to the reduction in the number of medium acid sites and in Cu surface area. While the yield of hydroca...