(A) study on CO hydrogenation over Pd/HZSM-5 catalysts and $CO_2$ hydrogenation over copper-SAPO hybrid catalysts

Abstract

Pd/HZSM-5 catalysts prepared by ion-exchange method using $Pd(NH_3)_4^{2+}$ were calcined and reduced at different temperatures to provide different metal dispersions. The effect of Pd dispersion on CO adsorption characteritics were investigated through FT-IR, TPD and isotopic tracer method. The C-O bond becomes stronger when the metal dispersion is higher, which favored the formation of oxygenates from CO and $H_2$ due to the inhibition of C-O dissociation. Methanol and dimethyl ether were main products in CO hydrogenation over Pd/HZSM-5 catalyst with small Pd particles on which CO was weakly adsorbed, while the selectivity to methane increased with metal sizes. FT-IR technique under reaction condition shows that formate species as well as methoxy species were the intermediates of methanol formation. Direct synthesis of hydrocarbon through carbon dioxide hydrogenation was investigated over hybrid catalysts composed of methanol synthesis catalysts (Cu/ZnO/$ZrO_2$ and Cu/ZnO/$Al_2O_3$) and molecular sieves(HZSM-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 $C_{2+}$ hydrocarbon. The high hydrocarbon yield seems to be due to the abundance of the weak- and medium-strength acid sites in SAPO, which could be evidenced through temperature-programmed desorption of ammonia. The product distribution of hydrocarbon product was altered by the acidity as well as 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. $CO_2$ conversion increased with reaction temperature, but maximum yield of $C_{2+}$ hydrocarbon was obtained at $340^\circ C$. An increase in contact time lowered the CO formation and increased the hydrocarbon formation. Addition of CO or $C_2H_4$ to the feed increased the yield of hydrocarbon. The reaction pathway to hydrocarbons is thought to be composed of met...