Pd/CNT-promoted Cu-ZrO2/HZSM-5 hybrid catalysts for direct synthesis of DME from CO2/H2

摘要

A type of bi-functional hybrid catalyst of Pd-decorated CNT-promoted Cu-ZrO2 admixed with HZSM-5 zeolite was developed, and displayed excellent performance for the direct synthesis of dimethyl ether (DME) from CO2/H2 via CH3OH in a single fixed-bed continuous flow reactor. Over a CuZr-PdCNTs/HZSM-5 hybrid catalyst under reaction conditions of 5.0 MPa, 523 K, V(H2)/V(CO2)/V(N2)=69/23/8, GHSV=25,000 mL_(STP)/(h g-hydr. catal.), the observed specific rate of CO2 hydrogenation-conversion reached 0.39 u,moI/(s m2-Cu°-SA), which was 1.22 times that of the corresponding Pd/CNTs-free counterpart CuZr/HZSM-5. The addition of a minor amount of the Pd-decorated CNTs into the CuZr/HZSM-5 host catalyst caused little change in the apparent activation energy for CO2 hydrdgenation, but led to an increase of metal Cu exposed area (catalytically active Cu surface-sites closely associated with the CO2 hydrogenation to methanol) and marked improvement of adsorption performance of the catalyst for H2 and CO2. The latter would help generate a micro-environment with higher concentration of active H and CO2 adspecies at the surface of the functioning catalyst, thus increasing the rate of the surface hydrogenation reactions. The present study also demonstrated that combining the methanol-synthesis and methanol-dehydration-to-DME processes in heterogeneous "one-pot" reactions by using the bi-functional hybrid catalyst could indeed enhance the driving force for CO2 hydrogenation conversion.