Effect of H-ZSM-5 zeolite morphology on the performance of bifunctional ZnCr₂O₄/H-ZSM-5 catalysts in the direct conversion of syngas into aromatics

A series of H-ZSM-5 zeolites with different morphologies including spherical, hollow, sheet and sponge-strip forms were hydrothermally synthesized through elaborately controlling the synthesis conditions and their crystal structural, textural and acidic properties were characterized by XRD, SEM, Py-FTIR, NH₃-TPD, ICP and N₂-sorption. The H-ZSM-5 zeolites of different morphologies and a spinel ZnCr₂O₄ oxide were then used to compose the bifunctional ZnCr₂O₄/H-ZSM-5 catalysts for the direct conversion of syngas into aromatics (STA). The effect of H-ZSM-5 morphology on the catalytic performance of ZnCr₂O₄/H-ZSM-5 in STA was then investigated. The results indicate that the morphology of H-ZSM-5 zeolites has significant influences on the catalytic performance of ZnCr₂O₄/H-ZSM-5 in STA.The selectivity to aromatics over the bifunctional catalysts with different H-ZSM-5 morphologies follows the order of sphere > sponge-strip > hollow > sheet. In particular, the ZnCr₂O₄/H-ZSM-5(sphere) catalyst composed of ZnCr₂O₄ and spherical H-ZSM-5 exhibits excellent performance in STA; under 350℃ and 3.0 MPa, a high selectivity of 68.8% to aromatics is achieved, with a CO conversion of 12.6%, whereas the selectivities to CH₄, C_2-4⁰ alkanes and CO₂ decrease to 1.3%, 14.3% and 41.4%, respectively. The isotropic and moderate particle size (about 350 nm) with appropriate pore length of spherical H-ZSM-5 zeolite are capable of avoiding the formation of lower hydrocarbons from early diffusion out of the acid zeolite channels and meanwhile yet conducive to the diffusion of aromatics, which can promote the aromatization of intermediates in STA and enhance the selectivity to aromatic products.