Catalytic Cracking of 1-Hexene to Propylene Using SAPO-34 Catalysts with Different Bulk Topologies

Three SAPO-34 catalysts, 100% SAPO-34, 30% SAPO-34, and meso-SAPO-34, with different bulk topologies were prepared. The catalysts were characterized by N2 adsorption, scanning electron microscopy, X-ray diffraction, and infrared spectroscopy techniques. The pore size, total acidity, and internal cage structure of the catalysts were almost identical, but they had different bulk appearances. The role of the bulk topology/structure of the catalysts was studied using 1-hexene cracking. On 30% SAPO-34, the surface acidity and diffusion rate decreased due to blocking by binder, which adversely affected catalytic activity. 100% SAPO-34 gave better cracking ability and higher propylene selectivity because of suitable acid sites and effective shape selectivity, respectively. In order to study the effect of diffusion, meso-SAPO-34 was used. The different bulk structure gave different feed conversion and selectivity profiles. A superior control of the stereochemistry was observed in the cracking by the meso-SAPO-34 and 100% SAPO-34 catalysts, in which enhanced diffusion mass transport played an appreciable role. Most of the propylene was produced by the direct cracking pathway by the β-scission carbenium ion mechanism. Hydrogen transfer reactions became significant at higher conversions. Decreasing the residence time to a certain extend is an appropriate way to obtain high propylene yield and selectivity. Activity and selectivity patterns for 1-hexene cracking to propylene were compared to justify superior SAPO-34 topology for 1-hexene cracking to propylene.