Synthesis of ZSM-22/SAPO-11 composite molecular sieve-based catalysts with small crystallites and superior n-alkane hydroisomerization performance

To improve oil quality, ZSM-22/SAPO-11 composite molecular sieves were synthesized by adding ZSM-22 into a synthetic gel of SAPO-11 for n-decane hydroisomerization. The mass ratios of ZSM-22/(ZSM-22+SAPO-11) in the composite molecular sieves were optimized and the optimal ZSM-22/SAPO-11 composite (ZS-9) was obtained. The electrostatic repulsions between the ZSM-22 precursors and the SAPO-11 crystalline nuclei produced small ZSM-22 and SAPO-11 crystallites in ZS-9, which increased the specific surface area and mesopore volume and thereby exposed more acid sites. In comparison with conventional SAPO-11, ZSM-22 and their mechanical mixture, ZS-9 with smaller crystallites and the optimal medium and strong Brønsted acid centers (MSBAC) content displayed a higher yield of branched C10 isomers (81.6%), lower cracking selectivity (11.9%) and excellent stability. The correlation between the i-C10 selectivity and the MSBAC density of molecular sieves indicated that the selectivity for branched C10 isomers first increased and then decreased with increasing MSBAC density on the molecular sieves, and the maximum selectivity (87.7%) occurred with a density of 9.6 × 10−2 μmol m−2.