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.