Hierarchical SAPO-11 prepared using SBA-15 as the silicon source and its application in n-dodecane hydroisomerization

SBA-15 with the removal of template agent, which served as both the silicon source and indirect template agent, was used to hydrothermally synthesize hierarchical SAPO-11 molecular sieve. The crystal structure, morphology, acidity and texture of the samples were characterized by XRD, SEM, FT-IR and N₂ physical adsorption. The results showed that the pure SAPO-11 can be obtained by using calcined SBA-15 as the silicon source. At the same time, SBA-15 was completely transformed in the synthesis system. The obtained SAPO-11 sample showed a hollow near-column shape with particle size of about 1-3 μm, which was composed of bar-shape structure with a width of about 100 nm. Compared with conventional SAPO-11 synthesized by adopting white carbon black or colloidal silica as the silicon source, the addition of SBA-15 successfully introduced mesoporous channels with pore size of 5-10 nm into SAPO-11 molecular sieve. Moreover, the proportion of moderate and strong Brønsted acid was larger than that of weak Brønsted acid. Finally, the catalytic behaviors of Pt/SAPO-11 bifunctional catalysts were investigated in the hydroisomerization of n-dodecane. The results indicated that the hierarchical catalyst synthesized with SBA-15 was much more active and selective. The excellent isomerization performance was closely related to the acidity and pore structure of the hierarchical SAPO-11 molecular sieve. The increase of moderate and strong Brønsted acidity contributed to the improvement of activity. Meanwhile, the mesopores were conducive to the significant increase of selectivity via accelerating diffusion of the isomerization products.