Controlling Secondary Porosity and Acidity in Micro–Mesoporous SAPO-11 via Silica Content for Efficient n-Hexadecane Hydroisomerization

Micropore diffusion limitations impair the performance of SAPO-11-supported bifunctional catalysts in the hydroisomerization of long-chain n-paraffins (C7+), reducing their activity, selectivity, and stability. These diffusion constraints can be mitigated by introducing secondary porosity and reducing the crystallite size. This study investigates the effect of the silica content (varied via the SiO2/Al2O3 molar ratio) on these parameters. The physicochemical properties of the synthesized samples were characterized by XRD, SEM, 29Si MAS NMR, NH3-TPD, and some other methods. Increasing the SiO2/Al2O3 ratio was shown to reduce the size of primary nanocrystals and generate a well-developed mesoporous structure. At SiO2/Al2O3 ratios ≥ 0.3, the extent of silicon incorporation and the concentration of acid sites reach a maximum, while a decrease in crystallinity was observed. The most developed hierarchical porous structure, featuring an external surface area of 67 m2/g and a mesopore volume of 0.19 cm3/g, was achieved at a SiO2/Al2O3 ratio of 0.1. In the hydroisomerization of n-hexadecane, the sample synthesized with a SiO2/Al2O3 ratio of 0.3—which exhibited an optimal combination of high acidity and small nanocrystal size—achieved the highest conversion and i-C16 selectivity. Therefore, fine-tuning the silica content is an effective strategy for the targeted design of high-performance hydroisomerization catalysts.