Tuning the Structural, Acidic, and Catalytic Properties of SAPO-11 by Varying the SiO2/Al2O3 Ratio in a Boehmite-Based Reaction Gel

The rational design of highly efficient bifunctional SAPO-11 catalysts for hydroisomerization of n-C16 requires unprecedented control over both acidic properties and diffusion characteristics. This work systematically investigates the influence of the SiO2/Al2O3 molar ratio (0.1–0.4) in the initial gel on the physicochemical and catalytic properties of SAPO-11. Using a combination of characterization techniques (XRD, SEM, TEM-SAED, 29Si MAS NMR, and IR-Py), it was established that this parameter serves as a simple tool for crystal engineering. The concentration of Brønsted acid sites and the external surface area demonstrate a non-linear dependency, reaching their maximum at SiO2/Al2O3 = 0.3. Further increase in silicon content reduces both crystallinity and acidity due to the transition to the dominant SM2 + SM3 incorporation mechanism and the formation of silicon islands. Notably, varying the SiO2/Al2O3 ratio enables control over crystal morphology—progressing systematically from truncated cones (SiO2/Al2O3 = 0.1) to flat prismatic platelets (SiO2/Al2O3 = 0.2) and ultimately hierarchical spherical aggregates (SiO2/Al2O3 = 0.4). In n-C16 hydroisomerization, the Pt/SAPO-11(0.2) catalyst demonstrated the highest yield of i-C16 compared to other samples reaching 81%. The platelet morphology ensures a minimal diffusion path (<100 nm), effectively suppressing secondary hydrocracking. This finding underscores that morphology optimization is more critical than maximizing acidity for achieving high selectivity in the context of n-C16 hydroisomerization over Pt/SAPO-11.