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

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.