Template/Al2O3 Ratio in Reaction Gels as a Tool to Control the Crystal Morphology, Crystal Dispersion, and Catalytic Performance in Hydroisomerization of n-Hexadecane over SAPO-11 Molecular Sieves

Abstract

Catalytic systems based on SAPO-11 molecular sieves are known for their superior selectivity in the hydroisomerization of higher (C₁₆₊) n-paraffins. However, further improvement of SAPO-11-based catalysts is impeded by the lack of effective tools for controlling the morphology and size of silicoaluminophosphate crystals. The present study investigates the effects of the DPA/Al₂O₃ molar ratio in reaction gels on the nature of intermediate phases and on the physicochemical properties of SAPO-11 molecular sieves. SAPO-11 has been found to crystallize through the formation of different phases depending on the DPA/Al₂O₃ ratio: an AlPO₄·2H₂O intermediate at DPA/Al₂O₃ = 1.0, a mixture of AlPO₄·2H₂O and a layered silicoaluminophosphate at DPA/Al₂O₃ = 1.4, and a layered silicoaluminophosphate alone at DPA/Al₂O₃ = 1.8. Proper adjustment of this ratio provides an effective tool to control the crystal morphology, crystal size, and the porous structure characteristics of the SAPO-11 molecular sieves. It has been identified that, at DPA/Al₂O₃ = 1.0, cubic and lamellar SAPO-11 nanocrystals 200–400 nm in size are formed. These nanocrystals have the following textural properties: S_BET = 286 m²/g; V_micro = 0.06 cm³/g; and V_meso = 0.21 m³/g. Furthermore, the effects of the morphology and size of SAPO-11 molecular sieve crystals on their catalytic performance in the hydroisomerization of n-hexadecane have been demonstrated. Promising catalytic systems based on SAPO-11 nanocrystals have been proposed for the hydroisomerization of C₁₆₊ n-paraffins.