Tailoring the Secondary Porous Structure and Acidic Properties of Micro-mesoporous SAPO-11 by the Silica Source for Efficient Hydroisomerization of n-hexadecane

Abstract The efficiency of hydroisomerization catalysts for higher n-paraffins based on SAPO-11 molecular sieve is significantly affected by diffusion limitations in the micropores, which reduces their activity, selectivity, and operational stability. To address this issue, this study investigates the influence of silicon sources with different dispersity (3, 20, 200 nm) on the physicochemical and catalytic properties of SAPO-11 synthesized using aluminum isopropoxide. The research employs a suite of characterization techniques, including X-ray fluorescence analysis, X-ray diffraction, 29Si magic-angle spinning nuclear magnetic resonance spectroscopy, scanning electron microscopy, nitrogen adsorption–desorption, ammonia temperature-programmed desorption, and infrared spectroscopy of adsorbed pyridine. It is demonstrated that a decrease in the size of the applied SiO2 particles leads to an increase in the total concentration of acid sites, changes in morphology, a reduction in crystal size, and an alteration of the secondary porous structure properties. The SAPO-11 samples synthesized using a SiO2 sol with an average particle size of 3 nm are characterized by the most developed hierarchical porous structure (SBET = 269 m2/g, SEX = 68 m2/g, Vmeso = 0.14 cm3/g). The bifunctional catalysts based on the synthesized SAPO-11 materials were tested in the hydroisomerization of n-hexadecane. The silicoaluminophosphates synthesized using the SiO2 sol with an average particle size of 3 nm provided the highest n-hexadecane conversion and C16 isomer selectivity among the studied samples. This performance enabled a total yield of C16 isomers exceeding 80%, which is attributed to a higher concentration and enhanced accessibility of strong acid sites. Thus, controlling the silicon source during the SAPO-11 synthesis is an effective strategy for the rational design of high-performance hydroisomerization catalysts for higher n-paraffins.