Sulfonic Acid-Functionalized KIT-6 Solid Acid Catalysts for Glycerol Acetylation

Abstract

Blending biodiesel fuel (BDF) with petrodiesel effectively reduces CO₂ emissions in the transportation sector. The carbon footprint can be further minimized by converting crude glycerol, a byproduct of the BDF industry, into high-value products like triacetin, which can serve as a biofuel additive. This study demonstrated the acetylation of glycerol with acetic acid using propylsulfonic acid-functionalized KIT-6 mesoporous silica (PA-KIT-6) as a solid catalyst to produce triacetin. A series of PA-KIT-6 catalysts was synthesized by post-synthesis (grafting) and direct-synthesis (co-condensation) methods with varying sulfonic acid loading, using 3-mercaptopropyltrimethoxysilane (MPTMS) and hydrogen peroxide as precursor and oxidant. In-situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectra confirm that all catalysts contained Brønsted acid sites, which increased with sulfonic acid loading. The P-20PA-KIT-6 catalyst, with 0.25 mmol g⁻¹ sulfonic acid loading, exhibited the best performance. Glycerol was rapidly consumed, and 52% selectivity for triacetin was achieved using a 1:9 glycerol-to-acetic acid molar ratio and 2.83 wt.% catalyst at 115 °C for 8 h. This catalyst was reusable, maintaining its activity and selectivity over four cycles. This study highlights the intrinsic activity of sulfonic acid-based solid acids with Brønsted acidity in glycerol acetylation, effectively producing triacetin.