Tungsten trioxide-modified zeolite-based catalysts in the esterification of lactic acid: the effect of Si/Al ratio and WO3 loadings

Zeolite-supported tungsten oxide catalysts were studied in the gas phase esterification of lactic acid with alcohols in different reaction conditions. Two different types of commercial zeolites pore topology and acidity strength were investigated. The Y & mordenite zeolite-supported catalysts were prepared by wet impregnation method by varying the Si/Al ratios in Y-zeolites (Si/Al = 60) and mordenite (CBV-20A). The X-ray diffraction and FT-IR spectroscopy examined the phase analysis with a degree of crystallinity, and modes of different chemical bonds and compounds existed in the zeolites. The textural and total support acidity properties were evaluated by the N 2 sorption isotherms and ammonia temperature-programmed desorption (NH 3 -TPD) techniques. The type of alcohol reactant, WO 3 loadings, and the SiO 2 /Al 2 O 3 ratio had a profound effect on the esterification reaction performance. The Y zeolites with different SiO 2 /Al 2 O 3 ratios exhibited varied total acidity and activities. Over 10 wt.% WO 3 -loaded Y-zeolite with SiO 2 /Al 2 O 3 = 60 was found to be the most promising catalyst in terms of high activity and selectivity in lactic acid esterification by butanol reactant. The optimized reaction conditions were determined over the most selective 10W-760 catalyst and at 175 ºC temperature with feed molar ratio of 1:3 (lactic acid to butyl alcohol ratio), and flow rate of 1.0 mL/h; a highest butyl lactate yield and selectivity was obtained. Over 10 wt.% WO 3 Y-zeolite catalysts exhibited both optimal amount of weak and moderate acid sites, which play an important role in the selective formation of butyl lactate. However, the strong acid sites enhance the selectivity of the secondary reactions, which led to the byproduct’s formation and thus decrease the selectivity of butyl lactate.