Surface amino-functionalization of Sn-Beta zeolite catalyst for lactic acid production from glucose

The catalytic conversion of glucose into lactic acid (LA) provides an alternative approach for LA production to bio-fermentation. Nevertheless, during the process, the dehydration of glucose and fructose into 5-hydroxymethylfurfural catalyzed by Brønsted acid exists as the main side reaction that leads to the decrease in LA yield. In order to promote the yield and selectivity of LA, a series of acid-base bifunctional Sn-Beta-NH₂ catalysts were prepared by post-grafting aminopropyl groups with the surface silanol groups of Sn-Beta zeolite. The catalysts were systematically characterized by X-ray diffraction, N₂ adsorption-desorption, elemental analysis, X-ray photoelectron spectroscopy, Fourier transform infrared spectra analysis following pyridine adsorption, and CO₂ temperature-programmed desorption. The as-prepared Sn-Beta-NH₂ catalysts exhibiting both Lewis acid and moderate base sites facilitated the conversion of glucose to LA in competition with undesirable side reactions. In addition, effects of reaction parameters including reaction temperature and time, catalyst dosage, and glucose concentration were investigated. A high LA yield up to 56% was achieved under optimized hydrothermal conditions (190 °C, 2 h), along with a complete conversion of glucose and a 5-hydroxymethylfurfural yield of 7%. The result indicated an alternative modification method of Sn-Beta zeolite for a more favorable LA yield.