Catalytic conversion of d-glucose into lactic acid with Ba(OH)2 as a base catalyst:mechanistic insight by NMR techniques

Abstract

Lactic acid (2-hydroxypropionic acid, LA) can be produced from abundant and renewable biomass resources via chemocatalytic processes, which exhibited high potential for meeting its tremendous demand in a wide range of applications. The basic Ba(OH)2 could efficiently catalyze the conversion of d-glucose into LA. In the present study, by utilizing various NMR techniques including, chemical shift titration, 2D diffusion ordered spectroscopy (DOSY) analysis, heteronuclear singular quantum correlation (1H–13C HSQC), the binding sites and catalytic interaction between d-glucose and catalyst were clarified. The experimental results by NMR and electrospray ionization mass spectrometry (ESI–MS) were used to explore the reaction mechanism. Experimental results disclosed that the catalyst, Ba(OH)2, may exist in the form of [Ba(OH)(H2O)2]+ under the reaction condition. The OH– of [Ba(OH)(H2O)2]+ is attached to the C5-O of d-glucose, and Ba2+ is connected to the C1-O of d-glucose. Herein, we suppose that Ba(OH)2 and d-glucose interact and form a complex, d-glucopyranose-[Ba(OH)(D2O)2]+. Furthermore, when the reaction was performed in D2O, we were surprised to find that LA underwent varying degrees of deuteration. The reaction mechanism, binding sites and deuteration of the product have shed light on the efficient conversion of biomass to LA.