Catalytic Mechanisms of Zirconium-Containing Active Sites over the SBA-15 Zeolite Framework for Xylose Conversion to Methyl Lactate
Zhou Huang, Lijuan Liu, Jinfeng Zhang, Hongmei Yang, Hongmei Yang, Shuai Fu, Ting‐Hao Liu, Hua‐Qing Yang, Hua‐Qing Yang, Changwei Hu
Abstract
Zr-containing SBA-15 zeolite (Zr-SBA-15) catalysts show good catalytic performance toward xylose conversion to methyl lactate (ML). Here, we report our study on the catalytic mechanisms for the conversion of β-xylopyranose and methanol to ML, glycolaldehyde, and water (1) over modeled −(SiO)3Zr(OH) ([Zr-1]) and −(SiO)2Zr(OH)2 ([Zr-2]) active sites on Zr-SBA-15. The gross reaction (1) mainly involves two kinds of reaction stages, that is, aldose–ketose or ketose–aldose tautomerization and retro-aldol condensation reaction. For the gross reaction (1), the rate-determining step is associated with the ketose–aldose tautomerization of deprotonated glycerosone to deprotonated glyceraldehyde, where the Lewis acidity of the Zr site plays a central role. For the retro-aldol condensation reaction, the rate-determining step is associated with the −C3–H bond formation, which relies on the ability of the H2O ligand providing the proton. Over the two active sites, the −OH group is in charge of the O–H bond cleavages, the resultant H2O ligand acts as not only a hydrogen resource for the C–O bond cleavages/formations and −C–H and −O–H bond formations but also a catalyst toward the keto–enol tautomerization, and the Lewis acidic Zr site is responsible for both aldose–ketose tautomerization and −C3–C4 bond cleavage in retro-aldol condensation of xylulose. [Zr-2] with two hydroxyls exhibits better catalytic activity than [Zr-1] with one hydroxyl, which stems from higher Lewis acidity of its Zr site. These findings may engineer the rational design and synthesis of environmentally friendly and recyclable heterogeneous catalysts toward the production of lactates from carbohydrates.