Catalytic Mechanisms of Zirconium-Containing Active Sites over the SBA-15 Zeolite Framework for Xylose Conversion to Methyl Lactate

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.