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

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.

Rational Positioning of Metal Ions to Stabilize Open Tin Sites in Beta Zeolite for Catalytic Conversion of Sugars

“…The results show the highest MLA yield (47.4 %) for the Sn‐only catalysts, which is achieved over Sn‐Beta‐F, while the highest MVG yield (17.9 %) is reached over Sn‐Beta‐PS. It is well known that both MLA and MVG are derived from the C3 and C4 (C2) products produced by the retro‐aldol condensation of fructose and glucose, respectively, and subsequently obtained by similar chemical transformations (e.g., dehydration, keto‐enol tautomerization, 1,2‐hydride shift, and internal Cannizzaro reaction) [25] . Early work has shown that the formation of MVG from C4 sugars can be stimulated by catalytic sites in confined space and operating at higher temperature [26] .…”

“…It is well known that both MLA and MVG are derived from the C3 and C4 (C2) products produced by the retro-aldol condensation of fructose and glucose, respectively, and subsequently obtained by similar chemical transformations (e.g., dehydration, keto-enol tautomerization, 1,2-hydride shift, and internal Cannizzaro reaction). [25] Early work has shown that the formation of MVG from C4 sugars can be stimulated by catalytic sites in confined space and operating at higher temperature. [26] The selectivity difference between these two products in this work, starting from glucose, may arise from the different glucose isomerization abilities of the catalysts.…”

Valorizing natural-abundant glucose to lactic acid using a MOF-808 catalyst under green hydrothermal conditions

“…Interestingly, the notably high yield of LA could be attributed to the accessible catalytic single-sites of UiO-66 with an ability to convert the C2 residues to LA. Several efforts towards the exploration of other Zr-based materials, such as ZrO 2 , 26 metal–organic frameworks (MOFs), 27 and Zr-doped zeolites, 28 have also been devoted to selectively producing LA from xylose. However, when considering large-scale production, utilizing xylose as a platform chemical is relatively high cost compared with the more abundant and economical C6 glucose.…”