Yingxia Li

Highly active tin(IV) phosphate phase transfer catalysts for the production of lactic acid from triose sugars

Lactic acid (LA) is an important intermediate in the fine chemical industry because it is utilized as a building block for the production of biodegradable plastics. In this study, a series of tin phosphate phase transfer catalysts modified with several surfactants have been prepared by a facile one-pot synthesis method and tested for the direct conversion of trioses to LA under hydrothermal conditions. Poly(ethylene glycol) (PEG) was identified as the most promising surfactant, and the product distribution closely depended on the reaction temperature, catalyst loading and substrate concentration. Complete DHA conversion and a good yield of up to 96.1% of LA were obtained at 140 °C after 4 h of reaction time. Pyridine FTIR demonstrated the presence of Bronsted and Lewis acid sites, which play crucial roles in the dehydration of DHA to pyruvaldehyde (PA) and the following isomerization of generated PA to LA. Furthermore, the isomerization of PA to LA was found to be the rate-determining step. A possible reaction mechanism was proposed: 1) the coordination between PEG and the metal ions caused a greater separation of the tin ions from the phosphate anions, making them more potent Lewis acid sites, and 2) the PEG behaved as a phase transfer catalyst during the reaction. This study paves the way for the further design of improved solid acid catalysts for aqueous phase production of LA from carbohydrates.

Siliceous tin phosphates as effective bifunctional catalysts for selective conversion of dihydroxyacetone to lactic acid

Methods to catalytically convert carbohydrates into lactic acid (LA), which is a versatile platform chemical, have been widely investigated. In this study, siliceous tin phosphates were utilized as reusable Bronsted–Lewis acid bifunctional catalysts during the conversion of 1,3-dihydroxyacetone (DHA) to LA under hydrothermal conditions. The product distribution closely depended on the reaction temperature, catalyst loading and substrate concentration. The highest LA yield of 93.8% was achieved with a complete DHA conversion at 140 °C after 5 h. The reaction was facilitated by the vast presence of Bronsted and Lewis acid sites that were confirmed by both pyridine FTIR and NH3-TPD analysis. The incorporation of silica significantly lowered the Sn content and improved the thermal stability of the tin phosphate catalysts. A possible reaction mechanism was proposed in that the Lewis and Bronsted acid sites synergistically catalyzed the conversion of pyruvaldehyde to LA, which was found to be the rate-determining step. The method allows for facile catalyst separation and recycling while expanding the applicability of silica in the field of biomass-to-chemical conversion.

Niobium phosphotungstates: excellent solid acid catalysts for the dehydration of fructose to 5-hydroxymethylfurfural under mild conditions

The efficient conversion of carbohydrates to 5-hydroxymethylfurfural (5-HMF) under mild conditions represents a very attractive and promising method of producing important building blocks. In this work, niobium phosphotungstates, with Nb/P molar ratios of 0.6, 1.0, 2.0 and 4.0 (NbPW-06, NbPW-1, NbPW-2, and NbPW-4, respectively) have been prepared by a facile, one-pot, alcohol-mediated thermal process and used for the direct conversion of fructose to 5-HMF. By adding a certain amount of Nb, the surface of the catalyst became enriched in P, and this enrichment was associated with the presence of surface P–OH groups that offered Bronsted acid sites that can activate superficial hydrogen species to facilitate 5-HMF generation. Pyridine-FTIR confirmed the presence of Bronsted and Lewis acid sites, which might play important roles in the dehydration of fructose to 5-HMF. Furthermore, polar aprotic solvents were well-suited for the conversion, and higher yields of 5-HMF were obtained in polar aprotic solvents than in nonpolar solvents. A 5-HMF yield of 96.7% with complete fructose consumption was obtained over NbPW-06 in DMSO at 80 °C after 90 min. In addition, NbPW-06 could be recycled several times without a significant decrease in the catalytic activity. A catalytic mechanism for this reaction was proposed. Moreover, this catalytic system can also be utilized for the dehydration of sucrose and inulin to 5-HMF in satisfactory yields. This study establishes an important platform for the further design of Nb-containing catalysts for the production of 5-HMF from carbohydrates under mild conditions.