Water-tolerant Lewis Acid Sites Research Articles

Generation and nature of water-tolerant Lewis acid sites in InxSn10−xOy/Al2O3 catalysts as active centers for the green synthesis of methyl lactate from glucose

The improvement of methyl lactate yield was achieved by constructing water-tolerant Lewis acid sites, which were generated by reducing hydroxyl groups and increasing coordinatively unsaturated sites.

Design of Nickel Supported on Water-Tolerant Nb2O5 Catalysts for the Hydrotreating of Lignin Streams Obtained from Lignin-First Biorefining

SummaryIn biomass conversion, Nb2O5 has attracted increasing attention as a catalyst support presenting water-tolerant Lewis acid sites. Herein, we address the design of Ni/Nb2O5 catalysts for hydrotreating of lignin to hydrocarbons. To optimize the balance between acidic and hydrogenating properties, the catalysts were first evaluated in the hydrotreating of diphenyl ether. The best catalyst candidate was further explored in the conversion of lignin oil obtained by catalytic upstream biorefining of poplar. As primary products, cycloalkanes were obtained, demonstrating the potential of Ni/Nb2O5 catalysts for the lignin-to-fuels route. However, the Lewis acidity of Nb2O5 also catalyzes coke formation via lignin species condensation. Thereby, an acidity threshold should be found so that dehydration reactions essential to the hydrotreatment are not affected, but the condensation of lignin species prevented. This article provides a critical “beginning-to-end” analysis of aspects crucial to the catalyst design to produce lignin biofuels.

A Density Functional Theory Study of the Mechanism of Direct Glucose Dehydration to 5‐Hydroxymethylfurfural on Anatase Titania

AbstractA new mechanism for glucose dehydration to HMF without the intermediate isomerization to fructose is discussed for surface models of anatase TiO2 using periodic density functional theory calculations. Activation of the glucose at glucose's C3‐OH position by titania starts the reaction resulting in adsorbed 3‐deoxyglucosone. The mechanistic study reveals two possible pathways for the acyclic form of glucose. Comparison of different surface models shows that the presence of tetrahedral Ti4+ species on a defective TiO2(101) anatase surface is essential for explaining the activity. Synergy between a strong Lewis acidic Ti site and a vicinal basic oxygen site of a TiOH group is essential to establish the direct conversion of glucose to HMF. IR spectroscopy of adsorbed chloroform in the presence of water confirms the presence of water tolerant Lewis acid sites in close proximity to basic sites. In conventional glucose dehydration, Lewis acid sites and proton donors act as catalytic sites for efficient sequential isomerization‐dehydration to HMF, while direct dehydration to HMF requires the cooperation of Lewis acid‐base pairs.

Discussing Lewis and Brønsted acidity on continuous pyruvaldehyde Cannizzaro reaction to lactic acid over solid catalysts

The eventual role played by Brønsted and Lewis acid sites on pyruvaldehyde Cannizzaro reaction to lactic acid was investigated in a flow reactor using different catalysts bearing different acidity. It was shown that Lewis acid sites on heterogeneous catalysts are crucial for the reaction while Brønsted sites play no role at all. The results revealed that catalytic activity is indeed directly related to catalyst acidity. TiO2, ZrO2 and Nb2O5 were the most promising catalysts and their activity was related to their water-tolerant Lewis acid sites. Nb2O5 was shown to be the most active and stable catalyst while ZnO underwent a fast deactivation process ascribed to the formation of amorphous carbonaceous materials as revealed by TG and laser Raman spectroscopy.

Highly Stable Nb2O5–Al2O3 Composites Supported Pt Catalysts for Hydrodeoxygenation of Diphenyl Ether

Various TiO2, SiO2, Al2O3, and Nb2O5–Al2O3 supported Pt catalysts have been prepared by urea precipitation method for catalytic hydrodeoxygenation (HDO) of diphenyl ether (DPE) as a 4-O-5 aryl-ether lignin model compound. The selectivity toward deoxygenated product cyclohexane increased obviously with Nb2O5·nH2O decorated, owing to the significant promotion effect of NbOx species and acid sites on C–O bond cleavage. At higher pressure (3.0 MPa H2), DPE underwent a HYD route, while direct hydrogenolysis route occurred at low pressure (0.1 MPa H2). In addition, the reaction rate constants and activation energies were obtained in the temperature range from 160 to 220 °C. Based on the Arrhenius law, the activation energy for the cleavage of the C–O bond in DPE was calculated to be 91.22 kJ/mol. It was noteworthy that the Pt/20Nb–Al2O3 showed higher stability than Pt/Al2O3 for hydrodeoxygenation of diphenyl ether, which can be attributed to its water-tolerant Lewis acid sites.

Photoassist-phosphorylated TiO2 as a catalyst for direct formation of 5-(hydroxymethyl)furfural from glucose.

Photo-assisted phosphorylation of an anatase TiO2 catalyst was examined to improve its catalytic performance for the direct production of 5-(hydroxymethyl)furfural (HMF), a versatile chemical platform, from glucose. In phosphorylation based on simple esterification between phosphoric acid and surface OH groups on anatase TiO2 with water-tolerant Lewis acid sites, the density of phosphates immobilized on TiO2 is limited to 2 phosphates nm-2, which limits selective HMF production. Phosphorylation of the TiO2 surface under fluorescent light irradiation increases the surface phosphate density to 50%, which is higher than the conventional limit, thus preventing the adsorption of hydrophilic glucose molecules on TiO2 and resulting in a more selective HMF production over photoassist-phosphorylated TiO2.

Cooperative action of Brønsted and Lewis acid sites of niobium phosphate catalysts for cellobiose conversion in water

The lively acidic properties in water of niobium phosphate (NBP) catalyst and NBP modified samples by acidic treatments in HCl solutions at 0.1, 1.0, and 10M concentration (NBP01, NBP1, and NBP10, respectively) have been exploited for the direct conversion reaction of cellobiose to hydroxymethylfurfural (HMF) at temperature of 80–130°C. Acidity of the samples was determined by liquid-solid titrations working in a modified HPLC line with basic solutions of phenylethylamine (PEA), used as a base probe. As solvents, an apolar-aprotic one, cyclohexane, was used for determining the intrinsic acidity and water for the effective acidity. In addition, pyridine/water co-adsorption FT-IR experiments complemented the acidity study, and the results obtained confirmed the presence of both water-tolerant Lewis acid sites (LAS) and some residual Brønsted acid sites (BAS) on all the NBP samples. On NBP, the LAS to BAS ratio was 1.69, measured in cyclohexane, and only slightly decreased to 1.30, when measured in water. Catalytic activity was studied in a liquid-solid reaction line with fixed bed flow reactor working in complete recirculation. The reaction of HMF formation from cellobiose consists of three distinct catalytic actions: hydrolysis of the β-1,4-glycosidic bonds of cellobiose, isomerization of the formed glucose monomers to fructose, and cyclo-dehydration of fructose to HMF. Comparative catalytic results obtained with a sulfonic acid resin (Amberlite IR 120) showed that it was not able to form any HMF from cellobiose in the range of temperature studied, only cellobiose hydrolysis to glucose can be successfully achieved on the protonic acid resin.

Formation of 5-(Hydroxymethyl)furfural by Stepwise Dehydration over TiO2 with Water-Tolerant Lewis Acid Sites

The reaction mechanism for the formation of 5-(hydroxymethyl)furfural (HMF) from glucose in water over TiO2 and phosphate-immobilized TiO2 (phosphate/TiO2) with water-tolerant Lewis acid sites was studied using isotopically labeled molecules and 13C nuclear magnetic resonance measurements for glucose adsorbed on TiO2. Scandium trifluoromethanesulfonate (Sc(OTf)3), a highly active homogeneous Lewis acid catalyst workable in water, converts glucose into HMF through aldose–ketose isomerization between glucose and fructose involving a hydrogen transfer step and subsequent dehydration of fructose. In contrast to Sc(OTf)3, Lewis acid sites on bare TiO2 and phosphate/TiO2 do not form HMF through the isomerization–dehydration route but through the stepwise dehydration of glucose via 3-deoxyglucosone as an intermediate. Continuous extraction of the evolved HMF with 2-sec-butylphenol results in the increase in the HMF selectivity for phosphate/TiO2, even in highly concentrated glucose solution. These results sugges...

Selective glucose transformation by titania as a heterogeneous Lewis acid catalyst

The Lewis acidity of phosphate-immobilized anatase TiO2 (phosphate/TiO2) has been studied to develop novel environmentally benign reaction systems. Fourier transform infrared (FT-IR) measurements suggested that most Lewis acid sites on bare and phosphate/TiO2 surface function even in water. phosphate/TiO2 exhibits high catalytic performance for selective 5-(hydroxymethyl)furfural (HMF) production from glucose in THF/water (90/10vol.%) solution. This is attributed to water-tolerant Lewis acid sites on phosphate/TiO2 that promote step-wise conversion of glucose into HMF. The catalyst was easily recovered from reaction solution by simple decantation or filtration, and can be used repeatedly without significant loss of original activity for subsequent reactions.

Nb2O5·nH2O as a Heterogeneous Catalyst with Water-Tolerant Lewis Acid Sites

Niobic acid, Nb(2)O(5)·nH(2)O, has been studied as a heterogeneous Lewis acid catalyst. NbO(4) tetrahedra, Lewis acid sites, on Nb(2)O(5)·nH(2)O surface immediately form NbO(4)-H(2)O adducts in the presence of water. However, a part of the adducts can still function as effective Lewis acid sites, catalyzing the allylation of benzaldehyde with tetraallyl tin and the conversion of glucose into 5-(hydroxymethyl)furfural in water.