Heterogeneous catalysis is a promising technology for the valorization of renewable biomass to sustainable advanced fuels and fine chemicals. Porosity and nanostructure are the most versatile features of heterogeneous solid catalysts, which can greatly determine the accessibility of specific active sites, reaction mechanisms, and the selectivity of desirable products. Hence, the precise tuning of porosity and nanostructure has been a potential strategy towards developing novel solid catalysts with indispensable characteristics for efficient biomass valorization. Herein, we present a timely and comprehensive review of the recent advances in catalytic biomass conversions over microporous zeolites, mesoporous silicas, and nanostructured metals/metal oxides. This review covers the catalytic processing of both edible (lipids and starch) and non-edible (lignocellulose) biomass as well as their derived compounds, along with a systematic evaluation of catalyst reusability/kinetic/mechanistic aspects in the relevant processes. The key parameters essential for tailoring particle size, morphology, porosity, acid–base, and redox properties of solid catalysts are emphasized, while discussing the ensuing catalytic effects towards the selective conversion of biomass into desirable chemicals. Special attention has been drawn to understand the role of water in liquid phase biomass conversions as well as the hydrothermal stability and the deactivation of nanoporous catalysts. We believe this comprehensive review will provide new insights towards developing state-of-the-art solid catalysts with well-defined porosity and nanoscale properties for viable biomass conversion.

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Advances in porous and nanoscale catalysts for viable biomass conversion.

Transition metal-zeolite composites are versatile catalytic materials for a wide range of industrial and lab-scale processes. Significant advances in fabrication and characterization of well-defined metal centers confined in zeolite matrixes have greatly expanded the library of available materials and, accordingly, their catalytic utility. In this review, we summarize recent developments in the field from the perspective of materials chemistry, focusing on synthesis, postsynthesis modification, (operando) spectroscopy characterization, and computational modeling of transition metal-zeolite catalysts.

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Engineering of Transition Metal Catalysts Confined in Zeolites.

Research efforts to find more sustainable pathways for the synthesis of adipic acid have led to the introduction of new catalytic processes for producing this commodity chemical from alternative resources. With a focus on the performance of oxygen and hydrogen peroxide as preferred oxidants, this minireview summarizes recent advances made in the selective oxidation of cyclohexene, cyclohexane, cyclohexanone and n-hexane to adipic acid. Special attention is paid to the exploration of catalytic pathways involving lignocellulosic biomass-derived chemicals such as 5-hydroxymethylfurfural, D-glucose, γ-valerolactone and compounds representative of lignin and lignin-derived bio-oils.

Emerging catalytic processes for the production of adipic acid

Carbohydrate epimerization is an essential technology for the widespread production of rare sugars. In contrast to other enzymes, most epimerases are only active on sugars substituted with phosphate or nucleotide groups, thus drastically restricting their use. Here we show that Sn-Beta zeolite in the presence of sodium tetraborate catalyses the selective epimerization of aldoses in aqueous media. Specifically, a 5 wt% aldose (for example, glucose, xylose or arabinose) solution with a 4:1 aldose:sodium tetraborate molar ratio reacted with catalytic amounts of Sn-Beta yields near-equilibrium epimerization product distributions. The reaction proceeds by way of a 1,2 carbon shift wherein the bond between C-2 and C-3 is cleaved and a new bond between C-1 and C-3 is formed, with C-1 moving to the C-2 position with an inverted configuration. This work provides a general method of performing carbohydrate epimerizations that surmounts the main disadvantages of current enzymatic and inorganic processes.

Sn-Beta zeolites with borate salts catalyse the epimerization of carbohydrates via an intramolecular carbon shift

This review article aims to cover the state-of-the-art of titanosilicate catalysts for selective oxidations developed within past seven years. Many elaborated materials (e.g., layered and pillared ...

Catalytic performance of advanced titanosilicate selective oxidation catalysts – a review

Hierarchical TS-1 with intracrystalline voids could be synthesized by post-modification with tetrapropyl ammonium hydroxide (TPAOH), however the process was supposed to be restrained by the charge balance effect. In order to release the constraint, NH3·H2O was introduced in the post-modification. The influences of the TPAOH/NH3·H2O modification, the TPAOH concentration and the modification time on the physiochemical properties were studied. The dissolution process and recrystallization process were observed in the combined modification, and both of them were intensified by the NH3·H2O introduced, indicating that the constraint on the OH− diffusion was released. Owing to the intensified processes, the TPAOH concentration and post-modification time could be reduced, and hierarchical TS-1 with relative high crystallinity, less defect sites and a larger secondary pore volume (about 0.18 cm3 g−1) was achieved in 4 h. However, the combined-modification exerted little influence on the chemical composition and acidity, and the microporous properties were almost the same with that of TS-1 modified with TPAOH only. The secondary porosity in the combined-modified samples were mainly intracrystalline voids, and grooves on the surface were also found. Although extra-framework titanium species were created by the post-modification, part of the titanium was still in the framework position. The catalytic activity was evaluated by phenol hydroxylation and 1-octene epoxidation, and better catalytic activity was achieved for the improved accessibility of the active sites.

Hierarchical TS-1 synthesized effectively by post-modification with TPAOH and ammonium hydroxide

The synthesis, characterization and catalytic activity of the hierarchical TS-1 with the intracrystalline voids and grooves

Titanium silicalite-1 (TS-1) has been shown to be a heterogeneous catalyst with remarkable efficiency and selectivity; however, the nature of the active Ti site in the MFI framework remains elusive. Here we report combined experimental and theoretical research on Ti distribution in the 12 crystallographically distinct T sites of the MFI framework in high-Ti-loaded TS-1 (2.7 wt % in TiO2). Using a multishell fit to extended X-ray absorption fine structure, we show that T4 is the most populated site, in marked contrast to the preferential substitution sites and the definitely excluded sites assumed hitherto by diffraction studies. The identification is supported by a good agreement between calculated and experimental X-ray absorption near-edge structure studies and by full periodic density functional theory (DFT) computation. In spite of having the identical most favored site, the preference order for the remaining sites predicted by DFT does not fully match the experimental results. This suggests that Ti d...

Peng Xinxin

Papers

Hierarchical TS-1 synthesized effectively by post-modification with TPAOH and ammonium hydroxide

The synthesis, characterization and catalytic activity of the hierarchical TS-1 with the intracrystalline voids and grooves

Toward a Unified Identification of Ti Location in the MFI Framework of High-Ti-Loaded TS-1: Combined EXAFS, XANES, and DFT Study

Hierarchical TS-1 synthesized via the dissolution-recrystallization process: Influence of ammonium salts

Confirmation of the isomorphous substitution by Sn atoms in the framework positions of MFI-typed zeolite