No abstract

Hydrothermal treatment of NH4[NbO­(C2O4)2(H2O)2]·nH2O in water at 448 K for 3 days produced crystalline Nb2O5 with a deformed orthorhombic structure and a high surface area (208 m2 g–1). Fourier-transform infrared spectroscopy measurements of pyridine adsorption revealed that the Nb2O5 catalyst has both high densities of Brønsted and Lewis acid sites that can work in the presence of water. One feature of the Nb2O5 catalyst is its high density of water-compatible Lewis acid sites (0.21 mmol g–1), which is much larger than that of Nb2O5·nH2O (0.03 mmol g–1). The Nb2O5 catalyst was studied as a solid acid catalyst for the formation of lactic acid from 1,3-dihydroxyacetone and pyruvaldehyde in water at 373 K, and was determined to be a highly active and selective catalyst, compared with typical acid catalysts (H2SO4, Sc­(OTf)3, and Nb2O5·nH2O). A high Lewis acid density with moderate acid strength is a crucial factor for the high catalytic performance exhibited for the former reaction. High densities of both Brønsted and Lewis acid sites in the catalyst promote the fast and selective production of lactic acid in the latter reaction. In addition, Such Lewis acidity of the Nb2O5 is also effective over conventional acid catalysts in xylose dehydration to furfural in water, with respect to reaction rate and furfural selectivity. The combination of aqueous-phase dehydration of xylose over the Nb2O5 and the continuous extraction of furfural with an immiscible organic solvent resulted in a high selectivity toward furfural of ∼78.5% with high xylose conversion (97%).

Aqueous-phase dehydration of xylose into furfural was studied in the presence of amorphous Nb2O5 with water-compatible Brønsted and Lewis acid sites. Nb2O5 was determined as a more active and selective catalyst for xylose dehydration than typical homogeneous Brønsted and Lewis acid catalysts including HCl and Sc­(OTf)3, and Nb2O5 converted 93% of xylose with 48% selectivity toward furfural in water at 393 K. No significant loss of the original catalytic activity was observed after Na+-exchange treatment, which indicates that the reaction proceeded only on Lewis acid sites. Isotope-labeling experiments using D2O and xylose suggested that furfural is formed through stepwise dehydration via a highly reactive dicarbonyl intermediate on Nb2O5, whereas typical Lewis acids such as CrCl3 and Sc­(OTf)3 convert xylose to furfural in water through hydride transfer and subsequent dehydration via xylulose as a ketose-type intermediate. The difference in the reaction mechanism accounts for the lower activation energy (83 kJ mol–1) with Nb2O5 than those with Sc­(OTf)3 and HCl (107–131 kJ mol–1). Continuous extraction of evolved furfural with toluene enabled a large increase in the selectivity toward furfural from 48% to 72% and prevented deactivation of the Lewis acid sites by covering with heavy byproducts, represented by humin.

Selective and economic conversion of lignocellulosic biomass components to bio‐based fuels and chemicals is the major goal of biorefineries, but low yields and selectivity for fuel precursors such as sugars, furanics, and lignin‐derived monomers pose significant disadvantages in process economics. In this Minireview we summarize the existing protection strategies used in biomass chemocatalytic conversion processes and focus the discussions on the mechanisms, challenges, and opportunities of each strategy. We introduce a concept of using analogous methods to manipulate biomass catalytic conversion pathways during the upgrading of carbohydrates to fuels and chemicals. This Minireview may provide new insights into the development of selective biorefining processes from a different perspective, expanding the options for selective conversion of biomass to fuels and chemicals.

Aerobic oxidation of biomass-derived furfural to furoic acid was studied with an N-heterocyclic carbene as a homogeneous catalyst. Carbene species generated in situ on 1,3-bis(2,4,6-trimethylphenyl) imidazolium chloride with a strong organic base (1,8-diazabicyclo[5.4.0]undec-7-ene) was highly active and selective for the formation of furoic acid in dimethyl sulfoxide at 40 °C. This reaction initiates the formation of a Breslow intermediate between an Nheterocyclic carbene and a furfural molecule and the subsequent activation of molecular O 2 . While the active carbene catalyst promoted furfural dimerization to afford furoin as a side reaction, furoin was decomposed into the Breslow intermediate and furfural through a reverse reaction, which were then converted quantitatively to furoic acid. Kinetic studies revealed that the apparent activation energy for this furfural oxidation was only 20 kJ mol −1 , which is significantly lower than that with a supported Au catalyst (30.4 kJ mol −1 ). The N-heterocyclic carbene catalyst can oxidize various furan-based aldehydes with high selectivity; however, the electronwithdrawing group bonded to the furan ring has a negative effect on the reaction rate. Furfural can also be oxidized selectively to furoic acid, even in the presence of byproducts that are formed by acid-catalyzed dehydration of xylose with Amberlyst-70. As a result, a sequential reaction system based on initial dehydration and subsequent aerobic oxidation was developed for the production of furoic acid from xylose, without the need for furfural purification, using Amberlyst-70 (a solid acid) and an Nheterocyclic carbene catalyst.

Depletion of oil reserves, increasing prices of petroleum products, and environmental concerns related to air pollution are the main driving forces for utilizing renewable energy resources to replace fossil fuels. Vegetable oils can be used to produce biodiesel. Biodiesel is obtained by transesterification of vegetable oil with alcohol using homogeneous or heterogeneous catalysts. In the industry, biodiesel is produced by heterogeneous catalysts due to high activity and selectivity, better reusability, reduction in processing steps, and wastes. The catalytic activity of catalysts depends on the strength and type of intrinsic basic or/and acid properties. Biodiesel production using heterogenous catalysts depends on the various reaction parameters such as reaction time, temperature, molar ratio of alcohol to oil, catalyst amount, and stirring speed. In this paper, the catalytic transesterification of various feedstocks such as edible oil, non‐edible oil, and waste using heterogeneous catalysts has been reviewed and optimization parameters for maximum biodiesel production have been summarized.

Thermal treatment induces a modification in the nanostructure of carbon nanospheres that generates ordered hemi‐fullerene‐type graphene shells arranged in a concentric onion‐type structure. The catalytic reactivity of these structures is studied in comparison with that of the parent carbon material. The change in the surface reactivity induced by the transformation of the nanostructure, characterized by TEM, XRD, X‐ray photoelectron spectroscopy (XPS), Raman, and porosity measurements, is investigated by multipulses of Cl2 in inert gas or in the presence of CH4 or CO. The strained CC bonds (sp2‐type) in the hemi‐fullerene‐type graphene shells induce unusually strong, but reversible, chemisorption of Cl2 in molecular form. The active species in CH4 and CO chlorination is probably in the radical‐like form. Highly strained CC bonds in the parent carbon materials react irreversibly with Cl2, inhibiting further reaction with CO. In addition, the higher presence of sp3‐type defect sites promotes the formation of HCl with deactivation of the reactive CC sites. The nano‐ordering of the hemi‐fullerene‐type graphene thus reduces the presence of defects and transforms strained CC bonds, resulting in irreversible chemisorption of Cl2 to catalytic sites able to perform selective chlorination.

The carbon-catalyzed reaction of Cl2 and CO constitutes the most important industrial route to phosgene. Although defects in carbon lead to surface chemical reactions, direct polarization of C-heteroatom bonds induces a more successful Cl2 catalytic activation, the rate-determining step in the overall catalytic cycle. The interplay between the electron-donating and -withdrawing ability of the incorporated nitrogen substituents on the formation and stabilization of active sites was examined by X-ray photoelectron and Raman spectroscopy. Mechanistic studies indicate that the polarized Cl2 induced by the direct interaction of Cl2 with a strongly electron-deficient carbon site in close proximity to a nitrogen substituent is essential for phosgene production. Nitrogen substitution into ordered carbon materials led to very active and stable carbon catalysts for COCl2 synthesis.