Performance, Structure, and Mechanism of ReOx–Pd/CeO2 Catalyst for Simultaneous Removal of Vicinal OH Groups with H2

Lattice oxygen insertion mechanism in CeO2-catalyzed reactions in water: nitrile hydration reaction

Cerium oxide (CeO2) exhibits prominent catalytic activity in various organic reactions owing to its unique acid-base and redox properties. One of the most interesting applications of pure CeO2-catalyzed organic reactions is the hydration of nitriles in water. The experimental results showed that the hydration of 2-cyanopyridine to picolinamide in water using CeO2 catalysts proceeds readily at low temperatures (30-100 °C) in high yields and that this reaction occurs exclusively on CeO2 among various metal-oxide catalysts. To elucidate the unique catalytic activity of CeO2, the reaction mechanism is dissected using the density functional theory-based molecular dynamics (DFT-MD) simulations. Based on the free energy analysis, it is demonstrated that the reaction proceeds with the involvement of the surface lattice oxygen, where the lattice oxygen atom is inserted into picolinamide. The involvement of the surface lattice oxygen is notably uncommon given the low temperatures of the reaction, and this computational prediction is verified by the two experiments using H2 18O solvent and 18O-exchanged CeO2 catalyst, where the introduction of surface lattice oxygen into picolinamide is confirmed. The inherent flexibility of the surface lattice oxygen and the unique acid-base properties of CeO2, which can favorably bind and activate both nitrile and water molecules, are key factors in the high reactivity for various organic reactions, which characterizes the outstanding catalytic activity of CeO2.

Highly Efficient Iridium-Iron-Molybdenum Catalysts Condensed on Boron Nitride for Biomass-Derived Diols' Hydrogenolysis to Secondary Monoalcohols

A trade-off of activity-selectivity exists in primary C-O hydrogenolysis of biomass-derived diols to secondary alcohols over bimetallic catalysts, especially the combination of noble metal and early transition metal in the metallic state and metal oxide state, respectively. Herein, the combination of high surface concentration of boron nitride (BN)-supported metals and the addition of Mo as third metal broke the trade-off. High yields (>50%) of secondary alcohols were obtained with robust productivity up to 25-fold based on Ir over Ir-Fe0.13-Mo0.08/BN (Ir = 20 wt %, Fe/Ir = 0.13, Mo/Ir = 0.08) than previously reported Ir-Fe catalysts. In contrast, simply increasing the loading amount of Ir-Fe catalysts or the addition of Mo species only enhanced the productivity by <2-4-fold. Various characterizations showed that large Ir loading enables the formation of condensed nanostructures (∼2 nm) on the BN support, which further alloy with Mo and Fe to form an face centred cubic (fcc)-type trimetallic alloy with surface enrichment of Fe. On the other hand, in Ir-Fe0.25-Mo0.08/BN with lower Ir (5 wt %) and lower Ir-based activity, the Mo species were rather bound on the support surface possibly as the MoBx state. These structures were formed by simple impregnation and reduction with H2 at the reaction temperature (453 K). The high activity of Ir-Fe0.13-Mo0.08/BN (20 wt % Ir) is derived from two aspects: (1) the formation of condensed nanostructures (∼2 nm) exposing more active sites and (2) alloying with Mo to modify the electronic state of Ir to enhance the H2 activation ability, as shown by the decreased Ea (82-84 → 67 kJ mol-1).

Synthesis of Pd-CoxOy Hybrid Nanostructure-Encapsulated Hollow Silica Nanospheres through Reverse Microemulsion Systems and Their Application as Efficient Hydrodechlorination Catalysts

Pd-Co_xO_y heteroaggregate-encapsulated hollow porous silica nanoreactors (Pd-Co_xO_y@HPSNs) were synthesized by a reverse microemulsion system. The key design of the developed reverse microemulsion system is to use poly(ethyleneimine) in the water droplets as the void templates for silica deposition and for anchoring the catalytic functionality inside the hollow silica nanospheres. The synthesized Pd-Co_xO_y@HPSNs contain ∼3 nm Pd-Co_xO_y hybrid nanostructures in ∼10 nm central cavities of silica nanospheres and illustrated a significantly promoted efficiency for hydrodechlorination of a series of chlorophenols into phenols under mild reaction conditions. The catalytic enhancement of Pd-Co_xO_y@HPSNs is ascribed to the synergistic effect between Pd and Co_xO_y and the protection of silica shells to the inner catalytic functionality.

Deoxydehydration of Biomass-Derived Polyols Over Silver-Modified Ceria-Supported Rhenium Catalyst with Molecular Hydrogen

Olefin production from polyols via deoxydehydration (DODH) was carried out over Ag-modified CeO₂ -supported heterogeneous Re catalysts with H₂ as a reducing agent. Both high DODH activity and low hydrogenation ability for C=C bonds were observed in the reaction of erythritol, giving a 1,3-butadiene yield of up to 90 % under "solvent-free" conditions. This catalyst is applicable to other substrates such as methyl glycosides (methyl α-fucopyranoside: 91 % yield of DODH product; methyl β-ribofuranoside: 88 % yield), which were difficult to be converted to the DODH products over the DODH catalysts reported previously. ReO_x -Ag/CeO₂ was reused 3 times without a decrease of activity or selectivity after calcination as regeneration. Although the transmission electron microscopy energy-dispersive X-ray spectroscopy and X-ray absorption fine structure analyses showed that Re species were highly dispersed and Ag was present as metal particles with various sizes from well-dispersed species (<1 nm) to around 5 nm particles, the catalysts prepared from size-controlled Ag nanoparticles showed similar performance, indicating that the catalytic performance is insensitive to the Ag particle size.

Titania-supported molybdenum oxide combined with Au nanoparticles as hydrogen-driven deoxydehydration catalyst of diol compounds

A heterogenous catalyst for deoxydehydration (DODH) reaction was developed using less expensive Mo than Re as the active center. Combination of Mo with anatase-rich TiO2 and Au as the support...

Transition metal-catalyzed deoxydehydration: missing pieces of the puzzle

Deoxydehydration (DODH) is a transformation that converts a vicinal diol into an olefin with the help of a sacrificial reductant.

Direct Deoxydehydration of Cyclic trans-Diol Substrates: An Experimental and Computational Study of the Reaction Mechanism of Vanadium(V)-based Catalysis*

The deoxydehydration of carbohydrates represents a key target to leverage renewable biomass resources chemically. Using a vanadium(V)-based catalyst, it was possible to directly deoxydehydrate cyclic trans-diol substrates. Accompanying mechanistic characterisation of this process by density functional calculations pointed to an energetically tractable route for deoxydehydration of cyclic trans-diol substrates involving stepwise cleavage of the diol C-O bonds via the triplet state; experimentally, this was supported by light dependence of the reaction. Calculations also indicated that cyclic cis-diols and a linear diol substrate could additionally proceed by a concerted singlet DODH mechanism. This work potentially opens a new and cost-effective way to efficiently convert carbohydrates of trans-diol stereochemistry into alkenes.

Recent Catalytic Advances in Hydrotreatment Processes of Pyrolysis Bio-Oil

Catalytic hydrotreatment (HT) is one of the most important refining steps in the actual petroleum-based refineries for the production of fuels and chemicals, and it will play also a crucial role for the development of biomass-based refineries. In fact, the utilization of HT processes for the upgrading of biomass and/or lignocellulosic residues aimed to the production of synthetic fuels and chemical intermediates represents a reliable strategy to reduce both carbon dioxide emissions and fossil fuels dependence. At this regard, the catalytic hydrotreatment of oils obtained from either thermochemical (e.g., pyrolysis) or physical (e.g., vegetable seeds pressing) processes allows to convert biomass-derived oils into a biofuel with properties very similar to conventional ones (so-called drop-in biofuels). Similarly, catalytic hydro-processing also may have a key role in the valorization of other biorefinery streams, such as lignocellulose, for the production of high-added value chemicals. This review is focused on recent hydrotreatment developments aimed to stabilizing the pyrolytic oil from biomasses. A particular emphasis is devoted on the catalyst formulation, reaction pathways, and technologies.

Selective Hydrogenolysis of Erythritol over Ir-ReOx /Rutile-TiO2 Catalyst

Partial hydrogenolysis of erythritol, which can be produced at large scale by fermentation, to 1,4-butanediol (1,4-BuD) is investigated with Ir-ReO_x /SiO₂ and Ir-ReO_x /rutile-TiO₂ catalysts. In addition to the higher conversion rate over Ir-ReO_x /TiO₂ than over Ir-ReO_x /SiO₂ , which has been also reported for glycerol hydrogenolysis, Ir-ReO_x /TiO₂ showed higher selectivity to 1,4-BuD than Ir-ReO_x /SiO₂ , especially at low conversion levels, leading to high 1,4-BuD productivity of 20 mmol_1,4-BuD  g_Ir ^-1  h^-1 at 373 K (36 % conversion, 33 % selectivity). The productivity based on the noble metal amount is higher than those reported previously, although the maximum yield of 1,4-BuD (23 %) is not higher than the highest reported values. The reactions of various triols, diols and mono-ols are tested and the selectivity and the reaction rates are compared between catalysts and between substrates. The Ir-ReO_x /TiO₂ catalyst showed about twofold higher activity than Ir-ReO_x /SiO₂ in hydrogenolysis of the C-OH bond at the 2- or 3-positions in 1,2- and 1,3-diols, respectively, whereas the hydrogenolysis of C-OH at the 1-position is less promoted by the TiO₂ support. Lowering the loading amount of Ir on TiO₂ (from 4 wt % to 2 or 1 wt %) decreases the Ir-based activity and 1,4-BuD selectivity. Similarly, increasing the loading amount on SiO₂ from 4 wt % to 20 wt % increases the Ir-based activity and 1,4-BuD selectivity, although they remain lower than those for TiO₂ -supported catalyst with 4 wt % Ir. High metal loadings on the support seem to be important.

H2 -Free Re-Based Catalytic Dehydroxylation of Aldaric Acid to Muconic and Adipic Acid Esters

As one of the most demanded dicarboxylic acids, adipic acid can be directly produced from renewable sources. Hexoses from (hemi)cellulose are oxidized to aldaric acids and subsequently catalytically dehydroxylated. Hitherto performed homogeneously, we present the first heterogeneous catalytic process for converting an aldaric acid into muconic and adipic acid. The contribution of leached Re from the solid pre-reduced catalyst was also investigated with hot-filtration test and found to be inactive for dehydroxylation. Corrosive or hazardous (HBr/H2 ) reagents are avoided and simple alcohols and solid Re/C catalysts in an inert atmosphere are used. At 120 °C, the carboxylic groups are protected by esterification, which prevents lactonization in the absence of water or acidic sites. Dehydroxylation and partial hydrogenation yield monohexenoates (93 %). For complete hydrogenation to adipate, a 16 % higher activation barrier necessitates higher temperatures.

Kinetics Study of the Hydrodeoxygenation of Xylitol over a ReOx-Pd/CeO2 Catalyst

In this study, we elucidate the reaction kinetics for the simultaneous hydrodeoxygenation of xylitol to 1,2-dideoxypentitol and 1,2,5-pentanetriol over a ReOx-Pd/CeO2 (2.0 weight% Re, 0.30 weight% Pd) catalyst. The reaction was determined to be a zero-order reaction with respect to xylitol. The activation energy was elucidated through an Arrhenius relationship as well as non-Arrhenius kinetics. The Arrhenius relationship was investigated at 150–170 °C and a constant H2 pressure of 10 bar resulting in an activation energy of 48.7 ± 10.5 kJ/mol. The investigation of non-Arrhenius kinetics was conducted at 120–170 °C and a sub-Arrhenius relation was elucidated with activation energy being dependent on temperature, and ranging from 10.2–51.8 kJ/mol in the temperature range investigated. Internal and external mass transfer were investigated through evaluating the Weisz–Prater criterion and the effect of varying stirring rate on the reaction rate, respectively. There were no internal or external mass transfer limitations present in the reaction.

Erythritol: Another C4 Platform Chemical in Biomass Refinery

The potential of erythritol as a platform chemical in biomass refinery is discussed in terms of erythritol production and utilization. Regarding erythritol production, fermentation of sugar or starch has been already commercialized. The shift of the carbon source from glucose to inexpensive inedible waste glycerol is being investigated, which will decrease the price of erythritol. The carbon-based yield of erythritol from glycerol is comparable to or even higher than that from glucose. The metabolic pathway of erythritol biosynthesis has become clarified: erythrose-4-phosphate, which is one of the intermediates in the pentose phosphate pathway, is dephosphorylated and reduced to erythritol. The information about the metabolic pathway may give insights to improve the productivity by bleeding. Regarding erythritol utilization, chemical conversions of erythritol, especially deoxygenation, have been investigated in these days. Erythritol is easily dehydrated to 1,4-anhydroerythritol, which can be also used as the substrate for production of useful C4 chemicals. C-O hydrogenolysis and deoxydehydration using heterogeneous catalysts are effective reactions for erythritol/1,4-anhydroerythritol conversion.

Reduction of sugar derivatives to valuable chemicals: utilization of asymmetric carbons

Recent progress on non-furfural routes from sugar derivatives to valuable chemicals including chiral chemicals was reviewed.

One-pot synthesis of 1-butylpyrrolidine and its derivatives from aqueous ammonia and 1,4-butandiol over CuNiPd/ZSM-5 catalysts

A straightforward and economical route for 1-butylpyrrolidine over CuNiPd/ZSM-5 catalysts.

Tungsten–zirconia-supported rhenium catalyst combined with a deoxydehydration catalyst for the one-pot synthesis of 1,4-butanediol from 1,4-anhydroerythritol

Biomass-derived 1,4-anhydroerythritol is reduced to 1,4-butanediol over a reusable mixture of heterogeneous catalysts, ReO_x–Au/CeO₂ and ReO_x/WO₃–ZrO₂.

Deoxydehydration of 1,4-anhydroerythritol over anatase TiO2(101)-supported ReOx and MoOx

Heterogeneously catalyzed deoxydehydration (DODH) ordinarily occurs over oxide supported ReO_x sites. A comparably high activity of MoO_x/TiO₂(101) suggests that it is a promising low-cost DODH catalyst that can replace Re-based catalysts.

A redox interaction-engaged strategy for multicomponent nanomaterials

The review article focuses on the redox interaction-engaged strategy that offers a powerful way to construct multicomponent nanomaterials with precisely-controlled size, shape, composition and hybridization of nanostructures.

Recent Advances in the Deoxydehydration of Vicinal Diols and Polyols

Deoxydehydration (DODH) is one of the most promising tools to reduce the oxygen content of biomass (sugars and polyols) and provide analogues of platform chemicals that are derived from fossil resources. This reaction converts a vicinal diol into an alkene and is typically catalyzed by high-oxidation-state metal-oxo compounds in the presence of a stoichiometric reductant, with examples of both homogeneous and heterogeneous systems. This minireview will highlight the developments in this field over the past 5 years and focus on efforts to solve the problems that currently prevent DODH being performed on a commercial scale, including the nature of the reductant, substrate scope and selectivity, and catalyst recovery and expense.

Preparation of Highly Active Monometallic Rhenium Catalysts for Selective Synthesis of 1,4-Butanediol from 1,4-Anhydroerythritol

1,4-Butanediol can be produced from 1,4-anhydroerythritol through the co-catalysis of monometallic mixed catalysts (ReO_x /CeO₂ +ReO_x /C) in the one-pot reduction with H₂ . The highest yield of 1,4-butanediol was over 80 %, which is similar to the value obtained over ReO_x -Au/CeO₂ +ReO_x /C catalysts. Mixed catalysts of CeO₂ +ReO_x /C showed almost the same performance, giving 89 % yield of 1,4-butanediol. The reactivity trends of possible intermediates suggest that the reaction mechanism over ReO_x /CeO₂ +ReO_x /C is similar to that over ReO_x -Au/CeO₂ +ReO_x /C: deoxydehydration (DODH) of 1,4-anhydroerythritol to 2,5-dihydrofuran over ReO_x species on the CeO₂ support with the promotion of H₂ activation by ReO_x /C, isomerization of 2,5-dihydrofuran to 2,3-dihydrofuran catalyzed by ReO_x on the C support, hydration of 2,3-dihydrofuran catalyzed by C, and hydrogenation to 1,4-butanediol catalyzed by ReO_x /C. The reaction order of conversion of 1,4-anhydroerythritol with respect to H₂ pressure is almost zero and this indicates that the rate-determining step is the formation of 2,5-dihydrofuran from the coordinated substrate with reduced Re in the DODH step. The activity of ReO_x /CeO₂ +ReO_x /C is higher than that of ReO_x -Au/CeO₂ +ReO_x /C, which is probably related to the reducibility of ReO_x /C and the mobility of the Re species between the supports. High-valent Re species such as Re⁷⁺ on the CeO₂ and C supports are mobile in the solvent; however, low-valent Re species, including metallic Re species, have much lower mobility. Metallic Re and cationic low-valent Re species with high reducibility and low mobility can be present on the carbon support as a trigger for H₂ activation and promoter of the reduction of Re species on CeO₂ . The presence of noble metals such as Au can enhance the reducibility through the activation of H₂ molecules on the noble metal and the formation of spilt-over hydrogen over noble metal/CeO₂ , as indicated by H₂ temperature-programmed reduction. The higher reducibility of ReO_x -Au/CeO₂ lowers the DODH activity of ReO_x -Au/CeO₂ +ReO_x /C in comparison with ReO_x /CeO₂ +ReO_x /C by restricting the movement of Re species from C to CeO₂ .

Mechanistic study of the selective hydrogenation of carboxylic acid derivatives over supported rhenium catalysts

The structure and performance of TiO₂-supported Re (Re/TiO₂) catalysts for selective hydrogenation of carboxylic acid derivatives have been investigated.