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.

Efficient Catalysts for the Green Synthesis of Adipic Acid from Biomass

Green synthesis of adipic acid from renewable biomass is a very attractive goal of sustainable chemistry. Herein, we report efficient catalysts for a two-step transformation of cellulose-derived glucose into adipic acid via glucaric acid. Carbon nanotube-supported platinum nanoparticles are found to work efficiently for the oxidation of glucose to glucaric acid. An activated carbon-supported bifunctional catalyst composed of rhenium oxide and palladium is discovered to be powerful for the removal of four hydroxyl groups in glucaric acid, affording adipic acid with a 99 % yield. Rhenium oxide functions for the deoxygenation but is less efficient for four hydroxyl group removal. The co-presence of palladium not only catalyzes the hydrogenation of olefin intermediates but also synergistically facilitates the deoxygenation. This work presents a green route for adipic acid synthesis and offers a bifunctional-catalysis strategy for efficient deoxygenation.

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.

One-Pot Conversion of Lignin into Naphthenes Catalyzed by a Heterogeneous Rhenium Oxide-Modified Iridium Compound

The direct transformation of lignin into fuels and chemicals remains a huge challenge because of the recalcitrant and complicated structure of lignin. In this study, rhenium oxide-modified iridium supported on SiO₂ (Ir-ReO_x /SiO₂ ) is employed for the one-pot conversion of various lignin model compounds and lignin feedstocks into naphthenes. Up to 100 % yield of cyclohexane from model compounds and 44.3 % yield of naphthenes from lignin feedstocks are achieved. 2 D HSQC NMR spectroscopy before and after the reaction confirms the activity of Ir-ReO_x /SiO₂ in the cleavage of the C-O bonds and hydrodeoxygenation of the depolymerized products. H₂ temperature-programmed reduction, temperature-programmed desorption of NH₃ , IR spectroscopy of pyridine adsorption, X-ray photoelectron spectroscopy, X-ray absorption fine structure analysis, and control experiments reveal that a synergistic effect between Ir and ReO_x in Ir-ReO_x /SiO₂ plays a crucial role in the high performance; ReO_x is mainly responsible for the cleavage of C-O bonds, whereas Ir is responsible for hydrodeoxygenation and saturation of the benzene rings. This methodology opens up an energy-efficient route for the direct conversion of lignin into valuable naphthenes.

Hydro(deoxygenation) Reaction Network of Lignocellulosic Oxygenates

Hydrodeoxygenation (HDO) is a key transformation step to convert lignocellulosic oxygenates into drop-in and functional high-value hydrocarbons through controlled oxygen removal. Nevertheless, the mechanistic insights of HDO chemistry have been scarcely investigated as opposed to a significant extent of hydrodesulfurization chemistry. Current requirements emphasize certain underexplored events of HDO of oxygenates, which include 1) interactions of oxygenates of varied molecular size with active sites of the catalysts, 2) determining the conformation of oxygenates on the active site at the point of interaction, and 3) effects of oxygen contents of oxygenates on the reaction rate of HDO. It is realized that the molecular interactions of oxygenates with the surface of the catalyst dominates the degree and nature of deoxygenation to derive products with desired selectivity by overcoming complex separation processes in a biorefinery. Those oxygenates with high carbon numbers (>C10), multiple furan rings, and branched architectures are even more complex to understand. This article aims to focus on concise mechanistic analysis of biorefinery oxygenates (C_10-35 ) for their deoxygenation processes, with a special emphasis on their interactions with active sites in a complex chemical environment. This article also addresses differentiation of the mode of interactions based on the molecular size of oxygenates. Deoxygenation processes coupled with or without ring opening of furan-based oxygenates and site-substrate cooperativity dictate the formation of diverse value-added products. Oxygen removal has been the key step for microbial deoxygenation by the use of oxygen-removing decarbonylase enzymes. However, challenges to obtain branched and long-chain hydrocarbons remain, which require special attention, including the invention of newer techniques to upgrade the process for combined depolymerization-HDO from real biomass.

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 .

Selective Cleavage of C-O Bonds in Lignin Catalyzed by Rhenium(VII) Oxide (Re2 O7 )

The selective cleavage of C-O bonds in typical model lignin β-O-4 compounds and deconstruction of a realistic lignin feedstock catalyzed by Re₂ O₇ is described. High yields of C-O cleavage products (up to 97.8 %) from model compounds and oils (76.3 %) from organosolv pinewood lignin were obtained under mild conditions. Evidence for the pathway of this catalytic process is also provided.

Selective Hydrogenolysis of Furfural Derivative 2-Methyltetrahydrofuran into Pentanediol Acetate and Pentanol Acetate over Pd/C and Sc(OTf)3 Cocatalytic System

It is of great significance to convert platform molecules and their derivatives into high value-added alcohols, which have multitudinous applications. This study concerns systematic conversion of 2-methyltetrahydrofuran (MTHF), which is obtained from furfural, into 1-pentanol acetate (PA) and 1,4-pentanediol acetate (PDA). Reaction parameters, such as the Lewis acid species, reaction temperature, and hydrogen pressure, were investigated in detail. ¹ H NMR spectroscopy and reaction dynamics study were also conducted to help clarify the reaction mechanism. Results suggested that cleavage of the primary alcohol acetate was less facile than that of the secondary alcohol acetate, with the main product being PA. A PA yield of 91.8 % (150 °C, 3 MPa H₂ , 30 min) was achieved by using Pd/C and Sc(OTf)₃ as a cocatalytic system and an 82 % yield of PDA was achieved (150 °C, 30 min) by using Sc(OTf)₃ catalyst. Simultaneously, the efficient conversion of acetic esters into alcohols by simple saponification was carried out and led to a good yield.

Supported Molybdenum Catalysts for the Deoxydehydration of 1,4-Anhydroerythritol into 2,5-Dihydrofuran

Efficient deoxygenation strategies are crucial for the valorization of renewable feedstocks. Deoxydehydration (DODH) enables the direct transformation of two adjacent hydroxyl groups into a double bond. Supported molybdenum-based catalysts were utilized for the first time in DODH. MoO_x /TiO₂ showed superior catalytic activity compared to common molybdenum salts. The catalyst efficiently converted 1,4-anhydroerythritol into 2,5-dihydrofuran in the presence of 3-octanol as reducing agent, showing high reproducibility and stability.

Selective Hydrogenolysis of Glycerol to 1,3-Propanediol: Manipulating the Frustrated Lewis Pairs by Introducing Gold to Pt/WOx

A highly dispersed Au and Pt catalyst supported on WO_x was developed for high performance in the selective hydrogenolysis of glycerol to 1,3-propanediol (1,3-PD) under very mild reaction conditions (81.4 % glycerol conversion, 51.6 % 1,3-PD selectivity at 413 K, 1 MPa H₂ ). The highly dispersed Au decreased the original surface Lewis-acid sites on Pt/WO_x but greatly increased its in situ generated Brønsted-acid sites with the assistance of H₂ through the formation of frustrated Lewis pairs. These in situ formed and spatially separated pairs of H⁺ and H^- function as the active sites in glycerol conversion to 1,3-PD.

Synthesis of 2-Butanol by Selective Hydrogenolysis of 1,4-Anhydroerythritol over Molybdenum Oxide-Modified Rhodium-Supported Silica

Rh-MoOx /SiO2 (Mo/Rh=0.13) is an effective catalyst for the hydrogenolysis of 1,4-anhydroerythritol (1,4-AHERY) and provides 2-BuOH in high yield of 51 %. This is the first report of the production of 2-BuOH from 1,4-AHERY by hydrogenolysis. 1,4-AHERY was more suitable as a starting material than erythritol because the 2-BuOH yield from erythritol was low (34 %). Based on the kinetics and comparison of reactivities of the related compounds using Rh-MoOx /SiO2 and Rh/SiO2 catalysts, the modification of Rh/SiO2 with MoOx leads to the high activity and high selectivity to 2-BuOH because of the generation of reactive hydride species and the strong adsorption of 1,4-AHERY on MoOx species. The reaction proceeds by main two routes, (I) the combination of single C-O hydrogenolysis with the desorption of intermediates, a usual route in hydrogenolysis, and (II) multiple C-O hydrogenolysis without the desorption of intermediates from the active site, and the reaction mechanism for Route (II) is proposed.

Hydrogenolysis of Glycerol to 1,3-propanediol under Low Hydrogen Pressure over WOx -Supported Single/Pseudo-Single Atom Pt Catalyst

Single/pseudo-single atom Pt catalyst was prepared on mesoporous WOx . The large surface area and abundant oxygen vacancies of WOx improve the Pt dispersion and stabilize the Pt isolation. This newly prepared catalyst exhibited outstanding hydrogenolysis activity under 1 MPa H2 pressure with a very high space-time yield towards 1,3-propanediol (3.78 g gPt (-1)  h(-1) ) in Pt-W catalysts. The highly isolated Pt structure is thought to contribute to the excellent H2 dissociation capacity over Pt/WOx . The high selectivity towards 1,3-propanediol is attributed to the heterolytic dissociation of H2 at the interface of Pt and WOx (providing specific Brønsted acid sites and the concerted dehydration-hydrogenation reaction) and the bond formation between glycerol and WOx , which favors/stabilizes the formation of a secondary carbocation intermediate as well as triggers the redox cycle of the W species (W(6+) ⇄W(5+) ).

Catalytic total hydrodeoxygenation of biomass-derived polyfunctionalized substrates to alkanes

The total hydrodeoxygenation of carbohydrate-derived molecules to alkanes, a key reaction in the production of biofuel, was reviewed from the aspect of catalysis. Noble metals (or Ni) and acid are the main components of the catalysts, and group 6 or 7 metals such as Re are sometimes added as modifiers of the noble metal. The main reaction route is acid-catalyzed dehydration plus metal-catalyzed hydrogenation, and in some systems metal-catalyzed direct CO dissociation is involved. The appropriate active metal, acid strength, and reaction conditions depend strongly on the reactivity of the substrate. Reactions that use Pt or Pd catalysts supported on Nb-based acids or relatively weak acids are suitable for furanic substrates. Carbohydrates themselves and sugar alcohols undergo CC dissociation easily. The systems that use metal-catalyzed direct CO dissociations can give a higher yield of the corresponding alkane from carbohydrates and sugar alcohols.

Hydrothermal synthesis of bimodal mesoporous MoS2 nanosheets and their hydrodeoxygenation properties

Bimodal mesopore MoS₂ nanosheets were successfully synthesized by adjusting the pH value and exhibited high HDO activity.