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).

Hydrodeoxygenation of Oxygen-Containing Aromatic Plastic Wastes to Liquid Organic Hydrogen Carriers.

To address the global plastic pollution issues and the challenges of hydrogen storage and transportation, we report a system, based on the hydrodeoxygenation (HDO) of oxygen-containing aromatic plastic wastes, from which organic hydrogen carriers (LOHCs) can be derived. We developed a catalytic system comprised of Ru-ReOx /SiO2 +HZSM-5 for direct HDO of polycarbonate (PC), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyphenylene oxide (PPO), and their mixtures, to cycloalkanes as LOHCs, with high yields up to 99 %, under mild reaction conditions. The theoretical hydrogen storage capacity reaches ca. 5.74 wt%. The reaction pathway involves depolymerization of PC into C15 aromatics and C15 monophenols by direct hydrogenolysis of the C-O bond between the benzene ring and ester group, and subsequent parallel hydrogenation of C15 aromatics and HDO of C15 monophenols. HDO of cyclic alcohol is the rate-determining step. The active site is Ru metallic nanoparticles with partially covered ReOx species. The excellent performance is attributed to the synergetic effect of oxophilic ReOx species and Ru metallic sites for C-O hydrogenolysis and hydrogenation, and the promotion effect of HZSM-5 for dehydration of cyclic alcohol. The highly efficient and stable dehydrogenation of cycloalkanes over Pt/γ-Al2 O3 confirms that HDO products can act as LOHCs.

Hydrogenation of n-octanoic acid over the MoPt alloy of Mo-Pt/SiO2 catalyst under solvent-free conditions.

SiO2-Supported bimetallic Mo and Pt worked as an effective heterogeneous catalyst for hydrogenation of n-octanoic acid at a low temperature of 373 K in the absence of solvent, providing 78% yield of hydrogenated products (62% yield of 1-octanol and 32% yield of octyl octanoate). Based on the catalyst characterization, MoPt alloy (Mo/Pt = 1) was formed by high-temperature reduction and was proposed to be the active site for the reaction.

Adsorption of Keggin-Type Polyoxometalates on Rh Metal Particles under Reductive Conditions.

The adsorption of POMs on Rh/SiO2 in water solvent under strongly reductive conditions was investigated. Aqueous solutions of α-Keggin type silicotungstate and silicovanadotungstates were mixed with Rh/SiO2 at 393-473 K under 1 MPa of H2. Monovanadium-substituted silicotungstate, α-SiVW11O405- (SiVW11), was more readily adsorbed than nonsubstituted silicotungstate, α-SiW12O404- (SiW12). After adsorption at 433 K, SiVW11 was desorbed from Rh/SiO2 by oxidation with Br2 water without change of the Keggin structure, as evidenced by 51V NMR. Trivanadium-substituted silicotungstate, α-1,2,3-SiV3W9O407-, was not stable, and the desorbed species from Rh/SiO2 by oxidation with Br2 did not maintain the Keggin structure. The very high temperature for adsorption (473 K) also led to the decomposition of the Keggin structure of SiVW11. An increase in the concentration of SiVW11 in the liquid phase gave a saturation of the amount of desorbable SiVW11, up to five SiVW11 anions per one Rh particle with a 3 nm size. The elemental analysis and W L3-edge extended X-ray absorption fine structure of Rh/SiO2 after the adsorption of SiVW11 showed that a part of SiVW11 was decomposed and irreversibly adsorbed as metallic W species incorporated into the surface of Rh metal particles. The amount of decomposed SiVW11 was almost the same as that of SiVW11 adsorbed as the original Keggin structure. The desorbable SiVW11 was probably bonded on the W atom incorporated on the Rh metal particles as the two-electron-reduced form (α-SiVIIIW11O407-).

CO2 Conversion with Alcohols and Amines into Carbonates, Ureas, and Carbamates over CeO2 Catalyst in the Presence and Absence of 2-Cyanopyridine.

Recent progress on the CeO2 catalyzed synthesis of organic carbonates, ureas, and carbamates from CO2 +alcohols, CO2 +amines, and CO2 +alcohols+amines, respectively, is reviewed. The reactions of CO2 with alcohols and amines are reversible ones and the degree of the equilibrium limitation of the synthesis reactions is strongly dependent on the properties of alcohols and amines as the substrates. When the equilibrium limitation of the reaction is serious, the equilibrium conversion of the substrate and the yield of the target product is very low, therefore, the shift of the equilibrium reaction to the product side by the removal of H2 O is essential in order to get the target product in high yield. One of the effective method of the H2 O removal from the related reaction systems is the combination with the hydration of 2-cyanopyridine to 2-picolinamide, which is also catalyzed by CeO2 .

Transformation of Sugars into Chiral Polyols over a Heterogeneous Catalyst.

Transformation of sugars, while maintaining the intrinsic stereochemical structure, is desirable. However, such a transformation requires multistep synthesis with protection and deprotection of the OH groups. Herein, a new method for selective transformation of sugar derivatives into chiral building blocks and a diol synthon, with retention of the intrinsic configuration (stereo- and regioselectively), is demonstrated. The method is based on the selective recognition of cis-vicinal OH groups in sugars and leads to the one-pot removal of the cis-vicinal OH groups, without protection of OH groups (except the OH group of the hemiacetal group), over a heterogeneous CeO2 -supported ReOx and Pd (ReOx -Pd/CeO2 ) catalyst by using H2 as a reducing agent.

Formation of a New, Strongly Basic Nitrogen Anion by Metal Oxide Modification.

Development of new hybrid materials having unique and unprecedented catalytic properties is a challenge for chemists, and heterogeneous-homogeneous hybrid catalysts have attracted much attention because of the preferable and exceptional properties that are highly expected to result from combination of the components. Base catalysts are widely used in organic synthesis as key materials, and a new class of base catalysts has made a large impact from academic and industrial viewpoints. Here, a principle for creating a new strong base by hybridization of homogeneous and heterogeneous components is presented. It is based on the modification of organic compounds with metal oxides by using the acid-base property of metal oxides. Based on kinetic and DFT studies, combination of CeO2 and 2-cyanopyridine drastically enhanced the basicity of 2-cyanopyridine by a factor of about 109 (∼9 by pKa (in CH3CN)), and the pKa was estimated to be ∼21, which locates it in the superbase category. 2-Cyanopyridine and CeO2 formed a unique adsorption complex via two interaction modes: (i) coordinative interaction between the Ce atom of CeO2 and the N atom of the pyridine ring in 2-cyanopyridine, and (ii) covalent interaction between the surface O atom of CeO2 and the C atom of the CN group in 2-cyanopyridine by addition of the lattice oxygen of CeO2 to the CN group of 2-cyanopyridine. These interactions established a new, strongly basic site of N- over the CeO2 surface.

Insight into the mechanism of hydrogenation of amino acids to amino alcohols catalyzed by a heterogeneous MoO(x) -modified Rh catalyst.

Hydrogenation of amino acids to amino alcohols is a promising utilization of natural amino acids. We found that MoOx -modified Rh/SiO2 (Rh-MoOx /SiO2 ) is an efficient heterogeneous catalyst for the reaction at low temperature (323 K) and the addition of a small amount of MoOx drastically increases the activity and selectivity. Here, we report the catalytic potential of Rh-MoOx /SiO2 and the results of kinetic and spectroscopic studies to elucidate the reaction mechanism of Rh-MoOx /SiO2 catalyzed hydrogenation of amino acids to amino alcohols. Rh-MoOx /SiO2 is superior to previously reported catalysts in terms of activity and substrate scope. This reaction proceeds by direct formation of an aldehyde intermediate from the carboxylic acid moiety, which is different from the reported reaction mechanism. This mechanism can be attributed to the reactive hydride species and substrate adsorption caused by MoOx modification of Rh metal, which results in high activity, selectivity, and enantioselectivity.

Combination of supported bimetallic rhodium-molybdenum catalyst and cerium oxide for hydrogenation of amide.

Hydrogenation of cyclohexanecarboxamide to aminomethylcyclohexane was conducted with silica-supported bimetallic catalysts composed of noble metal and group 6-7 elements. The combination of rhodium and molybdenum with molar ratio of 1:1 showed the highest activity. The effect of addition of various metal oxides was investigated on the catalysis of Rh-MoO x /SiO2, and the addition of CeO2 much increased the activity and selectivity. Higher hydrogen pressure and higher reaction temperature in the tested range of 2-8 MPa and 393-433 K, respectively, were favorable in view of both activity and selectivity. The highest yield of aminomethylcyclohexane obtained over Rh-MoO x /SiO2 + CeO2 was 63%. The effect of CeO2 addition was highest when CeO2 was not calcined, and CeO2 calcined at >773 K showed a smaller effect. The use of CeO2 as a support rather decreased the activity in comparison with Rh-MoO x /SiO2. The weakly-basic nature of CeO2 additive can affect the surface structure of Rh-MoO x /SiO2, i.e. reducing the ratio of Mo-OH/Mo-O- sites.

Redox properties of CeO2 at low temperature: the direct synthesis of imines from alcohol and amine.

We disclosed the redox properties of CeO2 in organic reactions at low temperature of 303 K. CeO2 works as the most effective heterogeneous catalyst for imine formation from benzyl alcohol and aniline at 303 K among various metal oxides and showed more than 38-fold higher activity than other simple metal oxides. CeO2 is applicable to the reaction of various alcohols and amines and gives high yields (80-98%) and high selectivities (89->99%). Kinetic measurements, MS, and FTIR analyses demonstrated that the high activity of CeO2 is a result of reactive oxygen species at the redox sites on CeO2. This discovery can help to create a new field in metal oxide catalysis.

Hydrodeoxygenation of vicinal OH groups over heterogeneous rhenium catalyst promoted by palladium and ceria support.

Heterogeneous ReOx-Pd/CeO2 catalyst showed excellent performance for simultaneous hydrodeoxygenation of vicinal OH groups. High yield (>99%), turnover frequency (300 h(-1)), and turnover number (10,000) are achieved in the reaction of 1,4-anhydroerythritol to tetrahydrofuran. This catalyst can be applied to sugar alcohols, and mono-alcohols and diols are obtained in high yields (≥85%) from substrates with even and odd numbers of OH groups, respectively. The high catalytic performance of ReOx-Pd/CeO2 can be assigned to rhenium species with +4 or +5 valence state, and the formation of this species is promoted by H2/Pd and the ceria support.

Selective hydrogenolysis of C-O bonds using the interaction of the catalyst surface and OH groups.

Hydrogenolysis of C-O bonds is becoming more and more important for the production of biomass-derived chemicals. Since substrates originated from biomass usually have high oxygen content and various kinds of C-O bonds, selective hydrogenolysis is required. Rhenium or molybdenum oxide modified rhodium and iridium metal catalysts (Rh-ReO(x), Rh-MoO(x), and Ir-ReO(x)) have been reported to be effective for selective hydrogenolysis. This review introduces the catalytic performance and reaction kinetics of Rh-ReO(x), Rh-MoO(x), and Ir-ReO(x) in the hydrogenolysis of various substrates, where selectivity is especially characteristic. Based the model structure of the catalysts and the reaction mechanism, the role of the oxide components is to make the interaction between the OH groups in the substrates and the catalyst surface, and the role of metal components is to dissociate hydrogen molecule heterolytically to give hydride and proton.

Role of Re species and acid cocatalyst on Ir-ReOx /SiO2 in the C-O hydrogenolysis of biomass-derived substrates.

The catalytic performance of ReOx -modified Ir metal catalyst in the hydrogenolysis of C-O bonds is strongly dependent on the choice of solvent. The acidic property of the Re species becomes obvious in the alkane solvent, and the hydrogenolysis reaction proceeds mainly by acid-catalyzed dehydration and the subsequent metal-catalyzed hydrogenation. The acidic property of the Re species is weakened in water; however, the hydrogenolysis reaction proceeds in water via a direct mechanism involving SN 2-like attack of a hydride species at the interface between Ir and ReOx on the adsorbed Re alkoxide species. This mechanism enabled the selective dissociation of the C-O bond neighboring the CH2 OH group.

Direct Esterification of Alkylcarbamic Acids with Alcohols over CeO2 Catalyst.

Alkylcarbamic acids, which are easily produced via chemical absorption of CO2 into amines, have a great potential to be substrates for producing value-added chemicals. In this research, the esterification of various alkylcarbamic acids with alcohols into alkyl N-alkylcarbamates was demonstrated by using a heterogeneous catalyst as well as the corresponding amine additives. In the model reaction, the esterification of benzylcarbamic acid (BZA-CA) and methanol (MeOH), the target product of methyl N-benzylcarbamate was obtained in 64 % CO2 -based yield at 413 K in 12 h over a CeO2 catalyst, which also exhibited good reusability. In this catalytic system, the corresponding amine additive (i. e., benzylamine for BZA-CA) had the important role in the improvement of CO2 -moiety-based balance, allowing the precise kinetic study, in contrast to the cases without such additive. The detailed kinetic study on the target catalytic system and control systems suggested that BZA-CA underwent the esterification by MeOH directly. The current catalytic system using the combination of CeO2 catalyst and corresponding amine additive was also demonstrated to be applicable to the synthesis of alkyl N-alkylcarbamates from alkylcarbamic acids and alcohols with short, linear alkyl chains (≤C3 ).

Formation of paired Ga sites in CHA-type zeolite frameworks via a transcription-induced method.

Paired Ga sites represented by the Ga-O-Si-O-Ga sequence were firstly formed intentionally in CHA-type zeolite frameworks via the transcription of pre-formed paired Ga species in a Ga-rich amorphous silica-gallia under seed-assisted hydrothermal conditions. Such paired Ga sites behaved as ion-exchange sites for capturing divalent cation, Co2+.

Comparative Study between 2-Furonitrile and 2-Cyanopyridine as Dehydrants in Direct Synthesis of Dialkyl Carbonates from CO2 and Alcohols over Cerium Oxide Catalyst.

The shift of equilibrium by removing water with nitrile dehydrants is crucial for CeO2 -catalyzed synthesis of dialkyl carbonates from CO2 and alcohols. Two nitriles - 2-cyanopyridine and 2-furonitrile - were previously found as effective dehydrants, yet their detailed comparison as well as exploration of potential of 2-furonitrile remain insufficient. Herein, the performance of 2-cyanopyridine and 2-furonitrile was compared in the synthesis of various dialkyl carbonates. 2-furonitrile was found to be superior to 2-cyanopyridine in the synthesis of dialkyl carbonates from CO2 and bulky or long-chain (≥C3) alcohols. Namely, the yield of diisopropyl carbonate (up to 50 %) achieved using CeO2 and 2-furonitrile is comparable to or even higher than previously reported ones. Meanwhile, 2-cyanopyridine acted as a better dehydrant than 2-furonitrile in the synthesis of dimethyl carbonate and diethyl carbonate. The adsorption experiments and density functional theory calculations have indicated that the better performance of 2-furonitrile compared to 2-cyanopyridine in the synthesis of dialkyl carbonates from bulky or long-chain alcohols is due to the weaker interaction of 2-furonitrile with the CeO2 surface. Such weak interaction of 2-furonitrile offers a larger reaction field on the catalyst surface for both CO2 and alcohols.

FTIR study of CO adsorption on Rh/MgO modified with Co, Ni, Fe, or CeO2 for the catalytic partial oxidation of methane.

The surface state of Rh/MgO catalysts modified with Co, Ni, Fe, or CeO(2) after the reduction and partial oxidation pretreatments as well as during the catalytic partial oxidation of methane has been investigated by FTIR of adsorbed CO. The results of CO adsorption on the reduced catalysts suggest the formation of Rh-M alloy on Rh-M/MgO (M = Co, Ni, Fe) and Rh particles partially covered with reduced ceria on Rh-CeO(2)/MgO. The strength of CO adsorption on Rh/MgO is weakened by the modification with Co, Ni, Fe, or CeO(2). Partial oxidation pretreatment of Rh/MgO leads to a significant decrease in the CO adsorption due to the oxidation of Rh. In contrast, on partially oxidized Rh-M/MgO (M = Co, Ni, Fe) and Rh-CeO(2)/MgO, the preferential oxidation of the surface M atoms or reduced ceria maintains the metallic Rh and preserves the CO adsorbed on the surface Rh atoms. The CO adsorption during the reaction of catalytic partial oxidation of methane on Rh/MgO and Rh-Ni/MgO is similar to that on the reduced catalysts. On the other hand, the CO adsorption during the reaction on Rh-Co/MgO, Rh-Fe/MgO, and Rh-CeO(2)/MgO is different from that on the reduced catalysts, and this is related to the structural change of these catalysts during the reaction.

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 ß-ribofuranoside: 88 % yield), which were difficult to be converted to the DODH products over the DODH catalysts reported previously. ReOx -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.

Porous nanographene formation on γ-alumina nanoparticles via transition-metal-free methane activation.

γ-Al2O3 nanoparticles promote pyrolytic carbon deposition of CH4 at temperatures higher than 800 °C to give single-walled nanoporous graphene (NPG) materials without the need for transition metals as reaction centers. To accelerate the development of efficient reactions for NPG synthesis, we have investigated early-stage CH4 activation for NPG formation on γ-Al2O3 nanoparticles via reaction kinetics and surface analysis. The formation of NPG was promoted at oxygen vacancies on (100) surfaces of γ-Al2O3 nanoparticles following surface activation by CH4. The kinetic analysis was well corroborated by a computational study using density functional theory. Surface defects generated as a result of surface activation by CH4 make it kinetically feasible to obtain single-layered NPG, demonstrating the importance of precise control of oxygen vacancies for carbon growth.

CeO2-Catalyzed Synthesis of 2-Imidazolidinone from Ethylenediamine Carbamate.

CeO2 acted as an effective and reusable heterogeneous catalyst for the direct synthesis of 2-imidazolidinone from ethylenediamine carbamate (EDA-CA) without further addition of CO2 in the reaction system. 2-Propanol was the best solvent among the solvents tested from the viewpoint of selectivity to 2-imidazolidinone, and the use of an adequate amount of 2-propanol provided high conversion and selectivity for the reaction. This positive effect of 2-propanol on the catalytic reaction can be explained by the solubility of EDA-CA in 2-propanol under the reaction conditions and no formation of solvent-derived byproducts. This catalytic system using the combination of the CeO2 catalyst and the 2-propanol solvent provided 2-imidazolidinone in up to 83% yield on the EDA-CA basis at 413 K under Ar. The reaction conducted under Ar showed a higher reaction rate than that with pressured CO2, which clearly demonstrated the advantage of the catalytic system operated at low CO2 pressure or even without CO2.