2,5-Furandicarboxylic (FDCA) is a potential substitute for petroleum-derived terephthalic acid, and aerobic oxidation of 5-hydroxymethylfurfural (HMF) provides an efficient route to synthesis of FDCA. On an activated carbon supported ruthenium (Ru/C) catalyst (with 5 wt% Ru loading), HMF was readily oxidized to FDCA in a high yield of 97.3% at 383 K and 1.0 MPa O2 in the presence of Mg(OH)2 as base additive. Ru/C was superior to Pt/C and Pd/C and also other supported Ru catalysts with similar sizes of metal nanoparticles (1–2 nm). The Ru/C catalysts were stable and recyclable, and their efficiency in the formation of FDCA increased with Ru loadings examined in the range of 0.5 wt%–5.0 wt%. Based on the kinetic studies including the effects of reaction time, reaction temperature, O2 pressure, on the oxidation of HMF to FDCA on Ru/C, it was confirmed that the oxidation of HMF to FDCA proceeds involving the primary oxidation of HMF to 2,5-diformylfuran (DFF) intermediate, and its sequential oxidation to 5-formyl-2-furancarboxylic acid (FFCA) and ultimately to FDCA, in which the oxidation of FFCA to FDCA is the rate-determining step and dictates the overall formation rate of FDCA. This study provides directions towards efficient synthesis of FDCA from HMF, for example, by designing novel catalysts more efficient for the involved oxidation step of FFCA to FDCA.

A fundamental understanding of the structural effects of supported metal catalysts at the molecular level is extremely important for developing high-performance catalysts that are widely used in industry, which is still a longstanding attractive but challenging topic in multidisciplinary fields. In this work, we report the strong effects of local coordination structures on the catalytic activity of subnanometric PtOx clusters over CeO2 nanowires in low-temperature CO oxidation as a probe reaction. Atoms and subnanometric clusters of Pt were deposited to form the coordination structure of PtOx on the well-defined CeO2 nanowires with mainly exposed (110) facets. The reactivity of active sites and the variation of the local coordination structures of the PtOx sites were deeply investigated with in situ spectroscopic experiments, assisted by density functional theory simulations. According to our observation, although the highly dispersed Pt sites at the subnanometric scale could provide increased accessible sites, some of the Pt sites did not show high activity for CO oxidation due to the increased surrounding oxygen that seemed to overstabilize the Pt atoms. An increased proportion of both adsorbed CO intermediates on oxidized Pt sites and the interfacial lattice oxygen of PtOx clusters tended to become inactive on samples with a high coordination number of oxygen bonded to Pt sites (CN(Pt–O)), leading to the loss of effective active sites and a decrease in the catalytic activity. A relatively small CN(Pt–O) value in the subnanometric PtOx/CeO2 NWs, which was found to be the appropriate structure for their catalytic performance, remarkably increased the activity by about 1/2 order of magnitude. We believe our investigation on the interfacial coordination structure effects of subnanometric PtOx clusters dispersed on CeO2 nanowires can provide some new basic chemical insights into the metal–support interfacial interactions of Pt/CeO2 catalysts for understanding their catalytic performance in some relevant reactions.Keywords: CeO2 nanowires; CO oxidation; in situ spectra; local coordination structure; PtOx clusters

Succinic acid is an important biomass-derived C4 building block and is ready to be converted into various value-added chemicals. Here we report that tunable selectivity for the formation of 1,4-butanediol, γ-butyrolactone and tetrahydrofuran from aqueous succinic acid hydrogenation could be achieved on FeOx-promoted Pd/C catalysts. Fe was found to be an efficient promoter for the succinic acid hydrogenation, which not only improved the activity of the catalysts but also tuned the product distribution. Succinic acid could be transformed into 1,4-butanediol with a yield of over 70% in the presence of the Pd–5FeOx/C catalyst under the relatively mild conditions of 200 °C and 5 MPa H2. The reaction pathway was also proposed according to the reaction and characterization results.

The water-soluble metal nanoparticles (NPs) stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) were prepared and examined as catalysts for the one-step selective hydrogenation of phenol to cyclohexanone in water. More than 99% conversion of phenol and selectivity to cyclohexanone was obtained at 90 °C and 1 atm H2 for 16 h over “soluble” Pd NPs that were reduced by NaBH4 and stabilized by PVP. These Pd NPs were stable, and no leaching or aggregation was detected after five successive runs, showing their advantage for catalyzing the efficient synthesis of cyclohexanone via the one-step selective hydrogenation of phenol under mild conditions.

•High acetol yields from cellulose, and also from glucose and fructose were obtained.•The mechanism of cellulose conversion to acetol on bimetallic Ni-SnOx/Al2O3 catalysts was studied.•SnOx facilitated the isomerizaiton of glucose to fructose and its subsequent CC bond cleavage.•The hydrogenation activity of the Ni-SnOx/Al2O3 catalysts depends on the Sn/Ni ratio.The direct conversion of cellulose into acetol was studied on SnOx-modified Ni/Al2O3 catalysts with different Sn/Ni atomic ratios in the range of 0–2.0. The selectivity to acetol strongly depended on the Sn/Ni ratios, which reached the highest value of 53.9% at the ratio of 0.5, compared at similar cellulose conversions (∼20%). On Ni-SnOx/Al2O3 (Sn/Ni = 0.5), cellulose, glucose and fructose converted to acetol in high yields of approximately 35%, 53% and 73%, respectively, at 210 °C and 6 MPa H2. The effects of the Sn/Ni ratios on the acetol selectivity appear to be related to their effects on the hydrogenation activity of the Ni-SnOx/Al2O3 catalysts that decreased with increase of the Sn/Ni ratios, and to the relative rate between the hydrogenation of C6 sugar intermediates (e.g. glucose and fructose) and their degradation intermediates (e.g. glyceraldeyde and dihydroxyacetone) involved in the cellulose reaction on the Ni particles and the isomerization of glucose to fructose and their CC bond cleavage by retro-aldol condensation on the SnOx domains. Comparison of SnOx with CeOx, ZnOx and AlOx supported on Al2O3 with different basicity suggested that the larger concentration of stronger basic sites on SnOx facilitated the isomerizaiton of glucose to fructose and its subsequent CC bond cleavage. These results and their understanding provide guidance for improving the acetol production from cellulose by tuning the catalytic functions required for the involved reactions of hydrogenation on the metal surfaces, and isomerization and CC bond cleavage on the basic sites.Cellulose directly converted to acetol on bimetallic Ni-SnOx/Al2O3 catalysts with a high efficiency.

Active center engineering at atomic level is a grand challenge for catalyst design and optimization in many industrial catalytic processes. Exploring new strategies to delicately tailor the structures of active centers and bonding modes of surface reactive intermediates for nanocatalysts is crucial to high-efficiency nanocatalysis that bridges heterogeneous and homogeneous catalysis. Here we demonstrate a robust approach to tune the CO oxidation activity over CeO2 nanowires (NWs) through the modulation of the local structure and surface state around LnCe′ defect centers by doping other lanthanides (Ln), based on the continuous variation of the ionic radius of lanthanide dopants caused by the lanthanide contraction. Homogeneously doped (110)-oriented CeO2:Ln NWs with no residual capping agents were synthesized by controlling the redox chemistry of Ce(III)/Ce(IV) in a mild hydrothermal process. The CO oxidation reactivity over CeO2:Ln NWs was dependent on the Ln dopants, and the reactivity reached the maximum in turnover rates over Nd-doped samples. On the basis of the results obtained from combined experimentations and density functional theory simulations, the decisive factors of the modulation effect along the lanthanide dopant series were deduced as surface oxygen release capability and the bonding configuration of the surface adsorbed species (i.e., carbonates and bicarbonates) formed during catalytic process, which resulted in the existence of an optimal doping effect from the lanthanide with moderate ionic radius.

Monoclinic zirconia (m-ZrO2) supported Ru, Rh, Pt, and Pd nanoparticles with controlled sizes were prepared and examined in glycerol hydrogenolysis to propylene glycol and ethylene glycol at similar conversions in the kinetic regime. Their activity (normalized per exposed surface metal atom, i.e., turnover rate) and selectivity depend sensitively on the nature of the noble metals and their particle size. At a similar size (ca. 2 nm), Ru exhibited a greater turnover rate than Rh, Pt, and Pd, and the rate decreased in the sequence Ru ≫ Rh > Pt > Pd by a factor of about 25 (from 0.035 to 0.0014 mol glycerol (mol surface metal·s)−1) at 473 K and 6.0 MPa H2. Following such activity sequence, Ru was more prone to catalyze excessive cleavage of C–C bonds, leading to the formation of ethylene glycol and methane, while Pd exhibited the highest selectivity to cleavage of C–O bonds to propylene glycol. Similarly, larger Ru particles possessed higher glycerol hydrogenolysis activity concurrently with higher selectivities to ethylene glycol and especially methane at the expense of propylene glycol in the range of 1.8–4.5 nm. Analysis of kinetics and thermodynamics for the proposed elementary steps involving kinetically relevant glycerol dehydrogenation to glyceraldehyde leads to expressions of glycerol hydrogenolysis rate and selectivity to cleavage of C–O bonds relative to C–C bonds. Together with different effects of reaction temperature and atmosphere of H2 and N2 on the activity and selectivity for Ru/m-ZrO2 and Pt/m-ZrO2, these results suggest that the observed difference for different noble metals and particle sizes can be attributed to the difference in the strength of adsorption of glycerol and glyceraldehyde, derived from their different availability of unoccupied d orbitals.Keywords: ethylene glycol; glycerol; noble metal; propylene glycol; selective hydrogenolysis; size effect

Cellulose is the most abundant source of biomass in nature, and its selective conversion into polyols provides a viable route towards the sustainable synthesis of fuels and chemicals. Here, we report the marked change in the distribution of polyols in the cellulose reaction with the Sn/Pt atomic ratios in a wide range of 0.1–3.8 on the SnOx-modified Pt/Al2O3 catalysts. Such a change was found to be closely related to the effects of the Sn/Pt ratios on the activity for the hydrogenation of glucose and other C6 sugar intermediates involved in the cellulose reaction as well as to the notable activity of the segregated SnOx species for the selective degradation of the sugar intermediates on the Pt–SnOx/Al2O3 catalysts. At lower Sn/Pt ratios of 0.1–1.0, there existed electron transfer from the SnOx species to the Pt sites and strong interaction between the catalysts, as characterized by temperature-programmed reduction in H2 and infrared spectroscopy for CO adsorption, which led to their superior hydrogenation activity (per exposed Pt atom), and in-parallel higher selectivity to hexitols (e.g. sorbitol) in the cellulose reaction, as compared to Pt/Al2O3. The hexitol selectivity reached the greatest value of 82.7% at the Sn/Pt ratio of 0.5, nearly two times that of Pt/Al2O3 at similar cellulose conversions (∼20%). As the Sn/Pt ratios exceeded 1.5, the Pt–SnOx/Al2O3 catalysts exhibited inferior hydrogenation activity (per exposed Pt atom), due to the formation of the crystalline Pt–Sn alloy, which led to the preferential conversion of cellulose to C2 and especially C3 products (e.g. acetol) over hexitols, most likely involving the isomerization of glucose to fructose and retro-aldol condensation of these sugars on the segregated SnOx species, apparently in the form of Sn(OH)2. These findings clearly demonstrate the feasibility for rational control of the cellulose conversion into the target polyols (e.g. acetol or propylene glycol), for example, by the design of efficient catalysts based on the catalytic functions of the SnOx species with tunable hydrogenation activity.

The selective hydrogenolysis of biomass-derived xylitol to ethylene glycol and propylene glycol was carried out on different catalysts in the presence of Ca(OH)2. The catalysts included Ru supported on activated carbon (C) and, for comparison, on metal oxides, Al2O3, TiO2, ZrO2 and Mg2AlOx as well as C-supported other noble metals, Rh, Pd and Pt, with similar particle sizes (1.6–2.0 nm). The kinetic effects of H2 pressures (0–10 MPa), temperatures (433–513 K) and solid bases including Ca(OH)2, Mg(OH)2 and CaCO3 were examined on Ru/C. Ru/C exhibited superior activities and glycol selectivities than Ru on TiO2, ZrO2, Al2O3 and Mg2AlOx, and Pt was found to be the most active metal. Such effects of the metals and supports are attributed apparently to their different dehydrogenation/hydrogenation activities and surface acid-basicities, which consequently influenced the xylitol reaction pathways. The large dependencies of the activities and selectivities on the H2 pressures, reaction temperatures, and pH values showed their effects on the relative rates for the hydrogenation and base-catalyzed reactions involved in xylitol hydrogenolysis, reflecting the bifunctional nature of the xylitol reaction pathways. These results led to the proposition that xylitol hydrogenolysis to ethylene glycol and propylene glycol apparently involves kinetically relevant dehydrogenation of xylitol to xylose on the metal surfaces, and subsequent base-catalyzed retro-aldol condensation of xylose to form glycolaldehyde and glyceraldehyde, followed by direct glycolaldehyde hydrogenation to ethylene glycol and by sequential glyceraldehyde dehydration and hydrogenation to propylene glycol. Clearly, the relative rates between the hydrogenation of the aldehyde intermediates and their competitive reactions with the bases dictate the selectivities to the two glycols. This study provides directions towards efficient synthesis of the two glycols from not only xylitol, but also other lignocellulose-derived polyols, which can be achieved, for example, by optimizing the reaction parameters, as already shown by the observed effects of the catalysts, pH values, and H2 pressures.

It is of great significance and challenge to achieve direct conversion of cellulose to specific polyols, e.g., ethylene glycol and propylene glycol. For such selective conversion, a novel one-pot approach was studied by combination of alkaline hydrolysis and hydrogenation on supported Ru catalysts. A wide range of bases including solid bases, e.g., Ca(OH)2 and La2O3, and phosphate buffers were examined in the cellulose reaction in water, and the cellulose conversions and polyol products depended largely on the basicity or pH values in the aqueous solutions. Ethylene glycol, 1,2-propanediol, and especially 1,2,5-pentanetriol were obtained with selectivities of 15%, 14% and 22%, respectively, at 38% cellulose conversion at pH 8 in phosphate buffer solution. These preliminary results provide potentials for efficient conversion of cellulose to targeted polyols by using the advantages of bases.

Seven soluble metal nanoparticle catalysts including Pt, Ru, Rh, Pd, Ir, Ag and Au were synthesized and studied for the aqueous-phase selective oxidation of non-activated alcohols under atmospheric pressure of O2. The effects of particle size were examined on the Pt catalysts with mean diameters of 1.5–4.9 nm. Pt nanoparticles efficiently catalyze the aerobic oxidation of alicyclic and aliphatic alcohols, in particular, primary aliphatic alcohols in the absence of any base. The particle sizes of the Pt catalysts strongly influence their activities, and the one of 1.5 nm exhibits much higher turnover frequencies. In comparison with the other metals examined in this work, it is concluded that Pt is the best metal of choice for the aerobic alcohol oxidation. Aliphatic primary alcohols reacts on the Pt catalysts more preferentially over their isomeric secondary alcohols with increasing their chain length or as they coexist. These steric effects, and the observed kinetic isotope effects with 1-C4H9OD and 1-C4D9OD are consistent with the general alcohol oxidation mechanism, which includes a sequence of elementary steps involving the formation of the alcoholate intermediates in quasi-equilibrated 1-C4H9OH dissociation on the Pt surfaces and the rate-determining hydrogen abstraction from the alcoholates. The inhibiting effects of hydroquinone, a typical radical scavenger, are indicative of the formation of radical intermediates in the H-abstraction step.

Pure monoclinic (m) and tetragonal (t) zirconia nanoparticles were readily synthesized from the reaction of inorganic zirconium salts (e.g., hydrated zirconyl nitrate) and urea in water and methanol, respectively, via a facile solvothermal method. The role of the solvents was crucial in the formation of the pure ZrO2 phases, whereas their purity was essentially insensitive to other variables, including reaction temperature, reactant concentration, pH, and zirconium salts. Water as the solvent led to the transformation of hydrous ZrO2 precipitates initially formed with tetragonal structures to thermodynamically more stable m-ZrO2 via the dissolution−precipitation process, whereas methanol favored the removal of water molecules from the precursors via their reaction with urea, consequently maintaining the tetragonal structures. The obtained tetragonal samples were found to possess superior hydrothermal stability compared to those reported previously, which provides the possibility for systematically studying the effects of ZrO2 phases on many catalytic reactions involving water as a reactant or product. Using these pure m- and t-ZrO2 phases as supports, dispersed MoOx catalysts were synthesized at MoOx surface densities of ∼5.0 Mo/nm2, which is close to one monolayer of coverage. Characterization by X-ray diffraction and Raman spectroscopy confirmed that the pure ZrO2 phases remained unchanged in the presence of the MoOx domains and the MoOx domains existed preferentially as 2D polymolybdate structures. The catalysts were subsequently examined for selective methanol oxidation as a test reaction. m-ZrO2 support led to 2-fold greater oxidation rates than for t-ZrO2 support, reflecting the higher intrinsic reactivity of the MoOx domains on m-ZrO2. This is consistent with their higher reducibility probed by temperature-programmed reduction with H2 (H2 TPR). These observed effects of the ZrO2 phases provide the basis for designing catalysts with tunable redox properties and reactivity.

Hydrogenolysis of biomass-derived glycerol is an alternative route to sustainable production of propylene glycol. Cu–ZnO catalysts were prepared by coprecipitation with a range of Cu/Zn atomic ratio (0.6–2.0) and examined in glycerol hydrogenolysis to propylene glycol at 453–513 K and 4.2 MPa H2. These catalysts possess acid and hydrogenation sites required for bifunctional glycerol reaction pathways, most likely involving glycerol dehydration to acetol and glycidol intermediates on acidic ZnO surfaces, and their subsequent hydrogenation on Cu surfaces. Glycerol hydrogenolysis conversions and selectivities depend on Cu and ZnO particle sizes. Smaller ZnO and Cu domains led to higher conversions and propylene glycol selectivities, respectively. A high propylene glycol selectivity (83.6%), with a 94.3% combined selectivity to propylene glycol and ethylene glycol (also a valuable product) was achieved at 22.5% glycerol conversion at 473 K on Cu–ZnO (Cu/Zn = 1.0) with relatively small Cu particles. Reaction temperature effects showed that optimal temperatures (e.g. 493 K) are required for high propylene glycol selectivities, probably as a result of optimized adsorption and transformation of the reaction intermediates on the catalyst surfaces. These preliminary results provide guidance for the synthesis of more efficient Cu–ZnO catalysts and for the optimization of reaction parameters for selective glycerol hydrogenolysis to produce propylene glycol.

The oxidation of 1-octanethiol to corresponding disulfide as a model reaction for mercaptan sweetening was examined on the bifunctional catalysts prepared by supporting cobalt phthalocyanine tetrasulfonate (CoPcTs) on mixed-oxide solid bases (designated as Mg(Al)O) derived from thermal decomposition of Mg–Al hydrotalcites with five Mg/Al molar ratios of 3.0, 4.9, 6.5, 9.0 and 13.2, and for comparison on Al2O3, MgO and K+-modified Mg(Al)O (Mg/Al = 3.0). For the five Mg(Al)O samples, at a similar CoPcTs dispersion, the oxidation rates increased in parallel with the number of the basic sites provided by the support surfaces, but their rates were greater than those of the corresponding MgO and K+-modified Mg(Al)O samples although MgO and K+-modified Mg(Al)O possessed more basic sites with stronger base strength compared to Mg(Al)O, showing the dependence of the oxidation rates not only on the numbers of the basic sites, but also on their nature and strength. In combination of the in situ infrared results for 1-hexanethiol adsorption on Mg(Al)O, effects of titration of the basic sites with CO2, BF3 and H2O on the reaction rates for CoPcTs/Mg(Al)O (Mg/Al = 3.0) led to the identification of the basic OH− rather than O2− ions as the active basic sites for the mercaptan oxidation. 1-propanethiol temperature-programmed desorption showed the effects of the base strength on the strength of the mercaptan adsorption on the Co2+ site of CoPcTs as a result of the effects on the electron density of the Co2+ site. Such effects led to the poor activities for the MgO and K+-modified Mg(Al)O samples, due to their strong base strength and consequently weaker mercaptan adsorption and activation on the Co2+ sites. It is thus clear that the OH− basic sites and medium base strength are required for the mercaptan oxidation, which provides the basis for the rational design of more efficient solid bases for the sweetening process.

The aerobic oxidation of glycerol provides an economically viable route to glyceraldehyde, dihydroxyacetone and glyceric acid with versatile applications, for which monometallic Pt, Au and Pd and bimetallic Au–Pt, Au–Pd and Pt–Pd catalysts on TiO2 were examined under base-free conditions. Pt exhibited a superior activity relative to Pd, and Au–Pd and Pt–Pd while Au was essentially inactive. The presence of Au on the Au–Pt/TiO2 catalysts led to their higher activities (normalized per Pt atom) in a wide range of Au/Pt atomic ratios (i.e. 1/3–7/1), and the one with the Au/Pt ratio of 3/1 exhibited the highest activity. Such promoting effect is ascribed to the increased electron density on Pt via the electron transfer from Au to Pt, as characterized by the temperature-programmed desorption of CO and infra-red spectroscopy for CO adsorption. Meanwhile, the presence of Au on Au–Pt/TiO2, most like due to the observed electron transfer, changed the product selectivity, and facilitated the oxidation of the secondary hydroxyl groups in glycerol, leading to the favorable formation of dihydroxyacetone over glyceraldehyde and glyceric acid that were derived from the oxidation of the primary hydroxyl groups. The synergetic effect between Au and Pt demonstrates the feasibility in the efficient oxidation of glycerol to the targeted products, for example, by rational tuning of the electronic properties of metal catalysts.Aerobic glycerol oxidation activity and selectivity to glyceraldehyde and dihydroxyacetone correlate with the electronic properties of Au–Pt/TiO2 catalysts tuned by their atomic ratios of Au/Pt. Download high-res image (82KB)Download full-size image

5-Hydroxymethylfurfural (HMF) is an important biomass-based platform chemical, and its selective aerobic oxidation to 2,5-diformylfuran (DFF) remains a formidable challenge. This work reports that activated carbon-supported Ru clusters (Ru/C) efficiently catalyzed HMF oxidation to DFF with a high yield of ∼96% at 383 K and 2.0 MPa O2 in toluene. Ru/C exhibited activities and DFF selectivities superior to those of Ru clusters with similar sizes (ca. 2 nm) on oxide supports, including Al2O3, ZSM-5, TiO2, ZrO2, CeO2, MgO, and Mg2AlOx, as a consequence of their different surface acidity–basicity and reducibility, which tend to facilitate degradation and polymerization of HMF and DFF. It was also superior to C-supported Pt, Pd, Rh, and Au at comparable sizes in the HMF oxidation to DFF. The effects of O2 and HMF concentrations on HMF oxidation were examined on Ru/C, showing near half-order dependence of the activities on them. Kinetic isotopic studies showed marked and no kinetic isotopic effects for two HMF molecules deuterated, respectively, at the methylene (kC–H/kC–D = 3.73) and hydroxyl (kO–H/kO–D = 1.09) groups. Taken together, these results are consistent with a Langmuir–Hinshelwood mechanism and a sequence of elementary steps involving kinetically-relevant H-abstraction from adsorbed alcoholate species using adsorbed atomic oxygen species, derived from the quasi-equilibrated dissociation of HMF and O2, respectively, on Ru surfaces. This reaction mechanism leads to a complex kinetic rate expression that accurately describes the measured HMF oxidation activities in a wide range of HMF and O2 concentrations.Graphical abstractRu/C is an effective and stable catalyst in the aerobic oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran, affording a high yield of ∼96% under mild conditions. Kinetic and isotopic studies suggest that this oxidation reaction proceeds by a Langmuir–Hinshelwood mechanism involving kinetically-relevant β-H elimination using dissociated oxygen species on the Ru surfaces.Download high-res image (34KB)Download full-size imageHighlights► Aerobic oxidation of 5-hydroxymethylfurfural achieves a 2,5-diformylfuran yield of 96% on Ru/C. ► Ru/C is superior to Ru supported on other supports and other noble metals on C. ► Metallic Ru is identified as the active catalytic species on C. ► Reaction mechanism and reaction elementary steps are proposed and verified. ► β-H elimination via the dissociated oxygen species is the rate-determining step.

•A 2,5-diformylfuran yield of 97.2% is achieved on OMS-2 for aerobic oxidation of 5-hydroxymethylfurfural (HMF).•OMS-2 is superior to other MnO2 catalysts with different morphologies.•A systematic study is carried out to identify the structural requirements for high HMF oxidation activity.•Reaction mechanism and reaction elementary steps are proposed and verified.•C–H bond cleavage and reoxidation of Mn3+ to Mn4+ are kinetically-relevant steps.A cryptomelane-type manganese oxide octahedral molecular sieve with a (2 × 2, 4.6 Å × 4.6 Å) tunnel size (OMS-2) efficiently catalyzed aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) with a high yield of 97.2% at 383 K and 0.5 MPa O2 in N,N-dimethylformamide. OMS-2 was superior to other MnO2 catalysts with different morphologies, including OMS-1, OMS-6, and OMS-7 with various tunnel sizes, amorphous MnO2 and birnessite-type MnO2, apparently due to its (2 × 2) tunnel structure and consequently high reducibility and oxidizability. Kinetic and isotopic studies on OMS-2 showed near half-order dependence of the activities on HMF and O2 concentrations and marked kinetic isotope effects for deuterated HMF at its methylene group. These results, together with the similar initial rates under aerobic and anaerobic conditions, suggest that HMF oxidation to DFF on OMS-2 proceeds via a redox mechanism involving kinetically-relevant steps of C–H bond cleavage in adsorbed alcoholate intermediate, derived from quasi-equilibrated dissociation of HMF, using lattice oxygen and reoxidation of Mn3+ to Mn4+ by dissociative chemisorption of O2.Download high-res image (133KB)Download full-size image

Au/Ce1–xZrxO2 catalysts (x = 0–0.8) were prepared by a deposition-precipitation method using Ce1–xZrxO2 nanoparticles as supports with variable Ce and Zr contents. Their structures were characterized by complimentary means such as X-ray diffraction, Raman, scanning transmission electron microscopy and X-ray photoelectron spectroscopy (XPS). These Au catalysts possessed similar sizes and crystalline phases of Ce1–xZrxO2 supports as well as similar sizes and oxidation states of Au nanoparticles. The oxidation state of Au nanoparticles was dominated by Au0 especially in CO oxidation. Their activities were examined in CO oxidation at different temperatures in the range of 303–333 K. The CO oxidation rates normalized per Au atoms increased with the increasing Ce contents, and reached the maximum value over Au/CeO2. Such change was in parallel with the change in the oxygen storage capacity values, i.e. the amounts of active oxygen species on Au/Ce1–xZrxO2 catalysts. The excellent correlation between the two properties of the catalysts suggests that the intrinsic support effects on the CO oxidation rates is related to the effects on the adsorption and activation of O2 on Au/Ce1–xZrxO2 catalysts. Such understanding on the support effects may be useful for designing more active Au catalysts, for example, by tuning the redox properties of oxide supports.

Succinic acid is an important biomass-derived C4 building block and is ready to be converted into various value-added chemicals. Here we report that tunable selectivity for the formation of 1,4-butanediol, γ-butyrolactone and tetrahydrofuran from aqueous succinic acid hydrogenation could be achieved on FeOx-promoted Pd/C catalysts. Fe was found to be an efficient promoter for the succinic acid hydrogenation, which not only improved the activity of the catalysts but also tuned the product distribution. Succinic acid could be transformed into 1,4-butanediol with a yield of over 70% in the presence of the Pd–5FeOx/C catalyst under the relatively mild conditions of 200 °C and 5 MPa H2. The reaction pathway was also proposed according to the reaction and characterization results.

Supported Ru clusters efficiently catalyzed the selective hydrogenolysis of sorbitol to ethylene glycol and propylene glycol in the presence of Ca(OH)2. Ru/C was more selective to the two target glycols than Ru catalysts on MgO, Al2O3, ZrO2 and TiO2 with similar Ru particle sizes (∼2 nm). The reaction parameters, including the amount of Ca(OH)2, H2 pressure and temperature, strongly influenced the activity and selectivity of Ru/C, which reflects the bi-functional requirements of sorbitol hydrogenolysis that involves the competitive Ru- and base-catalyzed reactions of ketose or aldose intermediates, derived primarily from sorbitol dehydrogenation. Kinetic isotopic studies with different deuterated sorbitols confirmed that such a sorbitol dehydrogenation step was kinetically relevant to the hydrogenolysis reaction, which most likely proceeded by preferential activation of its C(5)–H bond on the Ru surfaces, as indicated by the more noticeable kinetic isotope effect with sorbitol deuterated at its C-5 position than those at the other C positions. Moreover, it was found that the hydrogenolysis of hexitols with different configurations showed large differences in their activities and selectivities. Erythro sequences of the vicinal hydroxyl groups adjacent to the primary carbons in the hexitol molecules, compared to threo sequences, tended to facilitate the hydrogenolysis reaction and the formation of C3 products over C2 products, most likely as a result of the effects of the different sequences of the hydroxyl groups on the adsorption and C–H bond activation of hexitols on Ru. Clearly, these findings and understanding demonstrate the feasibility of effective synthesis of ethylene glycol and propylene glycol from sorbitol and other largely available polyols, for example, by rational design of more efficient catalysts and tuning of polyol adsorption configurations.