Haichao Liu

Selective hydrogenolysis of biomass-derived xylitol to ethylene glycol and propylene glycol on supported Ru catalysts

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.

Base-free aqueous-phase oxidation of non-activated alcohols with molecular oxygen on soluble Pt nanoparticles

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.

Promoting effect of SnOx on selective conversion of cellulose to polyols over bimetallic Pt–SnOx/Al2O3 catalysts

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.

Growth of Semiconducting Single-Walled Carbon Nanotubes by Using Ceria as Catalyst Supports

The growth of semiconducting single-walled carbon nanotubes (s-SWNTs) on flat substrates is essential for the application of SWNTs in electronic and optoelectronic devices. We developed a flexible strategy to selectively grow s-SWNTs on silicon substrates using a ceria-supported iron or cobalt catalysts. Ceria, which stores active oxygen, plays a crucial role in the selective growth process by inhibiting the formation of metallic SWNTs via oxidation. The so-produced ultralong s-SWNT arrays are immediately ready for building field effect transistors.

Aqueous-phase selective hydrogenation of phenol to cyclohexanone over soluble Pd nanoparticles

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.

Efficient production of methanol and diols via the hydrogenation of cyclic carbonates using copper–silica nanocomposite catalysts

Copper–silica nanocomposite catalysts with a uniform Cu dispersion prepared by a precipitation-gel method have been found to be highly efficient in the heterogeneous catalytic hydrogenation of CO2-derived cyclic carbonates, providing an indirect but practical approach for the transformation of CO2 to methanol with the co-production of diols under relatively mild conditions. The catalysts possessed remarkable stability in both batch and fixed-bed continuous flow reactors especially after promotion with B2O3. The reaction was found to depend sensitively on the Cu particle size, the surface acidity–basicity and the Cu valence of the catalysts. The synergetic effect between balanced Cu0 and Cu+ sites was considered to play a critical role in attaining high yields of methanol and diols.

Liquid-phase synthesis of methyl formate via heterogeneous carbonylation of methanol over a soluble copper nanocluster catalyst

Liquid-phase synthesis of methyl formate (MF) was achieved by green carbonylation of methanol with CO on a soluble copper nanocluster catalyst with high activities (e.g. 2.7–6.1 molMF molCu−1 h−1) and 100% MF selectivities under mild reaction conditions (353–443 K, 0.3–3.0 MPa CO), showing that the Cu nanoclusters can potentially replace the caustic catalysts of alkaline metal alkoxides (e.g. CH3ONa), required for the current carbonylation process in industry.

Aqueous-phase aerobic oxidation of alcohols by soluble Pt nanoclusters in the absence of base

A soluble Pt nanocluster catalyst (Pt-GLY) is efficient in the absence of base for aqueous-phase aerobic oxidation of, in particular, non-activated alcohols with high recyclability.

Total Syntheses of Festuclavine, Pyroclavine, Costaclavine, epi-Costaclavine, Pibocin A, 9-Deacetoxyfumigaclavine C, Fumigaclavine G, and Dihydrosetoclavine

A new approach for the divergent total synthesis of eight ergot alkaloids is reported. The approach allows the first total syntheses of pyroclavine, pibocin A, 9-deacetoxyfumigaclavine C, and fumigaclavine G and also enables the efficient synthesis of festuclavine, costaclavine, epi-costaclavine, and dihydrosetoclavine. The main feature of the synthesis is the use of an unprecedented Pd-catalyzed intramolecular Larock indole annulation/Tsuji-Trost allylation cascade to assemble the tetracyclic core in one step.