Construction of Oxygen Vacancy-Rich TiO2 Nanocrystals for Boosting the Ammonolysis of Caprolactam to 6-Aminocapronitrile.

Hexamethylene diamine, an important chemical intermediate for polyamides, can be synthesized through the two-step route of caprolactam (CPL) ammonolysis to 6-aminocapronitrile (ACN), followed by hydrogenation. This method has received increasing attention from academia and industry. However, studies on the catalyst structure-performance correlation in CPL ammonolysis are still sporadic. In this work, a series of anatase TiO2 with different oxygen vacancy concentrations was prepared by chemical reduction using NaBH4. The oxygen vacancy on TiO2 surface, presented as Ti3+ sites, substantially enhances the adsorption and activation of NH3, which are demonstrated as the key steps in ammonolysis. Owing to the synergistic effect of Ti3+ and Ti4+ species, the CPL conversion rate and ACN selectivity of 85 and 97%, respectively, are achieved within 250 h. Density functional theory calculations showed that the intermediates on oxygen vacancy-rich TiO2 had a more favorable adsorption energy compared to those on intact TiO2, which is in good agreement with the experimental results.

Selective methylation of toluene using CO2 and H2 to para-xylene.

Toluene methylation with methanol to produce xylene has been widely investigated. A simultaneous side reaction of methanol-to-olefin over zeolites is hard to avoid, resulting in an unsatisfactory methylation efficiency. Here, CO2 and H2 replace methanol in toluene methylation over a class of ZnZrO x -ZSM-5 (ZZO-Z5) dual-functional catalysts. Results demonstrate that the reactive methylation species (H3CO*; * represents a surface species) are generated more easily by CO2 hydrogenation than by methanol dehydrogenation. Catalytic performance tests on a fixed-bed reactor show that 92.4% xylene selectivity in CO-free products and 70.8% para-xylene selectivity in xylene are obtained on each optimized catalyst. Isotope effects of H2/D2 and CO2/13CO2 indicate that xylene product is substantially generated from toluene methylation rather than disproportionation. A mechanism involving generation of reactive methylation species on ZZO by CO2 hydrogenation and migration of the methylation species to Z5 pore for the toluene methylation to form xylene is proposed.

Tandem Hydrogenolysis-Hydrogenation of Lignin-Derived Oxygenates over Integrated Dual Catalysts with Optimized Interoperations.

The efficient hydrodeoxygenation (HDO) of lignin-derived oxygenates is essential but challenging owing to the inherent complexity of feedstock and the lack of effective catalytic approaches. A catalytic strategy has been developed that separates C-O hydrogenolysis and aromatic hydrogenation on different active catalysts with interoperation that can achieve high oxygen removal in lignin-derived oxygenates. The flexible use of tungsten carbide for C-O bond cleavage and a nickel catalyst with controlled particle size for arene hydrogenation enables the tunable production of cyclohexane and cyclohexanol with almost full conversion of guaiacol. Such integration of dual catalysts in close proximity enables superior HDO of bio-oils into liquid alkanes with high mass and carbon yields of 27.9 and 45.0 wt %, respectively. This finding provides a new effective strategy for practical applications.

Sulfur Moiety as a Double-Edged Sword for Realizing Ultrafine Supported Metal Nanoclusters with a Cationic Nature.

Heterogeneously and uniformly dispersed metal nanoclusters with high thermal stability and stable nonmetallic nature show outstanding catalytic performance. In this work, we report on the role of sulfur moieties in hydrochlorination catalysis over carbon-supported gold (Au/C). A combination of experimental and theoretical analyses shows that the -SO3H and derived -SO2H sulfur species in high oxidation states at the interface between Au and -SO3H at ≥180 °C give rise to high thermal stability and catalytic activity. By contrast, the grafted thiol group (-SH) and the derived low-valence sulfur species on carbon markedly destabilize the Au nanoclusters, promoting their rapid sintering into large Au nanoparticles and leading to the loss of their cationic nature. Theoretical calculations suggest that -SO3H favorably adsorbs and stabilizes cationic Au species. Compared to Au/C and Au-SH/C with the Auα+/Au0 atomic ratios of 1.02 and 0.24, respectively (α = 1 or 3), the activity and durability of acetylene hydrochlorination are remarkably enhanced by the interaction between the -SO3H moieties and cationic Au species that enables the high oxidation state of Au to be effectively retained (Auα+/Au0 = 3.82). These results clearly demonstrate the double-edged sword effect of sulfur moieties on the catalytic Au component in acetylene hydrochlorination. The double-edged sword effect of sulfur species in the stabilization/destabilization of metal nanoclusters is also applicable to other metals such as Ru, Pd, Pt, and Cu. Overall, this study enriches the general understanding of the stabilization of metal clusters and provides insight into a wet chemistry strategy for stabilizing supported ligand-free nanoclusters.

Intercalation of nanostructured CeO2 in MgAl2O4 spinel illustrates the critical interaction between metal oxides and oxides.

Heterogeneous catalytic oxidation arises from the prerequisite oxygen activation and transfer ability of metal oxide catalysts. Thus, engineering intercalated nanounits and heterophase metal oxide structures, and forming interstitial catalyst supports at the nanoscale level can drastically alter the catalytic performances of metal oxides. This is particularly important for ceria-based nanomaterial catalysts, where the interactions of reducible ceria (CeO2) and nonreducible oxides are fundamental for the preparation of enhanced catalysts for oxygen-involved reactions. Herein, we intercalated nanostructured CeO2 in the bulk phase of magnesium aluminate spinel (MgAl2O4, referred to as MgAl), produced the interstitial effect between CeO2 nanoparticles and MgAl crystallites, thus boosting their oxygen transfer and activation capability. This nanoscaled intercalation engineering significantly enhanced the number and quality of tight contact points between the nanostructured CeO2 and MgAl units. Therefore, the oxygen storage/release capability (OSC) is exceptionally improved as revealed by various characterizations and catalytic carbon oxidation reaction. A mechanism similar to the Mars-van Krevelen process at the nanoscale level was invoked to explain the catalytic oxidation mechanisms. The reactive oxygen species of gaseous O2 originate formed the bulk of the as-obtained nanomaterial, where strong interactions between the CeO2 and MgAl components occured, which were subsequently released and diffused to the catalyst-interface at elevated temperatures. Silver supported on Ce-MgAl produced an approximately 4-fold higher concentration of active oxygen species than Ag/MgAl, and gives the optimum low-temperature oxidation at 229 °C. This study verifies the importance of the redox performance of ceria-spinel with enhanced OSC, which validates that the arrangement of contacts at the nanoscale can substantially boost the catalytic reactivity without varying the microscale structure and properties of spinel.

Efficient low-temperature soot combustion by bimetallic Ag-Cu/SBA-15 catalysts.

In this study, the effects of copper (Cu) additive on the catalytic performance of Ag/SBA-15 in complete soot combustion were investigated. The soot combustion performance of bimetallic Ag-Cu/SBA-15 catalysts was higher than that of monometallic Ag and Cu catalysts. The optimum catalytic performance was acquired with the 5Ag1-Cu0.1/SBA-15 catalyst, on which the soot combustion starts at Tig=225°C with a T50=285°C. The temperature for 50% of soot combustion was lower than that of conventional Ag-based catalysts to more than 50°C (Aneggi et al., 2009). Physicochemical characterizations of the catalysts indicated that addition of Cu into Ag could form smaller bimetallic Ag-Cu nanolloy particles, downsizing the mean particle size from 3.7nm in monometallic catalyst to 2.6nm in bimetallic Ag-Cu catalyst. Further experiments revealed that Ag and Cu species elicited synergistic effects, subsequently increasing the content of surface active oxygen species. As a result, the structure modifications of Ag by the addition of Cu strongly intensified the catalytic performance.

Regioselective hydrogenolysis of aryl ether C-O bonds by tungsten carbides with controlled phase compositions.

Evenly dispersed tungsten carbides with controlled phase compositions that exhibit an impressive capacity to carry out the regioselective hydrogenolysis of inert aryl ether C-O bonds instead of aliphatic C-O bonds to produce aromatic compounds are reported.

Copper nanoparticles socketed in situ into copper phyllosilicate nanotubes with enhanced performance for chemoselective hydrogenation of esters.

Copper nanoparticles exsoluted in situ under a reducing atmosphere at elevated temperatures are socketed into the parent copper phyllosilicate nanotubes and exhibit excellent catalytic performance and superior stability for the selective hydrogenation of various esters to alcohols.

CH2 Linkage Effects on the Reactivity of Bis(aminophosphine)-Ruthenium Complexes for Selective Hydrogenation of Esters into Alcohols.

A novel ruthenium complex binding to two subtly different aminophosphine ligands, (o-PPh2C6H4CH2NH2)(o-PPh2C6H4NH2)RuCl2, was successfully isolated. This bis(aminophosphine)-ruthenium complex shows efficient activity in both dimethyl oxalate (DMO) and methyl benzoate (MB) hydrogenation. On the contrast, similar complexes (o-PPh2C6H4NH2)2RuCl2 and (o-PPh2C6H4CH2NH2)2RuCl2, can only effectively catalyze the hydrogenation of DMO and MB, respectively. Our experimental studies in combination of theoretical calculations reveal that the remarkable substrate selectivity in the hydrogenation of esters arises from the nonbonding interactions operated by the CH2 linkage of the ligand.

Surfactant-free nickel-silver core@shell nanoparticles in mesoporous SBA-15 for chemoselective hydrogenation of dimethyl oxalate.

Surfactant-free bimetallic Ni@Ag nanoparticles in mesoporous silica, SBA-15 prepared by simple wet co-impregnation catalyse hydrogenation of dimethyl oxalate to methyl glycolate or ethylene glycol in high yield.

Enhanced performance of lipase-catalyzed kinetic resolution of secondary alcohols in monoether-functionalized ionic liquids.

Several cationic monoether-functionalized ionic liquids (MEF-ILs) with different substituents were synthesized and used as media for kinetic resolution of secondary alcohols catalyzed by several lipases. The results indicate that Novozym 435 (an immobilized Candida antarctica Lipase B) had higher efficiency compared to other lipases in deracemization. The alkyl substituents at the 2- and 3-positions in the imidazolium ring of MEF-ILs were found to contribute to the increased enantioselectivity and enhancement of the reaction rate, respectively, while the higher stereo-hindrance of ether bonds decreased the activity. An enantioselectivity higher than 99% with 50% conversion of rac-1-phenylethanol was achieved using the catalyst system comprised of Novozym 435 and the MEF-IL 1-(3-ethoxypropyl)-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide. The catalytic system could be separated and reused without considerable activity loss. MEF-ILs can be a new class of enzyme-benign media suitable for lipase-catalyzed kinetic resolution of secondary alcohols.