Hydrotalcite-supported gold catalyst for the oxidant-free dehydrogenation of benzyl alcohol: studies on support and gold size effects

Gold nanoparticles with uniform mean sizes (≈3 nm) loaded onto various supports have been prepared and studied for the oxidant-free dehydrogenation of benzyl alcohol to benzaldehyde and hydrogen. The use of hydrotalcite (HT), which possesses both strong acidity and strong basicity, provides the best catalytic performance. Au/HT catalysts with various mean Au particle sizes (2.1-21 nm) have been successfully prepared by a deposition-precipitation method under controlled conditions. Detailed catalytic reaction studies with these catalysts demonstrate that the Au-catalyzed dehydrogenation of benzyl alcohol is a structure-sensitive reaction. The turnover frequency (TOF) increases with decreasing Au mean particle size (from 12 to 2.1 nm). A steep rise in TOF occurs when the mean Au particle size becomes smaller than 4 nm. Our present work suggests that the acid-base properties of the support and the size of Au nanoparticles are two key factors controlling the alcohol dehydrogenation catalysis. A reaction mechanism is proposed to rationalize these results. It is assumed that the activation of the β-C-H bond of alcohol, which requires the coordinatively unsaturated Au atoms, is the rate-determining step.     

Kinetic study of propane dehydrogenation and catalyst deactivation over Pt-Sn/Al2O3 catalyst

The kinetics of propane dehydrogenation and catalyst deactivation over Pt-Sn/Al₂O₃ catalyst were studied. Performance test runs were carried out in a fixed-bed integral reactor. Using a power-law rate expression for the surface reaction kinetics and independent law for deactivation kinetics, the experimental data were analyzed both by integral and a novel differential method of analysis and the results were compared. To avoid fluctuation of time-derivatives of conversion required for differential analysis, the conversion-time data were first fitted with appropriate functions. While the time-zero and rate constant of reaction were largely insensitive to the function employed, the rate constant of deactivation was much more sensitive to the function form. The advantage of the proposed differential method, however, is that the integration of the rate expression is not necessary which otherwise could be complicated or impossible. It was also found that the reaction is not limited by external and internal mass transfer limitations, implying that the employed kinetics could be considered as intrinsic ones.     

Additive-Free,Robust H2 Production from H2O and DMF by Dehydrogenation Catalyzed by Cu/Cu2O Formed In Situ

Hydrogen production from organic/inorganic hydrides is a promising strategy for the development of novel clean energy resources to replace fossil fuels and satisfy ever-increasing energy demands. Most current processes involve small flammable chemicals and are catalyzed by noble metals in basic media with the release of the greenhouse gas CO₂. Herein, we describe an alternative pathway for highly efficient and robust H₂ production through a dehydrogenation reaction between water and N,N-dimethylformamide catalyzed by Cu/Cu₂O catalysts formed in situ. The catalysts exhibit high and robust activity for H₂ production. Importantly, the formation of H₂ as the sole gas and the valuable by-product N,N-dimethylaminoformic acid make this process clean and valuable with 100 % atom economy.     

Synthetic scope and mechanistic studies of Ru(OH)x/Al2O3-catalyzed heterogeneous hydrogen-transfer reactions

Three kinds of hydrogen-transfer reactions, namely racemization of chiral secondary alcohols, reduction of carbonyl compounds to alcohols using 2-propanol as a hydrogen donor, and isomerization of allylic alcohols to saturated ketones, are efficiently promoted by the easily prepared and inexpensive supported ruthenium catalyst Ru(OH)x/Al2O3. A wide variety of substrates, such as aromatic, aliphatic, and heterocyclic alcohols or carbonyl compounds, can be converted into the desired products, under anaerobic conditions, in moderate to excellent yields and without the need for additives such as bases. A larger scale, solvent-free reaction is also demonstrated: the isomerization of 1-octen-3-ol with a substrate/catalyst ratio of 20,000/1 shows a very high turnover frequency (TOF) of 18,400 h(-1), with a turnover number (TON) that reaches 17,200. The catalysis for these reactions is intrinsically heterogeneous in nature, and the Ru(OH)x/Al2O3 recovered after the reactions can be reused without appreciable loss of catalytic performance. The reaction mechanism of the present Ru(OH)x/Al2O3-catalyzed hydrogen-transfer reactions were examined with monodeuterated substrates. After the racemization of (S)-1-deuterio-1-phenylethanol in the presence of acetophenone was complete, the deuterium content at the alpha-position of the corresponding racemic alcohol was 91%, whereas no deuterium was incorporated into the alpha-position during the racemization of (S)-1-phenylethanol-OD. These results show that direct carbon-to-carbon hydrogen transfer occurs via a metal monohydride for the racemization of chiral secondary alcohols and reduction of carbonyl compounds to alcohols. For the isomerization, the alpha-deuterium of 3-deuterio-1-octen-3-ol was selectively relocated at the beta-position of the corresponding ketones (99% D at the beta-position), suggesting the involvement of a 1,4-addition of ruthenium monohydride species to the alpha,beta-unsaturated ketone intermediate. The ruthenium monohydride species and the alpha,beta-unsaturated ketone would be formed through alcoholate formation/beta-elimination. Kinetic studies and kinetic isotope effects show that the Ru-H bond cleavage (hydride transfer) is included in the rate-determining step.     

Ru/g-al2o3 as reusable catalyst for dehydrogenation of alcohols without hydrogen acceptor

Summary Ru/g-Al₂O₃ catalyzed the dehydrogenation of alcohols to carbonyl compounds without employing hydrogen acceptor. The catalyst was readily recovered from the reaction mixture and could be reused.     

A dicationic ionic liquid-modified phosphotungstate hybrid catalyst for the heterogeneous oxidation of alcohols with H2O2

An ionic hybrid catalyst 1,1’-(butane-1,4-diyl)-bis(3-methylimidazolium) phosphotungstate(abbreviated [Dmim] 1.5 PW) has been prepared by anion-exchange of the divalent ionic liquid(IL) 1,1’-(butane-1,4-diyl)-bis(3-methylimidazolium) di(bromide) with the Keggin phosphotungstic acid H 3 PW 12 O 40,and characterized by IR,1 H NMR,13 C NMR,ESI-MS,TG,SEM,XRD,BET surface area measurements,elemental analysis,and melting point.The hybrid material was evaluated as a catalyst for the oxidation of alcohols with aqueous hydrogen peroxide under various conditions.The catalytic performance of [Dmim] 1.5 PW was also compared with related catalysts bearing other cations or anions.The new hybrid [Dmim] 1.5 PW proved to be an efficient liquid-solid heterogeneous catalyst for H2O2-based oxidation of alcohols,with the advantages of high conversion and selectivity,easy recovery,and quite good reusability.     

Optimization and statistical analysis of Au-ZnO/Al2O3 catalyst for CO oxidation

In our former work [Catal. Today 174 (2011) 127], 12 heterogeneous catalysts were screened for CO oxidation, and Au-ZnO/Al₂O₃ was chosen and optimized in terms of weight loadings of Au and ZnO. The present study follows on to consider the impact of process parameters (catalyst preparation and reaction conditions), in conjunction with catalyst composition (weight loadings of Au and ZnO, and the total weight of the catalyst), as the optimization of the process parameters simultaneously optimized the catalyst composition. The optimization target is the reactivity of this important reaction. These factors were first optimized using response surface methodology (RSM) with 25 experiments, to obtain the optimum: 100 mg of 1.0%Au-4.1%ZnO/Al₂O₃ catalyst with 220 °C calcination and 100 °C reduction. After optimization, the main effects and interactions of these five factors were studied using statistical sensitivity analysis (SA). Certain observations from SA were verified by reaction mechanism, reactivity test and/or characterization techniques, while others need further investigation.     

Synthetic scope of Ru(OH)x/Al2O3-catalyzed hydrogen-transfer reactions: an application to reduction of allylic alcohols by a sequential process of isomerization/Meerwein-Ponndorf-Verley-type reduction

Reduction of allylic alcohols can be promoted efficiently by the supported ruthenium catalyst Ru(OH)x/Al2O3. Various allylic alcohols were converted to saturated alcohols in excellent yields by using 2-propanol without any additives. This Ru(OH)x/Al2O3-catalyzed reduction of a dienol proceeds only at the allylic double bond to afford the corresponding enol, and chemoselective isomerization and reduction can be realized under similar conditions. The catalysis is truly heterogeneous and the high catalytic performance can be maintained during at least three recycles of the Ru(OH)x/Al2O3 catalyst. The transformation of allylic alcohols to saturated alcohols consists of three sequential reactions: oxidation of allylic alcohols to alpha,beta-unsaturated carbonyl compounds; reduction of alpha,beta-unsaturated carbonyl compounds to saturated carbonyl compounds; and reduction of saturated carbonyl compounds to saturated alcohols.     

gamma-1,2-H2SiV2W10O40] immobilized on surface-modified SiO2 as a heterogeneous catalyst for liquid-phase oxidation with H2O2

An organic-inorganic hybrid support has been synthesized by covalently anchoring an N-octyldihydroimidazolium cation fragment onto SiO2 (denoted as 1-SiO2). This modified support was characterized by solid-state 13C, 29Si, and 31P NMR spectroscopy, IR spectroscopy, and elemental analysis. The results showed that the structure of the dihydroimidazolium skeleton is preserved on the surface of SiO2. The modified support can act as a good anion exchanger, which allows the catalytically active polyoxometalate anion [gamma-1,2-H2SiV2W10O40]4- (I) to be immobilized onto the support by a stoichiometric anion exchange (denoted as I/1-SiO2). The structure of anion I is preserved after the anion exchange, as confirmed by IR and 51V NMR spectroscopy. The catalytic performance for the oxidation of olefins and sulfides, with hydrogen peroxide (only one equivalent with respect to substrate) as the sole oxidant, was investigated with I/1-SiO2. This supported catalyst shows a high stereospecificity, diastereoselectivity, regioselectivity, and a high efficiency of hydrogen peroxide utilization for the oxidation of various olefins and sulfides without any loss of the intrinsic catalytic nature of the corresponding homogeneous analogue of I (i.e., the tetra-n-butylammonium salt of I, TBA-I), although the rates decreased to about half that with TBA-I. The oxidation can be stopped immediately by removal of the solid catalyst, and vanadium and tungsten species can hardly be found in the filtrate after removal of the catalyst. These results rule out any contribution to the observed catalysis from vanadium and tungsten species that leach into the reaction solution, which means that the observed catalysis is truly heterogeneous in nature. In addition, the catalyst is reusable for both epoxidation and sulfoxidation without any loss of catalytic performance.     

Platinum‐Promoted Ga/Al2O3 as Highly Active,Selective, and Stable Catalyst for the Dehydrogenation of Propane

A novel catalyst material for the selective dehydrogenation of propane is presented. The catalyst consists of 1000 ppm Pt, 3 wt % Ga, and 0.25 wt % K supported on alumina. We observed a synergy between Ga and Pt, resulting in a highly active and stable catalyst. Additionally, we propose a bifunctional active phase, in which coordinately unsaturated Ga³⁺ species are the active species and where Pt functions as a promoter.     

离子交换法制备Pt-Sn_E/Mg(Al)O催化剂及其烷烃催化脱氢性能

以水滑石为载体,采用离子交换法制备了Pt-Sn_E/Mg(Al)O催化剂,并对其进行了X射线衍射、N2物理吸附、透射电镜等技术表征;考察了该离子交换法制备的Pt-SnE/Mg(Al)O催化剂对乙烷和丙烷脱氢的催化性能,并与浸渍法制备的Pt-SnI/Mg(Al)O催化剂进行了比较。结果表明,利用离子交换法制备的Pt-SnE/Mg(Al)O催化剂其反应活性和稳定性明显优于浸渍法制备Pt-SnI/Mg(Al)O催化剂的。在相同条件下反应2 h后,Pt-SnE/Mg(Al)O催化剂和Pt-SnI/Mg(Al)O催化剂的乙烷催化脱氢转化率分别为12.2%和3.1%,丙烷催化脱氢转化率分别为38.7%和26.4%。     

Core-shell structured nanospheres with mesoporous silica shell and Ni core as a stable catalyst for hydrolytic dehydrogenation of ammonia borane

Core-shell structured nanospheres with mesoporous silica shell and Ni core (denoted as Ni@meso-SiO₂) are prepared through a three-step process. Monodispersed Ni precursors are first prepared, and then coated with mesoporous SiO₂. Final Ni@meso-SiO₂ spheres are obtained after calcination. The products are characterized by X-ray powder diffraction, transmission electron microscopy and N₂ adsorption-desorption methods. These spheres have a high surface area and are well dispersed in water, showing a high catalytic activity with a TOF value of 18.5, and outstanding stability in hydrolytic dehydrogenation of ammonia borane at room temperature.     

Phase transfer catalyzed oxidation of ketones with borax-H2O2

Summary Borax forms peroxy species when dissolved in 30% hydrogen peroxide which can be transferred into the organic phase when biphase mixtures are agitated. The addition of a catalytic amount ofBTEAC promotes the transfer. This biphase system was used for theBaeyer-Villiger oxidation of several ketones insoluble in water. Effects of changing various parameters,e.g. temperature, time, amount of H₂O₂ etc. were investigated. At higher temperature (ca. 80°C), 100% conversion could be achieved in 2–4h. The results show that under appropriate conditions this reaction is of synthetic value for the oxidation of acid-sensitive ketones using inexpensive and easily available reagents.

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Oxidation von Ketonen mit Borax-H₂O₂ unter Phasentransferkatalyse

Zusammenfassung In Wasserstoffperoxid (30%) gelöstes Borax bildet Peroxyverbindungen, die durch Schütteln in die organische Phase zweiphasiger Systeme übergeführt werden können. Der Transferprozeß wird durch die Zugabe von katalytischen MengenBTEAC gefördert. Die zweiphasigen Systeme wurden zurBaeyer-Villiger — Oxidation einiger in Wasser unlöslicher Ketone eingesetzt und die Auswirkung der Variation verschiedener Parameter (z.B. Temperatur, Zeit, Menge an H₂O₂ etc.) untersucht. Unter dem Einfluß höherer Temperature (ca. 80°C) wurde vollständige Umsetzung innerhalb von 2–4 h erreicht. Die Ergebnisse zeigen, daß die genannte Reaktion unter geeigneten Bedingungen von synthetischem Wert zur Oxidation säureempfindlicher Ketone unter Verwendung billiger und leicht verfügbarer Reagenzien ist.