Qian He

Direct Catalytic Conversion of Methane to Methanol in an Aqueous Medium by using Copper‐Promoted Fe‐ZSM‐5

Iron copper zeolite (Fe-Cu-ZSM-5) with aqueous hydrogen peroxide is active for the selective oxidation of methane to methanol. Iron is involved in the activation of the carbon–hydrogen bond, while copper allows methanol to form as the major product. The catalyst is stable, re-usable and activates methane giving >90 % methanol selectivity and 10 % conversion in a closed catalytic cycle (see scheme).

Selective Oxidation of Glycerol by Highly Active Bimetallic Catalysts at Ambient Temperature under Base-Free Conditions†

Au–Pt alloy nanoparticles deposited on Mg(OH)2 (see STEM-HAADF image) show high activity in the selective oxidation of polyols using molecular oxygen as oxidant at mild and base-free conditions.

Synthesis of glycerol carbonate from glycerol and urea with gold-based catalysts

The reaction of glycerol with urea to form glycerol carbonate is mostly reported in the patent literature and to date there have been very few fundamental studies of the reaction mechanism. Furthermore, most previous studies have involved homogeneous catalysts whereas the identification of heterogeneous catalysts for this reaction would be highly beneficial. This is a very attractive reaction that utilises two inexpensive and readily available raw materials in a chemical cycle that overall, results in the chemical fixation of CO(2). This reaction also provides a route to up-grade waste glycerol produced in large quantities during the production of biodiesel. Previous reports are largely based on the utilisation of high concentrations of metal sulfates or oxides, which suffer from low intrinsic activity and selectivity. We have identified heterogeneous catalysts based on gallium, zinc, and gold supported on a range of oxides and the zeolite ZSM-5, which facilitate this reaction. The addition of each component to ZSM-5 leads to an increase in the reaction yield towards glycerol carbonate, but supported gold catalysts display the highest activity. For gold-based catalysts, MgO is the support of choice. Catalysts have been characterised by XRD, TEM, STEM and XPS, and the reaction has been studied with time-on-line analysis of products via a combination of FT-IR spectroscopy, HPLC, (13)C NMR and GC-MS analysis to evaluate the reaction pathway. Our proposed mechanism suggests that glycerol carbonate forms via the cyclization of a 2,3-dihydroxypropyl carbamate and that a subsequent reaction of glycerol carbonate with urea yields the carbamate of glycerol carbonate. Stability and reactivity studies indicate that consecutive reactions of glycerol carbonate can limit the selectivity achieved and reaction conditions can be selected to avoid this. The effect of the catalyst in the proposed mechanism is discussed.

Modified zeolite ZSM-5 for the methanol to aromatics reaction

Catalysts comprising zeolite ZSM-5 impregnated with precious metals including Ag, Cu, Ni, Pd, Ir and Ru, have been tested for the methanol to hydrocarbons reaction in a continuous flow fixed bed reactor. Comparison with the activity of unmodified ZSM-5 showed that Ag, Cu and Ni enhanced the selectivity to C6–C11 aromatic products by a factor of two or higher. Moreover, Ag/ZSM-5 showed improved selectivity for the C6–C7 fraction of aromatic products. Ni/ZSM-5 was found to be selective to naphthalene, while Cu/ZSM-5 was selective for C9–C11 aromatic products. It was ascertained that all the impregnated metals were present as metal oxides in the starting materials. It is therefore proposed that the enhanced selectivity to aromatic products is due to the interaction of the acid sites of the zeolite with the basic sites of the metal oxide at the edge of the zeolite crystals, as well as the possible coordination of propene molecules formed during the reaction, that are likely to be the building blocks for the formation of aromatics.

Gold, palladium and gold–palladium supported nanoparticles for the synthesis of glycerol carbonate from glycerol and urea

Supported Au–Pd nanoparticles are shown to be effective catalysts for the transformation of glycerol into glycerol carbonate. The reaction of glycerol with urea to form glycerol carbonate is a very attractive reaction that utilises two inexpensive and readily available raw materials in a chemical cycle that, overall, results in the chemical fixation of carbon dioxide. Previous reports are largely based on the utilisation of high concentrations of metal sulphates or oxides, which suffer from low intrinsic activity and selectivity and limited recoverability due to the dissolution of the catalyst in the reaction media. We now report that magnesium oxide is an excellent support for gold and bimetallic gold–palladium nanoparticles for this reaction. The preparation method and pre-treatment affect the catalytic performance and a colloidal preparation route produces the most active catalysts.

Aqueous Au-Pd colloids catalyze selective CH4 oxidation to CH3OH with O2 under mild conditions

A radical route from methane to methanol The conversion of methane into chemicals usually proceeds through high-temperature routes that first form more reactive carbon monoxide and hydrogen. Agarwal et al. report a low-temperature (50°C) route in aqueous hydrogen peroxide (H2O2) for oxidizing methane to methanol in high yield (92%). They used colloidal gold-palladium nanoparticles as a catalyst. The primary oxidant was O2; isotopic labeling showed that H2O2 activated methane to methyl radicals, which subsequently incorporated O2. Science, this issue p. 223 Methyl radicals generated catalytically from methane with aqueous hydrogen peroxide are converted to methanol with oxygen. The selective oxidation of methane, the primary component of natural gas, remains an important challenge in catalysis. We used colloidal gold-palladium nanoparticles, rather than the same nanoparticles supported on titanium oxide, to oxidize methane to methanol with high selectivity (92%) in aqueous solution at mild temperatures. Then, using isotopically labeled oxygen (O2) as an oxidant in the presence of hydrogen peroxide (H2O2), we demonstrated that the resulting methanol incorporated a substantial fraction (70%) of gas-phase O2. More oxygenated products were formed than the amount of H2O2 consumed, suggesting that the controlled breakdown of H2O2 activates methane, which subsequently incorporates molecular oxygen through a radical process. If a source of methyl radicals can be established, then the selective oxidation of methane to methanol using molecular oxygen is possible.

The Effect of Bromide Pretreatment on the Performance of Supported Au-Pd Catalysts for the Direct Synthesis of Hydrogen Peroxide

The effect of pretreating Au–Pd catalysts on MgO and C supports with aqueous bromide solution, prior to using them for the direct synthesis of hydrogen peroxide, has been investigated. These two supports were selected since the parent materials exhibit contrasting microstructures and activities. The carbon‐supported catalysts comprise homogeneous Au–Pd alloy nanoparticles, which give high activity, whereas the MgO‐supported catalyst has Au–Pd alloys with a Pd‐rich surface and a Au‐rich core, which result in lower activity. Pretreatment of these catalysts with bromide was found to enhance H2O2 productivity and the degree of enhancement was largely dependent on the nature of the Au–Pd nanoparticles. Whereas bromide pretreatment significantly enhanced H2O2 productivity over the MgO‐based catalysts, the carbon‐based catalyst only showed a subtle promotional effect. Very low loadings of bromide (0.00034–0.044u2005wtu2009%) were required to yield a significant positive effect. Higher bromide loadings (0.5–8.3u2005wtu2009%) proved deleterious. The promotional effect has been correlated to selective poisoning of sites responsible for H2O2 hydrogenation and decomposition. In view of the limited effect of bromide pretreatment on the yield of H2O2 coupled with the effective performance of the carbon supported Au‐Pd catalysts in the absence of halides, for practical processes the addition of halides is not considered advisable with this catalyst system.

Multifunctional supported bimetallic catalysts for a cascade reaction with hydrogen auto transfer: synthesis of 4-phenylbutan-2-ones from 4-methoxybenzyl alcohols

We report the one-pot tandem synthesis of 4-(4-methoxyphenyl)butan-2-one directly from 4-methoxybenzyl alcohol and acetone using a multifunctional supported AuPd nanoalloy catalyst. This one-pot synthesis involves dehydrogenation, aldol condensation and hydrogenation of CC. In this supported AuPd catalyst, the bimetallic sites catalyse the dehydrogenation and hydrogenation steps and, in combination with the support, catalyse the C–C coupling (aldol) process. This supported bimetallic catalyst is also effective in utilizing hydrogen from the dehydrogenation reaction for the hydrogenation of 4-(4-methoxyphenyl)but-3-en-2-one to 4-(4-methoxyphenyl)butane-2-one via a hydrogen auto transfer route. These multifunctional catalysts were characterised using transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy.

Highly active gold and gold-palladium catalysts prepared by colloidal methods in the absence of polymer stabilizers

Supported gold and gold–palladium nanoparticles were found to be effective catalysts for the selective oxidation of glycerol and benzyl alcohol. The properties and stabilities of catalysts are often sensitive to factors such as the dimensions, shape, and composition of the metal nanoparticles. Although colloidal methods provide an easy and quick way to synthesize supported metal catalysts, they typically involve the use of polymers such as polyvinyl alcohol and polyvinylpyrrolidone as steric stabilizers, which can sometimes be detrimental in subsequent catalytic reactions. Herein, we report the synthesis of supported gold and gold–palladium nanoparticles without the addition of stabilizing polymers. The catalysts prepared with and without the addition of polymers performed very similarly in the selective oxidation of glycerol and benzyl alcohol, which suggests that polymers are not essential to make active catalysts for these reactions. Thus, this new stabilizer‐free method provides a facile and highly effective way of circumventing the inherent problems of polymer stabilizers in the preparation of gold and gold–palladium catalysts.

The low temperature oxidation of propane using H2O2 and Fe/ZSM-5 catalysts; insights into the active site and enhancement of catalytic turnover frequencies

Fe‐containing ZSM‐5 catalysts are reported to be efficient catalysts for the partial oxidation of propane to oxygenated products at reaction temperatures as low as 50u2009°C in an aqueous phase reaction when using the green oxidant H2O2. It was previously proposed that extra framework Fe species at the exchange sites of the zeolite are responsible for activation of both the alkane and hydrogen peroxide. Through a systematic study of the influence of framework topology and exchange properties, it is now shown that this high catalytic activity is specific to the MFI‐type Brønsted acidic zeolite ZSM‐5. Furthermore, through a simple aqueous acid washing treatment, leaching of approximately 77u2009% of iron present within a Fe/ZSM‐5 catalyst only caused the relative propane conversion to decrease by 17u2009%; implying that most of the initially loaded Fe does not actually contribute to the catalytic activity. This small change in conversion after ‘excess’ Fe removal, amounts to a three‐fold increase in turnover frequency (TOF) (Fe) from 66u2005h−1 to 232u2005h−1 compared with the parent Fe/ZSM‐5 catalyst. By comparing these samples, it is shown by NH3 temperature‐programmed desorption, 27Al magic angle spinning NMR spectroscopy, X‐ray photoelectron spectroscopy and TEM analysis that surface iron oxide species are effectively spectators in the oxidation of propane with H2O2. This provides further insight as to the location and true nature of the catalytically active Fe species.