Selective oxidation reactions catalysed by gold

The oxidation of benzyl alcohol over supported gold, palladium and gold palladium catalysts was studied in a high-pressure stirred autoclave. Reaction conditions such as oxygen pressure and reaction temperature were varied to obtain optimum conditions for catalyst screening. Gold palladium catalysts supported on various oxides were prepared and screened with titania supported catalysts proving to be the most active. Different preparation methodologies were used to prepare titania supported bi-metallic catalysts, co-deposition precipitation methodology was found to give the most active and stable catalyst. Further gold, palladium and gold palladium catalysts were prepared supported on ceria that had been obtained from the calcination of cerium acetylacetate that had been precipitated into a supercritical carbon dioxide antisolvent. These catalysts proved to be significantly more active than those supported on the ceria form the simple calcination of the precursor. Reuse experiments showed that the supercritically prepared ceria precursor catalyst increased in activity with use, despite significant leaching of the metals, an effect not observed for the non-supercritically prepared ceria precursor catalyst. The ceria catalysts prepared by the supercritical antisolvent method displayed very high turn over frequencies for the oxidation of l-octen-3-ol, 2-octen-l-ol and cinnamyl alcohol. Gold and gold palladium catalysts supported on titania and graphite were used for the oxidation of 2,5-dimethyl furan to form hex-3-ene-2,5-dione. The effect of temperature, pressure and radical initiator were investigated and the highest selectivity was achieved at the lowest oxygen pressure. The oxidation of a-pinene to verbenone investigated, the reaction proceeded with highest conversion at high oxygen pressure with catalyst not required for the reaction to proceed. Gold palladium catalysts were used for the oxidation of isophorone and valencene using the optimized conditions established for the a-pinene reaction system. 2,6-lutidene, 2-picoline and 3-picoline were found to be poisons for gold palladium catalysts

A micropacked-bed multi-reactor system with in situ raman analysis for catalyst evaluation

AbstractA micropacked-bed multi-reactor platform with integrated portable Raman spectrometer is presented for fast evaluation of catalyst activity and stability for gas/liquid/solid reactions. The silicon-glass microreactor was designed and fabricated so that pockets containing the liquid reaction mixture were created after each packed bed, into which the laser could be directed for Raman spectral acquisition. Using the oxidation of benzyl alcohol as a model reaction, the Raman spectrum was found to be affected both by temperature and by the composition of the multiphase reaction mixture which was related to the reaction conversion. These effects were accounted for by calibrating the Raman spectra at the reaction temperature using mixtures produced by the reactors that were analysed independently by gas chromatography. Fourteen catalysts containing different combinations of Au, Pd and Pt supported on TiO2 prepared by sol-immobilisation (SI) and standard impregnation (SImp) techniques were tested. The results showed that the activity of the catalysts prepared by SI was overall higher than those prepared by SImp, while the activity sequence followed the same pattern: Pd>AuPd>AuPdPt>PdPt>(Au, Pt, AuPt). The Pd and AuPd catalysts from both SI and SImp were stable in 5-h testing, however, for the PdPt and AuPdPt catalysts prepared by SI deactivation was observed

The selective oxidation of n-butanol to butyraldehyde by oxygen using stable Pt-based nanoparticulate catalysts: an efficient route for upgrading aqueous biobutanol

Supported Pt nanoparticles are shown to be active and selective towards butyraldehyde in the base-free oxidation of n-butanol by O2 in an aqueous phase. The formation of butyric acid as a by-product promoted the leaching of Pt and consequently the activity of the catalysts decreased upon reuse. Characterisation showed that the degree to which Pt leached from the catalysts was related to both the metal–support interaction and metal particle size. A catalyst active and stable (<1% metal leaching) in the aqueous reaction medium was obtained when Pt nanoparticles were supported on activated carbon and prepared by a chemical vapour impregnation method. The presence of n-butanol in the aqueous medium is required to inhibit the over oxidation of butyraldehyde to butyric acid. Consequently, high selectivities towards butyraldehyde can only be obtained at intermediate n-butanol conversion

The role of Mg(OH)2 in the so-called 'base-free' oxidation of glycerol with AuPd catalysts

Mg(OH)2 and Mg(OH)2 containing materials can provide excellent performance as supports for AuPd nanoparticles for oxidation of glycerol in the absence of base, which is considered to be a result of additional basic sites on the support's surface. However, its influence on the reaction solution is not generally discussed. In this paper, we examine, in detail, the relationship between the basic Mg(OH)2 support and AuPd nanoparticles using four types of catalyst, where the physical interaction between Mg(OH)2 and AuPd was adjusted. It was found that the activity of the AuPd nanoparticles increased with the amount of Mg(OH)2 added under base-free conditions, regardless of its interaction with the noble metals. In order to investigate how Mg(OH)2 affected glycerol oxidation, detailed information about the performance of AuPd/Mg(OH)2, physically mixed (AuPd/C+Mg(OH)2) and (AuPd/C+NaHCO3) was obtained and compared. Furthermore, NaOH and Mg(OH)2 were added during the reaction using AuPd/C. All these results indicate that the distinctive and outstanding performance of Mg(OH)2 supported catalysts in base-free condition is in fact directly related to its ability to affect the pH during the reaction and as such, assists with the initial activation of the primary alcohol which is considered to be the rate determining step in the reactionperformance of Mg(OH)2 supported catalysts in base-free condition could be correlated to its ability to affect the pH during the reaction

The effect of ring size on the selective carboxylation of cycloalkene oxides

Carbon dioxide utilisation technology can contribute to the reduction of atmospheric CO2 levels both through its sequestration from flue gases and indirectly by relieving pressure on conventional feedstocks in chemical manufacturing. A promising approach is to employ CO2 to produce valuable cyclic carbonates (CCs) in reaction with suitable epoxides. This also has the advantage that carbon dioxide replaces toxic and hazardous reactants such as phosgene. In earlier work we have investigated the synthesis of epoxides from cycloalkenes using supported gold and gold–palladium nanoparticles as catalysts and oxygen from air as the oxidant under solvent free conditions. A strong dependence of epoxide selectivity on ring size was observed with C5 < C6 < C7 ≪ C8. In this study we extend this work to the investigation of cycloaddition of CO2 to different cycloalkene oxides with the ultimate aim of designing a process in which both epoxidation of an alkene and incorporation of CO2 could be achieved in a single process. However, we have found the opposite trend for the selectivity to carbonates: smaller ring cycloalkene oxides giving the highest carbonate selectivities while large rings do not yield CCs at all. The product distributions suggest that an alternative ring opening of the epoxides to yield alcohols and ketones is preferred under all the experimental conditions explored for larger ring systems. Additionally, the mechanism of the CC synthesis using a quaternary ammonium salt and ZnBr2 as the catalyst system was investigated using DFT methods. The results of the calculations support the experimental findings

Base-free oxidation of glucose to gluconic acid using supported gold catalysts

1%Au/TiO2 catalysts prepared by a range of preparation methods were studied for the base-free oxidation of glucose. The highest catalytic activity was observed with the catalyst prepared by the sol-immobilization method. Furthermore we have studied the effect of the post-synthesis treatments of treatment with water, or heating in air on the activity. The catalyst calcined at 250 °C showed optimal activity and selectivity. Additionally, we studied the effect of the amount of the stabilising ligand in the sol-immobilisation method and observed that this is a key parameter with respect to determining catalyst activity

Base-free glucose oxidation using air with supported gold catalysts

We report the selective oxidation of glucose to gluconic acid under mild conditions and show that if a basic support is used then the reaction can be carried out without the addition of sacrificial base or pH control. The use of sol-immobilisation prepared catalysts supported on magnesium oxide facilitates the use of ambient air as an oxidant source. These mild conditions resulted in an excellent selectivity towards gluconic acid. Different heat treatments result in an improvement in the activity of the catalyst, these improvements are discussed in terms of XRD, DRIFTD and TEM analysis of the catalysts, despite significant particle growth and phase segregation occurring during the thermal treatments

Inter-connected and open pore hierarchical TS-1 with controlled framework titanium for catalytic cyclohexene epoxidation

A post-synthesis method was developed to reduce the extra-framework titanium (Ti) in TS-1 zeolites (Si/Ti ratio = 50), in which tetrapropylamonium hydroxide (TPAOH) aqueous solution was used to promote the dissolution, redistribution and recrystallization processes, and hence to convert amorphous Ti species into zeolitic phases. It was found that TPAOH could effectively convert the extra-framework Ti into framework Ti, and the TPAOH concentration influenced the pore structure significantly. Under lower TPAOH concentration (i.e. 0.05–0.4 M), only closed meso-/macropores (grooves and hollow cavities) can be created in the TS-1 crystals. At an optimum concentration of 0.5 M TPAOH, open and connected hierarchical mesopores and macropores were created in the resulting TS-1 zeolites. Compared with the parent TS-1, the amount of extra-framework titanium was reduced significantly from 14.4% to 0.3% and the meso-/macropore volume was increased from 0.014 to 0.168 cm3 g−1 accordingly upon TPAOH post treatment. Along with the parent TS-1 zeolite, the developed hierarchical TS-1 zeolites were assessed in the catalytic epoxidation of cyclohexene. It was confirmed that the amount of framework titanium and hierarchical pore structure influenced the catalytic activity considerably. Closed porosity slightly improved the cyclohexene conversion, whereas the open-pore sample shows the optimum catalytic activity in cyclohexene conversion

Physical mixing of metal acetates: a simple, scalable method to produce active chloride free bimetallic catalysts

We have prepared supported gold, palladium and gold–palladium bimetallic catalysts by the physical mixing of the acetate salts of the metals followed by a simple heat treatment. The use of the acetates as the metal precursor eliminates chloride from the catalyst preparation step. Extensive characterisation shows the formation of bimetallic alloy particles. These catalysts are extremely active for alcohol oxidations and the direct formation of hydrogen peroxide