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In oxidation reactions catalysed by supported metal nanoparticles with oxygen as the terminal oxidant, the rate of the oxygen reduction can be a limiting factor. This is exemplified by the oxidative dehydrogenation of alcohols, an important class of reactions with modern commercial applications1–3. Supported gold nanoparticles are highly active for the dehydrogenation of the alcohol to an aldehyde4 but are less effective for oxygen reduction5,6. In contrast, supported palladium nanoparticles are less active than gold for dehydrogenation but offer high efficacy for oxygen reduction5,6. This imbalance can be overcome by alloying gold with palladium which gives enhanced activity to both reactions7,8; however, the electrochemical potential of the alloy is a compromise between that of the two metals meaning that although the oxygen reduction is improved in the alloy, the dehydrogenation activity is poorer. Here we show that by separating the gold and palladium components in bimetallic carbon-supported catalysts we can almost double the reaction rate beyond that achieved with a corresponding alloy catalyst. We demonstrate this using physical mixtures of carbon-supported monometallic gold and palladium and a bimetallic catalyst comprising separated gold and palladium regions. Furthermore, we demonstrate electrochemically that this enhancement is attributable to the coupling of separate redox processes occurring at isolated gold and palladium sites. The discovery of this novel catalytic effect, a cooperative redox enhancement (CORE), offers a new approach to the design of multi-component heterogeneous catalysts.

In this paper we aim to highlight the need to consider the possible role of autocatalysis in oxidation reactions when using molecular oxygen as the terminal oxidant. Oxygen in its ground state is a diradical, and depending on the reaction conditions, it can initiate oxidation through radical pathways through mechanisms which do not require the presence of a catalyst. Consequently, we contrast the oxidation of benzyl alcohol with oxidation of α-pinene. For benzyl alcohol oxidation the initial reaction is the oxidative dehydrogenation to form benzaldehyde, a non-radical process; but the subsequent over-oxidation to benzoic acid is a radical process. In this case the role of the autocatalysed reaction can be minimised. With α-pinene, the oxidation reaction is via radical pathways and now the autocatalysed reaction can be dominant and, indeed, can be the preferred pathway for the formation of high yields of the desired verbenone product. Graphic Abstract

Low temperature oxidation of alcohols over heterogeneous catalysts is exceptionally challenging, particularly under neutral conditions. Herein, we report on an efficient, base-free method to oxidise glycerol over a 0.5%Pd-0.5%Fe/SiO2 catalyst at ambient temperature in the presence of gaseous H2 and O2. The exceptional catalytic performance was attributed to the in situ formation of highly reactive surface-bound oxygenated species, which promote the dehydrogenation on the alcohol. The PdFe bimetallic catalyst was determined to be significantly more active than corresponding monometallic analogues, highlighting the important role both metals have in this oxidative transformation. Fe leaching was confirmed to occur over the course of the reaction but sequestering experiments, involving the addition of bare carbon to the reactions, confirmed that the reaction was predominantly heterogeneous in nature. Investigations with electron paramagnetic resonance spectroscopy suggested that the reactivity in the early stages was mediated by surface-bound reactive oxygen species; no homogeneous radical species were observed in solution. This theory was further evidenced by a direct H2O2 synthesis study, which confirmed that the presence of Fe in the bimetallic catalyst neither improved the synthesis of H2O2 nor promoted its decomposition over the PdFe/SiO2 catalyst.

In this work, the biomolecule-assisted microwave heating synthesis of ZnIn2S4, along with the ZnIn2S4/MoS2 composites and their photocatalytic applications, were studied. Well-defined flower-like ZnIn2S4 microspheres synthesized at microwave heating time of 1 h provided the highest surface area and total pore volume, which offered the highest H2 production rate (111.6 μmol h−1 g−1). Different amounts of MoS2 were loaded into the ZnIn2S4 microspheres to form ZnIn2S4/MoS2 composites aiming to improve the H2 production rate. Among the fabricated ZnIn2S4/MoS2 composites, the ZnIn2S4/MoS2-40% wt composite exhibited the highest H2 production rate (200.1 μmol h−1 g−1) under UV-visible light irradiation. In addition, for the first time, this composite was applied for the photoreduction reaction of Cr(VI) ion under visible light irradiation. It provided higher photoreduction efficiency than the single components, where the efficiency was improved in the acidic solutions over the levels recorded in the basic solution. The charge transfer pathway and photocatalytic mechanisms of the ZnIn2S4/MoS2-40% wt photocatalyst have been proposed based on the results obtained from UV-visible diffuse reflectance spectroscopy, photoluminescence spectroscopy, electrochemical impedance spectroscopy, Mott–Schottky measurements and the silver photo-deposition experiment.

The solar-driven reduction of nitrate to nitrogen has been studied in the presence of a formate hole scavenger, over a series of Au- and Ag-decorated TiO2 catalysts. In this study, the catalyst preparation protocol was found to influence the nitrate transformation in the order: incipient wetness impregnation > stabilizer-free sol immobilization > sol immobilization. However, the sequence of performing specific treatment steps such as drying, calcination and sieving had a less pronounced effect. Low-conversion conditions were utilized to study the photo-degradation of nitrate over a range of monometallic and bimetallic catalysts with metal concentrations in the range M = 0–1 wt% (M: Au, Ag, Pd, AuAg). Our findings demonstrate that selectively degrading nitrate to N2 over these co-catalysts is non-trivial and is metal content dependent. For Au-doped TiO2 catalysts, the highest activity was measured over 0.2 wt% Au/TiO2 while a higher metal loading of 0.4 wt% was required for the Ag/TiO2 photocatalyst. Product selectivity was also demonstrated to be dependent on metal and metal loading: approximately 22% nitrite selectivity was determined over a 0.1 wt% Ag-doped catalysts, however this product was not detected when utilising Au-doped catalysts. Total selectivity to dinitrogen was shown to be possible on both Au and Ag doped catalysts, and again this was dependent on the concentration of the metal (Ag > 0.3 wt%; 0.2 < Au ≥ 0.4 wt%).

In oxidation reactions catalysed by supported metal nanoparticles with oxygen as the terminal oxidant, the rate of the oxygen reduction can be a limiting factor. This is exemplified by the oxidative dehydrogenation of alcohols, an important class of reactions with modern commercial applications

The aerobic oxidation of cinnamyl alcohol in toluene under autoxidation conditions has been studied using a range of 1 wt% Au–Pd/TiO2 catalysts. The catalysts have been studied to determine the effect of preparation method (impregnation and sol immobilisation) and metal ratio on the conversion of cinnamyl alcohol and the selectivity to cinnamaldehyde. The catalysts prepared by sol-immobilisation demonstrate higher selectivity to the desired aldehyde than the analogous impregnation materials. The most active catalyst was found to be 0.75 wt% Au–0.25 wt% Pd/TiO2 prepared by sol-immobilisation and this demonstrates the importance of metal ratio optimisation in this catalytic process. Furthermore, this metal ratio was found to be most stable under the reactions conditions with little change observed over multiple uses.

Catalytic hydrogenation of levulinic acid to form γ-valerolactone was studied over Cu–ZrO2 catalysts doped with metal oxides from the first-row transition metals. The Cu–ZrO2 material was prepared by oxalate gel coprecipitation, and dopants were added by an incipient wetness approach. The addition of 1% Mn into Cu–ZrO2 significantly increases the yield of γ-valerolactone, and the catalytic activity of Mn/Cu–ZrO2 was found to be 1.6 times higher than that of the undoped Cu–ZrO2 catalyst. Catalyst characterization suggests that the Mn dopant improves the dispersion of Cu on the surface of ZrO2. Kinetic studies show that the reaction order with respect to the substrate concentration is approximately zero. However, the order of reaction with respect to the partial pressure of H2 is different for the Mn/Cu–ZrO2 and Cu–ZrO2 catalysts. Comparison of reaction products from reactions carried out in H2O and D2O solvents using 1H NMR and 13C NMR show that there is a pre-equilibrium keto–enol isomerization step under our reaction conditions. DFT calculations show that the enol isomers have a higher affinity for the Cu surface, which may improve the availability of the substrate in the hydrogenation step of the reaction.

The epoxidation of 2,5-dimethyl furan leads to the production of hex-3-ene-2,5-dione via a ring opening rearrangement reaction. A second epoxidation reaction could then enable a further ring closing rearrangement to form 4-hydroxy-2,5-dimethyl-3-furanone (furaneol). In this paper we report the use of gold and gold palladium supported on graphite and titania as catalysts for the ring opening reaction of 2,5-dimethyl furan. We show that by tuning the reaction conditions high selectivity towards hex-3-ene-2,5-dione can be achieved using green chemical methods and mild reaction conditions.

A 1% Ru/C catalyst prepared by the sol immobilization method showed a high yield of γ-valerolactone from levulinic acid. We performed an optimization of the catalyst by varying the preparation variables involved in the sol immobilization method and detremined that the ratio of PVA, NaBH4 to Ru and heat treatment conditions play a crucial role in the synthesis of active and selective catalysts. By varying these parameters we have identified the optimum conditions for catalyst preparation by providing well dispersed nanoparticles of RuOx on the carbon support that are reducible under low reaction temperature and in turn gave an enhanced catalytic activity. In contrast to a catalyst prepared without using a PVA stabiliser, the use of a small amount PVA (PVA/Ru = 0.1) provided active nanoparticles, by controlling the steric size of the Ru nanoparticles. An optimum amount of NaBH4 was required in order to provide the reducible Ru species on the surface of catalyst and further increase in NaBH4 was found to cause a decline in activity that was related to the kinetics of nanoparticle formation during catalyst preparation. A variation of heat treatment temperature showed a corresponding decrease in catalytic activity linked with the sintering and an increase in particle size.

We have investigated xNi–yCu–ZrO2 catalysts for the selective synthesis of γ-valerolactone from levulinic acid (LA). A series of xNi–yCu–ZrO2 catalysts with a consistent metal loading of 50% but varying Ni and Cu composition were prepared by an oxalate gel precipitation method and tested for LA hydrogenation. Ni-rich catalysts showed higher catalytic activity compared with Cu-rich formulations with a 45Ni–5Cu–ZrO2 composition yielding 76% γ-valerolactone after a reaction time of 30 min at 200 °C. Characterisation of the materials by XRD, surface area measurements and TPR allow us to attribute the differences in performance seen for different compositions to particle size and nanoparticle dispersion effects. DFT calculations also showed that a shift of d-band centre to higher energies with the mole fraction of Ni in Cu–Ni alloys would be expected to lead to improved hydrogen dissociation in Ni-rich catalysts and so aid hydrogenation activity.

BACKGROUND The selective conversion of cellulose to gluconic acid under mild conditions is challenging as it has abundant intra-and inter-molecular hydrogen bonds that protect the β-1,4-glycosidic bonds and make it intrinsically recalcitrant to deploymerize. RESULTS Au-Pd/TiO2 and Au-Pt/TiO2 catalysts prepared by sol immobilization method without pre-treatment are active and selective for the oxidation of glucose and cellobiose to gluconic acid under base-free conditions. Important preparation parameters are the pre-treatment of the catalyst and the metal ratio. CONCLUSION The optimized catalyst provided a good yield of gluconic acid from cellobiose and has opened up a new catalyst system for cellobiose conversion in terms of a heterogeneous catalyst.

Cu–ZrO2 catalysts were synthesized by the methanothermal (Me) and oxalate gel precipitation (Og) methods. Detailed characterization of the catalysts synthesized by the Me method shows that these contain only Cu substituted into the tetragonal ZrO2 lattice. For catalysts prepared using the Og method Cu is found not only in the tetragonal ZrO2 lattice but also in the form of CuO particles on the zirconia surface. When these materials were tested for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) it was found that Me materials show no catalytic activity, whereas GVL was formed using Og catalysts. A reduction treatment of the Og catalysts prior to use resulted in a marked increase in the catalytic activity, however, no activity increase was observed when the Me material was exposed to a similar treatment before testing. Based on these results and characterization data, we conclude that the catalytically active component of Cu–ZrO2 catalysts for the hydrogenation of LA is reduced Cu particles dispersed on the catalyst surface with strong interaction with the Cu incorporated zirconia support, while the role of Cu in the zirconia lattice is to improve the adhesion of these particles and maintain their dispersion.

A rapid microwave synthesis route for the fabrication of ZnIn2S4 powder and ZnIn2S4/WS2 composites is presented. Firstly, the effects of different sulfur sources – thioacetamide and L-cysteine – on the physicochemical properties and photocatalytic H2 production of the synthesized ZnIn2S4 were investigated. It was found that well-defined flower-like ZnIn2S4 microspheres obtained from L-cysteine facilitated a relatively higher H2 production rate. Then, different loadings of WS2 were introduced into the well-defined flower-like ZnIn2S4 microspheres aiming to improve its photocatalytic H2 production. Compared to pure ZnIn2S4 and WS2, all ZnIn2S4/WS2 composite photocatalysts exhibited enhanced photocatalytic H2 production in the presence of Na2S/Na2SO3 as sacrificial reagents under UV-visible irradiation, where the ZnIn2S4/WS2-40% wt composite had the highest photocatalytic activity. For this material, 293.3 and 76.6 μmol h−1 g−1 of H2 gas were produced under UV-visible and visible light irradiation, respectively. In addition, the photoreduction activity of hexavalent chromium (Cr(VI)) by ZnIn2S4/WS2-40% wt was also investigated under visible light irradiation and it was observed that 98.5% of Cr(VI) was reduced within 90 min at pH 4.

A novel pH gradient methodology was used to synthesise a series of Cu–ZrO2 catalysts containing different quantities of Cu and Zr. All of the catalysts were highly selective to the desired product, γ-valerolactone, and are considerably more stable than Cu–ZrO2 catalysts prepared by other co-precipitation methods for this reaction. Characterisation and further investigation of these catalysts by XRD, BET, SEM and XPS provided insight into the nature of the catalytic active site and the physicochemical properties that lead to catalyst stability. We consider the active site to be the interface between Cu/CuOx and ZrOx and that lattice Cu species assist with the dispersion of surface Cu through the promotion of a strong metal support interaction. This enhanced understanding of the active site and roles of lattice and surface Cu will assist with future catalyst design. As such, we conclude that the activity of Cu–ZrO2 catalysts in this reaction is dictated by the quantity of Cu–Zr interface sites.

Cyclic carbonates are valuable chemicals for the chemical industry and thus, their efficient synthesis is essential. Commonly, cyclic carbonates are synthesised in a two-step process involving the epoxidation of an alkene and a subsequent carboxylation to the cyclic carbonate. To couple both steps into a direct oxidative carboxylation reaction would be desired from an economical view point since additional work-up procedures can be avoided. Furthermore, the efficient sequestration of CO2, a major greenhouse gas, would also be highly desirable. In this work, the oxidative carboxylation of 1-decene is investigated using supported gold catalysts for the epoxidation step and tetrabutylammonium bromide in combination with zinc bromide for the cycloaddition of carbon dioxide in the second step. The compatibility of the catalysts for both steps is explored and a detailed study of catalyst deactivation using X-ray photoelectron spectroscopy and scanning electron microscopy is reported. Promising selectivity of the 1,2-decylene carbonate is observed using a one-pot two-step approach. Electronic supplementary material The online version of this article (10.1007/s11244-018-0900-y) contains supplementary material, which is available to authorized users.

By combining nanocatalysis and base-catalysis, a novel one-pot multistep process was found for the synthesis of substituted heterocycles of biological relevance from simple substrates. It is based on an initial Au/O2 oxidation of allylic alcohols followed by a base-catalysed tandem hetero-Michael/aldolisation/crotonisation with ortho-hydroxy or ortho-amino benzaldehydes. The flexibility of the reaction even allowed the benzaldehyde partner to be prepared in situ in an example of one-pot/5-steps process.

In this review we discuss the history of research into the use of gold for the acetylene hydrochlorination reaction, and describe the recent developments which have led to its commercialisation. We discuss the use of different precursors and the addition to gold of a secondary metal as methods which attempt to improve these catalysts, and consider the nature of the active gold species. The vast majority of poly vinyl chloride (PVC) produced globally still uses a mercuric chloride as a catalyst, despite the environmental problems associated with it. Due to the agreement by the Chinese government to remove mercury usage in the PVC industry over the course of the next few years there is an obvious need to find a replacement catalyst; the potential use of gold for this process has been well known for several decades and to date gold seems to be the best candidate for this, primarily due to its superior selectivity when compared to other metals.

A series of ruthenium catalysts supported on two different carbons were tested for the hydrogenation of lactic acid to 1,2-propanediol and butanone to 2-butanol. The properties of the carbon supports were investigated by inelastic neutron scattering and correlated with the properties of the ruthenium deposited onto the carbons by wet impregnation or sol-immobilisation. It was noted that the rate of butanone hydrogenation was highly dependent on the carbon support, while no noticeable difference in rates was observed between different catalysts for the hydrogenation of lactic acid.

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.

A series of Cu–ZrO2 catalysts prepared by a co-precipitation method were studied for the hydrogenation of levulinic acid to give γ-valerolactone (GVL). The effects of a range of catalyst preparation parameters, namely molar Cu/Zr ratio, calcination temperature and the ageing time of the precipitates, were systematically investigated. The molar Cu/Zr ratio was found to have a strong influence on the BET surface area of the material leading to a high activity for catalysts prepared with a Cu/Zr molar ratio of unity. Using this molar ratio the calcination temperature was varied from 300 °C to 800 °C, the material calcined at 400 °C showed the highest activity. Increasing the ageing time used in the catalyst preparation identified 6 h as the optimum to achieve the highest activity for LA conversion. Based on characterisation of all materials we conclude that the active Cu species is present in only low concentration suggesting that it should be possible to produce a catalyst of high activity with much lower Cu content.

A series of 1 wt% supported Au, Pd and AuPd nanoalloy catalysts were prepared via microwave assisted reduction of PdCl2 and HAuCl4 in a facile, one pot process. The resulting materials showed excellent activity for the direct synthesis of hydrogen peroxide from hydrogen and oxygen, with a synergistic effect observed on the addition of Au into a Pd catalyst. Detailed electron microscopy showed that the bimetallic particles exhibited a core–shell morphology, with an Au core surrounded by an Au–Pd shell, with a size between 10–20 nm. The presence of Au in the shell was confirmed by EDX studies, with corroborating data from XPS measurements showing a significant contribution of both Au and Pd in the spectra, with the Au signal increasing as the total Au content of the catalyst increased. No PdO was observed, suggesting a complete reduction of the metal chloride nanoparticles. Unlike similar catalysts prepared by sol-immobilisation methodology, the core–shell structures showed excellent stability during the hydrogen peroxide synthesis reaction, and no catalyst deactivation was observed over 4 reuse cycles. This is the first time the preparation of stable core–shell particles have been reported using microwave assisted reduction. The observation that these particles are core–shell, without the need of a complicated synthesis or high thermal treatment and form in just 15 minutes presents an exciting opportunity for this experimental technique.

In this study, we examine autoxidation and its role on the catalytic aerobic oxidation of cinnamyl alcohol using supported AuPd nanoparticles. We further report the crucial role of reaction conditions in the reaction pathway. Close attention is paid to the mechanism of the formation of benzaldehyde, a by-product formed in large quantities from the process of autoxidation. The activity of the catalysts depends on the preparation method with different sizes of nanoparticles obtained by the two methods of preparation. The catalysts can inhibit the autoxidation process or the two processes, namely catalytic oxidation and autoxidation, can coexist. In the case of oxidation of cinnamyl alcohol, 0.5%(wt) Au–0.5%(wt) Pd/TiO2 catalysts prepared by various methods allow different product distributions to be obtained. The catalyst prepared by the sol-immobilisation method seems to prevent autoxidation, leaving the catalytic process dominant. Catalysts prepared by the impregnation method seem to enable both the catalytic process and autoxidation to occur at the same time. We show that using the optimum catalyst for this reaction, the autoxidation process can be mitigated.

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.

Precious metal nanoparticles supported on magnesium-aluminum hydrotalcite (HT), TiO2 , and MgO were prepared by sol immobilization and assessed for the catalytic oxidation of octanol, which is a relatively unreactive aliphatic alcohol, with molecular oxygen as the oxidant under solvent- and base-free conditions. Compared with the TiO2 - and MgO-supported catalysts, platinum HT gave the highest activity and selectivity towards the aldehyde. The turnover number achieved for the platinum HT catalyst was3700 after 180 min under mild reaction conditions. Moreover, the results for the oxidation of different substrates indicate that a specific interaction of octanal with the platinum HT catalyst could lead to deactivation of the catalyst.

TiO2 with tailored anatase/rutile composition has been prepared from the supercritical antisolvent (SAS) precipitation of a range of titanium alkoxides. The calcination of the SAS TiO2 was monitored by in situ powder X-ray diffraction to determine the optimal calcination conditions for the formation of a mixed anatse/rutile phase TiO2. The SAS precipitated material calcined at 450 °C produced a predominantly anatase support while calcination at 750 °C resulted in a 90 wt% anatase and 10 wt% rutile TiO2. 5 wt% AuPd was added to the SAS TiO2 using an impregnation technique, with exceptional dispersion of the metals being observed by transmission electron microscopy. Mean metal particle sizes were determined to be below 1 nm for both anatase and anatase/rutile SAS TiO2 materials. These catalysts were found to be highly active for the selective oxidation of benzyl alcohol and the direct synthesis of hydrogen peroxide. In addition the anatase/rutile SAS TiO2 was found to have comparable activity to commercial anatase/rutile mixed phase TiO2 for the photocatalytic splitting of water for hydrogen production.

Pd catalysts supported on TiO2 functionalized with various amounts of 3-aminopropyltriethoxysilane (APTES) were prepared using a post-synthesis grafting method combined with electroless deposition of Pd. As revealed by the Fourier transformed infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) results, monolayer APTES grafting was obtained using 0.005 mmol APTES on 1.5 g TiO2 support. Excess amounts of APTES resulted in both multilayer and reversed attachment, which NH2 attached to the TiO2 surface rather than giving free NH2 termination. The catalytic activity in the solvent-free selective oxidation of benzyl alcohol was correlated well with the highest amount of Pd deposited as well as the formation of small and uniform Pd nanoclusters with narrow particle size distribution (average diameter 3.4 nm) on the 1%Pd/TiO2-0.005APTES. Increasing of surface basicity via the hydrolysis of amino groups (single bondNH2) is suggested to enhance the dehydrogenation of benzyl alcohol, and as a consequence the selectivity toward benzaldehyde increased for all the APTES-modified TiO2 supported Pd catalysts. In addition, the combination of metallic Pd0 and PdOx (Pd2+/Pd4+) species gave high catalytic activity in the benzyl alcohol oxidation, emphasizing that the reduction of PdOx species by the adsorbed benzyl alcohol is an essential step to form highly active metallic Pd0 sites.

The selective hydrogenation of furfuryl alcohol was investigated at room temperature by using supported palladium catalysts. The catalysts are very selective to the formation of 2-methylfuran. Furthermore, the addition of tin to palladium showed similar catalytic activity, but was more selective to tetrahydrofurfuryl alcohol. Variation of the Sn/Pd ratio has shown a considerable and interesting effect on the selectivity pattern. Addition of a small amount of Sn (1 wt %) shifted the selectivity towards tetrahydrofurfuryl alcohol and methyltetrahydrofuran, which are ring-saturated molecules. Increasing the tin ratio further decreased the catalytic activity and also showed very poor selectivity to either of these products.

The base-free selective catalytic oxidation of n-butanol by O2 in an aqueous phase has been studied using Au-Pd bimetallic nanoparticles supported on titania. Au-Pd/TiO2 catalysts were prepared by different methods: wet impregnation, physical mixing, deposition-precipitation and sol immobilisation. The sol immobilisation technique, which used polyvinyl alcohol (PVA) as the stabilizing agent, gave the catalyst with the smallest average particle size and the highest stable activity and selectivity towards butyric acid. Increasing the amount of PVA resulted in a decrease in the size of the nanoparticles. However, it also reduced activity by limiting the accessibility of reactants to the active sites. Heating the catalyst to reflux with water at 90 °C for 1 h was the best method to enhance the surface exposure of the nanoparticles without affecting their size, as determined by TEM, X-ray photoelectron spectroscopy and CO chemisorption analysis. This catalyst was not only active and selective towards butyric acid but was also stable under the operating conditions.

Mg(OH)2‐ and Mg(OH)2‐containing materials can provide excellent performance as supports for AuPd nanoparticles for the oxidation of glycerol in the absence of base, which is considered to be a result of additional basic sites on the surface of the support. However, its influence on the reaction solution is not generally discussed. In this paper, we examine the relationship between the basic Mg(OH)2 support and AuPd nanoparticles in detail using four types of catalyst. For these reactions, 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 the 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 reaction.

Catalysts containing bimetallic gold–palladium nanoparticles are extremely active and selective for the oxidation of alcohols to aldehydes and the direct synthesis of hydrogen peroxide from molecular hydrogen and oxygen. We show that the oxidation of benzyl alcohol can be carried out at 50 °C and below by generating hydrogen peroxide in situ. The oxidation of benzyl alcohol to benzaldehyde has been achieved with high selectivity (>85%) at temperatures where no reaction is observed with only molecular oxygen in an autoclave. The effect of temperature, catalyst support, and solvent are studied in an autoclave system and reactions were carried out in a fixed bed reactor at a range of gas flow rates where the catalysts demonstrated stable conversion and selectivity.

The oxidation of four butanediols under base-free conditions has been investigated using a set of Au, Pd and Pt catalysts prepared using sol-immobilization. The supported nanoparticles are found to be active with bimetallic alloys having much higher activity when compared with the monometallic counterparts. In general the AuPt catalysts are the most active and in all cases the corresponding C4 oxidation products were observed with high selectivity; sequential reaction of these products leads to the formation of acetic acid as an undesired by-product.

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.

The oxidation of cyclic alkenes (cyclopentene, cyclohexene, cycloheptene and cyclooctene) has been carried out under green solvent-free conditions using ambient air as the oxidant gas with a carbon-supported gold catalyst. We show that radical initiators are only required for this reaction when the cyclic alkene contains a stabilizer molecule, such as 6-bis(t-butyl)-4-methylphenol (BHT) or 3-octadecyl-(3,5-di-t-butyl-4-hydroxyphenyl)-propanoate (irganox 1076). We show that either removing the stabilizer chemically or sourcing the cycloalkane without the stabilizer the oxidation, both with and without radical initiator, is identical. This indicates that the only function of the radical initiator is to counteract the stabilizer. We have extended this work to show that stabilizer-free internal linear alkenes can be effectively epoxidised with air using the carbon-supported gold catalyst. However, unfortunately linear α-alkenes still require the addition of an initiator to achieve reaction. This work demonstrates that removal of a sacrificial reagent from a reaction is possible which adheres to the principles of green chemistry.

The selective hydrogenation of furfuryl alcohol into 2-methylfuran was investigated at room temperature using palladium supported catalysts. We have shown that Pd–TiO2 catalysts can be very effective for the synthesis of 2-methylfuran at room temperature and low pressure of hydrogen (1–3 bar). The effect of various reaction conditions (pressure, catalyst amount, and solvent) was studied.

Numerous environmental problems exist which may be solved using catalysis, in particular by supported precious metal nanoparticles. We focus here on the use of gold as a catalyst, which since the 1980s has become an important field of research. The use of gold-based catalysts for tackling environmental issues is discussed, within the context of projects currently being undertaken within the environment theme of the UK Catalysis Hub; the applications of gold catalysis for utilisation and valorisation of biomass derivatives, treatment of waste water using photo-catalysis and vehicle emissions control are considered… Read More: http://www.worldscientific.com/doi/abs/10.1142/9781786341228_0006

The effect of the reaction conditions and catalyst composition on the oxidation of 1,4-butanediol has been investigated. We have shown that gold and gold–palladium nanoparticles supported on titania can be effective catalysts for this oxidation reaction. We have demonstrated that the formation of butyrolactone, methyl-4-hydroxybutyrate and dimethyl succinate follow a sequential reaction pathway. We have also investigated the role of acid/base on the reaction and have shown a beneficial synergistic effect of alloying gold with palladium on both the conversion and selectivity towards dimethyl succinate.

The oxidation of 1,4-butanediol and butyrolactone have been investigated by using supported gold, palladium and gold-palladium nanoparticles. The products of such reactions are valuable chemical intermediates and, for example, can present a viable pathway for the sustainable production of polymers. If both gold and palladium were present, a significant synergistic effect on the selective formation of dimethyl succinate was observed. The support played a significant role in the reaction, with magnesium hydroxide leading to the highest yield of dimethyl succinate. Based on structural characterisation of the fresh and used catalysts, it was determined that small gold-palladium nanoalloys supported on a basic Mg(OH)2 support provided the best catalysts for this reaction.

In recent work, it was reported that changes in solvent composition, precisely the addition of water, significantly inhibits the catalytic activity of Au/TiO2 catalyst in the aerobic oxidation of 1,4-butanediol in methanol due to changes in diffusion and adsorption properties of the reactant. In order to understand whether the inhibition mechanism of water on diol oxidation in methanol is generally valid, the solvent effect on the aerobic catalytic oxidation of 1,3-propanediol and its two methyl-substituted homologues, 2-methyl-1,3-propanediol and 2,2-dimethyl-1,3-propanediol, over a Au/TiO2 catalyst has been studied here using conventional catalytic reaction monitoring in combination with pulsed-field gradient nuclear magnetic resonance (PFG-NMR) diffusion and NMR relaxation time measurements. Diol conversion is significantly lower when water is present in the initial diol/methanol mixture. A reactivity trend within the group of diols was also observed. Combined NMR diffusion and relaxation time measurements suggest that molecular diffusion and, in particular, the relative strength of diol adsorption, are important factors in determining the conversion. These results highlight NMR diffusion and relaxation techniques as novel, non-invasive characterisation tools for catalytic materials, which complement conventional reaction data.

Disproportionation of benzyl alcohol has been identified as the source of toluene formation in the solvent free oxidation of benzyl alcohol using supported gold palladium catalysts. There is a slight increase in the disproportionation reaction, and hence the toluene selectivity, when this reaction is performed in a continuous mode using a micro-packed bed reactor when compared to the same reaction performed in a conventional glass stirred batch reactor. Oxidation and disproportionation reactions respond slightly differently to the changes in reaction parameters, like oxygen concentration and pressure, when a micro packed bed reactor was used instead of a conventional glass stirred reactor. When MgO supported gold–palladium catalysts were used for this reaction, the toluene selectivity reduced substantially at the cost of conversion.

The use of bio-renewable resources for the generation of materials and chemicals continues to attract significant research attention. It is well established that glycerol is an excellent starting material for the production of 1,2-propanediol by dehydration/hydrogenation and that this can subsequently be oxidised to lactic acid, which has the potential to be used as a major chemical in the production of biodegradable polymers. Previous studies using gold catalysts for the oxidation of 1,2-propanediol have used elevated temperatures and pressures. We now show that the oxidation of 1,2-propanediol to form lactic acid can be carried out selectively under mild reaction conditions with gold-platinum catalysts prepared using a sol-immobilisation method, with activated carbon as the support. Carrying out the reaction at ambient temperature with air significantly improves the reaction in terms of its environmental impact and its industrial attractiveness, as lactic acid can be obtained with high selectivity. © 2012 Elsevier B.V. All rights reserved.

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 (

Gold and gold alloys, in the form of supported nanoparticles, have been shown over the last three decades to be highly effective oxidation catalysts. Mixed metal oxide perovskites, with their high structural tolerance, are ideal for investigating how changes in the chemical composition of supports affect the catalysts' properties, while retaining similar surface areas, morphologies and metal co-ordinations. However, a significant disadvantage of using perovskites as supports is their high crystallinity and small surface area. We report the use of a supercritical carbon dioxide anti-solvent precipitation methodology to prepare large surface area lanthanum based perovskites, making the deposition of 1 wt% AuPt nanoparticles feasible. These catalysts were used for the selective oxidation of glycerol. By changing the elemental composition of the perovskite B site, we dramatically altered the reaction pathway between a sequential oxidation route to glyceric or tartronic acid and a dehydration reaction pathway to lactic acid. Selectivity profiles were correlated to reported oxygen adsorption capacities of the perovskite supports and also to changes in the AuPt nanoparticle morphologies. Extended time on line analysis using the best oxidation catalyst (AuPt/LaMnO3) produced an exceptionally high tartronic acid yield. LaMnO3 produced from alternative preparation methods was found to have lower activities, but gave comparable selectivity profiles to that produced using the supercritical carbon dioxide anti-solvent precipitation methodology.

The effect of water on the catalytic oxidation of 1,4-butanediol in methanol over Au/TiO(2) has been investigated by catalytic reaction studies and NMR diffusion and relaxation studies. The addition of water to the dry catalytic system led to a decrease of both conversion and selectivity towards dimethyl succinate. Pulsed-field gradient (PFG)-NMR spectroscopy was used to assess the effect of water addition on the effective self-diffusivity of the reactant within the catalyst. NMR relaxation studies were also carried out to probe the strength of surface interaction of the reactant in the absence and presence of water. PFG-NMR studies revealed that the addition of water to the initial system, although increasing the dilution of the system, leads to a significant decrease of effective diffusion rate of the reactant within the catalyst. From T(1) and T(2) relaxation measurements it was possible to infer the strength of surface interaction of the reactant with the catalyst surface. The addition of water was found to inhibit the adsorption of the reactant over the catalyst surface, with the T(1)/T(2) ratio of 1,4-butanediol decreasing significantly when water was added. The results overall suggest that both the decrease of diffusion rate and adsorption strength of the reactant within the catalyst, due to water addition, limits the access of reactant molecules to the catalytic sites, which results in a decrease of reaction rate and conversion.

We report the solvent-free oxidation of 1-hexene with air by using supported gold catalysts with a catalytic amount of tert-butyl hydroperoxide (TBHP) as initiator. We confirm that gold supported on graphite is an effective catalyst for such oxidations and that graphite was the preferred support. Preparation of catalysts using modified sol-immobilisation was found to be effective, particularly when the PVA stabiliser was removed by a solvent treatment prior to the reaction.

The oxidative esterification of a homologous series of diols (1,3-propanediol,2-methyl-propanediol and 2,2-dimethyl-1,3-propanediol) with methanol has been investigated using titania-supported gold, palladium and gold–palladium catalysts using molecular oxygen. The gold– palladium catalysts showed the highest activity and 1,3-propanediol was the most reactive while the additional methyl groups decreased the reactivity. However, it is possible to achieve high selectivity to methyl 3-hydroxypropionate and 2-methyl-3-hydroxyisobutyrate by mono-oxidations.

Dehydration of glycerol was carried out using rubidium- and caesium-doped silicotungstic acid catalysts. These catalysts were prepared by varying concentration of the dopant metal cations while keeping the concentration of heteropoly acid unchanged. High acrolein selectivity (94–96%) was observed with unsupported caesium-doped silicotungstic acid and rubidium-doped silicotungstic acid with a dilute glycerol feed (0.5 wt.% in water). These catalysts were then supported on alpha-alumina and an alumina comprising a theta-delta mixture. Caesium-doped silicotungstic acid supported on theta-delta alumina gave a maximum selectivity of ca. 90% at 100% glycerol conversion for 90-h time online, with a 10 wt.% glycerol solution. With a more concentrated glycerol feed (20 wt.%), this catalyst achieved a space time yield of 210 . The catalyst was investigated further to determine the origin of the long-term stability. The binding strength of the partially doped silicotungstic acid on the alumina was found to be crucial to sustain the supported Keggin structure and hence the acidity of the active sites resulting in a high acrolein yield.

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. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Gold–palladium nanoparticles either in the form of colloids or supported nanoparticles have been extensively used as redox catalysts in recent years. These materials are very effective for the transformation of organic compounds to highly useful chemical products. The catalytic materials are usually prepared either using deposition-precipitation or impregnation techniques, but recently significant attention has been focused on the use of colloidal methods. Here we compare and contrast the preparation and catalytic reactivity of Au–Pd supported nanoparticles synthesised by impregnation, deposition-precipitation and colloidal methods. The catalyst materials have been evaluated for the oxidation of benzyl alcohol, as a model reaction. In addition, we have focused our attention on the utilisation of different types of reactors (autoclave versus glass reactors) and we now emphasise the possibility of using simplified types of reactors which have the advantage of significant cost savings and ease of application.