Heterogeneous gold catalysis

Catalysis is at the heart of many manufacturing processes and underpins provision of the goods and infrastructure necessary for the effective wellbeing of society; catalysis continues to play a key role in the manufacture of chemical intermediates and final products. There is a continuing need to design new effective catalysts especially with the drive toward using sustainable resources. The identification that gold is an exceptionally effective catalyst has paved the way for a new class of active heterogeneous and homogeneous catalysts for a broad range of reactions. As a heterogeneous catalyst gold is the most active catalyst for the oxidation of carbon monoxide at ambient temperature. It is also the most effective catalyst for the synthesis of vinyl chloride by acetylene hydrochlorination, and a gold catalyst has recently been commercialized in China for this reaction. In this outlook the nature of the active gold species for these two reactions will be explored

Selective oxidation using In situ-generated hydrogen peroxide

Hydrogen peroxide (H2O2) for industrial applications is manufactured through an indirect process that relies on the sequential reduction and reoxidation of quinone carriers. While highly effective, production is typically centralized and entails numerous energy-intensive concentration steps. Furthermore, the overhydrogenation of the quinone necessitates periodic replacement, leading to incomplete atom efficiency. These factors, in addition to the presence of propriety stabilizing agents and concerns associated with their separation from product streams, have driven interest in alternative technologies for chemical upgrading. The decoupling of oxidative transformations from commercially synthesized H2O2 may offer significant economic savings and a reduction in greenhouse gas emissions for several industrially relevant processes. Indeed, the production and utilization of the oxidant in situ, from the elements, would represent a positive step toward a more sustainable chemical synthesis sector, offering the potential for total atom efficiency, while avoiding the drawbacks associated with current industrial routes, which are inherently linked to commercial H2O2 production. Such interest is perhaps now more pertinent than ever given the rapidly improving viability of green hydrogen production. The application of in situ-generated H2O2 has been a long-standing goal in feedstock valorization, with perhaps the most significant interest placed on propylene epoxidation. Until very recently a viable in situ alternative to current industrial oxidative processes has been lacking, with prior approaches typically hindered by low rates of conversion or poor selectivity toward desired products, often resulting from competitive hydrogenation reactions. Based on over 20 years of research, which has led to the development of catalysts for the direct synthesis of H2O2 that offer high synthesis rates and >99% H2 utilization, we have recently turned our attention to a range of oxidative transformations where H2O2 is generated and utilized in situ. Indeed, we have recently demonstrated that it is possible to rival state-of-the-art industrial processes through in situ H2O2 synthesis, establishing the potential for significant process intensification and considerable decarbonization of the chemical synthesis sector. We have further established the potential of an in situ route to both bulk and fine chemical synthesis through a chemo-catalytic/enzymatic one-pot approach, where H2O2 is synthesized over heterogeneous surfaces and subsequently utilized by a class of unspecific peroxygenase enzymes for C–H bond functionalization. Strikingly, through careful control of the chemo-catalyst, it is possible to ensure that competitive, nonenzymatic pathways are inhibited while also avoiding the regiospecific and selectivity concerns associated with current energy-intensive industrial processes, with further cost savings associated with the operation of the chemo-enzymatic approach at near-ambient temperatures and pressures. Beyond traditional applications of chemo-catalysis, the efficacy of in situ-generated H2O2 (and associated oxygen-based radical species) for the remediation of environmental pollutants has also been a major interest of our laboratory, with such technology offering considerable improvements over conventional disinfection processes. We hope that this Account, which highlights the key contributions of our laboratory to the field over recent years, demonstrates the chemistries that may be unlocked and improved upon via in situ H2O2 synthesis and it inspires broader interest from the scientific community

The effect of polymer addition on base catalysed glycerol oxidation using gold and gold-palladium bimetallic catalysts

The oxidation of glycerol represents both a viable route to catalytic upgrading of biomass and has become a model reaction for catalytic polyol oxidation. Gold and gold–palladium nanoparticle catalysts prepared by colloidal methods involving polymer additives have been extensively studied. However, the effect of residual polymer at the catalyst surface on reaction pathways has not been decoupled from particle size effects. We show that when using catalysts prepared without polymer stabilisers the addition of either polyvinyl alcohol or polyvinylpyrrolidone to the reaction changes the reaction rate and results in a change in reaction selectivity. We conclude that the polymer additive has a significant effect on the reaction pathway and that these systems should be considered as a metal surface–polymer interface catalytic systems and properties should not be rationalised solely based on nanoparticle size

Plasmonic oxidation of glycerol using AuPd/TiO₂ catalysts

AuPd nanoparticles supported on P25 TiO2 (AuPd/TiO2) were prepared by a facile sol-immobilisation method and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions. The AuPd/TiO2 samples were characterized by UV-vis spectroscopy and transmission electron microscopy. The sol-immobilisation method readily permitted the Au : Pd molar ratio to be changed over a wide range whilst keeping the mean particle size of the AuPd nanoparticles at 3 nm. Visible light irradiation during the reaction has a beneficial effect on the conversion of glycerol with the most marked effect being observed with gold-rich catalysts and the increase of conversion on light irradiation increases linearly with the gold content of the nanoparticles. The reaction selectivity is also affected by the plasmon-assisted oxidation and glycolic acid, not observed during the dark reactions, was observed for all illuminated reactions due to the enhanced activity of these catalysts

Characterisation and activity of mixed metal oxide catalysts for the gas-phase selective oxidation of toluene

Mixed metal bi-component oxide catalysts, including Fe/Mo, U/Mo, U/W, Fe/U, U/V and U/Sb have been prepared, characterised and evaluated for gas phase selective toluene oxidation. Selective toluene oxidation activity to form benzaldehyde was exhibited by Fe/Mo, U/Mo and U/W mixed oxide catalysts. The Fe/Mo catalyst produced the highest benzaldehyde yield. Catalysts that formed benzaldehyde also produced a range of by-products, these were other partial oxidation and coupling products, and preliminary studies of benzaldehyde oxidation suggests they were formed from secondary reactions of benzaldehyde. The Fe/U, Sb/U and U/V catalysts produced only total oxidation to carbon oxides. Catalysts were characterised by X-ray diffraction, laser Raman spectroscopy and temperature programmed reduction. Single molybdate phases were identified for the Fe/Mo and U/Mo catalysts, and a mixture of uranium molybdate and WO3 was identified for the U/W catalyst. Results suggest that the formation of a molybdate phase is important for the selective oxidation of toluene. In contrast, the U/Fe catalyst was a mixture of U3O8 and V2O5, whilst the Fe/U catalyst was comprised of highly dispersed iron oxide on UO3. The presence of U3O8 was responsible for toluene total oxidation. The U/Sb catalyst did not exhibit selective toluene oxidation, but previous studies have demonstrated that the catalyst exhibits high activity for selective propene oxidation. Similar behaviour has been observed for the other catalysts in this study, and it is apparent that catalysts that were selective for toluene oxidation were not selective for propene/propane oxidation and vice versa

Direct synthesis of hydrogen peroxide in water at ambient temperature

The direct synthesis of hydrogen peroxide (H2O2) from hydrogen and oxygen has been studied using an Au–Pd/TiO2 catalyst. The aim of this study is to understand the balance of synthesis and sequential degradation reactions using an aqueous, stabilizer-free solvent at ambient temperature. The effects of the reaction conditions on the productivity of H2O2 formation and the undesirable hydrogenation and decomposition reactions are investigated. Reaction temperature, solvent composition and reaction time have been studied and indicate that when using water as the solvent the H2O2 decomposition reaction is the predominant degradation pathway, which provides new challenges for catalyst design, which has previously focused on minimizing the subsequent hydrogenation reaction. This is of importance for the application of this catalytic approach for water purification

Recent advances in the gold-catalysed low-temperature water-gas shift reaction

The low-temperature water–gas shift reaction (LTS: CO + H2O ⇌ CO2 + H2) is a key step in the purification of H2 reformate streams that feed H2 fuel cells. Supported gold catalysts were originally identified as being active for this reaction twenty years ago, and since then, considerable advances have been made in the synthesis and characterisation of these catalysts. In this review, we identify and evaluate the progress towards solving the most important challenge in this research area: the development of robust, highly active catalysts that do not deactivate on-stream under realistic reaction condition

Low temperature solvent-free allylic oxidation of cyclohexene using graphitic oxide catalysts

A range of graphitic oxides have been utilised as metal free carbocatalysts for the low temperature oxidation of cyclohexene. The activity of the catalysts was correlated with the amount of surface oxygen on the graphitic oxide. In the case of cyclohexene oxidation, major selectivity is observed to allylic oxidation products. This is in contrast to the epoxide being the major product in linear alkene oxidation. This selectivity was maintained over long reaction times and at a conversion of above 50 %. Only small amounts of epoxide were observed, which eventually decreases at higher conversion due to hydrolysis to cyclohexane diol. The similarity between the non-catalysed and the catalysed product distribution suggests that these catalysts act as a solid initiator, and the role of the graphitic oxide is to decrease the lengthy induction period observed in the blank non-catalysed reaction