The selective oxidation of methane to methanol and acetic acid represents a promising strategy for valorizing abundant natural gas into valuable chemicals. Using zeolite (ZSM-5)-supported Au nanoparticles from colloidal adsorption, deposition precipitation and wet impregnation methods, we demonstrate the selective oxidation of methane by cofeeding CH4, CO, O2 and steam in a high-pressure continuous flow reactor. Oxygenates (methanol, acetic acid and other trace chemicals.) and higher hydrocarbons (ethane) were produced in addition to carbon dioxide. The catalyst synthesis protocols showed a strong influence on the catalytic performance. Infrared spectroscopy and electron microscopy studies suggest that Au nanoparticles rather than ionic Au species are responsible for the active sites in the selective oxidation of methane.

Heterogeneous catalysts consistently suffer from the leaching of active components into the fluid phase, causing loss in activity and lifetime. Here we demonstrate, exemplified with the thermocatalytic oxidation of 1,2-propanediol, a metal stabilization mechanism arising from electrochemical coupling of half reactions between physically separated carbon-supported Au and Pd catalysts. When electrochemically coupled, polarization of each metal to an operating mixed potential occurs as a function of the Au:Pd molar ratio that, at pH 9-10, significantly reduces Pd leaching. At pH 14, where this stabilization no longer holds, we clearly show that an active redox mechanism can exist between disparate catalytic species in the liquid and solid phase, and that this leads to the cycling of Pd speciation as well as considerable enhancements in the rate of oxidation.

Decreasing our reliance on fossil carbon requires economically viable and scalable pathways to utilize sustainable sources, such as chitin. Current methods for converting chitin to high-value chemicals require treatment with strong acids and/or bases at high temperature, thereby giving complicated product mixtures with substantial, negative environmental impact. Here we present a chemo-enzymatic cascade in which H2O2, generated in situ over Pd-based nanoalloys, is used by a lytic polysaccharide monooxygenase (LPMO) to convert chitin into soluble oligosaccharide fragments. Our approach, which minimizes oxidative damage to the enzyme, eliminates the need for atom-inefficient and energy intensive approaches to chitin depolymerization, potentially achieving substantial environmental and economic savings. The simplicity of this chemo-catalyst/enzyme cascade has significant advantages for accessing chitin as a bio-carbon resource.

This contribution highlights the exceptional performance achievable via a chemo-enzymatic approach to the valorisation of chemical feedstocks, and outlines the key parameters which must be considered for such tandem systems to operate efficiently. Image partly generated using Google Gemini.

Abstract Direct conversion of CH4 into value-added chemicals is impeded by the inert C-H bonds and inefficient C-C coupling. We report a spatially separated Rh-O-Fe active-site architecture that decouples CH4 and H2O activation through a high-valent-metal mediated radical mechanism, enabling selective CH3COOH synthesis. In-situ infrared, operando Mössbauer spectroscopy, and quasi in-situ high-field EPR reveal that O2 oxidizes Rh and Fe to high valence states. Rh(III) activates CH4 to •CH3, while Fe(IV) = O dissociates H2O into •OH through a truncated water-gas shift pathway. •OH rapidly reacts with CO to form •COOH intermediates, which couples with •CH3 within the zeolite to yield CH3COOH. This dual-site strategy circumvents kinetic limits of conventional water-gas shift and CO insertion steps. The catalyst achieves 18.2 mmol gcat -1 h-1 CH3COOH with 92% selectivity and 100-hour stability in continuous operation. This study establishes radical decoupling enabled by high-valent metal sites as a design principle for selective alkane oxidation.

The selective oxidation of methane to form methanol and acetic acid has been studied using AuPd nanoparticles supported on the zeolite H-ZSM-5 in water at 240 degrees C using molecular oxygen as the terminal oxidant in the absence of any added coreductant. The addition of Pd to Au/ZSM-5 significantly increases the selectivity to acetic acid to levels approaching almost complete selectivity within the oxygenated products. However, we observe that the reaction conditions employed lead to the corrosion of the stainless-steel components of the autoclave reactor and also leaching of iron from the ZSM-5 zeolite, and hence the AuPd nanoparticles, on reaction, become coated or partially coated with an oxidic Fe shell. The presence of the oxidized iron coating hinders nanoparticle agglomeration preventing deactivation of the AuPd/ZSM-5 catalyst but does not adversely affect the observed catalysis.

The selective oxidation of alcohol to the corresponding aldehydes via in situ H 2 O 2 (and associated ROS) production offers an exciting, environmentally friendly alternative to the use of stoichiometric oxidants.

The formation of C-C bonds through coupling reactions is an important industrial process. The ability of Au to catalyze such reactions has been reported, with both homogeneous and heterogeneous catalyst examples. Previous work has shown that carbon-supported cationic and nanoparticulate Au are active for the homocoupling of phenylboronic acid to biphenyl. However, the stability of supported cationic Au is short-lived, and the formed nanoparticles were suggested to be the active species. Through the synthesis of two types of supported cationic Au catalysts, utilizing either aqua regia or acetone solvents, we show that both catalysts develop nanoparticulate Au species early in the reaction; however, only the aqua regia prepared catalyst is active. We ascribe the activity of the aqua regia prepared Au catalyst to excess Cl and the presence of C-Cl surface species in combination with Au. Carbon treated with aqua regia was inactive; however, when used as a support for Au deposited with acetone or via a sol immobilization method, activity was comparable to the aqua regia prepared catalyst. The role of C-Cl and Au nanoparticles is discussed with respect to their correlation to the biphenyl yield, which is shown to be significant only when the C-Cl species are present on the catalyst.

The development of titianosilicates is considered a major milestone in oxidative catalysis due to the ability of framework Ti sites to co-ordinate hydrogen peroxide/peroxy species. Herein, we demonstrate that interfacial Ti sites can be constructed through the vertical intergrowth of two MFI-type zeolite surfaces along [100] and [010] projections, with the assistance of UV-induced hydroxyl radicals. The application of these intergrown titanosilicalites as supports for Au species are observed to simultaneously offer a 2.1-fold and 3.0-fold increase in propene oxide (PO) formation rate and Au efficiency, respectively, when compared to standard Au/TS-1 catalysts. Mechanistic studies reveal that the intergrown interface Ti sites allow for lower-energy epoxidation pathways with more efficient activation of key oxygen-transfer intermediates. These results provide insights into the development of zeolite intergrown interface sites (e.g., titanosilicalite/silicalite-1/ZSM-5) and may allow for further advancements in the epoxidation of a range of key feedstocks.

The development of efficient strategies for the synthesis of levulinate esters is of significant current interest due to their potential as biofuels and fuel additives. Herein, we report a novel strategy to access levulinate esters derived from higher alcohols directly from levulinic acid through the in situ generation of lactone intermediates employing commercial heterogeneous catalysts, such as Amberlyst-15. This strategy employs a telescoped approach in which the lactonization/ring-opening reactions are combined into an operationally simple one-pot procedure. This strategy is advantageous as it employs a readily available and inexpensive catalyst and proceeds in short reaction times to produce excellent yields of higher levulinate esters with high selectivity. Furthermore, the Amberlyst-15 catalyst is fully recyclable and can be reused without loss of activity or selectivity.

Within this contribution, the combination of supported AuPd nanoalloys with horseradish peroxidase is demonstrated to offer high efficacy towards the one-pot oxidative polymerisation of the model wastewater contaminant phenol, via the chemo-catalytic supply of in-situ generated H2O2. Notably, the utilisation of AuPd alloyed formulations offered considerably improved cascade efficiencies, compared to that observed over monometallic analogues, with the optimal 0.5%Au-0.5%Pd/TiO2 catalyst achieving total conversion of phenol within 15 minutes when used in conjunction with the enzyme. Importantly, the in-situ chemo-enzymatic system was shown to offer good stability over successive reactions, and outperforms analogous approaches reliant on the use of preformed H2O2, while also avoiding the proprietary stabilising agents present in the commercial oxidant.

Chemical vapour impregnation gives a high level of control over palladium nanoparticle size and dispersion regardless of metal weight loading, with catalyst activity per active site being identical.

In this contribution, we outline the efficacy of Pd-based bimetallic catalysts toward the oxidative upgrading of benzyl alcohol via the in situ synthesis of H2O2 (and related reaction intermediates) from the elements. In particular, the formation of PdAu and PdFe nanoalloys is observed to be highly effective, offering high yields of benzaldehyde and near total selectivity to the desired product, with these catalysts outperforming alternative materials reported in the literature. Notably, the PdFe formulation also achieves high selective utilization of H-2, a key requirement if the in situ approach to chemical synthesis is to become economically viable. Correlative studies, focusing on the direct synthesis of H2O2 and further experiments utilizing preformed H2O2, coupled with Electron Paramagnetic Resonance (EPR) spectroscopy indicate that H2O2 itself is not primarily responsible for the observed catalysis, but rather, the performance of the PdAu and PdFe formulations can be related to the generation of reactive oxygen species (ROS). While the origin of these ROS is not fully understood, it is hypothesized that they are generated through a combination of (i) the desorption of reaction intermediates formed during H2O2 synthesis and (ii) through Fenton-mediated chemistry involving the synthesized H2O2, in the case of the PdFe-based materials. Importantly, our EPR studies also identify the noninnocent nature of the reaction solvent.

Nanoporous silicate materials, made by an operationally simple evaporation-induced self-assembly (EISA) approach, efficiently catalyze the formation of 2-substituted 2,3-dihydroquinazolin-4(1H)-ones from aldehydes and dimethyl acetals. This methodology can be extended to encompass the reaction of ketones and ketals to provide facile access to 2,2-disubstituted and spiro dihydroquinazolin-4(1H)-ones in high yields and short reaction times.

We report the selective liquid phase hydrogenation of furfural to 2-furfuryl alcohol using a Pt/TiO2 catalyst prepared by the wet-impregnation method under mild reaction conditions (30 degrees C and 3 bar H-2 pressure). The effect of heat treatment protocols on the catalyst structures and the resultant catalytic properties of 4.2%Pt/TiO2 and 0.6%Pt/TiO2 was investigated. For both Pt loadings, the calcined + reduced catalyst exhibited higher activity compared to the reduced only catalyst, with the difference in activity being more pronounced for 4.2%Pt/TiO2 than for 0.6%Pt/TiO2. For the 4.2%Pt/TiO2 catalyst, the reduced-only sample achieved 25% conversion with 90% selectivity for 2-furfuryl alcohol after 6 hours, while the calcined + reduced sample reached 99% conversion with 59% selectivity under identical reaction conditions. For the 0.6%Pt/TiO2 catalyst, the reduced-only sample showed 70% conversion with 96% selectivity for 2-furfuryl alcohol, whereas the calcined + reduced sample achieved 97% conversion and 95% selectivity after a 2-h reaction. Characterisation of the samples using X-ray photoelectron spectroscopy, CO chemisorption and scanning transmission electron microscopy revealed that direct high temperature reduction resulted in a mixture of large Pt particles (>5 nm) with irregular shapes, small Pt nanoparticles (ca. 2 nm) and some sub-nm clusters. In contrast, calcination + reduction produced uniformly distributed Pt nanoparticles (ca. 2 nm) for both Pt loadings. Despite the presence of strong metal support interaction (SMSI) in Pt/TiO2 catalysts, no spectroscopic evidence for such a strong interaction was found in this study. Therefore, the observed difference in catalytic activity is attributed to the variations in the shapes and sizes of the Pt nanoparticles. During the synthesis of Pt/TiO2 catalysts, the calcination + reduction activation procedure is more beneficial for enhancing both activity and selectivity compared to a reduction only procedure.

The behaviour of Pd deposited on Ga2O3 and In2O3 by CVI are compared for the hydrogenation of CO2. Ga2O3 alone is inactive, but In2O3 has good conversion, and selectivity as high as 89% to CH3OH. The addition of Pd to the catalysts had relatively little effect for In2O3, but in contrast, the addition of Pd to Ga2O3, has a very big effect, inducing high activity and selectivity. Both oxides form intermetallic Pd alloys - Pd2In3 and Pd2Ga. However, for the In catalysts there is also a thick (~3 nm) overlayer of the oxide, while for the Ga catalyst there was no such overlayer. Hence this is why addition of Pd to the Indium catalysts has relatively little effect on performance compared with Ga. Upon co-deposition of Pd and Zn, and after reduction, the Pd2In3 catalyst remains phase stable, whereas the Pd2Ga alloy is replaced by PdZn.

The behaviour of Pd deposited on Ga2O3 and In2O3 by CVI is compared for the hydrogenation of CO2 to methanol. Ga2O3 alone is inactive, but In2O3 has good conversion, and selectivity as high as 89% to CH3OH. The addition of Pd to the catalysts had relatively little effect for In2O3, but in contrast, the addition of Pd to Ga2O3, has a very big effect, inducing high activity and selectivity to methanol. Both oxides form Pd intermetallics - Pd2In3 and Pd2Ga. However, for the In catalysts there is also a thick (~3nm) overlayer of the oxide, while for the Ga catalyst there was no such overlayer. Hence this is why addition of Pd to the Indium catalysts has relatively little effect on performance compared with Ga. Furthermore, the effect of Pd and Zn co-deposition on Ga₂O₃ and In₂O₃ was investigated, as well as the effect of the support morphology. Upon co-deposition of Pd and Zn, and after reduction, the Pd2In3 catalyst remains phase stable, whereas the Pd2Ga alloy is replaced by PdZn, and is improved in methanol yield.

The selective oxidation of methane to methanol, using H2O2 generated in situ from H2 and O2 has been investigated using bimetallic gold-palladium catalysts, prepared via an industrially relevant wet co-impregnation protocol on a range of zeolite and metal oxide supports. The choice of catalyst support was found to drastically influence catalyst performance, through control of both nanoparticle dispersion and Pd speciation. Notably in the case of those formulations prepared on metal oxides a direct correlation between catalytic performance towards H2O2 synthesis and methane valorisation exists, whereas in the case of the zeolitic-based analogues, no clear correlation could be drawn between activity towards individual reaction pathways.

The selective oxidation of methane to methanol, using in situ generated H2O2 has been investigated using a series of TS-1 supported palladium-based catalysts, with the introduction of Au or Ni considerably improving catalytic performance.