Direct oxidation of methane to oxygenates remains challenging in heterogeneous catalysis. In this work, direct oxidative carbonylation of low-concentration methane to acetic acid has been investigated over Rh/zeolite catalysts. Formation of acetic acid over Rh/ZSM-5 is shown to be far superior compared to a range of other Rh/ zeolite catalysts, including Rh/Z-beta, Rh/Z-Y, Rh/Mor, Rh/Fer, and Rh/SSZ-13. The importance of the 3D channel structure of ZSM-5 is emphasised by a comparison with unidirectional Rh/ZSM-23 which showed much lower production of acetic acid. Acetic acid production is found to be maximum at a SiO2/Al2O3 ratio corresponding to ZSM5-50. Isolated Rh is identified as the active site for liquid oxygenate production with 0.09 wt% Rh/ZSM5 giving the highest acetic acid production. Higher Rh loading leads to a drop in production due to the gradual formation of Rh nanoparticles. Acetic acid production is shown to be strongly pressure dependent consistent with 2nd or higher order apparent kinetics compared to apparent 1st order for C1 products. Competing direct CO oxidation to CO2 was responsible for above 85 % of the total CO2 production. These findings highlight the critical role of zeolite topology in enhancing selective methane valorisation over Rh catalysts, and provide insights into developing practical catalytic processes forlow-carbon chemical manufacturing.

Zeolite-supported Rh catalysts have been shown to be highly effective in catalyzing the oxidative carbonylation of methane to produce acetic acid. However, most of the research reported so far employed high-pressure batch reactors with long residence times. Continuous flow reactors have been seldomly used, with the majority of earlier works focused on ambient pressure studies. In this contribution, we constructed a high-pressure continuous flow reactor with steam cofeeding to bridge the knowledge gap that exists between these two distinct approaches. The production of CH3OH, CH3COOH, and other oxygenates, using a reactant mixture of CH4, CO, O2, and steam, was observed over Rh/ZSM-5 catalysts. The primary oxygenates, CH3OH and CH3COOH, reached productivities of 45 and 102 mu mol/(gcat.center dot h), respectively. The influence of space velocity, Rh loading, Si/ Al ratio of the zeolite support, and CO partial pressure were investigated.

Nanostructured MnWO4 catalysts prepared with different initial pH exhibited systematic variation in the activity in the selective oxidation of cyclohexane. 2-propanol TPD showed that the ratio of Mn to W related sites increased with the initial synthesis pH. W sites were found to be mainly active in the formation of CHHP and dehydrogenation of cyclohexanol, while Mn sites appeared to be more active in decomposition of CHHP and deep oxidation, giving a significant selectivity to adipic acid. The high yield of KA oil at intermediate synthesis pH suggests an optimum balance of Mn and W surface sites.

The catalytic conversion of captured CO2 and H2 into fuels is recognised as an interesting option to decarbonise the transport sector in the short-midterm future. DME has been identified as an ideal diesel-substitute for heavy-duty vehicles due to its high cetane number and excellent combustion properties, but to be competitive with diesel a low-cost and low-carbon H2 production route is a key enabler. Recent developments indicate that methane pyrolysis has the potential to produce H2 at a similar cost compared to steam methane reforming, the main H2 production route nowadays, yet with no direct CO2 emissions. This paper presents an enviro-economic assessment of 12 life-cycle pathways for DME production. Our results show that DME produced using H2 from methane pyrolysis could be competitive with diesel, both economically and environmentally, but is highly dependent upon the utilisation of the carbon by-product.

We report a detailed density functional theory (DFT) study of the geometrical and electronic properties, and the growth mechanism of a Cun (n = 1-4) cluster on a stoichiometric, and especially on a defective CeO2(110) surface with one surface oxygen vacancy, without using pre-assumed gas-phase Cun cluster shapes. This gives new and valuable theoretical insight into experimental work regarding debatable active sites of promising CuOx/CeO2-nanorod catalysts in many reactions. We demonstrate that CeO2(110) is highly reducible upon Cun adsorption, with electron transfer from Cun clusters, and that a Cun cluster grows along the long bridge sites until Cu3, so that each Cu atom can interact strongly with surface oxygen ions at these sites, forming stable structures on both stoichiometric and defective CeO2(110) surface. Cu-Cu interactions are, however, limited, since Cu atoms are distant from each other, inhibiting the formation of Cu-Cu bonds. This monolayer then begins to grow into a bilayer as seen in the Cu3 to Cu4 transition, with long-bridge site Cu as anchoring sites. Our calculations on Cu4 adsorption reveal a Cu bilayer rich in Cu+ species at the Cu-O interface.

The catalytic performance of AuxPdy nanoparticles prepared by colloidal synthesis and immobilised on ceria nanorods (Ce-NR) in the selective oxidation of glucose has been studied under initially basic and relatively mild conditions. Activity was found to be strongly dependent on the bimetallic composition with Au-rich catalysts being more active in glucose oxidation. Catalyst recycling revealed negligible deactivation or metal loss from leaching, and continuing high selectivity to gluconic acid (?97.7%). Defects on the Ce-NR surface appear to serve as anchoring sites for Au-Pd NPs giving rise to small and very stable NPs.

Glucaric acid is being used increasingly as a food additive, corrosion inhibitor, in deicing, and in detergents, and is also a potential starting material for the production of adipic acid, the key monomer for nylon-66. This work describes a techno-economic analysis of a potential bio-based process for the production of pure glucaric acid from corn stover (biomass). Two alternative routes for oxidation of glucose to glucaric acid are considered: via heterogeneous catalytic oxidation with air, and by homogeneous glucose oxidation using nitric acid. Techno-economic and lifecycle assessments (TEA, LCA) are made for both oxidation routes and cover the entire process from biomass to pure crystalline glucaric acid that can be used as a starting material for the production of valuable chemicals. This is the first TEA of pure glucaric acid production incorporating ion exchange and azeotropic evaporation below 50°C to avoid lactone formation. The developed process models were simulated in Aspen Plus V9. The techno-economic assessment shows that both production routes are economically viable leading to minimum selling prices of glucaric acid of ∼$2.53/kg and ∼$2.91/kg for the heterogeneous catalytic route and the homogeneous glucose oxidation route respectively. It is shown that the heterogeneous catalytic oxidation route is capable of achieving a 22% lower environmental impact than the homogeneous glucose oxidation route. Opportunities for further improvement in sustainable glucaric acid production at industrial scale are identified and discussed.

The direct selective oxidation of methane by aqueous hydrogen peroxide over ZSM-5(50) has been investigated. Methyl hydroperoxide was confirmed as the initial product of methane oxidation. The decomposition of methyl hydroperoxide to formaldehyde is established as a key intermediate in an oxidation pathway to formic acid and ultimately CO2. Hydrogen was detected during the oxidation of methane over ZSM-5 zeolites. The hydrogen was evaluated in terms of its origins and possible role in the production of MeOH. The addition of a copper salt to the ZSM-5(50) zeolite was found to decrease the overall productivity of all methane oxygenates. The use of copper also promoted the selectivity for methyl hydroperoxide whilst removing formic acid. The role of hydroxymethyl-methyl hydroperoxide is considered as an intermediate in the decomposition of methyl hydroperoxide to methanol. A tentative proposal into the nature of the iron species responsible for the catalytic species in ZSM-5(50) is made.

Dymethyl ether (DME) is of industrial interest since it is used as a precursor in many other chemical processes and it can be used as fuel in diesel engines. Nowadays, the main route to produce DME is a two-step process in which a methanol dehydration unit is connected to a methanol synthesis plant (indirect synthesis). Combining methanol synthesis and dehydration in a single reactor (direct synthesis) has attracted significant attention in recent years as it offers a theoretically higher syngas conversion per pass but leads to a more challenging downstream separation. The main contribution of this paper is a model-based comparison between an indirect DME process and two direct DME processes: a standard reactor/separation/recycle process and a once-through configuration where the unreacted syngas is used to co-produce electricity. The key-performance indicators in our analysis are the break-even price of DME, the carbon efficiency, and the energy return on energy invested. The results suggest that indirect and direct DME synthesis have similar performances both in economic terms, and in carbon and energy efficiencies terms.

Polygeneration systems have been shown to be a flexible arrangement which can meet seasonal product demands for electric power, fuels and chemicals. This is especially relevant for small markets such as isolated small-medium size communities demanding electricity, and fuels. This work is focused on the feasibility of a polygeneration system based on a single-step DME plant to produce 200,000 tonnes/year of DME and 200 MW of electricity. An economic analysis is carried out to assess different configurations and feedstocks for syngas production. The DME-production was simulated at different possible recycle ratios and under the condition of potentially reduced catalyst performance. In all cases, the once-through polygeneration system showed significant improvement over all other configurations - up to 11.6% reduction in the amount of synthesis gas required to produce DME and power with a corresponding reduction in "wasted" feed leaving as effluent CO2. The flexibility of an integrated system meant that, in cases of decreased catalyst activity or selectivity, the advantages of the integrated system over standalone configurations are even greater - up to 18.6%. Moreover, polygeneration systems show further economic advantages depending upon the selling price of electricity for both fossil fuel and biomass sources of the syngas feedstock. Crown Copyright (C) 2019 Published by Elsevier Ltd. All rights reserved.

The dependence of the selective oxidation catalytic activity of Au-Pd supported on titanate nanotubes on the catalyst preparation method has been investigated. The most active Au-Pd/Ti-NT catalyst for the selective oxidation of benzyl alcohol is shown to be that prepared using colloidal synthesis and immobilization with PVA as a stabilizer, which has markedly superior catalytic activity compared to catalysts prepared by deposition-precipitation, adsorption, and dry impregnation methods. Au-Pd NPs stabilized by graphene oxide sheets and immobilized on Ti-NT has also been studied and while not optimum shows promising catalytic activity. The superior catalytic activity of the catalysts prepared by colloidal synthesis is attributed to the high metal dispersion on the external surfaces of Ti-NT, the narrow particle size distribution, and the high degree of Au-Pd alloying. This work also demonstrates that in the adsorption method of preparation using HAuCl4.3H(2)O and PdCl2 precursors, the uptake of Pd ions in solution by Ti-NT is proportional to the sodium content in Ti-NT, which implies that Na is involved in an ion-exchange reaction with Pd ions.

Three parameter estimation methods are compared for the discrimination between two kinetic models of methanol and DME synthesis over Cu/ZnO/Al2O3 catalysts. Two methods apply Bayes' rule and Monte Carlo sampling to approximate the posterior distribution, while the third one is the popular frequentist method of finding a maximum likelihood estimate and constructing ellipsoidal confidence regions. The credible regions obtained with either Bayesian methods are similar, and they are consistently smaller and more informative than the frequentist confidence regions. Both kinetic models suffer from some practical identifiability issues for the experimental data at hand, but the evidence derived from the Bayesian estimation strongly favors the kinetic model that accounts for direct CO hydrogenation.

The catalytic performance of Au-Pd nanoparticles (NP) prepared by colloidal synthesis and immobilised on titanate nanotubes (Ti-NT) in the selective oxidation of glucose to gluconic and glucaric acids has been studied under initially basic and relatively mild conditions. Catalysts with varying compositions displayed quite different product selectivity especially in relation to deeper oxidation. The yield of glucaric acid was found to be proportional to the atomic content of Au in the Au-Pd NPs: Au/Ti-NT exhibited the highest selectivity to glucaric acid, while Au15Pd85/Ti-NT displayed the highest selectivity to gluconic acid, S-GLO > 98%. Catalyst recycling revealed deactivation, which appears to be due to a combination of metal leaching, particle size changes and product adsorption, and is associated with the gradual fall in pH of the reaction mixture. Results suggest that the leached Au species play a significant role in the oxidation of gluconic to glucaric acid.

Activated carbons (ACs) show great potential for selective adsorption removal of sulfur (SARS) from hydrocarbon fuels but require improvements in uptake and selectivity. Moreover, systematic equilibria and kinetic analyses of ACs for desulfurization are still lacking. This work examines the influence of modifying a commercial-grade activated carbon (AC) by CO2 and steam treatment for the selective adsorption removal of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) at 323 K. An untreated AC and a charcoal Norit carbon (CN) were used for comparative purposes. Physicochemical characterization of the samples was carried out by combining N-2 -physisorption, X-ray diffractometry, microscopy, thermogravimetric and infrared analyses. The steam and CO2 treated ACs exhibited higher sulfur uptakes than the untreated AC and CN samples. The steam treated AC appears to be especially effective to remove sulfur, showing a remarkable sulfur uptake (similar to 24 mgS.gads(-1 )from a mixture of 1500 ppmw of DBT and 1500 ppm 4,6-DMDBT) due to an increased surface area and microporosity. The modified ACs showed similar capacities for both DBT and the sterically hindered 4,6-DMDBT molecules. In addition, they were found to be selective in the presence of sulfurfree aromatics and showed good multicycle stability. Compared to other adsorbents, the modified ACs exhibited relatively high adsorption capacities. The combination of batch and fixed bed measurements revealed that the adsorption sites of the samples are characterized as heterogeneous due to the better fit to the Freundlich isotherm. The kinetic breakthrough profiles were described by the linear driving force (LDF) model.

Bimetallic Au-Pd NPs with a mean particle size of ca. 1.7 nm were prepared by colloidal synthesis with PVA as a stabilizer and immobilized on various ceria and titania nanostructures. The catalysts were characterised by TEM, XRD, XPS, ICP-AES, and their catalytic performance assessed in the solvent-less selective oxidation of benzyl alcohol. It is shown that even with a technique such as sol-immobilization, the final particle size is sensitive to the physiochemical properties and morphology of the support. In particular, ceria nanostructures were found to be more effective at stabilizing colloidal Au-Pd NPs than titania. Among the ceria nanostructured supports investigated, Au-Pd/ceria nanorods exhibited the highest catalytic activity (TOF > 34,700 h(-1)) and highest benzaldehyde yield. The particle size of the supported Au-Pd was found to be correlated with the surface area and concentration of Ce3+ and oxygen vacancies in the ceria nanostructures. Overall, the catalytic activity of supported bimetallic Au-Pd catalysts is likely to be governed by a complex interplay of contributions from the particle size, and support morphology, structure and properties.

NH4-, H-, Na-, Cs-exchanged ZSM-5 zeolites have been investigated as catalysts for the selective oxidation of cyclohexane under mild conditions using molecular oxygen as oxidant. For comparison, Mn- and Fe-exchanged ZSM-5 zeolites were also studied, as Mn and Fe are well-known oxidation metals. It has been shown that the type of compensating cation in the zeolite framework is of extreme importance for the activity of these catalysts in this reaction. Surprisingly, superior selective oxidation performance was achieved with a commercial NH4-ZSM-5 zeolite. The ion-exchanged transition metals (Mn and Fe) were shown to have higher selective oxidation ability compared to the alkali metals, as expected owing to their better redox properties. The rate of cyclohexyl-hydroperoxides transformation into cyclohexanol and cyclohexanone also appears to depend on the ion-exchanged cation, being also much faster over the ammonium and transition metal-exchanged zeolites. Overall, this work has shown for the first time the potential of zeolites ion-exchanged with ammonium to catalyse the selective oxidation of cyclohexane, which in principle offers the possibility of avoiding or reducing the need for more expensive and less environmentally friendly transition metals.

Au–Pd colloidal NPs immobilised on ceria nanorods are highly active catalysts for selective oxidation.

Hydrotalcite-like compounds (HT) show potential as CO2 adsorbent materials for pre-combustion CO2 capture applications, but require improvements in stability, adsorption capacity and kinetics. In this study, HT/SBA15 hybrids (with different Mg/Al ratios varying from 0.3 to 3) have been synthesised using a two-stage grafting method to coat a mesoporous SBA15 with hydrotalcite layers. The HT/SBA15 hybrids showed significant improvement in intrinsic CO2 uptake (per mass of HT), initial uptake rate, and multicycle stability compared to unsupported HT. Compared to previously reported nanostructured carbon supports (e.g. CNF, MWCNTs), the HT/SBA15 hybrids were found to be more thermally stable and exhibit comparable adsorption uptake and rates. In particular, the use of SBA15 as a support is shown to prevent the gradual loss in weight from thermal decomposition observed for HT/MWCNT or HT/GO composites over extended cycling.