Methane Oxidation to Methanol.

The direct transformation of methane to methanol remains a significant challenge for operation at a larger scale. Central to this challenge is the low reactivity of methane at conditions that can facilitate product recovery. This review discusses the issue through examination of several promising routes to methanol and an evaluation of performance targets that are required to develop the process at scale. We explore the methods currently used, the emergence of active heterogeneous catalysts and their design and reaction mechanisms and provide a critical perspective on future operation. Initial experiments are discussed where identification of gas phase radical chemistry limited further development by this approach. Subsequently, a new class of catalytic materials based on natural systems such as iron or copper containing zeolites were explored at milder conditions. The key issues of these technologies are low methane conversion and often significant overoxidation of products. Despite this, interest remains high in this reaction and the wider appeal of an effective route to key products from C-H activation, particularly with the need to transition to net carbon zero with new routes from renewable methane sources is exciting.

Gas Phase Glycerol Valorization over Ceria Nanostructures with Well-Defined Morphologies.

Glycerol solutions were vaporized and reacted over ceria catalysts with different morphologies to investigate the relationship of product distribution to the surface facets exposed, particularly, the yield of bio-renewable methanol. Ceria was prepared with cubic, rodlike, and polyhedral morphologies via hydrothermal synthesis by altering the concentration of the precipitating agent or synthesis temperature. Glycerol conversion was found to be low over the ceria with a cubic morphology, and this was ascribed to both a low surface area and relatively high acidity. Density functional theory calculations also showed that the (100) surface is likely to be hydroxylated under reaction conditions which could limit the availability of basic sites. Methanol space-time-yields over the polyhedral ceria samples were more than four times that for the cubic material at 400 °C, where 201 g of methanol was produced per hour per kilogram of the catalyst. Under comparable glycerol conversions, we show that the rodlike and polyhedral catalysts produce a major intermediate to methanol, hydroxyacetone (HA), with a selectivity of ca. 45%, but that over the cubic sample, this was found to be 15%. This equates to a 13-fold increase in the space-time-yield of HA over the polyhedral samples compared to the cubes at 320 °C. The implications of this difference are discussed with respect to the reaction mechanism, suggesting that a different mechanism dominates over the cubic catalysts to that for rodlike and polyhedral catalysts. The strong association between exposed surface facets of ceria to high methanol yields is an important consideration for future catalyst design in this area.

Methane Oxidation to Methanol in Water.

Methane represents one of the most abundant carbon sources for fuel or chemical production. However, remote geographical locations and high transportation costs result in a substantial proportion being flared at the source. The selective oxidation of methane to methanol remains a grand challenge for catalytic chemistry due to the large energy barrier for the initial C-H activation and prevention of overoxidation to CO2. Indirect methods such as steam reforming produce CO and H2 chemical building blocks, but they consume large amounts of energy over multistage processes. This makes the development of the low-temperature selective oxidation of methane to methanol highly desirable and explains why it has remained an active area of research over the last 50 years.The thermodynamically favorable oxidation of methane to methanol would ideally use only molecular oxygen. Nature effects this transformation with the enzyme methane monooxygenase (MMO) in aqueous solution at ambient temperature with the addition of 2 equiv of a reducing cofactor. MMO active sites are Fe and Cu oxoclusters, and the incorporation of these metals into zeolitic frameworks can result in biomimetic activity. Most approaches to methane oxidation using metal-doped zeolites use high temperature with oxygen or N2O; however, demonstrations of catalytic cycles without catalyst regeneration cycles are limited. Over the last 10 years, we have developed Fe-Cu-ZSM-5 materials for the selective oxidation of methane to methanol under aqueous conditions at 50 °C using H2O2 as an oxidant (effectively O2 + 2 reducing equiv), which compete with MMO in terms of activity. To date, these materials are among the most active and selective catalysts for methane oxidation under this mild condition, but industrially, H2O2 is an expensive oxidant to use in the production of methanol.This observation of activity under mild conditions led to new approaches to utilize O2 as the oxidant. Supported precious metal nanoparticles have been shown to be active for a range of C-H activation reactions using O2 and H2O2, but the rapid decomposition of H2O2 over metal surfaces limits efficiency. We identified that this decomposition could be minimized by removing the support material and carrying out the reaction with colloidal AuPd nanoparticles. The efficiency of methanol production with H2O2 consumption was increased by 4 orders of magnitude, and crucially it was demonstrated for the first time that molecular O2 could be incorporated into the methanol produced with 91% selectivity. The understanding gained from these two approaches provides valuable insight into possible new routes to selective methane oxidation which will be presented here in the context of our own research in this area.

The interaction of CO with a copper(ii) chloride oxy-chlorination catalyst.

The interaction of CO with an attapulgite-supported, KCl modified CuCl2 catalyst has previously been examined using a combination of XANES, EXAFS and DFT calculations. Exposing the catalyst to CO at elevated temperatures leads to the formation of CO2 as the only identifiable product. However, phosgene production can be induced by a catalyst pre-treatment stage, where the supported CuCl2 sample is exposed to a diluted stream of dichlorine; subsequent CO exposure at ∼643 K then leads to phosgene production. This communication describes a series of FTIR based micro-reactor measurements, coupled with characterisation measurements utilising TEM, XRD and XPS to define the nature of the catalyst at different stages of the reaction coordinate. The CuCl2 catalyst is able to support Deacon activity , establishing this work with the possibility of utilising the oxy-chlorination of CO to produce phosgene. Continuous dosing of CO at elevated temperatures over the chlorine pre-dosed CuCl2 catalyst shows diminishing phosgene production as a function of time-on-stream, indicating surface chlorine supply to be rate-limiting under the reaction conditions studied. A pictorial reaction scheme is proposed to account for the surface chemistry observed.

A combined periodic DFT and QM/MM approach to understand the radical mechanism of the catalytic production of methanol from glycerol.

The production of methanol from glycerol over a basic oxide, such as MgO, using high reaction temperatures (320 °C) is a promising new approach to improving atom efficiency in the production of biofuels. The mechanism of this reaction involves the homolytic cleavage of the C3 feedstock, or its dehydration product hydroxyacetone, to produce a hydroxymethyl radical species which can then abstract an H atom from other species. Obtaining a detailed reaction mechanism for this type of chemistry is difficult due to the large number of products present when the system is operated at high conversions. In this contribution we show how DFT based modelling studies can provide new insights into likely reaction pathways, in particular the source of H atoms for the final step of converting hydroxymethyl radicals to methanol. We show that water is unlikely to be important in this stage of the process, C-H bonds of C2 and C3 species can give an energetically favourable pathway and that the disproportionation of hydroxymethyl radicals to methanol and formaldehyde produces a very favourable route. Experimental analysis of reaction products confirms the presence of formaldehyde. The calculations presented in this work also provide new insight into the role of the catalyst surface in the reaction showing that the base sites of the MgO(100) are able to deprotonate hydroxymethyl radicals but not methanol itself. In carrying out the calculations we also show how periodic DFT and QM/MM approaches can be used together to obtain a rounded picture of molecular adsorption to surfaces and homolytic bond cleavage which are both central to the reactions studied.

The formation of methanol from glycerol bio-waste over doped ceria-based catalysts.

A series of ceria-based solid solution metal oxides were prepared by co-precipitation and evaluated as catalysts for glycerol cleavage, principally to methanol. The catalyst activity and selectivity to methanol were investigated with respect to the reducibility of the catalysts. Oxides comprising Ce-Pr and Ce-Zr were prepared, calcined and compared to CeO2, Pr6O11 and ZrO2. The oxygen storage capacity of the catalysts was examined with analysis of Raman spectroscopic measurements and a temperature programmed reduction, oxidation and reduction cycle. The incorporation of Pr resulted in significant defects, as evidenced by Raman spectroscopy. The materials were evaluated as catalysts for the glycerol to methanol reaction, and it was found that an increased defect density or reducibility was beneficial. The space-time yield of methanol normalized to surface area over CeO2 was found to be 0.052 mmolMeOH m-2 h-1, and over CeZrO2 and CePrO2, this was to 0.029 and 0.076 mmolMeOH m-2 h-1, respectively. The inclusion of Pr reduced the surface area; however, the carbon mole selectivity to methanol and ethylene glycol remained relatively high, suggesting a shift in the reaction pathway compared to that over ceria. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Role of the Support in Gold-Containing Nanoparticles as Heterogeneous Catalysts.

In this review, we discuss selected examples from recent literature on the role of the support on directing the nanostructures of Au-based monometallic and bimetallic nanoparticles. The role of support is then discussed in relation to the catalytic properties of Au-based monometallic and bimetallic nanoparticles using different gas phase and liquid phase reactions. The reactions discussed include CO oxidation, aerobic oxidation of monohydric and polyhydric alcohols, selective hydrogenation of alkynes, hydrogenation of nitroaromatics, CO2 hydrogenation, C-C coupling, and methane oxidation. Only studies where the role of support has been explicitly studied in detail have been selected for discussion. However, the role of support is also examined using examples of reactions involving unsupported metal nanoparticles (i.e., colloidal nanoparticles). It is clear that the support functionality can play a crucial role in tuning the catalytic activity that is observed and that advanced theory and characterization add greatly to our understanding of these fascinating catalysts.

DFT-Assisted Spectroscopic Studies on the Coordination of Small Ligands to Palladium: From Isolated Ions to Nanoparticles.

A combination of experimental spectroscopies (UV-vis and Fourier-transform infrared) and computational modeling was used to investigate the coordination of small ligands (aminopropanol and propanediol) to Pd species during the metal nanoparticle formation process. Differences emerged between O- (propanediol) and N-containing (aminopropanol) ligands. In particular, a strong interaction between the NH amino group and Pd2+ ions could be inferred on the basis of spectroscopic evidences, which was corroborated by theoretical simulations, which confirmed the preferential coordination of aminopropanol through the NH group. This interaction seems to potentially cause the aminopropanol ligand to control the particle shape through a selective blocking of Pd(100) facets, which promote the growth on the Pd(111) facets.

The electronic properties of Au clusters on CeO2 (110) surface with and without O-defects.

We use density functional theory with Hubbard corrections (DFT+U) to understand the local electronic properties of Au adatom and Au2 dimer adsorption on the CeO2 (110) surface. We show that, based on the initial geometries, we can observe Au species in a variety of charge states including Au+, Au-, Auδ- and Auδ+-Auδ-. We present a detailed discussion using Bader charge analysis and partial density of states to support our observations. We also discuss the influence of solvent on the adsorption of Au adatoms adsorbed on top of an O-vacancy, which shows interesting geometrical and electronic properties.

Structural behaviour of copper chloride catalysts during the chlorination of CO to phosgene.

The interaction of CO with an attapulgite-supported Cu(ii)Cl2 catalyst has been examined in a micro-reactor arrangement. CO exposure to the dried, as-received catalyst at elevated temperatures leads to the formation of CO2 as the only identifiable product. However, phosgene production can be induced by using a catalyst pre-treatment where the supported Cu(ii)Cl2 sample is exposed to a diluted stream of chlorine. Subsequent CO exposure at ∼370 °C then leads to phosgene production. In order to investigate the origins of this atypical set of reaction characteristics, a series of X-ray absorption experiments were performed that were supplemented by DFT calculations. XANES measurements establish that at the elevated temperatures connected with phosgene formation, the catalyst is comprised of Cu+ and a small amount of Cu2+. Moreover, the data show that unique to the chlorine pre-treated sample, CO exposure at elevated temperature results in a short-lived oxidation of the copper. On the basis of calculated CO adsorption energies, DFT calculations indicate that a mixed Cu+/Cu2+ catalyst is required to support CO chemisorption.

Mechanistic Insights into Selective Oxidation of Polyaromatic Compounds using RICO Chemistry.

Ruthenium-ion-catalyzed oxidation (RICO) of polyaromatic hydrocarbons (PAHs) has been studied in detail using experimental and computational approaches to explore the reaction mechanism. DFT calculations show that regioselectivity in these reactions can be understood in terms of the preservation of aromaticity in the initial formation of a [3+2] metallocycle intermediate at the most-isolated double bond. We identify two competing pathways: C-C bond cleavage leading to a dialdehyde and C-H activation followed by H migration to the RuOx complex to give diketones. Experimentally, the oxidation of pyrene and phenanthrene has been carried out in monophasic and biphasic solvent systems. Our results show that diketones are the major product for both phenanthrene and pyrene substrates. These diketone products are shown to be stable under our reaction conditions so that higher oxidation products (acids and their derivatives) are assigned to the competing pathway through the dialdehyde. Experiments using 18 O-labelled water do show incorporation of oxygen from the solvents into products, but this may take place during the formation of the reactive RuO4 species rather than directly during PAH oxidation. When the oxidation of pyrene is carried out using D2 O, a kinetic isotope effect (KIE) is observed implying that water is involved in the rate-determining step leading to the diketone products.

A Kinetic Study of Methane Partial Oxidation over Fe-ZSM-5 Using N2 O as an Oxidant.

Catalytic methane oxidation using N2 O was investigated at 300 °C over Fe-ZSM-5. This reaction rapidly produces coke (retained organic species), and causes catalyst fouling. The introduction of water into the feed-stream resulted in a significant decrease in the coke selectivity and an increase in the selectivity to the desired product, methanol, from ca. 1 % up to 16 %. A detailed investigation was carried out to determine the fundamental effect of water on the reaction pathway and catalyst stability. The delplot technique was utilised to identify primary and secondary reaction products. This kinetic study suggests that observed gas phase products (CO, CO2 , CH3 OH, C2 H4 and C2 H6 ) form as primary products whilst coke is a secondary product. Dimethyl ether was not detected, however we consider that the formation of C2 products are likely to be due to an initial condensation of methanol within the pores of the zeolite and hence considered pseudo-primary products. According to a second order delplot analysis, coke is considered a secondary product and its formation correlates with CH3 OH formation. Control experiments in the absence of methane revealed that the rate of N2 O decomposition is similar to that of the full reaction mixture, indicating that the loss of active alpha-oxygen sites is the likely cause of the decrease in activity observed and water does not inhibit this process.

Oxidation of Polynuclear Aromatic Hydrocarbons using Ruthenium-Ion-Catalyzed Oxidation: The Role of Aromatic Ring Number in Reaction Kinetics and Product Distribution.

Oxidation of aromatic hydrocarbons with differing numbers of fused aromatic rings (2-5), have been studied in two solvent environments (monophasic and biphasic) using ruthenium-ion-catalyzed oxidation (RICO). RICO reduces the aromaticity of the polyaromatic core of the molecule in a controlled manner by selective oxidative ring opening. Moreover, the nature of the solvent system determines the product type and distribution, for molecules with more than two aromatic rings. Competitive oxidation between substrates with different numbers of aromatic rings has been studied in detail. It was found that the rate of polyaromatic hydrocarbon oxidation increases with the number of fused aromatic rings. A similar trend was also identified for alkylated aromatic hydrocarbons. The proof-of-concept investigation provides new insight into selective oxidation chemistry for upgrading of polyaromatic molecules.

Aqueous Au-Pd colloids catalyze selective CH4 oxidation to CH3OH with O2 under mild conditions.

The selective oxidation of methane, the primary component of natural gas, remains an important challenge in catalysis. We used colloidal gold-palladium nanoparticles, rather than the same nanoparticles supported on titanium oxide, to oxidize methane to methanol with high selectivity (92%) in aqueous solution at mild temperatures. Then, using isotopically labeled oxygen (O2) as an oxidant in the presence of hydrogen peroxide (H2O2), we demonstrated that the resulting methanol incorporated a substantial fraction (70%) of gas-phase O2 More oxygenated products were formed than the amount of H2O2 consumed, suggesting that the controlled breakdown of H2O2 activates methane, which subsequently incorporates molecular oxygen through a radical process. If a source of methyl radicals can be established, then the selective oxidation of methane to methanol using molecular oxygen is possible.

The adsorption of Cu on the CeO2(110) surface.

We report a detailed density functional theory (DFT) study in conjunction with extended X-ray absorption fine structure (EXAFS) experiments on the geometrical and local electronic properties of Cu adatoms and Cu(ii) ions in presence of water molecules and of CuO nanoclusters on the CeO2(110) surface. Our study of (CuO)n(=1,2&4) clusters on CeO2(110) shows that based on the Cu-O environment, the geometrical properties of these clusters may vary and their presence may lead to relatively high localization of charge on the exposed surfaces. We find that in the presence of an optimum concentration of water molecules, Cu has a square pyramidal geometry, which agrees well with our experimental findings; we also find that Cu(ii) facilitates water adsorption on the CeO2(110) surface. We further show that a critical concentration of water molecules is required for the hydrolysis of water on Cu(OH)2/CeO2(110) and on pristine CeO2(110) surfaces.

Gas phase stabiliser-free production of hydrogen peroxide using supported gold-palladium catalysts.

Hydrogen peroxide synthesis from hydrogen and oxygen in the gas phase is postulated to be a key reaction step in the gas phase epoxidation of propene using gold-titanium silicate catalysts. During this process H2O2 is consumed in a secondary step to oxidise an organic molecule so is typically not observed as a reaction product. We demonstrate that using AuPd nanoparticles, which are known to have high H2O2 synthesis rates in the liquid phase, it is possible to not only oxidise organic molecules in the gas phase but to detect H2O2 for the first time as a reaction product in both a fixed bed reactor and a pulsed Temporal Analysis of Products (TAP) reactor without stabilisers present in the gas feed. This observation opens up possibility of synthesising H2O2 directly using a gas phase reaction.

Selective oxidation of alkyl-substituted polyaromatics using ruthenium-ion-catalyzed oxidation.

Ruthenium-ion-catalyzed oxidation of a range of alkylated polyaromatics has been studied. 2-Ethylnaphthalene was used as a model substrate, and oxidation can be performed in either a conventional biphasic or in a monophasic solvent system. In either case the reaction rates and product selectivity are identical. The reaction products indicate that the aromatic ring system is oxidized in preference to the alkyl chain. This analysis is possible due to the development of a quantitative NMR protocol to determine the relative amounts of aliphatic and aromatic protons. From a systematic set of substrates we show that as the length of the alkyl chain substituent on a polyaromatic increases, the proportion of products in which the chain remains attached to the aromatic system increases. Larger polyaromatic systems, based on pyrene and phenanthrene, show greater reactivity than those with fewer aromatic rings, and the alkyl chains are more stable to oxidation.

Modelling analysis of the structure and porosity of covalent triazine-based frameworks.

Varying degrees of order have been found experimentally for a series of covalent triazine-based frameworks (CTFs) when synthesised under different reaction conditions. Here, we use molecular modelling to discuss the potential origins of this structural order by analysis of the node and strut building blocks. We use a combination of small model structures based on DFT optimised monomer units and more extended simulations using automated structure growth and molecular dynamics to discuss the influence of the strut structure on the local crystallinity of these materials.

Spectroscopic and atomic force studies of the functionalisation of carbon surfaces: new insights into the role of the surface topography and specific chemical states.

The utility of carbon materials in applications as diverse as drug delivery and photocatalysis is often undermined by the complexity of their surface chemistry; different sources of carbon give rise to a varied mixture of functional groups and hence different properties. Considerable efforts have been made to identify specific groups at these surfaces and elucidate the complex interactions that take place but even on materials such as graphene and carbon nanotubes there remains uncertainty about the nature of the components present and their role in the nucleation of other functional materials at the surface. The present study uses highly ordered pyrolytic graphite (HOPG) as a model on which the fundamental properties of specific functional groups and their interactions with deposited nanoparticles can be characterised. We have shown that treatment of HOPG surfaces with low concentrations of hydrochloric acid results in significant topographic changes to the surface and a low concentration of oxygen containing species. From selective derivatization and a comparison of their XP spectra, the latter can be unambiguously identified as surface hydroxyls. DFT calculations have shown that these groups are stable in close proximity to each other. Heating to 573 K leads to conversion of the hydroxyls to mixture of two states, one of which is identified as a ketone whilst the other is proposed to be an ether. Gold deposition on the surface from aqueous solutions of chloroauric acid is shown to be strongly influenced by the nature of the oxygen species present.

The effect of grafting zirconia and ceria onto alumina as a support for silicotungstic acid for the catalytic dehydration of glycerol to acrolein.

The effect of ceria and zirconia grafting onto alumina (α and θ-δ phases) as supports for silicotungstic acid for the dehydration of glycerol to acrolein was studied. 30 % Silicotungstic acid (STA) supported on 5 % zirconia/δ,θ-alumina was the best catalyst, producing 85 % selectivity to acrolein at 100 % glycerol conversion, and it showed stable activity without using oxygen as a co-feed. The catalyst produced a STA of 90 g(acrolein) kg(cat)(-1) h(-1), which was greater than the STA simply supported on δ,θ-alumina, which only demonstrated 75 % selectivity towards acrolein. The effect of grafting on the support material was investigated by means of nitrogen adsorption, ammonia temperature-programmed desorption, thermogravimetric analysis, Raman spectroscopy, and powder X-ray diffraction. A pulsed-field gradient (PFG) NMR technique was also used to study diffusion processes associated with the catalysts. Diffusion studies of the grafted catalysts showed that zirconia contributes to the formation of more tortuous pathways within the pore structure, leading to the diminution of acid strength and making the catalyst less susceptible to coke formation.