Enhancing Hydrogen Production from the Photoreforming of Lignin.

Photoreforming of lignocellulose biomass is widely recognised as a challenging but key technology for producing value-added chemicals and renewable hydrogen (H2 ). In this study, H2 production from photoreforming of organosolv lignin in a neutral aqueous solution was studied over a 0.1 wt % Pt/TiO2 (P25) catalyst with ultraviolet A (UVA) light. The H2 production from the system employing the lignin (~4.8 µmol gcat -1 h-1 ) was comparable to that using hydroxylated/methoxylated aromatic model compounds (i. e., guaiacol and phenol, 4.8-6.6 µmol gcat -1 h-1 ), being significantly lower than that from photoreforming of cellulose (~62.8 µmol gcat -1 h-1 ). Photoreforming of phenol and reaction intermediates catechol, hydroquinone and benzoquinone were studied to probe the mechanism of phenol oxidation under anaerobic photoreforming conditions with strong adsorption and electron transfer reactions lowering H2 production from the intermediates relative to that from phenol. The issues associated with catalyst poisoning and low photoreforming activity of lignins demonstrated in this paper have been mitigated by implementing a process by which the catalyst was cycled through anaerobic and aerobic conditions. This strategy enabled the periodic regeneration of the photocatalyst resulting in a threefold enhancement in H2 production from the photoreforming of lignin.

Direct Conversion of Methane to Ethylene and Acetylene over an Iron-Based Metal-Organic Framework.

Conversion of methane (CH4) to ethylene (C2H4) and/or acetylene (C2H2) enables routes to a wide range of products directly from natural gas. However, high reaction temperatures and pressures are often required to activate and convert CH4 controllably, and separating C2+ products from unreacted CH4 can be challenging. Here, we report the direct conversion of CH4 to C2H4 and C2H2 driven by non-thermal plasma under ambient (25 °C and 1 atm) and flow conditions over a metal-organic framework material, MFM-300(Fe). The selectivity for the formation of C2H4 and C2H2 reaches 96% with a high time yield of 334 µmol gcat-1 h-1. At a conversion of 10%, the selectivity to C2+ hydrocarbons and time yield exceed 98% and 2056 µmol gcat-1 h-1, respectively, representing a new benchmark for conversion of CH4. In situ neutron powder diffraction, inelastic neutron scattering and solid-state nuclear magnetic resonance, electron paramagnetic resonance (EPR), and diffuse reflectance infrared Fourier transform spectroscopies, coupled with modeling studies, reveal the crucial role of Fe-O(H)-Fe sites in activating CH4 and stabilizing reaction intermediates via the formation of an Fe-O(CH3)-Fe adduct. In addition, a cascade fixed-bed system has been developed to achieve online separation of C2H4 and C2H2 from unreacted CH4 for direct use. Integrating the processes of CH4 activation, conversion, and product separation within one system opens a new avenue for natural gas utility, bridging the gap between fundamental studies and practical applications in this area.

Microstructural and thermodynamic characterization of wormlike micelles formed by polydisperse ionic surfactant solutions.

For industrial applications of self-assembled wormlike micelles, measurement and characterization of a micellar material's microstructure and rheology are paramount for the development and deployment of new high-performing and cost-effective formulations. Within this workflow, there are significant bottlenecks associated with experimental delays and a lack of transferability of results from one chemistry to another. In this work, we outline a process to predict microscopic and thermodynamic characteristics of wormlike micelles directly from rheological data by combining a more robust and efficient fitting algorithm with a recently published constitutive model called the Toy Shuffling model [J. D. Peterson and M. E. Cates, J. Rheol. 64, 1465-1496 (2020) and J. D. Peterson and M. E. Cates, J. Rheol. 65, 633-662 (2021)]. To support this work, linear rheology measurements were taken for 143 samples comprising a common base formulation of commercial sodium lauryl ether sulfate, cocamidopropyl betaine, and salt (NaCl). The steady state zero shear viscosity evident in linear rheology was measured in duplicate via direct steady and oscillatory shear experiments. Fitting the collected data to the model, we found trends in the microstructural and thermodynamic characteristics that agree with molecular dynamics simulations. These trends validate our new perspective on the parameters that inform the study of the relationship between chemical formulation and rheology. This work, when implemented at scale, can potentially be used to inform and test strategies for predicting self-assembled micellar structures based on chemical formulation.

Heterogenised catalysts for the H-transfer reduction reaction of aldehydes: influence of solvent and solvation effects on reaction performances.

Heterogenisation of homogeneous catalysts onto solid supports represents a potential strategy to make the homogeneous catalytic function recyclable and reuseable. Yet, it is usually the case that immobilised catalysts have much lower catalytic activity than their homogeneous counterpart. In addition, the presence of a solid interface introduces a higher degree of complexity by modulating solid/fluid interactions, which can often influence adsorption properties of solvents and reactive species and, ultimately, catalytic activity. In this work, the influence of support and solvent in the H-transfer reduction of propionaldehyde over Al(OiPr)3-SiO2, Al(OiPr)3-TiO2 and Al(OiPr)3-Al2O3 heterogenised catalysts has been studied. Reaction studies are coupled with both NMR relaxation measurements as well as molecular dynamics (MD) simulations in order to unravel surface and solvation effects during the reaction. The results show that, whilst the choice of the support does not influence significantly catalytic activity, reactions carried out in solvents with high affinity for the catalyst surface, or able to hinder access to active sites due to solvation effects, have a lower activity. MD calculations provide key insights into bulk solvation effects involved in such reactions, which are thought to play an important role in determining the catalytic behaviour. The activity of the heterogenised catalysts was found to be comparable with that of the homogeneous Al(OiPr)3 catalysts for all supports used, showing that for the type of reaction studied immobilisation of the homogeneous catalyst onto solid supports is a viable, robust and effective strategy.

Mechanistic Study of Glucose Photoreforming over TiO2-Based Catalysts for H2 Production.

Glucose is a key intermediate in cellulose photoreforming for H2 production. This work presents a mechanistic investigation of glucose photoreforming over TiO2 and Pt/m-TiO2 catalysts. Analysis of the intermediates formed in the process confirmed the α-scission mechanism of glucose oxidation forming arabinose (Cn-1 sugar) and formic acid in the initial oxidation step. The selectivity to sugar products and formic acid differed over Pt/TiO2 and TiO2, with Pt/TiO2 showing the lower selectivity to formic acid due to enhanced adsorption/conversion of formic acid over Pt/TiO2. In situ ATR-IR spectroscopy of glucose photoreforming showed the presence of molecular formic acid and formate on the surface of both catalysts at low glucose conversions, suggesting that formic acid oxidation could dominate surface reactions in glucose photoreforming. Further in situ ATR-IR of formic acid photoreforming showed Pt-TiO2 interfacial sites to be key for formic acid oxidation as TiO2 was unable to convert adsorbed formic acid/formate. Isotopic studies of the photoreforming of formic acid in D2O (with different concentrations) showed that the source of the protons (to form H2 at Pt sites) was determined by the relative surface coverage of adsorbed water and formic acid.

Environmental assessment of a novel ionic-liquid based method for recycling of PVC in composite materials.

Waste PVC is scarcely recycled due to its high chlorine content and its use in composite materials, which reduces the applicability of conventional waste treatment methods, including thermal, mechanical and chemical recycling. For this reason, alternative treatment options are being developed to increase the recyclability of waste PVC. This paper focuses on one such option which utilises ionic liquids (ILs) for material separation and dehydrochlorination of PVC contained in composite materials. Taking blisterpacks used as a packaging for medicines as an example of a composite material, the paper presents for the first time the life cycle environmental impacts of this novel PVC recycling method, in comparison with thermal treatment (low-temperature pyrolytic degradation of PVC). Three ILs were considered for the PVC recycling process: trihexyl(tetradecyl)phosphonium chloride, bromide and hexanoate. The results suggested that the impacts of the process using the first two ILs were comparable, while the system with hexanoate-based IL had 7-229 % higher impacts. Compared to the thermal treatment of waste blisterpacks, the IL assisted process had significantly higher impacts (22-819 %) in all 18 categories considered due to the greater heat requirements and the IL losses. Reducing the latter would lower most impacts by 8-41 %, while optimising the energy requirements would reduce the impacts by 10-58 %. Moreover, recovering HCl would increase significantly the environmental sustainability of the process, resulting in net-negative impacts (savings) in most categories. Overall, these improvements would lead to lower or comparable impacts to those of the thermal treatment. The findings of this study will be of interest to the polymer, recycling and related industries, as well as to process developers.

Fracking wastewater treatment: Catalytic performance and life cycle environmental impacts of cerium-based mixed oxide catalysts for catalytic wet oxidation of organic compounds.

Water scarcity and the consequent increase of freshwater prices are a cause for concern in regions where shale gas is being extracted via hydraulic fracturing. Wastewater treatment methods aimed at reuse/recycle of fracking wastewater can help reduce water stress of the fracking process. Accordingly, this study assessed the catalytic performance and life cycle environmental impacts of cerium-based mixed oxide catalysts for catalytic wet oxidation (CWO) of organic contaminants, in order to investigate their potential as catalysts for fracking wastewater treatment. For these purposes, MnCeOx and CuCeOx were tested for phenol removal in the presence of concentrated NaCl (200 g L-1), which represented a synthetic fracking wastewater. Removal of phenol in pure ("phenolic") water without NaCl was also considered for comparison. Complete (100 %) phenol and a 94 % total organic carbon (TOC) removal were achieved in both the phenolic and fracking wastewaters by utilising MnCeOx (5 g L-1) and insignificant metal leaching was observed. However, a much lower activity was observed when the same amount of CuCeOx was utilised: 23.3 % and 20.5 % for phenol and TOC removals, respectively, in the phenolic, and 69.1 % and 63 % in the fracking wastewater. Furthermore, severe copper leaching from CuCeOx was observed during stability tests conducted in the fracking wastewater. A life cycle assessment (LCA) study carried out as part of this work showed that the production of MnCeOx had 12-98 % lower impacts than CuCeOx due to the higher impacts of copper than manganese precursors. Furthermore, the environmental impacts of CWO were found to be 94-99 % lower than those of ozonation due to lower energy and material requirements. Overall, the results of this study suggest that the adoption of catalytic treatment would improve both the efficiency and the environmental sustainability of both the fracking wastewater treatment and the fracking process as a whole.

Combined Superbase Ionic Liquid Approach to Separate CO2 from Flue Gas.

Superbase ionic liquids (ILs) with a trihexyltetradecylphosphonium cation and a benzimidazolide ([P66614][Benzim]) or tetrazolide ([P66614][Tetz]) anion were investigated in a dual-IL system allowing the selective capture and separation of CO2 and SO2, respectively, under realistic gas concentrations. The results show that [P66614][Tetz] is capable of efficiently capturing SO2 in preference to CO2 and thus, in a stepwise separation process, protects [P66614][Benzim] from the negative effects of the highly acidic contaminant. This results in [P66614][Benzim] maintaining >53% of its original CO2 uptake capacity after 30 absorption/desorption cycles in comparison to the 89% decrease observed after 11 cycles when [P66614][Tetz] was not present. Characterization of the ILs post exposure revealed that small amounts of SO2 were irreversibly absorbed to the [Benzim]- anion responsible for the decrease in CO2 capacity. While optimization of this dual-IL system is required, this feasibility study demonstrates that [P66614][Tetz] is a suitable sorbent for reversibly capturing SO2 and significantly extending the lifetime of [P66614][Benzim] for CO2 uptake.

High-Ionic-Strength Wastewater Treatment via Catalytic Wet Oxidation over a MnCeO x Catalyst.

Catalytic wastewater treatment has rarely been applied to treat high-ionic-strength wastewater (HISWW) as it contains large amounts of catalyst poisons (e.g., Cl-). This work investigates the catalytic wet oxidation (CWO) of phenol over a MnCeO x catalyst in the presence of high NaCl concentrations where the combination of MnCeO x and NaCl promoted the CWO of phenol. Specifically, in the presence of NaCl at a concentration of 200 g L-1 and MnCeO x at a concentration of 1.0 g L-1, phenol (initially 1.0 g L-1) and total organic carbon (TOC) conversions were ∼98 and 85%, respectively, after a 24 h reaction. Conversely, under the same reaction conditions without NaCl, the catalytic system only achieved phenol and TOC conversions of ∼41 and 27%, respectively. In situ Attenuated Total Reflection infrared spectroscopy identified the nature of the strongly adsorbed carbon deposits with quinone/acid species found on Ce sites and phenolate species on Mn sites in the single oxides and on MnCeO x . The presence of high concentrations of NaCl reduced the carbon deposition over the catalyst, promoting surface oxidation of the hydrocarbon and reoxidation of the catalyst, resulting in enhanced mineralization. Moreover, the used MnCeO x catalyst in the salt water system was efficiently regenerated via a salt water wash under the reaction conditions, showing the great potential of MnCeO x in practical HISWW treatment.

Photocatalytic Reforming of Biomass: What Role Will the Technology Play in Future Energy Systems.

Photocatalytic reforming of biomass has emerged as an area of significant interest within the last decade. The number of papers published in the literature has been steadily increasing with keywords such as 'hydrogen' and 'visible' becoming prominent research topics. There are likely two primary drivers behind this, the first of which is that biomass represents a more sustainable photocatalytic feedstock for reforming to value-added products and energy. The second is the transition towards achieving net zero emission targets, which has increased focus on the development of technologies that could play a role in future energy systems. Therefore, this review provides a perspective on not only the current state of the research but also a future outlook on the potential roadmap for photocatalytic reforming of biomass. Producing energy via photocatalytic biomass reforming is very desirable due to the ambient operating conditions and potential to utilise renewable energy (e.g., solar) with a wide variety of biomass resources. As both interest and development within this field continues to grow, however, there are challenges being identified that are paramount to further advancement. In reviewing both the literature and trajectory of the field, research priorities can be identified and utilised to facilitate fundamental research alongside whole systems evaluation. Moreover, this would underpin the enhancement of photocatalytic technology with a view towards improving the technology readiness level and promoting engagement between academia and industry.

Shielding Protection by Mesoporous Catalysts for Improving Plasma-Catalytic Ambient Ammonia Synthesis.

Plasma catalysis is a promising technology for decentralized small-scale ammonia (NH3) synthesis under mild conditions using renewable energy, and it shows great potential as an alternative to the conventional Haber-Bosch process. To date, this emerging process still suffers from a low NH3 yield due to a lack of knowledge in the design of highly efficient catalysts and the in situ plasma-induced reverse reaction (i.e., NH3 decomposition). Here, we demonstrate that a bespoke design of supported Ni catalysts using mesoporous MCM-41 could enable efficient plasma-catalytic NH3 production at 35 °C and 1 bar with >5% NH3 yield at 60 kJ/L. Specifically, the Ni active sites were deliberately deposited on the external surface of MCM-41 to enhance plasma-catalyst interactions and thus NH3 production. The desorbed NH3 could then diffuse into the ordered mesopores of MCM-41 to be shielded from decomposition due to the absence of plasma discharge in the mesopores of MCM-41, that is, "shielding protection", thus driving the reaction forward effectively. This promising strategy sheds light on the importance of a rational design of catalysts specifically for improving plasma-catalytic processes.

Effect of Ball-Milling Pretreatment of Cellulose on Its Photoreforming for H2 Production.

Photoreforming of cellulose is a promising route for sustainable H2 production. Herein, ball-milling (BM, with varied treatment times of 0.5-24 h) was employed to pretreat microcrystalline cellulose (MCC) to improve its activity in photoreforming over a Pt/TiO2 catalyst. It was found that BM treatment reduced the particle size, crystallinity index (CrI), and degree of polymerization (DP) of MCC significantly, as well as produced amorphous celluloses (with >2 h treatment time). Amorphous cellulose water-induced recrystallization to cellulose II (as evidenced by X-ray diffraction (XRD) and solid-state NMR analysis) was observed in aqueous media. Findings of the work showed that the BM treatment was a simple and effective pretreatment strategy to improve photoreforming of MCC for H2 production, mainly due to the decreased particle size and, specifically in aqueous media, the formation of the cellulose II phase from the recrystallization of amorphous cellulose, the extent of which correlates well with the activity in photoreforming.

Surfactant-free Synthesis of Spiky Hollow Ag-Au Nanostars with Chemically Exposed Surfaces for Enhanced Catalysis and Single-Particle SERS.

Spiky/hollow metal nanoparticles have applications across a broad range of fields. However, the current bottom-up methods for producing spiky/hollow metal nanoparticles rely heavily on the use of strongly adsorbing surfactant molecules, which is undesirable because these passivate the product particles' surfaces. Here we report a high-yield surfactant-free synthesis of spiky hollow Au-Ag nanostars (SHAANs). Each SHAAN is composed of >50 spikes attached to a hollow ca. 150 nm diameter cubic core, which makes SHAANs highly plasmonically and catalytically active. Moreover, the surfaces of SHAANs are chemically exposed, which gives them significantly enhanced functionality compared with their surfactant-capped counterparts, as demonstrated in surface-enhanced Raman spectroscopy (SERS) and catalysis. The chemical accessibility of the pristine SHAANs also allows the use of hydroxyethyl cellulose as a weakly bound stabilizing agent. This produces colloidal SHAANs that remain stable for >1 month while retaining the functionalities of the pristine particles and allows even single-particle SERS to be realized.

Arc Synthesis, Crystal Structure, and Photoelectrochemistry of Copper(I) Tungstate.

A little-studied p-type ternary oxide semiconductor, copper(I) tungstate (Cu2WO4), was assessed by a combined theoretical/experimental approach. A detailed computational study was performed to solve the long-standing debate on the space group of Cu2WO4, which was determined to be triclinic P1. Cu2WO4 was synthesized by a time-efficient, arc-melting method, and the crystalline reddish particulate product showed broad-band absorption in the UV-visible spectral region, thermal stability up to ∼260 °C, and cathodic photoelectrochemical activity. Controlled thermal oxidation of copper from the Cu(I) to Cu(II) oxidation state showed that the crystal lattice could accommodate Cu2+ cations up to ∼260 °C, beyond which the compound was converted to CuO and CuWO4. This process was monitored by powder X-ray diffraction and X-ray photoelectron spectroscopy. The electronic band structure of Cu2WO4 was contrasted with that of the Cu(II) counterpart, CuWO4 using spin-polarized density functional theory (DFT). Finally, the compound Cu2WO4 was determined to have a high-lying (negative potential) conduction band edge underlining its promise for driving energetic photoredox reactions.

Combined Experimental and Theoretical Study of the Competitive Absorption of CO2 and NO2 by a Superbase Ionic Liquid.

A superbase ionic liquid (IL), trihexyltetradecylphosphonium benzimidazolide ([P66614][Benzim]), is investigated for the capture of CO2 in the presence of NO2 impurities. The effect of the waste gas stream contaminant on the ability of the IL to absorb simultaneously CO2 is demonstrated using novel measurement techniques, including a mass spectrometry breakthrough method and in situ infrared spectroscopy. The findings show that the presence of an industrially relevant concentration of NO2 in a combined feed with CO2 has the effect of reducing the capacity of the IL to absorb CO2 efficiently by ∼60% after 10 absorption-desorption cycles. This finding is supported by physical property analysis (viscosity, 1H and 13C NMR, and X-ray photoelectron spectroscopy) and spectroscopic infrared characterization, in addition to density functional theory (DFT) calculations, to determine the structure of the IL-NO2 complex. The results are presented in comparison with another flue gas component, NO, demonstrating that the absorption of NO2 is more favorable, thereby hindering the ability of the IL to absorb CO2. Significantly, this work aids understanding of the effects that individual components of flue gas have on CO2 capture sorbents, through studying a contaminant that has received limited interest previously.

Atomically Dispersed Copper Sites in a Metal-Organic Framework for Reduction of Nitrogen Dioxide.

Metal-organic framework (MOF) materials provide an excellent platform to fabricate single-atom catalysts due to their structural diversity, intrinsic porosity, and designable functionality. However, the unambiguous identification of atomically dispersed metal sites and the elucidation of their role in catalysis are challenging due to limited methods of characterization and lack of direct structural information. Here, we report a comprehensive investigation of the structure and the role of atomically dispersed copper sites in UiO-66 for the catalytic reduction of NO2 at ambient temperature. The atomic dispersion of copper sites on UiO-66 is confirmed by high-angle annular dark-field scanning transmission electron microscopy, electron paramagnetic resonance spectroscopy, and inelastic neutron scattering, and their location is identified by neutron powder diffraction and solid-state nuclear magnetic resonance spectroscopy. The Cu/UiO-66 catalyst exhibits superior catalytic performance for the reduction of NO2 at 25 °C without the use of reductants. A selectivity of 88% for the formation of N2 at a 97% conversion of NO2 with a lifetime of >50 h and an unprecedented turnover frequency of 6.1 h-1 is achieved under nonthermal plasma activation. In situ and operando infrared, solid-state NMR, and EPR spectroscopy reveal the critical role of copper sites in the adsorption and activation of NO2 molecules, with the formation of {Cu(I)···NO} and {Cu···NO2} adducts promoting the conversion of NO2 to N2. This study will inspire the further design and study of new efficient single-atom catalysts for NO2 abatement via detailed unravelling of their role in catalysis.

Contrasting the EXAFS obtained under air and H2 environments to reveal details of the surface structure of Pt-Sn nanoparticles.

Understanding the surface structure of bimetallic nanoparticles is crucial for heterogeneous catalysis. Although surface contraction has been established in monometallic systems, less is known for bimetallic systems, especially of nanoparticles. In this work, the bond length contraction on the surface of bimetallic nanoparticles is revealed by XAS in H2 at room temperature on dealloyed Pt-Sn nanoparticles, where most Sn atoms were oxidized and segregated to the surface when measured in air. The average Sn-Pt bond length is found to be ∼0.09 Šshorter than observed in the bulk. To ascertain the effect of the Sn location on the decrease of the average bond length, Pt-Sn samples with lower surface-to-bulk Sn ratios than the dealloyed Pt-Sn were studied. The structural information specifically from the surface was extracted from the averaged XAS results using an improved fitting model combining the data measured in H2 and in air. Two samples prepared so as to ensure the absence of Sn in the bulk were also studied in the same fashion. The bond length of surface Sn-Pt and the corresponding coordination number obtained in this study show a nearly linear correlation, the origin of which is discussed and attributed to the poor overlap between the Sn 5p orbitals and the available orbitals of the Pt surface atoms.

Elucidating the role of H2O in promoting the formation of methacrylic acid during the oxidation of methacrolein over heteropolyacid compounds.

The involvement of water in the selective oxidation of MAL to MAA over a pure Keggin-type H3PMoO12O40 catalyst was investigated using an in situ DRIFTS reactor coupled with a mass spectrometer for the first time to elucidate the reaction pathway associated with water. Comparing the spectra and activity data using D2O instead of H2O during transient switching experiments has allowed us to evaluate the possible active sites where D2O is activated. It has been found that, during the cycling switches of D2O in and out of the MAL + O2 gas feed at 320 °C, the formation of MAA-OD product is increased and decreased when D2O is added and removed, respectively. This suggests that the deuterium from D2O is involved in the production of gas phase MAA-OD. In addition, the in situ DRIFTS-MS results obtained from the isotopic switches between D2O and H2O reveal changes in the characteristic infrared bands of the Keggin unit between 1200 and 600 cm-1. It is found that the isotopic exchange possibly occurs on the bridging oxygen of Mo-O-Mo unit, where water is activated for the formation of MAA. Based on the in situ DRIFTS-MS analysis from the transient switching experiments, the reaction mechanism associated with the effect of water on the selective oxidation of MAL to MAA over Keggin-type H3PMoO12O40 catalyst is proposed.

Industrial Applications of Ionic Liquids.

Since their conception, ionic liquids (ILs) have been investigated for an extensive range of applications including in solvent chemistry, catalysis, and electrochemistry. This is due to their designation as designer solvents, whereby the physiochemical properties of an IL can be tuned for specific applications. This has led to significant research activity both by academia and industry from the 1990s, accelerating research in many fields and leading to the filing of numerous patents. However, while ILs have received great interest in the patent literature, only a limited number of processes are known to have been commercialised. This review aims to provide a perspective on the successful commercialisation of IL-based processes, to date, and the advantages and disadvantages associated with the use of ILs in industry.