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Environmental assessments in green chemistry often rely on simplified metrics that enable comparisons between alternative routes but fail to shed light on whether they are truly sustainable in absolute terms, viz. relative to the Earth's ecological capacity. To expand our currently limited knowledge of the extent to which chemicals are environmentally sustainable, here we analyse 492 chemical products through the lens of seven planetary boundaries representing critical biophysical limits that should never be exceeded. We found that most of them transgress these environmental guardrails, mainly the ones strongly connected to greenhouse gas emissions (i.e., climate change, ocean acidification and biosphere integrity). However, their levels of transgression fail to correlate with their carbon footprints, currently the focus of most studies, implying that chemicals entailing higher greenhouse gas emissions are not necessarily less environmentally sustainable in absolute terms. Our work points towards the need to embrace absolute sustainability criteria in current environmental assessments, which will require agreeing on how to allocate shares of the planet's ecological capacity among anthropogenic activities, including chemicals' production. This work's absolute environmental sustainability assessment (AESA) method, which could complement standard life cycle assessment approaches, might help experimental researchers working in green chemistry develop truly 'green' products. The AESA method should be taken as a starting point to devise holistic approaches for quantifying the absolute environmental impact of chemicals to guide research and policymaking more sensibly. [ABSTRACT FROM AUTHOR]

Natural gas is widely considered as the key feedstock to enable the transition from the oil to the renewables era. Despite its vast reserves, the use of this resource to produce energy and chemicals does not match its full potential. The main reason lies in the nature of its wells, which are often found in remote locations around the globe, rendering access and transportation challenging. To aid this development, several technologies for energy and energy carrier production have been developed, all of which have in common the goal of upgrading natural gas directly at the source of extraction. Following this direction, this review firstly analyses the advances in process design towards decentralised generation of electricity and of liquefied natural gas. Subsequently, recent efforts in progress made in catalysed alkane transformations using heterogeneous catalysts are reviewed for small-scale chemicals and fuels production. The presented analysis identifies that techno-economic and life-cycle assessments should be widely performed to enable proper technological benchmarking of these technologies. The integration of these multidisciplinary fields is key to foster synergies between researchers in the areas of decentralised energy and energy carrier generation in view of developing effective and efficient processes for valorising natural gas directly on-site. [ABSTRACT FROM AUTHOR]

The efficient utilization of biomass, the only globally available, renewable and abundant carbon-neutral source, is of high significance in green and sustainable chemistry. Polyoxometalates (POMs) and POM-based composites have been widely applied in green catalytic reactions, due to their tunable Brønsted/Lewis-acidity and redox properties enabling high reactivity in a wide range of chemical transformations. This review covers recent advances in the chemocatalytic conversion of biomass into chemicals and fuels over POMs and POM–metal composites. For biomass valorisation over POMs, the advances of acid catalysis including hydrolysis, dehydration, etherification, alcoholysis, transesterification and esterification are summarised. Furthermore, applications in chemical oxidation for the synthesis of organic acids and furan chemicals are discussed. For biomass valorisation over metal–POM composites, an overview of tandem reactions (e.g. hydrolysis–hydrogenation, hydrolysis–oxidation, and hydrogenolysis–hydrodeoxygenation) is highlighted. The future prospects of biomass valorisation over POM-based catalysts are finally presented. [ABSTRACT FROM AUTHOR]

Catalytic conversion of CO2 to produce fuels and chemicals is attractive in prospect because it provides an alternative to fossil feedstocks and the benefit of converting and cycling the greenhouse gas CO2 on a large scale. In today's technology, CO2 is converted into hydrocarbon fuels in Fischer–Tropsch synthesis via the water gas shift reaction, but processes for direct conversion of CO2 to fuels and chemicals such as methane, methanol, and C2+ hydrocarbons or syngas are still far from large-scale applications because of processing challenges that may be best addressed by the discovery of improved catalysts—those with enhanced activity, selectivity, and stability. Core–shell structured catalysts are a relatively new class of nanomaterials that allow a controlled integration of the functions of complementary materials with optimised compositions and morphologies. For CO2 conversion, core–shell catalysts can provide distinctive advantages by addressing challenges such as catalyst sintering and activity loss in CO2 reforming processes, insufficient product selectivity in thermocatalytic CO2 hydrogenation, and low efficiency and selectivity in photocatalytic and electrocatalytic CO2 hydrogenation. In the preceding decade, substantial progress has been made in the synthesis, characterization, and evaluation of core–shell catalysts for such potential applications. Nonetheless, challenges remain in the discovery of inexpensive, robust, regenerable catalysts in this class. This review provides an in-depth assessment of these materials for the thermocatalytic, photocatalytic, and electrocatalytic conversion of CO2 into synthesis gas and valuable hydrocarbons. [ABSTRACT FROM AUTHOR]

New and enhanced processes will not be the only drivers toward a sustainable chemical industry. Implementing climate policies will impact all components of the chemical supply chain over the following decades, making improvements in energy generation, material extraction, or transportation contribute to reducing the overall impacts of chemical technologies. Including this synergistic effect when comparing technologies offers a clearer vision of their future potential and may allow researchers to support their sustainability propositions more strongly. Ammonia and methanol production account for more than fifty percent of the CO2 emissions in this industry and are, therefore, excellent case studies. This work performs a prospective life cycle assessment until 2050 for fossil, blue, wind, and solar-based technologies under climate policies aiming to limit the global temperature rise to 1.5 °C, 2 °C, or 3.5 °C. The first finding is the inability of fossil-based routes to reduce their CO2 emissions beyond 10% by 2050 without tailored decarbonisation strategies, regardless of the chemical and climate policy considered. In contrast, green routes may produce chemicals with around 90% fewer emissions than today and even with net negative emissions (on a cradle-to-gate basis), as in the case of methanol (up to −1.4 kg CO2-eq per kg), mainly due to the contributions of technology development and increasing penetration of renewable energies. Overall, the combined production of these chemicals could be net-zero by 2050 despite their predicted two to fivefold increase in demand. Lastly, we propose a roadmap for progressive implementation by 2050 of green routes in 26 regions worldwide, applying the criterion of at least 80% reduction in climate change impacts when compared to their fossil alternatives. Furthermore, an exploratory prospective techno-economic assessment showed that by 2050, green routes could become more economically attractive. This work offers quantitative arguments to reinforce research, development, and policymaking efforts on green chemical routes reliant on renewable energies. [ABSTRACT FROM AUTHOR]

Ammonia and methanol are essential to modern societies, but their production has been heavily reliant on natural gas, which contributes to supply disruptions and significant CO2 emissions. While low-carbon or green production routes have been extensively researched, their adoption has been hindered by higher costs, making them unsustainable. However, a recent energy crisis in Europe has created a unique opportunity to shift towards greener production technologies. Here we show that, green ammonia, produced through wind-powered water electrolysis, had the potential to outperform its fossil counterpart for six months as of December 2021, while methanol produced through CO2 capture and wind-based water electrolysis became an economically appealing alternative. With a coordinated effort from academia, industry, and policymakers, Europe can lead the grand transition towards more sustainable practices in the chemical industry. [ABSTRACT FROM AUTHOR]

Alkaline-assisted stannation is applied as a facile one-step post-synthetic approach for the preparation of the Lewis-acid Sn-BEA from a high-silica commercial beta (BEA) zeolite. The concentration of base, tin and protective additive tetrapropylammonium as well as the nature of the tin precursor are identified as key parameters of the metallation process determining the acidic and structural properties of the stannated zeolite and, in turn, its performance in the isomerisation of bio-based substrates. [ABSTRACT FROM AUTHOR]

The storage of renewable energy through the electrochemical reduction of CO2 (eCO2RR) is an attractive strategy to transform the current linear utilisation of carbon fuels (extraction–combustion–CO2 release) into an increasingly cyclic one. An electrochemical alternative for energy storage is the production of H2 from water splitting, studied for decades and commercially available up to the megawatt range. By comparing the technological similarities between these two processes, it is possible to extract both a global perspective and research directions for the eCO2RR. Herein, the main limiting phenomena affecting CO2 electrolysers and their causes are outlined first. This is followed by the derivation, for several eCO2RR products, of targets in terms of durability, current efficiency, energy efficiency, faradaic efficiency, and overvoltage, which must be achieved for the eCO2RR to reach a similar energy storage capability as the electrochemical production of H2. By comparing these figures of merit with the eCO2RR literature, we conclude that the conversion of carbon dioxide to carbon monoxide and formic acid lead the race towards practical use, although both still exhibit major performance gaps. The present energy efficiencies are low, mainly due to high overvoltages, and durability is not yet a developed research area. Besides the development of more efficient electrocatalysts, research advances on the fronts of electrode and electrolyser design, coupled with the optimisation of methods for the preparation of electrodes, are expected to push forward the electrochemical reduction of CO2 on its way to viability. [ABSTRACT FROM AUTHOR]

The catalytic conversion of renewable feedstocks into chemicals is pursued as a means to sustainably fulfil future societal needs. Due to the oxygen-rich nature of bio-derived substrates, isomerisation, transfer-hydrogenation and retro-aldol reactions have emerged as relevant transformations to produce commodity chemicals and polymer building blocks. In this context, porous materials containing Lewis-acid metals (e.g., Al, Ga, Sn, Ti, Zr) play an important role. Among these, tin-containing zeolites have demonstrated superior catalytic properties which have mainly been attributed to their hydrophobicity and crystallinity. This review evaluates the versatility and the scalability of bottom-up and top-down approaches to introduce Lewis-acid functionalities in zeolitic matrices. A precise characterisation is shown to be crucial to determine the structure, acidity and environment of the sites introduced. In this regard, we highlight the limitations of conventional techniques and the advantages of analytical and modelling tools recently applied to gain an improved understanding of these solids. Thereafter, property–performance relations and important aspects for the industrial amenability of new synthetic routes are exemplified through case studies. Finally, we put forward the need for gathering deeper knowledge of the site location, surface properties and stability to aid the design of next-generation Lewis-acid catalysts. [ABSTRACT FROM AUTHOR]

Progress in catalysis has been, is, and will always be motivated by societal needs (e.g. environment, energy, chemicals, fuels), with the ultimate aim of improving process efficiency on a technical scale. Technical catalysts are often complex multicomponent millimetre-sized bodies consisting of active phases, supports, and numerous additives in shaped forms suitable for their commercial application. They can differ strongly in composition, structure, porosity, and performance from research catalysts, i.e. laboratory-developed materials constituted by a single bulk or supported active phase in powder form, which are the predominant focus of academic investigations. The industrial manufacture of heterogeneous catalysts, encompassing the upscaled preparation, formulation, and structuring, is encircled by secrecy and is decisive for the overall process viability. Yet despite the tremendous relevance, understanding the added complexity of these multicomponent systems and the consequences for the respective structure–property–function relationships has been largely neglected. Accordingly, our review examines the intricacies of the scale up of heterogeneous catalysts. While emphasising the lack of fundamental knowledge we point out the multiple functions that additives could provide by enhancing the mass and heat transfer properties, acting as co-catalysts, or imparting improved chemical, mechanical, or thermal stability. Recent exemplary studies developing rational approaches to prepare, characterise, and evaluate technical catalysts are analysed in detail and new directions for research in this field are put forward. [ABSTRACT FROM AUTHOR]

The gas-phase hydrogenation of propyne to propene was investigated over a series of heterogeneous catalysts based on Pd (in the absence and the presence of CO), Ni, Au, Cu, and Cu–Ni. Catalytic tests in a continuous flow micro-reactor at ambient pressure and density functional theory (DFT) calculations were combined to understand complex structure–selectivity–activity relationships and their dependence on key experimental variables such as H2/alkyne and CO/H2ratios. The wide scope of our work enables the identification of similarities and differences between these systems and paves the way to design more selective catalysts. [ABSTRACT FROM AUTHOR]

Heterogeneous catalysts are vital to unlock superior efficiency, atom economy, and environmental friendliness in chemical conversions, with the size and speciation of the contained metals often playing a decisive role in the activity, selectivity and stability. This tutorial review analyses the impact of these catalyst parameters on the valorisation of biomass through hydrogenation and hydrodeoxygenation, oxidation, reforming and acid-catalysed reactions, spanning a broad spectrum of substrates including sugars and platform compounds obtained from (hemi)cellulose and lignin derivatives. It outlines multiple examples of classical structure sensitivity on nanoparticle-based materials with significant implications for the product distribution. It also shows how the recently emphasised application of metals in the form of ultrasmall nanoparticles (<2 nm), clusters and single atoms, while fulfilling superior metal utilisation and robustness, opens the door to unprecedented electronic and geometric properties. The latter can lead to facilitated activation of reactants as well as boosted selectivity control and synergy between distinct active sites in multifunctional catalysts. Based on the analysis conducted, guidelines for the selection of metals for diverse applications are put forward in terms of chemical identity and structure, and aspects that should be explored in greater depth for further improving the exploitation of metals in this research field and beyond are highlighted. [ABSTRACT FROM AUTHOR]

Commercial two-step processes to convert glycerol into epichlorohydrin are more benign compared to the predominant industrial route starting from propene in terms of materials requirements and CO2 emissions. Still, the use of alkali hydroxides in stoichiometric amounts in the second reaction, i.e., the dehydrochlorination of the dichloropropanol intermediate, leads to the formation of large amounts of salt wastes, thus limiting the greenness of the technology. Here, we show for the first time that the latter transformation can be selectively conducted in the gas phase in the presence of a heterogeneous hydrotalcite-derived mixed oxide of Al and Mg. Upon reaction, the lamellar solid is rehydrated to a hydrotalcite-like compound, which can effectively activate the alcoholic group of dichloropropanol owing to its strong Brønsted basic character and moderately high surface area. In-depth characterisation of the porous, compositional, structural and acid/base properties demonstrates that the HCl formed during the reaction causes the progressive exchange of interlayer OH groups by Cl atoms, thus gradually diminishing the reactivity of the material. Facile calcination restores the original mixed oxide structure and is shown to enable three equivalent consecutive reaction runs. Since the HCl evolved along with water upon regeneration can be recycled in the first step of the process, i.e., glycerol hydrochlorination, our approach paves the way for a waste-free and more atom efficient biobased epichlorohydrin production process. [ABSTRACT FROM AUTHOR]

Gamma-valerolactone (GVL), a versatile renewable compound listed among the top 10 most promising platform chemicals by the US Department of Energy, is produced via hydrogenation of levulinic acid (LA). The traditional high-loading ruthenium-on-carbon catalyst (5 wt% Ru) employed for this transformation suffers from low metal utilisation and poor resistance to deactivation due to the formation of RuOx species. Aiming at an improved catalyst design, we have prepared ruthenium nanoparticles modified with the water-soluble hexadecyl(2-hydroxyethyl)dimethylammonium dihydrogen phosphate (HHDMA) ligand and supported on TiSi2O6. The hybrid catalyst has been characterised by ICP-OES, elemental analysis, TGA, DRIFTS, H2-TPR, STEM, EDX, 31P and 13C MAS-NMR, and XPS. When evaluated in the continuous-flow hydrogenation of LA, the Ru-HHDMA/TiSi2O6 catalyst (0.24 wt% Ru) displays a fourfold higher reaction rate than the state-of-the-art Ru/C catalyst, while maintaining 100% selectivity to GVL and no sign of deactivation after 15 hours on stream. An in-depth molecular analysis by Density Functional Theory demonstrates that the intrinsic acidic properties at the ligand–metal interface under reaction conditions ensure that the less energy demanding path is followed. The reaction does not obey the expected cascade mechanism and intercalates hydrogenation steps, hydroxyl/water eliminations, and ring closings to ensure high selectivity. Moreover, the interfacial acidity increases the robustness of the material against ruthenium oxide formation. These results provide valuable improvements for the sustainable production of GLV and insights for the rationalisation of the exceptional selectivity of Ru-based catalysts. [ABSTRACT FROM AUTHOR]

Higher alcohols are important compounds with widespread applications in the chemical, pharmaceutical and energy sectors. Currently, they are mainly produced by sugar fermentation (ethanol and isobutanol) or hydration of petroleum-derived alkenes (heavier alcohols), but their direct synthesis from syngas (CO + H2) would comprise a more environmentally-friendly, versatile and economical alternative. Research efforts in this reaction, initiated in the 1930s, have fluctuated along with the oil price and have considerably increased in the last decade due to the interest to exploit shale gas and renewable resources to obtain the gaseous feedstock. Nevertheless, no catalytic system reported to date has performed sufficiently well to justify an industrial implementation. Since the design of an efficient catalyst would strongly benefit from the establishment of synthesis–structure–function relationships and a deeper understanding of the reaction mechanism, this review comprehensively overviews syngas-based higher alcohols synthesis in three main sections, highlighting the advances recently made and the challenges that remain open and stimulate upcoming research activities. The first part critically summarises the formulations and methods applied in the preparation of the four main classes of materials, i.e., Rh-based, Mo-based, modified Fischer–Tropsch and modified methanol synthesis catalysts. The second overviews the molecular-level insights derived from microkinetic and theoretical studies, drawing links to the mechanisms of Fischer–Tropsch and methanol syntheses. Finally, concepts proposed to improve the efficiency of reactors and separation units as well as to utilise CO2 and recycle side-products in the process are described in the third section. [ABSTRACT FROM AUTHOR]

Iron phosphates comprise an important class of materials with a wide range of applications. We have designed novel routes for the controlled synthesis of iron phosphate dihydrates of varying crystallographic phases (monoclinic and orthorhombic) and morphologies. Our approach comprises the phosphation of β-FeOOH nanorods with aqueous phosphoric acid solutions. Through a systematic parametric study coupled to an array of characterisation techniques, including XRD, TEM, HAADF-STEM, AAS, N2 sorption, TGA-MS, FTIR, UV-Vis, XPS, EXAFS, EPR, and 57Fe Mössbauer spectroscopy, we unravel the complex synthesis chemistry on both the macroscopic and microscopic levels. It is found that the formation of iron phosphates occurs exclusively under acidic conditions and in general involves the dissolution of β-FeOOH which, upon reaction with H3PO4, precipitates as FePO4·2H2O. The pH of the treatment solution determines the crystallographic phase of the resulting product by regulating the rate of β-FeOOH dissolution and of the precipitation of the iron phosphates, while the treatment time is decisive for the preservation of the morphology. The formation of the monoclinic phase entails a fast iron dissolution and subsequent precipitation in the solution. The generation of the orthorhombic analogue involves an interfacial reaction between H3PO4 and β-FeOOH, forming an amorphous layer of iron phosphate, which crystallises into a pure phase with increasing treatment time. The thermal transformation of hydrated to anhydrous iron phosphates is dependent on the phase and morphology of the precursors. The rod shape of iron-rich orthorhombic FePO4·2H2O can be preserved even after annealing at 923 K, with the formation of mesopores. These novel nanostructures may widen the applications of iron phosphates and the routes developed herein can be anticipated to guide the fabrication of other metal phosphates. [ABSTRACT FROM AUTHOR]

Herein, we report the preparation of small and narrowly distributed (2.1 ± 0.5 nm) Ag nanoparticles supported on passivated silica, where the surface OH groups are replaced by OSiMe3 functionalities. This synthetic method involves the grafting of silver(I) bis(trimethylsilyl)amide ([AgN(SiMe3)2]4) on silica partially dehydroxylated at 700 °C, followed by a thermal treatment of the grafted complex under H2. The catalytic performance of this material was investigated in the semi-hydrogenation of propyne and 1-hexyne and compared with that of 2.0 ± 0.3 nm Ag nanoparticles supported on silica. Whilst surface passivation slightly decreases the activity in both reactions (by a factor 2-3), probably as a result of the decreased alkyne adsorption properties or the presence of less accessible active sites on the passivated support, the AgNP@SiO2 catalysts demonstrate a remarkable selectivity for the production of alkenes. [ABSTRACT FROM AUTHOR]

The aqueous-phase heterogeneously catalysed isomerisation of bio-oil derived glyoxal is herein introduced as a novel route for the sustainable production of glycolic acid. While commercial ultra-stable Y zeolites displayed only moderate performance, their evaluation enabled us to highlight the crucial role of Lewis acidity in the reaction. Gallium incorporation into these zeolites boosted the glycolic acid yield, although the best catalytic results were obtained over tin-containing MFI-type zeolites, reaching 91% yield of the desired product at full conversion. These materials comprised hydrothermally-synthesised Sn-MFI as well as a novel catalyst obtained by the introduction of tin into silicalite-1 by means of a simpler and more scalable method, i.e. alkaline-assisted metallation. In-depth spectroscopic characterisation of these systems uncovered a substantial similarity of the tin centres obtained by the top-down and bottom-up synthetic approaches. NMR spectroscopic studies gave evidence that the reaction follows a 1,2-hydride shift mechanism solely catalysed by Lewis-acid sites. The Sn-MFI analogue could be reused in 5 cycles without the need for intermediate calcination, did not evidence any tin leaching, and demonstrated suitability for utilisation under continuous-flow operation. The tin-based zeolites exhibited remarkable performance also in alcoholic solvents, leading to the one-pot production of relevant alkyl glycolates. [ABSTRACT FROM AUTHOR]

Galliation of USY zeolites in alkaline media creates tetracoordinated Lewis-acid centres linked to the framework which display outstanding selectivity in the isomerisation of dihydroxyacetone to ethyl lactate and full reusability. This simple and affordable post-synthetic route enables to tune the amount of Lewis acidity in the zeolite and has great potential for application in related biomass conversions. [ABSTRACT FROM AUTHOR]

The conversion of layered metal hydroxides into a metal–organic framework is studied for the case of Fe–BTC (BTC = 1,3,5-benzenetricarboxylate). The synthesis occurs at room temperature as early as 5 min using a layered double hydroxide, green rust, as a precursor and yielding a product with high surface area. Monitoring of the Fe–BTC crystallisation kinetics as a function of the degree of crystallinity of the iron hydroxide reveals the dependence of MOF assembly on the presence of a layered structure in the precursor. Further insights into the evolution of the structure and Fe centers are provided by thermogravimetric analysis, infrared, Mössbauer and electron paramagnetic resonance spectroscopy. The obtained Fe–BTC exhibited a higher crystallinity and a higher content of accessible Lewis-acid sites compared to the commercial counterpart, which in turn resulted in a higher catalytic activity for Knoevenagel condensation. [ABSTRACT FROM AUTHOR]

A copper catalyst based on a delafossite precursor (CuAlO2) displays high activity and extraordinary lifetime in the gas-phase oxidation of HCl to Cl2, representing a cost-effective alternative to RuO2-based catalysts for chlorine recycling. [ABSTRACT FROM AUTHOR]