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.
Metal size and speciation strongly impact catalyst efficiency and robustness in biomass upgrading through redox and acid-mediated reactions, with the full potential of single atoms and low-nuclearity species still to be unlocked by future research.
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 + H
2
) 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 CO
2
and recycle side-products in the process are described in the third section.
We review synthetic, mechanistic and process aspects of the direct syngas conversion into higher alcohols to foster the identification of industrially-viable catalysts.
Abstract
Metal promotion in heterogeneous catalysis requires nanoscale-precision architectures to attain maximized and durable benefits. Herein, we unravel the complex interplay between nanostructure ...and product selectivity of nickel-promoted In
2
O
3
in CO
2
hydrogenation to methanol through in-depth characterization, theoretical simulations, and kinetic analyses. Up to 10 wt.% nickel, InNi
3
patches are formed on the oxide surface, which cannot activate CO
2
but boost methanol production supplying neutral hydrogen species. Since protons and hydrides generated on In
2
O
3
drive methanol synthesis rather than the reverse water-gas shift but radicals foster both reactions, nickel-lean catalysts featuring nanometric alloy layers provide a favorable balance between charged and neutral hydrogen species. For nickel contents >10 wt.%, extended InNi
3
structures favor CO production and metallic nickel additionally present produces some methane. This study marks a step ahead towards green methanol synthesis and uncovers chemistry aspects of nickel that shall spark inspiration for other catalytic applications.
Glycerol conversion into chemicals and fuel additives is pursued to valorise a burgeoning by-product in the bioenergy sector. To this aim, heterogeneous catalysts have been developed that enable, in ...many cases, efficient and green transformations. Still, the evaluation of the environmental and economic footprint that would be associated with their large-scale application has often been neglected, limiting their commercial attractiveness. Furthermore, the impact of integrating different glycerol upgrading routes within a biorefinery, which is highly instrumental to determine the effective sustainability and profitability of biodiesel production from vegetable oils, has not been assessed. Here, the manufacture of the most relevant chemical derivatives of glycerol is considered, i.e. , lactic acid, acrylic acid, glycerol carbonate, propanediols, epichlorohydrin and allyl alcohol. State-of-the-art catalysts for each upgrading route are briefly reviewed. Based on their performances, processes are rigorously modelled and relevant indicators, the global warming potential, the cumulative energy demand and the operating costs, quantified by life-cycle analysis. Glycerol-based processes are generally found more attractive than the conventional technologies nowadays applied for the production of the same chemicals, among which the paths to lactic acid and glycerol carbonate are particularly promising. In addition, the process variables mostly contributing to the environmental and cost metrics are identified. Accordingly, future studies should target further optimisation mainly in relation to selectivity, solvent volatility, reactants ratio and catalyst stability. Finally, the processes are integrated simulating a prospective glycerol biorefinery and the advantages deriving from the exchange of heat between the different routes quantified. If the glycerol feed is split equally among all routes the CO 2 emissions and energy requirements are decreased by 15 and 32%, respectively, and the profit is increased by 5% as compared to the sum of the individual glycerol-based processes. In order to minimise the ecological impact of the biorefinery, glycerol should be rather divided in an 80 : 20 mass ratio among 1,2-propanediol and glycerol carbonate production, which are expected to have a significant market size. The innovative approach outlined in this work holds potential to guide both fundamental chemical research and process design in the development of CO 2 and other bio-refineries.
Methanol synthesis by CO2 hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global ...warming. However, there still is a lack of efficient catalysts for such alternative processes. Herein, we unveil the high activity, 100 % selectivity, and remarkable stability for 1000 h on stream of In2O3 supported on ZrO2 under industrially relevant conditions. This strongly contrasts to the benchmark Cu‐ZnO‐Al2O3 catalyst, which is unselective and experiences rapid deactivation. In‐depth characterization of the In2O3‐based materials points towards a mechanism rooted in the creation and annihilation of oxygen vacancies as active sites, whose amount can be modulated in situ by co‐feeding CO and boosted through electronic interactions with the zirconia carrier. These results constitute a promising basis for the design of a prospective technology for sustainable methanol production.
Surface oxygen vacancies in indium oxide drive the selective hydrogenation of CO2 to methanol. Strong electronic interactions between this active phase and the ZrO2 carrier and further vacancy formation by CO co‐feeding lead to excellent catalytic activity.
Abstract
Metal promotion is broadly applied to enhance the performance of heterogeneous catalysts to fulfill industrial requirements. Still, generating and quantifying the effect of the promoter ...speciation that exclusively introduces desired properties and ensures proximity to or accommodation within the active site and durability upon reaction is very challenging. Recently, In
2
O
3
was discovered as a highly selective and stable catalyst for green methanol production from CO
2
. Activity boosting by promotion with palladium, an efficient H
2
-splitter, was partially successful since palladium nanoparticles mediate the parasitic reverse water–gas shift reaction, reducing selectivity, and sinter or alloy with indium, limiting metal utilization and robustness. Here, we show that the precise palladium atoms architecture reached by controlled co-precipitation eliminates these limitations. Palladium atoms replacing indium atoms in the active In
3
O
5
ensemble attract additional palladium atoms deposited onto the surface forming low-nuclearity clusters, which foster H
2
activation and remain unaltered, enabling record productivities for 500 h.
Abstract
Palladium promotion and deposition on
monoclinic
zirconia are effective strategies to boost the performance of bulk In
2
O
3
in CO
2
-to-methanol and could unlock superior reactivity if well ...integrated into a single catalytic system. However, harnessing synergic effects of the individual components is crucial and very challenging as it requires precise control over their assembly. Herein, we present ternary Pd-In
2
O
3
-ZrO
2
catalysts prepared by flame spray pyrolysis (FSP) with remarkable methanol productivity and improved metal utilization, surpassing their binary counterparts. Unlike established impregnation and co-precipitation methods, FSP produces materials combining low-nuclearity palladium species associated with In
2
O
3
monolayers highly dispersed on the ZrO
2
carrier, whose surface partially transforms from a
tetragonal
into a
monoclinic-
like structure upon reaction. A pioneering protocol developed to quantify oxygen vacancies using in situ electron paramagnetic resonance spectroscopy reveals their enhanced generation because of this unique catalyst architecture, thereby rationalizing its high and sustained methanol productivity.
Cuprous delafossites exhibit exceptional electrical, magnetic, optical, and catalytic properties. Through the application of a battery of in situ and ex situ characterization methods complemented by ...density functional theory (DFT) calculations, we gathered an in-depth understanding of the synthesis of CuMO2 (M = Al, Cr, Fe, Ga, Mn) by the solid-state reaction of Cu2O and M2O3 and of their stability against oxidative disproportionation to CuM2O4 and CuO. TGA-DTA and XRD studies of the synthesis revealed that the nature of the M3+ cation strongly impacts (i) the formation temperature of the delafossite phase, which occurred at a much lower temperature for CuCrO2 than for the other metals (1073 versus 1273–1423 K), (ii) the mechanism of formation of the CuMO2 in different atmospheres, which was found to comprise up to four steps in air and a single step in N2, and (iii) the kinetics of the process, which could be significantly accelerated upon mechanochemical activation of the precursors by ball milling. The identification of unstable intermediate phases and, thus, a proper description of the synthesis mechanism was only possible by the application of in situ XRD. Electron microscopy, nitrogen sorption, and mercury porosimetry analyses of the precursor oxide mixtures at different stages of the synthesis in air revealed that particle agglomeration took place prior to the solid-state reactions forming the intermediate spinel phase and the delafossite, respectively, and that these led to a substantial drop in porosity and specific surface area. On the basis of XRD and He pycnometry, the resulting CuMO2 samples exhibit pure delafossite phase with rhombohedral structure (R3̅m), except for CuMnO2 which features a monoclinic structure (C2/m). Upon heating in air, CuCrO2 retained its structure up to 1373 K, while all other delafossites decomposed, CuAlO2 at 1073 K, CuGaO2 at 873 K, CuFeO2 at 773 K, and CuMnO2 at 673 K. The DFT-calculated surface phase diagram of CuCrO2 and CuAlO2 indicated that, at elevated oxygen pressures, the terminations with 1/2 and 0 ML of Cu are the most stable for the (0001) facet. The formation enthalpy for interstitial oxygen species in the bulk is endothermic for both delafossites, while that for oxygen insertion in subsurface layers of these terminations is still endothermic for CuCrO2 but slightly exothermic for CuAlO2. These results provide an improved understanding of the chemistry of these mixed oxides, enabling their optimization for specific applications.
Optimal amounts of CO2 are added to syngas to boost the methanol synthesis rate on Cu‐ZnO‐Al2O3 in the industrial process. The reason for CO2 promotion is not sufficiently understood at the particle ...level due to the catalyst complexity and the high demands of characterization under true reaction conditions. Herein, we applied operando synchrotron X‐ray powder diffraction and modulated‐excitation infrared spectroscopy on a commercial catalyst to gain insights into its morphology and surface chemistry. These studies unveiled that Cu and ZnO agglomerate and ZnO particles flatten under CO/H2 and/or CO2/H2. Under the optimal CO/CO2/H2 mixture, sintering is prevented and ZnO crystals adopt an elongated shape due to the minimal presence of the H2O byproduct, enhancing the water‐gas shift activity and thus the methanol production. Our results provide a rationale to the CO2 promotion emphasizing the importance of advanced analytical methods to establish structure–performance relations in heterogeneous catalysis.
A methanol synthesis catalyst at work: Operando synchrotron X‐ray powder diffraction and modulated‐excitation infrared spectroscopy studies on Cu‐ZnO‐Al2O3 reveal that, under the optimal CO/CO2/H2 gas mixture, sintering of Cu and ZnO particles is prevented and ZnO adopts a preferred morphology which favors the water‐gas shift reaction. The results offer an explanation for the long‐debated promotional effect by CO2.
Recently, lactic acid has emerged as one of the most relevant platform molecules for the preparation of bio-chemicals. Due to the limited productivity of sugar fermentation, the dominant industrial ...technology practiced for its manufacture, new chemocatalytic processes are being developed in order to meet the expected demand for this intermediate. The Lewis-acid catalysed isomerisation of dihydroxyacetone has attracted particular interest. If the reaction is performed in water, lactic acid is attained directly, while if alcohol is used as the solvent, the desired product can be obtained upon subsequent hydrolysis of the alkyl lactates formed. Herein, we (i) demonstrate tin-containing MFI zeolites prepared by scalable methods as highly active, selective and recyclable catalysts able to operate in concentrated dihydroxyacetone aqueous and methanolic solutions, and (ii) reveal by life cycle analysis that a process comprising the enzymatic production of dihydroxyacetone from crude glycerol and its chemocatalytic isomerisation in methanol is advantageous for the production of lactic acid compared to glucose fermentation in terms of both sustainability and operating costs. In particular, we demonstrate that the reduced energy requirements and CO sub(2) emissions of the cascade process originate from the valorisation of a waste feedstock and from the high performance and recyclability of the zeolite catalyst and that the economic advantage is strongly determined by the comparably low market price of glycerol. It is also shown that the bio-/chemocatalytic route remains ecologically and economically more attractive even if the purity of glycerol is as low as 38%.
PDF
