Alberto V. Puga

Complete Photocatalytic Reduction of CO2 to Methane by H2 under Solar Light Irradiation

Nickel supported on silica-alumina is an efficient and reusable photocatalyst for the reduction of CO2 to methane by H2, reaching selectivity above 95% at CO2 conversion over 90%. Although NiO behaves similarly, it undergoes a gradual deactivation upon reuse. About 26% of the photocatalytic activity of Ni/silica-alumina under solar light derives from the visible light photoresponse.

Ionic Liquids Containing Boron Cluster Anions

The combination of different boron cluster anions and some of the cations typically found in the composition of ionic liquids has been possible by straightforward metathetic reactions, producing new low melting point salts; the imidazolium cations have been systematically studied, [C(n)mim]+ (when [C(n)mim]+ = 1-alkyl-3-methylimidazolium; n = 2, 4, 6, 8, 10, 12, 14, 16, or 18). Melting points increase in the anionic order [Co(C2B9H11)2]- < [C2B9H12]- < [B10Cl10]2- < [B12Cl12]2-. Nevertheless, alkyl chain length dramatically influences the thermal behavior, suggesting that packing inefficiency is the main cause of the existence of room temperature ionic liquids. The salts [C(n)mim][Co(C2B9H11)2] (n = 4, 6, 8, 10, 12 or 14) are liquids at room temperature, presenting strikingly low glass transition temperatures (> or = -34 degrees C). The salts [C(n)mim]2[X] ([X]2- = [B10Cl10]2- or [B12Cl12]2-, n = 16 or 18) show liquid crystal phases between the solid and liquid states. Tetraalkylphosphonium salts of [B10Cl10]2- have also been prepared. Physical properties, such as thermal stability, density, or viscosity, have been measured for some selected samples. The presence of the perhalogenated dianion [B12Cl12]2- in the composition of the imidazolium salts renders highly thermally stable compounds. For example, [C2mim]2[B12Cl12] starts to decompose above 480 degrees C in a dynamic TGA analysis under a dinitrogen atmosphere. Crystal structures of [C2mim][Co(C2B9H11)2] and [C2mim]2[B12Cl12] have been determined. 1H NMR spectra of selected imidazolium-boron cluster anion salts have been recorded from solutions as a function of the concentration, showing trends related to the cation-anion interactions.

Alkyltributylphosphonium chloride ionic liquids: synthesis, physicochemical properties and crystal structure.

A series of alkyltributylphosphonium chloride ionic liquids, prepared from tributylphosphine and the respective 1-chloroalkane, C(n)H(2n+1)Cl (where n = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12 or 14), is reported. This work is a continuation of an extended series of tetraalkylphosphonium ionic liquids, where the focus is on the variability of n and its impact on the physical properties, such as melting points/glass transitions, thermal stability, density and viscosity. Experimental density and viscosity data were interpreted using QPSR and group contribution methods and the crystal structure of propyl(tributyl)phosphonium chloride is detailed.

New ionic liquids from azepane and 3-methylpiperidine exhibiting wide electrochemical windows

New ionic liquids based on azepanium and 3-methylpiperidinium cations have been synthesised; they exhibit moderate viscosities and remarkably wide electrochemical windows, thereby showing promise, inter alia, as electrolytes and battery materials, and as synthetic media.

Carbon dioxide uptake from natural gas by binary ionic liquid–water mixtures

Carbon dioxide solubility in a set of carboxylate ionic liquids formulated with stoicheiometric amounts of water is found to be significantly higher than for other ionic liquids previously reported. This is due to synergistic chemical and physical absorption. The formulated ionic liquid/water mixtures show greatly enhanced carbon dioxide solubility relative to both anhydrous ionic liquids and aqueous ionic liquid solutions, and are competitive with commercial chemical absorbers, such as activated N-methyldiethanolamine or monoethanolamine.

Light-Promoted Hydrogenation of Carbon Dioxide—An Overview

Hydrogenation of carbon dioxide is considered as a viable strategy to generate fuels while closing the carbon cycle (heavily disrupted by the abuse in the exploitation of fossil resources) and reducing greenhouse gas emissions. The process can be performed by heat-powered catalytic processes, albeit conversion and selectivity tend to be reduced at increasing temperatures owing to thermodynamic constraints. Recent investigations, as summarised in this overview, have proven that light activation is a distinct possibility for the promotion of CO2 hydrogenation to fuels. This effect is particularly beneficial in methanation processes, which can be enhanced under simulated solar irradiation using materials based on metallic nanoparticles as catalysts. The use of nickel, ruthenium and rhodium has led to substantial efficiencies. Light-promoted processes entail performances on a par with (or even superior to) those of thermally-induced, industrially-relevant, commercial technologies.

Efficient Production and Separation of Biodegradable Surfactants from Cellulose in 1‐Butyl‐3‐Methylimidazolium Chloride

Alkyl glycoside biodegradable surfactants were produced from cellulose and 1-octanol or 1-dodecanol in a one-pot, two-step (hydrolysis-glycosidation) process in 1-butyl-3-methylimidazolium chloride. Both surfactant productivity and separation efficiencies have been strikingly enhanced compared to other previously reported ionic liquid processes. Production temperatures were decreased to limit the extent of glucose dehydration and further degradation processes, but the conversions remained high. Surfactant molar yields up to 72% were achieved by operating at 70 °C. Several separation procedures were tested to achieve high recoveries of both surfactant and ionic liquid. The use of a silica stationary phase was useful for isolation of the surfactant, whereas crystallization of the ionic liquid improved its separation efficiency. Finally, the precipitation of dodecyl glycosides in aqueous media was highly efficient for their isolation and for the recovery (>99%) of the ionic liquid by using only water as the solvent for separation.

Production of polyetheretherketone in ionic liquid media

Polyetheretherketones were successfully synthesised in an ionic liquid, namely 1-butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide, by polycondensation reactions of hydroquinone with 4,4′-dihalobenzophenones in the presence of potassium carbonate at elevated temperatures (up to 320 °C). The materials thus produced were similar to polymers prepared in the solvent usually employed in commercial processes, i.e. diphenyl sulfone, as suggested by spectroscopic techniques and X-ray crystallography. By replacing volatile diphenyl sulfone with the effectively involatile ionic liquid, the separation efficiency was significantly improved but the molecular weights were lower.

Hydrogenation of CO2 on Nickel–Iron Nanoparticles Under Sunlight Irradiation

Nickel–iron oxide nanoparticles were prepared by a simple mixed oxalate precursor decomposition method and used as catalysts for the sunlight-promoted CO2 hydrogenation reaction. The composition of the NiyFe1−yOx materials was designed to cover the entire Ni/Fe ratio range (y = 1, 0.9, 0.75, 0.5, 0.25, 0.1, 0). Characterisation was undertaken by means of elemental analyses, X-ray diffraction and high resolution transmission electron microscopy. The pure nickel material (NiOx) contained crystalline NiO nanoparticles. Upon introducing lower proportions of iron in Ni9FeOx and Ni3FeOx, NiO was the only crystalline phase, along with increasing amounts of amorphous iron oxides. Higher iron contents resulted in the co-existence of NiO and γ-Fe2O3 domains at the nanoscale in NiFeOx, NiFe3Ox and NiFe9Ox, whereas the pure iron material (FeOx) was composed of α-Fe2O3 as the only crystalline phase and a significant fraction of amorphous iron oxides. The hydrogenation of carbon dioxide was tested on the materials under simulated sunlight irradiation, and the activities and selectivities investigated as initial CO2 conversion rates and product distributions, respectively. The introduction of iron was beneficial for the activation of CO2, due to the known ability of this metal for promoting the reverse water–gas shift (rWGS) reaction. On the other hand, it was proven that nickel and iron favoured hydrogenation and chain growth processes, respectively. Moreover, the lack of hydrogenation sites in the pure iron material results in the expected preferential generation of olefins. Results for the entire compositional range draw a clear trend towards the enhanced formation of short-chain alkanes at middle iron contents, most likely owing to the existence of junctions between nickel and iron oxides at the nanoscale, and the related interfaces providing rWGS, chain growth and hydrogenation sites in close vicinity. The resulting hydrocarbon products, presumably produced by the efficient combination of thermal and photonic effects, can be considered as solar fuel replacements for natural gas and liquefied petroleum gases.Graphical Abstract

On the nature of active phases and sites in CO and CO2 hydrogenation catalysts

The production of hydrocarbons or oxygenates via the hydrogenation–polymerisation of either CO (Fischer–Tropsch) or CO2 as carbon sources has significant similarities from a catalytic perspective. However, strategically, the two processes differ. Fischer–Tropsch technologies, which have been commercially implemented for almost a century, rely mostly on fossil resources such as coal or natural gas after previous gasification to obtain syngas. Conversely, the direct transformation of waste CO2 using renewable hydrogen is in demand to partially compensate for its enormous emissions and to alleviate their impact on the global climate. The development of catalysts for Fischer–Tropsch processes is mature and has resulted in valuable knowledge; it also represents a valid starting point for the use of CO2 as a feedstock. In general terms, catalysts require active phases to promote (i) CO–CO2 interconversion via the water–gas shift (WGS) and its reverse (rWGS) reaction; (ii) CO activation via dissociation; (iii) hydrogenation; (iv) polymerisation leading to hydrocarbon chain growth; and (v) non-dissociative CO insertion for the formation of oxygenated moieties. Characterisation techniques have traditionally pointed to iron oxides as active phases for rWGS, to metallic cobalt or iron carbides as active phases for hydrogenation–polymerisation into hydrocarbons, and to copper, mixed cobalt–copper and rhodium as selective phases for oxygenate formation. However, the nature of these active phases and sites is still debated. Novel, improved in situ and operando investigations are challenging some previously accepted concepts by allowing the observation of metastable phases, most likely in the form of few-layer surface domains, which may play key roles. For example, the direct implication of cobalt carbides either as precursors of highly active Co(hcp) or as genuinely active phases has been proposed. Thus, the simplistic view of unique catalytically active sites is progressively being replaced with a more dynamic scenario wherein surface interconversion appears to be a common phenomenon. Herein, a perspective on the most enlightening of the recent discoveries regarding the nature of active COx hydrogenation phases and sites, providing valuable knowledge for future catalyst design, is presented.