V- and Nb-containing tungsten bronzes catalysts for the aerobic transformation of ethanol and glycerol. Bulk and supported materials

Niobium: The Focus on Catalytic Application in the Conversion of Biomass and Biomass Derivatives

The world scenario regarding consumption and demand for products based on fossil fuels has demonstrated the imperative need to develop new technologies capable of using renewable resources. In this context, the use of biomass to obtain chemical intermediates and fuels has emerged as an important area of research in recent years, since it is a renewable source of carbon in great abundance. It has the benefit of not contributing to the additional emission of greenhouse gases since the CO2 released during the energy conversion process is consumed by it through photosynthesis. In the presented review, the authors provide an update of the literature in the field of biomass transformation with the use of niobium-containing catalysts, emphasizing the versatility of niobium compounds for the conversion of different types of biomass.

Tungsten-niobium oxide bronzes: a bulk and surface structural study

Materials from the WO3-Nb2O5 system, presenting bronze-type crystal structures, display outstanding functional properties for several applications as thermoelectric materials, lithium-ion battery electrodes, or catalysts. In this work, a series of W-Nb-O oxide bronzes have been synthesized by the hydrothermal method (with Nb/(W + Nb) ratios in the range of 0-1). A combination of bulk and surface characterisation techniques has been applied to get further insights into: (i) the effect of thermal treatments on as-prepared materials and (ii) the surface chemical nature of W-Nb-O oxide bronzes. Thermal treatments promote the following structural changes: (i) loss of emerging long-range order and (ii) the elimination of NH4+ and H2O species from the structural channels of the as-synthesized materials. It has been observed that W-Nb-O bronzes with Nb at% of ca. 50% are able to retain a long-range order after heat-treatments, which is attributed to the presence of a Cs0.5[W2.5Nb2.5O14]-type structure. Increasing amounts of Nb5+ in the materials (i) promote a phase transition to pseudocrystalline phases ordered along the c-axis; (ii) stabilize surface W5+ species (elucidated by XPS); and (iii) increase the proportion of surface Lewis acid sites (as determined by the FTIR of adsorbed CO). Results suggest that pseudocrystalline oxides (with a Nb at% ≥ 50%) are closely related to NbO7 pentagonal bipyramid-containing structures. The stabilisation of Lewis acid sites on these pseudocrystalline materials leads to a higher yield of heavy compounds, at the expense of acrolein formation, in the gas-phase dehydration of glycerol.

Furfural Hydrogenation on Modified Niobia

In this study, niobia-based materials have been used as supports for Pt nanoparticles and used in the hydrogenation of furfural. The incorporation of dopants (W6+ and Ti4+) in the Nb2O5 structure induced modifications in the surface acidity of the support; in particular, the addition of W6+ increased the amount of Lewis acid sites, while the addition of Ti4+ decreased the number of Lewis acid sites. As a result, the catalytic activity towards the hydrogenation of furfural was affected; high surface acidity resulted in high catalytic activity. The selectivity of the reaction changed with the support acidity as well, with higher amount of furfuryl alcohol produced decreasing the Lewis acid sites.

Metal Oxides in Heterogeneous Oxidation Catalysis: State of the Art and Challenges for a More Sustainable World

This Review presents current knowledge, recent results, and challenges for the future in heterogeneous oxidation catalysis in liquid and gaseous phases on solid metal oxide catalysts. Metal oxides that are used as catalysts and their main structures and properties are summarized, as well as their catalytic properties in selective and total oxidation reactions, which were studied intensively, experimentally and theoretically, by Professor Jerzy Haber during his scientific life. Some emphasis is placed on the classical and unusual catalyst activation procedures for improving catalytic properties for better efficiency. For a more sustainable world, several examples are given of the oxidation of biomass derivatives to synthesize valuable chemicals and of other applications of metal oxides, such as depollution, photocatalysis, hydrogen production and fuel-cell components. The importance of metal oxide catalysis in environmental and green chemistry and sustainability is discussed, and challenges for the future are considered.

W–Nb–O oxides with tunable acid properties as efficient catalysts for the transformation of biomass-derived oxygenates in aqueous systems

The control of Nb incorporation in W–Nb–O system leads to tailored Brönsted/Lewis acid properties for the selective transformation of either glycerol or aqueous mixtures of short chain oxygenates into value added products.

General and Prospective Views on Oxidation Reactions in Heterogeneous Catalysis

In this review paper, we have assembled the main characteristics of partial oxidation reactions (oxidative dehydrogenation and selective oxidation to olefins or oxygenates, as aldehydes and carboxylic acids and nitriles), as well as total oxidation, particularly for depollution, environmental issues and wastewater treatments. Both gas–solid and liquid–solid media have been considered with recent and representative examples within these fields. We have also discussed about their potential and prospective industrial applications. Particular attention has been brought to new raw materials stemming from biomass, as well as to liquid–solid catalysts cases. This review paper also summarizes the progresses made in the use of unconventional activation methods for performing oxidation reactions, highlighting the synergy of these technologies with heterogeneous catalysis. Focus has been centered on both usual catalysts activation methods and less usual ones, such as the use of ultrasounds, microwaves, grinding (mechanochemistry) and photo-activated processes, as well as their combined use.