Today, there is growing interest in aqueous-phase catalytic conversions for the valorization of renewable biomass-based feedstocks for biorefineries to produce, in a sustainable way, biofuels, chemicals, power, energy, materials, pharmaceuticals and food. This is because of the highly polar nature of water which makes it an ideal medium to convert polar biomass-based lignocellulose (cellulose, hemicellulose, lignin), with high oxygen content, and their upgraded products such as hydrophilic carbohydrates, platform chemicals and their derivatives. Another reason which makes water the solvent of choice is that water itself is involved either as a reagent or as a byproduct even in large amounts in typical conversions for the valorization of biomass. The obtained intermediates further react in the aqueous medium, often without any separation and purification, to manufacture more valuable products. This results in substantial energy savings, lower emissions and economic benefits.

Furthermore, water could act as a catalyst in conversions of biomass-based feedstocks such as in liquefaction reactions under subcritical conditions. Moreover, novel types of catalytic reactivity have been observed in the aqueous solvent, not only with water-soluble transition metal catalytic complexes, but also with conventional heterogeneous catalysts and catalytic nanoparticles in a broad spectrum of different reactions such as, inter alia, aldol condensations and hydrogenation reactions. For example, in the aqueous-phase hydrogenation of the biomass-based key platform chemical levulinic acid into γ-valerolactone and beyond, employing heterogeneous catalysts and nanoparticles the presence of water has a beneficial effect and accelerates the reaction rates, whereas in organic solvents much lower activities were observed. This promotional effect of water in the hydrogenation of levulinic acid was proved by many experimental and theoretical studies using a broad spectrum of different types of catalytic systems.

The large heat capacity of water makes it an excellent medium to perform exothermic reactions such as hydrogenations and hydroformylations more safely and selectively. This is especially important in industrial scale exothermic catalytic processes. Aqueous-phase catalytic conversions of renewable biomass-based raw materials are consistent with four principles of Green Chemistry (use of safer solvents, renewable feedstocks, catalysts, and designing safer chemicals) thus possessing a high potential for the development of sustainable industrial catalytic processes for biorefineries in the future and contributing to an effective management of greenhouse gas emissions.

Based on these premises and the importance of biomass conversions, which decisively contributes to the transition from a fossil-based society to a renewable bio-resources based economy, the proposed Research Topic deals with a broad spectrum of catalytic biomass conversions employing homogeneous conventional and water-soluble transition metal catalytic complexes, biocatalysts and heterogeneous catalytic systems in aqueous monophasic and aqueous/organic two-phase systems which may include but are not limited to the following reactions:

· Hydrolysis, hydrogenolysis
· Reforming, hydrogenation, oxidation, dehydration, isomerization
· Ketonization, aldol condensation, esterification, hydroformylation, carbonylation, ring-openings, alkylation
· Telomerization, fermentation
· Hydrothermal liquefaction in sub- and supercritical water, hydrotreating i.e. hydrodeoxygenation, hydroisomerization, hydrodecarboxylation and hydrodecarbonylation

All aspects related to these reactions are within the scope of Research Topic such as novel catalyst development, characterization, hydrothermal stability, physico-chemical and mechanistic investigations, theoretical studies to kinetics, catalyst recycling, reaction engineering and scaling up.

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