Green catalytic approaches to lignocellulosic biomas upgrading. Synthesis of high value-added compounds

Resum

Nowadays, the enormous society’s needs for both energy and materials, together with the concerns about global warming and carbon-based fossil sources depletion, demand a switch in the productive model. The cornerstone of this change is based on the search of a new source of carbon atoms, capable of replacing carbon-based fossil sources (coal, crude-oil, gas) for both energy obtention and material manufacturing. Thus, lignocellulosic biomass has been highlighted as the most suitable candidate, due to is considered a renewable, easily accessible, and almost infinite feedstock. In this regard, the research work described in this thesis submitted to obtain a PhD degree in Chemistry is focused on the valorisation of different sources of lignocellulosic biomass, obtaining from them high value-added compounds by means of environmentally friendly processes. The motivation of this thesis is aimed to the future development of biorefineries based on lignocellulosic biomass, especially that coming from agro-industrial residues, as analogues of crude-oil refineries, from which a wide range of fuels and materials are obtained. To achieve this, two strategies for lignocellulosic biomass upgrading into high value-added compounds are studied. On the one hand, the cellulose depolymerisation and dehydration into platform compounds 5-hydroxymethylfurfural and levulinic acid, using either monophasic or biphasic system. On the other hand, an integral approach called reductive catalytic fractionation. This process is focused on obtain value from all lignocellulose fractions, and not only from the carbohydrate fraction as the traditional procedures do, by an in situ stabilisation of lignin just after its removal from the whole biomass. Moreover, valorisation of several biomass samples, such as Opuntia ficus-indica, and by-products from olive industry in Spain, namely table olive processing wastewaters and olive stone are performed. A full characterisation of feedstocks is carried out and then, the most suitable fractions of each one is submitted to thermochemical conversion under microwave radiation into 5-hydroxymethylfurfural and levulinic acid using the previously optimised conditions for cellulose conversion, or slightly modifications of this method. Finally, one of the products previously obtained, levulinic acid, has been transformed into a family of alkyl levulinates, compounds of high value-added with applications as fuel additives, solvents, or flavouring, among others, under environmentally friendly conditions.