Bifunctional niobia and zirconia catalysts for 5-hydroxymethylfurfural production from C6 sugars

Abstract

Biomass represents one of the most practical and environmentally friendly options available as an alternative to petroleum derived compounds. Amongst them, 5-hydroxymethylfurfural (HMF) stands as a potential platform chemical for the production of biofuels and other value added chemicals. Although HMF can be selectively produced from mono- and poly-saccharides with the assistance of organic solvents or ionic liquids, its formation in aqueous media under mild conditions remains a challenge due to undesired side reactions. In this thesis, the conversion of glucose and fructose to HMF has been studied over bulk and SBA-15 supported niobic acid catalysts synthesised through a recently reported peptization procedure which provides high surface area and strong Lewis and Brønsted acid densities. Sintering and crystallization induced by thermal treatment of bulk materials has been proven by XRD, TEM and Raman, provoking a worsening in the textural and acidic properties of the niobia based catalysts. The amorphous phase of nanoparticulate niobic acid is highly active for the aqueous phase isomerisation of glucose to fructose, and subsequent fructose dehydration to 5-HMF under mild reaction conditions, due to an adequate balance of Brønsted-Lewis acid sites. Additionally, another family of phosphated zirconia bifunctional catalysts has been successfully synthesised, fully characterized and employed for the production of HMF. Tunability of Brønsted-Lewis ratio was possible thanks to a controlled modification of surface acidity by phosphate loading. The presence of phosphate species inhibited the particle growth and crystallization of zirconia, as proved by Raman and XRD, which led to the enhancement of textural and acidic properties. Catalysts with low Brønsted-Lewis ratio and higher basicity show higher activity for glucose isomerization, while catalysts with high Brønsted-Lewis ratio and low basicity proved to be more suitable for the dehydration step to HMF.