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5-hydroxymethylfurfural (5-HMF) is an important platform molecule for the synthesis of high-added value products. Several synthesized clay materials, such as mesoporous hectorite and fluorohectorite, in addition to commercial montmorillonite K-10, have been acid modified by different methodologies to be applied as catalysts for the obtention of 5-HMF from glucose. The effects of the Brønsted and/or Lewis acidity, the reaction temperature and time, and the catalyst/glucose ratio on the conversion but especially on the selectivity to 5-HMF have been studied. By comparing the synthesized clays, the best selectivity to 5-HMF (36%) was obtained at 140 °C for 4 h with H-fluorohectorite because of the presence of strong Brønsted acid sites, although its conversion was the lowest (33%) due to its low amounts of Lewis acid sites. Different strategies, such as physical mixtures of montmorillonite K10, which contains high amounts of Lewis acid centers, with Amberlyst-15, which has high amounts of Brønsted acid sites, or the incorporation of rhenium compounds, were carried out. The best selectivity to 5-HMF (62%) was achieved with a mixture of 44 wt % Amberlyst-15 and 56 wt % of montmorillonite K10 for a 56% of conversion at 140 °C for 4 h. This proportion optimized the amount of Brønsted and Lewis acid sites in the catalyst under these reaction conditions.

It has previously been reported that the use of microwave heating, together with the presence of co-solvents, improves the efficiency of furfural production from biomass. Solid acid catalysts can be a good alternative to mineral acids, since they can prevent corrosion and can be reused. However, the formation of humines should be minimized. Several delaminated and fluorinated hectorites, with different types and strengths of acid sites, were synthesized and tested as catalysts for the production of furfural from commercial xylose and from an acid biomass extract of almond shells. A new methodology was developed to prepare crystalline fluorohectorite at 800 • C in just 3 h. The presence of F significantly increased the acidity strength in the protonated fluorohectorite (H-FH) taking into account its high ammonia desorption temperature (721 • C). Additionally, this sample had fourteen times higher total acidity by m 2 than the reference H-βeta acid catalyst. H-FH was the most efficient catalyst at short reaction times (1 h) for the transformation of xylose to furfural under microwaves using toluene as co-solvent, regardless of whether the xylose was commercial (20% furfural yield) or an extract of almond shells (60% furfural yield). However, the acidity of the extract affected the fluorohectorite structure and composition.

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