Impact of support oxide acidity in Pt-catalyzed HMF hydrogenation in alcoholic medium

Silica and four mixed silica oxides (silica-alumina, silica-niobia, and silica-zirconia) with nominally 5 wt.% of the added element (Al, Nb and Zr) were prepared and used as supports for dispersing monometallic Pt-nanoparticles. The presence of the second oxide component on the silica surface influenced some properties of the final samples, like surface area and acidity. The samples acidity was measured in a recirculation adsorption line with 2-phenylethylamine probe, by performing the titrations both in cyclohexane and in methanol to gather the intrinsic and effective acidity, respectively. The acid site density of silica-alumina was the highest compared with the other oxides; in general, an important decrease of acid sites density was determined in methanol. The order of the effective acidity in methanol was different from that determined in cyclohexane only for silica-zirconia and silica-niobia, confirming the peculiar acidity of Nb-oxide compounds in polar liquids: Colloidal spherical platinum nanoparticles were synthesized and then deposited (1 wt.%) on the oxide supports. The obtained metallic nanophases were studied in the reduction of 5-hydroxymethylfurfural (HMF) to valuable chemicals such as dimethylfuran, dimethyltetrahydro-furan, 2-hexanol. In particular, this study focused on the impact of the acidity of the oxide supports on reaction selectivity when 2-butanol is used as the solvent. When Pt is not present, Nb-doped silica is the most effective catalyst to di-hydroxymethyl furan diether (DHMFDE) derived from Meerwein-Ponndorf-Verley reaction, maintaining its Lewis character also in protic medium. In the presence of Pt, Nb-doped silica, however, presents the higher selectivity to hydrogenolysis products, 5-methyl furan (5-MF).