Acid-Functionalized SBA-15-Type Periodic Mesoporous Organosilicas and Their Use in the Continuous Production of 5-Hydroxymethylfurfural

The activity, selectivity, and stability of several supported acid catalysts were evaluated in tubular reactors designed to produce 5-hydroxymethylfurfural (HMF) continuously from fructose dissolved in a single-phase solution of THF and H2O (4:1 w/w). The reactors, packed with the solid catalysts, were operated at 403 K for extended periods, up to 190 h. The behaviors of three propylsulfonic acid-functionalized, ordered porous silicas (one inorganic SBA-15-type silica, and two ethane-bridged SBA-15-type organosilicas) were compared with that of a propylsulfonic acid-modified, nonordered, porous silica. The HMF selectivity of the catalysts with ordered pore structures ranged from 60 to 75%, whereas the selectivity of the nonordered catalyst under the same reaction conditions peaked at 20%. The latter was also the least stable, deactivating with a first-order rate constant of 0.152 h(-1). The organosilicas are more hydrothermally stable and maintained a steady catalytic activity longer than the inorganic SBA-15-type silica. The organosilica with an intermediate framework ethane content of 45 mol % was more stable, with a first-order deactivation rate constant of only 0.012 h(-1), than the organosilica containing 90 mol % ethane linkers in the framework. The catalysts were recovered and characterized after use by C-13 and Si-29 solid-state NMR, elemental analysis, nitrogen adsorption/desorption, X-ray diffraction, and SEM/TEM. Deactivation under flow conditions is caused primarily by hydrolytic cleavage of acid sites, which can be (to some) extent recaptured by the free surface hydroxyl groups of the silica surface.