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 H
2O (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 t
wo 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
鈥?. 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 frame
work ethane content of 45 mol %
was more stable,
with a first-order deactivation rate constant of only 0.012 h
鈥?, than the organosilica containing 90 mol % ethane linkers in the frame
work. The catalysts
were recovered and characterized after use by
13C and
29Si solid-state NMR, elemental analysis, nitrogen adsorption/desorption, X-ray diffraction, and SEM/TEM. Deactivation under flo
w 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.
Keywords:
5-hydroxymethylfurfural; continuous dehydration; packed-bed reactor; SBA-15; periodic mesoporous organosilicas; propylsulfonic acid; catalyst deactivation rate