Hydroxyl species in large-pore phenylene-bridged periodic mesoporous organosilica

Silanol species in phenylene-bridged periodic mesoporous organosilica (PMO), templated via tri-block copolymer Pluronic P123 and thus characterized by large pores and amorphous wall structure, have been characterized by means of FT-IR spectroscopy. Investigation has been carried out on both the naked sample outgassed at different temperatures and the sample when interacting with molecular probes able to form H-bonding (ammonia and carbon monoxide). After outgassing at 773 K, the material shows both isolated silanols and silanols engaged in "intraframework" H-bonding with the pi-cloud of structural aromatic rings. Interaction with ammonia showed that a fraction of these species is inaccessible, being probably located inside the pore walls. Thermal treatment above 673 K causes the appearance of SiO3(OH) species formed as a consequence of the cleavage of some Si-C bonds. The presence of hydroxyls slightly more acidic than isolated silanols has been evidenced: these are interpreted as perturbed geminal species.