Periodic mesoporous benzene-silicas prepared using boric acid as catalyst

Highly ordered periodic mesoporous benzene-silicas with 2D p6mm symmetry were synthesized through sol-gel self-assembly in the presence of a nonionic poly(ethylene oxide)-poly(dl-lactic acid-co-glycolic acid)-poly(ethylene oxide) (PEO-PLGA-PEO, EO₁₆(L₂₉G₇)EO₁₆) triblock copolymer template using boric acid (weak Lewis acid) as a catalyst instead of strong hydrochloric acid. 1,4-Bis(triethoxysilyl)benzene (BTEB) was used as an organosilica precursor, and the reaction and aging temperatures were set at 40 and 100 °C, respectively. The molar concentration of boric acid was varied from 0.5 to 3 M to find the minimum acidity required and to check the effect of this acidic catalyst on the formation of benzene-silica mesostructures. This study shows that the ordered mesostructure of benzene-silica was observed even at 1 M concentration of boric acid ([B(OH) ₃]/[BTEB] = 12), and highly ordered hexagonal mesostructures were obtained at higher concentrations of this acid. Thermal stability of the benzene-silica materials prepared in the presence of boric acid increased up to 570 °C depending on the amount of this catalyst and exceeded by 20 °C the degradation temperature of the analogous materials obtained with HCl acid. The specific surface areas and pore diameters of the benzene-silicas studied showed values in the range from 833 to 907 m² g^-1 and from 6.5 to 8.6 nm, respectively.