Role of aluminum salts in the synthesis of polymer-templated periodic mesoporous organosilicas

The effect of trivalent aluminum salts as well as mono- (KCl) and divalent (CaCl₂) salts was investigated in the synthesis of periodic mesoporous organosilicas (PMOs), ethane-silica, and benzene-silica, carried out in the presence of poly(ethylene oxide)-containing nonionic triblock copolymers. Polyethylene oxide)poly(DL-lactic acid-co-glycolic acid)-poly(ethylene oxide) (PEO-PLGA-PEO) triblock copolymer was employed as a soft template and aluminum chloride (AlCl₃), sodium aluminate (NaAlO₂), and aluminum butoxide (Al(O^sBt)₃) were used as auxiliary aluminum compounds. For comparison, analogous PMOs were prepared by using KCl and CaCl₂. It was shown that among additives used sodium aluminate had a pronounced effect on the porosity of 2D hexagonal (p6mm) ethane-silica mesostructures when adequate Al/ethane molar ratio (about 0.25) was used; namely, the mesopore diameter, BET surface area, and pore volume increased from 8.62 to 9.27 nm, from 897 to 1072 m² g^-1, and from 1.23 to 1.72 cm³ g^-1 respectively, and the wall thickness decreased from 3.50 to 2.05 nm. Importantly, highly ordered 2D hexagonal benzene-silica mesostructure was formed using AlCl₃ (Al/benzene ratio ≥1) instead of HCl; such improvement of the structure was not observed for the samples prepared in the presence of other salts, KCl and CaCl₂. Also, the crystallinity of benzene-silica mesostructures self-assembled in the presence of aluminum compounds was enhanced.