Role of Aluminum Salts in the Synthesis of Polymer-Templated Periodic Mesoporous Organosilicas

Abstract

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. Poly(ethylene 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(OsBt)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.