Effect of organosilane/polymer ratio on adsorption properties of periodic mesoporous ethane-silica

Abstract

Remarkable achievements in the area of pure siliceous mesoporous molecular sieves such as MCM-41, SBA-15, SBA-16 and FDU-1 paved the way for the discovery of periodic mesoporous organosilicas (PMOs). This work reports the effect of organosilane/polymer ratio on the synthesis of PMO with ethane bridging groups inside the SBA-15-type framework. This ethane-silica was obtained by self-assembly of 1,2-bis(triethoxysilyl)ethane (BTESE) precursor and poly(ethylene oxide)- block-poly(propylene oxide)- block-poly(ethylene oxide) triblock copolymer Pluronic P123 (EO 20PO 70EO 20) as structure directing agent under low acidic conditions. The amount of triblock copolymer P123 was increased progressively from 0.5 and 3 g, while the mass of BTESE in the synthesis gel mixture was kept constant. The polymeric template was removed via extraction with acidified ethanolic solution. The resulting PMO materials were characterized by powder X-ray diffraction, thermogravimetry and nitrogen adsorption at −196 °C. The latter was used to evaluate the specific surface area, mesopore width, total pore volume and the volume of complementary pores. The polymer/BTESE weight ratios of 0.73 and 1.46 favoured the formation of additional amorphous silica inside channel-like mesopores, which resulted in plugged hexagonal templated silicas (PHTS) with low mesopore volume, small mesopore size, wide pore size distribution (PSD), thick pore walls and low mesostructural ordering. A further increase in the polymer/BTESE ratio (2.2) led to a material with large pore diameter, narrow PSD, uniform pore openings and high structural ordering. On the other hand, high polymer/BTESE ratios (from 2.93 to 4.39) afforded PMOs with very broad PSD and high non-uniformity of pore entrances as well as low ordering. Furthermore, a complete removal of ethane bridging groups at elevated temperatures in air did not lead to the disappearance of mesostructural ordering in the OMS studied.