Tailoring the pore structure of SBA-16 silica molecular sieve through the use of copolymer blends and control of synthesis temperature and time

Abstract

SBA-16 mesoporous silicas with cubic Im3m structure were synthesized using Pluronic F127 poly(ethylene oxide)-poly(propylene oxide) -poly(ethylene oxide) triblock copolymer (EO106PO70EO106) and its blends with Pluronic P123 triblock copolymer (EO20PO70EO20) as supramolecular templates. The resulting materials were characterized using X-ray diffraction, transmission electron microscopy, and argon and nitrogen adsorption. Selected samples were also modified with series of organosilanes of gradually increasing sizes and the accessibility of the pore structure after the modification was assessed using argon adsorption, which allowed us to determine the diameter of entrances to the ordered mesopores. It was shown that the pore cage diameter in SBA-16 can be enlarged in a wide range not only by increasing the synthesis temperature and time, as previously known, but also by increasing the content of P123 copolymer in the polymer mixture. These three ways allowed us to synthesize SBA-16 with nominal mesopore diameters from similar to4.5 to 9 nm. Even more importantly, they were also suitable for the tailoring of the pore entrance diameter from similar to1 to at least 6 nm, although in this case, the effect of the copolymer mixture composition on the entrance size was relatively small. In the case of SBA-16 samples synthesized at low temperatures or short hydrothermal treatment times, there was evidence that the pore entrance size was as low as 0.4-0.7 nm, as argon atoms were capable of entering the pore structure, but trimethylchlorosilane appeared to be largely excluded from it. By varying the synthesis temperature, time, and template composition, SBA-16 samples with essentially the same mesopore cage diameter and with largely different pore entrance sizes were synthesized. The present work is a step forward in the synthesis of a mesoporous molecular sieve with independently tailored mesopore diameter and entrance size.