Expanding the Pore Size of MCM-41 Silicas: Use of Amines as Expanders in Direct Synthesis and Postsynthesis Procedures

Abstract

New opportunities are discussed for the pore-size engineering of mesoporous silicas using amines as swelling agents. It was shown that N, N -dimethylhexadecylamine (DMHA) is a convenient expander allowing for the synthesis of large-pore MCM-41 in mild conditions. Their pore size increased and the structural ordering decreased as the amount of the amine in the synthesis gel increased. The material with the largest pore size attained (7.7 nm) exhibited narrow pore-size distribution, very large primary mesopore volume, but a disordered pore system. Other amines, such as trioctylamine and tridodecylamine, were also found to be suitable for preparation of large-pore MCM-41 but not as efficient as DMHA. Addition of dimethyldecylamine (DMDA), dimethyloctylamine (DMOA), and small trialkylamines to the synthesis gel did not lead to the pore size enlargement but decreased the structural uniformity and in some cases resulted in development of microporosity. It was also demonstrated that it is possible to induce up to 3-fold enlargement of pore size of as-synthesized MCM-41 by treating it in an aqueous emulsion of an appropriate amine, such as DMHA, DMDA, and DMOA. This novel synthesis approach allowed us to prepare materials with pore sizes up to 11 nm, pore volumes up to 2.4 cm 3 /g, large surface areas (about 1000 m 2 /g), and narrow pore-size distributions. Although it was possible to achieve a limited pore-size increase (ca. 20%) with retention of the hexagonal structure, more significant enlargement was accompanied by loss of ordering. Structural features of materials restructured under different conditions, such as temperature, treatment time, and nature of the amine used, are described, and structural changes during the pore size enlargement are discussed.