New insights into pore-size expansion of mesoporous silicates using long-chain amines

The possibility of preparation of MCM-41 silicas with extra-large mesopore volume using either direct synthesis in the presence of long-chain amines or post-synthesis treatment was explored. It was shown that the use of large amounts of dimethyldecylamine (DMDA) to restructure MCM-41 with 3.5 nm pores prepared under low-temperature conditions (343 K) affords mesoporous silicas with up to 13.5 nm pores, relatively narrow pore-size distributions and extremely high total pore volumes up to 3.3 cm(3)/g. The application of large amounts of dimethylhexadecylamine (DMHA) was somewhat less effective, yet suitable for the preparation of materials with up to 8.5 nm pores and total pore volumes as large as 2.3 cm(3)/g. Large-pore silicas with narrow pore-size distributions were also obtained using direct synthesis in the presence of large amounts of DMHA along with cetyltrimethylammonium bromide in the gel. The current and earlier studies clearly demonstrate the feasibility of the preparation of disordered silicas with narrow pore-site distributions centered in the range from about 6 to 13.5 nm by a judicious choice of direct synthesis or restructuring procedures involving long-chain amines, such as DMDA or DMHA. Thermogravimetric analysis of as-prepared amine-expanded materials was shown to provide useful information about the template/silica mass ratio, which in turn was found to be related to the pore volume after calcination. Moreover, the weight-change derivatives provided some information regarding the organization of micelles inside the amine-swollen materials. (C) 2000 Elsevier Science B.V. All rights reserved.