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 o r 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 por es prepared under low-temperature conditions (343 K) affords mesoporous sil icas with up to 13.5 nm pores, relatively narrow pore-size distributions an d extremely high total pore volumes up to 3.3 cm(3)/g. The application of l arge amounts of dimethylhexadecylamine (DMHA) was somewhat less effective, yet suitable for the preparation of materials with up to 8.5 nm pores and t otal pore volumes as large as 2.3 cm(3)/g. Large-pore silicas with narrow p ore-size distributions were also obtained using direct synthesis in the pre sence of large amounts of DMHA along with cetyltrimethylammonium bromide in the gel. The current and earlier studies clearly demonstrate the feasibili ty of the preparation of disordered silicas with narrow pore-site distribut ions 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 show n to provide useful information about the template/silica mass ratio, which in turn was found to be related to the pore volume after calcination. More over, the weight-change derivatives provided some information regarding the organization of micelles inside the amine-swollen materials. (C) 2000 Else vier Science B.V. All rights reserved.