Fabrication of Mesoporous Polycarbosilane from SBA-15 Templated Polymethylsilane

Abstract

Mesoporous silica materials have been used as templates for syntheses of carbon, metal and polymer-based mesoporous replicas via nanocasting process. Since mesoporous silica templates have a narrow pore size distribution and uniform wall thickness, the replicas obtained by synthesis using mesoporous silica templates also have nanometersized pores with a narrow pore size distribution. Incorporation of hydrocarbon polymer through polymerization of monomer such as aniline, acrylonitrile, ethylene, methyl methacrylate and phenolformaldehyde within mesoporous MCM-41 could lead to the fabrication of organic nanofibers. Syntheses of mesoporous polymer networks with three dimensional continuity by the replication of mesoporous materials were reported. The preparation of highly ordered macroscopic silicon carbide, mesoporous silicon imido nitride and bimodal porous silicon oxycarbide from liquid preceramic polymer were also reported. In this communication, we report for the first time the preparation of mesoporous polycarbosilane from liquid polymethylsilane as a polymer precursor and SBA-15 as a template. The silica mesoporous material SBA-15 used as a template for synthesis of mesoporous polycarbosilane have been synthesized, following the procedure in the literature. The XRD (Rigaku D/Max 2200 diffractometer) pattern of calcined SBA-15 reflected the formation of high quality SBA-15 with hexagonal symmetry (not shown here). Figure 1(a) shows transmission electron micrograph (TEM, JEOL JEM-2010) image of rod-shaped calcined SBA-15 with the regular channel array in parallel along the crystallographic c-axis. The external morphology of SBA-15 is rod shape with the length of ca. 1 μm and the diameter of ca. 400 nm, as shown in Figure 1(a). N2 adsorption-desorption isotherm of calcined SBA-15 was measured using Micromeritics ASAP2010 instrument (Figure 2(a)). Brunauer-EmmettTeller (BET) surface area was 849 m/g, the average pore diameter, 5.1 nm, total pore volume, 1.073 cc/g, and micropore volume, 0.058 cc/g. The d100 spacing calculated from (100) peak at 1.03 of XRD pattern was 8.6 nm, the unit cell parameter (a0=2d100/ ), 9.9 nm and the wall thickness, 4.8 nm. The agent impregnated to the pore of SBA-15 for the synthesis of polycarbosilane-based mesoporous material was polymethylsilane prepared by polymerization via dehalocoupling of dichloromethylsilane using Na catalyst as following procedure in the literature. The number average molecular weight and the weight average molecular weight of polymethylsilane determined by gel permeation chromatography were 927 and 4120, respectively. The synthesis of mesoporous polycarbosilane was performed using SBA-15 as a host material and polymethylsilane as a guest material. 0.32 g of calcined SBA-15 was dried under vacuum of 10−3 torr. and impregnated with 0.28 g of polymethylsilane dissolved in 0.5 mL of toluene in the glove box filled with nitrogen atmosphere. The resultant composite was put into a fused quartz tube equipped with fritted disk. After removal of toluene under vacuum for 24 h at 160 C, the composite was heated to 250 C at a heating rate of 5 oCmin−1, cured at 250 C for 4 h and heated at 600 C for 2 h in Ar atmosphere. The heat-treated composite was dissolved with stirring for 2 h in 48% HF solution to remove SBA-15 template. The resulting mesoporous polycarbosilane was washed with ethanol/H2O solution and dried at 60 C for 12 h. The yield percentage based on the impregnated weight 3