Synthesis of [Si]-MCM-41 from TEOS and water glass: the water glass-enhanced condensation of TEOS under alkaline conditions

Abstract

Mesoporous [Si]-MCM-41 molecular sieve systems have been fabricated by aging silica gels in CTAB both at RT and under hydrothermal conditions with or without stirring. For the synthesis involving water glass as a SiO2 source, optimal conditions (100 °C, pH 10, crystallization time 2–3 days) gave reproducible, highly ordered [Si]-MCM-41 materials in multi gram scale with high lattice parameter values and BET surface areas. The synthesis with TEOS as silica source produced an inferior quality material. A novel synthesis route involving the use of mixtures of the two precursors gave [Si]-MCM-41 with improved structural (XRD) and microstructural (HRTEM) long-range order. Addition of small amounts of water glass to a TEOS-based synthesis gel under stirred conditions produced [Si]-MCM-41 with excellent mesoporosity, long-range microstructural order, XRD and SBET properties. These properties are reminiscent of those for materials obtained from water glass as a sole SiO2 source. The advantage of this route is that it gave high-quality materials at relatively lower temperatures (80 °C) and shorter crystallization times (6 h) after 21 h aging at RT. The variation of the XRD d100 peak position, BET and HRTEM properties with [TEOS]/[water glass] ratio revealed that a ratio = 3.95 gave optimal production of the highly ordered materials. The data suggests that water glass acts as a structure-directing agent for the TEOS and is incorporated into the final structure.