Mesoporous silica molecular sieves prepared by ionic and neutral surfactant templating: A comparison of physical properties

A series of mesoporous MCM-41 and HMS silica molecular sieves have been prepared by electrostatic and neutral assembly pathways, respectively, and their properties have been compared by a variety of physical techniques. Direct S+I- charge matching between catioinic quaternary ammonium ion surfactants (S+) and the anionic silicate precursors (I-) affords long-range hexagonal structures under hydrothermal synthesis conditions (100 degrees C), but the long-range order is greatly reduced when the synthesis is conducted at ambient temperature. Conversely, owing to weaker assembly forces, counterion-mediated S(+)X(-)I(+) and neutral (SI0)-I-0 pathways (where X(-) is halide. and S-0 is a primary amine) provide calcined products with the best long-range order when the syntheses are conducted at ambient temperature. In general, MCM-41 silicas formed by electrostatic S+I- or S(+)X(-)I(+) assembly exhibit greater long-range order than the HMS analogues prepared by the (SI0)-I-0 assembly. However, the framework-confined pore structures, though dependent on the size of the surfactant, remain uniform, regardless of the fidelity of the long-range hexagonal order. As-synthesized neutral HMS silicas are much more extensively cross-linked than the as-synthesized charged frameworks of MCM-41 analogues, as judged by the Si-29 MAS NMR Q(4)/Q(3) Si(O-Si)(4-x) connectivity ratios. The neutral and extensively cross-linked character of HMS frameworks allows for the efficient and environmentally benign recovery of the template by solvent extraction. In contrast, electrostatically templated MCM-41 derivatives require proton ion exchange or calcination for the complete removal of template from the framework pores. Also, relative to the S+I- and S(+)X(-)I(+) templated derivatives, HMS mesostructures possess thicker framework walls, superior thermal stability upon calcination in air, and a smaller crystallite size, which affords complementary textural mesoporosity for improved access to the frameswork-confined mesopores.