Mesoporous Silica Molecular Sieves Prepared by Ionic and Neutral Surfactant Templating:  A Comparison of Physical Properties

Abstract

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 cationic quaternary ammonium ion surfactants (S + ) and the anionic silicate precursors (I - ) affords long-range hexagonal structures under hydrothermal synthesis conditions (100 °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 S 0 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 S 0 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 29 Si 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 generally 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 framework-confined mesopores.