(222i) Multi-Scale Modeling of Periodic Mesoporous Silica Materials: Exploring the Role of Silica Oligomers

Authors: 
Monson, P. A., University of Massachusetts Amherst
Chien, S. C., University of Massachusetts
Auerbach, S. M., University of Massachusetts
Pérez-Sánchez, G., University of Porto
Cordeiro, M. N. D. S., University of Porto
Gomes, J. R. B., Universidade de Aveiro
Jorge, M., University of Strathclyde
Periodic mesoporous silicas (PMSs) have shown great potential in many applications such as catalysis, membrane separation, drug vehicles, etc. This kind of materials is usually fabricated through the self-assembly of surfactants and silica precursors in solutions, followed by the rearrangement of those species and thus forming different mesostructures. Several studies have been carried out to investigate various aspects of MCM-41 materials, one of the most popular PMSs, including their pore structure, pore properties, and formation mechanisms.[1] However, the elucidation of the formation process is hampered by the high complexity of the self-assembly between organic-inorganic species in solutions. To unravel the MCM-41 formation mechanism, a multi-scale modeling simulation approach is proposed herein to simulate mesostructure formation at large length and time scales.

Our simulations show that the silica oligomers play an important role during the MCM-41 synthesis. A substantial degree of condensation is required to promote the formation of hexagonal array. Those multiply charged silica oligomers are able to bridge adjacent micelles, thus allowing them to overcome their mutual repulsion and form aggregates. They also induce a phase separation that contains a dilute solution and a silica/surfactant-rich mesophase, which leads to MCM-41 formation at a very low surfactant content.[2] In addition, the system with a larger surfactant content shows that liquid crystal templating mechanism is not viable for MCM-41 synthesis. The hexagonal phase found in pure surfactant solution may collapse while the silica monomers are added, and it forms again after a higher degree of condensation of silica is reached. This study provides new insight into the formation mechanism of PMS materials, enabling tailored design of nanoporous materials using computational models.

[1] Beck, J. S., Vartuli, J. C., Roth, W. J., Leonowicz, M. E., Kresge, C. T., Schmitt, K. D., Chu, C. T. W., Olson, D. H., Sheppard, E. W., McCullen, S. B., Higgins, J. B., and Schlenker, J. L., J. Am. Chem. Soc. 1992, 114, 10834.

[2] Pérez-Sánchez, G., Chien, S.-C., Gomes, J. R. B., Cordeiro, M. N. D. S., Auerbach, S.M., Monson, P.A., and Jorge, M., Chem. Mater. 2016, 28, 2715.