(128e) Multi-Scale Modelling of the Synthesis of Nanoporous Silica Materials

Computational material design offers the possibility of tailoring the properties of materials to suit particular applications. To achieve this, however, one must surmount a major challenge - obtaining a fundamental, molecular-level understanding of the material synthesis process. Such knowledge would allow us to predict how each set of synthesis conditions relates to the structural and chemical properties of the final material. For the particular case of nanoporous silica materials, like zeolites and periodic mesoporous silica (PMS), the synthesis process relies on a delicate interplay of chemical reactions, self-assembly and phase equilibrium, taking place over a wide range of time and length scales. This makes it quite difficult, if not impossible, to achieve a unique view of the mechanism by experimental means alone. This talk will report recent advances by our group in the development of a new multi-scale modelling approach to describe the synthesis of nanoporous silica materials, that bridges time and length scales from the quantum to the coarse-grained levels.

Our simulations of the synthesis of MCM-41 [1], the archetypal example of a mesoporous silica material, have revealed unprecedented details of the molecular-level interactions that control the process. The addition of silica to an aqueous alkylammonium surfactant solution initially gives rise to micelle size increase and micelle fusion [2], promoting a shape transition from spherical to rod-like aggregates [3]. The strong adsorption of silicates at the micelle surface facilitates silica polymerisation. Once silica oligomers are present in the system, they act as multi-dentate binders inducing the formation of a hexagonally-ordered mesophase [4]. It is this mesophase that ultimately determines the pore network structure of MCM-41, upon further condensation and template removal. Before this takes place, however, it is possible to change the mesophase structure by altering the synthesis conditions (e.g. pH, temperature or composition). The results of our multi-scale model correctly describe experimental observations on this system, and make useful predictions for future design of these materials.

We have also extended our model to describe the synthesis of bioinspired silica. Inspired by the natural process of biosilicification, this strategy makes use of polyamine templates to produce porous silica materials under much more environmentally friendly conditions (ambient temperature and pressure, near-neutral pH) than currently employed in industry [5]. Compared to traditional templates used in industrial porous silica manufacture, polyamines bring the added complexity of being pH-responsive, given that their degree of ionisation changes with pH. We have applied our multi-scale model to the first example of bioinspired silica, which was heralded at the time as the first example of a “neutral templating route to porous silica” [6]. Crucially, our results show that such a hypothesised mechanism, based on hydrogen-bond interactions between silicates and templates, is not viable under the experimental synthesis conditions. Instead, the mechanism hinges on charge-matching interactions between precursor molecules [7]. Preliminary experimental data supports our interpretation of the synthesis mechanism. Our work suggests routes for increased control over the properties of this class of materials, paving the way for computational material design of bioinspired silica.

[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.; Schlenker, J. L. J. Am. Chem. Soc. 114 (1992) 10834.

[2] Jorge, M.; Gomes, J. R. B.; Cordeiro, M. N. D. S; Seaton, N. A. J. Am. Chem. Soc. 129 (2007) 15414.

[3] Pérez-Sánchez, G.; Gomes, J. R. B.; Jorge, M. Langmuir 29 (2013) 2387.

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

[5] Manning, J. R. H.; Yip, T.; Centi, A.; Jorge, M.; Patwardhan, S.V. ChemSusChem 10 (2017) 1683.

[6] Tanev, P. T. and Pinnavaia, T. J. Science 267 (1995) 865.

[7] Centi, A.; Jorge, M. Langmuir 32 (2016) 7228.