Could Mesophases Play a Role in the Nucleation and Polymorph Selection of Zeolites?

Zeolites and mesoporous silicas are
porous materials with important
applications in catalysis, gas storage, and separation. Zeolite crystals
form in the presence of cationic surfactants that act as structure
directing agents (SDAs). The way SDAs control the nucleation and polymorphs
selection in zeolites is not fully understood. The formation of mesoporous
silica is templated by liquid crystalline mesophases that result from
frustrated attraction between silicates and long-chain SDAs. Experiments
indicate that surfactants CnH2n+1(CH3)3N+ with n > 6 yield mesoporous silicas, and the one with n = 6 produces a zeolite. This suggests that the driving
force toward mesophase formation is also present for small organocations,
but is overcome by the ability of silica to wrap a crystal lattice
around them. Here we use molecular dynamics simulations to investigate
whether the existence of metastable mesophases can play a role in
the nucleation and polymorph selection of zeolitic crystals. As a
proof of concept, we investigate the phase behavior of simple mesogenic
mixtures of SDAs and a network former T that favors tetracoordinated
crystals. We represent the network-former T by Stillinger−Weber
models of water and silicon, in lieu of silica, because a computationally
efficient silica potential that would allow for the spontaneous nucleation
of zeolites in molecular dynamics simulations is not yet available.
The mixtures of T and SDA produce a rich phase diagram that encompasses
the sII clathrate and at least six zeolites, including sigma-2 (SGT).
We find that the nucleation of SGT is not assisted by a mesophase.
The nucleation of the other five zeolites of this study, however,
is facilitated by the existence of metastable mesophases that decrease
the nucleation barriers and direct the selection of the crystal polymorph.
Together with the experimental support for mesophases in mixtures
of silicates and SDAs, our results for model systems suggest that
metastable mesophases could play a prominent role in promoting the
nucleation and polymorph selection of some zeolites.