Lattice model for silica polymerization: Monte Carlo simulations of the transition between gel and nanoparticle...

We present Monte Carlo simulations of a lattice model describing silica polymerization with an emphasis on the transition between gel states and nanoparticle states as the pH and silica concentration are varied. The pH in the system is controlled by the addition of a structure-directing agent (SDA) of the type SDA(+)(OH(-)). The silica units are represented by corner-sharing tetrahedra on a body-centered cubic lattice and the SDA(+) species by single sites with near-neighbor repulsions. We focus on two systems: one with a low silica concentration with composition comparable to that of the clear solution silicalite-1 zeolite synthesis and a high silica concentration system that leads to gel states. In the dilute system, clusters have a core-shell structure, with the core predominantly comprised of silica with some SDA(+) cations, surrounded by a shell of only SDA(+) cations. Moreover, the average cluster size gradually decreases from 2 to 1.6 nm with increasing pH. The concentrated system forms a gel that remains stable to increasing pH up to about 9.2. At pH values in the range of 9.2-10, the gel transforms to nanoparticles of size around 1.0 nm, surprisingly smaller than those in the dilute system. We also study the evolution of the Q(n) distribution (a measure of the silica network structure) for both systems and obtain good agreement with (29)Si NMR data available for the concentrated system.