Understanding adsorption and transport of light gases in hierarchical materials using molecular simulation and effective medium theory

Ordered structures such as zeolites play an important role in a myriad of technological applications, from catalysis to adsorptive separations. Moreover, in recent years, a new class of zeolites with a "hierarchical" structure has been the subject of intense research, due to its potential to dramatically improve yield in those processes where the critical dimensions of the pore and the fluid are similar. In these hierarchical structures, a mesoporous network is purposefully embedded into the microporous framework; the presence of mesopores in the interior of microporous particles may significantly improve their transport properties. We analyze here our recent experiments on mass transfer in hierarchical CHA-like zeolite, microporous SAPO-34, having embedded mesopores as transport pathways providing access to intracrystalline micropores. We use effective medium theory (EMT), where no topological configuration of the phases is assumed other than they mix in an isotropic manner. Our results suggests that, while hierarchical materials yield significant improvement of mass transfer in nanoprorous crystals, the magnitude of such improvement is heavily influenced by the connectivity and availability of the transport pores. If the mesopore volume fraction is below the percolation threshold, there will exist microporous regions in the crystals where access of adsorbate cannot occur through an adjacent mesopore, slowing transport to a point where the difference between the diffusivity in the purely microporous and the mesopores-enhanced crystal will differ by no more than one or 2 orders of magnitude. Molecular simulation is used to predict the diffusion coefficients of ethane and propene in the microporous zeolite, providing results that compare well with the experimental data as long as the correct combination of framework/force field is chosen. In this sense, molecular simulation methods and EMT can potentially be combined to theoretically investigate hierarchical materials