Models of adsorption in zeolites and molecular sieves

The distribution of small adsorbates within the nanopores of zeolites and molecular sieves depends upon both the steric properties (pore size and shape) and the chemical properties of the adsorbent (the charge distribution among the framework atoms). Using both computer simulation and NMR experiment, this research has focused on the effects of steric properties - how the unique size and shape of the pore space in each zeolite or molecular sieve influences the placement energetics, and motion of adsorbates. Using computer simulations of simple models for the adsorbate and smooth adsorbent, we have deemed a low-loading regime of adsorption where the placement of the adsorbates is dictated by the energetics of the external pore potential. We have established the roles that pore size and shape play in adsorption in this regime. In the more interesting high-loading regime, the placement of adsorbates is not dictated by energetics but is rather primarily dominated by adsorbate packing. We have established the roles for pore size and shape in this regime as well and they are different than in the low-loading regime. We have applied the insights gained in the simple, smooth pores to pores with atomistic detail, more closely resembling real nanoporous materials.more » In the low-loading regime, cornputationally inexpensive potential energy maps of the pore space reveal the number, arrangement, and depth of adsorption sites, the accessible volume, and site-to-site activation energies. With knowledge of the lattice of adsorption sites, we have constructed models for diffusion, percolation, and adsorption in zeolite-A. Also, we have used simulations to demonstrate and explain binary adsorption selectivities as well as to predict the change in chemical shift of {sup 129}Xe with loading in several zeolites and molecular sieves.« less