Abstract
We present results of grand ensemble Monte Carlo simulation studies of the adsorption of nitrogen in Ca-chabazite containing two Ca ions per unit cell. The probable Ca ion locations were taken from previous work, and the simulations were made in relaxed but rigid framework structures. Ca ion sites included combination of A-sites (Ca in the hexagonal face of a D6R unit) and B-sites (Ca on one of the mutually orthogonal axes passing through the chabazite cavity. Interaction potentials were constructed on an all-atom basis, and were parameterised for the N–O, N–Si and N–Ca pairs with the aid of supplementary quantum mechanical calculations [Mol. Phys. 28 (2000) 1565]. At 77K, the zero coverage heat of adsorption (qst(0)) for nitrogen in the pure silica form is 15.9kJmol−1. When two A-site Ca cations per unit cell are included, qst(0) shows a large increase to 41.1kJmol−1. Analysis of the components of qst(0) gives induced, electrostatic and dispersive contributions of around 48, 50 and 2%, respectively. When loading is increased, the heat of adsorption decreases as a consequence of the screening of the strongly binding Ca-sites. Adsorption isotherms for a range of different Ca cation sites were found to have marked steps due to the heterogeneous nature of the material, where only a limited number of nitrogen molecules can bind very strongly to the cations. Experimental data for N2 and Ar data at 303K are qualitatively similar to the simulated isotherms at the same temperature, but the adsorption is weaker.