Gravimetric, neutron scattering and computational studies of the sorption and diffusion of gases in zeolites
Zeolites are a class of aluminosilicate materials with many important industrial applications as catalysts, ion exchange materials and sorbents. A novel dynamic Monte Carlo program has been written that allows both the kinetic and equilibrium properties of zeolitic systems such as CaNa zeolite A and Na zeolite Y to be simulated. Many of the characteristic features of the systems, such as the concentration dependence and ratio of the diffusion coefficients are observed. This technique provides a valuable bridge between traditional Molecular Dynamics and Metropolis et al algorithm Monte Carlo simulations as the evolution of the system can be followed over much longer timescales than for molecular dynamics. Gravimetry has been used to study the adsorption and desorption of water on Na zeolite Y in the temperature range 100-190°C. Significant hysteresis was observed in all cases, probably caused by the movement of cations away from their original positions. The sorption kinetics were controlled by the transport of heat away from sample. The concentration dependence of the ratio of the chemical diffusion coefficient Dc to the tracer diffusion coefficient Dt is consistent with that for a double Langmuir model isotherm. The form of the isotherms and the concentration dependence of the molar entropy of the sorbed phase suggests that a change of state from a state consisting of molecules sorbed on fixed sites to a mobile, fluid-like state occurs. The co-diffusion of water and methane in has been studied using incoherent quasi-elastic neutron scattering (QENS). For the water molecules, it was possible to distinguish three distinct modes of motion: a fast short-range component within the zeolite cage, a slower long-range component and a rotational component. The rotational component was found to be an activated process. Using coherent QENS from deuterated methane, a lower limit was placed on the values of Dc and Dt for methane in Na zeolite Y at room temperature. The feasibility of using neutron spin-echo spectroscopy to study the diffusion of water in zeolites at low Q has been demonstrated and the tracer diffusion coefficient of water in CaNa zeolite A measured at 150°C. Neutron diffraction studies of deuterated methane in Na zeolite Y and deuterated water in Na zeolite Y have been carried out for a range of loadings and temperatures. The change in lattice parameter with loading was measured. There was evidence for the increasing delocalisation of methane with temperature in agreement with simulation. The shift in the location of cations in Na zeolite Y and CaNa zeolite A as a function of water loading was observed.
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