MTBE adsorption and diffusion in silicalite-1

Vapor phase adsorption isotherms of methyl tertiary butyl ether (MTBE) in silicalite-1 at temperatures in the range of 298–600 K were measured using a gravimetric analysis system. A maximum saturation loading of 4 mol/u.c. was measured. The diffusion coefficients as a function of loading at each temperature were calculated from the differential adsorption uptake data. Diffusivities were observed to increase with loading up to about 1 mol/u.c., possibly due to significant adsorption on external surfaces of the particles. Above this value, diffusivities became independent of loading. Additionally, Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were carried out to estimate the adsorption isotherms and the diffusion coefficients, and the results were compared to those of the experiments. The GCMC simulation results were in reasonable agreement with the experimental data, especially at higher pressures and lower temperatures, where the contribution of interparticle and surface adsorption became marginal. From the MD simulations, MTBE molecules were seen to move very slowly inside the crystal since they spend time at the channel intersections. These simulations were also extended to loadings higher than 4 mol/u.c., in order to shed light on the higher adsorption capacities from aqueous solutions previously reported in the literature. The mobility of the MTBE molecules was observed to increase with loading beyond the value of 4 mol/u.c., which deserves further investigation.