Selectivities for binary mixtures of hydrogen/methane and hydrogen/carbon dioxide in silicalite and ETS-10 by Grand Canonical Monte Carlo techniques

Current environmental problems have created the need to search for cleaner fuels. Hydrogen has been proposed as a clean fuel because its combustion by-product is water. Unfortunately, hydrogen is not readily available in pure form. It is obtained from steam reforming (CH4 +2H2O→4H2 +CO2) and dry reforming (CH4 +CO2→2CO+2H2) [1] and has to be separated from methane, carbon dioxide and smaller amounts of other gases before the hydrogen can be used effectively in a fuel cell [2]. Current hydrogen separation membranes are made of palladium alloys or chemically and mechanically unstable organic polymer membranes [3]. Palladium membranes are costly and in limited supply. The use of zeolites and titanosilicate molecular sieves creates stable inorganic matrices and consequently stable membranes [3]. These nanoporous materials can be made into membranes that have the potential to separate hydrogen from methane.In this study the separation capabilities of silicalite and the titanosilicate molecularsieve ETS-10 for binary mixtures of hydrogen/methane and hydrogen/carbon dioxide wereevaluated by equilibrium molecular simulation techniques. This is the first molecularsimulation study that presents mixture adsorption isotherms of these components insilicalite and ETS-10, and determines selectivities based on the simulation results.Grand Canonical Monte Carlo (GCMC) simulations were carried out for pure componentsand binary mixtures for hydrogen/carbon dioxide and hydrogen/methane at 298K todetermine pure and mixture adsorption isotherms. The pure and mixture adsorptionisotherms were calculated up to pressures of approximately 2000 bar. The results of thisstudy indicate that the separation of hydrogen from methane or from carbon dioxide insilicalite would be successful, since hydrogen in a 50% bulk mixture does not adsorb unlessthe pressure is very high, on the order of 500 bar.In contrast, in ETS-10, hydrogen in a 50% bulk mixture adsorbs at a pressure near10 bar. Simulations of adsorption in ETS-10 showat low, intermediate and high pressures ahigher selectivity for the separation of carbon dioxide from hydrogen than the separation ofmethane from hydrogen. Simulations of adsorption in silicalite show a higher selectivity forthe separation of carbon dioxide from hydrogen than the methane/hydrogen separation athigh pressures only. Analysis of isosteric heat of adsorption information indicates thatsilicalite is energetically homogeneous with the adsorbates.In contrast, ETS-10 has energetic heterogeneity, as shown by the decrease of the heat ofadsorption with increasing loading. References[1] T.M. Nenoff, Defect-free thin film zeolite membranes for hydrogen separationand isolation, in: DOE/H2 Annual Review Meeting, May 19,2003.[2] S. Thomas, M. Zalbowitz, J. Cruz, Fuel Cells----Green Power, Los AlamosNational Laboratory, 1999.[3] M.E.Welk, T.M. Nenoff, F. Bonhomme, Studies in Surfaces and Catalysis,vol. 154, Elsevier B.V., 2004.