Molecular simulations of hydrogen and methane in inorganic molecular sieves composed of titanosilicates and zeolite ZSM-22.

摘要

Hydrogen as a fuel is a promising technology since its only combustion by-products are water and heat and it generates no pollution (greenhouse gases) as do hydrocarbon fuels. To develop this technology, hydrogen must be produced economically and it also must be stored safely in a manner that makes it easy to use.; Hydrogen can be produced by steam reforming and dry reforming but it has to be separated from methane and other species. Current separation technologies are very expensive and require extensive energy input. Nanoporous materials can be used in membranes to separate hydrogen from methane economically and to store hydrogen and methane at low pressures.; Zeolites are an important class of these nanoporous materials with pores 3--10 A in diameter, and have a sieving capacity which allows them to exclude certain molecules and adsorb others. Membranes fabricated from zeolitic materials are more stable than polymeric membranes. Membranes need to be stable in water and air, stable at high temperatures (>500°C) and mechanically strong to support transport and heat cycles.; This study examined three new materials called titanosilicates (Zorite, ETS-4 and ETS-10) in addition to a silica zeolite called ZSM-22. Our goal was to find which of these is the best material for hydrogen-methane separations. The criteria used to determine the best separation material was based on self-diffusion coefficients as well as pure and binary adsorption isotherms. We also determined molecular siting distributions for the adsorbates.; Based on Molecular Dynamics simulations (MD) and Grand Canonical Monte Carlo simulations (GCMC) it was found that hydrogen and methane can permeate faster in ETS-10 compared with the other materials, but the selectivity for hydrogen in ETS-10 is lower than in ETS-4 and Zorite. Hydrogen diffuses an order of magnitude faster in ETS-10 and has a higher saturation load of adsorbates in comparison to ETS-4, Zorite and ZSM-22. ETS-4 and Zorite would act as true molecular sieves, allowing only hydrogen to permeate but at the expense of lower hydrogen fluxes. The one-dimensional character of ZSM-22 renders it inappropriate to support high hydrogen permeation fluxes compared to ETS-4, ETS-10 and Zorite.