Resistance to the transport of H2 through the external surface of as-made and modified silicalite-1 (MFI)

The adsorption and the thermal desorption of hydrogen in as-made and surface-modified silicalite-1 (MFI) loose crystals has been studied to shed light on the relation between the outer surface barrier and the transport of mass through it. The four different surface modifications can be probed by the change in the contact angle of water, but do not produce any apparent change in the morphology and the structure of the crystals. The H2 adsorption isotherms in a Sievert's-type apparatus at 77 K and up to 8 MPa on the pristine and the modified silicalite-1 samples are almost overlapping, showing that the modifications involve the external surface of the samples. However the diffusion coefficients of H2, as derived from the sorption kinetics, evidence different sorption rates of H2 into the different silicalite-1 samples. In particular the unmodified silicalite-1 sample presents the lowest diffusion coefficient and yields the highest H2 desorption temperature in Thermal Desorption Spectra acquired at 0.03 K/s. The observed phenomena have been attributed to the surface silanols on the outer surface of the as-made silicalite-1 crystals, which are either eliminated or greatly reduced in number by the modifying agents. Polar and hydrophilic silanol groups bind frozen water molecules on the outer surface of as-made crystals which hinder the diffusion of hydrogen, and interact more strongly with the H2 molecules. This provides sorption sites with higher potential energy barriers, and at the same time physically obstructs the pore entrances of the molecular sieve. The surface modification of porous fillers is of interest for the manufacture of mixed matrix membranes, for the improvement of the performance of pressure swing adsorption processes, and for gas storage applications.