Hydrogen adsorption over Zeolite-like MOF materials modified by ion exchange

Novel porous Zeolite-like metal-organic framework (ZMOF) materials with Rho and Sod topologies are promising adsorbents for hydrogen storage due to their high surface area and, more importantly, to their capacity of being ion-exchanged, potentially changing their affinity for hydrogen. In this work, we have successfully synthesized both Rho and SodZMOF materials, optimizing experimental conditions for scaling-up the procedure already published to produce grams of material. The resultant materials were alkalinecation-exchanged, widely characterized and finally tested as hydrogen adsorbents. RhoZMOF is converted into an amorphous phase during some of the ion-exchange processes, whereas SodZMOF, whose ion-exchange capacity has not been investigated so far, always maintains its topology for any tested exchange cation and conditions. Additionally, thermogravimetric analyses and thermal treatments followed by in-situ powder X-ray diffraction analysis have evidenced a significantly higher thermal stability of both as-prepared and ion-exchanged SodZMOF materials in comparison to their Rho-structured homologues. Moreover, the thermal stability of the cation-exchanged ZMOF samples improves when methanol is the ion-exchange solvent rather than the reported ethanol-water mixture. Nitrogen and hydrogen adsorption isotherms at 77 K suggested that alkali-exchanged materials have lower affinity for hydrogen than the as-prepared samples compensated by imidazolium ion; however, due to the smaller size of Na+ or Li+ cations, their lower affinity is easily compensated by the inherent increase in surface area and pore volume as exchange degree increases. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.