(675e) Adsorption of Gases on Cu-Btc and Cr-Bdc Frameworks

Adsorption characteristics of metal organic frameworks are strongly affected by their physical properties like pore volume and surface area. This creates discrepancy when one tries to compare the isotherms on adsorbent material obtained from different laboratories and/or synthesis procedure. The discrepancy makes it difficult to use the existing experimental data for applications like adsorptive separations or energy storage. The objective of this work is to relate physical properties of the adsorbent to its adsorption characteristics, for several gases.

Cu-BTC was synthesized using two different procedures, to obtain materials with widely different surface areas (1482 and 857 m²/g). A completely different frame work viz. Cr-BDC with a surface area of about 2900 m²/g was also synthesized. Isotherms for seven permanent gases viz. N₂, O₂, Ar, CH₄, CO₂, C₃H₈ and SF₆ were measured on these three materials at two different temperatures and for pressures up to 8 bar. The selectivity between N₂, O₂ and Ar was close to unity in all cases. Henry constants increase in the order C₃H₈ >SF₆ > CO₂ >N₂. C₃H₈ and SF₆ seem to approach their monolayer coverage values. While CO₂ had the highest capacity on Cu-BTC, capacity for C₃H₈ was highest on Cr-BDC framework.

The isotherms on the two Cu-BTC samples can be readily related to one another through use of a scaling factor. This scaling factor is not unique but is a function of loading; however, it is independent of nature of the gas, temperature or pressure.