Adsorption dynamics of gases and vapors on the nanoporous metal organic framework material Ni-2(4,4 '-bipyridine)(3)(NO3)(4): Guest modification of host sorption behavior

This study combines measurements of the thermodynamics and kinetics of gues t sorption with powder X-ray diffraction measurements of the nanoporous met al organic framework adsorbent (host) at different adsorptive (guest) loadi ngs, The adsorption characteristics of nitrogen, argon, carbon dioxide, nit rous oxide and ethanol and methanol vapors on Ni-2(4,4'-bipyridine)(3)(NO3) (4) were studied over a range of temperatures as a function of pressure. Is otherm steps were observed for both carbon dioxide and nitrous, oxide adsor ption at similar to 10-20% of the total pore volume and at similar to 70% o f total pore volume for-methanol adsorption. The adsorption kinetics obey a linear driving force (LDF) mass transfer model for adsorption at low surfa ce coverage. At high surface coverage, both methanol and ethanol adsorption follow a combined barrier resistance/diffusion model. The rates of adsorpt ion in the region of both the carbon dioxide and methanol isotherm steps we re significantly slower than those observed either before or after the step . X-ray diffraction studies at various methanol loadings showed that the ho st structure disordered initially but underwent a structural change in the region of the isotherm step. These isotherm steps are ascribed to discrete structural changes in the host adsorbent, that are induced by adsorption on different sites. Isotherm steps were not observed for ethanol adsorption, which followed a Langmuir isotherm. Previous X-ray crystallography studies have shown that all the sites are equivalent for ethanol adsorption on Ni-2 (4,4'-bipyridine)(3)(NO3)(4), with the host structure undergoing a scissori ng motion and the space group remaining unchanged during adsorption. The ac tivation energies and preexponential factors for methanol and ethanol adsor ption were calculated for each pressure increment at which the linear drivi ng force model was obeyed. There was a good correlation between activation, energy and ln(preexponential factor), indicating a compensation effect. Th e results are discussed in terms of reversible adsorbate/adsorbent (guest/h ost) structural changes and interactions and the adsorption mechanism. The paper contains the first evidence of specific interactions between guests a nd functional groups leading to. structural change in flexible porous coord ination polymer frameworks.