Gas Storage and Diffusion through Nanocages and Windows in Porous Metal-Organic Framework Cu2(2,3,5,6-tetramethylbenzene-1,4-diisophthalate)(H2O)2

A novel nanoporous metal organic framework NPC-4 with excellent thermal stability was assembled from 2,3,5,6-tetramethylbenzene-1,4-diisophthalate (TMBDI) and the paddle-wheel secondary building unit (Cu-2(COO)(4)). The porous structure comprises a single type of nanoscale cage (16 angstrom diameter) interconnected by windows (5.2 X 6.3 angstrom), which give a high pore volume. CH4 (195-290 K), CO2 (198-303 K), N-2 (77 K), and H-2 (77 K) adsorption isotherms were studied for pressures up to 20 bar. NPC-4 exhibits excellent methane and carbon dioxide storage capacities on a volume basis with very high adsorbate densities, under ambient conditions. Isobars were investigated to establish the relationship for adsorption capacities over a range of storage temperatures. The isosteric enthalpies of adsorption for both CH4 and CO2 adsorption did not vary significantly with amount adsorbed and were similar to 15 and similar to 25 kJ mol(-1), respectively. The adsorption/desorption kinetics for CH4 and CO2 were investigated and activation energies, enthalpies of activation, and diffusion parameters determined using various kinetic models. The activation energies for adsorption obtained over a range of uptakes from the stretched exponential kinetic model were 5.1-6.3 kJ mol(-1) (2-13.5 mmol g(-1)) for CO2 and 2.7-5.6 kJ mol(-1) (2-9 mmol g(-1)) for CH4. The activation energies for surface barriers and diffusion along pores for both CH4 and CO2 adsorption obtained from a combined barrier resistance diffusion model did not vary markedly with amount adsorbed and were <9 kJ mol(-1). Comparison of kinetic and thermodynamic parameters for CH4 and CO2 indicates that a surface barrier is rate determining at high uptakes, while intrapartide diffusion involving diffusion through pores, consisting of narrow windows interconnecting with nanocages, being rate determining at very low uptakes. The faster CH4 intraparticle adsorption kinetics compared with CO2 for NPC-4 was attributed to faster surface diffusion due to the lower isosteric enthalpy of adsorption for CH4.