Effect of time, temperature, and kinetics on the hysteretic adsorption-desorption of H_2, Ar, and N_2 in the metal-organic framework Zn_2(bpdc)_2(bpee)

摘要

The intriguing hysteretic adsorption-desorption behavior of certain microporous metal-organic frameworks (MMOFs) has received considerable attention and is often associated with a gate-opening (GO) effect. Here, the hysteretic adsorption of N_2 and Ar to Zn_2(bpdc)_2(bpee) (bpdc = 4,4′-biphenyldicarboxylate; bpee = 1,2-bipyridylethene) shows a pronounced effect of allowed experimental time at 77 and 87 K. When the time allowed is on the order of minutes for N_2 at 77 K, no adsorption is observed, whereas times in excess of 60 h is required to achieve appreciable adsorption up to a limiting total coverage. Given sufficient time, the total uptake for N_2 and Ar converged at similar reduced temperatures, but the adsorption of Ar was significantly more rapid than that of N_2, an observation that can be described by activated configurational diffusion. N_2 and Ar both exhibited discontinuous stepped adsorption isotherms with significant hysteresis, features that were dependent upon the allowed time. The uptake of H_2 at 77 K was greater than for both N_2 and Ar but showed no discontinuity in the isotherm, and hysteretic effects were much less pronounced. N2 and Ar adsorption data can be described by an activated diffusion process, with characteristic times leading to activation energies of 6.7 and 12 kJ/mol. Fits of H_2 adsorption data led to activation energies in the range 2-7 kJ/mol at low coverage and nonactivated diffusion at higher coverage. An alternate concentration-dependent diffusion model is presented to describe the stepwise adsorption behavior, which is observed for N_2 and Ar but not for H_2. Equilibrium is approached very slowly for adsorption to molecularly sized pores at low temperature, and structural change (gate opening), although it may occur, is not required to explain the observations.