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)

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. N ₂ 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.