Effect of Time, Temperature, and Kinetics on the Hysteretic Adsorption鈥揇esorption of H2, Ar, and N2 in the Metal鈥揙rganic Framework Zn2(bpdc)2(bpee)

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• 作者：Sarmishtha Sircar ; Haohan Wu ; Jing Li ; Angela D. Lueking
• 刊名：Langmuir
• 出版年：2011
• 出版时间：December 6, 2011
• 年：2011
• 卷：27
• 期：23
• 页码：14169-14179
• 全文大小：1056K
• 年卷期：v.27,no.23(December 6, 2011)
• ISSN：1520-5827

文摘

The intriguing hysteretic adsorption鈥揹esorption behavior of certain microporous metal鈥搊rganic frameworks (MMOFs) has received considerable attention and is often associated with a gate-opening (GO) effect. Here, the hysteretic adsorption of N₂ and Ar to Zn₂(bpdc)₂(bpee) (bpdc = 4,4鈥?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 N₂ 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 N₂ and Ar converged at similar reduced temperatures, but the adsorption of Ar was significantly more rapid than that of N₂, an observation that can be described by activated configurational diffusion. N₂ and Ar both exhibited discontinuous stepped adsorption isotherms with significant hysteresis, features that were dependent upon the allowed time. The uptake of H₂ at 77 K was greater than for both N₂ 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 H₂ adsorption data led to activation energies in the range 2鈥? 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 N₂ and Ar but not for H₂. 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.