Department of Energy & Mineral Engineering, EMS Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Langmuir. 2011 Dec 6;27(23):14169-79. doi: 10.1021/la202842m. Epub 2011 Nov 1.
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.
某些微孔金属有机骨架（MMOFs）的有趣滞后吸附-解吸行为引起了相当大的关注，并且通常与开门（GO）效应有关。在这里，N(2)和 Ar 在 Zn(2)(bpdc)(2)(bpee)（bpdc=4,4'-联苯二甲酸酯；bpee=1,2-联吡啶乙烯）中的滞后吸附在 77 和 87 K 下表现出允许的实验时间的明显影响。当 N(2)在 77 K 下允许的时间为几分钟时，没有观察到吸附，而要达到相当大的吸附量，需要超过 60 小时的时间。在足够的时间内，N(2)和 Ar 的总吸收在相似的降低温度下收敛，但 Ar 的吸附速度明显快于 N(2)，这一观察结果可以用激活构象扩散来描述。N(2)和 Ar 都表现出具有显著滞后的不连续阶梯吸附等温线，这些特征取决于允许的时间。在 77 K 下，H(2)的吸收量大于 N(2)和 Ar，但在等温线中没有不连续性，滞后效应要小得多。N(2)和 Ar 的吸附数据可以用激活扩散过程来描述，特征时间导致 6.7 和 12 kJ/mol 的活化能。H(2)吸附数据的拟合导致在低覆盖度下的活化能在 2-7 kJ/mol 范围内，而在高覆盖度下为非活化扩散。提出了一种替代的浓度相关扩散模型来描述逐步吸附行为，该行为观察到 N(2)和 Ar，但未观察到 H(2)。在低温下，分子尺寸的孔吸附达到平衡非常缓慢，并且结构变化（开门）虽然可能发生，但不需要解释观察到的现象。
