Krishna Rajamani, van Baten Jasper M
Van't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
ACS Omega. 2021 Jun 2;6(23):15499-15513. doi: 10.1021/acsomega.1c02136. eCollection 2021 Jun 15.
Microporous crystalline adsorbents such as zeolites and metal-organic frameworks (MOFs) have potential use in a wide variety of separation applications. The adsorption selectivity is a key metric that quantifies the efficacy of any microporous adsorbent in mixture separations. The Ideal Adsorbed Solution Theory (IAST) is commonly used for estimating the value of , with unary isotherms of the constituent guests as data inputs. There are two basic tenets underlying the development of the IAST. The first tenet mandates a homogeneous distribution of adsorbates within the pore landscape. The second tenet requires the surface area occupied by a guest molecule in the mixture to be the same as that for the corresponding pure component. Configurational-bias Monte Carlo (CBMC) simulations are employed in this article to highlight several scenarios in which the IAST fails to provide a quantitatively correct description of mixture adsorption equilibrium due to a failure to conform to either of the two tenets underpinning the IAST. For CO capture with cation-exchanged zeolites and MOFs with open metal sites, there is congregation of CO around the cations and unsaturated metal atoms, resulting in failure of the IAST due to an inhomogeneous distribution of adsorbates in the pore space. Thermodynamic non-idealities also arise due to the preferential location of CO molecules at the window regions of 8-ring zeolites such as DDR and CHA or within pockets of MOR and AFX zeolites. Thermodynamic non-idealities are evidenced for water/alcohol mixtures due to molecular clustering engendered by hydrogen bonding. It is also demonstrated that thermodynamic non-idealities can be strong enough to cause selectivity reversals, which are not anticipated by the IAST.
微孔晶体吸附剂，如沸石和金属有机框架材料（MOFs），在各种各样的分离应用中具有潜在用途。吸附选择性是量化任何微孔吸附剂在混合物分离中效能的关键指标。理想吸附溶液理论（IAST）通常用于估算该值，将各组成客体的一元等温线作为数据输入。IAST的发展有两个基本原理。第一个原理要求吸附质在孔道内均匀分布。第二个原理要求混合物中客体分子占据的表面积与相应纯组分的相同。本文采用构型偏置蒙特卡罗（CBMC）模拟来突出几种情况，即由于不符合支撑IAST的两个原理中的任何一个，IAST无法对混合物吸附平衡提供定量正确的描述。对于用阳离子交换沸石和具有开放金属位点的MOF进行CO捕集，CO会在阳离子和不饱和金属原子周围聚集，导致孔空间内吸附质分布不均匀，从而使IAST失效。由于CO分子优先位于8元环沸石（如DDR和CHA）的窗口区域或MOR和AFX沸石的口袋内，也会出现热力学非理想性。对于水/醇混合物，由于氢键导致的分子聚集，热力学非理想性也很明显。还证明了热力学非理想性可能足够强，导致选择性反转，这是IAST所无法预测的。
