National Energy Technology Laboratory, United States Department of Energy, P.O. Box 10940, Pittsburgh, PA 15236, USA.
J Colloid Interface Sci. 2013 Mar 1;393:278-85. doi: 10.1016/j.jcis.2012.10.050. Epub 2012 Nov 2.
Mixed matrix membranes (MMMs) comprised of metal organic frameworks (MOFs) dispersed in organic polymers are popular materials under study for potential applications in gas separations. However, research on MMMs containing structurally dynamic sorbents known as flexible MOFs has only very recently appeared in the literature. The thermodynamic requirements of the structure transition between the low porosity and high porosity phases of flexible MOFs may provide a mechanism for high adsorption selectivity in these materials. A fundamental question in MMMs containing flexible MOFs is how the constraint of the polymer matrix on the intrinsic expansion of the flexible MOF particles that occurs during gas adsorption might affect the thermodynamics of this structural phase transition and influence the gas adsorption properties of the embedded MOF. To investigate the fundamental nature of this flexible MOF-polymer interface, thin films of ~20 um thickness were prepared using the flexible linear chain coordination polymer catena-bis(dibenzoylmethanato)-(4,4'bipyridyl)nickel(II) "Ni(Bpy)(DBM)(2)" embedded as 35 wt% dispersions in Matrimid®, polystyrene, and polysulfone. The adsorption of CO(2) in the polymers and embedded particles was studied using in situ ATR-FTIR spectroscopy and variable temperature volumetric CO(2) adsorption/desorption isotherms. Interestingly, no effect of the polymer matrix on the gas adsorption behavior of the embedded Ni(Bpy)(DBM)(2) particles was observed. The composite samples all showed the same threshold pressures for CO(2) absorption and desorption hysteresis associated with the structural phase change in the polymer embedded Ni(Bpy)(DBM)(2) particles as was observed in the pristine polycrystalline sample. The current results contrast those recently reported for a MMM containing the flexible MOF "NH(2)-MIL-53" where a significant increase in the threshold pressure for CO(2) adsorption associated with the structural phase change of the MOF was observed in the MMM as compared to the isolated MOF. The conflicting behaviors in these two systems are rationalized from the large differences in unit cell expansions between the two MOFs during the CO(2) adsorption process.
混合基质膜（MMM）由金属有机骨架（MOF）分散在有机聚合物中组成，是潜在气体分离应用的研究热门材料。然而，最近才在文献中出现了关于含有结构动态吸附剂的 MMM 的研究，这些吸附剂被称为柔性 MOF。柔性 MOF 的低孔隙率和高孔隙率相之间结构转变的热力学要求可能为这些材料中的高吸附选择性提供了一种机制。在含有柔性 MOF 的 MMM 中，一个基本问题是聚合物基质对气体吸附过程中柔性 MOF 颗粒固有膨胀的约束如何影响这种结构相变的热力学，并影响嵌入 MOF 的气体吸附性能。为了研究这种柔性 MOF-聚合物界面的基本性质，使用柔性线性链配位聚合物 catena-bis(dibenzoylmethanato)-(4,4'bipyridyl)nickel(II) "Ni(Bpy)(DBM)(2)" 作为 35wt%的分散体，在 Matrimid®、聚苯乙烯和聚砜中制备了约 20µm 厚的薄膜。使用原位 ATR-FTIR 光谱法和变温体积 CO2 吸附/解吸等温线研究了聚合物和嵌入颗粒中 CO2 的吸附。有趣的是，没有观察到聚合物基质对嵌入 Ni(Bpy)(DBM)(2)颗粒气体吸附行为的影响。复合样品都表现出相同的 CO2 吸收和解吸滞后的阈值压力，这与聚合物嵌入 Ni(Bpy)(DBM)(2)颗粒中结构相变化相关，与纯多晶样品中观察到的情况相同。目前的结果与最近报道的含有柔性 MOF "NH2-MIL-53"的 MMM 形成对比，在 MMM 中观察到与 MOF 结构相变化相关的 CO2 吸附的阈值压力显著增加，而在孤立的 MOF 中则没有。从这两种 MOF 在 CO2 吸附过程中单元胞膨胀的巨大差异，对这两种系统中的矛盾行为进行了合理化解释。
