School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive NW, Atlanta, Georgia 30332, United States.
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive North West, Atlanta, Georgia 30332-0100, United States.
J Phys Chem B. 2022 Aug 11;126(31):5920-5930. doi: 10.1021/acs.jpcb.2c01928. Epub 2022 Aug 3.
Vapor-phase infiltration, a postpolymerization modification process, has demonstrated the ability to create organic-inorganic hybrid membranes with excellent stability in organic solvents while maintaining critical membrane properties of high permeability and selectivity. However, the chemical reaction pathways that occur during VPI and their implications on the hybrid membrane stability are poorly understood. This paper combines quartz crystal microbalance gravimetry (QCM) and chemical characterization with first-principles simulations at the atomic scale to study each processing step in the infiltration of polymer of intrinsic microporosity 1 (PIM-1) with trimethylaluminum (TMA) and its co-reaction with water vapor. Building upon results from QCM experiments and SEM/EDX, which find TMA remains within PIM-1 even under long desorption times, density functional theory (DFT) simulations identify that an energetically stable coordination forms between the metal-organic precursor and PIM-1's nitrile functional group during the precursor exposure step of VPI. In the subsequent water vapor exposure step, the system undergoes a series of exothermic reactions to form the final hybrid membrane. DFT simulations indicate that these reaction pathways result in aluminum oxyhydroxide species consistent with XPS and FTIR characterization. Both NMR and DFT simulations suggest that the final aluminum structure is primarily 6-fold coordinated and that the aluminum is at least dimerized, if not further "polymerized". According to the simulations, coordination of the aluminum with at least one nitrile group from the PIM-1 appears to weaken significantly as the final inorganic structure emerges but remains present to enable the formation of the 6-fold coordination species. Water molecules are proposed to complete the coordination complex without further increasing the aluminum's oxidation state. This study provides new insights into the infiltration process and the chemical structure of the final hybrid membrane including support for the possible mechanism of solvent stability.
气相渗透是一种后聚合改性过程，它具有在有机溶剂中创造具有优异稳定性的有机-无机杂化膜的能力，同时保持高渗透性和选择性等关键膜性能。然而，气相渗透过程中发生的化学反应途径及其对杂化膜稳定性的影响还知之甚少。本文结合石英晶体微天平（QCM）和化学特性分析，以及原子尺度的第一性原理模拟，研究了聚集体微孔聚合物 1（PIM-1）与三甲基铝（TMA）的渗透过程中的每个处理步骤，以及其与水蒸气的共同反应。基于 QCM 实验和 SEM/EDX 的结果，这些结果发现 TMA 即使在长时间的解吸时间下仍保留在 PIM-1 中，密度泛函理论（DFT）模拟确定，在气相渗透的前体暴露步骤中，金属有机前体与 PIM-1 的腈基官能团之间形成了一种能量稳定的配位。在随后的水蒸气暴露步骤中，该体系经历了一系列放热反应，形成最终的杂化膜。DFT 模拟表明，这些反应途径导致形成最终杂化膜的铝氧氢氧化物物种与 XPS 和 FTIR 特性一致。NMR 和 DFT 模拟都表明，最终的铝结构主要是 6 配位的，并且铝至少是二聚的，如果不是进一步“聚合”的话。根据模拟结果，铝与 PIM-1 中至少一个腈基基团的配位似乎在最终无机结构出现时显著减弱，但仍然存在，以允许形成 6 配位物种。水分子被提出在不进一步增加铝的氧化态的情况下完成配位络合物。本研究提供了对渗透过程和最终杂化膜的化学结构的新见解，包括对溶剂稳定性可能机制的支持。
