Water–Hydrophobic Zeolite Systems

Water intrusion–extrusion in hydrophobic microporous AFI, IFR, MTW and TON pure silica zeolites (zeosils) has been investigated through molecular dynamics (MD) simulations. It was found that intruded water volumes correlate with the free volume of the zeosil unit cells. Calculated adsorption isotherms allowed us to estimate the amounts of water intruded, and deviations from experiments (lower experimental with respect to calculated intrusion pressures) have been explained in terms of connectivity defects in the synthesized materials. Water phase transitions in defectless zeosils occur in a narrow range at high pressure. On the basis of a simple model, we derived a thermodynamic equation that allows one to estimate the intrusion pressure with few parameters, which are easy to obtain, such as fractional free volume of zeosil and the intrusion pressure of a reference system. The structural properties of water clusters inside the zeosil micropores have been interpreted from the analysis of the MD simulations. Compact “bulk-like” clusters form in large channels such as those in AFI and IFR zeosils. The smaller channels of MTW and TON promote the formation of chain-like clusters, which, interestingly, are commensurate with the zeolite channel topology due to a coincidence between the distances of the crystallographic parameter, along the channel, and a maximum in the O–O radial distribution function of bulk water.针对疏水微孔AFI、IFR、MTW及TON型纯硅沸石（zeosils）的水侵入-挤出行为，本研究通过分子动力学（MD）模拟开展了系统探究。研究结果表明，侵入水体的体积与沸石晶胞的自由体积存在显著相关性。本研究借助计算得到的吸附等温线估算了侵入水量，实验观测的侵入压力低于计算结果，该偏差可通过合成材料中存在的连接缺陷予以解释。无缺陷沸石内的水相转变会在高压下的窄压力区间内发生。基于简化物理模型，本研究推导得到一条热力学方程，该方程仅需少量易于获取的参数即可估算侵入压力，例如沸石的分数自由体积以及参考体系的侵入压力。对沸石微孔内水团簇的结构特性的分析可依托分子动力学模拟结果展开：在AFI与IFR型沸石的大尺寸孔道中，会形成紧凑的“类本体”水团簇；而MTW与TON型沸石的较小孔道则会促进链状水团簇的形成，值得注意的是，由于孔道方向的晶体学参数间距与本体水的O-O径向分布函数峰值间距恰好吻合，此类链状团簇与沸石孔道拓扑结构高度匹配。