Yang Gang, Zhou Lijun
College of Resources and Environment &Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, 400715, Chongqing, China.
Sci Rep. 2014 Oct 13;4:6594. doi: 10.1038/srep06594.
Defects are often considered as the active sites for chemical reactions. Here a variety of defects in zeolites are used to stabilize zwitterionic glycine that is not self-stable in gas phase; in addition, effects of acidic strengths and zeolite channels on zwitterionic stabilization are demonstrated. Glycine zwitterions can be stabilized by all these defects and energetically prefer to canonical structures over Al and Ga Lewis acidic sites rather than Ti Lewis acidic site, silanol and titanol hydroxyls. For titanol (Ti-OH), glycine interacts with framework Ti and hydroxyl sites competitively, and the former with Lewis acidity predominates. The transformations from canonical to zwitterionic glycine are obviously more facile over Al and Ga Lewis acidic sites than over Ti Lewis acidic site, titanol and silanol hydroxyls. Charge transfers that generally increase with adsorption energies are found to largely decide the zwitterionic stabilization effects. Zeolite channels play a significant role during the stabilization process. In absence of zeolite channels, canonical structures predominate for all defects; glycine zwitterions remain stable over Al and Ga Lewis acidic sites and only with synergy of H-bonding interactions can exist over Ti Lewis acidic site, while automatically transform to canonical structures over silanol and titanol hydroxyls.
缺陷通常被认为是化学反应的活性位点。本文利用沸石中的多种缺陷来稳定在气相中不稳定的两性离子甘氨酸；此外，还展示了酸性强度和沸石通道对两性离子稳定化的影响。甘氨酸两性离子可通过所有这些缺陷得以稳定，且在能量上相比于铝和镓的路易斯酸位点，更倾向于标准结构，而非钛的路易斯酸位点、硅醇和钛醇羟基。对于钛醇（Ti-OH），甘氨酸与骨架钛和羟基位点存在竞争性相互作用，且前者的路易斯酸性占主导。从标准甘氨酸向两性离子甘氨酸的转变在铝和镓的路易斯酸位点上明显比在钛的路易斯酸位点、钛醇和硅醇羟基上更容易实现。发现通常随吸附能增加的电荷转移在很大程度上决定了两性离子的稳定化效果。沸石通道在稳定化过程中起着重要作用。在没有沸石通道的情况下，所有缺陷的标准结构占主导；甘氨酸两性离子在铝和镓的路易斯酸位点上保持稳定，而在钛的路易斯酸位点上仅在氢键相互作用的协同作用下才能存在，同时在硅醇和钛醇羟基上会自动转变为标准结构。
