Jameson C J, Lim H M, Jameson A K
Department of Chemistry, University of Illinois at Chicago 60607-7061, USA.
Solid State Nucl Magn Reson. 1997 Dec;9(2-4):277-301. doi: 10.1016/s0926-2040(97)00063-5.
We consider the role of polarization in the adsorption of Xe in zeolites of type A by direct comparative analysis of the adsorption isotherms, distributions of occupancies, and 129Xe NMR chemical shifts of Xe(n) in cages containing CaxNa12-2x ions per alpha cage (x = 0, 1, 2, 3, 5). We find that the qualitative trends in the adsorption isotherms, and in the progressions of Xe(n) chemical shifts (for n = 0-8 in cages with x = 0, 1 Ca2+ ions and for n = 0-5 in cages with x = 2, 3 Ca2+ ions) upon increasing the level of Ca2+ ion for Na+ ion substitution could only be accounted for by including polarization of the Xe atom by the zeolite framework and its ions. This system, which permits observation of individual Xe(n) peaks and of directly comparable adsorption isotherms in several cage types, provides a good model system for the interpretation of the more general case in which only the overall average 129Xe NMR chemical shift is observed in open network zeolites, arising from free exchange of Xe among cavities of variable occupancy and variable cation distribution.
通过对吸附等温线、占有率分布以及每个α笼中含有CaxNa12 - 2x离子（x = 0, 1, 2, 3, 5）的笼中Xe(n)的129Xe NMR化学位移进行直接对比分析，我们研究了极化在A型沸石中Xe吸附过程中的作用。我们发现，随着Ca2+离子取代Na+离子水平的增加，吸附等温线以及Xe(n)化学位移的变化趋势（对于x = 0、1个Ca2+离子的笼中n = 0 - 8，以及对于x = 2、3个Ca2+离子的笼中n = 0 - 5），只有通过考虑沸石骨架及其离子对Xe原子的极化才能得到解释。这个系统允许观察几种笼型中单个的Xe(n)峰以及直接可比的吸附等温线，为解释更一般的情况提供了一个良好的模型系统。在开放网络沸石中，由于Xe在占有率和阳离子分布可变的腔之间自由交换，只能观察到整体平均的129Xe NMR化学位移。
