A crystallographic study of Sr~(2+) and K~+ ion-exchanged zeolite Y (FAU, Si/Al = 1.56) from binary solution with different mole ratio of Sr~(2+) and K~+

摘要

To study the -Sr2+-ion selectivity of zeolite Y (Si/Al = 1.56) in binary solution with different -Sr2+ and -K+ concentration during exchange, two single- crystals of fully dehydrated, -Sr2+- and -K+-exchanged zeolites Y were prepared by the flow method using a mixed ion-exchange solution whose Sr(NO3)2:KNO3 mol ratios were 1:1 (crystal 1) and 1:100 (crystal 2), respectively, with a total concentration of 0.05 M, followed by vacuum dehydration at 723 K. Their crystal structures were determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group Fd3m, respectively, and were refined to the final error indices R1/wR2 = 0.0429/0.1437 and 0.0356/0.1239 for crystals 1 and 2, respectively. In the structure of |Sr28 K19|[Si117Al75O384]-FAU (crystal 1), 28 -Sr2+ ions per unit cell occupy four different crystallographically distinct sites; 15, 2, 2, and 9 are at sites I, I', II', and II, respectively, whereas, the -K+ ions occupy only one site: 19 are at site II. In the structure of |Sr17K41|[Si117Al75O384]-FAU (crystal 2), 17 -Sr2+ ions per unit cell occupy three equipoints; 10.5, 2, and 4.5 are at sites I, I', and II, respectively. The residual 41 K+ ions per unit cell are found at four different sites; 8, 24, 3, and 6 are at sites I', II, III'a, and III'b, respectively. The degrees of ion exchange are 74.7 and 45.3% for crystals 1 and 2, respectively. This result shows that the degree of -Sr2+ exchange decreased sharply by decreasing the initial -Sr2+ concentration and increasing the initial -K+ concentration in the given ion-exchange solution.