Crystal Structure of Fully Dehydrated Partially Cs+-Exchanged Zeolite X,Cs52Na40-X (The Highest Cs+-Exchanged Level Achieved by Conventional Method and Confirmation of Special Site Selectivity)

摘要

The crystal structure of fully dehydrated partially Cs+-exchanged zeolite X,[Cs52Na40Si100Al92O384],a = 24.9765(10) A,has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 at 21 °C.The crystal was prepared by flow method for 5 days using exchange solution in which mole ratio of CsOH and GsNO3 was 1 : 1 with total concentration of 0.05 M.The crystal was then dehydrated at 400 °C and 2 x 10~(-6) Torr for 2 days.The structure was refined to the final error indices,R1 = 0.051 and wR2 (based on F2) = 0.094 with 247 reflections for which F_o> 4 sigma(F_o).In this structure,about fifty-two Cs+ ions per unit cell are located at six different crystallographic sites with special selectivity;about one Cs+ ion is located at site I,at the centers of double oxygen-rings (D6Rs),two Cs+ ions are located at site I',and six Cs+ ions are found at site II'.This is contrary to common view that Cs+ ions cannot pass sodalite cavities nor D6Rs because six-ring entrances are too small.Ring-opening by the formation of -OH groups and ring-flexing make Cs+ ions at sites I,I',and II' enter six-oxygen rings.The defects of zeolite frameworks also give enough mobility to Cs+ ions to enter sodalite cavities and D6Rs.Another six Cs+ ions are found at site n,thirty-six are located at site III,and one is located at site HI' in the supercage,respectively.Forty Na+ ions per unit cell are located at two different crystallographic sites;about fourteen are located at site I,the centers of D6Rs and twenty-six are also located at site II in the supercage.Cs+ ions and Na+ ions at site II are recessed ca.0.34(1) A and 1.91(1) A into the supercage,respectively.In this work,the highest exchange level of Cs+ ions per unit cell was achieved in zeolite X by conventional aqueous solution methods and it was also shown that Cs+ ion could pass through the six-oxygen rings.