Crystal Structure of an Ethylene Sorption Complex of Fully Vacuum-Dehydrated Fully Ag~+-Exchanged Zeolite X (FAU).Silver Atoms Have Reduced Ethylene To Give CH_2~(2-) Carbanions at Framework Oxide Vacancies

摘要

The crystal structure of an ethylene sorption complex of fully vacuum-dehydrated fully Ag~+-exchanged zeolite X (FAU),a - 24.865(2) A,has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 at 21 deg C.It is very different from the ethylene complex of Ag_(92)-X that had been dehydrated at 400 deg C in flowing oxygen,as were the two dehydrated structures.The crystal was prepared by ion exchange in a flowing stream of aqueous 0.05 M AgNO_3 for 3 days,followed by dehydration at 400 deg C and 2 x 10~(-6) Torr for 2 days,followed by exposure to 300 Torr of zeolitically dry ethylene gas for 2 h at 21 deg C.The structure was determined in this atmosphere and was refined using all data to the final error indices (based upon the 534 reflections for which F_o>4a(F_o)) R_1=0.062 and wR_2=0.135.In this structure,per unit cell,14 Ag~+ ions were found at the octahedral site I (Ag-O=2.611(9) A),and 32 partially reduced Ag~+ ions fill two different site I' positions deep in the sodalite cavities (Ag-O=2.601(13) and 2.618(12) A).The sodalite cavities host two different cationic silver clusters.In about 47% of sodalite units,eight silver atoms form interpenetrating tetrahedra,Ag_8~(n+) (n=4 is suggested),with T_d symmetry.The other 53% of the sodalite units host cyclo-Ag_4~(m+) (m=2 is suggested) cations with near S_4 symmetry.These clusters are very similar to those in vacuum-dehydrated Ag_(92)-X.Thirty-two Ag~+ ions fill the single 6-rings,15 at site II' (Ag-O=2.492(10) A),and 17 at site II (Ag-O=2.460(9) A).The latter 17 lie in supercages where each forms a lateral pi-complex with an ethylene molecule.In turn,each C_2H_4 molecule forms two cis electrostatic hydrogen bonds to framework oxygens.The remaining 14 Ag~+ ions occupy three different II' sites.Vacuum dehydration had caused substantial decomposition:per unit cell,30 of the 92 Ag~+ ions were reduced and 15 of the 384 framework oxide ions were oxidized to O_2(g),leaving lattice vacancies.The sorption of C_2H_4 at 21 deg C reoxidized about 7 of the 30 Ag~0 atoms to Ag~+ and reduced 1.75 ethylene molecules to give CH_2~(2-) groups which refilled 3.5 of these 15 lattice vacancies.The remaining vacancies may have been filled with H_2C=C~(2-) ions.The unit cell formula,which originally contained 384 oxygen atoms,may be |Ag_(92)_(C_2H_4)_(17)|[Si_(100)Al_(92)O_(369)-(CH_2)_(3.5)] or |Ag_(92)H_(23)(C_2H_4)_(17)|[Si_(100)Al_(92)O_(369)(CH_2)_(3.5)(C_2H_2)_(11.5)].