Crystal structure of dehydrated CsZSM-5 (5.8A1): Evidence for nonrandom aluminum distribution

The crystal structure of dehydrated CS5.8Al5.8Si90.2O192 has been determined by Rietveld refinement of synchrotron powder diffraction data. The structure was refined in the Pnma space group to R-wp = 2.52% and R(F-2) = 5.32%. Cs ions were found with partial occupancy in three different positions. One Of these, Cs1, is in the channel intersection, near the sinusoidal channel 10-ring system and adjacent to a four-membered ring. The second, Cs2, is in the straight channel 10-ring. The third, Cs3, is 3.18 Angstrom from the Cs1 and also in the sinusoidal channel 10-ring area. The total Cs occupancy is 5.32(3) Cs ions, which compares well with the Cs unit cell content of 5.8 calculated from the chemical analysis. Modeling of the effect of aluminum siting on the position of Cs+ ions in the zeolite channel indicates that the Cs' ion has its closest interaction with an oxygen atom attached to the aluminum atom Tor Al substitution at each of the 12 ZSM-5 T-sites. From this result we conclude that Csi ion positioning provides information on aluminum atom distribution. The T sites 4. 7, 10, 11, and 12 have Cs-O distances less than similar to 3.55 Angstrom, indicating that they are possible sites for Al atoms. The oxygen atoms attached to T sites 2, 8, and 9 have no Cs-O distances shorter than 3.90 Angstrom, suggesting that there is a low probability that these sites contain Al atoms.