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

The crystal structure of dehydrated CS5.8Al5.8Si90.2O192 has been determine d by Rietveld refinement of synchrotron powder diffraction data. The struct ure 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. On e Of these, Cs1, is in the channel intersection, near the sinusoidal channe l 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 aluminu m 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 th is result we conclude that Csi ion positioning provides information on alum inum atom distribution. The T sites 4. 7, 10, 11, and 12 have Cs-O distance s less than similar to 3.55 Angstrom, indicating that they are possible sit es for Al atoms. The oxygen atoms attached to T sites 2, 8, and 9 have no C s-O distances shorter than 3.90 Angstrom, suggesting that there is a low pr obability that these sites contain Al atoms.