Detailed Determination of the Tl+ Positions in Zeolite Tl−ZSM-5. Single-Crystal Structures of Fully Dehydrated Tl−ZSM-5 and H−ZSM-5 (MFI, Si/Al = 29). Additional Evidence for a Nonrandom Distribution of Framework Aluminum

Abstract

The structures of the fully exchanged, fully dehydrated zeolites Tl−ZSM-5 (a = 20.064(1), b = 19.946(1), c = 13.416(1) A) and H−ZSM-5 (a = 20.079(1), b = 19.948(1), c = 13.419(1) A) have been determined crystallographically using modestly twinned single crystals and synchrotron X-radiation. They were refined in the orthorhombic space group Pnma to R1 = 0.071 and 0.066 with 3962 and 4278 reflections, respectively, for which Fo > 4σ(Fo). The geometry of the zeolite framework in both crystals (MFI, Al3.2Si92.8O192, Si/Al = 29) is very similar to that in earlier determinations. The ZSM-5 framework and Tl+ positions are described using the four nonequivalent partially overlapping 10-rings (R), the two channel systems (C), the channel intersection volume, and the cove of the ZC. The latter two comprise the conventionally defined cavity. The zigzag (sinusoidal) channel (ZC) passes through R1, R3, and R4; the straight channel (SC) passes through R2. A total of 3.3(2) Tl+ ions per unit cell were found scattered am...