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

The structures of the fully exchanged, fully dehydrated zeolites Tl-ZSM-5 (a = 20.064(1), b = 19.946(1), c = 13.416(1) angstrom) and H-ZSM-5 (a = 20.079(1), b = 19.948(1), c = 13.419(1) angstrom) have been determined crystallographically using modestly twinned single crystals and synchrotron X-radiation. They were refined in the orthorhombic space group Pnma to R-1 = 0.071 and 0.066 with 3962 and 4278 reflections, respectively, for which F-o > 4 sigma(F-o). 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 among 18 crystallographically distinct positions. Based on their positions near R1, R2, and R3, these positions may be classified into three groups (Gn, n = 1-3), each with two general cation sites (Sn and Sn'). Of the 18 Tl+ positions, six are in the intersection volume (at S2' in G2), 11 are elsewhere in the ZC (in G1 and G3), and one is elsewhere in the SC (at S2) just outside the intersection volume. Among the six general cation sites, S1' at the mouth of the cove has the highest Tl+ population, 28% of the total. Of the 18 Tl+ positions, 14 bond strongly to framework oxygen atoms with 2.68(4) < Tl-O < 2.87(5) angstrom (Tl+ + O2- radii = 2.79 angstrom); three other shortest distances range upward to 2.97(5) angstrom; one at 3.18(10) angstrom appears to be an unresolved position. The framework sites (T) that Al atoms preferentially occupy are identified from Tl+ occupancies and Tl-O bond distances. Using Tl-O < 2.70 angstrom, the order is T10 (most preferred) > T6. The preference of Al for the T10 site remains when longer, less reliable but more comprehensive, Tl-O bond length criteria are used. Using Tl-O < 2.80 angstrom, the order is T10 > T6 similar to T9 > T5 > T4 > T2 > T12 > T7 > T11, and with Tl-O < 2.90 angstrom, the preference order includes all 12 T sites: T10 > T3 > T2 > T12 > T6 similar to T9 > T5 > T4 > T8 > T7 > T11 > T1. T10 is a member of R4, the 10-ring that is common to the intersection volume and the cove.