Framework Sites Preferred by Aluminum in Zeolite ZSM-5. Structure of a Fully Dehydrated, Fully Cs+-Exchanged ZSM-5 Crystal (MFI, Si/Al = 24)

Abstract

The structure of a single crystal of fully Cs+-exchanged, fully dehydrated zeolite ZSM-5 (MFI, Cs3.8Al3.8Si92.2O192, Si/Al = 24) has been determined crystallographically using synchrotron X-radiation in the orthorhombic space group Pnma (a = 20.070(1), b = 19.913(1), c = 13.389(1) Å) and refined to R1 = 0.076 with 5443 reflections. A total of 3.5(2) Cs+ ions were found per unit cell scattered among 11 crystallographically distinct positions. Three are within the intersection volume (the volume that is both in the zigzag channel (ZC) and in the straight channel (SC)), two are in the cove at the side of the intersection volume, one is in the 10-ring that connects two intersection volumes, four lie elsewhere in 10-rings at the surface of the intersection volume, and one is in the unique segment of the ZC that is not a part of any of the above. The two most highly populated positions, with a total occupancy of 1.09(12) Cs+ ions per unit cell, are in the cove at the side of the intersection volume. The preference of Al atoms among the T sites of the ZSM-5 framework is identified from Cs+ occupancies and Cs–O bond lengths. Using all such bonds shorter than 3.35 Å, the preferred order for Al among the T sites appears to be T7 (most preferred) > T10 > T8 > T2. T7 is in the cove and on the wall of the intersection volume; T10 is in the cove and on the wall of just the ZC. A small Al presence is seen at T11, some may be present at T12, but there is no indication that Al atoms are present at T1, T3, T4, T5, T6, nor T9, fully half of the T positions in the crystal studied.