A novel pillared aluminum ethylene diphosphonate, showing reversible dehydration-rehydration

The novel aluminum ethylene diphosphonate Al-2(OH)(2)(H2O)(2)(O3PCH2CH2PO3) has been synthesized hydrothermally.. It crystallizes in the triclinic space group (P (1) over bar; a = 8.5977(10), b = 7.0499(3), c = 9.5290(4) Angstrom; alpha = 89.997(21), beta = 104.321(8), gamma = 114.225(11)degrees; V = 506.98(8)Angstrom(3)) with aluminophosphonate layers similar to those found in Al(OH)(H2O)CH3PO3 linked via ethylene (-CH2CH2-) groups. The solid loses water upon heating in two steps, the first at 100 degreesC and the second at 360 degreesC. The first water loss leaves a crystalline solid with a framework structure of Al-2(OH)(2)(O3PCH2CH2PO3) closely related to that of the parent (C2/ c; a = 15.3723(21), b = 6.7613(8), c = 9.7374(13) Angstrom; beta = 102.392(9)degrees; V = 988.4438 Angstrom(3)) whereas the second water loss results in major loss of crystallinity. X-ray powder diffraction has been used to determine and refine the structures of the parent and intermediate dehydrated phases. The main structural difference is the loss of one water molecule bound to aluminum, which changes the metal cation's coordination from octahedral to distorted square planar. The as-prepared and dehydrated samples have been simulated computationally. The transformation has been monitored by in-situ Al-27 MAS NMR spectroscopy; octahedral aluminum (delta(iso) = -7.1 ppm; asymmetry parameter, eta = 1.0; quadrupolar coupling constant, QCC = 5.4 MHz) is converted to 5-fold coordinated aluminum (delta(iso) = 20.7 ppm; eta = 0; QCC = 5.8 MHz) by heating at 130 degreesC for 1 h. The process is fully reversible.