Aluminum 1,4-benzenedicarboxylate Al(OH)[O(2)C-C(6)H(4)-CO(2)]. [HO(2)C-C(6)H(4)-CO(2)H](0.70) or MIL-53 as (Al) has been hydrothermally synthesized by heating a mixture of aluminum nitrate, 1,4-benzenedicarboxylic acid, and water, for three days at 220 degrees C. Its 3 D framework is built up of infinite trans chains of corner-sharing AlO(4)(OH)(2) octahedra. The chains are interconnected by the 1,4-benzenedicarboxylate groups, creating 1 D rhombic-shaped tunnels. Disordered 1,4-benzenedicarboxylic acid molecules are trapped inside these tunnels. Their evacuation upon heating, between 275 and 420 degrees C, leads to a nanoporous open-framework (MIL-53 ht (Al) or Al(OH)[O(2)C-C(6)H(4)-CO(2)]) with empty pores of diameter 8.5 A. This solid exhibits a Langmuir surface area of 1590(1) m(2)g(-1) together with a remarkable thermal stability, since it starts to decompose only at 500 degrees C. At room temperature, the solid reversibly absorbs water in its tunnels, causing a very large breathing effect and shrinkage of the pores. Analysis of the hydration process by solid-state NMR ((1)H, (13)C, (27)Al) has clearly indicated that the trapped water molecules interact with the carboxylate groups through hydrogen bonds, but do not affect the hydroxyl species bridging the aluminum atoms. The hydrogen bonds between water and the oxygen atoms of the framework are responsible for the contraction of the rhombic channels. The structures of the three forms have been determined by means of powder X-ray diffraction analysis. Crystal data for MIL-53 as (Al) are as follows: orthorhombic system, Pnma (no. 62), a = 17.129(2), b = 6.628(1), c = 12.182(1) A; for MIL-53 ht (Al), orthorhombic system, Imma (no. 74), a = 6.608(1), b = 16.675(3), c = 12.813(2) A; for MIL-53 lt (Al), monoclinic system, Cc (no. 9), a = 19.513(2), b = 7.612(1), c = 6.576(1) A, beta = 104.24(1) degrees.

The structure of AlPO4-CJ2 aluminophosphate has been reinvestigated by MAS, MQMAS (multiple quantum magic angle spinning), CP-MQMAS (cross polarization-MQMAS), and HETCOR (heteronuclear correlation) NMR spectroscopy. The CP-MQMAS method showed that the sample, when not allowed adequate time for crystallization, included a substantial concentration of amorphous species. The crystalline component was cleanly singled out by this technique. We discuss the relative populations and the distributions of F and OH groups within the structural building units (SBUs) and the distribution of various SBUs within the crystalline structure of AlPO4-CJ2. For the latter case, a mixture of SBUs is demonstrated from the 19F → 31P HETCOR spectrum. Prenucleation building units (PNBUs) are the principal objects used for efficient crystal formation. They may undergo an isomerization to reach the proper configuration to integrate into the solid network. The isomerization implies formation of a bridge within the PNBU, probably at random with respect to OH or F. As integration in the network is taking place parallel to isomerization, the network forms as domains containing a mixture of the different types of SBUs.

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