Layered double hydroxides: I. Synthesis by design. II. Catalysis by intercalated polyoxometalates

Layered double hydroxides (LDHs) have two predominant properties which can be exploited for materials applications. First, the hydroxylated lamellar framework can possesses the desired characteristics, as in their use as antacids. In the second approach, the exchangeable intergallery anions can be chosen for a specific function, with the framework serving merely as an inert support. The oxidation of alkenes by tungstate immobilized on an LDH intercalated with hydrophobic anions is an example which uses both the basicity of the LDH to enhance the selectivity of the epoxide over the diol product and the hydrophobic gallery anions to create a favorable environment for the organic substrata. 1; Taking advantage of the hydroxylated lamellar structure of the LDH intercalate, a method for making thin LDH films for application as sensors, devices, and flame retardant materials was developed. LDHs were synthesized in organic solvents, resulting in colloidal sized LDH particles which formed well oriented films less than a Micron thick, as judged by powder XRD and SEM studies. With this method, films with varying layer charge, intergallery anions, and framework cations are easily synthesized.; Catalysis by intercalated polyoxometalates (POMs) is an example of the second approach. Polyoxometalates are a promising class of anionic catalysts for intercalation into LDHs and much research has been invested In introducing gallery functionality by intercalation of the catalyst into LDH layers.; However, there are a few problems which complicate the LDH-POM chemistry. First, the synthesis of an LDH-POM is always accompanied by an impurity phase which results from the reaction of the basic LDHs with acidic polyoxometalates. This problem was to be addressed by using a more acidic Zn₂Al-LDH, which can be synthesized at a pH of 6.2. The second problem arose from discrepancies in the reported d spacing for otherwise identical polyoxometalate intercalates, namely 9.9 and 12.2 Å in the case of LDH-Mo₇O₂₄ 6−, and 12.2 and 14.6 Å for LDH-H₂W₁₂ O₄₀6− intercalates. In this case, Raman spectroscopy was used to unambiguously identify the molybdenum polyoxometalate intercalate. Finally, size selective catalysis implies pillaring of the gallery anions. Nitrogen adsorption was used to measure the pore volume of the LDH-POM systems. The catalytic activity of four catalytic systems, Mg₂Al-Mo ₇O₂₄6−, Mg₂Al-H₂W ₁₂O₄₀6−, and Zn₂Al-Mo ₇O₂₄6−, and Zn₂Al-H ₂W₁₂O₄₀6− was used for the peroxide oxygenation of cyclohexene to the corresponding epoxide and diol. However, our results indicate that only the surface bound anions are catalyticaly active and that the selectivity is a result of the basic nature of the LDH support.2,3 In addition, the impurity salt may contribute to the catalytic activity. The acidity of the LDH support is not the determining factor In the formation of the impurity salt, but rather the affinity of the POM or the layer cations.; 1Sels B. F., de Vos D. E., Jacobs P. A., Tet. Lett, 1996, 37(47), 8557. 2Gard E. A. and Pinnavaia T. J., Appl. Catal. A: General, 1998,167, 65. 3Gardner E. A., Yun S., and Pinnavaia T. J., Appl. Clay Sci., in press.