STEIN RESEARCH GROUP
POROUS SOLIDS — NANOCOMPOSITES — SELF-ASSEMBLED FRAMEWORKS
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Mesoporous Sieve Synthesis Based on Polyoxometalate Ions
(Including New Inorganic Anion-Exchange Materials)
A method developed in our lab for the synthesis of new mesoporous materials consists of pre-arranging metal oxide clusters in a surfactant salt and transforming the structure into a mesoporous product by introducing bridging groups between the cluster elements. This novel method permits the creation of certain mesoporous structures that can sometimes not be obtained by a direct hydrothermal syntheses. We were able to synthesize ordered mesoporous metallophosphates that displayed anion-exchange capabilities, properties which are rare among inorganic solids. The precursors for the non-lamellar mesoporous aluminophosphates and galloaluminophosphates, AlO4Al12(OH)24(H2O)127+ Al13 and GaO4Al12(OH)24(H2O)127+ (GaAl12) polyoxocations, were clusters which formed a layered mesoscopic salt with the anionic surfactant sodium dodecylsulfate. Upon reaction with a buffered hydrogenphosphate/dihydrogenphosphate solution the layers of clusters were transformed into a non-layered, nearly hexagonal mesoscopic phase. FT-IR, 27Al and 31P MAS NMR, as well as 27Al/31P cross-polarization experiments indicated that the phosphate treatment led to the formation of new bonds between aluminate and phosphate species. Removal of the surfactant from the alumino- and galloaluminophosphates by anion-exchange resulted in mesoporous materials with BET surface areas up to 630 and 455 m2/g, respectively. Subsequent anion-exchange was possible in these open mesoporous structures, e.g., with acetate, dichromate or napthol yellow S ions. Up to this point, very few inorganic anion-exchange materials had been synthesized. Anion-exchange capacities for chromate and several monoanionic and dianionic organic dyes fell in the range from approximately 1.3 - 1.6 meq/g. Higher uptake was observed for a dye which was capable of forming dimeric or polymeric aggregates. Control experiments with the neutral support MCM-41 and a nonionic dye indicated that anion adsorption in meso-AlPO was caused predominantly by electrostatic interactions rather than pure physisorption. The material exhibited some size-selectivity for anionic dyes. Multiple exchanges were possible if the support was regenerated by acid treatment at pH 4.3. The thermal stability of meso-AlPO exceeded that of typical ion exchange resins; thermal treatment of meso-AlPO did not lead to loss of the mesopore structure until the temperature exceeded 200 deg. C.
Linkage of Al13 clusters was also possible with silicate bridges, resulting in a less ordered mesoporous product with a surface area of 431 m2/g. Al/Si ratios, higher than in typical aluminosilicates could be achieved in this material. The high aluminum content was facilitated by the presence of octahedral aluminum in addition to tetrahedral aluminum. Small angle neutron scattering experiments were carried out on these materials at Argonne National Laboratories. Direct condensation by calcination of polyoxoaluminate cluster salts without additional linkers produced nanometer-sized one-dimensional strings.