From Natural Attapulgite to Mesoporous Materials: Methodology, Characterization and Structural Evolution

In this paper, we report the synthesis of hexagonally ordered aluminum-containing mesoporous silica, Al-MCM-41, from natural attapulgite (Al-substituted Si8O2Mg5(OH)(2)(H2O)(4)center dot 4H(2)O) without addition of silica or aluminum reagents. A pretreatment process involving sequential mechanical grinding and acid leaching is critical to the Successful use of attapulgite as a source of both Si and AI in the surfactant-templated hydrothermal synthesis of Al-MCM-41. The resulting mesophase had a surface area of 1030 m(2)/g and an average pore diameter of 3.7 nm with narrow pore size distribution. The influence of changes in processing parameters, such as grinding time, hydrothermal conditions, and calcination temperature, on the textural characteristics of the Al-MCM-41 products is studied. Investigations of the mechanism of structural evolution indicate that grinding of attapulgite results in amorphization and partial structural breakdown, transformation of the fibrous mineral bundles into rod-shaped particles, and partial displacement of octahedrally coordinated Al3+ ions into the Si-O tetrahedral framework. Subsequent acid etching dissolves the Mg-rich octahedral sheets to produce samples with variable texture due to modifications in the residual aluminum-containing silicate sheets and associated silica fragments. Solid-state magic-angle spinining NMR spectroscopy indicates that Al3+ ions are located in both octahedral and tetrahedral sites in the as-synthesized Al-MCM-41 samples, but that the calcined products consist primarily of Al3+ ions substituted in the tetrahedrally coordinated silica matrix of the ordered channel wall structure.