On the Porous Silicate HPM-5

Abstract

HPM-5, a porous aluminosilicate with an intricate and so far unsolved structure, has been synthesized by using 1,2,3-trimethylimidazolium in hydroxide medium in the absence of fluoride anions. It consistently displays an ill-defined XRD pattern with broad and overlapped reflections and contains a large concentration of defects, as evidenced by IR and 29Si MAS NMR spectroscopy. 1H MAS NMR spectroscopy confirmed the existence of highly deshielded protons (delta = -11.6 ppm) assigned to hydrogen atoms in relatively strong Si-O-H---O-Si hydrogen bonds (O---O distance = 2.64 A) The calcined material shows limited microporosity (0.07-0.09 cm3 g-1). The results of a high-resolution transmission electron microscopy study suggest HPM-5 may be far more complicated than "standard" layered zeolites and may consist of low-density layers profusely decorated by holes. The resulting lace-like layers may be too unstable to withstand reactions characteristic of more conventional layered materials (both delamination and expansion by silylation); although the derivatives show an increase in the specific surface area, the layers are apparently disassembled and, at most, only ribbons remain.