Synthesis and crystal structure of layered silicate PLS-3 and PLS-4 as a topotactic zeolite precursor

Abstract

New layered silicates PLS-3 and PLS-4 are synthesized by a solid-state reaction at 170 °C for 24 h using a protonated form of layered silicate kanemite as a silica source. Tetramethylammonium and diethyldimethylammonium hydroxides were used as structure directing agent for PLS-3 and PLS-4, respectively. Their crystal structures were analyzed by means of powder X-ray diffraction (XRD) and solid-state MAS NMR spectroscopy techniques. The crystal structure of PLS-3 (space group (S.G.) Pnnm) is similar to that of layered silicate PREFER although the interlayer distance is smaller (ca. 1.4 Å) than that of PREFER. On the other hand, PLS-4 (S.G. P2₁/c) is isostructural to layered silicate RUB-36. Both frameworks are composed of ferrierite layers, but the stacking sequences of the layers are different. Two kinds of SDA molecules are disorderly distributed in the interlayer of each layered silicate. ¹H MAS NMR spectroscopy elucidated that neighboring ferrierite layers are stabilized by strong hydrogen bonding (SiO–H⋯OSi) in both PLSs. The topotactic conversion of PLS-3 and PLS-4 to siliceous FER- and CDO-type zeolites, respectively, occurs via a dehydration-condensation reaction.