Structure Analysis of Si-Atom Pillared Lamellar Silicates Having Micropore Structure by Powder X-ray Diffraction

Four kinds of Si-atom pillared lamellar silicates (APZ-1−APZ-4) were prepared by thermal acid treatment in an autoclave at 443 K for 1−24 h using aqueous HCl. Layered silicates PLS-1, PLS-3, PLS-4, and PREFER, which consist of ferrierite layer sheets, could be converted into new open-framework microporous materials by pillaring the interlayers with SiO2(−OH)2 fragments. The fragments probably originated from the collapse of the crystalline structure, and interlayer silylation occurred with the migration of the fragments (dissolution−condensation). The four APZ crystal structures were investigated by means of powder X-ray diffraction, revealing topotactic microporous structures that derived from each original layered silicate. APZ-2 and APZ-4 have the large pore openings of a 12-membered ring, whereas APZ-1 and APZ-3 have the slightly smaller pore openings of a 10-membered ring. All the APZs are hydrophilic to approximately the same degree as the common hydrophilic zeolite, owing to hydroxyl groups of the Si atom pillar or the terminal silanols in the interlayer. Thus, large numbers of water molecules are adsorbed in the micropores. 29Si MAS NMR spectra showed Q2, Q3, and Q4 resonance peaks, whose Q2 signal indicated silanol groups of the Si atom pillar. All the APZ crystals showed high thermal stability and did not collapse upon heating to at least 873 K.