Preparation and crystal structure of RUB-18 modified for synthesis of zeolite RWR by topotactic conversion

Abstract The systematic study on the synthetic route of the siliceous zeolite RWR with a small 8-MR straight channel by topotactic conversion of crystalline layered silicate RUB-18 was carried out by using several topotactic precursors obtained from Na-RUB-18. Intermediate layered structures of topotactic precursors were designed by intercalation of amine molecules and acid treatment. Crystal structures of two kinds of TMAOH intercalated RUB-18s (TMA-RUB-18-poly1 and TMA-RUB-18-poly2) and their acidified TMA-RUB-18 were determined by ab initio structure analysis using X-ray powder diffraction data. The layered structure of TMA-RUB-18-poly1 and TMA-RUB-18-poly2 had tetragonal symmetry of space group P 4 2 / nmc , and the lattice parameters were estimated to be a  = 7.4121 A, c  = 22.615 A for TMA-RUB-18-poly1 and a  = 7.3718 A, c  = 29.342 A for TMA-RUB-18-poly2. Two silicate layers were involved in a unit-cell and TMA + cations were intercalated in the interlayer. The layer stacking sequence was completely different from that of Na-RUB-18. Structure of acidified TMA-RUB-18 had also tetragonal symmetry of space group I 4 1 / amd and lattice parameters a  = 7.4722 A, c  = 37.242 A with four silica layers. Stacking sequence in acidified TMA-RUB-18 was similar to that of Na-RUB-18 although a large shrinking of interlayer distance (ca. 2.0 A) was observed. In the acidified TMA-RUB-18, one-dimensional pseudo pore was constructed by semi-circular geometry derived from the framework structure and stabilized by formation of hydrogen bonding between the terminal silanol groups, which faced each other with atomic distance d (O–O) of ca. 2.5 A. By the dehydration–condensation of silanol groups during careful calcination, zeolite RWR could be successfully prepared from the acidified TMA-RUB-18.