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

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 P42/nmc, and the lattice parameters were estimated to be a = 7.4121 Å, c = 22.615 Å for TMA-RUB-18-poly1 and a = 7.3718 Å, c = 29.342 Å 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 I41/amd and lattice parameters a = 7.4722 Å, c = 37.242 Å 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 Å) 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 Å. By the dehydration–condensation of silanol groups during careful calcination, zeolite RWR could be successfully prepared from the acidified TMA-RUB-18.