Crystal structure analyses of two TMA silicates with ordered defects: RUB-20, a layered zeolite precursor, and RUB-22, a microporous framework silicate

Abstract

Abstract Synthesis experiments in the simple system TMAOH/SiO2/H2O led at 160°C to the crystallization of two new tetramethylammonium (TMA) silicates, RUB-20 and RUB-22. The common feature of the two silicates is the presence of ordered defects within the silicate network, necessary to compensate the charge of the occluded TMA cation. RUB-20, [(CH3)4N]1.7[Si18O35(OH)4], is a layered material with a0 = 10.493(2) Å, b0 = 13.988(2) Å, c0 = 7.411(1) Å, β = 97.80(2)° and space group P21/m. It consists of ferrierite type (fer) silicate layers which possess a significant amount of defects. These defects apply only to one particular Si–O–Si bridge which is hydrolyzed to about 50% to form two additional Si–OH/Si–O- groups which point to the interlayer region of the structure. These silanol groups together with the regular terminal silanol groups of neighbouring fer layers form a system of hydrogen bonds which binds the layers together. In addition, weak ionic bonds exist between the negatively charged silicate layers and the intercalated TMA cations. RUB-22 is a zeolite-like silicate possessing a microporous framework with double-cages, housing the TMA cations. The Rietveld structure analysis revealed a pseudo monoclinic unit cell with a0 = 13.6626(2) Å, b0 = 13.2400(2) Å, c0 = 12.3009(2) Å, α = 90.003(3)°, β = 112.700(1)°, γ = 90.005(3) and space group C 1 ¯ . $C\bar 1.$ RUB-22, [(CH3)4N]4[Si32O60(OH)12], possesses an interrupted silica framework of the same topology as RUB-10, [(CH3)4N]4[Si32B4O72] (zeolite framework type RUT), however, with the difference that defects replace the [BO4]-tetrahedra. The defects can be regarded as “hydroxyl nests” surrounding a vacancy □(OH-Si)3(-O-Si) instead of B(-O-Si)4 groups. Heating the as made materials at 600°C led in the case of RUB-20 to a highly disordered zeolite of the CDO type and in the case of RUB-22 to a distorted and defective RUT-type material.