Synthesis, Characterization And Properties Of Luminescent Microporous Lanthanide Silicates

Microporous materials with regular pore architectures, typically known as zeolites, constitute an important area of materials science because of their widespread applications in catalysis, separation, and host-guest assemblies. In recent decades, microporous silicates hosting lanthanide elements have attracted considerable attention because such materials have high thermal stability, uniform microporosity and tunable optical property that may have many important applications. A number of lanthanide silicates were prepared under high-temperature and high-pressure hydrothermal conditions to search for fast alkali ion. Recently, luminescent microporous lanthanide silicate materials containing stoichiometric amounts of lanthanide (Ln) in the frameworks have been successfully prepared under mild hydrothermal conditions in the presence of alkali metal cations. Notably, Rocha and co-workers first reported AV-5 (Ln=Ce³⁺) and AV-9 (Ln=Eu³⁺, Tb³⁺, Er³⁺) that are synthetic analogues of mineral montregianite, and AV-20 (Ln = Eu, Tb, Sm and Ce) that is closely related to hydrated calcium silicate minerals, known as tobermorites. Tsapatsis and co-workers reported Na_4.8Ce₂Si₁₂O₃0·4H₂O consisting of corrugated [Si₂O₅^2-]_∞silicate layers with 5-, 8-rings connected by CeO6. These microporous lanthanide silicate materials exhibit characteristic luminescent properties of lanthanide.This dissertation is focused on the systematic syntheses, structural characterizations and properties of a series of novel microporous lanthanide silicates with diverse framework structures and interesting luminescent properties. These materials are prepared under mild hydrothermal conditions.The main results in this dissertation are summarized as follows:1. A novel luminescent microporous terbium silicate Na₃TbSi₃O₉·3H₂O (denoted TbSiO-CJ1) have been successfully synthesized. TbSiO-CJ1 crystallizes in the enantiomorphic space group P2₁2₁2₁. Its structure is featured by aΛ-Co(en)₃³⁺-like chiral unit of [TbSi₆O₉], and contains helical sechser (six) silicate chains and 9-ring channels. The photoluminescent property of TbSiO-CJ1 has been also studied. The green ⁵D₄→⁷F₅ transitions of Tb³⁺ in TbSiO-CJ1 at 543 and 550 nm are the strongest, whereas the luminescence from higher excited states (e.g., ⁵D₃) is not detectable, implying very efficient nonradiative relaxation to the 5D4 level. The fluorescence decay curves of the ⁵D₄→⁷F₅ transitions for TbSiO-CJ1, excited at 266 nm, are well-fitted by a single-exponential function, yielding a lifetime value of 3.11 ms. This suggests the presence of a single Tb3+ environment in agreement with the single-crystal structure of TbSiO-CJ1.2. LnSiO-CJn (n = 2-5) have been synthesized from M₂O-SiO₂-Ln₂O₃-H₂O (M=K, Na, Ln = Ce, Eu, Tb, Nd, Sm, Gd, Dy, Er) system under mild hydrothermal conditions. Single-crystal structural analyses show that various lanthanide elements, have been successfully introduced into the frameworks of LnSiO-CJn (n = 2-5) with stoichiometric amounts. The framework of LnSiO-CJn (n = 2-5) consists of silicate single layers that are connected by six-coordinated LnO6 octahedra or seven-coordinated LnO₇ polyhedra to form a three dimensional (3D) open framework with 8-ring channels, in which Na+ ions and water molecules reside. In LnSiO-CJ2, the connection of SiO₄ tetrahedra results in a novel corrugated [Si₂O₅]_n^2n- silicate layer containing 4-, 5-, 6-, 8-rings parallel to the bc plane, denoted the 4, 5, 6, 8-net , in which each SiO₄ tetrahedron shares three oxygen atoms with other SiO₄ tetrahedra and the fourth oxygen atom is pointing up and down the sheet. EuSiO-CJ3 and EuSiO-CJ4 are both featured by [Si₂O₅]_n^2n- silica layers with 4, 6, 8-net, which is further connected by corner-sharing EuO₆ octahedra to form a 3D open framework structure. However, it is noted that the orientation of SiO₄ tetrahedra around the 4, 6, and 8-rings in the 4, 6, 8-net in each framework is different. Differently from LnSiO-CJn (n = 2-4), the connection of SiO4 tetrahedra results in a corrugated [Si₆O₁₆]_n^8n- silicate layer containing 6-, 8-, and 12-rings, denoted the 6, 8, 12-net. LnSiO-CJ2, CJ3, CJ4, and CJ5 are all very stable, whose intact crystalline structures can be kept up to 650,700 800,and 1000 oC, respectively. The photoluminescent properties of LnSiO- CJn (n = 2-5), have been studied. Their emission spectra are characteristic for lanthanide, and the lifetime measurements are consistent with the crystallographic data.3. LnSiO-CJ6 (Ln=Tb, Eu) have been synthesized from Na₂O-SiO₂-Ln₂O₃-H₂O system under mild hydrothermal conditions at 240℃. TbSiO-CJ6 crystallizes in the tetragonal noncentrosymmetric space group I-4. The structure of TbSiO-CJ6 is composed of macroanionic [TbSiO₄]^- framework, and Na+ ions residing in the channels compensate the negative charge of the framework. Its structure is built up from SiO4 tetrahedra and TbO₇(OH) polyhedra that share edges and/or corners to form an open-framework structure. The photoluminescent properties of LnSiO- CJ6 (Ln=Eu,Tb) have been studied in detail. LnSiO-CJ6 is stable up to 1000℃.To sum up, a series of alkali-metal-containing luminescent microporous lanthanide silicates have been synthesized under mild hydrothermal conditions. Various lanthanide elements have been introduced into the frameworks with stoichiometric amounts. These lanthanide silicates are composed of single silicate chains (LnSiO-CJ1), single silicate layers (LnSiO-CJ2, CJ3, CJ4, and CJ5) or singular tetrahedra of SiO4 (LnSiO-CJ6) interconnected by Ln-centered polyhedra (LnO6, LnO7 or LnO8) to form interesting 3-D open frameworks. Their structures contain 8-ring or 9-ring channels in which alkali ions (Na⁺ or K⁺), NO₃^- ions and water molecules reside. These materials show interesting photoluminescence properties. In addition, they also show ion-exchange capacity, ionic conductivity and magnetic properties. Further detailed investigation on their properties is under the way.Note: all formula for CJn TbSiO-CJ1 (Na₃TbSi₃O₉·3H₂O),CeSiO-CJ2 (Na2.4CeSi₆O₁₅·2.6H₂O),EuSiO-CJ2 (Na₂KEuSi₆O₁₅·2H₂O),EuSiO-CJ3 (K₃EuSi₆O₁₅·H₂O),EuSiO-CJ4 (K₃EuSi₆O₁₅·3H₂O),EuSiO-CJ5A (K₈Eu₃Si₁₂O₃₂(NO₃)·H₂O),EuSiO-CJ5B (K₈Eu₃Si₁₂O₃₂(OH)·H₂O),TbSiO-CJ6 (NaTbSiO₄·0.5H₂O),EuSiO-CJ6 (NaEuSiO₄·0.5H₂O)...