New 3D Zeolite Single Crystal Containing Si–O–B Bonds

Yu Han, University of Technology, Guangzhou, China, Xinxiong Li, Fuzhou University, China, Qichun Zhang, City University of Hong Kong, China, and colleagues have synthesized a 3D metal-free inorganic framework (H₂en)[Si(B₄O₉)], called CityU-11 (pictured), containing Si–O–B bonds. H₂en denotes ethanediamine, C₂H₄(NH₂)₂. The team studied the structure and physical properties using single-crystal X-ray diffraction (SCXRD), low-dose high-transmission electron microscopy (LD-HRTEM), ultraviolet-visible absorption spectroscopy, thermal gravity analysis (TGA), etc.

The single crystals of CityU-11 were grown by a mild solvothermal method, where the solvent is a mixture of mesitylene, 1,4-dioxane, and ethylenediamine. The raw materials, including SiO₂, H₃BO₃, and a tiny amount of NH₄F, were added to a Pyrex tube. The tube was degassed and flame-sealed under liquid nitrogen before reacting at 150 °C for five days.

CityU-11 has well-ordered Si⁴⁺ and B–O building blocks and belongs to the noncentrosymmetric space group of Pnn2 (the orthorhombic system). Its asymmetric unit contains one [B₄O₉]^6– cluster (labeled as B₄), one Si⁴⁺ cation, and two halves of (H₂en)²⁺ cations. Si is tetrahedrally coordinated to O atoms. The B₄ cluster is formed by connecting two BO₃ triangles and two BO₄ tetrahedra. Each B₄ cluster connects to eleven other B₄ clusters through four neighboring SiO₄ tetrahedra and vice versa. Thus, the B₄ clusters and SiO₄ tetrahedra are bridged by corner-sharing to form a [Si(B₄O₉)]_n^2n– anionic zeolitic framework with one-dimensional channels (diameter ∼5 Å) along the b-axis. Viewed along the c-axis, the framework shows straight channels with 8-membered rings and 11-membered rings. The framework shows a crb topology when B₄ and SiO₄ tetrahedra are simplified to 4-connected nodes, respectively. Two independent (H₂en)²⁺ cations are in two types of channels, connected to the anionic framework via H-bonding interactions.

CityU-11 has an interesting optical property with a moderate birefringence of 0.0258@550 nm. According to the researchers, their work provides valuable guidelines for the design and preparation of new borosilicate zeolites and their relevant applications.