A new microporous 12-ring zincosilicate THK-2 with many terminal silanols characterized by automated electron diffraction tomography

Abstract

A newly synthesized microporous zincosilicate THK-2 (estimated structural composition: |(H₂O)_6.7(C₆H₁₃N)_0.9|[Li_0.5Zn_3.1Si₃₂O_62.7(OH)_9.3]) was characterized by single-crystal electron diffraction using the automated electron diffraction tomography (ADT) approach in combination with powder X-ray diffraction. The lattice constants and space group of as-synthesized THK-2 were a = 2.50377(7) nm, b = 1.43866(4) nm, c = 0.505369(8) nm, and Pccn (no. 56) with orthorhombic symmetry. Because the crystal lattice was almost identical to a hexagonal lattice (), the first several peaks in its powder X-ray diffraction data severely overlapped, which suppressed the structural information to decide the framework topology. In order to overcome this intrinsic difficulty, the structure model of THK-2 was initially obtained by the direct method based on ADT data and refined by the Rietveld method. Its 3-dimensional framework structure was elucidated and it consisted of 4-, 5-, 6-rings of tetrahedral Si and Zn atoms and a one-dimensional straight channel with a 12-ring pore opening. Zn atoms were incorporated into the framework as four-coordinated [ZnO₄], although their distribution was confirmed to be disorderly. In the as-synthesized THK-2, the site occupancy of Zn was as low as 0.39; that is, more than 60% of the Zn sites were vacant. Hexamethyleneimine and water molecules were accommodated in the straight channel in a disordered manner. The material was stable upon calcination, and the BET specific surface area and micropore volume of calcined THK-2 were 240.6 m² g⁻¹ and 0.12 ml g⁻¹, respectively.