Study of Hydrothermal Stability and Water Sorption Characteristics of 3-Dimensional Zn-Based Trimesate

Abstract

Zinc trimesate material with high hydrothermal stability was studied for water adsorption/desorption dynamics, and we explained the reason for material’s stability in water. A zinc trimesate Zn2(BTC)(OH)(H2O)·1.67H2O with the three-dimensional framework contains [Zn2O6(OH)(H2O)] chains with ZnO2(OH)2 tetrahedra and ZnO4(OH)(H2O) octahedra corner-shared through μ3-OH group. Inorganic chains are linked with 1,3,5-benzenetricarboxylates forming two types of parallel channels (open and closed) containing adsorbed water in different environments. Closed channels are occupied by free water molecules connected through strong hydrogen-bonds with coordinated water, whereas open-type channels contain water molecules with partially occupied oxygen atom sites. The dynamics of water adsorption/desorption was evaluated by complementary techniques of thermogravimetric (TG), infrared (IR), water sorption, and different magic-angle spinning nuclear magnetic resonance (MAS NMR) techniques. The removal of water from open channels occurs below 100 °C, whereas hydrogen-bonded water molecules and coordinated water are expelled at higher temperatures. 2H MAS NMR was employed to prove that the removal of water from closed channels is not entirely simultaneous and that adsorbed water begins to diffuse at slightly lower temperatures than the coordinated one. The investigated material shows high hydrothermal stability and withstands 40-cycle hydrothermal-stability test without any significant loss of the structure integrity. It also shows complete structural reversibility upon dehydration/rehydration process at 200 °C. The reason for high stability in water mainly lies in the stabilization of the inorganic chains established by the interaction between the adsorbed water molecules and coordinated ones (framework water) via hydrogen bonds. The material also exhibits notable sorption capacity for water (208 mg·g–1) adsorbed in a stepwise process.