A novel zinc(ii) metal–organic framework with a diamond-like structure: synthesis, study of thermal robustness and gas adsorption properties

Abstract

A solvothermal reaction of Zn(II) salt with methanetetrabenzoic acid (H₄MTB) and 1,4,8,11-tetraazacyclotetradecane (cyclam, CYC) created a new microporous metal–organic framework {[Zn₂(μ₄-MTB)(κ⁴-CYC)₂]·2DMF·7H₂O}_n (DMF = N,N′-dimethylformamide). Single crystal X-ray diffraction showed that the complex exhibits a four-fold interpenetrated diamond-like structure topology with 1D jar-like channels with sizes about 14.1 × 14.1 and 2.4 × 2.4 Ų. The stability of the framework and activation conditions of the compound have been studied by high-energy powder X-ray diffraction during in situ heating, thermogravimetric analysis coupled with mass spectrometry and infrared spectroscopy performed at different temperatures. The gas adsorption behaviour of {[Zn₂(μ₄-MTB)(κ⁴-CYC)₂]·2DMF·7H₂O}_n was studied by adsorption of Ar, N₂, CO₂ and H₂. Nitrogen and argon adsorption showed that the activated sample exhibits Brunauer–Emmet–Teller (BET) specific surface areas of 644 m² g⁻¹ (N₂) and 562 m² g⁻¹ (Ar). The complex adsorbs carbon dioxide with a maximum storage capacity of 10.5 wt% at 273 K and 101 kPa. The observed hydrogen uptake was 1.27 wt% at 77 K and 800 Torr, which is the highest value reported for the compounds containing a MTB⁴⁻ linker. The adsorption heats of carbon dioxide and hydrogen, calculated according to the Clausius–Clapeyron equation, were in the range 22.8–22.4 kJ mol⁻¹ for CO₂ and 8.9–3.2 kJ mol⁻¹ for H₂, indicating weak interactions of the gases with the framework.