The influence of reaction temperature on the porosity of MOF-5 studied by nitrogen adsorption and positron annihilation lifetime spectroscopy

In this work, MOF-5, consisted of [Zn₄O]⁶⁺ clusters connected with organic linkers terephthalic acid (H₂BDC) to form a cubic network, was synthesized at different temperatures. Various measurement methods including powder X-ray diffraction patterns, scanning electron microscopy, thermos-gravimetric analysis, nitrogen adsorption-desorption isotherms at 77 K, and positron annihilation lifetime spectroscopy (PALS) were applied to investigate the properties of MOF-5. It was found that the crystallinity sizes, and porosity strongly depended on the synthesis temperature. Results showed that higher synthesis temperature was favorable to improve the porosity of MOFs because of the removal of residual ligand molecules. Remarkably, significant differences in pore radius distributions derived from nitrogen adsorption and PALS was obtained from MOF-5 synthesized at 110℃, suggesting that there exist closed pores due to the presence of ligand molecules in the inherent pores and/or pore entrances, where nitrogen molecules could not get in. (author)