Bis(carboxyphenyl)-1,2,4-triazole Based Metal–Organic Frameworks: Impact of Metal Ion Substitution on Adsorption Performance

This work presents the syntheses and comprehensive characterization of six paddlewheel based metal–organic frameworks (MOFs) with the general formula _∞³[M₂L₂] (M = Cu, Co, Zn; L = bis­(carboxyphenyl)-1,2,4-triazole) forming an isoreticular series with rutile (<b>rtl</b>) topology. These microporous materials are suitable for a systematic investigation of structure–property relationships based on the impact of the metal ion. Depending on the metal ion, the calculated porosities and the pore diameters reach from 58% to 61% and 300 to 750 pm, respectively. Simultaneous thermal analysis and temperature dependent PXRD studies reveal varying thermal behavior with stabilities up to 400 °C. In the case of syntheses with various Co²⁺/Cu²⁺, Co²⁺/Zn²⁺, and Cu²⁺/Zn²⁺ ratios, ICP-OES analyses and SEM-EDX studies confirm the formation of mixed metal MOFs and the metal ion distribution in the bulk samples as well as within the crystals. For the systematic investigation of CO₂ (298 K) and N₂ (77 K) adsorption properties, all materials were previously subjected to extraction with supercritical CO₂. Depending on the metal ion, this procedure causes different phase transitions for each compound. As a result, adsorption studies reveal varying network flexibility for these MOFs. This study is one of the rare examples demonstrating that targeted modification of gate opening pressure, hysteresis shape, and adsorbed amounts of CO₂ or N₂ are possible by choice of the metal ion. This finding is supported by adsorption studies on the mixed metal MOF _∞³[(Cu_0.48Co_0.52)₂(<i>p</i>-L)₂)], showing CO₂ adsorption/desorption characteristics of both homonuclear copper and cobalt materials, whereas N₂ does not induce gate opening of the framework, as observed for the cobalt MOF. Furthermore, catalytic studies reveal that _∞³[Cu₂(<i>p</i>-L)₂)] is a suitable catalyst for the oxidation of cyclohexene with <i>tert</i>-butylhydroperoxide (TBHP) with high conversion of the starting materials and good selectivity. Its robustness under the applied catalysis conditions leads to similar conversions in repetition measurements