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 (rtl) 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[(Cu0.48Co0.52)2(p-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(p-L)2)] is a suitable catalyst
for the oxidation of cyclohexene with tert-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.