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

Abstract

This work presents the syntheses and comprehensive characterization of six paddlewheel based metal− organic frameworks (MOFs) with the general formula ∞ 3 [M 2 L 2 ] (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 Co 2+ /Cu 2+ , Co 2+ /Zn 2+ , and Cu 2+ /Zn 2+ 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 CO 2 (298 K) and N 2 (77 K) adsorption properties, all materials were previously subjected to extraction with supercritical CO 2 . 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 CO 2 or N 2 are possible by choice of the metal ion. This finding is supported by adsorption studies on the mixed metal MOF ∞ 3 [(Cu 0.48 Co 0.52 ) 2 (p-L) 2 )], showing CO 2 adsorption/desorption characteristics of both homonuclear copper and cobalt materials, whereas N 2 does not induce gate opening of the framework, as observed for the cobalt MOF. Furthermore, catalytic studies reveal that ∞ 3 [Cu 2 (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.

A Series of Robust Copper-Based Triazolyl Isophthalate MOFs: Impact of Linker Functionalization on Gas Sorption and Catalytic Activity †

“…In addition, other Cu-containing MOFs show an overall higher selectivity for Cyhex-ene [42,44] and CyhexO [39,40] with TBHP or molecular oxygen as the oxidant. As can be further seen from Table 6, [Cu 2 L 2 ] (L = bis(carboxyphenyl)-1,2,4-triazole) [50] even shows an up to 1.5-fold higher activity in comparison to 5 at the same reaction conditions, although both reactions could occur on the outer surface of the catalysts. This could be again caused by the different coordination environment of the catalytically active sites resulting in different intrinsic activity of the Cu sites and, thus, different Lewis acidity.…”

“…In addition, low coordinated Cu sites at the outer surface of [Cu 2 L 2 ] probably cause the higher activity compared to 1 – 5 , as these active sites are easily accessible. Furthermore, Kobalz et al [50] concluded that the conversion of cyclohexene probably occurs at the outer surface of [Cu 2 L 2 ]. The previous comparison of the catalytic results of 1 – 5 to already published results (cf.…”

“…The influence of different functional groups within the triazolyl isophthalate linker on the gas sorption properties was already observed recently with a related series of MOFs based on the paddle wheel motif [48]. In addition, triazole-based MOFs were shown to be active catalysts in the selective oxidation of cyclooctene and cyclohexene with TBHP [36,50,51]. Accordingly, the selective oxidation of cyclohexene with TBHP in the liquid phase was chosen as a test reaction in the present work to investigate the influence of the triazole substituents on the catalytic activity of the MOFs.…”

“…The catalytic experiments were carried out as already described in a previous report [50]. Fifteen centimeters cubed of chloroform (99.8+%, Alfa Aesar, Karlsruhe, Germany) were loaded into a batch reactor followed by the addition of cyclohexene (2.054 g, 25 mmol, 99%, Sigma-Aldrich, Taufkirchen, Germany), tert -butyl-hydroperoxide (TBHP, 7.545 g, 50 mmol, 5.5 M solution in n -decane, Sigma-Aldrich) and chlorobenzene (2.814 g, 25 mmol, 99+%, Acros organics, Nidderau, Germany) as the internal analytical standard corresponding to a molar ratio of cyclohexene:TBHP:chloroform of 1:2:7.…”

Effect of the Metal within Regioisomeric Paddle‐Wheel‐Type Metal–Organic Frameworks

“…There are several reports about the relationshipsb etween flexibility (as well as breathing behavior and gate opening) and the metal cation on MOFs. [14][15][16][17][18][19] In particular, Kaskel and co-workers prepared as eries of DUT-8M OFs (DUT = Dresden University of Te chnology) with Ni, Cu, Co, and Zn and studied the magnitude of the changes in flexibility with different metal nodes. DUT-8 species are also DABCO-based pillared MOFs with as lightly tilted 2,6-naphthalenedicarboxylic acid).…”