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

The synthesis and characterization of an isomorphous series of copper-containing microporous metal-organic frameworks (MOFs) based on triazolyl isophthalate linkers with the general formula 3 ¥[Cu4(m3-OH)2(R1-R2-trz-ia)3(H2O)x] are presented. Through size adjustment of the alkyl substituents R1 and/or R2 at the linker, the impact of linker functionalization on structure-property relationships was studied. Due to the arrangement of the substituents towards the cavities, the porosity (pore fraction 28%–39%), as well as the pore size can be adjusted by the size of the substituents of the triazole ring. Thermal analysis and temperature-dependent PXRD studies reveal a thermal stability of the MOFs up to 230 C due to increasing framework stability through fine-tuning of the linker substitution pattern. Adsorption of CO2 (298 K) shows a decreasing maximum loading with increasing steric demand of the substituents of the triazole ring. Furthermore, the selective oxidation of cyclohexene with tert-butyl hydroperoxide (TBHP) is studied over the MOFs at 323 K in liquid chloroform. The catalytic activity increases with the steric demand of the substituents. Additionally, these isomorphous MOFs exhibit considerable robustness under oxidizing conditions confirmed by CO2 adsorption studies, as well as by the catalytic selective oxidation experiments

Mixed gas adsorption of carbon dioxide and methane on a series of isoreticular microporous metal-organic frameworks based on 2-substituted imidazolate-4-amide-5-imidates

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.In this work the adsorption of CO2 and CH4 on a series of isoreticular microporous metal–organic frameworks based on 2-substituted imidazolate-4-amide-5-imidates, IFP-1–IFP-6 (IFP = Imidazolate Framework Potsdam), is studied firstly by pure gas adsorption at 273 K. All experimental isotherms can be nicely described by using the Tòth isotherm model and show the preferred adsorption of CO2 over CH4. At low pressures the Tòth isotherm equation exhibits a Henry region, wherefore Henry's law constants for CO2 and CH4 uptake could be determined and ideal selectivity αCO2/CH4 has been calculated. Secondly, selectivities were calculated from mixture data by using nearly equimolar binary mixtures of both gases by a volumetric–chromatographic method to examine the IFPs. Results showed the reliability of the selectivity calculation. Values of αCO2/CH4 around 7.5 for IFP-5 indicate that this material shows much better selectivities than IFP-1, IFP-2, IFP-3, IFP-4 and IFP-6 with slightly lower selectivity αCO2/CH4 = 4–6. The preferred adsorption of CO2 over CH4 especially of IFP-5 and IFP-4 makes these materials suitable for gas separation application.DFG, SPP 1362, Poröse metallorganische Gerüstverbindunge

Developing a sustainable route to environmentally relevant metal-organic frameworks: ultra-rapid synthesis of MFM-300(Al) using microwave heating

NO2, SO2 and CO2 are major air pollutants causing significant environmental and health problems. Metal-organic frameworks (MOFs), in particular [Al2(OH)2(C16O8H6)](H2O)6 (trivial names: NOTT-300/MFM-300(Al)), have shown great promise for capturing these gases. However MOF syntheses often involve toxic solvents and long durations which are inherently energy intensive, an environmental burden, and have serious safety risks. There is a pressing need to develop environmentally-friendly routes to MOFs that require less energy and implement safer solvents particularly when considering scale-up beyond the laboratory for industrial application. We report the rapid synthesis of MFM-300(Al) in aqueous conditions and 10 minutes using microwave heating. This is the fastest reported synthesis of MFM-300(Al) to date with a 99.77 % reduction in reaction time compared to the current reported 3-day conventional heated route. The microwave synthesized sub-micron crystalline material exhibits gas uptake capacities of 8.8 mmol g-1 at 273 K and 1.0 bar for CO2, 8.5 mmol g-1 at 298 K and 0.17 bar for SO2, and 1.9 mmol g-1 at 298 K and 0.01 bar for NO2. These are 26 %, 70 %, and 90 % greater for CO2, SO2, and NO2, respectively, when compared to previously reported MFM-300(Al) materials produced via a 3-day conventionally heated route demonstrating the production of high quality materials at a fraction of the time with enhanced gas properties. Crucially, this offers an opportunity to move from batch to continuous processing owing to reduced reaction times underpinned by targeted heating

Pyrazolate-based cobalt(II)-containing metal-organic frameworks in heterogeneous catalytic oxidation reactions: elucidating the role of entatic states for biomimetic oxidation processes

Crystal structures of two metal–organic frameworks (MFU‐1 and MFU‐2) are presented, both of which contain redox‐active CoII centres coordinated by linear 1,4‐bis[(3,5‐dimethyl)pyrazol‐4‐yl] ligands. In contrast to many MOFs reported previously, these compounds show excellent stability against hydrolytic decomposition. Catalytic turnover is achieved in oxidation reactions by employing tert‐butyl hydroperoxide and the solid catalysts are easily recovered from the reaction mixture. Whereas heterogeneous catalysis is unambiguously demonstrated for MFU‐1, MFU‐2 shows catalytic activity due to slow metal leaching, emphasising the need for a deeper understanding of structure–reactivity relationships in the future design of redox‐active metal–organic frameworks. Mechanistic details for oxidation reactions employing tert‐butyl hydroperoxide are studied by UV/Vis and IR spectroscopy and XRPD measurements. The catalytic process accompanying changes of redox states and structural changes were investigated by means of cobalt K‐edge X‐ray absorption spectroscopy. To probe the putative binding modes of molecular oxygen, the isosteric heats of adsorption of O2 were determined and compared with models from DFT calculations. The stabilities of the frameworks in an oxygen atmosphere as a reactive gas were examined by temperature‐programmed oxidation (TPO). Solution impregnation of MFU‐1 with a co‐catalyst (N‐hydroxyphthalimide) led to NHPI@MFU‐1, which oxidised a range of organic substrates under ambient conditions by employing molecular oxygen from air. The catalytic reaction involved a biomimetic reaction cascade based on free radicals. The concept of an entatic state of the cobalt centres is proposed and its relevance for sustained catalytic activity is briefly discussed