Design of isostructural metal-imidazolate frameworks : application for gas storage

• Main Author: Mondal, Suvendu Sekhar
• Format: Doctoral Thesis
• Language: English
• Published: Universität Potsdam 2013
• Subjects: Metal-organic framework; Gas Sorption; Cobalt; Zinc; Ionic Liquid; metal-organic framework; gas sorption; cobalt; zinc; ionic liquid; Chemistry and allied sciences
• Online Access: http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-69692; http://opus.kobv.de/ubp/volltexte/2014/6969/

The sharply rising level of atmospheric carbon dioxide resulting from anthropogenic emissions is one of the greatest environmental concerns facing our civilization today. Metal-organic frameworks (MOFs) are a new class of materials that constructed by metal-containing nodes bonded to organic bridging ligands. MOFs could serve as an ideal platform for the development of next generation CO2 capture materials owing to their large capacity for the adsorption of gases and their structural and chemical tunability. The ability to rationally select the framework components is expected to allow the affinity of the internal pore surface toward CO2 to be precisely controlled, facilitating materials properties that are optimized for the specific type of CO2 capture to be performed (post-combustion capture, precombustion capture, or oxy-fuel combustion) and potentially even for the specific power plant in which the capture system is to be installed. For this reason, significant effort has been made in recent years in improving the gas separation performance of MOFs and some studies evaluating the prospects of deploying these materials in real-world CO2 capture systems have begun to emerge. We have developed six new MOFs, denoted as IFPs (IFP-5, -6, -7, -8, -9, -10, IFP = Imidazolate Framework Potsdam) and two hydrogen-bonded molecular building block (MBB, named as 1 and 2 for Zn and Co based, respectively) have been synthesized, characterized and applied for gas storage. The structure of IFP possesses 1D hexagonal channels. Metal centre and the substituent groups of C2 position of the linker protrude into the open channels and determine their accessible diameter. Interestingly, the channel diameters (range : 0.3 to 5.2 Å) for IFP structures are tuned by the metal centre (Zn, Co and Cd) and substituent of C2 position of the imidazolate linker. Moreover hydrogen bonded MBB of 1 and 2 is formed an in situ functionalization of a ligand under solvothermal condition. Two different types of channels are observed for 1 and 2. Materials contain solvent accessible void space. Solvent could be easily removed by under high vacuum. The porous framework has maintained the crystalline integrity even without solvent molecules. N2, H2, CO2 and CH4 gas sorption isotherms were performed. Gas uptake capacities are comparable with other frameworks. Gas uptake capacity is reduced when the channel diameter is narrow. For example, the channel diameter of IFP-5 (channel diameter: 3.8 Å) is slightly lower than that of IFP-1 (channel diameter: 4.2 Å); hence, the gas uptake capacity and Brunauer-Emmett-Teller (BET) surface area are slightly lower than IFP-1. The selectivity does not depend only on the size of the gas components (kinetic diameter: CO2 3.3 Å, N2 3.6 Å and CH4 3.8 ) but also on the polarizability of the surface and of the gas components. IFP-5 and-6 have the potential applications for the separation of CO2 and CH4 from N2-containing gas mixtures and CO2 and CH4 containing gas mixtures. Gas sorption isotherms of IFP-7, -8, -9, -10 exhibited hysteretic behavior due to flexible alkoxy (e.g., methoxy and ethoxy) substituents. Such phenomenon is a kind of gate effects which is rarely observed in microporous MOFs. IFP-7 (Zn-centred) has a flexible methoxy substituent. This is the first example where a flexible methoxy substituent shows the gate opening behavior in a MOF. Presence of methoxy functional group at the hexagonal channels, IFP-7 acted as molecular gate for N2 gas. Due to polar methoxy group and channel walls, wide hysteretic isotherm was observed during gas uptake. The N2 The estimated BET surface area for 1 is 471 m2 g-1 and the Langmuir surface area is 570 m2 g-1. However, such surface area is slightly higher than azolate-based hydrogen-bonded supramolecular assemblies and also comparable and higher than some hydrogen-bonded porous organic molecules. === Metallorganische Gerüstverbindungen (MOFs) sind eine neue Klasse von porösen Koordinationspolymeren, die aus Metall-Knoten und verbrückenden Liganden bestehen. MOFs können Gasgemische trennen und Gase speichern. Aufgrund ihres modularen Aufbaus können die MOF-Eigenschaften systematisch variiert werden. Ein wichtiges Ziel für das Design von MOFs ist die Synthese von Materialien, die eine hohe selektive Aufnahmefähigkeit und -kapazität für Kohlenstoffdioxid besitzen. Im Rahmen der Arbeit ist es gelungen sechs neue MOFs (IFP-5, -6, -7, -8, -9 und -10) zu synthetisieren. Diese MOFs tragen die Kurzbezeichnung IFP. IFP steht als Abkürzung für Imidazolat-Framework-Potsdam (Imidazolat-basierte Gerüstverbindung Potsdam). In diesen IFPs wurde der Metallknoten (Zink, Cobalt, Cadmium) und der Brückenligand, ein 2-substituiertes Imidazolat-amid-imidat, in der Position variiert, um gute und selektive Sorptionseigenschaften für Kohlenstoffdioxid zu erzielen. Von den synthetisierten Verbindungen hat das IFP-5 die besten Sorptionseigenschaften für Kohlenstoffdioxid. Es konnte weiter gezeigt werden, dass sich die IFP-Struktur bei der Wahl von geeigneten Substituenten 2, wie z.B. Methoxy und Ethoxy auch für das Design von gate-opening (Tür-öffnenden) Effekten eignet. Diese Effekte können wiederum genutzt werden, um selektiv Gasmischungen zu trennen. Wenn man das 4,5-Dicyano-2-methoxy-imidazol in Gegenwart von Zink- und Cobalt-Salzen unter solvothermalen Bedingungen zur Reaktion bringt, erhält man beispiellose supramolekulare Wasserstoffbrückenbindungen zu einem dreidimensionalen Netzwerk, die mit Kanälen verknüpft sind. Diese Kanäle können von Lösungsmittelmolekülen (Wasser und Dimethylformamid) befreit werden und Gase aufnehmen. Insgesamt besteht nun die neue MOF-Klasse der Imidazolat-basierten IFPs aus Vertretern. Das Potential der 2-substituierten 4,5-Dicyanoimidazole ist nicht nur auf die Bildung von porösen Koordinationspolymeren beschränkt, sondern kann auch für die Synthese von bisher unbekannten supramolekularen Strukturen genutzt werden.