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• W3145856027 abstract "Reducing the emission of toxic and greenhouse gases from burning of fossil fuels is an important global objective on the way towards a sustainable and green future. A significant fraction of fossil resources is consumed in combustion engines for transportation purposes (e. g. Germany: ≈ 55 % of crude oil [1]). The political will to drive the mobility sector towards cleaner technologies is reflected in a global trend towards increasingly strict emission and fuel specifications. However, a step change clearly requires implementation of new technologies that are not based on fossil energy sources. A very prominent and interesting option is the application of hydrogen as a fuel. Its combustion produces only water and is thus not of any environmental concern. Obviously, a truly renewable hydrogen production is required to make this technique sustainable. With the current renewable energy capacities this cannot be handled and a mobility sector fueled with hydrogen clearly has strategic character. However, with ongoing restructuring of the energy sector towards renewable technologies, capacities large enough to cover the hydrogen demand for transportation in a sustainable manner can be envisioned. A major challenge especially for mobile applications of hydrogen is its low volumetric energy density compared to liquid fuels (e. g. Hydrogen: 11 MJm-3; Gasoline: 31000 MJm-3). Large tank volumes and complex technologies involving high storage pressures are necessary to enable sufficient cruising ranges. This has implications for the overall car architecture. I. e. the tank is massively influencing car engineering with detrimental effects on competitiveness as regards comfort, design, passenger/cargo space and price as compared to conventional vehicles. Thus, the development of new technologies for more efficient hydrogen storage is one of the keys to enable the production of attractive hydrogen vehicles with low entry barriers into the market.A promising approach that has received considerable attention during the past ten years is the application of porous high surface area storage materials. The enrichment of hydrogen by physical adsorption on the material surface has the potential to enable higher storage densities compared to sole compression in an empty container. Among the group of porous materials, Metal Organic Frameworks (MOFs) were extensively probed for this application. Even in industry large efforts were devoted to this particular material class, in particular by BASF. But due to the rather weak host-guest interactions involved in physical adsorption, low temperatures are required to achieve performance improvements compared to gas compression. However, Metal-Organic Frameworks feature very unique chemical and structural diversity and offer huge potential to overcome this drawback by designing materials with proper host-guest interactions to arrive at applicable temperatures.Scope of the present work was the investigation of a particular approach for the modification of the surface properties of MOF materials for its feasibility and its impact on hydrogen storage properties, viz. the decoration of the internal surface with lithium alkoxide groups. Incentives for this route came from several theoretical investigations that showed the positive impact on hydrogen storage. However, little was known on the feasibility to synthesize these promising materials. Thus, as a first step a straightforward concept for the introduction of lithium alkoxide groups was developed comprising the treatment of hydroxy-functionalized MOFs with the selective base lithium diisopropylamide (LDA). The use of a sterically hindered harpoon base is an essential part of the concept, since other bases like lithium hydroxide or n-butyllithium will cause rapid framework degradation by nucleophilic displacement of the organic moieties from the metal centers. The approach implies that applicable hydroxy-functionalized MOFs have to be prepared at first. Little profound knowledge on this task was available at the beginning of the present work. Due to the nucleophilic properties of hydroxy groups and the strong interference with framework formation, synthesis of hydroxy-functionalized MOFs posed a massive challenge. Nevertheless, with the successful synthesis of a hydroxy-functionalized MIL 53(Al) analogue (MIL 53(Al) OH) an important milestone was achieved. Starting from the method for the un-functionalized MIL 53(Al) and employing 2 hydroxyterephthalic acid as the organic linker, the proper conditions were iterated by a rationale step-by-step approach. With this material in hand, the subsequent LDA treatment to form the lithium alkoxide was performed. The procedures non-destructive character was shown by a nearly complete retention of textural features. Analysis by 7Li NMR MAS spectroscopy, IR spectroscopy and chemical analysis gave clear evidence for the successful formation of the desired lithium alkoxide moieties. A thorough analysis of the hydrogen sorption properties confirmed the predicted improvements. The isosteric heat of adsorption was increased from 5 to 10 kJmol 1 along with higher hydrogen uptake at 1 bar.Following this proof-of-concept, attempts to extend the scope of the method by applying it to further MOFs were conducted starting with a hydroxy-functionalized analogue of CAU 1. However, the narrow pore openings of CAU 1 did not allow for transport of the bulky LDA molecule. Subsequent attempts involving lithium methanolate failed for similar reasons.To avoid mass transport limitations, further efforts were focused on the preparation of a hydroxy-functionalized analogue of MIL 101, a MOF material with large pores and apertures. Initial studies were performed with the iron based material employing 2 hydroxyterephthalic acid as the linker molecule. Starting from the conditions of the neat iron terephthalate, the synthesis parameters were varied over a wide range. However, it turned out that strong interference of the hydroxy group with the framework formation prevented the crystallization of MIL 101. These difficulties were finally overcome by development of a novel synthesis route based on ideas of the controlled secondary building unit (SBU) approach. Preassembly of the MIL 101 SBU by the monofunctional ligand benzoic acid prevented unwanted interactions with the hydroxyterephthalic acid linker and yielded a phase pure MIL 101 material with pendant hydroxy groups (MIL 101(Fe) OH). The method was successfully transferred to the chromium system demonstrating the power for the synthesis of MOFs involving linkers with additional functional groups that are strongly coordinating.As a benchmark for the lithium alkoxide functionalization experiments, the hydrogen sorption properties of MIL 101(Fe) OH were studied in detail involving hydrogen sorption equipment which was designed, built up and carefully calibrated within this work. A special feature of the unit comprises a closed cycle cryostat for an unchallenged accurate and precise sample temperature control down to temperatures of 12 K. Besides isosteric heat determination, this allows for hydrogen sorption experiments at the boiling point of hydrogen (20 K) to determine the available hydrogen specific surface area and pore volumes. The obtained values can be utilized to quantify a given storage materials efficiency in terms of utilization of the available porosity at a certain temperature above the critical temperature of hydrogen, e. g. at 77 K. This was exemplified with a reference material and then successfully transferred to MIL 101(Fe) OH.The hydrogen sorption on MIL 101(Fe) OH at 77 K and room temperature up to 30 bar was found to be lower than for MIL 101(Cr) at all conditions applied. This is in line with the reduced isosteric heat of adsorption of 7 kJmol 1 compared to 10 kJmol 1 for MIL 101(Cr). The SBU of MIL 101 features coordinatively unsaturated metal centers which are not accessible in MIL 101(Fe) OH, probably as a result of coordination/blockage by the pendant hydroxy groups.Lithium alkoxide formation experiments with MIL 101(Fe) OH were complicated by disintegration of the framework resulting in a substantial loss of porosity features and long range order. However, as proven by IR spectroscopy and chemical analysis this was not a result of nucleophilic displacements at the metal centers by LDA. A more likely explanation from the available data involves changes of the electron density in the vicinity of the iron centers by lithium alkoxide formation followed by framework collapse. Accordingly, the hydrogen sorption properties of the resulting materials did not show any prominent features. In this case, the stability of the MOF was too low to maintain the framework structure preventing the formation of accessible lithium alkoxide sites. However, it was found that the open porosity of the MIL 101 motif allowed for proper transport of LDA inside the framework and very efficient exchange of linker OH protons." @default.
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• W3145856027 date "2017-01-01" @default.
• W3145856027 modified "2023-09-23" @default.
• W3145856027 title "On the Synthesis and Hydrogen Sorption Properties of Hydroxy-functionalized Metal-Organic Frameworks and their Lithium Alkoxide Analogues" @default.
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