Synthesis of Metal-Organic Frameworks containing Heterocyclic Linkers for application in CO2 storage/utilization and Luminescent Sensing

Metal-organic frameworks (MOFs) are hybrid organic/inorganic 3D coordination polymers with open structures deriving from the self-assembly of polytopic linkers and metal ions or metal- based clusters. In more recent years, MOFs have emerged as promising alternative to all- inorganic materials (e.g. activated carbons and zeolites) in industrially and technologically relevant applications requiring porous compounds, such as gas adsorption or separation, heterogeneous catalysis and luminescence sensing. The tailored design of polydentate linkers (carboxylates, azolates) for MOFs construction is of fundamental importance to prepare well- performing materials for a target application. At present, a wide plethora of organic spacers has been prepared for this task. In many cases, the building units exploited are rigid aromatic six- membered fully carbocyclic rings (isolated or conjugated: benzene, 4,4'-biphenyl, terphenyl, naphthalene, anthracene, pyrene) decorated with carboxylate groups as coordinating units to metal centers. The introduction of heteroatoms or tailored functional groups within the linkers skeleton is generally beneficial to improve the MOF performance in a given process like gas adsorption/separation/transformation. One important example belonging to this category is that of carbon dioxide. The steadily increasing atmospheric concentration of carbon dioxide and the consequences of this phenomenon in terms of Earth’s global warming (the so-called greenhouse effect), ocean acidification, melting of polar ices and rising sea level are of primary concern for the scientific community. Carbon capture and storage (CCS) is a modern approach to solve this problem. CCS technologies based on CO2 adsorption by nano-porous adsorbents have become popular. MOFs have gained great attention in this context. The extraordinary versatility in MOFs design is the main reason for their applicative success. MOFs hold unique advantages, including control of their pore size and shape, high specific surface area, and the possibility to include proper functional groups on their linkers’ skeleton. An alternative approach to reduce CO2 concentration in the atmosphere and to mitigate its environmental effects moves from a radically new viewpoint, the so-called Carbon Capture and Utilization (CCU), where CO2 is no longer regarded as a simple waste but as a renewable resource to be harvested and recycled into C-containing products and feedstocks of added value. CCU sees CO2 as a safe and inexpensive C1 building block to yield expedient organic compounds. In a combined [CCS + CCU] perspective, MOFs represent excellent heterogeneous catalysts, showing great potentiality in terms of CO2 adsorption and its subsequent chemical conversion into useful chemicals (carbonates, methanol, methane). MOFs may have high CO2 adsorption, thus enhancing the local concentration of CO2 inside their pores and improving the catalytic efficiency. The introduction of basic heteroatoms in the MOF skeleton or the presence of basic functional groups further increases the concentration of CO2 at the catalytically active sites because of the enhanced interaction with acidic CO2. Indeed, various literature works have proved that MOFs built with heterocyclic linkers show a better CO2 uptake (in terms of mmol/g CO2 adsorbed) and a higher thermodynamic affinity (isosteric adsorption enthalpy, Qst) if compared with those made of fully carbocyclic rings. Another important applicative context from an environmental point of view is that of pollutants sensing in wastewaters. Pollutants as the so-called “contaminants of emerging concern” (CECs, i.e. a class of chemical and biological compounds such as pharmaceutical and personal care products that have been recognized as potentially harmful to human health and the environment) or cyanide anions (present in water sources as result of industrial processes and mining activity) are two classes of pollutants that can be particularly hazardous both to humans and the environment. Ideally, pollutants detection requires rapid, simple, cheap, sensitive and selective detection methods. Chemical sensors, i.e. analytical tools that provide information about the chemical composition of the environment in which they are introduced, are based on a variety of transduction mechanisms (e.g. optic, electronic, optoelectronic). Luminescent materials release energy in the form of electromagnetic radiation in the visible region in response to external stimuli. Recently, luminescent materials have gained the stage as chemical sensors, as luminescence is among the most desirable transduction mechanisms for its relative easiness of use, technical simplicity and broad adaptability. As chemical sensors, luminescent MOFs possess a number of advantages over other luminescent materials. Analyte adsorption within MOF pores allows for its pre-concentration, increasing sensor sensitivity. Selectivity in MOFs can be achieved by tuning pore dimension and/or by proper functionalization of the linkers. Fluorescent linkers typically bear π-conjugated electrons which can give emission upon irradiation. Linkers made of heterocyclic rings are particularly suitable for this application, since the presence of a heteroatom enhances the luminescent response.