Metal-organic frameworks based on phosphonate linkers

During the last few decades, Metal-Organic Frameworks (MOFs), also known as Coordination Polymers, have attracted worldwide research attentions due to their incremented fascinating architectures and unique properties. These multidimensional materials have been potential applications in distinct areas: gas storage and separation, ion exchange, catalysis, magnetism, in optical sensors, among several others. The MOF research group at the University of Aveiro has prepared MOFs from the combination of phosphonate organic primary building units (PBUs) with, mainly, lanthanides. This thesis documents the last findings in this area involving the synthesis of multidimensional MOFs based on four di- or tripodal phosphonates ligands. The organic PBUs were designed and prepared by selecting and optimizing the best reaction conditions and synthetic routes. The self-assembly between phosphonate PBUs and rare-earths cations led to the formation of several 1D, 2D and 3D families of isotypical MOFs. The preparation of these materials was achieved by using distinct synthetic approaches: hydro(solvo)thermal, microwave- and ultrasound-assisted, one-pot and ionothermal synthesis. The selection of the organic PBUs showed to have an important role in the final architectures: while flexible phosphonate ligands afforded 1D, 2D and dense 3D structures, a large and rigid organic PBU isolated a porous 3D MOF. The crystal structure of these materials was successfully unveiled by powder or single-crystal X-ray diffraction. All multidimensional MOFs were characterized by standard solid-state techniques (FT-IR, electron microscopy (SEM and EDS), solid-state NMR, elemental and thermogravimetric analysis). Some MOF materials exhibited remarkable thermal stability and robustness up to ca. 400 ºC. The intrinsic properties of some MOFs were investigated. Photoluminescence studies revealed that the selected organic PBUs are suitable sensitizers of Tb3+ leading to the isolation of intense green-emitting materials. The suppression of the O−H quenchers by deuteration or dehydration processes improves substantially the photoluminescence of the optically-active Eu3+-based materials. Some MOF materials exhibited high heterogeneous catalytic activity and excellent regioselectivity in the ring-opening reaction of styrene oxide (PhEtO) with methanol (100% conversion of PhEtO at 55 ºC for 30 min). The porous MOF material was employed in gas separation processes. This compound showed the ability to separate propane over propylene. The ionexchanged form of this material (containing K+ cations into its network) exhibited higher affinity for CO2 being capable to separate acetylene over this environment non-friendly gas.

Mercaptans extraction from jet-fuel streams using ionic liquids

Desulfurization is one of the most important processes in the refining industry. Due to a growing concern about the risks to human health and environment, associated with the emissions of sulfur compounds, legislation has become more stringent, requiring a drastic reduction in the sulfur content of fuel to levels close to zero (< 10 ppm S). However, conventional desulfurization processes are inefficient and have high operating costs. This scenario stimulates the improvement of existing processes and the development of new and more efficient technologies. Aiming at overcoming these shortcomings, this work investigates an alternative desulfurization process using ionic liquids for the removal of mercaptans from "jet fuel" streams. The screening and selection of the most suitable ionic liquid were performed based on experimental and COSMO-RS predicted liquid-liquid equilibrium data. A model feed of 1-hexanethiol and n-dodecane was selected to represent a jet-fuel stream. High selectivities were determined, as a result of the low mutual solubility between the ionic liquid and the hydrocarbon matrix, proving the potential use of the ionic liquid, which prevents the loss of fuel for the solvent. The distribution ratios of mercaptans towards the ionic liquids were not as favorable, making the traditional liquid-liquid extraction processes not suitable for the removal of aliphatic S-compounds due to the high volume of extractant required. This work explores alternative methods and proposes the use of ionic liquids in a separation process assisted by membranes. In the process proposed the ionic liquid is used as extracting solvent of the sulfur species, in a hollow fiber membrane contactor, without co-extracting the other jet-fuel compound. In a second contactor, the ionic liquid is regenerated applying a sweep gas stripping, which allows for its reuse in a closed loop between the two membrane contactors. This integrated extraction/regeneration process of desulfurization produced a jet-fuel model with sulfur content lower than 2 ppm of S, as envisaged by legislation for the use of ultra-low sulfur jet-fuel. This result confirms the high potential for development of ultra-deep desulfurization application.