Synthesis and properties of scandium carboxylate metal-organic frameworks

Abstract

This work investigated the synthesis, characterisation and properties of known and novel scandium carboxylate Metal-organic Frameworks (MOFs). The first part reports the performance of these Sc-MOFs as Lewis acid catalysts. The porous MOF scandium trimesate MIL-100(Sc) and the scandium terephthalates such as MIL-101(Sc), MIL-88B(Sc) and MIL-68(Sc) (prepared as the Sc-analogue for the first time), and scandium biphenyldicarboxylate MIL-88D(Sc) were prepared and tested as Lewis acid catalysts. Chromium MIL-101 and MIL-100 and scandium-exchanged zeolites were prepared for comparison. Moreover, successful encapsulation of the phosphotungstate polyoxometalates (POMs) in the cavities of MIL-101(Sc) enhanced the stability of this material. These scandium and chromium MOFs, POM-MOF composites and scandium-exchanged zeolite were tested as heterogeneous catalysts in the carbonyl ene reaction between α-methyl styrene and ethyl trifluoropyruvate. This showed that MIL-100(Sc) was the best catalyst for this reaction, achieving a conversion of 99% to the desired product. The stabilized MIL-101(Sc) was also very active, but less selective for this reaction. Acetalisation of acetaldehyde was also studied, and in this reaction the isoreticular MOFs MIL-88(B) and MIL-88D(Sc) were the most active and selective catalysts. For this reaction, the activity of MIL-100(Sc) was low, which was attributed to reduce pore size and blockage. Functionalisation of the range of scandium terephthalates such as MIL-101(Sc), MIL-88B(Sc), MIL-68(Sc), Sc₂BDC₃,and MIL-53(Sc) particularly with –NH₂ groups, made up the second main part of this research. Solvothermal synthesis were performed at lower temperatures and using mixed solvents to synthesize these amino-terephthalate MOFs, often for the first time, and their adsorption properties were studied, particularly for the adsorption of CO₂. The synthesis of pure Sc₂(NH₂-BDC)₃ and Sc₂(Br-BDC)₃ was achieved for first time by a solvothermal route, lower temperatures, and mixed solvents. This approach yielded large crystals suitable for single crystal diffraction and microcrystal IR spectroscopy. Post-synthetic modification (PSM) of Sc₂(NH₂-BDC)₃ was explored by incorporation of NO₂-groups into the framework by solvent-assisted ligand exchange. The adsorption properties of functionalised and post-modified materials were compared with those of Sc₂BDC₃ and Sc₂(NO₂-BDC)₃ for methanol and hydrocarbons This study demonstrated that Sc₂BDC₃ and Sc₂(NH₂-BDC)₃ give the higher uptakes while the –Br and –NO₂ forms display shape selectivity for n-alkanes over iso-alkanes. Amino-functionalised MIL-53(Sc) was prepared for the first time using a mixed ligand approach, so that 10-20% of a second functionalised terephthalate ligand (NO₂, Br, -(OH)₂) was required for successful single phase synthesis in addition to amino-terephthalic acid. The materials were characterised using PXRD, TGA and gas adsorption, which confirms the samples show a range of behaviour for CO₂ adsorption. Notably, the `breathing´ behaviour is strongly dependent on the type of functionalisation. Finally, the exploratory synthesis of novel scandium MOFs, using isophthalic acid (IA) and its amino and nitro- derivatives, 2,5-furandicarboxylate (FDA) and the porphyrin tetra(carboxyphenyl)porphyrin (TCPP) as linkers was carried out and six novel materials were synthesized, three of which gave crystals large enough for their structure to be determined by single crystal diffraction. Further characterisation was carried out by PXRD, TGA and solid-state NMR. Some of these materials have been shown to be porous to CO₂ and N₂.