Chemical and Mechanical Stability of Copper Carboxylate Metal-Organic Frameworks

Metal-Organic Frameworks (MOFs) are a new class of crystalline porous materials, formed by the union of metallic junctions and organic ligands. The wide pores present in such structures provide extremely high specific surface areas, larger than those typically observed in more common porous materials like zeolites or activated carbons. This peculiar property combined with the frequent presence of open-metal sites, makes them very promising for a broad range of applications, spacing from gas storage or separation, drug delivery, toxic air removal, chemical detectors, etc. However, the industrialization of MOFs is currently facing difficulties because of their low chemical stability to common substances like water or even air moisture, substances difficult to avoid in applications like those above mentioned. Another issue concerning MOF industrialization consists in the low packaging of the micrometric powder grains which typically constitute MOFs. The low mechanical stability of these materials seems to make difficult even to enhance the packaging by simple mechanical compaction. For this reason, in the last years, new techniques have been searched in order to improve the MOF performances without recurring to a direct compaction but also trying to synthesized them as monoliths or involving them in composite systems. In this thesis, both chemical and mechanical stability of MOFs are investigated by several experimental techniques and methods. Three different copper carboxylate MOFs, HKUST-1, STAM-1 and STAM-17-OEt, have been deeply investigated in order to establish the causes of damages induced in the framework upon application of mechanical pressures or interaction with compounds like water or ammonia. HKUST-1 is a well-known MOF, deeply studied in literature for its great potentialities as adsorbent system and for the ease with which it can synthesized in large scale, despite being poorly water-stable and having a very low bulk density. It is constituted by a pair of copper ions chelated by four carboxylate groups arising from 1,3,5-benzenetricarboxylate ligands. The metallic knot consisting in the two coppers and the four carboxylates is found also in the other two MOFs, STAM-1 and STAM-17-OEt, which then differ from HKUST-1 uniquely for the organic ligands. The copper ions confer to the three MOFs peculiar paramagnetic properties, which allowed us to perform experiment focused on their study by electron paramagnetic resonance (EPR) spectroscopy; despite scarcely employed in MOF research field, this experimental technique provided us essential information about the chemical bonds and the environment surrounding the open-metal sites of the three structures. Both chemical and mechanical stability of the three MOFs have been investigated by different experiments. In order to deepen the chemical stability limits of the three structures, we carried out a comparative study by exposing them to interaction with humidity for about 2 months; we find an initial interesting similar evolution in spite of the strongly different long-term water stability of the three materials investigated. This allowed us to find out which content of water molecules leads to the beginning of the hydrolysis process in HKUST-1 and to understand the reasons why, in contrast, STAM-1 and STAM-17-OEt remain stable to water for long time. In contrast, studying the effects induced in STAM-17-OEt when it interacts with ammonia molecules, we observed a degradation mechanism with strong similarities to the water hydrolysis occurring in HKUST-1. Concerning the mechanical stability, we individuated the main degradation mechanism involved when the crystalline lattice of a MOF is subject to mechanical compaction, finding that it strongly depends on the water content adsorbed in the pores of the framework at the moment of the compaction. Because of this meaningful result, we have been able to develop a protocol which can provide a stable HKUST-1 tablet obtained by simply applying mechanical pressure on commercial powders, preserving the structure of the pristine powders as well as the volumetric surface area, and reaching a high bulk density. For comparison, we also synthesized HKUST-1 in monolithic form, obtaining a material which shows an equally high surface area, high crystallinity degree e other properties comparable with those of commercial HKUST-1 powders. In order to find the most promising HKUST-1 product between powder, tablet and monolith, we conducted also for these materials a deep investigation about their water stability. For the first time we tried to produce also STAM-17-OEt tablets, finding that this MOF withstands relatively high pressures without losing crystallinity or lattice flexibility. Furthermore, we produced also a composite system involving STAM-17-OEt MOF, by synthesizing its grains directly within the large cavities of an activated carbon compound, combining the properties of the former with those of the latter. This composite system showed very interesting properties in toxic air removal.