Materiales autolimpiables basados en composites de polímeros de coordinación porosos

Resum

The fast-paced technological and industrial development of recent decades is leading to an uncontrolled emission of environmentally harmful gases, the effects of which can already be observed. The social alarm that this has aroused has led much of the scientific community to focus their efforts on developing effective systems to neutralize these emissions. One of the prototypical examples of toxic compounds are the so-called Chemical Warfare Agents (CWAs). Importantly, they are the most toxic known chemicals and pose a great social threat due to the relative ease with which terrorist groups or unscrupulous governments can carry out their manufacture and storage. Different types of CWAs include so-called nerve agents, organophosphonate compounds that damage the central nervous system; and blistering agents, organosulfur compounds that cause burns to the skin and mucous membranes. The origin of the toxicity of these compounds is related to the reactivity of the P-X and C-X bonds (X = F, Cl, O, S), their high volatility, and the low polarity of the organic skeleton that allows easy penetration through the skin and mucous membranes. Physisorption on activated carbons is the current method of protection against this type of toxic substances. However, physisorption alone is not adequate to prevent the contaminated adsorbent from becoming a secondary emitter. For this reason, there is great interest in the development of new porous materials that are not only capable of efficiently adsorbing these substances but are also capable of detoxifying them. In this sense, the hydrolysis of the P-X and C-X bonds is proposed as one of the most convenient pathways for detoxification, but it will only be possible in the presence of a suitable catalyst. Metal-Organic Frameworks, MOFs, are receiving great attention for the detoxification of this type of substances. This is attributed to the facile structural design of these materials through a proper selection of their constituents: metal ions or clusters and organic connectors. In this way, porous materials with advanced adsorbent and catalytic properties, optimized for the capture and catalytic detoxification of a given substance, can be prepared. One of the main disadvantages of these materials is that they are usually obtained in the form of microcrystalline powder, with consequently limited processability and low mechanical stability compared to other porous materials. One way to solve this problem is by the preparation of advanced materials based on MOFs combined with other classical materials (zeolites, activated carbons, etc.), so that they can be easily formed into tablets, tissues, membranes, etc. Zirconium oxohydroxocluster based MOFs [ZrO4(OH)4]12+ (Zr-MOFs) are of high interest due to their high thermal and chemical stability. Networks derived from this Secondary Building Unit (SBU) combined with a wide variety of organic ligands of different geometries result in an enormous variety of reticular topologies as well as a wide range of pore sizes and shapes. The efficiency of these systems in the hydrolytic degradation of nerve agents and their less toxic simulants, due to the combined action of Lewis acidity of the Zr4+ ions with the basicity of the O-/OH- bridge, is of interest for this project. This Thesis describes the synthesis and characterization of materials based on Zr-MOFs with improved adsorbent and catalytic properties towards the detoxification of chemical warfare agents (CWAs). Chapter 1 contains a brief introduction to the main characteristics and applications of porous materials and their composites. Emphasis is placed on the description of MOFs. Chapter 2 describes the synthesis, characterization, and catalytic application of Zr-MOFs doped with inorganic lithium and magnesium compounds. On the one hand, it is demonstrated that the simultaneous introduction of amino groups in the ligand; and of lithium alkoxides in the SBU of Zr-MOFs UiO-66 (Zr6O4(OH)4(1,4-benzenedicarboxylate)6) and UiO-67 (Zr6O4(OH)4(4,4'-biphenyl-dicarboxylate)6) gives rise to synergic effects in the catalytic detoxification of CWAs. This is attributed to the favourable effect in the microsolvatation environment around the cluster produced by the amino groups and to the extra basicity introduced by LiOtBu, linked to an O of the SBU. Regarding the preparation of these advanced materials, the possible decrease of accessibility to the porous structure produced by the steric effect of the amino groups was taken into account. In order to minimize this effect, two series of solid solutions of MOFs have been synthesized with mixed ligands (with and without amino group) UiO-66-xNH2, UiO-67-x(NH2)2 (where x represents the fraction of amino ligands). The materials resulting from doping with LiOtBu, UiO-66-xNH2@LiOtBu and UiO-67-x(NH2)2@LiOtBu, have also been studied. In a second stage, its effect on the catalytic detoxification of both simulants and real chemical warfare agents has been studied. The results show that the MOF doped UiO-66-0.25NH2@LiOtBu presents an adequate balance between chemical stability, accessibility to the porous network, basicity of LiOtBu residues and nucleophilic characteristics of the amino group giving rise to an optimal behaviour in the detoxification of CWAs. On the other hand, taking into account the diagonal analogy Li-Mg it has been shown that doping with basic magnesium species, in particular with Mg(OMe)2 under mild conditions (ambient temperature), results in materials with improved catalytic activity. A marked effect of pore size and network connectivity has been found and demonstrated by the use of three representative Zr-MOFs: UiO-66 (Zr6O4(OH)4(benzene-1,4-dicarboxylate)6, connectivity 12, microporous), NU-1000 (Zr6(OH)8(OH)8 (1,3,6,8-tetrakis(p-benzoate)pyrene)2) (connectivity 8, hierarchical microporous/mesoporous) and MOF-808 (Zr6O4(OH)4(benzene-1,3,5-tricarboxylate)2(HCOO)6) (connectivity 6, mesoporous). This study demonstrated: (i) functionalization with Mg(OMe)2 only takes place in the mesopores of these materials, due to the voluminous size of the reagent [Mg(OMe)2(MeOH)3]4 (1.2 nm) so that UiO-66 does not present appreciable reactivity; (ii) treatment with Mg(OMe)2 gives rise to the exchange of a Zr atom for an Mg atom in the SBU (giving rise to MgZr5O2(OH)6) as opposed to doping with LiOtBu on the periphery of the cluster; (iii) MOF-808@Mg(OMe)2 and NU-1000@Mg(OMe)2 doped materials show high chemical stability and improved catalytic activity in the hydrolytic degradation of P-F, P-O and P-S bonds (at room temperature in the absence of buffer) of both simulants and real nerve agents. The improvement of the catalytic activity is justified on the basis of the basicity introduced by Mg(OMe)2 and the increase in charge gradients in the heterometallic SBU. Finally, the doping of these materials has been studied by means of a mechanochemical green process. The mechanochemical reaction of Mg(OH)2 with representative Zr-MOFs UiO-66 and MOF-808 has been studied for this purpose. The results show that the composites UiO-66@Mg(OH)2 and MOF-808@Mg(OH)2 give rise to an important improvement of their catalytic activity due to a synergic effect of the two components. This methodology is of interest for practical application in decontamination equipment, due to the insolubility, low cost and non-toxicity of magnesium hydroxide, and simplicity of the mechanochemical process. Chapter 3 focuses on the preparation of composites of activated carbon materials (in the form of fabrics and spheres) doped with zirconium materials. On the one hand, the incorporation of thin layers of Zr-MOFs on the carbon material has been studied through a layer-by-layer growth process. The results show that a UiO-66/UiO-66-NH2@AC composite is obtained with a homogeneous distribution of crystalline MOF over the carbon material. UiO-66/UiO-66-NH2@CA composites are efficient in the degradation of chemical warfare simulants, thus avoiding the problem of secondary emission of activated carbons exposed to chemical warfare agents. Finally, the design, development and validation of a protective suit with adsorbent and catalytic properties against chemical warfare agents, is described. For this purpose and from an economically feasible point of view, the composite Zr(OH)4@LiOtBu, efficient in the degradation of CWAs simulants, was prepared on a large scale. This composite was integrated into polyurethane foam impregnated with activated carbon in collaboration with external companies. As a conclusion, this PhD Thesis demonstrates the rational design capability of advanced materials based on zirconium MOFs for the improvement of catalytic properties in the detoxification of CWAs. In addition, the high versatility of MOFs, together with the possibilities of post-synthetic functionalization or composite formation with other classical materials, open up an immense range of possibilities for the development of advanced functional materials.