New Alkaline-Earth Polymeric Frameworks as green materials for sorption and heterogeneous catalysis

  1. Ana Eva Platero Prats
Supervised by:
  1. Javier Gutiérrez Puebla Director

Defence university: Universidad Autónoma de Madrid

Fecha de defensa: 04 November 2011

  1. Santiago Alvarez Reverte Chair
  2. David Tudela Moreno Secretary
  3. Martín Martínez Ripoll Committee member
  4. Santiago García Granda Committee member
  5. Xiaodong Zou Committee member

Type: Thesis


Metal-Organic Frameworks (or MOFs) are porous organic-inorganic crystalline materials in which the metallic centers are joined through organic ligands via coordination bonds to give frameworks with different dimensionalities. Thus, through the wide choice of metals, and the unlimited choice and design of ligands, a broad range of properties (magnetic, electric, optical, and catalytic, among others) might be rationally incorporated into such materials. The increasing number of MOFs reported in the last years has offered a rich variety of new structural types to materials science. In this sense, Crystallography has played an important role, not only in the structure determination of novel materials, but also in the process of finding consistent strategies to classify them, then to point out possible structural tendencies and, as a result of the former, to possibly predict structures in the design of new materials. In this context, the role of new topological approaches in the crystal chemistry is becoming more and more important. The work presented in this thesis is focused on the obtaining of new MOFs using alkaline-earth elements as metal centers. The use of alkaline-earth metals to synthesize new MOFs remains a scientific challenge, mainly due to the inherent difficulties concerning their formation/crystallization and the variable coordination environments that these elements exhibit. In spite of all this, the development of alkaline-earth MOFs could represent a comparatively cheap, nontoxic and green alternative to conventional MOFs. Concerning the organic ligands used in this work, two different ligand functionalities have been evaluated: carboxylate and sulfonate groups. With regard to carboxylate ligands, flexible dipodal linkers have been considered to obtain new alkaline-earth MOFs. The used linker 4,4¿- hexafluoroisopropylidene)bis(benzoic) acid (H2L(1)) has been utilized before in our research group to synthesize both transition metals and rare-earth elements MOFs. These previous studies have shown that this flexibility can induce interesting materials phenomena like polymorphism, as well as the formation of a wide variety of networks with unexpected topologies. Continuing in this way, the role that play nonbinding ¿CF3 groups present in H2L(1) in the formation of some selected frameworks has been studied, compared to its counterpart with nonbinding ¿H groups (diphenylmethane- 4,4¿-dicarboxylic acid, H2L(2)). It is worth highlighting that most of the materials obtained using these linkers have shown a good performance as heterogeneous catalysts in hydrogenation and hydrosilylation reactions under mild conditions. In addition, a flexible calcium MOF with selective sorption properties (both in liquid and gas phases) has been obtained using H2L(1) . This material exhibits a reversible phase transition related to the remove/uptake of guest molecules that has been studied in detail. These results are presented in Chapter 4. Continuing with the flexible dicarboxylate H2L(1) ligand, in the case of Mg, an exhaustive study was performed to elucidate the effect of the introduction of nitrogenated chelating ancillary ligands in the formation of different supramolecular frameworks. Additionally, computational studies were performed to show the relative energy for the five obtained networks, allowing a better understanding of the processes that govern the formation of these Mg MOFs. These results are presented in Chapter 5. Finally, the other part of this work is based on the use of an anthraquinone derivate of disulfonate ligand (anthraquinone-2,6-disulfonate, 2,6-AQDS) to obtain new alkaline-earth MOFs. This rigid disulfonate linker has been before proved in our research group to be a suitable linker to obtain materials with high thermal stability. Moreover, as sulfonate groups can exhibit a wide variety of coordination modes, the use of this ligand can also lead to the formation of different alkaline-earth coordination environments. In this sense, novel materials with Mg, Ca, Sr and Ba have been successfully synthesized and tested as heterogeneous catalysts in hydrogenation and hydrosilylation reactions. These results are presented in Chapter 6.