Research

The group of Metal-Organic Frameworks led by Dr. Francesc X. Llabrés i Xamena, specializes in the study of crystalline porous materials, Metal-Organic Frameworks (MOFs), Covalent Organic compounds and Porous Organic Capsules, and their use in various technological applications, including heterogeneous catalysis. Within this context, our holistic approach is focused on three main topics:

The modular construction of MOFs provides an outstanding platform for designing a vast number of novel high-performance functional materials. It allows us fine-tuning the chemical composition, properties and pore architecture by smart selection of the metal and organic ligands forming the MOF and by controlling the way in which these sub-units are connected. However, sometimes it is very challenging to introduce the desired functionalities in a MOF by direct synthesis. In these cases, post-synthesis modification (PSM) come into play. We have developed various PSM strategies, mainly focused at introducing atomically precise catalytic sites on pre-formed MOFs. Those include the formation of salicylidene imine Schiff base metallic complexes or the use of oxalate auxiliary linkers.
Recently, Defect engineering (DE), or the deliberate creation of defects in MOFs, has emerged as a very effective tool for tailoring material properties in sorption and catalysis. One way of introducing defects during the synthesis of the MOF is by using a “defective-linker approach", whereby the regular linker for the synthesis of the MOF is mixed in appropriate amounts with a geometrically equivalent second linker featuring a reduced connectivity. This strategy has been successfully applied in our group to the preparation of Ru, Cu and Zr-containing MOFs with tunable catalytic properties.
Other relevant contributions from our group to MOF synthesis are the preparation of mixed-metal and mixed-ligand compounds with controlled spatial distribution within the crystalline framework, the encapsulation of monometallic and bimetallic core-shell metal nanoparticles, or the preparation of multi-functional MOFs containing two or more chemical functionalities for cooperative or multi-step catalytic transformations.

Precise understanding of the structural and physicochemical properties, as well as the local (intracavity) environment of the active sites in MOFs, is crucial to further develop their catalytic potential. Our goal is to contribute to the characterization of MOFs beyond routinely used laboratory techniques, by resorting to synchrotron based spectroscopies (such as EXAFS/XANES) and ab initio x-ray and electron diffraction. Collaborating with the group of Veronique Van Speybroeck (Ghent University), computational tools have allowed us to understand how these centers interact with the reaction substrates, thus assisting in the proposal of sound reaction mechanisms. We have further used advanced tomographic focused ion beam scanning electron microscopy (FIB-SEM) combined with 3D image reconstruction for the quantification of structural parameters of MOF-polymer composite membranes for gas separations.

No doubt, our primary effort has been focused on the development of highly efficient heterogeneous catalysts (and photocatalysts) based on MOFs. We started our research in this area as early as 2006, when the number of articles describing the use of MOFs in catalysis was still very scarce. Thus, our works have pioneered the study of important aspects in catalysis, such as catalyst stability and reusability, shape-selectivity, or the preparation of multi-functional MOFs for cooperative catalysis or multi-step cascade transformations. Throughout these years, our goal has been to engineer at the molecular level the active sites of a MOF and their chemical environment to maximize its performance for a give reaction. Common targeted reactions have been the synthesis of high added value fine chemical compounds and biomass derived products. These are usually complex molecules, featuring several functionalities, requiring highly chemo-, regio- and stereoselective catalysts for their synthesis. Beyond the structure of the active site, control of the pore dimensions and connectivity of the MOF, and the noncovalent host-guest interactions inside the pores, introduce highly valuable shape/size-selective properties to drive the reaction pathway through the desired outcome from various possible products.