V. Van Speybroeck

Zirconium terephthalate UiO-66 type metal organic frameworks (MOFs) are known to be highly active, stable and reusable catalysts for the esterification of carboxylic acids with alcohols. Moreover, when defects are present in the structure of these MOFs, coordinatively unsaturated Zr ions with Lewis acid properties are created, which increase the catalytic activity of the resulting defective solids. In the present work, molecular modeling techniques combined with new experimental data on various defective hydrated and dehydrated materials allow to unravel the nature and role of defective active sites in the Fischer esterification and the role of coordinated water molecules to provide additional Brønsted sites. Periodic models of UiO-66 and UiO-66-NH2 catalysts have been used to unravel the reaction mechanism on hydrated and dehydrated materials. Various adsorption modes of water and methanol are investigated. The proposed mechanisms are in line with experimental observations that amino groups yield a reduction in the reaction barriers, although they have a passive role in modulating the electronic structure of the material. Water has a beneficial role on the reaction cycle by providing extra Brønsted sites and by providing stabilization for various intermediates through hydrogen bonds.

We propose a methodology to derive pairwise-additive noncovalent force fields from monomer electron densities without any empirical input. Energy expressions are based on the symmetry-adapted perturbation theory (SAPT) decomposition of interaction energies. This ensures a physically motivated force field featuring an electrostatic, exchange repulsion, dispersion, and induction contribution, which contains two types of parameters. First, each contribution depends on several fixed atomic parameters, resulting from a partitioning of the monomer electron density. Second, each of the last three contributions (exchange-repulsion, dispersion, and induction) contains exactly one linear fitting parameter. These three so-called interaction parameters in the model are initially estimated separately using SAPT reference calculations for the S66x8 database of noncovalent dimers. In a second step, the three interaction parameters are further refined simultaneously to reproduce CCSD(T)/CBS interaction energies for the same database. The limited number of parameters that are fitted to dimer interaction energies (only three) avoids ill-conditioned fits that plague conventional parameter optimizations. For the exchange repulsion and dispersion component, good results are obtained for all dimers in the S66x8 database using one single value for the associated interaction parameters. The values of those parameters can be considered universal and can also be used for dimers not present in the original database used for fitting. For the induction component such an approach is only viable for the dispersion dominated dimers in the S66x8 database. For other dimers (such as hydrogen-bonded complexes), we show that our methodology remains applicable. However, the interaction parameter needs to be determined on a case-specific basis. As an external validation:, the force field predicts interaction energies in good agreement with CCSD(T)/CBS values for dispersion dominated dimers extracted from an HIV-II protease crystal structure with a bound ligand (indinavir). Furthermore, experimental second virial coefficients of small alkanes and alkenes are well reproduced.

In this Article, we present a molecular-level understanding of the experimentally observed loss of crystallinity in UiO-66-type metal–organic frameworks, including the pristine UiO-66 to -68 as well as defect-containing UiO-66 materials, under the influence of external pressure. This goal is achieved by constructing pressure-versus-volume profiles at finite temperatures using a thermodynamic approach relying on ab initio derived force fields. On the atomic level, the phenomenon is reflected in a sudden drop in the number of symmetry operators for the crystallographic unit cell because of the disordered displacement of the organic linkers with respect to the inorganic bricks. For the defect-containing samples, a reduced mechanical stability is observed, however, critically depending on the distribution of these defects throughout the material, hence demonstrating the importance of judiciously characterizing defects in these materials.

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http://pubs.acs.org/doi/abs/10.1021/acs.chemmater.6b01956