Nature of active sites on UiO-66 and beneficial influence of water in the catalysis of Fischer esterification

[EN] 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 Bronsted 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 Bronsted sites and by providing stabilization for various intermediates through hydrogen bonds. (C) 2017 The Authors. Published by Elsevier Inc.This work is supported by the Fund for Scientific Research Flanders (FWO) (project number 3G048612), the Research Board of Ghent University (BOF) and BELSPO in the frame of IAP/7/05. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 641887 (project acronym: DEFNET). Funding was also received from the European Union's Horizon 2020 research and innovation programme [consolidator ERC grant agreement no. 647755-DYNPOR (2015-2020)]. Computational resources (Stevin Supercomputer Infrastructure) and services were provided by Ghent University. Financial support from the Generalitat Valenciana (project AICO/2015/065), the Spanish Ministry of Economy and Competitiveness (program Severo Ochoa SEV20120267), and the Spanish Ministry of Science and Innovation (project MAT2014-52085-C2-1-P) is gratefully acknowledged.Caratelli, C.; Hajek, J.; García Cirujano, F.; Warroquier, M.; Llabrés I Xamena, FX.; Van Speybroeck, V. (2017). Nature of active sites on UiO-66 and beneficial influence of water in the catalysis of Fischer esterification. Journal of Catalysis. 352:401-414. https://doi.org/10.1016/j.jcat.2017.06.014S40141435