It remains difficult to understand the surface of solid acid catalysts at the molecular level, despite their importance for industrial catalytic applications. A sulfated zirconium-based metal–organic framework, MOF-808-SO4, was previously shown to be a strong solid Brønsted acid material. In this report, we probe the origin of its acidity through an array of spectroscopic, crystallographic and computational characterization techniques. The strongest Brønsted acid site is shown to consist of a specific arrangement of adsorbed water and sulfate moieties on the zirconium clusters. When a water molecule adsorbs to one zirconium atom, it participates in a hydrogen bond with a sulfate moiety that is chelated to a neighbouring zirconium atom; this motif, in turn, results in the presence of a strongly acidic proton. On dehydration, the material loses its acidity. The hydrated sulfated MOF exhibits a good catalytic performance for the dimerization of isobutene (2-methyl-1-propene), and achieves a 100% selectivity for C8 products with a good conversion efficiency.
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Synthetic and experimental procedures, as well as crystallographic, spectroscopic and computational data are provided in the Supplementary Information. Crystallographic data for the structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 1871192 (MOF-808), 1871193 (MOF-808-SO4) and 1871194 (MOF-808-SeO4). Copies of the data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. All other data supporting the findings of this study are available within the article and its Supplementary Information, or from the corresponding author upon reasonable request.
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This work, including the synthesis, characterization and crystal structure analysis, was funded by BASF SE and the US Department of Defense, Defense Threat Reduction Agency (HDTRA 1-12-1-0053). Work performed at the Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract no. DE-AC02-05CH11231. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. NMR work was supported as part of the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award no. DE-SC0001015. T.M.O.P. acknowledges funding from the NSF Graduate Research Fellowship Program. C.Y. acknowledges support from a Hewlett-Packard Stanford Graduate Fellowship. P.U. acknowledges the German Research Foundation (DFG, PU 286/1-1). M.J.K. is grateful for financial support through the German Research Foundation (DFG, KA 4484/1-1). We acknowledge B. Rungtaweevoranit for his assistance with electron microscopy, and S. Teat and L. McCormick for the synchrotron X-ray diffraction data acquisition support at beamline 11.3.1 of the Advanced Light Source, Lawrence Berkeley National Laboratory.
The authors declare no competing interests.
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Synthetic and experimental procedures; Crystallographic, spectroscopic and computational data; Supplementary Figures 1–30; Supplementary Tables 1–7; Supplementary References 1–14
CIF for MOF-808; CCDC reference: 1871192
Structure-factor file for MOF-808; CCDC: reference 1871192
CIF for MOF-808-SO4; CCDC reference: 1871193
Structure-factor file for MOF-808-SO4; CCDC reference: 1871193
CIF for MOF-808-SeO4; CCDC reference: 1871194
Structure-factor file for MOF-808-SeO4; CCDC reference: 1871194
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Trickett, C.A., Osborn Popp, T.M., Su, J. et al. Identification of the strong Brønsted acid site in a metal–organic framework solid acid catalyst. Nature Chem 11, 170–176 (2019) doi:10.1038/s41557-018-0171-z
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