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Ben Slater (University College London)


Characterising Defects in MOFs

Department of Chemistry, University College London, London WC1H 0AJ , United Kingdom

Keywords: defect, DFT, catalysis, vacancy

In this talk I will endeavour to summarise our efforts to provide a basic insight into defect formation energetics and characterisation of defects in MOFs, in the crystal interior and exterior. I will focus chiefly on UiO-66, mainly because defects are key to the catalytic behaviour of this material. As has been well documented, UiO-66 can tolerate exceptionally high concentrations of missing linker defects, where modulators such as formic acid replace the benzene dicarboxylate (BDC) groups. The nature of the acid sites associated with the defects has been disputed [1,2,3] but theory provides a consistent prediction of the nature of the defect centres, especially in the case when hydroxide acts as the capping/charge compensating species. Whilst the local structure of point defects in UiO-66 are well understood, the distribution of point defects and “missing cluster” defects as reported by Cliffe et al.[4] is less clear. Cliffe et al. postulated and produced evidence from diffuse scattering experiments which suggests that ordered missing cluster defects, corresponding to a nanodomain of reotopology, within the perfect UiO-66 fcutopology. I will show that we have been able to shed more light on the integrity of UiO-66 through a combination of theory, HRTEM and catalytic test reactions. Our work reveals that defective UiO-66 consists of fcu, reo and two newly identified nanodomains of bcu and scu. Intriguingly, the evolution of these nanodomains has been followed in two different modes of crystal growth: moderate concentration of modulator (50:1, formate: BDC)and high concentration of modulator (150:1, formate: BDC). In the former case, Ostwald ripening is observed and the crystals mature, containing a distribution of fcu, reo, bcu and scu to tend towards perfect fcu. The latter results in further ligand exchange resulting in increasing concentration of scu. Perhaps most interestingly, through performing catalytic test reactions at different points in the genesis of UiO-66, we make a clear distinction between their activity of UiO-66 that contains missing linkers only and that which also contains missing clusters. We find evidence that missing clusters are demonstrably more reactive than missing linkers, which points to the use of high modulator concentrations to engineer in potent active sites.

[1] Caratelli, C et al., J. Catal. 352, 401-414, 2017
[2] Ling, S. and Slater, B. Chem. Sci. 7, 4706-4712, 2016
[3] Trickett, C.A. et al., Angew. Chem., Int. Ed. 54, 11162-11167, 2015
[4] Cliffe, M.J. et al., Nat. Commun. 5, 4176, 2014