Modulator Control of Cation Distribution in Mixed-Metal UiO-66(Zr,Ce) Metal–Organic Frameworks

Abstract

The cation distribution and redox activity of the [(CexZr6-x)(OH)4O4]12+ clusters in bimetallic UiO-66(Ce/Zr) MOFs were modified by varying the choice of modulator during synthesis. Extended X-ray absorption spectroscopy (EXAFS) at the Ce and Zr K-edges, in situ Fourier-transform infrared (FTIR) spectroscopy, along with density functional theory (DFT) calculations reveal that using benzoic acid as a modulator leads to the preferential formation of the [(cis-Ce2Zr4)(OH)4O4]12+ cluster in UiO-66 (Ce/Zr) with higher Ce incorporation (33% Ce) than formic acid-modulated synthesis, which predominantly contains a [(CeZr5)(OH)4O4]12+ cluster (17% Ce). Analysis of the reaction solution using Ce K-edge EXAFS and Ce LIII-edge X-ray absorption near edge structure (XANES) shows how the formation of isolated Ce(III) in the synthesis solution limits the uptake of Ce(IV) into the UiO-66 structure for both modulators, but the effect is less for the benzoic acid modulator, explaining the enhanced overall incorporation of Ce. The local metal distribution of Ce and Zr in the M6 clusters is found to significantly impact the catalytic activity in the benzyl alcohol oxidation reaction using microwave-assisted heating, with the cis-Ce2Zr4 clusters exhibiting higher redox activity than CeZr5; this is rationalized by the greater possibility of removing oxide ions that bridge a pair of Ce(IV) in the cluster to mediate the catalytic transformation. The results show how the choice of modulator during MOF synthesis can influence local distribution of metal cations, providing a subtle way to fine-tune properties beyond the average crystal structure.