Metal-organic frameworks (MOFs) are highly ordered microporous materials that are of significant importance in applications as diverse as gas separation and storage, catalysis, and biomedicine. The design of new MOFs requires extensive, systematic studies under variation of numerous parameters, which will provide critical information in the design of new, technically important materials. In this work, we are focusing on the well-explored copper based MOF HKUST-1, bearing the 1,3,5-benzenetricarboxylate ligand, which was chosen due to its reproducible formation and technical importance. To this effect we have used time-resolved synchrotron small-angle X-ray scattering (SAXS), simultaneously combined with wide-angle X-ray scattering (WAXS), to study in situ the formation and the structural changes during mixing, nucleation/growth, and final crystallization of the copper-based MOF. In these experiments, which have been carried out at the Austro-SAXS beamline at the ELETTRA synchrotron (Trieste, Italy) – we used a batch reactor setup with constant stirring, pumping the solution of 1,3,5-benzenetricarboxylate in various solvent systems in a continuous flow and closed loop-mode through a quartz capillary centered in the X-ray beam-path. The time-dependent development of nano-particle size, morphology, and crystallinity could be extracted from the time-resolved SAXS-data. SAXS analysis was focused on the evaluation of the Guinier radius and the Porod invariant (nano-particle formation), whereas WAXS analysis (peak fitting of the emerging Bragg-peaks) gave us insight in the temporal process of crystallisation. The results (kinetics and structural model) of the in situ HKUST-1 formation using different copper salts and/or solvent systems will be presented.

In-situ formation of the Cu-based MOF HKUST-1 studied by time resolved S/WAXS

ACS Fall 2021

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