Continuous synthesis of UiO-66 in microreactor: Pursuing the optimum between intensified production and structural properties

The increasing demand for MOFs can only be satisfied by establishing robust and reproducible synthesis routes which allow a profitable scale-up of the requested materials while preserving their textural properties and stability. A scalable MOF synthesis may provide a technology platform for the further development and modification of the product, from both a chemical and an engineering perspective. And, above all, MOFs could be delivered to the market at a substantially reduced price. In this work UiO-66 was synthesized continuously by employing micro reactors and the concept of flow chemistry. The microreaction technology (MRT) process turned out to be a viable alternative to the time and energy consuming conventional synthesis processes, as significantly faster reaction rates could be achieved compared to the conventional batch processes (Zhou et al., 2014; Frameworks for Commercial Success, 2016). Microreactors are characterized in particular by high surface-to-volume ratios and channel dimensions in the submillimeter range, which offer a significant enhancement of heat and mass transfer within the reactor, additionally the chemical reactions can be accelerated by orders of magnitude and can thus be processed close to their kinetic optimum conditions. Different reaction parameters were studied in order to elucidate their influence on the product properties. All synthesized MOF crystals were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis and nitrogen adsorption measurements. The MRT approach allowed the continuous fabrication of MOF crystals with distinct morphological characteristics in a time-efficient manner. The obtained results represent a faster and energy-efficient route to continuously produce UiO-66, compared to the standard synthetic protocols. The materials prepared in this work have a lower coordination of the Zr sites in comparison to the UiO-66 that can be produced in batch (Cavka et al. 2008); nevertheless the presence of coordinative unsaturated sites is a preferred feature of MOFs for some specific application such as catalysis (Shearer et al., 2014).