In situ Raman spectroscopy for monitoring ZIF-8 synthesis using twin-screw extrusion Nikita Gugin, José A. Villajos, Michael Maiwald and Franziska Emmerling Federal Institute for Materials Research and Testing (BAM), Germany ZIF-8 or Zn(meIm) 2 (meIm = deprotonated 2-methylimidazole) is a prominent member of the zeolitic imidazolate frameworks (ZIFs) subfamily of MOFs which possesses high thermal, chemical, and mechanical stabilities [1] . Different routes have been explored to achieve cheaper synthetic approaches [2] but the large gap between its laboratory and commercial production persists. Aimed at developing scalable and greener production of ZIF-8, we adapted our recently-reported in-batch „mix and wait“ synthetic procedure [3] to continuous extrusion. To monitor the ZIF-8 formation over different experimental conditions and optimize the extrusion process, in-situ Raman spectroscopy was applied. The synthesis of ZIF-8 was performed using a twin-screw extruder ZE 12 HMI (Three-Tec GmbH, Switzerland). Automatic volumetric feeder ZD 12B (Three-Tec GmbH, Switzerland) and peristaltic pump BT-L (Lead Fluid, China) were used to supply the solid and liquid components to the extruder barrel. The process was monitored in six different zones along the barrel using Raman RXN1 TM analyser (Kaiser Optical Systems, France) with a non-contact probe head (working distance of 6 cm). PMMA screw-in parts, which are transparent to Raman laser radiation, were specially designed and manufactured to provide the laser focus close to the central conjunction of the two screws within the barrel, avoiding any dead volume between the screws and the probe where the extrudate can cumulate. Ex-situ powder X-Ray diffraction (PXRD) and thermogravimetric analysis (TGA) were used as complementary techniques to characterise the extrudates. The pre-mixed basic zinc carbonate (ZnCarb) and HmeIm were continuously fed in the inlet port at a rate of 1.5 g min -1 . To initiate the reaction, a catalytic amount of water at a rate of 0.15 mL min -1 was continuously pumped into the barrel. As evidenced by Raman spectra collected at the different extruder zones, the reaction takes place at 25 °C, and it progresses along the entire length of the barrel. The screws configured with one mixing and 15 conveying sections rotating at 10 rpm ensured a residence time of ca. 8 min, as was determined by monitoring a tracer (paracetamol) close to the extruder outlet. After washing the extrudate with EtOH, highly crystalline ZIF-8 with 93 % ZnCarb-to-ZIF-8 conversion was isolated. The effects of the liquid feeding rate, barrel length, and temperature on the product properties were further explored in order to increase the conversion of the Zn- source. These results provide highly valuable information for understanding the mechanism of ZIF-8 formation by extrusion, as well as for optimizing this process for economic and large-scale production. References 1. K. S. Park, Z. Ni, A. P. Cote, J. Y. Choi, R. Huang, F. J. Uribe-Romo, H. K. Chae, M. O'Keeffe, O. M. Yaghi, Proc Natl Acad Sci U S A 2006 , 103 , 10186-10191. 2. R. F. Wu, T. Fan, J. Y. Chen, Y. W. Li, Acs Sustainable Chemistry & Engineering 2019 , 7 , 3632-3646. 3. N. Gugin, J. A. Villajos, I. Feldmann, F. Emmerling, Rsc Advances 2022 , 12 , 8940-8944.
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