Application of Raman and chemometrics to the mechanochemical synthesis of 4,4-bipyridine/cobalt based MOF using twin screw extrusion

Abstract

The process optimization of 4,4-bipyridine based metal organic framework (MOF) synthesis by twin screw extrusion (TSE) is investigated using cobalt nitrate hexahydrate (Co(NO3)2·6H2O) as the metal precursor. The effects of operating parameters including liquid to solid ratio (L/S), screw speed and feeding rate on the product composition are examined. The collected MOF samples were characterized offline by powder X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, and scanning electron microscopy. It was found that the explored feeding and screw rates are insufficient to generate a pure MOF. However the MOF product was obtained using an optimum L/S ratio i.e., 0.5, which improved the wettability and mixing of reactants to allow effective interactions at a molecular level, facilitating the complete reaction inside the extruder. Partial least square based multivariate analysis of the collected Raman spectra was used to obtain predictive models to quantify MOF purity, which reached a maximum of 87.0 wt% using 0.5 L/S, 100 g/h feeding rate and 100 rpm screw speed.