In situ Monitoring of Double Metal Cyanide (DMC) Catalyst Synthesis by Raman Spectroscopy

Double metal cyanide (DMC) catalyst is widely used for the alkoxylation reaction to produce polyether polyol from ethylene/propylene/butylene oxides. It is challenging to optimize the synthesis process, due to the lack of real-time understanding of the speciation of the reaction mixture. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and actual alkoxylation reaction performance are effective ways for the evaluation of success of each synthesis, but it is difficult to guide process optimization. An in situ Raman method is developed in this study to monitor the DMC catalyst synthesis in real time to accelerate the process optimization. The synthesis involves the reaction of ZnCl2 and K3Co(CN)6 (KHCC) to form Zn3[Co(CN)6]2 (ZHCC). ZHCC is then converted to DMC in the presence of excess of t-butanol and ZnCl2 in the second step. Characteristic KHCC Raman peaks were observed at 2138 and 2153 cm-1, ZHCC at 2185 and 2206 cm-1, and DMC at 2203 and 2225 cm-1, respectively. This enables realtime tracking of both steps’ conversion. Both t-butanol and ZnCl2 concentrations were found to substantially influence the kinetics of DMC formation, but not the Raman spectra of the final DMC products. The reaction time could be adjusted from hours to minutes through the control of reactant concentrations.