Mechanistic insights into Schiff base incorporated double metal cyanide catalyst for the synthesis of colorless polyether polyols

Abstract

The organic complexing agents (CAs) with multidentate sites help to enhance the active sites (Zn) of double metal cyanide (DMC) catalysts and also control the metal leaching from the DMC catalysts. Schiff base ligands (SBLs) are versatile and impart the coordination ability to bind with transition metals to form metal complexes. In this study, various SBLs are synthesized, and their use as CAs in DMC catalysts is thoroughly investigated. The metal complex formation is confirmed by structurally sensitive techniques, including SEM, PXRD, FT-IR, TGA, UV–Vis, AAS, CHNS, DLS, 1H-13C, XPS, and 13C-CP MAS spectroscopy. The 13C-CP MAS spectroscopy revealed strong coordination from N and O to the Zn sites of the DMC catalysts. XPS survey confirmed the presence of three types of cyanide (CN) carbons (axial, coordinated, and uncoordinated), four types of nitrogen (three from CN and one from azomethine), and Zn in +2 oxidation state. The ring-opening polymerization (ROP) was carried out using the prepared DMC catalysts to produce high-quality colorless polyether polyols (PPG). At an optimized reaction condition of catalyst amount = 250 ppm, time = 50 min, pressure = 5 bar, temperature = 105 °C, PO : PEG-600 = 4400 : 1, a maximum PPG yield (>95 %) is obtained with turnover frequency (TOF) of 11,292 h−1 (mol-PO per mol-Zn per hour). Density functional theory (DFT) calculations were also performed to understand the nature of the active site and mechanistic insight into the polymerization reaction.