Analysis of propoxylation with zinc–cobalt double metal cyanide catalysts with different active surfaces and particle sizes

Abstract

The action of three different Co/Zn double-metal-cyanide (DMC) catalysts in the propoxylation of polyols was compared in a kinetic study, using pulse-wise feeding of propylene oxide (PO). Key insights are that the catalysis proceeds by an external attack of hydroxyl chain ends on coordinated PO and that the different diffusion rates of PO and polyols result in the broadening of the distribution. This can be visualized by the introduction of finger printing. The initial [PO] decay had a first order dependence on the catalyst, PO and hydroxyl concentrations. The temperature dependence of the characteristic product k_sK (k_s: rate constant for ring-opening; K: equilibrium constant for PO coordination) showed that the rate determining step is most compatible with a PO ring-opening by a direct external nucleophilic attack of a hydroxyl group or one modulated by the diffusion rate of PO. Consideration of the number of crystallites per unit volume (related to the crystal size) and diffusion leads to a consistent description of the catalysis in terms of the rate and polydispersity of the product.