Kinetic and Thermodynamic Requirements for Polyoxymethylene Dimethyl Ether Synthesis Catalyzed by Ion-Exchange Resin

Rate assessments and chemical uptake studies probe the kinetic and thermodynamic requirements for the synthesis of polyoxymethylene dimethyl ethers (OMEn, H3CO-(CH2O)(n)-CH3, n = 1-10) from methanol and formaldehyde molecular building blocks, derived from either 1,3,5-trioxane or paraformaldehyde, at the interface of 1,4-dioxane solvent-Amberlyst 15 ion-exchange resin. Starting from methanol and trioxane reagents, the initial C-O scission of trioxane restricts the overall turnovers, whereas the subsequent C-O formation reactions between methanol and CH2O building blocks that form OMEn are chemically equilibrated. Both methanol and 1,4-dioxane solvent inhibit trioxane turnovers by occupying Br & oslash;nsted acid sites (H+) and preferentially solvating the precursor state (acid sites and trioxane reagent) relative to the C-O scission transition state. Using paraformaldehyde instead of trioxane as the formaldehyde building blocks eliminates the C-O scission kinetic bottleneck, thus leading to much faster time scales toward attaining chemical equilibrium. Irrespective of the chemical origin of the formaldehyde building blocks, all oligomerization reactions via C-O bond formations are equilibrated. As a result, a coupled kinetic-thermodynamic parameter (Phi), defined as the instantaneous molar ratio of oxymethylene monomers and methanol throughout the reaction, is the sole descriptor of the observed selectivity trends for all reaction products. The interplay between kinetic and thermodynamic factors allows for a better understanding and modeling of OMEn synthesis reactions.