Reaction Equilibrium and Kinetics of Synthesis of Polyoxymethylene Dimethyl Ethers from Formaldehyde and Methanol

Abstract

Polyoxymethylene dimethyl ethers (PODE_n) are environmentally friendly diesel fuel additives. They belong to alkyl ethers that could reduce solid particulate matter formation and emissions of carbon monoxide and nitrogen oxide when added into diesel fuels. This work aimed to researching chemical equilibrium and reaction kinetics of the synthesis of polyoxymethylene dimethyl ethers from formaldehyde and methanol catalyzed by an ion-exchange resin at the reaction temperatures 313, 333, 343 and 353 K. In the reversible reaction, the K_{n ≥ 2}/K₂ ratio was equal to one. The reaction orders of methanol, formaldehyde, water and PODE_n were 0.2638, 0.1328, 0.1565 and 0.0048, respectively. At a 10 wt % dosage of H-SIR1 resin, the rate constants of the methylal (dimethoxymethane) formation and depolymerization were 1.04 × 10⁴ and 3.43 × 10⁶ min^–1, respectively, and the pre-exponential factor for the PODE_{n + 1} formation was 2.50 × 10³ min^–1. Activation energies for the methylal propagation and depolymerization and PODE_{n > 1} formation were 30.46, 48.40 and 27.10 kJ/mol, respectively. The results indicated that the equilibrium constants of PODE_{n > 1} formation reactions were consistent. The exothermic reaction of methylal formation was easier than the reverse reaction and more difficult than the formation of PODE_{n > 1}.