Kinetics of synthesis of polyoxymethylene dimethyl ethers from paraformaldehyde and dimethoxymethane catalyzed by ion-exchange resin

Polyoxymethylene dimethyl ethers (CH3-O-(CH2O)n-CH3, PODEn, n>1) are new concerned environmental benign alternative components for diesel fuels. This work aimed to investigate the kinetics of synthesis of PODEn from paraformaldehyde and dimethoxymethane catalyzed by ion-exchange resin NKC-9. Experiments were conducted in a designed space, namely reaction temperatures (60, 70, and 80 °C) vs. reaction times (2, 5, 10, 20, 30, 60, and 90 min), in a batch stirred autoclave. The transient molecular size distribution of PODEn compounds from paraformaldehyde and methylal followed Schulz-Flory distribution model. In this system, the concentration of formaldehyde in the homogenous solution (CF) was nearly constant. The sequential reversible reactions to produce PODEn were verified to follow a second-order kinetics for propagation and a first-order kinetics for depolymerization. The rate constants of propagation (kp) and depolymerization (kd) and the reaction equilibrium constant Kn were the same for the series of PODEn synthesis reactions. Respecting to 5 wt% dosage of NKC-9 resin catalyst, the pre-exponential factors Ap for propagation and Ad for depolymerization were 1.84×107 L mol-1 min-1 and 5.36×106 min-1, respectively. The activation energy Ep (39.52 kJ mol-1) for propagation was lower than Ed (52.01 kJ mol-1) for depolymerization, validating that the reversible reactions of producing PODEn compounds were exothermic.