Reaction Mechanisms for the Formation of Mono- And Dipropylene Glycol from the Propylene Oxide Hydrolysis over ZSM‑5 Zeolite

Stepwise and concerted mechanisms for the formation of mono- and dipropylene glycol over ZSM-5 zeolite were investigated. For the calculations, a T128 cluster model of zeolite was used with a QM/QM scheme to investigate the reaction mechanism. The active inner part of zeolite was represented by a T8 model and was treated at the DFT (BP86) level, including D3 Grimme dispersion, and the outer part of the zeolite was treated at the DFTB level. The solvent effects were taken into account by including explicitly water molecules in the cavity of the zeolite. The Gibbs energies were calculated for both mechanisms at 70 °C. In the case of the stepwise mechanism for the monopropylene glycol formation, the rate-limiting step is the opening of the epoxide ring. The activation energy for this process is 35.5 kcal mol^–1, while in the case of the concerted mechanism the rate-limiting step is the simultaneous ring opening of the epoxide and the attack by a water molecule. This process has an activation energy of 27.4 kcal mol^–1. In the case of the stepwise mechanism of the dipropylene glycol formation, the activation energy for the rate-limiting step is the same as for the monopropylene glycol formation, and in the case of the concerted mechanism, the activation energy for the rate-limiting step is 30.8 kcal mol^–1. In both cases (mono- and dipropylene glycol formation), the concerted mechanism should be dominant over the stepwise one. The barrier for monopropylene glycol formation is lower than that for dipropylene glycol formation. Consequently, our results show that the formation of the monopropylene glycol is faster, although the formation of dipropylene glycol as a byproduct cannot be avoided using this zeolite.