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

Abstract

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 ratelimiting 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.

Kinetic study of the hydration of propylene oxide in the presence of heterogeneous catalyst

“…investigated stepwise and concerted mechanisms for the formation of PG and DPG over ZSM-5 zeolite. They report that although the formation of the PG is faster, the formation of DPG as a byproduct cannot be avoided using ZSM-5 zeolite [14].…”

Liquid-Phase Epoxidation of Propylene with H2O2 over TS-1 Zeolite: Impurity Formation and Inhibition Study

“…The phenomenon is in line with the literature, which suggested the condensation of PO with 2M1P would lead to the generation of DPGME. 2 With respect to the formation of HAT, DPG, AA, PA, and AT, literature studies have mentioned that HAT might be formed by the oxidation of PG, 59 DPG by the condensation of PO with PG, 35 AA by the oxidation of propylene, 60 and PA and AT by the isomerization of PO. 61,62 Here, the variation trends of the relative content of these impurities with the HZSM-5 content of the blended "epoxidation catalysts" agree with these points of view.…”

Chirality Retention in Aqueous Propylene Oxide Hydration: Chirality of the Transition State

“…It is well known that with chiral catalysts, this reaction can be catalyzed with very high enantioselectivity. [9][10][11] Hydration occurs by opening of the epoxide ring and is known to happen slowly without the need for a catalyst. The epoxide ring opening can occur at either of the CÀ O bonds and depending on the acidic or basic conditions, primary or secondary alcohols are formed.…”