Theoretical Insights into the Reaction Mechanism of Direct Hydrogenation of Maleic Anhydride to Produce 1,4-Butanediol on the Cu–ZnO Surface

1,4-Butanediol is a crucial monomer for the production of biodegradable plastics such as polybutylene succinate (PBS) and polybutyleneadipate-co-terephthalate (PBAT). It is also utilized in the synthesis of derivatives such as γ-butyrolactone and tetrahydrofuran. The technology of direct hydrogenation of maleic anhydride to produce 1,4-butanediol on Cu-based catalysts has gained significant attention due to its short process, mild reaction conditions, cost-effective catalysts, and the ability to cogenerate various products. This makes it a promising avenue for the production of 1,4-butanediol. At present, the reaction mechanism for the direct hydrogenation of maleic anhydride to produce 1,4-butanediol on Cu–ZnO is not well understood, and the types and pathways of byproducts remain unclear. This lack of clarity hinders the modification and application of catalysts for the direct hydrogenation of maleic anhydride to produce 1,4-butanediol. This study systematically investigates the reaction mechanism of direct hydrogenation of maleic anhydride to produce 1,4-butanediol on Cu–ZnO using spin-polarized density functional theory. The adsorption properties of surface species were studied, revealing that key species succinic anhydride, γ-butyrolactone, 1,4-butanediol, tetrahydrofuran, and n-butanol exhibit more stability in adsorption at the Cu211–ZnO interface compared to noninterface regions. The optimal pathway for the main reaction of direct hydrogenation of maleic anhydride on the Cu211 surface is clarified as MA → C4H3O3 → SA → C4H5O3 → C4H4O2 → C4H5O2 → GBL. At the Cu211–ZnO interface, the optimal pathway for the main reaction of direct hydrogenation involves MA → C4H3O3 → SA → C4H4O3 → C4H5O3 → C4H4O2 → C4H5O2 → GBL → C4H6O2 → C4H7O2 → C4H8O2 → C4H9O2 → BDO. The study indicates that the Cu211–ZnO interface is more favorable for the main reaction of direct hydrogenation of maleic anhydride. The most probable pathway for the formation of byproduct butyric acid during the direct hydrogenation on Cu211–ZnO involves MA → C4H2O3 → C4H3O3 → C4H4O3 → C4H5O3 → C4H6O3 → C4H5O2 → C4H6O2 → C4H7O2 → C4H8O2. The optimal path for the production of butanol involves the process starting with butyric acid, i.e., C4H8O2 → C4H9O2 → C4H8O → C4H9O → BuOH. Propionic acid is most likely formed through MA, i.e., MA → C4H2O3 → C4H3O2 → C3H4O2 → C3H5O2 → C3H6O2. Propionic acid and propionaldehyde are more likely byproducts in the system, with propyl alcohol being difficult to generate due to its higher energy barrier. The most probable pathway for CO production involves SA → C4H4O3 → C3H4O2 + CO. In the presence of water, the rate-controlling step for the generation of maleic acid from MA is C4H2O3 + H → C4H3O3. SA is more inclined to generate succinic acid, succinaldehyde, and GBL, rather than γ-hydroxybutyric acid and BDO, with the rate-controlling step being C4H7O2 → C4H6O2 + H. Water is more likely to form through the combination of two OH groups. During the catalyst construction and modification processes, it is advisable to construct as many Cu211–ZnO interfaces as possible within a reasonable range to enhance the production of 1,4-butanediol. Suppressing the open-loop reaction of maleic anhydride can effectively inhibit the generation of byproducts such as butyric acid, butyraldehyde, butanol, propionic acid, propionaldehyde, and maleic acid. Timely removal of water generated in the system is essential to prevent the transformation of maleic anhydride into maleic acid and the conversion of succinic anhydride into succinic acid and succinaldehyde. This helps minimize raw material consumption and reduce the formation of byproducts. The elucidation of the reaction mechanism of direct hydrogenation of maleic anhydride on Cu–ZnO provides valuable insights and guidance for the construction and modification of catalysts, enhancement of 1,4-butanediol yield and purity, and optimization and improvement of the production process. It is hoped that this research can offer some suggestions and assistance for the improvement of the 1,4-butanediol and biodegradable plastics industry.