Zinc-based oxides as alternative cheap and stable catalysts for the production of adipates from cyclopentanone and dimethyl carbonate

Developing sustainable catalytic processes for specialty monomers is essential for advancing a renewable-based economy. Adipic acid esters, such as dimethyl adipate (DMA), are key intermediates in polyester and polyamide production and can be synthesized with complete atom economy (100 %) through the reaction of bio-based cyclopentanone (CPO) with dimethyl carbonate (DMC) or other CO2-derived carbonates. This study evaluates, for the first time, the catalytic performance of ZnO and Zn/Mg mixed oxides for this transformation. ZnO exhibited superior selectivity for DMA (53 %), compared to conventional MgO and CeO2 catalysts by effectively suppressing heavy by-product formation, thereby inhibiting CPO self-aldol condensation. This suppression minimizes catalyst fouling, allowing ZnO to be easily and readily recovered and reused without significant loss of activity. In contrast, Zn/Mg/O mixed oxides enhanced reaction rates but led to lower selectivity, primarily producing branched methylated adipates, which could serve as alternative polymer precursors. Optimization of catalyst composition and reaction conditions was critical for maximizing diester yields in this one-pot transformation, selectivity producing DMA over ZnO (up to 48 % yield after 7 h of reaction) or DMA and methylated DMAs (with yields higher than 40 % after 3 h of reaction in the presence of Zn/Mg/O). However, dedicated studies on the β-keto-ester intermediate revealed that methanol availability over the catalytic surface at the temperature required for the “one-pot” process (i.e. 260 °C) is a limiting factor. This constraint can reduce DMA yield and productivity due to the unselective decomposition of the intermediate, highlighting an area for further process refinement.