Carboxylation of Acetylene without Salt Waste: Green Synthesis of C4 Chemicals Enabled by a CO2-Pressure Induced Acidity Switch

The inherent formation of salt waste in C−H carboxylations is a key obstacle precluding the utilization of CO₂ as C₁ building block in the industrial synthesis of base chemicals. This challenge is addressed in a circular process for the production of the C₄ base chemical dimethyl succinate from CO₂ and acetylene. At moderate CO₂ pressures, acetylene is doubly carboxylated in the presence of cesium carbonate. Hydrogenation of the C−C triple bond stabilizes the product against decarboxylation. By increasing the CO₂ pressure to 70 bar, the medium is reversibly acidified, allowing an esterification of the succinate salt with methanol. The cesium base and the hydrogenation catalyst are regenerated and can be reused. This provides the proof of concept for a salt-free route to C₄ chemicals from biogas (CH₄/CO₂). The origin of this reversible acidity switch and the critical roles of the cesium base and the NMP/MeOH solvents were elucidated by thermodynamic modeling.