Valorization of biogas organic condensate via zeolite-catalyzed D-limonene ethoxylation: Kinetic and thermodynamic insights

Biogas condensate is rich in D-limonene and p -cymene, which are difficult to separate by distillation. This study explores an alternative approach, namely the zeolite-catalyzed selective liquid-phase ethoxylation of D-limonene with ethanol followed by the separation of the thus obtained α-terpinyl ethyl ether. The selected catalysts were characterized using X-ray diffraction, porosimetry, temperature-programmed NH 3 desorption, and d 3 -acetonitrile Fourier transform infrared spectroscopy and included hydrothermally synthesized BEA and MFI zeolites, a commercial MOR zeolite, and BEA samples with tuned acidity. The reaction mixtures formed at 45–75 °C were analyzed by gas chromatography-mass spectrometry and nuclear magnetic resonance spectroscopy, and the results revealed that D-limonene ethoxylation featured first-order kinetics. BEA showed high conversion and selectivity, whereas MFI and MOR were largely inactive. The optimization of BEA acidity was essential for reliable kinetic and equilibrium modeling. This work establishes a viable catalytic upgrading route for biogas condensates and supports more sustainable circular processing within anaerobic digestion systems. • BEA zeolite outperforms MFI and MOR in selective ethoxylation of d-limonene. • First kinetic model developed for d-limonene ethoxylation reaction. • Optimal acidity (~400 µmol g⁻¹) maximizes α-TEE yield and selectivity. • Process valorizes biogas condensate and supports circular economy and CO 2 savings.