The tandem cracking-oxidation of phthalate esters with high CO2 selectivity and reduced energy cost

Abstract

Gaseous phthalate ester (PAEs) pollutants pose a significantly high risk of environmental exposure and present a serious threat to human health. We developed the CuO@Hol-HZSM-5 catalyst with spatially isolated cracking-oxidation dual active sites for the oxypyrolysis treatment of di(2-ethylhexyl)/dioctyl/dibutyl phthalates (DEHP, DOP, DBP). DOP is an isomer of DEHP, and both of them exhibit the conversion and CO₂ selectivity of ca. 99 % at 375 ℃ and 20000 h⁻¹. DBP with smaller molecular size achieves comparable conversion and CO₂ selectivity even at a low temperature 350 °C and high space velocity 30000 h⁻¹. DFT studies, in-situ DRIFTS and GC–MS reveal the synergy of Brønsted acid sites and oxidation active sites for the DEHP decomposition. The reaction process is initiated by protonic acid-catalyzed hydrolysis over the CuO@Hol-HZSM-5 surface, yielding phthalic acid and 2-ethylhexanol as key intermediates. 2-Ethylhexanol undergoes a protonic acid-catalyzed tandem dehydration-β-scission reaction, generating low-molecular-weight alkenes such as ethylene and propylene. Low-molecular-weight alkenes and benzene from direct decarboxylation of phthalic acid or hydration-decarboxylation of phthalic anhydride are then converted to CO₂ and H₂O over CuO nanoparticles. Pilot-scale equipment was developed for the oxypyrolysis treatment of DEHP desorption gas. The exothermic energy from the cracking-oxidation reaction is re-utilized via an integrated heat exchanger to preheat the fresh air for thermal desorption. The COD of the outlet condensates was 26.86–45.96 mg/L, which is below the national wastewater discharge standards. The operation cost of the thermal desorption-oxypyrolysis process is €16.77/ton_sorbent, which outperforms the traditional method.