Simultaneous catalytic removal of complex sulfur-containing VOCs over Mn-based hydrotalcite-like compounds: Active sites and oxidation mechanism

Abstract

The biological fermentation industry faces a challenge in simultaneously purifying complex sulfur-containing volatile organic compounds (VOCs) and reducing tail gas emissions. This is due to the multi-step purification process required for sulfur-containing VOCs and the high energy usage involved. The adsorption performance and controllability of elements in hydrotalcite-like compounds are noteworthy and have the potential to enhance the catalytic purification of sulfur-containing VOCs. This study utilizes Mn-based hydrotalcite-like compounds as a highly efficient catalyst for the simultaneous catalytic removal of complex sulfur-containing VOCs such as ethanol (C2H5OH), acetaldehyde (CH3CHO), and diethyl sulfide (C4H10S). The study successfully achieves the simultaneous and efficient removal of sulfur-containing VOCs at a temperature of 325 °C. The removal rates of three types of gas pollutants can reach 100 % for 420 min, 450 min, and 360 min, respectively. Meanwhile, the VOCs breakthrough capacity is 426 mg/g. The competitive adsorption effect, adsorption sites, and reaction mechanism were investigated using experimental and theoretical calculation methods. The simultaneous adsorption of three gases resulted in competitive adsorption, with the priority order of adsorption being CH3CHO > C4H10S > C2H5OH. The various adsorption effects were attributed to the formation of Mg–O–H…O-C, O…O=C, and Mn–O–H…S-C, respectively. CH3CHO was initially adsorbed at the O-Top site, while C4H10S was adsorbed at the Mn-Top site. C2H5OH was adsorbed at the Mg-Top site. High-temperatures promote the oxidation of elemental S to sulfate. The primary reason for catalyst deactivation is the buildup and coating of sulfate on the surface of the catalyst. The main active sites responsible for the catalytic oxidation of C4H10S were Mn2+ and Mn3+. The oxygen defects act as active sites for the oxidation of C2H5OH and CH3CHO. C2H5OH undergoes gradual oxidation to form CH3CHO and CH3COOH (acetic acid), which are further oxidized to CO2 and H2O. C4H10S was oxidized to elemental S and eventually to sulfate. The experimental results in this work are consistent with the theoretical results, providing a theoretical basis for the simultaneous catalytic removal of sulfur-containing VOCs.