금속 이온 교환된 ZSM-5를 이용한 탄화수소 흡착 및 탈착 연구

Abstract

Exhaust hydrocarbon emissions from automotive engines are important sources of ambient air pollution and have been associated with harmful effect on public health. During a cold-start period of an automotive exhaust gas converters, a slow kinetics of three-way catalytic (TWC) reactions lead to emit a large amount of unburned hydrocarbons (70-80%) into the environment. Generally, adsorption technology has been known as one of the most effective methods employed for trapping hydrocarbons. However, a high water content in the exhaust gas causes a significant decrease of the activity of the adsorbent, especially at a high temperature. The objective of this research is to investigate hydrocarbon adsorption capacity and hydrothermal stability of both zeolites ZSM-5 and its modifications. Based on the commercial ZSM-5 zeolites with different SiO2/Al2O3 ratios of 23, 80, and 280, ion-exchanged processes are employed to produce M-ZSM-5 (M = La, K, Ag, or H). Ion-exchanged M-ZSM-5 are then evaluated by adsorption/desorption measurements of hydrocarbons which are identified as presentative of the major species present in the engine exhaust during cold start: propylene (olefin), n-butane (paraffin) and toulene (aromatic). Adsorption capacity is observed qualitatively by TPD of the hydrocarbons while the moles desorbed during the TPD experiments is relatively confirmed by integration of the area under the desorption curves. Under the hydrothermal treatment at 600℃, La or Ag or H ion-exchanged ZSM-5(SiO2/Al2O3=23) lead to enhance a strong selectivity for propylene and n-butane while K-ZSM-5(SiO2/Al2O3=80) improves a affinity with toluene. Increasing temperature of the hydrothermal treatment to 800℃, La-ZSM-5(SiO2/Al2O3=23) maintained a high adsorption capacity of 92% of propylene while K-ZSM-5(SiO2/Al2O3=80) kept toluene uptake by 62%. It is believed that an introduction of La ion not only creates much amount of acid sites but also forms a stable interaction inside structure of the zeolites. These account for their enhancements in high adsorption capacity of propylene/butane and hydrothermal stability. In case of K ion-exchange, the affinity between benzene ring of toluene (negative charge) and potassium ion (positive charge) is one possible cause for a high toluene uptake of K-ZSM-5 zeolite. In summary, La-ZSM-5(SiO2/Al2O3=23) and K-ZSM-5(SiO2/Al2O3=80) are potentially applicable for the capture of propylene and toluene in real operation.