Impact of the Zeolite Structure and Acidity on the Adsorption of Unburnt Hydrocarbons Relevant to Cold Start Conditions

Adsorption behavior of commercial zeolites were investigated for the trapping of a hydrocarbon mixture (propene, toluene and decane) mimicking Diesel cold start exhausts under dry conditions. The studied commercial zeolites, in most cases under protonated form, (with Si/Al ratio in the range 2-15) present different structures: FAU, *BEA, MOR, MFI, FER, and LTA. Textural and structural characterizations were carried out using N-2 adsorption at-196 degrees C and X-ray diffraction. In situ FTIR spectroscopy of adsorbed pyridine was used to probe quantitatively the acid sites. The methodology used in this study consisted of an adsorption phase at 35 degrees C followed by temperature-programmed desorption (TPD) under He at 10 degrees C/min, and three different adsorption mechanisms were highlighted according to the pore size and the host framework. MOR and MFI structures appear to be the less selective for any type of hydrocarbon and are thus the most suitable for cold start applications. Except for these types of zeolites presenting a single-file diffusion mechanism, propene often desorbs at low temperatures or is adsorbed very little. By contrast, heavier hydrocarbons such as toluene and decane desorb at temperatures that match with three-way catalyst (TWC) or Diesel oxidation catalyst (DOC) light-off temperatures.