Zeolite-based adsorbers for reducing light hydrocarbon emissions from engine exhaust

Adsorption of light hydrocarbons, such as ethane, ethylene, n-butane, isobutane and isobutene, on zeolites 5A [LTA] and H-Beta [BEA] was investigated, in order to evaluate the possible use of these zeolites as adsorbents for reducing cold start emissions at engine exhaust. Equilibrium isotherms at 25°C were determined and analysed by Langmuir and virial equations to estimate adsorption capacities at saturation and adsorption affinity for single hydrocarbons at low pressure. Good affinities of zeolites 5A and H-Beta for the tested light hydrocarbons were found either at saturation or at low pressure. Flow measurements were also carried out with a fixed-bed micro-reactor using a real engine exhaust gas. Reasonable performances of zeolite H-Beta in trapping hydrocarbons present in the emission at engine exhaust were obtained. Poor performances, obtained, on the contrary, with zeolite 5A, were interpreted as due to the preferential adsorption of water vapour, compared to hydrocarbons. On the basis of the present and of previously collected data, the possibility of use of zeolite H-Beta, coupled to zeolite H-Y or zeolite H-ZSM5, to realize a hydrocarbon adsorbent trap for reducing cold start emission is, at last, envisaged.