The effect of silica:alumina ratio and hydrothermal ageing on the adsorption characteristics of BEA zeolites for cold start emission control

摘要

Increasingly stringent legislation has led to the need for better control of vehicle cold start emissions.Research work has identified sodium-BEA zeolite with a silica:alumina ratio of 10 (Na-BEA-silica:alumina=10) as a potential trapping agent for hydrocarbons during vehicle cold start,with desorption occurring only at about the temperature of operation of a vehicle exhaust catalyst.Propene and toluene,used as probe molecules for vehicle emissions,were found to absorb under ambient conditions on all forms of BEA.There appear to be two adsorption mechanisms at play,one involving charge gradients in the zeolite framework and characterised by lower desorption temperatures (molecular adsorption) and the other related to zeolite acidity (oligomeric adsorption).Both the presence of water and simulated ageing reduced the adsorption capacity of the zeoites.Hydrothermal ageing was found to result in dealumination of the zeolite framework,a process that can be reduced by incorporation of lanthanum into the lattice.Water in the gas stream was found to block pores and reduce adsorption.A combination of a more hydrophilic zeolite with BEA zeolite is suggested to overcome the loss in capacity by the presence of water in car exhaust.