Molecular sieve bed/catalyst to treat automotive exhaust

Claim 1

A process for treating a cold-start engine exhaust gas stream containing hydrocarbons and other pollutants consisting of flowing said engine exhaust gas stream over a molecular sieve bed which preferentially adsorbs the hydrocarbons over water to provide a first exhaust stream, and flowing the first exhaust gas stream over a catalyst to convert any residual hydrocarbons and other pollutants contained in the first exhaust gas stream to innocuous products and provide a treated exhaust stream and discharging the treated exhaust stream into the atmosphere, the molecular sieve bed characterized in that it comprises at least one molecular sieve selected from the group consisting of molecular seives which have: 1) a framework Si:Al ratio of at least 2.4; 2) are hydrothermally stable; and 3) have a hydrocarbon selectivity (.sup.α HC-H 2 O) greater than 1 where .sup.α HC-H 2 O is defined by the following equation: ##EQU3## where X HC is the hydrocarbon co-loading on the molecular sieves in equilibrium with the hydrocarbon water vapor mixture in the gas phase over the molecular sieve adsorbent; X H .sbsb.2 O is the water co-loading on the molecular sieve in equilibrium with the water and hydrocarbon vapor mixture in the gas phase over the molecular sieve adsorbent; [H 2 O] is the concentration of water and [HC] is the concentration of hydrocarbon.