Use of a molecular sieve bed to minimize emissions during cold start of internal combustion engines

Claim 1

A process for treating an engine exhaust gas stream containing hydrocarbons and other pollutants comprising directing the engine exhaust gas stream over an adsorbent zone comprising a molecular sieve bed which preferentially adsorbs the hydrocarbons over water, to provide a first exhaust stream, flowing the first exhaust stream over a primary catalyst to convert substantially all the pollutants contained in the first exhaust stream to innocuous products, thereby providing a treated exhaust gas stream and discharging the treated exhaust stream into the atmosphere, said process being carried out for a time until the molecular sieve bed temperature is about 150° C., at which time the engine exhaust gas stream is diverted completely away from the adsorbent zone and routed directly over the primary catalyst until such time as the primary catalyst reaches its operating temperature, at which time the engine exhaust gas stream is divided into a major and minor portion, flowing the major portion of the engine exhaust gas stream over the primary catalyst and then discharging the treated exhaust gas stream from the primary catalyst into the atmosphere, flowing the minor portion of the engine exhaust gas stream over the adsorbent zone for a time sufficient to desorb substantially all the hydrocarbons adsorbed on the molecular sieve bed and provide a second exhaust gas stream containing desorbed hydrocarbons, flowing the second exhaust stream over the primary catalyst to provide a treated exhaust stream and discharging the treated exhaust stream to the atmosphere and after such time as necessary to desorb substantially all the hydrocarbons from the molecular sieve bed, the engine exhaust gas stream is completely directed over the primary catalyst to provide a treated exhaust stream and then discharging the treated exhaust stream to the atmosphere; the molecular sieve characterized in that it is selected from the group consisting of molecular sieves 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: ##EQU2## 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 2 O is the water co-loading on the zeolite 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.