The present invention relates to a process for reducing
cold start emissions in an
exhaust gas stream (such as from an
internal combustion engine) by contacting the exhaust
stream with a combination of molecular sieves comprising (1) a small pore crystalline
molecular sieve or mixture of molecular sieves having pores no larger than 8 membered rings selected from the group consisting of SSZ-13, SSZ-16, SSZ-36, SSZ-39, SSZ-50, SSZ-52 and SSZ-73
molecular sieve and having a
mole ratio at least 10 of (a) an
oxide of a first tetravalent element to (b) an
oxide of a trivalent element, pentavalent element, second tetravalent element which is different from said first tetravalent element or mixture thereof and (2) a medium-
large pore crystalline
molecular sieve having pores at least as large as 10 membered rings selected from the group consisting of SSZ-26, SSZ-33, SSZ-64,
zeolite Beta, CIT-1, CIT-6 and ITQ-4 and having a
mole ratio of at least 10 of (a) an
oxide of a first tetravalent element to (b) an oxide of a trivalent element, pentavalent element, second tetravalent element which is different from said first tetravalent element or mixture thereof.