sieve is heated to a temperature of about 350 u00b0 c to remove the ammonia the removal of na ions with nh 4 is more easily carried out than with multivalent ions as described below and these catalysts are generally moreactive but less stable to heat than the multivalent cation exchange forms mole sieves which have had their alkali metal reduced to low levels by partial treatment with nh 4 and partial multivalent metal cationexchange possess increased activity and increased stability in addition to mole sieves which are acidic according to the bronsted theory those mole sieves which exhibit acidic characteristics under the lewis theory for example calcium exchanged mole sieves are suitable for the present reaction by exchanging the univalent cations e g na with multivalent cation strong ionic activity is imparted the ratio of sio 2 al 2 o 3 valence and radius of the cation and the extent of exchange all affect the catalyst activity in general activity increases with 1 increased sio 2 al 2 o 3 ratio 2 decreased cation radius and an increase in cation valence the effect of replacing univalent ions e g na with bivalent e g ca ismuch greater than replacing the bivalent ions with cations of greater valence the various types of mole sieves having reduced alkali metal content are characterized as.