DRIFT study of molecular and dissociative adsorption of light paraffins by HZSM-5 zeolite modified with zinc ions:methane adsorption

摘要

DRIFT study of methane adsorption by ZnZSM-5 zeolites prepared by incipient wetness impregnation or by chemical reaction of the initial hydrogen form with zinc vapor at 770 K indicated an unusually broad DRIFT band with a maximum at 2805 cm~(-1).This frequency is more than 200 cm~(-1)lower than the asymmetric v_3 C-H stretching mode of gaseous methane and more than 100 cm~(-1)lower than the symmetric v_1 stretching frequency.Methane adsorption by zinc cations is much stronger than that by the bridging hydroxyl groups or sodium ions.At 473 K adsorbed methane is involved in heterolytic dissociation,resulting in formation of bridging hydroxyl groups and zinc alkyls.Such dissociative adsorption blocks the sites for subsequent molecular adsorption of methane or hydrogen.In accordance with our previous results on hydrogen adsorption,we conclude that the dissociative methane adsorption occurs on acid-base pairs with distantly separated Zn~2+ ions and basic oxygen atoms of the zeolite framework.Unusually large perturbation and dissociation of methane at the relatively low temperature are connected with only partial compensation of the positive electric charges of Zn~2+ ions at such sites.