Spectroscopic studies of adsorbed gases on transition metal exchanged zeolites

Abstract

Infrared studies of gases, C(_2)H(_4), trans-C(_2)D(_2)H(_2), C(_2)H(_2), C(_2)D(_2), C(_3)H(_6), NH(_3), CO and H(_2)S, adsorbed onto self supporting discs of various transition metal exchanged zeolites are reported (4000-1200cm(^-1)). C(_2)H(_4) and trans-C(_2)D(_2)H(_2) were adsorbed onto fully silver exchanged type A (AgA) zeolite. AgA samples were subjected to various dehydration treatments prior to adsorotion of C(_2)H(_4) and it has been shown that the adsorption behaviour is a function of pretreatment. Under all pretreatment conditions used two adsorption sites were observed. However, whilst AgA samples pretreated at higher temperature held ethylene equally strongly on both sites, those samples pretreated at the lower temperature held ethylene less strongly on one site than the other. To continue our study on AgA zeolite, we also adsorbed C(_2)H(_2) and C(_2) D(_2) onto AgA samples which had been degassed at temperatures of 543 and 673K for 2 hours. Acetylene was found to be adsorbed on the cations at two different sites, and were easily removed on evacuation. In addition part of the adsorbed acetylene lost hydrogen to form silver acetylide (HC=CAg). The liberated hydrogen formed both hydroxyl and hydronium ions within the framework. In another study, the adsorption behaviour of C(_2) H(_4) and C(_2)H(_2) on Cu(^II) and Cu(^I)Y zeolites was compared. Cu(^I)Y zeolite was prepared in situ by the reduction of Cu(_2)H(_2) zeolite in an atmosphere of CO with preadsorbed ammonia. It was found that the C(_2)H(_4) adsorbed on Cu(^II)Y was not rotating and that the symmetry of the adsorbed species was apparently preserved (D(_2h)), while, the symmetry of the adsorbed species in Cu(^I)Y zeolite was reduced, probably to C(_2v). In contrast to the results for C(_2)H(_2) adsorption, the adsorption behaviour of C(_2)H(_2) was found to be similar on both Cu(^II) and Cu(^I)Y zeolites. Acetylene was found to be adsorbed at two different sites with a 'side-on' interaction, and that one site held acetylene more strongly than the other. For the Cu(^I)Y sample, following the evacuation of either C(_2)H(_4) or C(_2)H(_2), GO was introduced. It was suggested that C(_2)H(_2) and CO were held with comparable strength by Cu(^+) ions and that the gases were adsorbed on equivalent sites. C(_2)H(_2) and CO, on the other hand, were adsorbed at two different sites. Partially Zn-, Ni-, and Cu- exchanged NaA zeolites were used to study the isomerization of cyclopropane to propene. ZnNaA samples were subjected to various pretreatment conditions before the adsorotion of cyclopropane. Water was found to promote the isomerization in this sample and that isomerization occurred via a, protonated cycloorooane intermediate. Bands due to cycloprooane and propene were observed at the same time in samples ZnNaA and NiNaA and it was suggested that the gases were adsorbed at two different sites. Propene was adsorbed on the cations more strongly than cycloprooane since propene could only be removed at 473K while cyclopropane could be removed easily by evacuation. For CuNaA, however, no interaction between the cations and cyclopropane was observed at room temperature. Upon heating the sample with 100 torr of cyclopropane, isomerization occurred at 473K. It was suggested that isomerization occurred on the external surface of the zeolite and that propene was not adsorbed on the cations within the framework. Finally, the adsorption of H(_2)S onto partially Ni-, Cu-, Zn-, Mn-, and Co- exchanged NaA zeolites were studied. No adsorption of H(_2)S onto the NiNaA and CuNaA samples was observed. On the other hand, two types of adsorption, dissociative and non- dissociative were observed in ZnNaA and MnNaA samples. Water was also formed. H(_2)S was found to adsorb weakly and molecularly onto CoNaA.