Vibrational spectroscopy of zeolite complexes

Abstract

Using infrared spectroscopy, studies of the interaction of small molecules {e.g. CO, C(_2)H(_2), CH(_3)CHO) with zeolites have been carried out. The work has resulted in the observation of both stable chemisorbed complexes and reacting species for which time-resolved measurements have been made. The isomerization of cyclopropane on ZnNa-A has been followed using time resolved spectroscopy. From the intensity of the v(C=C) stretch of the product, propene, the kinetics of the reaction have been followed and the reaction sites identified. The rate of isomerization was found to be very much faster than that previously reported for similar systems. In a separate study the adsorbed complex formed between propene and Na-A zeolite has been investigated. On adsorption of carbon monoxide on Cu{I) containing Y zeolite, three different locations for the adsorbed CO have been observed. These experiments have been repeated for a variety of sample pretreatment conditions and the results used to explain the inconsistency in the scientific literature, where a range of v(C=0) stretching frequencies have been pre sented by different authors. In essence, it has been shown that the sample pretreatment has a marked effect on this cation distribution within the zeolite, and hence on the number and intensity of CO bands that are observed, since each site has a characteristic carbonyl stretching frequency. Copper containing Y zeolites were also investigated through the adsorption of ammonia and the reduction of the Cu(II) form by CO and NH(_3). The rationale for this work was to attempt to better understand the reduction process. A study of ethyne adsorbed on ZnNa-A has revealed two adsorption sites within the framework. At one site C(_2)H(_2) was found to be weakly n-bonding to the Na cations. The second site was identified as the Zn cations, at which the C2H2 was observed to lose hydrogen and form Zn-acetylide. The Zn-acetylide was observed to react slowly with zeolitic water to form ethanal. In separate studies ethanal was shown to be formed when C2H2 was adsorbed on Ag-A zeolite but no chemical reaction was observed on the Na-A form. Ethanal adsorbed on some ion-exchanged type A zeolites (Zn, Na, Ag) has been observed to form complexes with the charge compensating cations and framework hydroxyl groups. On zeolite ZnNa-A three species were identified; a hydrogen bonding complex formed with the framework hydroxyl groups, and two coordinating complexes formed with the charge compensating cations; (one with the Na(I) ions and one with the Zn(II) ions).In contrast two complexes were found on Ag-A (one hydrogen bonding, the other with the Ag(^+) cation) and one on Na-A, formed with the Na(I) ions. When adsorbed ethanal was heated, under vacuum, on ZnNa-A and Na-A new species were formed. These were tentatively identified as carboxylate species. A study of ethene adsorbed on ZnNa-A has been made by infrared and inelastic neutron scattering spectroscopy (INS).The IR measurements clearly showed the formation of a TT-bonded adsorption complex, by the observation of the formally inactive V2(C=C) stretching mode of ethene but no v(C-H) stretching vibrations were observed. In the INS data a number of vibrations of the adsorbed C(_2)H(_4) with respect to the surface were seen, as well as a weak broad band due to the v(C-H) stretching modes. [brace not closed]