Structure-function relationships for basic zeolites

Van den Berg, M.W.E., 2004. Structure-function relationships for basic zeolites. PhD, Nottingham Trent University.

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Lewis basicity in zeolitic materials is thought to be a property of framework oxygen atoms. Zeolitic basicity is amongst other factors governed by Si/Al ratio, framework type and guest cation. It was speculated that lead(II) introduced into the zeolite through ion exchange would enhance the basic properties.

Three zeolite types (zeolite X, Y and Beta) were ion-exchanged with lead(II) acetate in a watery solution. As reference basic catalysts several potassium and caesium-exchanged zeolites were prepared. All ion exchanges resulted in crystalline zeolites. The amount of lead(II) introduced in Pb-Na-X zeolites varied from ~4 to ~42 wt%. Thermal treatment at high temperatures (973 K) of the zeolites in vacuum or in a helium atmosphere led to the emergence of a metallic lead phase in some Pb-Na-X zeolites, where a high lead loading (~42 wt%) resulted in the emergence of a metallic lead phase even at 773 K.

The dehydrogenation of isopropanol yielding acetone is thought to be catalysed by basic catalytic sites, whereas acidic sites catalyse the dehydration of isopropanol, yielding propene. Adsorption of probe molecules carbon dioxide, ammonia and nitromethane monitored by infrared spectroscopy was employed as further means of examining acido-basic character of the zeolites.

Results indicate for Pb-Na-X samples that at a temperature of ~523 K a selectivity to acetone of about 70 molar% was achieved in the dehydrogenation of isopropyl alcohol, at conversion levels of 1-2 molar%. In infrared spectra of adsorbed carbon dioxide, carbonate formation has been found over these samples, whilst with ammonia adsorption at 423 K very little acid sites were seen. In the infrared spectra of adsorbed nitromethane at room temperature, bands were found which might be assigned to isocyanate species. According to literature, the above results point to presence of basic sites in Pb-Na-X.

When comparing the Pb-Na-X samples to conventional basic materials such as K-Na-X, K-Na-Y and Cs-Na-X, in the isopropyl alcohol dehydrogenation it was noticed that most of these traditional catalysts produce acetone at a higher temperature than the Pb-Na-X materials. On Pb-Na-Y and Pb-H-Beta samples no (substantial) formation of acetone was detected. Known acidic materials, such as H-Y, produced propene only. It was found that washing K-Na-Y zeolites with water after the ion exchange had a negative effect on acetone production.

Item Type: Thesis
Creators: Van den Berg, M.W.E.
Date: 2004
ISBN: 9781369316162
Divisions: Schools > School of Science and Technology
Record created by: Linda Sullivan
Date Added: 25 Sep 2020 13:13
Last Modified: 23 Aug 2023 13:00

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