Jan B. Uytterhoeven

On the nature of the charged silver clusters in zeolites of type A, X and Y

The formation of charged silver clusters and the concomitant colour formation in zeolites of type A (NaAg-A) and faujasite-type zeolites was followed by X-ray powder structure determinations. Associations of three (or two) silver cations were located in faujasite-type zeolites with one silver cation in the centre of the hexagonal prism and two (or one) silver cation(s) in the adjacent sites after dehydration and oxygen treatment at 693 K. The number of such clusters increases with the Al content. The dimensions are comparable to analogous strings in A-type zeolites. The intensity of the colour (yellow) increases with the number of such clusters formed. The red colour in Ag-A type zeolites can be ascribed to two interaction clusters while in faujasite-type zeolites, no such interaction is possible.

Studies of the hydrogen held by solids: XVI. Infrared spectroscopy of X- and Y- type zeolites containing univalent and divalent cations

Abstract The IR spectra of Linde X- and Y- type zeolites, charge-balanced with different monovalent and divalent cations, were examined in the OH and NH regions after evacuation at various temperatures and when H 2 O or NH 3 was added back. Bands near 3650 and 3545 cm −1 were identified with decationated sites. With monovalent zeolites only the former band could be detected and it resulted from hydrolysis and corresponded to the base-exchange cation deficiency. With divalent zeolites both bands appeared with the Y zeolites and at high exchange levels with the X zeolites. These bands were too strong to attribute to a deficiency of cations and it is suggested that H 2 O molecules are dissociated by the strong electrostatic fields of the Me 2+ ions, resulting in the formation of decationated sites and ions of the type Me + -OH or Me + -O-Me + . The decationated sites reacted readily with NH 3 to form NH 4 + . A band near 3695 cm −1 appeared when small amounts of H 2 O were added back. This could be attributed mainly to H 2 O bound to the residual Na + ions of the preparations, but may have originated partly from a similar interaction with some of the divalent ions as well. Finally, a band near 3605 cm −1 was characterized, which could be assigned to Ca + -OH.

Properties of zeolites in relation to their electronegativity: Acidity, carboniogenic activity and strength of interaction in transition metal complexes

Abstract The Sanderson electronegativity model was used on order to explain changes in overall properties of molecules in the gas phase, adsorbed on zeolites, and of functional groups in zeolites. Using the Sandersons concepts, the changes in gas phase basicity of primary, secondary and tertiary alkylamines could be related to the changes in their electronegativity. In zeolites with different structure, the changes in stretching frequencies and integrated absorption coefficients of hydroxyl groups vibrating in the main pores of the structure could be rationalized in terms of the residual charge on the proton as calculated by the theory. Changes in overall acidity of these zeolites as measured by the rate of dehydration of isopropanol per active centre could be related to compositional changes of these materials. Also the changes of the symmetric deformation frequency of ammonia adsorbed on Ag + ions in zeolites with different structure and chemical composition could be rationalized using the electronegativity model of Sanderson.

Active sites in zeolites: I. Cumene cracking activity of NH4Y zeolites after different reactor pretreatments

Abstract The dealkylation of cumene was investigated in a pulse microreactor. The catalysts were zeolites Y in the hydrogen form and partially hydrolyzed HY pretreated at temperatures between 500 ° and 800 °C. The hydroxyl content of the samples was derived from infrared spectra taken from samples pretreated in identical conditions. Bronsted and Lewis sites were determined by adsorption of ammonia and pyridine, and poisoning experiments with pyridine were also carried out. A good correlation was found between the number of hydroxyls and the amount of pyridine needed for complete poisoning. Nevertheless, the initial activity after different pretreatments decreased much more slowly than the concentration of acidic hydroxyls. It was concluded that only a fraction of the hydroxyls are involved in the reaction. The activity varied with the pulse number. This was ascribed to the formation of a polymer by the propylene formed in the reaction. This polymer is catalytically active per se. On the HY samples the presence of the polymer resulted in a decrease of the initial activity. On the hydrolyzed sample the polymeric species had a favorable effect on the catalytic activity.

Some unusual properties of activated and reduced AgNaA zeolites

Carbon monoxide, oxygen and hydrogen were found to be chemisorbed on dehydrated AgA zeolites. This was investigated in detail using volumetric sorption and temperature programmed desorption techniques. Also i.r. and mass spectrometry were used to characterize the solid and the desorbed molecules.It was found that as a result of an auto-reductive process, colour centres are created upon degassing of the zeolite. These centres sorb hydrogen and oxygen dissociatively, while one molecule of carbon monoxide was chemisorbed per Ag ion available in the supercage. It is proposed that linear Ag+3 clusters are formed upon activation, the ends of which constitute chemisorption sites for hydrogen and oxygen.

Active sites in zeolites: Part 6. Alcohol dehydration over alkali cation-exchanged X and Y zeolites

2-Propanol and 2-butanol have been dehydrated over alkali cation-exchanged zeolites X and Y. The first reaction was carried out in a differential continuous-flow reactor, the second one in a recirculation reactor, using deuterated alcohols. From isopropanol, propylene and diisopropyl ether are formed as dehydration products. The kinetic data can be fitted to a Langmuir-Hinshelwood equation of the form: r = kKAPA(KAPA + KWPW)−1, where the rate of the reaction is slowed down by the desorption of water. The inter- and intramolecular dehydrations are parallel reactions catalyzed by weak hydroxyl groups. The selectivity is determined by the concentration and acidity of these groups. A detailed reaction mechanism (of the E1 type) is advanced which accounts for the observations.

Oxidation and reduction of silver in zeolite Y: a structural study

The influence of dehydration and repetitive oxidation and reduction steps on the structure of Ag-Y type zeolites was followed by X-ray powder methods. A conventional dehydration at 693 K results in the formation of external silver particles. Charged clusters are formed after a first oxidation of the fresh sample at 673 K or 873 K with Ag located in the sites I and I′. These clusters are destroyed upon reduction at 348 K while charged clusters inside the sodalite cage are formed with Ag in the sites I′ and site II′. These charged aggregates are not destroyed when heated to 723 K. A prolonged reoxidation is required to restore the original site I-site I′ charged clusters, which are responsible for the yellow colour.

Redox behaviour of transition metal ions in zeolites. Part 3.—Auto-reduction of cupric ions in Y zeolites

In CuY zeolites degassed at temperatures above 650 K cupric ions are reduced to some extent to cuprous ions.Evidence for this process of auto-reduction was taken from the decrease in weight of the sample, and the detection of a desorbing gas in a volumetric system. The latter was identified as oxygen. U.v. reflectance spectroscopy showed directly the decrease of the Cu2+ species upon outgassing and their increase after oxygen adsorption. Excitation spectra showed the presence of Cu+ ions after high temperature degassing. Infrared spectroscopic techniques showed that Lewis sites are also formed upon degassing. A mechanism is proposed that accounts for the observations.

Redox behaviour of transition metal ions in zeolites. Part 4.—Kinetic study of the reduction and reoxidation of copper–Y zeolites

The uptake of hydrogen and oxygen by (Cu, Na)–Y zeolites has been followed kinetically in a volumetric system. The temperature region investigated was between 273 and 773 K. The influence of several parameters on the reducibility of Cu2+ has been investigated: the degree of cation exchange, the presence of Cu2+ complexing ligands, the procedure of sample preparation and degassing.The formation of Cu+ ions is determined mainly by the mobility of Cu2+ ions in the zeolite. The preparational and degassing procedure has a drastic influence on the reducibility of the Cu2+ species. The rate of formation of Cu° is determined by the diffusion of the reactant through the pores.Cu+ ions and small Cu° clusters are reoxidized reversibly, the mobility of Cu+ ions determining the rate of the process. The reoxidation of Cu° external to the zeolite consists of a homothetic attack of oxygen on the Cu° particles.

Active sites in zeolites: II. Cumene cracking and toluene disproportionation activity of stabilized faujasite structures

Abstract The concentration of acidic hydroxyl groups, sorptive properties, and catalytic activity for cumene cracking and toluene disproportionation were compared for different stabilized catalysts, viz., “deep-bed” “ultrastable,” and aluminum-deficient Y zeolites. It was concluded that only a fraction of the acidic hydroxyls could be catalytically active. The toluene disproportionation reaction requires hydroxyl groups with higher acidity. Several structural properties have a favorable effect on the nature of the active sites for both reactions.