Number Of Strong Acid Sites Research Articles

Skeletal isomerization of cyclohexene on AlPO4 catalysts

The skeletal isomerization of cyclohexene, in a microcatalytic pulse reactor, was investigated by using aluminium orthophosphates (Al/P = 1) as catalysts. An attempt has been made to correlate the isomerization activity with the acid properties of the solids, measured by means of the irreversible adsorption of different organic bases in cyclohexane, at 298 K, using a spectrophotometric method.Apparent rate constants were calculated in terms of the kinetic model of Bassett and Habgood. Selectivity studies led to the conclusion that 1-MCP and 3-MCP were competitive products. Correlation between the kinetic selectivity factor, σ, and the number of strong acid sites has been sought.A possible parallel reaction pathway is proposed, which agrees with both the observed rates and selectivities.

Acid strength distribution and catalytic properties of H-ZSM-5: Effect of deammoniation conditions of NH 4-ZSM-5

Acid strength distribution, catalytic activity in cumene cracking and o-xylene isomerization reactions, and the distribution of hydrocarbons formed in methanol and ethanol conversion reactions on H-ZSM-5 zeolites obtained from NH 4-ZSM-5 under different deammoniation conditions (i.e., by varying temperature, period, atmosphere and heating rate in the deammoniation) have been studied. The catalytic properties of the zeolites are correlated to their acidity. The acid strength distribution, the cracking and isomerization activities, and the distribution of hydrocarbons in the alcohol conversion reactions on the zeolites are affected by the changes in the temperature, period, and atmosphere employed in the deammoniation, the effect produced by the higher deammoniation temperatures (> 873 K) being very drastic. The heating rate employed for attaining the deammoniation temperature has no significant effect on either the acidity distribution or the catalytic properties of the zeolite. The conversion of cumene, the xylene isomerization activity, and the extent of aromatization in the alcohol conversion reactions on the zeolites could be very satisfactorily correlated to the number of strong acid sites on which pyridine is adsorbed irreversibly at 673 K.

Acidity of dealuminated mordenites studied by infrared spectroscopy

The hydroxy groups of dealuminated mordenites having SiO2/Al2O3 molar ratios 14–44 have been studied by means of infrared spectroscopy. The intensity of the low-frequency (3609 cm–1) band decreases with increasing the SiO2/Al2O3 ratio and the intensity of the high-frequency (3738 cm–1) band increases. Using pyridine as adsorbate the acidity of this mordenite series has been determined and it was found that on dealumination the number of strong acid sites increases with a simultaneous decrease of the total number of acid sites. The SiOH groups which are formed during dealumination increase the strength of the remaining acid sites. A band at 1462 cm–1 is observed after pyridine desorption at ca. 673 K and is assigned as a characteristic band of pyridine adsorbed on stronger Lewis sites. Increased dealumination renders the Bronsted sites more accessible to pyridine at lower temperatures.

Isomerization of m-xylene on H-ZSM-5: Part I: Influence on catalytic activity and selectivity of si/al ratio, degree of cation exchange, deammoniation conditions and poisoning of stronger acid sites

Isomerization of m-xylene on H-ZSM-5 zeolites with different Si/Al ratios and degrees of cation exchange, and on the zeolites obtained at different deammoniation conditions from NH 4-ZSM-5 has been investigated in a pulse microreactor. The results are correlated to the number of strong acid sites as measured by the irreversible adsorption of pyridine at 673 K. The reaction on the poisoned zeolite has been studied by selectively blocking the stronger acid sites in the order of their strength with ammonia, quinoline and pyridine at different temperatures in the range 673–773 K. The catalytic activity and selectivity of the zeolite are seen to be strongly affected by the changes in its Si/Al ratio, degree of cation exchange and deammoniation conditions and also by the poisoning of the stronger acid sites of the zeolite by pyridine or quinoline. The selectivity in the Isomerization increases with the decrease in the number of strong acid sites on the zeolite and with the increase in the extent of poisoning of the stronger acid sites. The strength of the acid sites required for catalysing the various parallel reactions occurring in the overall conversion of m-xylene is in the following order:- isomerization < dealkylation < disproportionation.

Surface acidity relations of NiO-SiO 2

Solids composed of varying ratios of NiO to SiO 2 were prepared by mechanically mixing hydrated Ni(OH) 2 and silica hydrogel. The acid strengths and concentrations were estimated by the amine titration method on the products of thermal treatment at 500 °C, 600 °C and 800 °C. Most of the products showed an acid strength H o of +2.0 that was independent of the chemical nature or the chemical composition of the solids. However, most of the acid sites are very weak, with characteristic H o values of + 3.3 or less. At about 45 mol.% NiO the minimum acidity is obtained at 500 °C, but at 600 °C and above this solid is characterized by a maximum in the weak acidity and the presence of a small number of strong acid sites ( H o ⩽ + 2.0). A sample prepared by coprecipitating the two components did not differ much from the corresponding mechanically mixed solid, apart from the presence of an abundance of weak acid centres. Owing to the large separation of the moderate acid sites ( H o = + 2.0), which are calculated to have a distribution density of less than 1 site per 100 Å 2, it isdoubtful whether NiO-SiO 2 would be able to catalyse any reactions.

Heat of Adsorption from Solution. II. The Heat of Adsorption of Pyridine on Silica–Alumina from the Hexane Solution

Abstract The amounts of physisorbed and chemisorbed pyridine on silica–alumina in the hexane solution were measured at 30°C, and found to be 1.25 and 0.25 molecules/100 Å2 respectively. The number of strong acid sites determined by Benesi’s method are in good agreement with the chemisorbed molecules, which indicates that the ratio of the number of chemisorbed pyridine molecules to that of the strong acid sites on the surface of silica–alumina is equal. The adsorption calorimetry was carried out on the same system by means of a flow microcalorimeter which permits us to differentiate the heats of physisorption and chemisorption: the results showed that the heats of physisorption and chemisorption of pyridine are 11.3±0.3 and 22±4 kcal/mol respectively.

ChemInform Abstract: CHARACTERISTIC STUDIES ON DEALUMINATION OF FAUJASITE‐TYPE ZEOLITE

AbstractAl‐Ionen werden stöchiometrisch aus dem Gitter eines Zeoliths vom Faujasit‐Typ durch Behandlung mit EDTA extrahiert.

Characteristic Studies on Dealumination of Faujasite-type Zeolite

Abstract Aluminium ion was extracted stoichiometrically from the framework of the faujasite-type zeolite by EDTA treatment. Dealuminated zeolites with various silica/alumina ratios (from 4.80 to 10.29) were obtained. A dealuminated zeolite does not necessarily show the molecular-sieve-type adsorption isotherm of nitrogen. The lattice constant of zeolite decreases with the silica/alumina ratio. The zeolite with an increased silica/alumina ratio has a higher catalytic activity for a cumene-cracking reaction than the parent H-Y4.80 zeolite. It is suggested, on the basis of infrared spectroscopic and calorimetric measurements, that the total number of acid sites decreases with dealumination, while the number of strong acid sites increases. These strong aicd sites seem to contribute to the catalytic activity of dealuminated zeolite.

Structure and catalytic properties of thermally and hydrothermally treated zeolites: Acid strength distribution of REX and REY

Acidity distributions of REX and REY zeolites have been determined after pretreatment under various conditions. These acidities are correlated with the catalytic performance of the zeolites in gas oil cracking. It was found that hydrothermal treatment of the zeolites leads to a decreased number of strong acid sites, while the number of sites with intermediate and weak acid strength remain unchanged. The disappearance of the strong acid sites results in an increased cracking selectivity, especially for the REY type zeolites. This study shows that the selectivity in catalytic cracking for gasoline, gas and coke production is very much dependent on the total number of acid sites as also on the acidity distribution in the catalyst.

X, Y, aluminum-deficient, and ultrastable faujasite-type zeolites: III. Catalytic activity

The catalytic properties of faujasite-type zeolites have been studied in the cracking of iso-octane. The selectivity of the reaction depends mainly on the sodium ion content. The catalytic activity decreases with the number of strong acid sites. The active sites are linked to the aluminum sites which are related to strong acidity. They are removed only if the aluminum extraction exceeds 30% of the total aluminum content. Inactive and weak acid sites are related to the more easily removable fraction of the aluminum atoms. This distinction of aluminum atoms into two types correlates very well with the results obtained for zeolites subjected to deep-bed calcination or heated at 870 °C. The results point out the importance of AlO 4 − lattice aluminum with regard to the ultrastability.

Heats of adsorption of ammonia on acidic catalysts

A comparative calorimetric study has been made of the heat of chemisorption of ammonia at 30 °C on various preparations of alumina, silica, silica-alumina, and Linde molecular sieve X. Curves of heat of adsorption ( q) versus coverage (θ) for chi, eta, and gamma alumina all have the same general form. A small fraction of the chemisorbed ammonia is very strongly held. An initially high heat of 20–40 kcal mole −1 falls over the first few percent of a monolayer to a constant value between 11 and 13 kcal mole −1. The highest initial heat was observed for alumina prepared in the absence of alkali metal ions. Silica-alumina shows a significantly greater integral heat (up to 20% coverage) than alumina, but heats at very low coverage are not higher than those observed on pure alumina. This indicates an equivalent number of strong acid sites but a larger number of weaker acid sites on silica-alumina. The behavior is otherwise similar. The influence of composition on ammonia adsorption has been studied over the range 0–40% Al 2O 3 in silica-alumina. Calcium X behaves similarly to alumina, and has a higher heat of ammonia adsorption than the corresponding sodium zeolite at all coverages. No minima or maxima in the q- θ curves, suggesting the existence of strong attractive interactions in the chemisorbed ammonia layer, were found for any of the oxides investigated, contrary to certain earlier calorimetric observations. It is suggested that these effects may be due to differences in calorimetric practice.