Ashim K. Ghosh

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.

Characterization of zeolite acidity. An infrared study using ammonia, methylamine and n-butylamine

Characterization of the acidity of an HY and a number of mordenite zeolites (SiO2/Al2O3 ratios 8–26) has been studied by infrared spectroscopy using ammonia, methylamine and n-butylamine. The characteristic bands of adsorbed ammonia appeared at 1445 cm–1(Bronsted type) and 1623 cm–1(Lewis type), with the disappearance of the acidic OH band at 3609 cm–1 for mordenites and of the acidic OH bands at 3643 and 3568 cm–1 for HY. When methylamine and n-butylamine were adsorbed on the catalysts the acidic hydroxyls were found to be accessible to the amines and a band of δsym(NH+3)(RNH2 adsorbed on a Bronsted site) appeared in the 1500–1540 cm–1 region with a band at ca. 1610 cm–1, which is a combination of RNH2 on Lewis sites and δasym(NH+3). Bronsted acidity of the catalysts as determined from the band intensities of the adsorbed ammonia, methylamine and n-butylamine, although not in numerical agreement, showed the same trend. The alkylamines were decomposed into alkenes at elevated temperatures and the decomposition was prominent in the case of n-butylamine.

Characterization of zeolite acidity: a gas-chromatographic study using n-butylamine and ammonia

The acidity of a hydrogen mordenite and two dealuminated hydrogen mordenites has been determined by gas chromatography using ammonia and n-butylamine. In the case of hydrogen mordenite the results show a higher acidity value when ammonia was used than when n-butylamine was used as base, whereas for the dealuminated mordenites the results for both bases are in good agreement. However, in the case of all the mordenites the acidity values have been found to be higher than those determined by either calorimetry or the Benesi method and this has been interpreted as being due to the fact that the gas-chromatographic method includes some physical adsorption.

Characterization of zeolite acidity by n-butylamine titration. A comparative study using Hammett indicators and calorimetry

The acidity of three H-zeolites, a mordenite (HM), a dealuminated mordenite (DHM) and a HY zeolite, has been determined both by calorimetric titration and titration with non-aqueous n-butylamine solution using coloured Hammett indicators in the range H0=–3.0–6.8. Neither method is satisfactory for the mordenite (HM), and it is considered that the n-butylamine molecule blocks the entrance to the pores of this zeolite, preventing an accurate estimation of its acidity. The results for HY are in satisfactory agreement with published work. There is some uncertainty as to the acidity of DHM, as dealumination may have enlarged the opening of the channel system with pore blockage occurring further down from the entrance to the pore, or on the other hand it may have caused the formation of strong acid sites on the exterior surface.

Evaluation Of The Titration Method For Characterization Of Zeolite Acidity

The acidity of a number of mordenites and a zeolite Y has been determined by n-butylamine titration using indicators and thermometry. The results obtained by these methods are in agreement, hence confirming no error due to the indicators in the indicator method. The ir results show that when n-butylamine is contacted with HY preadsorbed with benzene at room temperature the base reacts with all the zeolite acid sites by displacing benzene but in the case of mordenites benzene blocks the channels and inhibits the diffusion of n-butylamine except at elevated temperatures.

An infrared study of adsorption of pyridine and n-butylamine on benzene preadsorbed zeolites Y and mordenites

When pyridine or n-butylamine was adsorbed on benzene-preadsorbed HY zeolite the bases reacted with all acid sites by displacing benzene molecules but not in the case of mordenites when benzene molecules block the mordenite channels.

Homoenolisation: a simple route to hirsutene

A sequence leading from dicyclopentadiene to the cis,anti,cis-tricyclopentane ring system of the hirsutanes is described in which the key step utilized rearrangement by homoenolisation to generate the carbon skeleton with the requisite stereochemistry.