ChemInform Abstract: The Role of Polyvalent Cations in Developing Strong Acidity: A Study of Lanthanum‐Exchanged Zeolites.

Abstract

ChemInform Abstract La3+ ions are introduced (ion-exchange) into a series of faujasite and ZSM-20 zeolites having a minimum amount of extraframework Al and a broad range of Si/Al ratios and the catalytic activity of the resulting materials in the hexane cracking reaction is measured. The cracking activity is found to increase with increasing La3+ loading up to a maximum and then decreases. Plots of the maximum activities of the different zeolites versus the framework Al content, Alf, show that the catalytic activity reaches a maximum at 35 Al/unit cell. The change in activity with respect to Alf parallels that observed with steam-dealuminated zeolites, but the absolute activities of the La-exchanged samples are lower. These results are consistent with a model of strong acidity in which both the Alf and the presence of polyvalent cations in the β-cage are important. Lanthanum ions in the form of (La(µ-OH)2La)4+ and La(OH)2+ appear to be responsible for a withdrawal of electrons from the framework hydroxyl groups resulting in a higher acidity of the protons.

Observation of a compensation relation for monomolecular alkane cracking by zeolites: the dominant role of reactant sorption

“…Similar to previous studies, mild steam dealumination of mordenite leads to increased conversion for monomolecular n-butane cracking up to a maximum of 2.7 times per Brønsted acid site [2,3,11,18,30]. Steam dealumination leads to an increase in the low coverage enthalpy of sorption by about 5-15 kJ/mol, depending on the degree of dealumination, which was correlated with the increased activity.…”

“…In addition to the number of acid sites, it is thought that the strength of the Brønsted acid sites is affected by the acid site density, i.e., the Si/Al ratio [7], the T-O-T bond angle [8,9], the presence of nonframework aluminum [2,3,8,[10][11][12][13][14][15][16][17][18], or charge-balancing cations [18,19]. Theoretical and experimental evidence suggests that the acid strength per aluminum is a maximum when the framework aluminum ions have no second nearest neighbor aluminum [20,21].…”

Imaging individual lanthanum atoms in zeolite Y by scanning transmission electron microscopy: Evidence of lanthanum pair sites

“…Zeolites are used extensively as catalysts in petrochemical processing and petroleum refining, with catalytic cracking of petroleum being the largest-scale application [1][2][3][4]. Many commercial cracking catalysts incorporate faujasite-type zeolites that contain rare earth ions such as lanthanum, because they significantly improve the catalytic activity [5][6][7] and hydrothermal stability [8,9] in many industrially relevant reactions. Characterizations of lanthanum-containing zeolites by X-ray diffraction crystallography, X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy [10][11][12] provide details of the zeolite structure and demonstrate that La 3+ ions hydrolyze water to form hydroxylated La 2+ species.…”

Acetalization of ketones on K-10 clay and rare earth exchanged HFAU-Y zeolites: A mild and facile procedure for the synthesis of dimethylacetals

“…Rare earth exchanged H-Y zeolites were obtained by contacting 2 g H-Y (Sud-Chemie (India) Ltd. Si/Al = 1.5) with 100 mL of 0.5 M rare earth nitrate (obtained from Indian Rare Earths Ltd. Udyogamandal, Kerala) solutions at 353 K for 24 h [20,21]. K-10 montmorillonite clay (Si/Al = 2.7) was procured from Aldrich Chemical Company, USA.…”