Rare-earth-exchanged Y zeolites

In the early 70 s, FCC formulations containing 10-40% CREY (calcined rare-earth exchanged Y zeolites) were widely employed because these catalysts offered improved chemical as well as thermal and hydrothermal stability over FCC compositions containing equivalent amounts of (low sodium) HY crystals (23-25). The... 

Moscou and Mone measured the acidity distribution of various rare earth-exchanged Y zeolites and correlated the results with the performance of the zeolite in cracking of gas-oil (224). They found that steaming the zeolite at 675°-750°C resulted in a significant decrease in the concentration of strongly acidic sites (H0 < -8.2), while the intermediate... 

In REHY and in calcined rare earths exchanged Y-zeolites (CREY) containing about 7.6% CeA, 4.0% La203, 2.8% Nd203 and 0.9 Pr203 crystals, it is believed that Ce+4 ions, present as an oxycerium complex, undergo a redox reaction with oxyvanadyl cations (VO+2) and form a stable orthovanadate (74) ... 

The reaction is known to proceed through activation of the olefin by the catalyst (a rare earth exchanged Y Zeolite In this work) and this activated olefin then reacts with benzene and alkylbenzene. However, since the concentration of activated olefin Is not known, the total concentration of olefin Is used In obtaining kinetic parameters according to the following reactions. 

Acidic and Oxidizing Properties of Rare Earth Exchanged Y Zeolites... 

Most of the published structural studies were done either on lanthanum or mixed rare earth exchanged Y zeolites. Few studies can be found on the effect of individual rare earths on the structural characteristics and2stabil ity of Y zeolites. 

In this paper, oi and Al MASNMP. spectroscopy is used in conjunction with crystallinity, surface area and unit cell size measurements to study individual rare earth exchanged Y zeolites in order to determine the effect of individual rare earths cations on their structure and stability. These methods are used to further probe rare earth induced structural changes that occur during hydrothermal treatment of the zeolites. The studies were extended to also establish the effect of different lanthanum-cerium mixtures on zeolite stability. The data presented and discussed are for lanthanum, cerium, praseodymium and neodymium exchanged Y zeolites, as well as for zeolites exchanged with different lanthanum-cerium mixtures. 

TABLE I. COMPOSITION AND SELECTED PROPERTIES OF RARE EARTH EXCHANGED Y ZEOLITES... 

Typical IR spectra for a mixed rare-earth-exchanged Y zeolite show hydroxyl bands at 3740, 3640 and 3530 cm-1 after evacuation at about 450°C, while they are observed at 3550 and 3618 cm-1 at lower temperatures (Ward 1976, Sousa-Aguiar et al. 1998). However, the frequency of the absorption band near 3500 cm-1 decreases with cation radius from 3522 cm-1 for La-Y to 3470 cm-1 for Yb-Y. 

Bolton (1971) suggested that some of the O-H present in rare-earth-exchanged Y zeolites result from direct exchange of Na+ ions by H30+ during the exchange with R3+ since this operation must be carried out under acidic conditions (pH 4.5) to avoid precipitation of hydrolysed R salts. 

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