Lanthanum-cerium-exchanged zeolites

In the case of cerium exchanged zeolites, the ionic exchanges were carried out under nitrogen, in order to avoid the oxidation of cerium (III) ions. Part of the cerium (III) exchanged zeolite was calcined under oxygen for 6 hours at 540°C in order to convert cerium (III) to cerium (IV) ions. Lanthanum-cerium exchanged zeolites were prepared with the following composition 75% La, 25% Ce 50% La, 50% Ce and 25% La, 75% Ce. 

Lanthanum (La), cerium (Ce), praseodymium (Pr) and neodymium (Nd) exchanged Y zeolites with different rare earth content, as well as mixed lanthanum-cerium exchanged. eolites, taute been prepared and characterized. sSi and 7A1 MASNMR spectra, surface area, crystallinity and unit cell size data were obtained for fresh and steamed zeolites. The stability of these zeolites toward steam has been investigated. An increase in lanthanum content from about 14 wt% La 03 to over 20 wt% enhances the stability of the Lar zeolite towards dealumination by steam. A similar effect was observed >r NdY and PrY zeolites. The resolution loss of Si- JR sigqia 1 s observed for paramagnetic jpns (Nd3, Pr, ... 

Cerium-, copper-cerium coexchanged ZSM-5, copper-MCM-22, copper- and cerium-EMT type zeolite, copper-FAU type zeolite and copper-Beta exhibit an activity of the same order as that of copper-ZSM-5 in NOx reduction under simulated Diesel exhaust conditions. Propene was used as the reducing agent. The catalysts were used in a powder form and their activities tested in a fixed-bed flow reactor at a space velocity of 50 000 H . Copper-SAPO-34 and cerium- and gallium-EMT type zeolite have a moderate activity under the same conditions. The presence of water vapor inhibits the activity of EMT-zeolites. When an ageing procedure is carried out on copper-EMT type zeolite, dealumination occurs. The increase of the Si/Al ratio of the zeolite does not limit the dealumination process. The exchange of the zeolite with lanthanum prevents the zeolite from dealumination but leads to a loss of the catalytic activity. 

In their use as catalyst components, rare earth Y zeolites are freauently prepared by ion exchange with commercial rare earth salt solutions. Such commercial salts are mixtures of different rare earths, in which the major components are lanthanum, cerium, praseodymium and neodymium (5). These rare earth elements therefore play a major role in determining the physico-chemical characteristics and stability of Y zeolites that are used in many commercial catalysts. 

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. 

The fresh RE,NaY samples all displayed spectra similar to a NaY sieve. For the lanthanum-cerium binary exchanges, only two peaks were well resolved at 94-98 and 100-103 ppm. Steam treatment of Hi-REY, similarly to RE,NH.Y, resulted in dealumination characterized by the appearance of a peak at 108-111 ppm. Substantial retention of the Y zeolite framework was observed for 100% La and 75% La/25% Ce exchanges indicating the stabilizing influence of lanthanum over cerium. 

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