Lanthanum exchanged zeolites

Large-pore zeolites such as Y zeolites are efficient for the hydroamination of several olefins. For example, propene reacts with NH3 over SK-500 (a pelleted lanthanum-exchanged zeolite) or La-Y or H-Y zeolites with 6-15% conversion to give i-PrNHj with high selectivity (95-100%) (Eq. 4.5) [50]. 

The deactivation of a lanthanum exchanged zeolite Y catalyst for isopropyl benzene (cumene) cracking was studied using a thermobalance. The kinetics of the main reaction and the coking reaction were determined. The effects of catalyst coke content and poisoning by nitrogen compounds, quinoline, pyridine, and aniline, were evaluated. The Froment-Bischoff approach to modeling catalyst deactivation was used. 

Lanthanum-exchanged zeolite-Y is an important catalyst used for fluid cracking in... 

The high catalytic activity of lanthanum-exchanged zeolites has been attributed to the presence of polyvalent lanthanum ions in the form of [La(OH)2La]4+ or La(OH)2 species in the large zeolite cavities, withdrawing electrons from the framework OH groups making the protons more acidic.23... 

Three-letter code of the International Zeolite Association for zeolite A Lanthanum-exchanged zeolite X Molecular dynamics... 

Unlike the mobility of imsaturated hydrocarbons, the mobihty of saturated hydrocarbons in Na-X is, in fact, essentially unaffected by the presence of sodium cations. This has been confirmed by PFG NMR diffusion studies with benzene and -heptane in zeohte Na-X and La-X [103,151]. Since the triva-lent lanthanum ions are predominantly localized at positions in the hexagonal prisms and sodalite units, the molecules adsorbed in lanthanum-exchanged zeolites are essentially without contact to the cations. As a consequence of the specific interaction between the cations and the unsaturated hydrocarbon, the benzene mobility in Na-X was found to be two orders of magnitude smaller than in La-X, while the -heptane diffusivities were the same. 

Wahlen, J. Hertogh De S. Vos, De D. E. et al. Disproportionation of hydrogen peroxide into singlet oxygen catalyzed by lanthanum-exchanged zeolites. J. Catal. 2005, 233,422-433. 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

Since heating conditions (32) and aluminum extraction leading to a high stability have been avoided, the increased stability of the catalysts seems related to the presence of exchanged lanthanum. A zeolite with high thermostability can be obtained by introducing only 3.7 La3+ ions/unit cell into a decationated zeolite. A higher content of lanthanum does not... 

The lanthanum exchanged Y zeolite (LaY) was made by contacting an ammonium Y (Linde type 31-200 powder) with an aqueous solution of lanthanum chloride. Approximately 60-70 percent of the ammonium ions were exchanged in the procedure. The resulting LaY powder was pressed into tablets, crushed and sieved to -60+80 mesh. 

The work discussed here compares the relative site occupancies for three series of hydrated Y zeolites ammonium, calcium and lanthanum exchanged. The data for these same three series of samples in the dehydrated forms are also presented and the assignment of the observed resonances is discussed. 

Figure 1. Sodium-23 MASNMR spectra of a series of hydrated, partially lanthanum exchanged Na-Y zeolites at 96 MHz. The level of lanthanum exchange is as indicated.

J.V. Smith, J.M. Bennett, and E.M. Flanigen, Dehydrated Lanthanum-exchanged Type Y Zeolite. Nature (London), 1967, 215, 241-244. 

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. 

Under heterogeneous conditions, the Knoevenagel reaction has been used as a well-adopted test reaction to check the activity of the basic sites of different solids, mainly basic zeolites such as alkali-exchanged zeolites [40] or zeolites containing occluded metal oxides [41]. The basic activity of alkali-containing MCM-41 [42] or binary cesium-lanthanum oxide supported on MCM-41 [43] has, moreover, also been evaluated in the Knoevenagel condensation. 

Lin and Chao (1991) studied the variation of the cation distribution with the lanthanum exchange level in hydrated La,Na-Y zeolites by two-dimensional 23Na nutation NMR spectra in which the chemical shift and the quadrupole interaction could be separated. They proposed that the mobility of both water molecules and sodium ions can be reduced by lowering the temperature of the sample, and the migration of lanthanum ions from supercages to small cages causes the redistribution of sodium after dehydration... 

Figure 2 Reaction of propionyl chloride (18 mmol) and toluene over lanthanum exchanged NaY zeolites, a) %Yield of para isomer, b) Distribution of isomers over La(70)Y.

Neutron diffraction study of protons in four lanthanum exchanged X and LSX zeolites... 

The corresponding Cu spectra for the copper exchanged zeolites obtained after hydrogen reduction were all identical (Figures 7A, 7B and 7C ). The satellite peak observed for the oxidized samples had disappeared, and one core line was observed at a BE of 931.9 eV. The spectra match that of either Cu° or Cu " 31). However, reduction with Hj at 600 C is expected to reduce copper completely to the metallic state (57). The presence of lanthanum does not protect the copper ions from reduction by hydrogen which is also corroborated by the surface enrichment of Cu during reduction (Table II). 

As well as readily exchanging cations in aqueous solution, it has been found that ion exchange also occurs upon bringing a cationic zeolite in contact with a metal salt, typically a chloride, and heating the intimate mixture. In particular, Beyer and co-workers have followed the reaction of protonated or ammonium forms of zeolites with metal halides (particularly volatile halides) at elevated temperatures (Scheme 6.5)." This results in hydrogen chloride and ammonium chloride removal and proton exchange by the metal cations. Many such examples have been reported, including zinc, iron(II) and lanthanum into zeolites Y, mordenite and ZSM-5. 

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