Zeolite exchanged with

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

The acidic character of 5A zeolite as a function of the calcium content has been explored by different techniques propylene adsorption experiments, ammonia thermodesorption followed by microgravimetry and FTIR spectroscopy. Propylene is chemisorbed and slowly transformed in carbonaceous compounds (coke) which remain trapped inside the zeolite pores. The coke quantities increase with the Ca2+ content. Olefin transformation results from an oligomerization catalytic process involving acidic adsorption sites. Ammonia thermodesorption studies as well as FTIR experiments have revealed the presence of acidic sites able to protonate NH3 molecules. This site number is also correlated to the Ca2+ ion content. As it has been observed for FAU zeolite exchanged with di- or trivalent metal cations, these sites are probably CaOH+ species whose vas(OH) mode have a spectral signature around 3567 cm"1. 

Scheme 4.4 Insertion of CO2 into epoxides and cleavage of cyclic carbonates. Step 1. Catalyst MgO, CaO. Step 2. Catalyst zeolites exchanged with alkah and/or earth metal ions.

FAU type zeolites exchanged with many different cations (Na, K, Ba, Cu, Ni, Li, Rb, Sr, Cs, etc.) have been extensively studied. The unit cell contents of hydrated FAU type zeolite can be represented as M,j(H20)y [A Sii92 0384] -FAU, where x is the number of A1 atoms per unit cell and M is a monovalent cation (or one-half of a divalent cation, etc.). The number of A1 atoms per cell can vary from 96 to less than 4 (Si/Al ratios of 1 to more than 50). Zeolite X refers to zeolites with between 96 and 77 A1 atoms per cell (Si/Al ratios between 1 and 1.5) and Zeolite Y refers to zeolites with less than 76 A1 atoms per cell (Si/Al ratios higher than 1.5). 

Zeolites can be ion-exchanged with cations or impregnated with various metals to modify their performance for use in applications such as separations, adsorption and catalysis. For example, faujasite zeolites exchanged with Na, Li, K, Ca, Rb, Cs, Mg, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd, Ag, Cd, In, Pt, H, Pb, La, Ce, Nd, Gd, Dy and Yb have been made and studied due to their use in separation and catalysis [135]. The ability to determine the distributions of these cations in the zeolitic structure is one of the key parameters needed in understanding adsorption mechanisms and molecular selectivities. Little has compiled an excellent reference... 

The dehydrated zeolites exchanged with various cations have been of catalytic interest in many reactions, among which cracking (259) and shape-selective catalysis (260) are most important. Other reactions include oxidation, carbonylation, and related reactions (261) as well as other nonacid catalytic reactions (262). 

Hi. Zeolites exchanged with transition metal ions. In the first row, scandium-, titanium-, cobalt-, and nickel-exchanged zeolites have been the most studied. Cobalt-exchanged zeolites are discussed in Section IV,E since they lead to oxygen adducts on adsorption of oxygen. There are several cases where copper and particularly iron ions are found as impurity cations which affect the oxygen adsorption properties of the zeolite. 

Surface acidity and catalytic activity. Faujasitic zeolites exchanged with multivalent ions demonstrate significant catalytic activity for reactions involving carbonium ion mechanisms, in contrast to the inactivity of the alkali metal ion-exchanged forms. Several possible sources of the observed activity were proposed initially. Rabo et al. (202, 214) suggested that electrostatic fields associated with the multivalent ions were responsible for the catalytic activity. Lewis acid sites were proposed as the seat of catalytic activity by Turkevich et al. (50) and by Boreskovaet al. (222). Br0nsted acid sites formed by hydrolysis of the multivalent metal ions were proposed as the catalytic centers by Venuto et al. (219) and by Plank (220). 

Despite the enormous importance of zeolites (molecular sieves) as catalysts in the petrochemical industry, few studies have been made of the use of zeolites exchanged with transition metal ions in oxidation reactions.6338- 634a-f van Sickle and Prest635 observed large increases in the rates of oxidation of butenes and cyclopentene in the liquid phase at 70°C catalyzed by cobalt-exchanged zeolites. However, the reactions were rather nonselective and led to substantial amounts of nonvolatile and sieve-bound products. Nevertheless, the use of transition metal-exchanged zeolites in oxidation reactions warrants further investigation. 

Crystallization and adsorption are both widely used to perform the separation distillation is not used (except for orthoxylene separation) because of too small differences between the boiling points (Table 10.1). Despite the still high importance of crystallization, adsorption becomes the most widely used technique because of its high efficiency. The adsorbents which are used for selective adsorption of paraxylene are X or Y zeolites exchanged with adequate cations. Liquid phase Simulated Counter Current adsorption, which is the most efficient process, is generally used (1). In addition to the complexity of this process, the choice of an adsorbent selective for paraxylene is the critical point. 

X and Y zeolites exchanged with potassium and barium cations and associated with an adequate solvent are efficient for paraxylene separation by adsorption. The design of the adsorbent as well as of the process are both critical to reach economical production of paraxylene. The complete system, adsorbent plus process with the chosen configuration - stand alone or hybrid (adsorption + crystallization) - has to be optimized. 

Dimerization presumably takes place on the transition metal-containing sites, and alkylation on the acidic sites of zeolltic surface. The sodium form of zeolite exchanged with transition metal cations Is capable of dimerization (and further polymerization), but does not practically exhibit alkylating capacity. This explains the composition of the product obtained from ethylene and Isobutane over this catalyst (Table V, column 3). 

Textural and chemical characterization of NaX zeolite exchanged with Zn(II) ions... 

The anion vacancy, RE(OH), or RE may all coordinate pyridine. Simultaneously, NaY zeolite exchanged with high-electron-aflBnity cations such as Cu and Ce acquires important oxidizing properties. The electron is transfered from the aromatic hydrocarbon to the ceric or cupric ion, whereas LaY zeolite behaves like a dehydroxylated NH4Y zeolite. It seems that the NaCe(IV)Y form also provides species such as PyH. ... 

In contrast to the previously discussed carbon-carbon bond rearrangements, these results clearly show that Lewis acid sites can also act as catalytically active sites for nucleophilic substitutions. Note that if catalysts without Bronsted acid sites are used (i.e., with zeolites exchanged with monovalent cations) the competitive side reaction leading to (i)-hydroxybutryonitrile via protonation of the acid amide can be completely suppressed (Scheme 6). 

The following sequence of overall acidity (density of Bronsted and Lewis acid sites) measured via t.p.d. of ammonia was found for dealuminated Y zeolites exchanged with alkali-metal ions H-D-Y Li-D-Y > Na-D-Y > Cs-D-Y > K-D-Y the strength of the acid sites was particularly low in the case of K-D-Y. The lower the overall acidity of the support, the lower was the observed average oxidation number of Mo. The zeolites with reduced overall acidity exhibited a higher ability to decompose ammonia. 

The present work describes the influence of reaction temperature on this reaction using two catalysts based on Y zeolite exchanged with Li and Na cations and tries to explain their catalytic properties using IR results on the benzene adsorption. These two zeolites are almost neutral and have neither protons nor strong basic sites 1 ). 

Figure 1.- XRD patterns of fresh Y-Zeolites exchanged with different concentration of REO. A) ZYO, B) ZY4, B) ZY7 and C) ZY12.

Y zeolites exchanged with rare earth cations are widely used as the active component for cracking catalysts in petroleum industry. Such cations improve the catalytic activity and the gasoline yield, lowering the gas production and the coke formation. They also promote the thermal and hydrothermal stability of the catalyst. 

Catalyst MgO. CaO. Step 2. Catalyst zeolites exchanged with alkali and/or earth... 

Aromatic molecules such as naphthalene will emit phosphorescence when adsorbed in zeolites exchanged with heavy ions such as thallium. This was exploited in a convenient, zeolite TlY-coated optical fiber format in order to detect naphthalene.[131]... 

We therefore engaged our effort toward the synthesis and the test of zeolites exchanged with metals such as cobalt, zinc or cerium. The results are presented in Table 8 ... 

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. 

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