Metal-Doped Zeolites

The A1 centers can be replaced by trivalent atoms such as B, Fe, Cr, Sb, As, and Ga, and the Si centers by tetravalent atoms such as Ge, Ti, Zr and Hf. Silicon enrichment up to a pure Si02 pentasil zeolite (sUicaUte) is also possible [4], 

Isomorphic substitution affects zeolite properties such as shape selectivity (influences on the framework parameters), acidity, and the dipersion of introduced components. The following sequence was found for the acidity of ZSM-5 zeolites  

Thus the weakly Bronsted acidic boron zeolites allow acid-catalyzed reactions to be carried out with high selectivity. Gallium substitution gives effective, sulfur-resistant catalysts for the synthesis of aromatics from lower alkanes, without the need for noble metal doping [8], The nonacidic titanium siUcalite exhibits very interesting properties in selective oxidation reactions with H2O2 [T32]. 

In addition, a completely new class of zeoUte-like materials has been synthesized from A1 and P compounds, namely the aluminophosphate (AIPO4) molecular sieves. 

In contrast to the zeolites, the frameworks of the aluminophosphates are electrically neutral, contain no exchangeable ions and are largely catalytically inactive. 

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TS-1 = metal doped zeolite Bovicelli. R Lupattelli. R Sanetti, A. Mincione, E. Tetrahedron Lett., 1994, 35, 8477... 

In 1968 Venuto and Landis reported on the use of sodium- and rare-earth metal-doped zeolite X in the rearrangement of ethylene oxide and propylene oxide to the corresponding aldehydes [17]. In addition to the desired propanal, acetone was formed with these catalysts because of the hydride shift induced by the intermediate carbocation consecutive reactions were also observed. Such catalysts also suffer from rapid deactivation. 

The bifunctionality of metal-doped zeolite catalysts is explained here for the important example of isomerization and hydrogenation. The metal content facilitates the hydrogenation and dehydrogenation steps, while the acid-catalyzed isomerization step takes place under the restricted conditions of the zeolite cavities (Scheme 7-1). 

Scheme 7-1 Bifunctionality of metal-doped zeolites isomerization and hydrogenation...

The search for new selectivity promoters will be improved, and more and more unusual elements such as Sc, Y, Ga, Hf, and Ta will be used. The zeolites have major potential, and it is expected that especially the pentasils and the metal-doped zeolites will achieve wider application in organic syntheses. The industrial application of aluminosilicates and sheet sihcates is also imminent. 

Other oxidizing systems based on metals that can carry out regioselec-tive oxidations of secondary alcohols on a catalytic quantity are a titanium-doped zeolite in the presence of H20243 and the hydrotalcite R.U-C0-AI-CO3 HT in the air 44... 

To improve process economics, further work is needed to improve catalyst lifetimes. A more stable system employed a noble metal-loaded potassium L-zeolite catalyst for the condensation of ethanol with methanol to produce a 1-propanol and 2-methyl-l-propanol (US patent no. 5,300,695) (18). However, yields were small compared with the large amounts of CO and C02 produced from the methanol. More recently, Exxon patented a noble metal-loaded alkali metal-doped mixed metal (Zr, Mn, Zn) oxide (US patent nos. 6,034,141 and 5,811,602) (19,20). The catalyst was used in a syngas atmosphere. As with other catalysts, the higher temperatures resulted in decomposition of methanol. Changes in catalyst composition were noted at higher temperatures, but the stability of the catalyst was not discussed. Recently, compositions including Ni, Rh, Ru, and Cu were investigated (21,22). 

Investigations into these topics are presented in this volume. Iron, nickel, copper, cobalt, and rhodium are among the metals studied as Fischer-Tropsch catalysts results are reported over several alloys as well as single-crystal and doped metals. Ruthenium zeolites and even meteo-ritic iron have been used to catalyze carbon monoxide hydrogenation, and these findings are also included. One chapter discusses the prediction of product distribution using a computer to simulate Fischer-Tropsch chain growth. 

As they are relatively cheap and are produced in the form of almost ultraviolet-transparent materials, rare-earth-doped zeolites have attracted growing interest as substitutes for more expensive phosphors which, applied in fluorescent lamps, should be able to efficiently convert UV into visible light. As shown by Borgmann et al. [92], Eu +-doped zeolite X excited with 254-nm radiation gave only weak emission. Additional insertion of molybdate caused absorption of UV radiation and subsequent transfer from the excited metalate LMCT state to the Eu + level increasing the quantum yield up to 7%. Upon thermal treatment at... 

So far in addition to uncharged metal clusters, charged species have also been present. Prototypes of the latter are the alkali metal ion clusters which can be formed in zeolites by exposure to high-energy radiation, by alkali metal doping or by introduction of molecular electron donors [ 109,134]. These seemingly very different procedures can be brought down to the common denominator of dis-... 

Aluminum silicate, alumina, mixed metal oxides, zirconia, metal doped oxides, zeolites Catalysts... 

The work presented in this chapter shows that the use of zeolites could be an interesting and complementary alternative to the current end-of-pipe gas cleaning techniques. Indeed, these catalysts present good properties towards the elimination of PCDD and PCDF precursors. If the addition of noble metals improves their efficiency, the kinetic studies showed that with concentrations in the range observed in the industry, non-doped zeolites are able to easily eliminate their precursors in the SCR temperature range. In regard to PAH reduction, acid zeolites are also very... 

Yokoyama, C. and Misono, M. Catalytic reduction of nitrogen oxides by propene in the presence of oxygen over cerium ion-exchanged zeolites II. Mechanistic study of roles of oxygen and doped metals. J. Catal, 1994, Volume 150,9-17. 

The abovementioned rate acceleration and selectivity enhancement brought about by catalysts are particularly marked when unactivated dienes and dienophiles are involved. Two molecules of 1,3-butadiene can react in a Diels-Alder reaction, one acting as diene and the other as a dienophile to produce 4-vinylcyclohexene (in 0.1% yield at 250°C in the absence of a catalyst). Cs+, Cu,+ and trivalent transition-metal exchanged montmorillonites534 as well as large-pore sodium zeolites (Na ZSM-20, NaY) and carbon molecular sieves,535 result in 20-35% yields with 95% selectivity. Large rate enhancement was observed when 1,3-cyclohexadiene underwent a similar cycloaddition536 in the presence of K10 montmorillonite doped with Fe3+ ... 

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