Zeolites rare-earth-doped

Zinner et al. (1993) observed that the alkylation of benzene with 1-dodecene over rare-earth-doped Y zeolite, R,Ca Na-Y (R = La, Ce, Nd or Gd) can be carried out at relatively low temperatures (80°C) forming selectively linear alkylbenzenes. This has been explained by the appearance of R hydroxyl cations in the zeolite cages after exchange and thermal treatment, resulting in the generation of Bronsted and Lewis acid sites. They observed that the catalytic activity decreases with the lanthanide contraction in the order La- > Ce > Nd- > Gd-. Although 2-phenyl dodecane is the most important reaction product, dodecane isomers (6-, 5-, 4- and 3-phenyl dodecane) are also observed. 

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... 

Rare earth compounds are also used in numerous catalytic reactions in petrochemical industry. One example is the use of rare earth salts to stabilize zeolites used for the catalytic cracking of crude oils to gasoline. Rare earth doping increases the activity of these zeolites with the consequence of higher gasoline yields. In addition, these rare earth-modified catalysts have found expanded application as a consequence of the refineries use of residual or heavy crude oils which contain high levels of nickel, vanadium, and sulfur which attack zeolites and reduce their activity rare earths are more resistant to these catalytic poisons. ... 

This process carries out the vapor phase oxychlorination of ethane, in the presence of oxygen or air enriched with oxygen, between 350 and 450°C, and between Oil and 10.10 Pa absolute. It employs a catalyst system based on silver doped by derivatives of manganese, cobalt or nickel, and possibly of rare earths (such as lanthanum), and which is employed in mass form or supported on a Y-type zeolite (offretite). 

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. 

Aluminosilicate zeolite ([Al]ZSM-5), borosilicate zeolite ([B]ZSM-5), and iron silicate zeolite ([Fe]ZSM-5), doped with transition metal or rare-earth or noble metal were tested as the catalysts. Different modifications of the zeolite do not improve the olefin conversion. Neither isomorphous substitution of Al atom in the zeolite structure by B or Fe, nor doping the zeolite with metals influences notably the conversion of olefin into the acid compared with [Al]ZSM-5 (Table 23). 

Another interesting feature of MOFs concerns the number of cations that can participate in the framework. Indeed, compared to inorganic ones [3], which are more based on a few cations (Si and A1 for zeolites, eventually doped with some transition metals, with the exception of titanosilicates [119] Zr, Al, Ga, In phosphates and arsenates, sometimes fuUy substituted by transition metals Ti [72], V [120], Fe [121,122], Co [123,124], Ni [125], Zn [126,127]), MOFs can accept almost all the cations of the classification, at least those which are di-, tri- (including rare earth) or tetravalent. Keeping in mind the tremendous number of species previously isolated in coordination chemistry, this provides a huge number of possibilities for creating new MOFs. 

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