Catalytic role, zeolites formation

Of the three possible types of shape selectivity [1] - due again to the ZSM structure — the predominant (but not exclusive) formation of para-xylene may indicate a moderate product shape-selectivity. The appearance of meta- (and also ort/io-xylene) points to the non-negligible catalytic role of the outer surface of presumable larger zeolite crystallites [1] with some Pt particles present on them [19]. 

The positive effect of Na in FAU zeolites in the formation of oxidation products has already been reported in the literature. Thus, pyrene trapped in the supercages of HFAU zeolites was shown to be totally oxidised above 400°C over NaY and only above 550°C over HFAU [18]. Moreover, during coke oxidation over a series of NaHFAU zeolites, the CO/CO2 ratio was found to decrease with increasing Na amount in the zeolite, the authors concluding that the effect was probably due to a catalytic role of Na cations in CO oxidation [18]. The easier formation of oxidation products which is observed here with NaY seems to confirm the positive role of Na cations in oxidation. 

A system that has received considerable interest in recent years is the catalytic conversion of methanol to gasoline. Numerous hypotheses have been advanced to explain the mechanism, and solid-state 13 C n.m.r. has played an important role in this respect, in that it can directly examine the organic species in the zeolite without any interference from the inorganic matrix. The initial formation of dimethyl ether from methanol over H-ZSM-5 was proposed by van Hooff 141 subsequent dehydration and methylation reactions lead to lower alkenes,142 which in turn oligomerize at ambient temperature to linear alkyl chains.143 At temperatures of about 373 K, branched alkyl chains are also formed. Many more promising applications can be anticipated in this area. 

The isomerization of n-butenes was used as a test reaction to follow the development of catalytic activity in Na-Y zeolite. The acidity was varied by the substitution of some of the Na " (0.3 to 5.7%) with Ca and by creating a Na deficiency (up to 0.94%) both series of catalysts were studied with and without water added as cocatalyst. A pure Na-Y zeolite containing no decationated sites was found cat-alytically inactive for this reaction. In contrast with silica-alumina catalysts, carbonaceous residues did not appear to play a role in the formation of the catalytic sites as long as H2O was used as cocatalyst. 

In recent publications there are contradictory points of view on the basis of unusual activity of CuH-ZSM-5 in both total oxidation of hydrocarbons and NO decomposition [3, 6, 8 - 11, 19, 20]. In our earlier work the decisive role of the low-coordinated square-planar Cu cations in alkane oxidation was demonstrated [11, 12, 14]. In our opinion, the results obtained agree well with these data the preservation of high catalytic activity of Co/CuH-ZSM-S/ gQO correlates well with conservation of just these the most unsaturated sites in Co/Cu-ZSM-S/ g. The physical mechanism of the stabilizing influence of cobalt additive on the conservation of Cu local topography in ZSM-5 matrix is unclear now. The migration of cobalt ions into zeolitic channels with formation of some kind of bi-cationic structures can not be excluded. 

Fluid Catalytic Cracking (FCC) is one of the most important process in oil refining. The evaluation of the catalysts in the laboratory scale is often carried out in a micro-reactor, the so called micro-activity test [1-3] (MAT). Coke formation plays an important role in the deactivation of FCC catalysts, which can be deactivated either permanently (loss of surface area, zeolite collapse, metals) or temporarily deactivated (coke). 

Cation Zeolites. Since zeolites are important in acid-catalyzed reactions, much effort has been spent determining if zeolites are acidic and what the nature of the acidity is. Although many reactions catalyzed by zeolites are those catalyzed by amorphous silica-alumina, early theories of their action (49) suggested that the electrostatic field of the zeolites was responsible for their catalytic action. There is evidence to support this concept since the activity of zeolites in several reactions increases with the calculated electrostatic field (49, 50, 63). However, further studies have suggested that if the electrostatic fields are involved, their role may be to promote the formation of acid sites. 

The predominant importance of the cations in zeolites is that they form so-called active sites for selective interaction with guest molecules in sorption and catalytic processes. From the point of view of advanced material science [47] they play a significant role in the formation of quantum-sized clusters with novel optical or semiconducting properties. As they give rise to cationic conductivity, zeolites can be used as solid electrolytes, membranes in ion-selective electrodes and as host structures in solid-state batteries. Organometallic compounds and coordination complexes can be readily formed on these cations within the larger cages or channels and applied to gas separation, electron-transport relays and hybrid as well as shape-selective catalysis [48]. 

The SAR value is expected to play a major role in the distribution of isomers 19, 20, and 21 in the reaction depicted in Scheme 3.10 (2-methoxynaphthalene/acetic anhydride ratio=0.5). In a comparative study with three HY zeolites, namely, HY(15), HY(40), andHY(lOO), an increase of the initial activity (TOP 1.72, 8.5, and 9.3m" , respectively) is obsawed with the increase of the SAR value and a consequent lowering of the surface acidity (0.65,0.15, and 0.062 meq HVg, respectively) [57]. These results confirm that the activity does not sten fiom an increased acid strength of the catalyst [58] and suggest, in agreement with some conclusions by Corma [59], that the catalytic activity can be bettCT related to a more hydrophobic character of dealuminated HY zeolites. In all cases, the reaction leads to the formation of compound 19 as the major product (95% yield), 21 (obtained only up to 4% yield), and traces of 20. 

The Si/Al ratio plays a significant role, since the aluminum atom is directly related to the acidic site and accounts for the formation of carbenium and/or carbonium ions or possibly cation radicals inside the zeolite. Dealumination processes can promote porous structure modifications, which may improve some interesting properties of zeohtes, like thermal and hydrothermal sta-bihty, acidity, catalytic activity, resistance to aging and low coking rate, and... 

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