TREATED ZEOLITES, CATALYTIC PROPERTIES

The incorporation of TiIV in the crystal lattice of silicalite has been attempted by the reaction of TiCl4 with dealuminated ZSM-5 (Kraushaar et al., 1988) or deborated borosilicalite (Carati et al., 1990). The same reaction has been used in the attempt to incorporate titanium in the crystal lattice of zeolite beta, morde-nite or zeolite Y. In many cases catalytic properties have resulted, but the way in which the incorporation takes place has been questioned. Because of its molecular dimensions, TiCl4 cannot enter or leave the pore system of ZSM-5. It has been shown that 89% of the OH groups present in the preformed zeolite as SiOH remain unreacted after treatment at 573 K with TiCl4. The incorporation of titanium must therefore be limited to the outer part of the crystals or proceed through a severe chemical attack with removal of silicon and formation of a secondary pore system (De Ruiter et al., 1993). Deposits of Ti02 on the outer part of the crystal treated with TiCl4 have indeed been observed (Carati et al., 1990), as has abnormal behavior in the oxidation of phenol (Section V.C.3.c). 

We believe the use of a direct gaseous phase fluorination process for modifying the surface and structure of zeolites to be a new process (18). The literature does contain references to the use of hydrogen fluoride (20, 22, 23), boron trifluoride (21, 24), aluminum monofluoride (sic) (19) and silica difluoride (sic) (19) to treat the surface of a zeolite to obtain higher catalytic activity. However, the use of fluorine gas to modify both surface and structure has not been reported before. The purpose of this paper is to report results of fluorination of zeolites and to describe the process involved in such a treatment. Detailed results on fluorine-treated zeolites and their unusual properties, both adsorptive and catalytic, will be discussed in forthcoming papers. 

It is unfortunate that in the published catalytic studies with heat-treated NH4-exchanged Y the catalyst substrate seems to be insufficiently characterized, and the assignment of structural details, which are most important for catalytic properties, are based on assumptions, usually drawn from comparisons with similar zeolite samples. Furthermore, the catalytic properties of zeolites which are more fully characterized usually are not reported in the literature. Consequently, comparison between catalysts prepared by different investigators is very difficult and risky since the catalytically important details depend on several chemical and structural characteristics, all of which usually are not described. As an example of this problem, the dehydroxylation of heat-treated NH4" -exchanged Y depends on the temperature, atmosphere, and time employed on activation, as well as on the Si/Al ratio, the degree of NH4" exchange 16), and the residual cations 108). 

Based on our experimental data, the following conclusions may be drawn (i) the incorporation of titanium into the framework of beta zeolite was achieved by treating Al-beta zeolite with ammonium titanyl oxalate solution and calcining the resultant material at 833 K for 6 h, (ii) the presence of Ti in tetrahedral framework positions was evidenced by various techniques, particularly UV-Vis, XPS and catalytic properties and (iii) Ti-P and Ox-Ti-P samples were active in the epoxidation of olefins. 

The most important catalyst systems involving rare earth elements are the oxides and intermetallics. Catalytic properties of rare earth oxides are described in section 4 and those of intermetallic compounds in section 6. Reports on surface reactivities of other binary rare earth compounds are only sparse, and this is mentioned in section 5. A very interesting class of catalyst systems comprises the mixed oxides of the perovskite structure type. As catalysis on these oxides is mainly determined by the d transition metal component and the rare earth cations can be regarded essentially as spectator cations from the catalytic viewpoint, these materials have not been included in this chapter. Instead, we refer the interested reader to a review by Voorhoeve (1977). Catalytic properties of rare earth containing zeolites are, in our opinion, more adequately treated in the general context of zeolite catalysis (see e.g. Rabo, 1976 Katzer, 1977 Haynes, 1978) and have therefore been omitted here. 

A detailed discussion of adsorption onto mesoporous solids is beyond the scope of this text, but certain features relevant to microporous solids should be described. Firstly, microporous solids can themselves contain mesoporosity. The most important example of this is observed in zeolites such as Y or mordenite that have been treated after synthesis to remove aluminium from the framework (Section 6.2.3). The migration of silica leaves mesopores that are evident from nitrogen adsorption isotherms and directly visible by electron microscopy. The presence of secondary mesopores enhances diffusion and catalytic properties. Conversely, mesoporous solids that are well ordered on the mesoscale can contain disordered micropores in their walls. The mesoporous channels of calcined SBA-15, for example, are connected by micropores that result from removal of block copolymer chains that run between the large channels in the as-synthesised material. This is observed from nitrogen... 

Moscou, L. and Mon6, R., "Structure and catalytic properties of thermally and hydrothermally treated zeolites. Acid strength distribution on REX and REY", J. Catal., 30, 471 (1973). 

This review focusses on the preparational aspects of dealuminated Y zeolites and will not treat the catalytic implications of the modification. The dealumination method using Si Cl 4 was selected because with this reagent, in principle, Si atoms should be supplied rapidly and be easily substituted in the lattice, thus avoiding defect and mesopore formation. Perfect siliceous and low-alumina faujasites are expected to be formed. An attempt is made to rationalize the literature data on the dealumination methods using S1C14, to compare methods and product properties and to treat the chemistry involved in the different steps of the dealumination processes. 

This paper addresses two issues concerning practical aspects of synthesis and utility of pillared zeolites. First it shows how to demonstrate that the observed mesoporous attributes of a particular preparation are not due to undesired M41S contamination. This possibility arises because of the similarity of synthesis regime in both cases aluminosilicate substrates treated with cationic surfactant at high pH and temperature. Second issue concerns the benefits of pillaring zeolite precursor as manifested via improved catalytic performance. Herein we compare the properties of MCM-41 and the pillared zeolite MCM-36 obtained with cetyltrimethylammonium cation as the swelling/templating surfactant. 

Acid-treated clay minerals were employed as cracking catalysts in the first commercial process, the Houdry process, widely used in the early petroleum industries to produce high-octane gasoline. The Houdry process catalysts had been discussed extensively by many investigators (2) but were eventually completely replaced by synthetic silica-alumina or zeolite catalysts. Recently, the need for new catalytic materials has revived special interest in the layer lattice silicates because of their ion-exchange properties and their expandable layer structures. 

The structures and properties of heteroatom zeolites vary due to the introduction of heteroatoms in the zeolite frameworks. When treated in high-temperature water vapor, zeolites Y, mordenite, and ZSM-5 are dealuminated to be ultra-stabilized, whereas heteroatom zeolites undergo demetallation that is, the heteroatoms are removed from the frameworks to form extraframework species, and the resulting zeolites become catalytically active for special reactions. For instance, when treated in water vapor at... 

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