Zeolite catalysts characteristics

However, some characteristics of the reactant and product (and solvent) molecules used in the reactions of fine chemical synthesis have to be considered their large size, their low thermal stability, and their polarity. The small size of the micropores limits the use of zeolites to the synthesis of relatively small molecules solutions to overstep this limitation have been found with the development of larger (mesoporous) molecular sieves and of nanocrystallite zeolites, the reaction then occurring on the large external surface. The low thermal stability of the molecules leads to operation at a low ternperamre, often in the liquid phase. Last, as will be shown in the first example (Section 12.2.3.1), the differences in polarity between reactant(s), product(s), and solvent molecules have to be considered for optimizing both the zeolite catalyst and the operating conditions. 

One of the most studied aspects of catalysis science is the relationship between structure and function. Some general themes are weU estabUshed by now, but specific connections between catalyst characteristics and performance attributes remain elusive in most cases. The crystalline geometry of zeolites makes them relatively more amenable to study by a variety of powerful modern characterization tools, but there remain many key unanswered questions in the catalytic application... 

Zeolite-Based Alkylation. Zeolites have the advantage of being noncot-rosive and environmentally benign. The Mobil-Badger vapor-phase ethylbenzene process was ihe lirsl zeolite-based process to achieve commercial success. It is based on a synthetic zeolite catalyst. ZSM-5. and has the desirable characteristics of high activity, low oligomerization, and low coke formation. See also Molecular Sieves. 

The formation of coke on acid zeolite catalysts depends on i) the characteristics of the acid sites and of the pore structure of the zeolite and ii) the nature of the feed and the operating conditions (T,P). 

It was found by Nis XPS studies of pyridine-adsorbed samples that after deactivation the surface acidic function changes in a different manner with the bulk acidity measured by infrared characteristic absorption bands of pyridine adsorbed samples [7], which would suggest different distributions of the acidic properties in the sample catalysts. The effects of additive elements on the overall acidic features of modified zeolite catalysts are dependent on sample pretreatment and/or reaction condition, which will contribute differently to the induced acidity on the surface and in bulk bifunctional properties, as examined by the reaction of n-heptane shown in Figure 1. 

It has been concluded that, in most cases, catalytic reactions over zeolites occur within their intracrystalline cages and channels. Zeolite catalysts can therefore be considered as a succession of nano or molecular reactors. The consequence is that the activity, selectivity, but also the stability of all the reactions carried out over zeolite catalysts, depend (slightly or significantly) on the shape and size of cages, channels and of their apertures, hence that shape selectivity is a general characteristic of zeolite catalyzed reactions. 

As stated above, shape selectivity due to molecular sieving depends on the relative rates of diffusion and reaction, hence on the respective sizes and shapes of molecules and pores and on the characteristics of active sites (e.g. concentration, nature and strength of acid sites). Obviously the diffusion rate, hence the selectivity depend also on the length of the diffusion path (i.e., on the size of the zeolite crystallites). The selectivity of a zeolite catalyst can be optimized by an adequate... 

UOP Cumene Benzene and propylene Process characteristics high yield and high quality cumene (99.95%+) using rugged, regenerate zeolitic catalyst 7 2001... 

Miskokzi et al. [8] stndied the cracking characteristics of low-density PE in the presence of different zeolite catalysts (Figure 27.10) H-ZSM-5, FCC catalysts and clinoptilolite... 

After evacuation of the Cu(II)zeolite samples at 973 K, the EPR signal assigned to the Cu species became very weak and could hardly be observed, indicating the reduction of Cu" to Cu. With the Cu(I)zeolite catalysts prepared in this way, as shown in Figs. 31 and 32, characteristic photoluminescence became observable at about 400-450 nm upon excitation at about 300 nm. As shown in Fig. 32, the relative intensity of the photolumincscence spectra attributed to the Cu monomer and the (Cu -Cu+) dimer species strongly depends on the type of zeolite. The photolumi-nescence band at about 430-460 nm is the major component for the Cu(I)ZSM-5 and Cu(I)mordenite. Moreover the Cu(I)Y zeolite exhibits two different intense photoluminescence bands at about 450 and 525 nm. 

Tn most applications of zeolites, it is necessary for molecules to be able to diffuse into or out of their fine pore structure, and in many of these applications, particularly catalysis, the counterdiffusion of at least 2 different kinds of molecules occurs. The rates of these diffusion processes can have a profound effect upon the apparent activity and selectivity of zeolitic catalysts (21) and upon such characteristics as dispersion and sharpness of separation in the use of zeolites in separation and purification processes. The state of knowledge of intracrystalline diffusion in zeolites is reviewed by Barrer in a paper for this symposium (4). Little is known about unidirectional diffusion in zeolites of substances of indus-... 

Active Sites in Zeolites. Based on the literature reviewed here, the main characteristics of carbonium ion type zeolite catalysts emerge in the following manner. 

For the fiiture of zeolite catalysis, the important question is to what extent zeolites or their future derivatives display some of the desired catalyst characteristics envisioned for advanced catalytic processes. In addition, entirely new, unforeseen catalytic materials may emerge with catalytic properties that lead to step change. Following is a description of the relevance of zeolites to the desirable catalyst features already described ... 

The effect of the acidity of polyfunctional zeolite catalysts on their activity in benzene alkylation by propene has been examined.Crystalline zeolites exist with a variety of characteristic pore and channel sizes. In the case of ZSM-5 zeolite, the interconnected channels formed by 10-membered rings of oxygen atoms allows certain benzene derivatives to fit rather closely, diffuse into the pores, and, after undergoing a reaction, diffuse out. This is the origin of the high para selectivity in reactions such as alkylation of toluene by methanol catalyzed by zeolites like ZSM-5 and related modifications. ... 

Since Cu ions on the zeolite surface exist in an isolated environment, they may interact with the sulfate species on the catalysts deactivated by SOj. The sulfate groups might partially surround the copper ions, as previously supested by Choi et al. [37] and Hamada et al. [38]. The sulfate may also have some characteristics of a coordinate covdent bond, where Cu ions and sulfate species may act as a Lewis acid and base, respectively [39], Ligands such as HjO, NHj, (C2Hj)3P, CO molecules and Cf, CN, OH , NOj, and C204 ions should at least contain a lone pair of electron to form a coordinate covalent bond between metal ions [40]. It should be noted that the sulfate catalyst species formed on the catalysts deactivated by SO2 contains lone electron pairs on O atoms which surround S atom of SO4 groups. Therefore, it is expected that the electrostatic interaction between Cu ions and sulfate species probably influences the local structure of Cu ions on the zeolite catalyst surface. 

The regeneration of Y-zeolite catalysts used in isobutane alkylation with C4 olefins was studied. The coke formed on these catalysts during this reaction needs temperatures higher than 500°C to be burnt off with air. Ozone was used in this study to eliminate most of the coke at a much lower temperature. After a treatment at 125 C with ozone, the small amount of coke remaining on the catalyst can be removed with air at 250°C. The ozone not only eliminates coke from the catalyst, but also modifies its burning characteristics as measured by Temperature Programmed Oxidation, shifting the peak to lower temperatures. This allows a combined treatment with ozone at 125°C followed by air at 250°C to restore the activity and stability of Y-zeolite catalysts for isobutane alkylation. 

The most characteristic feature of the Fe-treated Y-zeolite catalyst was that catalyst itself exhibits high activity and low coke deposition for toluene... 

The Co/zeolite catalyst (Fig. If) was reduced only to the extent of 60 wt% at 800°C. The unreduced cobalt is attributed to tetrahedral Co2+ species (phase III) since they exhibited the characteristic intense blue colour of tetrahedral Co2+ ions. The cobalt on magnesiun oxide (Fig. lg) also underwent reduction to only 60wt% up to 800°C. The shift of the phase II and III peaks to higher temperatures is evidence of a stronger metal-support interaction for these catalyst systems. Itoo reduction phases for cobalt on magnesium oxide have been... 

A great effort is now being made in order to substitute solid adds (in particular zeolites) for these polluting, corrosive, industrial catalysts (4-9). That is why it is important to understand how the alkylating activity, stability and selectivity of zeolite catalysts change as a function of their physico-chemical characteristics (pore structiu-e, aridity). The effect of these characteristics is investigated here on a model reaction, the alkylation of toluene with 1-heptene. The interest which this... 

Silica coatings are applied to particulate materials to modify surface characteristics that interfere with the exploitation of desired bulk properties, as with titania pigments that may photocatalyze the degradation of their vehicle, surfaces of selective zeolite catalysts that may promote undesired reactions, and fillers for plastics that may not disperse in their matrix. 

Platinum and palladium were used to facilitate the regeneration of zeolite catalysts used in isobutane alkylation. The TPO profiles in these cases display a characteristic peak, very sharp, that is not usually found in any other system, due to the presence of a large amount of hydrocarbonaceous deposits located very close to the metallic particles . ... 

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