Cracking and disproportionation

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

The typical operating conditions of xylene and EB isomerization processes are shown in Table 9.3. These conditions minimize the above side reactions. Pressure, temperature and H2/HC ratio are key parameters that define the partial pressure of C8 naphthenes intermediates for EB isomerization. Naphthene cracking and disproportionation/transalkylation are responsible for the C8 aromatics net losses that affect the overall pX yield. The C8 recycled stream from the isomerization unit to the separation unit is three times higher than the fresh feed stream (since there cannot be more than 24% of pX in the C8 aromatic cut after isomerization). This means that each percent of loss in the isomerization unit will decrease the pX yield by 3%. For example, when standard mordenite-based catalysts lead to 4% of net losses, the overall pX yield is roughly 88%. 

These catalysts are silica-aluminas whose cracking and disproportionation power has been altered by steam treatment, the use of an inhibitor, or of aluminas containing a halogenated compound or fluorine. They are very rugged, are employed without hydrogen and hence cannot isomerize ethylbenzene, which is therefore cracked or transformed by a disproportionation reaction into benzene and C10 aromatics. Consequently, they can only be used with feeds poor in ethylbenzene. However, no naphthenic hydrocarbons are formed. 

The mechanisms of alkylation reactions appear to be very complex. Analyses of typical alkylation products show that on basis of known reactions, secondary reactions of isomerization, cracking, and disproportionation, hydrogen transfer and polymerization must occur in the reaction. All these reactions are almost certain to occur by means of carbonium ion complexes including formation, addition, rearrangement, and proton and hydride ion transfer. The following reactions are at present beheved to occur as the main reactions in the alkylation of butene-1 with isobutane ... 

During the combustion process in gasoline engines, benzene, toluene, acetylene, acrolein, and aromatic aldehydes [94], of which the latter have a particularly high photochemical activity, are formed from volatile aromatics by cracking and disproportionation, and also by oxidation. 

Petroleum-derived benzene is commercially produced by reforming and separation, thermal or catalytic dealkylation of toluene, and disproportionation. Benzene is also obtained from pyrolysis gasoline formed ia the steam cracking of olefins (35). 

A fairly large number of patents has been issued describing the application of aluminum-deficient Y zeolites in different areas of catalysis. Ultrastable Y zeolites have been used in the preparation of catalysts applied in hydrocarbon cracking, e.g. (94,95) hydrocracking, e.g. (96,97) hydrotreating, e.g. (98) and disproportionation, e.g. (99). 

The involvement of carbocations accounts for the side reactions that accompany isomerization. Carbocations are known to undergo p scission to yield low-molecular-weight cracking products. They can also undergo proton elimination to form alkenes that, in turn, participate in condensation (oligomerization), cycli-zation, and disproportionation reactions. 

The process shown is thermal cracking and simultaneous hydrogen disproportionation, leading to aromatization of the hydroaromatic structure. A hydroaromatic unit was used in this example because such units are believed to have a predominant role in the coal structure. 

Much progress has been made in understanding the catalytic activity of zeolites for several type of reactions. The number of reactions catalyzed by zeolites has been extended, and new multi-component polyfunctional catalysts with specific properties have been developed. In addition to cracking and hydrocracking, reactions such as n-alkane isomerization, low temperature isomerization of aromatic C8 hydrocarbons, and disproportionation of toluene are industrially performed over zeolite-containing catalysts. Moreover, introduction of various compounds (C02, HCl) into reaction mixtures allows one to control the intensity and selectivity of the reactions. There are many reviews on the catalytic behavior of zeolites and even more original papers and patents. This review emphasizes the results, achievements, and trends which we consider to be most important. 

The reason why the minimum steam ratio goes down with temperature is not known with certainty. One possibility is that the competing reactions of carbon production and consumption have such kinetics that the rate of coke consumption increases faster with temperature than the rate of coke generation, which suggests that the carbon-steam reaction has a higher activation energy than the methane cracking and carbon monoxide disproportionation reaction. 

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

The molecular sieve zeolites have attained great technological importance for catalysis, gas separation and drying and many other applications. They are now used as industrial catalysts for such reactions as the cracking of paraffins and the isomerization and disproportionation of aromatic compounds (Thomas and Theocharis, 1989 Thomas, 1995, Martens etal., 1997). 

Various reactions have been studied on mixed rare earth and the La and Ce forms. These include ethylation of benzene 18), propylation of toluene 14), o-xylene isomerization 21), butane cracking 14), cracking of n-hexane, n-heptane, and ethylbenzene (8), and isomerization and disproportionation of 1-methy 1-2-ethylbenzene (7). Other reactions are summarized by Venuto and Landis 18). In several reports, an optimum calcination temperature for best catalytic performance has been demonstrated (7, 8, 14, 18, 21). 

The butanes show little tendency to crack or disproportionate (7) thus butane isomerization is fairly straightforward. However, the suppression of side reactions becomes more difficult as the molecular weight increases. With pentanes, disproportionation to isobutane and hexane is pronounced, amounting to as much as 63%. A typical composition of pentane disproportionation products is shown in Table III. Besides lowering the yield of isopentane, such side reactions shorten the life of the catalyst. Adding small amounts of cyclic hydrocarbons (7,15,18)... 

The cyclodimerization of cyclopropenes, a novel reaction, was foundto be catalysed by KA and NaX zeolites. Carbanion intermediates were proposed and the selectivity of the reaction was attributed to spatial constraints. Paraffin disproportionation, with isomerization, at about 500 K has been shownto occur over H-mordenite and HZSM-4 catalysts. Synthetic H-ferrierite is an active and very selective catalyst for n-paraffin cracking and hydrocracking. Palladium on zeolite L comparesfavourably with Pd/HY as a catalyst for pentane isomerization. 

Many industrial processes are based on acid/base catalysis (over 130). Examples include alkylation, etherification, cracking, dehydration, condensation, hydration, oligomerizations, esterification, isomerization and disproportionation. The dimensions of the processes range from very large scale in the field of refinery (thousand tons per day) to very small productions in fine and specialty chemical industries. In the latter case, adds and bases are often used in stoichiometric quantities, leading thus to large amounts of waste. 

Typical test reactions often used for the characterization of zeolites are the cracking of n-hexane - and disproportionation of ethylbenzene. The catalytic activity of a zeolite is determined by the concentration of protons and the acid strength. 

A potential MOGD kinetic scheme is described in Fig. 8 (ref. 19). Olefins react by oligomerization - for example C3= forms Cg=, C9=, C12=, etc. Olefins also undergo double-bond and skeletal isomerization, and disproportionation to produce intermediate olefins. Cracking occurs at the same time, and further reactions include cyclization and hydrogen transfer. 

In this paper, the cracking of n-hexane, n-dodecane and n-hexadecane on ZSM-5 zeolites at about atmosphere and temperatures of 260-400°C were studied. The results showed that both mono-molecular cracking and bimolecular reaction (disproportionation) for n-hexane cracking took place. A network for initial reactions was proposed, and the apparent kinetic parameters of the reactions were estimated. An examination for the factors affecting the product destribu-tion of n-hexadecane indicated that hydrogen transfer on the surface of HZSM-5 zeolites plays an important role in cracking reaction. 

Dewaxing, Methanol to gasoline, Methanol to olefins and products, FCC additive, Hydrocracking, Olefin cracking and oligomerisation, Benzene alkylation, Xylene isomerization, Toluene disproportionation, Aromatisation, NOx reduction, Oxidations, Hydration, Animation, Beckmann rearrangement, Cyclodimerisation,... 

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