Hydrocracking Dealkylation

The aromatization of propane on supported (Si02/Al203)Ga, Zn or Pt catalysts is limited due to hydrocracking, dealkylation and hydrogen transfer reactions which lead to the formation of methane and ethane as the major products, and ethene and propene. However, the use of a Pd membrane reactor increases the yield of aromatics (e.g. benzene, toluene, Cg-Ci2 aromatics) dramatically by effective separation of the produced hydrogen gas from the reaction mixture by utilizing the H2 permeability of Pd thin films. 

Dealkylation, fragmentation, and hydrogenation of substituted polynuclear aromatics may also occur. The following is a representative example of hydrocracking of a substituted anthracene. 

Toluene is dealkylated to benzene over a hydrogenation-dehydrogenation catalyst such as nickel. The hydrodealkylation is essentially a hydrocracking reaction favored at higher temperatures and pressures. The reaction occurs at approximately 700°C and 40 atmospheres. A high benzene yield of about 96% or more can be achieved ... 

Production of p-xylene via p-xylene removal, i.e., by crystallization or adsorption, and re-equilibration of the para-depleted stream requires recycle operation. Ethylbenzene in the feed must therefore be converted to lower or higher boiling products during the xylene isomerization step, otherwise it would build up in the recycle stream. With dual-functional catalysts, ethylbenzene is converted partly to xylenes and is partly hydrocracked. With mono-functional acid ZSM-5, ethylbenzene is converted at low temperature via transalkylation, and at higher temperature via transalkylation and dealkylation. In both cases, benzene of nitration grade purity is produced as a valuable by-product. 

Presently it is mostly used as a fuel, but as petroleum becomes increasingly scarce and expensive, proposals and experimental plants for using this material begin to appear. One such process uses fluid-bed hydrocracking and dealkylation to produce phenols and benzene. 

HYDROCRACKING HYDRODESULFURIZATION TOLUENE DEALKYLATION CYCLOHEXANE XYLENES ISOMERIZATION HYDROTREATING OF PYROLYSIS GASOLINE HYDROFINING... 

With alkyl aromatics, precious-metal H-mordenite catalysts are active for hydrogenation at low temperatures and hydrocrack at higher temperatures. Certain metal exchanged mordenites are effective for hydrogenation (30), dealkylation (7), transalkylation, disproportionation (31,38), and isomerization reactions (23). 

At 873K, under 2.0MPa and H2/Toluene=4mol/mol the toluene transformation over H-ZSM-5 was studied. The initial reactions rates of dealkylation, hydrocracking and condensation of toluene were correlated with the acidity of the zeolite catalysts. Schematic route for the transformation of toluene is presented and the mechanism of the particular reactions are discussed. 

Sample WHSV h Dealkylation Hydrocracking Yield % moll Condensation... 

In catalytic cracking many reactions take place simultaneously. Cracking occurs by C-C bond cleavage of paraffins, dealkylation etc. Isomerization and even condensation reactions take place. These reactions occur via positively charged hydrocarbon ions (carbocations). The nature of the carbocations is the subject of debate. For the cracking of paraffinic hydrocarbons it is usually assumed that carbenium ions are the crucial intermediates, which decompose via beta fission into olefins and (smaller) carbenium ions (see Chapter 4, Section 4.4). A typical reaction mechanism for catalytic cracking (and hydrocracking) imder the relatively mild conditions used in FCC is shown overleaf. 

This reaction probably involves the participation of more than two carbon atoms and the extension of a delocalized n-electron system. The results are readily explained by the unequal stabilities of the two possible intermediates, involving carbene formation on the methyl group and on the alkyl group, respectively (Scheme 74). In the first case, demethylation easily occurs by metallocarbyne formation, while in the second case, stabilization by carbene-olefin isomerization (very fast compared to hydrocracking) prevents dealkylation. However, when the a-carbon atom in the alkyl group cannot dehydrogenate to give a metallocarbene, as for 1-methyl-1-terf-butylcyclohexane, dealkylation prevails over demethylation. 

Alkylbenzenes with Side Chains of Three or More Carbons. The products from hydrocracking of alkylbenzenes containing side chains of three to five carbon atoms are relatively simple. Direct dealkylation is the primary cracking reaction. For example, ferf-amylbenzene gives benzene... 

The reactions of major importance in the octafining process are isomerization of naphthenes and aromatics, hydrogenation of aromatics, dehydrogenation of naphthenes, disproportionation of aromatics, dealkylation of aromatics, and hydrocracking of saturates (Figure 3). The last three reactions, of course, result in loss of product xylenes. These reactions, like the desired isomerization reactions, are carbonium-ion catalyzed. 

When dehydrogenation was carried out in a stream of hydrogen at 325°C, the yield of aromatic hydrocarbons formed in the dehydrogenation of cyclohexane and cyclohexene derivatives was 87—99%. As a result of side-reactions an insignificant amount of dealkylation products (benzene, toluene) was also formed. Decalin was most difficult to dehydrogenate, and in this instance, together with naphthalene, tetralin was also formed. Under these conditions isomerization of cyclopentane derivatives into cyclohexane hydrocarbons did not take place, and aromatic hydrocarbons were not formed from cyclopentane hydrocarbons. The process, however, was complicated by hydrocracking reactions. 

Houdry Litol Production of benzene from toluene 50 600 Co, Mo Hz Hydrogenation of unsaturated compounds hydrocracking of non-aromatics desulfurization, dealkylation and dehydrogenation of naphthenes lead to higher benzene yields... 

Figure 5. LHHW rate equation for irreversible reactions (C-C scission reactions such as parafiBn hydrocracking, opening of naphthene rings, and ring dealkylation).

Ring dealkylation reaction 2 Ring open reaction 3 Aromatic satjration 4. Paraffin hydrocracking reaction P paraffins N naphthenes A aromatics AN ring compound which has naphthene and aromatics... 

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