Hydrogenation-disproportionation process

After the cmde BTX is formed, by reforming in this case, a heart cut is sent to extraction. Actually, the xylenes and heavier components are often sent to downstream processes without extraction. The toluene produced is converted to ben2ene, a more valuable petrochemical, by mnning it through a hydrodealkylation unit. This catalytic unit operates at 540—810°C with an excess of hydrogen. Another option is to disproportionate toluene or toluene plus aromatics to a mixture of ben2ene and xylenes using a process such as UOP s Tatoray or Mobil s Selective Toluene Disproportionation Process (STDP) (36). 

Most organic free radicals have very short lifetimes, but certain structural features enhance stability. Radicals without special stabilization rapidly dimerize or disproportionate. The usual disproportionation process for alkyl radicals involves transfer of a hydrogen from the carbon P to the radical site, leading to formation of an alkane and an alkene ... 

Thus, in the latter case, the term ECE has to be abandoned and replaced by disp . (the SET occurs obviously via a disproportionation process). Finally, the strong base R formed after an overall two-electron reaction is protonated by the solvent or by any acidic impurity. Alternative mechanisms could be proposed taking into account that R or Ar may abstract hydrogen atoms from the solvent ... 

STDP [Selective toluene disproportionation process] A process for converting touene to mixed xylenes, predominately />-xylene. It takes place in the presence of hydrogen over a ZSM-5-type catalyst. Developed by Mobil in the 1980s and first operated by Enichem. 

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. 

In the same way as intramolecular displacement leads to particularly stable atomic groupings within a molecule, the disproportionation reactions between several molecules are attributable to the tendency for the more stable compound with the higher fluorine content to form. Reactions of this kind are sometimes used to obtain highly fluorinated compounds from products with lower fluorine contents, for example, the catalytic fluorination of chloroalkanes with hydrogen fluoride or with fluorination agents such as antimony(V) fluoride or antimony(III) fluoride. The chlorine compound formed as the second product of the disproportionation process is reused as the starting material for the preparation of the compound to be dispropor-tionated. 

The generation of di- and trimethylsilyl radicals by the reactions of hydrogen atoms with di- and trimethylsilane and their subsequent reactions have been described23. Three main products were observed in these reactions 1,1,2,2-tetramethyldisilane, pentamethyldisilane and hexamethyldisilane which are formed by both radical coupling and disproportionation processes. The authors describe a detailed kinetic analysis of the various reactions of interest but have difficulty in providing a mechanistic description that fully fits their observed data23. 

Or one radical can abstract a hydrogen from the carbon adjacent to the radical center of another radical in a disproportionation process ... 

An interesting way to increase pX production of an aromatics complex is to transform less valuable aromatics such as toluene and C9+ aromatics into xylenes. This is achieved either by toluene disproportionation (TDP) or by toluene C9+ aromatics transalkylation. There are two types of disproportionation processes Normal TDP produces a xylene cut where pX content is at thermodynamic equilibrium, while with STDP pX content is much higher (para-selectivity). This is possible when a selectivated zeolite (generally MFI) is used. For normal TDP as well as for transalkylation processes, mordenites doped with a hydrogenating metal to limit coking are used as catalysts. 

Certain zeo tes dbplay specific properties for the treatment of aromatics. Mobil Chemcd uses them as catalysts (desisted AF) to conduct the benzene dismut tm reaction in moderate < )etatiag conditioiis, by the Mobil LTD (Mobil Low Temperature Disproportionation) process. Conversion takes place in the liquid phase in the ateence of hydrogen, at a pressure of 45.10 Pa isolate, and a tenq)eratute that is rai progressively with time from 260 to 315 to maintain catalyst activity. 

Thus, a free a-position is kinetically favored for electrophilic attack in all cases, including the most reactive pyrrole. jS-Substituted derivatives are usually the thermodynamically favored products of further transformation primarily formed on replacement of an a-hydrogen atom. These transformations are intermolecular disproportionation processes (see stable thiophenium ions) or intramolecular 1,2-shifts more familiar for unstable cr-complexes formed on acylation or sulfonation. 

These disproportionations could proceed directly or by a 2-step mechanism involving prior transfer of a g-hydrogen to iron followed by reductive elimination. Similar disproportionation processes have been described with organocopper(l), organoman-ganese(II), and organoplatinum(II) complexes.(38)... 

The disproportionation of HOOH occurs via a concerted transfer of the two hydrogen atoms from a second HOOH to the Fe Cl3(HOOH) adduct. This dehydrogenation of HOOH is a competitive process with the Fe Cfi/substrate/HOOH reactions. The controlled introduction of dilute HOOH into the Fe Cfi/substrate solution limits the concentration of HOOH and ensures that the substrate/HOOH reaction can be competitive with the second-order disproportionation process. The substrate reaction efficiencies in Table 11 are proportional to the relative rates of reaction ( RH/kHoon)-The mode of activation of HOOH by Fe Cls is analogous to that of Fe (MeCN)4 + both are strong electrophiles in ligand-free dry MeCN and induce HOOH to monooxygenate organic substrates. 

Thus, the dominant characteristic of 02 in any medium is its ability to act as a strong Bronsted base via formation of HOO% which reacts with allylic hydrogens, itself, or a second 02 (Scheme 9). Such a proton-drive disproportionation process means that 02 can deprotonate acids much weaker than water (up to pXa 23). ... 

The processes considered in this section - hydrogenation, disproportionation, and isomerisation - are frequently encountered in the chemistry of the terpenes and steroids, but even with the simpler cycloalkenes many problems remain to be solved. It is unfortunate that no LEED or vibrational spectroscopy seems to have been performed on substituted cycloalkenes, so that stereochemical preferences cannot be related to adsorbed structure as defined for example by the na factor (Section 4.42). 

Fig. 14. Phase relation in Nd2pe,4B-H2 system (Nakamura et al. 1998). Dotted line (A— B— C) shows a typical treatment conditions during hydrogenation, disproportionation, and desorption processes.

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