Hydrogenolysis aromatic hydrocarbons

This residue is a mixture of stereoisomerio dicyclohexyl-18-crown-6 polyethers which may be contaminated with some unchanged dibenzo-18-crown-6 polyether and with alcohols arising from hydrogenolysis of the polyether ring. The submitter reports that this residue is sufficiently pure for many purposes such as the preparation of complexes with potassium hydroxide which are soluble in aromatic hydrocarbons. 

Pt-Sn Coimpregnation on A1203./ - / Reduced in H2. Hydrogenation, hydrogenolysis, aromatization of hydrocarbons. 

Benzene, toluene and / -xylene are the most industrially important aromatic hydrocarbons. Their relative proportions must be adjusted according to the needs of the market, which presently requires more benzene and xylene. Dealkylation of toluene can be carried out by hydrogenolysis with H2 using Cr, Mo or Pt oxides as catalyst precursors (Equation 4). Methyl redistribution in toluene is best carried out around 420°C on ZSM-5 zeolite. Under these conditions, xylene is mainly present as the para isomer (90% selectivity. Equation 5) due to the shape-selectivity of the zeolite. 

In catalytic hydrogenation, chlorine is replaced by hydrogen in chlorinated aromatic hydrocarbons (equations 41 and 42), phenols (equation 43), amines (equation 44), carboxylic acids (equation 45), and nitro compounds (equation 46). - Hydrogenolysis of chlorine in chloronitro compounds takes precedence over reduction of nitro groups, provided that contact with the halogen-free product is not too long. The reaction is achieved using palladium on carbon or tetrakis(triphenylphos-... 

Unlike some other group VIII metals (S), platinum does not promote hydrogenolysis of cyclohexanes, but dehydrogenation to aromatic hydrocarbons. Cycloheptanes undergo ring contraction and aromatization rather than hydrogenolysis (9-72) (Scheme 1). 

In the work of the author and his associates on bimetallic catalysts comprising various combinations of Group VIII and Group IB metals, it was discovered that the activity of the Group VIII metal for hydrogenolysis reactions of hydrocarbons was decreased markedly by the presence of the Group IB metal (11-15). It was shown that the inhibition of hydrogenolysis leads to improved selectivity for alkane isomerization reactions (11) and for reactions in which saturated hydrocarbons are converted to aromatic hydrocarbons (12,14,15). Interest in bimetallic catalysts increased markedly with the discovery of this selectivity phenomenon. 

Figure 5.1 Major reactions in catalytic reforming illustrated with specific examples (a) dehydrogenation of cyclohexanes to aromatic hydrocarbons (b) dehydroisomerization of alkylcyclopentanes to aromatic hydrocarbons (c) dehydrocyclization of alkanes to aromatic hydrocarbons (d) isomerization of n -alkanes to branched alkanes (e) fragmentation reactions (hydrocracking and hydrogenolysis) yielding low carbon number alkanes.

Phenol, polyphenols, and cresols yield aromatic hydrocarbons on hydrogenolysis over nickel catalysts at temperatures above 250°C. 

Rhodium and ruthenium are used if hydrogenolysis is to be avoided. The catalyst of choice changes somewhat with fused ring aromatic hydrocarbons. The relative order for reduction of naphthalene to tetralin (424) is ... 

Scheme 8.25. A path for hydrogenolysis of a phenol to an aromatic hydrocarbon. The para-methylphenol first reacts (via addition-elimination) with l-phenyl-5-chlorotetrazole to produce a dehydrohalogenated adduct. Catalytic reduction with hydrogen (H2) and palladinm on carbon powder (Pd-C) as the catalyst then produces methylbenzene (toluene) and hydroxytetrazole (which can be recycled to chlorotetrazole by treatment with phosphoryl chloride or thionyl chloride) (see Musliner.W. J. Gates, J.W. Jr. Org. Chem. Bm//., 1967,59,1).

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