Hydrogenation-disproportionation

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. 

The reaction of triethoxyvinylsilane, CH2=CHSi(OEt)3, with HSi(OEt)3 catalyzed by Ni(acac)2 gives a complex mixture of products, arising from dehydrogenative silylation, hydrogenation, disproportionation, and dimerization besides the normal hydrosilylation product, (EtO)3SiCH2CH2Si(OEt)356. The dimerization product is a mixture of (EtO)3Si(CH2)4Si(OEt)3 and (EtO)3SiCH(Me)(CH2)2Si(OEt)3. A possible mechanism that can accommodate the formation of these products is proposed, which includes key intermediates such as (EtO)3Si(CH2)2Ni—CH[Si(OEt)3]CH2Si(OEt)3 and Ni[(CH2)2Si(OEt)3]56. 

In the field of hydrocarbon conversions, N. D. Zelinskii and his numerous co-workers have published much important information since 1911. Zelinskii s method for the selective dehydrogenation of cyclohexanes over platinum and palladium was first applied to analytical work (155,351,438,439), but in recent years attempts have been made to use it industrially for the manufacture of aromatics from the cyclohexanes contained in petroleum. In addition, nickel on alumina was used for this purpose by V. I. Komarewsky in 1924 (444) and subsequently by N. I. Shuikin (454,455,456). Hydrogen disproportionation of cyclohexenes over platinum or palladium discovered by N. D. Zelinskii (331,387) is a related field of research. Studies of hydrogen disproportionation are being continued, and their application is being extended to compounds such as alkenyl cyclohexanes. The dehydrocyclization of paraffins was reported by this institute (Kazanskil and Plate) simultaneously with B. L. Moldavskil and co-workers and with Karzhev (1937). The catalysts employed by this school have also been tested for the desulfurization of petroleum and shale oil fractions by hydrogenation under atmospheric pressure. Substantial sulfur removal was achieved by the use of platinum and nickel on alumina (392). 

The further development of the theory of nonuniform surfaces in the U.S.S.R. was helped by the mathematical methods of Zel dovich and Roginskil (200,201,331). A. V. Frost analyzed some work on the subject (mostly Russian) in a recent review (10) and concluded that an equation derived by him on the assumption that the reactants are adsorbed on a uniform surface and that no significant interactions take place between the adsorbed molecules, satisfactorily described many reactions on non-uniform surfaces including cracking of individual hydrocarbons and petroleum fractions, hydrogen disproportionation, and dehydration of alcohols. From the experimental results it was concluded that the catalytic centers on the surface were not identical with the adsorption centers. The catalysts used consisted of different samples of silica-alumina and pure alumina. 

The kinetic expressions derived by Antipina and Frost have general applicability to monomolecular heterogeneous catalytic reactions which occur on a uniform surface. The expression can be made to describe the cracking of synthin or decomposition of octene over silica-alumina as well as hydrogen disproportionation of gasoline and cracking of gas oils over the silica-alumina. Numerous other applications are discussed. 

Petrov and Shchekin (297) showed that below the cracking temperature (250-316°C.) cyclohexene undergoes over silica-alumina hydrogen disproportionation and dimerization. Identical results were obtained from 1-methyl-l-cyclopentene. Ring expansion of lower alkylated cyclopentanes occurs simultaneously with polymerization. However, no bicyclic compounds with similar rings were formed. 

Pure sulfur compounds containing 9 or 10 carbon atoms were examined in the presence of silica-alumina at 250-300° (391). Liberation of hydrogen sulfide, destructive hydrogenation and hydrogen disproportionation took place forming disulfides, mercaptans, and olefins. 

Neither substance catalyzes hydrogen disproportionation reaction of cycloolefins. An essential difference was found in the action of these catalysts on alcohols. In the presence of vanadia, alcohols are hydro-genolyzed to the corresponding paraffins. At comparable conditions in the presence of chromia, alcohols undergo a dehydrogenation-condensation reaction with production of ketones. 

It was found that cyclohexene passed over vanadium trioxide catalyst at 250-450° in the presence of hydrogen shows no hydrogen disproportionation, but, depending on the temperature, a direct hydrogenation and dehydrogenation reaction approaching equilibrium values. 

Polymerization of the olefin probably proceeds via the reaction analogous to step 2. The olcfinic polymerization product may be isolated as such or it may undergo hydrogen transfer with the isoparaffin with the resultant saturation of the double bond. Also, hydrogen disproportionation between two molecules of the polymer may occur, yielding one molecule of paraffinic product and one of the highly unsaturated material found in the catalyst complex (cf. Bartlett, Condon, and Schneider, 15, p. 1536). 

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). 

HD HDDR Hydrogenation-disproportionation Hydrogenation disproportionation desorption recombination XMCD X-ray magnetic circular dichroism... 

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.

This point can be directly observed by TEM as shown in fig. 16. As the hydrogenation-disproportionation (HD) process develops, the main phase is changed into a lamellar structure of NdHa and a-Fe phases. At the front of the lamellar structure formation, there is the t-Fe3B phase, which contains the spherical NdH2 and Nd2Fei4B phases. The t-FeaB phase is decomposed into FeaB and a-Fe phases as the HD process develops. 

Bollero A, Gutfleisch O, Kubis M, Muller K-H, Schultz L (2000) Hydrogen disproportionation by reactive milling and recombination of Nd2(Fel-xCox)14B alloys. Acta Mater 48 4929—4934 Bychto L, Balaguer M, Pastor E, Chirvony V, Matveeva E (2008) Influence of preparation and storage conditions on photoluminescence of porous silicon powder with embedded Si nanocrystals. J Nanopart Res 10 1241-1249... 

Rosins are used little as such but are widely used in chemically modified forms. Most of the modifications involve the carboxyl group or the double bonds and are effected by transformations such as hydrogenation, disproportionation (the process is, to a significant part, a dehydrogenation), esterification, polymerization, salt formation, or reaction with maleic anhydride or formaldehyde. 

Modified rosins, however, are of greater importance now than the non-modified material. Typical modifications are hydrogenation-disproportionation, esterification, dimerization, and maleic/fumaric adducts. 

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