Isomerization of hydrocarbons

The isomerization of hydrocarbons in the petroleum industiy and the production of dodecyl benzene for detergents accounts for a further 10% each of the AICI3 used. 

Polyphosphoric acid supported on diatomaceous earth (p. 342) is a petrochemicals catalyst for the polymerization, alkylation, dehydrogenation, and low-temperature isomerization of hydrocarbons. Phosphoric acid is also used in the production of activated carbon (p. 274). In addition to its massive use in the fertilizer industry (p. 524) free phosphoric acid can be used as a stabilizer for clay soils small additions of H3PO4 under moist conditions gradually leach out A1 and Fe from the clay and these form polymeric phosphates which bind the clay particles together. An allied though more refined use is in the setting of dental cements. 

One of the ways to generate the tetramethylenethane-type diradical 350, an important reference compound in connection with non-Kekule hydrocarbons [147], consists in the thermal isomerization of hydrocarbon 34 at -100 °C [43, 148], Under the reaction conditions, the six-membered ring of 350 ruptures to yield [4]dendralene (3,4-bismethylene-1,5 -hexadiene) (351). 

Borchers, R Levsen, K. Isomerization of Hydrocarbon Ions. III. [CsHs], [CsHs], [CeHd, and [CsHs] Ions. Org. Mass Spectrom. 1975,10, 584-594. 

Levsen, K. Heimbach, H. Shaw, G.J. MUne, G.W.A. Isomerization of Hydrocarbon Ions. VIII. The Electron Impact Induced Decomposition of n-Dodecane. Org. Mass Spectrom. 1977, 72, 663-670. 

Transition metals are used as catalysts for a variety of reactions hydrogenation, hydrogenolysis and isomerization of hydrocarbons (group VIII metals), oxidation of... 

Mechanisms of Skeletal Isomerization of Hydrocarbons on Metals F. G. Gault... 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

Many of the petroleum refinery operations involve isomerization of hydrocarbons as part of the complex chemistry taking place (see Chapter 2). 

The /3-pinene fraction was used as a reference to determine the isomerization activity of the supports. Results given in Table 4 show that carbon VII is particularly inert with respect to /3-pinene. This behaviour is certainly related to the high content of this carbon in potassium (0.5 wt.-%). On the contrary, the CaO impurities present in carbon V seem to increase the isomerization activity of this carbon. It is well-known that the double bond shift isomerization of hydrocarbons can proceed via carbocation intermediates (protonic catalysis) or via allylic carbanion intermediates (acido-basic or purely basic catalysis) [Ref.7]. The results obtained with potassium-doped carbons show that in /3-pinene isomerization during HDS, the protonic mechanism predominates. 

We would expect peculiarities (1) and (2) to be also inherent in carbenium-ion-type intermediates of other reactions (such as the cracking, alkylation, and isomerization of hydrocarbons) occurring in the pores of solid acid catalysts. 

D. The Mechanism of Hydrogen Exchange and Isomerization of Hydrocarbons with Multiple Bonds... 

Reactions studied include dehydrations of alcohols, double bond shifts in olefins, isomerization of hydrocarbons, racemization of optically active compounds, etc.. In the literature a rather rigid separation is made between a Brested acid, which is actually a proton donor, and a Lewis acid, which works as a hydride abstractor. We may illustrate this difference by using the double bond shift in olefins as the model reaction. 

Synthetic zeolites are the most important materials used currently in industry for catalyst preparation. However, natural zeolites are not contemplated in catalyst manufacturing because of the impurities present in the natural raw materials nevertheless, in some reactions, such as the isomerization of hydrocarbons, this contamination does not affect the catalytic transformation therefore, acid natural zeolites can be used for this purpose [19]. Furthermore, acid clinoptilolites were tested for catalytic cracking with success [19,21,137-143], We have shown [19,21,138-143] that the acid clinoptilolite, used as catalyst in the reaction of ethanol dehydration, exhibits high selectivity for ethylene production due to steric restrictions imposed on the formation of diethyl ether. The scheme of the ethanol dehydration reaction is shown in Figure 9.18 [145],... 

Two reviews of isomerization reactions should be referred to as sources of information on Russian researches in the field. The book by Petrov (291) on isomerization of paraffins, olefins, diolefins, and acetylenes with up to 34 carbon atoms gives information based on Russian and non-Russian researches and was written by an author who has carried out with co-workers numerous studies of isomerization of olefins in the presence of various catalysts. The review by Danilov (55) was confined to Russian contributions to the field of isomerization during the period of 1917-1947 and covers isomerization of hydrocarbons of all classes, their halogenated derivatives, oxygenated compounds and heterocyclics. In view of these detailed publications, the discussion below is limited to highlights of the field. 

However frequently the support material does have a very important function. This is particularly so when the support acts as a (Bronsted) acid or a base. An example is in the catalytic cracking, alkylation, and isomerization of hydrocarbons (Section 5.2.6) The role of the transition metal is in oxidation or hydrogen transfer reactions while the support, for example acidic oxides such as aluminosilicates, act to protonate, rearrange and dehydrate organic species. 

Phosphates having these types of open structures can act as shape-selective acid catalysts, for example, for the cracking and isomerization of hydrocarbons. For examples of lamellar materials, see Section 5.3 and see Intercalation Chemistry). Microporous catalysts are described above and in (see Porous Inorganic Materials and Zeolites). Mesoporous AlPO materials have larger pores within a matrix of amorphous A1P04. ... 

Acidic zeolites are known for their excellent catalytic activity in cracking and isomerization of hydrocarbons (75). In the absence of metal, however, these catalysts rapidly deactivate due to the formation of carbonaceous products, usually referred to as coke. The carbonaceous residues are mainly formed via alkylaromatics and polyaromatics, which are the result of dehydrogenation, cyclization, and further alkylation processes. The coke deposits lower the catalytic activity by site poisoning and eventually also by pore blocking, which inhibits access of hydrocarbon molecules to the acid sites (286). 

M. V. Frash, V. B. Kanzansky, A. M. Rigby, and R. A. van Santen, /. Phys. Chem. B, 101, 5346 (1997). Density Functional and Hartree-Fock Calculations on the Cyclopropane Ring Intermediates Involved in the Zeolite-Catalyzed Skeletal Isomerization of Hydrocarbons and in the Carbon Isotope Scrambling in 2-Propyl Cation. 

In this chapter, an overview is presented of studies that deal with the electronic and chemical properties of Pd in bimetallic systems. We will focus on palladium for three main reasons. First, bimetallic catalysts that contain Pd or other Group-10 metals have many uses isomerization of hydrocarbons, olefin hydrogenation, CO oxidation, alcohol synthesis, acetylene trimerization, etc. [8,10,19-21]. Second, palladium is very sensitive to the formation of bimetallic bonds [22-24]. And third, there is a vast number of experimental and theoretical articles in the literature that examine the properties of Pd in bimetallic systems [14,15,19-23,25-44]. From this large volume of work, one can get a general idea of how deep is our knowledge about the basic nature of bimetallic bonding and how it affects the properties of a metal. 

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