Hydrocarbon transformations

Although the major emphasis of the work in the Loker Institute was and is directed in the broadest sense toward the study of the fundamental chemistry of hydrocarbons, substantial and increasing emphasis is also directed to the aspects of hydrocarbon transformations as well as of derived polymeric and varied synthetic materials. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

To explain how solid acids such as Nafion-H or HZSM-5 can show remarkable catalytic activity in hydrocarbon transformations, the nature of activation at the acidie sites of such solid acids must be eon-sidered. Nafion-H contains acidic -SO3H groups in clustered pockets. In the acidic zeolite H-ZSM-5 the active Bronsted and Tewis acid sites are in close proximity (—2.5 A). 

Brmnsted-Lewis Superacids. Conjugate Friedel-Crafts acids prepared from ptotic and Lewis acids, such as HCl—AlCl and HCl—GaCl ate, indeed, supetacids with an estimated value of —15 to —16 and ate effective catalysts in hydrocarbon transformation (217). 

M. L. Poutsma, "Mechanistic Gonsiderations of Hydrocarbon Transformations Gatalyzed by Zeohtes," in J. A. Rabo, ed.. Zeolite Chemisty and Catalysis, AGS Monograph 171, American Ghemical Society, Washington, D.G., 1976. 

The formation of protonated H+(H20)n species can affect the acidity of the non-solvated protonic sites. Therefore, as the acid strength of the protonic sites in zeolites plays a key role in the hydrocarbon transformation reactions, driving the rate of the hydrocarbon protonation [4-6], the presence of water vapor among the reactants can modify reaction rates of the individual reactions involving in the hydrocarbon transformations. 

The direct protonation of isobutane, via a pentacoordinated carbonium ion, is not likely under typical alkylation conditions. This reaction would give either a tertiary butyl cation (trimethylcarbenium ion) and hydrogen, or a secondary propyl cation (dimethylcarbenium ion) and methane (37-39). With zeolites, this reaction starts to be significant only at temperatures higher than 473 K. At lower temperatures, the reaction has to be initiated by an alkene (40). In general, all hydrocarbon transformations at low temperatures start with the adsorption of the much more reactive alkenes, and alkanes enter the reaction cycles exclusively through hydride transfer (see Section II.D). 

As few as six different kinds of adsorption have been proposed as being responsible for a great variety of hydrocarbon transformations over metal catalysts (14). We fully accept this approach—that the character of primary adsorption determines the structure of the product. One of the main points that will be stressed is that very different reactions may often be concealed behind the expression cyclization. ... 

The roles of carbocations in commercially important hydrocarbon transformations are still not perfectly understood. The same can be said for carbocations in biological systems. Significant questions concerning reactivity still need to be explained. Why do so many reactions of carbocations show constant selectivity, in violation of the reactivity-selectivity principle Is it possible to develop a unified scale of elec-trophilicity-nucleophilicity, in particular one that incorporates these parameters into the general framework of Lewis acidity and basicity. Finally, quite sophisticated synthetic transformations are being developed that employ carbocations, based upon insights revealed by the mechanistic studies. 

A study of the use of room temperature ionic liquids as a new class of nonaqneous solvents for 2-phase catalytic hydrocarbon transformations. The liquids investigated were mixtures of quartemary ammonium salts and organo-aluminum componnds. They were found to be very effective solvents for metal-catalyzed olefin dimerization and metathesis reactions. Their complexing ability and acidity can be tuned as re-... 

Further transformation (functionalization) reactions include varied additions,41 car-bonylative conversions42 acylations,47 5 substitutions,43,46-50 oxidations,51-60 and reductions 61-69 Major petroleum refining operations are discussed in Chapter 2, whereas Chapters 4-13 discuss the chemistry of prototype hydrocarbon transformation reactions. 

The carbocations involved in these reactions are trivalent carbenium ions, of which CH3+ is parent. It was Whitmore in the 1930s who first generalized their importance in hydrocarbon transformations based on fundamental studies by Meerwein, Ingold, Pines, Schmerling, Nenitzescu, Bartlett, and others. 

Transalkylation and Dealkylation. In addition to isomerizations (side-chain rearrangement and positional isomerization), transalkylation (disproportionation) [Eq. (5.56)] and dealkylation [Eq. (5.57)] are side reactions during Friedel-Crafts alkylation however, they can be brought about as significant selective hydrocarbon transformations under appropriate conditions. Transalkylation (disproportionation) is of great practical importance in the manufacture of benzene and xylenes (see Section 5.5.4) ... 

In addition to large-scale industrial applications, solid acids, such as amorphous silica-alumina, zeolites, heteropoly acids, and sulfated zirconia, are also versatile catalysts in various hydrocarbon transformations. Zeolites are useful catalysts in fine-chemical production (Friedel-Crafts reactions, heterosubstitution).165-168 Heteropoly compounds have already found industrial application in Japan, for example, in the manufacture of butanols through the hydration of butenes.169 These are water tolerant, versatile solid-phase catalysts and may be used in both acidic and oxidation processes, and operate as bifunctional catalysts in combination with noble metals.158,170-174 Sulfated zirconia and its modified versions are promising candidates for industrial processes if the problem of deactivation/reactivation is solved.175-178... 

Superacids Immobilized on Solid Supports. The considerable success of Magic Acid and related superacids in solution chemistry and interest to extend the scope and utility of acid-catalyzed reactions, particularly hydrocarbon transformations, logically led to the attempts to adopt this chemistry to solid systems allowing heterogeneous catalytic processes. 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

Rabo, J. A., "Zeolites Chemistry and Catalysis", A. C. S. Chemical Monograph Series, Chapter on "Mechanistic Considerations of Hydrocarbon Transformation Catalyzed by Zeolites", by M. L. Poutsma, in print. 

Fig. 17.71. Reduction of a conjugated tosylhydrazone. Since the diazene intermediate D cannot decompose via a diazene anion (cf. Figure 17.67-17.69) for lack of base and the radical decomposition mechanism of Figure 17.70 is not employed either, the third of a total of three mechanisms of the diazene —> hydrocarbon transformation is presented here.

To conclude this section, it is necessary to state that besides their application in catalytic cracking, amorphous silica-alumina acid catalysts have been applied in other hydrocarbon transformations, such as isomerization of olefins, paraffins, and alkyl aromatics, the alkylation of aromatics with alcohols and olefins, and in olefin oligomerization [55],... 

Turning now to the catalytic properties of EUROPT-1, very extensive measurements of all kinds of hydrocarbon transformations have been undertaken, and an impressive body of information is now available [6, 7, 12, 15, 16], Only the briefest of surveys is possible here the available reviews [6, 7] and the references cited therein give additional information. 

Novel organic syntheses that are possible in usual acidic media can be accomplished in superacids, including syntheses of economically important hydrocarbons. The remarkable ability of superacids to bring about hydrocarbon transformations can open up new fields in chemistry. In consideration of the exceptionally high activity of Hquid superacids, research was extended to prepare solid superacids. As for chemical appHcations of liquid superacids, efforts were made to attach them to soHd materials, and the results are found in extensive patent literature [10-13]. 

Ball H. A. and ReinhardM. (1996) Monoaromatic hydrocarbon transformation under anaerobic conditions at seal beach, California laboratory smdies. Environ. Toxicol. Chem. 15(2), 114-122. 

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