Isomerization of paraffin

Skeletal isomerization of n-Cs, Cs paraffins to corresponding isoparaffins is important for improving the octane number as they are mixed in gasoline. Since low temperature is favored for the equilibrium of this reaction, catalysts active at low temperatures are desirable. Noble metals loaded on zeolites such as Pt —Y zeolite with low Na content are effective and used at about 520 Fig 4.3 shows the effect of Na content of zeolite on the catalydc activity for hexane isomerization. As the acidity increases with decreasing Na content the optimum temperature of operadon b gready suppressed. 

The isomerization over noble metal-solid acid bifuncdonal catalysts proceeds by the combination of two funcdons The dehydrogenation-hydrogenation on metallic sites and the isomerization of olefin on acid sites. It has been pointed out that no- 

Run number Starting hydrocaiban 2,3-Dbncthylbutaaei 2- -Methylpentanes 3- -Methylpentanes  

Labeled carbons are indicated by solid circles. Percentage among the monolabeled isomers Most probably in run numbers 1 and 5. 

It was suggested that the interconversion between 2-methylpentane and 3-methylpentane occurs mainly by the 1,2- and 1,3-alkyl shifts in the hexyl cations which accompany rapid hydride shift. The rate of alkyl shift is in the order, 

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Catalysis. As of mid-1995, zeoHte-based catalysts are employed in catalytic cracking, hydrocracking, isomerization of paraffins and substituted aromatics, disproportionation and alkylation of aromatics, dewaxing of distillate fuels and lube basestocks, and in a process for converting methanol to hydrocarbons (54). 

A non-acidic isomerization catalyst system has unexpectedly emerged from recent studies by French workers [4] in the area of Mo-oxycarbides. Although at an early stage of development, these new materials exhibit high selectivities for the isomerization of paraffins such as n-heptane. An alternative non-carbenium ion mechanistic route to achieve isomerization of higher alkanes could potentially overcome some of the limitations of conventional solid acid based catalyst systems. 

Catalytic reforming rearranging hydrocarbon molecules in a gasoline-boiling-range feedstock to produce other hydrocarbons having a higher antiknock quality isomerization of paraffins, cyclization of paraffins to naphthenes (g.v.), dehy-drocyclization of paraffins to aromatics (g.v.). 

Catalytic cracking occurs at about 800°K (527°C, 980°F) and at these temperatures the formation of olefins and aromatics is favored while isomerization of paraffins to branched products is... 

Isomerization of paraffins using current octane catalysts under current conditions is favorably away from equilibrium. Additional isomerization activity would make more normal paraffins and a lower octane at FCC temperatures. A much more olefinic gasoline is a possibility. However, additional olefins above the current olefin levels of 10-30% would have decreased effectiveness, especially on the motor octane number. 

The octane number improvement obtained by isomerization of paraffin hydrocarbons is not great since the amounts of the more highly branched paraffins formed at equilibrium are small at the temperatures employed in catalytic reforming (5). Naphthene isomerization, on the other hand, plays a more important role in reforming. In most naphthas about 50% of the naphthene hydrocarbons are of the cyclopentane type (4) so that in order to obtain the maximum aromatic formation, isomerization of these rings to cyclohexane rings must be promoted by the catalyst. 

Isomerization of paraffins and naphthenes is a reversible first-order reaction limited by thermodynamic equilibria. It is slightly exothermic in nature and does not take place to any appreciable extent without a catalyst (4). Although the mechanism of the reaction... 

The isomerization of paraffinic heptanes presents still more difficult problems, and no successful method has been found for suppressing the side reactions. 

As in previous conferences, the section on catalysis contains the most papers. A general review of the different reactions which can be catalyzed by zeolites is presented by Kh. M. Minachev. H. W. Kouwenhoven discusses the isomerization of paraffins on zeolites. Cracking, isomerization, and electron transfer reactions are discussed in several papers. Correlations between particular activities and physicochemical properties are covered. Selectivities related to crystal size and molecular shapes are also studied. Most of the work is still done on modified Y zeolites, but mor-denite and erionite also receive attention. 

In the presence of hydrogen the isomerization of paraffins of five or more carbon atoms over dual function catalysts, such as amorphous silica-alumina supported platinum, can be described by the following scheme ... 

Catalysts Based on Mordenite. Isomerization of paraffins over H-mordenite based catalysts has been described (6, 7,14, 0, 21). Minachev (7) reports that cyclohexane isomerization activity of Na-H-mordenite catalysts increases linearly with H+ concentration in the zeolite for 25-94% exchange. He further observed that H-mordenite is deactivated by other cations such as Li, K, Mg, Cd, Zn, and Al. This agrees with Bryant s work (6) he reported that, compared with Pd-H-mordenite, samples in which hydrogen was partly replaced by Ca or Zn had an appreciably lower n-pentane isomerization activity. 

The paraffin alkylation process mentioned above has become a major refinery process and is still practiced today on a very large scale, as is the process for the isomerization of paraffins which originated in Herman Pines laboratory. It was these discoveries and many others that led to his 145 patents and 265 publications, all of which were based on skillful experimental work. 

Catalytic activity measurements and correlations with surface acidity have been obtained by numerous investigators. The reactions studied most frequently are cracking of cumene or normal paraffins and isomerization reactions both types of reactions proceed by carbonium ion mechanisms. Venuto et al. (219) investigated alkylation reactions over rare earth ion-exchanged X zeolite catalysts (REX). On the basis of product distributions, patterns of substrate reactivity, and deuterium tracer experiments, they concluded that zeolite-catalyzed alkylation proceeded via carbonium ion mechanisms. The reactions that occurred over REX catalysts such as alkylation of benzene/phenol with ethylene, isomerization of o-xylene, and isomerization of paraffins, resulted in product distribu-... 

Lewis Acid-Complexed Metal Salts. Mixtures of aluminum chloride and metal chloride are known to be active for the isomerization of paraffins at room temperature.178 Ono and co-workers179-183 have shown that the mixtures of aluminum halides with metal sulfates are much more selective for similar reactions at room temperature. 

The wide ranges of temperature and pressure employed for the hydrodesulfurization process virtually dictate that many other reactions will proceed concurrently with the desulfurization reaction. Thus, the isomerization of paraffins and naphthenes may occur and hydrocracking will increase as the temperature and pressure increase. Furthermore, at the higher temperatures (but low pressures) naphthenes may dehydrogenate to aromatics and paraffins dehydrocyclize to naphthenes, while at lower temperature (high pressures) some of the aromatics may be hydrogenated. 

Lebas E, Jullian S, Travers C, Capron P, Joly J-F, Thery M. Process for the isomerization of paraffins by reactive distillation. EP 787786, Institut Frangais du Petrole, 1997. 

Ayame and co-workers recently prepared a superacid of chlorinated alumina (199-203). A1203 was chlorinated by heat treatment with Cl2 gas at temperatures above 800°C in a circulation reactor the material was active for isomerization of paraffins such as butane, pentane, and cyclohexane (201). The chlorinated alumina showed a surface acidity due to the Lewis type of Ho - 14.52 (202, 203). 

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. 

Catalytic Reforming A catalytic reaction of heavy naphtha(1) used to produce high-octane gasoline. The byproducts are hydrogen and light hydrocarbons the primary reaction is dehydrogenation of naphthenes to produce aromatics. Some reshaping of paraffins to produce aromatics and some isomerization of paraffins to produce isoparaffins also occur. 

The numerical case studied is derived from a flowsheet given in Stanford Research Institute Report 91, Isomerization of Paraffins for Gasoline. Since no kinetic information is given in this report, only reactor inlet and exit conditions, we will assume two different types of kinetics. In Case 1 we consider that the reaction is irreversible. An activation energy of 30,000 Btudb mol is used, and the preexponential factor is adjusted to give the same conversion reported in the SRI report. In Case 2 we assume that the reaction is reversible. The equilibrium constant decreases with increasing temperature because the reaction is exothermic. We also increase the size of the reactor so that the effluent leaves essentially at chemical equilibrium. 

Added to these are the isomerization of paraffins to isoparaffins and cd alkylcydo-hexanes, conversions which supply the fotegoing reactions. 

Decomposition of alkylbenzene Ph-Me, Ph-Et, Ph- Pr Friedel-Crafts acylation acetylation, benzoylation, and so on Isomerization of paraffin open-chain C1-C7, cyclic Cs-Cn Esterification -> AcOH + MeOH, EtOH, and so on CgOH + phthalic acid, and so on Cationic polymerization Me, Et, Bu vinyl ether Oligomerization 3-pinene, 1-octene, 1-decene Others -> aldol condensation, and so on... 

Isomerization has long been familiar to organic chemists, but such reactions have become important in petroleum chemistry only in fairly recent years. Not until 1 33 did the first scientific paper on the isomerization of paraffinic hydrocarbons appear (17). 

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