Dehydrogenation to aromatics

Selecting the naphtha type can be an important processing procedure. For example, a paraffinic-base naphtha is a better feedstock for steam cracking units because paraffins are cracked at relatively lower temperatures than cycloparaffins. Alternately, a naphtha rich in cycloparaffins would be a better feedstock to catalytic reforming units because cyclo-paraffins are easily dehydrogenated to aromatic compounds. Table 2-5 is a typical analysis of naphtha from two crude oil types. 

Catalytic reformers. Catalytic reforming is an important step to improve the quality of gasoline. During the reforming process, naphthens are dehydrogenated to aromatics. As a representative example, hydrogen is produced by cyclohexane dehydrogenation to benzene as follows ... 

Isoquinolines are easily prepared by the reaction of an acyl derivative of a f5-phenylethylamine with a dehydrating agent, e.g. P2O5, then using a catalytic dehydrogenation to aromatize the intermediate 3,4-dihy droi soquinoline. 

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. 

The model compound studies confirmed that the molecular structure of the hydrogenated coal extract is of paramount importance in determining the product pattern hydroaromatics dehydrogenate to aromatics, which either survive or polymerize to tars and, eventually, to carbon naphthenes crack principally to BTX and ethylene aliphatics mainly give small unsaturates such as ethylene and butadiene. The abnormally low yield of BTX from mesitylene is attributed to its high symmetry and thermal stability. 

We shall not treat cracking processes here due to the complexity of these high-temperature (usually around 500°C) reactions. However, cycloalkane dehydrogenation to aromatics (Appendix A2.4.4), alkane isomerization and olefin alkylation (leading to branched alkanes from linear ones) occur via such carbocation rearrangements. 

Reforming is a process which, while not greatly altering the size of the molecules, increases their knock resistance. Among these processes are isomerization of straight chain alkanes to branched hydrocarbons, cycliz-ation and dehydrogenation to aromatics and removal of the side chains of aromatics. The process produces much of the H2 used elsewhere in the refinery. 

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). 

The dehydrogenation to aromatic compounds becomes more complex when groups such as hydroxy, carbonyl, and acid anhydride are attached to the ring (Table 1, Schemes 3-5) [6,29-35]. The reaction is then usually performed under milder conditions in the liquid phase by the use of a solvent. Common solvents are high boiling-point aliphatic ethers or esters and the typical catalyst is Pd on activated charcoal. When the conditions are optimized conversion and selectivity can be very high (> 95 %). Reaction 5 is an example of the use of a hydrogen-acceptor molecule (dimethyl maleate). The conditions for reaction 4(b) deseiwe a closer look. The reaction is performed in the gas phase with a low surface-area... 

Since the demethylation of methylcyclohexane with hydrogen is a potential route to cyclohexane, this reaction was studied under conditions where methane and cyclohexane were the principal products, and methylcyclopentane formation and dehydrogenation to aromatics were substantially avoided. 

The acetate-mevalonate pathway. In principle, we have already discussed this route. It is concerned with the formation of cyclic terpenes which can be dehydrogenated to aromatic systems. An example of such a terpene with aromatic character is thymol. This pathway is relatively unimportant in higher plants. Of the three pathways listed we have still to discuss pathways 1 and 2. 

Naphthenes Crack to olefins. Dehydrogenate to cyclic olefins. Isomerize to smaller rings. Furdier dehydrogenation to aromatics, by hydrogen transfer. 

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