Hydrocarbon processing gasoline isomerate

Isomerization is a small-volume but important refinery process. Like alkylation, it is acid catalyzed and intended to produce highly-branched hydrocarbon mixtures. The low octane C5/C6 fraction obtained from natural gasoline or from a light naphtha fraction may be isomerized to a high octane product. 

Cars and buses are fuelled by a volatile mixture of hydrocarbons. The mixture is called petrol in the UK, and gas (short for gasoline) in the USA. One of the main chemicals in petrol is octane, albeit in several isomeric forms. In the internal combustion engine, the carburettor first vaporizes the petrol to form an aerosol (see Section 10.2) comprising tiny droplets of petrol suspended in air (Figure 8.3). This vaporization process is similar to that which converts liquid perfume into a fine spray. 

Separation of the aromatics from each other and from other hydrocarbons by distillation is not economical because of the limited boiling-point differences and the formation of azeotropic mixtures. Instead, extractive or azeotropic distillation and liquid-liquid extraction are applied.234,235 The latter process is by far the most often used technique. The three processes are applied according to the aromatic content of the gasoline source. p-Xylene, the most valuable of the isomeric xylenes, is isolated by freezing (crystallization) or solid adsorption. 

One of the most significant stages in the development of zeolite catalysts was the synthesis by Mobil scientists (U.S. Patent 3,702, 866) of the zeolite now universally known as ZSM-5 (i.e. Zeolite Socony Mobil-5). This was the first - and most important - member of a new class of shape selective catalysts, which have made viable the production of synthetic gasoline . In this process, high-octane gasoline is produced by the catalytic conversion of methanol to a mixture of aromatic and aliphatic hydrocarbons (Derouane, 1980). Because of its unique combination of chemical nature and pore structure, ZSM-5 is a highly effective dehydration, isomerization and polymerization catalyst. 

The basic concept underlying alkylation reactions of aromatics is the formation of a stabilized carbocation able to attack nucleophilic substrates. Hydrocarbon cracking and hydrocracking, alkane isomerization, and olefin alkylation are important processes based on related alkane carbocation chemistry in the production of various types of hydrocarbons such as the branched ones for high octane gasolines. Zeolites and metal oxides are the preferred catalysts. 

In their protonated forms, zeolites are widely employed in the oil and petrochemical industries, in processes such as the conversion of alcohols to gasoline, catalytic cracking, isomerization and alkylations of hydrocarbons [3]. These chemical reactions most probably involve proton transfer from the acidic site of the zeolite to the organic substrate. In the case of hydrocarbons, this transfer gives rise to carbenium (I) or carbonium (II) ions as intermediates or transition states. 

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