Q&A on PTQ (Petroleum Technology Quarterly)

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In DHDT unit suppose benzene converted to cyclohexane and then cyclohexane converted to normal hexane. What is the mechanism of this reaction? How is aromatic converted to cyclohexane then how cyclohexane ring broken and converted to n-hexane?

Answers (excerpt)
19/08/2009 A: kavirayani R Murthy, paramount Limited , krmurthy_ipcl@yahoo.com
The catalyst used for the processes like Reforming and Hydrodesulphurization reactions are of bifunctional type. Bifunctional means there are two functions incorporated in to the same catalyst. The cooperation between the acid and metal functions is important requisite for the process to proceed as well as for better performance of the catalyst. In Reforming catalyst, metal function is performed by metals like platinum or platinum-rhenium whereas in the case of hydrodesulphurization catalyst this metal function is performed by metals like Nickel or Cobolt.
Metal function derived from the active metal present on the surface of the catalyst mostly performs hydrogenation and dehydrogenation reactions

Acid function derived from the catalyst supports like Gama alumina, eta alumina, zeolites and other catalyst supports having strong acid sites mostly performs acid related reactions like dehydrocyclization, dehydroisomerization, hydrocracking etc. These strong acid sites present in the catalyst support produces positively charged protons in the presence of moisture. These protons on migration to the carbon portion of the organic molecule will produce positively charged carbonium ions. This imbalance of charge helps in conducting various acid controlled reactions.

For example, in dehydrocyclization reaction normal hexane gets converted to cyclohexane by closing the ring due to the presence of strong acid sites in the support The cyclohexane thus formed on the acidic support, when comes in contact with active surface platinum atoms present in the metal portion of the catalyst gets converted to benzene by taking out the hydrogen from the molecule. Because of this excellent cooperation between these two functions, these catalysts are called bifunctional catalysts.

Like dehydrocyclization reaction, another important reaction is dehydroisomerization; this once again exhibits excellent cooperation between the acid and metal functions. In dehydroisomerization reaction say methyl cyclopentane gets converted to Benzene, Here the acid in support breaks the cyclopentane (the five member ring naphthene) and reforms into a six carbon ring naphthene like cyclohexane, the cyclohexanes thus formed gets dehydrogenated on the metal portion of the catalyst to form benzene.
Thus saturated paraffins may get converted to rings or saturated rings may get cracked and gets converted to normal paraffins like cyclohexanes in this case.

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