What does it mean by metallic and acidic side of a catalyst( e.g. Platinum)?
Answers
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13/03/2018
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A:
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Here is the best write-up I have seen on the subject. It is in my workshop manual:
Hydrotreating and Reforming Catalysts
A well-formulated catalyst allows the reaction to occur at a lower temperature, speeds up de- sired reactions, and slows down undesirable ones.
"The catalyst used for the processes like reforming and hydrodesulfurization reactions are of bifunctional type. Bifunctional means there are two functions incorporated into the same catalyst. The cooperation between the acid and metal functions is (an) 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 hydrodesulfurization catalyst this metal function is performed by metals like Nickel or Cobalt.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 (is) 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 (it) comes in contact with active surface platinum atoms present in the metal portion of the catalyst, gets converted to benzene by (removing) 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 (is) converted to benzene, here the acid in support breaks the cyclopentane (the five member ring naphthene) and reforms into a six carbon ring naphthene. The cyclohexanes thus formed gets (sic) 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 convert to normal paraffins like cyclohexane in this case."
Reference: Kavirayani R Murthy, PTQ Q&A, 24-11-2009.
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