Octane catalysts increased aromatics

The transformation of light alkanes (C2-C4) over H-ZSM-5 and Ga or Zn modified H-ZSM-5 catalysts to aromatic hydrocarbons has been studied intensively in recent years, since it would expand the raw material base for the manufacture of aromatics [1, 2]. The aromatics produced can be used as feed-stock for plastics, as chemical source for many chemical processes, as additives for increasing the octane number in gasoline, etc. 

Then came the idea to treat an entire gasoline fraction, well desulfurized, over a platinum catalyst. Various supports were made up and, as expected, we did convert a part of the naphthenes into aromatics, but the octane number increase was nothing sensational. When we moved up in temperature, the catalyst lay down and died. So we ran with hydrogen and applied a moderate pressure. The results were not particularly startling but at least the catalyst survived this ordeal. So we kept moving up and, sure enough, we did get better conversions. 

These new constraints on gasoline formulation focused attention on the decreased octane levels of gasoline produced with REY-zeolite cracking catalysts and the octane dip, or low octane number, of C6-C10 paraffins. To compensate, the aromatic content of the gasoline pool was increased from about 20% in 1973 to almost 40%, but the need for new octane catalysts was soon an important objective for refiners and catalyst producers. Octane catalysts require a suitable zeolite that can limit the hydrogen transfer reactions that convert olefins to paraffins. 

On sulfided metallic phases the hydrotreatment reactions also takes place. Noble metal catalysts usually include a zeolitic support. They are particularly used for fulfilling two different objectives, in the case of a gasoline oriented HCK their cracking and isomerization activity is the most important (increasing high octane and conversion yield). In a diesel HCK unit, the noble metal catalyst is mainly oriented to aromatic saturation and cetane improvement. However, in this latter case, also sulfided metal catalysts are used, especially NiW. 

A more desirable strategy is to prepare catalysts that will aromatize the gasoline without yield loss. Aromatization is thermodynamically favorable under FCC conditions of temperature (- SOO C) and pressure ( 2 atm). Blending studies show that the increase in research octane is about proportional to the amount of added aromatics. Figure 4. However, motor octane does not respond to initial increases in aromatic content in the blend but rises more sharply as aromatic content continues to increase. 

The octane number of gasolines obtained with the WS2 catalyst was somewhat low. The search for catalysts continued, therefore, with two objectives. The first was the development of a catalyst of the WS2 type with decreased hydrogenating activity and increased ability to produce branched-chain hydrocarbons. The second was the development of improved catalysts for the production of highly aromatic gasolines, that is, catalysts that give higher conversion and lower formation of gaseous hydrocarbons than those of the Mo-Zn-Mg type. 

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