Hydrocracking of alkylbenzenes and polycyclic aromatic hydrocarbons on acidic catalysts. Evidence for cyclization of the side chains

Abstract

The simple cyclization of alkyl side chains is accompanied by formation of hydrogen and is therefore a reaction that is repressed at high hydrogen pressure. It is shown that appreciable cyclization accompanied by cracking occurs when certain aromatic hydrocarbons are passed over a nickel sulfide on silica-alumina catalyst, even at hydrogen pressures as high as 75 atm. The products from cyclization are predominantly tetralins and indanes, of lower molecular weight than that of the reactant. For example, with n-decylbenzene, 39% of the reactant that cracks forms bicyclic hydrocarbons. The small extent of cyclization of side chains (without cracking) that is thermodynamically possible at 75 atm pressure of hydrogen is calculated. It is shown that this limitation can be exceeded if cyclization is accompanied by cracking or by some other reaction that consumes hydrogen. The hydrocracking pattern of hexaethylbenzene differs markedly from that of hexamethylbenzene and is characterized by de-ethylation as the initial reaction. Cyclization accompanied by cracking occurs as one of the subsequent reactions. The product distribution from the hydrocracking of phenanthrene is discussed in detail. The proposed mechanism based on cyclization accounts for the unexpected product distribution, namely, the formation of tetralin without the formation of attendant light alkanes. Product distributions from the hydrocracking of n-butylbenzene, tetralin, anthracene, and pyrene are discussed.