Preparation of a high-quality synthetic steamcracker feedstock from methylcyclohexane on acidic zeolite H-ZSM-5: influence of the hydrogen partial pressure

With the advent of the European Auto Oil Program and comparable legislation in other parts of the world, the aromatics content of gasoline has to be reduced considerably. Therefore, new outlets for surplus aromatics have to be found and, thus, the preparation of a synthetic steamcracker feed consisting mainly of ethane, propane and n-butane from cycloalkanes (which can easily be obtained from surplus aromatics) could become a useful process option. In this work, the influence of the hydrogen partial pressure (0.6–10 MPa) on the product composition is studied during the conversion of methylcyclohexane on acidic zeolite H-ZSM-5 at 250–400 °C. Deactivation by coking and aromatics formation can be suppressed by hydrogen partial pressures of 4 MPa and higher. On the other hand, upon increasing the hydrogen partial pressure, increasing amounts of isoalkanes are formed, partially at the expense of the desired n-alkanes. Best catalytic results, i.e. high yields of small linear alkanes, stable with time-on-stream, are, thus, obtained at medium pressures between 4 and 6 MPa. Mechanistically, these changes in the catalytic behavior observed with increasing pressure can be rationalized by: (i) an enhanced incorporation of molecular hydrogen into the hydrocarbons and (ii) a decreasing contribution of the so-called oligomeric cracking.