Aromatization of liquefied petroleum gas over metal containing MFI-type zeolite catalysts

Abstract

The conversion of liquefied petroleum gas (LPG) such as propane, butane and their mixtures to aromatic hydrocarbons was investigated over MFI-type (ZSM-5) catalysts containing various metals. It has been shown that zinc and gallium improved both the activity and selectivity of propane aromatization. Even with less amount of Zn or Ga loading, Zn or Ga exchanged MFI exhibited higher selectivity for aromatics, mainly benzene, toluene, and xylene (BTX), than that of Zn-silicate or Ga-silicate. This may be attributed to the presence of Al in Zn or Ga exchanged MFI which increases the catalyst acidity. Further development was done by preparing Zn.Al- or Ga.Al-silicates with purpose of minimizing the catalyst preparation procedure. It has been found that NH4-Zn.Al-silicate having an Si/Zn ratio of 150 and an Si/Al ratio of 40 and H-Ga.Al-silicate having an Si/Ga ratio of 155 and an Si/Al ratio of 40 exerted considerably high selectivity for aromatics, ca. 51% and 64% of BTX respectively. This selectivity was comparable to that of Zn or Ga exchanged MFI with the same amount of Zn or Ga loading. However, the bimetallosilicate catalysts can be prepared in only one step crystallization and thus minimizing the catalyst preparation procedure. The introduction of platinum into NH4-Zn.Al-silicate and H-Ga.Al-silicate by .ion-exchange did not give the beneficial effect to enhance the catalyst performance and the stability of catalyst. This was attributed to the presence of Al in zeolite framework which facilitates the hydrogen transfer to the coke precursors on the catalyst surface. In particular, such concerted role of Al in combination with Ga on coke prevention in H-Ga.Al-silicate can be expected to predominate the corresponding role of Pt, and thus the promoting role of Pt on coke prevention was not observed in this catalyst.