n-Octane aromatization on a Pt/KL catalyst prepared by vapor-phase impregnation

The aromatization of n-octane was investigated on a Pt/KL catalyst prepared by vapor phase impregnation (VPI), a preparation method that in previous studies was found to result in the highest Pt dispersion and maximum incorporation of Pt inside the channels of the zeolite compared to any other method. Although the aromatization of n-octane on Pt/KL has been investigated previously, those studies were conducted on nonoptimized catalysts. Our results show that, even on the Pt/KL catalyst prepared by the VPI method that exhibited optimum performance for the n-hexane aromatization, the activity for n-octane aromatization at 500 °C and 1 atm was low and it quickly dropped after a few hours on stream. The product distribution obtained from the n-octane conversion showed benzene and toluene as the dominant aromatic compounds, with small quantities of ethylbenzene (EB) and o-xylene (OX), which are the expected products from the direct closure of the six-member ring. The analysis of the product evolution as a function of conversion indicated that the benzene and toluene are secondary products resulting from the hydrogenolysis of ethylbenzene and o-xylene. Diffusional effects play a significant role in determining this product distribution. Since ethylbenzene and o-xylene are produced inside the channels of the zeolite, they are hydrogenolyzed before they can escape. By contrast, on the Pt/SiO2 catalyst used for comparison, ethylbenzene and o-xylene were the dominant aromatic products, although the overall aromatization activity was much lower than on the Pt/KL catalyst. The rapid deactivation found in the aromatization of n-octane on Pt/KL compared to that of n-hexane can also be explained in terms of the diffusional effects. The C8-aromatics produced inside the zeolite diffuse out of the system with much greater difficulty than benzene. Therefore, they form coke and plug the pores to a greater extent than benzene. Temperature programmed oxidation and sorption studies on spent samples demonstrate that the degree of pore blocking is much higher during n-octane aromatization than during n-hexane aromatization.