n-Alkanes aromatization over Pt-containing zeolite catalysts

Abstract

The aromatization of n-hexane was investigated on 1% Pt-containing L-zeolite catalysts under clean, sulfur-, and water-containing feeds whereas the aromatization of n-octane was studied only under clean feed on different Pt-containing large-pore zeolite catalysts. In the studies of n-hexane, PtKL and Pt/KL-containing rare earth (RE; Ce and Yb) catalysts were prepared by vapor phase impregnation (VPI) method. Pt/KL was also prepared by incipient wetness impregnation (IWI) method. The influence of RE promoters to catalytic performance under clean, sulfur-, and water-containing feeds was studied. It was found that the addition of rare earth element could enhance the catalytic acitivity in the presence of sulfur. Morphology of platinum clusters in the catalysts was investigated by a combination of characterization techniques i.e. FT-IR of adsorbed CO, hydrogen chemisorption, and EXAFS. The Pt/KL prepared by VPI method was also tested for the aromatization of n-octane. The results indicate that the activity was low and it quickly dropped after a few hours on stream. The product distribution obtained shows benzene and toluene as the dominant aromatic compounds, with small amount of ethylbenzene and o-xylene, which are the expected products from the direct closure of six-member ring. The analysis of the product evolution as a function of conversion indicates that the benzene and toluene obtained are secondary products resulting from the hydrogenolysis of ethylbenzene and o-xylene. Themperature programmed oxidation and sorption studies on spent catalysts demonstrate that the rapid deactivation is due to pore blocking by coke formation. The aromatization of n-octane was further studdied on 1%-Pt-containing other large pore zeolit catalysts, which are Pt/K-BEA, Pt/K-MAZ, and Pt/K-FAU catalysts. The Pt/KL, still, exhibits unique property among the zeolite catalysts. The results from FT-IR of adsorbed CO, hydrogen chemisorption and temperature programmed desorption of ammonia indicate that the superior property of Pt/KL is due to high Pt dispersion stabilized inside zeolite channel and lacking of high acid strength. Product distribution and coke formation were also discussed.