n-Octane aromatization on Pt-containing non-acidic large pore zeolite catalysts

Platinum catalysts supported on the potassium-form of different large-pore zeolites (i.e. K-LTL, K-BEA, K-MAZ, and K-FAU) have been tested for n-octane aromatization at 500 °C. All catalysts were prepared by the vapor phase impregnation (VPI) method. It was found that the Pt/K-LTL catalyst exhibit a better aromatization performance than the other zeolite catalysts. However, due to secondary hydrogenolysis, the C8 aromatics produced inside the zeolite are converted to benzene and toluene. By contrast, a non-microporous Pt/SiO₂ catalyst did not present the secondary hydrogenolysis. Therefore, despite a lower initial aromatization activity, Pt/SiO₂ results in higher selectivity to C8 aromatics than any of the other zeolite catalysts. All fresh catalysts were characterized by hydrogen chemisorption and FT-IR of adsorbed CO. In addition, the residual acidity of the supports was analyzed by temperature programmed desorption (TPD) of ammonia. In agreement with previous studies, it was found that after reduction at either 350 or 500 °C, the Pt/K-LTL showed much higher Pt dispersion than other catalysts. It is known that the structure of L zeolite can stabilize the small Pt clusters inside the zeolite channel. By contrast, FT-IR indicated that a large fraction of platinum clusters were located outside the zeolite channels in the case of Pt/K-BEA and Pt/K-MAZ catalysts.     

EFFECT OF ZEOLITE CRYSTALLITE SIZE ON Pt/KL CATALYSTS USED FOR THE AROMATIZATION OF n-OCTANE

The effects of varying zeolite crystallite size in n-octane aromatization over Pt/KL have been studied on a series of catalysts. Various KL zeolites were synthesized via microwave-hydrothermal treatment, which allows for good control of crystallite morphology. Zeolites with different crystallite sizes were prepared by varying aging time (17–24 h), amount of barium (0–445 ppm), and seeding (0–8 wt%). The results showed that higher aging time resulted in smaller zeolite crystallite size, whereas the addition of barium resulted in larger crystallite size. Moreover, the addition of seeding reduced the crystallite size from 1.47 to 0.94 μm. Pt supported on different zeolite catalysts (Pt/KL) was prepared by vapor phase impregnation (VPI). The fresh catalysts were characterized by DRIFTS of adsorbed CO and volumetric hydrogen chemisorption. The results indicated that Pt clusters are well dispersed inside the zeolite channel in all the catalysts prepared. The aromatization of n-octane was tested on the different catalysts at 500°C and atmospheric pressure. It was found that the catalytic activity of all catalysts dropped rapidly after about 200 min on stream due to coke plugging inside the pore of the KL zeolite. It was also observed that less ethylbenzene (EB) and o-xylene (OX) were obtained as the conversion increased because both EB and OX are converted to smaller molecules such as benzene, toluene, etc., by secondary hydrogenolysis. Furthermore, the EB/OX ratio increases with zeolite crystallite size due to an enhanced preferential conversion of the larger OX molecules compared to the narrower EB as their path through the pores is restricted.     

EFFECT OF ZEOLITE CRYSTALLITE SIZE ON Pt/KL CATALYSTS USED FOR THE AROMATIZATION OF n-OCTANE

The effects of varying zeolite crystallite size in n-octane aromatization over Pt/KL have been studied on a series of catalysts. Various KL zeolites were synthesized via microwave-hydrothermal treatment, which allows for good control of crystallite morphology. Zeolites with different crystallite sizes were prepared by varying aging time (17-24 h), amount of barium (0-445 ppm), and seeding (0-8 wt%). The results showed that higher aging time resulted in smaller zeolite crystallite size, whereas the addition of barium resulted in larger crystallite size. Moreover, the addition of seeding reduced the crystallite size from 1.47 to 0.94 μm. Pt supported on different zeolite catalysts (Pt/KL) was prepared by vapor phase impregnation (VPI). The fresh catalysts were characterized by DRIFTS of adsorbed CO and volumetric hydrogen chemisorption. The results indicated that Pt clusters are well dispersed inside the zeolite channel in all the catalysts prepared. The aromatization of n-octane was tested on the different catalysts at 500°C and atmospheric pressure. It was found that the catalytic activity of all catalysts dropped rapidly after about 200 min on stream due to coke plugging inside the pore of the KL zeolite. It was also observed that less ethylbenzene (EB) and o-xylene (OX) were obtained as the conversion increased because both EB and OX are converted to smaller molecules such as benzene, toluene, etc., by secondary hydrogenolysis. Furthermore, the EB/OX ratio increases with zeolite crystallite size due to an enhanced preferential conversion of the larger OX molecules compared to the narrower EB as their path through the pores is restricted.