Catalytic para-xylene maximization: IV. Hydroisomerization of meta-xylene on catalysts containing platinum on differently steamed H-ZSM-5 Original Research Article

The hydroconversion of meta-xylene was carried out using three series of catalysts containing 0.15, 0.30 or 0.60 wt.% Pt on unmodified and steam-modified H-ZSM-5 zeolite. The first series (Pt/H-ZSM-5 catalysts) contains Pt on the unmodified zeolite, whereas the second series (Pt/StH-ZSM-5) contains Pt on the steamed zeolite. The third series (StPt/H-ZSM-5) contains catalysts of the first series but treated with steam after Pt impregnation. The framework Al (AlF) and non-framework Al (AlNF) per unit cell (u.c.) in H-ZSM-5 and StH-ZSM-5 zeolites, as well as the XRD patterns for these zeolites were determined. The total pore volume of the catalysts was determined via thermogravimetry of the fully hydrated samples. The hydroconversion reaction was carried out using the mentioned catalysts in a pulse microreactor, atmospherically operated in a hydrogen flow at temperatures of 275–500 or 525°C. The reaction products were gas-chromatographically analyzed. The catalysts were characterized for acid sites strength distribution using temperature programmed desorption (TPD) of ammonia as well as for Pt dispersion in the support using hydrogen chemisorption and 1:1 stoichiometry. The total hydroconversiton activities of the catalysts, the selectivities for para- and ortho-xylenes production relative to the corresponding thermodynamic equilibrium values during the whole range of reaction temperature, the level of trimethylbenzenes formation as well as of the hydrodealkylation activities producing methane, benzene and toluene were evaluated and discussed. Correlations of the catalytic activities and selectivities with the characteristics of the catalysts were undergone.