(289ad) Influence of the Zeolite Structure, in Mo/Zeolite-Alumina Catalysts, on the Hydroconversion of a Model Mixture of N-Heptane-Methylcyclohexane-Toluene
The hydroconversion of a n-heptane (62 mol-%)-methylcyclohexane (11 mol-%)-toluene (27 mol-%) model mixture was carried out over Mo(3 wt-%)/zeolite(20 wt-%)-alumina catalysts under the following operating conditions: temperature: 588 K, pressure: 28 kg/cm2, LHSV: 1-10 h-1 and H2/hydrocarbon molar ratio: 2. It is well-known that the performance of Metal/zeolite hydroconversion catalysts depends on the shape selectivity properties of the zeolite, which in turn could modify the selectivity and product distribution of the process. Therefore in this study we analyze the effect of the zeolite structure by comparing the behavior of HZSM5 and HBeta, (SiO2/Al2O3 molar ratio=80) in Mo/zeolite-alumina catalysts, over the activity, selectivity and octane enhancement, during the hydroconversion of the n-heptane-methylcyclohexane-toluene model mixture. The results of this study indicated that the main reaction pathways which produce high octane hydrocarbons over the catalysts used here were isomerization and cracking of n-heptane and methylcyclohexane, dimerization-cracking of n-heptane, and aromatic alkylation reactions. It was also found that the zeolite structure has an important influence over the catalysts selectivity, thus modifying the relative importance of the main reaction paths. Since both HZSM5 and HBeta zeolites used here showed similar acid properties (pyridine FT-IR) and almost all of the structural aluminum is mainly in tetrahedral coordination (27Al-NMR), thus the changes observed in selectivity could be mainly associated with the different pore structure of each zeolite. While Mo/HZSM5-alumina catalyst produces a small amount of high-octane hydrocarbons, which is mainly due to the restricted structure of HZSM5 zeolite, Mo/HBeta-alumina catalyst leads to a higher production of bulky alkyl-aromatics and di-branched isoheptanes that contribute to the octane enhancement of the product.
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