Effect of Pore Geometry on Ring Closure Selectivities in Platinum L-Zeolite Dehydrocyclization Catalysts

• Published in: Journal of catalysis Vol. 163; no. 1; pp. 106 - 116
• Main Authors: Miller, J.T., Agrawal, N.G.B., Lane, G.S., Modica, F.S.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 15.09.1996; Elsevier
• Subjects: Catalysis; Catalysts: preparations and properties; Chemistry; Exact sciences and technology; General and physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Catalytic reaction; Hydrocarbon; Computer simulation; Pore structure; Transition metal; Selectivity; Zeolite; Experimental study; Supported catalyst; Molecular sieve; Dehydrocyclization; Heterogeneous catalysis; Platinum; Benzene; Alkane; Reaction mechanism; Hexane
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1006/jcat.1996.0309

Experimental evaluations of nonacidic supported platinum catalysts for the conversion ofn-hexane to benzene demonstrate that the support geometry plays a major role in controlling the primary amount of benzene formed relative to methylcyclopentane, or the 1–6 to 1–5 ring closure selectivity. For most catalysts including non-acidic zeolite and amorphous supports, the 1–6 to 1–5 ring closure selectivity is about 0.5. However, Pt/K–LTL catalysts exhibit a ring closure selectivity greater than 1.0. In addition, increasing the alkali level above the ion-exchange capacity of the zeolite increases the ring closure selectivity slightly. Although the C1–C5hydrogenolysis selectivity is determined primarily by the inherent properties of the metal, a small excess of alkali was also important for optimum reduction of the hydrogenolysis selectivity. The catalyst selectivities are discussed with respect to the models proposed in the literature which account for the high benzene yield of Pt/K–LTL compared to other catalysts. The experimental results are most consistent with the preorganization model, in which the reactant molecule is adsorbed in the zeolite channel in a conformation which is similar to the transition state, leading to a perference for 1–6 ring closure. The significance of 1–6 to 1–5 ring closure selectivity was demonstrated via computer simulations of the reaction pathway ofn-hexane to benzene. The simulations demonstrate that a high 1–6 to 1–5 ring closure selectivity leads to a higer apparent activity at highn-hexane conversions. In addition, at high conversion a catalyst with high ring closure selectivity gives almost double the benzene yield at constant C1–C5yield. These differences are important in the selection of a catalyst for commercial application and make the Pt/K–LTL catalyst with small platimum particles and a slight excess of alkali the clear choice.