Deactivation behavior of bifunctional Pt/H-zeolite catalysts during cyclopentane hydroconversion

Cyclopentane hydroconversion was used as a probe reaction to study the coking behavior of selected large-pore zeolites loaded with 0.5% Pt under identical conditions. The zeolites studied, namely, Y zeolite, zeolite beta, mordenite, LTL, and ZSM-12, were also tested at several different Si/Al ratios. The zeolite pore structure was the most important factor determining the deactivation behavior. Zeolites like Y and L that possess large cavities connected to smaller pore openings allow polyaromatic molecules to form and also trap these coke molecules to a great extent, resulting in rapid deactivation. This was confirmed by the analysis of soluble coke, which revealed the presence of large amounts of benzoperylene and coronene. Zeolite beta, which possesses an intersecting three-dimensional pore system, but no large cages, was the most stable. Apart from its pore structure, the excellent stability of the zeolite beta catalysts could also be partly attributed to the electronic state of platinum in the catalyst. The absence of aromatic compounds in the product stream and the aliphatic nature of coke indicated that the platinum in zeolite beta selectively opened the cyclopentane ring and did not dehydrogenate it, preventing the formation of cyclopentadiene that condenses via a Diels-Alder mechanism into polyaromatic coke. Mordenite and ZSM-12 showed reasonably good stability. Interestingly, the soluble coke in mordenite consisted of both polyaromatic and long-chain aliphatic compounds, indicating that the selectivity of the metal function changes with time. The amount of "hard coke" deposited in the zeolites was strongly related to the initial activity of the acid function, with the more active catalysts having a larger amount of coke. However, this trend was not followed in ZSM-12 which showed a shape-selective behavior that restricted the deposition of hard coke to a great extent. Apart from pore structure, variation of the acid site density (Si/Al ratio) affected metal dispersion and metal/acid balance and caused significant differences in the catalytic performance within a particular type of zeolite. However, the nature of the acid sites (Bronsted or Lewis) did not have a significant effect. Catalysts that had Si/Al ratios in the range 15 to 40 showed a minimum in the C(1)/C(3) ratio indicating a good balance between the metallic and acidic functions and showed good time stability. (C) 2002 Elsevier Science.