Decalin and Tetralin as Probe Molecules for Cracking and Hydrotreating the Light Cycle Oil

doi:10.1006/jcat.2001.3181

Journal of Catalysis

Volume 200, Issue 1, 15 May 2001, Pages 34-44

https://doi.org/10.1006/jcat.2001.3181 Get rights and content

Abstract

Cracking of tetralin and decalin was carried out over several zeolites to establish the effect of the pore topology of the catalyst on product distribution. These molecules were chosen as probe molecules, because they indicate which catalyst is the best for cracking or hydrotreating the light cycle oil (LCO) fraction, which is obtained directly from fluid catalytic cracking units. A set of zeolites with medium-sized (ZSM-5, MCM-22, ITQ-2), large (USY, Beta), and ultralarge pores (UTD-1), as well as a mesoporous MCM-41, were used as catalysts at 723 K. The results demonstrate that pore size and topology have a strong influence on diffusion, and consequently, on activity and selectivity in reactions such as ring opening, dealkylation, transalkylation, hydride transfer, and coke formation. UTD-1 generally has the highest activity per framework Al owing to the pore size and topology of this zeolite that enables flat molecules to diffuse easily inside the pores. According to the results, zeolites with medium-sized pores are adequate in combination with large-pore zeolites to crack naphthenes and fused aromatic–naphthenic rings, of the type present in LCO, to produce propene. Large-pore zeolites show good selectivity for naphthenic ring opening and appear to be better suited for hydrotreating LCO. Beta zeolite is a catalyst that is especially suitable for both processes.

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Featuring engineerable reaction pathway, ordered hierarchical cracking over core–shell structured zeolites (core@shell) plays a pivotal role in achieving the high-efficient conversion of low-value oil fractions into important petrochemical building blocks such as ethylene and propylene. However, the synthesis of core@shell zeolite composites with specific desirable textures is technically challenging. Herein, a novel embryo seed-breeding (ESB) approach was developed to synthesize polycrystalline ZSM-5@Beta core–shell composites (pZ@B) for boosted light olefin production. For the first time, pZ@B composites composed of a polycrystalline ZSM-5 core and a submicron-sized Beta shell was reported, which shows enhanced inner-diffusion properties and improved hydrothermal stabilities than that of conventional ZSM-5@Beta composites. In pZ@B-catalyzed reations, bulky molecules undergone a preliminary cracking into intermediate products within the Beta shell, followed by the direct diffusion into the micropores of the ZSM-5 core where they could be cracked selectively into ethylene and propylene. Compared with physically mixed zeolites and hierarchical ZSM-5, pZ@B composites showed the highest conversion and yield of ethylene and propylene in the cracking reaction of both decalin and practical industrial heavy oil. The superior catalytic activity of pZ@B can be attributed to the high accessibility of acid sites and the ordered hierarchical cracking space derived from the core–shell structure.
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Citation Excerpt :
Furthermore, abundant literatures have confirmed that the hydrocracking of PAHs on mono-functional acid zeolite involves many reactions, including ring-opening cleavage, hydrogen transfer, transalkylation, disproportionation and so on, resulting in producing >20 compounds and causing rapid deactivation of catalysts [17,18]. Corma et al. elaborated that the pore size and topology of zeolites have a great influence on the diffusion, activity and selectivity in the hydrocracking of decalin and tetralin [19]. Meanwhile, large-pore zeolites of Beta and Y are much easier to controllably open ring to reduce the number of aromatics and improve the CNs in comparison with the medium-pore zeolites (ZSM-5 and MCM-22) since medium-pore zeolites are prone to excessive cracking to generate small gas molecules like propylene.
The commercial Y zeolites are widely used in catalytic cracking and hydrocracking in industry, the efficient hydrocracking of polycyclic aromatics is a challenging task in petroleum and coal refining. The acidic zeolite in the hydrocracking bi-functional catalyst dominates largely the product selectivity, so understanding the independent effect of acid sites is favorable to the design of the catalyst. Herein, five industrial HY zeolites with different ratios of Si/Al were applied to tetralin hydrocracking to produce high-value benzene, toluene, and xylenes (BTX) in a high-pressure fixed bed reactor. The result indicated that mesoporous HUSY not only improved the catalytic activity but also exhibited longer-term stability compared with micropore HY. Interestingly, tetralin conversion in the hydrocracking showed a high-similar value when the Si/Al ratio of the zeolites is above 26 with almost identical porous structure, regardless of the distribution of acidity. And the yield of products is associated with the acidity of the HY zeolites, the lower the Si/Al ratio is, the more favorable the monocyclic aromatic hydrocarbons form. Therefore, it is beneficial for the industrial process to reduce the zeolite post-treatment procedure which achieves the high activity and BTX yield in the hydrocracking of tetralin and to abate the cost consumption, although it is still impractical to get rid of the trial-error scheme for the designing of an efficient catalyst in practice.

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Regular ArticleDecalin and Tetralin as Probe Molecules for Cracking and Hydrotreating the Light Cycle Oil

Abstract

J. Catal.

J. Catal.

Zeolites

J. Catal.

Microporous Mesoporous Mater.

J. Catal.

J. Catal.

Appl. Catal.

J. Catal.

Regular Article
Decalin and Tetralin as Probe Molecules for Cracking and Hydrotreating the Light Cycle Oil