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Studies of exfoliated molybdenum disulfide catalyst in hydrocracking and hydroprocessing reactions Tye, Ching Thian

Abstract

Studies of MoS₂ catalysts have demonstrated a close relationship between catalyst activity in hydroprocessing and the structure of the MoS₂. Most reports have focused on hydrodesulfurization and hydrogenation and some of these remain somewhat controversial. Moreover, the effect of MoS₂ structure on heavy oil hydrocracking has not been well studied. In this work, exfoliated MoS₂ was used to study the catalyst structure-activity relationship in Cold Lake heavy oil hydrocracking and model compound hydroprocessing. The exfoliated MoS₂ is dispersed directly in the oil eliminating the influence of catalyst support, and the laminar structure of exfoliated MoS₂ provides an interesting framework to study the relationship between catalyst activity and structure. In heavy oil hydrocracking at 415°C, exfoliated MoS₂ had similar liquid and coke yields but exhibited a better quality liquid product, especially in asphaltene and microcarbon residue removal, than MoS₂ derived from molybdenum naphthenate (MoNaph). In addition, the feasibility of recycling coke was confirmed as more than 90% of Mo resided in the coke after reaction and the spent catalyst-in-coke was active in Cold Lake heavy oil hydrocracking. The activity of exfoliated MoS₂ for hydroprocessing at 350°C was also determined using various model reactants. The reactants studied were naphthalene for hydrogenation, dibenzothiophene for hydrodesulfurization, quinoline and carbazole for hydrodenitrogenation, and phenol for hydrodeoxygenation. Again, the resulting catalyst activity was compared to MoNaph derived MoS₂. Exfoliated MoS₂ gave better overall hydrodesulfurization and hydrodeoxygenation as compared to MoNaph derived MoS₂. In contrast, MoNaph derived MoS₂ showed higher hydrogenation and hydrodenitrogenation activities. These results were a consequence of the morphological differences between the two catalysts in the model systems. The activity of exfoliated MoS₂ in hydrodesulfurization of dibenzothiophene was further compared to that of crystalline MoS₂, MoNaph, and ammonium heptamolybdate derived MoS₂. The prepared catalysts had significantly different morphologies, as described by the crystallite stack height and slab length. These data were used to estimate the fraction of rim and edge sites in the crystallites. The catalysts’ selectivity for hydrogenolysis and hydrogenation reactions were shown to correlate with the fraction of rim or edge sites. Selectivity for hydrogenolysis increased as the fraction of edge sites increased.

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