Investigation On Sulfur Tolerance Of Noble Metal Catalysts Supported On Y For Aromatic Hydrogenation

In recent years, a considerable attention had been focused on aromatics hydrogenation of diesel due to the increasingly stringent environmental legislation and the recognition of the importance of energy efficiency. Noble metal catalysts were highly active for aromatics hydrogenation. However,they were extremely sensitive to sulfur, it was paramount to improve the sulfur tolerance of the catalyst. In this work, fundamental research was conducted with tetralin hydrogenation utilizing USY supported noble metal catalysts in the presence of sulfur.The influence of sulfur compound on hydroconversion of tetralin and on product distribution over Pd-Pt/HDAY catalyst was investigated. The activity and thiotolerance for the hydrogenation of tetralin were examined in a high-pressure fixed-bed continuous-flow stainless steel catalytic reactor. The thiotolerance was tested with feeds containing 200 ppm sulfur in the form of 4, 6-dimethyldibenzothiophene and thiophene, respectively. It was found that thiophene was more effective for deactivating the catalyst than the former one. The diffusion barrier of the pores for the sulfur compound is likely the major reason for the different deactivation rate of catalysts.The products other than decalins were mainly decalin isomers (iso-DECS), ring-open (RO) hydrocarbons and their hydrogenated derivatives. The selectivities of RO and iso-DECS in the two feeds with sulfur are almost the same to each other and to that of the clean feed, while there are significant differences in the selectivities of cracking product. The differences in product distribution observed are explained by the different poisoning ability for acid sites on which secondary acid-catalyzed reactions, mainly cracking and dealkylation, leading to lighter products occurs.The effects of acid site amount on tetralin hydrogenation activity and sulfur tolerance were studied. USY supports with different acid amount were obtained by steaming dealumination, and the noble metal catalysts with same metal content and same metal dispersion were prepared by an ion-exchange method. Hydrogenation activity and sulfur tolerance of catalysts was examined. It was shown that Pd-Pt/USY1 catalyst with very high acid amount and Pd-Pt/USY4 catalyst with low acid amount showed noticeable decrease in sulfur tolerance. The low sulfur tolerance of Pd-Pt/USY4 was related to its low acid amount and insignificant electron-deficiency of metal particles. The noticeable decrease in sulfur tolerance of Pd-Pt/USY1 catalyst was because the acid sites were more easily poisoned by sulfur when acid amount increased.The contribution of acid sites to aromatic hydrogenation and influence to the distribution of products was studied. Neutralization with pyridine of acid sites on catalysts with different metal content was performed in order to evaluate the contributions of acid sites to activity. The activity and thiotolerance of fresh catalysts were higher than that poisoned by pyridine in same metal content, indicating that acid sites were favor to increasing the activity of catalysts. The contribution of acid site to turn over frequency (TOF) was roughly estimated by subtracting the TOF of the samples before and after acid site neutralization.Influence of metal content, support acidity and tetralin on stereoisomerization of decalin was explored. It was shown that support acidity played a predominate role in stereoisomerization of decalin. The mechanism for stereoisomerization of decalin on proton-form zeolites involved a carbenium ion.The deep hydrogenation of polycyclic arenes on Pd-Pt/USY catalyst has been investigated. It was shown that catalyst promoted deep hydrogenation of polycyclic arenas. Competitive hydrogenation of naphthalene and anthracene system as well as naphthalene and pyrene system was examined. For the naphthalene and anthracene with same kinetic diameter, a strong inhibition existed in this reaction system. The results showed that the polycyclic arenes inhibits the hydrogenation of arenas with less number of aromatic rings. However, for naphthalene and pyrene the hydrogenation activity of naphthalene was obviously higher than that of pyrene. It is because that pyrene can not diffuse to the supercage of Y. Although its hydrogenation may occur on the active sites in mesopores and on the outer surface of the crystallites, a low hydrogenating activity than naphthalene was reasonable. Inhibition of polycyclic arenas hydrogenation activity by sulfur was investigated. It showed that sulfur has stronger inhibiting for hydrogenation of deeply hydrogenated compounds than that with less number of aromatic rings.Modified USY by adding yttrium was prepared by an ion exchange method. TPR and NH₃-TPD were employed to investigate the character of catalysts and supports. At the same metal content and metal composition, activity and sulfur tolerance of catalysts were analysed. The results showed that the activity and sulfur tolerance of the catalyst could be improved by adding a low content of yttrium, while it could be decreased when content of yttrium was high.