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Investigation On Aromatics Hydrogenation Over Supported Pd Catalyst In The Presence Of Sulfur

Posted on:2006-04-06Degree:DoctorType:Dissertation
Country:ChinaCandidate:D P LiuFull Text:PDF
GTID:1101360182476062Subject:Industrial Catalysis
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In recent years, a considerable attention has been focused on aromaticshydrogenation of middle distillates due to the increasingly stringent environmentallegislation and the recognition of the importance of energy efficiency. Traditionalhydrotreating catalysts are not active enough to meet the requirement of deeparomatics hydrogenation. Therefore, a two-stage hydrogenation technique has beendeveloped and considered as the optimal approach to dearomatization for diesel. Inthe two stage process, a sulfide catalyst is employed in the first stage where sulfur andnitrogen containing species are mostly removed and meanwhile the aromatics aremildly saturated,followed by stripping to remove H2S and NH3. A highly active noblemetal catalyst is used in the second stage for aromatics hydrogenation. However,noble metals are extremely sensitive to sulfur, it is paramount to improve the sulfurtolerance of the catalyst. In this work, fundamental research was conducted withtetralin hydrogenation utilizing supported noble metal catalysts in the presence ofsulfur. The deactivation mechanism of the catalyst and the effect of the electronicstate of metal on sulfur tolerance are further discussed.The thermodynamic equilibrium characteristics of typical organic compounds inthe fuel, especially aromatic compounds were analyzed. Meanwhile, the effect oftemperature, pressure and hydrogen/hydrocarbon ratio on equilibrium compositions ofproduct(s) was also investigated. The results showed that the heterocyclic compounds(containing sulfur and nitrogen) have much higher equilibrium constant for openingof the C-X bond than that for aromatics hydrogenation under lower temperature so itis possible for the former to be removed during the first stage hydrogenation. Lowerreaction temperature, higher pressure and higher ratio of H2 to hydrocarbon arebeneficial to deep hydrogenation of aromatics, whereas higher reaction temperaturefavors isomerization and ring opening reaction of aromatics.Both the catalytic performance of a series of Beta zeolite supported Pd catalystand deactivation characteristics of ion exchanged Beta zeolite supported Pd catalystfor tetralin hydrogenation in the presence of thiophene were investigated. Manycharacterization techniques such as XRD, SEM, TEM, NH3-TPD, TG-DTA andGC-MS were employed. The results indicated that the catalytic activity of Pd/Betazeolite catalyst prepared by ion exchange (IE) is superior to that of the one preparedby incipient wetness impregnation (IWI). For all the IE catalysts, Pd/Beta80 (IE)catalyst has the best catalytic activity. Its tetralin conversion and decalin yield are75% and 55% respectively after 8 h reaction, while the activity of Pd/Beta60 (IE) andPd/Beta25 (IE) is lower, especially for Pd/Beta25 (IE). It deactivated rapidly, withtetralin conversion lowered to 10 % after 8 h reaction. The good catalytic performanceof Pd/Beta80 (IE) can be attributed to the presence of suitable amount of medium andstrong acid sites and small zeolite crystal size. Besides, addition of a small amount ofγ-Al2O3 to Pd/Beta25 (IE) promotes catalytic activity. It was found by TG-TDA andGC-MS, etc. that a considerable amount of coke composition existed on the usedcatalysts, in particular for used Pd/Beta25 (IE). It means that both mismatch of acidsite and metal site especially in presence of excess strong acid sites and sulfurpoisoning of metal are the important factors leading to catalyst deactivation.Nd3+ promoted and non promoted USY supported Pd catalysts were characterizedby TG-DTA and NH3-TPD, and the catalytic behaviors of them were evaluated fortetralin hydrogenation with benzothiophene. The electronic state of Pd cluster underdifferent environments and the sulfur poisoning of metal sites were simulated by aDFT method. It shows that the addition of small amount of Nd3+ ions change theacidity of zeolite and strengthen the interaction between metal and support, thusimproving the catalytic activity of the Pd catalyst. The simulation result indicates thatthe metal active sites become electron deficient as Nd3+ or H+ directly contact withthem, whereas if O-anion is attached to a metal site, excess electron would present onthe cluster. A H+ containing metal cluster is more difficult to be poisoned by sulfur.On the contrary, a O-containing metal site is sensitive to sulfur. Improvement ofcatalytic activity of noble metal catalyst via acid sites interaction with metal is thenattributed to the formation of electron deficiency of the metal site, which quite agreeswith the experimental observation. In conclusion, the sulfur tolerance of zeolitesupported noble metal catalysts can be enhanced by metal-support interaction causingmetal particle electron deficient.The USY support was modified by various transition metals, including Ti, Cr andMn. The physicochemical properties of various supports are characterized and thecatalytic performance of the Pd catalysts for tetralin hydrogenation with the presenceof benzothiophene was investigated. The results show that the modification of USYwith transition metal ions leads to more acid sites and higher strength of acidic siteson the support. However, the catalytic activity is not always enhanced. Compared tothe unmodified Pd/USY catalyst, only the Mn modified catalyst showed a higherhydrogenation activity, whereas addition of Ti and Cr decreased the activity after 6 htime on stream. The enhanced activity of Pd/MnUSY is attributed to a suitable acidamount and acid strength. Support effect during tetralin hydrogenation was alsoinvestigated by utilizing various carriers, i.e. Al-MCM-41, Si-MCM-41, USY andASA, herein Pd was used as metal active component. It was found that the catalyticactivity of these catalysts have a sequence as follows: Pd/ASA > Pd/USY >Pd/Al-MCM-41 > Pd/Si-MCM-41. The better catalytic activity of Pd/ASA andPd/USY was ascribed to a strong metal-support interaction. It was also found that anexcess amount of strong acid sites on USY induced isomerization and ring opening.Weak metal support interaction on Pd/Al-MCM-41 and Pd/Si-MCM-41 catalystsdecreased their hydrogenation activity, but enhanced the selectivity of aromaticssaturation.
Keywords/Search Tags:aromatics hydrogenation, Pd catalysts, sulfur tolerance, tetralin hydrogenation, catalyst deactivation, Beta zeolites, USY zeolites, modification by Nd3+, modification by transition metal.
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