Deep Hydrodesulfurization over Ni-Mo Catalysts Supported on the Mixed Molecular Sieves

DEEP HYDRODESULFURIZATION OVERNi-Mo CATALYSTS SUPPORTED ON THEMIXED MOLECULAR SIEVESJing Ren 1, Anjie Wang 1*, Xiang Li 1, Mohong Lu 1, LifengRuan 1, Jia Yan 1, Yongying Chen 1,Yongkang, Hu 21.State Key Laboratory of Fine Chemicals,Dalian University of Technology, Dalian 116012 China;2.Fushun Research Institute of Petroleum and Petrochemicals,Fushun, 113001 China * Corresponding author. Fax: +86-411-88993919. E-mail address: anjwang@ Introduction The increasing concerns of global air environment have stimulated the demand for cleaner and cleaner engine fuels. Hydrodesulfurization is a key process used in producing clean engine fuels. In the process, Mo and W sulfide catalysts supported on γ-Al 2O 3 and promoted by Co and Ni have been applied to the industrial hydrotreatment of petroleum and liquefied coal. 1 However, new environmental regulations require that the sulfur content in diesel fuel be significantly reduced to less than 0.05 %, which is not currently achievable with the γ-Al 2O 3-based catalysts. The conventional catalysts are active in converting thiophene and benzothiophenes, but not active enough to desulfurize efficiently the most refractory sulfur-containing polyaromatic compounds, e.g., dibenzothiophene (DBT) and its alky-substituted derivatives. Whitehurst et al. concluded, based on the composition of typical gas oil and the first-order kinetics for Hydrodesulfurization (HDS) of different sulfur-containing compounds, that the reaction time will have to be increased up to 4 times when using conventional catalysts to reduce the sulfur content from 2500 to 500 ppm. 2Thus deep HDS becomes of great research interest, aiming at removing DBT and its derivatives. Mesoporous MCM-41 has been the focus of much research interest since its discovery because it offers a uniform pore of 1.5 to 10.0 nm and a very high surface area. 3The unique properties of MCM-41 provide a potential for catalytic application to the bulky molecule involved reactions. The extremely high surface area favors a high dispersion of active species and is often conducible to high catalytic activity. The MCM-41-supported catalysts have been developed for a variety of reactions, 4including HDS .5-8Recently, Wang et al. studied the HDS of DBT oversiliceous MCM-41-supported Co-Mo and Ni-Mo.9,10 It isshown that siliceous MCM-41-supported catalysts weresuperior in HDS activity to the conventionalalumina-supported counterparts. It is assumed that the highperformance is due to the higher dispersion of the activespecies on the surface of MCM-41.Despite the extensive use of zeolite in catalysis, there hasbeen relatively little research work on their application inhydrotreating processes until recently. Mann et al. 11 studiedthe HDS and hydrodenitrogenation (HDN) of heavy gas oil over Ni-Mo sulfide catalysts supported on zeolite-alumina-silica, and found that over the catalysts 99 % sulfur-containing compounds and 86 % nitrogen-containingcompounds can be removed at 425 °C . Harvey and Pratt 12studied the HDN reaction of quinoline over catalysts of metal ion-exchanged Y zeolite mixed with a standard Ni-Mo/γ-Al 2O 3 and found that ruthenium-exchanged Y zeolite promotes the HDN activity of the standard Ni-Mo/γ-Al 2O 3 catalyst. More recently, Yumoto 13 reported a new type of γ-Al 2O 3-zeolite-based catalyst for deep HDS, and found that asmall amount of zeolite accelerates the hydrogenation rate of the DBT ring. Li et al. 14 studied HDS of DBT over Ni-Mosulfides catalysts supported on γ-Al 2O 3-USY , and found that the Ni-Mo catalysts supported on γ-Al 2O 3-USY zeolite havehigher HDS activity. In the present study, the HDS of DBT over Ni-Mo catalysts supported over the mixed molecular sieves has been investigated. The mixed molecular sieves consist of proton-exchanged MCM-41 and HY .2. Experimental 2.1 Materials. The materials used were sodium silicate hydrate (commercial GR grade), cetytrimethylammonium bromide (CTAB) (A.R.grade), 4 M H 2SO 4, 0.5 M HNO3. Both ammonium heptamolybdate tetrahydrate [(NH 4)6Mo 7O 24.4H 2O] and nickel nitrate hexahydrate [Ni(NO 3)2.6H 2O] were of commercial GR grade (Dalian Chemical reagents Co.). DBT was synthesized according to the method in the literature.9 Decalin was a product of Shanghai Chemical Reagents Co. HY zeolite was provided by FuShun Research Institute of Petroleum and Petrochemicals. 2.2 Catalysts. Siliceous MCM-41 and theproton-exchanged MCM-41 were prepared according to the procedure reported in the paper.15 Ni-Mo/HY-H-MCM-41 catalysts were prepared in the following way: the support wasmade by milling H-MCM-41 with HY . The aqueous solution of (NH 4)6Mo 7O 24.4H 2O and Ni(NO 3)2.6H 2O was dropped onsupport in vacuum condition. The mixture was stirred for 2 h at room temperature, then the water was evaporated and the solid product was dried at 120 °C overnight, and calcined at 450 °C for 5 h. A 20 wt % MoO 3 loading level was chosen for preparing this series of catalysts. The content of nickel was determined by varying the atomic ratio of Ni/Mo in the range of 0-1.0. The prepared catalysts was denoted asNi-Mo(α)/Y(β)-M, where α represents Ni/Mo atomic ratioand β represents the weight percent of HY in support.The temperature programmed reduction (TPR) patterns ofthe catalysts were measured on a Chembet-3000 analyzer.Before the measurement, the sample had been treated inhelium at 200 °C for 2 h. A gas mixture of H 2 and N 2 (10 vol % H 2 in nitrogen) was used as the reacting agent. The patterns of the catalysts were measured from room temperature to ca. 1000 °C at 10 °C /min. Nitrogen adsorption measurementswere performed using a Quantachrome adsorption analyzer, which reports adsorption isotherm, BET specific surface area, and pore volume automatically. The samples were outgassed for 4 h at 300 °C before the adsorption. 2.3 System and Procedure for HDS. HDS of DBT was carried out in a fixed-bed stainless steel tubular reactor. The HDS activities of the prepared catalysts were evaluated using a model fuel: 0.8 wt % DBT in decalin.The catalysts were presulfided before HDS reaction. The products were analyzed by an Agilent-6890+ gas chromatograph equipped with an FID detector using an HP-5 column.3. Result and discussion 3.1 Characterization of Supported Catalysts. The physical properties of the support and the supported Ni-Mo catalysts are summarized in Table 1. It is shown that both BET specific surface area and pore volume of catalysts were reduced after being deposited by the metal species. The decreases in specific surface area and pore volume might result partly from the density increase of the catalysts and partly from the pore blocking by the metal species.The TPR profiles of Ni-Mo(0.75)/Y(β)-M catalysts are illustrated in Fig 1. The main H 2 consumption peak temperatures are summarized in Table 2. Two TPR peaks are observed, one around 500 °C and the other around 800 °C . As HY is added, the low temperature (LT) peak of the Ni-Mo site shifted to lower temperature, but the high temperature (HT) peak changed a little. Moreover, when the HY exceeds 25 %, the LT peak shifted to higher temperature. Ni-Mo(0.75)/Y(0.25)-M showed the lowest consumption peak temperature, indicating that this catalyst has the highest reducibility. The shift of LT peak by introduction of HY might be attributed to accelerating the hydrogen spillover from Ni to Mo sites on the surface of the support.Table 1. Physical Properties of the Support and the Ni-Mo (α)/Y(β)-M CatalystsCatalyst BET surface area(m 2/g)Pore volume (cm 3/g) Ni-Mo (0.75)/Y (0)-M Ni-Mo (0.75)/Y (0.1)-M Ni-Mo (0.75)/Y (0.25)-M Ni-Mo (0.75)/Y (0.75)-M Ni-Mo (0.75)/Y (1.0)-M Ni-Mo (0)/Y (0.25)-M Ni-Mo (0.25)/Y (0.25)-M Ni-Mo (0.5)/Y (0.25)-M Ni-Mo (1.0)/Y (0.25)-M 366.6 361.3 462.8 353.7 411.7 380 414.3 434 433.8 0.52 0.47 0.44 0.28 0.21 0.54 0.47 0.42 0.41The TPR profiles of Ni-Mo(α)/Y(0.25)-M andNi/Y(0.25)-M catalysts are illustrated in Fig 2. The main H 2consumption peak temperatures are also summarized in Table2. Two peak temperatures were 420 °C and 519 °C forNi/Y(0.25)-M, and there were 578 °C and 935 °C forMo/Y(0.25)-M, indicating that the reducibility of Ni species is superior to that of Mo species.Table 2. The Main H 2 Consumption Temperatures in the TPRProfiles of Ni-Mo(0.75)/Y(β)-M Catalysts .Catalyst The LT peak temperature (°C )Ni-Mo (0.75)/Y (0)-M 501 Ni-Mo (0.75)/Y (0.1)-M 489 Ni-Mo (0.75)/Y (0.25)-M 461 Ni-Mo (0.75)/Y (0.75)-M 555 Ni-Mo (0.75)/Y (1.0)-M 576 Ni-Mo (0)/Y (0.25)-M 578 Ni-Mo (0.25)/Y (0.25)-M 492 Ni-Mo (0.5)/Y (0.25)-M 478 Ni-Mo (1.0)/Y (0.25)-M44220040060080010001200e d c b aT e m p e ratu re,¡æFigure 1. TPR profiles of Ni-Mo(0.75)/Y(β)-M catalysts.(a) Ni-Mo （0.75）/Y(0)-M, (b) Ni-Mo （0.75）/Y(0.1)-M, (c) Ni-Mo （0.75）/Y(0.25)-M, (d) Ni-Mo （0.75）/Y(0.75)-M, (e). Ni-Mo （0.75）/Y(1.0)-M,20040060080010001200T e m p e ra tu re ,¡æf e d c b aFigure 2. TPR profiles of Ni-Mo (α)/Y (0.25)-M catalysts(a) Ni-Mo (0)/ Y (0.25)-M, (b) Ni-Mo (0.25)/ Y (0.25)-M(c)Ni-Mo (0.5)/ Y (0.25)-M, (d) Ni-Mo (0.75)/ Y (0.25)-M,(e) Ni-Mo (1.0)/ Y (0.25)-M ，(f) Ni/ Y (0.25)-MTwo TPR peaks are also observed for Ni-Mo(α)/Y (0.25)-M catalysts, at about 500 °C and 800 °C. As the Ni/Mo atomic ratio increases, the LT peak temperature of the Mo site shifts to lower temperature, suggesting a strong interaction between Ni and Mo species. It is shown that Ni-Mo(0.75)/Y (0.25)-M shows the lowest consumption temperature, indicating that it has the highest reducibility. The shift of the LT reduction peaks by the introduction of Ni species may have resulted from the interaction among the metal species, andfrom the hydrogen spillover from Ni sites to Mo sites on the surface of the support.3.2 HDS of DBT3.2.1 HDS of DBT Over Ni-Mo(0.75)/Y(β)-M. In general, there two possible reaction pathways for removal of sulfur from DBT. The first pathway is direct extraction of the sulfur atom from the molecule yielding biphenyl (BP), the second pathway is prehydrogenation of one aromatic ring followed by extraction of the atom yielding cyclohexylbenzene (CHB).1 In the present study, the detectable compounds in the liquid samples are BP, CHB, and traces of benzene and cyclohexane. For the catalytic reaction, the conversion of DBT represents the activity of HDS for the catalysts.Fig.3 shows the conversion of DBT during HDS catalyzed by Ni-Mo(0.75)/Y(β)-M as a function of temperature. All the Ni-Mo(0.75)/Y(β)-M catalysts showed substantially high activity for converting DBT into hydrocarbons. Maximum activity was observed for the Ni-Mo(0.75)/Y(β)-M catalysts at β is 0.25.3.2.2 HDS of DBT Over Ni-Mo(α)/Y(0.25)-M. Fig.4 shows the conversion of DBT during HDS catalyzed by Ni-Mo(α)/Y(0.25)-M as a function of temperature. All the Ni-Mo(α)/Y(0.25)-M catalysts showed extremely high activity for converting DBT into hydrocarbons. Moreover, Ni-Mo(0)/Y(0.25)-M also showed relatively high HDS activity at high temperature. It is shown that Ni-Mo(0.75)/Y(0.25)-M catalyst exhibited the highest activity for DBT HDS. The optimal Ni-Mo atomic ratio for the mixed molecular sieves supported catalysts is different from that for conventional γ-Al2O3-supported catalysts, which generally falls in the range of 0.2-0.5.126028030032034036020406080100DBTConversion,%Temperature,¡æFigure 3. Variation of DBT conversion with temperatureduring HDS over Ni-Mo(0.75)/Y(β)-M(■)Ni-Mo(0.75)/Y(0)-M (●)Ni-Mo(0.75)/Y(0.1)-M(▲)Ni-Mo(0.75)/Y(0.25)-M(▼)Ni-M(0.75)/Y(0.75)-M (□) Ni-Mo (0.75)/Y (1.0)-M26028030032034036020406080100DBTConversion,%Temperature,¡æFigure 4. Variation of DBT conversion with temperature during HDS over Ni-Mo(α)/Y(0.25)-M(■)Ni-Mo(0)/Y(0.25)-M (●) Ni-Mo(0.25)/Y(0.25)-M (□)Ni-Mo(0.5)/Y(0.25)-M, (▼) Ni-Mo(0.75)/Y(0.25)-M (○) Ni-Mo(1.0)/Y(0.25)-M4.Discussion4.1 HDS Activity of DBT. It is generally accepted that small nickel suifide species are located at the MoS2 edges in the active phase. EXAFS measurements revealed that the Ni suifide species located at the MoS2 edges differ locally in size and ordering. It is believed that only the Ni-Mo-S structure is active in HDS. 9It is well accepted that high dispersion of Ni-Mo species on the support surface is essential for preparation of high-performance HDS catalysts and that the surface properties, such as acidity, affect the dispersion and local environment of the metal species. It was reported that siliceous MCM-41-supported Ni-Mo sulfides catalysts exhibited very high activity for DBT HDS.10 It is assumed that the high surface area and the mild acidity of the support may lead to a high dispersion of the metal species on the surface.The conversion of DBT for Ni-Mo(α)/Y(β)-M catalysts indicated that the optimal Ni/Mo atomic ratio is 0.75, higher than the conventional γ-Al2O3-supported catalysts. It is believed that a high dispersion of MoS2 and the promoters onthe surface of the support is essential to form a monolayer or tiny Ni-Mo-S crystallite structure so as to achieve a very high HDS activity. 24.2 Correlation of HDS Activity With ReducibilityAccording to the literature,15,16 a correlation exists between the reducibility of oxidic catalyst and the HDS activity of its sulfides. In order to correlate HDS activity and reducibility, both have to be quantified. HDS activity is easily represented by the conversion of DBT. Reducibility can be characterized in several ways, i.e., the temperature of reduction onset method or the average temperature method. Any of the above mentioned methods lead to the same conclusions.16 Thus, in this study, the reducibility is characterized by main H 2 consumption peak temperature. The DBT conversion at 280 °C is plotted against the main H 2 consumption peak temperature for the Ni-Mo(α)/Y(β)-M catalysts In Fig.5. Line M represents the Ni-Mo(0.75)/Y(β)-M series catalysts, and line N represents Ni-Mo(α)/Y(0.25)-M series catalysts. It is indicated that DBT conversion is well correlated with the main H 2 consumption peak temperature for the Ni-Mo(α)/Y(β)-M catalysts except Ni-Mo(1.0)/Y(0.25)-M catalysts. The exception may be due to the overloading of Ni on the surface, leading to the appearance of new species, which might be difficult to be reduced. It is indicated that HDS activity increased with the decrease of the main H 2 consumption peak temperature. This suggests that reduction of the transition metal compounds is a crucial step in the transformation of the oxidic precursor catalyst into the sulfided catalyst. The high DBT conversion for Ni-Mo (0.75)/Y(0.25)-M catalysts corresponds very well with its good reducibility. TPR thus appears to be a promising technique, which can be applied for a preliminary screening of the oxidic HDS catalysts.44046048050052054056058020406080100D B T C o n v e r s i o n ,%Main H 2consumption Temperature,¡æFigure 5. The DBT conversion at 280 ℃versus H 2 consumption temperature in the TPR profiles of the Ni-Mo(α)/Y(β)-M(M) Ni-Mo(0.75)/Y(β)-M,(N) Ni-Mo(α)/Y(0.25)-M, (a)Ni-Mo(0.75)/Y(1.0)-M,(b)Ni-Mo(0.75)/Y(0.75)-M,(c) Ni-Mo （0.75）/Y(0)-Mo(d) Ni-Mo （0.75）/Y(0.1)-M,(e) Ni-Mo （0.75）/Y(0.25)-M (f) Ni-Mo(0)/Y(0.25)-M, (g) Ni-Mo(0.25)/Y(0.25)-M, (h) Ni-Mo(0.5)/Y(0.25)-M, (i) Ni-Mo(1.0)/Y(0.25)-M,5. ConclusionIn summary, the mixed molecular sieve is a promising support for preparing deep HDS catalysts. The optimal weight percent of HY in the mixture was 25 %. A maximum HDSactivity was observed at Ni/Mo atomic ratio of 0.75. It is assumed that tiny Ni-Mo sulfide clusters might be created on the extremely high surface area, yielding considerably high activity for HDS of DBT. TPR data revealed that the H 2 consumption peak at low temperature shifted to lower temperature when the weight percent of HY in the support and the Ni/Mo atomic ratio increased. It is assumed that the shifting of the reduction peak might be attributed to the interaction between metal species and the support and to the synergetic effect of the binary metal species. A correlation exists between the reducibility of oxidic Ni-Mo(α)/Y(β)-M catalysts and the HDS activity of their sulfides. High reducibility corresponds to high HDS activity. TPR could be used as a time-saving preliminary screening technique the HDS catalysts.Acknowledgments. This research was partly supported by the Natural Science Foundation of China (Grant No. 20003002 and 20333030), by CNPC Innovation Foundation and by Education Ministry of China(20030141026). The authors are grateful to Mr Zequn Yin, Mr Guanwei Cao, Mr Yang Teng for their kind helps in the research work.References(1) Topsoe, H.; Clausen, B. S.; Massoth. “Hydrotreating Catalysis:Science and Technology” .Spinger-Verlag, Berlin. 1996,310 (2) Whitehurst, D. D.; Isoda. T. ; Mochida. I. Adv.Catal .1998,42,345.(3) Beck J. S.; Vartuli, J. C.; Roth, W. J. Leonowicz, M. E.; Kresge,C. T.; Schmitt, K.D.; Chu, C. T. W.; Olsen, D. H.; Sheppard,E. W.; McCullen, S. B., Higgins, J. B.; Schlenker, J. L. J.Am.Chem.Soc .1992, 114, 10834.(4) Ying, J. Y .; Mehnert, C.P.; Wong, M. S. Angew. Chem. Int. Ed.1999,36,56.(5) Coma, A.; Martinez, A.; Martinez-Soria V .; Monton, J.B. J.Catal .1995,153,25.(6) Song, C. S.; Reddy, K. M. Appl. Catalysis . 1999,176,1(7) Reddy, K. M; Wei, B. L; Song, C. S. 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