Energy & Fuels 1994,8, 249-257
249
Simultaneous Hydrodesulfurization and Hydrodenitrogenation of Model Compounds over Ni-Mo/ yA1203 Catalysts Umit S. Ozkan,* Shuangyao Ni, Liping Zhang, and Edgar Moctezumat Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 4321 0 Received June 24, 1993. Revised Manuscript Received November 2 , 1 9 9 9
In spite of the recognized importance of their industrial application, the questions regarding catalytic phenomena involved in hydrodenitrogenation (HDN) reactions still remain unanswered. In this study, y-Al203-supported Ni, Mo, and bimetallic Ni-Mo catalysts were used for investigating the pyridine HDN and the simultaneous pyridine HDN/thiophene HDS reactions. The purpose of this study was to investigate the role of Ni as a promoter in the Ni-Mo catalyst system and to examine the effect of the presence of sulfur-containing compounds in the gas phase on HDN reaction. The results showed the C5 yield to be a strong function of the Mo loading regardless of whether Ni was added to Moor not, whereas the piperidine yield showed no dependence on Mo loading. The findings of this study, when combined with the catalyst characterization results, lead us to suggest that Mo active species promote hydrogenolysis of piperidine while Ni-Mo species enhance the hydrogenation of pyridine. The experimental results also show the enhancement effect of the presence of sulfurcontaining compounds on HDN, especially when Ni is combined with Mo.
Introduction The importance of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) in petroleum refining processes to prevent poisoning of hydrocrackingand reforming catalysts and to produce environmentallyacceptable liquid fuels is well recognized. Although past emphasis has been on HDS catalysis, a growing need to process heavy fuels and low-quality stocks, both of which contain higher concentrations of nitrogen compounds than light petroleum stocks, has brought increased attention to acquiring a better understanding of the HDN catalysts. Generally, sulfided Co-Mo and Ni-Mo catalysts supported on y-A1203 are used in the industrial HDS and HDN processes. There is a large volume of Iiterature on Co-Mo catalysts, and especially on their application in HDS processes, as summarized in several extensive reviews on the subject.'" Due to the intensive research done over HDS catalysts, the reaction mechanism is fairly wellunderstood and usually explained in terms of one of the four structural models for Co-Mo catalysts."' Compared to HDS reactions, the catalytic phenomena involved in HDN reactions are not as well understood, with most of the conclusions from HDS studies being extrapolated to HDN reactions, although recent years have seen some important contributions in this area.1151a11
* To whom correspondence should be addressed.
t Present address: Facultad de Ciencias Quimicas, Universidad Autonoma de San Luis Potosi, San Luis Potosi, S.L.P., Mexico. *Abstract published in Aduance ACS Abstracts, December 1, 1993. (1)Schuit, G. C. A.;Gates, B. C. AlChE J. 1973,19,417. (2)Grange, P. Catal. Reu.-Sci. Eng. 1980,21,135. (3)Topsoe, H.;Clausen, B. S. Catal. Reu.-Sci. Eng. 1984,26,395. (4)Prins, R.;Beer, V. H. J. Catal. Reu.-Sci. Eng. 1989,31,1. (5)Ho,T.C. Catal. Rev.-Sci. Eng. 1988,30, 117. (6)Chianelli,R.R.;Pecoraro,T.A.;Halbert,T.R.;Pan, W.-H.;Stiefel, E. I. J. Catal. 1984,86,226. (7) Yermakov, Yu. I.; Startaev, A. N.; Burmistrov, V. A,; Shumilo, 0. N.;Bulgakov, N.N. Appl. Catal. 1985,18,33. (8)Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991,30,2021. (9)McIlvried, H.G. Znd.Eng. Chem. Process Des. Deu. 1971,10,125.
0887-0624/94/2508-0249$04.50/0
Table 1. Surface Areas of Supported Catalysts surface area
1.0 2.0 5.0 7.0 10.0
1.0 3.0 5.0 7.0 3.0 3.0
7.5 10.0 15.0 20.0 15.0 15.0 15.0 15.0 5.0
10.0 ?'-&03
(m2M 190.38 185.89 183.37 184.97 183.34 190.25 193.81 183.35 165.21 178.19 183.93 166.49 161.93 189.29 189.50 190.52
The pathway for pyridine hydrodenitrogenation reactions, which was proposed by McIlvriedg and which consists of hydrogenation of pyridine to form piperidine and C-N bond cleavage of piperidine (hydrogenolysis) to form C5 hydrocarbons, is widely accepted. Because the reaction pathway in HDN is different than that of HDS, the nature of catalytic sites involved in HDN reaction is different than those involved in HDS reaction, with HDN catalysts having to perform the dual function of hydrogenation and hydrogenolysis.l&l2 There are still several unresolved questions being debated in the literature related to these phenomena. Although Ni-Mo and Co-Mo catalysts supported on ?-A1203 remain as two of the most widely studied catalytic systems for HDN reaction, the role of the promoter (Ni or Co) and the actual catalytic steps involved over these bi(10)Hadjiloizou, G. C.; Butt, J. B.; Dranoff, J. S. Ind. Eng. Chem. Res. 1992,31,2503. (11)Satterfield, C. N.;Modell, M.; Mayer, J. F. AlChE J . 1976,21, 1100. (12)Yang, S. H.;Satterfield, C. N. J. Catal. 1983,81,168.
0 1994 American Chemical Society
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250 Energy & Fuels, Vol. 8, No. 1, 1994
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metallic catalysts are far from being resolved. While some researchers proposed a strong synergistic effect associated with the promoter,13J4 others reported no change in catalytic behavior due to the presence of the second metal.l5J6 Stanislaus et al." reported addition of Ni resulting in the formation of different acidic sites whereas Ledoux and Djelloulil4 found the synergistic effect of adding Ni to be limited to the presence of sulfur and concluded that addition of Ni promoted adsorption of undissociated, strongly acidic hydrogen sulfide which (13) Satterfield,C. N.;Modell,M.; Wilkens,A.Ind.Eng. Chem.Process Des. Dev. 1980, 19, 154. (14) Ledoux, M. J.; Djellouli, B. Appl. Catal. 1990, 67, 81. (15) Sobczak, J.; Vit, Z.; Zdrazil, M. Appl. Catal. 1988, 45, L23. J.;Cillo,D.L.Prepr.-Am. (16) Tischer,R.E.;Narain,N.K.;Stiegel,G. Chem. Soc., Diu. Petr. Chem. 1985, 30, 459. (17) Stanislaus, A.; Absi-Halabi,M.; Al-Dolama,K. Appl. Catal. 1989, 50, 237.
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Figure 2. Effect of temperature and nickel loading on (a) pyridine conversion rate, (b) piperidine yield, and (c) Cg yield over single-metal (Ni) catalysts. increased the rate of hydrogenolysis. Kherbeche et a1.18 found the hydrogen treatment temperature to have a strong effect on the behavior of Ni as a promoter. They suggested another reaction pathway where Ni sites promoted pyridine conversion to C5 directly, in addition to the pathway proposed by Mc1lvried.S Perot et al.19 found that a synergistic effect existed when hydrogenation was the rate-limiting step. Sobczak et al.15 proposed that the synergistic effect in HDN over sulfided bimetallic catalysts was not a general phenomenon as has been found in HDS and occurred only under special conditions. However, exactly what these conditions are still remains unclear. (18) Kherbeche, A,; Hubaut, R.; Bonnelle, J. P.; Grimblot, J. J. Catal. 1991, 131, 204. (19)Perot, G.; Brunet, S.; Hamze, N. In Proceedings of the I X International Congress on Catalysis, Calgary; Philips, M. J., Teman, M., Eds.; The Chemical Institute of Canada: Ottawa, 1988; Vol. 1, p 19.
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Figure 3. Effect of temperature and nickel loading on (a) pyridine conversion rate, (b) piperidine yield, and (c) CSyield over bimetallic (Ni-Mo) catalysts. Another important question that requires further study is the role of the presence of hydrogen sulfide or other sulfur-containingcompounds such as thiophene on HDN. Although it has been discussedextensively in the literature, some of the results are still controversial and most of the explanations lack supporting evidence from in situ or postreaction characterization of catalysts. Satterfield et al.13 reported that HDS is severely inhibited by the presence of nitrogen compounds, while thiophene exhibits a dual effect on pyridine HDN. At low temperatures, due to the competitive adsorption, the pyridine HDN is moderately inhibited; a t high temperatures, on the other hand, the rate of hydrogenolysis of C-N bond cleavage is enhanced by H2S formed by the HDS reaction. LaineZ0 (20) Laine, R. M. Catal. Rev.-Sci. Eng. 1983, 25, 459.
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Figure 4. Effect of temperature and molybdenum loading on (a)pyridine conversion rate, (b)piperidine yield, and (c) Ca yield over bimetallic (Ni-Mo) catalysts. proposed that HDN is enhanced by hydrogen sulfide because ring opening is promoted through a nucleophilic attack by HzS. Nelson and Levyz1 suggested that C-N bond cleavage involves Hofmann degradation which is promoted by H+formed by HzS dissociated on the catalyst surface. Satterfield et al.l3 summarized the enhancement of piperidine hydrogenolysis by sulfur-containing substances in several ways-by maintaining the catalyst in a fully-sulfided state, by improvingthe acidity of the catalyst itself, and by reducing the adsorptivity of the basic nitrogen compound on the catalyst. It is apparent that there remain several questions regarding the structure and catalytic performance of an effective HDN catalyst which would be able to balance the dual functionality involved in hydrogenation and (21)Nelson, N.; Levy, R. B. J. Catal. 1979,58, 485.
252 Energy & Fuels, Vol. 8, No. 1, 1994
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Figure 5. Variation of pyridine conversion rates with temperature and catalyst composition in the presence of thiophene. Catalyst Characterization. Surface area was measured by the BET method (Micromeritics 2100E Accusorb instrument) using nitrogen as the adsorbate. Laser Raman spectra were obtained using a SPEX 1403 h a l o g 9-1 spectrometer. The excitation source was the 514.5-nm line of an argon ion Laser (SpectraPhysics,Model2016). X-raypowder diffractionpatterns were obtained by a Scintag PAD-V diffractometer with Cu K a radiation (A = 1.5432 A) as the X-ray source. A Physical Electronics/Perkin Elmer (Model 550) ESCA/Auger spectrometer, operated at 15kV and 20 mA with Mg Ka radiation (1253.6 eV), was used to obtain X-ray photoelectron spectra of the samples. TPD experiments were carried out using helium as the carrier gas with a flow rate of 50 cm3(STP)/min. The heating rate was 5 OC/min. Postreaction characterization was performed through XPS and in situ TPD/TPR experiments. For XPS studies, spectra were obtained by transferring the contents of the reactor to a specially designed sample chamber within the spectrometer without exposing them to atmosphere. Details of catalyst characterization have been reported elsewhere.22 Reaction Studies. Presulfided Ni, Mo, and Ni-Mo catalysts with different ratios of Ni to Mo were used for performing HDN Experimental Section and simultaneousHDS/HDN reactionexperiments. The reaction was carried out in a 4 mm i.d. stainless steel reactor. The total Catalyst Preparation. Catalysts were prepared by wet cosurface area of the catalyst in the reactor was kept constant at impregnationof alumina(y-Al2O3)(Harshaw-Filtre)with aqueous solutions of ammonium heptamolybdate [(NHd~Mw0~4Hz01 50 m2 which gave a catalyst bed length of about 6 mm. The reactor temperature was measured with a type K thermocouple (Fisher) and nickel nitrate [Ni(N0&6HzO] (Mallinckrodt). welded to the external side of the reactor in the middle of the (NHdOH and HNOs were used to control the pH value during catalyst zone. The reactor was heated inside a tube furnace impregnation. After impregnation and drying under vacuum, (Lindberg,Model 55035)equipped with a temperature controller catalysts were calcined under a steady flow of oxygen at 500 O C (Eurotherm, Model 847). The reaction temperature was varied for 4 h. Catalystswere loadedinto the reactor without pelletizing. in the 320-400 OC range. The reaction pressure was maintained Before reaction, the catalysts were presulfided in situ at 400 "C at lo0psig for all reactions. Thehydrogen flowrate was controlled under a flow of 10% H&H2 for 10 h. The catalyst bed was, by a Tylan mass flow controller (ModelFC-280) at 70 cm*(STP)/ then, flushed with helium for 20 min before the reactants were min. The concentrations of pyridine, thiophene, and pyridine/ introduced. The catalyst compositions are reported as weight percentages of the oxide precursors, i.e., Moo3,NiO, following (22) Ozkan,U. S.;Ni, S.; Moctezuma,E.; Zhang,L.J.Cat& submitted for publication. the practice commonly used in HDS literature.
hydrogenolysis steps. In this study, y-Al203-supported Ni, Mo, and bimetallic Ni-Mo catalysts with different ratios of Ni/Mo were prepared to perform pyridine HDN, thiophene HDS, and simultaneous pyridine HDN/ thiophene HDS studies at different reaction conditions. The catalysts were characterized using surface area measurement, X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, temperatureprogrammed reduction (TPR), and temperature-programmed desorption (TPD) techniques. The effect of the catalyst composition, the reaction temperature, and the mutual interactive effect of the presence of sulfur and nitrogen compounds on catalytic activity and selectivity have been examined. Reaction data have been combined with in situ and postreaction characterization of the catalysts to acquire a better understanding of the phenomena involved in simultaneous HDN/HDS reactions.
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Figure 6. Comparison of Cg yields with and without thiophene in the gas phase (2' = 320 "C).
thiophene in the feed were adjusted by passinghydrogenthrough abubbler andcarefdycontrollingthetemperatureofthebubbler. The inlet thiophene and pyridine concentrations were kept constant at 0.87% each in Hz for all reactions. In experiments which involved pyridine hydrodenitrogenation in the presence of H a , the H2S concentration was also kept at 0.87 % . The reactant flow rate per unit surface area of the catalyst was 0.54 pmol/(m2-min). In reaction experiments where the effect of pyridine on thiophene HDS was examined,the flow rate per unit catalyst surface area was increased to 3.5pmol/(m2.min)to avoid complete conversion of thiophene. The feed and product streams were analyzed by an on-lineHP 5890 Agas chromatographwith both thermal conductivity (TCD) andflame ionization(FID)detectors. A60/80CarbopackB/4%Carbowax 20M/0.8% KOH column (8 ft. long and 1/8in. 0.d.) and a 60/80 mesh Hayesep Q column (2 ft. X 1/8 in.) in series with a 80/100mesh Carbopack C/O.19% picric acid column (18 ft. X 1/8 in.) were used to separate the products. The carboncontainingproducts produced by HDN and HDS/HDNreactions were C1,Cz, Ca, C,, Cghydrccarbons,and piperidine. Pentylamine, which was suggested as an intermediate in HDN reaction, was not detected in these experiments. The reactant conversionrates and product yields are expressed in units of pmol/(m2.min).The pyridine conversion levels were in the range of 0-50%. All reaction data were taken 4 h after the reactants were introduced into the catalyst bed. Thesteady-state conditions were achieved in less than 3 h and the criterion for steady state was established through monitoring the product distribution and the conversion level.
Results Catalyst Characterization. The BET surface areas of the catalyst samples used in this study are presented in Table 1. The surface areas of catalysts varied in the
range of 162-195 mZ/g. The X-ray diffraction patterns of NiO, MoO3, and NiO-Moo3 supported on ?-A1203 showed the strongest peaks of alumina diffraction pattern. Only 20% Moo3 showed an extra peak at a d spacing of 3.790 A which correspondsto the second strongest peak of Moo3 in JCPDS Files (3.810 A), indicating the formation of crystalline molybdenum trioxide at this loading level. Laser Raman spectra of the oxide samples showed the presence of crystalline MOOSfor 15 and 20 wt % singlemetal Mo catalysts through the appearance of bands at 820 and 996 cm-l. No crystalline Moo3 was detected for 3 % Ni0-15 % MOOScatalyst. However, the presence of surface polymolybdates was apparent in all these three samples as evidenced by a band around 952 cm-1. X-ray photoelectronspectra of sulfided catalysts showed that Ni in Ni-Molr-AlzO3 system existed in a different coordination environment from the supported bulk nickel sulfide. On the other hand, the XPS spectra of Mo in the bimetallic catalyst appeared to be similar to those obtained for supported MoSz catalyst. The details of characterization results have been presented elsewhere.% Reaction Studies. The effect of Moo3 loading on HDN without any sulfur compound present in the gas phase is presented in Figure 1. The results show that pyridine conversion rate and Cg yield increase with the Moo3 loading. The loading of Moo3 seems to have no evident effect on the piperidine yield. Also, shown in this figure is the effect of temperature on product distribution. While the Cg yield increases with increasing temperature at every loading level, the piperidine yield showsjust the opposite behavior, with highest piperidine yield being obtained at the lowest temperature used.
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254 Energy &Fuels, Vol. 8, No. 1, 1994
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5.0 10.0 15.0 5.0 7.0 NiO, w t % (15ZMo03) Mo03,wt.% (3ZNi0) Figure 7. Comparison of Csyields with and without thiophene in the gas phase (T= 360 "C).
3 .O
The effect of NiO loading on conversion and product distribution is presented in Figure 2. Pyridine conversion rate and C5 yield increase with the loading level although this increase seems to level off at higher loading levels. The higher temperatures seem to favor C5 formation whereas piperidine yield is higher at lower temperatures, suggesting the presence of thermodynamic equilibrium between pyridine-piperidine as reported earlier by Cocchetto and Satterfield.23 A comparison of Figures 1and 2 also shows molybdenum catalysts to be much more active than Ni catalysts in pyridine HDN. The results presented in Figure 3 are obtained by varying the NiO content at a constant loading level of 15% for Moos. At 320 "C, the addition of Ni increases the piperidine yield but has negligible effect at 360 and 400 "C. Figure 4 shows the effect of MOOSloading on HDN ' . The activity when NiO loading is kept constant at 3 % results exhibit similar trends to the ones seen in Figure 1such that the pyridine conversion and C5 yield increase with the loading of Moo3 while the piperidine yield rate shows little change with the changing Moo3 loading. A comparison of the results obtained over molybdena catalysts and bimetallic catalysts shows that the ratio of piperidine concentration to pyridine concentration in the product stream (CpipJCpp! increases while (Ccs/Cpip,r) decreases when Ni is combined with Mo. Figure 5 shows the effect of temperature and catalyst loading on pyridine conversion activity when there is thiophene present in the gas phase. One important point to note about these results is the fact that when there is (23) Cocchetto, J. F.;Satterfield, C.N. Jnd. Eng. Chen. Process Des. Dev. 1976, 15, 272.
thiophene present in the gas phase, piperidine completely disappears from the product stream, making the CSyield equal to the pyridine conversion rate. Monometallic Ni and Mo catalysts are less active for pyridine conversion than bimetallic catalysts. When monometallic catalysts are compared between themselves, it is seen that Mo catalysts are more active than Ni catalysts. Since in this case the pyridine conversion equals the C5 yield, this fact implies that monometallic Mo catalysts are more active than Ni in hydrogenation of pyridine. The HDN activity appears to be favored by higher laoding levels of Moo3 whereas it goes through a maximum with increasing nickel loading levels. It is also interesting to note that, while the activity increases with temperature over the monometallic catalysts, it goes through a maximum with increasing temperature over the bimetallic catalysts. Figures 6-8 present a comparison of the C5 yields in pyridine hydrodenitrogenation in the absence and in the presence of thiophene in the gas phase over different catalyst compositions. For all catalysts, the presence of thiophene in the gas phase seems to favor the C5 yield. The difference appears to be more pronounced over bimetallic catalysts than over single-metal catalysts. The enhancing effect of thiophene in the gas phase does not seem to be as strong when pyridine conversion rates are compared. Another important observation to make about these results is that the Cg yield goes through a maximum with increasing nickel concentration in bimetallic catalysts in the presence of thiophene. Figure 9 shows the effect of sulfurcompounds (thiophene and H2S) on C5 yields. As seen in the figure, H2S has a similar enhancement effect on C5 yield as thiophene, but
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66 0.3-
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it is less effective than thiophene over the Ni-Mo catalyst, suggesting that the enhancement role of thiophene may not be entirely due to the H2S formed during the hydrodesulfurization reaction of thiophene. I t is also seen that nickel catalysts have the lowest activity whereas the bimetallic catalysts seem to be superior to both of the monometallic catalysts. Figure 10 shows the effect of temperature and catalyst composition on thiophene conversion activity a t atmospheric pressure. The trends observed are similar to the ones reported earlier in the literature, with a strong synergistic effect observed over the bimetallic catalysts. A high-pressure comparison (100 psig) of the thiophene conversion rates a t 400 "C with and without pyridine being present in the gas phase is shown in Figure 11. The results point out to considerable inhibition of the thiophene hydrodesulfurization reaction by the presence of pyridine.
Discussion In the experiments presented here, the pentylamine and cracking products were negligibly small, suggesting a simplified pyridine HDN reaction pathway similar to the scheme suggested by McIlvriedgwhere there are two main successivesteps. In the first step, pyridine is hydrogenated to form piperidine and in the second step, hydrogenolysis of piperidine or C-N bond cleavage takes place to form C g and ammonia. Since hydrogenationand hydrogenolysis steps take place consecutively, the performance of the HDN catalysts should be considered from the vlewpoint of this dual functionality. From pyridine HDN thermodynamicequilibriumdata," we find that, a t the relatively low reaction pressure, the ratio of piperidine partial pressure to pyridine partial pressure ( P p i p p r / P p y ) in the product stream is very close to the thermodynamic equilibrium ratio a t temperatures above 350 "C. As the reaction temperature gets lower, the P p i p r / P p p ratio gets further away from the thermodynamic equilibrium ratio. On the other hand, the P c 5 / P p i F r and Pcs/PPyrratios in the product stream are far from equilibrium. The experimental data suggest that, a t high temperature and in the absence of sulfur-containing compounds in the gas phase, the rate of hydrogenation of pyridine is relatively fast and the overall HDN rate is determined by the hydrogenolysisrate of piperidine. These observations are in agreement with some of the earlier reports in the literatureg which concluded that the hydrogenation rate is faster than hydrogenolysis. When Figures 1 and 4, which show the effect of Moo3 loading on pyridine HDN rate are considered, we find (24) Satterfield,
C. N.; Cocchetto, J. F. AlChE J .
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256 Energy & Fuels, Vol. 8, No. 1, 1994
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I
10.0 15.0 Mo03,wt.96 (3%Ni0)
Figure 10. Effect of temperature and catalyst composition on thiophene conversion rates. that the overall HDN conversion rate and the Cg yield are strongly dependent on MOOSloading regardless of whether Ni is combined with Mo or not, while the piperidine yield shows no dependence on Moo3 loading. On the other hand, as seen in Figures 2 and 3, increasing the loading level of NiO increases the piperidine yield at lower temperatures where the hydrogenation reaction is not controlled by thermodynamic equilibrium. This behavior suggeststhat sites associated with Mo species are responsible for the hydrogenolysis of piperidine whereas the nickel species seem to play a more major role in the hydrogenation step a t lower temperatures where this step is driven catalytically. Another important feature that stands out in the results is the presence of a strong synergistic effect for the hydrogenation step when Ni and Mo species are brought together. Again considering the low-temperature end where the thermodynamic equilibrium is not yet established, combining molybdenum and nickel species seems to promote the hydrogenation of pyridine to piperidine, whereas at the higher temperatures where the ratedetermining step is the hydrogenolysis of piperidine, no increase in activity is observed when nickel is added to the molybdenum catalysts. This leads us to suggest that there is no promotional effect of Ni-Mo synergy in the hydrogenolysis rate of piperidine. Based on the above discussions, it can be concluded that, in the absence of sulfur-containing compounds, unpromoted Mo active species promote hydrogenolysisof piperidine while Ni-Mo active species enhance the hydrogenation of pyridine. At high temperatures and low pressures, combination of Ni with Mo does not promote the HDN rate because the thermodynamic equilibrium
limits the formation of piperidine and the rate-determining step is the hydrogenolysisof ~ i p e r i d i n e The . ~ ~ conclusion that the synergistic effect of adding Ni is in operation only when the hydrogenation is the rate-limiting step is also in agreement with the results reported by Perot et al.19 for other nitrogen-containing compounds. The fact that, in our pyridine HDN experiments, no piperidine was detected and that the Cg yield increased in the presence of thiophene or H2S (Figures 6-9) implies that thiophene or HzS enhances the hydrogenolysis of piperidine significantly. Another conclusion to be drawn from this observation is that, in the presence of sulfur compounds, the rate-limiting step is no longer the hydrogenolysis of piperidine, but probably the hydrogenation of pyridine, especially at higher temperatures. The results in Figures 6-8 also show that the enhancement effect due to the presence of thiophene observed on the HDN activity is much more pronounced when Ni is combined with Mo than when Mo is used alone. This fact indicates that the presence of thiophene may also have an enhancement effect on the hydrogenation step, since the higher overall HDN activity of the Ni-Mo catalyst compared to the monometallic Mo catalyst is due to the promoting effect of Ni on the hydrogenation step. Moreover, the results in Figure 9 show that, over the Ni-Mo catalyst, thiophene increased the HDN activity more than H2S did. The above findings imply that, under the conditions of our reaction studies, the enhancement effect of thiophene on the HDN rate is not only through H2S formed during the HDS of thiophene but also because of the interaction between thiophene and Ni-Mo synergistic species. This is supported by our postreaction XPS measurements which provided a better insight into the
Simultaneous HDS and HDN of Model Compounds
Energy & Fuels, Vol. 8, No. 1, 1994 257
0.50 0
4.01
HDSonly
X
HDS only HDS/HDN
0.40 3.0
-
0.30
-
2.0
0.20 0.10
-
0.00
1.0
/ I
I
I
5.0
7 .O
9.0
-
0.0 11.0
5.0
10.0
15.0
20.0
MoO3,wt.%
NiO, wt. % 5.0 El
HDS only
x
HDS/HDN
e
a
n
a
4.0
X
HDS only HDS/HDN
t
I
Q
3.O
2.0 1.o
1.o
0.0 3 .O
5 .O 7.0 NiO, wt.% (1596MO03)
I
5.0
10.0 15.0 MoO 3, wt,% (396NiO) Figure 11. Comparison of thiophene conversion rates with and without pyridine in the gas phase (T= 400 "C). relationship between HDN activity and the sulfidation level of the catalyst.22 The XPS results showed that thiophene leaves the Ni-Mo catalysts in a more "fullysulfided" state than H2S does. Although from our results it seems that a competitive adsorption between thiophene and pyridine has no inhibition effect on the HDN rate over the Ni-Mo catalysts, this competition seems to hinder the HDS rate significantly, suggesting that although both pyridine and
thiophene may be adsorbing on the same type of sites, the initial catalytic steps leading to HDN and HDS are different.
Acknowledgment. Financial support provided by the National ScienceFoundation through Grant EID-9023778, by the Exxon Corp., and by AMAX is gratefully acknowledged.
