Energy & Fuels 19948, 435-445
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Oxidative Regeneration of Spent Molybdate and Tungstate Hydrotreating Catalysts Y. Yoshimura,. T. Sato, H. Shimada, N. Matsubayashi, M. Imamura, and A. Nishijima National Institute of Materials and Chemical Research, Tsukuba 305,Japan
S. Yoshitomi Shibaura Institute of Technology, 3-9-4,Shibaura, Tokyo 108,Japan
T. Kameoka and H. Yanase Catalysts & Chemicals Industries Co., Ltd., Tokyo 108, Japan Received September 17, 1993. Revised Manuscript Received December 21, 1999
The oxidative regeneration of spent cobalt-molybdate and nickel-tungstate catalysts from hydrotreatment of petroleum vacuum gas oil and coal-derived oil was carried out in a fixed bed reactor. Temperature-programmed oxidation studies revealed that oxidation proceeded mainly in two steps (i.e., removal of sulfidic sulfur as SO2 around 500-600 K and removal of carbon as CO2 (CO) around 650-850 K). Carbonaceous materials on the spent NiW catalyst were less aromatic than those on the spent CoMo catalyst, but more severe oxidation conditions were needed for the NiW catalyst because of the lower oxidation activity of NiO and WO3 compared with Co304 and MOOS. For the NiW catalyst, EXAFS data revealed that WS2-like structures, which were laterally grown during the hydrotreatment run, were redispersed to nearly the same level as that of the fresh catalysts when carefully controlled oxidizing conditions were used @(02) = 1.5%). XPS data showed that surface compositions of Ni and W were recovered to almost the level of fresh catalysts, but the Ni/W ratio was slightly less than that of the fresh ones. Catalytic activities and selectivitieswere successfully recovered by low-temperature oxidation. On the contrary, for the CoMo catalyst on which MoSz-like sulfides were laterally grown, some of the Co aggregated to CosSg, and small amounts of Ni, Fe, and V were deposited, it was not possible to recover the same level of structural properties as those of the fresh catalysts. The catalytic activities and selectivitieswere almost recovered by low-temperature oxidation, while at higher regeneration temperatures there was a slight loss of hydrogenation activity and a large increase in the hydrocracking activity. The thermodynamic analyses were done to better understand the structural changes of the active components and their possible interactions with the deposited metals and the yA1203 support during oxidation and subsequent sulfidation.
Introduction Cobalt-molybdate catalysts are commonly used for residue and distillate hydrodesulfurization (HDS) because of their high activity for C-S hydrogenolysis with low hydrogen consumption. Nickel-molybdate catalysts are used for hydrogenation (HYG)and hydrodenitrogenation (HDN) oriented reactions. Tungstate catalysts are used for the hydrotreating/hydrocrackingof distillates containing low concentrations of sulfur. During use, the catalytic activity of these catalysts decreases due to carbonaceous and metal deposits as well as due to modification of the active metal sulfides by agglomeration and changes in their chemical state. Oxidative regeneration may be feasible for spent catalysts containing relatively small amounts of metal deposits. For severely deactivated spent catalysts containing large amounts of metal deposits, such as Ni, Fe, and V sulfides, an oxidative treatment may be necessary after the metal leaching/ recovery processes. Regeneration studies1I2have been reported that elucidated the kinetics of the carbon and sulfur removal Abstract published in Advance ACS Abstracts, February 1, 1994. (1) Mamoth, F. E. Fuel Process. Technol. 1981, 4 , 63.
mechanisms of spent molybdate catalysts, which are used for hydrotreating petroleum distillates and coal-derived oils. A temperature-programmed oxidation (TPO)technique was successfully utilized to characterize coke on the spent catalysts and to characterize sulfur on the freshly sulfided catalysts (sulfidic sulfur) and spent catalysts (sulfidic sulfur and organic sulfur).' Coke on the hydrotreating molybdate catalysts is less refractory than that on the FCC catalyst' and it had a higher H/C atomic ratio than those of heavy ends in a feedstock hydrocarbon2 due to the hydrogenation function of molybdate catalysts. Nalitham et al.3 formulated a kinetic model for removal of carbon, in which two types of coke (reactive and nonreactive cokes) were assigned depending on the oxidizing rates. The oxidation kinetics were dependent on the aging history of the catalysts, in particular the feed properties and processing conditions. On the contrary, little work was done to determine kinetic approaches for sulfur removal from spent hydrotreating catalysts. TPO methods were applied to characterized the sulfur removal (2) Furimsky, E.; Yoshimura, Y. Znd. Eng. Chem. Res. 1987,26,657. (3) Nalitham, R. V.; Tarrer, A. R.; Guin, J. A.; Curtis, C. W.Znd. Eng. Chem. Process Des. Dew 1986,24,160.
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behavior (non-steady-state behavior). From the TPO experiments of fresh sulfided CoMo/AleOs and NiMol A1203 catalysts, Yoshimura et a1.415 showed that sulfidic sulfur was removed mainly in two steps. A primary peak at around 423 K resulted from oxidation of sulfidic sulfur of MoSz and CogS8(Ni&), whereas the second peak at around 723 K resulted from oxidation of unreacted sulfides that were covered with Mo and Ni(Co) oxides and Ni(Co) sulfates. Interestingly, for the CoMo catalyst, the second SO2 peak disappeared after complexing agents were used in the impregnating solutions.6 Metal oxides such as Moo3 and V2O5 (if V was deposited), which were converted from the respective sulfides, would catalyze the carbon rem0val.I On the other hand, characterization studies have been done to elucidate the structural changes that occur with the active metals and supports during oxidative regeneration. Mo sulfides agglomerated during the hydrotreatment run, but the agglomerated Mo sulfide phases respread as Moo3phases over the 7-A1203surface during oxidative regeneration.8 Artegaet al.9showedthat Mo and Co phases on the spent CoMo/AlzO3catalyst were redispersed after regeneration and their surface concentration was enhanced compared with that of the fresh sulfided catalysts. Some of the Co tended to react with the ?-A1203 support to form CoA1204 above 773 K, which resulted in a reduction of the number of the active sulfided Co2+sites.1° After regeneration, formation of the sulfate species was indicated. The amount of sulfur remaining after oxidative regeneration can be reduced by using a low partial pressure of oxygen in the oxidizing g a ~ . If ~ spent ! ~ catalysts containing relatively small amounts of metal deposits were oxidized, we need to better understand the complex interactions among the deposited metals, the supported metals, and the 7-A1203support to qualify the regenerated catalysts. The sulfur removal mechanism was related to the structural changes that occur with the active metals during TPO of the fresh sulfided molybdate catalyst^.^,^ Tungstate catalysts are used for the distillates fraction containing no metal-containing compounds and a low amount of sulfur-containing compounds. The commercial utilization of the W catalysts, which are more expensive than Mo catalysts, is quite limited so far. Recently, we showed that NiW/A1203 catalysts showed higher HYGand HDN activities than NiMo/AlzOa catalysts in upgrading coal-derived oils containing a low amount of su1fur.l' Therefore, the recycle use of the spent NiW/A1203 catalysts via convenient oxidative regeneration might be useful in decreasing the catalyst cost. However, few studies have been reported on the oxidative regeneration of the spent NiW/A1203 catalysts. If the differences in the oxidation behavior between Mo and W spent catalysts were elucidated, and if we can better understand the regeneration (4)Yoshimura, Y.; Mataubayashi, N.;Yokokawa, H.; Sato,T.; Shimada, H.; Nishijima, A. Ind. Eng. Chem. Res. 1991,30, 1092. (5) Yoshimura, Y.; Yokokawa, H.; Sato,T.;Shimada, H.; Mataubayashi, N.; Nishijima, A. Appl. Catal. 1991, 73,39. (6) Yoshimura, Y.; Mataubayashi, N.; Sato, T.;Shimada,H.; Nishijima, A. Appl. Catal. 1991, 79,145.(7)VanDoorn, J.;Bosch,J.L.; Bakkum,R. J.; Moulijn,J. A. In Catalyst Deactiuation: Delmon, B., Froment, G. F., Eds., Elsevier: Amsterdam, 1987,p 391. (8)Sajkowski,D. J.;Pacheco, M. A.; Fleisch,T. H.; Meyers, B. L.Proc. 9th Int. Congr. Catal. 1988,1, 223. (9)Artega, A,; Fierro, J. L. G.; Delannay, F.; Delmon, B. Appl. Catal. 1986,26,227. (10)Artega, A,; Fierro, J. L. G.; Grange, P.; Delmon, B. Appl. Catal. 1987,34,89. (11)Nishijima, A.; Kameoka, T.; Yanase, H.; Sato, T.; Yoshimura, Y.; Shimada, H.; Mataubayashi, N. R o c . Int. Conf. Coal Sci.1991, 759.
Yoshimura et al. Table 1. Properties of the Fresh and Spent Catalysts. catalyst properties A B 4.2 NiO (wt %) coo (wt %) 4.5 MOO3 (wt %) 16.9 29.0 wo3 (wt %) 200 177 SA X 103 (m2/kg) 5.8 4.2 PV X lo4 (m3/kg) Trilobe 7.9 diameter X 104 (m) spent catalyst Ab B Properties of Carbonaceous Deposita 16.5 5.3 C (wt %) 0.85 0.51 H (wt %) 0.62 1.16 H/C 0.017 0.067 N/C 0.87 0.68 fa 130 141 SA X 103 (m2/kg) PV X 104 (mS/kg) 3.0 3.1 S A = surface area, PV = pore volume. fa = carbon aromaticity measured by 13CNMR. Amount of deposited metals: NiO = 0.06 wt %, V206 = 0.19 w t %, and Fez03 = 0.09 w t %.
behavior according to the same concepts, independent of the species of active components, supports, and deposited metals, it will be very useful to optimize the regeneration methods. The objectives of our work reported here were to elucidate the differencesin the mechanisms occurringwhen Mo and W spent catalysts are oxidized as well as to elucidate the structural changes that occur in the supported metals during oxidation and subsequent sulfidation. Furthermore, attempts were made to investigate the effectiveness of the thermodynamics to predict a priori the structural changes that occur in the supported metals and supports, as well as to understand some of the solidphase interactions that occur among the active metals, the deposited metals, and the 7-Al203 support during oxidative regeneration.
Experimental Section Catalysts. The properties of the fresh catalysts investigated in the present work are shown in Table 1. Catalyst A was a commercial catalyst that was used in a commercial plant to hydrotreat a vacuum gas oil. I t was used for 1 year under the usual operating conditions. Spent catalyst A contained metal deposits of NiO (0.06w t % ), V ~ 0 6(0.19w t %), and Fez03 (0.09 w t 96). No measurements were carried out for Na, Ca, Mg,and K. Catalyst B was prepared by the usual coimpregnation of 7-A1203using nickel nitrate and ammonium metatungstate. Catalyst B was used to hydrotreat coal-derived oil (CDO, gas oil fraction containing no ash). The time on stream was 973 h at 653 K under a Hz pressure of 11.8 MPa. The spent catalysts were extractedwithtoluene in a Soxhlet extractor. The properties of the spent catalysts are shown in Table 1. To evaluate the poisoning effect of metal deposition, catalyst A was doped with metals by impregnation with water-soluble salts such as NaN03, vanadium oxalate, and ferric ammonium citrate. The impregnated catalyst was calcined a t 773 K. The amounts of doped metals were as follows: NiO (1.0,2.0,4.0,8.0, and 14.0 wt %), Vz06 (1.0, 2.0,4.0,6.0,and 14.0 wt %), Fez03 (1.0,2.0,4.0, and 8.Owt %),andNaz03(0.5,1.0,2.0,4.0,and8.0wt%). Thefresh, regenerated, and metal-doped catalysts were sulfided with HzS/ Hz(5%/95%) gas prior to characterization and catalytic testa. Oxidative Regeneration. The regeneration experiments were carried out in a fixed bed reactor. A schematic diagram of the fixed bed system is shown in another report.2 A portion of catalyst (0.2-0.4g, 100-200 mesh) was mixed with quartz (2 g,
Oxidative Regeneration of Spent Catalysts 48-60 mesh) and was packed in a quartz reactor, and nitrogen gas (purity 99.999%) was flowed at room temperature for 10 min. Then the nitrogen gas was replaced by the oxidizing gas and the temperature increased at a heating rate of 5 K/min to 1023 K. The oxidizing gas contained nitrogen and oxygen, with oxygen levels of 1.5and 20 vol % . The compositionof the product gases was monitored using a quadrupole spectrometer. Quite small amounts of product gases were constantly withdrawn to a mass chamber, which was kept in the constant pressure of 1.33 X 1W Pa during TPO experiments. SO2 and C02 concentrations in the product gases were calibrated by using standard gases. To evaluate the catalytic activity of the regenerated catalysts, stepwise oxidationwas carried out at three different temperatures (details below), in which experiments catalyst were used as extrudate forms. Characterization of Catalysts. The EXAFS experiments were done at the Photon Factory of the National Laboratory for High Energy Physics in Tsukuba, Japan. The Mo K-edge and W Lm-edge measurements were done at room temperature in transmission mode using a channel-cut Si(311) double crystal monochromator at beam line BL-1OB. The Co K-edge EXAFS measurements were carried out at room temperature using a Si(ll1) double crystal monochromator. The synchrotron radiation rings was operated at 2.5 GeV and the beam current was between 240 and 350 mA. Photon energy was calibrated by using the pre-edge peak of Na2MoO4*2H20 at 2000.7 eV. The radial distribution function around Mo or Co was obtained by k3weighted Fourier transforms of the EXAFS data (Ak = 14.0 (3.15 < k < 17.15 k l ) for Mo, Ah = 14.0 (3.5 < k < 17.5 A-l) for W, and Ak = 10.0 (2.8 < k < 12.8 k 1 ) for Co). The phase shift and back-scattering amplitude (f(k))were obtained from the tables of Teo et al.12 The distance was finally corrected using standard samples: MoS2 (Sumitomo Metal Mining Co.) and WS2 (Nakaraitesku) for the Mo-S (W-S) and Mo-Mo (W-W) interatomic distances, Na2MoOr.2H20 (Nakaraitesku) for the Mo-Odistance, and COO(Nakaraitesuku)for the Co-0 distance. The Co-S and Co-Co distances were corrected using the Co-0 distance of COO. Detailed procedures for the EXAFS analysis are described elaewhere.13 The extrudate catalysts were pulverized to 100-200mesh and pressed into disks (1.0 cm diameter) in a glovebox under flowing nitrogen (purity 99.999 vol % ) in preparing the EXAFS samples (same as for the XPS samples). Sample disks for the EXAFS experiments were sealed in a polyethylene pouch in the same glovebox and mounted as such on the EXAFS sample holder. XPS measurements were performed using an XPS (PHI-5400MC). An XPS sample holder, on which a sample disk (pulverized sample) or three pieces of extrudates samples were mounted, was sealed in a polyethylene pouch in the same glovebox, and quickly transferred to the XPS degassing chamber out of that pouch. The sulfur and carbon contents of the sulfided, spent, and oxidized catalysts were measured by using sulfur and CHN analyzers. Activity of Regenerated Catalysts. The hydrogenation (HYG) and hydrocracking (HYC) activities of fresh, spent, regenerated, and metal-doped catalysts were evaluated by using the model compound diphenylmethane in a tube bomb reactor (volume 50 cm3) at 673 K under an initial hydrogen pressure of 6.9 MPa (0.3 g of catalyst and 10 mL of diphenylmethane). Reaction products were analyzed by using a gas chromatograph equipped with a capillary column. The HYG activity was calculated from the yields of dicyclohexylmethaneand benzylcyclohexane. The HYC activity was calculated from the yields of benzene and toluene. Thermodynamics. Chemical potential diagramslP16 were used to evaluate the stability/reactivity of the metal sulfides, (12) Teo, B.K.; Lee, P. A. J. Am. Chem. SOC.1979,101, 2815. (13) Mataubayashi, N.Ph.D. Thesis, Osaka University, 1986. (14) Yokokawa, H.;Kawada, T.; Dokiya, M. Denki Kagaku 1988,56 (9), 751. (15) Yokokawa, H.; Kawada, T.; Dokiya, M. J.Am. Ceram. SOC.1989, 72 (ll),2104.
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Figure 2. Profiles of SO2 andCO2formation during temperatureprogrammed oxidation. Spent catalyst B: SO2 (- - -), C02 (- -) and H20 (- - - -). Fresh catalyst B: SO2 (-). ~ ( 0 2 )= 20% and heating rate = 5 K/min. oxides, and sulfates during regeneration and subsequent sulfidation using the thermodynamic data base.17
Results and Discussion Temperature-ProgrammedOxidationof Spent Catalysts. T h e SO2 and CO2 formation trends during temperature-programmed oxidation of spent catalysts A a n d B are shown in Figures 1a n d 2, respectively. T h e SO2 formation trends for t h e respective fresh catalysts are also shown in these figures. SO2 Formation. For catalyst A, only one SO2peak was observed for t h e fresh catalyst, b u t after t h e hydrotreating run, t h e temperature for this peak shifted about 30K higher (549 K) and a second SO2 peak was observed at 707 K (Figure 1). T h e temperature for this second SO2 peak was close t o that for the primary CO2 peak (711 K). Two maxima were observed for H2O formation (573 a n d 710 K), where t h e temperature for t h e second H20 peak was almost the same as t h a t for t h e C02 peak. I n contrast, two maxima were observed for SO2 formation by the fresh and (16) Yokokawa, H.; Sakai, N.; Kawada, T.; Dokiya, M. J. Am. Ceram. SOC.1990, 73 (3), 649. (17) SOC.Calorim. Therm. Anal. Japan, ‘Thermodynamic Database
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spent catalyst B (Figure 2). After the hydrotreating run, the temperature of the primary SO2 peak shifted to about 30K higher (553 K) than that for the fresh catalyst (521 K), but little change was observed for the second SO2 peak. The temperature for this second SO2 peak was close to that for the primary COz peak (741 K). Two maxima were observed for H20 formation (565 and 740 K), where the ,temperature for the second H2O peak was almost the same as that for the C02 peak. The primary SO2peak originated mainly from oxidation of the MoS2- and WSz-like sulfides for fresh catalysts A and B, respectively. The second maximum for fresh catalyst B might originate mainly from the oxidation of tungsten sulfides which were not oxidized during the temperature range of the first maximum (discussed later). Formation of this second peak was accompanied with exothermic heat production, which was confirmed by the preliminary experiments using a TG/DTA, so there was little contribution from the decomposition of Ni sulfates, which is an endothermic reaction. The shift of the primary SO2 peak to higher temperature after the hydrotreating run for both the CoMo and NiW catalysts indicated that sulfur was less easily oxidized. However, it was uncertain whether carbonaceous deposits hindered the diffusion of oxygen to the metal sulfides and/or the size of the MoS2and WS2-like crystallites increased (i.e., agglomeration during hydrotreatment) to retard the oxidation. For spent catalyst A, the C02 formation was accompanied with a second small amount of SO2that was formed. This SO2 peak originates mainly from the oxidation of organic sulfur in the carbonaceous deposits (discussed later). COz and HzO Formation. During temperature-programmed oxidation, carbon was removed as C02 and CO via oxidative complex formation and subsequent decomposition.18 The temperature for the C02 peak of spent catalyst B was about 30 K higher than that of spent catalyst A, even though the carbon aromaticity of the carbonaceous deposits on the former catalyst was lower than that of the latter catalyst (Table 1). During oxidative regeneration of spent catalyst A, the amount of carbon removed as C02 up to a temperature of 1073 K was 1.1X mol/g. This was about 80% of the total amount of carbon measured by elemental analysis. The remaining 20% of the carbon might be removed as CO. For spent catalyst B (Figure 2), about 64% of the total carbon was removed as C02 and 36% of it as CO. This suggests that NiW catalysts have lower activity as oxidation catalysts than the CoMo catalysts. This difference in oxidation activities will be analyzed later by using thermodynamic analyses. Hydrogen in the carbonaceous deposits was removed as H2O during oxidation. Two maxima were observed for H2O formation (i.e., 573 and 710 K for spent catalyst A and 565 and 740 K (shoulderlike peak) for spent catalyst B). The second peak was almost the same as that for the C02 formation. This means that hydrogen in the carbonaceous deposits was converted to HzO faster than carbon was converted to COZ,which was a similar tendency to the hydrogen removal from coke on the FCC ~atalysts.'~ The preferential removal of hydrogen in the primary maximum region for spent catalyst B might be related to (18)Furimsky, E.; Yoshimura, Y. In Handbook of Heat and Mass Transfer; Cheremissinoff, N. P., Ed.;Gulf Publisher: Houston, 1988; Vol. 3, p 229. (19) Massoth, F. E.Ind. Eng. Chen. Process Des. Dev. 1967,6, 200.
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Figure 3. (top, left to right) Changes in XPS spectra of Mo 3d (a), Co 2p (b), and S 2p (c) for sulfided catalyst A during
temperature-programmed oxidation: (1) fresh sulfided, (2) oxidized up to 733 K, and (3) oxidized up to 773 K. (bottom,left to right) Changes in XPS spectra of W 4f (a), Ni 2p (b), and S 2p (c) for sulfided catalyst B during temperature-programmed oxidation: (1)fresh sulfided, (2) oxidized up to 293 K, and (3) oxidized up to 833 K. the lower aromaticity (Le., higher amount of aliphatic carbon and hydrogen) of carbonaceous deposits on spent catalyst B than that on spent catalyst A. Structural Changes in Active Components during Oxidation. XPS. The change in the chemical states of the active metals during oxidation were analyzed by XPS. Figure 3a shows the Mo 3d, Co 2p, and S 2p spectra for fresh sulfided (curve 1)and oxidized,fresh sulfided catalyst A (curve 2), which was oxidized up to 733 K @(02) = 20%) and subsequently quenched. The S 2p spectra for oxidized fresh-sulfided catalyst A, which was oxidized up to 773 K andsubsequently quenched, is also shown (curve 3). After sulfidation, Mo6+was converted to Mo4+and Mo6+ (mainly Mo4+),and cobalt into Cogs8 and Co2+ (mainly spinel structure). Almost all of the sulfur was sulfidic. After oxidation up to 733 K (curve 2), almost all of the Mo4+was converted into Mo6+and Co sulfide into Co2+. Almost all of the remaining sulfur existed as sulfate sulfur at this temperature, so these sulfates might originate mainly from Co sulfate. A small amount of S2+might originate from the small amount of unreacted Cogs8 and MoS2-like sulfides, but these S2+species disappeared at the oxidation temperature of 773 K and sulfate sulfur survived at this temperature. Figure 3b shows the W 4f, Ni 2p, and S 2p spectra for fresh sulfided (curve 1)and oxidized, fresh sulfided catalyst B (curve 21, which was oxidized up to 793 K @(02)= 20 % ) and subsequently quenched. The S 2p spectra for oxidized fresh-sulfided catalyst B, which was oxidized up to 773 K and subsequently quenched, is also shown (curve 3). After sulfidation, W6+ was converted to W4+and W6+ and nickel into Ni& and Ni2+(mainly spinel structure). Almost all of the sulfur was sulfidic. After oxidation up to 793 K (curve 2), almost all of the W4+was converted
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Figure4. Phase diagram for Mo-0-S and Co-0-S systems at 733 K Mo-0-S (-) and C0-0-S (- - -1.
Figure 5. Phase diagram for W-0-S and Ni-0-S systems at 793 K W-0-S (-) and Ni-0-S (- - -).
into We+, but a small amount of W4+still existed. This suggested that oxidation of the remaining W4+contribute to the secondary SO2 formation as shown in Figure 2. Almost all of the Ni sulfide was converted into Ni2+. Sulfidic and sulfate sulfur existed a t 793 K, so this sulfidic sulfur might mainly originate from W4+. These S2+species almost disappeared at 833K, but sulfate sulfur stillexisted. Thermodynamic Analysis of SO2 Formation and Structure Change. When we analyze the structural changes of dispersed phases during oxidation and sulfidation processes by using the thermodynamic data of the bulk phases, we should, strictly speaking, correct the equilibrium by taking into account the effect of the surface energy of the dispersed phases. However, there was little experimental data on the surface energies of the dispersed oxides and sulfides, and also little experimental data on the effect of particle size on the equilibria between sulfides and oxides (sulfates) during oxidation. On the contrary, from the equilibria between Ni and Ni& during the sulfidation, Okagami et al. reported that the equilibrium of the dispersed Ni phases on Kieselgur (0.5 nm Ni particles) shifted slightly from the bulk phase equlibria toward the equilibrium, thus, weaken the Ni-S bond strength20 (e.g., at 673 K log(p(HzS)/p(Hz)) = -3.9 for the dispersed phases and log(p(H2S)lp(H2)) = -4.2 for the bulk phases). However, the dependence of the equilibria on temperature were quite similar between the dispersed and bulk phases. According to the TEM analyses of sulfided catalysts A and B, the sizes of MoS2- and WS2like sulfides were around 5-15 nm, and these lateral sizes increased after hydrotreating runs (but no data on size of Ni and Co phases). Therefore, assuming that the oxidation tendency of the dispersed phases and bulk phases are similar that for sulfidation, we used the thermodynamic approach with bulk phase data for the regeneration of catalysts. Further elaboration of this thermodynamic approach that takes into account the particle size effects is required to increase the qualitative understanding. Figure 4 shows the phase diagram for the Mo-0-S and Co-0-S systems a t an oxidation temperature of 733 K. The y-axis indicates the partial pressure of SO2 and x-axis indicates the partial pressure of oxygen. Through this, we can understand the bulk phase structural changes
occurring during oxidation. If oxygen concentration in an oxidizing gas was 20% (Le., logp(O2) = -0.701, the log p(S02) value at 733K for fresh catalyst A was -4.7 (20 ppm of SO2 in Figure 1). This indicates that MoS2 and Co& in the sulfided catalyst are converted into Moos and CoS04, respectively. If we assume that there is an oxygen pressure gradient inside of the supported materials, then this graph suggests the coexistence of Cos04 and CsO4. Phase diagrams at 773 K also indicated that Cos04 survived even a t this temperature. Therefore, it was suggested thermodynamically that residual sulfate sulfur observed in Figure 3a might originate from Co sulfates. For regeneration of spent catalyst A, SO2 concentration was higher than that of fresh one at elevated temperature (e.g., 23 ppm of SO2 at 773 K, i.e., logp(SO2) = -4.6). This indicated that structural changes observed in oxidizing of spent catalyst A was similar to that of fresh catalyst A, if we did not take into account of the effect of the deposited metals on oxidation behavior. When we decrease the oxygen partial pressure in the oxidizinggas (i.e., decrease SO2 formation),theequilibrium shifta to left and down in the phase diagram. This indicates that the Cos04 formation will decrease with increasing CosOdCoO formation, i.e., decrease in residual amounts of sulfur remaining on the oxidized catalysts. Figure 4 also shows that the oxygen release from crystallite structures (i.e., from MOO3 to MoO2, and Cos04 to COO)will occur under a higher oxidizing atmosphere. Figure 5 shows the phase diagram for the W-0-S and Ni-O-S systems a t an oxidation temperature of 793 K. If oxygen concentration in an oxidizing gas was 20%, the log p(S02) value for the fresh catalysts B was -3.8 (176 ppm of SO2 in Figure 2). This indicates that WS2 and NiS/ Ni& are converted thermodynamically into WO3 and NiSO4, respectively. If we assume that there is an oxygen pressure gradient inside of the supported materials, then this graph suggests the coeaistence of NiSO4 and NiO. The phase diagram at 833 K also indicated that NiSO4 survived even a t this temperature. Therefore, it was suggested thermodynamically that residual sulfate sulfur observed in Figure 3b might originate from Ni sulfates. For regeneration of spent catalyst B, SO2 formation was similar to that of fresh one at elevated temperature. This indicated that structural changes observed in oxidizing of spent catalyst B was quite similar to that of fresh catalyst B thermodynamically, Le., decrease in the amount of
(2O).Okagami, A.; Hatano, Y.; Yemada, M.; Amano, A. Sekiyu
Gokkowhi 1990,33 (6), 360.
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440 Energy & Fuels, Vol. 8,No. 2, 1994
remaining NiS04. A decrease in t h e p ( 0 d in the oxidizing gas favors an increase in the formation of NiO. Oxygen release from W 0 3 crystallites occurs under more reducing conditions compared with Moo% Thermodynamic Analysis of COZFormation. Less aromatic and smaller amounts of carbonaceous materials were deposited on spent catalyst B than on spent catalyst A, but a higher oxidation temperature was necessary to remove the carbon from spent catalyst B. In Figures 4 and 5, the oxygen release from the W 0 3 crystal lattices was initiated around logp(0z) = -17. On the other hand, that from the Moo3 crystal lattice was around log p ( 0 z ) = -13 and that from the Co304 crystal lattice was around log p ( 0 z ) = -7. Taking into account the partial pressure gradients of 02 inside of the carbonaceous deposits and supported metal sulfides, which means different local equilibrium conditions than the log p ( 0 z ) = -0.7 value, the Cos04 and MOOSappear to act as oxidation catalysts at higher ~ ( 0 2 than ) do WO3 and NiO. This may be the main reason why we need more severe oxidative conditions for spent catalyst B than for spent catalyst A. Stepwise Oxidative Regeneration for Spent Catalysts. Based on the data from the temperature-programmed oxidations, and that on minimizing temperature runaway during oxidativeregeneration, the followingthreestage oxidation was adopted to evaluate the structural and catalytic properties of the regenerated catalysts.21i22 For spent catalyst A, the three steps were (1)573 K for 1h, (2) 673 K for 1h, and (3) at a set temperature in the range 713-773 K for 3 h. The heating rate was 5 K/min under an oxidizing gas of Oz/N2 (1.5%/98.5%), and the ratio of the total flow rate of oxidizing gas to the unit weight of catalyst was 1.0 L/(STP)g/min. For spent catalyst B, the oxidizing conditions were similar to those for catalyst A, except the third stage was done at a set temperature in the range 743-823 K for 3 h. In the first step, sulfidic sulfur was mainly removed, and in the third step, carbon was mainly removed. In the second step (Le., transition stage), less oxygen-accessible sulfidic sulfur and lighter fractions of carbonaceous deposits were removed. These regenerated catalysts were used for the characterization and catalytic tests after sulfidation. Characterization of Fresh, Spent, and Regenerated Catalysts. EXAFS. Figure 6 shows the Fourier transforms of the EXAFS spectra (Mo K-edge) for the freshsulfided, spent, oxidized (773 Kl-sulfided, and oxidized (673K) catalyst A. Two main peaks were observed at 0.24 and 0.32 nm for the sulfided and spent catalysts, which corresponds to the first Mo-S (0.241 nm) and Mo-Mo (0.316 nm) coordinations in MoSz crystallites, respectively. The intensities of both peaks and the peak at 0.64 nm (Mo-Mo bond in the second shell) were highest for the spent catalyst, which indicates that the size of the (002) basal planes of the MoSz-like crystallites increased during the hydrotreatment run.23 After oxidation (773K) of this spent catalyst and subsequent sulfidation, the agglomerated MoS2-like sulfides were redispersed, but the dispersion did not recover to the level of the fresh catalyst. On the contrary, after oxidation up to 673 K, a main peak was observed at 0.18 nm, which corresponds to the first (21) Tamayama, M. Kagaku Kogaku 1986,50 (9), 624. (22) Furimsky, E. Appl. Catal. 1988,44, 189.
(23) Nishijima, A.; Shimada,H.;Yoshimura,Y.; Sato,T.; Mataubayashi, N. Studies of Surface Science and Catalysis; Elsevier: Amsterdam, 1987; VOl. 34, p 39.
I
MO-S ,:
0
0.1
0.2
0.3
0.4
0.5 0.6
0.7
Distance (nm)
Figure 6. Fourier transforms of molybdenum EXAFS spectra for catalyst A (ka-weighted, Ak = 14.0 (3.16 k < 17.16 A-l)): fresh sulfided (-), spent (-), oxidized (773 K)-sulfided (- - -1, and oxidized (673 K) (- - -).
0
0.1
0.2 0.3 0.4 Distance (nm)
0.5
0.6
Figure 7. Fourier transforms of cobalt E M S spectra for catalyst A (k3-weighted, Ak = 10.0 (2.8< k < 12.8 A-l)): fresh sulfided (-), spent (- - -). Mo-0 (0.177 nm from NazMo04.2HzO) coordination of MoOa. No peak was observed at 0.24 nm, which might indicate that no MoSz crystallitesremained in this oxidized catalyst. This EXAFS data and also XPS data indicated that the secondary SOZ formation appeared in Figure 1 mainly originated from the oxidation of organic sulfur in the carbonaceous deposits. Figure 7 shows the Fourier transforms of the EXAFS spectra (Co K-edge) for the fresh-sulfided and spent catalyst A. For fresh catalyst A, the main peak was observed at 0.208 nm, which probably consists of the first Co-S (0.213,0.223 nm) and Co-Co (0.251 nm) coordinations for CqS824*26 and the first Co-0 (0.189 nm) coordination for CoA1204.26 The peak at 0.358 nm likely corresponds to that of the second Co-Co coordination of C09S8. The peak at 0.274 nm was close to that of the (24) Chiu, N.-S.; Johnson,M. F. L.; Bauer, S. H.J. Catal. 1988,113, 281. (25) Bouwens, S. M. A.M.; van Veen, J. A. R.; Koningsberger, D. C.; de Beer, V. H. J.; R. Prins, R. J . Phys. Chem. 1991,95, 123. (26) Yokoyama, Y.; Teranishi, K.; Nishijima, A.; Mataubayashi, N.; Nomura, M. Jpn. J. Appl. Phys. 1993,32,Suppl. 32-2,466. Shimada, H.;
Energy & Fuels, Vol. 8, No. 2,1994 441
Oxidative Regeneration of Spent Catalysts
Table 2. Surface Comporition of the Frerh, Regenerated, and Regenerated Suliided Catalyrtr
I i " ' 1 " " l ~ " ' l ' " '
w-s
catalyst A (CoMo)
fresh (S)a regen (733 K) regen (733 K)(S)
fresh (S) regen (793 K) regen (793 K)(S) a S = sulfided.
catalyst B (NiW) ti 'E a
e 0
0.1
0.2
0.3
0.4
0.5 0.6
0.7
AI 100 100 100
Mo CoIMo
S
Co 3.6 3.8 3.6 Ni
21.6 3.4 22.6
6.0 4.3 4.6
10.8 10.9 11.6
(27) Bouwens, 9. M. A.M.;de Beer V. H. J.; Koningsberger, D. C. J. Phys.Chem. 1980,94,3711. (28) Shimada,H.; Mataubayaehi,N.;Sa.to,T.;Yoehimura, Y.;Imamura, M.; Kameoka, T.;Yanase, H.;Nuhijima, A. Jpn. J. Appl. Phys.1998,92,
6.6 7.9 7.3
0.48 0.49
W
Ni/W 0.46 0.39
0.40
1
0
B q B
0.56
1.5
z 5
1
2.0
"O
- -).
Co-Mo (0.28 nm) coordination in the Co-Mo-S phases,27 but it was difficult to assign this peak based only on our experimental data. For spent catalyst A, the main peak became broader and the intensity of the peak a t 0.358 nm increased. Furthermore, a broad peak appeared at about 0.56 nm. This indicated that a relatively longrange order existed in the cobalt sulfide phases (Le., this suggests agglomeration of C09S8 during the hydrotreatment run). We could not measure the Co K-edge EXAFS of regenerated-sulfided catalysts, so we could not evaluate the redispersion of Co phase during regeneration. However, we can speculate that phase segregation (i.e., agglomeration of C09S8 and lateral growth of MoS2-like sulfides during the hydrotreatment run) might contribute to an irreversible dispersion of MoS2-like sulfides after regeneration and subsequent sulfidation. Figure 8 shows the Fourier transforms of the EXAFS spectra (W Lm-edge)for the fresh-sulfided,spent, oxidized (793 K)-sulfided, and oxidized (673 K) catalyst B. Two main peaks were observed at 0.24 and 0.32 nm for the sulfided and spent catalysts, which corresponds to the first W-S (0.241 nm) and W-W (0.315 nm) coordinations in WS2 crystallites.28 The intensity of both peaks and the peak at 0.64 nm (W-W bond in the second shell) were highest for the spent catalysts, which indicates that the size of the (002) basal planes of the WS2-like crystallites increased during the hydrotreatment run. After oxidation (793K) and subsequent sulfidation, the WS2 structure almost recovered to that of the fresh catalyst. On the contrary, after oxidation up to 673 K and subsequent quenching, a main peak was observed at 0.18 nm, which peak corresponds to the first W-0 (0.174-0.180 nm) coordination of WOa. A small peak was observed at 0.24 nm, which might indicate that a small amount of WS2 crystallites still remained in this oxidized catalyst. This EXAFS data and XPS data indicated that the secondary SO2 formation appeared in Figure 2 mainly originated from the oxidation of the remaining WS2-like sulfides. XPS. Table 2 shows the surface composition of the
Suppl. 32-2,463.
S 14.6 2.2 16.0
3.0
Dlstance (nm) Figure 8. Fourier transforms of tungsten EXAFS spectra for catalyst B (k*-weighted,Ak = 14.0 (3.6 C k C 17.6 A-l)): fresh sulfide (-), spent (e-), oxidized (793 K)-sulfided (- - -), and
oxidized (673 K)(-
AI 100 100 100
f
L
0
OS
1a
0
880
700
720
740
760
700
Temperature (K)
Figure 9. Effect of the fiial-stageoxidation tamperatureon the recovery of the catalyst activities of regenerated catalyst A and on the residual amount of carbon: catalyst activities were normalized to the activities of fresh catalyst A. fresh, regenerated (oxidic) and regenerated sulfided catalysts. The XPS spectra of the A1 28, S 2p, Mo 3d, W 4f, Ni 2p, and Co 2p peaks were measured for pulverized samples of the catalysts (Le., with an average composition between the exterior and interior of the extrudates). After sulfidation of the regenerated catalysts, the Co/Mo and Ni/W ratios did not recover to the level of the respective values of the fresh catalysts. Thii suggeststhat the amount of Co (Ni) located around MoS2 (WSz)-likesulfides might be less on the regenerated sulfided catalysts. The redispersion of Co relative to Mo was similar to Ni relative to W after oxidation and following sulfidation, which means that degree of recovery of the surface composition was similar for regenerated spent catalysts A and B in spite of a slight difference in the redispersion of MoS2- and WS2-like sulfides (as shown by the E M S data). The sulfur remaining on the regenerated catalysts originated mainly from Ni and Co sulfates. As shown in Figure 3b, a small amount of remaining WS2 contributed to an increase in the remaining sulfur for regenerated (oxidic) catalyst B. Evaluation of the Catalytic Recoverier of Regenerated Catalysts. Figure 9 shows the effect of the thirdstage oxidation temperature on the catalytic recovery and on the residual amount of carbon remaining on the regenerated catalyst A. The y-axis indicates the hydrogenation (HYG), hydrocracking (HYC), and conversion (conv) of diphenylmethane, in which activities were normalized to those of the fresh catalysts. Regenerated
442 Energy &Fuels, Vol. 8, No. 2, 1994
-
2.0
1
1
I
1
I
Yoshimura et
01.
11.0
e
PI 0.5 01
730
I
I
I
I
750
770
790
810
10
830
Temperature (K)
Figure 10. Effect of the final-stage oxidation temperature on the recovery of the catalytic activities of regenerated catalyst B and on the residual amount of carbon: catalyst activities were normalized to the activities of fresh catalyst B. catalyst A showed higher conversion than the fresh catalyst, and much higher hydrocracking activities were observed. The increase in HYC was significant around 710-730 K and approached an equilibrium value that was about 2.1 times higher than that of fresh catalyst. This suggests that some structural changes of the active components were dominant around 710-730 K. On the contrary, HYG recovery increased up to around 710 K, but thereafter HYG recovery decreased, which was compensated by the increase in HYC. For spent catalyst A, the HYG catalytic activity did not recover to the level of the fresh catalyst in contrast to an enhancement of the HYC activity, but the optimum temperature range for recovery of both of the catalytic activities was around 700720 K. This figure also suggested that the amount of residual carbon after oxidation would be at a level below 0.8wt 7%. Figure 10 shows the effect of the third-stage oxidation temperature on the catalytic recovery of regenerated catalyst B and the residual amount of carbon remaining on the regenerated catalyst. The conversions were almost recovered in these temperature ranges, but the recovery of HYG gradually decreased with increasing temperature. This was in contrast to the continual increase in the HYC activities. Because the HYG and HYC reactions were competitive in the diphenylmethane conversion, the increase in HYC relative to HYG indicated that the catalyst structures changed to more HYC active structures. The decrease in the surface Ni/W ratio for regenerated Catalysts might contribute to this because sulfided WO3 (29 w t 7% ) /A1203 catalyst (without Ni promotion) showed higher hydrocracking selectivity (HYC/HYG = 30). Therefore, an oxidation temperature around 750-770 K might be the most suitable for this catalyst. Figure 9 also suggests that the amount of residual carbon will be on the level of 0.7 wt 7% (i.e., it is not necessary to completely remove the carbon). This data suggests that if no metals were deposited on the spent catalysts, optimization of the oxidative conditions will be easier. The recovery of the catalytic activities of the spent catalysts that were oxidized under a partial pressure of oxygen of 20 vol 76 was lower than that with a partial pressure of 1.5 vol 7%. The exothermic heat production and sulfate formation, which resulted in irreversible structural changes, were significant
1
0
0
1.0
0
3.0
2.0
Metal/Mo (mol/mol)
Figure 11. Effect of metal doping on the hydrogenation activity (HYG)of catalyst A: Na (O),Ni (O), Fe (A),and V (V).
g
2.0
h
5!I 1.o . ..
L
0 , 0
Na
O
1.o
2.0
3.0
Metal/Mo (mol/mol)
Figure 12. Effect of metal doping on the hydrocrackingactivity (HYC) of catalyst A: Na (O),Ni (O),Fe (A),and V (V). under higher ~(02) conditions. These structurally irreversible changes might result in a lower catalytic recovery. Effect of Deposited Metals on Catalytic Recovery. As shown in Figure 9, regenerated CoMo catalysts showed higher HYC activity, but at the expense of less HYG activity. Assuming that some of these effects were caused by metal deposits, the specific metals were doped on the fresh CoMo catalyst to evaluate the effect of metal deposits on the catalytic activities. Figure 11shows the inhibitory effect of metal doping on the hydrogenation activity of catalyst A. The HYG activities were normalized to that of the non-metal-doped fresh catalyst. The x-axis indicates the amount of doped metals relative to Mo. The inhibitory effect was as follows: V L Fe > Na. Fe and V poisoned the HYG active sites significantly. Ni showed some promotion effect, however, which might be due to the formation of new hydrogenation-active NiMo-S phases. Figure 12 shows the inhibitory effect of metal doping on the hydrocracking activity of catalyst A. The HYC activities were normalized to that of the non-metal-doped
Oxidative Regeneration of Spent Catalysts
Energy & Fuels, Vol. 8, No. 2, 1994 443
Table 3. Composition of the Active Components and Deposited Metals on the Exterior Surface and the Interior of the Extrudates of Catalyst A position Al S Co Mo Fe V Ni Co/Mo V/Mo Fe/Mo Ni/Mo 100 0 4.1 8.8 0 0 0 fresh exteriop 0.47 0 0 0 4.0 8.6 0 0 0 100 0 0.47 0 0 0 (oxidic) interiorb 14.3 8.1 13.3 3.3 1.5 1.77 100 9.8 0.41 1.64 regen (693K) exterior 0.19 0.48 7.8 nil nil nil 100 2.8 3.8 (oxidic) interior 8.9 18.7 3.4 2.2 1.74 regen (773K) exterior 100 5.8 15.5 0.38 2.10 0.24 0.47 100 0.9 3.3 7.0 nil nil nil (oxidic) interior 0 Average composition at points A, B, and C in the extrudate. See text. Pulverized samples (i.e., average between the exterior and the interior).
fresh catalyst. The x-axis indicates the amount of doped metals relative to Mo. The inhibitory effect was as follows: Na >> Ni >Fe. Depending on the amount of doped metals, however, a promotion effect was observed as follows: V > Fe (Fe/Mo < 1.2) > Ni (Ni/Mo < 0.7). This indicates that the deposition of Fe and V on the CoMo/AlzO3 catalyst resulted in an additional hydrocracking oriented activities, but that the HYG activities were diminished. To estimate the contribution of metal deposits on the catalytic recoveries of spent-regenerated catalysts, on which about a 10 times lower amount of metal was deposited than that in the metal-doping experiments, the deposited metals were analyzed by an XPS. Table 3 shows the composition of deposited metals on the exterior surface of extrudates and on the interior surface (average between the interior and exterior surfaces) of extrudates of fresh (oxidic) and regenerated catalysts. A
After regeneration up to 693 K, the concentration of Co and Mo in the interior slightly decreased, but the Co/Mo ratio of the interior was almost the same as that of fresh oxidic catalysts. On the exterior surface, however, Fe, V, and Ni selectively deposited, where the amount of the deposits was in the order of Fe >> V > Ni. The higher concentration of Co at the exterior surface with relative less in the interior indicated the migration of Co toward the exterior surface. This might be related to the agglomeration of Co& during the hydrotreatment runs as shown by the EXAFS data in Figure 7. After regeneration up to 773 K, the concentrations of Co and Mo decreased to about 20% of those of the fresh catalysts, but the Co/Mo ratio at the interior showed little change. The concentration of Fe, V, and Ni as well as the active components of Co and Mo on the exterior surface increased slightly after increasing the oxidation temperature. The increase in the exterior surface concentration of Co and Mo with increasing oxidation temperature from 693 to 773 K was opposite to the decrease in their concentrations in the interior. This suggested that some of the Co304 and Moo3 in the interior migrated toward the exterior surface at 773 K. As shown by the S 2p XPS spectra, most of the sulfur originated from sulfate sulfur. The amount of sulfur remaining on the oxidized catalysts decreased with increasing temperature. The high amount of sulfur on the exterior surface might originate mainly from Co and Fe sulfates.
s
0 X
.-c0 ?!
.-0
E 1.0
c
m 0
z 3
0
Me/Mo (atomic ratio) (Theoretical) Figure 13. Surface composition of doped Fe and V components on the exterior and interior of the extrudate of catalyst A.
To correlate the concentration of deposited metals, particularly on the exterior, with the equivalent concentration of doped metals, the surface concentrations of Fe and V in metal-doped catalysts were measured by XPS (Figure 13). Because the inhibitory effect of Ni was small (as shown in Figures 11 and 12) and the deposited amount was low, no measurement was done for the Ni-doped catalysts. The x-axis shows the atomic Fe/Mo and V/Mo ratios calculated from the doped metal amounts, and the y-axis shows the measured values. For both of the metaldoped catalysts, the concentrations on the exterior surface were about 20 7% higher than those of the interior. From the V/Mo and Fe/Mo ratios on the exterior surface of the regenerated catalyst (773 K) shown in Table 3, the equivalent amounts of V205 and Fez03 on the CoMo/AlzO3 catalysts were about 5-6 wt 7% (i.e., V/Mo = 0.5-0.6 in Figures 11 and 12) and 12-14 wt 7% (i.e., Fe/Mo = 1.6-1.8 in Figures 11 and 121, respectively. Therefore, though the total amount of deposited metals on the regenerated catalysts was more than 10 times lower than those of the metal-doped catalysts, the exterior parts of the extrudates contribute the changes in the catalytic performances at the level of the doped metal amounts. It is very difficult to estimate the catalytic activities by taking into account the radial distribution of the deposited metals in the interior of the extrudate. However, if we assume the catalytic performances on the interior and exterior surfaces were additive, we can qualitatively explain the catalytic recoveries trends of the regenerated catalyst as related to the effect of metal deposition. According to studies on the temperature-programmed desorption of ammonia, the doping metal strongly affects
444 Energy & Fuels, Vol. 8, No. 2, 1994 201
,
1
,
I
Yoehimura et al.
I
10
8
.
% 6
U al
m
-m0
4
2
I1
-25
-20
-15
II
1
-10
I
1
I
-5
0
5
0
log P ( 0 2 ) (atm)
Figure 14. Phase diagram for the Mo-Fe-0-S system at 773 K:
log p(SO2)latm = -5.
the acidity of metal-doped NiMo/AlzOa~ a t a l y s t s .Alkali ~ and alkaline earth metals decreased the amount and strength of the acid. On the contrary, V and Fe increased the amount and strength of the acidity. This can be attributed to the formation of double oxides/oxisulfides. These solid-phase interactions to form new acidic sites probably occur between Mo and doped metals, so similar tendencies are expected for the metal-doped CoMo/AlnO3 catalysts. Thermodynamicsoft he Interactionbetween Active Components and Deposited Metals. Because the concentration of the deposited metals was quite small, we did not get clear evidence for formation of double oxides between the deposited metals and the active components, and between the depositedmetals and the 7 - A 1 2 0 3 support, particularly over 733 K. However, to optimize the regeneration procedures it would be best to know in advance the possible solid phase reactions that might occur among the various components. Therefore, we did thermodynamic analyses of the possibility of double oxide formation during r e g e n e r a t i ~ nparticularly ,~~ for Fe, V, and CoMoly-Al203. Figure 14 shows the phase diagram of the Mo-Fe-0-S system at 773 K and logp(S02) = -5 (i.e., 10 ppm of SO& As we did not measure the SO2 concentration in the stepwise regeneration, 10 ppm of SO2 was temporarily adopted from Figure 1. Under a medium pressure ofp(Oz), ferromolybdate (Le.,FeMoO4) was formed during oxidation of a mixture of MoS2 and FeSdFeS. Thermodynamically, with increasing p ( 0 2 ) ,FeMoO4 is decomposed into Moo3 and Fez03 The horizontal width of the stability area represents the stability against the oxidative and reducing decomposition, whereas the vertical one represents the stability from the constituent binary oxides.le Taking into account of oxygen partial pressure gradient in solid phases and assuming the local equilibrium, however, it is possible for FeMo04 to survive after oxidation under oxygen pressure in the oxidizing gas (Le., log ~ ( 0 2 = ) -1.82). Figure 15 shows the phase diagram of the Mo-Fe-0-S system at 673 K under conditions where log@(H~S)/p(Hz)) (29) Yoshimura, Y.; Endo, S.; Yoshitomi, S.; Sato, T.; Shimada, H.; Matsubayashi, N.; Nishijima, A. Fuel 1991, 70, 733. (30) Yoshimura, Y .;Sato,T.; Shimada,H.; Mataubayashi,N.; Imamura,
M.; Nisbijima,A.;Yoshitomi,S.Am. Chem. SOC.,Prepr.-Diu. Pet. Chem. 1993,38,32.
Figure 15. Phase diagram for the Mo-FA-S system at 673 K: log@(HzS)/p(H2))= -1.28.
-
-40
-30
-20
-10
0
log P ( 0 2 ) (atm)
Figure 16. Phase diagram for the Co-Fe-O-S eyetem at 673 K: log@(HzS)/p(Hz))= -1.28. = -1.28 (i.e., H2S/H2 (5%/95%)sulfiding gas). During sulfidation, iron and molybdenum oxides are finally converted into FeS and MoS2, respectively, under the reducing/sulfiding conditions (left side of Figure 15). However, depending on the degree of sulfidation, other oxides (e.g., FeMoO4, Fe304, etc.) were also indicated to coexist. We cannot rule out the formation of oxysulfides because of a lack of thermodynamic data. Sometimes, these double oxides and oxysulfides on the sulfided catalysts were more acidic than simple oxides and sulfides. Figure 16 shows the phase diagram of the Co-Fe-0-S system at 673 K under conditions where log@(HzS)lp(Hz)) = -1.28 (Le., H2S/H2 (5%/95%)sulfiding gas). Cobaltiron double oxide (ie., CoFezOr),which was formed during oxidation of sulfides, could survive during medium sulfidation. The stability area of this oxide was larger than that of FeMoO4, so there is a much higher possibility that CoFe204 can coexist with the respective sulfides FeS and
Co9Sa.
On the other hand, deposited Fereacted with the y-AlzO3 support to form the spinel of FeA1204 during regeneration. The stability region of FeA1204 was larger than that of CoFe204 at 673 K under conditions where log(p(HzS)/ p(H2)) = -1.28. This means that, under the condition
Oxidative Regeneration of Spent Catalysts
where CoFezOr decomposed into FeS and CO&B(around log ~ ( 0 2 )= -311, FeA1204 could survive (until more reducing condition around log ~ ( 0 2 = ) -38). Therefore, if deposited iron selectively reacted with the 7-A1203and is trapped there, the effect of the deposited Fe on the formation of double oxides (e.g., FeMoO4 and CoFe204) would be little. However, some of the iron deposited on the supported components would react more selectively with Co and Mo. At the present, little is known about how much of the deposited Fe reacted with Co and Mo, but these results indicate that some of deposited iron poisons Co as well as Mo sites, and that the poisoning effect is more harmful for Co than Mo. According to the phase diagram of the Mo-V-O-S system at the same sulfidation conditions as those used for iron, V205 will be reduced to V203 and finally converted into V2S3 at more severe reducing conditions (i.e., logp(O2) = -38). This indicates that MoS2 and vanadium oxides such as V203 may coexist under the same reducing1 sulfidation conditions for which Fe and Ni will exist as FeS and NiS, respectively. Thermodynamic data showing the mixed phase formation between Mo and V as well as between V and 7-A1203 is not available yet. It can be speculated, however, that some double oxides/oxysulfides affect the formation of acid sites. By using these thermodynamic analyses, we can predict a priori the formation of double oxides between active components and deposited metals during oxidative regeneration. For molybdate catalysts, deposited metals such as Na, Ca, Mg, As, Fe, and V may form the following compounds during oxidation at 773 K and p(02)= 1.5 vol % in the oxidizing gas: Mo: NanMoO4 (-31.8), CaMoO4 (-29.91, MgMoO4 (-26.3), AsMoO4 (unstable), FeMoO4 (-20.31, and V-Mo double oxide. Co: CoFeO4 (-30.6), CoaAszO~(-24.8). Y-Al203: FeA1204 (-38.4), NiA1204 (-27.51, AlAs04 (-30), MgA1204 (-46.3), CaA407 (-39.7), NaAlO2 (-37.51, Na2A122034 (-49.5). Here, numbers in parentheses indicate the critical log ~ ( 0 2 )values where double oxides are decomposed into the respective sulfides, or sulfides and 7-A1203 under sulfidation (log (P(H2S)I p(H2)) = -1.28) at 673 K. The lower the values of log ~ ( 0 2 are, ) the higher the stability of double oxides under reducing conditions. Most of the double oxide formation between the deposited metals and the active components might result in a decrease in the amount of active Co-Mo-S sites,3I (31) Topsae, H.; Clausen, B. S.;Candia, R.; Wivel, C.; Merrup, S. Bull. SOC.Chim. Belg. 1981, 90,1189.
Energy & Fuels, Vol. 8, No. 2, 1994 446
some of which might be active for hydrogenation. On the contrary, some of these double oxides may cause new acidic sites (e.g., via V-Mo and Fe-Mo double oxides29). Most of the alkali and alkaline earth metals selectivelydecreased the catalyst acidity by neutralization. These thermodynamic analyses clearly suggest that careful control of the oxidation temperature and partial pressure of oxygen in the third stage is quite important not only to remove carbon and sulfur from spent catalysts but also to minimize the solid-phase interactions among the active metals, the deposited metals, and the y-A1203 supports.
Conclusions The following conclusions can be drawn regarding the oxidative regeneration of spent CoMo and NiW catalysts. 1. Temperature-programmed oxidation of spent Mo and W catalysts showed that oxidation proceeded mainly in two oxidation steps: removal of sulfidic S as SO2 in the first step and removal of carbon as CO2 (and CO) in the second step. A small amount of SO2 was also formed in the second step, the formation of which might originate from organic sulfur and residual sulfides that were covered with oxides and sulfates. 2. Carbonaceous materials on the spent NiW catalyst were less aromatic than those of the spent CoMo catalyst, but more severe oxidation conditions were needed for the NiW catalyst. This might be due to the lower oxygen donating ability (Le., lower oxidative catalytic effect) of NiO and WOa than MOO3 and Co304, which is indicated based on the thermodynamic data. 3. Catalytic activities and selectivities successfully recovered by low-temperature oxidation, while at higher oxidation temperatures there was a slight loss of hydrogenation activity and a large increase in hydrocracking ) the oxidizing gas was carefully activity, when the ~ ( 0 2 in controlled to a low level ( P ( 0 2 ) 1.5%). 4. Thermodynamic analyses indicated a priori the trends in the structural changes of the active components and the trends in the interactions among the active Components,the deposited metals, and the ?-A1203 support during oxidationlsulfidation.
-
Acknowledgment. We thank the Sunshine Project Promotion Headquarters of AISTIMITI for their financial support. The EXAFS work was performed under the approval of the Photon Factory Program Advisory Committee.
