Znd. Eng. Chem. Res. 1991,30, 2365-2371
2365
KINETICS AND CATALYSIS
Effect of Fluorine on Hydrodesulfurization and Hydrogenation Activity of Doubly Promoted (Zn + Co) Molybdena-Alumina Catalysts Jose F. Cambra, Pedro L.Arias,* M. Belgn Guemez, and Juan A. Legarreta Departamento de Ingenieria Quimica y del Medio Ambiente, Escuela de Ingenieros, Almda. de Urquijo sln, 48013 Bilbao, Spain
Jose L. G. Fierro Znstituto de Catdisis y Petroleoqulmica, CSIC, Serrano, 119, 28006 Madrid, Spain
Fluorinated alumina (0-2.0 wt % F) has been used as a carrier for promoted (Co + Zn) molybdate catalysts. The preparations consisted of constant molybdenum (12 wt ?’% Moo3) and promoter (COO + ZnO) loading (4 wt % and atomic ratio Co/Zn = 1). The sulfided catalyst activity has been studied in the hydrodesulfurization of thiophene. It has been observed that the thiophene conversion attained a maximum at 0.0-0.4 wt 7% F. In addition to this, the thiophene conversion changed only slightly and linearly with H2 pressure. Textural and chemical characterization results revealed that A1203 carrier becomes attacked by (NH4)HF2solution yielding pores of greater sizes and plugging those of lower diameter, and also some A13+ ions are solubilized, which then precipitate as Al(OH)3during the drying step over fluorinated alumina surface. As both Mo and promoter are incorporated in further impregnation steps, it is expected that a fraction of these ingredients can be lost through strong interaction with the resulting A1203 coating.
Introduction The influence of fluorine incorporation in aluminasupported molybdenum-based catalysts on their surfaces and catalytic properties has been studied recently (Boorman et al., 1982, 1985, 1986, 1987; Tejuca et al., 1983; Muralidhar et al., 1984; Fierro et al., 1985; Jiratova and Kraus, 1986; Papadopoulou et al., 1988, Matralis et al., 1988). The interest in these studies arose from the industrial requirements for more effective catalysts for hydroprocessing heavy crude oil fractions containing high heteroatoms (N, S, 0)proportions. It is well established that fluorination of alumina increases its surface acidity and therefore it enhances acid-catalyzed reactions (Tejuca et al., 1983; Boorman et al., 1986; Hughes et al., 1969; Scokart et al., 1979). However, most of the reported studies on fluorine-containing hydroprocessing catalysts have focused on both cracking and hydrocracking (Boorman et al., 1982, 1985, 1986, 1987) as well as on hydrodesulfurization (HDS) (Boorman et al., 1982; Muralidhar et al., 1984; Papadopoulou et al., 1988) activities. Apart from this, relatively little attention has been paid to the effect of fluorine in hydrogenation (Boorman et al., 1987; Jiratova and Kraus, 1986). For instance Boorman et al. (1987) reported that F incorporation to CoMo/A1203 yielded high cyclohexene hydrogenation, while cyclohexene isomerization was inhibited. In contrast with this study, Jiratova and Kraus (1986) reported that the addition of 3% F to the raw CoMo/A1203 catalyst increased substantially both hydrogenation and isomerization of cyclohexene as well as thiophene hydrodesylfurization. In agreement with this latter finding, Ramirez et al. (1990) showed that F incorporation enhanced appreciably and moderately the hydrogenation activity on Mo and CoMo catalysts, respectively; this increase being only roughly related to Mo dispersion in both types of catalysts. Other studies related to the effect of doping cobalt-
molybdenum catalysts with minor amounts of other elements have been reported (Trifiro et al., 1973; Kibby and Swift, 1976; Fierro et al., 1984). The structural and catalytic effects of doping alumina with Mg2+and Zn2+prior to Co and Mo incorporation were interpreted as due to partial displacement of Co2+from the alumina lattice to the surface by Mg2+and Zn2+ions (Kibby and Swift, 19761, thus increasing the proportion of octahedrally coordinated cobalt. Following the same line of reasoning, Fierro et al. (1984) studied the activity for gas-oil HDS of a series of ZnCo-Mo/A1203 catalysts, in which molybdena content was fxed (8 w t %Y Mood and overall promoter content was constant (3 wt 7’0 ZnO + COO) but the Zn/(Zn + Co) atomic ratio varied between 1 and 0, and found a very important HDS activity enhancement at Zn/(Zn + Co) = 0.5. The increase of catalyst activity at this composition was interpreted in a similar manner: the structural effect of Zn is largely to decrease formation of inactive CoA1204 phase and to increase the dispersion of Co and the interaction Co-Mo phase. In this respect, we consider of great importance to investigate systematically the effect of fluorine on the thiophene HDS activity on a series of doubly promoted ZnCo-Mo/Al,O, catalysts in which the proportions of Zn and Co promoters are the same. For this purpose, the results of thiophene HDS activity and characterization of the sulfide state of the catalysts by IR spectroscopy of the adsorbed NO molecule and X-ray photoelectron spectroscopic analysis are presented.
Experimental Section Preparation of Catalysts. A series of alumina-supported catalysts was prepared by multistep impregnation. The carrier was a 7-A1203 (Girdler T-126, SBET = 188 m2/g, pore volume = 0.39 cm3/g). Because the original size of the support was too large (cylinders: 6.35 mm long,
0 1991 American Chemical Society OSSS-5S85/91/2630-2365~02.50/0
2366 Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 Table I. Catalyst Composition, BET Areas, and Hg Porosimetry Data Moo3, wt % 12 12 12 12 12 12 12
CMA412 CZMA2212 CZMFA22120.4 CZMFA22120.7 CZMFA22121.O CZMFA22121.3 CZMFA22122.0
COO,w t % 4 2 2 2 2 2 2
ZnO, wt % 0 2 2 2 2 2 2
6.35" diameter) to get an homogeneous profile of metal atom concentration across the particle (Fierro et al., 1987; Blanco et al., 1987), it was ground and screened, and the fraction between 0.42 and 0.75 mm was collected to manufacture the catalysts. The first step of preparation was the incorporation of the acidic function to the support. For this purpose, A1,03 was impregnated according to the incipient wetness procedure, with aqueous solutions of NHIHFz whose concentrations were selected to obtain fluorine contents between 0 and 2 wt % . The preparations were maintained at room temperature for 6 h and then dried in a rotavapor at 353 K to dryness and after that in an oven at 383 K for 16 h, and finally calcined in two steps: first at 623 K for 2.5 h and second at 773 K for 4 h. Under this process the fluorine ion is anchored to the alumina surface, possibly following the scheme on o o o OH F O O F A'! !A'
A '!
'A{
+
2"
*
+ w20
+ !A'
A '!
A '!
The precursor fluorinated aluminas so prepared where then impregnated simultaneously, using the same procedure, with aqueous solutions of cobalt and zinc nitrates of adequate concentrations to obtain an oxide ratio CoO/ZnO = 1.0 and an overall metal oxide content of 4.0 wt % in the calcined catalysts. The last step in the preparation was the impregnation with ammonium heptamolybdate solutions at pH = 5; their concentrations were selected in order to obtain a 12 wt % Moos in the final catalysts. Drying and calcination where the same as above. They will be referenced to thereafter as CZMA22122, where CZMA notation refers to cobalt, zinc, and molybdenum oxides supported on alumina, whose compositions are 2,2, and 12%, respectively, and x denotes the fluorine content. A blank zinc-free catalyst, CMA412, was also used for comparative purposes. Their compositions and BET areas are summarized in Table I. Materials. The model compound used in the hydrodesulfurization experiments was thiophene (Merck for synthesis). Cyclohexane was used as solvent (Probus). The reaction mixture was made by weighting the necessary amounts to get the desired proportion of sulfur in the solution. Argon, hydrogen, and hydrogen sulfide were used as received. Activity Tests. The hydrodesulfurization runs were performed in a conventional high-pressure bench-scale plant described elsewhere (Cambra, 1989). The reactor was a stainless steel (AIS1 316) tube (12.7-mm external diameter, 1.65 mm thick), with a thermowell along ita axis to introduce a thermocouple in order to measure the temperature at any position in the catalytic bed and in the preheating section. This reactor was filled with glass beads (1-mm diameter) to the desired level for the catalytic bed. The bed, which was between two thin layers of Sic, was made of 0.2 g of catalyst diluted in 0.6 g of Sic of the same size fraction as the catalyst. Once the reactor was filled, it was placed in the benchscale plant, the following step consisted in the sulfidation of the catalyst. For this purpose the reactor was heated
F,wt 70 0.0 0.0
0.4 0.7 1.0
1.3 2.0
SBET, m2g-' 175 174 172 157 162 158 155
intrusion vol, cm3 g-l
pore area, m2g-'
0.34
96.87
0.26
73.99
0.33
71.56
up to the sulfidation temperature (673 K) in an Ar flow, at atomospheric pressure; when the temperature was reached a flow of a gas mixture Hz/HzS (10/1 mol/mol) was passed through the bed for 4 h. After that, Ar was passed through again for 0.5 h, without cooling, to desorb the H a adsorbed onto the catalytic particles. After cooling in the same gas flow to room temperature, the system was pressurized (3-7 MPa) and was ready for the HDS test. The catalytic bed was heated up to the reaction temperature in an Ar flow. When the temperature was reached, hydrogen and model compound solution flows were passed through the reactor. The reaction products were analyzed by gas chromatography, in a KONIK chromatograph Model KNK-3000-HRGC, with a thermal conductivity detector (TCD) and temperature programs. Gas and liquid were analyzed. Two parameters were determined: the first was the conversion defined as x = 100(Fo-F)/Fo, where Fois the molar flow of thiophene feed and F the molar flow of the same compound in the product stream. The second parameter was the selectivity defined as S = (100)butane/(butane + butenes) in the product flow. Also, the pseudo-first-order kinetic constant was calculated, taking into account that a variation of the number of moles exists (Levenspiel, 1981). It was done with the expression kwCo/Fo = -(1 + t) In (1 - x ) - tx, where k is the pseudo-first-order kinetic constant, w the catalyst weight, Co the concentration of thiophene in the feed, x the conversion, and t the relative volume variation with the conversion. With the experimental system and the procedure described above, a space velocity of 14 g of thiophene/(g of cat. h) was tried, the pressure was varied from 3 to 5 and 7 MPa, and four temperatures were employed (573, 598,623,648 K). Catalyst Characterization. BET surface area measurements were computed from the adsorption isotherms of Nz at 77 K, obtained on an ACCUSORB 2100E interfaced to a data station. Pore volume and pore size distributions were obtained by mercury intrusion on a MICROMERITICS 9310 analyzer. Adsorption of NO was carried out volumetrically in a conventional Pyrex glass high-vacuum system equipped with an MKS pressure transducer. Sulfided samples of ca. 0.4 h were placed in a flow microreactor and purged with He at 673 K to remove adsorbed water and then resulfided in situ with a 10% H,S/H, mixture at 673 K for 1 h. After this, the samples were outgassed at 773 K for 1 h and then cooled to room temperature. NO was contacted with samples at increasing pressures, and a first isotherm was constructed. The amount of NO reversibly adsorbed was determined by a second isotherm obtained by outgassing the catalyst previously saturated by NO (Fierro and Garcia de la Banda, 1986). The difference between both isotherms gave the extent of NO irreversibly adsorbed. For infrared spectroscopy measurements self-supporting wafers of the catalyst of ca. 10 mg cm-, thickness were placed in a special cell that allowed either dynamic or static
Ind. Eng. Chem. Res., Vol. 30,No. 11, 1991 2367 Table 11. €IDS with SV
14 h-' and €I, Flow = 10 L (NPTVh ~~
~
CZM412 CMFA2212 CZMFA22120.4 CZMFA22120.7 CZMFA22121.0 CZMFA22121.3 CZMFA22122.0
573 K 31 26 20 13 15 15 13
conversion, % 598 K 623 K 50 71 48 69 37 59 28 49 32 58 34 60 31 53
Table 111. HDS with SV = 14 h-*and T = 623 K conversion, 5% CIH10/CIH8 3MPa 5MPa 7MPa 3MPa 5MPa 7MPa 5.5 CMA412 71 79 85 1.8 3.2 81 1.4 2.4 4.3 czMA2212 69 76 CZMFA22120.4 59 3.4 76 1.1 1.9 67 66 0.9 1.2 1.9 CZMFA22120.7 49 58 CZMFA22121.0 58 67 74 0.7 1.0 1.6 2.5 CZMFA22121.3 60 69 77 1.0 1.6 72 1.0 1.5 2.3 CZMFA22122.0 53 63
treatments. The samples were resulfided in situ according to the same procedure as above. NO at a pressure of 30 Torr was contacted with samples for 0.5 h. After the gas phase was condensed in a liquid nitrogen trap, spectra were recorded at room temperature. X-ray photoelectron spectra were obtained by using a Leybold LHS 10 spectrometer equipped with a Mg Ka X-ray excitation source. The powdered samples were sulfided separately, according to the same procedure as for IR measurements, and then collected under isooctane and pressed into small holders under the meniscus of isooctane to prevent oxidation by air. The holders were fixed on a long rod, and the samples were outgassed down to 10" Pa before they were moved into the analysis chamber. Each spectral region was signal-averaged for a number of scans to obtain good signal-to-noise ratios. Althouh surface charging was observed in all preparations, accurate binding energies (BE) could be determined by charge referencing with the Al(2p) line a t 74.7 eV.
Results Activity Data. (a) Influence of Temperature. The space velocity (SV) of 14 g of thiophene/(g of cat. h) was selected to get a higher difference in conversions with the temperature. Some other tests with increased space velocities showed that 14 is a good value to distinguish between conversions for different catalysts, even if they were too high for other objectives. Activity results for this space velocity and 3.0 MPa are shown in Table 11. For all the temperatures, the catalysts with 0 and 0.4 w t 70 fluorine exhibit the maximum hydrodesulfurization activity. The catalyst with 0.7 wt % fluorine shows the minimum in hydrodesulfurization activity. As can be seen in Table 11, the thiophene conversion increases with temperature, although a certain tendency to saturation occurs in all cases at 623 K. At all the temperatures the most active catalyst is the Yconventional"cobalt-molybdenum catalyst, except at the highest temperature at which the Zn, F-free catalyst has the biggest activity. (b) Influence of Pressure. As experimental evidence exists on the influence of pressure on catalyst activity (Betteridge and Burch, 1986; Qusro and Massoth, 1987), activity runs varying the pressure were carried out. Selected pressures were 3,5, and 7 MPa, at two temperatures (598 and 623 K), thiophene space velocity of 14 g/(g of cat. h), and hydrogen flow of 10 L(NTP)/h. The results are shown in Table I11 (623 K, example case) and Figure 1
CIHl0/CIH8 598 K 623 K 52 64
573 K 33
648 K 77
-598
K
-8-623K
"
648 K 71
/
' U
I
1
t
P(MPa)
8
6
Figure 1. CZMFA22120.4 catalyst. Effect of pressure. Table IV. Kinetic Coefficients and Activation Enernies K,L/(h g) E., 573 K 598 K 623 K 648 K kJ/g-mol CMA412 3.1 6.0 11.0 13.6 74.3 85.1 2.6 5.8 10.7 CMFA2212 15.2 1.9 4.1 8.0 CZMFA22120.4 12.6 85.7 94.2 1.3 3.0 6.2 CZMFA22120.7 9.8 7.9 11.6 104.6 1.4 3.4 CZMFA22121.0 104.7 1.4 3.7 8.3 CZMFA22121.3 11.7 7.0 10.1 109.8 1.1 3.3 CZMFA22122.0 ~~
(catalyst CZMFA22120.4, example case). It is seen that an increase in total pressure leads to an increase in conversions and selectivities, probably because the global process implies a reduction in the total number of moles in the system. Besides that, the increase of thiophene conversion seems to be relatively independent of temperature, since the slope is very similar for both temperatures. Selectivity, however, increases more readily at the higher temperature (623 K). Thus, thiophene conversion changes only slightly and linearly with H2 partial pressure, but these changes are independent of temperature. Conversely, hydrogenation reactions are very sensitive to hydrogen partial pressure. One may explain the increase of hydrogenation activity by the variations in hydrogen partial pressure that alter the equilibrium sulfurized active phases-hydrogen-hydrogen sulfide. At high H2pressure sulfur-deficient active centers are formed, with more metallic character, where H2is easily dissociated and hence more active in hydrogenation reactions. Kinetic Analysis. A simple kinetic fitting of the results obtained with a thiophene space velocity of 14 g/(g of cat. h), total pressure of 3 MPa, and hydrogen flow of 10 L(NTF')/h was made. T h e values for the kinetic coefficients calculated following the method exposed above are summarized in Table IV. Kinetic results of the values obtained for the catalyst CZMFA22121.0, and the quadratic minimum regression line, using only the values at the three lowest temperatures, are plotted in Figure 2. Notice that at the highest temperature, diffusional transport phenomena within the catalyst porous structure became important. Besides that increasing conversion implies higher H2Spartial pressure and its inhibition effect could explain
2368 Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991
t
-1
\ 1.50
1.57
\
1.65 loOO/T
1.72
1.80
Table V. Extent of NO Adsorption on Sulfided Catalysts NO adsomtion mmol/g relative 12% MOO" 0.093 0.139 4% c o o 0.281 CMA 412 CZMA2212 0.294 1.OOO CZMFA22120.4 0.296 1.007 CZMFA22120.7 0.310 1.054 CZMFA22121.0 0.333 1.133 CZMFA22121.3 0.331 1.126 CZMFA22122.0 0.297 1.010
Figure 2. Arrhenius plot (CZMFA22121).
the relatively low activity at very high temperatures. The values for the three lowest temperatures satisfactorily fit the Arrhenius plot. Thus, this kinetic model fits quite well the kinetic results obtained under these operating conditions. The activation energy calculated for these three temperatures (Table IV) are similar in magnitude order to those published by other authors (Bouyssieres et al., 1986). Finally, as these values are very similar, one may infer that the differences in catalyst activity could be related to a variation in the number of active centers of the catalysts.
Catalyst Characterization The catalysts were characterized by several physicochemical techniques, namely, surface area (BET method), pore size distribution (mercury penetration), X-ray photoelectron spectroscopy, NO chemisorption, and the combination of NO chemisorption with IR spectroscopy. BET Surface Area Determinations. From the BET data in Table I it is seen that the catalyst with the highest fluorine content exhibits the lowest surface area in the series. This fact may be an indication of the destruction of the alumina matrix because of the anion incorporation (Ghosh and Kydd, 1985). Fluorine contents up to 0.4 wt % have no effect on the BET surface area, with the decrease being important at 0.7 wt % fluorine. In contrast with the work of Matralis and Lycourghiotis (Matralis et al., 1988), which showed a slight increase of surface area when the fluorine content reaches 2 wt %, our results clearly indicate that the surface area continuously decreases by increasing fluorine content. This different behavior could be due to the differences in the catalyst preparation method. Pore Volume and Pore Size Distribution. These measurements were carried out on three representative catalysts: fluorine-free, 1 w t % fluorine, and 2 wt % fluorine. Table I summarizes these data. While the mercury volume introduced into the 2% F catalyst is only slightly lower than in the F-free catalyst, the difference in surface area accessible to mercury is higher. This could be due to a decrease of low-diameter porm, since they have a higher surface/volume ratio (Table I). Comparing the F-free and 1% F catalysts, a low pore volume accessible to mercury and a slight surface area decrease are observed in the latter. Moreover, evidence exists that when alumina is treated with ammonium fluoride solutions, they attack the solid; thus alumicum ions are present in the solution (Cambra, 1989; Ramirez et al., 1990). The AP+ ions precipitate as Al(OH), over the support surface during drying; upon calcination this hydroxide undergoes a new alumina phase that may also be responsible for the surface area and pore size and volume variations observed. Chemisorption of NO. The chemisorption with NO as probe molecule over sulfided catalysts has been em-
I
1
I
os
'
1.0
15
20
Jo
%F
Figure 3. NO adsorption and IR absorbance.
ployed to study the type and proportion of centers generated after sulfidation. The results of the extent of irreversibly adsorbed NO on sulfided catalysts are summarized in Table V. For comparative purposes the data of the reference 12 wt 7'% Mo03/A1203and 4 w t % CoO/A1203preparations are also included in the same table. As can be observed, the catalyst containing only Co shows a larger extent of adsorbed NO than that containing only Mo, still with a 3 times higher active phase content. In addition, the complex catalysts containing both Co and Mo show a larger extent of adsorbed NO than that expected from an additive behavior. Assuming an 1:l N0:Me stoichiometry in the sulfided state, the number of Mo4+ions titrated by NO is very low (NO/Mo = 0.112). The number of Co2+ions titrated by NO is, however, more than 2 times higher. This indicates that the sulfided Co species are much more exposed than that of Mo. Comparing the data of Table V and Figure 3, it results that the binary (Co-Mo) or ternary (Co-Mo-Zn) catalysts adsorbed a larger amount of NO than that expected from an additive behavior of the individual components, a fact interpreted as an increase of the dispersion of promoter. It is important to note that the catalyst containing both promoters shows an adsorption of NO slightly higher than that found on the Co-Mo catalyst containing twice Co loading. Since Zn2+ions do not adsorb NO at a significant extent (Gil Llambias et al., 1979; Fierro et al, 1984), it implies that these differences reside in the dispersion of Co2+ions. It should be also noted that cobalt incorporation to Mo-alumina preparations leads to a well-known Co-Mo-S phase (Wive1 et al., 1981) in which Co atoms are mostly located at the edges of MoS2slabs, where Mo ions with a certain coordinative unsaturation exist. On the basis of this model, it seems that the adsorption of NO is essentially concentrated on Co ions exposed on the edges of MoSz crystals, because most of the Mo4+sites able to adsorb NO are covered by Co promoter atoms. Infrared Spectroscopy. This kind of measurement allow establishment of the different surface exposure of
Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 2369 HoOd
I
I
I
2000
1900
\
1
1800
no0
em -9
I
I
235
230
I
22 5 BE (cV)
Figure 5. XPS spectra. Mo(3d) zone.
Figure 4. IR spectra.
the Co2+and Mod+(a = 4) ions in the sulfided state. This was feasible because of the lack of Zn2+ions to adsorb NO and also because of the different frequencies at which NO-Co and NO-Mo do adsorb IR radiation. The IR spectra of adsorbed NO on sulfided catalysts are shown in Figure 4. The bands at 1800 and 1860 cm-' are characteristic of dinitrosyl species adsorbed on the same Co2+ ions surrounded by sulfide ions. Upon adsorption of NO the Mo4+ions form a similar doublet but shifted to lower wavenumbers: one band overlaps with that of NO on Co2+ at 1800 cm-' while the second appears at 1700 cm-'. It is rather well known that NO adsorbs at the edges of MoSz crystallites where coordinative unsaturation sites exist. However, the promoter (Co) atoms have a high tendency to be deposited in the same sites (Co-Mo-S model). If the amount of promoter atoms is enough to saturate Mo4+sites in those locations, no NO adsorption should be observed. Reexamining again the spectra in Figure 4, it is clear that a very weak band at ca. 1700 cm-' is observed, thus indicating that Co ions are mostly placed on those Mo sites that adsorb NO, as would be expected from the presence of a Co-Mo-S phase. In a conventional Co-Mo catalyst containing about 4% COOa fraction of the Mo4+ions in the sulfided state still remain unsaturated by Co atoms. On the other hand, the substitution of 50% of Co atoms by Zn atoms does not produce significant changes on the Co atom exposure. This apparent contradiction lies in the fact that in the case of high Co content a fraction of these atoms is lost because of the interaction at the alumina surface to give a nonactive CoA1204spinel-like compound. This explains why the catalyst in which half of the Co has
been substituted by Zn shows almost the same absorbance as that on the unsubstituted partner, as the Zn displays a much higher tendency than Co to react at the alumina surface. The fluorine content is another important factor to be considered. As observed in Figure 3, the absorbance of the band at ca. 1800 cm-' depends markedly on the fluorine content of the catalysts: a maximum of nearly 1 wt % fluorine content is attained. Although the IR data are semiquantitative, they follow the same trend as the extent of NO adsorption. To summarize this findings, it can be concluded that an optimized exposure of the active Co promoter occurs in the doubly promoted (Co-Zn) preparations containing nearly 1 wt 9% fluorine. XPS Results. The Mo(3d) spectra have been recorded for all sulfided catalysts. The Mo(3d) doublet is rather well resolved (Figure 5), and the BE values for the Mo(3d,/,) and Mo(3d3 2) peaks at ca. 228.4 and 231.4 eV, respectively,-fit weh with the literature data for MoSz species (Ramirez et al. (1990) and references cited therein). Although a small S(2s) peak is observed on the lower BE side of the Mo(3d6,d peak, no further attention will be paid to this peak. Figure 5 and Table VI show that the BE values of Mo(3d) peaks remain almost constant, whatever the fluorine catalyst content. As can be seen in Table VI, the BE of (Co(2p3/,) and Zn(2p3/,) are essentially unchanged. The average value of 781.4 eV for the C0(2p3/2) level is slightly lower than that observed for oxidic preparations (Declerck-GrimBeet al., 1978);therefore it can be assigned to sulfided cobalt. The broad C0(2p3/2)peaks and the appearance of very small satellites, characteristic of paramagnetic Co2+ ions, indicate the presence of non-
2370 Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 Table VI. Binding Energies (eV) and Surface Atomic Ratios of the Catalysts
CZMA2212 CZMFA22120.4 CZMFA22120.7 CZMFA22121.0 CZMFA22121.3 CZMFA22122.0
228.6 228.4 227.9 227.6 228.1 227.9
781.4 781.1 781.8 781.6 781.7 781.5
1022.9 1022.6 1022.8 1022.9 1022.9 1022.9
.
513K
.
x 623K
-
-
0.0
0.2
0.4
0.6
0.8
1.0
598K A
648K
1.2
F/AI
Figure 6. Kinetic constant vs F/Al ratio.
sulfided cobalt, viz., C0A1204phase. The situation is much clearer for zinc. The BE value close to 1022.9 eV for the Z n ( 2 ~ ~level , ~ ) is characteristic of nonsulfided ZnA1204,at least in the experimental conditions followed in this work. To get an idea of the surface proportion of Mo and promoters (Co and Zn) ions as well as of the possible changes introduced by fluorine, the XPS M/Al (M = F, Mo, Co, Zn) ratios have been calculated and summarized in Table VI. In general, surface F increases with F loading, and the Mo- and promoter-to-A1 ratios give a maximum for the catalyst F (0.7) and then decrease at higher F contents. As the XPS F/A1 ratio is a measure of the fluorine concentration at the alumina surface, both Mo and promoter (Co and Zn) exposure do not follow the same trend.
Discussion Activity results show clearly that fluorine incorporation in the catalysts leads to a decrease in HDS activity. As can be seen in Figure 6, the HDS activity decreases monotonously with the XPS F/Al ratio and the slope of the straight lines increases slightly with reaction temperature. In principle, one would expect that dispersions of the Mo and promoters (Co + Zn) follow the same trend, but this is not the case. As summarized in Table VI, the XPS Mo/Al, Co/Al, and Zn/Al ratios seem to attain a maximum for F contents in the range 0.7-1 wt % and then decrease at higher F contents. Therefore, the absence of correlation between HDS activity and dispersion of the active components in their sulfided state, evaluated either by XPS measurements or by the extent of chemisorption of NO, implies that factors other than dispersion would be responsible for the activity changes. It must be emphasized that important textural changes of the A1203carrier occur along the genesis of catalysts. These changes are mainly induced in the F-incorporation step (Cambra, 1989; Ramirez et al., 1990). As SBET areas and Hg porosimetry have revealed, A1203carrier treated with (NH4)HF2solutions becomes attacked, yielding pores of greater sizes and plugging those of a lower diameter. Some solubilization of A13+ ions is, indeed, expected because the impregnation pH is very low (Cl), and specially when F concentrations are high. Thus, the A13+ ions remain in solution, filling the pores, but they precipitate as
684.8 685.3 685.1 685.5 685.8
0.395 0.896 0.835 0.812 1.147
1.896 2.217 3.200 1.742 1.333 1.260
4.001 3.392 6.432 3.641 3.959 3.541
1.117 1.419 2.281 0.979 1.196 1.200
Al(OH), phase on the surface of the fluorinated alumina during the drying step, which upon calcination provokes the appearance of very small M203particles on the F-Al2O3 substrate. I t is evident that both concentration and size of such A1203particles are directly related to (NH4)HF2 concentration during impregnation; viz., for the 2 wt % F containing catalyst, most of the F-modified alumina surface may be largely covered by an A1203coating. As the promoters (Co and Zn) and Mo are incorporated in further steps, it is expected that higher proportions of these ingredients are lost, through formation of nonsulfidable ZnA1204 and CoA1204phases or a difficult to sulfide molybdate-alumina strongly interacting phase, with respect to their F-free counterparts. If a comparison is made between the two F-free promoted (4 wt % COO and 2 wt % COO + 2 w t % ZnO) catalysts, it resulb that HDS activity is only slightly higher in the former, for twice the Co content, than in the latter. This is mainly due to the fact that practically all Zn2+ions in the doubly Co-Zn promoted catalyst saturate tetrahedrally sites of the A1203surface forming the very stable ZnA1204phase (Fierro et al., 1984) while in the singly Co promoted catalyst those alumina sites are saturated by Co2+ions to form inactive CoA1204phase at the expenses of the overall Co loading. The effect of Zn addition is very interesting in the case of gas-oil HDS. Previous work reveals that the doubly promoted catalyst shows a HDS activity higher than that of the singly promoted one (Cambra et al., 1991). This fact is in agreement with other works (Fierro et al., 1984). Many references establish that this kind of catalyst shows also a considerable hydrogenation activity (viz., Massoth and Muralidhar, (1982)). Moreover, evidence indicating different active centers for HDS and hydrogenation reactions exists (Ramachadran and Massoth, 1981). As in the HDS activity measurements, the hydrogenation activity decreases with the fluorine incorporation. To investigate relative hydrogenation/HDS activity, a differential reactor must be used. In this work high conversions (>30%) were obtained due to the operating conditions selected. A tentative discussion could be supported by inspection of Figure 7 where C4HI0/(C4HB+ C4Hlo) ratios are plotted as functions of HDS conversions. It can be observed that different catalysts show significant differences in hydrogenation activity for the same level of HDS conversion. The minimum in hydrogenation activity corresponds to the catalyst with 1w t % F content. The differences become negligible for the lowest HDS conversions. The lowest HDS activity is for 0.7 wt % F content. One possible explanation would be that the great dispersion of active phases measured by XPS in the catalyst with 0.7 wt % F content could be related to Co and Mo well dispersed but with low interaction; this would explain its low HDS activity (low Co-Mo-S phase content). Conclusions As a resume of results obtained the following can be concluded: Zn is a good promoter, by optimization of Co utilization, yielding a higher formation of Co-Mo-S phase active in
Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 2371
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Mo/Alz03 Catalysts. An X-ray Photoelectron Sepctroscopy Study. J. Phys. Chem. 1978,82,885. Fierro, J. L. G., Garcia de la Banda, J. F. Chemisorption of Probe Molecules on Metal Oxides. Catal. Reo-sci. Eng. 1986, 28, 265-333.
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Receioed for review May 16, 1991 Accepted June 16,1991
