I n d . Eng. C h e m . Res. 1993,32, 1573-1578
1573
Hydrodesulfurization Reactions of Atmospheric Gas Oil over CoMo/ Alumina-Aluminum Borate Catalysts Chiuping Li,’J Yu-Wen Chen,’J Shien-Jen YangJ and Jung-Chung Wut Refining & Manufacturing Research Center, Chinese Petroleum Corporation, Chia- Yi 60036, Taiwan, and Department of Chemical Engineering, National Central University, Chung-Li 32054, Taiwan
A precipitation technique at constant pH value was used to prepare a series of alumina-aluminum borates (AABs) with various Al/B atomic ratios. These materials were used as the supports of Co-Mo catalysts. Hydrodesulfurization (HDS) of Kuwait atmospheric gas (AGO) oil was carried out over these presulfided catalysts in a bench-scale trickle bed reactor at 400 psi and 340 “C. All CoMolAAB catalysts are much more active than the conventional CoMo/A1203 catalyst on HDS reactions. A correlation exists between the acidity and the HDS activity of the catalysts. The high activities of the CoMo/AAB catalysts can be rationalized on the presence of boron. On one hand, it can increase the metal dispersions and hydrogenation capabilities. On the other hand, it can enhance the acidities and cracking abilities of the catalysts. The desulfurization data can be fitted with a pseudo-second-order rate equation. The activation energy for desulfurization is found to be 26 kcal/mol.
Introduction The demand for low-sulfur distillates is increasing due to the increasing environmental concern of diesel engine emissions. It is known that low-sulfur distillate fuels (0.05 wt % S) can significantly reduce the contribution of sulfates to particulates from diesel engine emissions (Eastwood and van de Venne, 1990). Conventional hydrotreating processes can desulfurize petroleum distillates to low-sulfur contents. Cobalt-promotedmolybdenum catalysts on alumina are commercial catalysts for hydrodesulfurization (Ohtsuka, 1977;Stanislaus et al., 1988). The chemistry and structure of the CoMo/AlzO3 catalysts are well studied. Several models have been proposed to describe the interaction between MoS2 and the Copromoter. These models include the ‘monolayer”, “Co-Mo-S complex”,“intercalation”,and “contact synergy”or “remote control”models (Topsoeand Clausen, 1984,1986). Recently, the “Co-Mo-S” edge sites are generally believed to be the active sites for the CoMo/ A1203 catalysts (Topsoe et al., 1981; Topsoe, 1983; Farragher, 1977; Pollack et al., 1979; Delannay, 1985). Besides alumina, other supports such as carbon, silica, zirconia, and zeolite have been used to prepare CoMo HDS catalysts (Thomas et al., 1983; Cid et al., 1987). For example, Mo-based carbon-supported catalysts have been reported to have much higher HDS activity than the corresponding AlzO3-supported catalysts (Schmidt et al., 1976; Duchet et al., 1983). The use of alumina-aluminum borate (AAB) as a catalyst support has received less attention than the use of other supports. It has been reported that aluminaboria can be used as a cracking catalyst (Tanabe, 1970). Pine (1976) found that catalysts prepared with aluminaboria in combination with zeolite, nickel oxide, and molybdenum were active for hydrocracking of petroleum feedstocks. Sat0 et al. (1987) reported that aluminasupported boria exhibited a high activity for vapor-phase Beckmann rearrangementreaction of cyclohexanoneoxide. It has been reported (Peil et al., 1989; Wang and Chen, 1991) that alumina-aluminum borate is highly acidic.
* To whom all correspondence should be addressed. + Chinese Petroleum Corp. t
National Central University.
The influence of boron on catalyst activity has been investigated. The hydrogenolysisactivity of Ni-Mo/AlgOs catalysts (Lditau et al., 1976) and the CO hydrogenation activity of Ru/A1203 catalysts (Okuhara et al., 1985) were increased as a result of boron support modification. An increase in the activity of Pt/A1203 liquid phase hydrogenation catalysts modified with boron has also been reported (Chen and Li, 1992). In a previous paper (Tsai et al., 1991) the authors have shown that alumina-aluminum borate (AAB) is a good support material for Co-Mo catalysts in hydrotreating of heavy residue oils. The present study was extended to desulfurization of lighter feedstock, Le., atmosphere gas oil (AGO). A series of CoMo/AAB catalysts with various Al/B ratios and nearly constant metal contents were prepared and tested for their HDS activity of AGO. A conventional CoMo/AlzOs catalyst was included for comparison. Kinetic studies of HDS reactions were also carried out in a trickle bed reactor. Effects of boron content and surface acidity on desulfurization activity were examined. Most of the published data on hydrotreating reactions were obtained with a noncontinuous batch reactor using model compounds as feeds. This makes it difficult to interpret the performance results and to correlate them to commercial operations. Therefore, it is our intention to describe the results obtained by using AGO as a feedstock in a continuous trickle bed reactor, which closely mimics a commercial operation.
Experimental Section Catalyst Preparation. Alumina-aluminum borate supports were prepared from the common solution of aluminum nitrate and boric acid, using an ammonium hydroxide solution (pH = 10.00 i 0.02) as a precipitant. A well-stirred container was originally charged with distilled water as buffer. Solutions of acid (aluminum nitrate and boric acid) and base (ammonium hydroxide) were slowly added to the distilled water to maintain the pH value of the solution at 8.0 f 0.1. The resulting AAB precipitates were filtered, washed with distilled water, and dried overnight at 100OC. The dried samples were ground to powder and calcined at 550 OC for 5 h. By altering the relative amount of aluminum nitrate and boric acid, a desired AAB composite could be obtained.
0888-588519312632-1573$04.00/0 0 1993 American Chemical Society
1574 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993
The oven-dried AAB material was ground to powder, and then the solution of ammonium heptamolybdate tetrahydrate and cobalt nitrate hexahydrate in an appropriate amount of distilled water was impregnated successfully,where Moo3 content is 12 wt % and Co/Mo atomic ratio is 0.6. Then, the CoMo/AAB powders were extruded by adding an appropriate amount of water to 1/16-in. cylindrical extrudates. The wet extrudates were dried at 100 OC for 12 h, and calcined at 550 "C for 5 h. We denote CoMo/alumina-aluminum borate catalyst as CoMolAABx, where x represents the Al/B atomic ratio. Characterization. Surface areas and pore volumes of catalyst samples were determined by nitrogen adsorption at 77 K, i.e., the BET method, using a Micromeritics 2600 surface area analyzer. The pore size distributions for all catalysts were measured by the nitrogen adsorption method with a Micromeritics Digisorb 2500 instrument. This enables us to measure pore size distributions in the radius range 10-300A. The measurements were performed on the oxide form of the catalyst samples. Since the average pore sizes of these catalysts were not large, the mercury penetration method was not valid in the present study (Lee et al., 1991). The mechanical strengths of catalysts were measured by a Toyama Sangyo TH-203CP tablet hardness tester. The Co and Mo concentrations of catalysts were determined by means of inductively coupled plasma-atomic emission spectrometry (ICP-AES) performed on a Thermo Jarrell Ash 1100 spectrometer. In a typical analysis, 0.1 g of catalyst was dissolved in approximately 3 mL of a mixture of nitric and hydrochloric acid (volumetric ratio 1:3). The mixture was then heated to its boiling point and maintained at that temperature for 1 h. Water was added after the solution had cooled down to make it up to 100 mL. The Co and Mo contents of the solution were subsequently measured by ICP-AES. The spectrometer was calibrated with 1, 10, and 20 ppm solutions of Co or Mo metal. Thermal gravimetric analysis (TGA) was carried out on a Perkin-Elmer TGS-2 analyzer. All catalysts prepared in this study were characterized by X-ray diffraction to investigate their crystallographic properties. X-ray diffraction patterns were obtained by using a Siemens D-500 diffractometer operated at 30 kV and 20 mA with Cu Ka radiation. Temperature-Programmed Desorption (TPD) of Ammonia. For the characterization of surface acidity, in regard to both the amount and strength of acid sites, TPD of preadsorbed ammonia has become every popular. The CoMo/AAB catalyst (0.1g),which was contained in a fused silica tube, was dried at 120 "C for 1 h in a stream of nitrogen (5 mL/s) and equilibrated with ammonia at 100 "C. Then the sample was heated to 800 "C by an increment of 5 "C/min in a nitrogen flow (5 mL/s). The amount of NH3 desorbed from the sample was determined by bubbling the exit gas through an acid solution and using 0.0288 N sulfuric acid as a titrant to maintain the solution at a constant pH value of 4.9. HDS Activity Measurement. Hydrodesulfurization (HDS) of Kuwait atmospheric gas oil was conducted in a stainless steel cocurrent downflow trickle bed reactor. The properties of Kuwait atmospheric gas oil are shown in Table I. The cylindrical catalyst of diameter 1.5 mm and length 4 mm was used in this study. A stainless steel tube reactor of internal diameter 14.3 mm, outer diameter 25.4 mm, and length 430 mm was applied. In a typical run, the reactor was loaded with 12 g of catalyst extrudates. The catalyst bed was diluted with ceramics. Ceramics (40,4, 4, 19 g) and catalysts (3, 4, 5 g) were loaded one after another in order to reduce the dispersion effect and to create a more homogeneous thermal distribution in the reactor. The reactor was heated and controlled by means
Table I. Properties of Kuwait Atmospheric Gas Oil OAPI. 60 O F 34.2 ASTM distillation. O C sulfur, wt % 1.45 IBP density, g/cms 0.854 5% composition, w t % 10% saturate 62.3 20% monoaromatic 14.9 30% diaromatic 16.5 40% polyaromatic 6.3 50% 60% 70% 80% 90% 95% FBP
153 253.8 272.5 290.2 298.8 303.7 308.2 312.8 317.4 323.6 334.3 345.1 348.1
Table 11. Physical Properties of Co-Mo Catalysts
AVB 1 2 2.5 3 3.5 5 8 20 A l p 0 8 surf.area(ma/g) 167 193 212 251 270 244 232 229 284 porevol (mL/g) 0.63 0.33 0.33 0.35 0.36 0.37 0.40 0.42 0.58 avporediam (A) 150 68 62 56 53 61 69 73 82
of three electric resistances, and the temperature over the reaction zone was kept constant. The reaction temperature was monitored with three thermocouples. One of the thermocouples was set in the center of tube reactor; the other two were located at the entrance and exit of the reactor. It has been found that, under the reaction conditions, channeling, axial dispersion, wall heat-transfer effects, and axial heat condition effects are negligible. The catalyst was presulfided with gas oil spiked with dimethyl disulfide (DMDS). Normal operating conditions were 4 WHSV, 400 psi, and 300 mL/min once-through hydrogen flow rate. Reactor temperature profiles were quite isothermal with 2-5 "C. The analysis indicated that the flow pattern of the reactor in this work is indeed in the trickling flow mode. The sulfur contents of feed and products were measured by X-ray fluorescence (Oxford, Lab-2000). The aromatic contenb were analyzed by high-performance liquid chromatography with an ultraviolet detector (Yoes and Asim, 1987). More details of the experimental methods have been described in our previous paper (Chen et al., 1990). All reaction runs were at least duplicated. In general, the deviations in conversions were within 2 wt %, and the average values are reported here. Coke on the spent catalyst was measured using thermal gravimetric analysis (Perkin-Elmer TGS-2) with 02 as the carrier gas. The spent catalyst was washed by dimethylbenzene and then heated a t 60 "C and 1 psi for 72 h.
Results and Discussion Catalyst Characterization. Physical properties of the CoMo/AAB catalysts are strongly affected by the Al/B ratio of the catalysts. Surface areas, pore volumes, and average pore diameters of all CoMo/AAB catalysts studied are listed in Table 11. The average pore diameter was calculated by the common definition of 4VdS, for cylindrical pore, where V, and S, represent the pore volume and internal surface area of catalyst, respectively. As shown in Table 11,the incorporation of a small amount of boron into the alumina matrix resulted in a significant decrease in both surface area and pore volume. However, further incorporation of boron into the alumina matrix resulted in a slight increase in surface area and reached a maximum surface area at an AVB ratio of 3.5. In contrast, pore volumes of the CoMo/AAB catalysts decreased slightly with the boron content, except for the CoMo/ AABl catalyst which has the highest boron content among
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 1576 I 0.8
3001
CoMo/A1203 0.7
5
2
0
I
a 2 Q)
0
5 1 v)
1.50
I\ = IS.\ E
v
5
0
10
15
20
0.5 25
AI/B Ratio
Figure 1. Surface areas and pore volumes versus Al/B ratios for CoMo/AAB catalysts.
CoMo/AAB2 1
100
Pore Diameter
(A)
Figure 2. Pore size distributions of CoMo/AAB catalysts.
the catalysts examined. The CoMo/AABl catalyst has a relatively high pore volume as shown in Figure 1. Figure 1 shows the BET surface areas and the pore volumes versus Al/B atomic ratios of CoMo/AAB catalysts. It clearly shows that a desired surface area could be obtained easily by varying the AVB atomic ratio. In addition, the pore size distributions of the CoMo/AAB catalysts shifted to smaller pores as compared to the CoMo/ A1203 catalyst (Figure 2). The peak diameter, Le., the pore size for dVp/drp maximum in the pore size distribution, has a value between 35 and 40 A. The other information that can be extracted from the pore size distribution is the lack of any bimodal pore distribution as would be expected for materials consisting a simply a mixture of alumina and aluminum borate. This implies that it is a true microcomposite structure material rather than a mixture of the individual materials. These results clearly indicate that the interaction between alumina and boron exists during the formation of the AAB materials, although the chemistry of this process can be quite complex. The results suggest that the AAB materials may exist in several phases such as A1203 + 2A1203.B203 (phase I), 2A1203-B203+ 9A120$&03 (phase 11),and 9A1& 2B2O3 + Bz03 (phase 111)as reported by Gielisse and Foster (1962). It is conceivable that physical properties of the CoMo/AAB catayst are dedicated by the distributions of these phases. Table I11lists the mechanical strengths of the catalysts. The results indicate that the CoMo/AAB catalysts have a much stronger mechanical strength than CoMo/Al203. This implies that the incorporation of boron into the aluminaframework can increase the interactions between
0
CoMo/AAB
I
.I
2 0.6
AI/B Ratio Figure 3. Total acid amounts versus Al/B ratios. Table 111. Mechanical Strength for AAB Supports mech strength mech strength samples (kg/cm2) samples (kg/cm2) AAB8 12.83 AABl 7.70 11.16 10.17 AABl2 AAB2 AAB2.5 10.28 AAB16 8.23 AAB3.5 11.63 AAB20 8.89 AAB5 &os 7.92 10.89
particles. Since no other binder or lubricant additives except distilled water were used during the course of extruding process, it is, therefore, hypothesized that the main bonding types between particles are A1-0-B, Al0-Al, and B-0-B. These chemical bonds between particles may be formed by means of calcination. This indicates that the probabilities of forming the A1-0-B and B-0-B chemical bonds will rise as the boron is added and finally result in a stronger mechanical strength of the catalyst. The XRD results showed that all catalysts demonstrate an amorphous structure. This indicates that the thermal stability of AAB prepared in this study is higher than that of alumina. In addition, no characteristic peaks of Moo3 and COOwere observed for all catalysts, indicating that Co and Mo are highly dispersed on the support. It is hypothesized that microcrystals are formed. However, the crystals are too small to be detected by XRD. Stranick et al. (1987) have reported that the presence of boron on alumina can increase the metal dispersion. Acidity of the Catalyst. Because ammonia is basic, it can be adsorbed on the surface acidic sites of CoMo/ AAB catalysts. Thus, one can apply the temperatureprogrammed-desorption (TPD) method to measure the acid strength of these catalysts, and from the titration amount of desorbed ammonia gas, the surface acid amount can be determined. Wang and Chen (1991) have shown that the acid concentration increases with the boron content of the AAB. Sat0 et al. (1987) also pointed out that combination of boron and alumina produces strong acidic sites, although too much boron occupies the acidic sites or blocks the pore and leads to decrease of the acidity. Figure 3 shows the total acid amounts of the CoMo/AAB catalysts as a function of Al/B atomic ratios. Data plotted in Figure 3 exhibit that the total acid amount of the catalyst increases with decreasing Al/B ratio of the catalyst until it reaches the maximum at the Al/B atomic ratio of 3.5. This is possibly due to the large surface area of CoMo/ AAB3.5 catalyst. Wang (1987) has demonstrated that the acidic sites on CoMo/AABs are mainly related to the Brransted acid. Figure 4 shows that the maximum desorption temperatures of ammonia of CoMo/AAB catalysts are in the range of 180-190 OC, indicating that these catalysts belong to medium-strength acids. The other information we can obtain is that the acid strengths of
1576 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 190 I
1
X
0 CoMo/AAB
170
l-
160 0
0
CoMo/Al,O,
5
10
@I
15
20
40
25
0
AI/B Ratio Figure 4. Maximum desorption temperatures of NHs for CoMo/
I.
0
I
20
10
30
40
50
60
Time on Stream (hours) Figure 5. Gas oil HDS activity versus time on stream. Conditions: 340 "C, 400 psi, Hz flow rate = 300 cm3/min, flow rate of gas oil = 48 crns/h.
CoMolAAB catalysts are much stronger than that of CoMo/AlzO3. The results indicated that the addition of boron in even a small amount could significantly enhance the acid strengths of the catalysts. CoMo/AAB3.5 gives the highest acid strength among all the catalysts. HDS Activity. The percentage of sulfur removal in a particular activity tests is given by %S =
[S,l - [S,,l [Si,]
10
15
20
25
AI/B Ratio
AAB catalysts.
h
5
x 100%
where [Si,] and [S,J are the experimentally measured inlet and outlet sulfur concentrations. Since all the catalysts tested were advanced HDS catalysts, under normally recommended operation conditions all samples gave greater than 90% conversion of the sulfur compounds to hydrogen sulfide. In order to magnify the difference in activity, the tests were carried out under conditions that were less severe for the catalysts than those used in practice. Under the conditions used in this study, the different catalysts showed differences of conversions varying over the range 40-80%. The relation between HDS activity of Kuwait atmospheric gas oil and time on stream with various CoMo/ AAB catalysts is plotted in Figure 5. The decrease in activity during the initial period of time on stream for atmospheric gas oil is much slighter as compared with that of residue oil (Tsai et al., 1991). Indeed, further tests showed that the activity was maintained at a fairly constant level over a period of 200 h. The formation of coke on the catalyst during the reaction has been determined by TGA with air flow to be reasonably negligible. For freedom
Figure 6. HDS activity versus Al/B ratio of CoMo/AAB catalysts.
from the influence of deactivation, the reaction data at the 40th hour on stream were used for comparison. However, it holds the same trend even at other times on stream. Figure 6 shows the percentage of sulfur removal as a function of Al/B atomic ratio. It should be noted that the activities of all CoMo/AAB catalysts are higher than the activity of CoMo/AlzO3, in spite of CoMo/AlzOs having a higher surface area and pore volume than CoMo/AAB catalysts. The high HDS activities of CoMo/AABcatalysts can be interpreted in terms of the presence of highly dispersed boron on the support surface (Stranick et al., 1987). Since boron has a higher electronegativity than aluminum, B-OH has a stronger acidic strength than AlOH on the surface of AAB support. This leads to Mo,02d6 linking with B3+ rather than with A13+. Therefore, it is speculative that more Co-Mo-0 groups (which are the precursors of the Co-Mo-S groups) exist on the surface of AAB than on the surface of &03. This will produce more HDS active sites, generate higher hydrogenation capability, and make higher HDS activity (Topsoe et al., 1981; Topsoe, 1983; Farragher, 1977; Pollack et al., 1979; Thomaset al., 1981;Delannay, 1985). Stranicket al. (1987) reached the same conclusion. They reported that the presence of boron exhibited an increase in metal dispersion compared to boron-free sample. The second explanation for the HDS activity behavior is based on the acidity of the catalyst. The high acidity of the CoMo/AAB could enhance the cracking capability which would result in the increase of HDS activity. Both hydrogenation and cracking capabilities of the catalysts are beneficial to the HDS activity. The HDS activity expresses a progressive increase as Al/B ratio decreases until it reaches a maximum at Al/B = 3.5. This catalyst has highest surface area and surface acidity. The relation between HDS activities and the total acid amounts of the catalysts is plotted in Figure 7. A linear correlation exists between the acid amounts and the HDS activities of the catalysts. However, the results of CoMo/ AAB2 and CoMo/AlzO3 do not fit the correlation line. The acidity of the catalyst has been believed to be an important factor for HDS reaction. The stronger acid sites could enhance the cracking capability which would result in the increase of HDS activity. The low activity of CoMo/AAB2 is not surprising if one considers that it has the lowest surface area and acid strength among all the CoMo/AAB catalysts. The low activity of CoMo/AlzOs is due to the low acid strength. Reaction Kinetics. Since the sulfur compounds in oils are not simple, the HDS mechanism becomes complex. For AGO, a wide range of a series of different sulfur
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 1877 ," 2.4
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55 'c
co''i
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Total Acidic Amount (m mol/g) Figure 7. HDS activity versus total acid amount of CoMo/AAB catalysts (CMAABx denotes CoMo/AABs, where x represents the Al/B atomic ratio).
1.50
1.55
1.60
1.65
1.70
lOOO/T (1/K) Figure 9. Data fit for Arrhenius reaction rate formula (CoMo/ AAB3.5). Table IV. Aromatic Saturation in the Product Stream saturation monoaromatic Co-Mo polynuclear (wt %) aromatic (wt % ) catalysts (wt%) AGO feed 62.3 14.9 22.8 AAB2 65.0 17.9 17.1 AAB2.5 65.9 17.9 16.2 AAB3.5 65.4 18.5 15.9 AAB5 65.7 18.4 15.9 AAB8 64.4 17.4 18.2 AAB20 64.6 17.3 18.1 &Os 64.0 17.9 18.1
0.0
0.1
0.2
0.3
0.4
0.5
1/WHW (g hr/crn3) Figure 8. Data fit for second-order rate equation (CoMo/AAB3.5).
compounds exists. non-asphaltene reactivity is high and asphaltene reactivity is very low. Therefore, the reaction order for AGO depends on its composition. Generally speaking, the higher the boiling point of the oil is, the more difficult it is to perform HDS reaction. For the first-order kinetics of HDS reaction, the rate equation can be described as h(CdCl) = h[l/(l-X,)] = kl/WHSV
(2)
For the second-order kinetics, the rate equation is XJ(1- X,) = k,CdWHSV (3) where COis the sulfur concentration of feed, C1 is the sulfur concentration of product, X, is the desulfurization conversion, kl is the rate constant for first-order reaction, k~ is the rate constant for second-order reaction, and WHSV is the weight hourly space velocity. Fitting the experimental data of this study to the above two equations, one obtained a well correlated pseudo-second-order reaction profile for HDS of AGO as shown in Figure 8. In a previous paper (Tsai et. al., 1991), the authors also reported a second-order kinetics for HDS of heavy residue oils over CoMo/AAB catalysts. Although the desulfurization rate of difficult individual sulfur compounds follows the firstorder rate equation, when these compounds are simultaneously desulfurized the combined results provide an apparent second-order behavior (Schuit and Gates, 1973). Reaction activation energy for desulfurization (E,) can be calculated from the Arrhenius rate equation
k, = A exp(-Ea/R7') (4) where R is the gas constant, A is the preexponential factor, and T is the temperature in K. As shown in Figure 9, E a calculated from the slope is 26.0 kcal/mol. The Arrhenius rate formula in this work is
k, = 2.31 X 10" exp(-26.O/RT)
(5)
In a previous paper (Tsai et. al.,1991))the authors r e p o d an activation energy of 29.0 kcal/mol for HDS of heavy residue oils over the same catalyst. Aromatics Saturation. A summary of the available results allowinga quantitative comparison of the reactivity of the various catalysts for aromatics saturation is listed in Table IV. Since the aromatic ring saturation reactions require more severe processing conditions,the CoMoIAAJ3 catalysts express a low activity for aromatic saturation under our operation condition. However, it is expected that the aromatic saturation will be increased to a large extent by increasing temperature, increasing hydrogen partial pressure, and using NiMo as the active component. In general, CoMoIAAB catalysts show better aromatic saturation capabilities than that of CoMo/AlzOa. This reconfirms our previous conclusion that the presence of boron will increase the hydrogenation ability of the catalyst. The increase in monoaromatic content in the product compared with that of AGO feed is caused by hydrogenation and hydrogenolysis of polynuclear aromatics (Girgis and Gates, 1991). Conclusions The results presented in this paper have shown that a new family of catalyst in which CoMo is dispersed on the alumina-aluminum borate support appears to be well suited for HDS reactions of AGO. The HDS activities of
1678 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993
all CoMo/AAB catalysts are higher than that of CoMo/ A1203. The catalyst with Al/B atomic ratio of 3.5 gives the highest activity. The results of TPD of ammoniaof CoMo/AABs indicate that the total acid amountsof CoMo/AABsshow a volcano plot with the maximum a t Al/B ratio = 3.5. Since boron has a higher electronegativity than alumina, B-OH has a stronger acid strength than A1-OH on the surface of AAB support. This leads to that M07024~linking with B3+ rather than with A13+. Therefore, it is speculative that more Co-Mo-0 groups (the precursor of Co-Mo-S) exist on the surface of AAB than on Al2O3. This produces more HDS active sites and makes higher HDS activity. A correlation exists between the acidity and the HDS activity of the catalyst. The higher the acidity, the higher is the HDS activity. The HDS activities of catalysts on AAB supports can be rationalized on the presence of boron. On one hand, boron can increase the metal dispersions and the hydrogenation capabilities of the catalysts. On the other hand, it can enhance the surface acidities and the cracking abilities of the catalysts. Both are beneficial to the HDS reactions of AGO. The kinetics of HDS activity can be described by a pseudo-second-order rata equation. The activation energy for desulfurization is found to be 26.0 kcal/mol. The Arrhenius reaction rate formula is 2.31 X 1O1O exp(-26.0/
RT). Acknowledgment This research is supported by the Chinese Petroleum Corporation of the Republic of China.
Nomenclature A = preexponential factor CO= feed sulfur concentration, wt % E, = activation energy, kcal/mol kl = rate constant for first-order reaction kz = rate constant for second-order reaction, mol/(g.s) R = gas constant, 1.987 cal/(mol.K) rp = pore radius of catalyst, A S, = internal surface area of catalyst, m2/g T = temperature, K V , = pore volume of catalyst, cmVg WHSV = weight hourly space velocity, h-1 X, = desulfurizationconversion, wt %
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