Ind. Eng. Chem. Res. 1994,33,2040-2046
2040
Hydrodesulfurization of Residue Oils over NiMo/Alumina-Aluminum Borate Catalysts: Effect of Boria Content? Yu-Wen Chen' and Ming-Chang Tsai Department of Chemical Engineering, National Central University, Chung-Li, 32054 Taiwan
Chiuping Li' Refining & Manufacturing Research Institute, Chinese Petroleum Corporation, Chia- Yi, 60036 Taiwan
A series of alumina-aluminum borates (AAB) with various boria contents were prepared by a coprecipitation technique. They were characterized with respect to surface area, pore volume, and pore size distribution. The temperature-programmed-reduction (TPR)method was used to monitor molybdenum oxide-support interaction. T h e results showed that both the Tmaxand hydrogen consumption of low-temperature-reduction peak increase with increasing B203 content. Hydrodesulfurization of heavy residue oils over NiMo/AAB catalysts was carried out in a trickle bed reactor a t 663 K and 7.6 MPa. T h e results revealed that these catalysts are much more active than the conventional NiMo/Alz03catalysts. The dispersion of active sulfide phase as well as the hydrogenation ability of NiMo/A1203 catalyst is increased by the incorporation of adequate boria content. An optimum Bz03content which gives the highest activity was found in the vicinity of 4 w t % ' .
Introduction Given the importance of the hydrodesulfurization (HDS) process in petroleum refining, coal liquefaction, and environmental problems, great efforts have been made in the study of this problem in the past half century. Hydrotreating of heavy residue oils is generally carried out over AlzO3-supported catalysts to disperse an active phase composed of mixed sulfides such as Ni (Co)-Mo (W)-S. Changing the nature of the carrier is an interesting line of research to achieve more thorough hydrodesulfurization (HDS). Until now, various supports such as carbon (Topsoe et al., 1986),silica (Spozhakinaet al., 1988),zeolite (Cid et al., 1987),zirconia (Duchet et al., 19911, and mixed oxides (Wang and Chang, 1989; Daly, 1989) have been used to prepare hydrotreating catalysts. A detailed review of support effects on hydrotreating catalysts has been published by Breysse et al. (1991). Mixed oxides, particularly alumina-based binary oxides, are well-suited catalytic materials. In a previous paper (Chen et al., 1990)the authors demonstrated that CoMo/ alumina-aluminum phosphates possess high catalytic HDS activity, and the maximum activity is located at atomic ratio P/(Al+ P)= 0.1. However, less attention has been devoted to alumina-aluminum borate (AAB). Tanabe (1970) and Pine (1976) have shown that alumina-boria can be used as cracking and hydrocracking catalysts, respectively. Sat0 (1987)reported that alumina-supported boria exhibited a high conversion of cyclohexanone oxime to caprolactam. An optimum boria content to effect the most active and selective Beckmann rearrangement reaction of cyclohexanone oxime occurred between 20 and 25 wt 5%. They suggested that boria, in excess of the optimum value, might have been deposited on active sites on alumina, so lowering the activity of the catalyst, since boria itself showed little activity in the absence of alumina. Later, the same experiment over alumina-boria was performed by Curtin et al. (1992);a correlation was observed between the concentration of surface acidic sites of catalyst and
* To whom correspondence should be addressed.
Part of the results have been presented at Asian Pacific Confederation of Chemical Engineers, Melbourne, Australia, September 1993. t
the selectivity to caprolactam. Peil et al. (1989) prepared AAB by a coprecipitation method. They reported that this material is highly acidic. Wang and Chen (1991) also reached the same conclusion. Hydrothermal stability of AAB was examined by Tsai and Chen (1990) under 750 "C for up to 48 h. AAB exhibited a very good hydrothermal stability compared to alumina. Recently, Chen and Li (1992) investigated liquid-phase hydrogenation of cyclohexene over Pt/AAB catalyst and found that high activity can be attributed to high metal dispersion and high turnover frequency as a result of boron support modification. The hydrogenolysis activity of NiMo/AlzOa catalyst (Lafitau et al., 1976) and the CO hydrogenation activity of Ru/A1203catalyst (Okuhara et al., 1985)were increased as a result of boron support modification. This boronmodified catalyst effect has also been revealed by bulk and surface spectroscopictechniques for Co/A1203catalyst (Stranick et al., 1987). In addition, Wendlandt and Unger (1990) investigated the properties and applications of boron-containing pentasil-type zeolites, and observed a higher stability and a more advantageous product selectivity than conventional ZSM-5 containing catalysts during the course of reforming and hydroisomerization reactions. In our previous studies (Tsai et al., 1991;Li et al., 1993), AAB was reported to be a good support material for CoMo catalysts in hydrotreating of heavy residue oils and atmospheric gas oils. The study reported here is a continuation of our previous work on hydrotreating reactions of heavy residue oils over NiMo/AAB catalysts. Conventional NiMo/AlzOa catalyst was also included for comparison. The aim of this study mainly focused on the effect of boria content.
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 f 0.02) as a precipitant. Both the acidic solution and the basic solution were slowly added to a well-stirred vessel containing distilled water as a stirring medium. The pH value was maintained at 8.00 f 0.02throughout the precipitation process. The resulting precipitate was filtered and washed with distilled water
OSSS-5SS5/94/2633-2040$04.50/00 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 2041 until no nitrate ions could be detected. The cake was dried at 120 "C overnight and then calcined a t 500 "Cfor 5 h. By altering the relative amount of aluminum nitrate and boric acid, a desired AAB composite could be obtained. After calcination, dry clump AAB was well ground and then extruded by addition of an appropriate amount of distilled water to 1/16-in. (1 in. = 2.54 cm) cylindrical extrudates. The wet extrusion was dried in air at room temperature, dried at 120 "C overnight, and calcined at 500 "C for 6 h. NiMo/AABcatalysts were prepared by incipient wetness impregnation of supports in the form of extrusion with solutions of ( N H ~ ) M O ~ O ~ . Vand ~ H ZNi(NOs)r6HzO, O successively in distilled water, where Moo3 content is 12 w t 7% and Ni/Mo atomic ratio is 0.6. Mo was added first, and catalyst was dried and calcined before addition of Ni. The first impregnation (Mo) was dried at atmospheric pressure and room temperature overnight and at 120 "C for 6 h, and then calcined at 500 "C for 6 h. The second impregnation (Ni) was dried and calcined in the same way. This calcination temperature has been reported to give optimal HDS activity (Stanislaus et al., 1988). Characterization. Surface areas, pore volumes, and pore size distributions for all catalysts were obtained from the analysis of nitrogen adsorption-desorption isotherms at -176 "C with a Micromeritics ASAP 2400 instrument. 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. The Ni and Mo contents 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 3 mL of a mixture of a nitric and hydrochloric acid (volumetric ratio 1:3). The mixture was then heated to its boiling point and maintained at that temperature for 1h. Water was added after the solution had cooled to make it up to 100 mL. The Ni and Mo contents of the solution were subsequently measured by ICP-AES. The spectrometer was calibrated with 1, 10, and 20 ppm solutions of Ni or Mo metal. All catalysts prepared in this study were characterized by X-ray diffraction to investigate their crystallographic properties. X-ray powder diffraction (XRD) patterns were carried out with a Siemens D-500 diffractometer operated at 35 kV and 20 mA with Ni-filtered Cu K a radiation (A = 1.5418 A). Finely ground samples were scanned at a speed of 2 deg min-1 over the diffraction angle range 28 = 10"-60". Temperature-Programmed Reduction (TPR). TPR was used to monitor molybdenum oxide-support interaction. NiMo/AAB catalyst was placed in a U-type quartz reactor tube. The sample (0.2 g) was heated in a Ar flow at 400 "C for 2 hand then cooled to ambient temperature. The sample was then heated in a HdAr reducing gas mixture (HdAr = 1/9) from room temperature to 900 "C by a heating rate of 5 "Urnin and retained at 900 "C for 1 h. Water, formed by reduction or dehydration, was trapped in a 4A molecular sieve column. After the gas mixture had passed through the molecular sieves, its composition was monitored continuously with a thermal conductivity detector. Presulfiding Treatment. A spiked feedstock method was used for the presulfiding of the catalysts, where dimethyl disulfide (DMDS) was used as a spiking agent. Before sulfiding, a Nz flow purge was flowing through the reactor at 120 "C overnight to desorb water. A
5
Figure 1. Schematic diagram of HDS test unit. 1, ATB tank; 2, diesel tank; 3, DMDS tank; 4, stop valve; 5, feed pump; 6, check valve; 7, reactor; 8, furnace; 9,separator; 10,surge tank; 11, back pressure regulator; 12, receiver; 13, condenser; 14, absorber; 15, product stock tank;16,sampling valve; 17,compressor; 18,three way valve; 19,needle valve; 20,mass flow controller; 21,wet test meter. Table 1. Properties of Kuwait Atmospheric Tower Bottom Residue Oil
OAPI, 60 O F sulfur, mass % nitrogen, mass % Conradson carbon residue, mass % nickel, mg/kg vanadium, mg/kg H/Cratio ASTM distillation, K 5% 10% 20% 30% 40% 50% 60% 65%
16.8 3.72 0.21 10.1 14.0 53.0 0.146 554 584 633 673 704 743 794 811
hydrogen stream (7582 kPa and 300 mL (STP)/min;STP, standard temperature and pressure) was then switched in. The diesel oil feedstock doped to a level of 1 wt 7% sulfur by addition of DMDS was passed through the reactor (1mL/min) with a temperature program. The following temperature program was applied: heated from room temperature to 175 "C and retained for 2 h, and then increased to 250 "C and held for 4 h; after that, increased to 320 "C and retained for 4 h. HDS Activity Measurement. Hydrodesulfurization of Kuwait residue oil was conducted in a stainless steel cocurrent downflow trickle bed reactor contained in a fluidized sand bath. The reactor was made of a 318-in.i.d. 316 stainless steel pipe and was equipped with a calibrated feed buret, a pump, a gas-liquid separator, and a product collector. The schematic diagram of HDS test unit is shown in Figure 1. In a typical run, the reactor was loaded with 20 g of catalyst extrudates. The catalyst bed was diluted with ceramics (50-70 mesh). Ceramics (40,4, 4,4,20 g) and catalyst (3,5,6,6 g) were loaded one after another in order to reduce the dispersion effect and to create a more homogeneous thermal distribution in the reactor (Chen et al., 1990). The feedstock used was Kuwait atmospheric tower bottom (ATB) residue oil. Some of its properties are listed in Table 1. The feed was passed through the reactor (0.5 mL/min) at 663 K, 7.6 MPa, and weight hourly space velocity = 1.5. In all runs a large excess of hydrogen was used in order to prevent inhibition effect of HzS. The hydrogen flow rate was 300 mL(STP)/ min. Temperature profiles with 2 "C variations were typically observed. At an appropriate time, the liquid samples were withdrawn from the separator with a sampling valve. The sulfur content of the liquid product was measured by a X-ray fluorescence spectrometer
2042 Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 Table 2. Physical Properties of AAB S U D D O ~ ~ S nominal AI/B 0.5 1 2 3.5 5 10 8 12 20 a
SUDDOrt measd measdBzOs, AI/B wt% 27.0 1.3 14.9 3.9 9.5 4.8 5.5 9.2 4.6 10.6 2.0 26.4 29.3 1.9 30.5 1.8 1.0 38.0 0
S B ~ , PV, cm3/g m2/g 0.45 98 0.55 158 229 0.51 0.43 225 0.73 308 0.40 229 0.40 222 0.58 270 0.36 222 0.41 203
PD,” nm 18.4 13.9 8.9 7.6 9.5 7.0 7.2 8.6 6.5 8.1
Calculated from 4Pv/sBm for cylindrical pore.
(Oxford,Lab-X2000). It should be noted that the amount of light ends was found to be very small; the liquid yields for all the runs were around 100% by weight because the reaction conditions are relatively mild. All the details of the experiment have been reported in the previous papers (Chen et al., 1990; Tsai et al., 1991; Li et al., 1993). The analysis indicated that the flow pattern of the reactor in this work is indeed in the trickling flow mode. All reaction runs were at least duplicated. In general, the deviations in conversions were within 2 wt % ,and the average values are reported here.
Results and Discussion Catalyst Characterization. Surface areas, pore volumes, and average pore diameters of all prepared AAB supports and NiMoIAAB catalysts are listed in Tables 2 and 3, respectively. It can be seen in Tables 2 and 3 that the incorporation of a small amount of boron into the alumina structure resulted in a slight increase in both surface area and pore volume. However, further incorporation of boron into the alumina framework resulted in a decrease in surface area and a striking increase in pore volume. In addition, it is worth mentioning that the measured A11B atomic ratios are approximately 2-3 times those of nominal ones (as shown in Tables 2 and 3). This indicates that some of boron is not precipitated at pH = 8 in this study. The other important information that can be extracted from these results is that the surface areas of NiMoIAAB catalysts are greater than those of AAB supports, especially for the higher Al/B atomic ratios materials. It is postulated that the impregnated Ni species will cause more “defect” across the internal surface area of catalyst and, thus, increase the total surface area. Of course, the phenomenon of pore plugging of small pores caused by impregnation will occur simultaneously and result in a decrease in surface area. Therefore, the obtained catalyst surface area will be influenced by the abovementioned two opposite effects. A comparison of the BET surface areas of AAB supports with those reported by Peil et al. (1989) was made. It has clearly shown that the surface areas of aluminum borate materials prepared in this study are always smaller than those presented by Peil et al. (1989). It should be noted that their surface area of pure alumina (i.e., Al/B = infinite) is 303 m2/g; however, our result is 203 m2/g. Therefore, the distinct results may be due to different chemical sources. Figure 2 showsthe BETsurface areas and average pore diameters versus Bz03contents of aluminum borates. The effect of boria content on the pore structure is unclear. However, all AAB supports demonstrate a high surface area and a small average pore diameter, except at very high boria content (>25 wt %).
The results of pore volume distribution measurements of AAB and NiMo/AAB samples are shown in Tables 4 and 5, respectively. They showed the same trend as the surface areas and pore volumes, i.e., shifted toward small pores after pore impregnation of Ni and Mo. By examination of the pore volume distributions of NiMoIAAB, one could reach the following general conclusions. The medium pore radius, i.e., the pore radius for dVldr maximum in the pore size distribution, has avalue between 2 and 3 nm. An important piece of information which can be extracted from the pore size distributions is the lack of any bimodal pore distribution as would be expected for materials consisting of simply a mixture of alumina and boria oxides. This result indicates that these materials are not just mixtures of alumina and boria. To be more specific, the monodispersed pore size distribution of AAB implies that it is a true microcomposite structure material rather than a mixture of individual materials. These results also clearly indicate that the interaction between aluminum and boron exists during the formation of AAB materials, although the chemistry of this process can be quite complex. XRD. The XRD results showed that all NiMo/AAB catalysts demonstrate an amorphous structure, in agreement with previous results of CoMo/AAB catalysts (Tsai et al., 1993). 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 NiO were observed for all catalysts, indicating that Ni 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. In a previous paper (Wangand Chen, 1991),one of the authors has shown that AAB has a higher concentration of OH groups than alumina. From the fact that, in supported catalysts, the monolayer phase is formed by a strong chemical interaction between OH groups of the support surface and the supported metal oxide precursors present in the impregnated solution (Lakshmi et al., 19931, it is expected that more Ni-Mo-0 groups (which are the precursors of Ni-Mo-S groups) exist on the surface of AAB than on the surface of alumina. The more Ni-Mo-0 groups means high dispersions of Mo on the AAB surface. TPR. The TPR patterns of active metals Ni and Mo supported on various aluminum borate supports demonstrate a two-peak profile for all samples (Figure 3). The low-temperature-reduction peak T,, in TPR spectra increases dramatically from ca. 470 to 703 OC. Yet the trend of the high-temperature-reduction peak T,, with various samples is irregular. It has been widely accepted that the low-temperature-reduction peak is the octahedrally surrounded Mo6+multilayer reduction peak (Thomas et al., 1982; Arnoldy et al., 1985). Multilayer Moo3 is the precursor and responsible for HDS activity. Figure 4 shows the low-temperature-reduction peak T,,, and hydrogen consumption versus B203 content of NiMo/AAB catalysts, where the amounts of hydrogen consumed were estimated from the low-temperature-reduction peak area. Two linear correlations were established. This indicates that both the T,, and hydrogen consumption increase as the Bz03 loading is increased. It has been reported (Arnoldy et al., 1985) that in the oxidic catalyst the Mo0-A1 links polarize the Mo-0 bonds, making them more difficult to reduce. In this study, AP+ in the BO4% environment has a stronger ability to polarize Mo-0 bonds
Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 2043 Table 3. Physical Properties of NiMo/AAB Catalysts
catalyst measd AI/B 2.7 5.5 10.0 11.7 32.5 34.1 65.1
nominal
AUB ~
~
~~
1 2
3.5 5 8 12
20 NiMo/AlzOa a
measd BzOs, wt%
&ET, m2/g
11.3 6.6 3.8 2.7 1.3 1.0 0.7 0
118 207 252 252 271 306 279 250
PV, cmYg 0.43 0.52 0.32 0.41 0.36 0.23 0.31 0.32
PD,a nm 14.6 10.0 5.1 6.5 5.3 3.0 4.4 5.1
Ni, w t % 3.52 3.23 3.92 3.81 3.52 4.03 4.26 4.11
Mo, w t % 9.49 8.85 10.74 10.06 9.86 11.15 11.32 11.10
Ni/Mob 0.61 0.60 0.60 0.62 0.58 0.59 0.61 0.60
Calculated from 4PV/SBm for cylindrical pore. Atomic ratio. 320
I 20
I
0 IO
Po 280
p)
-0
200 0
0-
0
0
5
10
15
20
25
30
content, w t . % Figure 2. BET surface area and average pore diameter versus Al/B atomic ratio.
than in pure Alz03. This would make Moo3 more difficult to reduce on the AAB supports than on the alumina support. The relatively high Tm, indicates a stronger interaction between the carrier and the Mo species. More hydrogen consumption implies not only the more active sites of Mo species but also stronger hydrogenation ability. Although the reducibility of NiMo/ AAB samples cannot be facilitated by incorporation of boron into the alumina matrix, the amounts of active sites and the hydrogenation ability could be enhanced even when a little boron was incorporated (Chen et al., 1993). The more active sites and the stronger hydrogenation abilities may be attributed to the contribution of the high concentrations of OH groups, which can yield high dispersion of Mo metal. HDS Activity. The percentage of sulfur removal in a particular activity test is given by %S=
[S,l - [S,,I [Si,]
x 100%
where [Sd and [S,J are the experimentally measured inlet and outlet sulfur concentrations. All the catalysts were very active and gave greater than 90% conversion of the sulfur compounds to hydrogen sulfide under normal recommended operation conditions. 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. The relation between HDS activity of residue oils and time on stream with various NiMo/AAB catalysts is plotted in Figure 5. The decrease in activity during the initial period of time on stream is essentially due to coke formation (Tsai et al., 1991). For freedom from the influence of deactivation, the reaction data a t 40th h on stream were used for comparison. However, the same trend holds even a t other times on stream. The results
showed that all NiMo/AAB catalysts are much more active than NiMo/AlzO3 except the sample with Al/B ratio of 2.7, whose activity and stability are poor. HDS Activity vs Boria Content. In order to investigate the influence of boria content on HDS activity, all the correlations were plotted against Bz03 content. Figure 6 demonstrates HDS activities as a function of BzO3 contents of NiMo/AAB catalysts. The results revealed a volcano plot, and the optimum Bz03 content is in the vicinity of 4 w t %. This can be interpreted in terms of the interaction between multilayer Moo3 and support, the amounts of active sites, and the hydrogenation ability of the catalyst itself. Because sulfidability of catalyst, especially multilayer MoO3, was generally used to account for the HDS activity, the weaker interaction means that multilayer Moo3 is open to sulfide and results in a higher HDS activity. In addition, the amounts of active sites and hydrogenation ability increase with Bz03 content. It is clear that the molybdenum species react preferentially with the B-OH groups on the AAB surface. In addition, the presence of boron may influence the other surface hydroxyls so that they are more reactive toward molybdenum (Lewis and Kydd, 1992). Both are beneficial to the dispersion of MOOS. Molybdenum on the AAB supports, particularly that bonded to B-OH groups, is more easily sulfided/reduced compared to that on A1203support. However, due to their too strong interaction between Moo3 and support in excess BzO3 content, Moo3 would become more difficult to reducelsulfide and result in the decrease of activity. It should be noted that sulfidability depends not only on interaction with the support but also on the dispersion. HDS Activity vs TPR T-. It has been widely accepted that, in the NiMo supported catalysts, the first peak at low temperature in the TPR profile is the octahedrally surrounded Mo6+ “multilayer” reduction peak, and the second peak at high temperature is the octahedrally and tetrahedrally surrounded Mos+ “monolayer” reduction peak (Arnoldy et al., 1985). However, the monolayer species has little contribution to HDS activity, and only the multilayer Moo3 is the precursor of active site. Therefore, it is meaningful to find a possible correlation between T- of the low-temperature-reduction peak and HDS activity. The HDS activity versus TPR T,, is presented in Figure 7. The line shown in Figure 7 is not necessarily a straight line. It is just an indication of the general trend. The results show that HDS activity is strongly influenced by the TPR Tm,. The higher the T,, is, the lower is the HDS activity. The results are in agreement with our previous results for CoMo/AAB catalysts (Tsai et al., 1991). The TPR patterns showed that when BzO3 content is greater than 10 wt % , the interaction between Moo3 and AAB support becomes very strong. A strong interaction decreases the covalent character of the Mo-S bond and
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Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994
Table 4. Pore Volume Distribution of AAB Supports
support AB1.4 AB2.9 AB4.8 AB8.7 AB9.2 AB9.8 AB19.8 AB22.9 AB26.4 AB29.3 AB30.5 AB35.8 AB38.0
2-3 0.010 0.167 0.138 0.194 0.218 0.206 0.239 0.278 0.293 0.129 0.243 0.250 0.111
1-2
0.008 0.017 0.012 0.062 0.037 0.049 0.057 0.044 0.076 0.036 0.006 0.147 0.052
3-4 0.009 0.202 0.121 0.072 0.062 0.069 0.048 0.048 0.024 0.203 0.138 0.023 0.177
pore radius range, nm 4-5 0.019 0.039 0.075 0.018 0.006 0.018 0.011 0.010 0.003 0.001 0.003 0.007 0.001
5-7.5 0.098 0.021 0.026 0.014 0.001 0.008 0.003 0.009 0.008 0.001 0.001 0.006 0.002
7.5-10 0.109 0.005 0.001 0.003 0.001
10-20 0.144 0.009 0.002
0 0 0
0 0 0 0 0 0 0 0
0.004 0.001
0.004 0.001
0 0
0.004
Table 5. Pore Volume Distributions of NiMo/AAB Catalysts
catalyst NiMo/AB5.5 NiMo/AB10.0 NiMo/AB11.7 NiMo/AB32.5 NiMo/AB34.1 NiMo/AB65.1
1-2 0.161 0.110 0.133 0.126 0.040 0.138
2-3 0.043 0.141 0.144 0.044 0.131 0.129 Bz03
3-4 0.022 0.010 0.044 0.004 0.006 0.003
content
pore radius range, nm 4-5 0.004 0.011 0.021 0.002 0.008 0.003
5-7.5 0.006 0.003 0.002 0.003 0.001 0.003
7.5-10 0.001 0.002 0.001 0.002 0.001 0.003
10-20 0
0.001 0.001 0.001 0.001 0.007
, 2.0
973 7
073 --
0 wt% h
5 10.5
1. 8 wt%
E \
1 . 9 wt% 2 . 3 wt%
rn
Y
'
1
4 . 0 wt% 4 . 8 wt%
\11.
6 wt%
80
-A
3
< -+-
P
2 1 . 9 wt% -773 573
1173 973
I- isothermal
Temperature (K) Figure 3. TPR patterns of active metals Ni and Mo supported on various supports.
results in the reduction of anion vacancy. The anion vacancies on Mo are the active sites for HDS reaction; in other words, the strong support interaction hinders the formation of active sulfide phase and results in poor HDS activity. HDS Activity vs Mo Surface Coverage. To further investigate the effect of Mo on HDS activity, the surface coverage of Mo on the support was used for comparison and the result is presented in Figure 8. The definition of surface coverage is the number of Mo atoms over the AAB surface area. Ni is assumed as a promoter, and it is excluded in the calculation for active metals. The calculated surface coverage values indicated that all the catalysts have high coverages of Mo metal. This would
6 0 g
0
E a, IY L
40-
3
NiMo/AB2.7 0 NiMo/AB10 - - 0 NiMo/AB34 1 -- A NIMo/AI,O~
- &a s--p:--
YAG--\
l Q a a
'c 3
m 20 1
1
-a
Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 2045
r-----l
6o
30
10 content (wt.%)
15
5
0 B,O,
Sulfur removal versus B203 content.
sulfide phase; increase of the hydrogenation ability and the amount of active site. All are beneficial to the HDS reactions. 3. The HDS activity of NiMoIAAB is a function of boria content. I t shows avolcano plot. An optimum BzO3 content which gives the highest activity was observed at 4 wt % or so. 4. A correlation exists between HDS activity and TPR T-. The higher is the T-, the lower is the activity. The high TPR T- indicates a strong Mo03-support interaction. The strong support interaction hinders the formation of active sulfide phase and results in poor HDS activity. 5. The HDS activity of NiMo/AAB is increased with decreasing the surface coverage of Mo on the AAB support. It implies that the greater surface area possesses more opportunity to provide better dispersion of active sulfide phase and results in higher HDS activity. Acknowledgment This research is supported by the National Science Council (NSC-82-0402-EOOS-012)and Chinese Petroleum Corporation of the Republic of China. Literature Cited
700
I
I
1
800
900
1000
Sulfur removal versus TPR T-.
1
a
i!
B
4 m
30 1.5
3.0
I
I
J
4.5
6.0
7.5
Mo-content, atom/-
e
Figure 8. Sulfur removal versus Mo surface coverage.
possesses more opportunity to provide better dispersion of active sulfide phase and results in a higher HDS activity. Because the TPR results cannot well account for low HDS activity for NiMo/AlzOs catalyst under high reducibility surroundings, the low dispersion of this catalyst probably makes up for the TPR deficiency for interpreting the results of HDS activity. Consequently, the dispersion of active sulfide phase as well as the amounts of active sites and hydrogenation ability of NiMo/AlaOa catalysts is enhanced by incorporation of adequate BzO3 content. Conclusions 1. The TPR results indicated that both the T- and hydrogen consumption of low-temperature-reduction peak increase with increasing B2O3 content. 2. The HDS results revealed that the NiMo/AAB catalysts are much more active than NiMo/AlzOs. This can be rationalized in the presence of boron. Several effects may concur in NiMo/AAB catalysts and facilitate their HDS activities: enhancement of the dispersion of active
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* Abstract published in Advance A C S Abstracts, August 1, 1994.
