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Alumina-Titania Mixed Oxide Used as Support for Hydrotreating Catalysts of Maya Heavy CrudesEffect of Support Preparation Methods S. K. Maity,* J. Ancheyta, Mohan S. Rana, and P. Rayo Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Col. San Bartolo Atepehuacan, Me´ xico D. F. 07730 ReceiVed August 15, 2005. ReVised Manuscript ReceiVed January 5, 2006
The main objective of this work is to study the effect of preparation methods of supports on hydroprocessing of Maya heavy crude. In this respect, we have used different procedures to prepare alumina-titania binary mixed oxide supports. A wetness incipient impregnation method is used to make PCoMo catalysts by using these binary supports. The catalysts are characterized by XRD and pore size distribution and the hydroprocessing activities of these catalysts are evaluated for a microreactor and batch reactor. Maya heavy crude and a mixture of Maya crude and hydrodesulfurized diesel are used as feedstock in the batch reactor and microreactor, respectively. The supports prepared by sodium aluminate and titanium chloride salts have higher pore volume and bigger average pore diameter compared with supports prepared by the other methods in this investigation. XRD results reveal that the titanium oxide is well dispersed over alumina in the supports prepared by the above said method. The catalysts supported on binary oxide prepared by this method exhibit higher activities and stability with time-on-stream. The activity results obtained from the microreactor are more or less comparable with those obtained from the batch reactor. The rate of deactivation is faster in the initial period of the reaction and after that it is slowed down. It is also found that the rate of hydrodemetalation deactivation is faster than that of hydrodesulfurization.
Introduction The use of titania as a support to prepare hydrotreating catalysts is well-known due to its higher intrinsic activity. This oxide has also suitable morphology and higher reducibility compared with alumina-supported catalyst. Several authors reported the beneficial effect of this oxide for use in hydrotreating catalyst.1-4 However, its disadvantage in commercial application is due to its low surface area and low thermal stability. To avoid these disadvantages the use of binary oxides, such as alumina-titania and zirnonia-titania, is an attractive option. Several methods have been employed to prepare aluminatitania support, like coprecipitation, grafting, and vapor deposition. In general, higher surface area binary oxides are obtained in these methods compared with just TiO2. Rodenas et al.5 reported that Al2O3-TiO2 oxide prepared by urea hydrolysis was less acidic compared with the binary oxide prepared by ammonia hydrolysis. Zhabobin et al.6 have used various methods to prepare mixed oxides supports. They have found that the grafting method is the best one to obtain better dispersion of titania over alumina. In our earlier study,7 different preparation methods such as urea hydrolysis, pH swing, and a combined * Corresponding author. Tel: 52-55-91758422. Fax: 52-55-91758429. E-mail:
[email protected]. (1) Ramirez, J.; Fuentes, S.; Diaz, G.; Vrinat, M.; Breysse, M.; Lacroux, M. Appl. Catal. A 1989, 52, 211. (2) Breysse, M.; Portefaix, J. L.; Vrinat, M. Catal. Today 1991, 10, 489. (3) Nag, N. K. J. Phys. Chem. 1987, 91, 2324. (4) Maity, S. K.; Rana, M. S.; Bej, S. K.; Ancheyta, J.; Murali Dhar, G.; Prasada Rao, T. S. R. Appl. Catal. A: Gen. 2001, 205, 215. (5) Rodenas, R.; Yamaguchi, T.; Hattori, K. J. Catal. 1981, 69, 434. (6) Zhaobin, W.; Qin, X.; Xienxian, G.; Sham, E. L.; Grange, P.; Delmon, B. Appl. Catal. 1990, 63, 305.
method of urea hydrolysis and pH variation were used to prepare alumina-titania binary oxides. It was found that the support obtained by the combined method gave bigger pore diameter and a higher pore volume support. Various authors studied the effect of Al2O3-TiO2 binary oxide as a support for hydrodesulfurization (HDS) of thiophene or benzothiophne.6-13 Muralidhar and co-workers8-11 have reported the thiophene activity of MoO3 and WO3 catalysts supported on this oxide. They have studied the effect of compositions of alumina and titania in mixed oxides. It was found that catalyst supported on alumina-titania (1:1 by weight) showed the highest HDS activity compared with catalysts supported on alumina-titania of other compositions. The results were also compared with pure TiO2 and pure Al2O3 supported catalysts. Catalyst supported on Al2O3-TiO2 (1:1) showed 5 times higher activity than Al2O3-supported catalysts and 2 times higher than TiO2-supported catalysts. From low-temperature oxygen chemisorption and from temperature-programmed re(7) Maity, S. K.; Ancheyta, J.; Soberanis, L.; Alonso, F.; Llanos, M. E. Appl. Catal. A: Gen. 2003, 244, 141. (8) Srinivas, B. N.; Maity, S. K.; Prasad, V. V. D. N.; Rana, M. S.; Kumar, M.; Murali Dhar, G.; Prasada Rao, T. S. R. Stud. Surf. Sci. Catal. 1998, 113, 497. (9) Murali Dhar, G.; Rana, M. S.; Maity, S. K.; Srinivas, B. N.; Prasada Rao, T. S. R. Chemistry of Diesel Fuels; Song, C., Hsu, S., Mochida I., Eds.; Taylor & Francis Publishers: Philadelphia, 2000; Chapter 8. (10) Srinivas, B. N.; Rana, M. S.; Maity, S. K.; Chiranjeevi, T.; Murali Dhar, G.; Prasada Rao, T. S. R. DiV. Pet. Chem., Am. Chem. Soc. 2000, 45 (2), 361. (11) Murali Dhar, G.; Srinivas, B. N.; Rana, M. S.; Kumar, M.; Maity, S. K. Catal. Today 2003, 86, 45. (12) Pophal, C.; Kameda, F.; Hocino, K.; Yoshinaka, S.; Segawa, K. Catal. Today 1997, 39, 21. (13) Ramirez, J.; Castillo, P.; Ceden˜o, L.; Cuevas, R.; Castillo, M.; Palaeiso, J. M. A.; Lopez, A. Appl. Catal. A: Gen. 1995, 132, 317.
10.1021/ef0502610 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/03/2006
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Table 1. Different Salts and Hydrolyzing Agents Used To Prepare Al2O3-TiO2 Supports support
salts used
method (hydrolyzing agent)
AT1 AT2 AT3 AT4 AT5
aluminum sulfate, sodium aluminate, titanium isopropoxide aluminum sulfate, sodium aluminate, titanium isopropoxide aluminum sulfate, titanium chloride sodium aluminate, titanium chloride sodium aluminate, titanium chloride
urea + pH variation urea + pH variation, slurry is aged 7 days in ammonium hydroxide solution urea + ammonium hydroxide urea + HCl HCl
duction results, they have found the presence of more reducible MoO3 species on binary oxide compared with Al2O3-supported catalyst, and it might be the reason the mixed oxide supported catalysts show higher activity. Zhabobin et al.6 reported that modification of Al2O3 by TiO2 improved hydrodesulfurization and hydrogenation activities. Pophal et al.12 found that aluminatitania-supported catalysts were more efficient for hydrodesulfurization of 4,6-dimethyldibenzothiophene. Ramirez and co-workers13 studied the detailed characterization of hydrotreating catalysts supported on Al2O3-TiO2. They observed that intrinsic HDS activity was a maximum at a TiO2/(TiO2+Al2O3) ratio of around 0.95. Maity et al.7 have observed that catalyst supported on Al2O3-TiO2 showed higher activities compared with catalysts supported on Al2O3 and Al2O3-SiO2. Maya heavy crude with desulfurized naphtha was used as feedstock for their study. They have concluded that the catalyst supported by Al2O3-TiO2 showed higher activity due to the presence of more reducible species in this catalyst. In a recent review, Bej et al.14 have discussed the beneficial effect of this oxide in deep hydrodesulfurization. It was reported that removal of sulfur atom from the refractory compounds was easy on this mixed oxide supported catalysts. In this present investigation, different methods are used to prepare alumina-tinania supports. These binary oxide supports are employed to prepare catalysts for hydrotreating Maya heavy crude. The effect of preparation methods is correlated with physicochemical characteristics of the catalysts. Experimental Section Preparation of the Supports. Different methods were used to prepare alumina-titania (95:5 wt %, AT) binary mixed oxide supports. Alumina-titania (AT1) mixed oxide was prepared by a combination method of urea hydrolysis and pH variation. Three solutions of aluminum sulfate, titanium isopropoxide, and sodium aluminate were made in distilled water. A sufficient amount of urea was taken into a reactor vessel and a clear solution was made with water. The urea solution was heated at 90 °C with continuous stirring. The titanium solution was added to the reactor. The aluminum sulfate solution was added to the mixture until the pH of the mixture reached at 4. The mixture was aged for 10 min. A basic solution of sodium aluminate was added until the pH of the mixture reached 9, and then the mixture was aged for 10 min. The addition of acid and basic solutions was repeated three times in total. The pH of the mixture was varied from 4 to 9 by adding acid and basic solutions of aluminum salts. The slurry was allowed 10 min between addition of acid and basic solutions. Finally, base solution was added and solution mixture was aged for 3 h at pH of 9. The slurry was then filtered and washed. The cake was used for extruding. The extrudate was allowed to dry overnight at room temperature and it was then dried at 160 °C for 1 day and calcined at 600 °C for 5 h. In the second preparation method, alumina-titania (AT2) support was prepared by the same method as mentioned above; the only difference in this method was that the precipitate was aged in ammonium hydroxide solution for a period of 7 days. The extrudate was dried and calcined at the above said temperature. (14) Bej, S. K.; Maity, S. K.; Turaga, U. T. Energy Fuel 2004, 18, 1227.
In the third procedure, alumina-titania (AT3) mixed support was prepared from alumina sulfate and titania chloride salt solutions. Appropriate amounts of aluminum sulfate (Baker) and titanium tetrachloride (Aldrich) were dissolved into distilled water. A sufficient amount of urea was taken into the reactor vessel and a clear solution was made with water. The urea solution was heated at 95 °C with continuous stirring. Sulfate and chloride solutions were added into the reactor vessel. The solution mixture was heated at 90 °C with continuous stirring for a duration of 4 h. A solution of ammonium hydroxide was added to the mixture to achieve pH 9. The solution mixture was heated another 2 h at same temperature. The slurry was filtered and washed subsequently. The cake was extrudate having 1 mm diameter. The extrudate was dried and calcined as a similar procedure as employed for preparation of AT1. In the forth preparation method, alumina-titania (AT4) support was prepared from aluminum sodium aluminate and titanium chloride (Aldrich) salt solutions. Appropriate amounts of sodium aluminate and titanium chloride salts were dissolved into distilled water. A sufficient amount of urea was taken into the reactor vessel and a clear solution was made with water. The urea solution was heated at 95 °C with continuous stirring. Aluminate and chloride solutions were added into the reactor vessel. A diluted HCl solution was added to the reactor to reduce the pH of the mixture at around 9. The solution mixture was heated at 90 °C with continuous stirring for a duration of 4 h. The precipitate was aged overnight in mother liquid. The slurry was filtered and washed subsequently. The cake was extrudate having 1 mm diameter. The extrudate was dried and calcined by following a similar procedure as employed for preparation of AT1. The support AT5 was prepared by similar procedure as used in the preparation of AT4; the only difference was that urea was not used in this method. The different methods for support preparation are tabulated in Table 1 to easily distinguish the procedures used in this investigation. Preparation of Catalysts. The catalysts in the present investigation were prepared by coimpregnation. Appropriate amounts of ammonium heptamolybdate salt, cobalt nitrate, and phosphoric acid were dissolved into distilled water and made a clear solution. The amount of water taken for making the salt solution was just sufficient to fill up the pore of the support. The solution was impregnated on the dry support, and impregnated samples were allowed to dry overnight at room temperature. The samples were then dried for 7 h at 120 °C and calcined at 450 °C for 5 h. All these catalysts contain 10 wt % of MoO3, 3 wt % of CoO, and 1.8 wt % of P2O5 on catalyst basis. Characterization of Catalyst. X-ray diffractograms were recorded on a SIEMENS D-500 model using a Cu KR radiation. BET specific surface area, pore volume, and pore size distribution of fresh and spent catalysts were measured by nitrogen adsorption at 77 K (Automatic Micromeritics ASAP 2100). The metal contents of the spent catalysts were determined by atomic absorption spectrometery. The percentage of carbon was also measured on spent catalysts. The spent catalysts were washed with hot toluene by Soxhlet extraction and dried at 110 °C before carbon and metal analyses. Coke is defined in this work as being the carbon content on spent catalyst. Presulfiding of Catalyst. The oxide catalysts were sulfided in situ before an actual run was started. Ten milliliters of oxide catalyst was loaded with an equal volume of diluent, carborandum (0.2 mm size). Both catalyst and carborandum were mixed and divided into five parts. Each part of the mixture was loaded into the reactor at a time with intermittent tapping. The catalyst was then dried for 2
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Table 2. Properties of Maya Crude properties
Maya
C, wt % H, wt % N, wt % S, wt % metals (wppm) Ni V gravity, °API Ramsbottom carbon, wt % asphaltenes (in C7), wt % density 20/4°C
86.9 5.3 0.3 3.52 49.5 273 20.99 11.01 11.2 0.9251
h at atmospheric pressure at 120 °C. After drying, the catalyst was allowed to soak for 2 h at 150 °C. Light gas oil (LGO) was used for soaking. This light gas oil contains 1.7 wt % of sulfur. The actual sulfiding agent was introduced after soaking. The sulfiding agent was light gas oil with dimethyl disulfide (DMDS, 1 wt %). Sulfidation was done at 28 kg/cm2 pressure at two different temperatures. The first sulfidation was done at 260 °C for 3 h and finally the catalyst was sulfided at 320 °C for 5 h. Catalyst Activity Test. Catalytic tests were performed in a highpressure fixed-bed microreactor in up flow mode. The experimental conditions are as follows: total pressure, 54 kg/cm2; reaction temperature, 380 °C; liquid hourly space velocity (LHSV), 1.0 h-1; hydrogen-to-hydrocarbon ratio, 356 m3/m3. A mixture of Maya heavy oil with hydrodesulfurized diesel (50/50 wt/wt) was used for catalyst activity tests. The first balance was taken after a stabilization period of 9 h. The catalytic activity was also studied in a batch reactor by using Maya heavy crude as feed. The properties of this feed are given in Table 2. The weighted Maya heavy crude was taken into the reactor (1-L capacity). The sulfided catalyst was transferred into the reactor at nitrogen atmosphere very quickly so that catalyst would not contact with air for a long time. The reactor vessel was ensured to be tight and checked for leaks. The reactor vessel was then purged two or three times with hydrogen gas so that there is no air left inside the reactor. Heating was started from room temperature to the required temperature at the rate of 3 °C/min. Stirring was started when temperature reaches the set point and the time was noted at that point as the beginning of the reaction. The experimental conditions of the batch reactor were as follows: temperature, 380 °C; total pressure, 100 kg/cm2; catalyst weight, 4 g; Maya crude, 200 g; duration of reaction, 6 h; and stirring speed, 750 rpm. Analysis of Feed and Products. Total metals in the feed and products were measured by Atomic Absorption (ASTM D-5863). Sulfur and nitrogen were analyzed by X-ray fluorescence (ASTM D-4294) and chemiluminescence, respectively. Asphaltene is defined as the insoluble fraction in n-heptane (ASTM D-2007).
Results and Discussion Physical Properties of the Catalysts. The XRD diagrams of the five different supports are presented in Figure 1. Two broad peaks are observed, and these peaks correspond to γ-alumina. Titania has two crystalline phases, anatase (2θ ) 25.3°) and rutile (2θ ) 27.4°). In the first three supports (AT1AT3) we also observe a very low intensity peak at 2θ equal to 25.3°, which corresponds to the presence of anatase titania.4 However, for the supports prepared by sodium aluminate (i.e. AT4 and AT5), no titania phase is noticed, indicating that titania is well-dispersed upon the alumina matrix in these two supports. The XRD diffractograms of supported catalysts are presented in Figure 2. We do not observe any noticeable change in these diffractograms compared to diffractograms of corresponding supports. It shows that molybdenum oxides are well dispersed into the supports. The physical properties of the fresh and the spent catalysts were measured by nitrogen adsorption, and the results are given
Figure 1. XRD diagrams of alumina-titania supports.
Figure 2. XRD diagrams of alumina-titania supported catalysts.
Figure 3. Pore size distribution of five different catalysts.
in Table 3. The physical properties of the spent catalysts were measured after washing them with hot toluene and subsequent drying. The pore size distribution of the fresh catalysts is also presented in Figure 3. Table 3 shows that the catalysts (D and E) supported on alumina-titania, prepared from sodium aluminate/TiCl4 salts, have comparatively higher total pore volume and average pore diameter. Catalyst C supported on AT3, which is prepared from alumina sulfate and TiCl4 by using urea and NH4OH, has the lowest surface area and total pore volume and smaller average pore diameter. Support AT4, prepared by urea hydrolysis using sodium aluminate and titania chloride, is bimodal in nature; i.e., it has pores in the smaller pore diameter region as well as in the higher one. Several factors like, size, shape and agglomeration of alumina particles are important variables to control the pore structure of alumina. The fine particles of amorphous alumina are formed with crystalline pseudo-boehmite during normal preparation of alumina. These fine particles contribute smaller pores in the pore size distribution. Ono et al.15 reported that the formation of amorphous alumina could be controlled by variation of the pH during precipitation of aluminum hydroxides. Compared (15) Ono, T.; Ohguchi, Y.; Togari, O. Preparation of Catalyst III; Poncelet, G., Grange, P., Jocobs, P. A., Eds.; Elsevier: New York, 1983; p 631.
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Table 3. Physical Properties and Coke and Vanadium Depositions on Catalysts PCoMo/AT1 (A)
PCoMo/AT2 (B)
PCoMo/AT3 (C)
PCoMo/AT4 (D)
PCoMo/AT5 (E)
properties
fresh
spent
fresh
spent
fresh
spent
fresh
spent
fresh
spent
SSA (m2/g) TPV (mL/g) APD (Å) PSD (vol %) >1000 Å 1000-500 Å 500-200 Å 200-100 Å 100-50 Å D > B > A > C, which is quite similar to that observed in the microreactor. This result also shows that the catalyst C has lower HDM activity. Figure 6 also shows that the catalysts (D and E) prepared on supports AT4 and AT5, respectively, have higher HDS activity. These two supports are prepared from sodium aluminate salt. The catalysts A and B, which are prepared on supports AT1 and AT2, respectively, show moderate HDS activity. These two supports are prepared by an almost similar procedure. We do not observe any great differences in HDAsp activity of the five catalysts. The HDN conversion of the catalysts shows a different trend: A > D = C > B > E. From the above activity results it can be said that the results obtained from the batch reactor could be comparable with those obtained from the microreactor. It should be mentioned that feeds in both reactor systems are different. Maya crude is used as feed in batch reactor, whereas a mixture of Maya crude with desulfurized diesel is employed as feed in the microreactor. Diesel has no metal compounds, and therefore, similar HDM trends are shown in both systems. However, a little difference is observed in HDS activities in the batch reactor compared to the microreactor. It is worth mentioning here that both feeds are not same with respect to the sulfur contained. The feed for the microreactor has more easily removed sulfur compounds compared with the feed for the batch reactor, which may be the reason for the different trends in HDS activity. The supports AT4 and AT5 prepared by sodium aluminate salt have favorable pore structure, which is the reason for the higher activity of the catalysts supported on these mixed oxides. It is also observed that the anatase phase of titania is welldispersed over the alumina matrix on AT4 and AT5 supports. These dispersed phases of titania may be more effective to
Alumina-Titania Mixed Oxide Supports
Figure 7. Rate of HDM deactivation for different catalysts.
enhance activity compared with the corresponding crystalline phases. Among all five catalysts, the total pore volume and average pore diameter of the catalyst C are the lowest and result in poor activities. Catalyst Deactivation. The coke and metal contents in the deactivated catalysts are given in Table 3. These elements are measured on the spent catalysts after 60 h of operation. The table shows that coke and metal depositions are the highest on the catalyst AT5. Around 13 wt % of coke is deposited on this catalyst. BET specific surface area, total pore volume, and pore size distribution of the spent catalysts are measured, and the results are compared with the respective fresh catalyst in Table 3. These textural properties of fresh catalysts are totally changed in the corresponding spent catalysts. Decreases of these properties are observed in the spent catalysts compared with the fresh ones, mainly due to coke and metal deposition. Drastic changes of pore structure have been observed in catalyst C. The percentages of conversion of HDM and HDS are plotted against hours of operation in Figures 4 and 5, respectively. Figure 4 shows that the HDM activity decreases with time-onstream, however, the decreasing tendency is different for different reactions and different catalysts. Similar results are also obtained in the case of the HDS reaction in Figure 5. The rate of deactivation for all five catalysts is also calculated by using the following equation16,17 -btn
Xt ) X0e
where Xt is the conversion at time t, X0 is the initial conversion, b is the deactivation rate constant, and n ) 1 (used according to the ref 17). -ln(Xt/X0) is plotted against time of operation for HDM and HDS reactions in Figures 7 and 8, respectively. Though the total reaction time is low (60 h), even within this time different deactivation rates are noticed. The deactivation rate is faster at the initial period and after that it slows down and is more prominent in the case of the HDM reaction. The metal compounds in the heavy crude oils are very large complex molecules. On the other hand, there are two types of sulfur(16) Voorhies, A. Ind. Eng. Chem. 1945, 37, 318. (17) Chen, Y. W.; Hsu, W. C. Ind. Eng. Chem. Res. 1997, 36, 2526.
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Figure 8. Rate of HDS deactivation for different catalysts.
containing molecules in heavy fractions: one is bigger in size and attached with asphaltene structure, and other is smaller in size and non-asphaltene. During the hydroprocessing reaction, the solubility of the asphaltenes is reduced by removing their aliphatic chains; particularly at the initial period when the catalyst activity is very high, asphaltenes precipitate from the product and hence they are deposited on the catalysts. As a result, catalyst is deactivated due to pore mouth blockage by this deposition. Therefore, the deactivation rate for HDM reaction is faster than that for HDS, because metals are associated with asphaltene moiety. The smaller size of the nonasphaltenic sulfur compounds still leaves a path to enter into the pore cavity, and hence, the deactivation rate is slower. The deactivation rates for the HDM reaction are different for different catalysts. Catalyst E not only shows higher activity but also more stability with run of operation. The rate of HDM deactivation is the highest for the catalyst C. The deactivation rate for the HDS reaction is also different for different catalysts. Catalysts A and C are deactivated comparatively faster than the other three catalysts. The pore structure of catalyst C is changed drastically during the hydrotreating. Even its specific surface area decreases to 18 m2/g from 126 m2/g. It is the reason for the rapid deactivation of this catalyst. Conclusions In this present investigation, different methods are used to prepare alumina-titania supports, and it is found that supports AT4 and AT5 prepared by sodium aluminate and titanium chloride salts have higher pore volume and bigger pore size. It is also observed that in this method titania phases are welldispersed upon the alumina matrix. These physicochemical properties are responsible for showing higher activities of the catalysts D and E. Moreover, the catalysts D and E not only show higher activities but also have good stability with timeon stream. The deactivation rate is faster at the initial period of operation and becomes slower with the run progresses. The deactivation rate for HDM reaction is faster than that for HDS reaction. Acknowledgment. The authors would like to thank Instituto Mexicano del Petroleo for the financial support. EF0502610
