Energy Fuels 2009, 23, 4860–4865 Published on Web 09/04/2009
: DOI:10.1021/ef900248g
CoMo/Ti-MCM-41/Alumina Catalysts: Properties and Activity in the Hydrodesulfurization (HDS) of Dibenzothiophene (DBT) P. Schacht, S. Ramı´ rez, and J. Ancheyta* Instituto Mexicano del Petr oleo, Eje Central L azaro C ardenas Norte 152, Mexico City 07730, M exico Received March 23, 2009. Revised Manuscript Received July 17, 2009
The objective of this work is to study the experimental synthesis of Co-Mo catalysts supported on Ti-MCM-41/alumina. The mixing process of alumina and Ti-MCM 41 and the resulting effects on mechanical and textural properties of the final catalyst were studied. MCM-41 was synthesized according to methods previously reported in the literature. An alumina matrix was obtained from the thermal treatment of pseudo-boehmite. Cylindrical extrudate support was prepared by physical mixture of TiMCM-41 and boehmite. Co and Mo were incorporated into the support by simultaneous impregnation. The catalysts were characterized by textural properties, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Catalytic activity tests were conducted in a differential reactor using the hydrodesulfurization (HDS) of dibenzothiophene (DBT) as the model reaction. Reaction products were analyzed via online gas chromatography (GC). The integration of Ti-MCM-41 with alumina resulted in a support with suitable textural and mechanical properties, such as those required for commercial application. The results of HDS of DBT indicated that Co-Mo supported on Ti-MCM-41/alumina catalyst exhibits activity that is very similar, in comparison, to that of a typical commercial catalyst.
DBT, etc.) is needed. Regarding catalyst formulation, an almost 3-fold increase in activity, with respect to current catalysts, is required to obtain such a hydrodesulfurization (HDS) level. The combination of using the best-available catalyst, tailoring feed quality by end-point reduction, and reducing operating cycles due to higher reaction severity operation may provide temporary capability to produce ULSD.3 However, the long-term solution will certainly need added reactor volume (another approach to reduce LHSV without detriment of diesel fuel production) and quench capability (to process high-hydrogen-consuming feeds and keep the hydrocarbon under a more-favorable hydrogenrich atmosphere), or even investment in new hydrotreating plants.4,5 In addition to all this, the production of light petroleum is declining and heavy and extra-heavy crude oils will be compulsory to refine.6 This situation adds more inconvenience, because these new feeds are more contaminated and their refining is more complicated and costly.7 As for the development of new highly active and selective catalysts, most of the effort is being made in support composition. Various mixed oxides have been studied and reported
1. Introduction Currently, everybody recognizes that actual hydrotreating (HDT) technologies that are used in worldwide refineries to reduce the sulfur content of straight-run distillates are not adequate to produce diesel fuel with an almost-zero level of this impurity (the so-called ultralow sulfur diesel, ULSD). Some years ago, when refiners faced the problem of producing 500 wppm sulfur diesel, HDT units were capable of achieving that goal by increasing reaction severity, by either increasing the reactor temperature or decreasing the feed flow rate (i.e., a reduction in the liquid hourly space velocity, LHSV) or incorporating lighter distillates (e.g., kerosene) to the usual feedstock, among other strategies.1 Then, the problem got more complicated because the demand for diesel increased and the common feed to HDT plants (straight-run gas oil) was limited, and other streams such as light cycle oil (LCO) from fluid catalytic cracking (FCC) units, were necessary to add in the regular feed to HDT plants.2 These new HDT feed components are highly refractory in nature and require even more-severe reaction conditions for the desired sulfur removal. In addition, they consume large amounts of hydrogen, because of their elevated aromatics content. However, the problem continues and more-stringent specifications for sulfur in diesel fuel are necessary (i.e. 200 mesh > 100 mesh. To confirm this qualitative observation, the catalysts were analyzed to determine mechanical strength. The results of bulk density (BD) and crush strength (CS) are presented in the last columns of Table 1. It is noticed that CS of the prepared catalysts decreases in the aforementioned order. BD values follow the same tendency, which indicate that the degree of agglomeration can be as high as 0.8492. High BD
(14) Beck, S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834–10843. (15) Lee, J. S.; Rhee, C. H. Catal. Today 1997, 38 (2), 213–219.
(16) Ramı´ rez, J.; Rana, M. S.; Ancheyta, J. Characteristics of Heavy Oil Hydroprocessing Catalysts. In Hydroprocessing of Heavy Oil and Residua; Ancheyta, J., Speight, J. G., Eds.; CRC Press, and Taylor & Francis: New York, 2007; Chapter 6, pp 121-190.
hexadecane as solvent (0.5 wt % sulfur as DBT). Prior to reaction, the catalyst was dried with nitrogen at a flow of 100 mL/min and 423 K during 0.5 h. The catalyst then was sulfided in situ at atmospheric pressure with a mixture of H2S/H2 (10/90 (vol/vol)) at a rate of 60 mL/min for 4 h at 673 K. After sulfidation, the catalyst was purged with nitrogen and kept overnight under a nitrogen flow of 5 mL/min. Before the experimental runs, the hydrogen pressure was increased to 5.6 MPa, and the reactor was heated to the reaction temperature of 573 K. Product samples were recovered every 2 h, and analyses were performed online with a Perkin-Elmer gas chromatograph that was equipped with a 50 m HP Ultra 2 capillary column and a flame ionization detection (FID) device. Conversion in each experiment was defined as 100 minus the unconverted DBT (in terms of weight percent). Product selectivities were calculated by dividing each product yield by conversion.
3. Results and Discussion
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Energy Fuels 2009, 23, 4860–4865
: DOI:10.1021/ef900248g
Schacht et al.
Figure 6. Punctual microanalysis of the 200 mesh catalyst by SEM: (a) Mo detection, (b) Co detection, (c) P detection.
Figure 3. Morphological analysis by stereoscopic microscopy of extruded catalysts: (a) 100 mesh, (b) 200 mesh, and (c) 325 mesh. Figure 7. Punctual microanalysis of the 325 mesh catalyst by SEM: (a) Mo detection, (b) Co detection, (c) P detection. Table 2. Semiquantitative Chemical Analysis of Extrudate Catalysts Composition (wt %)
Figure 4. Punctual microanalysis of catalysts by scanning electron microscopy (SEM): (a) 100 mesh, (b) 200 mesh, and (c) 325 mesh.
catalyst
Al
Si
P
Co
Mo
total
100 mesh support 200 mesh support 325 mesh support
6.86 5.81 6.85
13.39 21.65 12.53
2.52 2.07 2.75
8.63 4.23 8.67
15.26 11.25 15.79
46.66 45.01 46.59
roughness on the transversal cut in the 100 mesh sample. This fact implies that the metallic particles will be more finely divided and distributed on the support on the 200 and 325 mesh samples. According to these results, it is concluded that Mo, Co, and P impregnation was performed in a more homogeneous manner with 325 mesh- and 200 mesh-supported catalysts, which, similarly, presented lesser roughness on its surface. 3.5. Microanalysis by Scanning Electron Microscopy (SEM). The global microanalysis by SEM practiced at the surface of the transversal cut of each extrudate provides a semiquantitative estimation of the composition of the catalysts.18,19 The catalyst compositions determined by this analysis are reported in Table 2. Catalysts prepared with 100 mesh and 325 mesh supports presented an apparent superior level of impregnation of the active phases, compared with the 200 mesh supported catalyst. From these values, it is important to highlight that this is a semiquantitative estimate and does not correspond to the entire composition of the catalysts; it only represents, in a local section of the material, the ratio in which Mo, Co, and P are present in the analyzed surface of the materials. 3.6. Catalytic Activity. HDS of DBT was chosen as the model reaction, whose main products are unconverted dibenzotiophene (DBT), partially hydrogenated DBTs (TH-DBT (tetrahydrodibenzotiophene) and HH-DBT (hexahydrodibenzotiophene)), biphenyl (BP), cyclohexylbenzene (CHB), and bicyclohexyl (BCH).20
Figure 5. Punctual microanalysis of the 100 mesh catalyst by SEM: (a) Mo detection, (b) Co detection, and (c) P detection.
values may be possible due to the amount of Ti added and the agglomeration degree achieved, as mentioned. Both CS and bulk density of the catalyst prepared with 325 mesh support were found to be higher than those of the commercial catalyst. 3.4. Punctual Microanalysis. Punctual microanalyses that were performed on 100 mesh, 200 mesh, and 325 mesh supported catalysts are shown in Figure 4 for transversal cuts. It is possible to see the approximate porosity of the material from the transversal surface cut of each extrudate.17 In certain zones of these images, it seems that the elements are not present, which is not the case, because the apparent absence of metals is due to relief variations along the analyzed surface. In such a way, the characteristic X-ray radiation emitted by areas below an imaginary horizontal plane that was considered as reference was unable to reach the detection system. Mapping of Mo, Co and P on the transversal cut of one particle of catalyst are depicted in Figures 5, 6, and 7 respectively. Depending on the size of solid particles used to obtain the extruded support, it is possible to see higher
(18) Nersisyan, H.; Lee, J. H.; Won, C. W. J. Mater. Res. 2002, 17, 2859–2864. (19) Selvaraj, M.; Sinha, P. K.; Lee, K.; Ahn, I.; Pandurangan, A.; Lee, T. G. Microporous Mesoporous Mater. 2005, 78, 139–149. (20) Houalla, M.; Broderick, D. H.; Sapre, A. V.; Nag, N. K.; J. de Beer, V. H.; Gates, B. C.; Kwart, H. J. Catal. 1980, 61 (2), 523–527.
(17) Duprey, E.; Beaunier, P.; Springuel-Huet, M. S.; Bozon-Verduraz, F.; Fraissard, J.; Manoli, J.; Bregeault, J. M. J. Catal. 1997, 165, 22–32.
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Energy Fuels 2009, 23, 4860–4865
: DOI:10.1021/ef900248g
Schacht et al.
Figure 10. Selectivity toward cyclohexylbenzene: (- - -) Co-Mo/ Al2O3, (O) 100 mesh Co-Mo/Ti-MCM-41/Al2O3, (b) 200 mesh Co-Mo/Ti-MCM-41/Al2O3, and (0) 325 mesh Co-Mo/Ti-MCM41/Al2O3.
published that involve the HDS of DBT and 4,6-DMDBT on several compositions of Co, Mo, and Co-Mo on alumina, MCM-41, and Ti MCM-41. The reported activity and selectivity results are different and have been explained in terms of the specific properties of each catalyst. For example, Song and Reddy22 showed that a catalyst supported on MCM-41 is more active for the HDS of 4,6-DMDBT. Similarly, Ramirez et al.12 determined that MCM-41 allows a lower interaction among Co, Mo, and the support, which leads to high activity on HDS on DBT. In our previous work, we have also reported that TiO2 that has been stabilized with lanthanum and cerium for the HDS of DBT23 is more active than alumina-supported catalysts. However, the interaction of titanium, MCM-41, and alumina with supported Co-Mo is different in every case and must have specific properties, with respect to each one of its components. Figure 9 reports the BP selectivity of all catalysts. It is evident that the commercial catalyst follows the route of breaking the C-S bond to desulfurize the DBT molecule. The Ti-MCM-41/γ-Al2O3 catalysts show very low BP selectivity, indicating that the desulfurization mechanism is different. Selectivity toward CHB for all catalysts is presented in Figure 10. With these results and those shown in Figure 9, it is possible to observe that BP exhibits a faster formation rate than that of TH-DBT, indicating that the reaction followed mainly the direct HDS route, which is an appropriate behavior for Co-promoted catalysts. Catalyst prepared with 100 mesh support showed lower activity and selectivity compared with 200 and 325 mesh supported catalysts, which may be attributed to less-dispersed active metals. It is also important to mention that the commercial catalyst exhibited a change in selectivity from hydrogenolysis to direct desulfurization in a short period of time (1 h). The observed tendency on Ti-MCM-41/Al2O3-based catalyst samples is quite different, because the selectivity changes gradually and requires a longer period of time (8 h). Similar behavior regarding changes in selectivity in the evaluation of experimental catalysts has been reported by others.24 It has been suggested that this behavior is a result of the change, creation, or destruction of different metallic sites, including sulfide metallic site from the sulfidation process. This fact may indicate that our catalysts require more activation (i.e., sulfidation) time than commercial catalyst during
Figure 8. Total DBT conversion of the catalysts: (- - -) Co-Mo/ Al2O3, (O) 100 mesh Co-Mo/Ti-MCM-41/Al2O3, (b) 200 mesh Co-Mo/Ti-MCM-41/Al2O3, and (0) 325 mesh Co-Mo/Ti-MCM41/Al2O3.
Figure 9. Selectivity toward biphenyl: (- - -) Co-Mo/Al2O3, (O) 100 mesh Co-Mo/Ti-MCM-41/Al2O3, (b) 200 mesh Co-Mo/TiMCM-41/Al2O3, and (0) 325 mesh Co-Mo/Ti-MCM-41/Al2O3.
Every two hours, a small sample was taken out from the reactor and the products were identified using gas chromatography-mass spectroscopy (GC-MS). Benzene (BZ), cyclohexane (CH), and some other cracked products (light components) were also detected in small amounts, which is in agreement with other reports.21 This implies that the HDS of DBT undergoes the two well-known path reactions of HDS: via C-S bond hydrogenolysis to form BP, and via hydrogenation to form hydrogenated DBTs (TH-DBT and HH-DBT). All these products are then desulfurized to form CHB and finally BCH by hydrogenation. DBT conversions for different catalysts are presented in Figure 8. It can be observed that the catalyst prepared with 325 mesh support is the one that approximates to the conversions obtained with γ-Al2O3 supported catalyst. DBT conversions of the 100 mesh supported catalyst were determined to be less than those obtained with 200 mesh and 325 mesh supported catalysts. Among the three Ti-MCM41/γ-Al2O3 catalysts, the 325 mesh supported catalyst showed superior activity, but it was slightly lower than commercial catalyst activity. Several papers have been
(22) Song, C.; Reddy, K. M. Appl. Catal., A 1999, 176, 1–10. (23) Schacht, P.; Hernandez, G.; Cede~ no, L.; Mendoza, J. H.; Ramı´ rez, S.; Garcia, L. A.; Ancheyta, J. Energy Fuels 2003, 17, 81–86. (24) Reddy, K. M.; Wei, B.; Song, C. Catal. Today 1998, 43, 261–272.
(21) Whitehurst, D.; Farag, H.; Nagamatsu, T.; Sakanishi, K.; Mochida, I. Catal. Today 1998, 45 (1), 299–305.
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: DOI:10.1021/ef900248g
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HDS of DBT, and once activation has been completely achieved all catalysts (lab-prepared and commercial) would have very similar behavior. We indeed know that an experimental catalyst exhibiting lower activity than a commercial catalyst at the laboratory-scale level is, in principle, distant from that of commercial applications, and perhaps the results may be not worthy of discussion. Also, if the criteria for deciding which research is more important than others are functions of the activity of the prepared samples, compared with commercial ones, most of the already published experimental results of catalytic prototypes would not deserve attention, because, for instance, the top commercial HDS catalysts (e.g., NEBULA) exhibit much higher activity than some other reported catalysts. However, what we would like to highlight in this contribution is that Ti-MCM-41 combined with γ-alumina can be a promising material for possible commercial application in the HDS of middle distillates of petroleum. To work in this direction, more experiments (not only with DBT, but also with more refractory molecules, e.g., 4,6-DMDBT, and
petroleum distillates) in continuous fixed-bed reactors are mandatory, which will certainly provide much more information about the possible application of Ti-MCM-41/ Al2O3-based catalysts. Apart from improving activity and selectivity of Ti-MCM-41/alumina based catalysts it is very important to conduct thermal stability studies of this material, since it changes its framework structure during the different steps of catalyst preparation. 4. Conclusion It is possible to integrate Ti-MCM-41 into an aluminum matrix to synthesize Ti-MCM-41/Al2O3 catalytic supports with adequate textural and mechanical properties. The prepared Co-Mo/Ti-MCM-41-γ-alumina catalyst shows competitive activity, in comparison to typical commercial catalysts, which makes this material suitable for possible industrial application. Acknowledgment. The authors thank Instituto Mexicano del Petroleo for its financial support.
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