Ind. Eng. Chem. Res. 2007, 46, 3877-3883
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Platinum Doped Hydrotreating Catalysts for Deep Hydrodesulfurization of Diesel Fuels Ste´ phanie Pessayre,† Christophe Geantet,*,† Robert Bacaud,† Michel Vrinat,† Thanh Son N’Guyen,† Yvonne Soldo,‡ Jean Louis Hazemann,§ and Miche` le Breysse# Institut de Recherches sur la Catalyse, UPR CNRS 5401, 2 AVenue Albert Einstein, 69626 Villeurbanne Cedex, France, Laboratoire d’Electrochimie et Physico-chimie des Mate´ riaux et des Interfaces, UMR CNRS, INPG-UniVersite´ Joseph Fourier, 38402 St Martin d’He` res, France, Laboratoire de Cristallographie, UPR CNRS, BP 166, 38043 Grenoble, France, and Laboratoire de Re´ actiVite´ de Surfaces Paris, UMR CNRS, UniVersite´ Pierre et Marie Curie-Paris6, 75252 Paris Cedex, France
Two-stage processes are possible solutions for reaching 10 ppm sulfur in gas oil without severe running conditions. First, most of the sulfur is removed by using a CoMo conventional catalyst. Second, after H2S removal, the treated oil, with a remaining S content below 500 ppm, is again hydrotreated. In this latter case, the use of noble metal catalysts can be envisaged. In the present study dealing with this second stage reactor, we examined a catalytic system based on the addition of a low content of Pt to a commercial sulfide catalyst. An enhancement in catalytic activity can be obtained in the conversion of model molecules (tetralin and dibenzothiophene) as well as a hydrotreated straight run gas oil. The promotion effect strongly depends on the preparation procedure which requires the impregnation of Pt on a presulfided commercial HDT catalyst. Advanced EXAFS and HRTEM characterizations were used to characterize the active phases. Introduction The primary goal of the recently proposed regulations by the European Community, as well as those which have appeared in United States or Japan, is to reduce the sulfur content in transportation fuels in order to minimize air pollution and prevent the poisoning of exhaust treatment catalysts. For instance, present EC or US regulation for the sulfur contents of diesel fuels is 50 ppm and is expected to be 10 ppm in 2009.1 Thus, sulfur removal is still a major problem to be solved2 and several approaches are proposed.3 As a result of the tightening of these regulations, there has been a great interest in the research and development of refining processes and suitable catalysts. Even if the preparation of conventional hydrotreating catalysts has been improved and their reactivity enhanced by the use of P or organic additives, the severe operating conditions required to meet the objectives drastically reduces the catalyst life time. Thus, two-stage processes have been proposed and developed.4-6 The objective of the first step is to reduce the sulfur content of the crude in order to decrease the H2S partial pressure in the second reactor. As a result, only organic compounds difficult to desulfurize (mostly substituted dibenzothiophenes) remain. The conversion of these sterically hindered molecules mainly proceeds via the hydrogenation route instead of the direct desulfurization route which is dominating for molecules like dibenzothiophene.2,7,8 Therefore, a way to overcome the low reactivity of 4,6DMDBT would be to favor the hydrogenation pathway. Noble metal supported on silica-alumina or NiW sulfide supported on alumina catalysts were found to be promising systems for the desulfurization of a straight run gas oil containing 760 ppm * To whom correspondence should be addressed. Tel.: (0)4 72 44 53 36. Fax: (0)4 72 44 53 90. E-mail:
[email protected]. † Institut de Recherches sur la Catal. ‡ Laboratoire d’Electrochimie et Physico-chimie des Mate´riaux et des Interfaces. § Laboratoire de Cristallographie. # Laboratoire de Re´activite´ de Surfaces Paris.
of S.9,10 Besides, by doping conventional systems with small amounts of Pt, a benefit in catalytic activity could be expected. This has been done for improving the catalytic activities of MoS2 with Pt either as a dopant11-14or as a catalyst mixture (MoS2/ Al2O3 + Pt/Al2O3).15 Some of these authors pointed out the importance of an impregnation of Pt on already sulfided Mo on alumina catalysts.13,14 EXXON patented the preparation of a Pt doped NiMo supported on alumina catalysts,16 the Pt being introduced with a S-containing precursor with ligands such as dithiocarbamates. More recently the use of mixed clusters such as [Pt(NH3)4](Mo6S8)S,x(H2O/MeOH)] was also proposed.17 A favorable effect of the addition of Pt or Rh on CoMo catalysts was also reported by Vanhaeren.18 The objective of the present study was to examine the effect of the addition of Pt on three industrial sulfide catalysts, i.e., CoMo, NiMo, and NiW supported on alumina, in the context of ultra-deep desulfurization, i.e., in the presence of a low partial pressure of H2S. Hydrogenation properties and hydrodesulfurization properties were characterized by means of test reactions: tetralin hydrogenation and dibenzothiophene desulfurization. Moreover, the conversion of gas oils in a trickle bed reactor was also examined. The catalysts were characterized mainly by X-ray absorption spectroscopy. Experimental Section Catalysts. Commercial NiMo, CoMo (14 wt % MoO3, 3 wt % NiO or CoO), or NiW (25.7 wt % WO3, 3.82 wt % NiO) on alumina catalysts were used. The Pt catalyst was prepared by impregnation of a 250 m2/g alumina with H2PtCl6 (0.3 wt %), then calcined for 1 h at 773 K, and reduced under pure H2 during 6 h at 573 K. Mixed PtNiW/Al2O3 or PtCo(Ni)Mo/Al2O3 catalysts (0.15 < Pt wt % < 0.7) were obtained by deposition of H2PtCl6 precursor either on the oxidic form of the commercial catalyst (so-called Pt/NiW ox, Pt/NiMo ox, or Pt/CoMo ox) or on the sulfided one (so-called Pt/NiW sulf, Pt/NiMo sulf, Pt/ CoMo sulf). In both cases, the impregnated catalysts were dried at 393 K. Then, the commercial samples and mixed catalysts
10.1021/ie060932x CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006
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Table 1. Composition and Physical Properties of Straight Run Hydrotreated Gas Oil Feeds (A and B)
density (288 K) g/L sulfur content (ppm) aromatics (wt %)
gas oil A
gas oil B
846 515 36
852 142 30
Table 2. Catalytic Activities in Gas Oil Conversion of Conventional Alumina Supported Sulfidesa catalyst
CoMo
NiMo
NiW
gas oil 515 ppm S kapp (103 g1,9 mmol-1,9 h-1) gas oil 142 ppm S kapp (106 g2,8 mmol-2,8 h-1)
13.2
14.6
14.2
16
16.3
15.4
a
Initial wt % S content: (a) 515 ppm S and (b) 142 ppm S.
Figure 1. Stability in the conversion of tetralin of NiMo and NiW sulfide catalysts under reducing conditions.
were sulfided using gas flows of 5 vol % H2S in H2 at 673 K for 4 h (heating rate 10 K/min). After cooling down to room temperature under the same sulfiding mixture and flushing with nitrogen for 30 min, the catalysts were transferred under argon and kept in sealed vessels. Catalytic Activity. The catalysts were tested by means of three reactions: the hydrogenation of tetralin, the hydrodesulfurization of dibenzothiophene (DBT), and the straight run (SR) gas oil conversion. Experiments on model molecules were carried out in a microreactor running in the dynamic mode, in the vapor phase. The operating conditions were the following for tetralin hydrogenation: reaction temperature, 573 K; total pressure, 43 × 105 Pa; tetralin partial pressure, 8886 Pa; H2S partial pressure, 550 ppm or 0 (to check the stability of the catalysts). For DBT conversion, the reaction temperature was kept at 523 K with a total pressure of 34 × 105 Pa, a DBT partial pressure of 480 Pa, and no H2S addition. According to the model of the integral reactor, the rate constant k of a reaction can be expressed as follows:
k)
F0 ln(1 - x) (L s-1 g-1) mC0
x ) conversion, m ) mass of catalyst (g), F0 ) molar flow of reactant (mol/s), C0 ) concentration of reactant (mol/L). When the conversion is less than 15%, the above formula can be simplified, giving the following reaction rate:
r)
F0 x (mol s-1 g-1) m
Figure 2. Catalytic activities in tetralin hydrogenation at 45 KPa (500 ppm H2S) of (a) NiW, (b) CoMo, and (c) NiMo on alumina, Pt on alumina, and mixed catalysts prepared by impregnation of Pt on the sulfided form (sulf) or oxidic form (ox).
The rates were measured when the steady state of the catalyst was reached after 15 h on-stream. The accuracy is within (5%. Catalytic rates were also measured in a trickle bed reactor described in references 19 and 20 using hydrotreated gas oils as feeds. The reactions were performed at 613 K, with a 2 < LHSV < 10 h- 1 (LHSV: liquid hourly space velocity), and a total pressure of 3 Mpa (catalyst weight 0.5< m < 1 g). The ground catalysts were placed between two layers of alumina in an up-flow tubular reactor (2 cm3). The composition of the starting gas oil feeds is given in Table 1. A periodic sampling of the liquid effluent was performed, and the total sulfur content was determined by X-ray fluorescence (HORIBA SLFA-1800H analyzer), careful attention being paid on matrix effects.21 Kinetic orders n were obtained from the fitting of f(n) ) 1/LHSV according to the following expression (for n * 1):
1 n-1
(( ) ( ) ) 1 So
(n-1) -
1 Si
(n-1)
)
kapp LHSV
with Si and So corresponding to S concentration at the entrance and at the exit of the reactor, respectively. Then, rate constants kapp were calculated according to the expression above. Catalyst Characterization. Chemical analysis was used to control the metal contents of the catalysts. The dispersion of the catalysts was determined by TEM performed on a JEOL
Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 3879 Table 3. Catalytic Activities in DBT Conversion and Gas Oil Conversiona of NiW on Alumina, Pt on Alumina, and Mixed PtNiW on Alumina Catalysts
a
catalyst
Pt(0.3)/Al2O3
NiW/Al2O3
Pt(0.3)NiW sulf
Pt(0.3)NiW ox
DBT k (10-3 L g-1 s-1) gas oil kapp (106 g2,8 mmol-2,8 h-1)
0.8
2.5
3.5
0.7
13
10
19
9
Initial wt % S content: 142 ppm.
Figure 3. Effect of Pt loading on the catalytic activities (or conversion) of PtNiW on alumina catalysts in tetralin conversion (HYD) and gas oil conversion.
Figure 4. HRTEM image of a Pt(0.3)NiW on alumina catalysts prepared from impregnation of Pt on the sulfided catalyst.
2010-FEG instrument equipped with a Link-Isis EDS detector. Catalyst grains were dispersed in pure ethanol, the suspension stirred in an ultrasonic bath, and one drop deposited on a carbon coated copper grid. X-ray absorption spectroscopy (XAS) measurements were performed using BM32 beam line at the ESRF (French Collaborative Research Group). The storage ring operated at 6 GeV in the multi-bunch mode (2/3 filling) with a 200 mA current. Experiments were performed at Pt LIII edge in the fluorescence
detection mode performed with a 30-element solid-state detector (Canberra). The sulfidation procedure was carried out in a dedicated in situ furnace adapted for fluorescence detection from the cell described in reference 22. This activation process was carried out in a way similar to the laboratory procedure under an H2/H2S (5%) flow from room temperature up to 673 K (rising temperature 4 K/min, gas flow 50 mL/min). Standard analysis of the EXAFS spectra (normalization, background removal, Fourier transformation, and curve fitting) were carried out using
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Figure 5. Normalized absorption spectra at Pt LIII edge of the catalysts after impregnation of H2PtCl6 and drying on (a) oxidic CoMo and (b) sulfided CoMo catalysts.
(∆E0) were used as variables in the fitting procedure. Scale factors S02 were fixed at 0.8. Results
Figure 6. Magnitude of the Fourier transformed k3-weighted data on (a) oxidic CoMo and (b) sulfided CoMo catalysts (top) and after sulfidation with a 5 vol % H2S in H2 gas mixture.
the SEDEM software23 with FEFF624 theoretical phase and amplitude functions. The curve fitting procedure was performed in R-space. Fourier transformation of the normalized k3-weighted EXAFS signal was performed over the 2.5-15 Å-1 k-range with Kaiser window functions. Coordination numbers (N), interatomic distances (R), Debye-Waller parameters (σ2), and energy shifts
Catalytic Properties of Conventional Catalysts. Conventional industrial sulfide catalysts were investigated in the conversion of the two SR gas oil hydrotreated feeds containing, respectively, 515 and 142 ppm of S. By varying the LHSV, we determined for each reacting oil the kinetic order with respect to S and found it respectively equal to 2.9 and 3.8. These kinetic orders express the wide reactivity scale of the various refractory S compounds remaining in the desulfurized oil .25 It can be seen in Table 2 that the three catalysts present almost the same activity. However, the stability of the sulfided catalyst is also an important factor and depends on the partial pressure of H2S. It is known that CoMo catalysts segregate under reducing conditions (pure H2)26 and therefore are less stable than in the presence of H2S. The stability of the other industrial catalysts, i.e., NiW and NiMo, was checked in the conversion of tetralin. After stabilization of the activity under 500 ppm of H2S, the addition of this gas was suppressed and the evolution of the conversion was measured as illustrated in Figure 1. It can be seen that the NiW system is the most stable catalyst in the absence or with low partial pressure of H2S in agreement with previous reported in the literature.27 Catalytic Properties of Pt Based Systems. In order to improve the catalytic performance of conventional industrial catalytic systems and favor the hydrogenation pathway of the desulfurization scheme, these catalysts were doped with a small amount of Pt. The activity of Pt on alumina, NiW, NiMo, and CoMo on alumina catalysts alone or doped with 0.3 wt % of Pt were measured in tetralin conversion (Figure 2). Among commercial catalysts, NiW is the most active one. In all cases, an improvement in the catalytic hydrogenation activity is observed when Pt is added after the sulfidation of the commercial catalyst. In that case, the catalytic properties seem to correspond to the addition of the properties of each component. On the contrary, when Pt is impregnated on the oxidic form of the catalyst a sharp decrease in the activity is observed. The NiW system was then studied in the conversion of DBT (see Table 3).
Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 3881 Table 4. Structural Parameters Obtained from EXAFS Fitting for Impregnated and in Situ Sulfided PtCoMo Catalysts Prepared from the Oxidic or Sulfidic State sample
a
M neighbor
Kmin (Å-1)
Kmax (Å-1)
PtCoMo ox PtCoMo sulf PtCoMo sulf PtCoMo sulf
O Cla Cl Sa
3 3 3
Impregnated State 13.7 13.7 13.7
PtCoMo ox PtComo sulf
S S
3 3
Sulfided State 13.5 13.5
RPt-M
NPt-M
E0 (eV)
σ2 (Å2)
2.06 2.33 2.31 2.33
4 1.5 3.6 4
12 16 9.2 8.3
0.008 0.002 0.005 0.0049
2.34 2.33
4 4
8.2 7.8
0.005 0.005
S or Cl cannot be distinguished by XAS.
A similar trend, i.e., addition of the properties of each component, is observed when Pt is added on a presulfided NiW on alumina catalysts. Thus, we can expect that the conversion of a real feed can be increased by the use of such a mixed system. Table 3 gives the rate constants in the conversion of gas oil B. Again, we notice an enhancement of the rate for the mixed catalysts as compared to the unpromoted one. As compared to tetralin hydrogenation, Pt on alumina catalyst exhibits a much higher activity with respect to NiW catalyst. This effect can be ascribed to a weaker inhibiting effect of H2S due to the lower sulfur content of the feed. Under these specific conditions, Pt is less inhibited by H2S and the feed is composed of refractory alkyl DBT which are mainly converted by the hydrogenation route.2 The variations of the catalytic properties with the amount of Pt added to the NiW catalyst were studied in both tetralin and gas oil conversion reactions (see Figure 3). Although these variations are not exactly similar, it appears that a loading of 0.25-0.3 wt % of Pt gives the best improvement in the catalytic activity. Nanoscale Characterization of the Catalysts. Characterization by HRTEM. Both types of catalysts were characterized after sulfidation by HRTEM. For sulfided Pt(0.3)/NiW sulf, HRTEM pictures show NiW sulfide slabs and small particles with an average size of 0.8 nm (see Figure 4) composed of Pt as revealed by 1 nm probe analysis with EDS. Due to the background emission of neighboring (Ni)WS2 particles and the radiolytic effect of the small electron probe, the presence of S on these particles (and the stoichiometry) cannot be determined. Nevertheless, it can be concluded from these examinations that sulfide NiW slabs and Pt containing particles are separated systems. The sizes of the WS2 slabs, i.e., average length 3.1 nm, and average number of layers, i.e., 2, are not very different from those reported previously for NiW catalysts,28,29 but a slight decrease in the length can be noticed. The dispersion of the reference catalyst composed by 0.3 wt % of Pt on alumina was also determined by HRTEM, the average size of the particles being close to 1.5 nm by comparison to 0.8 nm for the mixed catalyst. This means that both phases “NiWS” and Pt are highly dispersed. By contrast, Pt cannot be observed by HRTEM in sulfided Pt(0.3)/NiW ox catalyst, which can be due either to the small size of the Pt containing particles (less than the resolution limit of the TEM, i.e., 0.8 nm) or to their close vicinity to dense WS2 particles. The morphology of this last phase (length and stacking) is similar to that of sulfided Pt(0.3)/ NiW sulf. XAS Characterization of the Precursor and Sulfided States of the Catalysts. Further information on the chemical nature of the noble metal nanoparticles as well as on the role of the preparation procedure was obtained by means of XAS
performed before and after sulfidation under in situ conditions (5 vol % H2S in H2 at 673 K, for 30 min, heating rate 5 K/min). In order to avoid the overlap of adsorption edges (Pt LIII: 11564 eV and W LII: 11544 eV), the CoMo sulfide system was chosen instead of the NiW one. The XANES spectra of the Pt(0.3)/ CoMo ox catalyst and Pt(0.3)/CoMo sulf after impregnation by hexachloroplatinic acid and drying at 393 K are presented in Figure 5. For Pt(0.3)/CoMo ox, some Pt-O bonds are formed as already reported for Pt/Al2O3 catalysts .30,31 The presence of these Pt-O bonds gives rise to a strong white line. By contrast, the Pt(0.3)/CoMo sulf sample presents a different electronic configuration indicating another nature of the Pt-neighboring atoms bonds as compared to the previous sample. The XANES spectra is similar to the one obtained on the sulfided catalyst (either Pt(0.3)/CoMo ox or sulf) suggesting that the electronic configuration of Pt is close the one obtained after in-situ sulfidation. The Fourier transforms at Pt LIII edge are presented in Figure 6 (top) and the results deduced from the EXAFS fitting summarized in Table 4. After impregnation and drying, the Pt neighborhood for Pt(0.3)/CoMo ox sample consists of O and Cl atoms. For the Pt(0.3)/CoMo sulf sample, there is no O shell contribution but XAS is not able to distinguish between Cl or S neighboring atoms. Thus, either the platinum chloride precursor remains unmodified at the surface of alumina or, more probably, interacts with S surface atoms (SH entities) and a partial substitution of Cl by S atoms occurs in the neighboring shell of Pt atoms (both cases are presented in Table 4). These Pt-S bonds are rather stable, and they are not expected to be strongly oxidized by the drying procedure.32 After sulfidation, the Pt local structure is schematized in Figure 6 (bottom) by the magnitude of the Fourier transform, as a function of real space separation R. For both samples, only one main contribution is visible, and data fitting indicates that Pt is surrounded by four sulfur atoms like in the PtS structure. However, we were not able to introduce a second Pt-M shell. This is indicative of the high dispersion of PtS particles as observed in the case of sulfided Pt(0.3)/CoMo sulf sample by HRTEM. Due to the high disorder of the Pt phase, it has not been possible either to characterize any interaction with WS2 for sulfided Pt(0.3) CoMo ox as proposed above for explaining the HRTEM data. Discussion The calcined state of CoMo, NiMo, and NiW supported on alumina catalysts has been studied by many techniques.33 It was proposed by several authors that at this precursor stage an interaction between the promoter, Co or Ni, and the polymolybdate or polytungstate phase already exists. This interaction would prevent the formation of MoO3 or WO3 at high Mo or W loading and would lead to a high dispersion of the promoter.
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The interaction of Pt with molybdenum or a tungsten oxidic phase has also been addressed previously. In a comprehensive study of PtW catalysts, De Penguilly observed several kinds of interaction between both components depending on W concentration.34 At high W loadings, Pt can be either grafted on the free alumina space left after polytungstates formation or linked to the tungstate phase by hydrogen bonding. Such a kind of interaction between Pt and W would explain HRTEM results since it has not been possible to detect distinct Pt particles in sulf Pt(0.3) NiW ox, probably because they are in close vicinity of the dense WS2 sulfide phase either in a kind of decoration or between alumina and WS2 slabs. In accordance with EXAFS data, the Pt phase is very well dispersed. However, whatever the nature of the mixed catalysts, CoMo or NiW, a large decrease in the catalytic properties was observed when Pt was added on the oxide form of the catalyst precursor. The close neighboring of Pt and WS2 may prevent the formation of some highly active so-called “CoMoS” (or “NiWS”) entities. In the case of sulfided Pt(0.3)/NiW sulf catalyst, the isolated PtS particles independently contribute to the catalytic activity providing an additive effect. The benefit of the sulfidation for the formation of “CoMoS” is kept even after impregnation of the Pt, indicating that the negative effect originates from oxidic interactions. However in order to keep the additive effect, only a small content of Pt (0.2 < wt % Pt < 0.4) must be added. We can suggest that a higher Pt content gives rise to larger and less active particles. Conclusion In order to achieve ultra-deep desulfurization, improved catalysts are needed. In the framework of a two-stage process, a tentative solution is provided by the addition of a small amount of noble metal to a conventional catalyst. This allows a 20% to 40% increase in the activity as compared to the unpromoted sulfide catalyst. The benefit originates from the addition of the catalytic properties of each individual system. However, such an effect requires the impregnation of Pt on a sulfided NiW on alumina catalyst otherwise, if Pt addition is performed on the oxidic catalyst, detrimental interactions between Pt and “NiWS” or “CoMos” occur. Acknowledgment This work received support from TotalFina, ELF, IFP, Procatalyse, and CNRS-Ecodev. We thank the French-CRG committee for providing machine-time. F. Bourgain and M. Cattenot are gratefully acknowledged for the realization of the in situ cell. We thank also J. F. Lambert for fruitful discussion on the Pt-W system. Literature Cited (1) European Union, EU directive 98/70/EC, 1998. EPA, Control of air pollution from new motor vehicles amendment to the tier-2/gasoline sulfur regulations, U.S. Environmental Protection Agency, April 13, 2001. (2) Breysse, M.; Djega-Mariadassou, G.; Pessayre, S.; Geantet, C.; Vrinat, M.; Perot, G.; Lemaire, M. Deep desulfurization: reactions, catalysts and technological challenges, Catal. Today 2003, 84, 129-138. (3) Song, C. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today 2003, 86, 211-263. (4) Sie, S. T. Reaction order and role of hydrogen sulfide in deep hydrodesulfurization of gas oils: consequences for industrial reactor configuration. Fuel Process. Technol. 1999, 6, 149-171.
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ReceiVed for reView July 18, 2006 ReVised manuscript receiVed October 17, 2006 Accepted October 18, 2006 IE060932X
