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Langmuir 1999, 15, 5800-5806
Effects of Support Surface Chemistry in Hydrodeoxygenation Reactions over CoMo/Activated Carbon Sulfided Catalysts† G. de la Puente,*,‡ A. Gil,§ J. J. Pis,‡ and P. Grange| Instituto Nacional del Carbo´ n, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain, Departamento de Quı´mica Aplicada, Universidad Pu´ blica de Navarra, Campus de Arrosadı´a s/n, 31006 Pamplona, Spain, and Unite´ de Catalyse et Chimie des Mate´ riaux Divise´ s, Universite´ Catholique de Louvain, Place Croix du Sud 2/17, 1348 Louvain-la-Neuve, Belgium Received September 10, 1998. In Final Form: January 28, 1999 The influence of the nature of the surface of activated carbon supports on the activity of CoMo/activated carbon sulfided catalysts for hydrodeoxygenation of model compounds representative of the composition of pyrolysis oils has been studied. For this purpose, an activated carbon support was subjected to oxidative treatments with HNO3 at various temperatures to modify its surface chemistry. Supported sulfided CoMo catalysts on the resulting activated carbons were prepared. These catalysts were tested for hydrodeoxygenation of model compounds, 4-methylacetophenone (4MA), ethyl decanoate (ED), and 2-methoxyphenol (GUA), representative of the oxygenated functions that should be eliminated for improving oil stability. The activities of the various CoMo catalysts for the hydrogenation of the carbonyl group of 4MA were very similar, and the conversion reached 100% in less than 120 min at 280 °C and 7 MPa. Introduction of oxygen-containing functional groups to the carbon supports led to higher decarboxylation in the conversion of the carboxyl group of ED and higher phenol/catechol selectivity from 2-methoxyphenol. The surface chemistry of the support determined the precursor/support interaction and, hence, the nature of the sulfided phases present in the catalyst after activation. The different natures of the metal sulfides formed from the oxide precursors that were bound or not to the oxygen groups on the carbon supports could be responsible for the differences in selectivity displayed by these catalysts. Almost no coking reactions were observed. The results suggest that catalytic conversion of oils obtained from biomass pyrolysis can be controlled and modified by appropriate modifications of the surface chemistry of the activated carbon supports.
Introduction Because of the reactivity of oxygenated groups, the main problems of the oils obtained from biomass pyrolysis (biooils) are thermal instability and some corrosivity, as well as a poor heating value. The elimination of these oxygenated groups by catalytic hydrotreatment has been described as an interesting way of upgrading bio-oils.1-3 Several authors4-9 have reported the activity of sulfided CoMo and NiMo catalysts as well as their modifications, with alkaline and precious metals, for the hydrodeoxygenation of model molecules representative of bio-oils. These authors observed the tendency of these molecules, because of their instability, to form coke that could render * To whom correspondence should be addressed. E-mail: gema@ muniellos.incar.csic.es. Fax: +(34) 985297662. † Presented at the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland, August 9-16, 1998. ‡ INCAR-CSIC. § Universidad Pu ´ blica de Navarra. | Universite ´ Catholique de Louvain. (1) Baker, E. G.; Elliot, D. C. In Research in Thermochemical Biomass Conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier: Amsterdam, 1988; p 883. (2) Bridgwater, A. V.; Bridge, S. A. In Biomass Pyrolysis Liquids. Upgrading and Utilisation; Bridgwater, A. V., Grassi, G., Eds.; Elsevier: Amsterdam, 1991; p 11. (3) Grange, P.; Laurent, E.; Maggi, R.; Centeno, A.; Delmon, B. Catal. Today 1996, 29, 297. (4) Laurent, E.; Delmon, B. Appl. Catal. 1994, 109, 77. (5) Laurent, E.; Delmon, B. Appl. Catal. 1994, 109, 97. (6) Hurff, S.; Klein, M. Ind. Eng. Fundam. 1983, 22, 426. (7) Centeno, A.; Laurent, E.; Delmon, B. J. Catal. 1995, 154, 288. (8) Elliot, D. C.; Baker, E. G. Biotechnol. Bioeng. Symp. 1984, 14, 159. (9) Furimsky, E. Catal. Rev.sSci. Eng. 1983, 25, 421.
the low-temperature stabilization of bio-oils more difficult and limit the life of the catalysts. To avoid the formation of polymerization products, catalysts less active for this unwanted reaction must be used. Alumina-supported catalysts are traditionally used for the hydrodeoxygenation of carbonyl, carboxylic, and methoxyl groups. The polycondensation products that can be formed from the thermal treatment of the above model compounds can be strongly adsorbed by the support, leading to formation of coke. The preparation of catalysts with a support less active for coke formation would be an interesting objective to improve the catalytic system. In this sense, carbon is of particular interest, since it is has been shown that carbon supports have a lower coking propensity than alumina supports10,11 and should thus maintain their activity over extended periods of operation.7 On the alumina support, the exposed surface hydroxyl groups act as adsorption sites for the metal ions. A more complex situation is encountered in activated carbons due to the nature of the carbon surface. The presence of various acidic or basic oxygenated groups on the carbon surface confers on them an amphoteric character.12 When activated carbons are used as support for metallic catalysts, the performance is expected to be strongly influenced by the nature and concentration of the functional groups on the carbon surface. The initial metallic dispersion, its resistance to sintering, and the catalytic behavior are (10) de Beer, V. H. J.; Derbyshire, F. J.; Groot, C. K.; Prins, R.; Scaroni, A. W.; Solar, J. M. Fuel 1984, 63, 1095. (11) Scaroni, A. W.; Jenkins, R. G.; Walker, P. L., Jr. Appl. Catal. 1985, 14, 173. (12) Radovic, L. R.; Rodrı´guez-Reinoso, F. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1996; Vol. 25, p 243.
10.1021/la981225e CCC: $18.00 © 1999 American Chemical Society Published on Web 04/02/1999
Effects of Support Surface Chemistry on Catalysts
sensitive to the state of the carbon surface.13-18 In previous work,19-21 it was observed that the modification of the activated carbon surface by oxidation treatments could create oxygen functionalities producing a minimum change in the textural properties. It would then be possible to tune the adsorption sites on the surface of the support that were accessible to the metal precursor. It was confirmed22 that interactions were established between molybdenum species and oxygen functional groups generated on support surfaces. These interactions could influence the nature of active centers of sulfided CoMo/C catalysts. The aim of the present work is to study the influence of the nature of the surface of the activated carbon supports on the activity, selectivity, and coking tendency of sulfided CoMo/C catalysts for hydrodeoxygenation of model compounds representative of the composition of bio-oils. Experimental Section The supports used for the preparation of catalysts were an activated carbon based on coconut shell (Merck-9631) (sample K0) and a series of samples of this carbon subjected to an oxidative treatment with HNO3 at various temperatures (25, 60, and 90 °C and reflux at 110 °C) during 3 h. Details of the activated carbon treatments are given elsewhere.19 These supports will be referred to as N25, N60, N90, and NR, respectively. Since the main concern of this study is to evaluate the influence of the carbon surface on the catalyst behavior, all the samples were prepared so as to keep the support loading constant. Activated carbon supported CoMo catalysts were prepared by incipient wetness impregnation procedures, with the required amounts of aqueous solutions of ammonium heptamolybdate tetrahydrate (Merck, p.a.) and cobalt nitrate (Merck, p.a.) in order to obtain catalysts with a nominal composition of 15 wt % MoO3 and 3 wt % CoO. The samples were subsequently dried at 130 °C for 16 h in a stream of N2 (Air Liquide, 99.995%). These samples will be referred to as CoMo/supports (CoMo/K0, CoMo/ N25, CoMo/N60, CoMo/N90, and CoMo/NR, respectively). The precursors were activated using a standard reduction-sulfidation procedure. After ≈1.2 g of catalyst was dried under a flow of 100 cm3/min of argon at 130 °C for 1 h, reduction-sulfidation was done by a mixture of 15 vol % H2S in H2 (100 cm3/min). The temperature was raised to 400 °C at a rate of 3 °C/min, and the reduction-sulfidation procedure was continued for 3 h. The catalysts were then cooled to room temperature, and the gas was switched back to argon. The model reactants used in this work were 4-methylacetophenone (4MA) (5 g), ethyl decanoate (ED) (5 g), and guaiacol (2-methoxyphenol) (GUA) (5 g). They were dissolved in p-xylene (135 g). Pentadecane (2.5 g) was added to this solution as an internal standard for the chromatographic analysis. CS2 (0.25 cm3) was added as a precursor of H2S in order to generate a partial pressure under reaction test conditions (approximately 0.05 MPa, H2S/H2 ≈ 7 × 10-3) to maintain the catalysts in the sulfided form during the reaction. (13) Ehrburger, P.; Mahajan, O. P.; Walker, P. L., Jr. J. Catal. 1976, 43, 61. (14) Richard, D.; Gallezot, P. In Preparation of Catalysts IV; Delmon, B., Grange, P., Jacobs, P. A., Poncelet, G., Eds.; Elsevier: Amsterdam, 1987; p 71. (15) van Dam, H. E.; van Bekkum, H. J. Catal. 1991, 131, 335. (16) Suh, D. J.; Park, T.-J.; Ihm, S.-K. Carbon 1993, 31, 427. (17) Martı´n-Gullo´n, A.; Prado-Burguete, C.; Rodrı´guez-Reinoso, F. Carbon 1993, 31, 1099. (18) Roma´n-Martı´nez, M. C.; Cazorla-Amoro´s, D.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C.; Yamashita, H.; Anpo, M. Carbon 1995, 33, 3. (19) Gil, A.; de la Puente, G.; Grange, P. Microporous Mater. 1997, 12, 51. (20) de la Puente, G.; Pis, J. J.; Mene´ndez, J. A.; Grange, P. J. Anal. Appl. Pyrolysis 1997, 43, 125. (21) de la Puente, G.; Centeno, A.; Gil, A.; Grange, P.; Delmon, B. In Characterisation of Porous Solids IV; Royal Society of Chemistry: Cambridge, U.K., 1998; p 327. (22) de la Puente, G.; Centeno, A.; Gil, A.; Grange, P. J. Colloid Interface Sci. 1998, 202, 155.
Langmuir, Vol. 15, No. 18, 1999 5801 A 570 cm3 closed batch reactor was used for the tests. The solution of reactants occupied 170 cm3. Under reaction conditions, the solution was vigorously agitated by means of a magnetically operated stirrer. The reactor was provided with a thermocouple, and a line ending in a stainless steel filter was immersed in the liquid phase. The reactor was sealed immediately after the addition of the sulfided catalyst to the reaction mixture. Air was evacuated by pressurization-depressurization cycles with nitrogen and subsequently with hydrogen. Taking into account that the pressure requirements of the hydrotreating of pyrolytic oils were reported as being in the range 3-10 MPa,3 in this work, 7 MPa was used as the reaction pressure. These conditions were also applied at the laboratory scale by several authors.1,23,24 The mixture was then heated to 280 °C at 4.5 °C/min under slight agitation and low hydrogen pressure (1 MPa), because of safety reasons, and then the pressure was increased to 7 MPa. After reaching the reaction temperature, the sampling line was purged by withdrawing 7 cm3 of the reactant solution. A first sample of 0.5 cm3 was then taken and was considered to correspond to time zero of the reaction. The concentration of reactants in this sample was conventionally considered to be 100%, neglecting the slight conversion reached at the end of the heating period. The pressure was increased to 7 MPa and the agitation was set at 900 rpm. Liquid samples (0.5 cm3) were taken every 10 min during the first hour of reaction and every 20 and 30 min in the second and third hours, respectively. The liquid sampling line was always flushed with 3 cm3 of the reacting mixture before taking a sample. H2 (Air Liquide, 99.9%) was added when necessary to maintain the pressure at 7 MPa. Blank test experiments with the support alone were performed in order to check the activity of the bare supports. The liquid samples were analyzed by gas chromatography in a Chrompack CP 9000-1 gas chromatograph equipped with a split injector and a flame ionization detector (FID), both working at 280 °C. The different compounds were separated in a 25 m DB-5 capillary column. The chromatographic peaks were identified by GC-MS analysis and by comparison of the retention times with those of known compounds. Response factors of the reactants and products were determined experimentally using pure compounds and served as a basis for the determination of the molar balances and selectivities. Representative subsamples of the oxide catalysts were dispersed in double-sided adhesive graphite on a sample holder and examined by scanning electron microscopy (SEM) at 2000× magnification and using the backscattered signal for phase contrast. The microscope (Zeiss DSM-942) was also equipped with an energy-dispersed X-ray detector (EDX) (Oxford LINK PENTAFET model) that allowed the determination of the chemical compositions of the different phases. X-ray photoelectron spectroscopy (XPS) analyses of oxide, freshly sulfided, and used catalysts were carried out. A spectrometer Surface Science Instrument SSX-100, model 206, with a monochromatic Al KR source (1486.6 eV), operating at 10 kV and 15 mA was used. The samples were outgassed under a minimum vacuum of 5 × 10-7 Torr before being placed in the analysis chamber. The C 1s, O 1s, N 1s, S 2s, Mo 3d, and Co 2p lines were investigated. A nonlinear, Shirley-type25 baseline and an iterative least-squares fitting algorithm were used for curve fitting. The curves were taken as 85% Gaussian and 15% Lorentzian. Surface atomic concentration ratios were calculated as the ratio of the corresponding peak areas, corrected with theoretical sensitivity factors based on Scofield’s photoionization cross sections.26
Results and Discussion Activated carbon supports and molybdenum catalysts have been widely characterized by means of nitrogen adsorption at 77 K, ammonia chemisorption, temperature(23) Churin, E.; Grange, P.; Delmon, B. In Research in Thermochemical Biomass Conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier: Amsterdam, 1988; p 896. (24) Laurent, E.; Pierret, C.; Keymeulen, O.; Delmon, B. In AITBC; Bridgwater, A. V., Ed.; 1994; p 1404. (25) Shirley, D. A. Phys. Rev. 1972, 35, 4709. (26) Scofield, J. H. Electron. Spectrosc. Relat. Phenom. 1976, 8, 129.
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Table 1. Surface Composition (XPS), Texture Properties (N2 Adsorption), and Acidic Properties (NH3 Chemisorption) of the Activated Carbon Supports
As (m2/g)
Vp (cm3/g)
acidity NH3 sorption (µmol/g)
1242 1116 1075 842 129
0.645 0.582 0.551 0.449 0.081
126 404 480 569 638
texture propertiesa surface compn (atom %) support C O S N K0 N25 N60 N90 NR
95.4 90.3 88.1 80.4 76.0
4.3 8.4 10.6 18.1 22.1
0.3 0.2 0.2 0.1 0.1
1.1 1.1 1.4 1.7
a A ) specific total surface area, Langmuir equation; V ) specific s p total pore volume.
Table 2. Quantitative XPS Analysis of As-Prepared Impregnated Catalysts (Atom %) sample
C
O
Mo
Co
N
S
CoMo/K0 CoMo/N298 CoMo/N333 CoMo/N363 CoMo/NR
88.14 86.12 83.75 76.70 68.31
8.80 10.70 13.09 18.76 26.77
1.24 1.27 1.27 1.38 1.45
0.26 0.36 0.36 0.38 0.46
1.25 1.35 1.33 2.65 2.92
0.31 0.20 0.20 0.13 0.09
programmed desorption (TPD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy. Details of the characterization of the activated carbons are given elsewhere19-21 and are summarized briefly in Table 1. When the results obtained from nitrogen adsorption analyses of the parent activated carbon (sample K0) and its acid-treated derivatives with HNO3 (samples N25, N60, N90, and NR) were compared, a continuous loss of the textural properties of the activated carbon supports with the increase of the treatment temperature was observed. The comparison of the nitrogen adsorption isotherms of the activated carbon supports and molybdenum catalysts22 indicated that there was no restriction of the accessibility of nitrogen due to the blocking of micropores as a consequence of the fixation of molybdenum at the entrance of micropores. Nevertheless, it was also observed that the incorporation of cobalt into the molybdenum-activated carbon catalysts caused no important modifications of the nitrogen adsorption isotherms and of the textural properties. The thermal treatment with HNO3 of the parent activated carbon not only modified its textural properties but also produced an increase of the surface oxygen groups. It has been reported that these new groups play an important role in the preparation of activated carbon supported catalysts.27-29 The surface compositions of the as-prepared impregnated catalysts evaluated by XPS are summarized in Table 2. Quantitative XPS analysis showed that the relative amount of Mo on catalyst surfaces increased in the order corresponding to the increase of surface oxygen content of the supports. SEM micrographs of the oxide catalysts CoMo/K0 and CoMo/N90 are shown in Figure 1. As can be seen, there are substantial differences in both the sizes of the crystallite units and their distribution. In the case of the nontreated support, carbon K0, with a small amount of oxygen surface groups, relatively coarse clusters are formed (Figure 1a). Oxygen functionalities are thought to act as nucleation centers from which the crystal could grow. So, the large number of these surface groups in the oxidized support, carbon N90, could favor the formation (27) Prado-Burguete, C.; Linares-Solano, A.; Rodrı´guez-Reinoso, F.; Salinas-Martı´nez de Lecea, C. J. Catal. 1989, 115, 98. (28) Prado-Burguete, C.; Linares-Solano, A.; Rodrı´guez-Reinoso, F.; Salinas-Martı´nez de Lecea, C. J. Catal. 1991, 128, 397. (29) Calafat, A.; Laine, J.; Lo´pez-Agudo, A.; Palacios, J. M. J. Catal. 1996, 162, 20.
Figure 1. Representative SEM micrographs showing the differences in crystal size and distribution of the oxide phases on the precursor catalysts prepared with various supports: (a) nontreated activated carbon, CoMo/K0; (b) activated carbon oxidized with HNO3 at 90 °C, CoMo/N90. Table 3. Quantitative XPS Analysis of Sulfided Catalysts before and after Reaction atom % sample
C
O
Mo
Co
N
S
S/Mo
CoMoS/K0 CoMoS/N298 CoMoS/N333 CoMoS/N363 CoMoS/NR
87.54 86.78 86.24 84.51 82.11
Before Reaction 7.68 1.08 0.25 8.27 1.05 0.24 8.41 1.07 0.26 9.97 1.14 0.26 11.5 1.29 0.31
0.59 0.84 0.94 1.00 1.06
2.86 2.82 3.08 3.12 3.73
2.6 2.7 2.9 2.7 2.9
CoMoSar/K0 CoMoSar/N298 CoMoSar/N333 CoMoSar/N363 CoMoSar/NR
88.45 85.23 82.87 82.00 80.64
After Reaction 7.77 0.89 0.23 10.15 0.93 0.29 11.12 1.07 0.34 12.68 1.00 0.22 13.44 1.23 0.28
0.42 0.57 0.74 0.99 1.10
2.24 2.83 3.86 3.11 3.31
2.5 3.0 3.6 3.1 2.7
small crystal units and a good dispersion of the oxide phase (see Figure 1b). In a previous work,22 FTIR, TPD, and XPS analyses confirmed that oxygen groups acted as chemical anchorage centers for the oxide species. The surface chemistry of the support determines the precursor/ support interaction and could influence the nature of the active sulfided phases in the catalyst and therefore their activity and selectivity. The surface compositions of the sulfided catalysts before and after the reaction are presented in Table 3. At the
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Langmuir, Vol. 15, No. 18, 1999 5803
Figure 2. XPS spectra of the Mo 3d doublet in catalysts prepared on (1) nontreated activated carbon, CoMo/K0, and (2) activated carbon oxidized at 110 °C, CoMo/NR: (a) oxide catalysts; (b) sulfided catalysts before reaction; (c) sulfided catalysts after reaction.
temperatures used for activation (400 °C), some of the surface groups of the activated carbon support are not stable (CO2-desorbing groups). This could cause some molybdenum mobility and sintering during activation. The relative amount of Mo on activated catalyst surfaces increased in the same order as the oxygen content of the supports, indicating that the dispersion of the metal precursor on the support surface was maintained during catalyst activation. The distribution of Mo oxidation states was estimated by curve fitting of the Mo 3d region. The Mo 3d5/2-Mo 3d3/2 doublet was fitted so that each peak had the same Gaussian line shape. The relative area ratios of spin-orbit doublet peaks are given by the ratio of their respective degeneracy (2j + 1), I(3d5/2)/I(3d3/2) ) 3/2. A splitting energy of 3.2 eV was used. Before activation, the Mo 3d spectra of the catalysts, displayed in Figure 2a, showed no evidence of splitting or broadening, indicating that the Mo was present in a single environment. They presented two well-resolved spectral lines at 232.8 and 236.0 eV corresponding to the Mo 3d5/2 and Mo 3d3/2 peaks, respectively. This species is assigned to MoVI.30 After the catalysts were activated, the Mo spectra, Figure 2b, (30) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. In Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp., Physical Electronics Division: Eden Praire, MN, 1979.
showed a shift to lower binding energies, indicating that MoVI was transformed to lower oxidation states. A species with Mo 3d5/2 and Mo 3d3/2 binding energies of 232.1 and 228.9 eV, respectively, was identified. These peaks can be assigned to MoIV. The peak appearing at a binding energy of 226.1 eV corresponds to sulfur. No important differences were observed between the various catalysts used. Most of Mo was transformed to MoIV after the reductionsulfidation process (see Figure 2), indicating that, under the experimental conditions used, activation took place in an efficient way. After the reaction tests, Mo remained as MoIV, as may be observed in Figure 2c. The various activated carbon catalysts were tested for hydrodeoxygenation of model compounds, 4-methylacetophenone, ethyl decanoate, and 2-methoxyphenol, representative of the composition of bio-oils. The mechanisms of the hydrodeoxygenation of the reactants used in this work confirmed those established in previous studies,4,7 and they are reported in Scheme 1. 4-Methylacetophenone (4MA) gives ethylmethylbenzene (EMB) as the only product. The intermediate products, methylbenzyl alcohol and methylstyrene, were not observed in the GC analyses. The carboxylic group of ethyl decanoate (ED) reacts by following two paths. One is the hydrogenation to a CH3 group; the final product is decane. The other
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Scheme 1. Conversion of Carbonyl, Carboxylic Ester, and 2-Methoxyphenol Groups with CoMo Sulfided Catalyst
Figure 3. Evolution of the molar fraction of 4-methylacetophenone (4MA, open symbols) and its product of reaction, ethylmethylbenzene (EMB, full symbols), with the time of reaction for various sulfided catalysts: (0, 9) CoMo/K0; (4, 2) CoMo/N60; (], [) CoMo/NR.
path corresponds to the decarboxylation and leads finally to nonane. The carboxylic ester group may also react intermediately to form a carboxylic acid group. Concerning 2-methoxyphenol (GUA), hydrogenolysis of the methyloxygen bond leads to catechol. Hydrogenolysis of the aromatic carbon-oxygen bonds of catechol gives phenol and benzene, and the subsequent aromatic ring hydrogenation gives cyclohexane as the final product. Phenol can also be directly obtained by hydrogenolysis of the carbon-oxygen bond of 2-methoxyphenol. The conversions of the three reactants in the solution of model compounds under the standard conditions used in this work (280 °C, 7 MPa, 180 min) for the blank test and support indicated that conversions obtained are 17% for 4MA, 2% for ED, and 5% for GUA. A similar behavior was observed for the various activated carbon supports. For all the reactants, the conversions with the bare supports were similar to those obtained without any solid in the reaction mixture. These results indicate that, during the catalytic test, some thermal conversion takes place, but the supports do not present catalytic activity. The evolution of the molar fractions in the conversion of 4MA for the various activated carbon catalysts as a function of the reaction time is presented in Figure 3. The activities of the various catalysts for the hydrogenation of the carbonyl group are very similar, and as can be seen in the figure, 4MA is rapidly converted in the presence of CoMo/C catalysts. The conversion reached 100% in 120 min at 280 °C. Ethylmethylbenzene accounts for nearly all the converted 4MA, indicating that heavy products are not formed. The absence of intermediate products such as methylbenzyl alcohol and methylstyrene indicates that alcohol dehydration and olefin hydrogenation reactions are both very rapid under the experimental conditions (280 °C, 7 MPa). Alcohol and olefin formed in the reaction are unstable, and 100% conversion to EMB is obtained during the first minutes of reaction. Previous work7 showed similar high activities for CoMo/Al2O3 and CoMo/C catalysts, despite the different acidities of the respective supports. The active sites for the conversion of 4MA can be located on the metal sulfides and/or the support. The hydrogenation of the carbonyl group should be affected
Figure 4. Conversion of ethyl decanoate (ED) as a function of the reaction time for the sulfided catalyst prepared with various activated carbon supports: (0) CoMo/K0; (]) CoMo/N25; (4) CoMo/N60; (×) CoMo/N90; (9) CoMo/NR.
by the dispersion of the sulfide phases,31,32 and the dehydration step should be accelerated by the acidity of the support. Several possibilities were described to increase the metal dispersion on the activated carbon surface.33 The generation of oxygen functional groups that increase the acidity properties of the supports and can chemisorb elements and the presence of small pores that stabilize highly dispersed particles by forming physical barriers that suppress surface migration are two of them. The activated carbon supports are catalytically inert but can promote a good dispersion of metallic sulfides. The behavior can therefore be attributed to the dispersion of the sulfide phases and not directly to the acidity properties of the activated carbon supports. For the hydrodeoxygenation of ethyl decanoate, two types of data were taken into account: (i) the variation of total conversion and (ii) the variation of selectivity, namely the promotion of decarboxylation or hydrogenation reactions. The evolution of the conversion of ethyl decanoate as a function of reaction time for the various catalysts is shown in Figure 4. There is a marked difference in activity, showing a progressive decrease with the increasing acidity of the supports. The final conversion, after 3 h of reaction, ranges from 36% for the CoMo/K0 catalyst, prepared with the nonoxidized activated carbon support, to only 10% in the case of CoMo/NR, obtained from the support oxidized with HNO3 under reflux conditions. The thermal treat(31) Gajardo, P.; Grange, P.; Delmon, B. J. Phys. Chem. 1979, 83, 1771. (32) Gajardo, P.; Grange, P.; Delmon, B. J. Phys. Chem. 1979, 83, 1780. (33) Rodrı´guez-Reinoso, F. In Porosity in Carbons: Characterization and Applications; Patrick, J. W., Ed.; Edward Arnold: London, 1995; p 253.
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Langmuir, Vol. 15, No. 18, 1999 5805
Figure 5. Product yields from ethyl decanoate hydrodeoxygenation with sulfided CoMo catalyst supported on nontreated activated carbon.
ments with HNO3 produce an important increase of the oxygen functional groups but also a decrease in the textural properties.19 This reduction of the textural properties modifies the metal dispersion and can be the fact responsible for the lower activity of the catalysts prepared on the more oxidized activated carbon supports. The conversion of the carboxylic ester in the standard mixture leads to a series of products identified by GC analysis (see Scheme 1), whose molar fraction evolution is shown in Figure 5 as a function of ethyl decanoate conversion for the catalyst prepared with the nonoxidized activated carbon support. The carboxylic acid is the product that appears first. Decane is the final product obtained when the hydrogenation reaction occurs on the carboxylic carbon of the molecule. Nonane is the result of the elimination of the carboxylic group. Decanol and decene were also identified at long reaction times. Both are intermediate products of the hydrogenation of the carboxylic acid to decane. The carboxylic acid behaves as an intermediate product and does not accumulate in the reaction medium, presenting a maximum concentration at an ED conversion of 20%. Decane and nonane are observed only after an ED conversion of about 10%, indicating that the main way to obtain these products with respect to the hydrogenation and elimination of the carboxylate group is from the carboxylic acid. Selectivity for the hydrodeoxygenation of the carboxylic ester group may be expressed as a decarboxylation percentage. It is calculated as the ratio between the concentrations of the decarboxylation product, nonane, and the hydrogenation products, decane, decanol, and decene, according to
Sdecarb )
Cnon × 100 Cdec + Cdecol + Cdecene
(1)
The decarboxylation selectivities calculated at the same conversion of ethyl decanoate (20%) for the various catalysts are displayed in Figure 6 as a function of the oxygen content on the surface of the supports. The selectivity results indicate that the more oxidized the support, the higher the decarboxylation selectivity. The advantage of obtaining a high ratio of decarboxylation products in the hydrotreatment of pyrolysis oils is that it leads to elimination of CO2 and lower hydrogen consumption when compared to hydrogenation. The low activity of the pure activated carbon showed that the support itself is catalytically inert. This is logical since it is known that active sites for these reactions are situated on the metal sulfides, and the differences in selectivity must therefore be attributed to the differences in the state of the active phases formed on the various supports with different surface properties, and not to the support itself. Laurent and Delmon5,7 related the decarboxylation reaction,
Figure 6. Decarboxylation selectivity (O) in the conversion of ethyl decanoate and phenol/catechol selectivity (0) in the conversion of 2-methoxyphenol for the catalysts prepared with the various modified supports with different oxygen contents.
synthesis of nonane, with the Bro¨nsted acid sites present on the metal sulfides bound to the alumina support. It was reported in a previous work22 that interactions were established between the metallic phase and the oxygen groups formed on the carbon surface as a consequence of nitric acid treatment:
Figure 1 provides evidence for the precursor/support interaction which causes significant differences in morphology. The surface chemistry of the support determines the precursor/support interaction and, hence, the nature of the sulfide phases in the catalyst. Active sites in the nonmodified carbon-supported catalyst should correspond to metal sulfides formed from precursor molecules that are not bound to the carbon support, due to the lack of oxygen groups to serve as anchorage centers for the metallic phase. Active sites in the oxidized carbonsupported catalyst should correspond to metal sulfides produced from metal precursor molecules that are bound to oxygen groups of the carbon support. The different nature of these two kinds of active sites could be the fact responsible for the increasing decarboxylation selectivity observed for the catalysts prepared with the more oxidized activated carbons. Variations with the time of reaction of the molar fraction of 2-methoxyphenol are shown in Figure 7 for the catalysts CoMo/K0 and CoMo/NR. The material balances calculated indicated that almost no coking reactions were produced. Guaiacol yielded catechol (2-hydroxyphenol) and phenol as major products. Traces of benzene and cyclohexane were also detected at long reaction times. One of the reaction schemes proposed to explain guaiacol HDO (hydrodeoxygenation) considers that hydrogenolysis of the methyl-oxygen bond of the methoxyl group to form catechol and methane is the first stage, followed by a second stage leading to the elimination of one of the hydroxyl groups to produce phenol and H2O.34 It was suggested that methyl-oxygen bond hydrogenolyses take (34) Bredenberg, J. B-son.; Huuska, M.; Toropaien, P. J. Catal. 1989, 120, 401.
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Figure 7. Temporal variation of the main product yields from 2-methoxyphenol hydrodeoxygenation and material balance for the sulfided catalysts prepared on the nontreated activated carbon support CoMo/K0 and on the oxidized support CoMo/ NR.
place on both the support surface and the metal sulfides, each by a different mechanism. In the case of activated carbon supported catalysts, active sites for the reaction might be situated on metal sulfides, because carbon as such is an inert material. Another possible mechanism suggests the direct elimination of the methoxyl group by hydrogenolysis of the aromatic carbon-oxygen bond.35 This mechanism is favored by the presence of metal sulfides.36,37 Some authors38 have also proposed the existence of two different active sites on the CoMo sulfided catalysts, one type for the breaking of C-O bonds and the other for hydrogenation. Concerning the conversion of guaiacol, the differences in activity for the various catalysts tested in this work are less marked than those for the carboxylic ester group. A slight decrease of the final conversion of guaiacol with the increasing acidity of the support was observed. The final conversion, after 3 h of reaction, ranges from 35% for CoMo/ K0 to 30% for CoMo/NR catalysts. In all the experiments, catechol appears in first place, and its concentration increases with reaction time, not reaching a maximum yield in the considered period. The phenol/catechol ratio is not perfectly linear, showing a slight increase with the conversion, and can be interpreted as due to some catechol conversion to phenol. The present data do not allow a decisive assignment of phenol as the primary or secondary product that appears through catechol decomposition. The selectivity in the conversion of guaiacol is reported as the ratio between phenol and catechol:
Sphe/cat )
Cphe × 100 Ccat
(2)
The phenol/catechol selectivities calculated at the same conversion of guaiacol (20%) for the various catalysts are (35) Vuori, A.; Helenius, A.; Bredenberg, J. B-son. Appl. Catal. 1989, 52, 41. (36) Petrocelli, P. F.; Klein, M. T. Fuel Sci. Technol. 1987, 5, 25. (37) Train, P. M.; Klein, M. T. Fuel Sci. Technol. 1991, 9, 193. (38) Gevert, B. S.; Ericksson, M.; Ericksson, P.; Massoth, F. E. Appl. Catal. 1994, 117, 115.
presented in Figure 6. The phenol/catechol ratio slightly decreases with increasing oxygen content on the surface of the support. On the nonmodified carbon-supported catalyst, a higher amount of phenol is produced. These results could be interpreted as due to some direct elimination of the methoxyl group by the hydrogenolysis of the aromatic carbon-oxygen bond of guaiacol. Active sites for this reaction might be situated on metal sulfides formed from precursor molecules that were bound to the carbon support surface. The increase of oxygenated groups in the HNO3-treated supports causes an increase in the amount of metal precursor molecules bound to the carbon support,22 and active sites produced from them may be responsible for the increasing phenol/catechol selectivity. The presence of molecules with two oxygen-containing substituents of benzene is thought to contribute to coke formation.6,37,39 It has been reported7,40 that these reactions during the hydrodeoxygenation of guaiacol take place on acid sites. In the experiments of this work, even when catechol was the major product, results indicate that very small amounts of additional products were formed. Coke reactions were negligible in the catalyst CoMo/K0, and an average material balance closure of about 98% was found in the catalyst prepared with the most acidic support, CoMo/NR. The absence of coke deposition indicates that carbon offers good perspectives for the HDO of pyrolysis oils. Conclusions Various supported CoMo sulfided catalysts were prepared by impregnation of activated carbons obtained from oxidative treatments with HNO3 at various temperatures. The introduction of surface oxygen groups to the supports gave rise to important differences of activity and selectivity in the hydrodeoxygenation of model compounds representative of the composition of pyrolysis oils. For the hydrogenation of the carbonyl group of 4-methylacetophenone, carbon-supported catalysts showed good activity. Concerning the conversion of the carboxyl group of ethyl decanoate, it was observed that the more oxidized the support, the higher the decarboxylation selectivity. The phenol/catechol selectivity decreases with increasing oxidation temperature of the support, indicating that introduction of oxygen groups to the support surface enhances catechol production. The surface chemistry of the support determines the precursor/support interaction and, hence, the nature of the metallic phases present in the catalysts and their selectivity. Metal sulfides produced from metal precursor molecules bound to the oxygen groups on carbon supports could be active sites for decarboxylation of carboxylic group and production of catechol from 2-methoxyphenol. The catalytic conversion can be controlled and modified by modifying the surface chemistry of the carbon support, leading to an optimized catalytic system. The obtained results suggest that catalysts supported on carbon offer good perspectives for the HDO of pyrolysis oils. Coking reactions were negligible, and thus the potentially detrimental effect of guaiacols on the stability of the catalytic system during hydrotreating could be avoided by using activated carbon supports. LA981225E (39) Bredenberg, J. B-son.; Ceylan, R. Fuel 1983, 62, 342. (40) Mare´cot, P.; Martı´nez, H.; Barbier, J. J. Catal. 1992, 138, 474.
