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γ-Al2O3 during the Hydrodeoxygenation of Bio-Oils: Influence of a High Water Pressure
B. Delmon and G.F. Froment (Eds.) Catalyst Deactivalion 1994 Studies in Surface Science and Catalysis, Vol. 88 0 1994 Elsevier Science B.V. All rights rcserved.
459
Deactivation of a Sulfided NiMo/y-A1203 during the Hydrodeoxygenation of Bio-Oils: Influence of a High Water Pressure E. Laurent and B. Delmon Unite? de Catalyse et Chimie des M&riaux Divids, Universit6 Catholique de Louvain, Place Croix du Sud 2/17,1348Louvain-la-Neuve (Belgium).
ABSTRACT The influence of a high pressure of water on the hydrodeoxygenation activity and physico-chemical characteristics of a sulfided NiMoly-AlzO3 catalyst was studied in a batch reactor. Water caused a decrease of the catalytic activity to one-third the activity of the fresh catalyst but did not change the hydrogenation-hydrogenolysisselectivity. Water was also the cause of a small loss of the specific surface area conjugated to some crystallisation of the yalumina support in a hydrated boehmite phase, but this phenomenon cannot account entirely for the deactivation. On the other hand, the metal content, the dispersion, and the sulfidation state were not specifically affected by water. The deactivation would rather be related to the appearance of oxidised nickel species in the water treated samples. Two interpretations are proposed: either the formation of an inactive sulfate layer, or the consumption of nickel as nickel aluminare. 1. INTRODUCTION
Solid biomass is an attractive source of energy in the contexts of energy supply and environmental problems. Thermochemical processes were developed for its efficient energetic use. Especially, liquefaction by fast pyrolysis presents interesting potentialities [lI. Nevertheless, if the objective is the obtaining of good quality fuels, the pyrolysis oils must be upgraded in order to exceed the drawbacks originating from their high organic oxygen and water content [2.3]. Hydrotreating (Hydrodeoxygenation) with traditional sulfided catalysts has proven to be adapted for producing hydrocarbon fuels [4,5].The process is closely related to the processes of petroleum hydrodesulfurisation and hydrodenitrogenation as similar reaction conditions and catalysts may be used. Nevertheless, the composition and properties of the biomass derived feeds have to be carefully considered for the technical development of this hydrodeoxygenation route. Potential problems to face are related, for example, to the thermal instability (polymerisation tendency) of the oils, the diversity of the chemical groups, the high heteroatom content and the high water content. One particularly important aspect of these problems is related to the fact that catalysts need to be highly resistant to deactivation. However, there has been, until now, nearly no focus on the catalyst stability during bio-oils hydromating. It was reported only recently that hydrotreatment tests of fast pyrolysis oils could not last more than seven days because of reactor plugging [6]. The present paper is focused on the potential action of water for deactivating a classical NiMoly-Al203 catalyst. Actually, the general consensus in the literature is that water may be detrimental to the activity and to the chemical stability of the active sulfided phases, but strong evidences are stiu lacking. Water may perturb directly the activity of the catalysts by competitive adsorption on active sites. In that case, the inhibition will be a function of the adsorption strength. It was reported in previous papers [7,81that the inhibition by oxygenated compounds is moderate. Water caused
460
only very weak inhibition on the hydrodeoxygenation of phenolic [7], ketonic and carboxylic groups [8]. This indicated a low adsorptivity of water on the sulfide phases responsible for the reactions. As a whole, water was much less poisoning than other small molecules like hydrogen sulfide or ammonia [7]. On the other hand, water may adsorb relatively strongly on y-alumina and perturb reactions occurring on this support, such as hydrolysis of carboxylic esters [81. But water may also deactivate sulfided hydrotreatment catalysts by acting on their structure or chemical composition. The present paper focuses on this aspect. Several authors reported on the oxidation of active phases by oxygenated compounds during the hydrotreating of low sulfur content feeds as a probable cause of catalyst deactivation [9-131. Furimsky [9] observed that, during the hydrodeoxygenation of teaahydrofuran in the absence of sulfur compounds, sulfided catalysts lost sulfur, which was supposed to be replaced by oxygen. Yoshimura et al. [lo-121 studied the deactivation occurring during the hydrotreating of low sulfur containing coal derived feed stocks (which contain molecules similar to those of biomass derived oils). They observed by XPS the oxidation of molybdenum sulfide to Mo&, of nickel sulfide to unspecified oxidised nickel species mi++) and of sulfidic sulfur to sulfate anions. The extent was a function of the oxygen and sulfur content of the feeds [lo, 121. On the base of thermodynamicalphase diagrams. they deduced that molybdenum and nickel have a much higher affmity with oxygen than with sulfur (M-0 >> M-S) and that molybdenum has a higher affinity with sulfur or oxygen than nickel (Mo-X >> Ni-X) [12]. But strong evidences of the deactivation role of water in opposition to organic oxygenated compounds are still lacking. Vogelzang et al. reported that water (added as methanol) caused a rapid decrease of the catalytic activity for the conversion of naphtol[13]. This observation was interpreted as due to structural modifications of the catalyst, especially oxidation by water. On the other hand, Yoshmura et al. observed that the deactivating and oxidising strength of water was only very weak in comparison to benmfuran and molecular oxygen, Beside chemical modifications, water could be the cause of other deactivation phenomena In particular, sulfate species formed even in low quantities could be quickly dissolved in water, thus rapidly decreasing the metal content. On the other hand, there is a danger of structural changes of the alumina support due to the action of water. The present paper reports, for the first time, on the deactivation and modifications induced by the action of water in the absence of other oxygenated compounds. A sulfided nickel molybdenum catalyst was subjected to typical hydrotreating conditions in a closed batch reactor during a five days period with or without added water. Catalyst samples withdrawn each day were characterised for their BET surface area, crystallinity (XRD), bulk composition and surface chemical composition. The catalytic activity of samples subjected to each treatment condition was evaluated. 2. EXPERIMENTAL 2.1. Catalyst An industrial nickel molybdenum catalyst supported on y-alumina (F'rocatalyse HR 346) was used throughout this study. The size of the catalyst particles was between 0.315 and 0.5 mm. Prior to use, the catalyst in its oxide form (as dehvered) was activated by a standard laboratory sulfidation procedure with a mixture of 15 % H2S / 85 % H2 at 400 OC. 2.2. Treatments Twenty grams of sulfided catalyst were exposed to three different conditions for studying the effect of water : a blank test in an inert solvent dodecane alone (Blank run (x)). the solvent plus water (H20 (x)) and the simultaneous addition of water and hydrogen sulfide (HzO-H~S The suffix (x) corresponds to the sample number or the number of days of treatment. The conditions of treatment were a total pressure of 7 ma,a tempexature of 360 "C and the treatment time was between from 1 to 5 days. The nomenclature of the different samples is resumed in Table 1.
46 1
TABLE 1 Nomenclatute of the treated catalyst samples and corresponding feed ACRONYM FEED Mink run (x) 170 mi dodecane H20 (x) 170 ml dodecane + 10 ml distilled water H20-H2S (x) 170 ml dodecane + 10 ml distilled water + 0.25 ml CS2 The experimental procedure was the following. The activated catalyst was quickly poured into a 0.57 dm3 batch reactor containing 170 ml of dodecane. Ten ml of water and 0.25 ml of CS2 (which is quickly decomposed in H2S under treatment conditions) were added, depending on the experiment. The reactor was sealed and air was carefully evacuated. The temperature was increased to 360 "C under vigorous mechanical stirring and mild hydrogen pressure. The total pressure was then fixed at 70 bars by adding hydrogen. These conditions were maintained for about 12 hours. The heating was then stopped and the reactor cooled down overnight. The day after, the reactor was opened and approximately two grams of catalyst were sampled. Care was taken that the catalyst did not come in contact with air by keeping it wetted by dodecane during sampling and pouring it immediately in iso-octane. The catalyst particles were washed three times with iso-octane. In the experiment with added CSa0.25 ml was introduced in the reactor after each sampling opemion. The reactor was then closed, and the whole cycle realised again. The operation was repeated five times (five days). The calculated pressure of H2S was 1.1 bar. The water pressure, determined experimentally, was around 25 bars under treatment conditions. Note that the saturation pressure of water is around 170 bars at the treatment temperature. 2.3. Catalytic activity of treated catalysts
Q 6' -Q
Some of the catalyst samples recovered were further subjected to a standard reaction test in batch reactor. The catalyst particles, protected from air by iso-octane, were poured into the reactor containing a standard reaction mixture composed of 4-methyl Me phenol, 2-ethyl phenol and dibenzofuran, each at 4.5 wt%, in dodecane. 0.25 ml of CS2 was also added. The tests were performed at 340 "C and 70 bars. Liquid samples were taken at various times during the tests and analysed by gas chromatography. More details about the reaction procedure may be found in reference [7]. The catalyst was quantitatively lvle recovered after the reaction, washed with acetone, Me dried and weighed. The activities are arbitrarily Figure 1. Simple reaction scheme of reported as pseudo first order rate constants for the 4-methyl phenol hydrodeoxygenation. conversion of one Of the reactants: 4-methyl phenol. The conversion proceeds through two parallel pathways, one being the hydrogenation of the aromatic ring, the other being the direct elimination of the OH group (hydrogenolysis). The selectivity is expressed as the ratio of corresponding products: methyl cyclohexane to toluene (MCHROL) (Fig. 1). The standard deviation of the rate consrants and selectivities was estimated to k 7 % and k 10 %, respectively. 2.4. Physico-chemical characterisations
The recovered catalyst particles were never crushed before physico-chemical characterisation in order to avoid contamination by atmospheric oxygen. Iso-octane was generally evacuated by passing a flow of argon at 130 "C during three hours except for the XPS analysis. The BET surface area of the catalysts was determined using an automated nitrogen adsorption apparatus Micromentics ASAP 2000.
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The X-ray diffraction spectra were recorded on a Philips PW-1130/90 apparatus using the copper KCI radiation (2. = 1.5405 A). The Ni and Mo content of the catalysts were determined by ICP-AES and the sulfur content thanks to an automated Str6hlein Coulomat 702. The catalyst samples were equilibrated in atmosphere previous to measurement. The XPS analyses of the catalyst samples were performed on a Surface Science Instruments spectrometer (SSX 100) with a resolution (FWHM Au 4f7/2) of 1.0 eV. The X-ray beam was monochromatized A1 K a radiation (1486.6 eV). It was focalized and lighted an elliptical surface of 1.74 mm on 1.37 mm. Charging effects were avoided thanks to the isolation of the samples and the utilisation of a charge neutraliser (flood gun)adjusted at an energy of 10 eV. Binding energies were referenced to the binding energy of C1, considered to be at 284.8 eV. 3. RESULTS 3.1 Catalytic Activity The activity and selectivity of the freshly sulfided catalyst and several catalyst samples treated in the presence of the solvent dodecane alone (Blank run (5)). added water (H20 (5)) and water plus hydrogen sulfide (HzO-H~S(11, H20-HzS (3). HzO-HzS ( 5 ) ) are reported in Table 2. TABLE 2 Activity and selectivity for the HDO of 4-methyl phenol and BET surface area of catalyst samples mated under different conditions Rate constant Selectivity BET Surface area (cm3 min-1 gcata-1) MCWTOL m2/g Fresh 2.65 20.0 181 (1) Blank run (5) 2.30 20.5 172 0.90 19.0 133 H20 (5) H20-H2S (1) 1.43 23.3 170 H20-H2S (3) 0.82 19.5 156 0.92 23.0 150 H20-H2S (5) ( l ) The BET surface area corresponds to the fresh catalyst in its oxide form The catalyst treated in the presence of dodecane alone retains nearly all the activity of the freshly sulfided catalyst. The catalyst samples treated in the presence of water are both deactivated. They lost approximately two thirds of the initial activity. The evolution of the activity for the catalyst treated in the presence of water and hydrogen sulfide (H20-H2S(,)) indicates that 46 % of the activity are lost after 12 hours of treatment and that a stabilisanon occurs between 12 and 36 hours. The selectivity for hydrogenation of the aromatic ring relative to direct elimination of phenolic OH group (hydrogenationhydrogenolysis) does not vary significantly for any catalyst. 3.2 BET Surface Area The BET surface area of the various tested catalysts is presented in the last column of Table 2. Compared to the fresh catalyst, the presence of water during the matment induced a decrease of the surface area of 17 and 26 % in the sample treated in the presence of water and hydrogen sulfide and water alone, respectively. On the other hand, the Blank run catalyst sample five lost only a very slight fraction of its surface area. Experimental values of the samples in this series fluctuate between 170 and 176 m2/g.
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3.3 X-Ray Diffraction The X-ray diffraction patterns of the last sample of the various treated catalysts and of the freshly sulfided catalyst are shown in Fig. 2. The diffraction spectra of the fresh and dodecane-treated catalysts (A and D) do not present any sharp diffraction lines except a peak at 28 = 44.6'. Broad diffraction lines corresponding to molybdenum sulfide and y-alumina are visible. The catalyst samples treated in the presence of water present other diffraction peaks (spectra B and C). These appear at 28 values of 14.6, 28.3, 38.4, 49.2 '. Their position and relative intensity are characteristic of the presence of the hydrated alumina called boehmite [14]. This phase is produced continuously as a function of the treatment time. 3.4 Elemental composition
The weight content of nickel, molybdenum and sulfur as well as the ratios S/(Ni+Mo) and Ni/(Ni+Mo) in the last sample of the differently treated catalysts is presented in Table 3. I I No clear modification of the composition of 70 60 50 40 30 20 10 0 the catalyst can be observed depending of the Degree 28 treatment conditions. The sulfidation state S/(Ni+Mo) is maximum for the freshly sulfided Fimre 2. : XRD spectra of (A) freshly catalyst. It is the lowest for the catalyst treated in sulfided, (B) H20-HzS (5). (C)H20 ( 5 ) dodecane alone although still indicative of welland (D) Blank run ( 5 ) . The arrows sulfided phases. It is intermediate in the water treated catalysts. The ratio of active metals did not indicate the peaks of (V)MoS2, (a) yvary, indicating that the treatments did not lead to a A203 and (0)boehmite. selective removal of one of the metal.
TABLE 3.
Weight percentage of nickel, molybdenum and sulfur and atomic ratios of sulfur and nickel to total metal content of the fmhly sulfided and of the differently treated samples Ni Mo S S/(Mo+Ni) Ni/(Mo+Ni) Fresh 1.98 0.30 2.25 8.71 8.18 1.66 0.30 7.40 2.42 9.36 Blank run (5) 1.79 0.30 7.05 2.16 8.21 H20 (5) 1.87 0.30 7.73 6.90 H20-HzS (5) 2.03
3.5 XPS Analysis Figures 3 , 4 and 5 present respectively the S2p, M03d and Ni2 3/2 spectra of the freshly sulfided and of the fourth sample of the differently treated catalysts. & these figures. the peaks obtained from pure nickel sulfide (a-NiS), nickel sulfate (NiSO4) and the oxidic NiMo catalyst are also reported.
4 64
175
171
167
163
159
Binding Energy (eV)
155
241
237
233
229
225
Binding Energy (eV)
221
Figure 3, : XPS SzP spectra of (A) NiSO4, (B) Fimre 4, : XPS Mo3d spectra of (A) freshly a-NiS, (C) freshly sulfided NMo, (D) Blank sdfided Nwov (B) Blank run (4). (c) H2OH2S (4) and @) H2O (4)(4). (E)H20-H2S (4) and (F) H20 (4). The S2p peaks appear at a binding energy of 161.8 eV in pure nickel sulfide and at 168.7 eV in nickel sulfate (Fig. 3). No indication of the presence of sulfatespecies can be observed in the freshly sulfided and Blank tested samples. In the samples treated in the presence of water, a peak correspondingto sulfate can be observed. Its intensity is one order of magnitude lower than the intensity of the sulfide peak. This peak did not show any clear evolution as a function of the treatment time. The Mo3d spectrum of the freshly sulfided catalyst corresponds to the doublet of M036n and Mo3d3~in the +4 oxidation state (228.7 eV and 23 1.9 eV) (Fig. 4). Mo in the +5 (230.0 eV and 233.2 eV) or +6 (231.9 eV and 235.1 eV) oxidation state is not present in sensible amount. The spectra of the catalyst treated in the solvent dodecane alone is very similar. No evident sign of an oxidation of the molybdenum sulfide phase can be observed for the water treated samples. Only extremely weak signals which could correspond to Mo"3d3~ can hardly be noted in some spectra. For the nickel species, only the region corresponding to the Ni2p3/2 bands was recorded. The Nizp3n peaks are more complex because of the presence of satellites (Fig. 5). Pure nickel sulfide exhibits its main peak at 853.0 eV. The peaks in nickel sulfate and in the oxidic NiMo catalyst are broader and the satellites are more marked. The principal peak is present at 856.5 eV and 856.4 eV, respectively. The freshly sulfided catalyst has a Ni2p3/2 band very similar to the one of pure nickel sulfide although broader. The maximum of the main peak is at 853.2 eV. In the water treated samples, the Nizp3p is made of two main peaks at 853.2 eV and 856.5 eV. The fmt can reasonably be attributed to nickel sulfide species. The second peaks could be attributed to nickel sulfate or nickel oxide. The literature reports also that the Ni2 312 peak of nickel hydroxide appears at 855.8 [15] and 856.6 eV [16] and nickel aluminate at d 6 . 0 eV [la]. These binding energies are too close for attributing the second peak of the water samples specifically to one of these species. Nevertheless, the simultaneous presence of sulfate tends to indicate that this second peak corresponds partially to nickel sulfate. In the Blank run samples, this second peak does not appear in appreciable quantity.
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The XPS quantitative results are not reported because they do not bring very useful information. The data were relatively scattered and no influence of water on the dqersion could be detected The scattering certainly resulted from the fact that the X-Ray beam had a size comparable to the size of the catalyst particles and consequently indicates probably inhomogeneities in the preparation. 4. DISCUSSIONS
The partial pressure of water was around 25 bars during the treatments. This is a high pressure compared to the water pressure that could exist during the hydroueatments of petroleum or coal derived feeds but is representative of the conditions which will exist during the hydrotreatment of bio-oils containing up to 30 wt% of > water and 40 wt% of structural oxygen. On the other hand, the treatment time (around 60 hours) was limited d compared to the normal life of an industrial catalyst. .9 The presence of water during the treatments 2 compared to the Blank run in dodecane alone induceds Y some modifications. The catalytic activity decreases to one third in the first hours of treatment independently of the presence of hydrogen sulfide while the hydrogenation - hydrogenolysis selectivity is not modified. This deactivation should be attributed to a physico-chemical modification of the catalyst samples. The presence of water induces a crystallisation of the alumina support in the hydrated boehmite phase. A sm 1 decrease of the BET surface area is conjugated to this observation. These modifications are in full agreement with the literature which indicates that yalumina is metastable under hydrothermal conditions and transforms into boehmite in the temperature range 140 380 OC [ 171. Nevertheless, although these phenomena can play a major role in long term catalyst deactivation, 867 863 859 855 851 847 they cannot be entirely connected to the observed deactivation because the decrease of activity is much Binding Energy (eV) higher than the decrease of surface area. A priori, the deactivation could be due to the loss Figure 5; XPS spectra of (A) of active elements nickel, molybdenum or sulfur. But the elemental analysis does not show important modifications a-NiS’ (B) Oxidic NiMo’ NiS04’ “lfided NMo* of the composition of the catalysts. The water treated (E) (4)1 (F)H20-H2S (4)1 samples have a slightly lower Ni and Mo content but the (G)H20 (4). Ni/(Ni+Mo) ratio is unmodified. Moreover, no decrease could be measured by quantitative XPS. The sulfur content is slightly lower in all the treated samples but this cannot be the cause of the deactivation because it is the lowest in the dodecane treated sample. Moreover, the addition of hydrogen sulfide during the treatment had no effect This leads to envisage that a more subtle modification led to the deactivation. The deactivation could, according to literature [9-131, originate from an oxidation of the sulfide phases. No oxidation of the molybdenum sulfide phase was discerned in our experiments! This is in general agreement with the results of Yoshimura et al. [12] who did not observe an additional oxidation due to water when it was added to a coal liquid feed. These authors substantiated their observations by drawing thermodynamical diagrams which indicate e that no oxidation of MoS2 and NiS, has to be expected under practical water p ~ s s u r conditions. On the other hand, a limited oxidation of the sulfidic sulfur into sulfate and a clear oxidation of the nickel sulfide species were observed in our experiments when water was present. The b
-4
s
4 66
species are rapidly produced and do not evolve as a function of the treatment time. The deactivation should be related to these modifications. Two interpretations may be proposed. The first is that nickel sulfide is oxidised in nickel sulfate and that these species form an inactive layer at the surface of nickel sulfide and possibly molybdenum sulfide particles. Nickel species decorating the edge planes of MoS2 could be especially sensitive. The formation of a protective sulfate layer above sulfide was reported by several authors [18-201. A second plausible explanation could be that the oxidised nickel species correspond to nickel aluminate. The effect of water would be to favour the migration of nickel atoms into the alumina lattice. A positive effect of water on the formation of cobalt aluminate during regeneration was reported by Arteaga [21]. The observed deactivation would originate from a decrease of the catalytic synergy between nickel and molybdenum phases due to modification of the optimum Ni/Mo ratio.
ACKNOWLEDGEMENT
This work was accomplished in the frame of CEC contract Joub-0055. REFERENCES 1. E. Laurent and B. Delmon, in "Proceedings of the 7th European Conference on Biomass for
Energy and Environment; Agriculture and Industry". Florence, 1991, in press. 2. A. V. Bridgwater and S. A. Bridge, in "Biomass Pyrolysis Liquids Upgrading and Utilization" (A. V. Bridgwater and G. Grassi, Eds.), Elsevier, London, 1991, P. 11. 3. R. Maggi and B. Delmon, Fuel, in press 4. E. Churin and B. Delmon, in "Pyrolysis and Gasification" (G. L. Ferrero, K. Maniatis, A. Beukens and A. V. Bridgwater, Eds.), Elsevier, London, 1898, p. 342. 5. E. G. Baker and D. C. Elliott, in "Research in Thermochemical Biomass Conversion", Phoenix, 1988 (A. V. Bridgwater, J. L. Kuester, Eds.), Elsevier, London, 1988, p. 883. 6. W Baldauf, in "Proceedings of the 7th European Conference on Biomass for Energy and Environment; Agriculture and Industry", Florence, 1991, in press. 7. E. Laurent and B. Delmon, Ind. Eng. Chem. Res., 32 (1993), p. 2516. 8. E. Laurent and B. Delmon, Appl. Catal., 109 (1994), p. 99. 9. E. Furimsky, Ind. Eng. Chem. Prod. Res. Dev.. 22 (1983), p. 31. 10. A. Nishijima, H. Shimada, Y. Yoshimura, T. Sat0 and N. Matsubayashi, in "Catalyst Deactivation (Studies in Surface Sciences and Catalysis, Vol. 34)" (B. Delmon and G. F. Froment, Eds.), Elsevier, Amsterdam, 1987, p. 39. 11. Y. Yoshimura, H. Shimada, T. Sato, M. Kubota and A. Nishijima, Appl. Catal., 29, (1987). p. 125. 12. Y.Yoshimura, T. Sato, H. Shimada, N. Matsubayashi, and A. Nishijima, Appl. Catal., 73 (1991), p. 55. 13. M. W.Vogelzang,C.-L. Li, G. C. A. Schuit, B. C. Gates and L. Petrakis, J. Catal., 84 (1983), p. 170. 14. National Bureau of Standards, "Powder diffraction data", Joint Committee on Powder Diffraction Standards, Swarthmore, Pennsylvania, 1976. 15. P. Dufresne, E. Payen, J. Grimblot, J. P. BoMelle, J. Phys. Chem., 85 (1981), p. 2344. 16. C. P. Li, A. Proctor and D. M. Hercules, Applied Spectroscopy, 38 (1984), p. 880. 17. G. Ervin and E. F. Osbom, J. Geol., 59 (1950), p. 381. 18. Y. Yoshimura, H. Yakokawa, T. Sato, N. Shimada, N. Matsubayashi and A. Nishijima, Appl. Catal., 73 (1991), p. 39. 19. M. Montes, M. Genet, D. K. Hodnett, W. E. Stone and B. Delmon, Bull. SOC.Chim. Belg., 95 (1986), p. 1. 20. D. Lichtman, J. H. Vraig, V. Sailer, M. Drinkwine, Appl. Surf. Sci., 7 (1981), p. 325. 21. A. Arteaga, J. L. G. Fieno, P. Grange and B. Delmon, Preprints, ACS Div. Petrol. Chem., 32 (2) (1987), p. 339.
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Deactivation of a Sulfided NiMo/y-A1203 during the Hydrodeoxygenation of Bio-Oils: Influence of a High Water Pressure E. Laurent and B. Delmon Unite? de Catalyse et Chimie des M&riaux Divids, Universit6 Catholique de Louvain, Place Croix du Sud 2/17,1348Louvain-la-Neuve (Belgium).
ABSTRACT The influence of a high pressure of water on the hydrodeoxygenation activity and physico-chemical characteristics of a sulfided NiMoly-AlzO3 catalyst was studied in a batch reactor. Water caused a decrease of the catalytic activity to one-third the activity of the fresh catalyst but did not change the hydrogenation-hydrogenolysisselectivity. Water was also the cause of a small loss of the specific surface area conjugated to some crystallisation of the yalumina support in a hydrated boehmite phase, but this phenomenon cannot account entirely for the deactivation. On the other hand, the metal content, the dispersion, and the sulfidation state were not specifically affected by water. The deactivation would rather be related to the appearance of oxidised nickel species in the water treated samples. Two interpretations are proposed: either the formation of an inactive sulfate layer, or the consumption of nickel as nickel aluminare. 1. INTRODUCTION
Solid biomass is an attractive source of energy in the contexts of energy supply and environmental problems. Thermochemical processes were developed for its efficient energetic use. Especially, liquefaction by fast pyrolysis presents interesting potentialities [lI. Nevertheless, if the objective is the obtaining of good quality fuels, the pyrolysis oils must be upgraded in order to exceed the drawbacks originating from their high organic oxygen and water content [2.3]. Hydrotreating (Hydrodeoxygenation) with traditional sulfided catalysts has proven to be adapted for producing hydrocarbon fuels [4,5].The process is closely related to the processes of petroleum hydrodesulfurisation and hydrodenitrogenation as similar reaction conditions and catalysts may be used. Nevertheless, the composition and properties of the biomass derived feeds have to be carefully considered for the technical development of this hydrodeoxygenation route. Potential problems to face are related, for example, to the thermal instability (polymerisation tendency) of the oils, the diversity of the chemical groups, the high heteroatom content and the high water content. One particularly important aspect of these problems is related to the fact that catalysts need to be highly resistant to deactivation. However, there has been, until now, nearly no focus on the catalyst stability during bio-oils hydromating. It was reported only recently that hydrotreatment tests of fast pyrolysis oils could not last more than seven days because of reactor plugging [6]. The present paper is focused on the potential action of water for deactivating a classical NiMoly-Al203 catalyst. Actually, the general consensus in the literature is that water may be detrimental to the activity and to the chemical stability of the active sulfided phases, but strong evidences are stiu lacking. Water may perturb directly the activity of the catalysts by competitive adsorption on active sites. In that case, the inhibition will be a function of the adsorption strength. It was reported in previous papers [7,81that the inhibition by oxygenated compounds is moderate. Water caused
460
only very weak inhibition on the hydrodeoxygenation of phenolic [7], ketonic and carboxylic groups [8]. This indicated a low adsorptivity of water on the sulfide phases responsible for the reactions. As a whole, water was much less poisoning than other small molecules like hydrogen sulfide or ammonia [7]. On the other hand, water may adsorb relatively strongly on y-alumina and perturb reactions occurring on this support, such as hydrolysis of carboxylic esters [81. But water may also deactivate sulfided hydrotreatment catalysts by acting on their structure or chemical composition. The present paper focuses on this aspect. Several authors reported on the oxidation of active phases by oxygenated compounds during the hydrotreating of low sulfur content feeds as a probable cause of catalyst deactivation [9-131. Furimsky [9] observed that, during the hydrodeoxygenation of teaahydrofuran in the absence of sulfur compounds, sulfided catalysts lost sulfur, which was supposed to be replaced by oxygen. Yoshimura et al. [lo-121 studied the deactivation occurring during the hydrotreating of low sulfur containing coal derived feed stocks (which contain molecules similar to those of biomass derived oils). They observed by XPS the oxidation of molybdenum sulfide to Mo&, of nickel sulfide to unspecified oxidised nickel species mi++) and of sulfidic sulfur to sulfate anions. The extent was a function of the oxygen and sulfur content of the feeds [lo, 121. On the base of thermodynamicalphase diagrams. they deduced that molybdenum and nickel have a much higher affmity with oxygen than with sulfur (M-0 >> M-S) and that molybdenum has a higher affinity with sulfur or oxygen than nickel (Mo-X >> Ni-X) [12]. But strong evidences of the deactivation role of water in opposition to organic oxygenated compounds are still lacking. Vogelzang et al. reported that water (added as methanol) caused a rapid decrease of the catalytic activity for the conversion of naphtol[13]. This observation was interpreted as due to structural modifications of the catalyst, especially oxidation by water. On the other hand, Yoshmura et al. observed that the deactivating and oxidising strength of water was only very weak in comparison to benmfuran and molecular oxygen, Beside chemical modifications, water could be the cause of other deactivation phenomena In particular, sulfate species formed even in low quantities could be quickly dissolved in water, thus rapidly decreasing the metal content. On the other hand, there is a danger of structural changes of the alumina support due to the action of water. The present paper reports, for the first time, on the deactivation and modifications induced by the action of water in the absence of other oxygenated compounds. A sulfided nickel molybdenum catalyst was subjected to typical hydrotreating conditions in a closed batch reactor during a five days period with or without added water. Catalyst samples withdrawn each day were characterised for their BET surface area, crystallinity (XRD), bulk composition and surface chemical composition. The catalytic activity of samples subjected to each treatment condition was evaluated. 2. EXPERIMENTAL 2.1. Catalyst An industrial nickel molybdenum catalyst supported on y-alumina (F'rocatalyse HR 346) was used throughout this study. The size of the catalyst particles was between 0.315 and 0.5 mm. Prior to use, the catalyst in its oxide form (as dehvered) was activated by a standard laboratory sulfidation procedure with a mixture of 15 % H2S / 85 % H2 at 400 OC. 2.2. Treatments Twenty grams of sulfided catalyst were exposed to three different conditions for studying the effect of water : a blank test in an inert solvent dodecane alone (Blank run (x)). the solvent plus water (H20 (x)) and the simultaneous addition of water and hydrogen sulfide (HzO-H~S The suffix (x) corresponds to the sample number or the number of days of treatment. The conditions of treatment were a total pressure of 7 ma,a tempexature of 360 "C and the treatment time was between from 1 to 5 days. The nomenclature of the different samples is resumed in Table 1.
46 1
TABLE 1 Nomenclatute of the treated catalyst samples and corresponding feed ACRONYM FEED Mink run (x) 170 mi dodecane H20 (x) 170 ml dodecane + 10 ml distilled water H20-H2S (x) 170 ml dodecane + 10 ml distilled water + 0.25 ml CS2 The experimental procedure was the following. The activated catalyst was quickly poured into a 0.57 dm3 batch reactor containing 170 ml of dodecane. Ten ml of water and 0.25 ml of CS2 (which is quickly decomposed in H2S under treatment conditions) were added, depending on the experiment. The reactor was sealed and air was carefully evacuated. The temperature was increased to 360 "C under vigorous mechanical stirring and mild hydrogen pressure. The total pressure was then fixed at 70 bars by adding hydrogen. These conditions were maintained for about 12 hours. The heating was then stopped and the reactor cooled down overnight. The day after, the reactor was opened and approximately two grams of catalyst were sampled. Care was taken that the catalyst did not come in contact with air by keeping it wetted by dodecane during sampling and pouring it immediately in iso-octane. The catalyst particles were washed three times with iso-octane. In the experiment with added CSa0.25 ml was introduced in the reactor after each sampling opemion. The reactor was then closed, and the whole cycle realised again. The operation was repeated five times (five days). The calculated pressure of H2S was 1.1 bar. The water pressure, determined experimentally, was around 25 bars under treatment conditions. Note that the saturation pressure of water is around 170 bars at the treatment temperature. 2.3. Catalytic activity of treated catalysts
Q 6' -Q
Some of the catalyst samples recovered were further subjected to a standard reaction test in batch reactor. The catalyst particles, protected from air by iso-octane, were poured into the reactor containing a standard reaction mixture composed of 4-methyl Me phenol, 2-ethyl phenol and dibenzofuran, each at 4.5 wt%, in dodecane. 0.25 ml of CS2 was also added. The tests were performed at 340 "C and 70 bars. Liquid samples were taken at various times during the tests and analysed by gas chromatography. More details about the reaction procedure may be found in reference [7]. The catalyst was quantitatively lvle recovered after the reaction, washed with acetone, Me dried and weighed. The activities are arbitrarily Figure 1. Simple reaction scheme of reported as pseudo first order rate constants for the 4-methyl phenol hydrodeoxygenation. conversion of one Of the reactants: 4-methyl phenol. The conversion proceeds through two parallel pathways, one being the hydrogenation of the aromatic ring, the other being the direct elimination of the OH group (hydrogenolysis). The selectivity is expressed as the ratio of corresponding products: methyl cyclohexane to toluene (MCHROL) (Fig. 1). The standard deviation of the rate consrants and selectivities was estimated to k 7 % and k 10 %, respectively. 2.4. Physico-chemical characterisations
The recovered catalyst particles were never crushed before physico-chemical characterisation in order to avoid contamination by atmospheric oxygen. Iso-octane was generally evacuated by passing a flow of argon at 130 "C during three hours except for the XPS analysis. The BET surface area of the catalysts was determined using an automated nitrogen adsorption apparatus Micromentics ASAP 2000.
462
The X-ray diffraction spectra were recorded on a Philips PW-1130/90 apparatus using the copper KCI radiation (2. = 1.5405 A). The Ni and Mo content of the catalysts were determined by ICP-AES and the sulfur content thanks to an automated Str6hlein Coulomat 702. The catalyst samples were equilibrated in atmosphere previous to measurement. The XPS analyses of the catalyst samples were performed on a Surface Science Instruments spectrometer (SSX 100) with a resolution (FWHM Au 4f7/2) of 1.0 eV. The X-ray beam was monochromatized A1 K a radiation (1486.6 eV). It was focalized and lighted an elliptical surface of 1.74 mm on 1.37 mm. Charging effects were avoided thanks to the isolation of the samples and the utilisation of a charge neutraliser (flood gun)adjusted at an energy of 10 eV. Binding energies were referenced to the binding energy of C1, considered to be at 284.8 eV. 3. RESULTS 3.1 Catalytic Activity The activity and selectivity of the freshly sulfided catalyst and several catalyst samples treated in the presence of the solvent dodecane alone (Blank run (5)). added water (H20 (5)) and water plus hydrogen sulfide (HzO-H~S(11, H20-HzS (3). HzO-HzS ( 5 ) ) are reported in Table 2. TABLE 2 Activity and selectivity for the HDO of 4-methyl phenol and BET surface area of catalyst samples mated under different conditions Rate constant Selectivity BET Surface area (cm3 min-1 gcata-1) MCWTOL m2/g Fresh 2.65 20.0 181 (1) Blank run (5) 2.30 20.5 172 0.90 19.0 133 H20 (5) H20-H2S (1) 1.43 23.3 170 H20-H2S (3) 0.82 19.5 156 0.92 23.0 150 H20-H2S (5) ( l ) The BET surface area corresponds to the fresh catalyst in its oxide form The catalyst treated in the presence of dodecane alone retains nearly all the activity of the freshly sulfided catalyst. The catalyst samples treated in the presence of water are both deactivated. They lost approximately two thirds of the initial activity. The evolution of the activity for the catalyst treated in the presence of water and hydrogen sulfide (H20-H2S(,)) indicates that 46 % of the activity are lost after 12 hours of treatment and that a stabilisanon occurs between 12 and 36 hours. The selectivity for hydrogenation of the aromatic ring relative to direct elimination of phenolic OH group (hydrogenationhydrogenolysis) does not vary significantly for any catalyst. 3.2 BET Surface Area The BET surface area of the various tested catalysts is presented in the last column of Table 2. Compared to the fresh catalyst, the presence of water during the matment induced a decrease of the surface area of 17 and 26 % in the sample treated in the presence of water and hydrogen sulfide and water alone, respectively. On the other hand, the Blank run catalyst sample five lost only a very slight fraction of its surface area. Experimental values of the samples in this series fluctuate between 170 and 176 m2/g.
463
3.3 X-Ray Diffraction The X-ray diffraction patterns of the last sample of the various treated catalysts and of the freshly sulfided catalyst are shown in Fig. 2. The diffraction spectra of the fresh and dodecane-treated catalysts (A and D) do not present any sharp diffraction lines except a peak at 28 = 44.6'. Broad diffraction lines corresponding to molybdenum sulfide and y-alumina are visible. The catalyst samples treated in the presence of water present other diffraction peaks (spectra B and C). These appear at 28 values of 14.6, 28.3, 38.4, 49.2 '. Their position and relative intensity are characteristic of the presence of the hydrated alumina called boehmite [14]. This phase is produced continuously as a function of the treatment time. 3.4 Elemental composition
The weight content of nickel, molybdenum and sulfur as well as the ratios S/(Ni+Mo) and Ni/(Ni+Mo) in the last sample of the differently treated catalysts is presented in Table 3. I I No clear modification of the composition of 70 60 50 40 30 20 10 0 the catalyst can be observed depending of the Degree 28 treatment conditions. The sulfidation state S/(Ni+Mo) is maximum for the freshly sulfided Fimre 2. : XRD spectra of (A) freshly catalyst. It is the lowest for the catalyst treated in sulfided, (B) H20-HzS (5). (C)H20 ( 5 ) dodecane alone although still indicative of welland (D) Blank run ( 5 ) . The arrows sulfided phases. It is intermediate in the water treated catalysts. The ratio of active metals did not indicate the peaks of (V)MoS2, (a) yvary, indicating that the treatments did not lead to a A203 and (0)boehmite. selective removal of one of the metal.
TABLE 3.
Weight percentage of nickel, molybdenum and sulfur and atomic ratios of sulfur and nickel to total metal content of the fmhly sulfided and of the differently treated samples Ni Mo S S/(Mo+Ni) Ni/(Mo+Ni) Fresh 1.98 0.30 2.25 8.71 8.18 1.66 0.30 7.40 2.42 9.36 Blank run (5) 1.79 0.30 7.05 2.16 8.21 H20 (5) 1.87 0.30 7.73 6.90 H20-HzS (5) 2.03
3.5 XPS Analysis Figures 3 , 4 and 5 present respectively the S2p, M03d and Ni2 3/2 spectra of the freshly sulfided and of the fourth sample of the differently treated catalysts. & these figures. the peaks obtained from pure nickel sulfide (a-NiS), nickel sulfate (NiSO4) and the oxidic NiMo catalyst are also reported.
4 64
175
171
167
163
159
Binding Energy (eV)
155
241
237
233
229
225
Binding Energy (eV)
221
Figure 3, : XPS SzP spectra of (A) NiSO4, (B) Fimre 4, : XPS Mo3d spectra of (A) freshly a-NiS, (C) freshly sulfided NMo, (D) Blank sdfided Nwov (B) Blank run (4). (c) H2OH2S (4) and @) H2O (4)(4). (E)H20-H2S (4) and (F) H20 (4). The S2p peaks appear at a binding energy of 161.8 eV in pure nickel sulfide and at 168.7 eV in nickel sulfate (Fig. 3). No indication of the presence of sulfatespecies can be observed in the freshly sulfided and Blank tested samples. In the samples treated in the presence of water, a peak correspondingto sulfate can be observed. Its intensity is one order of magnitude lower than the intensity of the sulfide peak. This peak did not show any clear evolution as a function of the treatment time. The Mo3d spectrum of the freshly sulfided catalyst corresponds to the doublet of M036n and Mo3d3~in the +4 oxidation state (228.7 eV and 23 1.9 eV) (Fig. 4). Mo in the +5 (230.0 eV and 233.2 eV) or +6 (231.9 eV and 235.1 eV) oxidation state is not present in sensible amount. The spectra of the catalyst treated in the solvent dodecane alone is very similar. No evident sign of an oxidation of the molybdenum sulfide phase can be observed for the water treated samples. Only extremely weak signals which could correspond to Mo"3d3~ can hardly be noted in some spectra. For the nickel species, only the region corresponding to the Ni2p3/2 bands was recorded. The Nizp3n peaks are more complex because of the presence of satellites (Fig. 5). Pure nickel sulfide exhibits its main peak at 853.0 eV. The peaks in nickel sulfate and in the oxidic NiMo catalyst are broader and the satellites are more marked. The principal peak is present at 856.5 eV and 856.4 eV, respectively. The freshly sulfided catalyst has a Ni2p3/2 band very similar to the one of pure nickel sulfide although broader. The maximum of the main peak is at 853.2 eV. In the water treated samples, the Nizp3p is made of two main peaks at 853.2 eV and 856.5 eV. The fmt can reasonably be attributed to nickel sulfide species. The second peaks could be attributed to nickel sulfate or nickel oxide. The literature reports also that the Ni2 312 peak of nickel hydroxide appears at 855.8 [15] and 856.6 eV [16] and nickel aluminate at d 6 . 0 eV [la]. These binding energies are too close for attributing the second peak of the water samples specifically to one of these species. Nevertheless, the simultaneous presence of sulfate tends to indicate that this second peak corresponds partially to nickel sulfate. In the Blank run samples, this second peak does not appear in appreciable quantity.
465
The XPS quantitative results are not reported because they do not bring very useful information. The data were relatively scattered and no influence of water on the dqersion could be detected The scattering certainly resulted from the fact that the X-Ray beam had a size comparable to the size of the catalyst particles and consequently indicates probably inhomogeneities in the preparation. 4. DISCUSSIONS
The partial pressure of water was around 25 bars during the treatments. This is a high pressure compared to the water pressure that could exist during the hydroueatments of petroleum or coal derived feeds but is representative of the conditions which will exist during the hydrotreatment of bio-oils containing up to 30 wt% of > water and 40 wt% of structural oxygen. On the other hand, the treatment time (around 60 hours) was limited d compared to the normal life of an industrial catalyst. .9 The presence of water during the treatments 2 compared to the Blank run in dodecane alone induceds Y some modifications. The catalytic activity decreases to one third in the first hours of treatment independently of the presence of hydrogen sulfide while the hydrogenation - hydrogenolysis selectivity is not modified. This deactivation should be attributed to a physico-chemical modification of the catalyst samples. The presence of water induces a crystallisation of the alumina support in the hydrated boehmite phase. A sm 1 decrease of the BET surface area is conjugated to this observation. These modifications are in full agreement with the literature which indicates that yalumina is metastable under hydrothermal conditions and transforms into boehmite in the temperature range 140 380 OC [ 171. Nevertheless, although these phenomena can play a major role in long term catalyst deactivation, 867 863 859 855 851 847 they cannot be entirely connected to the observed deactivation because the decrease of activity is much Binding Energy (eV) higher than the decrease of surface area. A priori, the deactivation could be due to the loss Figure 5; XPS spectra of (A) of active elements nickel, molybdenum or sulfur. But the elemental analysis does not show important modifications a-NiS’ (B) Oxidic NiMo’ NiS04’ “lfided NMo* of the composition of the catalysts. The water treated (E) (4)1 (F)H20-H2S (4)1 samples have a slightly lower Ni and Mo content but the (G)H20 (4). Ni/(Ni+Mo) ratio is unmodified. Moreover, no decrease could be measured by quantitative XPS. The sulfur content is slightly lower in all the treated samples but this cannot be the cause of the deactivation because it is the lowest in the dodecane treated sample. Moreover, the addition of hydrogen sulfide during the treatment had no effect This leads to envisage that a more subtle modification led to the deactivation. The deactivation could, according to literature [9-131, originate from an oxidation of the sulfide phases. No oxidation of the molybdenum sulfide phase was discerned in our experiments! This is in general agreement with the results of Yoshimura et al. [12] who did not observe an additional oxidation due to water when it was added to a coal liquid feed. These authors substantiated their observations by drawing thermodynamical diagrams which indicate e that no oxidation of MoS2 and NiS, has to be expected under practical water p ~ s s u r conditions. On the other hand, a limited oxidation of the sulfidic sulfur into sulfate and a clear oxidation of the nickel sulfide species were observed in our experiments when water was present. The b
-4
s
4 66
species are rapidly produced and do not evolve as a function of the treatment time. The deactivation should be related to these modifications. Two interpretations may be proposed. The first is that nickel sulfide is oxidised in nickel sulfate and that these species form an inactive layer at the surface of nickel sulfide and possibly molybdenum sulfide particles. Nickel species decorating the edge planes of MoS2 could be especially sensitive. The formation of a protective sulfate layer above sulfide was reported by several authors [18-201. A second plausible explanation could be that the oxidised nickel species correspond to nickel aluminate. The effect of water would be to favour the migration of nickel atoms into the alumina lattice. A positive effect of water on the formation of cobalt aluminate during regeneration was reported by Arteaga [21]. The observed deactivation would originate from a decrease of the catalytic synergy between nickel and molybdenum phases due to modification of the optimum Ni/Mo ratio.
ACKNOWLEDGEMENT
This work was accomplished in the frame of CEC contract Joub-0055. REFERENCES 1. E. Laurent and B. Delmon, in "Proceedings of the 7th European Conference on Biomass for
Energy and Environment; Agriculture and Industry". Florence, 1991, in press. 2. A. V. Bridgwater and S. A. Bridge, in "Biomass Pyrolysis Liquids Upgrading and Utilization" (A. V. Bridgwater and G. Grassi, Eds.), Elsevier, London, 1991, P. 11. 3. R. Maggi and B. Delmon, Fuel, in press 4. E. Churin and B. Delmon, in "Pyrolysis and Gasification" (G. L. Ferrero, K. Maniatis, A. Beukens and A. V. Bridgwater, Eds.), Elsevier, London, 1898, p. 342. 5. E. G. Baker and D. C. Elliott, in "Research in Thermochemical Biomass Conversion", Phoenix, 1988 (A. V. Bridgwater, J. L. Kuester, Eds.), Elsevier, London, 1988, p. 883. 6. W Baldauf, in "Proceedings of the 7th European Conference on Biomass for Energy and Environment; Agriculture and Industry", Florence, 1991, in press. 7. E. Laurent and B. Delmon, Ind. Eng. Chem. Res., 32 (1993), p. 2516. 8. E. Laurent and B. Delmon, Appl. Catal., 109 (1994), p. 99. 9. E. Furimsky, Ind. Eng. Chem. Prod. Res. Dev.. 22 (1983), p. 31. 10. A. Nishijima, H. Shimada, Y. Yoshimura, T. Sat0 and N. Matsubayashi, in "Catalyst Deactivation (Studies in Surface Sciences and Catalysis, Vol. 34)" (B. Delmon and G. F. Froment, Eds.), Elsevier, Amsterdam, 1987, p. 39. 11. Y. Yoshimura, H. Shimada, T. Sato, M. Kubota and A. Nishijima, Appl. Catal., 29, (1987). p. 125. 12. Y.Yoshimura, T. Sato, H. Shimada, N. Matsubayashi, and A. Nishijima, Appl. Catal., 73 (1991), p. 55. 13. M. W.Vogelzang,C.-L. Li, G. C. A. Schuit, B. C. Gates and L. Petrakis, J. Catal., 84 (1983), p. 170. 14. National Bureau of Standards, "Powder diffraction data", Joint Committee on Powder Diffraction Standards, Swarthmore, Pennsylvania, 1976. 15. P. Dufresne, E. Payen, J. Grimblot, J. P. BoMelle, J. Phys. Chem., 85 (1981), p. 2344. 16. C. P. Li, A. Proctor and D. M. Hercules, Applied Spectroscopy, 38 (1984), p. 880. 17. G. Ervin and E. F. Osbom, J. Geol., 59 (1950), p. 381. 18. Y. Yoshimura, H. Yakokawa, T. Sato, N. Shimada, N. Matsubayashi and A. Nishijima, Appl. Catal., 73 (1991), p. 39. 19. M. Montes, M. Genet, D. K. Hodnett, W. E. Stone and B. Delmon, Bull. SOC.Chim. Belg., 95 (1986), p. 1. 20. D. Lichtman, J. H. Vraig, V. Sailer, M. Drinkwine, Appl. Surf. Sci., 7 (1981), p. 325. 21. A. Arteaga, J. L. G. Fieno, P. Grange and B. Delmon, Preprints, ACS Div. Petrol. Chem., 32 (2) (1987), p. 339.