Energy Fuels 2010, 24, 4728–4737 Published on Web 02/19/2010
: DOI:10.1021/ef901270h
Hydrodeoxygenation of 4-Methylphenol over Unsupported MoP, MoS2, and MoOx Catalysts† Victoria M. L. Whiffen and Kevin J. Smith* Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada Received October 31, 2009. Revised Manuscript Received January 16, 2010
A study of the hydrodeoxygenation (HDO) of 4-methylphenol over unsupported, low-surface-area MoS2, MoO2, MoO3, and MoP catalysts is reported. With the exception of MoO3, the catalysts had the same physicochemical properties before and after the 5 h reaction at 623 K and 4.40 MPa H2. The used MoO3 was partially reduced to a mixed oxide containing Mo4O11, MoO2, and Mo. Compared to the unused MoO3, the used MoO3 CO uptake increased by a factor of 100 following the reaction. The partially reduced Mo oxide catalyst had a high conversion for the HDO of 4-methylphenol because of Brønsted acid sites and the formation of anionic vacancies. The catalyst turnover frequency (TOF) based on CO uptake for the HDO of 4-methylphenol decreased in the order MoP > MoS2 > MoO2 > MoO3, while the activation energy increased in the order of MoP < MoS2 < MoO2 < MoO3. The activity trends correspond to the increased electron density of the Mo among the catalysts. Two primary reactions, C-O hydrogenolysis to yield toluene and saturation of 4-methylphenol followed by rapid dehydration to produce 4-methylcyclohexene, were identified. The catalysts differed in their hydrogenation and isomerization capabilities. The MoP catalyst displayed the highest selectivity toward hydrogenated products, suggesting that the rate-limiting step over MoO3, MoO2, and MoS2 was the saturation of 4-methylphenol to produce 4-methylcyclohexanol.
hamper the success of introducing bio-oil as a fuel to the market that is both competitive and comparable to petroleum oil. The oxygen can be removed from the oxygenated hydrocarbon species via a hydroprocessing reaction called hydrodeoxygenation (HDO). Much research on the removal of oxygen by HDO is available on conventional supported metal sulfide catalysts as well as supported noble metal catalysts, which are known to be active for sulfur removal from crude oils using high-pressure hydrogen.5-11 Noble metals are expensive and have high selectivity for hydrogenation reactions that consume H2. Although sulfided metal catalysts are very active for oxygen removal, oxidation of the active catalyst phase can occur during the hydrotreating of bio-oil because of the high oxygen content of the oil. The addition of a sulfiding
1. Introduction The increase in awareness of global pollution and the depletion of natural oil resources have increased the need for sources of alternative energy. As early as the 1940s, experimental data from Berl1 supported the concept of oil production from biomass using an alkali catalyst and water. Since the oil crisis in the mid 1970s, considerable efforts have been made to convert wood biomass to liquid fuels.2 Bio-oil derived from pyrolyzed biomass has several environmental advantages over fossil fuels as a clean source of energy. Biooils are approximately CO2- and greenhouse gas (GHG)neutral; no SOx emissions are generated upon combustion of the bio-oil; and bio-oil fuels generate less than half the NOx emissions of diesel oil in gas turbines.2 Bio-oils derived from the pyrolysis of biomass contain high amounts of oxygen (45-50 wt %) because of the presence of phenols, furans, carboxylic acids, ethers, and aromatic alcohols.2,3 The oxygenated hydrocarbons are responsible for the high viscosity, poor stability, non-volatility, and immiscibility with fossil fuels and low heating value of the bio-oil.4 The undesirable properties of these compounds
(5) Bejblova, M.; Zamostny, P.; Cerveny, L.; Cejka, J. Hydrodeoxygenation of benzophenone on Pd catalysts. Appl. Catal., A 2005, 296 (2), 69–175. (6) Bunch, A.; Ozkan, U. Investigation of the reaction network of benzofuran hydrodeoxygenation over sulfided and reduced Ni-Mo/ Al2O3 catalysts. J. Catal. 2002, 206 (2), 177–187. (7) Laurent, E.; Delmon, B. Influence of oxygen-, nitrogen-, and sulfur-containing compounds on the hydrodeoxygenation of phenols over sulfided CoMo/γ-Al2O3 and NiMo/γ-Al2O3 catalysts. Ind. Eng. Chem. Res. 1993, 32 (11), 2516–2524. (8) Senol, O.; Viljava, T.; Krause, A. Hydrodeoxygenation of aliphatic esters on sulphided NiMo/γ-Al2O3 and CoMo/γ-Al2O3 catalyst: The effect of water. Catal. Today 2005, 106 (1-4), 186–189. (9) Viljava, T.; Komulainen, R.; Krause, A. Effect of H2S on the stability of CoMo/Al2O3 catalysts during hydrodeoxygenation. Catal. Today 2000, 60 (1-2), 83–92. (10) Yang, Y.; Tye, C.; Smith, K. Influence of MoS2 catalyst morphology on the hydrodeoxygenation of phenols. Catal. Commun. 2008, 9 (6), 1364–1368. (11) Zhang, S.; Yongjie, Y.; Li, T.; Ren, Z. Upgrading of liquid fuel from pyrolysis of biomass. Bioresour. Technol. 2004, 96 (5), 545–550.
† This paper has been designated for the Bioenergy and Green Engineering special section. *To whom correspondence should be addressed. E-mail: kjs@ interchange.ubc.ca. (1) Berl, E. Production of oil from plant material. Science 1944, 99, 309–312. (2) Mohan, D.; Pittman, C.; Steele, P. Pyrolysis of wood/biomass for bio-oil: A critical review. Energy Fuels 2006, 20 (3), 848–889. (3) Furimsky, E. Catalytic hydrodeoxygenation. Appl. Catal., A 2000, 199 (2), 147–190. (4) Elliott, D. Historical developments in hydroprocessing bio-oils. Energy Fuels 2007, 21 (3), 1792–1815.
r 2010 American Chemical Society
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: DOI:10.1021/ef901270h
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agent, such H2S or CS2, is required to maintain the catalyst sulfur content,12 but this is undesirable. Hence, there is a need to move from traditional metal sulfide catalysts to alternative catalysts for HDO processes. Sulfided Mo catalysts that are known to be active and selective for C-S scission in hydrodesulfurization (HDS)13 have also demonstrated high activity and selectivity for C-O scission in HDO.10 Similarly, other catalysts that are known to be active for HDS are candidates for HDO, such as metal phosphides. Recently, several authors have shown that metal phosphides are active and selective in HDS.14-16 Furthermore, because of the presence of high oxygen concentrations in bio-oil, oxidation of the active catalyst phase may occur during the hydroprocessing reaction and the oxidized catalyst may also play a role in the HDO. Metal oxides can be reduced to form surface defects, such as anionic vacancies that can catalyze hydrogenolysis and hydrogenation reactions at typical HDO conditions.17,18 Hence, metal oxides are also candidates as alternative HDO catalysts. The furans and phenols account for 3-10 wt % of all of the compounds in bio-oil,3 and because these are the most refractory species present in the bio-oil, they have been used as model compounds to represent bio-oil feedstocks in past studies of catalytic hydrotreatment.6,11,18-20 In the present paper, a systematic study of the activity and selectivity of different Mo catalysts for the HDO of 4-methylphenol is reported. Unsupported MoS2, MoP, MoO2, and MoO3 catalysts, used to eliminate support interaction effects as well as internal mass-transfer limitations, have been examined for the HDO of 4-methylphenol.
salts dissolved in water. A Mo/P molar ratio of 1:1 was obtained by dissolving 4 g of ammonium heptamolybdate [(NH4)6Mo7O24 3 4H2O] and a corresponding amount of diammonium hydrogen phosphate [(NH4)2HPO4] in 30 mL of deionized water. The precursor solution was aged in a covered beaker held at 363 K for 24 h in a water bath, dried in an oven at 397 K for 24 h, and calcined at 773 K for 5 h at 5 K/min. The calcined catalyst precursor was ground to a powder (dP < 0.053 mm) and converted to the active metal phosphide by temperature-programmed reduction (TPR) in H2 at a flow rate of 160 mL [standard temperature and pressure (STP)]/min [gas hourly space velocity (GHSV) = 1530 h-1] and a heating rate of 5 K/min to 573 K, followed by a heating rate of 1 K/min to 923 K. The final temperature was held for 2.5 h. The sample was then cooled to room temperature in He and passivated in a flow of 1 mol % O2/He for 3 h prior to removal from the reactor. Passivation was applied to the pyrophoric phosphide catalyst to prevent bulk oxidation. 2.2. Catalyst Characterization. Catalyst characterization was performed on the samples directly after preparation, and these samples are referred to herein as the “unused” catalysts. Characterization was also performed on the catalyst samples following the HDO reaction for 5 h at 623 K and 4.40 MPa, and these samples are referred to as “used” catalysts. The used catalysts were recovered from the reactor by decanting and filtering the reactor product solution. The recovered solids were washed with methylene chloride followed by vacuum filtration to remove excess liquid. Powder X-ray diffraction (XRD) spectra were collected on both the unused and used catalysts using a Rigaku diffractometer with a Cu KR X-ray source of wavelength 1.54 A˚. The analysis was performed using a 40 kV and 20 mA source and a scan range of 10-80° with a step size of 2°/min. The phase identification was carried out after subtraction of the background using standard software and powder diffraction files (PDFs) for reference. Crystallite size (dc) estimates were made using the Scherrer equation, dc = Kλ/β cos θ, where the constant K was taken to be 0.9, λ is the wavelength of radiation, β is the peak width in radians, and θ is the angle of diffraction. TPR of the unused and used Mo oxide samples was performed in H2. TPR was also performed on the MoP-calcined catalyst precursor and the passivated MoP. TPR experiments were carried out using a Micromeritics AutoChem II 2920 automated catalyst characterization flow system. Prior to the TPR, a known amount of catalyst was pretreated in Ar [50 mL (STP)/min] at 723 K for 1 h and then cooled to room temperature. Subsequently, the Ar flow was switched to 9.5 mol % H2/Ar [50 mL (STP)/min], and the sample was heated from room temperature to 923 K at 5 K/min in accordance with the MoP preparation method. TPR was also performed on the unused and used MoO3 and the unused MoO2 at a heat up rate of 5 K/min to 998 K. The H2 consumption was measured by a thermal conductivity detector (TCD). The average oxidation state of molybdenum in the oxide catalysts was determined by performing temperature-programmed oxidation (TPO) on the used and unused Mo catalysts. Reoxidation of the used catalyst was performed using a Micromeritics Autochem II 2920 unit. Prior to the TPO experiment, the catalyst was pretreated in He [50 mL (STP)/min] at 723 K for 1 h and then cooled to room temperature. Subsequently, the He flow was switched to 10 mol % O2/He [50 mL (STP)/min], and the sample was heated to 873 K for 2 h at a rate of 5 K/min. The O2 consumption was measured by a TCD. Thermal gravimetric analysis (TGA) was also performed in air on the used MoO3 catalyst to monitor the change in mass during re-oxidation in the TGA. Prior to analysis, the sample was treated in N2 [50 mL (STP)/min] at 393 K for 1.5 h. The gas flow was then changed to a 40 mol % air/N2 mixture at 70 mL (STP)/min, and the temperature was ramped to 873 K at 10 K/ min and held for 3 h. The TPR, TPO, and TGA profiles were differentiated and de-convoluted using Origin 8 software.
2. Experimental Section 2.1. Catalyst Preparation. Molybdenum disulfide (99%), molybdenum dioxide (99%), and molybdenum trioxide (99.5þ%) were purchased from Sigma-Aldrich, sieved to a particle size of dP < 0.053 mm, and used as catalysts without further treatment. The MoP was prepared following the method of Stinner and Prins21 and Wang and Smith,22 using precursor (12) Wang, W.; Yang, Y.; Bao, J.; Luo, H. Characterization and catalytic properties of Ni-Mo-B amorphous catalysts for phenol hydrodeoxygenation. Catal. Commun. 2009, 11 (2), 100–105. (13) Tye, C.; Smith, K. Hydrodesulfurization of dibenzothiophene over exfoliated MoS2 catalyst. Catal. Today 2006, 116 (4), 461–468. (14) Oyama, S.; Wang, X.; Requejo, F.; Sato, T.; Yoshimura, Y. Hydrodesulfurization of petroleum feedstocks with a new type of nonsulfide hydrotreating catalyst. J. Catal. 2002, 209 (1), 1–5. (15) Yang, S.; Liang, C.; Prins, R. Preparation and hydrotreating activity of unsupported nickel phosphide with high surface area. J. Catal. 2006, 241 (2), 465–469. (16) Abu, I.; Smith, K. The effect of cobalt addition to bulk MoP and Ni2P catalysts for the hydrodesulfurization of 4,6-dimethyldibenzothiophene. J. Catal. 2006, 241 (2), 356–366. (17) Furimsky, E.; Mikhlin, J.; Jones, D.; Adley, T.; Baikowitz, H. On the mechanism of hydrodeoxygenation of ortho substituted phenols. Can. J. Chem. Eng. 1986, 64 (6), 982–985. (18) Massoth, F.; Politzer, P.; Concha, M.; Murray, J.; Jakowski, J.; Simons, J. Catalytic hydrodeoxygenation of methyl-substituted phenols: Correlations of kinetic parameters with molecular properties. J. Phys. Chem. 2006, 110 (29), 14283–14291. (19) Vuori, A.; Helenius, A.; Bredenberg, J. Influence of sulphur level on hydrodeoxygenation. Appl. Catal. 1989, 52 (1-2), 41–56. (20) Lee, L.; Ollis, D. Interactions between catalytic hydrodeoxygenation of benzofuran and hydrodesulfurization of dibenzothiophene. J. Catal. 1984, 87 (2), 332–338. (21) Stinner, C.; Prins, R.; Weber, T. Formation, structure, and HDN activity of unsupported molybdenum phosphide. J. Catal. 2000, 191 (2), 438–444. (22) Wang, R.; Smith, K. Hydrodesulfurization of 4,6-dimethyldibenzothiophene over high surface area metal phosphides. Appl. Catal., A 2009, 361 (1-2), 18–25.
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Brunauer-Emmett-Teller (BET) analysis was performed on the used and unused catalysts. Surface areas were measured by N2 adsorption at 77 K using a volumetric unit (Micromeritics Flowsorb II 2300). Prior to N2 adsorption, the samples were degassed at 423 K in 30 mol % N2/He [15 mL (STP)/min] for 2 h to remove moisture. The CO uptake of the unused and used catalysts was measured by a Micromeritics AutoChem II 2920 unit using pulsed chemisorption. The passivated MoP catalyst was pretreated to remove the passivation layer by passing 50 mL (STP)/min of 9.5 mol % H2/Ar while heating to 923 K at 5 K/min and maintaining the final temperature for 1 h. The flow was then switched to He [50 mL (STP)/min] at 923 K for 1 h to remove any adsorbed species. All other catalysts were pretreated in Ar [50 mL (STP)/min] at 723 K for 1 h and then cooled to room temperature. After this treatment, 0.5 mL pulses of CO were injected into a flow of He [50 mL (STP)/min] and the CO uptake was measured using a TCD. CO pulses were repeatedly injected until no further CO uptake was observed after consecutive injections. The acid sites of the catalysts were titrated using n-propyl amine (n-PA). The acidity analyses were performed using a Micromeritics AutoChem II 2920 unit. The catalyst was pretreated in the same way as that used for the CO uptake measurement. A flow of He [50 mL(STP)/min] saturated at room temperature with n-PA (Aldrich, 99.8%) was then injected onto the pretreated catalyst in 0.5 mL pulses. The adsorption was performed at 298 K, and a TCD was used to quantify the amount of n-PA adsorbed. Pulses of n-PA were repeatedly injected until no further uptake was observed after consecutive injections. A Leybold Max200 X-ray photoelectron spectrometer was used for X-ray photoelectron spectroscopy (XPS) studies. Al KR was used as the photon source generated at 15 kV and 20 mA. The pass energy was set at 192 eV for the survey scan and 48 eV for the narrow scan. The used and unused catalysts were analyzed. All XPS spectra were corrected to the C 1s peak at 285.0 eV. Deconvolution of the XPS profiles was performed using XPS Fit software. 2.3. Catalyst Activity. The HDO of 4-methylphenol was measured in a 300 mL stirred-batch autoclave reactor operated at 598, 623, and 648 K. Initially, 100 mL of 2.96 wt % 4-methylphenol (Aldrich, 97%) in decalin (Sigma-Aldrich, 98%) was added to the reactor and slurried with 11 000 ppm of the molybdenum catalyst. The 4-methylphenol reactant was also used in past studies by Yang et al.,10 Gevert et al.,23 and Laurent and Delmon7 to represent bio-oil. The reactant concentration employed was similar to that used by Odebunmi and Ollis24 for HDO studies. The reactor was first purged in 55 mL (STP)/min N2 for 1 h. Finally, the reactor was pressurized with ultra-high purity (UHP) hydrogen to 2.41 MPa and then heated to the desired temperature at 10.8 K/min and a stir rate of 1000 rpm. Subsequently, the H2 pressure increased to 4.14, 4.40, or 4.83 MPa following the heat-up phase to 598, 623 or 648 K. During this heat-up phase, the catalyst was activated and minimal reaction occurred. The reaction during the heat-up phase was accounted for by analyzing the liquid in the reactor once the reactor temperature was reached and setting this measured concentration as the concentration at time = 0 min. The H2 pressure, stirrer speed, and temperature were continuously monitored during the experiment. Small volumes (90% and a number of the experiments were repeated to ensure repeatability of the data. The decomposition of 4-methylphenol was modeled assuming a pseudo-first-order reaction (eq 1) lnð1 - xÞ ¼ -kCcat t
ð1Þ
where t is the HDO reaction time (s), Ccat is the concentration of the catalyst in the reactor at the reaction conditions (molMo/mL), x is the conversion, and k is the rate constant (mL s-1 molMo-1). The absence of internal and external mass-transfer effects on the measured reaction kinetics was confirmed experimentally and by theoretical analysis. The volumetric liquid side masstransfer coefficient was estimated according to Dietrich et al.25 for stirred tank benchtop reactors operated in slurry phase. The Sherwood number was calculated using a correlation by Albal et al.26 The mass-transfer coefficient was at least 10 orders of magnitude higher than the observed reaction rate. Furthermore, repeating the reactions at stirrer speeds of 600 and 1000 rpm showed that the measured reaction rates were within 10% of the average values. Internal mass-transfer effects were minimal because of the small catalyst particle size, and this was confirmed by operating the reactor with catalysts of particle size of 0.053 and 0.180 mm. In both cases, the measured reaction rates were within 10% of the average values.
3. Results and Discussion 3.1. Catalyst Characterization. The X-ray diffractogram of the unused MoP after passivation, presented in Figure 1, confirmed that phase pure MoP had been successfully prepared and that, following the 5 h reaction at 623 K and 4.40 MPa, the structural integrity of the MoP was maintained. (25) Dietrich, E.; Mathieu, C.; Delmas, H.; Jenck, J. Raney-nickel catalyzed hydrogenations: Gas-liquid mass transfer in gas-induced stirred slurry reactors. Chem. Eng. Sci. 1992, 47 (13-14), 3597–3604. (26) Albal, R.; Shah, Y.; Carr, N.; Bell, A. Mass transfer coefficients and solubilities for hydrogen and carbon monoxide under FischerTropsch conditions. Chem. Eng. Sci. 1984, 39 (5), 905–907.
(23) Gevert, B.; Otterstedt, J.; Massoth, F. Kinetics of the hydrodeoxygenation of methyl-substituted phenols. Appl. Catal. 1987, 31 (1), 119–131. (24) Odebunmi, E.; Ollis, D. Catalytic hydrodeoxygenation I. Conversions of o-, p-, and m-cresols. J. Catal. 1983, 80 (1), 56–64.
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Table 1. Main Diffraction Peaks and Crystallite Sizes of Unused and Used Mo Catalysts angle (plane)
crystallite size (nm)
angle (plane)
crystallite size (nm)
Unused MoP 42.9° (101) 32.0° (100) 27.9° (110)
Used MoP 22.6 22.5 26.3
43.0° (101) 32.0° (100) 27.9° (110)
139.1 46.4 39.1
14.4° (002) 39.6° (103) 49.8° (105)
23.3 24.2 28.3 Used MoS2
Unused MoS2 14.4° (002) 39.6° (103) 49.8° (105)
129.6 45.9 36.6 Used MoO2
Unused MoO2 26.1° (-111) 37.0° (-211) 53.5° (-312)
74.7 26.4 27.5
26.0° (-111) 37.0° (-211) 53.5° (-312)
73.4 24.7 26.1 Used MoO3
Unused MoO3 25.7° (040) 39.0° (103) 27.4° (021) a
a
91.1 87.6 200.7
b
26.2° (-111) (211) 36.7° (200)a (1000)b 53.6° (-222)a (1311)b
38.7 15.9 17.8
Plane for monoclinic MoO2. b Plane for monoclinic Mo4O11.
Table 2. Surface Area, CO Uptake, and Total Acidity of Mo Catalysts
sample
SBET (m2/g)
CO uptake (mmol/mol of Mo)
MoP MoS2 MoO2 unused MoO3 used MoO3
8.80 4.30 4.80 0.30 3.50
0.07 0.14 0.19 10. On the basis of the work of Matsuda et al.,28 we assume that, under the reaction conditions, the in situ reduction of the unused crystalline MoO3 began preferentially at the shear planes. As reduction proceeded at these planes, the metal lattice contracted and fractured the crystal, decreasing its size and creating micropores because of oxygen removal.28 The measured physicochemical properties of the catalysts are compared in Table 2. The BET surface area, CO chemisorption,
Figure 2. X-ray diffractogram of unused and used MoO2 and unused and used MoO3: (2) MoO2, (0) Mo4O11, and (b) MoO3.
Table 1 also shows that the crystallite sizes of the unused and used MoP, estimated by XRD line broadening, were of similar dimensions (20-30 nm, Table 1). Figure 1 shows similar results for the used and unused MoS2 with comparable crystallite sizes in the range of 35-130 nm (Table 1). The X-ray diffractogram of the unused MoO2 (Figure 2) showed the presence of monoclinic MoO2, Mo4O11, and MoO3 phases, suggesting that air exposure resulted in some oxidation of MoO2. The proportion of MoO3 in the catalyst was minimal ( MoO3. Similarly, the activation energies were found to increase in the order of MoP < MoS2 < MoO2 < MoO3. The high TOF of the MoP was likely due to the electronic properties of this catalyst. Mo (3d) of the MoP had the highest electron density (lowest BE; Table 6) among all catalysts tested. Because the lowest unoccupied molecular orbital (LUMO) of C-O is antibonding, surfaces that are able to transfer electron density to this orbital facilitate the dissociation of the C-O bond. Therefore, the increased electron density of Mo may account for the higher TOF experienced over the MoP catalyst. The electron density of the catalysts decreased in the order MoP > MoS2 > MoO2 > MoO3, which is in (38) Moreau, C.; Aubert, C.; Durand, R.; Zmimta, N.; Geneste, P. Structure-activity relationships in hydroprocessing of aromatic and heteroaromatic model compounds over sulphided NiO-MoO3/Al2O3 and NiO-WO3/Al2O3 catalysts: Chemical evidence for the existence of two types of catalytic sites. Catal. Today 1988, 4 (1), 117–131. (39) Vogelzang, M.; Li, C.; Schuit, G.; Gates, B.; Petrakis, L. Hydrodeoxygenation of 1-naphthol: Activities and stabilities of molybdena and related catalysts. J. Catal. 1983, 84 (1), 170–177. (40) Weigold, H. Behaviour of Co-Mo-Al2O3 catalysts in the hydrodeoxygenation of phenols. Fuel 1982, 61 (10), 1021–1026.
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hydrogenolysis reactions, while the rim planes (top edge planes) are active for hydrogenation.41,42 In contrast to the rim-edge model, Hensen et al.43 reported that increasing MoS2 layer stacking increases hydrogenation because of a less hampered planar adsorption of reactants. Bunch and Ozkan6 suggest that hydrogenation sites are associated with metal anionic vacancies, whereas hydrogenolysis sites are believed to be Brønsted acid centers associated with the adsorption and dissociation of H2S or similarly H2O, because it is likely that OH- groups operate in a similar manner to SH- groups. The O2-, OH-, and Hþ groups present on the surface of oxidic catalysts exhibit Brønsted acid character and are formed by dissociative adsorption of H2O on the oxygen vacancies or similarly S2- and SH- groups over sulfided catalysts. The dissociation of H2 also converts O2or S2- groups into OH- or SH- groups.44 In the present study, we suggest that Hþ involved in the nucleophilic substitution reaction for the hydrogenolysis of C-O is primarily associated with Brønsted acid sites of the catalyst generated by the adsorption and dissociation of H2O or H2 to produce OH- or SH- groups on the oxide or sulfide catalyst, respectively. The OH- or SH- groups also provide Brønsted acidity for acid-catalyzed reactions. Therefore, the MoO3 catalyst displaying the highest Brønsted acidity displays the highest HDO (per unit mass of catalyst). The primary reaction for the production of 4-methylcyclohexene from 4-methylcyclohexanol may be by an E2 elimination reaction, which causes the dehydration of the protonated alcohol under acidic conditions to produce a π bond. Senol et al.44 suggest that hydrogenation reactions occur on anionic vacancies and the degree of saturation of the anionic vacancies for the hydrogenation reactions are different from those for the hydrogenolysis reaction. Therefore, the CUSs for hydrogenolysis reactions are more electrophilic (i.e., more positively charged) than those for hydrogenation reactions.6,7,45,46 Thus, in the present study, the hydrogenation of 4-methylphenol to 4-methylcyclohexanol is limited over the MoO3, MoO2, and MoS2 catalysts because of the abundance of very electrophilic CUSs that cause a high selectivity toward hydrogenolysis products. The source of the acid sites over the MoP catalyst is likely a consequence of the incomplete reduction of phosphate species, which can result in Brønsted acidity that carries out protonation for hydrogenation reactions (PO-H). However, the measured surface acidity of the phosphide catalyst was low. Therefore, any acidic sites present on the phosphide catalyst were less electrophilic than those of any other catalyst, which accounted for its high hydrogenation abilities. The high selectivity toward hydrogenation observed over MoP was also a consequence of the noble-metal-like properties of the transition-metal phosphide.47 Thus, MoP displayed bifunctional acidic and metallic properties with
Both MoO2 and MoS2 catalysts displayed a similar degree of hydrogenolysis but differed in their hydrogenation and isomerization selectivity, with MoO2 having a higher selectivity for complete hydrogenation to methylcyclohexane, while MoS2 had a high selectivity for partially saturated products, where 36% of the hydrogenated product was methylcyclohexane, 31.7% was 4-methylcyclohexene, and 32.3% was 1-methylcyclohexene after 5 h at 648 K. Furimsky et al.17 found conflicting results over CoMo/Al2O3 oxide and sulfide, reporting that the sulfided catalysts produced a higher degree of hydrogenated products in comparison to the oxide catalyst.17 Gevert et al.23 observed similar selectivities for the decomposition of 4-methylphenol over sulfided CoMo/Al2O3 at 573 K and 5 MPa as the MoS2 catalyst of the present study. The MoO2 catalyst, with the higher surface acidity, also displayed a higher selectivity toward isomerization products. At 648 K, the MoO2 product selectivity toward completely hydrogenated product was similar to that of MoO3, which could indicate the formation of anionic vacancies over the MoO2 catalyst at higher temperatures. After the 5 h reaction at 648 K, the selectivity toward hydrogenated products over MoO2 increased, where nearly 80% of the hydrogenated product was methylcyclohexane as opposed to only 38 and 16% methylcyclohexane at 623 and 598 K, which could indicate the formation of anionic vacancies. Two parallel reactions for the decomposition of 4-methylphenol were observed over the phosphide catalyst to produce toluene and hydrogenated products. Of all catalysts tested, MoP displayed the highest selectivity toward hydrogenated products, where 100% of the hydrogenated product was completely saturated to methylcyclohexane at all reaction temperatures. No methylcyclohexenes were detected over MoP. Small amounts of isomerization products were detected at 623 and 648 K, indicative of the presence of an acidic phosphate phase. As the temperature over the MoP catalyst increased, the selectivity toward toluene increased, thereby displaying the high activation energy for the formation of this product. On the basis of these results, hydrogenolysis to produce toluene and the coupled ring saturation/rapid dehydration to produce 4-methylcyclohexene were found to be the primary reactions for the HDO of 4-methylphenol. Toluene was found to be a stable product, while 4-methylcyclohexene was hydrogenated and cracked to produce saturated and isomerization products. This reaction scheme is similar to that proposed by Laurent and Delmon7 and suggests that the rate-limiting step over MoO3, MoO2, and MoS2 was the saturation of the 4-methylphenol ring to produce 4-methylcyclohexanol, which was then rapidly dehydrated to produce 4-methylcyclohexene. 3.4. Active Catalytic Sites. Coordinatively unsaturated sites (CUSs) on the oxide and sulfided catalysts display Lewis acid behavior, and these anionic vacancies catalyze the reactions in hydroprocessing. On MoS2, CUSs are located at the edge planes and are believed to be the sites for catalytic
(44) Senol, O.; Viljava, T.; Krause, A. Effect of sulphiding agents on the hydrodeoxygenation of aliphatic esters on sulphided catalysts. Appl. Catal., A 2007, 326 (2), 236–244. (45) Bataille, F.; Lemberton, J.; Michaud, P.; Perot, G.; Vrinat, M.; Lemaire, M.; Schulz, E.; Breysse, M.; Kasztelan, S. Alkyldibenzothiophenes hydrodesulfurization;Promoter effect, reactivity, and reaction mechanism. J. Catal. 2000, 191 (2), 409–422. (46) Bunch, A.; Zhang, L.; Karakas, G.; Ozkan, U. Reaction network of indole hydrodenitrogenation over NiMoS/γAl2O3 catalysts. Appl. Catal., A 2000, 90 (1,2), 51–60. (47) Lee, Y.; Oyama, S. Bifunctional nature of a SiO2-supported Ni2P catalyst for hydrotreating: EXAFS and FTIR studies. J. Catal. 2006, 239, 376–389.
(41) Daage, M.; Chianelli, R. Structure-function relations in molybdenum sulfide catalysts: The rim-edge model. J. Catal. 1994, 149 (2), 414–427. (42) Kasztelan, S.; Toulboat, H.; Grimblot, J.; Bonnelle, J. A geometrical model of the active phase of hydrotreating catalysts. Appl. Catal. 1984, 13 (1), 127–159. (43) Hensen, J.; Kooyman, P.; van der Meer, Y.; van der Kraan, A.; de Beer, V.; van Veen, J.; van Santen, A. The relation between morphology and hydrotreating activity for supported MoS2 particles. J. Catal. 2001, 199 (2), 224–235.
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Energy Fuels 2010, 24, 4728–4737
: DOI:10.1021/ef901270h
Whiffen and Smith
strong hydrogenating capabilities similar to Pd and Pt on acidic supports.48
exception of MoO3, which underwent reduction to a mixed oxide containing Mo4O11, MoO2, and Mo phases. This partially reduced Mo oxide was found to have high activity for the decomposition of 4-methylphenol because of the formation of anionic vacancies and Brønsted surface acidity. Both MoO2 and MoS2 displayed similar conversions, selectivities, and activation energies for the decomposition of 4-methylphenol. The TOF was found to be highest over the MoP catalyst, which also displayed the lowest activation energy and highest selectivity toward completely hydrogenated products. Therefore, potential lies in using this catalyst for the HDO of pyrolysis-derived bio-oils.
4. Conclusions The results of the present study demonstrate that unsupported low-surface-area MoO3, MoO2, MoS2, and MoP catalysts are active for the HDO of 4-methylphenol. The catalysts were stable at the reaction conditions, with the (48) Talukdar, A.; Bhattacharyya, K.; Sivasanker, S. Hydrogenation of phenol over supported platinum and palladium catalysts. Appl. Catal., A 1993, 96 (2), 229–239.
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