Energy Fuels 2010, 24, 2052–2059 Published on Web 01/20/2010
: DOI:10.1021/ef901222z
Catalytic Activities of NiMo Carbide Supported on SiO2 for the Hydrodeoxygenation of Ethyl Benzoate, Acetone, and Acetaldehyde Wei Zhang,†,‡ Ye Zhang,*,† Liangfu Zhao,*,† and Wei Wei† †
Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, People’s Republic of China and ‡Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China Received October 27, 2009. Revised Manuscript Received December 31, 2009
Bimetallic NiMo carbide supported on SiO2 has been synthesized by means of a temperature-programmed reaction. Characterization was performed by elemental analysis, Brunauer-Emmett-Teller (BET) surface area analysis, temperature-programmed reduction (H2-TPR), temperature-programmed oxidation (TPO), temperature-programmed desorption of NH3 (NH3-TPD), and X-ray diffraction (XRD). Elemental analysis and TPO characterization indicated that carbon was successfully introduced into the lattice of NiMo carbide by a temperature-programmed reaction with a mixture of H2 and CH4. Ethyl benzoate was used as a model molecule to investigate the hydrodeoxygenation (HDO) activities of NiMo carbide. As a comparison, HDO reactions of ethyl benzoate were also investigated over Mo carbide as well as CoMo sulfide. The results indicated that NiMo carbide was the most stable catalyst for HDO among the samples. On the basis of the hydrodeoxygenation (HDO) evaluation of ethyl benzoate and characterization of passivated and used NiMo carbide, it can be deduced that the changes of catalytic activity of NiMo carbide, during HDO reactions, may be ascribed to oxygen accumulation as well as coke deposition on the surface of the catalyst. HDO reactions of acetone and acetaldehyde were also investigated over NiMo carbide. The results indicated that NiMo carbide was a highly active and stable catalyst for HDO of acetone and acetaldehyde. hydrocarbons changed with time.6 Great efforts have been devoted to enhance the stability of these sulfide catalysts, but the results are not yet satisfactory.4,6-10 In recent years, mono- and bimetallic carbides and nitrides based on molybdenum have drawn considerable interests because of their potential use for a variety of catalytic processes.11-17 However, up to now, there were few reports on the use of molybdenum carbide for HDO. In this research, Ni-promoted molybdenum carbides were synthesized and used for HDO reactions to determine the potential usefulness of carbide on HDO. Neutral silica was used as a support to probe catalytic properties of the carbide better. The bio-oil contains hundreds of organic oxygenates, including acids, esters, alcohols, aldehydes, furans, phenols, etc.1 Its reactivity varies according to the type and amount of oxygen-containing compounds. Therefore, HDO of the different oxygen-containing compounds present in bio-oil
Introduction Bio-oil, as one of the renewable, environmentally friendly, and sustainable sources of liquid fuels, has been given more and more attention as a result of declining petroleum resources and increasing environmental concerns.1 However, bio-oil is a complex mixture including hundreds of organic oxygenates, in which the oxygen content can be as high as 50%. These oxygenates lead to some deleterious properties of bio-oil, including high viscosity, thermal and chemical instability, corrosiveness, poor heating value, and immiscibility with hydrocarbon fuels.2-4 Great efforts have been paid to the upgrading of bio-oil by hydrodeoxygenation (HDO) over hydrotreating catalysts, in which conventional sulfided CoMo/γ-Al2O3 and NiMo/γ-Al2O3 were investigated most extensively. These catalysts have been observed to possess a high initial activity on HDO, but they deactivated rapidly because of coke deposition5 or modification of the sulfide structure.4 For example, Viljava et al. found that the conversion of phenol on sulfided CoMo/γ-Al2O3 decreased from 72 to 52% in 6 h at 300 °C.5 In a study on HDO of aliphatic esters on sulfided CoMo/γ-Al2O3 and NiMo/γ-Al2O3, Senol et al. observed that, in the absence of a sulfiding agent, the catalysts deactivated and the selectivity to different
: (6) Senol, O. I.; Viljava, T.-R.; Krause, A. O. I. Catal. Today 2005, 106, 186–189. (7) Ferrari, M.; Bosmans, S.; Maggi, R.; Delmon, B.; Grange, P. Catal. Today 2001, 65, 257–264. (8) Viljava, T.-R.; Komulainen, R. S.; Krause, A. O. I. Catal. Today 2000, 60, 83–92. (9) Laurent, E.;: Delmon, B. Appl. Catal., A 1994, 109, 97–115. (10) Senol, O. I.; Viljava, T.-R.; Krause, A. O. I. Appl. Catal., A 2007, 326, 236–244. (11) Oyama, S. T. Catal. Today 1992, 15, 179–200. (12) Kojima, R.; Aika, K. Chem. Lett. 2000, 514–515. (13) Boisen, A.; Dahl, S.; Jacobsen, C. J. H. J. Catal. 2002, 208, 180–186. (14) Hada, K.; Tanabe, J.; Omi, S.; Nagai, M. J. Catal. 2002, 207, 10–22. (15) Trawczynski, J. Appl. Catal., A 2000, 197, 289–293. (16) Kojima, R.; Aika, K. Appl. Catal., A 2001, 219, 141–147. (17) He, H.; Dai, H. X.; Ngan, K. Y.; Au, C. T. Catal. Lett. 2001, 71, 147–153.
*To whom correspondence should be addressed. Telephone: þ86351-4041526. Fax: þ86-351-4041526. E-mail:
[email protected] (L.Z.);
[email protected] (Y.Z.). (1) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044– 4098. (2) Czernik, S.; Bridgwater, A. V. Energy Fuels 2004, 18, 590–598. (3) Mohan, D.; Pittman, C. U., Jr.; Steele, P. H. Energy Fuels 2006, 20, 848–889. (4) Furimsky, E. Appl. Catal., A 2000, 199, 147–190. (5) Viljava, T.-R.; Komulainen, R. S.; Selvam, T.; Krause, A. O. I. Stud. Surf. Sci. Catal. 1999, 127, 145–152. r 2010 American Chemical Society
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needs to be thoroughly investigated for a full understanding and optimization of the upgrading process conditions. In this study, ethyl benzoate, acetone, and acetaldehyde were used as model molecules to investigate the HDO reactions of various oxygenated groups (carboxylic, ketonic, and aldehyde) over carbide. The proposed reaction network of these oxygenates was hypothesized respectively according to the intermediates produced during the reaction.
the system was adjusted to the desired value. The liquid feed was then introduced by a high-pressure pump into the reactor. The hydrogen flow rate was controlled by a mass flow controller during the experiment. The liquid samples from the reactor were collected at intervals of 12 h and analyzed quantitatively by gas chromatography (GC) with a capillary column of DB-1 and a flame ionization detector. Cumene was used as the internal standard in the analysis of HDO of ethyl benzoate. Methyl ethyl ketone was used as the internal standard in the analysis of HDO of acetone and acetaldehyde. The gas outlet stream of the flow reactor was quantitatively analyzed during the runs by an online gas chromatograph equipped with a HP-INNOWAX capillary column. As a comparison, Mo carbide supported on SiO2 was synthesized by carburization of Mo oxide (30 wt % MoO3) with the same method adopted in the synthesis of NiMo carbide. Sulfided CoMo/γ-Al2O3 was also synthesized by sulfidation of CoMo oxide (4.5 wt % CoO þ 15 wt % MoO3) with a 10% H2S in H2 (by volume) at 400 °C for 4 h under atmospheric pressure. The catalytic activity and lifetime of Mo carbide and sulfided CoMo/γ-Al2O3 for HDO of ethyl benzoate were measured and compared to those of NiMo carbide. Conversion (%) and selectivity (%) are defined as follows:
Experimental Section Catalyst Preparation. The supported bimetallic (Mo with Ni) oxide precursor was prepared by co-impregnating SiO2 with aqueous solution containing (NH4)6Mo7O24 and Ni(NO3)2. The wet product was then dried at 100 °C for 12 h and calcined at 550 °C for 6 h. The concentration of active metallic oxide was 4.5 wt % NiO and 15 wt % MoO3, respectively. Supported bimetallic (Mo with Ni) carbide was synthesized by the temperature-programmed reaction of the oxide precursor with a flow of 20% CH4 and 80% H2. A typical carburization procedure was as follows: initially, 5 mL of oxide precursor was packed into a stainless reactor and heated from room temperature to 350 °C at a rate of 3 °C/min in flowing Ar (flow rate of 100 mL/min). At this temperature, the flow was switched to carburization gases (20% CH4/80% H2), and then the temperature was ramped at 1 °C/min from 350 to 700 °C in flowing carburization gases (flow rate of 100 mL/min). After the temperature was kept at 700 °C for 3 h, the carburization gases were switched to Ar. The sample was quenched to room temperature in flowing Ar and then passivated in flowing mixed gases (1% O2/Ar) for 2 h. The particle size of the carbide is 0.60-0.85 mm. Catalyst Characterization. Powder X-ray diffraction (XRD) measurements were carried out using Cu KR radiation and Ni filter in the 2θ range of 5-80° on a Rigaku D/max-2500 diffractometer. The amounts of carbon in samples were measured by elemental analysis using VARIO EL. Brunauer-Emmett-Teller (BET) surface areas were measured in a Micromeritics ASAP 2000 instrument based on the static volumetric principles, using the multiple-point method and nitrogen as the adsorption gas. H2-temperature-programmed reduction of samples was carried out in a U-shaped quartz tube reactor. Typically, 0.2 g of sample was placed in the U-shaped quartz tube reactor, heated from room temperature to 500 °C in Ar, and maintained for 1 h to remove impurities adsorbed on the surface of the sample. After the sample was cooled to 40 °C in flowing Ar, it was heated from 40 to 1030 °C at a rate of 10 °C/min in flowing mixed gases (30 mL/min) of 5% H2/90% Ar and the temperature-programmed reduction (TPR) profile was recorded by a thermal conductivity detector (TCD). Temperature-programmed oxidation was carried out on a TG instrument. Typically, 10 mg of sample was heated from room temperature to 900 °C at a rate of 10 °C/min in flowing mixed gases (50 mL/min) of 20% O2/80% He. The weight change caused by oxidation of the sample was recorded. Temperature-programmed desorption of ammonia (NH3TPD) was measured on a self-designed apparatus equipped with a TCD. Typically, 0.2 g of sample was first pretreated in flowing Ar at 500 °C for 1 h, then cooled to 40 °C, and saturated at this temperature with ammonia. After adsorption reached equilibrium, the sample was heated from 40 to 500 °C at a rate of 10 °C/min in flowing Ar. The NH3-TPD profile was recorded by a TCD. Reaction Studies. Reactions were carried out in a highpressure stainless fixed-bed reactor. A total of 5 mL of sample of oxide precursor was packed into the reactor between two layers of quartz sand. Prior to the reaction test, the sample was carburized in situ at atmospheric pressure. After the temperature of the catalyst bed dropped to the reaction temperature, the gases were switched from Ar to pure H2 and the total pressure of
C ð%Þ ¼
nR, 0 - nR 100 nR, 0
S ð%Þ ¼
ni 100 nR, 0 - nR
where nR,0 is the moles of model molecular in feed, nR the moles of model molecular in product, and ni is the moles of model molecular converted to selected products. The HDO conversion was the fraction of the reactant converted to hydrocarbons.
Results and Discussion General Remarks. A comparison of repeated experiments showed the experiments to be highly reproducible: the values of conversion and selectivity were within 3% units. For ethyl benzoate, carbon balances were calculated on the basis of liquid product analyses. For acetone and acetaldehyde, carbon balances were calculated on the basis of liquid and gas product analyses. The values were about 95% on average. Catalysts Characterization. Figure 1 shows the XRD patterns of passivated Mo and NiMo carbides prepared from the corresponding oxide precursors. For NiMo carbide, No peaks other than those associated with SiO2 support were observed in XRD patterns, presumably because of the high dispersion of the carbide on SiO2. For Mo carbide, the XRD pattern shows peaks at 34.6°, 37.9°, 39.5°, 61.8°, and 75.1°, which correspond to the diffraction of β-Mo2C. Table 1 lists the carbon content and BET surface areas (SBET) of samples. It is obvious that, after carburization, carbon was successfully introduced into the carbides. In the study by Sundaramurthy et al.,18 Mo2C was observed to be the main active phase in NiMoC/γ-Al2O3. As shown in Table 1, the Mo/C ratio of passivated NiMo carbide synthesized in this study was 2.12 and close to the stoichiometric value of Mo2C, suggesting the existence of Mo2C in the sample. There are two forms of carbon in carbide. One is lattice carbon, which exists in the bulk phase, and the other is carbonaceous residue, which deposited on the surface of carbide. To identify the relative content of different carbon forms, TPO was carried out. As shown in Figure 2, there is a (18) Sundaramurthy, V.; Dalai, A. K.; Adjaye, J. Appl. Catal., B 2006, 68, 38–48.
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Figure 1. XRD patterns of (a) passivated Mo carbide and (b) passivated NiMo carbide. (/) β-Mo2C.
Figure 3. Total conversion of ethyl benzoate over the (a) Mo carbide, (b) NiMo carbide, and (c) CoMo sulphide.
Table 1. Surface Areas and Carbon Content of Samples
diffusion-controlled reaction. For Mo carbide, there is an obvious weight loss above 600 °C, which may be ascribed to the oxidation of carbon residues.19 This suggests that, during carburization, some carbon was incorporated into the lattice of Mo carbide, while the others would deposit as carbon residues on the surface of the sample. For NiMo carbide, some weight loss occurs above 600 °C, which may also be ascribed to the oxidation of carbon residues. HDO of Ethyl Benzoate. The HDO reactions of ethyl benzoate over NiMo carbide as well as Mo carbide and sulfided CoMo/γ-Al2O3, used as reference, were studied at 300 °C and 5 MPa. The liquid feed containing 18 wt % ethyl benzoate in n-hexane was introduced by a high-pressure pump into the reactor at a rate of 0.22 mL/min. The hydrogen flow rate was maintained at 100 mL/min by a mass flow controller during the experiment. Products were collected at intervals of 12 h and analyzed by GC and GC/mass spectrometry (MS). The results are shown in Figure 3. It can be seen that the total conversion of ethyl benzoate on the presulfided CoMo/γ-Al2O3 catalyst decreased from 95 to 73% in 48 h and then deactivated rapidly. In comparison to CoMo sulfide, Mo and NiMo carbides exhibited a higher activity and stability on HDO of ethyl benzoate. With Ni promotion, the stability of molybdenum carbide was improved further. Therefore, further experiments were performed with NiMo carbide to determine the potential usefulness of it on HDO. To obtain reaction intermediates and identify the product distributions with time on stream, experiments were carried out with a liquid feed containing 25 wt % ethyl benzoate in n-hexane and NiMo carbide under the present conditions. Table 2 shows the reaction products detected by GC/MS. The hypothesized reaction network was depicted according to intermediates in Scheme 1. The conversion of ethyl benzoate and the selectivity of the products at different reaction times are listed in Table 3. It can be seen that NiMo carbide exhibited high initial activities for HDO of ethyl benzoate. With the increase of the reaction time, the total conversion of ethyl benzoate decreased rapidly from 98% at 24 h to 71% at 48 h and then decreased gradually. In comparison to the total conversion, the HDO conversion decreased more rapidly because of the formation of oxygencontaining intermediates. The selectivity of cyclohexane and
sample
carbon content (wt %)
Mo/C
SBET (m2/g)
CoMo sulfide passivated Mo carbide passivated NiMo carbide used NiMo carbide
1.28 0.59 1.21
1.94 2.12 1.03
172.64 169.36 191.49 80.14
Figure 2. TPO profiles of (a) passivated Mo carbide, (b) passivated NiMo carbide, and (c) used NiMo carbide.
weight loss up to 150 °C, because of the desorption of H2O that adsorbed on the surface of carbides. Above 150 °C, the weight of Mo and NiMo carbides begins to rise because of the oxidation of lattice carbon. As shown in Figure 2, the Mo carbide shows one-stage oxidation between 150 and 510 °C and the NiMo carbide shows an oxidation between 150 and 390 °C. Both Mo and NiMo carbides show a broad stage of oxidation. This may be due to the limitation of the pore structure, which leads the oxidation of carbides to be a (19) Bouchy, C.; Pham-Huu, C.; Heinrich, B.; Derouane, E. G.; Derouane-Abd Hamid, S. B.; Ledoux, M. J. Appl. Catal., A 2001, 215, 175–184.
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Table 2. Products Detected in HDO Experiments of Ethyl Benzoate aliphatic products aromatic products
hydrocarbon
oxygen-containing compounds
cyclohexane, methylcyclohexane, 4-methyl-1-cyclohexene, ethylcyclohexane, 1-ethyl-2-methyl-cyclohexane, 1-ethyl-4-methyl-cyclohexane benzene, toluene, ethylbenzene, 1-ethyl-2-methyl-benzene, 1-ethyl-4-methyl-benzene
ethanol, cyclohexane-carboxylic acid, cyclohexane-methanol benzyl alcohol, benzoic acid
Scheme 1. Reaction Network of Ethyl Benzoate
methylcyclohexane (aliphatic products) decreased gradually and became negligible, with a simultaneous increase in selectivity of benzene and toluene (aromatic products). This suggests that, during HDO reactions of ethyl benzoate, the hydrogenating activity of NiMo carbide decreased with time on stream. With the deactivation of NiMo carbide, the selectivity of benzoic acid and benzyl alcohol increased, while the selectivity of cyclohexane-carboxylic acid and cyclohexanemethanol increased first and then decreased. With the increase of the reaction time, some alkylation products, which may be catalyzed by acid sites, including ethylbenzene, 1-ethyl-2-methylbenzene, ethylcyclohexane, 1-ethyl-2-methyl-cyclohexane, etc., were formed. Characterization of Used NiMo Carbide. SiO2-supported NiMo carbide was characterized after the HDO reactions with 25 wt % ethyl benzoate. As shown in Table 1, in comparison to the passivated sample, the used NiMo carbide contained more carbon, which may be ascribed to coke deposition during HDO reactions. Because of pore blockage by coke, the BET surface area of the used sample is apparently lower than that of the passivated sample. The TPO pattern of used NiMo carbide is given in Figure 2c. It can be seen that the used NiMo carbide also presents a weight increase between 150 and 390 °C, which indicates a maintenance of the basic carbide structure after HDO reactions. However, the weight loss, above 600 °C, indicates the existence of coke deposition formed during HDO reactions. This was in good accordance with the results of elemental analysis. The fresh molybdenum-based carbides are active and readily oxidized by oxygen. To prevent the bulk oxidation, a passivation step is required before these materials are exposed to air. Because of their high reactivity with oxygen, it is possible that oxygen in the model molecule will
Table 3. Total Conversion of Ethyl Benzoate and Selectivity to Different Products over NiMo Carbide reaction time (h) conversion (%) total HDO selectivity (%) cyclohexane and methylcyclohexane benzene and toluene cyclohexane-carboxylic acid benzoic acid cyclohexane-methanol benzyl alcohol others
12
24
36
48
60
72
99 99
98 91.7
85 73.5
71 59.6
70 61.3
67 57.2
95.2
74.9
37.3
trace
trace
trace
1.8 trace
12.4 3.3
42.1 5.4
75.7 trace
81.2 trace
77.9 trace
1.3 3.4 trace 10.5
9.1 trace 5.9 9.3
7.3
8.4
3
trace 2.9 trace 6.5
4.2 7.3
4.7 9
accumulate on the surface of the carbide during HDO reactions. H2-TPR profiles of the passivated, used NiMo carbide together with the corresponding oxide precursor are shown in Figure 4. The oxide precursor shows a three-peak spectrum. The peaks at 535 and 770 °C may be assigned to the reduction of NiMoO4,20 while the shoulder peak at about 630 °C may be due to the reduction of some intermediate species. Both passivated and used samples show a broad peak at a range of 100-650 °C in the H2-TPR spectrum, which may be assigned to the reduction of surface oxygen. For the used sample, there is a dip in the baseline at high temperatures. This may be due to a low level desorption of H2, which was adsorbed on the surface of the used sample. The similarity of the spectrum of passivated and used samples indicates that, during the HDO reactions, oxygen (20) Rodriguez, J. A.; Chaturvedi, S.; Hanson, J. C. J. Phys. Chem. B 1999, 103, 770–781.
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Figure 5. NH3-TPD profiles of used NiMo carbide.
Figure 4. H2-TPR spectra of passivated and used carbides as well as corresponding oxide precursor: (a) NiMo oxide, (b) passivated, and (c) used.
important role on HDO reactions of acetone and acetaldehyde. HDO of Acetone over NiMo Carbide. The HDO reactions of acetone over NiMo carbide were studied with pure acetone as liquid feed. Prior to the reaction test, the fresh NiMo carbide was pretreated in situ at 300 °C and 3 MPa with a liquid feed rate of 0.3 mL/min. After the activity of the catalyst reached stability, the test started. The effect of the temperature on HDO of acetone over NiMo carbide was investigated at 3 MPa and a temperature range of 220-300 °C. To obtain reaction intermediates, acetone was introduced at a high rate of 0.3 mL/min. The hydrogen flow rate was kept at 100 mL/min. The reaction products detected by GC and GC/MS are as follows: methane, ethene, ethane, propylene, propane, isopropanol, 2-methyl-pentane, 2-pentanone, 2-ethoxy-propane, methyl isobutyl ketone, 4-methyl-2-pentanol, 2,6-dimethyl-3-heptene, 2,4-dimethyl-heptane, 2,6-dimethyl-heptane, 1,1,3-trimethylcyclohexane, 2-methyl-4-heptanone, 4-methyl-2-heptanone, 6-methyl-2-heptanol, 2,6-dimethyl-4-heptanone, 4,6-dimethyl2-heptanone, and 2,6-dimethyl-4-heptanol as well as some heavier ones. The proposed reaction network was depicted in Scheme 2. One major reaction pathway of acetone is direct hydrogenation, which is catalyzed by hydrogenating sites and yield isopropanol as a primary product. The self-aldol condensation of acetone is another pathway. In this pathway, acetone undergoes self-aldol condensation on acid sites to form an aldol alcohol, which then undergoes dehydration on acid sites to form an R-β unsaturated ketone, followed by hydrogenation to yield methyl isobutyl ketone. The methyl isobutyl ketone may undergo further aldol condensation with acetone to yield a C9 ketone. These ketones undergo hydrogenation and dehydration to yield corresponding alkene and water. The alkene hydrogenates further to yield corresponding alkane. In addition, there were a little amount of ethene, ethane, as well as 2-ethoxy-propane that may have resulted from the pathway of decarbonylation of acetone. Table 4 shows the effect of the temperature on the total conversion of acetone and distribution of products. As expected, the total conversion of acetone increased from 68.5 to 75.2% in the temperature range. With the increase of the temperature, the activity of dehydration catalyzed by acid sites increased and lead to a rapid increase of HDO conversion and C3 selectivity. Meanwhile, the selectivity of
accumulation occurred on the surface of NiMo carbide. This result is in agreement with the proposal. It was reported by Yuanzhi et al.21 that the oxidation states and their relative content of Mo species on the surface of molybdenum nitride could be changed by electronic modification by oxygen, which played a decisive role on the change of selectivity and activity for hydrogenation. The Mo species with a lower oxidation state had higher hydrogenation activity than Mo species with a higher oxidation state. During the HDO reactions, the total conversion of ethyl benzoate decreased from 98% at 24 h to 71% at 48 h, while the selectivity of aliphatic products decreased gradually and aromatic products increased correspondingly. This result may be ascribed to the oxygen accumulation on the surface of NiMo carbide during HDO reactions, which leads to the change of the metallic oxidation state and then affected the activity of NiMo carbide. The coke deposition, during HDO reactions, may also be responsible for the changes of the catalytic activity of NiMo carbide. The oxygen accumulation would lead to the formation of oxygen-modified NiMo carbide. The oxynitrides and oxycarbides are expected to be useful catalysts because of their bifunctional properties.22-24 Miga et al. reported the bifunctional catalytic sites of metallic sites (Mo atom) and acid sites (Mo-OH group) over Mo oxynitride.25 Nagai et al. observed the existence of Bronsted as well as Lewis acid sites on the surface of the passivated Mo nitride.26 In this study, the NH3-TPD profile of the used sample is presented in Figure 5. It can be seen that the ammonia desorption spectra exhibits a strong peak and two shoulder peaks at about 160, 240, and 310 °C, respectively, which indicates the existence of weak acid sites on the surface of the catalyst. The following study indicates that these acid sites play an (21) Yuanzhi, L.; Yining, F.; Jie, H.; Boling, X.; Hanpei, Y.; Jianwen, M.; Yi, C. Chem. Eng. J. 2004, 99, 213–218. (22) Nagai, M. Appl. Catal., A 2007, 322, 178–190. (23) Lamic, A.-F.; Pham, T. L. H.; Potvin, C.; Manoli, J.-M.; Djega-Mariadassou, G. J. Mol. Catal. A: Chem. 2005, 237, 109–114. (24) Santos, J. B. O.; Valenca, G. P.; Rodrigues, J. A. J. J. Catal. 2002, 210, 1–6. (25) Miga, K.; Stanczyk, K.; Sayag, C.; Brodzki, D.; Djega-Mariadassou, G. J. Catal. 1999, 183, 63–68. (26) Nagai, M.; Goto, Y.; Irisawa, A.; Omi, S. J. Catal. 2000, 191, 128–137.
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Scheme 2. Reaction Network of Acetone
Table 4. Effect of the Temperature on the Total Conversion of Acetone and Products Distribution over the NiMo Carbide selectivity (%)a reaction temperature (°C)
conversion (%)
HDO conversion (%)
C3
IP
oxygenates (C6)
oxygenates (C9)
aliphatic hydrocarbon (C6 þ C9)
others
220 240 260 280 300
68.5 68.3 71.9 73.6 75.2
3.0 7.8 20.9 36.0 43.6
2.3 5.8 21.6 39.3 45.1
53.6 40.5 17.7 5.1 1.9
29.7 33.5 36.2 32.1 26.4
6.9 8.9 9.7 8.8 8.1
2.1 5.6 7.5 9.6 12.9
5.4 5.7 7.3 5.1 5.6
a C3 is propene and propane. IP is isopropanol. C6 oxygenates are methyl isobutyl ketone and 4-methyl-2-pentanol. C9 oxygenates are 2,6-dimethyl-4heptanone, 4,6-dimethyl-2-heptanone, and 2,6-dimethyl-4-heptanol. Aliphatic hydrocarbons (C6 þ C9) are 2-methyl-pentane, 2,6-dimethyl-3-heptene, 2,4-dimethyl-heptane, 2,6-dimethyl-heptane, and 1,1,3-trimethyl-cyclohexane. Others are mainly over-condensation products.
isopropanol decreased rapidly. The self-aldol condensation of acetone was also promoted by the increase of the temperature. The selectivity of C6 and C9 oxygenates increased and reached a maximum at 260 °C. Beyond 260 °C, the selectivity of C6 and C9 oxygenates decreased, which may be attributed to the enhanced formation of corresponding aliphatic hydrocarbons. The system pressure also showed a significant impact on the HDO of acetone. Experiments were carried out under the following conditions: 300 °C, 0.1-4 MPa, liquid feeding rate of 0.3 mL/min, and hydrogen flow rate of 100 mL/min. As presented in Table 5, both the total conversion and HDO conversion increased with the increase of the pressure. Because of almost complete dehydration, the selectivity of isopropanol was kept at low values all along and the
selectivity of C3 increased with the increase of the acetone conversion. With the increase of the pressure, the selectivity of C6 and C9 oxygenates decreased, which may be also attributed to the enhanced formation of corresponding aliphatic hydrocarbons. HDO of Acetaldehyde over NiMo Carbide. The effect of the temperature and system pressure on the conversion of acetaldehyde over NiMo carbide was also studied with 40 wt % acetaldehyde solution as liquid feed under the same conditions adopted in the HDO of acetone. According to the results of GC and GC/MS, reaction products were identified as methane, ethene, ethane, ethanol, ethanol acetate, acetic acid, 1-butanol, 1,1-diethoxy-ethane, butyl acetate, ethyl butyrate, and 1,1-diethoxy-butane. The proposed reaction network was depicted in Scheme 3. Similar to 2057
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Table 5. Effect of the Pressure on the Total Conversion of Acetone and Products Distribution over the NiMo Carbide selectivity (%) pressure (MPa)
conversion (%)
HDO conversion (%)
C3
IP
oxygenates (C6)
oxygenates (C9)
aliphatic hydrocarbon (C6 þ C9)
others
0.1 1 2 3 4
40.1 63.4 69.2 75.2 83.4
8.5 23.2 34.4 43.6 52.5
13.9 26.5 40.4 45.1 49.3
0.4 1.3 1.8 1.9 2.6
39.9 35.6 29.1 26.4 21.2
25.8 18.2 11.7 8.1 6.9
7.4 10.1 9.3 12.9 13.7
12.6 8.3 7.7 5.6 6.3
Scheme 3. Reaction Network of Acetaldehyde
Table 6. Effect of the Temperature on the Total Conversion of Acetaldehyde and Products Distribution over the NiMo Carbide selectivity (%)a reaction temperature (°C)
conversion (%)
HDO conversion (%)
C2
ethanol
ethanol acetate
1,1-diethoxy-ethane
others
220 240 260 280 300
47.3 65.3 85.9 87.7 91.4
0.3 1.4 8.9 19.7 33.3
0.6 2.1 10.4 22.5 36.4
88.5 84.8 73.5 61.1 45.1
3.8 4.5 5.7 5.8 6.3
2.9 2.6 3.3 3.2 5.3
4.2 6.0 7.1 7.4 6.9
a
C2 is ethene and ethane.
acetone, the major step involved in the HDO of acetaldehyde is the direct hydrogenation to form alcohol as a primary product. The self-aldol condensation of acetaldehyde, which is catalyzed by acid sites, is also existed as a side reaction. In this pathway, acetaldehyde undergoes self-aldol condensation to form an aldol alcohol, which then undergoes dehydration and hydrogenation to yield butyl aldehyde. Methane may be formed from the decarbonylation of acetaldehyde. In addition, acetaldehyde and butyl aldehyde may react with water to produce acetic and butyric acids, which then react with ethanol and butanol to form a corresponding ester. Table 6 shows the effect of the temperature on the conversion of acetaldehyde and distribution of products. It can be seen that the total conversion of acetaldehyde increased with an increase of the reaction temperature. Similar to the HDO of acetone, the HDO conversion of acetaldehyde increased more rapidly than the total conversion because
of the enhanced dehydration activity of carbide with the reaction temperature. During HDO of acetaldehyde, there are only two main products: C2 and ethanol. With the increase of the temperature, the selectivity of ethanol decreased with a simultaneous increase in selectivity of C2. Some ethanol was free of dehydration and remained at high temperatures. This indicates that NiMo carbide only possesses a weak acidity and presents a low activity for the dehydration of ethanol. Apart from C2 and ethanol, there are two major side products: ethanol acetate and 1,1-diethoxy-ethane. The total selectivity of ethanol acetate and 1,1-diethoxy-ethane was around 10% and increased slightly with the temperature. The effect of the system pressure on the HDO of acetaldehyde was also investigated. As shown in Table 7, with the increase of the pressure, the total conversion of acetaldehyde increased greatly. In comparison to the temperature, the pressure has less of an effect on the HDO conversion of acetaldehyde and 2058
Energy Fuels 2010, 24, 2052–2059
: DOI:10.1021/ef901222z
Zhang et al.
Table 7. Effect of the Pressure on the Total Conversion of Acetaldehyde and Products Distribution over the NiMo Carbide selectivity (%) pressure (MPa)
conversion (%)
HDO conversion (%)
C2
ethanol
ethanol acetate
1,1-diethoxy-ethane
others
0.1 1 2 3 4
60.1 68.8 87.9 91.4 95.2
29.7 30.1 33.8 33.3 34.1
49.5 43.7 38.5 36.4 35.8
29.1 37.7 42.5 45.1 47.9
7.7 6.5 7.1 6.3 5.9
6.4 6.3 5.4 5.3 5.1
7.3 5.8 6.5 6.9 5.3
Conclusion In this research, bimetallic NiMo carbide supported on SiO2 has been synthesized by a temperature-programmed reaction. Elemental analysis and TPO characterization indicated that, after carburization, carbon was successfully introduced into the lattice of NiMo carbide. On the basis of the HDO evaluation of ethyl benzoate and characterization of passivated and used samples, it can be deduced that the changes of catalytic activity of NiMo carbide, during HDO reactions, may be ascribed to oxygen accumulation as well as coke deposition on the surface of the catalyst. The oxygen accumulation will lead to the formation of oxygen-modified NiMo carbide, a new type of bifunctional catalyst. NH3-TPD measurement indicated the existence of weak acid sites over this oxygen-modified NiMo carbide. HDO reactions of acetone and acetaldehyde were also investigated over NiMo carbide. According to the proposed reaction network, the acid sites on the surface of the NiMo carbide played an important role on the HDO reactions of acetone and acetaldehyde. Bio-oil is a complex mixture including hundreds of organic oxygenates. Complete HDO of bio-oil means a high consumption of hydrogen. In this study, NiMo carbide presents a high activity and stability on the HDO of acetone and acetaldehyde. This may provide a way for the stabilization of bio-oil with lower H2 consumption by incomplete HDO, in which active oxygen-containing groups will be removed under relatively moderate conditions.
Figure 6. Stability test of NiMo carbide for the HDO of acetone and acetaldehyde.
the total selectivity of ethanol acetate and 1,1-diethoxyethane. The selectivity of ethanol increased and C2 decreased with the increase of acetaldehyde conversion. Stability Test. The stability test of NiMo carbide was carried out at 300 °C, 3 MPa, liquid feeding rate of 0.3 mL/min, and hydrogen flow rate of 100 mL/min. The liquid feed contained 50 wt % acetone, 20 wt % acetaldehyde, and 30 wt % water. The results, displayed in Figure 6, demonstrate that NiMo carbide is an active and stable catalyst for the HDO of acetone and acetaldehyde under the present conditions.
Acknowledgment. We are grateful to the National Key Technology R&D Program (2007BAB24B04) and The Innovation Program of Institute of Coal Chemistry, Chinese Academy of Sciences.
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