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Nickel and potassium co-modified β-Mo2C catalyst for CO conversion
Journal of Natural Gas Chemistry 18(2009)183–186
Nickel and potassium co-modified β -Mo2C catalyst for CO conversion Minglin Xiang1,2∗ , Juan Zou1,2 , Debao Li2 ,
Wenhuai Li2 ,
Yuhan Sun2∗ ,
Xichun She1
1. Hunan Changling Petrochemical S&T Developing Co. Ltd, Yueyang 414012, Hunan, China; 2. State Key Laboratory of Coal Conversion, Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China [ Received January 7, 2009; Revised February 26, 2009; Available online June 11, 2009 ]
Abstract Nickel and potassium co-modified β-Mo2 C catalysts were prepared and used for CO hydrogenation reaction. The major products over β-Mo2 C were C1 –C4 hydrocarbons, only few alcohols were obtained. Addition of potassium resulted in remarkable selectivity shift from hydrocarbons to alcohols at the expense of CO conversion over β-Mo2 C. Moreover, it was found that potassium enhanced the ability of chain propagation with a higher C2+ OH production. Modified by nickel, β-Mo2 C showed a relatively high CO conversion, however, the products were similar to those of pure β-Mo2 C. When co-modified by nickel and potassium, β-Mo2 C exhibited high activity and selectivity towards mixed alcohols synthesis, and also the whole chain propagation to produce alcohols especially for the stage of C1 OH to C2 OH was remarkably enhanced. It was concluded that the Ni and K had, to some extent, synergistic effect on CO conversion. Key words β-Mo2 C; nickel; potassium; CO conversion
1. Introduction Considerable interest remains in CO hydrogenation reactions for either the synthesis of hydrocarbons, methanol or higher alcohols. In these synthesis reactions, efforts have been made to limit the selectivity to methane in preference to other products such as higher alcohols, heavy hydrocarbons or alkenes. Although there are several catalytic systems developed for CO hydrogenation, they still suffer from the lack of selectivity and activity. Thus, the study of new catalytic material for CO conversion is still a good pursuit. In the field of C1 chemistry, the study of molybdenum carbide has attracted more attention because of its recent development. Molybdenum carbide exhibits catalytic properties similar to those of noble metals, and it has been shown to be promising catalysts for CO hydrogenation [1−3]. According to the literature, methane and CO2 are the major products found under the reaction conditions of 573 K and 2.0 MPa over the molybdenum carbides [1]. The authors also analyze for possible CO2 in the product stream and so observe the water gas shift activity. Lee and coworkers [3,4] have studied the effect of potassium promotion on CO hydrogenation over molybdenum carbide catalysts. The results also show that in the absence of potassium mainly C1 –C5 paraffins (in ∗
which methane is the major product) are produced by molybdenum carbide, whereas at 1.0 MPa an increased selectivity to C2 –C5 olefins is observed with the addition of potassium. Increasing the pressure to 8.0 MPa an improved selectivity to mixed alcohols synthesis in the range of C1 –C7 is obtained, and these enhanced selectivities, however, are achieved at the expense of CO conversions. Our previous studies also reveal that molybdenum carbides (both β-Mo2 C and α-MoC1−x ) produce mainly light hydrocarbons, together with little alcohols under conditions of 573 K, 8.0 MPa, GHSV = 2000 h−1 , and n(H2 )/n(CO) = 1.0. Addition of potassium as a promoter, over both β-Mo2 C and α-MoC1−x , results in remarkable selectivity shift from hydrocarbons to alcohols [5]. GribovalConstant et al. [2] have studied the cobalt or ruthenium promoted molybdenum carbide for Fischer-Tropsch reaction. The results reveal that molybdenum carbide gave light hydrocarbons, alcohols and CO2 , and the addition of Ru decreased the alcohol production whereas Co increased the formation of heavy hydrocarbons. Therefore, the distribution of products over the molybdenum carbide can be controlled and tuned by addition of different promoters; it appears that molybdenum carbides could be potential catalysts for CO conversion. In this paper, nickel and potassium co-modified β-Mo2 C were prepared and studied for CO conversion. The results revealed that nickel and potassium rendered the catalysts highly
Corresponding author. Tel: +86-730-8452441; Fax: +86-730-8478962; E-mail: [email protected]; [email protected] This work was supported by the National Key Project for Basic Research of China (973 Project) (No. 2005CB221400)
Copyright©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(08)60103-6
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active and selective towards the synthesis of mixed alcohols, especially for the C2+ OH production. 2. Experimental 2.1. Sample preparation The preparation of molybdenum carbide could be found elsewhere [5,6]. Typically, β-Mo2 C and Ni/β-Mo2 C (Ni/Mo = 1/8) with the hexagonal close packed (HCP) structure were prepared by direct carburization of MoO3 and NiMo oxide precursor through temperature-programmed-reaction (TPR) method, respectively. The NiMo oxide precursor for NiMo bimetallic carbide was prepared by mechanically mixing a stoichiometric amount of Ni(NO3 )2 ·6H2 O with (NH4 )6 Mo7 O24 ·4H2 O, and this mixture was then calcined at 773 K for 4 h [7]. TPR was carried out under atmospheric pressure in a flow of 20 v/v% CH4 /H2 gas mixture. The temperature was linearly increased at the rate of 60 K/h from room temperature to 973 K. Then the samples were quenched to room temperature in a flow of argon and gradually passivated with 1.0 v/v% O2 /N2 before exposure to air. K2 CO3 modification (K/Mo = 1/5) was accomplished by a post-doping procedure in which K2 CO3 was added after the synthesis of the final carbide [8]. The alkalinization was carried out by physically mixing K2 CO3 with the final carbide, which was calcined at 773 K.
Mo2 C-based catalysts all had definitive phase of the HCP β-Mo2 C [9] (2θ = 34.4o, 38.0o, 39.4o, 52.1o, 61.5o, 69.6o, 74.6o and 76.0o for β-Mo2 C [100], [002], [101], [102], [110], [103], [112] and [201], respectively). Modified by potassium, the intensity of peaks assigned to β-Mo2 C weakened, yet new diffraction peaks at 2θ values of 49.5o and 66.7o appeared; “K-Mo-C” entities could be envisaged in our case, and it was also believed that “K-Mo-C” entities were analogous to “KMo-S” in the K/MoS2 catalysts, but more studies are needed. Modified by nickel, diffraction peaks corresponding to the metallic Ni [10] were detected, with 2θ values at 44.5o and 72.9o respectively. However, co-modified by potassium and nickel, the intensity of peaks assigned to β-Mo2 C was weakened, and the intensity of peaks assigned to “K-Mo-C” entities or nickel species became weaker simultaneously.
2.2. Characterization methods X-ray powder diffraction (XRD) patterns of the tested catalysts were obtained on a Rigaku D/Max 2500 powder diffractometer using Cu Kα radiation as the X-ray source.
Figure 1. XRD patterns of the catalysts. (1) β-Mo2 C, (2) K/β-Mo2 C, (3) Ni/β-Mo2 C, (4) K/Ni/β-Mo2 C
2.3. CO hydrogenation
3.2. Ef fect of potassium and nickel on the performance of CO conversion
The CO hydrogenation reaction was conducted with a fixed-bed, stainless steel micro-reactor with 2.0 ml of catalysts (pellets, 40−60 mesh). All catalysts were used without any pretreatment. The products were analyzed by 1790-GC, H2 , CO, CH4 and CO2 were analyzed by thermal conductivity detector (TCD) equipped with a TDX-101 column; the water and methanol in liquids were also detected by TCD with a GDX-401 column; the alcohols and hydrocarbons were analyzed by flame ionization detector (FID) with a Porapak-Q column. The mass balance was based on carbon and the error of the balance of oxygen and hydrogen was within 5%. The activity of synthesis of alcohols was expressed as space-timeyield (STY), g/L-cat·h. 3. Results and discussion 3.1. Structural properties of β-Mo2 C-based catalysts The XRD pattern (see Figure 1) showed that β-
The measured stable catalytic activity for CO conversion after 72 h is shown in Table 1 and Figure 2. It could be seen that for β-Mo2 C catalyst, the CO conversion was much higher than that of K/β-Mo2 C, and hydrocarbons were the main products. Only few alcohols was obtained. Among the liquid products, the methanol was the main one. Another feature of all β-Mo2 C-based catalyst was high carbon dioxide yields (∼50 C%) instead of water as a carbon monoxide product, which represented high water gas shift activities of molybdenum carbide as frequently reported for unpromoted catalysts [1,11,12]. Modified by potassium, the CO conversion over βMo2 C catalyst was sharply reduced from 58.6 C% to 23.4 C%, yet the products shifted remarkably from hydrocarbons to alcohols, and the STY of alcohol reached 122.1 g/(L·h). Among the alcohols distribution, the methanol was suppressed and the C2+ OH production was greatly increased. Combined with XRD analysis, it was seen that “K-Mo-C” entities might favor the alcohols synthesis. Modified by nickel, the CO conversion reached 88.1 C%. The main products still were C1 –C4
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hydrocarbons, and only few alcohol was formed, which was similar to those on pure β-Mo2 C. However, co-modified by nickel and potassium, the β-Mo2 C catalyst showed highly active and selective towards mixed alcohols synthesis in CO hydrogenation. The CO conversion achieved 73.0 C%, which was even much higher than that of pure β-Mo2 C, and the STY of alcohol was 324.2 g/(L·h), the selectivity to alcohols slightly decreased whereas hydrocarbons increased compared with those on K/β-Mo2 C catalyst. Thus, high activity of CO conversion over K/Ni/β-Mo2 C catalyst might be attributed to the Ni species in the sample, but more study is needed. It could be concluded that in the presence of Ni and K, the βMo2 C showed highly active and selective to higher alcohols synthesis, and the Ni and K promoter might have synergistic effect on CO conversion. Table 1. Catalytic performance of β -Mo2 C-based catalysts in CO hydrogenationa Catalyst β-Mo2 C K/β-Mo2 C Ni/β-Mo2 C K/Ni/β-Mo2 C
CO conv. (C%) 58.6 23.4 88.1 73.0
Alcohol STY (g/L·h) 15.7 122.1 23.5 324.2
Selectivity (C%) ROH CO2 CHx 1.8 51.8 46.4 26.5 49.6 23.9 1.2 45.7 53.1 23.3 50.9 25.8
a Reaction conditions: T = 573 K, p = 8.0 MPa, n(H )/n(CO) =1.0, 2 GHSV = 2000 h−1
Figure 2. Distribution of alcohols for β-Mo2 C-based catalysts
The change in CO conversion and alcohol selectivity with time on stream is shown in Figure 3 for potassium modified β-Mo2 C as an example. Other catalysts also showed similar trends. Initially, the catalysts showed high CO conversion and relatively low selectivity to alcohols. The CO conversion decreased and alcohol selectivity increased with time on stream, and a steady state was obtained after 72 h. Generally, molybdenum-based catalysts for mixed alcohols synthesis all tended to have an induction period during the CO hydrogenation. For molybdenum carbide, a part of the surface oxygen on the catalyst probably reacted with carbidic carbon and was removed from the uppermost surface layer during this induction period [2]. It was also believed that during the induction, some surface carbon atoms of carbide and some deposited carbon species were eliminated [13].
Figure 3. Changes in CO conversion and alcohol selectivity over K/βMo2 C with time on stream (selectivity calculated from CO2 free basis)
3.3. Ef fect of potassium and nickel on chain propagation The distributions of alcohols and hydrocarbons over the β-Mo2 C-based catalysts are shown in Figure 4, as an Anderson-Schultz-Flory (A-S-F) distribution: Yn = P n−1 (1−P ), where Yn was the mole fraction of the alcohol with a carbon number of n and P was the chain growth probability [14]. For β-Mo2 C catalyst, the distribution of hydrocarbons and alcohols had similar linear A-S-F plots, the chain-growth probability P reached 0.62 and 0.26, respectively. Modified by potassium, two different types of plots were obtained. The hydrocarbons still obeyed the linear A-S-F plot, and the chain-growth probability P of hydrocarbons dropped remarkably from 0.62 to 0.48. However, the alcohols showed a remarkable deviation for methanol, indicating a different reaction mechanism for alcohols formation over K/β-Mo2 C catalysts. Except for methanol, it was worth noting that the C2 –C4 alcohols still followed the linear distribution plot and the chain-growth probability P of 0.29 was obtained, which was higher than that of β-Mo2 C catalyst. It meant that though potassium promoter exerted an effective function on the whole chain propagation to produce alcohols, the main difference of reaction mechanism over β-Mo2 C and K/β-Mo2 C catalyst lied in chain propagation of C1 OH to C2 OH. For K modified β-Mo2 C catalyst, the formation of “K-Mo-C” phase was considered to be the active step for the alcohols synthesis, which exerted a prominent function on the whole chain propagation to produce alcohols especially for C1 OH to C2 OH. Lee and co-workers also considered that the K2 CO3 promoter readily removed its counter anions under the reaction conditions to form a new complex which could be assigned to a carboxylic species, and this carboxylic species was proposed to be an active intermediate for alcohol synthesis from CO-H2 [4]. Modified by nickel, the distributions of alcohols and hydrocarbons were similar to those on pure β-Mo2 C, namely, linear A-S-F plots were obtained. However, the ability of chain propagation was greatly reduced, 0.11 and 0.33 for alcohols and hydrocarbons, respectively. As known to all, Ni was an excellent methanation promoter, combined
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with XRD results, the nickel species might be responsible for the high methane and methanol content. When co-modified by nickel and potassium, β-Mo2 C showed similar types of AS-F plots of hydrocarbons and alcohols compared with those on K modified β-Mo2 C catalyst. However, the chain-growth probability P of hydrocarbons was 0.58, which was much higher than that on K modified β-Mo2 C catalyst, and was comparable to that on β-Mo2 C catalysts. It could also be seen
that methanol was further suppressed to 25.8 C% (see Table 1), the P of alcohols greatly increased to 0.44, which was much higher than those of other catalysts. Compared with K or Ni modified β-Mo2 C catalyst, the Ni and K co-modified β-Mo2 C enhanced the ability of formation of alcohols and hydrocarbons simultaneously, and again this would suggest that the Ni and K promoters have synergistic effect on CO conversion.
Figure 4. A-S-F plots of alcohols and hydrocarbons over (a) β-Mo2 C, (b) K/β-Mo2 C, (c) Ni/β-Mo2 C and (d) K/Ni/β-Mo2 C catalysts. (P denotes the probability of chain growth)
4. Conclusions Novel Ni and K co-modified β-Mo2 C catalysts were prepared through TPR method and tested for CO conversion. Potassium shifted the products over β-Mo2 C remarkably from hydrocarbons to alcohols, whereas nickel changed β-Mo2 C to a methanation catalyst together with a high CO conversion. However, co-modified by nickel and potassium, βMo2 C showed highly active and selective for alcohols synthesis, and the STY of alcohol was greatly increased. It was concluded that nickel and potassium promoter might have synergistic effect on CO conversion. References [1] Patterson P M, Das T K, Davis B H. Appl Catal A: Gen, 2003, 251: 449
[2] Griboval-Constant A, Giraudon J M, Leclercq G, Leclercq L. Appl Catal A: Gen, 2004, 260: 35 [3] Woo H C, Park K Y, Kim Y G, Namau I S, Chung J S, Lee J S. Appl Catal, 1991, 75: 267 [4] Lee J S, Kim S, Kim Y G. Top Catal, 1995, 2: 127 [5] Xiang M, Li D, Li W, Zhong B, Sun Y. Fuel, 2006, 85: 2662 [6] Lee J S, Oyama S T, Boudart M. J Catal, 1987, 106: 125 [7] Xiao T C, York A P E , Al-Megren H, Williams C V, Wang H T, Green M L H. J Catal, 2001, 202: 100 [8] Kojimi R, Aika K. Appl Catal A: Gen, 2001, 219: 141 [9] Oshikawa K, Nagai M, Omi S. J Phys Chem B, 2001, 105: 9124 [10] Nagai M, Zahidul A M, Matsuda K. Appl Catal A, 2006, 313: 137 [11] Saito M, Anderson R B. J Catal, 1980, 63: 438 [12] Kijima I, Miyazaki E. J Catal, 1984, 89: 168 [13] Wu W C, Wu Z L, Liang C H, Chen X W, Ying P L, Li C. J Phys Chem B, 2003, 107: 7088 [14] Li X G, Feng L J, Liu Z Y, Zhong B, Dadyburjor D B, Kugler E L. Ind Eng Chem Res, 1998, 37: 3853
Nickel and potassium co-modified β -Mo2C catalyst for CO conversion Minglin Xiang1,2∗ , Juan Zou1,2 , Debao Li2 ,
Wenhuai Li2 ,
Yuhan Sun2∗ ,
Xichun She1
1. Hunan Changling Petrochemical S&T Developing Co. Ltd, Yueyang 414012, Hunan, China; 2. State Key Laboratory of Coal Conversion, Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China [ Received January 7, 2009; Revised February 26, 2009; Available online June 11, 2009 ]
Abstract Nickel and potassium co-modified β-Mo2 C catalysts were prepared and used for CO hydrogenation reaction. The major products over β-Mo2 C were C1 –C4 hydrocarbons, only few alcohols were obtained. Addition of potassium resulted in remarkable selectivity shift from hydrocarbons to alcohols at the expense of CO conversion over β-Mo2 C. Moreover, it was found that potassium enhanced the ability of chain propagation with a higher C2+ OH production. Modified by nickel, β-Mo2 C showed a relatively high CO conversion, however, the products were similar to those of pure β-Mo2 C. When co-modified by nickel and potassium, β-Mo2 C exhibited high activity and selectivity towards mixed alcohols synthesis, and also the whole chain propagation to produce alcohols especially for the stage of C1 OH to C2 OH was remarkably enhanced. It was concluded that the Ni and K had, to some extent, synergistic effect on CO conversion. Key words β-Mo2 C; nickel; potassium; CO conversion
1. Introduction Considerable interest remains in CO hydrogenation reactions for either the synthesis of hydrocarbons, methanol or higher alcohols. In these synthesis reactions, efforts have been made to limit the selectivity to methane in preference to other products such as higher alcohols, heavy hydrocarbons or alkenes. Although there are several catalytic systems developed for CO hydrogenation, they still suffer from the lack of selectivity and activity. Thus, the study of new catalytic material for CO conversion is still a good pursuit. In the field of C1 chemistry, the study of molybdenum carbide has attracted more attention because of its recent development. Molybdenum carbide exhibits catalytic properties similar to those of noble metals, and it has been shown to be promising catalysts for CO hydrogenation [1−3]. According to the literature, methane and CO2 are the major products found under the reaction conditions of 573 K and 2.0 MPa over the molybdenum carbides [1]. The authors also analyze for possible CO2 in the product stream and so observe the water gas shift activity. Lee and coworkers [3,4] have studied the effect of potassium promotion on CO hydrogenation over molybdenum carbide catalysts. The results also show that in the absence of potassium mainly C1 –C5 paraffins (in ∗
which methane is the major product) are produced by molybdenum carbide, whereas at 1.0 MPa an increased selectivity to C2 –C5 olefins is observed with the addition of potassium. Increasing the pressure to 8.0 MPa an improved selectivity to mixed alcohols synthesis in the range of C1 –C7 is obtained, and these enhanced selectivities, however, are achieved at the expense of CO conversions. Our previous studies also reveal that molybdenum carbides (both β-Mo2 C and α-MoC1−x ) produce mainly light hydrocarbons, together with little alcohols under conditions of 573 K, 8.0 MPa, GHSV = 2000 h−1 , and n(H2 )/n(CO) = 1.0. Addition of potassium as a promoter, over both β-Mo2 C and α-MoC1−x , results in remarkable selectivity shift from hydrocarbons to alcohols [5]. GribovalConstant et al. [2] have studied the cobalt or ruthenium promoted molybdenum carbide for Fischer-Tropsch reaction. The results reveal that molybdenum carbide gave light hydrocarbons, alcohols and CO2 , and the addition of Ru decreased the alcohol production whereas Co increased the formation of heavy hydrocarbons. Therefore, the distribution of products over the molybdenum carbide can be controlled and tuned by addition of different promoters; it appears that molybdenum carbides could be potential catalysts for CO conversion. In this paper, nickel and potassium co-modified β-Mo2 C were prepared and studied for CO conversion. The results revealed that nickel and potassium rendered the catalysts highly
Corresponding author. Tel: +86-730-8452441; Fax: +86-730-8478962; E-mail: [email protected]; [email protected] This work was supported by the National Key Project for Basic Research of China (973 Project) (No. 2005CB221400)
Copyright©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(08)60103-6
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active and selective towards the synthesis of mixed alcohols, especially for the C2+ OH production. 2. Experimental 2.1. Sample preparation The preparation of molybdenum carbide could be found elsewhere [5,6]. Typically, β-Mo2 C and Ni/β-Mo2 C (Ni/Mo = 1/8) with the hexagonal close packed (HCP) structure were prepared by direct carburization of MoO3 and NiMo oxide precursor through temperature-programmed-reaction (TPR) method, respectively. The NiMo oxide precursor for NiMo bimetallic carbide was prepared by mechanically mixing a stoichiometric amount of Ni(NO3 )2 ·6H2 O with (NH4 )6 Mo7 O24 ·4H2 O, and this mixture was then calcined at 773 K for 4 h [7]. TPR was carried out under atmospheric pressure in a flow of 20 v/v% CH4 /H2 gas mixture. The temperature was linearly increased at the rate of 60 K/h from room temperature to 973 K. Then the samples were quenched to room temperature in a flow of argon and gradually passivated with 1.0 v/v% O2 /N2 before exposure to air. K2 CO3 modification (K/Mo = 1/5) was accomplished by a post-doping procedure in which K2 CO3 was added after the synthesis of the final carbide [8]. The alkalinization was carried out by physically mixing K2 CO3 with the final carbide, which was calcined at 773 K.
Mo2 C-based catalysts all had definitive phase of the HCP β-Mo2 C [9] (2θ = 34.4o, 38.0o, 39.4o, 52.1o, 61.5o, 69.6o, 74.6o and 76.0o for β-Mo2 C [100], [002], [101], [102], [110], [103], [112] and [201], respectively). Modified by potassium, the intensity of peaks assigned to β-Mo2 C weakened, yet new diffraction peaks at 2θ values of 49.5o and 66.7o appeared; “K-Mo-C” entities could be envisaged in our case, and it was also believed that “K-Mo-C” entities were analogous to “KMo-S” in the K/MoS2 catalysts, but more studies are needed. Modified by nickel, diffraction peaks corresponding to the metallic Ni [10] were detected, with 2θ values at 44.5o and 72.9o respectively. However, co-modified by potassium and nickel, the intensity of peaks assigned to β-Mo2 C was weakened, and the intensity of peaks assigned to “K-Mo-C” entities or nickel species became weaker simultaneously.
2.2. Characterization methods X-ray powder diffraction (XRD) patterns of the tested catalysts were obtained on a Rigaku D/Max 2500 powder diffractometer using Cu Kα radiation as the X-ray source.
Figure 1. XRD patterns of the catalysts. (1) β-Mo2 C, (2) K/β-Mo2 C, (3) Ni/β-Mo2 C, (4) K/Ni/β-Mo2 C
2.3. CO hydrogenation
3.2. Ef fect of potassium and nickel on the performance of CO conversion
The CO hydrogenation reaction was conducted with a fixed-bed, stainless steel micro-reactor with 2.0 ml of catalysts (pellets, 40−60 mesh). All catalysts were used without any pretreatment. The products were analyzed by 1790-GC, H2 , CO, CH4 and CO2 were analyzed by thermal conductivity detector (TCD) equipped with a TDX-101 column; the water and methanol in liquids were also detected by TCD with a GDX-401 column; the alcohols and hydrocarbons were analyzed by flame ionization detector (FID) with a Porapak-Q column. The mass balance was based on carbon and the error of the balance of oxygen and hydrogen was within 5%. The activity of synthesis of alcohols was expressed as space-timeyield (STY), g/L-cat·h. 3. Results and discussion 3.1. Structural properties of β-Mo2 C-based catalysts The XRD pattern (see Figure 1) showed that β-
The measured stable catalytic activity for CO conversion after 72 h is shown in Table 1 and Figure 2. It could be seen that for β-Mo2 C catalyst, the CO conversion was much higher than that of K/β-Mo2 C, and hydrocarbons were the main products. Only few alcohols was obtained. Among the liquid products, the methanol was the main one. Another feature of all β-Mo2 C-based catalyst was high carbon dioxide yields (∼50 C%) instead of water as a carbon monoxide product, which represented high water gas shift activities of molybdenum carbide as frequently reported for unpromoted catalysts [1,11,12]. Modified by potassium, the CO conversion over βMo2 C catalyst was sharply reduced from 58.6 C% to 23.4 C%, yet the products shifted remarkably from hydrocarbons to alcohols, and the STY of alcohol reached 122.1 g/(L·h). Among the alcohols distribution, the methanol was suppressed and the C2+ OH production was greatly increased. Combined with XRD analysis, it was seen that “K-Mo-C” entities might favor the alcohols synthesis. Modified by nickel, the CO conversion reached 88.1 C%. The main products still were C1 –C4
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hydrocarbons, and only few alcohol was formed, which was similar to those on pure β-Mo2 C. However, co-modified by nickel and potassium, the β-Mo2 C catalyst showed highly active and selective towards mixed alcohols synthesis in CO hydrogenation. The CO conversion achieved 73.0 C%, which was even much higher than that of pure β-Mo2 C, and the STY of alcohol was 324.2 g/(L·h), the selectivity to alcohols slightly decreased whereas hydrocarbons increased compared with those on K/β-Mo2 C catalyst. Thus, high activity of CO conversion over K/Ni/β-Mo2 C catalyst might be attributed to the Ni species in the sample, but more study is needed. It could be concluded that in the presence of Ni and K, the βMo2 C showed highly active and selective to higher alcohols synthesis, and the Ni and K promoter might have synergistic effect on CO conversion. Table 1. Catalytic performance of β -Mo2 C-based catalysts in CO hydrogenationa Catalyst β-Mo2 C K/β-Mo2 C Ni/β-Mo2 C K/Ni/β-Mo2 C
CO conv. (C%) 58.6 23.4 88.1 73.0
Alcohol STY (g/L·h) 15.7 122.1 23.5 324.2
Selectivity (C%) ROH CO2 CHx 1.8 51.8 46.4 26.5 49.6 23.9 1.2 45.7 53.1 23.3 50.9 25.8
a Reaction conditions: T = 573 K, p = 8.0 MPa, n(H )/n(CO) =1.0, 2 GHSV = 2000 h−1
Figure 2. Distribution of alcohols for β-Mo2 C-based catalysts
The change in CO conversion and alcohol selectivity with time on stream is shown in Figure 3 for potassium modified β-Mo2 C as an example. Other catalysts also showed similar trends. Initially, the catalysts showed high CO conversion and relatively low selectivity to alcohols. The CO conversion decreased and alcohol selectivity increased with time on stream, and a steady state was obtained after 72 h. Generally, molybdenum-based catalysts for mixed alcohols synthesis all tended to have an induction period during the CO hydrogenation. For molybdenum carbide, a part of the surface oxygen on the catalyst probably reacted with carbidic carbon and was removed from the uppermost surface layer during this induction period [2]. It was also believed that during the induction, some surface carbon atoms of carbide and some deposited carbon species were eliminated [13].
Figure 3. Changes in CO conversion and alcohol selectivity over K/βMo2 C with time on stream (selectivity calculated from CO2 free basis)
3.3. Ef fect of potassium and nickel on chain propagation The distributions of alcohols and hydrocarbons over the β-Mo2 C-based catalysts are shown in Figure 4, as an Anderson-Schultz-Flory (A-S-F) distribution: Yn = P n−1 (1−P ), where Yn was the mole fraction of the alcohol with a carbon number of n and P was the chain growth probability [14]. For β-Mo2 C catalyst, the distribution of hydrocarbons and alcohols had similar linear A-S-F plots, the chain-growth probability P reached 0.62 and 0.26, respectively. Modified by potassium, two different types of plots were obtained. The hydrocarbons still obeyed the linear A-S-F plot, and the chain-growth probability P of hydrocarbons dropped remarkably from 0.62 to 0.48. However, the alcohols showed a remarkable deviation for methanol, indicating a different reaction mechanism for alcohols formation over K/β-Mo2 C catalysts. Except for methanol, it was worth noting that the C2 –C4 alcohols still followed the linear distribution plot and the chain-growth probability P of 0.29 was obtained, which was higher than that of β-Mo2 C catalyst. It meant that though potassium promoter exerted an effective function on the whole chain propagation to produce alcohols, the main difference of reaction mechanism over β-Mo2 C and K/β-Mo2 C catalyst lied in chain propagation of C1 OH to C2 OH. For K modified β-Mo2 C catalyst, the formation of “K-Mo-C” phase was considered to be the active step for the alcohols synthesis, which exerted a prominent function on the whole chain propagation to produce alcohols especially for C1 OH to C2 OH. Lee and co-workers also considered that the K2 CO3 promoter readily removed its counter anions under the reaction conditions to form a new complex which could be assigned to a carboxylic species, and this carboxylic species was proposed to be an active intermediate for alcohol synthesis from CO-H2 [4]. Modified by nickel, the distributions of alcohols and hydrocarbons were similar to those on pure β-Mo2 C, namely, linear A-S-F plots were obtained. However, the ability of chain propagation was greatly reduced, 0.11 and 0.33 for alcohols and hydrocarbons, respectively. As known to all, Ni was an excellent methanation promoter, combined
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with XRD results, the nickel species might be responsible for the high methane and methanol content. When co-modified by nickel and potassium, β-Mo2 C showed similar types of AS-F plots of hydrocarbons and alcohols compared with those on K modified β-Mo2 C catalyst. However, the chain-growth probability P of hydrocarbons was 0.58, which was much higher than that on K modified β-Mo2 C catalyst, and was comparable to that on β-Mo2 C catalysts. It could also be seen
that methanol was further suppressed to 25.8 C% (see Table 1), the P of alcohols greatly increased to 0.44, which was much higher than those of other catalysts. Compared with K or Ni modified β-Mo2 C catalyst, the Ni and K co-modified β-Mo2 C enhanced the ability of formation of alcohols and hydrocarbons simultaneously, and again this would suggest that the Ni and K promoters have synergistic effect on CO conversion.
Figure 4. A-S-F plots of alcohols and hydrocarbons over (a) β-Mo2 C, (b) K/β-Mo2 C, (c) Ni/β-Mo2 C and (d) K/Ni/β-Mo2 C catalysts. (P denotes the probability of chain growth)
4. Conclusions Novel Ni and K co-modified β-Mo2 C catalysts were prepared through TPR method and tested for CO conversion. Potassium shifted the products over β-Mo2 C remarkably from hydrocarbons to alcohols, whereas nickel changed β-Mo2 C to a methanation catalyst together with a high CO conversion. However, co-modified by nickel and potassium, βMo2 C showed highly active and selective for alcohols synthesis, and the STY of alcohol was greatly increased. It was concluded that nickel and potassium promoter might have synergistic effect on CO conversion. References [1] Patterson P M, Das T K, Davis B H. Appl Catal A: Gen, 2003, 251: 449
[2] Griboval-Constant A, Giraudon J M, Leclercq G, Leclercq L. Appl Catal A: Gen, 2004, 260: 35 [3] Woo H C, Park K Y, Kim Y G, Namau I S, Chung J S, Lee J S. Appl Catal, 1991, 75: 267 [4] Lee J S, Kim S, Kim Y G. Top Catal, 1995, 2: 127 [5] Xiang M, Li D, Li W, Zhong B, Sun Y. Fuel, 2006, 85: 2662 [6] Lee J S, Oyama S T, Boudart M. J Catal, 1987, 106: 125 [7] Xiao T C, York A P E , Al-Megren H, Williams C V, Wang H T, Green M L H. J Catal, 2001, 202: 100 [8] Kojimi R, Aika K. Appl Catal A: Gen, 2001, 219: 141 [9] Oshikawa K, Nagai M, Omi S. J Phys Chem B, 2001, 105: 9124 [10] Nagai M, Zahidul A M, Matsuda K. Appl Catal A, 2006, 313: 137 [11] Saito M, Anderson R B. J Catal, 1980, 63: 438 [12] Kijima I, Miyazaki E. J Catal, 1984, 89: 168 [13] Wu W C, Wu Z L, Liang C H, Chen X W, Ying P L, Li C. J Phys Chem B, 2003, 107: 7088 [14] Li X G, Feng L J, Liu Z Y, Zhong B, Dadyburjor D B, Kugler E L. Ind Eng Chem Res, 1998, 37: 3853