Investigation on the Performance of Supported Molybdenum Carbide for the Partial Oxidation of Methane

Transcription

1 Journal of Natural Gas Chemistry 12(2003)23 30 Investigation on the Performance of Supported Molybdenum Carbide for the Partial Oxidation of Methane Quanli Zhu, Jian Yang, Jiaxin Wang, Shengfu Ji, Hanqing Wang State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou , China [Manuscript received September 21, 2002; revised January 23, 2003] Abstract: The performance of supported and unsupported molybdenum carbide for the partial oxidation of methane (POM) to syngas was investigated. An evaluation of the catalysts indicates that bulk molybdenum carbide has a higher methane conversion during the initial stage but a lower selectivity to CO and H 2/CO ratio in the products. The rapid deactivation of the catalyst is also a significant problem. However, the supported molybdenum carbide catalyst shows a much higher methane conversion, increased selectivity and significantly improved catalytic stability. The characterization by XRD and BET specific area measurements depict an improved dispersion of molybdenum carbide when using alumina as a carrier. The bulk or the supported molybdenum carbide exists in the β-mo 2C phase, while it is transformed into molybdenum dioxide postcatalysis which is an important cause of molybdenum carbide deactivation. Key words: molybdenum carbide, molybdenum oxide, partial oxidation of methane, syngas, deactivation 1. Introduction Methane is the predominant component of natural gas, which is widely distributed all over the world. Natural gas is considered to be an important future alternative to petroleum due to its extensive reserves. It is currently difficult for natural gas to be widely utilized for commercial purposes because its reservoirs are situated in remote areas far from the locations of highest energy consumption combined with the high cost of its compression and transportation. It is a challenging task for scientists, especially chemists, to make utilization of natural gas economically feasible. The chemical utilization of natural gas can be achieved through two pathways. The first involves direct oxidative conversion (e.g. the oxidative coupling of methane to ethylene, selective oxidation to methanol or formaldehyde). The other is an indirect conversion in which natural gas is initially converted to syngas (CO+H 2 ) and then transformed into liquid fuel or other chemicals [1]. Indirect conversion is currently the most predominant, thus, that natural gas is changed into syngas becomes a keystep. The syngas can be produced via three pathways: (a) steam reforming, (b) dry reforming, (c) catalytic partial oxidation of methane (POM). Compared to the first two, the POM technique has several advantages. (1) Among the products, the H 2 /CO ratio is nearly 2:1, which is suitable for the methanol industry and F- T synthesis. (2) Syngas can be produced at great gas hourly space velocity (GHSV) and autoexothermicity, which results in smaller production scope, less energy consumption, less investment and cost of production with the same output. (3) A traditional facility can be used while increasing the output. Considering these merits, more and more attention is being paid to the POM to syngas. Certain catalysts (e.g. noble metal, nickel, etc.) can provide excellent catalytic performance for the POM to syngas. Noble metals have good activity and Corresponding author.

2 24 Quanli Zhu et al./ Journal of Natural Gas Chemistry Vol. 12 No anti-coking ability, but their rareness limits employment opportunities. Nickel has an activity comparable to that of a noble metal but also promotes coking, which, in addition to its disappearance under the reaction conditions, causes it to rapidly deactivate. Nickel sulphide has a better coking-resistance, but its activity is limited [2] within a lower level. Thus, to a great extent, the aim of studies on the POM to syngas is to search for a kind of catalyst with higher activity and better anti-coking capacity due to the importance of catalysis in the POM process. Since Levy and Boudart et al. [3] reported in 1973 that tungsten carbide has catalytic performance similar to that of noble metal, a great deal of interest has focused on early transition metal carbide, in particular molybdenum or tungsten carbide. Molybdenum or tungsten carbide exhibits excellent catalytic properties in reactions including hydrodenitrogenation (HDN) and hydrodesulphurization (HDS) [4 7], isomerization of hydrocarbon [8 12], hydrogenation of carbon monoxide and F-T synthesis [13,14], as well as electro-catalytic reactions on the electrode of a fuel cell [15,16]. Recently, Claridge et al. [17,18] investigated the catalytic performance of molybdenum and tungsten carbide for the POM to syngas and obtained encouraging results. However, no detailed information on carbide use for the POM to syngas at ambient pressure has been reported in any literature except for the few words by Claridge et al. [18]. Therefore, out motivation is to attempt to employ molybdenum carbide as a catalyst for the POM to syngas at ambient pressure and present details on its performance. 2. Experimental 2.1. Catalyst preparation There are several methods that can be used for the catalyst preparation [19]. Carbide catalysts have usually been prepared using the temperatureprogrammed reaction method (TPRe) since it was introduced by Oyama et al. [20,21] for the preparation of nitride and later by Boudart et al. [22,23] for the preparation of carbide. Catalysts prepared using this technique have a much higher specific surface area. The samples used in our experiments were prepared according to procedures set up in previous research [19 23] that can be roughly divided into two steps. The goal of the first step was to prepare the oxide precursor of the molybdenum carbide samples. Molybdenum trioxide was obtained from ammonium heptamolybdate (A.R) heated at 500 for 3 hours. An appropriate amount of alumina ( mesh, Research Institute of Lanzhou Chemical Industry Corporation, Lanzhou, China) was added into the correct amount of ammonium heptamolybdate and ammonia (A.R.) solution while stirring. The stirring was stopped after half an hour, and the obtained solution was laid on a table overnight at room temperature. The sample was then evaporated to dry in a water bath at and transferred into a temperature-controlled oven and held at 115 for 12 hours. The final step before obtaining the oxide precursor of the catalyst supported on alumina was to calcine the sample at 500 for 4 hours. The second method aimed to transform the oxide precursor into carbide. The above oxide precursor was carburized with 20 (v/v)% flowing methane (nominal purity, 99.99%) in hydrogen (nominal purity, 99.99%) at a rate of 1.47 mmol/min. This process was carried out in a 4 mm i.d. quartz tube loaded with ca. 150 mg of sample and heated at 10 /min from room temperature to 300, 1 /min from 300 to 850 and maintained at the final temperature for 2 hours. After carburization, the sample was pretreated with H 2 (1.18 mmol/min) for half an hour at 850. The tube furnace was controlled by a PID controller with an accuracy of ±1. The prepared carbide samples were evaluated in situ or cooled in helium (nominal purity, 99.99%) to room temperature and then passivated with 1(v/v)% O 2 (nominal purity, 99.9%) in helium for 12 hours for the purpose of characterization Catalyst evaluation Catalyst samples were evaluated in situ after carburization and pretreatment. Reactant gases with a 275 mmol/(h g) total flow were controlled with a mass controller. The methane and oxygen gas mixture, with a 2.05:1 molar ratio (when the ratio of mixture became stable), was switched to the quartz tube equipped with coarse quartz wool near its central section. The products were analyzed using a SC-8 model Chromatograph with 5A zeolite and Porapak Q packed columns, a thermal conductivity detector (TCD) and He as the carrier gas. Calculation of conversion and selectivity was carried out according to the general equations below:

3 Journal of Natural Gas Chemistry Vol. 12 No Conversion of x = C[x] = % conversion of x into all products Selectivity of CO = [amount of CO in all products] [amount of C in all products] carrier Al 2 O 3 and that the size of the molybdenum carbide particles that existed on the surface increased with the loading and vice versa. This conclusion was consistent with that of XRD. The characteristics of the samples are presented in Table 1. These results also prove that alumina as a carrier can improve the dispersion of molybdenum carbide. Selectivity of H 2 = 2 [amount of H 2 in all products] [amount of H in all products] The deposited carbon was not taken into account, and the carbon balances were better than 95 % in all cases Catalyst characterization The crystalline components of the materials were identified by X-ray diffraction (XRD) using a D/max- RB model X-ray diffractometer with Cu K α radiation (λ=0.154,18 nm). The morphology of the passivated samples was observed using scanning electron microscopy (SEM) on a JSM-5600LV microscope. The specific surface areas of the passivated samples were measured on an ASAP 2010 (Micrometrics Instrument Corp.) using the N 2 adsorption method. TPSR experiments were carried out using an apparatus based on gas chromatography with a TCD. 3. Results 3.1. Identification of crystal structure The XRD patterns of the samples are shown in Figure 1. The results (2θ= 34.4 o, 38.0 o, 39.4 o, 61.5 o and 69.6 o ) indicated that molybdenum carbide prepared under the above conditions was β-mo 2 C (hexagonal-close-packed, h.c.p.). When supported on alumina with a lower loading, molybdenum carbide existed in smaller particles judging from the full width at the maximum height, which disclosed that molybdenum carbide was highly dispersed at the lower loading. At higher loadings, such as 35.4wt% in Figure 1, the XRD pattern was similar to that of bulk molybdenum carbide (unsupported), indicating that bulk molybdenum carbide was abundant on the surface. The diffraction peaks at 2θ=26.1 o, 37.1 o and 53.5 o showed the appearance of MoO 2 after a 5-hour reaction. The SEM micrographs of several samples are shown in Figure 2. These prove that the dispersion of molybdenum was improved by the introduction of Figure 1. XRD patterns of the passivated samples Carburized at 850 and pretreated with H 2 for half an hour. (1) 10.6wt% Mo 2 C/Al 2 O 3, (2) 24.8wt% Mo 2 C/Al 2 O 3, (3) 35.4wt% Mo 2 C/Al 2 O 3, (4) 42.5wt% Mo 2 C/Al 2 O 3, (5) Mo 2 C, (6) 35.4wt% Mo 2 C/Al 2 O 3 after the reaction for 5 h Table 1. Characteristics of samples BET surface Average pore Pore Sample area volume diameter (m 2 /g) (cm 3 /g) (nm) Mo 2 C wt%Mo 2 C/Al 2 O wt%Mo 2 C/Al 2 O wt%Mo 2 C/Al 2 O wt%Mo 2 C/Al 2 O Evaluation of catalysts The methane conversion versus time on stream for the POM to syngas is shown in Figure 3. The bulk molybdenum carbide provided rather high catalytic activity during the initial stage of the reaction but rapidly deactivated with time on stream. However, when supported on alumina, the deactivation became markedly slower. This trend had two distinct

4 26 Quanli Zhu et al./ Journal of Natural Gas Chemistry Vol. 12 No Figure 2. SEM micrographs of the passivated samples (a) 10.6wt%Mo 2 C/Al 2 O 3, (b) 35.4wt%Mo 2 C/Al 2 O 3, (c) 46.0wt%Mo 2 C/Al 2 O 3, (d) Mo 2 C characteristics: one was that catalyst 10.6wt% Mo 2 C/Al 2 O 3 loading had a lower activity but was rather stable; the other was that activity and stability varied with loading, in creasing with the loading at first, and then becoming stable. The best activity and stability occurred at a loading of about 35.4wt% (no detectable loss of activity after 10 hours, which was not given in the paper). However, the activity and stability began to decline at different rates depending on the molybdenum carbide loading when the Mo 2 C loading exceeded 35.4wt%. The selectivity to CO versus time on stream is shown in Figure 4. For bulk molybdenum carbide, the selectivity to CO increased with time on stream. The significant change for supported catalysts was the improved stability. All samples initially had a lower CO selectivity that became stable over a period of about 30 minute and increased with loading. The catalyst with a loading of 35.4wt% had the best selectivity. However, when the loading of Mo 2 C is over the loading, the stability decreased to some extent depending on loading, though the selectivity was maybe a little higher. The key aspect of the product composition is the H 2 /CO ratio, and the molar ratio of H 2 to CO versus time on stream on is presented in Table 2. During the initial stage, there was higher H 2 /CO ratio which then decreased depending on the sample. Table 2. The H 2 /CO molar ratio in products vs time on stream Reaction time (min) Mo 2 C wt%Mo 2 C/Al 2 O wt%Mo 2 C/Al 2 O wt%Mo 2 C/Al 2 O wt%Mo 2 C/Al 2 O wt%Mo 2 C/Al 2 O wt%Mo 2 C/Al2O wt%Mo 2 C/Al 2 O wt%Mo 2 C/Al 2 O

5 Journal of Natural Gas Chemistry Vol. 12 No Figure 3. Conversion of methane vs time on stream Reaction conditions: 850, GHSV=6,850 ml/(h g), CH 4 :O 2 =2.05:1, catalyst: (1) Mo 2 C, (2) 10.6wt%Mo 2 C/Al 2 O 3, (3) 17.7wt%Mo 2 C/Al 2 O 3, (4) 24.8wt%Mo 2 C/Al 2 O 3, (5) 31.9wt%Mo 2 C/Al 2 O 3, (6) 35.4wt%Mo 2 C/Al 2 O 3, (7) 39.0wt%Mo 2 C/Al 2 O 3, (8) 42.5wt%Mo 2 C/Al 2 O 3, (9) 46.0wt%Mo 2 C/Al 2 O 3 Figure 4. Selectivity to CO vs time on stream (legend same as in Figure 3)

6 28 Quanli Zhu et al./ Journal of Natural Gas Chemistry Vol. 12 No Characterization by TPSR The temperature-programmed surface reaction measurements with methane (CH 4 -TPSR) in situ for the samples just after carburization are shown in Figure 5. The figure indicates that methane can be decomposed over the surface of supported molybdenum carbide even though methane can also be decomposed over the surface of alumina. Judging from the initial decomposition temperature, molybdenum carbide catalyzes the decomposition of methane. Figure 5. CH 4 -TPSR of samples Sample weight, 100 mg; the flow rate of methane, 30 ml/min at room temperature; ramped rate of temperature, 10 /min; (1) Al 2 O 3, (2) 35.4wt%Mo 2 C/Al 2 O The analysis of surface elements The data from surface element analysis by XPS are listed in Table 3. The molybdenum was lost after 5 hours on stream, and this loss was more serious for the sample with a lower loading. 4. Discussions The bulk or supported catalysts have high activity for the conversion of methane. The bulk molybdenum carbide prepared under the condition mentioned above has such a small specific surface area that there are few pores in it, and the reactions have to take place on limited areas. Based on this observation, it is reasonably inferred that molybdenum carbide has high capacity to activate methane. The low initial CO selectivity means that transformation of methane occurs via different pathways. If methane firstly reacts via the total combustion pathway, the initial conversion could not be as high and the H 2 /CO ratio would be much lower than 2:1. This is indeed not the case. The data shown in Figure 3(1) and Table 2 imply that the conversion of methane takes place via the other pathway. The TPSR profiles in Figure 5 indicate that catalytic pyrolysis of methane can occur over the molybdenum carbide and explains the Table 3. Atomic Ratios of Mo/Al on the surface for sample pre- and post- catalysis 10.6(wt)%Mo 2 C/Al 2 O (wt)%Mo 2 C/Al 2 O 3 Sample Pre-catalysis Post-catalysis (5 h) Pre-catalysis Post-catalysis (5 h) Mo/Al (atomic ratio) high methane conversion and high ratio of H 2 to CO during the initial stage. However, the low selectivity to CO may be ascribed to the low capacity of molybdenum carbide to activate oxygen. As the reaction proceeds, the conversion of methane decreases, while the selectivity to CO increases. Claridge and coworkers [18] pointed out that the transformation of carbide into oxide is one of the important reasons for carbide catalyst deactivation, and our experiments observed the same phenomenon (Figure 1(6)). When carbide is transformed into oxide, the molybdenum species are easily washed off with stream due to the lower melting point (e.g. 795 for MoO 3, while Mo 2 C has melting point as high as 2,640 ). The data in Table 3 illustrates the loss of molybdenum species. These reasons all account for the decreasing methane conversion with time on stream. There are also reasons for the increase in CO selectivity. With the decreasing methane conversion and transformation of carbide into oxide, there is more and more methane in the gas phase, and less and less methane is converted via the catalytic decomposition pathway. Meanwhile, the unreacted methane can react with CO 2 to form CO, which is favorable to the CO selectivity. Ignoring the deposited carbon in the calculation also increases the selectivity to CO, and the transformation of carbide also favors the production of CO. When molybdenum carbide is dispersed on alumina, it generally becomes more stable because of the strong interaction between metal and support (SIMS). Oxide precursor is almost dispersed in a monolayer at lower loadings, and when it is transformed into carbide, the carbide particles become more stable due

7 Journal of Natural Gas Chemistry Vol. 12 No to SIMS. Thus it should be of very stable activity, as shown in Figure 3(2). At high temperatures (e.g. as high as 850 ) it is natural and unavoidable for molybdenum carbide to agglomerate, and the molybdenum carbide particles therefore stably exist as much larger particles. In light of this, the MoO 3 is dispersed almost in monolayer, for example, the precursor of catalyst 10.6wt%Mo 2 C/Al 2 O 3 in Figure 3, while the corresponding molybdenum carbide particles are sporadically dispersed due to agglomeration after they are carburized. In other words, molybdenum carbide particles in catalyst 10.6wt%Mo 2 C/Al 2 O 3 do not cover the whole surface of carrier. When loading increases, the bulk MoO 3 appears on the surface. During carburization, the bulk MoO 3 is transformed via two pathways: one is carburization in situ, which leads to the formation of larger particles or particles that weakly interact with the support; the other involves migration onto the vacant surface of carrier, which leads to the formation of particles that strongly interact with the support. The density of active sites increases with the density of particles on the surface. The increase in active site density results in an increase in activity, while only an increase in the density of particles that strongly interact with the support results in a stability enhancement (Figure 3 and Figure 4 from (2) to (6)). When the loading reaches a certain value, for example, 35.4wt% as in shown Figure 3(6), there is a small increase in the number of particles stacked on surface, but almost no increase in the number of particles that strongly interact with the support. In other words, the new particles are those that weakly interact with the support and are stacked on the outer layer. This particle increase is hardly favorable to the activity because the density of active sites did not increase. On the contrary, it is unfavorable to the stability because the number of unstable particles that weakly interact with the support increased as shown in Figure 3 (8) and (9) and in Figure 4(9). The selectivity increase in Figure 4 from (6) to (9) can be, at least partly, ascribed to the reasons mentioned above. In terms of the products, the most attention is focused on the H 2 /CO ratio. The data listed in Table 2 indicate that it approaches the optimal value of 2:1 when supported, but it is away from 2:1 when unsupported due to the transformation of carbide. This proves that the support can also improve the product composition. For the supported catalysts, there is always lower CO selectivity during the initial stage and a relatively high methane conversion, no matter what pretreatment (e.g. hydrogen or carbon dioxide pretreatment), is carried out or the length of time before evaluation. This tempts us to speculate that surface reconstruction occurs before the POM is carried out. The active sites just after carburization and pretreatment are favorable to activate methane but unfavorable to the production of CO. After they are reconstructed over a period of time, the active sites become suitable for the POM to syngas. During reconstruction, oxygen may take an important role because the selectivity to CO increases which is involved in oxygen. The POM to syngas may be a result of synergetic functions between two kinds of active sites, but the exact mechanism requires further investigation. 5. Conclusions It can be primarily concluded from the above fact that molybdenum carbide has a high capacity to activate methane but is usually unstable because of its transformation into oxide. The activity and selectivity to CO and H 2 are greatly improved. Particularly, the catalytic performance is stabilized when supported on alumina, and the components of the products are also kept relatively stable which is favorable to the production of downstream processes. Acknowledgments This work has been financially supported by Foundation of National Fundamental Research and Development. We are grateful to Madam Li and Madam He (Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences) for the BET specific area and XRD measurements, respectively. References [1] Gradassi M J, Green N W. Fuel Process Technol, 1995, 42: 65 [2] Rostrup-Nielsen J R. J Catal, 1984, 85: 31 [3] Levy R B, Boudart M. Science, 1973, 181: 547 [4] Schlater J C, Oyama S T, Metcalfe III J E et al. Ind Eng Chem Res, 1988, 27: 1648 [5] Lee J S, Boudart M. Appl Catal, 1993, 19: 207 [6] Abe H, Bell A T. Catal Lett, 1993, 18: 1 [7] Choi J-G, Brenner J R, Thompson L T. J Catal, 1995, 154: 33 [8] Iglesia E, Riberio F H, Boudart M et al. Catal Today, 1992, 15: 307

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