Thread Content
Effect of Preparation Method on the Performance of Molybdenum-Based Sulfur-Resistant Methanation Catalysts Li Zhenhua, Zhang Xiaoshan, Qu Jianglei, Wang Weihan, Wang Baowei, Ma Xinbin (School of Chemical Engineering, Tianjin University, Key Laboratory of Green Synthesis and Transformation, Ministry of Education, Tianjin 300072) Abstract: Syngas methanation is one of the main processes in coal-to-natural gas production. Compared to traditional nickel-based catalysts, molybdenum-based catalysts used for sulfur-tolerant methanation can eliminate the need for precise desulfurization and water-gas shift processes, offering certain technical and cost advantages. However, molybdenum-based catalysts have relatively low activity, especially their activity at low temperatures and thermal stability need to be improved. A comparative study was conducted on the sulfur-tolerant methanation performance of MoO3/ZrO2 catalysts prepared by different methods in a fixed-bed reactor, and it was found that the catalysts prepared by the solution combustion method exhibited higher sulfur-tolerant methanation activity under the same conditions. At an space velocity of 5000 h-1, a reaction pressure of 3 MPa, and reaction temperatures of 300°C and 400°C, the CO conversion rates reached 26% and 79%, respectively. Characterization results such as N2 physisorption, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy of the catalyst show that the catalyst prepared by the solution combustion method has a smaller ZrO2 grain size and a larger specific surface area, with the active Mo species being more uniformly dispersed on the ZrO2 support. The MoO3/ZrO2 catalysts prepared by the coprecipitation and impregnation methods exhibited varying degrees of Mo species agglomeration, resulting in lower activity for sulfur-tolerant methanation. Keywords: catalyst preparation method, carbon monoxide ； Methane ； Molybdenum trioxide ； Zirconia Introduction Coal-to-natural gas is an important aspect of the coal-based gaseous energy industry; it aligns with China’s energy structure characterized by abundant coal, limited oil reserves, and some natural gas resources, and it can help alleviate the imbalance between supply and demand for natural gas. This process not only features high thermal energy efficiency but also reduces environmental pollution caused by the traditional combustion of coal. Methanation catalysts are a key technology in the coal-to-natural gas process, and can be divided into two main categories. One category is nickel-based indirect methanation catalysts, which are widely used in industry. However, nickel-based catalysts exhibit high activity only when the H2/CO ratio is greater than 3, and they tend to become deactivated in sulfur-containing atmospheres. Therefore, it is necessary to adjust the H2/CO ratio to above 3 through steam reforming before feeding the syngas in, and moreover, a costly desulfurization process is required to reduce the sulfur content in the syngas to below 1×10-6 g·m-3 ； Another category consists of molybdenum-based direct methanation catalysts, which exhibit good catalytic activity under low H2/CO ratios and in the presence of sulfur; as a result, there is no need for steam reforming or desulfurization of the gases exiting the gasifier. The direct methanation process developed, represented by the U.S. Gas Research Institute, uses molybdenum-based catalysts, which simplifies the process flow and reduces production costs. However, in methanation reaction systems, the activity of molybdenum-based catalysts is generally lower than that of nickel-based catalysts; therefore, it is particularly important to improve the activity of molybdenum-based catalysts. The methanation reaction is a reversible exothermic reaction; from a thermodynamic perspective, lower temperatures are more favorable for its progression. Since the methanation reaction is a highly exothermic reaction, in order to make the most of the heat generated by this reaction, it is generally carried out in an adiabatic reactor; this requires the catalyst to possess both high activity at low temperatures and good stability at high temperatures. The higher the low-temperature activity of the catalyst, the less impact the adiabatic temperature rise has on the methanation catalyst at high temperatures; this is more conducive to prolonging its service life under high-temperature conditions. There are few reports in the literature regarding the activity of this catalyst at low temperatures; most reports concern its activity under high-temperature conditions. Therefore, it is of great significance to develop a sulfur-tolerant methanation catalyst with high activity at low temperatures. The active phase of sulfur-tolerant methanation catalysts is MoS2. MoS2 has been widely used in hydrogenation reaction systems involving sulfur-containing atmospheres, such as hydrodesulfurization, hydrodenitration, and hydrodeoxygenation. MoS2 is typically obtained by the reduction and sulfidation of the oxide MoO3 in an H2S/H2 atmosphere at a certain ratio. In supported molybdenum-based catalysts, the dispersion of MoO3 on the surface of the support and the interaction between MoO3 and the support have a significant impact on the morphology and catalytic performance of the MoS2 formed via reductive sulfidation. ZrO2 is a P-type semiconductor material with excellent chemical stability; it readily generates oxygen holes. It possesses weak acid-base properties, redox capabilities, as well as good mechanical strength, thermal stability, and resistance to acid and alkali corrosion, making it widely used as a carrier for various catalysts. Fu et al. studied the methanation performance of MoS2 catalysts supported on different carriers and found that ZrO2 exhibited higher methanation activity as a carrier. The preparation method has a significant impact on the performance of the catalyst. The MoO3/ZrO2 catalyst prepared by the coprecipitation method by Li et al. exhibits high resistance to sulfur methanation. As a new and advanced method for synthesizing materials, solution combustion has received increasing attention in recent years. Compared with traditional catalyst preparation methods, the solution combustion method features simple equipment and processes, high product purity, and no pollution. Ying et al. prepared a Ni/Al2O3 catalyst using the solution combustion method, which exhibits high activity and stability in the syngas methanation reaction. However, there are few reports on the study of molybdenum-based catalysts prepared by the solution combustion method in low-temperature methanation reactions; thus, further research is warranted. In this study, MoO3/ZrO2 catalysts with the same loading were prepared using three different methods: solution combustion, coprecipitation, and equal-volume impregnation. The sulfur-tolerant methanation performance of these three catalysts under low-temperature conditions was investigated, and a series of characterization analyses were conducted on the prepared MoO3/ZrO2 catalysts to explore the influence of catalyst structure on reaction performance. 1 Experimental Section 1.1 Catalyst Preparation The 15% (by mass) MoO3/ZrO2 catalysts were prepared using the solution combustion method, co-precipitation method, and equal-volume impregnation method, respectively. Solution combustion method: A mixture of 0.6395 g of ammonium paramolybdate (AR, Tianjin Cosmo Chemical Reagents Co., Ltd.) and 12.1193 g of zirconium nitrate (AR, Tianjin Cosmo Chemical Reagents Co., Ltd.) was dissolved at room temperature in a mixture of 21 ml of ethylene glycol (AR, Tianjin Cosmo Chemical Reagents Co., Ltd.) and 21 ml of distilled water. The mixture was stirred in a water bath at 50°C until it formed a paste, which was then placed in a tubular furnace and reacted at 550°C for 4 hours to yield the corresponding catalyst. The resulting catalyst was sieved to obtain particles with a particle size of 0.425–0.850 mm, which were set aside for use. Co-precipitation method: The specific steps for the co-precipitation method are described in the literature. Equal-volume impregnation method: Dissolve 12.1193 g of zirconium nitrate in a mixture of 21 ml of ethylene glycol and 21 ml of distilled water. Stir the mixture in a water bath at 50°C until it becomes paste-like. Then, place it in a tube furnace and react at 550°C for 4 hours to obtain the corresponding ZrO2 carrier. 0.6395 g of ammonium paramolybdate was dissolved in an appropriate amount of distilled water at room temperature; ZrO2 particles were obtained through equal-volume impregnation. The mixture was allowed to dry naturally at room temperature for 24 hours, then dried at 120°C, and finally calcined at 600°C for 4 hours to yield the catalyst with the desired composition. The resulting catalyst was compressed into tablets and crushed to a particle size of 0.425–0.850 mm for later use. 1.2 Catalyst evaluation The low-temperature sulfur-tolerant methanation performance of the catalyst was evaluated using a pressurized fixed-bed reactor (Tianjin University Beiyang Chemical Engineering Experimental Equipment Co., Ltd.). The inner diameter of the stainless steel reactor was 12 mm, and the amount of catalyst loaded was 2 ml. Before the determination of reactivity, the catalyst must be activated by in-situ sulfidation and reduction using a 3% (volume fraction) H2S/H2 gas mixture. Evaluation conditions for the catalyst: The H2/CO ratio in the reaction gas is 1, with 10% (by volume) N2 used as the internal standard gas; the H2S concentration is 0.24%. The reaction temperatures are 300°C and 400°C respectively, the pressure is 3 MPa, and the space velocity is 5000 h-1. The quantitative analysis of the reaction products was carried out using a 3420A gas chromatograph (Beifen Rayleigh) for on-line analysis. The chromatograph was equipped with a thermal conductivity detector (TCD) and a hydrogen flame ionization detector (FID) as detection systems, and the content of each component was determined quantitatively by the external standard method. The calculation formulas for the main indicators of catalyst performance are as follows. file:///C:/Users/muzil/AppData/Local/Temp/msohtmlclip1/01/clip_image001.png CO conversion rate file:///C:/Users/muzil/AppData/Local/Temp/msohtmlclip1/01/clip_image002.png CH4 selectivity. Here, x represents the conversion rate of the reactant gas ； s is the selectivity of the product ； n is the molar flow rate, in mol·min-1; the subscript in denotes the reactor inlet, and out denotes the reactor outlet. 1.3 Catalyst characterization: Physical N2 adsorption characterization was carried out using a TriStar 3000 adsorptometer from Micromeritics in the United States. The procedure is as follows: Weigh a certain mass of the sample to be tested; under vacuum degassing conditions, dry it at 90°C for 1 hour, then raise the temperature to 300°C and dry it for another 4 hours. Finally, record the mass of the sample after degassing. The sample is placed in a sample tube and loaded into the adsorption analyzer for analysis. The analysis temperature is that of liquid nitrogen (-195.8°C). The amount of N2 adsorption was determined at an N2 partial pressure of 0–0.3 MPa, followed by N2 adsorption-desorption experiments at room temperature. The specific surface area of the sample is calculated using the Brunauer-Emmett-Teller (BET) equation. A field-emission transmission electron microscope (TEM) of the Tecnai G2F20 model from Philips in the Netherlands was used (with a point resolution of 0.248 nm, a line resolution of 0.102 nm, and a magnification capability of up to 1 million times). A small amount of the sample was placed in a small beaker containing an appropriate amount of anhydrous ethanol, and then placed in an ultrasonic oscillator for ultrasonic treatment. Once the sample was evenly dispersed, a small drop of the suspension was placed on a copper mesh; after drying, it was observed. A Japanese Rikaku Rigaku D/max2500V/PC X-ray diffractometer was used, with a CuKα radiation source (λ= 0.154056 nm), an operating voltage of 40 kV, and an operating current of 100 mA. Graphite monochromator, slit SS 1°, RS 0.15 mm, counter SC, scanning range 2θ from 5° to 90°. In the experiment, powder X-ray diffraction (XRD) analysis was employed to examine the XRD diffraction patterns of the catalysts, and the PDF-2004 card in Jade5.0 was used to identify the crystals corresponding to the diffraction peaks. The InVia-Reflex laser microconfocal Raman spectrometer from the British company Renishaw was used, with an Nd:YAG laser as the light source; the intensity of the laser light was 6 mW, and the excitation wavelength was 532 nm. Raman analysis studies structural information such as molecular vibrational and rotational energy and symmetry by analyzing the vibrational and rotational scattering spectra of molecules. 2 Experimental Results and Discussion file:///C:/Users/muzil/AppData/Local/Temp/msohtmlclip1/01/clip_image003.png 2.1 Effect of Different Preparation Methods on the Sulfur-Tolerant Methanation Activity of MoO3/ZrO2 Catalysts The evaluation results of the sulfur-tolerant methanation performance of MoO3/ZrO2 catalysts prepared by different methods are shown in Figure 1. As can be seen from Figure 1, at different reaction temperatures, the order of CO conversion rates for the MoO3/ZrO2 catalysts prepared by the three methods is combustion method > coprecipitation method > impregnation method, and the gap widens as the reaction temperature increases. At a reaction temperature of 300°C, the CH4 selectivity of the MoO3/ZrO2 catalysts prepared by the three methods shows little difference ； However, at a reaction temperature of 400°C, the order of CH4 selectivity among different MoO3/ZrO2 catalysts is combustion method > co-precipitation method ≈ impregnation method. The evaluation results show that the catalysts prepared by the combustion method exhibit the best low-temperature resistance to sulfur-methanation activity. Analysis suggests that different preparation methods may affect the structure of the MoO3/ZrO2 catalyst as well as the dispersion of the active Mo species on the surface of the ZrO2 support, thereby influencing the catalyst’s resistance to sulfur-mediated methanation and its CH4 selectivity. 2.2 Catalyst characterization results 2.2.1 N2 adsorption-desorption characterization of the catalyst file:///C:/Users/muzil/AppData/Local/Temp/msohtmlclip1/01/clip_image004.png N2 adsorption-desorption characterization was performed on each sample, and the results are shown in Table 1. As can be seen from Table 1, there are significant differences in the textural properties of the MoO3/ZrO2 catalysts prepared by the three methods. The order of their specific surface areas is: combustion method > co-precipitation method > impregnation method. The order of pore volumes is: co-precipitation method > combustion method > impregnation method. The order of average pore sizes is: impregnation method > co-precipitation method > combustion method. file:///C:/Users/muzil/AppData/Local/Temp/msohtmlclip1/01/clip_image005.png The pore size distribution of the catalysts prepared by different methods is shown in Figure 2. As can be seen from Figure 2, the pore sizes of the MoO3/ZrO2 catalysts prepared by the combustion method and co-precipitation method are mainly distributed in the range of 2–5 nm. Among them, the pore size distribution of the catalyst prepared by the combustion method is relatively narrow, while that of the catalyst prepared by the impregnation method has a relatively wider range. The MoO3/ZrO2 catalyst prepared by the impregnation method has a low specific surface area of only 29 m2·g-1, indicating that a certain degree of sintering occurred during the calcination process of the catalyst, which is consistent with the results reported in the literature. During the preparation of catalysts by combustion, the burning of ethylene glycol releases a large amount of heat and gases rapidly; the high rate of temperature increase along with the large volume of escaping gases contribute to the formation of numerous pores with small pore sizes. Compared with the impregnation and coprecipitation methods, the MoO3/ZrO2 catalysts prepared by the combustion method have the largest specific surface area, which is more conducive to the uniform dispersion of the active Mo species. 2.2.2 TEM characterization results of the catalysts file:///C:/Users/muzil/AppData/Local/Temp/msohtmlclip1/01/clip_image006.png Figure 3 shows the TEM characterization results of the MoO3/ZrO2 catalysts prepared by different methods. As can be seen from Figure 3, the particle size of the MoO3/ZrO2 catalysts prepared by the combustion method is significantly smaller than that of the catalysts obtained using the other two methods, and they contain more small pores. It is evident that MoO3 is dispersed on the surface of the ZrO2 support, with no signs of clustered accumulation. This is also consistent with the results of N2 adsorption-desorption characterization; it is precisely because the MoO3/ZrO2 catalysts prepared by the combustion method have small particle sizes that they possess a large specific surface area, and the average pore size formed by the stacking of these particles is small. The MoO3/ZrO2 catalyst prepared by the co-precipitation method in Figure 3(b) exhibits some agglomeration and stacking phenomena. There are also many pores in the catalyst; however, the pore sizes are significantly larger than those of the catalyst prepared by the combustion method. This is consistent with the data on the average pore size obtained from the BET results. The MoO3/ZrO2 catalyst prepared by the impregnation method shown in Figure 3(c) exhibits no obvious pore structure; moreover, there is extensive agglomeration of the catalyst particles. This is likely due to the uneven distribution of the active Mo species during the impregnation process. To further determine the distribution of Mo and Zr elements in the catalyst, a planar scanning characterization of the MoO3/ZrO2 catalyst prepared by the combustion method was conducted using TEM. The results are shown in Figure 4. Figures 4(c) and 4(d) show the distribution of Zr and Mo elements in the region, respectively. It is evident from these figures that both Zr and Mo elements are uniformly distributed in the tested area, further indicating that the Mo and Zr elements are evenly dispersed in the MoO3/ZrO2 catalyst prepared by the combustion method. file:///C:/Users/muzil/AppData/Local/Temp/msohtmlclip1/01/clip_image007.png The XRD characterization results of catalyst 2.2.3 are shown; the XRD characterization results of the MoO3/ZrO2 catalysts prepared by different methods are shown in Figure 5 at file:///C:/Users/muzil/AppData/Local/Temp/msohtmlclip1/01/clip_image008.png. As shown in the figure, the characteristic peak corresponding to MoO3 is not present in the results; Li et al. believe that the MoO3 crystalline phase is highly dispersed on the ZrO2 support. The ZrO2 grain sizes in the MoO3/ZrO2 catalysts prepared by different methods were calculated using the Schemer equation, and the results are shown in Table 1. It can be clearly seen from Table 1 that the grain size of ZrO2 in the catalysts prepared by the combustion method is significantly smaller than that in the other two methods. In the MoO3/ZrO2 catalysts prepared by the combustion method and co-precipitation method, the ZrO2 is present in the tetragonal phase (T-ZrO2). However, the particle size of ZrO2 in the catalysts prepared by the combustion method is significantly smaller than that in those prepared by co-precipitation. In the XRD pattern of the MoO3/ZrO2 catalyst prepared by impregnation, both the characteristic peaks of tetragonal ZrO2 and those of the monoclinic phase (M-ZrO2) are present; moreover, a characteristic peak of ZrMo2O8 appears at 2θ = 23.20°. This is because the loading amount of MoO3 exceeds the single-layer saturation loading capacity of the ZrO2 carrier, which is consistent with the single-layer saturation loading amount of 15% by mass for the MoO3/ZrO2 catalysts obtained by impregnation method by El-Sharkawy et al. The appearance of ZrMo2O8 at this point is presumably due to the uneven distribution of the active Mo species on the surface of the ZrO2 carrier during the impregnation process; calcination then leads to the formation of a stable solid solution between Mo and Zr. This is also the reason for the low activity of catalysts prepared by the impregnation method. It can be seen that the MoO3/ZrO2 catalysts prepared by the combustion method and co-precipitation method possess great advantages. 2.2.4 Raman spectroscopic characterization of the catalysts file:///C:/Users/muzil/AppData/Local/Temp/msohtmlclip1/01/clip_image009.png To further verify the form in which MoO3 exists on the ZrO2 support, Raman spectroscopy was performed on MoO3/ZrO2 catalysts prepared by different methods; the results are shown in Figure 6. The 900–1000 cm-1 region mainly contains characteristic peaks of Mo=O bonds, which primarily indicate the Mo species on the surface of the ZrO2 carrier. The Raman spectra in the range of 700–1100 cm-1 show that the MoO3/ZrO2 catalysts obtained by the three different preparation methods all exhibit a characteristic peak corresponding to crystalline MoO3 at 819 cm-1. However, in the Raman spectrum of the catalyst prepared by the impregnation method, this peak is sharper and its intensity is significantly higher than that of the other two catalysts. This is mainly due to the smaller specific surface area of the ZrO2 carrier used in the impregnation method, resulting in a higher concentration of MoO3 species on its surface, which is consistent with the results obtained from BET analysis. The MoO3/ZrO2 catalysts prepared by the combustion method and coprecipitation method both exhibited two broad characteristic peaks of MoO3 at 800–900 cm-1 and 950–1000 cm-1, indicating the presence of highly dispersed crystalline MoO3 phases in the catalysts. The MoO3/ZrO2 catalyst prepared by co-precipitation exhibits a weak peak at 1000 cm-1, indicating that some Mo species form highly distorted compounds with the surface of the ZrO2 support at this location. This makes it difficult to reduce and sulfide certain Mo species. This may be the reason why the activity of the MoO3/ZrO2 catalyst prepared by co-precipitation is lower than that prepared by the combustion method. The Raman spectrum of the MoO3/ZrO2 catalyst prepared by impregnation method shows distinct characteristic peaks at 748, 945, and 1000 cm-1 corresponding to ZrMo2O4; no significant characteristic peaks of MoO2 are observed in the range of 920–980 cm-1, which is consistent with the results obtained from XRD analysis. The MoO3/ZrO2 catalysts prepared by the impregnation method have the poorest activity, as the formation of a stable ZrMo2O4 compound makes it difficult to generate the active component MoS2 during the reduction of sulfides. As can be seen from the Raman spectra in the 100–700 cm⁻¹ range shown in Figure 6, the MoO₃/ZrO₂ catalyst prepared by the impregnation method exhibits characteristic double peaks of the monoclinic phase M-ZrO₂ at 180, 340, and 550 cm⁻¹, respectively. Meanwhile, the characteristic peaks of the tetragonal phase T-ZrO₂ near 148 and 644 cm⁻¹ are relatively weak. This indicates that the ZrO₂ in the catalyst obtained via the impregnation method is predominantly in the monoclinic phase, with only a small amount of tetragonal phase present. This finding is consistent with the previous XRD characterization results. In contrast, the MoO3/ZrO2 catalysts prepared by the combustion and coprecipitation methods exhibited only the characteristic T-ZrO2 peaks near 148 and 644 cm-1, which is consistent with previous XRD analysis results. 3 Conclusions Compared with the coprecipitation and impregnation methods, the 15% (by mass) MoO3/ZrO2 catalyst prepared by the combustion method exhibits a large specific surface area and small pore sizes. Moreover, the components in the catalyst are evenly distributed, which gives it high activity for sulfur-methanation at low temperatures. The 15% (by mass) MoO3/ZrO2 catalyst prepared by coprecipitation exhibited lower methanation activity, due to its lower specific surface area and larger pore sizes, which result in fewer attachment sites available for the active Mo species. The 15% (by mass) MoO3/ZrO2 catalysts prepared by the impregnation method have the lowest specific surface area. The loading amount of MoO3 exceeds the single-layer saturation limit on the surface of the ZrO2 support, and due to the strong interaction between Mo and Zr, the stable compound ZrMo2O8 is formed. This results in a lower amount of the active species MoS2 being produced during the sulfidation process, thereby leading to poorer activity of the catalysts obtained by this impregnation method. References: GAO J J, LIU Q, GU F N, et al. Recent advances in methanation catalysts for the production of synthetic natural gas. RSC Advances, 2015, 5(29):22759-22776. HAPPEL J, HNATOWM, BAIARS L. Methods of making high activity transition metal catalysts: US4491639. 1985-01-01. LI Z H, TIAN Y, HE J, et al. High CO methanation activity on zirconia-supported molybdenumsulfide catalyst. Journal of Energy Chemistry, 2014, 23(5): 625-632. FU Y L, LU WJ, IIUANG Z G. Study of methanation and O2 chemisorption with several supported sulfide molybdenum catalysts. Journal of China University of Science and Technology, 1989, 19(2): 171-177. ZHAO A M, YING WY, ZHANG H T, et al. Ni-Al2O3 catalysts prepared by solution combustion method for syngas methanation. Catalysis Communications, 2012, 17(1): 34-38. SRINIVASAN T K K. Raman spectroscopy of monolayer-type catalysts: supported molybdenum oxides. Catalysis Reviews, 1998, 40(4):451-570. El-SHARILAWY EA, KHDER A S, AHMED A I. Structural characterization and catalytic activity of molybdenum oxide-supported zirconia catalysts. Microporous & Mesoporous Materials, 2007, 102(1/2/3): 128-137.