Hydrothermal preparation of Fe-Mn catalyst for light olefin synthesis from CO hydrogenation

Transcription

1 Product distribution (wt.%) Article Hydrothermal preparation of Fe-Mn catalyst for light olefin synthesis from CO hydrogenation Jianli Zhang 1 *, Shipeng Lu 1, Subing Fan 1, Tian-sheng Zhao 1 *, Kan Zhang 2 1 State Key Laboratory Cultivation Base of Natural Gas Conversion, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, China 2 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, China Abstract: Potassium modified Fe-Mn catalysts were prepared by hydrothermal procedure using glucose as carbon source and incipient wetness impregnation method. The catalysts were highly active for CO hydrogenation and light olefins production. The prepared sample particles were uniform carbon spheres in the range of nanometer scale. Partial reduction of the samples was confirmed during the hydrothermal process. The bulk structures of Fe 3 O 4 and iron-manganese spinel phase of the samples were observed before reaction, while MnCO 3 was formed in the bulk after reaction. CO adsorption on the samples was significantly enhanced compared with the sample prepared by co-precipitation method, which improved light olefin selectivity and promoted the formation of C 2 -C 4 olefin products. Under the given reaction conditions, the catalyst sample (Fe:Mn:C 6 = 3:1:5) with 1wt% potassium carbonate promotion showed the CO conversion of 95.98% and olefin productivity of 68.79g/Nm 3 (H 2 +CO). The selectivity of CH 4 and CO 2 lowered compared with that over Fe-Mn catalyst by co-precipitation method. Keywords: Hydrothermal method; Fe-Mn catalyst; CO hydrogenation; Light olefins Received 2 Nov. 2014, Revised 18 Dec. 2014, Accepted 8 Jan * Corresponding Author: Jianli Zhang, zhangjl@nxu.edu.cn; Tiansheng Zhao, zhaots@nxu.edu.cn S 1 CH 4 C 2 = -C 4 = C 2 0 -C 4 0 C 5 + Fe-Mn-K catalyst for syngas to light olefins S 1 -S 5 : hydrothermal method : co-precipitation method S 2 S 4 S 5 1. Introduction In response to inadequate petroleum supplies for ever-increasing fuel and chemical demands, direct production of light olefins from coal-derived syngas (syngas to olefins, STO), as one of the promising non-petroleum processes, has received renewed interests in recent years as well as Fischer-Tropsch synthesis (FTS) to hydrocarbons. At the present time, this route is still a challenge in the chemical industry. How to improve the selectivity to olefins and the stability of its catalysts are still hot research topics. Iron-based catalysts have extensively been studied as the main components for improving light olefin selectivity, [1-3] in spite of the limitation of Anderson-Schulz-Flory (A-S-F) distribution in FTS and the impacts of secondary reactions such as hydrogenation and polymerization of primary olefin products. However, under typical FTS conditions, most of the catalysts prepared by the conventional preparation method show a broader spectrum of products and high heavier hydrocarbons (C 5 + ) with high CO conversion. Development of effective catalysts for controlling the product distribution is still a focus. [4] Factors which influence the behavior of FTS catalysts involve the chemical state, the size and the microenvironment of active phases, promoters, and so on. [5] Recently STO catalysts based on new preparation methods and promotion of additive have progressed and improved light olefin production. [1-3, 6-8] In particular, the FTS activity and olefin selectivity could be greatly enhanced by improving the dispersion of iron species [9]. It was reported that nanoparticle catalysts with special structure and homogeneous size can be synthesized by hydrothermal method and exhibited better catalytic activity for FTS than the catalysts prepared by the conventional method. [10] This special structure could also remarkably improve the dispersion of metal components. [11] Although iron-manganese catalysts have been investigated and been found to be active catalyst components for light olefin synthesis from syngas, [12-15] up to now few research has been conducted on iron-manganese catalysts prepared by hydrothermal method and their properties for the light The American Computational Science Society. 15, 2015, 1, 15-19

2 olefin synthesis from CO hydrogenation. In this paper, the preparation and optimization of alkali modified Fe-Mn catalysts via hydrothermal-incipient wetness impregnation (H-IWI) method were carried out. The catalysts exhibited excellent properties for the production of C 2 -C 4 olefins in CO hydrogenation compared with the catalysts prepared by co-precipitation method. The relationship between the structure and properties of the catalysts was discussed by means of the physicochemical property characterization. 2. Experimental 2.1. Catalyst preparation The catalyst precursor of Fe-Mn samples were prepared by the hydrothermal-incipient wetness impregnation (H-IWI) method using glucose (C 6 H 12 O 6 ) as carbon source. The molar ratio of Fe/Mn/C 6 was 3:1:5. First, desired amount of Fe(NO 3 ) 3 9H 2 O, Mn(NO 3 ) 2 (50 %wt) and glucose aqueous solution were mixed together and dissolved in certain amount of deionized water, and the concentration of glucose was controlled to be 0.5, 1.0, 1.5, 2.0 and 2.5 mol/l, respectively, for different samples. Then, the mixed solution was put into a 500 ml Teflon-lined stainless steel autoclave, being kept at 180 ºC for 4h. Next, the products were filtered, washed and dried at 120 ºC for 12 h. The obtained powder samples were further impregnated with the aqueous solution of potassium carbonate (1wt %), and dried at 120 ºC for 8 h. The prepared samples were named as S 1, S 2,, S 4 and S 5, respectively. Reference sample by co-precipitation method [16] was named as Catalyst characterization The microscopic morphology configuration was observed on a JEOL JSM-6360LV Scanning Electron Microscope (SEM) equipped with a JED-2300 Energy Dispersive Spectroscopy (EDS) surface element analysis system and a JEOL JFC-1600 Auto Fine Coater for sample preparation. Transmission electron microscopy (TEM) analysis was conducted on a HITACHI H-7650 microscope. Samples were ultrasonically dispersed in acetone prior to the analysis. The bulk structure of the catalysts was analyzed on a Rigaku D/MAX-2200PC X-ray diffratrometer with Cu Kα radiation, 40 kv, 30 ma at scanning speed of 8 /min. H 2 temperature-programmed reduction (H 2 -TPR) measurement of the fresh catalysts was carried out on a chemisorption analyzer (TP5000). Reducing gas (25 ml/min) was composed of 5% H 2 and 95% N mg sample (20-40 mesh) was pretreated in He at 350 ºC for 1 h, then cooled to room temperature. H 2 consumption was recorded as the temperature ramped from room temperature to 900 ºC at a heating rate of 10 ºC/min. CO temperature-programmed desorption (CO- TPD) measurement was performed on the TP mg sample was loaded in the quarts sample tube and was pretreated in H 2 /CO (2:1) (30 ml/min) at 280 ºC for 12 h. Then the sample was cooled to room temperature and purged with He (50 ml/min) for 1 h. Next the sample was treated with CO at 150 ºC until saturation, following He purge for 1 h to remove the weakly adsorbed species. Finally, the TPD profiles were recorded as the temperature rose from room temperature to 700 ºC at a heating rate of 10 ºC /min Catalytic activity tests The CO hydrogenation reaction was performed in a micro stainless flow-type pressurized fixed-bed reactor with 2 ml of catalysts (20-40 mesh) and the ratio of height to diameter (H/D) of the catalyst bed was 5. The typical reaction conditions were as follows: H 2 /CO=2/1 (molar ratio), GHSV=500 h -1, T=320 ºC and P=1.5 MPa. The catalyst sample was pre-reduced for 12 h in the flow of H 2 /CO=2, GHSV=500 h -1, 0.1 MPa and 280 ºC. The gas products were analyzed using an online gas chromatograph (GC-9560-I), with a 2 m TDX-01 column for C 1 component via TCD detector, and a 50 m Al 2 O 3 column for C 1 -C 5 hydrocarbons via FID detector, respectively. The liquid products were analyzed using an off-line GC-9560-II, with a 2 m GDX-401 column for aqueous phase products via TCD detector and a 30 m SE-30 column for oil phase products via FID detector, respectively. The results were calculated in terms of activity by CO conversion, light olefin selectivity (wt.) by olefin fraction in overall hydrocarbon products and olefin to paraffin ratio (O/P) in the C 2 -C 4 fraction. 3. Results and Discussion 3.1 Structure of the catalysts Fig.1 shows the SEM images of sample by hydrothermal-incipient wetness impregnation (H-IWI) method and sample by co-precipitation method. The sample shaped in regular sphere with uniform size. Carbon spheres embedded with highly dispersed iron nanoparticles by hydrothermal method could be easily synthesized at mild conditions. [10] The sample was irregularly shaped and showed large particle size. [16] Figure 1. SEM images of the samples. The TEM micrographs of samples are shown in Fig.2. It could be seen that all the samples shaped The American Computational Science Society. 16, 2015, 1, 15-19

3 Intensity (a.u.) spherical particles as observed by SEM. As the glucose concentration was increased from 0.5 mol/l to 2.5 mol/l, the average particle size decreased from about 600 nm to 80 nm. It was reported that the metal components are in a homogeneous dispersion state in the carbon matrix, and this special structure could improve the dispersion of metal components and provides a qualified, shape-selective environment. [11,18] The confinement effect of the carbonaceous matter could also inhibit the sintering of the iron active component during the FTS reaction, [10] which then further improved the activity and stability of the catalysts. with iron-containing species, could produce a synergistic effect, which could disperse iron active centers and prevent the carbon chain growth, and then increase the olefin selectivity and the production of light hydrocarbons during CO hydrogenation. [19] 3.2 Reduction behavior of the catalysts As shown in Fig.4, the co-precipitation catalyst sample was reduced via three steps by H 2. The reduction curve was similar to the reported Fe-Mn catalyst. [9,16] The first occurred at low temperature of around ºC, assigned to the reduction of a-fe 2 O 3 and a-mn 2 O 3 to Fe 3 O 4 and Mn 3 O 4. The second peak at around ºC could be attributed to the reduction of (Fe,Mn) 3 O 4 mixed spinel phase to manganowustite. [16] The third peak at high temperature of around ºC, corresponding to the reduction of FeO to form Fe metal. [9] However, the H 2 consumption was greatly decreased and the initial reduction temperature shifted to higher temperature for the sample prepared by H-IWI method, indicating that the sample was partially reduced during the hydrothermal process. The disappearance of low temperature reduction peak for the sample further confirmed the pre-reduction. The negative peak at about 640 ºC may be caused by the decomposition of incomplete carbonized organic complexes during the hydrothermal process. Figure 2. TEM images of the samples The XRD patterns of the sample are shown in Fig.3. It could be found that before reaction, weak diffractive peaks of Fe 3 O 4 and iron-manganese spinel phase were identified, implying that the sample was gradually reduced during the hydrothermal preparation, which could be confirmed by H 2 -TPR measurement in Fig.4. The diffractive peaks of MnCO 3 were observed after reaction, and the intensity of iron-manganese spinel phase and carbon phase were enhanced. The newly formed MnCO 3 phase, intimately contacting Fe 3 O 4 FeO-MnO MnCO 3 C Figure 3. XRD patterns of sample. after reaction before reaction H2 consumption (a.u.) T/ C Figure 4. H 2 -TPR profiles of the samples. 3.3 Surface adsorption behavior of the catalysts The CO-TPD profiles of the sample and are shown in Fig.5. Both two samples were pre-reduced in the flow of H 2 /CO (2:1). It was clear that no desorption peak was found at low temperature (< 300 ºC). The CO desorption temperature was mainly at ºC for the two samples. Obviously, the desorption peak of the sample prepared by H-IWI method was larger while the desorption temperature was slightly lower than that of the co-precipitation sample. This is agreeing with that the accessibility to the reactants for the reduced samples prepared by the hydrothermal method. [10] The different CO adsorption behavior of the samples by different preparation methods would result in different activities during CO hydrogenation. The American Computational Science Society. 17, 2015, 1, 15-19

4 CO desorption (a.u.) hydrothermal method had played an important role in improving the product distribution in CO hydrogenation. The CO conversion of 95.98% and the C 2 -C 4 olefin yield of 68.79g for each standard cubic meter of syngas (g/nm 3 (CO+H 2 )) were obtained under the given reaction conditions for the sample, in which the glucose concentration for the H-IWI preparation was 1.5 mol/l. Catalyst S1 S2 S3 S4 S5 S T/ C Figure 5. CO-TPD profiles of the samples. 3.4 CO hydrogenation activity The catalytic activity of the catalyst samples for CO hydrogenation is shown in Table 1. As mentioned above, samples S 1 -S 5 were prepared by the H-IWI method using glucose of different concentrations while sample by conventional co-precipitation method. All the samples showed high activity and light olefin selectivity, and exhibited good stability with the increase of time on stream. It was obvious that the activity of the catalysts prepared by the H-IWI method was significantly affected by the glucose concentration. As the glucose concentration increased, the CH 4 and CO 2 selectivity decreased. The C = 2 -C = 4 content in C 2 -C 4 fraction (O/P) was first increased and then decreased. The C + 5 weight content for the catalysts sample prepared by the H-IWI method was slightly increased compared with the sample by co-precipitation method. This may be caused by the increase in CO adsorption and thus the promotion on the chain propagation ability. That is, the increase of CO adsorption reduced the H 2 /CO molar ratio on the active sites during reaction, which improved the olefin selectivity, reduced the CH 4 selectivity and promoted the formation of heavy hydrocarbons. It could also be seen from Table 1 that the catalysts prepared by the H-IWI method in proper glucose concentration at mol/l, favored the formation of C 2 -C 4 hydrocarbons with high olefin selectivity. The ratio of olefin to paraffin in C 2 -C 4 fraction for sample could reach In addition, both the CO 2 and CH 4 selectivity for most of the samples were much lower than the catalyst prepared by co-precipitation method. It was believed that the presence of carbon with suitable content contacting with the active sites provided a hydrophobic surface, which could restrain the adsorption of water and inhibit the WGS reaction. [17] Increasing the carbon content favored to suppress the formation of CO 2 but excessive carbon covered the active centers and lowered the activity. Obviously, the improved dispersion of active components on iron-manganese catalyst prepared by Carbon Sel.% Products (wt. %) Productivity g/[nm 3 (CO+H2)] CO Conv.% CO CH CH C2= = C C5 C2= = Table 1. CO hydrogenation activity of the samples. Reaction conditions: H 2 /CO = 2, GHSV = 500 h -1, T = 320 ºC, p = 1.5 MPa, time-on-stream = 150 h 4. Conclusion Iron-manganese catalyst prepared by the hydrothermal procedure with glucose as carbon sources showed uniform particle size in certain glucose concentration. The hydrothermal process significantly influenced the structure, reduction behaviors, the adsorption behaviors and then the catalytic performance of the catalyst samples. The active components were distributed by carbon in a homogeneous dispersion state, which increased CO adsorption and then enhanced light olefin selectivity. For CO hydrogenation, the catalyst samples with suitable carbon content, prepared in a glucose concentration of mol/l, could reduce CO 2 and CH 4 selectivity and favor the formation of C 2 -C 4 olefins. Acknowledgments The authors gratefully acknowledged the financial support from the Key Project of Natural Science Foundation of Ningxia (NZ13010) and the Western Light Talent Culture Project of Chinese Academy of Sciences. References [1] H.M. Torres Galvis, J.H. Bitter, C.B. Khare, M. Ruitenbeek, A. Iulian Dugulan, K.P. de Jong, Science 335 (2012) [2] W. Chen, Z.L. Fan, X.L. Pan, X.H. Bao, Journal of the American Chemical Society 130 (2008) The American Computational Science Society. 18, 2015, 1, 15-19

5 [3] C.F. Wang, X.L. Pan, X.H. Bao, Chinese Science Bulletin 55 (2010) [4] Q.H. Zhang, J.C. Kang, Y. Wang, ChemCatChem 2 (2010) [5] Q.H Zhang, W.P. Deng, Y. Wang, Journal of Energy Chemistry 22 (2013) [6] S.H. Kang, J.W. Bae, P.S. Sai Prasad, S.J. Park, K.J. Woo, K.W. Jun, Catalysis Letters 130 (2009) [7] H.M. Torres Galvis, K.P. de Jong, ACS Catalysis 3 (2013) [8] H.M. Torres Galvis, A.C.J. Koeken, J.H. Bitter, T. Davidian, M. Ruitenbeek, A. Iulian Dugulan, K.P. de Jong, Journal of Catalysis 303 (2013) [9] J.B. Li, H.F. Ma, H.T. Zhang, Q.W. Sun, W.Y. Ying, D.Y. Fang, Reaction Kinetics Mechanisms and Catalysis 112 (2014) [10] G.B. Yu, B. Sun, Y. Pei, S. Yan, M.H. Qiao, K.N. Fan, X.X. Zhang, B.N. Zong, Journal of the American Chemical Society 132 (2010) [11] X.M. Sun, Y.D. Li, Angewandte Chemie-international Edition 43 (2004) [12] J.L. Zhang, L.H. Ma, S.B. Fan, T.S. Zhao, Y.H. Sun, Fuel 109 (2013) [13] Y. Yang, H.W. Xiang, R.L. Zhang, B. Zhong, Y.W. Li, Catalysis Today 106 (2005) [14] T. Herranz, S. Rojas, F.J. Pérez-Alonso, M. Ojeda, P. Terreros, J.L.G. Fierro, Applied Catalysis A: General 311 (2006) [15] T.Z. Li, H.L. Wang, Y. Yang, H.W. Xiang, Y.W. Li, Journal of Energy Chemistry 22 (2013) [16] J.L. Zhang, K.G. Fang, K. Zhang, W.H. Li, Y.H. Sun, Korean Journal of Chemical Engineering 26 (2009) [17] M.L. Cubeiro, H. Morales, M.R. Goldwasser, M.J. Pérez-Zurita, F. González-Jiménez, N.C. Urbina de, Applied Catalysis A: General 189 (1999) [18] X.M. Sun, J.F. Liu, Y.D. Li, Chemistry-A European Journal 12 (2006) [19] X.G. Li, B. Zhong, S.Y. Peng, Q. Wang, Catalysis Letters 23 (1994) Professor Tian-Sheng Zhao is executive director at State Key Laboratory Cultivation Base of Natural Gas Conversion, Ningxia University, People's Republic of China. He received his Ph.D in applied catalysis (1999) from State Key Laboratory of Coal Conversion, Chinese Academy of Sciences and started work at Ningxia University. He did a visiting research fellow at Venture Business Laboratory, University of Toyama, Japan ( ). His current interests involve development of structural catalysts for directional transformation of carbon oxides/methanol, hydrogenation, interconversions of hydrocarbons/alkyl halides/silanes. Jianli Zhang received his Ph.D in industrial catalysis from State Key Laboratory of Coal Conversion, Chinese Academy of Sciences in 2009 and then entered Ningxia University. He is now an associate professor at State Key Laboratory Cultivation Base of Natural Gas Conversion, Ningxia University, People's Republic of China. His research interest focuses on the synthesis and characterization of catalysts in Fischer-Tropsch Synthesis. The American Computational Science Society. 19, 2015, 1, 15-19