Effect of zinc oxide on the properties and Fischer-Tropsch synthesis activities of the Co/ZrO 2 catalyst prepared by flame spray pyrolysis

Transcription

1 Paper Code: cr006 TIChE International Conference 2011 Effect of zinc oxide on the properties and Fischer-Tropsch synthesis activities of the Co/ZrO 2 catalyst prepared by flame spray pyrolysis Phattiya Phuttipong *, Patita Tangjitsirirat, Thunrawee Kierdwarunpunya, Choowong Chaisuk Department of Chemical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom, 73000, Thailand * far_princess-of-art@hotmail.com Abstract In this research to study the effect to ZnO promote on the catalytic properties and Fischer Tropsch synthesis activity of the cobalt catalyst on Zr support prepared by two techniques, wet impregnation and FSP-incorporation. The influence of the promoter on the reducibility particle dispersion, surface area of the catalysts and cobalt particle size was studied by different techniques including N 2 adsorption, X-Ray diffraction (XRD), temperature programmed reduction (TPR), and H 2 chemisorptions were used to characterize the catalysts. Addition of ZnO affected significantly the reducibility of catalyst peaks indicating two steps of reaction for all ZnO-modified catalyst. The presence of ZnO promoted strong interaction of cobalt and zirconia.the catalytic performances were tested by FTS in a fix-bed reactor. It shows that the methane selectivity and heavy hydrocarbon increasing. Keyword: Co/ZrO 2 catalyst/ Fischer-Tropsch synthesis/ ZnO/ flame spray pyrolysis/ syngas cr006-1

2 1. Introduction The Fischer Tropsch synthesis was a catalyzed chemical reaction to convert synthesis gas (CO+H 2 ) to liquid hydrocarbons of various forms [1]. Metallic cobalt is one of the most commonly used catalysts for low temperature FT synthesis because of its high activity, high selectivity to linear C5+ hydrocarbons, low activities for water gas shift reaction, and lower price compared to ruthenium [2-5]. Use of zirconia as a cobalt catalyst support has showed the high activity and selectivity of long chain hydrocarbon in FTS [6]. The metal oxide added in bimetallic form of catalyst can be used to modified the support leading to increased dispersions, the result of this promotion was increase in the number of active sites and catalyst activity. The metal oxide was useful in FTS catalyst such as MgO, K 2 O, CaO, MnO 2, Ni 2 O 3, ZnO, Mo 2 O 3 and La 2 O 3. Addition of zinc oxide shows increased activity of cobalt catalyst, reduced methane content, enhanced Co reducibility and increased the cobalt dispersion [7-8]. In this research, ZnO doping by two techniques, wet impregnation and FSP-incorporation, were focused on Co-ZrO 2 based catalysts for testing the Fischer-Tropsch synthesis Catalyst preparation 2. Experimental In this research, the flame spray pyrolysis (FSP) and impregnation techniques were used to prepare the catalyst. All the catalysts containing cobalt (20 wt%) were promoted by calcium oxide (5 wt%). Some cobalt and/or calcium oxide was partially incorporated with ZrO 2 during the FSP step whereas some of it was impregnated. The six types of catalysts were shown in table 1. Table 1. The symbol of the catalysts prepared by the different methods Catalyst preparation method Symbol 20 wt% Co incorporated with ZrO 2 by FSP 20Co-Z 20 wt% Co impregnated on ZrO 2 by FSP 20Co/Z 20 wt% Co and 5 wt% CaO were incorporated with ZrO 2 by FSP 5 wt% CaO was impregnated on 20 wt% Co incorporated with ZrO 2 by FSP 20 wt% Co was impregnated on 5 wt% CaO incorporated with ZrO 2 by FSP 20 wt% Co and 5 wt% CaO were impregnated on ZrO 2 prepared by FSP Preparation by Flame Spray Pyrolysis 20Co-5CaO-Z 5CaO/20Co-Z 20Co/5CaO-Z 20Co-5CaO/Z The FSP was technique used for preparation of support and catalysts. 80 wt% of zirconium n-butoxide and cobalt naphthenate were used as a catalyst precursor and diluted with xylene to a 0.5 M solution. The precursor was fed in the center of the FSP nozzle by syringe pump at 5 ml/min. Dispersing the precursor by oxygen was fed at rate 5 ml/min. The pressure drop at the nozzles was held constant at 1.5 bar by adjusting the orifice gap area at the nozzle. The catalyst powder was collected on a glass microfiber filter (Whatman) with the aid of a vacuum pump Metal loading by wet impregnation The metal oxide promoted cobalt catalyst on zirconia support with different position were prepared by wetimpregnation using cobalt (II) nitrate hexahydrate [(Co(NO 3 ) 2 ).6H 2 O], Calcium nitrate [Ca(NO 3 ) 2. 4H 2 O] or Zinc acetate [(CH 3 COO) 2 Zn. 2H 2 O]. The aqueous precursors were droplet above until flooding on support prepared by flam spray pyrolysis. After wet impregnation the catalyst precursors were stirred under 70 C for 6 h and subsequently the samples were dried at 110 C overnight. All catalysts were calcined from room temperature to 500 C by temperature rate 10 C /min and kept at 500 C for 4 h under air flow. The catalysts which succeed from first calcinations were introduced to the second impregnation. The second aqueous precursor was used to wet impregnation again and followed by the second calcination in the same condition. 2.2 Catalyst characterization XRD measurement The XRD measurement was used to identification and quantification of minerals, compounds and other crystalline phases of each catalyst in an X-ray diffractometer SIEMENS D-5000, X-ray diffractometer of Cu Kα connected with a computer with Diffract ZT cr006-2

3 version 3.3 program for fully control of the XRD analyzer. The XRD patterns were recorded in the range 2θ from 20 to 80. The crystallite sizes were concluded from XRD data using the Scherrer equation. The Scherrer equation appears or immediately following: d (1) B cos Where d is the average crystallite diameter, k is the peak length ( A ), B is the half peak width of Co 3 O 4 from Temperature Programmed Reduction (TPR) Phase and support of each catalyst were examined by using temperature programmed reduction (TPR) technique. The TPR experiment was carried out using a Micrometritics Pulse Chemisorb 2700 instrument. The catalyst (ca. 0.1g) was placed in a quartz tubular reactor was initially heated under 25 ml/min of nitrogen atmosphere flow at a heating rate of 10 C/min to 250 C and held at 250 C for 1 h to eliminate the adsorbed water and then reduced in the 10% H 2 in Ar 0f 25 ml/min from room temperature to 800 C at the ramp heating rate of 10 C/min. The effluent gas was monitored by a TCD chromatographs (GC) connecting with GC-FID and GC- TCD detector. 3. Result and discussion 3.1 The Phase Analysis by X-ray Diffraction (XRD) Fig. 1.presented the catalysts were prepared by flam spray pyrolysis was tetragonal phase, while monoclinic and tetragonal take place with impregnated catalysts. The catalysts with addition of zinc oxide was analyzed, the result was presented in Fig. 2.The impregnation cobalt on catalysts are contain with the monoclinic and tetragonal phase of zirconia. The FSP catalysts reduce the monoclinic phase of zirconia in 5ZnO/20Co-Z H 2 chemisorption The active metallic cobalt surface area of cobalt metal on each catalyst samples are estimated from a 0.1 g of a catalyst sample was packed in a glass U-tubular reactor. The catalyst sample was reduced in hydrogen heated to 350 o C under atmosphere pressure of hydrogen with a flow rate 50 ml/min and held at 350 o C for 2 h. After that reactor temperature was cooled down to room temperature by nitrogen at a flow rate of 30 ml/min.the metal active sites were measured when catalyst was heated to 100 o C. The purity hydrogen gas was injected into the injection port to adsorb on the metal surface of the catalyst sample. Injection of hydrogen was continuously repeated until saturation. Figure 1. XRD patterns of the 20Co-Z and the 20Co/Z 2.3 Catalyst evaluation The catalytic activity and product selectivity of all samples were tested by Fischer-Tropsch synthesis in a fixed-bed reactor. Prior the reaction, 0.2 ml of catalyst was reduced by H 2 with flow rate of 30 ml/min at 350 C for 2 h. After the reduction, the reactor temperature was decreased to 220 C under atmospheric pressure, the syngas (molar ratio of H 2 /CO = 2.33 and GHSV = 9000 h -1 ) was introduced. The total volumetric flow rate was 30 ml/min. The composition of hydrocarbons in the product was analyzed by two on-line gas Figure 2. XRD patterns of the Co based catalysts with bimetallic zinc oxide. cr006-3

4 3.2 The Reduction Characteristics by TPR Zinc oxide also addition to FSP catalysts. Figure3 presents the reducibility of 20Co-5ZnO-Z and 5ZnO/20Co-Z catalysts. The reduction peak of 20Co- 5ZnO-Z is similar to the 20Co-Z catalyst but occur with higher temperature for the both peaks. Other that, the first peak of 5ZnO/20Co-Z catalyst taken place at higher temperature than 20Co-Z but in the second peak, CoO reduced to Co metal at lower temperature. However, addition of ZnO by wet impregnation between supports. The reducibility of impregnation cobalt catalyst with addition of zinc oxide is exhibits in Figure 4. Zinc oxide in 20Co/5ZnO-Z and 20Co-5ZnO/Z catalysts gave the effect to the reducibility peak, the reduction of cobalt metal in catalysts finish at the temperature higher than 450 C. bulk could transform to Co metal. After that, hydrogen gas was absorbed into site of Co metal. The active sites of catalysts were calculated by the remained of hydrogen absorbed.the catalyst which gave lowest amount of H 2 chemisorption was is 5ZnO/20Co-Z. Its might be the particles of ZnO loading by impregnate is cover on 20Co-Z. Thus, this affect to difficultly absorbed atom of H 2 into cobalt active site. 3.4 Fischer-Tropsch synthesis The result of FTS test for the 20Co-5ZnO-Z and the 5ZnO/20Co-Z shows in Figure 5 the low CO conversion and high selectivity to heavy hydrocarbon due to the cobalt reducibility was decrease and the cobalt particle size was increase. The 5ZnO/20Co-Z showed the low CO conversion and high selectivity to methane and heavy hydrocarbon due to the cobalt reducibility was decrease and very large cobalt particles. The iso-paraffin was product of both catalysts. Figure 6 presents the low CO conversion of the 20Co/5ZnO-Z and the 20Co-5ZnO/Z because the cobalt reducibility was decrease. The both catalysts were the same selectivity of heavy hydrocarbon due to the cobalt particle size was decrease when addition of ZnO. Iso-paraffin was product of the both catalysts Figure 3.TPR patterns of the Co/ZrO 2 catalysts prepared by FSP and addition ZnO bimetallic Figure 5.The selectivity of products for effect of zinc oxide loading on cobalt catalyst at 180 min 4. Conclusions Figure 4. TPR patterns of the Co/ZrO 2 catalysts prepared by wet impregnation and addition ZnO bimetallic. 3.3 The Cobalt Metallic Sites by H 2 Chemisorption The active sites of cobalt metals on catalyst samples were measured by Hydrogen Chemisorption. Initial, the catalysts were reduced by hydrogen gas. The Co 3 O 4 The effect of promoter ZnO on catalytic property. Addition of ZnO with impregnation catalysts gave higher reduction temperature than 20Co/Z catalyst. ZnO might be improved the interaction between support and active site, it difficultly to reduced. It gave strongly interaction between active site and support, the reduction peak of TPR is highest. However, addition of ZnO can improve the reducibility of the catalyst at the reduce step of CoO to Co metal. And increased the cr006-4

5 selectivity of methane and heavy hydrocarbon. The catalysts can change the product distribution and promote product of isoparaffin. References [1] Y. Liu, T. Hanaoka, T. Miyazawa, K. Murata, K. Okabe, K. Sakanishi, Fischer Tropsch synthesis in slurry-phase reactors over Mn- and Zr-modified Co/SiO2 catalysts, Fuel Tech, 90 (2009) 901. [2] P. Dutta, B.C. Dunn, E.M. Eyring, N. Shah, G.P. Huffman, A. Manivannan, M.S.Seehra, Chem. Mater. 17 (2005) [3] M.E. Dry, Catal. Today 71 (2002) [4] A.Y. Khodakov, W. Chu, P. Fongarland, Chem. Rev. 107 (2007) [5] [F. Diehl, A.Y. Khodakov, Oil Gas Sci. Technol. Revue de l IFP 64 (2009) [6] T. Ma, Y. Huang, J. Yang, J. He, L. Zhao. Preparation of spherical zirconia powder in microemulsion system and its densification behavior, Mater. Design, 25 (2004) 515. [7] N. N. Madikizela, N. Covile, The influence of Zn on Co/TiO 2 catalysts for Fischer-tropsch synthesis. J.Catal., 181 (2002) 128. [8] N. Escalona, C. Medina, R. Garcıa, P. Reyes, Fischer Tropsch reaction from a mixture similar to biosyngas. Influence of promoters (Ru, Re, Cu, Zn) on surface and catalytic properties of Co/SiO2 catalysts, Catal. Today, 143 (2009) 78 cr006-5