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ScienceDirect Energy Procedia 79 (2015) 163 – 168

2015 International Conference on Alternative Energy in Developing Countries and Emerging Economies

Effect of Manganese Promoter on Cobalt Supported Magnesia Catalyst for Fischer-Tropsch Synthesis Watis Warayanona,c, Sabaithip Tungkamania,c*, Hussanai Sukkathanyawata,b,c, Monrudee Phongaksorna,c, Tanakorn Ratanaa,c, Thana Sornchamnid a

Department of Industrial Chemistry, Faculty of Applied Science, King Mongkut’s University of Technology North Bangkok, Bangkok 10800, Thailand. b Faculty of Science Energy and Environment, King Mongkut’s University of Technology North Bangkok (Rayong Campus), Rayong 21120, Thailand. c Research and Development Center for Chemical Engineering Unit Operation and Catalyst Design (RCC), Science and Technology Research Institute, King Mongkut’s University of Technology North Bangkok, Bangkok 10800, Thailand. d PTT Research and Technology Institute, PTT Company Limited, Ayutthaya 13170, Thailand.

Abstract Fischer-Tropsch synthesis (FTS) is a key performance in the gas-to-liquid (GTL) process. This research is focused on the effect of Mn promoter on cobalt supported MgO catalyst in Fischer Tropsch reaction. 30 wt.%Co/MgO (30CM) and 30 wt.%Co/MgO-x wt.%Mn (30CM-x%Mn) were synthesized as an unpromoted and a promoted catalyst, respectively. The effect of Mn promoter and its loading on physical and chemical properties of catalysts were investigated by BET, H2-TPR, H2-TPD and TPSR techniques. The catalytic performance of all catalysts was explored in FTS. The addition of Mn promoter deliberately promotes the FTS activity as well as selectivity towards long chain hydrocarbons in the range of diesel product. Mn promoter beneficially improves the cobalt dispersion and decorating cobalt surface. This behavior is elucidated that Mn is functionalized in term of structural and electronic promoter. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

© 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility the Organizing Committee of 2015 AEDCEE. responsibility of theofOrganizing Committee of 2015 AEDCEE Peer-review under

Keywords: Magnesia; Manganese promoter; Cobalt based catalyst; Fischer-Tropsch synthesis

* Corresponding author. Tel.: +66-2555-2000 ext. 4822; fax: +66-2587-8251. E-mail address: [email protected] or [email protected].

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Organizing Committee of 2015 AEDCEE doi:10.1016/j.egypro.2015.11.456

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1. Introduction Fischer-Tropsch synthesis (FTS) is a key performance of the gas-to-liquid (GTL) process which transforms natural gas into environmental friendly liquid fuels. FTS converts syngas, the mixture of hydrogen (H2) and carbon monoxide (CO) primarily, to clean hydrocarbon product over solid catalysts. In many practical applications, cobalt-based catalysts have been used as a commercial catalyst due to the selectivity towards linear paraffin with low activity for water-gas shift reaction [1, 2]. However, the activity and stability of this catalyst must be improved. This research has developed a cobalt supported catalyst by using magnesia (MgO) as a support in order to increase carbon resistance properties. To enhance the catalytic activity, the catalyst was promoted by manganese (Mn). The reducibility, transient CO hydrogenation and FTS activity including product selectivity over synthesized Co/Mn/MgO and Co/MgO catalyst were investigated and discussed. 2. Experimental 2.1. Catalyst preparation 30wt.%Co/MgO (30CM) catalyst was prepared by sol-gel method. The catalyst was dried and calcined at 450 ºC for 4 h. 30 wt.%Co/MgO-x wt.%Mn (30CM-x%Mn) catalyst was prepared by impregnation of aqueous manganese solution onto 30CM followed by dried and calcined at 450 ºC for 4 h. 2.2. Catalyst characterization Surface area, pore volume and average pore diameter of catalyst samples were analyzed by N2 adsorption/desorption isotherm at -196 °C, using BELSORP-mini II. Prior to the analysis, samples were degassed at 350 °C in N2 flow for 4 h. The reducibility of all catalysts was studied using temperature programmed reduction of H2 (H2-TPR) in BELCAT-Basic system. The measurement was obtained over 0.05 g of a sample, placed in a quartz Utube reactor, fed with a 5%H2/Ar flowing at 50 mL/min and heated at the rate of 10 °C/min from 40 °C to 900 °C. The H2 consumption was analyzed with thermal conductivity detector (TCD). The metal dispersion, surface area of metal and size of metal particle for all samples were determined by temperature programmed desorption of H2 (H2-TPD) using a BELCAT-Basic system. The quartz Utube reactor was loaded with 0.05 g of a sample. The samples were reduced in situ at 500 °C in H2 flow (50 mL/min) for 2 h using a heating rate of 10 °C /min followed by cooling to 100 °C in Ar flowing at 50 mL/min. Subsequently, an isothermal adsorption of H2 was performed at 100 °C for 0.5 h. Then, the temperature was cooled to ambient temperature in Ar flow (50 mL/min). Then, H2-TPD was measured from 40 °C to 900 °C at a ramping rate of 10 °C/min using Ar flowing at 50 mL/min. The H2-TPD was recorded by TCD detector. Temperature programmed surface reaction (TPSR) of CO-hydrogenation was conducted in a stainless steel tubular reactor (in-house system) loaded with 0.2 g of a catalyst. Before TPSR measurement, the sample was reduced in H2 flow (30 mL/min) at 500 °C for 2 h using a temperature ramp of 10 °C/min. The sample was then cooled to ambient temperature in flowing He (30 mL/min), CO was adsorbed at ambient temperature by flowing 10%CO/He (30 mL/min) for 30 min. After adsorption, the samples were flushed in He flow (30 mL/min) for 30 min. The transient CO-hydrogenation was performed from 40 °C to 900 °C in H2 flowing at 30 mL/min and heated at the rate of 10 °C/min. The CO-hydrogenation activity is represented as a methanation monitored by flame ionization detector (FID) in Agilent 6820-GC.

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2.3. FTS reaction The FTS test was carried out using a fixed-bed reactor. Catalyst was in-situ reduced at 750 ºC for 24 h under H2 flow and the catalyst bed was cooled down to the reaction temperature under inert gas. FTS was performed at 200 ºC in 80 mL/min flow of syngas (H2:CO of 2:1) for 24 h. Liquid product was trapped and analyzed by gas chromatograph (Bruker 430-GC) while gas products were detected by online gas chromatograph (Bruker 450-GC) using FID and TCD detector. 3. Results and discussion 3.1. Characterization of the catalysts 3.1.1. N2 adsorption measurement Textural properties of support and all catalysts are summarized in Table 1. MgO prepared by sol-gel method shows a total surface area as high as 188 m2/g with a pore volume of 0.76 cm3/g and a pore diameter of 16.4 nm. Lower values of surface area, pore volume and pore diameter are found for catalyst samples (30CM and 30CM-x%Mn) due to pore blocking effect. Consequently, these values of promoted catalysts (30CM-x%Mn) are smaller than those of unpromoted catalysts. Table 1. Textural properties of support and catalysts. Sample Support MgO 30CM 30CM-0.25%Mn 30CM-0.5%Mn 30CM-1%Mn

Surface area (m2/g)

pore volume (cm3(STP)/g)

Average pore diameter (nm)

187 134 117 112 88

0.76 0.50 0.34 0.35 0.39

16.4 15.0 11.8 12.5 17.7

3.1.2. Temperature programmed reduction (TPR) TPR profiles (Figure 1) of cobalt based catalysts show that Co 3O4 bulk was reduced in two-steps. The first peak is assigned to the reduction of Co3O4 to CoO and the second peak represents a reduction of CoO to Co0 [3]. However, the broad peak observed at high temperature is attributed to a reduction peak of cobalt oxide which has strong interaction with support. Compared to unpromoted catalyst (30CM), the reduction profile of promoted catalyst (30CM-x%Mn) is slightly shifted to lower temperature, indicating a higher reducibility. A presence of Mn presumably influences on the cobalt dispersion by preventing the agglomeration of the Co particles [4]. Moreover, manganese oxide particles decorating Co surface assist the reduction of cobalt oxide by electronic effect.

Watis Warayanon et al. / Energy Procedia 79 (2015) 163 – 168

TCD Response (a.u.)

166

(d)

(c)

(b)

(a) 0

100

200

300 400 500 600 o Temperature ( C)

700

800

900

Fig. 1. TPR profiles of the catalysts (a) 30CM; (b) 30CM-0.25%Mn; (c) 30CM-0.5%Mn; (d) 30CM-1%Mn.

3.1.2. Temperature programmed desorption of hydrogen (H2-TPD) Metal dispersion, surface area of metal and size of metal particle calculated from the H2-TPD measurement are reported in Table 2. It indicates that Mn acts as a structural promoter because the addition of small quantity of Mn enhances the metal dispersion, surface area of metal and size of metal particle. However, this feature is decreased as Mn loading is increased due to the pore blocking effect resulting into lowering total surface area [5]. Table 2. Metal dispersion, surface area of metal and size of metal particle of all catalysts calculated from H2-TPD measurement. Catalyst 30CM 30CM-0.25%Mn 30CM-0.5%Mn 30CM-1%Mn

%Metal dispersion

Surface area of metal (m2/g)

Size of metal particle (nm)

7.64 9.55 8.22 5.65

81.34 101.70 87.45 60.18

8.28 6.62 7.70 11.20

3.1.3. Temperature programmed surface reaction (TPSR) TPSR technique was used to determine catalyst activity of CO-hydrogenation (Equation 1) at transient state. CO + 3H2

o

CH4 + H2O

(1)

For comparison, temperature at the maximum for hydrogenation of pre-adsorbed CO over 30CMx%Mn is lower than 30CM. The amount of methane product for 30CM-Mn also increases with enhancement of Mn loading (Table 3). The results disclose that Mn promoter significantly improves CO hydrogenation due to Mn increases electron density of CO chemisorptions sites [6, 7].

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FID Response (a.u.)

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(d)

(c) (b) (a)

0

100

200

300

400

500

600

700

800

900

Temperature (oC) Fig. 2. TPSR profiles of the catalysts (a) 30CM; (b) 30CM-0.25%Mn; (c) 30CM-0.5%Mn; (d) 30CM-1%Mn. Table 3. CH4 formation calculated using area under TPSR profiles. Catalyst

CH4 formation (Pmol/gcat)

30CM 30CM-0.25%Mn 30CM-0.5%Mn 30CM-1%Mn

6.04 20.17 30.36 49.35

3.2. Catalytic performance The results obtained from FTS reaction at 200 ºC for 24 h are summarized in Table 4. The appearance of small quantity of Mn decreases the CH4 formation but increases the long-chain hydrocarbon product selectivity. The reason is that Mn promoter decreases the hydrogenation rate leading to higher long-chain hydrocarbon product [8]. Table 4. FT catalytic performance after 24 h reaction at 1 bar and 200 ºC. Catalyst 30CM 30CM-0.25%Mn 30CM-0.5%Mn 30CM-1%Mn

CO Conversion (%) 9.85 9.89 9.00 8.20

CH4 33.61 16.77 18.79 24.33

Hydrocarbon selectivity (%) C2-C4 C5-C8 C9-C15 34.25 13.33 14.58 25.90 23.39 30.76 27.27 19.11 34.73 29.33 12.70 33.26

C16-C24 4.28 3.18 0.10 0.38

4. Conclusions The effect of different addition the Mn on the catalyst and catalytic performance was investigated. In this work, 30CM catalyst were prepared by sol-gel method while the 30CM-Mn was prepared by impregnated 30CM with Mn-solution. Furthermore, the effect of Mn promoter on the reducibility and transient CO hydrogenation were investigated by physical and chemical properties. Finally, the effect of

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Mn promoter on the catalytic performance was investigated by FTS reaction. It was found that 30CM0.25%Mn catalyst was shown the better catalytic performance than the other catalysts. Acknowledgements The authors would like to thank the Research and Development Center for Chemical Engineering Unit Operation and Catalyst Design (RCC) and PTT Research and Technology Institute for instrument and financial support. References [1] Ali A. Mirzaei, Mostafa Faizi, Razieh Habibpour. Effect of preparation condition on the catalytic performance of cobalt manganese oxide catalyst for conversion of the synthesis gas to light olefins. Applied Catalysis A: General 2006;306:98-107. [2] M. Asalanfar, A.A. Mirzaei, H.R. Bozorgzadeh, A. Samimi, R. Ghobadi. Effect of support and promoter on the catalytic performance and structural properties of the Fe-Co-Mn catalysts for Fischer-Tropsch synthesis. Journal of Industrial and Engineering Chemistry 2014;20:1313-1323 [3] Ahmed Tavasoli, Reza M. Malek Abbaslou, Marine Trepanier, Ajay K. Dalai. Fischer-Tropsch synthesis over cobalt catalyst supported on carbon nanotubes in slurry phase reactor. Applied Catalysis A: General 2008;345:134-142. [4] Theresa E. Feltes, Leticia Espinosa-Alonso, Emiel de Smit, Lawrence D'Souza, Randall J. Meyer, Bert M. Weckhuysen, John R. Regalbuto. Selective adsorption of manganese onto coblat for optimized Mn/Co/TiO2 Fischer-Tropsch catalysts. Journal of Catalysis 2010;270:95-102. [5] Yuping Li, Xinxin Qin, Tiejun Wang, Longlong Ma, Lungang Chen, Noritatsu Tsubaki. Fischer-Tropsch synthesis from H2deficient biosyngas over Mn added Co/SiO2 catalysts. Fuel 2014;136:130-135. [6] J. Thiessen, A. Rose, J. Meyer, A. Jess, D. Curulla-Ferré. Effect of manganese and reduction promoters on carbon nanotube supported cobalt catalysts in Fischer-Tropsch synthesis. Microporous and Mesoporous Materials 2012;164:199-206. [7] Mostafa Feyzi, Ali A. Mirzaei. Catalytic behaviors of Co-Mn/TiO2 catalysts for Fischer-Triosch synthesis. J Fuel Chem Technol 2012;40:1435-1443. [8] Fernando Morales Carno. Manganese promotion in titania-supported cobalt Fischer-Tropsch catalysis. Utrecht University; 2006.