Fischer–Tropsch synthesis over Pd promoted cobalt based mesoporous supported catalyst

The present study focuses on the catalytic conversion of syngas (CO + H2) through Fischer–Tropsch (FT) route using two identically prepared 0.1 wt.% palladium promoted Mesoporous Alumina (MA) and SBA–15 supported Co (15 wt.%) catalysts. The Fischer–Tropsch activity is performed in a fixed bed tubular reactor at temperature 220 °C and pressure 30 bar with H2/CO ratio ~2 having Gas Hourly Space Velocity (GHSV) of 500 h−1. Detail characterizations of the catalysts are carried out using different analytical techniques like N2 adsorption-desorption, Temperature-programmed reduction with hydrogen (H2-TPR), Temperature-programmed desorption with NH3 (NH3-TPD), X-Ray Diffraction (XRD), and Transmission Electron Microscopy (TEM). The results show that the SBA–15 supported catalyst exhibits higher C6–C12 selectivity (57.5%), and MA supported catalyst facilitates the formation of higher hydrocarbons (C13–C20) having a selectivity of 46.7%. This study attributes the use of both the support materials for the production of liquid hydrocarbons through FT synthesis.


Introduction
Skyrocketing energy demand for enhancement of human lifestyle, development of industries, and transportation sectors across the globe is a matter of great concern, especially the markets for specified liquid fuels like gasoline, jet, and diesel [1]. It is a challenging and demanding task to develop alternative and environmentally friendly routes to produce liquid fuel. Fischer-Tropsch (FT) synthesis and associated technologies like Gas-To-Liquid (GTL), Coal-To-Liquid (CTL) and Biomass-To-Liquid (BTL) routes are a promising alternative for the production of substitutes of petroleum crude. They can positively contribute to the world energy security and supplies. Although FT was discovered about a century before, still it is an attractive and alternative source of environmentally benign liquid hydrocarbon fuels with near-zero sulfur and aromatic compounds. FT synthesis is a catalytic reaction in which syngas (a mixture of CO and H 2 ) is converted to liquid hydrocarbon fuels [2,3]. FT synthesis has received considerable worldwide attention in both industrial as well as in academic domains. Catalyst is considered as the heart of this process. Preparation technique, the composition of the catalyst, and other operating parameters like temperature, pressure, etc. play crucial roles towards FT synthesis, activity of the catalyst, and product distribution. So, the development of FT catalysts with higher activity, selectivity, and stability is of utmost importance for better performance. Among the transition metals, ruthenium, nickel, cobalt and iron are the most common metals used for FT catalyst. Cobalt is an active metal for the synthesis of long-chain hydrocarbons due to its high activity and selectivity towards paraffins, low Water-Gas Shift (WGS) activity, lower CO 2 selectivity, and less oxygenates formation [4][5][6]. Many researchers have studied the effect of different oxide supports (titania, silica, alumina, zirconia, etc.) on the activity/selectivity of Co catalysts [7,8], and these supports have considerable influence on the dispersion and reducibility of active metal as well. Al 2 O 3 , SiO 2 , and TiO 2 are the most extensively used support materials for Co catalysts. In Co/Al 2 O 3 catalyst, the limited reducibility of Co is due to the strong interaction between the support and the Co [9][10][11]. Due to the weaker interaction with silica, dispersion of Co is low, enhancing the reducibility of Co oxides. Reducibility of cobalt plays a vital role in catalytic activity of the supported catalyst. Promoters such as Pd, Pt, Re, and Ru are added to the Co catalysts to facilitate better reduction of Co as it has strong interaction with Al 2 O 3 than other supports [12,13]. It is reported by many researchers that Pd promoter enhances adsorption sites for H 2 on Co catalyst and thus increasing the hydrogenation rate in FT synthesis [14][15][16]. Guczi et al. [17] showed that Pd as a promoter to Co/SiO 2 increased the relative fraction of alkanes in FT synthesis reaction. Many researchers utilized Pd to improve the performance of zeolite and FT synthesis hybrid catalysts for C 5 -C 11 iso-paraffins synthesis [18,19]. Researchers of the Centre for Applied Energy Research (CAER), UK [20], observed that Pd is an effective promoter of cobalt oxide reduction in Co/c-Al 2 O 3 FT catalysts. Pd can improve the reduction and dispersion of cobalt-based FT synthesis catalysts [21]. Among bimetallic catalysts, Co-Pd systems are of high interest for several reactions, such as hydrogenation reactions [22] and FT synthesis [23,24]. Furthermore, the presence of Pd can prevent the formation of hard wax which causes catalyst deactivation [25]. It is observed that Pd promoter produces a variety of effects on the structure of cobalt catalysts which modifications could affect cobalt reducibility and/or cobalt dispersion, formation of bimetallic particles, and formation of barely reducible cobalt support mixed compounds [23][24][25].
In this research paper, the role of Pd promotion on SBA-15 and MA supported cobalt catalysts is systematically evaluated for liquid hydrocarbon synthesis from syngas (CO + H 2 ). Catalytic performance including CO conversion and selectivity of the catalysts has been evaluated under similar conditions in a fixed bed tubular reactor. The effects of catalysts on FT product spectrum have also been studied using chain growth probability (a) of Anderson-Schulz-Flory (ASF) equation.

Preparation of Mesoporous Alumina (MA)
Mesoporous Alumina is synthesized through hydrothermal technique with molar compositions of 1.0 Al:0.5 CTAB:5.28 Urea:70 H 2 O [26]. In this synthesis process, 74 g of Al (NO 3 ) 3 Á9H 2 O and 62.15 g of urea are dissolved in 550 mL of de-ionized water and stirred vigorously for 30 min at 35°C and then 35.75 g of CTAB is added to the solution with continued stirring till homogeneous solution is formed. Thereafter, mixture is poured into a Teflon autoclave bottle and heated at 120°C for 24 h. The white precipitate thus obtained is filtered using Whatman filter paper and washed with deionised water. Finally, the sample is dried at 120°C in an air oven for 12 h and calcined at 750°C @ 5°C/min for 5 h.

Preparation of SBA-15
The SBA-15 is synthesized using the method as reported by Ding et al. [27]. In this synthesis process, 16 g Pluronic (P-123), 576 mL deionised water and 18.3 mL concentrated HCl (35 wt.%) are taken in a beaker and stirred continuously until P-123 is fully dissolved. As and when the solution becomes homogeneous, 35.74 mL Tetraethyl Orthosilicate (TEOS) is added to it and stirred continuously for 24 h at 35°C temperature. The mixture is then transferred to a polypropylene bottle and it is kept under static condition for 24 h at 100°C in an air oven. The resulting precipitate is filtered by using Whatman (No. 42) filter paper, washed with deionised water and dried at 80°C for 16 h and then calcined at 540°C @ 1°C/min for 16 h. The prepared sample is kept in a desiccator.

Characterization
Brunauer-Emmett-Teller (BET) surface area measurements are performed in a Micromeritics TriStar 3000 instrument by N 2 adsorption-desorption isotherms. Pore volumes and average pore diameter of the catalysts are also measured with the same instrument. SAXS (Small Angle X-ray Scattering) and WAXRD (Wide Angle X-Ray Diffraction) patterns are recorded using a Rigaku Smartlab instrument.
The phases of the fresh calcined catalysts are analyzed and crystallite size of Co 3 O 4 is determined from the XRD pattern using Scherrer equation mentioned below: where, d is the average crystallite size, k is the wavelength of the X-ray, b is the Full Width at Half Maximum (FWHM) due to crystallite size broadening and h is the position of strongest Bragg profile in the diffractogram. Catalysts are also analyzed using Transmission Electron Microscopy (TEM) (JEOL, JEM 2100 instrument) to study the morphology. The Temperature-Programmed Reduction (TPR), and Temperature-Programmed Desorption (TPD) profiles of the catalysts are measured by using ChemiSorb 2720 (Micrometrics, USA) instrument equipped with a Thermal Conductivity Detector (TCD). The temperatureprogrammed reduction profiles are determined with the flow 10% H 2 in Ar at a flow rate of 20 mL/min and the temperature is increased linearly from room temperature to 1000°C @ rate of 10°C/min. Prior to analysis, 0.1-0.2 g of calcined catalyst is taken in a U-tube quartz reactor and dried at 150°C for 2 h under flow of Argon gas at 20 mL/min. The temperature-programmed desorption of the catalysts is measured by passing NH 3 (0.4% NH 3 in He) over the catalysts at room temperature for 30 min. Then adsorbed gas is desorbed in the flow of He @ rate of 20 mL/min and temperature is increased from room temperature to 1000°C at a rate of 10°C/min.
The FT experiments are performed in a fixed-bed stainless steel reactor (internal diameter: 10 mm, Volume: 10 mL). The schematic diagram of the reactor set up is shown in Figure 1 and experimental parameter is mentioned in Table 1. 3.0 mL powder calcined catalyst is charged in the reactor. Activity of the catalysts is evaluated at pressure of 30 bar and GHSV of 500 h À1 . The catalysts are reduced insitu with H 2 flow (GHSV of 400 h À1 ) at temperature 350°C for 12 h. After reduction, the temperature is lowered to 180°C under the flow of H 2 , the gas flow is switched to syngas (H 2 : 28.63%, N 2 : 46.83%, CO: 14.27%, CO 2 : 9.27% and CH 4 : 0.997%, H 2 /CO: 2, GHSV: 500 h À1 ) with simultaneous increase of temperature up to 220°C with an increment of 5°C/min. The flow of all the gases is controlled independently by mass flow controllers (Brooks). During testing of Fischer-Tropsch activity, the condensed vapors are collected in bottom catch pot. The uncondensed gas mixture is continuously monitored by GC (Model-GC 1000; Make: M/s Chemito Technologies Ltd, India) equipped with TCD and FID detectors.

N 2 adsorption-desorption isotherm
The adsorption-desorption isotherms of both the catalysts and support materials are shown in Figures 2 and 3, respectively. N 2 adsorption-desorption isotherms of both the support materials as well as catalysts showed hysteresis characteristics of Type IV, indicating formation of mesoporous structures according to IUPAC classification [28]. BET surface area and pore size of catalysts are presented in Table 2. The surface areas of the catalysts are decreased by 28% and 30% respectively after Co impregnation for Co-Pd/MA and Co-Pd/SBA-15 catalysts. Correspondingly, the pore volume and the pore diameter have also decreased by 45% and 24% respectively for both the catalysts. The sharp decrease in the BET surface area and total pore volume is due to impregnation of Co and Pd into the mesoporous supports indicating that a portion of the metal oxide particles have entered into the pores and at the mouths of the supports leading to partial blockage by formation of agglomerates [29]. The Pore Size Distribution (PSD) study of the catalysts shows that Co-Pd/MA has wider and larger pore diameter as compared to Co-Pd/ SBA-15.

X-Ray Diffraction (XRD)
The SAXS patterns of MA and SBA-15 (Fig. 4) show no peak for MA, whereas SBA-15 shows one peak between 0.5 and 3° [30]. This indicates that the MA has different morphology. The MA and Co-Pd/MA are characterized by wide angle X-ray diffraction as shown in Figure 5.   [33]. However, there is no peak for Pd oxide in both the catalyst due to very low content of Pd [15,34]. Scherrer equation is used to calculate the crystallite size of Co 3 O 4 at 2h of 36.94°in both the catalysts and results are presented in

Temperature-Programmed Reduction (TPR)
TPR profiles of Co-Pd/MA and Co-Pd/SBA-15 catalysts are shown in Figure 7. In Co-Pd/MA catalyst, the two peaks are observed at 178°C and 300°C that correspond to the two-step reduction. The first peak is attributed to the reduction of Co 3 O 4 to CoO and the second peak signifies the reduction of the CoO to Co. Hossain [35] observed reduction profile of Co/Al 2 O 3 catalyst (without Pd promotion) where the reduction completed in two steps; firstly, Co 3 O 4 to CoO and then CoO to Co in temperature range of 300-400°C and 400-600°C respectively. Khodakov et al. [36] observed peaks at 600°C and higher    [37]. Yuanyuan et al. [38] interpreted the first peak at 305°C to the reduction of Co 3 O4 to CoO, second peak at 442°C to subsequent reduction of CoO to Co for the TPR profile of Co/SBA-15 (without Pd promotion) catalyst. In another study, the broad peak at higher temperature 626°C is assigned to the reduction of cobalt silicate (Co 2 SiO 4 ) formed by a strong interaction between cobalt and the support [39,40]. However, in present work the reduction peak above 400°C is not observed, signifying enhancement of reducibility by Pd promotion [41].

Temperature-Programmed Desorption (TPD)
The quantity and strength of acidic sites of the catalyst are measured by NH 3 -TPD (Fig. 8). NH 3 desorption peak at 100°C and very small peaks at high temperature 367°C and 700°C are observed for Co-Pd/MA catalyst. The Co-Pd/SBA-15 shows the peak at 90°C, owning to desorption of the physisorbed NH 3 from the Si-OH sites (weak acid sites) of SBA-15 [42]. The Co-Pd/SBA-15 catalyst contains more week acid sites however Co-Pd/MA catalyst contains more strong acid sites (Tab. 4).

FT products distribution
The catalytic performance of both catalysts are investigated for Low Temperature Fischer-Tropsch (LTFT) synthesis at pressure 30 bar and temperature 220°C using syngas. The characterization of the liquid products is done by Simdist Analyser GC (M/s Thermo Scientific TRACE 1110 GC). The carbon number distribution of C 5+ hydrocarbons (mole %) for each catalyst is presented in Figures 9 and 10.     [45]. The conversion of syngas is higher for Co-Pd/MA catalyst, but selectivity towards the middle distillate product is higher for Co-Pd/SBA-15. Large pore properties of Co-Pd/MA provide longer residence time for reactant and lead to formation of long-chain hydrocarbons inside the mesopores [46]. In addition, larger pore size of the catalyst facilitates higher hydrocarbon selectivity [37]. Product distribution of FT synthesis is presented by classical Anderson Schulz-Flory (ASF) equation, which is a kinetic model to describe the product distribution. Slope obtained from this plot log(Wn/n) versus carbon number (where "n" is carbon number, "Wn" is the mole percentage of the component) referred as chain growth probability (a), which is given by equation (2) [47]. Figure 11 shows non-linear nature of the plot signifying non-ASF FTS product distribution. However, a line of best fit to the data for carbon numbers C 10 -C 20 is used to obtain chain growth probability (a) for the catalysts (Tab. 5). The value of a is 0.80 for Co-Pd/MA, indicating formation higher molecular mass products like gasoline, kerosene and diesel [48], log W n n ¼ n log a þ log ð1 À aÞ 2 a : ð2Þ

Conclusion
The Fischer-Tropsch synthesis experiments have been carried out over Co-Pd/MA and Co-Pd/SBA-15 catalysts. The addition of 0.1 wt.% quantity of palladium not only improved the reduction in cobalt oxides, but also promoted the reduction in cobalt silicate/cobalt aluminate interactions. CO conversion is achieved 64.76% for Co-Pd/MA while 59.27% for Co-Pd/SBA-15. Pd/SBA-15 has resulted higher selectivity of lower hydrocarbon, in contrast Co-Pd/ SBA-15 catalyst shows higher selectivity towards middle distillate. Hence, the selection of an appropriate support with optimum surface area, pore size, acidity, and basicity is essential for syngas conversion studies. The present study thus provides a better understanding of FT synthesis over mesoporous supports for formation of liquid hydrocarbon from syngas.