Effect of Lanthanum as a Promoter on Fe-Co/SiO2 Catalyst

Iron-Cobalt catalyst is well known from both operational and economical aspects for Fischer-Tropsch synthesis. Effort to increase the efficiency of this kind of catalyst is an important research topic. In this work, the effect of lanthanum on characteristic behavior, conversion and selectivity of a Fe-Co/SiO2 Fischer-Tropsch catalyst was studied. The Fe-Co-La/SiO2 catalysts were prepared using an incipient wetness impregnation method. These catalysts were then characterized by XRF-EDAX, BET and TPR techniques, and their performances were evaluated in a lab-scale reactor at 250 oC, H2/CO = 1.8 of molar ratio, 16 barg pressure and GHSV = 600 h-1. TPR analysis showed that the addition of La lowered the reduction temperature of Fe-Co catalyst, and due to a lower temperature, the sintering of the catalyst could be mitigated. Furthermore, from these tests (about 4 days), it was found that lanthanum promoted catalyst had higher selectivity toward hydrocarbons, and lower selectivity toward CO2. ©2014 BCREC UNDIP. All rights reserved


Introduction
Fischer-Tropsch (FT) process converts syngas (mixture of H2 and CO) into liquid hydrocarbons. Because of increasing in transportation fuels demand, it is an attractive and economical method which has high potential to produce such fuels. In the FT process, the choice of the catalyst to synthesis hydrocarbons is very important. Cobalt and iron catalysts are the most widely used in FT process [1]. Among other FT catalysts such as nickel and ruthenium, only cobalt and iron appear to be economically feasible at an industrial scale [2]. Iron-based catalysts are more selective toward olefin and gasoline range hydrocarbons, and produce CO2 whereas cobalt-based catalysts produce wide range of hydrocarbons and water as a major product [3]. It has been shown that using a Fe-Co catalyst would allow to control the hydrocarbon product spectrum [4] by varying structure, electronic properties and metal particle size of the catalysts [5]. During the last two decades, Fe-Co catalysts have been used by many authors to investigate their property. Some reports showed that the use of Fe-Co catalysts lead to increase in selectivity, especially for small olefins and alcohols [6][7][8][9].
It is reported that the activity and hydrocarbon distribution of FT catalysts are also strongly depended on the addition of other metals as promoters. Some researchers have studied the effect of lanthanum and ruthenium as promoters on the activity of the FT catalysts [10][11][12]. In their studies, they have found that lanthanum and ruthenium could increase the activity and stability of the Co catalyst. In other studies, the effect of ruthenium and niobium as promoters on the activity of iron catalysts was studied [13][14][15][16][17], and it was found that these promoters could increase the C 5+ product. Ma et al. [18] studied the effect of adding potassium and zirconium on the activity of Fe-Co catalyst, and found that these promoters could enhance the activity of the catalyst and increase the CO conversion.
Up to now, few studies have been done on the effect of lanthanum on the Fe-Co catalysts for FT process. The aim of the present research was to investigate the effects of lanthanum on Fe-Co catalysts yield for this purposes.

Material used
SiO2 used as catalyst support was purchased from Fluka (

Fe-Co/SiO2 catalyst
The Fe-Co/SiO2 catalyst was prepared by wet-impregnation method. Amount of 5 g of SiO2 particles with the mesh size between 30 and 40 were calcined at 500 °C for 6 h, then put in a rotary vacuum evaporator, and kept at 80 °C. Meanwhile, 2.12 g of Fe(NO3)2.9H2O and 2.9 g of Co(NO3)2.6H2O were dissolved in 4.63 g of deionized water at 80 °C. The solution was then added to silica particles in the rotary vacuum evaporator, and kept at 80 °C at a specified time. The prepared catalyst was dried at 120 °C for 2 h, and kept at room temperature for 24 h. Temperature was then increased to 80 °C under vacuum condition, and the catalyst was calcined at 400 °C for 6 h.

La-Fe-Co/SiO2 catalyst
To study the effect of lanthanum on Fe-Co catalyst characteristic, La-Fe-Co/SiO2 was prepared by wet-impregnation method. 5 g of SiO2 particles with the mesh size between 30 and 40 was calcined at 500 °C for 6 h, then put in a rotary vacuum evaporator, and kept at 80 °C. 2.12 g of Fe(NO3)2.9H2O, 2.9 g of Co(NO3)2.6H2O and 0.193 g of La(NO3)3.6H2O were dissolved in 4.63 g of deionized water at 80 °C. The solution was then added to silica particles in the rotary vacuum evaporator, and kept at 80 °C at a specified time. The prepared catalyst was dried at 120 °C for 2 h, and kept at room temperature for 24 h. Temperature was then increased to 80 °C under vacuum condition, and the catalyst was calcined at 400 °C for 6 h.

Characterization
The BET specific surface area of the catalysts was determined by nitrogen adsorptiondesorption isotherms employing a Quantasorb surface area analyzer (Quantachrome Corporation) using a 3:7 ratio of a N2/He mixture. Typically, 0.1 -0.3 g of sample was used for the measurement, and it was out-gassed at 300 ºC for 2 h prior to N2 adsorption at -196 °C; then, specific surface area values were calculated by the BET equation. The chemical compositions of catalysts were measured by Perkin-Elmer 2380 atomic absorption spectrophotometer. Moreover, the reducibility of the metal oxides was determined by temperature program reduction (TPR) experiment which was carried out using a Micromeritics TPD/TPR 2900 Analyzer. About 0.15 g freshly prepared catalyst was loaded into the quartz reactor and treated with a mixture of 6 % H2/Ar. The temperature was increased from 27 to 827 °C at a rate of 5 °C/min. H2O formed during TPR were removed by the cold trap, and the effluent gas was monitored by a thermal conductivity detector (TCD). Finally, a Varian gas chromatograph device (model CP-3800), equipped with a TCD was used to analyze the feed and products during catalyst activity test.

Activity test
The study of catalyst activity was carried out in a laboratory scale reactor with 1.25 cm internal diameter and 60 cm height. 2.5 g of catalyst with average particle size of 0.4-0.6 mm (mesh 30-40), diluted with 10 cm 3 of milled quartz particles, was loaded in the reactor. The catalyst bed was heated to 420 °C with the tem- , 9 (1), 2014, 24 Copyright © 2014, BCREC, ISSN 1978-2993 perature ramp of 2 °C/min under hydrogen flow with GHSV = 1000 h -1 and pressure of 5 bars, and kept at 420 °C for 16 h. The temperature was then decreased to 100 °C and hydrogen flow was stopped. A mixture of H2/CO with the molar ratio of 1.8 and GHSV = 600 h -1 was introduced to the reactor at 16 bar. The temperature was slowly increased to 250 °C, and kept at this temperature to study catalyst activity. During the activity test, the product was analyzed by GC. The test was carried out for 4 days.

Results and Discussion
The chemical compositions of the prepared catalysts are shown in Table 1 BET surface areas of the prepared catalysts are shown in Table 2. These results show that the surface area of the Fe-Co catalyst is slightly higher than the catalyst promoted with lanthanum.
Temperature programmed reduction (TPR) profiles of prepared catalysts are shown in Figures 1 and 2. In Figure 1, the TPR profile of Fe-Co/SiO2 shows that this catalyst has a multistep reduction temperature in which each peak corresponds to the reduction temperature of a different compound. At 330 o C reduction of Co3O4 to CoO and also partial reduction of Fe2O3 to Fe3O4, at 410 o C complete reduction of Fe2O3 to Fe3O4, at about 500 o C reduction of CoO to Co, at 680 o C reduction of Fe2O3 to Fe, and finally at about 840 o C the cobalt and iron phases which are hard to reduce.
In Figure 2, it can be seen that lanthanum loading slightly decreases the reduction temperatures of the catalyst. The decrease in reduction temperature lowers the partial oxidation, and therefore catalyst sintering.
Test conditions and product specifications for the performance evaluation of Fe-Co/SiO2 , 9 (1), 2014, 25 Copyright © 2014, BCREC, ISSN 1978-2993 Table 3. Test conditions and product compositions for the Fe-Co/SiO2 catalyst  Table 3. The product selectivity and CO conversion for this catalyst was evaluated at interval time of 1 day for a period of 4 days. With the same reaction conditions as shown in Table 3, Fe-Co-La/SiO2 catalyst was tested, and no liquid product was in the reactor effluent (i.e. the indication of FT synthesis catalyst performance). Therefore, reaction conditions such as temperature and GHSV were changed until the liquid products were obtained (see Table 4).

Bulletin of Chemical Reaction Engineering & Catalysis
Comparison the results shown in Table 3 and Table 4 showed that the presence of La in the catalyst results in: (1). CO2 selectivity considerably was decreased leading to a lower CO conversion which affected the waste products; , 9 (1), 2014, 26 Copyright © 2014, BCREC, ISSN 1978-2993 (2). The C2-C4 and C5+ selectivity of the gas product was increased; (3). While GHSV increased by 300 %, the percentage of the organic components in the liquid product was not changed. This showed that the productivity of Fe-Co-La/SiO2 catalyst was much higher than Fe-Co/SiO2 catalyst; (4). Lower operating temperature results in lower coke deposition on the catalyst, which was an important operation factor.

Bulletin of Chemical Reaction Engineering & Catalysis
From the above results, it is seen that the presence of lanthanum promoter in the catalyst can improve the productivity of the Fe-Co catalyst, and probably increases the catalyst life.

Conclusions
The presence of lanthanum as a promoter on the behavior of Fe-Co/SiO2 for Fischer-Tropsch synthesis i.e. selectivity and conversion were studied. Two types of catalysts, i.e. Fe-Co/SiO2 and Fe-Co-La/SiO2 were prepared, and their characteristics were studied by BET and TPR techniques. Their performances were evaluated in a fixed-bed micro-reactor, and results showed that the lanthanum had a slight effect on the catalyst surface area, but had more effect on catalyst reduction temperatures.
The results showed that addition of lanthanum resulted in catalyst to have better performance in terms of increasing C1-C4 and C5 + formations in the gas product, increasing the liquid productivity, decreasing the CO2 selectivity and decreasing the operating temperature of the process. The latter could be important in decreasing catalyst deactivation rate.