Article pubs.acs.org/IECR
Effect of Carbon Porosity and Cobalt Particle Size on the Catalytic Performance of Carbon Supported Cobalt Fischer−Tropsch Catalysts Tingjun Fu, Jing Lv, and Zhenhua Li* Key Lab for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China S Supporting Information *
ABSTRACT: Cobalt catalysts supported on activated carbon and on carbon nanotubes (CNTs) with different porosities were prepared by an incipient wetness impregnation method and characterized by a series of methods. The samples were reduced and then evaluated in a fixed-bed reactor for Fischer−Tropsch (FT) synthesis. The porosity of the carbon support greatly influenced the microstructure, the reducibility, the dispersion, and the FT performance of the cobalt catalysts. The carbon structure and the cobalt dispersion determined CO conversion. CNTs with larger pore sizes were more stable at high temperature in a H2 atmosphere. The cobalt particle size impacted the CO turnover frequency (TOF) and the C5+ selectivity. Larger cobalt particles (up to 7 nm) resulted in higher TOF and C5+ selectivity; for cobalt particles larger than 7 nm, no such increase in these parameters was seen. The carbon support influenced the C5+ selectivity and the C5+ hydrocarbon distribution. Interestingly, the olefin/paraffin ratio of C2 was lower than that of C3 or C4 and a positive relationship existed between the C2−C4 olefin/paraffin ratio and the C5+ selectivity.
1. INTRODUCTION Diminishing petroleum reserves, sharp fluctuations of crude oil prices, increasingly stringent environmental regulations, and the global demand for a decreased dependence on petroleum for the production of fuels and chemicals are the main driving force for the recent renewed interest in Fischer−Tropsch (FT) synthesis in both academia and industry.1 FT synthesis is a heterogeneous catalytic polymerization process that can produce superclean fuels or valuable chemicals at economically feasible costs from syngas. Syngas can be derived from nonpetroleum feedstock such as natural gas, coal, and biomass through steam re-forming, partial or autothermal oxidation, or gasification processes.2 Among the reported FT catalysts, cobalt is the preferred catalyst for the production of long-chain paraffins because of its high activity, low water-gas shift activity, and comparatively low price compared to ruthenium catalysts.1,2 In order to obtain a high cobalt dispersion and reduce cost, various supports for cobalt catalysts have been used in this reaction, including silica, alumina, titanium, and carbon.3 The structure of these supports and their properties, such as average pore diameter, pore volume, and surface area, can significantly influence the cobalt dispersion, the reducibility, and the activity/selectivity of the catalyst for FT synthesis. A better understanding of the relationship between the porous characteristics of the support and the catalytic performance of the cobalt catalyst is important for designing a better cobalt catalyst for FT synthesis.4,5 The effect of the support and its porosity on supported cobalt catalysts and FT synthesis has been reported for traditional oxide supports such as silica or alumina. Song and Li6 studied Co/SiO2 catalysts with SiO2 pore sizes ranging from 2.4 to 15.8 nm. They found that larger SiO2 pore sizes resulted in the formation of larger Co3O4 crystallites and greater reducibility, but the larger sizes also resulted in less Co © 2014 American Chemical Society
dispersion. The catalysts with pore sizes of 6−10 nm displayed higher FT activity and C5+ selectivity, which is due to the suitable particle sizes and the moderate adsorption of CO on the catalysts. A comparison of the FT performance of Co catalysts supported on silicas with different average pore diameters (2, 4, 6, 10, or 15 nm) showed that both the C5+ selectivity and CO conversion were at a maximum when the average pore size was 10 nm. Except for the sample with the smallest pore size, both the degree of reduction and the Co particle size increased with pore size.7 Borg et al.8 carried out a systematic study on Co catalysts promoted with Re using a series of γ-Al2O3 supports with average pore diameters varying from 5.9 to 26.7 nm. The degree of reduction, the Co particle size, and the C5+ selectivity all increased with the pore size of γAl2O3. Unlike the above Co/SiO2 catalyst, the maximum C5+ selectivity was not achieved at a medium pore size. Mesoporous materials9,10 such as MCM-41 and SBA-15 with ordered porous channels and different pore diameters have also been selected to evaluate the influence of the support porosity on cobalt dispersion, reducibility, and FT performance. Compared with conventional oxide supports, carbon materials display special properties such as high purity, high mechanical strength, good electrical conductivity, high thermal stability, and large surface area,11−15 and they are increasingly attracting more attention as potential supports for cobalt-based FT catalysts. Tavasoli et al.16 compared carbon nanotubes (CNTs) with oxide supports and found that the metal−support interactions in the Co/CNTs were much weaker than those in the oxide supports and hence the reducibility was significantly Received: Revised: Accepted: Published: 1342
July 5, 2013 October 26, 2013 January 10, 2014 January 10, 2014 dx.doi.org/10.1021/ie402128y | Ind. Eng. Chem. Res. 2014, 53, 1342−1350
Industrial & Engineering Chemistry Research
Article
degassed at 300 °C for 4 h under 50 mTorr. The adsorption of N2 was done at −196 °C to get the adsorption isotherms. The pore distribution and specific surface areas of the AC and Co/ AC were determined by nitrogen adsorption at −196 °C using a Micromeritics ASAP-2020 analyzer. Prior to the measurement, the samples were degassed at 300 °C for 16 h. X-ray diffraction (XRD) characterization of the supports and catalysts was performed using a RigakuD/max-2500 diffractometer with Cu Kα radiation (40 kV, 200 mA). The scan rate was 8°/min from 2θ = 5 to 90°. The average Co3O4 size was calculated using the Scherrer equation. Both temperature programmed reduction of H2 (H2-TPR) and temperature programmed desorption of H2 (H2-TPD) were performed with a Micromeritics AutoChem 2910 system. H2-TPR was performed by heating the catalyst sample (0.1 g) to 900 °C with a ramp rate of 10 °C/min in a gas mixture of 10% H2 in Ar using a flow rate of 30 mL/min. Co dispersion was estimated by H2-TPD. The catalyst (0.1 g) was reduced at 400 °C for 5 h under H2 (flow rate = 30 mL/min) and then cooled to 100 °C (During this process, the H2 adsorption on the catalyst was finished). At 100 °C, the H2 flow was switched to Ar and maintained for 1 h to remove the weakly adsorbed H2. Then the temperature was raised to 400 °C at a rate of 10 °C/min under Ar (flow rate = 30 mL/min) and maintained for 1.5 h. The TPD profile was used to determine the Co dispersion (D) using the following formula.
improved. This greatly improved the FT performance. In addition, the influence of confinement in CNTs,17 acid treatment of CNTs, 18 the preparation method,19 and promoters20 on the FT performance of Co/CNTs catalyst have all been reported, leaving no doubt that Co/CNTs is a promising FT catalyst. Yang et al.21 prepared supported Co catalysts on ordered mesoporous carbon by simply tuning the content of the carbon precursor (furfuryl alcohol). This method to some extent resolved the normal trade-off between dispersion and reducibility. In addition, carbon materials have been used as supports to study the intrinsic properties of cobalt particles, the structure−activity relationship, and the FT synthesis mechanism. Bezemer et al.22 and den Breejen et al.23 prepared a Co catalyst using graphitic carbon nanofibers (CNFs) with large pore volumes and surface areas as a support. They discussed the influence of the cobalt particle size on FT synthesis using Co/CNF as a model catalyst and found a direct influence of Co particle size on specific surface activity and CH4 selectivity. Xiong et al.3 also observed a positive relationship between the cobalt particle size and the C5+ selectivity for both Co/CNTs and Co/carbon-spheres catalysts. However, previous studies of carbon supported Co catalysts have mainly focused on a carbon support. The effect of the porosity of different carbon supports on the FT performance of supported cobalt catalysts is not yet fully understood. Also, it is unclear if there is a size effect of Co particles supported on carbon supports with different porosities, and whether this effect is similar on silica, alumina, or single carbon support. Here three commercially available CNTs with different outer diameters (
