Synthesis and application of CNT-supported MoO3-K2O nanocatalyst using microemulsion technique: role of nanoparticle size on catalyst activity and selectivity in higher alcohol synthesis

Tavasoli et al. International Journal of Industrial Chemistry 2013, 4:21 http://www.industchem.com/content/4/1/21 RESEARCH Open Access Synthesis and application of CNT-supported MoO3-K2O nanocatalyst using microemulsion technique: role of nanoparticle size on catalyst activity and selectivity in higher alcohol synthesis Ahmad Tavasoli*, Saba Karimi and Morteza Shoja Abstract Background: Alkalized MoO3 nanocatalyst supported on carbon nanotubes (CNTs) is prepared using microemulsion technique. The nanocatalyst was extensively characterized by different methods, and its activity and selectivity in higher alcohol synthesis (HAS) have been assessed in a fixed bed micro-reactor. The physico-chemical properties and performance of the nanocatalyst were compared with the catalyst prepared by impregnation method. Results: The transmission electron microscopy images showed that small Mo nanoparticles are confined inside the CNTs (1 to 5 nm) and on the outer surface of the CNTs (1 to 7 nm). Using microemulsion technique, the average MoO3 particle sizes decreased from 7 to 4.8 nm, the percentage of dispersion increased from 51 to 59 and the percentage of reduction increased by 37%. Also, the chemical interactions between K-Mo-O species increased, enhancing the conditions for the formation of alcohols. Conclusion: The percentage of CO conversion increased from 27.7% to 43.8%. The hydrocarbon selectivity decreased from 32.4% to 24.5%, and alcohol selectivity increased from 45.3% to 61.4%. Keywords: Higher alcohols, Molybdenum, Carbon nanotubes, Microemulsion, Particle size Background Higher alcohol synthesis from syngas, an economically attractive method for making fuels and chemicals, is of interest due to the enhancement of petroleum price, environmental concerns, and additive for gasoline to increase octane number [1]. There are three different methods for making higher alcohols from syngas: (1) using modified catalysts for synthesis of methanol including Cu-, Zn-, and Cr-based catalysts, (2) using modified catalysts for Fisher-Tropsch synthesis including Fe-, Ni-, and Co-based catalysts, and (3) using promoted molybdenum sulfide catalysts [2-9]. Among them, molybdenum-based catalysts are more attractive due to their high activity for the watergas shift reaction [10]. Molybdenum disulfide (MoS2) catalyst mainly produces hydrocarbons, but when it is * Correspondence: School of Chemistry, College of Science, University of Tehran, Tehran 1417614411, Iran promoted with alkali metals, it can produce alcohols from syngas [11,12]. The function of alkali metals is to reduce the hydrogenation ability of alkyl species to form alkanes and increase the sites active for the formation of alcohols [13]. Some investigations have been carried out to study the influence of catalyst physico-chemical properties on higher alcohol synthesis and to have a better understanding of the structure-sensitive effects in higher alcohol synthesis (HAS) catalysis [14-16]. A sub-category of the structure-sensitive reactions regards the dependence of both catalytic activity and selectivity on catalytic metal particle size. Microemulsion, a novel technique for catalyst preparation, enables the control of metal particle size with a narrow particle size distribution, regardless of metal content [17-20]. Briefly, a microemulsion consists of nanosized water droplets surrounded by an oil phase, which is stabilized by a surfactant [20]. The size of the metal © 2013 Tavasoli et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Tavasoli et al. International Journal of Industrial Chemistry 2013, 4:21 http://www.industchem.com/content/4/1/21 particles formed in water-in-oil (w/o) microemulsions is controlled by changing the micelle size (the water-to-surfactant ratio) [20]. Hayashi et al. found that the activity of a Fe/SiO2 catalyst prepared by microemulsion was higher than that of the catalyst prepared by incipient wetness impregnation with the same metal loading [21]. However, it has been also shown that metal nanoparticles obtained by microemulsion techniques interact strongly with oxygen-carrying ceramic supports (such as Al2O3, SiO2, and TiO2). Such interactions lead to a decrease of the catalyst reduction efficiency [17,22,23]. Therefore, oxidic carriers impose serious limitations to the investigation of structure-sensitive effects in catalysis because of the co-existence of incompletely reduced metal phase caused by strong interaction with the oxidic carrier [17,22,24]. Specifically, nanoparticles synthesized by microemulsion are known to be more difficult to reduce on oxidic carriers. The control of particle size is another advantage for nanoparticle preparation in microemulsion system. The ratio of water to surfactant is one of the size-determining key parameters [25]. The size of the active metal particles depends on the size of the droplets in the microemulsion. The droplet size will be influenced by the water-to-surfactant ratios. An increase of this ratio at constant concentration of surfactant will increase the average diameter of the droplets. Lisiecki and Pileni reported that the size of Cu nanoparticles prepared in a system consisting of bis(2-ethylhexyl) sulfosuccinate (aerosol-OT) surfactant, cyclohexane, and water increased from 2 to 10 nm as water-to-surfactant ratios changed from 1 to 10 [26]. In our previous works, we used impregnation method to prepare the carbon nanotube-supported MoO3-K2O higher alcohol synthesis catalyst [27,28]. We showed that CNTs, when used as HA synthesis catalyst support, allow a better metal dispersion and control and minimize the metalsupport interactions (formation of mixed compounds). In the present work, we intend to compare the proposed microemulsion technique for MoO3-K2O catalyst preparation with the incipient wetness impregnation method for the control of metal particle size using CNTs as catalyst support. The influences of molybdenum particle size on the catalyst physico-chemical properties, activity, and selectivity were assessed and reported. Page 2 of 11 and n-hexanol as the oil phase. The concentrations of molybdenum and potassium were adjusted using aqueous solutions of (NH4)6Mo7O24·4H2O, (Merck, Whitehouse Station, NJ, USA) and K2CO3 as 15 wt.% molybdenum and 8 wt.% potassium. The water-to-surfactant molar ratio (w/s) was selected as 2. After 15 min of vigorous stirring, a microemulsion was obtained. Hydrazine was added to develop nanoparticle formation in the core of the micelles by reducing the metal oxides. Then, the purified carbon nanotubes were added under stirring. Afterward, tetrahydrofurane was added dropwise (1 ml/min). The mixture was left to mature and settle slowly overnight and then decante (...truncated)