Residue catalyst support removal and purification of carbon nanotubes by NaOH leaching and froth flotation
Introduction
Since the discovery of carbon nanotubes (CNTs) in the form of multi-walled carbon nanotubes (MWNTs) in 1991 [1] followed by the discovery of single-walled carbon nanotubes (SWNTs) in 1993 [2], [3], CNTs have become of great interest and have been investigated extensively because they have outstanding electrical, mechanical, and thermal properties [4], [5]. Currently, there are three main processes to produce CNTs—arc-discharge [2], [3], laser vaporization [6], and heterogeneous catalytic reaction [7], [8], [9], [10], [11], [12], [13]. The first two processes are limited for scaling up to commercial production due to the requirement of high temperatures and the complicated development for a continuous system, while the heterogeneous catalytic process not only requires moderate temperatures but it is also easy to set up. Therefore, the production of CNTs by using the heterogeneous catalytic reaction has been considered to be a promising approach for a large-scale production like fluidized bed reactor system [14], [15], [16], [17], [18], [19]. However, the purity of CNTs, especially SWNTs produced from this process is not yet satisfied since the as-prepared CNTs still contain a large fraction of the catalyst support [20]. To scale up the heterogeneous catalytic reaction process for a commercial production, the purification of the CNTs produced by this technique has to be considered as a crucial step. Typically, there are several methods to eliminate the impurities in the as-prepared CNTs such as the use of oxidation to remove carbonaceous impurities and to expose enclosed catalytic metal surface [21], acid treatment to remove metal catalyst [22], high temperature annealing to melt the metal and eliminate the defects [23], [24], and ultrasonication, grinding, ball milling and high speed shearing to separate the particles [25]. In this research, the SWNTs produced from the CO disproportionation over a Co-Mo catalyst using a silica support were used in the purification experiment. One interesting method to purify SWNTs is to oxidize the spent catalysts and then to follow by acid treatment such as with HCl [26], [27] or HNO3 [28], [29] for metallic catalysts removal. HF or NaOH was also used for removing silica support [30]. Even though HF is the most effective chemical to dissolve silica, it is extremely dangerous and very difficult to handle because it can burn the eyes, skin, and mucous membranes [31]. Therefore, it is not practical to use HF in a large-scale SWNT purification. The use of a membrane is also considered to be a potentially viable technique to concentrate SWNTs. However, the membrane filtration technique also has some drawbacks, such as high material cost, and fouling build up.
Froth flotation is a separation technique that uses a surfactant as a means of separation; called a surfactant-based separation process [32]. It has been widely used in the area of mineral processing [33]. In a froth flotation operation, air is introduced at the bottom of a flotation column to generate froth, which is the critical issue in this technique [34] since SWNTs can co-adsorb preferentially at the bubble surface rising to the top of the column. A surfactant added to the solution plays an important role to generate froth as well as to stabilize the produced froth on which both solid particles and surfactant concentrate. The reasons for selecting the froth flotation technique to selectively separate and concentrate SWNTs are its outstanding features of rapid operation, low space requirement, high removal efficiency, and low cost of operation [35].
In our previous work [36], froth flotation was applied to separate carbon black, a model of SWNTs, from silica gel. In that study, the separation of carbon black from silica gel was done by using a nonionic surfactant (ethoxilated alcohol) because the point of zero charges (PZCs) of these two solid particles are not significantly different (the PZC of carbon black is 3.5 and the PZC of silica gel is 4.1). It was reported that a higher than 70% separation efficiency of carbon black was achieved by using froth flotation. However, carbon black was easily separated from silica gel by froth flotation because carbon black and silica gel were physically blended together. In contrast to the mixture of carbon black and silica gel, there are strong interactions between the SWNTs and the silica support in the as-prepared SWNT sample since the SWNTs were synthesized by growing on the silica surface [37]. Therefore, in the present study, NaOH was used for silica dissolution prior to the step of SWNT recovery by froth flotation. An ethoxilated alcohol nonionic surfactant was selected again for running the froth flotation experiments.
Section snippets
Metal precursors and catalyst support
Cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O) with a purity above 98% and ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O) with 81.0–83.0% as MoO3 were used as catalyst precursors. Silica gel (SiO2) was used as a catalyst support and having particle sizes in the range of 70–230 mesh (62–210 μm) with an average pore diameter of 6 nm, a BET surface area of 480 m2/g, and a pore volume of 0.75 cm3/g. All the catalyst precursors and the catalyst support were supplied by Sigma–Aldrich Co. Ltd. NaOH
Results and discussion
In this work, the carbon content of the SWNT sample is defined as the weight of the combustible fraction as a percentage of the total solid, where the combustible fraction in the total solid was calculated from the peak area under the TPO curve [38]. From the TPO results, the carbon content of the as-prepared SWNT sample was measured to be around 4%, in which it contained a disordered carbon fraction of 0.25 and an SWNT fraction of 0.75 (see details in Section 3.6). The carbon content of the
Conclusions
Single-walled carbon nanotubes are of great interest due to their unique physical and chemical properties. Most applications of SWNTs need to have a very high purity of SWNTs; hence, the purification of SWNTs was focused on in this study. In this research, the first step of removing silica by concentrated NaOH and the second step of recovering NaOH-treated SWNTs by froth flotation were studied. To achieve a high removal efficiency of froth flotation, a surfactant dosage of 30 mg/l was selected
Acknowledgements
The Thailand Research Fund (TRF) is acknowledged for providing a research grant and the Royal Golden Jubilee Ph.D. fellowships. The National Excellence Center for Petroleum, Petrochemical and Advanced Materials, under The Ministry of Education, Thailand, and The Research Unit: Applied Surfactants for Separation and Pollution Control, under The Ratchadapiseksompote Fund, Chulalongkorn University, are also acknowledged for providing research facilities and funding, respectively.
References (50)
- et al.
Purification and isolation of SWNTs
Carbon
(2004) - et al.
Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite films
Carbon
(2003) - et al.
Catalytic growth of single-walled nanotubes by laser vaporization
Chem. Phys. Lett.
(1995) - et al.
Controlled production of single-wall carbon nanotubes by catalytic decomposition of CO on bimetallic Co-Mo catalysts
Chem. Phys. Lett.
(2000) - et al.
Gas-phase catalytic growth of single-walled carbon nanotubes from carbon monoxide
Chem. Phys. Lett.
(1999) - et al.
Large-scale synthesis of single-wall carbon nanotubes by catalytic chemical vapor deposition (CCVD) method
Chem. Phys. Lett.
(2000) - et al.
Continuous production of aligned carbon nanotubes: a step closer to commercial realization
Chem. Phys. Lett.
(1999) - et al.
The large-scale production of carbon nanotubes in a nano-agglomerate fluidized-bed reactor
Chem. Phys. Lett.
(2002) - et al.
Effect of adding nickel to iron-alumina catalysts on the morphology of as-grown carbon nanotubes
Carbon
(2003) - et al.
Relationship between the structure/composition of Co-Mo catalysts and their ability to produce single-walled carbon nanotubes by CO disproportionation
J. Catal.
(2001)
A treatment method to give separated multi-walled carbon nanotubes with high purity, high crystallization and a large aspect ratio
Carbon
99.9% purity multi-walled carbon nanotubes by vacuum high-temperature annealing
Carbon
Purification and characterization of zeolite-supported single-walled carbon nanotubes catalytically synthesized from ethanol
Chem. Phys. Lett.
Synergism of Co and Mo in the catalytic production of single-wall carbon nanotubes by decomposition of CO
Carbon
Flotation of soot particles from a sandy soil sludge
Colloids Surf. A
A simple combinatorial method to discover Co-Mo binary catalysts that grow vertically aligned single-walled carbon nanotubes
Carbon
Helical microtubules of graphitic carbon
Nature
Single-shell carbon nanotubes of 1-nm diameter
Nature
Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls
Nature
CVD synthesis and purification of single-walled carbon nanotubes on aerogel supported catalyst
Appl. Phys. A
Improvement of Fe/MgO catalysts by calcination for the growth of single- and double-walled carbon nanotubes
J. Phys. Chem. B
CO2-assisted SWNT growth on porous catalysts
Chem. Mater.
Single-walled carbon nanotubes of controlled diameter and bundle size and their field emission properties
J. Phys. Chem. B
Tailoring (n,m) structure of single-walled carbon nanotubes by modifying reaction conditions and the nature of the support of CoMo catalysts
J. Phys. Chem. B
Production of carbon nanotubes in a packed bed and fluidized bed
AIChE J.
Cited by (18)
Effect of MWCNTs surface functionalization group nature on the thermoelectric power factor of PPy/MWCNTs nanocomposites
2022, Synthetic MetalsCitation Excerpt :Except for the pyrrole, which is distilled before use, all chemicals are used as received. The received crude MWCNTs was purified [16,17]; briefly, 5 g of crude MWCNTs were mixed with 500 ml of 12 M NaOH solution in a round-bottomed flask. The above suspension was refluxed under constant agitation at 170 °C for 12 h.
Transition metal impurities in carbon-based materials: Pitfalls, artifacts and deleterious effects
2020, CarbonCitation Excerpt :The solubility of commonly-used supports in alkalis implies that hydroxide treatment can remove such impurities. For instance, silica-supported SWCNTs were purified using a concentrated NaOH aqueous solution and minimal CNT structural damage was reported [556]. Obviously, unlike aqueous NaOH solution washing, rousting of CNTs with NaOH at temperatures of ca. 700 °C resulted in serious damage to the structure of MWCNTs [557].
A novel approach for development of graphene structure in mesoporous carbon of high specific surface area
2016, CarbonCitation Excerpt :However, the complexity, low yield of such process and the remaining of the metallic impurities are considered as three main disadvantages of such approach. In addition, complete chemical removal of these graphitizing agents could lead to the formation of severe structural defects [23,24] which would adversely affect the electrical conductivity of the prepared materials. Here for the first time, to the best of our knowledge, we report a novel strategy based on soft template route for the preparation of MC powders of high electrical conductivity (up to 6.25 S/cm) and rather high specific surface area (∼625 m2/g), using water-soluble planar polyaromatic hydrocarbons (PAH) as a group of non-catalytic graphitizing agent at rather low carbonization temperature.
Enhanced graphitization of c-CVD grown multi-wall carbon nanotube arrays assisted by removal of encapsulated iron-based phases under thermal treatment in argon
2014, Applied Surface ScienceCitation Excerpt :A variety of physical and chemical strategies was developed to purify and/or reduce the number of structural defects in CNTs and hence a different emphasis on removal of catalyst residues versus graphitization was laid. The applied methods were frequently dependent upon nature/content of the contaminant, e.g. specificity toward encapsulated catalyst [3] as well as a type of nanotubes (single-, double or multi-wall CNTs) [4]. Among chemical methods of purification, acidic [5,6], halogen [7,8], molten salts [9] or oxidative [10] treatments were frequently used, but due to restricted access to the catalyst particles they were time-consuming, required environmentally unfriendly conditions and/or multi-step co-procedures.
Motor oil removal from water by continuous froth flotation using extended surfactant: Effects of air bubble parameters and surfactant concentration
2009, Separation and Purification TechnologyA review of carbon nanotube purification by microwave assisted acid digestion
2009, Separation and Purification Technology