HiPCo process

Single-wall carbon nanotubes (SWNTs) can be prepared by laser-vaporization of a graphite source. A newer process uses carbon monoxide as the source of the carbon and is called the HiPco process. The catalyst is generated in situ from iron carbonyl. The SWNTs from the HiPco process are characterized by a smaller diameter and exhibit greater reactivity with organic reagents. 

Derivatized products such as alkyl-, aryl-, alkoxyl- and amino-nanotubes and cross-linked nanotubes are obtained (Scheme 1.2). F-SWCNTs prepared by the HiPCO process exhibited a higher degree of alkylation using alkyllithium reagents than fluorinated SWCNTs from the laser-oven method. Dealkylation occurred at 500 °C. 1-Butene and n-butane were formed during the thermolysis... 

Ceo = Fullerene SWNTs = Single-walled carbon nanotubes MWNTs = Multiwalled carbon nanotubes DWNTs = Double-walled carbon nanotubes CNTs = carbon nanotubes TEM = Transmission electron microscopy HRTEM = High-resolution transmission electron microscopy SEM = Scanning electron microscopy AFM = Atomic force microscopy Ch = Chiral vector CVD = Chemical vapor deposition HiPco process = High-pressure disproportionation of CO RBM = Radical breathing vibration modes DOS = Electronic density of states. 

The SWNTs produced from the HiPco process exhibit a smaller average diameter and, hence, a mild wet-air oxidation is done to avoid damages to the tubes. The oxidation of SWNTs in the presence of wet-air is found to selectively oxidize the nontubular carbon impurities without affecting the small diameter tubes. In this procedure, the oxidation is done in the presence of air saturated with water vapor at 225 °C for 24 h. This is followed by sonication for 15 min or... 

Carbon nanotubes and nested fullerenes were also prepared by reductive carbonization of CO2 with magnesium metal. Motiei et al. [104] reported on the production of nanotubes in ca. 0% yield and nested fullerenes in ca. 12% yield at 1000°C by the reaction of dry ice and magnesium. The reductive carbonization of CO2 is reminiscent of the HiPco process [105, 106], based on disproportionation of carbon monoxide (the Boudouard reaction) ... 

Bronikowski, M.J. Willis, P.A. Colbert, D.T. Smith, K.A. Smalley, R.E. Gas-phase production of carbon single walled nanotubes from carbon monooxide via the HIPCO process a parametric study. J. Vac. Sci. Technol. A 2001, 19, 1800-1805. 

The preparation of single-walled nanotubes succeeds more easily by the so-called HiPCo-process that was published for the first time in 1998. The name is deduced from high-pressure carbon monoxide and signifies a crucial aspect of the method Here the carbon source is not a hydrocarbon, but carbon monoxide that does not suffer pyrolysis at the relevant temperatures. The formation of carbon material is based on the Boudouard equilibrium (3.9) ... 

The feasibility of continuous operation counts among the major advantages of the HiPCo-process over most alternative methods. The catalytic decomposition of carbon monoxide can thus be considered as a potential way of scaling up SWNT-production from a few grams to kilograms or even tons. 

The thermal decomposition of organic compounds can also be employed to generate small carbon clusters or atoms. The borderline with chemical vapor deposition (CVD) as presented in the next section is not really fix. In both cases, the method is based on the thermal decomposition of organic precursors. Processes both with and without catalyst have been reported. Contrary to the chemical vapor deposition, however, the catalyst (if applied) is not coated onto a substrate, but the substance or a precursor is added directly to the starting material ( floating catalyst ). The resulting mixture is then introduced into the reactor either in solid or in liquid state by a gas stream. From this point of view the HiPCo-process could also be considered a pyrolytic preparation of SWNT, but due to its importance it is usually regarded as autonomous method. 

The HiPco process for preparing SWCNTs developed by R.E. Smalley and co-workers [Nikolaev et al. Chem Phys Lett 313 91 1999] involves formation of the catalysts by in situ thermal decomposition of Fe(CO)5 in a heated flow of CO in the gas phase at 1-lOatm and SOO-HOO C. By adjusting the processing parameters (best at 1-lOatm and 800-1200°C), SWCNTs were produced in high yields (79 mole%, i.e. 44 wt%) with narrowest tubes (as small as 0.7mn in diameter, same as in C50 fullerene) at the rate of 1.24mg/hour. The structures were confirmed by TEM, SEM, EDX and TGA. An important advantage of this procedure is that it is a continuous-flow process which can be used for bulk production of narrow SWCNTs. See below for pmification of HiPco tubes. 

Nikolaev P. Gas-phase production of single-walled carbon nanotubes from carbon monoxide a review of the HiPco process. J Nanosci Nanotechnol 2004 4 307-16. 

CNTs can be made by means of arc-discharge, laser ablation, high pressure carbon monoxide decomposition (HiPCO), and chemical vapor decomposition (CVD) processes. The HiPCO process delivers high quality SWCNTs but the upscaling is difficult. Arc-discharge, laser ablation and CVD systems are currently used to produce SWCNTs as well as MWCNTs in laboratory scales. However, until now only CVD processes have been upscaled successfully to produce MWCNTs in commercially relevant amounts. For this purpose the synthesis is usually performed in fluidized beds [10-12]. Currently (2011), industrial grades of MWCNTs can be purchased for less than 130 USD/kg. 

Five methods of synthesis of CNTs are (1) arc discharge, (2) laser ablation, (3) CVD, (4) the HIPCO process, and (5) surface-mediated growth of vertically aligned tubes. CNTs can have different morphologies such as SWNT, DWNT, MWNT, nanoribbon, nanosheet, nanopeapods, linear and branched CNTs, conically overlapping bamboo-like Y-shaped tubules, nanopores, nanovoids, nanowire, and nanofiber. 

The tensile properties of films prepared from poly(vinyl alcohol), PVA, and SWNT are shown in Table 13.6 (73). The SWNT were prepared by the HiPCO process. Further, the SWNT were coated with poly(vinyl pyrrolidone) and sodium dodecyl sulfate to allow good dispersion in the PVA. Flowever, as studied, there was no orientation of the film or the SWNT. 

Chiang IW, Brinson BE, Huang AY, Willis PA, Bronikowski MJ, Margrave JL, Smalley RE, Hauge RH (2001) Purification and characterization of single-wall carbmi nanotubes (SWNTs) obtained from the gas-phase decomposition of CO (HiPco process). J Phys Chem B 105 8297-8301... 

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