2007 NSTI Nanotechnology Conference and Trade Show - Nanotech 2007 - 10th Annual

MWCNT synthesis by fluidized bed pyrolysis of polyethylene-terephtalate: effect of reactor temperature

M.L. Mastellone and U. Arena
Second University of Naples, IT

Keywords:
multi-wall carbon nanotubes; pyrolysis; CNT production; plastics

Abstract:
The study refers to a method for MWCNT production by means of a continuous fluidized bed pyrolysis process of virgin or recycled polymers (1, 2). The technique produces, in a relatively large quantity and at low cost, carbon nanotubes having quality similar to MWCNTs available on the market even when the starting materials were polymers coming from post-consumers waste collection and recycling. Polyethylene terephtalate has been continuously fed to an atmospheric bubbling fluidized bed pyrolyser operated at 600 and 800°C. Experiments were carried out with the aim to quantify the yield of produced gas, liquids and solids. The solid phase has been characterized by means of different methods: TG-DTG allowed to determine the thermal stability of the compounds present in the solid phase and to give a preliminary indication about their nature; SEM and TEM microscopy, coupled with EDAX analysis, allowed to investigate the morphology of solid structures and to recognise the presence of some elements (e.g. metals included into the structures). The peculiar interaction between polymers and bed materials, in a fluidized bed reactors, promotes a very fast thermal cracking of polymer chain with production of light and heavy hydrocarbons, waxes and carbon nanostructures, like multi-wall nanotubes (3, 4). The very fast progress of heating and melting of the polymer in contact with bed material leads to the cracking of the carbon–carbon bonds that starts when the polymer already covered the bed particles. Therefore, cracking, radicalic and graphitization reactions always related to a layer of polymer which coats the external surfaces of bed particles (2). A first series of experiments were carried out at 600°C. At this temperature the interaction between the bed material (quartz sand) and the oils produced by pyrolysis induces a layering effect that leads to the worsening of fluidization and eventually to defluidization [3]. After a 1 hour test the solid phase produced by the process has been collected and characterized by SEM observations and TG-DTG measurements. Figure 1 shows a detail of a bundle of nanostructures adhering to the bed particle surface while Figure 2 shows two pictures obtained in the same tests. TG-DTG curve reported in Figure 3 shows the presence of a peak around 600°C, which has been recognized as typical of MWCNTs (5). Investigations carried out with other techniques (TEM, X-Ray, Raman) demonstrated that these nanostructures can be classified as nanotubes and nanofibers having diameters between 10 and 150nm (1). A second series of tests have been carried out at 800°C. Under this condition the yield of solids increased but the nanostructure morphology was different. Both SEM/TEM (Figure 4) and TG (Figure 5) investigations showed that quasi-spherical structures were produced together with few MWCNTs. These results were similar to those obtained in investigations carried out with PE and PP at 900°C (5). These type of nanostructures have, as demonstrated by TG analysis, a higher thermal stability (up to 680°C in air). An interesting further result is that, for tests at 800°C longer than 1 hour, a progressive reduction of solid yield (and of MWCNTs) and a parallel increase of liquid products were observed. A possible explanation is the interaction between carbon monoxide produced by PET pyrolysis and metal catalysts present in the reactor. In presence of a CO+H2 environment it is in fact possible that metals (iron and nichel) carbonyls form. Other Authors studied the performance of metal carbonyls as catalysts for CNTs synthesis and concluded that the size of metal particles was too large to allow their growing [6, 7]. 1.Arena U., Mastellone M.L., Camino G., Boccaleri E., Polymer Degradation and Stability, 2006, 91: 763-768 2.Arena U. and Mastellone M.L., chapter 16 in Feedstock Recycling and Pyrolysis of Waste Plastics, J. Scheirs and W. Kaminsky (eds.) J. Wiley&Sons Ltd, pp. 435-474, 2006 3.Arena U. and Mastellone M.L., Chemical Engineering Science, 2000, 55, 2849-2860 4.Mastellone, M.L. and Arena U., AIChE Journal, 2002, 48:7, 1439-1447 5.Arena U. and Mastellone M.L., Fluidization XII, F.Berruti, X.Bi, T.Pugsley (Eds), Vancouver, May 2007 6.Moisala, A., Nasibulin, A.G., Brown, D.P., Jiang, H., Khriachtchev, L., Kauppinen, E. I., Chemical Engineering Science, 61: 4393-4402, 2006 7.Rohmund F., L. K.L. Falck, E. E. B. Campbell, Chemical Physics Letter, 328 (2000)

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