WO2009081362A1

WO2009081362A1 - Process for recycling organic materials with the production of carbon nanotubes

Info

Publication number: WO2009081362A1
Application number: PCT/IB2008/055447
Authority: WO
Inventors: Simone Musso; Marco Zanetti; Alberto Tagliaferro; Maria Paola Luda
Original assignee: Politecnico Di Torino; Universita' Degli Studi Di Torino
Priority date: 2007-12-20
Filing date: 2008-12-19
Publication date: 2009-07-02
Also published as: ITTO20070923A1

Abstract

A process for the recycling of organic waste materials, particularly waste plastics materials comprising a stage of pyrolysing the materials and a stage of converting the gaseous pyrolysis products into carbon nanotubes, in which conversion into nanotubes takes place through vapour phase chemical deposition in an inert gas with the help of an organometallic catalyst, particularly ferrocene, is described. The process therefore makes it possible to convert pyrolysis products which also comprise hydrocarbons of a toxic and harmful nature that would otherwise have to be removed in conventional treatments for recycling plastics materials by pyrolysis into a raw material of high value (nanotubes).

Description

Process for recycling organic materials with the production of carbon nanotubes

This invention relates to a process for disposing of and recycling organic waste materials, particularly waste plastics materials which is suitable for the conversion of such materials into reusable materials of high added value; in particular the invention relates to a process which uses waste plastics materials and converts them into carbon nanotubes.

Since the discovery of the two new allotropic forms of carbon, fullerenes and nanotubes, in the last twenty years, research in the field of carbon nanotubes has developed exponentially thanks to the special and exceptional chemical/physical/ mechanical/electrical properties of this material. In comparison with other forms of carbon this material is in fact potentially applicable to a great variety of fields (nanoelectronics, field emission display, reinforcing fibres for plastics or ceramics materials, etc.). At the present time the only limitation on the development of this technology lies in the processes for the synthesis of nanotubes, which do not allow large-scale production with high yield/efficiency.

Research in the sector of processes for the synthesis of nanotubes has also developed in connection with processes through which carbon-containing materials of low cost or originating from recycled materials can be converted into nanotubes.

EP-A-I 522 355 describes a process for obtaining carbon-containing material, essentially carbon black, from the pyrolysis of wastes. The carbon black so obtained can be converted into activated carbon and carbon nanotubes by conventionally known methods, such as electric arc and laser ablation.

US 6 444 864 describes a process for the recycling of polymer waste materials with the production of nanotubes through the Plasma Enhanced Chemical Vapour Deposition (PECVD) technique. In this process use is made of the plasma effect produced between two electrodes by means of radio frequencies or microwaves to bring about decomposition of the carbon-containing precursors and as a consequence, under suitable conditions, the formation of nanotubes. The process described there makes it necessary to reduce the polymer waste to a powder, and this is delivered in the powdered state to the electric arc where pyrolysis and nanotube formation take place simultaneously. One disadvantage of this procedure lies in the fact that the polymer, as often happens, contains fillers, and these remain mixed with the nanotubes, making it necessary to use particularly expensive and complex procedures to recover the nanotubes.

Also, in general, the PECVD process has disadvantages such as the inability to operate on a large scale, the need to operate with quite stringent process parameters which do not allow any flexibility in production/conversion, and also a reagent conversion yield and a rate of deposition which are unsatisfactory.

One specific process and apparatus for the production of nanotubes by PECVD is described in GB- A-2 380 494.

US 2006/062 714 describes a process in which the carbon nanotubes are produced by burning an organically-modified montmorillonite (organoclay) composite in air in the presence of a mesoporous nickel-based catalyst, that is a supported catalyst.

The use of supported catalysts nevertheless has many disadvantages such as low contact surface area, high sensitivity to poisoning reagents and also, because these catalysts become inactive in a short time, only a low efficiency and low flexibility batch process can be developed. The abovementioned process also requires the polymer to be premixed with organoclay and catalyst, which cannot be applied to thermosetting plastics materials (such as epoxy resins, polyurethanes, etc.), that is to those materials which are most advantageous for pyrolytic recycling. Premixing with extruders constitutes an additional cost. In addition to this, large quantities of hydrofluoric acid, which is highly toxic, and sulphuric acid need to be used in order to recover the carbon nanotubes, making the method potentially highly polluting.

WO2006/033457 describes a process for the production of nanotubes which uses a vegetable oil as a source of carbon, in which the oil is sprayed onto a metal catalyst supported on a silica gel, aluminium, magnesia, silica-alumina and zeolite support. One object of the invention is to provide a technically and economically advantageous process for converting waste plastics materials into carbon nanotubes.

Another object of the invention is that of providing a process which can be used in the context of conventional processes for the heat treatment of plastics and organic wastes which makes it possible to use advantageously products resulting from the aforesaid heat treatment, with the production of a material such as carbon nanotubes having a high added value.

Another object of the invention is to provide a process which can be operated continuously, in a controllable and flexible way, and with a high efficiency of conversion of the reagents into carbon nanotubes.

With these aims in view, the object of the invention is a process for recycling waste plastics materials comprising a stage of pyrolysing the said plastics materials and a stage of converting the gaseous pyrolysis products into carbon nanotubes, characterised in that the stage of converting the gaseous products is carried out through vapour phase chemical deposition in inert gas with the help of unsupported organometallic catalyst.

Further features of the process according to the invention will be described in the following claims.

The pyrolysis stage may be carried out using conventional techniques for the treatment of wastes, using temperatures between 300°C and 800°C, to convert the material from the solid state into gaseous substances comprising aromatic and aliphatic compounds, and liquid products (pyrolysis oil), with the formation of a waste which depending upon the nature of the plastics materials used essentially comprises carbon black, inorganic fillers (calcium carbonate, talc, etc.) and metals.

In the process according to the invention the pyrolysis is preferably performed in a nitrogen atmosphere, and the temperature conditions are determined according to the nature of the waste plastics materials used with a view to maximising the generation of gaseous compounds of an aromatic nature. In particular the pyrolysis conditions may be selected to maximise the production of aromatic polycyclic hydrocarbons which in conventional processes for the pyrolysis of organic wastes constitute an undesired component, in that they are of a toxic nature, while their presence in the process according to the invention is instead advantageous in that it is useful for improving the yield of carbon nanotubes.

Although in conventional processes for the pyrolysis of wastes aromatic hydrocarbon compounds must generally be removed from the synthesis gas obtained, with a consequent increase in the cost of the process, the process according to the invention has the considerable advantage that it uses such hydrocarbons as a useful and advantageous raw material for the production of nanotubes.

Preferably the pyrolysis temperature lies between 300°C and 800°C. For example, temperatures between 350°C and 45O⁰C are preferable for polymer wastes in which epoxy resins constitute the fraction of greatest relative weight.

Optionally the gaseous hydrocarbon flow obtained from pyrolysis may be subjected to a stage of catalytic reforming before being delivered to the stage of conversion into nanotubes, in order to increase the aromatic component.

The stage of vapour phase chemical deposition for producing nanotubes can be carried out continuously, feeding the carbon-containing gases obtained into an environment maintained at the pyrolysis temperature for such carbon-containing gases, preferably through a carrier flow of inert gas.

An unsupported organometallic catalyst is injected into the flow of carbon-containing gas or directly into the environment in which these gases are converted into nanotubes. The organometallic catalyst used is typically ferrocene in the liquid state, or may be ferrocene in a liquid organic solvent.

Because of the high temperatures of the gases, the catalyst is vaporised. Because the catalyst particles form in the gas phase, the contact surface area of the catalyst is extremely high, with a consequent increase efficiency of converting the reagents into nanotubes. The conversion reactor may be a continuous reactor, for example a tubular reactor (of the horizontal flow type, for example), in which the nanotubes produced are deposited on a solid substrate, for example a substrate of silica (SiO₂) or silicon (Si).

Alternatively a continuous flotation reactor may be used.

The conversion temperature is generally between 650°C and 900°C.

It has been found that at temperatures of between 650°C and 75O⁰C the rate of deposition is relatively low (for example 100 nm/s at 700⁰C), while the conversion yield is low (30% by weight), but the intrinsic quality of the nanotubes is better, as confirmed by HR-TEM, SEM and RAMAN analyses. At higher temperatures, for example between 850⁰C and 900⁰C, the rate of deposition is higher (500 nm/s at 900⁰C), the conversion yield is high (50% by weight), but the intrinsic quality of the nanotubes is poorer, as confirmed by the analyses indicated above.

Unconverted gases, mainly consisting of aliphatic hydrocarbons, are removed from the conversion reactor. These gases can be used as a fuel in a heat generator, where the heat produced may be used to heat the conversion reactor and/or the pyrolysis reactor.

The process according to the invention makes it possible to convert organic wastes into nanotubes, particularly: a) wastes of polymeric materials of a thermoplastic nature, also including mixtures thereof b) wastes of polymeric materials of a thermosetting nature, elastomers included, also including mixtures thereof c) materials of a wood-cellulose nature, deriving from wastes from the processes for upgrading biomasses

Polymer wastes in which the thermosetting fraction predominates are preferred.

For example, the polymer wastes may comprise: a) polymer wastes deriving from electronic wastes, having for example the following formulation as a percentage by weight:

60% epoxy resins

20% aromatic polyesters (PET and PBT)

10% polyamides

10% inorganic fillers, mainly silica, b) polymer wastes deriving from electronic wastes having for example the following percentage formulation by weight:

20% polycarbonate

5% polyphenylene oxide

30% acrylonitrile butadiene styrene copolymer

10% polypropylene

5% polyester

20% epoxy resins

10% inorganic fillers, mainly silica c) polymer wastes deriving from urban solid wastes, having for example the following percentage formulation by weight:

67% polyethylene and polypropylene 13% polystyrene and derivatives 10% PVC 6% PET 4% others d) wastes deriving from the recovery of used tyres, having for example the following percentage formulation by weight:

45% isoprene rubber 15% plasticisers 38% carbon black 2% silica

Example

Pyrolysis zone The reagents come from a pyrolysis chamber where a sample obtained by shredding printed circuits has been heated to 600°C in a flow of nitrogen (100 ml/min).

Deposition zone

A vertical deposition chamber in which reagents flow from bottom to top and the catalyst floats. Operating conditions: T = 85O⁰C, P - 1.1 atm (N₂), 5% by weight of catalyst dispersed in gaseous reagents. Reagent conversion: 50% by weight. The nanotubes are collected through filtering or a separating cyclone.

Claims

1. Process for the treatment and recycling of organic waste materials, particularly waste plastics materials, comprising a stage of pyrolysing the said materials and a stage of converting the gaseous pyrolysis products into carbon nanotubes, characterised in that the stage of converting the gaseous products is carried out through vapour phase chemical deposition in inert gas, with the help of an unsupported organometallic catalyst.

2. Process according to Claim 1, characterised in that the said pyrolysis stage is performed under conditions such as to maximise the formation of aromatic hydrocarbons from the said materials.

3. Process according to Claim 1 or 2, characterised in that the pyrolysis stage is carried out in a nitrogen atmosphere at a temperature of between 300°C and 800°C.

4. Process according to any one of Claims 1 to 3, characterised in that the gases produced in the pyrolysis stage are subjected to a catalytic reforming stage to increase the aromatic hydrocarbon fraction before the said stage of conversion into nanotubes.

5. Process according to any one of Claims 1 to 4, characterised in that the said stage of conversion into nanotubes is carried out continuously in inert gas at a pyrolysis temperature of the gaseous products deriving from the pyrolysis stage.

6. Process according to any one of Claims 1 to 5, characterised in that the said stage of conversion into nanotubes takes place at a temperature of between 650°C and 900°C.

7. Process according to any one of Claims 1 to 6, characterised in that the said stage of conversion into nanotubes is carried out with the help of an unsupported ferrocene catalyst.

8. Process according to Claim 7, characterised in that the said ferrocene catalyst is injected into the flow of gaseous products fed to the stage of conversion into nanotubes at a temperature such as to cause the ferrocene to volatilise.

9. Process according to any one of Claims 1 to 8, characterised in that the said stage of conversion into nanotubes is carried out in a continuous flotation reactor.

10. Process according to any one of Claims 1 to 8, characterised in that the said stage of conversion into nanotubes is carried out in a tubular reactor with the nanotubes being deposited on a substrate selected from silica (SiO₂) or silicon (Si).

11. Process according to any one of the preceding claims, characterised in that the said waste plastics materials comprise polymer wastes deriving from electronic wastes comprising epoxy resins, aliphatic polyesters, polyamides and their mixtures.

12. Process according to any one of Claims 1 to 10, characterised in that the said waste plastics materials comprise polymer wastes deriving from electronic wastes comprising polycarbonate, polyphenyl oxide, acrylonitrile butadiene styrene copolymer, polypropylene, polyester, epoxy resins and their mixtures.

13. Process according to any one of Claims 1 to 10, characterised in that the said plastics materials comprise polymer wastes deriving from urban solid wastes and comprise polypropylene, polyethylene, polystyrene, polyvinyl chloride, polyethylene terephthalate and their mixtures.

14. Process according to any one of Claims 1 to 10, characterised in that the said waste plastics materials comprise wastes deriving from recovery of used tyres.

15. Process according to any one of Claims 1 to 10, characterised in that the said waste plastics materials comprise epoxy resins as the fraction of greater relative weight and in which the pyrolysis stage is carried out at a temperature between 350⁰C and 450°C.