WO2012156297A2

WO2012156297A2 - Method for synthesis of functionalised carbon nanotubes by cycloaddition under continuous flow conditions and apparatus for the method

Info

Publication number: WO2012156297A2
Application number: PCT/EP2012/058746
Authority: WO
Inventors: Michele Maggini; Enzo MENNA; Tommaso Carofiglio; Emiliano ROSSI; Alessandro Pace; Patrizio SALICE
Original assignee: Universita' Degli Studi Di Padova
Priority date: 2011-05-13
Filing date: 2012-05-11
Publication date: 2012-11-22
Also published as: WO2012156297A3; EP2707328A2; ITPD20110153A1

Abstract

A method is described for functionalising carbon nanotubes by 1,3-dipolar cycloaddition of azomethine ylides under semi-continuous flow conditions in solvents of high boiling point. The method necessitates an apparatus for flow reactions having technical characteristics suitable for its implementation. The method has proved to be efficient and sustainable and capable of producing single-wall (SWNT), double-wall (DWNT) or multiple-wall (MWNT) carbon nanotubes, functionalised in less time and with the same characteristics as carbon nanotubes functionalised using a batch-wise synthesis process.

Description

Method for synthesis of functionalised carbon nanotubes by cycloaddition under continuous flow conditions and apparatus for the method

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Field of the invention

The invention relates to a method for chemical functionalization of single-wall (SWNT), double-wall (DWNT), or multi-wall (MWNT) carbon nanotubes by 1 ,3- dipolar cycloaddition of azomethine ylides under semi-continuous flow conditions and to an apparatus for in-flow reactions for implementation of said method.

State of the art

The particular chemico-physical and mechanical characteristics of carbon nanotubes (CNTs) render these materials highly advantageous in respect of their applications. Indeed, CNTs have considerable qualities of resistance and flexibility, combined with electronic and conductive properties, both electrical and thermal, that are especially important for the use of these materials as metallic conductors or semiconductors, insulators or materials having high mechanical resistance. They can be, therefore, used in electronic and optoelectronic equipment, for example for electrical and electronic microcircuits, diodes, transistors, sensors, in composite or even polymeric materials of high electrical, thermal and mechanical resistance. However, for all these numerous applications it is necessary to have available materials that are finely dispersible in solvents or polymeric matrices, and this is a crucial requirement for pristine carbon nanotubes, namely those industrially produced according to known processes. This has led to the development of different approaches to the synthesis of CNT derivatives and, of these, the method that has been more widely used is the reaction of 1 ,3-dipolar cycloaddition of azomethine ylides, generated in situ from thermal condensation between an cc-amino acid and an aldehyde, a reaction that leads to formation of a pyrrolidine derivative of CNTs (Georgakilas V. et al., J. Am. Chem. Soc, 2002, 124, 760-761 ). This method of synthesis has found wide applications, not only for single-wall, double-wall and multi-wall carbon nanotubes, but also for other carbon materials different in shape from nanotubes, such as "nanohorns" (Cioffi, C. et al., Chem. Comm., 2006, pp. 2129-2131 ), "nanoonions" (Cioffi, C. et al., Chemistry - A European Journal, 2009, 15, 4419-4427), and graphene (Quintana M. et al., ACS Nano, 2010, 4, 3527-3533). The functionalised derivatives of CNTs and the like have proved to be soluble, or at least readily dispersible, in many organic solvents and at the same time readily defunctionalisable and/or usable for several applications, thus rendering them key products for a number of uses.

Nevertheless, all the synthetic processes adopted are limited by the elevated, virtually total insolubility and dispersibility of CNTs, implying considerably longer reaction times as compared with analogous reactions conducted on common organic molecules and treatments with vigorous sonication, which can lead to structural defects of the CNTs. Furthermore, their complex structure can signifi- cantly influence the homogeneity of distribution of the functionalization on the surface of these carbon materials.

Summary

Synthesis under continuous flow conditions can solve these problems by enabling more precise control of parameters that are critical for the whole process. Therefore, it is a purpose of the present invention to provide a method for synthesising functionalised CNTs under continuous or semi-continuous flow conditions, which combines a use of solvents suitable for dispersing and disaggregating the nanotubes with fine control of the parameters critical for the synthesis of these functionalised CNTs.

An object of the invention is, therefore, a method for functionalising carbon nanotubes by 1 ,3-dipolar cycloaddition of azomethine ylides, wherein said functionalisation is carried out under semi-continuous flow conditions and comprises at least the steps of:

- transferring a reaction mixture consisting of a dispersion of carbon nanotubes in a solvent of amide type and azomethine ylide precursors consisting of an a-amino acid and an aldehyde into a first fluid circuit consisting of a loading channel connected to a loading pump and controlled by two switching valves;

- flowing said reaction mixture by means of an organic carrier solvent delivered by a flow pump from the loading channel to a flow reactor thermostated at a temperature comprised between 120 and 180°C and regulated by a backpressure valve; - maintaining the reaction mixture thermostated in the flow reactor for a residence time comprised between 15 and 60 min, and preferably 30 min;

- collecting the reaction products and separating the obtained functionalized carbon nanotubes from the reaction mixture.

Optionally, the method may provide for repetition of the loading and transfer steps, with re-introduction into circulation of the reaction mixture comprising the functionalised nanotubes, further adding to said mixture, prior to transfer into the loading channel, azomethine ylide precursors.

Another object of the invention is a flow-reaction apparatus for the method for functionalising carbon nanotubes by 1 ,3-dipolar cycloaddition of azomethine ylides under semi-continuous flow conditions, comprising:

a first fluid circuit comprising a first vessel (1 ), into which the reaction mixture is loaded, where said first vessel is connected by means of a first inlet channel (2) and a first switching valve (3) to a loading channel (4), whose outlet is in turn downstream connected, by means of a second switching valve (5) and a first outlet channel (6) on which a loading pump (7) is placed, to a second vessel (8), in which any residual carrier solvents are collected; and

- a second fluid circuit comprising a third vessel (9), into which a carrier solvent is loaded, where said third vessel (9) is connected by means of a second inlet channel (10), on which a second principal flow pump (1 1 ) is placed, the first switching valve (3), the loading channel (4), the second switching valve (5) and a second outlet channel (12) downstream of the latter, said channel (12) being connected to a flow reactor (13) downstream connected to a backpressure regulator (14) and to a fourth vessel (15), in which the reaction mixture comprising the obtained functionalized carbon nanotubes is collected.

The aims and advantages of the method for functionalising carbon nanotubes under semi-continuous flow conditions, and of the apparatus for in-flow reactions for the said method will be better understood from the detailed description below in which, by way of non-limiting illustration of the invention, example preparations of carbon nanotubes will be described, which nanotubes have been functionalised according to the semi-continuous flow method that is the object of the invention and the flow reaction apparatus for carrying out said method.

Brief description of the drawings

Figure 1. The figure shows a configuration of the flow reaction apparatus usable for the method for functionalising carbon nanotubes by 1 ,3-dipolar cycloaddition of azomethine ylides according to the invention.

Figure 2. The figure shows a configuration of the flow reaction apparatus during the steps of loading the reaction mixture, namely the first fluid circuit of said flow reaction apparatus.

Figure 3. The figure shows a configuration of the flow reaction apparatus during the reaction steps, namely the second fluid circuit of said flow reaction apparatus.

Figure 4. The figure shows in detail a flow diagram of the method for functionalising carbon nanotubes by 1 ,3-dipolar cycloaddition of azomethine ylides according to the invention using the flow reaction apparatus described.

Figure 5. The figure shows the UV-vis absorption spectrum of a maximum concentration solution of functionalised (solid line) and pristine (dotted line) single- wall carbon nanotubes (SWNTs) in dimethylformamide (DMF).

Figure 6. The figure shows the dynamic light scattering (DLS) graph showing the distribution of the hydrodynamic volumes in DMF of functionalised (solid line) and pristine (dotted line) single-wall carbon nanotubes.

Figure 7. The figure shows the Raman spectrum of the functionalised (solid line) and pristine (dotted line) single-wall carbon nanotubes (laser excitation: 633 nm). The panel demonstrates the increase in intensity of band D.

Figure 8. The figure shows the thermogram (TGA) of the functionalised (solid line) and pristine (dotted line) single-wall carbon nanotubes.

Figure 9. The figure shows the UV-vis absorption spectrum of a solution in DMF at maximum concentration of functionalised (solid line) and pristine (dotted line) double-wall carbon nanotubes (DWNTs). Figure 10. The figure shows the UV-vis absorption spectrum of a solution in DMF of maximum concentration of functionalised (solid line) and pristine (dotted line) multi-wall carbon nanotubes (MWNTs).

Figure 11. The figure shows the graph showing the hydrodynamic volumes distribution in DMF of the functionalised (solid line) and pristine (dotted line) DWNTs.

Figure 12. The figure shows the graph showing the hydrodynamic volumes distribution in DMF of the functionalised (solid line) and pristine (dotted line) MWNTs.

Figure 13 The figure shows the Raman spectrum of the functionalised (solid line) and pristine (dotted line) DWNTs.

Figure 14. The figure shows the Raman spectrum of the functionalised (solid line) and pristine (dotted line) MWNTs.

Figure 15. The figure shows the thermogram of the functionalised (solid line) and pristine (dotted line) DWNTs.

Figure 16. The figure shows the thermogram of the functionalised (solid line) and pristine (dotted line) MWNTs.

Detailed description of the invention

The method for functionalising carbon nanotubes under semi-continuous flow conditions according to the invention can be conducted in suitable fluidic apparatus, the essential configuration of which will be described in detail in what follows with the aid of Figures 1 -3.

The semi-continuous flow reaction apparatus shown schematically in Fig. 1 comprises two fluidic circuits having a common part in co-division consisting of a loading channel (4), said common part being defined and controlled at the inlet by a first three-way switching valve (3) and at the outlet at by a second three-way switching valve (5).

In detail as shown in Fig. 1 , the apparatus for the semi-continuous flow reaction is composed of a loading channel (4), consisting substantially of a tubular serpentine of a polymeric plastic material, for example polytetrafluoroethylene (PTFE), connected to a first inlet channel (2) or to a second inlet channel (10) by means of a first T-shaped three-way switching valve (3), and to a first outlet channel (6) or to a second outlet channel (12) by means of a second T-shaped three-way switching valve (5). For the purposes of the present invention, the loading channel (4) has an internal diameter comprised between 0.8 and 5 mm, and preferably 2 mm, and a length comprised between 20 and 750 cm, and preferably 350 cm, for a loading volume comprised between 1 and 15 ml_, and preferably 1 1 ml_.

The first inlet channel (2) draws from a first vessel (1 ) equipped with stirring means, said stirring being preferably achieved by means of a bar magnet inside the vessel and a magnetic agitator underneath the same; the second inlet channel (10) is connected to a flow pump (1 1 ), and said second inlet channel (10) in turn draws from a second vessel (9); the first outlet channel (6) is connected to a loading pump (7), and said first outlet channel (6) is connected to a third vessel (8); the second outlet channel (12) is connected into the inlet of a flow reactor (13), the outlet of which is connected, by means of a backpressure regulating valve (14), to a collecting fourth vessel (15). The flow reactor (13) is substantially a serpentine in which the reaction mixture is held for a certain residence time, calculated on the basis of the total volume of the flow reactor ( V) and the rate of flow (F). In particular, the residence time (f) of the reaction mixture is given by the equation (a):

ί = V/F (a)

and an overall flow time ir is given by the equitation (b):

tr= Vm/F (b)

where Vm is the volume of the reaction mixture.

The flow reactor (13) preferably consists of a microtube of a polymeric plastic material, for example polytetrafluoroethylene (PTFE), forming a serpentine and having an outer diameter comprised between 0.8 mm and 5 mm, and preferably of 1 .58 mm, and an internal diameter comprised between 0.5 and 4 mm, and preferably of 0.8 mm, and having a length of at least 100 cm and preferably between 400 and 800 cm for a residence volume comprised between 1 and 10 ml_, and preferably of 2 ml_.

These features of the flow reactor (13), as also those of the loading channel (4), can be adapted using a conventional technique in accordance with the volumes of reaction fluid involved in the residence time observed according to equations (a) and (b) indicated above.

The flow reactor (13) is thermostated at a temperature comprised between 120 and 180°C, and preferably 140°C. The flow reactor (13) is preferably thermostated by immersion in a thermostatic oil bath to at least 400 cm of effective length of the serpentine, corresponding to a preferred residence volume thereof of at least 2.0 ml_.

In detail, pristine carbon nanotubes loaded in vessel are dispersed in an amide- type organic solvent, having a boiling point higher than 120°C and preferably selected from dimethylformamide (DMF), 1 -methyl-2-pyrrolidone (NMP) and 1 - cyclohexyl-2-pyrrolidone (NCP). Since the said carbon nanotubes are known to be insoluble in any solvent, even organic ones, they are subjected to mild sonication. Said dispersion of CNTs can be alternatively first prepared and then loaded into the first loading vessel (1 ) or prepared directly in the first vessel (1 ). To the CNT dispersion the reagent precursors of an azomethine ylide, that is an cc-amino acid and an aldehyde, are added. The reaction mixture is then transferred from the first vessel (1 ) to the loading channel (4) by switching the three-way valves (3) and (5) so as to connect the loading channel (4) to the first inlet channels (2) and to the first outlet channels (6) and to form the first fluid circuit. The flow reaction apparatus is then configured as shown in Fig. 2 with both the switching the three- way valves (3) and (5) commutated in a way to form the first fluid circuit (configuration 1 of the apparatus). For transferring the reaction mixture from the first vessel (1 ) to the loading channel (4), the loading pump (7) placed on the first outlet channel (6) is then actuated. This pump operates under suction until all the reaction mixture has been transferred into the loading channel (4). Any solvent present into the channel (6) for priming the pump (7) before the loading of the reaction mixture into loading channel (4) is discharged and collected in vessel (8). At the end of the step of loading the reaction mixture into the loading channel (4), the valves (3) and (5) are switched so as to connect the loading channel (4) to the first inlet channels (10) and to the second outlet channels (12) and to form the second fluid circuit. The apparatus is then configured as shown in Fig.3 with both the switching the three-way valves (3) and (5) commutated in a way to form the second fluid circuit (configuration 2 of the apparatus). For the step of flowing of the reaction mixture from the loading channel (4) to the reactor (13), the flow pump (1 1 ) drawing into vessel (9) containing an organic carrier solvent, for example dimethylformamide (DMF), is then actuated. This solvent is pushed by the principal flow pump (1 1 ) into the loading channel (4), and causes the reaction mixture to flow through the flow reactor (13) thermostated at the reaction temperature, preferably 140°C. The flow rate of the reaction mixture through the reactor (13) is regulated on the basis of the overall volume of said flow so as to achieve a residence time of the reaction mixture in the reactor (13) of at least 15 minutes and preferably 30 minutes. The flow rate is preferably comprised between 2 and 6 mL/h, and preferably 4 mL/h, for a residence volume comprised between 1 and 10 ml_ and the total flow time of the whole loaded reaction mixture is comprised between 10 minutes and 7.5 hours.

During this step, the backpressure regulator (14) is set in operation in such a way as to have a constant pressure inside the reactor (13) not greater than 2 atm, and preferably between 1 .4 and 1 .7 atm. In this way, any pressure variations due to the development of gas and vapours during the functionalisation reaction are avoided. The flow of the carrier solvent is maintained until all the mixture has flowed into and been collected in vessel (15).

With regard to the step of isolation of the functionalised carbon nanotubes from the reaction mixture, said nanotubes can be separated according to techniques known to the person skilled in the art, for example by means of centrifugation and/or extraction with organic solvents.

The method for functionalising carbon nanotubes under semi-continuous flow conditions, object of the invention, is substantially implementable in its entirety as shown in the flow diagram of Fig. 4, and comprises the steps of:

a) loading into a first vessel (1 ) a dispersion of carbon nanotubes in an organic solvent of amide type, which dispersion is preferably obtained with the aid of ultrasonication, by keeping the mixture under stirring; b) adding to the mixture contained in vessel (1 ) the reagent precursors of an azomethine ylide consisting of an cc-amino acid and an aldehyde; c) switching the valves (3) and (5), so as to connect the loading channel (4) to the first inlet channel (2), and to the first outlet channel (6) into/out of said loading channel (4), so forming a first fluid circuit (i.e. configuration 1 );

d) actuating a loading pump (7), placed on the first outlet channel (6), until the reaction mixture is transferred from the first vessel (1 ) to the loading channel (4), and any solvent previously present in the first outlet channel (6) for priming the pump is discharged into a second vessel (8); e) switching the valves (3) and (5), so as to connect the loading channel (4) to the second inlet channel (1 0), and to the second outlet channel (1 2) into/out of the loading channel (4), so forming the second fluid circuit comprising the flow reactor (1 3) (i.e. configuration 2);

f) actuating a flow pump (1 1 ), placed on the second inlet channel (1 0), which flow pump draws a carrier solvent into a third vessel (9) and pushes it into the loading channel (4) to cause the reaction mixture to flow through the flow reactor (1 3) thermostated to the reaction temperature ( 7);

g) regulating the flow rate (F) on the basis of the volume ( V) of the reactor, so as to achieve a residence time {t = V/F) of at least 15 minutes;

h) simultaneously starting the backpressure regulator (14) in operation, so as to maintain a constant pressure (P) within the reactor by opposing any variations due to the development of gas and vapours within the mixture;

i) maintaining the flow until all the reaction mixture has flowed into the fourth collecting vessel (1 5) ;

j) isolating the products (functionalised nanotubes) from the reaction mixture by means of centrifugation procedures and/or extraction with solvents.

The method can optionally provide for repetition of steps b) to i) with re- introduction into circulation, by means of transfer into vessel (1 ) of the reaction mixture collected in vessel (1 5) during step i), adding additional azomethine ylide precursors to this mixture (step b).

Compared to batch synthesis reported in the literature (Georgakilas V. et al., J. Am. Chem. Soc, 2002, 124, 760-761 ), this method allows to obtain CNT derivatives with similar physicochemical properties and yields, but in much shorter reaction times.

The method for functionalising carbon nanotubes under semi-continuous flow conditions, according to the invention is described in greater depth with the aid of the examples given below, which are provided by way of non-limiting illustration. Example 1. Preparation of functionalised single-wall carbon nanotubes (SWNTs) by means of 1,3-dipolar cycloaddition reaction of azomethine ylides under continuous flow conditions in DMF

The reaction in continuous flow was conducted in apparatus as described above and shown schematically in Fig. 1 .

The carbon nanotubes SWNT (10 mg; purity 70-80% by weight, diameter 1 .2-1 .4 nm, length 1 -5 μιτι; NanoCarblab) were previously dispersed in the reaction solvent Ν,Ν-dimethylformamide (DMF; 10 ml_; Sigma Aldrich) by means of sonification (Sonicator 300, Misonix) conducted using 3-sec pulses (on/off) with a power of 15-20 Watt. The SWNT dispersion was introduced into vessel (1 ) (step a), and the two precursors of azomethine ylide (step b) 2(2-(2- methoxyethoxy)ethoxy)acetaldehyde 1 , synthesised as described by Marcus Week et al. (J. Org. Chem, 2005, 70, (14), 5550-5560) (60 mg, 0.36 mmol), and an amino acid ester, ethyl 2-(benzylamino)acetate 2 (70 mg, 0.36 mmol; Sigma Aldrich) were then added. A bar magnet arranged within the vessel maintained the suspension under vigorous stirring.

Valves (3) and (5) were switched as in the configuration in Fig. 2 (step c), and the mixture was transferred into the loading channel (4) by means of aspiration using the pump (7) (step d). Valves (3) and (5) were then switched as in Fig. 3 (step e) and the pump (1 1 ) was actuated so as to draw the carrier solvent (DMF) from vessel (9) and push it into the loading channel (4) to cause the reaction mixture to flow into the flow reactor (13), thermostated at the temperature T= 140°C (step f). Given the total volume of the flow reactor ( V) equal to 2.0 ml_, with a flow rate (F) of 4.0 ml_/h a residence time (f) of the reagents of 30 min was achieved, and an overall flow rate (tr = Vm/F; where Vm = 10 ml_ is the volume of the reaction mixture) of 2.5 h per run. During this step the backpressure regulator was set so as to keep a pressure (P) of approx. 1 .7 bar within the reactor (13). Once all the reaction mixture was collected in vessel (15), the pump (1 1 ) was stopped and the mixture was transferred to vessel (1 ). The cycle formed by steps b) to i) was repeated another 2 times, each time adding the same quantities of reagents 1 and 2, for a total residence time of 90 minutes and an overall process time of 7.5 h. The reaction mixture obtained was then centrifuged at 3500 rpm for 3 min and, after removing the supernatant, the solid was washed with 5 portions of 7ml toluene and then dried in a vacuum (0.2 mbar) at 80 °C for 4 h.

9.5 mg of functionalised SWNT was obtained.

Example 2. Preparation of single-wall carbon nanotubes functionalised by means of 1,3-dipolar cycloaddition reaction of azomethine ylides under continuous flow conditions in NMP

The functionalisation of single-wall carbon nanotubes was conducted as described in Example 1 , using 1 -methyl-2-pyrrolidone (NMP) as the reaction solvent. Starting with the same quantities of SWNT, and of reagents.

9.8 mg of functionalised SWNT was obtained.

Example 3. Preparation of single-wall carbon nanotubes (SWNT) functionalised by means of 1,3-dipolar cycloaddition reaction of azomethine ylides under continuous flow conditions in NCP

The functionalisation of single-wall carbon nanotubes was conducted as described in Example 1 , using 1 -cyclohexyl-2-pyrrolidone (NCP) as reaction solvent. Starting with the same quantities of SWNT, and of reagents.

10.3 mg of functionalised SWNT was obtained.

Example 4. Preparation of double-wall carbon nanotubes (DWNT) functionalised by means of 1,3-dipolar cycloaddition reaction of azomethine ylides under continuous flow conditions in NCP

The functionalisation of double-wall carbon nanotubes (purity 90% by weight, outer diameter 2-4 nm, length 5-30 μιτι; CheapTubes) was conducted as described in Example 1 , starting with 10.1 mg of DWNT, 3 x 28 mg of reagent 1 , 3 x 34 mg of reagent 2 in NCP.

9.1 mg of functionalised DWNT was obtained.

Example 5. Preparation of multi-wall carbon nanotubes (MWNT) functionalised by means of 1,3-dipolar cycloaddition reaction of azomethine ylides under continuous flow conditions in NCP

The functionalization of multi-wall carbon nanotubes (purity 90% by weight, outer diameter 10-15 nm, internal diameter 2-6 nm, length 0.1 -10 μιη; Sigma Aldrich) was conducted as described in Example 1 , starting with 1 1 .3 mg of MWNT, 3 x 35 mg of reagent 1 , 3 x 41 mg of reagent 2 in NCP.

10.8 mg of functionalised MWNT was obtained.

The functionalised carbon nanotubes were dispersed in 1 -methyl-2-pyrrolidone (NMP) and subjected to the following characterisations:

- absorption spectra between 280 and 1400 nm, at intervals of 0.5 nm, scanning rate of 300 nm/min., SBW: 2 nm by spectrophotometer (Varian

Cary 5000), at room temperature;

- dynamic light scattering (DLS) measurements using a Zetasizer Nano S (Malvern Instruments) at 20°C, setting 20 runs of 10 sec for each measurement;

- Raman spectra on film obtained for "drop-casting" on a microplate (Corning) and heated to 100°C, recorded by Raman microspectrophotometry (Invia Renishaw, 50 x objective), using a 633 nm laser beam from an He-Ne laser at room temperature with a low-power laser;

- Thermogravimetric (TGA) analyses in a nitrogen atmosphere with thermal ramping between 100 and 1000°C at 10°C/min.

Functionalised SWNTs inflow had a mean solubility of 0.1 mg/ml, 8 times greater than that of non-functionalised SWNTs, obtained by measurements of UV-visibile absorbance (Fig. 5). The occurrence of functionalization is confirmed by the increase in the hydrodynamic volume of the particles due to the superficial organic residues, which interact with the solvent, as confirmed by measurements of Dynamic Light Scattering (DLS) (Fig. 6). Furthermore, the Raman spectrum depicted in Fig. 7 shows an increase in band D following functionalization, evidence of the expected increase in the number of sp³ carbon atoms due to the formation of new bonds. The thermogram (TGA) in Fig.8 shows that, whereas the pristine SWNTs are characterised by weight loss (up to 1000°C) equal to 91 .5%, indicating the presence of 8.5% of inorganic impurities in the commercially available sample, for the functionalised SWNTs the residual mass above 800 °C is nil, as confirmed by the fact that in-flow functionalisation enables this type of impurity, which is normally present in SWNTs, to be effectively eliminated. Furthermore it is possible to estimate the degree of functionalisation of SWNTs on the basis of the weight loss below 400 °C (40%), corresponding to the organic fraction of the product, introduced with in-flow synthesis. It is therefore possible to estimate the presence of an organic functionality every 25 atoms carbon of the nanotube.

An analogous characterisation performed on functionalised DWNTs and MWNTs shows that in both cases, that the carbon nanotubes functionalised with the method of the present invention are more soluble than the pristine carbon nanotubes (Figs. 9 and 10), have greater hydrodynamic volumes (Figs. 1 1 and 12), and their Raman spectrum shows an increase in band intensity links to the presence of functions covalently bound to their structure (Figs. 13 and 14). In addition their thermogram shows a weight loss corresponding to the organic fraction of 7% and 20% respectively (Figs. 15 and 16).

Claims

1 . A method for carbon nanotube functionalization by 1 ,3-dipolar cycloaddition of azomethine ylides, wherein said functionalization is carried out under semi- continuous flow conditions and comprises at least the steps of:

transferring a reaction mixture consisting of a dispersion of carbon nanotubes in a solvent of amidic type and azomethine ylide precursors consisting of an a-amino acid and an aldehyde into a first fluidic circuit, comprising of a loading channel connected to a loading pump and controlled by two switching valves;

flowing said reaction mixture into a second fluidic circuit by means of an organic carrier solvent delivered by a flow pump from the loading channel to a flow reactor thermostated at a temperature comprised between 120 and 180°C and regulated by a backpressure valve; - maintaining the reaction mixture thermostated in the flow reactor for a residence time comprised between 15 and 60 min. ;

collecting the reaction mixture comprising the functionalized carbon nanotubes; and

separating the obtained functionalized carbon nanotubes from the reaction mixture.

2. The method for carbon nanotube functionalization by 1 ,3-dipolar cycloaddition of azomethine ylides according to claim 1 , wherein the functionalization reaction is repeated at least once on the reaction mixture comprising functionalized carbon nanotubes by re-cycling and adding to the same further amounts of azomethine ylide precursors before transfer into the loading channel.

3. The method for carbon nanotube functionalization by 1 ,3-dipolar cycloaddition of azomethine ylides according to claim 1 , wherein the solvent of amidic type has a boiling point higher than 120 °C.

4. The method for carbon nanotube functionalization by 1 ,3-dipolar cycloaddition of azomethine ylides according to claim 1 , wherein the solvents of amidic type are selected from dimethylformamide, 1 -methyl-2- pyrrolidone and 1 -cyclohexyl-2-2-pyrrolidone.

The method for carbon nanotube functionalization by 1 ,3-dipolar cycloaddition with azomethine ylides according to claim 1 , wherein the flow reactor is thermostated at a temperature of 140 °C.

The method for carbon nanotube functionalization by 1 ,3-dipolar cycloaddition of azomethine ylides according to claim 1 , wherein the residence time is 30 minutes.

The method for carbon nanotube functionalization by 1 ,3-dipolar cycloaddition of azomethine ylides according to claim 1 , wherein during functionalization time the pressure is maintained under 2 atm.

The method for carbon nanotube functionalization by 1 ,3-dipolar cycloaddition of azomethine ylides according to claim 1 , comprising the following steps:

a) loading a dispersion of carbon nanotubes in a solvent of amidic type, obtained with an ultrasonication, into a first vessel and maintaining the mixture under stirring;

b) adding to the carbon nanotube dispersion the azomethine ylide precursor reactants consisting of an a-amino acid and an aldehyde; c) switching the switching valves so as to connect the vessel to a loading channel by means of an inlet channel and an outlet channel to/from the same, so as to form a first fluidic circuit;

d) operating a loading pump until all the reaction mixture is transferred from the vessel to the loading channel ;

e) switching the switching valves so as to connect the loading channel to the inlet and outlet channels of a second fluidic circuit comprising a flow reactor;

f) operating a flow pump drawing a carrier solvent from a further vessel and delivering it into the loading channel to make the reaction mixture flow from the loading channel through a flow reactor thermostated at the reaction temperature;

g) adjusting the flow speed (F) according to the volume ( V) of the flow reactor so as to obtain a residence time (t = VI F) of at least 1 5 minutes; h) simultaneously setting a backpressure valve so as to maintain a constant pressure in the flow reactor;

i) maintaining the flow until all the reaction mixture has flowed into a further collecting vessel;

j) isolating the functionalized nanotubes from the reaction mixture by means of centrifugation and/or solvent extraction procedures;

k) optionally repeating the steps from b) to i) by transferring into the first vessel the reaction mixture collected in the collecting vessel during step i), and by adding further azomethine ylide precursors to said mixture. 9. An apparatus for flow reactions for a method for carbon nanotube functionalization according to claim 1 , comprising:

a second fluid circuit comprising a third vessel (9), into which a carrier solvent is loaded, where said third vessel is connected by means of a second inlet channel (10), on which a second principal flow pump (1 1 ) is placed, the first switching valve (3), the loading channel (4), the second switching valve (5) and a second outlet channel (12) downstream of the latter, said channel (12) being connected to a flow reactor (13) downstream connected to a backpressure regulator (14) and to a fourth vessel (15), in which the reaction mixture comprising the obtained functionalized carbon nanotubes is collected.