Reaction of silica-supported fullerene C60 with nonylamine vapor

Carbon 41 (2003) 2339–2346

Reaction of silica-supported fullerene C 60 with nonylamine vapor Elena V. Basiuk (Golovataya-Dzhymbeeva)a , *, Vladimir A. Basiuk b , ´ Flores a , Vladimir P. Shabel’nikov c , Vitaliy G. Golovatyi c , J. Ocotlan Jose´ M. Saniger a a

´ ´ ´ , Universidad Nacional Autonoma de Mexico , Circuito Exterior C.U., Centro de Ciencias Aplicadas y Desarrollo Tecnologico ´ 04510 Mexico D.F., Mexico b ´ ´ de Mexico , Circuito Exterior C.U., A. Postal 70 -543, Instituto de Ciencias Nucleares, Universidad Nacional Autonoma ´ 04510 Mexico D.F., Mexico c L.V. Pisarzhevsky Institute of Physical Chemistry, National Academy of Sciences of Ukraine, Prospekt Nauki 31, 03039 Kiev, Ukraine Received 28 January 2003; accepted 6 June 2003

Abstract The reaction of silica-supported [60]fullerene with vaporous nonylamine at 150 8C produces a mixture of addition products. Quantum chemical calculations, at the B3LYP/ STO-3G level of theory, support that the addition reaction most likely takes place across the 6,6 bonds of C 60 pyracyclene units (and not across the 5,6 bonds). Numerous peaks were found in high-performance liquid chromatograms, apparently due to a large number of possible isomers. According to elemental analysis data (C:N ratio), the number of nonylamine molecules attached to C 60 is 3 on average. Thermogravimetric analysis of the nonylamine adduct showed two weight losses, one between 360 and 590 8C due to thermal decomposition of nonylamine moieties, and one between 725 and 840 8C due to decomposition of the remaining fullerene-derived carbonized material. Field-desorption mass spectrometric study revealed a number of molecular and fragment ions corresponding to the adducts with up to six nonylamine moieties attached to [60]fullerene; some of them were observed as multiply-charged ions. The temperature behavior of these peaks is similar to that for TGA, with maxima shifted to lower temperatures due to the cooperative effect of the strong electric field. C 60 can be partially regenerated by pyrolysis of the nonylamine adduct, although at very low yields (below 1%, after heating at 350 8C under air for 2 h).  2003 Elsevier Ltd. All rights reserved. Keywords: Fullerene; Chemical treatment; Heat treatment; Infrared spectroscopy; Mass spectrometry

1. Introduction Among the whole rich chemistry of fullerenes, their interactions with amines gained special attention [1–10]. A decade ago, an interesting reaction of [60]fullerene (C 60 ) with amines was discovered [1,2]. Both primary and secondary amines (which are all neutral nucleophiles) add onto C 60 at room temperature by reacting with fullerene dissolved in liquid amines or in their solutions in DMF, DMSO, chlorobenzene, etc. [1–5]. The reaction stoichiometry varies significantly depending on the size of the amine *Corresponding author. Fax: 152-55-56-22-86-20. E-mail address: [email protected] (E.V. Basiuk).

reactant molecule. For smaller amine molecules such as 2-methylaziridine, the average amine:C 60 ratio can reach 10:1 [5]. For long-chain aliphatic amines it decreases dramatically; the lowest average number of amine molecules added, .1, was reported for dodecylamine [1]. Our interest in this reaction is associated with a wider use of the gas-phase technique for chemical derivatization of inorganic oxides [11–15] and carbon materials [16]. Some attractive features of this procedure are that it is relatively fast (usually |1 h) due to the use of elevated temperatures (.150 8C), and that an excess of derivatizing reagent is easily removed from the reaction zone by simple pumping out. Recently, the gas-phase derivatization procedure was employed for direct (i.e. without chemical

0008-6223 / 03 / $ – see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00255-0

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activation of terminal carboxylic groups) amidization of oxidized single-walled carbon nanotubes with simple aliphatic amines. The procedure included treatment of the nanotubes with amine vapors under reduced pressure and a temperature of 160–170 8C for 1 h [16], and produced carbon nanotubes soluble in organic solvents. In the present paper, we tested the applicability of the gas-phase technique to the reaction of fullerene C 60 with nonylamine, a representative of long-chain aliphatic primary amines. Since reactivity of crystalline C 60 is too low as compared to that in solution, we improved it by impregnation of C 60 onto porous silica, which is widely used as a support and catalyst in synthetic organic chemistry [17]. Another aspect of our interest is the thermal behavior of the addition product: in particular, to what extent the reaction with amine is reversible, that is, can C 60 be regenerated by pyrolysis, or does the high-temperature treatment produce some other carbon material?

2. Experimental and computational details

2.1. Materials Nonylamine, silica gel (60–200 mesh), toluene (99.8%, HPLC grade) from Aldrich, methanol (EM Science, HPLC grade) and fullerene C 60 (99.5%1 purity) from MER were used as received, without further purification.

2.2. C60 impregnation onto silica After C 60 (100 mg) was dissolved in toluene, silica gel (0.55 g) was added. Toluene was removed by boiling at atmospheric pressure. The C 60 :silica proportion employed provides only a few-layer coverage of the support material with fullerene, and thus ensures easy accessibility of C 60 molecules for the amine reagent.

moved, and the reactor cooled and disconnected from the manifold. Before unloading the solid material, the upper reactor part with some condensed residual nonylamine was wiped with ethanol-wetted cotton wool. The addition product was extracted from silica gel by toluene. After removal of the solvent, a brown amorphous solid was produced. To ensure complete elimination of unreacted nonylamine, the crude product was additionally heated at 150 8C under vacuum. Elemental analysis (QTI, NJ, USA), wt.%: found C 83.42, H 7.87, N 3.37.

2.4. Infrared spectroscopy Infrared (IR) absorption spectra were recorded in KBr pellets on a Nicolet 5SX FTIR spectrometer.

2.5. High-performance liquid chromatography For high-performance liquid chromatography (HPLC) analyses, the samples were dissolved in toluene. The resulting solutions were chromatographed on an Agilent 1100 Series instrument, equipped with an Alltima (Alltech) reversed-phase C 18 column, 3-mm particle size, and dimensions 50 mm34.6 mm I.D. UV detection was performed at 330-nm wavelength. As a mobile phase, we used 1:1 and 1:4 mixtures (by volume) of toluene and methanol, at 0.5 ml min 21 . C 18 columns show a good separation of different fullerenes (e.g. C 60 and C 70 [18]) and their derivatives [19].

2.6. Thermogravimetric analysis The TGA measurements were performed on a DuPont Thermal Analyzer 951, with a heating ramp of 10 8C 21 21 min until 1000 8C, under air flow of 100 ml min .

2.3. Reaction with nonylamine 2.7. Field-desorption mass spectrometry The procedure was performed using the custom-made Pyrex glass vacuum manifold described in detail elsewhere [16]. The impregnated material (containing 0.55 g SiO 2 and 100 mg C 60 ) was placed into the reactor. To remove volatile residues (toluene, water, etc.), the reactor was pumped out to a vacuum of |10 22 Torr, and its bottom was heated for 0.5 h at 100–120 8C by means of a heating mantle. Then the reactor was cooled to room temperature, opened and nonylamine (400 mg) was added directly to the silica-supported fullerene at the reactor bottom. After pumping the reactor out to |1 Torr at room temperature, its valve was closed, and the bottom was heated at |150 8C for 1 h. During this procedure, nonylamine evaporated and reacted with C 60 . After finishing the treatment, the reactor valve was opened again for |1 h to pump out most unreacted nonylamine. Then the heating mantle was re-

Field-desorption mass spectrometric (FDMS) [20] measurements were performed on a modified MI1201 instrument (Electron, Sumy, Ukraine). A custom-made ion source was used [21], where samples were deposited onto tungsten filament emitters coated with gold dendrites. A strong electrostatic field (typically 10 9 V m 21 ) applied to the emitter, allows ‘soft’ ionization of analyte molecules, with less extensive fragmentation as compared to that caused by electron impact. In addition, by applying an electric current (up to |10 22 mA) to the filament it is possible to vary its temperature, facilitating desorption of less volatile compounds (the degree of their fragmentation increases as well). Fullerene samples were deposited onto the emitter from suspensions in ethanol. Maximum heating temperature was 600 8C.

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2.8. Quantum chemical calculations The Gaussian 98W suite of programs was used [22]. Geometry optimization was performed by Becke’s threeparameter hybrid method [23] with the exchange functional of Lee, Yang and Parr [24] (B3LYP), in conjunction with the STO-3G basis set. All optimizations met the default convergence criteria set in Gaussian 98W.

3. Results and discussion The elemental analysis data are difficult to reconcile with a rational formula, a typical situation for adducts of this type [1], which points to the formation of a mixture of nonylamine-derivatized [60]fullerenes. If based on the major element content (carbon), the average number of nonylamine molecules added is 6; the H:N ratio (as suggested by Wudl et al. [1]) gives a value of 5 to 6 nonylamine moieties. From the C:N ratio it follows that the average number of nonylamine molecules added is 3. Analysis of the products by HPLC reveals numerous peaks, apparently due to the existence of a large number of isomers (Fig. 1). Small peaks with a retention time of approximately 3.4 (Fig. 1b) and 40 min (Fig. 1d) indicate the presence of a negligible amount of unreacted C 60 . It was not possible to obtain a good resolution and peak shape for the nonylamine derivatives and C 60 simultaneously. When the product peaks are better resolved (Fig. 1d) by using a toluene:methanol volume ratio of 1:4, the C 60 peak overlaps with some of them. With the 1:1 solvent mixture (Fig. 1b), the products are eluted soon after the void volume, but C 60 is better separated from them and can be quantified. The IR spectrum (Fig. 2a) shows a series of bands at 525, 575, 723, 1100, 1380, 1460, 1660, 2850, 2920 and 3450 cm 21 . The first two correspond to characteristic vibration modes of C 60 [25,26]. Vibrations of the nonyl hydrocarbon moiety are represented by symmetric and asymmetric nCH bands at 2850 and 2920 cm 21 , as well as by symmetric and asymmetric CH bending modes (dCH ) at 1380 and 1460 cm 21 , respectively, and by CH 2 rocking vibrations at 723 cm 21 . A broad intense band around 1660 cm 21 can be assigned to nC=C in alkenes, overlapping with the dNH absorption, whereas that at 3450 cm 21 corresponds to nNH vibrations. A broad maximum between 1000 and 1200 cm 21 is commonly observed in amorphous carbon materials (with mixed sp 2 and sp 3 hybridization states), and is assigned to different vibrations such as nC – C and nH – C – C in polynuclear condensed structures [27–29]. The addition reaction most likely takes place across the 6,6 bonds of C 60 pyracyclene units, and not across the 5,6 bonds. We calculated the energies of formation (relative to the reactant level, that is to the sum of energies of separated C 60 and nonylamine) of the two possible isomeric monoadducts (their optimized structures are shown

Fig. 1. HPLC chromatograms of (a) starting C 60 , (b,d) product of nonylamine addition before and (c) after heating at 350 8C under air for 2 h. Mobile phase: (a–c) 1:1 and (d) 1:4 mixture (by volume) of toluene and methanol. Arrows show the position of the C 60 peak.

in Fig. 3), at the B3LYP/ STO-3G level of theory. The difference between the calculated values is very large: 21.7 kcal mol 21 for the 5,6 hypothetical isomer, and 223.0 kcal mol 21 for the 6,6 adduct. The liquid-phase addition [1] occurs stepwise: electron transfer precedes covalent bond formation, as supported by spectrophotom-

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Fig. 2. Infrared spectra (in KBr pellet) of the product of nonylamine addition on C 60 (a), after heating under air at 100 (b), 200 (c), 350 (d), 400 (e), 450 (f), 500 (g), and 600 8C (h).

etry and electron spin resonance spectroscopy. In our case, the adsorbed layer on silica gel can act as a liquid polar medium (due to the presence of surface silanol groups ≡Si–O–H and strongly adsorbed water molecules), and the stepwise mechanism is possible as well. Thermogravimetric analysis shows significant differences in the behavior of the addition product and starting C 60 (Fig. 4). While the latter exhibits only one sharp and uniform weight decrease after 500 8C (Fig. 4a), the nonylamine adduct first shows a few-percent gain in weight (up to 360 8C), then two steps of weight loss: |70% between 360 and 590 8C, and |15% between 725 and 840 8C (Fig. 4b). To interpret this decomposition pattern, we submitted the adduct (diluted in a KBr pellet) to gradual heating under air, and recorded its IR spectra. As seen from Fig. 2b–h, the intensities of the hydrocarbon and amine bands (at 723, 1380, 1460, 1660, 2850, 2920 and 3450 cm 21 ) gradually decrease to total (or almost total) disappearance at 600 8C, thus suggesting that the first, largest weight loss in TGA is mainly due to thermal decomposition of nonylamine moieties. The band at 1100 cm 21 , however, shows less significant changes in its

intensity, consistent with a higher thermal stability of the fullerene cage structure, and according to TGA, the remaining fullerene-derived carbonized material decomposes after 600 8C. As already mentioned, another interesting aspect was to what extent the reaction with amine is reversible, that is, can C 60 be regenerated by pyrolysis (at least partially), or does the high-temperature treatment produce only other carbon material? We therefore heated a sample of the nonylamine adduct at 350 8C under air atmosphere for 2 h, and then analyzed its composition (fraction soluble in toluene) by HPLC. As can be seen from Fig. 1c, there are still poorly resolved components present with short retention times. In addition to that, a peak appears due to C 60 at |3.4 min. According to semiquantitative estimates, C 60 recovery is below 1% (a low peak resolution did not allow precise measurements). One can expect that this value would be significantly higher for thermal decomposition in an inert atmosphere. To provide more detailed information on the thermal behavior of the nonylamine addition products, we employed the technique of field-desorption mass spectrometry

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Fig. 4. Thermogravimetric curves for (a) C 60 and (b) product of nonylamine addition.

˚ Fig. 3. Geometry (with most important interatomic distances in A; in italics, corresponding values for the starting C 60 ) and formation energy (relative to the reactant level), calculated at the B3LYP/ STO-3G level of theory, for possible products of nonylamine addition across 5,6 (a) and 6,6 bond (b) of the pyracyclene unit.

[20,21]. A strong electrostatic field applied to the emitter allows ‘soft’ ionization of the analyte molecules, with less extensive fragmentation as compared to that caused by electron impact; this allows observation of molecular ions. At the same time, by applying an electric current to the filament, it is possible to vary its temperature and to study thermal effects in the compounds under study. An example of recorded FD mass spectra is presented in Fig. 5a (at the emitter temperature of 190 8C). A number of different ions were detected. One of the most abundant ones has m /z 863, and thus can be assigned to molecular ions of the

product of single nonylamine addition (Fig. 5b). Its most likely fragmentation pattern is due to cleavage of the linear side chain, which undergoes a loss of different (C 1 to C 9 ) hydrocarbon species. Five possible resulting fragments for the monoadduct were detected as singly charged ions (with m /z 778, 792, 806, 834 and 848). An interesting feature of FDMS (pp. 169–170 in Ref. [18]) is the presence of multiply charged ions, especially in those compounds where a charge delocalization is possible (fullerenes are typical examples of such compounds). In many cases, the abundance of singly- and doubly-charged ions turns out to be of the same order of magnitude; sometimes only the latter species can be observed. In the present case, we detected a series of peaks attributed to doubly-charged ions, in particular those at m /z 390, 411, 437, 449, 464, 477, 493, 537 and 577. The first one represents the case when both singly- and doubly-charged ions are observed for the same fragment, namely for C 60 (H)–NH(CH 2 ) 3 . The peak at m /z 411 can be assigned to ion C 60 (H)–NH(CH 2 ) 11 , whereas a singly-charged ion 6 was not detected for this fragment. Fragments of the addition products of more than one nonylamine molecules were identified as well in the FD mass spectra. Although molecular ions for isomeric products with two amines attached, that is C 60 (H 2 )– (NH(CH 2 ) 8 CH 3 ) 2 , were not found, neither as singly- nor as doubly-charged ions, these compounds manifest themselves as peaks at m /z 890 and 918. The latter ion can be assigned to singly-charged ions of C 60 (H 2 )– (NH(CH 2 ) 8 CH 3 ) 2 , which have lost in total six C-atoms and 16 H-atoms; different combinations are possible, for example –(C 3 H 8 1C 3 H 8 ), –(C 2 H 6 1C 4 H 10 ), etc. Similarly, the former is the product of C 8 H 20 elimination, as –(C 4 H 10 1C 4 H 10 ), –(C 3 H 8 1C 5 H 12 ), etc. The group of peaks around m /z 890, however, is particularly wide, so

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Fig. 5. (a) Field desorption mass spectrum for nonylamine addition product at the filament temperature of 190 8C; (b) fragmentation pattern for the monoadduct.

that elimination of additional hydrogen atoms is possible as well. The products of triple nonylamine addition, C 60 (H 3 )– (NH(CH 2 ) 8 CH 3 ) 3 , have a molecular weight of 1149. This value goes beyond the limit of reliable determination of mass numbers for the instrument employed. Nevertheless, a group of peaks detected around m /z 577 most likely corresponds to the doubly-charged molecular ions. In addition to that, the peak at m /z 535–537 can be due to their fragmentation with a loss of C 6 H 10 (again, different combinations are possible: –2C 3 H 5 , –C 2 H 4 –C 4 H 6 , etc.) to C 6 H 12 (–2C 3 H 6 , –C 2 H 5 –C 4 H 7 , –CH 3 –C 5 H 9 , etc.). Less abundant peaks at m /z 437, 449, 464, 477 and 493 apparently correspond to multiply-charged ions of higher addition products, with four and more nonylamine molecules attached to C 60 . For example, a triply-charged molecular ion of C 60 (H 5 )–(NH(CH 2 ) 8 CH 3 ) 5 (MW 1435) would have m /z 478: found m /z 477. Then the peaks at m /z 464, 449 and 437 might be due to a gross loss of C 3 H 7 , C 6 H 16 and C 9 H 16 , respectively. Furthermore, although there is no peak which could correspond to a triply-charged molecular ion of C 60 (H 6 )– (NH(CH 2 ) 8 CH 3 ) 6 (MW 1578), that at m /z 493 matches the hexa-adduct which has lost C 7 H 15 . A peak due to C 60 at m /z 720 was detected as well under certain conditions. There are two possibilities: either it is due to unreacted fullerene impurity, or it forms as a result of fragmentation of the nonylamine adducts. A way to verify the origin of this peak is to monitor how its abundance and that of other peaks changes when the temperature of the emitter is increased. For pure C 60 , the ion appears below 100 8C (Fig. 6a). Its intensity rapidly

grows to a maximum at 190 8C, then drops even more sharply to total disappearance at 250 8C. In the FD mass spectra of the adduct recorded below 200 8C (Fig. 5a), this peak can be barely distinguished above a noise level, that is, when it already decreases in the spectrum of pure C 60 . Upon further heating it passes through maximum intensity at 350 8C (Fig. 6b), disappearing completely at |500 8C. The behavior of all other fragments is totally different. Those with higher mass numbers (m /z.700) appear at lower temperatures, with maxima below 200 8C; those having m /z,600 just appear at |200 8C, showing maximum abundances at |300 8C. In other words, thermal decomposition here begins with the cleavage of hydrocarbon side chains, which disappear after 350 8C. Upon further heating, the material remaining on the emitter is converted (at least in part) to the starting fullerene. Although all the maxima in FDMS are shifted to lower temperatures due to the cooperative effect of the strong electric field, this trend is consistent with the TGA curve observed above (Fig. 4b).

4. Conclusions (1) The reaction of silica-supported [60]fullerene with vaporous nonylamine at 150 8C produces a mixture of addition products. (2) The addition reaction most likely takes place across the 6,6 bonds of C 60 pyracyclene units, and not across the 5,6 bonds. (3) Numerous peaks were found in high-performance liquid chromatograms, apparently due to a large number of

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Fig. 6. Plots of intensities versus temperature for (a) C 60 molecular ion (m /z 720), and (b) characteristic peaks in the mass spectra of nonylamine addition product. Only peaks with peak-to-noise ratio of .2:1 were taken into consideration.

possible isomers. According to elemental analysis data (C:N ratio), the number of nonylamine molecules attached to C 60 is 3 on average. (4) TGA of the nonylamine adduct shows two weight losses, one between 360 and 590 8C due to thermal decomposition of nonylamine moieties, and one between 725 and 840 8C due to decomposition of the remaining fullerene-derived carbonized material. (5) A similar trend can be observed by field-desorption mass spectrometry: the C 60 fragment ion is detected at temperatures of 300–500 8C, whereas hydrocarbon-containing fragments evolve and disappear at significantly lower temperatures. FDMS shows the presence of adducts

with up to six nonylamine moieties attached to [60]fullerene. (6) C 60 can be partially regenerated by pyrolysis of the nonylamine adduct, although at very low yields (below 1%, after heating at 350 8C under air for 2 h).

Acknowledgements The authors acknowledge financial support from the National Council of Science and Technology of Mexico (grants CONACYT-36317-E and -40399-Y) and from the

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National Autonomous University of Mexico (grants DGAPA-IN100402-3 and -IN102900).

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