On the Synthesis of FullerenesN.R. Conley and J.J. Lagowski, Department of Chemistry and Biochemistry, The University of Texas at Austin,

Austin, TX 78717; Telephone and fax: 512/471-3288, e-mail: Lagarto2@aol.com

Abstract

In July of 1991, the potential of fullerene combustion synthesis made itself known to the world. Chemists working at the Massachusetts Institute of Technology reported fullerene yields up to 9% using low-pressure, premixed benzene/oxygen/argon laminar flames [3]. Continuing the work of J.B. Howard, J.T. McKinnon, Y. Markarovsky, A. Lafleur and M. Johnson, we have investigated fullerene production in high-temperature flames at atmospheric pressure using various hydrocarbons, heterocyclic compounds, and ferrocene. Mass spectral data is presented.

Introduction

Since the introduction of Kratschmer’s graphite vaporization method in 1990, the first procedure for macroscopic synthesis of fullerenes, chemists have worked diligently to modify these new fullerene structures--trapping atoms, adding elements, and attaching functional groups. Despite all of the hard work, several “ideal” modifications have eluded organic chemists for years. It has come to the attention of the authors that fullerene combustion synthesis offers the advantage of flame-doping with the possibility of modifying the fullerene products, an area which has not been thoroughly explored. Below is a table with the compounds we introduced into a hot, non-sooting oxy-acetylene flame, the method by which we introduced them, and the products we intended to form.

C60, also known as “buckyball” C70, slightly larger than “buckyball”

Procedure

To achieve the high temperature necessary for fullerene combustion synthesis, we used a Victor Medalist XL oxy-acetylene torch with a brazing tip. The hydrocarbons/heterocyclic compounds were injected, 10 mL per experiment, into the non-sooting oxy-acetylene flame using a glass syringe. Preliminary experiments in benzene combustion revealed the optimal oxy-acetylene flame conditions at atmospheric pressure: 19 cm. flame length, a cone 4 cm. in length, and flame/stainless steel interaction at 14 cm. below the torch tip. Soot from these experiments was collected on a 19 cm. x 24 cm. x ¾ cm. stainless steel plate that was placed on top of a water-cooled brass block. The cooling block allowed the stainless steel plate to remain below 800 K, the temperature at which C60 sublimes. Because of its low boiling point and its ability to dissolve fullerenes, CS2 was chosen as a solvent. After the experiments, the soot was scraped off of the stainless steel plate, 10 mL of CS2 were added, and the samples were placed in an ultrasonic bath for at at least thirty minutes. Following ultrasonication, the samples were filtered and the filtrate was collected. After removing enough solvent to concentrate the samples, they were sent for mass spectral analysis using chemical ionization.

Experimental

The following table provides a list of the doping compounds, the amounts used for soot formation, the total amount of soot formed, and the corresponding mass spectrum numbers for analysis of the aromatic soot.

FLAME-DOPING COMPOUNDS

METHOD OF INTRODUCTION

DESIRED PRODUCTS

benzene (C6H6) Injection into flame C60, C70

dicyclopentadiene (C10H12) Injection into flame C60, C70

pyridine (C5H5N) Injection into flame Azafullerene (CxNy)

thiophene (C4H4S) Injection into flame Thiofullerene (CxSy)

ferrocene (C10H10Fe) Dissolved in benzene, injection into flame

Iron encapsulated fullerene (Fe@C60)

Ø Neither spectrum is provided.

+ Only the positive CI spectrum is provided.

- Only the negative CI spectrum is provided.

× The high-resolution mass spectrum is provided.

SPECTRA NOT PUBLISHED ON POSTER ARE AVAILABLE FOR VIEWING UPON REQUEST.

Results

  

Discussion

oxy-acetylene torch/injector set-up

FLAME-DOPING COMPOUNDS

RATIOS AND AMOUNTS TOTAL MASS OF SOOT

COLLECTED

MASS SPECTRUM NUMBER

benzene (C6H6) 10 mL  0.032 g 1 (-)

dicyclopentadiene (C10H12) 10 mL  0.478 g 2 (-)

dicyclopentadiene/benzene 3 mol dicycopentadiene:10 mol benzene; 10 mL total

 0.174 g 2.1 (-)

pyridine (C5H5N) 10 mL 0.029 g 3a (-), 3b (+, ×)

pyridine/benzene 1 mol pyridine:1 mol benzene; 10 mL total

0.070 g 

  1 mol pyridine:5 mol benzene; 10 mL total

 0.063 g 3.2 (Ø)

thiophene (C4H4S) 10 mL 0.013 g  4 (Ø)

thiophene/benzene 1 mol thiophene:1 mol benzene; 10 mL total

0.020 g  4.1a (+), 4.1b (-)

  1 mol thiophene:5 mol benzene; 10 mL total

0.028 g  4.2 (Ø)

ferrocene (C10H10Fe) 10 mL of saturated benzene solution

 0.060 g 5 (-)

3.1 (Ø)

1(-) 2 (-)

2.1 (-) 3a (-)

3b (+) 3b (×)

4.1a (+) 4.1b (-)

5 (-)

We have identified C60 and C70 in soot from a benzene/oxy-acetylene flame

(spectrum #1). While we had no device to measure the temperature, NMDO models predict the window of thermodynamic stability for C60 to be between

2200 and 2600 K at one atmosphere [2]. We have also identified C60 and C70 in

soot from the combustion of dicyclopentadiene (spectrum #2). However, the lower peak intensities suggest that fullerenes are formed to a lesser extent in the combustion of dicyclopentadiene. When we injected a mixture of benzene and dicyclopentadiene in the stoichiometric ratio of ten moles of benzene to three moles of dicyclopentadiene, resembling the 20 hexagon to 12 pentagon ratio of C60, we also produced fullerenes. A wide band of peaks appeared around 655 on

this spectrum (# 2.1). In spectrum #3a, obtained from the combustion of pyridine, a similar band of peaks appears around 653. In addition, smaller groups of peaks are observed at masses 722, 799, and 877. Spectrum #3b, the CI+ spectrum of pyridine soot, reveals a large peak at 680. High-resolution mass spectroscopic analysis resolves this peak to 679.5988. Elemental composition suggests the presence of the compound C40H72N9 in this sample. There is a

relatively low-intensity peak at 679 on spectrum #4.1b, the negative CI spectrum of soot formed from a mixture of thiophene and benzene. In the combustion of a benzene solution saturated with ferrocene, there are no peaks indicative of an iron-encapsulated fullerene (spectrum #5).

Conclusion

Until these experiments can be carried out at lower pressures, producing larger fullerene yields that will allow for separation [2], it is difficult to make conclusions concerning the products that are formed. However, peak intensities from spectra #1 and #2 suggest that it is easier to form the five-membered carbon rings necessary in the mechanism for fullerene formation from six-membered carbon rings than it is to form six-membered carbon rings from five-membered carbon rings. The majority of the products in these experiments are large polycyclic aromatic hydrocarbons (PAH), often referred to soot. PAH are believed to be precursors in fullerene formation [3]. The large peak at 655 in spectrum #2.1 may represent a stable PAH intermediate in the formation of fullerenes. In the spectra of soot obtained from the combustion of pyridine or thiophene, there are no peaks with an isotopic distribution that would suggest a fullerene structure, although the presence of these large PAH indicates the potential for their formation. The ferrocene-doping of a benzene flame seems to suppress the formation of C60 and C70. Again however, due

to the presence of these PAH, it is not possible to rule out the potential of combustion synthesis in forming an iron-encapsulated fullerene.

References

1. Howard, J.B., McKinnon, J.T., Makarovsky, Y., Lafleur, A.L., and Johnson, M.E., Nature 352: 139-141 (1991).

2. McKinnon, J.T., and Bell, W.L., Combustion and Flame 88: 102-112 (1992).

3. Bachmann, M., Wiese, W., and Homann K.-H., Twenty Sixth Symposium (International) on Combustion, The Combustion Institute, 1996, pp. 2259-2267.

Special Thanks

The authors would like to extend special thanks to all of those involved in this project. First and foremost, we acknowledge Robin Rogers, Longfei Jiang, and Dr. Mehdi Moini in the Department of Mass Spectrometry for their diligent work in sample analysis. Also, we would like to thank Robert A. Lewandowski III for his unequivocally skillful glassblowing and all of the machinists whose insight and hard work has made an enormous impact on our continuing research. We are thankful for the help of Michael Klysik, who has assisted in a portion of this research. We are forever indebted to Rita Wilkinson for her timely filing of all forms necessary to make possible our trip to San Francisco, CA. Finally, we would like to recognize the Welch Foundation for their funding of this research.