Joint Synthesis of Small Carbon Molecules (C3-C11), Quasi-Fullerenes (C40, C48, C52) and their Hydrides
1Frantsevich Institute for Problems of Materials Science of NASU, Krzhyzhanovsky St. Kiev, Ukraine
2Taras Shevchenko National University of Kiev, Volodymyrs'ka St. Kiev, Ukraine
Earlier small carbon molecules C2- C13 as cations and аnions in hot carbon plasma and flame gases were detected only. From large amount of quasi-fullerenes (C20 < n < 60) revealed in mass spectra of carbon vapor only C20 and C36 were synthesized. Therefore problem of creation of new methods for synthesis of carbon molecules is considered extremely important. By us new reactionary conditions of pyrolysis of hydrocarbons, in particular benzene is developed. A main distinctive feature of this pyrolysis is opportunity of separate localization of condensed products and soot. Mass spectra of toluene and ethanol solutions of obtained products contain intensive peaks with m/z values appropriate anions of small molecules (C3 – C20), them hydrides (C5H2, C10Н4, C14H4, C16H8, C18Н2) and cations of molecules (C6, C7, C15, C17). For the first time in products of pyrolysis quasi-fullerenes C40, C48, C52, C54, C56 and C58 are found out. Thus, small carbon molecules and quasi-fullerenes in reactionary conditions excluding carbon evaporation can be formed.
At a glance: Figures
Keywords: small carbon molecules, quasi-fullerenes, hydrides, synthesis, pyrolysis, benzene
Chemical Engineering and Science, 2013 1 (3),
Received December 26, 2012; Revised June 07, 2013; Accepted June 08, 2013Copyright © 2014 Science and Education Publishing. All Rights Reserved.
Cite this article:
- Kharlamov, Alexey, et al. "Joint Synthesis of Small Carbon Molecules (C3-C11), Quasi-Fullerenes (C40, C48, C52) and their Hydrides." Chemical Engineering and Science 1.3 (2013): 32-40.
- Kharlamov, A. , Kharlamova, G. , Bondarenko, M. , & Fomenko, V. (2013). Joint Synthesis of Small Carbon Molecules (C3-C11), Quasi-Fullerenes (C40, C48, C52) and their Hydrides. Chemical Engineering and Science, 1(3), 32-40.
- Kharlamov, Alexey, Ganna Kharlamova, Marina Bondarenko, and Veniamin Fomenko. "Joint Synthesis of Small Carbon Molecules (C3-C11), Quasi-Fullerenes (C40, C48, C52) and their Hydrides." Chemical Engineering and Science 1, no. 3 (2013): 32-40.
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After discovery of fullerene C60  and its higher homologues (C70, C76, C78, C84 and more) [2, 3] the synthesis of quasi-fullerenes C20 < n < 60 (in which isolated pentagons are absent) and small carbon molecules Cn < 20 is considered as the basic challenge to modern chemical community. The fate of small carbon molecules C2-C20 and quasi-fullerenes found out simultaneously with clusters C60 and C70 in carbon vapors [1, 2] by means of a mass spectrometric method  has appeared less bright in comparison with fullerenes C60 and C70. Only C20  of many quasi-fullerenes detected in mass spectra [6, 7] in preparative amounts was synthesized. Quasi-fullerenes C28 and C50 are obtained only as their derivative: endohedral metallofullerene (MC28)  and decachlorofullerene (C50Cl10) . The fate of guasi-fullerene C36 synthesized by Piskoti  by arc-discharge method is still unclear because in  by the same reactionary conditions only the hydrides (C36H4, C36H6) and oxyhydrides (C36H4O, C36H6O) as main products were revealed. Despite intensive signals in mass spectra [6, 7] molecular clusters C26, C30, C34, C38, C42, C46, C48, C52, C54, C56, C58 and their endohedral derivatives were not found out yet. Moreover, assumed as most stable clusters C32 and C44, as well as C50, which are referred to the category «magic» in a molecular form also are not synthesized yet.
The small carbon molecules such as C2 and C3, C4 and C5 together with polyynes (HCnN) (n ≤ 11) only in the circumstellar medium  are found out. In laboratory conditions these carbon molecules only in solid argon about 15 °К [13, 14, 15, 16, 17] were isolated. However a time of life of such frozen clusters is extremely small (~ 10 ms). Even and odd ions of small carbon molecules in laboratory conditions in mass spectra of carbon vapour are detected only. It is considered  that the hot carbon plasma contains linear chains C1 - C10, where C3 cluster is contained in the greatest amount (~ 70 %) . These chains C1-C10 as a chemical radicals to radical polymerization are extremely inclined and can be stabilized owing to the interaction of trailer atoms of carbon with H, N or CN with formation of relatively more stable polyynes or cyanopolyynes [18, 20]. Therefore problem of creation of a new method for synthesis of carbon molecules is considered extremely important. The absence of appreciable progress in synthesis Cn (n < 20) and quasi-fullerenes is probably determined by domination of hypothesis that the atoms of carbon are necessary always for growth of carbon molecules and their generating is possible only under carbon sublimation.
Here we first represent a new method of generation of small carbon molecules and quasi-fullerenes as well as their hydrides in which the stage of high-temperature sublimation of carbon is completely excluded. In mass spectra of products of benzene pyrolysis the ions of all kinds of carbon molecules are found out:
- small carbon molecules (C3 – C20);
- quasi - fullerenes C21, C23, C33, C40, C48, C52, C54,C56 and C58;
- hydrides of small carbon molecules, for example, C5Н2, C10Н4, C14Н4, C16H8 and C18Н2;
- hydrides of quasi - fullerenes (C25Н2, C27Н2, C31Н4, C33Н4, C37Н6, C39Н6, C43Н8, C45Н8, C47Н10 and C49Н10).
2. Materials and Methods
Experiments on the typical apparatus for pyrolysis of hydrocarbon vapors were carried out. Tests by means of changing of temperature in the most highly temperature zone A of reactionary space, the concentration and flow rate of introduced in the horizontal tubular quartz reactor reagent (benzene (ESSO Deutschland GmbH)) were realized. The maximum (T2) and minimum (T1) temperatures in zone A were 1100 and 1000°C respectively. Concentration of reagent (benzene) was varied in the range of 0.05-0.2% and the flow rate of carrier gas (Ar) range 100-300ml / min. Products B and C condensed and deposited in low temperature zones B and C respectively reactionary space for the study were used. The temperatures in the zones B and C were 400 and 70°C, respectively. Products of several (8-10) tests obtained at given temperature taken from zones B and C were mixed. Condensed substances by toluene from products zone B were extracted. Product of zone C in ethanol was dissolved. By a method of matrix-activated laser (nitrogen, 337nm) desorption/ionization (MALDI) (Bruker Daltonics flexAnalysis) obtained extracts were investigated. The extract on a metal substrate is located and after evaporation of the solvent to a laser irradiation was exposed.
XPS valence-band and core-level spectra of the product synthesized were measured using the UHV-Analysis-System assembled by SPECS Surface Nano Analysis Company (Germany). The system is equipped with a PHOIBOS 150 hemispherical analyzer. A base pressure of a sublimation ion-pumped chamber of the system was less than 5x10-10 mbar during the present experiments. The Mg Kα radiation (E=1253.6eV) was used as a source of XPS spectra excitation. The XPS spectra were measured at the constant pass energy of 25eV. The energy scale of the spectrometer was calibrated by setting the measured Au 4f7/2 and Cu 2p3/2 binding energies to 84.00±0.05eV and 932.66±0.05eV, respectively, with regard to EF. For investigated sample, all the spectral features are attributed to the constituent element core-levels or Auger lines.
The composition of volatile products of thermal decomposition of product B2 was investigated by a method temperature-programmed desorbtion mass spectrometry (TPDMS). Тhermodesorption measurement was carried out on monopole mass spectrometer МХ-7304А (Sumy, Ukraine) with impact electron ionization (EI). A sample at the bottom of molibdenium-quarts ampoule was evacuated at room temperature up to 5∙10-5 Pa. The linear heating of a sample up to 650°C was carried out with speed 0.15K∙s-1. The volatile thermolysis products passed through a high-vacuum valve (5.4mm in diameter) into the ionization chamber of the mass-spectrometer The ion currents of the desorption and thermolysis products were recorded with a secondary-electron multiplier VEU-6. Mass spectra were registered in a range 1-210amu.
3. Results and Discussion
Shortly after discovery  and synthesis C60 by the electro-arc method  method of heat treatment of hydrocarbons was used as an alternative [21, 22]. However fullerenes under usual method of pyrolysis (continuous flow pyrolysis, CFP)  and under burning of hydrocarbons  in small amounts are formed only. Products obtained at heat treatment (< 1200°C) of naphthalene C10H8 , its dimer and corannulene [22, 23] as well as benzene, cyclopentadiene , ethylene and acetylene , up to ~ 1% C60 contain only. Besides fullerene C70 at the presence of metallic (nickel) catalyst is formed only . Hence, known methods of hydrocarbons pyrolysis are low effective. Though the obtained experimental results convincingly demonstrate that the formation of C60 and C70 can be realized in the absence of atoms (or clusters) of carbon from hot carbon gas.
Earlier by us [27, 28] it was proposed the polycondensation mechanism of carbon molecules and nanostructures growth at hydrocarbons pyrolysis according to which the escalating of bonds C-C with formation of graphene net can be fulfilled due to a reaction of dehydropolymerization (polycondensation) mainly of benzene molecules and products of their partial decomposition. The curvature of graphene net is created at the expense of trailing, peripheral sp3 - hybridization atoms of carbon. At pyrolysis (as well as at cracking) of hydrocarbons the escalating of bonds C-C is realized also according to reaction of polymerization with formation of polyaromatic hydrocarbons. The reactions of polycondensation and polymerization are competitive in the given process and, therefore, one of them is obviously possible for optimizing by means of a variation of reactionary conditions.
By the systematic investigation [27, 28, 29, 30] we have studied the influence of various technological parameters and reactionary factors on obtained product composition and, in particularly, on the composition of condensed products. As a result of these researches the new method of heat treatment of organics vapours was developed. This method differs from two already of known processes of pyrolysis which are used long ago and for different purposes. Flash vacuum pyrolysis (FVP) [31, 32] is used mainly to prepare small size, highly reactive objects. Continuous flow pyrolysis (CFP) [22, 31] is employed for obtaining both carbon nanostructures of different morphologies and large polyaromatic molecules. Novel method of pyrolysis allows obtain simultaneously not only carbon nanostructures but also fullerene C60 and fullerenes hydrides [30, 33, 34, 35].
Distinctive feature of our method is the possibility of partial division of products of deposition and condensation, their subsequent localization in different zones of reactionary space. A part of condensation substances and pyrolytic soot are taken out in a gas reactionary flow from a zone A and are located in more low temperature zone В. Other part of vapour-like products (sometimes along with very small amount of pyrolytic soot) is condensed in the special zone C cooled reactionary space. The composition of products zones B and C and their amounts on temperature in a zone A depend on essentially. The results of study of two products (B1 and B2) and one product C condensed in zones B and C respectively are presented. A product B1 at more low temperature in a zone A, then product B2 was obtained. The reaction of polymerization in comparison with competitive reaction of polycondensation in the thermodynamic relation should be more preferable at lower temperature. Therefore according to the mechanism, assumed by us, the product В2 should contain mainly carbon clusters and product В1 are hydrides of carbon clusters.
Toluene extract from В1 by method of mass spectrometry matrix-assisted laser desorption/ionization (MALDI) was investigated. It is visible (Figure 1) that mass spectra of positive and negative ions cardinally differ.
In group of positively charged clusters (Figure 1 (b)), beginning from the cluster with value m/z 228 and finishing by cluster with value m/z 696, there is a precise periodicity in growth of their size. Every subsequent cluster on 24 or 26 units grows only. Clusters with values m/z 228, 252, 276, 624, 648, 672 and 696, probably, correspond to molecules C19, C21, C23, C52, C54, C56 and C58. (Usually believe that clusters C52, C54 and C56, C58 which detected in mass spectra of C60 are products of its decomposition) . Peaks with m/z from 302 up to 598 can belong both as a little hydrogenated odd (C25H2 - C27H2 - … - C31H4 - C33H4 - … C37H6 - C39H6 - … C43H8 - … - C45H8 - … C47H10 - C49H10) as even clusters, but with the large contents of hydrogen (C24H12 - C26H12 - … - C48H22). Closing one in the given group of peaks is a cluster with m/z 748 which corresponds to hydride C60H28. It is possible, that positively charged cluster with m/z 123 correspond to hydride (C10Н4) of small molecule C10.
In a spectrum of negative ions (Figure 1 (a)) peaks with m/z 172 and 326 considerably dominate and they can correspond to partially hydrogenated molecules C14H4, and C27H2. The less intensive peaks with m/z 62, 200 and 96, probably, also display presence of hydrides C5H2, C16H8 and molecule C8 at a product В1. At a spectrum also there is a group of peaks, m/z values which differ on 24 or 26 units. Among them there is a distinct peak with value m/z 480 that can correspond to quasi-fullerene С40. Only cluster C27H2 is detected in both spectra. Positively charged cluster C52 (m/z 624) is shown in a spectrum of negative ions as hydride C52H4 (m/z 628). Remarkable, that negatively charged clusters (C30H20 and C34H22) are, as a rule, more hydrogenated than appropriate positively charged (C30H16 and C34H16).
From a product В2 obtained at the more high temperature T2 in a zone A soluble in toluene substances were extracted and deposited by ethanol. Deposited powder В2 was again dissolved in toluene and mass spectra of this solution are submitted in Figure 2 (anions) and Figure 3 (cations). In a spectrum of negative ions (Figure 2) there is a group of peaks, in which the periodicity of change of m/z values makes 12 units. Such m/z values as 36, 48, 60, 72, 84, 96, 108, 120, 132, 144, 156, 168 and 180 (Figure 2 (b)), undoubtedly, concern to small carbon molecules C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14 andC15 accordingly. It is considered ,  that in the carbon vapour formed line and ring Cn (2 > n < 20) clusters with carbon atoms in the sp-hybridization. Clusters with m/z 193, 205, 217, 229, 241, 255 and 326 can correspond to anions C16H, C17H, C18H, C19H, C20H, C21H3, C27H2 which at ionization by laser of minimally hydrogenated cyclic molecules (or linear polyynes  C16H2 … C27H2 are formed.
In the spectrum of negative ions the peaks with m/z 720 and 576 are present, which may correspond to fullerene (C60) and quasi-fullerene C48. The peak with m/z 326 (C27H2) was also detected in the mass spectrum of the product B1 (Figure 1). Note, that in the mass spectrum of high-resolution (Figure 2 (a), inset) it can be seen that the molecules C5, C9 and C11 are hydrogenated, because for these molecules the ration, that is characteristic for the natural isotopic distribution of carbon , is a garbled. The formation of small carbon clusters as a result of degradation of C60 is unlikely. It is likely that the clusters C3-C15 and their hydrides are new products of the pyrolysis of benzene, which are formed at a temperature of 1000°C, eliminating the evaporation of carbon.
Spectrum of positive ions (Figure 3) contains three intensive peaks with m/z 84, 133 and 219, which can display molecules C7, С11 (or C11H) and C18H3. Just these clusters detected by mass spectrometric method as cations as anions, are, probably, most stable among small carbon molecules.
Cation mass spectra of high resolution (Figure 3, inset) demonstrates that odd cluster C7 partially hydrogenated up to C7H2 but cluster with even number carbon atoms (n=18) is only hydrogenated.
So, the products B1 and B2 deposited together with soot contain small carbon molecules from C3 up to C15, quasi-fullerenes C21, C23, C40, C48, C52, C54, C56, C58, fullerene C60 and hydrides of carbon molecules of different sizes, particularly, C14H2, C16H8, C18H2, C20H2. If the small carbon molecule can only be a cyclic structure with polyynic (even molecules C2n) or cumulenic bonds, the dihydrides of these molecules can be both cyclic and linear (polyynes) structures.
The hydrogen as can see from a curve of thermodesorption begins allocation from a powdery product B2 already at room temperature (in vacuum) and in enough large amount. It is improbable that PAHs in a similar way can be decomposed. Mass spectrum (EI) at 200°C of a powdery product B2 on Figure 4 is presented. Intensive peaks with 18, 28 and 44 amu correspond to molecular ions desorbed water, nitrogen and carbon dioxide, respectively. Ions with 31 and 45 amu as fragments of decomposition of molecules of the solvent (ethanol) are characteristic for EI mass spectra. The basic products of thermodesorption are carbon clusters С3, С4 and С5Н2 with molecular mass 36, 48 and 64 accordingly. It is possible that detected on MALDI mass spectra of a product B2 carbonaceous and hydrogenated molecules are thermo unstable already at low temperatures.
Product of C represents of white crystalline transparent plates (Figure 5) which are formed at any regime of heat treatment of benzene vapors. Sometimes these plates are located on a surface of deposited in a reactionary C zone soot and are taken with the help of optical microscope.
White plates are easily dissolved in many solvents (ethanol, benzene, acetone). A study of white plates by the XPS method was shown (Figure 6 (a, b, c)) that they consist of carbon (96.5%) only. Broadened peak of oxygen at 532.2eV indicates on its adsorbed state.
Mass spectra of an ethanol solution of this product are submitted in Figure 7 (anions) and Figure 8 (cations). It is visible, that the spectrum of negative ions (Figure 7 (a, b)) contains mainly a group of the most intensive peaks, which, as well as in mass – spectrum of a product В2, correspond to anions of small carbon molecules from C3 up to C20. In a spectrum peak with m/z 576 distinctly also is seen, which thin structure (Figure 7 (a), inset) a somewhat differs from isotopic distribution of carbon. Probably, this cluster as quasi-fullerene C48 partially hydrogenated up to C48H2. There is a peak with m/z 846, characteristic for C70H6 at a spectrum also.
At mass spectrum of positive ions (Figure 8 (a, b)) peaks with m/z 85, 97 and 109 are present, which can correspond to ionizing by proton molecules C7, C8 and C9. Four distinct peaks at m/z 72, 120, 180 and 204 correspond to molecules C6, C10, C15 and C17. Note, that in the mass spectrum of high-resolution (Figure 8 (a), inset) it can be seen that the molecules C15 and C17 are minimally hydrogenated, because for these molecules the ration, that is characteristic for the natural isotopic distribution of carbon , is a bit garbled. Thin structure of C18 (Figure 8 (a), inset) demonstrate that this molecule is hydrogenated only. Hence, the small carbon molecules are detected mainly as anions but their hydrides are as cations. Only molecules C6, C10, C15 and C17 are detected in both spectra.
It is unlikely that in the products B2 and C contain practically the full spectrum of small carbon molecules. It is possible that the product B2 contains mainly molecule C7, C11 and C18, which are recorded in the mass spectra in the form of cations (Figure 3) and anions (Figure 2). Under the impact of the laser from these molecules anions mostly of smaller molecules of carbon are formed. In a product C mainly molecules C15, C17, C18 and C20 can break up at laser ablation.
In this paper, we report the experimental results obtained at realization a new method of benzene pyrolysis which for the first time are demonstrate that the small carbon molecules (C3 - C20), quasi-fullerenes C40, C48, C52, C54, C56 and C58 can be obtained in reactionary conditions excluding evaporation of carbon. Earlier such carbon molecules as ions in hot carbon plasma and in flame gases under burning of hydrocarbons were detected only.
First the method of joint synthesis of carbon molecules and their hydrides is developed. Usually the hydrides fullerenes exclusively by hydrogenation of carbon molecules are obtained.
First solid substances are synthesized, which mass-spectra clusters, appropriate to small carbon molecules contain. Before such molecules in circumstellar medium and in a matrix of solid argon at the temperatures ~ 15-24°K were detected.
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