ABSTRACT

Firstly the method of joint synthesis of carbon molecules and their hydrides is developed. The stage of high-temperature sublimation of carbon in a new method of generation of carbon molecules is completely excluded. By mass spectrometric method the condensation products of new method of pyrolysis (NMP) benzene are studied. Firstly clusters (C₃-C₁₇), typical for carbon vapour, in substances obtained under pyrolysis of hydrocarbons were detected. Fullerene C₆₀ and its hydrides, quasi-fullerenes C₄₈ and C₃₃ in products of benzene pyrolysis are detected also. Firstly it is shown what clusters C₃-C₅ can be generated at so low (100˚C-200˚C) temperatures of decomposition of substance. Obtained experimental results firstly demonstrate that the small carbon molecules can be generated in reactionary conditions excluding evaporation of carbon. Dehydrogenation and destruction of hydrocarbon molecules is the first stage on a route of the transformation of benzene to carbon molecules.

1. Introduction

It is considered, that precursor of formation of carbon molecules are carbon clusters, generated at high-temperature evaporation processes of carbon or materials on its basis. One year early to discovery of fullerene C₆₀ by Kroto et al. [1], Rofling [2] obtained the unique mass spectrum of carbon clusters that were created at laser evaporation of graphite in a flow of inert gas. In mass spectrum alongside with even and odd carbon clusters of the small sizes С_n (1 < n < 30) were detected even clusters С_n (n > 30), including clusters, appropriate to fullerenes opened later. Kroto et al. [1] have created the special conditions of increased collision (clusterization) of carbon clusters and firstly have obtained mass spectrum of carbon vapour, which contained mainly clusters С₆₀ and С₇₀. Isolation of many others clusters, observable in Rofling mass spectrum [2], from carbon vapour has appeared immeasurably more difficult task. Only after 6 years (in 1990) after detection of fullerenes С₆₀ and С₇₀ by mass spectrometric method, Kretchemer [3] succeeded to create conditions of the arc-discharge, which have allowed to fulfill not only the clusterization of generated carbon vapour with primary formation of С₆₀ and С₇₀, but also to locate them in appreciable amounts in obtained fullerene soot. The research by mass spectrometric method of a powder [3], obtained at evaporation of benzene extract from fullerene soot, has shown presence of positive ions with m/z 60 and 70 in the ratio 10:1 in mass spectrum.

The further researches of benzene extracts from fullerene soot have shown [4] that in carbon vapour generated by the arc-discharge method can be realized raising clusterization with the formation of larger than С₆₀ and С₇₀ molecules. It was possible to allocate fullerenes С₇₆, С₈₄, С₈₆, С₉₀ and С₉₄ [5], and also С₇₈ [6] from o-xylene (or o-dichlorobenzene) extracts of fullerene soot by a chromatography method (Al₂O₃, toluene). However some of “high” fullerenes such as С₇₄ and С₈₀ is not possible to isolate because of extremely high propensity of the given molecules with not coupled electrons [7] to polymerization.

In [8] carbon clusters, earlier detected only in carbon plasma, were found out in gases of a flame of incomplete combustion of benzene by means of mass spectrometric method. This method has appeared most effective for obtaining of fullerenes [9]. Presence of С₆₀ and С₇₀ is found out also in mass spectra of products of heat treatment of benzene and acetylene [10], naphthalene С₁₀Н₆ and corannulene [11-13].

Thus, from large amount clusters generated at super high temperatures of the evaporations and burning of carbon and benzene accordingly are synthesized only fullerene С₆₀ and its homologues as more stable molecules with isolated pentagons. Only С₃₆ was synthesized from smaller carbon molecules with adjacent pentagons, quasi-fullerenes [14]. (Though in [15] there is information about obtaining by an arc-discharge method hydrides (C₃₆H₄, C₃₆H) and oxy-hydrides (C₃₆H₄O, C₃₆H₆O) only, but not molecules С₃₆). Unsuccessful attempts to synthesize quasi-fullerenes are explained by their low stability because of presence of adjacent pentagons at the structure. However quasi-fullerene С₂₀, which molecule consists of only pentagons, is easily formed at an irradiation of polythene by a beam of ions Ar⁺ [16] and laser аblation of diamond [17]. On the other hand, quasifullerenes С₂₈ and С₅₀ with smaller number of adjacent pentagons are formed only as their derivative: endofullerene M@C₂₈ (М-Ti, Zr, Hf or U) [18] and decahlorofullerene C₅₀Cl₁₀ [19].

As to ions of small clusters C_n (n < 20), which always present in carbon plasma [2] and flame of a benzene/ oxygen stream [8,9], the molecules, appropriate to them, for example С₂ and С₃, С₄ and С₅ are found out only in circumstellar medium [20]. From carbon vapour the chains C₁-C₁₀ stabilize in solutions of methanol or acetonitrile due to a connection to trailer atoms of carbon H, N or CN with formation of relatively more stable polyynes [21] or cyanopolyynes [22]. The technology of matrix isolation of carbon vapour in solid argon (or neon) at 25 - 14 K [23] allows to keep carbon chains, but time of life of such frozen clusters is extremely small (~10 ms) [24].

The results of study by a mass spectroscopic method of the condensed products of a new method of pyrolysis (NMP) of benzene are presented and discussed in the report. Firstly in mass spectra of several solid products of condensation as and in carbon plasma (or in flame gases) small carbon clusters, new carbon molecules (quasi-fullerenes) as well as fullerene С₆₀ and it hydrides are detected simultaneously. The stage of high-temperature sublimation of carbon in a new method of generation of carbon molecules of the different size is completely excluded.

2. Experimental Results and Discussion

Earlier [25-28] we have been systematically studied the influence of various technological parameters on composition of obtained products and, in particular, condensed substances formed at heat treatment of hydrocarbons. On the basis of the experimental results the new method of pyrolysis (NMP) of organic vapours was developed [28-31]. This method differs from two already known processes of pyrolysis [32]. Flash-pyrolysis (FP) [32,33] is used for obtaining of highly active objects of very small size. Flowing continuous pyrolysis (FCP) [32,34] is applied to obtain carbon nanostructures and polyaromatic hydrocarbons (PAH). NMP allows obtaining simultaneously not only carbon nanostructures but also practically all carbon clusters detected in carbon plasma. Distinctive feature of NMP is an opportunity of partial division of products deposition and condensation. The time of stay of reagents in the most high temperature (~1000˚C) zone A of reaction can be changed in a wide interval that allows generating of intermediate products. A part of condensed substances and pyrolytic soot are taken out from a zone A and are located in more lowtemperature zones B and D. Vapour-like products (sometimes together with traces of soot) also is condensed in the special zone C of cooled reactionary space. Products of several (8 - 10) experiences obtained at given temperature taken from zones B, C and D were blended. Results of study of the condensed products B, C and D of the heat treatment of benzene vapours by a method of matrix-assisted laser (nitrogen, 337 nm) desorption/ionization (MALDI) (Bruker Daltonics Flex Analysis) are submitted here. The extract (ethanol, toluene or water) was located on a metal substrate and after evaporation of the solvent was exposed to a laser irradiation.

2.1. Products of Zone B

From a product, located in a zone B, condensed substances were extracted serially by toluene В₁ and then ethanol В₂. After evaporation of ethanol from a solution В₂ a deposit В₃ have obtained as conglomerates from transparent white crystals. The deposit В₃ is easily dissolved in water В₄. In mass spectrum of negative ions of a water solution В₄ (Figure 1) there are peaks which correspond to values m/z (48, 60, 72, 84, 96, 108, 120 and 132) differing precisely on 12 units (Figure 1, inset). The similar periodicity is connected to different number of atoms of carbon in detected clusters: С₄, С₅, С₆, С₇, С₈, С₉, С₁₀ and С₁₁. The spectrum of cations contains some peaks of small intensity with m/z: 429 and 219.

Mass spectra of cations and anions of a sample В₂ contain two general peaks with m/z 72 (cluster С₆) and m/z 144 (cluster С₁₂). In mass spectrum of anions (Figure 2) the periodicity already marked for В₄ is observed among the most intensive peaks: 8 clusters with consecutive (in 12 units) increasing of number of carbon atoms from С₃ up to С₁₄ (Figure 2, inset). Also peaks with m/z 219 and 429 are contained in a spectrum of cations, those were found out in a spectrum of a product В₄. Therefore it is possible to assume that in ethanol and water the substance (or substances) is dissolved which is exposed to destruction with formation of small carbon clusters from С₃ up to С₁₂ under action of a laser beam. Though, probably that separate small carbon molecules are stabilized in ethanol and in water as well. Earlier a similar range of carbon clusters was detected only in carbon plasma [2], in gases of benzene combustion [9] and at a

laser irradiation of fullerene soot [35]. Probably, in fullerene soot formed at оligomerization of carbon clusters not only soluble in toluene С₆₀ and С₇₀ can be condensed as products by their increased clusterization. Precursors of С₆₀ and С₇₀ also can be condensed in this soot with formation of fixed (or otherwise deactivated) radicals (molecules), mainly soluble in alcohol and water.

In mass spectra of anions and cations of an extract В₁ there are groups of peaks with m/z 720, 696, 672, 648 and 624 (Figure 3(a)), which, as it is accepted to consider, are characteristic for fullerene С₆₀ and clusters С₅₈,

С₅₆, С₅₄ and С₅₂. Really, from thin structure of peaks С₆₀ (Figure 3(a), inset), and С₅₈ (Figure 3(b)) follows that the isotope distribution in these peaks completely corresponds to natural distribution of isotopes of carbon in molecules С₆₀ and С₅₈. In a spectrum of anions distinctly (against to a spectrum of cations) the periodicity in a range of peaks with m/z from 48 up to 120 also is visible which was observed in mass spectra of negative ions of products soluble in water and ethanol. It is accepted to consider that clusters group closest on the size to С₆₀ and group of smallest clusters are formed at destruction С₆₀ or its derivatives (La@C₆₀, C₆₀O [36]) only at powerful laser irradiation. It is possible that substances soluble in water and ethanol are dissolved as well in toluene. Hydrogenated fullerenes С₆₀Н₆, С₆₀Н₁₆ and С₆₀Н₂₀ which peaks distinctly are visible in spectra of both anions and cations are dissolved as well in toluene. According to thin structure of peak with m/z 736 (Figure 3(c)) the isotope distribution in it completely corresponds to calculated ratio of isotopes for a molecule С₆₀Н₁₆.

2.2. Products of Zone С

Product C contains mainly transparent light particles. According to the data of the X-ray microanalysis (X-ray microanalyzer Camebax SX-50) the product consists only of carbon. The product C is practically completely dissolved in ethanol. Mass spectrum of anions (Figure 4) of an ethanol solution С₁ contains a group of the most intensive peaks with m/z 36, 48, 60, 72, 84, 96, 108, 120, 132, 144 and 156, which can correspond to anions of small carbon molecules from С₃ up to С₁₃ (Figure 4, inset). Peaks with m/z 169 and 181, it is possible, correspond to hydrogenated molecules. According to thin structure of peak with m/z 576 isotope distributions in it correspond to a molecule С₄₈.

Mass spectrum of cations (Figure 5) contains a group of peaks which the values m/z (85, 97, 109, 133 and 193) can correspond to protonated molecules C₇, C₈, C₉, С₁₁ and C₁₆ (Figure 5(b)). Three distinct peaks with m/z 72, 180 and 396 can correspond to molecules С₆, С₁₅ and С₃₃. Though, from thin structure of peak with m/z 396 (Figure 5(a), inset) follows that a part of molecules С₃₃ are partially hydrogenated. Hence, the molecules С₇, С₈ and С₉ are detected either as anions, or as protonated clusters. Only molecule С₆ is detected in both spectra.

2.3. Products of Zone D

Soluble in toluene substances from a product D were extracted and deposited by ethanol. Deposited red-brown powder D₁ was dissolved in acetone. Mass spectra of anions (Figure 6) and cations (Figure 7) of an acetone extract D₁ essentially differ. First of all, in a spectrum of anions a peak with m/z 576 is distinctly seen which, as is marked earlier, presents in mass spectrum of a product С₁ dissolved in ethanol. In a spectrum the peak with m/z 576 is distinctly seen as well, the isotope distribution in which ((Figure 6(a), inset) differs from natural isotope distribution of carbon in a molecule С₄₈. It is probably, molecule С₄₈ is partially hydrogenated (up to С₄₈Н₂). Hence, quasi-fullerene С₄₈ is located in different zones of reactionary space and is easily dissolved both in ethanol and in acetone.

In a spectrum of anions (Figure 6(b)) there is a large group of very intensive peaks with relatively small values m/z: 36, 48, 60, 72, 84, 96, 108, 120, 132, 144, 156, 168 and 180. Periodicity of occurrence of these peaks is 12 units that can demonstrate the belonging of these peaks to clusters from С₃ up to С₁₅. The similar structure of mass spectrum of anions was found out and for a product С₁ (Figure 4, inset), dissolved in ethanol. The very intensive peak with m/z 255 can correspond to hydrogenated molecule С₂₁Н₃ (or С₂₀Н₁₅). Just the peak with m/z 255

allows to see some difference in mass spectra of аnions of products С₁ (transparent light particles) and D₁ (red - brown powder). However, mass spectra of cations testify to essential distinctions of these products (Figure 5, Figure 7). In mass spectrum D₁ most intensive peak with m/z 133 as well as rather intensive peaks with m/z 85 and 219 is contained. It is possible to believe, that peaks with m/z 85 and 133 correspond to minimally hydrogenated molecules. The peak with m/z 219 can correspond also to protonated molecule С₁₈Н₃ (Figure 7, inset). It is possible, that the product D₁ contains stabilized by other products of benzene pyrolysis molecules С₇ and С₁₁, which are decomposed on clusters mainly of smaller size С₃-С₅ under laser beam.

Thus, clusters С₃-С₁₅ as well as С₆₀ or С₄₈ are detected as anions in all three products of benzene pyrolysis. In a spectrum of cations the molecules С₆-С₉ and С₁₁ as well as С₁₅ and С₃₃ are detected only.

2.4. Products 1B of Zone B

It is necessary to note that the composition of products of benzene pyrolysis, in particular, located in a zone B essentially depends on a regime of synthesis. From a product 1В obtained at lower temperature of pyrolysis were extracted by toluene the condensed substances and some of them deposited at addition of ethanol. The obtained deposit 2В as red (wax-similar) film was again dissolved in toluene and its mass spectra are submitted in Figure 8. Mass spectrum of anions (Figure 8(a)) contains the most intensive peak with m/z 168 as well as less intensive peaks with m/z 96, 132 and 216. From thin structure of peaks with m/z 96 and 168 follows, that the distribution of isotopes of carbon in them corresponds to molecules С₈ and С₁₄. At the same time, from the extended spectrum it is possible to see that some of small carbon molecules, for example С₁₃, are hydrogenated essentially. The spectrum of cations (Figure 8(b)) testifies a high degree of hydrogenation of a product 2В. The most intensive peak with m/z 139 according to its thin structure corresponds to a hydrogenated molecule С₁₁Н₇. The peaks with the large values m/z also correspond to hydrogenated molecules of carbon or thermostable polyaromatic hydrocarbons (PАHs) which can be intermediate at the formation С₆₀ and its hydrides.

According to the chemical analysis a red product 2В consists of carbon, hydrogen (up to 4.2% mass.) and oxygen (up to 3.1% mass.). The composition of volatile products of thermal decomposition 2В 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) [37]. A sample 2В at the bottom of molibdeniumquarts 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 0.15 K∙s⁻¹ [37]. The volatile thermolysis products passed through a high-vacuum valve (5.4 mm 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 - 210 amu. The hydrogen as can see from a curve of thermodesorption (Figure 9(a)) begins allocation from a sample 2В already at room temperature (in vacuum) and in enough large amount. It is improbable that PAHs in a similar way can be decomposed. It should be noted that the intensive peaks of hydrogen is also observed in the MALDI mass spectra of water, ethanol and toluene solutions of the product B₄ (Figure 1, inset), B₂ (Figure 2, inset) and B₁ (Figure 3(a), inset) respectively. Mass spectrum EI at 200˚C on Figure 9(b) 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 carboneous (С₈ and С₁₄) and hydrogenated molecule (С₁₁Н₇) are thermo unstable already at low temperatures.

Thus, firstly clusters, distinctive for carbon plasma, are generated at destruction of substances obtained at hydrocarbon pyrolysis. Pyrolysis is carried out at temperatures excluding evaporation of carbon therefore small carbon clusters (С₃-С₅) can be formed only due to dehydrogenation and destruction of benzene molecules. The destruction of a molecule С₆Н₆ can be proceeded its complete dehydrogenation with formation of a linear or ring molecules С₆ of polyynic or cumulenic structure. Clusters С₇-С₁₁ and С₁₅ can be products of clusterization С₃- С₆. The formation of molecules С₆₀, С₄₈ and С₃₃ can be realized owing to polycondensation of molecules С₆Н₆, while hydrides (С₆₀Н₆, С₆₀Н₁₆, С₆₀Н₂₀) are formed because of reactions of polymerization of molecules С₆Н₆ or hydrogenation of the formed molecules С₆₀. It is possible, that the radicals, for example С₆Н₅, are formed at partial dehydrogenation of molecules С₆Н₆ which further, as well as С₆, can be precursors of carbon molecules and clusters. However absence of biphenyl (С₆Н₅-С₆Н₅) in products of benzene pyrolysis as product of the first stage of reaction of polycondensation С₆Н₆, but presence of clusters С₁₂ (m/z 144) and С₁₈ (m/z 216) as dimer and trimer С₆ accordingly, can testify to preferable formation С₆₀ and С₄₈ from carbon clusters, instead of from hydrocarbons radicals.  

Very important question connected to the detailed mechanism of the formation С₆₀ and С₄₈, remains, open: whether clusters С₆ of an initial molecule С₆Н₆ accept participation in formation of molecules С₆₀ and С₄₈? Or these large carbon molecules are formed only due to increase of clusterization of fragments (С₃-С₅) of disintergration С₆?

Pyridine (C₅NH₅) is heterocyclic analogue of benzene (С₆Н₆) therefore from precursors C₅N or С₃-С₅ should be formed accordingly heteroatomic or monoatomic fullerenes and quasi-fullerenes. Our preliminary researches [29,30] have shown that at pyridine pyrolysis large nitrogen-carbon containing molecules are formed which further study represents not only scientific but also practical interest.

3. Conclusion

New method of organics pyrolysis for generation of carbon clusters as an alternative powerful laser (or arc-discharge) evaporation of graphite is developed. Condensation products obtained at new method of pyrolysis of benzene vapours by mass spectrometric method are studied. In products of all kinds of carbon molecules and some hydrides are detected. Obtained experimental results firstly demonstrate that the small carbon molecules can be generated in reactionary conditions excluding evaporation of carbon. The first stage of the transformation of benzene molecules to carbon molecules is their dehydrogenation and destruction. Firstly fullerene С₆₀ and quasifullerenes С₄₈ and С₃₃ as well as small carbon molecules and some hydrides molecules in different substances are detected simultaneously.

REFERENCES

NOTES

^*Corresponding author.