Hydrogen and Hydrocarbon Gases, Polycyclic Aromatic Hydrocarbons

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Hydrogen and Hydrocarbon Gases, Polycyclic Aromatic Hydrocarbons, and Amorphous Carbon Produced by Multiple Shock Compression of Liquid Benzene up to 27.4 GPa Koichi Mimura, and Tamihito Nishida J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b06627 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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The Journal of Physical Chemistry

Hydrogen and Hydrocarbon Gases, Polycyclic Aromatic Hydrocarbons, and Amorphous Carbon Produced by Multiple Shock Compression of Liquid Benzene up to 27.4 GPa

Koichi Mimura*a and Tamihito Nishidaa

a

Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan

*

To whom correspondence should be addressed. E-mail: [email protected] (K. Mimura), TEL: 81-52-789-3030

ACS Paragon Plus Environment

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ABSTRACT Phase diagrams of benzene have been reported on the basis of data mainly obtained from static compression at various pressure-temperature, P-T, conditions. However, there are few data in the high pressure and high temperature region of the phase diagram. To understand the physical and chemical behavior of benzene in that region, multiple shock compression of benzene was evaluated by a recovery experimental system that directly analyzed the shocked samples. The shocked samples were composed of the remaining benzene, gases (H2, CH4, C2H4, C2H6, C3H6, and C3H8), polycyclic aromatic hydrocarbons with molecular weights from 128 (naphthalene) to 300 (coronene), and amorphous carbon. The abundances of these chemical species varied according to the P-T conditions induced by shock compression. Samples in the lower pressure and lower temperature region of the a-C:H phase in the phase diagram contained a significant amount of benzene as well as amorphous carbon. In the higher pressure and higher temperature region of the a-C:H phase, benzene was mostly converted into amorphous carbon (H/C = 0.2), H2, and CH4. Therefore, the amorphous carbon in the present study was produced by a different pathway than in previous studies that have detected hydrogenated amorphous carbon (H/C = 1). For earth sciences, the present study can provide basic information on the delivery to the early earth of extraterrestrial organic materials related to the origin of life.


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1. INTRODUCTION Benzene is a fundamental aromatic hydrocarbon with a π-bonded, 6-membered planar ring structure. The delocalized π-orbitals in the planar ring cause the stability of benzene and polycyclic aromatic hydrocarbons (PAHs). Due to this stability, benzene and PAHs are acknowledged as the most abundant organic materials in nature.1,2 Benzene and PAHs are present in the deep interior of the earth as well as on the surface.3,4 Benzene has also been detected throughout the universe, in meteorites,5 the atmospheres of Jupiter,6 Saturn,7 and Titan,8 and the circumstellar medium.9 Benzene reacts to form other materials by pressure-induced overlap of its molecules because of its π-bonded planar ring structure. The behavior of benzene at different pressure-temperature, PT, conditions was studied by static compression experiments10-13 and molecular-dynamics simulations.14-16 Using these results, the phase diagram of benzene was compiled.17,18 At 883K and 4 GPa, benzene shows an irreversible chemical transformation and changes into amorphous carbon.10 Shinozaki et al.19 reported that the static compression of benzene up to 16 GPa at room temperature produced naphthalene, biphenyl, phenylhexadiene, and a white insoluble material similar to the materials reported in previous studies.13,20,21 Fitzgibbons et al.22 synthesized sp3bonded carbon nanothreads from benzene by static high pressure at room temperature. However, there are few data in high-pressure and high-temperature region of the phase diagram. Shock compression of benzene was performed to examine the behavior of benzene under high pressure and high temperature conditions.15,23-30 Shock compression is grouped into two types: single shock compression and multiple shock compression. Single shock compression elevates the pressure of the material by one propagation of a shock wave. In contrast, multiple shock compression increases the pressure by the reverberation of a shock wave in the material. The


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multiple shock compression can achieve lower temperature conditions than the single shock compression does for the compressed materials to reach the same shock pressure condition. In single shock compression experiments, the slope of the benzene Hugoniot curve changes at 13.3 GPa, suggesting that a chemical transformation of benzene occurred at this pressure condition.15,25,28 This was consistent with the results of molecular dynamics simulations.15,16,31,32 In contrast, multiple shock compression29,30,33,34 was carried out to determine the behavior of benzene under high pressure and lower temperature conditions compared with single shock compression. Shock recovery experiments of benzene showed that the shocked benzene contained various PAHs.33,34 However, these studies did not clarify the behavior of benzene in the phase diagram. Root and Gupta29 showed that liquid benzene was not transformed into solid up to 13 GPa, at which benzene should be solid in the phase diagram. Root and Gupta30 reported an irreversible chemical change of benzene at 24.5 GPa and 1065 K corresponding to the P-T condition of the amorphous carbon phase in the phase diagram. These data are fundamentally important to make the phase diagram more reliable. Multiple shock compression is not a special phenomenon in nature. The impact of celestial bodies is a ubiquitous natural event, e.g., the impact of meteorites and comets on the earth. When meteorites and comets impact on the earth, the generated shock wave propagates the inside of the bodies. Some meteorites and comets contain not only minerals but also organic materials.35-37 Because the organic materials are present between the minerals which have higher shock impedance than the organic materials, the organic materials are multiply compressed by shock wave reverberation. Organic materials in meteorites and comets experience shock compression and chemical change according to the P-T condition. To examine the behavior of organic materials in these bodies, shock recovery experiments of organic materials were carried out and


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showed that the starting materials changed into complicated organic materials useful for the emergence of life.38-42 Regarding the origin of life, many researchers43-45 proposed that meteorites and comets delivered organic materials as raw materials for the first life on the earth. The study of shock compression of organic materials can provide important knowledge regarding chemical evolution. In this study, we performed multiple shock compression of benzene under a wide range of P-T conditions and directly analyzed the recovered samples. In this paper, we describe the shock experimental conditions (shock pressure, shock temperature, and post-shock temperature) and the detailed chemical composition of the shock products. We also discuss the physical and chemical behavior of benzene in the phase diagram, especially in the high-pressure and hightemperature region. 2. METHODS 2.1. Shock Experiment. Liquid benzene was used as the starting material (99.5% purity, Kishida Chemical Co., Ltd) and was purified by distillation before use. The distilled benzene was packed in a reactor, which consisted of a capsule (SUS 304), a plug (SUS 304), a gasket (Pb), and a screw (SUS 304) (Figure 1). In the plug, a pit (2 mm in diameter and 0.3 mm deep) was dug. After the pit was filled with the benzene, the plug was pushed into the capsule by the screw with 40 Nm of torque. Then, the gasket was deformed and sealed the benzene. To be certain of the seal, the screw and the capsule were welded. In the welding procedure, the reactor was tightly held into a copper block cooled with circulating water. By cooling, the temperature of benzene was controlled below 310 K. After setting the reactor into a capsule holder (HPM1: a pre-hardened steel to 39 HRC; Hitachi Metals, Ltd.), a momentum trap (SUS 304), which was


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similar to the reactor in weight, was attached on the lower side of the reactor (Figure 1). Before use, the reactors were washed with distilled acetone and distilled dichloromethane.

Figure 1. A schematic view of the experimental configuration just prior to impact. The projectile for the experiment consisted of a flyer plate (SUS 304) and a sabot (polycarbonate) (Figure 1). The flyer plate was 15 mm in diameter and 2 mm thick. The projectile was accelerated by a vertical powder gun and intercepted two laser beams irradiating photo diodes just before impacting the reactor (Figure S1 in the Supporting Information). The projectile velocity was calculated using the interval of the laser beams (100 mm) and the time difference of interception. The projectile impacted the upper side of the reactor, and the generated shock wave compressed the benzene. 2.2. Recovery and Analyses of Shocked Samples. 2.2.1. Recovery and Chemical Analyses of Gases. The shocked reactor with the capsule holder was flattened by a lathe and set on a recovery container (Figure S2 in the Supporting Information). After adding a certain volume of internal standard gas (n-butane) to the container with a syringe though an injection port, the wall of reactor was drilled. Then, the produced gases were released in the container and mixed with the standard gas.


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Hydrogen and methane in the gas mixture were analyzed by gas chromatography with a thermal conductivity detector (GCTCD) (GC-2014; Shimadzu Co.). The GC column (3 m x 3 mm I.D.) was filled with molecular sieve 5A (GL Sciences Inc.). The temperatures of the injection port, column, and detector were held at 50 °C. The carrier gas was He, and the flow rate was 15 mL/min. Organic species in the gas mixture were analyzed by GC with a flame ionization detector (GCFID) (GC-2014; Shimadzu Co.). The GC column was 25 m x 0.32 mm I.D., with a 5 µm layer of CP-PoraBOND Q (Agilent Technology Co.). The column conditions were as follows: maintained at 40 °C for 10 min, then increased to 140 °C at 6 °C/min and up to 215 °C at 15 °C/min, and finally held at 215 °C for 4 min. The temperatures of the injection port and detector were held at 220 °C. The carrier gas was He, and the flow rate was 1.5 mL/min. The abundances of gas products were determined by calibration curves of each gas, which were plotted using purchased gases. The recovery containers were washed with distilled acetone and distilled dichloromethane before use. 2.2.2. Recovery and Chemical Analyses of Soluble and Insoluble Materials. After the shocked reactor with the capsule holder was put on the recovery container, a certain volume of internal standard solution (a dichloromethane solution containing C13H26O2: methyl laurate, C19H38O2: methyl stearate, and C31H62O2: methyl triacontanoate) was injected into the recovery container. Then, the shocked sample was drilled out and was diffused in the solution of the container. The soluble and insoluble materials were collected with a pipette and were filtered with a glass filter. The carbon and hydrogen contents of the insoluble materials were determined with an elemental analyzer (vario EL cube; Elementar Analysensysteme GmbH). The filtrate was concentrated and analyzed by GC with mass spectrometry (JMS-K9; JEOL Co.). The column was 30 m x 0.25 mm I.D., with a 0.25 µm layer of HP-5 (Agilent Technology Co.). The


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temperature program was as follows: maintained at 120 °C for 10 min, then increased up to 210 °C at 3.5 °C/min and up to 320 °C at 15 °C/min, and finally held at 320 °C for 10 min. The temperatures of the injection port and interface were set to 325 °C and 290 °C, respectively. The carrier gas was He, and the flow rate was 1.5 mL/min. The ionization voltage of MS was set to 70 eV. The scan range of m/z was from 50 to 500. We made a calibration curve for each soluble product to determine the product abundance. Because reagents corresponding to P1, P2, and P3 were not available, the calibration curves of 1methylnaphthalene, 1-phenylnaphthalene, and benzo(a)anthracene were substituted for the curves of P1, P2, and P3, respectively. Glass filters and glass apparatus were precombusted at 450 °C for 4 hours. 2.2.3. Raman Analyses of Insoluble Materials. The insoluble materials were rinsed with distilled dichloromethane and distilled acetone three times. The materials were put on a glass plate and were analyzed with a Raman reflex microscope (inVia Reflex; Renishaw plc.) after laser breaching for 60 s. The laser light was supplied by a laser system (JUNO 532 nm J150GS; Showa Optronics) and was focused with a 100x objective lens. The wavelength and the laser power were 532 nm and 1.5 mW, respectively. The obtained spectra were processed with specialized software (WiRE 3.5; Renishaw plc). 2.3. Calculation of Shock Pressure and Shock Temperature. Multiple shock compression causes the increase of pressure and temperature in a different way from single shock compression. In the present experiments, the shock wave generated by a projectile impact propagates through the upper wall of the reactor and sequentially enters the benzene sample. Because benzene has a lower shock impedance than SUS 304, the shock wave in the benzene reverberates and compresses the benzene multiple times. In this process, pressures were


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estimated by cross points of SUS 304 Hugoniot and benzene Hugoniot (Figure S3 in the Supporting Information). The first passage of the shock wave compressed the benzene sample from point A to point B in Figure 2 and Figure S3 of the Supporting Information. After the shock wave reached the lower boundary of the sample and the plug, the wave reflected and compressed the sample from point B to point C in Figure 2 and Figure S3 of the Supporting Information. The step by step increase (C to D, D to…) in pressure continued until a rarefaction wave reached the sample. When the rarefaction wave propagated into the sample, the pressure of the sample immediately decreased to the ambient pressure. The pressure achieved by the shock wave reverberation is called the shock pressure. In contrast, the pressure achieved in the projectile and the reactor wall is the peak shock pressure (see point P of Figure S3 in the Supporting Information). The duration of the shock pulse is the period during which the shock wave reverberated through the sample.41,46 In the calculation of shock pressure, we used onedimensional impedance matching method. The real pressure might be made lower than the calculated pressure due to the rarefaction wave from the flyer plate/sabot interface of the projectile and due to the reflection of shock wave from the side wall of the pit.


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Figure 2. Profiles of shock pressure and shock temperature. Solid lines show calculated pressure history (a) and temperature history (b) at a point in the center of a sample (B13C). The points A, B, C, and D correspond to the points of A, B, C, and D in Figure S3 in the Supporting Information. Dashed lines show the pressure and temperature histories, which were estimated without the calculation. The temperature at the shock pressure was obtained by calculating the temperature at each step of pressure. The temperature at the shock pressure is called the shock temperature. Because previous studies29,30 indicated that benzene remained a liquid under multiple shock compression up to 25 GPa, we did not consider latent heat for the calculation of the shock temperature. We also calculated the post shock temperature,47 which the shocked sample experienced after the pressure returned zero. The post shock temperature was caused by the increase of entropy during


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shock compression. The calculation based on thermal diffusion shows that it takes less than a second for the post shock temperature to become the ambient temperature. Table 1. Summary of Experimental Details Sample number B01C B02C B03C B04C B05C B06C B07C B08C B09C B10C B11C B12C B13C B01G B02G B03G B04G B05G B06G B07G B08G B01R B02R B03R

Projectile velocity (m/s) 0 312 485 595 709 729 830 856 1000 1020 1020 1130 1240 0 561 759 793 1000 1080 1190 1260 1000 1120 1240

Peak shock pressure*1 (GPa) 5.9 9.5 11.8 14.3 14.7 17.0 17.6 21.0 21.5 21.5 24.2 26.9 11.1 15.4 16.2 21.0 23.0 25.7 27.5 21.0 23.9 26.9

Duration of shock pulse*2 (ns) 729 724 711 700 690 644 680 625 618 618 623 591 732 676 660 625 639 605 586 625 626 591

Shock pressure*3 (GPa) 4.4 8.6 11.2 13.9 14.4 16.7 17.4 20.8 21.3 21.3 24.1 26.8 10.5 15.1 15.8 20.8 22.8 25.6 27.4 20.8 23.8 26.8

Shock temperature*4 (K) 590 737 812 883 896 951 969 1050 1061 1061 1125 1191 797 913 931 1050 1097 1161 1203 1050 1120 1191

Post shock temperature*5 (K) 384 449 483 516 522 548 557 596 601 601 633 665 477 530 538 598 619 650 671 598 630 665

*1 : Pressure of reactor achieved by impact of the projectile. *2 : Duration of shock wave reverberation in sample. *3 : Pressure of sample achieved by the shock wave reverberation in the sample. *4 : Temperature of sample achieved by the shock wave reverberation in the sample. *5 : Temperature of sample, just after the shock pressure was released. The experimental run conditions are shown in Table 1. Figure 3 shows the data for shock pressure and shock temperature in the phase diagram of benzene summarized by previous studies.17,18 In the present study, we used the Hugoniot data of benzene (C0 = 1.50 km/s, S = 1.67, ρ0 = 877 kg/m3, γ0 = 1.2)48 and SUS 304 (C0 = 4.58 km/s, S = 1.49, ρ0 = 7896 kg/m3, γ0 =


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2.17).49 To simplify the calculation, we used the data for benzene Hugoniot below 13.3 GPa, although the Hugoniot data changed according to the shock pressure range.25 In the calculation of temperature, we used the equation (4) in Lacina and Gupta50 for the specific heat at a constant volume (Cv) and used the approximation of γ0 / V0 = γ / V = constant.

Figure 3. Shock pressures and shock temperatures of samples (open circle: B02C-B13C, open square: B01R-B03R, and closed triangle: B02G-B08G) plotted in a phase diagram of benzene. The diagram was redrawn based on Ciabini et al. (2007). 3. RESULTS AND DISCUSSION 3.1. Chemical Species in Shocked Benzene. In the shocked samples, benzene, gases, PAHs, and insoluble materials were detected (Figure 4a-b). The benzene in the shocked samples should be a part of the starting material that apparently survived the chemical reaction. The gases consisted of mainly H2 and CH4 and contained C2H2, C2H4, C2H6, C3H6, and C3H8. The PAHs contained many species with molecular weights ranging from 128 (naphthalene) to 300 (coronene). The insoluble materials were black and soot-like, showing clusters with indeterminate forms. Abundances of benzene, gases, PAHs, and insoluble materials in the shocked samples were estimated lower than they should be because there were many unidentified PAHs in the shocked samples, and there is a possibility that the shocked samples were recovered incompletely.


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Figure 4. (a) The remaining ratio of benzene (open circle) and the yields of PAHs (closed triangle) and insoluble materials (open diamond) in shocked samples. Benzene data in B03C (8.6 GPa) was not available because of inadequate adjustments of the GCMS. (b) The yield of total gas (closed diamond) in shocked samples. 3.2. Remaining Benzene. The abundance of benzene gradually decreased to 20 GPa and rapidly fell to below 1 wt% at 26.8 GPa (Figure 4a). The shocked samples below 20 GPa depleted a part of the starting benzene, e.g., approximately 15 wt% of the starting benzene was lost at 13.9 GPa (B05C). The lost benzene reacted to produce gases, PAHs, and insoluble materials. Our results show that the shock-induced reaction of benzene occurred at 14 GPa and was completed at 26.8 GPa. Root and Gupta30 performed multiple shock compression of benzene, and monitored the molecular and chemical changes in situ by Raman spectroscopy.


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They observed the characteristic Raman feature for benzene up to 20 GPa, but the feature was not observed at 24.5 GPa. They concluded that the chemical transformation did not occur at 20 GPa but did at 24.5 GPa. Their observation seems to be inconsistent with our results up to 20 GPa. The inconsistency may be attributed to the detection limit of benzene by Raman spectroscopy. Whether the chemical transformation occurs or not, the analysis system would show the typical Raman feature of benzene if the shocked samples contained a significant amount of benzene above the detection limit of the analysis system. In the present study, shocked samples below 20 GPa contained 60 wt% of the starting benzene, which was probably sufficient to be detected by Raman spectroscopy. Otherwise, the decomposition of benzene may occur after the monitoring time of the Raman analysis. Single shock compression of benzene showed that the chemical transformation of benzene begins at 13.3 GPa and is completed at 19.4 GPa.25,48 Single shock compression causes higher shock temperature than multiple shock compression at comparable shock pressure. The shock temperatures of benzene by single shock compression were calculated as 1280 K at 13.3 GPa and 1530 K at 19.4 GPa.15 We estimated the shock temperatures by multiple shock compression as 870 K and 1020 K, respectively. At 20 GPa in our experiments, benzene remained at over 60 wt%, indicating that the shock temperature is an important factor for the shock reaction. 3.3. Gas Species. Hydrogen and hydrocarbon gases were detected in the shocked benzene samples. The major gas products were H2 and CH4. The carbon number of hydrocarbon gases ranged from 1 to 3, including saturated and unsaturated hydrocarbon gases (Table 2). The yields of gases elevated with increasing shock pressure. The yields of hydrocarbon gases decreased with increasing carbon number. The unshocked benzene sample (B01G) as a control contained a


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small amount of H2 and no carbon-bearing gases (Table 2), suggesting that metal shavings during the sample recovery process reacted with water vapor in the air and produced hydrogen. Table 2. Yields (mmol / mol) of Gas Products Sample number Pressure (GPa) H2 CH4 C2H2 C2H4 C2H6 C3H6 C3H8

B01G 0 0.9 -

B02G 10.5 24.3 4.8 0.1 0.3 0.7 0.1 0.2

B03G 15.1 66.5 101.2 0.8 6.1 11.6 2.8 2.7

B04G 15.8 78.8 106.6 0.9 6.6 11.4 2.6 2.7

B05G 20.8 360.5 584.0 0.3 28.7 37.9 10.8 3.7

B06G 22.8 414.1 618.1 0.7 30.1 31.6 9.5 2.7

B07G 25.6 517.5 665.0 0.4 21.3 24.8 5.2 1.8

B08G 27.4 309.3 679.6 21.4 31.6 8.2 2.8

- : under detection limit (< 0.1 mmol / mol) H2 and CH4 were major gas products in our shock experiments, and C2H2 was a minor product. Many researchers reported that H2 and C2H2 were major products of pyrolyses of benzene, e.g., pyrolysis of benzene using a shock tube51-53 and pyrolysis of benzene using a flow system.54,55 Benzene pyrolysis under high temperature conditions is mainly controlled by reactions (1) – (3).51,52 C6H6 → C6H5˙ + H˙

(1)

C6H6 + H˙ → C6H5˙ + H2 C6H5˙ → C4H3˙ + C2H2

(2) (3)

However, our results of shock compression are not consistent with the previous reports of benzene pyrolyses. The main factor of inconsistency would be the phase of benzene when the reaction occurs. Initially, generated C2H2 molecules would react with other molecules close to the C2H2 molecules in the condensed phase and would subsequently produce PAHs and soot-like materials. A previous study34 of the shock recovery of benzene reported that shocked benzene contained gas species consisting of H2, light alkanes from C1 to C3, light alkenes from C2 to C3, and C2


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alkyne C2H2. This chemical composition of gas products is similar to the one obtained by the present study. However, the two studies differ widely in the gas yield. The total gas yield of the present study was much higher (~104 times) than the previous study. The difference of yield is mainly attributed to the shape of reactor used in the studies. In the previous study, the inner space (40 mm in diameter) of the reactor for benzene was wider than the projectile (15 mm in diameter). After the projectile impacted the reactor, the generated shock wave propagated into the benzene only under the impact point of the projectile. The previous shock experiments compressed a part of the benzene sample in the reactor. In the present study, the inside space (2 mm in diameter) of the reactor was narrow compared with the projectile (15 mm in diameter). The generated shock wave by projectile impact compressed the entire benzene sample. Therefore, the present study can provide the appropriate information about the chemical behavior of benzene at the various P-T conditions realized by shock compression. 3.4. Polycyclic Aromatic Hydrocarbons (PAHs). Shocked samples contained PAHs with molecular weights ranging from 128 (naphthalene) to 300 (coronen) (Table S1 in the Supporting Information). Major PAHs products were naphthalene, biphenyl, and their methyl derivatives. Numbered peaks in the chromatogram of Figure 5 were identified and quantified using internal standards and purchased chemical reagents. Although unnumbered peaks in the chromatogram were not identified due to the lack of reagents, the mass fragmental patterns of the peaks showed the typical feature of PAHs.


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Figure 5. The gas chromatogram of PAHs from a shocked sample (B09C). The peak numbers correspond to the peak numbers in Table S1 in the Supporting Information. I.S.(1), I.S.(2), and I.S.(3) are internal standards of methyl laurate (C13H26O2), methyl stearate (C19H38O2), and methyl triacontanoate (C31H62O2), respectively. The produced PAHs contain fused PAHs, in which rings share one side or more, e.g., naphthalene, phenanthrene, and chrysene, and bound PAHs, in which rings are connected with C-C bonds, e.g., biphenyl and terphenyl. The ratio of fused PAHs against total PAHs elevates with increasing pressure, but the ratio of bound PAHs decreases (Figure 6). The amount of bound PAHs became lower than that of the fused PAHs above 20 GPa. This result suggests that bound PAHs are easily produced and stable under low pressure and low temperature conditions. Under high pressure and high temperature conditions, bound PAHs probably convert to fused PAHs by phenyl addition and cyclization or by hydrogen abstraction and C2H2 addition. This reaction mechanism is the same as the mechanism of hydrogen abstraction and acetylene addition (HACA) reported by previous studies56-58 on the pyrolysis of benzene.


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Figure 6. Molar ratios of the bound PAHs (closed circle) and the fused PAHs (open square) to the total PAHs. Some shocked samples contain PAHs that were not reported as products in the thermal reaction of benzene. The PAHs correspond to compounds of P1, P2, and P3 in Figure 5. Figure 7a-c show mass fragmental patterns and possible structures for these PAHs, although we did not determine these PAHs using purchased chemical reagents. Compounds corresponding to P119, P2 and P3 (a private communication of Shinozaki) were detected in benzene samples which were statically compressed at room temperature. A similar compound to P2 and P3, which shows a very low intensity molecular ion peak in mass fragmental pattern, was also reported in naphthalene samples statically compressed at room temperature (Figure 4 in Shinozaki et al.59). The previous results suggested that the formation of these PAHs is dominantly induced by pressure. The yields of these PAHs drastically decreased from 20.8 GPa to 21.3 GPa, while the yields of other PAHs increased. Because shock compression raises not only pressure but also temperature, the reactions during the shock compression should contain both products of pressure-induced and thermal effects. The production of PAHs above 21 GPa would be controlled not by the pressure-induced reaction, but by the thermal reaction. Then, the


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compounds P1, P2, and P3 should be converted into other heat-resistant PAHs by the thermal reaction.

Figure 7. Representative mass patterns and possible structures of P1, P2, and P3. 3.5. Insoluble Materials. Insoluble materials were detected in the samples that were compressed above 13.7 GPa (Figure 4a). The weights of insoluble materials were estimated using the contents of hydrogen and carbon obtained by the elemental analyzer. Regarding the samples below 21.3 GPa, the hydrogen contents of insoluble materials were too low to be distinguished from the analytical blank. In these samples, the contents of insoluble materials were estimated using only the carbon contents. We determined hydrogen of the insoluble materials in only two samples: B12C (24.2 GPa) and B13C (26.9 GPa). The insoluble materials of B12C and B13C showed H/C atomic ratios of 0.2. These materials showed lower H/C values than the starting materials, indicating that the shock compression caused dehydrogenation. Pyrolysis of benzene under high temperature and atmospheric or lower pressures also caused


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dehydrogenation and formed soot-like materials.51,54,55,60 The high temperature generated by shock compression can contribute to the formation of insoluble materials.

Figure 8. Representative first-order spectra (a) and second-order spectra (B) of shocked samples.


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Figure 8 shows representative Raman spectra of insoluble materials in B01R (20.8 GPa), B02R (23.8 GPa), and B03R (26.8 GPa). The insoluble materials have two broad peaks for the firstorder Raman spectra (Figure 8a) and not well-resolved peaks for the second-order Raman spectra (Figure 8b). The first-order spectra were fit with a band combination which includes four Lorentzian bands (G, D1, D2, and D4) and a Gaussian band (D3).61 The positions of the G, D1, D2, D3, and D4 bands for the three shocked samples were at approximately 1590, 1335, 1605, 1505, and 1190 cm-1, respectively (Table S2 in the Supporting Information). Values of full width at half maximum (FWHM) for the G and D1 bands ranged from 58 cm-1 to 61 cm-1 and 154 cm-1 to 177 cm-1, respectively. Peak area ratios of D1/G were approximately 3. The first-order Raman spectra of the insoluble materials can be separated into five wide bands, showing that the insoluble materials can be recognized as amorphous carbon. The Raman features resemble those of diesel soot,61 industrial carbon black,61 and thermally decomposed benzene,60 which consist of amorphous carbon comprising polycyclic aromatic compounds with over 80 wt% carbon content. The spectra of B01R (20.8 GPa) showed rises in the baseline with increasing Raman shift. The rises of the baseline became gentle with increasing pressure and were not observed in B03R (26.8 GPa). The rises of the baseline suggest that the insoluble materials contained fluorescent substances, e.g., PAHs with high molecular weights. The fluorescent substances should change into other materials as the shock pressure and the shock temperature become higher. This is supported by the results showing that many kinds of PAHs were produced at approximately 20 GPa (B10C), but the production of PAHs became very low at 27 GPa (B13C). The shocked sample of 24.1 GPa in the present study contained 37 wt% amorphous carbon, which is shown in the Raman spectra in Figure 8. However, the previous study30 did not detect


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the spectra of amorphous carbon in the 24.5 GPa sample by Raman spectroscopy. The analysis system for amorphous carbon may not be sensitive enough that 37 wt% of amorphous carbon could be detected. Otherwise, the formation of amorphous carbon may occur after the monitoring time of the analysis system. Previous studies13,20,62 on the static compression of benzene reported that the formation of products occurred during the depression process. Carbon-bearing insoluble materials were also reported in the static compressed benzene at room temperature and were called hydrogenated amorphous carbon (a-C:H).13,19,21,63,64 The formation mechanism of these materials was explained by the pressure-induced transformation of benzene, which involved opening benzene rings leading to a highly cross-linked polymer.13 Fitzgibbons et al.22 observed white insoluble materials in compressed benzene up to 20 GPa at room temperature and identified the white insoluble product as carbon nanothreads with 1:1 C/H stoichiometry dominated by sp3 bonding. They claimed that the nanothreads should have higher strength and stiffness than the conventional high-strength polymers. On the other hand, the amorphous carbon in the shocked benzene samples was black, not white, and showed a deficiency of hydrogen molecules caused by dehydrogenation. Therefore, the amorphous carbon products are dissimilar to the insoluble materials produced by the static compression of benzene. The difference was ascribed to a slow decompression of the sample but could also be due to the distinct temperature conditions of the experiments. The reaction by shock compression is accompanied by high temperature, whereas the reaction of static compression occurred at room temperature. 3.6. Shock Compression Data in Phase Diagram of Benzene. Shocked samples inevitably experience the post shock temperature, which is maintained after the passage of shock wave and falls to ambient temperature during a sub-second period. The shock recovery experiments cannot


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isolate the reaction during the passage of the shock wave from the reaction occurring under the post shock temperature conditions. For that reason, the shock recovery experiments may be acknowledged to be unsuitable for revealing the reaction during shock compression. In our experiments, the post shock temperature was calculated to be from 384 K to 671 K, which is very low compared with the shock temperature from 590 K to 1203 K (Table 1). These post shock temperatures do not alter the results caused by the reaction of benzene during the passage of the shock wave. Figure 3 is the phase diagram of benzene in which shock pressures and shock temperatures were plotted. Shocked samples should experience the liquid phase, the solid phase, and the aC:H phase. Compressed samples below 11.2 GPa are placed in the P21/c phase. B03C (8.6 GPa) and B04C (11.2 GPa) contained a small amount of PAHs, 90 wt% or more of the starting benzene, and no amorphous carbon. The chemical transformation of benzene hardly occurred in this P-T condition. B05C (13.9 GPa) - B11C (21.3 GPa) are placed in the a-C:H phase. They contained amorphous carbon (3.8 wt% to 20 wt%) and preserved 28 wt% or more of the starting benzene. In the pressure range from 13.9 GPa to 21.3 GPa, benzene does not change into the chemical form expected from the phase diagram, suggesting that the duration of the shock reaction was too short to finish the chemical change of benzene. B12C (24.1 GPa) and B13C (26.8 GPa) contained amorphous carbon (37 wt% to 77 wt%) and lost 93 wt% or more of the starting benzene. The features of these two samples closely match the expected chemical form from the phase diagram of benzene. In this P-T condition, benzene mainly should change into H2, CH4, and amorphous carbon (H/C = 0.2). The amorphous carbon in the present study is distinguished from the hydrogenated amorphous carbon detected in static compressed benzene at high pressure and room temperature.


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Figure 9. Relative abundances of chemical species (B: benzene; G: hydrogen and hydrocarbon gases; P: PAHs; A: amorphous carbon) in compressed benzene. Benzene, Poly, a-C:H show names of phase reported by previous studies.17,18 To understand the behavior of benzene at various pressures, we calculated the relative abundances of chemical species (benzene, hydrogen and hydrocarbon gases, PAHs, and amorphous carbon) in compressed benzene up to 27 GPa (Figure 9). Using the fitting curve of each species in Figure 4a-b (Figure S4 in the Supporting Information), the relative abundances were obtained. In Figure 9, previously reported boundaries were also drawn at 10 GPa (P21/c polymers boundary) and 13 GPa (polymers - a-C:H boundary). Figure 9 shows that benzene partly reacts to produce amorphous carbon and gases at 10 GPa and completely changes at 27 GPa. In the pressure range between the two chemical boundaries, there is a phase (which we call a transition phase) in which shocked benzene samples contained benzene, hydrogen and hydrocarbon gases, PAHs, and amorphous carbon. Benzene chemically changed along the P-T path of the present experiments and experienced, consecutively, the benzene phase, transition phase, and amorphous carbon, H2, and CH4 phase. In the previous studies on static compression, the polymers phase was reported, but the transition phase was not reported. Reaction rates for the formation of polymers from benzene would be too low to complete reaction during shock compression (sub-microsecond) at the P-T condition of the transition phase. If the duration of


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shock compression was longer, the transition phase may become the polymers phase and the amorphous carbon, H2, and CH4 phase. 4. CONCLUSIONS We performed multiple shock compression of benzene and directly analyzed the recovered samples. Shock pressures and temperatures ranged from 4.4 GPa to 27.4 GPa and from 590 K to 1203 K, respectively. Shocked samples contained the remaining benzene, gases (H2, CH4, C2H2, C2H4, C2H6, C3H6, and C3H8), PAHs with molecular weights ranging from 128 (naphthalene) to 300 (coronene), and amorphous carbon. Abundances of these compounds varied according to the P-T conditions. Benzene chemically changed along the P-T path and experienced, consecutively, the benzene phase, the transition phase, and the amorphous carbon, H2, and CH4 phase. Samples compressed into the benzene phase (the P-T conditions of the P21/c phase) were mainly composed of benzene. Samples in the transition phase (the low-pressure region of the a-C:H phase) consisted of the remaining benzene, gases, PAHs, and amorphous carbon. Samples in the amorphous carbon, H2, and CH4 phase (the high-pressure region of the a-C:H phase) consisted of amorphous carbon (H/C = 0.2), H2, and CH4. The amorphous carbon in the shocked samples was produced by dehydrogenation of benzene and was different from the hydrogenated amorphous carbon (H/C = 1) produced by static compression at room temperature. In the a-C:H phase, there would be several types of amorphous carbon, e.g., the high pressure and high temperature type and the high pressure and ambient temperature type. For earth sciences, the production of various PAHs by shock compression, which can change to amino acids by aqueous alteration,65 suggests that meteorite and comet impacts are important events for the emergence of life on the earth. ASSOCIATED CONTENT Supporting Information


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Additional figures about the shock gun, recovery container, shock impedance matching, and Tables of PAH yields and parameters for Raman spectra are given in PDF format. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (K. M.) ORCID Koichi Mimura: 0000-0001-9195-1861 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors thank H. Hidaka and K. Sakuma for support with the Raman microscopy analysis. We also thank A. Kokubu, Y. Mimura, and A. Nakamura for supporting the calculations of shock temperature and the development of the shock experimental system. This study was supported by a JSPS KAKENHI Grant No. 25287174. REFERENCES (1) Ehrenfreund, P.; Charnley, S. B. Organic Molecules in the Interstellar Medium, Comets, and Meteorites: A Voyage from Dark Clouds to the Early Earth. Annu. Rev. Astron. Astrophys. 2000, 38, 427-483.


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Hydrogen and Hydrocarbon Gases, Polycyclic Aromatic Hydrocarbons

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