Polycyclic aromatic hydrocarbons at high pressures: oligomerization, carbonization, compressibility and thermal expansion.
High-pressure study of polycyclic aromatic hydrocarbons (PAHs) is extremely important for modern science: Earth and planetary dynamics, meteoritics, organic and fundamental basic chemistry. PAHs are believed to be the most abundant free organic molecules in the Universe. Electron delocalization over their carbon skeleton makes them remarkably stable. PAHs play a central role in the gas phase chemistry. PAHs are supposed to be an important component of C-O-H fluid in the deep Earth. PAHs have been found in natural samples such as mantle-derived garnet, olivine, and diamond from kimberlite pipes. Naphthalene, phenanthrene, pyrene, benzopyrene, and benzoperylene were identified as the primary constituents in these inclusions. Theoretical calculations of the equations of state for heavy hydrocarbons indicate their enhanced stability in the deep Earth’s mantle. In addition, PAHs were also identified in carbonaceous chondrites such as Murchison meteorite, and Antarctic Martian meteorites. PAHs-bearing carbonaceous chondrites could play a significant role to the delivery of extraterrestrial organic prebiotic materials to the early Earth.
Our high-pressure studies have been focused on the most abundant PAHs: naphthalene (C10H8), anthracene (C14H10), pyrene (C16H10), and coronene (C24H12). High-pressure multianvil experiments of PAHs at room and low temperatures up to 773 K revealed significant oligomerization by matrix-assisted laser desorption/ionization (MALDI) measurements of quenched products. We detected intensive signals from synthesized dimers, trimers, and tetramers of selected PAHs at 7 GPa and 773 K. Higher oligomers (up to eicosamers) were detected by MALDI, but the signal intensities of these compounds did not exceed 1% of the maximum signal. Moreover the MALDI analyses of the experimental products at higher pressure (16 GPa) and room temperature revealed the minor oligomerization of selected PAHs. The number of oligomers increased with increasing initial PAH size. Oligomer formation might occur via PAH dehydrogenation and successive fusion of the initial hydrocarbon molecules through C–C bond formation. PAH oligomerization at high pressure and temperature is extremely important for PAH chemistry in space. A range of PAHs found in meteorites, cometary comae, interstellar clouds, and planetary nebulas could be explained by high pressure (shock) oligomerization.
PAH carbonization at 1.3–4 GPa was defined at 773–973 K, at 7-9 GPa - 873–1073 K, at 15.5 GPa - 873–973 K in high-pressure multianvil experiments using in situ X-ray synchrotron radiation. Melting was identified for naphthalene at 727 – 730 K and 1.5 GPa. Quenched products were analyzed by Raman spectroscopy. The PAH decomposition products consist of nanocrystalline graphite and graphite at 1.3 – 4 GPa and 773–973 K, amorphous hydrogenated carbon at 7–9 GPa and 873–973 K and diamond and trans-polyacetylene lying in grain boundaries at 15.5 GPa and 973 K. Determined decomposition temperatures for PAHs at 6-7 GPa (873–1073 K) are much lower than known Earth's geotherms and subduction slab P–T profiles. Thus, PAHs found in fluid inclusions in mantle-derived garnets, olivines, and diamonds could not be formed at mantle conditions. The possible origin of these PAHs can be explained as a result of polycondensation of reduced C–O–H fluid under the influence of natural catalysts through Fischer-Tropsch type reactions upon its cooling after or at the final stages of kimberlite eruption at moderate residual pressures. The possible candidates involved in catalytic reactions could be transition metals, carbon, H2O, and CO2. Due to the limited temperature stability of PAHs at high pressure, proposed polycondensation of reduced C–O–H fluid could be occurred only in the pressure range of 0 – 1.5 GPa and temperature lower than 900 K.
Defined PAH decomposition parameters are extremely important for understanding the planet accretion by carbonaceous chondrites such as the Murchison meteorite. Recent shock-wave experiments revealed PAH dehydrogenation at 17.9 – 36.5 GPa and 940 – 1660 K. Therefore, assuming that carbonaceous PAH-bearing chondrites accreted on and formed the Earth, they should have dehydrogenated at a point where the growing Earth reached a radius of 1525 - 3060 km and most of the hydrogen delivered to the Earth had been discharged into the atmosphere by the end of the accretion. In our static experiments we defined that carbonaceous chondrites such as the Murchison meteorite accreted on the early Earth could not be dehydrogenated until at a point where the growing Earth reached a radius at least of 1375 - 1575 km. Further growth and the heating of the Earth led to the decomposition of accumulated PAHs to carbon and hydrogen in the interiors. However, smaller celestial bodies, also formed by the collision of meteorites, should preserved in their depths complex polycyclic hydrocarbons. Indeed, PAHs could not survive via possible heating episodes and formation of global magma oceans in the early Earth.
Recent high-pressure diamond anvil cell experiments defined compressibility curves of naphthalene, anthracene and coronene at room temperature up to 13 – 14 GPa. Our determined thermal expansion data for PAHs are consistent with the appropriate compressibility curves. We found that PAHs possess very low temperature effect on increasing the unit cell volumes at pressures 1.3 – 7.9 GPa for the temperature range of 298–873 K. Such a diminishing of thermal effects with the pressure increase is apparently a specific feature of the high-pressure behavior of molecular crystals like benzene and PAHs. This may be related to low energy of intermolecular interactions and corresponding lattice phonons, which can be easily suppressed by the applied pressure.