Raman Spectroscopy Study on Chemical Transformations of Propane at High Temperatures and High Pressures

This study is devoted to the detailed in situ Raman spectroscopy investigation of propane C3H8 in laser-heated diamond anvil cells in the range of pressures from 3 to 22 GPa and temperatures from 900 to 3000 K. We show that propane, while being exposed to particular thermobaric conditions, could react, leading to the formation of hydrocarbons, both saturated and unsaturated as well as soot. Our results suggest that propane could be a precursor of heavy hydrocarbons and will produce more than just sooty material when subjected to extreme conditions. These results could clarify the issue of the presence of heavy hydrocarbons in the Earth’s upper mantle.


Methods
Propane (Linde Gas Polska), with a purity of 99.99%, was used in the experimental procedure without any additional purification. In our experiment, propane was subjected to cooling by liquid nitrogen and subsequent cryogenic loading in symmetric BX-90-type diamond anvil cells (DACs) equipped with synthetic, CVD-type IIa diamonds with a culet size of 250 μm. The rhenium gaskets were indented to a thickness of 25 μm. Pressure chambers with a step (Fig. S1 in supplementary) were prepared in the gaskets by combination of laser ablation and drilling. Thin (~1-2 μm) gold foil act as heat absorber in experiments with laser heating.
The Raman spectra were excited using a He-Ne laser (632.8 nm excitation). Then the the acquisition of the spectra was made via the use of a LabRam spectrometer with a 2 cm −1 spectral resolution. If possible, the pressure was determined by a calibration of propane high-pressure behavior 16 , or else the pressure was determined by the first-order peak of the diamond. The uncertainties in the Raman peak positions were ±1 cm −1 . Raman spectra were collected at several points of the heating areas to ensure that the transformations being investigated, actually occurred. The Raman spectra of propane and the products of the reaction were measured before and after heating under the required thermobaric conditions.
In some cases, the green Ar + -laser (514.5 nm) the LabRam spectrometer (2 cm −1 spectral resolution) was equipped with was also employed for the in situ analysis.
The laser heating of the samples was performed using a home-laboratory laser heating setup at the Bayreuth Geoinstitut 26 . This system could be described as transferable double-sided laser heating setup for diamond-anvil cells with the possibility of in situ temperature determination and precise heating of the samples inside a cell. Using high magnification and low working distance infinity corrected laser focusing objectives provided the opportunity of the laser beam size decrease less than 5 μm as well as achievement of the 320 times optical magnification.
Heating of the sample is carried out by two YAG lasers (1064 nm central wavelength). For temperature measurements the thermal emission spectra of the heated area is guided into an IsoPlane SCT 320 spectrometer with a 1024 × 256 PI-MAX 4 camera. The temperature was determined by fitting the black body radiation spectra of the heated area in a given wavelength range (570-830 nm) to the Plank radiation function. Liquid and solid propane are optically transparent and do not absorb well at the central wavelength of the YAG laser. This means that it is important to find a way to heat the sample and eliminate the catalytic influence of the absorber that could appear because of the usage of noble metals such as Ir. For these reasons, gold foil was employed as the absorber of the laser radiation to dissipate heat to the sample. The Raman spectra were measured at the hot points, near the hot points (marked as "near" on the several spectra), as well in the cold sample areas to facilitate a deeper understanding of propane's behavior.

Results and Discussion
3GPa. The main chemical transformation of propane at 3 GPa ( Fig. 1) observed at the temperatures displayed by Fig. 1 is a reaction with the prevalent formation of a sooty material. This material has very similar Raman spectra to the typical black solid compound obtained during the thermal and catalytic petrochemical processes or as a by-product of combustion according to the reaction: www.nature.com/scientificreports www.nature.com/scientificreports/ = + C H 3C 4H 3 8 2 During such processes, there is also the possibility of obtaining a graphite, which could be either disordered or highly ordered. However, the character of the presented spectra of the propane reaction products indicates the presence of disordered phases of graphite or soot 27 . The highly ordered Raman spectra of graphite exhibits only one band (first-order G-bands) at 1580 cm −1 at ambient temperature. On the contrary, the disordered structure of graphite has the presence of additional first-order bands (D-bands) at 1360 and 1620 cm −1 depending on the ambient conditions 28 . The bands in the region of 2800-3500 cm −1 could also correspond to the combinational models of D and G-bands 26,27 . The signals of the graphite and soot are hard to distinguish, however, there is evidence that the soot itself has broader peaks 27,28 . The spectra of propane under pressure of 3 GPa and at an ambient temperature of 900 K (±100 K) show no major changes in the display of any of the bands that are typical for propane 16 . The stability of propane at a temperature of ~900 K is in good correspondence with the earlier experiments of Kolesnikov et al. 12 on methane and ethane, showing the same behavior of propane. The absence of the hydrogen peaks in the region of 500-800 cm −1 (the region was not shown on the graph) can be explained as being due to the high rate of hydrogen diffusion through the rhenium gasket or through the reaction products. The appearance of the intense peak at ~3000 cm −1 at temperatures of 1420 and 1500 K (±100 K) could be attributed to C-H vibrations of various aliphatic hydrocarbons due to the well-known radical reaction mechanism resulting in the formation of methyl and ethyl radicals. These radicals could subsequently react via various pathways leading to the formation of hydrocarbon compounds 29 : CH  CH  CH  CH  CH  CH  CH  CH   3  3  3  3  2  ethyl radical   3  methyl radical   3  2  3  4  methane   3  2  2  propyl radical   3  2  2  3  3  2  2  3  n butane   3  2  3  2  3  3  3  ethane   3  2  2   3  2  3  3  2  3  2  2 Above mentioned reaction pathway may also explain why we do not observe pure hydrogen in the system. Hydrogen could be consumed in the reactions with other hydrocarbons: 6GPa. Heating at 940 K does not affect Raman spectra of propane (Fig. 2). www.nature.com/scientificreports www.nature.com/scientificreports/ The spectra collected after heating at higher temperatures characterized by presence of bands at ~3100-3200 cm −1 (may be attributed to formation of unsaturated compounds [2][3][4]12 ), and peak at ~3000 cm −1 (probably due to saturated hydrocarbon(s)). It is impossible to attribute these peaks to an individual compound or group of compounds due to high fluorescence in C-C stretching region or due to formation of ultradisperse diamonds 30 . We hypothesize that due to complicated thermal mechanisms of propane transformations the products of polymerization or aromatization were obtained, for instance, via allyl-radical reaction: It is important to notice, that in the works of Kolesnikov et al. 31,32 the formation of unsaturated hydrocarbons wasn't reported. Propane is heavier than methane by mass, and thus propane can easier decompose and produce larger number of hydrocarbon compounds. www.nature.com/scientificreports www.nature.com/scientificreports/ 8GPa. With the increase of the pressure to 8 GPa only methane could be identified among all of the hydrocarbons along with the formation of complex compounds with hydrogen at 1230 K (Fig. 3). These are formed due to the escape of hydrogen from the reacting system and the consequent formation of Van Der Waals bonds 33 .
Unfortunately, the region of the hydrocarbon footprint lacks the characteristic peaks overlapped by the fluorescence. However, the C-H of aliphatic and unsaturated fragments in the region of valence vibrations suggests the presence of various hydrocarbon compounds. With the increase of the temperature up to 2000 K and higher, the formation of graphite-like systems could be seen with the total disappearance of C-H and C-C vibrations at 2350 K.   [2][3][4]12 , for unsaturated compounds the reference peaks were obtained from 31,32 , and for graphite (soot) modes, the reference peaks were obtained from 27,28 . The reference peaks for C-C stretching and C-C bending of hydrocarbons were obtained for ethane from 12 , the peaks for propane were obtained from 12,37 , the peaks for n-butane from 12,37 , the peaks for n-pentane from 39 , and the peaks for n-hexane were obtained from 38 . The propane remained stable at 930 K. The spectra of untouched propane are in good correspondence with the previous experiments we carried out 37  www.nature.com/scientificreports www.nature.com/scientificreports/ Another possible mechanism of hydrocarbon generation as well as hydrogen could be the interaction of various forms of carbon and hydrocarbons. There is evidence that C-H fluids could be in contact with carbon 34 in the Earth's mantle, which could lead to certain chemical reactions of hydrogenation. For example, the hydrogen generation in our case could be explained not only by thermal decomposition of hydrocarbons, but by the catalytic effect of the carbon in the form of graphite or soot 35,36 . 11 and 14 GPa. The reference peaks for C-H valence of saturated hydrocarbon compounds were obtained from 2-4,12 , for unsaturated hydrocarbons from 31,32 , for graphite (soot) modes, they were obtained from 27,28 . The reference peaks for C-C stretching and C-C bending of hydrocarbons were obtained for ethane from 12 , for propane from 12,37 , for n-butane from 12,37 , for n-pentane from 38 , and for n-hexane from 38 . The propane remained stable at temperatures of 840 K and lower. The spectra of untouched propane are in good correspondence with the previous experiments we carried out 37 .
The formation of C 1 -C 6 hydrocarbons at 11 and 14 GPa (Figs. 4 and 5 respectively) starting from 900 K corresponds with the previous results for methane of Kolesnikov 12 and is in good agreement with the results from simulation experiments 2 . The spectra of 11 and 14 GPa have a main, obvious difference-the presence of hydrogen. The absence of hydrogen at 11 GPa is because of hydrogen diffusion or secondary reactions of hydrocarbons or graphite. The C-C vibrations of n-butane 37 , n-hexane 39 , and n-pentane 38 were detected in the spectra. In the case of the n-pentane and n-hexane, they were never detected in such types of experiments. This result proposes the complicated condensation mechanism that has a radical character, as in the case of industrial processes of pyrolysis. These series of reactions could be described with a classic radical-polymerization mechanism, because   www.nature.com/scientificreports www.nature.com/scientificreports/ of a certain regularity in the decreasing intensity of the hydrocarbon peaks with the consequent increase in the molecular mass. By modifying the reaction of 12 , it is reasonable to assume that we obtain the following result: The most important fact that was observed during the experiments at 14 GPa is the contemporaneous presence of hydrogen, graphite, and other hydrocarbons during the laser heating, which could be interpreted as the equilibrium state. However, with the pressure increase it is hard to distinguish the particular hydrocarbon signal. After 1500 K only graphite and C-H valence vibrations could be seen.

Summary of the Results
The Table 1 and Fig. 8 summarize observations described above. Our experiments demonstrate that at pressure and temperature conditions relevant for wide range of depth within the Earth pure propane (without any catalyst) can transform in to different hydrocarbons, both saturated and unsaturated. Under these conditions propane is reacting via two simultaneous and competing pathways -(1) the growing of the hydrocarbon chain via condensation or polymerization mechanisms with the formation of higher hydrocarbons and (2) destruction via the cleavage of C-C and C-H bonds with the formation of lighter hydrocarbons and also graphite (or sooty material). Our observations suggest that propane, if subducted in to the mantle, undergoes complex transformations and may be source of more complex organic compounds.The issue of presence of heavy hydrocarbon compounds in the Earth's mantle was thoroughly described and examined in these works 40,41 . conclusion The observations of propane chemical transformations under a wide range of high pressures and temperatures that are also present in the Earth's mantle and in subduction environments, provides an insight into the fate of the carbon-bearing fluids fate deep in the Earth's interior. The thermodynamic stability of propane under the pressures of 3-14 GPa and temperatures less than 900 K have been shown. At temperatures greater than 900 K, over a full range of pressures, propane transformation led to the formation of complex hydrocarbon systems and soot. At pressures of 11 and 14 GPa it was possible to identify the product mixture which includes light hydrocarbons, methane, and ethane and heavy hydrocarbons such as n-butane, n-pentane, and n-hexane.
We also have shown that the formation of heavier alkanes from propane at temperatures in the range of ~1000-2000 K and under pressures from 6 to 22 GPa is possible without any catalysts, and corresponds to the reactions leading to the formations of similar compounds occurring at depths of more than 130 km beneath the Earth's surface.