Raman Spectroscopy Study on Chemical Transformations of Propane at High Temperatures and High Pressures
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-01-30 |
| Journal | Scientific Reports |
| Authors | D. A. Kudryavtsev, Timofey Fedotenko, Egor Koemets, Saiana Khandarkhaeva, Vladimir Kutcherov |
| Institutions | University of Bayreuth, Gubkin Russian State University of Oil and Gas |
| Citations | 11 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study details the high-pressure, high-temperature (HPHT) chemical transformations of propane (C3H8) using in situ Raman spectroscopy in laser-heated diamond anvil cells (LH-DACs).
- Extreme Conditions Tested: Propane stability and reactivity were mapped across a wide thermobaric range: 3 to 22 GPa and 900 to 3000 K.
- Transformation Observed: Above 900 K, propane undergoes complex reactions, forming a mixture of saturated hydrocarbons (C1-C6 alkanes), unsaturated hydrocarbons, and solid carbon (soot/graphite).
- Heavy Hydrocarbon Precursor: The results confirm that propane can act as a precursor for heavier hydrocarbons (up to n-hexane, C6H14) under mantle-relevant conditions (11 and 14 GPa).
- Reaction Pathways: Two simultaneous, competing pathways were identified: (1) condensation/polymerization via radical mechanisms leading to heavier alkanes (e.g., n-pentane, n-hexane), and (2) destruction/cleavage leading to lighter alkanes (methane, ethane) and solid carbon.
- Geological Significance: The findings provide crucial insight into the fate of carbon-bearing fluids deep within the Earthâs interior, supporting the hypothesis that complex organic compounds can exist in the upper mantle.
- Stability Limit: Propane was found to remain chemically stable only at temperatures less than 900 K across the entire pressure range investigated.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Pressure Range (P) | 3 to 22 | GPa | Range of HPHT investigation. |
| Temperature Range (T) | 900 to 3000 | K | Range where chemical transformation occurs. |
| Propane Purity | 99.99 | % | Purity of starting material (Linde Gas Polska). |
| Diamond Anvil Cell Type | Symmetric BX-90 | - | High-pressure apparatus used. |
| Diamond Type | Synthetic, CVD-type IIa | - | Used for the DACs. |
| Diamond Culet Size | 250 | ”m | Size of the diamond tip. |
| Gasket Material | Rhenium (Re) | - | Used for pressure containment. |
| Gasket Thickness (Indented) | 25 | ”m | Thickness of the Re gasket. |
| Heat Absorber Material | Gold Foil | ~1-2 | ”m |
| Heating Laser Wavelength | 1064 | nm | Central wavelength of two YAG lasers. |
| Raman Excitation (Primary) | 632.8 | nm | He-Ne laser excitation source. |
| Raman Spectral Resolution | 2 | cm-1 | Resolution of the LabRam spectrometer. |
| Temperature Measurement Range | 570-830 | nm | Wavelength range used for black body fitting. |
| Minimum Stable T (Propane) | less than 900 | K | Temperature threshold for reaction onset. |
| Identified Products (Max C-chain) | C6H14 | - | n-Hexane (detected at 11 GPa). |
Key Methodologies
Section titled âKey MethodologiesâThe study utilized advanced in situ Raman spectroscopy coupled with laser-heated diamond anvil cells (LH-DACs) to monitor chemical changes in real-time under extreme conditions.
- Cryogenic Sample Loading: Propane (C3H8) was loaded into the DACs under cryogenic conditions (liquid nitrogen cooling) to ensure high density and purity within the pressure chamber.
- DAC Preparation: Symmetric BX-90 DACs equipped with 250 ”m culet CVD-type IIa diamonds were used. Rhenium gaskets were pre-indented to 25 ”m, and pressure chambers were created via laser ablation/drilling.
- Laser Heating System: A home-laboratory double-sided laser heating setup was employed, utilizing two YAG lasers (1064 nm). A thin gold foil (1-2 ”m) served as the heat absorber to minimize catalytic effects from noble metals.
- Temperature Measurement: Sample temperature was determined in situ by fitting the thermal emission spectra (570-830 nm) of the heated area to the Planck radiation function.
- Pressure Measurement: Pressure was determined either by calibrating the high-pressure behavior of propane or by measuring the shift of the diamond first-order peak.
- In Situ Raman Analysis: Raman spectra were collected using a LabRam spectrometer (2 cm-1 resolution), primarily excited by a He-Ne laser (632.8 nm). Spectra were measured at hot points, near hot points, and cold areas to confirm transformation occurrence.
- Product Identification: Reaction products were identified by analyzing characteristic C-H valence vibrations (~2800-3200 cm-1) and C-C stretching/bending regions, comparing them against known reference peaks for C1-C6 saturated and unsaturated hydrocarbons, and solid carbon (D and G bands for graphite/soot).
Commercial Applications
Section titled âCommercial ApplicationsâThe fundamental understanding of hydrocarbon chemistry under extreme thermobaric conditions has relevance across several engineering and industrial sectors:
- Petrochemical Engineering:
- Optimization of industrial thermal cracking and pyrolysis processes, particularly those involving light alkanes (C3H8) at high temperatures (500-900 °C), by understanding radical-polymerization and condensation pathways.
- Design of high-pressure chemical reactors where complex hydrocarbon mixtures are synthesized or processed.
- Deep Earth Energy and Resources:
- Informing models related to abiogenic hydrocarbon formation and the stability of carbon-bearing fluids in the Earthâs mantle (depths > 130 km, relevant to subduction zones).
- Advanced Carbon Materials Synthesis:
- Developing novel methods for synthesizing specific carbon allotropes or precursors (soot, disordered graphite) by controlling the decomposition of light hydrocarbons under extreme pressure and temperature.
- High-Pressure Instrumentation:
- Refinement and calibration of in situ spectroscopic techniques (Raman, IR) used in LH-DACs for analyzing chemical reactions in extreme environments, crucial for materials science research.
- CVD Diamond Technology (Indirect):
- The study contributes to the fundamental knowledge base regarding the behavior of hydrocarbon precursors (like C3H8) at high temperatures, which is relevant to the chemistry governing Chemical Vapor Deposition (CVD) diamond growth processes.