Phase Relation and Reactivity of 1-Hexene under High-Pressure and High-Temperature Conditions

The phase changes and chemical reactions of 1-hexene under high-pressure and high-temperature (HPHT) conditions were investigated by using diamond anvil cells and Raman spectroscopy. The slow crystallization of 1-hexene was observed after compression to approximately 3 GPa at room temperature. Solid 1-hexene was dominant up to 20.7 GPa without any phase changes. Under HPHT conditions, the intensities of the Raman peaks related to sp2 carbon were selectively reduced, and a new C═C vibration mode appeared, indicating an irreversible reaction. The reaction threshold temperature decreased with an increase in pressure to approximately 100 °C at 3 GPa, where the reaction progressed from liquid 1-hexene. In contrast, the reaction threshold temperature increased with increasing pressure above 3 GPa, at which point the reaction progressed from a solid 1-hexene. Oligomers of 1-hexene up to at least 8mers were detected from gas chromatography–mass spectrometry analysis of the reaction products recovered from 120 and 240 °C at 1.5 GPa by further experiments using a piston–cylinder-type high-pressure and high-temperature apparatus. The major dimers in the reaction products were linear and branched alkenes bearing C12H24.