Studies of circumstellar envelopes (CSEs) and nebulae associated with evolved stars indicate that gaseous materials, high-temperature conditions and the relatively high molecular density environment support rich and complex chemistry. Over sixty different organic chemical compounds have been identified in CSEs and many of these are exotic species with more than eight atoms. There is evidence indicating that in oxygen-rich envelopes, olivine-type silicates (OTS) are present as dust grains. The inner part of CSEs is warm (the temperature can be up to 1,500 K) and the number density could be as high as 1010 molecule/cm-3). Given the presence of gaseous carbon-containing molecules, particularly methanol, warm solid grains as catalysts, and favorable physical conditions in CSEs and some mixed-chemistry objects such as the Red Rectangle, it is expected that catalytic reactions can be of significance in such astrophysical environments. We have performed a series of catalytic reactions with olivine-type silicates as catalysts for the conversion of methanol to small organic molecules, and also polycyclic aromatic hydrocarbons (PAHs). The time-of-flight mass spectrometry (TOF-MS) technique has been used to detect in situ the small-to-medium-sized gas-phase compounds such as hydrocarbons, monoxides of carbon, and PAHs. Solid deposition on the surface of the catalyst was examined using high-resolution transmission electron microscopy (HRTEM) and gas-chromatography mass spectrometry (GC-MS) techniques. Our TOF-MS results indicate that C2H4, C3H3+, CH3O, CO2, naphthalene, anthracene, pyrene, etc. were produced in the catalytic conversion of methanol. The formation of these compounds was confirmed by using deuterated methanol. The solid deposition on the catalytic surface was found to be graphitic and amorphous carbon. It is likely that the laboratory catalytic reactions mimic similar chemical processes in suitable astrophysical environments.

Laboratory astrochemistry: Catalytic conversions of methanol to organic molecules over olivine-type silicates

254th National Meeting (2017)

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