Catalytic conversion of acetylene to polycyclic aromatic hydrocarbons over particles of pyroxene and alumina

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are known to be present in many astrophysical objects and environments, but our understanding of their formation mechanism(s) is far from satisfactory. In this paper, we describe an investigation of the catalytic conversion reaction of acetylene gas to PAHs over pyroxene and alumina. Crystalline silicates such as pyroxenes (with general formula [Mg, Fe]SiO 3 ) and alumina (Al 2 O 3 ) are observed astrophysically through their infrared spectra and are likely to promote grain surface chemical reactions. In the experiments reported here, gas-phase PAHs were produced by the catalytic reaction of acetylene over crystalline silicates and alumina using a pulsed jet expansion technique and the gaseous products detected using time-of-flight mass spectrometry. In a separate experiment, the catalytic formation of PAHs from acetylene was further confirmed with acetylene gas at atmospheric pressure flowing continuously through a fixed-bed reactor. The gas effluent and carbonaceous compounds deposited on the catalysts were dissolved separately in dichloromethane and analysed using gas chromatography–mass spectrometry. Among the samples studied, alumina showed higher activity than the pyroxene-type grains for the acetylene reaction. It is proposed that formation of the PAHs relies on the Mg 2+ ions in the pyroxenes and Al 3+ ions in alumina, where these ions act as Lewis acid sites. X-ray diffraction, Fourier transform infrared and high-resolution transmission electron microscopy techniques were used to characterize the structure and physical properties of the pyroxene and alumina samples.

Catalytic conversion of methanol to larger organic molecules over crystalline forsterite: Laboratory study and astrophysical implications

“…4b and 4c), similar to the observation of acetylene conversion to PAHs over SiC grains (Zhao et al 2016). As reported in our earlier publication (Tian et al 2013) Figure 4d shows the thermal gravimetric (TG) profiles of the carbon deposit. An analysis of the TG weight loss of carbon deposition indicates (i) trace amounts of physically adsorbed water (200 0 C), (ii) hydrogenated carbon of the form CxHy including large PAHs (0.5%, 345 0 C), (iii) graphene or graphitic structures (0.5%, 455 0 C) and (iv) carbon nanotubes (CNTs) (1%, above 600 0 C) (Guo et al 2014); the aforementioned carbon species on the forsterite surface were oxidized into CO2 and removed from the surface at a different temperatures.…”

“…The time-of-flight mass-spectrometer (TOF-MS) used in this work has been discussed in detail in earlier publications (Tian et al 2012(Tian et al , 2013Zhao et al 2016) so only a brief description of the equipment and experimental conditions is given here. This home-built instrument consists of a source chamber with a quartz tube fixed-bed reactor, an ionization chamber, a flight tube and a detection chamber.…”

“…Kress & Tielens (2001), using kinetic model calculations, confirmed that Fischer-Tropsch catalysis plays an important role in solar nebula chemistry by converting CO to less volatile hydrocarbons. Our own earlier laboratory studies (Tian et al 2012(Tian et al , 2013Zhao et al 2016) have shown that olivine-type silicates, pyroxene, alumina and SiC particles can promote the conversion of acetylene gas to polycyclic aromatic hydrocarbons (PAHs).…”

Prevalence of non-aromatic carbonaceous molecules in the inner regions of circumstellar envelopes

“…There are several models to account for the presence of aromatic species in the ejecta of evolved stars; one involves the polymerization of acetylene to benzene 5 and further growth by the hydrogen abstraction acetylene addition (HACA) mechanism 22,46 . Other scenarios involve the processing of SiC dust grains 47 or the catalytic activity of C 2 H 2 on silicate surfaces 48 . Although the HACA mechanism has been invoked to unravel the synthesis of PAHs in the outflows of carbon-rich AGB stars 22,46 , our results suggest an alternative scenario.…”

The Moon as a Recorder of Organic Evolution in the Early Solar System: A Lunar Regolith Analog Study

“…The JSC-1 lunar analogue consists of a mixture of different mineral types, the main constituents being feldspar, pyroxene, and olivine. The catalytic properties of these minerals have been investigated in a number of industrial and natural settings (e.g., Rapagnà et al, 2000;Demiral and S xensöz, 2008;Tian et al, 2013). However, for this study we simply focused on the overall effects of the natural JSC-1 mineral assemblage as a representative of reactions that may occur in the lunar regolith.…”