(521bx) Plasma-Assisted Approaches for the Direct Conversion of Natural Gas to Liquid Products

In the pharmaceutical, agrochemical and fine chemical industries, nitrogen functional groups (i.e. nitriles, amines, N-containing rings) are prevalent and serve important functions.¹ However, the synthesis of these compounds is challenging; they require high temperatures and pressures, are multistep, and often require ammonia as the source of nitrogen_, which is a hazardous chemical that is produced through an energy intensive process.^1,2 Nonthermal plasmas (NTPs) have emerged as an alternative reactive environment comprised of high energy electrons, radicals, ions, and excited species that can perform reactions at lower temperatures and pressures compared to thermal reactions.³ In this work, we demonstrate a process that couples N₂ directly with natural gas (i.e., methane, ethane, and propane) to produce valuable nitrogen-containing liquid products using NTP excitation.

Specifically, we have studied reactions between nitrogen and light hydrocarbons under NTP stimulation to form NH_3, C_xH_y, and C_xH_yN_z­­ species. In order to simulate a natural gas mixture in the laboratory, a representative mixed feed (CH₄/C₂H₆/C₃H₈/N₂) was fed to the reactor. The observed products varied as a function of gas composition, plasma power, and bulk gas temperature. With nitrogen-rich feeds, the formation of liquid and solid products was observed, and the liquid products were characterized by a variety of analytical techniques to determine how N was incorporated in the diverse product slate. Chain growth (-CH₂- and -HCN-) pathways and in situ H₂ saturation pathways were postulated based on the products identified from electrospray ionization mass spectrometry (ESI-MS). Further, N/C ratios were determined for each product, showing a range of nitrogen incorporation is possible through control of the reaction conditions. Characterization of the bulk liquid products by ATR-FTIR, XPS, and various NMR techniques show that the nitrogen is being incorporated as nitrile and amine functionalities. Overall, we showcase the ability of NTP stimulation to activate hydrocarbon and nitrogen mixtures for the production of valuable compounds. Ongoing efforts target identification of intermediate species for liquid formation, which can be introduced in the feed in order to improve liquid production rates.

References:

(1) Ruiz-Castillo, P.; Buchwald, S. L. Rev. 2016, 116(19), 12564−12649.

(2) Torborg, C.; Beller, M. Synth. Catal. 2009, 351(18), 3027-3043.

(3) Mehta, P.; Barboun, P.; Go, D. B.; Hicks, J. C.; Schneider, W. F. ACS Energy Lett. 2019, 4(5), 1115–1133.