Catalysis Science & Technology COMMUNICATION Cite this: Catal. Sci. Technol., 2018, 8, 6302 Received 30th June 2018, Accepted 5th November 2018 DOI: 10.1039/c8cy01355a rsc.li/catalysis Microwave catalytic synthesis of ammonia from methane and nitrogen† Xinwei Bai, a Sarojini Tiwari, a Brandon Robinson, a Casey Killmer, a Lili Li * b and Jianli Hu * a This study presents our recent findings that under microwave irradiation and/or microwave plasma conditions, nitrogen can react with methane to form ammonia and other value-added by- products, hydrogen and carbon nanotubes, at atmospheric pres- sure. Microwave catalysis alters current industrial processes to en- able most difficult reactions to take place under mild conditions. Since the microwave effect was discovered in the 1980s, the effect of microwave irradiation (MW) on synergizing chemical reactions, including oxidative/non-oxidative methane coupling, aromatization, etc., has been reported. 1–9 The Haber–Bosch ammonia synthesis, developed by Fritz Haber and Carl Bosch, has been commercialized for more than 100 years and this process has achieved a single-pass-conversion of 15%. 10 The Haber–Bosch process requires high tempera- tures (400–570 °C) and high pressures (100–300 atm) in the reactor, which dramatically increase the capital and operating costs. Therefore, novel ammonia synthesis processes have been studied worldwide to devise a sustainable energy effi- cient methodology. To date, most of the research is focused on ammonia synthesis using nitrogen and hydrogen. In al- most all of these previous studies, the activation of nitrogen (N 2 ) is a huge barrier due to its N–N triple bond. Several acti- vation schemes have helped to overcome this hurdle such as the use of nitrogen activating catalysts and non-thermal plasma. 11–15 In industry, natural gas reforming (or coal gasifi- cation) is the main source of hydrogen (H 2 ). For instance, about 50% of the cost in ammonia plants is on the hydrogen production from methane steam reforming. Direct natural gas conversion to value-added products without steam reforming can lead to a huge economic impact. As a result, direct catalytic ammonia synthesis from CH 4 and N 2 is an at- tractive alternative to not only better utilize CH 4 but also pro- duce H 2 as a valuable by-product. However, there has been al- most no investigation about this process due to the high chemical stability of N 2 and CH 4 molecules. One valuable study which mentioned CH 4 conversion co-fed with N 2 was reported in 1994. 16 Although only trace amounts of ammonia were reported, it inspired the nitrogen activation by micro- wave plasma. In a traditional fixed-bed reactor without cata- lysts, either the methane conversion or ammonia selectivity is low. 17 Using plasma without a catalyst seems to result in low ammonia selectivity which increases the separation cost downstream. 18 Driven by the demand for high selectivity of CH 4 /N 2 am- monia synthesis under atmospheric pressure, we used a microwave-assisted catalytic reactor system to investigate the reaction between these two stable molecules. The main goal of this paper is to synthesize ammonia directly from methane and nitrogen and simultaneously produce valuable by-prod- ucts, crystalline carbon nanotubes, to increase the process economics. Cobalt and cobalt–iron supported on gamma alumina (γ-Al 2 O 3 ) were used as catalysts. As a catalyst support, γ-Al 2 O 3 was experimentally observed to have better absorption of microwave energy. The simplified diagram that represents the microwave reaction system is shown in Fig. 1. Fig. 2 illustrates the reaction performance for all four ex- periments, and Table S1 (ESI†) numerically summarizes the experimental result. For all four experiments, the reaction was carried out at 600 °C and 1 atm, with a weight hourly 6302 | Catal. Sci. Technol., 2018, 8, 6302–6305 This journal is © The Royal Society of Chemistry 2018 a Chemical & Biomedical Engineering Department, West Virginia University, Morgantown, WV, USA. E-mail: john.hu@mail.wvu.edu b College of Life Science and Agronomy, Zhoukou Normal University, Zhoukou, Henan, China. E-mail: 13672165360@163.com † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c8cy01355a Fig. 1 Simplified diagram of the microwave-catalysis reaction system.