NTP reactor for a single stage methane conversion to methanol: Influence of catalyst addition and effect of promoters
Graphical abstract
Introduction
Increasing environmental concerns due to the excessive usage of fossil fuels led to the search for alternative fuels. In this context, methanol has drawn a significant interest as a renewable energy resource due to its high energy density and economic benefits [1], [2]. In addition, it is also considered to be a promising hydrogen carrier and a clean energy source [3]. In this regard methane has been drawing the attention as a desirable energy source for methanol production due to its high calorific value and worldwide availability [4], [5]. But the current large-scale synthesis of methanol from methane proceeds via steam reforming process followed by gas to liquid conversion is an energy demanding process [6], [7], [8]. Although various attempts are made on direct methane partial oxidation to methanol over solid catalysts, the methanol yield is not satisfactory [9], [10]. Therefore, the development of an energy efficient process for the direct methanol synthesis from methane is highly desirable. Thermocatalytic conversion is not so preferred mainly due the formation of total oxidation products. One of the alternate routes is the non-thermal plasma (NTP) activation of methane under ambient conditions. NTP has been receiving attention due to the presence of high energetic electrons (1–10 eV) that are capable of initiating the reaction under ambient conditions. [11], [12], [13]. Moreover, NTP has a great potential to overcome the thermodynamic barrier of conventional catalytic methane partial oxidation to methanol.
Even though a few attempts have been made on plasma-catalytic MPOM with various oxidants like O2, zero air, H2O, CO2, N2O and NO2, but most of them are non-catalytic approaches [14], [15], [16], [17], [18], [19]. However, it is known that plasma-catalysis improves the performance of NTP. Earlier research by Indarto showed that plasma catalytic methane conversion to methanol with copper-zinc-alumina catalyst is effective in improving methanol selectivity by two times than plasma process alone [20]. MPOM was investigated in a post-plasma catalytic system by Chen et al. where a copper promoted iron oxide showed ∼45% higher selectivity to methanol than that of non-catalytic plasma system [21]. Recently, Wang et al proposed a single stage plasma-catalytic CO2 hydrogenation to methanol under ambient conditions. Their findings showed that a combination of the plasma with Cu/γ-Al2O3 or Pt/ γ-Al2O3 significantly enhanced the CO2 conversion as well as methanol yield [22]. When compared to conventional catalytic methane partial oxidation to methanol, a limited numbers of catalysts have been tried in plasma-catalytic MPOM. Therefore, there is a lack of proper understanding regarding the plasma-chemical reaction pathways and as well as the role of the appropriate catalysts employed. Cu based catalysts have attracted considerable interest towards methanol synthesis and Cu/ZnO/Al2O3 is a well-established commercial catalyst in this regard [23], [24]. In CuZnAl catalyst, ZnO is considered as promoter which provides active sites either for hydrogen spillover and/or improves the dispersion of the cupper particles over the Al2O3 support [25]. Extensive works have also been reported on modifying the catalysts with various supports (Al2O3, ZrO2, SiO2, zeolites, ZnO etc.) and promoters (Zn, Ce, Zr, K, Cr etc.) [25], [26], [27]. Although several attempts are made on this context, but the major problem arises is that the catalytic activity reduces significantly at higher temperature range and thus the process suffers from a kinetic limitation [28]. Therefore, single stage catalyst combined plasma reactor system operating at ambient condition could be a suitable choice to overcome the thermodynamic and kinetic barriers.
In the present study, a dielectric barrier discharge reactor has been employed as the non-thermal plasma source, which is combined with γ-Al2O3 supported Cu catalyst modified with different promoters. DBD is favorably chosen as the NTP reactor because of its uniform distribution of the filamentary microdischarges all over the discharge volume which could be able to hold the consistency in energy distribution throughout the process. Both the plasma discharge property and the physiochemical property of the catalysts were studied. Various parameters like discharge power, specific input energy, feed gases composition have also been investigated with a clear objective of obtaining high selectivity to methanol.
Section snippets
Experimental section
A schematic representation of the experimental set-up is presented in Fig. 1. DBD reactor consists of a cylindrical quartz tube (23 mm outer × 20 mm inner diameter) with a stainless steel (SS) rod (11 mm) placed at the center of the quartz tube acts as the inner electrode which is also connected to a high voltage source. A SS mesh wrapped over the quartz tube, served as the outer electrode which was grounded through a capacitor of 0.4 µF. The total discharge length of the DBD reactor is 11 cm
X-ray diffraction analysis
Fig. 2 shows the XRD pattern of the prepared catalysts. The XRD pattern of the γ-Al2O3 support shows three major diffraction peaks placed at 2θ = 46.1°, 66.5° corresponding to the (4 0 0) and (4 4 0) planes of crystalline γ-Al2O3 (JCPDS-290063). All the catalysts show a broad diffraction signal characteristic of CuO species at 2θ = 35.4°. Two weak diffraction peaks of the CuO which actually look like humps, also appear at 2θ = 32.4° (1 1 0), 38.9° (2 0 0) respectively in catalyst CuAl
Conclusions
Catalytic methane partial oxidation to methanol was demonstrated successfully in a DBD plasma reactor operating under ambient condition. The reactor performance is strongly dependent on the discharge parameters, catalyst integration and feed gases composition. When compared to the DBD reactor, integration of catalysts with plasma leads to higher methane conversion and methanol selectivity. Addition of promoters ZnO, ZrO2 and MgO increased the performance of CuO/γ-Al2O3 catalyst, probably due to
Acknowledgements
The author greatly thankful to the MHRD, India for providing junior research fellowship.
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