Elsevier

Chemical Engineering Journal

Volume 372, 15 September 2019, Pages 638-647
Chemical Engineering Journal

NTP reactor for a single stage methane conversion to methanol: Influence of catalyst addition and effect of promoters

https://doi.org/10.1016/j.cej.2019.04.172Get rights and content

Highlights

  • An in-plasma catalytic DBD reactor was employed for single stage MPOM process.

  • γ-Al2O3 supported Cu catalysts modified with different promoters (ZnO, ZrO2 and MgO).

  • The plasma-catalyst synergy strongly improves the efficiency of the reactor system.

Abstract

Partial oxidation of methane to methanol is one of the best routes for liquid oxygenates preparation. The present study describes the application of a non-thermal plasma reactor operated under dielectric barrier discharge mode with/without catalyst addition for a single stage methane conversion to methanol. Air has been chosen as the oxidant for methane partial oxidation. It is found that both the reactant conversion and product distribution are strongly dependent on the reactor configuration, feed gases composition and also catalyst addition. A series of γ-Al2O3 supported Cu catalyst with metal oxide promoters (ZnO, ZrO2 and MgO) were integrated with plasma zone as to obtain in-plasma catalytic reactor. Typical results show that the synergistic effect due to plasma activation and catalytic action, significantly improves both CH4 conversion and CH3OH selectivity. The best methanol selectivity of ∼28% is achieved over the CuZrAl catalyst with a CH4 conversion of ∼11%, while plasma reactor provides only ∼18% CH3OH selectivity. The possible reaction mechanism of methanol formation inside the plasma reactor has been discussed, which highlights that the catalyst facilitates the adsorption of plasma excited species and improves the performance of the reactor.

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.

References (42)

Cited by (25)

  • Recent progress in plasma-catalytic conversion of CO<inf>2</inf> to chemicals and fuels

    2023, Catalysis Today
    Citation Excerpt :

    Even, the CH4 selectivity was improved by the Ni-Ag bimetallic catalyst although Ag alone is selective towards CO production. Plasma discharges can generate UV radiation [53] and visible light [191] without using any extra light sources (e.g. UV lamps), due to the decay of the short-lived excited species in plasma. Hence, the energy efficiency of plasma catalysis can be further improved if we can utilize the light emitted, which means that the photocatalysts are potentially useful in plasma system as well.

View all citing articles on Scopus
View full text