High yield of aromatics from CH4 in a plasma‐followed‐by‐catalyst (PFC) reactor

Abstract

The oxygen-free dehydroaromatization of methane is of great scientific importance and industrial benefit for the effective utilization of natural gas and biogas to provide alternative petrochemical feed stocks and COx-free hydrogen production for fuel cells.1 Methane oxygen-free conversion into aromatics and hydrogen is not thermodynamically favorable at low temperatures ( 773 K); the equilibrium CH4 conversion at 973 K is 12%. At this temperature, benzene and naphthalene, in approximately equimolar amounts, are the thermodynamically preferred hydrocarbon products.2 The thermodynamic limitation is the main barrier for this process. To enhance the conversion of methane into aromatics beyond thermodynamic equilibrium, studies using H2-permeation membrane catalytic reactors are already underway by several research groups.3,4 However, preliminary reports on this approach are not very optimistic, and catalyst deactivation was more pronounced in a membrane catalytic reactor than in a fixed-bed reactor because of continuous withdrawal of the coproduced H2. Atmospheric nonthermal equilibrium plasma has been used as an efficient method for oxygen-free conversion of methane into C2 hydrocarbons (mainly acetylene) and H2 using pulsed corona/streamer discharges, or pulsed spark discharges.5-10 Only small amounts of C3-C5 hydrocarbons and trace amounts of aromatics were usually coproduced. Recently, Heintze and Magureanu reported the conversion of CH4 into aromatics in pulsed microwave plasma at atmospheric pressure in the presence of a heterogeneous catalyst.11 In their work, the authors suggested that methane aromatization is very likely catalyzed on coke surfaces and highly unlikely by the trimerization of acetylene. The most abundant aromatic formed was benzene, and 5% of the maximum benzene yield was obtained. Herein, we show a two-stage plasma-followed-bycatalyst (PFC) reactor for the oxygen-free conversion of methane into aromatic-rich hydrocarbons and coproduced H2. Methane is converted to acetylene by plasma in the first stage, with the second stage for acetylene trimerization on nickel-loaded HZSM-5 catalyst. Oxygen-free conversion of methane was performed at atmospheric pressure in a two-stage PFC reactor, as shown in Figure 1. The first stage of the PFC reactor, for generating pulsed spark discharge, was located in the upper part of a quartz tube (i.d. 10 mm) and consisted of a stainless steel wire (2 mm diameter) as the high-voltage electrode and a perforated iron plate (9 mm diameter) as the ground electrode. The distance between the two electrodes was 10 mm. The high-voltage power supply source for generating pulsed spark discharge was described before.10 The second stage of the PFC reactor was filled with 0.5 g of HZSM-5 or Ni/HZSM-5 pellets of 20-40 mesh. The Ni/HZSM-5 catalysts were prepared by the conventional impregnation method using HZSM-5 (SiO2/Al2O3 25, Nankai University, China) and aqueous solution of nickel nitrate. The impregnated samples were dried at 393 K for 4 h, calcined at 723 K for 4 h in air, and reduced at 673K for 2 h in hydrogen. The flow rates for CH4, H2, and N2 were controlled through a mass flow controller. The feed gas mixture of CH4 (99.99%) and H2 (99.99%) with a molar ratio of CH4/H2 1 was introduced into the PFC reactor at a flow rate of 9.2 cm/min. 2.2 cm/min of N2 (99.99%) was used as an internal standard for analysis and introduced into the effluent gas from the PFC reactor to avoid conversion of N2 in the plasma. Hydrocarbon products were sampled by a six-way valve heated to 473 K and analyzed on-line by a gas chromatograph (Agilent 1790F) with a flame ionization detector (FID) and using a 2 mm (i.d.) 3m Porapak-P column with N2 as the carrier gas. N2 and CH4 were analyzed on-line by another gas chromatograph (Agilent 1790T) with a thermoconductive deY. Xu and A.-M. Zhu are also affiliated with State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China. Correspondence concerning this article should be addressed to A.-M. Zhu at amzhu@dlut.edu.cn.