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Ind. Eng. Chem. Res. 2008, 47, 6568–6572
Methane Conversion to Higher Hydrocarbons by UV Irradiation Alan R. Derk, Hans H. Funke, and John L. Falconer* Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309-0424
The potential for using UV photolysis to convert methane to higher hydrocarbons with high yields was demonstrated with a radio-frequency- (RF-) powered Kr/Ar discharge lamp (λ ) 116.5 and 123.6 nm, flux ) 3.3 × 1014 photons/s) at pressures of 47 and 93 kPa and at ambient temperature. The highest conversion was 39%. Photon yields for carbon-carbon bond formation were as high as 4.5. The main reaction products were hydrogen, ethane, propane, and n- and i-butane, but smaller amounts of ethylene and higher alkanes were also detected. The hydrocarbon products followed a Flory distribution. Introduction The conversion of methane from natural gas to higher hydrocarbons could provide new feedstocks for chemical processes and allow for more economical transport. Because methane is stable, however, energy requirements and capital investments are high for current large-scale technologies such as syngas/Fischer-Tropsch processes or partial oxidation.1 Photolysis with high-energy UV light (vacuum UV, VUV) is an alternative method for converting methane and might be economically feasible where modular design and smallerscale operation are desired. For example, large quantities of methane are present in remote gas wells and offshore sites, but these sources are not utilized because on-site liquefaction of the natural gas for transport is not cost-effective and access to pipelines is not available.2 Even though the energy requirements for UV photon formation are high, a fraction of the extracted methane or the hydrogen formed as a byproduct of the photolysis can be used to generate electricity to operate the lamps. Methane photochemistry has been studied extensively to obtain a fundamental understanding of the reaction path ways3–20 and more recently to gain insight into the atmospheric chemistry of planets and moons that have methanerich atmospheres.21–26 The mechanism of hydrogen elimination was the target of the majority of these investigations, but higher hydrocarbons were also detected by some groups.4–9 The excitation of CH4 electronic transitions that result in photodissociation requires a photon energy above about 8.9 eV (wavelength ≈ 140 nm).15 The light sources used included cyclotron radiation,15,27,28 frequency-multiplied lasers,28 and discharge lamps. Common window materials strongly absorb VUV radiation, so that its intensity is lowered inside the reactor. Only MgF2 and LiF seem to provide acceptable transmission for wavelengths below 180 nm. In one study with synchrotron radiation, the low-pressure reaction mixture was exposed directly to the UV source by differentially pumping the absorption chamber.27 Walker et al. described a helium curtain for a windowless design.5 Another option for low pressures are thin aluminum windows, which are somewhat transparent at VUV frequencies.10 Gondal et al. used lower-energy photons and multiphoton absorption at the high light intensity of laser sources to photodissociate methane.29,30 In the first step of photolysis, hydrogen and methane fragments form * Corresponding author. E-mail:
[email protected]. Fax: 303 492 4341.
CH4 + hν f CH3 + H f CH2 + H2 f CH2 + H + H f CH + H2 + H
(1)
Some groups have interpreted deuterium labeling experiments to indicate that molecular elimination of H2 is the dominant process,4,8,9,14 whereas other studies have concluded that atomic H fission is the primary first step.17–19 The presence of ionic species such as CH2+ was also observed at photon energies above the ionization potential of CH4 (12.99 eV, λ < 94.6 nm).5,20 Wang et al.28 concluded that CH2 from the first photodissociation has ionization potentials of 10.0 and 10.3 eV, and therefore, CH2+ could be present even below the ionization potential of CH4 as a result of ionization of the primary photofragments. Investigations with deuterium-labeled methane indicated that higher hydrocarbons result from insertion of CHx fragments into CH bonds,4,7 such as CH2 + CH4 f C2H6
(2)
or further photochemical reactions of the products. The net reaction is nCH4 f CnH2n+2 + (n - 1)H2
(3)
The enthalpy of reaction 3 increases with chain length (ethane, 65 kJ/mol; propane, 121 kJ/mol; butane, 161 kJ/mol). The larger hydrocarbons that form could also decompose to CH4 or C2 molecules. Such reverse processes have been seen for C4 and C5 alkanes under UV radiation.31,32 Most previous measurements of CH4 photolysis have been carried out at low pressure, and few studies have focused on pressures above 13 kPa.4,6 Overall conversions have typically been in the low percent6 and subpercent7 ranges, and quantitative conversion has not been attempted. The low conversions were a result of low light intensity or short exposure times that were selected to minimize the influence of reaction products for mechanistic studies of the primary processes. The present work, in contrast, shows that CH4 conversions that are several times higher than those previously reported can be achieved using longer exposure times and high light intensities at pressures of 47 and 93 kPa. The product distribution indicates a chain-growth mechanism. The effect of conversion on the product distribution was determined, and photon efficiencies were measured. Experimental Methods Methane was exposed to UV radiation in a stainless-steel reactor with an attached radio-frequency (RF) Kr discharge lamp
10.1021/ie0712840 CCC: $40.75 2008 American Chemical Society Published on Web 08/06/2008
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Figure 2. Product distribution for the photolysis of methane at 93 kPa.
Figure 1. Schematic of experimental setup.
(Resonance Ltd., Ontario, Canada). The lamp had emissions in the 120-nm range (Kr lines at 116.5 and 123.6 nm) and a 1-cm aperture sealed with a LiF window. Aluminum spacers were placed in the reactor to reduce dead volume and thus increase conversion. Two mass flow controllers (Aera) controlled argon and methane flows, and the system pressure was measured with two electronic pressure transducers. A schematic of the experimental setup is shown in Figure 1. A gas chromatograph (model 6890, Agilent) equipped with a 180-cm packed silica column, an automated 10-port sample valve, a thermal conductivity detector (TCD), and a flame ionization detector (FID) was used to analyze the reaction products. Argon was used as the carrier gas to improve hydrogen sensitivity for the TCD. The FID was calibrated with a 1% hydrogen/helium standard mixture, 1000 ppm gas standards for C1-C6 linear alkanes and C2-C6 linear alkenes (Alltech), and pure i-butane. Acetylene, 2-methylbutane, 2,2-dimethlybutane, neopentane, and 3-methylpentane were used as references to identify peaks observed at chain lengths below C6. The C6 and C7 isomers were not separated, and they were all treated as linear alkanes. The FID response, which increased linearly with carbon number, was extrapolated to include heptanes. Carbon dioxide/ carbon monoxide mixtures, which were used for photon flux calibration, were analyzed with a Carbosphere column, helium carrier gas, and the TCD. The methane was research-grade (Airgas), and gas chromatography (GC) analysis verified that other hydrocarbons were below the GC detection limits (
