Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 156 (2016) 118–122
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Possibility of methane conversion into heavier hydrocarbons using nanosecond lasers H.A. Navid a,b, E. Irani b, R. Sadighi-Bonabi b,⁎ a b
Department of laser and Optical Engineering, University of Bonab, Bonab, Iran Department of Physics, Sharif University of Technology, P.O. Box 11365-9567, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 28 February 2015 Received in revised form 17 August 2015 Accepted 20 November 2015 Available online 21 November 2015 Keywords: Dissociation Conversion mechanism Methane Nanosecond lasers Ion-neutral reactions
a b s t r a c t Effect of nanosecond lasers on the methane dissociation is experimentally studied by using three different laser wavelengths at 248 nm, 355 nm and 532 nm. C2H2 generation is measured as a major reaction product in experiments and the energy consumptions in production of this component are measured as 5.8 MJ/mol, 3.1 MJ/mol and 69.0 MJ/mol, for 355 nm, 532 nm and 248 nm wavelengths, respectively. The mechanism of conversion and production of new stable hydrocarbons is also theoretically investigated. It is found that in theoretical calculations, the ion-molecule reactions should be included and this leads to a unique approach in proper explanation of the experimental measurements. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Strong laser field interaction with molecules is a very exciting subject for many aspects, especially controlling chemical reactions [1,2]. Irradiation of molecules by strong optical field readily leads to breaking one or more molecular bonds and molecular processes such as ionization and conversion of fragments into the favorite products [3–5]. Several mechanisms are used to explain the photo-dissociation of molecules in intense laser fields. Some methods such as multiphoton ionization (MPI), above threshold ionization (ATI), field ionization (FI), coulomb explosion (CE), and explosive photo-dissociation are proposed [6–10]. At the ultra-intense elliptically polarized laser fields, the molecular alignment and angular dependence of the ionization probability present a major effect on the ionization processes [11–13]. The old dream of comprehension of the molecular dynamics and controlling of reactions towards desired valuable products among other competing components became transparent by real time observations of the transition-state region between reagents and products in 1988 [14]. Using the advanced intense femtosecond laser pulses with pulse shaping technology, the objective of controlling chemical reactions and optical spectroscopy with coherent light are implemented [15–18]. The capability of ultrashort laser pulses allowed the probe of reaction dynamics at a real time of nuclear motion and the direct observation of molecular orientation [19–20].
⁎ Corresponding author. E-mail address: [email protected] (R. Sadighi-Bonabi).
http://dx.doi.org/10.1016/j.saa.2015.11.018 1386-1425/© 2015 Elsevier B.V. All rights reserved.
Methane, the most stable hydrocarbon at room temperature, is one of the major greenhouse gases with very high global warming potential. Indeed, the global warming potential of CH4 is very high for UV absorption and IR emission and on a mass basis it is 25 times that of CO2 over a 100-year time horizon [21]. Although, methane is a clean and primary fossil fuel, the conversion of methane into higher hydrocarbons and hydrogen gas hydrocarbons is the goal of many nations including the oil and gas fields [22,23]. In addition, the transportation of methane across distant and remote geographic regions is a main challenge, which may be resolved by conversion methods. Moreover, mechanism of onsite liquefaction of methane can prevent contributing to the generated greenhouse gases [24,25]. The conversion of natural gas to hydrocarbons has not yet been successfully economized in an inexpensive process. In general, there are two types of indirect and direct techniques for methane conversion to hydrocarbons. The indirect route for methane conversion (e.g. Fischer–Tropsch) requires the production of synthesis gases (CO and H2) from methane by expensive and inefficient process of steam reforming. The direct ways are not accepted since to date it has not been possible to achieve considerable conversion yield to heavier hydrocarbons, directly [26–28]. The photo-catalytic conversion techniques have some advantages including partially selecting and low temperature processing of some desired chemical products which has motivated the scientist attention to solve the problems of direct methods in conversion processes [29,30]. Experimental irradiations of CH4–C2H2 gas mixtures using UV lasers have been carried out in order to test the catalytic scheme of dissociation of methane via the photolysis of acetylene [31]. By analyzing the dependence of the stable fragments on laser
H.A. Navid et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 156 (2016) 118–122
parameters such as irradiation time, it is shown that the stable products of C2H2, C2H4, and C2H6, appear in constant amounts in the different irradiation times. However, this is not a cost effective approach and valuable catalysts waste during the process. As a result, some other techniques should be explored. One of the important parameters in the conversion method is the energy consideration which determines the cost effectiveness of the process. Since femtosecond lasers with unique capabilities of delivering the minimum required energy for breaking the favorite bond, as mentioned above, may be the best candidate in this regard. The experimental setup is supplied by a complex system and computer equipped with the genetic algorithm code. Although, great improvements are achieved in controlling chemical reactions; by these methods, initial experimental data for genetic and learning is needed and is not an economical method. However, In spite of the femtosecond lasers capabilities, nanosecond lasers become a powerful tool in dissociation of chemical reactions. These lasers are inexpensive, accessible sources and can have optimum energy consumption in the conversion process. Due to these valuable features there have been considerable attentions on the application of these lasers in this field. Few fundamental studies are concentrated on the interaction of intense laser fields with molecules using nanosecond lasers [24,32–34]. Since, the photo-dissociation of methane has been studied extensively based on the theoretical approaches [35, 36]. Some studies focused on recombination mechanisms in production of higher hydrocarbons [37]. In earlier reports it is supposed that the laser induced methyl and methylene radicals constitute the building blocks of higher hydrocarbons [38,39]. Following our recent studies on dissociation of methane [35,40], in the present work the conversion process of methane is experimentally implemented by using three different wavelengths of 248 nm, 355 nm and 532 nm and a reaction theoretical model is proposed. The main aim is to present a comprehensive conversion mechanism for production of the stable valuable products. Basically, lasers do not considerably increase the temperature inside the reaction chamber and can be an appropriate candidate economically. To the best of our knowledge there is no complete reported consistent conversion model experimentally in this field. Furthermore, a useful theoretical model is proposed which properly explained the experimentally obtained yields. This is explained in detail in the following. 2. Experimental setup Fig. 1 shows a schematic experimental setup which is similar to the earlier reported system [40]. The reaction chamber is a cylindrical aluminum chamber with an internal diameter of 3 cm and a length of 10 cm, which gives about 71 cm3 volume. All of the chamber volume was filled with methane in 870 mbar for 248 nm and 355 nm laser experiments; however, the effective volume was decreased to 10 cm3 for 532 nm in order to decrease the laser energy consumption. It should be noted that the decrease of reaction volume does not affect the
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Fig. 2. The geometrical properties of plane-convex lens used in experimental setup for focusing laser beam. The focal length of the lens is about 50.0 mm and the direction of incident beam is as shown in the figure.
consumed specific energy per mole (J/mol) of produced components, because the reactions occur in a small volume around the focal point. The laser beam is focused by fused silica lens of Fig. 2, longitudinally through quartz window at the center of the cell. Two mass flow controllers are used to evacuate the reaction chamber by a vacuum pump in order to fill it by proper methane gas. A gas chromatograph (GC) series B 5890 equipped with packed column and a Flame Ionization Detector (FID) is used to analyze the reaction products. Helium was used as a carrier gas and acetylene, ethylene and ethane were used as references. The reactor was evacuated to below 0.001 Pa and then pressurized with research-grade (99.999%) methane gas. Following the laser exposure, the reactor content was injected into the packed column of GC and the concentrations of final stable products were measured.
3. Experimental results In conversion process three different nanosecond lasers were used in experiments. The characteristics of lasers are listed in Table 1. Focal spot sizes of the beams are estimated using the ZEMAX optical software package. Fig. 3 depicts the fluorescence spectrum of photo-dissociation of methane as a function of wavelength in the 200–1000 nm region induced by second harmonic Nd:YAG laser at 532 nm irradiation. The existence of 532 nm wavelength is due to the scattering of laser light into detector. Other lines correspond to the unstable products formed after laser absorption process.
Fig. 1. Schematic view of the experimental setup.
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H.A. Navid et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 156 (2016) 118–122
Table 1 Laser parameters used in experiments of conversion of methane. Laser
Second harmonic of Nd:YAG laser
third harmonic of Nd:YAG laser
KrF laser
Wavelength Pulse duration Laser beam cross section (incident on lens) Pulse energy (after input window) Laser repletion rate Average irradiation power
532 nm 10 ns 2.0 cm2 100 mJ 1 Hz 0.1 W
355 nm 10 ns 0.28 cm2 150 mJ 10 Hz 1.5 W
248 nm 20 ns 0.75 cm2 100 mJ 15 Hz 1.5 W
The results of 532 nm laser photo-conversion of methane into stable products are shown in chromatography graph in Fig. 4. From this figure one can see that C2H2 is the most abundant product among the other stable produced components. C2H6 density is lower than the detection limit of GC (0.1%). In Table 2 the production yields are listed for three mentioned wavelengths of 532 nm, 355 nm, and 248 nm. In order to have the amount of energy used in conversion process the amount of energy used in production of one mole of C2H2 is measured and summarized in this table. All experiments were performed at atmospheric pressure. Considering the nonlinear interaction of lasers with methane the quality of focused beam has a direct effect on the amount of used energy. It should be realized that beside the dependence of different amounts of consumed energy on wavelengths, it depends on laser beam quality.
4. Mechanism of dissociation and conversion process Dissociation processes occur as a result of laser interaction with methane. The multiphoton absorption moves CH4 to higher unstable states and causes disintegration. Although the results of methane conversion were different for three different laser wavelengths with various quantities, they have similar qualitative results. Apart from quantitative considerations, the detected hydrocarbons were C2H2, C2H4 and C2H6 by radiating all of the mentioned wavelengths. Furthermore, the concentration of C2H2 is higher than the other produced stable components and C2H6 has the minimum concentration. These results are different from outcomes reported with conventional UV lamps [41]. Based on the same dissociation and recombination mechanism assumption for all three wavelengths, the qualitative experimental description for this process was reported [40]. According to that approach the chemical evolutions initiate from uni-molecular dissociation reactions of methane as a consequence of multiphoton absorption process. Unstable components react with each other and also other components inside the cell and produce chain reactions. It seems that the dissociation mechanism is the challenging problem, because the
Fig. 3. Fluorescence spectrum of photo-dissociation of methane in the region of 200– 800 nm using 532 nm, 10 ns and 100 mJ/pulse laser irradiation. The wavelength of 532 nm is observed due to the scattering of the laser light into the detector.
chain reactions occur as a result of natural activities which manifest in rate constants. That means after the first step of dissociation, the chain reactions would be predictable by available reaction rate constants. Now for the first step it can be supposed that methane is dissociated according to references [42,43]. 8 CH3 þ H > > > > CH 2 þ H 2 < nhv CH 4 → 1 CH 2 þ H þ H > > > 3 CH 3 þ H þ H > : CH þ H 2 þ H
ðaÞ ðbÞ ðcÞ ðdÞ ðeÞ
ð1Þ
The above dissociation channels have been confirmed experimentally for UV sources [41]. In a simplified model we considered only components up to two carbon atoms. The experimental results of UV sources agree very well with our calculations. This justifies our simple model. However, assuming the same dissociation channels for multiphoton absorption process in nanosecond laser irradiation of methane creates a wide discrepancy between theory and experiments even in qualitative analysis [40]. Fig. 5 shows the calculated results for evolution of density of stable hydrocarbons as a function of time for 248 nm and 355 nm lasers. These calculated results are obtained using the laser parameters listed in Table 1 and the dissociation channel coefficients of UV sources. Two and three photon absorption coefficients of methane are used in absorption process for 248 nm and 355 nm lasers [44]. According to calculated results in Fig. 5 the concentration of C2H2 is less than the other stable components, while the experiments show different results of dominance of C2H2 as a produced component. Therefore some other approaches should be sought. In the following based on the present unique approach, the other reaction mechanisms have been explored that could lead to proper results. In this method, based on the confirmed earlier experiments of nanosecond laser interaction with methane it is necessary to introduce the ion-molecule reactions. It should be realized that recently in irradiation of methane by nanosecond pulses the ionized components of C+,
Fig. 4. Chromatogram of conversion of methane into higher hydrocarbons using 532 nm laser, 1 Hz repetition rate, 100 mJ per pulse energy, 10 ns duration, 860 mbar pressure for 90 min irradiation times. The peak values correspond to methane, acetylene and ethylene, respectively. The horizontal and vertical axes represent the GC residence time in minutes and the number of carbon atoms/min of produced components in arbitrary unit, respectively.
H.A. Navid et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 156 (2016) 118–122 Table 2 The results of product efficiency by 355 nm laser, 150 mJ/pulse, 1 hour irradiation time and 10 Hz repetition rate (Exp1), the results of radiation of 248 nm laser, 10 Hz repetition rate, 100 mJ/pulse, and 1 hour irradiation time (Exp2) and the results of conversion with 532 nm laser, 1 Hz repetition rate and 90 min irradiation time (Exp3) are summarized. The amount of laser energy used in production of one mole of C2H2, in MJ/mol, is also listed in the table. Experiment number Components
Exp1
Exp2
Exp3
CH4 C2H2 C2H4 C2H6 Specific consumed energy for C2H2 (MJ/mol)
62% 33.5% 3.0% 1.4% 5.8
96.8% 2.2% 0.5% 0.6% 69.0
65.6% 33.2% 1.2% b0.1% 3.l
+ CH+, CH+ 2 and CH3 were detected [32]. Therefore, the dissociation channels should be modified as:
8 > >
CH2 þ þ H 2 þ e > : CH 3 þ þ H þ e
ðaÞ ðbÞ ðcÞ ðdÞ
ð2Þ
In order to complete the differential equations governing the time evolution of the components, the ion-neutral reactions are added to the previous neutral-neutral reactions [45,46]. For simplifying the calculations the components containing up to two carbon atoms were considered as before. By assumption of arbitrary values for different dissociation channels, the differential equations are solved using fourth order Runge–Kutta method. Unfortunately, none of them could produce a similar result with the experiments and were not compatible even qualitatively. In all these calculations, C2H6 was the main product. Investigating the mechanism of C2H6 production clarifies that the CH3 radicals play an important role in this process as an example in the reaction of CH3 + CH3 + M → C2H6 + M. Moreover, the CH3 radical efficiently produces in the cell, as a consequence of chain reactions. For example the reaction of CH4 + H → CH3 + H2 is inevitable and cannot be effected by varying the dissociation coefficients in set of reaction (2). The inclusion of ion-radical reactions for CH3 is imperative in order to find the possible experimentally consistent outcomes. Therefore, the reactions that can consume CH3 and produce C2H2 are
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explored. One of the important reactions in interstellar chemistry studies is the electron-ion recombination [47]: C 2 H 3 þ þ e→C 2 H 2 þ H;
ð3Þ
In this case the electron-ion recombination reaction (3) could lead to the C2H2 production. In the following the possibility of C2H+ 3 production + by ion-radical reactions of CH3 with C+, CH+, CH+ 2 and CH3 is investigated. The possible reactions are: 8 þ C → C 2 H3 þ > > < CH þ → C 2 H3 þ þ H CH 3 þ > C 2 H3 þ þ H2 CH2 þ → > : CH3 þ → C 2 H3 þ H2 þ H:
ð4Þ
It should be noticed that the effect of each of the above reactions is examined on the final results of calculations by varying the rate constants. Among these reactions only the first one produces considerable amount which fortunately has good agreement with the above experimentally measured data. This is explained in the following by considering the production of C+ and then the possible reaction is: CH 3 þ C þ þ M→C 2 H3 þ þ M;
ð5Þ
M is a molecule that in inelastic collision can absorb the excess energy of C2H+ 3 . The net result of reactions (3) and (5) can be written as CH 3 þ C þ þ e→C 2 H 2 þ H:
ð6Þ
This is a promising result, because it converts the CH3 radical into C2H2, and can lead to obtained experimental results. To the best of our knowledge the ion-radical reactions are not measured and should be calculated theoretically. In this regard, the rate of reaction (5) is investigated in more details. It is actually composed of two step bimolecular reactions as: CH 3 þ C þ → C 2 H3 þ
ð7Þ
C 2 H 3 þ þM→C 2 H3 þ M;
ð8Þ
where (C2H+ 3 )* is supposed an excited component that exceeds the bond dissociation energy of C2H+ 3 . The rate of reaction (5) can be calculated as [48]: kðT Þ ¼ K 1 ðT Þk2 ðT Þ;
ð9Þ
where K1(T) is the equilibrium constant for reaction (7) and k2(T) is the rate constant for reaction (7). For the reaction (7), the equilibrium constant can be calculated using the equation K 1 ðT Þ ¼ ð1=cÞ expðΔG=RT Þ;
ð10Þ
where cis the background gas concentration and ΔG is the free Gibbs energy difference between products and reactants that can be calculated using Gaussian software package optimization at the level of uhf/631g(d) for C+, CH3 [49]. ⁎ For the (C2H+ 3 ) it is assumed the molecule is in the first electronic excited state and is optimized at the level of CIS/6-31g (d) using Gaussian package. The calculated results are summarized in Table 3.
Fig. 5. Numerical results of densities of C2H2, C2H4 and C2H6 as a function of time for about 5 min, in the cell. Primed and unprimed notations are used for Nd:YAG and KrF laser irradiation results, respectively. The calculated results are obtained using data of Table 1 at atmospheric pressure and for dissociation channels of UV irradiation. Two and three photon absorption coefficients of methane are used in the absorption process for 248 nm and 355 nm lasers.
Table 3 Output of Gaussian package used in ΔG calculation. C+ Sum of electronic and thermal energies (Hartree/molecule)
CH3
⁎ (C2H+ 3 )
−37.301238 −39.546913 −76.856240
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H.A. Navid et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 156 (2016) 118–122
According to data of Table 3 and using equation (10) the equilibrium constant is obtained asK1 = 9.7 × 10−9cm3/molecule. For the reaction (8) at the first approximation it is assumed that the reaction proceeds according to the collision frequency in hard sphere model [47]: 2
1=2
k2 ðT Þ ¼ πd ð8kB T=πμ Þ
package the required constants are calculated. The measured amounts in these experimental arrangements are in good agreement with the results of the introduced theoretical approach. Acknowledgments
;
ð11Þ
where d is the sum of the two colliding partner radii, T is the temperature and μ is the reduced mass. According to this equation k2(T) = 1.35 × 10− 8cm3/(s . mole cule) is obtained, with d = 2.33 Å, T = 298.15 °K and μ = 10.05 g/mol, which are calculated using Gaussian software package. Substituting the obtained data in equation (9) the rate constant is calculated as k = 1.3×10−12 cm6/molecule for temperature of 298.15 °K. Including reaction (6) with calculated rate constant to the neutral– neutral and ion–neutral reactions complete equations were obtained. In order to complete the numerical differential equations the rate of absorption of photons for methane must be taken into account. It is supposed that the dissociation of methane occurs based on the rate of three photon absorption process. Fig. 6 shows the result of numerical calculations based on new proposed reactions. In accordance with Fig. 6, the qualitative agreement between theory and experiments has been established and the discrepancy has been removed, introducing the ion–radical reactions. Therefore, it is shown that introducing the ion–molecule reactions in conversion mechanism leads to fruitful results. The use of hard sphere collision theory to estimate the ion molecule reaction (8) gives the lower limit to the rate constant, because the ion molecule reaction proceeds via the long range capture and likely exceeds the hard sphere rate constant. This point does not concern us, because the use of reaction rates higher than the obtained value for reaction (8) does not alter the order of the components and leads to more consistent results. That is the concentration of C2H2 becomes much more than the other two components. 5. Conclusions In this work, a combined experimental and theoretical study of the conversion of methane into higher hydrocarbons has been carried out by using nanosecond lasers at wavelengths 355 nm, 532 nm and 248 nm. The energy consumption in production of one mole C2H2 is measured as 5.8 MJ/mol, 3.1 MJ/mol and 69.0 MJ/mol, for the above mentioned wavelengths, respectively. On the theoretical study the density of new stable products is obtained using 4th order Runge–Kutta method. According to the experiments, C2H2 is the main product in all measurements which is in contrast with the yields of conventional UV sources in which C2H6 is the dominant product. Based on the introduced unique ion–radical reaction mechanism and by using Gaussian software
The authors want to thank Pars oil and gas company of ministry of oil for their support for this project through contract number PT131. We also acknowledge the research deputy of Sharif University of Technology. References: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]
Fig. 6. Numerical results of densities for stable hydrocarbons as a function of time in the cell at room temperature and atmospheric pressure. The calculated results are obtained by assumption of the dissociation rate 1.3 × 1012 molecule/s of methane, that is calculated for three photon absorption process and for data of Table 1 for 355 nm wavelength. The density of C2H6 is lower than C2H2 and C2H4 that is absent in the figure.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Possibility of methane conversion into heavier hydrocarbons using nanosecond lasers H.A. Navid a,b, E. Irani b, R. Sadighi-Bonabi b,⁎ a b
Department of laser and Optical Engineering, University of Bonab, Bonab, Iran Department of Physics, Sharif University of Technology, P.O. Box 11365-9567, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 28 February 2015 Received in revised form 17 August 2015 Accepted 20 November 2015 Available online 21 November 2015 Keywords: Dissociation Conversion mechanism Methane Nanosecond lasers Ion-neutral reactions
a b s t r a c t Effect of nanosecond lasers on the methane dissociation is experimentally studied by using three different laser wavelengths at 248 nm, 355 nm and 532 nm. C2H2 generation is measured as a major reaction product in experiments and the energy consumptions in production of this component are measured as 5.8 MJ/mol, 3.1 MJ/mol and 69.0 MJ/mol, for 355 nm, 532 nm and 248 nm wavelengths, respectively. The mechanism of conversion and production of new stable hydrocarbons is also theoretically investigated. It is found that in theoretical calculations, the ion-molecule reactions should be included and this leads to a unique approach in proper explanation of the experimental measurements. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Strong laser field interaction with molecules is a very exciting subject for many aspects, especially controlling chemical reactions [1,2]. Irradiation of molecules by strong optical field readily leads to breaking one or more molecular bonds and molecular processes such as ionization and conversion of fragments into the favorite products [3–5]. Several mechanisms are used to explain the photo-dissociation of molecules in intense laser fields. Some methods such as multiphoton ionization (MPI), above threshold ionization (ATI), field ionization (FI), coulomb explosion (CE), and explosive photo-dissociation are proposed [6–10]. At the ultra-intense elliptically polarized laser fields, the molecular alignment and angular dependence of the ionization probability present a major effect on the ionization processes [11–13]. The old dream of comprehension of the molecular dynamics and controlling of reactions towards desired valuable products among other competing components became transparent by real time observations of the transition-state region between reagents and products in 1988 [14]. Using the advanced intense femtosecond laser pulses with pulse shaping technology, the objective of controlling chemical reactions and optical spectroscopy with coherent light are implemented [15–18]. The capability of ultrashort laser pulses allowed the probe of reaction dynamics at a real time of nuclear motion and the direct observation of molecular orientation [19–20].
⁎ Corresponding author. E-mail address: [email protected] (R. Sadighi-Bonabi).
http://dx.doi.org/10.1016/j.saa.2015.11.018 1386-1425/© 2015 Elsevier B.V. All rights reserved.
Methane, the most stable hydrocarbon at room temperature, is one of the major greenhouse gases with very high global warming potential. Indeed, the global warming potential of CH4 is very high for UV absorption and IR emission and on a mass basis it is 25 times that of CO2 over a 100-year time horizon [21]. Although, methane is a clean and primary fossil fuel, the conversion of methane into higher hydrocarbons and hydrogen gas hydrocarbons is the goal of many nations including the oil and gas fields [22,23]. In addition, the transportation of methane across distant and remote geographic regions is a main challenge, which may be resolved by conversion methods. Moreover, mechanism of onsite liquefaction of methane can prevent contributing to the generated greenhouse gases [24,25]. The conversion of natural gas to hydrocarbons has not yet been successfully economized in an inexpensive process. In general, there are two types of indirect and direct techniques for methane conversion to hydrocarbons. The indirect route for methane conversion (e.g. Fischer–Tropsch) requires the production of synthesis gases (CO and H2) from methane by expensive and inefficient process of steam reforming. The direct ways are not accepted since to date it has not been possible to achieve considerable conversion yield to heavier hydrocarbons, directly [26–28]. The photo-catalytic conversion techniques have some advantages including partially selecting and low temperature processing of some desired chemical products which has motivated the scientist attention to solve the problems of direct methods in conversion processes [29,30]. Experimental irradiations of CH4–C2H2 gas mixtures using UV lasers have been carried out in order to test the catalytic scheme of dissociation of methane via the photolysis of acetylene [31]. By analyzing the dependence of the stable fragments on laser
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parameters such as irradiation time, it is shown that the stable products of C2H2, C2H4, and C2H6, appear in constant amounts in the different irradiation times. However, this is not a cost effective approach and valuable catalysts waste during the process. As a result, some other techniques should be explored. One of the important parameters in the conversion method is the energy consideration which determines the cost effectiveness of the process. Since femtosecond lasers with unique capabilities of delivering the minimum required energy for breaking the favorite bond, as mentioned above, may be the best candidate in this regard. The experimental setup is supplied by a complex system and computer equipped with the genetic algorithm code. Although, great improvements are achieved in controlling chemical reactions; by these methods, initial experimental data for genetic and learning is needed and is not an economical method. However, In spite of the femtosecond lasers capabilities, nanosecond lasers become a powerful tool in dissociation of chemical reactions. These lasers are inexpensive, accessible sources and can have optimum energy consumption in the conversion process. Due to these valuable features there have been considerable attentions on the application of these lasers in this field. Few fundamental studies are concentrated on the interaction of intense laser fields with molecules using nanosecond lasers [24,32–34]. Since, the photo-dissociation of methane has been studied extensively based on the theoretical approaches [35, 36]. Some studies focused on recombination mechanisms in production of higher hydrocarbons [37]. In earlier reports it is supposed that the laser induced methyl and methylene radicals constitute the building blocks of higher hydrocarbons [38,39]. Following our recent studies on dissociation of methane [35,40], in the present work the conversion process of methane is experimentally implemented by using three different wavelengths of 248 nm, 355 nm and 532 nm and a reaction theoretical model is proposed. The main aim is to present a comprehensive conversion mechanism for production of the stable valuable products. Basically, lasers do not considerably increase the temperature inside the reaction chamber and can be an appropriate candidate economically. To the best of our knowledge there is no complete reported consistent conversion model experimentally in this field. Furthermore, a useful theoretical model is proposed which properly explained the experimentally obtained yields. This is explained in detail in the following. 2. Experimental setup Fig. 1 shows a schematic experimental setup which is similar to the earlier reported system [40]. The reaction chamber is a cylindrical aluminum chamber with an internal diameter of 3 cm and a length of 10 cm, which gives about 71 cm3 volume. All of the chamber volume was filled with methane in 870 mbar for 248 nm and 355 nm laser experiments; however, the effective volume was decreased to 10 cm3 for 532 nm in order to decrease the laser energy consumption. It should be noted that the decrease of reaction volume does not affect the
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Fig. 2. The geometrical properties of plane-convex lens used in experimental setup for focusing laser beam. The focal length of the lens is about 50.0 mm and the direction of incident beam is as shown in the figure.
consumed specific energy per mole (J/mol) of produced components, because the reactions occur in a small volume around the focal point. The laser beam is focused by fused silica lens of Fig. 2, longitudinally through quartz window at the center of the cell. Two mass flow controllers are used to evacuate the reaction chamber by a vacuum pump in order to fill it by proper methane gas. A gas chromatograph (GC) series B 5890 equipped with packed column and a Flame Ionization Detector (FID) is used to analyze the reaction products. Helium was used as a carrier gas and acetylene, ethylene and ethane were used as references. The reactor was evacuated to below 0.001 Pa and then pressurized with research-grade (99.999%) methane gas. Following the laser exposure, the reactor content was injected into the packed column of GC and the concentrations of final stable products were measured.
3. Experimental results In conversion process three different nanosecond lasers were used in experiments. The characteristics of lasers are listed in Table 1. Focal spot sizes of the beams are estimated using the ZEMAX optical software package. Fig. 3 depicts the fluorescence spectrum of photo-dissociation of methane as a function of wavelength in the 200–1000 nm region induced by second harmonic Nd:YAG laser at 532 nm irradiation. The existence of 532 nm wavelength is due to the scattering of laser light into detector. Other lines correspond to the unstable products formed after laser absorption process.
Fig. 1. Schematic view of the experimental setup.
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Table 1 Laser parameters used in experiments of conversion of methane. Laser
Second harmonic of Nd:YAG laser
third harmonic of Nd:YAG laser
KrF laser
Wavelength Pulse duration Laser beam cross section (incident on lens) Pulse energy (after input window) Laser repletion rate Average irradiation power
532 nm 10 ns 2.0 cm2 100 mJ 1 Hz 0.1 W
355 nm 10 ns 0.28 cm2 150 mJ 10 Hz 1.5 W
248 nm 20 ns 0.75 cm2 100 mJ 15 Hz 1.5 W
The results of 532 nm laser photo-conversion of methane into stable products are shown in chromatography graph in Fig. 4. From this figure one can see that C2H2 is the most abundant product among the other stable produced components. C2H6 density is lower than the detection limit of GC (0.1%). In Table 2 the production yields are listed for three mentioned wavelengths of 532 nm, 355 nm, and 248 nm. In order to have the amount of energy used in conversion process the amount of energy used in production of one mole of C2H2 is measured and summarized in this table. All experiments were performed at atmospheric pressure. Considering the nonlinear interaction of lasers with methane the quality of focused beam has a direct effect on the amount of used energy. It should be realized that beside the dependence of different amounts of consumed energy on wavelengths, it depends on laser beam quality.
4. Mechanism of dissociation and conversion process Dissociation processes occur as a result of laser interaction with methane. The multiphoton absorption moves CH4 to higher unstable states and causes disintegration. Although the results of methane conversion were different for three different laser wavelengths with various quantities, they have similar qualitative results. Apart from quantitative considerations, the detected hydrocarbons were C2H2, C2H4 and C2H6 by radiating all of the mentioned wavelengths. Furthermore, the concentration of C2H2 is higher than the other produced stable components and C2H6 has the minimum concentration. These results are different from outcomes reported with conventional UV lamps [41]. Based on the same dissociation and recombination mechanism assumption for all three wavelengths, the qualitative experimental description for this process was reported [40]. According to that approach the chemical evolutions initiate from uni-molecular dissociation reactions of methane as a consequence of multiphoton absorption process. Unstable components react with each other and also other components inside the cell and produce chain reactions. It seems that the dissociation mechanism is the challenging problem, because the
Fig. 3. Fluorescence spectrum of photo-dissociation of methane in the region of 200– 800 nm using 532 nm, 10 ns and 100 mJ/pulse laser irradiation. The wavelength of 532 nm is observed due to the scattering of the laser light into the detector.
chain reactions occur as a result of natural activities which manifest in rate constants. That means after the first step of dissociation, the chain reactions would be predictable by available reaction rate constants. Now for the first step it can be supposed that methane is dissociated according to references [42,43]. 8 CH3 þ H > > > > CH 2 þ H 2 < nhv CH 4 → 1 CH 2 þ H þ H > > > 3 CH 3 þ H þ H > : CH þ H 2 þ H
ðaÞ ðbÞ ðcÞ ðdÞ ðeÞ
ð1Þ
The above dissociation channels have been confirmed experimentally for UV sources [41]. In a simplified model we considered only components up to two carbon atoms. The experimental results of UV sources agree very well with our calculations. This justifies our simple model. However, assuming the same dissociation channels for multiphoton absorption process in nanosecond laser irradiation of methane creates a wide discrepancy between theory and experiments even in qualitative analysis [40]. Fig. 5 shows the calculated results for evolution of density of stable hydrocarbons as a function of time for 248 nm and 355 nm lasers. These calculated results are obtained using the laser parameters listed in Table 1 and the dissociation channel coefficients of UV sources. Two and three photon absorption coefficients of methane are used in absorption process for 248 nm and 355 nm lasers [44]. According to calculated results in Fig. 5 the concentration of C2H2 is less than the other stable components, while the experiments show different results of dominance of C2H2 as a produced component. Therefore some other approaches should be sought. In the following based on the present unique approach, the other reaction mechanisms have been explored that could lead to proper results. In this method, based on the confirmed earlier experiments of nanosecond laser interaction with methane it is necessary to introduce the ion-molecule reactions. It should be realized that recently in irradiation of methane by nanosecond pulses the ionized components of C+,
Fig. 4. Chromatogram of conversion of methane into higher hydrocarbons using 532 nm laser, 1 Hz repetition rate, 100 mJ per pulse energy, 10 ns duration, 860 mbar pressure for 90 min irradiation times. The peak values correspond to methane, acetylene and ethylene, respectively. The horizontal and vertical axes represent the GC residence time in minutes and the number of carbon atoms/min of produced components in arbitrary unit, respectively.
H.A. Navid et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 156 (2016) 118–122 Table 2 The results of product efficiency by 355 nm laser, 150 mJ/pulse, 1 hour irradiation time and 10 Hz repetition rate (Exp1), the results of radiation of 248 nm laser, 10 Hz repetition rate, 100 mJ/pulse, and 1 hour irradiation time (Exp2) and the results of conversion with 532 nm laser, 1 Hz repetition rate and 90 min irradiation time (Exp3) are summarized. The amount of laser energy used in production of one mole of C2H2, in MJ/mol, is also listed in the table. Experiment number Components
Exp1
Exp2
Exp3
CH4 C2H2 C2H4 C2H6 Specific consumed energy for C2H2 (MJ/mol)
62% 33.5% 3.0% 1.4% 5.8
96.8% 2.2% 0.5% 0.6% 69.0
65.6% 33.2% 1.2% b0.1% 3.l
+ CH+, CH+ 2 and CH3 were detected [32]. Therefore, the dissociation channels should be modified as:
8 > >
CH2 þ þ H 2 þ e > : CH 3 þ þ H þ e
ðaÞ ðbÞ ðcÞ ðdÞ
ð2Þ
In order to complete the differential equations governing the time evolution of the components, the ion-neutral reactions are added to the previous neutral-neutral reactions [45,46]. For simplifying the calculations the components containing up to two carbon atoms were considered as before. By assumption of arbitrary values for different dissociation channels, the differential equations are solved using fourth order Runge–Kutta method. Unfortunately, none of them could produce a similar result with the experiments and were not compatible even qualitatively. In all these calculations, C2H6 was the main product. Investigating the mechanism of C2H6 production clarifies that the CH3 radicals play an important role in this process as an example in the reaction of CH3 + CH3 + M → C2H6 + M. Moreover, the CH3 radical efficiently produces in the cell, as a consequence of chain reactions. For example the reaction of CH4 + H → CH3 + H2 is inevitable and cannot be effected by varying the dissociation coefficients in set of reaction (2). The inclusion of ion-radical reactions for CH3 is imperative in order to find the possible experimentally consistent outcomes. Therefore, the reactions that can consume CH3 and produce C2H2 are
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explored. One of the important reactions in interstellar chemistry studies is the electron-ion recombination [47]: C 2 H 3 þ þ e→C 2 H 2 þ H;
ð3Þ
In this case the electron-ion recombination reaction (3) could lead to the C2H2 production. In the following the possibility of C2H+ 3 production + by ion-radical reactions of CH3 with C+, CH+, CH+ 2 and CH3 is investigated. The possible reactions are: 8 þ C → C 2 H3 þ > > < CH þ → C 2 H3 þ þ H CH 3 þ > C 2 H3 þ þ H2 CH2 þ → > : CH3 þ → C 2 H3 þ H2 þ H:
ð4Þ
It should be noticed that the effect of each of the above reactions is examined on the final results of calculations by varying the rate constants. Among these reactions only the first one produces considerable amount which fortunately has good agreement with the above experimentally measured data. This is explained in the following by considering the production of C+ and then the possible reaction is: CH 3 þ C þ þ M→C 2 H3 þ þ M;
ð5Þ
M is a molecule that in inelastic collision can absorb the excess energy of C2H+ 3 . The net result of reactions (3) and (5) can be written as CH 3 þ C þ þ e→C 2 H 2 þ H:
ð6Þ
This is a promising result, because it converts the CH3 radical into C2H2, and can lead to obtained experimental results. To the best of our knowledge the ion-radical reactions are not measured and should be calculated theoretically. In this regard, the rate of reaction (5) is investigated in more details. It is actually composed of two step bimolecular reactions as: CH 3 þ C þ → C 2 H3 þ
ð7Þ
C 2 H 3 þ þM→C 2 H3 þ M;
ð8Þ
where (C2H+ 3 )* is supposed an excited component that exceeds the bond dissociation energy of C2H+ 3 . The rate of reaction (5) can be calculated as [48]: kðT Þ ¼ K 1 ðT Þk2 ðT Þ;
ð9Þ
where K1(T) is the equilibrium constant for reaction (7) and k2(T) is the rate constant for reaction (7). For the reaction (7), the equilibrium constant can be calculated using the equation K 1 ðT Þ ¼ ð1=cÞ expðΔG=RT Þ;
ð10Þ
where cis the background gas concentration and ΔG is the free Gibbs energy difference between products and reactants that can be calculated using Gaussian software package optimization at the level of uhf/631g(d) for C+, CH3 [49]. ⁎ For the (C2H+ 3 ) it is assumed the molecule is in the first electronic excited state and is optimized at the level of CIS/6-31g (d) using Gaussian package. The calculated results are summarized in Table 3.
Fig. 5. Numerical results of densities of C2H2, C2H4 and C2H6 as a function of time for about 5 min, in the cell. Primed and unprimed notations are used for Nd:YAG and KrF laser irradiation results, respectively. The calculated results are obtained using data of Table 1 at atmospheric pressure and for dissociation channels of UV irradiation. Two and three photon absorption coefficients of methane are used in the absorption process for 248 nm and 355 nm lasers.
Table 3 Output of Gaussian package used in ΔG calculation. C+ Sum of electronic and thermal energies (Hartree/molecule)
CH3
⁎ (C2H+ 3 )
−37.301238 −39.546913 −76.856240
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According to data of Table 3 and using equation (10) the equilibrium constant is obtained asK1 = 9.7 × 10−9cm3/molecule. For the reaction (8) at the first approximation it is assumed that the reaction proceeds according to the collision frequency in hard sphere model [47]: 2
1=2
k2 ðT Þ ¼ πd ð8kB T=πμ Þ
package the required constants are calculated. The measured amounts in these experimental arrangements are in good agreement with the results of the introduced theoretical approach. Acknowledgments
;
ð11Þ
where d is the sum of the two colliding partner radii, T is the temperature and μ is the reduced mass. According to this equation k2(T) = 1.35 × 10− 8cm3/(s . mole cule) is obtained, with d = 2.33 Å, T = 298.15 °K and μ = 10.05 g/mol, which are calculated using Gaussian software package. Substituting the obtained data in equation (9) the rate constant is calculated as k = 1.3×10−12 cm6/molecule for temperature of 298.15 °K. Including reaction (6) with calculated rate constant to the neutral– neutral and ion–neutral reactions complete equations were obtained. In order to complete the numerical differential equations the rate of absorption of photons for methane must be taken into account. It is supposed that the dissociation of methane occurs based on the rate of three photon absorption process. Fig. 6 shows the result of numerical calculations based on new proposed reactions. In accordance with Fig. 6, the qualitative agreement between theory and experiments has been established and the discrepancy has been removed, introducing the ion–radical reactions. Therefore, it is shown that introducing the ion–molecule reactions in conversion mechanism leads to fruitful results. The use of hard sphere collision theory to estimate the ion molecule reaction (8) gives the lower limit to the rate constant, because the ion molecule reaction proceeds via the long range capture and likely exceeds the hard sphere rate constant. This point does not concern us, because the use of reaction rates higher than the obtained value for reaction (8) does not alter the order of the components and leads to more consistent results. That is the concentration of C2H2 becomes much more than the other two components. 5. Conclusions In this work, a combined experimental and theoretical study of the conversion of methane into higher hydrocarbons has been carried out by using nanosecond lasers at wavelengths 355 nm, 532 nm and 248 nm. The energy consumption in production of one mole C2H2 is measured as 5.8 MJ/mol, 3.1 MJ/mol and 69.0 MJ/mol, for the above mentioned wavelengths, respectively. On the theoretical study the density of new stable products is obtained using 4th order Runge–Kutta method. According to the experiments, C2H2 is the main product in all measurements which is in contrast with the yields of conventional UV sources in which C2H6 is the dominant product. Based on the introduced unique ion–radical reaction mechanism and by using Gaussian software
The authors want to thank Pars oil and gas company of ministry of oil for their support for this project through contract number PT131. We also acknowledge the research deputy of Sharif University of Technology. References: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]
Fig. 6. Numerical results of densities for stable hydrocarbons as a function of time in the cell at room temperature and atmospheric pressure. The calculated results are obtained by assumption of the dissociation rate 1.3 × 1012 molecule/s of methane, that is calculated for three photon absorption process and for data of Table 1 for 355 nm wavelength. The density of C2H6 is lower than C2H2 and C2H4 that is absent in the figure.
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