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APPLIED CHEMISTRY Density Functional Theory Study of Methanol Conversion via Cold Plasmas You Han, Jian-guo Wang, Dang-guo Cheng, and Chang-jun Liu* Key Laboratory of Green Chemical Technology, School of Chemical Engineering, Tianjin UniVersity, Tianjin 300072, People’s Republic of China
A density functional theory (DFT) study has been conducted in this work to investigate the mechanism of methanol conversion using cold plasmas. All pathways for initial methanol dissociation were analyzed. The feasibility of the production of various products, including hydrogen, carbon monoxide, and ethylene glycol (EGL), was discussed. The present DFT study confirmed that the major obstacle of methanol conversion to either hydrogen or EGL is the dissociation of methanol. After the cold plasma provides necessary energy for methanol dissociation, hydrogen and EGL can be easily obtained without any thermodynamic obstacles. The calculation shows that EGL can be synthesized from the coupling of CH2OH• radicals, which can be principally generated from methanol dissociation and from the reaction of H• + CH3OH. If the energy of electrons within cold plasmas could be controlled to be no more than 100 kcal/mol, the further dissociation of CH2OH• radicals, the re-decomposition of EGL, and the production of H2 and CO through complex steps would be effectively reduced. By this way, the yield of EGL can be increased. 1. Introduction Hydrogen production from methanol is an interesting and promising option for the energy supply of fuel cells and other applications. Compared to the developed hydrogen storage technologies, the use of methanol as an effective storage of hydrogen is much safer and less expensive. The developed hydrogen production technologies from methanol include steam reforming,1,2 partial oxidation,3 oxidative steam reforming,4 and methanol decomposition.5,6 The decomposition of methanol provides a better alternative for hydrogen production. However, catalytic methanol decomposition is normally conducted at elevated temperatures, which induces a problem during the cold startup and transients with hydrogen production from catalytic methanol decomposition. Recently, several investigations on methanol decomposition using cold plasmas have been reported.7-10 The cold plasma methanol decomposition can be operated even under ambient conditions with simple setup. A very high one-pass conversion (up to 80%) can be achieved with an input power in the range of ca. 0.04 Wh/(Ncm3 H2)8 to 6.40 Wh/(Ncm3 H2).7 An important characteristic of cold plasma is its high electron temperature (as high as 105 K), whereas the bulk gas temperature remains as low as room temperature. This characteristic makes it suitable for the activation of small molecules, such as methanol and methane,7-13 at or near ambient conditions. On the other hand, during our investigation on methanol decomposition using corona discharge,8,9 as one of the coldplasma phenomena, we achieved a coproduction of ethylene glycol (EGL). EGL is one of the top chemicals produced, with a world production value of approximately 6.5-7.0 billion U.S. dollars per year,14,15 for the production of many chemicals and materials. The global demand of EGL is growing over 2% each year.15 The manufacture of EGL on an industrial scale currently involves two steps: (1) gas-phase oxidation of ethylene to * To whom correspondence should be addressed. E-mail: ughg_cjl@ yahoo.com.
produce ethylene oxide; (2) hydration of ethylene oxide to yield EGL, with diethylene, triethylene, and tetraethylene glycols as coproducts. However, there are some drawbacks with this commercial production of EGL, including (1) the production is an energy-intensive process, with which most of the energy is consumed during the cracking process for the production of the raw ethylene; (2) the oxidation of ethylene to produce ethylene oxide is not a process-friendly or environmentally friendly reaction; and (3) ethylene oxide is a dangerous chemical and requires special care during the production. Moreover, the world petroleum reservoir would be consumed in the next decades. It is emergent to develop alternative processes to produce EGL. Because the EGL molecule contains two hydroxymethyl (CH2OH•) groups and CH2OH• can be generated from methanol dissociation, it will be very promising if EGL could be produced from the coupling of methanol. This will not only be a simple and direct production but also use methanol as feedstock, which can be much more easily obtained from many resources, including coal, natural gas, oil, refinery gas, and, especially, renewable resources (such as wood, municipal solid wastes, and sewage). However, there is still no other reported theoretical and experimental works on EGL synthesis directly from methanol, to the knowledge of the author. Although the present yield of EGL from plasma methanol conversion was still less than that of H2,8,9 the possibility of EGL synthesis directly from methanol has been demonstrated. In this work, we attempt to perform a density functional theory (DFT) study to analyze the reaction mechanism or pathways of methanol conversion using cold plasmas. All the possibilities of methanol dissociation have been considered, to further improve either the hydrogen production or the EGL yield using cold plasmas. 2. Computational Details All the calculations were performed using the Materials Studio DMol3 program16-18 (version 3.1) from Accelrys. A double numerical plus polarization (DNP) basis set has been
10.1021/ie060132m CCC: $33.50 © 2006 American Chemical Society Published on Web 04/06/2006
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3461 Table 1. Theoretical and Experimental Energies for Selected Species, Relative to Methanol Theoretical Energy (kcal/mol) species CH3OH CH3O• + H• CH2OH• + H• CH3• + OH• CH2O + H2 trans-HCOH + H2 cis-HCOH + H2 1CH + H O 2 2 vdw CH2O + 2H• TS1 TS2 TS3 TS4 TS5 TS6 TS7
This E0K 0.00b 100.56 94.35 89.54 17.9 72.98 76.56 88.72 82.33 122.31 88.53 86.71 84.55 83.88 131.11 129.7 126.81
Worka E298.15K
Walch23,24
Harding et al.25
Chang et al.26
Xia et al.27
0.00c 100.78 94.57 81.51 10.89 65.93 69.50 78.15 80.93 122.96 88.62 85.02 84.45 83.60 131.40 129.04 126.78
0.00
0.00
0.00 104.9 96.2 91.9 18.1 70.5 74.8 90.3 82.7
18.1 71.1f
16.4 71.1f
0.00 100.6 92.9 88.3 18.3 70.6f
88.8
94.9
93.3
91.9
96.5
119.4 90.9
85.0f 82.3
91.0f 80.7
85.5f 104.9 131.6 130.96 124.5
experimental energy at 0 K (kcal/mol) 0.00 96.2d 90.2e 18.7e 89.6e,g
90.6 88.1 85.5 84.1
a Zero-point energies are included. b The total energy of methanol at 0 K is -115.86 hartree. c The total energy of methanol at 298.15 K is -115.89 hartree. d From ref 30. e From ref 28. f The energies correspond to HCOH. g From ref 29.
used to describe the valence orbitals of the O, C, and H atoms. The accuracy of the DNP basis set has been analyzed in detail by Delly.16 The nonlocal exchange and correlation energies of the reactant, intermediates, transition states, and products of methanol conversion were calculated with the Perdew-Wang (PW91) functional19,20 of the generalized gradient approximation (GGA). The convergence criteria consisted of threshold values of 1 × 10-5 hartree, 0.002 hartree/Å, and 0.005 Å for energy, force, and displacement convergence, respectively, whereas a self-consistent-field (SCF) density convergence threshold value of 1 × 10-6 hartree was specified. A Fermi smearing of 0.005 hartree was used to improve computational performance. The geometries of all stationary points were fully optimized at this level. Frequency analysis at the same level determines the nature of the stationary points, e.g., minimum structures without imaginary frequencies, and each transition state with one imaginary frequency. To be consistent with the experimental temperature in our previous work,8,9 zero-point vibrational energies and thermal contributions to the free energy at 298.15 K were also calculated. The linear synchronous transit (LST) and quadratic synchronous transit (QST) methods21 were used to study the transition state. The transition state was confirmed using the nudged elastic band (NEB) method.22 3. Results and Discussion 3.1. Initiation of Methanol Dissociation. The relative energies of the reactant, intermediates, and transition states of methanol dissociation with zero-point energy corrections are exhibited in Table 1. The table includes not only the present calculated values, but also the recent theoretical and experimental results reported by others.23-30 It shows that the present calculated energies, relative to methanol, agree well with the experimental data and the calculated results that have been reported by others. The difference between the present calculated energies and experimental data is less than (2 kcal/mol. These indicate that the energies calculated at the GGA/PW91/DNP level are reasonable. There are seven initial reaction channels of methanol dissociation, according to the previous studies:24,26,27,31,32
CH3OH f CH3• + OH•
(1)
f CH2OH• + H•
(2)
f CH3O• + H•
(3)
f 1CH2 + H2O
(4)
f trans-HCOH + H2
(5)
f cis-HCOH + H2
(6)
f CH2O + H2
(7)
The pathways based on the ground state are also shown in Figure 1. The three reaction channels (reactions 1-3) directly generate CH3• + OH•, CH2OH• + H•, and CH3O• + H•, respectively. The other four pathways have transition states. Particularly, a van der Waals (vdw) complex is first formed from methanol via TS4 in channel 4 before the production of 1CH + H O. In addition to 1CH , CH has another low-lying 2 2 2 2 state, 3CH2.24 For 1CH2, the two C lone-pair orbitals are singletpaired, whereas 3CH2 has two orthogonal high-spin coupled singly occupied orbitals. Compared to 1CH2 + H2O, the production of 3CH2 + H2O by the singlet to triplet surface crossing in the vicinity of the vdw complex can be neglected.27 Previous studies have shown that the CH3• + OH• channel is dominant at moderate temperature (ca. 1000 K)32,33 and the trans-HCOH+H2 channel becomes important when the temperature increases.30 Xia et al.27 also reported that the product branching ratios are strongly pressure-dependent and the production of CH3• + OH• and 1CH2 + H2O is dominant under high pressures (P > 103 Torr) and low pressures (P < 5 Torr), respectively. However, in our previous work,8,9 methanol is dissociated by using corona plasma under ambient conditions. Because there are a large number of electrons in corona discharges with energies in the range of 23.06-230.61 kcal/ mol, it is not difficult to induce the decomposition of methanol to CH3• + OH•, CH2OH• + H•, CH3O• + H•, 1CH2 + H2O, trans-HCOH + H2, cis-HCOH + H2, and CH2O + H2 through the seven channels that have been mentioned previously. Among these products, the energy of the CH3O• + H• mechanism is the highest, which is followed by that of the CH2OH• + H• mechanism. The third is the energy of the CH3• + OH•
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Figure 1. Initial reaction pathways of methanol dissociation (at 298.15 K).
Figure 2. Trans-HCOH dissociation channels on the CH2O ground-state potential energy surface (at 298.15 K).
mechanism, which is approximately equal to that of 1CH2 + H2O and is slightly higher than that of TS1, TS2, and TS3, which correspond to the translation states in channels 7, 6, and 5 (see Figure 1). These results indicate that the production of trans-HCOH + H2 is the most energetically favored, whereas the least energetically favored mechanism is CH3O• + H•. Furthermore, the methoxy radical is readily converted to hydroxymethyl via TS5. Moreover, CH3O• and CH2OH• can be also dissociated to formaldehyde, via TS7 and TS6, respectively, and the energy requirement for them is 26.00 and 35.37 kcal/mol (at 298.15 K), respectively. In contrast, the decomposition reactions require much more energy:
CH3O• f CH3• + O •
CH2OH f CH2 + OH
(-105.96 kcal/mol at 298.15 K) (8) •
(-114.94 kcal/mol at 298.15 K) (9)
Thus, CH3• + O and CH2 + OH• are much more difficult to form than CH2O + H• from the dissociation of CH3O• and CH2OH• radicals.
3.2. Pathways for the Formation of Carbon Monoxide and Hydrogen. The aforementioned analysis shows that a great amount of trans-HCOH, cis-HCOH, and CH2O are generated from the initial dissociation of CH3OH through channels 5, 6, and 7. In addition, some of the CH2O is produced from the secondary dissociation of CH3O• and CH2OH•. Furthermore, trans-HCOH, cis-HCOH, and CH2O can be decomposed to carbon monoxide and hydrogen through the pathways shown in Figure 2. The relative energies of these species that appear in these pathways are shown in Table 2, which also include the reported experimental and theoretical results.34-39 Be aware that trans-HCOH, cis-HCOH, and CH2O are isomers of each other and rearrangement reactions could occur:
trans-HCOH f CH2O
(10)
trans-HCOH f cis-HCOH
(11)
The calculated barriers for reactions 10 and 11 via TS8 and TS9 are 27.03 and 21.93 kcal/mol (at 298.15 K), respectively. Then, cis-HCOH and CH2O are both dissociated to CO and H2 via TS10 and TS11 with barriers of 47.21 and 81.03 kcal/mol
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3463 Table 2. Theoretical and Experimental Energies for Selected Species, Relative to Formaldehyde Theoretical Energy (kcal/mol) This
Worka
species
E0K
E298.15K
Bauerfeldt et al.34
Jalbout et al.35
CH2O trans-HCOH cis-HCOH H2 + CO HCO• + H• CO + 2H• TS8 TS9 TS10 TS11
0.00b 56.37 59.92 5.22 86.03 99.65 83.44 78.35 107.2 81.16
0.00c 56.31 59.83 3.75 86.39 101.20 83.34 78.24 107.04 81.03
0.00
0.00 52.57 56.75 1.04
experimental energy (kcal/mol) 0.00 54.0d 86.4e
86.6 84.9
83.00 79.06 104.27 79.19
86.7
80.6f
Zero-point energies are included. The total energy of formaldehyde at 0 K is -114.67 hartree. The total energy of formaldehyde at 298.15 K is -114.69 hartree. d From ref 36. e From ref 37. f From refs 38 and 39. a
b
c
Figure 3. Structures of the transition states (yellow lines denote the bonds that are broken and formed during reactions).
(at 298.15 K), respectively. Besides, another main product from formaldehyde dissociation is H• + HCO•, which further produces
CO and H•, as shown in Figure 2. All structures of the transition states are shown in Figure 3.
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Figure 4. Further dissociation pathways of the methyl radical (at 298.15 K).
In addition, there is another pathway to produce CO and H2 from CH2 and O:
Table 3. Enthalpy of Reaction (∆H) for the Synthesis of EGL and Other Products PW91/DNPa
CH2 + O f CO + H2 (+188.92 kcal/mol at 298.15 K) (12) The O atoms can be mainly generated from reaction 8, whereas the source of CH2 is more complex: (1) from the initial CH3OH dissociation through reaction channel 4; (2) from the further decomposition of the methyl radical (see Figure 4); (3) from the dissociation of CH2OH• via reaction 9. Because of the limitation of the oxygen amount and the presence of the competitive recombination reactions for CH2 and other radicals such as OH• and CH3• (which will be discussed in a later section), the yield of carbon monoxide and hydrogen from reaction 12 would be much less than that from the pathways shown in Figure 2. There is a considerable number of H• radicals produced from many dissociation reactions such as reactions 2, 3, and so on. Therefore, the recombination of the H• radicals to H2 is another major source of hydrogen products. 3.3. Pathways for the Formation of Ethylene Glycol. During methanol decomposition using corona discharges,8,9 ethylene glycol (EGL) is mainly formed from the combination (or coupling) of two CH2OH• radicals:
CH2OH• + CH2OH• f HOCH2-CH2OH (+66.43 kcal/mol at 298.15 K) (13) As mentioned in the previous section, CH2OH• can be generated from the initial dissociation of methanol. In addition, a great amount of H• radicals exist in the discharge region. These hydrogen radicals are produced from the initial dissociation reactions and can also react with methanol to generate CH2OH• radicals:
CH3OH + H• f CH2OH• + H2 (+10.62 kcal/mol) (at 298.15 K) (14) This is an exothermic reaction via TS12, and the activation barrier for it is 11.78 kcal/mol (at 298.15 K). The reported studies40-42 on the reactions between H• radicals and methanol
∆H0K reaction
(kcal/mol)
∆H298.15K (kcal/mol)
CH2OH• + CH2OH• f HOCH2-CH2OH CH3CH2• + OH• f CH3CH2OH CH3• + CH2OH• f CH3CH2OH CH3CH2CH2• + OH• f CH3CH2CH2OH CH3CH2• + CH2OH• f CH3CH2CH2OH CH3• + CH3• f CH3CH3 CH3CH2• + CH3• f CH3CH2CH3 CH3O• + CH3• f CH3OCH3 CH• + CH• f C2H2 CH2 + CH2 f C2H4 C2H4 + CH2 f C3H6
-77.12 -94.27 -84.58 -94.58 -80.11 -90.63 -87.00 -77.92 -235.48 -173.56 -101.80
-66.43 -85.83 -74.64 -82.86 -69.17 -81.22 -77.07 -67.74 -227.53 -164.66 -92.10
a
Zero-point energies are included.
indicated that H• normally attacks the methyl hydrogen rather than the alcoholic hydrogen in methanol. Thus, CH2OH• is the main product of the hydrogen abstracting reaction of CH3OH + H•. The reaction enthalpy for the synthesis of EGL through reaction 13 is negative, as shown in Table 3, which indicates that the reaction route is thermodynamically favorable. Furthermore, there is no energy barrier present for reaction 13. This suggests that it is feasible to synthesize EGL directly from methanol, if CH2OH• can be effectively and efficiently generated from the dissociation of methanol. 3.4. Pathways for the Formation of Other Products. Based on the aforementioned analyses, many H2O and CH3• radicals are produced from the initial dissociation of methanol, and there are also some CH3• radicals from reaction 8. Under the collision of the high-energy electrons within corona discharge plasmas, H2O and methyl radicals can be further dissociated to CH2, OH•, and H•:
H2O f OH• + H• CH3• f CH2 + H•
(-118.73 kcal/mol at 298.15 K) (15) (-101.62 kcal/mol at 298.15 K) (16)
Of course, CH2 can be further dissociated to CH• and even
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Figure 5. Scheme of the entire process of methanol dissociation. Atoms are as given in Figure 3.
carbon. However, the amount of CH• and carbon is much less than that of CH2, because the formation of CH• and carbon requires greater energy input and more reaction steps, as shown in Figure 4. According to the radical chain mechanism, the small fragments (e.g., OH•, CH3•, CH2, CH3O•, and so on) in the discharge region can be recombined with each other to generate higher hydrocarbons, alcohols, and ethers (such as ethane, propane, ethylene, propylene, ethanol, propanol, and dimethyl ether), as shown in Table 3. It is suggested that these recombination reactions are thermodynamically favored. 3.5. Factors Influencing the Product Distribution. The scheme of the entire process of methanol dissociation (as shown in Figure 5) shows that there are many pathways to produce CO and H2. Moreover, these reaction channels occur easily under the conditions of cold plasmas that possess many electrons with energies of 23.06-230.61 kcal/mol. Thus, H2 and CO would be the major product of the methanol decomposition under the collision of these high-energy electrons. Because EGL is mainly generated from CH2OH• radicals and the reaction is exothermic, the EGL synthesis directly from methanol is feasible under the conditions of cold plasmas. The stability of CH2OH• and EGL in the discharge region is the key to decide the yield of EGL from methanol. In our previous experimental studies,8,9 we found that the waveforms affected the generation rates of the products. When the direct current (DC) corona discharge was used to decompose methanol, the positive DC corona discharge (DC-pos) produced much more EGL, ethanol, and propanol than the negative DC corona plasma (DC-neg),
Table 4. Effect of Waveforms on CH3OH Decompositiona Generation Rate (mmol/min)
Production Rate (µmol/min)
waveform
H2 yield (%)
H2
CO
ethylene glycol, EGL C2H5OH C3H7OH
DC-pos DC-neg sinusoid sinusoid triangle
38.5 46.3 70.4 73.5
0.42 0.57 0.64 0.70
0.17 0.25 0.33 0.39
0.01 trace 1.87 0.02
0.06 0.01 3.63 0.04
0.78 0.12 8.65 0.03
a Data taken from ref 9. Methanol feed concentration ) 20%; argon flow rate ) 40 NmL/min; and average AC voltage, 0.80 kV.
whereas the generation of H2 and CO is less than that observed when using DC-neg, as shown in Table 4. The reason is that the energy of the electrons within DC-pos is less than that in DC-neg. Therefore, the big fragments (such as CH2OH•) and the large EGL molecules are not easily broken with it. The production rate of EGL using DC-pos then is much more than that using DC-neg. The same phenomena occurred with using different alternating current (AC) corona discharge. Because the sinusoid (AC-sin) waveform possesses a longer discharge time than the sinusoid triangle (AC-sin-tri) waveform in every cycle, the discharge voltage of the former is lower, which means the electrons in the AC-sin corona plasma has lower energies than that in the AC-sin-tri corona plasma. Thus, the production of EGL using AC-sin is much greater than that using AC-sin-tri. These results demonstrated that the energies of the electrons
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within the plasma have an important effect on the stability of CH2OH•, which further influences the yield of EGL. Our results show that the production of CH2OH• from methanol dissociation requires 94.57 kcal/mol at 298.15 K (see Table 1), and the experimental30 and other calculated values26,27,43,44 are all
