Pyrolysis of mixtures of methane and ethane: activation of methane with the aid of radicals generated from ethane

Abstract

Direct chemical conversion of methane (CH₄) has been actively researched in order to use natural gas as a chemical resource. However, the high stability of CH₄ molecules hinders the chemical conversion of CH₄. In this study, we investigated pyrolysis of mixtures of CH₄ and ethane (C₂H₆) at 973–1073 K. Even though CH₄ alone did not react in the temperature range, mixtures of CH₄/C₂H₆ and of Ar/C₂H₆ showed different pyrolysis behaviours; the co-existence of CH₄ significantly increased yields of propylene (C₃H₆), propane (C₃H₈) and toluene. Mass spectrometry analysis using ¹³C-labeled CH₄ revealed that carbon contained in CH₄ was incorporated into the pyrolysis products. The results suggested that CH₄ was activated with the aid of C₂H₆. We assumed that CH₄ was attacked by radical species generated from pyrolysis of C₂H₆ and was converted into methyl radicals. The CH₄-derived methyl radicals were incorporated into pyrolysis products via radical reactions. This study clarified that CH₄ can be activated by radicals generated from co-existing molecules without the help of catalysts or extremely high temperature.

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Introduction

Innovations in shale gas extraction technology are boosting the use of natural gas in various fields. Natural gas is expected to play an active role as a raw material for the chemical industry. This is because petroleum, which is a fossil resource that plays a major role in the current chemical industry, is depleting rapidly. Methane (CH₄), a main component of natural gas, is industrially used to produce synthesis gas via steam reforming. In addition, ethane (C₂H₆), a secondary component of natural gas, can be converted to ethylene (C₂H₄) via thermal cracking. Steam reforming and thermal cracking are typical examples of methods of chemical conversion of natural gas, however, although industrial interest in natural gas conversion has intensified, most natural gas is presently used as fuel for thermal power generation etc. In other words, natural gas plays a more limited role in the current chemical industry compared with petroleum.

The reason is attributed to the high stability of the CH₄ molecule; the high stability of CH₄ hampers the chemical conversion processes for natural gas.¹ Thus, the development of technology to convert natural gas into essential chemicals is being intensively researched. In particular, direct conversion processes of CH₄ such as coupling to produce lower olefins^2–7 and aromatization^8,9 are being rigorously explored.

In contrast, Periana et al. developed a catalyst for the selective oxidation of mixtures of CH₄, C₂H₆ and C₃H₈ to alcohol esters.¹⁰ Such direct conversion of CH₄-based hydrocarbon mixtures is interesting from the viewpoint of direct utilization of natural gas because natural gas is a CH₄-based hydrocarbon mixture (C₂H₆ and C₃H₈ are present as secondary and tertiary components). In related works, dehydrogenative conversion of lower olefins containing CH₄ using zeolite-supported catalysts has also been investigated.^10–15

However, the chemical conversion of CH₄-based hydrocarbon mixtures is not easy because the reactivities of CH₄ and other hydrocarbons are quite different; in brief, CH₄ is much more stable than other hydrocarbons. Pyrolysis of CH₄ has been intensively investigated,^16–20 and among various hydrocarbons, CH₄ requires extreme high temperatures (>approx. 1473 K) for pyrolysis reactions.^1,18 However, such high temperatures are too severe for other hydrocarbons. For most hydrocarbons, coke should be formed by deep dehydrogenation under the conditions appropriate for pyrolysis of CH₄. Conversely, reaction conditions suitable for pyrolysis of other hydrocarbons such as C₂H₆ and C₃H₈ are too mild to activate CH₄ molecules, indicating CH₄ does not react.

So far, most studies have focused on catalysts that enable the direct activation of CH₄. However, indirect activation routes would be feasible; for example, OH radicals generated from H₂O on a molten salt catalyst activate CH₄ molecules and promote the coupling of CH₄.^21,22 In addition, CH₄ activation by gas phase atomic clusters has been investigated.²³ Thus, if active radical species can be generated, CH₄ would be activated with the aid of the radicals. In this study, pyrolysis of mixtures of CH₄ and C₂H₆ was examined. We clarified that CH₄ was activated by C₂H₆-derived radicals; as a result, even under mild conditions where CH₄ alone did not react, CH₄ molecules played a role in the pyrolysis reaction and were incorporated into pyrolysis products.

Experimental

Pyrolysis reactions

Schematic diagram of reactor system is shown in Fig. 1. Pyrolysis reactions were carried out with a quartz reactor (i.d. = 10 mm, o.d. = 12 mm) that was heated with an electric furnace. The temperature profile for the furnace is shown in Fig. 1. Flow rates of CH₄ (99.999%), C₂H₆ (99.7%), and Ar (99.99%) were controlled by using mass flow controllers. The reactor was purged by flowing Ar for 10 min, and then heated to reaction temperature (973–1073 K). By introducing mixtures of CH₄ and C₂H₆ or of Ar and C₂H₆, pyrolysis was carried out for 1 h. Hereafter, mixtures of CH₄ and C₂H₆ and of Ar and C₂H₆ will be denoted as CH₄/C₂H₆ and Ar/C₂H₆, respectively. Volume fractions of C₂H₆ in the gas mixtures were adjusted to 0.17, 0.25, and 0.50, and flow rates of the gas mixtures were controlled at 10, 30, and 50 mL min⁻¹. The flow rates of the outlet gas were measured by a soap film flowmeter.

Fig. 1 Schematic diagram of reaction system and the temperature profile for the furnace.

During the pyrolysis reaction, outlet gas of 0.5 mL was collected with a gas-tight syringe and injected into gas chromatographs every 15 min. For H₂, CH₄, and C₂H₆, a gas chromatograph (Shimadzu GC-8A, TCD) equipped with a packed column (Active carbon) was used at 473 K (injection/detector) and 443 K (column) under flowing Ar as a carrier gas. For C₂H₄, C₂H₂, and C₃ hydrocarbons, a gas chromatograph (Shimadzu GC-8A, FID) equipped with a packed column (Unibeads 1S) was used at 453 K (injection/detector) and 383 K (column) under flowing N₂ as a carrier gas. To quantify gaseous products, calibration curves for all products were prepared by injecting different volume of the gases. The gases were collected from a gas cylinder (99.999% H₂, 99.999% CH₄, and 99.7% C₂H₆) or a gas can (GL Science Inc.; 99.5% C₂H₄, 0.100% C₂H₂, 99.5% C₃H₆, and 99.5% C₃H₈). Formation rates of the products were calculated based on the flow rates and the GC analysis.

Aromatics (benzene, toluene, styrene, and naphthalene) formed by the pyrolysis were collected in a glass trap cooled with a dry ice/ethanol bath. After the reaction, the trapped aromatics were dissolved in acetonitrile and then 30 mM butyl acetate in acetonitrile (1 or 2 mL) was added as an internal standard. The obtained solution was injected into a gas chromatograph (Shimazu, GC-18A) equipped with a capillary column (Shinwa chemical industries ltd., ULBON HR-1, 0.25 mm i.d., 30 m) under flowing N₂ as a carrier gas. Temperature for injection/detector was settled at 523 K and temperature for column was raised from 313 K to 473 K at 10 K min⁻¹. To quantify the aromatics, solutions containing different amounts of benzene (99.5%, Kanto Chemical Co., Inc.), toluene (99.5%, Kanto Chemical Co., Inc.), styrene (99.0%, Kanto Chemical Co., Inc.), naphthalene (98.0%, Kanto Chemical Co., Inc.), and butyl acetate (99.0%, FUJIFILM Wako Pure Chemical Corporation) in acetonitrile (99.5%, Kanto Chemical Co., Inc.) were prepared and calibration curves were prepared by injecting the solutions into the GC.

Conversions of CH₄ and C₂H₆ were calculated based on gas composition analysed by GC and flow rates of mixture gas. Mass balance was calculated from ratios of carbon atoms for components of feed and outlet gases and trapped solutions. Equilibrium conversion and standard free energy of formation for hydrocarbons was calculated using HSC Chemistry (Outotec).

Pyrolysis reactions using ¹³C-labeled CH₄

Pyrolysis reactions using ¹³C-labeled CH₄ (¹³CH₄; Watari Co. Ltd.) were carried out on a closed gas-circulation system (224 mL) equipped with an electric furnace. The closed gas-circulation system was mainly made of Pyrex glass and a reactor that was heated with an electric furnace was made of quartz glass. After evacuating the closed gas-circulation system, ¹³CH₄/C₂H₆ (20 kPa/20 kPa) or He/C₂H₆ (20 kPa/20 kPa) were introduced. The reactor was heated to 1073 K under circulating the gas mixtures and maintained for 2 h. During the reaction, the aromatics were collected in a glass trap cooled with a dry ice/ethanol bath. After the reaction, the trapped aromatics were dissolved in acetonitrile and mass spectra of the aromatics were obtained with GC-MS (Bruker, SCION SQ) equipped with a capillary column (Bruker, BR-5 ms, 0.25 mm i.d., 30 m).

Results and discussion

In this study, pyrolysis reactions of two different gas mixtures (i.e., CH₄/C₂H₆ and Ar/C₂H₆) were carried out by controlling three reaction parameters: (1) reaction temperature, (2) gas flow rate, and (3) gas composition. Consequently, 55 experimental results were obtained and are summarized in Table 1. In these reactions, the pyrolysis products were hydrogen (H₂), CH₄, C₂H₄, acetylene (C₂H₂), propylene (C₃H₆), C₃H₈, benzene (C₆H₆), toluene (C₇H₈), styrene (C₈H₈), and naphthalene (C₁₀H₈). Coke was also produced depending on the reaction conditions. Note that CH₄ conversions under CH₄/C₂H₆ conditions were calculated based on the amount of CH₄ in the inlet and the outlet gases, therefore, when CH₄ was formed from C₂H₆ (eqn (1) and (2)), the amount of CH₄ in the outlet gas increased so that the CH₄ conversion became negative.

C₂H₆ → 2CH₃˙ (1)

CH₃˙ + C₂H₆ → CH₄ + C₂H₅˙ (2)

Table 1 All results of pyrolysis reactions in this study

Entry	Reaction temp./K	Reactant gas	Volume fraction of C2H6	Flow rate/mL min^−1	Yield/μmol h^−1											Conv.^b/%		Mass balance/%
					H2	CH4^a	C2H6	C2H4	C2H2	C3H8	C3H6	C6H6	C7H8	C8H8	C10H8	CH4	C2H6
a CH4 yields for CH4/C2H6 conditions contained not only formed CH4 but also feed CH4. b Average value in pyrolysis for 1 h. c Although C2H6 and C2H4 was formed by dehydrogenation reactions, H2 could not be detected because hydrocarbon was analysed by using GC-FID at the highest sensitivity but H2 was analysed by using GC-TCD. d The CH4 conversion was calculated on the basis of the yields of C2H6 and C2H4.
1	973	Ar/C2H6	0.5	10	5011	731	6978	4258	11	4	76	20	2	1	0.3	n/a	43.9	94.2
2	973	CH4/C2H6	0.5	10	5138	13 356	7444	4496	13	12	109	16	2	1	0.4	−1.7	42.3	96.7
3	1023	Ar/C2H6	0.5	10	7457	2010	3360	5418	28	7	121	182	14	14	12	n/a	73.1	85.1
4	1023	CH4/C2H6	0.5	10	7900	15 746	3607	5668	29	16	181	202	15	12	12	−20.1	72	93.2
5	1073	Ar/C2H6	0.5	10	8764	5831	1238	3882	57	3	60	508	20	31	62	n/a	90.1	80.5
6	1073	CH4/C2H6	0.5	10	9648	18 610	1524	4204	54	7	109	522	30	34	58	−44.7	88	89.8
7	973	Ar/C2H6	0.25	10	2898	319	3346	2678	11	1	32	6	0.4	n.d.	n.d.	n/a	47.5	97.9
8	973	CH4/C2H6	0.25	10	2710	19 680	3393	2481	12	8	63	5	0.4	n.d.	n.d.	2.1	46.8	96.0
9	1023	Ar/C2H6	0.25	10	4494	1559	1157	3364	26	2	52	88	5	6	7	n/a	79.6	89.5
10	1023	CH4/C2H6	0.25	10	4590	20 406	1485	3331	25	9	123	86	8	5	5	−1.2	76.8	94.2
11	1073	Ar/C2H6	0.25	10	5280	1819	311	2463	50	1	28	239	8	14	30	n/a	94.3	73.0
12	1073	CH4/C2H6	0.25	10	6029	21 788	612	2672	47	4	90	251	17	15	24	−8.4	90.4	92.7
13	973	Ar/C2H6	0.17	10	1916	165	1953	1771	10	0.4	15	4	0.3	n.d.	n.d.	n/a	53.7	91.1
14	973	CH4/C2H6	0.17	10	1873	22 490	2259	1619	9	6	47	2	0.3	n.d.	n.d.	1.2	46.1	97.2
15	1023	Ar/C2H6	0.17	10	3071	811	693	2238	24	1	25	60	2	2	4	n/a	83.5	85.3
16	1023	CH4/C2H6	0.17	10	3404	23 104	947	2382	26	6	103	47	6	4	3	−0.9	77.4	97.3
17	1073	Ar/C2H6	0.17	10	3599	1571	209	1667	47	0.2	14	129	3	6	14	n/a	95	75.7
18	1073	CH4/C2H6	0.17	10	4568	23 411	334	1984	48	3	81	154	12	9	16	−2.2	92	94.4
19	973	Ar/C2H6	0.5	30	9882	473	27 007	8797	25	2	68	1	n.d.	n.d.	n.d.	n/a	26.2	98.7
20	973	CH4/C2H6	0.5	30	10 149	39 915	26 705	8386	25	16	103	1	n.d.	n.d.	n.d.	−1.5	27.3	97.9
21	1023	Ar/C2H6	0.5	30	20 085	3170	13 316	17 459	80	17	332	132	5	3	3	n/a	61.7	96.0
22	1023	CH4/C2H6	0.5	30	21 066	41 303	13 881	16 799	82	43	450	123	12	7	3	−5.9	61.3	94.8
23	1073	Ar/C2H6	0.5	30	27 284	7769	5463	17 287	209	13	314	756	17	24	34	n/a	84.9	81.9
24	1073	CH4/C2H6	0.5	30	27 551	47 205	5678	18 353	206	33	537	736	24	27	28	−20.6	84.1	92.1
25	973	Ar/C2H6	0.25	30	5687	227	12 796	5395	24	1	28	n.d.	n.d.	n.d.	n.d.	n/a	31	98.8
26	973	CH4/C2H6	0.25	30	5152	59 286	13 357	4871	24	16	68	n.d.	n.d.	n.d.	n.d.	1.9	30.1	97.7
27	1023	Ar/C2H6	0.25	30	12 473	1368	6223	10 941	74	5	133	68	7	5	2	n/a	65.7	101.4
28	1023	CH4/C2H6	0.25	30	11 930	59 433	6637	9604	64	29	299	46	6	3	0.8	1.3	65.1	95.0
29	1073	Ar/C2H6	0.25	30	16 412	4165	2047	10 793	199	3	130	292	3	6	12	n/a	88.6	89.8
30	1073	CH4/C2H6	0.25	30	17 353	62 212	2355	10 650	185	18	385	252	7	7	19	−3.3	87.5	93.2
31	973	Ar/C2H6	0.17	30	3817	137	8570	3838	23	0.4	16	n.d.	n.d.	n.d.	n.d.	n/a	32.7	98.4
32	973	CH4/C2H6	0.17	30	3214	64 877	9227	3024	22	11	47	n.d.	n.d.	n.d.	n.d.	−0.4	27.8	99.4
33	1023	Ar/C2H6	0.17	30	8998	1045	3574	7723	68	3	23	46	3	3	1	n/a	76.8	88.8
34	1023	CH4/C2H6	0.17	30	9164	64 408	4536	7323	62	22	263	33	4	2	0.5	−0.8	68.9	98.3
35	1073	Ar/C2H6	0.17	30	11 468	2966	987	7602	185	1	19	202	2	4	15	n/a	99.1	77.5
36	1073	CH4/C2H6	0.17	30	13 015	64 175	876	8141	167	13	353	180	6	5	10	−0.7	94.3	94.0
37	973	Ar/C2H6	0.5	50	12 156	415	50 438	11 555	41	1	53	n.d.	n.d.	n.d.	n.d.	n/a	20.5	98.4
38	973	CH4/C2H6	0.5	50	12 481	67 289	50 763	11 319	38	20	84	n.d.	n.d.	n.d.	n.d.	−0.3	19.3	99.4
39	1023	Ar/C2H6	0.5	50	32 276	2931	29 434	28 272	112	20	412	49	3	1	0.6	n/a	53.3	94.9
40	1023	CH4/C2H6	0.5	50	33 106	69 008	29 123	27 750	112	70	596	49	2	1	0.6	−2.8	53.4	96.0
41	1073	Ar/C2H6	0.5	50	46 302	8525	11 065	34 253	391	25	599	611	10	12	20	n/a	82.3	83.6
42	1073	CH4/C2H6	0.5	50	48 454	76 300	12 148	34 998	390	64	972	594	14	14	20	−13.6	80.7	92.4
43	973	Ar/C2H6	0.25	50	7165	215	25 122	7180	37	0.5	25	n.d.	n.d.	n.d.	n.d.	n/a	23.7	98.5
44	973	CH4/C2H6	0.25	50	6892	98 540	24 831	5894	37	20	61	n.d.	n.d.	n.d.	n.d.	1.1	24.1	97.1
45	1023	Ar/C2H6	0.25	50	19 571	1796	12 678	16 951	101	7	171	37	2	2	0.5	n/a	61.5	94.0
46	1023	CH4/C2H6	0.25	50	18 142	100 294	14 487	13 050	85	46	333	20	2	1	n.d.	−0.6	55.8	94.9
47	1073	Ar/C2H6	0.25	50	27 145	5836	4560	19 518	315	7	239	278	2	4	11	n/a	86.1	86.6
48	1073	CH4/C2H6	0.25	50	28 294	103 283	5319	19 925	312	39	722	231	5	4	7	−5.4	83.3	98.0
49	973	Ar/C2H6	0.17	50	4429	148	14 111	4620	36	0.2	12	n.d.	n.d.	n.d.	n.d.	n/a	32.2	90.4
50	973	CH4/C2H6	0.17	50	3599	105 133	16 386	3948	34	15	95	n.d.	n.d.	n.d.	n.d.	2.7	23.7	96.4
51	1023	Ar/C2H6	0.17	50	12 528	936	7243	10 989	87	3	81	23	0.7	1	0.3	n/a	65.2	91.0
52	1023	CH4/C2H6	0.17	50	11 820	109 066	9741	10 206	73	37	305	8	0.3	n.d.	n.d.	−0.5	54.9	99.0
53	1073	Ar/C2H6	0.17	50	18 047	3318	1941	13 027	303	2	113	192	0.9	2	10	n/a	90.6	85.0
54	1073	CH4/C2H6	0.17	50	19 563	109 098	3235	13 611	266	24	595	125	3	2	4	−0.7	85	96.2
55	1073	CH4	0	10	n.d.^c	n/a	3	1	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.	0.002^d	n/a	102.0

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Before considering the main subject (i.e., activation of CH₄ with the aid of C₂H₆), the basic effects of the reaction parameters on the pyrolysis behaviour will be considered.

Effect of reaction temperature

To discuss the effect of reaction temperature, we used several results of Ar/C₂H₆ conditions as examples (entries 1, 3, 5 in Table 1). C₂H₆ conversions increased to 44, 73, and 90% as reaction temperature increased to 973, 1023, and 1073 K, which is because the pyrolysis reactions are endothermic. Fig. 2a shows C₂H₄ and benzene yields at different reaction temperatures. Because C₂H₄ and benzene were the main products in the reaction, their yields may become higher with increasing C₂H₆ conversion with the reaction temperatures. While the benzene yield increased with the reaction temperatures, the C₂H₄ yield dropped at 1073 K. As for the formation of H₂, dehydrogenation of C₂H₆ to C₂H₄ mainly contributed at 973 K. For example, in entry 1, the yields of C₂H₄ and H₂ was 4258 and 5011 μmol h⁻¹; the amounts C₂H₄ and H₂ were similar. At higher temperature, the formation of benzene and coke also contributed to the formation of H₂.

Fig. 2 C₂H₄ and benzene yields for Ar/C₂H₆ condition at different (a) reaction temperatures, (b) flow rates and (c) volume fractions of C₂H₆. (d) C₂H₆ conversion at different flow rates and volume fractions. (a) flow rate: 10 mL min⁻¹, volume fraction of C₂H₆: 0.5; (b) reaction temperature: 1023 K, volume fraction of C₂H₆: 0.25; (c and d) reaction temperature: 973 K.

According to previous studies on CH₄ pyrolysis,^24,25 benzene is formed by the recombination of propargyl radicals (C₃H₃˙), which are generated via H abstraction reactions of C₂H₆.

C₂H₆ + H˙ → C₂H₅˙ + H₂ (3)

C₂H₅˙ → C₂H₄ + H˙ (4)

C₂H₄ + H˙ → C₂H₃˙ + H₂ (5)

C₂H₃˙ → C₂H₂ + H˙ (6)

C₂H₂ + CH₃˙ → C₃H₄ + H˙ (7)

C₃H₄ + H˙ → C₃H₃˙ + H₂ (8)

C₃H₃˙ + C₃H₃˙ → C₆H₆ (9)

In addition, there is another model that cyclopentadienyl radical is an important intermediate.^26,27

Considering that C₂H₄ is the primary product in the pyrolysis of C₂H₆,²⁸ C₂H₄ concentration should increase at high reaction temperatures because C₂H₆ conversion increased with the reaction temperature. Furthermore, as shown in eqn (3)–(9), benzene is formed via C₂H₄; therefore, it is likely that benzene formation is enhanced with increasing C₂H₄ concentration at high temperature. Consequently, the C₂H₄ yield was suppressed apparently by consuming C₂H₄ to form benzene. This is the reason why the C₂H₄ yield dropped at 1073 K.

Mass balance for carbon atoms was shown in Table 1. At 973 K, the mass balance was approx. 100%, while the mass valance decreased as increasing reaction temperature. This is because the formation of coke, indeed, the wall of the quartz reactor became black after the pyrolysis reaction at high temperature.

Effect of gas flow rate

The effect of gas flow rate on the pyrolysis of Ar/C₂H₆ was considered by using entries 9, 27, 45 in Table 1 as typical examples. As the flow rate of the reactant gas increased to 10, 30, and 50 mL min⁻¹, C₂H₆ conversions decreased to 80, 66, and 62%, respectively. The high flow rate means a short residence time of reactant gases in the reactor, which should result in the decrease in C₂H₆ conversion. Yields of lower hydrocarbons such as C₂ and C₃ increased when the flow rate was high, and conversely, yields of aromatics decreased. As typical examples, the effect of the flow rate on yields of C₂H₄ and benzene is shown in Fig. 2b. With increasing gas flow rates, the C₂H₄ yields increased and the benzene yields decreased. As described above, benzene is formed via the recombination of propargyl radicals that are successively formed by H abstraction reactions of C₂H₆. Such a successive reaction is likely to be affected by the residence time of reactant gases; long residence time is favourable for the formation of benzene. Therefore, high flow rate, that is, short residence time, resulted in increasing yield of the intermediate product, namely C₂H₄.

Effect of volume fraction of C₂H₆

The volume fraction of C₂H₆, that is the concentration of C₂H₆, is a reaction parameter in this study. As shown in Table 1, product yields became higher with higher C₂H₆ concentrations, and product distributions were almost the same regardless of the C₂H₆ concentration. Fig. 2c shows C₂H₄ and benzene yields at different volume fraction of C₂H₆ and flow rates. C₂H₄ and benzene yields tended to increase as a function of the volume fraction of C₂H₆. As described above, H abstraction from C₂H₆ provides C₂H₄ and benzene. Thus, it is reasonable that increase in volume fraction of C₂H₆ contributed to the formation of C₂H₄ and benzene. In addition, similar to Fig. 2b, as increasing flow rates, C₂H₄ yield increased and benzene yield decreased regardless of volume fraction of C₂H₆.

Fig. 2d shows C₂H₆ conversion at different volume fractions of C₂H₆ in Ar. C₂H₆ conversion was in inverse proportion to the volume fraction of C₂H₆. The dominant reaction in the pyrolysis of C₂H₆ is the dehydrogenation of C₂H₆ into C₂H₄ (C₂H₆ → C₂H₄ + H₂). The equilibrium conversions for the reaction are as follows: 71, 67, and 62% for the volume fractions of 0.17, 0.25, and 0.5, respectively. This order is in accordance with the tendency of C₂H₆ conversion shown in Fig. 2d.

Pyrolysis of CH₄/C₂H₆ mixtures

A CH₄ molecule is so stable that CH₄ alone did not react under the reaction conditions in this study. Even under the most severe condition (i.e., the highest temperature and the lowest flow rate), the CH₄ conversion was approximately 0% (entry 55 in Table 1). Thermodynamic data also suggest that CH₄ is much more stable than other hydrocarbons (Fig. 3). Thus, in this study, CH₄ is expected to behave as an inert molecule, such as Ar and He. In other words, results in pyrolysis of CH₄/C₂H₆ and Ar/C₂H₆ are assumed to be the same, however, their pyrolysis results were quite different. Fig. 4a shows C₃H₆ yields under CH₄/C₂H₆ and Ar/C₂H₆ conditions at different temperatures and flow rates. Interestingly, the C₃H₆ yields were greatly increased in the presence of CH₄. For example, under the condition at 1073 K and 50 mL min⁻¹, C₃H₆ yields for Ar/C₂H₆ and CH₄/C₂H₆ were 0.11 and 0.60 mmol h⁻¹, respectively, indicating the co-existing CH₄ promoted the C₃H₆ formation by more than five times. The same trend was observed for C₃H₈ formation.

Fig. 3 Standard free energy of formation of hydrocarbons as a function of temperature.

Fig. 4 (a) C₃H₆ yield, (b) toluene yield, (c) C₂H₆ conversion, and (d) C₂H₄ and benzene yields for CH₄/C₂H₆ or Ar/C₂H₆ condition at different reaction temperatures, different flow rates and C₂H₆ volume fraction of 0.17. Flow rate for (c and d): 30 mL min⁻¹.

Fig. 4b shows the effect of co-existing CH₄ on toluene yields. Similar to the C₃ hydrocarbons, co-existing CH₄ significantly enhanced toluene formation. For example, under the condition at 1073 K and 10 mL min⁻¹, toluene yield increased by more than four times (from 2.9 to 12.5 μmol h⁻¹).

From the perspective of the transition from petrochemical to natural gas chemistry, it is necessary to reconsider the production process of C₃H_6, which is an essential molecule in the chemical industry. In petrochemicals, C₃H₆ (and benzene, toluene, xylene) are obtained as by-products of naphtha cracking to produce C₂H₄. In contrast, C₂H₄ synthesis from C₂H₆ in natural gas produces poor by-products. In this regard, the enhancement of C₃H₆ formation by co-feed of CH₄ to C₂H₆ would be worthwhile from the viewpoint of natural gas utilization.

Fig. 4c shows C₂H₆ conversion for CH₄/C₂H₆ and Ar/C₂H₆ conditions. As described earlier, C₂H₆ conversion increases with reaction temperatures, and the co-existence of CH₄ slightly suppressed the C₂H₆ conversion. Probably, thermal cracking of C₂H₆ to form CH₄ would be inhibited by the presence of CH₄, which can be a reason why C₂H₆ conversion slightly decreased under CH₄/C₂H₆ conditions. In addition, the slight decrease in C₂H₆ conversion in the presence of CH₄ indicates that the increase in C₃ hydrocarbons and toluene is irrelevant to C₂H₆ conversion. Fig. 4d shows C₂H₄ and benzene yields for CH₄/C₂H₆ and Ar/C₂H₆ conditions. As expected from Fig. 4c, C₂H₄ and benzene were less formed under CH₄/C₂H₆ conditions because C₂H₆ conversion was suppressed in the presence of CH₄. As described later, we considered that methyl radicals were generated from CH₄ with the aid of radicals formed from C₂H₆. Fig. 4 shows that while C₂H₄ yield was hardly affected by CH₄, the formation of C₃ hydrocarbon was enhanced in the presence of CH₄. From the results, we presumed that methyl radicals derived from CH₄ promoted the recombination reaction of C₂H₃˙ (or C₂H₅˙) and CH₃˙ to form C₃ hydrocarbons.

Fig. 5 shows the effect of volume fractions of C₂H₆ on yields of lower olefins. Regardless of reaction conditions, all the product yields became higher with increasing C₂H₆ concentration. Considering that products were mainly formed from the pyrolysis of C₂H₆, it is likely that the product yields were proportional to the concentration of C₂H₆. As shown in Fig. 5a, it appeared that the C₂H₄ yields slightly decreased with the presence of CH₄. The co-existence of CH₄ contributed to the decrease in C₂H₆ conversion (Fig. 4c), thus, it is possible that the yield of the main product (i.e., C₂H₄) decreased with the presence of CH₄. On the other hand, the C₃H₆ yield was significantly increased by the co-existing CH₄ regardless of the C₂H₆ concentration. This tendency is the same as shown in Fig. 4a. Fig. 5b shows yields of aromatics at various volume fractions of C₂H₆. Like the lower olefins, aromatics yields also increased with the concentration of C₂H₆. Comparing CH₄/C₂H₆ and Ar/C₂H₆ conditions, we can see that there were no significant effects of CH₄ on the benzene, styrene and naphthalene yields, while only the toluene yield increased with the CH₄/C₂H₆ conditions, which is similar to the results shown in Fig. 4b.

Fig. 5 Products yield for CH₄/C₂H₆ or Ar/C₂H₆ condition at different volume fractions of C₂H₆ in the mixture gases. (a) Reaction temperature: 1023 K, flow rate: 30 mL min⁻¹, (b) reaction temperature: 1073 K, flow rate: 10 mL min⁻¹.

Based on the above results, the effects of co-existing CH₄ on the pyrolysis reactions are summarized as follows:

• C₃H₆, C₃H₈ and toluene yields increased significantly in the presence of CH₄.

• C₂H₆ conversion was slightly lowered in the presence of CH₄.

• The influence of co-existing CH₄ on main products (i.e., C₂H₄ and benzene) yields was small.

Mass spectrometry analysis using ¹³C-labeled CH₄

It is well-known that CH₄ molecules are significantly stable because of both their structural symmetry and strong C–H bond.¹ Indeed, pyrolysis of CH₄ hardly took place when only CH₄ was heated to 1073 K (entry 55 in Table 1). Thus, it is not surprising that CH₄ molecules in CH₄/C₂H₆ behaved the same as an inert gas, however, CH₄/C₂H₆ showed different pyrolysis behaviour from Ar/C₂H₆. This result strongly suggests that CH₄ molecules in C₂H₆ played a role in the pyrolysis reaction. In order to understand the role of CH₄, pyrolysis using ¹³CH₄ was carried out. If CH₄ molecules were activated and converted into pyrolysis products, mass spectra of the pyrolysis products should be changed by incorporating ¹³C into them.

Fig. 6 shows mass spectra of the aromatic compounds formed by pyrolysis of ¹³CH₄/C₂H₆ or He/C₂H₆ at 1073 K (mass spectra of C₂ and C₃ hydrocarbons could not be obtained due to the limitation of separation in the GC-MS system). It was no wonder that typical mass spectra of benzene and toluene were observed under the He/C₂H₆ condition (Fig. 6a and b). On the other hand, it is interesting that mass spectra of benzene and toluene formed from ¹³CH₄/C₂H₆ (Fig. 6c and d) were different from those of He/C₂H₆; benzene and toluene from ¹³CH₄/C₂H₆ had higher m/z values. The m/z values at the highest peaks were 78 (benzene from He/C₂H₆), 80 (benzene from ¹³CH₄/C₂H₆), 91 (toluene from He/C₂H₆) and 94 (toluene from ¹³CH₄/C₂H₆). The increase in m/z of benzene and toluene in the presence of ¹³CH₄ strongly suggested that ¹³C derived from CH₄ was incorporated into benzene and toluene during the pyrolysis reactions.

Fig. 6 Mass spectra of (a and c) benzene and (b and d) toluene formed by dehydrogenation of (a and b) He/C₂H₆ and (c and d) ¹³CH₄/C₂H₆ at 1073 K.

Reaction mechanism

The mass spectra shown in Fig. 6 indicated that CH₄ was incorporated into the products in the presence of C₂H₆ even though CH₄ alone did not react, which implies CH₄ molecules were activated with the aid of C₂H₆. Considering that no catalysts were used in this study, we assumed that radical species generated from C₂H₆ would play a role to activate the stable CH₄ molecules.

It is known that various radical species are formed from C₂H₆ at above 773–873 K.²⁹ If the radicals abstract H˙ from CH₄ molecules, CH₄ molecules generate radical species (mainly methyl radicals):

CH₄ + H˙ → CH₃˙ + H₂ (10)

CH₄ + C₂H₅˙ → CH₃˙ + C₂H₆ (11)

Consequently, the methyl radicals derived from CH₄ molecules can convert into C₃H₆ and C₃H₈ by the following radical reactions:

C₂H₃˙ + CH₃˙ → C₃H₆ (12)

C₂H₅˙ + CH₃˙ → C₃H₈ (13)

In the above recombination reactions, CH₄ molecules are incorporated into C₃ hydrocarbons. For example, by combining eqn (5), (10), and (12), we know that a CH₄ molecule is consumed to form a C₃H₆ molecule (CH₄ + C₂H₄ + 2H˙ → C₃H₆ + 2H₂).

Also, when the methyl radical reacts with a phenyl radical, toluene is formed:

C₆H₅˙ + CH₃˙ → C₇H₈ (14)

The significant increases in C₃ hydrocarbons and toluene yields under CH₄/C₂H₆ conditions can be explained by the above mechanism; the increase in the concentration of methyl radicals promoted the formation of C₃ hydrocarbons and toluene. As shown in the mass spectra, CH₄-derived carbon was also incorporated into benzene, which can be explained by the recombination of propargyl radicals that is formed from ¹³CH₃˙ and C₂H₂ (eqn (7)–(9)).

Current strategies for CH₄ activation can roughly be categorized into (1) the development of catalysts that efficiently activate C–H bonds in CH₄ and (2) pyrolysis of CH₄, where coupling of methyl radicals proceeds with the aid of extreme high temperature (>approx. 1473 K). In this study, an alternative route for the chemical conversion of CH₄ was proposed: radicals generated from co-existing molecules (in this study, C₂H₆) activate CH₄ molecules.

Conclusions

Pyrolysis reactions of mixtures of CH₄/C₂H₆ or Ar/C₂H₆ were carried out at 973–1073 K. Even though CH₄ alone did not react in the temperature range, the pyrolysis behaviour was quite different in the presence of CH₄; the formation of C₃ hydrocarbons and toluene was enhanced. In contrast, C₂H₄ and benzene yields were not affected by the co-existing CH₄. Mass spectrometry analysis using ¹³C-labeled CH₄ revealed that CH₄-derived carbon was incorporated into the pyrolysis products, indicating CH₄ molecules were activated and played a role in the pyrolysis reactions. Probably, radicals generated from C₂H₆ abstracted H˙ from CH₄ so that CH₄-derived methyl radicals would be formed. By reacting the methyl radicals with C₂H₃˙ and C₆H₅˙, C₃H₆ and toluene were formed. In other words, the increase in the concentration of methyl radicals enhanced the formation of C₃ hydrocarbons and toluene. This study clarified that CH₄ can be activated by radicals generated from co-existing molecules without the help of catalysts or extreme high temperature.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the technology development project carried out in Japan Petroleum Energy Center (JPEC) under the commission of the Ministry of Economy, Trade and Industry (METI) and also by JST CREST, Grant Number JPMJCR15P4. We appreciate technical support of Mr. Ohshima (Technical Support Center, Saitama University) for pyrolysis reactions on the closed gas-circulation system and of Mr. Niimi (Comprehensive Analysis Center for Science, Saitama University) for GC-MS analyses.

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