Methane conversion on cobalt-added liquid-metal indium catalysts

Hiroki Tajima , Hitoshi Ogihara *, Miru Yoshida-Hirahara and Hideki Kurokawa
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan. E-mail: ogihara@mail.saitama-u.ac.jp

Received 29th July 2020 , Accepted 7th September 2020

First published on 23rd September 2020


Non-oxidative methane (CH4) conversion was carried out using a silica-supported cobalt-added indium liquid metal catalyst (In–Co/SiO2). In–Co/SiO2 showed different activity from In/SiO2 and Co/SiO2. In–Co/SiO2 promoted the formation of aromatics (e.g., benzene) from CH4. The ethylene conversion reaction implied that Co present in indium liquid metal was effective for the aromatization reaction. We propose bifunctional catalysis by In–Co for CH4 conversion: (1) indium liquid metal activates CH4 molecules to form C2 hydrocarbons by CH4 coupling, and (2) a portion of C2 hydrocarbons is converted to aromatics by Co species present in the liquid metal. This work was compared to reported systems such as supported catalytically active liquid metal solutions (SCALMS).


Petroleum is a raw material in the production of basic chemicals, such as olefins and aromatic compounds, but faces a depletion problem in the future. Therefore, natural gas is expected to be used for the production of essential chemicals, and recent development of the shale gas extraction technology also boosts the use of natural gas in chemical industries. However, methane (CH4), the main component of natural gas, is difficult to convert to useful chemicals because of its high stability that is attributed to strong C–H bonds and a symmetrical molecular structure.1 For several decades, chemical conversion of CH4 has been intensely investigated,2–5 and a dehydrogenative route is one of the most promising processes for CH4 conversion.6–13 In the dehydrogenative process, CH4 is converted into hydrogen, C2 hydrocarbons, and aromatics. However, catalyst deactivation due to coke deposition is a crucial problem. Therefore, the design of catalysts with high coking resistance is necessary. So far, various catalysts have been reported, and liquid metal (LM) is currently being paid attention to as a new catalyst for CH4 conversion. Molten metal catalysts that can diminish the deactivation by coke deposition have been examined in the complete dehydrogenation of CH4 to form hydrogen and separable carbon.14 Recently, it is reported that low melting-point metals, such as indium (In) and bismuth (Bi), are effective for dehydrogenative CH4 conversion, where In and Bi LMs catalyze CH4 activation, and C2 hydrocarbons and aromatics are produced.7,15,16

On the other hand, Wasserscheid and coworkers recently developed the idea of supported catalytically active liquid metal solutions (SCALMS); SCALMS presumably have homogeneously distributed metal atoms at the surface of a liquid metallic phase.17–19 For example, a liquid mixture of Ga (melting point is 303 K) and Pd deposited on porous glass was active for butane dehydrogenation, and was insensitive to deactivation by coke deposition.17 In addition, for propane dehydrogenation in the presence of CO, Ga37Pt/Al2O3 SCALMS showed higher conversion than Pt/Al2O3 because of its high tolerance to the poisoning effect of CO adsorption and coke.18 These studies also suggested that the noble metals (Pd or Pt) added to Ga are present as isolated noble-metal atoms at the surface of a liquid Ga matrix, where it is considered that Ga LM is inert for catalytic reactions, and the isolated noble-metal atoms act as unique catalysts. Furthermore, it is reported that liquid metal galinstan containing Rh catalysts is effective for hydroformylation of olefins.20

As described above, researches on catalysis by LM are categorized into two concepts: (1) the catalysis by LMs themselves (e.g. molten metals and low melting point metals) and (2) the catalysis by isolated metal atoms on SCALMS. In this study, CH4 conversion was performed on an In–Co catalyst in which Co was added as a second component to In LM. In–Co acted as a bi-functional catalyst to convert CH4 into value-added molecules: In LM converted CH4 into C2 hydrocarbons, and successively the added Co species promoted the aromatization of the formed C2 hydrocarbons. The synergy effect of LM and the secondary component (Co) has been considered, and it may help shed light on catalysis in hydrocarbon conversion processes.

All catalysts were prepared by a typical impregnation method. Loading of In was 5 wt% in all catalysts. The loading of Co in Co/SiO2 was 0.5 wt%. In–Co/SiO2 catalysts with 1, 2, 9, 17, 50 atomic% of Co were prepared, and hereafter denoted as In–Co(1), In–Co(2), In–Co(9), In–Co(17), and In–Co(50)/SiO2, respectively. The Co loadings for In–Co(1), In–Co(2), In–Co(9), In–Co(17), and In–Co(50)/SiO2 were 0.026, 0.051, 0.26, 0.51, and 2.6 wt%, respectively. The detail in CH4 and ethylene (C2H4) conversion reactions, a schematic of reaction system (Fig. S1, ESI) and catalyst characterization are shown in ESI.

Table 1 summarizes the reaction results for dehydrogenative conversion of CH4 on various catalysts. Products for CH4 conversion were C2 hydrocarbons (C2H6, C2H4, and C2H2), propylene (C3H6), aromatics (C6H6, C7H8, and C10H8), and coke. For all catalysts, except for SiO2, the main hydrocarbon product was C2H4. Time courses for CH4 conversion and formation rate for gaseous products are shown in Fig. S2 (ESI). When using only SiO2 (entry 1), CH4 hardly converted, which indicated CH4 is so stable that pyrolysis did not proceed at 1073 K. As previously reported, In/SiO2 was active for the reaction;7 the yield of C2H4 was much higher than SiO2 (entries 1 and 3). For Co/SiO2 (entry 2), the reaction proceeded, but the amount of formed hydrocarbon was inferior to In/SiO2. For In–Co/SiO2, the yields of the main product (C2H4) were higher than In/SiO2 and Co/SiO2, regardless of the In–Co ratio. Interestingly, the formation of aromatics (i.e., benzene, toluene, and naphthalene) was observed with In–Co/SiO2, while aromatics were not detected for In/SiO2 and Co/SiO2. The results suggest that In–Co/SiO2 has different catalytic functions than In/SiO2 and Co/SiO2; In–Co/SiO2 had higher activities for CH4 coupling and aromatization reactions than In/SiO2 and Co/SiO2. TG/DTA analysis for In–Co(17)/SiO2 after reaction was carried out, and we can see that the weight loss for coke combustion was 3–4 wt% (Fig. S3, ESI).

Table 1 Reaction results of CH4 conversion at 1073 K. Catalyst mass: 0.2 g; flow rate: 10 mL min−1; p(CH4): 1 bar; reaction time: 2 h
Entry Catalysts Yield/μmol h−1 gcat−1 Coke/mg h−1 gcat−1 Conv./% Carbon balance/%
H2 C2H6 C2H4 C2H2 C3H6 C6H6 C7H8 C10H8
1 SiO2 200 19.1 8 n.d. 1 n.d. n.d. n.d. 1 0.1 98.5
2 Co/SiO2 665 51.1 60 2.0 6 n.d. n.d. n.d. 3 0.4 99.3
3 In/SiO2 3025 74.7 207 6.0 13 n.d. n.d. n.d. 15 1.6 100.8
4 In–Co(1)/SiO2 4250 55.3 248 6.1 19 1.4 n.d. n.d. 22 2.1 99.5
5 In–Co(2)/SiO2 4486 53.3 261 5.9 21 2.7 0.7 0.5 23 2.2 97.7
6 In–Co(9)/SiO2 5281 49.1 262 4.8 22 4.0 1.2 0.6 28 2.6 99.1
7 In–Co(17)/SiO2 5648 47.5 253 4.1 20 4.9 1.4 0.6 30 2.7 99.6
8 In–Co(50)/SiO2 5874 42.5 229 3.7 19 5.3 1.4 0.7 32 2.8 100.0


Table 2 shows product yields for the conversion of C2H4 in the presence of H2 at 1023 K on In–Co(17)/SiO2, In/SiO2, Co/SiO2, and SiO2. Note that the reaction temperature for C2H4 was lower than CH4 conversion (Table 1) and H2 was coexisted in C2H4 because C2H4 has higher reactivity than CH4. Time courses for C2H4 conversion and formation rate for gaseous products are shown in Fig. S4 (ESI). In the reactions, CH4 (a cracking product), C2H6 (a hydrogenation product), trace amounts of C3 hydrocarbons, aromatics, and coke were formed. For SiO2 and In/SiO2 (entries 9 and 11), similar reaction results were observed, which indicated that In LM has no catalytic activity for converting C2H4. Compared to In/SiO2, Co/SiO2 promoted the formation of CH4, C2H6, C2H2, and aromatics (entry 10). Interestingly, In–Co(17)/SiO2 (entry 12) showed higher yields of CH4, C2H6 and aromatics than Co/SiO2 and In/SiO2. Here, it should be noted that In–Co(17)/SiO2 showed the highest aromatics yields for C2H4 conversion in Table 2. The results shown in Table 2 indicated that the aromatization is strongly promoted on Co interacted with In LM.

Table 2 Reaction results of C2H4 conversion at 1023 K. Catalyst mass: 0.5 g; flow rate of mixture of Ar/H2/C2H4: 47/3/3 mL min−1; reaction time: 2 h
Entry Catalysts Yield/μmol h−1 gcat−1 Coke/mg h−1 gcat−1 Conv./% Carbon balance/%
CH4 C2H6 C2H2 C3H6 C6H6 C8H8 C10H8
9 SiO2 91 243 133 26 0.9 n.d. n.d. 3 3.8 95.6
10 Co/SiO2 114 290 228 29 4.5 n.d. 0.3 9 6.6 97.5
11 In/SiO2 26 185 183 5 n.d. n.d. n.d. 3 3.1 92.7
12 In–Co(17)/SiO2 572 340 86 31 15.9 0.6 1.5 31 13.4 91.4


In/SiO2 is reported as an effective catalyst for dehydrogenative coupling of CH4 to form C2 hydrocarbons.7 The structure of In catalysts was examined using temperature programmed XRD,7 first-principle molecular dynamics calculation,15 and in situ X-ray absorption fine structure analysis,16 revealing that In is present as LM and the liquid-state is the origin for the catalysis of CH4 coupling. In this study, we used Co added In/SiO2, and according to the phase diagram of In–Co,21 a few percent of Co dissolves in In LM at reaction temperature (1073 K). Therefore, it is considered that a part of added Co dissolved in In LM particles under reaction condition and it is likely that the dissolved Co showed different catalytic activity from Co supported on SiO2. However, the LM state of In and the dissolution of Co in In LM at reaction conditions were not directly observed in this work and assumed based on the previous works.7,15,16

In this regard, the effect of Co content in In–Co/SiO2 was investigated. The results were shown in Table 1 (entries 3–8). As the Co ratio increased from 0 to 17 at%, the yields of aromatics increased, indicating Co species played a role in the aromatization. The formation of aromatics seemed to occur equally at 17 and 50 at% of Co (entries 7 and 8). As mentioned above, Co dissolves in In LM at reaction temperature and the amount of dissolved Co is a few percent. We will discuss the catalysis by the dissolved Co later in detail; briefly, we consider that the dissolved Co promoted the aromatization reaction. The dissolved Co species increased with the amount of added Co, which would promote the formation of aromatics. It is considered that not all of the added Co necessarily dissolved in the In particles, so that a part of Co was supported on the SiO2 support apart from In LM. Therefore, when Co exceeding the solubility limitation was added, the dissolution of Co in In LM reached the limitation. Considering the result that the aromatics yields were almost the same for 17 and 50 at% of Co, we assume that the addition of 17 at% of Co maximized the dissolution of Co in In LM and leveled off the aromatics formation.

Fig. 1 shows the XRD patterns of catalysts. For In/SiO2, diffraction lines attributed to In metal were observed in before and after reactions. For In–Co(17)/SiO2, similar XRD patterns to In/SiO2 were observed. The phase diagram indicates that In–Co solid solution was not formed at low temperature,21 therefore, phase separation cannot be avoided at room temperature. This is because the XRD patterns of In/SiO2 and In–Co(17)/SiO2 were the same. For Co/SiO2, no diffraction lines were detected because the loading of Co was quite small (0.5 wt%).


image file: d0nj03808c-f1.tif
Fig. 1 XRD patterns of (a) In–Co(17)/SiO2 after CH4 conversion reaction, (b) In/SiO2 after CH4 conversion reaction, (c) In/SiO2 after reduction with H2, and (d) Co/SiO2 after CH4 conversion reaction.

The specific surface areas of catalysts were as follows: SiO2 (186 m2 g−1), In/SiO2 (171 m2 g−1), Co/SiO2 (163 m2 g−1), and In–Co(17)/SiO2 (173 m2 g−1). Fig. 2a shows a SEM image of In–Co(17)/SiO2 after the CH4 dehydrogenation reaction. Spherical particles with a diameter of approx. 0.5 μm were observed. EDX analysis confirmed that the spherical particles consisted of In (Fig. 2b and c). The SEM and EDX analysis suggested that In was present as spherical LM under the reaction conditions. At the same time, EDX analysis for Co was performed, but information on the In–Co solid solution was not obtained, because the low loading of Co in the catalysts (e.g. 0.51 wt% for In–Co(17)/SiO2) made it difficult to confirm Co species using EDX analysis. TEM images also indicated that large particles are present on SiO2 support and an STEM image suggested the particles are metal (Fig. 2d–f). Temperature programmed reduction (TPR) profiles for In/SiO2, Co/SiO2, and In–Co(17)/SiO2 after calcination at 1073 K are shown in Fig. S5 (ESI). For In–Co(17)/SiO2, two reduction peaks were observed at 700 and 920 K. Taking account of TPR profile of Co/SiO2 (reduction was observed at 600–800 K) into account, the first peak would be the reduction of cobalt oxide. In addition, similar reduction peaks were observed for In–Co(17)/SiO2 and In/SiO2, indicating the reduction of In2O3. The reduction peak for In–Co(17)/SiO2 was slightly shifted to higher temperature than In/SiO2. The difference in reduction behaviour implies the interaction between In and Co.


image file: d0nj03808c-f2.tif
Fig. 2 (a and b) SEM, (c) EDX mapping of In, (d and e) TEM, and (f) STEM images for In–Co(17)/SiO2 after CH4 conversion reaction.

Based on the above results, the reaction route of CH4 conversion over In–Co/SiO2 and roles of the catalysts were proposed as shown in Scheme 1. We considered that CH4 dehydrogenation proceeds by sequential reactions. At first, a CH4 molecule is activated by abstracting a hydrogen atom and the CH3 species couple to form C2H6 (step (a) in Scheme 1). For step (a), the activation mechanism of CH4 by In LM was proposed on the basis of density functional theory calcinations, indicating low-coordinated In atoms continuously appear on In LM surface, and promote the dissociation of C–H bond in CH4 molecule.15 Furthermore, it is reported that In LM works as a catalyst for C2 hydrocarbons formation from CH4.7 Indeed, the results in Table 1 suggested that In/SiO2 showed higher CH4 coupling activity than Co/SiO2 and SiO2. In–Co/SiO2 produced almost the same amount of C2 hydrocarbons as In/SiO2, indicating that In LM of In–Co/SiO2 also acted as a coupling catalyst.


image file: d0nj03808c-s1.tif
Scheme 1 The proposed reaction route for dehydrogenative conversion of CH4 on In–Co/SiO2 catalyst.

Because C2H6 readily converts to C2H4 in gas phase without catalysts at 1073 K (step (b) in Scheme 1),22 the formation of C2H4 mainly occurred without the aid of catalysts. In Table 1, In–Co/SiO2 showed slightly higher C2H4 yield than In/SiO2, implying a part of C2H4 was produced via the dehydrogenation of C2H6 on Co present in LM. The final step is the aromatization of C2 hydrocarbons. It should be noted again that C2 hydrocarbons yields in CH4 conversion were similar with In/SiO2 and In–Co/SiO2, but aromatics were observed only for In–Co/SiO2 (Table 1). In addition, as shown in Table 2, In–Co/SiO2 had higher catalytic activity for aromatization of C2H4 than In/SiO2 and Co/SiO2. Taking these reaction results into account, we considered that C2H4 formed via the coupling of CH4 converted to aromatics by the In–Co catalyst. We presumed that Co dissolved in LM showed catalytic activity for the aromatization reaction. Although the origin of catalysis due to Co present in LM is not clear at the present, it is reported that the recombination of propargyl radicals (C3H3˙) provides benzene during CH4 pyrolysis,23,24 therefore, Co in LM might promote the formation of propargyl radicals from C2H4.

Previous reports of LM catalysts have focused on the concept of catalysis by LM itself. Furthermore, for SCALMS system, isolated metal atoms on LM are the catalysts, where LM itself does not work as catalysts. This study is different from the previous works. We proposed a new bifunctional catalyst based on the LM system: In LM activated CH4 molecules along with additional Co present in LM was effective for the aromatization of C2 hydrocarbons. For In–Co/SiO2, both LM and the added metal acted as catalysts.

Conclusions

For the dehydrogenative conversion of CH4, In–Co/SiO2 showed different activity from In/SiO2 and Co/SiO2. As previously reported, In/SiO2 catalyzed the coupling of CH4 to form C2 hydrocarbons. For In–Co/SiO2, not only the coupling activity of CH4, but also the aromatization ability was exhibited. Examining the catalytic activity for C2H4, which is the main product of CH4 conversion, it was revealed that In/SiO2 has no activity for C2H4, while In–Co/SiO2 catalyzed the aromatization of C2H4. SEM and EDX analysis implied that LM was present in In–Co/SiO2. Based on the results, we propose a unique catalytic mechanism of In–Co/SiO2 for the conversion of CH4: LM assists CH4 coupling and Co present in LM promotes the aromatization. Such bifunctional mechanism is proposed for the first time in an LM catalyst system.

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 the technical support of Comprehensive Analysis Center for Science (Saitama University) for FE-SEM, EDX, and XRD analyses. We thank the technical support of Mr Usui for nitrogen adsorption measurements of catalysts. A part of this work was supported by NIMS microstructural characterization platform as a program of “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj03808c

This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020