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Catalytic Mechanism of Liquid-Metal Indium for Direct Dehydrogenative Conversion of Methane to Higher Hydrocarbons.

ABSTRACT: There is a great interest in direct conversion of methane to valuable chemicals. Recently, we reported that silica-supported liquid-metal indium catalysts (In/SiO2) were effective for direct dehydrogenative conversion of methane to higher hydrocarbons. However, the catalytic mechanism of liquid-metal indium has not been clear. Here, we show the catalytic mechanism of the In/SiO2 catalyst in terms of both experiments and calculations in detail. Kinetic studies clearly show that liquid-metal indium activates a C-H bond of methane and converts methane to ethane. The apparent activation energy of the In/SiO2 catalyst is 170 kJ mol-1, which is much lower than that of SiO2, 365 kJ mol-1. Temperature-programmed reactions in CH4, C2H6, and C2H4 and reactivity of C2H6 for the In/SiO2 catalyst indicate that indium selectively activates methane among hydrocarbons. In addition, density functional theory calculations and first-principles molecular dynamics calculations were performed to evaluate activation free energy for methane activation, its reverse reaction, CH3-CH3 coupling via Langmuir-Hinshelwood (LH) and Eley-Rideal mechanisms, and other side reactions. A qualitative level of interpretation is as follows. CH3-In and H-In species form after the activation of methane. The CH3-In species wander on liquid-metal indium surfaces and couple each other with ethane via the LH mechanism. The solubility of H species into the bulk phase of In is important to enhance the coupling of CH3-In species to C2H6 by decreasing the formation of CH4 though the coupling of CH3-In species and H-In species. Results of isotope experiments by combinations of CD4, CH4, D2, and H2 corresponded to the LH mechanism.

SUBMITTER: Nishikawa Y

PROVIDER: S-EPMC7643202 | biostudies-literature | 2020 Nov

REPOSITORIES: biostudies-literature

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Catalytic Mechanism of Liquid-Metal Indium for Direct Dehydrogenative Conversion of Methane to Higher Hydrocarbons.

Nishikawa Yuta Y Ohtsuka Yuhki Y Ogihara Hitoshi H Rattanawan Rattanawalee R Gao Min M Nakayama Akira A Hasegawa Jun-Ya JY Yamanaka Ichiro I

ACS omega 20201022 43

There is a great interest in direct conversion of methane to valuable chemicals. Recently, we reported that silica-supported liquid-metal indium catalysts (In/SiO<sub>2</sub>) were effective for direct dehydrogenative conversion of methane to higher hydrocarbons. However, the catalytic mechanism of liquid-metal indium has not been clear. Here, we show the catalytic mechanism of the In/SiO<sub>2</sub> catalyst in terms of both experiments and calculations in detail. Kinetic studies clearly show t ...[more]

PMID: 33163798

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Project description:A ≥ 200 °C fraction (CT200F) of low-temperature coal tar was prepared by a rotary film evaporator. The catalytic conversion experiments of CT200F and six model compounds were conducted on the pyrolysis gas chromatography-mass spectrometer. The yields of catalytic conversion products benzene, toluene, xylene, and naphthalene (BTXN) were analyzed by semi-quantitative analysis according to the chromatographic peak areas. Additionally, the possible formation pathways and mechanisms of the target products BTXN generated over different catalysts were investigated. The results show that the yield of aromatic hydrocarbons increases and the yield of acid compounds decreases during CT200F pyrolysis over ZSM-5, HY, USY, and β-zeolite compared with that of its non-catalytic pyrolysis, especially the yields of BTXN obtained over USY and β-zeolite increase by 128 and 108%, respectively. The pore structure of ZSM-5 is suitable to produce BTX, while the suitable acidity and pore structure of USY, HY, and β-zeolite are more beneficial for the selective preparation of naphthalene than that of ZSM-5. The conversion pathways of six model compounds into BTXN over zeolites were obtained, and the following conclusions can be drawn: The dehydroxylation effect of zeolites shows the order of ZSM-5 > HY > USY > β-zeolite. The catalytic effect of zeolites on the cracking and ring opening of PAHs in CT200F shows the order of β-zeolite > USY > HY > ZSM-5. The catalytic effect of catalysts on the cracking and aromatization of aliphatic compounds shows the order of ZSM-5 > β-zeolite > USY > HY. β-zeolite has an outstanding catalytic performance in the conversion of PAHs into naphthalene. ZSM-5 and HY can effectively remove phenolic hydroxyl groups in phenol and naphthol. During the catalytic conversion processes of the coal tar fraction and model compounds, the catalytic effect of the pore constructions of zeolites is more important than their acidities, which determines whether large molecules can enter and whether acid sites in non-micropores can be effectively utilized.

Dataset Information

Catalytic Mechanism of Liquid-Metal Indium for Direct Dehydrogenative Conversion of Methane to Higher Hydrocarbons.

Publications

Catalytic Mechanism of Liquid-Metal Indium for Direct Dehydrogenative Conversion of Methane to Higher Hydrocarbons.

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