Visible-light-driven methane formation from CO2 with a molecular iron catalyst

Abstract

Summary

A photochemical reduction of CO₂ to methane via CO was shown using the iron porphyrin complex Fe(pTMAPP)Cl5 as a catalyst in combination with the iridium-based photosensitizer Ir(ppy)3. Turnover numbers (TONs) up to 367 for CO and 159 for CH₄ and selectivities of 79% for CO and 87% for CH₄ were reached in acetonitrile. The experiments were conducted under visible-light irradiation (λ > 420 nm) using triethylamine as sacrificial electron donor (see section SEDs below).

Advances and special progress

The first example of the visible-light-catalyzed eight-electron reduction of CO₂ to methane using an iron tetraphenylporphyrin catalyst is reported.

Additional remarks

An additional experiment with Ru(bpy)3 yielded no CH₄, instead CO and H₂ were obtained.

Content of the published article in detail

The article contains results for the reduction of CO₂ to CO and CH₄ as well as H₂. The catalytic system performs best (referring to the TON of CH₄ production) for CO as a feedstock; the highest TON for the conversion of CO₂ to CO and CH₄ was obtained in acetonitrile without additives.

Catalysts

Fe(pTMAPP)Cl5 Fe(DHPP)Cl

Photosensitizer

Ir(ppy)3

Investigation

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	solvent A	additives	.	λexc [nm]	.	TON CO	TON CH4	TON H2	.
1.	Fe(pTMAPP)Cl5	0.002			TEA	0.05	MeCN			> 420		33
2.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN			> 420		198	31	24
3.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN	TFE		> 420		240	66	73
4.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN			> 420		367	79	26
5.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN	Argon gas		> 420				43
6.			Ir(ppy)3	0.2	TEA	0.05	MeCN			> 420		3		1
7.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2			MeCN			> 420		5
8.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	TEA			dark
9.	Fe(DHPP)Cl	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN			> 420		139	26	15

Experiment-Name: Table 1Export

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	solvent A	additives	.	λexc [nm]	.	TON CH4	TON H2	.
1.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN			> 420		89	18
2.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN			> 420		140	28
3.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN	TFE		> 420		159	34
4.			Ir(ppy)3	0.2	TEA	0.05	MeCN			> 420
5.	Fe(pTMAPP)Cl5	0.002	Ir(ppy)3	0.2	TEA	0.05	MeCN			dark

Experiment-Name: Table 2 CO gasExport

Sacrificial electron donor

In this study, the experiments were done with the sacrificial electron donor TEA (100505).

Additives

In this study, trifluoroethanol (100618) was used as an additive. A control experiment employing Argon gas was also conducted.