136
the development of frustrated Lewis pair in homogeneous catalysis of carbon dioxide hydrogenation to methanol. Based on previous findings, Ashley and co-workers
(2009) firstly reported a process of heterolytic activation of hydrogen and subsequent insertion of carbon dioxide into a B–H bond in the homogenous catalytic
conversion of carbon dioxide to methanol. The isolation of methanol by solvent
removal produced a yield of 17–25%.
The N-heterocyclic carbenes are well established as organocatalysts and ligands
in organic synthesis, and N-heterocyclic carbenes behave as nucleophiles with the
lone pair of carbene electrons. Riduan and co-workers (2009) reported first hydrosilylation of carbon dioxide using a stable N-heterocyclic carbene catalyst and reducing carbon dioxide ultimately to methoxide under very mild conditions while
allowing the use of air as a feedstock. The yield in methanol typically reached over
90%, based on silane.
5.4.4 Photocatalytic Hydrogenation of Carbon Dioxide
to Methanol in the Liquid Phase
Photochemical, photoelectrochemical carbon dioxide reduction to fuels could be an
important alternative to address current environmental and energy challenges of the
depletion of fossil fuels. However, advances in carbon dioxide photoreduction with
homogeneous transition metal catalysts are limited to carbon monoxide or formic
acid or formate production (Chen et al. 2011; Hull et al. 2012; Morris et al. 2009).
Only a few studies related to reduction to methanol are reported. From the work of
Morris and co-workers (2011) and Seshadri and co-workers (1994) on selective
electrocatalytic conversion of carbon dioxide to methanol using pyridine molecule,
Boston and co-workers (2013) reported the first example of homogeneous photochemical reduction of carbon dioxide to formate and methanol in an aqueous system containing a chromophore, ascorbic acid, and pyridine. In the system, carbon
dioxide-reducing catalyst was pyridine; the chromophore was ruthenium(II) phenanthroline, [Ru(C 12 H 8 N 2 ) 3 ]
2+
; and ascorbic acid was the sacrificial donor. Methanol
production was related to the pyridine to chromophore ratio while no methanol
detected after 6 h irradiation using excess of the chromophore. For example, approximately 30 μM of methanol was detected when pyridine was in large excess, e.g., 1
to 100, but only around 6 μM at the ratio of 1 to 1 or 2 to 1. The turnover numbers
was around 0.15 methanol per [Ru(phen) 3 ]
2+
or 0.9 electrons per [Ru(C 12 H 8 N 2 ) 3 ]
2+
based on the six-electron stoichiometry in the reaction, wherein phen means phenanthroline. The ruthenium complexes [Ru(phen) 2 (ptpbα)]
2+
(Ruα) and
[Ru(phen) 2 (ptpbβ)]
2+
(Ruβ), wherein ptpbα = pyrido[2,3,5,6]pyrazino[2,3-f][1,10]
phenanthroline and ptpbβ = pyrido[3,4,5,6]pyrazino[2,3-f][1,10]phenanthroline,
are shown as electrocatalysts and photocatalysts for the hydrogenation of carbon
dioxide into formate, formaldehyde, and methanol (Boston et al. 2014).
D. P. Minh et al.
the development of frustrated Lewis pair in homogeneous catalysis of carbon dioxide hydrogenation to methanol. Based on previous findings, Ashley and co-workers
(2009) firstly reported a process of heterolytic activation of hydrogen and subsequent insertion of carbon dioxide into a B–H bond in the homogenous catalytic
conversion of carbon dioxide to methanol. The isolation of methanol by solvent
removal produced a yield of 17–25%.
The N-heterocyclic carbenes are well established as organocatalysts and ligands
in organic synthesis, and N-heterocyclic carbenes behave as nucleophiles with the
lone pair of carbene electrons. Riduan and co-workers (2009) reported first hydrosilylation of carbon dioxide using a stable N-heterocyclic carbene catalyst and reducing carbon dioxide ultimately to methoxide under very mild conditions while
allowing the use of air as a feedstock. The yield in methanol typically reached over
90%, based on silane.
5.4.4 Photocatalytic Hydrogenation of Carbon Dioxide
to Methanol in the Liquid Phase
Photochemical, photoelectrochemical carbon dioxide reduction to fuels could be an
important alternative to address current environmental and energy challenges of the
depletion of fossil fuels. However, advances in carbon dioxide photoreduction with
homogeneous transition metal catalysts are limited to carbon monoxide or formic
acid or formate production (Chen et al. 2011; Hull et al. 2012; Morris et al. 2009).
Only a few studies related to reduction to methanol are reported. From the work of
Morris and co-workers (2011) and Seshadri and co-workers (1994) on selective
electrocatalytic conversion of carbon dioxide to methanol using pyridine molecule,
Boston and co-workers (2013) reported the first example of homogeneous photochemical reduction of carbon dioxide to formate and methanol in an aqueous system containing a chromophore, ascorbic acid, and pyridine. In the system, carbon
dioxide-reducing catalyst was pyridine; the chromophore was ruthenium(II) phenanthroline, [Ru(C 12 H 8 N 2 ) 3 ]
2+
; and ascorbic acid was the sacrificial donor. Methanol
production was related to the pyridine to chromophore ratio while no methanol
detected after 6 h irradiation using excess of the chromophore. For example, approximately 30 μM of methanol was detected when pyridine was in large excess, e.g., 1
to 100, but only around 6 μM at the ratio of 1 to 1 or 2 to 1. The turnover numbers
was around 0.15 methanol per [Ru(phen) 3 ]
2+
or 0.9 electrons per [Ru(C 12 H 8 N 2 ) 3 ]
2+
based on the six-electron stoichiometry in the reaction, wherein phen means phenanthroline. The ruthenium complexes [Ru(phen) 2 (ptpbα)]
2+
(Ruα) and
[Ru(phen) 2 (ptpbβ)]
2+
(Ruβ), wherein ptpbα = pyrido[2,3,5,6]pyrazino[2,3-f][1,10]
phenanthroline and ptpbβ = pyrido[3,4,5,6]pyrazino[2,3-f][1,10]phenanthroline,
are shown as electrocatalysts and photocatalysts for the hydrogenation of carbon
dioxide into formate, formaldehyde, and methanol (Boston et al. 2014).
D. P. Minh et al.