179
conducted selective CH 4 oxidation with CO 2 in a fixed-bed reactor on variously supported metal oxides within 500 to 700 °C and 1 atm. Among supported catalysts,
they found that V 2 O 5 /SiO 2 catalyst exhibited the selective CH 4 oxidation to formaldehyde, while CH 4 dry reforming, rather than selective CO 2 oxidation, for yielding
CO and H 2 was observed on SiO 2 -supported NiO and Fe 2 O 3 catalysts. They also
investigated CO 2 partial pressure effect on HCHO yield and reported that increasing
CO 2 partial pressure from about 20 to 70 kPa gradually enhanced the yield of formaldehyde on 2%V 2 O 5 /SiO 2 catalysts. Additionally, they concluded that CO 2 could
act as a selective oxidizing agent for converting CH 4 to HCHO via lattice oxygen
on V 2 O 5 .
CH
CO
HCHO CO H O
4
2
2
2
2
+
→
+
+
(6.14)
Recently, homogeneous catalysts have drawn significant attention for CO 2 conversion to value-added chemicals. These catalysts have been largely implemented
for hydrogenation of CO 2 to methanol (Bontemps and Sabo-Etienne 2013). The
Ru-pincer complex with –NH functionalities showed high turnover frequency for
the direct methanol production from CO 2 reduction (Bontemps and Sabo-Etienne
2013). In this regard, the synthesis of methanol via CO 2 hydrogenation would not
only be the major interest but also could extend to the production of other industrially value-added chemicals. In a series of different C1 molecules derived from the
hydrogenation of CO 2 , formaldehyde generation has been rarely conducted in
homogeneous catalytic reactions (Bontemps and Sabo-Etienne 2013). In this context, the bis(tricyclopentylphosphine) complex [RuH 2 (H 2 ) 2 (PCyp 3 ) 2 ] and
[RuH 2 (H 2 ) 2 (PCy 3 ) 2 ] (where Cyp and Cy refers to cyclopentyl and cyclohexyl,
respectively) were employed for direct CO 2 reduction to formaldehyde (Bontemps
and Sabo-Etienne 2013; Bontemps et al. 2014). The pinacolborane was used as the
reductant and oxygen scavenger. The reaction was conducted in a closed nuclear
magnetic resonance (NMR) tube for 30 min on 10% of ruthenium complex with
Cyp ligand.
The in situ mechanistic studies revealed that formaldehyde can be selectively
trapped by in situ condensation with an amine to imine under moderate conditions
(Bontemps et al. 2014). Most importantly, 0.5 mol% of the polyhydride ruthenium
[Ru] catalyst afforded to yield 54% of formaldehyde in 5 h as seen in Fig. 6.9. The
mechanistic insights were further studied by using the density functional theory
(DFT) calculations for hydrogenation of CO 2 to formaldehyde (Dong et al. 2017).
In addition, the transitions states and electronic structures of the reaction intermediates were observed by using natural bond orbital theory. The metal complexes of
Ru, Fe, and Os with bis(tricyclopentylphosphine) were used to study reaction mechanisms by DFT calculations. They found that the formation of Ru–O, C–H, and
O–B bonds, as well as Fe–O bond cleavage were the rate-determining steps of reactions. The results also revealed that the low-cost iron catalyst with the total free
energy barrier about 24 kcal mol
−1
could be a potential candidate for CO 2 reduction
to formaldehyde at mild conditions.
6 Conversion of Carbon Dioxide into Formaldehyde
conducted selective CH 4 oxidation with CO 2 in a fixed-bed reactor on variously supported metal oxides within 500 to 700 °C and 1 atm. Among supported catalysts,
they found that V 2 O 5 /SiO 2 catalyst exhibited the selective CH 4 oxidation to formaldehyde, while CH 4 dry reforming, rather than selective CO 2 oxidation, for yielding
CO and H 2 was observed on SiO 2 -supported NiO and Fe 2 O 3 catalysts. They also
investigated CO 2 partial pressure effect on HCHO yield and reported that increasing
CO 2 partial pressure from about 20 to 70 kPa gradually enhanced the yield of formaldehyde on 2%V 2 O 5 /SiO 2 catalysts. Additionally, they concluded that CO 2 could
act as a selective oxidizing agent for converting CH 4 to HCHO via lattice oxygen
on V 2 O 5 .
CH
CO
HCHO CO H O
4
2
2
2
2
+
→
+
+
(6.14)
Recently, homogeneous catalysts have drawn significant attention for CO 2 conversion to value-added chemicals. These catalysts have been largely implemented
for hydrogenation of CO 2 to methanol (Bontemps and Sabo-Etienne 2013). The
Ru-pincer complex with –NH functionalities showed high turnover frequency for
the direct methanol production from CO 2 reduction (Bontemps and Sabo-Etienne
2013). In this regard, the synthesis of methanol via CO 2 hydrogenation would not
only be the major interest but also could extend to the production of other industrially value-added chemicals. In a series of different C1 molecules derived from the
hydrogenation of CO 2 , formaldehyde generation has been rarely conducted in
homogeneous catalytic reactions (Bontemps and Sabo-Etienne 2013). In this context, the bis(tricyclopentylphosphine) complex [RuH 2 (H 2 ) 2 (PCyp 3 ) 2 ] and
[RuH 2 (H 2 ) 2 (PCy 3 ) 2 ] (where Cyp and Cy refers to cyclopentyl and cyclohexyl,
respectively) were employed for direct CO 2 reduction to formaldehyde (Bontemps
and Sabo-Etienne 2013; Bontemps et al. 2014). The pinacolborane was used as the
reductant and oxygen scavenger. The reaction was conducted in a closed nuclear
magnetic resonance (NMR) tube for 30 min on 10% of ruthenium complex with
Cyp ligand.
The in situ mechanistic studies revealed that formaldehyde can be selectively
trapped by in situ condensation with an amine to imine under moderate conditions
(Bontemps et al. 2014). Most importantly, 0.5 mol% of the polyhydride ruthenium
[Ru] catalyst afforded to yield 54% of formaldehyde in 5 h as seen in Fig. 6.9. The
mechanistic insights were further studied by using the density functional theory
(DFT) calculations for hydrogenation of CO 2 to formaldehyde (Dong et al. 2017).
In addition, the transitions states and electronic structures of the reaction intermediates were observed by using natural bond orbital theory. The metal complexes of
Ru, Fe, and Os with bis(tricyclopentylphosphine) were used to study reaction mechanisms by DFT calculations. They found that the formation of Ru–O, C–H, and
O–B bonds, as well as Fe–O bond cleavage were the rate-determining steps of reactions. The results also revealed that the low-cost iron catalyst with the total free
energy barrier about 24 kcal mol
−1
could be a potential candidate for CO 2 reduction
to formaldehyde at mild conditions.
6 Conversion of Carbon Dioxide into Formaldehyde