5 Dehydrogenation of Formic Acid
Formic acid is one of the best studied compounds in the field of dehydrogenation
catalysis. Early attempts of formic acid dehydrogenation under the conditions of
homogeneous catalysis were reported in the 1960s–1970s, shedding light onto the
potentially high catalytic performance of Ir complexes [143–145]. This dehydrogenation is a thermodynamically favored process, producing an equimolar mixture of
H 2 and CO 2 (Scheme 75) [146]. The reverse hydrogenation can be accomplished
under pressurized conditions in the presence of bases. Therefore, formic acid has
recently attracted considerable attention as an organic hydrogen carrier for hydrogen
storage applications [147–149]. The liquid nature of formic acid under ambient
conditions as well as its low toxicity and good gravimetric hydrogen capacity
(4.4 wt%) makes this compound a suitable hydrogen carrier.
The catalytic dehydrogenation reactions of formic acid in the presence of Ir
complexes under homogeneous conditions are summarized in Table 4, and the
structures of Ir complexes utilized for this purpose are depicted in Fig. 4. Most
catalytic systems employ aqueous conditions in the temperature range of ambient100
C. Basic additives such as formates and Et 3 N are utilized in some systems to
enhance catalytic performance by pH control. Various Cp*Ir complexes such as 35
and 76–101 with chelating ancillary ligands have been developed and shown to
exhibit excellent catalytic activities, achieving TONs of up to ten million and initial
TOFs of three million [150–174]. Recently, the incorporation of nitrogen-based
coordination sites into polymer scaffolds has been implemented to render Cp*Ir
complexes suitable for heterogeneous catalysis [175, 176]. Other types of Ir complexes 102–109 bearing phosphine or NHC ligands have also been investigated
[177–184].
Scheme 74 Dehydrogenation of an alcoholic moiety in lignin model compound
Scheme 75 Dehydrogenation of formic acid affording H 2 and CO 2
Iridium-Catalyzed Dehydrogenative Reactions
43
Formic acid is one of the best studied compounds in the field of dehydrogenation
catalysis. Early attempts of formic acid dehydrogenation under the conditions of
homogeneous catalysis were reported in the 1960s–1970s, shedding light onto the
potentially high catalytic performance of Ir complexes [143–145]. This dehydrogenation is a thermodynamically favored process, producing an equimolar mixture of
H 2 and CO 2 (Scheme 75) [146]. The reverse hydrogenation can be accomplished
under pressurized conditions in the presence of bases. Therefore, formic acid has
recently attracted considerable attention as an organic hydrogen carrier for hydrogen
storage applications [147–149]. The liquid nature of formic acid under ambient
conditions as well as its low toxicity and good gravimetric hydrogen capacity
(4.4 wt%) makes this compound a suitable hydrogen carrier.
The catalytic dehydrogenation reactions of formic acid in the presence of Ir
complexes under homogeneous conditions are summarized in Table 4, and the
structures of Ir complexes utilized for this purpose are depicted in Fig. 4. Most
catalytic systems employ aqueous conditions in the temperature range of ambient100
C. Basic additives such as formates and Et 3 N are utilized in some systems to
enhance catalytic performance by pH control. Various Cp*Ir complexes such as 35
and 76–101 with chelating ancillary ligands have been developed and shown to
exhibit excellent catalytic activities, achieving TONs of up to ten million and initial
TOFs of three million [150–174]. Recently, the incorporation of nitrogen-based
coordination sites into polymer scaffolds has been implemented to render Cp*Ir
complexes suitable for heterogeneous catalysis [175, 176]. Other types of Ir complexes 102–109 bearing phosphine or NHC ligands have also been investigated
[177–184].
Scheme 74 Dehydrogenation of an alcoholic moiety in lignin model compound
Scheme 75 Dehydrogenation of formic acid affording H 2 and CO 2
Iridium-Catalyzed Dehydrogenative Reactions
43