A catalytic cascade
system for CO2 hydrogenation to
MeOH under acidic conditions is described. The reaction uses three
catalysts which promote stepwise formation and conversion of formic
acid and formate ester intermediates. The activities and decomposition pathways of different catalyst
candidates for each step were investigated. The combination Ru(H)2[P(CH2CH2PPh2)3]/Sc(OTf)3/Ir(tBuPCP)(CO) was found to be the
most active for CO2 hydrogenation to MeOH. An overall TON
of 428 was achieved after 40 h at 155 °C in EtOH, resulting in
a high concentration of MeOH (1.07 M). Catalyst speciation studies
upon completion of the reaction indicated that the carbonyl complexes
[Ru[P(CH3CH2PPh2)3](H)(CO)](OTf),
Ir(tBuPCP)(H)(CO)(OTf), and [Ir(tBuPCP)(H)(CO)2](OTf) were formed as the major metal-containing species.
Notably, [Ru[P(CH3CH2PPh2)3](H)(CO)](OTf) was found to be inactive for CO2 hydrogenation,
limiting the productivity of the reaction.
The source of hydrogen atoms will determine the reaction hazard, cost, and reaction conditions. Hydrogen sources can be classified into two main groups: hydride or molecular hydrogen sources. [3][4][5][6]
Half-sandwich iridium bipyridine
complexes catalyze the hydrogenation
of esters and lactones under base-free conditions. The reactions proceed
with a variety of ester and lactone substrates. Mechanistic studies
implicate a pathway involving rate-limiting hydride transfer to the
substrate at high pressures of H2 (≥50 bar).
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