138
reactor design and scale-up. Depending on the types of homogeneous catalysts
employed, various reaction mechanisms have been proposed in literature.
Tominaga et al. (1995) conducted the first homogenous metal-catalyzed carbon dioxide hydrogenation to methanol using Ru 3 (CO) 12 , molecular hydrogen,
and potassium iodide additive in a N-methyl-2-pyrrolidone solution using the
molar ratio of hydrogen to carbon dioxide of 3 to 1, at 240 °C and 80 bar. These
authors found that in the absence of potassium iodide, vigorous carbon dioxide
methanation was observed instead of methanol formation due to Ru 3 (CO) 12
decomposition to ruthenium metal. The authors also proposed that carbon dioxide was initially converted to the primary carbon monoxide intermediate product
via the reverse water- gas shift reaction at 200 °C followed by hydrogenation of
carbon monoxide to methanol at 240 °C with the usage of a halide. Both carbon
monoxide and methanol were reportedly not detected below 160 °C, while at
temperatures higher than 240 °C, methanol product could be subsequently hydrogenated to undesirable methane. The mechanistic steps of carbon dioxide hydrogenation to methanol using this catalyst system can be thoroughly explained as
illustrated in Fig. 5.7.
As seen in Fig. 5.7, after the initial conversion of carbon dioxide to carbon monoxide catalyzed by [H 2 Ru 4 (CO) 12 ]
2˗
anion, this tetranuclear ruthenium cluster in the
presence of added iodide and produced carbon monoxide was converted to
[Ru(CO) 3 I 3 ]
˗
and [HRu 3 (CO) 11 ]
˗
, which in turn interacted and hydrogenated carbon
monoxide into methanol. Nevertheless, the weakness associated with this homogeneous catalytic process is low selectivity and high operating temperatures required.
In 2011, Milstein’s research group used the soluble and well-defined rutheniumbased pincer complexes, commonly known as phosphorus–nitrogen–nitrogen pincer ligands or PNN ligands, which originated from pyridine- and bipyridine-based
tridentate ligands, as catalysts for carbon dioxide hydrogenation to methanol
(Balaraman et al. 2011b). This reaction occurred via two sequential steps, in which
carbon dioxide was first converted to carbonates, formates, and carbamates and then
to methanol. In contrast to previous studies, carbon dioxide hydrogenation catalyzed by Ru-PNN pincer catalysts was conducted at mild temperature and pressure
conditions but exhibited high efficiency and selectivity without any unwanted
wastes and by-products, e.g., carbon monoxide and methane formation. Notably,
this mechanistic pathway is significantly different from the Tominaga’s proposed
mechanism involving the reduction of formed carbon monoxide intermediate.
As mentioned in these previous studies, attention has been focused on using
single catalysts which are capable of initiating multiple steps of carbon dioxide
reduction to methanol. Huff and Sanford (2011), however, used a series of three different homogeneous catalysts for producing methanol from carbon dioxide hydrogenation in a single pot. The benefits of this cascade catalysis strategy include no
requirement of isolating chemically unstable intermediates and flexible manipulation of rate and selectivity for each step.
D. P. Minh et al.
reactor design and scale-up. Depending on the types of homogeneous catalysts
employed, various reaction mechanisms have been proposed in literature.
Tominaga et al. (1995) conducted the first homogenous metal-catalyzed carbon dioxide hydrogenation to methanol using Ru 3 (CO) 12 , molecular hydrogen,
and potassium iodide additive in a N-methyl-2-pyrrolidone solution using the
molar ratio of hydrogen to carbon dioxide of 3 to 1, at 240 °C and 80 bar. These
authors found that in the absence of potassium iodide, vigorous carbon dioxide
methanation was observed instead of methanol formation due to Ru 3 (CO) 12
decomposition to ruthenium metal. The authors also proposed that carbon dioxide was initially converted to the primary carbon monoxide intermediate product
via the reverse water- gas shift reaction at 200 °C followed by hydrogenation of
carbon monoxide to methanol at 240 °C with the usage of a halide. Both carbon
monoxide and methanol were reportedly not detected below 160 °C, while at
temperatures higher than 240 °C, methanol product could be subsequently hydrogenated to undesirable methane. The mechanistic steps of carbon dioxide hydrogenation to methanol using this catalyst system can be thoroughly explained as
illustrated in Fig. 5.7.
As seen in Fig. 5.7, after the initial conversion of carbon dioxide to carbon monoxide catalyzed by [H 2 Ru 4 (CO) 12 ]
2˗
anion, this tetranuclear ruthenium cluster in the
presence of added iodide and produced carbon monoxide was converted to
[Ru(CO) 3 I 3 ]
˗
and [HRu 3 (CO) 11 ]
˗
, which in turn interacted and hydrogenated carbon
monoxide into methanol. Nevertheless, the weakness associated with this homogeneous catalytic process is low selectivity and high operating temperatures required.
In 2011, Milstein’s research group used the soluble and well-defined rutheniumbased pincer complexes, commonly known as phosphorus–nitrogen–nitrogen pincer ligands or PNN ligands, which originated from pyridine- and bipyridine-based
tridentate ligands, as catalysts for carbon dioxide hydrogenation to methanol
(Balaraman et al. 2011b). This reaction occurred via two sequential steps, in which
carbon dioxide was first converted to carbonates, formates, and carbamates and then
to methanol. In contrast to previous studies, carbon dioxide hydrogenation catalyzed by Ru-PNN pincer catalysts was conducted at mild temperature and pressure
conditions but exhibited high efficiency and selectivity without any unwanted
wastes and by-products, e.g., carbon monoxide and methane formation. Notably,
this mechanistic pathway is significantly different from the Tominaga’s proposed
mechanism involving the reduction of formed carbon monoxide intermediate.
As mentioned in these previous studies, attention has been focused on using
single catalysts which are capable of initiating multiple steps of carbon dioxide
reduction to methanol. Huff and Sanford (2011), however, used a series of three different homogeneous catalysts for producing methanol from carbon dioxide hydrogenation in a single pot. The benefits of this cascade catalysis strategy include no
requirement of isolating chemically unstable intermediates and flexible manipulation of rate and selectivity for each step.
D. P. Minh et al.