134
(1993). The coproducts included carbon monoxide, methane, and a minimal amount
of ethane. It was later noted that when transition metal carbonyl complexes other
than Ru 3 (CO) 12 were used with potassium iodide at 200 °C, there was either no
formation of methanol with Ir 4 (CO) 12 , W(CO) 6 , Mo (CO) 6 , and Co 2 (CO) 8 or minimal formation of methanol with catalysts such as Rh 4 (CO) 12 and Fe 2 (CO) 9 (Tominaga
et al. 1995).
In 2011, Huff and Sanford (2011) developed a cascade catalysis strategy using a
combination of catalysts including ruthenium complex catalyst and scandium triflate for the hydrogenation of carbon dioxide to methanol. This method involved a
sequence of the carbon dioxide reduction steps including (1) hydrogenation of carbon dioxide to formic acid; (2) esterification to generate a formate ester; and (3)
hydrogenation of the ester to methanol in one pot and the use of a series of homogeneous catalysts to promote these steps. The sequence achieved 21 turnovers of
methanol from carbon dioxide under optimal conditions. Rezayee and co-workers
(2015) combined dimethylamine, NH(CH 3 ) 2 , with a homogeneous ruthenium
hydrogenation catalyst to transform hydrogen and carbon dioxide to a mixture of
dimethylformamide and methanol using a single batch reactor. They obtained up to
96% conversion of carbon dioxide. Since this catalytic process was carried out
under basic conditions, rather than acidic conditions, it should be compatible with
other carbon dioxide capture schemes and makes this pathway amenable to a huge
potential for practical use.
In 2012, Wesselbaum and co-workers (2012) also demonstrated the hydrogenation of carbon dioxide to methanol using a single homogeneous transition metal
catalyst, e.g., ruthenium–phosphine, under relatively mild reaction conditions. They
investigated (i) systems comprising ruthenium(III) acetylacetonate, e.g., Ru(acac) 3 ,
and the tridentate ligand Triphos, wherein Triphos is 1,1,1-tris(diphenylphosphino
methyl)ethane, in the presence of additional organic acid and (ii) ruthenium(II)complex [(Triphos)Ru-(TMM)] 2 (TMM = trimethylenemethane) in the presence of
additional organic acid as an efficient catalyst system. Their work showed that only
a slight excess of acid is required for catalyst activation and the counterion introduced through the acid may significantly affect the catalyst performance. The results
also indicated the enhancing effect of weakly coordinating anions and the intermediate cationic ruthenium complex as probable catalytic active species. In 2015,
Wesselbaum and co-workers (2015) demonstrated that a multistep transformation
can take place directly on the ruthenium–Triphos. So, methanol can be obtained
from carbon dioxide and hydrogen by using this single molecular organometallic
catalyst.
Milstein’s group developed (i) dearomatized ruthenium(II) pincer complexes
derived from pyridine- and bipyridine-based tridentate ligands to homogeneously
catalyze hydrogenation of organic formates (Balaraman et al. 2011b) and (ii) tridentate Ru(II) pincer complexes based on pyridine and acridine backbones to homogeneously catalyze hydrogenation of urea derivatives (Balaraman et al. 2011a),
respectively, to methanol. Since organic formates and urea derivatives are carbon
dioxide-derived compounds, these works are representation of a mild, two-step
hydrogenation of carbon dioxide to methanol. Han and co-workers (2012) effiD. P. Minh et al.
(1993). The coproducts included carbon monoxide, methane, and a minimal amount
of ethane. It was later noted that when transition metal carbonyl complexes other
than Ru 3 (CO) 12 were used with potassium iodide at 200 °C, there was either no
formation of methanol with Ir 4 (CO) 12 , W(CO) 6 , Mo (CO) 6 , and Co 2 (CO) 8 or minimal formation of methanol with catalysts such as Rh 4 (CO) 12 and Fe 2 (CO) 9 (Tominaga
et al. 1995).
In 2011, Huff and Sanford (2011) developed a cascade catalysis strategy using a
combination of catalysts including ruthenium complex catalyst and scandium triflate for the hydrogenation of carbon dioxide to methanol. This method involved a
sequence of the carbon dioxide reduction steps including (1) hydrogenation of carbon dioxide to formic acid; (2) esterification to generate a formate ester; and (3)
hydrogenation of the ester to methanol in one pot and the use of a series of homogeneous catalysts to promote these steps. The sequence achieved 21 turnovers of
methanol from carbon dioxide under optimal conditions. Rezayee and co-workers
(2015) combined dimethylamine, NH(CH 3 ) 2 , with a homogeneous ruthenium
hydrogenation catalyst to transform hydrogen and carbon dioxide to a mixture of
dimethylformamide and methanol using a single batch reactor. They obtained up to
96% conversion of carbon dioxide. Since this catalytic process was carried out
under basic conditions, rather than acidic conditions, it should be compatible with
other carbon dioxide capture schemes and makes this pathway amenable to a huge
potential for practical use.
In 2012, Wesselbaum and co-workers (2012) also demonstrated the hydrogenation of carbon dioxide to methanol using a single homogeneous transition metal
catalyst, e.g., ruthenium–phosphine, under relatively mild reaction conditions. They
investigated (i) systems comprising ruthenium(III) acetylacetonate, e.g., Ru(acac) 3 ,
and the tridentate ligand Triphos, wherein Triphos is 1,1,1-tris(diphenylphosphino
methyl)ethane, in the presence of additional organic acid and (ii) ruthenium(II)complex [(Triphos)Ru-(TMM)] 2 (TMM = trimethylenemethane) in the presence of
additional organic acid as an efficient catalyst system. Their work showed that only
a slight excess of acid is required for catalyst activation and the counterion introduced through the acid may significantly affect the catalyst performance. The results
also indicated the enhancing effect of weakly coordinating anions and the intermediate cationic ruthenium complex as probable catalytic active species. In 2015,
Wesselbaum and co-workers (2015) demonstrated that a multistep transformation
can take place directly on the ruthenium–Triphos. So, methanol can be obtained
from carbon dioxide and hydrogen by using this single molecular organometallic
catalyst.
Milstein’s group developed (i) dearomatized ruthenium(II) pincer complexes
derived from pyridine- and bipyridine-based tridentate ligands to homogeneously
catalyze hydrogenation of organic formates (Balaraman et al. 2011b) and (ii) tridentate Ru(II) pincer complexes based on pyridine and acridine backbones to homogeneously catalyze hydrogenation of urea derivatives (Balaraman et al. 2011a),
respectively, to methanol. Since organic formates and urea derivatives are carbon
dioxide-derived compounds, these works are representation of a mild, two-step
hydrogenation of carbon dioxide to methanol. Han and co-workers (2012) effiD. P. Minh et al.