Homogeneous WGS Catalysts

The earliest examples of homogeneous WGS were reported in 1943 using aqueous solutions of potassium bicarbonate [20]. Potassium carbonate and sodium carbonate were examined as catalysts in WGS as well. In all mechanisms proposed, a formate species was postulated in correlation with heterogeneous catalysis. However, these basic catalyst systems required rather harsh conditions (200-350°C, 70-150bar) and resulted in severe corrosion of standard reactor materials [21-23]. 

In the 1970s, several research groups started investigating transition-metal complexes as WGS catalysts. Several metal carbonyls were studied by Pettit at elevated pressures of 25 bar in the temperature range between 100 and 180 °C, as listed in Table 16.1 [24, 25]. 

Conditions 0.05 mmol of catalyst in a 300 ml stirred autoclave combining 22 ml of 25% aqueous trimediylamine, 78 ml ofTHF, and 350psi 25 bar of CO. 

Under significantly milder reaction conditions, Ford et al. studied a variety of ruthenium complexes for their activity in the WGS in the presence of KOH as base [26-28]. As a result, much lower TOFs were reported, as summarized in Table 16.2. 

Laine et al. studied the WGS mechanism of these systems by in situ Fourier transform infrared (FT-IR) experiments [33-35]. From the assigned intermediates in the ruthenium carbonyl-catalyzed WGS, a possible involvement of formic acid or formate was discussed and the occurrence of polymeric ruthenium clusters was reported. 

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An example of a homogeneous WGS catalyst is Rht(CO)i2 in aqueous pyridine.51 Analogous reactions with CO and alcohols instead of water are also known.52 Some important reactions of CO, H2, and methanol are shown in Fig. 22-5. 

It is not surprising that homogeneous WGS catalysts are in two categories—those which operate in acidic and those in basic media. Of the acid-based systems, the most active are the rhodium carbonyl iodide combinations ", the PtCl4 /SnCl3 preparation, and the system based on the ruthenium carbonyl cluster catalyst precursors Ru3(CO)i2 and H4Ru4(CO),2 . The Rh carbonyl iodide system under more vigorous conditions (185°C, 23 atm ) shows a catalytic rate of 400 tumovers/h. 

Immobilization of homogeneous WGS catalysts was reported by Doi ct al. [41] for ruthenium carbonyl complexes modified with phosphine ligands as linkers to silica support material, resulting in a low activity of 0.5 h . RuClj-hydrate immobilized on silica was investigated by means of FT-IR. Upon recrystallization, the formation of dimeric [Ru(CO)3Cl2]2 and [Ru2(CO)i Cl4(H20)] was seen on the surface. Silica was also used to immobilize RuClj [42]. 

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