Methanol catalysts, ruthenium complexes

Most of the catalysts employed in PEM and direct methanol fuel cells, DMFCs, are based on Pt, as discussed above. However, when used as cathode catalysts in DMFCs, Pt containing catalysts can become poisoned by methanol that crosses over from the anode. Thus, considerable effort has been invested in the search for both methanol resistant membranes and cathode catalysts that are tolerant to methanol. Two classes of catalysts have been shown to exhibit oxygen reduction catalysis and methanol resistance, ruthenium chalcogen based catalysts " " and metal macrocycle complexes, such as porphyrins or phthalocyanines. ... 

Knifton reported that the combination of ruthenium complex/phosphonium salt, such as Ru02/Bu4PBr and Ru(acac)3/Bu4PBr, is a good catalyst for the synthesis of ethylene glycol together with methanol and ethanol [12]. 

Homologation is the one-carbon extension reaction of organic compounds such as alcohols and carboxylic esters, and is very important. Cobalt, rhodium, and ruthenium complexes are known to be efficient catalysts. Methanol and methyl ester can be converted to ethanol and ethyl ester, respectively, using Ru/F [28] and Ru/Co [29] catalysts (Eq. 11.9). 

In 1970 the transition metal catalyzed formation of alkyl formates from CO2, H2, and alcohols was first described. Phosphine complexes of Group 8 to Group 10 transition metals and carbonyl metallates of Groups 6 and 8 show catalytic activity (TON 6-60) and in most cases a positive effect by addition of amines or other basic additives [26 a, 54-58]. A more effective catalytic system has been found when carrying out the reaction in the supercritical phase (TON 3500) [54 a]. Similarly to the synthesis of formic acid, the synthesis of methyl formate in SCCO2 is successful in the presence of methanol and ruthenium(II) catalyst systems [54 b]. 

FIGURE 2 Comparison of methanol carbonylation rates as a function of water concentration for rhodium-complex, iridium-complex, and iridium/ruthenium-complex catalysts (190 °C, 28 bar, 30% MeOAc (w/w), 8.4% Mel (w/w), 1950 ppm Ir or equimolar Rh). Adapted with permission from Figure 1 in reference [125], copyright 2004, American Chemical Society. 

Catalytic oxidation of electron rich alkenes such as styrene with ruthenium complexes gave mainly benzaldehyde rather than the expected epoxide. When styrene was oxidised with CHP as the oxidising agent benzaldehyde was the major detectable product. Polymerisation of styrene occurs in solvents like methanol. Table-4 shows the oxidation of styrene with CHP. Substitution of methyl groups in the pyridine ring increases the yield of benzaldehyde. When RuCl2(4-Mepy)4 was the catalyst, negligible amounts of styrene oxide was also detected. However with other catalysts benzaldehyde was the only detectable product. Other oxidation products like phenyl acetaldehyde and acetophenone were not detected [10,11]. 

Pu [23] prepared soluble irregular copolymers, and Fan [24] regular ones (9), of BINAP and BINOL that were incorporated into the polymer chain [Eq. (7)]. The ruthenium complexes were used for catalytic hydrogenation of arylacrylic acid [Eq. (8)]. The maximum ee of 88% (er= 16) with TTN = 100 is lower than with BINAP imder comparable conditions. No recycling of the hydrogenation catalyst is reported, whereas for diethylzinc addition to benzaldehyde the catalyst was recycled by precipitation with methanol. 

Genet and co-workers reported the synthesis of a P EG (molecular weight = 5000)-attached BINAP ligand 14 [Eq. (15)] [30]. The ruthenium complex was used for the hydrogenation of 3-ketobutanoic (methyl aceroacetate) acid methyl ester with a quantitative yield and ee = 98% (er = 99) in methanol [Eq. (16)]. The catalyst was recycled three times by precipitation and successive filtration, giving an overall TTN of400. Whereas conversion in each run was still quantitative, the ee dropped from 98% (er = 99) in the first two runs to 96% (49) and 95% (39) in the next two runs. 

Ruthenium catalysts have also been applied as effective catalysts. The selective oxidation of 12 could be achieved by using terpyridine-derived ruthenium complexes, with catalyst loadings below 1 mol% and the addition of a phase transfer catalyst (PTC an ammonium, phosphonium, or sulfonium salt) in biphasic aqueous systems, but also in methanol without PTC, and without the need for adding... 

The above oxidation protocol was adapted to the formation C -CN bond of tertiary amines using RuCl3.nH20 as the ruthenium catalyst and oxygen in the presence of acetic acid as the oxidant at 60 C in a 3 1 methanol/acid acetic media (Scheme 33) [24]. Other ruthenium complexes were tested and RuCls, K2[RuCl5(H20)] and Ru2(OAc)4Cl were found to be excellent catalysts for the... 

The Water-gas Shift Reaction.—This reaction is catalysed by M(CO) (activity M = W>Mo>Cr) in the presence of base and under phase-transfer conditions these carbonyls, in common with MS(CO)i2 (M =Ru or Os), are also active in the presence of sodium sulphide. The most active catalysts reported are Fe(CO)6 in basic methanol (turnover No. 2000 per day at 180 °C ) and Rh6(CO)i6 with diamine co-catalysts (e.g., en, turnover No. a 25 h at 100 C). Photolysis of [RuCl(CO)(bipy)a]Cl in water under CO produces COa and catalytically the CO2 is produced in a thermal step, whereas the formation of Ha is photo-initiated. Water-gas has also been used to hydroformylate pent-1-ene in the presence of ruthenium complexes similarly, water-gas is used in reaction (9), which is catalysed by a variety of Group VIII metal complexes... 

In 1977 Ford and co-workers showed that Ru3(CO)12 in the presence of a ca. fiftyfold excess of KOH catalyzes the shift reaction at 100°C/1 bar CO (79). The effectiveness of the system increased markedly as temperature was increased (rate of hydrogen formation approximately quadrupled on raising the temperature from 100° to 110°C), and over a 30-day period catalyst turnovers of 150 and 3 were found for Ru3(CO)12 and KOH, respectively. Neither methane nor methanol was detected in the reaction products. Although the nature of the active ruthenium species could not be unambiguously established, infrared data indicated that it is not Ru3(CO)12, and the complexity of the infrared spectrum in the... 

For each case we will also present catalytic analogues, namely (1) the activation of methane to form methanol with platinum, the reaction of certain aromatics with palladium to give alkene-substituted aromatics, and (2) the alkylation of aromatics with ruthenium catalysts, and the borylation of alkanes and arenes with a variety of metal complexes. 

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