Hydrogenation, of CO, to methanol

With the recent development of zeolite catalysts that can efficiently transform methanol into synfuels, homogeneous catalysis of reaction (2) has suddenly grown in importance. Unfortunately, aside from the reports of Bradley (6), Bathke and Feder (]), and the work of Pruett (8) at Union Carbide (largely unpublished), very little is known about the homogeneous catalytic hydrogenation of CO to methanol. Two possible mechanisms for methanol formation are suggested by literature discussions of Fischer-Tropsch catalysis (9-10). These are shown in Schemes 1 and 2. 

The formation of metal-oxygen bonds has previously been found to occur for the stoichiometric hydrogenation of CO to methanol with metal hydrides of the early transition metals (20). Moreover, in ruthenium-phosphine catalyzed hydrogenation (with H2) of aldehydes and ketones, metal-oxygen bonded catalytic intermediates have been proposed for the catalytic cycle and in one case isolated (21,22). 

It is assumed diat this reaction is an important step in the catalytic hydrogenation of CO to methanol ... 

Another example of potassium as a promoter is in the hydrogenating of CO to give methanol directly, as mentioned earlier [M. Maack, H. Friis-Jensen, S. Sckerl, J. H. Larsen and I. Chorkendorff Top. Catal. 22 (2003) 161]. Here it works as a promoter for CO hydrogenation, but with conventional methanol synthesis great efforts are made to avoid the presence of alkalis in the catalyst as they tend to ruin the selectivity by promoting the production of higher alcohols, i.e. the surface becomes too reactive. Thus great care has to be exercised to achieve the optimal effects. 

The CO reductions generally could likely proceed through formyl intermediates, probably at a multinuclear site (420) hydride migration to a coordinated CO [e.g., as in the hypothetical scheme outlined in Eq. (72)] has not yet been observed, although metal formyl complexes have been synthesized via other methods (422-425). A ir-bonded formyl also seems plausible (426), since 7r-bonded acyl groups have been demonstrated (427). A stoichiometric hydrogen reduction of CO to methanol under mild conditions via a bis(pentamethylcyclopentadienyl)zirconium complex is considered to go through a formyl intermediate (428, 429) ... 

In a very recent example (Figure 1.53), INEPT DOSY has been applied to confirm the dinudear nature of the unstable Zirconium intermediate 141 in the reaction of CO with [ZrHCl(Cp)], 140 [344], a model reaction for the heterogeneously catalyzed hydrogenation of CO2 to methanol [345]. As was expected for a binuclear compound, the diffusion coefficient of intermediate 141 is smaller than for the mononuclear 142. 

Also of potential interest is the direct hydrogenation of CO to isobutanol as it was practiced by BASF (13-14) or still is done in the German Democratic Republic. Again, a mixture of alcohols is obtained with isobutanol amounting up to 30 % (. The latter one can be dehydrated to isobutene (chemical usage) or converted with methanol to fuel usage (MTBE). 

With the established success of heterogeneous catalysts in the hydrogenation of CO via methanol synthesis, methanation, and F-T synthesis, it is justifiable to question the interest in investigating these reactions under... 

Figure 1.1.7 Equilibrium calculation for the hydrogenation of CO2 to methanol at 50 bar pressure. The parameter is the fraction of CO in CO2 facilitating the hydrogenation.

Examination of the power-law exponents presented here show that the rate of hydrogenation of CO to hydrocartx)ns and oxygenate is inhibited by CO over Rh/Al203 but not for methanol formation over Rh-Mo/Al203. Interestingly, the inhibition for CH4 and higher alcohols formation remains. The implication is that the mechanism of the rate determining step for methanol differs fiom methane and that the latter is dependent on the Rh. 

In this mechanism the intermediates are boimd to the surface through oxygen. Support for this assmnption is provided by the fact that CO is known to react with strongly basic hydroxides such as NaOH to form formate ions. The methanol catalyst contains the strongly basic component ZnO. Fmthermore, it is known that copper is a highly active catalyst for the hydrogenation of formate to methanol. 

In addition to the CO dissociation paths discussed in the previous subsection, the presence of coadsorbed hydrogen offers a third potential reaction path for the activation of the CO bond. CO bond activation, which proceeds through interaction with the metal surface, becomes more difficult for Group VIII metals at the bottom-right corner of the periodic table. In contrast, the weaker M-CO bonds tend to help promote the hydrogenation of CO to form formaldehyde or methanol. This reaction proceeds through the formation of surface formyl (CHO) intermediates. 

Carbon Monoxide Reduction and the Water Gas Shift Reaction A review of the role of metal cluster catalysts in solution and attached to supports for CO hydrogenation is optimistic about the use of cluster-derived heterogeneous catalysts on industrial processes.Selective reduction of CO to methanol occurs on Pt cathodes coated with K2Fe2(CN)6 in methanol solutions containing Fe(III) or Cr(III) complexes. 

Non-metallic homogeneous catalyst systems were also reported for methanol synthesis. Recently, Ashley et al. [49] demonstrated the selective hydrogenation of COj to methanol using a FLP-based nomnetal mediated procedure at low pressures (1-2 atm). N-Heterocyclic carbine (NHC) was found to be an elFective organic catalyst for methanol synthesis from CO2 reduction with silane. Compared to transition metal catalyst, NHC is more efficient at ambient reaction conditions [50,51]. Table 5.1 lists catalytic activities of different heterogeneous catalysts employed for methanol synthesis from CO. It shows that maximum CO conversion of 25.9%, methanol selectivity of 99.5% and methanol yield of378 mg/g-cat h could be achieved. The space velocities were tried between 1800 and 18,000 h and the temperature from 170 to 270 C. 

Production of Methanol Remarkable progress has been made in the last 10 years in terms of catalyst development for the hydrogenation of CO2 to methanol, such that 100% selectivity and high TOP have been observed. The excellent performance, most hkely due to a different reaction mechanism [139] than with CO, compensates for the extra amount of dihydrogen needed for CO2 reduction (Eq. 27) with respect to CO (Eq. 26), that is currently used ... 

Non-stoichiometric Zn/Cr and Cu/Cr mixed oxides are one of the principal examples of these unusual solids. They have applications as both solid state gas sensors (5) and catalysts for hydrogenation reactions (of CO to methanol and/or methanol-higher alcohol mixtures, and of many organic molecules) (6-12). These systems have been widely investigated over the last few years, and results obtained show that their peculiar catalytic properties may be associated with the presence of non-stoichiometric phases (with a M /M ratio higher than 0.5, M= metal), in which some of the zinc or copper ions are present in octahedral positions, i.e., with an unusual coordination. However, until now very few data have been reported regarding the changes in structure and reactivity as a function of the composition in ternary systems (for instance Cu/Zn/Cr). 

Dehydrogenation processes in particular have been studied, with conversions in most cases well beyond thermodynamic equihbrium Ethane to ethylene, propane to propylene, water-gas shirt reaction CO -I- H9O CO9 + H9, ethylbenzene to styrene, cyclohexane to benzene, and others. Some hydrogenations and oxidations also show improvement in yields in the presence of catalytic membranes, although it is not obvious why the yields should be better since no separation is involved hydrogenation of nitrobenzene to aniline, of cyclopentadiene to cyclopentene, of furfural to furfuryl alcohol, and so on oxidation of ethylene to acetaldehyde, of methanol to formaldehyde, and so on. 

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