CO 2 and CO activation

Transcription

1 2 and activation Most organic chemicals are currently made commercially from ethylene, a product of oil refining. Itispossiblethatinthenextseveraldecadeswemayhavetoshifttowardothercarbonsources for these chemicals as depletion of our oil reserves continues. Either coal or natural gas (methane) can be converted into / 2 mixtures with air and steam, and it is possible to convert such mixtures, variously called water gas or synthesis gas to methanol and to alkane fuels with various heterogeneous catalysts. In particular, the Fischer Tropsch reaction converts synthesis gas to a mixture of long-chain alkanes and alcohols using heterogeneous catalysts(typically Co, or Ru).

2 Water-gas shift reaction It is often useful to change the : 2 ratio in synthesis gas, and this can be accomplished by the water gas shift reaction, which can be catalyzed heterogeneously ( 3 4 or Cu/Zn) or byavarietyofhomogeneouscatalysts,suchas() 5 orpt(i-pr 3 P) 3. The reagents and products have comparable free energies; the reaction can therefore be run ineitherdirectionandthiscanberegardedasbothand 2 activation. In the mechanism proposed forthe homogeneous iron catalyst, binds to the metal and so becomesactivatedfornucleophilicattackby ionatthecarbon. Decarboxylation of the resulting metalacarboxylic acid probably does not take place by β elimination because this would require prior loss of to generate a vacant site; instead, deprotonation may precede loss of 2, followed by reprotonation at the metal to give ()4.

3 substitution 2 - nucleophilic addition C electrophilic addition elimination (decarboxylation)

4 Using a platinum catalyst for the same water-gas shift reaction is perhaps more interesting in thatitactivatesboththewaterandthe,sonoaddedbaseisneeded. This happens because the platinum complex is sufficiently basic to deprotonate the water, leading to a cationic hydride complex. The cationic charge activates the for nucleophilic attack by hydroxide ion to give the metalacarboxylic acid(m ).

5 Alkene hydroformylation (oxo process) The catalysis of alkene hydroformylation(roelen 1938), with 4 million tonnes per year, is one of the most industrially relevant applications of organometallic chemistry. The aldehyde products are further reacted via hydrogenation for alcohol synthesis, which are used for detergent, plastics and carboxylic acids(via further oxidation) production. The irreversibility of the overall hydroformylation cycle is due to the irreversibility of the final reductive elimination step, releasing an aldehyde from the intermediate metal-hydride-formyl species.thelatterbeinggeneratedbyoxidativeadditionof generated by addition of 2.

6 binuclear oxidative addition Co Co, 2 Co - C 3 reductive elimination n-butanal Co addition Co Co C 3 oxidative addition 2-2 1,2-insertion Co - Co addition& 1,1'-insertion

7 Alkene hydroformylation (Reppe reaction) The Reppe reaction uses the water gas shift reaction to generate 2 / for subsequent hydroformylation of the substrate alkene to give an aldehyde, followed by hydrogenation to give an alcohol. Withthe() 5 /basecatalystmentionedabove,theproductisthelinearalcohol. Thealkeneisbelievedto insert intoan bond of the active catalyst, 2 () 4, followed by migratory insertion to give (RC 2 C 2 )() 3, which in turn reductively eliminates the aldehyderc 2 C 2 C. Thisaldehydeisthenhydrogenatedtothealcoholwith() 4 ascatalyst. By itself, () 5 is not a hydroformylation catalyst because 2 cannot displace to form 2 ()4,hencetheneedforthebasetoremove the.

8 .postulated mechanism: nucleophilic addition - C 2 elimination ( decarboxylation) substitution electrophilic addition n-butanal C 3 addition + 2 C 3 C 3 1,1'-migratory insertion C 3 1,2-migratory insertion

9 Monsanto acetic acid process ver two million tons of acetic acid a year are produced by carbonylation of methanol, which happens in >99% selectivity with a rhodium catalyst. The active catalyst is Theneteffectisthecleavage ofthemethanol C bond andinsertionofa. To be carbonylated, the methanol has to bind to the catalyst and this requires adding a certain amount of I to the system, which produces an equilibrium concentration of MeI, which can in turn oxidatively add to the metal in the turnover limiting step. nce we have the rhodium methyl, migratory insertion can take place with to give an acetylrhodium iodide. Reductive elimination of the acyl iodide completes the cycle. Thefreeacyliodideishydrolyzed by themethanolto give methylacetate andcanbeultimately converted to acetic acid with water. The resulting acetic acid can be entirely derived from synthesis gas.

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11 In a very closely related reaction, C 3 Me can be carbonylated to acetic anhydride (C 3 ) 2...postulated mechanism: 3 C 3 C C 3 C 3 reductive elimination I Rh oxidative addition I I I I Rh C 3 C 3 1,1'-insertion &addition I 3 C Rh C 3

12 The Monsanto process for making acetic acid is replacing the older route that goes from ethylene by the Wacker process to acetaldehyde, which is then oxidized to acetic acid in a second step. This example shows how important it is that chemical companies carry out research into possible alternative ways to make a compound, even though the current route is working well; otherwise their competitors may discover a better one. An improved process based on iridium has been developed by BP-Amoco.

13 Wacker oxidation (Nucleophilic addition of 2 at ethylene) Alkene complexes undergo nucleophilic attack to give metal alkyls, which can often rearrange to give other products. This is the basis of an important industrial process, the Wacker process, now used to make about 4 million tons of aldehydes from alkenes annually. The fact that aqueous PdCl 2 oxidizes ethylene to acetaldehyde had been known (although not understood) since the nineteenth century. The reaction consumes the PdCl 2 and deposits metallic Pd(0). It took considerable imagination to seethatsuchareactionmightbeusefulonanindustrialscalebecausepdcl 2 isfartooexpensiveto use as a stoichiometric reagent in the synthesis. The key is catalysis, which allows the Pd to be recycled almost indefinitely. J. Smidt of Wacker Chemie realized in the late 1950 s that it is possible to intercept the Pd(0) before it has a chance to precipitate by using CuCl 2, which reoxidizes the palladium and is itself reduced to cuprous chloride. This is air sensitive and is reoxidized back to Cu(II). The resulting set of reactions are an elegantly simple solution to the problem and resemble the coupled reactions of biochemical catalysis.

14 2 substitution 2 Cl 2 Pd Cl nucleophilic addition ( hydroxypalladation) 2 Cl Pd Cl 2 1/ Cl 2 2CuCl 2CuCl Cl Pd Cl 2 2 reductive elimination 2 Cl 2 2 Pd Pd 2 2 Cl 2 Cl Pd elimination Cl substitution acetaldehyde 2 2 Cl Pd elimination

15 A related process, 2 reduction, is receiving increased attention towards mitagation of greenhouse gases and production of C-1 feedstocks such as methanol. 2 is so thermodynamically stable that only a very few potential products can be made from 2 byexothermicprocesses. ne could reduce it to with hydrogen by the water gas shift, and then use chemistry to makevariouscarboncompounds,exceptthat 2 isveryexpensive. Indeed, the current methods of making 2 involve the consumption of either coal or natural gas, which are valuable carbon sources.

16 Utilization of 2 in chemical industry today Intermediates or fine chemicals for industry R-C()- R ; acids, esters, lactones R--C()-R ; carbonates RN-C()R ; carbamic esters -N; isocyanates R 2 N-C()-NR 2 ; ureas Use as solvent Energy rich products, C 3 ydrocarbons and their derivatives The amount of 2 (115 Mt) used annually by the chemical industry is less than 0.5 % of the total annual 2 emissions----t MUC 2 T NVERT T CEMICALS! M. Arestaet al., Chem. Rev. 1998

17 (lecture 8 metal hydrides and dihydrogen complexes) An example of an isolable late transition metal hydride complex [(η 6 -hmb)ru II bpy] + and its aqua derivative [(η 6 -hmb)ru II ( 2 )bpy ] 2+ (hmb = hexamethylbenzene) ayashi et al. J. Am. Chem. Soc., 2003, 125 (47), go et al. rganometallics, 2002, 21 (14),

18 Applied in catalysis to promote ketone reduction e.g. acetone to isopropanol. The formate ion ( 2 ) is used as a source of with liberation of 2.

19 The same catalyst can be used for reduction of 2 under the appropriate conditions.