Direct Routes from Synthesis Gas to Ethylene Glycol 6. D. Dornbek Union Carbide Corporation, South Charleston, WV 25303
Ethylene glycol is an important industrial chemical that is used mainly as a component in polyester resin, fiber, and automobile antifreeze. This chemical is currentlv manufactured from ethylene. A putenrially more economical starting material is syn~hesisgas (syngas), R mixture of carbon monoxide and hvdroeen that can~beobtained from natural -eas., . heavy petroleum residues, coal, or even biomass. Although there are manv ~ossiblewavs to obtain ethvlene elvcol - " from syngas, adirectioute (eq 1);s theoretically the most efficient and simplest route.
-
2CO
+ 3H2
-
HOCHzCHzOH
(I)
As written, this is a remarkable reaction involving the combination of the simple molecules CO and Hz to yield a product having C-C, C-H, and O-H bonds not in the starting materials. In fact, there are homogeneous catalysts which can perform this conversion quite effectively (I).(Although heterogeneous catalysts hydrogenate CO, as in the FischerTropsch process or the methanol synthesis reaction, these catalysts are not known to produce ethylene glycol or other ~olvalcohols.)I would like to describe here some of the iesearch effort devoted to improving and understanding the action of the oreanometallic catalvsts involved in this remarkable reaction. Homogeneous~catalystsfor the direct conversion of syngas to organic chemicals have not yet been operated commercially, so this chemistry should be viewed less as an industrial application than as an approach to an important industrial G l .
hydroformylation (8.9) (or hvdroaenation) of formaldehvde and areshown in Figurr l.'l'he ke; step in this reaction isihe insertion ut iormaldehyde into the metal-hydrogen bond to yield either a methoxy~ligand(which may lead to methanol or methyl formate) or a bydroxymethyl ligand (which can generate elvcolaldehvde and thus ethvlene elvcol). - " , The direction of this insertion presumably depends on a number of factors. some of which mav be related to the aciditv of the hydride ligand as will be discussed below. Small amounts of alvcerol and hieher - ~ o l v o l sare sometimes observed in re&ons which produce ethylene glycol (I). These products can apparentlv he formed because an aldehyde inivrmediate suchas glycolaldehyde can be hydrogenated (by u process analogous ro r h a ~of Fig. 1) as well as hydrofurmylared. Amounrs uf these higher products mav he low, howrver, because of a steric preference for hydride addition to the aldehyde carbon in these higher aldehydes.
-.
-
Rhodium Catalysts Rhodium catalysts were found to be more active and selective than cobalt catalvsts for homoeeneous svneas conversion and particularl; for ethyleneuglycol formation (10). These catalysts are most effective in the presence of certain additives, or promoters, which appear to act as reducing agents by virtue of their proton basicity (I). Effective promoters, which include amines and alkali metal carboxylates, convert the rhodium to anionic carbonyl complexes, including a variety of cluster complexes. The amount of promoter employed is critical in this process. With no promoter, the rhodium exists mainlv a s RhlO) , , comnlexes such ~~~~~-as ~~RhdC0)16; too large an.imount of promoter converts much of the rhodium to [Rh(CO)&, in the Rh(1-) oxidation state. The optimum amount of promoter appears to be in the vicinitv of a Rh/~romoterratio of 6:l. The com~lexesproduced-in such mixtures are composed of rhodium id an average oxidation state between 0 and -1, and the species observed are clusters such as [Rhs(CO)lsl-, [Rh&0)3oI2-, ( x = 2,3). I t is not known whether and IR~I~(CO)UH,~(S-')these rlustersare themselves thecatalysts forsyngas conversion, or whether they are in equilil)rium with other species which are thr catalytirally acrive complexes; the kinetic dependences are romplex and uninformative on his point. This rhodium-containing catalytic system suffers from relatively low stability, apparently because the metal clus~
Cobalt Catalysts Catalytic experiments a t Du Pont reported in patents of the earlv 1950's showed that cobalt catalvsts under hieh (-3000 atm) converted syngas to organic includine alvcol. methanol, and other alcohols (2). -ethvlene . More recent researc'inder lower pressures (where the reaction is markedly slower) has provided much information about how this reaction proceeds (3, 4). The major species present under reaction conditions is HCo(C0)4. The rate equation observed for product formation (first-order dependences on Hz pressure and cobalt concentration) suggests that the reaction proceeds by intramolecular migration of the hydride in this complex from the metal to a carbonyl ligand (eq 2). Analogous migrations of alkyl groups from a metal atom to a carbonyl ligand are, of course, well known (5). Formyl ligand formation by hydride migration, however, is rarely observed under normal conditions, apparently because of the thermodynamic instability of most metal formyl complexes (6). (Many metal formyl complexes have been prepared bv other methods, usuallv bv reaction of metal-caibonyl complexeg with p ~ ~ e rhidride f ~ i donors such as borohydrides (7).) Subsequent reaction of the cobalt formyl intermediate with hydrogen (probably the actual rate-limiting step) could convert the formyl ligand to formaldehyde, a reactive intermediate which may not leave the metal before undergoing further reaction.
Pathways by which such a formaldehyde complex could react are presumed to be similar t o those involved in the 210
Journal of Chemical Education
A~
~
~~
Figure 1. General mechanism proposed to account for the formation of major products observed in homogeneous CO hydrogenation reactions.
ters present grow to even larger clusters under the rather severe conditions required for catalysis (temperatures of 200 OC and above). These large clusters have lower solubility, and thus the rhodium can be lost from solution. Ruthenium Catalysts Simple ruthenium catalysts, such as that derived from R u ~ ( C O in ) ~organic ~ solvents, also possess syngas conversion activitv" (11. . . 12). . The stable species observed under catalytic conditions is the mononuciear carbonyl Ru(CO)s, u,hich is known to react with H? to vield the reactive hvdride HZRu(CO),. Kinetic evidence supports areaction scheme for this complex which is closely analogous to that outlined above for the cobalt hydride (I). Indeed, studies on this system have contributed much to an understanding of the basic processes involved in product formation (13). A major difference, however, is seen in the products formed by this ruthenium svstem. No ethvlene elvcol .. . is observed:. onlv. methanoland methyl formate are produced. This may occur because the hvdride invol\,ed at the formaldehvde incorporation step is iess acidic than in the cobalt systed, so most of the reaction mav Droceed throueh a methoxv intermediate. as shown by Figure 2. Efforts to improve the selectivitv and activity of this system led us to
