Homogeneous Catalysis-Industrial Applications Jiirgen Falbe and Helmut Bahrrnann Ruhrchemie, AG., Mulheim (Ruhr), West Germany Translator: Hans-Georg Glldei Marietta College, Marietta, OH 45750 The use of catalysts with their selective and effective transformation paqsibilities appears in a new light as a result of the increased price of oil, a raw material and an energy source so imoortant to the chemical industrv. While heterogeneous catalysts have been used for some time by the chemical industrv, the laree chemical Drocesses with homogeneous catalystshave been applied on& in recent times. Now more than ever in the development and planning of new processes i t is important, if both systems are applicable to compare carefully the advantages and disadvantages of both catalvsts. ,~ ~A comparison of important properties of the two types of catalysts is found in'rable 1. There are some ohvious advantages of homogeneous catalysts. First, hecause surface effech are lacking, homogeneous catalysts are more effective and have a higher specificity than heterogeneous catalysts. Moreover. homoeeneous catalvsts can he made more reDroducihly A d witcdefinite stoiihiometry and structure. h r thermore.. thev- not onlv work under milder reaction conditions than heterogeneous ckalysts, but also the constancy of homo~eneous-catalvzedreactions can be sustained better over a longer reaction;nterval and the material requirements are as a rule minimal. The catalvst separation. however..in eeneral is easier with heterogeneous catalysts. In spite of these advantages, the homogeneous catalysts could displace the heterogeneous catalysts only in part. The important large-scale productions of chemical raw materials from oil-like hydrocracking in the production of ethylene, propylene, butylene, and aromatics as well as the reforming processes-are still exclusively heterogeneously catalyzed. The reason for this is that to date no one has succeeded in finding a homogeneous catalyst that sufficiently activates the C-H bond. Significant future prospects, however, will be in the chemistry of synthesis gas which is based on coal and will be greatly expanded in the coming years. ~
'
Permission to publish a translated version of this raticle, which was published originally in Chemie in unsererzeit, (15, 37 (1981)], was granted by Verlag Chemie and Is hereby gratefully acknowledged. The banslator thanks Mrs. H. C. Giide and Mrs. J. Dunn for their assistance in the preparation of the manuscript.
A central urohlem in the technical use of catalvsts is the contact-separation. As Table 2 indicates, the homogeneous catalysts can he separated by distillation, extraction with chemical decomposition, and subsequent filtration. These operations are in general not quantitative so that in a continuing process, in contrast toheterogeneous catalysts, the return of the catalyst necessitates an additional purification. The separation and re-use of the catalyst are made additionally difficult by the small catalyst concentrations. Kxtensive care must be used to avoid losses since often expensive noble metals are employed. In summary, however, the advantages of homogeneous catalysts outweigh the disadvantages because the greater variety of possible and specifically designed reactions promise a bright future. Industrially Established, Homogeneously Catalyzed Reactions nydroformylaltio (0x0 SynthesidRoelen Reaction) One of the first major industrial reactions to use homogeneous catalysts was the 0x0 synthesis, which has been exteusively investigated. In 1978the 0x0 capacity totaled 4.4 million tons with 2.3 million tons in Western Europe and 1.4 million tons in North America. This process converts olefins in the presence of cobalt hydridocarbonyl, carbon monoxide, and water to aldehydes. Generally the cobalt catalysts are produced in silu from cobalt compounds and synthesis gas. The synthesis procceedsat approximately 140' to 180°Cand with a pressure of 2W:iOO atm. As illustrated in Figure 1, the active catalyst I reacts with the olefin to form the T complex 11which then yields the alkyl complex 111. Following coordination and insertion of CO, compound IV and the acy1 complex V result. Then, upon addition of hvdroeen. . - . the aldehvde is formed and releases-the active catalyst I. The technical importance of this reaction derives from the ease with which the primaryoxo product, thealdehyde, can be convened into a multitude of industriallv important secondary products like alcohols, acids, diols; amines, or es-
"-.".
Propylene is used primarily as the olefin. The resulting n-butyraldehyde can be transformed either to n-butanol via
Table 1. Comparlsm of Homogeneous and Heterogeneous Catalysts
Table 2. Separation of Catalysts
Catalyst
Actiie centers Concentration Diffusion problems
Homogeneous Effectiveness all metal atoms small not present
Heterogeneous
Separatim sMfilceatoms only high
present
Reprcducibilityof catalyst prepratlon Sbucture known unknown Stoichiometiy known unknown Modification possibllny high small Reaction condnions mild severe Catalyst separation in part costly easy Applications limited wide
Addnional apparatus expenditure Catalyst return
Mmogeneous catalyst finration after chemical decomposition distillation extraction yes possible
Cost for catalyst high 106s Catalyst concent. small In the waduct
Heterogeneous catalyst suspension solid suppolt filtration no problems
S ~ I I
no
easiw possible small
not required
small
high
...
Volume 61 Number 11 November 1984
961
hydrogenation or, following aldol condensation and hydrogenation, to 2-ethylhexanol, the most important alcohol for plasticizers. Starting with higher olefins, one obtained alcohols and acids which are converted further t o detereents. lubrieating oils, plasticizers, and solvents. The largest 0x0 plants are operated by companies such as BASF, ICI, Kuhlmann, Mitsubishi, Ruhrchemie, Shell, and UCC.
2CH&H,CH2CH0 hutyraldehyde
I
ddol
CH,CH,CH,CH=C-CHO
mndemtion
pCH,
Using the cobalt catalyst system as a basis, Tahle 3 shows the steps, such as catalyst separation and return, in the different variations of the technically decisive processes. The catalytically active cobalt thydridocarhonyl is generally decomposed thermally or chemically. In the Ruhrchemie process, for example, the solid cobalt-containing phase that is formed is separated mechanically and returned to the reaction. In the BASF process the cobalt carhonyl is first oxidized and then returned as an aqueous cobalt acetate solution. Depending upon the starting olefin and desired reaction product, every process has its own specific advantages. In rontrast to other proresses, Shell utilizes hydridocarhonyltrialkylphosphine catalyst, HCotCO)yPR:,. Due to the high thermalstability of the catalyst, it is possible to separate the
1
Cobalt~ compound
R-CHs-CHFCHO straight-chain aldehyde
+
0
1
~
products by distillation. T o do this, the following disadvantages have to he accepted. Despite higher temperature (180°C),the reaction rate is slower by a factor of 5 as compared with unmodified cobaltcarbonyland requires, therefore,a 5-10 times larger reaction volume with correspondingly higher investment cos& The higher reducing activity of the catalyst results in a loss of 10-156 of the starting olefin;reaction products are predominantly alcohols. Reppe Reactions According to Reppe, a base HX with a mobile hydrogen can he added to an olefinic or acetylenic C-C bond with simultaneous insertion of CO (Fig. 2). The insertion of CO is also possible on an activated carbon atom, e.g., in alcohols. Table 3. Process
Catalyst Separation and Return for DUlerent Oxowoceases Separation Method
Ruhrchemie* Ihermal decomposIion BASFs chemical dewmposition (CtbCOOH 02) Mkubishia chemical dewmposition (carboxylic acid) Kuhlmann* extraction under pressure (10a h ) with dilute NaHCOs solution UCCs chemical decomposition (H2S04.CH3COOH) Shell distillation
HC--CH
I
HCo(C0)r
+H@
cat1co
cat./co
HZC=CHZ + H @
R-CH-CHO branched-chain aldehyde Cw(C0)s
Hzl PC0
solid cobalt compounds cobalt acetate solution
+
~
CH3
Returned as
H~C@
cst/co
Catalyst: HCo(C0)4 Ni(C0)r Fe(CO)s
oilaoluble cobalt soaps H W C O k (after release wim HISO,) in me insertion olefin W O W , (afterprecipitation with base) di~tillatlonresidue
H-CH=CH-COB H-CH~CH~-CC@
H3C-CC@
@= -OH -0-Alkyl -0-ACYI
-NRz ete. Figure 2.
Elementary Reppe transformation
X = Halogen Figure 1.
962
Mechanism of the hydrofamylationwiIh cobalt catalysts.
Journal of Chemical Education
Figure 3.
Mecbnism of the Reppe acrylic acid synwsls
labla 4. Capacltler ot the Rappe Carbonylatlon Process (1976) lr)80rtiar awn1
Acetylene
Capaciiy (in Product
awlate and acrylic acid
hcess BASF (W.Gsrmany
and U.S.A.);
Rohm ard Haas (U.S.A.); Toagasei Wlem. Ird. Co. Elhylsm Ropylene MemenOl
pr~plmlcacld BASF buianol Japan Butanal Co. Lid. (BASF-license) acetic acid BASF Borden Chemicals (U.S.A.) (BASF-license)
tons/vearl
Hoechst-Wacker Process
Soluble metal-complex salts are the catalysts for the oxidation of ethylene to acetaldehyde. As shown in Figure 5, the
132.000 30.000 140.000
36,000 40.000
30.000 40.000
52.000
I
Hs=C I
1. HzSOaCO 2. ROH
I
&C-C--COOR
kHa isohutylene
I
I ~ H ~
Pivalic acid (or ester) R=H or Alkyl
Figure 3 shows the mechanism of the acrylic acid synthesis from acetylene, carbon monoxide, and water with activated nickel carhonyl catalyst. The active catalyst [HNi(C0)3X] adds to the triple bond of the acetylene, and the CO is inserted in the formed Ni-alkyl bond. The complex thus formed is subsequently decomposed hy alcohol (or water) to yield ester (or acid) and the active catalyst. Table 4 shows the capacities of the Reppe carhonylation installations. The major producer is BASF, which produces acrylate from acetylene and alcohol, propionic acid from ethylene and water, and acetic acid from methanol. Although the RepDe svnthesis involves a neater auantitv of starting reagentsandreaction products &d altho&h it has progresed in development, the capacities, compared with the basic BASF techniaui. have been &naller than those of the 0x0 svnthesis for the'fo~owingreasons The starting agent acetylene is diilicult to handle: the transport especially is problemaric. The kppe p r e s s must compete with otber processes that utilize ethylene and that are often less costly Lewis Acid Catalyzed Transformations The simplest homogeneous catalyst is the proton. Lewis acid (H2SOa, BFa HsP04) catalyzed processes are employed industrially to a large extent in alkylation reactions, e.g., in the production of alkylated gasoline from isobutylene as well as of cumene, xylene, and styrene from benzene.
A variation of a CO-insertion known as the Koch synthesis is shown in Figure 4. Olefins or other compounds that in the presence of acid catalyst readily convert to carhocationsreact with carbon monoxide and water to produce carboxylic acid or with alcohol to yield the corresponding ester. The starting reagent (olefin, alcohol, or aldehyde) reacts thereby with a proton to form a carbocation VI which then adds CO. The generated acylium ion VII, after addition of alcohol, forms the oxonium complex VIII. This complex decomposes into the desired carboxylic acid or into the corresponding ester whereby the active catalyst is again freed. The most im~ortantstartine olefins are isobutvlene and diisohutylene as well as mixtures of olefins in the r&e of Cg to Clo (from oil). The highly branched carboxylic acids formed (e.g., pivalic acid) are starting materials lor resins, lacquers, and svnthetic lubricatine oils. The reaction conditions are relatiiely mild (pressures up to 70 atm and temperatures to a maximum of 70°C); however, expensive and corrosion-resisting material must be saved. Main producers are Shell, Enjay Chemical Co., and DuPont. The total world production is estimated to he -150,000 tons per year.
ROH
Figve 4. Uachanism of lbs Koch symhesis.
Figure 5. Ethylem banshmatiar wing palladium catalysts (Wacker prccess). Volume 81 Number 11 November 1984
963
Alfen mthesis (Zieeler) (see Fie. 6). the catalvst triethvl" aluminhm, A~(c~H&,, ;e&s w%h ethylene in-a so-called Aufbau reaction by insertion of the ethylene into the aluminum-carbon bond. The aluminum complex thus formed can accept additional ethylene molecules until finally the growing alkyl chain is set free as an a-olefm and the tri-alkylaluminum is regenerated. The Aufbau reaction and disolacement reactionare favored by low temperature and high pressure (90120°C and 100 atm) and high temperatures and low Dressure (200400°C and 50 atm), r&pect&ely. The isomer formation, besides being dependent on the reaction conditions, can be highly influenced by the ligands on the aluminum. This is generally true for all homogeneous catalysts. An impressive example is the oligomeriz&ion of dienes according to Wilke. Active catalvsts for such reactions are nickel(0) . ~comnlexes ~-~~~ . with little sterk hinderance (no ligands) such as represented bv the bis-?r-allvlcomoound XI1 (Fie. 7a). This comoound can accept an addikmal Gutadiene molecule with the subsequent formation of XIII. Further addition of butadiene leads to cyclododeca-1,5,9-trieneand the free nickel catalyst. When one of the coordination sites on nickel is blocked by a ligand (L) as in XIV (Fig. 7b), the result is almost entire formation of eight-carbon compounds. A highly basic ligand converts XIV to XV since the central atom in XV can better accommodate the extra electrons of the lieands L. In the displacement reaction with new butadiene, XIV reacts to form 1,s-cuclooctadiene while XV leads LO 4-vinvlcvclohexene. he long-chain a-olefins are important staking reagents for the production of lubricating- materials.. olasticizers. de. tergents, and polyamides. Of special importance are mixed catalyst systems since a higher degree of selectivity can be achieved witb these. Processes of industrial significance for the ethylene oligomerization are the Esso process with alkylaluminum chloride and titanium tetrachloride, the Shell process witb nickel-phosphine catalysts, and the Ethyl process with the classicalZiegler trialkylaluminum catalyst. In 1975 H d s began the production of cyclododecatriene from hutadiene with a timnium-tetrachloride/diethylaluminum chloride catalyst. Total capacity of the oligomerization process is about 800,000 tons per year of which approximately 90%consists of the a-olefins. ~
.~~-~~
~
.
n(CHz==CHd Figwe 6.
Mechanism of the Allen synihesis (Ziegler).
u
reaction forms T-complex IX, the coordination complex of ethylene and palladium(II), and upon nucleophilic attack by water converts to X. I t is still unclear as to how the hydroxvlated ethyl -mouo - in compound X converts to acetaldehyde and how the complex breaks down. A reasonable path is the transfer of hydrogen from the p-carbon atom in X to palladium with the formation of the vinylalcohol complex XI. The palladium(0) is continually regenerated to PdClz in situ by the redox system Cu(I)C1/Cu(Il)Cl~,HCI, and oxygen. The reaction conditions are mild (e.g., the two-step Wacker process operates at 10.5 atm and 125' to 130°C). The process can be carried out either in two steps with air or in one step with oxveen. 95%. The , " The -~ vield ~ in both cases is ao~roximatelv .. application of these two variations is de~erminedmore by fattors such as cost of oxveen than bv maior differences. This process, known only since 1960, h a i by i979 reached a production of approximately 2.24 million tons per year.
~.
~~~
-
~
~
~
~~
~
~~~~~
~~~~
Olioomerization of Olefins and Dienes
Homogeneous Oxidation Reactions in the Liouid Phase
Olefins and dienes can be oligomerized by means of homogeneous catalysts under mild conditions. According to the
Of great importance also is the oxidation of various substances by homogeneous catalysts, for example cyclohexane
L is weakly harir
xni 18)
\l 1.5-Cgclo~ctadiene
4-Vinylcyclohexene
lbl Figure 7. Mechanism of the cyciwiigomerlrationof butadlene (a) to cyckdeca-1.5,9-hiene using a nickei(0hr-bisaliyicatalyst. (b)to 1,Scycloocladiene (when L is of low basicity) or Cvinylcyclohexene (when L is shongiy basic) using nlckei(0t~-bisallyl-or nickel(0)-T-a-bisallyi catalysts, respectlveiy.
964
Journal of Chemical Education
to adipic acid (USA, 1977; 700,000 tons), butane to vinegar (USA, 1977; 986,000 tons), propylene to propylene oxide and especially p-xylene to terephthalic acid (USA, 1977; 2.277 million tons). The assumption for example is that in the oxidation of p-xylene, the hiemediate hydroperoxide converts the Co2+ and Mn2+ ions to 3+ ions which then abstract a benzylic hydrogen atom inducing further oxidation.
These ions also aid in the decomposition of the intermediate hydroperoxides. I t is still not cle& if the metal ions also play a role in the activation of oxygen through complex formation. With xylene as the startingagent the major products are terephthalic acid and dimethyl terephthalate. For the technical production different processes were developed by Amoco, Toray, and Dynamit Nobel. Hydrocyanation(DuPonQ The direct addition of hydrocyanic acid to butadiene can he accomplished with nickel(0) phhsphine or phosphite comolexes a t standard oressure and 30-150°C in oxveen"containing solvents such as tetrahydrofuran. A plant producine 100.000 tonslvr has been in ooeration in Texas since 1977. kdditional pl&ts are being built.
CO + Hz synthesisgas
-
CHsOH 2 C H ~ C O ~ H
Compared with the older BASF process, which was likewise homogeneously cataly~edand marked by hieh reaction temperatures of -250' and by pressures around 750 atm, the Monsanto process operates under considerably milder reaction conditions. Applied to methanol, a selectivity of 99% is achieved and with the cobalt process, 90%. Because of the high price of rhodium, a complete recovery of the catalyst is absolutely essential if the process is to be economically efficient. A similar example for pressure reduction is the rhodiumcatalyzed UCC-oxo-process. Triphenylphhsphine, P(CsH& is used as co-catalyst. The mechanism is largely like the known 0x0 proceas which utilizes the hydridocobaltcarhony1 catalyst. The advantages of the rhodium process compared with the classical cobalt process are reduced pressure (10-25 atm) better proportion of nonnal to isoaldehyde in the final product (90:lO as opposed to 30:20). On the other hand, the cobalt process distinguishes itself by greater flexibility with regard to starting olefins (Cz to Czo) and reaction produets (aldehydes, alcohols), the psihility of improved energy recovery since high ternperaturea are used. Critical evaluation of the efficienciesof both variations from all viewpoints holds them to be about equal. Future Developments in Homogeneous Catalysis Further development in homogeneous catalysis will center around the following
Newer Industrial Developments
Besides the established processes summarized above, there exist new developments which only recently have been introduced into major production. With Wilkinson's metal chloride, complex tris(tripheny1phosphine)rhodium ClRh[P(C,jH&]3, it is possible to selectively hydrogenate biologically active substrates like steroids. The value of this catalyst is based upon the fact that the hydrogen is transferred specifically to the cis positions. Figure 8 shows a typial course of a reaction of a homogeneously catalyzed hydrogenation. Monsanto, for example using this reaction, converts cinnamic acid derivative XVJ by asymmetric hydrogenation into the levo-rotating precursor of L-dopa (3,4-dihydroxyphenylalanine).L-dopa is formed after cleavage of the acetyl protecting group from nitrogen.
variation of the central atoms. introduction of new Iigands. variation in the aooliration ohape. and new chemical reaeiions. .
[Rh(diene)Lzlf Diene [RhHzLzlf
H-RhL"
Aa a drug, L-dopa is effective against Parkinson's disease. Monsanto's rhodium-catalyzed acetic acid process is an example of the effort to replace the conventionalhigh pressure processes with low pressure ones and to substitute coal for oil in the production of basic chemicals. Methanol, the starting agent for the synthesis as well as CO from synthesis gas can be produced from coal.
\ Figure 8. Reaction mechanism of a homogenearsly catalyzed reduction of Olefins with a modium catalyst.
Volume 61 Number 11 November 1984
965
Variation of the Central Atoms
It is known that certain transition metals are especially suitable for specific reactions. For example, under hydroformvlation conditions stvrene with cobalt carbonvl catalvsts is almost exclusively hydrogenated, whereas with the rhodium catalvst the reaction leads almost auantitativelv t o hvdroform;lation. It is generally recognizedthat cohalt hasagieater tendency to hydrogenation than rhodium. It seems that col~alt. as a hard me-tal in the sense of the theory of hard and soft bases, eases the oxidation addition of the hard water in the key step of the reaction. Correspondingly, the initially formed aldehydes are partially reduced to alcohols by cobalt hut not so by rhodium. Cohalt and nickel catalysts are quite advantageous in the oligomerization of conjugated diolefins, whereas titanium, vanadium, and chromium catalysts are more suitable for a-olefins. Variation of the central atom permits a coarse adjustment of the selectivity of the homogeneously catalyzed process, while the fine tuning can be achieved through modification of the ligands as well as the reaction conditions. New Ligands
Ligands on the transition metal catalyst influence the electronic and steric structure and thereby facilitate a derived steering of homogenously catalyzed process. In the future, through ligand modification, the selectivity of many processes will be further unproved. A prime example of the desired modification of ligands is the development of the stereoseledive catalysts for asymmetric hydrogenation, where the basic principles of "catalyst tailoring" are clearly evident. A planned variation of the ligands of the rhodium complex led finally to the ligands "dipamp."
corresponds to the surface of a heteroaeneous metal catalvst and, as such, is the basis for the hypotIhesis that the catal&c behavior of these two should be the same. In the multiphase catalysis, the catalyst is dissolved in a e this solvent in which the substrate is insoluble. An e x a m ~ l of technique is the ahove-mentioned successful ~li~omerization prtress. In this Shell AC process the ethylene Dasses in bead form through adiol phase which contaiis the nickel catalyst (Fig. 10) and is rhere oligomerized to higher a-olefins: these leave the catalyst phase without going into solution. The product and catalyst solution is thus easily separated through simple phase separation. Similar processes have been developed for hydration and hydroformylation. In the broadest sense the application of fused salts helongs to multiphase catalysis. These can serve as reaction media or as catalytically active species. The special properties of such melts offer certain advantages high chemical stability (make high temperature reactions easier), lower vapor pressure (eases high temperature reactions), good thermal and dectrolytir ronductivity, and dissolving rapacity for oxides, hydridpx, metals, and carbides. Corrosion causes a technical problem, which is intensified additionally hy the relatively high temperatures (4W700°C). A few examples of applications of salt melts to catalytic reactions are the chlorination of methane to a mixture of chlorinated methanes (CuC121KCImelt), the chlorination of ethane to vinyl chloride (CuO/CuC12/KCI), the selective hydration of polyenes ( P ~ C ~ ~ / ( C Z H S ) ~ N Shydration ~ C L ) , of coal (ZnCWKCl), and the production of monosilane, SiH4(LiC1/ KC1). he transformation of synthesis gas is a focus in the search for new reactions that could be homogeneously catalyzed. I t
Rhodium complexes with this ligand are capable of hydrogenating amino acid precursors to optically active amino acid with up to 95% optical purity (This corresponds to an enantiomeric ratio of 98:2). The Monsanto amino acid process makes use of this in the production of L-dopa. Variation in the Application Phase
At present the major areas of investigation are the following the immobilizationof homogeneous catalysts, the catalyst on metal clusters, the multiphase catalysis, and the development of newer chemical reactions. As in the peptide synthesis, the attempt in the immohilization is to fix the active, homogeneous combination of the central atom and ligands onto a heterogeneous matrix. An example is shown in Fiaure 9. In aeneral. the union of the active homogeneous corn-bination ;f central atom and ligands to the matrix isachieved viacoordinatc or covalent hondine. The basic problem of thiscatalyst type lies in thedivergenJe between the required high stabilitu of the fixed catalvst and the required lability of the ligand sphere around thecentral atom for catalytic activity. Thus, a relatively stable catalyst with minimal activity exists or there is the reverse, an active catalyst with concomitant loss of metal through elution. Variants of the immobilized catalysts are the metal cluster catalysts. By joining several single nuclear complexes t o multinuclear clusters via metal-metal bonds, an insoluble heterogeneous catalyst finally forms. This is also of interest scientifically, in that the surface of the clusters, in principle, 966
Journal of Chemical Education
D = Donor Atom (P,As, N, Sh)
Figure 9. A mcdium complex cwrdimte cwalentty banded lo a solid suppm of polyshlrenedivinylbenzenecopolymer.
L(
75-120' 53 bsr ~ i e k adfa d
CHaCHz
+ CHzCHz + nCH=CH2
Catalyst solution
less dense phase phase more deme
Ffgure 10. Ollgomerization of elhylene In a twophase catalysis.
is in this area that new scientific knowledge could accelerate the substitution of coal for petroleum Methane formation CO+3Hz-Cl&+HzO
Glycol synthesis 2 CO
+ -
+ 3 Hz
Hydration of coal C
Hz
HOCHzCHzOH CCHzS
Fischer-Tropsch synthesis Formation of homologous alcohols ROH
+ CO + 2 Hz
Methanol synthesis
-
RCHzOH + Hz0
Osmium and iridium carbonyls should be catalytically active in the methane formation. A homogeneous titanium complex reacts stoichiometrically under mild conditions. In the glycol synthesis (important quantity-wise) the discovery of a rhodium carhonyl cluster catalyst that is active with synthesis gas could open the possibility of replacing the ethylene with synthesis gas as the raw material. 2 CO + Hz
synthesis gas
( C O ) 2101
0.53Yz
HOCHzCHIOH
glycol
For the Fischer-Tropsch synthesis as well as for the hydrogenation of coal, a salt-melt variation has been developed.
For the hydrogenation of coal a ZuC12KCI melt is used; in the Fischer-Tropxch synthesis the catalyst Irl(C0),2 is in a NaCI/AICIq melt at 180°C and 2 atm or nreasure. A decisive breakthrouph, however, will be possible oily when newer, very noncorrosive materials are develooed. In the svnthesis of homologous alcohols the synthesis g& can serve a s k e starting base for methanol, ethanol, and higher alcohols. Outlook Homogeneous catalysis bas, within a few years, captured an important part of the technically applied processes in chemistry. Additional processes t h a t could be industrially utilized include: the introduction of new catalysts with new ligands, the discovery of new reactions (for example the activation of C-C and C H bonds), the improvement of existing processes, and the variation of the application phase. In the future energy and raw material considerations will increasinelv determine the direction of world-wide develonment nf traditional industrial organic chemistry. Since homogeneous catalysts are generally active a t low temperatures, new developments can be expected in this area. Great advances will occur where processes based on petroleum can be replaced by synthesis gas technology based on coal or where carbon dioxide as the end product in the hydrocarbon cycle can be recycled as a starting agent. Of special interest in the long term are the homogeneously catalyzed processed based on water and air in which the requried energy could be s u ~ d i e hv d the sun, an ever-available energy source. ~ x a m ~ l e s - such-applications df exist: the reduction of atmospheric nitrogen to hydrazine, the activation of oxygen for fuel cells, and the photochemical splitting of water into hydrogen and oxygen. In these processes the carbonyl catalysts as well as the cluster compounds could make a decisive contribution.
A Useful Model for the "Lock and Key" One of the most generally useful analogies for the specificityof enzymes toward substrates is the well-known "lock and key" hypothesis' originally introduced by Emil Fischer in 1894. A useful model that nicely illustrates this principle is the "SOMA" puzzle cuhe2(see figure). By using differently colored sets it is possihle to arrange them as in the figure. Manipulationsquickly demonstrate that the "enzyme" cavity will only be filled with one of the two similarly shaped "substrates." This demonstration can be extended to include an a n a l w concerning the differencesin physiological properties possessed by many enantiomeric pairs (e.g., flavor-enhancingproperties of MSG, (monosodium glutamate) versus its enantiomer). As can be quickly seen, the two "substrates" are indeed enantiomeric and only one will completely fill the "enzyme" cavity. The demonstration is made clear to a small class by using the models in the front of the class. By using either single 35-mm slides or lap-dissolve projection,3 the concepts can easily he conveyed to any size class. Another use of this relationship can he made concerning the odor differThis clearly emphasizes the conences that exist between R and S car~one.~ troversy concerning the stereochemical versus the other theories of odor recognition.
'Jencks, W. P.. "Catalysis In Chemistryand Enzymology." McGrsw-HillBwk Company. New Y a k . 1989, p. 288.
"SOMA by Parker Brothers, Inc., P.O. Box 900, Salem, MA., 01970. Harpp, D. N. and Snyder. J. P.. J. CmM. E m . , 54,68 (1977):Fine. L. W.. m p . D. N.. Krakower. E., and Snydar. J. P.. J. CmM. E m . .
54, 72 (1977).
'Russell, G. F. and Hills, J. I., Science, 172, 1043(1971);Friedman, L. and Miller, J. G., SclenC.3, 172, 1045 (1971). Arlel E. Fenster John Abbon College St. Anne de Bellwe. Que., Can&
David N. Harpp McGill University Montreal. Oue.. H3A 2K6 Canada
Joseph A. Schwarcz Vanier College Montreal. Que.. H3X 2N9 Canada
Volume 61 Number 11
November 1984
967
