a comprehensive mechanism for the fischer-tropsch synthesis

8/2/2019 A Comprehensive Mechanism for the Fischer-Tropsch Synthesis

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CI", ev. 1981, 81,447-474 447

A Comprehensive Mechanism for the Fischer-Tropsch Synthesis

Contents1.M"11. Viewpoints on Chemical Mechanisms111. The MechanismA. Adsorption Steps1. Hydrogen Adsorption

2. Carbon Monoxide Adsorption3. Carbon Monoxide D i s s oc i a t hE. lnitlai Reactions amona Chemisorbed

44744 845 04 5 04 5 045 14 5 1454Species

1. Reactions of C and 0 with H 4552. Reactions of Undissociated CO 4 5 6457IntermediatesC. Reactions of Hydrocarbon and H-C-046046 046 246 2463

cheryl K. ~ o ~ w ~ e ~ o o r t weceived an A.B.mea fr~m~ l p o nCollege, Ripon. WI. in 1963, and an M.S. from the Unlvenky of. Organic Products2. Water California at Berkeley in 1964. For most of th e time since then,3. CarbonDbxide 4 6 2 sh e has been a staf f member at the Los Alamos ScientificL a b4. Unreactive Carbon ratory. She has published papers onthe photochemistry of uranylcompounds in organic solvents and is currentty studying laser py-rolysis of coal and kwopn to elucidate structureand combustion. Metal Carbides and Oxkles

D. Product Formation

IV. Comparison with Previously Proposed 463 and gasification characteristics.MechanismsA. Carbide MechanismB. Bureau of Mines MechanismC. Pichler-Schuiz MechanismD. Mechanisms Based on Complex ChemktryE. Ponec MechanismF. The Fischer-Tropsch Synthesis a s aPolymerization ReactionV. Extension to Other SystemsA. Carbon Monoxide DisprqmrWnationB. WaterGas Shill ReactionC. MethanationD. Isosynthesis and Koibei-EngeihardtSynthesisE. Homogeneously Catalyzed ReactlonsVI. Summary and Prospects for Future ResearchVII. AcknowledgmentsVIII. References

48 446 44654654664 6 64 6 746746746 846 948 94 7 04 7 14 7 1

I . IntroductionHydrocarbons and oxygenated compounds can beproduced catalytically from syn thesis gas (CO + H 3 a tatmospheric pressure or above and a t a few hundreddegrees Celsius. T he pro ductio n of chemical feeds tocksor motor fuels from any com bustible carbon-containingsource, including coal, biomass, or garbage, would b ean attractive alternative to politically unstab le petro-leum supplies. In addition, the Fischer-Tropsch syn-thesis has th e potential for producing chemical feeds-tocksor motor fuels without the production of the en-vironmentally h armful compounds enco untered in di-rect hydrogenation. However, the Fischer-Tropsch0009-2685181107616447S08.0010 e

synthesis is now utilized on a large scale only in t heSouth African Sasol plants.T he proceas could be improved in sev eral areas. Theproduction of a range of compounds indicates tha t th esynthesis might be used to supply several chemicalfeedstocks, bu t it also requires extensive refining of th eprodu ct stream. Catalyst deterioration is another sig-nificant d ifficulty. A more detailed understan ding ofthe process should lead to greater flexibility of appli-cation.A more theoretical approach m ay be of use in furtherimproving the Fische r-Trop ch synthesis. Although thesynthesis has been stud ied for more tha n 50 years,'-17no unde rstanding of i ts mechanism exists that is suf-ficient to predict products un der various conditions orto unify the observations in a d etailed way.Many of the features of the syn thesis indicate tha tthe mechanism is not simple. Extensive reaction, in-cludin g dissociation to adsorbed atoms, must tak e placeto transform the reactants, CO and Hz,o compoundscontain ing several C-C and C-H bonds. Numerousprodu cts are obtained th at vary in both carbon chainlength and ty pe of functional g r o u p ~ ; ~ ~ ~ ~ ~ * ~ * ~he dif-ferent functional groups imply different paths anddifferent intermediates. Relative amo unts of theproducts vary with the catalyst , temperature, andp r e ~ s u r e ; ~ * ~ ~ ~ ~ ~ ~ ~ 'ifferent paths must be favored bydiffe rent sets of conditions.Large amount8 of data have been generated onproduct d istribution for different operating conditionsand on experiments intended to elucidate the mecha-) 1981 A m n hemical Society


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448 Chemical Reviews, 1981, Vol. 81 , No. 5nism. T he sheer volume of data makes correlationsdifficult, with an enormous number of possible com-binations of catalyst, tem peratu re, and pressure. Sev-e ra l r eac t ion mechan i sms have been p ro -posed.3,7~8~10~12~14-16,1824ost are constructed around asingle critical interm ediate or reaction,3~8~12~18-20~23~24l-though multiple paths have also been consid-ered.7,10,14-16,21.22ll of the intermediates proposed haveevidence to support them, b ut contradictory data alsoexist for all of th em .The question in formulating a mechanism for theFischer-Tropsch synth esis appe ars to be less whichintermediate is the critical intermediate than to whatdegree each type of intermediate contributes. Th edifference between th ese two q uestions is the differencebetween two approaches to chemical mechanisms. Thefirst approach has been predom inant in previous stud-ies. T he second will be pursu ed in this review.Th e second approach leads to a set of elem entaryreactions tha t describe a network of paths, whereas thefirst approach leads to a more linear system. Most ofthe ste ps and interm ediates tha t have been proposedby other workers are included, some new steps havebeen ad ded , and seve ral step s have been proposed aspossibilities for fu tu re inclusion.Th e mechanism proposed here provides a preliminarybasis for unifying the many observations relating to th eFischer-Tropsch synthesis; a definitive stat em ent of allelementary rea ctions is not possible at this time. Step scan be added or subtracted as new information becomesavailable. This mechanism provides a structu re bywhich stu die s of various sorts of model systems (metalcomplexes, surfaces) can be related to each othe r andby which relationships among several synthesis-gasprocesses may be explored. It is also a starting pointfor calculational analysis of the m echanism. Many ofthe elementary reactions are of such a form th at rateconstants can be calculated or estim ated; the mecha-nism as a w hole is treatable by kinetics codes and sen-sitivity analysis codes. Th e calculation of relative sta-bilities of intermediates can also contribute to andbenefit from this type of mechanistic treatment.Th e elementary reactions that make up this mecha-nism are those tha t have some support in experiment.Th e types of evidence t ha t will be considered are, indecreasing order of relevance to the mechanism: directobservation of reactions or interme diates on surfaces,kinetic evidence for elementary reac tions on surfaces,kinetic evidence and product distributions for combi-nations of e lemen tary reactions on surfaces, direct ob-servation of reactions or intermediates in complexes,kine tic evidence from complexes, and analogy to organicreactions.The metals that will be considered as substrates,again in o rder of decreasing relevance, are iron, cobalt,nickel, ruthenium (the metals commonly used asFischer-Tropsch catalysts); other group 8 metals; group7 an d 6 metals; and other transition m etals, lanthanides,and actinides. If sufficient evidence is available fromthe more relevant categories to support a n elementaryreaction, ev idence from th e less relevant c ategories willnot be included.Th e relevance of complex che mistry to heterogeneouscatalysis has been discussed in some andopinion as to its usefulness as a model system differs.

Rofer-DePoorterI will assume tha t the ob servation of a reaction or anintermediate in a m etal complex implies that the re-action or interm ediate can occur on a surface, althoughrates may differ.Evidence for th e reactions will be reviewed with em -phasis on that published from 1975 through 1980.Man y of th e subtop ics touched on h ere have themselvesbeen the subject of reviews, which willbe referenced tothe grea test extent possible. Prim ary references willalso be used extensively. Th e capabilities and lim ita-tions of the experimental techniques cannot be dis-cussed here; specialized reviews will be referenced asappropriate.Th e Fischer-Tropsch synthesis will be defined hereas the catalytic polymerization and hydrogenation ofCO to give hydrocarbons and oxygenated productshaving various chain len gths; COzand HzO are the sideproducts. This definition agrees with some tha t haveappeared recently15J6 and differs slightly from oth-

Th is discussion will be focused on the mechanism ofthe heterogeneous Fischer-Tropsch synthesis, becausehomo geneous react ions giving Fischer-Tropsch prod -ucts may be m echanistically distinct; this point is notnow clear.17 Sim ilarly, mechanism s for zeolite cat a-lysts will no t be discussed, although it seems likely t ha tthe mechanism for them is similar to tha t for the nor-mal heterogeneous synthesis, with spatial lim itationsimposed on the products by the zeolite.31Th e approach to chemical reaction mechanisms usedhere will first be discussed in some detail. Ne xt, themechanism a nd sup porting evidence for the elementaryreactions will be prese nted . Th e reactio ns of th emechanism are grouped by a time sequence in thesynthesi s, in th e sense of following a single CO moleculethrough its possible reactions from absorption on t hesurface to the formation of stable product molecules.Under steady-state conditions, however, all elementaryreactions proceed simultaneously. The relation of thismechanism to other proposed mechanisms will be dis-cussed, and its relation to other synthesis-gas reactionswill be exp lored.

ers.5910,12

II . Vie wpoinfs on Chemical MechanismsThe mechanism of a chemical reaction can be un-derstood in a number of ways, all of them reducing tothe same elem ents. Th e extremes of viewpoint can beillustrate d by two definitions:Th e mechanism of a reaction is simply the pa ththe molecules follow in going from reactant toproduct.32Th e mechanism of a complex reaction is the listof elementary chemical reactions postulated to ex-plain the observed rates and products.33aObviously, the pa th t he m olecules follow can be rep-resented by appropriate elementary chemical reactions,while the list of elementary chemical rea dio ns is arriveda t by examining path s th at th e molecules might followin going from reactant to product. Although the twoviewpoints may refer to th e same set of facts, each givesa different kind of insight. Th e first has been emph a-sized in previous discussions of Fischer-Tropschmechanisms, usually with identification of a probableintermediate or reaction and its implications for apossible path. In this pap er, a set of elementary


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Mechanism for Fischer-Tropsch Synthesischemical reactions will be proposed. This approach hasbeen used for other catalytic system^.^^^^^ However, ithas not been applied in detail to the Fischer-Tropschsynthesis. T he set of elementary reactions presentedhere can explain the products; further developmentshould allow an analysis of observed rates.In this m echanism, some intermediates are produc edby more than one path, and an intermediate may yieldmore than one product. Th us, the appropriate spatialimage for the mechan ism is a network of paths. Pre-viously, when m ultiple paths have been considered forFischer-Tropsch mecha nisms, the image has been ofparallel paths or of forking late in the mechanism toproduce multiple products from a few intermedi-ates.7J4J5J6v213 nfortunately, it is much more difficultto gain an intuitive feeling for the outcome of a ne tworkof reactions tha n for parallel paths or forks. A thoroughanalysis of such a system can only be done by com-puting th e solutions of the appro priate set of differ entialequations. Th is complexity is the reason th at fewsystematics have emerged from the study of theFischer-Tropsch synthesis.For exa mple, the de termina tion of Arrhenius param-eters for th e overall synthesis has shed little light onthe mechanism.1 The re are two possible reasons forthis. First, the heterogeneou s reactions of adsorptionand desorption may be slow enough relative to chemicalreactions on the surface tha t they dominate the overallkinetics, although th is can usu ally be recognized fromthe form of the Arrhenius parameters an d appears notto be the case here. Second, and more likely in thiscase, the concept of a rate-determining ste p may notbe as useful in such a complex mechanism.33b Th e in-ability to reduce the kinetics to a single rate-deter-mining step can lead to the fractional concentrationdependences observed in the Arrhenius analyses ofFischer-Tropsch syntheses,1 or to misinterp retationsof seemingly interpretable data.33bTh e com plexity ofthe isotope effect when D2 is substituted for H2 nm e t h a n a t i ~ n ~ ~nd in the Fischer-Tropsch synthesis37may also derive fro m this ty pe of nonsimplifiability.3p3T he co ncept of a zero effective relative rate m ay bemore useful in understanding the Fischer-Tropschmechan ism. For a set of eleme ntary reactions, it ispossible th at un der particular con ditions the rates ofsome reactions will effectively become zero relative tothe rates of others. Concentrations of intermediatesmay change, or the rates themselves may chang e, to giveeffective relative rates of zero. The n the products fromthat part of the network will disappear or decrease,depending on th e num ber a nd efficiency of other p athsto them. It is impo rtant t o recognize tha t the disap-peara nce of a produ ct indicates an effective relative rateof zero, bu t does not require the removal of th at ste por pa th from the mechanism. Th e rates for the ele-men tary reactions of th at pa th may remain what theywere for conditions leading to those products, but therates of elementary reactions in other paths may haveincrease d, or the r at e of a single reaction leading to acrucial intermediate may have decreased. Conceptually,however, the elementary reactions m ust remain in themechanism, although their contributions to th e differ-ential equations m ay become negligible. O nly by theirretention can furthe r changes in conditions be under-stood.

Chemical Reviews, 198 1, Vol. 8 1, No. 5 449Th e complexity of th e mechanism an d th e possibilityof zero effective relative rates lead to difficulties indisproving a particular reaction path . Und er a givenset of conditions, a given reaction pa th m ay have a noverall effective rate of nearly zero, and therefo re th eexperiment will appear to disprove the participation ofa particular intermediate. However, th at result mayapply only to that set of co nditions and is not a disproofof th at interme diate for all conditions. Man y investi-gators have recognized this limitation, but it is oftenlost in later discussion of mechanisms. For exam ple,

I 4C tracer experiment^^^ have often been cited asproving tha t m ethylene intermediates do not participatein carbon-carbon bond formation. Th e conclusiondrawn by the authors of th at study, however, is tha t"the mechanism of hydrocarbon production proposedby Fischer a nd his co-w orkers plays only a minor ro lein the synthesis of hydrocarbo ns unde r the con ditionsand over the catalyst studied in the present work,provided one assumes that the surface is uniformlyactive in the synthesis".39 The qualifications are a t leastas impor tant as the conclusion in this case.Relative rates of the elementary reactions have b eenmanipulated reasonably successfully in the empiricaldevelopment of the indu stria l Fischer-Tropschsyntheses producing motor fuels. Th e rates for thepath s in the reaction network leading to oxygenatedproducts a nd extremely long carbon chains have beenreduced relative to the rates of the paths leading tohydrocarbon s. In most applications it would be desir-able to reduce rates for paths leading to methane andto carbon dep osition on the catalyst to a greater degreethan has been achieved empirically. If the Fischer-Tropsch sy nthesis is to be m odified to produce indus-trial feedstocks from coal, then relative rates of theelementary reactions leading to oxygenated pro ductsor alkenes will need to be enha nced ; the relative ratesof paths to other products will need to be reduced.Cases in which a particular relative rate is effectivelyzero can be used to extract information on the m echa-nism from otherwise hard to handle data. For example,the existing data on hydrocarbon production representa network in which the relative rates for the pathsleading to oxygen ated products ar e zero. Com parisonsof these systems with those producing oxygenates invarious quan tities may lead to an un derstan ding of th etypes of intermediates and changes in rates that areleading to the different product distributions. This typeof analysis has been done in an empirical way in theindustrial developm ent of th e synthesis. It should alsobe useful in reconsidering some of the subsidiary ex-periments th at have been done to elucidate the m ech-anism. A comp uter code incorporating this mechanismwould be the most helpful developm ent in this type ofdata analysis.If the multitudinous phenomena gathered under th eFischer-Tropsch umbrella are to be represented by asingle mechanism , it will have to be a set of elem entaryreac tions, some of which will have effective relative ratesof zero unde r some conditions. Sub sets of these reac-tions may be written for particular co nditions, bu t thisapproach will lead to a different mechanism (set ofelementary reactions) for every pressure, temperature,or catalyst condition tha t gives different produ cts. Thismore limited approach cannot give a unified under-


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45 0 Chemical Reviews, 1981, Vol. 81,No. 5standing of the Fischer-Tropsch synthesis and relatedsynthesis-gas reactions. Sub sets of th e elementary re-actions presented here can also represent other syn-thesis-gas reactions, such as carbon m onoxide dispro-portionation, the water-gas shift reaction, methanation,and other sy ntheses closely related to Fischer-Tropsch.III. The Mechanism

In order to write elementary reactions involving asurface, certain conventions beyond ordinary chemicalnotatio n are necessary. For this discussion, it will bedesirable to indicate which atom of an adsorbed in-termediate is bound t o the surface, but n ot the detailsof its bonding, such as th e type of active site to whichit is bound. Mo st recent experim ental findings showonly small energy differences for chemisorption ondifferent plane s of t he same metal (see, for example, ref40-42). Under the high coverage conditions typical ofthe Fischer-Tropsch synthesis, these differences arelikely to be even sma ller. Other aspe cts of the bon dingof the intermediate to the surface will be ignored oremphasized as appropriate; these will be noted as heyoccur. It is also desirable to have a com pact notationth at lends itself to th e writing of e lementary reactionsin a linear form. Me tal surface atoms will be indicatedby M; the chemisorptive bond will be represented bya hyphen, which may represent any bond order. Inmost cases, no attempt will be made to representmultiple bonding to the surface, which may be to oneor more surface atoms. Proxim ity of chemisorbedspecies is indicated by bonding to a single metal a tomor is understood as being necessary for two interme-diates to react; thu s, H-M-CO is equivalent to M-H+ M-CO; no necessary relation t o metal com plexes isintended by this notation.All reactions will be assumed to proceed in both di-rections, by the principle of microscopic reversibility,and therefore the reverse reactions will not be w rittenexplicitly, although rates for the forward and reversereactions can be expected to differ.A. Adsorption Steps1. Hydrogen Adsorption

on most transitionDihydrogen is first physisorbed and then dissociatedH z + M e M-H2 (physisorbed) (1)M-H2 (physisorbed) ~t H-M-H (2 )

No distinction will be m ade between hydrogen adsorbedin a bridged mode (between two metal atoms) and hy-drogen adsorbed atop a single metal atom.Bonding of hydrogen atoms to a transition-metalsurface appears t o involve charge transfer from them etal to hydrogen;41 he hydrogen atom s are hydridic.The degree of participation of metal electrons in thebond differs from one metal to another. Ultravioletphotoelec tron spectra show th at bond ing of H to nickeland iron involves the d b ands of the metals less thand oe s t he bo nd in g of H t o p alla diu m an d p l a t i n ~ m . ~ ~ , ~ ~Mo st investigations of hydrogen adsorp tion have beencarried out for pure m etals. However, Fischer-Tropschcatalysts are generally supp orted or promoted metals,th at is, systems of more than one metal. Th e catalyticproperties of promoted and unpromoted metals differ

Rofer-DePoortersufficiently that studies of adsorption on promotedmetals and of the difference between supported andunsupported metals are important for understandingthe synthesis.Potassium carbonate on iron was one of the earliestcatalysts studied by Fischer a nd Tropsch,2 and thepotassium-iron system is the only one in which th eeffect of a prom oter on hydrogen adso rption has beenstudied. A potassium adlayer on a Fe(100) surfacedisplayed greater binding energy for chemisorbed hy-drogen than a clean Fe(100) surface.47An earlier studyof KzO adsorbed on iron gave the op posite result: hy-drogen was m ore weakly adsorbed.55 It has been sug-g e ~ t e d ~ ~ha t the difference was due to a layer of oxideon the iron surface in the earlier study. This differenceillustrates both the difficulty of characterizing metal-promoter systems an d the sensitivity of their ca talyticproperties.Th e effect of th e suppo rt on hydrogen adsorption hasbeen more studie d, although in mos t cases not in directrelation to the Fischer-Tropsch synthe sks6 Undersome conditions, hydrogen atoms chemisorbed on asupported metal can migrate to the support, which thenacts as a h ydrogen atom reservoir even though i t is itselfincapab le of dissociating H2 he po ssibility of this so rtof in teraction was recognized by Sin felt, Lucch esi, andCarter57958 nd was nam ed spillover by Bo udart,V annice, and B e n s ~ n . ~ ~xperim ental evidence relatingt o hydrogen spillover has been extremely difficult tointerpret.s6 An apparently unambiguous proof of suchhydrogen was given by the comparison of tempera-ture-programm ed desorption curves for hydrogen fromalumina-su pported metals (platinum a nd nickel) withcurves for hydrogen atoms deposited on alum ina by ahigh-frequency discharge.60Th e mechanism of spillover is unclear. W ater oranother hydroxylic solvent appears to be necessary forit to occur.61 This finding suggests that hydrogen istransferred to th e support as a proton, which is con-sisten t with the form of hydrogen ad sorp tion most likelyon metal oxides. Infrared evidence62 ndicates thepresence of add itional hydroxyl groups on alumina re-sulting from hydrogen spillover. However, since hy-drogen chemisorbed on metals is hydridic, it should berepelled by t he electron density around the oxygens ofthe support. An oxidation-reduction couple betweenhydrogen chemisorbed to the m etal and oxidized metalat the interface between the metal and the support63or a cell-like situation involving the metal, th e hydrogen,the water, and the supporta may account for the con-version of hydridic hydrogen to a proton. In a relatedexperiment, support and promoter effects were com-bined. Mossbauer effect studies showed th at an equi-librium among Hz , Fez+,H +, and FeO can e xist on ironcatalysts supported on alumina, but this equilibriumis suppressed by a potassium prom oter.@ Migration ofhydrogen atoms from m etal to support also has someexperimental In any case, the presence ofmobile protons on the support and mobile hydridichydrogens on the metal might be expected to lead toa rapid recombination to H z at the interface (reverseof reaction 2) , although this is not observed.For later steps in the m echanism, both hydridic andprotonic hydrogen atoms will be assumed t o be availa-ble.


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Mechanism for Fischer-Tropsch Synthesis2. Carbon Monoxide Adsorption

In con trast t o the relatively simple situation of dis-sociative chem isorption for hydrogen, CO may be ad-sorbed both molecularly and dissociatively, and t her eis reason to believe that more than one molecularlyadsorbed species is impo rtant in the Fischer-Tropschsynthesis. A physisorption step is first:CO + M e M-CO (physisorbed) (3)

This species is rapidly converted to the chemisorbedspecies:M-CO (physisorbed) + M-CO (4)

Carbon m onoxide is predominantly adsorbed on them etal in a carbonyl-like mode, bonded t o one or moremetal atom s through the carbon by donation of CO 5aelectrons to the m etal and backbonding by the d onationof metal d electrons into the CO 27r antibonding or-bital.e8-72 As more metal atoms participate in thebonding, the metal-carbon bond will strengthen and theC-O bond will weaken, making the CO m ore susceptibleto hydrogenation and other reactions, although thisspecies appears to be difficult to hydrogenate from t heevidence of cluster com pounds.73A more strongly adsorbed form of CO has been sug-gested to be bonded side-on to the m etal by Muetter-and Masters.16 T his species should be moreactivated toward hydrogenation and carbon-carbonbond for ma ti or^.'^*^^ A side-on species seems likely tobe an intermediate in CO dissociation:

Chemical Reviews, 1981, Vol. 81, No. 5 45 1atoms, and t he oxygen to a m etal or metalloid atom inthe oxide support. This type of CO adsorption may berelated t o the strong metal-support interactions ob-served in catalyst and to differences inFischer-Tropsch activity observed in system s th a tmight be capable of these interaction^.^'-^' If th eside-on CO complexed between two dissimilar m etalsis particularly active in carbon-carbon bond form ation,then the catalytic activity of a m etal that, unsupported,is a methanation catalyst should be shifted towardFischer-Tropsch activity (m ore two-carbon and heavierhydrocarbons) by support on the oxide of an earlytransition m etal, with em pty d orbitals to complex th ecarbonyl oxygen. Th is shift has been observed fornickel on a T i0 2 and other combinationsof this type have shown enhanced carbon-carbon bondf o r m a t i ~ n . ~ ~ ~ ~hen a compound is formed betweenth e metal and s ~ p p o r t , ~ * ~ ~O adsorption would beexpected to be suppressed because the metals aresharing electrons directly rather tha n through a com-plexed CO.A few studies have been performed on CO adsorptionon potassium -prom oted iron.47g55*93 higher bindingenergy is observed tha n for iron alone, and the satura -tion coverage of CO increases. Th e CO appears to beadsorbed end-on to the iron through the carbon, al-though no conclusions were drawn abou t the partici-pation of the potassium in the active site.93A study of CO interaction with potassium-promotedalumina% howed tha t, although CO is poorly adsorbed,it can exchange oxygen with the alumina, probablythrough th e formation of a C 02 - intermediate.Carbo nization of the metal surface, which may occurduring the synthesis, weakens the b onding of CO to th esurface in the case of nickelg5 bu t causes a side-onbonding with interactions of the m etal with both C and0 in th e case of osmium.76 This contradictory behaviorin two group 8 metals may be significant in their dif-fering activity for the synthesis.A species of CO molecularly adsorbed end -on throughthe oxygen has been postulated% o explain some of theobservations relating to the Fischer-Tropsch synthesis,but the re appears to be no experimental or theoreticalevidence for the existence of this species.The two types of molecularly adsorbed CO will bedistinguished only where there appears to be goodreason t o write separate reactions for th e two types.3. Carbon Monoxide Dissociation

Dissociation of CO on the me tal surface is possiblefor the common Fischer-Tropsch catalyst metals at thetem peratures of t he s y n t h e s i ~ : ~ ~ ~ ~ ~ - " ~

C0M-111 + M M-C + M- 0 (6 )

Th e side-on CO must be th e precursor to dissociation,because breaking of the C -0 bond in end-on CO shouldlead to loss of th e oxygen to the gas phase, which doesnot seem to occur.The metals used as Fischer-Tropsch catalysts occupya border ar ea in the transition series;g7 or these metals,both dissociative and molecular CO chemisorption occura t temp eratures near room temperature. For metals tothe left in the periodic chart, CO chemisorption isdissociative a t room temp erature, and for metals to th e

M-CO ( 5 )Th e more strongly chemisorbed CO can interact withtwo or more metal atoms, which may belong to thecatalyst metal. An infrared band a t 1620 cm-' has beenassigned to CO coordinated to a t least three nickel at-oms, one of which is coordinated to the oxygen.74A COstretch of 1520cm-' has been observed for CO adsorbedat steps on a nickel surface,75which would be suitablesites for side-on adsorption, perhaps combined withend-on interactions. Ultraviolet photoelectron spec-troscopy of CO adsorbed on a carbonized osmium sur-face is consistent w ith side-on bonding.76 Cluster com-pounds of iron77 n d manganese78have been preparedin which CO is coordinated en d-on to one metal atomand side-on to another.Carbon monoxide may also be complexed betweentwo dissimilar metal atoms, which, in a synthesis cat-alyst, might be in the catalyst and th e promoter or inth e catalyst and the support. Bimetallic complexeshave been prepared in which the carbon of the CO iscomplexed to a m etal capab le of don ating electrons andthe oxygen complexed to a metal capable of acceptinge l e ~ t r o n s . ~ * ~ lhe CO stretching frequency in thesecompounds is lowered relative to its value in the car-bonyl complexes. Com plexes with CO coordinatedthrough both carbon and oxygen are summarized inTable I. Carbocations can also react with th e oxygenof carbonyl CO in a Lewis acid-base fashion. Bo thiron82and cobalt83carbonyls have been reacted withcarbocations to produce new complexes.For a m etal on an oxide support, the carbon of theCO could be coordinated to one or more of the metal

M-[


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452 Chemical Reviews, 1981, Vol. 81, No. 5 Rofer-DePoorterTABLE I. C - a n d 0 - C o o r d i n a t e d Carbon M o n o x i d e

bond ing s to ich iome t ry CO s t re t chpa ren t me ta l complex to c Lewis ac id (ac id/co mple x) for -CO- ref

Fe -Ni compounds(q 5-C5H5)Ni(CO), -F e ( v 5 - C 5 H 5 ) ( C O )

C o c o m p o u n d sc03(c0)10

Mn( CO) ,(q -C,H4Me)Mn(CO),( q5 - C 5 H 4 M e ) M n ( C O ) ,

H+BF3BCl,BCI,BBr,BBr,

Mg(C5H13Al(i-C4H9),A1(C2H5)3A1Br3

AlBr,A1Br3Sm(q5-C5H4Me) ,Sm (q 5-C5H5 3Ho(q -C,H,Me),Gd(q 5-C5H4Me),

Al(CzH5 1 3A1(CZH5)3Al( i -CaH9)3Er(q5-C5H4Me),Sm(q5 - C 5 H 4 M e ) ,AIBr,Al(i-C,H, )3

1121121120.5214123411222211110.511111110 .5110 .510 .52111112120. 51112

1 3 6 5i;:E IiEl1iE

1 4 6 31 2 9 21 4 3 81 3 2 01 3 6 51 7 1 11 6 8 21 6 8 01 5 2 71 3 9 2{E }1 4 1 5

I :E IiE; t1 4 7 31 5 4 81 7 0 01700

1 7 3 817 38

1 7 5 11 6 0 0

1 7 8 1

1 6 3 71 7 6 117611 7 8 01 7 8 01 5 3 51 6 7 91 7 2 11 8 6 81 8 6 81 8 6 81 8 6 5

abbbbbbbbb, d - ffebbbbbbgg

c

gghi-khhibI-nm

1c

0

P,PrPPPPgs

fffggbf, tcgggL


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Mechanism for Fischer-Tropsch SynthesisTABLE I ( C o n t i n u e d )

Chemical Reviews, 1981, Vol. 81, No. 5 453

bonding stoichiometr y CO stretchparent metal comp lex to c Lewis acid (acid/com plex) for -CO- ref~~ Mo co m po unds 0 . 5 1 6 6 4 C1 6 6 5 U2 1 6 3 3

(r l 5 -C,H , )Mo (CO ) , Mo Mg (c,H, N 14(q - P h , P C , H , ) M ~ ( O ) , MO Al(CH313

Al(&C,H,), 2 16 33 UM o( 5 , 6 - d m p h e n ) - M o A 1 ( C 2 H 5 ) 3 2 1 6 2 7 VM o Al(C*H,)3 2 1565 UMo (phen) , (CO ) ,

Uo(phen)(PPh,) , (CO), M o

(PPh3 12 ("),W co m po und

(I) 5-C,H,)W(CO)3 W Al( c4H, 0 P 113 16701 6 0 5 W1 5 7 0Hodali, H. A. ; Shriver, D. F .; Ammlung, C. A. J. A m . C h e m . S O C.1978, 100, 2 3 9 .Znorg. Chem. 1974, 3, 4 9 9 .4 4 6 9 . Kim, N. E.; Nelson, N. J.; Shriver, D. F. Znorg. Chim. Acta 1973, 7, 3 9 3 . e Nelson, N . J .; Kim, N. E.; Shriver, D.F. J . A m . C h e m S O C.1969, 1, 5 1 7 3 . f Alich, A .; Nelson, N. H.; Strope, D.; Shriver, D. F . Inorg. Chem. 1972, 1, 2 9 7 6 .

1 4 9 . Schmid, G . ; Stut te , B . Zbid. 1972, 7, 3 7 5 . * Mann, C.D. M. ; Cleland, A. J.; Fieldhouse, S. A. ; Freeland, B. H.; O'Brien, R. J.Zbid. 1970,24, 61. Nicholson, B. K.; S impson,J. Zbid. 1978, 155, 3 7 . S tu t te ,B.; Batzel, V.; Boese, R.; Schmid, G . C h e m B er . 1978,111, 6 0 3 . Q Schmid, G . ;Batzel, V.; Stut te , B . J. Organomet .Che m . 1976, 113, 7 . * Schmid, G . ; Stut te , B.; Boese, R. Chem. Ber. 1978, 2 3 9 .Che m . 1978, 155, 0 3 .Turnipseed, C. D. Zbid. 1970, 1 . " Shriver, D. F .; Alich, A . Znorg. Chem. 1972, 1, 2 9 8 4 .J. J.; Wan, C.; Burlitch, J. M.; Hughes, R. E. J . A m . C h em . SO C .1971, 3, 3 5 3 2 .

Kristoff, J. S.; Shriver, D. F.Ulmer, S. W.; Skarstad, P. M. ; Burlitch, J. M.; Hughes, R. E. J. A m . C h e m . SO C .1973, 95,Crease, A. E .; Legzdins, P. J. Che m . SO C. ,Che m . Co m m un. 1972, 6 8 ; J . Che m . S O C ,Dalton Trans. 1973, 5 0 1 .Schmid, G.; atzel, V.; Etzrodt, G . ;Pfeil , R. J. Organomet . Chem. 1975, 6, 2 5 7 . ' Schmid, G . ; Batzel, V. Ib id . 1972,46,Batzel, V .; Muller, U.; Allman, R. Zbid. 1975, 102, 1 0 9 .

Ingle, W. M.; Preti, G . ; MacDiarmid, A. G . J. Che m . SO C. ,C h e m . C o m m u n . 1973, 9 7 .Fieldhouse, S. A .; Cleland, A. J.; Freeland, B. H.; Mann, C. D. M.; O'Brien, R. J. J. Che m . SO C.A 1971, 5 3 6 .Stutte, B .; Schmid, G . J. Organomet .Petersen, R. B.; Stezowski,Alich, A.; Nelson, N. J.; Shriver, D. F . J. Chem. SOC.,Che m . Co m m un . 1971, 5 4 . " K o tz , J. C.;

right, it is molecular. Related to th is observation is thefact th at dissociation becomes more likely as the he atof adsorption for molecular CO increase^.^^*^^ Theserelationships have been exp lained in terms of the hea tsof formation of the transition-metal carbides and ox-In th is borderline group of metals, a sm all change inenergy can change the mode of adsorption from mo-lecular to dissociative; adsorption m ay be molecular onone crystal plane of a m etal and dissociative on anoth er.Increasing temperature also promotes CO dissociation.However, although adsorption m ay be predominantlymolecular or dissociative a t different tempe ratures, atemperature range will exist over which both m olecularand dissociated CO w i l l be present on th e m etal surface.For exam ple, in th e case of iron, CO adsorbs molecularlya t 100 to 300 K and slowly dissociates; at 350 K an dabove, it is predominantly dissociated both on themeta1101-104 nd o n iro n supported on alumina.10s Evenin the case of predominantly dissociative adsorption,however, molecularly adsorbed CO will be present asan intermediate between gaseous CO an d surface car-bon an d oxygen atoms. Furth er, the side-on CO willbe present as an intermed iate which can be interceptedbetween adsorp tion a nd dissociation if the ra tes of itsreactions with hydrogen and other intermediates allow.Th e kinetics of interconversion of the various types ofadsorbed CO will determine th e path s leading to thepr od ud s of the overall synthesis, and d ifferent balancesof intermed iates wil l be favored by different conditions.For example, higher surface coverage (higher gaspressure) increases the am oun t of mo lecularly adsorbe dCO relativ e to dissociated C0.1137114Carbon monoxide adsorption on ruthenium may bean exception to the generalization of dissociation atsynthesis temperatures. Some question exists as towhether CO is dissociatedlmor molecu la rly ad ~ 0r be d . l~ ~Although th is question is not now resolved, two types

ides.11lJ12

of adsorbed CO are known o exist on ruthenium, an dthey may correspond to the two types of molecularlyadsorbed CO discussed in section II.A.2.Other factors affect the dissociation of CO on m etalsurfaces. Irregularities in th e surface prom ote disso-ciation; steps and kinks on a p latinum surface promotedissociation of adsorbed CO, even though platinumsurfaces with low M iller indices exhibit m olecular ad-sorption e x ~ l u s i v e l y . ~ ~ ~ ~ " ~ ~ ~ ~ ~imilarly, CO adsorbed a tsteps an d kinks on a nickel surface has a lowered C -Ostretching frequency75 nd dissociates more readily tha nCO adsorbed on low-index planes.l18 Silica-supportedruthenium -platinum bimetallic clusters with a clustercomposition of 10atom % ruthenium dissociate C0.'lsThe effect of supports and promoters on CO disso-ciation has been little studied. Th e probability of COdissociation is greater on potassium -prom oted iron thanon clean iron, but the temperature of dissociation isunchangedeg3Supports are sometimes used in inves-tigations of CO dissociation, bu t their e ffect (or lack ofeffect) has not been stud ied systematically. Investiga-tions of supp ort effects on CO dissociation w ould beuseful, particularly in view of recent observations ofme tal-support interactions affecting CO adsorption-and ~ a t a l y s i s ~ ~ ~ ~ ~ ~ ~ ~ ~ ~nd earlier work indicatingchanges in th e stretch ing frequencies of adsorbed COwith changes in support.lZ6Coad sorption of hydrocarbons, which are producedby the synthesis and can be expected to be present onthe catalyst, with CO on nickel and platinum shifts th eCO vibration t o lower frequencies, indicating a weak-enin g of the C-0 bond.ln A carbon layer on rhodium128or osmium76 urfaces also appears t o promote dissoci-ation of CO, although the formation of a carbide layeron cobaltlmJ1Or nickel% nhibi ts the dissociation of CO.Th e disproportionation of CO to COz and su rfacecarbon is som etimes suggested as an alternative or ad -ditional source of surface carbon. However, because the


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45TABLE 11. Adsorption Reactions andIntermediates Produced

Chemical Reviews, 1981, Vol. 81, No. 5

reaction eq no.H, + M P M-H, (phyfsorbed) 1

3M-H, (physisorbed) t H-M-H 2C O + M Z -CO (physisorbed)M-CO (physisorbed)2 M-CO 4CM-CO f M-Ill 5M-Ill + M 2 M-C + M -0 6C 00

IntermediatesM-H, (physisorbed) M-CO M-HM-CO (physisorbed) M-Ill M-C

M - 0C0

Rofer-DePoorter

disproportionation is heterogeneous and takes placeunder conditions similar to those for the Fischer-Tropsch synthesis, the chemisorption and dissociationof CO are probably a part of its Inother words, disproportionation of C O is a single pos-sible path in the overall mechanism of the Fischer-Tropsph synthesis. This path will be discussed in sec-tion V.A.For later steps in the mechanism, both M-C andM-O wi l l be assumed to be available for reaction. TableI1 summarizes the physisorption and chemisorptionreactions and the intermediates produced.B. Initial Reactions among ChemisorbedSpecies

The division between this section and the next issomew hat arbitrary; the word initial refers only to th efact that the reactants for the elementary reactionsdiscussed in this section are the interme diates generatedin the adsorption steps of section 1II.A. and result inintermediates rather th an stable products. Theseinitial reactions will, in fact, be taking placethrou gho ut a Fischer-Tropsch synthesis. Similar re-

actions have been proposed previously; these reac tionsand the type of evidence on which they have b een basedare given in Table 111. In some cases in Table 111,individual elementary reactions have been combined;all reactions have been translated into t he notation usedhere to facilitate comparisons. Not all of th e previouslypropose d reactions will be discussed individua lly. T hediscussion in the rest of section I11 will be directedtoward establishing those reactions that are best sup-ported by th e curren tly available experim ental evidence.In order for reactions to take place among adsorbedspecies, those species mu st be m obile on the surface.Little direct evidence is available for or against themobility of adsorbed species of the type of interesthere.136 CO and H ligands are extremely mobile onm etal cluster c o m p o u n d ~ , ~ ~ J ~nd evidence is availablefor the mobility of a combination of CO, H, and hy-drocarbon ligands in a ruthenium complex.138 It is notclear, however, whe ther these o bservations ar e trans-ferable to s ~ r f a c e s . ~ ~ ~ ~ ~recent study indicates littlemobility for CO on tungsten.139 However, the presenceof a ttractive po tentials among adsorbed species or forparticular sites on the surface may change the situation.Carbon m onoxide appears to diffuse toward coordina-tively unsaturated sites (kinks and steps) on a platinumsurface.l13 For CO or H adsorbed alone on a metalsurface, the in teractions among the ad sorbates appearto be repulsive, as indicated by the common observationof decrea sing adsorption energy with increasing cover-age. Th e interaction between adsorbed CO and H ap -pears to be slightly attractive on the Fischer-Tropschcatalyst metals1e149 an d different from their interac-tions on other transition metals.141J50J51 or example,the interaction on rhodium appears to be repulsiveenough that the CO an d H form separate islands a tlow pressures.151 T he ex ten t an d significance of th esedifferences among the metals are not yet clear.Most adsorption studies are necessarily done atpressures of adsorbin g gases well below 1atm , while theoperating conditions of the Fischer-Tropsch synthesisTABLE 111. Previously Proposed R eactions Corresponding to Initial Reactions of Chemisorbed Species

react ion type of ev idence refM-C + H , Z -CH, product distribution 1 9product distributions, kinetics 20CO adsorption experiments 131CO, H, adsorption propert ies 13 3 ,3 53t ra ns ie nt e x pe r im e n ts 2 2 , 1 3 5 , 1 6 7 , 1 7 0isotope tracer experiments 17 2k i n e t i c s 1 6 5 , 1 7 6combination 7 , 14 , 37M-C + H,O 2 M-CHOH combination 23M -0 + H , Z M + H,O CO adsorption experiments 13 1

combination 37M -0 + M-H 2 M-OH + M CO adsorption experiments 353CO, H, adsorption experiments 133kinet ics 165transient experim ents 17 0M-CO + M-H,- Z -HCOH + M + e - combination 299M-CO + M-H, 2 M-CH,O (surface comb ination 6complex with undef ined structure) kinet ics 121M-CO + M-H 2 M-CHO + M complex chemistrycombination 7M-CO + M-H Z -COH + M kinet ics 135M-CO + 2M-H 2 M-CHOH + M complex chemistry 3M-CHO + M-H 2 M-CH,=O + M comp lex chemistry 1 2M-CH,=O + M-H Flc M-CH,OH + M complex chemistry 1 2M-CHO + M-H t -CH,O-M combination 7

M-C + 2H , t M + CH ,M-C + M-H 2 M-CH + M

1 2 , 16, 21

M-CHO + H , 2 M-CH,OH complex chemistry 1 8


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Mechanism for Fischer-Tropsch Synthesisare 1atm or above. Differences exist between the tworegimes, the most im portan t of which is th at thermo-dynamics favor the formation of hydrocarbons fromsynthesis gas at the higher pressures, but not at thelower pressures.128 Th e forma tion of some of the in-termediates may also be thermodynam ically unfavora-ble a t low pressures. Kinetic studies of dynamic cata-lytic systems a t atmospheric pressure and above arebeing d ~ n e , ~ ~ ~ ~ J ~ ~ut detailed monitoring of surfacespecies under rea ction con ditions is still not possible.Com binations of these technique s with some of t he newlaser light-scattering techniques, perhaps with Fouriertransform analysis,153may eventually prove helpful inelucidating th e na ture of surface species under synthesisconditions. Several techniques are available for exam-ining surface species (for example, see ref 154-160);intermediates in slow reactions have been observed.161JB2However, not a l l of the ob served pheno men a in systemsof th is ty pe are w ell u n d e r s t ~ o d . ' ~ ~ J ~It may be helpful a t this point to visualize th e catalystsurface under the conditions of the synthesis. Imm e-diately next to (and bonded to ) the m etal surface is themost active layer, containing C, 0, and H atoms anda t least two types of adsorbed CO; next to these is alayer of physisorbed CO, Ha, and desorbing products.T he interm ediates to be discussed in this section andthe next are also present in the surface layer, as aresome physisorbed CO, H2,and product molecules. T hephysisorbed layers will be several molecules deep, re-quiring diffusion through them for transfer between thegas phase and the c atalyst surface. Th e molecules inthe physisorbed layers, in electronic structure andtherefore in chem ical reactivity, resemble m olecules inthe gas phase, although they w ill be perturbed by vande r Waals interactions and o ther liquid-like forces. Nodistinction will be made here between physisorbed andgas-pha se molecules, since energy differen ces are likelyto be small.

Chemical Reviews, 1981, Voi. 81, No. 5 455nate over reaction with an unadsorbed m olecule.Adsorbed carbon atoms are known to exist on catalystsurfaces and to react readily with hydrogen. In addi-tion, other types of surface carbon may contribute tothis reaction. Four types of surface carbon have beenshown to exist on nickel supported on alumina.le5 Indecreasing order of reactivity toward hydrogen , they areidentified as: chemisorbed carbon atoms, bulk nickelcarbide, amorphous carbon, an d crystalline (graphitic)elemental carbon. Th e more reactive carbons are con-verted to the less reactive carbons by heating.Other studies have identified a reactive carbon thatmay be adsorbed carbon atoms. Auger electron spec-troscopy indicates tha t a readily hydrogenated form ofcarbon re mains on t he surface of iron foils1% and ofpolycrystalline rhodium128 uring th e sy nthesis of hy-drocarbons. Infrared studie s of silica-supported ru-thenium led to similar conclusion^.'^^ An active carbonlayer was also identified on ruthenium and nickel byA uger electron s p e c t r o s ~ o p y . ~ ~ ~ J ~ ~inetic evidencefrom transient-method studies also implicates a reactivecarbon intermediate for fused iron170and supportediron171 ataly sts. Oth er kine tic evidence131J33J35J72J73and isotopic d i s t r i b ~ t i o n s l ~ ~ J ~ ~mply that adsorbedcarbon atoms can be readily hydrogenated. Isotopicstudies indicate th at carbon deposited by dispropor-tionation of CO on ruthenium and cobalt hydrogenatesto m ethane, but ano ther intermediate is implicated inaddition for the reaction on ruthenium.174A carbon species identified as a surface carbide mayalso contrib ute to th e hydrogenation reactions. Bulkcarbides of iron and nickel form unde r synthesis con-ditions165~170J75~176nd can be hydrogenated to meth-ane.165J72J77However, it is not clear tha t the carbonsometimes referred to as carbidic is different from w hathas been discussed here as adsorbed atomic carbon,following the classification of McCarty and Wise.laAuger studies of carbon deposits on iron178J7ghow acarbidic phase th at behaves similarly to w hat has beendiscussed here as adsorbed atomic carbon, and a readilyhydrogenated superficial carbide of n i c k e P is depos-ited by CO disproportionation, a m ethod concluded byother workers to deposit adsorbed carbon atoms.131This apparently semantic difficulty arises from a dif-ficult chemical distinction. Is there a difference be-tween an adsorbed carbon atom and a surface carbide?There a re no simple answers to this question, and itis possible th at no distinction can be made.% However,both terms carry implicit assumptions about the natureof the bond between the carbon and the metal. Aclassification of surface species th at m ight h elp to clarifythis qu estion has been suggested by M adix,ls1 bu t m oreinforma tion is necessary on the chemical state s of th evarious types of surfac e carbon before this classificationcan be applied.The amorphous and crystalline carbon species appearto act m ainly as poisons to h y d r o g e n a t i ~ n ; ' ~ J ~ ~ J ~ ~h a tis, their effective rates of hy drogena tion will be zerorelative to hydrogenation of the other form s of carbon.If the elemental carbon is in intimate contact with metalparticles, its rate of hydrogenation is increased,la2perhaps du e to surface interactions forming the moreactive types of carbon.Another form of carbon that has been observed indepos i t s on ca ta lys t s inc ludes hydr~gen . '~~ . '~~his

1. Reactions of C and 0 with Hwith adsorbed hydrogen:Carbo n atom s from the dissociation of CO can react

M-C + M-H + M-CH + M (7 )M-CH + M-H + M-CH2 + M (8)M-CH2 + M-H + M-CH3 + M (9)

In this case, and in some of the others following, anadsorbed reactive species could also react with di-hydrogen:M-C + H2 F! M-CH + H (10)

Th e hydrogen atom liberated in this reaction could thenreact w ith other m olecules in the p hysisorbed layer ina chain fashion; the chain would be terminated by re-action of a radical w ith the surface to give one of theadsorbe d species or by recom bination of t he radical withanother radical or a surface species to give a pro ductmolecule. Th is type of reaction would give unp redic t-able products, depending largely on the proximity ofvarious molecules in the physisorbed layers. However,it may well play a minor or insignificant p art, since inth e reactions tha t have been researched in detail, re-action between adsorbed species is found to predom i-


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45 6 Chemical Reviews, 1981, Vol. 81, No. 5depo sit also hydrogenates rap idly, althoug h it is fairlystable to the hydrogen isotopic substitution t ha t mightbe expected of M-CH, species. It is not clear whetherthe hy drogen in the se deposits is chemically bonded tothe carbon or physically included in the deposit.Several types of evidence point to the existence ofhydrocarbon-like intermed iates, although some of thisevidence may res ult from carbonaceous deposits con-taining hydrogen or from adsorbed products rather th anfrom true intermediates. Infrared spectroscopy showedC-H stretch es for coadsorbed CO and H2 heated ons ilic a-sup po rted i r ~ n , I @ J ~ ~nd ruthenium167and for alumina-supported ruthenium.lE5 However,in er tn es s to iso to pic s u b ~ t i t u t i o n l ~ ~ J ~nd the bandgrowth structure185 mply th at these bands arise fromadsorbed p roducts as well as intermediates. The re-action of alkenes with surface intermediates on ru the-nium gives evidence for the presen ce of alkyl an d al-kylidene groups as intermediates.la7Few metal complexes that give reactions similar toreactions 7-10 have been prepared,15although numer-ous complexes containing hydrocarbon ligands areknown. Alkylmetal complexes areAlky1idenelxJg3 nd alk ylidyne lg4 omplexes have beenstudied mainly for the earlier transition metals, butcobal t alkylidynes are w el l - kn ~w n, '~ ~ron alkylidenecomplexes have been observed spectroscopically1% ndi s ~ l a t e d , ' ~ ~ - ~ ~ ~obalt iron alkylidynes have been iso-lated,m and an osmium alkylidyne has been isolated.m1Several cluster complexes whose chemistry may proveto be mo re analogous to reactions 7-10 than th at of thecomplexes previously known have recently been syn-

Adsorbed oxygen atoms should react with adsorbedhydrogen atoms (or with H2):M -0 + M-H + M-OH + M (11)

Th e forma tion of hydroxyl is known to proceed rapidlyon nickel from atomic oxygen,21o lthough it is not clearwhether H2or the adsorbed atom is the reactive species.Th e reaction of oxygen and hydrogen on rheniumj211i r i d i u m , 2 1 2 l a t i n ~ m , ~ ' ~ - ~ ' ~nd palladium216 nd of ox-ygen and deuterium on platinum21 7 ppear s to proceedas in reaction 11 . The re is some indication th at othermechanisms may contribute a t temperatures below tha tof the synthesis.2181220 ydrox yl group s have been ob-served by infrared spectroscopy on platinum, iridium,rhenium , nickel, cobalt, and iron as a result of the re-action of oxygen and hydrogen.221 Under synthesisconditions, this reaction appe ars to be very rap id, sinceoxygen is observed to be removed rap idly from iron178and rhodium,167 iving water as a product.2. Reactions of Undissociated CO

Chem isorbed mo lecular CO'67 an d CO in m etal car-b o n y l ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ppear not to react directly with di-hydrogen. Therefore, only reactions with adsorbedhydrogen atom s will be considered here. Mo lecularlyadsorbed CO should yield two intermediates upon ad-dition of a hydrogen atom. For hydridic hydrogen,M-CO + M-H * M-CHO + M (12)

For protonic hydrogen,M-CO + M-H M-COH + M (13)

Th e M-CHO interm ediate would have the formyl

thesized.1WO2-209

Rofer-DePoorterstructure I, and the M-COH intermediate, an alcoholicstructure (11). The triple bond may in fact represent

HM-C=O M=COHI I1bonds to three metal atoms. Analogy to the aldolequilibrium in organic compounds suggests that the twointermediates should be able to interconvert tautom -erically, althoug h no metal com plex analogue or surf acereaction has been observed:

M-CHO P M-COH (14)The evidence for the addition of hydridic and pro-tonic hydrogen to CO in metal com plexes has been re-viewed recen tly.2n T he reverse of reaction 12 has beenobserved in metal com plexes, bu t th e equilibrium favorsth e left-h and side of t ha t r e a c t i ~ n . ' ~ ~ ~ ~ ~omplexing ofthe carbonyl oxygen to a Lewis acid (d iscussed in sec-tion III.A.2; see also ref 224) activates the carbonylcarbon to nucleophilic attack, for example, by hydride,to give the M-CHO intermed iate. For this reason, asecond reaction, or rate constant for reaction 12, maybe necessary to represent the reaction of the more

strongly adsorb ed CO. Because the carbonyl oxygencan act as a Lewis base, attack by a proton is possiblea t tha t point to give an M-COH intermediate.82-225v226A study of m ethanation over alumina-supported me talcarbonyls also suggests tha t protonic hydrogen m ay beactive in hydrogenation of undissociated C0.227Similar reactions have been observed for CO com-plexed to more electron-deficient metals, such as zir-coniumm and the actinides,m and have been postulatedfor titanium.230Th e relatively empty d orbitals of thesemetals allow the formation of 'IT bonds between themetal and th e product formy l ligand. However, thesemetals are sufficiently different from the Fischer-Tropsch catalyst metals to raise a question of the re-lation of these reactions to the Fischer-Tropsch syn-thesis.Further hydrogenation of I and 11,which will be as-sumed to be by adsorbed atomic hydrogen rather tha nby H2 , yields alcoholic intermediates:

M-COH + M-H M-CHOH + M (15)M-CHO + M-H + M-CHOH + M (16)

M-CHOH + M-H + M-CH20H + M (17)Both h y d r i d i ~ ~ ~ ~ i ~ ~ ~ - ~ ~ ~nd protonic237hydrogen canparticipate in reactions of this typ e in metal complex-es.15 Carbon m onoxide in rhenium complexes has beenreduced to M-CHO, M -CHOH, a nd M-CH2 0H lig-a n d ~ . ~ ~ ~eduction of a formyl ligand to methyl hasbeen reported.231wA formyl ligand has been identifiedspectroscopically in the reduction of an osmium car-bonyl complex to a carbene, and a reaction sequenceanalogous to reactions 16 and 17 was postulated.2sReduction to hy droxymethyl has also been reported bytwo methods, a disproportionation and one involvinghydride.231i233i234strongly acidic solution of an ironcluster compound known to protonate at a carbonyloxygen gave methane as a this may includesteps 15 and 17 .Adsorbed intermediates containing C, H, and 0 havebeen produced by the interaction of CO and H2 onrutheniuml6 and nickel,'& although their stru ctures are


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458 Chemical Reviews, 1981. Vol. 81 , No. 5 Rofer-DePoorterTABLE V. Previously ProDosed Reac tions CorresD ondina to Reactions of Hydrocarbon and H-C-0 ntermediates

reaction type of evidence refM-CH + M - C H Z M-CHCH + MM-CH, + M-CH, Z M-CH,CH,-MM-CH, t M-CH, 2 M-CH,CH, + MM-R + M-CH, 2 M-CH,R + MM-CH, + M-CH, 2 M-CHCH, + MM - C H R + M-CH, t -CHCH,R + MM-CHR + M-H 2 M-CH,R + MM-CH, + R C H = C H , -Fc M-CH,CH,CHRM-CH, + M-CO t -C(O)CH, I

M-R + M-CO "c M - C ( O ) RM-C(O)CH, + mM-H t ,O + M-CH,-,CH, + mMM-C(O)CH, + H, t -CH(OH)CH,M-C(O)CH, + 2M-H 2 M-CH(OH)CH, + 2MM-C(0)R + M- H d M-CH(OH)CH,M - C ( O ) R + 2M-H t -OCH,RM-C( O)R + M- H + H,O 2 M-C( R)( 0H)O -MM - C ( O ) R + M-H 2 M-CH(R)O-MM-CH(OH)CH, + H, t -CH,CH, + H, OM-CH(OH)CH, + 2M-H t -CH,CH, + H,OM-CHOH + H, d M-CH, + H,OM-C(OH), + H, 2 M-CHOH + H,OM-CHOH + nM-H t -CH, + H,OM-COH + M-H 2 M-C + M + H,OM-CH(OH)CH, P M - C H = C Y + H,OM-OR + M-CO 2 M-0,CR2M-CHOH 2 M-CHC(OH)-M + H,OM-CHC(0H)-M + 2M-H 2 M-C(OH)CH, + 3MM - C ( 0 H ) R + M-CHOH d M-C(R)C(OH)-M

struc ture of most of these intermediates has not beendeterm ined beyond evidence of C-H and sometimesC-C bonds; however, an ethylidene interm ediate hasbeen identified on an ahmina-supported rhodiumcatalyst by tunneling spectroscopyB2as a result of theadso rption of CO, although th e source of hydrog en wasnot evident.I t is clear, however, th at carbon-carbon bond for-mation can occur without th e participation of an oxy-genated intermed iate. Carbon deposited on Fischer-Tropsch catalyst metals has been hydrogenated toethane, propane, and butane in the absence of CO.131~134~167~172Carbon-carbon bond formation has alsobeen observed between surface intermediates on a ru-thenium catalyst and added olefins,187 process thatmay ta ke place similarly to reaction 18,with the olefinadsorbed on the surface as an alkylidene; olefins alsoadd to an iron methylene complex.lW T he fact th atrelatively short-chain pro ducts are observed in theseexperiments raises the question of whether n and r inreaction 18 are limited in some way or whether thereaction is a hydrogenation of a short-chain carbonfragment.An ethynyliron complex has been reduced to a neo-pen tylidene complex by addition of methyl from methylfluorosulfonate; interme diates have been isolated.259The reaction of diazomethane and hydrogen ontransition-metal surfaces to produce hydrocarbonszmhas been interpreted as evidence in favor of this typeof carbon -carb on bond formation. However, the surfaceintermediates have n ot been exp erimentally identified,and this approach is subject to the uncertainties in-herent in the study of back reactions of stable product

surface studieskinetics, product distributionkinetics, product distributiondiazomethane reactioncombinationisotopic studiessurface chemistrysurface chemistrycombinationcomplex chemistrycomplex chemistrytransient experimentscombinationcomplex chemistryir spectroscopy of surfacescombinationcombinationcomplex chemistrytransient experimen tscombinationcombinationcombinationcombinationcombinationtransient experimentscombinationcombinationkine ticskineticsorganic dehydration reactioncombinationcombinationcombinationcombination

3531 9 , 2 01 9 , 2 03 7 , 1 7 2

2 601 7 21 6 71 6 718, 21257 , 258

1 61 7 02 12 8 3

7 , 1 27, 1414, 221 2 , 1 81 7 0212 8 1771 21 7 0

2 9 92991 2 113518283 , 2973, 2333molecu les (see discussion of reaction 31)plus the furtherdifficulty th at diazomethane is not a Fischer-Tropschproduct. Th is evidence, therefore, falls short of thecriteria listed in the Introduction, although it appear sto supp ort reactions of th e class repre sented by reaction18 .A special case of reaction 18 is for m an d s both equalto 0:

M-C + M-C P M-CC-M (20)Th is is probably a step in th e formation of carbon de-posits, which will be discussed in section IIJ.D.4.Complex chemistry offers evidence for the insertionof CO into an alkyl-metal bond (or migration of thealkyl to CO) as a way of forming carbon-carbonbonds:7,12J4-16

M-R + M-CO M-C(0)R + M (21)Wh en the m echanism of alkyl migration is consideredin detail, it has been treated 88 bond-break ing betweenthe alkyl carbon and th e metal accompanied by bond-making between th e alkyl and carbonyl carbons:261

RiHz2 2 )RCH2'iHz= ,,", --M-CO M-CO M-COIf CO is complexed side-on, the mechanism can bewritten in analogy to the mechanisms proposed forZiegler-Natta p olymerization. Analogy to Cossee'smechanismz2 gives

R R. R. 'I .I c j "CM-Ill 0 M-Ill 0 M-ll 0


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Mechanism for Fischer-Tropsch SynthesisAnalogy to th e m echanism of Ivin e t al.263 ives a dif-ferent type of carbonyl insertion:

RCH.

Chemical Reviews, 1981, Vol. 81, No. 5 459oxygen,16,286o th at reaction probably takes place fromth e q2-acyl. A possible sequence is7

CRM -1 + M-H t -CH(R)O-MM-CH(R)O-M + M-H = -CH(R)OH + M (28)

M-CH(R)OH + M-H M-CH(R)OHz + M (29)(2 7 )

M-CH(RIOH2 M-CHR + H 2 0 (30)Rhe nium acyls have been protonated to give hydroxy-carbene ligands,287n alternative to reaction 27, yieldingM-C (R)O H. Alkoxy intermediates may also be formedunder th e conditions of th e synthesis, bu t the evidencefor their formation is of a different type than givenearlier. Th e formation of alkoxy intermed iates migh tbe expected to occur by the reaction of an alkyl inter-mediate w ith an adsorbed oxygen atom:

M-R + M -0 + M-OR + M (31)However, this reaction ha s not been directly observedin either the forward or reverse direction. M ost of theevidence comes indirectly from studie s of alcohol ad-sorption on surfaces. Stable m etal alkoxide complexesare well-known,288 ut the y are n ot form ed by reactionsanalogous to reaction 31. Although alcohols adsorbedon iron ,m nicke1,290*1 and s ilica-supp orted iron , coba lt,and can give alkoxy intermediates, these ob-servations supp ort reaction 31 only in an indirect way.Somew hat less indirect is the observation tha t alcoholsadsorbed on Fe(100) gave CO, H2 , aldehydes, an d hy-d r o c a r b o n ~ . ~ ~he formation of the reactants of theFischer-Tropsch synthesis and hydrocarbon p rod uc bsuggests tha t alkoxy intermed iates can be consideredto play a part in the Fischer-Tropsch synthesis andconstitutes evidence for reaction 31 by an argumentbased on the principle of microscopic reversibility.Th e adsorption reaction can be considered to be thereverse of the reaction forming alcohols as products(section III.D.l). If hydrocarbons are formed throughthe alkoxy intermediates, the oxygen mu st be lost atsome point. One path t o oxygen loss th at would leadto alkanes would be formation of alkyl intermediatesthrough the reverse of reac tion 31, followed by produ ctformation. However, ethan e may be a product of theadsorption of ethanol on iron, but its presence could notbe ~ o n f i r m e d . ~ ~thylene was confirmed as a product.Elimination of OH from alkoxy intermediates wouldyield alkenes, but this reaction has no other support.Another route to alkoxy intermediates would be hy-drogen ation of th e product of reaction 27 with hydridichydrogen:M-CH(R)O-M + M-H M-OCH2R + M (32)Again, the evidence is not inco nsistent with th e reverseof this reaction. A difficulty in interpreting reactionsin which products of the synthesis are inserted intosynthesis conditions is in the network of paths open tothem and the intermediates formed from them and inthe cu rrent uncertainty of the natu re of the interm e-diates corresponding to a late stage of reaction. Al-though alcohols added to the feed stream have beenshown to participate in chain growth to a greater extenttha n do added alkenes, these results canno t simplybe interpreted in terms of a single interme diate or path.Th e relative participations of the two add ed products

Th e product acyl in this case would be expected to ben bonded to th e metal through th e acyl double bond:CM-R + M - i M - i R

0Th e q2-acylcomplexwill then be able to convert to th ea-bonded form:264

M-RR _t M-C(0)R ( 2 6 )0Carbon monoxide insertion is a w ell-known reactionin complex chemistry189J91*265*2Mnd has been studiedtheoretically in some detail.261However, it has n ot beenobserved unambiguously on surfaces. Th e reaction of

the more strongly adsorbed form of CO (reaction 25)would be expected to be more rapid tha n th at of theend-on CO (reaction 21), from several types of evidence.Alkyl migration in an iron complex has been found tobe catalyzed by Lewis acids such as alkali metal cat-ionsm and protonsm which complex the oxygen of thecarbonyl ligand. For manganese complexes, A B r3 ha sbeen shown to play th e same role as the alkali metalcations; an intermediate has been isolated that containsa Mn-CO-A1-Br ring.2699270urther, th e same sort ofinteraction a ppears to tak e place between the manga-nese complexes and an alumina surface, with oxygenfrom the alumina taking the place of bromine in thering.271As was he case for hydrogenation of C O, the earliertransitio n metals also facilitate alkyl migration to COcomplexed to them . Zirconium complexes have beenthe most studied,228 ut facile alkyl migration has beenobserved for complexes of uranium and t h ~ r i u m , ~ ~ ~ ~ ~ ~t i t a n i ~ m , ~ ~ ~ ? ~ ~ ~afnium,276 nd tantalum.277 Most ofthese reactions result in q2-acylproducts, as might beexpected from reaction 25.An q2-acyl derivative of ruthen ium is also known,278an d acyls with oxygen and carbon coordinated to twodifferent metals have also been ch a ra c te r i ~ e d ,~ ~n onecase having been prepared by CO insertion into analkyl-metal bond.280

Acyl intermediates have been formed by the inter-action of CO and alkenes adsorbed on metal sur-faces.281-2&1 owever, isotopic evidence suggests thatth e formation of th e oxygenated species is by oxygentransfer rather than alkylThe acyl intermediate must be hydrogenated to analkyl to continue chain growth. Th e probable mecha-nism is through an alcoholic interm ediate with loss ofwater. Several sequences of hydrogen addition can bewritten for this reaction; the overall reaction from acylt o alkyl has been observed in a c o m p l e ~ , ~ ~ , ~ ~ ~ut thedetailed mechanism has not been elucidated. Someanalogous interme diates have been isolated in the caseof reduction of formyl ligands (section III.B.2). Th ereduction appears to require coordination of the acyl

-


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460 Chemical Reviews, 1981, Vol. 81, No. 5 Rofer-DePoortermay rather be related to the rates of the reactions th atgive the chain-forming intermediates from the addedproducts.Evidence for the participation of carboxyl interme-diates suffers from the same difficulties. Adsorbedcarboxyl species have been observed in the coadsorptionof alkenes and C 02 = and in the adsorption of acet-aldehyde.241Carboxylic acids adsorb on Fe(100) to givecarboxylate intermediates which decompose to CO, H2,and C02,296 ut the same difficulty arises here in in-terpreting these experiments as in the alcohol adsorp-tion experiments. Another possible path to carboxylateintermediates may be alkyl migration t o chemisorbed

Th e roles of alkoxy and carbox yl interme diates in thesynthesis need to be clarified with more direct evidencewith respect to the mechanism of their formation andtheir furth er reactions. Th ey have been suggested tobe the predominant intermediates in the Fischer-Tropsch s y n t h e s i ~ , ~ ~ ~ ~ ~ ~ ~ ~ ~ut th e present evidence istoo slender to include their reactions in this mechanism.Numerous other reactions canbe written, particularlyfor elimination of water from oxygenated intermediates .Some reactions can be removed from consideration bythe assum ption th at chains grow only at one end, andth at end is the precursor of the functional groups in theproducts; the end not bonded to th e m etal is an alkyl.Th is assump tion is com monly made in discussions ofthe Fischer-Tropsch mechanism, but it is not the onlyone possible. T he predominance of unbranched hy-drocarbon chains with functional groups m ost often inthe 1-position isconsistent with chain growth at one endonly. However, there a ppe ars to be no a priori reasonwhy reactions of the typecould not take place. Th en further reaction, such aselimination of water or readsorption of the function algroup on the surface, could take place at the free endas well. However, the formation of reactive loci in thechain would produce more branched products andproducts with functional groups in positions other thanthe 1-position. Since some of these prod ucts are seen,these typ es of reactions cannot be completely ruled o ut,although the fact th at these are minor products arguesthat these paths are minor. A t present, there is nodirect evidence for or against double-ended chaingrowth.Th e last several reactions in Table V also have nodirect evidence and relatively little indirect evidenceth at can be interpr eted in their favor. Most of themare base d on analogies to well-known organic reactions;thu s, they remain possibilities. Th e hydroxycarbenepolymerization mechanism proposed by Storch, Go-lumbic, and Anderson3 is not supported by observedsurface species; few metal hydroxycarbene complexeshave been identified.223>287ome authors have postu-lated intermediates of this type to explain kinetics ofm e t h a n a t i ~ n , ~ ~ ~ ~ ~ ~ ~ ~ ~ut the evidence as a whole ap-pears not to su ppor t these reactions sufficiently t o in-clude them in this mechanism.Th e reactions proposed in this section and th e re-sulting intermediates are summarized in Table VI.These reactions include two types of carbon-carbonbond-forming steps; the hydrogenation steps are anal-ogous to the reactions of Table IV.

co2.254

M-CH2 + M-CH20H e M-CH2CH20H

TABLE VI . Reac t io n s of Hy d ro carb o n an dH-C-0 In termediatesreaction eq no .

M-C,H, + M-C,H, t -C,+,H,+, + MM-R + M-CO t -C(O)R + M

181921M-C,H, + M-H Z M-C,H,+,C CR' 'M-R + M-Ill 2 M-II0 0C R0M- II t -C(0)R

252 6

C R0M- I1 t M-H 'Tt M-CH( R)O-M 27M-CH(R)O-M + M-H t M-CH(R)OH + n1 28M C H ( R ) O H + M-H 2 M-CH(R)OH, t M 29M-CH(R)OH, 'Tt M-CHR + H,O 30

In te rmed ia tesM-CH (CH, ),CH, M-CH( R)O-MM-CH,(CH, ),CH, M-CH( R) OHM-C(O)R M-CH(R\OH,

D. Product FormationAlthough some of the previously proposed mecha-nisms have been discussed in terms of primary andsecondary products, this distinction becomes somew hatarbitrary in a mechanism of the type proposed here.Th e custom ary distinction is that primary produc b arethose produced directly by th e reactions of the m ech-anism, and th e secondary products are those producedby readsorption and further reaction of the primaryproducts. However, if all the reactions, including th eproduct desorption reactions, are reversible, as theymust be by the principle of microscopic reversibility,then the readsorption of stable produ ct molecules willgive intermed iates th at are indistinguishable from in-termediates th at have not undergone desorption and

readsorption and therefore will lead to the sam e kindsof products. A distinction can be made on the basis ofrelative amounts of different products with differentconversions, bu t this d istinction seem s not to be usefulin this discussion. Although some products may beobserved to form earlier th an others in the synthesis,this may simp ly result from the k inetics of the form a-tion of the various products.In addition t o the reactions giving the organic prod-ucts, the generation of the side products HzO, CO2,graphite, and metal carbides and oxides will also bediscussed in this section. In every case where a produ ctis formed, a desorption reaction can also be written, butthe desorption reactions will be omitted here. Previ-ously proposed product-forming reactions are given inTable VII.7. Organic Products

methan e and other alkanes:Two reactions are possible for the formation of(33)(34)

M-CH3 + M-H F! 2M + CHIM-CH3 + M-CH3 F! 2M + CH3CHs

or, more generally,M-CH2R + M-H F! 2M + RCH3 (35)

M-R + M-R' 2M + RR ' (36)


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Mechanism for Fischer-Tropsch SynthesisTABLE VII. Previously Proposed Prod uct-Forming R eactions

Chemical Reviews, 1981, Vol. 81, No. 5 461

reactionM-C + 2H , t M + CH ,N i , C d + 2H , t 3N i + CH ,M-CH, + H, t M + CH ,M-CH, + H, t M + CH ,M-CH, + M-H t 2M + CH ,

M-CHR + 2M-H t 3M + RCH,M-CH,R + H, Z M + RCH,M-CH,R + H, t M-H + RCH,M-CH,R + M-H t 2M + RCH,M-(CH,),-M t 2M + R C H = C H ,M-CH=CHR + H, t M + R C H = C H ,M-CH,CH,R t M-H + n-M-CH,=CHRn-M-CH,=CHR t M + R C H = C H ,M-CH,CH,R t M-H + R C H = C H ,M-OR + M-H t 2M + R O HM - C H ( 0 H ) R + H, t M + RCH,OHM - C H ( 0 H ) R + M-H 2 2M + RCH,OHM - C ( 0 ) R + M-H + t 2M + R C ( 0 ) HM - 0 + H, t + H,OM-OH + M-H t M + H,OM - O + C O t M + CO,M - 0 + M-CO t M + CO ,nM-C t nM + C,(graphite)nM-C t nM + C,(amorphous)C,(amorphous) F*t C,(graphite)M-C + CO + H, t M-CC + H, OM-C + 2C O t M-CC + CO ,M-C + 2 Fe t Fe,CM-C + 3N i Z Nil%&Chwst&ial+ 3N i t Ni,C,w

ty p e of evidencetransient exper imentssurfac e speciesco mb in a t io nkineticsco mb in a t io nsurface specieskineticstransient exper imentsco mb in a t io nco mb in a t io ncomplex chemistrykinetics , p roduct d is tr ib utionco mb in a t io ntransient exper imentsk inetics , p roduct d is tr ibutioncomplex chemistryco mb in a t io nco mb in a t io ncomplex chemistryco mb in a t io ncomplex chemistrytransient exper imentscomplex chemistryco mb in a t io nco mb in a t io ntransient exper imentsH, , CO absorption proper t iesco mb in a t io nco mb in a t io nt r an s ien t ex p er imen tsCO absorption proper t iest r an s ien t ex p er imen tsco mb in a t io nsurface speciessurface speciessurface speciesco mb in a t io nco mb in a t io nt r an s ien t ex p er imen tssurface specieshydrogenation of carb ides

re f1 3 11 6 51 8 01 2 11 6 71 7 81 3 51 7 01 3 31 6 7182 01 7 0201 8

7 , 1 6 , 2 2 , 3 7

7 , 2 17 , 2 17 , 2 11 2181 7 01 22 11 6 6 , 1 6 71 3 11 3 41 3 31 3 5 , 1 6 6 , 1 6 71 3 11 3 41 7 01 3 51 7 81 6 51 6 51 6 61 6 61 7 01 6 51 8 0

The formation of 2-methyl-substitutedalkanes probablytakes place throug h a succession of two reaction s of th eform of reaction 36 , where R' is methyl:Alkene formation can occur from alkyl intermediatesby @-hydrogene l i m i n a t i ~ n : ' ~ ~ J ~

M-CHZCHZR a M-H + RCH=CHZ (39)M-CHR + M-CH3 M-CH(CH3)R (37)

M-CH(CH3)R + M-CH3 2M + RCH(CH3)Z (38)T he evidence for hydrogenation (reaction 33) has beendiscussed in section III.B.1. In m etal alkyl complexes,reactions 35 and 36 are called reductive eliminationsand have been observed for numerous met als .le sJm ~- mDihydrogen has also been observed to react with metalcarbene complexes to cleave the M=C bond an d givehydrocarbons.lssm*- Reactio ns between iron dim ersand methane,307nickel clusters and pentane,308andcopper atoms and methanem in low-temperature ma-trices to give the me tal alkyls may be t he reverse ofreaction 33. Th e apparen t reverse of th e hydrogenationof carbon atoms to methane (reactions7-9,33) has beenobserved in the adsorption of methane on nickel sin-gle-crystal Th e lack of significant branchin gin the carbon chains beyond 2-methyl must be ex-plained in this mechanism by competing rates of hy-drogenation and carbon-carbon bond formation andwill be discussed in detail in section 1V.B.

or from less saturated intermediates by rearrangement:Ethylene has been shown by electron energy lossspectroscopy, interp reted in term s of adso rbate vibra-tional spectra, to adsorb on platinum as a -CHCH3the reverse of reaction 40. Results fromthe adsorption of ethylene on Fe(100) are consistentwith the platinum results, but the structu re of the in-termediate was not identified conclusively.m Th eproduct from reaction 40 may be formed as a ?r com-p l e x . ' ~ ~ ~Alcohols, like alkanes, can be formed by h ydrogena-tion of the appropriate intermediates:More generally,Reactions of thistype have been observed for chrom iumcomplexes in acidic aqueous solution, protons being thesource of hydrogen.313 If alkoxy intermediates arepresent, they m ay be hydrogenated to alcohols:

M-CHCHZR M + RCH=CHZ (40)

M-CHZOH + M-H a 2M + CHSOH (41)M-CH(0H)R + M-H * 2M + RCHzOH (42)


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48 2 Chemical Reviews, 1981, Vol. 81, No . 5M-OR + M-H + 2M + ROH (43)

Th e reverse of this reaction has been observed on iron,nickel, and cobalt surface^.^^^^^^^ However, the forma-tion of alkoxy intermediates has not been well sup-ported (section III.C), and their role in the synthesisis not clear. Reaction of alkyl intermed iates with sur-face hydroxyls may also occur:Certain similarities and differences among reactions 42,43 , and 44 remain to be understood. Reaction 44 is anucleophilic attack of hydroxyl at the alkyl carbonbonded to th e surface; reaction 42 might be expectedto occur through nucleophilic attack by hydride, but th esingle exp erimen tal observation is of electrophilic attac kby a proton. Hyd rogenation of alkoxy intermed iates(reaction 43) should also require protonic hydrogen.For th e form ation of aldehydes, kinetic evidence foran a-hydroxymethyl intermediate in a reaction ofmanganese complexes to give benzaldehyde314 n d th etherm al decompo sition of a hydroxym ethylene rheniumcomplex to give acetaldehyde287 uggest that a prod-uct-forming reaction is

M-CH(0H)R F? M-H + RCHO (45)orM-C(0H)R + M f RCHO (46)

Aldehydes may also be formed by hydrogenation of acylintermediates:How ever, no direct evidence is available for this reac-tion.T he form ation of acids can also be accounted for byreaction of the available intermediates:

Esters could be form ed through an analogous reaction,with alkoxy intermediates replacing hydroxyl.2. Water

T he evidence for the forma tion of surface hydroxyl(section III.B.l) also supports the reaction of chemi-sorbed intermediates to form waterM-OH + M-H 2M + H20 (49)

rather tha n th e reaction of H 2 with adsorbed oxygenatoms. Water may also be formed in t he hydrogenationof acyls to alkyls (rea ction 30).3. Carbon Dioxide

The reaction between CO and 0 to produce C02probably takes place through the adsorbed species onmetal s u ~ f a c e s : ~ ~ ~ ~ ~ ~M-CO + M -0 F? 2M + C02 (50)

rather than through a collision of an unadsorbed COmolecule with an adsorbed oxygen atom. The kineticsof the reaction have been studied mainly on rhodium,palladium , iridium, and platinum. Of the Fischer-Tropsch m etals, only ruthenium and cobalt have beenstudied. Th e results for ruthenium did not eliminatee ith er mech an ism f ro m c ~ n s i d e r a t i o n , ~ ~ ~nd the reac-tion appears to have som e different characteristics fromth a t on the other metals s t ~ d i e d . ~ ~ ~ * ~ ~ ~ * ~ ~ ~he results

M-CH2R + M-OH F? 2M + RCH20H (44)

M-C(0)R + M-H * 2M + RCHO (47)

M-C(0)R + M-OH + 2M + RCOOH (48)

Rofer-DePoorterfor cobalt321were consistent with reaction 50.Me tal oxides can also serve as substrate s for CO ox-idation. On mixed cobalt magnesium,323s24ndmixed nickel magnesium3%oxides, the oxidation takesplace a t the metal ions, through ionic intermediates.This path is also a possibility in supported catalystsystems in w hich the metal a t the interface betweenme tal and su ppo rt is oxidized and for systems in whichmetal oxides are formed during the synthesis.4. Unreactive CarbonCarbon deposition is one of the major routes ofdeactivation of th e Fischer-T ropsch c a t . a l y ~ t s . ~ ~ JCarbon blocks adsorption of H2and C0,390v331nd someforms are difficult to hydrogenate (section III.B.l). Inaddition, carbon deposited on the catalyst is removedfrom paths to desired products.Carbon atoms are produced by dissociation of CO(reaction 6). A t least four types of carbon have beenidentified on the surface of catalysts, some of themreactive enough to be considered interme diates (sectionIII.B.1). Th e carbon to be discussed here is the inertcarbon that is effectively an end product of the syn-thesis. Th e formation of this carbon may be directlyfrom adsorbed atoms:178*332

nM-C F! nM + C,(graphite) (51)Th e transforma tion of surface carbon atoms to graphitehas also been observed to proceed through am orphouscarbon?

nM-C nM + C,(amorphous) ( 52 )C,(amorphous) a CJgraphite) (53)

Th e amorph ous carbon may also be a poison, becauseit hydrogenates slowly, although n ot a s slowly as th egraphitic carbon.l@ On th e other hand, it app ears tooxidize more rapidly than carbidic carbon.333Stud ies of the dispro portionation of CO over iron,=catalysts and studies of cokeformation on dehydrogenation (iron-chromium) cata-l y s t ~ ~ ~ ~ndicate that graphite deposits can also beformed by the breakdown of an intermediate metalcarbide. Although these studie s were carried out athigher temperatures than typical synthesis conditions,this path should also be considered as a possibility forformation of unreactive carbon.Because the s trengths of the metal-carbon bonds forthe various m etals should influence the rate s of reac-tions 51 and 52, it has been speculated t ha t one of thefunctions of prom oters is to decrease the formation ofgraph ite deposits. How ever, prom oters have been

shown to have little or no effe ct on the specific activityof iron catalysts for carbon deposition.326Carbon deposition has also been studied by pyrolysisof hydrocarbons over catalysts.310 Although thesestudies are typically carried out a t higher tem peraturestha n those of the Fischer-Tropsch synthesis, this pa thshould also be considered for carbon deposition. Itwould be represented by th e reverse of th e hydro carbonformation reactions and hydrogenation reactions andby th e reactions for the fo rmation of unreactive carbondiscussed in this section; thus, these studies can alsocontribute to the understanding of carbon depositionin th e Fischer-Tropsch synthesis, but will be subjectto som e of th e sam e types of difficulties in interpre ta-

an d


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Mechanism for Fischer-Tropsch Synthesistion discussed in section 1II.C with respect to alkoxyintermediates.5. Metal Carbides and Oxides

Although the definition of a catalyst requires th at itbe unchanged a t the end of a reaction, the Fischer-Tropsch catalysts have been found to have been con-verted from the metal to the carbide or the oxide at theend of a synthesis. In addition, the metal oxides andcarbides themselves possess catalytic activity for variousparts of the synthesis and can participate in product-forming reactions. Therefore, their forma tion duringa synthesis may change rate constants for elementaryreactions.T h e formation of meta l oxides may be slightly ben-eficial in removing oxygen by another path th an reac-tion with Hz or CO, although this cannot be a largecontribution, in view of the relative amountsof catalystand reactants. Th e formation of me tal carbides, unlessit provides new catalytic paths or improved rate con-stants, will be detrim ental in removing carbon from theproduct-forming paths, although, again, only to a smalldegree.

The formation of carbides and oxides of iron andnickel is thermodynamically favorable under the con-d itio ns of t he Fischer-T ropsch s y n t h e ~ i s . ~ ~ ~ - ~ ~ ~ow-ever, the kinetics of the processes involved may be slowenough relative to other reactions in the m echanism3%th at th e reactionsM-C + MC(carbide) (54)M -0 F! MO(oxide) (55)

may be sufficient to indicate the rem oval of carbon an doxygen from other paths.T he behavior of th e particular me tal in the catalystwill need to be taken intoconsideration to rebresent themechanism over a particular catalyst. Whereas theme tal may chang e the rates of earlier reactions by themetal-intermediate bond strengths, the formation ofoxides and carbides proceeds through different com-pounds with different stoichiometries and phases, andtherefore m ust be represented by different sets of ele-mentary reactions. Reactions 54 an d 55 will be regardedas sufficient here. How ever, the available literatur erelevant to the k inetics of carbide and oxide formationunder Fischer-Tropsch conditions will be reviewed hereto give a starting point for further studies to show theam oun t of detail needed for including these reactionsin the mechanism.A com plete set of elem entary reactions describing thetransformation of oxygen or carbon into a metal oxideor carbide, from surface atom to bulk com pound, is notavailable for any system. Nickel has been the m osts t ~ d i e d . ~ ~ ~ ~ ~ ~ * ~ 'he sequence app ears to be surfaceatom, dissolved atoms th at diffuse into the bulk, andforma tion of compound. This sequence has been pos-t ul at ed fo r n ic ke l c arb id e f ~ r m a t i o n , ' ~ ~ @ " ~ ~ ~nd i t ha ssome support in k i n e t i ~ s . ~ ~ ~ ~ ~ ~Metallic iron,327*328va4uthenium:% an d iron- ruthe-niumsB catalysts, silica-supported iron,175*a5-a7 ick-el,34et347nd i r o n - n i ~ k e l ' ~ ~ v ~ + ~ ~ ~atalysts, and iron in-tercalated in graphite3&have been studied by severalmethods to determ ine the p hases of oxides and c arbidesformed und er synthesis conditions. Several phases areobserved, some of them a t different times in the syn-

Chemical Reviews, 1981, Vol. 81, No. 5 463TABLE VIII. Pro d u c t -Fo rmin g Reac t io n s

reaction ea no.M-CH,R + M-H t M + RCH,M-R + M-R' t M + R R 'M-CH.CH.R t M-H + RCH=CH,

353639M - C H ~ H , Rz + R C H = C H , * 40M-CH(OHlR + M-H t M + RCH.OH 42M - C H ( 0 H j R P M-H + RCHOM - C ( 0 H ) R t M t RCHOM-OH + M-H 2 2M + H, OM-CO t M -0 t M + CO ,nM-C 3 nM t C,(amorphous)C,(amorphous) t ,(graphite)M-C t MC (carb ide)M -0 t MO (o x id e)

454 649505 25 35 45 5

thesis, indicating th at the kinetics of th e transforma-tions from one phase to another may be significant inunderstanding th e synthesis. Carbide formation ap-pears to increase overall synthesis rates an d to shift theac t iv i ty toward fo rmat ion o f h igher hydro -c a r b o n ~ ; ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~-iron carbide is the most activecatalytic species a t low temperature^.^^The cobalt (1012) face has been found to allow oxygenfrom dissociation of CO to diffuse into it rapidly, whileforming a surface carbide phase that may be C O ~ C . ~ ~ JTh is surface layer is capable of adsorbing C O, but n otof dissociating it,109J10 nd may therefore change thebalance of reactions occurring.Ruthenium does not form carbides and dissolvescarbon over a limited range of composition.349Adsorbedoxygen atoms have been found to diffuse into th e Ru-(001)plane in an early step in the formation ofTh e carbides and oxides themselves can participatein further reactions, mainly to produce CH I, H 20 ,CO,C0 2 ,and unreactive ~ a r b o n . ~ ~ * ~ lhese reactions canbe represented by elementary reactions already in-cluded in this mechanism, starting w ith the reverse ofreactions 54 an d 55 to form the reactive adsorbed car-bon an d oxygen atoms from the bulk carbides and ox-ides. Th en the earlier hydrogenation and oxygenationreactions can take place.Th e reactions proposed in this section are summ a-rized in Table VIII. Th e hydrogenation and recom-bination reactions are analogous to the reactions ofTables IV and VI. In addition, th e formation of alkenesand aldehydes may proceed through elimination reac-tions.I V . Comparison with Prevlous& ProposedMechanisms

Parts of the mechanism for the Fischer-Tropschsynthesis have been discussed widely, but only fivecomplete mechanisms have been proposed the carbidemec hanism ,l9a the B ureau of M ines mechanism,@ thePichler-Schulz mechanisms based onsteps observed in complex chemistry,12J8and a com-bined mechanism recently advanced by Ponec.l42 Onlythe complete mechanisms will be discussed here. Allof th e m echanisms overlap to some degree. T he objectof th is section is to provide an overview of t he rela-tionship among the mechanisms.In general, the mechanism proposed h ere differs fromprevious mechanisms in tha t it includes more possiblesteps; in particular, two types of carbon-carbon bondformation (reactions 18,21 , and 25) are included here,whereas the others include either a hydrocarbon in-


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464 Chemical Reviews, 1981, Vol. 81 , No. 5termediate recombination reaction or an alky l migrationto carbonyl, but not b oth. In addition , most of the othermechan isms do not include path s for the production ofH 2 0 , C 0 2 ,unreactive carbon, and the metal carbidesand oxides. Th e mechanism proposed here resemblesthe

a comprehensive mechanism for the fischer-tropsch synthesis

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