Activation of C-H, C-C, and C-O Bonds of Oxygenates on Rh(111

Chapter 24

Activation of C - H , C - C , and C - O Bonds of Oxygenates on Rh(111)

Downloaded by UCSF LIB CKM RSCS MGMT on November 28, 2014 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0517.ch024

N. F. Brown and M . A. Barteau Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, DE 19716

Decarbonylation of oxygenates to liberate CO, H2, and volatile or adsorbed hydrocarbons has been observed on single crystal surfaces of a number of Group VIII metals. The details of this chemistry hold the promise of providing new insights into catalytic processes which are essentially its reverse, e.g., higher oxygenate synthesis from C O and H2, and heterogeneous hydroformylation of olefins. The results of our studies of ten C2 and C3 oxygenates on Rh(111) demonstrate, however, that the decarbonylation network for these probe molecules is far more complicated than previously recognized. Higher aliphatic aldehydes release CO plus alkyl groups via decarbonylation on Rh(111), these alkyls are hydrogenated to volatile alkanes. Higher alcohols do not dehydrogenate to aldehydes, and do not release volatile hydrocarbon products. Parallels between the reactions of alcohols and epoxides suggest that both form oxametallacycle intermediates on the surface. The divergence of alcohol and aldehyde decarbonylation pathways persists for the unsaturated oxygenates acrolein and allyl alcohol; although both ultimately deposited CO, H, and CH3C≡species on the surface, only acrolein liberated volatile C2 hydrocarbons. Of the ten oxygenates examined, decarbonylation reactions must release at least five different hydrocarbon ligands to the surface in order to account for the range of behavior observed in TPD and HREELS experiments. This suggests that the identities of the primary products of CO insertion on metal catalysts may depend on the identities of the surface hydrocarbon ligands undergoing insertion.

There can be little doubt that oxygen-containing hydrocarbons (oxygenates) will play an increasingly prorninent role as components of motor fuels and, potentially, as raw materials for other petrochemical-based processes. The last decade has seen increasing use of oxygenates, including alcohol blends and methyl tertiary butyl ether (MTBE), as octane

0097-6156/93/0517-0345$06.00/0 © 1993 American Chemical Society

In Selectivity in Catalysis; Davis, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

SELECTIVITY IN CATALYSIS


346

enhancers in gasoline. Current trends include the introduction of "environmental gasolines," reduction of some high octane hydrocarbons including butanes and aromatics, and legislative requirements for increased oxygen content as a means of improving air quality (7). The last decade and a half have seen a great deal of attention devoted to the synthesis of higher molecular weight products from CO and H2 with supported transition metal catalysts. Strategies for the production of higher oxygenates have centered primarily around two classes of catalysts: supported group VTJI metals (typically rhodium) and modified methanol synthesis catalysts (based on Cu^nO/AkQs). The operative mechanisms of higher oxygenate synthesis on these two classes of materials are generally proposed to be different. That for group VIII metals has been suggested by numerous workers to involve hydrocarbon chain growthfromC H units on the surface, with oxygenates formed via termination of chain growth by C O insertion and subsequent hydrogénation (2 -4). On modified methanol catalysts chain growth is viewed as the result of various condensation reactions of oxygen-containing intermediates (5-7). While these sequential reaction networks involving competing chain growth and termination steps are adequate to produce the typical Anderson-Schultz-Flory distribution of product chain lengths, it is not clear that they can adequately describe the selectivity to various products having the same number of carbon atoms. For example, it is typically suggested in the above network for metal-catalyzed reactions that C +i oxygenates are formed by addition of CO to a C hydrocarbon ligand on the metal to form an acyl; the sequential hydrogénation of this intermediate leads in succession to aldehyde and alcohol products. Evidence for the sequential hydrogénation of aldehydes to alcohols in the CO+H2 reaction on rhodium has been provided by Underwood and Bell (8)> but conflicting evidence can be found in isotopic tracer studies by Takeuchi and Katzer (9) and Orita et al. (10). In the latter case, ethanol and acetaldehyde were observed to have different distributions of labelled carbon, suggesting that they are not formed by a common mechanism. If so, then the identity of the oxygenate product would depend on the identity of the hydrocarbon ligand to which CO is added, rather than on die extent of hydrogénation of the product after CO insertion. A second conundrum is evidentfromthe lack of successful exploitation of the analogy between homogeneous and heterogeneous hydrocarbon hydroformylation catalysis. The scheme usually proposed (3-5) and outlined above for supported metal-catalyzed oxygenate synthesis is essentially the hydroformylation of surface hydrocarbon ligands derivedfromCO and H2. The exact identity of these hydrocarbon species (alkyl, alkylidene, olefin, etc.) has been much debated. In contrast to the state of affairs in heterogeneous catalysis of higher oxygenate synthesis, homogeneously catalyzed carbonylation and hydroformylation reac tions are both well understood and commercially practiced. For example, the essential step in commercial Rh-catalyzed olefin hydroformylation and methanol carbonylation processes is CO insertion into a metal-alkyl bond. The mononuclear rhodium complexes cycle between the +1 and +3 oxidation state in both systems (77). Putative analogies between homogeneous and heterogeneous catalysis have led various workers to postulate and to search for cationic transition metal species on active and selective oxygenate synthesis catalysts. Both the Tamaru (12) andPonec (75) groups proposed, for example, thatzero-valentPd wouldnotlead to methanol synthesis over supported Pd catalysts. The latter group demonstrated a positive correlation between methanol synthesis activity and the concentration of P d ions on the catalyst extractable with acetyl acetonate. These conclusions have since been called into question by other workers (14-16). Moreover, the Ponec group demonstrated a negative correlation between higher oxygenate activity and R h extractable from supported rhodium X

n

n

+1

+1



24. BROWN & BARTEAU

C-H, C-C, and C-0 Bonds of Oxygenates

347

catalysts (7 7). Perhaps it is not surprising that the hydroformylation analogy has not advanced much further. Surface science studies of oxygenate chemistry permit one, in principle, to enter these complex reaction networks via the products and to trace the sequence of reactions back to proposed mtermediates and reactants. For example, studies of higher oxygenate decomposi tion on metals such as Ni (18), Pd (79), and Pt (20) have all demonstrated that the principal reaction pathway is decarbonylation to CO, H(ad), and a hydrocarbon one unit shorter than the parent, in excellent agreement with the picture above of C O insertion as a chaintemiination step. On Pd( 111 ) it can even be shown that the decarbonylation of higher alcohols leads precisely through the sequence of steps suggested above; from alcohol to alkoxide to aldehyde to acyl to CO plus a hydrocarbon one unit shorter. On Rh( 111) however, in keeping with the confusion noted above, a purely sequential reaction network cannot account for the difference in the decarbonylation behavior of higher oxygenates. This paper describes the complexity of decarbonylation networks on Rh( 111 ), and the menagerie of surface hydrocar bon ligands required to account for them. Experimental Temperature Programmed Desorption (TPD) and High Resolution Electron Energy Loss Spectroscopy (HREELS) studies have been carried out after adsorption of three different C2oxygenates and seven different C3-oxygenates on the R h ( l l l ) surface in the apparatus described previously (27). A l l adsorption experiments described below were carried out to saturation of the first monolayer on a Rh(l 1 l)-oriented single crystal, prepared as described (22), with the surface held between 90 Κ and 100 K. TPD experiments involved ramping the sample temperature at a rate of 4 K s" while monitoring the desorbing products with a colhmated ΙΓΠ100 C mass spectrometer. HREEL spectra were all acquired at the adsorption temperature; spectra characteristic of the adlayers at higher temperatures were obtained after brief heating to the desired temperature followed by cooling to the original temperature. A variety of oxygen-containing reactants were examined including ethanol, acetaldehyde, ethylene oxide, propanal, 1-propanol, acrolein, allyl alcohol, acetone, and 2-propanol; each was dosed onto the surface from the vapor phase via a needle doser connected to a valve external to the chamber. A l l exposures are reported in Langmuirs (1 L = 10" torr* sec). A l l other experimental procedures were as previously described (27). 1

6

Results While adsorbed primary alcohols on the Pd(l 11) surface dehydrogenate sequentially to form the corresponding adsorbed aldehyde and acyl species prior to their decarbonylation (23), we have found no evidence for aldehyde formation from primary alcohols on Rh(l 11) (27,24). Instead, alcohol and aldehyde decarbonylation pathways on R h ( l l l ) appear to be nonintersecting. This surprising divergence of reaction pathways for such closely related molecules is demonstrated by two critical observations: 1. Volatile hydrocarbon products one unit shorter than the parent were consistendy produced by decarbonylation of aldehydes, but not of alcohols. 2. CO elimination from ethanol and 1-propanol occurred at lower temperature than from acetaldehyde and propanal, thus the latter cannot be intermediates in the reactions of the former. Primary alcohols dissociate to form stable alkoxides below 150 Κ on Rh(l 11) (212224). The range of possible subsequent bond scission steps is fairly limited, especially for smaller


348


molecules. For example, if the ethoxide does not undergo C - H scission at the α position to form acetaldehyde, the only alternative C - H scission would be at the β position. β-CH activation would form not an aldehyde, but an oxametallacycle of the form: CH

2

CH


Rh There are numerous precedents for such proposed mtermediates for mononuclear metal complexes in solution (25), and such cyclic adsorbates on an extended metal surface should experience less ring strain than their mononuclear counterparts in solution. In order to test the hypothetical formation of surface oxametallacycles, we have examined the decarbonylation reactions of epoxides such as ethylene oxide ( C H C H p ) and propylene oxide (CH^ÇHCH^Ç) on the R h ( l l l ) surface. Simple ring opening of these molecules by C - 0 scission should produce oxametallacycles analogous to those postulated for the alkoxides; a 1,2 hydrogen shift accompanied by ring opening would produce the aldehydes which are isomers of these epoxides. In fact, as shown by Figures 1 and 2, the decarbonylation of these epoxides parallels that of the primary alcohols and not that of the isomeric aldehydes. Methane desorbed from the R h ( l l l ) surface during the course of acetaldehyde decarbonylation but not during those of ethanol or ethylene oxide. Similarly, propanal released ethane, but no hydrocarbons desorbed during TPD experiments with either 1 -propanol or propylene oxide. Taking account of the different yields of CO and H from the 2

2

2

Mgthane anrtalftehyrte QthaflQl ethylene oxide

xl.5 acetaldehyde ethanol ethylene oxide. 100 200

300 400 500 600 Temperature (K)

700

800

Figure 1. A comparison of product TPD spectra obtained after exposure of 1.5 L acetaldehyde, 1.1 L ethanol, or 3.9 L ethylene oxide to the clean Rh(l 11) surface at ca. 91K.


24. BROWN & BARTEAU

C-H, C-C, and C-O Bonds of Oxygenates

349

Ethane

x3 1-propanol propylene oxide


propanal

propylene oxide

x3 1-propanol propylene oxide 100

200

300 400 500 600 Temperature (K)

700

800

Figure 2. A comparison of product TPD spectra obtained after exposure of 1.6 L propanal, 2.3 L 1-propanol, or 2.1 L propylene oxide to the clean Rh(l 11) surface at ca. 91 K . alcohols and epoxides, and the coverage dependence of the kinetics of reactions such as H atom recombination, the TPD spectra for the primary alcohols and epoxides were in excellent agreement, and were distinguishable from those of the aldehydes of the same chain length. For example, the CO desorption peak from propanal was broadened and shifted to lower temperature relative to those forCOfromthecleanRh(l 11) surface orforCOeliminated from 1 -propanol or propylene oxide. This CO peak shift has been shown to be characteristic of CO adsorbed in the presence of ethylidyne (CH3O) species (26) and, indeed, ethylidyne formation was observed by HREELS from propanal but not from 1-propanol or propylene oxide (24). Thus the common decarbonylation routes for epoxides and for primary alcohols support the intermediacy of the proposed oxametallacycles in each. The formation of such multiply coordinated surface intermediates would be expected to be enhanced by adsorption of multi-functional reagents, e.g., oxygenates with hydrocarbon chains more reactive than saturated alkyl ligands. To test this hypothesis, we have also examined the adsorption and reaction of allyl alcohol (CH2=CH-CH20H) and acrolein (CH2=CH-CHO) on the R h ( l l l ) surface. While these molecules do exhibit evidence for interaction with the surface via both their oxygen and vinyl functions, and while they appear to preserve the divergence of decarbonylation pathways observed for their aliphatic counter parts, their reactivity patterns add yet another layer of complexity to the puzzle of oxygenate decarbonylation.


350


Temperature programmed desorption experiments with acrolein and allyl alcohol were qualitatively consistent with the results for the other C3-oxygenates depicted in Figure 2. Acrolein decarbonylation gave rise to ethane at ca. 260 Κ (27) as did that of propanal (24), but small amounts of ethylene were also formed from acrolein (27). Allyl alcohol, like its aliphatic counterparts, produced no volatile hydrocarbon products. However, it differed from 1-propanol in the identity of the hydrocarbon ligand deposited on the surface. As shown by the HREEL spectra of Figure 3, decarbonylation of propanal, acrolein, and allyl alcohol led to the formation of recognizable surface ethylidyne intermediates (fingerprinted by the pattern of peaks at 980, 1135, and 1370 cm ), but 1-propanol did not. Thus for these four C3 oxygenates, four different reactivity patterns (and four different decarbonylation tempera tures) were observed, as summarized in Table I. Propanal produced ethane and ethylidynes, acrolein produced these plus ethylene, allyl alcohol produced ethylidynes but no ethane or ethylene, and 1-propanol produced neither ethylidynes nor volatile C2 hydrocarbons. These observations suggest that the divergence of reaction pathways is not just two-fold but four fold: all four of these C3 oxygenates follow a different path to CO, producing a different hydrocarbon moiety from each.


-1

Table I. Summary of C - 0 Decarbonylation Reactions 3

Reactant

Decarbonylation Temperature (K)

Volatile Hydrocarbon Product

Adsorbed Hydrocarbon Intermediate

CH3CH2CHO

251

C H6

CH3O

CH =CHCHO

201

C2H6 + C2H4

CH te

CH =CHCH OH

278

None

CH3O

CH3CH2CH2OH

226

None

CH

2

2

2

2

3

X

Discussion Of the ten different C2 and C3 oxygenates examined on the Rh(l 11) surface to date, at least five and as many as seven different hydrocarbon ligands must be involved as initial elimination products to explain the diversity of their behavior in TPD and HREELS experiments. A summary of these observations and proposed sequences of bond activation is contained in Table Π; the logic behind this proposal is described below. Aldehyde decarbonylation on R h ( l l l ) liberates volatile hydrocarbons one unit shorter than the parent. Previous experiments have demonstrated that there is no kinetic isotope effect observed in the kinetics of CH3CHO vs. CD3CDO decarbonylation, but CH3D can be formed cleanly by decarbonylation of CH3CHO in the presence of adsorbed deuterium atoms (27). These two results strongly suggest that alkyl groups are eliminated intact in the decarbonylation of aliphatic aldehydes; these add one Η from the surface to form the corresponding alkanes or, in the case of ethyl ligands, undergo parallel dehydrogenation to form stable ethylidynes. The C - C bond scission step liberating alkyl ligands does not


24. BROWN & BARTEAU


351


1-Propanol 258 Κ

i

-200 200 600 1000 1400 1800 2200 2600 3000 3400 Energy Loss (cm ) 1

Figure 3. HREEL spectra observed after exposure of 2.2 L propanal, 2.6 L acrolein, 1.3 L allyl alcohol, or 2.3 L 1-propanol to the clean Rh(l 11) surface and annealing to 301 K, 247 K , 304 K, and 258 K, respectively, to complete the decarbonylation reaction.

necessarily occur for the intact aldehyde, but likely involves acyl ligands formed from aldehydes via an initial C - H bond scission step. C - C scission in the unsaturated acyl formed from acrolein would release vinyl (CH2=CH-) ligands to the surface. Isomerization of these would lead to stable ethylidynes, hydrogénation, to volatile ethylene and ethane. The observation that acrolein decarbonylates at lower temperature than does propanal suggests that these hydrogénation steps must follow C - C scission; propanal cannot be an intermediate in the acrolein decarbonylation sequence. Oxametallacycle formation from aliphatic alkoxides could occur via C - H scission at the β position, orfromepoxides by C - O scission to open the ring. The subsequent C - H and C - C bond activation steps are less clear than those of the aldehydes above. For the oxametallacycle formed from the ethoxide, C - C scission must release CH2 or perhaps C H



352

Table Π. Summary of Bond Activation Sequences for C and C Oxygenates on Rh(l 11) 2


Reactant

3

Volatile Adsorbed Proposed Proposed Hydrocarbon Hydrocarbon Ligand Bond Activation Product Product Eliminated Sequence (Greek Observed Observed letters designate C - H bonds at that position)

CH3CH2OH β α

none

CH

X

CH

2

OH, β, α, C - C

CH3CHO

CH

4

CH

3

CH

3

α, C - C

none

CH

X

CH

2

C-O, a, C - C

CH CH CH OH γ β α

none

CH

X

CH CH CHO γ β α

C H

CH CHCH 0

β

α

CHoCHoO L_J L β α 3

2

3

2

2

3

γ

β

C H

none

CH

CCH

none

CCH3

HCCH

CCH3

CHCH

6

X

2

0-H,fra,fry,C-C

2

CCH3

2

2

CCH

a, C - C

5

C-O, α, β, γ, C - C

2

α

CH =CHCH OH γ β α 2

2

CH =CHCHO γ β α

C H , C H

2

2

6

2

4

0-H,y,C-C a, C - C

2

CH3CHOHCH3 β α β

none

CH

X

CH

0-H^x4,C-Cx2

CH3COCH3 β α β

none

CH

X

CH

β x4, C-C χ 2

groups to the surface. These do not appear to be hydrogenated to methyl groups or methane under the U H V conditions prevailing in these surface studies. The situation for 1-propanol decarbonylation is less certain. Clearly, additional C - H scissions are necessary to account for the absence of C H6 or ethylidyne formation. Direct C - C scission in the intermediate oxametallacycle would release ethylidene ligands (CH3CTH almost any reaction or rear rangement of which should lead to proscribed species such as ethane, ethylene, vinyl or ethylidyne. Thus it appears likely that substantial dehydrogenation occurs along the hydro carbon tail of the oxametallacycle prior to C - C scission, and the C hydrocarbon eliminated can possess no more than two hydrogen atoms if intersection with the products of propanal 2

2


24.

BROWN & BARTEAU


353

or acrolein dehydrogenation is to be avoided. Allyl alcohol produces ethylidynes, unlike 1propanol, but does not liberate volatile C hydrocarbons, unlike acrolein. Possible ligands which might be eliminated from allyl alcohol include ethylidyne, C H 3 O , vinyl, CH2=CH-, acetylene, H - O C - H , or vinylidene, CH2=C=. Objections can be raised with respect to each of these possibilities. Ethylidyne elirnination would require prior H shifts along the hydrocarbon tail, for which there is litde evidence among any of the other oxygencontaining adsorbates. Vinylidene elirnination would put allyl alcohol and propanol decarbonylation on a common path, but stable ethylidynes were produced only from the former. Acetylene satisfies all of the observations, and has been shown to form ethylidyne and ethylene by hydrogénation on Rh(l 11), but the absence of volatile C2H2 or C2H4 products from allyl alcohol is then somewhat troubling. Vinyl ehrninationfromallyl alcohol would put allyl alcohol and acrolein decarbonylation on a common path, and would appear to be in conflict with the observation of volatile hydrocarbons in one case but not the other. However, it should be noted that release of CO from allyl alcohol occurs above 275 Κ while that from acrolein begins at 200 K. Thus elimination of a vinyl moiety in both cases could still be feasible, provided that the activation barrier for isomerization to ethylidyne is greater than that for hydrogénation to ethylene; the isomerization/hydrogenation selectivity would then increase with increasing temperature. In any case the minimum number of different hydrocar bon ligands required to account for the behavior of thefirsteight reactants in Table Π is five: CH

Activation of C-H, C-C, and C-O Bonds of Oxygenates on Rh(111

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