Bifunctional Condensation Reactions of Alcohols on Basic Oxides Modified by Copper and ...

18 Pages·1998·0.52 MB·English

JOURNALOFCATALYSIS176,155–172(1998)
ARTICLENO.CA982009
Bifunctional Condensation Reactions of Alcohols on Basic Oxides
Modified by Copper and Potassium
MarceloJ.L.GinesandEnriqueIglesia1
DepartmentofChemicalEngineering,UniversityofCaliforniaatBerkeley,Berkeley,California94710
ReceivedSeptember9,1997;revisedJanuary12,1998;acceptedJanuary20,1998
(1–5). These catalysts produce isobutanol with relatively
Alcohol dehydrogenation and condensation reactions are low rates and selectivity at lower temperatures and pres-
involved in chain growth pathways on Cu/MgCeO promoted sures. Alkali-modified methanol synthesis catalysts pro-
x
with potassium. These pathways lead to the formation of isobu- duce higher alcohols from CO/H mixtures at lower tem-
2
tanol with high selectivity via reactions of higher alcohols with perature (583 K) and pressures (7.6 MPa) (6–9). These
methanol-derived C species in reaction steps also relevant to
1 catalystsproduceamixtureoflinearandbranchedalcohols
higheralcoholsynthesisfromCO/H mixturesathigherpressures
2 andmaximumisobutanolproductivitiesare48.6g/kgcat-h
onK–Cu/MgCeO catalysts.EthanolreactionsonK–Cu Mg CeO
x y 5 x (8).Recently,K–Cu–Mg–CeO materialshavebeenshown
showthatbothCuandbasicsitesparticipateinalcoholdehydro- x
to catalyze higher alcohol synthesis at low temperatures
genationandaldolcondensationstepsleadington-butyraldehyde
andpressures(10–13).Thesebifunctionalcatalystsconsist
and acetone. Chain growth occurs by condensation reactions
involvingametal-basebifunctionalaldol-typecouplingofalcohols. ofCucrystallitessupportedonhighsurfaceareabasicox-
Reactions of 12C H OH–13C H O mixtures show that direct idesbasedonK-modifiedMgO.
2 5 2 4
condensationreactionsofethanolcanoccurwithoutrequiringthe Isobutanol synthesis requires the initial formation of
intermediate formation of gas phase acetaldehyde. Reactions of methanol and higher linear alcohols and their subse-
C2H5OH/D2mixturesshowthatCusitesincreasetherateofaldol quent chain growth leading to 2-methyl-branched alco-
condensation by introducing recombinative desorption sites that hols with high selectivity (3 and references therein). Cu
remove hydrogen atoms formed in C–H activation steps leading
metalsitescatalyzemethanolsynthesisandhydrogenation–
to the unsaturated aldol-type species required for chain growth.
dehydrogenation reactions of aldehydes and alcohols (14,
Reactionsofacetaldehydeand13C-labeledmethanolleadpredomi-
15).SolidsbasedonMgOcatalyzecondensationreactions
nantlyto1-13C-propionaldehydeand2-13C-isobutyraldehyde,both
of alcohols and aldehydes with high selectivity (16, 17).
of which lead to isobutanol during CO/H reactions. Mixtures of
2
propionaldehydeand13C-labeledmethanolleadtosingly-labeled Also, MgO is an active and selective catalyst for Guerbet
isobutyraldehyde.ChaingrowthtoC2Calcoholsoccursviaaddition condensation reactions of primary C2–C5 alcohols with
of a methanol-derived C species to adsorbed oxygen-containing methanol(18,19).
1
intermediates.Thegradualappearanceof13Cintheunlabeledre- ThemechanismofmethanolsynthesisfromCO2/CO/H2
actantwithinthesemixturesshowsthataldolcouplingreactionsare mixtures has been extensively studied (20–25). Methanol
reversible.Reversealdolcondensationreactionsafterintramolecu- synthesis involves the formation of a formate species via
larhydridetransferleadtotheformationofacetonefromethanol. the reaction of CO with H-adatoms, followed by a rate-
2
Isobutyraldehyde is a preferred end-product of aldol-type chain
determining hydrogenolysis step (22–25). Bowker et al.
growthreactionsofalcoholsbecauseitlacksthetwofi-hydrogens
(22) reported that the mechanism by which methanol is
requiredforsubsequentchaingrowth. (cid:176)c 1998AcademicPress madeonCu/ZnO/Al O fromCO /H /HeandCO /CO/H
2 3 2 2 2 2
feeds is by the co-adsorption of hydrogen and carbon
1. INTRODUCTION dioxide on the copper component of the catalyst, form-
ing surface formate species, which undergo hydrogena-
Isobutanol is a useful intermediate in the synthesis of tion/hydrogenolysis reactions to yield methanol. Robbins
methyl-tert-butyl-ether after its dehydration to isobutene et al. (24) showed that from CO/CO /H feeds the rate of
2 2
and further etherification with methanol. Currently avail- methanol formation on Cu/SiO catalysts is proportional
2
able catalysts for the synthesis of isobutanol from CO/H2 to Cu surface area, suggesting that metallic Cu is the ac-
mixtures with high productivity (>100 g/kg cat-h) require tive catalytic component. In CO -free feeds, the synthe-
2
hightemperatures(750–800K)andpressures(10–30MPa) sis rate, however, is not proportional to the Cu surface
area, indicating that other types of catalytic sites are in-
1Correspondingauthor.E-mailaddress:[email protected]. volved. They suggested that surface hydroxyls might be
155
0021-9517/98$25.00
Copyright(cid:176)c 1998byAcademicPress
Allrightsofreproductioninanyformreserved.
156 GINESANDIGLESIA
required for the initial conversion of CO to adsorbed
formate. Using diffuse reflectance infrared Fourier trans-
form (DRIFTS) and temperature-programmed reaction
spectroscopies(TPRS),Neophytidesetal.(25)foundthat
methanol synthesis on a Cu/ZnO/Al O commercial cata-
2 3
lyst occurs via the rate-limiting hydrogenation of copper
formate.Recently,ithasbeenproposedthatmethanolsyn-
thesis from carbon oxide/hydrogen mixtures involves the
generation of a formate species on the copper surface,
the formate species then migrates onto ZnO, and is sub-
sequently hydrogenated to methanol (26). Temperature-
programmedinfrared(TPIR)spectroscopyofcopperfor-
mate/ZnOshowedthemigrationofsurfaceformatespecies
fromcoppertoZnOattemperaturesbelow430Kandthe
beginning of copper formate decomposition above 430 K
(26).
TheformationofC2ClinearalcoholsduringCOhydro-
genationremainstheleastunderstoodstepandtherateof
formation of ethanol appears to limit the overall rate of
chain growth (4, 10). Several mechanisms have been pro- SCHEME1. Homogenousaldolcondensationofaldehydes(33).
posedfortheformationoftheinitialC–Cbond:
in the C to C step (Scheme 3) (7, 8). The retention of
2 3
(a) C–C bond formation by CO insertion into metal– oxygen associated with the C intermediate is believed to
1
carbonbondsinadsorbedformaldehydespecies,observed becausedbythestrongbondingofthealkyloxygeninthe
onZnOandThO2catalysts(27); di-oxygenated RCH(H2CO¡)CHO intermediate anion to
(b) Coupling of two C1 aldehydic intermediates on car- CsC cations, which prevents hydrogenation of the H2CO¡
bides(CaC2,Na2C2,CeC2,andLaC2)(28); group,whileallowingcompletehydrogenationofthefree
(c)Methanolandethanolformationfromacommonin- –CHOgroupto–CH (7).
3
termediateonCuO/ZnO/Al2O3(29); Here, we report kinetic and isotopic tracer studies
(d)DirectcarbonylationofsurfaceC1species(CH3OH, of alcohol dehydrogenation and condensation reactions,
H2CO, and CH3O¡) on Cu/ZnO/Al2O3 and Cs–Cu/ZnO with emphasis on chain growth pathways on Cu/MgCeOx
catalysts(6,30). solidspromotedwithpotassium.Ourresultsshowthatdi-
Ourrecentexperimentsusing13CO/CH OH/H mixtures rect condensation reactions of ethanol occurs without the
3 2
intermediateformationofgasphaseacetaldehyde,thatCu
have shown that ethanol is formed directly from CO at
sites increase the rate of aldol condensation by introduc-
short residence time on K–Cu Mg CeO catalysts (31),
0.5 5 x
ingrecombinativedesorptionsitesthatdisposalhydrogen
butviamethanolcouplingreactionsonCs–Cu/ZnO/Al O
2 3
catalysts(6,32).
After initial C–C bond formation steps, higher alcohol
mixturesformviaaldolcondensationreactionsofsurface-
boundaldehydicspecies(10).Aldol-typecondensationre-
actions of alcohols in the liquid phase proceed via the
followingreactionsteps(Scheme1):alcoholdehydrogena-
tion to form aldehydes (or ketones), fi-hydrogen abstrac-
tiontoformenolatespecies,nucleophilicadditionofeno-
late to carbonyl species followed by dehydration to form
fi,fl-unsaturatedaldehyde(orketone),andhydrogenation
of the C==C and C==O bonds to form the corresponding
alcohol(33).
OnCuO/ZnO/Al O ,linear1-alcoholscontainingncar-
2 3
bons condense mainly to esters with 2n carbon atoms,
ketones with 2n-1 carbons, and ketones with 2n atoms
(Scheme2)(34).OnCu/ZnO,thealdol-condensationreac-
tionsoccurviathenormalmechanism,butonCs–Cu/ZnO
theoxygenassociatedwiththeC1carbonsourceisretained SCHEME2. Pathwaystohigheroxygenates(34).
BIFUNCTIONALCONDENSATIONREACTIONS 157
N O/pulse)intoaHestream.Thenumberofchemisorbed
2
oxygenatomswasobtainedfromtheamountofN Ocon-
2
sumed or N produced. A saturation coverage of 0.5:1
2
O:Cu was used to estimate Cu surface area and disper-
s
sion(35–37).Cudispersionisdefinedastheratioofsurface
Cu(Cu)tothetotalnumberofCuatomsinthecatalyst.
s
The density of basic sites was determined using a
13CO /12CO exchangemethodat573K.Thismethodpro-
2 2
vides a direct measure of the number of sites that can
reversibly interact with CO at typical reaction tempera-
2
tures. Detailed descriptions and experimental procedures
andmethodsarereportedelsewhere(13,38).
2.3. CatalyticEvaluationandMechanisticStudies
SCHEME 3. Normal and reversal aldol condensation according to 2.3.1. Ethanol reactions. Ethanol reactions were car-
Nunanetal.(7). riedoutinagradientlessrecirculatingbatchreactorat573K
and 101.3 kPa. Catalyst samples (18.0 mg) were first re-
atoms formed in C–H activation steps, and that chain ducedinH (10%H /He,175cm3/min,5K/min)at623K
2 2
growthtoC2CalcoholsoccursviaadditionofaC1species for0.5h.Thetemperaturewasloweredto573Kandethanol
to adsorbed oxygenates. Based on these findings, we pro- was introduced along with a small amount of methane,
pose that chain growth occurs by condensation reactions which was used as an unreactive internal standard in or-
involvingametal-basebifunctionalmechanism. dertoensureaccuratemassbalances(C H OH/CH /HeD
2 5 4
4.0/2.7/94.6 kPa; C H OH, Fisher Scientific, A.C.S. certi-
2 5
2. EXPERIMENTALMETHODS fied;CH4,Matheson,ultrahighpurity).H2waspurifiedby
passing through a catalytic purifier (Matheson, Model 64-
2.1. CatalystSynthesis 1008).TracesofwaterwereremovedfromH andhelium
2
Cu–Mg–CeO sampleswerepreparedbycoprecipitation using molecular sieves (Matheson, Model 452:4A). CH4
x
wasusedwithoutfurtherpurification.Thereactantmixture
ofmixedmetalnitratesolutionswithanaqueoussolutionof
was circulated continuously through the catalyst bed and
KOH(2M)andK CO (1M)at338KandaconstantpHof
2 3
reactionproductsweresampledbysyringeextractionafter
9inawell-stirredthermostatedcontainer(13).Thesolids
formedwerefiltered,washedthoroughlywith300–500cm3 various contact times. Reactants and products were ana-
lyzed using a dual detector gas chromatograph (Hewlett-
ofdeionizedwaterat333K,anddriedat353Kovernight.
Packard, Model 5890 II Plus; TCD/FID detectors). One
Driedsamplesweretreatedinflowingairat723Kfor4h
samplewasinjectedintoa5%phenyl-methyl-siliconecap-
in order to form the corresponding mixed metal oxides.
illarycolumn(HP-1,50m,0.32mmdiameter,1.05„mfilm
Residualpotassiuminalldrysampleswaslessthan0.1wt%
thickness)andtheotherintoapackedcolumn(PorapakQ,
before alkali impregnation. K was introduced by incipi-
ent wetness of the oxidized samples using 0.25M K CO
2 3
aqueous solutions (K CO : Fisher Scientific, A.C.S. Certi- TABLE1
2 3
fied).Thepropertiesofthesynthesizedsamplesarelistedin
Composition,SurfaceArea,andBasicSiteDensity
Table1.
ofMixedMetalOxides
2.2. CatalystCharacterization BET
Cu Ka surface Exchangeable
PowderX-raydiffraction(XRD)spectrawereobtained
content content area Cub CO at573K
2
usingaD5000SiemensDiffractometerandmonochromatic Sample (wt%) (wt%) (m2/g) dispersion (10¡6mol/m2)
Cu-Kfi radiation. Total surface areas were determined us-
ing the single point BET method by measuring N phy- Mg5CeOx 0 0.01 188 — 0.95
2
MgCeO 0 0.8 155 — 1.7
sisorptionat77KusingacontinuousflowQuantasorbSur- 5 x
Cu MgCeO 7.7 0.1 167 0.23 1.2
0.5 5 x
faceAreaAnalyzer(QuantachromeCorp.).Bulkcatalyst
Cu MgCeO 7.7 1.0 147 0.14 2.3
0.5 5 x
compositions were measured by atomic absorption spec- Cu MgCeO 49 1.2 92 0.047 3.3
7.5 5 x
troscopy(AAS).
aBulkcompositionmeasuredbyatomicabsorption.
Cusurfaceareasinreducedsamplesweremeasuredby
bDispersion calculated from the ratio of the number of surface Cu
titratingCusurfaceatomswithN O(Matheson,ultrahigh
2 (determined by N2O decompositon at 363 K (35, 36, 37)) to the total
purity)at363K.N2Owasintroducedaspulses(1.60„mol numberofCuatomsinthecatalyst.
158 GINESANDIGLESIA
1.8mlength,0.32cmdiameter).Athermalconductivityde- spectrometryusingamatrixdeconvolutionmethodthatac-
tector(TCD)wasusedtomeasuretheconcentrationofCO, countsforthenatural13Cabundanceandmassfragmenta-
CO ,andC H OHintheeffluentfromthepackedcolumn. tionpatternsofeachmolecule(39).
2 2 5
A flame ionization detector (FID) was used to measure
2.3.4. ReactionsofC H OH/D mixtures. Reactionsof
2 5 2
the concentrations of all organic compounds eluting from
C H OH/D mixtureswerecarriedoutinagradientlessre-
2 5 2
thecapillarycolumn.Productswerealsoanalyzedbymass
circulatingbatchreactorinordertoprobetherateofrecom-
spectrometry after capillary chromatography in order to
binative hydrogen desorption during ethanol coupling re-
confirmthechemicalidentityandtodeterminetheisotopic
actions.Catalystsamples(20.0mg)weretreatedinH (10%
2
compositionsofreactantsandproducts(Hewlett-Packard, H /He,175cm3/min,5K/min)at623Kfor0.5h.Thetemper-
2
Model5890IIPlusGC;Hewlett-Packard,Model5972Mass
aturewasthendecreasedto573Kandthereactantmixture
SelectiveDetector).
wasintroducedalongwithasmallamountofmethane(reac-
Reactionratesarereportedasturnoverrates,definedas tionmixture:C H OH/CH /D /HeD3.6/2.4/27.0/68.3kPa;
2 5 4 2
themolesofreactantconvertedtoproductsperunittime
C H OH, Fisher Scientific, A.C.S. certified; CH , Mathe-
2 5 4
and per g-atom surface site. The local slope of turnover-
son, ultra high purity; D , C.P. Matheson). Products were
2
timeplotsgivestheturnoverratefortheformationofeach
sampledbysyringeextractionfromtherecirculatingstream
product. Turnovers (for reactants) or site-yields (for each
atdifferentcontacttimesandanalyzedasdescribedinSec-
product) are defined as the moles of reactant converted
tion 2.3.1. The deuterium content in reactants and prod-
to a given product per gram-atom surface sites in the re-
uctswasmeasuredbymassspectrometryusingpreviously
actor. When divided by the fractional Cu dispersion, this
reporteddeconvolutionmethods(39).
becomesasite-timeyieldforeachproduct,atleastforre-
2.3.5. Cross-couplingreactionsof13CH OH/C H Oand
actions with rate-determining elementary steps requiring 3 2 4
13CH OH/C H O mixtures. Cross-coupling reactions
Cusurfaceatoms.Also,thetotalsurfacearea,theMgCeO 3 3 6
x of 13CH OH (13C, 99%,18O, <1%, Icon Services Inc.)
surfacearea(estimatedfromthedifferencebetweentheto- 3
with acetaldehyde (Fisher Scientific, Reagent Grade)
talsurfaceareaandthecoppersurfacearea)andthenum-
and propionaldehyde (Aldrich, 97%) were carried out
ber of basic sites from CO exchange were used in order
2 using the following reactant mixture: 12C H O(12C H O)/
to normalize reaction rates. The choice among these val- 2 4 3 6
13CH OH/CH /HeD7.3/4.0(4.0)/2.7/87.3kPa. 13CH OH
ues depends on which sites or surfaces are required for 3 4 3
was purified by several freeze-pump-thaw cycles before
the rate-determining elementary step in the overall cata-
catalyticexperiments.Reactionswereconductedat573K
lyticsequence.
and 101.3 kPa in a recirculating reactor unit. Mass spec-
2.3.2. Reactionsofacetaldehyde. Condensationandhy-
trometry analysis after chromatographic separation was
drogenation reactions of acetaldehyde were examined on
used to confirm the identity of each reaction product and
0.8 wt% K–Mg5CeOx, 1.0 wt% K–Cu0.5Mg5CeOx, and to determine 13C content, position, and distributions in
1.2wt%K–Cu Mg CeO .Thecatalyst(33.0mg)wasfirst
7.5 5 x reactantsandproducts.
reduced in H (10% H /He) at 623 K for 0.5 h. The reac-
2 2
tion was carried out at 573 K. The reactant mixture was: 3. RESULTSANDDISCUSSION
C H O/CH /HeD6.6/2.7/92.0 kPa (C H O, Fisher Scien-
2 4 4 2 4
3.1. StructuralandTexturalProperties
tific, Reagent Grade; CH , Matheson, ultra high purity).
4
ofK–Cu–Mg–CeO Catalysts
Reaction products were analyzed by the procedures de- x
scribedintheprevioussection. Thecomposition,surfacearea,Cudispersion,anddensity
2.3.3. Reactions of C H OH/13C H O mixtures. Com- ofbasicsitesformixedmetaloxidesareshowninTable1.
2 5 2 4
petitive reactions between 12C H OH and 13C H O were Copper dispersion decreased with increasing K loading
2 5 2 4
examined on 0.8 wt% K–Mg CeO , 1.0 wt% K–Cu (Table 1), because the presence of alkali decreases
5 x 0.5
Mg CeO , and 1.2 wt% K–Cu Mg CeO catalysts in or- Mg CeO surface area, blocks surface Cu atoms, and in-
5 x 7.5 5 x 5 x
dertoprobedirectpathwaysfromethanoltocondensation hibitsCu2CreductiontoCumetal(13).TheadditionofK
products and the role of acetaldehyde intermediates. The alsodecreasedthetotalsurfaceareaofCu Mg CeO sam-
0.5 5 x
catalyst (22.0 mg) was reduced in 10% H (balance He) ples (Table 1). The copper dispersion in Cu Mg CeO is
2 0.5 5 x
at 623 K for 0.5 h. Reactions were conducted at 523 K 0.23 (Table 1); this value is among the highest reported
and 101.3 kPa in the recirculating reactor unit. The reac- in the literature for Cu-based methanol and higher al-
tantmixturewasintroducedalongwithasmallamountof cohol synthesis catalysts. The density of basic sites ob-
methane (reaction mixture: 13C H O/12C H OH/H /CH / tained by isotopic CO exchange methods on MgO at
2 4 2 5 2 4 2
HeD2.9/1.1/2.9/2.4/92.0 kPa; 13C H O: Isotec Inc., 13C: 573K is 0.38£10¡6 mol/m2; this is lower than the value
2 4
99%; C H OH: Fisher Scientific, A.S.C. Certified; CH : of 18.3£10¡6 mol/m2 reported for the surface density of
2 5 4
Matheson,ultrahighpurity).13Ccontentandisotopicdis- oxygenatomsinMgO(40).Thisisnotunexpectedbecause
tributionsinreactantsandproductswereanalyzedbymass the13CO /12CO isotopicswitchmethoddetectsonlythose
2 2
BIFUNCTIONALCONDENSATIONREACTIONS 159
basicsitesthatinteractreversiblywithCO at573Kintyp- n-butyraldehyde, and other oxygenates. Acetaldehyde
2
icalcatalyticturnovertimes(13,38).Thebasicsitedensity reachesamaximumconcentrationatintermediatecontact
increasedfrom1.2to2.3£10¡6mol/m2withincreasingK times during ethanol reactions and then decreases grad-
loadingfrom0.1to1.0wt%.SmallamountsofCeO inMgO ually. The acetaldehyde concentration at this point corre-
x
(Mg/CeD5at.)increasedthedensityofbasicsites(Table1). spondstotheacetaldehydeconcentrationcalculatedfrom
Mg(OH) andCe(OH) /CeO phaseswereidentifiedby thethermodynamicequilibriumofthechemicalreaction:
2 4 2
X-ray diffraction on dried precipitate samples. The poor
C H OH(cid:192)C H OCH :
samplecrystallinityandthecoincidenceofdiffractionlines 2 5 2 4 2
forCeoxideandhydroxidephasesmakeitdifficulttode- The small decrease in acetaldehyde concentration at long
termine conclusively the form of Ce in dried samples. No contact times could be caused by further reaction of
layered hydrotalcite-type structures were detected by X- acetaldehydetosecondaryproductsand/orbythereversal
ray diffraction. Samples calcined at 723 K exhibited sep- oftheethanol-acetaldehydeequilibriumasethanoliscon-
arate MgO and CeO2 phases. Diffraction lines provided verted to chain growth products. Acetone and n-butyral-
no evidence for MgCeOx mixed oxides. CuO crystallites dehydewerethepredominantcondensationproducts.The
were not detected, suggesting that the Cu component is non-zero initial slopes on acetone and n-butyraldehyde
well dispersed, either as small crystallites or as a solid so- curves (Fig. 1) are inconsistent with their formation
lutionwithintheCeO2lattice.MixedCu/Ceoxidesolidso- only in sequential reactions of gas phase acetaldehyde
lutions can form during co-precipitation and remain after intermediatessuchas
hightemperatureoxidativetreatments(41,42).
C H OH(cid:192)C H O!Condensationproducts:
2 5 2 4
3.2. EthanolReactions
Therefore,condensationproductsareeitherformeddirect-
Ethanol dehydrogenation and coupling reactions were lybyethanolself-condensationorfromacetaldehyde,butat
studied in order to probe reaction pathways involved reactionratesindependentofacetaldehydeconcentration.
in branched and linear higher alcohol syntheses from Methyl-ethyl ketone (by acetaldehyde self-condensation
methanol,ethanol,andpropanolintermediatesformedini- with intramolecular hydride shift before dehydration)
tially during CO hydrogenation reactions at high pres- (7, 8), 2-pentanone (by acetaldehyde-acetone condensa-
sures. These studies also address the roles of Cu, K, and tion),2-propanol(byhydrogenationofacetone),1-butanol
Mg–Ce oxides in ethanol coupling reactions. The experi- (by hydrogenation of n-butyraldehyde), and ethyl acetate
mentswerecarriedoutonMg5CeOx,0.8wt%K–Mg5CeOx, (by ethanol-acetaldehyde) were also detected in much
Cu0.5Mg5CeOx, 1.0 wt% K–Cu0.5Mg5CeOx, and 1.2 wt% smaller concentrations than n-butyraldehyde and acetone
K–Cu7.5Mg5CeOxatatmosphericpressure. amongreactionproducts.
Acetaldehydewasthepredominantproductofreactions The formation of all detected products of ethanol
of pure ethanol on 1.0 wt% K–Cu0.5Mg5CeOx (Fig. 1). reactions on K–Cu–Mg5CeOx catalysts can be described
Dehydrogenation reactions occur much faster than chain by the reaction pathways shown in Scheme 4. Acetalde-
growth;thelatterreactionleadstotheformationofacetone, hyde is formed via ethanol dehydrogenation (step I).
Self-condensation of acetaldehyde (or surface acetalde-
hyde formed from dissociative adsorption of ethanol and
sequential fi-hydrogen abstraction) leads to the forma-
tion of n-butyraldehyde (steps II–IV) and methyl-ethyl
ketone (steps II–V) (34). Enolate species can convert to
the keto form via intramolecular H transfer (step VI);
methyl-ethyl ketone can then be formed by dehydration-
dehydrogenation reactions of this keto form (step VII).
Acetone can form via two pathways: (i) by reaction of
aldolintermediateswithsurfacelatticeoxygentoformthe
fl-keto acid followed by decarboxylation (steps VIII–IX)
and/or(ii)byreversealdolreactionsoftheketoformofre-
actionintermediates(stepX).Formaldehydeformedinthe
latterreactiondecomposesrapidlytoCOandH .Surface
2
lattice oxygen consumed by reaction (IX) can be replen-
ishedbywaterandCO formedincondensationreactions;
2
2-pentanone is formed by condensation of aldol-type
FIG.1. Site yields as a function of contact time on 1.0 wt%
K–Cu MgCeO inethanolreactions(573K,101.3kPatotalpressure, intermediates formed in acetone-acetaldehyde reactions
0.5 5 x
4.0kPaethanol,balanceHe). (steps XI–XII). Diethyl ether was only detected in trace
160 GINESANDIGLESIA
SCHEME4. ReactionschemeforethanolreactionsonK–Cu–MgCeO catalysts.
5 x
amounts on Mg CeO catalysts, and it is formed on CeO andthecoppersurfacearea),thenumberofbasicsitesfrom
5 x x
acidsitesbydehydrationofethanol.Ethylacetatecanform CO exchange,ortheCumetalsurfaceareaofeachsample
2
byreactionsofethanolwithacetaldehyde(stepXIII)(34). inordertonormalizereactionrates.
Ethanol reaction rates and the selectivities to acetalde- Initial dehydrogenation rates were very low on both
hyde, n-butyraldehyde, and acetone were measured on Mg CeO and0.8wt%K–Mg CeO catalystsandethanol
5 x 5 x
K–Cu–MgCeO catalysts with a range of Cu and K con- conversion reached a limiting value of about 9% after
x
tentsinordertoexaminetheroleofCuandbasicsiteson 0.33 h; this conversion value is much lower than the
dehydrogenation and condensation reactions. The results predicted equilibrium conversion (90%) at these reaction
are shown in Table 2. Initial reaction rates were obtained conditions. The decrease in ethanol conversion rates with
fromtheslopeofthesite-yieldplotsatshortcontacttimes increasing conversion may reflect (1) the formation of
usingthetotalsurfacearea,theMgCeO surfacearea(es- surfacepolymericspeciesfromacetaldehydeand/or(2)an
x
timatedfromthedifferencebetweenthetotalsurfacearea inhibitioneffectofCO formedinethanolcondensationon
2
TABLE2
EffectsofCu-andK-loadingonEthanolConsumptionandProductFormationonMg CeO
5 x
Ratesofformation
Ethanoldehydrogenation AcetoneC2-Propanol ButyraldehydeC1-Butanol
Cu K Arealrate Turnoverrate Arealrate Turnoverrate Arealrate Turnoverrate
(wt%) (wt%) ra rb rc rd rc rd
1 1 2 2 3 3
0 0 4.0£10¡9 — 7.5£10¡11 7.9£10¡5 2.8£10¡11 2.9£10¡5
0 0.8 3.4£10¡9 — 9.7£10¡11 5.6£10¡5 1.6£10¡10 9.4£10¡5
7 0.1 3.6£10¡7 0.24 3.0£10¡9 2.2£10¡3 4.5£10¡10 3.4£10¡4
7 1.0 2.4£10¡7 0.23 4.3£10¡9 1.8£10¡3 7.6£10¡10 3.2£10¡4
49 1.2 9.4£10¡7 0.24 4.4£10¡8 1.0£10¡2 1.2£10¡9 2.8£10¡4
ar istherateofethanolconsumption,inmol/m2total¢s.
1
bTurnoverratesperCusurfaceatom,ins¡1.
cr andr aretheratesofproductformation,inmol/m2MgCeO ¢s.
2 3 x
dTurnoverratesperaccessiblesitefrom13CO/12CO inmolCHOH/molbasicsite¢s;basicsitesmeasuredby13CO/12CO isotopicswitchmethod
2 2 2 5 2 2
(13,38).
BIFUNCTIONALCONDENSATIONREACTIONS 161
base-catalyzed dehydrogenation reactions on Mg CeO . Dehydrogenation rates were much higher on Cu Mg CeO ,
5 x 0.5 5 x
1.0wt%K–Cu Mg CeO ,and1.2wt%K–Cu Mg CeO catalyststhanonMg CeO ,suggestingthatCusitescatalyze
0.5 5 x 7.5 5 x 5 x
ethanol dehydrogenation to acetaldehyde. The presence of K decreased areal ethanol dehydrogenation rates, because
KdecreasedCudispersion.Ethanoldehydrogenationturnoverrates(normalizedbythenumberofexposedCuatoms)
on Cu Mg CeO , 1.0 wt% K–Cu Mg CeO , and 1.2 wt% K–Cu Mg CeO catalysts were 0.24, 0.23, and 0.24 s¡1,
0.5 5 x 0.5 5 x 7.5 5 x
respectively.Thus,ethanoldehydrogenationstepsrequireexposedCuatomsfortherate-determiningstep.SurfaceCu
atomsbecomeunavailableforbothN OdecompositionandalcoholdehydrogenationwhencoveredwithKspeciesoras
2
Cucrystallitesbecomelarger.DehydrogenationturnoverrateswerenotaffectedbyCucrystallitesizeorbythetitration
ofafractionoftheCusurfacewithKO species.
x
AldolcouplingchaingrowthratesareloweronMg CeO thanonCu Mg CeO becauseoftheloweracetaldehyde
5 x 0.5 5 x
concentrationswhenCusitesarenotpresent.Kincreasestherateofbase-catalyzedaldolcouplingreactionstoacetone
and butyraldehyde (Table 2), even though steady-state acetaldehyde concentrations on Cu Mg CeO and 1.0 wt%
0.5 5 x
K–Cu Mg CeO catalystsaresimilaronthesetwocatalysts.Thus,itappearsthatthehigherbasicsitedensitiesdetermined
0.5 5 x
by13CO /12CO isotopicswitchmeasurementsindeedleadtohigherratesofbase-catalyzedaldolcondensationreactions.
2 2
The total rates of base-catalyzed aldol coupling reactions, when normalized by the number of accessible basic sites,
are similar on Cu Mg CeO (2.5£10¡3) and K–Cu Mg CeO (2.2£10¡3). Aldol condensation rates on 1.2 wt%
0.5 5 x 0.5 5 x
K–Cu Mg CeO (49wt%Cu,0.047Cudispersion)are,however,muchhigherthanoncatalystswithlowerCucontent,
7.5 5 x
eventhoughthedensityofbasicsitesissimilaronthesetwosamples.Thedifferenceincondensationratessuggeststhat
Cusitesareinvolvedinrate-determiningstepsrequiredforcondensationreactions,eventhoughsuchCusitesarepresent
insufficientnumbertoensureethanol-acetaldehydethermodynamicequilibrium.Theloweraldolcouplingchaingrowth
reactionratesobservedonCu-freecatalysts,whichiscausednotonlybythelowconcentrationofrequiredacetaldehyde
intermediatesbutalsobytheabsenceofCusitesrequiredincondensationsteps,isconsistentwiththisproposal.
Asequentialreactionscheme,whereethanoldehydrogenatestoformgasphaseacetaldehyde,whichthenundergoes
self-condensationreactionsintoproducts,isnotconsidered,becauseitisnotconsistentwiththesharpinitialincreasein
productsite-yieldcurves(Fig.1).OnCucontainingcatalysts,thesteady-stateacetaldehydeconcentrationsaresimilar.
The effect of copper suggests a bifunctional mechanism for aldol condensation on metal-promoted basic oxides; this
mechanisticproposalisdiscussedindetailbelow.
TheproposedmechanisminvolvestheinitialdissociativeadsorptionofethanolonMgOtoformethoxideandhydrogen
species(43,44).HydrogenspeciescanthenberemovedbymigrationtoCusites,recombinationwithanotherhydrogen
adatom,anddesorptionasH :
2
AdditionalC–Hbondcleavageeventsinethoxidespeciescanthenoccurandthehydrogenatomsformedaretransferred
tooxygenionsandformsurfaceacetaldehydicspecies:
In this scheme, hydrogen atoms migrate from basic to Cu sites and recombine to form H . As a result, basic oxygen
2
sites become available for another hydrogen abstraction step. H–H recombination rates increase with increasing ratio
of surface Cu atoms to basic sites, which is 0.31 on K–Cu Mg CeO and 0.07 on K–Cu Mg CeO , at similar basic
7.5 5 x 0.5 5 x
162 GINESANDIGLESIA
sitedensity.ThehighratioofCutobasicoxidesurfaceareaonK–Cu Mg CeO leadstohigheraldol-condensationrates
7.5 5 x
thanoncatalystswithlowerratios,withoutadetectableeffectonthe(equilibrium)concentrationofacetaldehyde.
Adsorbedacetaldehydespeciescandesorbasacetaldehydeorreactwithothersurfacespeciestoformaldolconden-
sationproducts:
Surface aldol species can undergo condensation reactions via the pathways shown in Scheme 4. In these reactions,
the presence of Cu sites increases the rate of hydrogen removal and reintroduction and provides the hydrogen atoms
required for hydrogenation of unsaturated aldehyde species (steps IV, VII, and XII, Scheme 4). The non-zero initial
slopesofacetoneandn-butyraldehydeturnover-timeplots(Fig.1)areconsistentwiththedirectcondensationofethanol
reactants.Thesebifunctionalcondensationpathwaysarequalitativelysimilarinkineticstructuretothoseproposedfor
bifunctionalC–Hactivationofalkanesoncation-modifiedzeolites(45–47).
The ratio of C to C oxygenates measured on the different catalysts gives the selectivity of the decomposition of
3 4
keto species to form acetone (Scheme 4, step X) compared to aldol dehydration–hydrogenation of such species to
form n-butyraldehyde (Scheme 4, steps III and IV). On Cu-free catalysts this ratio is<3, and the total condensation
rate is »10¡10 mol/m2-s. At high Cu concentrations, the C /C selectivity ratio increases to 37, while the total conden-
3 4
sation rate increases to »10¡8 mol/m2-s. Thus, in addition to increasing the surface concentration of aldol species via
removalofadsorbedhydrogenatoms,Cuappearstoenhancehydrogentransferstepsrequiredforenol-ketointercon-
version (Scheme 4, step VI); as a result, Cu sites increase the concentration of keto species and the rate of step X
(Scheme4).
3.3. ReactionsofAcetaldehyde
InSection3.2,weproposedthatcondensationreactionscouldproceed(i)viadirectreactionsofethanolwithoutthe
intermediateformationofgasphaseacetaldehydemolecules,or(ii)throughconsecutivereactionsincludingtheformation
ofgas-phaseacetaldehyde,withzeroorderkineticsinacetaldehydeconcentration.Inordertotestthesecondpossibility,
catalyticactivityandproductyieldsforacetaldehydereactionson1.0wt%K–Cu Mg CeO weremeasuredatseveral
0.5 5 x
acetaldehydepressures.
Acetone and crotonaldehyde are the predominant products of acetaldehyde reactions (Fig. 2). Carbon dioxide,
n-butyraldehyde, and 2-pentanone were also detected in small concentrations among reaction products. Ethanol
forms by hydrogenation of acetaldehyde using H-adatoms formed in C–H activation steps or H molecules des-
2
orbed by Cu sites after C–H activation. Aldol-type condensation of acetaldehyde forms crotonaldehyde, which then
hydrogenates to form n-butyraldehyde. Acetaldehyde can also polymerize to give dimers and trimers (Scheme 5)
and/orbysequentialaldolcondensationsleadingtohighmolecularweightaldehydes(48).Thesepolymericspeciesare
likelytoremainonthecatalystsurfaceandmayaccountforthepooroverallmassbalancesobservedintheseexperiments
BIFUNCTIONALCONDENSATIONREACTIONS 163
FIG.2. Acetaldehydereactions.Siteyieldsasafunctionofcontact FIG.3. Acetaldehydereactions.Effectoftheacetaldehydeinitialpar-
timeon1.0wt%K–Cu MgCeO (573K,101.3kPatotalpressure,8.0kPa tialpressureovertheinitialrates.Acetaldehydeconversionasafunction
0.5 5 x
acetaldehydepressure,balanceHe). ofcontacttimeon1.0wt%K–Cu0.5Mg5CeOx(573K,101.3kPatotalpres-
sure,3–15kPaacetaldehydepressures,balanceHe).
(carbonselectivitytotalslessthan45%).Hydrogenisalso
less pronounced than for reactions of ethanol. These re-
producedbydecompositionoftheformaldehydeformedin
sults show that Cu sites also increase rates of C–H bond
retro-aldolreactionsleadingtoacetone(Scheme4).
activationinacetaldehyde.
Initialreactionratesatdifferentinitialpartialpressures
Acetone is a condensation product, while ethanol
ofacetaldehydeareshowninFig.3.Condensation,hydro-
is a hydrogenation product. The molar ratio between
genation,andtotalconversionratesareapproximatelyfirst
ethanol/acetone is a measurement of the catalyst ability
order in acetaldehyde concentration. These results show
to remove H-atoms from the surface by recombinative
that the initial reaction nonzero slope for condensation
desorptioncomparedtoitsabilitytocatalyzecondensation
products in ethanol reactions (Fig. 1) cannot reflect zero-
reactions. On Mg CeO catalysts, ethanol is produced
order kinetics for aldehyde condensation. Instead, it sug- 5 x
selectively over acetone. The catalyst surface is saturated
gests that direct reactions of ethanol account for a signif-
with H-atoms from the abstraction of fi-hydrogens from
icant fraction of the condensation products formed from
acetaldehydeonbasicsites.WithoutCu,itismoreprobable
ethanolreactants.
Figure4showsthemolarratioofethanoltoacetoneand
ofC oxygenates(crotonaldehydeandn-butyraldehyde)to
4
acetoneamongthereactionproductsandtheinitialconden-
sationratesforacetaldehydereactionsonK–Cu Mg CeO
y 5 x
catalysts. Aldol coupling chain growth rates increase with
increasing copper surface area. The increase, however, is
FIG.4. C to C and C to C molar ratios on acetaldehyde reac-
2 3 4 3
tionsandinitialacetaldehydecondensationratesasafunctionofCusur-
SCHEME5. Formationofdimerandtrimercompoundsfromacetalde- faceareaonK–CuMgCeO catalysts(573K,101.3kPatotalpressure,
y 5 x
hyde. 6.6kPaacetaldehyde,balanceHe).
164 GINESANDIGLESIA
FIG.5. 13C-content in reactants and reaction products (13C2H4O– FIG.7. 13C-content in reactants and reaction products (13C2H4O–
C2H5OH–H2reactionmixtureson0.8wt%K–Mg5CeOx)(523K,101.3kPa C2H5OH–H2 reaction mixtures on 1.2 wt% K–Cu7.5Mg5CeOx) (523 K,
totalpressure,2.9kPaacetaldehydepressure,1.1kPaethanolpressure, 101.3kPatotalpressure,2.9kPaacetaldehydepressure,1.1kPaethanol
2.9kPahydrogenpressure,balanceHe). pressure,2.9kPahydrogenpressure,balanceHe).
that the H-atoms are used to hydrogenate acetaldehyde 3.4. ReactionsofC2H5OH/13C2H4OMixtures
to ethanol. When Cu sites are present, ethanol selectivity
As discussed in Section 3.2, it appears that ethanol
decreases markedly because these H-atoms recombine to
undergoes condensation reactions directly, without re-
formH .Ontheotherhand,acetoneisformedselectivity
2 quiring its intermediate conversion to gas phase acetal-
on Cu-containing catalysts instead of n-butyraldehyde or
dehyde. Also, condensation rates increase with increas-
crotonaldehyde. This shows that reverse aldol reaction
ing acetaldehyde concentration. Chung et al. (49), using
stepsfromC4intermediates(Scheme4)becomelesslikely CH 13CH OH/C O H mixtures on Cu/ZnO catalysts,
3 2 2 4
asCuisintroducedintothecatalysts.
showedthatacetonecanbeproducedbyC–Cbondcleav-
age in C species formed from two aldehydic species and
4
n-butyraldehyde by C–O bond cleavage in C species
4
formed from ethanol and acetaldehyde. In order to ex-
amine the role of ethanol and to detect available di-
rectpathwaysforethanolcondensation,competitivereac-
tions between 12C H OH and 13C H O were carried out
2 5 2 4
on 0.8 wt% K–Mg CeO , 1.0 wt% K–Cu Mg CeO , and
5 x 0.5 5 x
1.2 wt% K–Cu Mg CeO catalysts. The carbon-13 con-
7.5 5 x
tentsinreactantsandproductsasafunctionofcontacttime
areshowninFigs.5,6,and7.
Onallcatalysts,the13C-contentinbothethanolandac-
etaldehydechangeswithcontacttime,astheproductsreach
chemical and isotopic equilibrium. The rate of ethanol-
acetaldehyde equilibration is much higher on 1.2 wt%
K–Cu Mg CeO than on the other two catalysts. When
7.5 5 x
copper is present, hydrogenation–dehydrogenation reac-
tions occur more rapidly, in agreement with the results
shown in Section 3.2. On Cu-free catalysts (Fig. 5), the
13C-contentinbothethanol(30%)andacetaldehyde(80%)
reach values that differ markedly from those predicted at
FIG.6. 13C-content in reactants and reaction products (13CHO–
2 4 isotopicequilibrium(70%).Thisisconsistentwiththerapid
CHOH–H reaction mixtures on 1.0 wt% K–Cu MgCeO) (523 K,
2 5 2 0.5 5 x
deactivationobservedduringethanolreactionsonCu-free
101.3kPatotalpressure,2.9kPaacetaldehydepressure,1.1kPaethanol
pressure,2.9kPahydrogenpressure,balanceHe). catalysts(Sections3.2,3.3).