1. IntroductionIn the past years there has been a growing demand
for solid ca ta lyst s efficien t in the par t ia l oxida t ion ofa lcohols for the product ion of fine and specia ltychemica ls. The use of stoich iomet r ic inorganic re-agents, though decreasing, is st ill widespread. Thepresent st r ingent ecologica l standards increase thepressure to develop new, environmenta lly benignmethods. In many instances, homogeneous ca ta lysisprovides powerfu l solu t ions, bu t on an indust r ia lsca le the problems rela ted to cor rosion and pla t ingout on the reactor wall, handling, recovery, and reuseof the ca ta lyst represen t limita t ions of these pro-cesses.
* To whom correspondence should be addressed. Tel.: +41(1)-632-5515. E-mail: malla [email protected] .
AlfonsBaiker (born 1945) studied chemical engineering and received hisPh.D. at ETHZurich in1974. Afterward, hespent several yearsat variousuniversities abroad. In 1980, he completed his habilitation thesis onammonia synthesis kinetics at Stanford University, California, where hewas also involved in teaching courses in kinetics and catalysis. Afterreturning to ETH, he started his own research group, focusing onheterogeneous catalysis and reaction engineering. In 1989 he becameassociate professor and in 1990 full professor at the ETH. His mainresearch interests are centered around catalyst design, mechanismandkineticsof catalytic surfaceprocesses, asymmetrichydrogenation, selectiveoxidation and environmental catalysis, in situ spectroscopy, and theapplication of supercritical fluids in catalysis. His goal is to further thescientific basis required for developing environmentally benign chemicalprocesses which make optimal use of rawmaterials and energy.
Tamas Mallat received an M.S. degree in 1972 in chemical engineeringand a Ph.D. in 1976 in technical chemistry, both from the TechnicalUniversity of Budapest, Hungary. Hewas apostdoctoral associate underG. C. Bondat Brunel University, London, from1978 to1979. Hecontinuedhis academic carrier as a scientist at the Organic Chemical TechnologyResearch Group of the Hungarian Academy of Sciences. In 1990, hemoved to theSwissFederal Institute of Technology (ETH), Zurich, wherehe is currently a senior scientist. He was awarded a D.Sc. degree in1994by theHungarianAcademyof Sciences. Hismain scientific interestis heterogeneous catalytic oxidation and enantioselective hydrogenationin fine chemistry.
Applica t ion of solid ca ta lyst s for the gas- or vapor-phase oxidat ion of simple, small-chain alcohols to thecor responding carbonyl compounds is well estab-lished.1-3 An impor tan t requirement is the reason-able vola t ility and thermal stability of reactan t andproductsa st rong limita t ion in the synthesis ofcomplex molecules.Solid ca ta lyst s act ive in the liqu id phase under
mild condit ions have a much broader applica t ionrange.4,5 A major cha llenge in liqu id-phase oxida t ionwith solid ca ta lyst s is to prevent leaching of theact ive species.6 The reactan ts and par t icu la r ly thecarboxylic acid-type (by)products a re frequent ly ex-cellen t chela t ing agents and accelera te dissolu t ionand deact iva t ion of the ca ta lyst s.The a im of th is review is to provide an overview
on the var ious types of mater ia ls tha t have beenapplied for the heterogeneous ca ta lyt ic oxida t ion ofa lcohols in the liquid phase and extract some genera lconclusions, where possible. Our in ten t was not tobe a ll-inclusive; ra ther we focused on the develop-ment in the past 10-15 years. Only those methodswill be considered which apply the technica lly a t -t ract ive and environment fr iendly oxidant , molecularoxygen . The numerous ca ta lyt ic systems using mo-lecu la r oxygen in combina t ion with addit ives, suchas reducing agents or radica l scavengers, a re notdiscussed here due to the environmenta l impact ofthe addit ive or the coproduct formed.There are differen t opin ions in heterogeneous
ca ta lysis whether compar ison of the per formance ofgrea t ly differen t ca ta lyt ic mater ia ls is fa ir underident ica l condit ions or under the best condit ionsiden t ified for each ca ta lyst separa tely. The formerapproach is commonly used due to it s simplicity.Choosing a differen t set of condit ions, however , caneasily reverse the act ivity or select ivity order , asillust ra ted schemat ica lly in Figure 1. Obviously, a
randomly chosen set of react ion condit ions cannotprovide a suitable basis for catalyst comparison. Herethe ca ta lyst s a re compared on the basis of their bestrepor ted performance. We assumed that publica t ionsrepor t a good set of parameters and took the bestresu lt s for compar ison tha t , we hope, a re close to theopt imum.The review is divided into three major sect ions. The
first two include oxidat ions over in t r insica lly hetero-geneous (solid) and heterogenized meta l complex
ca ta lysts. The th ird par t compares the best ca ta lystsusing some representa t ive and frequent ly repor tedtest react ions. We hope tha t th is collect ion will be ava luable reference source to chemist s search ing foreffect ive solid ca ta lyst s and “green” oxida t ion meth-ods.
2. Intrinsically Solid Catalysts2.1. Supported Platinum-Group MetalsOxida t ion of a lcohols and polyols over suppor ted
noble meta l ca ta lyst s has been thoroughly invest i-ga ted in the past years. Deta ils of the ear ly work, inpar t icu la r the oxida t ion of carbohydra tes and theirder iva t ives (sugar a lcohols, a ldonic acids, etc.), willnot be discussed here. There are numerous books andreviews ava ilable on the topic, including the genera lru les of chemo-, regio-, and stereoselect ive oxida t ionof these complex molecules.7-15
2.1.1. Catalysts and Reaction ConditionsPt-group meta ls can act iva te a lcohols and molec-
u la r oxygen under close to ambient condit ions andproduce the cor responding carbonyl compounds orcarboxylic acids in high yields. Today, var ious bi- andmult imeta llic ca ta lyst s a re applied tha t a re moreact ive, more select ive, and less prone to deact iva t ionthan monometa llic ca ta lyst s.11-20The most commonly used ca ta lyst s consist of Pt or
Pd as act ive components and Bi or Pb as promoters,on carbon and a lumina suppor t s. Ru and Rh areusua lly applied without promoters.21-29 Besides Biand Pb, a var iety of promoter meta ls have beensuggested, including Cd,30,31 Co,30,31 Cu,32 Se,33 Ce,33,34Te,35 Sn,36,37 Au,36,38 and Ru.36,39 The non-noble meta lpromoters are inact ive under react ion condit ions, andtheir deposit ion onto the act ive sites should resu ltin lower oxida t ion ra tes. St ill, promot ion of Pt or Pdmay lead to a considerable rate enhancement (Scheme1)40-42 and to a remarkable sh ift in the productdist r ibu t ion (Scheme 2).32,37,43
The bimeta llic ca ta lysts can be prepared by simul-taneous deposit ion and reduct ion of the meta l pre-cursors onto a su itable suppor t .33,44,45 The mostcommonly used method is, however , the deposit ionand reduct ion of promoter onto a suppor ted Pt or Pdca ta lyst . A var ia t ion of th is method is the in situmodifica t ion ; i.e., the promoter meta l sa lt is simplyadded to the slur ry conta in ing the suppor ted Pt -group meta l ca ta lyst , and the meta l ion is reducedto meta l by the a lcohol reactan t in the ear ly stage ofthe react ion.32,46,47 During promoter deposit ion, metal
Figure 1. Compar ison of ca ta lyst s: a t condit ions A andB the ca ta lyst s I and III a re the best , respect ively, thoughthey are infer ior to ca ta lyst II when their per formance iscompared under the best condit ions for each ca ta lyst .
Scheme 1. An Example on the Application ofP romoted P t-Group Metal Catalysts ;41(Se le ctiv it ie s are 100%)
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ada toms (submonolayer deposit ion) and small par -t icles (mult ilayer deposit ion) are formed on thesurface of the Pt or Pd part icles. In addit ion, par t iclesconta in ing only the promoter meta l (M0 or Mn+) maydevelop on the support , as illustrated in Figure 2. The
promoter influences the performance of the neighbor-ing Pt -group meta l sit es on ly; thus, the promoter -con ta in ing par t icles on the suppor t a re on ly spec-ta tor species. As the non-noble meta l promoter a loneon the suppor t is unstable in the presence of oxy-gen and dissolves easily with a su itable an ion orchela t ing agent , th is cont r ibu t ion should be mini-mized.48 In cont rast , meta l ada toms on the sur faceof Pt -group meta ls a re fa r more resistan t aga instoxida t ion and dissolu t ion than the cor respondingbulk meta ls.49-52In some cases, good resu lt s were achieved with
organic compounds as modifiers. These N- and P-conta in ing compounds are simply added to the reac-t ion mixture; they adsorb st rongly on the meta lsur face and improve the ra te or select ivity. In theoxida t ion of L-sorbose to 2-keto-L-gulonic acid, theselect ivity of Pt could be doubled by addit ion of t raceamounts of amines53,54 or phosphines55,56 (Scheme 3).The oxida t ions are commonly car r ied out a t 330-
370 K and ambient pressure or sligh t ly above witha ir or oxygen . Besides the usua l applica t ion of ast ir red ba tch (slur ry) reactor , there are successfu lexamples of cont inuous opera t ion .57-61An advantage of the method is tha t the ca ta lyst s
a re act ive and select ive in water , though organicsolvents,62-64 ion ic liqu ids,65 and dense (“supercr it i-ca l”) CO2 have a lso been used.61,66-70 A water-
detergent system may be the choice for water -insoluble reactants when flammable organic solventsa re to be avoided.41,43,71 Nonaqueous solvents andrela t ively high tempera ture for the rapid removal ofthe coproduct water a re required for the synthesisof a ldehydes; these condit ions disfavor the hydra t ionof a ldehyde to geminal diol and the subsequent rapiddehydrogena t ion to acid 72-75 (Scheme 4).
When the ta rget product is a carboxylic acid,precise cont rol of the a lka line pH dur ing react ion iscr it ica l to achieve good select ivity, enhance the reac-t ion by facilit a t ing a ldehyde hydra t ion and productdesorpt ion from the act ive sites, and avoid meta lleaching favored a t h igh pH.60,76-79 The inorganicbase can be replaced by a st rongly adsorbing N-basewhen the reactan t or product is unstable in basicmedium (Figure 3). This loca lized basifica t ion a t themeta l sur face increases the ra te of a lcohol dehydro-gena t ion without sign ifican t ly influencing the pH ofthe solu t ion .53The C/S mass ra t io var ies in a broad range; it may
be as low as 0.01 for a ca ta lyst conta in ing 1-5 wt %Pt-group metal,41 but in extreme cases the mass ra t iocan exceed 1.80 Simila r ly, it is difficu lt to give acharacter ist ic range of react ion ra te. The TOF canapproach 10 000 h-1, but there are examples showinga lso va lues below 1 h-1.81,82 Besides the role ofreactan t st ructure, a probable reason for the big
Scheme 2. The Best Yie lds of Hydroxyace tone orPyruvic Acid Ach ieved by Promotion of a 5%Pt/Graph ite Catalyst in the Oxidation ofP ropylene Glycol (333 K, w ater, pH 8)37
Figure 2. Schemat ic represen ta t ion of the st ructure of abimeta llic ca ta lyst prepared by deposit ion /reduct ion of thepromoter meta l (gray and black) onto a suppor ted Pt-groupmeta l (white) ca ta lyst .
Scheme 3. Oxidation of L-Sorbose to2-Ke to-L-gu lon ic Acid over P t Modified by StronglyAdsorbing N- and P -Contain ing Compounds (323K, H2O, pH 7.3, O2, 1 bar, Se le ctiv ity De termined at50%Convers ion )
Scheme 4. Effe ct of Water on the Reactiv ity of theAldehyde In termediate on P t-Group MetalCatalysts
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var ia t ion is the inappropr ia te choice of ca ta lystcomposit ion. For example, a 1 wt % Pt/C cata lyst wascompletely inact ive in the oxida t ion of phenoxyetha-nol, bu t addit ion of lead and cadmium nit ra te pro-moters to the aqueous solu t ion a llowed the fast andalmost quant ita t ive t ransformat ion to phenoxyacet icacid (TOF ) 840 h-1).46,83
2.1.2. Catalyst DeactivationA commonly observed deact iva t ion is the “over -
oxida t ion” of the act ive sites.21,47,73,84-92 It was ear lydiscovered tha t the ra te of a lcohol oxida t ion is muchhigher on a reduced meta l sur face than on theoxidized sur face.72,93,94 An impor tan t consequence ofth is difference is tha t the ca ta lyst has to be pre-reduced before a lcohol oxida t ion by hydrogen or bythe reactant itself in an iner t a tmosphere.93 Fur ther-more, the reactor should be opera ted in the oxygen-t ranspor t -limited region; i.e., the ra te of oxygensupply should be lower than the actual ra te of alcoholdehydrogena t ion . In the kinet ic region , the act ivesites a re successively oxidized and their act ivitydrops. In a ba tch reactor , the ra te of a lcohol dehy-drogena t ion decreases with react ion t ime due to adecrease of a lcohol concent ra t ion ; thus, the ra te ofoxygen supply has to be adjusted by, e.g., decreasingthe oxygen concent ra t ion or increasing the temper-a ture toward the end of the react ion . More deta ilson the engineer ing aspects of a lcohol oxida t ion canbe found elsewhere.19,59,90,95-101Format ion of st rongly adsorbed byproducts dur ing
a lcohol oxida t ion on Pt -group meta ls has been fre-quent ly repor ted.43,80,85,89,102,103 Typica l side react ionsare the aldol condensat ion and oligomeriza t ion of thecarbonyl compound product104-109 and the decomposi-t ion of a lcohols main ly via the carbonyl compoundformed in situ , a ffording adsorbed CO and carbon-aceous species (CxH y) (Scheme 5).97,110-125 Even de-hydrogena t ion of the simplest secondary a lcohol,2-propanol, may poison a Pt -group meta l a lready a troom tempera ture (Scheme 6).126-130In contrast to blocking of the act ive sites by oxygen
or byproducts, sin ter ing of meta l par t icles and meta l
dissolut ion (leaching) results in irreversible deact iva-t ion .19,42,131,132 Sin ter ing (increase of average meta lpar t icle size) a t close to ambient tempera ture isa t t r ibuted to an atomic migrat ion process, the t rans-por t of sur face meta l a toms “ext racted” by chela t ingmolecules (Ostwald r ipening).133 Metal dissolut ion asMn+ species is facilita ted by acidic or strongly alkalinepH,134,135 h igh ca ta lyst poten t ia l (h igh oxygen cover -age), and the presence of complexing agents.30,76,136,137In a milder case, the dissolved species redeposit ontola rger , thermodynamica lly more stable meta l par -t icles, resu lt ing in par t icle size enla rgement and aloss of act ive sur face area .48,102,138-140 Meta l dissolu-t ion can be minimized by applying a low oxygenconcent ra t ion a t the end of the react ion when thea lcohol (“reducing agent”) concent ra t ion is low.141St ill, leach ing remains a limita t ion in the synthesisof (poly)hydroxy acids, a t radit ional applica t ion of Pt-group meta l ca ta lyst s.
2.1.3. Reaction MechanismThe mechanist ic proposa ls may be divided in to
three groups. According to the classica l dehydroge-na t ion mechanism (model A),11,13,21,73,142,143 the ad-sorbed a lcohol dehydrogena tes in two elementarysteps (Scheme 7). The O-H bond of a lcohol breaks
upon adsorpt ion on the sur face sit es, a ffording anadsorbed a lkoxide and hydrogen .144 In the adsorbedalkoxide, the !-C-H bond is weaker than other C-Hbonds due to the elect ron-withdrawing effect ofthe oxygen a tom, leading to the preferen t ia l break-ing of the !-C-H bond in the ra te-determiningstep.145 Dehydrogena t ion is genera lly ca ta lyzed bybases.143,146,147 Adsorbed oxygen (or sur face OH spe-cies in alkaline medium)148 is necessary to oxidize the
Figure 3. Rate of oxidat ion of L-sorbose to 2-keto-L-gulonicacid over 5% Pt /Al2O3 modified by st rongly adsorbingamines and ammonium hydroxides (323 K, 1 bar , O2,water , modifier /sorbose, 590 ppm). Modifiers: (1) pyr idine,(2) diazabicyclooctane, (3) t r ibutylamine, (4) t r iethylamine,(5) quinuclidine, (6) tet rabutylammonium hydroxide, and(7) tet ramethylammonium hydroxide.
Scheme 5. Simplified General Route for theTransformation (Decomposition ) of a P rimaryAlcohol on the P t-Group Metal Surface 110
Scheme 6. Simplified Route for theTransformation (Decomposition ) of 2-P ropanol ona Pd(111) Surface 129
Scheme 7. “Class ica l” DehydrogenationMechan ism of Alcohol Oxidation over P t-GroupMetal Catalysts (Mode l A)
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coproduct hydrogen and thus sh ift the equilibr iumtoward the carbonyl compound and accelera te thereact ion by libera t ing free surface meta llic sites. Theinfer ior role of oxygen in this mechanism is supportedby the observa t ions tha t oxygen can be replaced bya hydrogen acceptor (e.g., an olefin)149-153 and tha tthe meta l ca ta lyst is in a reduced sta te dur ingoxida t ion , as demonst ra ted by elect rochemica l andEXAFS measurements.91,95,154-158 A fur ther suppor tfor th is model is tha t a lcohol oxida t ion may beaccompanied by hydrogena t ion and hydrogenolysis-type side react ions, displaying a significant H-cover-age of the act ive sites (Scheme 8).125
Another class of mechanist ic models assumes tha tthe ra te-determining step involves direct in teract ionof the adsorbed oxidizing species with the adsorbedreactant or its part ia lly dehydrogenated intermediate(model B).16,19,57,159 This in terpreta t ion is suppor tedby numerous kinet ic studies revea ling a Langmuir-Hinshelwood-type behavior ,87,92,103,160,161 and elec-t rochemica l36,41,43,71,92,95,102,162 and X-ray absorpt ionspect roscopic70 studies tha t indica te a par t ia l oxygencoverage on the meta l surface dur ing some react ions.On the basis of recent da ta , we favor a th ird ap-
proach (model C) adopt ing the classica l dehydroge-na t ion mechanism for the a lcohol f carbonyl t rans-format ion and presuming tha t the key role of oxygenis to suppress ca ta lyst deact iva t ion due to st rongadsorpt ion of byproducts.125 According to th is model,the oxida t ive clean ing of the sur face sites, or evenprevent ion of the decomposit ion pa thway,128 is thepr imary role of oxygen and not the oxida t ion of thecoproduct hydrogen. Par t ia l regenera t ion of the cata-lyst s by oxida t ive removal of st rongly adsorbed poi-soning species can lead to a dramat ic ra te accelera -t ion compared to anaerobic dehydrogenat ion (Figure4),125 and thus to the (fa lse) conclusion tha t oxygen
is direct ly involved in the ra te-determining step ofthe react ion . According to th is model, some react ionsrun only on a par t ia lly oxygen-covered meta l sur facebecause chemisorbed oxygen is necessary to eliminate(oxidize) some sur face impur it ies and libera te act ivesites for a lcohol dehydrogena t ion .76,163,164Another field of cont radictory opin ions is the role
of meta l promoters. Some feasible explana t ions forthe ra te and select ivity enhancement are collectedbelow.(i) Geometr ic blocking of a fract ion of act ive sites
may be responsible for the improved cata lyst per for -mance.36,43,131,134,165 This so-ca lled “ensemble” effectis t raced back to the bigger act ive site ensemblesnecessary for the format ion of poisoning in termedi-a tes (by, e.g., C-C bond cleavage), compared to thesite requirement of the a lcohol dehydrogena t ionreact ion . This in terpreta t ion is in line with model Cof a lcohol oxida t ion and suppor ted by numerouselect roca ta lyt ic studies.166-168(ii) The high regioselect ivity in the oxida t ion of
polyfunct iona l a lcohols in the 2-posit ion has beena t t r ibu ted to complex format ion between the reac-tan t , a sur face Pt or Pd a tom, and a neighbor ingposit ively charged Bi or Pb promoter (F igure5a).11,38,60,79,85,135,169 A simila r proposa l is the complex
format ion between a 1,2-diol and a Sn IV species onthe Pt surface, result ing in the preferent ia l oxidat ionof the secondary OH group (Figure 5b).37 Although
Scheme 8. Reaction Ne tw ork for theTransformation of t r a ns-Cinnamyl Alcohol over5%Pd/Al2O3 (Air, 1 bar, 338 K) Figure 4. Cata lyst deact iva t ion dur ing conversion of
cinnamyl a lcohol over a 5% Pd/Al2O3 ca ta lyst in tolueneat 338 K. The init ia l TOF of 450 h-1 in Ar decreased rapidlyto 10 h-1 due to decarbonyla t ion-type side react ions (seeScheme 8). In t roduct ion of a ir enhanced the TOF to 2850h-1. Format ion of CO and its oxidat ive removal by air wereconfirmed by in situ ATR-IR spect roscopy.
Figure 5. Models assuming complex format ion with thesubst ra te as the role of meta l promoters in select iveoxida t ions.
Catalytic Oxidation of Alcohols with Molecular Oxygen Chemical Reviews, 2004, Vol. 104, No. 6 3041
the specia l role of neighbor ing Pt -group meta l-promoter sur face sites in the adsorpt ion and dehy-drogena t ion of the reactan t is plausible, the (par -t ia lly) oxidized sta te of the promoter cont rast s toelect roca ta lyt ic, LEED, XPS, and EXAFS stud-ies.52,158,170(iii) The ra te accelera t ion and the sh ift in product
dist r ibut ion may be at t r ibuted to bifunct ional cata ly-sis, assuming tha t oxygen or OH radica ls adsorbedon the promoter a tom are involved in the a lcoholoxida t ion react ion .36,111,171 This assumpt ion conformswith model B of alcohol oxidat ion. It may be general-ized assuming tha t the promoter facilit a tes theoxida t ive removal of sur face impur it ies, thus sup-por t ing model C of a lcohol oxida t ion . Another prob-able explanat ion for bifunct ional ca ta lysis is tha t thepromoter metal adatoms change the adsorpt ion prop-er t ies of hydrogen and OH radicals (oxidizing species)due to elect ron ic modifica t ion of the sur face Pt or Pda toms.52,170(iv) An ordered a lloy (in termeta llic compound)
formed between the Pt -group meta l and Bi, Pb, orTe promoters may be the rea l act ive site,35,80,132,172,173or the bimeta llic site may influence the select ivity,174though the observa t ions are cont radictory.(v) It has been shown tha t the promot ing effect of
Bi is not due to leaching as Bi3+ species.173,175-178 Thepresence of a la rge amount of Bi3+ complexes insolu t ion can even inhibit the oxida t ion react ion ,probably due to extensive reduct ion of Bi3+ to (inac-t ive) Bi0 onto the Pt sur face by the reactan t a lcohol.
2.1.4. Application Range of Pt-Group Metal CatalystsThe applica t ion range of suppor ted Pt -group met-
a ls, with and without promoters, is very broad. Upto 100% yields have been achieved in the oxida t ionof var ious pr imary a lcohols to the cor respondingcarboxylic acids and of secondary a lcohols to ke-tones. The poten t ia l of these ca ta lyst s is reflected bythe select ive oxida t ion of only one OH funct ion incomplex molecules such as a ldonic acids (Scheme9)38,84,85,136,169,179 and steroida l a lcohols.62,180
The select ive oxida t ion of an a lcoholic OH grouphas been a t ta ined in the presence of the followingother funct ions: a lkoxy and aryloxy groups,83,181,182phenolic hydroxyl,183-185 CdC bond (a llylic and iso-la ted),63,186 a romat ic a ldehyde,143 amino group,105qua ternary ammonium group,187,188 acetamino func-t ion,8 n it ro group,29 n it r ile group,8 aromat ic Cl,32,46,189t r ifluoromethyl group,64 N- and S-conta in ing het -eroaromat ic r ing,29 and cyclopropyl group.29 Gener -a lly, keto-carbonyl and carboxylic groups are resis-tan t to fur ther oxida t ion .
An impor tan t limita t ion of these ca ta lyst s is theoxidat ion of aliphat ic a lcohols to aldehydes.16 Hydra-t ion of a lipha t ic a ldehydes and dehydrogena t ion ofthe gemina l diol a re fast on the Pt -group meta lsur face, even in nonaqueous solvents (Scheme 4).Rapid removal of the coproduct water by azeot ropicdist illa t ion improves the yield only modera tely.190 Incont rast , hydra t ion of aromat ic and R,!-unsa tura teda ldehydes is minor ,191,192 and their fur ther oxida t ionby oxygen inser t ion requires sign ifican t ly h igherca ta lyst poten t ia l than the dehydrogena t ion step.43This is the explana t ion for the good yields in thesyn thesis of a romat ic, heteroa romat ic, and R,!-unsa tura ted a ldehydes, even in aqueous media .Not only the oxida t ion of a lipha t ic a lcohols to
a ldehydes but genera lly a lso the synthesis of act i-va ted carbonyl compounds is a demanding task dueto the poor stability (rapid decarbonyla t ion) of theproduct . Representa t ive examples for these difficu l-t ies and some good solu t ions are the oxida t ion oflact ic acid to pyruvic acid (Scheme 10),80,172 L-sorbose
to 2-keto-L-gulonic acid (Scheme 3),56,76,140,193-196 pan-toyl lactone to ketopantoyl lactone,75,197 and R,R,R-t r ifluoromethyl a lcohols to ketones.64,75To sum up, suppor ted Pt -group meta l ca ta lyst s
possess a very broad applica t ion range. The tech-n ica l impor tance of the method, including the ad-vantage of oxida t ions in aqueous media , is reflectedby the la rge number of pa ten ts. The composit ion ofproven ca ta lyst s and the range of pract ica lly usefu lcondit ions are much extended. A nega t ive aspect ofth is var iety is the demanding opt imiza t ion pro-cess198,199sa promising area for combinator ia l chem-ist ry.27,200
2.2. Supported Gold and SilverIn recent years, the unexpectedly high act ivity of
Au as a low-tempera ture CO oxidat ion cata lyst 201-204has in it ia ted in tensive research in the use of Aunanopar t icles for the liqu id-phase oxida t ion of a lco-hols. Genera lly, the adsorpt ion and cata lyt ic proper-t ies of Au st rongly depend on the par t icle size, whichcan be cont rolled by the prepara t ion method and theca ta lyst suppor t .205-213 A crucia l and not fu lly un-derstood quest ion is why small Au nanopar t iclesexhibit behavior radica lly differen t than tha t of bulkAu.214-222The applica t ion range of suppor ted gold in a lcohol
oxida t ion is not (yet ) broad, and under mild condi-t ions (320-350 K and 1-3 bar oxygen) th is oxidat ionrequires a st rongly a lka line aqueous medium.223,224The aqueous a lka line medium may represent ast rong limita t ion of the method, as var ious sidereact ions can diminish the select ivity (keto-enolequilibra t ion , Cannizzaro react ion , oxida t ive decar -
Scheme 9. Se le ctive Aqueous-Phase Oxidation ofSodium Gluconate at 6- or 2-Pos itions (H2O, O2,333-338 K)38,169
Scheme 10. Effe ct of P romoter in the Oxidation ofLactic Acid to Pyruvic Acid (H2O, pH 8, 363 K)80
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bonyla t ion).205,224 Another drawback is the format ionof carboxylates instead of carboxylic acids, though Pt-group meta ls have the same limita t ion in the oxida-t ion of pr imary a lipha t ic a lcohols. In acidic media ,gold oxidizes a ldehydes to carboxylic acids but isinact ive in the t ransformat ion of a lcohols, and meta lleach ing may a lso be sign ifican t .225 An in terest ingsolu t ion is the combina t ion of Au with a Pt -groupmeta l. A 1% Au-0.1% Pd/Al2O3 bimeta llic ca ta lystwas used for the t ransformat ion of 6-hydroxyhexa-noic acid to adipic acid.226 Both the react ion ra te andthe yield (97%) were high in water , even in theabsence of a base, though only a t eleva ted tempera-ture (433 K).Gold is h ighly act ive and select ive in the t ransfor -
mat ion of var ious diols205,206,208,218,224,227-230 and glyc-erol223,231 to monocarboxyla tes (F igure 6). Transfor -
mat ion of amino a lcohols to amino acids is a lsoselect ive, but good yields have been repor ted only fora lan ine.228The unusua lly h igh subst ra te specificity of sup-
ported gold is illustrated by the following comparison.Oxida t ion of ethylene glycol to glycolic acid was fast(TOF ) 1290 h-1), and complet ion of the react ionrequired a C/S mass ra t io of only 0.03.230 In cont rast ,in the t ransformat ion of 2-hydroxybenzyl a lcohol tosa licyla ldehyde, the TOF was only around 5 h-1, a tan ext raordinary C/S mass ra t io of over 7.232,233Despite having some simila r it ies to suppor ted
Pt -group meta ls, Au is a dist inct ly differen t ca ta -lyst . In cont rast to Pt -group meta ls, Au cannotca ta lyze a lcohol dehydrogena t ion in the absence of ast rong base. Fur ther oxida t ion of a ldehydes to car -boxylic acids is an oxygen inser t ion react ion , andth is step runs smooth ly on Au, even in organicmedia .234 In compara t ive studies of aqueous-phaseaerobic oxida t ion of a lcohols with suppor ted Pt , Pd,and Au, the la t ter was the most select ive and theleast prone to meta l leaching, to over -oxida t ion byoxygen , and to self-poisoning by st rongly adsorbedbyproducts.205,227,231,235 The higher select ivity andabsence of ca ta lyst deact iva t ion are probably due toweaker adsorpt ion of oxygen, hydrogen, reactant , andproducts on Au.119,123,236A deta iled mechanism of a lcohol oxida t ion on Au
nanopar t icles has not been repor ted yet . On the basisof the poor act ivity of Au in the absence of a base, itwas suggested tha t the ra te-limit ing step would beH+ abst ract ion from a pr imary OH group of glyc-erol.223,231 A simila r mechanism was proposed many
years ago for a lcohol oxida t ion on Pt /C, involving asecond step, the t ransfer of a hydr ide ion to the Ptsur face (Scheme 11).8,87,237
We consider it more feasible tha t the ra te-deter -min ing step is the cleavage of the C-H bond at theR-carbon atom. A similar mechanism is now generallyaccepted for Au electrodes (Scheme 12).238 Despite the
st ructura l differences between Au nanopar t icles andan extended Au elect rode sur face, there are a lsosimila r it ies, such as the cr it ica l role of aqueousalkaline medium and the absence of deact ivat ion dueto decomposit ion product s (CO and CxH y fr ag-ments).239,240An impor tan t quest ion is the na ture of act ive sites
on Au nanopar t icles. Elect rooxida t ion of ethanol onAu nanopar t icles suppor ted on glassy carbon re-quired the par t ia l coverage of Au surface by oxides.241Another analogy might be the model proposed for COoxida t ion .219,242,243 According to th is suggest ion , theact ive site consist s of an ensemble of meta llic Auatoms and a ca t ion ic Au+ species with a neighbor ingOH group (Figure 7).
Silver -based ca ta lyst s a re well established in thegas-phase oxida t ion of a lcohols and diols1,3,244-247 butra rely used in the liqu id phase. Recent ly, the quan-t it a t ive t ransformat ion of benzyl a lcohol to benza l-dehyde in oxygen was achieved over a 0.6% Ag/pumice ca ta lyst under mild condit ions (348 K, 1bar ).248 The mater ia l showed reasonably good act iv-ity, though the react ion over a simila r ly prepared0.27% Pd/pumice ca ta lyst was a lways faster . Thesynergic effect observed with Pd-Ag/pumice bimetal-lic ca ta lyst s and with mechanica l mixtures of themonometa llic ca ta lysts was at t r ibuted to coopera t ionbetween the Pd0 and Ag0 act ive sites; cont r ibu t ion ofthe a lloy phase detected by EXAFS was excluded.The infer ior per formance of carbon-suppor ted Ag,
compared to that of various supported Pt-group metal
Figure 6. Examples on the synthesis of monocarboxyla tesfrom alcohols in aqueous a lka line medium on suppor tedAu nanopar t icles.205,223,228
Scheme 11. Alcohol Oxidation on P t (and Au)Involv ing Deprotonation of the Alcoholic OHFunction by Adsorbed Hydroxyl Ions
Scheme 12. Rate -De termin ing Step in theOxidation of Ethane -1,2-dio l on Au Electrode inAqueous Alkaline Solu tion
Figure 7. Model of the act ive site in the oxida t ion of COby suppor ted Au nanopar t icles.219
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ca ta lyst s, has been concluded a lso in the oxida t ionof geran iol to cit ra l.28
2.3. Oxides and Mixed Oxides2.3.1. Ru-Containing OxidesHydra ted ru thenium dioxide (RuO2‚xH2O) is a
stoich iomet r ic oxidant tha t can be applied a lso as areusable ca ta lyst .249-254 The amount of water (x) isusua lly in the range 1-1.3 equiv and decreasesrapidly with increasing tempera ture of hea t t rea t -ment .255 The mater ia l has a two-dimensiona l st ruc-ture of independent chains, in which RuO6 octahedraare connected by pairs of O-br idges. Hydrous ru the-n ium dioxide is a mixed elect ron-proton conductor ,and the coordinat ively unsaturated surface atoms canact iva te molecular oxygen.256,257 The oxida t ion act iv-ity of ru thenium dioxide has been a t t r ibu ted to thein teract ion of st rongly bound reactan t and weaklybound oxygen species.258 The completely dehydra tedoxide is inact ive in a lcohol oxida t ion , and var ia t ionin the degree of hydra t ion , which can be cont rolledby the synthesis procedure, st rongly influences thestructure and the catalyt ic act ivity.259,260 The hydrousoxide RuO2‚xH2O easily t ransforms in to a mixtureof Ru and RuO2 upon thermal t rea tment a t eleva tedtempera ture.253Hydra ted ru thenium dioxide afforded over 98%
yield to cinnamaldehyde in the slow oxida t ion ofcinnamyl alcohol.261 A drawback of the process is that2,6-di-tert-bu tyl-p-cresol addit ive was necessary toprevent fur ther oxidat ion to cinnamic acid. React ivityof pr imary and secondary a llylic a lcohols, R-ketoa lcohols, R-hydroxy-lactones, and sa tura ted a lcoholsdecreased in th is order . Oxida t ion of pantoyl lactoneto ketopantoyl lactone a t eleva ted tempera ture wasquant ita t ive, and the performance of RuO2‚xH2O wassimilar to tha t of 5% Ru/C (Scheme 13).197 Obviously,
cont inuous format ion of the coproduct water is suf-ficient to mainta in the hydra ted sta te and the oxida-t ion act ivity of ru thenium dioxide, even a t a round450 K.A novel approach is the synthesis of ru thenium
oxide nanoclusters in the supercages of fau jasitezeolite (10% RuO2/FAU).262 On average, the ru the-nium oxide clusters homogeneously distr ibuted in ther igid zeolite framework contained only five Ru atoms,and their size was about 1.3 nm. St rong in teract ionsbetween RuO2 chains and framework Si -O functionsstabilized the oxide nanopar t icles dur ing the hydro-thermal synthesis (Figure 8). The recyclable ca ta lystwas act ive (TOF ) 0.5-8.5 h-1) and highly select ive(>99%) in the oxida t ive synthesis of benza ldehyde,cyclohexanone, 2-cyclohexen-1-one, crotona ldehyde,
1-heptanal, and 2-heptanone. In comparat ive exper i-ments, in the oxida t ion of benzyl a lcohol, 10% RuO2/FAU afforded 6-fold higher yield to benza ldehydethan bulk RuO2‚xH2O. This ra te enhancement ispar t icu la r ly impressive when consider ing the prob-able diffusion limita t ion inside the zeolite par t icles.It was demonst ra ted tha t the nar row channels offaujasite zeolite around the ruthenium oxide clustersst rongly influenced the react ivity of bulky subst ra tes(shape select ivity). In the compet ing oxida t ion ofbenzyl a lcohol and 9-hydroxyfluorene, only benzyla lcohol was oxidized, though bulk RuO2‚xH2O af-forded the facile oxida t ion of both subst ra tes. Theseexper iments provide addit ional evidence for the loca-t ion of ru thenium oxide par t icles inside the zeolitesupercages.Synthesis of mixed oxides by co-precipita t ion is
another approach to tune the ca ta lyt ic proper t ies ofru thenium oxide. A remarkable synergism was ob-served in the oxida t ion of cinnamyl a lcohol by addi-t ion of Co (Co/Ru atomic ra t io 1.5; TOF up to 38h-1).254 Pr imary a lipha t ic a lcohols were less react ive(TOF ) 1.3-5 h-1), and fur ther oxida t ion of a lde-hydes to carboxylic acids could be suppressed onlyby addit ion of a radica l scavenger . Cobalt oxide alonewas inact ive under the same condit ions, and the syn-ergic effect of Co was a t t r ibu ted to act iva t ion of oxy-gen in reoxida t ion of the ru thenium hydr ide speciesformed after dehydrogena t ion of the subst ra te.A simila r mult icomponent ca ta lyst is Ru0.3Co2Ce
oxide.263 The black powder synthesized by co-precip-ita t ion was act ive (TOF ) up to 13 h-1) and highlyselect ive in the oxida t ion of aromat ic and a llylica lcohols. Over 99% yields were obta ined in var iousreact ions, even in the synthesis of bulky aliphat ic andaromat ic a ldehydes and ketones (e.g., 2-adaman-tanone, diphenyl ketone, 1-pyrenecarboxa ldehyde;TOF ) 5-11 h-1). An in terest ing fea ture of th isthree-meta l oxide is the oxida t ion of pr imary a li-pha t ic a lcohols to carboxylic acids, and R,ω-pr imarydiols to lactones, in good yields (Y ) 64-97%; TOF) 0.7-2.5 h-1). Addit ion of a radica l scavengerprevented a ldehyde oxida t ion , and a lipha t ic a lde-hydes could be prepared in quant ita t ive yields.Oxidat ion of 1,4-pentanediol (which contains primaryand secondary OH funct ions) a fforded methyl γ-bu-tyrolactone in 87% yield.Other effect ive Ru-based mixed oxides developed
for a lcohol oxida t ion include Ru0.35MnFe1.65O4264 and
Ru0.45MnFe1.4Cu0.15Ox.265 The cata lysts were designedusing fer r ite spinel (Fe2MnO4) as a model, in whichFe is par t ia lly subst itu ted by Ru and Cu. The mul-
Scheme 13. Oxidation of Pantoyl Lactone in1,2-Dich lorobenzene w ith RuO2‚xH2O (at 453 K)and 5%Ru/C (at 446 K)
Figure 8. Structure of RuO2 in the supercages of faujasitezeolite.262
3044 Chemical Reviews, 2004, Vol. 104, No. 6 Mallat and Baiker
t icomponent cata lysts were prepared by co-precipita-t ion from aqueous solu t ions. Composit iona l var ia -t ions proved tha t Ru is by far the most act ive com-ponent . The yields with Ru0.35MnFe1.65O4 were excel-len t in the oxida t ion of benzylic, heteroaromat ic,a llylic, and bulky cycloaliphat ic a lcohols to aldehydesand ketones.264 In a ll react ions, including the oxida-t ion of simple a lipha t ic a lcohols such as 1-octanol,the select ivit ies were a t least 99%. The ca ta lyst wasreusable without significant loss in act ivity or selec-t ivity. The TOFs were modera te (0.2-4.1 h-1), and aC/S mass rat io over 1 was used, though the unusuallymild react ion condit ions (room tempera ture, a ir ) a reprobably far from the opt imum.Successfu l applica t ion of conduct ing lead-ru the-
n ium and bismuth-ru thenium pyrochlore oxides inelect roca ta lyt ic react ions266,267 in it ia ted their syn-thet ic applica t ion in a lcohol oxida t ion .268 The ca ta -lyst s have an expanded la t t ice pyrochlore st ructurewith the genera l composit ion A2+xRu2-xO6.5-7, whereA ) Bi or Pb and 0 < x < 1. Genera lly, an increasingsubst itu t ion (x) enhances the ca ta lyt ic act ivity.269Oxida t ive stability of Ru-pyrochlore oxides dependson the amount of oxygen vacancies: a la rge numberof oxygen defects decreases the stability.270 The blackmacroporous mater ia ls funct ion only in st ronglya lka line aqueous media .269,271Ruthenium pyrochlore oxides were act ive under
mild condit ions in the oxida t ive cleavage of 1,2-cy-clohexanediol.268,269 Select ivity to adipic acid (70-99%) increased a t h igher C/S ra t ios, which was bestach ieved in a cont inuous-flow tr ickle-bed reactor . Alimita t ion of the method is tha t the ca ta lyst s a react ive in severa l other oxida t ion react ions, includingthe oxida t ion of ketones, pr imary a lcohols, and ole-fins. Hence, it is understandable that Ru-pyrochloreoxides were not select ive in the oxida t ive cleavage ofcarbohydra tes and polysacchar ides.272,273The probable mechanism of diol cleavage over a
Pb-Ru pyrochlore oxide involves a chela ted diolintermediate coordinated to a PbIV site at the cata lystsur face (Scheme 14).274 Reoxida t ion of the PbII sit eis media ted by the neighbor ing RudO lat t ice site.
2.3.2. Other OxidesMost of the oxides commonly used in gas-phase
oxida t ions are barely act ive ca ta lysts with molecularoxygen under mild condit ions, and repor t s on theirsuccessful applica t ions in the liquid phase are scarce.An example is V2O5 suppor ted on TiO2, a well-knownca ta lyst for gas-phase aerobic oxida t ion of var ioussubst ra tes.275-277 The modera tely act ive mater ia la fforded good yield in the oxida t ion of 5-(hydroxy-methyl)furfural to 2,5-furandicarboxaldehyde (Scheme15).278 The prefer red ca ta lyst conta ined mult ilayerV2O5 on the suppor t , a st ructure close to tha t of bulk
V2O5. For compar ison , suppor ted Pt -group meta lcata lysts produced the formyl-carboxyl or dicarboxylder iva t ives in good yields.22,143A broad range of oxides and mixed oxides were
tested in the synthesis of ethyl pyruva te from ethyllacta te.279 Many of them were surpr isingly act ivea lready at 403 K and conver ted ethyl lacta te a t a C/Smass ra t io of only 0.0002. No byproducts formed onSnO2 and ZrO2 at low conversions, but the select ivitydropped rapidly with increasing conversion, and eventhe best ca ta lyst , SnO2-MoO3, gave only 50% yield.MnO2 is commonly used as an oxidizing agent in a
la rge excess, bu t u lt rasonica t ion of the act iva tedoxide (MnO2‚xH2O) provided a ca ta lyt ica lly act ivemater ia l.280 Diphenylmethanol was t ransformed tobenzophenone with only 0.1 molar equiva len t ofact ive manganese dioxide a t 363 K in oxygen , butoxida t ion of cinnamyl a lcohol was sluggish , and noreact ion occurred with 2-octanol. It was assumed thatthe sur face OH groups were the act ive sites on thehydra ted ca ta lyst .Hydrous PdO was modera tely act ive and select ive
in the oxida t ion of benzyl and cinnamyl a lcohols toa ldehydes.281 Synthesis of binary oxides by addit ionof Co3+, Fe3+, or Mn3+ afforded far more efficien thydra ted binary oxides (TOF ) 19-25 h-1). None ofthe added components were act ive a lone, in theabsence of PdO. A disadvantage of these co-precipi-ta ted mixed oxides is the slow oxida t ion of a ldehydeto carboxylic acid, a side react ion tha t could not beelimina ted even by the addit ion of a radica l scaven-ger , 2,6-di-tert-bu tyl-p-cresol. Other impor tan t sidereact ions, the format ion of benzene by decarbonyla-t ion of benzaldehyde and that of 3-phenyl-1-propanolby t ransfer hydrogena t ion of cinnamyl a lcohol, indi-ca ted tha t the subst ra te a lcohol presumably reducedPdO to meta llic Pd in the binary oxides and theactua l ca ta lyst s were oxide-suppor ted Pd0.281
2.4. Molecular SievesVar ious subst itu ted zeolites, a lso termed “redox
molecular sieves”, have been tested in a lcohol oxida-t ion in the liqu id phase, bu t most of them require aperoxide as oxidant .5,282-288 An except ion is a Cr-
Scheme 14. Oxidative Cleavage of a Vic inal Dio l byRuthen ium Pyroch lore Oxide
Scheme 15. Oxidation of 5-Hydroxymethylfurfuralw ith Air over 15%V2O5/TiO2,278 5%Ru/C, and 5%Pt/Al2O322 Catalysts
Catalytic Oxidation of Alcohols with Molecular Oxygen Chemical Reviews, 2004, Vol. 104, No. 6 3045
subst ituted aluminophosphate, CrAlPO-5,289,290 whichca ta lyzed a lcohol oxida t ion with oxygen in the pres-ence of a t least 0.1 equiv of TBHP (tert-bu tyl hydro-peroxide). However , a thorough invest iga t ion re-vea led tha t the rea l act ive species was Cr , leachedin t race amounts in the presence of oxidant .6Recent ly, a synthet ic cryptomelane-like manganese
oxide octahedra l molecula r sieve, K-OMS-2, wasproposed as a select ive ca ta lyst tha t can ut ilizemolecula r oxygen .291 This mater ia l, with a composi-t ion of KMn8O16‚nH2O, conta ins one-dimensiona ltunnels with MnO6 octahedra l building blocks andK+ as tunnel ca t ion . Rela t ively simple synthesismethods afford up to 250 m2 g-1 BET surface area ,though the pore size is in the range 0.45-0.7nm291,292sa st rong limita t ion in the t ransformat ionof bulky subst ra tes. Oxida t ion could be accelera tedby par t ia l replacement of K+ by H+ ions to formBrønsted acidic H-K-OMS-2. The act ivity of H-K-OMS-2 was, however , st ill low in the oxida t ion ofvarious simple aliphat ic, cycloaliphat ic, aromatic, andallylic a lcohols (TOF ) 0.035-0.5 h-1).292-294 Besidesthe probable mass t ranspor t limita t ion in the narrowpores, the low act ivity might be due to ca ta lystpoisoning by water , the only coproduct of the reac-t ions. The major advantage of the recyclable ca ta lystis the excellen t select ivity: a lways 100% to thecor responding a ldehydes and ketones.The la rge kinet ic isotope effect observed in the
oxida t ion of benzyl a lcohol implies tha t removal ofhydrogen from the R-C-a tom is the ra te-cont rollingstep.293 As concerns the overa ll process, the react ionobeys the Mars-van Krevelen mechanism295,296soneof the few examples of th is mechanism in liqu id-phase oxida t ion react ions. Removal of two protonsfrom the organic subst ra te is accompanied by a two-elect ron reduct ion of the Mn4+-type Lewis acidic sites(Scheme 16). The Mn2+ sites a re reoxidized by mo-
lecula r oxygen , and facile decomposit ion of the in-termedia te H2O2 on the molecula r sieve produceswater , the fina l coproduct .293
2.5. HydrotalcitesHydrota lcites (“HT”), tha t is, Mg- and Al-hydroxy-
carbonates with the formula Mg6Al2(OH)16CO3‚4H2O,are rela t ively new mater ia ls in cata lysis.297-301 Theseanionic clays consist of ca t ion ic Brucite layers withanionic compounds in the inter layer . Two- and three-va len t ca t ions can be in t roduced in to the Brucitelayer , and the anions can also be var ied. The genera lformula of hydrota lcite-like compounds (a lso termed“layered double hydroxides”, LDH) is [MIIMIIIA-HT)MII
1-xMIIIx(OH)2]An-
x/n‚mH2O, where A is an anion.A recyclable ca ta lyst , Ru-Mg-Al-HT {Ru0.5Mg6-
Al2(OH)16CO3}, showed modera te act ivity (TOF )0.3-1.9 h-1) in the oxida t ion of act iva ted a lcohols
with molecu la r oxygen .302 The C/S mass ra t io wasra ther h igh , a round 1, but the select ivity to carbonylcompounds was usually high, and quantita t ive trans-format ion of some aromat ic a lcohols was achieved.Further oxidat ion of aromatic aldehydes to carboxylicacids was not detected. Pr imary alcohols were t rans-formed faster than secondary a lcohols. The ca ta lysthaving 7.3 wt % Ru in the Brucite layer was super iorto mater ia ls in which other t ransit ion meta ls (Ni, Fe,Mn, V, or Cr ca t ions) were in t roduced as act ivespecies. Var ia t ion of the anion in the in ter layer ofRu-Mg-Al-HT showed tha t the ca ta lyst with car -bonate ions was the most act ive. It has been assumedtha t the act ive sites a re hydroxyl groups associa tedto Ru ca t ions (Scheme 17).300,302 In the fir st step, a
basic OH group ca ta lyzes the format ion of Ru-a lkox-ide, and the subsequent !-elimina t ion gives the car -bonyl compound. The redox cycle is completed by re-oxida t ion of the reduced Ru species with molecula roxygen .Fur ther subst itu t ion of Mg ions with Co or Mn ions
in Ru-Mg-Al-HT led to a prominent synergic effect .303The most act ive and select ive ca ta lyst was Ru-Co-Al-HT, which oxidized a var iety of aromat ic anda lipha t ic a lcohols to a ldehydes and ketones in goodto excellen t yields (TOF ) 0.8-14 h-1). Ru-Co-Al-HTwas act ive a lso in the oxygena t ion of aromat ic com-pounds at benzylic posit ions.300 For example, fluoreneand xanthene were t ransformed at 343 K to thecorresponding ketones in 93-98% yield. The synergiceffect between Ru and Co ions was a t t r ibu ted to theformat ion of h igher oxida t ion sta te Ru ions.300,303In teract ion between Ru and Co was evidenced byXPS by a shift in the elect ron ic sta te of Ru.A similar catalyst , Ru-Cu-Al-HT, showed low activ-
ity (TOF < 1 h-1) with molecula r oxygen in theabsence of a co-oxidant , such as PhIO or tet rabutyl-ammonium per ioda te.304 On the basis of XPS analy-sis, an in teract ion was assumed between the tworedox couples Cu0/Cu2+ and Ru4+/Ru6+ during alcoholoxida t ion .There are several hydrotalcite-like mater ia ls which
were developed for the aerobic oxida t ion of a lcoholsand do not conta in Ru as the act ive species. Forexample, the recyclable cata lyst Ni-Al-HT305 affordedgood yields in the t ransformat ion of aromat ic anda llylic a lcohols to the cor responding carbonyl com-pounds, bu t the react ions were slow, even a t a C/Smass ra t io above 2. Secondary benzylic a lcohols were
Scheme 16. Mechan ism of Benzyl AlcoholOxidation on Manganese Oxide OctahedralMolecu lar Sieve , K-OMS-2
Scheme 17. Mechan ism of Alcohol Oxidation overRu-Co-Al-HT300
3046 Chemical Reviews, 2004, Vol. 104, No. 6 Mallat and Baiker
more react ive than pr imary alcohols, and subst itu tedbenzylic a lcohols reacted faster than benzyl a lcohol,independent of the elect ron-withdrawing or elec-t ron-releasing character of the funct ion . The cata lystwas poor ly act ive in the oxida t ion of pr imary andsecondary a lipha t ic a lcohols. The ca ta lyt ica lly act ivespecies is probably Ni in associa t ion with a luminumoxide.305
Numerous hydrota lcite-like mater ia ls conta in ingCo, Cu, Cr , Fe, Mn, Ni, and Zn were tested in thesolvent-free oxidat ion of benzyl alcohol.306 At the highreact ion tempera ture of 483 K, benzaldehyde forma-t ion was only about 5 t imes faster with the mostact ive ca ta lyst , Cu-Cr-HT, than the unca ta lyzedoxida t ion react ion .
2.6. PhosphatesHydroxyapa t ites307,308 ga in increasing in terest in
ca ta lysis due to their un ique proper t ies.309-311 Hy-droxyapat ites can be synthesized by various methods,including simple co-precipitat ion,308,312-315 and a greatvar iety of new mater ia ls can be developed by surfacemodifica t ion316-318 or by using the ion-exchange abil-ity of apat ites.319-321 A Ru-conta ining hydroxyapat ite(RuHAp), prepared by par t ia l replacement of Ca2+ions with Ru3+, gave high yields in the synthesis ofaromat ic, hetereoaromat ic, R,!-unsatura ted, and ali-pha t ic carbonyl compounds.322 The stable and recy-clable ca ta lyst was modera tely act ive; the C/S massra t io was around 1, despite the high Ru conten t (17wt %). In terest ingly, a t 333 K, RuHAp oxidized1-octanol to octana l in 94% yield without fur theroxida t ion to carboxylic acid, and a t 353 K, octanoicacid formed in 82% yield. Pr imary a lipha t ic a lcoholswere preferen t ia lly oxidized in the presence of sec-ondary alcohols. This chemoselect ivity resembles theproper t ies of RuCl2(PPh3)3323 and cont rast s those of“bulk” Ru ca ta lyst s (RuO2, Ru/Al2O3), indica t ing thepresence of isola ted Ru ca t ions as act ive species inRuHAp.322 The act ive sites a re probably monomer icRu ca t ions sur rounded by O and Cl,322 though no Clhas been found recent ly in a simila r ca ta lyst .324 Ruis st rongly coordinated to the apat ite framework, andth is in teract ion prevents leaching of the act ive spe-cies. The st ructure shown in Figure 9 expla ins why
the authors classified the mater ia l as a suppor ted Rucomplex.322 According to their model, the oxida t ionis in it ia ted by a ligand exchange between the a lcoholand the chlor ine species a t the Ru site. The carbonylcompound product is formed by !-hydride eliminat ionfrom the Ru-a lcohola te. In terest ingly, the applica-t ion range of RuHAp is quite broad: it is act ive and
select ive in the aerobic oxida t ion of pr imary aminesto nit r iles325 and organosilanes to silanols,326 and inthe racemiza t ion of chira l a lcohols.324A simila r ca ta lyst , Pd-hydroxyapa t ite, was pre-
pared by in teract ing stoich iomet r ic hydroxyapa t iteCa10(PO4)6(OH)2 with a solu t ion of PdCl2(PhCN)2.327The ca ta lyst conta in ing only 0.3 wt % Pd was farmore act ive than the Ru ana logue (TOFs up to 500h-1) and afforded good yields to var ious a ldehydesand ketones. The unused ca ta lyst conta ined mono-meric PdCl2 species grafted by chemisorpt ion onto theapat ite sur face (Figure 10). Analysis of the st ructure
of the recovered sample revea led, however , tha tdur ing react ion Pd2+ was reduced to Pd0 by thereactant a lcohol and the actual cata lyt ic species wereapa t ite-suppor ted Pd nanopar t icles with an averagediameter of 4 nm. Accordingly, th is ca ta lyst belongsto the class of suppor ted Pt -group meta ls discussedin sect ion 2.1.Pd2+ was resistan t aga inst reduct ion when a non-
stoich iomet r ic, Ca-deficien t hydroxyapa t it e, Ca 9-(HPO4)(PO4)5(OH), was applied for ion-exchangeusing the same method as descr ibed above. In terest -ingly, th is ca ta lyst was act ive in Heck and Suzukireact ions but not in a lcohol oxida t ion with molecularoxygen .327Recent ly, Cu2(OH)PO4 and Cu4O(PO4)2 have been
proposed for the aqueous-phase aerobic oxida t ion ofbenzyl alcohol and cyclohexanol.328 The react ion rateswere low, and the highest yield of the cor respondingcarbonyl compounds was only 12%.
3. Heterogenized Metal ComplexesFor immobiliza t ion of soluble ca ta lyst s act ive
with molecu la r oxygen , var ious convent iona l tech-n iques329-333 have been applied, including incorpo-ra t ion of meta l ions in to la t t ices, adsorpt ion on ahigh sur face area suppor t , ion exchange, cova len tbonding via tethers, and ent rapment in to a poroussuppor t . As concerns the stability of these hybr idca ta lyst s, simple adsorpt ion on a suppor t can rarelyprevent leaching of the act ive species, and the oxida-t ive stability of polymer ic suppor t s is frequent lyinsufficien t .In some cases, new mater ia ls a re designed and
descr ibed as suppor ted meta l complexes, bu t thepublished da ta revea l tha t , dur ing prepara t ion , themeta l ions were reduced to meta l (M0). For example,a polyt itazane-pla t inum complex on magnesiumoxide suppor t was suggested as a suppor ted complexca ta lyst for a lcohol oxida t ion .334-337 The black colorof the fina l mater ia l and par t icu la r ly the XPS data
Figure 9. Isola ted Ru3+ species with the nearest oxygenson the sur face of hydroxyapa t ite, as ident ified by XANES,XPS, and EDX analysis.
Figure 10. Structure of as-synthesized Pd-exchangedhydroxyapa t ite (left ) and it s rest ructur ing to a suppor tedmeta l ca ta lyst (r igh t ) under a lcohol oxida t ion condit ions.
Catalytic Oxidation of Alcohols with Molecular Oxygen Chemical Reviews, 2004, Vol. 104, No. 6 3047
indica te, however , tha t the ca ta lyst likely conta inedsmall Pt par t icles dispersed in the organic-inorganicmat r ix. Accordingly, these mater ia ls belong to sup-por ted Pt -group meta ls (sect ion 2.1) and will not bediscussed here.
3.1. Ru-Based CatalystsA simple method, adsorpt ion on act iva ted carbon ,
was applied for the heterogeniza t ion of Ru(PPh3)3-Cl2.338 The suppor t enhanced the react ivity of thecomplex in the oxida t ive cleavage of vicina l diols toa ldehydes, and fa ir to good yields a t modera te ra tes(TOF ) 1.5-3 h-1) were achieved in hydrophobicsolvents. The highest select ivity to a ldehydes wasmeasured in the oxida t ion of termina l diols, thoughthe t ransformat ion of in terna l vicina l diols to a l-dehydes and cyclic diols to R,ω-dia ldehydes wasa lso efficien t . A disadvantage of the method is thehigh C/S mass ra t io (up to 4), and it is not knownwhether the ca ta lyst is t ru ly heterogeneous andrecyclable. Format ion of a dia lkoxyru thenium com-plex leading to a six-membered t ransit ion sta te wasproposed for the react ion mechanism, as illust ra tedin Scheme 18.
A genera l deficiency of the adsorpt ion technique istha t the meta l in it s h ighest oxida t ion sta te hasthe lowest a ffin ity to the suppor t , resu lt ing in con-siderable leaching of the act ive species dur ing oxida-t ion .332 This limita t ion is illust ra ted by the oxida-t ion of a lcohols with another carbon-suppor ted Rucomplex, [RuCl2(p-cymene)]2.339 The ca ta lyt ic prop-er t ies of the immobilized complex remained sim-ila r to those of the homogeneous complex.340,341 Ben-zylic and a llylic a lcohols were oxidized in the pres-ence of 5 mol % base, Cs2CO3. Reasonable ra tes (TOF) 1.6-14 h-1) and good yields of the carbonylderivat ives were achieved, but the C/S mass rat io waswell above 1, and more than 30% of the complexleached out of the suppor t dur ing a 9-fold ca ta lystrecycling test .Polyoxometa la tes342-346 a re proven ca ta lyst s of
oxida t ion and dehydrogena t ion react ions. They aresufficien t ly stable, possess h igh oxida t ion poten t ia l,and are reoxidized by molecular oxygen. An effect iveheterogeneous oxida t ion ca ta lyst , Ru-subst itu tedsilicotungsta te, [(n -C4H9)4N]4H[SiW11Ru(H2O)O39]‚2H2O, was synthesized by the react ion of RuCl3 withthe polyoxometa la te in an organic solvent .347 Theca ta lyst was insoluble in var ious weakly pola r or -ganic solvents and could be recycled without anysignifican t loss in act ivity. A st rong synergic effectwas observed between the Ru3+ act ive site and the
polyoxometa la te [SiW11O39]8-, a llowing fast react ion(TOF ) 15-23 h-1) even a t low C/S ra t ios of only afew weight percent . Var ious a liphat ic, cycloaliphat ic,and aromat ic ketones were prepared in 67-99%yield. Oxida t ion of pr imary a lipha t ic and aromat ica lcohols was slower and gave a mixture of a ldehydeand carboxylic acid. Oxida t ion of a llylic a lcoholsbarely proceeded. In cont rast , the ca ta lyst was ac-t ive in the oxida t ion of some alkanes, such as ada-mantane.At tach ing ligands to an insoluble polymer suppor t
is a popular st ra tegy to heterogenize soluble com-plexes, bu t it has never worked very well. A recentexample is the immobiliza t ion of a Ru(III)-Schiffbase complex on the sur face of a chloromethyla tedstyrene-divinylbenzene cross-linked copolymer viasequent ia l a t tachment of ethylenediamine, sa licyla -ldehyde, and RuCl3 (F igure 11).348,349 No byproduct
was detected in the oxida t ion of benzyl a lcohol tobenza ldehyde, but about 35% of Ru was lost dur ingthe leaching test with in less than 6 h.Per ru thena te anchored to an Amber list an ion-
exchange resin via qua ternary ammonium groupswas or igina lly developed as a clean oxidant .350 La terit was shown tha t it can a lso be used as a ca ta lystwith oxygen for the t ransformat ion of pr imary a lco-hols to pure a ldehydes (Scheme 19).351 Nonact iva ted
and secondary a lcohols reacted more slowly. Fur theroxida t ion of a ldehydes to carboxylic acids was notdetected, even in the oxida t ion of a lipha t ic a lcohols.Oxida t ion of ter t ia ry amino a lcohols and epoxy-a lcohols to a ldehydes gave 90-95% yields. In thefastest react ion , in the oxida t ion of benzyl a lcohol, aTOF of 20 h-1 was achieved, though at a C/S massra t io of over 9sa value character ist ic of stoichiomet-r ic oxidants ra ther than ca ta lyst s. A fur ther limita -t ion to pract ical applicat ion is the difficulty of catalystrecycling, likely due to oxida t ive degrada t ion of thepolymer ic suppor t .352In a sophist ica ted approach , per ru thena te was
immobilized as a tethered complex on a more stableinorganic mater ia l, mesoporous silica lit e MCM-41(Figure 12).352 The ca ta lyst was recyclable withoutany detectable meta l leaching or decline in theca ta lyt ic per formance. Var ious aromat ic and a llylica lcohols were oxidized quant ita t ively to a ldehydes at
Scheme 18. Cleavage of 1,2-Dio ls by Ru(PPh3)3Cl2/Cvia a Six-Membered-Ring Trans ition State 338
Figure 11. Polymer-anchored Ru(III)-Schiff base com-plex.349
Scheme 19. Oxidation of Alcohols by aPerru thenate Supported on a Polymer (Amberlis tAn ion -Exchange Res in )351
3048 Chemical Reviews, 2004, Vol. 104, No. 6 Mallat and Baiker
353 K, a t a C/S mass ra t io of only 0.1. The presenceof CdC bonds and Cl, MeO, BnO, NO2, CF3, or Ffunct ions in the aromat ic r ing did not disturb thereact ions. Secondary a liphat ic a lcohols and alkenols,such as cyclohexanol and cyclohexenol, did not react .Though the applica t ion range is not broad, th is is themost act ive heterogenized homogeneous ca ta lyst forthe oxida t ion of a lcohols with molecula r oxygen ,affording TOF values up to 170 h-1. Even distr ibut ionof the act ive species on the suppor t sur face and theabsence of mass t ranspor t limita t ion in the meso-porous st ructure of MCM-41286 seem to be cr it ica l tothe good per formance.A versa t ile route to prepare solid ca ta lyst s is
entrapment of a soluble transit ion-metal complex intoorganically modified sol-gel silica (“ORMOSIL”).353,354The key st ructura l proper t ies of doped sol-gel ma-ter ia ls, e.g., hydrophilic-hydrophobic character andpore size dist r ibu t ion , can be ta ilored according tothe requirement of the react ion . A genera l limita t ionof the method is tha t wide pores cannot preventleaching of the act ive species, while too narrow poreshamper the diffusion of bulky subst ra tes to the ac-t ive sit es. Recen t ly, t et ra -n -propylammonium per -ru thena te (TPAP) was embedded in to methyl-modi-fied hydrophobic silica , using methylt r imethoxysi-lane as a sol-gel precursor .355 The leach-proof ca ta -lyst possessed h igh sur face area and a nar rowpore size dist r ibut ion of around 1.2 nm. Transforma-t ion of a lcohols to carbonyl compounds was faston ly in the case of benzyl a lcohol (TOFs in the range1-13 h-1).
3.2. Pd-Based CatalystsThere is an increasing interest in the synthesis and
ca ta lyt ic applica t ion of (heterogenized) ligand-stabi-lized giant clusters that may be considered as modelsof suppor ted meta l ca ta lyst s, or as a link betweenhomogeneous and heterogeneous cata lysis.356-361 Thegian t Pd clusters dissolve in common solvents andact as homogeneous ca ta lyst s,362 though they can beimmobilized on a support to produce a heterogeneous,easily recyclable ca ta lyst .In the fir st applica t ion to heterogeneously ca ta -
lyzed oxida t ion of a lcohols with molecula r oxygen , afive-shell Pd cluster , Pd561phen60(OAc)180 (phen )1,10-phenant roline), was immobilized on TiO2 whilepreserving the or igina l st ructu re and size (ca . 3nm) of the precursor .363,364 The cluster con ta ined
Pd0 a toms in the inner par t and cat ionic species (Pd+and Pd2+) on the sur face, stabilized by phen ligandsand OAc anions. The ca ta lyst was reusable, withsome loss in yield and select ivity. Var ious pr imarya llylic a lcohols were oxidized in AcOH to R,!-un-sa tura ted aldehydes in up to 99% yield, a t good ra tes(TOF ) 0.3-15 h-1). The react ions were slower inbenzene, but ester ifica t ion of geraniol could be elimi-na ted.The react ion mechanism is assumed to be dehy-
drogena t ion of the a llylic a lcohol coordina ted to aca t ion ic Pd species, followed by oxida t ion of thesur face hydr ide with oxygen to form water . Thesuppor ted cluster showed low act ivity for the oxida-t ion of secondary a llylic and benzylic a lcohols, whichwas a t t r ibu ted to ster ic crowdedness around the Pdca t ions. Isomer iza t ion and hydrogena t ion of thesubstrate on reduced surface sites were also observed.A hydrogen t rea tment in acetonit r ile removed thephen ligands from the surface of Pd561phen60(OAc)180/TiO2 and decreased the react ivity of the ca ta lystby a factor of 5.364 Clea r ly, the phen ligands playan impor tan t yet unknown role in the oxida t ionprocess.Trea tment of Pd4phen2(CO)2(OAc)4 with Cu(NO3)2
in the presence of oxygen gave a monodispersedeight -shell nanocluster with the approximate com-posit ion of Pd2060(NO3)360(OAc)360O80 and an aver -age size of 3.8 nm.365 The mater ia l, immobilized onTiO2, a fforded high yields in the oxida t ion of pr i-mary aromat ic a llylic a lcohols (TOF up to 20 h-1)and benzylic a lcohols (TOF ) 3 h-1) to the cor re-sponding a ldehydes. Elect ron-withdrawing groupsreta rded the react ion , and oxida t ion of sa tura teda lcohols (1- and 2-octanol) barely occur red. A com-par ison of Pd2060(NO3)360(OAc)360O80 and Pd561phen60-(OAc)180 nanoclusters revealed significant differencesin their react ivity toward a llylic and benzylic a lco-hols.365A pa lladium-pyr idine complex, {Pd(OAc)2(py)2},
was immobilized on the externa l sur face of hydro-ta lcite by a simple adsorpt ion step.366-369 The hydro-talcite-supported Pd(II) catalyst was act ive in a broadrange of a lcohol oxida t ion react ions (TOF up to 10h-1) and gave good yields to a ldehydes and ketones.The cata lyst was, however , ra ther unstable, and Pd2+was easily reduced to Pd0 par t icles when the a lcoholconcent ra t ion (reducing agent ) or the react ion tem-pera ture exceeded a cer ta in limit .369 Besides, theoxida t ion was sluggish and incomplete without ad-dit ion of a t least 4 equiv of pyr idine to Pd(II).Apparen t ly, th is heterogen iza t ion method was notfu lly successfu l.
3.3. V- and Mo-Containing Catalysts(Heteropolyoxometalates)The importance of polyoxometalates and heteropoly-
acids in ca ta lyt ic oxida t ion react ions342-346 has beenbr iefly discussed in a previous sect ion in connect ionwith Ru-based ca ta lyst s. The presence of Ru is not arequirement for oxida t ion act ivity. A heterogenized,stable, and reusable ca ta lyst was prepared by im-pregna t ion of a mixed-addenda heteropolyanion sa lton to act iva ted carbon (10 wt % Na5PMo2V2O40/C).346
Figure 12. Perru thena te immobilized on a mesoporoussilica lit e MCM-41.
Catalytic Oxidation of Alcohols with Molecular Oxygen Chemical Reviews, 2004, Vol. 104, No. 6 3049
Some aromat ic pr imary and secondary a lcohols wereselect ively oxidized to carbonyl compounds in 93-98% yield in toluene without fur ther oxida t ion toacids (Scheme 20). It was assumed tha t reoxida t ion
of the ca ta lyst and not the dehydrogenat ion step wasra te-limit ing. Aromat ic a lcohols with elect ron-donat-ing or -withdrawing subst ituen ts reacted a t simila rra tes (TOF ) 4.5 h-1), except hydroxybenzyl a lco-hols (no yield). Oxida t ion of a lipha t ic secondarya lcohols was slow and incomplete, and pr imarya lcohols were iner t . Only 1 mol % of the immobi-lized cata lyst rela t ive to the substra te was used. Thisra t io, however , cor responds to a C/S mass ra t io over
1 due to the low molar concent ra t ion of the act ivespecies in the cata lyst . The cata lysts were act ive alsoin the dehydrogena t ion of benzylic amines to a lde-hydes and ketones in two steps via the Schiff basein termedia te.The same st ra tegy, graft ing an ammonium molyb-
dovanadophospha te onto carbon , provided an effec-t ive and t ru ly heterogeneous ca ta lyst , (NH4)5H6PV8-Mo4O40/C.370 The ca ta lyst gave 46-92% yields in theoxida t ion of a llylic and benzylic a lcohols to carbonylcompounds under condit ions simila r to those de-scr ibed above.346 In terest ingly, in a cont rol react ionthe unsuppor ted heteropolyoxometa la te was inac-t ive (Scheme 21). Limita t ions to the applica t ion of
th is ca ta lyst as a select ive a lcohol oxida t ion ca ta -lyst a re the necessary high C/S mass ra t io (up to 3)and the good act ivity of the mater ia l in the oxida-t ion of amines, a lkyl-subst itu ted phenols, and a l-kanes.370
Table 1. Oxidation of Aliphatic Alcohols to Aldehydes
subst ra te ca ta lyst T , K solvent TOF, h-1 Y , % S , % ref1-octanol 5%Pt-1%Bi/Al2O3 333 PhCH3 142 76 85 1891-octanol Ru0.35MnFe1.5Cu0.15Ox 295 PhCH3 0.2 51 100 2641-octanol Ru3+-hydroxyapa t ite 333 PhCH3 0.4 94 99 3221-octanol Pr 4N+RuO4
Scheme 20. Generalized Scheme for AlcoholOxidation w ith Carbon-SupportedNa5PMo10V2O40346
Scheme 21. Effe ct of Carbon Support on theOxidation Activ ity of (NH4)5H6PV8Mo4O40370
3050 Chemical Reviews, 2004, Vol. 104, No. 6 Mallat and Baiker
4. Oxidation of Alcohols on Various Catalysts: AComparisonIt is somet imes difficu lt to judge the ca ta lyt ic per -
formance of a new mater ia l. One way is to comparethe var ious ca ta lyt ic mater ia ls to the sta te-of-the-ar tca ta lysts in some frequent ly used test react ions. Thecompar ison based on the data in Tables 1-16 should
be considered only as semiquantita t ive, since in manycases the react ion condit ions applied are obviouslyfa r from the opt imum.
4.1. Primary Alcohols to AldehydesSelect ive oxida t ion of a lipha t ic a lcohols to a lde-
hydes with molecular oxygen is presumably the mostdemanding t ransformat ion . The major ity of knowncatalysts are poorly act ive or possesses low select ivityin these react ions, and deta iled resu lt s a re ra relyrepor ted. In the oxida t ion of 1-octanol and 1-dode-canol (Table 1), P t and par t icu la r ly a Pt /Al2O3promoted by Bi are by far the most act ive mater ia ls,though they are not very select ive. Even the moder-a te yield of 76-77% to an a lipha t ic a ldehyde is nottypica l for Pt -group meta ls, since these ca ta lyst s a rehighly act ive in the dehydrogenat ion of the hydra teda ldehyde and usua lly produce the carboxylic acid inhigh yield (Scheme 4).68 When high select ivity toa ldehyde is cr it ica l, var ious Ru-conta in ing ca ta lyst soffer a bet ter choice.Pr imary aromat ic (benzylic) a lcohols can be readily
transformed to aldehydes that are relat ively resistantto fur ther oxida t ion , a t least under mild condit ions.For example, oxida t ion of benzyl a lcohol, the mostcommonly repor ted test react ion , is character ized byexcellen t select ivit ies and yields on a lmost a ll ca ta -lyst s (Table 2). Outstanding ca ta lyst s a re Pd/Al2O3,Pd/hydroxyapa t ite, and per ru thena te suppor ted onmesoporous silica te MCM-41. Per ru thena te is par -t icular ly at t ract ive when fur ther oxidat ion to benzoicacid has to be avoided, though synthesis of theca ta lyst via tether ing is demanding.Transformat ion of 2-hydroxybenzyl a lcohol to sa li-
cyla ldehyde is of pract ica l impor tance, and th isreact ion is the fastest among all a lcohol oxida t ionreact ions over solid ca ta lysts (Table 3). The cata lystsand react ion condit ions are discussed main ly in thepa ten t lit era ture. The best TOFs are close to 10 000h-1 over Pt /C promoted by Bi or Pb; the promot ion iscar r ied out by simple addit ion of the promoter meta lsa lt to the react ion mixture. Suppor ted gold nano-
Table 3. Oxidation of 2-Hydroxybenzyl Alcohol toSalicy la ldehyde
Catalytic Oxidation of Alcohols with Molecular Oxygen Chemical Reviews, 2004, Vol. 104, No. 6 3051
par t icles a lso per form well, though the average TOFcould not be ca lcu la ted.233More demanding test react ions are the par t ia l
oxida t ion of the heteroaromat ic a lcohols 2-(hydroxy-methyl)pyr idine (Table 4) and 2-(hydroxymethyl)-th iophene (Table 5). The yields are h igh , but theact ivity of the cata lysts is usually low. Most cata lystsconta in ionic or metallic Ru as the act ive species. Thegood performance of Ru/Al2O3 and Pd/hydroxyapat itein the synthesis of 2-formylth iophene is surpr isingin the ligh t of the well-known poisoning effect ofsu lfur compounds on Pt -group meta ls.373-375 A com-par ison of the da ta in Tables 2-5 shows tha t rela -t ively small st ructura l changes can lead to dramat i-ca lly differen t ca ta lyst per formance. Hence, assess-ment of the cata lysts on the basis of these simple testreact ions may be misleading.Synthesis of R,!-unsa tura ted a ldehydes from the
cor responding a lcohols is a facile react ion with mostsolid ca ta lyst s, simila r to the oxida t ion of pr imarybenzylic a lcohols. Many ca ta lyst s give high yields a ta reasonable ra te in the oxida t ion of, for example,cinnamyl a lcohol (Table 6) and geran iol (Table 7). Inboth react ions, the most act ive ca ta lyst s a re sup-por ted Pt -group meta ls. Among the nonmeta llic ca t -a lyst s, Ru-Co mixed oxide is a good choice, thoughsome other Ru-based cata lysts give higher select ivityto the R,!-unsa tura ted a ldehyde. Oxida t ion of ge-ran iol to trans-cit ra l (and of nerol to cis-cit ra l) isusua lly h ighly stereoselect ive (E/ Z or Z/ E > 95/5,respect ively),23,263,264 though the values are not alwaysment ioned.
4.2. Primary Alcohols to AcidsThere are very few test react ions commonly used
to compare differen t types of ca ta lyst s in the a lcohol
f carboxylic acid t ransformat ion . Addit iona lly, fur -ther oxida t ion of a ldehydes to acids is frequent lyconsidered as an undesired side react ion , and theca ta lyst s and condit ions are opt imized in order toavoid th is t ransformat ion . Table 8 compares someca ta lyst s in the oxida t ion of 1-octanol and 2-phen-oxyethanol. The Ru-based mixed oxide is not veryact ive but a ffords the free carboxylic acid in highyield without byproduct format ion in the nonaque-ous medium. Pt -group meta ls a re act ive and selec-t ive, though only in aqueous a lka line media ; in theabsence of base, oxidat ion of the aldehyde intermedi-a te is slow and the act ive sites a re par t ia lly blockedby the st rongly adsorbing carboxylic acid.Promoted Pt -group meta ls a re the most commonly
used cata lysts for the oxidat ive synthesis of carboxy-lic acids (as sa lt s), including the oxida t ion of a broadrange of funct iona lized a lcohols32,46,63,83,376 and poly-hydroxy compounds.11-15 A recent indust r ia lly im-por tan t example is the oxida t ion of a choline sa lt orhydroxide over 5 wt % Pd/C to give 90% yield tobeta ine:187,188,377
The select ivity of suppor ted Au is a lso excellen t inthe oxida t ion of amino a lcohols to amino acids.228
4.3. Secondary AlcoholsOxidat ion of secondary alcohols to ketones is highly
select ive, and quant ita t ive t ransformat ion withoutany byproduct format ion under mild condit ions is notunusua l. Representa t ive test react ions for a lipha t icand cycloa lipha t ic a lcohols are the oxida t ions of2-octanol (Table 9), cyclohexanol (Table 10), cyclo-pentanol (Table 11), and 2-adamantanol (Table 12).Ru/Al2O3 seems to be the most act ive ca ta lyst (Table9, TOF ) 300 h-1), though the outstanding act ivitymight be due to the eleva ted react ion tempera tureand solvent -free condit ions. Other Ru-conta in ingcata lysts (e.g., Ru-silicotungsta te) and suppor ted Ptand Pd also per form well.The oxida t ion of secondary aromat ic a lcohols is
illust ra ted by the transformations of 1-phenylethanol(Table 13) and diphenylmethanol (Table 14). Theact ivity of suppor ted Pt -group meta ls is remarkablyhigher than those of other ca ta lyst s; the yields areexcellen t with a ll ca ta lyst s.29,41,327
Table 8. Oxidation of Aliphatic Alcohols to Carboxylic Acids
subst ra te ca ta lyst T , K solvent a TOF, h-1 Y , % S , % ref1-octanol 5% Pt-1% Bi/Al2O3 333 H2O 29 93 97 1891-octanol Pd/resin 373 H2O 0.1 90 - 821-octanol Ru0.3Co2CeOx 333 PhCF3 2.5 97 97 2632-phenoxyethanol 1% Pt /C + Cd(OAc)2 + Pb(OAc)2 343 H2O 840 98 - 462-phenoxyethanol 10% Pd/C + Bi(NO3)3 343 H2O 82 100 100 46a Water is used together with some base, and the product is the carboxylic acid sa lt .
[Me3N+CH2CH2OH]OH
- + O2 fMe3N
+CH2COO- + 2H2O (1)
3052 Chemical Reviews, 2004, Vol. 104, No. 6 Mallat and Baiker
4.4. Diols and TriolsPar t ia l oxida t ion of var ious diols and glycerol to
hydroxyacids and lactones, ketoacids, and hydroxy-ketones is a field of indust r ia l in terest . The react ionmedium is water , and the precise cont rol of pH orthe amount of base is cr it ica l to achieve high selec-t ivit ies. The per formance of these ca ta lyst s is com-pared below in the oxida t ion of ethylene glycol andglycerol to monocarboxyla tes.Ethylene glycol can be t ransformed quant ita t ively
to glycolic acid by Pt /C when only 1 equiv of base isadded and the actua l oxygen concent ra t ion a t themeta l sur face is kept low by working a t refluxtemperature in air (Table 15).146 Supported Au seemsto be a bet ter choice: it combines an excellen t ra tewith h igh select ivity.205,208,227-230 The super ior selec-t ivity of gold has been shown also in the oxida t ion ofpropane-1,2-diol to lacta te.228Suppor ted Au nanopar t icles are the ca ta lyst of
choice also for the oxidat ion of glycerol to glyceric acid(Table 16).223,231 The act ivity of Pt /C is simila r , bu tthe select ivity is lower .
5. ConclusionsIn this review, we have explored the heterogeneous
catalyt ic methods available for the select ive oxidat ionof a lcoholic OH groups to carbonyl or carboxyl func-t ional groups, using molecular oxygen as the only oxi-dant . A great number of new catalysts have been sug-gested in recent years for the “clean” oxidat ion of alco-hols with molecular oxygen. Many of them are basedon Ru and, to a smaller exten t , on Pd species invar ious forms.There are several cata lysts that provide high yields
in each class of the rela t ively simple test react ionsdiscussed here. The select ion of an appropr ia te ca ta -lyst may be based on other factors, such as the easysynthesis of the cata lyst and its stability, sufficient lylow C/S mass ra t io, and high react ion ra te (TOF). Acloser inspect ion of the published da ta revea ls tha tsome cata lysts possess surpr isingly low act ivity, andfor most react ion types they do not offer a real a ltern-at ive to the conventional supported Pt-group metalssmater ia ls whose act ivity was discovered in the 19thcentury. The la rge number of pa ten ts reflect s aconsiderable indust r ia l in terest in the applica t ion ofsuppor ted (and promoted) Pt -group meta l ca ta lyst s,
Catalytic Oxidation of Alcohols with Molecular Oxygen Chemical Reviews, 2004, Vol. 104, No. 6 3053
dominant ly in aqueous media . There are encouragingexamples on the indust r ia l acceptance of th is greentechnology, though severa l other project s have beenabor ted due to unsa t isfactory resu lt s. Obviously,replacement of stoichiometr ic and homogeneous cata-lyt ic oxida t ion methods by heterogeneous ca ta lyt icoxida t ions with molecular oxygen has a lready begunin the indust ry, though accelera t ion of th is processnecessita tes more efficien t solid ca ta lyst s.Consider ing the topic from the poin t of view of
subst ra te st ructure, the fast and select ive oxida t ionof a lipha t ic and cycloa lipha t ic a lcohols is cer ta in lyan unsolved problem. The difficu lt ies a re mult ipliedwhen more complex structures, such as polyfunct ion-a lized and thermolabile a lcohols, have to be oxidized.This area may be the most promising and rewardingfield for fu ture development .
6. AbbreviationsA% M/Y meta l (M) suppor ted on Y, meta l content (A)
in wt %A% M1-B%M2/X
bimeta llic ca ta lyst suppor ted on a solid (X);meta l conten t (A , B ) in wt %
A% MOx/Y suppor ted oxide; oxide conten t (A) in wt %AxByCz...On mixed oxide conta in ing elements A, B, C
C/S ca ta lyst /subst ra te mass ra t ioHAp hydroxyapa t iteHT hydrota lciteMn+-X solid mater ia l (X) conta ining Mn+ ions in the
mat r ixOMS octahedra l molecula r sieveS select ivity, in mol %TOF turnover frequency; molar ra t io of conver ted
subst ra te to the act ive component of theca ta lyst , per unit t ime (h-1); it refers tothe average ra te a t h igh conversion
X conversion , in mol %Y yield, in mol %
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