ceria based solid catalysts for organic chemistry

DOI: 10.1002/cssc.201000054

Ceria-Based Solid Catalysts for Organic ChemistryLaurence Vivier* and Daniel Duprez[a]

654 � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2010, 3, 654 – 678

1. Introduction

Ceria is a negative semiconductor oxide in which oxygen va-cancies can be created at high temperatures in vacuum or inan inert gas [Equation (1)] , or at moderate temperatures in thepresence of a reductor [H2, CO, hydrocarbons; Equation (2) iswritten with CO):[1, 2]

2 CeO2 ðsÞ ! 2 CeO2�y ðsÞ þ y O2 ðgÞ ð1Þ

2 CeO2 ðsÞ þ 2y CO ðgÞ ! 2 CeO2�y ðsÞ þ 2y CO2 ðgÞ ð2Þ

The nonstoichiometric phase CeO2�y can be better describedas: [CeIV

1�2yCeIII2y][O2�y(VO88)y] , where VO88 represents an oxygen

vacancy.[3] Equation (2) can be rewritten in a more stoichiomet-ric manner, showing that one O out of four is involved in theCeIV/CeIII reduction process:

2 CeIV þ 4 O�II þ CO ðgÞ ! 2 CeIII þ 3 O�II þ VO�� þ CO2 ðgÞ ð3Þ

Ceria is widely used in catalytic converters for exhaust gasesbecause of its exceptional redox properties. The material isable to store oxygen during the lean phase (i.e. , excess ofoxygen) and to give oxygen back to metal particles during therich phase (when there is virtually no O2 in the gas phase); thisis the so-called oxygen storage capacity (OSC) of ceria. The useof OSC components was first proposed by Ghandi et al. in1976.[4] Since the pioneering works of Yao and Yu Yao[2] and ofSu et al. ,[5, 6] many studies have been devoted to improvingknowledge of the kinetics of OSC[7–9] and of the mechanismsimplicated in surface and bulk oxygen mobility in ceria and re-lated compounds,[10–14] with a special insight into CeZrOx

oxides.[15–19] Oxygen diffusivity in these materials can be mea-sured by 18O/16O isotopic exchange,[20–22] while oxygen species(e.g. , superoxides, peroxides) that might be involved in the dif-fusion process can be investigated by electron spin resonance(ESR)[23–27] or FTIR.[18, 28–31] ESR studies have shown that the pres-ence of a metal (Pt, Rh) drastically changes the nature of theoxygen species; the metal favoring the formation of O� ionsinstead of superoxide species.[26] The presence of certain im-purities in ceria, such as chloride ions, can also affect thenature and amounts of surface oxygen species.[24] Superoxidesspecies give a sharp IR band at 1126 cm�1, while on reducedceria samples surface peroxide species can be recorded at

880 cm�1. OSC measurements are currently carried out by thedynamic technique, in which CO or H2 pulses are injected overthe preoxidized sample at regular time intervals.[32, 33] As men-tioned above, it is generally accepted that one surface oxygenatom out of four is involved in the redox process shown inEquation (2), which represents a theoretical OSC value of5.4 mmol O m�2 for a mean surface density of13.1 mmol O m�2.[22, 33] Reduced ceria is able to dissociatewater[34–38] or carbon dioxide[34, 36] according to Equation (4) orthe reverse of Equation (2), respectively.

2 CeO2�y ðsÞ þ 2y H2O ðgÞ ! 2 CeO2 ðsÞ þ 2y H2 ðgÞ ð4Þ

Ceria also possesses versatile acid–base properties, depend-ing on the nature and temperature of the pretreatment. Ceriacan chemisorb pyrrole, a proton donor, and CO2, an electronacceptor, which is characteristic of strong Lewis-base sites.[30]

These properties are relatively insensitive to the state of ceria(i.e. , reduced or unreduced). Reductive pretreatment may how-ever change the distribution of carbonate species at the ceriasurface (bridged, bidentate, monodentate, polydentate). Onthe basis of CO2 chemisorption studies, Martin and Duprezfound the following scale for the density of basic sites of se-lected oxides (expressed in mmol CO2 m�2): CeO2 (3.23)>MgO(1.77)>ZrO2 (1.45)>10 % CeO2–Al2O3 (0.44)>Al2O3 (0.18)>SiO2 (0.02).[39] These values were obtained by adsorption atroom temperature. At higher temperatures, the amounts ofchemisorbed CO2 decrease. Li et al. have shown that theamount of CO2 that remains chemisorbed at 100 8C would be0.67 mmol m�2 on ceria and 1.40 mmol m�2 on zirconia.[40] Thisproves that ceria possesses a high number of basic sites ofweak or medium strength. Binet et al. also observed that ceriacan chemisorb CO or pyridine, but the band positions stronglysuggest that the Lewis acidity of ceria is significantly lowerthan that of zirconia or titania.[30] In constrast to Lewis basicity,the Lewis acidity would decrease upon reduction of ceria.

Ceria has been the subject of thorough investigations, mainlybecause of its use as an active component of catalytic convert-ers for the treatment of exhaust gases. However, ceria-basedcatalysts have also been developed for different applications inorganic chemistry. The redox and acid–base properties of ceria,either alone or in the presence of transition metals, are impor-tant parameters that allow to activate complex organic mole-cules and to selectively orient their transformation. Pure ceriais used in several organic reactions, such as the dehydration ofalcohols, the alkylation of aromatic compounds, ketone forma-

tion, and aldolization, and in redox reactions. Ceria-supportedmetal catalysts allow the hydrogenation of many unsaturatedcompounds. They can also be used for coupling or ring-open-ing reactions. Cerium atoms can be added as dopants to cata-lytic system or impregnated onto zeolites and mesoporous cat-alyst materials to improve their performances. This Reviewdemonstrates that the exceptional surface (and sometimesbulk) properties of ceria make cerium-based catalysts very ef-fective for a broad range of organic reactions.

[a] Dr. L. Vivier, Prof. Dr. D. DuprezLACCO Laboratoire de Catalyse en Chimie OrganiqueCNRS—Universit� de Poitiers40 Avenue du Recteur Pineau, 86022 Poitiers Cedex (France)Fax:(+33) 5 49453499E-mail : [email protected]

ChemSusChem 2010, 3, 654 – 678 � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemsuschem.org 655

Ceria-Based Solid Catalysts for Organic Chemistry

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All these acid–base or redox surface properties enable ceriato catalyze numerous organic reactions that require thesetypes of active centers. Although it is often difficult to ascribethe catalytic activity to a unique type of site, this Review willbe organized into three parts: reactions preferentially catalyzedon acid–base sites (dehydration and ketonization), reactionspreferentially catalyzed by redox centers (reduction and oxida-tion of organic compounds), and finally reactions that may re-quire both acid–base and redox sites (addition, substitution,isomerization, or ring opening).

2. Dehydration of Alcohols

2.1. Dehydration of 4-methyl-2-pentanol

The dehydration of 4-methyl-2-pentanol by cerium-based cata-lysts could represent an alternative route to the preparation of4-methyl-1-pentene; a monomer for the manufacture of ther-moplastic polymers of superior technological properties(Scheme 1). The unsupported mixed oxides CeO2-ZrO2

[41–43] andCeO2-La2O3,[42, 44, 45] and CeO2-ZrO2 supported on SiO2

[46–48] havebeen used for this reaction, in the vapor phase under normalatmospheric pressure of N2 between 250 8C and 400 8C. It wasobserved that these catalysts exhibit high and stable catalyticactivities. Their acid–base properties govern the competitionbetween dehydration into the desired 1-alkene, the formation

of other undesired alkenes, and parasitic dehydrogenation. Theproduct distribution gives a detailed picture of the acid–baseproperties of the material.

4-methyl-1-pentene is the most abundant product of 4-methyl-2-pentanol conversion. By dehydration, 4-methyl-2-pen-tene and trace amounts of skeletal isomers of C6 alkenes arealso formed. Dehydrogenation leads to 4-methyl-2-pentanone,and high-molecular-weight ketones are formed only in traceamounts. A maximum in 4-methyl-1-pentene selectivity is ob-served with the ceria-rich catalysts, and this selectivity decreas-es with increasing reaction temperature.

An E1cB mechanism, which needs balanced concentrationsof the acid and base sites, as well as a higher strength of thelatter, is probably operating on these catalysts. The selectivityin 1-alkene has been already shown during the dehydration of1-butanol and 2-butanol on CeO2-based catalysts.[49]

2.2. Dehydration of diols

Allylic alcohols can be selectively produced by the vapor-phasedehydration of 1,3-diols over CeO2 between 300 8C and 425 8C(Scheme 2).[50–56] 1,3-Diols are more reactive than other diols

and monoalcohols over CeO2. 2-Propen-1-ol was producedfrom 1,3-propanediol over pure CeO2 with a maximum selectiv-ity of 98.9 % at 325 8C. In the dehydration of 1,3-butanediol, 2-buten-1-ol and 3-buten-2-ol were produced with a sum selec-tivity >99 %. 3-Penten-2-ol was also produced selectively from2,4-pentanediol.[50]

2-methyl-1,3-propanediol is less reactive than 1,3-butanediolor 1,3-propanediol : the methyle group obstructs adsorption onthe surface because of steric hindrance. The corresponding al-lylic alcohol was produced with lower selectivity: decomposi-tion proceeds simultaneously.[51]

Theoretical investigations have indicated an interaction be-tween the oxygen atoms in butan-1,3-diol and cerium cations:butane-1,3-diol preferentially adsorbs on the oxygen-defectsite of the CeO2 (111) surface and is dehydrated at the defectsite. Indeed, in the dehydration of 1,3-butanediol and of 1,4-butanediol into unsaturated alcohols, the activity increased

Laurence Vivier obtained her Ph.D. in

chemistry from the University of Poiti-

ers (France) in 1991. Following a post-

doctoral stay at the University of

Swansea (UK), she returned to Poitiers

at the Laboratoire de Catalyse en

Chimie Organique as an Assistant Pro-

fessor. Her research focuses on hydro-

teatment on sulphide catalysts. In

2008, she joined the team of Dr.

Duprez to pursue her research inter-

ests in the use of biomass for renewa-

ble fuels.

Daniel Duprez obtained his Ph.D. from

Nancy Polytechnicum (France). After a

two-year stay at the Elf Research

Center at Solaize (near Lyon, France),

he joined the Laboratoire de Catalyse

en Chimie Organique de Poitiers

(France) in 1978. He developed several

projects on the use of isotopic ex-

change for measuring oxygen and hy-

drogen mobilities on supported metal

catalysts, with applications in H2 pro-

duction from biomass resources, H2 purification, oxidation, and

DeNOx reactions and water purification processes (CWAO). Rare-

earth oxides are frequently used in these catalytic applications,

either alone or as “active” supports of metals.

Scheme 1. Transformation of 4-methyl-2-pentanol on ceria-based catalysts.

Scheme 2. Dehydration of 1,3-diols into allylic alcohols.

656 www.chemsuschem.org � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2010, 3, 654 – 678

L. Vivier and D. Duprez

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with increasing the CeO2 particle size. The CeO2 (111) facets,more numerous on larger particles, have active sites for the de-hydration reactions.[56]

In the dehydration of 1,4-butanediol, 3-buten-1-ol is pro-duced over CeO2 between 375 8C to 450 8C.[56–58] The better se-lectivity (68.1 %) was observed at 400 8C. Side reactions, suchas isomerization of the initial product, but-3-en-1-ol, hydroge-nation, and dehydrogenation proceed, together with the cycli-zation of 1,4-butanediol to tetrahydrofuran (Scheme 3).

Among the various lanthanide oxides, CeO2 shows proper-ties that differ from those of the other members of the lantha-nide series.[58] In the reaction of 1,5-pentanediol, CeO2 cata-lyzed only undesirable side reactions, such as dehydrogena-tion, as well as dehydration;[59] d-valerolactone and cyclopenta-none being the major products. The authors speculated thatthe redox cycle of Ce4+/Ce3+ on the surface of CeO2 plays akey role in the activation of 1,5-pentanediol. Although 1,3-bu-tanediol is readily activated on CeO2 at 325 8C, higher tempera-tures are needed to activate 1,4-butanediol and 1,5-pentane-diol on CeO2. Thus, both the reactivity and the selectivity overCeO2 decrease in the order of 1,3->1,4->1,5-diols.

Triols, such as 1,2,3-propanetriol (glycerol) and 1,2,3- and1,2,4-butanetriols have been dehydrated to afford the corre-sponding hydroxyketones, while 1,2-propanediol was dehydro-genated to form hydroxyacetone over both ceria-supportedand nonsupported Cu-based catalysts.[60]

2.3. Other dehydrations

4-Hydroxy-2-butanone has been converted into 3-buten-2-oneover various oxide catalysts at 160 8C. Ceria shows a relativelyhigh initial activity but is rapidly deactivated. This deactivationis probably caused by the strong interaction of 3-buten-2-onewith acid sites to form carbon species on the catalyst surface;however, the results of NH3-temperature programmed desorp-tion (TPD) could not explain this deactivation; CeO2 showing amuch lower acidity than other oxides such as Al2O3 and SiO2/Al2O3. In this case, the redox nature of CeO2 may be the causeof this deactivation.[61]

3. Ketonization

3.1. Acid condensation

An important route for ketone production is decarboxylativecondensation of two carboxylic acids. Symmetrical, nonsym-metrical, and arylalkylketones have been obtained by ketoniza-tion of carboxylic acids in the gas phase over ceria-based cata-lysts under flowing conditions, proceeding according to thegeneral equation:

R0COOHþ RCOOH! R0CORþ CO2 þ H2O ð5Þ

Usually, the ketonization by acid condensation was carriedout in the presence of ceria-based catalysts (10–20 % CeO2 sup-ported on SiO2, TiO2, or Al2O3) between 300 and 450 8C.[62–70]

Symmetrical ketones such as 3-pentanone, 6-undecanone,and 7-tridecanone have been obtained from ketonization ofthe appropriate acids.[62, 65] This method was applied to the syn-thesis of nonsymmetrical ketones used as raw materials forpesticides and pharmaceutical products.[66–68] For example,methylcyclopropylketone and methylnonylketone were pro-duced by the condensation of acetic acid with cyclopropane-carboxilic acid and decanoic acid, respectively.[66] In the reac-tion of propanoic acid, the reactivity of the carboxylic acidslightly decreased as its chain length was increased, andbranched acids were less reactive than linear ones.[68]

Aromatic ketones were obtained from aromatic carboxylicacids and acetic acid over CeO2/Al2O3, chosen as an industrialcatalyst for the preparation of the 2-methylacetophenone be-cause of its higher productivity, longer catalyst life, and the lift-ing of legal restrictions on catalyst handling. The catalystsystem can also be applied to the preparation of acetophe-none, nitroacetophenone, and chloroacetophenone.[63] Also,carboxylic acids were selectively reduced to aldehydes by con-densation with formic acid.[70] Moreover, the literature men-tions some patents relating to the synthesis of nonsymmetricalketones from carboxylic acids over CeO2/Al2O3 at 350–580 8C[71–73] and CeO2/TiO2 at 440 8C.[74]

The condensation of acetic acid to acetone[75] or of propano-ic acid to 2-pentanone[56] was also carried out over pure CeO2

from 300 8C. The catalytic sites were suggested to be Lewisacid–base pair sites, with the Lewis acid sites (Ce4+) being re-ducible.[75] The activity towards acid condensation increasedwith increasing particles size, because CeO2 (111) facets arepredominant on larger particles and have active sites for thecondensation reaction of propanoic acid.[56] Stubenrauch et al.had shown previously by TPD that acetone is produced duringthe decomposition of acetic acid only on the CeO2 (111) sur-face.[76]

More recently kinetic factors have been investigated for ke-tonization upgrading processes over a Ce0.5Zr0.5O2 catalyst from175 to 350 8C.[77] This material showed desirable catalytic prop-erties for ketonization of carbohydrate-derived carboxylic acidsin the presence of other monofunctional oxygenated species,such as alcohols or ketones.[78] Under these conditions two dif-ferent reactions take place, esterification and ketonization.

Scheme 3. Transformation of 1,4-butanediol over pure CeO2.



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Both consume hexanoic acid, used as model molecule. Directketonization of esters does not take place in the presence ofacids.

3.2. Ketonization of esters

Glinski and collaborators have studied the ketonization of vari-ous aliphatic or aromatic esters over 20 wt % MOx/S, whereM = Mn, Ce, Zr, or Th and S = Al2O3 or SiO2, in the gas phasebetween 300 and 425 8C.[79–81] The ketonization of ethyl ester inthe presence of these oxide catalysts proceeds according tothe general equation:

2 RCOOC2H5 ! RCORþ CO2 þ H2Oþ 2 C2H4 ð6Þ

In these studies, the highest yields in ketones were alwaysobtained with manganese-based catalysts. 3-Pentanone and 7-tridecanone were formed in ketonization of pure aliphaticesters, ethyl propanoate and ethyl heptanoate. Unfortunately,from pure ethyl benzoate, benzene was obtained instead of di-phenylketone. Dialkyl- and arylalkyl-ketones were obtainedfrom the cross-ketonization of a mixture of aliphatic and aro-matic esters.[80]

The reactivity of tert-butyl heptanoate was higher than thatobserved for ethyl heptanoate over all MOx/Al2O3 catalysts,with M = Mn, Ce, or Zr. Thus, various alkyl heptanoates(C6H13COOR, with R = Me, Et, nPr, iPr, nBu, iBu, sBu, and tBu)were used in the ketonization reaction in the presence of themost active catalytic system, MnO2/Al2O3. In the case of n-alkylheptanoates, the reactivity increased with the elongation ofthe n-alkyl chain.[81]

In the cycloketonization of diethylhexanedioate, only moder-ate yields of cyclopentanone (<35 % over MnO2/Al2O3) wereachieved, accompanied by various amounts of byproducts.[79]

Cycloketonization of dimethylhexanedioate was investigatedover pure CeO2 between 350 and 475 8C.[82] The conversion ofdiethylhexanedioate increased with increasing reaction tem-perature whereas the selectivity to cyclopentanone decreased.This decrease at high conversion was mainly caused by a con-secutive reaction of cyclopentanone into 2-methylcyclopenta-none due to alkylation with methanol, produced by the cyclo-ketonization.

Long-carbon-chain ketones (C17H35COC17H35, C15H31COC15H31,CH3COC17H35, CH3COC15H31) were also obtained from methylesters of fatty acids (essentially C17H35COOCH3 andC15H31COOCH3), in methanol at atmospheric pressure at 385 8C(optimal temperature), over catalysts containing Sn�Ce�Rhoxides in a molar ratio 90:9:1 (total yield: 63 %, conversion:96 %).[83] A similar catalyst was used to transform methyl lau-rate (C11H23COOCH3) to 12-tricosanone (C11H23COC11H23).[84]

3.3. Dimerization of alcohols

During the alkylation of phenol with 1-propanol over CeO2/MgO catalysts under atmospheric pressure of helium, Satoet al. observed that the formation of propanal and 3-penta-none and the conversion of 1-propanol to 3-pentanone in-

creased with increasing CeO2 content.[85] Elsewhere, Plint et al.studied the reaction of a series of primary and secondary alco-hols containing n carbon atoms under oxidative conditions.Symmetrical ketones with 2n�1 carbon atoms were producedin the presence of O2 over 40 % CeO2/MgO (450 8C, 1 atm).There was no dimerization reaction with 2-methyl-2-propanol.The yield of ketone increased with chain length from C2–C4

and then reached a maximum for the C4–C7 reactant; the con-version being consistently high (>90 %).[86, 87]

A reaction mechanism in which alcohol is oxidized to alde-hyde and then carboxylic acid [Equation (7)] following by thecoupling of two equivalents of acid to give the symmetricalketone and CO2 [Equation (8)] , is proposed according to thegeneral scheme:

RCH2OH! RCHO! RCO2H ð7Þ

2 RCO2H! RCORþ CO2 þ H2O ð8Þ

The reaction of a 1:1 mixture of 1-hexanol and 1-heptanolproduced a statistical yield of the three expected ketones.[87]

Under nonoxidative conditions, 1-propanol was preferentiallyconverted into 3-pentanone over CeO2-Fe2O3 catalysts at450 8C and propanal, 3-hydroxy-2-methylpentanal, and n-propyl-propionate were observed as by-products.[88] The addi-tion of Fe2O3 to CeO2 enhances the ability of CeO2 for the cata-lytic dehydrogenation of 1-propanol to propanal, withoutlosing the ability to dimerize propanal. The formation of 3-pen-tanone from 1-propanol over CeO2-Fe2O3 proceeds via aldoladdition of propanal into 3-hydroxy-2-methylpentanal, fol-lowed by decomposition into 3-pentanone, while n-propylpro-pionate is formed as a mere by-product [Equation (9)] .

2 RCHO! aldol! RCORþ H2 þ CO ðor CO2Þ ð9Þ

This reaction was applied to the cyclization of 1,6-hexanediolinto cyclopentanone, an useful intermediate for medical andperfume products.[89] The cyclization of 1,6-hexanediol was se-lectively catalyzed by CeO2-MnOx with a Mn content of 10–30 mol %: cyclopentanone was produced with a selectivity of80 mol % at 450 8C. A possible reaction pathway over ceria-based catalysts is illustrated in Scheme 4.

Elsewhere, Sn�Ce�Rh oxides already used in the condensa-tion of methyl esters of fatty acids have shown high activityand selectivity at relatively low temperatures in the ketoniza-tion of n-butanol. At 350 8C, 4-heptanone was obtained with

Scheme 4. Probable cyclization reaction pathway over ceria-based catalysts.



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89 % selectivity and 88 % n-butanol conversion.[90, 91] Introduc-ing a cerium dopant to the tin basic dioxide structure causedthe appearance of strong acidic centers of Lewis type at thesurface. As the selectivity to 4-heptanone increases with thepresence of cerium, the Lewis acidic sites become more in-volved in the mechanism of alcohol, aldehyde, or acid conden-sation.

4. Reduction

Ceria is able to change reversibly from Ce4+ under oxidizingconditions to Ce3+ under reducing conditions. Oxygen atomsin CeO2 units are very mobile and easily leave the ceria lattice,giving rise to a large variety of nonstoichiometric oxides withthe two limiting cases: CeO2 and Ce2O3.

4.1. Hydrogenation of C=C bonds

4.1.1. Hydrogenation of phenol to cyclohexanone

Cyclohexanone is a key raw material in the production of bothcaprolactam for Nylon-6 and adipic acid for Nylon-6,6. Industri-ally, cyclohexanone is produced either by the oxidation of cy-clohexane or by the hydrogenation of phenol to cyclohexanol,followed by dehydrogenation of cyclohexanol. Selective hydro-genation of phenol to cyclohexanone is attractive in terms ofcapital cost and energy saving.

The selective hydrogenation of phenol to cyclohexanonewas carried out in gas phase over supported Pd catalysts.[92, 93]

The catalytic performance was improved by a modification ofthe electronic surroundings of Pd, induced by a promoter orby a modification of the acid–base characteristics of the sup-port, leading to a change in the adsorption–desorption equilib-rium of reactants and products. Similar to La2O3, CeO2 as sup-port provides a better activity and a good stability to cata-lysts.[92]

The high surface area mesoporous oxide support gives riseto well dispersed and stable metal particles on the surface andthen has some beneficial effect on the catalytic performance.The vapor-phase hydrogenation of phenol, at atmosphericpressure, over 3 % Pd supported on mesoporous CeO2 (Pd/CeO2-Ms) at 180 8C produced a mixture of cyclohexanone(about 50 %), cyclohexanol (35 %), and cyclohexane (15 %) witha phenol conversion of about 80 %. The selectivity dependedon the modes of phenol adsorption, which are governed bythe nature of the support. On the Pd/CeO2-Ms catalyst, underthe reported experimental conditions, there was a significantreduction of the CeO2 surface, which resulted in the formationof nonstoichiometric CeO2 creating acid and basic sites. Overbasic sites, nonplanar-adsorbed phenol led to the formation ofcyclohexanone, while coplanar-adsorbed phenol led to the for-mation of cyclohexanol.[93]

More recently, the hydrogenation of phenol to cyclohexa-none was carried out in the liquid phase with ethanol as sol-vent in order to improve the selectivity, because the reactioncould be performed at relatively low temperature.[94, 95] Themaximum phenol selectivity for cyclohexanone over 5.8 % Pd-

Ce-B supported on hydrotalcite reached 80 %, with a phenolconversion of 82 %.[94] A similar conversion and selectivity wereobtained by the same authors on Ce-doped Pd-B amorphousalloy catalysts.[95] The promoting effect of the Ce-dopant onthe catalytic performance could be attributed to stabilizationof the amorphous structure of the Pd-B alloy by cerium; theelectron-enriched Pd active sites, owing to the electron-dona-tion from cerium; and the increase of surface basicity resultingfrom the formation of Ce2O3.

4.1.2. Hydrogenation of 1,3-butadiene

The hydrogenation of 1,3-butadiene was carried out in the gasphase at atmospheric pressure between 47 to 107 8C. The pres-ence of CeO2 in Pd-CeO2 supported on Al2O3 catalysts favoredhydrogenation at the 1,2 position of the 1,3-butadiene mole-cule, increasing the selectivity towards 1-butene.[96] The ab-sence of butane as reaction product indicated that the adsorp-tion strength of 1,3-butadiene was reduced by the presence ofcerium, which modifies the electronic structure of Pd, therebyavoiding total hydrogenation.

4.1.3. Hydrogenation of acrylonitrile

Selective gas-phase hydrogenation of acrylonitrile (CH2=CH�CN) over low-loaded Pd (0.05 wt %)/Al2O3 catalysts doped withvarious contents of cerium gave propionitrile with a selectivitynearly 100 %. The addition of cerium improved the activity andthe stability of these catalysts.[97] The sintering of the dispersedpalladium particles was retarded by the addition of cerium andthe catalytic activity was preserved. Elsewhere, the presence ofPd facilitated the reduction of Ce4+ to Ce3+ to form new activesites.[97]

4.1.4. Hydrogenation of mesityl oxide

Mesityl oxide (4-methyl-2-penten-2-one) was selectively hydro-genated in the gas phase at 175 8C over 2 CuO-CeO2 and 2 Cu-CeO2 reduced catalysts to produce methyl isobutyl ketone (4-methylpentan-2-one), an important chemical used at the in-dustrial level.[98] The reduced sample was more active than theoxidized one; the selectivities being 93 % and 100 %, respec-tively. Binary copper–cerium intermetallic compounds (CeCu2)are interesting precursors to provide new supported coppermaterials.

4.1.5. Hydrogenation of sunflower oil

The selective hydrogenation of ethyl esters of traditional sun-flower oil, a mixture comprising linoleic acid C18:2 (9,12) Z,Z(60.11 %), oleic acid C18:1 (9) Z (27.49 %), and stearic acidC18:0 (3.72 %), was carried out at low temperature (40 8C) inethanol as solvent in the presence of supported palladium cat-alysts (Scheme 5). The aim was to selectively hydrogenate lino-leic acid C18:2 (9,12) Z,Z towards oleic acid C18:1 (9) Z, avoid-ing Z–E isomerization, position isomerization, and complete hy-drogenation. The use of CeO2 as oxide support to deposit pal-



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ladium did not improve the selectivity toward the C18:1 (9) Zor (12) Z compared to Pd/SiO2.[99]

4.1.6. Hydrogenation of acetylene

By using Au/CeO2 as catalyst in acetylene hydrogenation in thegas phase at 300 8C, a selectivity of 100 % toward ethylene wasobtained with a high conversion, which depended on the H2/C2H2 ratio. Above 300 8C, the only byproduct was methane,formed by carbene intermediates that also polymerized, givingrise to deactivation. The high selectivity could be explained bythe difference in the strength of adsorption onto the gold sur-face between acetylene and ethylene.[100]

4.1.7. Hydrogenation of benzene

The hydrogenation of benzene in the gas phase at atmospher-ic pressure can be used to characterize the size of metallic par-ticles of supported ceria-based catalysts.[101–104] With this reac-tion it is possible to determine the amount of the accessiblemetallic atoms of low-loaded supported rhodium cata-lysts.[102, 104]

A Ni/CeO2 catalyst exhibited a different behavior from thatobserved with Al2O3 or SiO2 supports. The modification of thecatalytic properties of Ni/CeO2 catalysts with reduction pre-treatment is correlated to a transformation of the CeO2 sup-port and to strong interactions between theses species andmetal particles.[101] In another study, nickel catalysts supportedon ceria, synthesized by g-radiolysis, were tested in the hydro-genation of benzene. At 100 8C, the catalyst completely con-verted benzene to cyclohexane and remained stable for atleast 20 h. The high catalytic performance of Ni/CeO2 was at-tributed to the high dispersion of nickel and to the promoterrole of the support, through the formation of Ni�Ce phases.[103]

The partial hydrogenation of benzene to cyclohexene is ofgreat industrial interest: cyclohexene can be used in the syn-thesis of various organic compounds.[105, 106] Ru/CeO2 catalystsare very promising systems for cyclohexene formation throughpartial benzene hydrogenation in a three-phase medium, in

the presence of water and TiCl3. The maximum yield in cyclo-hexene (about 17 %) was obtained with Ru/CeO2 catalysts,noncalcinated and reduced at 500 8C or 750 8C.[105]

The partial hydrogenation of benzene to cyclohexene wasalso carried out over Ru–Ce catalysts supported on siliceousmaterials such as SBA-15, in the presence of ZnSO4 in aqueoussolution. The existence of the CeIII species decreased thenumber of exposed Ru atoms, increased the number of elec-trons on metallic Ru, and enhanced the hydrophilicity of thecatalyst. The maximum yield of cyclohexene (53.8 %) was ob-tained on a RuCe/SBA-15 catalyst, with a molar ratio Ce/Ruequal to 0.4.[106]

4.1.8. Hydrogenation of biphenyl

Hexagonal mesoporous silica (HMS) was used as support forthe preparation of Au catalysts, and was tested in the liquid-phase hydrogenation of biphenyl at 5 MPa and 215 8C (in a so-lution of n-tetradecane, with about 10 % n-hexadecane). Modi-fication of HMS by Ce led to Au-supported catalysts that weremore stable with respect to sintering. In the Au/HMS–Ce cata-lyst, Ce was not incorporated into the framework of HMS, lead-ing to clusters of Au and CeO2. However, further investigationson the nature of the Au–CeO2 interaction could yield more ex-planations with regard to the performance of Au/HMS–Ce cat-alyst.[107]

4.2. Hydrogenation of C=O bonds

4.2.1. Hydrogenation of a-b-unsaturated aldehydes

The selective hydrogenation of a-b-unsaturated aldehydes tounsaturated alcohols is an important reaction in the produc-tion of many pharmaceutical, agrochemical, and fragrancecompounds. The hydrogenation of the C=C bond is thermody-namically more favorable than the C=O hydrogenation, andlow yields of the desired product are obtained with the con-ventional hydrogenation catalysts.

Cerium-based platinum catalysts have been extensively stud-ied for the hydrogenation of crotonaldehyde (CH3�CH=CH�CHO)[108–118] and citral ((CH3)2C=CH�(CH2)2�C(CH3)=CH-CHO).[119, 120] The activation of the carbonyl bond is induced bythe presence of oxygen vacancies sites located at the interfacebetween ceria and the platinum particles.

Indeed, the selective hydrogenation of a-b-unsaturated alde-hydes has been used as probe reaction to study the existenceof a “strong metal–support interaction” (SMSI) effect in ceria-supported and -promoted noble catalysts. Ceria is able to formoxygen vacancies and intermetallic compounds after reductiontreatment at relatively high temperatures.

Touroude and collaborators have studied the selective hy-drogenation of crotonaldehyde on Pt/CeO2 in the gas phase atatmospheric pressure.[108–110] On chlorine-free Pt/CeO2, thecrotyl alcohol selectivity increased up to more than 80 % (con-version: 45 %) when the reduction temperature of the catalystsreached 700 8C. The presence of chlorine, during the catalystsynthesis, preserves the catalytic properties of platinum metal

Scheme 5. Hydrogenation and isomerization of linoleic acid C18:2 (9,12) Z,Z.



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for the hydrogenation of C=C bond; chlorine atoms aroundplatinum particles inhibit the diffusion of cerium atoms insidethe metal particles and prevents the formation of CePt5 alloy.By controlling the nanostructure, size, and morphology of sup-ported platinum particles, the authors showed that it is possi-ble to orientate the selectivity in the hydrogenation of croto-naldehyde.[110]

The presence of zinc facilitated the reduction of surfaceceria, thereby increasing the potential number of Ce3+–Ptmetal interface sites, particularly in cases where small Pt parti-cles were located in ceria rich zones of the support.[113–115] Inthe case of vapor-phase hydrogenation of crotonaldehyde, theoverall catalytic activity increased significantly after reductionat 500 8C in the zinc-containing catalyst, and furthermore, theselectivity toward the hydrogenation of carbonyl bond is im-proved. Pt on mesostructured CeO2 nanoparticles embebbedwithin ultrathin layers of highly structured SiO2 binder showedthe highest reported activity, with 80 % selectivity for the che-moselective hydrogenation of crotonaldehyde.[118] By increasingthe reduction temperature, the number of Pt-CeO2�x interfacialsites, which are responsible for activating the carbonyl bond,increased.

The hydrogenation of citral was carried out in the liquidphase at 50 8C in ethanol[119] or at 70 8C in isopropanol.[120] Theformation of geraniol (E isomer) has been observed as the soleproduct on Pt/CeO2 and has been attributed to the influenceof the SMSI state in the selective hydrogenation of C=Obond.[119]

On a Pt/C catalyst promoted with highly dispersed ceria, themain products of the hydrogenation of citral were citronellal(hydrogenation of the conjugated C=C bond), the unsaturatedalcohols geraniol and nerol, and the saturated alcohol citronel-lol (by hydrogenation of the C=O bond of citronellal).[120] Onthe one hand, the creation of new Pt–CeOx sites at the metal/support interface act as Lewis acid sites able to activate the C=

O bond of the citral molecule; on the other hand, the exis-tence of an electronic interaction between the reduced ceriaparticles and the active metal leads to an increase in electrondensity on the platinum particles, with subsequent weakeningof the adsorption of citral via the C=C bond. Moreover, whentin is added, the ceria reducibility is increased. The presence ofSnn+ species, also able to act as Lewis acid sites, on the surfaceof platinum particles and/or in their close vicinity could ac-count for the increase in selectivity to unsaturated alcoholswith reduction temperature. The increase of conversion afterreduction at high temperature in these catalysts could be alsoexplained by the creation of new Pt�Snn+ sites active for hy-drogenation of the C=O bond in the citral molecule.

Previously, Barrault et al. have studied the hydrogenation ofcinnamaldehyde (C6H5�CH=CH-CHO) in the liquid phase withpropylene carbonate as solvent over cobalt catalysts supportedon activated carbon, and showed that the addition of ceriumto cobalt increased the selectivity to unsaturated alcohol with-out decreasing of the activity.[121] Elsewhere, the use of ceriumon Ru-based catalysts supported on alumina and activatedcarbon increased the selectivity to the unsaturated alcohols inhydrogenation of crotonaldehyde (in gas phase) and of citral

(in liquid phase with isopropanol as solvent).[122] However, theobserved activities were very low.

More recently, Campo et al. studied the influence of the spe-cific surface area of the support on the selective hydrogena-tion of crotonaldehyde on Au/CeO2.[123–126] They showed thatthe high surface area catalyst (Au/HAS-CeO2, with 240 m2g)�1

was active and highly selective towards the hydrogenation ofthe C=O bond either at 120 8C and atmospheric pressure[123–125]

or at 80 8C in the liquid phase (solvent: isopropyl alcohol).[126]

Other Au/CeO2 catalysts with lower specific surface areasshowed a rather low selectivity. The high selectivity is an intrin-sic characteristic of gold particles (particle size lower than4 nm on Au/HAS-CeO2 and higher than 9 nm on other Au/CeO2 catalysts), though ceria plays an important role as aresult of its redox and acid–base properties.

The selectivity to the unsaturated alcohol is governed by dif-ferent factors: the nature of the active metal, metal particlessize, support effects, and presence of promoters or bimetallicphases.

4.2.2. Enantioselective hydrogenation

Mixed nickel–cerium oxides, using tartaric acid as modifier,were used for studying the enantioselective hydrogenation ofmethylacetoacetate in methyl 3-hydroxybutyrate (Scheme 6).

X-ray photoelectron spectroscopy (XPS) and FTIR analysis evi-denced that the modifying agent reacts with the metallicnickel to give nickel tartarate. In the presence of this modifier,the reduction of Ce4+ to Ce3+ was improved. The tartarate saltis stabilized by these Ce3+ species in close vicinity of Ni2+,forming a complex with methyl acetoacetate. Thus, a stablesix-membered ring complex is formed between a hydroxylgroup of tartarate, the reactive C=O bond, and the methylenicgroup (acidic H) of methylacetoacetate and gives rise to asingle product, having a specific configuration depending onthe absolute configuration of the modifying agent.[127]

Another, similar reaction concerned the enantioselective hy-drogenation of 1-phenyl-1,2-propanedione[128] (Scheme 7). Thisreaction was carried out in the liquid phase (cyclohexane wasused as solvent) at 25 8C and a dihydrogen pressure of 40 barover an iridium-supported catalyst promoted by ceria, in thepresence of cinchonidine as modifier. The presence of Ce in

Scheme 6. Hydrogenation of methylacetoacetate.

Scheme 7. Hydrogenation of 1-phenyl-1,2-propanedione.



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the Ir/SiO2 produced a slight increase in both the activity andthe enantioselectivity (formation of (R)-2-hydroxy-1-phenyl-1-propanone). The cerium oxide species were mainly present onthe silica support and contributed to the formation of Ird+ spe-cies that were responsible for C=O bond polarization and reac-tion rate enhancement.

4.2.3. Hydrogenation of carboxylic acids to aldehydes

For the hydrogenation of aromatic carboxylic acids, variousmetal oxides such as CeO2 have shown high activity and selec-tivity to corresponding aldehydes.[129–133] The hydrogenation ofbenzoic acid over CeO2 proceeds at up to 350 8C in gas phase;the selectivity to benzaldehyde was more than 95 % and theactivity was controlled by the number of oxygen vacanciesthat are produced under the reaction conditions. The carboxyl-ic acid deoxygenates with the help of an oxygen vacancy ac-cording to the Mars–Van Krevelen mechanism, forming an acy-lium ion that is hydrogenated to aldehydes. An enhancementof the catalytic activity of CeO2 at low temperatures could beachieved by addition of the promoters, as Mn, Zr, In, and Pboxides.[130] However, ceria catalysts show little deactivation instability test owing to coke formation and the valence changesof Ce over the catalyst.[132, 133]

To limit the use of CeO2, owing to its high cost, some au-thors have used mixed CeO2–Al2O3 oxides for the hydrogena-tion of benzoic acid to benzaldehyde. To improve their per-formances, the simultaneous addition of Mn and K to thesesoxides is necessary.[134] For hydrogenation of aliphatic carboxyl-ic acids that have two a-hydrogen atoms, CeO2 shows a lowselectivity because undesirable ketonization occurs.[131]

4.2.4. Hydrogenation of aldehydes to alcohols

The reduction of benzaldehyde has been carried out at 300 8Cin a helium or dihydrogen atmosphere over simple metaloxides as CeO2. CeO2 was not proved to be the better catalystfor either the Cannizzaro reaction (under helium) to producebenzyl alcohol and benzoate or direct hydrogenation (underdihydrogen) when compared with other oxides.[135]

Under helium, the bare CeO2 support was more active thanthe corresponding copper-supported catalyst, which is due tothe lesser amount of active hydroxyl groups in the surface ofsupported catalyst than of bare support.[136] Indeed, the Canni-zzaro reaction consumes surface OH groups to form benzyl al-cohol and benzoate surface species.

Under dihydrogen, a higher activity and a higher selectivityto benzyl alcohol were obtained on an irreducible support. Ona reducible support such as CeO2, the activity was lower andcould be explained by a strong metal–support interaction.Indeed, only traces of benzyl alcohol were observed on Cu/CeO2 ; the selectivity to toluene being above 90 %. More re-cently, a significant amount of benzyl alcohol was producedon Ni/CeO2 under atmospheric pressure of H2 at only a lowtemperature (70 8C). The selectivity to toluene increased withincreasing temperature. Toluene is the product of consecutivebenzyl alcohol hydrogenolysis. This mechanism could involve

hydride species, which could be formed on nickel metal sup-ported on a reducible support such as CeO2.[137]

Elsewhere, Ce-doped Ni�B amorphous alloy catalysts haveexhibited excellent selectivity to furfural alcohol during liquid-phase hydrogenation of furfural (ethanol used as solvent). Thepromoting effect of Ce dopant could be interpreted with amechanism of activation and hydrogenation of C=O bond.[138]

The cerium in a low-valent state (Ce3+) on the surface couldact as Lewis adsorption sites, which have a strong affinity forthe oxygen atom of a carbonyl function and cause a polariza-tion of the C=O bond. This polarization favors nucleophilicattack of the carbon atom by hydrogen dissociatively adsorbedon the neighboring Ni active sites.

4.2.5. Hydrogenation of esters

Diols can be produced from the hydrogenation of esters overRuSn-based catalysts supported on various oxides in liquidphase with dioxane as solvent. Unfortunately, CeO2 was notproved to be the better support. However, an enhancement ofthe activity could be attributed to an interaction of the C=Obond with the exposed cations of the reducible oxide, that is,an SMSI effect.[139]

4.2.6. Hydrogen transfer reactions

The vapor-phase hydrogen transfer reaction of cyclohexanonewith isopropyl alcohol as hydrogen donor was carried out onmixed oxides of CeO2 and ZnO with a high surface area, to in-vestigate the effect of rare earth oxide on the activity of ZnO.Addition of ceria into zinc oxide was found to increase the cat-alytic activity. Cyclohexanol was the only product observed inthis reaction, with a selectivity greater than 98 %. The CeO2–ZnO materials exhibited excellent redox and moderate acid–base properties. The addition of ceria to ZnO influenced theparticle morphology, surface area, and acid–base proper-ties.[140–142]

4.3. Hydrogenation of C�N bonds

Catalytic hydrogenation of nitriles is an important route to pro-duction of amines, which are of practical importance, in partic-ular primary amines, as chemicals and intermediates.

The gas-phase hydrogenation of acetonitrile over variousPd-based catalysts gives a mixture of ethylamine, diethylamine,and triethylamine. The use of a CeO2 support is significant forpreparing PdZn, PdGa, or PdIn alloy species on its surface. Theactivity can then be enhanced while maintaining high selectivi-ty to ethylamine (97 % at 170 8C over Pd/ZnO/CeO2). Ceria facil-itates the reduction of ZnO to Zn, which is then alloyed withPd.[143]

The formation of primary amines from nitriles on copper cat-alysts is normally followed by the formation of secondaryamines. Therefore, the supported copper–lanthanide oxidesare active and very selective for the propionitrile gas phase hy-drogenation to n-propylamine; the 2Cu-CeO2 being the moreactive. The basicity of the copper–lanthanide oxides seems to



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play a key role in this reaction, and the selective formation ofprimary amine is due to the lack of acid sites, which areknown to catalyze condensation reactions that lead to secon-dary or tertiary amines.[144, 145]

4.4. Other hydrogenation reactions

Highly dispersed copper promoted by nickel or cobalt support-ed on CeO2 catalysts exhibit high conversion (96.6 %) for thegas-phase hydrogenation of ortho-chloronitrobenzene toortho-chloroaniline with a high selectivity (98 %) and withoutfast deactivation.[146] The high activity and stability of these bi-metallic catalysts was attributed to a better dispersion and theformation of smaller particles as well as to a high specific sur-face area of the catalyst.

5. Oxidation

The oxidation of organic compounds is one of the most impor-tant reactions for synthesis of fine chemicals. They are manyexamples of oxidation reactions involving chromium-, manga-nese-, or vanadium-based compounds that are used in stoi-chiometric quantities and present serious disadvantages be-cause they are expensive, toxic, and produce equimolar quanti-ties of waste that are often difficult to separate from the de-sired products. Owing to ceria’s high oxygen storage capacityand good catalytic properties, the use of ceria-based materialshas been intensively investigated and applied in the catalyticoxidation reaction.

5.1. Aerobic oxidation of alcohols

Oxidation of alcohols to the corresponding carbonyl com-pounds is an important transformation in organic synthesis; al-dehydes and ketones being an important class of compoundsin organic chemistry. The oxidation of primary alcohols to alde-hydes is an interesting process in perfumery industry. The aero-bic oxidation of ortho- and para-hydroxybenzyl alcohol selec-tively produced the corresponding aldehydes with good yields,while the selectivity of meta-hydroxybenzyl alcohol was moretowards the corresponding acid at the expense of aldehyde.The selectivity was improved by reaction in aqueous methanoland the addition of CeCl3. The catalytic role of CeCl3 in the cat-alyst system Pt/C–CeCl3–Bi2(SO4)3 is not clear, but it might pro-tect the generated aldehyde from further oxidation by acetali-zation.[147]

The reaction of ethanol on unreduced and H2-reduced CeO2

and 1 wt % Pd/CeO2 has been investigated by steady state re-actions, temperature programmed desorption (TPD), andin situ FTIR spectroscopy. Steady-state reactions have shown azero-reaction-order dependency for dioxygen. The conversionof ethanol was increased by the addition of Pd, from 15 and30 % on CeO2 and H2-reduced CeO2, to 71 and 63 % on Pd/CeO2 and H2-reduced Pd/CeO2, respectively. Ethanol was con-verted into acetaldehyde, which in turn can react to give vari-ous compounds (e.g. , acetone, crotonaldehyde, CO, CO2, meth-ane, benzene). Benzene formation was detected only on Pd/

CeO2 catalysts, with the H2-reduced Pd/CeO2 catalyst decreas-ing benzene formation to almost negligible amounts. The H2-reduction of the oxide surface inhibited the b-aldolizationroute owing to a considerable decrease of the Lewis-basesites, oxygen anions.[148]

A catalyst of ruthenium combined with cobalt hydroxideand cerium oxide (Ru–Co(OH)2–CeO2) exhibited a high activityfor the oxidation of various alcohols in liquid phase with ben-zotrifluoride as solvent, in the presence of dioxygen. Allylic,benzylic, and secondary alcohols gave high yields of the corre-sponding carbonyl compounds.[149] The oxidation of primaryaliphatic alcohols led to the formation of corresponding car-boxylic acids. a,w-Primary diols were selectively transformedinto the corresponding lactones. In the case of 1,4-pentanediolhaving primary and secondary hydroxyls, methyl-g-butyrolac-tone was obtained with 87 % yield by an intramolecular com-petitive oxidation (Scheme 8).[149, 150]

With Ce-free Ru catalysts, the oxidation of primary alcoholsled to the formation of aldehydes. The oxidation of this inter-mediate to carboxylic acid was slow. Moreover, when 2,6-di-tert-butyl-para-cresol was added in the oxidation of 1-octanolas a radical scavenger, octanal was formed without formationof octanoic acid. The high activity of the Ru–Co(OH)2–CeO2 cat-alyst might be due to the high oxidation state of the Ru spe-cies (RuIV), arising from the Co atoms in the vicinity of the CeO2

particles. The radical process of the aldehyde oxidation mightbe facilitated by synergism between the Ru, Co, and Ce com-ponents.[149, 150]

Au/CeO2 catalysts were also effective for the selective oxida-tion of primary alcohols (benzyl alcohol) to aldehydes, undersolvent-free conditions at 100 8C in the presence of O2. Formore acidic supports, such as Fe2O3, subsequent oxidation ofaldehydes to the corresponding acids occurred.[151]

Gold nanoparticles supported on ceria are excellent generalheterogeneous catalysts for the aerobic oxidation of alco-hols.[152–156] The combination of small-crystal-size gold (2–5 nm)and nanocrystalline ceria (ca. 5 nm) led to a highly active, se-lective, and recyclable catalyst for the oxidation of alcoholsinto aldehydes or ketones using dioxygen at atmospheric pres-sure as oxidant, in the absence of solvent and base. The nano-meter-scale ceria surface stabilized the positive oxidationstates of gold by creating Ce3+ and oxygen-deficient sites inthe ceria. Aliphatic primary alcohols were more reluctant to un-dergo oxidation in the absence of solvent. Notably, they pre-dominantly gave the corresponding ester with high selectivity.The esters were directly formed by the hemiacetal intermedi-ate, which was dehydrogenated (Scheme 9).[152]

Abad et al. proposed a mechanism for the aerobic oxidationof alcohols over Au/CeO2, where the alcohol is adsorbed on

Scheme 8. Oxidation of 1,4-pentanediol to methyl-g-butyrolactone.



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Lewis-acid sites to give a metal alkoxide that subsequently un-dergoes a rapid hydride transfer from C�H to Ce3+ and Au+ togive the ketone and Ce�H and Au�H. In the presence of dioxy-gen, cerium-coordinated superoxide (Ce�OO·) species areformed and, by hydrogen abstraction from Au�H, becomecerium hydroperoxide, which is responsible for the formation,after reduction of CeIV, of the initial Au+ species. The absenceof gold would render this step impossible and lead to a deple-tion of CeIII.[152]

An allylic alcohol, such as 1-octen-3-ol, undergoes a chemo-selective oxidation in the presence of Au/CeO2 catalysts to thecorresponding ketone without oxidizing or isomerizing the C=

C double bond. In contrast to this, Pd/CeO2 catalysts promotea considerable degree of C=C double bond isomerization (1,2-migration) with the formation of 3-octanone as the final prod-uct.[153, 154] Au/CeO2 is a good catalyst for the oxidation of allylicalcohols without solvent or in organic media (toluene as sol-vent). The chemoselectivity of Pd or Au�Pd for the solventlessoxidation of this family of alcohols is very low when comparedwith Au catalysts and this selectivity can be correlated to thestability and concentration of metal hydrides, Au�H and Pd�H.[154] Primary aliphatic alcohols are selectively oxidized to thecorresponding aliphatic aldehydes up to moderate conver-sions. When the conversion increased, the selectivity towardsthe aldehyde decreased significantly, owing to overoxidationof the aldehyde to the corresponding carboxylic acid. The ac-tivity of a gold catalyst for the aerobic oxidation of alcohols in-volves the presence of a high density of positive gold atomsthat could act as Lewis acid sites that coordinate with alcoholsto form gold alcoholates and also accept hydrides. In thisregard, the role of the support should be, on the one hand, toprovide stability for positive gold species by interfacial gold–support interactions, and on the other hand, to facilitateoxygen activation to promote the reoxidation of metal hy-drides.[156]

The oxidation state of gold particles deposited on differentsupports such as CuO-CeO2 and CeO2 was investigated duringthe liquid-phase catalytic aerobic oxidation of 1-phenylethanol(in toluene) using in situ XAFS combined with FTIR for productanalysis. A correlation between the oxidation state of Au andcatalytic activity was observed for Au/CuO–CeO2. The 1-phenyl-ethanol conversion increased with concomitant reduction ofAu species. Different behaviors were observed for Au/CeO2,with the activity decreasing simultaneously with the reductionof Au species. However, this deactivation is not directly relatedto reduction of the gold species.[157]

Au/CeO2 catalysts with various gold particle sizes showed amoderate catalytic activity and high selectivity in the liquid-phase oxidation of benzyl alcohol to benzaldehyde in mesity-lene or in toluene. The catalytic behavior of this reaction wasaffected by the gold particle sizes, showing highest activity forthe catalyst containing gold particles of 6.9 nm averagesize.[158] The effect of the adsorption of the two thiols, n-octa-decanethiol (ODT) and mercaptoacetic acid (MAA), has beenstudied on CeO2-supported gold catalysts with different Auparticle sizes (2.1 and 6.9 nm). Upon addition of 10 mol %thiol/Autotal, an almost complete loss of activity in the aerobicoxidation of benzyl alcohol was observed when the Au cata-lysts were poisoned by ODT, while at the same concentration,the MAA adsorption had relatively little influence on activity.ODT first binds to crystal facets of the Au particles and laterforms reversibly bound species on the surface, likely adsorbingon edge and corner sites. On the other hand, MAA stronglybinds to the edge and corner sites on the supported Au. Theadsorption of MAA on crystal facets is thermodynamically theleast stable configuration.[159]

A mesoporous CeO2 crystalline film used as support wasloaded with gold particles of about 5 nm. The resulting Au/CeO2 composite showed a good catalytic activity and stabilityfor benzyl alcohol aerobic oxidation in absence of solvent andbase.[160]

The catalysts ruthenium hydroxide and manganese oxidesupported on cerium oxide Ru/MnOx/CeO2 show a high catalyt-ic activity for the oxidation of alcohols to the correspondingcarbonyl compounds in liquid phase with a,a,a-trifluoroto-luene as solvent. Nonactivated aliphatic alcohols requiredlonger reaction time for their oxidation than the other activat-ed benzylic and allylic alcohols. A primary aliphatic alcohol, 1-octanol, was less reactive than a secondary alcohol, 2-octanol.The particular advantage of this catalyst is the smooth oxida-tion of alcohols at 27 8C under dioxygen atmosphere. Suchhigh catalytic activities are attributable to cooperative actionamong the Ru species, MnOx, and CeO2 in the catalyst.[161] Thecatalytic activity could significantly be improved by depositionof the ternary RuMnCe oxidic mixture on redox-active sup-ports, especially on ceria.[162]

Recently, silver catalysts were proposed as a promising alter-native, being less expensive than Au or Pt catalysts and appli-cable to a wide variety of alcohols.[163] A Ag/SiO2 catalyst thatacts as efficient catalyst (but only in the presence of ceria:10 wt % Ag/SiO2 mixed with ceria in a ratio of 2:1) gave thebest catalytic performance in the selective liquid-phase oxida-tion of various benzyl alcohols (solvent: toluene). The negativeeffects of electron-withdrawing groups in the benzyl alcoholoxidation suggest a mechanism where metallic silver acts asmain component for the dehydrogenation via a cationic inter-mediate. The role of cerium can rather be ascribed to the acti-vation of molecular oxygen.

5.2. Dehydrogenation of alcohols

An infrared spectroscopy study of the adsorbed species andthe gas-phase products was reported for the transformation of

Scheme 9. Oxidation of aliphatic primary alcohols over Au/CeO2.



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2-propanol over CeO2 catalysts calcined at various tempera-tures in the gas phase.[164] The dehydrogenation to give ace-tone started to take place at 150 8C. The acetone, through fur-ther interaction with the surface, became involved in anotherreaction to give isobutene and methane when the reactiontemperature increased to 250 8C and the dehydration, whichled to propene, had begun. The increase of the dehydrationactivity of ceria, upon calcination, was due to an increase ofthe acidity of Brønsted acid sites and an increase of thenumber of Lewis acid sites. The alcohol dehydrogenation reac-tion is controlled by the electronic mobility of the catalyst sur-face and decrease with calcination. The aldol condensation re-action can also occur over CeO2 catalysts.[164]

Reactions of ethanol on Cu–Mg5CeOx in the gas phase at300 8C led to the formation of acetaldehyde, n-butyraldehyde,and acetone as predominant products. The initial rate of etha-nol dehydrogenation increased linearly with the Cu surfacearea. Acetaldehyde concentrations increased rapidly to equilib-rium levels that became independent of Cu content inCuyMg5CeOx catalysts. Dehydrogenation and condensation re-actions occured after binding of ethanol to an acid–base sitepair present in basic oxides. These reactions were influencedby the presence of Cu metal crystallites. Basic sites may inter-act with Cu sites via migration of hydrogen atoms.[165]

The conversion of cyclohexanol[39, 142, 166, 167] or of 2-propa-nol[168–175] allows the characterization of the acid–base surfaceproperties of the oxides. The dehydration of alcohol leading toalkene would be catalyzed by the acid centers, whereas its de-hydrogenation leading to ketone would be catalyzed both byacid and basic sites. The dehydration activity could be relatedto the surface acidity, whereas the ratio between the activity indehydrogenation and the activity in dehydration (AONE/AENE)would represent the surface basicity.

At 300 8C, CeO2 presents an acidic activity and on the otherhand a basic activity. At 200 8C, the presence of ceria support-ed on Al2O3 (12 % CeO2/Al2O3) decreased the acidity in compar-ison with Al2O3: the ratio AONE/AENE increased (0.2 to 0.8).[39] Theconversion of cyclohexanol increased with the temperaturewith increasing selectivity to cyclohexene on pure CeO2.[166]

The addition of a CeO2 component to a ZnO catalyst (up to40 % of CeO2), enhanced the activity of the catalyst with morethan 90 % selectivity to cyclohexanone. In CeO2–ZnO, Ce4+ ionsof different degrees of coordination unsaturation will act asLewis-acid sites for cyclohexanol dehydrogenation. The ab-straction of hydride ions is more efficient at the Ce4 + ion sitethan at the Zn2 + ion site.[142, 166]

At 300 8C, on CeO2, in the presence of helium and dihydro-gen, propene was formed during the transformation of 2-prop-anol. The temperature increase favored dehydration, whichoccurs on acid–base pair sites that consist of coordinatively un-saturated Ce4+ and O2� ions. At 150 8C, 2-propanol undergoesa dehydrogenation to give acetone. The active sites wereagain described as Ce4+–O2� ion pairs, however, the acidic siteis probably different from that involved in the dehydration re-action. When the temperature increased to up to 300 8C, ace-tone molecules, previously produced, took part in a bimolecu-lar reaction to give isobutene and methane and an acetate sur-

face species. The surface reactivity was associated with Ce4+–OH� pair sites.[168]

Under air, ceria is more active and the selectivity in acetoneis more important than under helium or dihydrogen.[169] Theselectivity of Cu/CeO2/CNF (CNF: carbon nanofiber) was depen-dent on the fraction of CeO2 and on the temperature. High ac-tivity and selectivity were achieved with the Cu12Ce5/CNF cata-lyst. However, excess CeO2 enhanced the dehydration activityand thereby reduced the selectivity. The presence of CeO2 en-hanced the reduction and dispersion of Cu.[171]

For a Au/CeO2 catalyst, the good oxidation performanceswere explained by a combination of the oxidation capability ofgold atoms with the redox properties of the ceria phase.[173] Infact, the dehydrogenation reaction required redox sites, ratherbasic sites. The alcohol transformation cannot be a simple testof acidity. Particularly, on ceria, its redox property and its highlability of lattice oxygen contribute to products formations, in-volving oxygen vacancies.

5.3. Hydrogen transfer reactions

Ceria-supported Cu, Ir, and Pd catalysts have shown a veryhigh activity for liquid-phase transfer dehydrogenation of cy-clohexanol and 2-octanol to cyclohexanone and 2-octanone,respectively, using styrene as the hydrogen acceptor, but forthe primary alcohols, the reaction rates were much lower;however, with a good selectivity for aldehydes. The Cu/CeO2

and Pd/CeO2 catalysts were more active than the previously re-ported Cu and Pd catalysts supported on Al2O3, and Ir/CeO2

catalyst exhibited extremely high activity. The synergistic effectbetween metals and CeO2 might be responsible for the highcatalytic activity. Pretreatment of the catalysts by hydrogencaused partial reduction of ceria and thus led to the genera-tion of Ce3+ species on the catalyst surface. This species wouldenhance the adsorption of alcohols through the coordinationbetween the Ce3+ cation and the hydroxyl group, which favorsthe dehydrogenation of alcohols. Meanwhile, the in situ re-moval of hydrogen would take place on the nearby metal par-ticles through the hydrogenation of styrene.[176]

5.4. Oxidation of hydrocarbons

Partial oxidation of toluene to benzaldehyde via gas phaseprocess is one of the current challenges in the field of catalysis.The oxidation of toluene was carried out on Ag1.2V3CeyO8+x cat-alysts between 360 and 460 8C. The catalytic effect of Ce ismainly to increase the selectivity to benzaldehyde and benzoicacid.[177]

Series of Ce–Mo catalysts have been prepared for the partialoxidation of toluene to benzaldehyde. The highest yield ofbenzaldehyde is obtained when the composition of the ultra-fine particles reaches the vicinity of Ce/Ce+Mo = 0.5, corre-sponding to a catalyst comprising both CeO2 and Ce2(MoO4)3.The excess CeO2 works as a promoter by releasing its latticeoxygen to the oxygen vacant sites formed on the Ce2(MoO4)3

species during reaction.[178]



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A simple and efficient method for the synthesis of 3-nitro-phthalic acid by the oxidation of 1-nitronaphthalene has beenreported by Rajiah et al. (Scheme 10). Selective oxidation has

been achieved by the one-step reaction of 1-nitronaphthalenewith 5 % CeO2/g-Al2O3 catalyst in acetonitrile in presence ofaqueous acid at 90 8C producing 3-nitrophthalic acid in80 mol % yield with 98 % selectivity.[179]

Elsewhere, the oxidation products of ethylbenzene arewidely used as intermediates in organic chemistry. Various sup-ported vanadia catalysts exhibit efficient catalytic activity inthe selective oxidation of ethylbenzene using H2O2 in liquidphase (solvent: acetonitrile), producing essentially acetophe-none. The oxidation activity of V2O5/CeO2 catalysts could becorrelated to the amount of the vanadia loaded and the struc-ture of the species. The CeVO4 formation associated with in-creased concentration of vanadia on ceria is related to the for-mation of 2-hydroxyacetophenone.[180] These catalysts were al-ready used in the partial oxidation of benzene to phenol.[181]

Selective formation of phenol can be attributed to the pres-ence of highly dispersed active sites of vanadia over the sup-port.

The hydroxylation of benzene was used also as test reactionto characterize catalysts, as MxCe1�xVO4 (with M = Li, Ca, andFe), for the degradation by photocatalysis of different dyes andorganic compounds. This reaction is associated to the oxida-tion of cyclohexane to produce cyclohexanol and cyclohexa-none in liquid phase with chloroform as solvent.[182]

Liquid phase oxidation of cyclohexane to cyclohexanol wascarried out under mild reaction conditions over mesoporousCe-MCM-41 catalysts using aqueous hydrogen peroxide (30 %)as oxidant and acetic acid as solvent. MCM-41, without the in-corporation of Ce as a catalyst under the same conditions withthose used for Ce-MCM-41, did not exhibit any significant ac-tivity. Furthermore, even incorporating other metal ions, suchas, Fe-MCM-41 exhibited significantly lower activity than Ce-MCM-41.[183] The Ce present in the framework structure of Ce-MCM-41 can impart dual catalytic activity to the catalyst andcan form labile oxygen vacancies and the relatively high mobi-lity of bulk oxygen species. A complex with peroxy acetic acidwas possibly formed in the pores of Ce-MCM-41 which is rela-tively more hydrophobic and stable than hydrogen peroxide.The synergistic effects among doped cerium, mesoporousframework of MCM-41, acetic acid and hydrogen peroxidemake Ce-MCM-41 an effective catalyst for the oxidation of cy-clohexane.[183] Previously, the hydroxylation of 1-naphthol wascarried out with aqueous H2O2 on this kind of catalysts.[184]

Elsewhere, in cyclohexene and cyclohexanol oxidation withH2O2 in acetonitrile, the catalytic activity depends on thecerium amount in Ce-silica mesoporous SBA-15 materials andmetal atom coordination. Thus, in cyclohexene oxidation thetotal yield of oxidative products and selectivity of cyclohexeneoxide (epoxy-) increase with the increase of cerium amount upto 2 wt % and then tend to decrease. Similar correlation is ob-served in cyclohexanol oxidation. Probably, the surface OHgroups and state of cerium sites influence the catalytic activityof Ce-SBA-15. Thus, the sorption value of cyclohexene is highwhen content and density of Si�OH groups are low. Cyclohex-ene is sorbed on the surface coordinatively with unsaturated(cus) oxygen or on the surface lattice oxygen anion and isstrongly inhibited if ceria is slightly reduced due to the de-creasing of available (cus) oxygen and surface oxygen species.Then the cyclohexene adsorption value decreases with increas-ing of cerium content in Ce-SBA-15.[185]

Ce-SBA-15 catalysts are also active for the oxidative cleavageof cyclohexene to adipic acid using aqueous H2O2 as oxidantunder solvent-free conditions.[186] The coordination of ceriumions in mesoporous materials can affect the catalytic propertiesbecause the incorporation of cerium atoms into the walls ofmesoporous material allows creation of Lewis and Brønstedacid sites and preparation of materials with various acidi-ties.[185]

The epoxidation of cyclohexene was carried out also in thepresence of Fe/CeO2 catalysts using aqueous hydrogen perox-ide (30 %) as the oxidizing agent.[187]

Direct oxidation of propane to acrolein could be an interest-ing alternative to propylene oxidation. Bi-Mo based catalystshave been heavily studied for the selective oxidation (ammoxi-dation) of propylene to acrolein via acrylonitrile at 500 8C. In Bi-CeVMoO catalysts, bismuth may be substituted by a lowamount of cerium while the structure of BiVMoO remains un-changing. With the increasing of Ce (Ce/Ce+Bi>0.15), newphases of CeVO4 and Ce2(MoO4)3 formed. The cerium promotesthe forming of acrolein, and the selectivity to acrolein in-creased to a maximum at Ce/Ce+Bi atomic ratio equal to 0.15(45 mol % at about 30 mol % propane conversion). Further in-creasing Ce content results in further oxidation of acrolein toCOx due to the strong oxidative ability of catalysts.[188]

5.5. Oxidative dehydrogenation of hydrocarbons

5.5.1. Dehydrogenation of light paraffins

The dehydrogenation of light paraffins has acquired more im-portance owing to the growing demand for light olefins suchas propylene[189–191] and isobutene.[192–196] The dehydrogenationof light alkanes to alkenes is a highly endothermic reaction,and conversion is limited by a thermodynamic equilibrium.Thus, high operating temperatures (500–750 8C) are requiredto obtain an acceptable level of alkane conversion. Underthese conditions undesirable side reactions, such as hydroge-nolysis and isomerization, occur with the formation of byprod-ucts and coke deposits, thus producing catalyst deactivation.

Scheme 10. Synthesis of 3-nitrophthalic acid by oxidation of 1-nitronaphtha-lene.



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The introduction of Zr4+ in a CeO2 lattice leads to significantvariations in the chemical physical features of ceria, and im-proves the selectivity to isobutene in the oxidative dehydro-genation of isobutane between 300 and 400 8C. This enhance-ment has been attributed to an increased oxygen mobility andto an increased activity for the Ce4+/Ce3+ redox couple, occur-ring as a consequence of the creation of surface and bulk de-fects in the solid solution.[192] The dehydrogenation was carriedout at 450 8C over cerium oxide in the presence of tetrachloro-methane to obtain propene, with a selectivity of up to 80 %.Without tetrachloromethane, carbon dioxide is the principalproduct. The enhancement of conversion and selectivity topropene was shown to be dependent upon the presence ofchlorine, in whatever form, in the surface region of the cata-lyst.[189]

The Ce–Ni–O catalytic system is active and selective in oxida-tive dehydrogenation of propane to propene at 300 8C. Theyield of propene increased with the increase in the Ni loadingup to a Ni/Ce atomic ratio equal to 1 and decreased at higherloadings.[190] Ceria was also found to be a good support forchromium oxide catalysts in the oxidative dehydrogenation ofisobutane.[193, 194] Recently, chromium oxide was supported onnanometer-sized Ce0.60Zr0.35Y0.05O2 for the same application.[196]

Elsewhere, platinum catalysts have been widely used foralkane dehydrogenation. Although a wide variety of catalystformulations have been reported in the literature, most plati-num-based catalysts are characterized by the simultaneouspresence of tin. Pt-Sn/Ce-Al2O3 catalysts, with cerium loadingsin the range of 1.1–3.3 wt %, exhibit a highly efficient perfor-mance for propane dehydrogenation to propylene at 576 8C.The presence of Ce in the Pt-Sn/Ce-Al2O3 catalysts could notonly stabilize the active states of Pt, Sn, and the support, butcould also suppress the coke accumulation on the catalystduring reaction.[191]

Pt-Sn/20 wt % CeO2–C catalysts, with different Sn/Pt atomicratios, showed good performance in the dehydrogenation ofisobutane at 500 8C. Cerium plays an important role in activity,inhibiting tin reduction and maintaining the amount of alloyedplatinum at an adequate level. A catalyst with Sn/Pt = 0.5showed the best isobutene yield.[195]

5.5.2. Dehydrogenation of ethylbenzene

The industrial demand for styrene is growing, and its produc-tion via dehydrogenation of ethylbenzene is gaining impor-tance. The reversible conversion of ethylbenzene to styreneand dihydrogen is highly endothermic: C6H5CH2CH3!C6H5CH=

CH2+H2 ; DH = 125 kJ mol�1. Conversion is favored by low pres-sures and high temperatures. Industrially, the reaction is carriedout over potassium-promoted iron oxide at temperatures rang-ing from 550 to 650 8C and pressures from sub-atmospheric to2 atm, with a selectivity of about 90 % in styrene at a conver-sion of 50�60 %. Ceria is a key component of the catalyst for-mulation for the dehydrogenation of ethylbenzene to sty-rene.[197]

Activated-carbon-supported cerium catalysts (Ce/AC) exhibita high styrene yield (about 40 %) with over 80 % selectivity at

550 8C in the presence of carbon dioxide. The dehydrogenationof ethylbenzene to styrene proceeds via two reaction paths.One is the simple dehydrogenation and an oxidation reactionof dihydrogen formed with carbon dioxide (reverse water–gasshift reaction; CO2+H2!CO+H2O). The other is the oxidativedehydrogenation of ethylbenzene through the redox cycle.[198]

V2O5-based catalysts supported on ZrO2–SiO2,[199] Al2O3,[200]

and TiO2-ZrO2[201, 202] doped with CeO2 exhibit high activities in

oxidative dehydrogenation reaction of ethybenzene. Ceria-con-taining materials suppress catalyst deactivation by preventingcoke formation during the reaction.

Elsewhere, the activity of CeO2–ZrO2/SBA-15 catalyst for thedehydrogenation of ethylbenzene to styrene in the presenceof CO2 revealed that mesoporous silica SBA-15 is one of thepromising support materials for the development of highlyactive and selective CeO2–ZrO2 mixed metal oxide catalysts.[203]

5.6. Dehydroisomerization

a-Limonene can be easily dehydrogenated to para-cymene,which is an important starting material for the production ofintermediates such as para-cresol and can also be used in themanufacturing of fragrances, herbicides, and pharmaceuticals.Ce-promoted Pd/ZSM5 catalysts gave higher selectivities thannonpromoted catalysts for the hydroisomerization of a-limo-nene to para-cymene at 300 8C (Scheme 11).[204, 205]

A chemical interaction between CeO2 and Pd particles withvery small size exists inside ZSM5 cavities. A bifonctional mech-anism had been proposed for the conversion of a-limonene.An acid-catalyzed shift of the double bond from the isopro-penyl group into the cyclohexene ring is followed by dehydro-genation on a palladium site.

5.7. Oxidation of aldehydes to acids

Clean aerobic oxidation of various aldehydes to the corre-sponding carboxylic acid was carried out over Ru–Co(OH)2–CeO2 (already used in a previously reported study[149]) at roomtemperature, in liquid phase with benzotrifluoride as sol-vent.[206] The aliphatic and aromatic aldehydes were rapidlyoxygenated. However for allylic aldehydes such as cinnamalde-hyde, no conversion was observed even at 60 8C. The authorsproposed a reaction mechanism via a free-radical process:there was no conversion of octanal in the presence of a radicalscavenger (2,6-di-tert-butyl-para-cresol).[206]

Gold supported on nanocrystalline or on meso-structurednanocrystalline CeO2 supports was also highly active and selec-

Scheme 11. Hydroisomerization of a-limonene to para-cymene.



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tive for the aerobic oxidation of aliphatic and aromatic alde-hydes at 25 8C and 50 8C in liquid phase with acetonitrile as sol-vent.[207] With this catalyst, the oxidation of cinammaldehyde at65 8C was highly selectivite towards carboxylic acid (77.5 %) atmoderate conversion (40.7 %). The activity was attributed tothe nanometric particle size of Au and CeO2.

5.8. Other oxidation reactions

5.8.1. Oxidation of 2,3-dim�thylphenol

2,6-dimethyl-1,4-benzoquinone is a key intermediate for thesynthesis of a number of medicines and physiologically activesubstances such as 2,3,6-trimethyl-para-benzoquinone, an in-termediate in the industrial production of Vitamin E. Theliquid-phase oxidation of 2,6-dimethylphenol to 2,6-dimethyl-1,4-benzoquinone (Scheme 12) was carried out at 20 8C, using

ethanol as solvent and aqueous hydrogen peroxide as a cleanoxidizing agent in the presence of TiO2–CeO2 mixed xero-gels.[208] The 2,6-dimethylphenol conversion was 100 % in 6 h,and the yields of 2,6-dimethyl-1,4-benzoquinone achievedwere 85–96 % when using the TiO2–CeO2 mixed xerogels ascatalysts, while the yield was 49 % when a titania catalyst with-out cerium was used.

5.8.2. Ammoxidation

Kanta Rao et al. reported on the use of ceria–titania (rutile andanatase) catalysts for the ammoxidation of 3-methylpyridine or4-methylpyridine to their corresponding nitriles in gas phase at410 8C (Scheme 13).[209] The authors noted a marked steric

effect in their reactivity. The best results were obtained on20 % ceria on anatase, showing a 4-methylpyridine conversionof 89 % (37 % for 3-methylpyridine) and a selectivity to 4-meth-ylpyridine of 77 % (45 % for 3-methylpyridine). The ammoxida-tion reaction was highly active when the catalysts were synthe-sized by dispersing ceria on suitable supports. Indeed, interact-ed CeO2 species of supported catalysts have shown increased

O2 uptakes as well as increased conversions and selectivities inthe ammoxidation reactions.[209]

5.8.3. Dehydrogenation of amines

Au(OAc)3 preadsorbed onto CeO2 was applied as an effectivecatalyst of the selective oxidation of dibenzylamine to dibenzy-limine using molecular oxygen as the only oxidant in the liquidphase with toluene as solvent (Scheme 14).[210, 211] The authors

developed a very simple route for the synthesis of gold cata-lysts for the oxidation of amines. The catalyst precursor,Au(OAc)3, and an oxide support, CeO2, were simply added tothe reaction mixture and the active gold nanoparticles on thesupport were formed in situ. During the transformation of di-benzylamine to dibenzylimine; benzonitrile, benzylamine, andbenzaldehyde were formed in amounts of 0.5 %, 0.4 % and7.8 %, respectively. The latter two byproducts are the result ofthe hydrolysis of dibenzylimine with the coproduct water,while benzonitrile is formed by oxidative dehydrogenation ofbenzylamine. The low benzylamine/benzaldehyde ratio can beexplained the coupling and oxidative dehydrogenation of ben-zylamine to dibenzylimine. The small amount of benzonitrileindicates that the direct oxidation of benzylamine to benzoni-trile is very slow.[211]

5.8.4. Oxidation of oximes

Gold supported on ceria (Au/CeO2), usually used in the oxida-tion of alcohols[152–156] or aldehydes,[207] is a highly active andselective catalyst for the liquid phase aerobic oxidation ofoximes to the corresponding carboxylic compounds.[212] For ex-ample, for the aerobic oxidation of keto- and aldoximes(Scheme 15), Au/CeO2 (with 0.72 wt % of Au) in a mixture of

ethanol water (1:1) or toluene is very efficient to produce ace-tophenone and benzaldehyde (conversion ca. 85–99 % with100 % selectivity, except for benzaldehyde in the presence ofwater where there was overoxidation of benzaldehyde to ben-zoic acid). Another, more elaborate catalyst, a core/shell alloyof gold and palladium supported on nanoparticulated ceria,

Scheme 12. Oxidation of 2,6-dimethylphenol to 2,6-dimethyl-1,4-benzoqui-none.

Scheme 13. Ammoxidation of 3-methylpyridine or 4-methylpyridine.

Scheme 14. Selective oxidation of dibenzylamine to dibenzylimine.

Scheme 15. Oxidation of keto- and aldoximes.



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was also efficient in toluene (conversion 99 %, with a selectivityof 99 %) but the drawback was a difficult preparation.[212]

6. Addition Reactions

6.1. Synthesis of carbonates

The development of environmental processes based on theutilization of naturally abundant carbon resources such ascarbon dioxide has gained considerable attention in recentyears. Organic carbonate synthesis using carbon dioxide is oneof the promising reactions in this respect. Organic carbonatecompounds have been used as both a reactive intermediateand an inert solvent.

Dimethyl carbonate (DMC) can be synthesized from epoxidecompounds such as ethylene oxide or propylene oxide by atwo-step reaction (Scheme 16). In the first step the epoxidereacts with CO2, producing a corresponding cyclic carbonate.In the second step, the carbonate is transesterified with metha-nol to DMC and a corresponding glycol.

CeO2 studied with several metal oxide does not appear tobe a good catalyst for this reaction. The basic metal oxide cata-lysts give high activity and selectivity for the reaction of epox-ides and CO2 to the corresponding cyclic carbonates and forthe transesterification with methanol. Among the catalysts ex-amined, MgO is the best catalyst, active for both these two re-actions.[213]

Corma and collaborators[214] have shown that ceria nanocrys-tallites are a moderately active catalyst for the transalkylationof propylene carbonate by methanol. The presence of goldnanoparticles on ceria in appropriate loading significantly in-creases the activity and selectivity towards transalkylation.

DMC can be also synthesized by reaction between methanoland CO2 in the presence of catalysts with acidic and basicproperties such as ZrO2 and CeO2–ZrO2 solid solutions(Scheme 17). CeO2–ZrO2 catalyst appear to be very effective forthe selective synthesis of DMC from CH3OH and CO2.[215]

Although the selectivity of DMC syntheses over CeO2–ZrO2

catalysts with Ce/(Ce+Zr) = 0.2 was very high (100 %) underthe employed reaction conditions, unfortunately the methanolconversion was very low because the equilibrium of the reac-

tion was largely shifted to the left. However, when H2O is re-moved from the reaction system, it is possible to drastically en-hance the methanol conversion. H2O removal can be achievedby reaction with acetals such as 2,2-dimethoxypropane (DMP;Scheme 18).[216] These catalysts are also effective to the directsynthesis of cyclic carbonate from CO2 and diols such as ethyl-ene glycol and propylene glycol.[217, 218]

The synthesis of DMC from CH3OH and CO2 was investigatedon CeO2 prepared with various kinds of precursors under vari-ous calcination temperatures. The formation rate of DMC wasalmost proportional to the BET surface area of the catalysts.This suggests that the active site of this reaction is on a stablecrystal surface of CeO2, such as (111).[219]

CeO2 has been reported to catalyze the direct carboxylationof methanol to dimethylcarbonate. Nevertheless, the catalystlifetime was quite short as after the first cycle the activity de-creased and went to zero after a few cycles. This deactivationis mainly due to a surface modification produced by the reduc-tion of CeIV to CeIII during catalysis and to crystal conglomera-tion. The modification of ceria by loading alumina strongly re-duces the oxidation of methanol and the consequent reduc-tion of CeIV to CeIII, with increase of both the life of the cata-lysts and their selectivity.[220, 221]

6.2. Aldol condensation

Condensation reactions of aldehydes and ketones are widelyused in organic synthesis, mainly because they lead to C�Cbond formation. These reactions are generally catalyzed bybases. The condensation over solid bases leads mainly to aldolor ketol and/or a,b-unsaturated carbonyl compounds.

The aldol condensation of acetone was studied over solidbase catalysts such as Ca(OH)2, La(OH)3, ZrO2, and CeO2 in thevapor phase between 200 and 400 8C. The condensation ofacetone 1 gives diacetone alcohol 2, which is dehydrated tomesityl oxide 3. Various secondary products are formed by nu-merous secondary reactions, such as further aldolization andMichael condensation (Scheme 19). At 200–400 8C, 1 over CeO2

led to 74–97 % conversion. CeO2 promoted the formation of1,3,5-trimethylbenzene 7 with a selectivity of 47 % at 400 8C,but also produced large amounts of higher condensationproducts (38.9 % at 300 8C). The formation of 7 is favored withdecreasing basic strength. Indeed, the basicity and basicstrength, respectively, of the catalysts decreased in the orderCa(OH)2>La(OH)3>CeO2>ZrO2.[222]

Reduced CeO2 was observed to be active for the cross-re-ductive-coupling reaction between acetaldehyde and benzal-dehyde to form 1-phenylpropene (C6H5�CH=CH�CH3).[223]

Scheme 16. Two-step synthesis of dimethyl carbonate.

Scheme 17. Synthesis of dimethyl carbonate from CH3OH and CO2.

Scheme 18. Reaction of H2O with 2,2-dimethoxypropane.



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Elsewhere, the reaction of acetaldehyde was studied onCeO2-based catalysts. CeO2 was chosen as support because itsreducibility and basicity favor aldolization reactions.[224, 225]

Three C�C bond formation reactions from acetaldehyde wereobserved: aldolization to crotonaldehyde and crotyl alcohol(more prominent on CeO2 alone), ketonization to acetone, andreductive coupling to form butenes and butadiene.[224]

4-Methyl-2-pentanone or methylisobutylketone (MIBK) wassynthesized from 2-propanol in one pot on bifonctional metal/acid–base catalysts. The synthesis of MIBK from 2-propanol in-volves the dehydrogenation of 2-propanol to acetone which isconverted to mesityl oxide (3, Scheme 19) via an aldol conden-sation reaction and consecutive dehydration of the aldol inter-mediate, diacetone alcohol (2, Scheme 19). MO is hydrogenat-ed on the metallic site to MIBK by H2 generated during 2-prop-anol dehydrogenation. One of the most selective catalysts forthe formation of MIBK is CuCe4Ox that presents the higher den-sity of base sites and lower density of acid sites than CuAl16Ox

catalyst.[226, 227]

The aldol condensation/hydrogenation reaction of 2-hexa-none was carried out over a Pd/CeZrOx catalyst at tempera-tures between 300 and 400 8C, and pressures of 5–26 bar. Theprimary product of aldol condensation/hydrogenation is C12

ketone, with the formation of C9 and C18 ketones as secondaryproducts. The CeZrOx support was selected because it possess-es a high lattice oxygen mobility and because of its ability tointeract strongly with supported metals.[228, 229]

The retro-aldolization of diacetone alcohol (2, Scheme 19) toacetone was considered as a test reaction that allowed thesemiquantitative assessment of basic centers.[230] Ceria (and ti-tania) were found to exhibit considerable activity in the de-composition of diacetone alcohol.

6.3. Knoevenagel condensation reaction

The Knoevenagel condensation reaction is a cross-aldol reac-tion between an aldehyde or ketone and an methylene com-pound, activated by two electron-withdrawing groups, such asmalononitrile, cyanoesters, b-ketoesters or malonates, in the

presence of base. Ceria–zirconiashows interesting catalytic per-formances in the Knoevenagelcondensation between benzal-dehyde and malononitrile(Scheme 20) with ethanol as asolvent at 80 8C.[231]

The direct correlation betweenthe concentration of acidic sitesand the yield of the products in-dicated that a higher concentra-tion of acidic sites gives moreproducts in the reaction even ifthe presence of basic sites re-mains obligatory. CexZr1�xO2 cat-alysts can be interesting alterna-

tives to soluble bases in view of the following advantages:(1) high catalytic activity under mild reaction conditions,(2) easy separation of the catalyst after the reaction, and (3) re-usability of the catalyst.[231]

6.4. Synthesis of acetals

Besides the interest of acetals as protecting groups of carbonylcompounds during organic synthesis, many of them havefound direct applications as fragrances in cosmetics, food andbeverage additives, pharmaceuticals, and polymer chemistry.

The synthesis of dimethyl acetals of carbonyl compoundssuch as cyclohexanone (Scheme 21), acetophenone or benzo-phenone has succefully been carried out by the reaction be-tween ketones and methanol using different solid acid cata-lysts.[232]

Among various rare-earths-exchanged Mg–Y zeolites, CeMg–Y and Ce-montmorillonite were revealed to be the most effi-cient catalysts for the acetalization reactions. Acetalization ofcyclohexanone reached equilibrium within 60 min and theyields of acetal were 66.7 % with CeMg–Y zeolite and 69.8 %with Ce–montmorillonite. The yields then slightly increased to

Scheme 20. Knoevenagel condensation reaction between benzaldehyde andmalononitrile.

Scheme 19. Condensation of acetone and secondary reactions.

Scheme 21. Acetalization of cyclohexanone with methanol.



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80.5 % and 98.8 % respectively. The Ce3+ cation acted as aLewis-acid site and activated the carbonyl group by coordina-tion, on the order of 1 kJ mol�1 as measured by FTIR.[233]

6.5. Synthesis of benzimidazole derivatives

Corma and collaborators[234] have developed an effective strat-egy for the rapid and efficient one-pot synthesis of benzimida-zoles involving a new environmentally friendly catalytic proce-dure. Benzimidazole derivatives were prepared by a four-stepprocess with gold and/or palladium catalysts and dioxygen(Scheme 22). The four steps are (1) oxidation of the benzylalco-

hol to benzaldehyde, (2) cyclocondensation of the aldehydewith ortho-phenylenediamine, (3) oxidation of carbon–nitrogenbond, and (4) an N-alkylation reaction. The highest activity andselectivity were achieved when gold was deposited onto CeO2.

With electron-acceptor substituents on the aromatic diaminethe cyclization/oxidation reaction proceeded more slowly and,accordingly, lower yields of the desired benzimidazole wereobtained. The same effect was observed when the electron-withdrawing substituent was at the aromatic alcohol. On theother hand, 1-butanol afforded very poor yields of the corre-sponding heterocycle provided this aliphatic alcohol hardlyconverted to the corresponding aldehydes. In striking contrastthe conjugated alcohol 2-octen-1-ol converted up to 80 % tothe corresponding aldehyde but the latter hardly reacted withthe diamine to afford the desired heterocycle.

6.6. Mannich-type reactions

A Mannich-type reaction is an organic reaction that consists ofan amino alkylation of an acidic proton placed next to a car-bonyl functional group with aldehyde and ammonia or any pri-mary or secondary amine to lead a b-amino-carbonyl com-pound. A sulfated CexZr1�xO2 catalyst was found to exhibitsolid-super-acidity and good catalytic activity for synthesis ofb-amino ketones by a three-component Mannich-type reactionin the liquid phase under solvent-free conditions at ambienttemperature. The reaction between benzaldehyde, aniline, andcyclohexanone (Scheme 23) proceeded in liquid phase (mixtureof reactants) to afford 82 % of product, with a d :l ratio of82:18. The sulfation of ceria-zirconia mixed oxide can lead tothe formation of super-acidic sites in the catalyst, while the un-promoted ceria-zirconia mixed oxide possesses only a broaddistribution of weak acid sites.[235]

6.7. Biginelli-type reaction

A Biginelli-type reaction is a multiple-component chemical re-action that creates 3,4-dihydropyrimidin-2(1 H)-ones from b-ke-toesters, aldehydes, and urea. This reaction is generally cata-lyzed by Brønsted acids and/or by Lewis acids. Thus, the reac-tion between benzaldehyde, ethyl acetoacetate, and urea(Scheme 24) was performed in water at 80 8C for 4.5 h in thepresence of ceria nanoparticles supported on vinylpyridinepolymer, to give 92 % of product. The catalyst was recoveredeasily and reused without loss of its activity.[236]

6.8. Coupling reactions

Coupling reactions are of particular interest because they are apowerful and versatile tool in synthetic organic chemistry forthe formation of carbon–carbon bonds. Gold or palladium sup-ported on CeO2 are active and extremely selective in perform-ing the homocoupling of arylboronic acids in liquid phase withtoluene as solvent (Scheme 25),[237–239] the Suzuki–Miyaura

cross-coupling reaction of arylboronic acids and arylbromidesin liquid phase with a mixture ethanol water as solvent(Scheme 26),[240] and the Sonogashira cross-coupling reactionof aryliodides and alkynes in N,N-dimethylformamide (DMF) assolvent (Scheme 27).[241]

Scheme 24. Biginelli-type reaction.

Scheme 25. Homocoupling reaction of phenylboronic acid.

Scheme 22. Synthesis of benzimidazole.

Scheme 23. Mannich-type reaction.

Scheme 26. Cross-coupling reaction of phenylboronic acid and arylbromide.



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The catalytic activity of Au/CeO2 for the homocoupling of ar-ylboronic acids is directly proportional to the concentration ofAuIII surface species, and nanocrystalline CeO2 is able to stabi-lize surface AuIII species on the surface.[238] Under the same re-action conditions, supported palladium catalysts are less activethan supported gold catalysts.[239] The palladium catalystsshowed a selectivity of about 75 % towards biphenyl with for-mation of benzene and phenol as byproducts. When support-ed gold was used for the reaction, a selectivity of 100 % to-wards biphenyl is obtained.

Pd/CeO2 behaves as an efficient catalyst in the Suzuki–Miyaura coupling reaction starting from aryl bromides with dif-ferent electronic substituents at room temperature, in air, in anenvironmentally friendly solvent such as ethanol/water mix-ture.[240] By comparison of isoelectronic PdII and AuIII supportedon ceria, the authors have found that the latter selectively pro-motes homocoupling, while the former catalyzes the cross-coupling reaction.[240]

Previously, Corma and collaborators have reported that aAu/CeO2 catalyst, active for performing the Sonogashira cross-coupling reaction, contains Au0, AuI, and AuIII species. Thecross-coupling reaction was catalyzed by AuI, while the homo-coupling reaction was catalyzed by AuIII.[241]

7. Substitution Reactions

7.1. Alkylation of aromatic compounds

The alkylation of aromatic rings, called Friedel–Crafts alkylation,is a reaction of very broad scope. The most important alkylat-ing reagents are alkyl halides, alcohols, and olefins. These reac-tions are usually catalyzed by Lewis acids or also by Brønstedacids. These conventional catalysts are homogeneous and gen-erate corrosive and nonrecyclable waste, and are thus not en-vironmentally friendly. The solid catalysts do not exhibit thesedisadvantages.

CeO2[242] or CeO2-MgO[85, 243] were found to exhibit excellent

catalytic activities for the vapor-phase ortho-alkylation ofphenol with methanol[242, 243] and with 1-propanol.[85] The au-thors speculated that the reaction mechanism of the ortho-pro-pylation over the CeO2–MgO catalyst proceeds by the perpen-dicularly adsorption of phenol on weak basic sites on the cata-lyst. These species are selectively alkylated in ortho position by1-propanol, which is possibly activated in the form of 1-hy-droxypropyl radical rather than propyl cation. The redox prop-erties of Ce4+–Ce3+ are probably attributed with the activationof 1-propanol to produce 1-hydroxypropyl radical. Moreover,neither 2-n-propylphenol nor 2-isopropylphenol is producedduring the alkylation of phenol with 2-propanol in the sameconditions. This fact suggests that isopropyl cation cannot be

produced on the CeO2–MgO at temperatures lower than500 8C.[85]

Sn–Ce–Rh oxide monophase system, already used in ketoni-zation of esters[83, 84] or alcohols[90, 91] was found to be an activeand selective catalyst for the ortho-alkylation of phenol withmethanol.[244] Elsewhere, alkylation of aromatics compoundswith alcohols or alkenes was performed over cerium modifiedmicroporous materials such as zeolites or silicoaluminophos-phate (SAPO).[245–251] The impregnation of cerium leads to thedeactivation of external acid sites of H-mordenite: the selectivi-ty of 2,6-diisopropylnaphthalene in the isopropylation of naph-thalene was enhanced without significant decrease of catalyticactivity.[245] Ceria thereby prevents non-regio-selective reactionon external surfaces with improvement of the selectivity. Simi-lar results were observed over various SAPO for the isopropyla-tion of biphenyl to produce 4,4’-diisopropylbiphenyl,[247] overH-ZSM-5 zeolite for the ethylation of ethylbenzene to 1,4-di-ethylbenzene[249] and over H-mordenite zeolite for the tert-bu-tylation of toluene to 4-tert-butyltoluene.[250, 251]

Ce–Al–MCM-41-type mesoporous silicate materials wasfound to exhibit catalytic activities for isopropylation of naph-thalene[252] and benzylation of toluene.[253] Both the densityand the strength of the acid sites were considerably higher inthe samples containing both Ce and Al than in the sampleswith only one of these substituents.

7.2. Synthesis of coumarins

The synthesis of coumarins that starts from phenol, called thePechmann reaction, requires concentrated sulphuric acid ascatalyst and involves corrosion problems. Reddy et al. reportedan efficient method for the preparation of coumarins using sul-fated CexZr1�xO2 solid catalyst under solvent-free conditions at120 8C (Scheme 28).[254] With 10 wt % of catalyst, the condensa-tion between activated phenols with alkyl acetoacetate led tohigh yields of products (superior to 70 %) within a short periodtime (shorter than 143 min).

7.3. Synthesis of anisaldehyde

Hydroxymethylation of anisole has been carried out overSnO2–CeO2 catalysts in the gas phase in the temperature range350–450 8C.[255] Anisaldehyde (methoxybenzaldehyde) and con-densation products (Scheme 29) were formed, along withminor quantities of methoxybenzyl alcohol, ortho-cresol,phenol, and 2,6-xylenol. A maximum anisaldehyde selectivityof 64 % was obtained at 350 8C at an anisole conversion of

Scheme 28. Pechmann condensation.

Scheme 27. Cross-coupling reaction of alkyne and aryliodide.



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46 %. Catalytic activity was ascribed to the presence of weakacid sites and redox metal sites. Stronger acid sites lead to theformation of condensation products.

7.4. Nitration of aromatic compounds

The liquid-phase nitration of toluene was carried out in thepresence of sulphated titania promoted by a ceria catalyst atambient temperature and atmospheric pressure without sol-vent.[256] It is an attractive method for the environmentallyfriendly synthesis of nitroaromatic compounds. Moreover, onlymononitrotoluenes were detected in the products, and theratio of para-nitrotoluene and ortho-nitrotoluene was approxi-mately 1:1. A maximum yield of about 11.4 % was achieved formononitrotoluenes in 3 h with SO4

2�/TiO2 doped with CeO2

catalyst.

7.5. Acylation of alcohols, amines, or thiols

The acylation of alcohols, amines, phenols, and thiols is an im-portant and frequently used organic transformation as it notonly provides an efficient and inexpensive route for protectinghydroxy, amino, phenolic, and thiol groups, but also producesimportant organic intermediates in multistep synthetic pro-cesses that are widely used in the synthesis of fine chemicals,pharmaceuticals, perfumes, plasticizers, cosmetics, and chemi-cal auxiliaries. Some of the solid-acid catalysts have been inves-tigated as potential replacements for mineral acids in theesterification reaction.

Ce–MCM-41[252] and ceria–yttria catalysts[257] were found tobe good catalysts for the acylation of alcohols, amines, andthiols with acetic anhydride without solvent[252] or in the pres-ence of acetonitrile.[257] Ceria–yttria catalysts exhibit strongLewis acid properties.[257]

S2O82�/ZrO2–CeO2 catalysts[258] or SO4

2�/ZrO2 promoted byCe2O3

[259] were also used as superacid catalysts in the esterifica-tion reactions. The incorporation of Ce into the catalyst wasbeneficial to the formation of sole tetragonal ZrO2 and effec-tively prevented the formation of monoclinic ZrO2, and re-strained the loss of sulfated species.

7.6. Transesterification

The preparation of monoglycerides from fatty acids or fattymethyl esters and glycerol can be carried out in the presenceof acidic or basic catalysts. The use of solid basic catalystscould limit secondary reactions leading to product degrada-tion. A comparison of various basic oxide solids has shownthat the more significant the intrinsic basicity is, the moreactive the catalyst is.[260] The comparison of the catalytic results

between CeO2 and MgO shows that even if they have similarintrinsic basicity and surface area, their initial activity in this re-action are different. MgO is the most active solid which couldbe due to the presence of stronger basic sites. But the selectiv-ities to the monoglycerides are similar and only depend on thereagent conversion.

The transesterification of b-keto esters has been also studiedin the presence of Lewis acid catalysts as ceria–yttria basedcatalyst.[261] The authors have previously reported the applica-tion of this catalyst for the acylation reactions.[257] This catalystis also an efficient catalyst for the transesterification of b-ketoesters by a variety of alcohols.

7.7. Oxidative esterification

5-Hydroxymethyl-2-furfural (HMF) has been selectively convert-ed into 2,5-dimethylfuroate (DMF) (99 mol % yield) under mildconditions (65–130 8C, 10 bar O2) in the absence of any base,by using gold nanoparticles on nanoparticulated ceria(Scheme 30)[262] usually used in oxidation of alcohols.[152–156]

DMF is a valuable biomass derivative that can be used as poly-mer precursor to replace terephthalate in PET polymers. Thereaction kinetics show that the oxidative pathway encountersits limiting step for the oxidation of the alcohol to aldehyde.Once the aldehyde is formed, the corresponding hemiacetal isobtained, which is rapidly oxidized into the ester.

8. Isomerization or ring opening

8.1. Isomerization of alkanes

Branched alkanes are very important for high-octane gasolines.They are produced by isomerization of normal paraffins; anacid-catalyzed chain reaction that is preferably performed atlow temperatures in order to avoid cracking and aromatizationproducts.

Isomerization of hexane over WOx/CeO2 catalysts leads tomono- and dialkylated hydrocarbons and methylcyclopen-tane.[263] In the absence of dihydrogen, a relatively rapid deacti-vation of the catalysts occurs. The introduction of hydrogenimproves the stability and modifies the selectivity : the mono-and dialkylated hydrocarbons predominate. In these condi-tions, the alkenes are less abundant and, consequently, the ac-tivity is decreased. These results suggest that the key point isthe formation of alkenes. In a first step, an oxidative hydrideabstraction occurs on Lewis sites associated with W=O species,followed by isomerization either as a cooperative effect on theBrønsted acid sites created by tungsten, or even on the sameLewis site from which the hydride ion was abstracted.

Scheme 30. Oxidative esterification of 5-hydroxymethyl-2-furfural into 2,5-di-methylfuroate.

Scheme 29. Synthesis of anisaldehyde.



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8.2. Ring opening

The hydrogenolysis of methylcyclobutane (MCB) is a processthat is well-known to be SMSI-sensitive, and is suited for study-ing the effect of low- and mid-temperature reduction on skele-tal hydrocarbon reactions. The hydrogenolytic ring opening ofmethylcyclobutane occurs easily at 100 8C and below on Ptand Rh nanoparticles supported on CeO2.[264] At low tempera-ture and under dihydrogen excess only the ring opening prod-ucts n-pentane and isopentane are formed. The progressiveloss of activity observed on ceria-supported Pt and Rh catalystswith low surface areas upon reduction below 450 8C is mostlikely due to electronic perturbations at the interface betweenthe metal nanoparticles and the increasingly reduced ceriasupport.

In upgrading highly aromatic fractions such as light cycle oil(LCO) from FCC, the hydrogenation of aromatic compoundsmay not always be sufficient to increase the cetane numberand the opening of at least one of the naphthenic rings is nec-essary. Nyl�n et al. have shown that ceria appears to be thebest support among several one as Al2O3, SiO2-Al2O3, ZrO2,MgO and SiO2.[265, 266] The higher activity and selectivity towardsring opening of indane are obtained with a 2 wt % Pt5Ir95/CeO2

catalyst. The desired products are 2-ethyltoluene and n-propyl-benzene, when the naphthenic ring has been cleaved onlyonce. However, consecutive dealkylation occurs irrevocablyand products such as ortho-xylene, ethylbenzene, toluene, ben-zene, and light products (<C6) are formed. This may be attrib-uted to the electron-deficient character of the metals whensupported on acid materials. The amphoteric properties mayhave impact on limiting secondary cracking reactions promot-ed by solely acid support materials and therefore increasingthe selectivity towards valuable ring-opening products.

8.3. Isomerization of alkenes

The catalytic isomerization of isoprenol by the shifting ofdouble bond to prenol (Scheme 31) is a process applied in thelarge-scale manufacture of solvents, dyes, surface coatings,

paints, and pesticides. Silica-supported palladium catalyst pro-moted by selenium and cerium (0.5 %Pd-0.05 %Se-0.3 %Ce/SiO2

catalyst) shows higher performance, among a large variety ofprepared catalysts, in the liquid-phase isomerization of isopre-nol to prenol in the presence of dihydrogen (45 % conversionwith 93 % selectivity). Addition of cerium improves the disper-sion of Pd species affecting catalyst activity. Selenium, beingan electronic modifier, is responsible for stabilization of Pdn+

species. This species determines the formation of p-complexesupon isoprenol adsorption, manifesting extended performance

in the double bond shift and depressing hydrogenation activi-ty.[267]

9. Conclusion and Perspectives

Over the last decade (more than 75 % of the references report-ed in this Review), ceria-based catalysis has demonstrated highefficiency in a variety of chemical transformations widely usedfor the synthesis of fine chemicals and specialties. Convention-ally, these reactions are carried out with homogeneous cata-lysts or with stoichiometric reactants, which are not environ-mentally benign methods. Heterogeneous catalysts should bepreferred to conventional synthesis methods because theyhave the advantages of simple removal from the product andrecyclability. They also provide greater selectivity and en-hanced reaction rates. For these reasons, cerium-based cata-lysts can contribute to new attempts to develop “clean andgreen” chemistry.

The redox ability and the acid–base properties of CeO2,either alone or in the presence of transition metals, are impor-tant parameters that allow to activate complex organic mole-cules and to selectively orient their transformation.

Pure ceria is used in the dehydration of alcohols, particularlyin the selective dehydration of diols to allylic alcohols, in theortho-selective alkylation of aromatic compounds with alco-hols, in ketone formation through dimerization of esters andcarboxylic acids, in aldolization, in the reduction of carboxylicacid to aldehydes and of aldehydes to alcohols and in the re-verse reaction, the dehydrogenation of alcohols, and it is ableto dehydrogenate isobutane to isobutene or ethylbenzene tostyrene.

The acid–base or redox properties of ceria can be modifiedby involving other oxides (ZrO2, La2O3, MnOx, ZnO, MoO4,V2O5,…), increasing the scope of the reactions. Ceria can alsobe supported on polymers for the Biginelli reaction.

Ceria-supported metal catalysts allow the hydrogenation ofC=C, C=O, or C�N bonds, the selective hydrogenation of a-b-unsaturated aldehydes (to unsaturated alcohols), b-keto esters(to b-hydroxy esters), and fatty esters. They are also used incoupling reactions or ring opening, in the synthesis of benzi-midazole, and in the oxidation of oximes. Cerium atoms havealso been added as promoters to catalytic systems or impreg-nated onto zeolites and mesoporous materials to improve theperformance of these catalysts.

In the near future, the very rich chemistry of cerium oxidesshould boost research on new catalysts with better propertiesfor organic syntheses. The great variety of cerium-based mixedoxides allows to adjust acid–base and redox properties and tomodulate both the number and strength of active sites for thedesired reaction. New developments in the synthesis of ceriananocrystals of controlled shapes (nanorods, nanocubes, poly-hedras, and others)[268, 269] should also lead to new catalystswith higher activities and selectivities in organic chemistry andcatalysis.

Scheme 31. Isomerization of isoprenol to prenol.



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Acknowledgements

The authors thank Mrs. Dani�le Mesnard, who began this work.

Keywords: cerium · heterogeneous catalysis · hydrogenation ·oxidation · synthetic methods

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ceria based solid catalysts for organic chemistry

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oxygen diffusivity

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