Ceria has been the subject of thorough investigations, mainly because of its use as an active component of catalytic converters for the treatment of exhaust gases. However, ceria-based catalysts have also been developed for different applications in organic chemistry. The redox and acid–base properties of ceria, either alone or in the presence of transition metals, are important parameters that allow to activate complex organic molecules and to selectively orient their transformation. Pure ceria is used in several organic reactions, such as the dehydration of alcohols, the alkylation of aromatic compounds, ketone formation, and aldolization, and in redox reactions. Ceria-supported metal catalysts allow the hydrogenation of many unsaturated compounds. They can also be used for coupling or ring-opening reactions. Cerium atoms can be added as dopants to catalytic system or impregnated onto zeolites and mesoporous catalyst materials to improve their performances. This Review demonstrates that the exceptional surface (and sometimes bulk) properties of ceria make cerium-based catalysts very effective for a broad range of organic reactions.
Ceria is a negative semiconductor oxide in which oxygen vacancies can be created at high temperatures in vacuum or in an inert gas [Equation (1)], or at moderate temperatures in the presence of a reductor [H2, CO, hydrocarbons; Equation (2) is written with CO):[1, 2]((1)), ((2))
The nonstoichiometric phase CeO2−y can be better described as: [CeIV1−2yCeIII2y][O2−y(VO°°)y], where VO°° represents an oxygen vacancy.3 Equation (2) can be rewritten in a more stoichiometric manner, showing that one O out of four is involved in the CeIV/CeIII reduction process:((3))
Ceria is widely used in catalytic converters for exhaust gases because of its exceptional redox properties. The material is able to store oxygen during the lean phase (i.e., excess of oxygen) and to give oxygen back to metal particles during the rich phase (when there is virtually no O2 in the gas phase); this is the so-called oxygen storage capacity (OSC) of ceria. The use of OSC components was first proposed by Ghandi et al. in 1976.4 Since the pioneering works of Yao and Yu Yao2 and of Su et al.,5, 6 many studies have been devoted to improving knowledge of the kinetics of OSC7–9 and of the mechanisms implicated in surface and bulk oxygen mobility in ceria and related compounds,10–14 with a special insight into CeZrOx oxides.15–19 Oxygen diffusivity in these materials can be measured by 18O/16O isotopic exchange,20–22 while oxygen species (e.g., superoxides, peroxides) that might be involved in the diffusion process can be investigated by electron spin resonance (ESR)23–27 or FTIR.18, 28–31 ESR studies have shown that the presence of a metal (Pt, Rh) drastically changes the nature of the oxygen species; the metal favoring the formation of O− ions instead of superoxide species.26 The presence of certain impurities in ceria, such as chloride ions, can also affect the nature and amounts of surface oxygen species.24 Superoxides species give a sharp IR band at 1126 cm−1, while on reduced ceria samples surface peroxide species can be recorded at 880 cm−1. OSC measurements are currently carried out by the dynamic technique, in which CO or H2 pulses are injected over the preoxidized sample at regular time intervals.32, 33 As mentioned above, it is generally accepted that one surface oxygen atom out of four is involved in the redox process shown in Equation (2), which represents a theoretical OSC value of 5.4 μmol O m−2 for a mean surface density of 13.1 μmol O m−2.22, 33 Reduced ceria is able to dissociate water34–38 or carbon dioxide34, 36 according to Equation (4) or the reverse of Equation (2), respectively.((4))
Ceria also possesses versatile acid–base properties, depending on the nature and temperature of the pretreatment. Ceria can chemisorb pyrrole, a proton donor, and CO2, an electron acceptor, 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 however change the distribution of carbonate species at the ceria surface (bridged, bidentate, monodentate, polydentate). On the basis of CO2 chemisorption studies, Martin and Duprez found the following scale for the density of basic sites of selected oxides (expressed in μmol 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 at room temperature. At higher temperatures, the amounts of chemisorbed CO2 decrease. Li et al. have shown that the amount of CO2 that remains chemisorbed at 100 °C would be 0.67 μmol m−2 on ceria and 1.40 μmol m−2 on zirconia.40 This proves that ceria possesses a high number of basic sites of weak or medium strength. Binet et al. also observed that ceria can chemisorb CO or pyridine, but the band positions strongly suggest that the Lewis acidity of ceria is significantly lower than that of zirconia or titania.30 In constrast to Lewis basicity, the Lewis acidity would decrease upon reduction of ceria.
All these acid–base or redox surface properties enable ceria to catalyze numerous organic reactions that require these types of active centers. Although it is often difficult to ascribe the catalytic activity to a unique type of site, this Review will be organized into three parts: reactions preferentially catalyzed on acid–base sites (dehydration and ketonization), reactions preferentially catalyzed by redox centers (reduction and oxidation of organic compounds), and finally reactions that may require 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 catalysts could represent an alternative route to the preparation of 4-methyl-1-pentene; a monomer for the manufacture of thermoplastic polymers of superior technological properties (Scheme 1). The unsupported mixed oxides CeO2-ZrO241–43 and CeO2-La2O3,42, 44, 45 and CeO2-ZrO2 supported on SiO246–48 have been used for this reaction, in the vapor phase under normal atmospheric pressure of N2 between 250 °C and 400 °C. It was observed that these catalysts exhibit high and stable catalytic activities. Their acid–base properties govern the competition between dehydration into the desired 1-alkene, the formation of other undesired alkenes, and parasitic dehydrogenation. The product distribution gives a detailed picture of the acid–base properties of the material.
4-methyl-1-pentene is the most abundant product of 4-methyl-2-pentanol conversion. By dehydration, 4-methyl-2-pentene and trace amounts of skeletal isomers of C6 alkenes are also formed. Dehydrogenation leads to 4-methyl-2-pentanone, and high-molecular-weight ketones are formed only in trace amounts. A maximum in 4-methyl-1-pentene selectivity is observed with the ceria-rich catalysts, and this selectivity decreases with increasing reaction temperature.
An E1cB mechanism, which needs balanced concentrations of the acid and base sites, as well as a higher strength of the latter, is probably operating on these catalysts. The selectivity in 1-alkene has been already shown during the dehydration of 1-butanol and 2-butanol on CeO2-based catalysts.49
2.2. Dehydration of diols
Allylic alcohols can be selectively produced by the vapor-phase dehydration of 1,3-diols over CeO2 between 300 °C and 425 °C (Scheme 2).50–56 1,3-Diols are more reactive than other diols and monoalcohols over CeO2. 2-Propen-1-ol was produced from 1,3-propanediol over pure CeO2 with a maximum selectivity of 98.9 % at 325 °C. In the dehydration of 1,3-butanediol, 2-buten-1-ol and 3-buten-2-ol were produced with a sum selectivity >99 %. 3-Penten-2-ol was also produced selectively from 2,4-pentanediol.50
2-methyl-1,3-propanediol is less reactive than 1,3-butanediol or 1,3-propanediol: the methyle group obstructs adsorption on the surface because of steric hindrance. The corresponding allylic alcohol was produced with lower selectivity: decomposition proceeds simultaneously.51
Theoretical investigations have indicated an interaction between the oxygen atoms in butan-1,3-diol and cerium cations: butane-1,3-diol preferentially adsorbs on the oxygen-defect site of the CeO2 (111) surface and is dehydrated at the defect site. Indeed, in the dehydration of 1,3-butanediol and of 1,4-butanediol into unsaturated alcohols, the activity increased with increasing the CeO2 particle size. The CeO2 (111) facets, more numerous on larger particles, have active sites for the dehydration reactions.56
In the dehydration of 1,4-butanediol, 3-buten-1-ol is produced over CeO2 between 375 °C to 450 °C.56–58 The better selectivity (68.1 %) was observed at 400 °C. Side reactions, such as isomerization of the initial product, but-3-en-1-ol, hydrogenation, and dehydrogenation proceed, together with the cyclization of 1,4-butanediol to tetrahydrofuran (Scheme 3).
Among the various lanthanide oxides, CeO2 shows properties that differ from those of the other members of the lanthanide series. In the reaction of 1,5-pentanediol, CeO2 catalyzed only undesirable side reactions, such as dehydrogenation, as well as dehydration;59 δ-valerolactone and cyclopentanone being the major products. The authors speculated that the redox cycle of Ce4+/Ce3+ on the surface of CeO2 plays a key role in the activation of 1,5-pentanediol. Although 1,3-butanediol is readily activated on CeO2 at 325 °C, higher temperatures are needed to activate 1,4-butanediol and 1,5-pentanediol on CeO2. Thus, both the reactivity and the selectivity over CeO2 decrease in the order of 1,3->1,4->1,5-diols.
Triols, such as 1,2,3-propanetriol (glycerol) and 1,2,3- and 1,2,4-butanetriols have been dehydrated to afford the corresponding hydroxyketones, while 1,2-propanediol was dehydrogenated to form hydroxyacetone over both ceria-supported and nonsupported Cu-based catalysts.60
2.3. Other dehydrations
4-Hydroxy-2-butanone has been converted into 3-buten-2-one over various oxide catalysts at 160 °C. Ceria shows a relatively high initial activity but is rapidly deactivated. This deactivation is probably caused by the strong interaction of 3-buten-2-one with acid sites to form carbon species on the catalyst surface; however, the results of NH3-temperature programmed desorption (TPD) could not explain this deactivation; CeO2 showing a much lower acidity than other oxides such as Al2O3 and SiO2/Al2O3. In this case, the redox nature of CeO2 may be the cause of this deactivation.61
3.1. Acid condensation
An important route for ketone production is decarboxylative condensation of two carboxylic acids. Symmetrical, nonsymmetrical, and arylalkylketones have been obtained by ketonization of carboxylic acids in the gas phase over ceria-based catalysts under flowing conditions, proceeding according to the general equation:((5))
Usually, the ketonization by acid condensation was carried out in the presence of ceria-based catalysts (10–20 % CeO2 supported on SiO2, TiO2, or Al2O3) between 300 and 450 °C.62–70
Symmetrical ketones such as 3-pentanone, 6-undecanone, and 7-tridecanone have been obtained from ketonization of the appropriate acids.62, 65 This method was applied to the synthesis of nonsymmetrical ketones used as raw materials for pesticides and pharmaceutical products.66–68 For example, methylcyclopropylketone and methylnonylketone were produced by the condensation of acetic acid with cyclopropanecarboxilic acid and decanoic acid, respectively. In the reaction of propanoic acid, the reactivity of the carboxylic acid slightly decreased as its chain length was increased, and branched acids were less reactive than linear ones.68
Aromatic ketones were obtained from aromatic carboxylic acids and acetic acid over CeO2/Al2O3, chosen as an industrial catalyst for the preparation of the 2-methylacetophenone because of its higher productivity, longer catalyst life, and the lifting of legal restrictions on catalyst handling. The catalyst system can also be applied to the preparation of acetophenone, nitroacetophenone, and chloroacetophenone. Also, carboxylic acids were selectively reduced to aldehydes by condensation with formic acid. Moreover, the literature mentions some patents relating to the synthesis of nonsymmetrical ketones from carboxylic acids over CeO2/Al2O3 at 350–580 °C71–73 and CeO2/TiO2 at 440 °C.74
The condensation of acetic acid to acetone75 or of propanoic acid to 2-pentanone56 was also carried out over pure CeO2 from 300 °C. The catalytic sites were suggested to be Lewis acid–base pair sites, with the Lewis acid sites (Ce4+) being reducible.75 The activity towards acid condensation increased with increasing particles size, because CeO2 (111) facets are predominant on larger particles and have active sites for the condensation reaction of propanoic acid.56 Stubenrauch et al. had shown previously by TPD that acetone is produced during the decomposition of acetic acid only on the CeO2 (111) surface.76
More recently kinetic factors have been investigated for ketonization upgrading processes over a Ce0.5Zr0.5O2 catalyst from 175 to 350 °C.77 This material showed desirable catalytic properties for ketonization of carbohydrate-derived carboxylic acids in the presence of other monofunctional oxygenated species, such as alcohols or ketones.78 Under these conditions two different reactions take place, esterification and ketonization. Both consume hexanoic acid, used as model molecule. Direct ketonization of esters does not take place in the presence of acids.
3.2. Ketonization of esters
Glinski and collaborators have studied the ketonization of various aliphatic or aromatic esters over 20 wt % MOx/S, where M=Mn, Ce, Zr, or Th and S=Al2O3 or SiO2, in the gas phase between 300 and 425 °C.79–81 The ketonization of ethyl ester in the presence of these oxide catalysts proceeds according to the general equation:((6))
In these studies, the highest yields in ketones were always obtained with manganese-based catalysts. 3-Pentanone and 7-tridecanone were formed in ketonization of pure aliphatic esters, ethyl propanoate and ethyl heptanoate. Unfortunately, from pure ethyl benzoate, benzene was obtained instead of diphenylketone. Dialkyl- and arylalkyl-ketones were obtained from the cross-ketonization of a mixture of aliphatic and aromatic esters.80
The reactivity of tert-butyl heptanoate was higher than that observed 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 the most active catalytic system, MnO2/Al2O3. In the case of n-alkyl heptanoates, the reactivity increased with the elongation of the n-alkyl chain.81
In the cycloketonization of diethylhexanedioate, only moderate yields of cyclopentanone (<35 % over MnO2/Al2O3) were achieved, accompanied by various amounts of byproducts.79 Cycloketonization of dimethylhexanedioate was investigated over pure CeO2 between 350 and 475 °C.82 The conversion of diethylhexanedioate increased with increasing reaction temperature whereas the selectivity to cyclopentanone decreased. This decrease at high conversion was mainly caused by a consecutive reaction of cyclopentanone into 2-methylcyclopentanone due to alkylation with methanol, produced by the cycloketonization.
Long-carbon-chain ketones (C17H35COC17H35, C15H31COC15H31, CH3COC17H35, CH3COC15H31) were also obtained from methyl esters of fatty acids (essentially C17H35COOCH3 and C15H31COOCH3), in methanol at atmospheric pressure at 385 °C (optimal temperature), over catalysts containing SnCeRh oxides in a molar ratio 90:9:1 (total yield: 63 %, conversion: 96 %).83 A similar catalyst was used to transform methyl laurate (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, Sato et al. observed that the formation of propanal and 3-pentanone and the conversion of 1-propanol to 3-pentanone increased with increasing CeO2 content. Elsewhere, Plint et al. studied the reaction of a series of primary and secondary alcohols containing n carbon atoms under oxidative conditions. Symmetrical ketones with 2n−1 carbon atoms were produced in the presence of O2 over 40 % CeO2/MgO (450 °C, 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 conversion being consistently high (>90 %).86, 87
A reaction mechanism in which alcohol is oxidized to aldehyde and then carboxylic acid [Equation (7)] following by the coupling of two equivalents of acid to give the symmetrical ketone and CO2 [Equation (8)], is proposed according to the general scheme:((7)), ((8))
The reaction of a 1:1 mixture of 1-hexanol and 1-heptanol produced a statistical yield of the three expected ketones.87 Under nonoxidative conditions, 1-propanol was preferentially converted into 3-pentanone over CeO2-Fe2O3 catalysts at 450 °C and propanal, 3-hydroxy-2-methylpentanal, and n-propyl-propionate were observed as by-products.88 The addition of Fe2O3 to CeO2 enhances the ability of CeO2 for the catalytic dehydrogenation of 1-propanol to propanal, without losing the ability to dimerize propanal. The formation of 3-pentanone from 1-propanol over CeO2-Fe2O3 proceeds via aldol addition of propanal into 3-hydroxy-2-methylpentanal, followed by decomposition into 3-pentanone, while n-propylpropionate is formed as a mere by-product [Equation (9)].((9))
This reaction was applied to the cyclization of 1,6-hexanediol into cyclopentanone, an useful intermediate for medical and perfume products.89 The cyclization of 1,6-hexanediol was selectively catalyzed by CeO2-MnOx with a Mn content of 10–30 mol %: cyclopentanone was produced with a selectivity of 80 mol % at 450 °C. A possible reaction pathway over ceria-based catalysts is illustrated in Scheme 4.
Elsewhere, SnCeRh oxides already used in the condensation of methyl esters of fatty acids have shown high activity and selectivity at relatively low temperatures in the ketonization of n-butanol. At 350 °C, 4-heptanone was obtained with 89 % selectivity and 88 % n-butanol conversion.90, 91 Introducing a cerium dopant to the tin basic dioxide structure caused the appearance of strong acidic centers of Lewis type at the surface. As the selectivity to 4-heptanone increases with the presence of cerium, the Lewis acidic sites become more involved in the mechanism of alcohol, aldehyde, or acid condensation.
Ceria is able to change reversibly from Ce4+ under oxidizing conditions to Ce3+ under reducing conditions. Oxygen atoms in CeO2 units are very mobile and easily leave the ceria lattice, giving rise to a large variety of nonstoichiometric oxides with the two limiting cases: CeO2 and Ce2O3.
4.1. Hydrogenation of CC bonds
4.1.1. Hydrogenation of phenol to cyclohexanone
Cyclohexanone is a key raw material in the production of both caprolactam for Nylon-6 and adipic acid for Nylon-6,6. Industrially, cyclohexanone is produced either by the oxidation of cyclohexane or by the hydrogenation of phenol to cyclohexanol, followed by dehydrogenation of cyclohexanol. Selective hydrogenation of phenol to cyclohexanone is attractive in terms of capital cost and energy saving.
The selective hydrogenation of phenol to cyclohexanone was carried out in gas phase over supported Pd catalysts.92, 93 The catalytic performance was improved by a modification of the electronic surroundings of Pd, induced by a promoter or by a modification of the acid–base characteristics of the support, leading to a change in the adsorption–desorption equilibrium of reactants and products. Similar to La2O3, CeO2 as support provides a better activity and a good stability to catalysts.92
The high surface area mesoporous oxide support gives rise to well dispersed and stable metal particles on the surface and then has some beneficial effect on the catalytic performance. The vapor-phase hydrogenation of phenol, at atmospheric pressure, over 3 % Pd supported on mesoporous CeO2 (Pd/CeO2-Ms) at 180 °C produced a mixture of cyclohexanone (about 50 %), cyclohexanol (35 %), and cyclohexane (15 %) with a phenol conversion of about 80 %. The selectivity depended on the modes of phenol adsorption, which are governed by the nature of the support. On the Pd/CeO2-Ms catalyst, under the reported experimental conditions, there was a significant reduction of the CeO2 surface, which resulted in the formation of nonstoichiometric CeO2 creating acid and basic sites. Over basic sites, nonplanar-adsorbed phenol led to the formation of cyclohexanone, while coplanar-adsorbed phenol led to the formation of cyclohexanol.93
More recently, the hydrogenation of phenol to cyclohexanone was carried out in the liquid phase with ethanol as solvent in order to improve the selectivity, because the reaction could be performed at relatively low temperature.94, 95 The maximum phenol selectivity for cyclohexanone over 5.8 % Pd-Ce-B supported on hydrotalcite reached 80 %, with a phenol conversion of 82 %.94 A similar conversion and selectivity were obtained by the same authors on Ce-doped Pd-B amorphous alloy catalysts.95 The promoting effect of the Ce-dopant on the catalytic performance could be attributed to stabilization of the amorphous structure of the Pd-B alloy by cerium; the electron-enriched Pd active sites, owing to the electron-donation from cerium; and the increase of surface basicity resulting from the formation of Ce2O3.
4.1.2. Hydrogenation of 1,3-butadiene
The hydrogenation of 1,3-butadiene was carried out in the gas phase at atmospheric pressure between 47 to 107 °C. The presence of CeO2 in Pd-CeO2 supported on Al2O3 catalysts favored hydrogenation at the 1,2 position of the 1,3-butadiene molecule, increasing the selectivity towards 1-butene.96 The absence of butane as reaction product indicated that the adsorption strength of 1,3-butadiene was reduced by the presence of cerium, which modifies the electronic structure of Pd, thereby avoiding total hydrogenation.
4.1.3. Hydrogenation of acrylonitrile
Selective gas-phase hydrogenation of acrylonitrile (CH2CHCN) over low-loaded Pd (0.05 wt %)/Al2O3 catalysts doped with various contents of cerium gave propionitrile with a selectivity nearly 100 %. The addition of cerium improved the activity and the stability of these catalysts.97 The sintering of the dispersed palladium particles was retarded by the addition of cerium and the catalytic activity was preserved. Elsewhere, the presence of Pd facilitated the reduction of Ce4+ to Ce3+ to form new active sites.97
4.1.4. Hydrogenation of mesityl oxide
Mesityl oxide (4-methyl-2-penten-2-one) was selectively hydrogenated in the gas phase at 175 °C 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 industrial level.98 The reduced sample was more active than the oxidized one; the selectivities being 93 % and 100 %, respectively. Binary copper–cerium intermetallic compounds (CeCu2) are interesting precursors to provide new supported copper materials.
4.1.5. Hydrogenation of sunflower oil
The selective hydrogenation of ethyl esters of traditional sunflower oil, a mixture comprising linoleic acid C18:2 (9,12) Z,Z (60.11 %), oleic acid C18:1 (9) Z (27.49 %), and stearic acid C18:0 (3.72 %), was carried out at low temperature (40 °C) in ethanol as solvent in the presence of supported palladium catalysts (Scheme 5). The aim was to selectively hydrogenate linoleic acid C18:2 (9,12) Z,Z towards oleic acid C18:1 (9) Z, avoiding Z–E isomerization, position isomerization, and complete hydrogenation. The use of CeO2 as oxide support to deposit palladium did not improve the selectivity toward the C18:1 (9) Z or (12) Z compared to Pd/SiO2.99
4.1.6. Hydrogenation of acetylene
By using Au/CeO2 as catalyst in acetylene hydrogenation in the gas phase at 300 °C, a selectivity of 100 % toward ethylene was obtained with a high conversion, which depended on the H2/C2H2 ratio. Above 300 °C, the only byproduct was methane, formed by carbene intermediates that also polymerized, giving rise to deactivation. The high selectivity could be explained by the difference in the strength of adsorption onto the gold surface between acetylene and ethylene.100
4.1.7. Hydrogenation of benzene
The hydrogenation of benzene in the gas phase at atmospheric pressure can be used to characterize the size of metallic particles of supported ceria-based catalysts.[101–104] With this reaction it is possible to determine the amount of the accessible metallic atoms of low-loaded supported rhodium catalysts.102, 104
A Ni/CeO2 catalyst exhibited a different behavior from that observed with Al2O3 or SiO2 supports. The modification of the catalytic properties of Ni/CeO2 catalysts with reduction pretreatment is correlated to a transformation of the CeO2 support and to strong interactions between theses species and metal particles. In another study, nickel catalysts supported on ceria, synthesized by γ-radiolysis, were tested in the hydrogenation of benzene. At 100 °C, the catalyst completely converted benzene to cyclohexane and remained stable for at least 20 h. The high catalytic performance of Ni/CeO2 was attributed to the high dispersion of nickel and to the promoter role of the support, through the formation of NiCe phases.103
The partial hydrogenation of benzene to cyclohexene is of great industrial interest: cyclohexene can be used in the synthesis of various organic compounds.105, 106 Ru/CeO2 catalysts are very promising systems for cyclohexene formation through partial benzene hydrogenation in a three-phase medium, in the presence of water and TiCl3. The maximum yield in cyclohexene (about 17 %) was obtained with Ru/CeO2 catalysts, noncalcinated and reduced at 500 °C or 750 °C.105
The partial hydrogenation of benzene to cyclohexene was also carried out over Ru–Ce catalysts supported on siliceous materials such as SBA-15, in the presence of ZnSO4 in aqueous solution. The existence of the CeIII species decreased the number of exposed Ru atoms, increased the number of electrons on metallic Ru, and enhanced the hydrophilicity of the catalyst. The maximum yield of cyclohexene (53.8 %) was obtained on a RuCe/SBA-15 catalyst, with a molar ratio Ce/Ru equal to 0.4.106
4.1.8. Hydrogenation of biphenyl
Hexagonal mesoporous silica (HMS) was used as support for the preparation of Au catalysts, and was tested in the liquid-phase hydrogenation of biphenyl at 5 MPa and 215 °C (in a solution of n-tetradecane, with about 10 % n-hexadecane). Modification of HMS by Ce led to Au-supported catalysts that were more stable with respect to sintering. In the Au/HMS–Ce catalyst, Ce was not incorporated into the framework of HMS, leading to clusters of Au and CeO2. However, further investigations on the nature of the Au–CeO2 interaction could yield more explanations with regard to the performance of Au/HMS–Ce catalyst.107
4.2. Hydrogenation of CO bonds
4.2.1. Hydrogenation of α-β-unsaturated aldehydes
The selective hydrogenation of α-β-unsaturated aldehydes to unsaturated alcohols is an important reaction in the production of many pharmaceutical, agrochemical, and fragrance compounds. The hydrogenation of the CC bond is thermodynamically more favorable than the CO hydrogenation, and low yields of the desired product are obtained with the conventional hydrogenation catalysts.
Cerium-based platinum catalysts have been extensively studied for the hydrogenation of crotonaldehyde (CH3CHCHCHO)108–118 and citral ((CH3)2CCH(CH2)2C(CH3)CH-CHO).119, 120 The activation of the carbonyl bond is induced by the presence of oxygen vacancies sites located at the interface between ceria and the platinum particles.
Indeed, the selective hydrogenation of α-β-unsaturated aldehydes has been used as probe reaction to study the existence of a “strong metal–support interaction” (SMSI) effect in ceria-supported and -promoted noble catalysts. Ceria is able to form oxygen vacancies and intermetallic compounds after reduction treatment at relatively high temperatures.
Touroude and collaborators have studied the selective hydrogenation of crotonaldehyde on Pt/CeO2 in the gas phase at atmospheric pressure.108–110 On chlorine-free Pt/CeO2, the crotyl alcohol selectivity increased up to more than 80 % (conversion: 45 %) when the reduction temperature of the catalysts reached 700 °C. The presence of chlorine, during the catalyst synthesis, preserves the catalytic properties of platinum metal for the hydrogenation of CC bond; chlorine atoms around platinum particles inhibit the diffusion of cerium atoms inside the metal particles and prevents the formation of CePt5 alloy. By controlling the nanostructure, size, and morphology of supported platinum particles, the authors showed that it is possible to orientate the selectivity in the hydrogenation of crotonaldehyde.110
The presence of zinc facilitated the reduction of surface ceria, thereby increasing the potential number of Ce3+–Pt metal interface sites, particularly in cases where small Pt particles were located in ceria rich zones of the support.113–115 In the case of vapor-phase hydrogenation of crotonaldehyde, the overall catalytic activity increased significantly after reduction at 500 °C in the zinc-containing catalyst, and furthermore, the selectivity toward the hydrogenation of carbonyl bond is improved. Pt on mesostructured CeO2 nanoparticles embebbed within ultrathin layers of highly structured SiO2 binder showed the highest reported activity, with 80 % selectivity for the chemoselective hydrogenation of crotonaldehyde.118 By increasing the reduction temperature, the number of Pt-CeO2−x interfacial sites, which are responsible for activating the carbonyl bond, increased.
The hydrogenation of citral was carried out in the liquid phase at 50 °C in ethanol119 or at 70 °C in isopropanol.120 The formation of geraniol (E isomer) has been observed as the sole product on Pt/CeO2 and has been attributed to the influence of the SMSI state in the selective hydrogenation of CO bond.119
On a Pt/C catalyst promoted with highly dispersed ceria, the main products of the hydrogenation of citral were citronellal (hydrogenation of the conjugated CC bond), the unsaturated alcohols geraniol and nerol, and the saturated alcohol citronellol (by hydrogenation of the CO bond of citronellal).120 On the one hand, the creation of new Pt–CeOx sites at the metal/support interface act as Lewis acid sites able to activate the CO bond of the citral molecule; on the other hand, the existence of an electronic interaction between the reduced ceria particles and the active metal leads to an increase in electron density on the platinum particles, with subsequent weakening of the adsorption of citral via the CC bond. Moreover, when tin is added, the ceria reducibility is increased. The presence of Snn+ species, also able to act as Lewis acid sites, on the surface of platinum particles and/or in their close vicinity could account for the increase in selectivity to unsaturated alcohols with reduction temperature. The increase of conversion after reduction at high temperature in these catalysts could be also explained by the creation of new PtSnn+ sites active for hydrogenation of the CO bond in the citral molecule.
Previously, Barrault et al. have studied the hydrogenation of cinnamaldehyde (C6H5CHCH-CHO) in the liquid phase with propylene carbonate as solvent over cobalt catalysts supported on activated carbon, and showed that the addition of cerium to cobalt increased the selectivity to unsaturated alcohol without decreasing of the activity.121 Elsewhere, the use of cerium on Ru-based catalysts supported on alumina and activated carbon increased the selectivity to the unsaturated alcohols in hydrogenation of crotonaldehyde (in gas phase) and of citral (in liquid phase with isopropanol as solvent).122 However, the observed activities were very low.
More recently, Campo et al. studied the influence of the specific surface area of the support on the selective hydrogenation of crotonaldehyde on Au/CeO2.123–126 They showed that the high surface area catalyst (Au/HAS-CeO2, with 240 m2g)−1 was active and highly selective towards the hydrogenation of the CO bond either at 120 °C and atmospheric pressure123–125 or at 80 °C in the liquid phase (solvent: isopropyl alcohol).126 Other Au/CeO2 catalysts with lower specific surface areas showed a rather low selectivity. The high selectivity is an intrinsic characteristic of gold particles (particle size lower than 4 nm on Au/HAS-CeO2 and higher than 9 nm on other Au/CeO2 catalysts), though ceria plays an important role as a result of its redox and acid–base properties.
The selectivity to the unsaturated alcohol is governed by different factors: the nature of the active metal, metal particles size, support effects, and presence of promoters or bimetallic phases.
4.2.2. Enantioselective hydrogenation
Mixed nickel–cerium oxides, using tartaric acid as modifier, were used for studying the enantioselective hydrogenation of methylacetoacetate in methyl 3-hydroxybutyrate (Scheme 6). X-ray photoelectron spectroscopy (XPS) and FTIR analysis evidenced that the modifying agent reacts with the metallic nickel to give nickel tartarate. In the presence of this modifier, the reduction of Ce4+ to Ce3+ was improved. The tartarate salt is stabilized by these Ce3+ species in close vicinity of Ni2+, forming a complex with methyl acetoacetate. Thus, a stable six-membered ring complex is formed between a hydroxyl group of tartarate, the reactive CO bond, and the methylenic group (acidic H) of methylacetoacetate and gives rise to a single product, having a specific configuration depending on the absolute configuration of the modifying agent.127
Another, similar reaction concerned the enantioselective hydrogenation of 1-phenyl-1,2-propanedione128 (Scheme 7). This reaction was carried out in the liquid phase (cyclohexane was used as solvent) at 25 °C and a dihydrogen pressure of 40 bar over an iridium-supported catalyst promoted by ceria, in the presence of cinchonidine as modifier. The presence of Ce in the Ir/SiO2 produced a slight increase in both the activity and the enantioselectivity (formation of (R)-2-hydroxy-1-phenyl-1-propanone). The cerium oxide species were mainly present on the silica support and contributed to the formation of Irδ+ species that were responsible for CO bond polarization and reaction rate enhancement.
4.2.3. Hydrogenation of carboxylic acids to aldehydes
For the hydrogenation of aromatic carboxylic acids, various metal oxides such as CeO2 have shown high activity and selectivity to corresponding aldehydes.[129–133] The hydrogenation of benzoic acid over CeO2 proceeds at up to 350 °C in gas phase; the selectivity to benzaldehyde was more than 95 % and the activity was controlled by the number of oxygen vacancies that are produced under the reaction conditions. The carboxylic acid deoxygenates with the help of an oxygen vacancy according to the Mars–Van Krevelen mechanism, forming an acylium ion that is hydrogenated to aldehydes. An enhancement of the catalytic activity of CeO2 at low temperatures could be achieved by addition of the promoters, as Mn, Zr, In, and Pb oxides. However, ceria catalysts show little deactivation in stability test owing to coke formation and the valence changes of Ce over the catalyst.132, 133
To limit the use of CeO2, owing to its high cost, some authors have used mixed CeO2–Al2O3 oxides for the hydrogenation of benzoic acid to benzaldehyde. To improve their performances, the simultaneous addition of Mn and K to theses oxides is necessary. For hydrogenation of aliphatic carboxylic acids that have two α-hydrogen atoms, CeO2 shows a low selectivity because undesirable ketonization occurs.131
4.2.4. Hydrogenation of aldehydes to alcohols
The reduction of benzaldehyde has been carried out at 300 °C in a helium or dihydrogen atmosphere over simple metal oxides as CeO2. CeO2 was not proved to be the better catalyst for either the Cannizzaro reaction (under helium) to produce benzyl alcohol and benzoate or direct hydrogenation (under dihydrogen) when compared with other oxides.135
Under helium, the bare CeO2 support was more active than the corresponding copper-supported catalyst, which is due to the lesser amount of active hydroxyl groups in the surface of supported catalyst than of bare support.136 Indeed, the Cannizzaro reaction consumes surface OH groups to form benzyl alcohol and benzoate surface species.
Under dihydrogen, a higher activity and a higher selectivity to benzyl alcohol were obtained on an irreducible support. On a reducible support such as CeO2, the activity was lower and could 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 recently, a significant amount of benzyl alcohol was produced on Ni/CeO2 under atmospheric pressure of H2 at only a low temperature (70 °C). The selectivity to toluene increased with increasing temperature. Toluene is the product of consecutive benzyl alcohol hydrogenolysis. This mechanism could involve hydride species, which could be formed on nickel metal supported on a reducible support such as CeO2.137
Elsewhere, Ce-doped NiB amorphous alloy catalysts have exhibited excellent selectivity to furfural alcohol during liquid-phase hydrogenation of furfural (ethanol used as solvent). The promoting effect of Ce dopant could be interpreted with a mechanism of activation and hydrogenation of CO bond.138 The cerium in a low-valent state (Ce3+) on the surface could act as Lewis adsorption sites, which have a strong affinity for the oxygen atom of a carbonyl function and cause a polarization of the CO bond. This polarization favors nucleophilic attack of the carbon atom by hydrogen dissociatively adsorbed on the neighboring Ni active sites.
4.2.5. Hydrogenation of esters
Diols can be produced from the hydrogenation of esters over RuSn-based catalysts supported on various oxides in liquid phase with dioxane as solvent. Unfortunately, CeO2 was not proved to be the better support. However, an enhancement of the activity could be attributed to an interaction of the CO bond 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 cyclohexanone with isopropyl alcohol as hydrogen donor was carried out on mixed oxides of CeO2 and ZnO with a high surface area, to investigate the effect of rare earth oxide on the activity of ZnO. Addition of ceria into zinc oxide was found to increase the catalytic activity. Cyclohexanol was the only product observed in this 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 the particle morphology, surface area, and acid–base properties.140–142
4.3. Hydrogenation of CN bonds
Catalytic hydrogenation of nitriles is an important route to production of amines, which are of practical importance, in particular primary amines, as chemicals and intermediates.
The gas-phase hydrogenation of acetonitrile over various Pd-based catalysts gives a mixture of ethylamine, diethylamine, and triethylamine. The use of a CeO2 support is significant for preparing PdZn, PdGa, or PdIn alloy species on its surface. The activity can then be enhanced while maintaining high selectivity to ethylamine (97 % at 170 °C over Pd/ZnO/CeO2). Ceria facilitates the reduction of ZnO to Zn, which is then alloyed with Pd.143
The formation of primary amines from nitriles on copper catalysts is normally followed by the formation of secondary amines. Therefore, the supported copper–lanthanide oxides are active and very selective for the propionitrile gas phase hydrogenation to n-propylamine; the 2Cu-CeO2 being the more active. The basicity of the copper–lanthanide oxides seems to play a key role in this reaction, and the selective formation of primary amine is due to the lack of acid sites, which are known to catalyze condensation reactions that lead to secondary or tertiary amines.144, 145
4.4. Other hydrogenation reactions
Highly dispersed copper promoted by nickel or cobalt supported on CeO2 catalysts exhibit high conversion (96.6 %) for the gas-phase hydrogenation of ortho-chloronitrobenzene to ortho-chloroaniline with a high selectivity (98 %) and without fast deactivation.146 The high activity and stability of these bimetallic catalysts was attributed to a better dispersion and the formation of smaller particles as well as to a high specific surface area of the catalyst.
The oxidation of organic compounds is one of the most important reactions for synthesis of fine chemicals. They are many examples of oxidation reactions involving chromium-, manganese-, or vanadium-based compounds that are used in stoichiometric quantities and present serious disadvantages because they are expensive, toxic, and produce equimolar quantities of waste that are often difficult to separate from the desired products. Owing to ceria′s high oxygen storage capacity and good catalytic properties, the use of ceria-based materials has been intensively investigated and applied in the catalytic oxidation reaction.
5.1. Aerobic oxidation of alcohols
Oxidation of alcohols to the corresponding carbonyl compounds is an important transformation in organic synthesis; aldehydes and ketones being an important class of compounds in organic chemistry. The oxidation of primary alcohols to aldehydes is an interesting process in perfumery industry. The aerobic oxidation of ortho- and para-hydroxybenzyl alcohol selectively produced the corresponding aldehydes with good yields, while the selectivity of meta-hydroxybenzyl alcohol was more towards the corresponding acid at the expense of aldehyde. The selectivity was improved by reaction in aqueous methanol and the addition of CeCl3. The catalytic role of CeCl3 in the catalyst system Pt/C–CeCl3–Bi2(SO4)3 is not clear, but it might protect the generated aldehyde from further oxidation by acetalization.147
The reaction of ethanol on unreduced and H2-reduced CeO2 and 1 wt % Pd/CeO2 has been investigated by steady state reactions, temperature programmed desorption (TPD), and in situ FTIR spectroscopy. Steady-state reactions have shown a zero-reaction-order dependency for dioxygen. The conversion of ethanol was increased by the addition of Pd, from 15 and 30 % on CeO2 and H2-reduced CeO2, to 71 and 63 % on Pd/CeO2 and H2-reduced Pd/CeO2, respectively. Ethanol was converted into acetaldehyde, which in turn can react to give various compounds (e.g., acetone, crotonaldehyde, CO, CO2, methane, benzene). Benzene formation was detected only on Pd/CeO2 catalysts, with the H2-reduced Pd/CeO2 catalyst decreasing benzene formation to almost negligible amounts. The H2-reduction of the oxide surface inhibited the β-aldolization route owing to a considerable decrease of the Lewis-base sites, oxygen anions.148
A catalyst of ruthenium combined with cobalt hydroxide and cerium oxide (Ru–Co(OH)2–CeO2) exhibited a high activity for the oxidation of various alcohols in liquid phase with benzotrifluoride as solvent, in the presence of dioxygen. Allylic, benzylic, and secondary alcohols gave high yields of the corresponding carbonyl compounds.149 The oxidation of primary aliphatic alcohols led to the formation of corresponding carboxylic acids. α,ω-Primary diols were selectively transformed into the corresponding lactones. In the case of 1,4-pentanediol having primary and secondary hydroxyls, methyl-γ-butyrolactone was obtained with 87 % yield by an intramolecular competitive oxidation (Scheme 8).149, 150
With Ce-free Ru catalysts, the oxidation of primary alcohols led to the formation of aldehydes. The oxidation of this intermediate to carboxylic acid was slow. Moreover, when 2,6-di-tert-butyl-para-cresol was added in the oxidation of 1-octanol as a radical scavenger, octanal was formed without formation of octanoic acid. The high activity of the Ru–Co(OH)2–CeO2 catalyst might be due to the high oxidation state of the Ru species (RuIV), arising from the Co atoms in the vicinity of the CeO2 particles. The radical process of the aldehyde oxidation might be facilitated by synergism between the Ru, Co, and Ce components.149, 150
Au/CeO2 catalysts were also effective for the selective oxidation of primary alcohols (benzyl alcohol) to aldehydes, under solvent-free conditions at 100 °C in the presence of O2. For more acidic supports, such as Fe2O3, subsequent oxidation of aldehydes to the corresponding acids occurred.151
Gold nanoparticles supported on ceria are excellent general heterogeneous catalysts for the aerobic oxidation of alcohols.152–156 The combination of small-crystal-size gold (2–5 nm) and nanocrystalline ceria (ca. 5 nm) led to a highly active, selective, and recyclable catalyst for the oxidation of alcohols into aldehydes or ketones using dioxygen at atmospheric pressure as oxidant, in the absence of solvent and base. The nanometer-scale ceria surface stabilized the positive oxidation states of gold by creating Ce3+ and oxygen-deficient sites in the ceria. Aliphatic primary alcohols were more reluctant to undergo oxidation in the absence of solvent. Notably, they predominantly gave the corresponding ester with high selectivity. The esters were directly formed by the hemiacetal intermediate, which was dehydrogenated (Scheme 9).152
Abad et al. proposed a mechanism for the aerobic oxidation of alcohols over Au/CeO2, where the alcohol is adsorbed on Lewis-acid sites to give a metal alkoxide that subsequently undergoes a rapid hydride transfer from CH to Ce3+ and Au+ to give the ketone and CeH and AuH. In the presence of dioxygen, cerium-coordinated superoxide (CeOO⋅) species are formed and, by hydrogen abstraction from AuH, become cerium hydroperoxide, which is responsible for the formation, after reduction of CeIV, of the initial Au+ species. The absence of gold would render this step impossible and lead to a depletion of CeIII.152
An allylic alcohol, such as 1-octen-3-ol, undergoes a chemoselective oxidation in the presence of Au/CeO2 catalysts to the corresponding ketone without oxidizing or isomerizing the CC double bond. In contrast to this, Pd/CeO2 catalysts promote a considerable degree of CC double bond isomerization (1,2-migration) with the formation of 3-octanone as the final product.153, 154 Au/CeO2 is a good catalyst for the oxidation of allylic alcohols without solvent or in organic media (toluene as solvent). The chemoselectivity of Pd or AuPd for the solventless oxidation of this family of alcohols is very low when compared with Au catalysts and this selectivity can be correlated to the stability and concentration of metal hydrides, AuH and PdH.154 Primary aliphatic alcohols are selectively oxidized to the corresponding aliphatic aldehydes up to moderate conversions. When the conversion increased, the selectivity towards the aldehyde decreased significantly, owing to overoxidation of the aldehyde to the corresponding carboxylic acid. The activity of a gold catalyst for the aerobic oxidation of alcohols involves the presence of a high density of positive gold atoms that could act as Lewis acid sites that coordinate with alcohols to form gold alcoholates and also accept hydrides. In this regard, the role of the support should be, on the one hand, to provide stability for positive gold species by interfacial gold–support interactions, and on the other hand, to facilitate oxygen activation to promote the reoxidation of metal hydrides.156
The oxidation state of gold particles deposited on different supports such as CuO-CeO2 and CeO2 was investigated during the liquid-phase catalytic aerobic oxidation of 1-phenylethanol (in toluene) using in situ XAFS combined with FTIR for product analysis. A correlation between the oxidation state of Au and catalytic activity was observed for Au/CuO–CeO2. The 1-phenylethanol conversion increased with concomitant reduction of Au species. Different behaviors were observed for Au/CeO2, with the activity decreasing simultaneously with the reduction of Au species. However, this deactivation is not directly related to reduction of the gold species.157
Au/CeO2 catalysts with various gold particle sizes showed a moderate catalytic activity and high selectivity in the liquid-phase oxidation of benzyl alcohol to benzaldehyde in mesitylene or in toluene. The catalytic behavior of this reaction was affected by the gold particle sizes, showing highest activity for the catalyst containing gold particles of 6.9 nm average size.158 The effect of the adsorption of the two thiols, n-octadecanethiol (ODT) and mercaptoacetic acid (MAA), has been studied on CeO2-supported gold catalysts with different Au particle sizes (2.1 and 6.9 nm). Upon addition of 10 mol % thiol/Autotal, an almost complete loss of activity in the aerobic oxidation of benzyl alcohol was observed when the Au catalysts 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 later forms reversibly bound species on the surface, likely adsorbing on edge and corner sites. On the other hand, MAA strongly binds to the edge and corner sites on the supported Au. The adsorption of MAA on crystal facets is thermodynamically the least stable configuration.159
A mesoporous CeO2 crystalline film used as support was loaded with gold particles of about 5 nm. The resulting Au/CeO2 composite showed a good catalytic activity and stability for benzyl alcohol aerobic oxidation in absence of solvent and base.160
The catalysts ruthenium hydroxide and manganese oxide supported on cerium oxide Ru/MnOx/CeO2 show a high catalytic activity for the oxidation of alcohols to the corresponding carbonyl compounds in liquid phase with α,α,α-trifluorotoluene as solvent. Nonactivated aliphatic alcohols required longer reaction time for their oxidation than the other activated 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 oxidation of alcohols at 27 °C under dioxygen atmosphere. Such high catalytic activities are attributable to cooperative action among the Ru species, MnOx, and CeO2 in the catalyst.161 The catalytic activity could significantly be improved by deposition of the ternary RuMnCe oxidic mixture on redox-active supports, especially on ceria.162
Recently, silver catalysts were proposed as a promising alternative, being less expensive than Au or Pt catalysts and applicable to a wide variety of alcohols.163 A Ag/SiO2 catalyst that acts as efficient catalyst (but only in the presence of ceria: 10 wt % Ag/SiO2 mixed with ceria in a ratio of 2:1) gave the best catalytic performance in the selective liquid-phase oxidation of various benzyl alcohols (solvent: toluene). The negative effects of electron-withdrawing groups in the benzyl alcohol oxidation suggest a mechanism where metallic silver acts as main component for the dehydrogenation via a cationic intermediate. The role of cerium can rather be ascribed to the activation of molecular oxygen.
5.2. Dehydrogenation of alcohols
An infrared spectroscopy study of the adsorbed species and the gas-phase products was reported for the transformation of 2-propanol over CeO2 catalysts calcined at various temperatures in the gas phase.164 The dehydrogenation to give acetone started to take place at 150 °C. The acetone, through further interaction with the surface, became involved in another reaction to give isobutene and methane when the reaction temperature increased to 250 °C and the dehydration, which led to propene, had begun. The increase of the dehydration activity of ceria, upon calcination, was due to an increase of the acidity of Brønsted acid sites and an increase of the number of Lewis acid sites. The alcohol dehydrogenation reaction is controlled by the electronic mobility of the catalyst surface and decrease with calcination. The aldol condensation reaction can also occur over CeO2 catalysts.164
Reactions of ethanol on Cu–Mg5CeOx in the gas phase at 300 °C led to the formation of acetaldehyde, n-butyraldehyde, and acetone as predominant products. The initial rate of ethanol dehydrogenation increased linearly with the Cu surface area. Acetaldehyde concentrations increased rapidly to equilibrium levels that became independent of Cu content in CuyMg5CeOx catalysts. Dehydrogenation and condensation reactions occured after binding of ethanol to an acid–base site pair present in basic oxides. These reactions were influenced by the presence of Cu metal crystallites. Basic sites may interact with Cu sites via migration of hydrogen atoms.165
The conversion of cyclohexanol39, 142, 166, 167 or of 2-propanol168–175 allows the characterization of the acid–base surface properties of the oxides. The dehydration of alcohol leading to alkene would be catalyzed by the acid centers, whereas its dehydrogenation leading to ketone would be catalyzed both by acid and basic sites. The dehydration activity could be related to the surface acidity, whereas the ratio between the activity in dehydrogenation and the activity in dehydration (AONE/AENE) would represent the surface basicity.
At 300 °C, CeO2 presents an acidic activity and on the other hand a basic activity. At 200 °C, the presence of ceria supported on Al2O3 (12 % CeO2/Al2O3) decreased the acidity in comparison with Al2O3: the ratio AONE/AENE increased (0.2 to 0.8).39 The conversion of cyclohexanol increased with the temperature with increasing selectivity to cyclohexene on pure CeO2.166 The addition of a CeO2 component to a ZnO catalyst (up to 40 % of CeO2), enhanced the activity of the catalyst with more than 90 % selectivity to cyclohexanone. In CeO2–ZnO, Ce4+ ions of different degrees of coordination unsaturation will act as Lewis-acid sites for cyclohexanol dehydrogenation. The abstraction of hydride ions is more efficient at the Ce4+ ion site than at the Zn2+ ion site.142, 166
At 300 °C, on CeO2, in the presence of helium and dihydrogen, propene was formed during the transformation of 2-propanol. The temperature increase favored dehydration, which occurs on acid–base pair sites that consist of coordinatively unsaturated Ce4+ and O2− ions. At 150 °C, 2-propanol undergoes a dehydrogenation to give acetone. The active sites were again described as Ce4+–O2− ion pairs, however, the acidic site is probably different from that involved in the dehydration reaction. When the temperature increased to up to 300 °C, acetone molecules, previously produced, took part in a bimolecular reaction to give isobutene and methane and an acetate surface species. The surface reactivity was associated with Ce4+–OH− pair sites.168
Under air, ceria is more active and the selectivity in acetone is more important than under helium or dihydrogen.169 The selectivity of Cu/CeO2/CNF (CNF: carbon nanofiber) was dependent on the fraction of CeO2 and on the temperature. High activity and selectivity were achieved with the Cu12Ce5/CNF catalyst. However, excess CeO2 enhanced the dehydration activity and thereby reduced the selectivity. The presence of CeO2 enhanced the reduction and dispersion of Cu.171
For a Au/CeO2 catalyst, the good oxidation performances were explained by a combination of the oxidation capability of gold atoms with the redox properties of the ceria phase. In fact, the dehydrogenation reaction required redox sites, rather basic sites. The alcohol transformation cannot be a simple test of acidity. Particularly, on ceria, its redox property and its high lability of lattice oxygen contribute to products formations, involving oxygen vacancies.
5.3. Hydrogen transfer reactions
Ceria-supported Cu, Ir, and Pd catalysts have shown a very high activity for liquid-phase transfer dehydrogenation of cyclohexanol and 2-octanol to cyclohexanone and 2-octanone, respectively, using styrene as the hydrogen acceptor, but for the 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 reported Cu and Pd catalysts supported on Al2O3, and Ir/CeO2 catalyst exhibited extremely high activity. The synergistic effect between metals and CeO2 might be responsible for the high catalytic activity. Pretreatment of the catalysts by hydrogen caused partial reduction of ceria and thus led to the generation of Ce3+ species on the catalyst surface. This species would enhance the adsorption of alcohols through the coordination between the Ce3+ cation and the hydroxyl group, which favors the dehydrogenation of alcohols. Meanwhile, the in situ removal of hydrogen would take place on the nearby metal particles through the hydrogenation of styrene.176
5.4. Oxidation of hydrocarbons
Partial oxidation of toluene to benzaldehyde via gas phase process is one of the current challenges in the field of catalysis. The oxidation of toluene was carried out on Ag1.2V3CeyO8+x catalysts between 360 and 460 °C. The catalytic effect of Ce is mainly to increase the selectivity to benzaldehyde and benzoic acid.177
Series of Ce–Mo catalysts have been prepared for the partial oxidation of toluene to benzaldehyde. The highest yield of benzaldehyde is obtained when the composition of the ultrafine particles reaches the vicinity of Ce/Ce+Mo=0.5, corresponding to a catalyst comprising both CeO2 and Ce2(MoO4)3. The excess CeO2 works as a promoter by releasing its lattice oxygen to the oxygen vacant sites formed on the Ce2(MoO4)3 species during reaction.178
A simple and efficient method for the synthesis of 3-nitrophthalic acid by the oxidation of 1-nitronaphthalene has been reported by Rajiah et al. (Scheme 10). Selective oxidation has been achieved by the one-step reaction of 1-nitronaphthalene with 5 % CeO2/γ-Al2O3 catalyst in acetonitrile in presence of aqueous acid at 90 °C producing 3-nitrophthalic acid in 80 mol % yield with 98 % selectivity.179
Elsewhere, the oxidation products of ethylbenzene are widely used as intermediates in organic chemistry. Various supported vanadia catalysts exhibit efficient catalytic activity in the selective oxidation of ethylbenzene using H2O2 in liquid phase (solvent: acetonitrile), producing essentially acetophenone. The oxidation activity of V2O5/CeO2 catalysts could be correlated to the amount of the vanadia loaded and the structure of the species. The CeVO4 formation associated with increased concentration of vanadia on ceria is related to the formation of 2-hydroxyacetophenone.180 These catalysts were already used in the partial oxidation of benzene to phenol.181 Selective formation of phenol can be attributed to the presence of highly dispersed active sites of vanadia over the support.
The hydroxylation of benzene was used also as test reaction to characterize catalysts, as MxCe1−xVO4 (with M=Li, Ca, and Fe), for the degradation by photocatalysis of different dyes and organic compounds. This reaction is associated to the oxidation of cyclohexane to produce cyclohexanol and cyclohexanone in liquid phase with chloroform as solvent.182
Liquid phase oxidation of cyclohexane to cyclohexanol was carried out under mild reaction conditions over mesoporous Ce-MCM-41 catalysts using aqueous hydrogen peroxide (30 %) as oxidant and acetic acid as solvent. MCM-41, without the incorporation of Ce as a catalyst under the same conditions with those used for Ce-MCM-41, did not exhibit any significant activity. Furthermore, even incorporating other metal ions, such as, 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 and can form labile oxygen vacancies and the relatively high mobility of bulk oxygen species. A complex with peroxy acetic acid was possibly formed in the pores of Ce-MCM-41 which is relatively more hydrophobic and stable than hydrogen peroxide. The synergistic effects among doped cerium, mesoporous framework of MCM-41, acetic acid and hydrogen peroxide make Ce-MCM-41 an effective catalyst for the oxidation of cyclohexane.183 Previously, the hydroxylation of 1-naphthol was carried out with aqueous H2O2 on this kind of catalysts.184
Elsewhere, in cyclohexene and cyclohexanol oxidation with H2O2 in acetonitrile, the catalytic activity depends on the cerium amount in Ce-silica mesoporous SBA-15 materials and metal atom coordination. Thus, in cyclohexene oxidation the total yield of oxidative products and selectivity of cyclohexene oxide (epoxy-) increase with the increase of cerium amount up to 2 wt % and then tend to decrease. Similar correlation is observed in cyclohexanol oxidation. Probably, the surface OH groups and state of cerium sites influence the catalytic activity of Ce-SBA-15. Thus, the sorption value of cyclohexene is high when content and density of SiOH groups are low. Cyclohexene is sorbed on the surface coordinatively with unsaturated (cus) oxygen or on the surface lattice oxygen anion and is strongly inhibited if ceria is slightly reduced due to the decreasing of available (cus) oxygen and surface oxygen species. Then the cyclohexene adsorption value decreases with increasing of cerium content in Ce-SBA-15.185
Ce-SBA-15 catalysts are also active for the oxidative cleavage of cyclohexene to adipic acid using aqueous H2O2 as oxidant under solvent-free conditions.186 The coordination of cerium ions in mesoporous materials can affect the catalytic properties because the incorporation of cerium atoms into the walls of mesoporous material allows creation of Lewis and Brønsted acid sites and preparation of materials with various acidities.185
The epoxidation of cyclohexene was carried out also in the presence of Fe/CeO2 catalysts using aqueous hydrogen peroxide (30 %) as the oxidizing agent.187
Direct oxidation of propane to acrolein could be an interesting alternative to propylene oxidation. Bi-Mo based catalysts have been heavily studied for the selective oxidation (ammoxidation) of propylene to acrolein via acrylonitrile at 500 °C. In BiCeVMoO catalysts, bismuth may be substituted by a low amount of cerium while the structure of BiVMoO remains unchanging. With the increasing of Ce (Ce/Ce+Bi>0.15), new phases of CeVO4 and Ce2(MoO4)3 formed. The cerium promotes the forming of acrolein, and the selectivity to acrolein increased to a maximum at Ce/Ce+Bi atomic ratio equal to 0.15 (45 mol % at about 30 mol % propane conversion). Further increasing Ce content results in further oxidation of acrolein to COx 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 importance owing to the growing demand for light olefins such as propylene189–191 and isobutene.192–196 The dehydrogenation of light alkanes to alkenes is a highly endothermic reaction, and conversion is limited by a thermodynamic equilibrium. Thus, high operating temperatures (500–750 °C) are required to obtain an acceptable level of alkane conversion. Under these conditions undesirable side reactions, such as hydrogenolysis and isomerization, occur with the formation of byproducts and coke deposits, thus producing catalyst deactivation.
The introduction of Zr4+ in a CeO2 lattice leads to significant variations in the chemical physical features of ceria, and improves the selectivity to isobutene in the oxidative dehydrogenation of isobutane between 300 and 400 °C. This enhancement has been attributed to an increased oxygen mobility and to an increased activity for the Ce4+/Ce3+ redox couple, occurring as a consequence of the creation of surface and bulk defects in the solid solution. The dehydrogenation was carried out at 450 °C over cerium oxide in the presence of tetrachloromethane to obtain propene, with a selectivity of up to 80 %. Without tetrachloromethane, carbon dioxide is the principal product. The enhancement of conversion and selectivity to propene was shown to be dependent upon the presence of chlorine, in whatever form, in the surface region of the catalyst.189
The Ce–Ni–O catalytic system is active and selective in oxidative dehydrogenation of propane to propene at 300 °C. The yield of propene increased with the increase in the Ni loading up to a Ni/Ce atomic ratio equal to 1 and decreased at higher loadings.190 Ceria was also found to be a good support for chromium oxide catalysts in the oxidative dehydrogenation of isobutane.193, 194 Recently, chromium oxide was supported on nanometer-sized Ce0.60Zr0.35Y0.05O2 for the same application.196
Elsewhere, platinum catalysts have been widely used for alkane dehydrogenation. Although a wide variety of catalyst formulations have been reported in the literature, most platinum-based catalysts are characterized by the simultaneous presence of tin. Pt-Sn/Ce-Al2O3 catalysts, with cerium loadings in the range of 1.1–3.3 wt %, exhibit a highly efficient performance for propane dehydrogenation to propylene at 576 °C. The presence of Ce in the Pt-Sn/Ce-Al2O3 catalysts could not only stabilize the active states of Pt, Sn, and the support, but could also suppress the coke accumulation on the catalyst during reaction.191
Pt-Sn/20 wt % CeO2–C catalysts, with different Sn/Pt atomic ratios, showed good performance in the dehydrogenation of isobutane at 500 °C. Cerium plays an important role in activity, inhibiting tin reduction and maintaining the amount of alloyed platinum at an adequate level. A catalyst with Sn/Pt=0.5 showed the best isobutene yield.195
5.5.2. Dehydrogenation of ethylbenzene
The industrial demand for styrene is growing, and its production via dehydrogenation of ethylbenzene is gaining importance. The reversible conversion of ethylbenzene to styrene and dihydrogen is highly endothermic: C6H5CH2CH3→C6H5CHCH2+H2; ΔH=125 kJ mol−1. Conversion is favored by low pressures and high temperatures. Industrially, the reaction is carried out over potassium-promoted iron oxide at temperatures ranging from 550 to 650 °C and pressures from sub-atmospheric to 2 atm, with a selectivity of about 90 % in styrene at a conversion of 50±60 %. Ceria is a key component of the catalyst formulation for the dehydrogenation of ethylbenzene to styrene.197
Activated-carbon-supported cerium catalysts (Ce/AC) exhibit a high styrene yield (about 40 %) with over 80 % selectivity at 550 °C in the presence of carbon dioxide. The dehydrogenation of ethylbenzene to styrene proceeds via two reaction paths. One is the simple dehydrogenation and an oxidation reaction of dihydrogen formed with carbon dioxide (reverse water–gas shift reaction; CO2+H2→CO+H2O). The other is the oxidative dehydrogenation of ethylbenzene through the redox cycle.198
V2O5-based catalysts supported on ZrO2–SiO2,199 Al2O3,200 and TiO2-ZrO2201, 202 doped with CeO2 exhibit high activities in oxidative dehydrogenation reaction of ethybenzene. Ceria-containing materials suppress catalyst deactivation by preventing coke formation during the reaction.
Elsewhere, the activity of CeO2–ZrO2/SBA-15 catalyst for the dehydrogenation of ethylbenzene to styrene in the presence of CO2 revealed that mesoporous silica SBA-15 is one of the promising support materials for the development of highly active and selective CeO2–ZrO2 mixed metal oxide catalysts.203
α-Limonene can be easily dehydrogenated to para-cymene, which is an important starting material for the production of intermediates such as para-cresol and can also be used in the manufacturing of fragrances, herbicides, and pharmaceuticals. Ce-promoted Pd/ZSM5 catalysts gave higher selectivities than nonpromoted catalysts for the hydroisomerization of α-limonene to para-cymene at 300 °C (Scheme 11).204, 205
A chemical interaction between CeO2 and Pd particles with very small size exists inside ZSM5 cavities. A bifonctional mechanism had been proposed for the conversion of α-limonene. An acid-catalyzed shift of the double bond from the isopropenyl group into the cyclohexene ring is followed by dehydrogenation on a palladium site.
5.7. Oxidation of aldehydes to acids
Clean aerobic oxidation of various aldehydes to the corresponding carboxylic acid was carried out over Ru–Co(OH)2–CeO2 (already used in a previously reported study149) at room temperature, in liquid phase with benzotrifluoride as solvent.206 The aliphatic and aromatic aldehydes were rapidly oxygenated. However for allylic aldehydes such as cinnamaldehyde, no conversion was observed even at 60 °C. The authors proposed a reaction mechanism via a free-radical process: there was no conversion of octanal in the presence of a radical scavenger (2,6-di-tert-butyl-para-cresol).206
Gold supported on nanocrystalline or on meso-structured nanocrystalline CeO2 supports was also highly active and selective for the aerobic oxidation of aliphatic and aromatic aldehydes at 25 °C and 50 °C in liquid phase with acetonitrile as solvent.207 With this catalyst, the oxidation of cinammaldehyde at 65 °C was highly selectivite towards carboxylic acid (77.5 %) at moderate conversion (40.7 %). The activity was attributed to the nanometric particle size of Au and CeO2.
5.8. Other oxidation reactions
5.8.1. Oxidation of 2,3-diméthylphenol
2,6-dimethyl-1,4-benzoquinone is a key intermediate for the synthesis of a number of medicines and physiologically active substances such as 2,3,6-trimethyl-para-benzoquinone, an intermediate in the industrial production of Vitamin E. The liquid-phase oxidation of 2,6-dimethylphenol to 2,6-dimethyl-1,4-benzoquinone (Scheme 12) was carried out at 20 °C, using ethanol as solvent and aqueous hydrogen peroxide as a clean oxidizing agent in the presence of TiO2–CeO2 mixed xerogels. The 2,6-dimethylphenol conversion was 100 % in 6 h, and the yields of 2,6-dimethyl-1,4-benzoquinone achieved were 85–96 % when using the TiO2–CeO2 mixed xerogels as catalysts, while the yield was 49 % when a titania catalyst without cerium was used.
Kanta Rao et al. reported on the use of ceria–titania (rutile and anatase) catalysts for the ammoxidation of 3-methylpyridine or 4-methylpyridine to their corresponding nitriles in gas phase at 410 °C (Scheme 13). The authors noted a marked steric effect in their reactivity. The best results were obtained on 20 % ceria on anatase, showing a 4-methylpyridine conversion of 89 % (37 % for 3-methylpyridine) and a selectivity to 4-methylpyridine of 77 % (45 % for 3-methylpyridine). The ammoxidation reaction was highly active when the catalysts were synthesized by dispersing ceria on suitable supports. Indeed, interacted CeO2 species of supported catalysts have shown increased O2 uptakes as well as increased conversions and selectivities in the ammoxidation reactions.209
5.8.3. Dehydrogenation of amines
Au(OAc)3 preadsorbed onto CeO2 was applied as an effective catalyst of the selective oxidation of dibenzylamine to dibenzylimine using molecular oxygen as the only oxidant in the liquid phase with toluene as solvent (Scheme 14).[210, 211] The authors developed a very simple route for the synthesis of gold catalysts for the oxidation of amines. The catalyst precursor, Au(OAc)3, and an oxide support, CeO2, were simply added to the reaction mixture and the active gold nanoparticles on the support were formed in situ. During the transformation of dibenzylamine to dibenzylimine; benzonitrile, benzylamine, and benzaldehyde were formed in amounts of 0.5 %, 0.4 % and 7.8 %, respectively. The latter two byproducts are the result of the hydrolysis of dibenzylimine with the coproduct water, while benzonitrile is formed by oxidative dehydrogenation of benzylamine. The low benzylamine/benzaldehyde ratio can be explained the coupling and oxidative dehydrogenation of benzylamine to dibenzylimine. The small amount of benzonitrile indicates that the direct oxidation of benzylamine to benzonitrile is very slow.211
5.8.4. Oxidation of oximes
Gold supported on ceria (Au/CeO2), usually used in the oxidation of alcohols152–156 or aldehydes,207 is a highly active and selective catalyst for the liquid phase aerobic oxidation of oximes to the corresponding carboxylic compounds.212 For example, 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 acetophenone and benzaldehyde (conversion ca. 85–99 % with 100 % selectivity, except for benzaldehyde in the presence of water where there was overoxidation of benzaldehyde to benzoic acid). Another, more elaborate catalyst, a core/shell alloy of gold and palladium supported on nanoparticulated ceria, was also efficient in toluene (conversion 99 %, with a selectivity of 99 %) but the drawback was a difficult preparation.212
6. Addition Reactions
6.1. Synthesis of carbonates
The development of environmental processes based on the utilization of naturally abundant carbon resources such as carbon dioxide has gained considerable attention in recent years. Organic carbonate synthesis using carbon dioxide is one of the promising reactions in this respect. Organic carbonate compounds have been used as both a reactive intermediate and an inert solvent.
Dimethyl carbonate (DMC) can be synthesized from epoxide compounds such as ethylene oxide or propylene oxide by a two-step reaction (Scheme 16). In the first step the epoxide reacts with CO2, producing a corresponding cyclic carbonate. In the second step, the carbonate is transesterified with methanol to DMC and a corresponding glycol.
CeO2 studied with several metal oxide does not appear to be a good catalyst for this reaction. The basic metal oxide catalysts give high activity and selectivity for the reaction of epoxides and CO2 to the corresponding cyclic carbonates and for the transesterification with methanol. Among the catalysts examined, MgO is the best catalyst, active for both these two reactions.213
Corma and collaborators214 have shown that ceria nanocrystallites are a moderately active catalyst for the transalkylation of propylene carbonate by methanol. The presence of gold nanoparticles on ceria in appropriate loading significantly increases the activity and selectivity towards transalkylation.
DMC can be also synthesized by reaction between methanol and CO2 in the presence of catalysts with acidic and basic properties such as ZrO2 and CeO2–ZrO2 solid solutions (Scheme 17). CeO2–ZrO2 catalyst appear to be very effective for the 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 %) under the employed reaction conditions, unfortunately the methanol conversion was very low because the equilibrium of the reaction was largely shifted to the left. However, when H2O is removed from the reaction system, it is possible to drastically enhance the methanol conversion. H2O removal can be achieved by reaction with acetals such as 2,2-dimethoxypropane (DMP; Scheme 18). These catalysts are also effective to the direct synthesis of cyclic carbonate from CO2 and diols such as ethylene glycol and propylene glycol.217, 218
The synthesis of DMC from CH3OH and CO2 was investigated on CeO2 prepared with various kinds of precursors under various calcination temperatures. The formation rate of DMC was almost proportional to the BET surface area of the catalysts. This suggests that the active site of this reaction is on a stable crystal surface of CeO2, such as (111).219
CeO2 has been reported to catalyze the direct carboxylation of methanol to dimethylcarbonate. Nevertheless, the catalyst lifetime was quite short as after the first cycle the activity decreased and went to zero after a few cycles. This deactivation is mainly due to a surface modification produced by the reduction of CeIV to CeIII during catalysis and to crystal conglomeration. The modification of ceria by loading alumina strongly reduces the oxidation of methanol and the consequent reduction of CeIV to CeIII, with increase of both the life of the catalysts and their selectivity.220, 221
6.2. Aldol condensation
Condensation reactions of aldehydes and ketones are widely used in organic synthesis, mainly because they lead to CC bond formation. These reactions are generally catalyzed by bases. The condensation over solid bases leads mainly to aldol or ketol and/or α,β-unsaturated carbonyl compounds.
The aldol condensation of acetone was studied over solid base catalysts such as Ca(OH)2, La(OH)3, ZrO2, and CeO2 in the vapor phase between 200 and 400 °C. The condensation of acetone 1 gives diacetone alcohol 2, which is dehydrated to mesityl oxide 3. Various secondary products are formed by numerous secondary reactions, such as further aldolization and Michael condensation (Scheme 19). At 200–400 °C, 1 over CeO2 led to 74–97 % conversion. CeO2 promoted the formation of 1,3,5-trimethylbenzene 7 with a selectivity of 47 % at 400 °C, but also produced large amounts of higher condensation products (38.9 % at 300 °C). The formation of 7 is favored with decreasing basic strength. Indeed, the basicity and basic strength, respectively, of the catalysts decreased in the order Ca(OH)2>La(OH)3>CeO2>ZrO2.222
Reduced CeO2 was observed to be active for the cross-reductive-coupling reaction between acetaldehyde and benzaldehyde to form 1-phenylpropene (C6H5CHCHCH3).223
Elsewhere, the reaction of acetaldehyde was studied on CeO2-based catalysts. CeO2 was chosen as support because its reducibility and basicity favor aldolization reactions.224, 225 Three CC bond formation reactions from acetaldehyde were observed: aldolization to crotonaldehyde and crotyl alcohol (more prominent on CeO2 alone), ketonization to acetone, and reductive coupling to form butenes and butadiene.224
4-Methyl-2-pentanone or methylisobutylketone (MIBK) was synthesized from 2-propanol in one pot on bifonctional metal/acid–base catalysts. The synthesis of MIBK from 2-propanol involves the dehydrogenation of 2-propanol to acetone which is converted to mesityl oxide (3, Scheme 19) via an aldol condensation reaction and consecutive dehydration of the aldol intermediate, diacetone alcohol (2, Scheme 19). MO is hydrogenated on the metallic site to MIBK by H2 generated during 2-propanol dehydrogenation. One of the most selective catalysts for the formation of MIBK is CuCe4Ox that presents the higher density of base sites and lower density of acid sites than CuAl16Ox catalyst.226, 227
The aldol condensation/hydrogenation reaction of 2-hexanone was carried out over a Pd/CeZrOx catalyst at temperatures between 300 and 400 °C, and pressures of 5–26 bar. The primary product of aldol condensation/hydrogenation is C12 ketone, with the formation of C9 and C18 ketones as secondary products. The CeZrOx support was selected because it possesses a high lattice oxygen mobility and because of its ability to interact strongly with supported metals.228, 229
The retro-aldolization of diacetone alcohol (2, Scheme 19) to acetone was considered as a test reaction that allowed the semiquantitative assessment of basic centers.230 Ceria (and titania) were found to exhibit considerable activity in the decomposition of diacetone alcohol.
6.3. Knoevenagel condensation reaction
The Knoevenagel condensation reaction is a cross-aldol reaction between an aldehyde or ketone and an methylene compound, activated by two electron-withdrawing groups, such as malononitrile, cyanoesters, β-ketoesters or malonates, in the presence of base. Ceria–zirconia shows interesting catalytic performances in the Knoevenagel condensation between benzaldehyde and malononitrile (Scheme 20) with ethanol as a solvent at 80 °C.231
The direct correlation between the concentration of acidic sites and the yield of the products indicated that a higher concentration of acidic sites gives more products in the reaction even if the presence of basic sites remains obligatory. CexZr1−xO2 catalysts can be interesting alternatives 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) reusability of the catalyst.231
6.4. Synthesis of acetals
Besides the interest of acetals as protecting groups of carbonyl compounds during organic synthesis, many of them have found direct applications as fragrances in cosmetics, food and beverage additives, pharmaceuticals, and polymer chemistry.
The synthesis of dimethyl acetals of carbonyl compounds such as cyclohexanone (Scheme 21), acetophenone or benzophenone has succefully been carried out by the reaction between ketones and methanol using different solid acid catalysts.232
Among various rare-earths-exchanged Mg–Y zeolites, CeMg–Y and Ce-montmorillonite were revealed to be the most efficient catalysts for the acetalization reactions. Acetalization of cyclohexanone reached equilibrium within 60 min and the yields of acetal were 66.7 % with CeMg–Y zeolite and 69.8 % with Ce–montmorillonite. The yields then slightly increased to 80.5 % and 98.8 % respectively. The Ce3+ cation acted as a Lewis-acid site and activated the carbonyl group by coordination, on the order of 1 kJ mol−1 as measured by FTIR.233
6.5. Synthesis of benzimidazole derivatives
Corma and collaborators234 have developed an effective strategy for the rapid and efficient one-pot synthesis of benzimidazoles involving a new environmentally friendly catalytic procedure. Benzimidazole derivatives were prepared by a four-step process with gold and/or palladium catalysts and dioxygen (Scheme 22). The four steps are (1) oxidation of the benzylalcohol to benzaldehyde, (2) cyclocondensation of the aldehyde with ortho-phenylenediamine, (3) oxidation of carbon–nitrogen bond, and (4) an N-alkylation reaction. The highest activity and selectivity were achieved when gold was deposited onto CeO2.
With electron-acceptor substituents on the aromatic diamine the cyclization/oxidation reaction proceeded more slowly and, accordingly, lower yields of the desired benzimidazole were obtained. The same effect was observed when the electron-withdrawing substituent was at the aromatic alcohol. On the other hand, 1-butanol afforded very poor yields of the corresponding heterocycle provided this aliphatic alcohol hardly converted to the corresponding aldehydes. In striking contrast the conjugated alcohol 2-octen-1-ol converted up to 80 % to the corresponding aldehyde but the latter hardly reacted with the diamine to afford the desired heterocycle.
6.6. Mannich-type reactions
A Mannich-type reaction is an organic reaction that consists of an amino alkylation of an acidic proton placed next to a carbonyl functional group with aldehyde and ammonia or any primary or secondary amine to lead a β-amino-carbonyl compound. A sulfated CexZr1−xO2 catalyst was found to exhibit solid-super-acidity and good catalytic activity for synthesis of β-amino ketones by a three-component Mannich-type reaction in the liquid phase under solvent-free conditions at ambient temperature. The reaction between benzaldehyde, aniline, and cyclohexanone (Scheme 23) proceeded in liquid phase (mixture of reactants) to afford 82 % of product, with a D:l ratio of 82:18. The sulfation of ceria-zirconia mixed oxide can lead to the formation of super-acidic sites in the catalyst, while the unpromoted ceria-zirconia mixed oxide possesses only a broad distribution of weak acid sites.235
6.7. Biginelli-type reaction
A Biginelli-type reaction is a multiple-component chemical reaction that creates 3,4-dihydropyrimidin-2(1 H)-ones from β-ketoesters, aldehydes, and urea. This reaction is generally catalyzed by Brønsted acids and/or by Lewis acids. Thus, the reaction between benzaldehyde, ethyl acetoacetate, and urea (Scheme 24) was performed in water at 80 °C for 4.5 h in the presence of ceria nanoparticles supported on vinylpyridine polymer, to give 92 % of product. The catalyst was recovered easily and reused without loss of its activity.236
6.8. Coupling reactions
Coupling reactions are of particular interest because they are a powerful and versatile tool in synthetic organic chemistry for the formation of carbon–carbon bonds. Gold or palladium supported on CeO2 are active and extremely selective in performing the homocoupling of arylboronic acids in liquid phase with toluene as solvent (Scheme 25),237–239 the Suzuki–Miyaura cross-coupling reaction of arylboronic acids and arylbromides in liquid phase with a mixture ethanol water as solvent (Scheme 26),240 and the Sonogashira cross-coupling reaction of aryliodides and alkynes in N,N-dimethylformamide (DMF) as solvent (Scheme 27).241
The catalytic activity of Au/CeO2 for the homocoupling of arylboronic acids is directly proportional to the concentration of AuIII surface species, and nanocrystalline CeO2 is able to stabilize surface AuIII species on the surface.238 Under the same reaction conditions, supported palladium catalysts are less active than supported gold catalysts.239 The palladium catalysts showed a selectivity of about 75 % towards biphenyl with formation of benzene and phenol as byproducts. When supported gold was used for the reaction, a selectivity of 100 % towards biphenyl is obtained.
Pd/CeO2 behaves as an efficient catalyst in the Suzuki–Miyaura coupling reaction starting from aryl bromides with different electronic substituents at room temperature, in air, in an environmentally friendly solvent such as ethanol/water mixture.240 By comparison of isoelectronic PdII and AuIII supported on ceria, the authors have found that the latter selectively promotes homocoupling, while the former catalyzes the cross-coupling reaction.240
Previously, Corma and collaborators have reported that a Au/CeO2 catalyst, active for performing the Sonogashira cross-coupling reaction, contains Au0, AuI, and AuIII species. The cross-coupling reaction was catalyzed by AuI, while the homocoupling 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 alkylating reagents are alkyl halides, alcohols, and olefins. These reactions are usually catalyzed by Lewis acids or also by Brønsted acids. These conventional catalysts are homogeneous and generate corrosive and nonrecyclable waste, and are thus not environmentally friendly. The solid catalysts do not exhibit these disadvantages.
CeO2242 or CeO2-MgO85, 243 were found to exhibit excellent catalytic activities for the vapor-phase ortho-alkylation of phenol with methanol242, 243 and with 1-propanol.85 The authors speculated that the reaction mechanism of the ortho-propylation over the CeO2–MgO catalyst proceeds by the perpendicularly adsorption of phenol on weak basic sites on the catalyst. These species are selectively alkylated in ortho position by 1-propanol, which is possibly activated in the form of 1-hydroxypropyl radical rather than propyl cation. The redox properties of Ce4+–Ce3+ are probably attributed with the activation of 1-propanol to produce 1-hydroxypropyl radical. Moreover, neither 2-n-propylphenol nor 2-isopropylphenol is produced during the alkylation of phenol with 2-propanol in the same conditions. This fact suggests that isopropyl cation cannot be produced on the CeO2–MgO at temperatures lower than 500 °C.85
Sn–Ce–Rh oxide monophase system, already used in ketonization of esters83, 84 or alcohols90, 91 was found to be an active and selective catalyst for the ortho-alkylation of phenol with methanol. Elsewhere, alkylation of aromatics compounds with alcohols or alkenes was performed over cerium modified microporous materials such as zeolites or silicoaluminophosphate (SAPO).[245–251] The impregnation of cerium leads to the deactivation of external acid sites of H-mordenite: the selectivity of 2,6-diisopropylnaphthalene in the isopropylation of naphthalene was enhanced without significant decrease of catalytic activity.245 Ceria thereby prevents non-regio-selective reaction on external surfaces with improvement of the selectivity. Similar results were observed over various SAPO for the isopropylation of biphenyl to produce 4,4′-diisopropylbiphenyl,247 over H-ZSM-5 zeolite for the ethylation of ethylbenzene to 1,4-diethylbenzene249 and over H-mordenite zeolite for the tert-butylation of toluene to 4-tert-butyltoluene.250, 251
Ce–Al–MCM-41-type mesoporous silicate materials was found to exhibit catalytic activities for isopropylation of naphthalene252 and benzylation of toluene.253 Both the density and the strength of the acid sites were considerably higher in the samples containing both Ce and Al than in the samples with only one of these substituents.
7.2. Synthesis of coumarins
The synthesis of coumarins that starts from phenol, called the Pechmann reaction, requires concentrated sulphuric acid as catalyst and involves corrosion problems. Reddy et al. reported an efficient method for the preparation of coumarins using sulfated CexZr1−xO2 solid catalyst under solvent-free conditions at 120 °C (Scheme 28). With 10 wt % of catalyst, the condensation between activated phenols with alkyl acetoacetate led to high yields of products (superior to 70 %) within a short period time (shorter than 143 min).
7.3. Synthesis of anisaldehyde
Hydroxymethylation of anisole has been carried out over SnO2–CeO2 catalysts in the gas phase in the temperature range 350–450 °C.255 Anisaldehyde (methoxybenzaldehyde) and condensation products (Scheme 29) were formed, along with minor quantities of methoxybenzyl alcohol, ortho-cresol, phenol, and 2,6-xylenol. A maximum anisaldehyde selectivity of 64 % was obtained at 350 °C at an anisole conversion of 46 %. Catalytic activity was ascribed to the presence of weak acid sites and redox metal sites. Stronger acid sites lead to the formation of condensation products.
7.4. Nitration of aromatic compounds
The liquid-phase nitration of toluene was carried out in the presence of sulphated titania promoted by a ceria catalyst at ambient temperature and atmospheric pressure without solvent.256 It is an attractive method for the environmentally friendly synthesis of nitroaromatic compounds. Moreover, only mononitrotoluenes were detected in the products, and the ratio of para-nitrotoluene and ortho-nitrotoluene was approximately 1:1. A maximum yield of about 11.4 % was achieved for mononitrotoluenes in 3 h with SO42−/TiO2 doped with CeO2 catalyst.
7.5. Acylation of alcohols, amines, or thiols
The acylation of alcohols, amines, phenols, and thiols is an important and frequently used organic transformation as it not only provides an efficient and inexpensive route for protecting hydroxy, amino, phenolic, and thiol groups, but also produces important organic intermediates in multistep synthetic processes that are widely used in the synthesis of fine chemicals, pharmaceuticals, perfumes, plasticizers, cosmetics, and chemical auxiliaries. Some of the solid-acid catalysts have been investigated as potential replacements for mineral acids in the esterification reaction.
Ce–MCM-41252 and ceria–yttria catalysts257 were found to be good catalysts for the acylation of alcohols, amines, and thiols with acetic anhydride without solvent252 or in the presence of acetonitrile.257 Ceria–yttria catalysts exhibit strong Lewis acid properties.257
S2O82−/ZrO2–CeO2 catalysts258 or SO42−/ZrO2 promoted by Ce2O3259 were also used as superacid catalysts in the esterification reactions. The incorporation of Ce into the catalyst was beneficial to the formation of sole tetragonal ZrO2 and effectively prevented the formation of monoclinic ZrO2, and restrained the loss of sulfated species.
The preparation of monoglycerides from fatty acids or fatty methyl esters and glycerol can be carried out in the presence of acidic or basic catalysts. The use of solid basic catalysts could limit secondary reactions leading to product degradation. A comparison of various basic oxide solids has shown that the more significant the intrinsic basicity is, the more active the catalyst is.260 The comparison of the catalytic results between CeO2 and MgO shows that even if they have similar intrinsic basicity and surface area, their initial activity in this reaction are different. MgO is the most active solid which could be due to the presence of stronger basic sites. But the selectivities to the monoglycerides are similar and only depend on the reagent conversion.
The transesterification of β-keto esters has been also studied in the presence of Lewis acid catalysts as ceria–yttria based catalyst.261 The authors have previously reported the application of this catalyst for the acylation reactions.257 This catalyst is also an efficient catalyst for the transesterification of β-keto esters by a variety of alcohols.
7.7. Oxidative esterification
5-Hydroxymethyl-2-furfural (HMF) has been selectively converted into 2,5-dimethylfuroate (DMF) (99 mol % yield) under mild conditions (65–130 °C, 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 polymer precursor to replace terephthalate in PET polymers. The reaction kinetics show that the oxidative pathway encounters its limiting step for the oxidation of the alcohol to aldehyde. Once the aldehyde is formed, the corresponding hemiacetal is obtained, 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; an acid-catalyzed chain reaction that is preferably performed at low temperatures in order to avoid cracking and aromatization products.
Isomerization of hexane over WOx/CeO2 catalysts leads to mono- and dialkylated hydrocarbons and methylcyclopentane.263 In the absence of dihydrogen, a relatively rapid deactivation of the catalysts occurs. The introduction of hydrogen improves the stability and modifies the selectivity: the mono- and dialkylated hydrocarbons predominate. In these conditions, the alkenes are less abundant and, consequently, the activity is decreased. These results suggest that the key point is the formation of alkenes. In a first step, an oxidative hydride abstraction occurs on Lewis sites associated with WO species, followed by isomerization either as a cooperative effect on the Brønsted acid sites created by tungsten, or even on the same Lewis site from which the hydride ion was abstracted.
8.2. Ring opening
The hydrogenolysis of methylcyclobutane (MCB) is a process that is well-known to be SMSI-sensitive, and is suited for studying the effect of low- and mid-temperature reduction on skeletal hydrocarbon reactions. The hydrogenolytic ring opening of methylcyclobutane occurs easily at 100 °C and below on Pt and Rh nanoparticles supported on CeO2.264 At low temperature and under dihydrogen excess only the ring opening products n-pentane and isopentane are formed. The progressive loss of activity observed on ceria-supported Pt and Rh catalysts with low surface areas upon reduction below 450 °C is most likely due to electronic perturbations at the interface between the metal nanoparticles and the increasingly reduced ceria support.
In upgrading highly aromatic fractions such as light cycle oil (LCO) from FCC, the hydrogenation of aromatic compounds may not always be sufficient to increase the cetane number and the opening of at least one of the naphthenic rings is necessary. Nylén et al. have shown that ceria appears to be the best support among several one as Al2O3, SiO2-Al2O3, ZrO2, MgO and SiO2.265, 266 The higher activity and selectivity towards ring opening of indane are obtained with a 2 wt % Pt5Ir95/CeO2 catalyst. The desired products are 2-ethyltoluene and n-propylbenzene, when the naphthenic ring has been cleaved only once. However, consecutive dealkylation occurs irrevocably and products such as ortho-xylene, ethylbenzene, toluene, benzene, and light products (<C6) are formed. This may be attributed to the electron-deficient character of the metals when supported on acid materials. The amphoteric properties may have impact on limiting secondary cracking reactions promoted by solely acid support materials and therefore increasing the selectivity towards valuable ring-opening products.
8.3. Isomerization of alkenes
The catalytic isomerization of isoprenol by the shifting of double bond to prenol (Scheme 31) is a process applied in the large-scale manufacture of solvents, dyes, surface coatings, paints, and pesticides. Silica-supported palladium catalyst promoted by selenium and cerium (0.5 %Pd-0.05 %Se-0.3 %Ce/SiO2 catalyst) shows higher performance, among a large variety of prepared catalysts, in the liquid-phase isomerization of isoprenol to prenol in the presence of dihydrogen (45 % conversion with 93 % selectivity). Addition of cerium improves the dispersion of Pd species affecting catalyst activity. Selenium, being an electronic modifier, is responsible for stabilization of Pdn+ species. This species determines the formation of π-complexes upon isoprenol adsorption, manifesting extended performance in the double bond shift and depressing hydrogenation activity.267
9. Conclusion and Perspectives
Over the last decade (more than 75 % of the references reported in this Review), ceria-based catalysis has demonstrated high efficiency in a variety of chemical transformations widely used for the synthesis of fine chemicals and specialties. Conventionally, these reactions are carried out with homogeneous catalysts or with stoichiometric reactants, which are not environmentally benign methods. Heterogeneous catalysts should be preferred to conventional synthesis methods because they have the advantages of simple removal from the product and recyclability. They also provide greater selectivity and enhanced reaction rates. For these reasons, cerium-based catalysts can contribute to new attempts to develop “clean and green” chemistry.
The redox ability and the acid–base properties of CeO2, either alone or in the presence of transition metals, are important parameters that allow to activate complex organic molecules and to selectively orient their transformation.
Pure ceria is used in the dehydration of alcohols, particularly in the selective dehydration of diols to allylic alcohols, in the ortho-selective alkylation of aromatic compounds with alcohols, in ketone formation through dimerization of esters and carboxylic acids, in aldolization, in the reduction of carboxylic acid to aldehydes and of aldehydes to alcohols and in the reverse reaction, the dehydrogenation of alcohols, and it is able to dehydrogenate isobutane to isobutene or ethylbenzene to styrene.
The acid–base or redox properties of ceria can be modified by involving other oxides (ZrO2, La2O3, MnOx, ZnO, MoO4, V2O5,…), increasing the scope of the reactions. Ceria can also be supported on polymers for the Biginelli reaction.
Ceria-supported metal catalysts allow the hydrogenation of CC, CO, or CN bonds, the selective hydrogenation of α-β-unsaturated aldehydes (to unsaturated alcohols), β-keto esters (to β-hydroxy esters), and fatty esters. They are also used in coupling reactions or ring opening, in the synthesis of benzimidazole, and in the oxidation of oximes. Cerium atoms have also been added as promoters to catalytic systems or impregnated onto zeolites and mesoporous materials to improve the performance of these catalysts.
In the near future, the very rich chemistry of cerium oxides should boost research on new catalysts with better properties for organic syntheses. The great variety of cerium-based mixed oxides allows to adjust acid–base and redox properties and to modulate both the number and strength of active sites for the desired reaction. New developments in the synthesis of ceria nanocrystals of controlled shapes (nanorods, nanocubes, polyhedras, and others)268, 269 should also lead to new catalysts with higher activities and selectivities in organic chemistry and catalysis.
The authors thank Mrs. Danièle Mesnard, who began this work.
Laurence Vivier obtained her Ph.D. in chemistry from the University of Poitiers (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 Professor. Her research focuses on hydroteatment on sulphide catalysts. In 2008, she joined the team of Dr. Duprez to pursue her research interests in the use of biomass for renewable 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 exchange for measuring oxygen and hydrogen mobilities on supported metal catalysts, with applications in H2 production 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.