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
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.
5.4. 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