1 Introduction
Hydroxyl groups are widely present in natural biomass raw materials. The activation of hydroxyl groups plays an important role in modifying natural products, which also benefits the synthesis of many high-value-added chemicals.
Alkylation of phenols is an important kind of reaction in the field of hydroxyl group activation, and the products are widely used in producing raw materials and intermediates for detergents, plastics, dyes, perfumes, etc. [1⇓⇓⇓~5]. Alkylation of phenols is a reaction in which hydrogen is replaced by an alkyl group, and the products formed depend on the substitution position. If the hydrogen atom on the benzene ring is substituted, a C-alkylated product is achieved; if the hydrogen atom on the phenolic hydroxyl group is substituted, an alkyl ether will be formed, which is an O-alkylated product. At present, most of the research are focused on the C-alkylation of phenols, while fewer researches focused on the O-alkylation of phenols. Representatively, the product anisole, obtained from the O-methylation of phenol can be widely used in the organic synthesis and as a solvent, perfume and insect repellent [4]. However, the high reaction temperature is a key problem that should be solved in the alkylation of phenols. Furthermore, oxidative esterification of alcohols is also an important reaction of hydroxyl groups activation. The product organic carboxylic acid ester is an important organic chemical product, which is widely used in food, perfume, cosmetics, coatings, medicine, polymer materials and organic synthesis. The traditional preparation methods of organic carboxylic acid esters generally include direct esterification of acids with alcohols, cross-coupling of halogenated hydrocarbons with alcohols, esterification of acyl halides with alcohols, and esterification of aldehydes and alcohols. However, strong acids are present as reactant leading to low selectivity of products and the formation of by-products, in addition, the equipments are corroded severely. Esterification of halogenated hydrocarbons or acyl halides with alcohols will generate halides, which are harmful to the environment. At present, most research focuses on the esterification of aldehydes and alcohols to carboxylic acid esters, but aldehydes are generally prepared by the oxidation of alcohols. Therefore, in view of the atomic economy, the one-step synthesis of carboxylic acid esters by alcohols directly will achieve the effective utilization of energy. Moreover, alcohol as a raw material displays obvious advantages such as low toxicity, green, pollution-free and easy preparation. Thus, the oxidative esterification of alcohols to carboxylic acid esters shows great industrial application value, but the high temperature and long reaction time still exist as the bottleneck problem that should be solved.
The catalytic O-alkylation of phenols and oxidative esterification of alcohols can be divided into homogeneous acid-base catalysis and heterogeneous acid-base catalysis, and heterogeneous catalysis is widely studied due to its advantage of catalyst recovery. In the review, we compared the catalytic performance of various catalysts in the alkylation of phenols, mainly including metal oxide, ionic liquids and molecular sieves. The synergistic catalysis at the atomic level and choice of alkyl carbonates as alkylating agents can achieve excellent activity under mild reaction conditions. Besides, the benzene ring of phenols adsorbed on the catalyst surface in a vertical manner and the strong acidic sites in the bi-activity sites are beneficial in forming the O-alkylated products at a lower temperature. In addition, the heterogeneous catalysts in oxidative esterification of alcohols are mainly metal-supported catalysts: Au-supported catalysts, Co-supported catalysts, Pd-supported catalysts, Ru-supported catalysts, Pt-supported catalysts, and alloy-supported catalysts. Among them, metal is generally the catalytic active site, but a large amount of research shows that the support also plays a pivotal role, which inspired us that the study of the structural relationship in these catalytic processes should receive full attention. Thus, we mainly highlight the mechanisms of heterogeneous catalytic. Besides, the application of photocatalysis in the oxidative esterification of alcohols and photocatalytic mechanisms are discussed to improve the catalytic efficiency. Finally, the challenges and development prospects of catalytic conversion of hydroxyl compounds in future development are summarized.
2 Activation of phenols hydroxyl group: alkylation of phenols
2.1 Catalysts
2.1.1 Homogeneous catalyst
Homogeneous catalysts for phenols alkylation mainly include protonic acid catalysts such as sulfuric acid, hydrofluoric acid, and phosphoric acid, and a series of organic base catalysts such as quaternary ammonium salts, quaternary phosphorus salts, nitrogen compounds, and some metal halides such as AlCl3, FeCl3, SnCl4, TiCl2, SbCl5, TeCl4, BiCl3, etc. Alkylation of phenols usually yields O-alkylated products and C-alkylated products, or mono-alkylated products and di-alkylated products, and is always accompanied by the formation of some isomers. Therefore, it is important to design efficient catalysts with high selectivity. Bryner et al. [6] reported the alkylation of phenol with the mixture of 5% isobutylene and 91% n-butene using sulfuric acid as a catalyst at 373 K, and found that p-tert-butylphenol with a selectivity of 80% was obtained. If the AlCl3-boric acid were used as the catalyst, p-tert-butylphenol, with only a selectivity of 64%, can be obtained. If ZrCl4 was selected to catalyze the alkylation of phenol with methanol or isobutylene, the 100% selectivity of para-alkylation products could be achieved. Thiébaud et al. [7] found that using tetrabutylammonium bromide as a catalyst for the methylation of phenol with dimethyl carbonate (DMC), the O-alkylation product anisole with a yield of 100% could be obtained at 403 K. Besides, Sennyey et al. [8] used various guanidines as the basic catalysts in the methylation of phenol and DMC. The excellent catalytic activity with the high selectivity of O-alkylated products was achieved when the 2-methyl-1,1,3,3-tetrabutylguanidine was used as a catalyst. Therefore, it can be concluded that the formation of C-alkylated products is favorable under the acidic catalytic conditions and the formation of O-alkylated products is favorable under the basic conditions. Although the homogeneous catalytic system generally displayed high catalytic activity, the strong nucleophilicity of phenols is prone to generate some side reactions such as disproportionation or intramolecular rearrangement under strong acid and strong base conditions. In addition, the reaction equipment will be corroded in the strong acid and base catalytic system. In particular, the difficulty in separating catalysts and products makes continuous production difficult. Thus, the heterogeneous catalysts with the characteristics of little pollution, easy separation, and re-usability have gradually attracted attention.
2.1.2 Heterogeneous catalyst
2.1.2.1 Metal oxide catalysts
Metal oxides are the most widely used catalysts for phenols alkylation, mainly including the monometallic oxides and metal composite oxides.
2.1.2.1.1 Monometallic oxides
The alkylation of phenol with methanol over MgO has been studied by Woo et al. [9]. The conversion of phenol at 733 K was 64.5%, and the main products were C-alkylated products: 2,6-xylenol and o-cresol with a selectivity of 39.1% and 30.1%, respectively. The alkylation of phenol with methanol catalyzed by Al2O3 has been studied by Santacesar et al. [10] A series of alkylated products such as anisole and o-cresol were formed at the same time, but with poor selectivity. It is concluded that MgO displayed lower catalytic activity and product selectivity in the alkylation of phenol, even if carried out at high temperatures. Although Al2O3 can catalyze the alkylation of phenols at a relatively low temperature, the product selectivity is still poor. Thus, monometallic oxides are gradually eliminated as the alkylated catalysts. The metal composite oxides, by compounding two or more metallic oxides, perform superior catalytic activity.
2.1.2.1.2 Metal composite oxides
The first metal composite oxides used in the alkylation of phenols are Al2O3-SiO2 catalysts. In the alkylation of phenol with isobutylene on the Al2O3-SiO2 catalyst, not only the conversion of phenol is much higher than that of γ-Al2O3, the selectivity of para-alkylated product is also higher than that of γ-Al2O3. This is because the Lewis acid site is the main active site in γ-Al2O3 alone, and the Bronsted acid is the main active site in SiO2. When adding a proper amount of SiO2 into Al2O3, the increased activity was achieved due to the presence of Lewis acid site and Bronsted acid site in the Al2O3-SiO2 composite oxides [11]. A V2O5-modified ZrO2 composite oxide was prepared and used as the catalyst in the alkylation of phenol with methanol[12]. It was found that the conversion of phenol was increased from 30.2% on the ZrO2 to 91.5% on the metal composite oxide catalyst. Besides, Zhang et al. [13] prepared a modified MnO2 catalyst and used it as the catalyst in the alkylation of phenol with methanol. The conversion of phenol was improved when a small amount of SiO2 was added to MnO2. When the ratio of Si to Mn was 0.02, the conversion of phenol reached 99.8% at 703 K, and the selectivity of alkylated product 2,6-xylenol was 76.3%. Besides, it is noted that tri-metal and even multi-metal composite oxide catalysts were also used in the alkylation of phenols. A V-Mn-MgO catalyst showed excellent catalytic activity in the alkylation of phenol with methanol, has been studied by Choi et al. [9]. Compared with the double metal composite oxide V-MgO, the conversion of phenol was increased from 64.5% to 91.7% under the same reaction conditions, and the selectivity of C-alkylated product 2,6-xylenol was increased from 39.1% to 47.6% at 733 K.
It can be seen that the alkylation of phenols was performed at a high reaction temperature, whether in the bi-metal composite oxide catalytic systems or multi-metal composite oxide catalytic systems, and most of the products were C-alkylated products. According to the preparation process of these metal composite oxides, it was found that only simple recombination between metal oxides was performed and did not achieve synergistic catalysis at the atomic level. Hydrotalcite-based nanomaterials, as a class of two-dimensional materials, are a kind of unique layered anionic compounds. Hydrotalcite is generally composed of divalent and trivalent metal cations located on the laminate and anions located in the layers, and metal ions are not limited to divalent and trivalent. Monovalent and tetravalent can also be introduced, such as Sn4+, Li+, Zr4+, and Ti4+. Thus, the metal composite oxides based on hydrotalcite display excellent characteristics such as adjustable composition and controllable ratio. Layered double oxides (LDOs) can be obtained by calcining the hydrotalcite, which contains two or more metal ions at the same time. Such metal composite oxides can achieve synergistic catalysis at the atomic level. Sun et al. [14] prepared magnesium-aluminum layered double oxide (MgAl-LDOs) by calcining magnesium- aluminum hydrotalcite (MgAl-LDHs) in a nitrogen atmosphere at 723 K for 8 h, and then introducing F− into LDOs through the memory effect of LDHs to obtain the catalyst F/MgAl-LDOs, which was used to catalyze the alkylation of phenol with dimethyl carbonate (DMC). The best catalytic activity was obtained when using the catalyst F/MgAl-LDOs with a 2.93 ratio of Mg to Al, the conversion of phenol was 99.3%, and the selectivity of O-alkylated product anisole reached 100% at 473 K for 8 h. Jyothi et al. [15] directly calcined the MgAl-LDHs at 723 K to achieve the magnesium-aluminum composite oxide, then applied it to the alkylation of catechol with DMC. When the reaction was performed at 598 K for 2 h, the conversion of catechol reached 94%, and the selectivity of the O-alkylated product was 79.1%, the selectivity of the C-alkylated product was only 5.3%. Pitchumani et al. [16] also prepared the MgAl-LDOs by calcining MgAl-LDHs, then various amino acids were inserted into the hydrotalcite layer through the memory effect of MgAl-LDHs to prepare amino acid intercalated hydrotalcite (AA-LDHs), which were used as the catalyst in the alkylation of phenol or thiophenol with DMC. It was found that the yield of O-alkylated products over Leu-LDHs, Glu-LDHs, Arg-LDHs, Lys-LDHs and Met-LDHs catalysts were more than 90% at 453 K for 6 h, and no C-alkylated products were formed. MgAl-LDOs as a catalyst for the alkylation of catechol with DMC has also been reported by Jyothi et al. [17]. The conversion of catechol was 96.0% and the selectivity of O-methoxyphenol was 84.0% at 573 K.
2.1.2.2 Ionic liquid catalysts
An ionic liquid catalyst is a new type of alkylated catalyst that overcomes the shortcomings of strong inorganic acid and strong base catalysts and can also avoid the high reaction temperature in the alkylation of phenols. It is a kind of composite salt composed of anions and cations. The cations generally include quaternary ammonium salts, pyridines, imidazoles, etc. The anions include PF6-, ZnCl3−, C3F7COO−, N(CF3SO2)2−, C4H9SO3−, NTF2−, SbF6−, BF4−, etc. The physical characteristics of being liquid at room temperature, wide temperature range, non-flammable, non-toxic, non-volatile and strong dissolving ability are displayed in the ionic liquid. The synergy effect between Bronsted acidic sites and Lewis acidic sites shows excellent catalytic activity and product selectivity, and the large adjustable range of anions and cations makes it able to be designed for certain types of reactions. In addition, the immiscibility of ionic liquids with some organic solvents allows ionic liquids to be reused. Deng et al. [18] prepared a proton pyridine salt ionic liquid ([2-MPyH] OTf) catalyst, which exhibited excellent catalytic activity in the alkylation of phenol with TBA. The phenol conversion was 92% and product (2,4-DTBP) selectivity reached 80% for 6 h at 373 K.
2.1.2.3 Molecular sieve catalysts
A molecular sieve catalyst is an important type of heterogeneous catalyst. The high controllability and catalytic activity under mild conditions make it widely used in the alkylation of phenol. The basic skeletal structure of molecular sieves is composed of SiO4 and AlO4 tetrahedrons, which form a three-dimensional network structure via the combination of shared oxygen atoms. This combination forms a uniform pore size at the molecular level. According to the different pore sizes, it can be divided into microporous molecular sieves (smaller than 2 nm), mesoporous molecular sieves (between 2~50 nm), and macroporous molecular sieves (larger than 50 nm). After dehydration, the molecular sieve will form a pore structure with an internal specific surface area of more than 1000 m2·g-1, which is beneficial to improve the adsorption of reactant molecules. Besides, the powerful electrostatic field formed in the pores and the uniformly distributed pore structure make it exhibit high catalytic activity and excellent product selectivity. The widely used molecular sieves in the alkylation of phenols including HZSM-5, ZMS-11, Al-MCM-41, H-USY zeolite, HY, β zeolite, etc. The results of the alkylation of phenol with methanol on a series of molecular sieve catalysts are shown in Fig. 1 . It is found that a single type of molecular sieve as a catalyst displayed poor catalytic activity and low selectivity.
The surface modification of the molecular sieve has gradually become a method to improve the catalytic activity and product selectivity. Obviously, the synergistic catalysis of the acidic sites and basic sites at the atomic level improved the catalytic activity in a mild condition. A series of modified molecular sieves were prepared by introducing active components into the molecular sieves in different ways to regulate the framework, pores and surface properties. Kim et al. [19] introduced alkali metals (Li, Na, K, Cs) into the pores of X molecular sieve to catalyze the alkylation of phenol with methanol and found that the selectivity of O-alkylated product anisole was more than 90%. When the temperature was increased to 563 K, the conversion of phenol achieved 98%. Zhou et al. [20] used the modified mesoporous molecular sieve SBA-15-SO3H to catalyze the gas-phase alkylation of phenol with methyl tert-butyl ether. Under the optimal reaction conditions, the conversion of phenol was 88.7% and the selectivity of butylphenol was increased to 83.5%.
2.2 Alkylating agent
The alkylating agents of phenols alkylation mainly include alcohols, halogenated alkanes, olefins, alkyl sulfates and alkyl carbonates, but alkyl carbonates exhibit excellent performance (Table 1 ). When alcohols and olefins were used as the alkylating agent, low conversion of phenols and poor selectivity of O-alkylated products were performed. Jyothi et al. [17] reported that when using methanol as the alkylating agent, the conversion of catechol was only 14%, the selectivity of O-alkylated product orthomethoxyphenol was 42.3%, and the selectivity of C-alkylated product was 31.6%. However, when DMC was used as the alkylating agent, the conversion of catechol was as high as 94%, and the selectivity of the O-alkylated product was as high as 79.1% under the same reaction conditions. When halogenated alkanes and sulfates were used as alkylating agents, not only the poor selectivity of O-alkylated products was achieved, but also the highly corrosive toxic substances such as HCl, H2SO4, etc., were formed [19,21⇓~23]. However, alkyl carbonate exhibit the advantages of high phenols conversion and green process. Li et al. [24] used KNO3/NaY as the catalyst and diethyl carbonate (DEC) as the alkylating agent in the alkylation of phenol. The conversion of phenol was up to 96.1% at 593 K, and the selectivity of product ether was up to 98.3%. Besides, when using ethanol as an alkylating agent, the conversion of phenol decreased to 9.3% under the same reaction conditions. In addition, Khusnutdinov et al. [25] used W(CO)6 as the catalyst and dimethyl carbonate as the alkylating agent in the alkylation of phenol and found that the yield of anisole was up to 95% at 453 K for 1 h. The alkylation of phenol with DEC over the fly ash-based zeolite catalyst has been reported by Gjyli et al. [26] The conversion of phenol was 95%, and the selectivity of phenyl ether was 85% at 573 K.
表1 不同烷基化剂在酚烷基化反应中性能的比较Table 1 Activity comparison of different alkylating agents used in the alkylation of phenols |
| Phenols | Alkylating agent | Conv. (%) a) | Sel. (%) b) | Ref |
|---|---|---|---|---|
| catechol | methanol | 14 | 42.3 | 16 |
| DMC c) | 94 | 79.1 | ||
| phenol | DEC d) | 96.1 | 98.3 | 23 |
| ethanol | 9.3 | - | ||
| phenol | DMC | 100 | 95 | 24 |
| phenol | DEC | 95 | 85 | 25 |
a). conversion of phenols, b). selectivity of O-alkylated product, c). dimethyl carbonate, d). diethyl carbonate |
2.3 Catalytic mechanism of phenols alkylation
Although the synergistic catalysis at the atomic level and the alkyl carbonate as alkylating agents can achieve excellent activity under mild reaction conditions, the catalytic mechanism should be understood to improve the controllability of catalysts and the predictability of activity. The mechanism of different catalysis systems will be different, but the adsorbed method of reactants on the catalyst surface, the strength and the types of acidic sites and basic sites on the catalyst surface are the crucial factors.
2.3.1 Acidic sites on the catalyst surface as active sites
In the initial research on phenol alkylation, the mechanisms were focused on the acidic sites as the active sites. Padro et al. [27] used a series of molecular sieves as catalysts for the alkylation of m-cresol with methanol, and the mechanism was investigated. It is found that the strength of acidic sites on the catalyst surface is the main factor affecting the conversion of m-cresol. Al-MCM-41 molecular sieve with medium acid strength is conducive to forming the O-alkylated products, while ZnY and HMCM22 molecular sieves with strong Lewis acid and Bronsted acid are conducive to forming the C-alkylated products (Fig. 2 ). Reddy et al. [28] found that the rare earth metal phosphate modified by cesium catalyst with the weak acidity performed the high selectivity for the O-alkylated products. It is concluded that the weak acidic sites are beneficial to the O-alkylation of phenols. Bhattacharyya et al. [29] chose the Al-MCM-41 molecular sieve as the catalyst for the alkylation of phenol with methanol. It was also found that the weak acidic catalyst is beneficial to the formation of O-alkylated products, and the strong acidic molecular sieve is conducive to the formation of C-alkylated products by changing the ratio of SiO2 to Al2O3. The catalytic mechanism is that the alkylating agent methanol molecules were adsorbed on the surface of the Al-MCM-41 molecular sieve to promote its dehydrogenation, and then the activated CH3 of methanol molecular electrophilic attack at the hydrocarbyl hydrogen of phenol molecule to form an O-alkylated product anisole, the C-alkylated products and reformed products were also formed by the subsequent reaction of anisole itself as shown in Fig. 3 . In addition, there are several researches reported that the Bronsted acidic site on the surface of the solid acid catalyst is a benefit to the formation of O-alkylated products, as shown in Fig. 4 : Firstly, the alkylating agent alcohol was adsorbed on the acidic site of the solid acid. At the same time, the Bronsted acidic site promoted the formation of carbocation in the phenol molecule. Then the п-ligand was formed. Finally, the п-ligand dehydrated and dehydrogenated to form the O-alkylated products [30,31].
图2 多种分子筛催化间甲酚与甲醇烷基化反应的催化性能研究（反应温度：523 K）Fig. 2 Catalytic results for the alkylation of m-cresol with methanol over various molecular sieves, reaction temperature: 523 K |
According to the above-described catalytic activity, although the weak and medium acid strength Bronsted acidic site is prone to form O-alkylated products, the conversion and the selectivity were low and the reaction temperature was very high. With the deepening research on phenols alkylation, acid-base synergistic catalysis has been further developed to obtain efficient catalytic performance, and the structural relationship in the catalytic processes is proposed.
2.3.2 Acidic and basic sites on the catalyst surface as active sites
A large number of studies have reported that the benzene ring of phenols tends to adsorb on the active site of the catalyst in a horizontal manner when the catalyst surface displays strong acidic sites or basic sites. The main products of this adsorption type are a mixture of C-alkylated products and O-alkylated products, as shown in Fig. 5 . In addition, the benzene ring tends to adsorb on the catalyst surface in a vertical manner when the catalyst surface displays both acidic sites and basic sites. The O-alkylated products are mainly formed when the reaction temperature is low, and the C-alkylated products are mainly formed when the reaction temperature is higher. Thus, the catalyst surface with both acidic and basic sites and the benzene ring of phenols adsorb on the catalyst in a vertical manner is beneficial to achieve O-alkylated products in a mild condition, as shown in Fig. 6 . Jyothi et al. [14] used MgAl-LDO as the catalyst in the alkylation of catechol with DMC. The main product was an O-alkylated product at 523~623 K. It was found that the acid-base pair on the catalyst surface was the active site. The mechanism is shown in Fig. 7 : catechol was adsorbed on the basic sites of the catalyst surface by phenolic hydroxyl hydrogen, DMC was adsorbed on the acidic site of the catalyst surface by carbonyl oxygen, and the phenoxy anion nucleophilically attacked at the methyl carbon of DMC to form the product. Besides, our research group reported the layered double oxides (MgAl-LDO) catalyst was prepared for the O-alkylation of phenols to ethers under light irradiation [32]. The synergism of phenol molecules absorbing light to reach the first excited states with the acid-base pairs of MgAl-LDO was beneficial to accelerate the catalytic activity. And the yield of phenetole was up to 91% in the presence of light at 423 K for 12 h.
图5 苯酚分子的苯环以水平方式吸附于催化剂表面的酸性位点Fig. 5 Parallel orientation of the aromatic ring of phenol molecule on the acidic site of catalyst surface |
图6 苯酚分子的苯环以垂直方式吸附于催化剂表面的酸性位点Fig. 6 Vertical orientation of the aromatic ring of phenol molecule on the acidic site of catalyst surface |
Although the adsorbed method of substance on the acidic sites and basic sites affects the catalytic activity, the strength of acidic sites and basic sites on the catalyst surface are also crucial factors. Velu et al. [33] calcined hydrotalcite with different divalent and trivalent ions at 723 K to obtain a layered double oxide (LDO) catalyst, which was used as the catalyst in the alkylation of phenol with methanol. The conversion of phenol was 100% at 673 K, and the main product was the O-alkylated product anisole. When Cr3+ and Fe3+ were used to replace the Al3+, the conversion of phenol was decreased under the same conditions, and the main product was the C-alkylated product o-cresol. It can be seen that the main factor affecting the distribution of products is the acid-base pair on the catalyst surface. When the catalyst surface displays both acidic sites and basic sites, the strong basic sites and acidic sites are conducive to forming the O-alkylated products, while the strong basic sites and weak acid sites are conducive to forming the C-alkylated products. Cavani et al. [34] also used the layered double oxide by calcining hydrotalcite as the catalyst for the methylation of phenol. It was found that the yield of the C-alkylated product ortho-cresol was gradually increased with the increase of the Mg/Al ratio in the layered double oxide, while the yield of the O-alkylated product anisole gradually decreased, indicating that the medium basic sites are prone to the C-alkylation of phenols, and the relatively weaker basic sites and relatively stronger acidic sites are prone to the O-alkylation of phenols. It is concluded that the strong acidic sites in the bi-activity sites are benefit to form the O-alkylated products.
In summary, for the alkylation of phenols, the synergistic catalysis of the acidic sites and basic sites and the choice of alkyl carbonates as alkylating agents determined the catalytic performance in a mild condition. Thus, reasonable control of the strength and types of acidic sites and basic sites on the catalyst surface by changing ions or calcined temperature of the catalyst can improve the catalytic performance and lower the reaction temperature.
3 Activation of alcohols hydroxyl group: oxidative esterification of alcohols
3.1 Homogeneous catalyst
The direct oxidative esterification of alcohols to synthesize carboxylic acid esters has not been extensively studied. Basically, it is mostly studied around homogeneous catalysts. The typical homogeneous catalyst is concentrated sulfuric acid, but it has been gradually replaced by other milder homogeneous catalysts because its strong acidity causes equipment corrosion and environmental pollution. The high alcohol conversion and product selectivity were achieved when the precious metal Pd was used as a catalyst, which became a representative type of homogeneous catalyst in the oxidation esterification of alcohols. In 2011, Lei et al. [35] used ([PdCl2 (CH3CN)2]) as the catalyst in the oxidative esterification of benzyl alcohol with methanol. It was found that the yield of product methyl benzoate was up to 74% at 318 K for 12 h when NaOtBu was used as an additive. Besides, the catalyst can also be used in the oxidative esterification of the homologues of benzyl alcohol with methanol, such as different substituent groups and substitution positions or benzyl alcohol-like molecular structure materials such as furan methanol, as shown in Fig. 8 . Even the oxidative esterification of benzyl alcohol with various long-chain aliphatic alcohols also can be achieved when used ([PdCl2(CH3CN)2]) as the catalyst, and the corresponding product yields were considerable. Subsequently, Xu et al. [36] used ([Pd(OAc)2]) as the catalyst for the oxidative esterification of benzyl alcohol with methanol, and found that the conversion of benzyl alcohol was more than 99%, and the selectivity of methyl benzoate was as high as 100% by adding the additives Na2CO3 and polymethylpolysiloxane (PMHS) at 313 K for 24 h. In addition, the oxidative esterification of homologues of benzyl alcohol with methanol could also be achieved under the same catalytic condition. Moreover, Beller et al. [37] chose ([Pd(OAc)2]) as the catalyst and n-butyl bis (1-adamantyl) phosphine as the ligand. The yield of methyl benzoate was 88% at 333 K for 24 h, and the universality of this catalytic reaction system was also considerable in the presence of AgPF6 and K2CO3 additives.
In addition, there are many reports of homogeneous catalytic reaction systems using precious metals Au and Ru as catalysts for the oxidative esterification of alcohols. Gao et al. [38] used NaAuCl4·H2O as the catalyst for the oxidative esterification of benzyl alcohol with methanol, and the yield of methyl benzoate was up to 97% by using K2CO3 as an additive and N-heterocyclic carbenes salt as a ligand at 353 K for 5 h. Pan et al. [39] selected ([Ru(p-cymene)Cl2]2) as the catalyst in the oxidative esterification of benzyl alcohol with p-nitrophenol as shown in Fig. 9 , and the yield of the corresponding product was 90% at 403 K for 24 h by using CsCO3 as the additive. Williams et al. [40] used ([Ru(PPh3)4H2]) as the catalyst and 4,5-bis(diphenylphosphino)-9, 9-dimethylxanthene as the ligand in the oxidative esterification of methanol, and an 83% yield of methyl benzoate was achieved at 383 K for 48 h. Milstein et al. [41] also used Ru complex as a catalyst and KOH as an additive in the oxidative esterification of alcohol. It was found that the yield of the corresponding product ester was up to 99.5% at 388 K for 72 h. In addition, Itoh et al. [42] chose the non-noble metal homogeneous catalyst tetrabromomethane (CBr4) to catalyze the oxidative esterification of benzyl alcohol with methanol, and the yield of methyl benzoate was 94% in 20 h under the irradiation of a xenon lamp. Wu et al. [43] used non-noble metal ZnBr2 as a homogeneous catalyst for the oxidative esterification of alcohol and H2O2 as a co-agent. The conversion of alcohol was 100% in 16 h at room temperature, but the yield of methyl benzoate was only 30%.
It is indicated that there are some disadvantages in the homogeneous catalytic system of alcohol oxidative esterification, such as some suitable ligands, high temperature, or a long reaction time needed to achieve a considerable yield of carboxylic acid esters (Table 2 ). Moreover, the difficulty in the recycling of homogeneous catalysts has always hindered its further development. Therefore, the research on heterogeneous catalysts with the advantages of the separation of products and catalysts recyclability has gradually appeared.
表2 不同均相催化剂在苯甲醇与甲醇氧化酯化反应中性能的比较Table 2 Comparison of different homogeneous catalysts activity in the oxidative esterification of benzyl alcohol with methanol |
| Catalysts | Ligands | Additives | Temp. (K) | Time (h) | Yield (%) a) | Ref |
|---|---|---|---|---|---|---|
| [PdCl2 (CH3CN)2] | - | NaOtBu | 318 | 12 | 74 | 33 |
| [Pd (OAc)2] | - | Na2CO3 | 313 | 24 | 99 | 34 |
| [Pd (OAc)2] | b) | K2CO3 | 333 | 24 | 88 | 35 |
| NaAuCl4·H2O | c) | K2CO3 | 353 | 5 | 97 | 36 |
| [Ru(p-cymene)Cl2]2 | - | CsCO3 | 403 | 24 | 90 | 37 |
| [Ru (PPh3)4H2] | d) | - | 383 | 48 | 83 | 38 |
| Ru complex | - | KOH | 388 | 72 | 99.5 | 39 |
| ZnBr2 | - | H2O2 | 303 | 16 | 30 | 41 |
a) Yield of methyl benzoate; b) n-butyl bis (1-adamantyl) phosphine; c) N-heterocyclic carbenes salt; d) 4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene |
3.2 Heterogeneous catalyst and catalytic mechanism
The supported metal heterogeneous catalysts can reduce the reaction temperatures and shorten the reaction times. The reports of supported metal catalysts for alcohol oxidation esterification mainly focus on Au, Co, Pd, Ru, Pt and alloy-supported catalysts, and their specific catalytic mechanisms mainly depend on the metal nanoparticles and support.
3.2.1 Role of metal nanoparticles and catalytic mechanism
The Au-supported catalysts are the most commonly used catalysts for direct oxidative esterification of alcohols to carboxylic acid esters, which display obvious advantages such as high activity, high selectivity and cycle stability. Wang et al. [44] chose MgO as the support to prepare a supported Au nanoparticles catalyst, and used it in the oxidative esterification of methacrolein with methanol. The conversion of methacrolein was up to 98%, and the selectivity of product ester was up to 99% at 343 K for 2 h when used 0.45 wt% Au/MgO as the catalyst without the addition of a base. In addition, by prolonging the aging time and changing the added amount of chloroauric acid, Au/MgO catalysts with different Au particle sizes can be prepared. The catalytic activity was related to the particle size of Au particles. Au particles with a particle size of 2.2 nm performed the highest catalytic activity. Au/MgO catalyst was also used for the oxidative esterification of benzyl alcohol derivatives with methanol, and the conversion was considerable. Besides, the oxidative esterification of octanol with methanol could also be achieved by prolonging the reaction time. The conversion of octanol was 89%, and the selectivity of the corresponding ester was 98% at 343 K for 12 h. It is clear that the catalytic activity of supported Au catalysts is closely related to the size of metal particles.
In addition to the Au-supported catalysts, Pd and Ru-supported catalysts also achieved excellent catalytic activity. In 2017, Stahl et al. [45] prepared an activated carbon-supported Pd catalyst for the oxidative esterification of alcohol. When 5 wt% Pd/C was used as a catalyst, the conversion of alcohol and the yield of ester were both up to 100% at 333 K for 2 h with the addition of potassium methoxide, and Bi(NO3)2 and Te were introduced into the reaction system as a co-catalyst. Similarly, by changing the reaction conditions appropriately, the oxidative esterification of alcohols with the different benzene ring substituents and methanol, as well as the oxidative esterification of long-chain aliphatic alcohols with methanol can be achieved. Besides, Sánchez et al. [46] used an MCM-41 molecular sieve as support to successfully prepare a Ru-supported complex with the nitrogen-containing heterocycle [(NHC)NN-Ru]. The self-esterification of n-hexanol was chosen as a probe reaction, using KOH as a base additive; the ester yield was 88% and the TOF value was 5 h−1 at 383 K for 24 h. Ru2+ serves as the catalytic activity site, and the catalyst mechanism is shown in Fig. 10 : Firstly, the active Ru complex (Ru-H) with the nitrogen-containing heterocycle was prepared by treating the inactive complex with potassium t-butoxide to remove Cl, then alcohol molecules were dehydrogenated by Ru-H activation to form phenolic oxygen species. The oxygen of the phenolic oxygen species interacted with the unsaturated coordinated Ru in the complex. The base in the system promoted the removal of α-H forms an intermediate aldehyde. At the same time, the complex Ru species changed from Ru-H to Ru-H2, and then a molecule of H2 was removed to restore the unsaturated coordination environment around Ru. Besides, aldehyde was condensated with alcohol to form an acetal species. The interaction of the acetal species with the unsaturated coordination Ru promoted the removal of the second α-H to form the product ester. It was concluded that the metal particles as the activity sites of alcohol molecules and O2 molecules achieved excellent catalytic activity, but the high cost used the noble metals hindered the pace of industrialization.
To reduce the cost of noble metal catalysts, non-noble metal Co-supported catalysts are used for direct oxidative esterification of alcohols to carboxylic acid esters. In 2013, Beller et al. [47] reported that Co-supported catalysts were used as the catalyst in the oxidation esterification of alcohols to form esters for the first time (Fig. 11 ) and the preparation method of the catalyst was relatively simple. The catalyst Co3O4-N@C was obtained by mixing Co(OAc)2·4H2O, nitrogen-containing ligand and carbon powder, then calcining at 1073 K under an argon atmosphere for 2 h. It showed excellent catalytic activity in the oxidative esterification of benzyl alcohol with methanol. When using 3 wt% Co3O4-N@C as the catalyst and K2CO3 as the base additive, the conversion of benzyl alcohol was up to 99%, and the selectivity of methyl benzoate was 97% at 333 K for 24 h. In addition, the catalyst also performed excellent activity in the direct esterification of the homologues of benzyl alcohol with the longer carbon chains aliphatic alcohols, and it could be used as the catalyst in the self-esterification of aliphatic alcohols. Besides, Jain et al. [48] used polyaniline (PANI) as the substrate, and introduced cobalt acetate to obtain a nitrogen-doped carbon- supported Co-type catalyst by high-temperature calcining, which was used as a catalyst in the oxidative esterification of benzyl alcohol with methanol. When using K2CO3 as a base additive, and ([2.5 mol% CoOx-N@C, PANI]) as the catalyst, the conversion of benzyl alcohol was 92%, and the yield of product ester was 90% at 333 K for 24 h. The catalytic activity remained unchanged after the catalyst was recycled six times. This catalyst can also be applied to the oxidative esterification of benzyl alcohol homologues with methanol and the self- esterification of ethanol or benzyl alcohol. In addition, the mechanism of the catalytic reaction was investigated: Firstly, the benzyl alcohol molecule was activated by Co to form the intermediate benzaldehyde, then the nucleophilic addition of benzaldehyde with methanol molecule to form the final product methyl benzoate under the addition of a base. In the same year, Li et al. [49] also used ZIF-67 as a substrate calcined at high temperature to obtain Co@CN(800) as a catalyst for the oxidative esterification of p-nitrobenzyl alcohol with methanol (Fig. 12 ). The conversion of p-nitrobenzyl alcohol was higher than 99%, and the yield of product ester was higher than 99% at 298 K for 96 h without the addition of base additives. It is clear that the metal Co, as the activity sites of alcohol molecules achieved the high yield of product ester, but the reaction time is long or the reaction conditions are harsh.
图11 Co3O4-N@C催化苯甲醇与甲醇的氧化酯化反应方程式Fig. 11 Reaction formulas of the oxidative esterification of benzyl alcohol with methanol on the Co3O4-N@C catalyst |
图12 不添加碱的条件下，Co@CN(800)催化对硝基苯甲醇与甲醇的氧化酯化反应示意图[49]Fig. 12 Schematic illustration of the oxidative esterification of p-nitrobenzyl alcohol with methanol on Co@CN(800) catalyst without the addition of base. Reprinted (adapted) with permission from ref 49. Copyright (2015) American Chemical Society |
Thus, although single metal-supported catalysts are widely used in the direct oxidation esterification of alcohols, problems such as large metal particles, poor dispersion on the surface of support, and harsh reaction conditions are still present. The different natures of different metal atoms often result in some unique surface structure composition, which will perform an excellent catalytic activity. Therefore, research on alloy catalysts in the direct oxidation esterification of alcohols has gradually attracted attention. The AuPd alloy nanoparticle catalyst is the most widely used alloy-supported catalyst. Mullins et al. [50] directly used the alloy PdAu as a catalyst for self-esterification of ethanol and performed excellent activity. From the results of DTF theoretical calculations, it was found that oxygen was pre-adsorbed on the bimetallic interface of the PdAu(111) plane. The pre-covered oxygen and hydroxyl promoted the dehydrogenation of OH and α-H of ethanol molecule (Fig. 13 ) to form acetyl intermediates and alkoxide, which subsequently produced acetaldehyde. Then acetaldehyde dehydrogenation produced the ethyl acetate. This conclusion was verified by the isotope labeling method. In addition, Hutchings et al. [51] prepared a series of AuPd catalysts using the reduction method of sodium borohydride: AuPd/TiO2, AuPd/SiO2, AuPd/Fe2O3, AuPd/CeO2. The 0.5% Au0.5%Pd/CeO2 catalyst was used as a catalyst in the self-esterification of 1,2-propanediol, the selectivity of 2-hydroxypropionic acid methyl ester was 71.1%, and the conversion of 1,2-propanediol was 39.6% at 373 K for 36 h. In comparison, when a single metal catalyst 1% Au/CeO2 was used, the selectivity of methyl 2-hydroxypropionate was up to 83.5% under the same reaction conditions, but the conversion of 1,2-propanediol was only 18.1%. When 1% Pd/CeO2 was used, the conversion of 1,2-propanediol was 56.0% under the same reaction conditions, but the selectivity of methyl 2-hydroxypropionate was only 54.0%. It is concluded that the alloy nanoparticle catalyst performs the best catalytic activity. Wang et al. [52] prepared the nitrogen-doped carbon black (NCB)-supported PdBi alloy nanocatalyst for the oxidative esterification of alcohols. A yield of 96% for methyl benzoate could be achieved at 333 K for 2 h with the addition of the K2CO3. It was concluded that the PdBi alloy nanostructure displays a synergistic effect between individual components, which benefits the catalytic process. Besides, Stahl et al. [53] prepared PdBiTe/C by doping Bi and Te in Pd as a catalyst for the oxidative esterification of benzyl alcohol with methanol. It was found that the introduction of Bi and Te metals greatly improved the catalytic activity. If Pd/C was used as the catalyst, only benzaldehyde was obtained, and the yield was only 63%. While using modified PdBiTe/C as the catalyst, the conversion of benzyl alcohol was up to 90% for 2 h, and methyl benzoate was the main product with a yield of 80%. It was found that BiTe could promote the activation of oxygen, and it could also inhibit the oxidation and inactivation of Pd during the reaction. The reaction mechanism is shown in Fig. 14 : In the first step, Bronsted basic sites promoted the adsorption of alcohol molecules on the metal surface and activated the alcohol molecules to alcohol oxygen species. In the second and third steps, O2 was activated by the metal BiTe and then interacted with the alcohol oxygen species to promote the elimination of α-H of alcohol generated benzaldehyde. In the fourth step, benzaldehyde is condensed with methanol to form hemiacetal species. In the fifth step, the hemiacetal species were activated by oxygen species to form methyl benzoate and HOO species. In the sixth step, the HOO species combined with the hydrogen of the methanol molecule to generate H2O2, which decomposed into H2O and O2. It was concluded that alloy nanoparticle catalysts not only achieved the superior conversion of alcohols but also improved the selectivity of the target product esters.
图13 PdAu合金催化乙醇的自酯化反应示意图[50]Fig. 13 Self-esterification of ethanol over PdAu alloy catalyst. Reproduced with permission from. Reprinted (adapted) with permission from ref 50. Copyright (2019) American Chemical Society |
According to the above reports, it is concluded that metal nanoparticles can act directly as the active site to activate the reactant molecule (alcohols and O2). The smaller metal nanoparticles’ size and the alloy catalysts exhibited superior performance in the direct oxidation esterification of alcohols.
3.2.2 Role of support and catalytic mechanism
Although the metal nanoparticles are usually the active sites, the support also plays an important role in the metal-supported catalyst for oxidative esterification of alcohols. Lots of research shows that the support can also be used as an active site for the activation of reactant molecules and performs excellent catalytic activity in a mild reaction condition. In the catalytic process, the acidity and basicity of the support are crucial for adsorbing and activating the reactant molecules. In 2012, Gusevskaya et al. [54] prepared an Au/MgO catalyst by co-precipitation method for the oxidative esterification of benzyl alcohol with methanol. The selectivity of methyl benzoate was 58% at 373 K without a base additive, and the TOF value was 155 min−1. The most important feature of this research is that the reaction system does not require the addition of base additives. It can be found that MgO as support displays more basic sites than SiO2. In 2014, Shimizu et al. [55] selected a series of metal oxides (SnO2, ZrO2, CeO2, Nb2O5, Al2O3, MgO, SiO2) as support to prepare the Pt-supported catalysts. It was found that when using 1 mmol% Pt/SnO2 as the catalyst for the oxidative esterification of n-octanol, the ester yield was up to 98% at 453 K for 36 h without the solvent and base added. SnO2 was the Lewis acidic site to activate the carbonyl oxygen of the intermediate aldehyde molecule. The catalytic mechanism is shown in Fig. 15 : Firstly, the alcohol molecule was dehydrogenated to form an aldehyde activated by the metal Pt, and H2 was released from the Pt active site. Then, the aldehyde molecule was adsorbed on the Sn4+ (Lewis acidic site) of SnO2 support by carbonyl oxygen, which promoted another alcohol molecule to nucleophilically attack the aldehyde and form the intermediate species acetal. Finally, the acetal was dehydrogenated by Pt ion to form the ester. In 2015, Jain et al. [56] used Fe-doped graphene oxide as a support to prepare a Pd-supported catalyst in the oxidation esterification of alcohols, in which palladium existed as divalent ions. When a catalyst with 1 mol% Pd loading was used and the appropriate amount of potassium carbonate was added, the conversion of benzyl alcohol was up to 100%, and the selectivity of methyl benzoate was 90% at 333 K for 6 h. Besides, the separation of the product and the catalyst can be promoted by the introduction of Fe in graphene oxide because of the magnetism of the catalyst. In addition, it was found that the graphene oxide support could adsorb and activate the reactant molecules and O2 molecules. In the same year, Simakov et al. [57] used CeO2 and Al2O3 mixed oxides as support to obtain the Au/CeO2-Al2O3 catalyst. The optimal catalyst Au/Ce (30)-Al was obtained by adjusting the ratio of Ce to Al for the oxidative esterification of benzyl alcohol with methanol. Without the addition of base additives, the conversion of benzyl alcohol was 85%, the selectivity of methyl benzoate was 84%, and the TON was up to 9130 at 383 K for 10 h under the oxygen atmosphere. The catalytic activity was better than that of Au/CeO2 or Au/Al2O3 catalyst due to the oxygen storage capacity of the mixed oxide was better than CeO2 and Al2O3. The catalytic mechanism is that mixed oxides can store a large amount of oxygen and eventually promote Au to activate more oxygen molecules to improve the rate of the entire reaction. Besides, Jiang et al. [58] used the N-doped porous carbon material formed after the stripping of MOFs material as a support to prepare the catalyst Co-CoO@NC for the oxidation esterification of benzyl alcohol with methanol. When 32.9% Co-CoO@NC-700-3h was used as the catalyst, and K2CO3 was also used as the base additive, the conversion of benzyl alcohol was as high as 100%, and the selectivity of methyl benzoate was 100% at 353 K for 12 h. The support N-doped porous carbon material not only stabilizes the Co nanoparticles, but also promotes the activation and desorption of the reactant molecules on the active site.
In addition, the support can not only be used as an active site for the activation of reactant molecules but also have great effects on the size and surface charge of metal particles. The synergistic effect of the metal and the support drive the reaction and achieve superior catalytic activity in mild reaction conditions. In 2011, Gusevskaya et al. [59] used mesoporous SiO2 (HMS) as a support to prepare Au-supported catalysts for the oxidative esterification of benzyl alcohol with methanol. When Au/HMS was used without modification as the catalyst and K2CO3 as an additive, the TOF value was 744 h−1, and the selectivity of methyl benzoate was 48% at 383 K. The catalyst activity obviously increased when the catalyst was modified by Fe, Ce and Ti, and the catalytic activity of Au/HMS-Ce was the best. The TOF was up to 995 h−1 and the selectivity of methyl benzoate was as high as 94% under the same reaction conditions. It was concluded that when HMS was modified by Fe, Ce and Ti, the smaller Au particle clusters could be achieved, and the surface of these small clusters would become negative charge due to the interaction with oxygen molecules, which could used as Lewis basic sites to activate the reactant molecules. In 2013, Zhaorigetu et al. [60] synthesized a series of metal oxide-supported Au nanoparticle catalysts for the oxidative esterification of benzyl alcohol with methanol. Compared with Au/TiO2, Au/CeO2, Au/Al2O3 and Au/ZnO, the catalytic activity of Au/ZrO2 was the best. When 3 wt% Au/ZrO2 was used as the catalyst, the conversion of benzyl alcohol was up to 95%, and the selectivity of methyl benzoate was 95% at 303 K for 24 h. It was found that the Au particle size in Au/Al2O3 was significantly bigger than the others, and this could be the main reason for the low catalytic activity of Au/Al2O3. It can be seen that the support had a great effect on the size of Au particles during the synthesis process, which was crucial to the catalytic activity. In 2014, Han et al. [61] synthesized a stable porous ionic liquid-hydrogel by the induction of inorganic salts and loading the metal nanoparticles successfully for the first time. This method can prepare metal catalysts with a particle size of less than 1 nm, such as Au/SiO2, Ru/SiO2, Pd/Cu-MOF, and Au/PAM. The 1.0 wt% Au/SiO2 catalyst performed excellent catalytic activity in the oxidative esterification of benzyl alcohol with methanol. The conversion of benzyl alcohol was above 99%, and the methyl ester selectivity was also above 99% when using K2CO3 as an additive at 323 K for 24 h. The small metal particle size prepared by the porous ionic liquid-hydrogel one-step method and the porous characteristics of the support were the main reasons for its excellent catalytic activity. The support plays an important role in promoting the adsorption of reactant molecules and the mass transfer of intermediate species. It can be seen that the supported metal catalyst should include two active sites: metal particles and support. The synergistic effect of these two active sites promotes the reaction, and it also proves that Au particles with smaller particle sizes were beneficial to the direct oxidation of alcohol to esters. Li et al. [62] synthesized a series of Au nanoparticle catalysts using different metal oxides as supports. Catalyst Au/CeO2 and Au/ZrO2 performed higher catalytic activity than Au/TiO2, Au/HT and Au/Al2O3 in the oxidative esterification of benzyl alcohol with methanol. When using 2.8 wt% Au/CeO2 as the catalyst and Cs2CO3 as the base additive, the conversion of benzyl alcohol was up to 99.7%, and the selectivity of methyl benzoate was 99.4% at 298 K for 6 h. When using 2.9 wt% Au/ZrO2 as a catalyst, the benzyl alcohol conversion of 100% and a methyl benzoate selectivity of 99% were achieved at 298 K for 6 h. It was not only because of the different sizes of Au nanoparticles prepared by different supports but also because of the different interactions of support and reactant molecules. The reaction mechanism is shown in Fig. 16 : Firstly, the benzyl alcohol molecule was adsorbed on the surface of the support. The hydrogen atom of the benzyl alcohol hydroxyl interacted with the oxygen atom of the support. The hydroxyl oxygen atom interacted with the metal ion of the support, promoting the cleavage of OH. The desorbed H was captured by the carbonate ion of Cs2CO3 to form HCO3−. At the same time, the oxygen molecule was activated by Au particles to O=O, and then the α-H of alcohol oxygen species was attracted by the oxygen active species O=O, which caused the cleavage of α-H bond to form the intermediate species benzaldehyde. Then, the nucleophilic addition of methanol molecule and benzaldehyde formed a hemiacetal species, and the second α-H of the alcoholic oxygen species was also attracted by the oxygen-active species O=O, leading to the cleavage of the bond. Finally, the product molecule and OH- were formed. The role of support in the catalytic process was discussed in-depth in this work. In 2023, Ishida et al. [63] synthesized a gold nanoparticle deposited on the cation and anion-substituted hydroxyapatites (Au/sHAPs) catalyst for the oxidative esterification of octanal or 1-octanol with ethanol to obtain ethyl octanoate. It was found that the metal-support interaction (SMSI) plays an important role in catalytic activity, in which the SMSI not only strengthened the cationic properties of Au but also achieved stronger basic sites after Au loading. Besides, the sHAP support can also adsorb and activate the substrate in the catalytic process. And the Au/sHAPs catalyst performs excellent catalytic activity in the oxidative esterification of octanal with ethanol. The ethyl octanoate yield was 84% at 373 K for 3 h.
In summary, the supported metal heterogeneous catalysts exhibit excellent catalytic activity in the direct oxidation esterification of alcohols, especially, and can reduce reaction temperatures and shorten reaction times compared to the homogeneous catalysts. The metal nanoparticles can be used as the active sites for the adsorption and activation of reactant molecules and O2 molecules during the catalytic reaction process. And the size of the metal nanoparticles directly affects the catalytic activity or the product’s selectivity. The smaller the metal particles, the better the catalytic activity. In addition, the support also plays an important role: 1) the small particle size metals can be obtained by choosing different supports or regulating the support surface; 2) the acidity and basicity of the support affect the adsorption and activation of the reactant molecules. Therefore, the synergy between the metal and the support is beneficial in achieving excellent catalytic performance in the direct oxidation esterification of alcohols under mild conditions.
4 Photocatalytic oxidative esterification of alcohols
At present, the application of photocatalysis in the oxidative esterification of alcohols mainly focuses on the plasmon resonance metal-supported photocatalysts, which greatly improve the catalytic activity. In particular, the catalytic conversion of alcohols in an air atmosphere, without the addition of base additives and at room temperature, can be achieved by introducing light into the catalytic process. The metal as a light absorption site, which generated the LSPR effect by the electronic excitation, synergized with the acid-base sites of support to activate the reactant molecules.
Zhu et al. [64] chose MgAl-LDHs as the support to prepare Au-Pd alloy catalyst for the direct self-esterification and oxidative esterification of alcohols under visible light irradiation. PO43 − was introduced as an interlayer anion in the MgAl-LDHs by the memory effect of hydrotalcite, so that the excellent conversion of alcohol hydroxyl groups was achieved without the addition of base additives. The conversion of 1-Octanol was up to 94%, and the selectivity of the corresponding product ester was 76% at 328 K for 24 h under visible-light irradiation. In the photocatalytic process, the α-H bond was activated by the free electrons excited by the LSPR effect of Au-Pd alloy. The reaction mechanism is shown in Fig. 17 . Firstly, the benzyl alcohol molecules are adsorbed on the Au-Pd alloy nanoparticles, the light promotes the dehydroxylation of the benzyl alcohol molecules, and the support basic sites also facilitate the cleavage of the OH in alcohol molecules. Then, the LSPR effect of the alloy is excited to form the free electrons, which promotes the cleavage of α-H bonds to form intermediate benzaldehyde molecules. At the same time, the basic sites of the support can also reduce the reaction energy barrier for the removal of α-H from alcoholic oxygen species. Finally, the benzaldehyde molecules condense with another molecule of alcohol to form a hemiacetal. The oxidative dehydrogenation of hemiacetal forms the final product ester.
Li et al. [65] prepared a series of supported alloy nanoparticles with different metal proportions by the impregnation-co-reduction method. The effect of light on the catalytic performance of AgPd alloy was investigated by the oxidative esterification of benzyl alcohol, and the results showed that when the molar ratio of Ag/Pd was in the range of 1 : 0.6 to 1 : 1.1, visible light could significantly improve the conversion of benzyl alcohol to 94% and the selectivity of methyl benzoate to 99% in an air atmosphere at 323 K for 2 h, and not oxygen atmosphere. The reaction mechanism was proposed (Fig. 18 ): Firstly, the nanoalloy surface was capable of producing Oα species by the activation of light-induced electrons. Then, the oxygen atom interacted with the H+ of alcoholic hydroxyl, and the H atoms of α-C, which were activated by the LSPR effect of AgPd alloy, formed the benzaldehyde molecules. Finally, an oxidative addition of benzaldehyde with methanol formed ester.
In 2020, our research group reported the UiO-66 supported Au nanoparticles photocatalyst for the oxidative esterification of benzyl alcohol at room temperature [66]. The defects were generated in UiO-66 by introducing HCl. The conversion of benzyl alcohol was up to 94.3%, and the selectivity of methyl benzoate was 99.2% under visible-light irradiation. The proposed mechanism is as follows (Fig. 19 ). Firstly, the defect sites in UiO-66 facilitate the adsorption of alcoholic hydroxyl groups and finish the dehydrogenation of alcoholic hydroxyl. Then, O2 is activated to O2•− by the LSPR effect of Au nanoparticles, which promotes the breaking of C-H to form benzaldehyde. Finally, the benzaldehyde molecule condenses with methanol to form a hemiacetal intermediate, and oxidative dehydrogenation is completed to form methyl benzoate.
Besides, our research group prepared the Layered double hydroxide supported Au-Cu alloy nanoparticles photocatalyst and used for the oxidative esterification of benzyl alcohol. The reaction temperature is as low as 30 °C under the visible-light irradiation, which maintained the conversion and selectivity higher than 95% [67]. It is indicated that the application of photocatalysis in the oxidative esterification of alcohols can effectively improve the catalytic performance in mild reaction conditions.
5 Conclusions and outlook
Catalytic conversion of hydroxyl-containing compounds has been studied based on the catalysis of phenols alkylation, and alcohols direct oxidative esterification. Although the homogeneous catalytic system performs a high conversion of hydroxyl-containing compounds, the intense corrosive and low selectivity of target products limits its application. The heterogeneous catalysis of hydroxyl activation with the advantages of catalyst recyclability and mild reaction conditions has attracted great interest.
In the alkylation of phenols, the difference in acidity and basicity strength on the different catalyst surfaces, the adsorption method of organic reactant molecules on the catalyst surface, the kind of alkylating agents, and the type of acidic site and basic site will affect the catalytic activity and product distribution ultimately. When the benzene ring adsorbs on the catalyst surface in a vertical manner, and the catalyst surface displays strong acidic sites in the bi-activity sites, O-alkylated products are mainly formed. MgAl-LDOs catalyst has displayed higher catalytic activity in the alkylation of phenols and benefited from obtaining the O-alkylated products with high selectivity due to its acid-base sites on the surface. In addition, alkyl carbonate, as the alkylating agent, displays the advantages of high activity and a green process.
In the oxidative esterification of alcohols, the supported alloy catalysts have gradually attracted attention due to a unique surface structure composition, which improves the catalytic activity under mild conditions. The size of the metal nanoparticles directly affects the catalytic activity, and it is found that the smaller the metal particles, the better the catalytic activity. In addition, the support also plays an important role in obtaining small particle-size metals and promoting the adsorption and activation of the reactant molecules due to their acidity and basicity. Therefore, the synergy between the metal and the support is the main reason for the excellent performance in a mild condition. It is worth noting that the application of photocatalysis in the activation of alcoholic hydroxyl groups gradually attracts great interest because of the milder reaction conditions that can be achieved. The metal as a light absorption site generated the LSPR effect synergized with the acid-base sites of support to promote the reaction.
It is reasonable to assume that the heterogeneous catalysts exhibit great potential in the activation of hydroxyl due to the cost-effective and environmentally benign catalytic process. Besides, plasmonic-metal- supported photocatalysis is a new way of catalytic conversion of hydroxyl-containing compounds, which efficiently utilizes solar energy. Although significant progress has been made in the photocatalysis of hydroxyl activation, the light quantum efficiency is still low, which hinders the expansion of production capacity. The conversion path of phenols and alcohols is complex, leading to the low selectivity of ethers and esters, and the reaction mechanism has not been fully elucidated. These problems are great challenges that still exist. It is hopeful that this review can stimulate new insights and creative methods in the catalytic conversion of hydroxyl-containing compounds.