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.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 ZrCl₄was selected to catalyze the alkylation of phenol with methanol or isobutylene,the 100%selectivity of para-alkylation products could be achieved.Thiébaud et al.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.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^[6][7][8]。

In the initial research on phenol alkylation,the mechanisms were focused on the acidic sites as the active sites.Padro et al.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.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.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 SiO₂to 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 CH₃of 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 theп-ligand was formed.Finally,theп-ligand dehydrated and dehydrogenated to form the O-alkylated products^{[27][28][29][30,31]}。

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。

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.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^[14][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。

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.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.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^[33][34]。

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。

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.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^[44]。

In addition to the Au-supported catalysts,Pd and Ru-supported catalysts also achieved excellent catalytic activity.In 2017,Stahl et al.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)₂and 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.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⁻¹at 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 ofα-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 H₂was 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 secondα-H to form the product ester.It was concluded that the metal particles as the activity sites of alcohol molecules and O₂molecules achieved excellent catalytic activity,but the high cost used the noble metals hindered the pace of industrialization^[45][46]。

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.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 K₂CO₃as 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.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 K₂CO₃as a base additive,and([2.5 mol%CoO_x-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.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^[47][48][49]。

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.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 andα-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.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/CeO₂catalyst 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/CeO₂was 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/CeO₂was 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.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.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,O₂was activated by the metal BiTe and then interacted with the alcohol oxygen species to promote the elimination ofα-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^{[50][51][52][53]}。

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 。

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.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.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/SnO₂as 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.SnO₂was 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 H₂was released from the Pt active site.Then,the aldehyde molecule was adsorbed on the Sn⁴⁺(Lewis acidic site)of SnO₂support 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.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 O₂molecules.In the same year,Simakov et al.used CeO₂and Al₂O₃mixed oxides as support to obtain the Au/CeO₂-Al₂O₃catalyst.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/CeO₂or Au/Al₂O₃catalyst due to the oxygen storage capacity of the mixed oxide was better than CeO₂and 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.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 K₂CO₃was 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^{[54][55][56][57][58]}。

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.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 K₂CO₃as 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⁻¹and 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.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/Al₂O₃and Au/ZnO,the catalytic activity of Au/ZrO₂was the best.When 3 wt%Au/ZrO₂was 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/Al₂O₃was 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.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/SiO₂catalyst 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 K₂CO₃as 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.synthesized a series of Au nanoparticle catalysts using different metal oxides as supports.Catalyst Au/CeO₂and Au/ZrO₂performed higher catalytic activity than Au/TiO₂,Au/HT and Au/Al₂O₃in the oxidative esterification of benzyl alcohol with methanol.When using 2.8 wt%Au/CeO₂as the catalyst and Cs₂CO₃as 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/ZrO₂as 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 Cs₂CO₃to form HCO₃⁻.At the same time,the oxygen molecule was activated by Au particles to O=O,and then theα-H of alcohol oxygen species was attracted by the oxygen active species O=O,which caused the cleavage ofα-H bond to form the intermediate species benzaldehyde.Then,the nucleophilic addition of methanol molecule and benzaldehyde formed a hemiacetal species,and the secondα-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.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^{[59][60][61][62][63]}。

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 O₂molecules 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 。