Homogeneous basic catalysts have generally shown good catalytic performance for the production of lactic acid^{[17⇓⇓~20]}。 in 2005,Kishida et al.Reported the role of inorganic bases In the hydrothermal conversion of glycerol^[17]。 When the reaction temperature is 300℃and the concentration of NaOH is 1.25 mol/L,the yield of lactic acid is as high as 90 mol%.Almost no lactic acid was produced without the addition of NaOH.The reaction mechanism is based on the first oxidative dehydrogenation of glycerol to methylglyoxal,the release of H₂by hydrogen transfer to neighboring hydrogen,and the conversion of methylglyoxal to lactate ion by benzilic acid rearrangement 。

Shen et al.Further demonstrated the feasibility of the hydrothermal conversion of glycerol to lactic acid catalyzed by alkali metal hydroxides(KOH,NaOH,LiOH)and alkaline earth metal hydroxides（Ba（OH）₂,Sr（OH）₂,Ca（OH）₂,Mg（OH）₂）as well as amphoteric hydroxide Al(OH)₃^[18]。 Except for Al(OH)₃,the above alkaline catalysts can catalyze glycerol to produce lactic acid or lactate,and the effect of alkali metal hydroxides is better than that of alkaline earth metal hydroxides.The effect of KOH is better than that of NaOH at lower concentration or shorter reaction time,indicating that the alkali metal ion species affect the reaction behavior.Sharning Hausen et al.Compared the performance of KOH,LiOH and NaOH,and found that KOH was the best catalyst for the formation of lactic acid^[19]。 Chen et al.Used CaO as a solid base for the conversion of glycerol to lactic acid,and the reaction path is shown in Figure 1^[21]。 Glycerol is first oxidized to glyceraldehyde,after which glyceraldehyde is dehydrated to 2-hydroxyacrolein,which is readily converted to methylglyoxal by keto-enol isomerism.At the same time,2-hydroxyacrolein and methylglyoxal may undergo hydrogenation reaction with the intermediate product H₂to produce a small amount of propylene glycol.Finally,under the catalysis of CaO,methylglyoxal readily undergoes benzilic acid rearrangement to form calcium lactate(Figure 1A).In addition,the oxide ion may also evolve into an oligomer during its formation(fig.1B).These oligomers are poorly soluble or are adsorbed on the catalyst surface,resulting in decreased reaction efficiency.Therefore,the reaction activity of solid base catalyst alone is low,and its further application is limited 。

Hydrothermal reaction system has harsh reaction conditions,large waste emissions,and high cost of product and catalyst separation,which makes it difficult to promote industrial application.At present,the conversion of glycerol to lactic acid mainly focuses on the development of heterogeneous catalytic systems。

In addition to the hydrothermal conversion of glycerol,the introduction of oxidant and catalyst is another feasible way to produce lactic acid from glycerol.At present,most researchers mainly focus on the selective oxidation of glycerol catalyzed by noble metal(Au,Pt,Pd)catalysts under mild conditions^[22,23]。 However,this system usually needs to add a certain amount of base(NaOH,LiOH or KOH)to increase the pH of the reaction system,couple the catalytic dehydrogenation and dehydration ability of the base under hydrothermal conditions,and improve the catalytic activity and selectivity.In 2010,Shen et al.Reported the efficient one-pot conversion of glycerol to lactic acid using Au-Pt/TiO₂as a catalyst,and the reaction path is shown in Fig.2^[15]。 Glycerol is first oxidatively dehydrogenated to Dihydroxyacetone(DHA)or Glyceraldehyde(GLA)in the presence of O₂,DHA and GLA can be converted to each other,then DHA is catalytically dehydrated to Pyruvialdehyde(PA),and finally PA is rearranged to LA by benzilic acid.The equilibrium between DHA and GLA depends on pH,and the oxidation products without alkali are GLA,DHA and Glyceric acid(GLYA),indicating that the presence of alkali promotes the oxidation of glycerol to lactic acid catalyzed by Au-Pt,which is consistent with the results reported by Kishida et al^[17]。 the presence of base on the one hand promotes the deprotonation of the glycerol hydroxyl group and on the other hand accelerates the conversion of the intermediate(DHA)to lactic acid。

As mentioned earlier,the alkali solution can lead to the formation of lactate,and the production of lactic acid requires additional reaction and separation steps,which not only increases the production cost,but also produces a large amount of salt waste.Therefore,the selective oxidation of glycerol to lactic acid in alkali-free system has more application potential.It has been shown that,similar to bases,Lewis acids are also able to catalyze dehydration and isomerization in glycerol conversion.Xu et al.Also used AuPt/TiO₂as a catalyst and found that glycerol could selectively oxidize glycerol directly to lactic acid in the presence of Lewis acid AlCl₃under alkali-free conditions^[24]。 The reaction path to lactic acid is as follows:glycerol is oxidatively dehydrogenated to DHA and glyceraldehyde intermediates,and H⁺and Al³⁺cooperatively catalyze the dehydration and rearrangement of these intermediates to LA.However,the hydration of Al³⁺and the toxicity of Cl^-often affect their practical application.In 2014,Cho et al.Proposed a bifunctional catalyst strategy,using Pt/Sn-MFI catalyst,to realize the direct oxidation of glycerol to lactic acid(Fig.3 )^[25]。 Under mild reaction conditions,glycerol was first oxidized to DHA or GLA on the active site of Pt metal,and then GLA was rapidly converted to more stable DHA in the presence of Lewis acid of Sn-MFI molecular sieve,DHA was dehydrated to PA,and PA was added to water by intramolecular hydrogen transfer to LA.Tao et al.Proposed to introduce Lewis acid sites Al³⁺or Cr³⁺into polyoxometalates(POMs)by proton exchange with metal ions to prepare bifunctional catalysts(AlPMo or CrPMo)with Lewis acid active sites and oxidation active sites to directly catalyze the oxidation of glycerol to lactic acid^[26]。 It was found that Lewis acid metal ions had a significant effect on the redox potential of POMs,and the stronger the Lewis acid,the higher the redox potential,thus increasing the glycerol conversion.At the same time,stronger Lewis acid catalysts improve the chemoselectivity of DHA,and the dehydration of DHA and the conversion of PA to LA also require Lewis acid sites.In addition,the higher catalytic efficiency of AlPMo or CrPMo is also related to its hydrophobicity.Insoluble AlPMo or CrPMo can quickly release the water produced from the secondary structure,promote the dehydration of DHA to PA,and show the reaction characteristics of rapid production of LA.Feng et al.Prepared Pt/L-Nb₂O₅bifunctional catalyst with layered L-Nb₂O₅as the carrier,and also realized the direct conversion of glycerol to lactic acid^[27]。 Glycerol is first converted to DHA and GLA catalyzed by Pt nanoparticles,DHA is dehydrated to form intermediate PA catalyzed by Brønsted acid and Lewis acid,and finally lactic acid is produced by 1,2-hydride shift catalyzed by Lewis acid.It has been shown that the high catalytic activity of L-Nb₂O₅is most likely due to its large number of Lewis acid sites and Brønsted acid sites from the deformed orthorhombic structure of a and B axes and the additional layer of C axis.The reaction mechanism of other catalytic glycerol conversion under alkali-free conditions is also similar to the above process^[28,29]。

Since methanol is inevitably present in the crude glycerol for biodiesel production,it is theoretically simple and easy to directly convert glycerol to alkyl lactate(methyl lactate)in a"one-pot"process without additional separation and esterification steps.At present,the direct conversion of glycerol to methyl lactate or ethyl lactate in methanol or ethanol has become a hot topic.Similar to the reaction pathway of glycerol to lactic acid in aqueous solution,the conversion of glycerol to methyl lactate involves three consecutive reactions:(1)oxidation of glycerol to DHA and GLA^[16,30]; (2)DHA and GLA are dehydrated to form pyruvic aldehyde;(3)addition and isomerization of pyruvic aldehyde to methyl lactate.Lu et al.Developed Au/Sn-USY bifunctional catalyst for one-pot conversion of glycerol to methyl lactate in methanol solution^[16]。 On the one hand,the interaction between Au and extra-framework SnO_xaround the DeAl-USY mesopores promotes the dispersion of Au particles,which is beneficial to the oxidation of glycerol into DHA and GLA.On the other hand,the framework Sn changes the Lewis acid property of the catalyst,giving it a unique carbonyl activation ability,which is essential for the formation of methyl lactate 。

Although there are different reaction mechanisms,the conversion of glycerol to lactic acid will produce a certain amount of by-products,such as glyceric acid,oxalic acid,acetic acid,formic acid and other C-C cleavage products^[31]。 Therefore,it is still a challenge to construct alkali-free catalytic systems with high activity and selectivity.the development of bifunctional or multifunctional catalysts is an inevitable choice,and The oxidation active sites(noble metals),dehydration and isomerization active sites(acid sites or basic sites,metal active sites)of hydroxyl groups need to be considered^[30]。 noble metals have been proved to be the best choice for glycerol activation,and how to reduce the amount of Noble metals while avoiding their leaching deactivation In the liquid phase reaction is the main challenge.in addition,more and more attention has been paid to the improvement of oxidation activity and selectivity through the synergistic effect of L acid and B acid^[32,33]。

Compared with the traditional thermal catalytic oxidation,the electrocatalytic oxidation can not only realize the selective oxidation of glycerol to lactic acid under mild conditions,but also couple with the cathode to produce hydrogen。

Lux et al.Reported the electrocatalytic oxidation of crude glycerol to DHA and GLA,followed by base-catalyzed conversion of DHA or GLA to lactic acid^[34]。 Dai et al.Prepared an AuPt bimetallic nanocatalyst to produce lactic acid by electrocatalytic oxidation of glycerol at normal temperature and pressure,and the reaction path is shown in Figure 4^[35]。 glycerol was first oxidized to DHA or GLA by AuPt catalyst and alkali,then dehydrated to 2-hydroxyacrolein or pyruvic aldehyde by alkali catalysis,and finally rearranged to lactic acid by Cannizzaro.Recently,Yan et al.Reported a photo-assisted electrocatalytic strategy to realize the selective oxidation of Glycerol to lactic acid over gold nanowires(Au NWs)catalyst with 80%selectivity of lactic acid^[36]。 In situ Fourier transform infrared spectroscopy(FTIR)revealed that irradiation could promote the adsorption of glycerol on the surface of Au NWs,especially the secondary hydroxyl group of glycerol,which contributed to the production of DHA intermediate and further conversion to lactic acid by base catalysis,while the side reaction of GLA intermediate to GLYA or C1~C2 products could be significantly inhibited。

At present,the electrocatalytic oxidation method is still limited by the low current density of the battery,which hinders its practical application.On the other hand,the development of low-cost,highly selective and stable electrocatalysts is another major challenge for electrocatalytic oxidation。

Au-based catalysts generally exhibit excellent low-temperature catalytic activity and selectivity for hydroxyl groups^{[39⇓⇓~42]}。 Meng et al.Reported that the activation energy of the secondary hydroxyl group of glycerol on the Au(111)crystal face is 112.3 kJ/mol,which is lower than that of the primary hydroxyl group(169.2 kJ/mol),and the selective activation ability of the secondary hydroxyl group is strong^[43]。 In addition,Au-based catalysts are not prone to deactivation due to excessive oxidation and poisoning,and have high stability and reusability^[44,45]。 Table 1 shows the research results of glycerol oxidation to lactic acid catalyzed by some Au-based catalysts。

In the aqueous phase oxidation system of glycerol,the presence of base can improve the activity of Au-based catalyst^[46]。 on the one hand,the combination of hydroxide on the catalyst surface can significantly reduce the activation energy of Au(111)crystal face,on the other hand,it can promote theβ-hydride elimination reaction of aldehyde or ketone intermediates formed on the catalyst surface to produce the corresponding carboxylic acid^[40]。 Demirel-G Gülen et al.Found that Au catalyst can effectively catalyze glycerol oxidation only under alkaline conditions^[11]。 With the increase of alkali content,the glycerol oxidation was kinetically more favorable,indicating that the Au catalyst needs the assistance of alkali to achieve the glycerol oxidation reaction.Redina et al.Found that when the ratio of NaOH to glycerol was 2,the conversion and selectivity of glycerol to lactic acid were the best,while too high ratio of NaOH to glycerol could easily lead to a large number of carbonates,resulting in the deactivation of Au catalyst^[47]。 Zope et al.Used¹⁸O₂and H₂¹⁸O isotope labeling experiments to prove that the oxygen for the oxidation of the hydroxyl group to the carbonyl or carboxyl group in the glycerol oxidation reaction comes from the hydroxide ion rather than the oxygen atom of molecular oxygen^[48]。 Density functional theory(DFT)calculations further show that molecular oxygen participates in the catalytic cycle not through metal site dissociation to atomic oxygen,but through the formation of H₂O₂intermediates to catalyze the decomposition to form hydroxide ions.In addition to basicity,increasing the reaction temperature can also improve the selectivity of lactic acid over Au-based catalysts.Purushothaman et al.Investigated the effect of reaction temperature on the oxidation of glycerol to lactic acid,and found that when the temperature increased from 140℃to 180℃,the conversion rate of glycerol increased from 70%to about 80%,while the selectivity of lactic acid increased from 24%to 60%,indicating that high temperature was conducive to the formation of lactic acid^[49]。 Xu et al.Also found that the oxidation of glycerol to lactic acid required a higher temperature under acidic conditions^[24]。 the conversion rate of glycerol was very low at 100℃,and no lactic acid was produced.When the temperature was increased to 140℃,the selectivity of lactic acid was 57.5%.When the temperature was further increased to 160°C,the selectivity of lactic acid increased slightly,while the glycerol conversion increased significantly。

the oxidation of glycerol molecules on Au-based catalysts is considered to be a structure-sensitive reaction,and the size of Au nanoparticles is an important factor in controlling the selective oxidation of glycerol.Bianchi et al.Found that the activity of Au/C catalyst reached the maximum when the average diameter of Au particles was 7~8 nm^[45]。 Lakshmanan et al.Found that the formation of lactic acid was closely related to the size of Au particles^[23]。 When the Au content in the Au/CeO₂catalyst is 1%,the selectivity to lactic acid reaches the maximum with the increase of Au particle size to~6 nm;However,when the Au content in Au/CeO₂catalyst was 3%and 5%,the medium-size Au particles(~8 nm)showed the highest selectivity for lactic acid,while the smaller or larger Au particles showed poor selectivity for lactic acid.It can be seen that in the reaction of glycerol oxidation to lactic acid,the optimal size of Au nanoparticles should be 5~10 nm,and the selectivity of lactic acid is high.However,a large number of hydrogen and oxygen reactions using Au nanoparticles have been reported,and the size range of Au nanoparticles with excellent activity and selectivity should be smaller(1~5 nm).This difference is related to the formation and conversion path of DHA and GLA(Figure 3)in the process of glycerol conversion to lactic acid:small-scale Au nanoparticles have more beneficial oxidative dehydrogenation ability,which can quickly oxidize and dehydrogenate GLA to GLYA,and accordingly,the path of GLA isomerization to DHA is inhibited 。

Alloying is another effective strategy to improve the activity and selectivity of Au-based catalysts for glycerol oxidation to lactic acid.In the oxidation system in the presence of alkali,Purushothaman et al.Compared the effect of monometallic Au/nCeO₂catalyst and bimetallic Au-Pt/nCeO₂catalyst on the reaction performance^[31]。 The monometallic Au/nCeO₂TOF is about 1100 mol·mol^-1·h^-1,and the lactic acid selectivity is 60%.Bimetallic Au-Pt/nCeO₂catalyst with higher TOF（1350 mol·mol^-1·h^-1）and lactic acid selectivity(80%).It should be noted that Pt metal has poor activity and selectivity in the alkaline system suitable for Au catalyst.However,by alloying it with Au to form bimetallic Au-Pt,both activity and selectivity are improved.This promotion is related to the synergistic effect of metal alloying and is also affected by the strong metal-support interaction.In this study,the reducible CeO₂was coupled with Pt metal,which was suitable for the effect of strong metal-support interaction:during the reduction or activation process of the catalyst,the CeO₂at the interface between the support and the metal was partially reduced,forming Ce³⁺and oxygen deficient sites,while some Ce species could migrate to cover the metal surface and inhibit metal sintering.In addition,due to the strong interaction between metal and support,Ce⁴⁺ions also change the chemical valence of Pt and Au metals,and the surface active sites of Au⁰decrease,which increases new surface active species such as Au³⁺and Au⁺,which are beneficial to the activation of polar bonds such as C—O and C=O.Shen et al.Compared the glycerol oxidation performance of Au-Pt/TiO₂catalysts with different Au/Pt atomic ratios(1/3–7/1),and found that the activity was the highest when the Au/Pt atomic ratio was 3∶1(TOF was 524 h^-1）^[50]。 The promotion effect further shows that the electron density of Pt is increased by the electron transfer from Au to Pt through alloying,the chemical valence of Au and Pt active metal on the surface is changed,the oxidation of glycerol secondary hydroxyl is promoted,and the formation of DHA is better than that of glyceraldehyde.The synergistic effect between Au and Pt indicates that glycerol can be efficiently oxidized to lactic acid by rationally adjusting the electronic properties of the metal catalyst.In the alkali-free reaction system,Xu et al.Found that AuPd/TiO₂catalyst together with AlCl₃could selectively oxidize glycerol directly to lactic acid without alkali,and Au-Pd alloy could increase the selectivity of lactic acid from 47.7%to 58.5%^[24]。 The results of X-ray photoelectron spectroscopy(XPS)and transmission electron microscopy(TEM)characterization prove that there is an interaction between Au and Pd。

the Au catalyst also has excellent performance in the glycerol reaction system with methanol as the solvent.Purushothaman et al.Used Au/USY as a catalyst,and the yield of methyl lactate was 73%when the glycerol conversion was 95%^[30]。 Tang et al.Used metal oxide-supported Au nanoparticles and Sn-MCM-41 to form a multifunctional catalytic system by physical mixing,and found that the physical mixing of Au/CuO and Sn-MCM-41 had the best performance,with glycerol conversion of 95%and methyl lactate yield of 63%^[51]。 Mechanistic studies showed that the Au-based catalyst promoted the glycerol dehydrogenation step,converting glycerol to DHA and GLA,and the solid acid Sn-MCM-41 with strong Lewis acid and weak Brønsted acid isomerized the intermediate DHA to methyl lactate.the research team further found that the multifunctional catalytic system composed of Au-Pd bimetallic supported on carbon nanotubes(CNTs)and Sn-MCM-41 could increase the conversion of glycerol to 96%and the yield of methyl lactate to 85%^[52]。 Zhou et al.Physically mixed Au/CuO and Sn-Beta to form a bifunctional catalyst,which realized the efficient conversion of glycerol to methyl lactate^[53]。 Under the low temperature of 90℃,the glycerol conversion of 86%and the methyl lactate yield of 60%can be obtained.Au/CuO provides oxidation active sites for the oxidative dehydrogenation of glycerol to DHA,while Sn-Beta provides Lewis acid sites for the further conversion of DHA to methyl lactate.Lu et al.Investigated the effects of reaction temperature,reaction time and other conditions on the catalytic performance of Au/Sn-DeAl-USY^[16]。 It was found that the glycerol conversion and methyl lactate yield increased gradually in a certain temperature range(100~140℃),while higher reaction temperature(>180℃)led to an increase in over-oxidation products.Similarly,the reaction process can be effectively inferred according to the product distribution at different reaction times。

Compared with Pt and Pd noble metal catalysts,Au has a weaker ability to adsorb and activate oxygen,so its reactivity is significantly affected by the acidity and alkalinity of the system.Moreover,Au is easy to sinter or run off.How to construct multifunctional active sites and improve the activity and stability of Au by introducing carriers or additives is the focus of current research on Au-based catalysts。

the main challenges of Pt-based catalysts in glycerol oxidation are to improve the resistance to oxidative poisoning and the directional activation of hydroxyl groups.Table 2 shows the research results of glycerol oxidation to lactic acid catalyzed by some Pt-based catalysts。

The support has a significant effect on the performance of Pt-based catalysts.Bruno et al.Prepared Pt catalysts supported on different oxides（Al₂O₃,ZnO,MgO)by wet impregnation method,and found that the selectivity of Pt/Al₂O₃,Pt/ZnO,Pt/MgO catalysts to lactic acid was 45%,60%and 55%,respectively^[54]。 The Pt/Al₂O₃catalyst has the lowest selectivity to lactic acid,which is generally considered to be caused by its low alkaline density（0.3μmol CO₂m^-2）.However,the basicity（35.8μmol CO₂m^-2）of Pt/ZnO catalyst is lower than that of Pt/MgO catalyst（60.9μmol CO₂m^-2）,but the selectivity and yield of lactic acid over Pt/ZnO catalyst are better than those over Pt/MgO.This fully shows that the selectivity of lactic acid does not depend entirely on the acidity and basicity of the carrier,but also on the structure of the carrier.As an n-type semiconductor,ZnO is the active component of syngas to methanol in the early stage,which shows that it has excellent performance in activating polar bonds such as C—O or C=O.Pt/ZnO catalyst,ZnO can not only be used as a carrier to support the high dispersion of Pt,but also the Zn²⁺itself has Lewis acid characteristics,which can activate the C—O bond in DHA and promote the conversion of DHA to PA.This is corroborated by the results of Ftouni et al.,who compared the glycerol oxidation performance of Pt/C,Pt/TiO₂,Pt/ZrO₂catalysts^[55]。 Lactic acid selectivity of Pt/C catalyst is 60%,while that of catalysts with n-type semiconductor TiO₂or ZrO₂is more than 80%.Checa et al.Further compared the glycerol conversion performance of Pt catalysts supported on different reducible metal oxides（TiO₂,ZnO,SnO₂,and ZrO₂）supports^[56]。 It was found that the Pt catalysts supported on ZnO and SnO₂exhibited the highest oxidation activity due to the Zn or Sn atoms at the interface between the metal and the support,because of the strong metal-support interaction,resulting in the formation of Pt-Zn or Pt-Sn alloy structure,which promoted glycerol conversion.Komanoya et al.Demonstrated that the combined catalyst of Pt nanoparticles and TiO₂could realize the conversion of glycerol to lactic acid under oxygen atmosphere^[28]。 The yield of lactic acid reached 63%even in the absence of base.Among them,the Lewis acid site on TiO₂facilitates the dehydration and rearrangement of the intermediate to produce lactic acid with high efficiency.Wu et al.Found that the support itself could participate in the reaction,improve the activity of the catalyst and affect the selectivity of the product^[57]。 Pt/CuO,Pt/TiO₂and Pt-Sol catalysts were prepared by Sol-gel method for glycerol oxidation,and combined with the results of in situ infrared spectroscopy and reaction kinetics,it was found that Pt/CuO and Pt/TiO₂catalysts showed different reaction paths(Fig.5).The O atom on the surface of Pt/CuO catalyst has a strong interaction(βinteraction)with the secondary hydroxyl group of glycerol through hydrogen bonding,and the oriented oxidation of the secondary hydroxyl group of glycerol is realized by reducing the electron cloud density of the C—O bond of the secondary hydroxy group of glycerol in the reaction process.However,the Ti atom on the surface of the Pt/TiO₂catalyst forms an alkoxide with the primary hydroxyl group of glycerol,resulting in a strong metal-oxygen interaction(γinteraction),which reduces the electron cloud density of the C—O bond of the primary hydroxyl group of glycerol during the reaction,thus improving the selectivity of the oxidation of the primary hydroxy group of glycerol 。

In conclusion,coupling the reducible n-type semiconductor oxide support with metal Pt can improve the thermal stability and anti-sintering stability of metal Pt by using the strong metal-support interaction,and the activity and selectivity of glycerol conversion can be significantly improved by using the interfacial reaction to generate a new active phase。

in addition to the support effect,the structure and size of Pt metal nanoparticles are also important factors affecting the catalytic activity.Zhang et al.Prepared a series of Pt/AC catalysts with Pt particle size ranging from 10.2 nm to 3.8 nm by changing the precipitation-deposition temperature(0~80℃),and studied the size effect of Pt particles in the reaction of glycerol to lactic acid catalyzed by Pt/AC catalysts in alkaline solution^[58]。 Among them,the Pt/AC-30 catalyst with medium Pt particles(average particle size of 7.9 nm)had the highest TOF value,and the selectivity to lactic acid reached 42.9%at 64.1%glycerol conversion.However,the selectivity of lactic acid was not significantly affected by the Pt particle size effect.In addition,the particle size effect is also related to the type of alkali,and the effect of Pt metal particle size on glycerol conversion is more obvious in LiOH solution than in KOH solution.Oberhauser et al.Prepared 1.5 nm Pt nanoparticles and supported them on carbon materials,and the resulting Pt@C exhibited superior catalytic activity,（780 h^-1）,and lactic acid selectivity(94% )^[59]。

the terminal hydroxyl group of glycerol is preferentially activated on the Pt catalyst,so the Pt catalyst is used in the oxidation reaction of glycerol and is easy to produce glyceric acid^{[60⇓⇓~63]}。 In order to improve the selectivity of lactic acid,an alloy or bimetallic catalyst is usually formed by introducing a second metal into Pt metal.Zhang et al.Introduced Cu into Pt/AC catalyst and found that the introduction of Cu improved Pt dispersion^[64]。 The strong interaction between Cu and Pt enhanced the glycerol oxidation activity,and the lactic acid yield of 0.5%Cu-1.0%Pt/AC catalyst was 2.4 times higher than that of 1.0%Pt/AC catalyst.Further study showed that the valence state of Cu species in the catalyst changed with the Pt/Cu ratio.Among them,Cu⁺and Cu⁰species are beneficial to glycerol activation and lactic acid production,while Cu²⁺（large CuO particles)are beneficial to glyceric acid formation.Zhang et al.Prepared a bimetallic Pt-Co/CeO_xcatalyst and found that electrons were transferred from Co⁰to Pt⁰,while Pt-Ce interaction occurred^[65]。 Both glycerol and NaOH need to be activated on the Pt-Co surface,and the unique electronic coupling effect between Pt-Co and the CeO_xlattice distortion caused by Co incorporation make the catalyst have excellent performance.The activity（1533.9 h^-1）and selectivity(87.7%)of glycerol oxidation to lactic acid at 200℃were significantly increased.Li et al.Prepared Ni-promoted Pt_mNi_nO_x/TiO₂catalyst,and found that compared with Pt/TiO₂,NiO_xcooperated with metal Pt to catalyze glycerol oxidation,thus significantly improving the glycerol conversion activity of the catalyst,and the selectivity of lactic acid was 62.6%at glycerol conversion of 99.1%^[66]。

Pt-based catalysts have excellent oxidative dehydrogenation performance,and the size of metal particles and support is the key factor affecting the reaction process.Although Pt-based catalysts show good catalytic activity and selectivity in glycerol oxidation,their stability is poor,and they are prone to oxygen poisoning and metal leaching.By introducing the second metal to modify the catalyst,the alloy can be formed and the number of active sites can be increased,which can not only improve the reaction activity,but also greatly improve the stability of the catalyst。

in addition to Au and Pt catalysts,Pd,Ru,and Ag catalysts have also been used for glycerol oxidation.the selectivity of Pd catalyst to glyceric acid is higher,and it is necessary to add alkali In the reaction system to improve the activity^[67]。 Marques et al.Used Pd/AC catalyst for the oxidation of glycerol to lactic acid.Under the conditions of 230℃and NaOH/glycerol molar ratio of 1.1,the conversion of glycerol was 99%and the selectivity of lactic acid was 46%^[68]。 Arcanjo et al.Studied the activity of activated carbon supported Pd catalyst,and the catalytic performance was significantly affected by the catalyst metal loading,NaOH/glycerol molar ratio,temperature and other parameters^[69]。 At a reaction temperature of 230°C,the 10%Pd/C catalyst gave a glycerol conversion of about 99%and a selectivity to lactic acid of 68%,but with a high Pd loading.Shen et al.Prepared Pd₃/HAP catalyst by 3%Pd supported on hydroxyapatite(HAP),and the selectivity of lactic acid was as high as 95%at glycerol conversion of 99%under the conditions of NaOH/glycerol molar ratio of 1.1 and reaction temperature of 230°C^[70]。 The Pd₃/HAP catalyst was separated from the solution by centrifugation,and the glycerol conversion decreased from 99%to 93%and the lactic acid selectivity decreased from 95%to 94%after five repeated experiments.The slight decrease in the catalytic activity and selectivity of the Pd₃/HAP catalyst is due to the leaching of trace Pd from the surface of the support into the solution.Recently,Ten et al.Prepared a bimetallic Pd,Bi@Uio-66 catalyst for the cascade conversion of glycerol to lactic acid,and the unique structure and tunability of the hybrid material allowed 58%selectivity to lactic acid at 19%glycerol conversion without the addition of any base^[71]。 it can be seen that It is difficult for Pd catalyst to obtain high activity under alkali-free conditions。

Ru has a high oxidation and coordination number,so Ru metal complexes usually have good oxidation reactivity.Li et al.Proposed a method for hydrogen production from glycerol and selective synthesis of lactic acid using pincer Ru complexes^[72]。 using less than 1 ppm of Ru-MACHO catalyst for 2 H at 125°C,an unprecedented TOF value was obtained with 67%yield of lactic acid.in addition,the analysis of glycerol dehydrogenation products showed that improving the decarboxylation efficiency is the key to further reforming process.It is worth noting that there is a problem in the production of lactic acid from glycerol Using Ru-based catalysts.Ru is easy to break the C—C bond in hydrocarbons.Therefore,when Ru nanoparticles catalyze the conversion of glycerol to lactic acid under alkaline conditions,the decomposition of lactic acid to formic acid inevitably leads to the formation of methane and carbon dioxide^[73]。 To solve the above problem,Jiang et al.Reported a hydroxyapatite(HAP)supported trimetallic Ru-Zn-Cu^I/HAP catalyst to convert glycerol into lactic acid^[74]。 The results show that the Ru particles(<2 nm)are uniformly dispersed on the HAP.Cu^Ican effectively inhibit the cleavage of C—C bond,and the selectivity of lactic acid is significantly improved.Meanwhile,the presence of Zn²⁺enhanced the isomerization of methylglyoxal as an intermediate.Compared with Ru/HAP catalyst(63.7%)and Ru-Zn/HAP catalyst(70.9%),the selectivity of Ru-Zn-Cu^I/HAP catalyst to lactic acid increased to 82.7%.Pemmana et al.Studied the application of activated carbon supported Ru-V bimetallic catalyst in the conversion of glycerol to lactic acid^[75]。 The Ru/V₂O₅（Ru-V/AC）bimetallic catalyst was superior to the monometallic Ru and V catalysts,and the Ru-V/AC bimetallic catalyst showed the best catalytic performance under mild reaction conditions,with a glycerol conversion of 98.7%and a lactic acid yield of 75.5% 。

Unlike other noble metals,Ag-based catalysts showed the weakest activity for glycerol oxidation.Lari et al.Proposed a conversion process of glycerol to lactic acid using methylglyoxal as an intermediate,namely,glycerol gas phase oxidative hydration to methylglyoxal,and continuous liquid phase conversion of methylglyoxyal to lactic acid or methyl lactate^[76]。 Tao et al.Prepared silver-exchanged phosphomolybdic acid catalyst Ag_xPMo₁₂O₄₀（x=1,2,3）and evaluated its performance in the conversion of glycerol to lactic acid^[77]。 The results show that the addition of Ag metal can increase the redox potential of H₃PMo₁₂O₄₀,which is beneficial to the conversion of glycerol to DHA rather than GLA,while the Lewis acid site of Ag is beneficial to the further dehydration of DHA to lactic acid.The suitable redox potential,Lewis acid sites and hydrophobicity enable high conversion and selectivity.In the absence of any base,the selectivity of Ag₃PMo₁₂O₄₀to lactic acid reached 93%at 99%glycerol conversion.At the same time,Ag₃PMo₁₂O₄₀as a heterogeneous catalyst remained highly active after 12 cycles of reaction 。

the high cost and limited reusability of noble metals make the development of low-cost catalysts for glycerol conversion an attractive direction in the future.Therefore,the introduction of non-noble metal active phases into catalysts is the focus of future catalyst design。

Carbon materials have large specific surface area,good conductivity,acid and alkali resistance,and can realize the recovery of precious metals through high temperature combustion,so they are often used to support precious metal catalysts^[78]。

activated carbon(AC)is a general term for carbon materials with strong adsorption capacity.the specific surface area and pore size distribution of Activated carbon will affect the activity of the catalyst.Zhang et al.Studied the catalytic performance of AC supported Pt catalysts with different particle sizes in the selective oxidation of glycerol to lactic acid,and found that Pt nanoparticles were highly dispersed on the surface of AC,with an average particle size of 2.8~5.0 nm.These highly dispersed Pt nanocatalysts showed high activity in the oxidation of glycerol under alkali-free conditions^[79]。 the decrease of AC particle size increases the accessibility of Pt nanoparticles to reactants,and thus increases the reactivity。

N-doping of carbon materials can introduce surface functional groups,including pyridine N,pyrrole N and graphite N,which can anchor metal particles and improve the dispersion of metals^[80]。 in addition,the doped N-containing groups interact electronically with the metal nanoparticles,resulting In a novel catalytic behavior.Chen et al.Found that the incorporation of N atoms into carbon nanotubes(CNTs)increased the surface basicity of the catalyst,thereby accelerating the activation of glycerol hydroxyl groups^[81]。 The activity of Pt/N-CNTs catalyst doped with N atoms for glycerol oxidation in alkali-free aqueous solution was much higher than that of Pt/CNTs catalyst without N atoms.The reasons for the improvement of catalytic activity by doping N are:(I)electron transfer from N to Pt;(II)accelerating the activation of molecular oxygen;(III)The increase of surface defect sites promotes the adsorption of glycerol and O₂;(IV)The increase of surface basicity makes the—OH group in glycerol more easily activated.It is also pointed out that these catalysts can be reused many times,but the catalysts are subject to oxygen poisoning(formation of PtO_xspecies)and need to be regenerated at H₂.Zhang et al.Used N-doped multi-walled carbon nanotubes(MWCNTs)as the support of Pt nanoparticles for glycerol oxidation reaction under alkali-free conditions^[82]。 the results are consistent with the conclusion of Chen et al.,and the incorporation of N is beneficial to the high dispersion of Pt species,while the Pt nanoparticles maintain the electron-rich state^[81]。

Compared with other supported catalysts,molecular sieve-supported metal catalysts have many advantages,such as the formation of ultra-small and stable metal species,excellent thermal/hydrothermal stability in catalytic reactions(especially under harsh reaction conditions),and unique shape-selective catalytic characteristics^[83]。 At present,the main carriers for glycerol catalytic conversion to lactic acid are BEA molecular sieve,MCM molecular sieve,MWW molecular sieve and MFI molecular sieve.Among them,Sn heteroatom-containing molecular sieves(Sn-β,Sn-MCM,Sn-MWW,Sn-MFI)have attracted much attention as multifunctional Lewis acid catalysts^[84,85]。

the combination of Lewis acid sites and Brønsted acid sites in Sn-containing molecular sieves can greatly improve The catalytic activity.Asgar et Al.Synthesized Sn-Al-Beta molecular sieves containing L acid sites(framework Sn)and B acid sites(framework Al)and mixed with AuPd/CNTs as a multifunctional catalytic system for one-pot conversion of glycerol to methyl lactate^[86]。 At 140℃,the conversion of glycerol was 29%and the selectivity of methyl lactate was 67%。

Due to the large molecular size of glycerol,the intragranular diffusion limitation of molecular sieves can also affect the catalytic effect.the construction of hierarchical pore structure molecular sieves can significantly improve the diffusion performance.Tang et al.Constructed core-shell Snβ@mesoporous silica(MS)composites by oriented assembly of mesoporous silica on microporous Snβzeolite^[87]。 the shell mesoporous channel is vertically aligned with the Snβinner core to achieve sufficient mass transfer between the shell and the inner core.On this basis,the Au/Snβ@MS catalyst was prepared(Fig.6).the introduction of mesoporous silica can effectively change the ratio of L acid to B acid of Snβ@MS,and also promote the formation of highly dispersed Au nanoparticles.In addition,the core-shell structure of the catalyst can effectively protect Au nanoparticles from sintering,thus showing high stability and recyclability.Cho et al.Successfully synthesized hierarchical pore Sn-MFI molecular sieve with high mesopore ratio by adding three-dimensional ordered mesoporous carbon template^[88]。 In addition,the weak acid sites produced by the silicon hydroxyl defects can promote the activation of reactants。

metal oxide support has the advantages of adjustable composition,controllable morphology,and strong interaction with the supported metal,which can inhibit the agglomeration and loss of active metal,accelerate electron transfer,and thus increase the activity and stability of the catalyst.In addition,the metal oxide itself has the acidity and basicity,oxygen adsorption activity and so on,which may also play the role of promoter。

Metal oxides,such as TiO₂,CuO,Al₂O₃,CeO₂,ZnO,and SnO₂,are commonly used to enhance the Lewis acid strength in the oxidation of glycerol to lactic acid.However,the limited surface area of the oxide support limits the dispersion of the noble metal.To solve this problem,Douthwaite et al.Synthesized a high specific surface area mesoporous TiO₂material（110 m²/g）by nanocasting using SBA-15 as a hard template,and then prepared a AuPt/TiO₂catalyst^[89]。 The nano-cast TiO₂supported AuPt catalyst was found to exhibit higher glycerol conversion(99%)and lactic acid selectivity(82%).It is believed that the large amount of Si remaining on the surface of TiO₂after SBA-15 etching will form B acid sites,and the reaction with NaOH will form L acid sites,both of which can promote the dehydration of primary hydroxyl groups of glycerol,thus improving the selectivity of lactic acid 。

the metal-support strong interaction(SMSI)exists in the noble metal-reducible oxide system,which can change the geometric and electronic structure of the metal^[90]。 Pan et al.Prepared Au/ZnO catalyst by precipitation deposition(DP)and controlled the air heat treatment condition（Au/ZnO_DP-Air）,which produced strong SMSI between the ZnO layer and Au nanoparticles^[91]。 The results show that ZnO migrates to the surface of Au nanoparticles during air heat treatment,forming Au-O-Zn interfaces as well as a large number of oxygen vacancies.The secondary hydroxyl group of glycerol can be adsorbed and activated on the generated interfacial site,resulting in higher DHA selectivity(Fig.7).Specifically,the secondary hydroxyl groups of glycerol are dissociatively adsorbed on the oxygen vacancy defects at the Au/ZnO interface to form adsorbed(R₂)CHO^*and H^*species.Therefore,the preparation of this interfacial structure is an effective strategy to achieve the selective conversion of glycerol 。

Metal oxide perovskites,with their high structural tolerance,are ideal materials for studying how changes in the chemical composition of the support affect the performance of catalysts.Evans et al.Used LaBO₃perovskite(where B=Cr,Mn,Fe,Co or Ni)as a support to prepare bimetallic AuPt nanocatalysts for glycerol oxidation^[92]。 By changing the B site,the catalytic activity and selectivity of the catalyst change significantly with the oxygen adsorption capacity.For example,the selectivity of the AuPt/LaMnO₃catalyst to glyceric acid was 70%,while the selectivity of the AuPt/LaCrO₃catalyst to lactic acid was 86%under the same reaction conditions 。

In recent years,two-dimensional layered materials such as layered bimetallic hydroxides(LDHs)and layered rare earth hydroxides(LREHs)have also been explored as carriers for the aerobic oxidation of glycerol^[93][94]。

LREHs are a new type of rare earth materials with two-dimensional layered structure,which are composed of host layers of rare earth metal hydroxides rich in positive charges and guest anions between layers^[95]。 At present,there are few reports on the use of LREH materials in the field of catalysis,which needs to be further developed.Wang et al.Prepared a series of uniform LREH(RE=Y,La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er and Tm)nanosheets by reverse microemulsion method,and then loaded Au catalyst(Au/LREO)by precipitation-deposition method^[94]。 The catalytic activity and structural characterization of Au/LREO showed that the type of rare earth ions played a key role in the particle size,valence and reducibility of Au,which was an important factor affecting the catalytic activity of Au in the conversion of glycerol.Au/LPrO showed the best performance,including the highest glycerol conversion(68.2%),lactic acid selectivity(66.8%),and C3 product selectivity(90.6%).In fact,the ultra-small size(∼3 nm),high Au⁰component content,and high reducibility of gold particles are the necessary prerequisites for achieving the excellent catalytic performance of Au/LPrO 。