Controllable synthesis of novel nanoporous manganese oxide catalysts for the dir

时间：2024-05-22

Fushan Chen ,Songlin Zhao ,Tao Yang,Taotao Jiang,Jun Ni,Houfeng Xiong,Qunfeng Zhang,*,Xiaonian Li

1 Industrial Catalysis Institute of Zhejiang University of Technology,Hangzhou 310014,China

2 Jiangxi Province Engineering Research Center of Ecological Chemical Industry,Jiujiang University,Jiujiang 332005,China

3 School of Pharmaceutical and Chemical Engineering,Taizhou University,Taizhou 318000,China

Keywords:Oxalate route Controllable synthesis Manganese oxide Imine synthesis Heterogeneous catalysis Aerobic oxidation

ABSTRACT A novel template-free oxalate route was applied to synthesize different mesoporous manganese oxides(amorphous manganese oxide(AMO),Mn5O8,Mn3O4,MnO2)in the narrow temperature range from 350°C to 400°C by controlling the calcination conditions,which were employed as the efficient catalysts for the oxidative coupling of alcohols with amines to imines.The chemical and structural properties of the manganese oxides were characterized by the methods of thermogravimetry analysis and heat flow(TG-DSC),X-ray diffraction(XRD),nitrogen sorption,scanning electron microscope(SEM),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),H2temperature-programmed reduction(H2-TPR),and inductively coupled plasma optical emission spectrometry(ICP-OES)techniques.The structures of different manganese oxides were confirmed by characterization.The M-350(AMO)presented the maximum surface area,amorphous nature,the lowest reduction temperature,the higher(Mn3++Mn4+)/Mn2+ratio,and the higher adsorbed oxygen species compared to other samples.Among the catalysts,M-350 showed the best catalytic performance using air as an oxidant,and the conversion of benzyl alcohol(BA)and the selectivity of N-benzylideneaniline(NBA)reached as high as 100%and 97.1%respectively at the lower reaction temperature(80°C)for 1 h.M-350 had also the highest TOF value(0.0100 mmol·mg-1·h-1)compared to the other manganese oxide catalysts.The catalyst was reusable and gave 95.8%conversion after 5 reuse tests,the XRD pattern of the reactivated M-350 did not show any obvious change.Lattice oxygen mobility and(Mn3++Mn4+)/Mn2+ratio were found to play the important roles in the catalytic activity of aerobic reactions.

1.Introduction

Imines are well-known to be key N-containing intermediates for the synthesis of pharmaceutically,various biological,agricultural compounds,and other chemicals[1].The C=N bond in imines can go through versatile transformations such as reduction,addition,condensation,and multicomponent reaction[2-4].Therefore,the diverse synthesis of imines has enormous value in the aspect of the organic synthesis.In general,imines are synthesized via the condensation reaction of amines with carbonyl compounds in the presence of Lewis acid catalysts,dehydrating agents,etc.[5,6].Therefore,this process is actually limited in terms of application and environment.

Recently,several new methods have been reported,such as the selfcoupling of primary amines[7],the oxidative coupling of alcohols and amines[8-11],the oxidative dehydrogenation of secondary amines[12],and N-alkylation of amines with alcohols[13,14].Among these methods,the oxidative coupling of alcohols and amines seems to be one of the most promising approaches,because alcohols are less toxic,readily available,more stable,and inexpensive.Various heterogeneous catalysts had been reported for the oxidative coupling of alcohols and amines,such as catalysts based on supported Au[15],Pd[16],Pt[17],and Ru[18].However,most reaction systems required base additives,high temperature,longer reaction time and usage of precious metal species,which limited their large-scale application in imine synthesis.Therefore,it will be a significant breakthrough if an inexpensive and effective heterogeneous catalyst is used for the synthesis of imines from alcohols and amines at ambient,atmospheric reaction conditions in the absence of base.

Fig.1.XRD patterns of wide angle(10°-80°)of M-t and M-380-L.

Among the various kinds of the heterogeneous catalysts for alcohols and amines to imines,manganese oxide had a wide range of catalytic applications due to its tunable redox properties,diverse crystal structures[7,8,19,20]，and could be effective in the presence of other elements[21].Some established catalytic systems possessed excellent catalytic performance for the oxidative coupling of alcohols and amines to imines,such as MnOx/HAP[10],MnCo2O4-500[9],OMS-2[8],Cs/MnOx[22,23].Manganese-based catalysts not only acted as in the process to oxidize the alcohols to the corresponding carbonyl compounds,but also formed the imines as a Lewis acid.The oxidation of the alcohols in the first step was the rate determining step.Among these catalysts,the valence state change of manganese and the mobility of lattice oxygen,and so on were the decisive factors.However,these systems often suffered from low activity,high temperature(＞100°C),and tedious catalyst preparation process.

From the industrial point of view,precipitation method is one of the most preferred catalyst preparation methods.Recently,a novel template-free oxalate route was applied to synthesize mesoporous manganese oxides with high surface area,well-defined mesopore,which showed better catalytic activity for deep oxidation[24-27].We had recently found that copper-doped MnOxby oxalate route had the excellent catalytic effect on the oxidative dehydrogenation of 1,2,3,4-tetrahydroquinoline[28].However,manganese oxides was prone to very easy phase change during low temperature calcination,and it was very difficult to obtain a pure manganese oxide crystal phase[29-32].Manganese oxides with different crystal phases possessed different valence states of manganese and lattice oxygen mobility,and so on[25].In this work,four types of nearly pure crystal manganese oxides were simply synthesized by the method of manganese oxalate calcining in the narrow temperature range from 350°C to 400°C,which were applied to the imines from alcohols and amines.Bifunctional manganese oxide catalysts possessed the ability to catalyze oxidation of alcohols and Lewis acidity.Catalytic oxidation of alcohols played a key role in the synthesis of imines from alcohols and amines because the oxidation of the alcohols in the first step was the rate determining step.It had been investigated in detail that four manganese oxides had the effect on the catalytic performance of imine synthesis.Among these manganese oxides,M-350 possessed maximum surface area,easy lattice oxygen mobility,higher(Mn3++Mn4+)/Mn2+ratio,which were crucial to achieving high catalytic activity.The catalytic process was done under low temperature(80°C),and air used as the sole oxidant.Moreover,no base additives and dehydrating agents were needed.

2.Experimental

2.1.Catalyst synthesis

All chemicals were of analytical grade and used without further purification including Mn(NO3)2(50 wt%)solution(Aladdin Reagent Cop.),(NH4)2C2O4·H2O(99.9%)(Aladdin Reagent Cop.),absolute alcohol(99.9%)(Tianjin Damao Reagent Cop.).High purity water(15 MΩ·cm-1)was used to dissolve chemicals and wash products.Manganese oxides had been synthesized according to reported procedures[25,28].In a typical synthesis,96 mmol(NH4)2C2O4was dissolved in 400 ml water.The resulting clear solution was quickly added to the 400 ml manganese ion solution with a mole concentration of 0.2 mol·L-1under strong stirring at room temperature.The oxalate precipitate formed quickly during the reaction period.After stirring for 40 min,the precipitate(precursor)was filtered,subsequently washed by high purity water and absolute ethanol several times,then dried at 80°C for 20 h.The sample was transferred into a crucible without the lid,heated at 2°C·min-1up to a certain calcination temperature(350°C-400°C)in a muffle furnace and then treated for further 5 h.The products were denoted M-t(t represents the different calcination temperatures of 350°C,380°C,and 400°C).Under the same calcination conditions,another sample in a crucible with the lid was calcined at 380°C and the product was denoted M-380-L.

2.2.Catalyst characterization

The samples were characterized by thermogravimetry analysis and heat flow(TG-DSC),X-ray diffraction(XRD),nitrogen sorption,scanning electron microscope(SEM),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),H2temperatureprogrammed reduction(H2-TPR),inductively coupled plasma optical emission spectrometry(ICP-OES).The details of these characterization methods could be found in supplementary Information.

Fig.2.(a)Nitrogen sorption isotherms of M-t and M-380-L,(b)BJH desorption pore size distributions of M-t and M-380-L.

Table 1 Surface areas,average pore size,and pore volume from N2sorption analysis and average primary particle/crystallite sizes

2.3.Catalytic activity measurements

The aerobic oxidative coupling of alcohols and amines was performed in a one-necked flask equipped with a condenser at atmospheric pressure.In a typical reaction,a mixture of alcohols(0.5 mmol),amines(1.0 mmol),catalyst(50 mg)and toluene(5.0 ml)were added.The reaction mixture was heated to 80°C under vigorous stirring(700 r·min-1)for the required time under an air balloon.After reaction,the mixture was cooled and the catalyst was removed by centrifugation.The product mixture was identified by GC-MS(Thermo Fisher TRACE1310-TSQ8000Evo)or standard substances.The conversion and selectivity were analyzed by GC(Agilent 6890N,FID)based on the ratio of alcohols to imines.The conversion of benzyl alcohol(BA)and the selectivity of N-benzylideneaniline(NBA)were determined using internal standard method with dodecane as internal standard.The conversion and selectivity of the others were determined using area normalization method.

3.Results

3.1.XRD

The XRD pattern of the precursor was shown in Fig.S1,which could be indexed to MnC2O4·3H2O(JCPDS PDF 32-0648).This was consistent with TG-DSC result(Fig.S2).The catalysts prepared by an oxalate route were shown in Fig.1.When the calcination temperature was 350°C,the diffraction pattern of M-350 showed no obvious peaks,representing amorphous nature or poor crystallinity(Fig.2,M-350).Interestingly,when the calcination temperature was increased to 380 °C,the few of weak peaks appear in the wide angle XRD patterns(Fig.1,M-380).M-380 pattern could be indexed as the pure metastable intermediate Mn5O8phase(JCPDS PDF 39-1218).At the same calcination temperature,the main diffractions of M-380-L could be indexed to main Mn3O4phase(JCPDS PDF 24-0734)and the very small proportion of Mn5O8phase(Fig.1,M-380-L).When the calcination temperature was further raised to 400 °C,the diffraction peaks clearly appeared and corresponded to the tetragonal MnO2phase(JCPDS PDF 24-0735),and a small part of emerging cubic Mn2O3phase(JCPDS PDF 41-1442).

3.2.Nitrogen sorption

Porous manganese oxides were analyzed by N2sorption measurement to study the textural property and pore size distributions.The N2adsorption and desorption isotherms of four samples(Fig.2a)displayed type IV isotherms(IUPAC)[33].M-350,M-380,M-380-L followed by a typical type hysteresis loop related to narrow micropores and mesopores in the structures of the materials,and the steep uptakes of N2at low P/P0(＜0.02)in the isotherms were due to N2filling of few micropores(Fig.2a,M-350,M-380,M-380-L)[24].However,the hysteresis loop began from P/P0(=0.7)in M-400,indicating that the pore structure was evolved into mesoporous scope and micropores no longer existed.BJH desorption pore size distributions of four samples were displayed in Fig.2b,and the pore structure parameters were summarized in Table 1.The average pore size of M-350 was calculated as 3.1 nm,the maximum surface area was 298.8 m2·g-1(calculated by BET method),and the pore volume was 0.19 cm3·g-1from the desorption branch of the isotherm(Table 1,M-350)[33].The pore size distributions of the material became wider,and an increment of pore size(3.1→13.2 nm)was observed with the rising calcination temperature(Table 1).The surface areas rapidly decreased(298.8→33.0 m2·g-1),and the pore volumes also went down(0.19,0.23→0.14 cm3·g-1)(Table 1).M-350,M-380,M-380-L simultaneously possessed mesoporous and micropores.The pore size distribution of M-400 was only mesoporous.

Fig.3.SEM images of(a)precursor,(b)M-350,(c)M-380,(d)M-380-L,and(e)M-400.

Fig.4.HRTEM images of(a)M-350,(b)M-380,(c)M-380-L,and(d)M-400.

3.3.SEM and TEM

SEM images of the uncalcined sample and the calcined samples were depicted in the Fig.3.It was observed that the precursor was micro rods of different sizes.Though the high total weight loss reached about 57%from the TG curve in Fig.S2,the shapes of calcined samples had generally kept their original morphologies except for partial rupture and agglomeration.It was speculated that the materials should had the significant porosity.

To further analyze the morphologies and microstructures of the samples,Fig.4 showed HRTEM images of different manganese oxides.Fig.4a clearly showed the amorphous nature of M-350.This was also confirmed by XRD(Fig.1,M-350).Fig.4b-d showed the crystalline texture of M-380,M-380-L,M-400.The lattice fringes of 0.49 nm and 0.28 nm(Fig.4b)could correspond to the crystal phase(200)and(-311)of the Mn5O8.Fig.4c showed the lattice fringes of 0.25 nm,matching well with(310)plane of Mn3O4.The HRTEM image(Fig.4d)showed that the lattice fringe was 0.24 nm,0.31 nm,which could be indexed to the(101)and(110)diffraction planes of MnO2.It was speculated that these samples except for M-350 were mainly single crystals.This result was in agreement with the XRD data.

3.4.XPS

Fig.5.Mn 2p and O 1s XPS spectra of the samples.

Table 2 Summary of molar ratio of different elemental components on surface of the samples from Mn 2p and O 1 s XPS spectra

Valence state information and elemental components about the manganese oxide samples were further investigated by XPS characterization.Fig.5 showed the Mn 2p and O 1s XPS spectra of the manganese oxide samples.The spectra of Mn 2p3/2and Mn 2p1/2were known to be useful to distinguish the oxidation states of Mn element.The Mn 2p3/2XPS spectra exhibited three characteristic peaks at 640.6 eV,641.7 eV and 643.0 eV,which could be ascribed to Mn2+,Mn3+and Mn4+ions on the surface of the samples,respectively(Fig.5a)[25,34].An energy separation of(11.5±0.2)eV was shown between Mn 2p3/2and Mn 2p1/2states.Table 2 showed surface element contents of the samples from Mn 2p and O 1s XPS spectra.It was shown in Table 2 that M-350 possessed the contents of Mn3+(0.418),Mn4+(0.377),and the higher(Mn3++Mn4+)/Mn2+ratio(3.88)compared with other samples.The M-400 contained the highest content of Mn4+(0.566),the lowest content of Mn2+(0.084),and possessed the highest(Mn3++Mn4+)/Mn2+ratio(10.90)(Table 2,M-400).The O 1s spectra(Fig.5b)with the shoulder peak were very broad and asymmetrical,this was because of the superposition of various oxygen species.O 1s XPS spectra could be fitted with three peaks at 529.8 eV,531.3 eV,and 533.0 eV.The peak at 529.8 eV corresponded to the lattice oxygen atoms(O2-,denoted as Olatt);the peak at 531.3 eV was attributed to the surface adsorbed oxygen species(O2-,O22-,O-,denoted as Oads);the small peak at 533.0 eV was attributed to the adsorbed OH groups,molecular water and carbonate species(denoted as Osuf)[25,34].M-350 and M-400 had the more adsorbed oxygen species(Table 2),implying the better catalytic activity for the organic oxidative reactions due to the mobility and availability of the surface adsorbed oxygen species[35].By comparing Table 2,it was found that the higher the(Mn3++Mn4+)/Mn2+ratio,the higher the content of surface adsorbed oxygen species was.This suggested that there was the positive correlation between the adsorbed oxygen contents and the higher valence manganese contents,which was also confirmed by the literature[25].

3.5.H2-TPR

Fig.6.H2-TPR profiles of M-t and M-380-L from 30°C to 600°C.

Table 3 N-Benzylideneaniline(NBA)synthesis from benzyl alcohol(BA)and aniline over the as prepared catalysts under various conditions

On the basis of the reduction of Kapteijn et al.[36],manganese oxides could be described by successive processes:Mn2O3(MnO2,Mn5O8)→Mn3O4→MnO.MnO was the final state from 30 °C to 600 °C.The reduction of Mn3O4should show one reduction peak.Each further reduction process would need the higher reaction temperature,which gave us an effective method to determine the accurate composition of the catalysts.The H2-TPR profiles of the four manganese oxides under study were presented in Fig.6.The H2-TPR profile of M-350 sample showed three reduction peaks appearing at 274°C,293°C,and 429°C(Fig.6,M-350).The former could correspond to the reduction of MnO2to Mn3O4,the middle could be assigned to Mn2O3to Mn3O4,and the latter was ascribed to the reduction of Mn3O4to MnO[25,37].The three reduction peaks of M-380 similar to M-350 shifted to high temperature(Fig.6,M-380),which could be indexed as the pure intermediate Mn5O8(MnO2·2Mn2O3)phase by XRD characterization[36,38].The reason shifting to high temperature might be attributed to the crystalline nature of the materials and the increasing of particle size(2.6 nm→22.8 nm)in Table 1.The broad peak at 369 °C could correspond to the reduction of Mn3O4to MnO(Fig.6,M-380-L).This result was in good accordance with the literature[36].The two non-obvious weak peaks at 230°C and 324°C(Fig.6,M-380-L)were attributed to the reduction of MnO2and Mn2O3in Mn5O8(MnO2·2Mn2O3).This was also in agreement with the XRD and TEM results.M-400 showed two-step reduction at 326 °C and 432°C(Fig.6,M-400).The former could be ascribed to the first reduction of MnO2to Mn3O4.The latter could be attributed to the reduction of Mn3O4to MnO.The area ratio of the lower temperature peak to the higher temperature peak was about 2.This result was consistent with the reduction of MnO2in the literature[36].The lattice oxygen mobility on the manganese oxides could be linked to the material reducibility.The reducibility of the samples was in the order:M-350＞M-380＞M-400＞M-380-L under the low temperature zone.The nature of the material and the particle size(Table 1)were the two main factors affecting the shift of the reduction temperature.The amorphous M-350 sample showed the lowest reduction temperature in the low temperature zone,suggesting the easy lattice oxygen mobility[39].The lattice oxygen mobility of catalysts would play important role in the catalytic activity of aerobic reactions[25,37,40].

Fig.7.Catalytic activity of M-350 dependent on reaction temperature.Reaction conditions:BA(0.5 mmol),aniline(1.0 mmol),catalyst(50 mg),toluene(5.0 ml),reaction time(1 h),air balloon.

Fig.8.Catalytic activity of M-350 dependent on the reaction time.Reaction conditions:BA(0.5 mmol),aniline(1.0 mmol),catalyst(50 mg),toluene(5.0 ml),80°C,air balloon.

3.6.Catalytic performance

The synthesis of BA with aniline to NBA was chosen as a model reaction for filtrating the catalytic efficiency of the manganese oxide catalysts.The evaluated results were summarized in Table 3.When the reaction was initially carried out in the absence of catalyst,no reaction was observed(Table 3,Entry 1).Mn(NO3)2,Mn(C2O4)·3H2O also showed no any catalytic activity(Table 3,Entries 2-3).To our delight,M-350 catalyst shows the highest conversion(100%)of BA and the highest selectivity(97.1%)of NBA among these as-prepared manganese oxide catalysts(Table 3,Entry 4).Benzaldehyde was detected as the only by-product from BA in toluene.Other manganese oxide catalysts gave the low conversions(Table 3,Entries 5-7).These results clearly suggested M-350(AMO)was the most effective catalyst.M-350 had the highest TOF value(0.0100 mmol·mg-1·h-1)compared to the other manganese oxide catalysts.

After confirming M-350 as the most effective catalyst,the reaction conditions of M-350 were further investigated in order to obtain a more comprehensive understanding.Concerning the solvent,both polar and nonpolar solvents were used to test(Table 3,Entries 4,8-11).Full conversion of BA and high selectivity to NBA were observed in toluene and n-octane.The results implied that the cheap toluene was the best solvent.The reaction was performed at different gas atmosphere in order to confirm the effect of oxidant.The results were shown in Table 3(Entries 4,12-13).M-350 also showed the conversion(99.3%)and the selectivity(94.9%)under oxygen atmosphere at 80°C for 40 min(Table 3,Entry 12).When the reaction was carried out under inert(N2)atmosphere,the conversion(27.4%)was achieved(Table 3,Entry 13).The lower conversion was attributed to the oxidation role of adsorbed oxygen and labile lattice oxygen on the catalyst surface[37],implying that O2was essential for the oxidation of BA.From the economic point of view,it was suggested that the air was chosen as the final oxidant.The reaction was performed at different temperatures to find the effect of temperature on the catalytic performance.The result showed the conversion of BA and selectivity of NBA increased with the reaction temperature raising(Fig.7).The highest conversion(100%)and selectivity(97.1%)were achieved at 80°C.The conversion and selectivity still remained nearly unchanged at 110°C.The dependence of catalytic activity on different reaction time was investigated,and the result showed the conversion of BA could reach 100%after 60 min,and the selectivity increased with the raising of the conversion(Fig.8).

Table 4 The synthesis of imines from various alcohols and amines by the M-350 catalyst

3.7.Reaction scope

Having analyzed the optimum reaction conditions for the synthesis of NBA from BA and aniline,the scope and limitations of M-350 were investigated using different types of amines and alcohols(Table 4).First,BA was tested with various amines with different functional groups(electron withdrawing and electron donating)as the coupling partner to afford the desired imines(Table 4,Entries 1-8).BA with ndodecylamine(Table 4,Entry 4)gave the highest conversion(100%)and the highest selectivity(100%)for imine after only 1 h.p-Anisidine showed the lowest conversion(88.1%)and the lower selectivity(79.5%)for the corresponding imine(Table 4,Entry 5).p-Nitroaniline gave the lower conversion(91.6%)and the lowest selectivity(65.1%)(Table 4,Entry 8).The other anilines with different functional groups(o-Me-,m-Me-,p-OMe-,p-Cl-),and cyclohexylamine gave excellent conversion and high selectivity for the corresponding imines(Table 4,Entries 1-3,6,7).Second,BA with different functional groups(electron withdrawing and electron donating)and aniline also showed excellent conversion and selectivity for the corresponding imines(Table 4,Entries 1,9,15,21,23).However,aliphatic alcohols including n-butanol and n-octanol exhibited extremely low conversion and selectivity(Table 4,Entries 25,26).Finally,the BAs with different functional groups and the amines with different functional groups as the coupling partners gave pleasant conversion and selectivity as well.(Table 4,Entries 10-14,16-20,22,24).

3.8.Reusability and heterogeneity

Fig.9.Reusability of the M-350 catalyst.Reaction conditions:BA(0.5 mmol),ndodecylamine(1.0 mmol),catalyst(50 mg),toluene(5.0 ml),80°C,reaction time(1 h),air balloon.

Stable reusability and negligible leaching of active species are the two important factors of the heterogeneous catalysts.The reusability was explored by recycling the used catalyst in succession for the catalytic oxidative coupling of BA and n-dodecylamine by M-350 catalyst.After the reaction,the catalyst was separated by centrifuging the reaction mixture,washed with ethanol,and then reactivated at 250°C for 1 h.The aim of reactivation removed the adsorbed species from the catalyst surface.The regenerated catalyst showed scarcely any appreciable loss of its high catalytic performance compared to the fresh catalyst(100%conversion,100%selectivity),even after the fifth reuse(95.8%conversion)(Fig.9).The hot filtration test was also performed to investigate any possible leaching of active species from the catalyst surface.The reaction did not take place after filtering off the catalyst at 73.4%conversion(Fig.10).After the first use of the catalyst,the filter liquor was analyzed by ICP-OES and trace Mnn+ion(1.28 mg·L-1)was detected.This suggested that there was almost no leaching of Mnn+ion on the surface of the catalyst during the reaction.It could be seen from the XRD pattern in Fig.11 that the regenerated catalyst(after third reuse)also preserved the amorphous nature compared with the fresh catalyst.All of these results showed the M-350 catalyst was truly reusability and heterogeneous nature.

Fig.10.Hot filtration test of M-350.Reaction conditions:BA(0.5 mmol),aniline(1.0 mmol),catalyst(50 mg),toluene(5.0 ml),80°C,air balloon.

Fig.11.XRD of M-350 catalysts before and after third reuse.

4.Discussion

In this work,a novel template-free oxalate route was applied to synthesize different mesoporous manganese oxides.Surprisingly,MnC2O4·3H2O precursor was decomposed into four types of mesoporous manganese oxide(AMO,Mn5O8,Mn3O4,MnO2)by controlling the calcination conditions from 350°C to 400°C.Calcination temperature and atmosphere were two important factors to get different types of manganese oxide from MnC2O4·3H2O[24,29-31,41].Calcination atlow temperature(350°C)resulted in amorphous manganese oxide(M-350)under sufficient air atmosphere[24,25].Calcination at 380°C led to the main crystallized phase of Mn3O4(M-380-L)under deficient air atmosphere.At 380°C,Mn3O4transformed into the metastable Mn5O8phase(M-380)under sufficient air atmosphere.Mn2+and Mn3+in Mn3O4were oxidized to a certain proprotion of Mn3+and Mn4+to form Mn5O8(MnO2·2Mn2O3)[29].Surprisingly,the main crystal phase of MnO2(M-400)was obtained at 400°C under sufficient air atmosphere.This might be that Mn3+was oxidized to Mn4+under the calcination condition.

Table 5 Comparison of catalytic performance of various Manganese-based heterogeneous catalyst for the direct synthesis of NBA from BA and aniline

M-350 displayed the highest catalytic activity for the synthesis of NBA.The M-350 catalyst exhibited the highest specific surface area,the proper proportion of micropores and mesopores,amorphous nature.Large surface area increased contact area of reactant molecules.Amorphous nature of M-350 improved the low temperature reducibility of the catalyst and promoted the easy lattice oxygen mobility.The mobility of lattice oxygen was directly related to the catalytic activity of aerobic reactions[42].M-350 showed the higher(Mn3++Mn4+)/Mn2+ratio among the four types of manganese oxide catalysts.Mn3+and Mn4+produced more surface adsorption oxygen,implying the better catalytic activity for the organic oxidative reactions[35].Therefore,(Mn3++Mn4+)/Mn2+ratio were also found to play the important role in the aerobic oxidation[7,9,25,37,43].Although M-400 had the highest(Mn3++Mn4+)/Mn2+ratio,the specific surface area and lattice oxygen mobility of M-400 were significantly reduced.Therefore,M-400 displayed the lower catalytic activity than M-350,but the much higher than other manganese oxide catalysts.

In general,imines from alcohols and amine were synthesized by two obvious reaction steps.Firstly,alcohols were oxidized to the corresponding aldehydes in the presence of air.Secondly,aldehydes in situ generated condensed with amines to form the imines.Tables S1-2 showed that the slower reaction rate of BA oxidation(63.1%conversion in 20 min)than condensation(94.5%conversion in 5 min).These results implied that the initial oxidation of BA was the rate determining step,which was consistent with the literature[23].Fig.9 showed that the conversion of BA with aniline for NBA was 73.4%in 20 min,suggesting NBA forming also promoted oxidation of BA.

The catalytic system was applied to study different alcohol and amine substrates(Table 4).Different alcohols and amines gave excellent conversion and selectivity.The presence of electron donating or withdrawing groups in the aromatic ring played important roles in determining the conversion and selectivity.The lowest selectivity of p-nitroaniline was that electron withdrawing groups decreased the electron density in the nitrogen atom,which resulted in a relatively low rate of nucleophilic attack on the carbonyl carbon to form the imines.P-methoxyaniline had the lower conversion and selectivity,mainly due to the conversion of p-methoxyaniline to azobenzene by-product.

When carried out without the catalyst under air atmosphere,no product was obtained(Table 3,Entry 1).This indicated that the oxidation of BA required the catalyst.Table 3(Entries 2-3)showed the reaction did not happen using Mn(NO3)2or Mn(C2O4)·3H2O as the catalyst,suggesting that Mn2+was not the active site of the catalyst.However,both M-350 and M-400 with the higher(Mn3++Mn4+)/Mn2+ratio showed the higher catalytic activity(from XPS results).According to Mars-Van-Krevelen mechanism[44],a multi-electron redox process was anticipated(Scheme S1).The results showed that Mn3+and Mn4+were the active site of the catalyst,which was also confirmed by the literature[8,23].

Based on experiments and the literature,two step mechanism was anticipated for NBA synthesis from BA and aniline by the manganese oxide catalysts[7,8].First,a Mars-Van-Krevelen mechanism was proposed for BA oxidation over manganese oxides[44].This implies a multi-electron redox process according to this mechanism in the liquid phase(Scheme S1).Second,nucleophilic attack was carried out between carbonyl in situ generated and amine.On the basis of the literature[45],manganese oxides had a dominant Lewis acidity.When carbonyl oxygen acted with Lewis acidic sites,the effect promoted that the amine group attacked the carbonyl carbon to form NBA(Scheme S2).

For the sake of comparison,Table 5 summarized the catalytic performance of various heterogeneous manganese-based catalysts for the synthesis of NBA from BA and aniline.Compared to literature reported studies,M-350 was the most effective catalyst for NBA synthesis.M-350 not only gave high conversion and selectivity,but also had the highest TOF(0.0100 mmol·mg-1·h-1).

5.Conclusions

In summary,It had been reported that MnC2O4·3H2O precursor was successfully decomposed into four types of mesoporous manganese oxide(AMO,Mn5O8,Mn3O4,MnO2)by controlling the calcination conditions.Calcination temperature and atmosphere were two important factors to get different types of manganese oxide.Manganese oxides serving as a bifunctional catalyst catalyzed two distinct processes(oxidation followed by imine formation)under the same reaction conditions.M-350 catalyst showed the best catalytic performance to form NBA without the need for any additives or promoters.Diverse types of substituted imine could be also obtained under the mild conditions.Our investigation revealed that catalytic performance mainly depended on(Mn3++Mn4+)/Mn2+ratio and lattice oxygen mobility.Further investigations of the catalyst are underway.