Zhiping Du*,Lihua Xiong,Zhikun Lin,Xuli Li,Yigang Ding,Yuanxin Wu

Catalysis,Kinetics and Reaction Engineering

Oxidative Carbonylation of Methanol to Dimethyl Carbonate Over Cu(II)-1,10-Phenanthroline Bromide Complexes☆

Zhiping Du*,Lihua Xiong,Zhikun Lin,Xuli Li,Yigang Ding,Yuanxin Wu

Key Laboratory for Green Chemical Process of Ministry of Education,Hubei Key Laboratory of Novel Chemical Reactor and Green Chemical Technology,Wuhan Institute of Technology,Wuhan 430073,China

A R T I C L EI N F O

Article history:

Dimethyl carbonate

Cu(phen)Br2

Methanol

Oxidative carbonylation

In order to develop the catalysts with low corrosiveness for the oxidative carbonylation of methanol to dimethyl carbonate(DMC),CuBr2was selected as the metal source to prepare Cu coordination compounds,Cu(phen)Br2, [Cu(phen)2Br]Br and[Cu(phen)3]Br2(phen=1,10-phenanthroline).These complexes were characterized by thermogravimetric analysis and temperature-programmed reduction.Their catalytic performances were investigated.It was found that the metal coordination environments and thermal stability of the complexes playedanimportant role intheircatalytic activities.Cu(phen)Br2exhibitedthehighest activity due tothelowest sterichindrance,themostpositionsoccupiedbythebromideionsandthehighestthermalstability.Theturnover numberwasupto47.6DMC mol·(Cumol)?1withselectivityof92.8%underconditionsof120°C,ratioofpartial pressure of CO to O2of 19:1(below the explosion limit of CO)and catalyst concentration of 0.011 mol·L?1. Furthermore,a plausible reaction mechanism was suggested on the basis of the experimental data.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

As an environmentally benign chemical,dimethyl carbonate(DMC) canbeusedasthecarbonylationreagentandthemethylationreagentto replace highly toxic phosgene or dimethyl sulfate[1-4].DMC has been drawingattentionasa safesolvent andanadditive in lithium-ion batteries.DMCcanalsobeusedpotentiallyasafueladditivetoreplacemethyltert-butylether(MTBE)duetoitshighoxygencontent,goodblending octane,rapid biodegradability and low toxicity[5,6].

Amongseveralphosgene-freemethodsforthesynthesisofDMC,the oxidative carbonylation of methanol is the most perspective route,in which CuCl is used as a catalyst[7-9].However,this catalytic system suffers from some disadvantages,such as the losing activity due to the decomposition of CuCl,the corrosiveness caused by the chlorine ions and the redox reaction of copper.

In order to overcome these drawbacks,the inf l uence of N-ligand on CuCl was studied[10-14].It was found that 1,10-phenanthroline (Phen)wasthemosteffectivepromoterintermsofthecatalyticactivity and the corrosion inhibition.Xiong and co-workers reported that Phen and N-methylimidazole(NMI)exhibited a synergic effect on the catalyticactivityofCuCl[15].WhenN-butylpyridiniumtetraf l uoroborate ([BPy]BF4)wasusedasreactionmedia,thesolubilityofCuClinmethanol increasedfromabout0.0044g·g?1to0.022g·g?1,andtheconversionof methanolandtheselectivityofDMCwereenhancedfrom9.0%and97.3% to 17.8%and 97.8%,respectively[16].Considering the low solubility of CuCl in methanol,CuCl2is selected to catalyze the oxidative carbonylation of methanol as well.Raab and co-workers found that the addition of NMI could enhance the catalytic activity of CuCl2dramatically and inhibit the corrosion to stainless steel autoclaves[17].

The inf l uence of the above ligands on the catalytic performance of CuClx(x=1or2)isnotinvestigatedbytheCucoordinationcompounds as the catalyst,but by the direct mixture of the ligand and CuClxas the catalyst.It is very diff i cult to determine the amount and structure of the Cu coordination compound,which is formed in the oxidative carbonylation of methanol.As a result,it is diff i cult to understand the essence of the reaction.

Recently it is found that Cu(phen)Cl2exhibited the higher activity than the equimolar mixture of CuCl2and Phen in the oxidative carbonylation of methanol[18].Considering that the corrosiveness of Cl?to metallic vessels is very high,it is replaced with Br?[19,20].In this paper,the copper coordinations with different structures,Cu(phen) Br2,[Cu(phen)2Br]Br and[Cu(phen)3]Br2,were prepared through the reactionof1,10-phenanthrolinewithcupric bromide inthemixed solution of methanol and ethanol,and their catalytic performances were evaluated.Furthermore,a plausible reaction mechanism was proposed.

2.Experimental

2.1.Materials

AllreagentswithA.R.puritywerepurchasedfromlocal manufactures and used without further purif i cation.Cu(phen)Br2,[Cu(phen)2Br]Br and[Cu(phen)3]Br2were synthesized accordingtothe procedures described by Antunes[21],and were characterized by Fourier-transform infrared spectra.

4 mmol of 1,10-phenanthroline(Phen)in 15.0 ml of ethanol was added dropwise to a solution containing 4 mmol of CuBr2and 15.0 mL of methanol.Then the mixture was stirred for 30 min at 25°C. Cu(phen)Br2was isolated by f i ltration,washed with methanol(3× 20 ml)and dried at 45°C for 24 h in vacuum.The same procedure was conducted for the synthesis of[Cu(phen)2Br]Br and Cu(phen)3Br2using 2 and 3 equivalents of Phen with CuBr2,respectively.[Cu(phen)3]Br2was obtained with the evaporation of the solvent at room temperature.

Cu[(phen)3]Br2:Grass green solid,yield 75.2%.IR(KBr,cm?1): 3054.0,1622.4,1581.4,1513.9,1422.7,1340.1,1142.4,1098.9,851.7, 773.5,721.4,426.4.[Cu(phen)2Br]Br:Dark green solid,yield 82.4%.IR (KBr,cm?1):3082.4,1623.4,1603.8,1583.5,1513.2,1493.1,1422.7, 1223.1,1142.4,1102.1,854.8,781.4,721.4,429.5.Cu(phen)Br2:Brickred solid,yield 85.2%.IR(KBr,cm?1):3052.0,1623.4,1606.9,1583.1, 1513.8,1422.8,1346.2,1145.5,1105.1,851.7,775.2,718.3,430.2.

2.2.Catalyst characterization

Thermogravimetric analysis(TGA)was carried out on a TGA Q50 analyzer under nitrogen at a heating rate of 10°C·min?1from room temperature to 900°C.

2.3.Measurement of catalytic activity and selectivity

TheoxidativecarbonylationofmethanolwithCOandO2wascarried out in a 250 ml stainless steel autoclave equipped with an adjustable speed stirrer.50 ml of methanol and 0.011 mol·L?1of the catalyst were loaded into the autoclave.The autoclave was purged three times with O2,and then pressurized to 4.0 MPa with CO and O2(PCO/PO2= 19:1)at room temperature.The system was heated to 120°C and kept for4h.Afterthereaction,thereactorwascooleddowntoroomtemperature.The reaction mixture was analyzed by a Shimadzu GC-2014 equipped with a Rtx-50 capillary column(30 m×0.32 mm×0.25 μm) and f l ame ionization detector.

3.Results and Discussion

3.1.Catalytic performances

The activities of Cu complexes were evaluated in the oxidative carbonylation of methanol.As a comparison,the activities of CuBr and CuBr2and the equimolar mixture of Phen and CuCl2were also tested. The results are listed in Table 1.When CuBr is used as the catalyst,the turnovernumber(TON)is4.4DMCmol(Cumol)?1with95.2%selectivity of DMC.The result might be ascribed to the low solubility of CuBr in methanol,resultingin thedecrease of the active centers.CuBr2could be solvedinmethanol,butTONisonly5.9DMC mol·(Cumol)?1becauseof its low activity,and dimethoxymethane(DMM)as a by-product is detected with 27.4%selectivity.With the addition of the equimolar amount of Phen,TON and the selectivity of DMC are enhanced to 12.3 DMC mol·(Cu mol)?1and 92.7%,respectively.The result might arise from the formation of the σ-π coordination bond between Phen and Cu(II),which can improve the catalytic activity of CuBr2.However, when the mixture catalyst is replaced with Cu(phen)Br2,TON is increased to 39.9 DMC mol·(Cu mol)?1with 92.5%selectivity of DMC. The results show that:(1)Cu(phen)Br2is an active species,(2)it is essential for the synthesis of Cu(phen)Br2from Phen and CuBr2to maintain enough time,and(3)it is disadvantageous for the in-situ formation of Cu(phen)Br2at the reaction temperature of 120°C.

The literatures have reported that the number of ligand affects the activities of the Cu complexes in the oxidation of cyclohexane and toluene[21,22].The inf l uence of the structures of Cu complexes on the activities was also investigated.As shown in Table 1,TON is only 1.8 DMC mol·(Cu mol)?1over[Cu(phen)3]Br2,in which copper is hexacoordinated with six Cu-N bonds[23].The low activity might be ascribed to the following factors:(1)the steric hindrance from three ligands blocks the coordination of Cu(II)with methanol or CO;(2)the saturation of the f i rst coordination sphere inhibits the access of the reacting molecules to the metal center.With the decrease of the ligand number,[Cu(phen)2Br]Br,where copper is pentacoordinated,has the lowly steric hindrance and labile positions occupied by Br?,so TON is enhanced to 4.3 DMC mol·(Cu mol)?1.The mono-Phen complex might be formulated as Cu(phen)Br2in solution,where copper is tetracoordinated[23].TON is up to 39.9 DMC mol·(Cu mol)?1due to the lowest steric hindrance and the most labile positions occupied by Br?.However,its DMC selectivity is lower than that of[Cu(phen)2Br]Br and[Cu(phen)3]Br2.

Compared with Cu(phen)Cl2[18],the activity of Cu(phen)Br2is slightly high.The literatures have reported that the insertion of CO in monocarbonyl species(Cu(CO)Cl)into the Cu-O bond in the cupric methoxychlide is a key step in the catalytic cycle[24,25],and thus the stability of Cu(CO)Cl is very important.It is well known that CO is an excellent π acceptor,and the Cu-(CO)bond is stabilized by π backbonding interaction between Cu and CO.Because the electronegativity of Cl(3.0)is higher than that of Br(2.8),the Cu-(CO)bond in Cu(CO) Br is more stable than that in Cu(CO)Cl.Thus,the insertion is easier, resulting in the higher activity for Cu(phen)Br2.

It could be seen from Table 1 that Cu(phen)Br2exhibits the far higher activity than(C3H7)4NBr/CuBr2,and the reaction conditions are more moderate(the partial pressure of oxygen is below the explosive limit)[20].

Table 1 Effect of different catalysts on oxidative carbonylation of methanol(reaction conditions: VMeOH=50 ml,CCu=0.011 mol·L?1,T=120°C,p=4 MPa,pCO:pO2=19:1,5 h.)

3.2.Effect of the thermal stability of the complex on the catalytic activity

The temperature in the oxidative carbonylation of methanol was usually controlled at 110-160°C,thus the thermal stability of Cu(II) complexes was characterized.As shown in Fig.1(a),the mass loss of Cu(phen)Br2is started at 300°C.This shows that Cu(phen)Br2is stable in the oxidative carbonylation of methanol.The mass loss is 42.5%from 300 to 470°C.It is probably caused by the decomposition of Phen because the mass loss is close to 44.7%of Phen in Cu(phen)Br2.The mass loss above 470°C is attributed to the decomposition of CuBr2.

As listed in Fig.1(b),the 3.5%mass loss at about 130°C is due to the removalofcrystalwaterin[Cu(phen)2Br]Br.Themasslossabove270°C is caused by the decomposition of[Cu(phen)2Br]Br.It is shown that [Cu(phen)2Br]Br is also stable in the reaction.The mass loss at 100°C, as shown in Fig.1(c),arose from the elimination of crystal waters in [Cu(phen)3]Br2.Its thermal degradation appears above 160°C.At the same time,it is found that the stability of these catalysts in methanol is similar to that in the nitrogen atmosphere.

Fig.1.TG/DTG patterns of Cu(phen)Br2(a),[Cu(phen)2Br]Br(b)and[Cu(phen)3]Br2(c).

The initial decomposing temperature and the TON value of the complexes are listed in Table 2.When the oxidative carbonylation of methanol is catalyzed by the Cu complex,its activity is relevant to the σ-πcoordinationbondbetweenPhenandCu(II).Assoonasthedecomposition of the catalyst takes place,the σ-π coordination bond is destroyed,thus the catalytic activity decreases.Among the complexes, thethermalstabilityofCu(phen)Br2isthehighest.Theeffectofthetemperature on the σ-π coordination bond in Cu(phen)Br2is the least,and thus it exhibits the highest activity.As for[Cu(phen)2Br]Br,its thermal stability is lower than that of Cu(phen)Br2,implying that the inf l uence of thetemperature on the σ-π coordination bondis enhanced,and consequently,itsactivityisreduced.Thethermalstabilityof[Cu(phen)3]Br2is the least,and thus its activity is also the lowest.

Table 2Initial decomposing temperature and TON of the complexes

Fig.3.Effect of temperature on the carbonylation(VMeOH=50 ml,CCu=0.011 mol·L?1, T=4 h,p=4 MPa,pCO:pO2=19:1).

Fig.2.The chemical equation of methanol,O2and CO.

3.3.Effect of temperature on the reaction

DMC and H2O can be synthesized by the oxidative carbonylation of methanol,meanwhile DMC can be hydrolyzed.This process is a consecutive reaction.Methanol can be oxidized to DMM as well.In a few words,the reaction includes a consecutive reaction and a parallel reaction as shown in Fig.2.In the reaction,the selectivity of DMM and the hydrolysis of DMC can be controlled through the selection of the catalyst.On the other hand,the yield of DMC can be improved by the study on the reaction kinetics.The effect of temperature was tested over Cu(phen)Br2.The results are exhibited in Fig.3.

The generation rate of DMC is faster than that of DMM and the hydrolysis rate of DMC between 100 and 120°C,and thus TON isgradually enhanced.Especially,TON is rapidly increased from 5.5 to 39.9 DMC mol·(Cu mol)?1with a rise of the temperature from 110°C to 120°C.Above 120°C,the synthesis of DMC is inhibited because the carbonylation is exothermic.On the contrary,the generation rate of DMM and the hydrolysis rate of DMC are accelerated.Thereby,TON andthe DMCselectivityare graduallyreduced.Insummary,theoptimal temperature is 120°C.

3.4.Effect of time on the reaction

The inf l uence of time on the carbonylation was tested.As shown in Fig.4,TON is only 7.8 DMC mol·(Cu mol)?1within 3 h.The result implies that there is an induction period in the early stage.Along with the extension of the time,the induction period is broken,and TON is rapidly enhanced and reached to 47.6 DMC mol·(Cu mol)?1at 5 h. Above 5 h,both TON value and DMC selectivity are decreased because the generation rate of DMM and the hydrolysis rate of DMC are improved.

Fig.4.Effect of time on the carbonylation(VMeOH=50 ml,CCu=0.011 mol·L?1, T=120°C,p=4 MPa,pCO:pO2=19:1).

Fig.5.Proposed catalytic reaction cycle for the oxidative carbonylation.

3.5.Reaction mechanism

In the oxidative carbonylation of methanol,the key intermediate, the Cu(II)methoxycarbonyl species,is usually formed from a Cu(I,II)-ligand-bridged cluster[24,25],which stems from a cupric methoxy derivative and a cuprous carboxyl species.Therefore,it is necessary for the coexistence of Cu(II)and Cu(I).By detection,not any peak is found on the H2-TPR curve of Cu(phen)Br2.It is demonstrated that the reduction of Cu(phen)Br2cannot take place under the reaction conditions.The Cu(II)methoxycarbonyl might stem from the direct inset of CO in the Cu(II)-OCH3bond and the rate is very slow.Along with the formation of the Cu(II)methoxycarbonyl,Cu(I)can be generated by the reductive elimination of the Cu(II)methoxycarbonyl and the cupric methoxy.Hereafter,the formation of the Cu(II)methoxycarbonyl is easier due to the coexistence of Cu(II)and Cu(I).Thus,the catalytic cycle is accelerated,resulting in the rise of TON.As a result,there is the induction period in the early stage.

Based on the previous mechanism and our own experimental data, the proposed mechanism is shown in Fig.5.DMC and Cu(phen)Br are generated through the reductive elimination of Cu(phen)Br(OCH3) and Cu(phen)Br(COOCH3).Because Cu(phen)Br(COOCH3)is hard to form via the inset of CO in the Cu(II)-OCH3bond in the early reaction (route A is shown in Fig.5),there exists the induction period.After the formation of Cu(phen)Br,the induction period is broken and route B is the main catalytic cycle.Parts of Cu(phen)Br are oxidized to Cu(phen)Br2,further forming the Cu(phen)Br(OCH3)species.The other parts of Cu(phen)Br are reacted with CO to form Cu(phen) Br(CO)species because of Phen rendering Cu(I)more rich electron.Then,Cu(phen)Br(OCH3)and Cu(phen)Br(CO)can form Cu(phen) Br(COOCH3)through the Cu(I,II)-ligand-bridged cluster.Finally,DMC is synthesized through the reductive elimination of Cu(phen) Br(COOCH3)and Cu(phen)Br(OCH3).At the same time,Cu(phen)Br is produced.

4.Conclusions

(1)TheCucoordinationcompound,Cu(phen)Br2,showedthehigher activity than the equimolar mixture of CuBr2and Phen in the oxidative carbonylation of methanol to DMC.

(2)In comparison with[Cu(phen)2Br]Br and[Cu(phen)3]Br2, Cu(phen)Br2exhibited the highest activity due to the lowest sterichindrance,thegreatestcoordinationchanceandthehighest thermostability.TON arrived to 47.6 DMC mol·(Cu mol)?1with 92.8%selectivity under the standard batch reaction conditions ([Cu(II)]=0.011 mol·L?1,0.2 MPa O2,3.8 MPa CO,120°C and 5 h).

(3)Theinductionperiodintheearlystagemightstemfromthedirect

inset of CO in the Cu(II)-OCH3bond.

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29 May 2013

☆Supported by the National Natural Science Foundation of China(20936003, 21276201).

*Corresponding author.

E-mail address:dzpxyhry@163.com(Z.Du).

http://dx.doi.org/10.1016/j.cjche.2014.08.005

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Received in revised form 13 September 2013

Accepted 8 October 2013

Available online 19 August 2014