The influence mechanism of solvent on the hydrogenation of dimethyl oxalate☆

Shi Yin,Lingjun Zhu*,Xiaoliu Wang,Yingying Liu,Shurong Wang*

State Key Laboratory of Clean Energy Utilization,Zhejiang University,Hangzhou 310027,China

Keywords:Hydrogenation Dimethyl oxalate Solvent Higher alcohols Decomposition

A B S T R A C T Thehydrogenation ofdimethyloxalate(DMO)for theproducingofC2-C4 alcoholswith methanolassolvent was researchedatthetemperatureof270 °Cto310 °C.Ethyleneglycol(EG)wasthemainproductatlowtemperature and the selectivity of which was 61.9%at 230°C.However,EG selectivity decreased sharply with the increase of temperature while ethanol became the main liquid products with the selectivity of 43.5%at 270°C.It can be ascribed to a thorough hydrogenation of DMO at a high temperature.In addition,the promotion of Guerbet reaction led to the production of propanol and butanol.Simultaneously,the amount of gas products including CO,CO2and dimethyl ether(DME)also increased,which became a competition factor in the conversion of DMOtoliquidproductsincludingC2-C4alcohols.Theblanktestwascarriedoutwithpuremethanolasfeedstock with and without Cu/SiO2catalyst,which revealed that methanol was involved in the formation of gas products and higher alcohols on Cu-based catalyst,and the main gas product was CO.

1.Introduction

The growing problem of energy shortage and environmental pollution required humans to exploit an eco-friendly utilization of coal resources and to synthesize compounds with high value-added[1,2].Theuseofethanolcansignificantlyreducetheairpollutionandtheconsumption of fossil fuel[3].Nowadays,the fermentation of agricultural feedstocks,includingboth starch-contained biomass and lignocellulosic biomass,is the main production method for ethanol[4,5].In recent years,indirect ethanol synthesis from syngas has been raised where esters such as methyl acetate(MA)and dimethyl oxalate(DMO)are often the intermediates.MA can be synthesized through the carbonylation of dimethyl ether(DME),and then ethanol was produced by the hydrogenationofMA[6,7].ThepathwithDMOasintermediateincludes the DMO formed by the coupling of CO with nitrite esters and then the subsequenthydrogenation of DMO to ethanol[8].Generally,thehydrogenation of DMO could produce three main products including methyl glycolate(MG),ethylene glycol(EG)and ethanol according to the extent of hydrogenation.Among of them,EG have enormous potential applications,such as the precursor of polyesters,coolants,antifreezes,fuel cells as well as the synthesis of fine chemicals.Therefore,great efforts have been undertaken to improve the efficiency of EG synthesis in the past decades[9].In the study of DMO hydrogenation,C3-C4 alcohols were also obtained at a high reaction temperature[10,11].As for the industrial application,these higher alcohols can be easily blended into gasoline which would reduce engine knocking[12,13].

Due to the good property to adsorb carbonyl in organic compounds,Cu-based catalysts are intensively investigated in the hydrogenation of esters like DMO[14].Previous researches on Cu catalyst supported on SiO2or SBA-15 indicated that there is a significantinfluenceof the crystallinesizeanddispersionofCuonitsactivityinDMOhydrogenation,as well as a great effect of the interaction of Cu and support on its stability[7,15,16].However,the agglomeration of copper particles under high temperature is a major drawback for Cu-based catalysts in industrialized applications[17].Many supports such as ZnO,La2O3,SiO2,and Al2O3were applied to prepare the supported Cu catalysts[18-20].It was found that Cu/SiO2exhibited an excellent catalytic performance for its support having neutral property.In addition,many methods including ion-exchange,impregnation,sol-gel and precipitation deposition by urea hydrolysis and ammonia evaporation respectively were researched for the preparation of Cu-based catalysts[21,22].It was found that the urea hydrolysis method is beneficial for the Cu dispersion,since Cu active species can be precipitated slowly and uniformly by this method.As a result,Cu/SiO2catalyst exhibited a high DMO conversion and EG selectivity[15].

In the case of DMO hydrogenation,alcohols and ethers were usually used as the solvents for their relative good solubility.Methanol was the most widely selected solvent for DMO hydrogenation to produce MG,EG and high alcohols[5,11].For instance,Kong et al.[23]applied methanol as solvent in the hydrogenation of DMO to EG over Mg promoted Cu catalyst;Fan et al.[24]used methanol as solvent in the research of DMOhydrogenationtoMGonRu-basedcatalyst;Songetal.[11]studied the synthesis of C3-C4 alcohols by the hydrogenation of DMO in the solution of methanol over Cu catalyst.Besides,Zhu et al.[8]designed copper inlaid mesoporous Al2O3catalysts for ethanol synthesis from DMO hydrogenation with 1,4-dioxane as solvent.On the other hand,temperature can influence the products distribution of DMO hydrogenation.Whenethanolisthetargetproduction,thereactiontemperature of DMO hydrogenation for the Cu-based temperature is around 270°C[25].However,DMO and methanol are both the chemicals that are easily decomposed at high temperature.The decomposition of methanol can release CO which would influence the valence state of Cu species.In addition,the released CO would also form chemisorption on Cu active sites which may also change the ratio of Cu+/Cu0active sites[25].

Herein,the alcohols formation and the gas production from the hydrogenation of DMO with methanol as solvent at 230-310°C was detailed investigated on the Cu/SiO2catalyst which was prepared by deposition precipitation method using urea as the precipitant.Especially,the change of gas products composition was detected with the temperature increasing.And the blank test was also investigated to further reveal the influence of methanol solvent on the wholereactionsystemincludingtheliquidproductsandthegasproducts.

2.Experimental

2.1.Preparation of catalysts

The Cu/SiO2catalyst was prepared by deposition precipitation method in the following procedure.14.26 g of Cu(NO3)2·3H2O and 10.63 g of urea was dissolved in 300 ml deionized water to obtain a uniform solution.Then 60 g silica sol was added slowly with a peristaltic pump under magnetic stirring.After that,the suspension was kept at 90°C until pH=6.5,and a light green precipitate was obtained.After filtering and washing,the precipitate was dried at 120 °C for 12 h and then calcined at 450 °C for 4 h.The calcinated sample were crushed and sieved to obtain particles with the size of 0.30-0.45 mm.As a result,Cu/SiO2catalyst with the nominal loading of 20 wt%was prepared.

2.2.Catalyst characterization

The BET surface area and pore structure of the catalyst was detected by N2physisorption method using a Quantachrom-Autosorb-1-C apparatus.Powder X-ray diffraction(XRD)analysis of the catalyst was carried out on a D/max-Ra X-ray diffractometer with a CuKαradiation source operated at 40 kV and 30 mA.Transmission electron microscopy(TEM)was used to analyze the particle size and distribution of Cu species on Tecnai G2 F30 supported by Philips-FEI.

2.3.Activity test and product analysis

The hydrogenation of DMO was carried out in a continuous-flow fix bed reactor.Briefly,3 ml catalyst was sandwiched with quartz sands in a stainless-steel reactor with an inner diameter of 8 mm.Before the reaction,Cu catalyst was activated by pure H2at 350°C for 3 h.During the reaction of DMO hydrogenation,the system pressure was kept by pure H2and controlled at 2.0 MPa by a back-pressure valve.The reactant with15wt%DMOinmethanolsolutionwasfedintothereactorbyahighpressure pump.The hydrogen to ester mole ratio was 40 and the liquid hour space velocity(LHSV)was 1.5 h?1.The liquid products were collected and analyzed by an Agilent 7890A GC and GC-MS,both of which equipped with INOWAX capillary column.The gas products were analyzed on an online Huaai 9560 GC equipped with Porapak N and TDX-01 packed columns for the separation of H2,CO and CO2,as well as HPPlot Q capillary column for the separation of hydrocarbons.The flow rate of effluent gas was measured by an electronic flowmeter(Agilent,AMD1000).TheDMOconversionandproductsselectivitywerecalculated as the following equations:

3.Results and Discussion

3.1.Morphology and physicochemical features of Cu/SiO2

The textural properties of Cu/SiO2catalysts were determined by the N2physisorption method.It was found that the Cu/SiO2catalyst has a BET surface area of 317 m2·g?1.The pore volume and average pore size are 0.79 cm3·g?1and 9.9 nm respectively.Additionally,Fig.1 shows the pore size distribution and N2adsorption-desorption isotherm of the catalyst.The catalyst exhibits Langmuir type IV isotherms which can be ascribed to a typical mesoporous material.The pore size distribution curve derived from the desorption branch shows that the pore size distribution is relatively concentrated at 3 nm.

Fig.1.N2adsorption-desorption isotherm and pore size distribution of the prepared catalyst.

Fig.2 shows the TEM images of the reduced Cu/SiO2catalyst,the light gray and dark spherical particles are assigned to silica and copper species respectively.The copper particle size is about 2-3 nm which indicated that the copper species were not sintering during the reduction process.The interplanar crystal spacing value of 0.244 nm can be assigned to the presence of Cu2O(111),while the value of 0.207 nm belonging to the Cu particles with metallic state.It indicates the coexistence of Cu2O and Cu in the reduced catalysts.

Fig.3 illustrates the XRD patterns of the calcinated and reduced Cu/SiO2catalysts,respectively.Fortheas-preparedcatalyst,onlydiffraction peak of amorphous SiO2was observed.The absence of diffraction peaks from copper oxide species indicates the highly dispersed of copper.The catalyst reduced at 350°C exhibits a main peak corresponding to Cu2O at 2θ =36.4°[25].However,there was no visible Cu0peak in the reduced catalyst,which might be due to the quite small and well dispersed Cu0particles.

Fig.2.TEM images of reduced Cu/SiO2catalyst.

Fig.3.XRD patterns of Cu/SiO2catalysts after calcination and reduction.

3.2.Catalytic hydrogenation of DMO

3.2.1.The production of C2-C4 alcohols

Fig.4.Influence of temperature on the DMO conversion and products selectivities.

ThehydrogenationofDMOtoC2-C4alcoholswascarriedoutoverthe Cu/SiO2catalyst at different temperature.The reaction pressure was 2.0 MPa,the LHSV was 1.5 h?1,and the hydrogen to ester mole ratio was 40.As shown in Fig.4,although the hydrogen to ester mole ratio is low,DMO has almost completely transformed and the DMO conversion was 98.3%at 230°C.It indicates that Cu/SiO2catalyst was efficient for the hydrogenation of DMO.As shown in the Figs.2 and 3,Cu+and Cu0were detected in the TEM analysis and the Cu+was also detected obviouslyintheXRD analysis.Accordingtothe reports,theintrinsic highsurfaceofCu0andCu+areimportantfortheexcellentcatalyticperformance in ester hydrogenation[26,27].It has been suggested that Cu0sites could not only activate H2,but also facilitate the adsorb of hydroxyl group and thesubsequentdehydrationreactionofmonobasicalcoholsorpolyhydric alcohols[27].The Cu+species can facilitate the stable of the surface methoxy and acylspecies in MA hydrogenation,which are intermediates in DMO hydrogenation[28].On the other hand,Cu+sites can polarize carbonyl group in esters,thus improving the activity of ester group[29].

The selective catalytic hydrogenation of DMO toethanol involves severalreactions.Firstly,DMOunderwenthydrogenationtoformMG,which was further hydrogenated to form EG.Finally,the ethanol would be obtained by the hydrogenation of EG[30].Temperature has obvious influence on the selectivity of products.At 230°C,the selectivity of EG was 61.9%,and then decreased sharply with temperature increase,so the hightemperaturecanpromotetheconversionofEGtoethanol.Theselectivity of ethanol increased initially and then decreased.At 270°C,the maximumethanolselectivityof43.5%wasobtained.Butwiththeincrease oftemperature,theethanolselectivitydecreasedto21.0%at310°C.Asfor theotheralcohols,thecontentofn-propanolandisobutanolarerelatively high.Whenthetemperatureincreasedfrom230 °Cto310 °C,theselectivity of n-propanol increased from 0.7%to 14.8%and isobutanol from 0.5%to 6.4%.The reason for the increase of the C3-C4 alcohols selectivity may be ascribed to the Guerbet reaction.Veibel et al.[31]proposed the path of the Guerbet reaction as follows.Firstly,the alcohols are dehydrated to thecorrespondingaldehydes,andsubsequently,aldolcondensationreaction occurs to the formed aldehydes,as well as hydrogenation of the unsaturated condensation products to produce the higher alcohols.Accordingly,the formation of C3-C4 alcohols can be ascribed to the Guerbet reaction of oxalic acid derived from DMO with alcohols and the Guerbetreactionofethanolwithmethanol.Theoxalicacidisthemaindecomposition product of DMO under high temperature and the oxalate ions can absorb on the metal oxide with two modes,the one is physisorptionthroughelectrostaticforces,andtheotherischemisorption through coordination to metal atoms[32,33].As reported earlier,the C＝O group are necessary for the Guerbet reaction,thus the C＝O of the adsorbed oxalic acid make the Guerbet reaction occur easily[34].Hilmen et al.[35]found that the adsorbed acetaldehyde tend to participate in the formation of MA and n-propanol.Carlo et al.[36]also got higher alcohols with high yield in the research of Guerbet condensation of ethanol with methanol catalyzed by copper-based component and CH3ONa.In addition,stability is an important parameter for a catalyst in a practical application.As shown in Fig.5,the stability of Cu/SiO2catalyst in DMO hydrogenation was studied at LHSV=1.5 h?1,P=2 MPa,H2/DMO=40 mol·mol?1,T=270 °C.A full conversion of DMO could be achieved without any deactivation even after 100 h of time on stream with the alcohol selectivities maintaining its initial state which indicated a good stability of the catalyst.

Fig.5.Stability of Cu/SiO2catalyst in DMO hydrogenation(LHSV=1.5 h?1,P=2 MPa,H2/DMO=40(mol·mol?1),T=270 °C).

3.2.2.The competing reaction to gas products

At present,although many studies were carried out about the hydrogenation of DMO,nearly all the studies were concerned on the liquid products such as EG,ethanol and C3-C4 alcohols,but there are also many gas products detected in the experiment,mainly including CO,CO2,DME,and hydrocarbons.As shown in the Fig.6,with the temperature increase,the amount of gas products increased sharply.When the temperature increased from 230 to 310°C,the yield of the all the gas products increased from 0.003 mol·h?1to 0.03 mol·h?1.The CO,CO2and DME were the main products with a little amount of hydrocarbon.At 270°C,the yield of thegas products have exceeded thenumberof carbon moles of DMO in the feed and the amount of liquid collected decreased sharply,which indicates that the methanol was involved in the decomposition reaction.Besides,when the temperature was below 270°C,the concentration of CO2was higher than that of CO,but when the temperature exceeded 270°C,the concentration of CO was much higher than that of CO2.The main reason is that CO was produced from the decomposition of methyl formate that formed from the decomposition of methanol[37].In situ FTIR characterization of methanol stream onCu/SiO2catalyst at200°C showedthatmethylformate,formaldehyde,COandCO2arethemainproductsofmethanoloxidation[24].Besides,the selectivityof CO is higherthanthat of CO2,probablydue totheveryweak adsorptionof CO onCu0,whichled tothedesorptionofCOintotheoutlet gas easily at higher temperatures[38].

Fig.6.The yield of gas products with the mixture of DMO and methanol as the feedstock under different temperature.

3.3.The behavior of methanol solvent

In order to having further understand the influence of methanol solvent on the hydrogenation of DMO,two blank tests,namely methanol thermo-decomposition with and without Cu/SiO2catalyst,were carried out respectively under the same reaction conditions as DMO hydrogenation.The results are shown in Fig.7.As for the methanol streamingontheCu/SiO2catalyst,therewas noliquidproductgenerated.While the yield of gas products increased obviously with the increase of temperature,especially for CO.When the temperature was 250°C and the mixture of DMO and methanol as feedstock,the yield of CO was only 0.001 mol·h?1,but when the feedstock was replaced with pure methanol,the yield of CO increased to 0.02 mol·h?1.On the contrary,the yield of CO2decreased from 0.0045 mol·h?1to 0.001 mol·h?1.It indicated that CO was generated from the decomposition of methanol and the presence of DMO can inhibit the decomposition of methanol into CO.Besides,the gas product was only DME when the catalyst was unloaded,and no obviously change of DME yield was found in the two blank tests with and without catalysts.

Fig.7.The yield of the gas products with pure methanol as the feedstock under different temperature(left column:with catalyst;right column:without catalyst).

By analyzingtheDMO hydrogenation and themethanol decomposition with and without catalysts,the reaction pathways of DMO hydrogenation with methanol as the solvent can be described as Fig.8.During the different stages of DMO hydrogenation,the liquid products mainly include EG,ethanol,and C3-C4 alcohols.The EG and ethanol was formed through the hydrogenation of DMO.The C3-C4 alcohols were synthesized through the Guerbet reaction of methanol and ethanol.As for the gas products,it can be deduced that the decomposition of methanol to CO may require the participation of copperbased catalyst,while the presence of DMO competes with the methanol adsorption on the copper active sites.Additionally,the yield of CO2decreased from 0.0045 mol·h?1to 0.001 mol·h?1when the solvent was replaced with methanol at 250°C,which indicated that the CO2was mainly derived from the decomposition of DMO.Therefore,it can be deduced that the CO was from the intramolecular catalytic reaction ofmethanolandtheDMEwasfromtheintermolecularthermalreaction of methanol.The Cu-based catalyst not only promoted the hydrogenation of DMO,but also promoted the decomposition of methanol at high temperature.

Fig.8.The reaction pathways of DMO hydrogenation with methanol as the solvent.

4.Conclusions

The gas-phase hydrogenation of DMO to higher alcohols with methanol as solvent over the Cu/SiO2catalysts was evaluated,and the gas products were also analyzed.The products distribution would be affected by temperature obviously.As for the liquid products,the maximum ethanol selectivity of 43.5%was obtained at 270°C.The content of propanols and butanols increased with the increase of temperature due to the Guerbet reaction,where methanol contributing the formation of propanols.As for the gas products,CO,CO2,DME were the main products.In order to get insight the effect of methanol on the formationofgasproducts,theblanktestwascarriedoutwithpuremethanol as feedstock with and without Cu/SiO2catalyst.The results showed that CO was derived from the decomposition of methanol on the Cubased catalyst while CO2was mainly formed by the decomposition of DMO.Additionally,the DME was produced by the intermolecular dehydration of methanol.