Direct Synthesis of Dimethyl Carbonate from CO2 and Methanol by Mg-Doped Ceria Monolithic Catalyst

YAN Yue-YingLI YueDENG JieZHAO XiTA NaCHEN Yong-Dong＊,

(1College of Chemistry and Chemical Engineering,Southwest Petroleum University,Chengdu 610500,China)(2State Key Laboratory of Catalysis,Dalian Institute of Chemical Physics,Chinese Academy of Sciences,Dalian,Liaoning 116023,China)

Abstract:In this paper,Ce1-xMgxO2(x=0.05,0.10,0.15,0.20)solid solution catalytic materials with different molar ratios were successfully synthesized by co-precipitation method.These materials were characterized by transmission electron microscope(TEM),X-ray diffraction(XRD),nitrogen adsorption-desorption test,Raman spectroscopy,X-ray photoelectron spectroscopy(XPS),CO2temperature-programmed desorption(CO2-TPD)and other techniques.It was found that the particle size,specific surface area,surface defects,etc.of the prepared Ce1-xMgxO2catalytic materials can be tuned by regulating the content of Mg in the CeO2lattice.Among them,Ce0.90Mg0.10O2exhibited the best surface properties,with the smallest average particle size of about 5.8 nm,the largest specific surface area of about 136 m2·g-1,and the highest surface oxygen content(31.98%).Ce1-xMgxO2catalytic material was coated on the cordierite honeycomb ceramic to make a monolithic catalyst,and its catalytic performance for the direct synthesis of dimethyl carbonate from CO2and CH3OH was investigated.Under the conditions of 140℃,2.4 MPa,and 2 h reaction,the yield of dimethyl carbonate on Ce0.90Mg0.10O2monolith catalyst was as high as 20.21%,and the catalytic activity was significantly higher than that of CeO2and other Ce1-xMgxO2(x=0.05,0.15,0.20)catalytic materials.

Keywords:CO2conversion;dimethyl carbonate;oxygen vacancies;monolithic catalyst;magnesium-cerium oxides

0 Introduction

Dimethyl carbonate(DMC)has been widely applied as a fuel additive,in electrochemistry and organic synthesis due to its environmental-friendly properties[1-3].Although many methods have been applied for DMC synthesis,such as phosgene method,transesterification method,urea alcoholysis method,epoxy alkane method,methanol,and CO2direct synthesis method,etc.[4-6].Direct synthesis of DMC from CO2and methanol has attracted great attention(Scheme 1)[7].The utilization of CO2as the carbon source instead of fossil feedstock may promote the sustainability of the chemical industry and terminate the greenhouse effect caused by excessive CO2emission.However,there are still some vital challenges such as low yield,deactivation of the catalyst,and thermodynamic limitations for this route[6,8].Thus,designing novel catalysts and developing efficient water removing methods from the reaction mixture are crucial to overcoming the thermodynamic equilibrium of the reaction.

Scheme 1 Direct synthesis of DMC from CO2and methanol

Ceria-based nanomaterials have been widely studied in the direct synthesis of DMC from CO2and methanol.This is mainly due to its fascinating CO2capture ability which significantly affects the reaction efficiency.Inert CO2molecular in the gas phase needs to be adsorbed and activated by the surface oxygen vacancy sites and then can react with methanol to generate DMC[9-12].Doping metal ions while maintaining the fluorite crystalline structure of ceria is one of the effective ways to enhance the concentration of surface oxygen vacancy of CeO2[9,13-14].Because the impurity ions can reduce the crystalline size,generate more surface defects and boost the reducibility of surface oxygen[15-16].On another hand,the surface acid-base property of CeO2can be mediated by the doping method,which will further favor the formation of DMC to improve the selectivity[9].According to Scheme 1,the reaction equilibrium can shift toward the right side by water removal[17].Usually,inorganic dehydrating agents are introduced to physically remove water with limited effect due to the low dehydration capacity at reaction temperatures[18-21].While organic dehydrating agents are applied to remove water by chemical reactions which may form lots of by-products complicating the entire process[22-25].Coating the catalyst powder on the surface of cordierite honeycomb ceramics can improve the phase-phase mass transfer performance[26-28].Therefore,it is reasonable to expect an enhanced efficiency for water removal using a honeycomb structure catalyst,which will improve the DMC yield in return.

In this contribution,Ce1-xMgxO2(x=0.05,0.10,0.15,0.20)solid solutions with a variation of magnesium content were prepared by the co-precipitation method to find an optimal ratio.Mg ions doping in CeO2lattice adjusted the surface acid-base property and the surface oxygen vacancies.Among all the obtained catalytic materials,Ce0.90Mg0.10O2was found to show the best catalytic activity in the direct synthesis of DMC from methanol and carbon dioxide.Using a unique structure,monolithic catalyst produced by coating powder on cordierite honeycomb ceramics showed high effective and stable catalytic performance.At 140℃,2.4 MPa,and 2 h continuous reaction,the yield of DMC over Ce0.90Mg0.10O2monolithic catalyst was the highest(20.21%).

1 Experimental

1.1 Materials preparations

The preparation of Ce0.90Mg0.10O2by the coprecipitation method is described as an example.We weighed 15.000 0 g(NH4)2Ce(NO3)6,0.779 5 g Mg(NO3)2·6H2O,and 70.000 0 g urea(CH4N2O)and dissolved them completely with 500 mL deionized water under stirring.The mixture was transferred to a 1 000 mL three-neck flask and gradually heated to 90℃under mechanical stirring(600 r·min-1)for 5 h.After the reaction,the product was cooled to room temperature naturally,the precipitate was filtered and washed with water(over 4 000 mL)and absolute ethanol(about 300 mL),dried overnight at 80℃,and calcined at 400℃for 4 h in the air to obtain the target product.The obtained Ce1-xMgxO2powder was ground with the required deionized water to obtain a slurry,which was coated on a cordierite honeycomb ceramics(64 cells per cm2,Φ:10 mm,L:25 mm).The load was maintained at 0.5 g,and the excess slurry was blown away.Finally,the coated catalyst was dried overnight at 80℃and calcined at 400℃for 4 h in the air to obtain a Ce0.90Mg0.10O2monolithic catalyst.The preparation method of Ce0.95Mg0.05O2,Ce0.85Mg0.15O2,and Ce0.80Mg0.20O2monolithic catalysts were the same as above,only the mass of Mg(NO3)2·6H2O was changed.

1.2 Catalytic tests

The catalytic activity of the prepared catalyst for the direct synthesis of DMC from CO2and methanol was evaluated in a continuous fixed-bed reactor.Water was the main disadvantageous factor for the formation of DMC in the synthesis reaction.The flow of the reaction system can remove the water vapor well and detect the reaction products online.A typical procedure was to place the prepared Ce1-xMgxO2monolith catalyst in a stainless steel reaction tube.The reactor was sealed and purged with a CO2stream for 30 min to drain the internal air.When the reaction system reached the required temperature,a mixed gas stream of CH3OH(0.145 mL·min-1)and CO2(40 mL·min-1)(nCH3OH∶nCO2=2∶1)was introduced.Then the reaction was carried out at 140℃,2.4 MPa,and 2 880 h-1of gas hourly space velocity(GHSV).The outlet component after the reaction was analyzed online using gas chromatography(Agilent 7890B)equipped with a hydrogen flame ionization detector.The calculation formula for CH3OH conversion and DMC selectivity is as follows:Wherecirepresents the concentration of a component(i).

2 Results and discussion

2.1 Characterization of as-prepared solid solutions

Fig.1 shows the X-ray diffraction(XRD)patterns of the prepared Ce1-xMgxO2composite oxides(Detailed characterization conditions can be found in Supporting Information).CeO2samples showed typical diffraction lines of cubic fluorite structure(PDF No.43-1002).Besides,it can be seen that the catalyst doped with Mg2+still maintained the characteristic peak of cubic fluorite ceria after calcination,no diffraction line representing MgO or any other impurities was detected.Compared with pure CeO2,the(111)plane peak shifted to a higher angle with increased Mg concentration(Fig.1b),indicating a lattice contraction.The calculated lattice constant decreased from 0.541 8 nm for CeO2to 0.540 6 nm for Ce0.80Mg0.20O2(Table 1)because the ionic radius of Mg2+(0.089 nm)is smaller than that of Ce4+(0.097 nm).The XRD patterns imply that the Mg2+incorporate into the CeO2lattice forming no MgO species and part of them substitutes the Ce4+leading to lattice contraction.These results are in good agreement with previous reports[15,29-30].The calculated grain size from(111)for all samples ranges from 5.8 to 6.1 nm and the specific surface area is basically the same,indicating that the addition of Mg has little influence on the micro-textural property.

Table 1 Structural and textural properties of Ce1-xMgxO2composite oxides

The N2adsorption-desorption isotherms and pore size distributions of Ce1-xMgxO2catalyst are shown in Fig.S1.As shown in Fig.S1,all catalysts obtained typeⅣ isotherms with clear H3 hysteresis lines,indicating typical mesoporous materials.In Fig.S2,all catalysts contain mesopore pore size distributions with pore sizes ranging from 2 to 20 nm.The above results show that the Mg2+content has a significant effect on the pore size distribution.The BET(Brunauer-Emmett-Teller)surface area and pore volume of the synthesized Ce1-xMgxO2catalyst are summarized in Table 1.It can be observed that Ce0.90Mg0.10O2composite oxide possesses the highest specific surface area of 136 m2·g-1and pore volume of 0.188 cm3·g-1.

Transmission electron microscope(TEM)images(Fig.2)of as-prepared Ce1-xMgxO2composite oxides indicated that all samples were in irregular spherical shape exposing no specific facets.The average particle size of as-prepared Ce1-xMgxO2is consistent with the grain size.

Fig.2 (a-e)TEM images of Ce1-xMgxO2composite oxides;(f)Size distribution of Ce0.90Mg0.10O2

There are two bands observed in Raman spectra(Fig.3).The vibration peak around 461 cm-1can be attributed to theF2gvibrational mode of Ce—O,which usually shows a sharp and symmetric band at 466 cm-1[9,31].Considering the high specific surface area of the prepared material,the peak shifted to low frequency and showed asymmetric character,which are mainly attributed to the small particle size.Compared with asprepared CeO2nanoparticles,theF2gband gradually blue-shifted with increased Mg2+content,which demonstrates the decreased average length of Ce—O bondand lattice contraction further.Therefore,it is reasonable to deduce that smaller Mg2+cations substitute some Ce4+ions in the fluorite lattice.It is also noted that the intensity ofF2gdecreased with increased Mg2+content,revealing structural distortion[32-33].Another band near 596 cm-1is related to the oxygen vacancies caused by the Ce3+ion in the CeO2lattice(Fig.3b)[34].The intensity of this mode increased with an increase of Mg2+content,pointing at increased intrinsic oxygen vacancy concentration.No Raman shifts of MgO were observed in Ce1-xMgxO2,which further infers Ce1-xMgxO2prefers a solid solution state.

To elaborate on changes in the CeO2chemical state after Mg doping,X-ray photoelectron spectroscopy(XPS)analysis was carried out.The XPS spectra of Ce3d(Fig.4a)exhibit complex features with eight peaks.U and V represent spin-orbits of Ce3d3/2and Ce3d5/2,respectively.Spin-orbit doublet(V‴ca.898.3 eV and U‴ca.916.8 eV,V″ca.888.9 eV and U″ca.907.4 eV,Vca.882.4 eV and Uca.900.9 eV)are attributed to the Ce4+species,while(V′ca.884.9 eV and U′ca.903.4 eV)are assigned to the Ce3+species[29,31].Then the concentration of Ce3+can be estimated by taking the ratio of the area of the integrated peak corresponding to Ce3+to the total area of fitted peaks.It is shown that Mg doping has enhanced the concentration of Ce3+on the surface remarkably,and the maximum ratio(19.42%)has been obtained when 10% Mg doping.The O1sXPS spectra(Fig.4b)of Ce1-xMgxO2composite oxides can be deconvoluted into 3 surface oxygen species:lattice oxygen(OLca.529.3 eV),surface oxygen vacancies(OVca.530.5 eV);and chemisorption oxygen species(OC)at the highest binding energy(ca.532.2 eV)[35].The intensity ratio of surface oxygen vacancies to the sum of all oxygen species was summarized in Table 2.It was observed that the incorporation of Mg2+can effectively increase the number of surface oxygen species(OV+OC).These results confirm that there are enhanced mobility and availability of lattice oxygen species due to the synergistic effect between MgO and CeO2.

Table 2 Relative ratio of Ce3+species and oxygen vacancies on the surface

Fig.5 shows the temperature-programmed reduction by hydrogen(H2-TPR)profile of as-prepared Ce1-xMgxO2composite oxides.The TPR of pure CeO2showed a broad peak starting at 500℃and one peak at 825℃,representing the surface and the bulk reduction process,respectively.The surface reduction initiated around a lower temperature 500℃after Mg2+ions(less than 20%)were introduced,which means the reducibility of surface oxygen species has been significantly improved.Meanwhile,the area of this broad peak increased gradually with higher Mg concentration as well,indicating the lattice oxygen in bulk can move to the surface and participate in chemical reactions at a relatively lower temperature.Thus not only the reducibility of surface oxygen but also the mobility of lattice have activated due to Mg2+introduction,resulting in more oxygen vacancies,probably by reducing the interaction between Ce—O with a distorted crystalline structure.This feature will facilitate chemical reactions whose reactants would be activated by oxygen vacancies.According to the related literature,the oxygen vacancy is crucial for activating carbon dioxide in the direct synthesis of DMC from CO2and methanol[11,34,36].

Fig.5 H2-TPR profiles of CeO2and Ce1-xMgxO2composite oxides

2.2 Catalytic performance

Fig.6a illustrates photographs of as-prepared monolithic catalyst.A scanning electron microscope(SEM)image(Fig.6)revealed that the average thickness of the catalyst coating wasca.60 μm.Well uniform coating layers were found,as evidenced in the corner,inner,and frontal channel views from the energy dispersion X-ray spectrum (EDS) mappings of Ce0.90Mg0.10O2-coated monolithic catalyst.The abnormal distribution of Mg is due to a small amount of Mg in cordierite.It also demonstrates that Ce0.90Mg0.10O2-coated monolithic catalyst can be insufficient contact with the reaction gas stream to promote the conversion and the yield of the product[37].Catalyst activity of monolithic and particulate(40-60 mesh)Ce0.90Mg0.10O2catalyst was comparatively studied(Fig.7).It is easy to conclude this monolithic do have enhanced the DMC yield and methanol conversion even though both were carried out in the same fixed bed reactor.Therefore,it is probable that the unique structure of the monolithic catalyst accelerates the water removal and shifts the reaction equilibrium successfully.Fig.8 shows the performance of Ce1-xMgxO2monolithic catalysts on direct DMC synthesis.The optimum temperature and optimum pressure can be obtained from Fig.S4 and S5.The activity of the catalyst was Ce0.90Mg0.10O2＞ Ce0.95Mg0.05O2＞CeO2＞ Ce0.85Mg0.15O2＞ Ce0.80Mg0.20O2.Whenx=0.10,the yield of DMC reached the maximum of 20.21% and decreased with a higher doping concentration.It is mainly reflected in the decrease of DMC selectivity and the increase of HCHO and DME selectivity.

Fig.6 (a)Photographs,(b)SEM image,and(c-e)EDS element mappings on Ce0.90Mg0.10O2-coated monolithic catalyst

Fig.7 Catalytic activity of monolithic and particulate Ce0.90Mg0.10O2catalyst

Fig.8 Catalytic performance of Ce1-xMgxO2monolithic catalysts

According to our previous studies,there are the following reaction processes in this process:（Ⅰ）2CH3OH → CH3OCH3+H2O;（Ⅱ） 2CH3O+CO2→HCHO+CO+H2O[35].It can be seen that the doping of Mg can promote the process of（Ⅰ） and（Ⅱ）,which leads to a decrease in the selectivity of DMC.

Fig.9 Durability test of Ce0.90Mg0.10O2monolithic catalyst

To provide referable information for the industry,we examined the stability of Ce0.90Mg0.10O2monolithic catalyst at 140℃and 2.4 MPa.There is little deactivation for this catalyst(DMC yield from 20.21% to 19.56%)during the 50 h durability test implies it is a quite promising application for the direct synthesis of DMC from CO2and methanol.

3 Conclusions

In conclusion,doping Mg in CeO2lattice can enhance the catalytic performance on the direct formation of DMC from methanol and CO2.Since Mg2+ions play an important role in the activation of oxygen species in CeO2lattice,which favors the oxygen vacancies formation.At the same time,the honeycomb structure of the monolithic catalyst greatly improves the removal of reaction products,overcoming thermodynamic limitations to some extent.Consequently,the yield of DMC and the stability of the catalyst can be improved.

Supporting information is available at http://www.wjhxxb.cn

Acknowledgments:We acknowledge XIAO Yong-Li,JIANG Lan for their aid in this work.