MgO-SBA-15 Supported Pd-Pb Catalysts for Oxidative Esterif i cation of Methacrolein with Methanol to Methyl Methacrylate☆

Li Jiang,Yanyan Diao,Junxing Han,Ruiyi Yan,Xiangping Zhang,*,Suojiang Zhang,*

Catalysis,Kinetics and Reaction Engineering

MgO-SBA-15 Supported Pd-Pb Catalysts for Oxidative Esterif i cation of Methacrolein with Methanol to Methyl Methacrylate☆

Li Jiang1,2,Yanyan Diao1,Junxing Han1,Ruiyi Yan1,Xiangping Zhang1,*,Suojiang Zhang1,*

1Key Laboratory of Green Process and Engineering,Beijing Key Laboratory of Ionic Liquids Clean Process,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China
2University of Chinese Academy of Sciences,Beijing 100049,China

A R T I C L EI N F O

Article history:

Methacrolein

Methyl methacrylate

Oxidative esterif i cation

Pd-Pb bimetal

SBA-15

Novel MgO-SBA-15 supported catalysts were prepared for oxidative esterif i cation of methacrolein(MAL)with methanol to methyl methacrylate(MMA).The MgO-SBA-15 supports were synthesized with different magnesia loadingsfromdifferentmagnesiumprecursorsandhydrochloricacidmolarconcentrations.TheMgO-SBA-15supports and Pd-Pb/MgO-SBA-15 catalysts were characterized by several analysismethods.The results revealed that the addition of MgO improvedthe ordered structure of SBA-15 supports and providedsurface alkalinity of SBA-15 supports.TheaveragesizeofthePd3Pbparticles on magnesia-modif i edPd-Pb/MgO-SBA-15catalystswassmaller than that on the pure silica-based Pd-Pb/SBA-15 catalysts.The experiments on catalyst performance showed that the magnesia-modif i ed Pd-Pb/MgO-SBA-15 catalysts had higher activity than pure silica-based Pd-Pb/SBA-15 catalysts,showing thestrong dependenceof catalyticactivity on the averagesizeofactiveparticles.The difference ofactivitybetweenPd-Pb/SBA-15catalystsandPd-Pb/MgO-SBA-15catalystswasduetothediscrepantstructural propertiesandsurfacealkalinityprovidedbyMgO,whichledtothedifferentPd3Pbparticlesizesandthenresulted in the different number of active sites.Besides magnesia loadings,other factors,such as hydrochloric acid molar concentration and magnesium precursors,had considerable inf l uences on the catalytic activity.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

Methyl methacrylate(MMA)is one of the most important chemicals to produce plexiglass,acrylic coatings,polymeric dispersants for paints and so on.Oxidative esterif i cation of methacrolein(MAL)with methanol toMMAinthepresenceofoxygenhastheadvantagesofshortroutesand highatomeff i ciency.Ingeneral,thecatalystsforoxidativeesterif i cationof aldehydes are mainly supported noble metal catalysts.Among them,palladium catalysts are one of the most active catalysts.In general,the most commonly used support is alumina,but the pore of alumina is irregular and the specif i c surface area is relatively small.Mesoporous silica materials have some advantages over alumina such as reproducible and scalable synthesis,and signif i cantly larger surface areas and more f l exible pore architectures[1-3].The large surface area of mesoporous silica materials helps in attaining higher metal dispersions compared with lower surface area non-ordered supports.Additionally,the regular and tunable pore size offered by ordered mesoporous silica materials can impart this uniformity to the hosted catalytic nanoparticles under controlled synthesisconditions[4-7].Aboveall,theintroductionofmesoporescanstrongly affect mass transport to in-pore active sites and increase palladium dispersion[8-10].Therearesomanyadvantagesofmesoporoussilicamaterials over alumina,and thus,we want to use them in oxidative esterif i cation of MAL with methanol to MMA.

SBA-15 is one of the most popular mesoporous silica materials due to its highsurfacearea and largetunable pores[11].Its thick pore walls provide enhanced mechanical stability,and can be modif i ed to tailor their properties and achieve specif i c purposes[12-14].However,the inherent lack of adequate acid and basic sites limits their applications in catalysis. Several metals such as Al,Zr,Ti,Cr and B[15-18]have been incorporated into SBA-15 to broaden its applications.SBA-15 based solid bases have been widely studied including amino[19],magnesium[20,21],and calcium[22]modif i edSBA-15.MgOisregardedasatypicalsolidbasematerial duetoitselectrondonationability.ItisfoundthatMgOacceleratesthereaction rate of oxidative esterif i cation of MAL with methanol in the presence of oxygen[23].By adding magnesium salts into the initial mixture of raw materials,basic mesoporous silica materials can be achieved.The addition of magnesium salts will lead toa better order,eitherin structure ormorphology[2].Itisbelievedtoberelatedtothespecialinteractionbetweensurfactantheadgroupsandmagnesium[2].As farasweknow,the investigations on MgO-SBA-15 supported Pd-Pb catalysts for oxidative esterif i cation of MAL with methanol have not been reported so far.

In this work,a series of MgO-SBA-15 supports was prepared with different magnesia loadings,hydrochloric acid molar concentration and precursors of magnesium salts.MgO-SBA-15 supported Pd-Pbcatalysts,preparedbyco-impregnationmethods,wereappliedtooxidativeesterif i cationofMALwithmethanol(Fig.1).Theeffectsofmagnesia loadings on the structural properties of supports,surface alkalinity of catalystsandtheparticlesizedistributionofactivesiteswereinvestigated.The effects of hydrochloric acid molar concentration on the particle size distribution of active sites and the inf l uences of precursors on the structuralpropertiesofsupportsandcatalystswerealsostudied.Finally, the catalytic activity of catalysts prepared from the above factors was examined.

Fig.1.Oxidative esterif i cation of MAL with methanol to MMA.

2.Experimental

2.1.Preparation of supports and catalysts

MgO-SBA-15 supports were synthesized according to the literature with some modif i cations[2].Typically,4.0 g of Pluronic P123 (EO20PO70EO20,Aldrich)and a calculated amount of magnesium salts from one of MgCl2,Mg(NO3)2and Mg(CH3COO)2(AR,Sinopharm Chemical Regent Co.,Ltd.)were dissolvedin 150 g hydrochloric acid solution(AR,Beijing Chemical Works).Then,8.5 g of tetraethoxysilane (TEOS,XilongChemicalCo.,Ltd.)was added.Themolarratioof themixture TEOS:P123:Mg2+:HCl:H2O was 1:0.02:X:Y:192,where X was 0, 0.015,0.075,0.150,0.300 and 0.450 for the preparation of samples with the mass ratio of MgO/SBA-15 of 0%,1%,5%,10%,20%,and 30% and Y was 6,3,and 1.5 with the hydrochloric acid molar concentrations of 1.6 mol·L?1,0.8 mol·L?1,and 0.4 mol·L?1.It was denoted as MgOSBA-15(X-Cl)-Y,MgO-SBA-15(X-N)-Y and MgO-SBA-15(X-A)-Y respectively.The resulting mixture was stirred for 24 h at 313 K and then heated at 373 K for another 24 h without stirring.The liquid was evaporated with stirring at 353 K.Finally,the solids were dried at 353 K and calcined at 823 K for 6 h to remove the template.Palladium chloride(Beijing Cuibolin Non-Ferrous Technology Developing Co., Ltd.)and lead nitrate(AR,Xilong Chemical Co.,Ltd.)were impregnated simultaneously.The loading value of Pd and Pb was both 5%(by mass) of MgO-SBA-15 supports.The obtained samples were reduced by hydrazine hydrate.

2.2.Experiments on catalytic performance

The reaction was performed at 353 K and 0.3 MPa of oxygen for 2 h over 0.013 g·ml-1of the supported catalysts under stirring.The molar ratio of methanol to MAL was 20.Catalytic results were expressed as conversion(x,%)and selectivity(S,%)determined by gas chromatography(GC)using ethanol as external standard.Parameters were def i ned as

where CM0AL(mol·L?1)was the molar concentration of MAL at the beginning mixture,CMAL(mol·L?1)the molar concentration of MAL after 2 h and CMMA(mol·L?1)was the molar concentration of MMA after 2 h.

2.3.Catalyst characterization

Several analysis methods were applied to characterize the supports and catalysts in this work.The nitrogen adsorption and desorption isotherms were measured using a TriStarII3020 system at 77 K.The samples were degassed at 473 K for 6 h.The Brunauer-Emmett-Teller (BET)specif i c surface area was calculated using adsorption data in the relativepressurerangeof 0.04 to0.20.Thetotalpore volumewasdetermined from the amount adsorbed at a relative pressure of about 0.99. The mean pore diameter was calculated by the Barrett-Joyner-Halenda (BJH)method using the desorption curves.X-ray diffraction patterns (XRD)were recorded in the 2 theta range of 0.5°to 5.0°and 10°to 90°respectively on a X'Pert PRO diffractometer equipped with a copper anode generating Cu Kαradiation(λ=0.15418 nm).Transmission electron microscopy(TEM)measurements were obtained on a JEM2010 electron microscope operated at 120 kV.MgO contents of catalysts were determined by an inductively coupled plasma optical emission spectroscopy(ICP-OES)(IRIS Intrepid II XSP).In CO2-TPD experiments,30 mg samples were heated in a f l ow of helium at a rate of10K·min?1to773Kandkeptfor2h,thencooledto323Ktoadsorb CO2.AfterthephysicallyadsorbedCO2waspurgedbyheliumfor2h,the sample was then heated at the rate of 10 K·min?1up to 773 K and the liberated CO2was detected by a thermal conductivity detector(TCD).

3.Results and Discussion

Table 1Textural characteristics of supports

3.1.Characterization

3.1.1.Textural characteristics

The textural characteristics of Pd-Pb/MgO-SBA-15 catalysts and MgO-SBA-15 supports were characterized by the N2adsorption-desorption method.The specif i c surface area(SBET),pore volume(Vtotal) and mean pore diameter(dp)of supports and catalysts are summarized in Tables 1 and 2,respectively.Firstly,the specif i c surface area,porevolume and mean pore diameter of MgO-SBA-15 supports decrease with an increase in the initial added amounts of magnesium acetate [Table 1,Entries b-f].When the content of MgO was 10%[Table 1, Entry d]or less[Table 1,Entries b and c],the specif i c surface area and pore volume of MgO-SBA-15 are higher than those of SBA-15.Meanwhile,the mean pore diameter is the same value of 6.3 nm,which indicates that MgO is highly dispersed on the surface of SBA-15.But the specif i c surface area,pore volume and mean pore diameter of supports MgO-SBA-15(0.300-A)and MgO-SBA-15(0.450-A)[Table 1,Entries e and f]are reduced.It means that the redundant MgO occupies some of the total pore volumes.The above results show that 10%content of MgO loadings is a transition point of the mesostructural change.Secondly,the pore volume and mean pore diameter of MgO-SBA-15 supports decrease with reducing hydrochloric acid molar concentration [Table 1,Entries d,g and h].Finally,the pore volume and mean pore diameter of MgO-SBA-15 supports synthesized by magnesium chloride [Table 1,Entry i]are the smallest and MgO-SBA-15 supports synthesized by magnesium acetate[Table 1,Entry d]are the largest.

The specif i c surface area of catalysts enhances with the increase in theaddedamountsofmagnesiafrom1%to30%andreachedthehighest of 713.5 m2·g?1[Table 2,Entry f].However,the pore volume decreases from 0.79 ml·g?1[Table 2,Entry b]to 0.49 ml·g?1and the mean pore diameter reduces sharply from 10.2 nm to 3.1 nm.Compared with supports MgO-SBA-15(0.300-A)and MgO-SBA-15(0.450-A)[Table 1,Entries e and f],the pore volume and mean pore diameter of catalysts [Table 2,Entries e and f]decrease sharply due to the active particles thatoccupypartofthechannels.Someoftheactiveparticles arelocated on the external surface of the supports.The specif i c surface area and pore volume also decrease after palladium and lead particles deposit on the supports synthesized by different hydrochloric acid molar concentrations[Table 2,Entries d,g and h].Interestingly,the mean pore diameter increases as palladium and lead particles deposit on MgO-SBA-15 supports.The reason is that the active particles are formed in the channels of MgO-SBA-15 supports and occupied some pore spaces. The specif i c surface area,pore volume and mean pore diameter of catalysts synthesized from magnesium chloride[Table 2,Entry i]are a little lower than those of catalysts synthesized from magnesium acetate and nitrate[Table 2,Entries d and j].

3.1.2.XRD characterization

Fig.2 displays the small angle XRD patterns of MgO-SBA-15 supports with varied amounts of magnesia,different magnesium precursors and hydrochloric acid molar concentrations.All supports exhibit one intense peak at 0.86°-0.96°along with two weak peaks at 1.49°-1.55°and 1.73°-1.78°,corresponding to 100,110 and 200 ref l ections of SBA-15,respectively.They were the characterization of the typical two-dimensional hexagonal structure.It indicated that MgO-SBA-15 supports had highly ordered mesoporous structures[10].It could be seen that the intensity of(100)in MgO-SBA-15 supports is obviously stronger than that of the pure SBA-15[Fig.2(a)]with the exception of MgO-SBA-15(0.150-N)[Fig.2(d)].It clearly elucidated that the addition of magnesium salts could improve the ordered structure of SBA-15 due to the salt effect of magnesium precursors[2].The corresponding unit cell parameters(α0)and the wall thickness(τ) are provided in Table 3.The thickest wall of MgO-SBA-15(0.150-A) is 6.1 nm attained at the hydrochloric acid molar concentration of 0.8 mol·L-1[Table 3,Entry b].

Table 2Textural characteristics of catalysts

Fig.2.XRD patterns of supports.a—MgO-SBA-15(0-A);b—MgO-SBA-15(0.150-A);c—MgO-SBA-15(0.450-A);d—MgO-SBA-15(0.150-N);e—MgO-SBA-15(0.150-Cl);f—MgOSBA-15(0.150-A)-0.8 mol·L?1;g—MgO-SBA-15(0.150-A)-0.4 mol·L?1.

Table 3Structure properties of MgO-SBA-15

Fig.3 shows the XRD patterns of Pd-Pb/MgO-SBA-15(0.150-A)catalysts and MgO-SBA-15(0.150-A)supports.In the XRD pattern of Pd-Pb/MgO-SBA-15(0.150-A)catalysts[Fig.3(a)],the peak at 38.6°could be assigned to Pd3Pb[24].No MgO crystallization is detected in the XRD patterns of MgO-SBA-15(0.150-A)supports[Fig.3(b)],implying the good dispersion of MgO on SBA-15.This further facilitates the formation of long range ordered domains of mesostructure.

3.1.3.Surface alkalinity measurement

From the results of the textural properties of the catalysts,it is concluded that the mesoporous Pd-Pb/MgO-SBA-15(X)catalysts have different surface properties.Therefore,CO2-TPD is used to measure their surface basic properties.Fig.4 shows only one asymmetric broad peak on the prof i les of the different added amounts of MgO.When the support is pure SBA-15,the catalysts[Fig.4(a)]almost shows no alkalinity. The maximum temperature of CO2desorption peaks increases from 361.8 K to 379.5 K with an increase in added amount of MgO from 1% [Fig.4(b)]to 30%[Fig.4(f)].Furthermore,the number of surface basic sites on MgO-SBA-15(X-A)supports corresponding to the areas of CO2desorption peaks are different.The dosage of alkaline increases from 145.68 μmol·g?1[Table 4,Entry b]to 573.06 μmol·g?1[Table 4, Entry f]when the added amount of MgO is elevated from 1%to 30%. Both the strength and the dosage of the alkaline enhance with the increase of MgO deposited on SBA-15 supports.3.1.4.TEM observation

Fig.3.XRD patterns of(a)Pd-Pb/MgO-SBA-15(0.150-A)catalysts and(b)MgO-SBA-15(0.150-A)supports.

The morphology and particle size distribution of Pd3Pb particles on different MgO-SBA-15 supports are shown in Fig.5.The particle size distribution is manually measured.The average size of Pd3Pb particles varies with the change of supports.The location of Pd3Pb particles is also an important issue in metal/mesoporous silica catalysts.

The major size distribution of Pd3Pb particles on pure silica-based SBA-15 supports[Fig.5(a)]is between 12-14 nm and few of them are larger than 20 nm.Furthermore,most of the Pd3Pb particles are on the externalsurfaceofsupportsbecauseoftherelativelysmallerporediameter.Pd3Pb particles supported on SBA-15 are larger aggregates than those supported on MgO-SBA-15.For example,Pd3Pb particles on MgO-SBA-15(0.150-A)supports[Fig.5(b)]are around 6 nm,much smaller than those on supports MgO-SBA-15(0-A)[Fig.5(a)].Elevating the added amounts of MgO on SBA-15 supports,Pd3Pb particles [Fig.5(c)]are enlarged,but they are still smaller than the particles on pure SBA-15 supports.It indicates that MgO on the surface of SBA-15 improves the dispersion and decreases the average size of Pd3Pb particles compared with SBA-15 supports.

Compared with catalysts synthesized from different hydrochloric acid molar concentration,Pd3Pb particles on MgO-SBA-15(0.150-A)-0.8 mol·L?1catalysts[Fig.5(d)]are highly dispersed alloy particles with the particle size of approximately 4.5 nm.It appears that Pd3Pb particles on MgO-SBA-15(0.150-A)-0.8 mol·L?1are primarily located inside the channels,although the possibility of Pd3Pb particles located on the external surface could not be ruled out.Pd3Pb particles on MgO-SBA-15(0.150-A)-1.6 mol·L?1catalysts[Fig.5(b)]are merely a little larger than catalysts synthesized at 0.8 M.But Pd3Pb particles supported on MgO-SBA-15(0.150-A)-0.4 mol·L?1[Fig.5(e)]are about 10 nm,which are the largest and the most uneven dispersion.Slightly lower hydrochloric acid molar concentration than usually preparation concentration[25]could also reduce Pd3Pb particle size.

3.2.Catalytic performance

3.2.1.Effects of Mg precursors on catalytic activity

The conversion of MAL with Pd-Pb/MgO-SBA-15(0.150-A)catalysts[Table 5,Entry a]is 68%which is the same as Pd-Pb/MgOSBA-15(0.150-N)catalysts[Table 5,Entry b].The selectivity of MMA of Pd-Pb/MgO-SBA-15(0.150-N)catalysts is 67%,which is the same as Pd-Pb/MgO-SBA-15(0.150-Cl)catalysts[Table 5,Entry c].Even when there is only a little difference among the catalysts prepared from different Mg precursors,the catalytic activity follows theorderofPd-Pb/MgO/SBA-15(0.150-A)＞Pd-Pb/MgO/SBA-15(0.150-Cl)＞Pd-Pb/MgO/SBA-15(0.150-N).The catalysts prepared from magnesium chloride have the lowest specif i c surface area,pore volume and mean pore diameter,which drop the diffusion speed of reactants.The ordered mesostructure of catalysts prepared from magnesium nitrate is the weakest.Finally,magnesium acetate is selected as magnesium precursors.

Table 4Basicity of catalysts of different MgO loadings

Table 5Effects of Mg precursors on catalytic activity

Table 6Effects of MgO loadings on catalytic activity

Fig.5.TEM imagesand particlesize distributionsfor(a)Pd-Pb/MgO-SBA-15(0-A),(b)Pd-Pb/MgO-SBA-15(0.150-A);(c)Pd-Pb/MgO-SBA-15(0.450-A),(d)Pd-Pb/MgO-SBA-15(0.150-A)-0.8 mol·L?1and(e)Pd-Pb/MgO-SBA-15(0.150-A)-0.4 mol·L?1.

3.2.2.Effects of MgO loadings on catalytic activity

Table 6 summarizes the effects of the amounts of MgO added on the conversion of MAL and selectivity of MMA.When there is no MgO on SBA-15 supports[Table 6,Entry a],the conversion of MAL and selectivity of MMA are 33.1%and 49.5%,respectively,which are lower than those of Pd-Pb/MgO-SBA-15[Table 6,Entries b-f].Compared with the catalysts supported on MgO-SBA-15,the specif i c surface area of catalysts supported on pure silica-based SBA-15 is much smaller and there is barelysurfacealkalinity.Butaboveall,Pd3Pb particlesonSBA-15 supportsaretoolarge.Therefore,thenumberofPd3Pb particlesisrelatively low when the loading amounts of palladium and lead are about the same.When the supports changed to MgO-SBA-15(0.150-A)[Table 6, Entry d],the conversion of MAL increases to 67.9%and the selectivity of MMA reaches a maximum of 71.7%.Compared with Pd-Pb/SBA-15 catalysts,thespecif i c surfacearea andporevolume ofcatalystssupported on MgO-SBA-15(0.150-A)are higher and the surface alkalinity increases.Most of all,the Pd3Pb particles supported on MgO-SBA-15(0.150-A)are at least 6 nm smaller than those supported on SBA-15,leading to more active sites and higher activity.The conversion of MAL increases to 82.7%with an increase in the added amounts of MgO to 30%[Table 6,Entry f],while the selectivity of MMA decreases to52.5%,which resulted from relatively large Pd3Pb particle size.The pore volume and mean pore diameter of catalysts supported on MgO-SBA-15(0.450-A)decrease signif i cantly,which might result from the blocking pore entrances.Pd3Pb particles supported on MgO-SBA-15(0.450-A)are enlarged,but they are still smaller than the Pd3Pb particles supported on SBA-15.The surface alkalinity continues to increase but the catalytic activitydecreases.Therefore,thecatalysts for this reaction need appropriate alkalinity.

It was known that the activity of catalysts was mainly attributed to Pd3Pb particles even when the catalytic reaction mechanism was not yet very clear[26].Therefore,the small Pd3Pb particles presented homogeneously on the surface of the supports should exhibit the high activity for oxidative esterif i cation of MAL with methanol.The dispersion and particle size distribution of Pd3Pb particles were inf l uenced by the amountsofMgOdopedonSBA-15supports.Itisnoticedthatanoptimal added amount of MgO is 10%[Table 6,Entry d].

3.2.3.Effects of hydrochloric acid molar concentration on catalytic activity

Table 7 summarizes the results of oxidative esterif i cation of MAL with Pd-Pb/MgO-SBA-15 catalysts synthesized with different hydrochloric acid molar concentrations.There is no signif i cant difference of selectivity of MMA for the catalysts prepared at hydrochloric acid molar concentration of 1.6 mol·L?1[Table 7,Entry a]and 0.8 mol·L?1[Table 7,Entry b].But the conversion of MAL obviously increases 10% when the hydrochloric acid molar concentration reduces from 1.6 mol·L?1to 0.8 mol·L?1.In addition,the conversion of MAL is 77% for the catalysts prepared at hydrochloric acid molar concentration of 0.4 mol·L?1[Table 7,Entry c]which is the same as the catalysts of 0.8 mol·L?1.Buttheselectivitysharplydecreasesto58.1%athydrochloric acidmolar concentrationof 0.4 mol·L?1.As we could seefrom Fig.2, the ordered texture is weak and the wall of the supports is the thinnest whenthehydrochloricacidmolarconcentrationis0.4 mol·L?1.Inaddition,the specif i c surface area,pore volume and mean pore diameter of the catalysts synthesized at hydrochloric acid molar concentration of 0.4 mol·L?1are smaller than those of another two catalysts.Furthermore,TEM images show that the average particle size of Pd3Pb particles follows the order of 0.8 mol·L?1＜1.6 mol·L?1＜0.4 mol·L?1,which is the same order as the catalytic activity.Low hydrochloric acid molar concentration leads to non-uniform dispersion and large particle size of Pd3Pb.In summary,the catalysts prepared at hydrochloric acid molar concentration of 0.8 mol·L?1present the best conversion of MAL and the best selectivity of MMA,which are 78.5%and 72.5%, respectively.

Fig.5(continued).

Table 7Effects of hydrochloric acid molar concentration on catalytic activity

4.Conclusions

In conclusion,a series of Pd-Pb/MgO-SBA-15 catalysts was prepared.It was found that the addition of MgO enhanced the ordered structure and increased the surface alkalinity of SBA-15 supports, which facilitated the dispersion of Pd3Pb particles.The average size of the Pd3Pb particles on Pd-Pb/MgO-SBA-15 catalysts was smaller than that on Pd-Pb/SBA-15 catalysts.Therefore,Pd-Pb/MgO-SBA-15 catalysts showed higher activity than Pd-Pb/SBA-15 catalysts,indicating strongdependenceof catalytic activity ontheaveragesizeof active particles.Evenwhentheexcessamounts of addedMgOincreased theaverage size ofPd3Pb particles,they were still smaller than particles on SBA-15 supports.Besides magnesia loadings,hydrochloric acid molarconcentration and magnesium precursors affected the catalytic activity as well.Low hydrochloric acid molar concentration led to non-uniform dispersion and large particle size of Pd3Pb.Through tailoring the mass ratio of MgO/SBA-15 and hydrochloric acid molar concentration, catalysts with uniform dispersion and small size Pd3Pb particles could be successfully synthesized.The optimal preparation conditions of MgO-SBA-15 supports were as follows:magnesium acetate precursors, 10%MgOloadings,and0.8 mol·L-1hydrochloricacidmolar concentration.

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26 February 2013

http://dx.doi.org/10.1016/j.cjche.2014.08.002

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

Received in revised form 15 April 2013

Accepted 7 May 2013

Available online 20 August 2014