Zhang 2012 Chemical Engineering Journal

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Template-free synthesis of Cu@Cu2O core–shell microspheres and their application

as copper-based catalysts for dimethyldichlorosilane synthesis

Zailei Zhang a, Hongwei Che a, Yingli Wang a,⇑, Jiajian Gao a, Yuan Ping a, Ziyi Zhong b, Fabing Su a,⇑

a State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, Chinab Institute of Chemical Engineering and Sciences, AÃStar, 1 Pesek Road, Jurong Island, Singapore 627833, Singapore

h i g h l i g h t s

" Cu@Cu2O core–shell microspheres were prepared via a template-free solvothermal method." The effect of synthesis parameters on the morphology of products were investigated." The formation mechanism of Cu@Cu2O core–shell structure was tentatively proposed." Cu@Cu2O exhibited an enhanced catalytic activity for dimethyldichlorosilane synthesis.

a r t i c l e i n f o

Article history:

Received 19 May 2012Received in revised form 12 September2012Accepted 24 September 2012Available online 5 October 2012

Keywords:

CopperCuprous oxideCore–shell microspheresDimethyldichlorosilane synthesis

a b s t r a c t

We report the synthesis of Cu@Cu2O core–shell microspheres via a facile template-free solvothermalmethod. The resulting products were characterized by X-ray diffraction, scanning electron microscopywith energy-dispersive spectroscopy, transmission electron microscopy, temperature-programmedreduction, and thermogravimetric analysis. It is found that, Cu2O microspheres were firstly formedthrough the reduction of copper acetate by glutamic acid, and then, the reduction started inside themicrospheres due to the higher surface energies of inner Cu2O particles, resulting in the formation of

Cu@Cu2O core–shell structure. The content of Cu core in the composite microspheres increased withthe reaction time and temperature. The as-prepared Cu@Cu2O core–shell microspheres exhibited a bettercatalytic performance for dimethyldichlorosilane synthesis than pure Cu2O and Cu, and even superior tothe physically mixed Cu and Cu2O microspheres possibly because of the synergistic catalytic effect. TheseCu@Cu2O core–shell microspheres will have potential application in organosilicon industry as copper-based catalysts.

Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction

Recently, there has been increasing interest in the controllablesynthesis of core–shell inorganic micro/nanostructures because of their special geometries, unique properties[1–3], andwideapplica-

tions [4,5]. It is reported that core–shell structures containing metal[6,7], polymer [8], semiconductor [9], inorganic material[10], metaloxide@C [11], and carbon/inorganic hybrid [12] have been synthe-sized and used in optics [13], pharmaceutics [14], semiconductor[15], catalysis [16], sensors [17], and Raman scattering [18]. Amongvarious core–shell structures, the ones with metal core and metaloxide shell are of particular interest, as they can be used in COgas absorption [19], magnetic [20], electronic [21], and catalyticapplications [22,23]. One particular example is the Cu-core and itsoxide-shell structures. In the last years, Cu/Cu2O core–shell and

hollow Cu2O nanodendrites [18], mesoporous Cu/Cu2O [24], Cu/Cu2O cermets [25], Cu–Cu2O heterogeneous architectures [26],Cu/Cu2O nanoparticles [27], Cu@Cu2O core–shell nanoparticles[19], Cu/Cu2O hollow microspheres [28], and Cu/Cu2O hollownanocubes [29] have been synthesized using electrodeposition

[18], co-deposition [19], milling method [25], two-step synthesis[29] withtoxicreducing agent [27], respectively, because of the factthat copper (Cu) [30] and cuprous oxide (Cu2O) [31,32] are widelyused in catalysis. However, it still remains a bigchallenge to synthe-size these core–shell nanostructures in a green way, thus develop-ing facile, template-free, one-step process is highly desirable.

Since Rochow discovered the direct synthesis route to producemethylchlorosilanes (MCSs) using Si particles to react with gaschloromethane (MeCl) over the copper-based catalysts in 1940s[33], named Rochow reaction, this reaction is still the most eco-nomical route in organosilane industry for direct synthesis of methylchlorosilanes (MCSs) [34]. Metallic copper and its com-pounds including Cu2O [35], CuO [36], CuCl [37], Cu3Si [38], and

1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2012.09.095

⇑ Corresponding authors. Tel.: +86 10 82544850; fax: +86 10 82544851.

E-mail addresses: [email protected](Y. Wang), [email protected] (F. Su).

Chemical Engineering Journal 211–212 (2012) 421–431

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Cu–Cu2O–CuO composite [39] have been demonstrated to be ac-tive for Rochow reaction. In general, the copper-based catalystsemployed both in the organosilane industry [40] and in fundamen-tal studies [41,42] are densely packed and irregular in morphology,while it is reported that core–shell catalysts can result in enhancedcatalytic activity [43,44]. Therefore, it would be interesting toexplore the performance of the novel core–shell copper-basedcatalysts for the organosilane synthesis.

Our previous work investigated the catalytic properties of theporous cubic Cu [45], mesoporous Cu2O [35], and flower-like CuOmicrospheres [46] as one-component model copper-basedcatalysts, as well as the effect of urchin-like ZnO microspheres aspromoters [47] for dimethyldichlorosilane synthesis via theRochow reaction. Herein, we report a facile template-free solvo-thermal synthesis of two-component core–shell Cu@Cu2O micro-spheres and their catalytic performance in the same reaction.Various synthetic parameters such as solvothermal reaction time,temperature, and reactant amounts are varied to investigate theireffects on the formation of the core–shell Cu@Cu2O microspheres.Compared with the pure Cu2O and Cu, and the physically mixed Cuand Cu2O microspheres, the core–shell Cu@Cu2O microspheresshow a better catalytic performance with a higher selectivity todimethyldichlorosilane, demonstrating the occurrence of a syner-gistic catalytic mechanism.

2. Experimental

2.1. Material synthesis

All the reagents were of analytical grade and purchased fromSinopharm Chemical Reagent Co., Ltd. Table 1 compiles the synthe-sis conditions used for preparing the samples. In a typical synthe-sis, 0.005 mol of copper acetate (Cu(CH3COO)2ÁH2O) was dissolvedin the solution containing 10.0 ml of deionized water, 50.0 ml of absolute alcohol (CH3CH2OH), and 20.0 ml of glycol (C2H6O2). Then

0.035 mol of sodium hydroxide and 0.0133 mol of glutamic acidwere added to the above solution. After stirred for 10 min, theresulting solution was transferred into a 100 ml stainless-steelautoclave lined with poly(tetrafluoroethylene) (PTFE, Telfon). Theautoclave was sealed and maintained at 130 °C for 18 h, and thencooled down to room temperature. The resultant precipitate solidwas collected by centrifugation, followed by washing with distilledwater and absolute ethanol, and finally dried in vacuum at 60 °Cfor 8 h. 10.0 ml of deionized water, 50.0 mL absolute alcohol, and20.0 mL glycol were constant for all sample synthesis.

2.2. Characterization

X-ray diffraction (XRD) patterns of the samples were recordedon a PANalytical X’Pert PRO MPD using the Ka radiation of Cu(k = 1.5418 Å). The crystallite size of the samples was calculatedusing the Debye–Scherrer equation. The microscopic features of the samples were observed with field-emission scanning electronmicroscopy (SEM) with energy-dispersive spectroscopy (EDS)(JSM-6700F, JEOL, Tokyo, Japan) and transmission electron micros-copy (TEM) (JEM-2010F, JEOL, Tokyo, Japan). Thermal gravimetric(TG) analysis was carried out on EXSTAR TG/DTA 6300 (SeikoInstruments, Japan) with a heating rate of 10 °C/min in air(200 mL/min). Temperature programmed reduction (TPR) mea-surements were carried out on an automated chemisorption ana-lyzer (ChemBET pulsar TPR/TPD, Quantachrome). 0.10 g of Cu@Cu2O was initially loaded in a quartz U-tube. Prior to the mea-

surement, the sample was degassed at 200 °C for 30 min under he-lium. When the temperature was dropped to 20 °C, the gas was

switched to 9.9% H2/Ar. Finally, the sample was heated from 20to 800 °C at 10 °C/min under the flow of 9.9% H2/Ar at 30 mL/min.

2.3. Measurement of catalytic property

The evaluation of the catalyst was carried out with a typicalMCS lab fixed-bed reactor [36]. The scheme of the catalytic reactorsystem is shown in Fig. 1.

10.0 g of Si powder (20–50 mesh, provided by Jiangsu HongdaNew Material Co., Ltd.) and 1.0 g of Cu@Cu2O, together with 0.1 gof Zinc (Zn, A.R., Sinopharm Chemical Reagent Co., Ltd.) as thepromoter, were ground homogeneously to form a contact mass,which was then loaded into the glass reactor. The reactor systemwas purged with purified N2 for 0.5 h and then heated to 325 °Cwithin 1 h under a N2 flow rate of 25 mL/min. Subsequently, N2

was turned off and MeCl gas with a flow rate of 25 mL/min wasintroduced into the bed to react with Si at 325 °C. After a givenperiod of 24 h, the reaction was stopped. The gaseous productwas condensed with a circulator bath controlled at 5 °C via a pro-grammable thermal circulator (GDH series, Ningbo xinzhi biolog-ical technology Co., Ltd.), and the collected liquid was analyzedwith Agilent Technologies 7890A GC System. The spent contactmass (solid residue after reaction) containing unreacted Si pow-der and Cu + Zn compounds was weighed to calculate Si conver-sion. The Rochow reaction is described in the following equation:

ðR1Þ

In this reaction, the obtained products mainly consist of methyltri-chlorosilane (CH3SiCl3, M1), dimethyldichlorosilane ((CH3)2SiCl2,M2), and trimethylchlorosilane ((CH3)3SiCl, M3), which are ac-counted for more than 95 wt% of the total reaction products[40]. Among them, M2 is the highly desired organosilane mono-mer. To simplify the calculation, other trace products and thechange of catalyst are not accounted for, and thus Mi ( i = 1,2,3)selectivity and Si conversion are calculated using the followingformulas:

Selectivity of Mi ðSMiÞ ¼

moleMiP

3i¼1moleMi Â 100% ð

i¼ 1

;

2;

3Þ ð1Þ

Table 1

Synthesis conditions used for preparing samples.

Sample Copperacetate(mol)

Sodiumhydroxide(mol)

Glutamicacid(mol)

ReactionT (°C)

Reactiontime (h)

Product

S1 0.005 0.035 0.0133 130 3 Cu2OS2 0.005 0.035 0.0133 130 8 Cu@Cu2OS3 0.005 0.035 0.0133 130 12 Cu@Cu2O

S4 0.005 0.035 0.0133 130 18 Cu@Cu2OS5 0.005 0.035 0.0133 130 27 Cu@Cu2OS6 0.005 0.035 0.0133 130 36 Cu@Cu2OS7 0.005 0.035 0.0133 130 48 CuS8 0.005 0.035 0.0133 80 18 Cu2OS9 0.005 0.035 0.0133 100 18 Cu2OS10 0.005 0.035 0.0133 150 18 Cu@Cu2OS11 0.005 0.035 0.0133 180 18 CuS12 0.001 0.007 0.0027 130 18 CuS13 0.0025 0.0175 0.0067 130 18 Cu@Cu2OS14 0.010 0.070 0.0266 130 18 Cu@Cu2OS15 0.020 0.140 0.0533 130 18 Cu2O

422 Z. Zhang et al. / Chemical Engineering Journal 211–212 (2012) 421–431



Conversionof Si ðCSiÞ ¼weightcontactmass before reaction À weightcontactmassafter reaction

weightSibeforereaction

Â 100%

ð2Þ

3. Results and discussion

3.1. Effect of reaction time

Fig. 2a shows the XRD patterns of the samples obtained at dif-ferent reaction times. For the sample S1 obtained after the reactionfor 3 h, diffraction peaks at 2h values of 36.5°, 42.2°, 61.3°, and73.3° are observed, which are corresponded to the lattice planesof (11 1), (2 00), (2 20), and (31 1) respectively in Cu2O with cubicsymmetry (JCPDS No. 05-0667). When the reaction time was pro-longedto 8, 12, 18, 27, and 36 h, besides the above Cu2O diffractionpeaks, several other peaks at 2h values of 43.3°, 50.5°, and 74.2°,which correspond to the lattice planes of (111), (200), and(220) of cubic symmetry Cu (JCPDS No. 01-070-3039), are also ob-served, suggesting that samples S2, S3, S4, S5 and S6 are composed

of Cu2O and Cu phases. When the reaction time was increased to48 h, the sample S7 is only composed of pure metal Cu. Fig. 2band c exhibit the SEM images of samples S1 and S2. It can be seenthat the microspheres have a size of 1–3 lm. With the increase of the reaction time to 12 h, some pores on the surfaces of micro-spheres are observed for S3. Further prolonging the reaction timeto 18 h, typical Cu@Cu2O core–shell microspheres are formed forS4 as shown in Fig. 2e. As the reaction time is further extend to27 h and 36 h, the shells of Cu@Cu2O core–shell microspheres be-come thinner and the cores become larger as shown in Fig. 2f and g. Meanwhile, it is noted that this evolution process is accom-panied by the increase of Cu content in Cu@Cu2O composites,which is proved by the XRD pattern (Fig. 2a). When the reactiontime is as long as 48 h, some Cu micro-particles with irregular mor-

phologies appear in the resulting products, which is shown inFig. 2h, and the metallic Cu phase is confirmed by Fig. 2a. Thus,

Cu@Cu2O core–shell microspheres can be synthesized by adjustingthe reaction time.

Fig. 3a reveals the SEM image of Cu@Cu2O core–shell micro-spheres of the sample S4. The EDX spectrum demonstrates thatthe core of Cu@Cu2O is made of Cu and O (Fig. 3b), and the atomicratio of Cu to O is approximately 12:1, corresponding to the com-pound of most of Cu and little Cu2O, the shell is made of Cu and O

(Fig. 3c), and the atomic ratio of Cu to O is approximately 2:1, cor-responding to the compound of Cu2O. Therefore, these resultsprove that core–shell microspheres comprise Cu as the core andCu2O as the shell for S4.

As shown in Fig. 4a, the sample S1 obtained at 3 h exhibitsdense microspheres with a size of about 2 lm. By increasing thereaction time to 12 h (S3), some core–shell structures are devel-oped gradually as shown in Fig. 4b. With further prolongation of the reaction time to 18 h (S4), the microspheres display typical hol-low structure as shown in Fig. 4c (the Cu core flowed out the Cu2Oshell). When the reaction time reaches 36 h, the shell of the core–shell microsphere become thinner, as is shown in Fig. 4d, confirm-ing the transformation process from dense microspheres towardthe core–shell structures with the increase of the reaction time.

Fig. 4e–h shows the HRTEM images obtained from the core andshell of one microsphere according to the TEM image (Fig. 4e) of S4. The lattice spacing of about 0.18 nm between adjacent latticeplanes corresponds to the interplanar spacing of the (200) planeof the cubic phase of Cu (JCPDS No. 01-070-3039), suggesting thatthe core of the core–shell microsphere is composed of nanocrystal-line Cu as shown in Fig. 4f. The lattice spacing of about 0.24 nm be-tween adjacent lattice planes of the nanocrystals in the shell can beindexed to the (111) plane of the cubic Cu2O structure (JCPDS No.05-0667), as shown in Fig. 4g and h, revealing that the shell of thecore–shell microsphere is composed of Cu2O nanocrystals.

3.2. Effect of reaction temperature

Fig. 5a shows XRD patterns of the samples formed at differentreaction temperatures. When the reaction is conducted at 80 °C

Fig. 1. The scheme of the catalytic reactor system.

Z. Zhang et al. / Chemical Engineering Journal 211–212 (2012) 421–431 423



(S8) and 100 °C (S9) for 18 h, the diffraction peaks at 2h values of 36.5°, 42.2°, 61.3°, and 73.3° correspond to the lattice planes of (111), (200), (220), and (311), respectively, indicating the pres-ence of the pure cubic symmetry Cu2O (JCPDS No. 05-0667). Asthe reaction temperature is raised to 130 (S4) and 150 °C (S10)respectively, besides the Cu2O diffraction peaks, new peaks at 2h

values of 43.3°, 50.5°, and 74.2°, corresponding to the lattice planesof (111), (200), and (220) of the cubic symmetry Cu (JCPDS No.

01-070-3039), are developed. This suggests that S4 and S10 areboth composed of Cu2O and Cu phases. After the reaction temper-ature was increased to 180 °C (S11), all the diffraction peaks of Cu2O phase disappeared, accompanying with the appearance of the diffraction peaks of Cu phase, indicating that the product isconverted to Cu. Fig. 5b reveals the SEM image of the sample S8 ob-

tained at 80 °C for 18 h, which possesses non-uniform micro-spheres with a size of 1–6 lm. Fig. 5c shows the SEM image of

Fig. 2. XRD patterns of samples at different reaction times (a), and SEM images of products formed at different reaction times: (b) 3 h (S1), (c) 8 h (S2), (d) 12 h (S3), (e) 18 h(S4), (f) 27 h (S5), (g) 36 h (S6), and (h) 48 h (S7) (insets are their high-magnification SEM images).




the sample S9 obtained at 100 °C, exhibiting microspheres with adiameter of 2–5 lm. By further increasing the reaction tempera-ture to 130 °C (S4), typical core–shell microspheres with a size of 1–3lm are formed as shown in Fig. 5d. These core–shell micro-spheres comprise Cu2O and Cu phase, which is confirmed by the

above XRD pattern (Fig. 5a). When the reaction temperaturereaches 150 °C (S10), only porous microspheres are observed asshown in Fig. 5e, accompanying with an increase in metallic Cuphase content as proved in Fig. 5a. Finally, as the reaction temper-ature is increased to 180 °C, irregular microparticles are generatedin S11, which contains pure Cu phase only. As mentioned above,adjusting the reaction temperature to 130 °C is favorable for thesynthesis of Cu@Cu2O core–shell microspheres.

3.3. Effect of reactant amounts

Fig. 6a shows XRD patterns of the samples formed at differentreactant amounts. For sample S12, the diffraction peaks at 2h val-ues of 43.3°, 50.5°, and 74.2° correspond to the lattice planes of

(11 1), (200), and (22 0) respectively, confirming the formationof pure cubic symmetry Cu (JCPDS No. 01-070-3039). However,

for samples S13, S4 and S14, besides the presence of Cu phase, cu-bic symmetry Cu2O is also formed, as evidenced by the observationof the diffraction peaks at 2h values of 36.5°, 42.2°, 61.3° and 73.3°,which correspond to the lattice planes of (111), (200), (220), and(311) of Cu2O (JCPDS No. 05-0667). For sample S15, it is composed

of pure Cu2O phase. Fig. 6b shows the SEM images of porous Cuwith a size of 1–3 lm when the reaction amount of copper acetate,sodium hydroxide and glutamic acid is 0.001, 0.007 and0.0027 mol (S12), respectively. Increasing the correspondingamount of copper acetate, sodium hydroxide and glutamic acidto 0.0025, 0.0175 and 0.0067 mol, non-uniform core–shell micro-spheres are present for S13 (Fig. 6c). In these samples, the Cu phaseaccounts for the majority in the composite microspheres accordingto their XRD patterns (Fig. 6a). As the amount of copper acetate, so-dium hydroxide, and glutamic acid amount is further raised to0.005, 0.035 and 0.0133 mol (S4), respectively, typical Cu@Cu2Ocore–shell microspheres with a size of 1–3 lm are displayed forS4as shown in Fig. 6d. Once the amounts of copper acetate, sodiumhydroxide, and glutamic acid become 0.010, 0.070, and 0.0266 mol,

respectively, non-uniform Cu@Cu2O microspheres with a size of 1–6 lm are revealed for S14 as shown in Fig. 6e, and all of them are

Fig. 3. (a) SEM image of Cu@Cu2O core–shell microsphere (S4), (b) the core of EDX spectrum of Cu@Cu 2O microsphere and (c) the shell of EDX spectrum of Cu@Cu2Omicrosphere.

Fig. 4. TEM images ofproductsformed at different reactiontimes: (a)3 h (S1), (b) 12 h (S3), (c)18 h (S4), (d)36 h (S6); TEM images of singleCu@Cu2O core–shell microsphereS4 (e), Cu core of S4 (f), and the edge of Cu 2O shell of S4 (g and h).




mainly composed of Cu2O phase. When the amounts of copper ace-tate, sodium hydroxide and glutamic acid amounts reach 0.020,0.0140 and 0.0533 mol, respectively, non-uniform Cu2O micropar-ticles are obtained, as shown in Fig. 6f and confirmed by the XRDpattern of Fig. 6a.

3.4. Formation mechanism of Cu@Cu 2O core–shell microspheres

On the basis of the above characterization, we thus tenta-tively propose a formation process for the Cu@Cu2O core–shellmicrospheres as illustrated in Fig. 7. The possible chemicalreactions occurred in the system are expressed in equations(Eqs. (2-4)). Initially, Cu2+ could react with glutamic acid to forma complex (Eq. (2)) [48]. Cu2O microspheres are produced via thereduction of Cu2+ via the decomposition of glutamic acid mole-cules at 130 °C and further aggregation of the formed nanoparti-cles (Eq. (3)) [49]. When glutamic acid are not added to thesolution, the products is only Cu2O at 130 °C after reaction18 h in our previous work [45]. With increase of the reaction

time, inner Cu2O nanoparticles are further reduced due to theanchored glutamic acid molecules onto the nanoparticles (Eq.

(4)). Then, these inner Cu nanoparticles aggregated to formmicrospheres. Consequently, Cu@Cu2O core–shell microsphereswere obtained. With further increasing the reaction time, the in-side-out reduction under base environment due to their highersurface energies than those of outer nanoparticles was continu-

ously performed for Cu@Cu2O core–shell microspheres, resultingin the thinner shells and larger cores. Finally, Cu2O phases werecompletely transformed to Cu phases, accompanying with theformation of Cu microspheres. This evolution process is differentfrom that reported by Ai et al. [16]. They synthesized Cu@Cu2Ocore–shell microspheres using the interfacial hydrothermalmethod. As for the formation process, Cu microspheres were firstformed through the reduction of copper (II) acetylacetonate, andthen surface Cu nanoparticles were oxidized to Cu2O nanoparti-cles, resulting in the formation of Cu@Cu2O core–shell micro-spheres. With prolonging the reaction time, the shells becamethicker and the cores became smaller arising from the out-insideoxidation for Cu@Cu2O core–shell microspheres. They used out-inside oxidation for Cu@Cu2O core–shell microspheres, but we

used inside-out reduction to perform Cu@Cu2O core–shellmicrospheres.

Fig. 5. XRD patterns (a) of samples at different reaction temperature for 18 h, SEM images of samples formed at different reaction temperature: (b) 80 °C (S8), (c) 100°C (S9),(d) 130 °C (S4), (e) 150 °C (S10), and (f) 180 °C (S11).




Fig. 7. Illustration of the possible formation process for Cu@Cu2O core–shell microspheres.

Fig. 6. XRD pattern (a) of samples at different reactant amounts, SEM images of samples formed at copper acetate, sodium hydroxide, and glutamic acid amounts at: (b)0.001, 0.007 and 0.0027 mol (S12), (c) 0.0025, 0.0175 and 0.0067 mol (S13), (d) 0.005, 0.035 and 0.0133mol (S4), (e) 0.010, 0.070, and 0.0266 mol, and (f) 0.020, 0.0140 and0.0533 mol (S15).

ðR2Þ




3.5. The oxidability and reducibility of the products

Fig. 8a shows the H2–TPR curves of the products (S1–S7 in Table1). It can be seen that the reduction temperature increases andhydrogen consumption decrease with the increase of the Cu con-tent in the products of S1–S7. However, the Cu@Cu2O core–shellmicrospheres reveal two main reduction peaks between 350 and400 °C. Assuming the hydrogen consumption of S1 (Cu2O) is 100,the calculated hydrogen consumptions of S2, S3, S4, S5, S6, andS7 from the TPR peak areas are about 92, 86, 66, 40, 15 and 2, sug-gesting S2, S3, S4, S5, S6 and S7 contain about 92%, 86%, 66%, 40%,15% and 2% Cu

2O respectively. Fig. 8b shows the TG curves of the

products (S1–S7) measured in air. The oxidation of the products(S1–S7) started at about 200 °C, and the complete oxidation tem-perature and oxygen consumption increased with the increase of the Cu content in the S1–S7 products. The weight of S1 reachesabout 110.4% after the oxidation, slightly less than its theoreticalvalue (111.1%) assuming it is pure Cu2O, suggesting the presenceof trace amount of Cu on its the surface. In this TG curve, the initialweight loss of S1 in the low temperature range should be attrib-uted to the adsorbed water [50]. For the samples ofS2 and S3, theirweights increase to about 111.3% and 112.4% respectively after the

oxidation, slightly higher than the theoretical value of pure Cu2O(111.1%), confirming the presence of little Cu in the core of them.Similarly, the TG analysis shows that the Cu and Cu2O content inS4 is 29% and 71%, 57% and 43% in S5, 79% and 21% in S6, 99.4%and 0.6% in S7, respectively, well consistent with the above TPR results.

3.6. Catalytic test

Table 2 shows the catalytic performance of Cu2O (S1), Cu@Cu2Ocore–shell microspheres (S4 and S5), Cu (S7), the physical mixtureof S1 (Cu

2O 70%) + S7 (Cu 30%), and S1 (40% Cu

2O) + S7 (60% Cu),

together with some commercial Cu compounds for the Rochowreaction. It can be seen that, at 325 °C, similar Si conversions (Csi)are observed on the above catalysts, namely, 25.2% on S1, 28.6%on S4, 29.2% on S5, 30.5% on S7, 27.8% on S1 (Cu 2O 70%) + S7 (Cu30%), and 29.3% on S1 (40% Cu2O) + S7 (60% Cu), which are higherthan commercial Cu (15.0%) and commercial Cu2O (16.5%). How-ever, higher M2 selectivity (SM2) is obtained on the Cu@Cu2Ocore–shell catalysts. For example, SM2 is 83.3% for S4, and 77.6%for S5, while that of Cu2O (S1), Cu (S7), and the two physical mix-tures of S1 (Cu2O 70%) + S7 (Cu 30%) and S1 (40% Cu2O) + S7 (60%

f

e

d

c

b

a

H 2 - T P R S i g n a l

Temperature (oC)

g

(a)

200 300 400 500 600 100 200 300 400 500 600 700 800

95

100

105

110

115

120

125 a

bc

d

e

f

g

(b)

T G W e i g h t ( w t % )

Temperature(oC)

Fig. 8. H2–TPR(a) and TG(b)curvesof samples obtainedat different reactiontimes: (a) 3 h (S1), (b) 8 h (S2), (c) 12h (S3), (d) 18h (S4), (e) 27h (S5), (f) 36h (S6), and (g) 48h(S7).

ðR4Þ

ðR3Þ




Cu) are 65.2%, 63.2%, 76.1% and 70.5%, respectively, which are allhigher than that of commercial Cu (55.0%) and commercial Cu2O(56.7%). According to the TPR results, the Cu2O content in S4 andS5 is about 66% and 40% respectively. Likely, the higher Cu2O con-tent is favorable for higher M2 selectivity.

The enhanced catalytic performance observed on the Cu@Cu2Ocore–shell spheres should be attributed to the synergistic effect be-tween Cu and Cu2O. Similar phenomenon was observed on themixture of commercial Cu2O and Cu, which showed better catalyticperformance than either the commercial Cu2O or the commercialCu under our reaction conditions (not shown here). Interestingly,although S4 and S5 contain similar composition of Cu2O and Cuas that of the two physical mixtures of S1 (Cu2O 70%) + S7 (Cu30%) and S1 (40% Cu2O) + S7 (60% Cu) (S4 contains about 66%

Cu2O and 34% Cu, and S5contains ca. 40% Cu2O and 60% Cu), higherSM2 is obtained on S4 and S5, indicating that the synergistic effectis enhanced due to formation of this core–shell structure. It isknown that M2 is the most desired product in the organosilaneindustry. The above results show that the synthesized Cu@Cu2Ocore–shell microspheres are able to produce more M2 under thesame reaction conditions.

The above catalysts before and after the reaction are character-ized with XRD as shown in Fig. 9. The XRD patterns of the freshcontact masses (S1 + Si, S4 + Si, S5 + Si, and S7 + Si) are shown inFig. 9a, and an enlarged view in the 2 Theta angle range of 30–70° is displayed in Fig. 9b. Both the Cu2O and Cu phases are iden-tified in S4 and S5. However, after the reaction, only Cu species areobserved in all the samples as shown in Fig. 9c and d. The forma-tion of Cu is because of removal of oxygen from the Cu2O catalystsby reaction with chlorosilane, and the final products are siloxanesand copper chlorides [36]. The higher M2 selectivity may be be-cause Cu@Cu2O core–shell microspheres of Cu core and Cu2O shellsynergism enhanced M2 selectivity, associated with that the cata-lytic property of commercial Cu2O and Cu composites is betterthan pure commercial Cu2O and pure commercial Cu in our labconditions. In our study, the M2 selectivity of Cu@Cu2O core–shellmicrospheres is higher than that of Cu and Cu2O alone, even thancommercial copper catalysts Cu–Cu2O–CuO (81.5%) [51], whoseM2 selectivity is more than 80% [39], although the Si conversionis a little less. This is because the commercial copper catalysts con-tains other copper components such as CuO, together with other

Table 2

Catalytic performance of different copper-based catalysts.

Samples SM1 (%) SM2 (%) SM3 (%) CSi (%)

S1(Cu2O) 31.8 65.2 3.0 25.2S4(Cu@Cu2O) 15.5 83.3 1.2 28.6S5(Cu@Cu2O) 20.8 77.6 1.6 29.2S7(Cu) 33.2 63.2 3.6 30.5S1(70%) + S7(30%) 22.1 76.1 1.8 27.8

S1(40%) + S7(60%) 17.5 70.5 2.0 29.3Commercial Cu 48.2 55.0 1.8 15.0Commercial Cu2O 42.2 56.7 1.1 16.5Commercial catalyst 15.7 81.5 2.8 41.8

Fig. 9. XRD patterns of the contact masses beforethe reaction(a), and an enlarged view in the2 Thetaanglerangeof 30–70° (b), after the reaction(c), andan enlarged view inthe 2 Theta angle range of 40–80° (d) (a: S1 + Si, b: S4+ Si, c: S5+ Si, and d: S7+ Si).




promoters such as Sn and P [52], which may increase the Si conver-sion. The same contents of Cu@Cu2O core–shell spheres (S4 and S5)show a higher M2 selectivity than simple mixture of Cu and Cu2O,suggesting the occurrence of a synergistic catalytic mechanism be-tween Cu and Cu2O compounds. Based on our experience, Cu maybe related to a short inductionperiod, Cu2O may be helpful for highselectivity of M2, and CuO could promote the conversion of Si rawmaterial. However, the true synergism catalytic mechanism is notfully understood yet. The optimized ratio and interaction amongthese three Cu compounds is the concern of our future work andthe organosilane industry.

4. Conclusions

In summary, we have demonstrated a facile solvothermal meth-od to prepare Cu@Cu2O core–shell microspheres, and their compo-sition or molar ratio of Cu/Cu2O is tunable by varying the syntheticparameters such as the reactant amounts, solvothermal reactiontemperature and time. Compared to the single component of Cu2O or Cu samples, the Cu@Cu2O core–shell microspheres exhibita better catalytic property for dimethyldichlorosilane synthesis via

the Rochow reaction. Meanwhile, the Cu@Cu2O core–shell micro-spheres with higher Cu2O content yields higher M2 selectivity,and are supervisor to physically mixed Cu and Cu2O having thesame Cu/Cu2O, indicating the occurrence of a synergistic functionamong different Cu compounds. It is expected that these new find-ing will be very helpful to design high efficient catalysts and under-standing the catalytic mechanism for the Rochow reaction.

Acknowledgements

The authors gratefully acknowledge the financial supports fromNational Natural Science Foundation of China (Nos. 21206172,51272252, and 21031005), State Key Laboratory of MultiphaseComplex Systems (No. MPCS-2011-D-14), the Hundred Talents

Program of the Chinese Academy of Sciences (CAS), and CAS-Locality Cooperation Program (No. DBNJ-2011-058).

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