Effects of ethanol on the in situ synthesized Cu/SiC》2 catalyst: Texture, structure, and the catalyt

Availableonlineatwww.sciencedirect.coCHINESE° Science DirectCHEMICALLETTERSELSEVIERhinese Chemical Letters 22(2011)362-365www.elsevier.com/locate/ccletEffects of ethanol on the in situ synthesized Cu/Sio2 catalystTexture, structure, and the catalytic performance in hydrogenationdimethyl oxalate to ethylene glycolShu rong Wang", Ling Jun Zhu, Ying Ying Zhu, Xiao Lan Ge, Xin Bao LiState Key Laboratory of clean Energy Unilization, Zhejiang University, Hangzhow 310027, ChinaReceived 21 June 2010Available online 22 December 2010AbstractThe Cu/SiO catalysts were in situ synthesized by the hydrolysis of tetraethyl orthosilicate (TEOS)in one phase solution usingethanol as co-solvent or TEOS/H2o two phases solution, followed by the precipitation of copper on Sio2 by ammonia evaporation.In the hydrogenation of dimethyl oxalate, the catalyst prepared by one phase hydrolysis exhibited higher activity and ethyleneglycol (EG)selectivity at lower temperature than that of two phases due to its larger BET surface area and multimodal poredistribution. At 488-503 K, the catalyst prepared in one phase solution with water/ethanol (W/E) volume ratio of 3: 1 exhibited 90-95% EG selectivity, while catalyst prepared by two phase hydrolysis reached 90% EG selectivity only atC 2010 Shu Rong Wang. Published by Elsevier B V on behalf of Chinese Chemical Society. All rights reserved.Keywords: Ethanol; Cu/SiO2: TEOS; Hydrogenation; Dimethyl oxalate; Ethylene glycolEthylene glycol (EG), an important chemical widely used in polyester manufacture and as antifreeze, is mainlyroduced by the hydrolysis of ethylene oxidation generated from petroleum in the present industrial approach. Sinceour dependence on fossil fuels and the diminishing of petroleum resources have brought environmental and politicalproblem, the indirect synthesis of EG from biomass-derived syngas is a promising new process to sustainableproduction of EG. The process involves the coupling of Co with nitrite esters to oxalates, followed by thehydrogenation of oxalates to EG [1-4]. Since Cu shows high activities for the esters hydrogenation, and is much lessexpensive than the noble Ru, several types of Cu based catalysts have been prepared and evaluated in thehydrogenation of dimethyl oxalate(DMO)to EG. Although a high yield of EG was obtained on CuCr based catalyst[1, 5, 6], the toxicity of Cr severely limits its practical application. Therefore, the research of Cr-free Cu-based catalystsattracts more and more interest, among which, the Cu/Sio2 catalyst[7-12]exhibited the highest catalytic performancein the hydrogenation of dMo to EG due to the weak acidic and basic properties of SiO2In the above-mentioned Cu/SiO2 catalysts, Cu supported on tetraethyl orthosilicate(TEOS)based amorphous Sio2[7] and mesoporous materials, i.e. HMs [10, 11] and SBA-15 [12], by deposition-precipitation and incipient wetimpregnation, have been studied. Recently, we prepared Cu/Sioz catalyst via a convenient new method, one-potsynthesis using TEOS as a Sior source and without surfactant, which exhibited a high activity of DMo conversion ands Corresponding author.中国煤化工E-mailaddress:srwang@zju.edu.cn(SR.Wang).CNMHG1001-8417/S-see front matter C 2010 Shu Rong Wang. Published by Elsevier B. on behalf of Chinese Chemical Society. All rights reserved.doi:10.106clet.2010.10006S.R. Wang et aL /Chinese Chemical Letters 22(2011)362-365EG selectivity. In order to have an insight to the influence of the hydrolysis solution on the structure and performance ofthe catalysts, we studied the Cu/Sio2 catalysts in situ prepared in the different water/ethanol (W/Eratio in this work.1. ExperimentalThe Cu/SiO2 catalyst was prepared by in situ synthesis as described below. Typically, under gentle stirring, therequired amounts of TEOS with the Cu/Si molar ratio of 0.3 was added dropwise to the copper ammonia complexsolution with the pH value of 12-13 and with ethanol as a co-solvent, then stirred for another 4 h at room temperatureAfter aged for 24 h, the suspension was evaporated at 363 K till the pH value of 6.5, and then filtrated. The filtrate waswashed with deionized water fully and dried at 393 K overnight. The catalyst precursors were calcined in static air at723 K for 4 h, crushed, seized 40-60 meshes, and denoted as CuSio-W,Ey where x and y stand for the W/E volumeratio. The reduced catalysts were denoted as Cusi-WEyThe Brunauer-Emmett-Teller(BET) surface area(SBEr)and other textural properties were measured on aMicromeritics TriStar 3000 apparatus at 77 K. Powder X-ray diffraction(XRD)was executed on a PANalytical X'PertPRO X-ray diffractometer using Cu Ka radiation(=0. 15418 nm). The tube voltage was 40 kV and the current was30 mA. The surface morphologies were observed by scanning electron microscopy(SEM, FEI Model SIRION-100)Theparticle size and distribution were observed by transmission electron microscopy (TEM; Philips-FEL, Tecnai G2 F30)4. The reactions were carried out in a continuous-flow fix-bed reactor. In each experiment, 3 mL catalyst wassandwiched with quartz sand and packed in a stainless steel tubular reactor with an inner diameter of 8 mm. Before thereaction, the catalyst was activated with hydrogen of 50 mL/min at 350C for 4 h. After cooling to the reactiontemperature, a 12.5 wt% DMO/methanol solution was fed into the preheater by a syringe pump and mixed with therequired amount of H,. The liquid products were condensed and analyzed by an agilent 6820 GC equipped with anINNOWAX capillary and fame ionization detector (FID)2. Results and discussionAll the catalysts show a type IVisotherm( Fig. la)typical of particulate adsorbents with low porosity in the range ofmicro-and mesopores [13]. The BJH plots reveal that there are three kinds of pore size distribution in the catalystsprepared in one phase solution, with the pore size of about 3.5, 10, 70 and 200 nm, respectively, which can be attributedto the framework mesopore, textural mesopore and textural macropore, respectively [14, 15]. It is known that thetextural pore would facilitate mass transport to the framework pore and thus indicate a potentially excellent catalyticactivity of the catalyst with multimodal pore size distributionTable 1 summarizes the physicochemical properties of the catalysts after calcination. It is found that, withincreasing the W/E ratio, the BEt surface area and pore volume increased, and the average pore diameter decreased.However, the tendency changed with the further increases in the W/E ratio to the absence of ethanol, suggesting thatTEOS hydrolysis in one phase solution lead to a larger BET surface area and pore volume than that of TEOS/H20 twophases On the other hand, TEOS hydrolysis in different W/Eratios has no clearly effect on the Cu particle size, and theparticle size is about 4-5 nm according to TEMV中国煤化工CNMHGFig. I. N2 adsorption-desorption isotherms(A)and coresponding BJH pore size distributions(B)for Cu/Sio catalysts after calcination:(a)CuSio-W1oo:(b)CuSio-W7sEas;(c)CuSio-Ws(d)CuSio-W2s EnsSR Wang ef aL/Chinese Chemical Letters 22 (2011)362-365Table 1Physicochemical properties of the Cu/Sio catalysts after calcinationsSBeT(m/g(cmis)CuSio-W2sErs4.5CuSio-WsoEso129CuSio-W7sE2s21.61964.16.100284.7Cu particle size determined by TEM.The morphological differences between the CuSio-WisE2s and CuSio-W1o0 catalysts are readily visible fromSEM(not show here). For CuSio-Wns Es catalyst prepared in one phase solvent, silica is constituted by polydispersedspherical particles. While for CuSio-Wloo catalyst prepared in TEOS/H2O two phases condition, silica is constructedby sticky-linked plate-like particles stacking, indicating that the addition of ethanol as the co-solvent, i.e. One phasesolvent, lead to increase in the TEOS hydrolysis rate.Fig. 2 depicts the XRD patterns of calcined and reduced Cu/SiO2 catalysts. For the catalysts after calcination, onlya broad peak assignable to amorphous SiO2 at 20 of around 22 was found, indicating that Cu was highly dispersed onSiO, and was too small to be detected, which is in accordance with the result of TEM. For the catalysts after reduction,two weak diffraction peaks at 28 of 43.30 and 50. 4@ characteristic of fcc Cu (CPDS 04-0836)were detected. The Cucrystallite size is 3 and 4 nm for CuSi-W7sE2s and CuSi-W1oos respectivelyFig. 3 shows that Cu/SiO2 catalysts prepared in one phase solvent exhibited high activity owing to the highlydispersed Cu species [16], the conversion of DMO reached to 100%under the identical reaction conditions. It also showsthat, with increasing the W/E ratio from 1: 3 to 3: 1, the EG selectivity increased to 95%, and the ethanol selectivitydecreased. Interestingly, the absence of ethanol lead to the catalytic activity and EG selectivity decrease sharply,indicating the reduction of BET surface area and single modal pore distribution not facilitate the DMo hydrogenation,Fig. 4 compares the catalytic performance of the CuSi-W7sE2s and Cusi-W1oo catalysts versus reactiontemperature. With the increase of reaction temperature, the DMO conversion increased monotonously on the Cusi-W1oo catalyst. While for CuSi-W7sE2s catalyst, the catalytic activity reached 100% under all the identical reactionconditions. On the other hand, the CuSi-WIsE2s catalyst exhibited 90-95% EG selectivity at 488-503 K, whereascatalyst prepared by two phase hydrolysis reached 90% EG selectivity only at 498-503 K, confirming the bettercatalytic performance of the catalyst prepared in one phase solution with the ethanol as the co-solvent.In conclusion, the Cw/SiO2 catalyst in situ synthesized in one phase solution using ethanol as co-solvent is superiorto that of TEOS/H20 two phases, i.e. the absence of ethanol. On the Cusi-WrsE2s catalyst, a DMO conversion of100% and EG selectivity of 90-95% was obtained at the temperature of 488-503 K. The higher catalytic activity and中国煤化工CNMHGterms of the calcined and reduced Cu/SiOz catalysts:(a)CuSio-W1oo; (b)CuSio-W7sE2s: (c)CuSiO-WsoEso; (d) CuSio-W2segs(e)CuSi-W7sE2s and (f) Cusi-lS.R. Wang et aL / Chinese Chemical Letters 22(2011)362-365365e DMO Cod--Ehaol se←EGsFig. 3. The catalytic performance of the Cu/SiO2 catalysts prepared in different W/E volume ratio solution in the hydrogenation of DMO to EG at503 K. 2 MPa, H/DMO molar ratio of 260 and LHSV of 0.8h-.Fig 4. The catalytic performance of the CuSi-WisE2s(solid symbol)and CuSi-Wroo(hollow symbol)catalysts versus reaction temperature in thehydrogenation of DMO to EG 2 MPa, H/DMO molar ratio of 260 and LHSV of 0.8hEG selectivity on catalyst prepared in one phase solution is attributed to the higher BEt surface area and multimodalpore distribution facilitate the mass transport and increase the amounts of the active sites probably, and as a resultimprove the catalytic performance, since the absence of ethanol has no clear effect on the Cu particle dispersionAcknowledgmentThis work was supported by the International Science and Technology Cooperation Program (No.2009DFA61050), National High Technology Research and Development Program of China(863 program)(Nos2007AA05Z334& 2009AA05Z407), and National Basic Research Program of China(No. 2007CB210200References[1]T Susumu, F. Kozo, N. Keigo, et al. EP 0046983( 1981)[2] G.S. Chen, B. Xue, H.M. Yan, et al. Chin J Catal. 13(4)(1992)29[3] G H. Xu, Y.C. 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