Promoting effect of polyoxyethylene octylphenol ether on Cu/ZnO catalysts for low-temperature methan

Available online at www.sciencedirect.comJOURNAL:OFScienceDirectNATURAL GASCHEMISTRYELSEVIERJournal of Natural Gas Chemistry 18(2009) 375- -378www.elsevier.com/locate/jngc .Promoting effect of polyoxyethylene octylphenol ether on Cu/ZnOcatalysts for low-temperature methanol synthesisLing Liu，Tiansheng Zhao*，Qingxiang Ma, Yufang ShenKey Laboratory of Energy Resources & Chemical Engineering of Ningxia, Ministry-Province Co-Cuitivated State Key Laboratory Base,Ningxia University, Yinchuan 750021, Ningxia, China[ Received December 29, 2008; Revised July 9, 2009; Available online August 21, 2009 ]AbstractCu/ZnO catalysts were prepared by the co precipitation method with the addition of OP- 10 (polyoxyethylene octylphenol ether) and werechemically and structurally characterized by means of XRD, BET, H2-TPR, CO-TPD and N2O-titration. The effect of OP- 10 addition on theactivity of Cu/ZnO for the slurry phase methanol synthesis at 150 °C was evaluated. The results showed that Cu/ZnO prepared with additionof 8% OP- 10 (denoted as C8) exhibited the promoted activity for the methanol synthesis. The conversion of CO and the STY (space timeyield) of methanol were 42.5% and 74.6% higher than thosepf Cp/ZnO prepared pvithout addition of OP-10 (denoted as C), respectively. Theprecursor of C8 contained more aurichalcite and rosasite,nd the concerted CEct of Cu-Zn in C8 was found to be stronger than that in CO.Compared with CO, C8 showed smaller particle size, lowerKey words .surfactant; Cu/ZnO; low-temperature methanol synthesis1. Introductionway of methanol synthesis at 443 K and under 50 bar fromCO2-containing syngas was reported [6- 10] as the follow-Methanol is an important bulk chemical, a kind of trans-ing steps (3)- (7), where Cu represented the catalytic site ofportation fuel and energetic material for fuel cells. DurativeCu/ZnO and ROH is the accompanying alcohol. CO2 andattention has been paid to methanol synthesis from syngas atH2O were cycled during the reaction.low temperature owing to thermodynamic advantage. BNLCO+ H2O→CO2+ H2(3)reported an effective route for low-temperature methanol syn-thesis, however, syngas needed to be free of CO2 and H2O inCO2 + 1/2H2 + Cu→HCOOCu(4order to maintain the activity of the basic catalysts. Methanolsynthesis at low-temperature via concurrent two steps of car-HCOOCu + ROH→HCOOR + CuOHbonylation and hydrogenolysis as shown below was inten-sively studied[1-3].HCOOR + 2H2→ROH+ CH;OHCH3OH + CO→HCOOCH3CuOH + 1/2H2→H2O + Cu(7)HCOOCH3 + 2H2→2CH3OHIn all CO+ 2H2→CH3OHHomogeneous alkali metal methoxides and modifiedmetal copper are used as catalysts for steps (1) and (2), re-Conventional preparation of Cu/ZnO is via cospectively. Alkali metal methoxides are vulnerable to CO2 precipitation of metal nitrates by bases, and some copper-and H2O, therefore, syngas is required to be free of both CO2 and zinc-containing phases are produced, and CuO/ZnO mix-and H2O or with low content of them, which causes high costtures were formed after calcination. When the precursor con-of syngas preparation.tained more aurichalcite (CuZn)s(CO3)2(OH)6 and rosasite(CuZn)2CO3(OH)2, Cu and ZnO showed homogenous distri-synthesis and is also used in water-gas shift reaction [4,5].bution and stronger concerted interaction. More active centersWith the aid of alcohol solvents, a novel and more practicalthen generated through thermal decomposition [11,12]. The* Corresponding author. E-mail: zhaots @ nxu.edu.cnThis work was supported by the Chinese Ministry of Science & Technology (2005CCA000), Spr中国煤化工。n for New CenturyExcellent Talents in University (NCET 08-0872) from the Chinese Ministry of Education are acknowledgMHCNM HGCopyright@2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi:10. 1016/S1003-9953(08)60121-8376Ling Liu et al./ Journal of Natural Gas Chemistry Vol. 18 No. 32009interaction between Cu and ZnO [13] along with Cu surfaceXco(%) =Aco(im)/Arin) - Aco(ou)/Ar(ou) x 100 .area [14] played a vital role in the development of catalystsAco(in)/Ar(i)with high activity. One of the crucial points for Cu/ZnO prepa-ration was to produce the fine interdispersion of the metallicXco2(%) =Acor(n)/Ar(in) - AcO2(oun)/AAr(ou)x 100elements [15].Aco2(in)/Ar(i)In recent years, several researchers focus on Cu-basedAco(in)catalyst preparation with high surface area, fine particle andXTotalc(%)= XcoXAco(in) + Aco2(in)high dispersion in order to improve the activity for methanolsynthesis. Bao et al. reported that Cu/ZnO prepared by+Xco2 xsol-gel method exhibited large surface area, even elementAco(in) + AcO2(in)distribution and various mesoporous sizes in the range ofNCHsOH-x1002- 10 nm [16]. However, fine particles with high surface ar-Scri:OH(%)=nirH.OH+noyby-productseas are instable and tend to agglomerate. Chang et al. pre-Where, X,A,S and n stood for the conversion, peak area,pared ultrafine Cu/ZnO/Al2O3 catalyst via the promoted co-selectivity and molar number of reactants or products, respec-precipitation by using surfactants. The resulting catalysts pos-sessed smaller crystallite size and showed higher catalytic ac-tively.tivity in methanol synthesis [17,18].In this work， Cu/ZnO was prepared by the co-2.3. Catalyst characterizationprecipitation with addition of surfactant (OP- 10). The effectsof preparation and the additive on the activity for methanolPowder X-ray diffraction (XRD) analysis was carried outsynthesis at low temperature were investigated. The structureon a Rigaku D/Max -2200 using Cu K a radiation at a scanningof the catalysts was characterized by means of XRD, BET,speed of 8 0/min. BET surface areas were determined on anH2-TPR, CO-TPD and N2O-titration.ASAP 2010M nitrogen automatic adsorption instrument. Cat-alysts were pretreated at 250 °C for 6 h. H2 TPR was carried2. Experimentalout on a TP- 5000 system.50 mg of CuO/ZnO was placed in aquartz reactor and pretreated at 350 °C for 1 h in stream of He.2.1. Catalyst preparationThen it was heated linearly at a rate of 5 C/min from roomtemperature to 400 °C with H2 flow of 5% (30 ml/min). ForAn aqueous solution of Cu(NO3)2-6H2O andCO-TPD, 200 mg of CuO/ZnO was firstly reduced by 5% H2Zn(NO3)2-6H2O with Cu/Zn molar ratioof 1 : 1 and an aque-at 240 °C for 2h. Finally, it was pretreated at 350°C for 1 hous solution of sodium carbonate with the desired surfactantin stream of He and pulse-adsorbed by CO at 150 °C until sat-were simultaneously added to distilled water under vigoriousuration. Then it was heated at a rate of 10°C/min from roomstirring at 65°C and pH of 8. The precipitates were agedtemperature to 600°C. Cu specific surface area and particleovernight at room temperature, after that washed with 65 °Cdispersion were determined by N2O adsorption according todistilled water, then dried at 120 °。C for 6 h, followed by cal-the reaction: 2Cu(s) + N2O(g)-→Cu2O(s) + N2(g). 200 mgcination in air at 350°C for 1 h. The precursors pressed intoof CuO/ZnO was pretreated as the CO-TPD procedure and20- 40 meshes were reduced in 5% H2 (50 m/min) at 240°Cpulse saturated by N2O in stream of He (40 ml/min), and thefor 10 h, and then passivated by 1% 02. The catalysts withouteffluent gas was synchronously analyzed by TCD.or with addition of OP- 10 were denoted as Cn,n=0,2, 5, 8,Dcu(%) = N(ecalCoc)where n stands for the weight percentage of added OP- 10.Acu(m2/g) = 4nr2Qou(foc)/w2.2. Activity testsWhere, D, Nusuface), Na(otal), Acu, r and w stood for theA flow-type semi-batch autoclave reactor with an innerdispersion of Cu, molar number of the surface, total Cu atom,volume of 100 ml was employed to test the catalytic activ-surface area of Cu, the radius of Cu atom (0.1278 nm) and theity, which connected to a high-pressure gas flow controllingsample weight, respectively.system and an auto- analysis system. Typical test conditions3. Results and discussionwere as follows: Vco/Vco,/Vq/VAr = 31.51/2.00/63.49/3.00;reaction temperature, 150 °C; reaction pressure, 5.0 MPa andstirring speed, 850 r/min.4g catalyst in 40 ml ethanol was3.I. Activity of the catalystsground and transferred into the reactor. The reactor waspurged three times by feed gas before the reaction. TheThe methanol synthesis activities at 150°C of Cu/ZnOeffuent gas was analyzed by an on-line TCD (GC-920) with prepared with and withp. dlitinn of AP are shown ina 2 m AC column and the liquid products analyzed by TCDTable 1. Cu/ZnC中国煤化Iitation without(GC-16A) with a 2 m Porapak Q column (with n-propanol asOP addition showMHCNMHGnand100%ofinternal standard). Result calculation was as follows:methanol selectivity. Compared with the lterature [7] results,Journal of Natural Gas Chemistry Vol. 18 No.3 2009377Table 2. XRD intensity of CuO/ZnO precursorstive conversion of CO2 indicated that CO2 was generated dur-MalachiteAurichalciteRosasiteCatalysting the reaction related to water-gas shift reaction. The hydro-34.2generation of CO2 (as shown in the reaction (5)) might be notCO3040.04106.62466.6fast enough at lower reaction temperature.C83018.34431.62698.2When the surfactant OP was used in the catalyst prepa-ration, the CO conversion and STY of methanol were in-Figure 2 shows the XRD patterns of the reduced CO andcreased. As the content of OP- 10 increased from 2% to 8%,C8. After the reduction, both CO and C8 produced ZnO phasethe CO conversion and STY of methanol of C8 were 42.5%(20 = 31.2° (100), 34.40 (002) and 36.3° (101)) and metal Cu .and 74.6% higher than those of CO, respectively. In addition,phase (20 = 43.30 (111), 50.40 (200) and 74.50 (00)). Thethe selectivity of methanol was 100% and the CO2 conversion calculated average sizes of Cu metal particles of CO and C8similar to that of CO, indicating the OP- 10 addition enhancedwere 13.9 nm and 2.4 nm, respectively, indicating that C8the synthesis reaction while the performance of catalysts re-showed smaller particle size and better Cu dispersion.quires further improvement.Table 1. Activity of Cu/ZnO with OP-10 additionConversion (%)Selectivity of STY of CH3OHCo CO2 TotalC CH;OH(%) (g.kg-'h-l)_， CuC17.9- 49.2 1 2.91022.9- 65.4 16.127.924.2- - 51.318.6_C25.5 -41.6 20.534.4E||C83.2. The phase of the catalystsThe XRD patterns of CO and C8 precursors are2(314(67(8①shown in FigureBoth C0 and C8 showed the20/(° )feature diffractive peaks of three phases:malachite,Figure 2. XRD patterns of Cu/ZnOCu2(CO3)2(OH)2, 2θ= 24.30, 32.40 and 36.20; aurichalcite,(Cu, Zn)s(OH)6(CO3)2, 20 = 13.0, 27.40 and 34.10; and ros-asite, (Cu, Zn)2CO3(OH)2, 20= 29.90.3.3. H2-TPR and CO-TPD of the catalystsThe H2-TPR profiles of CuO/ZnO (C0 and C8) are shownin Figure 3. Both C0 and C8 produced two H2-consumptionCu2(CO)2(OH)2peaks. The low-temperature peak was assigned to the re-， (Cu,Zn),CO(OH)2duction of surface CuO, while the high-temperature pealwas assigned to the reduction of bulk CuO [19]. The high-temperature peaks for C0 and C8 located at 200 °C and184 °C, respectively. The former was 16 °C higher than thelatter, indicating that with the addition of OP- 10 in the prepa-ration, CuO/ZnO was easily reduced.wwMMwwiw8200Figure 1. XRD patterns of CuO/ZnO precursors;CThe peak intensities of C0 and C8 precursors are shown inTable 2, where the intensity I2o was defined as the multiply-ing of peak height and half peak width. C8 showed more au-中国煤化工二richalcite than CO. Aurichalcite can decompose into copper-MHCNMHG50 400zinc alloy, which is the main crystal phase of highcatalysts [14,121030" 4050 Figure 3. H2-16Qofles of CuO/Zno7 0802秒/(o)378Ling Liu et al./ Journal of Natural Gas Chemistry Vol. 18 No. 3 2009of methanol of C8 were 42.5% andThe reduced CO showed two desorption peaks of CO at around of CO, respectively. The precursor of C8 contained more au-237 °C and 466 °C, while, the reduced C8 produced three des-richalcite and rosasite phase, and the concerted effect of Cu-orption peaks at around 70°C, 136°C and 438 °C, respec-Zn in C8 was found to be stronger than that in CO. With the .tively. The low-temperature and high- temperature desorptionuse of OP- 10 in the Cu/ZnO preparation, C8 showed smallerpeaks of C8 were lower than those of C0, but C0 showedparticle size, lower reduction temperature, higher Cu disper-higher peak areas and total area of desorption peak than C8.sion and specific surface area, which were responsible for theThese results suggested that the OP-10 addition reduced theimproved activity in methanol synthesis.adsorption intensity of Co of the catalysts and lowered theCO adsorption amount [19].AcknowledgementsFund supports from the Chinese Ministry of Science & Tech-nology (2005CCA00700), Spring Scenery Plan (2006) and Program66for New Century Excellent Talents in University (NCET-08-0872)from the Chinese Ministry of Education are acknowledged.References[1] Liu Z, Tierney J W, Shah Y T, Wender I. Fuel Process Technol,371989, 23(238[2] Palekar V M, Jung H, Tierney J w, Wender I Appl Catal A,8入70.136[3] Ohyama s. 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Ranliao HuaxueCu/ZnO prepared by co-precipitation method with the ad-Xuebao (J Fuel Chem Technol), 2005, 33(4): 479dition of appropriate OP-10 improved the activity in the low- [19] Huang L H, Chu w, Long Y, CiZ M, Luo S z. Catal Lelt, 2006,temperature methanol synthesis. The CO conversion and STY108(1-2): 113中国煤化工3 00HCNMH G、600010040U5U0Temerat。Q