Study of CO2 Hydrogenation to Methanol over Cu-V/γ-Al2O3 Catalyst

Avilable online at www.sciencedirect.comScienceDirectJoumal of NaturalGas ChenisryJournal of Natural Gas Chemistry 16(2007)12-15SCIENCE PRESSArticleStudy of CO2 Hydrogenation to Methanol overCu-V/r-Al2O3 CatalystYiping Zhangl.2*，Jinhua Fei'，Yingmin Yu，Xiaoming Zheng'1. Institute of Catalysis, Xizi Campus, Zhejiang University, Hangzhou 310028, Zhejiang, China;2. Department of Chemistry, Zhejiang Education Institute, Hangzhou 310012, hejiang, China[ Manuscript received September 4, 2006; revised November 3, 2006 ]Abstract:The effect of vanadium addition to Cu/~-Al2O3 catalyst used in the hydrogenation of CO2to produce methanol was studied. It was found that the catalytic performance of the Cu-based catalystimproved after V addition. The influence of reaction temperature, space velocity and the molar ratio ofH2 to CO2 on the performance of 12%Cu-6%V/r-Al2O3 catalyst were also studied. The results indicatedthat the best conditions for reaction were as follows: 240 °C, 3600 h-1 and a molar ratio of H2 to CO2of 3:1. The results of XRD and TPR characterization demonstrated that the addition of V enhancedthe dispersion of the supported CuO species, which resulted in the enbanced catalytic performance ofCu-V/~-Al2O3 binary catalyst.Key words: CO2; hydrogenation; methanol; copper; vanadium1. Introductionhydrogenation of CO2 [7]. In this work, we studiedthe influence of vanadium addition on the catalyticCarbon dioxide can be regarded as the most abun-performance of Cu/y-Al2O3 as a catalyst used for hy-dant carbon source on earth. The massive dischargesdrogenation of CO2 to produce methanol.of CO2 have not only degraded the resources but alsopolluted the environment, causing the global green-2. Experimentalhouse effect, therefore, the utilization of CO2 hasdrawn considerable interests in recent years.2.1. Catalyst preparationAs the important C1 chemical product, methanolcan be used not only as fuel but also as organic rawCopper-based catalysts used in this work con-material. Recently, because the scopes of methanolsisted of 12wt% Cu [8] with varying loadings of Vutilization are widened day by day, people have paidsupported on ~-Al2O3. The catalysts were preparedmore and more attention to its synthesis. In the stud-by impregnating r-Al2O3 support (20 - 40 mesh) withies related to CO2 fixation, it is one of the focusesaqueous solutions of vanadyl oxalate and copper ni-to synthesize methanol directly from CO2 and H2.trate, respectively. Then the samples were dried atSeveral catalysts have been developed for synthesis of110 °C, followed by calcination at 500°C for4 h. Themethanol at low temperature, such as Cu-based cat-catalysts that were obtained were named as 12%Cu-alyst to which are added Zn, Cr, Mn, Ce, Mg, Ga,x%中国煤化工cent of copper andetc [1-6]. Beltramini et al. reviewed the recent ad-vanaYH.CNMHGwithrespettothevances in catalysts for the synthesis of methanol viaweiguu Jt 1-21r2U3 ouppuiuj.。Corresponding author. Tel: 0571-88213093; E-mail: zhangyiping9578@yahoo.com.cnThis work is supported by the Natural Science Foundation of Zhejiang Province (No. 405088)Journal of Natural Gas Chemistry Vol. 16 No. 1 200732.2. Catalytic reaction of CO2 hydrogenationity results of Cu-based catalysts with various V load-ings. The reaction was carried out at T-=240 °C,The reaction of CO2 hydrogenation to formP=3.0 MPa, GHSV= 3600 h-1 and H2:CO2= 3:1 (mo-methanol was carried out in a high pressurelar ratio). The carbon-containing products that weremicroreactor -chromatograph system (MRCS 8004).detected are CO, CH4 and CH3OH. It can be seenFor each experiment, 2 ml catalyst was pretreated byfrom Figure 1 that the conversion of CO2 increasedH2 at a temperature of 300 °C for 3 h before thefirst, then decreased with the increase of V loading,reactant gas was fed in. Analyses of product mix-whereas the selectivity for CH:OH increased all along.tures were carried out in an on-line HP5890-II modelThe highest yield of CH3OH was obtained over thegas chromatograph with thermal conductivity detec-12%Cu-6%V/~-Al2O3 catalyst.tor (TCD), in which Porapak Q was used to separatethe products.3.2. Effect of reaction temperature on CO2hydrogenation over 12%Cu-6%V/r- Al2O3 cat-2.3. Catalyst characterizationalystMeasurement of XRD (X-ray diffraction) wasThe effect of reaction temperature on the bydro-conducted by using a Thermo ARL X'TRA X-raygenation of CO2 over 12%Cu-6%V/r- Al2O3 catalystpowder diffractometer with CuKo radiation foris shown in Figure 2. In our experiment, there was noanalysis of the crystalline phase.involvement of methanol but some CO and CH4 wereH2-TPR was performed by using a AMI-2000 cat-produced when the reaction temperature was 180 °C.alyst characterization system. A total of 150 mg sam-This is because the temperature was too low to acti-ple was pretreated in Ar at a temperature of 100 °Cvate and dissociate the C-0 bond of CO2. Figure 2for 30 min, then the reducing gas of H2-Ar (5:95) mix-clearly shows that along with the elevation of reac-ture was introduced, and finally, the temperature wastion temperature, the CO2 conversion also increasedprogrammed so as to rise the temperature at a rampunceasingly but the CH3OH selectivity was reducedrate of 10 °C/min to 750 °C. The amount of H2 con-over 12%Cu-6%V/~-Al2O3 catalyst. Therefore, thesumned was measured by a thermal conductivity de-catalyst showed the highest yield of CH3OH at thetector (TCD).reaction temperature of 240 °C. Because the reactionof CO2 and H2 to produce CHgOH is an exothermic3. Results and discussionreaction(AH298K=- 49.43 kJ/mol), raising tempera-ture is unfavorable for the production of CH3OH.3.1. Effect of V loading of the 12%Cu/r-Al2O3catalyst on CO2 hydrogenation70Figure 1 shows the catalytic activity and selectiv-80CO, conversion- + co setivity_。CH sletivity6040T.CH- CH,OH setivit- + CO2 conversionCo seletivity台3010- CH4 selectivity- t CH2OH slectivity208280中国煤化工(C)Content of V (wt%)Figure1. Effect of V loading of the 12%Cu/r-FigurCNMHGratureonCO2hy-Al2Og catalyst on COz hydrogenationarogenation over 1270Uu-6%V /7- Al2OgReaction conditions: temperature 240 °C, pressure 3.0 MPa,Reaction conditions: pressure 3.0 MPa, GHSV=3600 h~1,GHSV 3600 h-1, H2:CO2=3:1(molar ratio)H2:COz=3:1(molar ratio)14Yiping Zhang et al./ Journal of Natural Gas Chemistry Vol. 16 No.1 20073.3. Effect of space velocity on CO2 hydro-CO and CH4 are produced as the by-products asgenation over 12%Cu-6%V/r-Al2Og catalystshown in Equations 2 and 3:CO2+H2→CO + H2O(2)Figure 3 shows the experimental results of hydro-CO2+4H2→CH4+2H2O(3)genation of CO2 over 12%Cu-6%V/r-Al2O3 catalystunder various space velocities. The experiment wasThe coefficient ratio of H2 to CO2 in reaction (1)carried out at a temperature of 240 °C, a pressureproducing CH3OH is 3:1, therefore, the maximum se-of 3.0 MPa, and the molar ratio of H2 to CO2 oflectivity for CH3OH is achieved at a molar ratio of3:1. The results indicated that along with the riseH2 to CO2 of 3:1. And the coefficient ratio of H2 toof space velocity, conversion of CO2 and selectivity ofCO2 in reaction (2) producing CO is 1:1, whereas itCO were reduced, whereas the selectivity for CH3OHis 4:1 in reaction (3) producing CH4，therefore, thewas increased. The yield of CH3OH was highest when selectivity for CH4 increases, whereas the selectivityGHSV was 3600 h-1.for CO decreases with the increase in the molar ratioof H2 to CO2. To explain the effect of the addition of区V to the Cu/r-Al2O3 catalyst on the catalytic perfor-60mance of CO2 hydrogenation, physical and chemical- -CO, conversioncharacterizations, such as XRD and TPR were used.soE- + CO selectivity十CH sletivity10040Fτ CH,OH seetivityt口CO, conversion3080z0 CO selectivity5 CH, selectivityCH2OH secivity20上4(1800360054007200Space velocity (h^ ')Figure 3. Effect of space velocity on CO2 hydro-genation over 12%Cu-6%V /~-Al2Og cat-8talystReaction conditions: temperature 240 °C, pressure 3.0 MPa,1: I3: 15: 110: 1H2:CO2=3:1 (molar ratio)Molar ratio of H,to CO2Figure 4. Effect of molar ratio of H2 to CO2 on reac-tion behaviors over 12%Cu-6%V/~-Al2Og3.4. Effect of molar ratio of H2 to CO2 on theReaction conditions: temperature 240 C, pressure 3.0 MPa,reaction behavior over 12%Cu-6%V/r-Al2OzGHSV 3600 h-catalyst3.5. XRD analysisFigure 4 shows the effect of molar ratio of theThe XRD patterns of Cu-V/r-Al2O3 catalystsreactant gas on catalyst activity and selectivity overwith various V loadings are shown in Figure 5. The12%Cu-6%V/r-Al2O3 catalyst. From Figure 4, it isdiffraction peaks that were detected at about 37°,seen that when the molar ratio of H2 to CO2 in-creased from 1:1 to 10:1, the conversion of CO2 also469, and 670 repectively over all the catalysts wereincreased. The selectivity for CH3OH was the highestascribed to Al2O3. Obvious diffraction peaks of CuOat H2:CO2=3:1, subsequently it was reduced to somewere observed for the original 12%Cu/r-Al2O3 cat-extent. It can also be seen that with the increase inalyst. With the addition of V and the increase in Vmolar ratio of H2 to CO2, the selectivity for CH4 ele-loadings, the CuO diffraction peaks vanished, whereasvated unceasingly whereas the selectivity for CO wasthe diffraction peaks of V2Os were not observed, andonly weak diffraction lines of Cu2 V2O7 phase werereduced.The reaction of CO2 hydrogenation to CH3OH ishau)bsu中国煤化工he additionof V en-described in Equation (1):YHCNM H G_whereas V becameoratedinto copperto form a new phase over Cu-V binary oxide cata-CO2+3H2→CH3OH+H2O( 1 l) lysts. We believe that these are the factors associatedJournal of Natural Gas Chemistry Vol. 16 No.1 200715with better performance of the Cu-V/y-Al2O3 cata-observed on profile (5) of 6%V/r-Al2O3 catalyst.lysts compared to that of Cu/r-Al2O3. This can be :Two reduction peaks were observed over binary Cu-illustrated by the H2-TPR analysis of the catalysts.V/r-Al2O3 catalysts (profiles 2，3， 4)， and thepeak positions lay between the reduction peak ofCuO species over the original Cu/~-Al2O3 and V2Osspecies over the V/r-Al2O3 catalyst. Apparently, the= Cu2V,O,position of the lower temperature peak shifted to thehigher temperature with the increase of V loadings,and the peak area became larger. And the intensityof the higher temperature peak decreased with the in-crease of V loadings. When the results of XRD werecombined, we infer that the two peaks can be assignedto the highly dispersed CuO and the newly formedCu2V2O7 species, respectively.4. ConclusionsAfter addition of V, both the catalytic activity ofCu-based catalysts and the selectivity for methanol3(407080were improved. The best conditions for reaction for20/(0 )Figure 5. XRD patterns of Cu-V/r-Al2O3 catalyststhe hydrogenation of CO2 were as follows: 240 。C,with various V loadings3600 h-1 and a molar ratio of H2 to CO2 of 3:1. The(1) 12%Cu/r-Al2O3, (2) 12%Cu-3%V/r- Al2O3, (3) 12%Curesults of XRD and TPR showed that the addition of6%V/~Al2O3, (4) 12%Cu-9%V/r-Al2O3V was beneficial in enbancing the dispersion of thesupported CuO species, which was responsible for the3.6. H2-TPR analysisenchanced catalytic performance of Cu-based catalystIn the TPR profiles of CuV/r- Al2O3 catalystsfollowing V addition.with various V loadings (Figure 6), two reductionAcknowledgementspeaks were observed on profile (1) of 12%Cu/~-Al2O3The authors wish to express their gratitude to Dr Yi-catalyst, and they were ascribed to the reduction offan Zheng from Zhejiang University of Technology for per-the highly dispersed CuO species and the bulk-likemitting us to use the XRD facility.CuO phases [9], which were denoted as a and β peak,respectively. 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