Methanol Synthesis From Co2

Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

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JIEC-2432; No. of Pages 7

Methanol synthesis from CO2 hydrogenation over Cu/g-Al2O3 catalystsmodified by ZnO, ZrO2 and MgO

Hong Ren, Cheng-Hua Xu *, Hao-Yang Zhao, Ya-Xue Wang, Jie Liu, Jian-Ying Liu

Air Environmental Modeling and Pollution Controlling Key Laboratory of Sichuan Higher Education Institutes, Chengdu University of Information Technology,

Chengdu 610225, PR China

A R T I C L E I N F O

Article history:

Received 17 October 2014

Received in revised form 2 March 2015

Accepted 2 March 2015

Available online xxx

Keywords:

CO2 hydrogenation

Methanol

Cu-based catalyst

Modification

A B S T R A C T

Cu/g-Al2O3 catalysts for methanol synthesis from CO2 are prepared and modified by metal oxides via

impregnation in the present work. Results indicate that promoters modification leads to the formation of

small Cu0 particles with a high dispersion, improves catalytic performance of Cu-based catalysts in

methanol synthesis. Moreover, the activation temperature is another important factor on affecting the

Cu0 dispersion and particle size. The investigation on CO2 hydrogenation shows that methanol is mainly

from hydrogenation of activated CO2 with active hydrogen on Cu0 particles, which is inhibited by high

reaction temperature due to improvement on reverse water-gas shift reaction and methanation.

� 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

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Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

Introduction

Carbon dioxide (CO2) has been considered as one of the maingreenhouse gases, and its increasing emissions arguably lead to theglobal warming and climate changes. Simultaneously, the emittedCO2 is also a cheap, nontoxic and abundant C1 feedstock. Therefore,the utilization of CO2 as a raw material in the synthesis of valuablechemicals has attracted more and more attentions as a technologyto control its emissions, but also provide a grand challenge inexploring new concepts and opportunities for catalytic andindustrial development [1–3]. Among them, CO2 catalytic hydro-genation to methanol (MeOH) has been recognized as one of themost effective and economical ways to fix and utilize the emittedCO2. It is mainly because that MeOH can be used as a fuel additiveor clean fuel, also converted as a starting feedstock to high-octanegasoline, aromatics, olefins and other chemicals such as formalde-hyde, acetic acid and methyl tertiary-butyl ether so on [1–3].

As well known, Cu-ZnO-Al2O3 catalysts have been widelyapplied in MeOH synthesis from syngas [4–6]. In Cu-ZnO-basedcatalysts, ZnO is regarded to provide active sites for hydrogenspillover, or as a structure-directing support controlling thedispersion of metallic copper particles [7–9]. The synergeticinteractions of Cu and ZnO can improve the hydrogenation of

* Corresponding author. Tel.: +86 28 85967101; fax: +86 28 85966089.

E-mail address: [email protected] (C.-H. Xu).

Please cite this article in press as: H. Ren, et al., J. Ind. Eng. Chem. (

http://dx.doi.org/10.1016/j.jiec.2015.03.001

1226-086X/� 2015 The Korean Society of Industrial and Engineering Chemistry. Publis

syngas containing H2, CO and a small amount of CO2. Therefore, thesimilar catalysts system has been also used in CO2 directhydrogenation to MeOH [10,11]. However, it has been found thatthe Cu-ZnO exhibits a poor activity for CO2 conversion and MeOHformation, the space-time yield of MeOH is less than9 g kgCat

�1 h�1 [11–13]. Other researchers [8,10,14,15] havediscovered that Cu-based catalysts supported on ZnO-Al2O3 mixedoxides exhibit a high catalytic activity for MeOH synthesis fromCO2. Moreover, it has been also found that the introduction of ZrO2

can further improve the Cu dispersion and catalytic performance ofCu-Zn-Al catalysts [16,17]. Therefore, ZnO-ZrO2 mixed oxides havebeen directly used as the supports for Cu-based catalysts, and theobtained catalysts exhibit an excellent catalytic performance inCO2 hydrogenation to MeOH [2,10,15,18,19].

Among the above investigations, it is generally agreed that theCu0 nano-particles in catalysts are active phase for CO2 hydro-genation, and the metal oxide supports can disperse the activecopper species on the surface of catalysts. And most of the reportedCu-based catalysts for CO2 hydrogenation are almost preparedthrough co-precipitation. Recently, Urakawa and co-workers [20]have adopt g-Al2O3 as support to prepare Cu-based catalysts via

impregnation and investigate the effect of promoters such as K andBa on the catalytic performance of catalysts in MeOH synthesis.The impregnation is a well-known process with an easy operationand controlling for catalysts preparation. It can distribute theintroduced components over the support surface with a lowamount and no loss. Therefore, the present work uses commercial

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H. Ren et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx2

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g-Al2O3 particles as support to prepare Cu-based catalysts via

impregnation. And the promoters such as ZnO, ZrO2 or alkalispecies MgO are introduced by the same method to furtherdisperse copper species in order to obtain the MeOH synthesiscatalysts with a high catalytic performance.

Experimental

Catalysts preparation

Cu/g-Al2O3 catalysts modified by ZnO, ZrO2 and MgO wereprepared via the impregnation method. Typically, ZrOCO3 was firstsolved in 10% nitric acid aqueous solution, in which the desiredamount of Cu(NO3)2�3H2O, Zn(NO3)2�6H2O Mg(NO3)2�6H2O andwater were then added to obtain about 165 ml mixture liquidcontaining Cu2+, Zn2+, Zr4+ and Mg2+ ions with a molar ratio of2:1:0.9:0.1. 150 g of dried g-Al2O3 (20–60 mesh) particles werethen impregnated in the above-obtained metal ions aqueoussolution for 2 h at room temperature. The used amount of metal Cuwas 10 wt% relative to the weight of support Al2O3. The obtainedsamples were dried at 80 8C in an oven, calcined at 600 8C for 6 h inair, and denoted as CZZMA. For comparison, other four catalystssuch as CZZA (ions solution contained 2Cu:1Zn:1Zr), CZrA(2Cu:1Zr), CZnA (2Cu:1Zn) and CA (only containing Cu) wereprepared according to the above method, and their Cu amount wasalso 10 wt%.

Catalysts characterization

BET Surface area and pore volume of samples were obtainedfrom N2 adsorption–desorption isotherms (�196 8C) on an SSA-4200 micromeritics instrument (Builder Co., Beijing). X-raydiffraction (XRD) patterns of catalysts were recorded on aDX-1000 powder diffractometer (Dandong Fangyuan Co., China)operated at 40 kV and 25 mA and a scan step of 0.06 8C min�1,using Cu Ka radiation (l = 0.15406 nm).

H2 temperature-programmed reduction (H2-TPR) of catalysts(0.2 g) was performed on a TP-5080 adsorption instrument(Tianjing Xianquan Co., China) with a 5% H2–N2 mixture gas(30 ml min�1) in the temperature range 20–800 8C at rate of10 8C min�1, the H2 consumption was monitored with a thermalconductivity detector (TCD). H2 or CO2 temperature-programmeddesorption (H2-TPD or CO2-TPD) characterizations of catalystswere performed on the above apparatus from 30 to 850 8C. 0.4 gsamples were first pre-reduced with H2 at 350 8C for 30 min, andthen cooled to room temperature, following that a H2 or CO2

stream was introduced for adsorption (30 min). After adsorption,the examined samples were flushed with Ar (H2-TPD) or He (CO2-TPD) stream (30 ml min�1) for 30 min to remove weakly adsorbedH2 or CO2, and at last they were heated from 20 to 700 8C at a rate of10 8C min�1. The desorbed H2 or CO2 was also detected by TCD.

The dispersion and metallic surface area of Cu were determinedby N2O-H2 titration on the same instrument as TPR. 0.2 g catalystswere first reduced in H2 for 1 h at 350 8C, then cooled to 90 8C in Ar

Table 1Physiochemical properties of Cu-based catalysts supported on g-Al2O3.

Samples BET surface areaa (m2 g�1) Pore volumea (cm3 g�1) Cu

g-Al2O3 230.7 0.51 –

CA 234.3 0.45 22.

CZrA 224.8 0.40 26.

CZnA 196.4 0.39 20.

CZZA 181.8 0.35 33.

CZZMA 189.7 0.40 67.

a Tested by N2 adsorption–desorption at �196 8C.b Calculated from N2O-H2 titration of catalysts reduced at 350 8C.


stream and isothermally exposed to 30 ml min�1 of N2O for 30 minto ensure complete oxidation of Cu0 to Cu+. The samples were thenflushed with Ar to remove the physically-adsorbed N2O, afterwhich a pulse of pure H2 was passed over samples to reduce thesurface Cu+. The dispersion, particle size and metallic surface areaof Cu on catalysts were calculated by quantifying the consumed H2

amount according to methods reported in references [10,21].

Catalytic test

Activity measurements of catalysts in CO2 hydrogenation wereperformed in a high-pressure fixed-bed reactor. 5 g catalysts(about 7.2 ml) were placed in a stainless steel tube reactor(øin = 12 mm). Prior to reaction, the catalysts were activated at adesired temperature in an 80 ml min�1 of pure H2 for 6 h underatmospheric pressure. The reactor was then cooled to roomtemperature. CO2 hydrogenation was carried out under reactionconditions of 230–310 8C, 16–32 atm, n(H2): n(CO2) = 3: 1, gashourly space velocity (GHSV) = 1400–5000 h�1. The steady-stateactivity measurements were taken after at least 8 h on the stream.The produced H2, CO, CH4 and CO2 were quantitatively analyzed byusing gas chromatograph (GC) equipped with a TCD and TDX-01column. MeOH in liquids were quantitatively analyzed by usinganother GC with an Agilent CAM capillary column(30 m � 0.32 mm � 0.25 mm film thickness) and flame ionizationdetector. CO2 conversion and carbon-based selectivity values forthe hydrogenated products such as CO and CH4 were calculated byan internal normalization method. The space-time yield of MeOH(STYMeOH) was defined as the amounts (grams) of MeOH producedper kg catalyst per hour.

Results and discussion

Physiochemical properties

N2 adsorption–desorption results (Table 1) of catalysts indicatethat CuO/g-Al2O3 (CA) has a similar BET surface area (234.3 m2 g�1)to support g-Al2O3 (230.7 m2 g�1), however its pore volume isdecreased. It shows that the introduced copper species first occupythe inner surface of support and further interact with supportstrongly to possibly form copper sites with a low catalytic activity.From Table 1, it is also observed that the introduced metal oxidespromoters such as ZrO2, ZnO and MgO can further decrease thesurface area and pore volume of catalysts.

N2O-H2 titration indicates that copper species on Cu/Al2O3

exhibit a low dispersion, it is possibly because that the stronginteraction between metal and support leads to the migration of afraction of introduced copper species into support structure tounreactive copper species. However, the introduction of promoterscan reduce this interaction, the dispersion and metallic surfacearea of Cu0 particles on catalysts surface are improved in someextent. Especially, the simultaneous modification of ZnO, ZrO2 andMgO will lead to the highest Cu0 dispersion (67.5%) and the biggestmetallic surface area (45.4 m2 g�1).

dispersionb (%) Cu0 particle sizeb (nm) Cu0 surface areab (m2 g�1)

– –

8 4.56 15.3

0 4.00 17.5

3 5.12 13.4

8 3.08 22.7

5 1.54 45.4



Fig. 1. XRD patterns of CuO/Al2O3 catalysts modified by different promoters.

0 100 200 300 400 500 600 700

CZZA

CZZMA

CZrA

CA

TC

D s

igna

l (m

V)

Heating temperature (oC)

CZnA

(a)

H. Ren et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx 3

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From XRD results (Fig. 1), it is found that the supports of allsamples exhibit g-phase Al2O3 (Pdf No. 1-1308). Both CuO/Al2O3

and CuO-ZnO/Al2O3 catalysts give weak diffraction signalscharacteristics of CuO species (Pdf No. 1-1117) at 2u of 35.88,37.48 and 49.18, respectively. However, the modification of ZrO2,ZrO2-ZnO and ZrO2-ZnO-MgO can make these diffraction peaksweaken or disappear. The presence of ZrO2 leads to thedisappearance of diffraction peaks characteristic of ZnO (Pdf No.1-1136) at 2u of 31.88. But no diffraction signals assigned to ZrO2 orMgO species are observed for CuO/Al2O3 samples modified by ZrO2,ZrO2-ZnO and ZrO2-ZnO-MgO. It is possible because that theintroduced MgO species is too low, and ZrO2 species are dispersedin the state of amorphous phase. No matter whether ZrO2, ZnO andMgO can be detected, it is certain that the modification of ZrO2,ZrO2-ZnO, and especially ZrO2-ZnO-MgO will promote the forma-tion of amorphous CuO species with a high dispersion. It is possiblydue to the fact that the introduced alkaline MgO first interacts withthe acidic sites of support g-Al2O3, and both ZrO2 and ZnO speciesact as dispersing agents. These can give rise to the decrease on theamount of CuO species strongly interacted with support.

H2-TPR profiles of five calcined CuO/Al2O3 catalysts (CA, CZnA,CZrA, CZZA and CZZMA) are presented in Fig. 2. For CuO/Al2O3, amain reduction peak and a wide shoulder peak are observed atabout 200 8C and 250 8C, respectively. The former peak is assignedto the reduction of the highly dispersed CuO phases [20,22,23], andthe latter one is attributed to that of CuO species located in thesupport structure due to the strong interaction between metal andsupport [20,24]. It is also discovered that the high-temperaturereduction peak becomes weak for samples modified by ZnO and

Fig. 2. H2-TPR profiles of the calcined CuO/Al2O3 catalysts modified by different

promoters.


ZrO2 (CZnA, CZrA), and even disappears for these modified byZnO-ZrO2 and ZnO-ZrO2-MgO mixed oxides (CZZA, CZZMA).However, the intensities of the low-temperature reduction peaksare increased slightly. It is further proved that the modification ofthese promoters can lead to the improvement on the dispersion ofCuO species on catalysts surface through decreasing the interac-tion between CuO and support, which prevents the migrationmetal Cu species into support structure. Simultaneously, thereduction temperature of the highly-dispersed CuO species forCZnA, CZrA and CZZA is slightly higher than that for CA. It ispossibly because that the introduced copper oxides exhibitaggregation phenomenon besides migration. And the peaksslightly shift to lower temperature as the following promotersorder: ZnO < ZrO2 < ZnO-ZrO2 < ZnO-ZrO2-MgO, and their peakintensities also increase successively. It indicates that themodification of promoters especially ZnO-ZrO2-MgO can give riseto the increase on the amount of highly-dispersed CuO species[16,25]. It is just the reason that CZZMA exhibits the highest copperdispersion and the biggest metallic Cu0 surface area, which is inaccordance with N2O-H2 titration results.

H2-TPD (a) and CO2-TPD (b) profiles of five catalysts arepresented in Fig. 3. From Fig. 3(a), it is found that all samplesdisplay a H2 desorption peak in the range of 30–100 8C, which isassigned to the desorption of atomic hydrogen adsorbed on thesurface of metallic Cu0 sites [16,26,27]. And another strong H2

desorption peak located in the range of 400–600 8C, is also discoveredfor the samples except Cu/Al2O3. It represents the desorption ofstrongly-adsorbed hydrogen on either the ZnO or ZrO2 surfacethrough spillover from Cu0 to promoters [16,26,28]. Meanwhile,

0 100 20 0 300 400 500 600 700 800

CA

CZrA

CZnA

CZZA

TC

D s

igna

l (m

V)


(b)

CZZMA

Fig. 3. H2-TPD (a) and CO2-TPD (b) results of CuO/Al2O3 catalysts modified by

different promoters.




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CZZMA gives two weak broader H2 desorption peaks in the range of100–180 8C and 185–310 8C, respectively. They are possibly fromdesorption of spilled-over hydrogen. These results imply thatCu/Al2O3 modified by ZnO-ZrO2-MgO will probably exhibit a goodcatalytic activity in MeOH synthesis from CO2.

From CO2-TPD results (Fig. 3(b)), it can be observed thatCu/Al2O3 only exhibits a CO2 desorption peak at about 80 8C, whichis assigned to the weakly-adsorbed CO2. The addition of promoterssuch as ZnO, ZrO2 or MgO increases the amount of weakly-adsorbed CO2, and also leads to the emergence of high-tempera-ture desorption peak in the range of 400–700 8C, which is assignedto the strongly-adsorbed CO2 on the strong basic sites[16,27,29]. From Fig. 3(b), it is clearly found that the simultaneousmodification of ZnO, ZrO2 and MgO makes Cu/Al2O3 catalystexhibit the strongest CO2 desorption peaks both in the low-temperature range and in the high-temperature one, respectively.It will be advantageous to the activation of CO2 during thehydrogenation to MeOH.

Comparison on catalytic performance

From the catalytic test of five reduced catalysts (Table 2), it canbe found that Cu/Al2O3 catalyst gives rise to a low CO2 conversion(about 9%), a poor MeOH selectivity and STY (only 13.4% and8 g�kgCat

�1�h�1, respectively). It seems that the modification of ZnOor ZrO2 has no obvious effect on improving CO2 conversion, andhowever can slightly improve the selectivity and STY of producedMeOH. Moreover, the simultaneous introduction of ZnO, ZrO2 intoCu/Al2O3 catalyst can obviously improve MeOH formation alongwith a small increase on CO2 conversion. According to the abovecharacterization results, it can be deduced that the improvementon the catalytic performance of Cu/Al2O3 is due to the increase onthe copper dispersion, metallic Cu0 surface area, adsorptioncapacity of H2 and CO2 after the modification of promoters. FromTable 2, Cu-ZnO-ZrO2-MgO/Al2O3 exhibits the smallest Cu0

particles (1.54 nm) with the biggest metallic surface area, highestdispersion and strongest adsorption capacity toward H2 and CO2,also gives the highest CO2 conversion (12.1%), MeOH selectivity(26.0%) and STY (31.0 g�kgCat

�1�h�1). It shows that smaller Cu0

particles are main catalytic active sites for MeOH synthesis. Andfrom Table 1 and 2 the catalytic performances of Cu-basedcatalysts in CO2 hydrogenation to MeOH are observed to exhibit alinear relationship with copper dispersion, particle size and surfacearea of metallic Cu0, the similar results have been also discovered

0.0 0.1 0.2 0.3 0.412

16

20

24

28

32

STYMeOH

CCO2

SCO

SCH4

SMeOH

Molar rati o of Mg/(Zr+Mg)

ST

YM

eOH (

g•kg

Cat

-1•h

-1)

0

10

20

30

4060

70

80

Con

vers

ion

& s

elec

tivi

ty (

%)

Fig. 4. Effect of MgO content on catalytic properties of Cu-ZnO-ZrO2-MgO/Al2O3

reduced at 350 8C for CO2 hydrogenation at 250 8C, H2/CO2 = 3 and pressure 20 atm.


on Cu-based catalysts from co-precipitation reported in references[12,16,21,26,30].

MgO content

From above results, MgO species in Cu-based catalysts arefound to exhibit an important role on improving MeOH synthesisfrom CO2. Therefore, the effect of MgO content on catalyticperformance of Cu/Al2O3 modified by ZnO-ZrO2-MgO is furtherinvestigated. From results (Fig. 4), it is discovered that CO2

conversion, selectivity and STY of MeOH first increase, and thendecrease with the increasing MgO content in catalysts. However,CO selectivity exhibits a reverse trend. Cu/Al2O3 catalyst modifiedby mixed oxides with a Cu: Zn: Zr: Mg molar ratio of 2: 1: 0.9:0.1 gives the highest CO2 conversion, selectivity and STY of MeOH.H2-TPR profiles of Cu-ZnO-ZrO2-MgO/Al2O3 with different MgOcontents (Fig. 5) indicate that all samples exhibit an obvious H2

consumption peak at about 200 8C due to the reduction of Cu2+ toCu0. The presence of MgO species will make this peak shift towarda lower temperature, however too much MgO species(Mg/(Mg + Zr) molar ratio > 0.1) will give rise to the difficultreduction of Cu2+. It is possibly because that the introduced MgOspecies are easily adsorbed on the acid surface of support Al2O3,which is helpful to the dispersion of copper species. However, theabundant MgO will possibly promote the strong interactionbetween CuO and the introduced oxides promoters such as ZnO,ZrO2 and even MgO [13,22]. It is disadvantageous to the formationof highly-dispersed active Cu0, the MeOH synthesis is henceinhibited. On the other hand, a high MgO content gives rise toformation of more alkali sites on catalysts, which can prevent theCH4 formation. It seems difficult that methanation carries out onalkali sites of catalysts. Anyway, the present data show thatCu/Al2O3 catalyst modified by ZnO-ZrO2-MgO (CZZMA) with a Cu:Zn: Zr: Mg molar ratio of 2: 1: 0.9: 0.1 is the optimum catalyst forMeOH synthesis from CO2. Therefore, the effect of hydrogenationparameters on the catalytic performance of CZZMA is furtherinvestigated in the following work.

Gas hourly space velocity (GHSV)

Fig. 6 shows the effect of GHSV on catalytic performance ofCZZMA in MeOH synthesis from CO2 hydrogenation. It isdiscovered that both CO2 conversion and MeOH selectivity onCZZMA catalyst slightly decrease with the increasing GHSV,however CO selectivity exhibits a reverse trend. It is possibly

100 200 300 400 500 600

(2)(1)

(3)

TC

D s

igna

ls (

a.u.

)


(6)

(5)

(4)

195 oC

Fig. 5. H2-TPR of Cu-ZnO-ZrO2-MgO/Al2O3 catalyst with a different Mg/(Mg + Zr)

molar ratio of (1) 0, (2) 0.05, (3) 0.1, (4) 0.2, (5) 0.3, (6) 0.5.



Fig. 6. Effect of GHSV on catalytic properties of CZZMA reduced at 350 8C for CO2

hydrogenation at 250 8C, H2/CO2 = 3 and pressure 20 atm.


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because that a high GHSV will give a short contact time of reactantgas with catalytic active sites. It is found by some researchers[12,16,21,26,30] that MeOH formation in CO2 hydrogenation has alinear relationship with metallic Cu0 dispersion and catalystsbasicity. From above results, it is also found that STYMeOH exhibits alinear relationship with copper dispersion and metallic Cu surfacearea in some extent (Tables 1 and 2). Moreover, the presence ofalkali MgO species in Cu-based catalysts is helpful to improveSTYMeOH. Therefore, it is proposed that CO2 during hydrogenationis first activated on basic sites to form the formates intermediatesespecially HCOOH*, the intermediates are then hydrogenated withthe activated H* on active Cu0 particles to directly produce MeOH.It has been also proved theoretically that the formation of theseintermediates through activating CO2 needs a lower energy thanthat of CO from CO2 hydrogenation according to density functionaltheory (DFT) [31,32]. A short contact time will decrease the ratio ofactivating CO2 to formates intermediates, which is disadvanta-geous to CO2 conversion to MeOH. It can be proved by the decreaseon selectivity and STY of MeOH with increasing GHSV. However,CO formation is possibly from the CO2 reaction with the activatedhydrogen on Cu0, which will be still controlled by internaldiffusion. It can be proved by the relationship between COformation and GHSV. Although CO selectivity is discovered toincrease with increasing GHSV, and however it can be found fromFig. 6 that its yield relative to CO2 conversion give a decreasedtrend (from 7.47% to 5.90%). Therefore, it is deduced that the MeOHformation is carried out mainly through the formates pathway.

From Fig. 6, it can be clearly found that STYMeOH over CZZMAcatalyst exhibits an obvious increase with the increasing GHSV. It is

Table 2Catalytic properties of five catalysts in CO2 hydrogenation to methanol.a

Catalysts CO2 conversion (%) Selectivity (%) STYMeOH

(g kgCat�1 h�1)

CO CH4 MeOH

CA 8.98 83.37 3.19 13.44 8.00

CZrA 8.85 82.60 3.65 13.75 8.75

CZnA 9.34 80.22 2.41 17.37 11.02

CZZA 10.87 66.41 11.2 22.44 19.94

CZZMA 12.12 61.61 2.41 35.98 31.00

a Catalytic hydrogenation conditions: 5 g catalysts (about 7.2 ml) reduced by

pure H2 at 350 8C, H2/CO2 molar ratio 3, GHSV 1400 h�1, hydrogenation temperature

250 8C and pressure 20 atm.


mainly because that GHSV has no obvious effect on both CO2

conversion and MeOH selectivity. However, the high GHSV willgive rise to the obvious increase on the fed amount of reactants.

Catalyst activation temperature

The effect of catalyst activation temperature on MeOHsynthesis from CO2 over CZZMA is presented in Fig. 7. It can beeasily found that both selectivity and STY toward MeOH increasewith the increasing activation temperature, and however COselectivity exhibits a reverse trend at <250 8C. From H2-TPR results(Fig. 2), the reduction of copper oxides to Cu0 mainly occurs atabout 200 8C, showing that it is difficult for the completeconversion of Cu2+ to Cu0 through the activation by H2 at170 8C. It can be concluded that the active centers for CO2

hydrogenation to MeOH are mainly the metallic Cu0 particles.Therefore, a high temperature is needed for the activation ofcopper oxides to metallic Cu0. From Table 3, it can be found that theactivation at 210 8C gives rise to a smaller metallic Cu0 particle sizeand a higher Cu dispersion and metallic surface area than that at250 8C. However, it is found that the CZZMA catalyst activated at210 8C does not exhibit the highest catalytic activity, it is possiblybecause that the formed metallic Cu0 particles are not too stable inthe present high-temperature reaction atmosphere (250 8C), canbe aggregated to larger one. It can be proved by the fact that thedetected Cu dispersion is decreased from 81.22% to 69.56% alongwith an increase of Cu particle size (from 1.28 nm to 1.50 nm)according to N2O-H2 titration results.

From Fig. 7, it can be also observed that both selectivity and STYof MeOH decrease, however CO selectivity increases with theincreasing activation temperature at >250 8C. It is possibly due tothe fact that the higher temperature during activation by H2 cangive rise to the aggregation of metallic Cu0 particles, resulting inthe increase of particle size and decrease on Cu dispersion andmetallic surface area (see Table 3) which is in accordance with theresults obtained in our previous work [21]. The bigger Cu0 particlesgive a low activity on adsorbing and activating hydrogen, which isdisadvantageous to MeOH synthesis through hydrogenation offormates intermediates especially HCOOH*. However, it is easythat RWGS reaction occurs on these bigger Cu0 particles. Therefore,no obvious change is observed for CO2 conversion with theincreasing activation temperature.

Fig. 7. Effect of activation temperature on catalytic properties of CZZMA in CO2

hydrogenation at 250 8C, H2/CO2 = 3, GHSV = 3000 h�1 and pressure 20 atm.



Table 3Physical and catalytic properties of CZZMA catalysts activated at different temperatures.

Activation temperature (8C) Cu dispersiona (%) Cu0 particle sizea (nm) Cu0 surface areaa (m2 g�1) MeOH selectivity (%)b STYMeOH (g kgCat�1 h�1)b

210 81.22 1.28 54.57 35.48 53.50

250 74.82 1.39 52.78 37.99 66.25

290 70.72 1.47 47.67 36.78 60.90

350 67.52 1.54 45.41 34.82 55.25

390 56.66 1.84 38.07 20.86 34.00

a Calculated from N2O-H2 titration.b Catalytic reaction conditions: H2/CO2 molar ratio 3, GHSV 3000 h�1, 5 g catalysts (about 7.2 ml), hydrogenation temperature 250 8C and pressure 20 atm.

Fig. 8. Effect of hydrogenation temperature on catalytic properties of CZZMA

activated at 250 8C in CO2 hydrogenation under the conditions of H2/CO2 = 3,

GHSV = 3000 h�1 and pressure 20 atm.


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Hydrogenation temperature

The present work also investigates the effect of hydrogenationtemperature on the catalytic performance of CZZMA catalystactivated by H2 at 250 8C. From Fig. 8, it can be observed that theincrease on CO2 conversion with the increasing hydrogenationtemperature is rapid in the low-temperature region (<270 8C) andbecomes slow in the high-temperature one (>270 8C). It indicates

Fig. 9. Effect of hydrogenation pressure on catalytic properties of CZZMA activated

at 250 8C in CO2 hydrogenation at 270 8C, H2/CO2 = 3, GHSV = 3000 h�1.


that the high reaction temperature is helpful to the CO2

hydrogenation, however possibly lead to the aggregation ofmetallic Cu0 particles to a larger size [1,2,12,16,30]. From Fig. 8,it can be also found that CO selectivity exhibits a slow increase,however CH4 selectivity gives a rapid increase with the increasingreaction temperature in the high temperature range. It indicatesthat the formation of Cu0 particles with larger size is helpful to theRWGS reaction and methanation possibly through the furtherhydrogenation of CO.

Hydrogenation pressure

The effect of hydrogenation pressure on MeOH synthesis fromCO2 over CZZMA activated by H2 at 250 8C, is present in Fig. 9. It isfound that CO2 conversion increases slowly, both STY andselectivity of MeOH are increased rapidly with the increasingreaction pressure. It is because that a high pressure promotes theformation of formates intermediates on the alkali sites of catalystssurface [15,20], which will accelerate the formation of MeOHthrough the further hydrogenation with the activated H2 onmetallic Cu0 particles. Moreover, it is also found that the reactionpressure has no obvious influence on the CH4 formation throughmethanation, and a high pressure can inhibit RWGS reaction.When reaction pressure reaches up to 28 atm, CZZMA catalystgives STY and selectivity toward MeOH of 121.5 g kgCat

�1 h�1 and46.19%, respectively. Further increasing hydrogenation pressureseemingly has no obvious function on improving MeOH formation.

From all above results, it can be deduced that during CO2

hydrogenation the MeOH formation is mainly from the interactionof formates intermediates produced by the activated CO2 withactive hydrogen adsorbed on the metallic Cu0 particles with aproper size; small part of CO produced from RWGS reaction can bedirectly converted by H2 to MeOH [20,33]. The latter is a slowprocess for MeOH synthesis. Simultaneously, direct CO2 hydro-genation to MeOH, RWGS reaction and methanation are carried outas parallel reactions on the corresponding active sites of catalysts.The metallic Cu0 particles with larger size exhibit a positivefunction for RWGS and methanation reactions.

Conclusions

The simultaneous modification of ZnO, ZrO2 and MgO is foundto be able to increase the copper dispersion and metallic Cu0

surface area, promote the formation of Cu0 particles with a smallsize for Cu/Al2O3 catalyst prepared by impregnation. The Cu0 siteson the Cu-based catalysts are the catalytic active centers for CO2

hydrogenation to MeOH. The activation temperature is also animportant factor affecting the metallic Cu0 particle size. Theinvestigation on the catalytic hydrogenation to MeOH synthesisover Cu-ZnO-ZrO2-MgO/Al2O3 catalyst shows that a higherreaction temperature will inhibit MeOH synthesis, and howeverincrease the reverse water-gas-shift and methanation reactions.The high gas hourly space velocity and reaction pressure areadvantageous to the increase on the space-time yield of MeOH.




G Model


Acknowledgement

This work was supported by the Major Natural Science Project ofSichuan Provincial Department of Education under Grant No. 13Z171

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