Reaction Kinetics, Mechanisms and Catalysis

Reaction Kinetics, Mechanisms and Catalysis Correlation between the porosity of γ-Al2O3 and the performance of CuO-ZnO-Al2O3 catalysts for CO2 hydrogenation into methanol --Manuscript Draft-Manuscript Number:

REAC-D-17-00483R1

Full Title:

Correlation between the porosity of γ-Al2O3 and the performance of CuO-ZnO-Al2O3 catalysts for CO2 hydrogenation into methanol

Article Type:

Full Paper

Corresponding Author:

Nguyen LE PHUC, Ph.D Vietnam Petroleum Institute HochiMinh city, VIET NAM

Corresponding Author Secondary Information: Corresponding Author's Institution:

Vietnam Petroleum Institute

Corresponding Author's Secondary Institution: First Author:

Nguyen LE PHUC, Ph.D

First Author Secondary Information: Order of Authors:

Nguyen LE PHUC, Ph.D TRI V TRAN, Master PHUONG THUY NGO, MASTER LUONG HUU NGUYEN, PhD THUAT THANH TRINH, PhD

Order of Authors Secondary Information: Funding Information: Abstract:

Influence of the porosity of γ-Al2O3 on the performance of CuO-ZnO-Al2O3 catalysts for methanol synthesis from H2 + CO2 mixture was studied. Various types of γ-Al2O3 with different surface areas (from 130 to 280 m2/g) and pore sizes (from 3 to 11 nm) were investigated. N2 adsorption, XRD, TPR studies and Grand Canonical Monte Carlo simulation were utilized to determine the correlation between their physicochemical properties and catalytic performance. It was shown that crystallite size of CuO (determined by XRD) and BET surface area of supports are not the key factors for methanol productivity. TPR profiles of catalysts demonstrated a direct relationship between CuO-ZnO interaction with their catalytic performance. Interestingly, samples with the uniform pore size of 5 nm exhibit a higher CuO-ZnO interaction and the highest methanol yield. In addition, at this pore size, simulation results showed that the ratio of H2 and CO2 inside the γ-Al2O3 pore was 1.5, which could be an appropriate feed ratio for high methanol productivity.

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Author's Response to Reviewer Comments made by the reviewers and editor 1. Editor On an editorial note, let me point out that our journal has published ... articles in this field recently: 1. Heondo Jeong, Churl Hee Cho, Tae Hwan Kim. Reac Kinet Mech Cat (2012) 106:435–443 DOI 10.1007/s11144-012-0441-5 2. Hania Ahouari, Ahcene Soualah, Anthony Le Valant, Ludovic Pinard, Patrick Magnoux, Yannick Pouilloux. Reac Kinet Mech Cat (2013) 110:131–145 DOI 10.1007/s11144-013-0587-9 3. Taraknath Das, Siddhartha Sengupta, Goutam Deo. Reac Kinet Mech Cat (2013) 110:147–162. DOI 10.1007/s11144-0130592-z Please check the references and consider citing them in the revised version if you find them relevant. Modify the argument based on TOF values

Author’s Response

We cited ref 1 and 2 as your requested: +ref 1 [22]: Introduction +ref 2 [21]: section 2.1 and 3.2 2.1. To synthesize CuO/ZnO/Al2O3 catalyst, CuO and ZnO were deposited on alumina by co-precipitation method, which was similar to the procedure described by Ahouari [21] with some modifications. 3.2: It has been also demonstrated that the catalytic activity of the CuO-ZnO/Al2O3 catalyst depends on both the metallic copper surface area and the interaction between copper and zinc oxide [21]. + ref 3 related to another catalytic system and no influence of different type of alumina was investigated

Corrected.

Reviewer #1: In the present manuscript of REAC-D-17-00483 (Correlation between the porosity of gamma-Al2O3 and the performance of CuO-ZnO-Al2O3 catalysts for CO2 hydrogenation into methanol) various types of gamma-Al2O3 were investigated for CO2 hydrogenation to methanol. The topic of this manuscript is very important, especially due to the utilization of carbon dioxide. The manuscript has enough scientific quality, and the reviewer concludes that the manuscript is sufficient for publication in Reaction Kinetics, Mechanisms and Catalysis journal. However, below I am listing major comments, those might be helpful for the authors to improve the current manuscript. 1. There are few small mistekes in the Corrected manuscript, like deficiency of prepostions or lack of dots in to the end of the sentence. 2. In the line no. 23 "samples containig (...) CZ/Al-X (Table 1) where Al-X stand for different types of alumina: which are named as CZ/Al-x" while under Al-X could be: Al-M;

the table 1 and in the tambe 1 these samples Al-TB1, Al-TB2; Al-N… are named as "CZ/Al-TBx". 3. Section "Catalyst preparation" need a Corrected: The obtained solid was washed several times with hot clarification. The obtained solid was water and 3 times with ethanol washed with mixtre of wather and ethanol or artelnately? 4. At the end of the Figure 2 and Table 3 Corrected and Figure 4 is a dot.

Reviewer #2: this paper gives a set of results dealing on the effect of textural properties on the performances of CuO-ZnO alumina catalyst for CO2 hydrogenation into methanol. my comments : - the preparation method is quite confusing for me since CuO - ZnO is first precipitated and Al2O3 is subsequently added. not the best way to favor an homogeneous dispersion of metals in the alumina porosity (mesh of alumina ?)

The co-precipitation method used in this study was similar to the procedure described by Ahouari [Methanol synthesis from CO2 hydrogenation over copper based catalysts Hania Ahouari et al, Reac Kinet Mech Cat (2013) 110:131–145] with some modifications. The idea of adding gamma alumina after having co-precipitation mixture of copper and zinc components is to enhance the Cu-Zn interaction.

- page 6 : the discussion on the effect of the pore size on the CuO particle size is too speculative . many others factors may have an influence, such as the surface properties of the oxide (OH, CUS sites ...) , which govern the reactivity of metallic salt and the particle size at the end. Moreover , DRX does not provides Particle Size Distribution (only the size of the biggest cristallised domain) MET would give more info

We modified the discussion based on your comment: “The larger pore size of Al-N (11 nm) would allow the active phase CuO to disperse more deeply into its porous structure, leading to a significant decrease of CuO crystallite size (17 nm), even if this parameter should be not the only one. The surface properties of aluminas (acidity, number of OH- sites…) could have an influence on the reactivity of copper or zinc element abd the particle size at the end.” For the CuO-ZnO/Al2O3 catalyst system, TEM was not widely used. The reason is that CuO and ZnO content are very high compared with other supported catalyst. In the literature, XRD is still used to calculate dCu based on Scherrer equation. For example: Weijie Cai, Pilar Ramirez de la Piscina, Jamil Toyir, Narcis Homs, CO2 hydrogenation to methanol over CuZnGa catalysts prepared using microwave-assisted methods, In Catalysis Today, Volume 242, Part A, 2015, Pages 193-199 Xiaosu Dong, Feng Li, Ning Zhao, Fukui Xiao, Junwei Wang, Yisheng Tan, CO2 hydrogenation to methanol over Cu/ZnO/ZrO2

catalysts prepared by precipitation-reduction method, In Applied Catalysis B: Environmental, Volume 191, 2016, Pages 8-17 Xiaoming Guo, Dongsen Mao, Guanzhong Lu, Song Wang, Guisheng Wu, Glycine–nitrate combustion synthesis of CuO–ZnO– ZrO2 catalysts for methanol synthesis from CO2 hydrogenation, In Journal of Catalysis, Volume 271, Issue 2, 2010, Pages 178-185 - Pores size distribution is given based on the BJH analysis of the desorption branch (I guess) Nevertheless, desorption branch does not give the "real" PSD but the size distribution of the pore controlling the desorption process (pore draining). in order to have a clear picture of the texture of the support and catalysts, one should consider the porous volume and surface developped (at adsorption branch).

Thank you for your advice. We think that the PSD based on BJH is still using at present. We have checked some recent papers and the corresponding editor accepted the PSD based on BJH. For example: - Tingting Liu, Linqing Ju, Yasong Zhou, Qiang Wei, Sijia Ding, Wenwu Zhou, Xiujuan Luo, Shujiao Jiang, Xiujuan Tao, Effect of pore size distribution (PSD) of Ni-Mo/Al2O3 catalysts on the Saudi Arabia vacuum residuum hydrodemetallization (HDM), In Catalysis Today, Volume 271, 2016, Pages 179-187 - Tokarova, Venceslava & Stavova, Gabriela & Novakova, Jana & Stiborova, Stanislava & Kasparek, Ales & Zukal, Arnost. (2017). Synthesis of beta zeolite with mesopores from a milk containing precursor and its performance in naphthalene isopropylation. Reaction Kinetics, Mechanisms and Catalysis. - Xiaosu Dong, Feng Li, Ning Zhao, Fukui Xiao, Junwei Wang, Yisheng Tan, CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts prepared by precipitation-reduction method, In Applied Catalysis B: Environmental, Volume 191, 2016, Pages 8-17 In our study, the PSD results have been used to raised a relationship between the pore size and/or pore size distribution of studied catalysts with catalytic activity. Then we have pointed out that the pore size in the range of 5 nm is probably an appropriate size to have high dispersion of active phases as well as interactions between the active phases CuO and ZnO to form catalytic active sites for CO2 hydrogenation to methanol over CuO-ZnO/Al2O3.

- TPR could be exploited in terms of reducibility rate of Cu (total and by type I II or III) to have a quantitative , relative comparison between samples

Corrected. We have added: Table 4. Relative areas of TPR profiles in studied catalysts Figure 4. Correlation between the STY (gCH3OH.gcat-1.h-1) and the total area of reduction peaks T1 and T2. T1 and T2 are reduction peaks of CuO particles which were in a direct contact with ZnO. And discussion for this modification is page 10.

- table 3 , CO2 conversion in the caption instead of MeOH conversion - what about deactivation ?

Corrected These catalysts exhibit quite high stability. The STY of CZ/TB2 during 60h time on stream is presented in following figure:

After 60 h time on stream, the loss of STY is less than 10%. However, we think that these results are not so important for this manuscript. - what is the origin of the variation of H2/CO2 with pore size , confinement ? (H2 "solubility" or concentration is supposed to increased when pore decreased as shown in litterature)

The variation of H2/CO2 ratio inside the pore was obtained by Monte Carlo simulations. We cannot give any comment on the increase of H2/CO2 ratio with pore size since several factors were taken into account during simulation process. As described in section 2.4: The adsorption simulation was carried out at 280 oC and a total gas pressure of 5 bar. The crystal face (110) of γ-Al2O3 was taken as slit pore model. All simulations were done with Lammps package. A fix model of alumina surface was used, while TraPPE potential was used for H2 and CO2. The interactions between atoms was described by the generic CLAYFF potential. This approach produced a good description of gas adsorption on the surface.

- conclusions which mainly ascribe the the performances to the pore size is too simplistic . in my opinion, the lower the pore size (higher S area at constant porous volume) will lead to higher metal support interaction and lower reducibility of Cu as TPR seems to show, and at the end a lower catalytic activity

We have slightly modified the conclusion. However, we could not add your comment since samples have pore size smaller or larger than 5nm exhibit lower activity.

Reviewer #3 General Comment: This manuscript reports the catalytic evaluation and characterization of catalysts CuO-ZnO supported on 5 different types of γ-Al2O3, to investigate hydrogenation of CO2 to methanol. Catalytic hydrogenation of CO2 into methanol is operated mostly on Cu catalyst for its excellent hydrogenation activity and inexpensive price. Additionally, the increasing of Cu activity in the presence of ZnO is very good

known in the literature and has been studied intensely in the past few decades. Although the catalytic results (CO2 conversion and selectivity to methanol) obtained by different scientific groups were often better than these presented in this manuscript despite similar reaction conditions, I can see the some elements of novelty. Therefore, my recommendation is giving to the authors the chance of resubmitting manuscript after major revision. Authors listed limited papers of catalytic hydrogenation of CO2 into methanol. I recommend adding additional papers to strengthen the background. Some recent examples of the papers are strongly recommended, especially Journal of the Taiwan Institute of Chemical Engineers 78 (2017) 416-422 containing results for really similar catalysts.

Corrected. Some recent articles were added: - Ahouari, H., Soualah, A., Le Valant, A. et al. Reac Kinet Mech Cat (2013) 110: 131. https://doi.org/10.1007/s11144-013-0587-9. - Jeong, H., Cho, C.H. & Kim, T.H. Reac Kinet Mech Cat (2012) 106: 435. https://doi.org/10.1007/s11144-012-0441-5 - Tursunov O, Kustov L, Tilyabaev Z (2017), J Taiwan Inst Chem Eng 78:416-422. - Ren H, Xu C.H, Zhao H.Y, Wang Y.X, Liu J (2015), J Ind Eng Chem, 28:261-267

Catalysts characterization is strongly limited. Application of additional characterization techniques such as XPS and TEM is suggested.

For the CuO-ZnO/Al2O3 catalyst system, TEM was not widely used. The reason is that CuO and ZnO content are very high compared with other supported catalyst. For the XPS technique, in Vietnam, we haven’t got any XPS analyzer yet. However, we have read carefully the XPS result reported in: -

Ren H, Xu C.H, Zhao H.Y, Wang Y.X, Liu J (2015), J Ind Eng Chem, 28:261-267; H. Lei, R.F. Nie, G.Q. Wu, Z.Y. Hou. J Fuel, 154 (2015), pp. 161-166 P. Gao, F. Li, H.J. Zhan, F.K. Xiao, W. Wei, L.S. Zhong, et al., J Catal, 298 (2013), pp. 51-60 “Journal of the Taiwan Institute of Chemical Engineers 78 (2017) 416-422”

and we think that despite the lack of the XPS, the most important characterization method for our study is H2-TPR. We also added table 4 to give the relative areas of TPR profiles and Figure 5 in order to have a better exploitation of TPR results. The space time yield (STY) calculation is questionable. I do not see the reason of the calculation per 1kg of the catalyst. Calculation of STY according to Journal of the Taiwan Institute of Chemical Engineers 78 (2017) 416-422 strongly recommended.

STY was changed to the amounts of MeOH produced per gram catalyst per hour (gCH3OH.gcat-1.h-1) as described in Journal of the Taiwan Institute of Chemical Engineers 78 (2017) 416-422

In the manuscript is written: The reaction was conducted at a temperature of 280 oC, pressure of 5 bar (page 4 line 34). Based on the literature data the pressure values seems to be approximately 10 times lower than in the earlier studies. Are Authors sure that the value of pressure is only 5 bar?

Yes, the value of pressure in only 5 bar. In this study. The objective of this study is to investigate the influence of the nature of gamma alumina to the catalytic activity of CuO-ZnO/alumina catalyst. Then, we have used our in-house testing system which can go to maximum of 7 bars. Then, we have evaluated the STY (gCH3OH.gcat-1.h-1) of CZ/Al-TB3 sample at 20 and 35 bars in another commercialize testing system. The results of these tests are: STY (gCH3OH.gcat-1.h-1) at 5 bar: 0.116 STY (gCH3OH.gcat-1.h-1) at 20 bar: 0.168 STY (gCH3OH.gcat-1.h-1) at 35 bar: 0.371 However, I think that these results are not so important in this manuscript.

Manuscript

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Click here to view linked References 1 2 3 4 5 6 7 8 9 10 Correlation between the porosity of γ-Al2O3 and the performance of CuO-ZnO11 Al2O3 catalysts for CO2 hydrogenation into methanol 12 13 Nguyen Le-Phuca,*, Tri Van Trana, Phuong Ngo Thuya, Luong Huu Nguyena, Thuat Thanh Trinhb. 14 15 a Catalysis Research Department, PetroVietnam Research & Development Center for Petroleum Processing 16 (PVPro), Vietnam Petroleum Institute, Block E2b, D1 Street, High Tech Park, Tan Phu Ward, District 9, 17 18 Ho Chi Minh City, Vietnam 19 b 20 Department of Chemistry, Norwegian University of Science and Technology, Trondheim, Norway 21 22 Email: [email protected] Telephone number: +84902666482 23 24 Abstract 25 Influence of the porosity of γ-Al2O3 on the performance of CuO-ZnO-Al2O3 catalysts for methanol 26 27 synthesis from H2 + CO2 mixture was studied. Various types of γ-Al2O3 with different surface areas (from 28 130 to 280 m2/g) and pore sizes (from 3 to 11 nm) were investigated. N2 adsorption, XRD, TPR studies and 29 Grand Canonical Monte Carlo simulation were utilized to determine the correlation between their physico30 chemical properties and catalytic performance. It was shown that crystallite size of CuO (determined by 31 XRD) and BET surface area of supports are not the key factors for methanol productivity. TPR profiles of 32 33 catalysts demonstrated a direct relationship between CuO-ZnO interaction with their catalytic performance. 34 Interestingly, samples with the uniform pore size of 5 nm exhibit a higher CuO-ZnO interaction and the 35 highest methanol yield. In addition, at this pore size, simulation results showed that the ratio of H2 and CO2 36 inside the γ-Al2O3 pore was 1.5, which could be an appropriate feed ratio for high methanol productivity. 37 38 Keywords: CO2 hydrogenation, γ-Al2O3, CuO-ZnO interaction, TPR, pore size distribution, Monte Carlo. 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 1 57 58 59 60 61 62 63 64 65

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Introduction The increasing CO2 concentration in the atmosphere is one of the main cause of climate change. On the other hand, fossil fuels are now depleting because of the high demand [1-2]. These reasons motivated in recent years the development of process to use CO2 as “carbon source” for petrochemical industry [3-4]. Among products of CO2 conversion, methanol is a valuable product which can be used as a raw material for chemicals production, an alternative energy source for transportation as well as a medium for the storage and transportation of hydrogen [5, 6, 7, 8, 9, 10]. It has been known that CuO-ZnO based catalyst is one of the most effective catalyst systems for the catalytic hydrogenation of CO2 to methanol [5, 6, 7, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,22, 23, 24]. Recent researches on Cu/ZnO-based catalysts showed influences of Cu and Zn components on their structural, morphological properties and catalytic performance [11, 15, 20, 21, 23, 24, 25, 26] whereas investigation on the effects of Al component was rarely performed. Guo Xian-ji et al. [13] have stated that Al component acts as a structural promoter, and restrains the active component grains from enlarging during the calcination of catalyst precursors. Witoon et al. [27] have found out that macro-pores of the hierarchical porous alumina are capable of enhancing diffusion of methanol out of the porous structure of Cu-based catalysts. As a result, the selectivity of methanol is significantly increased. However, influence of porosity of -Al2O3 on the catalytic performance of the ternary Cu-Zn-Al systems has rarely been reported in the literature. The specific surface area, pore size volume and pore size distribution of -Al2O3 may contribute to the catalytic activity. Therefore, objective of this study was to investigate the influence of porosity of -Al2O3 on the structure, morphology, and activity of the Cu/Zn/Al based catalyst for methanol synthesis from H2 + CO2 mixture. Experimental 1.1

Catalyst preparation To synthesize CuO/ZnO/Al2O3 catalyst, CuO and ZnO were deposited on alumina by co-

precipitation method, which was similar to the procedure described by Ahouari [21] with some modifications. First, a mixture of appropriate quantities of aqueous solutions of Cu(NO 3)2.3H2O and Zn(NO3)2.4H2O was stirred in a 250-ml beaker at room temperature. Then, (NH4)2CO3 was trickled into this solution to provoke precipitation of Cu2+ and Zn2+ ions. During the precipitation, the solution was maintained at its pH value of 7 and temperature of 70 oC under a constant stirring speed for 30 minutes. Subsequently, a required amount of alumina was added into the beaker. Different types of alumina were used (see Table 1), including -Al2O3 Merck (commercial product from Merck, denoted as Al-M), in-house -Al2O3-TBx and -Al2O3-N synthesized from commercial Al(OH) 3 and Al(NO3)3 by precipitation method, respectively. The mixture was then heated up to 90 oC at a constant stirring speed for 1 h. Afterward, the raw product was dried at 110 °C for 12 hours and calcined at 500 °C for 6 h. -Al2O3-TBx was synthesized as follows: a calculated amount of Al(OH) 3 (from TAN BINH Chemical Factory, Viet Nam) was dissolved in a 30% NaOH solution for 30 min prior to being stirred for

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15 h and filtered. The precipitation step was carried out by slowly adding 30% H2SO4 into the solution until the pH reached 9. After 30 min, the mixture was aged at 80-90 oC for a duration varied between 24 and 72 h at room temperature. The solid separated from the mixture was washed 3 times with hot water, and dried at 100 oC for 5 h. The solid was then calcined at 500 oC for 5 h. These aluminas are denoted Al-TBi depending on the aging time after precipitation step. The aging time were 24, 48 and 72 h for Al-TB1, AlTB2 and Al-TB3 samples, respectively. -Al2O3-N was prepared by co-precipitation method in which, first of all, Al(NO3)3 precursor and 5% NH3 were slowly added into a beaker (2 ml/min) until the pH of the mixture reached 8-9. The mixture was then stabilized for 12 h before the solid was separated out of the liquid by using centrifugation (2000 rpm). The obtained solid was washed several times with hot water and 3 times with ethanol before being dried at room temperature for 12 h and at 100 oC for 5 h. The calcination step was conducted at 500 oC for 5 h to convert Al(OH)3 solid into Al2O3. It is denoted Al-N. To clarify the correlation between the porosity of alumina and the catalytic performance, we focused on samples containing 30 wt.% CuO- 30 wt.% ZnO- 40% wt.% Al2O3, which are named as CZ/Alx X (Table 1) where Al-X stand for different types of alumina. Some additional samples as 50C50Z, 40C40Z/Al-M and Cu/Al-M were also prepared. A complete list of the names and the compositions of the studied catalysts is shown in Table 1.

Table 1. Chemical composition of studied materials prepared with different alumina source Target composition (wt.%) Samples

Actual chemical composition (wt.%)*

Alumina source**

CuO

ZnO

Al2O3

CuO

ZnO

Al2O3

CZ/Al-M

30

30

40

31.12

30.87

37.81

Al-M

Cu/Al-M

30

-

70

29.87

-

70.03

Al-M

40C40Z/Al-M

40

40

20

39.54

40.87

19.51

Al-M

CZ/Al-TB1

30

30

40

29.65

31.03

39.18

Al-TB1

CZ/Al-TB2

30

30

40

30.39

29.32

40.21

Al-TB2

CZ/Al-TB3

30

30

40

30.33

30.87

38.77

Al-TB3

CZ/Al-N

30

30

40

31.12

29.02

39.65

Al-N

50C50Z

50

50

-

51.67

48.23

-

-

*: The chemical compositions of the samples were determined by ICP-OES technique **: The sample designating are: Al-M stands for alumina Merck, Al-TBx (x = 1, 2, 3) stand for aluminas prepared from Al(OH)3 Tan Binh and Al-N stands for alumina prepared from Al(NO3)3

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1.2

Catalyst characterization X-ray powder diffraction (XRD) was performed in the 20o ≤ 2θ ≤ 70o range on a Bruker D8

Advance diffractometer using scintillation counter detector and CuK radiation generated at 40 kV and 40 mA, to investigate the crystalline structures and phase compositions of samples. The crystalline phases were identified by comparison with ICDD database files, and Rietveld refinement was carried out using TOPAS software. The BET surface areas (SA), pore volume (PV) and pore size distribution (PSD) of the samples were determined from the N2 adsorptiondesorption isotherms at −196ºC on a TRISTAR 3020 Micromeritics apparatus. Prior to the measurements, all samples were degassed at 260°C for 4h under vacuum. TPR experiments were carried out in an AMI – 902 (Altamira) automatic analyzer equipped with a TCD detector. Prior to the TPR test, all samples were pre-treated under Ar at 400 °C for 30 minutes and cooled down to 50 oC. The reduction step was performed from 50 oC up to 800 oC under a gas mixture containing 1 % H2 in Ar, using a heating rate of 5 K.min-1.

1.3

Catalyst testing for CO2 hydrogenation The catalytic activities of prepared samples were evaluated in a fixed bed reactor equipped with a

thermocouple placed directly in the catalytic bed. Before any reaction, 0.5 g catalyst (particle size: 100-150 μm) was loaded into a stainless-steel reactor tube (3/8 inch i.d.) and reduced in a flow of 30% H2/N2 at 400 o

C under atmospheric pressure for 2 h. The reaction was conducted at a temperature of 280 oC, pressure of

5 bar, and H2/CO2 ratio of 3 as well as WHSV of 36,000 (L.Kgcat-1.h-1). All catalytic tests were free of mass transfer limitations at this rather high WHSV (GHSV > 30,000 h-1). The catalytic activity was evaluated via the space time yield (STY), which is the amount of formed methanol (g methanol) per 1 kg catalyst for 1h of reaction (gmethanol.kgcat-1.h-1).

%Me: concentration of methanol in the product mixture (mole %) analyzed by GC- FID; F: Total flow rate of product mixture (L/h); Vm is the molar volume of an ideal gas at STP (= 22.414 L.mol−1); MMe: molecular mass of methanol (g/mol); mcat: amount of catalyst (kg); The products were analyzed after 2 h on stream by two gas chromatographies, an Agilent Technologies 7890 GC equipped with TCD detector for detecting H2, CO, CO2 and a HP 6890 Plus GC with FID detector for CH4, CH3OH and dimethyl ether analysis.

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1.4

Monte Carlo simulation In order to determine the ratio of H2 and CO2 inside the γ-Al2O3 pore, we performed a Grand

Canonical Monte Carlo simulation (GCMC). The H2/CO2 ratio in the gas phase was initiated at 3:1. The adsorption simulation was carried out at 280 oC and a total gas pressure of 5 bar. The crystal face (110) of γ-Al2O3 was taken as slit pore model [28]. All simulations were done with Lammps package [29]. A fix model of alumina surface was used, while TraPPE potential [30] was used for H2 and CO2. The Interactions between atoms was described by the generic CLAYFF potential [31]. This approach produced a good description of gas adsorption on the surface [32-33].

Results and discussions 1.5

XRD and nitrogen adsorption Fig 1A presents XRD diffractograms of alumina samples. XRD peaks assignable to gamma alumina

(using gamma alumina Merck as reference) were detected for all prepared alumina. The XRD (Fig. 1B) patterns of all catalyst samples show a typical crystalline structure of monoclinic CuO (JCPDS 65-2309: 2 = 35.6o, 38.7o, 49o, 61.8o) and ZnO phase (JCPDS 36-1451: 2 = 31.7o, 34.5o, 36.5o, 47.5o, 56.8o, 63o, 68o). Based on the XRD diffractograms, the composition of different crystalline phases in the catalyst samples and the crystallite size of CuO was calculated by Rietveld Refinement using TOPAZ software. Generally, the simulation results are acceptable when the Rwp (Weighted R Profile) and GOF (Goodness of Fit) values are lower than 10% and 2%, respectively [34-35].

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Figure 1. X-ray diffractograms of of studied alumina samples (A) and all catalyst samples (B)

The results of Rietveld Refinement analysis (Table 2) demonstrate that all samples exhibit a similar composition of crystalline phases. The crystallite size of active phase CuO (dCuO) in the samples can also be obtained from Rietveld Refinement analysis (Table 2), in which the crystallite size of CuO in CZ/Al-M and CZ/Al-TBx samples are in the range of 25-30 nm, whereas that in CZ/Al-N is smaller. It is noticeable that there is a correlation between the crystallite size of CuO phase and the specific surface area of alumina (Table 3). A high surface area of the support allows a high dispersion of the active phase, leading to a decrease in its crystallite size. The difference in CuO crystallite size of the two catalysts, namely CZ/Al-TB3 and CZ/Al-N, which have a similar specific surface area could be interpreted through the existence of a larger pore size of Al-N (Table 3). The larger pore size of Al-N (11 nm) would allow the active phase CuO to disperse more deeply into its porous structure, leading to a significant decrease of CuO crystallite size (17 nm), even if this parameter should be not the only one. The surface properties of aluminas (acidity, number of OH- sites…) could have an influence on the reactivity of copper or zinc element abd the particle size at the end..

Table 2. Result of Rietveld Refinement analysis of studied catalysts dCuO

% mol

% mol

(nm)

CuO

ZnO

CZ/Al-M

30

36% ± 4

CZ/Al-TB1

28

CZ/Al-TB2

27

Sample

Rwp, %

GOF

37% ± 4

9.56

1.25

35% ± 4

36% ± 4

7.56

1.05

34% ± 4

35% ± 4

9.21

1.32

6

Formatted: Superscript

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

CZ/Al-TB3

25

35% ± 4

35% ± 4

8.65

1.16

CZ/Al-N

17

36% ± 4

35% ± 4

6.71

1.21

The pore size distributions of aluminas and corresponding catalysts obtained from N2 adsorptiondesorption measurement (BJH method) are presented in Fig. 2. The non-alumina sample (50C50Z) only shows a large pore size of around 50 nm while 40C40Z/Al-M has two types of pores, including small (5 nm) and large ones (50 nm) (Fig. 2C). Since the Al-M sample have a uniform pore size distribution centered at 5 nm, the large pore (50 nm) of 40C40Z/Al-M is corresponding to CuO-ZnO species which separated from Al-M, and the small pore (5 nm) is belonged to CuO-ZnO species dispersed on Al-M, respectively. However, the deposition of lower CuO and ZnO loading on alumina (CZ/Al-TBx or CZ/Al-M samples) only causes a decrease in the porous volume of the samples compared to parent alumina but still keeps the pore size distribution pattern unchanged (Fig. 2A and 2B). Hence, for CZ/Al-TBx or CZ/Al-M samples, the CuO and ZnO phases were deposited inside the pore of alumina.

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Figure 2. Pore size distribution of studied aluminas (A), 30%CuO-30%ZnO catalysts (B) and 40C40Z/AlM, 50C50Z (C).

3.2. Methanol synthesis measurements Table 3 shows the methanol space time yield (STY, gmethanol.Kgcat-1.h-1), CO2 conversion (%) and methanol selectivity (%) of studied catalysts at 280 oC, 5 bar. In the blank test without catalyst, no CO2

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conversion is observed under the reaction conditions. All experiments were repeated twice, and the standard deviations were below 8%. In all experiments, the C-products obtained contain mainly unreacted CO2, CO and CH3OH with minor amounts of CH4. At low pressure (5 bar), CZ/Al-x show 21.4-31.5% selectivity to methanol. At high WHSV (36,000 L.Kgcat-1.h-1) and low pressure (5 bar), all CuO-ZnO/Al2O3 catalysts give around 3% of CO2 conversion. Then, the comparasion of different catalyst performance are based on the STY and methanol selectivity. It was reported that the small CuO crystallite size had a positive effect on its catalytic performance, since a higher surface area of the active phase facilitated methanol formation [5, 12]. Nevertheless, the obtained results in this study (Table 3) are not in agreement with the literature. It can be seen that the STY and methanol selectivity of CZ/Al-M is higher than that of CZ/Al-TB1 although these two samples have similar crystallite sizes of CuO. In addition, the smallest CuO crystallite size of CZ/Al-N does not exhibit a positive impact on its catalytic performance. Table 3. Methanol STY, methanolCO2 conversion (%), methanol selectivity (%) at 280 oC, 5 bar,

Formatted: Subscript

H2/CO2 = 3, WHSV = 36,000 L.Kgcat-1.h-1, SBET and pore size of catalysts, CuO crystallite size.

Sample

Methanol STY (gCH3OH.gcat1 .h 1 mCH3OH(g). Kgcat-1.h-1)

CZ/Al-M

0.105

3.1

28.6

130

64

5.1

30

CZ/AlTB1

0.073

2.9

21.4

213

78

3.2

28

CZ/AlTB2

0.110

3.1

29.7

242

84

5.0

27

CZ/AlTB3

0.116

3.1

31.5

280

89

4.9

25

CZ/Al-N

0.083

2.8

24.5

283

98

10.1

17

Cu/Al-M

0.0012.5

0.2

8.4

130

92

5.1

35

CO2 Methanol SBET of conversion selectivity alumina (%) (%) (m2/g)

SBET of Pore size of dCuO catalysts catalysts (nm) (m2/g) (nm)

Formatted Table

Interestingly, the catalysts possessing high STY (ranging from 0.105-0.116 gCH3OH.gcat-1.h-1) and high selectivity to methanol such as CZ/Al-M, CZ/Al-TB2, CZ/Al-TB3 have similar pore size distributions and in the range of 5 nm (Fig. 2), whereas those of less active catalysts such as CZ/Al-TB1 and CZ/Al-N are outside of that range. This result has not been reported in the literature. Song et al. [36] found that the formation rate of CH3OH was a function of the average pore diameter of the silica support, following the order: MCM-41 > SBA-15 > MCM-48 > MSU-F ~ amorphous SiO2. The highest activity was reported for

9

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the sample having an average pore diameter at around 3 nm (MCM-41 as the support). However, based on the constant TOFs calculated from the hydrogen chemisorption results, the high activities of Ca-Pd/MCM41 and Ca-Pd/SBA-15 were caused by the high dispersion of Pd inside the support mesopores. It was stated that the small mesopores of MCM-41 and SBA-15 would work as templates, which facilitated the small Pd nanoparticles formation. However, it should be noted in our study, the smaller CuO crystallite size does not lead to a higher activity. In order to observe if a correlation exists between the pore size distributions –the Formatted: Subscript

methanol productivity behavior and the redox properties, the samples characterized by H 2-TPR. TPR-H2 profiles of studied catalysts are shown in Fig. 3. All curves were deconvoluted using a Gaussian-type function. In order to have a better understanding of the Cu-Zn interactions, the CuO supported on alumina-Merck catalyst (Cu/Al-M) is also studied. The deconvoluted plots show that Cu/AlM was reduced in 2 steps, one at 256 oC and the other at 282 oC. They could be assigned to the reduction of highly dispersed CuO particles (peak at 256 oC) and large CuO particles (peak at 282 oC). After the addition of ZnO, the reduction of CuO occurred at a lower temperature, indicating the promotional effect of ZnO on CuO reduction [37]. Fierro and coworkers [38] reported that ZnO played the role of H2 activator and CuO dispersive agent. Then, the deconvolution of TPR profiles of CuO-ZnO based catalysts shows that copper oxide sites could be divided into 3 main species depending on the ease of reducibi lity (peak (1), (2), (3) in figure 3). According to M. Bahmani and coworker [39], the first peak (peak (1)) corresponded to the reduction of highly dispersed CuO particles, which were in a direct contact with ZnO. The second peak (peak (2)) belonged to the reduction of large CuO particles that contacted with ZnO. Considering the third peak (peak (3)), this reduction peak can be assigned to the reduction of CuO without ZnO contact, which could be verified in the TPR profile of Cu/Al-M. Moreover, this peak is only observed in CZ/Al-TB1 and CZ/Al-N. The TPR profiles of CZ/Al-TB2, CZ/Al-TB3 and CZ/Al-M catalysts only show two peaks, (1) and (2), indicating that CuO species in these samples are in contact with ZnO. It was reported that CuO species that interacted with ZnO were more reactive than those without ZnO interaction [16, 17, 21, 16, 39]. According to Natesakhawat et al. [40], the interaction of Zn-Cu can create an active site at the Cu/ZnO interface that acts as an adsorption site of CO2. It has been also demonstrated that the catalytic activity of


the CuO-ZnO/Al2O3 catalyst depends on both the metallic copper surface area and the interaction between


copper and zinc oxide [21]. In this study, the very low STY and methanol selectivity of Cu/Al-M (Table 3)


clearly show that CuO alone is not sufficiently active for the reaction. Therefore, the higher activities of CZ/Al-TB2, CZ/Al-TB3, and CZ/Al-M compared to those of CZ/Al-M and CZ/Al-TB1 can be attributed to the better interactions between CuO and ZnO species or better reducibility of CuO (Fig. 3). These results are supported by those of Tursunov et al. [23]. The latter authors also show that the catalytic activity of CuO–ZnO/Al2O3 catalyst was related tightly to the reducibility of the copper oxide particles. Table 4 shows the reduction temperature and relative reduction peak area of studied catalysts. We can see that samples having higher total area of peaks T1 and T2 exhibit higher STY (Fig. 4). In our study, Thisthe result seems to indicate that the supports having a pore size of 5 nm exhibit a better CuO-ZnO interaction compared with those having a smaller or larger pore size (3 nm or 10 nm).

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11

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Figure 3. TPR-H2 results of studied catalysts Formatted: Font: (Default) Times New Roman, Bold

Table 4. Relative areas of TPR profiles in studied catalysts T1a

Area1b

T2c

Area2d

CZ/AlTB3

177 and 218

5099

242

1291

CZ/AlTB2

214

2344

238

3909

CZ/AlTB1

218

1916

242

CZ/Al-N

215

2456

CZ/Al-M

214

3755

Catalyst

Cu/Al-M

T3e

Area3f

2805

257

826

244

2574

265

1189

234

1806 256

3359

T4g

Area4h

282

2964

a

: Reduction temperature of highly dispersed CuO particles, which were in a direct contact with ZnO

b

: Area of highly dispersed CuO, which were in a direct contact with ZnO, reduction profile

c

Formatted: Font: (Default) Times New Roman

Formatted: Font: Italic

: Reduction temperature of large CuO, which were in a direct contact with ZnO

d

: Area of large CuO, which were in a direct contact with ZnO, reduction profile

Formatted: Font: Italic, Superscript

: Reduction temperature of highly dispersed CuO without ZnO contact


: Area of highly dispersed CuO, without ZnO contact, reduction profile


g

: Reduction temperature of large CuO particles without ZnO contact


h

: Area of large CuO particles, without ZnO contact, reduction profile


e f

Formatted: Font: Italic Formatted: Font: Italic, Superscript Formatted: Font: Italic Formatted: Font: Italic, Superscript Formatted: Font: Italic Formatted: Font: (Default) Times New Roman, Italic

Figure 4. Correlation between the STY (gCH3OH.gcat-1.h-1) and the total area of reduction peaks T1 and T2. T1 and T2 are reduction peaks of CuO particles which were in a direct contact with ZnO

Formatted: Centered, Indent: First line: 0" Formatted: Font: Not Italic

The distribution of H2 and CO2 molecules inside the alumina supports with different pore sizes


was further investigated using Monte Carlo simulations. Fig. 4 5 shows the increasing of H2/CO2 ratio


12

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when the pore size increases from 3 nm to 10 nm. The optimum performance of catalyst at 5 nm corresponds to a ratio H2/CO2 of 1.5 inside the pore. Then, the reaction pathway of H2+CO2 conversion to methanol over CuO-ZnO/alumina catalyst might be related to the distribution of H2 and CO2 inside the Formatted: Font: (Default) Times New Roman, Vietnamese

pore. However, more details of micro kinetic model require a further investigation.


Figure 5. Molar ratio of H2/CO2 inside the pore at different pore size using Monte Carlo simulations.

In our opinion, pore size in the range of 5 nm is probably an appropriate size in terms of steric configuration, adsorption-desorption of reactants or products, dispersion of active phases inside the porous structure as well as interactions between the active phases CuO and ZnO to form catalytical active sites for CO2 hydrogenation to methanol over CuO-ZnO/Al2O3 [39, 41 ]. To confirm the hypothesis, another aluminas, namely, Al-TB4 and Al-TB5 were prepared using Al(OH)3 as a precursor with the abovedescribed procedure but at 36h and 84h of post-stabilization time. The average pore size of Al-TB4 and AlTB5 are respectively 5.6 nm and 7.4 nm. The deposition of 30 wt.% CuO and 30 wt.% ZnO over Al-TB4 and Al-TB5 causes a slight decrease in pore size to 5.1 nm and 7.1 nm. The obtained catalysts are labeled as CZ/Al-TB4 and CZ/Al-TB5. They exhibit BET surface area of 81 m2.g-1 and 101 m2.g-1, respectively. Fascinatingly, CZ/Al-TB4 shows a high catalytic performance with STY reaching a value of 0.108 gmethanol.Kgcat-1.h-1 and 28.8% methanol selectivity. These values are closed to the STY and methanol selectivity of those catalysts with the approximate average pore size of 5 nm. In opposition, CZ/Al-TB5, which has an average pore size of 7.1 nm, exhibits a lower STY (0.091 gmethanol.Kgcat-1.h-1) whereas this sample has the highest surface area.

Conclusion In this study, severalsfive types of γ -alumina with different surface areas and pore size distributions were used to prepare CuO-ZnO/Al2O3 catalysts. These catalysts were tested for methanol synthesis from

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CO2 and H2. It was shown that the crystallite size of CuO (determined by XRD) and the BET surface area of catalysts were not the key factors affecting the methanol productivity. However, it was clearly demonstrated that the pore size of aluminas/catalysts played an important role regarding to the CuO-ZnO interaction and methanol productivity. TPR profiles of CuO species show that the uniform pore size of 5 nm seems to be an appropriate size to increase the possibility that CuO species are in direct contact with ZnO to form active sites, leading to a better reducibility of CuO and higher methanol productivity. Smaller of larger pore size of aluminas/catalysts resulted in a loss of the catalytic activity. This observation has not yet been described in the literature. It is necessary to conduct a more in-depth investigation to offer a complete picture about the nature of this phenomenon. Nevertheless, the results obtained from this study are still very helpful to orient our future research to control the pore size of catalysts to produce highly efficient catalysts for methanol synthesis from H 2+CO2.

Acknowledgement This work was carried out at PVPro, VPI and supported by Vietnam National Oil and Gas Group (03/NCCB(PVPro)/2012/HĐ-NCKH) and the Ministry of Industry and Trade of Vietnam (DT.03.12/NLSH). Additional Information: Formatted: Font: Not Bold

On behalf of all authors, the corresponding author states that there is no conflict of interest.

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Formatted: Not Superscript/ Subscript Formatted: Vietnamese Formatted: Not Superscript/ Subscript Formatted: Subscript


Copyright Transfer Statement Copyright Transfer Statement Statement on publishing and usage license with Green Open Access extension Author name: LE-PHUC Nguyen Address: Vietnam Petroleum Institute, D1 Street, High Tech Park, Tan Phu Ward, District 9, Ho Chi Minh City, Vietnam

Publisher: Akadémiai Kiadó Zrt. Prielle Kornélia u. 21-35, H-1117 Budapest, Hungary

E-mail [email protected] Article title: Correlation between the porosity of γ-Al2O3 and the performance of CuO-ZnO-Al2O3 catalysts for CO2 hydrogenation into methanol Journal title: Reaction Kinetics, Mechanisms and Catalysis Co-Authors: Tri Van Tran, Phuong Ngo Thuy, Luong Huu Nguyen, Thuat Thanh Trinh

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Date:

September 25, 2017

Author’s Signature:_____________________________ Signature:_____________________________ LE-PHUC Nguyen

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