Al2O3 catalyst for methanol and DME synthesis via CO2 hydrogenation

Journal of CO₂ Utilization 36 (2020) 82–95

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Enhanced catalytic performance of Zr modified CuO/ZnO/Al2O3 catalyst for methanol and DME synthesis via CO2 hydrogenation

T

Shoujie Rena, Xiao Fana, Zeyu Shanga, Weston R. Shoemakera, Lu Mab, Tianpin Wub, Shiguang Lic, Naomi B. Klinghofferc, Miao Yud, Xinhua Lianga,* a

Department of Chemical and Biochemical Engineering, Missouri University of Science and Technology, Rolla, MO 65409, United States X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL, 60439, United States c Gas Technology Institute, 1700 South Mount Prospect Road, Des Plaines, IL 60018, United States d Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, United States b

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2hydrogenation Methanol Dimethyl ether CuO/ZnO/ZrO2/Al2O3 (CZZA) Stability

Zirconium (Zr) modified CuO/ZnO/Al2O3 (CZA) catalysts with different aluminum (Al) and Zr contents were synthesized by the co-precipitation method. The synthesized CuO/ZnO/ZrO2/Al2O3 (CZZA) catalysts were comprehensively characterized and studied for methanol synthesis via CO2 hydrogenation. The CZZA catalyst showed the highest methanol yield of 12.4 % at 220 °C and 2.76 MPa with an optimized catalyst composition of Cu/Zn/Zr/Al (atomic ratio) at 4:2:1:0.5. The CZZA catalyst showed better activity than that of the CZA catalyst and a superior stability for methanol synthesis. There was no decrease in the BET surface area and very little coke formation for the spent CZZA catalyst, after 300 h of methanol synthesis. Bifunctional catalysts, composed of CZZA and HZSM-5, were investigated for dimethyl ether (DME) synthesis directly from CO2 hydrogenation, and a maximum DME yield of 18.3 % was obtained at a reaction temperature of 240 °C and a pressure of 2.76 MPa. The stability of the bifunctional CZZA and HZSM-5 catalyst during the DME synthesis also significantly improved, as compared to that of the CZA and HZSM-5. A significant decrease in the BET surface area and an increase in coking on the spent CZZA catalyst were observed after 100 h of DME synthesis, indicating a detrimental effect on CZZA stability when a HZSM-5 catalyst was present. The changes in structural properties (e.g., BET surface area and crystallinity) and coking for HZSM-5 could be responsible for the deactivation of the bifunctional catalyst.

1. Introduction Dimethyl ether (DME) has been considered to be a promising alternative fuel and an important intermediate chemical used for the production of other chemicals, such as methyl acetate [1,2]. The catalytic conversion of syngas (CO and H2) to DME has been widely investigated during the last decade and a good yield has been achieved [1,3,4]. Due to its extensive availability and a motivation to reduce green-house gas emissions, CO2 has been selected as a building block in producing useful chemicals and fuels [5–10]. CO2 hydrogenation with H2 to produce DME is becoming of increasing interest to researchers [11–13]. Compared to DME production from syngas, production with CO2 involves a reverse water gas shift reaction, as a side reaction, as well as CO2 hydrogenation to methanol and methanol dehydration to DME. A conventional catalyst used for methanol synthesis from syngas is

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CuO/ZnO/Al2O3 (CZA), which has high activity [14–16]. This catalyst is also the primary choice for CO2 hydrogenation to methanol and has been investigated during the last several years [11,14,17,18]. Although good activity has been achieved, the stability of CZA for CO2 hydrogenation has not been sufficient [19–22]. Due to the hydrophilic property of Al in the CZA catalyst, water generated in CO2 hydrogenation to methanol and reverse water gas shift reactions can be adsorbed onto the catalyst, which will reduce the activity. Large amounts of produced water could accelerate catalyst deactivation. To improve the stability of the hydrogenation catalyst, particularly with respect to water resistance, some promoters, such as Fe, Pd, and Zr have been investigated [23–26]. Zr, as a structural promoter, can prevent the aggregation of CuO and ZnO, and favor the dispersion of CuO, thereby improving the structural properties of catalysts [27,28]. Li et al. [29] synthesized Zr doped catalysts and investigated the stability of the catalysts for methanol synthesis from CO2. The Zr doped catalysts

Corresponding author. E-mail address: [email protected] (X. Liang).

https://doi.org/10.1016/j.jcou.2019.11.013 Received 31 May 2019; Received in revised form 24 October 2019; Accepted 8 November 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

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solutions (total metal concentration of mixed aqueous at 0.1, 0.4, 0.7, and 1.0 M) and aqueous sodium carbonate solution (0.1, 0.4, 0.7, and 1.0 M) were used for catalyst synthesis and denoted as CZZA-x, in which x stands for the concentration of solutions (the same concentrations of nitrates and sodium carbonates) used for catalyst synthesis. Bifunctional catalysts were prepared by physically mixing CZZA powder and HZSM-5 powder. CZZA-0.1 was used for all experiments, except when testing the effects of precursor concentration. For a catalytic activity test, 0.5 g of CZZA powder and 0.5 g of HZSM-5 were used for bifunctional catalyst preparation. For a HZSM-5 loading test, bifunctional catalysts were prepared by fixing CZZA powder at 0.5 g and varying HZSM-5 from 0.1 g to 0.5 g. In order to determine the deactivation mechanism of CZZA and HZSM-5 in DME synthesis, the bifunctional catalyst used for the stability test was prepared by physically mixing pelletized CZZA (40–60 mesh) and HZSM-5 (40–60 mesh).

showed an excellent tolerance against water vapor. It was also pointed out that Zr could prevent the crystal growth of CuOx and reduce the irreversible deactivation of catalysts. Solid acid catalysts are generally used for methanol dehydration. In direct CO2 hydrogenation to DME, bifunctional catalysts composed of hydrogenation catalysts (e.g., CZA) and solid acid catalysts (e.g., HZSM5 or γ-Al2O3), which are similar to those used for syngas conversion to DME, have been investigated [20,22,30–32]. However, detrimental interactions, such as an ion exchange between hydrogenation and dehydration catalysts, have been reported and the presence of water could enhance this undesirable effect. Catalyst deactivation and low product yields have been significant challenges in the industrial application of bifunctional catalysts [20,21,32]. Zr promoted catalysts have shown an improvement in CO2 conversion to methanol. However, in direct DME synthesis from CO2 by bifunctional catalysts, the methanol dehydration reaction produces a significant amount of water, in addition to the water produced by CO2 hydrogenation to methanol and the reverse water gas shift reactions. There have only been a few reports about the effects of Zr modified CZA catalysts on the activity and stability of bifunctional catalysts in DME synthesis from CO2, and the deactivation mechanism of bifunctional catalysts composed of Zr-modified CZA and HZSM-5 was not fully understood. In this study, we modified a CZA catalyst by Zr and tested the activity and stability for CO2 conversion to methanol and DME. The Zr loading and Al loading in the catalyst were optimized. A long-term stability test for methanol and DME synthesis was conducted to determine the effect of Zr on catalyst stability. The spent catalysts were also characterized to reveal the deactivation mechanism of the catalysts.

2.3. Catalyst characterization Nitrogen adsorption and desorption of catalysts was determined by a Quantachrome Autosorb-1. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific area of samples in a relative pressure range of 0.05–0.25 of nitrogen adsorption and desorption isotherms. The reducibility of a CZZA catalyst was determined by hydrogen temperature-programmed-reduction (H2-TPR) analysis, which was performed using a Micromeritics Autochem II 2920 (Micrometritics Instrument Corporation) under the following conditions: 25 mg sample, 10 % H2-Ar, with a flowrate of 20 mL/min, and heating rate of 10 °C/ min. NH3 temperature-programmed desorption (NH3-TPD) was performed with a Micromeritics Autochem II 2920 using the following procedures. A 100 mg sample was pretreated at 350 °C, with a He flow of 30 mL/min for 1 h, and then cooled to 50 °C. Ammonia (5 % NH3/ He) was introduced into the catalyst for 60 min at 50 °C. After that, the sample was flushed with helium for 60 min to remove weakly adsorbed NH3 and, then, the temperature was programmed to increase to 700 °C at a rate of 10 C/min. The amount of ammonia in the effluent was measured by TCD and quantitatively calibrated by NH3 pulses. The metallic copper surface area, copper dispersion, and particle size were determined by performing N2O-chemisorption using a Micromeritics Autochem II 2920. The catalyst was first degassed at 250 °C for 2 h in pure He, then reduced at the same temperature for 2 h in 10 % H2 in He. After the reduction, pure He was used to cool down the catalyst to 50 °C. Then, gas was changed to a mixture of 1 % N2O in He to start N2O-chemisorption. The stoichiometry of Cu:N2O was assumed to be 2:1. The textural structure and powder X-ray diffraction (XRD) patterns of fresh and spent catalysts were determined by a Philips X'Pert PRO PW3050 X-ray diffractometer, equipped with a Cu Kα radiation and a graphite generator. The tube voltage and the current were at 45 kV and 40 mA, respectively. The scan range was 20–90°, with a scan rate at 0.5°min-1. X-ray absorption spectroscopy (XAS) was performed by using the 9-BM beamline of the Advanced Photon Source at Argonne National Lab, USA. Images of reduced catalysts and spent catalysts were taken by a FEI Tecnai F30 TEM, operated at 300 kV, to determine the microstructure. Energy Dispersive X-Ray Spectroscopy (EDS) was used for element mapping. Coking content of spent catalysts was determined by a TA Instruments Q50 thermogravimetric analyzer. In order to remove moisture and absorbed CO2, catalysts were first heated to 100 °C, at a heating rate of 10 °C/min, and kept at 100 °C for 60 min, with a N2 flow rate of 100 mL/min. Then, the catalyst was heated to 1000 °C at the same heating rate with air (100 mL/min) to determine the coking amount.

2. Materials and methods 2.1. Materials A Zr-modified CuO/ZnO/Al2O3 (CZZA) catalyst was synthesized by the co-precipitation method. The precursors, including copper nitrate (Cu(NO3)2 ·3H2O, 99 %), zirconium dinitrate oxide hydrate (ZrO (NO3)2·xH2O), 99.9 %), zinc nitrate (Zn(NO3)2·6H2O, 98 %), aluminum nitrate (Al(NO3)3·9H2O, 98 %), and sodium carbonate (Na2CO3, 99 %), were purchased from Alfa Aesar (USA) and used to synthesize the CZZA catalyst. Ammonia-ZSM-5 (molar ratio of SiO2/Al2O3 = 23:1, with a surface area of 425 m2/g) was purchased from Alfa Aesar and used as a precursor for the dehydration catalyst. Before being used, the ammoniaZSM-5 was calcined at 500 °C for 5 h to obtain HZSM-5. Admixed gas of CO2 and H2, with a volume ratio of 1:3, was purchased from Airgas Inc. (USA), and was used as feed gas for reactions. 2.2. CuO/ZnO/ZrO2/Al2O3 synthesis and bifunctional catalyst preparation The co-precipitation method was used to synthesize a CZZA catalyst [22]. A certain amount of metal nitrates (copper nitrate, zinc nitrate, zirconium nitrate, and aluminum nitrate) were mixed and diluted with deionized (DI) water to a final volume of 500 mL. The aqueous solution of mixed metal nitrates and an aqueous solution of sodium carbonate were simultaneously added dropwise into 400 mL of preheated DI water (65−70 °C), under vigorously stirring at 400 rpm. The temperature was kept at 65−70 °C and the pH value was kept at 6.5–7.0 during the coprecipitation process. The pH value was immediately adjusted to 7.0 by an aqueous solution of sodium carbonate at the end of co-precipitation. Precipitates were then aged at 70 °C, for 30 min under vigorous stirring at 400 rpm. After aging, the precipitates were filtered under reduced pressure and rinsed several times with warm DI water. Next, the catalyst (solid residues from filtration) was dried in an oven, at 110 °C overnight, then calcined in air at 400 °C for 4 h, at a heating rate of 2 °C/min. To investigate the effects of precursor concentration on catalytic performance, different concentrations of mixed aqueous nitrate 83

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Cu/Zn/Zr/Al atomic ratio of 4:2:1:0.5 has the highest Cu surface area and Cu dispersion, but the smallest Cu particle size. Increasing Al loading slightly decreased Cu surface area and Cu dispersion, while increased the Cu particle size. Decreasing Zr loading to 0.5 or increasing Zr loading to 1.5 from 1.0 also resulted in a decrease in Cu surface area and Cu dispersion, and an increase in Cu particle size. These results indicate that Cu/Zn/Zr/Al atomic ratio of 4:2:1:0.5 is the optimal for CZZA catalyst. The BET surface area and N2O chemisorption results of catalysts prepared by different concentrations are shown in Table 1. Similar to the results of specific surface areas, the largest Cu surface area and Cu dispersion, and the smallest Cu particle size were 44.3 m2/g, 18.9 %, and 4.6 nm, respectively, for the catalyst synthesized with the lowest precursor concentration of 0.1 M. This suggested that the low precursor concentration could provide increased Cu surface area and dispersion, and a reduced Cu particle size. Increasing the precursor concentration from 0.1 M to 0.4 M resulted in a significant decrease in Cu surface area and dispersion and an increase in Cu particle size. With the further increase of precursor concentration, a slight decrease in the Cu surface area and dispersion and an increase in Cu particle size were observed. Compared to a CZA catalyst, synthesized using the same method [22], the addition of Zr reduced the Cu surface area and Cu particle size, while it increased Cu dispersion, especially with a low precursor concentration. This can be explained by the fact that the addition of Zr in a CZZA catalyst reduced the Cu content, as compared to that of CZA and, therefore, improved the dispersion of Cu in the catalyst, which led to a decrease of particle size. Meanwhile, due to the lower Cu content in the CZZA catalyst, compared to that of the CZA, the Cu surface area, per gram of catalyst, was reduced. For a catalyst prepared with a precursor concentration of 0.1 M, the Cu surface area, Cu dispersion, and Cu particle size were 59.3 m2/g, 16.3 %, and 5.8 nm for CZA, and 44.3 m2/ g, 18.9 %, and 4.6 nm for CZZA. Compared to other published results [33–35], our synthesized catalysts, especially those using the lowest precursor concentrations, had good properties, such as high specific surface area, high Cu surface area and Cu dispersion, and small Cu particle size. This could be attributed to the large amount of initial water and low precursor concentration during the co-precipitation process, which could improve particle dispersion and reduce precipitate aggregation during co-precipitation [22]. The results of acidity characterization of bifunctional catalysts are shown in Table S2. The peaks observed for bifunctional catalysts at 175−190 °C, 320−330 °C, and 500−520 °C corresponded to weak, medium, and strong acidic sites. These acidic sites of bifunctional catalysts were attributed to the presence of HZSM-5. CZZA-0.1 had more medium acidic sites, as compared to other catalysts, that were prepared by high precursor concentrations. However, the weak and medium acidic sites of these catalysts contributed to over 92 % of total acidity, which could be helpful in improving DME selectivity in methanol dehydration. TEM and HRTEM images were taken and EDS mapping was done to reveal the morphological properties and atomic distribution of reduced CZZA-0.1 (Fig. 1a, b, and c). Previous reports identified that Zr improved Cu dispersion and reduced the aggregation of Cu clusters [36]. In this study, a TEM image of reduced CZZA-0.1 shows nanoscale particles (ca. 15 nm) with uniform distributions. Crystal fringes, corresponding to Cu (111), were observed and Cu nanoparticles were dispersed homogeneously in the reduced catalysts. In addition, as seen from EDS mapping, the elements of Zn, Zr, Al, and O in reduced CZZA0.1 were very well distributed, indicating that these elements were homogeneously dispersed in the reduced catalyst. These results agree well with previous findings [36,37]. XRD patterns of a reduced catalyst are shown in Fig. 2a. Diffraction peaks at 43.3°, 50.5°, and 74.1°, corresponding to (111), (200), and (220) planes of copper were observed. In addition, diffraction peaks at 31.8°, 34.4°, 36.3°, and 56.6°, corresponding to (100), (002), (101), and

2.4. Catalytic testing of methanol and DME synthesis The activity of the synthesized catalysts was tested using a fixed-bed reactor system as described in our previous reports [22,31]. To evaluate the catalytic activity for methanol production, the CZZA powder catalyst (0.5 g) was diluted by quartz sand (0.5 g, particle size of 60–120 mesh), with a mass ratio of 1:1. CZZA-0.1 was used for all experiments except testing the effects of precursor concentration. The catalysts were reduced at 250 °C for 10 h, with 20 mL/min of pure H2. The reactions were performed at 200−260 °C, with pressure of 2.76 MPa, 6 mL/min CO2, 18 mL/min H2, and space velocity of 823 h-1. The long-term stability tests for methanol synthesis were conducted at 220 °C, 2.76 MPa, 6 mL/min CO2, 18 mL/min H2, and 823 h-1 space velocity. A bifunctional catalyst, composed of CZZA and HZSM-5, was used for DME synthesis from CO2 hydrogenation. The bifunctional catalysts were reduced at the same conditions as those used for methanol synthesis. To optimize the reaction conditions for DME synthesis, reaction temperatures of 220−280 °C and pressure of 2.76 MPa were investigated. All reactions were tested with 6 mL/min CO2, 18 mL/min H2, and 472−917 h-1 space velocity, depending on the HZSM-5 loading. Long-term stability tests for DME synthesis were conducted at 240 °C, 2.76 MPa, 6 mL/min CO2, 18 mL/min H2, and 472 h-1 space velocity. The reaction products were analyzed by an on-line gas chromatography (SRI 8610C), as described previously [22]. CO2 conversion ( XCO2 ), product selectivity (SCO , SMeOH , and SDME) and yield (YCO , YMeOH , and YDME ) were calculated using equations described in a previous report [22]. 3. Results and discussion 3.1. Catalyst characterization Specific surface areas of synthesized catalysts from different precursor concentrations were measured, based on the BET method, and the results are presented in Table 1. The largest surface area of 109 m2/ g was obtained for a CZZA catalyst that was prepared with a precursor concentration of 0.1 M. Increasing the precursor concentration, from 0.1 M to 0.4 M, resulted in a decrease in specific surface area. When the precursor concentration was increased further, the specific surface area did not decrease any more. Compared to the specific surface area of CZA (i.e., 128 m2/g for a CZA catalyst prepared with a precursor concentration of 0.1 M) [22], the specific surface area of CZZA decreased with Zr modification. The reducibility of CZA catalysts and CZZA catalysts, prepared by 0.1 M concentration of precursor solutions, is shown in Fig. S1 in Supplementary Material. The catalysts CZA-0.1 and CZZA-0.1 showed similar H2-TPR profiles, with one single and clear reduction peak. The reduction peak of CZA was at around 220 °C, while the peak of CZZA was at around 245 °C, which were lower than that of pure CuO [29]. Compared to the CZA catalyst, Zr loading decreased the reducibility of CuO. Cu surface area, Cu dispersion, and Cu particle size of catalysts prepared by different atomic ratios were calculated according to N2Ochemisorption and the results are shown in Table S1. The catalyst with Table 1 BET surface area and N2O chemisorption results of CuO/ZnO/ZrO2/Al2O3 catalysts prepared with different concentrations of precursors. Catalyst

BET surface area (m2/g)

Cu surface area (N2O) (m2/g)

Cu dispersion (%)

Cu particle size (nm)

CZA-0.1 CZZA-0.1 CZZA-0.4 CZZA-0.7 CZZA-1.0

128 109 99 99 101

59.3 44.3 33.7 33.5 31.1

16.3 18.9 12.2 12.1 11.2

5.8 4.6 7.2 7.2 7.7

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Fig. 1. TEM, HRTEM, and EDS mapping images of reduced fresh CuO/ZnO/ZrO2/Al2O3 catalyst: (a) TEM, (b) HRTEM, and (c) EDS mapping.

3.2.1. Effects of Al content in CZZA on methanol synthesis According to our previous report [22], the optimized atomic ratio of Cu and Zn for a CuO/ZnO based catalyst was 2:1. In this study, in order to investigate the effects of Al and Zr loadings, we fixed the composition of Cu/Zn at a 4:2 molar ratio for catalyst synthesis. Al, as a structural promoter present in a catalyst, can improve the dispersion of Cu and enhance the synergistic effects between Cu and Zn [42]. The atomic ratio of Cu/Zn/Zr was fixed at 4:2:1 and Al loading varied from 0.5 to 1.5. The catalytic performance of the catalysts is shown in Fig. 3. CO2 conversion increased with the increase in reaction temperature, while the methanol selectivity decreased. Although increasing the reaction temperature enhanced the CO2 conversion, the highest methanol yield was achieved at a relatively low reaction temperature (220 °C), due to the relatively high CO2 conversion and high methanol selectivity at that temperature. At reaction temperatures of 200 °C and 220 °C, CO2 conversion decreased when the Al loading increased from 0.5 to 1 and, then, increased when the Al loading increased from 1 to 1.5. When the reaction temperature increased to 240 °C and 260 °C, the amount of Al had little effect on CO2 conversion. The methanol selectivity decreased with the increase in Al loading. The highest selectivity for methanol was 64.9 % with an Al loading of 0.5 at 200 °C. Methanol yield also decreased with an increase of Al loading. The highest methanol yield was 12.4 %, with Al loading of 0.5 at 220 °C. In contrast to methanol, the CO

(110) planes of zincite were observed. There were no peaks corresponding to zirconium oxides and aluminum oxides, indicating that zirconium oxides and aluminum oxides were either too highly dispersed to form very small particles or existed as an amorphous phase in the reduced CZZA catalysts. Previous studies pointed out that alloying of Zn with Cu could improve CO2 hydrogenation [38,39]. Therefore, we performed a XAS analysis of both reduced CZA and CZZA catalysts to determine the interaction between Cu and Zn. However, we did not observe any alloyed CuZn. As shown in Fig. 2b and c, there was no significant shift in k space, indicating that Cu was not alloyed with any other metals in either CZA or CZZA. This could have been due to the low reduction temperature (250 °C), which could not result in bulk alloy formation [40,41]. 3.2. Catalytic activity for methanol synthesis Fig. S2 shows the effect of space velocity on CO2 conversion, methanol selectivity and yield. CO2 conversion, methanol selectivity, and yield decreased with the increase of space velocity from 823 to 1235 h1 . This was attributed to the decrease in contact time between reactants and catalysts, which resulted in insufficient hydrogenation of CO2. In this study, a fixed flow rate of 24 sccm (mixture of H2 and CO2 at a molar ratio of 3:1) was used for all catalytic activity tests.

Fig. 2. XRD analysis (a, d) of CZZA and HZSM5, and extended x-ray absorption fine structure (EXAFS) analysis (b, c) of reduced CZA and CZZA catalysts in (b) k space and (c) R together with EXAFS reference spectra of Cu foil. Spent CZZA_100 h and spent HZSM-5_100 h were spent catalysts after 100 h of reaction in DME synthesis; spent CZZA_300 h was the spent catalyst after 300 h of reaction in methanol synthesis.

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Fig. 3. Effects of Al content in CZZA catalysts on methanol synthesis and equilibrium data at the same reaction conditions: (a) CO2 conversion, (b) MeOH selectivity, (c) MeOH yield, (d) CO selectivity, and (e) CO yield. Atomic ratios are presented as Cu/Zn/Zr/Al in charts. Equilibrium data were evaluated based on references [43,44].

reaction temperature, a similar trend of CO2 conversion for different Zr loadings was observed, indicating that Zr loading did not significantly impact CO2 conversion. A similar or slight increase in methanol selectivity and yield were observed when Zr loadings increased. At the lowest Zr loading, the highest CO selectivity and yield were observed. Increasing the Zr loading to 1, the CO selectivity and yield decreased. Further increasing the Zr loading did not decrease the CO selectivity and yield. Considering the effects of different Al loadings and Zr loadings, the optimized composition of CZZA was Cu/Zn/Zr/Al = 4:2:1:0.5 at atomic ratio, which is consistent to the results of N2O chemisorption for CZZA catalysts.

selectivity and yield increased with the increase in reaction temperature. Increasing Al loading resulted in an increasing trend in CO selectivity and yield. The best Al loading was found to be 0.5 based on these results. 3.2.2. Effects of Zr content in CZZA on methanol synthesis Fig. 4 shows that CO2 conversion increased with an increase in reaction temperature, from 200 °C to 220 °C, and then remained constant when the reaction temperature increased further. The methanol selectivity was similar when reaction temperatures were at 200 °C and 220 °C; however, it significantly decreased with the further increase in the reaction temperature. Due to the low CO2 conversion, the methanol yield at 200 °C was lower than that at 220 °C. Methanol yield significantly decreased when the reaction temperature increased to 240 °C and 260 °C, as compared to that at 220 °C. Similar CO selectivity was observed at 200 °C and 220 °C. However, it significantly increased with the increase in reaction temperature. The CO yield also increased with the reaction temperature. These results indicated that the optimum reaction temperature for methanol synthesis was 220 °C. At each

3.2.3. Effects of precursor concentration during CZZA synthesis on methanol synthesis The effects of precursor concentration on CZZA catalysts for methanol synthesis were also studied. As shown in Fig. 5a, precursor concentration had no obvious effects on CO2 conversion. However, precursor concentration influenced the methanol and CO selectivity. The methanol selectivity decreased with increasing precursor 86

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Fig. 4. Effects of Zr content in CZZA catalysts on methanol synthesis: (a) CO2 conversion, (b) MeOH selectivity, (c) MeOH yield, (d) CO selectivity, and (e) CO yield. Atomic ratios are presented as Cu/Zn/Zr/Al in charts.

obtained in this study.

concentration, while the CO selectivity increased. There were only minor effects on product yield; the CO yield increased slightly by an absolute value of 1 % (from 7.5 to 8.5 %), with an increase of precursor concentration from 0.1 M to 1.0 M, while methanol yield decreased slightly by an absolute value of 1.3 % (from 12.4 to 11.1 %). These changes could be attributed to the decrease in Cu surface area, when the precursor concentration increased. To understand the effects of catalyst chemo-physical properties on catalytic performance, we did further regression analysis and plotted graphics to show the relationship between catalytic activity and catalyst chemo-physical properties (i.e., BET surface area, Cu surface area, Cu dispersion, and Cu particle size) in both methanol synthesis and DME synthesis. As shown in Fig. 5b-e, there was no linear relationship between catalyst chemo-physical properties and catalytic activity (CO2 conversion, methanol selectivity, and yield) at the confidence level of 90 %. However, Cu surface area, Cu dispersion, and Cu particle size weighed more on methanol selectivity (p < 0.2) than those on CO2 conversion and methanol yield. We have summarized the literature results of methanol synthesis from CO2 in Table S3 and made a comparison with the best results we

3.3. Catalytic activity for DME synthesis 3.3.1. Effects of Al content in CZZA on DME synthesis As discussed in Section 3.2, the optimized atomic ratio of Cu/Zn/Zr/ Al for a methanol synthesis catalyst was 4:2:1:0.5. In order to understand the effects of Al loadings on catalyst performance, we investigated DME synthesis further. The CZZA catalyst composition had a fixed atomic ratio of Cu/Zn/Al at 4:2:1 and Al loading varied from 0.5 to 1.5. CZZA (0.5 g) and HZSM-5 (0.5 g), at a mass ratio of 1:1, was mixed to form a bifunctional catalyst for DME synthesis. As shown in Fig. 6, for all catalysts with different Al loadings, CO2 conversion and DME yield first increased with an increase in reaction temperature from 220 °C to 240 °C, and then decreased with the increase in reaction temperature from 240 °C to 280 °C. A relatively high DME selectivity was observed at 220 °C and 240 °C, indicating that low reaction temperature favored DME selectivity. When the reaction temperature increased further, CO2 conversion, DME selectivity, and yield greatly decreased, which was similar to our observation of DME 87

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Fig. 5. Effects of precursor concentration on (a) methanol synthesis and (b, c, d, e) regression analysis between chemo-physical properties (i.e., (b) BET surface area, (c) Cu surface area, (d) Cu dispersion, and (e) Cu particle size) and catalyst activity (i.e., CO2 conversion, methanol selectivity, and yield) in methanol synthesis.

yield than that of 0.5 and 1.

synthesis from CO2 using CuO/ZnO/Al2O3 [22]. On the contrary, CO selectivity and yield increased significantly with the increase in reaction temperature. Similar methanol selectivity and yield were observed at each reaction temperature, indicating that the reaction temperature had no significant effects on methanol production. Al loading had a significant effect on the CO2 conversion, product selectivity, and yield. As shown in Fig. 6, CO2 conversion, DME selectivity, and yield decreased with the increase in Al loading, from 0.5 to 1.5, for all synthesized catalysts at reaction temperatures of 220 °C–260 °C. At 280 °C, CO2 conversion remained similar, with Al loading at 0.5 and 1.0, but greatly decreased when Al loading increased to 1.5. DME selectivity and yield greatly decreased, first with the increase in Al loading to 1.0 and, then, remained similar with a further increase in Al loading to 1.5. The highest DME yield was 18.3 %, with a CO2 conversion of 26.5 % and DME selectivity at 69.2 %, when the Al loading was at 0.5 and the reaction temperature was 240 °C. CO selectivity and yield increased with the increase in Al loading from 0.5 to 1 and, then, remained similar, when the Al loading increased further from 1 to 1.5. Methanol selectivity and yield remained similar, with Al loading at 0.5 and 1, and then increased with an Al loading of 1.5 at reaction temperatures of 220 °C–260 °C. At reaction temperature of 280 °C, the Al loading of 1 showed a higher methanol selectivity and

3.3.2. Effects of Zr content in CZZA on DME synthesis As discussed in Section 3.2.1, the optimized Al loading was 0.5. We further investigated the effects of Zr loadings on catalyst performance for DME synthesis. The CZZA catalyst composition had a fixed atomic ratio of Cu/Zn/Al at 4:2:0.5, and we varied Zr loading from 0.5 to 1.5. CZZA (0.5 g) and HZSM-5 (0.5 g), at a mass ratio of 1:1, was mixed to form a bifunctional catalyst for DME synthesis. As shown in Fig. 7, the amount of Zr affected catalytic performance and volcano trends for CO2 conversion, DME selectivity and yield were observed with an increase in Zr loading at each reaction temperature, except at 220 °C. The highest CO2 conversion, DME selectivity, and yield were obtained when Zr loading was 1.0. On the contrary, CO and methanol selectivity and yield showed reversed volcano trends with the increase of Zr loading at each reaction temperature. Low CO selectivity and yield were observed with Zr loading of 1. Reaction temperature also had significant effects on CO2 conversion, product selectivity, and yield. Our previous study of CZA catalyst performance indicated that the optimum reaction temperature for DME was 240 °C [22]. Similarly, the optimum reaction temperature for CZZA was also 240 °C, and the highest DME yield achieved was 18.3 % when 88

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Fig. 6. Effects of Al content in a CZZA catalyst on DME synthesis with equilibrium data at the same reaction conditions: (a) CO2 conversion, (b) DME selectivity, (c) DME yield, (d) CO selectivity, (e) CO yield, (f) MeOH selectivity, and (g) MeOH yield. Atomic ratios are presented as Cu/Zn/Zr/Al in charts. Equilibrium data were evaluated based on references [43,44].

reaction temperature.

the catalyst was composed of Cu/Zn/Zr/Al at 4:2:1:0.5 (atomic ratio). The DME yield obtained was higher than those reported in previous papers [12,22,25]. At reaction temperatures of 220 °C and 240 °C, relatively low CO selectivity and yield were obtained, indicating that the low reaction temperature was unfavorable for CO production. Methanol selectivity and yield showed a slight decrease trend with the increase in

3.3.3. Effects of precursor concentration during CZZA synthesis on DME production Similar to those of methanol synthesis (discussed in Section 3.2), the CO2 conversion, DME selectivity, and DME yield slightly decreased with 89

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Fig. 7. Effects of Zr content in CZZA catalyst on DME synthesis: (a) CO2 conversion, (b) DME selectivity, (c) DME yield, (d) CO selectivity, (e) CO yield, (f) MeOH selectivity, and (g) MeOH yield. Atomic ratios are presented as Cu/Zn/Zr/Al in charts.

We also did a regression analysis and plotted graphics to show the relationship between catalytic activity and catalyst chemo-physical properties in DME synthesis. As shown in Fig. 8b–f, the catalytic activity in DME synthesis had no significant correlation with the BET surface areas of the catalysts. However, there was a strong linear relationship between catalyst chemo-physical properties (Cu surface area, Cu dispersion, Cu particle size, and total acid) and CO2 conversion/DME

the increase in precursor concentration, while the CO selectivity increased (Fig. 8 a). This could be due to the decreases in the BET surface area, Cu surface area, and Cu dispersion with increasing precursor concentration for CZZA synthesis. Since HZSM-5 in bifunctional catalysts can efficiently convert methanol to DME, only a very small amount of methanol was detected. Methanol selectivity and yield remained similar for catalyst synthesis by different precursor concentrations. 90

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Fig. 8. Effects of precursor concentration on (a) DME synthesis and (b, c, d, e, f) regression analysis between chemo-physical properties (i.e., (b) BET surface area, (c) Cu surface area, (d) Cu dispersion, (e) Cu particle size, and (f) total acid) and catalyst activity (i.e., CO2 conversion, DME selectivity, and yield) in DME synthesis.

yield. At a confidence level of 90 %, the Cu surface area and Cu particle size had significant effects on both CO2 conversion and DME yield. Cu dispersion and total acid also had a significant effect on CO2 conversion. These results suggested that the Cu properties (Cu surface area, Cu dispersion, and Cu particle size) on the catalyst surface and the acidity of the bifunctional catalyst were critical in determining CO2 conversion to DME. The literature results of DME synthesis from CO2, using bifunctional catalysts, are summarized in Table S4. Compared to these literature data, our synthesized bifunctional catalyst, composed of CZZA and HZSM-5, showed a better performance than others. Again, the very good chemo-physical properties of our synthesized CZZA could be a major factor contributing to the excellent performance. 3.3.4. Effects of HZSM-5 loading on DME synthesis The effects of HZSM-5 loadings on catalytic performance were also investigated. Previous studies showed that HZSM-5 loading significantly affected DME production; the DME production showed a volcano pattern with HZSM-5 loading, when an optimum HZSM-5 loading was at 50 wt.% of a bifunctional catalyst [45]. However, in this

Fig. 9. Effects of HZSM-5 loading (i.e., 0.1 g, 0.2 g, 0.3 g, and 0.5 g) on DME synthesis.

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3.4.2. Catalyst stability for DME synthesis To further understand the stability of a Zr-modified CZA catalyst for DME synthesis, CZZA was mixed with HZSM-5 to form a bifunctional catalyst and tested for 100 h. As shown in Fig. 11, the bifunctional catalyst, prepared by Zr modified CZA (CZZA-0.1) and HZSM-5, was more stable than CZA and HZSM-5, which was similar to the performance for methanol synthesis. After 100 h of reaction, CO2 conversion reduced from 27.1 to 24.1 % for the CZZA plus the HZSM-5 catalyst, while CO2 conversion for the CZA plus HZSM-5 catalyst reduced from 26.2 to 22.0 %. After 100 h of reaction, the DME yield reduced from 18.5 to 14.1 % for CZZA plus the HZSM-5 catalyst, and the DME selectivity was still over 58.7 %. These results suggested that Zr modification of the CZA catalyst could improve stability, which was consistent with a previous report [49].

study, HZSM-5 loading did not affect CO2 conversion, as shown in Fig. 9. HZSM-5 loading had a very small impact on CO and DME selectivity. As a result, no significant changes in CO yield, methanol yield, and DME yield were observed, even when the HZSM-5 loading was reduced to less than 20 wt.% of the bifunctional catalyst. This indicated that the effects of HZSM-5 loading on DME synthesis were not significant when HZSM-5 loadings were between 0.1 to 0.5 g for a short reaction time. The HZSM-5 loading could impact the long term stability of the catalyst. These results were similar to our observations on DME synthesis, using a bifunctional catalyst of CZA and HZSM-5 [31].

3.4. Catalyst stability for methanol and DME synthesis 3.4.1. Catalyst stability for methanol synthesis Catalyst deactivation is a major issue in the commercialization of a catalyst. In previous studies, the CZA catalyst has showed good activity, but the stability of CZA was still undesirable [14,15,46,47]. The presence of Al in the catalyst made the catalyst hydrophilic, which resulted in a decrease in catalytic activity due to water adsorption [48]. In this study, a Zr-modified CZA catalyst (CZZA-0.1) showed improved stability. After 300 h of reaction, CO2 conversion, methanol selectivity and yield decreased from 20.8 to 20.0 %, from 62.1 to 58.5 %, and from 13.0 to 11.7 %, respectively. In contrast, the CZA catalyst (using 0.1 M precursor concentration with Cu/Zn/Al atomic ratio of 6/3/1), prepared in our lab, showed a considerable decrease in CO2 conversion, methanol selectivity, and yield (Fig. 10); after 100 h of reaction, the CO2 conversion, methanol selectivity, and methanol yield decreased from 21.3 to 18.9 %, from 64.9 to 56.8 %, and from 12.8 to 10.8 %, respectively. These results indicated that the CZZA catalyst was much more stable than the CZA catalyst. That could be attributed to the hydrophobic property of Zr, which can repel produced water away from the active sites of a catalyst and, thus, reduce the opportunity for Cu oxidation by water [29].

3.4.3. Characterization of spent catalysts Coking and Cu sintering have been considered as major reasons for catalyst deactivation. Previous studies pointed out the presence of detrimental interactions, such as migration of metal ions between hydrogenation and dehydration catalysts, which could block acidic sites [20,21,31,32]. To understand the deactivation mechanism of Zr modified CZA catalysts and HZSM-5 during CO2 conversion to DME, the spent catalysts used in methanol synthesis (300 h) and DME synthesis (100 h) were characterized. Before characterization, the bifunctional catalyst used in DME synthesis was separated as spent CZZA and spent HZSM-5 in order to clearly understand the deactivation of each catalyst. As shown in Table 2, after 300 h of methanol synthesis, the specific surface area of the spent CZZA was still similar to that of the fresh catalyst; however, the Cu surface area and dispersion had significantly decreased, while the Cu particle size had increased. For the spent CZZA catalyst used in DME synthesis, the specific surface area had decreased significantly after 100 h on stream; the Cu surface area and dispersion had also significantly decreased and the Cu particle size had increased for the CZZA catalyst used in DME synthesis. Compared to the spent CZZA used in methanol synthesis, the spent CZZA catalyst used in DME

Fig. 10. Comparison of catalyst stability between CZZA and CZA for methanol synthesis: (a) CO2 conversion, (b) MeOH selectivity, and (c) MeOH yield. 92

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Fig. 11. Comparison of the catalyst stability of CZZA and CZA for DME synthesis: (a) CO2 conversion, (b) DME selectivity, and (c) DME yield.

study, after 287 °C, the curves of the spent CZZA were almost parallel to those of the fresh catalyst. The weight loss of spent CZZA (after 300 h of methanol synthesis) between 287 °C and 900 °C in TGA analysis was only about 0.05 %. For spent CZZA, after 100 h of reaction in DME synthesis, the weight loss was about 0.15 % in the same temperature range of TGA analysis, which was three times that for the spent CZZA used in methanol synthesis. This result indicated that more coking could be formed for the CZZA catalyst during DME synthesis. The methoxy ions were intermediates in the formation of methanol and could also be intermediates in the formation of coke precursors [52]. More methoxy ions were formed in DME synthesis than methanol synthesis from CO2, which could result in more coke formation. However, the amounts of coking for both spent CZZA catalysts were very small and, therefore, were unlikely to significantly impact performance, as compared to the over 2 % coking on spent CZA catalysts reported in the literature [50]. The amount of coking for spent HZSM-5 was about 1.87 % (Fig. 12b), which was much higher than that of the spent CZZA catalyst. These results indicated that the coking on the HZSM-5 could be responsible for the deactivation of the bifunctional catalyst. After 300 h of methanol synthesis and 100 h of DME synthesis, the active Cu sites in both spent CZZA catalysts were changed (Fig. 2a). Peaks were observed 35.5° and 38.7° that corresponded to (002) and (111) planes of tenorite, indicating that the copper active sites were oxidized during long-term reaction. The comparison of XRD patterns between fresh and spent HZSM-5 catalysts are shown on Fig. 2d. The intensity of spent HZSM-5 reduced after 100 h of reaction for DME synthesis, indicating that the crystallinity of HZSM-5 had decreased.

synthesis showed a much lower Cu surface area and Cu dispersion, and larger Cu particle size. These results indicated that the CZZA catalyst tended to be less stable in DME synthesis, when mixed with HZSM-5. As more water was generated in DME synthesis, this could have resulted in hydrothermal sintering of Cu, thereby reducing the Cu surface area and increasing Cu particle size, leading to the deactivation of CZZA. The surface area of spent HZSM-5 also showed a significant decrease, as compared to the fresh as-prepared powder catalyst (392 m2/g), which could be one of major reasons for the deactivation of bifunctional catalysts. TGA analysis for these spent catalysts was also performed and the results are shown in Fig. 12a. Due to Cu oxidation in the reduced fresh CZZA, the TGA curve for fresh CZZA first increased until it reached 287 °C; after that, a slight decrease was observed, which could have been due to the presence of a residual precursor during calcination (360 °C). Compared to fresh CZZA, the spent CZZA (used in methanol synthesis and DME synthesis) decreased with increasing temperature, when the temperature was lower than 287 °C. One reason was that some of the Cu in the spent catalyst had already been oxidized during the reaction. Previous studies also found that the type of coke formed depended on the oxidation temperature. Some small carbon molecules formed unstable coke, which could be burned at a relatively low temperature (less than 300 °C) [50,51]. Therefore, in this study the weight loss before 287 °C could be attributed to these small carbon molecules. Some more stable coke could be burned off at temperatures over 300 °C, which was the primary reason for the catalyst deactivation. This coke was hard to remove at relatively low regeneration temperatures. In this

Table 2 BET surface area and N2O chemisorption of spent CZZA and spent HZSM-5 catalysts. Catalyst

Reaction

BET surface area (m2/g)

Cu surface area (N2O) (m2/g)

Cu dispersion (%)

Cu particle size (nm)

CZZA CZZA CZZA HZSM-5 HZSM-5

Fresh Methanol synthesis, 300 h DME synthesis, 100 h Fresh DME synthesis, 100 h

109 106 76 392 261

44.3 17.9 14.1 – –

18.9 6.5 5.1 – –

4.6 13.5 17.1 – –

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Fig. 12. TGA analysis of spent catalysts: (a) CZZA and (b) HZSM-5. Spent CZZA_100 h and spent HZSM-5 were catalysts after 100 h of reaction in DME synthesis at 240 °C and 2.76 MPa; spent CZZA_300 h was the catalyst after 300 h of reaction in methanol synthesis at 220 °C and 2.76 MPa.

Fig. 13. TEM, HRTEM, and EDS mapping images of spent CuO/ZnO/ZrO2/Al2O3 catalyst: (a) TEM, (b) HRTEM, and (c) EDS mapping.

Declaration of Competing Interest

The peaks of spent HZSM-5 also shifted, to a high degree, as compared to those of fresh HZSM-5. This could be due to the unit cell dimension change for the crystal induced by coking and the operating conditions of high reaction temperature and pressure. TEM and HRTEM images for spent catalysts showed an aggregation of particles (Fig. 13a, 13b, and 13c). EDS elemental mapping also showed that Cu on the surface had partially aggregated. These results indicated that the Cu sintering occurred after a long-term reaction, which led to the reduction of the Cu surface area and dispersion. This observation was consistent with the results of N2O chemisorption for Cu.

The authors declare no competing financial interest. Acknowledgements This work was supported by the U.S. Department of Energy through contract DE-AR0000806. We thank DOE ARPA-E Program Director, Dr. Grigorii Soloveichik, for his assistance and support. Use of the Advanced Photon Source (9-BM) by the Argonne National Laboratory, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. We also thank Dr. Xiaoqing He at the Electron Microscopy Core Facility, University of Missouri-Columbia for TEM and EDS analysis.

4. Conclusion

Appendix A. Supplementary data

In this study, Zr modified CZA catalysts were synthesized by the coprecipitation method. The effects of Al and Zr loading were investigated in both methanol and DME synthesis from CO2 hydrogenation. The optimized catalyst composition for methanol and DME synthesis was 4:2:1:0.5 for an atomic ratio of Cu/Zn/Zr/Al. The catalyst showed a maximum methanol yield of 12.4 % at a reaction temperature of 220 °C and a pressure of 2.76 MPa, and a maximum DME yield of 18.3 % at a reaction temperature of 240 °C and pressure of 2.76 MPa, which were much higher than those of the CZA catalyst. A long-term stability test indicated that the CZZA catalyst had superior stability during methanol synthesis and a significant improvement in stability for DME synthesis, as compared to CZA. Characterization of the spent catalysts indicated that the CZZA was less stable during DME synthesis than for methanol synthesis. A significant decrease in the BET surface area and higher coking in the HZSM-5 catalyst could be a primary reason for the deactivation of the bifunctional catalyst for DME synthesis via CO2 hydrogenation.

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