Al2O3 Catalyst Prepared by Mechanical ... - ACS Publications

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Article

CuO/ZnO/Al2O3 Catalyst Prepared by Mechanical-Force-Driven Solid-State Ion Exchange and Its Excellent Catalytic Activity under Internal Cooling Condition Wangyang Wu, Kai Xie, Dalin Sun, Xiaohong Li, and Fang Fang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01464 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on July 1, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

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

Industrial & Engineering Chemistry Research

CuO/ZnO/Al2O3 Catalyst Prepared by Mechanical-Force-Driven Solid-State Ion Exchange and Its Excellent Catalytic Activity under Internal Cooling Condition Wangyang Wua, Kai Xiea, Dalin Suna, Xiaohong Lib,*, and Fang Fanga,* a b

Department of Materials Science, Fudan University, Shanghai 200433, China

Faculty of Environmental Engineering, The University of Kitakyushu, Kitakyushu, Fukuoka 808-0135, Japan

* *

Corresponding author. E-mail: [email protected] Corresponding author. E-mail: [email protected] 1

ACS Paragon Plus Environment


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

ABSTRACT CuO/ZnO/Al2O3 catalysts were prepared by a mechanical-force-driven solid-state ion exchange method, and their catalytic performance for methanol synthesis was investigated in a manufactured reactor with an internal cooling system. With the increasing of milling speed during ball-milling, the ion exchange between Cu2+ and Zn2+ in catalyst precursors is enhanced. After calcination, CuO nanoparticles are neighboring to ZnO nanoparticles and ZnO nanoparticles serve as spacers to prevent the agglomeration of CuO nanoparticles, leading to a cross-distribution of CuO and ZnO in catalysts. The as-prepared catalysts exhibit excellent catalytic activities and the highest CO2 conversion and CH3OH yield at 240 °C and 4 MPa can reach 59.5% and 43.7%, respectively. The extraordinary catalytic performance can be attributed to both the cross-distribution of CuO and ZnO nanoparticles caused by solid-state ion exchange and the promotion of reversible CO2 hydrogenation reaction towards methanol synthesis by the internal cooling system. Keywords: Cu/ZnO catalysts, ion exchange, internal cooling system, CO2 hydrogenation, methanol synthesis

2


Page 2 of 23

Page 3 of 23

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


1. INTRODUCTION Increasing carbon dioxide (CO2) concentration in the atmosphere has been considered as a major anthropogenic source for global warming and climate change due to the greenhouse effect.1-4 The chemical hydrogenation of CO2 to methanol under the catalysis of Cu-Zn catalysts has been recognized as one of the most effective and economical ways to keep CO2 out of the atmosphere because methanol shows significant economic value as a fuel or to produce a wide variety of chemicals, such as olefins and gasoline.5-15 Currently, commercial Cu-Zn catalysts that also contain a small amount of Al2O3 are usually prepared by the co-precipitation method via several steps: precipitation, aging, filtration, drying, calcination, and reduction. Their catalytic activity depends on such factors as their components, active surface area, CuO and ZnO distribution.16-21 Among these influence factors, the distribution of CuO and ZnO has been proven to be one of the most important factors, which is mainly controlled during the aging step. During the aging process, the mutual substitution between Cu2+ in malachite (Cu2CO3(OH)2) and Zn2+ in hydrozincite (Zn5(OH)6(CO3)2) occurs in the mother liquor, resulting in the formation of Cu/Zn hydroxy carbonate precursors, i.e., zincian malachite ((Cu,Zn)2CO3(OH)2) and/or aurichalcite ((Cu,Zn)5(OH)6(CO3)2),22-27 which leads to an outstanding catalytic activity after calcination. However, this ion exchange between Cu2+ and Zn2+ to form Cu/Zn hydroxy carbonate precursors is very susceptible to the aging conditions, such as the pH value of mother liquor, aging temperature, aging duration and stirring velocity.22,25,27-29 For example, Farahani et al.29 reported that at a constant aging time of 5 h, the catalytic activity of a prepared Cu-Zn catalyst for methanol synthesis first increases, passes through a maximum, and then decreases with increasing aging temperature from 40 °C to 80 °C and the highest activity was obtained at 60 °C. In addition, the fact that the Cu/Zn hydroxy carbonate precursors were obtained from the solid phase of the raw material, dissolved to the liquid state and then recrystallized to solid state via tedious multistep processing is very inconvenient and uneconomical. Therefore, a simple preparation method for solid Cu/Zn hydroxy carbonate precursors is highly desired to reduce the influence of environmental conditions and avoid dissolution and recrystallization processes. Recently, we found that cations between solid-phase materials can be mutually replaced during the mechanical-force-driven ball-milling process. For instance, Ikeda et al.30 reported that the LixNa1-xMgH3 and Li1-xNaxH (0≤x≤0.5) phases were formed by the mechanical-force-driven 3



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

exchange between Na+ and Li+ during ball-milling of NaMgH3 and LiH. Inspired by their work, if the mutual substitution between Cu2+ and Zn2+ could be realized by mechanical-force-driven solid-state ion exchange, the Cu/Zn hydroxy carbonate precursors could be simply prepared to avoid sensitive and complex co-precipitation processes. Moreover, during the chemical hydrogenation of CO2, two reversible reactions, i.e. methanol synthesis reaction (Equation 1) and reverse water-gas shift reaction (RWGS, Equation 2), can occur simultaneously.

CO2+3H2 CH3 OH+H2 O

CO2 +H2 CO+H2 O

(1) (2)

The methanol yield is limited by the equilibrium conversion of both methanol synthesis reaction and unavoidable RWGS side reaction. From the Equations 1 and 2, it is noted that the mole number of products decreases from 4 to 2 by 50% toward the methanol formation in equation (1) but remains constant in equation (2), indicating that the higher the partial pressure of CO2 and H2, the higher the methanol yield. Therefore, two feasible strategies are proposed to increase the partial pressure of CO2 and H2. One is to directly increase the pressure of the reactant gas of CO2 and H2.16 The other is to remove methanol and water from the reversible reaction to indirectly increase the partial pressure of the reactant gases. However, the former requires an energy-intensive gas compression and brings on a series of security issues owing to the high pressure. Hence, the latter is believed to be more feasible and valuable. Since the liquefaction temperatures of 64.7 °C and 100 °C for methanol and water are far higher than -78.5 °C and -252.8 °C for CO2 and H2, transferring methanol and water from the gas phase to the liquid phase to separate them from the reversible reaction is an effective and feasible method to indirectly increase the partial pressure of the reactant gases and thus promote the hydrogenation of CO2 to methanol. In this study, CuO/ZnO/Al2O3 catalysts were first prepared by mechanical-force-driven solid-state ion exchange under different milling speeds and the mutual substitution between Cu2+ and Zn2+ was also investigated in detail. Subsequently, the catalytic performance of the prepared CuO/ZnO/Al2O3 catalysts was evaluated in a specially manufactured reactor with an internal cooling system. The results showed that the complete mutual substitution between Cu2+ and Zn2+ 4


Page 4 of 23

Page 5 of 23

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


during mechanical ball-milling resulted in an cross-distribution of nano-sized CuO and ZnO in CuO/ZnO/Al2O3 catalysts. With the help of the internal cooling system to transfer methanol and water to liquid phase, the prepared CuO/ZnO/Al2O3 catalysts exhibited fantastic catalytic performances. The highest CO2 conversion and CH3OH yield could reach 59.5% and 43.7% at the reaction conditions of 240 °C and 4 MPa, respectively. 2. EXPERIMENTAL SECTION 2.1 Catalyst Preparation (a) Cu/Zn hydroxy carbonate precursors. The Cu/Zn hydroxy carbonate precursors with a Cu/Zn atomic ratio of approximately 70/30 were prepared by the mechanical ball-milling approach. The malachite (Cu2CO3(OH)2) and hydrozincite (Zn5(OH)6(CO3)2) were loaded into a stainless steel vial and about 3 mL CH3COCH3 was also added to the vial to facilitate the diffusion of Cu2+ and Zn2+. The mechanical ball-milling process was performed in a planetary mill (QM-1SP2, Nanjing University, China) with a 15:1 ball-to-powder ratio for 10 h at speeds of 200, 300 or 400 rpm. Moreover, the pristine malachite and hydrozincite with ca. 3 mL CH3COCH3 were milled for 10 h at 400 rpm as a reference sample, respectively. (b) CuO/ZnO/Al2O3 catalysts. It was known that the addition of Al2O3 as a structure promoter can effectively improve the catalytic performance of a CuO/ZnO catalyst.19,31,32,33 Therefore, the obtained Cu/Zn hydroxy carbonate precursors and Al2O3 powder were further milled with a 58:25:17 molar ratio of Cu:Zn:Al for 1 h at corresponding milling speeds of 200, 300 and 400 rpm, respectively. Next, the precursors with Al2O3 were calcined in air at 320 °C for 3 h to form CuO/ZnO/Al2O3 catalysts. The final CuO/ZnO/Al2O3 catalysts were designated as CZAx (C = CuO, Z = ZnO, A = Al2O3, x = milling speed), respectively, and the corresponding Cu/Zn hydroxy carbonate precursors were indicated as CZAx precursors. For the purpose of comparison, a CuO/ZnO/Al2O3 sample with same molar ratio of Cu:Zn:Al was prepared by traditional co-precipitation method (see experimental detail in Supporting Information) and was termed as CZAcp. 2.2 Catalyst Characterization The phase structure of the samples was characterized by powder X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer) with a scanning angle (2θ) of 10°–50°. The decomposition behavior of the CZAx precursors was characterized by thermogravimetry (TG) 5



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

coupled with mass spectrometry (MS) (Netzsch STA 409 PC connected to a GSD 301T). The morphology of CZA400 was observed by transmission electron microscopy (TEM) using a Shimadzu SUPERSCAN SSX-550 instrument. 2.3 Catalytic Performance The catalytic activity was measured in the range of 220–260 °C and at 2–4 MPa in a continuous-flow, fixed-bed reactor equipped with a specially manufactured internal cooling system, as shown in Figure 1. The internal cooling water system consisted of a pump and copper tube that was made in a spiral form to increase the efficiency of heat exchange. Samples (5.5 g, 40–60 mesh) diluted with 4.5 g quartz sand were loaded into the reactor. Prior to the activity measurements, the samples were heated at 280 °C for 3 h in a flow of 10 vol.% H2/N2 (200 mL/min) at atmospheric pressure to reduce CuO. After the reduction, the temperature was reduced to room temperature. Subsequently, the reactant gas (CO2:H2:Ar = 24.04:72.95:3.01) was introduced at a flow rate of 80 mL/min and the gas pressure was controlled at 2–4 MPa by a pressure controller (PC in Figure 1). The reactor was heated to 220–260 °C to measure the catalytic activity. During the activity measurement, cooling water flowed through the copper tube inside the reactor with a constant rate of 200 mL/min to condense the generated H2O and methanol from the gaseous products. Effluent gases, such as CO, CO2, H2 and Ar, were analyzed on line with a gas chromatograph (Shimadzu GC-8A) equipped with a thermal conductivity detector (TCD) and a packed column of activated charcoal. The liquid products, such as methanol and water, were analyzed by a gas chromatograph (Shimadzu GC-2014) equipped with a flame ionization detector (FID) and a capillary column of Porapak-Q after finishing the catalyst tests. The CO2 conversion, the selectivity of methanol and CO and the CH3OH yield were calculated by the internal standard method and the calculation details are provided in Supporting Information.

3. RESULTS AND DISCUSSION 3.1 Structural Evolutions of CZAx Precursors with Increasing Milling Speed (a) Phase composition. The XRD patterns of CZAx precursors (x = 200, 300 and 400) are shown in Figure 2. For comparison, the XRD results of ball-milled malachite and hydrozincite at the milling speed of 400 rpm are also presented in Figure 2d and 2e, respectively. From Figure 2(a–c), it can be seen that the diffraction peaks corresponding to malachite and hydrozincite are 6


Page 6 of 23

Page 7 of 23

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


gradually weaken with the increase of milling speed from 200 to 300 rpm and disappear when the milling speed reaches 400 rpm, suggesting the amorphization of raw materials themselves by ball-milling or the formation of amorphous Cu/Zn hydroxy carbonate by the mutual substitution between Cu2+ and Zn2+. However, from Figure 2d and 2e, ball-milled malachite is amorphous and obvious diffraction peaks belonging to hydrozincite can still be found in ball-milled hydrozincite. Therefore, it is reasonable to conclude that the mechanical-force-driven mutual substitution between Cu2+ and Zn2+ occurs during ball-milling and is enhanced by increasing the ball-milling speed, leading to a formation of amorphous Cu/Zn hydroxy carbonate. To further confirm this point, the thermal decomposition of the CZAx precursors is studied by TG-MS. (b) Thermal properties. The decomposition behavior of CZAx precursors (x = 200, 300 and 400) is depicted in Figure 3. From the TG curves, the overall mass loss of approximately 27.8% was observed for all samples, which was in good agreement with the theoretical value of 28.2% for the complete transformation from raw materials into corresponding metal oxides. These weight losses definitely suggest that raw materials and/or the formed precursors do not decompose into CO2 and H2O during the ball-milling process. Furthermore, detailed analysis of the mass loss via the MS curves shows that the total weight loss can be divided into three regions: low (25–150 °C), medium (150–370 °C) and high (370–550 °C) temperature regions. First, in the low-temperature region, the mass loss is mainly attributed to the elimination of interlayer or physically adsorbed water molecules for all CZAx precursors. Second, in the medium temperature region, two peaks at 273 °C and 321 °C are observed in both the CO2 and H2O MS curves in Figure 3a for the CZA200 precursor, corresponding to simultaneous emissions of CO2 and H2O. However, when the milling speed increases to 300 rpm and 400 rpm, H2O emissions of CZA300 and CZA400 precursors shift to a lower temperature and occur at a wide temperature range from 150 to 370 °C. In addition, only a single peak for CO2 emission in the medium temperature region is observed for CZA300 and CZA400 precursors and its intensity decreases with increasing milling speed. Third, in the high temperature region, no CO2 and H2O are emitted from CZA200 precursor, while an obvious peak for CO2 emission presents at 421 °C in Figure 3b for CZA300 precursor and shifts to 449 °C along with an intensity enhancement in Figure 3c for CZA400 precursor. Therefore, from Figure 3, three features can be summarized as follow: i) compared with the decomposition temperatures of 270 °C for milled hydrozincite and 325 °C for milled malachite in Figure S1, the same 7



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

temperature for CO2 and H2O emissions in Figure 3a indicates that CZA200 precursor mainly consists of hydrozincite and malachite and the mutual cation substitution between Cu2+ and Zn2+ hardly occurs during the ball-milling process, which is in accordance with the observation of the XRD pattern in Figure 2d; ii) the CO2 emission above 400 °C is attributed to the decomposition of the so-called high-temperature carbonate species (HT-CO3), which have been proved to be present only in the Cu/Zn hydroxy carbonate rather than the raw materials of hydrozincite and malachite.34–36 Therefore, the intensity enhancement of the decomposition peak of HT-CO3 with increasing milling speed from 200 to 400 rpm proves that the mechanical-force-driven mutual substitution between Cu2+ and Zn2+ is enhanced to form more Cu/Zn hydroxy carbonate with the increase of milling speed; and iii) most of CO2 is emitted from the decomposition of HT-CO3 in CZA400, suggesting that most of the hydrozincite and malachite have been successfully changed to Cu/Zn hydroxy carbonate precursors after mechanical ball-milling at 400 rpm. 3.2. Structure of CZAx catalysts After the addition of Al2O3 and calcination, the phase structures of CZAx were characterized by XRD and the results are shown in Figure 4. It can be found that broad peaks at 35.5°, 38.7° and 48.7° are attributed to the CuO phase, and the small peaks at 31.8°, 34.4° and 36.3° belong to the ZnO phase. With increasing milling speed from 200 rpm to 400 rpm, the CuO and ZnO peaks broaden and decrease in intensity, indicating the increasingly poor crystallinity of CuO and ZnO phases with increasing milling speed. Moreover, no Al2O3 peaks can be found in any of the three samples. The absence of the Al2O3 phase is consistent with previous reports by Baltes22 and Plyasova37 and may be attributed to the added Al2O3 being changed to an amorphous state after ball-milling. To further investigate the distribution of CuO, ZnO and Al2O3 particles, the prepared CZA400 as a typical sample was characterized by TEM. TEM micrographs of the CZA400 catalyst are shown in Figure 5. From Figure 5a and 5b, it can be seen that black and gray particles with a particle size ranging from 5 to 20 nm cover the light gray substrate. The EDS mapping results of Cu, Zn, Al and O in Figure S2 prove that this light gray substrate mainly consists of an Al2O3 phase (marked by the blue oval in Figure 5a and S2a) and the area covered by black and gray particles contains CuO, ZnO and Al2O3 phases (marked by the white oval in Figure 5a and S2a), demonstrating that CuO and ZnO nanoparticles are supported and dispersed by relative large Al2O3 particles. Furthermore, the HRTEM image in 8


Page 8 of 23

Page 9 of 23

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


Figure 5b shows that ZnO nanoparticles serve as spacers to prevent the agglomeration of CuO nanoparticles, resulting in an alternating fashion of CuO and ZnO nanoparticles. Combined with the XRD and TG results, this alternating fashion of CuO and ZnO nanoparticles is an exact result of the complete mutual substitution between Cu2+ and Zn2+ in the precursors. 3.3. Catalytic performance with internal cooling system (a) Influence of milling speed on methanol synthesis. The prepared CZAx are first reduced for 3 h and then the catalytic performance with an internal cooling system is measured at 240 °C and 3 MPa to investigate the influence of milling speed on methanol synthesis. Table 1 shows the catalytic performance of CZAx. For comparison, the catalytic performance of CZAcp is also compared in Table 1. From Table 1, the CO2 conversion and the methanol selectivity increase from 31.3% and 50.7% for CZA200, 40.1% and 58.1% for CZA300 to 50.0% and 64.2% for CZA400, respectively, which means that the methanol yield increases from 15.8% for CZA200 to 23.3% for CZA300 to 32.1% for CZA400. Obviously, the catalytic performance is significantly improved with increasing milling speed from 200, 300 to 400 rpm. According to the characterization results in sections 3.1 and 3.2, the improvement of catalytic performance is ascribed to increasingly enhanced mutual substitution between Cu2+ and Zn2+ with increasing milling speed. In addition, the CO2 conversion, CH3OH selectivity and CH3OH yield for CZAcp are 52.7%, 69.4% and 36.6%, respectively. Compared with the catalytic performance of CZAcp, CZA400 shows slightly poorer catalytic performance, which would be further discussed in section 3.4. (b) Influence of reaction temperature on methanol synthesis. Figure 6 shows the catalytic performance of CZAx with an internal cooling system at 3 MPa and 220–260 °C. For comparison, the equilibrium yields of CH3OH at 3 MPa and 220–260 °C without internal cooling system were also calculated theoretically and presented in Figure 6c. From Figure 6a and 6b, it can be seen that with the rise of reaction temperature, the CO2 conversion of CZAx gradually increases while the CH3OH selectivity slightly decreases. For instance, the CO2 conversion of CZA400 rises from 34.1% at 220 °C, to 42.1% at 240 °C to 58.5% at 260 °C while the CH3OH selectivity decreases from 67.2% at 220 °C, to 64.2% at 240 °C to 62.2% at 260 °C. These suggest that with internal cooling system, both methanol synthesis reaction (equation 1) and RWGS (equation 2) are promoted by increasing reaction temperature, but the latter is done more effectively. Although 9



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

CH3OH selectivity slightly decreases, the CH3OH yield in Figure 6c increases distinctly with increasing reaction temperature for all three samples due to the substantial increasing of CO2 conversion. Compared with the decrease of the calculated equilibrium CH3OH yield without internal cooling system (hollow square in Figure 6c) from 19.9% at 220 °C, 16.2% at 240 °C to 13.2% at 260 °C, the much higher CH3OH yield as well as the increase trend with temperature rise under the help of internal cooling system demonstrates that the designed internal cooling system can efficiently promote the reversible methanol synthesis reaction (equation 1) in the direction of methanol formation. (c) Influence of reaction pressure on methanol synthesis. Figure 7 shows the catalytic performance of CZAx with the internal cooling system at 240 °C and 2−4 MPa. For comparison, the equilibrium yields of CH3OH at 240 °C and 2−4 MPa without internal cooling system were also calculated theoretically and are compared in Figure 7c. From Figure 7, both the CO2 conversion and CH3OH selectivity increase with increasing reaction pressure from 2 to 4 MPa, leading to a significant improvement of the CH3OH yield for all samples. Compared with the calculated equilibrium CH3OH yield without internal cooling system in Figure 7c (hollow square), the increase extent of CH3OH yield with increasing reaction pressure is much greater under the help of the internal cooling system. For instance, when the reaction pressure increases from 2 MPa to 4 MPa, the calculated equilibrium CH3OH yield rises from 11.5% to 19.6% by 70.4% while the CH3OH yield for CZA400 increases from 21.4% to 43.6% by 103.7%. From the above results, it can be found that with an increase in milling speed from 200 rpm to 400 rpm, the mutual substitution between Cu2+ and Zn2+ during ball milling is promoted, resulting in increasingly improved catalytic performance of CZAx. Furthermore, with the help of the internal cooling system, the water vapor and methanol gas are continually cooled to the liquid phase, leading to a high partial pressures of syngas to constantly promote the equilibrium reaction (equation 1) to produce methanol. Therefore, the catalytic performance of CZAx with internal cooling system breaks the limitation of reaction equilibrium for methanol synthesis without internal cooling system, giving rise to a much higher methanol yield. The highest CO2 conversion and CH3OH selectivity reach 59.5% and 73.4% for CZA400 at 240 °C and 4 MPa, respectively, resulting in a fantastic CH3OH yield of 43.7%, which is 2.2 times higher than the calculated equilibrium yield of 19.6% without internal cooling system under the same reaction conditions. 10


Page 10 of 23

Page 11 of 23

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


3.4. Catalytic performance without internal cooling system To further highlight the effect of the internal cooling system, the catalytic performance of CZAx and CZAcp without internal cooling system was investigated at 240 °C and 3 MPa and the results are summarized in Table 2. It can be seen that the CO2 conversion and CH3OH selectivity of CZA200 are 13.9% and 39.3%, respectively, leading to a very low CH3OH yield of 5.5%. By increasing the milling speed to 300 rpm, both CO2 conversion and CH3OH selectivity are slightly improved and an enhancement of the CH3OH yield to 6.6% can be obtained for CZA300. For CZA400, the CO2 conversion and CH3OH selectivity further increase to 19.3%, and 47.6%, respectively, resulting in a CH3OH yield of 9.2%. The improvement in catalytic performance from CZA200, CZA300 to CZA400 without the internal cooling system proves again that the more complete substitution between Cu2+ and Zn2+ by increasing the milling speed would improve the catalytic performance of CZAx. Moreover, limited by the reaction equilibrium of methanol synthesis without internal cooling system, the catalytic performance is much inferior to that with internal cooling system, demonstrating a great potential for the application of the internal cooling system to the hydrogenation of carbon dioxide to methanol. In addition, as shown in Table 2, the CO2 conversion, CH3OH selectivity and CH3OH yield for CZAcp are 19.7%, 50.3% and 9.9%, respectively, indicating a slightly better catalytic performance than CZA400. The slightly poorer catalytic performance of CZA400 as compared to CZAcp with and without internal cooling system could be ascribed to the following two reasons: i) since Al2O3 is introduced after the mutual substitution between Cu2+ and Zn2+, the dispersity and uniformity of Al2O3 in CZAx is not optimized; ii) the mole ratio of Cu2+, Zn2+ and Al3+ of 63:27:10 is adopted according to the optimized mole ratio reported by Behrens using the co-precipitation method.19,38,39 However, there may be a difference in the optimized mole ratio between the co-precipitation method and the solid-state ion exchange method. Nevertheless, the catalytic performance of CZA400 has proved that mechanical-force-driven solid-state ion exchange is an effective method to prepare Cu-Zn catalysts without sensitive and complex co-precipitation processes.

4. Conclusions A mechanical-force-driven solid-state ion exchange method has been developed to prepare CuO/ZnO/Al2O3 catalysts. During mechanical ball milling, the mutual substitution between Cu2+ 11



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

and Zn2+ was enhanced with increasing milling speed to form Cu/Zn hydroxy carbonate precursors, which were further used to produce CuO/ZnO/Al2O3 catalysts with an cross-distribution of nano-sized CuO, ZnO and Al2O3. The catalytic performance of the prepared catalysts under different reaction conditions was investigated with an internal cooling system. The results show that: i) the catalytic performance is significantly improved due to the increasingly enhanced mutual substitution between Cu2+ and Zn2+ with increasing milling speed from 200 to 400 rpm; ii) with the rise of reaction temperature from 220 °C to 260 °C, the CO2 conversion substantially increases while the CH3OH selectivity slightly decreases, leading to a gradual increase of CH3OH yield with the help of an internal cooling system, which shows an opposite variation trend compared with the calculated equilibrium CH3OH yield without internal cooling system; iii) the catalytic performance, including CO2 conversion, CH3OH selectivity and CH3OH yield, is comprehensively and significantly improved with increasing reaction pressure from 2 MPa to 4 MPa. The highest CO2 conversion and CH3OH selectivity reach 59.5% and 73.4% for CZA400 at 240 °C and 4 MPa, respectively, resulting in a fantastic CH3OH yield of 43.7%. The CuO/ZnO/Al2O3 catalysts prepared by this mechanical-force-driven solid-state ion exchange method demonstrate a great practical application potential for chemical hydrogenation of CO2 to methanol.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. Experimental details for co-precipitation method; calculation details for CO2 conversion, the selectivity of methanol and CO and the CH3OH yield; TG-MS traces of milled malachite and hydrozincite; TEM image for CZA400 and corresponding EDS map scan results.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Tel: 81-93-6953286 (Xiaohong Li). *E-mail: [email protected]. Tel: 86-21-65642873 (Fang Fang). ORCID 12


Page 12 of 23

Page 13 of 23

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


Fang Fang: 0000-0003-4274-7578 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51471052, 51571063, 51671058), and the Science and Technology Commission of Shanghai Municipality (No. 14JC1490200, 15YF1401300).

REFERENCES (1) Olah, G.A. After oil and gas: methanol economy. Catal. Let. 2004, 93, 1–2. (2) Centi, G.; Perathoner, S. Opportunities and prospects in the chemical recycling of carbon dioxide to fuels. Catal. Today 2009, 148, 191–205. (3) Wang, W.; Wang, S.P.; Ma, X. B.; Gong, J. L. Recent advances in catalytic hydrogenation of carbon dioxide. Chem. Soc. Rev. 2011, 40, 3703–3727. (4) Ali, K. A.; Abdullah, A. Z.; Mohamed, A. R. Recent development in catalytic technologies for methanol synthesis from renewable sources: A critical review. Renew. Sust. Energ. Rev. 2015, 44, 508–518. (5) Tisseraud, C.; Comminges, C.; Belin, T.; Ahouari, H.; Soualah, A.; Pouilloux, Y.; Valant, A. L. The Cu–ZnO synergy in methanol synthesis from CO2, Part 2: Origin of the methanol and CO selectivities explained by experimental studies and a sphere contact quantification model in randomly packed binary mixtures on Cu–ZnO coprecipitate catalysts. J. Catal. 2015, 330, 533–544. (6) Grabow, L. C.; Mavrikakis, M. Mechanism of methanol synthesis on Cu through CO2 and

13



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

CO hydrogenation. ACS Catal. 2011, 1, 365–384. (7) Meunier, F. C. Mixing copper nanoparticles and ZnO nanocrystals: a route towards understanding the hydrogenation of CO2 to methanol. Angew. Chem. Int. Ed. 2011, 50, 4053– 4054. (8) Liao, F.; Huang, Y.; Ge, J.; Zheng, W.; Tedsre, K.; Collier, P.; Hong, X.; Tsang, S. C. Morphology–dependent interactions of ZnO with Cu nanoparticles at the materials’ interface in selective hydrogenation of CO2 To CH3OH. Angew. Chem. Int. Ed. 2011, 50, 2162–2165. (9) Gao, P.; Li, F.; Zhan, H.; Zhao, N.; Xiao, F.; Wei, W.; Zhong, L.; Wang, H.; Sun, Y. Influence of Zr on the performance of Cu/Zn/Al/Zr catalysts via hydrotalcite–like precursors for CO2 hydrogenation to methanol. J. Catal. 2013, 298, 51–60. (10) Li, M. M.; Zeng, Z. Y.; Liao, F. L.; Hong, X. L.; Tsang, S. C. E. Enhanced CO2 hydrogenation to methanol over CuZn nanoalloy in Ga modified Cu/ZnO catalysts. J. Catal. 2016, 343, 157–167. (11) Raudaskoski, R.; Turpeinen, E.; Lenkkeri, R.; Pongrácz, E.; Keiski, R. L. Catalytic activation of CO2: use of secondary CO2 for the production of synthesis gas and for methanol synthesis over copper–based zirconia-containing catalysts. Catal. Today 2009, 144, 318–323. (12) Liu, X. M.; Lu, G. Q.; Yan, Z. F.; Beltramini, J. Recent advances in catalysts for methanol synthesis via hydrogenation of CO and CO2. Ind. Eng. Chem. Res. 2003, 42, 6518–6530. (13) Palo, D. R.; Dagle, R. A.; Holladay, J. D. Methanol steam reforming for hydrogen production. Chem. Rev. 2007, 107, 3992–4021. (14) Saeidi, S.; Amin, N. A. S.; Rahimpour, M. R. Hydrogenation of CO2 to value-added products-A review and potential future developments. J. CO2 Util. 2014, 5, 66–81. (15) Milani, D.; Khalilpour, R.; Zahedi, G.; Abbas, A. A model-based analysis of CO2 utilization in methanol synthesis plant. J. CO2 Util. 2015, 10, 12–22. 14


Page 14 of 23

Page 15 of 23

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


(16) Bansode, A.; Urakawa, A. Towards full one-pass conversion of carbon dioxide to methanol and methanol-derived products. J. Catal. 2014, 309, 66–70. (17) Kobl, K.; Thomas, S.; Zimmermann, Y.; Parkhomenko, K. Power-law kinetics of methanol synthesis from carbon dioxide and hydrogen on copper–zinc oxide catalysts with alumina or zirconia supports. Catal. Today 2016, 270, 31–42. (18) Fujitani, T.; Nakamura, J. The effect of ZnO in methanol synthesis catalysts on Cu dispersion and the specific activity. Catal. Let. 1998, 56, 119–124. (19) Behrens, M.; Zander, S.; Kurr, P.; Jacobsen, N.; Senker, J.; Koch, G.; Ressler, T.; Fischer, R. W.; Schlögl, R. Performance improvement of nanocatalysts by promoter-induced defects in the support material: methanol synthesis over Cu/ZnO:Al. J. Am. Chem. Soc. 2013, 135, 6061–6068. (20) Arena, F.; Italiano, G.; Barbera, K.; Bonura, G.; Spadaro, L.; Frusteri, F. Basic evidences for methanol-synthesis catalyst design. Catal. Today 2009, 143, 80–85. (21) Behrens, M. Meso- and nano-structuring of industrial Cu/ZnO/(Al2O3) catalysts. J. Catal. 2009, 267, 24–29. (22) Baltes, C.; Vukojević, S.; Schüth, F. Correlations between synthesis, precursor, and catalyst structure and activity of a large set of CuO/ZnO/Al2O3 catalysts for methanol synthesis. J. Catal. 2008, 258, 334–344. (23) Porta, P.; Rossi, S. D.; Ferraris, G.; Jacono, M. L.; Minelli, G.; Moretti, G. Structural characterization of malachite-like coprecipitated precursors of binary CuO-ZnO catalysts. J. Catal. 1988, 109, 367–377. (24) Himelfarb, P. B.; Simmons, G. W.; Klier, K.; Herman, R. G. Precursors of the Copper-zinc 15



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

oxide methanol synthesis catalysts. J. Catal. 1985, 93, 442–450. (25) Zhang, Q. C.; Liu, Z. W.; Zhu, X. H.; Wen, L. X.; Zhu, Q. F.; Guo, K.; Chen, J. F. Application of microimpinging stream reactors in the preparation of CuO/ZnO/Al2O3 catalysts for methanol synthesis. Ind. Eng. Chem. Res. 2015, 54, 8874–8882. (26) Behrens, M.; Schlögl, R. How to prepare a good Cu/ZnO catalyst or the role of solid state chemistry for the synthesis of nanostructured catalysts. Anorg Z. Allg. Chem. 2013, 639, 2683–2695. (27) Li, Z.; Yan, S. W.; Fan, H. Enhancement of stability and activity of Cu/ZnO/Al2O3 catalysts by microwave irradiation for liquid phase methanol synthesis. Fuel 2013, 106, 178–186. (28) Kiener, C.; Kurtz, M.; Wilmer, H.; Hoffmann, C.; Schmidt, H. W.; Grunwaldt, J. D.; Muhler, M.; Schüth, F. High-throughput screening under demanding conditions: Cu/ZnO catalysts in high pressure methanol synthesis as an example. J. Catal. 2003, 216, 110–119. (29) Farahani, B. V.; Rajabi, F. H.; Bahmani, M.; Ghelichkhani, M.; Sahebdelfar, S. Influence of precipitation conditions on precursor particle size distribution and activity of Cu/ZnO methanol synthesis catalyst. Appl. Catal. A 2014, 482, 237–244. (30) Ikeda, K.; Nakamori, Y.; Orimo, S. Formation ability of the perovskite-type structure in LixNa1-xMgH3( x = 0, 0.5 and 1.0). Acta Mater. 2005, 53, 3453–3457. (31) Li, J. L.; Inui, T. Characterization of precursors of methanol synthesis catalysts, copper/zinc/aluminum oxides, precipitated at different pHs and temperatures. Appl. Catal. A 1996, 137, 105–117. (32) Jensen, J. R.; Johannessen, T.; Wedel, S.; Livbjerg, H. A study of Cu/ZnO/Al2O3 methanol catalysts prepared by flame combustion synthesis. J. Catal. 2003, 218, 67–77. (33) Saito, M.; Fujitani, T.; Takeuchi, M.; Watanabe, T. Development of copper/zinc oxide-based multicomponent catalysts for methanol synthesis from carbon dioxide and hydrogen. Appl. 16


Page 16 of 23

Page 17 of 23

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


Catal. A 1996, 138, 311–318. (34) Bems, B.; Schur, M.; Dassenoy, A.; Junkes, H.; Herein, D.; Schlögl, R. Relations between synthesis and microstructural properties of Copper/Zinc hydroxycarbonates. Chem. Eur. J. 2003, 9, 2039–2052. (35) Behrens, M.; Girgsdies, F.; Trunschke, A.; Schlögl, R. Minerals as model compounds for Cu/ZnO catalyst precursors: structural and thermal properties and IR spectra of mineral and synthetic (zincian) malachite, rosasite and aurichalcite and a catalyst precursor mixture. Eur. J. Inorg. Chem. 2009, 10, 1347–1357. (36) Schur, M.; Bems, B.; Dassenoy, A.; Kassatkine, I.; Urban, J.; Wilmes, H.; Hinrichsen, O.; Muhle,r M.; Schlögl, R. Continuous coprecipitation of catalysts in a micromixer: nanostructured Cu/ZnO composite for the synthesis of methanol. Angew. Chem. Int. Ed. 2003, 42, 3815–3817. (37) Plyasova, L. M.; Yureva, T. M.; Kriger, T. A.; Makarova, O. V.; Zaikovskii, V. I.; Soloveva, L. P.; Shmakov, A. N. Formation of a catalyst for methanol synthesis. Kinet. Catal. 1995, 36, 425–433. (38) Behrens, M.; Girgsdies, F. Z. Structural effects of Cu/Zn substitution in the malachite– rosasite system. Z. Anorg. Allg. Chem. 2010, 636, 919–927. (39) Behrens, M.; Kasatkin, I.; Kuhl, S.; Weinberg, G. Phase-pure Cu,Zn,Al hydrotalcite-like materials as precursors for copper rich Cu/ZnO/Al2O3 catalysts. Chem. Mater. 2010, 22, 386-397.

17



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

Page 18 of 23

Tables Table 1. Catalytic performance of all prepared catalysts for methanol synthesis with internal cooling water

CO2 conversion

Product selectivity (%)

Methanol yield

Catalysts (%)

CO

CH3OH

(%)

CZA200

31.3

49.3

50.7

15.8

CZA300

40.1

41.9

58.1

23.3

CZA400

50.0

35.8

64.2

32.1

CZAcp

52.7

30.6

69.4

36.6

Reaction conditions: T = 240 °C, P = 3 MPa, water flow rate = 200 mL/min.

Table 2. The catalytic performance of CZAx (x = 200, 300, 400) and CZAcp catalysts without internal cooling system

CO2 conversion

Product selectivity (%)

Methanol yield

Catalysts (%)

CO

CH3OH

(%)

CZA200

13.9

60.7

39.3

5.5

CZA300

15.4

57.1

42.9

6.6

CZA400

19.3

52.4

47.6

9.2

CZAcp

19.7

49.7

50.3

9.9

Reaction conditions: T = 240 °C, P = 3 MPa.

18


Page 19 of 23

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


Figures

Figure 1. Schematic setup for methanol synthesis from CO2 hydrogenation

Figure 2. XRD patterns of the CZAx precursors (x = 200 (a), 300 (b) and 400 (c)) and ball-milled raw materials (hydrozincite (d) and malachite (e)).

19



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

Figure 3. TG curves (black) and MS traces for CO2 (red) and H2O (blue) of the CZAx precursors (x = 200 (a), 300 (b) and 400 (c)).

Figure 4. The XRD patterns of CZAx catalysts (x = 200 (a), 300 (b) and 400 (c)).

Figure 5. TEM (a) and HRTEM (b) images of the CuO/ZnO/Al2O3 catalyst prepared at 400 rpm.

20


Page 20 of 23

Page 21 of 23

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


Figure 6. Effect of temperature on the conversion of CO2 (a), selectivity (b) and yield (c) of CH3OH at 3 MPa for CZAx (x = 200, 300 and 400).

21



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

Figure 7. Effect of pressure on the conversion of CO2 (a), selectivity (b) and yield (c) of CH3OH at 240 °C for CZAx (x = 200, 300 and 400).

22


Page 22 of 23

Page 23 of 23

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


Table of Contents

23


Al2O3 Catalyst Prepared by Mechanical ... - ACS Publications

Recommend Documents