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ZnO catalyst being dried by supercritical CO2 for low-temperature methanol synthesis
Fuel 268 (2020) 117213
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Full Length Article
Urea-derived Cu/ZnO catalyst being dried by supercritical CO2 for lowtemperature methanol synthesis
T
Peipei Zhanga, Yuya Arakia, Xiaobo Fenga, Hangjie Lia, Yuan Fanga, Fei Chena, Lei Shid, ⁎ Xiaobo Pengc, Yoshiharu Yoneyamaa, Guohui Yanga,b, Noritatsu Tsubakia, a
Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China c National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305 0044, Japan d Institute of Industrial Chemistry and Energy Technology, Shenyang University of Chemical Technology, Shenyang 110142, PR China b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Methanol synthesis Supercritical phase CO2 drying Homogeneous precipitation Urea hydrolysis Syngas
Methanol synthesis from syngas is a crucial process in the transformation of coal, natural gas and biomass into high-value added products. The high CO conversion and methanol yield are challenging in low-temperature methanol synthesis, thus improving catalyst activity is necessary. Cu/ZnO nanoparticles with a narrow size distribution were synthesized via homogeneous precipitation method using urea as the precipitant, being followed by supercritical CO2 drying treatment (named as CZhp-S). The analysis results rendered that CZhp-S had a lower reduction temperature, higher Cu0 specific surface area and smaller crystal sizes compared with those of the catalyst prepared by co-precipitation method with conventional drying process (named as CZcp-H). Due to the large specific surface area and the enhanced amount of active sites, the CZhp-S with optimized supercritical CO2 drying treatment condition demonstrated a maximum CO conversion of 52.7% and STY of methanol 87.6 gMeOH/kgcatalyst.h−1 for low-temperature methanol synthesis from syngas, which were much higher than those of CZcp-H (CO conversion 36.5%, STY of methanol 62.9 gMeOH/kgcatalyst.h−1).
1. Introduction Methanol is a crucial feedstock to produce fine chemicals, such as olefins, hydrocarbons, DME, and aromatics [1–3]. As methanol is a ⁎
platform molecule and can be generated from syngas conversion and CO2 hydrogenation, it receives rising attention as a renewable energy storage and carrier [4,5]. Converting CO2-added syngas (a mixture of CO/CO2/H2) to methanol is conducted under a harsh reaction condition
Corresponding author. E-mail address: [email protected] (N. Tsubaki).
https://doi.org/10.1016/j.fuel.2020.117213 Received 26 June 2019; Received in revised form 30 December 2019; Accepted 26 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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(250–400 °C, 10–20 MPa) in industrial plants based on Cu/ZnO/Al2O3 catalyst with around 50 million tons of methanol production annually all over the world [6,7]. However, the efficiency of methanol production is limited by the thermodynamics due to the exothermicity of reaction (CO + 2H2 = CH3OH, ΔH298K = -90.8 KJ/mol). Many efforts were devoted to improving CO conversion and methanol production in low temperature methanol synthesis. Matsumura et al reported ultrafine Pd/CeO2 catalyst can selectively synthesized methanol from syngas at a reaction temperature as low as 170 °C [8]. However, the high onepass CO conversion requires a high Pd content, which was not economy. Christiansen et al proposed a process for methanol production from CO/H2 by alkyl formate route at lower temperature, involving the carbonylation of methanol to methyl formate, and the subsequent hydrogenolysis of methyl formate to methanol. The Brookhaven National Laboratory (BNL, USA) then developed a very strongly basic catalyst (a mixture of NaH, alcohol, and acetate), realizing methanol synthesis at 100–130 °C and 1.0–5.0 MPa. However, trace amounts of carbon dioxide and water in the feed syngas seriously poison the basic catalyst [9–11]. Recently, a novel reaction route using alcohols as catalytic reaction medium at low temperature (150–170 °C) for methanol synthesis from syngas containing CO2 is proposed by us [12–15]. This new route uses syngas containing CO2 and/or H2O directly from an industrial methane reformer or coal gasifier, which overcomes the drawbacks of BNL method. Furthermore, use of alcohols as catalytic solvent in the methanol production from syngas not only alters the reaction path to low-temperature direction but also exhibits high activity. The interaction between Cu catalyst and oxide support plays a vital role in controlling the activity and selectivity for numerous catalytic reactions. It is well known that ZnO promotional role to copper has been identified by various mechanisms, such as electronic and structural support or a hydrogen reservoir by spillover [1,6,16]. This synergistic effect of Cu and ZnO can contribute to the high activity of methanol synthesis from syngas or CO2 hydrogenation. Moreover, the addition of a second metal or cation (Ce, Zn, Zr, La, Ga) to form bimetallic catalysts with Cu based catalyst can also enhance the ratio of Cu+/Cu0, which can promote the reaction of methanol synthesis and methanol steam reforming [17–23]. Li and his co-workers demonstrated that the introduction of La or Ga species on La or Ga-modified Cu/SiO2 catalysts improved the dispersion of both Cu0 and Cu+ on the catalyst and enhanced the catalytic activity and long-term stability [17,19]. Cu/ZnO catalysts are extensively applied in methanol synthesis from CO and/or CO2 hydrogenation reaction and synthesized by a typical co-precipitation (cp) method using sodium carbonate precipitant [24–26]. However, it is difficult to control the homogeneity and the pH of the solution, which are major problems to affect high dispersion and uniform particle size of final catalyst. It is well known that the size, dispersion and surface area greatly affect the activity, selectivity and stability of the catalyst [27–29]. Therefore, modulating these properties during the catalyst preparation is significantly important. At present, to enhance the catalytic activities, various kinds of synthetic approaches have been extensively studied: precipitation, decomposition, hydrothermal synthesis, solid synthesis with acid and sol–gel techniques [29–36]. Being different with other methods, the homogeneous precipitation (hp) method for preparation of solid metal catalyst has been developed in order to achieve smaller particles and larger surface area of catalysts, including SnO2, Fe2O3 and Ni/MgAl2O4 [37–39]. Importantly, the hydrolysis rate of urea can be controlled by reaction temperature. The gradually released hydrolysate will directly affect the rate of precipitation production and makes the active component dispersed more homogeneously, since there is no gradient in concentration of precipitants in the solution [40]. Drying process is one of the fundamental steps in almost all of catalyst synthesis procedures, and has a large impact on the distribution of active phase. The conventional drying process leads to some unavoidable irreversible shrinkage and cracks in porous materials since
the existence of the vapor–liquid interface [41]. Recently, using supercritical CO2 (scCO2) discloses that the drying steps can also exert a huge influence on the structure and distribution of active species [42,43]. Epple et al used the scCO2 technology to extract solvent from CuZnAl aerogels, yielding a higher specific surface area and higher Cu surface area [32]. When scCO2 is used to dry and extract the solvent instead of conventional heating process, the scCO2 fluid maintains the structure upon removal of the solvents or water and hence preventing collapse by eliminating surface tension. Therefore, using scCO2 drying treatment can perfectly retain the microstructure and significantly maintain the high surface area of Cu/ZnO catalyst. In this study, the CZhp-S catalyst was synthesized by homogenous precipitation method using urea as precipitator, followed by scCO2 drying treatment. To comparison, CZhp-H catalyst was also fabricated by the homogenous precipitation method with heating treatment instead of scCO2 drying treatment, while CZcp-H catalyst was prepared by conventional co-precipitation method with heating treatment. All catalysts were evaluated for low-temperature methanol synthesis from CO2-added syngas (CO/CO2/H2) at 170 °C and 5.0 MPa in a flow-type semi-batch reactor using 2-butanol as solvent. The relevant characterization methods, including STEM, TEM, SEM, XRD, XPS, N2 adsorption–desorption, TG, and H2-TPR were carried out to determine the relationship between catalytic activity and structure of CZhp-S catalyst. 2. Experimental 2.1. Catalyst preparation CZhp-S catalyst preparation The CZhp-S catalyst with Cu/Zn molar ratio of 1:1 was prepared by a homogeneous precipitation method using urea as precipitator, followed by supercritical CO2 drying treatment. An aqueous solution containing 8.34 g Cu(NO3)2·3H2O, 10.26 g Zn(NO3)2·6H2O, and 41.44 g urea were added simultaneously into 1480 ml deionized water under constant stirring at 95 °C for 2 h. The obtained precipitate was aged for 24 h, filtrated, washed with deionized water. The precipitate was then sealed into an autoclave, and the supercritical CO2 was flowed by a high-pressure pump into the autoclave for 3–12 h at a temperature of 35–80 °C with 7.5–9.0 MPa to dry the precursor. Finally the dried precipitate was calcined in air at 350 °C for 1 h (Scheme 1). CZhp-H catalyst preparation To compare with CZhp-S catalyst, the catalyst dried by conventional heating progress under air at 120 °C for 12 h instead of the supercritical drying processing was also prepared. The remaining procedures of homogeneous precipitation method for CZhp-H catalyst were same to those of CZhp-S catalyst. The precipitate was aged 24 h, filtrated, washed with deionized water and dried on oven in 120 °C for 12 h, then the precipitate was heated at a rate of 2 °C/min in air up to 350 °C for 1 h. CZcp-H catalyst preparation The CZcp-H catalyst with Cu/Zn molar ratio of 1:1 was prepared by co-precipitation method. 8.34 g Cu (NO3)2·3H2O and 10.26 g Zn(NO3)2·6H2O were firstly dissolved in deionized water. The above mixture solution and 0.05 mol/L Na2CO3 aqueous solution were dividedly added into one beaker by dropwise under constant stirring at 75 °C for 1 h. The pH value was kept at 7.5. The deposit was aged for 24 h at room temperature. Then precipitate was filtrated, washed with hot water and dried in the heating progress at 120 °C and finally calcined in air at 350 °C for 1 h. 2.2. Characterizations X-ray diffraction (XRD) patterns was measured using a Rigaku RINT 2400 diffractometer with Cu-Kα radiation (λ = 0.154 nm) to determine the crystalline of the catalysts. The operations were operated at 40 kV and at 20 mA, and all the samples were scanned in the range of 20–80°. The crystallite size of Cu and ZnO was calculated from the reflections of Cu (1 1 1) and ZnO (1 0 0) planes in the XRD using the Scherrer 2
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Scheme 1. Illustration for preparation of CZhp-S catalyst by homogeneous precipitation through scCO2 drying treatment.
equation. The scanning electron microscopy (SEM) images were observed with JSM-6360 LV (JEOL, Japan) at an accelerating voltage of 15 kV. The samples were per-treated in vacuum at 70 °C for 10 h and sprayed platinum at 10 kV for 120 s before examination. The scanning transmission electron microscopy images (STEM) and transmission electron microscopy images (TEM) were performed on a JEM-3200Fs (JEOL) at an acceleration voltage of 200 kV. The specific surface area of all catalysts was measured by N2 adsorption–desorption using a Micromeritics NOVA2200e surface area and porosimetry analyzer. The surface areas were calculated according to the method of Brunauer, Emmett and Teller (BET). The Cu and Zn molar ratios of different catalyst were analyzed with a Varian 715-ES inductively coupled plasmaatomic emission spectrometer (ICP-AES). Specific metallic copper surface areas were determined by N2O adsorption method. 30–50 mg sample was pretreated at 150 °C in He flow for 1 h, then reduced in 5% H2/Ar at 220 °C for 2 h and finally 10% N2O in He was introduced for 1 h while temperature was cooling down to 60 °C. H2 temperature programmed reduction (H2-TPR) profiles of the samples were tested by a catalyst analyzer BELCAT-B-TT (BEL Japan Co. Ltd.) with a thermal conductivity detector (TCD). 30–50 mg catalysts were pretreated at 150 °C in He flow for 1 h, followed by temperature programmed reduction in H2 with flow rate of 30 ml/min from room temperature to 600 °C with a ramping rate of 10 °C/min. The X-ray photoelectron spectroscopy (XPS) analysis was done by Thermo Fisher Scientific ESCALAB 250Xi multifunctional X-ray photoelectron spectroscope equipped with H2 reduction pretreatment chamber. Thermogravimetry (TG) measurement was carried out over 10 mg spent catalysts in air with flow rate of 50 ml/min using TA-60WS thermal analyzer (Shimadzu). The program was performed at the heating rate of 10 °C/ min from room temperature to 900 °C.
3.0) was flowed into reactor and the reaction was carried at 170 °C under 5.0 MPa for 20 h. The gas product from the reactor outlet through the ice trap was determined by an online gas chromatograph (Shimadzu, GC-8A) equipped with thermal conductivity detector (GC-TCD). Meanwhile, the liquid product was collected from both reactor and ice trap after reaction and analyzed by gas chromatograph (Shimadzu, GC14B) with flame ionization detector (FID), in which 1-propanol was employed as the internal standard. The CO, CO2, and total carbon conversion were calculated with the followed equations, using Ar as an inner standard. CCO = (XCO,feedgas-XCO,effluent)/(XCO,feedgas) × 100%
(1)
CCO: Conversion of CO, % XCO, feedgas: Mole fraction of CO in feedgas XCO, effluent: Mole fraction of CO in effluent gas CCO2 = (XCO2,feedgas-XCO2,effluent)/(XCO2,feedgas) × 100%
(2)
CCO2: Conversion of CO2, % XCO2, feedgas: Mole fraction of CO2 in feedgas XCO2, effluent: Mole fraction of CO2 in effluent gas Totalcarbonconv = CCO × XCO,feedgas/(XCO,feedgas + XCO2,feedgas) + CCO2 × XCO2,feedgas/(XCO,feedgas + XCO2,feedgas) (3) 3. Results and discussion 3.1. Characterization CZhp-S catalyst fabricated by homogenous precipitation synthesis method using urea as precipitator through drying with scCO2 treatment was tested for low-temperature methanol synthesis from syngas. SEM image shows that CZhp-S displays numerous uniform pores with sheetlike structure (Fig. 1a), due to formation of homogenous mixture solution of Cu-Zn hydroxide during the homogenous precipitation procedure [40]. The average size of CZhp-S particle calculated by TEM was 9.3 nm with a narrow distribution (Fig. 1b). A HR-TEM image of CZhp-S sample demonstrates a lattice spacing of 0.25 nm and 0.28 nm, corresponding to the (1 1 1) lattice plane of CuO and (1 0 0) lattice plane of ZnO (Fig. 1c). STEM-EDS mapping results clearly render that Cu and Zn elements are homogenously distributed in CZhp-S sample (Fig. 1d). The surface structure of CZhp-H and CZcp-H, as reference catalysts prepared by the homogeneous and co-precipitation method with the same heating drying process respectively, exhibits the massive agglomerates (Fig. S1). It is implied that homogeneous precipitation method with scCO2 treatment effectively prevents the particles
2.3. Catalytic activity measurements In this study, the CZhp-S catalyst prepared by homogenous precipitation method through scCO2 drying treatment was evaluated for low-temperature methanol synthesis from syngas containing a little amount of CO2 at 170 °C and 5.0 MPa in a flow-type semi-batch reactor using 2-butanol as catalytic solvent. Typically, the low-temperature methanol synthesis was conducted out in a flow-type semi-batch autoclave reactor with an inner volume of 85 ml [40]. Prior to reaction, the catalysts were firstly reduced in the flow of 5% H2/N2 at 220 °C for 10 h and then passivated in 1% O2/N2 atmosphere for 4 h to get passivated surface of catalyst after the reduction temperature being cooled to the room temperature. 3.0 g reduced catalyst and 40 ml 2-butanol as catalytically active solvent were sealed into reactor concurrently with constant stirring. Then the syngas (CO/CO2/H2/Ar = 29.5/4.9/62.6/ 3
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scCO2 fluid drying leads to an easier reduction due to the small particles and high dispersion of CZhp-S catalyst. The abundant oxygen vacancies in the catalyst play vital roles in methanol synthesis. XPS technology is carried out to determine the state of the surface element and the existence of oxygen vacancies on CZhp-S, CZhp-H and CZcp-H reduced catalyst before reaction. As displayed in Fig. 3a, the survey XPS spectrum confirms the primary peaks of carbon (C1s), oxygen (O1s), copper (Cu2p) and zinc (Zn2p) peak in these catalysts. After exposing to hydrogen at 220 °C, the primary peaks with central position at 952.0 and 932.2 eV related to Cu 2p1/2 and 2p3/2 species respectively, imply the compete reduction of CuO into metallic copper, as in Fig. 3b. This is consisted with XRD results. For Zn 2p spectrum, the peaks at 1044.8 and 1021.7 eV are related to Zn 2p1/2 and Zn 2p3/2 respectively, rendering that the oxidation state of Zn element in both catalysts is Zn2+ (Fig. 3c). In addition, in the deconvoluted O spectra, three peaks at binding energies of 531.8, 530.7 and 530.0 eV are clearly observed, which are associated with the chemically adsorbed oxygen, oxygen vacancy and lattice oxygen (Fig. 3d) [46,47]. The relative concentration of deconvoluted O peaks is shown in Table S1. The ratio of oxygen vacancy to the lattice oxygen for CZhp-S catalyst reaches the highest value (0.799), compared to that of CZhp-H (0.681) or CZcp-H (0.594) catalysts. As reported by previous literatures, the higher oxygen vacancy content is beneficial for the methanol synthesis activity [48,49]. From these results, we can conclude that the preparation method for Cu/ZnO catalyst influences oxygen vacancy distribution.
Fig. 1. (a) SEM image, (b) TEM image (the inset shows particle distribution of CZhp-S catalyst), (c) HR-TEM image and (d) STEM-EDS mapping of CZhp-S catalyst.
3.2. Catalytic results The scCO2 drying technique is an extremely attractive technology for the preparation of porous materials and it is defined as a single fluid phase which occurs when the temperature and pressure of CO2 are above its critical temperature and pressure conditions (Tc = 31.2 °C, Pc = 7.38 MPa) [43]. Herein we examined the effects of supercritical temperature, pressure and drying time on the catalyst performance in the low temperature methanol synthesis (Fig. 4). There is no significant variation in specific surface area and activities of catalyst with varying of scCO2 temperatures and pressures. Although temperature and pressure influence the mass transfer, solubility of solvent and vapor pressure, which affect the property of catalyst during the drying progress, the effect is more complicated [43,50]. However, both CO conversion and methanol yield demonstrated highest value at near the critical point of CO2 (T = 35 °C and P = 7.5 MPa). This phenomenon is ascribed to clustering effect which increases the solvent density in the adjacent vicinity of the solute molecules compared to the bulk fluid density, in the zone near the critical pressure and temperature [43]. The detailed catalytic performance is listed in the Table S2. With the increase of supercritical drying time, it is exhibited an upward trend of the CO conversion and methanol yield. When the reserved time of drying is increased to 12 h, the CO conversion and total carbon conversion reach the highest value of 52.7% and 46.0% separately. The solvent in the most inner part of the porous catalyst need sufficient time to approach a drying equilibrium value corresponding to the complete dry. Hence, the longtime of 12 h is enough to replace solvent to scCO2 fluid until to complete dry. This higher catalytic behavior is related to the generation of small crystal size and high specific surface area of Cu0, which are important factors to control the catalyst activity in methanol synthesis reaction (Table S3). The catalytic behaviors of different catalysts for low-temperature methanol synthesis are compared in Fig. 5a. CO conversion over the CZhp-S catalyst is as high as 52.7%, which is improved remarkably, compared to that of CZcp-H catalyst (36.5%). Moreover, the CZhp-S catalyst realized STY of methanol 87.6 gMeOH/kgcatalyst.h−1, whereas CZcp-H catalyst illustrated STY of methanol 62.9 gMeOH/kgcatalyst.h−1. The by-product was mainly 2-butyl formate with trace amount of formate, and there is no methane produced in the CZhp-S catalyst (Table
aggregation and maintains high nanoparticle dispersion. The uniform porous structure of CZhp-S catalyst may boost surface area and catalytic activities. The XRD patterns of the calcined and reduced catalysts, prepared by different methods are shown in Fig. 2a and 2b. Although the morphologies of CZhp-S, CZhp-H and CZcp-H are distinctly different, they have identical crystal diffractions both in calcined and reduced catalysts. XRD patterns of reduced catalysts render three diffraction peaks located at 2θ value of 43.3, 50.4 and 74.1°, which correspond to reflections of (1 1 1) (2 0 0) (2 2 0) crystal planes of Cu metal (JCPDS file no. 04–0836). It is suggested that CuO was totally reduced to Cu0 after reduction process, while the ZnO phases still maintain after reduction treatment, as displaced in Fig. 2b. The crystallite size of Cu and ZnO for different reduced catalysts calculated by the XRD Scherrer equation is listed in Table 1. The Cu0 size of CZhp-S catalyst reaches the smallest value. As reported by the literatures, the Cu0 is the primary active site for low temperature methanol synthesis [15,40]. Thus the smaller Cu0 size may be responsible for improving catalytic performance. The CZhp-S catalyst illustrates higher BET surface area and Cu0 surface area than CZcp-H catalyst from the results calculated by BET method and N2O adsorption method, respectively. Furthermore, the specific surface area of metallic copper for CZhp-S catalyst reaches the highest value of 31.8 m2/g by N2O adsorption (Table 1), proving that there is strong synergy between Cu and ZnO [44,45]. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst activity. N2 adsorption–desorption isotherms and corresponding Barrett-Joyner-Halenda (BJH) pore-size distribution of three samples are illustrated in Fig. S2. The obtained high surface area is ascribed to the uniform porous structure of CZhp-S catalyst, according to the SEM observation. The reduction behaviour for all catalysts is investigated by H2-TPR technology. The reduction peak for the CZhp-S catalyst shifts to the lower temperature (main reduction peak of 220.4 °C) than the other two catalysts (CZhp-H: 225.6 °C and CZcp-H: 233.9 °C), as shown in Fig. 2c. It is suggested that the homogeneous synthesis method with 4
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Fig. 2. XRD patterns of (a) calcined and (b) reduced catalysts, (c) H2-TPR profiles of the different catalysts.
authors are as follows: (1) CO2 formed by water–gas shift reaction; (2) formic acid produced from CO2 hydrogenation; (3) esterification of formic acid by ROH to generate HCOOR; (4) hydrogenation of HCOOR to methanol and returning ROH, as shown in Scheme 2. In this reaction, ROH, as both catalytically-active solvent and promoter, facilitates the esterification reaction with HCOOH and accelerates methanol production at low reaction temperature [12,13]. The hydrogenation of CO2 to formic acid or ester appears to be the rate-limiting step for Cu-based catalyst, based on the mechanism and pathways of reaction [51]. The strong synergy between Cu and ZnO and the presence of oxygen defects on the CZhp-S catalyst are of benefit for hydrogen adsorption, and thus boosting formic acid formation from CO hydrogenation. Subsequently, formic acid easily reacts with alcohol by the catalyzed esterification reaction. Therefore, the overall CO conversion rates are significantly enhanced over the CZhp-S catalyst.
Table 1 Textural properties of different catalysts. Catalysts
CZcp-H CZhp-H CZhp-S a b c d
Cu/Zna (molar %)
1.00/0.91 1.00/0.91 1.00/0.92
SBETb (m2/g)
60.2 63.3 72.7
SCuc (m2/g)
28.5 30.2 31.8
Particle sized (nm) Cu
ZnO
9.0 8.6 8.5
9.6 8.7 8.3
Measured by ICP-AES. Calculated by BET method. Determined by N2O adsorption method. Calculated by XRD Scherrer equation.
S4). For comparison, the catalytic performance of CZhp-H catalyst is also presented in Fig. 5a. The CO conversion and methanol production of CZhp-H catalyst are higher than those of the CZcp-H catalyst, while lower than those of CZhp-S catalyst. It’s indicated that the supercritical dried catalyst demonstrates better catalytic performance in low-temperature methanol synthesis than that of conventional heating process due to high specific Cu surface area and small Cu crystal size. The high CO conversion of CZhp-S catalyst is ascribed to the obtained large specific surface area and highly dispersed copper nanoparticles from homogenous precipitation method. It is suggested that this approach, combining homogenous precipitation and scCO2 drying treatment, is a hopeful preparation method to synthesize effective Cu/ZnO catalyst for the low-temperature methanol reaction. The primary reaction equations included in the alcohol-assisted low temperature methanol synthesis reaction invented by the present
CO + H2O → CO2 + H2
(4)
CO2 + H2 → HCOOH
(5)
HCOOH + ROH → HCOOR + H2O
(6)
HCOOR + 2H2 → CH3OH + ROH
(7)
The whole reaction equation: CO + 2H2 → CH3OH
ΔH298K = -90.8KJ/mol
(8)
To study the stability of the CZhp-S, low-temperature methanol synthesis reaction was performed for 50 h at a reaction temperature of 170 °C and pressure of 5.0 MPa. Fig. 5b illustrates the time-on-stream changes of CO conversion, CO2 conversion and total carbon conversion over CZhp-S catalyst. With reaction time, the CO and total carbon conversion are enhanced rapidly at the initial stage of reaction, then 5
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Fig. 3. XPS curves of the (a) survey, (b) Cu, (c) Zn and (d) deconvoluted O of different catalysts.
Fig. 4. The catalytic performance of CZhp-S catalyst prepared under different temperature, pressure and time of scCO2 drying treatment, Reaction conditions: 3.0 g of catalyst; 170 °C; 5.0 MPa; solvent: 2-butanol, 40 ml; CO/CO2/H2/Ar = 29.5/4.9/62.6/3.0; Fsyngas = 20 ml/min. STY means space time yield.
increased slowly, and finally maintained stable after 16 h on stream. The catalyst yields 51.0% of methanol with 52.7% of CO conversion after 50 h on stream. Whereas, the CO2 conversion is initially negative due to the water–gas shift reaction, and finally becomes about 17.0%. Furthermore, the CZhp-S catalyst is extremely stable and there is no significant carbon deposition on the catalyst after 50 h reaction, as presented in Fig. S4. Fig. 5c and 5d list SEM images of the CZhp-S catalyst before and after 20 h reaction. The CZhp-S catalyst was reduced at 220 °C in the hydrogen atmosphere before reaction and its nanostructured morphology reveals sheetlike structure after reduction, as in Fig. 5c. The SEM image of spent CZhp-S catalyst remains the original structure of asreduced catalyst and there is no apparent particles aggregation. It is indicated that CZhp-S catalyst effectively stays against thermal sintering and prevents particles aggregation during reaction.
4. Conclusions The CZhp-S catalyst demonstrates excellent catalytic performance with CO conversion of 52.7% and STY of methanol 87.6 gMeOH/ kgcatalyst.h−1 for low temperature methanol synthesis, which is rather higher than CZcp-H catalyst (CO conversion of 35.6%, STY of methanol 62.9 gMeOH/kgcatalyst.h−1). The high activities of the catalysts may be ascribed to the low reduction temperature, highly dispersed Cu metal particles and the high accessibility of Cu metal particles to methanol synthesis, while the homogeneous precipitation method by urea hydrolysis with scCO2 drying progress guarantees these to form uniform size of CZhp-S catalyst with superior catalytic performance in lowtemperature methanol synthesis. Our work offers a promising approach to developing effective Cu/ZnO catalyst and extends the application of scCO2 drying process. 6
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60
CO
CO2
Total C.
Methanol yield
(b)
60
60 40
50
40
40
30
30
20
20
10
10
-20
0
-40
0
CZcp-H
CZhp-H
Conversion (%)
50
Methanol yield (%)
Conversion (%)
(a)
CZhp-S
20 0
CO CO2 Total Carbon
0
10 20 30 Time on stream (h)
40
50
(d)
(c)
Fig. 5. (a) The catalytic performance of CZcp-H, CZhp-H and CZhp-S catalyst, (b) the time on stream of the CZhp-S catalyst for low temperature methanol synthesis, the SEM images of the CZhp-S catalyst (c) before and (d) after reaction.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2020.117213. References [1] Nielsen DU, Hu X, Daasbjerg K, Skrydstrup T. Chemically and electrochemically catalysed conversion of CO2 to CO with follow-up utilization to value-added chemicals. Nat Catal 2018;1:244–54. [2] Xu S, Zheng A, Wei Y, Chen J, Li J, Chu Y, et al. Direct observation of cyclic carbenium ions and their role in the catalytic cycle of the methanol-to-olefin reaction over chabazite zeolites. Angew Chem Int Ed 2013;52:11564–8. [3] Olsbye U, Svelle S, Bjørgen M, Beato P, Janssens TVW, Joensen F, et al. Conversion of methanol to hydrocarbons: how zeolite cavity and pore size controls product selectivity. Angew Chem Int Ed 2012;51:5810–31. [4] Kasipandi S, Bae JW. Recent advances in direct synthesis of value-added aromatic chemicals from syngas by cascade reactions over bifunctional catalysts. Adv Mater 2019:1803390–408. [5] Zhang P, Tan L, Yang G, Tsubaki N. One-pass selective conversion of syngas to paraxylene. Chem Sci 2017;8:7941–6. [6] Behrens M, Studt F, Kasatkin I, Kühl S, Hävecker M, Abild-pedersen F, et al. The active site of methanol synthesis over Cu/ZnO/Al2O3 industrial catalysts. Science 2012;759:893–8. [7] Kattel S, Ramirez P, Chen JG, Rodriguez JA, Liu P. Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts. Science 2017;355:1296–9. [8] Matsumura Y, Shen WJ, Ichihashi Y, Okumura M. Low-temperature methanol synthesis catalyzed over ultrafine palladium particles supported on cerium oxide. J Catal 2001;197:267–72. [9] Jeong Y, Kim I, Kang JY, Yan N, Jeong H, Park KJ, et al. Effect of the aging time of the precipitate on the activity of Cu/ZnO catalysts for alcohol-assisted low temperature methanol synthesis. J Mole Catal A 2016;418:168–74. [10] Haggin J. Chem Eng News 1986;21:;Aug:4. [11] Brookhaven National Laboratory, US patent, 461479, 4619946, 4623634, 4613623 (1986), 4935395 (1990). [12] Tsubaki N, Ito M, Fujimoto K. A new method of low-temperature methanol synthesis. J Catal 2001;197:224–7. [13] Yang R, Fu Y, Zhang Y, Tsubaki N. In situ DRIFT study of low-temperature methanol synthesis mechanism on Cu/ZnO catalysts from CO2-containing syngas using ethanol promoter. J Catal 2004;228:23–35. [14] Shi L, Yang G, Tao K, Yoneyama Y, Tan Y, Tsubaki N. An introduction of CO2 conversion by dry reforming with methane and new route of low-temperature
Scheme 2. Reaction pathway for the alcohol-assisted low temperature method synthesis reaction over the CZhp-S catalyst.
Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is supported by JST-CREST of Japan Science and Technology Agency (Grant Number, 17-141003297). Research fund (2017AA001) from Xinjiang-Tianye Co. Ltd. and Xinjiang government is greatly appreciated. 7
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P. Zhang, et al. methanol synthesis. Acc Chem Res 2013;46:1838–47. [15] Meng F, Zhang Q, Yang G, Yang R, Yoneyama Y, Tsubaki N. Structural and kinetical studies on the supercritical CO2 dried Cu/ZnO catalyst for low-temperature methanol synthesis. Chem Eng J 2016;295:160–6. [16] Kattel S, Ramriez P, Chen J, Rodriguez J, Liu P. Active sites for CO2 hydrogenation to methanol on Cu/ZnO Catalysts. Science 2017;355:1296–9. [17] Wang X, Ma K, Guo L, Tian Y, Cheng Q, Bai X, et al. Cu/ZnO/SiO2 catalyst synthesis by reduction of ZnO-modified copper phyllosilicate for dimethyl ether steam reforming. App Catal A, General 2017;540:37–46. [18] Wang X, Ma K, Guo L, Ding T, Cheng Q, Tian Y, et al. Catalytic performance for hydrogen production through stream reforming of dimethyl ether over silica supported copper catalysts synthesized by ammmonia evaporation method. Acra Phys Chim Sin 2017;33:1699–708. [19] Huang J, Ding T, Ma K, Cai J, Sun Z, Tian Y, et al. Modification of Cu/SiO2 catalysts by La2O3 to quantitatively tune Cu+-Cu0 dual sites with improved catalytic activities and stabilities for dimethyl ether steam reforming. ChemCatChem 2018;10:3862–71. [20] Ma K, Cui Z, Zhang Z, Huang J, Sun Z, Tian Y, et al. Alloy-mediated ultra-low CO selevtivity for steam reforming over Cu-Ni bimetallic catalysts. ChemCatChem 2018;10:4010–7. [21] Huang J, Cai J, Ma K, Ding T, Tian Y, Zhang J, et al. Ca2O3-modified Cu/SiO2 catalysts with low CO selectivity for catalytic steam reforming. Acra Phys Chim Sin 2019;35:431–41. [22] Tan L, Zhang P, Suzuki Y, Li H, Guo L, Yoneyama Y, et al. Bifunctional capsule catalyst of Al2O3@Cu with strengthened dehydration reaction field for direct synthesis of dimethyl ether from syngas. Ind Eng Chem Res 2019. https://doi.org/ 10.1021/acs.iecr.9b04864. [23] Zhu C, Fang Y, Luo Z, Zhang C, Zhang X, Li J, et al. Direct dimethyl ether synthesis over mesoporous Cu-Al2O3 catalyst via CO hydrogenation. Res Chem Intermed 2019;45:5863–76. [24] Behrens M, Schlögl R, Anorg Z. How to prepare a good Cu/ZnO catalyst or the role of solid state chemistry for the synthesis of nanostructured catalysts. Allg Chem 2013;639:2683–95. [25] Cai M, Palcic A, Subramanian V, Moldovan S, Ersen O, Valtchev V, et al. Direct dimethyl ether synthesis from syngas on copper-zeolite hybrid catalysts with a wide range of zeolite particle sizes. J Catal 2016;338:227–38. [26] Smith PJ, Kondrat SA, Chater PA, Yeo BR, Shaw GM, Lu L, et al. A new class of Cu/ ZnO catalysts derived from zincian georgeite precursors prepared by coprecipitation. Chem Sci 2017;8:2436–47. [27] Munnik P, De Jongh PE, De Jong KP. Control and impact of the nanoscale distribution of supported cobalt particles used in fischer-tropsch catalysis. J Am Chem Soc 2014;136:7333–40. [28] Dong C, Lian C, Hu S, Deng Z, Gong J, Li M, et al. Size-dependent activity and selectivity of carbon dioxide photocatalytic reduction over platinum nanoparticles. Nat Commun 2018;9:1–11. [29] Galvis HMT, Bitter JH, Khare CB, Ruitenbeek M, Dugulan AI, de Jong KP. Supported iron nanoparticles as catalysts for sustainable production of lower olefins. Science 2012;335:835–8. [30] Shi L, Shen W, Yang G, Fan X, Jin Y, Zeng C, et al. Formic acid directly assisted solid-state synthesis of metallic catalysts without further reduction: as-prepared Cu/ ZnO catalysts for low-temperature methanol synthesis. J Catal 2013;302:83–90. [31] Dong X, Li F, Zhao N, Xiao F, Wang J, Tan Y. CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts preparedby precipitation-reduction method. Appl Catal B
Environ 2016;191:8–17. [32] Guo Y, Meyer-Zaika W, Muhler M, Vukojević S, Epple M. Cu/Zn/Al xerogels and aerogels prepared by a sol-gel reaction as catalysts for methanol synthesis. J Inorg Chem 2006;23:4774–81. [33] Darr JA, Zhang J, Makwana NM, Weng X. Continuous hydrothermal synthesis of inorganic nanoparticles: applications and future directions. Chem Rev 2017;117:11125–238. [34] Sun J, Yu J, Ma Q, Meng F, Wei X, Sun Y, et al. Freezing copper as a noble metal-like catalyst for preliminary hydrogenation. Sci Adv 2018;4:3275–6. [35] Ma K, Tian Y, Zhao Z, Cheng Q, Ding T, Zhang J, et al. Achieving efficient and robust catalytic reforming on dual-sites of Cu species. Chem Sci 2019;10:2578–84. [36] Sun Z, Tian Y, Zhang P, Yang G, Tsubaki N, Abe T, et al. Sputtered Cu-ZnO/γ-Al2O3 bifunctional catalyst with ultra-low Cu content boosting dimethyl ether steam reforming and inhibiting side reactions. Ind Eng Chem Res 2019;58:7085–93. [37] Song KC, Kang Y. Preparation of high surface area tin oxide powders by a homogeneous precipitation method. Mater Lett 2000;42:283–9. [38] Ocana M, Morales MP, Serna CJ. Homogeneous precipitation of uniform a-Fe2O3 particles from iron salts solutions in the presence of urea. J Colloid Interf Sci 1999;323:317–23. [39] Szijjártó GP, Pászti Z, Sajó I, Erdohelyi A, Radnóczi G, Tompos A. Nature of the active sites in Ni/MgAl2O4-based catalysts designed for steam reforming of ethanol. J Catal 2013;305:290–306. [40] Fan R, Kyodo M, Tan L, Peng X, Yang G, Yoneyama Y, et al. Preparation and application of Cu/ZnO catalyst by urea hydrolysis method for low-temperature methanol synthesis from syngas. Fuel Process Technol 2017;167:69–77. [41] Lekhal A, Glasser BJ, Khinast JG. Impact of drying on the catalyst profile in supported impregnation catalysts. Chem Eng Sci 2001;56:4473–87. [42] Mizushima Y, Hori M. Alumina aerogel catalysts prepared by two supercritical drying methods used in methane combustion. J Mater Sci 1995;30:1551–5. [43] Sui R, Charpentier P. Synthesis of metal oxide nanostructures by direct sol-gel chemistry in supercritical fluids. Chem Rev 2012;112:3057–82. [44] Behrens M, Zander S, Kurr P, Jacobsen N, Senker J, Koch G, et al. Performance improvement of nanocatalysts by promoter-induced defects in the support material: methanol synthesis over Cu/ZnO:Al. J Am Chem Soc 2013;135:6061–8. [45] Fujitani T, Nakamura J. The chemical modification seen in the Cu/ZnO methanol synthesis catalysts. Appl Catal A 2000;191:111–29. [46] Santos VP, Pereira MFR, Órfão JJM, Figueiredo JL. The role of lattice oxygen on the activity of manganese oxides towards the oxidation of volatile organic compounds. Appl Catal B Environ 2010;99:353–63. [47] Zhu M, Zhang Z, Zhong M, Tariq M, Li Y, Li W, et al. Oxygen vacancy induced ferromagnetism in Cu-doped ZnO. Cream Int 2017;43:3166–70. [48] Studt F, Behrens M, Kunkes EL, Thomas N, Zander S, Tarasov A, et al. The mechanism of CO and CO2 hydrogenation to methanol over Cu-based catalysts. ChemCatChem 2015;7:1105–11. [49] Fichtl MB, Schumann J, Kasatkin I, Jacobsen N, Behrens M, Schlögl R, et al. Counting of oxygen defects versus metal surface sites in methanol synthesis catalysts by different probe molecules. Angew Chem Int Ed 2014;53:7043–7. [50] Unlusu B, Sunol AK. Modeling of equilibration times at high pressure for multicomponent vapor-liquid diffusional processes. Fluid Phase Equilib 2004;226:15–25. [51] Schumann J, Eichelbaum M, Lunkenbein T, Thomas N, Galván MCÁ, Schlögl R, et al. Promoting strong metal support interaction: doping ZnO for enhanced activity of Cu/ZnO: M (M = Al, Ga, Mg) catalysts. ACS Catal 2015;5:3260–70.
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Full Length Article
Urea-derived Cu/ZnO catalyst being dried by supercritical CO2 for lowtemperature methanol synthesis
T
Peipei Zhanga, Yuya Arakia, Xiaobo Fenga, Hangjie Lia, Yuan Fanga, Fei Chena, Lei Shid, ⁎ Xiaobo Pengc, Yoshiharu Yoneyamaa, Guohui Yanga,b, Noritatsu Tsubakia, a
Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China c National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305 0044, Japan d Institute of Industrial Chemistry and Energy Technology, Shenyang University of Chemical Technology, Shenyang 110142, PR China b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Methanol synthesis Supercritical phase CO2 drying Homogeneous precipitation Urea hydrolysis Syngas
Methanol synthesis from syngas is a crucial process in the transformation of coal, natural gas and biomass into high-value added products. The high CO conversion and methanol yield are challenging in low-temperature methanol synthesis, thus improving catalyst activity is necessary. Cu/ZnO nanoparticles with a narrow size distribution were synthesized via homogeneous precipitation method using urea as the precipitant, being followed by supercritical CO2 drying treatment (named as CZhp-S). The analysis results rendered that CZhp-S had a lower reduction temperature, higher Cu0 specific surface area and smaller crystal sizes compared with those of the catalyst prepared by co-precipitation method with conventional drying process (named as CZcp-H). Due to the large specific surface area and the enhanced amount of active sites, the CZhp-S with optimized supercritical CO2 drying treatment condition demonstrated a maximum CO conversion of 52.7% and STY of methanol 87.6 gMeOH/kgcatalyst.h−1 for low-temperature methanol synthesis from syngas, which were much higher than those of CZcp-H (CO conversion 36.5%, STY of methanol 62.9 gMeOH/kgcatalyst.h−1).
1. Introduction Methanol is a crucial feedstock to produce fine chemicals, such as olefins, hydrocarbons, DME, and aromatics [1–3]. As methanol is a ⁎
platform molecule and can be generated from syngas conversion and CO2 hydrogenation, it receives rising attention as a renewable energy storage and carrier [4,5]. Converting CO2-added syngas (a mixture of CO/CO2/H2) to methanol is conducted under a harsh reaction condition
Corresponding author. E-mail address: [email protected] (N. Tsubaki).
https://doi.org/10.1016/j.fuel.2020.117213 Received 26 June 2019; Received in revised form 30 December 2019; Accepted 26 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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(250–400 °C, 10–20 MPa) in industrial plants based on Cu/ZnO/Al2O3 catalyst with around 50 million tons of methanol production annually all over the world [6,7]. However, the efficiency of methanol production is limited by the thermodynamics due to the exothermicity of reaction (CO + 2H2 = CH3OH, ΔH298K = -90.8 KJ/mol). Many efforts were devoted to improving CO conversion and methanol production in low temperature methanol synthesis. Matsumura et al reported ultrafine Pd/CeO2 catalyst can selectively synthesized methanol from syngas at a reaction temperature as low as 170 °C [8]. However, the high onepass CO conversion requires a high Pd content, which was not economy. Christiansen et al proposed a process for methanol production from CO/H2 by alkyl formate route at lower temperature, involving the carbonylation of methanol to methyl formate, and the subsequent hydrogenolysis of methyl formate to methanol. The Brookhaven National Laboratory (BNL, USA) then developed a very strongly basic catalyst (a mixture of NaH, alcohol, and acetate), realizing methanol synthesis at 100–130 °C and 1.0–5.0 MPa. However, trace amounts of carbon dioxide and water in the feed syngas seriously poison the basic catalyst [9–11]. Recently, a novel reaction route using alcohols as catalytic reaction medium at low temperature (150–170 °C) for methanol synthesis from syngas containing CO2 is proposed by us [12–15]. This new route uses syngas containing CO2 and/or H2O directly from an industrial methane reformer or coal gasifier, which overcomes the drawbacks of BNL method. Furthermore, use of alcohols as catalytic solvent in the methanol production from syngas not only alters the reaction path to low-temperature direction but also exhibits high activity. The interaction between Cu catalyst and oxide support plays a vital role in controlling the activity and selectivity for numerous catalytic reactions. It is well known that ZnO promotional role to copper has been identified by various mechanisms, such as electronic and structural support or a hydrogen reservoir by spillover [1,6,16]. This synergistic effect of Cu and ZnO can contribute to the high activity of methanol synthesis from syngas or CO2 hydrogenation. Moreover, the addition of a second metal or cation (Ce, Zn, Zr, La, Ga) to form bimetallic catalysts with Cu based catalyst can also enhance the ratio of Cu+/Cu0, which can promote the reaction of methanol synthesis and methanol steam reforming [17–23]. Li and his co-workers demonstrated that the introduction of La or Ga species on La or Ga-modified Cu/SiO2 catalysts improved the dispersion of both Cu0 and Cu+ on the catalyst and enhanced the catalytic activity and long-term stability [17,19]. Cu/ZnO catalysts are extensively applied in methanol synthesis from CO and/or CO2 hydrogenation reaction and synthesized by a typical co-precipitation (cp) method using sodium carbonate precipitant [24–26]. However, it is difficult to control the homogeneity and the pH of the solution, which are major problems to affect high dispersion and uniform particle size of final catalyst. It is well known that the size, dispersion and surface area greatly affect the activity, selectivity and stability of the catalyst [27–29]. Therefore, modulating these properties during the catalyst preparation is significantly important. At present, to enhance the catalytic activities, various kinds of synthetic approaches have been extensively studied: precipitation, decomposition, hydrothermal synthesis, solid synthesis with acid and sol–gel techniques [29–36]. Being different with other methods, the homogeneous precipitation (hp) method for preparation of solid metal catalyst has been developed in order to achieve smaller particles and larger surface area of catalysts, including SnO2, Fe2O3 and Ni/MgAl2O4 [37–39]. Importantly, the hydrolysis rate of urea can be controlled by reaction temperature. The gradually released hydrolysate will directly affect the rate of precipitation production and makes the active component dispersed more homogeneously, since there is no gradient in concentration of precipitants in the solution [40]. Drying process is one of the fundamental steps in almost all of catalyst synthesis procedures, and has a large impact on the distribution of active phase. The conventional drying process leads to some unavoidable irreversible shrinkage and cracks in porous materials since
the existence of the vapor–liquid interface [41]. Recently, using supercritical CO2 (scCO2) discloses that the drying steps can also exert a huge influence on the structure and distribution of active species [42,43]. Epple et al used the scCO2 technology to extract solvent from CuZnAl aerogels, yielding a higher specific surface area and higher Cu surface area [32]. When scCO2 is used to dry and extract the solvent instead of conventional heating process, the scCO2 fluid maintains the structure upon removal of the solvents or water and hence preventing collapse by eliminating surface tension. Therefore, using scCO2 drying treatment can perfectly retain the microstructure and significantly maintain the high surface area of Cu/ZnO catalyst. In this study, the CZhp-S catalyst was synthesized by homogenous precipitation method using urea as precipitator, followed by scCO2 drying treatment. To comparison, CZhp-H catalyst was also fabricated by the homogenous precipitation method with heating treatment instead of scCO2 drying treatment, while CZcp-H catalyst was prepared by conventional co-precipitation method with heating treatment. All catalysts were evaluated for low-temperature methanol synthesis from CO2-added syngas (CO/CO2/H2) at 170 °C and 5.0 MPa in a flow-type semi-batch reactor using 2-butanol as solvent. The relevant characterization methods, including STEM, TEM, SEM, XRD, XPS, N2 adsorption–desorption, TG, and H2-TPR were carried out to determine the relationship between catalytic activity and structure of CZhp-S catalyst. 2. Experimental 2.1. Catalyst preparation CZhp-S catalyst preparation The CZhp-S catalyst with Cu/Zn molar ratio of 1:1 was prepared by a homogeneous precipitation method using urea as precipitator, followed by supercritical CO2 drying treatment. An aqueous solution containing 8.34 g Cu(NO3)2·3H2O, 10.26 g Zn(NO3)2·6H2O, and 41.44 g urea were added simultaneously into 1480 ml deionized water under constant stirring at 95 °C for 2 h. The obtained precipitate was aged for 24 h, filtrated, washed with deionized water. The precipitate was then sealed into an autoclave, and the supercritical CO2 was flowed by a high-pressure pump into the autoclave for 3–12 h at a temperature of 35–80 °C with 7.5–9.0 MPa to dry the precursor. Finally the dried precipitate was calcined in air at 350 °C for 1 h (Scheme 1). CZhp-H catalyst preparation To compare with CZhp-S catalyst, the catalyst dried by conventional heating progress under air at 120 °C for 12 h instead of the supercritical drying processing was also prepared. The remaining procedures of homogeneous precipitation method for CZhp-H catalyst were same to those of CZhp-S catalyst. The precipitate was aged 24 h, filtrated, washed with deionized water and dried on oven in 120 °C for 12 h, then the precipitate was heated at a rate of 2 °C/min in air up to 350 °C for 1 h. CZcp-H catalyst preparation The CZcp-H catalyst with Cu/Zn molar ratio of 1:1 was prepared by co-precipitation method. 8.34 g Cu (NO3)2·3H2O and 10.26 g Zn(NO3)2·6H2O were firstly dissolved in deionized water. The above mixture solution and 0.05 mol/L Na2CO3 aqueous solution were dividedly added into one beaker by dropwise under constant stirring at 75 °C for 1 h. The pH value was kept at 7.5. The deposit was aged for 24 h at room temperature. Then precipitate was filtrated, washed with hot water and dried in the heating progress at 120 °C and finally calcined in air at 350 °C for 1 h. 2.2. Characterizations X-ray diffraction (XRD) patterns was measured using a Rigaku RINT 2400 diffractometer with Cu-Kα radiation (λ = 0.154 nm) to determine the crystalline of the catalysts. The operations were operated at 40 kV and at 20 mA, and all the samples were scanned in the range of 20–80°. The crystallite size of Cu and ZnO was calculated from the reflections of Cu (1 1 1) and ZnO (1 0 0) planes in the XRD using the Scherrer 2
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Scheme 1. Illustration for preparation of CZhp-S catalyst by homogeneous precipitation through scCO2 drying treatment.
equation. The scanning electron microscopy (SEM) images were observed with JSM-6360 LV (JEOL, Japan) at an accelerating voltage of 15 kV. The samples were per-treated in vacuum at 70 °C for 10 h and sprayed platinum at 10 kV for 120 s before examination. The scanning transmission electron microscopy images (STEM) and transmission electron microscopy images (TEM) were performed on a JEM-3200Fs (JEOL) at an acceleration voltage of 200 kV. The specific surface area of all catalysts was measured by N2 adsorption–desorption using a Micromeritics NOVA2200e surface area and porosimetry analyzer. The surface areas were calculated according to the method of Brunauer, Emmett and Teller (BET). The Cu and Zn molar ratios of different catalyst were analyzed with a Varian 715-ES inductively coupled plasmaatomic emission spectrometer (ICP-AES). Specific metallic copper surface areas were determined by N2O adsorption method. 30–50 mg sample was pretreated at 150 °C in He flow for 1 h, then reduced in 5% H2/Ar at 220 °C for 2 h and finally 10% N2O in He was introduced for 1 h while temperature was cooling down to 60 °C. H2 temperature programmed reduction (H2-TPR) profiles of the samples were tested by a catalyst analyzer BELCAT-B-TT (BEL Japan Co. Ltd.) with a thermal conductivity detector (TCD). 30–50 mg catalysts were pretreated at 150 °C in He flow for 1 h, followed by temperature programmed reduction in H2 with flow rate of 30 ml/min from room temperature to 600 °C with a ramping rate of 10 °C/min. The X-ray photoelectron spectroscopy (XPS) analysis was done by Thermo Fisher Scientific ESCALAB 250Xi multifunctional X-ray photoelectron spectroscope equipped with H2 reduction pretreatment chamber. Thermogravimetry (TG) measurement was carried out over 10 mg spent catalysts in air with flow rate of 50 ml/min using TA-60WS thermal analyzer (Shimadzu). The program was performed at the heating rate of 10 °C/ min from room temperature to 900 °C.
3.0) was flowed into reactor and the reaction was carried at 170 °C under 5.0 MPa for 20 h. The gas product from the reactor outlet through the ice trap was determined by an online gas chromatograph (Shimadzu, GC-8A) equipped with thermal conductivity detector (GC-TCD). Meanwhile, the liquid product was collected from both reactor and ice trap after reaction and analyzed by gas chromatograph (Shimadzu, GC14B) with flame ionization detector (FID), in which 1-propanol was employed as the internal standard. The CO, CO2, and total carbon conversion were calculated with the followed equations, using Ar as an inner standard. CCO = (XCO,feedgas-XCO,effluent)/(XCO,feedgas) × 100%
(1)
CCO: Conversion of CO, % XCO, feedgas: Mole fraction of CO in feedgas XCO, effluent: Mole fraction of CO in effluent gas CCO2 = (XCO2,feedgas-XCO2,effluent)/(XCO2,feedgas) × 100%
(2)
CCO2: Conversion of CO2, % XCO2, feedgas: Mole fraction of CO2 in feedgas XCO2, effluent: Mole fraction of CO2 in effluent gas Totalcarbonconv = CCO × XCO,feedgas/(XCO,feedgas + XCO2,feedgas) + CCO2 × XCO2,feedgas/(XCO,feedgas + XCO2,feedgas) (3) 3. Results and discussion 3.1. Characterization CZhp-S catalyst fabricated by homogenous precipitation synthesis method using urea as precipitator through drying with scCO2 treatment was tested for low-temperature methanol synthesis from syngas. SEM image shows that CZhp-S displays numerous uniform pores with sheetlike structure (Fig. 1a), due to formation of homogenous mixture solution of Cu-Zn hydroxide during the homogenous precipitation procedure [40]. The average size of CZhp-S particle calculated by TEM was 9.3 nm with a narrow distribution (Fig. 1b). A HR-TEM image of CZhp-S sample demonstrates a lattice spacing of 0.25 nm and 0.28 nm, corresponding to the (1 1 1) lattice plane of CuO and (1 0 0) lattice plane of ZnO (Fig. 1c). STEM-EDS mapping results clearly render that Cu and Zn elements are homogenously distributed in CZhp-S sample (Fig. 1d). The surface structure of CZhp-H and CZcp-H, as reference catalysts prepared by the homogeneous and co-precipitation method with the same heating drying process respectively, exhibits the massive agglomerates (Fig. S1). It is implied that homogeneous precipitation method with scCO2 treatment effectively prevents the particles
2.3. Catalytic activity measurements In this study, the CZhp-S catalyst prepared by homogenous precipitation method through scCO2 drying treatment was evaluated for low-temperature methanol synthesis from syngas containing a little amount of CO2 at 170 °C and 5.0 MPa in a flow-type semi-batch reactor using 2-butanol as catalytic solvent. Typically, the low-temperature methanol synthesis was conducted out in a flow-type semi-batch autoclave reactor with an inner volume of 85 ml [40]. Prior to reaction, the catalysts were firstly reduced in the flow of 5% H2/N2 at 220 °C for 10 h and then passivated in 1% O2/N2 atmosphere for 4 h to get passivated surface of catalyst after the reduction temperature being cooled to the room temperature. 3.0 g reduced catalyst and 40 ml 2-butanol as catalytically active solvent were sealed into reactor concurrently with constant stirring. Then the syngas (CO/CO2/H2/Ar = 29.5/4.9/62.6/ 3
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scCO2 fluid drying leads to an easier reduction due to the small particles and high dispersion of CZhp-S catalyst. The abundant oxygen vacancies in the catalyst play vital roles in methanol synthesis. XPS technology is carried out to determine the state of the surface element and the existence of oxygen vacancies on CZhp-S, CZhp-H and CZcp-H reduced catalyst before reaction. As displayed in Fig. 3a, the survey XPS spectrum confirms the primary peaks of carbon (C1s), oxygen (O1s), copper (Cu2p) and zinc (Zn2p) peak in these catalysts. After exposing to hydrogen at 220 °C, the primary peaks with central position at 952.0 and 932.2 eV related to Cu 2p1/2 and 2p3/2 species respectively, imply the compete reduction of CuO into metallic copper, as in Fig. 3b. This is consisted with XRD results. For Zn 2p spectrum, the peaks at 1044.8 and 1021.7 eV are related to Zn 2p1/2 and Zn 2p3/2 respectively, rendering that the oxidation state of Zn element in both catalysts is Zn2+ (Fig. 3c). In addition, in the deconvoluted O spectra, three peaks at binding energies of 531.8, 530.7 and 530.0 eV are clearly observed, which are associated with the chemically adsorbed oxygen, oxygen vacancy and lattice oxygen (Fig. 3d) [46,47]. The relative concentration of deconvoluted O peaks is shown in Table S1. The ratio of oxygen vacancy to the lattice oxygen for CZhp-S catalyst reaches the highest value (0.799), compared to that of CZhp-H (0.681) or CZcp-H (0.594) catalysts. As reported by previous literatures, the higher oxygen vacancy content is beneficial for the methanol synthesis activity [48,49]. From these results, we can conclude that the preparation method for Cu/ZnO catalyst influences oxygen vacancy distribution.
Fig. 1. (a) SEM image, (b) TEM image (the inset shows particle distribution of CZhp-S catalyst), (c) HR-TEM image and (d) STEM-EDS mapping of CZhp-S catalyst.
3.2. Catalytic results The scCO2 drying technique is an extremely attractive technology for the preparation of porous materials and it is defined as a single fluid phase which occurs when the temperature and pressure of CO2 are above its critical temperature and pressure conditions (Tc = 31.2 °C, Pc = 7.38 MPa) [43]. Herein we examined the effects of supercritical temperature, pressure and drying time on the catalyst performance in the low temperature methanol synthesis (Fig. 4). There is no significant variation in specific surface area and activities of catalyst with varying of scCO2 temperatures and pressures. Although temperature and pressure influence the mass transfer, solubility of solvent and vapor pressure, which affect the property of catalyst during the drying progress, the effect is more complicated [43,50]. However, both CO conversion and methanol yield demonstrated highest value at near the critical point of CO2 (T = 35 °C and P = 7.5 MPa). This phenomenon is ascribed to clustering effect which increases the solvent density in the adjacent vicinity of the solute molecules compared to the bulk fluid density, in the zone near the critical pressure and temperature [43]. The detailed catalytic performance is listed in the Table S2. With the increase of supercritical drying time, it is exhibited an upward trend of the CO conversion and methanol yield. When the reserved time of drying is increased to 12 h, the CO conversion and total carbon conversion reach the highest value of 52.7% and 46.0% separately. The solvent in the most inner part of the porous catalyst need sufficient time to approach a drying equilibrium value corresponding to the complete dry. Hence, the longtime of 12 h is enough to replace solvent to scCO2 fluid until to complete dry. This higher catalytic behavior is related to the generation of small crystal size and high specific surface area of Cu0, which are important factors to control the catalyst activity in methanol synthesis reaction (Table S3). The catalytic behaviors of different catalysts for low-temperature methanol synthesis are compared in Fig. 5a. CO conversion over the CZhp-S catalyst is as high as 52.7%, which is improved remarkably, compared to that of CZcp-H catalyst (36.5%). Moreover, the CZhp-S catalyst realized STY of methanol 87.6 gMeOH/kgcatalyst.h−1, whereas CZcp-H catalyst illustrated STY of methanol 62.9 gMeOH/kgcatalyst.h−1. The by-product was mainly 2-butyl formate with trace amount of formate, and there is no methane produced in the CZhp-S catalyst (Table
aggregation and maintains high nanoparticle dispersion. The uniform porous structure of CZhp-S catalyst may boost surface area and catalytic activities. The XRD patterns of the calcined and reduced catalysts, prepared by different methods are shown in Fig. 2a and 2b. Although the morphologies of CZhp-S, CZhp-H and CZcp-H are distinctly different, they have identical crystal diffractions both in calcined and reduced catalysts. XRD patterns of reduced catalysts render three diffraction peaks located at 2θ value of 43.3, 50.4 and 74.1°, which correspond to reflections of (1 1 1) (2 0 0) (2 2 0) crystal planes of Cu metal (JCPDS file no. 04–0836). It is suggested that CuO was totally reduced to Cu0 after reduction process, while the ZnO phases still maintain after reduction treatment, as displaced in Fig. 2b. The crystallite size of Cu and ZnO for different reduced catalysts calculated by the XRD Scherrer equation is listed in Table 1. The Cu0 size of CZhp-S catalyst reaches the smallest value. As reported by the literatures, the Cu0 is the primary active site for low temperature methanol synthesis [15,40]. Thus the smaller Cu0 size may be responsible for improving catalytic performance. The CZhp-S catalyst illustrates higher BET surface area and Cu0 surface area than CZcp-H catalyst from the results calculated by BET method and N2O adsorption method, respectively. Furthermore, the specific surface area of metallic copper for CZhp-S catalyst reaches the highest value of 31.8 m2/g by N2O adsorption (Table 1), proving that there is strong synergy between Cu and ZnO [44,45]. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst activity. N2 adsorption–desorption isotherms and corresponding Barrett-Joyner-Halenda (BJH) pore-size distribution of three samples are illustrated in Fig. S2. The obtained high surface area is ascribed to the uniform porous structure of CZhp-S catalyst, according to the SEM observation. The reduction behaviour for all catalysts is investigated by H2-TPR technology. The reduction peak for the CZhp-S catalyst shifts to the lower temperature (main reduction peak of 220.4 °C) than the other two catalysts (CZhp-H: 225.6 °C and CZcp-H: 233.9 °C), as shown in Fig. 2c. It is suggested that the homogeneous synthesis method with 4
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Fig. 2. XRD patterns of (a) calcined and (b) reduced catalysts, (c) H2-TPR profiles of the different catalysts.
authors are as follows: (1) CO2 formed by water–gas shift reaction; (2) formic acid produced from CO2 hydrogenation; (3) esterification of formic acid by ROH to generate HCOOR; (4) hydrogenation of HCOOR to methanol and returning ROH, as shown in Scheme 2. In this reaction, ROH, as both catalytically-active solvent and promoter, facilitates the esterification reaction with HCOOH and accelerates methanol production at low reaction temperature [12,13]. The hydrogenation of CO2 to formic acid or ester appears to be the rate-limiting step for Cu-based catalyst, based on the mechanism and pathways of reaction [51]. The strong synergy between Cu and ZnO and the presence of oxygen defects on the CZhp-S catalyst are of benefit for hydrogen adsorption, and thus boosting formic acid formation from CO hydrogenation. Subsequently, formic acid easily reacts with alcohol by the catalyzed esterification reaction. Therefore, the overall CO conversion rates are significantly enhanced over the CZhp-S catalyst.
Table 1 Textural properties of different catalysts. Catalysts
CZcp-H CZhp-H CZhp-S a b c d
Cu/Zna (molar %)
1.00/0.91 1.00/0.91 1.00/0.92
SBETb (m2/g)
60.2 63.3 72.7
SCuc (m2/g)
28.5 30.2 31.8
Particle sized (nm) Cu
ZnO
9.0 8.6 8.5
9.6 8.7 8.3
Measured by ICP-AES. Calculated by BET method. Determined by N2O adsorption method. Calculated by XRD Scherrer equation.
S4). For comparison, the catalytic performance of CZhp-H catalyst is also presented in Fig. 5a. The CO conversion and methanol production of CZhp-H catalyst are higher than those of the CZcp-H catalyst, while lower than those of CZhp-S catalyst. It’s indicated that the supercritical dried catalyst demonstrates better catalytic performance in low-temperature methanol synthesis than that of conventional heating process due to high specific Cu surface area and small Cu crystal size. The high CO conversion of CZhp-S catalyst is ascribed to the obtained large specific surface area and highly dispersed copper nanoparticles from homogenous precipitation method. It is suggested that this approach, combining homogenous precipitation and scCO2 drying treatment, is a hopeful preparation method to synthesize effective Cu/ZnO catalyst for the low-temperature methanol reaction. The primary reaction equations included in the alcohol-assisted low temperature methanol synthesis reaction invented by the present
CO + H2O → CO2 + H2
(4)
CO2 + H2 → HCOOH
(5)
HCOOH + ROH → HCOOR + H2O
(6)
HCOOR + 2H2 → CH3OH + ROH
(7)
The whole reaction equation: CO + 2H2 → CH3OH
ΔH298K = -90.8KJ/mol
(8)
To study the stability of the CZhp-S, low-temperature methanol synthesis reaction was performed for 50 h at a reaction temperature of 170 °C and pressure of 5.0 MPa. Fig. 5b illustrates the time-on-stream changes of CO conversion, CO2 conversion and total carbon conversion over CZhp-S catalyst. With reaction time, the CO and total carbon conversion are enhanced rapidly at the initial stage of reaction, then 5
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Fig. 3. XPS curves of the (a) survey, (b) Cu, (c) Zn and (d) deconvoluted O of different catalysts.
Fig. 4. The catalytic performance of CZhp-S catalyst prepared under different temperature, pressure and time of scCO2 drying treatment, Reaction conditions: 3.0 g of catalyst; 170 °C; 5.0 MPa; solvent: 2-butanol, 40 ml; CO/CO2/H2/Ar = 29.5/4.9/62.6/3.0; Fsyngas = 20 ml/min. STY means space time yield.
increased slowly, and finally maintained stable after 16 h on stream. The catalyst yields 51.0% of methanol with 52.7% of CO conversion after 50 h on stream. Whereas, the CO2 conversion is initially negative due to the water–gas shift reaction, and finally becomes about 17.0%. Furthermore, the CZhp-S catalyst is extremely stable and there is no significant carbon deposition on the catalyst after 50 h reaction, as presented in Fig. S4. Fig. 5c and 5d list SEM images of the CZhp-S catalyst before and after 20 h reaction. The CZhp-S catalyst was reduced at 220 °C in the hydrogen atmosphere before reaction and its nanostructured morphology reveals sheetlike structure after reduction, as in Fig. 5c. The SEM image of spent CZhp-S catalyst remains the original structure of asreduced catalyst and there is no apparent particles aggregation. It is indicated that CZhp-S catalyst effectively stays against thermal sintering and prevents particles aggregation during reaction.
4. Conclusions The CZhp-S catalyst demonstrates excellent catalytic performance with CO conversion of 52.7% and STY of methanol 87.6 gMeOH/ kgcatalyst.h−1 for low temperature methanol synthesis, which is rather higher than CZcp-H catalyst (CO conversion of 35.6%, STY of methanol 62.9 gMeOH/kgcatalyst.h−1). The high activities of the catalysts may be ascribed to the low reduction temperature, highly dispersed Cu metal particles and the high accessibility of Cu metal particles to methanol synthesis, while the homogeneous precipitation method by urea hydrolysis with scCO2 drying progress guarantees these to form uniform size of CZhp-S catalyst with superior catalytic performance in lowtemperature methanol synthesis. Our work offers a promising approach to developing effective Cu/ZnO catalyst and extends the application of scCO2 drying process. 6
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60
CO
CO2
Total C.
Methanol yield
(b)
60
60 40
50
40
40
30
30
20
20
10
10
-20
0
-40
0
CZcp-H
CZhp-H
Conversion (%)
50
Methanol yield (%)
Conversion (%)
(a)
CZhp-S
20 0
CO CO2 Total Carbon
0
10 20 30 Time on stream (h)
40
50
(d)
(c)
Fig. 5. (a) The catalytic performance of CZcp-H, CZhp-H and CZhp-S catalyst, (b) the time on stream of the CZhp-S catalyst for low temperature methanol synthesis, the SEM images of the CZhp-S catalyst (c) before and (d) after reaction.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2020.117213. References [1] Nielsen DU, Hu X, Daasbjerg K, Skrydstrup T. Chemically and electrochemically catalysed conversion of CO2 to CO with follow-up utilization to value-added chemicals. Nat Catal 2018;1:244–54. [2] Xu S, Zheng A, Wei Y, Chen J, Li J, Chu Y, et al. Direct observation of cyclic carbenium ions and their role in the catalytic cycle of the methanol-to-olefin reaction over chabazite zeolites. Angew Chem Int Ed 2013;52:11564–8. [3] Olsbye U, Svelle S, Bjørgen M, Beato P, Janssens TVW, Joensen F, et al. Conversion of methanol to hydrocarbons: how zeolite cavity and pore size controls product selectivity. Angew Chem Int Ed 2012;51:5810–31. [4] Kasipandi S, Bae JW. Recent advances in direct synthesis of value-added aromatic chemicals from syngas by cascade reactions over bifunctional catalysts. Adv Mater 2019:1803390–408. [5] Zhang P, Tan L, Yang G, Tsubaki N. One-pass selective conversion of syngas to paraxylene. Chem Sci 2017;8:7941–6. [6] Behrens M, Studt F, Kasatkin I, Kühl S, Hävecker M, Abild-pedersen F, et al. The active site of methanol synthesis over Cu/ZnO/Al2O3 industrial catalysts. Science 2012;759:893–8. [7] Kattel S, Ramirez P, Chen JG, Rodriguez JA, Liu P. Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts. Science 2017;355:1296–9. [8] Matsumura Y, Shen WJ, Ichihashi Y, Okumura M. Low-temperature methanol synthesis catalyzed over ultrafine palladium particles supported on cerium oxide. J Catal 2001;197:267–72. [9] Jeong Y, Kim I, Kang JY, Yan N, Jeong H, Park KJ, et al. Effect of the aging time of the precipitate on the activity of Cu/ZnO catalysts for alcohol-assisted low temperature methanol synthesis. J Mole Catal A 2016;418:168–74. [10] Haggin J. Chem Eng News 1986;21:;Aug:4. [11] Brookhaven National Laboratory, US patent, 461479, 4619946, 4623634, 4613623 (1986), 4935395 (1990). [12] Tsubaki N, Ito M, Fujimoto K. A new method of low-temperature methanol synthesis. J Catal 2001;197:224–7. [13] Yang R, Fu Y, Zhang Y, Tsubaki N. In situ DRIFT study of low-temperature methanol synthesis mechanism on Cu/ZnO catalysts from CO2-containing syngas using ethanol promoter. J Catal 2004;228:23–35. [14] Shi L, Yang G, Tao K, Yoneyama Y, Tan Y, Tsubaki N. An introduction of CO2 conversion by dry reforming with methane and new route of low-temperature
Scheme 2. Reaction pathway for the alcohol-assisted low temperature method synthesis reaction over the CZhp-S catalyst.
Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is supported by JST-CREST of Japan Science and Technology Agency (Grant Number, 17-141003297). Research fund (2017AA001) from Xinjiang-Tianye Co. Ltd. and Xinjiang government is greatly appreciated. 7
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P. Zhang, et al. methanol synthesis. Acc Chem Res 2013;46:1838–47. [15] Meng F, Zhang Q, Yang G, Yang R, Yoneyama Y, Tsubaki N. Structural and kinetical studies on the supercritical CO2 dried Cu/ZnO catalyst for low-temperature methanol synthesis. Chem Eng J 2016;295:160–6. [16] Kattel S, Ramriez P, Chen J, Rodriguez J, Liu P. Active sites for CO2 hydrogenation to methanol on Cu/ZnO Catalysts. Science 2017;355:1296–9. [17] Wang X, Ma K, Guo L, Tian Y, Cheng Q, Bai X, et al. Cu/ZnO/SiO2 catalyst synthesis by reduction of ZnO-modified copper phyllosilicate for dimethyl ether steam reforming. App Catal A, General 2017;540:37–46. [18] Wang X, Ma K, Guo L, Ding T, Cheng Q, Tian Y, et al. Catalytic performance for hydrogen production through stream reforming of dimethyl ether over silica supported copper catalysts synthesized by ammmonia evaporation method. Acra Phys Chim Sin 2017;33:1699–708. [19] Huang J, Ding T, Ma K, Cai J, Sun Z, Tian Y, et al. Modification of Cu/SiO2 catalysts by La2O3 to quantitatively tune Cu+-Cu0 dual sites with improved catalytic activities and stabilities for dimethyl ether steam reforming. ChemCatChem 2018;10:3862–71. [20] Ma K, Cui Z, Zhang Z, Huang J, Sun Z, Tian Y, et al. Alloy-mediated ultra-low CO selevtivity for steam reforming over Cu-Ni bimetallic catalysts. ChemCatChem 2018;10:4010–7. [21] Huang J, Cai J, Ma K, Ding T, Tian Y, Zhang J, et al. Ca2O3-modified Cu/SiO2 catalysts with low CO selectivity for catalytic steam reforming. Acra Phys Chim Sin 2019;35:431–41. [22] Tan L, Zhang P, Suzuki Y, Li H, Guo L, Yoneyama Y, et al. Bifunctional capsule catalyst of Al2O3@Cu with strengthened dehydration reaction field for direct synthesis of dimethyl ether from syngas. Ind Eng Chem Res 2019. https://doi.org/ 10.1021/acs.iecr.9b04864. [23] Zhu C, Fang Y, Luo Z, Zhang C, Zhang X, Li J, et al. Direct dimethyl ether synthesis over mesoporous Cu-Al2O3 catalyst via CO hydrogenation. Res Chem Intermed 2019;45:5863–76. [24] Behrens M, Schlögl R, Anorg Z. How to prepare a good Cu/ZnO catalyst or the role of solid state chemistry for the synthesis of nanostructured catalysts. Allg Chem 2013;639:2683–95. [25] Cai M, Palcic A, Subramanian V, Moldovan S, Ersen O, Valtchev V, et al. Direct dimethyl ether synthesis from syngas on copper-zeolite hybrid catalysts with a wide range of zeolite particle sizes. J Catal 2016;338:227–38. [26] Smith PJ, Kondrat SA, Chater PA, Yeo BR, Shaw GM, Lu L, et al. A new class of Cu/ ZnO catalysts derived from zincian georgeite precursors prepared by coprecipitation. Chem Sci 2017;8:2436–47. [27] Munnik P, De Jongh PE, De Jong KP. Control and impact of the nanoscale distribution of supported cobalt particles used in fischer-tropsch catalysis. J Am Chem Soc 2014;136:7333–40. [28] Dong C, Lian C, Hu S, Deng Z, Gong J, Li M, et al. Size-dependent activity and selectivity of carbon dioxide photocatalytic reduction over platinum nanoparticles. Nat Commun 2018;9:1–11. [29] Galvis HMT, Bitter JH, Khare CB, Ruitenbeek M, Dugulan AI, de Jong KP. Supported iron nanoparticles as catalysts for sustainable production of lower olefins. Science 2012;335:835–8. [30] Shi L, Shen W, Yang G, Fan X, Jin Y, Zeng C, et al. Formic acid directly assisted solid-state synthesis of metallic catalysts without further reduction: as-prepared Cu/ ZnO catalysts for low-temperature methanol synthesis. J Catal 2013;302:83–90. [31] Dong X, Li F, Zhao N, Xiao F, Wang J, Tan Y. CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts preparedby precipitation-reduction method. Appl Catal B
Environ 2016;191:8–17. [32] Guo Y, Meyer-Zaika W, Muhler M, Vukojević S, Epple M. Cu/Zn/Al xerogels and aerogels prepared by a sol-gel reaction as catalysts for methanol synthesis. J Inorg Chem 2006;23:4774–81. [33] Darr JA, Zhang J, Makwana NM, Weng X. Continuous hydrothermal synthesis of inorganic nanoparticles: applications and future directions. Chem Rev 2017;117:11125–238. [34] Sun J, Yu J, Ma Q, Meng F, Wei X, Sun Y, et al. Freezing copper as a noble metal-like catalyst for preliminary hydrogenation. Sci Adv 2018;4:3275–6. [35] Ma K, Tian Y, Zhao Z, Cheng Q, Ding T, Zhang J, et al. Achieving efficient and robust catalytic reforming on dual-sites of Cu species. Chem Sci 2019;10:2578–84. [36] Sun Z, Tian Y, Zhang P, Yang G, Tsubaki N, Abe T, et al. Sputtered Cu-ZnO/γ-Al2O3 bifunctional catalyst with ultra-low Cu content boosting dimethyl ether steam reforming and inhibiting side reactions. Ind Eng Chem Res 2019;58:7085–93. [37] Song KC, Kang Y. Preparation of high surface area tin oxide powders by a homogeneous precipitation method. Mater Lett 2000;42:283–9. [38] Ocana M, Morales MP, Serna CJ. Homogeneous precipitation of uniform a-Fe2O3 particles from iron salts solutions in the presence of urea. J Colloid Interf Sci 1999;323:317–23. [39] Szijjártó GP, Pászti Z, Sajó I, Erdohelyi A, Radnóczi G, Tompos A. Nature of the active sites in Ni/MgAl2O4-based catalysts designed for steam reforming of ethanol. J Catal 2013;305:290–306. [40] Fan R, Kyodo M, Tan L, Peng X, Yang G, Yoneyama Y, et al. Preparation and application of Cu/ZnO catalyst by urea hydrolysis method for low-temperature methanol synthesis from syngas. Fuel Process Technol 2017;167:69–77. [41] Lekhal A, Glasser BJ, Khinast JG. Impact of drying on the catalyst profile in supported impregnation catalysts. Chem Eng Sci 2001;56:4473–87. [42] Mizushima Y, Hori M. Alumina aerogel catalysts prepared by two supercritical drying methods used in methane combustion. J Mater Sci 1995;30:1551–5. [43] Sui R, Charpentier P. Synthesis of metal oxide nanostructures by direct sol-gel chemistry in supercritical fluids. Chem Rev 2012;112:3057–82. [44] Behrens M, Zander S, Kurr P, Jacobsen N, Senker J, Koch G, et al. Performance improvement of nanocatalysts by promoter-induced defects in the support material: methanol synthesis over Cu/ZnO:Al. J Am Chem Soc 2013;135:6061–8. [45] Fujitani T, Nakamura J. The chemical modification seen in the Cu/ZnO methanol synthesis catalysts. Appl Catal A 2000;191:111–29. [46] Santos VP, Pereira MFR, Órfão JJM, Figueiredo JL. The role of lattice oxygen on the activity of manganese oxides towards the oxidation of volatile organic compounds. Appl Catal B Environ 2010;99:353–63. [47] Zhu M, Zhang Z, Zhong M, Tariq M, Li Y, Li W, et al. Oxygen vacancy induced ferromagnetism in Cu-doped ZnO. Cream Int 2017;43:3166–70. [48] Studt F, Behrens M, Kunkes EL, Thomas N, Zander S, Tarasov A, et al. The mechanism of CO and CO2 hydrogenation to methanol over Cu-based catalysts. ChemCatChem 2015;7:1105–11. [49] Fichtl MB, Schumann J, Kasatkin I, Jacobsen N, Behrens M, Schlögl R, et al. Counting of oxygen defects versus metal surface sites in methanol synthesis catalysts by different probe molecules. Angew Chem Int Ed 2014;53:7043–7. [50] Unlusu B, Sunol AK. Modeling of equilibration times at high pressure for multicomponent vapor-liquid diffusional processes. Fluid Phase Equilib 2004;226:15–25. [51] Schumann J, Eichelbaum M, Lunkenbein T, Thomas N, Galván MCÁ, Schlögl R, et al. Promoting strong metal support interaction: doping ZnO for enhanced activity of Cu/ZnO: M (M = Al, Ga, Mg) catalysts. ACS Catal 2015;5:3260–70.
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