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Cite This: Ind. Eng. Chem. Res. 2018, 57, 4526−4534
An Effective CuZn−SiO2 Bimetallic Catalyst Prepared by Hydrolysis Precipitation Method for the Hydrogenation of Methyl Acetate to Ethanol Yujun Zhao,* Bin Shan, Yue Wang, Jiahua Zhou, Shengping Wang, and Xinbin Ma
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Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China
ABSTRACT: A highly active CuZn−SiO2 catalyst with excellent stability in hydrogenation of methyl acetate (MA) was fabricated by the hydrolysis precipitation (HP) method. The HP method caused notable high dispersion of both copper and zinc species due to the strong interaction between metal species and silica support caused by the pseudo-homogeneous reaction during the mixing process. According to the linear correlation between the TOF (Cu+) and Zn/Cu ratio, the Cu+ was considered the predominant active site, and the Cu+ site derived from the strong interaction between copper and zinc species has higher activity than those Cu+ species generated from copper phyllosilicate. Compared to the CuZnO/Al2O3 and Cu−SiO2 catalysts, the Cu9Zn1−SiO2 catalyst displayed the highest catalytic activity on account of the high dispersion of active species and the enhanced ability of Cu+ species on ester adsorption by stronger Cu-ZnO interaction.
1. INTRODUCTION As an important chemical raw material, ethanol is widely used as solvent or in the synthesis of acetaldehyde, ether, ethylamine, and so on. In particular, it is also considered as a reliable alternative to petroleum as it is environmentally friendly and can reduce dependence on oil resources.1 Ethanol is now mainly produced by the fermentation of biomass and ethylene hydration. Recently, a new ethanol synthesis route which consists of carbonylation of dimethyl ether (DME) and hydrogenation of methyl acetate (MA) has attracted wide attention because of its high atom economy and environmental friendliness.2 Previous studies on this new route showed that a copper-based catalyst was generally used as the main catalyst in the hydrogenation of MA due to its prominent performance in selective hydrogenation of carbonyl and relatively weak ability in scissoring the C−C bond.3,4 On the basis of previous studies on Cu-based catalysts, the conclusion could be drawn that the synergy of copper species plays a vital role in the hydrogenation of MA.5 Furthermore, it is widely accepted that Cu0 sites facilitate the decomposition of H2 and Cu+ species absorbed methoxy and acyl groups. Moreover, the catalytic performance of Cu/SiO2 catalysts was promoted essentially after modification by Ce,6 Mg,7 B,8 La,9 © 2018 American Chemical Society
which enhanced the dispersion of active species or increased the proportion of Cu+. Recently, Zn-modified Cu/SiO2 catalysts have been applied in multifarious reaction process including esters hydrogenation, the water−gas shift reaction, methanol steam reforming, methanol synthesis, dimethyl ether synthesis, and so on.10−17 The interaction between copper and zinc species has been studied for several decades in CuZn/Al2O3 catalyst for methanol synthesis. Up to now, two major viewpoints about the ZnO effect are generally accepted: (1) the interface between Cu and ZnO provide the active site,18 (2) the formation of Cu/Zn alloy creates the active site.19,20 Although both viewpoints have been confirmed by experiment and theoretical calculation in methanol synthesis, the doubt about the role of ZnO in the Cu-based catalyst still exists in the esters hydrogenation system. Zhu et al.21 found that different ZnO loadings can influence the relative dispersion of Cu species, and the Cu/Zn ratio is vital to the catalytic performance of Cu−Zn/ Received: Revised: Accepted: Published: 4526
December March 15, March 16, March 16,
29, 2017 2018 2018 2018 DOI: 10.1021/acs.iecr.7b05391 Ind. Eng. Chem. Res. 2018, 57, 4526−4534
Article
Industrial & Engineering Chemistry Research
molar ratio was kept at 20. The products were condensed in air condenser, and the liquid samples were analyzed by an Agilent Micro GC 6820 equipped with a FID. The byproduct is mainly ethyl acetate. Catalyst Characterization. The pore structure properties of catalysts were determined by a Micromeritics Tristar II 3000 analyzer instrument. Before the analysis, the sample powder of catalysts was degassed under vacuum at 573 K for 4 h. The specific surface area, the pore volume, and pore diameter were separately determined by Brunauer-Emmett-Teller and Barrett−Joyner−Halenda approach. The copper and zinc loading of calcined catalysts were obtained by an inductively coupled plasma optical emission spectrometry (ICP-OES) (VISTA-MPX, Varian). The operating conditions are high frequency emission power at 1.5 kW and plasma airflow of 15.0 mL/min. Before the measurement, the catalyst was dissolved in hydrofluoric acid (HF) for 30 min, followed by the addition of a certain amount of boric acid to form a complex with HF. The reducibility of the catalysts was tested by H 2 temperature-programmed reduction (H2-TPR) experiments on the Micromeritics Autochem II 2920. A certain amount of catalyst samples were placed in the U-tube and pretreated in Ar flow at 473 K for 2 h before reduction. After being cooled to room temperature, the catalysts were heated to 1073 K in 10% H2/Ar with a heating rate of 10 K/min. A TCD was used to quantify the hydrogen consumption during the reduction, and the water vapor had been condensed in the cold trap before this process. The X-ray diffraction (XRD) patterns of catalysts were obtained by a Rigaku model C/max-2500 diffractometer with a radiation source of Cu Kα at ambient temperature. The data was acquired from 2θ = 10° to 80° with the step scanning rate of 8°/min. To avoid being oxidized by air, the prereduced samples were prudently gathered into a plastic bottle filled with pure Ar before analysis. The particle size of the metallic Cu crystal was calculated by the Scherrer equation by an X-ray broadening technique. Transmission electron microscopy (TEM) was conducted on a Philips Tecnai G2 F20 system electron microscope to observe the morphology and structure of the samples. Before the experiments, the samples were ground into powder and then dispersed in ethanol with the assistance of ultrasound. After that, the suspension was allowed to stand a while and then the supernatant was dropped onto the ultrathin carbon film. The surface copper and zinc species of the catalysts were detected by X-ray photoelectron spectroscopy (XPS) and Al Kα (1486.6 eV) was used as the X-ray source. The ratio between different copper species were obtained by Auger electron spectroscopy (XAES). Before the analysis, the catalyst samples were prereduced in pure hydrogen flow. The energies of C1s were used to calibrate the binding energies of the catalyst samples. The CO2 temperature-programmed desorption experiment was performed to study the basicity of the catalysts by using a Micromeritics Autochem II 2920.25,26 First, 100 mg of catalysts was introduced into the U-shaped tube and reduced at 573 K in pure hydrogen flow, then purged with helium for a while. After being cooled to 323 K, the sample was pretreated by 10% CO2/ He for 1 h in order to the basic sites were occupied sufficiently. After that, the samples were heated to 873 K in the pure He flow. The quantity of CO2 was detected by TCD.
Al catalyst for ethyl acetate hydrogenation. In our previous studies, it was found that ZnO can promote the dispersion of Cu species and influence the ratio of Cu+/Cu0 in CuZnO/SBA15 catalysts.22 Furthermore, the Cu+ species originated from Cu-ZnOx species are more active than that from Cu−O−Si structures in the activation of MA. Obviously, the highly dispersed Cu and ZnO species are favorable for realizing the strong interaction between Cu and ZnO, which is responsible for the excellent performance of Cu/ZnO catalysts. However, as far as we know, how to further prompt the dispersion of Cu and ZnO species is still a challenge especially for the traditional coprecipitation synthesis method21 as well as the solid grinding method.22 In this study, a range of highly dispersed CuZn−SiO2 catalysts were synthesized by the hydrolysis precipitation method23 and the activity was examined in the hydrogenation of MA. The ratio between Cu and Zn was modulated, and the influence of ZnO on the copper species was investigated. Furthermore, the interaction between Cu and ZnO species and its roles on the intrinsic activity was deeply investigated.
2. EXPERIMENTAL SECTION Catalysts Preparation. The HP method was used to prepare the Cu−SiO2 and CuZn−SiO2 catalysts (Cu/Zn molar ratio = 14:1, 12:1, 9:1, 7:1) whose metal loading is 30 wt %. An appropriate amount of copper(II) nitrate and (zinc(II) nitrate) were added in distilled water to form the salt solution A. A required amount of TEOS was added into the ethanol to prepare the organic solution B. Then the above two solution were mixed well to form mixture C. To precipitate the copper (zinc) and silica support, 0.25 mol/L (NH4)2CO3 solution and the mixture C were dripped under stirring conditions. The pH of solution in this process is about 7, and the temperature was maintained at 353 K. After that, the suspension was aged for 18 h at 353 K. Then, a precipitate was obtained from filtration, washing, drying, and calcination by the same method as described in our previous work.23 The catalysts were denoted as CuxZny−SiO2, and the x and y stand for the molar ratio between copper and zinc loading. The Zn−SiO2 catalyst was also prepared for a comparison. For a comparison, a CuZn/Al2O3 catalyst was prepared by the coprecipitation method.24 In this method, an aqueous solution of copper(II) nitrate, zinc(II) nitrate, and aluminum(III) nitrate (total concentration of metal ions is 1 mol/L) and an aqueous solution of sodium carbonate (1 mol/L) were simultaneously dripped to 100 mL of deionized water under stirring conditions. The pH in this process was maintained at 7.5 ± 0.2 by adjusting the dropping rate of Na2CO3 aqueous solution. After that, the reaction mixture was aged for a while. The temperature of this process was maintained at 338 K. The precipitate was filtered and washed, then dried in air at 393 K overnight. Finally, the precipitate was treated by calcination. The obtained catalyst was named CuZn/Al2O3. Catalytic Performance Examination. The activity of the Cu−SiO2, CuxZny−SiO2, Zn−SiO2, and CuZn/Al2O3, catalysts for the hydrogenation of MA was evaluated in a continuous flow reactor packed with 0.55 g of catalyst. First, all catalysts were pretreated in H2 flow at 573 K. After that, the reaction was conducted at 493 K under 2.0 MPa of pressure. To evaluate the catalytic activity of these catalysts, a stream of MA was injected through a high-pressure pump. The MA and H2 were mixed in evaporator and then the homogeneous mixture was continuously fed into the reactor. The H2/MA 4527
DOI: 10.1021/acs.iecr.7b05391 Ind. Eng. Chem. Res. 2018, 57, 4526−4534
Article
Industrial & Engineering Chemistry Research Table 1. Physicochemical Properties of Cu−SiO2 and CuxZny−SiO2 Catalysts
a
catalysts
Cu loadinga (%)
Zn loadinga (%)
SBETb (m2/g)
Vpb (cm3/g)
Dpb (nm)
XCu+c (%)
SCu+d (m2/g)
SCu0 (m2/g)
SCu (m2/g)
Cu−SiO2 Cu14Zn1−SiO2 Cu12Zn1−SiO2 Cu9Zn1−SiO2 Cu7Zn1−SiO2
30.23 26.50 27.20 27.96 25.27
0 2.71 3.37 4.56 4.68
484 467 489 497 495
0.90 1.10 1.00 0.98 1.08
5.56 6.80 6.36 6.01 6.77
50.83 47.37 46.83 40.87 29.25
33.81 41.60 39.20 31.72 19.90
38.80 46.18 44.69 45.99 48.10
72.61 87.78 83.89 77.71 68.00
Content of copper and zinc determined by ICP-OES analysis. bDetermined by nitrogen adsorption. cCu+/ (Cu0+Cu+) obtained by XAES spectra. Calculated from the results of the N2O−CO titration and XCu+.
d
Figure 1. Cu−SiO2 and CuxZny−SiO2 catalysts with different Cu/Zn ratio: (a) N2 adsorption−desorption isotherm and (b) pore distribution curves.
Figure 2. XRD patterns of (a) calcined and (b) reduced Cu−SiO2 and CuxZny−SiO2 catalysts.
by Cu XAES, NA is the Avogadro constant, NCu is the quantity of Cu0 or Cu+ atoms per unit surface area (1.46 × 1019).
The surface area of copper species was determined by combining N2O oxidation and CO temperature-programmed desorption (CO-TPD) using a Micromeritics Autochem II 2920.27 First, 100 mg catalyst samples were introduced into the U-tube and reduced for 2 h at 573 K in 10% H2/Ar mixture gas. After the catalysts were cooled down to 363 K in He flow, pure N2O was introduced to oxidize the surface Cu0 species and oxygen vacancies. After that, the samples were continually cooled to 323 K at the atmosphere of pure He and then purged with 10.1% CO/He for 1 h. Finally, the CO-TPD experiments were conducted in the He flow from 323 to 1073 K. The quantity of CO released from the samples was counted by TCD. The surface area of copper species (SCu0 or SCu+, m2/g) was calculated as follows: SCu =
NXNA NCu
3. RESULTS AND DISCUSSION Physicochemical Properties of Cu−SiO2 and CuxZny− SiO2. The physicochemical properties of Cu−SiO2 and CuxZny−SiO2 catalysts are shown in Table 1. The copper and zinc loadings of the calcined CuxZny−SiO2 catalysts were determined by ICP-OES. As described in Table 1, the actual metal loadings are slightly lower than the design value due to the loss of metallic ions in the washing process. The profiles of pore structure properties of catalysts are exhibited in Figure 1. Although the ratio between Cu and Zn loadings increase from 7:1 to 14:1, the surface areas still maintain at the same level as Cu−SiO2 catalyst. However, the pore column and pore diameter of catalysts had a significant improvement after the modification by zinc species, which may be attributed to the less obstruction of pores by the aggregation of copper species. As shown in Figure 1a, all the catalysts
(1)
where N is the quantity of CO desorption (mol/g), X is the molar fraction of surface Cu0 or Cu+ on the catalyst calculated 4528
DOI: 10.1021/acs.iecr.7b05391 Ind. Eng. Chem. Res. 2018, 57, 4526−4534
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Industrial & Engineering Chemistry Research
from 14:1 to 7:1, the crystallinity of the lamella decrease as well, which may be owed to the fact that the introduction of ZnO inhibits the formation of copper phyllosilicate. To observe the morphology and the particle size of the catalysts doped by ZnO, the HRTEM characterization of reduced catalysts was also conducted. Figure 4 shows that the tubular structure of catalysts disappeared after reduction. When the catalysts were modified by ZnO, the particle size of copper species was inhibited remarkably. With further increase of ZnO loadings, the particle size of copper species decreases first and then increases, which agrees well with the trend of SCu listed in Table 1. H2-TPR Profiles. The H2-TPR experiments were conducted to investigate the reducibility of CuxZny−SiO2 catalysts. Figure 5 shows that the reduction peak of Cu−SiO2 catalyst is centered at 203 °C, which results from the reduction of welldispersed CuO and copper phyllosilicate.33,34 But when the catalysts were modified by ZnO, the reduction peak shifted toward to a high temperature. Figure 5 shows that all the catalysts with different Cu/Zn molar ratios display a sharp but not symmetrical reduction peak centered at about 220 °C, which is much higher than that of Cu−SiO2. Moreover, compared with the Cu−SiO2, CuxZny−SiO2 catalysts present a new weak reduction peak at about 290 °C, which may be due to the strong interaction between Cu species and ZnO. What is more, there is no obvious evidence to support the existence of large CuO crystallites from TPR profiles, which is consistent with the results of the XRD patterns. CO2-TPD Profiles. Due to the interaction between copper and ZnO species, the basic sites on the surface of the catalysts will be changed.35 Therefore, the interaction between copper and ZnO species can be determined by studying the basic sites of the CuxZny−SiO2 catalysts. The CO2-TPD experiments were performed, and the CO2 desorption profiles are displayed in Figure 6. It is obvious that all the catalyst samples contain a large amount of surface basic sites. The profile of Cu−SiO2 can be divided into two Gauss peaks, the weak peak (α peak) was assigned to the silicon hydroxyl, and another peak (β peak) could be ascribed to the moderately basic sites provided by the metal−oxygen pairs. However, a stronger peak (γ peak) appears in the catalysts after the modification of the zinc species, which could be ascribed to the strongly basic sites provided by lowcoordination oxygen atoms.35 According to the results of peak deconvolution, the proportion of strongly basic sites gradually increases with the increase of ZnO. It could be interpreted that some ZnOx species would migrate on the surface of copper species during the process of reduction, leading to the formation of low-coordination oxygen.25 Therefore, a low Cu/Zn molar ratio is more favorable to the formation of Cu− ZnO interaction in this HP method. XPS. To evaluate the surface chemical state of the copper species and the roles of ZnO, the reduced CuxZny−SiO2 catalysts were characterized by XPS and XAES measurements. As shown in Figure 7, the peaks centered at 932.5 and 952.8 eV are assigned to the binding energy of Cu 2p3/2 and Cu 2p1/2.36 Meanwhile, there is no obvious peak assigned to well-dispersed CuO between 942 and 944 eV, which indicates that the Cu2+ had been reduced to Cu+ and Cu0 completely.37 Although the reduced copper species (Cu0 and Cu+) were hard to discriminate by XPS spectra on account of their similar binding energy, the proportion of surface Cu+ and Cu0 can be acquired by XAES spectra.38 As shown in Figure 8, an asymmetrical and broad peak was observed in reduced
exhibit typical Langmuir type IV isotherms featured by the H1hysteresis loop, indicating the existence of mesoporous structure in all the samples. Figure 1b shows that the catalysts have two kinds of pore size within the range of 3 to 10 nm. With the increase of ZnO loadings, the size of the larger pore was slightly enlarged, and the small peak at 3 nm could be attributed to the existence of copper phyllosilicate in catalysts.28 XRD Patterns. The XRD patterns in Figure 2a evidence the structures of calcined Cu−SiO2 and CuxZny−SiO2 catalysts. The low signals at 2θ = 30.8° and 35.0° confirm the existence of Cu2SiO5(OH)2.29 No diffraction peaks for the CuO phase were observed in all catalyst samples, probably in virtue of the high dispersion of the metal species generated by this method. Beyond that, the diffraction peaks of ZnO were also not observed in XRD patterns, implying the homogeneous distribution of ZnO in CuxZny−SiO2 catalysts. The XRD patterns of reduced Cu−SiO2 and CuxZny−SiO2 catalysts are shown in Figure 2b. Besides the diffraction peak ascribed to amorphous SiO2, the diffraction peaks centered at 2θ = 36.4° and 43.3° belong to Cu2O (JCPDS05−0667) and Cu0 (JCPDS04−0836), respectively.30,31 Although the diffraction peaks of copper species in all reduced samples were observed, the sizes of the Cu crystallites (2−3 nm, based on the calculations by using Scherrer equation) have little difference among different catalysts. According to the results of XRD patterns, the copper and zinc species have excellent dispersion on the reduced samples. To further confirm the dispersion of the copper species, the HRTEM images were examined to characterize the particle size of the catalysts. TEM Images. Figure 3 shows the TEM images to analyze the effect of the Cu/Zn molar ratio on the morphology of the calcined CuxZny−SiO2 catalysts prepared by the HP method. All the catalysts exhibit a large amount of silicate nanotubes which can be attributed to the intrinsic structures of copper phyllosilicates.32 Meanwhile, the morphology varies with the Cu/Zn molar ratio on the catalysts. When the ratios decrease
Figure 3. TEM images of calcined catalysts with different Cu/Zn molar ratio: (a) Cu14Zn1−SiO2, (b) Cu12Zn1−SiO2, (c) Cu9Zn1−SiO2, (d) Cu7Zn1−SiO2. 4529
DOI: 10.1021/acs.iecr.7b05391 Ind. Eng. Chem. Res. 2018, 57, 4526−4534
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Figure 4. HRTEM images of reduced catalysts (a) Cu−SiO2, (b) Cu14Zn1−SiO2, (c) Cu9Zn1−SiO2.
Figure 7. Cu 2p XPS spectra of reduced CuxZny−SiO2 catalysts.
Figure 5. H2-TPR profiles of catalysts with different Cu/Zn molar ratio.
Figure 6. CO2-TPD profiles of reduced Cu−SiO2 and CuxZny−SiO2 catalysts.
Figure 8. Cu LMM XAES spectra of reduced Cu−SiO2 and CuxZny− SiO2 catalysts.
CuxZny−SiO2 catalysts, and it can be divided into two peaks centered at about 572.9 and 570.1 eV, corresponding to the Cu+ and Cu0 species, respectively.14 Table 1 shows that the Cu+/(Cu0 + Cu+) molar ratio decreases with the increase of the ZnO loadings. These results explained that the Cu/Zn ratio has a serious influence on the molar ratio of Cu+/(Cu0 + Cu+) on the surface. Ji et al. reported that the introduction of ZnO could influence the amount of Cu+ on Cu−SiO2 catalyst for the dehydrogenation of cyclohexanol, suggesting the presence of the interactions between Cu and ZnO species.39 So the formation of copper phyllosilicate and the interaction between
copper and zinc species could be the key reasons for the change of XCu+. The surface area of the copper species was calculated according to the N2O−CO titration and XCu+ results, assuming that all copper species occupy identical atomic areas. As listed in Table 1, obviously, the total surface area of the copper species (SCu) was improved significantly by the addition of the zinc species in the catalysts, which implied that the zinc species can facilitate the dispersion of the copper species. This is consistent with the study by Sun et al.40 on Cu-ZnO/SiO2 4530
DOI: 10.1021/acs.iecr.7b05391 Ind. Eng. Chem. Res. 2018, 57, 4526−4534
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Figure 9. Activity of Cu−SiO2, CuxZny−SiO2, Zn−SiO2, and CuZnO/Al2O3 catalysts: (a) conversion of MA, (b) selectivity of EtOH, (c) conversion of MA and selectivity of EtOH on different CuxZny−SiO2 catalysts. Reaction conditions: P = 2.0 MPa, T = 493 K, H2/MA = 20 (inset: conversion of MA and the selectivity of EtOH on Cu9Zn1−SiO2 catalyst at different temperatures). (d) Effects of Zn/Cu molar ratio on the TOF of Cu+ species.
indicates that ZnO is not the active site for this reaction. For Cu−SiO2 and CuxZny−SiO2 catalysts, both the conversion of MA and the selectivity to EtOH exhibit a decreasing trend with the increase of LHSV. Meanwhile, when decreasing the molar ratio of Cu/Zn, the conversion and selectivity to EtOH present a similar volcanic curve, and the Cu9Zn1−SiO2 catalyst shows the highest activity among all the catalyst samples (Figure 9c). Additionally, it was noteworthy that the as-prepared Cu9Zn1− SiO2 catalyst achieved much higher activity (STY = 1.024 gEtOH gcat−1 h−1) than CuZnO/Al2O3 catalyst (STY = 0.775 gEtOH gcat−1 h−1) prepared by the traditional coprecipitation method. Generally, it is considered that the catalytic performance was influenced by the surface copper species on the Cu-based catalyst.41 Li et al.23 reported that the superior surface areas of Cu species can promote the activity of catalyst in the hydrogenation of dimethyl oxalate. According to the results in Table 1, the better catalytic performance of the catalysts modified by ZnO should be attributed to the improved surface areas of copper species. Beyond that, since all the CuxZny−SiO2 catalysts showed the similar surface area of Cu0, the amount of Cu+ should play the key role in the hydrogenation of MA.
catalysts for the dehydrogenation of methanol to methyl formate. Moreover, the Cu+ surface area gradually drops with the decrease of the Cu/Zn ratio in the reduced CuxZny−SiO2 catalysts. It might have occurred because the copper phyllosilicate was inhibited by the addition of zinc species, which has also been confirmed by TEM results (Figure 3). Although the Cu/Zn ratio ranged from 14:1 to 7:1, the Cu0 surface area maintained at a similar level. Since Cu0 was derived from the reduction of the CuO in Cu/SiO2 or CuZn/SiO2,33,34 the identical surface areas of Cu0 on CuxZny−SiO2 catalysts suggested that the CuO species can be highly dispersed during the HP preparation process. However, in our previous studies on the CuZn/SBA-15 catalysts prepared by the solid state grinding method, the surface areas of Cu0 dropped markedly. It can be deduced that the HP method should be a superior method for the preparation of bimetallic catalyst. Catalytic Performance. The hydrogenation of MA to EtOH was conducted in a continuous flow reactor to examine the activity of the Zn−SiO2, Cu−SiO2, and CuxZny−SiO2 catalysts. As shown in Figure 9a,b, the Zn−SiO2 catalyst has nearly no catalytic activity in the hydrogenation of MA, which 4531
DOI: 10.1021/acs.iecr.7b05391 Ind. Eng. Chem. Res. 2018, 57, 4526−4534
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Industrial & Engineering Chemistry Research However, it is noteworthy that the Cu14Zn1−SiO2 catalyst showed the worst catalytic performance among the CuxZny− SiO2 catalysts, even though it possessed the highest surface area of Cu+. So, according to the above analysis results, it can be concluded that neither the surface area of Cu0 nor that of Cu+ is the determining factor for the catalytic activity of CuxZny−SiO2 catalysts. Here, the TOF (Cu+) was correlated with the Zn/Cu molar ratio as shown in Figure 9d. Obviously, the TOF (Cu+) linearly increases with the increment of Zn/Cu, indicating that the activity of Cu+ would be readily influenced by the zinc species. This is consistent with our previous study on CuZn/ SBA-15 for MA hydrogenation, in which it was proposed that the Cu+ derived from Cu−O−Zn species has higher ability in adsorbing the ester groups.22 But too high Zn loading will inevitably lead to the decrease of the surface copper species and the lower activity, which agrees with the results reported by Wang et al.14 on Cu/ZnO/SiO2 catalyst for the dimethyl ether steam reforming. Then, it is not hard to understand the superior catalytic performance of Cu7Zn1−SiO2 than of Cu− SiO2, although the latter possessed more Cu+ sites. All in all, a significantly high dispersion of copper and zinc species was realized by the HP method, which is also beneficial for the interaction between Cu and ZnO. Moreover, the increase of Zn/Cu molar ratio could enhance the formation of Cu-ZnOx species, which benefits more the generation of high active Cu+ sites than Cu−O−Si species. The stability of Cu9Zn1−SiO2 catalyst was evaluated in the hydrogenation of MA. As depicted in Figure 10, at the
to EtOH. The HP method realized the significant high dispersion of copper and zinc species and the formation of the strong interaction between Cu and ZnO. Compared with the CuZnO/Al2O3 catalyst (STY = 0.775 gEtOH gcat−1 h−1) prepared by traditional coprecipitation method, the Cu9Zn1− SiO2 catalyst achieved significant higher catalytic performance with a STY of about 1.024 gEtOH gcat−1 h−1 in the MA hydrogenation. It was also found that the introduction of ZnO inhibits the formation of copper phyllosilicate, which is the cause of the decrease of Cu+ specific surface area. Meanwhile, the linear relationship between TOF (Cu+) and Zn/Cu molar ratio indicated that the increase of Zn/Cu molar ratio could enhance the formation of Cu-ZnOx species, which benefits more the generation of highly active Cu+ sites than Cu−O−Si species. This work could also provide a concept for the synthesis of other kinds of bimetallic catalysts.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yujun Zhao: 0000-0002-3135-9281 Yue Wang: 0000-0002-2362-3345 Shengping Wang: 0000-0001-7918-8891 Xinbin Ma: 0000-0002-2210-0518 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This contribution was identified by Dr. Maocong Hu (New Jersey Institute of Technology) as the Best Presentation in the “Carbon Management: Advances in Carbon Efficiency, Capture, Conversion, Utilization & Storage” session of the 2017 ACS Fall National Meeting in Washington, D.C. We are grateful to the financial support from the National Nature Science Foundation of China (U1510203, 21276186, 21325626, 91434127), and the Tianjin Natural Science Foundation (13JCZDJC33000).
■ Figure 10. Stability data of Cu9Zn1−SiO2 catalyst. Reaction condition: P = 2.0 MPa, H2/MA = 20.
beginning of the reaction, the catalyst can achieve a high catalytic performance at the condition of 2 h−1 and 220 °C. Even if it has been evaluated under the higher LHSV and temperature, the catalyst could go back to the original activity. Therefore, it can be confirmed that the Cu9Zn1−SiO2 catalyst can maintain an outstanding stability no matter how the reaction conditions change within limits.
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NOMENCLATURE TOF(Cu+) = the frequency conversion of MA normalized by Cu+ site (h−1) Vp = pore volume (cm3/g) SBET = BET surface area (m2/g) Dp = average pore diameter (nm) XCu+ = the ratio of Cu+/ (Cu++Cu0) (%) SCu0 = specific surface area of Cu0 (m2/g) SCu+ = specific surface area of Cu+ (m2/g) SCu = specific surface area of Cu species (m2/g) REFERENCES
(1) Farrell, A. E.; Plevin, R. J.; Turner, B. T.; Jones, A. D.; O’Hare, M.; Kammen, D. M. Ethanol can contribute to energy and environmental goals. Science 2006, 311 (5760), 506−508. (2) San, X.; Zhang, Y.; Shen, W.; Tsubaki, N. New Synthesis Method of Ethanol from Dimethyl Ether with a Synergic Effect between the Zeolite Catalyst and Metallic Catalyst. Energy Fuels 2009, 23 (5), 2843−2844. (3) Xia, S.; Yuan, Z.; Wang, L.; Chen, P.; Hou, Z. Hydrogenolysis of glycerol on bimetallic Pd-Cu/solid-base catalysts prepared via layered double hydroxides precursors. Appl. Catal., A 2011, 403 (1−2), 173− 182.
4. CONCLUSION In this work, hydrolysis precipitationan effective method has been utilized for the first time to prepare the supported bimetallic catalysts, and the prepared CuxZny−SiO2 catalyst presented a predominant activity in the hydrogenation of MA 4532
DOI: 10.1021/acs.iecr.7b05391 Ind. Eng. Chem. Res. 2018, 57, 4526−4534
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DOI: 10.1021/acs.iecr.7b05391 Ind. Eng. Chem. Res. 2018, 57, 4526−4534
