Bimetallic Nano CuâCo Based Catalyst for Direct Ethanol Synthesis

Article pubs.acs.org/IECR

Bimetallic Nano Cu−Co Based Catalyst for Direct Ethanol Synthesis from Syngas and Its Structure Variation with Reaction Time in Slurry Reactor Qilei Yang,†,‡ Ang Cao,†,‡ Na Kang,†,‡ Hongyan Ning,†,‡ Jiaming Wang,†,‡ Zhao-Tie Liu,§ and Yuan Liu*,†,‡ †

Tianjin Key Laboratory for Applied Catalysis Science & Technology, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, P. R. China ‡ Collaborative Innovation Center of Chemical Science & Engineering (Tianjin), Tianjin 300072, P. R. China § Key Laboratory of Applied Surface & Colloid Chemistry, School of Chemical Engineering & Technology, Shaanxi Normal University, Xi’an 710062, Shaanxi, P. R. China ABSTRACT: The catalytic performance of Cu−Co/Al2O3 in the slurry reactor for direct ethanol synthesis from syngas was investigated. The reduced catalyst and catalyst reacted for 100, 300, 500, and 750 h were characterized by using XRD, TEM, EDS, BET, and ICP-MS techniques. Compared with that in the fixed-bed reactor, the catalyst in the slurry reactor exhibited higher selectivity to ethanol, which was above 70 wt % in the total alcohols. The structure variation of the bimetallic Cu−Co with reaction time is that CuCo2O4 was transformed into Cu− Co alloy after reduction, and Cu−Co alloy was decomposed into Cu and Co after reaction for 100 h. After reaction for 300 h, Co2C started to generate and Cu@Co@Co2C was formed as reacted for 500 h. All of Cu−Co alloy, Cu@Co, and Cu@ Co@Co2C are highly selective to ethanol, which was maintained for 750 h. Meanwhile, the slurry solvent of PEG-400 is another reason for the high ethanol selectivity.

1. INTRODUCTION Ethanol has been widely used as a potential alternative synthetic fuel and as a renewable fuel component for transportation sectors, blended with gasoline.1 Ethanol can be produced by the following three processes. (a) The biological fermentation process through a sugar platform, which consumes a great deal of grain. The pity is that the lignin component in the biomass cannot be converted by the current fermentation process. (b) Hydration of petroleum-based ethylene, which has been abandoned in many areas such as China, due to the high price of oil and the dependence on imported oil. (c) Chemical biomass to liquid process through a syngas platform. Syngas can also be obtained from coal and natural gas.2,3 The direct ethanol synthesis from syngas is developed from higher alcohols synthesis (HAS). The catalysts for HAS can be divided into four classes of (a) methanol synthesis catalysts modified with alkali promoters, over which methanol still is the main product;2,4 (b) Mo-based catalysts, which are resistant to sulfur poisoning while the reaction conditions are very strict;5,6 (c) Rh-based catalysts, which show high selectivity to ethanol while suffering sintering and high price;7−10 and (d) modified Fischer−Tropsch catalysts, including Cu−Fe and Cu−Co based catalysts. © XXXX American Chemical Society

As for Cu−Co based catalysts, Cu activates CO nondissociatively2,11 and Co enables the dissociative adsorption of CO and hydrogenation to produce CHx,12−14 and then the nondissociatively adsorbed CO and the CHx interact to generate ethanol. It is critical for the synergistic catalyzing between Cu and Co, because Cu and Co alone would generate methanol and hydrocarbons, respectively.15−17 Thus, the preparation of bimetallic Cu−Co is crucial for ethanol formation, and the alloy of Cu−Co nanoparticles (NPs) is optimized for ethanol synthesis. Previously, we prepared Cu−Co alloy catalyst by using layered double hydroxides (LDHs) as the precursors for HAS from syngas in a fixed-bed reactor, which showed high activity, good stability, and very high selectivity to ethanol and propanol. LDHs are a kind of synthetic two-dimensionally nanostructured layered material.18,19 In a LDH, the metallic ions are uniformly mixed at the atomic level and the uniform mixing would be maintained in the resultant mixed metal oxides after calcination, which is attributed to the topological effect of LDHs.20,21 The Received: December 1, 2016 Revised: February 5, 2017 Accepted: February 27, 2017

A

DOI: 10.1021/acs.iecr.6b04664 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

for 3 h in air and named (Cu1Co2)2Al-CLDHs. And then the calcined catalyst was reduced at 450 °C for 3 h in hydrogen and named (Cu1Co2)2Al-Red. 2.3. Catalyst Characterization. X-ray diffraction (XRD) measurements were performed on a Bruker D8-Focus X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 0.154 06 nm). Transmission electron microscopy (TEM) images and energy dispersive spectroscopy (EDS) line scans were obtained on a JEOL JEM-2100F microscope field-emission transmission electron microscope. After ultrasonic dispersion of the samples in absolute ethanol, the well-dispersed samples were dropped and dried on a Mo grid with a layer of the holey carbon film. Specific surface areas of the catalysts were evaluated by N2 adsorption according to the BET method on a Quantachrome QuadraSorb SI instrument. The samples were pretreated in vacuum at 300 °C for 4 h before the experiments. The content of elements in the catalyst after reduction and reaction were determined on an inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700X). 2.4. Catalyst Performance Testing. The catalytic performance tests were conducted on a 500 mL stirred slurry autoclave reactor. At first, 5 g of (Cu1Co2)2Al-CLDHs catalyst was crushed to 140 mesh grain size and was activated in the fixed-bed reactor under a flow of H2 at 450 °C for 3 h at a heating rate of 2 °C min−1. Subsequently, the reactor was cooled to room temperature, and then the catalyst was taken out from the fixed-bed reactor, mixed with 250 mL of polyethylene glycol-400 (PEG-400) solvent and loaded into the slurry reactor. The reaction conditions were set at 3 MPa by feeding the syngas mixture of H2/CO/N2 = 8:4:1, where N2 was used as the internal standard gas for analyzing the reacted gases, the GHSV of 720 mL (gcat h)−1, the stirring rate of 1000 r min−1 and the temperature of 240 °C with a heating rate of 5 °C min−1. The reaction conditions were kept for 24 h to reach steady state; then data collection was begun. At the reaction times of 100, 300, 500, and 750 h, the reactor was cooled to room temperature, and 0.5 g of catalyst was taken out from the slurry reactor. Then the reaction was run again under the same reaction conditions. The liquid products were analyzed per 24 h in the all reaction. A blank test with no catalyst in the slurry reactor was made to maintain the reaction conditions stable. A gas chromatograph with two packed columns was used to analyze the products. The gas products of CO, H2, CH4, CO2, and N2 were separated online using a TDX-01 packed column (2 m) connected to a TCD detector. After separation by condensation, liquid products and hydrocarbons were analyzed off-line using a Porapak Q column (3 m) connected to a FID detector. CO conversion (XCO), the product selectivity (Si), and the mass fraction of a certain alcohol in the total alcohols (Wi) were calculated according to the following equations:

uniform distributions of the metal species are beneficial and crucial for the formation of nanometallic alloy. The synthesis of ethanol from syngas is a highly exothermic reaction, hot spots would be formed in the fixed-bed reactor, which favors the generation of hydrocarbons. Meanwhile, it is difficult to remove the heat on a pilot plant or at commercial scale.22−24 Compared with the fixed-bed reactor, the slurry reactor has advantages in heat transfer and heat removing.22,25 The reports up to now on ethanol synthesis or HAS in the slurry reactor are as follows. Yu et al.26 prepared Cu-ZnO/ Al2O3 catalyst by a complete liquid-phase method for direct synthesis of ethanol, over which CO conversion of 17.0% and ethanol selectivity of 24.5% were obtained at 250 °C, 5.0 MPa, and H2/CO = 1 and under the gas hourly space velocity (GHSV) of 360 mL (gcat h)−1. Ishida et al.27 prepared alkalimetal-modified cobalt catalyst for synthesis of higher alcohols in the slurry reactor, and over the catalyst Na−Co/SiO2, CO conversion of 75.6%, total alcohol selectivity of 14.6%, and the mass diffraction of ethanol in the total alcohols of 14.3% were shown at 245 °C, 6.0 MPa, H2/CO = 1, and GHSV of 6000 mL (gcat h)−1. Zhang et al.28 reported Fe2O3/Al2O3 catalyst for HAS from syngas in the slurry reactor, and the catalyst showed very high selectivity to alcohols, which reached as high as 95 wt % and, where the C2+ alcohols (mainly ethanol) are, more than 40 wt % at 250 °C, 3.0 MPa, H2/CO = 2, and GHSV of 360 mL (gcat h)−1. Roberts et al.25,29,30 reported a series of studies about zinc chromite catalyst in a slurry reactor using different liquids as the slurry medium, changing the solvent composition and using cesium-promoted zinc chromite catalyst. However, methanol was still the major product of more than 90 mol % in the alcohols. Cu−Co based catalysts are one of the most promising for HAS and ethanol synthesis, whereas, to the best of our knowledge, there is no report on Cu−Co based catalyst in a slurry reactor for direct ethanol synthesis or HAS from syngas. In this work, the catalytic performance and its structure variation with reaction time of Cu−Co/Al2O3 catalyst prepared by using a LDH as the precursor was investigated in a slurry reactor for direct ethanol synthesis from syngas. It was found that the catalytic performance is much different from that in a fixed-bed reactor, the selectivity to ethanol is very high, and the structure variation is interesting.

2. EXPERIMENTAL SECTION 2.1. Materials. Analytical grade chemicals of Cu(NO3)2· 3H2O, Co(NO3)2·H2O, Al(NO3)·9H2O, NaOH, and Na2CO3 were purchased from Aladdin and used without further purification. Analytical grade chemical of PEG-400 was purchased from Kermel. Deionized water was used in all the experimental processes. 2.2. Preparation of Cu−Co Catalysts. The catalyst preparation is same as in our previous work.18 Briefly, Cu(NO3)2·3H2O, Co(NO3)2·6H2O, and Al(NO3)3·9H2O were dissolved in deionized water with [Cu2+] + [Co2+] + [Al3+] = 1.0 M and [Cu2+] + [Co2+]/[Al3+] = 2 M. NaOH and Na2CO3 were dissolved in deionized water with [NaOH] = 1.5 M and [Na2CO3] = 2[Al3+]. The two solutions were simultaneously added into deionized water keeping pH = 9.5 ± 0.1 under vigorous stirring. As the precipitation was completed, the slurry was aged at 60 °C for 12 h, and then filtered, washed with distilled water until pH = 7, and dried at 60 °C for 12 h. The prepared catalyst was named (Cu1Co2)2AlLDHs. And then the prepared catalyst was calcined at 500 °C

XCO =

B

COin − COout × 100% COin

Si =

nCi × 100% ∑ nCi

Wi =

mi × 100% ∑ mi DOI: 10.1021/acs.iecr.6b04664 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


Figure 1c shows the XRD pattern of (Cu1Co2)2Al-Red. After reduction, the diffraction peaks of CuCo2O4 disappeared, meaning that the CuCo2O4 structure was destroyed and the cations of copper and cobalt were reduced. Meanwhile, the broad diffraction peak at 43.9° between Co [111] of 44.2° and Cu [111] of 43.3° should be attributed to the bimetallic Co− Cu alloy.31 After reduction, CuCo2O4 disappeared and weak diffraction peaks of Al2O3 at 36.5 and 65.8° could be seen, which were overlapped by the stronger peaks of CuCo2O4 before reduction. Figure 1d shows the XRD pattern of the (Cu1Co2)2Al-Red after reaction for 100 h. The diffraction peaks of Cu−Co alloy disappeared and the diffraction peaks of Cu at 43.3° and Co at 44.2° appear, meaning that the Cu−Co alloy was decomposed into Cu and Co after reaction for 100 h. Figure 1e shows the XRD pattern of the (Cu1Co2)2Al-Red after reaction for 300 h. Compared with Figure 1d, the diffraction peaks of Cu at 43.3 and 50.5° and Co at 44.2° become sharper, indicating that the size of the nanoparticles (NPs) increased and the size of copper increased from 16.8 to 25.2 nm, which were calculated with the Scherrer equation and are listed in Table 1. Figure 1f shows the XRD pattern of the catalyst reacted for 500 h. Compared with Figure 1e, new weak diffraction peaks at 41.4, 42.7, and 45.7° can be seen, which is attributed to the diffraction peak of Co2C. Meanwhile, the diffraction peaks of Cu at 43.3 and 50.5° became stronger, and the size of copper increased from 25.2 to 41.6 nm as listed in Table 1. The diffraction peak corresponding to Co at 44.2° became weaker and Co2C at 42.7° became stronger and sharper, suggesting that Co transformed to Co2C under the syngas and the average particle size of Co2C is 31.9 nm. No diffraction peaks of Co2C could be detected for the catalyst reacted for 300 h, indicating that Co2C was formed mainly in the reaction period between 300 and 500 h. Figure 1g shows the XRD pattern of the catalyst reacted for 750 h. The diffraction peaks of Co2C at 41.4, 42.7, and 45.7° became obviously stronger, indicating that more Co was transformed to Co2C and the size of Co2C increased from 31.9 to 59.5 nm. Note that the size of Cu and Co NPs have only a little change, which may be due to the fact that Co2C was formed as a shell and the shell can protect the copper from sintering. The structure of the bimetallic Cu−Co transformation derived from XRD is summarized as follows. The uniformly mixed copper and cobalt ions in the layer of LDHs precursors were converted into highly dispersed CuCo2O4 supported on alumina matrix during the calcination process, and the resulting

where COin and COout are the moles of CO in the feed gases and off-gases, respectively; n and Ci represent the number of carbon atoms in a molecule of the products and moles of a carbon-containing product, respectively; and mi is the weight of a certain alcohol in the products.

3. RESULTS AND DISCUSSION 3.1. XRD. Figure 1a shows the XRD pattern of (Cu1Co2)2Al-LDHs, which exhibited sharp and symmetric

Figure 1. XRD patterns of (a) (Cu1Co2)2Al-LDHs, (b) (Cu1Co2)2AlCLDHs, (c) (Cu1Co2)2Al-Red, (d) after reaction for 100 h; (e) after reaction for 300 h; (f) after reaction for 500 h, and (g) after reaction for 750 h. Symbols: (Φ) LDHs; (&) Al2O3; (#) CuCo2O4; (O) Cu− Co alloy; (□) Cu; (●) Co; (◆) Co2C.

diffractions at 2θ values of 11.8, 23.8, and 34.8°, corresponding to the crystalline planes of (003), (006), and (009) for LDHs, indicating that a well-crystallized hydrotalcite structure is formed. Furthermore, no detectable impurity phases can be observed, showing the purity of LDHs structure is very high.18 Figure 1b shows the XRD pattern of (Cu1Co2)2Al-CLDHs. Compared with Figure 1a, the diffraction peaks corresponding to LDHs disappeared, replaced by the diffraction peaks of metal oxides (CuCo2O4), indicating that the LDHs structure was completely destroyed and transformed into CuCo2O4 after calcination. This agrees with ref 18 because the copper and cobalt ions are uniformly distributed at the atomic level in the layers of LDHs, which is beneficial for forming CuCo2O4, the formation of CuCo2O4 structure is easy, and the ratio of Cu/Co is 1:2 in the sample.

Table 1. Surface Areas, Pore Sizes, Crystal Sizes, and the Elemental Analysis of the Catalyst under Different Treatments in Slurry Reactor atomic ratio specific to Cua time (h)

SBET (m2 g−1)

pore volume (cm3 g−1)

pore diameter (nm)

Co

Cu

0 100 300 500 750

108.8 93.9 88.3 66.5 60.5

0.5 0.5 0.5 0.4 0.4

8.8 9.8 11.8 12.9 13.0

1.93 1.82 1.74 1.68 1.67

1.00 1.00 1.00 1.00 1.00

crystal sizeb (nm) Cu 16.8 25.2 41.6 44.8

Co 4.7 6.8 5.4 5.1

Co2C

Cu−Co alloy 5.4 (4.8)c (6.2)d

31.9 59.5

a

The theoretical specific values of Co and Cu are 2.00 and 1.00, respectively. bCalculated by X-ray diffraction with the Scherrer equation. cCrystal size of Cu−Co alloy in a fixed-bed reactor. dCrystal size of Cu−Co alloy in a fixed-bed reactor after a 200 h stability test. C


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Figure 2. continued

D


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Figure 2. TEM pictures and the corresponding EDS line scanning profiles for Cu and Co elements of (a, a′) (Cu1Co2)2Al-Red; (b, b′) (Cu1Co2)2 after reaction for 100 h; (c, c′) (Cu1Co2)2 after reaction for 300 h; (d, d′) (Cu1Co2)2 after reaction for 500 h; (e, e′) (Cu1Co2)2 after reaction for 750 h.

of Co2C NPs increased markedly, which is very consistent with the XRD results listed in Table 1. This means that metallic cobalt was continually transformed into Co2C in syngas atmosphere during the reaction period from 500 to 750 h. Meanwhile, the insets in Figure 2a−e show the representative EDS line scanning curves for the catalyst after reduction and after reaction for 100, 300, 500, and 750 h, where the red lines are the scanning areas. Figure 2a shows the same variation tendency of the two elements of copper and cobalt for the reduced catalyst, meaning the uniform mixing of copper and cobalt. Namely, copper and cobalt were in forms of homogeneous alloy, which is consistent with the XRD and TEM results. The EDS line scanning curve in Figure 2b shows that copper and cobalt elements are in coexistence; where there is copper, there is cobalt. To see the metallic NPs more clearly, part of the EDS line was magnified; namely, the inset in below of Figure 2b is the enlarged profile of the above inset in the range 120− 140 nm. It can be seen that the intensities of the copper and cobalt peaks are not consistent, but they are still in close contact, indicating that many small NPs of copper and cobalt were evenly mixed. The EDS line scanning curve in Figure 2c shows that sintering of copper is obvious and, comparatively, the sintering of cobalt is light. In the scanning line, a stronger peak should be a nanocrystallite composed of closely packed metallic atoms, and the comparatively broad peaks are congregated in small NPs in which exist interspaces between the NPs. XRD results indicate that the average size of Cu NPs is 25.2 nm and the average size of Co NPs is 6.8 nm. The line scanning suggests that NPs of copper are surrounded by smaller NPs of cobalt. A core−shell structure of Cu@Co can be seen from the scanning line in Figure 2d for the catalyst reacted for 500 h, while copper seems not well covered. At this stage, sintering of copper is severe; the large black dots in the TEM image are the bimetallic Cu−Co. Similar is the catalyst reacted for 750 h, where sintering and congregation of NPs are severe. In the inset in Figure 2e of a representative scanning line, the bimetallic NPs are in the core−shell composites of Cu@Co. From the TEM results, Figure 2d.d′, for the catalyst used for 500 h, Cu NPs are surrounded by Co NPs and on the outside is Co2C NPs. Seen from the lattice fringes, the thickness of Co2C is small. In panels e and e′ of Figure 2 for the catalyst used for 750 h, the lattice fringes of Co could hardly be found, indicating

CuCo2O4 were reduced to Cu−Co alloy after reduction. Cu− Co alloy was decomposed into metallic Cu and Co after reaction for 100 h and sintering of copper is obvious in the reaction period from 100 to 300 h. After reaction for 300 h, Co2C started to generate and the sizes of Co changed little, whereas from 500 to 750 h, the thickness of Co2C increased markedly. 3.2. TEM. Seen from Figure 2a of a TEM image of (Cu1Co2)2Al-Red, there are some black dots on the gray background, which are attributed to the metallic NPs. To study the phase composition, a representative HRTEM image is shown in Figure 2a′. The lattice parameters for the [111] planes of Cu and Co are 0.208 and 0.205 nm, respectively. So the lattice distance of 0.207 nm observed from the HRTEM images is attributed to the [111] planes of the Cu−Co alloy, meaning the formation of Cu−Co alloy, which agrees with the XRD results. Panels b and b′ of Figure 2 show the TEM images of (Cu1Co2)2Al-Red reacted for 100 h. Two obvious lattice distances can be observed. The lattice distance of 0.208 nm is in accordance with the lattice parameter for the [111] planes of Cu; the lattice distance of 0.205 nm is in accordance with the lattice parameter for the [111] planes of Co, indicating that the Cu−Co alloy was decomposed into Cu and Co, which also agrees with the XRD results in Figure 1d. It can be seen that although the Cu−Co alloy converted to Cu and Co NPs, the copper and cobalt crystallites are in close contact, which is critical for direct ethanol synthesis from syngas. Panels c and c′ of Figure 2 show the TEM images of (Cu1Co2)2Al-Red after reaction for 300 h. The lattice distance of copper (0.208 nm) and cobalt (0.205 nm) can be seen clearly, and it seems that NPs of copper are surrounded by smaller NPs of cobalt. Panels d and d′ of Figure 2 show the TEM images of the catalyst reacted for 500 h. Observed from the HRTEM results in Figure 2d′, besides the lattice distance of Cu and Co, a new lattice distance of 0.211 nm was detected, which is in accordance with the lattice parameter for the [111] planes of Co2C, indicating that Co2C was generated under syngas after reaction for 500 h. Panels e and e′ of Figure 2 show the TEM images of the catalyst reacted for 750 h. Seen from the HRTEM results in Figure 2e′, close to the [111] planes of copper, the [111] planes of Co2C could be more evidently observed, and the size E


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Industrial & Engineering Chemistry Research Scheme 1. Structure Variation of (Cu1Co2)2Al-LDHs with Reaction Time

Figure 3. N2 adsorption−desorption curves (I) and pore size distributions (II) of (a) (Cu1Co2)2Al-Red; (b) (a) after reaction for 100 h; (c) (a) after reaction for 300 h; (d) (a) after reaction for 500 h; (e) (a) after reaction for 750 h.

the surface energy of Co markedly; therefore, cobalt enriched onto the surface to form nanocomposite of a Cu NP surrounded by smaller Co NPs, which is somewhat like the core−shell structure of Cu@Co. During the reaction period of 100−300 h, sintering of metallic copper and enriching of cobalt onto the surface were going on, generating a larger nanocomposite of Cu@Co. Meanwhile, carbonizing the surface metallic cobalt to form Co2C began. During the reaction period of 300−500 h, sintering of Cu and Co NPs was going on. At the same time, a large portion of metallic cobalt was transformed to Co2C. Co2C was the product of the reaction of CO with metallic cobalt, which should be formed on the surface of metallic cobalt, meaning that Co2C was located on the surface of metallic cobalt. Thus, Cu@Co@Co2C was forming. During the reaction period of 500−750 h, in Cu@Co@ Co2C, Cu NP was wrapped by Co and Co2C and could not contact with other Cu NPs, so the size of Cu NPs no was longer increasing. Meanwhile, Co was also wrapped by Co2C and was no longer being lost. But Co would transform into Co2C continually; therefore, the thickness of Co decreased a little and the thickness of Co2C increased with increasing reaction time. The nanocrystallite sizes of Cu−Co alloy, Cu, Co, and Co2C are listed in Table 1. As for (Cu1Co2)2Al-Red, the size of the nanocrystallites of Cu−Co alloy is about 5.4 nm. As for the catalyst reacted for 100, 300, 500, and 750 h, the sizes of the nanocrystallites of Cu increased to 16.8, 25.2, 41.6, and 44.8

the thickness of Co became thin. Meanwhile, the lattice parameter of Co2C can be seen obviously, and the lattice parameter of Co2C shows that the thickness of Co2C is much bigger than that of the catalyst used for 500 h. Combined with the EDS results, the signal of Co after reaction for 750 h can be contributed mainly to Co2C, which is around 50 nm, in agreement with the XRD results. According to the XRD, TEM, and EDS results above, bimetallic Cu−Co is in the structure Cu@Co and Co2C was generated from the surface Co of Cu@ Co, therefore resulting in Cu@Co@Co2C. Thus, on the basis of the XRD, TEM, and EDS results, the structure variation of the catalyst with reaction time is shown in Scheme 1. CuCo2O4 was transformed into Cu−Co alloy supported on Al2O3 after reduction; then Cu−Co alloy was decomposed into Cu and Co NPs in a syngas atmosphere during the reaction period before 100 h. Then, larger NPs of copper would be surrounded by smaller NPs of cobalt, and a nanocomposite somewhat like Cu@Co would be formed with ongoing reaction, attributed to the following reasons. First, copper and cobalt elements in Cu−Co alloy are uniformly mixed at the atomic level. As a result, the copper and cobalt species from the Cu−Co alloy are uniformly mixed, and the uniformly mixed Cu and Co NPs would be congregated. Second, the congregated Cu and Co NPs would restructure and sinter in the syngas atmosphere, and the sintering of Cu NPs was more serious. Third, the adsorption energy of CO on Co is much higher than that on Cu and the adsorption of CO on Co is much stronger than that of H2.15,32−34 The adsorption of CO on Co decreased F


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Industrial & Engineering Chemistry Research nm, respectively, indicating that the sintering of copper NPs are severe before 500 h and in a period of 500−750 h the sintering is slight. Meanwhile, the size of cobalt NPs are 4.7, 6.8, 5.4, and 5.1 nm, respectively. The sizes of Co2C are 31.9 and 59.5 nm for the catalyst reacted for 500 and 750 h, respectively. Cobalt NPs were dispersed on the surface of copper NPs before the reaction time of 300 h; therefore, the size of cobalt NPs is much smaller than that of copper NPs. During 300−750 h, NPs of cobalt began to sinter while at the same time metallic cobalt was transferring to Co2C. A a result, the thickness of metallic cobalt was not increased while the thickness of Co2C increased markedly. This agrees well with Scheme 1. Maybe this technique can be used for preparing core−shell nanocomposites. 3.3. N2 Adsorption−Desorption Isotherms and SBET. Curves a−e of Figure 3 show the N2 adsorption−desorption isotherms of (Cu1Co2)2Al-Red and the catalyst reacted for 100, 300, 500, and 750 h. The surface areas, pore volumes, and pore diameters calculated from the adsorption isotherms are listed in Table 1. All of the isotherms present the typical IV shape with a H3 hysteresis loop. The distribution of mesoporosity sizes ranged from 5 to 13 nm and the porosity structure was maintained in the 750 h reaction period, indicating the framework of the alumina matrix had very good stability. During the reaction period from the beginning to 500 h, the BET surface area and the pore volume decreased from 108.8 to 66.5 m2 g−1 and 0.48 to 0.43 cm3 g−1, respectively, and the pore diameter increased from 8.8 to 12.9 nm, due to the sintering of the active metallic NPs and the congregation of alumina support. However, during the reaction period from 500 to 750 h, the surface area, pore volume, and pore diameter nearly did not change. 3.4. Catalytic Performance of Cu−Co/Al2O3 Catalyst in the Slurry Reactor. Parts a and b of Figure 4 show the catalytic performance of (Cu1Co2)2Al-Red. To correlate the catalytic performance with the structure of the catalyst, the discussion was divided into four stages corresponding to reaction periods 0−100, 100−300, 300−500, and 500−750 h. Ethanol accounts for above 70 wt % in the total alcohols and the selectivity to alcohols is around 40%, which is maintained in the 750 h running. During the period from the beginning to 100 h, CO conversion and the selectivities varied significantly. The CO conversion decreased from 61.3 to 40.6%, the selectivity to alcohols decreased from 56.8 to 51.0%, the selectivity to CO2 increased from 10.8 to 17.8%, and the mass fraction of methanol in total alcohols increased from 5 to 8%. To correlate with the structure shown in Scheme 1, the following can be seen. (1) The phase separation of Cu−Co alloy and the sintering of Cu led to the decrease of CO conversion. (2) The decrease of the selectivity to alcohols and the increase of the selectivity to methanol and to CO2 should be ascribed to the decomposition of Cu−Co alloy into Cu and Co. It is known that higher alcohols are generated by synergistic catalysis of copper and cobalt, and the synergistic catalysis works better on the Cu−Co alloy than on the NPs of copper and cobalt;15,35 therefore, the decomposition of the Cu−Co alloy resulted in the decrease of the selectivity to higher alcohols. Accordingly, the selectivities to methanol and CO2 increased, because copper tends to catalyze the formation of both, which is accordance with the fact that Cu is more active than Co for the water gas shift reaction.15 The formations of alcohols and hydrocarbons are accompanied by the production of water, and

Figure 4. Stability tests of (Cu1Co2)2Al-LDHs at T = 240 °C, P = 3 MPa, r = 1000 r/min, GHSV = 720 mL (gcat h)−1, and H2/CO/N2 = 8:4:1. (a) Conversion of CO and the selectivities of CO2, hydrocarbons and total alcohols. (b) Distribution of alcohols.

water reacted with CO via the water gas shift reaction, generating CO2.36−38 During 100−300 h, CO conversion and the selectivity to hydrocarbons became comparatively stable, being decreased at a speed of 1.6 and 0.8% per 100 h, respectively. The decrease of CO conversion is mainly due to the sintering of metallic Cu and Co, and the decrease of the selectivity to hydrocarbons is attributed to carbonization of cobalt. For the catalyst used for 500 h, the diffraction peaks attributed to Co2C could be observed evidently. For the catalyst used for 300 h, the weak diffraction peak of Co2C could be seen in the inset in Figure 1. Co2C is not active for hydrocarbon formation.39−41 During 100−300 h, the selectivity to total alcohols decreased from 51.0 to 40.2%, which should be due to the sintering of copper and covering of Cu NPs by smaller cobalt NPs, as it is known that copper is crucial for alcohol formation. Meanwhile, the mass fraction of C4+OH in the total alcohols increased significantly, in accordance with the size increase of metallic cobalt. Metallic Co is the active site for C-chain growth and the larger size of cobalt favors the formation of higher carbon containing alcohols. The selectivity to CO2 increased a lot, which may be due to the formation of Co2C. Dong et al.42 pointed out that CO preferred to molecularly adsorb on the Co2C surface derived from the density functional theory (DFT) calculations.15 In the Co2C−Co pair, higher alcohols could be obtained by the synergistic catalyzing, where Co2C works like Cu in the pair of Cu−Co. Therefore, Co2C may be active for WGSR, generating CO2. During 300−500 h, CO conversion decreased markedly, the selectivity to hydrocarbons decreased a small amount, and the selectivities to alcohols and CO2 remained relatively stable. In Scheme 1, in this period, the core−shell nanocomposite of Cu@Co was converting to Cu@Co@Co2C, i.e., the shell of metallic cobalt was transforming to Co2C. G


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Table 2. Catalytic Performance of CO Hydrogenation over (Cu1Co2)2Al-LDHs in the Slurry Reactor and Fixed-Bed Reactor alcohol distribution (%) reactor

T (°C)

t (h)

XCO (%)

SCO2 (%)

SRH (%)

SCOH (%)

C1

C2

C3

C4+

fixed-bed

250

slurry

240

24 200 24 100

50.8 53.6 61.3 40.6

2.0 2.2 10.8 17.8

57.5 58.6 32.4 31.2

40.5 39.2 56.8 51.0

8.2 9.0 5.5 7.0

49.2 46.2 80.2 75.6

41.8 43.6 12.0 15.3

0.8 1.2 2.3 2.1

Reaction conditions: P = 3 MPa, GHSV = 3900 mL (gcat h)−1, H2/CO/N2 = 8:4:1. bReaction conditions: P = 3 MPa, GHSV = 720 mL (gcat h)−1, H2/CO/N2 = 8:4:1, r = 1000 r min−1.

a

Xu et al.43 and Lebarbier et al.44 pointed out that Co2C species played an important role in CO insertion and favored the formation of higher alcohols, whereas the activity of Co2C is lower than that of the Cu−Co alloy. They also pointed out that the formation of Co2C resulted in a decrease in the number of Co0, which is the active site for C-chain growth, resulting in a lower CO conversion and the selectivity decrease to hydrocarbons. Actually, this viewpoint is accepted extensively, as reported in refs 39 and 45. The selectivity to alcohols had little change, which is due to the synergistic catalysis working between the close contacted Co and Co2C. During 500−750 h, both CO conversion and the selectivity to hydrocarbons decreased a lot, and the selectivities to alcohols and CO2 remained comparatively stable. During this period, more metallic cobalt was transformed to Co2C, meaning that a lot of active sites of Co0 for hydrocarbon formation were transformed to the active sites of Co2C for alcohol formation. At the same time, sintering of Co2C was observed; therefore, the reaction rate for alcohol formation did not increase. In conclusion, the catalytic performance changes are very consistent with the structure variation of the catalyst, supporting the structure variation framework in Scheme 1. 3.5. Comparing with in Fixed-Bed Reactor. The differences of catalytic performances over Cu−Co/Al2O3 in the slurry reactor and in the fixed-bed reactor are listed in Table 2. In the slurry reactor, CO conversion reached 61.3%, the total alcohol selectivity reached 56.8%, and C2+OH accounts for 94.5 wt % and ethanol accounts for 80 wt % in the total alcohols in the initial 24 h at 240 °C, 3 MPa, and GHSV of 720 mL (gcat h)−1 with stirring rates of 1000 r min−1, which are evidently higher than those in the fixed-bed reactor. In the fixed-bed reactor, CO conversion is 50.8%, the total alcohol selectivity is 40.5%, and 91.8 wt % to C2+OH and 49 wt % of C2+OH to ethanol at 250 °C, 3 MPa, and GHSV of 3900 mL (gcat h)−1. In the slurry reactor, the slurry solvent is critical for the selectivity and distribution of products, as pointed by Mahajan and Vijayaraghavan that the oxygenated solvents favor the formation of oxygenated products.28,46 The oxygenated solvent can remove the oxygenates from the surface of the catalyst in time and block the oxygenates being further hydrogenated to hydrocarbons because oxygenates can be dissolved in the oxygenated solvent. Zhang et al.36,47,48 showed an optimal route of ethanol formation on Cu-based catalyst, starting with the first process of CO + H → CHO + H → CH2O+ H → CH3O + H → CH3 + OH to produce CH3. Subsequently, CO insertion into CH3 leads to CH3CO, followed by successive hydrogenation to form ethanol, in which the desorption of ethanol from the catalyst surface is the key step to the synthesis of ethanol. In this work, the oxygenated solvent of PEG-400 was used as the slurry solvent to suppress the hydrocarbon production and promote alcohol production and to suppress C-chain growth to form propanol and C4+OH so that the

generated ethanol on the catalyst surface would be dissolved in PEG-400 in time. However, on the bimetallic Cu−Co, CH3O + H to CH3 is favored over CH3OH, leading to low methanol selectivity.35 However, the catalyst’s stability is poor in the slurry reactor. The Cu−Co alloy separated into Cu and Co, and sintering of copper is severe after reaction for 100 h in the slurry reactor. Although there was no separation of Cu−Co alloy after reaction for 200 h in the fixed-bed reactor and the sintering of the Cu−Co alloy NPs is not severe, the average size of the Cu− Co alloy NPs increased from 4.8 to 6.2 nm, Table 1. The poor stability of the catalyst in the slurry reactor should be due to the liquid medium of the slurry, which intensified the mass transferring for the Cu−Co alloy decomposition, sintering of metallic NPs, and Co2C generation.49 Zhai et al.50 reported Cubased catalysts for the slurry-phase methanol synthesis and pointed out that the main cause of the catalyst deactivation is sintering of the active metallic NPs due to the intensified mass transfer in the liquid phase. What is interesting is that the high selectivity to ethanol was stable in the 750 h reaction period although CO conversion decreased a lot. The deactivation process of the catalyst is the variation process of the catalyst structure, where the active component varied from Cu−Co alloy to the core−shell of Cu@ Co and then to the core−shell structure of Cu@Co@Co2C with Co−Co2C on the surface. All of the structures are highly active for producing higher alcohols, assisted by the PEG-400 slurry to suppress C-chain growth, leading to maintenance of the high ethanol selectivity. 3.6. ICP-MS. The content of Cu and Co in (Cu1Co2)2AlRed and the catalyst reacted for 100, 300, 500, and 750 h were measured by ICP-MS, and the results are listed in Table 1. The molar ratio of Co/Cu is 1.93 in the reduced catalyst, which is close to the theoretical value of 2.0. The ratios of Co/Cu are 1.82, 1.74, 1.68, and 1.67 for the catalyst reacted for 100, 300, 500, and 750 h, respectively. The loss of cobalt species can contribute to the formation of volatile Co-carbonyls, as pointed in ref 51, and Co loss should be one of the reasons for the catalyst deactivation during the reaction period from the beginning to 500 h. During the reaction period of 500−750 h, nearly no Co loss was observed, which should be owing to the protection of the Co2C shell in Cu@Co@Co2C.

4. CONCLUSIONS Cu−Co/Al2O3 was prepared by using a layered double hydroxide as the precursor for direct ethanol synthesis from the syngas in the slurry reactor, and it showed high selectivity to ethanol, the mass fraction of ethanol in total alcohols kept above 70 wt % in the 750 h running. Compared with the fixedbed reactor, the catalyst is much more selective to ethanol while with poorer stability in the slurry reactor. H


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(10) Haider, M.; Gogate, M.; Davis, R. Fe-promotion of supported Rh catalysts for direct conversion of syngas to ethanol. J. Catal. 2009, 261, 9−16. (11) Grabow, L. C.; Mavrikakis, M. Mechanism of Methanol Synthesis on Cu through CO2and CO Hydrogenation. ACS Catal. 2011, 1, 365−384. (12) Van Der Laan, G. P.; Beenackers, A. A. C. M. Kinetics and Selectivity of the Fischer−Tropsch Synthesis: A Literature Review. Catal. Rev.: Sci. Eng. 1999, 41, 255−318. (13) Storsæter, S.; Chen, D.; Holmen, A. Microkinetic modelling of the formation of C1 and C2 products in the Fischer−Tropsch synthesis over cobalt catalysts. Surf. Sci. 2006, 600, 2051−2063. (14) Xiao, K.; Bao, Z.; Qi, X.; Wang, X.; Zhong, L.; Lin, M.; Fang, K.; Sun, Y. Unsupported CuFe bimetallic nanoparticles for higher alcohol synthesis via syngas. Catal. Commun. 2013, 40, 154−157. (15) Xiang, Y.; Barbosa, R.; Kruse, N. Higher Alcohols through CO Hydrogenation over CoCu Catalysts: Influence of Precursor Activation. ACS Catal. 2014, 4, 2792−2800. (16) Xiao, K.; Bao, Z.; Qi, X.; Wang, X.; Zhong, L.; Fang, K.; Lin, M.; Sun, Y. Structural evolution of CuFe bimetallic nanoparticles for higher alcohol synthesis. J. Mol. Catal. A: Chem. 2013, 378, 319−325. (17) Wang, J.; Chernavskii, P. A.; Khodakov, A. Y.; Wang, Y. Structure and catalytic performance of alumina-supported copper− cobalt catalysts for carbon monoxide hydrogenation. J. Catal. 2012, 286, 51−61. (18) Cao, A.; Liu, G.; Yue, Y.; Zhang, L.; Liu, Y. Nanoparticles of CuCo alloy derived from layered double hydroxides and their catalytic performance for higher alcohol synthesis from syngas. RSC Adv. 2015, 5, 58804−58812. (19) Cao, A.; Liu, G.; Wang, L.; Liu, J.; Yue, Y.; Zhang, L.; Liu, Y. Growing layered double hydroxides on CNTs and their catalytic performance for higher alcohol synthesis from syngas. J. Mater. Sci. 2016, 51, 5216−5231. (20) Wang, J.; Chernavskii, P. A.; Khodakov, A. Y.; Wang, Y. Structure and catalytic performance of alumina-supported coppercobalt catalysts for carbon monoxide hydrogenation. J. Catal. 2012, 286, 51−61. (21) Mo, X.; Tsai, Y.-T.; Gao, J.; Mao, D.; Goodwin, J. G., Jr. Effect of component interaction on the activity of Co/CuZnO for CO hydrogenation. J. Catal. 2012, 285, 208−215. (22) Roberts, G. W.; Márquez, M. A.; McCutchen, M. S. Alcohol synthesis in a high-temperature slurry reactor. Catal. Today 1997, 36, 255−263. (23) Breman, B. B.; Beenackers, A. A. C. M.; Schuurman, H. A.; Oesterholt, E. Catalysis in Multiphase ReactorsKinetics of the gasslurry methanol-higher alcohol synthesis from CO/CO2/H2 over a Cs Cu/ZnO/Al2O3 catalyst, including simultaneous formation of methyl esters and hydrocarbons. Catal. Today 1995, 24, 5−14. (24) Chen, H. Y.; Lau, S. P.; Chen, L.; Lin, J.; Huan, C. H. A.; Tan, K. L.; Pan, J. S. Synergism between Cu and Zn sites in Cu/Zn catalysts for methanol synthesis. Appl. Surf. Sci. 1999, 152, 193−199. (25) Sun, X. L.; Roberts, G. W. Synthesis of higher alcohols in a slurry reactor with cesium-promoted zinc chromite catalyst in decahydronaphthalene. Appl. Catal., A 2003, 247, 133−142. (26) Yu, S. R.; Chen, Y. E.; Gao, S. S.; Wang, X. D.; Huang, W. Ethanol Synthesis from Synthesis Gas Over Complexing Agent Modified Cu-ZnO/Al2O3 Prepared by Complete Liquid-Phase Technology. Energy Sources, Part A 2013, 35, 955−961. (27) Ishida, T.; Yanagihara, T.; Liu, X. H.; Ohashi, H.; Hamasaki, A.; Honma, T.; Oji, H.; Yokoyama, T.; Tokunaga, M. Synthesis of higher alcohols by Fischer−Tropsch synthesis over alkali metal-modified cobalt catalysts. Appl. Catal., A 2013, 458, 145−154. (28) Zhang, X. B.; Li, Z.; Guo, Q. H.; Zheng, H. Y.; Xie, K. C. Selective synthesis of mixed alcohols from syngas over catalyst Fe2O3/ Al2O3 in slurry reactor. Fuel Process. Technol. 2010, 91, 379−382. (29) Roberts, G. W.; Marquez, M. A.; Haney, C. A. Liquid catalyst interactions in slurry reactors: methanol synthesis over zinc chromite. Appl. Catal., A 1999, 183, 395−410.

The NPs of the Cu−Co alloy supported on Al2O3 were obtained after reduction. The Cu−Co alloy would decompose into Cu and Co NPs after reaction for 100 h. After reaction for 300 h, Co2C started to generate and the core−shell structure of Cu@Co@Co2C supported on Al2O3 matrix was formed as reacted for 500 h. Then, during 500−750 h, more metallic cobalt was transformed to Co2C and Co2C protected the Cu@ Co@Co2C composite from cobalt loss and from the sintering of copper NPs. The high selectivity to ethanol in the whole reaction is mainly because all of the Cu−Co alloy, Cu and Co NPs in close contact, and Co2C in contact with Co are highly selective to ethanol. Meanwhile, the slurry solvent PEG-400 as the liquidphase medium favors the synthesis of ethanol from the syngas and avoids C-chain growth. However, the phase separation, the sintering of active metals, and Co loss accelerated the rate of catalyst deactivation.

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AUTHOR INFORMATION

Corresponding Author

*Yuan Liu. E-mail address: [email protected]. Tel: +86 022 87401675. ORCID

Yuan Liu: 0000-0001-9785-8799 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of this work by National Natural Science Foundation of China (NSFC) (Nos. 21576192 and 21376170) is gratefully acknowledged.

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REFERENCES

(1) Du, X.; Carriquiry, M. A. Flex-fuel vehicle adoption and dynamics of ethanol prices: lessons from Brazil. Energy Policy 2013, 59, 507− 512. (2) Spivey, J. J.; Egbebi, A. Heterogeneous catalytic synthesis of ethanol from biomass-derived syngas. Chem. Soc. Rev. 2007, 36, 1514− 1528. (3) Subramani, V.; Gangwal, S. K. A review of recent literature to search for an efficient catalytic process for the conversion of syngas to ethanol. Energy Fuels 2008, 22, 814−839. (4) Heracleous, E.; Liakakou, E. T.; Lappas, A. A.; Lemonidou, A. A. Investigation of K-promoted Cu-Zn-Al, Cu-X-Al and Cu-Zn-X (X = Cr, Mn) catalysts for carbon monoxide hydrogenation to higher alcohols. Appl. Catal., A 2013, 455, 145−154. (5) Surisetty, V. R.; Dalai, A. K.; Kozinski, J. Deactivation Studies of Alkali-Promoted Trimetallic Co−Rh−Mo Sulfide Catalysts for Higher Alcohols Synthesis from Synthesis Gas. Energy Fuels 2011, 25, 580− 590. (6) Zaman, S.; Smith, K. J. A Review of Molybdenum Catalysts for Synthesis Gas Conversion to Alcohols: Catalysts, Mechanisms and Kinetics. Catal. Rev.: Sci. Eng. 2012, 54, 41−132. (7) Chen, G.; Guo, C.-Y.; Huang, Z.; Yuan, G. Synthesis of ethanol from syngas over iron-promoted Rh immobilized on modified SBA-15 molecular sieve: Effect of iron loading. Chem. Eng. Res. Des. 2011, 89, 249−253. (8) Ning, X.; An, Z.; He, J. Remarkably efficient CoGa catalyst with uniformly dispersed and trapped structure for ethanol and higher alcohol synthesis from syngas. J. Catal. 2016, 340, 236−247. (9) Abdelsayed, V.; Shekhawat, D.; Poston, J. A., Jr.; Spivey, J. J. Synthesis, characterization, and catalytic activity of Rh-based lanthanum zirconate pyrochlores for higher alcohol synthesis. Catal. Today 2013, 207, 65−73. I


Article

Industrial & Engineering Chemistry Research (30) Sun, X. L.; Jones, N. W.; Gesick, J. C.; Xu, L. L.; Roberts, G. W. Liquid/catalyst interactions in slurry reactors: changes in tetrahydroquinoline composition during methanol synthesis over zinc chromite. Appl. Catal., A 2002, 231, 269−280. (31) Su, J.; Mao, W.; Xu, X. C.; Yang, Z.; Li, H.; Xu, J.; Han, Y. F. Kinetic study of higher alcohol synthesis directly from syngas over CoCu/SiO2 catalysts. AIChE J. 2014, 60, 1797−1809. (32) Couble, J.; Bianchi, D. Heats of adsorption of linearly adsorbed CO species on Co2+ and Co° sites of reduced Co/Al2O3 catalysts in relationship with the CO/H2 reaction. Appl. Catal., A 2012, 445−446, 1−13. (33) Liao, K.; Fiorin, V.; Gunn, D. S.; Jenkins, S. J.; King, D. A. Single-crystal adsorption calorimetry and density functional theory of CO chemisorption on fcc Co{110}. Phys. Chem. Chem. Phys. 2013, 15, 4059−65. (34) Carenco, S.; Tuxen, A.; Chintapalli, M.; Pach, E.; Escudero, C.; Ewers, T. D.; Jiang, P.; Borondics, F.; Thornton, G.; Alivisatos, A. P.; Bluhm, H.; Guo, J.; Salmeron, M. Dealloying of Cobalt from CuCo Nanoparticles under Syngas Exposure. J. Phys. Chem. C 2013, 117, 6259−6266. (35) Prieto, G.; Beijer, S.; Smith, M. L.; He, M.; Au, Y.; Wang, Z.; Bruce, D. A.; de Jong, K. P.; Spivey, J. J.; de Jongh, P. E. Design and Synthesis of Copper−Cobalt Catalysts for the Selective Conversion of Synthesis Gas to Ethanol and Higher Alcohols. Angew. Chem. 2014, 126, 6515−6519. (36) Zhang, R.; Wang, G.; Wang, B. Insights into the mechanism of ethanol formation from syngas on Cu and an expanded prediction of improved Cu-based catalyst. J. Catal. 2013, 305, 238−255. (37) Zaman, S. F.; Smith, K. J. Synthesis gas conversion over a Rh-KMoP/SiO2 catalyst. Catal. Today 2011, 171, 266−274. (38) Li, S.; Wang, H.; Li, W.; Wu, X.; Tang, W.; Chen, Y. Effect of Cu substitution on promoted benzene oxidation over porous CuCobased catalysts derived from layered double hydroxide with resistance of water vapor. Appl. Catal., B 2015, 166−167, 260−269. (39) Jiao, G.; Ding, Y.; Zhu, H.; Li, X.; Li, J.; Lin, R.; Dong, W.; Gong, L.; Pei, Y.; Lu, Y. Effect of La2O3 doping on syntheses of C-1C-18 mixed linear alpha-alcohols from syngas over the Co/AC catalysts. Appl. Catal., A 2009, 364, 137−142. (40) Lebarbier, V. M.; Mei, D.; Kim, D. H.; Andersen, A.; Male, J. L.; Holladay, J. E.; Rousseau, R.; Wang, Y. Effects of La2O3 on the Mixed Higher Alcohols Synthesis from Syngas over Co Catalysts: A Combined Theoretical and Experimental Study. J. Phys. Chem. C 2011, 115, 17440−17451. (41) Wang, Z.; Kumar, N.; Spivey, J. J. Preparation and characterization of lanthanum-promoted cobalt-copper catalysts for the conversion of syngas to higher oxygenates: Formation of cobalt carbide. J. Catal. 2016, 339, 1−8. (42) Dong, W. D.; Liu, J. X.; Zhu, H. J.; Ding, Y. J.; Pei, Y. P.; Liu, J.; Du, H.; Jiang, M.; Liu, T.; Su, H. Y.; Li, W. X. Co-Co2C and CoCo2C/AC Catalysts for Hydroformylation of 1-Hexene under Low Pressure: Experimental and Theoretical Studies. J. Phys. Chem. C 2014, 118, 19114−19122. (43) Xu, X.; Scholten, J. J. F.; Mausbeck, D. Stability of copper/cobalt catalysts for the synthesis of higher alcohols from syngas. Appl. Catal., A 1992, 82, 91−109. (44) Lebarbier, V. M.; Mei, D. H.; Kim, D. H.; Andersen, A.; Male, J. L.; Holladay, J. E.; Rousseau, R.; Wang, Y. Effects of La2O3 on the Mixed Higher Alcohols Synthesis from Syngas over Co Catalysts: A Combined Theoretical and Experimental Study. J. Phys. Chem. C 2011, 115, 17440−17451. (45) Volkova, G. G.; Yurieva, T. M.; Plyasova, L. M.; Naumova, M. I.; Zaikovskii, V. I. Role of the Cu−Co alloy and cobalt carbide in higher alcohol synthesis. J. Mol. Catal. A: Chem. 2000, 158, 389−393. (46) Mahajan, D.; Vijayaraghavan, P. Selective synthesis of mixed alcohols catalyzed by dissolved base-activated highly dispersed slurried iron. Fuel 1999, 78, 93−100. (47) Zhang, R. G.; Wang, G. R.; Wang, B. J.; Ling, L. X. Insight into the Effect of Promoter Mn on Ethanol Formation from Syngas on a

Mn-Promoted MnCu(211) Surface: A Comparison with a Cu(211) Surface. J. Phys. Chem. C 2014, 118, 5243−5254. (48) Zheng, H. Y.; Zhang, R. G.; Li, Z.; Wang, B. J. Insight into the mechanism and possibility of ethanol formation from syngas on Cu(100) surface. J. Mol. Catal. A: Chem. 2015, 404, 115−130. (49) Yang, Y.; Qi, X.; Wang, X.; Lv, D.; Yu, F.; Zhong, L.; Wang, H.; Sun, Y. Deactivation study of CuCo catalyst for higher alcohol synthesis via syngas. Catal. Today 2016, 270, 101−107. (50) Zhai, X. F.; Shamoto, J.; Xie, H. J.; Tan, Y. S.; Han, Y. Z.; Tsubaki, N. Study on the deactivation phenomena of Cu-based catalyst for methanol synthesis in slurry phase. Fuel 2008, 87, 430−434. (51) Liu, G.-L.; Niu, T.; Cao, A.; Geng, Y.-X.; Zhang, Y.; Liu, Y. The deactivation of Cu−Co alloy nanoparticles supported on ZrO2 for higher alcohols synthesis from syngas. Fuel 2016, 176, 1−10.

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