Two mechanisms for acetic acid synthesis from ethanol and water

Journal of Catalysis xxx (xxxx) xxx

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Two mechanisms for acetic acid synthesis from ethanol and water Guilherme G. Gonzalez a,b, Priscila C. Zonetti a, Erika B. Silveira a, Fabiana M.T. Mendes a, Roberto R. de Avillez c, Carlos R.K. Rabello d, Fatima M.Z. Zotin b, Lucia G. Appel a,⇑ a

Divisão de Catálise e Processos Químicos, Instituto Nacional de Tecnologia, Av Venezuela 82, 518, 20081-312 Rio de Janeiro, RJ, Brazil Laboratório de Catálise, Petróleo e Meio Ambiente, Universidade do Estado do Rio de Janeiro, Av. São Francisco Xavier, 524, Pavilhão Haroldo Lisboa da Cunha, Maracanã, 20550-900 Rio de Janeiro, RJ, Brazil c Departamento de Engenharia Química e de Materiais, Pontifícia Universidade Católica do Rio de Janeiro, PUC-Rio, Rua Marquês de São Vicente 225, Gávea, 22451-900 Rio de Janeiro, RJ, Brazil d CENPES/Petrobras, Ilha do Fundão, Quadra 7, Cidade Universitária, 21949-900 Rio de Janeiro, RJ, Brazil b

a r t i c l e

i n f o

Article history: Received 4 June 2019 Revised 16 August 2019 Accepted 16 September 2019 Available online xxxx Keywords: Acetic acid Water dissociation Zinc oxide Zirconia Copper Ethanol

a b s t r a c t Two catalysts, Cu/ZnO/Al2O3 and Cu/ZrO2/Al2O3, were employed for acetic acid synthesis from ethanol and water. They were characterized by X-ray fluorescence, N2 physisorption, N2O titration, in situ X-ray diffraction refined by the Rietveld method, in situ X-ray photoelectron spectroscopy, temperature-programmed reduction followed by X-ray absorption near-edge structure, and temperature-programmed desorption of ethanol, H2O, CO2, and NH3. The Zn-based catalyst is composed of ZnO nanoparticles, Cu0, CuZn alloy, and Al2O3, whereas the Cu/ZrO2/Al2O3 is composed of Cu0, ZrO2, and Al2O3. Both catalysts show the same Cu0 metallic surface area. On one hand, the spectra of the H2O temperature-programmed desorption show that the ZnO nanoparticles of Cu/ZnO/Al2O3 promote water dissociation and, consequently, the redox properties of this catalyst. On the other hand, the Zr-based catalyst shows low activity for H2O dissociation and higher acidity and basicity than for the Zn-based catalyst. These different properties lead to different mechanisms for acetic acid synthesis from ethanol. Taking the redox mechanism of the WGS reaction into account, the steps of the acetic acid synthesis promoted by the Zn-based catalyst can be described as follows: first, ethanol is dehydrogenated alloy to acetaldehyde on Cu0 and CuZn; then, this aldehyde is oxidized on the ZnO–Cu0 interface to acetate, which desorbs, forming acetic acid; finally, H2O dissociates on the O vacancies of the ZnO–Cu0 interface and reoxidizes the oxide. The ZnO–CuZn alloy interface should also be considered in this mechanism. The second mechanism, which occurs mainly on the Zr-based catalyst, is related to the acetaldehyde and ethanol condensation forming ethyl acetate. This ester is hydrolyzed, synthesizing acetic acid and ethanol. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Acetic acid is employed in the generation of vinyl acetate monomer (VAM), which is very important for the production of paints, textiles, paper, and many other products. It is also an intermediate in the industrial synthesis of many chemicals, such as ethyl acetate, acetic anhydride, isobutyl acetate, methoxypropanol acetate, and terephthalic acid (PTA) [1]. Nowadays, huge amounts of acetic acid are produced by the methanol carbonylation reaction. Methanol is synthesized from natural gas, a fossil fuel raw material. According to the literature [2–4], it is possible to obtain acetic acid by the selective oxidation of ethanol, a renewable feedstock. The oxidizing agent is oxygen from the air. High yields of acetic ⇑ Corresponding author.

acid can be reached when the right catalyst is employed. However, to avoid ethanol combustion, this biofuel should be employed at a low concentration in air (4.3%) [5]. This constraint affects the economic feasibility of the process. To overcome this problem, H2O can be used instead. In this case, it will be possible to employ much higher ethanol concentrations. Another possibility is to synthesize this acid from ethanol using a mechanism in which it is not necessary to directly oxidize the alcohol. Voss et al. [6] showed that acetic acid could be synthesized from ethanol using a Cu/SiO2 catalyst and H2O as the oxidizing agent. They proposed that H2O is dissociated on Cu0 sites. Acetaldehyde, which is generated by the dehydrogenation of ethanol, is oxidized to acetic acid by the oxygen from the H2O dissociation. In studying the acetone synthesis from ethanol, our group observed that the Cu/ZnO/Al2O3 + ZrO2 physical mixture generates this ketone by an oxidative mechanism [7]. Water dissociates on

E-mail address: [email protected] (L.G. Appel). https://doi.org/10.1016/j.jcat.2019.09.031 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

Please cite this article as: G. G. Gonzalez, P. C. Zonetti, E. B. Silveira et al., Two mechanisms for acetic acid synthesis from ethanol and water, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.031

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G.G. Gonzalez et al. / Journal of Catalysis xxx (xxxx) xxx

the Cu sites and oxidizes acetaldehyde to acetate species. These steps are very similar to the ones proposed by Voss et al. [6]. Finally, the acetate species condense to acetone on the ZrO2 oxide. According to Voss et al. [6], acetate species are also precursors of acetic acid. It is worth stressing that Cu/ZnO/Al2O3 is a wellknown catalyst employed in methanol synthesis. On the other hand, Brei et al. [8] and Inui et al. [9–11], employing catalysts based on Cu, ZnO, Al2O3, and ZrO2 in ethanol conversion, proposed that acetic acid is generated from ethanol and H2O in the following steps: first, ethanol dehydrogenation generates acetaldehyde; after that, it condenses, producing ethyl acetate; finally, this ester is hydrolyzed to acetic acid and ethanol. In this case, there is no oxidation step. The objective of this contribution is to determine the correlation between the physicochemical properties of the catalysts and their catalytic behavior and to employ these data to describe the steps of acetic acid synthesis from ethanol and H2O using catalysts based on Cu0, ZnO and ZrO2.

2. Materials and methods 2.1. Catalyst preparation The CuO/ZnO/Al2O3 (CuZnAl) and CuO/ZrO2/Al2O3 (CuZrAl) catalysts were prepared by co-precipitation from aqueous solutions of Cu(NO3)2, Zn(NO3)2, Al(NO3)3, ZrO(NO3)2, and Na2CO3 following the preparation method used by Melián-Cabrera [12]. The suspension obtained was aged for 18 h at room temperature. Next the precipitate was washed, filtered, and dried for 12 h at 120 °C. Then the precursors were calcined in air for 4 h at 400 °C.

2.2. Characterization The catalysts’ chemical composition was determined by X-ray fluorescence spectroscopy by wavelength dispersion (WD-XRF) in an S8 Tiger Bruker spectrometer equipped with an Rh tube operating between 30 and 60 kV. Specific area measures were obtained by the Brunauer–Emme tt–Teller (BET) method in a Micromeritics ASAP 2420. The samples were dried at 100 °C for 24 h and then heated in situ under vacuum until 350 °C at 5 °C min 1 until 12 lmHg was achieved. Nitrogen physisorption occurred at 196 °C. The copper metallic surface area measured by N2O titration follows Van der Grift’s methodology [13] (see the Supporting Information). This analysis was performed on a Micromeritics AutoChem 2920 equipped with a thermal conductivity detector. First, 200 mg of the catalysts were pretreated for 1 h with a 10% H2/Ar mixture flow at 300 °C. Then, the catalysts were oxidized for 1 h with a 5% O2/He mixture flow at 300 °C, and after that, they were cooled to room temperature. The samples were heated to 300 °C (at a rate of 10 °C min 1) under a 10% H2/Ar mixture flow (TPR1). Next, the reactor was cooled to 90 °C and the catalysts were exposed for 1 h to a 1% N2O/He mixture flow. Finally, the catalysts were cooled to room temperature and TPR2 was performed in the same way as TPR1. X-ray diffraction analyses (XRD) were performed by a Bruker D8 Advance X-ray diffractometer equipped with a CuKa radiation source (1.5406 Å) and a Ni filter at 40 kV and 40 mA. The analysis conditions were 0.02° step, 0.5 s/step, and 2h range 10°–85°. The precursors and the samples calcined and reduced in situ were analyzed. The reduced samples were dried under a N2 flow at 130 °C for 30 min and then reduced under a 2%H2/N2 mixture flow at 300 °C for 1 h before the analysis. Collected data were refined by the Rietveld method.

Ethanol and H2O temperature-programmed desorption (ethanol TPD and H2O TPD) were carried out employing a Micromeritics AutoChem 2920 coupled to a Pfeiffer Omni Star mass spectrometer. The samples were dried under a He flow at 150 °C for 30 min. Then they were reduced under a 10% H2/Ar flow at 300 °C for 1 h. Ethanol or H2O adsorption was carried out at 40 °C. The vapors were generated by passing He through a saturator. Desorption was carried out from 40 to 300 °C, under He flow (80 mL min 1) at 20 °C min 1, and then the samples were kept at 300 °C for 1 h. The main fragments followed up during the analyses were m/z = 2 (H2), m/z = 18 (H2O), and m/z = 32 (O2) for H2O TPD and m/z = 2 (H2), m/z = 18 (H2O), m/z = 28 (ethene), m/z = 29 (acetaldehyde), m/z = 31 (ethanol), m/z = 32 (O2), m/z = 43 (acetic acid, ethyl acetate, acetone), and m/z = 44 (CO2) for ethanol TPD. The intensities of these fragments were mathematically treated to eliminate contributions of more than one species. NH3 temperature-programmed desorption (NH3 TPD) was performed in a multipurpose unit with a thermal conductivity detector. The catalysts were previously dried for 30 min at 130 °C under N2 flow. After that they were reduced for 1 h under a 10% H2/He mixture flow at 300 °C. Finally, the catalysts were reoxidized for 30 min with air at 300 °C. The NH3 adsorption was carried out for 1 h at 100 °C under a 4% NH3/He mixture flow. The desorption started from 100 to 300 °C under a He flow (80 mL min 1) at 10 °C min 1 and then the samples were kept at 300 °C for 30 min. The TPD profiles were deconvoluted in Gaussian curves in order to quantify the weak, medium, and strong acid sites. Peaks where the maximum was observed at temperatures lower than 200 °C were assigned to weak strength acid sites. The ones between 200 and 300 °C were assigned to medium and strong acid sites. CO2 temperature-programmed desorption (CO2 TPD) was performed in a multipurpose unit with a thermal conductivity detector. The catalysts were dried for 30 min under N2 flow at 130 °C. Then they were reduced for 1 h under a 10% H2/He mixture flow at 300 °C. After that, the catalysts were reoxidized for 30 min with air at 300 °C. Carbon dioxide was adsorbed for 1 h at room temperature. Desorption was carried out from room temperature to 300 °C under a He flow (80 mL min 1) at 10 °C min 1. The samples were kept at 300 °C for 30 min. The TPD profiles were also deconvoluted in Gaussian curves in order to quantify the weak, medium, and strong basic sites. Peaks where the maximum was observed at temperatures lower than 150 °C were assigned to weak basic sites, and the ones between 150 and 300 °C were assigned to mediumstrength basic sites. The X-ray absorption near-edge structure (XANES) spectra obtained in the D06A-DXAS beam line at the Brazilian Synchrotron Light Laboratory (LNLS) were collected during the thermalprogrammed reduction (TPR) analyses. A Si monochromator (1 1 1) was used to select the X-ray beam of the synchrotron light produced by the electron storage ring of 1.37 GeV with a maximum current of 200 mA. The absorption spectra at the Cu K-edge were recorded in the transmission mode in a photon energy range between 8800 and 9200 eV using a CCD camera. This spectrometer is able to collect full absorption spectra through a parallel detection scheme over an extended range of photon energies without any mechanical movement. This configuration permits detecting very weak signals for tracking chemical reactions that are timedependent [14]. The measurements were also performed on a dispersive beamline and not using a conventional XAS beamline. The XANES spectra obtained under these conditions show very good time resolution. Four spectra were acquired per minute. The samples were diluted in a binder (boron nitride) and placed in a capillary reactor, where they were reduced under a 5% H2/He mixture flow (10 mL min 1) from 25 to 300 °C (10 °C min 1). The surface composition of catalysts was determined by X-ray photoelectron spectroscopy (XPS). This analysis was performed

Please cite this article as: G. G. Gonzalez, P. C. Zonetti, E. B. Silveira et al., Two mechanisms for acetic acid synthesis from ethanol and water, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.031

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using SPECS equipment, with a PHOIBOS-150 spherical analyzer and an AlKa X-ray source (ht = 1486.6 eV). The C1s energy (284.6 eV) was used as reference. The samples were dried for 30 min under a N2 flow at 130 °C and then reduced for 2 h under a 5%H2/N2 mixture flow at 300 °C in a HPC prechamber. Then the samples were moved under vacuum to the analysis chamber. The results were analyzed employing the Casa XPS software. 2.3. Thermodynamics calculation The thermodynamic calculation of the reaction of ethanol and water to form acetic acid was done with Thermo-Calc software, version 2015, using the SSUB3 database. 2.4. Catalytic tests Catalytic tests were performed in a multipurpose unit using a conventional system with a fixed-bed reactor at atmospheric pressure. The reagents and products were analyzed on line using a GC Agilent HP6890 equipped with two detectors (thermal conductivity detector and flame ionization detector). The column employed was a Porapak-Q/60 using He as the carrier gas. The samples were analyzed every 23 min during 6 h on stream. The catalysts were previously dried at 130 °C with N2 for 30 min and then reduced by a 10% H2/N2 flow at 300 °C for 1 h. The gas stream composition was N2:H2O:C2H5OH = 89:10:1 mol.%. Ethanol and H2O vapor was generated by passing N2 through two saturators, one at 4.6 °C and the other at 60.5 °C. Reactions were performed at different temperatures (200, 225, 250, 275, and 300 °C) in order to observe the evolution of the product selectivity. The catalyst mass and the GHSV used were 300 mg and 8000 mL gcat1 h 1, respectively. Isoconversion tests were also performed to compare selectivities at the same conversion. The catalyst mass and gas flow were changed to reach 70% conversion at 225 °C. Catalytic tests at high ethanol concentrations were also performed with gas mixture composition, catalyst mass, and flow rate of N2:H2O:C2H5OH = 85:10:5 vol%, 300 mg, and 40 mL min 1, respectively. The ethanol conversion was defined as the ratio of the moles of ethanol consumed to the moles of ethanol introduced into the feed. The definition of the selectivity to one specific compound is the ratio of the number of carbon moles consumed to synthesize this compound to the total number of carbon moles consumed. The turnover frequency (TOF) was calculated based on the assumption that each surface Cu atom is one site for ethanol dehydrogenation. The amount of Cu on the surface was obtained using N2O titration. Only acetaldehyde was observed as a product under the experimental conditions employed (225 °C). 3. Results and discussion Table 1 shows the specific area, metallic copper area, and Cu, Zn, Zr, Al, and O concentrations of the two prepared catalysts. As can be observed, in spite of the different Cu concentrations and specific areas, both catalysts show the same Cu0 metallic surface area. Fig. 1a and b display the catalytic tests of CuZnAl and CuZrAl, respectively. Fig. 1a shows that as the temperature increases, the selectivity for acetaldehyde decreases, whereas that for acetic acid increases. Thus, acetaldehyde might be an intermediate of acetic acid generation. Ethanol conversion reaches 100% at 250 °C. At

the same temperature, the selectivity for acetic acid reaches its maximum (52%). At higher temperatures, the selectivity for acetone increases whereas that for acetic acid decreases. Silva-Calpa et al. [15] and Lima et al. [16] showed that acetate species condense and generate acetone. Voss et al. [6] suggested that these same species are intermediates of acetic acid synthesis from ethanol. Fig. 1a shows that at high temperature the selectivity for acetone is higher than that for acetic acid. Thus, it can be inferred that acetate condensation is faster than its desorption as acetic acid at high temperatures. Carbon dioxide, a byproduct of this ketone synthesis, was also observed during the catalytic tests (not shown). At low temperatures, ethyl acetate is synthesized at low selectivity. When the temperature increases, the selectivity for this ester decreases. Fig. 1b shows that the Zr-based catalyst synthesizes the same compounds as CuZnAl. Moreover, acetaldehyde exhibits almost the same behavior as described in the case of the Zn-based catalyst. Ethanol conversion reaches 100% at a higher temperature (275 °C) than for the Zn-based catalyst. At this temperature, the acetic acid selectivity reaches its maximum (58%). This value is higher than the acetic acid selectivity maximum of CuZnAl. At 300 °C, the acetic acid selectivity almost does not change, whereas the acetone selectivity increases due to acetaldehyde consumption. Both catalysts also generate ethyl acetate; however, the selectivity for CuZrAl is higher. This ester exhibits behavior similar to that observed in the case of acetaldehyde, which is described above: as the temperature increases its selectivity decreases, whereas that for acetic acid increases. Thus, ethyl acetate might also be an intermediate for acetic acid synthesis. Both reactions also generate H2 (not shown). Table 2 exhibits the catalytic behavior of CuZnAl and CuZrAl at isoconversion (70%) at 225 °C. As can be observed, these catalysts synthesize acetic acid, ethyl acetate, and acetaldehyde. From comparing the values of the catalyst mass, it can be inferred that CuZnAl is slightly more active than CuZrAl. It can also be observed that CuZrAl shows higher selectivity to acetic acid and ethyl acetate at these conditions. These results are consistent with those of Fig. 1a and b. Considering that these two catalytic systems show the same Cu0 metallic area, it can be suggested that the differences observed in their catalytic behavior can be associated with the presence of ZnO or ZrO2 in the catalyst formulation. Table 3 depicts the catalysts’ precursors (samples before calcination) and Fig. S1 in the Supporting Information shows their XRD patterns. The CuZrAl precursor only exhibits a malachite phase (Cu2CO3(OH)2). Zirconium crystalline compounds were not observed. A double-layer hydroxide (LDH), which contains both Cu and Zn, and a malachite phase can be identified in the CuZnAl precursor diffractogram. The LDH phase, which is similar to the mineral hydrotalcite (Mg6Al2(OH)16CO3
4H2O) [17], shows the highest concentration. The XPS results described below show that ZnO and ZrO2 partially cover the Cu surface after reduction. The decrease in Cu concentration at the surface of the catalyst can be associated with the SMSI (strong metal–support interaction) phenomenon [18,19]. Both Cu catalysts show the same Cu0 metallic surface area, despite the differences in Cu concentration and specific area depicted in Table 1. This might be associated with the SMSI phenomenon. Indeed, to reach the same Cu0 area for the two catalysts, some preparation attempts were carried out.

Table 1 Specific area (S, m2gcat1), metallic copper area (ACu, m2Cugcat1), and Cu, Zn, Zr, Al, and O mass concentrations (wt.%). Catalyst

S

ACu

Cu

Zn

Zr

Al

O

CuZnAl CuZrAl

105 190

14 14

46 39

22 –

– 29

8 6

24 26

Please cite this article as: G. G. Gonzalez, P. C. Zonetti, E. B. Silveira et al., Two mechanisms for acetic acid synthesis from ethanol and water, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.031

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Fig. 1. Ethanol conversions and selectivities of (a) CuZnAl and (b) CuZrAl vs. temperature. The gas mixture composition, catalyst mass, and flow rate are N2:H2O: C2H5OH = 89:10:1 vol%, 300 mg, and 40 mL min 1, respectively.

Table 2 Acetic acid, acetaldehyde and ethyl acetate selectivities at isoconversion (70%). Catalyst

CuZnAl CuZrAl

Selectivity (%) Acetic acid

Acetaldehyde

Ethyl acetate

15 19

57 54

12 18

Notes: The experimental conditions were as follows: gas mixture flow 40 mL min 1, composition N2:H2O:C2H5OH = 89:10:1 vol%, and temperature 225 °C, respectively. The CuZnAl and CuZrAl catalysts masses were 210 and 300 mg, respectively.

Table 3 Phase distribution of precursor and calcined and reduced samples analyzed by XRD.

a b c

Catalyst

Precursor

Calcined

Reduced

CuZnAl CuZrAl

LDHa (62 wt%);c malachiteb (38 wt%)c malachiteb

CuO CuO

Cu0; ZnO Cu0

+3 +x Layered double hydroxide (LDH), [(Cu,Zn)+2 1- x Al x(OH)2] (CO3) Cu2CO3(OH)2. Quantified by the Rietveld method.

2 x/2
mH2O.

The two calcined samples show very similar diffractograms (Fig. S2 and Table 3). Their reflections are associated with the CuO oxide only. Peaks related to the Zn, Al, and Zr compounds were not observed. These XRD data were analyzed by the Rietveld refinement method (Table S1 in the Supporting Information). Compared with pure CuO, both catalysts show lattice parameter changes, cell volumes increase, and crystallite sizes decrease. Furthermore, the presence of structural defects can be suggested for both calcined samples, since the oxygen occupancy factor values are lower than 1. These results indicate the formation of mixed oxides. However, it is not possible to rule out the presence of Zr, Zn, and Al oxides, which might not be identified by XRD due to the detection limit of this technique. Indeed, the presence of ZnO nanoparticles, not detectable by XRD in the CuZnAl catalyst, is highlighted in the literature [20,21]. Moreover, according to some authors [22–24], an amorphous ZrO2 phase might occur when it is prepared by precipitation or coprecipitation. The XRD analysis of the (in situ) reduced CuZrAl catalyst (Fig. S3 and Table 3) exhibits only Cu0 (cubic phase) whereas the reduced CuZnAl catalyst diffractogram shows reflections related to Cu0 and also small peaks associated with ZnO. Table 4 shows the lattice parameters, cell volume, and Cu0 crystallite size of the reduced catalysts. These data were obtained by

the Rietveld refinement method. Both catalysts, when reduced, exhibit reflections associated with Cu0 (cubic phase). The cell volume of the reduced Zr-based catalyst shows a slight decrease when compared with Cu0 (reference), whereas the CuZnAl catalyst showed two different Cu0 phases, Cu-a and Cu-b. On one hand, the lattice parameters and cell volume of Cu-a (Table 4) are very similar to those of Cu0 (reference). On the other hand, the Cu-b phase exhibits an increase in the lattice parameter values and cell volume as compared with the reference. According to Spencer et al. [25] and Günter et al. [26], Zn atoms can replace Cu atoms in the Cu0 structure, forming a CuZn surface alloy at low reduction temperatures. Thus, the Cu-b phase described in Table 4 might be related to the Zn insertion in the Cu0 phase. It is worth stressing that reduced CuZnAl shows low-intensity ZnO reflections, which are impossible to analyze accurately employing the Rietveld method. These peaks might be associated with ZnO nanoparticles (Fig. S3). Table 5 shows the Cu2p3/2, Zn2p3/2, Zr3d5/2, and Al2p binding energies (BE), Cu LMM kinetic energy (KE) and their assigned species. These data are related to the XPS analysis employing the reduced catalysts (in situ). Fig. 2 and S4 depict the Zn2p3/2, Cu2p3/2, and Cu LMM spectra (reduced catalysts). The Cu2p3/2 binding energies of the reduced catalysts can be assigned to Cu0 and/or Cu+ [27]. The Auger modified parameter, which is the sum of the Cu2p3/2 binding energy and the Cu LMM kinetic energy, is used to determine the presence of Cu0 or Cu+. In the case of the CuZnAl catalyst, the value of this parameter is 1851.12 eV, and for CuZrAl, it is 1851.00 eV. Thus, according to the literature, it is possible to infer that there is a predominant presence of Cu0 in both catalysts [28–30]. The Zr3d spectrum of the reduced CuZrAl catalyst can be associated with ZrO2 [31]. The Zn2p3/2 spectrum (Fig. 2) of the reduced CuZnAl catalyst exhibits a maximum at 1021.4 eV and one shoulder at 1017.4 eV. The former can be associated with ZnO [31] and the latter with metallic Zn [32]. This shoulder is also observed in the Zn2p1/2 spectrum (Fig. 2, arrow). These data suggest the presence of a CuZn alloy [25,26,33] as observed in the Rietveld refinement. The binding energy observed for Al2p can be assigned to Al2O3 [31] for both catalysts (see spectra, Fig. S5) Thus, the reduced CuZnAl catalyst shows ZnO nanoparticles, CuZn alloy, and Cu0, whereas the reduced CuZrAl catalyst is composed of ZrO2, Cu0, and Al2O3. From comparing the Cu/Zn superficial atomic ratio of the oxidized (4.3) and the reduced (2.0) CuZnAl catalysts, it can be inferred that the Zn concentration on the surface increases after

Please cite this article as: G. G. Gonzalez, P. C. Zonetti, E. B. Silveira et al., Two mechanisms for acetic acid synthesis from ethanol and water, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.031

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G.G. Gonzalez et al. / Journal of Catalysis xxx (xxxx) xxx Table 4 Crystallographic parameters of the reduced samples determined by Rietveld refinement of XRD data. Crystallographic parameters GOF Rpb

a

Cu reference

CuZnAl

CuZrAl

1.66 2.09

1.23 1.98

1.26 1.95

Cu0 Space group: Fm-3m

Lattice parameters Cell volume Cu0 crystallite size

a (Å) (Å3) (nm)

3.614895(32) 47.2375(12) 71.2(15)

Cu-a

Cu-b

3.61491(28) 47.238(11) 20.78(96)

3.61634(84) 47.294(33) 4.419(57)

3.61419(30) 47.210(12) 31.2(20)

Note: Numbers in parentheses are related to the fitting error. a GOF = goodness of fit. b Rp = profile residual factor.

Table 5 XPS analysis of the reduced CuZnAl and CuZrAl catalysts. CuZnAl

Cu2p3/2 Cu LMM Zn2p3/2 Zr3d5/2 Al2p

CuZrAl

BE

Species

BE

Species

931.9 919.3a 1021.41017.9 – 73.9

Cu0/Cu1+ Cu0 ZnO ZnCu – Al2O3

932.1 918.9a – 182.3 74.1

Cu0/Cu1+ Cu° – ZrO2 Al2O3

Notes: Table shows binding energy (BE, eV) of Cu2p3/2, Zn2p3/2, Zr3d5/2, and Al2p and Cu LMM kinetic energy (KE, eV) and species assigned to these energies. a KE.

Fig. 2. (a) Zn2p spectra of the reduced CuZnAl catalyst and (b) zoomed Zn2p3/2 spectra.

reduction, which could be related to the ZnO particles’ migration to the surface during this process. This migration is well described in the literature and it could be an important step in the formation of the CuZn surface alloy [25,32,33]. In the case of the Zr-based catalyst, similar behavior is observed. However, only ZrO2 is observed in this case. The TPR profiles followed by XANES at Cu K-edge are depicted in Fig. 3. Both catalysts show similar profiles, Cu2O being an intermediate of CuO reduction. In the case of the Zr-based catalyst, the reduction starts at 210 °C, whereas the reduction of the Zn based catalyst begins at 245 °C. Moreover, the peak related to the Cu2O of CuZnAl is larger than that of CuZrAl. Therefore, these catalysts show different behavior in the reduction process, indicating that the CuO reducibility is affected by the presence of the ZnO and ZrO2 oxides. Possibly, the slightly lower reducibility of CuZnAl

compared with the Zr-based catalyst can be associated with CuZn alloy formation and migration of ZnO nanoparticles to the Cu0 surface. It is important to notice that at the end of the analysis, CuO is completely reduced for both catalysts, in agreement with the XRD and XPS results. The XANES spectra are depicted in Fig. S6. The number of acid and basic sites obtained by the NH3 TPD and CO2 TPD are shown in Table 6. The profiles are depicted in Figs. S6, S7. The CuZrAl catalyst shows not only a larger number of weak and medium acid sites but also a larger number of weak and medium basic sites than CuZnAl. This might occur due to the amphoteric properties of ZrO2 [22]. Fig. 4a shows the H2, H2O, and CO2 spectra of the ethanol TPD of CuZrAl. An m/z = 43 small peak is also observed, which can be associated with acetic acid, ethyl acetate, or acetone (the most intense fragment of these three compounds). As this signal shows low intensity, it is not possible to employ other fragments to discriminate these species. At low temperature (171 °C), H2 is generated, which can be related to acetaldehyde synthesis via ethanol dehydrogenation. Considering that this aldehyde and its derivatives are almost not observed, it can be suggested that acetaldehyde might have condensed, generating heavy compounds, H2, H2O, and CO2. Therefore, the heavy compounds would most probably have been deposited on the catalyst surface. Inui et al. [11] also observed ethanol condensation reactions in which the byproducts were mainly CO2, H2, and H2O. Fig. 4b exhibits the spectra of the ethanol TPD of CuZnAl. At low temperature (130 °C), a huge H2 peak is observed, which can also be associated with the dehydrogenation of ethanol. Taking into account that this temperature is lower than the one observed for the Zr-based catalyst, CuZnAl seems to be more active in the dehydrogenation of ethanol. As both catalysts show the same metallic area, it can be suggested that the CuZn alloy promotes this dehydrogenation. However, this H2 band is less intense than that of CuZrAl. This might occur due to the ethanol dehydrogenation versus the dehydration competition. In spite of ethylene not being a

Please cite this article as: G. G. Gonzalez, P. C. Zonetti, E. B. Silveira et al., Two mechanisms for acetic acid synthesis from ethanol and water, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.031

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Fig. 3. Copper phase distribution during TPR XANES of (a) CuZnAl and (b) CuZrAl.

Table 6 H2 amount generated during H2O TPD (A, lmol g basic sites (lmol g 1).

1

), number of weak (NWA) and medium and strong (NMSA) acid sites (lmol g

1

), and number of weak (NWB) and medium (NMB)

Catalyst

A

NWA

NMSA

NWB

NMB

CuZnAl CuZrAl

15 2

35 77

122 271

23 92

0 68

of acetone synthesis [7,15,16,34] and desorb at the same temperature as m/z = 43, this fragment can be assigned mainly to acetone. Fig. 5 depicts the H2 (m/z = 2) spectra of H2O TPD, and Table 6 also shows the amount of H2 formed during H2O TPD. The CuZnAl catalyst generates much more H2 than the Zr-based catalyst. Some authors have observed that Cu0 is an active site for this reaction [6,35,36]. However, as both catalysts show the same metallic area, it can be inferred that the presence of ZnO nanoparticles and/or the CuZn alloy promotes H2O dissociation. It is well known that nanoparticles of metal oxides are prone to form oxygen vacancies [18,37], which are active in H2O dissociation [38]. Moreover, the number of ZnO nanoparticles is much higher than for the CuZn alloy on the surface (Fig. 2). Thus, the H2O dissociation behavior of the Zn-based catalyst seems to be mainly associated with the presence of ZnO nanoparticles. It can be suggested that acetaldehyde and H2 (ethanol TPD, Fig. 4b) are generated on Cu0 and CuZn alloy sites. Ethylene and H2O are synthesized on pairs of ZnO Lewis acid and basic sites

Fig. 4. Spectra of ethanol TPD on (a) CuZrAl and (b) CuZnAl.

byproduct of the acetic acid synthesis, this olefin and H2O are generated in the ethanol TPD. Probably this occurs because of the experimental conditions, which are not the same as the ones employed in the catalytic tests. Indeed, acetaldehyde, ethylene, m/z = 43, and H2O show peaks at the same temperature, 189 °C. The m/z = 43 peak can be associated with ethyl acetate, acetic acid, or acetone. Carbon dioxide, a byproduct of this ketone synthesis, is not observed at this temperature. Therefore, the m/z = 43 peak can be assigned to acetic acid and ethyl acetate. Acetaldehyde, ethylene, m/z = 43, H2O, CO2, and H2 peaks can be observed at 289 °C. Once again, ethyl acetate, acetone, and acetic acid can be associated with the m/z = 43 spectrum. As CO2 and H2 are byproducts

Fig. 5. H2 (m/z = 2) spectra of H2O TPD on the CuZnAl and CuZrAl catalysts.

Please cite this article as: G. G. Gonzalez, P. C. Zonetti, E. B. Silveira et al., Two mechanisms for acetic acid synthesis from ethanol and water, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.031

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[39] (Table 6). Since the CuZnAl catalyst is reduced before the ethanol TPD, H2O may be decomposed promptly on ZnO oxygen vacancies, generating H2 and filling the vacancies with O (Fig. 4b, peak at 189 °C). Then, acetaldehyde is immediately oxidized to acetate species employing the O of the ZnO lattice by the MVK mechanism [15,16,40]. Some acetate species desorbs as acetic acid (first peak of m/z = 43). Oxygen vacancies, which are strong basic sites [41], are then generated. Increasing the temperature (peaks at 289 °C), strong basic sites promote the condensation (ketonization) of the remaining acetate species adsorbed on the catalyst surface, forming acetone, CO2, and H2. More ethylene and H2O are synthesized as acid sites are released due to the acetone synthesis. The condensation of ethanol (or ethoxide species) with acetaldehyde can also be suggested for ethyl acetate generation at low temperature (m/ z = 43). The main point of the ethanol TPD is the acetone formation, which shows the redox properties of the CuZnAl catalyst. Since the beginning of the twentieth century, Cu/ZnO catalysts have been employed in methanol synthesis. Many research groups have studied the Cu–Zn interaction. On one hand, the mechanism of this reaction is not completely established [42–44]. On the other hand, the description of the Cu/ZnO/Al2O3 catalyst during the reaction is now accepted. The most critical points are the presence of a CuZn alloy [25,32,33] and the ZnO nanoparticles [18,37]. Both phases were observed in this work. The Cu/ZnO/Al2O3 catalyst is used in the WGS (water gas shift) reaction as well. The redox and associative mechanisms are the main ones proposed for this reaction [45]. According to Aranifard et al. [45], the former can be described as following: first, carbon monoxide adsorbs on the metal; then, H2O decomposes on an oxygen vacancy of the metal–oxide interface, generating hydrogen and leaving oxygen in the vacancy; next, carbon monoxide migrates to the metal–oxide interface and abstracts the oxygen of the oxide lattice recovering the vacancy; finally, carbon dioxide desorbs. This mechanism shows that the redox properties of the oxide are associated with the dissociation of H2O. Considering the ethanol TPD and H2O TPD of Cu/ZnO/Al2O3 and the characterization data and chemical similarity between carbon monoxide and acetaldehyde, the following mechanism can be proposed for the synthesis of acetic acid from ethanol: first, ethanol is dehydrogenated to acetaldehyde on Cu0 and the CuZn alloy; next, this aldehyde, adsorbed on the metal–oxide interface is oxidized to acetate species by the oxygen of the ZnO nanoparticles on the ZnO–Cu interface and ZnO–CuZn alloy interface and generating

7

oxygen vacancies; then, the acetate species are desorbed as acetic acid; next, acetate species can also condense on the ZnO surface, forming acetone; finally, the oxygen vacancies are eliminated by H2O dissociation on the ZnO–Cu interface and the ZnO–CuZn interface. The H2O TPD and ethanol TPD spectra show that the CuZrAl catalyst seems not to show relevant redox properties. Thus, acetic acid synthesis occurs on the metallic, acid, and basic sites of CuZrAl. Some authors [11,46] describe the mechanism of ethyl acetate synthesis from ethanol as follows: first, acetaldehyde is synthesized by the ethanol dehydrogenation on Cu0; then, ethanol adsorbs onto ZrO2 and the hydrogen of the hydroxyl species is eliminated, forming ethoxide species adsorbed onto the acid sites of ZrO2; after that, the aldehyde spillover from the metal to the oxide (ZrO2) or it is desorbed and adsorbed on ZrO2; next, acetaldehyde reacts with the ethoxide species via a nucleophilic attack (basic site), forming a hemiacetal; then, this compound is dehydrogenated to ethyl acetate; finally, H2O in the reagent mixture decomposes the ester and acetic acid is generated. Hemiacetal synthesis can also occur by the Eley–Rideal mechanism, in which the acetaldehyde adsorbed on acid sites reacts with ethanol (not adsorbed). Scheme 1 depicts the mechanisms of acetic acid generation from ethanol without the presence of air. The redox properties are essential for the mechanism promoted by CuZnAl and might be supplied by the ZnO nanoparticles. However, taking into account that H2O can also be dissociated on Cu and CuZn alloy, some contribution of the Langmuir–Hinshelwood mechanism proposed by Voss et al. [6] should also be considered. Taking into account that CuZrAl is also able to dissociate H2O (see Fig. 5) and that CuZnAl synthesizes ethyl acetate (low selectivity; Fig. 1a), it can be suggested that with different intensities both mechanisms might occur on both catalysts. The TOF values of CuZnAl and CuZrAl obtained at 225 °C (differential conditions) are 7  10 3 s 1 and 5  10 3 s 1, respectively. These values are related to the first step of these syntheses, i.e., the acetaldehyde generation. It can be inferred that the Zn-based catalyst is slightly more active for ethanol dehydrogenation than the CuZrAl catalyst. This result is in line with those for ethanol TPD and can be associated with the presence of the CuZn alloy in the Zn based catalyst. Fig. S9 shows the Gibbs energy of formation for acetic acid synthesis from ethanol and H2O. As can be observed, its value is positive at temperatures lower than 290 °C. However, Fig. 1a and b

Scheme 1. Reaction pathways of acetic acid synthesis from ethanol: oxidative (blue) and ethyl acetate hydrolysis (green).

Please cite this article as: G. G. Gonzalez, P. C. Zonetti, E. B. Silveira et al., Two mechanisms for acetic acid synthesis from ethanol and water, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.031

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show that this reaction truly occurs. It was also experimentally verified that with decreasing H2O concentration in the gaseous mixture the ethanol conversion drops. Thus, it is proposed that the reaction equilibrium shifts toward acetic acid due to the high concentration of H2O employed in the catalytic tests (see Materials and Methods). Moreover, the calculated equilibrium constant value is close to 1 above 280 °C, consistent with the observed maximum formation of acetic acid (Fig. 1b). Finally, it should be noted that the complete equilibrium between water and ethanol calculated with the same composition relationship used in the present research forms H2, CH4, and CO2. Both catalysts were analyzed at a higher concentration of ethanol (beyond explosive limits) than under the experimental conditions of Fig. 1. When acetic acid synthesis occurs by ethanol oxidation employing air, these experimental conditions should be avoided for safety reasons. The results are depicted in Figs. S8a and S8b. As can be observed, the selectivities are lower than those in Fig. 1a and b. Therefore, the formulation of these two catalysts should be improved in order to reach higher productivity. To describe the role of Al2O3 in the CuZnAl catalyst, Cu/ZnO was prepared by coprecipitation. When these catalysts are compared, the latter shows lower selectivity to acetic acid and a very low surface area. Taking these results into account, it can be suggested that Al2O3 might improve the thermal stability of the Cu-based catalyst. These data are in line with the literature [47]. The CuZrAl catalyst is the most selective to acetic acid, whereas CuZnAl is the most active (Table 2). The catalytic behavior and mechanisms suggested are directly associated with the physicochemical properties of these catalysts. The redox mechanism is related to the presence of ZnO nanoparticles, which promote not only H2O dissociation on the O vacancies (Fig. 5), but also acetaldehyde oxidation, the previous step of acetic acid formation. These steps might occur at the interface of Cu and CuZn alloy with ZnO. The mechanism via ethyl acetate is promoted by the acid and basic properties of the CuZrAl catalyst, which makes the ester generation and hydrolysis feasible [46]. Last but not least, Cu0 and the CuZn alloy are associated with ethanol dehydrogenation to acetaldehyde, the first step of both mechanisms. The Zn-based catalyst is slightly more active in ethanol dehydrogenation. This behavior can be associated with the presence of the CuZn alloy. 4. Conclusions This contribution sheds light on two one-pot mechanisms for the generation of acetic acid from ethanol and H2O. The Zn-based catalyst is composed of ZnO nanoparticles, Cu0, CuZn surface alloy, and Al2O3 whereas CuZrAl shows Cu0, ZrO2, and Al2O3. Both catalysts exhibit the same Cu0 surface. The interfaces of Cu0 with ZnO and CuZn alloy with ZnO are able to dissociate H2O, promoting the redox properties of the CuZnAl catalyst, whereas the Zr-based catalyst almost does not show these properties. The CuZrAl catalyst exhibits higher acidity and basicity than the Zn-based catalyst. These different properties lead to different mechanisms for acetic acid synthesis. Taking the redox mechanism of the WGS reaction into account, the steps of acetic acid synthesis promoted by the Zn-based catalyst can be described as follows: first, ethanol is dehydrogenated on Cu0 and also on CuZn alloy to acetaldehyde; then, this aldehyde is oxidized at the ZnO–Cu0 interface to acetate species, which desorb, forming acetic acid; finally, H2O dissociates on the O vacancies of the ZnO-Cu0 interface and reoxidizes the oxide. The ZnO and CuZn alloy interface should also be considered in the steps of this synthesis. The second mechanism is related to the acetaldehyde and ethanol condensation forming ethyl acetate. This ester is hydrolyzed, synthesizing acetic acid and ethanol.

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Please cite this article as: G. G. Gonzalez, P. C. Zonetti, E. B. Silveira et al., Two mechanisms for acetic acid synthesis from ethanol and water, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.031