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Acetone from ethanol employing ZnxZr1−xO2−y
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Acetone from ethanol employing Znx Zr1−x O2−y Leydi del R. Silva-Calpa a , Priscila C. Zonetti b , Daniela C. de Oliveira c , Roberto R. de Avillez a , Lucia G. Appel b,∗ a Departamento de Engenharia Química e de Materiais, Pontifícia Universidade Católica do Rio de Janeiro, PUC-Rio, rua Marquês de São Vicente 225, Gávea, Rio de Janeiro, 22451-900, RJ, Brazil b Divisão de Catálise e Processos Químicos, Instituto Nacional de Tecnologia, INT, Av. Venezuela 82/518, Rio de Janeiro, 21081-312, RJ, Brazil c Laboratório Nacional de Luz Síncrotron, LNLS, rua Giuseppe Máximo Scolfaro 1000, Campinas, 13083-970, SP, Brazil
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Article history: Received 25 May 2016 Received in revised form 4 August 2016 Accepted 9 September 2016 Available online xxx Keywords: Acetone Ethanol Redox Solid solution Zinc Zirconium
a b s t r a c t The main purpose of this work is to contribute to the description of the acetone synthesis from ethanol employing Znx Zr1-x O2-y based catalysts. The catalytic behavior of these solids was evaluated (isoconversion) in the acetone synthesis. The most active catalyst and m-ZrO2 (used as a reference) were characterized by the following techniques: pyridine adsorption, TPD of NH3 , TPD of CO2 , TPD of ethanol followed by IR (DRIFTS)/MS, TPD of ethanol followed by XANES at the Zr K-edge and Zn K-edge and XRD in situ. The present study suggests that the main steps of the acetone generation from ethanol are the following: firstly, ethoxide species are generated and, then, they are dehydrogenated to acetaldehyde. Both steps are related to strong basic and acid sites. Acetaldehyde reacts with the O of the solid solution generating acetate species and vacancies on the catalyst surface. These carboxylate species condensate (strong basic sites) and generate acetone and CO2 . Water dissociates on the vacancies of the catalyst and reoxidizes the its surface, closing the catalytic cycle. All these steps might occur on Zn+2 and on the species in its vicinity (XANES). © 2016 Elsevier B.V. All rights reserved.
1. Introduction Acetone, a water-white volatile and flammable liquid, is employed in the production of acetone cyanohydrins (ACH), which is a precursor of methyl methacrylate (MMA). This compound is employed in the production of polymers which have innumerous applications. The second largest use of acetone is the manufacture of bisphenol-A, which is used in the polycarbonate sector. Acetone is also employed in the production of methyl isobutyl ketone, isophorone and diacetone alcohol/hexylene glycol. Nowadays, the cumene process is in charge of almost all the acetone production, which is a co-product to phenol. Thus, the production of acetone is dependent on the market demands for phenol. Moreover, the cumene route, a three-step process, not only uses fossil feedstock and shows low yields, but also is highly energy-consuming [1]. Ethanol is produced in huge amounts as a fuel from sugar-cane, corn (1G ethanol) and agriculture residues (2G ethanol). Ethanol is also considered a special platform molecule which is able to generate many “drop-in” chemicals in one-step processes [2,3].
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (L.G. Appel).
Since the first decades of the last century, it is known that acetone can be produced from ethanol [4]. This ketone is an intermediate of the isobutene, isopropanol and propene production from ethanol. Nevertheless, the steps of the acetone synthesis from ethanol are not well established in the literature. In other words, the reaction system related to the ethanol transformation into acetone is a topic for discussion. 2CH3 CH2 OH + H2 O → CH3 COCH3 + CO2 + 4H2 Rodrigues et al. [5] studied the acetone synthesis employing a physical mixture composed of Cu/ZnO/Al2 O3 (CZA) and m-ZrO2 . They proposed the following steps for this synthesis: firstly, ethanol adsorbs on both m-ZrO2 and CZA generating ethoxide species. These species adsorbed on m-ZrO2 migrate to the CZA surface and are dehydrogenated on Cuo to acetaldehyde. This aldehyde migrates to m-ZrO2 and is oxidized to acetate by the superficial O of the oxide. These carboxylates condense producing acetone, CO2 and H2 . Finally, H2 O is dissociated on the Cuo (CZA) surface producing H2 and oxidizing species which then migrate to m-ZrO2 recovering its surface which was reduced by the carboxylates species synthesis. Thus, the role of CZA is not only the generation of acetaldehyde, but also the dissociation of H2 O. This system shows the relevance
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of the redox steps, i.e., the oxidation of acetaldehyde and the H2 O dissociation. Employing Sc/In2 O3 or Y2 O3 -CeO2 , Iwamoto [6] suggested different reactional steps for each catalyst for the acetone synthesis from ethanol. On the one hand, employing Sc/In2 O3 , acetaldehyde is oxidized by water or by the surface hydroxyl groups on this catalyst. The acetic acid generated condenses producing acetone and CO2 . On the other hand, using Y2 O3 -CeO2 , acetaldehyde is converted into ethyl acetate (condensation) and then this ester is decomposed and produces acetic acid and ethene. After that, acetone is generated by the ketonization of acetic acid. Sun et al. [7,8], suggested that acetone is an intermediate of the isobutene generation from ethanol. They employed a Znx Zry Oz mixed oxide, which is very active, not only for the isobutene generation, but also for the acetone synthesis. The authors suggested that acetone is generated by the dehydrogenation of ethanol and aldol-condensation of acetaldehyde. Recently, our group synthesized a superficial solid solution composed of Zr and Zn on m-ZrO2 , i.e., Znx Zr1−x O2−y /ZrO2 , by placing a Zn(NO3 )2 solution in contact with m-ZrO2 . After that, the suspension was filtered, dried and calcined [9]. The XRD, XPS and Raman spectroscopy analyses showed that ZnO was not synthesized. Instead, Zn diffused into the first layers of the m-ZrO2 lattice. The XRD and EPR results indicate the generation of O vacancies when Zn is added to m-ZrO2 . These results showed that Zn replaces Zr in the m-ZrO2 lattice. This occurs due to the similar size of these ions and also the different valences of these elements (Zr+4 and Zn+2 ). It was verified that these O vacancies promote the O mobility, which increases the reducibility of m-ZrO2 and the redox properties of this oxide. All in all, the main purpose of this work is to contribute to the description of the acetone synthesis steps analyzing the role of the redox, basic and acid properties of the catalysts when employing a Znx Zr1−x O2−y system and m-ZrO2 as a reference.
ried out at 400 ◦ C. The catalyst mass was changed in order to reach the isoconversion. In order to identify the intermediates of the reaction the 0.7Zn catalyst was tested at 400 ◦ C using different masses of catalyst (changing the residence time). Some catalytic tests were also carried out employing this same catalyst (50 mg) using different temperatures (350, 375, 400, 425, 450 and 475 ◦ C). All of the others parameters used in these tests are the same as described above. The ethanol conversion was defined as the ratio of the moles of ethanol consumed to the moles of ethanol introduced in 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 performances of m-ZrO2 and 0.7Zn catalysts in the watergas shift reaction (WGS) were evaluated measuring the CO consumption rates under differential conditions (CO conversion <10%) at 450 ◦ C, 1 atm and H2 O/CO = 1. Reagents and products were analyzed by on-line gas chromatography.
2. Experimental
2.3.3. Pyridine adsorption The samples acid properties were evaluated by the adsorption of pyridine followed by IR spectroscopy. The spectra were recorded from thin (∼20 mg) self-supporting wafers using a Nicolet Magna 2000 FT-IR spectrophotometer. The samples were pretreated at 450 ◦ C for 2 h under vacuum and exposed to high vacuum for 30 min (10−7 Torr). Pyridine were adsorbed at 25 ◦ C for 1 h at 2 Torr. Spectra were collected after desorption at 150 ◦ C for 30 min under high vacuum. The absorption at 1445 cm−1 was employed for the calculation of the Lewis acid sites density. The FTIR spectra were divided by the mass of the wafers in order to normalize the results.
2.1. Catalysts synthesis A suspension composed of m-ZrO2 supplied by NORPRO (5 g) and a 75 mL of aqueous solution of Zn(NO3 )2 ·6H2 O were heated at 70 ◦ C under constant stirring for 4 h. After that, these solids were filtered, dried and calcined at 450 ◦ C for 12 h under synthetic air flow. A sample of m-ZrO2 calcined at 450 ◦ C for 12 h was used as a reference. Three different concentrations (0.04, 0.08 and 0.12 M) of the Zn precursor were employed in order to prepare three catalysts, 0.4Zn, 0.6Zn and 0.7Zn, respectively. The numbers 0.4, 0.6 or 0.7 refers to the wt.% of zinc in the catalysts. 2.2. Catalytic tests Catalytic tests were performed 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 PorapakQ/60 using He as the carrier gas. The samples were analyzed every 23 min during 12 h on stream. The catalysts were previously dried at 130 ◦ C with N2 (90 mL min−1 ) for 30 min. Then, the samples were reduced under 10% H2 /N2 flow (100 mL min−1 ) at 450 ◦ C for 1 h. The gas stream composition and flow rate were N2 :H2 O:C2 H5 OH = 91:8:1 mol% and 70 mL min−1 , respectively. Ethanol and H2 O vapors were generated by passing N2 through two saturators, one at 4.9 ◦ C and the other at 53.6 ◦ C, respectively. The catalytic tests at isoconversion (∼ 40%) were car-
2.3. Characterization 2.3.1. Chemical analyses Chemical analyses of Zn and Zr were performed by an inductively- coupled plasma – atomic emission spectrometer (ICPAES), Optima 300DV, Perkin Elmer Instruments, employing the following conditions: plasma air, auxiliary air and Ar for the nebulization flow rates of 15 L min−1 , 0.2 L min−1 , 0.60 L min−1 , respectively and 1400 W of power. 2.3.2. Specific surface area The analyses were conducted employing a Micrometrics ASAP2010. The samples were pre-treated at 100 ◦ C for 24 h, and then submitted to an in situ treatment under vacuum at 150 ◦ C for 2 h. The N2 adsorption occurred at −196 ◦ C.
2.3.4. TPD-CO2 The density of basic sites was determined by temperature programmed desorption of CO2 (TPD-CO2 ). The experiments were carried out using a micro reactor system coupled to a QMS200 Balzers mass quadrupole spectrometer. The samples were treated at 130 ◦ C for 30 min under N2 flow (30 mL min−1 ) and reduced under 10% H2 /N2 flow (50 mL min−1 ) at 450 ◦ C for 1 h. After that, the catalysts were oxidized under 20% O2 /He flow (40 mL min−1 ) at 450 ◦ C for 1 h. The CO2 adsorption was conducted at room temperature for 1 h (25 mL min−1 ). The desorption was performed by heating (10 ◦ C min−1 ) the sample from room temperature up to 450 ◦ C under He flow (50 mL min−1 ). The fragment m/z = 44 was continuously monitored by mass spectrometer. The TPD profiles were decomposed employing Gaussian curves in order to quantify the strength of the basic sites, as previously described by Carvalho et al. [10] The basic weak sites were attributed to a curve, which shows a maximum at temperature lower than 170 ◦ C, medium sites between 170 ◦ C and 270 ◦ C, and strong sites above 270 ◦ C.
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2.3.5. TPD-NH3 The density of acid sites of m-ZrO2 and 0.7Zn was determined by the temperature programmed desorption of ammonia (TPD-NH3 ). The samples (600 mg) were dried at 130 ◦ C for 30 min under N2 flow (90 mL min−1 ). After that, they were reduced under 10% H2 /N2 flow (100 mL min−1 ) at 450 ◦ C (10 ◦ C min−1 ) for 1 h, purged and oxidized under synthetic air flow (90 mL min−1 ) at 450 ◦ C for 30 min. The adsorption of 4% NH3 /He was conducted at 100 ◦ C for 30 min (30 mL min−1 ). The desorption was performed at 10 ◦ C min−1 , under He flow (30 mL min−1 ) from 100 ◦ C to 450 ◦ C. The TPD profiles were decomposed in Gaussian curves in order to quantify the weak, medium and strong acid sites [11]. The weak strength sites of the samples are related to a curve which shows a maximum at temperatures lower than 200 ◦ C, the ones between 200 and 350 ◦ C as medium; and finally, above 350 ◦ C as strong ones. 2.3.6. TPD-ethanol followed by IR and mass spectroscopies The temperature programmed desorption of ethanol (TPDethanol) followed by infrared (DRIFTS) and mass (MS) spectroscopies were carried out using a Nicolet iS50 FT-IR Spectrometer equipped with a MCT/B detector and a diffuse reflectance chamber (Harrick) with ZnSe window. The samples were dried at 130 ◦ C for 30 min under He flow (20 mL min−1 ). Then, they were reduced at 450 ◦ C for 1 h under 22% H2 /He (2 mL min−1 ) flow and purged under He flow for 10 min. After that, the catalysts were oxidized at 450 ◦ C for 30 min under 21% O2 /He flow (20 mL min−1 ). Passing He through an ethanol saturator maintained at 10 ◦ C generated vapor of ethanol. The adsorption of ethanol was conducted at 50 ◦ C for 1 h using an ethanol/He flow (10 mL min−1 ). Ethanol desorption was performed under He flow (40 mL min−1 ) at 20 ◦ C min−1 from 50 ◦ C to 450 ◦ C. The compounds were continuously analyzed by an on-line Dycor Mass Spectrometer, Ametek Process Instruments. The DRIFTS spectra were obtained using 4 cm−1 and 64 scans. They were collected at each 50 ◦ C in the range 50–450 ◦ C. The main fragments followed up during the analyses were: m/z = 2 (H2 ), m/z = 18 (H2 O), m/z = 26 (ethene), m/z = 29 (acetaldehyde), m/z = 31 (ethanol), m/z = 44(CO2 ) and m/z = 43 (acetone). The intensities of these fragments were mathematically treated in order to eliminate contributions of more than one species. 2.3.7. TPD-ethanol followed by XANES X-ray Absorption Near Edge Spectroscopy (XANES) in situ were carried out employing 0.7Zn at the Brazilian Synchrotron Light Laboratory (LNLS) on XAFS2 beamline. The spectra were collected at the Zn K-edge and Zr K-edge employing a double crystal Si 111 monochromator. The sample (100 mg) was dried at 130 ◦ C under He flow (95 mL min−1 ) for 30 min. Then, the catalyst was reduced under 5% H2 /He flow (100 mL min−1 ) at 450 ◦ C for 1 h. After that, the catalyst was purged under He flow and then oxidized at 450 ◦ C for 30 min under 5% O2 /He flow (50 mL min−1 ). The ethanol adsorption was carried out for 1 h at 30 ◦ C using an ethanol/He flow (10 mL min−1 ). The vapor of ethanol was generated passing He through a saturator at 10 ◦ C. The TPD of ethanol was performed under He flow (38 mLmin−1 ) at 3 ◦ C min−1 from 30 ◦ C to 450 ◦ C. The spectra were collected in step-scan mode during the reduction, oxidation, ethanol adsorption and the ethanol desorption steps. The data processing and analysis was performed by the Demeter software package [12]. 2.3.8. TPD-H2 O The temperature programmed desorption of H2 O was carried out using a micro reactor system coupled to a mass spectrometer QMS200 Balzers. The samples were dried at 130 ◦ C for 30 min under He flow (30 mL min−1 ). After that they were reduced at 450 ◦ C for 1 h, 5% H2 /N2 (50 mL min−1 ) and then purged with He for 30 min at 450 ◦ C. The H2 O adsorption was carried out at room temperature
Fig. 1. Selectivities of m-ZrO2 and prepared samples at isoconversion (∼40%). The flow rate, the gas mixture composition and temperature are 70 mL min−1 , N2 :H2 O:C2 H5 OH = 91:8:1 mol% and 400 ◦ C, respectively. The symbol O.7Zn WW is related to the catalytic test carried out employing 0.7Zn at the same conditions described above with adding H2 O (N2 :C2 H5 OH = 99:1 mol%).
for 1 h. The H2 O vapors were generated by passing He (50 mL min−1 through a water saturator at 40 ◦ C. The desorption was carried out employing a He flow (50 mL min−1 ) at 10 ◦ Cmin−1 from 25 to 450 ◦ C. The fragments, m/z = 2 (H2 ), m/z = 32 (O2 ) and m/z = 18 (H2 O) were continuously monitored during the analyses. 2.3.9. XRD (in situ) X-ray diffraction (XRD) was performed using a D8 Advance Bruker diffractometer equipped with an Anton Paar chamber, a CuK␣ radiation source (1.5418 Å) and a Ni filter operated at 40 kV and 40 mA. The angular range varied from 5◦ to 90◦ , with increments of 0.05◦ and 2 s per step. The samples (m-ZrO2 and 0.7Zn) were dried at 130 ◦ C under N2 flow (15 mL min−1 ) for 30 min and reduced under 2% H2 /N2 flow (15 mL min−1 ) at 450 ◦ C for 2 h. After that the catalyst was purged under N2 flow down to room temperature. The diffraction patterns were collected at room temperature after drying and reduction. The diffraction patterns were analyzed using the Rietveld method with fundamental parameters as implemented in the software TOPAS Academic v. 4.1.11. 3. Results and discussion The Zn loading of the three prepared catalysts were analyzed employing ICP-AES. The Zn concentrations of the 0.4Zn, 0.6Zn and 0.7Zn catalysts are 0.4, 0.6 and 0.7 wt.%, respectively. The specific surface areas of both m-ZrO2 and Zn based catalysts were also measured. All the values obtained are in the proximity of 98 m2 g−1 . 3.1. Catalytic tests The selectivities to acetone, ethene, acetaldehyde and CO2 at isoconversion of the Zn catalysts and m-ZrO2 samples are depicted in Fig. 1. Methane and propene were also observed despite their very low concentration. Each one of the four samples generates the same compounds. Nevertheless, m-ZrO2 shows a much higher selectivity to ethene than the others. This fact brings to light that Zn added to m-ZrO2 change the catalytic behavior of this oxide. In spite of the small increase of the acetaldehyde selectivity, which is observed when the Zn concentration increases from 0.4 to 0.6 wt.%, it can be
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Fig. 2. Ethanol conversion and selectivities of 0.7Zn versus temperature. The flow rate, the gas mixture composition and the weight of 0.7Zn are 70 mL min−1 , N2 :H2 O:C2 H5 OH = 91:8:1 mol%, 50 mg, respectively.
inferred that 0.4Zn, 0.6Zn and 0.7Zn show similar selectivities to ethylene, acetone, CO2 and acetaldehyde. The isoconversion tests of the prepared samples (0.4Zn, 0.6Zn and 0.7Zn and m-ZrO2 ) were carried out employing the same experimental conditions except for the catalysts mass. In order to reach 40% of conversion, 50, 40 and 33 mg of 0.4Zn, 0.6Zn and 0.7Zn, were used, respectively. Thus, the lower the Zn concentration, the higher the mass used in these tests. Indeed, the Zn/Zr atomic ratio obtained by chemical analysis and the mass employed in these experiments show a linear correlation (r2 = 0.995). This result reveals a direct association between the catalysts behavior and their Zn content, i.e., the higher the amount of zinc, the most active the catalyst is. The selectivities data of these catalysts suggest that adding Zn to m-ZrO2 there is an increase in the number of active sites without changing significantly the catalytic properties of the catalysts. The 0.7Zn was also tested at isoconversion (∼40%, 33 mg of catalyst) without adding H2 O to the feed (Fig. 1, 0.7Zn WW). The temperature, ethanol concentration and flow rate values were the same which were used in the tests described above. It can be noted that the main products are ethene and acetaldehyde. The selectivity to acetone and CO2 is very low. This result shows that without H2 O the acetone synthesis does not occur. As 0.7Zn showed the highest activity when it was compared with the other catalysts, it was chosen to be analyzed by the techniques described above. Besides that, m-ZrO2 is used as a reference. The ethanol conversion and selectivities versus the reaction temperature for 0.7Zn are shown in Fig. 2. Increasing the temperature the ethanol conversion increases and the selectivity to acetaldehyde decreases (from 375 ◦ C), whereas the one to acetone (until 425 ◦ C) and CO2 increase. The selectivity to acetone shows a maximum at 425 ◦ C. The selectivity to isobutene rises above 425 ◦ C. According to Sun et al. [7] this last compound is generated by the condensation of three molecules of acetone. Some experiments were also carried out increasing the space velocity of the reaction (Fig. 3). It was observed that as the residence time decreases, the conversion of ethanol and the selectivity to acetone and CO2 decrease, whereas the selectivity to acetaldehyde increases. These results indicate that ethanol generates acetaldehyde and then this aldehyde synthesizes acetone, CO2 and H2 . These data highlight that acetaldehyde is an intermediate of this reaction
Fig. 3. Ethanol conversion and selectivities of 0.7Zn versus WHSV. The gas mixture composition and temperature are N2 :H2 O:C2 H5 OH = 91:8:1 mol%, 400 ◦ C, respectively.
and are in line with the results of Fig. 2 and the ones of Rodrigues et al. [5]. 3.2. The acid and basic properties Table 1 exhibits the acid and basic sites densities of the catalysts. On the one hand, when NH3 is used as probe molecule, it can be observed that adding Zn to m-ZrO2 , the density of the strong acid sites decreases considerably, whereas the one of the medium and weak sites increases. On the other hand, the density of the strong basic sites increases, whereas the one of the medium and weak sites decreases (TPD-CO2 ). Only Lewis acid sites were observed when pyridine is employed as a probe of acid sites [13]. In this case, the acid sites density of mZrO2 is around sevenfold higher than the one of the Zn catalyst. This result is similar to the ratio of the strong acid sites densities of mZrO2 to 0.7Zn measured by NH3 . It indicates that the data generated by pyridine are mainly related to the strong acid sites. This might occur due to the basic strength of these probe molecules and also the experimental conditions employed. It is worth stressing that the changes in the acid and basic properties of m-ZrO2 introduced by Zn might contribute to modify the catalytic behavior of this oxide. Acetaldehyde can be synthesized by dehydrogenation of ethanol [14]. The first step of this reaction occurs on an acid-strong base pair site which breaks the O H bond of ethanol and generates an ethoxide species. After that the ␣-hydrogen of the ethoxide group is abstracted by another strong basic site producing acetaldehyde and H2 [14]. Table 1 exhibits that m-ZrO2 and 0.7Zn show strong basic sites and also acid sites. Thus, these oxides are able to dehydrogenate ethanol. Acetaldehyde can also be produced by the oxidative dehydrogenation of ethanol (ODH). This reaction might also contribute to the synthesis of this aldehyde. The density of strong basic sites and redox properties are very important for this synthesis, being the former more relevant [15]. Moreover, according to Rodrigues et al. [5] strong basic sites are also very relevant for the acetate condensation which generates acetone and CO2 . 3.3. TPD – ethanol followed by IR spectroscopy The m-ZrO2 and 0.7Zn were analyzed employing the TPD of ethanol followed by IR and mass spectroscopies. Figs. 4 and 5
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Table 1 Density of weak (W), medium (M), strong (S) and total (T) basic sites and acid sites. Sample
m-ZrO2 0.7Zn
Basic sites density (mol CO2 g−1 )
Acid sites density (mol NH3 g−1 )
Acid sites density (ua g−1 , pyridine)
W+M
S
T
W+M
S
T
T
191 147
46 69
237 216
18 28
25 5
43 33
182 26
Fig. 4. TPD spectra of ethanol over m-ZrO2 (in situ DRIFTS): (a) 150 ◦ C, (b) 250 ◦ C, (c) 300 ◦ C, (d) 350 ◦ C, (e) 400 ◦ C, and (f) 450 ◦ C, respectively.
m-ZrO2 . Both figures also show a band around 1159 cm−1 and a tiny absorption at 1076 cm−1 at low temperature. These bands can be associated with v(CO) vibrations of monodentate and bidentate ethoxide species, respectively. As the temperature increases the high intensity band shifts from ∼1159 cm−1 to ∼1144 cm−1 (see Figs. 4, 5). This indicates that the adsorption sites of the ethoxide species are changing. Other vibrations, 1447 and 1381 cm−1 , related to the ethoxide species can be assigned to ␦as (CH3 ) and ␦s (CH3 ), respectively. On the one hand, the intensities of the ethoxide species bands decrease as the temperature increases. On the other hand, increasing the temperature, the intensity of the bands around 1587, 1442, 1420 cm−1 , assigned to as (OCO), ␦as (CH3 ), s (OCO) of the acetate species, respectively, increase as well [16], being this effect more intense in the case of 0.7Zn. Thus, it can be proposed that ethoxide species are dehydrogenated to acetaldehyde (Fig. 3), which is then oxidized to acetate species. These species condensate promoted by strong basic sites and generate acetone and CO2 [5,17]. Moreover, the bands around 1554 and 1356 cm−1 can the associated with bidentate carbonates [16]. Thus, the acetate species condensate or oxidize producing CO2 , which react with the catalysts producing carbonates. The TPD of ethanol employed He as the carrier gas. Therefore, according to the Mars and Van Krevelen mechanism, the O of the m-ZrO2 or 0.7Zn lattice is the oxidant agent of the generation of acetates and carbonates species during these experiments. Thus, the O consumption of the lattice changes the catalysts surface. Binet et al., [18] showed that the v(CO) vibrations of the monodentate ethoxide species adsorbed on CeO2 shift to lower frequencies when oxygen is eliminated from the surface. Therefore, the shifts observed in the case of the v(CO) vibrations of the monodentate ethoxide species (∼1159 cm−1 to ∼1144 cm−1 ) can be associated with removal of the superficial O of the m-ZrO2 and 0.7Zn surfaces. Moreover, as well known, DRIFTS measurements are not quantitative. However, it can be observed that the intensity of the acetate species vibrations of 0.7Zn is much higher than the ones of m-ZrO2 . The insertion of Zn in the m-ZrO2 lattice promotes its redox properties rendering 0.7Zn more active for the oxidation of the oxygenate species (see Introduction). It is worth stressing that ethene is not observed in the m-ZrO2 DRIFTS spectra due to the low intensity of its absorption band.
3.4. TPD – ethanol followed by mass spectroscopy
Fig. 5. TPD spectra of ethanol over 0.7Zn (in situ DRIFTS): (a) 150 ◦ C, (b) 250 ◦ C, (c) 300 ◦ C, (d) 350 ◦ C, (e) 400 ◦ C, and (f) 450 ◦ C, respectively.
exhibit the DRIFTS spectra of 0.7Zn and m-ZrO2 , respectively. These figures show bands at 1256 cm−1 . These peaks can be assigned to ␦as (OH) of ethanol. As the temperature increases, the intensity of these bands decrease. This effect is more relevant in the case of 0.7Zn due to the fact that this catalyst is more active than
The spectra of ethanol, acetone, acetaldehyde, ethene, H2 and CO2 of the TPD of ethanol followed by mass spectrometry (MS) are depicted in Fig. 6. Both catalysts show spectra which exhibit similar shapes except for the one of acetaldehyde. In spite of the different catalytic behavior of 0.7Zn and m-ZrO2 (Fig. 1), these results suggest that the phenomena that occur on these catalysts during the TPD of ethanol are almost the same. The first step of the acetone synthesis is the generation of acetaldehyde [5] (Fig. 3). The spectra of this aldehyde for both catalysts depict that, at low temperature (temperature < ∼280 ◦ C), it is entirely consumed generating acetone and CO2 . It is worth stressing that at this temperature there is ethanol adsorbed on the catalysts. Therefore, the generation of acetalde-
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ARTICLE IN PRESS L.d.R. Silva-Calpa et al. / Catalysis Today xxx (2016) xxx–xxx Table 2 Amount of H2 produced during the TPD-H2 O and CO consumption rate of WGS at 450 ◦ C (-rCO ).
Fig. 6. TPD spectra of ethanol over m-ZrO2 and 0.7Zn (analyzed by MS).
hyde seems to be the slower step of the acetone synthesis at these low temperatures for both catalysts. Some catalytic tests were carried out employing m-ZrO2 and 0.7Zn at the same experimental conditions used in the isoconversion tests (Fig. 1), except for the high values of the residence times and low temperatures (400 mg, 40 mL min−1 , 250 ◦ C). Both catalysts generate almost only acetaldehyde at very low conversions, which are much smaller than the equilibrium conversions (thermodynamic data of the dehydrogenation of ethanol [19]. These results suggest that the acetaldehyde synthesis is a slow step under these conditions (at low temperature). Both the acetone and CO2 spectra of the 0.7Zn catalyst show two peaks (Fig. 6). The same occurs for the acetone and CO2 spectra of the m-ZrO2 catalyst. The first peaks of the CO2 and acetone spectra are almost at the same temperature for both catalysts. However, when the ratio of the intensities of CO2 to the acetone peaks for the 0.7Zn catalyst (low temperature peaks) is compared with the same ratio for the m-ZrO2 catalyst, it can be observed that the former is much higher. The same result can be verified for the bands of CO2 and acetone at high temperature. It can be proposed that the 0.7Zn catalyst promotes the total oxidation of the acetates (CO2 high intensity peaks), which is in line with its improved redox properties (see Introduction, [9]). Oxygen was not admitted in the system during the TPD of ethanol. Thus, it can be suggested that the O of the 0.7Zn and mZrO2 catalysts surfaces are consumed by the synthesis of acetone and total oxidation. The ethoxide species shifts exhibited by the DRIFTS spectra (see Figs. 4 and 5) are related to the O elimination from the catalysts. Moreover, after the acetone and CO2 desorption first peaks, as the surface of the catalysts are reduced, the ketone synthesis is almost extinguished. Zirconia synthesizes a larger amount of ethene at a slightly lower temperature than 0.7Zn (see MS spectra). This result is in line with the catalytic results (Fig. 1). According to Di Cosimo [14], ethene can be synthesized by the E2 mechanism. It is related to the simultaneous abstraction of OH and -H by a pair of acid and basic Lewis sites, respectively. It is known that the higher the strength of the acid sites, the higher the rate to the ethylene synthesis. Both m-ZrO2 and 0.7Zn show strong acid sites (Table 1). However, the one with Zn exhibits a much lower density of these sites. Thus, the catalytic behavior related to the ethylene synthesis of these two catalysts (Fig. 1) is associated with their density of strong acid sites depicted on Table 1. The dehydration of ethanol generates ethene and H2 O. The H2 O dissociation on CeO2 (reduced) is well described in the literature
samples
H2 molgcat −1
m-ZrO2 0.7Zn
0.68 1.33
−rCO molgcat −1 min−1 20 75
[20,21]. When H2 O is adsorbed on the O vacancies it generates two hydroxyl species, which react producing H2 and two O on the surface of CeO2 , thus, oxidizing its surface. Previous work of our group showed that 0.7Zn and m-ZrO2 are also able to dissociate H2 O, being the 0.7Zn catalyst the most efficient one due to its higher amount of vacancies [9]. The TPD of water was carried out employing these samples, 0.7Zn and m-ZrO2 . Two broad peaks of H2 were observed with their maxima around 100 and 160 ◦ C for 0.7Zn and m-ZrO2 , respectively. Table 2 depicts that 0.7Zn generates much more H2 than m-ZrO2 . Therefore, the H2 spectra of the TPD-ethanol of both catalysts can be associated with the dissociation of H2 O. Moreover, the H2 peak intensity of 0.7Zn is much higher than the one of m-ZrO2 , which is in line with Table 2 results. Thus, after the first peaks of acetone and CO2 , the H2 O dissociation reoxidizes the catalysts and the reaction occurs again (CO2 and acetone peaks). Fig. 1 exhibits that without H2 O there is a very low selectivity to acetone synthesis. This happens because without the reoxidation of the catalyst, the catalytic cycle does not close and the reaction cannot occur. Both samples also show that the generation of H2 occurs before the ethene desorption (see the dots). This occurs because the dehydrogenation of acetaldehyde also generates H2 [14]. The different shapes of the acetaldehyde spectra might be associated with the different consumption rates of this aldehyde as an intermediate species. The acetaldehyde MS signal of m-ZrO2 emerges at 345 ◦ C, between the two peaks of CO2 or acetone peaks. At this temperature the surface of the catalyst is reduced. As a consequence, the acetaldehyde oxidation rate is very low and this aldehyde is desorbed. In the case of the 0.7Zn spectrum, a shoulder can be observed at around this same temperature. However, the maximum of the acetaldehyde spectrum is at 450 ◦ C, which is the same temperature of the CO2 and acetone desorption peaks (high temperature). At the temperature of 450 ◦ C acetaldehyde might be also synthesized by the oxidative dehydrogenation reaction. This result suggests that at high temperature the acetaldehyde generation is not the slower step of the acetone synthesis, but rather the oxidation of acetaldehyde. 3.5. The WGS reaction as a model reaction The redox [22] and associative (carboxyl [23] and formate [24]) mechanisms have been proposed for the water gas shift (WGS) reaction employing Pt or other metals supported on reducible oxides. Zonetti et al. [25] verified that some oxides (without metal) promote the reverse water gas shift reaction (RWGS) and follow the redox mechanism. Considering the principle of the microscopic reversibility, this might also occur in the case of the WGS reaction. Thus, the WGS reaction on oxides can be described as following: initially, CO is oxidized to CO2 by the O of the oxide lattice; then, this catalyst is reduced generating O vacancies; finally, H2 O dissociates on this vacancies, re-oxidizing the oxide and producing H2 . The slower step of this mechanism is the reduction of the catalyst. Indeed, the oxidation step of acetaldehyde to acetate species is very similar to the WGS reaction (CO + H2 O → CO2 + H2 ). Both are related to the oxidation of C O species using H2 O as an oxidant agent. Thus, it can be proposed that the WGS reaction can be employed as a model reaction for probing the redox properties of the catalysts employed in the acetone synthesis.
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Fig. 7. TPD spectra of ethanol over 0.7Zn (XANES at Zn K-edge): (a) initial sample, (b) sample at the end of the reduction, (c) sample at the end of the oxidation, (d) sample at the end of ethanol adsorption, (e) sample at the end of ethanol desorption, (f) ZnO reference and (g) Zn foil reference; [inset] figure shows the relative intensity on the white line region.
Table 2 exhibits the CO consumption rates of m-ZrO2 and 0.7Zn in the WGS reaction. The 0.7Zn catalyst shows a higher consumption rate of CO. It is almost fourfold higher than m-ZrO2 . It oxidizes CO faster because it shows higher reducibility [25]. Considering the similarity of the CO oxidation with the acetaldehyde oxidation, it can be suggested that 0.7Zn is more active for this acetaldehyde oxidation. This result is in line with the TPD-ethanol that shows that 0.7Zn is very active for acetaldehyde selective and total oxidation. 3.6. TPD – ethanol followed by XANES Fig. 7 shows the experimental XANES at Zn K-edge of 0.7Zn during the TPD of ethanol (after normalization procedure). The spectra a, b, c, d, e, f and g are related to (a) initial sample, (b) sample at the end of the reduction, (c) sample at the end of the oxidation, (d) sample at the end of the ethanol adsorption, (e) sample at the end of the ethanol desorption, (f) ZnO reference and (g) Zn foil reference. The [inset] figure shows the relative intensities of the spectra in the white line region. The position of the absorption edge in the XANES spectra reveals the oxidation state of Zn. The Zn◦ atoms (Fig. 7g) exhibits the edge at 9659 eV, whereas the one of Zn2+ (ZnO, Fig. 7f) at 9662 eV, which is ascribed to a 1s → 4p electronic transition of Zn2+ ions [26,27]. The Zn K-edge position of 0.7Zn is observed at ∼9662.4 eV at the TPD of ethanol. No energy shifts were verified during the TPD. The value of the absorption edge is equivalent to the one of ZnO (zincite), indicating that Zn oxidation state in this catalyst is Zn2+ . Zinc in the zincite (Zn2+ ions) is in a tetrahedral geometry. However, the spectrum shape of 0.7Zn (Fig. 7a–e) cannot be designated as ZnO (see Fig. 7f). Thus, it can be suggested that Zn2+ is a component of a solid solution or it is in the hydrozincite phase, which shows a mixture of tetrahedral and octahedral sites [28]. Indeed, this result is in line with our previous study which proposed that: firstly, Zn+2 is in a solid solution in the superficial layers of m-ZrO2 (see Introduction) and secondly that there is no ZnO in the 0.7Zn sample [9]. The TPD of ethanol on 0.7Zn spectra indicated that the superficial O of 0.7Zn is employed to oxidize ethanol to acetone or more specifically acetaldehyde to acetate species and CO2 . However, the XANES spectra show that Zn+2 is not reduced. The spectra depicted in Fig. 7a–e are very similar. However, some changes can be observed on the white line intensity. When
Fig. 8. XANES spectra at Zr K-edge collected at the beginning (a) and at the end (b) of the TPD of ethanol.
0.7Zn is reduced during the pretreatment (Fig. 7b), a decrease in the white line intensity when compared with Fig. 7a is observed. According to Waychunas et al. [28], this change is associated with the protonation of Zn (Zn-O → Zn-OH). Recently, Schimming et al. [29] proved that the CeO2 vacancies are able to dissociated H2 . Thus, it might be suggested that H2 can also be dissociated on the 0.7Zn catalyst vacancies generating hydroxyl species on the remaining O of the 0.7Zn surface. The catalyst reoxidation (Fig. 7c) slightly decreases the protonation of its surface. The spectra depicted on Fig. 7d and e were collected after the ethanol adsorption and desorption (TPD of ethanol), respectively. Both spectra show a decrease of the white line intensity which can be associated with the adsorption of oxygenate species on Zn+2 [30,31]. The XANES spectra at the Zr K-edge of 0.7Zn were also collected during the TPD of ethanol on the 0.7Zn catalyst (Fig. 8). These spectra show features that can be assigned to the monoclinic m-ZrO2 [31]. The XANES spectra at the Zr K-edge of 0.7Zn catalyst did not change after ethanol adsorption and desorption procedures. On the one hand, the XANES at Zn K-edge of 0.7Zn spectra show changes in the intensity of the white line which are associated with the interaction of some species with Zn during or after the reduction step and TPD of ethanol. On the other hand, XANES at Zr K-edge spectra of the same catalyst submitted to these same experimental procedures do not change. This result shows that the TPD −ethanol occurs on Zn+2 and its vicinity and also that Zr+4 species show very low activity. Indeed, Fig. 1 shows that a small amount of Zn is able to drop the selectivity of ethylene from 68% to 12%. This behavior can be associated not only with changes in the acidity of the catalyst promoted by Zn, but also to the increase of redox properties which promote the acetone synthesis. This result shows that the active sites of 0.7Zn are Zn+2 and its vicinity. Indeed, the catalytic test (isoconversion tests) are in line with the XANES results which shows that adding Zn to m-ZrO2 increases in number of active sites without changing significantly the catalytic properties of the catalysts. Moreover, XANES at Zr K-edge and XANES at Zn K-edge spectra not only confirm the solid solution formation, but also exhibit that it
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is stable during the TPD of ethanol. Some spectra collected during the acetone synthesis (reaction) confirm the stability of the solid solution.
3.7. The XRD “in situ” The catalysts 0.7Zn and m-ZrO2 both oxidized and reduced in situ were analyzed by XRD (see Table S1, Supporting information). All the four samples show m-ZrO2 and small amount of t-ZrO2 . The ZnO was not observed. The addition of Zn to m-ZrO2 significantly changes the lattice parameters of this oxide, which can be better observed considering the cell volume values. The Rietveld refinement is able to model the substitution of Zr by Zn atoms. It was also observed that there is an increase of the oxygen vacancies concentration when Zn is added to m-ZrO2 . Considering the very small amount of tetragonal phase in the samples, it was not possible to infer if Zn substitutes Zr in this phase. Table S1 showed that reduction treatments at temperatures lower than 450 ◦ C did not change the crystallographic characteristics of the studied samples. This result is in line which the XANES ones.
4. Conclusion The present study suggests that the main steps of the acetone generation from ethanol are the following: firstly, ethoxide species are generated and, then, they are dehydrogenated to acetaldehyde. Both steps are related to strong basic and acid sites. According to the Mars and Van Krevelen mechanism, acetaldehyde reacts with the O of the solid solution surface generating acetate species and vacancies on the catalyst surface. These carboxylate species condensate (strong basic sites) and generate acetone and CO2 . Water dissociates on the vacancies of the catalyst and reoxidizes the catalyst surface, closing the catalytic cycle. All these steps might occur on Zn+2 and on the species in its vicinity. Acknowledgments The authors are indebted to Eng. Johnatan Celnik and the LACAT/INT staff for their assistance in the experimental work. Brazilian Synchrotron Light Laboratory (LNLS) is gratefully acknowledged for the use of XAFS2 beamline installation (Project no. XAFS1-17747). The financial support of FAPERJ, CNPq and CAPES (Brazil) is also acknowledged. Appendix A. Supplementary data
3.8. Final comments On the one hand the XANES and XRD results show that the reduction of 0.7Zn by H2 or acetaldehyde does not significantly change the monoclinic structure. On the other hand, it is possible to infer considering the TPD-ethanol results that superficial O of 0.7Zn is consumed by the redox process. According to McFarland and Metiu [32], the majority of the oxidation reactions on oxides follow the Mars-Van Krevelen mechanism. It can be described as following: reductant is oxidized by the oxygen atoms from the oxide surface layer; this compound is then desorbed and oxygen vacancies are generated. It is well known that the reoxidation of the surface is fast. Thus, the rate-limiting step is the oxidation of the reductant by the O of the oxide surface (the reduction of the surface). According to these authors, any modification which can facilitate the removal of the surface oxygen will increase the activity of the catalyst. Since oxygen is an electrophilic species, it can be easily removed from the surface layer when an electron deficit at the surface is created. This deficit can be produced by the generation of solid solutions, i.e., replacing cations of the host oxide with others with lower valence. When Zn is added to m-ZrO2, the phenomenon described by McFarland and Metiu might occur. The MS and IR spectra of the TPD of ethanol indicate that the O of the catalyst lattice is employed in the oxidation of acetaldehyde to acetate species. Adding Zn to the m-ZrO2 lattice, the rate of the redox step is improved due to the increase of the reducibility of the catalyst. Increasing the acetaldehyde consumption, the ethanol conversion to this aldehyde increases as well. The decrease of the strong acid sites density when Zn is added to m-ZrO2 leads to a lower selectivity to ethylene promoting the generation of acetaldehyde at high temperatures. The Znx Zr1−x O2−y (solid solution) vacancies are in charge of the H2 O dissociation which reoxidizes the catalyst. Strong basic sites are very important for the acetaldehyde generation and the condensation of acetate species. The XANES at Zn K-edge spectra not only confirm the synthesis of the solid solution, but also indicate that Zn and the species at its vicinity are the sites where the reaction steps might occur. The XRD and XANES at Zn K-edge spectra show that during the TPD of ethanol, despite the elimination of the oxygen of the catalyst surface, there are no changes in the crystalline structure of this solid solution.
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Acetone from ethanol employing Znx Zr1−x O2−y Leydi del R. Silva-Calpa a , Priscila C. Zonetti b , Daniela C. de Oliveira c , Roberto R. de Avillez a , Lucia G. Appel b,∗ a Departamento de Engenharia Química e de Materiais, Pontifícia Universidade Católica do Rio de Janeiro, PUC-Rio, rua Marquês de São Vicente 225, Gávea, Rio de Janeiro, 22451-900, RJ, Brazil b Divisão de Catálise e Processos Químicos, Instituto Nacional de Tecnologia, INT, Av. Venezuela 82/518, Rio de Janeiro, 21081-312, RJ, Brazil c Laboratório Nacional de Luz Síncrotron, LNLS, rua Giuseppe Máximo Scolfaro 1000, Campinas, 13083-970, SP, Brazil
a r t i c l e
i n f o
Article history: Received 25 May 2016 Received in revised form 4 August 2016 Accepted 9 September 2016 Available online xxx Keywords: Acetone Ethanol Redox Solid solution Zinc Zirconium
a b s t r a c t The main purpose of this work is to contribute to the description of the acetone synthesis from ethanol employing Znx Zr1-x O2-y based catalysts. The catalytic behavior of these solids was evaluated (isoconversion) in the acetone synthesis. The most active catalyst and m-ZrO2 (used as a reference) were characterized by the following techniques: pyridine adsorption, TPD of NH3 , TPD of CO2 , TPD of ethanol followed by IR (DRIFTS)/MS, TPD of ethanol followed by XANES at the Zr K-edge and Zn K-edge and XRD in situ. The present study suggests that the main steps of the acetone generation from ethanol are the following: firstly, ethoxide species are generated and, then, they are dehydrogenated to acetaldehyde. Both steps are related to strong basic and acid sites. Acetaldehyde reacts with the O of the solid solution generating acetate species and vacancies on the catalyst surface. These carboxylate species condensate (strong basic sites) and generate acetone and CO2 . Water dissociates on the vacancies of the catalyst and reoxidizes the its surface, closing the catalytic cycle. All these steps might occur on Zn+2 and on the species in its vicinity (XANES). © 2016 Elsevier B.V. All rights reserved.
1. Introduction Acetone, a water-white volatile and flammable liquid, is employed in the production of acetone cyanohydrins (ACH), which is a precursor of methyl methacrylate (MMA). This compound is employed in the production of polymers which have innumerous applications. The second largest use of acetone is the manufacture of bisphenol-A, which is used in the polycarbonate sector. Acetone is also employed in the production of methyl isobutyl ketone, isophorone and diacetone alcohol/hexylene glycol. Nowadays, the cumene process is in charge of almost all the acetone production, which is a co-product to phenol. Thus, the production of acetone is dependent on the market demands for phenol. Moreover, the cumene route, a three-step process, not only uses fossil feedstock and shows low yields, but also is highly energy-consuming [1]. Ethanol is produced in huge amounts as a fuel from sugar-cane, corn (1G ethanol) and agriculture residues (2G ethanol). Ethanol is also considered a special platform molecule which is able to generate many “drop-in” chemicals in one-step processes [2,3].
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (L.G. Appel).
Since the first decades of the last century, it is known that acetone can be produced from ethanol [4]. This ketone is an intermediate of the isobutene, isopropanol and propene production from ethanol. Nevertheless, the steps of the acetone synthesis from ethanol are not well established in the literature. In other words, the reaction system related to the ethanol transformation into acetone is a topic for discussion. 2CH3 CH2 OH + H2 O → CH3 COCH3 + CO2 + 4H2 Rodrigues et al. [5] studied the acetone synthesis employing a physical mixture composed of Cu/ZnO/Al2 O3 (CZA) and m-ZrO2 . They proposed the following steps for this synthesis: firstly, ethanol adsorbs on both m-ZrO2 and CZA generating ethoxide species. These species adsorbed on m-ZrO2 migrate to the CZA surface and are dehydrogenated on Cuo to acetaldehyde. This aldehyde migrates to m-ZrO2 and is oxidized to acetate by the superficial O of the oxide. These carboxylates condense producing acetone, CO2 and H2 . Finally, H2 O is dissociated on the Cuo (CZA) surface producing H2 and oxidizing species which then migrate to m-ZrO2 recovering its surface which was reduced by the carboxylates species synthesis. Thus, the role of CZA is not only the generation of acetaldehyde, but also the dissociation of H2 O. This system shows the relevance
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of the redox steps, i.e., the oxidation of acetaldehyde and the H2 O dissociation. Employing Sc/In2 O3 or Y2 O3 -CeO2 , Iwamoto [6] suggested different reactional steps for each catalyst for the acetone synthesis from ethanol. On the one hand, employing Sc/In2 O3 , acetaldehyde is oxidized by water or by the surface hydroxyl groups on this catalyst. The acetic acid generated condenses producing acetone and CO2 . On the other hand, using Y2 O3 -CeO2 , acetaldehyde is converted into ethyl acetate (condensation) and then this ester is decomposed and produces acetic acid and ethene. After that, acetone is generated by the ketonization of acetic acid. Sun et al. [7,8], suggested that acetone is an intermediate of the isobutene generation from ethanol. They employed a Znx Zry Oz mixed oxide, which is very active, not only for the isobutene generation, but also for the acetone synthesis. The authors suggested that acetone is generated by the dehydrogenation of ethanol and aldol-condensation of acetaldehyde. Recently, our group synthesized a superficial solid solution composed of Zr and Zn on m-ZrO2 , i.e., Znx Zr1−x O2−y /ZrO2 , by placing a Zn(NO3 )2 solution in contact with m-ZrO2 . After that, the suspension was filtered, dried and calcined [9]. The XRD, XPS and Raman spectroscopy analyses showed that ZnO was not synthesized. Instead, Zn diffused into the first layers of the m-ZrO2 lattice. The XRD and EPR results indicate the generation of O vacancies when Zn is added to m-ZrO2 . These results showed that Zn replaces Zr in the m-ZrO2 lattice. This occurs due to the similar size of these ions and also the different valences of these elements (Zr+4 and Zn+2 ). It was verified that these O vacancies promote the O mobility, which increases the reducibility of m-ZrO2 and the redox properties of this oxide. All in all, the main purpose of this work is to contribute to the description of the acetone synthesis steps analyzing the role of the redox, basic and acid properties of the catalysts when employing a Znx Zr1−x O2−y system and m-ZrO2 as a reference.
ried out at 400 ◦ C. The catalyst mass was changed in order to reach the isoconversion. In order to identify the intermediates of the reaction the 0.7Zn catalyst was tested at 400 ◦ C using different masses of catalyst (changing the residence time). Some catalytic tests were also carried out employing this same catalyst (50 mg) using different temperatures (350, 375, 400, 425, 450 and 475 ◦ C). All of the others parameters used in these tests are the same as described above. The ethanol conversion was defined as the ratio of the moles of ethanol consumed to the moles of ethanol introduced in 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 performances of m-ZrO2 and 0.7Zn catalysts in the watergas shift reaction (WGS) were evaluated measuring the CO consumption rates under differential conditions (CO conversion <10%) at 450 ◦ C, 1 atm and H2 O/CO = 1. Reagents and products were analyzed by on-line gas chromatography.
2. Experimental
2.3.3. Pyridine adsorption The samples acid properties were evaluated by the adsorption of pyridine followed by IR spectroscopy. The spectra were recorded from thin (∼20 mg) self-supporting wafers using a Nicolet Magna 2000 FT-IR spectrophotometer. The samples were pretreated at 450 ◦ C for 2 h under vacuum and exposed to high vacuum for 30 min (10−7 Torr). Pyridine were adsorbed at 25 ◦ C for 1 h at 2 Torr. Spectra were collected after desorption at 150 ◦ C for 30 min under high vacuum. The absorption at 1445 cm−1 was employed for the calculation of the Lewis acid sites density. The FTIR spectra were divided by the mass of the wafers in order to normalize the results.
2.1. Catalysts synthesis A suspension composed of m-ZrO2 supplied by NORPRO (5 g) and a 75 mL of aqueous solution of Zn(NO3 )2 ·6H2 O were heated at 70 ◦ C under constant stirring for 4 h. After that, these solids were filtered, dried and calcined at 450 ◦ C for 12 h under synthetic air flow. A sample of m-ZrO2 calcined at 450 ◦ C for 12 h was used as a reference. Three different concentrations (0.04, 0.08 and 0.12 M) of the Zn precursor were employed in order to prepare three catalysts, 0.4Zn, 0.6Zn and 0.7Zn, respectively. The numbers 0.4, 0.6 or 0.7 refers to the wt.% of zinc in the catalysts. 2.2. Catalytic tests Catalytic tests were performed 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 PorapakQ/60 using He as the carrier gas. The samples were analyzed every 23 min during 12 h on stream. The catalysts were previously dried at 130 ◦ C with N2 (90 mL min−1 ) for 30 min. Then, the samples were reduced under 10% H2 /N2 flow (100 mL min−1 ) at 450 ◦ C for 1 h. The gas stream composition and flow rate were N2 :H2 O:C2 H5 OH = 91:8:1 mol% and 70 mL min−1 , respectively. Ethanol and H2 O vapors were generated by passing N2 through two saturators, one at 4.9 ◦ C and the other at 53.6 ◦ C, respectively. The catalytic tests at isoconversion (∼ 40%) were car-
2.3. Characterization 2.3.1. Chemical analyses Chemical analyses of Zn and Zr were performed by an inductively- coupled plasma – atomic emission spectrometer (ICPAES), Optima 300DV, Perkin Elmer Instruments, employing the following conditions: plasma air, auxiliary air and Ar for the nebulization flow rates of 15 L min−1 , 0.2 L min−1 , 0.60 L min−1 , respectively and 1400 W of power. 2.3.2. Specific surface area The analyses were conducted employing a Micrometrics ASAP2010. The samples were pre-treated at 100 ◦ C for 24 h, and then submitted to an in situ treatment under vacuum at 150 ◦ C for 2 h. The N2 adsorption occurred at −196 ◦ C.
2.3.4. TPD-CO2 The density of basic sites was determined by temperature programmed desorption of CO2 (TPD-CO2 ). The experiments were carried out using a micro reactor system coupled to a QMS200 Balzers mass quadrupole spectrometer. The samples were treated at 130 ◦ C for 30 min under N2 flow (30 mL min−1 ) and reduced under 10% H2 /N2 flow (50 mL min−1 ) at 450 ◦ C for 1 h. After that, the catalysts were oxidized under 20% O2 /He flow (40 mL min−1 ) at 450 ◦ C for 1 h. The CO2 adsorption was conducted at room temperature for 1 h (25 mL min−1 ). The desorption was performed by heating (10 ◦ C min−1 ) the sample from room temperature up to 450 ◦ C under He flow (50 mL min−1 ). The fragment m/z = 44 was continuously monitored by mass spectrometer. The TPD profiles were decomposed employing Gaussian curves in order to quantify the strength of the basic sites, as previously described by Carvalho et al. [10] The basic weak sites were attributed to a curve, which shows a maximum at temperature lower than 170 ◦ C, medium sites between 170 ◦ C and 270 ◦ C, and strong sites above 270 ◦ C.
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2.3.5. TPD-NH3 The density of acid sites of m-ZrO2 and 0.7Zn was determined by the temperature programmed desorption of ammonia (TPD-NH3 ). The samples (600 mg) were dried at 130 ◦ C for 30 min under N2 flow (90 mL min−1 ). After that, they were reduced under 10% H2 /N2 flow (100 mL min−1 ) at 450 ◦ C (10 ◦ C min−1 ) for 1 h, purged and oxidized under synthetic air flow (90 mL min−1 ) at 450 ◦ C for 30 min. The adsorption of 4% NH3 /He was conducted at 100 ◦ C for 30 min (30 mL min−1 ). The desorption was performed at 10 ◦ C min−1 , under He flow (30 mL min−1 ) from 100 ◦ C to 450 ◦ C. The TPD profiles were decomposed in Gaussian curves in order to quantify the weak, medium and strong acid sites [11]. The weak strength sites of the samples are related to a curve which shows a maximum at temperatures lower than 200 ◦ C, the ones between 200 and 350 ◦ C as medium; and finally, above 350 ◦ C as strong ones. 2.3.6. TPD-ethanol followed by IR and mass spectroscopies The temperature programmed desorption of ethanol (TPDethanol) followed by infrared (DRIFTS) and mass (MS) spectroscopies were carried out using a Nicolet iS50 FT-IR Spectrometer equipped with a MCT/B detector and a diffuse reflectance chamber (Harrick) with ZnSe window. The samples were dried at 130 ◦ C for 30 min under He flow (20 mL min−1 ). Then, they were reduced at 450 ◦ C for 1 h under 22% H2 /He (2 mL min−1 ) flow and purged under He flow for 10 min. After that, the catalysts were oxidized at 450 ◦ C for 30 min under 21% O2 /He flow (20 mL min−1 ). Passing He through an ethanol saturator maintained at 10 ◦ C generated vapor of ethanol. The adsorption of ethanol was conducted at 50 ◦ C for 1 h using an ethanol/He flow (10 mL min−1 ). Ethanol desorption was performed under He flow (40 mL min−1 ) at 20 ◦ C min−1 from 50 ◦ C to 450 ◦ C. The compounds were continuously analyzed by an on-line Dycor Mass Spectrometer, Ametek Process Instruments. The DRIFTS spectra were obtained using 4 cm−1 and 64 scans. They were collected at each 50 ◦ C in the range 50–450 ◦ C. The main fragments followed up during the analyses were: m/z = 2 (H2 ), m/z = 18 (H2 O), m/z = 26 (ethene), m/z = 29 (acetaldehyde), m/z = 31 (ethanol), m/z = 44(CO2 ) and m/z = 43 (acetone). The intensities of these fragments were mathematically treated in order to eliminate contributions of more than one species. 2.3.7. TPD-ethanol followed by XANES X-ray Absorption Near Edge Spectroscopy (XANES) in situ were carried out employing 0.7Zn at the Brazilian Synchrotron Light Laboratory (LNLS) on XAFS2 beamline. The spectra were collected at the Zn K-edge and Zr K-edge employing a double crystal Si 111 monochromator. The sample (100 mg) was dried at 130 ◦ C under He flow (95 mL min−1 ) for 30 min. Then, the catalyst was reduced under 5% H2 /He flow (100 mL min−1 ) at 450 ◦ C for 1 h. After that, the catalyst was purged under He flow and then oxidized at 450 ◦ C for 30 min under 5% O2 /He flow (50 mL min−1 ). The ethanol adsorption was carried out for 1 h at 30 ◦ C using an ethanol/He flow (10 mL min−1 ). The vapor of ethanol was generated passing He through a saturator at 10 ◦ C. The TPD of ethanol was performed under He flow (38 mLmin−1 ) at 3 ◦ C min−1 from 30 ◦ C to 450 ◦ C. The spectra were collected in step-scan mode during the reduction, oxidation, ethanol adsorption and the ethanol desorption steps. The data processing and analysis was performed by the Demeter software package [12]. 2.3.8. TPD-H2 O The temperature programmed desorption of H2 O was carried out using a micro reactor system coupled to a mass spectrometer QMS200 Balzers. The samples were dried at 130 ◦ C for 30 min under He flow (30 mL min−1 ). After that they were reduced at 450 ◦ C for 1 h, 5% H2 /N2 (50 mL min−1 ) and then purged with He for 30 min at 450 ◦ C. The H2 O adsorption was carried out at room temperature
Fig. 1. Selectivities of m-ZrO2 and prepared samples at isoconversion (∼40%). The flow rate, the gas mixture composition and temperature are 70 mL min−1 , N2 :H2 O:C2 H5 OH = 91:8:1 mol% and 400 ◦ C, respectively. The symbol O.7Zn WW is related to the catalytic test carried out employing 0.7Zn at the same conditions described above with adding H2 O (N2 :C2 H5 OH = 99:1 mol%).
for 1 h. The H2 O vapors were generated by passing He (50 mL min−1 through a water saturator at 40 ◦ C. The desorption was carried out employing a He flow (50 mL min−1 ) at 10 ◦ Cmin−1 from 25 to 450 ◦ C. The fragments, m/z = 2 (H2 ), m/z = 32 (O2 ) and m/z = 18 (H2 O) were continuously monitored during the analyses. 2.3.9. XRD (in situ) X-ray diffraction (XRD) was performed using a D8 Advance Bruker diffractometer equipped with an Anton Paar chamber, a CuK␣ radiation source (1.5418 Å) and a Ni filter operated at 40 kV and 40 mA. The angular range varied from 5◦ to 90◦ , with increments of 0.05◦ and 2 s per step. The samples (m-ZrO2 and 0.7Zn) were dried at 130 ◦ C under N2 flow (15 mL min−1 ) for 30 min and reduced under 2% H2 /N2 flow (15 mL min−1 ) at 450 ◦ C for 2 h. After that the catalyst was purged under N2 flow down to room temperature. The diffraction patterns were collected at room temperature after drying and reduction. The diffraction patterns were analyzed using the Rietveld method with fundamental parameters as implemented in the software TOPAS Academic v. 4.1.11. 3. Results and discussion The Zn loading of the three prepared catalysts were analyzed employing ICP-AES. The Zn concentrations of the 0.4Zn, 0.6Zn and 0.7Zn catalysts are 0.4, 0.6 and 0.7 wt.%, respectively. The specific surface areas of both m-ZrO2 and Zn based catalysts were also measured. All the values obtained are in the proximity of 98 m2 g−1 . 3.1. Catalytic tests The selectivities to acetone, ethene, acetaldehyde and CO2 at isoconversion of the Zn catalysts and m-ZrO2 samples are depicted in Fig. 1. Methane and propene were also observed despite their very low concentration. Each one of the four samples generates the same compounds. Nevertheless, m-ZrO2 shows a much higher selectivity to ethene than the others. This fact brings to light that Zn added to m-ZrO2 change the catalytic behavior of this oxide. In spite of the small increase of the acetaldehyde selectivity, which is observed when the Zn concentration increases from 0.4 to 0.6 wt.%, it can be
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Fig. 2. Ethanol conversion and selectivities of 0.7Zn versus temperature. The flow rate, the gas mixture composition and the weight of 0.7Zn are 70 mL min−1 , N2 :H2 O:C2 H5 OH = 91:8:1 mol%, 50 mg, respectively.
inferred that 0.4Zn, 0.6Zn and 0.7Zn show similar selectivities to ethylene, acetone, CO2 and acetaldehyde. The isoconversion tests of the prepared samples (0.4Zn, 0.6Zn and 0.7Zn and m-ZrO2 ) were carried out employing the same experimental conditions except for the catalysts mass. In order to reach 40% of conversion, 50, 40 and 33 mg of 0.4Zn, 0.6Zn and 0.7Zn, were used, respectively. Thus, the lower the Zn concentration, the higher the mass used in these tests. Indeed, the Zn/Zr atomic ratio obtained by chemical analysis and the mass employed in these experiments show a linear correlation (r2 = 0.995). This result reveals a direct association between the catalysts behavior and their Zn content, i.e., the higher the amount of zinc, the most active the catalyst is. The selectivities data of these catalysts suggest that adding Zn to m-ZrO2 there is an increase in the number of active sites without changing significantly the catalytic properties of the catalysts. The 0.7Zn was also tested at isoconversion (∼40%, 33 mg of catalyst) without adding H2 O to the feed (Fig. 1, 0.7Zn WW). The temperature, ethanol concentration and flow rate values were the same which were used in the tests described above. It can be noted that the main products are ethene and acetaldehyde. The selectivity to acetone and CO2 is very low. This result shows that without H2 O the acetone synthesis does not occur. As 0.7Zn showed the highest activity when it was compared with the other catalysts, it was chosen to be analyzed by the techniques described above. Besides that, m-ZrO2 is used as a reference. The ethanol conversion and selectivities versus the reaction temperature for 0.7Zn are shown in Fig. 2. Increasing the temperature the ethanol conversion increases and the selectivity to acetaldehyde decreases (from 375 ◦ C), whereas the one to acetone (until 425 ◦ C) and CO2 increase. The selectivity to acetone shows a maximum at 425 ◦ C. The selectivity to isobutene rises above 425 ◦ C. According to Sun et al. [7] this last compound is generated by the condensation of three molecules of acetone. Some experiments were also carried out increasing the space velocity of the reaction (Fig. 3). It was observed that as the residence time decreases, the conversion of ethanol and the selectivity to acetone and CO2 decrease, whereas the selectivity to acetaldehyde increases. These results indicate that ethanol generates acetaldehyde and then this aldehyde synthesizes acetone, CO2 and H2 . These data highlight that acetaldehyde is an intermediate of this reaction
Fig. 3. Ethanol conversion and selectivities of 0.7Zn versus WHSV. The gas mixture composition and temperature are N2 :H2 O:C2 H5 OH = 91:8:1 mol%, 400 ◦ C, respectively.
and are in line with the results of Fig. 2 and the ones of Rodrigues et al. [5]. 3.2. The acid and basic properties Table 1 exhibits the acid and basic sites densities of the catalysts. On the one hand, when NH3 is used as probe molecule, it can be observed that adding Zn to m-ZrO2 , the density of the strong acid sites decreases considerably, whereas the one of the medium and weak sites increases. On the other hand, the density of the strong basic sites increases, whereas the one of the medium and weak sites decreases (TPD-CO2 ). Only Lewis acid sites were observed when pyridine is employed as a probe of acid sites [13]. In this case, the acid sites density of mZrO2 is around sevenfold higher than the one of the Zn catalyst. This result is similar to the ratio of the strong acid sites densities of mZrO2 to 0.7Zn measured by NH3 . It indicates that the data generated by pyridine are mainly related to the strong acid sites. This might occur due to the basic strength of these probe molecules and also the experimental conditions employed. It is worth stressing that the changes in the acid and basic properties of m-ZrO2 introduced by Zn might contribute to modify the catalytic behavior of this oxide. Acetaldehyde can be synthesized by dehydrogenation of ethanol [14]. The first step of this reaction occurs on an acid-strong base pair site which breaks the O H bond of ethanol and generates an ethoxide species. After that the ␣-hydrogen of the ethoxide group is abstracted by another strong basic site producing acetaldehyde and H2 [14]. Table 1 exhibits that m-ZrO2 and 0.7Zn show strong basic sites and also acid sites. Thus, these oxides are able to dehydrogenate ethanol. Acetaldehyde can also be produced by the oxidative dehydrogenation of ethanol (ODH). This reaction might also contribute to the synthesis of this aldehyde. The density of strong basic sites and redox properties are very important for this synthesis, being the former more relevant [15]. Moreover, according to Rodrigues et al. [5] strong basic sites are also very relevant for the acetate condensation which generates acetone and CO2 . 3.3. TPD – ethanol followed by IR spectroscopy The m-ZrO2 and 0.7Zn were analyzed employing the TPD of ethanol followed by IR and mass spectroscopies. Figs. 4 and 5
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Table 1 Density of weak (W), medium (M), strong (S) and total (T) basic sites and acid sites. Sample
m-ZrO2 0.7Zn
Basic sites density (mol CO2 g−1 )
Acid sites density (mol NH3 g−1 )
Acid sites density (ua g−1 , pyridine)
W+M
S
T
W+M
S
T
T
191 147
46 69
237 216
18 28
25 5
43 33
182 26
Fig. 4. TPD spectra of ethanol over m-ZrO2 (in situ DRIFTS): (a) 150 ◦ C, (b) 250 ◦ C, (c) 300 ◦ C, (d) 350 ◦ C, (e) 400 ◦ C, and (f) 450 ◦ C, respectively.
m-ZrO2 . Both figures also show a band around 1159 cm−1 and a tiny absorption at 1076 cm−1 at low temperature. These bands can be associated with v(CO) vibrations of monodentate and bidentate ethoxide species, respectively. As the temperature increases the high intensity band shifts from ∼1159 cm−1 to ∼1144 cm−1 (see Figs. 4, 5). This indicates that the adsorption sites of the ethoxide species are changing. Other vibrations, 1447 and 1381 cm−1 , related to the ethoxide species can be assigned to ␦as (CH3 ) and ␦s (CH3 ), respectively. On the one hand, the intensities of the ethoxide species bands decrease as the temperature increases. On the other hand, increasing the temperature, the intensity of the bands around 1587, 1442, 1420 cm−1 , assigned to as (OCO), ␦as (CH3 ), s (OCO) of the acetate species, respectively, increase as well [16], being this effect more intense in the case of 0.7Zn. Thus, it can be proposed that ethoxide species are dehydrogenated to acetaldehyde (Fig. 3), which is then oxidized to acetate species. These species condensate promoted by strong basic sites and generate acetone and CO2 [5,17]. Moreover, the bands around 1554 and 1356 cm−1 can the associated with bidentate carbonates [16]. Thus, the acetate species condensate or oxidize producing CO2 , which react with the catalysts producing carbonates. The TPD of ethanol employed He as the carrier gas. Therefore, according to the Mars and Van Krevelen mechanism, the O of the m-ZrO2 or 0.7Zn lattice is the oxidant agent of the generation of acetates and carbonates species during these experiments. Thus, the O consumption of the lattice changes the catalysts surface. Binet et al., [18] showed that the v(CO) vibrations of the monodentate ethoxide species adsorbed on CeO2 shift to lower frequencies when oxygen is eliminated from the surface. Therefore, the shifts observed in the case of the v(CO) vibrations of the monodentate ethoxide species (∼1159 cm−1 to ∼1144 cm−1 ) can be associated with removal of the superficial O of the m-ZrO2 and 0.7Zn surfaces. Moreover, as well known, DRIFTS measurements are not quantitative. However, it can be observed that the intensity of the acetate species vibrations of 0.7Zn is much higher than the ones of m-ZrO2 . The insertion of Zn in the m-ZrO2 lattice promotes its redox properties rendering 0.7Zn more active for the oxidation of the oxygenate species (see Introduction). It is worth stressing that ethene is not observed in the m-ZrO2 DRIFTS spectra due to the low intensity of its absorption band.
3.4. TPD – ethanol followed by mass spectroscopy
Fig. 5. TPD spectra of ethanol over 0.7Zn (in situ DRIFTS): (a) 150 ◦ C, (b) 250 ◦ C, (c) 300 ◦ C, (d) 350 ◦ C, (e) 400 ◦ C, and (f) 450 ◦ C, respectively.
exhibit the DRIFTS spectra of 0.7Zn and m-ZrO2 , respectively. These figures show bands at 1256 cm−1 . These peaks can be assigned to ␦as (OH) of ethanol. As the temperature increases, the intensity of these bands decrease. This effect is more relevant in the case of 0.7Zn due to the fact that this catalyst is more active than
The spectra of ethanol, acetone, acetaldehyde, ethene, H2 and CO2 of the TPD of ethanol followed by mass spectrometry (MS) are depicted in Fig. 6. Both catalysts show spectra which exhibit similar shapes except for the one of acetaldehyde. In spite of the different catalytic behavior of 0.7Zn and m-ZrO2 (Fig. 1), these results suggest that the phenomena that occur on these catalysts during the TPD of ethanol are almost the same. The first step of the acetone synthesis is the generation of acetaldehyde [5] (Fig. 3). The spectra of this aldehyde for both catalysts depict that, at low temperature (temperature < ∼280 ◦ C), it is entirely consumed generating acetone and CO2 . It is worth stressing that at this temperature there is ethanol adsorbed on the catalysts. Therefore, the generation of acetalde-
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Fig. 6. TPD spectra of ethanol over m-ZrO2 and 0.7Zn (analyzed by MS).
hyde seems to be the slower step of the acetone synthesis at these low temperatures for both catalysts. Some catalytic tests were carried out employing m-ZrO2 and 0.7Zn at the same experimental conditions used in the isoconversion tests (Fig. 1), except for the high values of the residence times and low temperatures (400 mg, 40 mL min−1 , 250 ◦ C). Both catalysts generate almost only acetaldehyde at very low conversions, which are much smaller than the equilibrium conversions (thermodynamic data of the dehydrogenation of ethanol [19]. These results suggest that the acetaldehyde synthesis is a slow step under these conditions (at low temperature). Both the acetone and CO2 spectra of the 0.7Zn catalyst show two peaks (Fig. 6). The same occurs for the acetone and CO2 spectra of the m-ZrO2 catalyst. The first peaks of the CO2 and acetone spectra are almost at the same temperature for both catalysts. However, when the ratio of the intensities of CO2 to the acetone peaks for the 0.7Zn catalyst (low temperature peaks) is compared with the same ratio for the m-ZrO2 catalyst, it can be observed that the former is much higher. The same result can be verified for the bands of CO2 and acetone at high temperature. It can be proposed that the 0.7Zn catalyst promotes the total oxidation of the acetates (CO2 high intensity peaks), which is in line with its improved redox properties (see Introduction, [9]). Oxygen was not admitted in the system during the TPD of ethanol. Thus, it can be suggested that the O of the 0.7Zn and mZrO2 catalysts surfaces are consumed by the synthesis of acetone and total oxidation. The ethoxide species shifts exhibited by the DRIFTS spectra (see Figs. 4 and 5) are related to the O elimination from the catalysts. Moreover, after the acetone and CO2 desorption first peaks, as the surface of the catalysts are reduced, the ketone synthesis is almost extinguished. Zirconia synthesizes a larger amount of ethene at a slightly lower temperature than 0.7Zn (see MS spectra). This result is in line with the catalytic results (Fig. 1). According to Di Cosimo [14], ethene can be synthesized by the E2 mechanism. It is related to the simultaneous abstraction of OH and -H by a pair of acid and basic Lewis sites, respectively. It is known that the higher the strength of the acid sites, the higher the rate to the ethylene synthesis. Both m-ZrO2 and 0.7Zn show strong acid sites (Table 1). However, the one with Zn exhibits a much lower density of these sites. Thus, the catalytic behavior related to the ethylene synthesis of these two catalysts (Fig. 1) is associated with their density of strong acid sites depicted on Table 1. The dehydration of ethanol generates ethene and H2 O. The H2 O dissociation on CeO2 (reduced) is well described in the literature
samples
H2 molgcat −1
m-ZrO2 0.7Zn
0.68 1.33
−rCO molgcat −1 min−1 20 75
[20,21]. When H2 O is adsorbed on the O vacancies it generates two hydroxyl species, which react producing H2 and two O on the surface of CeO2 , thus, oxidizing its surface. Previous work of our group showed that 0.7Zn and m-ZrO2 are also able to dissociate H2 O, being the 0.7Zn catalyst the most efficient one due to its higher amount of vacancies [9]. The TPD of water was carried out employing these samples, 0.7Zn and m-ZrO2 . Two broad peaks of H2 were observed with their maxima around 100 and 160 ◦ C for 0.7Zn and m-ZrO2 , respectively. Table 2 depicts that 0.7Zn generates much more H2 than m-ZrO2 . Therefore, the H2 spectra of the TPD-ethanol of both catalysts can be associated with the dissociation of H2 O. Moreover, the H2 peak intensity of 0.7Zn is much higher than the one of m-ZrO2 , which is in line with Table 2 results. Thus, after the first peaks of acetone and CO2 , the H2 O dissociation reoxidizes the catalysts and the reaction occurs again (CO2 and acetone peaks). Fig. 1 exhibits that without H2 O there is a very low selectivity to acetone synthesis. This happens because without the reoxidation of the catalyst, the catalytic cycle does not close and the reaction cannot occur. Both samples also show that the generation of H2 occurs before the ethene desorption (see the dots). This occurs because the dehydrogenation of acetaldehyde also generates H2 [14]. The different shapes of the acetaldehyde spectra might be associated with the different consumption rates of this aldehyde as an intermediate species. The acetaldehyde MS signal of m-ZrO2 emerges at 345 ◦ C, between the two peaks of CO2 or acetone peaks. At this temperature the surface of the catalyst is reduced. As a consequence, the acetaldehyde oxidation rate is very low and this aldehyde is desorbed. In the case of the 0.7Zn spectrum, a shoulder can be observed at around this same temperature. However, the maximum of the acetaldehyde spectrum is at 450 ◦ C, which is the same temperature of the CO2 and acetone desorption peaks (high temperature). At the temperature of 450 ◦ C acetaldehyde might be also synthesized by the oxidative dehydrogenation reaction. This result suggests that at high temperature the acetaldehyde generation is not the slower step of the acetone synthesis, but rather the oxidation of acetaldehyde. 3.5. The WGS reaction as a model reaction The redox [22] and associative (carboxyl [23] and formate [24]) mechanisms have been proposed for the water gas shift (WGS) reaction employing Pt or other metals supported on reducible oxides. Zonetti et al. [25] verified that some oxides (without metal) promote the reverse water gas shift reaction (RWGS) and follow the redox mechanism. Considering the principle of the microscopic reversibility, this might also occur in the case of the WGS reaction. Thus, the WGS reaction on oxides can be described as following: initially, CO is oxidized to CO2 by the O of the oxide lattice; then, this catalyst is reduced generating O vacancies; finally, H2 O dissociates on this vacancies, re-oxidizing the oxide and producing H2 . The slower step of this mechanism is the reduction of the catalyst. Indeed, the oxidation step of acetaldehyde to acetate species is very similar to the WGS reaction (CO + H2 O → CO2 + H2 ). Both are related to the oxidation of C O species using H2 O as an oxidant agent. Thus, it can be proposed that the WGS reaction can be employed as a model reaction for probing the redox properties of the catalysts employed in the acetone synthesis.
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Fig. 7. TPD spectra of ethanol over 0.7Zn (XANES at Zn K-edge): (a) initial sample, (b) sample at the end of the reduction, (c) sample at the end of the oxidation, (d) sample at the end of ethanol adsorption, (e) sample at the end of ethanol desorption, (f) ZnO reference and (g) Zn foil reference; [inset] figure shows the relative intensity on the white line region.
Table 2 exhibits the CO consumption rates of m-ZrO2 and 0.7Zn in the WGS reaction. The 0.7Zn catalyst shows a higher consumption rate of CO. It is almost fourfold higher than m-ZrO2 . It oxidizes CO faster because it shows higher reducibility [25]. Considering the similarity of the CO oxidation with the acetaldehyde oxidation, it can be suggested that 0.7Zn is more active for this acetaldehyde oxidation. This result is in line with the TPD-ethanol that shows that 0.7Zn is very active for acetaldehyde selective and total oxidation. 3.6. TPD – ethanol followed by XANES Fig. 7 shows the experimental XANES at Zn K-edge of 0.7Zn during the TPD of ethanol (after normalization procedure). The spectra a, b, c, d, e, f and g are related to (a) initial sample, (b) sample at the end of the reduction, (c) sample at the end of the oxidation, (d) sample at the end of the ethanol adsorption, (e) sample at the end of the ethanol desorption, (f) ZnO reference and (g) Zn foil reference. The [inset] figure shows the relative intensities of the spectra in the white line region. The position of the absorption edge in the XANES spectra reveals the oxidation state of Zn. The Zn◦ atoms (Fig. 7g) exhibits the edge at 9659 eV, whereas the one of Zn2+ (ZnO, Fig. 7f) at 9662 eV, which is ascribed to a 1s → 4p electronic transition of Zn2+ ions [26,27]. The Zn K-edge position of 0.7Zn is observed at ∼9662.4 eV at the TPD of ethanol. No energy shifts were verified during the TPD. The value of the absorption edge is equivalent to the one of ZnO (zincite), indicating that Zn oxidation state in this catalyst is Zn2+ . Zinc in the zincite (Zn2+ ions) is in a tetrahedral geometry. However, the spectrum shape of 0.7Zn (Fig. 7a–e) cannot be designated as ZnO (see Fig. 7f). Thus, it can be suggested that Zn2+ is a component of a solid solution or it is in the hydrozincite phase, which shows a mixture of tetrahedral and octahedral sites [28]. Indeed, this result is in line with our previous study which proposed that: firstly, Zn+2 is in a solid solution in the superficial layers of m-ZrO2 (see Introduction) and secondly that there is no ZnO in the 0.7Zn sample [9]. The TPD of ethanol on 0.7Zn spectra indicated that the superficial O of 0.7Zn is employed to oxidize ethanol to acetone or more specifically acetaldehyde to acetate species and CO2 . However, the XANES spectra show that Zn+2 is not reduced. The spectra depicted in Fig. 7a–e are very similar. However, some changes can be observed on the white line intensity. When
Fig. 8. XANES spectra at Zr K-edge collected at the beginning (a) and at the end (b) of the TPD of ethanol.
0.7Zn is reduced during the pretreatment (Fig. 7b), a decrease in the white line intensity when compared with Fig. 7a is observed. According to Waychunas et al. [28], this change is associated with the protonation of Zn (Zn-O → Zn-OH). Recently, Schimming et al. [29] proved that the CeO2 vacancies are able to dissociated H2 . Thus, it might be suggested that H2 can also be dissociated on the 0.7Zn catalyst vacancies generating hydroxyl species on the remaining O of the 0.7Zn surface. The catalyst reoxidation (Fig. 7c) slightly decreases the protonation of its surface. The spectra depicted on Fig. 7d and e were collected after the ethanol adsorption and desorption (TPD of ethanol), respectively. Both spectra show a decrease of the white line intensity which can be associated with the adsorption of oxygenate species on Zn+2 [30,31]. The XANES spectra at the Zr K-edge of 0.7Zn were also collected during the TPD of ethanol on the 0.7Zn catalyst (Fig. 8). These spectra show features that can be assigned to the monoclinic m-ZrO2 [31]. The XANES spectra at the Zr K-edge of 0.7Zn catalyst did not change after ethanol adsorption and desorption procedures. On the one hand, the XANES at Zn K-edge of 0.7Zn spectra show changes in the intensity of the white line which are associated with the interaction of some species with Zn during or after the reduction step and TPD of ethanol. On the other hand, XANES at Zr K-edge spectra of the same catalyst submitted to these same experimental procedures do not change. This result shows that the TPD −ethanol occurs on Zn+2 and its vicinity and also that Zr+4 species show very low activity. Indeed, Fig. 1 shows that a small amount of Zn is able to drop the selectivity of ethylene from 68% to 12%. This behavior can be associated not only with changes in the acidity of the catalyst promoted by Zn, but also to the increase of redox properties which promote the acetone synthesis. This result shows that the active sites of 0.7Zn are Zn+2 and its vicinity. Indeed, the catalytic test (isoconversion tests) are in line with the XANES results which shows that adding Zn to m-ZrO2 increases in number of active sites without changing significantly the catalytic properties of the catalysts. Moreover, XANES at Zr K-edge and XANES at Zn K-edge spectra not only confirm the solid solution formation, but also exhibit that it
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is stable during the TPD of ethanol. Some spectra collected during the acetone synthesis (reaction) confirm the stability of the solid solution.
3.7. The XRD “in situ” The catalysts 0.7Zn and m-ZrO2 both oxidized and reduced in situ were analyzed by XRD (see Table S1, Supporting information). All the four samples show m-ZrO2 and small amount of t-ZrO2 . The ZnO was not observed. The addition of Zn to m-ZrO2 significantly changes the lattice parameters of this oxide, which can be better observed considering the cell volume values. The Rietveld refinement is able to model the substitution of Zr by Zn atoms. It was also observed that there is an increase of the oxygen vacancies concentration when Zn is added to m-ZrO2 . Considering the very small amount of tetragonal phase in the samples, it was not possible to infer if Zn substitutes Zr in this phase. Table S1 showed that reduction treatments at temperatures lower than 450 ◦ C did not change the crystallographic characteristics of the studied samples. This result is in line which the XANES ones.
4. Conclusion The present study suggests that the main steps of the acetone generation from ethanol are the following: firstly, ethoxide species are generated and, then, they are dehydrogenated to acetaldehyde. Both steps are related to strong basic and acid sites. According to the Mars and Van Krevelen mechanism, acetaldehyde reacts with the O of the solid solution surface generating acetate species and vacancies on the catalyst surface. These carboxylate species condensate (strong basic sites) and generate acetone and CO2 . Water dissociates on the vacancies of the catalyst and reoxidizes the catalyst surface, closing the catalytic cycle. All these steps might occur on Zn+2 and on the species in its vicinity. Acknowledgments The authors are indebted to Eng. Johnatan Celnik and the LACAT/INT staff for their assistance in the experimental work. Brazilian Synchrotron Light Laboratory (LNLS) is gratefully acknowledged for the use of XAFS2 beamline installation (Project no. XAFS1-17747). The financial support of FAPERJ, CNPq and CAPES (Brazil) is also acknowledged. Appendix A. Supplementary data
3.8. Final comments On the one hand the XANES and XRD results show that the reduction of 0.7Zn by H2 or acetaldehyde does not significantly change the monoclinic structure. On the other hand, it is possible to infer considering the TPD-ethanol results that superficial O of 0.7Zn is consumed by the redox process. According to McFarland and Metiu [32], the majority of the oxidation reactions on oxides follow the Mars-Van Krevelen mechanism. It can be described as following: reductant is oxidized by the oxygen atoms from the oxide surface layer; this compound is then desorbed and oxygen vacancies are generated. It is well known that the reoxidation of the surface is fast. Thus, the rate-limiting step is the oxidation of the reductant by the O of the oxide surface (the reduction of the surface). According to these authors, any modification which can facilitate the removal of the surface oxygen will increase the activity of the catalyst. Since oxygen is an electrophilic species, it can be easily removed from the surface layer when an electron deficit at the surface is created. This deficit can be produced by the generation of solid solutions, i.e., replacing cations of the host oxide with others with lower valence. When Zn is added to m-ZrO2, the phenomenon described by McFarland and Metiu might occur. The MS and IR spectra of the TPD of ethanol indicate that the O of the catalyst lattice is employed in the oxidation of acetaldehyde to acetate species. Adding Zn to the m-ZrO2 lattice, the rate of the redox step is improved due to the increase of the reducibility of the catalyst. Increasing the acetaldehyde consumption, the ethanol conversion to this aldehyde increases as well. The decrease of the strong acid sites density when Zn is added to m-ZrO2 leads to a lower selectivity to ethylene promoting the generation of acetaldehyde at high temperatures. The Znx Zr1−x O2−y (solid solution) vacancies are in charge of the H2 O dissociation which reoxidizes the catalyst. Strong basic sites are very important for the acetaldehyde generation and the condensation of acetate species. The XANES at Zn K-edge spectra not only confirm the synthesis of the solid solution, but also indicate that Zn and the species at its vicinity are the sites where the reaction steps might occur. The XRD and XANES at Zn K-edge spectra show that during the TPD of ethanol, despite the elimination of the oxygen of the catalyst surface, there are no changes in the crystalline structure of this solid solution.
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