Synthesis of Propene from Ethanol: A Mechanistic ... - ACS Publications

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The synthesis of propene from ethanol: a mechanistic study Caio Matheus, Luciano Honorato Chagas, Guilherme Gonzalez, Eduardo Falabella Sousa-Aguiar, and Lucia G. Appel ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01727 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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The synthesis of propene from ethanol: a mechanistic study Caio R. V. Matheus1,2, Luciano H. Chagas1, Guilherme G. Gonzalez1,3, Eduardo Falabella S. Aguiar2, Lucia G. Appel1* 1

Instituto Nacional de Tecnologia, Divisão de Catálise e Processos Químicos, Laboratório de

Catálise, Rio de Janeiro, RJ, 20081-312, Brazil. 2

Escola de Química, Centro de Tecnologia, UFRJ, Rio de Janeiro, Rj, 21941-972, Brazil.

3

Universidade Estadual do Rio de Janeiro, Departamento de Engenharia Química e Petróleo,

Laboratório de Catálise, Processos e Meio Ambiente Rio de Janeiro, RJ, 20550-900, Brazil. * mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Physical mixtures comprising AgCeO2 and t-ZrO2 or MgO were employed as catalysts for the generation of propene from ethanol in presence of water. The catalysts were characterized by means of several techniques such as XRD, N2 physical adsorption, isopropanol conversion and ethanol to acetone MPV model reactions, NH3-TPD, CO2-TPD and ethanol-TPD followed by DRIFTS and MS. Acid and strong basic sites, redox properties and the capacity to conduct both water dissociation and MPV reduction are important characteristics for this reaction. Water is the oxidant agent; however, water may also affect the acid-basic sites of the oxides, as demonstrated when MgO was used. The physical mixture comprising t-ZrO2 and AgCeO2 is active on the ethanol conversion to propene, presenting the following steps: firstly, ethanol is oxidized to acetaldehyde; after that, it is oxidized to acetate species, which condensate producing acetone. This ketone reacts with ethanol (MPV) generating isopropanol and acetaldehyde, which is oxidized to acetate and undergoes the same sequence described above. Finally, isopropanol is dehydrated to propene. At low temperatures (~300 ºC), generation of acetone seems to be the rate determining step of the reaction, while at higher temperatures (>450 ºC), MPV becomes the determining step. At these high temperatures dehydration predominates. Thus, there is an optimum temperature range for propene synthesis. The acetaldehyde synthesis via MPV and its participation on the principal reaction path makes propene synthesis a special cascade reaction. This paper presents broader perspective for the propene generation from ethanol, shedding light on the mechanism of this rather complex process.

KEYWORDS: propene; ethanol; MPV; acetone; ZrO2, CeO2


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1. Introduction The shift from fossil raw materials to renewable ones is one of the most important themes of research in heterogeneous catalysis nowadays1–3. Employed mainly as biofuel and gasoline additive, ethanol is produced in huge amounts in Brazil and in the USA4. It is widely known that the production of chemicals is associated with that of fuels and, in this context, ethanol can be very relevant. Indeed, it is a platform molecule, suitable to produce many chemicals in one-pot employing multifunctional catalysts4 (Scheme 1). Many of these compounds are named “dropin”, which means they can be readily replaced in the chemical industry, substituting for those produced from fossil feedstocks5.

Scheme 1. Chemicals from ethanol Propene is the second most important petrochemical commodity. It is employed in the synthesis of many chemicals, such as propylene oxide, n-butanol, isobutanol, butanediol, isopropanol, acrylic acid, acrylate esters, oxo-alcohols, acrylonitrile, cumene and, especially, polypropylene6–9. It is worth stressing that the propene production growth rate has been sustained


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at about 4% a year10. Although traditional processes (mainly steam cracking and catalytic fluid cracking) have been capable of meeting its demand so far, the change of the feedstock of these processes from naphtha or liquefied petroleum gas (LPG) to shale gas, mainly in the US, may compromise the production of this olefin. Its growing demand and the concern about environmental issues are driving forces for the development of new sustainable processes focused on the propene production6,7. Some routes for the propene synthesis are under investigation, such as ethanol to olefins (ETO)11 and the metathesis of 2-butene and ethene, both produced from ethanol12. Nevertheless, some problems are of usual concern in these processes, such as the high production of other olefins and heavier compounds, as well as the coke formation with the consequent blockage of the active sites leading to catalyst deactivation13. Iwamoto´s2,8,14,15pioneer studies related to the generation of propene from ethanol pointed to a new more promising path: the acetone route (Scheme 2). At first, ethanol undergoes dehydrogenation, producing acetaldehyde. This aldehyde is converted into acetate species which condensate (ketonization reaction) to produce acetone. Finally, this ketone is hydrogenated and then dehydrated to yield propene. Two active and selective catalysts were proposed for the propene synthesis: Sc/In2O314 and Y2O3 and CeO2 solid solution8. Our group studied the first step of this synthesis i.e., the one-pot acetone generation from ethanol. Some promising catalytic systems were identified such as the Cu/Zn/Al2O3+ZrO2 physical mixture16, ZrO2 doped with Zn17 and CeO2 doped with Ag (AgCeO2)1. The last one is the most active and selective to acetone, being its performance associated with redox properties. The first step of the propene synthesis is the dehydrogenation7 or the oxidative dehydrogenation of ethanol to acetaldehyde1. The second one, the synthesis of acetate species,


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may be carried out by the acetaldehyde oxidation, which might follow the Mars Van Krevelen mechanism (MVK)1,14,18. An oxygen of the catalyst lattice oxidizes the acetaldehyde generating an oxygen vacancy. After that, water dissociates on this vacancy reoxidizing the catalyst. According to some authors, acetate species can also be synthesized by the acetaldehyde condensation

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leading to ethyl acetate. This ester may be decomposed into ethene and acetate

species 2,7,8 or hydrolyzed to ethanol and acetate species19. It was also proposed that acetaldehyde can be oxidized by hydroxyl species of the catalyst surface7,19. After that, hydrogenation may occur either by hydrogen addition2,7 or by a hydrogen transfer from ethanol to acetone known as the Meerwein-Ponndorf-Verley (MPV) reduction7,8. Finally, the isopropanol formed is dehydrated to propene8. As can be verified, despite the information already available, many reaction steps related to the acetone synthesis from ethanol have not been well established yet.

Scheme 2. Reaction steps in the ethanol conversion to propene


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Recent results obtained in our laboratory show that a physical mixture containing AgCeO2 and t-ZrO2 converts ethanol into propene at high selectivities with an important feature: a low production of ethene. It was also observed that acetone is an intermediate of this synthesis. The composition of this physical mixture is based on the Iwamoto´s works which are related to the MPV hydrogenation7,15 and performance of the AgCeO2 catalyst in the acetone synthesis1. Physical mixtures of catalysts are simple and effective tools to describe cascade reactions. These systems were employed by our group in several reactions, such as: the one-step ethyl acetate synthesis from ethanol20, the methanation reactions21 and the dimethyl ether synthesis22. Thus, the main objective of this work is to describe the steps of the synthesis of propene from ethanol employing a physical mixture of AgCeO2 and t-ZrO2 or MgO. Also, the physicochemical properties of the catalysts which are relevant for this synthesis will be presented. Eventually, the thermodynamics analysis of the system, model reactions and ethanolTPD followed by FTIR and MS will be used in order to support the proposed mechanism of the reaction.

2. Experimental Section

Catalysts The AgCeO2 catalyst was synthesized by precipitation using [(NH4)2Ce(NO3)6] (SigmaAldrich) as precursor. A strong base (NH4OH – Sigma-Aldrich) aqueous solution was added dropwise to a [(NH4)2Ce(NO3)6] solution under continuous stirring at room temperature in order to synthesize CeO2. The solid obtained was filtered and washed with distilled water until neutral pH. Finally, it was calcined at 500 °C for 1 h at 10 °Cmin−1. The resulting powder was submitted to a wetness impregnation using an aqueous solution of AgNO3 (Sigma-Aldrich) in order to


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produce a catalyst with 0.02% w/w of Ag. The solid was dried overnight at 100 °C, calcined at 500 °C for 4 h under flow of synthetic air at 3 °Cmin-1 and milled and sieved with a 90 µm mesh. Tetragonal zirconia (t-ZrO2) was supplied by Saint-Gobain NorPro. It was calcined at 400 °C for 4 h and at 500 °C for 20 h at 10 °Cmin-1 under air flow (30 mLmin-1), milled and sieved with a 90 µm mesh. MgO was prepared by the precipitation technique. A solution containing 1.6 M of both K2CO3 and KOH (Sigma-Aldrich) was slowly added to a solution of Mg(NO3)2 (Sigma-Aldrich) and deionized water under stirring. The system was kept with a thermal bath at 70 °C. After precipitation, the precipitate was allowed to stand at ambient temperature to age, washed and dried at 120 °C overnight. Magnesia was calcined at 400 °C for 4 h (10 °Cmin-1) under air flow (30 mLmin-1), milled and sieved with a 90 µm mesh. Binary physical mixtures comprising AgCeO2 and t-ZrO2 or MgO were also employed in this research. Different amounts of the sieved oxides were gently mixed in a small becker with a glass stick. These mixtures were named PM1X-C/Z, when employing AgCeO2 and t-ZrO2, and PM2-C/M, when containing AgCeO2 and MgO. The letters C, Z and M represent, respectively, the masses (g) of AgCeO2, t-ZrO2 and MgO in the physical mixtures, while X represents the different mixtures compositions. This way, four systems were prepared: PM11-50/25, PM1250/50, PM13-100/25 and PM2-50/25. Another system named DB, comprising a layer of AgCeO2 over t-ZrO2, separated by a glass wool in a double bed configuration, was also employed in the catalytic test.

Characterization


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The BET specific surface areas (SBET) of the catalysts were obtained by N2 physisorption at -196 °C employing a Micromeritics ASAP 2010. The samples were dried at 100°C, 12h and then treated in situ under vacuum at 350°C for 4h. X-ray diffraction (XRD) was performed using a Bruker D8 Advance instrument equipped with a LynxEye detector, Ni filter and CuKα (1.5418 Å) radiation. Data were collected at room temperature, from 10° to 90°, with a step size of 0.02° and 1 s per step. Before performing the TPD of NH3 (NH3-TPD) and of CO2 (CO2-TPD), the TPD-MS and the TPD-DRIFTS of ethanol, the isopropanol and the MPV model reactions and finally, the catalytic tests, the samples were firstly dried at 150 °C under N2 flow (30 mLmin−1) for 30 min. After that, reduction proceeded at 500 °C (10 °Cmin−1) for 1 h under 10% H2/N2 flow, followed by a purge with N2 and oxidation with synthetic air (90 mLmin−1) at 500 °C for 30 min. Exceptions were the TPD-DRIFTS of ethanol and the MPV model reaction, where catalysts were reduced and oxidized at 400 °C and the catalytic tests, with reduction at 400 °C and no following oxidation. The carrier gas was also different in the TPD-MS and TPD-DRIFTS analysis, where He was used instead of N2. Besides that, when the catalytic tests were performed under different temperatures, the reduction and oxidation temperatures were 500 °C. The density of acid sites was determined by the NH3-TPD-. Ammonia was adsorbed at 30 °C for 30 min under 4% NH3/He flow (30 mLmin−1). Desorption was conducted at 20 °Cmin−1, under He flow (50 mLmin−1) until 500 °C. The TPD profiles were decomposed in Gaussian curves in order to quantify the weak, medium and strong acid sites. Peaks occurring at temperatures below 200 °C were assigned to weak, acid sites, whereas peaks between 200 and 350 °C were assigned to medium acid sites; and finally, peaks above 350 °C were assigned to strong acid sites.


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The density of basic sites of the prepared samples was determined by CO2-TPD of. The adsorption was conducted at room temperature for 1 h (30 mLmin−1). The CO2-TPDmeasurements were carried out at 20 °Cmin−1, under He flow (50 mL min−1) up to 450 °C. The TPD profiles were deconvoluted in Gaussian curves in order to quantify the weak, medium and strong basic sites. Peaks at temperatures lower than 170 °C were assigned to weak basic sites, the ones between 170 °C and 270 °C were assigned to medium basic sites and those above 270 °C were assigned to strong basic sites. Both TPD of NH3 and CO2 were carried out using a micro reactor coupled to a multipurpose analytical system. Aiming at testing the catalysts performance for intermediate steps, two model reactions were used: the isopropanol dehydrogenation/dehydration reaction and the hydrogenation of acetone by ethanol (MPV reduction). The tests were performed in a fixed-bed reactor and monitored online by gas chromatography with flame ionization (FID) and thermal conductivity (TCD) detectors. Both experiments were conducted under differential conditions at 200°C and atmospheric pressure. The reactant vapors were generated by passing N2 through saturators with isopropanol (for its dehydrogenation/dehydration) or ethanol and acetone (for the MPV reduction). The TPD-MS of ethanol was carried out using a micro reactor system coupled to a mass spectrometer QMS200 Balzers. The ethanol adsorption was conducted at room temperature for 1 h. Ethanol vapors were generated by passing He (30 mLmin−1) through a saturator at 40 °C. Desorption was carried out employing a He flow (80 mLmin−1) at 10 °C min−1 from 40 to 450 °C. The fragments m/z = 2 (H2), 16 (CO2), 18 (H2O), 26 (ethene), 29 (acetaldehyde), 31 (ethanol), 42 (propene), 55 (isobutene) and 43 (acetone) were continuously monitored during the


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analyses. The intensities of these fragments were mathematically treated in order to eliminate contributions of more than one species. The TPD of ethanol followed by infrared spectroscopy (DRIFTS) were carried out employing a Vertex 70 spectrometer (Bruker) equipped with a LN-MCT detector, a diffuse reflectance assembly chamber (HVC-DRP-4/Harrick) with a ZnSe window. The ethanol adsorption was carried out at 50 °C for 1 h. Ethanol vapors were generated by passing He (20 mLmin-1) through a saturator at 10 °C. Desorption was carried out from 50 °C until 400 °C under He flow (30 mLmin-1) at 10 °C min-1. The spectra were collected every 100 °C with a spectral resolution of 16 cm-1 and 384 scans. A reference spectrum was collected after pretreatment.

Catalytic tests The ethanol conversion to propene in presence of water vapor was evaluated in a fixedbed reactor under atmospheric pressure, at 400°C using physical mixtures and the individual catalysts AgCeO2, t-ZrO2 and MgO (smaller than 90 µm particle size) under isoconversion conditions (catalysts mass/ gas flow x 10³ (gsmL-1 = 90 (DB), 54 (t-ZrO2), 60 (AgCeO2), 124 (MgO), 91 (PM11-50/25) and 92 (AgCeO2 + MgO PM) gscm-3). Ethanol and water vapors were generated by passing N2 (50 mLmin-1) through two saturators: one at 57.2 °C and the other at 5.3 °C, respectively. The reaction products were analyzed online during 17 h using a GC Agilent HP6890 instrument equipped with FID and TCD detectors. DB was employed in order to evaluate the migration of molecules between the catalysts. The PM11 system was also evaluated at different temperatures, ratios and contact times. The ethanol conversion was defined as the ratio of the moles of ethanol consumed to the moles of ethanol in the feed. The definition of selectivity to one specific compound is the ratio of the number of carbon moles consumed to


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synthesize this compound to the total number of carbon moles consumed. Considering the propene synthesis reaction (Scheme 2), 75% is the highest selectivity that can be obtained.

Thermodynamic analysis A thermodynamic analysis of the equilibrium constant of the probable reaction steps and of the global reaction was conducted in order to better understand the equilibrium of these reactions and how does temperature affect them. Microsoft Excel 2013 was used for the calculations, while data were obtained from National Institute of Standards and Technology23, 14 and Apendix A from The Properties of Gases and Liquids24. The equations regarding the thermodynamic calculations are listed in the support information.

3. Results and Discussion 3.1 Catalysts characterization Data regarding the textural properties of the catalysts as well as their acid-base properties are exhibited in Table 1. Moreover, the XRD patterns of t-ZrO2, MgO and AgCeO2 can be observed in Figure S1.


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Table 1. Specific surface area and density of acid and basic sites, represented as weak + medium and strong acid sites and weak + medium and strong basic sites Sa

Dab x 10³

DAc x10³

Dbd x10³

DBe x10³

/m²g-1

/µmolNH3m-2

/µmolNH3m-2

/µmolCO2m-2

/µmolCO2m-2

MgO

205

-

-

166 (54%)

141 (46%)

t-ZrO2

156

974 (45%)

1192 (55%)

2154 (86%)

353 (14%)

AgCeO2

39

2308 (100%)

-

5103 (77%)

1538 (23%)

Catalyst

a

S specific surface area

b

Da density of weak and medium acid sites

c

DA density of strong acid sites

d

Db density of weak and medium basic sites

e

DB density of strong basic sites

In parentheses percentage of the contribution of each kind of basic or acid site. Both t-ZrO2 and MgO show specific areas much higher than AgCeO2. The t-ZrO2 oxide is the only one among the three oxides which shows strong acid sites, whereas the three of them exhibit strong basic sites. The MgO oxide shows the higher concentration of strong basic sites, being followed by AgCeO2 and t-ZrO2. It is interesting to mention that the presence of both acid and basic sites on t-ZrO2 has been described as the reason for the versatility of this oxide in promoting many reactions25. Furthermore, the adsorption with pyridine was not employed as the presence of Brønsted acid sites is expected on none of the oxides1,3,26.

3.2 Model reactions


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3.2.1 The isopropanol conversion In order to better describe the propene synthesis from ethanol, the catalytic behavior of the physical mixtures components was analyzed employing two model reactions: the isopropanol conversion and the acetone reduction by ethanol (MPV). It is worth stressing that these reactions are also suggested as steps of the propene synthesis from ethanol (Scheme 2). The isopropanol reaction (Scheme 3) was employed to describe the acid and basic properties of the catalysts. However, in this work the objective is to assess the competition between the dehydrogenation and the dehydration of alcohols employing AgCeO2, t-ZrO2 and MgO. This is an important issue for the propene synthesis as it occurs twice in the system. The first one is related to the acetaldehyde generation from ethanol. The dehydration of ethanol is undesirable as it consumes the reactant for a side reaction, whereas the ethanol dehydrogenation generates acetaldehyde, a key intermediate of the propene synthesis. The second one is the isopropanol conversion to propene or acetone in the end of the reaction system (Scheme 2). The isopropanol dehydration is essential for the propene generation whereas its dehydrogenation generates acetone as byproduct or intermediate.

Scheme 3. Isopropanol model reaction: dehydration to propene and dehydrogenation to acetone.

Table 2 depicts the rates of both isopropanol (Ri) and acetaldehyde (Ra) formations for the three oxides studied in this paper in order to describe the catalytic behavior of the samples in the MPV model reaction, which employs ethanol and acetone as reactants. It also presents both


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dehydration (rp) and dehydrogenation (ra) rates of isopropanol over AgCeO2, t-ZrO2 and MgO catalysts.

Table 2. Isopropanol and acetaldehyde generation rates of the ethanol and acetone MPV model reaction. Propene and acetone generation rates of the isopropanol model reaction. The reactions were carried out at 200 °C and conversion ~10%. Ria

R ab

rpc

rad

/µmolgcat-1h-1

/µmolgcat-1h-1

/µmolgcat-1h-1

/µmolgcat-1h-1

MgO

2.5

2.7

0.01

0.25

AgCeO2

0.0

0.4

0.01

0.55

t-ZrO2

1.9

1.7

0.65

0.05

Catalyst

a

Ri isopropanol generation rate

b

Ra acetaldehyde generation rate

c

rp formation rate of propene

d

ra acetone generation rate

3.2.2 The Meerwein-Ponndorf-Verley reduction The Meerwein-Ponndorf-Verley reduction (MPV – Scheme 4) has been described as an efficient way to selectively hydrogenate an aldehyde or a ketone using an alcohol as hydrogen donor (usually a secondary alcohol), avoiding the undesirable hydrogenation of other groups, such as unsaturated carbon moieties27–29.


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Scheme 4. MPV hydrogenation: hydrogenation of acetone to isopropanol and dehydrogenation of ethanol to acetaldehyde

According to Ivanov et al.30, the MPV mechanism involves the ethoxide formation by the adsorption of the alcohol on a Lewis acid site and an H-abstraction by a neighbor basic site. After that, the ethoxide interacts with the aldehyde or ketone through the carbonyl on the same acid site. A six-atom ring intermediate is formed and the hydrogen transfer occurs (Scheme 5-A). Weak acidic and strong basic surface pairs can also efficiently promote the formation of the sixatom cyclic intermediate required for the MPV mechanism29,30 (Scheme 5-B). Recently, Komanoya et al.31 employing ZrO2, proposed a mechanism involving Lewis acid and basic sites. Their observations pointed to the interaction of the latter with the α-hydrogen from the alkoxide as a way to activate the H-transfer, after the adsorption of the reactants on the Lewis acid sites (Scheme 5-C).


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Scheme 5. Proposed mechanism for the MPV reduction on strong acid (A), weak acid-strong basic (B) and strong acid-strong basic (C) sites, represented by ZrO2 (A and C) and MgO (B).

On one hand, AgCeO2 is not active in the MPV reaction. It exhibits the isopropanol (Ri) and acetaldehyde formation (Ra) lowest rates. On the other hand, MgO shows the highest ones. Moreover, both MgO rates show similar values, meaning that side reactions are not important, as this is a 1:1 reaction. Other products were not observed, confirming this statement. It is interesting to observe that in spite of MgO not showing acidity (Table 1), it is able to adsorb oxygenates, which are necessary for the reaction. Considering the B mechanism (Scheme 5-B)


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and the acid and basic properties of AgCeO2, this catalyst should be active in the MPV hydrogenation. However, this does not occur. Therefore, it can be suggested that the suface atomic arrangement of CeO2 might not be suitable for this reaction32. The t-ZrO2 oxide also exhibits high and similar rates for acetaldehyde and isopropanol formation although lower than the ones observed for MgO. As it displays both acid and basic sites (Table 1), both A and C mechanisms are likely to be followed (Scheme 5-A and 5-C). Both MgO and AgCeO2 show high acetone (ra) and low propene formation rates (rp), whereas t-ZrO2 showed a much higher rp and a low ra values (Table 2). According to Vedrine33, the correlation of the acid-basic properties and the isopropanol conversion is not straightforward. However, the catalytic behavior of these oxides can be roughly associated with their acid and basic properties, i. e., high acidity leads to high dehydration rates while high basicity, to high activity toward dehydrogenation32,34. The AgCeO2 and MgO catalytic behaviors in the isopropanol conversion are in line with this statement. The same happens to t-ZrO2, prevailing dehydration, though this oxide is able to promote dehydrogenation too.

3.3 Thermodynamics analysis A thermodynamic analysis of the propene synthesis steps (Scheme 1) was performed and is depicted in Figure 1. The thermodynamics calculations and the analysis of reactions involving the acetaldehyde conversion into acetate species are described in the Support Information.


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thermodynamic equilibrium constant

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10

20

10

15

10

10

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5

10

0

10 10

-5

350

400

450

500

o

temperature / C

CH3CH2OH

CH3CHO + H2 (a)

CH3CH2OH

CH2CH2 + H2O (b)

2CH3CHO + H2O

CH3COCH3 + CO2 + 2H2 (c)

CH3CH2OH + CH3COCH3 CH3COCH3 + H2 CH3CHOHCH3 2CH3CH2OH

CH3CHO + CH3CHOHCH3 (d)

CH3CHOHCH3 (e) CH3CHCH2 + H2O (f)

CH3CHCH2 + CO2 + 3H2 (g)

Figure 1. Thermodynamic equilibrium constants of the possible reactions involved in the propene synthesis from ethanol. The reactions investigated are the following: ethanol dehydrogenation (a), ethanol dehydration (b), acetone generation (c), acetone reduction (MPV) (d), acetone hydrogenation (e), isopropanol dehydration (f) and propene synthesis from ethanol (g). The ethanol dehydrogenation and dehydration, acetone generation, isopropanol dehydration and the synthesis of propene from ethanol are favored by thermodynamics (Figure 1). As Scheme 2 indicates, acetone should be hydrogenated and dehydrated to generate propene. Hence, acetone can be hydrogenated by the addition of H2 (e) or by the MPV reduction (d). As one can see (Figure 1), the former is not a thermodynamically favorable reaction as its equilibrium constant value (K) is lower than 1 within the temperature range analyzed. In a different way, despite being close to the equilibrium in the experimental conditions considered,


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the MPV reaction presented a negative variation of the Gibbs free energy, what brings about a K value higher than 1, suggesting the presence of this step in the propene synthesis. Thus, the addition of hydrogen to acetone (e) should not be considered as a step for the propene synthesis at these experimental conditions. The steps mentioned above and the reaction representing the propene synthesis (a, c, d, f and g) show that their equilibrium is shifted toward the products. It is worth mentioning that the MPV step, due to its thermodynamics nature, brings about the requirement of shifting the equilibrium toward the products. This can be achieved by dehydrating the isopropanol to propene, what is quite convenient. Some authors8 also propose the condensation of ethanol and acetaldehyde generating ethyl acetate, which can be hydrolyzed or decomposed producing not only acetic acid but also ethanol and ethene, respectively (Scheme 2) . The acetic acid undergoes the ketonization reaction to generate acetone, which is hydrogenated and dehydrated. The condensations of acetaldehyde and ethanol show equilibrium limitations, although it can occur in a lower extent at the experimental conditions considered. The hydrolysis of ethyl acetate is the least favored reaction when compared with the ones of Figure 1 (except e), being limited by thermodynamic equilibrium, whereas the decomposition of ethyl acetate is favored. However, it is worth stressing that the propene synthesis from ethanol seems to be a cascade reaction. Thus, all these steps can be shifted from the equilibrium limitation by the product formation (Figure S2).

3.4 Catalytic tests


19

ACS Catalysis

The catalytic performances of the three oxides were analyzed individually, in physical mixtures and in a double bed system at approximately 37 % conversion. The results are depicted in Figure 2.

selectivities / %

80

acetaldehyde ethene

carbon dioxide acetone

propene

60

40

20

0

t-ZrO2

AgCeO2

AgCeO2 +

MgO (PM2-50/25)

AgCeO2 +

t-ZrO2

(PM11-50/25)

AgCeO2 and

t-ZrO2 (DB)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 2. Physical mixtures and catalysts selectivities at isoconversion (37 %). The gas mixture molar composition, temperature and flow rate are N2:H2O:C2H5OH = 91:8:1, 400 oC and 50 mLmin-1, respectively.

The MgO oxide exhibits very low activity. It could not reach the level of isoconversion under the experimental conditions, synthesizing only acetaldehyde (not shown). Furthermore, the high rate of alcohol dehydrogenation of MgO exhibited in Table 2 is not reproduced under the propene synthesis conditions. The synthesis of acetaldehyde is associated with the presence of strong basic sites and also weak acid sites32. Thus, this result might be related to the presence of water, competing with ethanol for the few MgO adsorption sites (very weak acid sites), reducing this oxide activity.


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Acetone is the main product when AgCeO2 is employed as catalyst with the consequent generation of CO2. This observation corroborates previous results published by Lima et al.1 This behavior can be associated with the acid, basic and redox characteristics of this oxide (see introduction and Table 1). Figure 2 shows that the t-ZrO2 oxide synthesizes ethene and acetaldehyde. According to its acid and basic properties (Table 1), it is able to dehydrate and dehydrogenate alcohols32. However, the results exhibited in Figure 2 are not in line with the isopropanol conversion, where one could observe a much higher activity toward dehydration than dehydrogenation. It should be highlighted, though, that ethanol is a primary alcohol while isopropanol, a secondary one. Thus, the latter is more prone to undergo dehydration than the first. Furthermore, the presence of water may lead to a reduction in the number of available acid sites making the dehydration reaction more difficult to occur8 and favoring the acetaldehyde formation. This oxide also shows poor redox properties. Thus, it does not promote the acetone synthesis. The AgCeO2 and t-ZrO2 physical mixture (PM11-50/25) generates acetone, carbon dioxide, acetaldehyde, ethene and propene. The last one presented the highest selectivity. It is interesting to emphasize that the behavior of this mixture is not a linear combination of the performance described above for the two individual components. The catalytic performance of the physical mixture comprising AgCeO2 and MgO (PM250/25) is similar to the AgCeO2 one, showing acetone and CO2 as major products. Despite the high activity of MgO in the MPV reduction, no significant amount of propene or isopropanol is detected in the products of the reaction. Considering the results already exposed for MgO, it can be suggested that this oxide seems to play no role in the physical mixture under the tests conditions. This might occur due to the presence of water, as proposed before.


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In an attempt to better describe this reaction system, a double bed reactor (DB, Figure 2) was employed. The gaseous mixture reaches firstly the AgCeO2 catalyst and then the t-ZrO2 oxide. It can be observed that the selectivity to acetaldehyde and acetone increases when compared with the physical mixture. Moreover, this system keeps its capacity to produce propene, however to a lower extent when compared with PM11-50/25. In the first bed, acetaldehyde, acetone, carbon dioxide are generated and desorbed. After that, acetone can be adsorbed on t-ZrO2, undergoing with ethanol the MPV hydrogenation on this oxide. Isopropanol (not observed) and acetaldehyde are then generated, being the latter dehydrated to propene. The acetaldehyde generated by the MPV hydrogenation in the case of DB desorbs (selectivity increases compared with PM11-50/25) from t-ZrO2 without undergoing oxidation and ketonization, as this catalyst presents poor redox features. Hence, the selectivity to propene decreases compared with PM11-50/25. This result shows that both AgCeO2 and t-ZrO2 must keep a close contact in order to promote this complex system. It is important to highlight that no ethyl acetate is observed under reaction conditions, indicating that either it is not produced or that the hydrolysis or decomposition of this ester is too fast. Furthermore, diffusional tests (not shown) were performed, evidencing that diffusional problems are not important for the AgCeO2 and t-ZrO2 physical mixture.


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conversion and selectivity / %

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100 80 60 40

conversion propene acetone acetaldehyde carbon dioxide ethene isopropanol

20 0 PM12-50/50

PM11-50/25

PM13-100/25

Figure 3. Selectivities and ethanol conversion of the physical mixtures with different AgCeO2/t-ZrO2 mass ratios. The gas mixture composition, temperature and flow rate are: N2:H2O:C2H5OH = 91:8:1 mol%, 400oC, 50 mLmin-1, respectively. The catalysts PM11-50/25, PM12-50/50 and PM13-100/25 were employed using the following total masses: 75, 100 and 125 mg, respectively.

Figure 3 depicts the catalytic behavior of the physical mixtures with different AgCeO2 and t-ZrO2 mass ratios. Adding more t-ZrO2 to PM11-50/25 (PM12-50/50) brings about an increase in the ethanol conversion. As previously presented (Figure 1), not only AgCeO2 but also t-ZrO2 generates acetaldehyde, what contributes with the main reaction. Nevertheless, a small rise in the ethene selectivity also occurs, whereas the acetone selectivity slightly decreases and propene keeps almost unaltered. These changes are associated with the acid and basic properties of t-ZrO2. Adding more AgCeO2 to PM11-50/25, resulting in PM13-100/25, increases the conversion of ethanol at a higher intensity when compared with PM12-50/50. Moreover, a slight increase of acetone and isopropanol selectivities can be observed, whereas the selectivity to


23

ACS Catalysis

ethene decreases. These changes are associated with the acid and basic properties of AgCeO2. However, the propene selectivity does not change. When the mass of t-ZrO2 or AgCeO2 increases in the PM catalyst, more acetaldehyde is generated. The aldehyde synthesized on zirconia migrates to ceria oxide. Then, acetaldehyde generated by both oxides is oxidized and condensed to acetone, which is hydrogenated and dehydrated to propene. It is interesting to observe that the increase of the ethanol conversion does not lead to an increase in the selectivity of the final product, i.e., propene. Thus, this result suggests that there is an optimum AgCeO2 / t-ZrO2 mass ratio associated with the propene generation. 80 conversion acetaldehyde propene etene acetone

conversion acetaldehyde propene ethene acetone

100

conversion and selectivities (%)

60 conversion and selectivities (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

80 40

20

20

0

0 50

100

150

360 -3

250 300 350 400 450 500 3

ratio masscat /flowgas (gscm ) x 10

Figure 4. Ethanol conversions and selectivities of Figure

o

temperature / C

5.

Ethanol

conversions

and

PM11-50/25 versus the ratio of catalyst mass/ gas selectivities of PM11-50/25 versus the flow rate. The gas mixture composition and temperature. The gas mixture composition, temperature are N2:H2O:C2H5OH = 91:8:1 mol%, catalyst

mass

and

flow

rate

are:


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ACS Catalysis

400 °C, respectively.

N2:H2O:C2H5OH = 91:8:1 mol%, 75mg, 50 mLmin-1 respectively.

Figure 4 depicts an increase in the ethanol conversion as the contact time rises. The acetone selectivity exhibits a maximum at low residence time. Moreover, as the contact time increases, the selectivity toward acetone decreases, whereas the one to propene increases. This result shows that this ketone is an intermediate of the reaction. Ethene selectivity almost does not change with contact time, whereas the one toward propene increases. This means that at this temperature the rate of propene generation is much higher than the one of the ethanol dehydration. Furthermore, ethyl acetate is not observed as an intermediate. Comparing the results of Figure 4 with those of Figure 3, it can be inferred that the AgCeO2/t-ZrO2 mass ratio around 2 may be used to increase the selectivity to propene without substantially changing that of ethene. This shows that there is an optimal mass ratio between these two oxides in order to promote the generation of propene. Raising the temperature of the reaction, the conversion rises as well (Figure 5). Acetaldehyde and acetone selectivities exhibit maxima around 250 (not shown) and 340oC, respectively, suggesting that they are intermediates of this synthesis. The propene selectivity also exhibits a maximum around 425oC. Higher temperatures promote a decrease in the propene selectivity, whereas the one to ethylene increases. The selectivity toward acetone increases, since there is less ethanol to react with this ketone. Thus, in order to reach high selectivities to propene this synthesis should be carried out at a specific temperature range. Increasing the water concentration in the reactant mixture enhances both the acetone and propene selectivities, whereas the selectivity to ethylene and ethanol conversion decreases. This


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result shows that water not only promotes the oxidation step but also competes with the oxygenate species for the acid sites.

3.5 Ethanol-TPD -followed by IR and MS TPD followed by IR of ethanol on t-ZrO2 (Figure 6A-B) was conducted from 50°C to 400°C. The vibrations 1481, 1385, 1144 cm-1 can be assigned, respectively, to δ(CH2), δs(CH3) and ν(C-O) of ethoxide species adsorbed on t-ZrO235 (white numbers and lines). These species are quite stable, as they can be observed from 50 to 300oC. The vibration at 1267cm-1 can be assigned to the adsorption of molecular ethanol (italic numbers)32. Absorptions at 1570, 1462, 1416 and 1334 cm-1 (underlined numbers) can be assigned to νas(OCO), δas(CH3), νs(OCO) and δs(CH3) of acetate species, respectively. The first one, the most intense, can be observed at low temperatures, suggesting that ethanol can be oxidized to acetate species on t-ZrO2 at low temperature. At high temperatures (400oC) the intensity of the acetate species adsorptions decrease and vibrations related to carbonate species emerge (1516 and 1340 cm-1, black numbers). Thus, acetate species might condensate and generate acetone and CO2, which reacts with the oxide generating carbonate species (1,35). Indeed, Figure 2 exhibits that this oxide can generate small amounts of propene. TPD of ethanol on the PM11-50/25 followed by IR is exhibited in Figure 7A-B. The vibrations at 1148, 1057 and 1109 cm-1 can be assigned to ν(C-O) vibrations of ethoxide species, being the first one related to the adsorption of this species on t-ZrO2 and the others to AgCeO2 (white numbers and lines). The vibrations 1384, 1481 cm-1 can also be assigned to δs(CH3), δ(CH2) 1,16,18,36–38 of ethoxide species (white numbers) adsorbed on both oxides, respectively. As the temperature rises, the intensity of the ethoxide species bands decrease. The ethoxide species adsorbed on t-ZrO2 are much more stable, i.e., they remain adsorbed at higher temperatures


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compared with the ethoxides on AgCeO2. The ethoxides adsorbed on ZrO2 might be associated with the MPV reduction. Indeed, propene is observed at temperatures higher than 300oC. At low temperature (50oC) it is possible to observe vibrations associated with acetate species (see black arrows), showing that it is easy to oxidize ethanol to acetate species. At higher temperatures, above 250oC (Figure 5), acetone and carbon dioxide are generated producing carbonate species. As the temperature rises, acetate bands become more intense and broader. Carbonate species emerge at the same frequency range, making difficult the assignment of these species. The vibrations around 1030, 1311, 1446 and 1557 cm-1 might be are associated to ρ(CH3), δs(CH3), νs(OCO), νas(OCO) of acetate species1,16,36,37, respectively. Moreover, it can be suggested that the bands at 1568, 1425 cm-1 can be associated with bidentate and symmetric carbonate, respectively. Comparing the t-ZrO2 (Figure 6) and AgCeO21 spectra with the ones of the PM1150/25 physical mixture, it can be observed that the physical mixture is not a linear combination of the others. Indeed, at temperatures higher than 200oC, the PM11-50/25 spectra are very similar to the ones of AgCeO2, showing that acetate and carbonate adsorbed on AgCeO2 are predominant. This result is in line with the redox properties of this oxide. It is interesting to observe that the intensity of acetate species bands of PM11-50/25 increase at temperatures higher than 200oC, whereas the intensity of the vibrations assigned to the ethoxide species adsorbed on t-ZrO2 decrease. Indeed, the propene synthesis begins to be observed at these conditions (see Figure 5). The ethoxide on t-ZrO2 may participate in the MPV step.


27

B

e

13 40

14

1144

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16

1516 1462

d

13

34

157

A

1385

e

c

/ a.u.

/ a.u

d c

1267

1481

b

b

a

a

1600

1400

/ cm

1200

-1

1200

900

-1

/ cm

Figure 6A and B. Spectra of the TPD of ethanol on t-ZrO2, 50 °C (a), 100°C (b), 200°C (c), 300°C (d) and 400°C (e). Underlined, black, white and italic numbers are related to acetate,

1109 1057

1030

B

6 144 1425 1384 1311

1557

B A

1148

carbonate, ethoxide species and ethanol adsorbed, respectively.

e

c

1481

d c

e

d

/ a.u

/ a.u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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b

b

a

a 1800

1600

1400

1200

1200

-1

1000 -1

/ cm

/ cm


28

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Figure 7A and B. Spectra of the TPD of ethanol on PM11-50/25, 50 °C (a), 100°C (b), 200°C (c), 300°C (d) and 400°C (e). Underlined, black, white and italic numbers are related to acetate, carbonate, ethoxide species and ethanol adsorbed, respectively.

In order to better understand how the reactions take place on the catalysts, ethanol-TPDMS were taken for both individual catalysts and the physical mixture, being depicted in Figures 8A-C. C

B H2(x0.4)

H2O (x0.1) ethene (x0.1)

/ a.u.

CO2 (x4)

acetaldehyde (x2)

H2 (x0.1)

H2(x0.2)

CO2 (x0.8)

CO2

H2O (x0.15)

H2O (x0.2)

ethene (x4)

/ a.u.

A

/ a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ethene

acetaldehyde (x8)

acetaldehyde(x4)

ethanol (x2)

ethanol (x4)

propene (x8)

propene

ethanol (x8) propene (x8) isobutene (x8) acetone (x8)

isobutene (x8)

isobutene (x2)

acetone (x2)

acetone (x4)

isothermal 0 150 300 450 600 750

isothermal 0 150 300 450 600 750 o temperature / C

o

temperature / C

0 150 300 450 600 isothermal 750 temperature / oC

Figure 8. Spectra of the TPD-MS of ethanol on t-ZrO2 (A), AgCeO2 (B) and PM11-50/25 (C).

Figure 8A shows the TPD of ethanol on t-ZrO2. Water and ethene (334oC) desorption are related to ethanol dehydration, whereas acetaldehyde and hydrogen are generated by dehydrogenation of ethanol on t-ZrO2 at the same temperature. This behavior is in agreement with data on Tables 1, 2 and Figure 2.


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The TPD of ethanol on AgCeO2 is exhibited in Figure 8B. At low temperature, it can be observed that water desorption (182oC) might be related to the oxidative dehydrogenation of ethanol, generating acetaldehyde and reducing the catalyst (the oxygen of the CeO2 lattice is consumed). As can be observed, this aldehyde does not desorb. The hydrogen desorption observed at 230oC can be related to the water dissociation on AgCeO2 vacancies, what reoxidizes the catalyst1. Ethene desorption may be observed as temperature increases (280oC). This catalyst generates less ethene than t-ZrO2, corroborating results from Tables 1, 2 and Figure 2. Water desorption observed at high temperatures might be related to ethanol dehydration. The acetone desorption spectrum shows a peak with the maximum at 304oC. At this same temperature isobutene, propene and acetaldehyde exhibit very small and broad bands. Although isobutene is not observed as a byproduct (Figure 2), it is observed at the ethanol-TPD. This occurs because the experimental conditions of the ethanol-TPD are different from the ones of the catalytic tests. According to Zaki et al., acetone condensates producing diacetone alcohol, which dehydrates to mesityl oxide and decomposes into isobutene and acetaldehyde39. Thus, acetaldehyde band at around 304oC may be generated by both propene (Scheme 2) and isobutene. It is interesting to see that t-ZrO2 keeps ethanol adsorbed, whereas higher amounts of ethanol desorbs from AgCeO2. This behavior can be associated to the number of acid sites of these oxides. Figure 8C exhibits the TPD of ethanol on PM11-50/25. Water and hydrogen show bands at low temperature as observed in the case of AgCeO2. Thus, the syntheses of acetaldehyde by oxidative dehydrogenation as described above can be also suggested. Higher amounts of propene, isobutene and acetaldehyde, compared with the ones observed for AgCeO2, are desorbed at 304oC, which is the same temperature for the acetone desorption in Figure 8B. This result suggests that the acetone hydrogenation and dehydration to propene are very fast. The


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isobutene synthesis is also fast. The acetone spectrum (Figure 8C) shows a broad band with a maximum around 306-358oC. Indeed, this shape might be related to at least two types of acetone generation: one from acetaldehyde oxidation/condensation described above (Scheme 2) and the other one from acetaldehyde, which is the byproduct of the MPV reaction and isobutene synthesis. Moreover, one can also suggest that acetone can also be generated by the dehydrogenation of isopropanol. This acetone generation at high temperature (after the MPV hydrogenation) makes this reaction system special, i.e., it is not a simple cascade reaction.

3.6 Final remarks Acetate species are observed by FTIR at low temperature (Figure 7A, ethanol-TPD followed by FTIR). However, neither acetone nor propene is generated at these conditions (Figure 5). A considerable raise in the acetate species concentration is observed on the catalysts surface when the temperature increases from 200oC to 300oC (Figure 7A). Indeed, acetone and propene are synthesized at temperatures higher than 300oC (Figure 5, Figure 8B, C). Thus, the condensation of acetate species occurs at temperatures in which their concentration on the surface is high (above 300oC). Assuming that the MPV reduction of acetone and dehydration of isopropanol are fast steps (Figure 8C), these data suggest that the slower step of the propene synthesis might be the acetone generation at this temperature range. At temperatures higher than 400oC the selectivity to acetone increases, i.e., this ketone desorbs (Figure 5). This result suggests that the MPV rate is lower than the one of the ethanol dehydration. Indeed, Figure 5 exhibits the competition between ethanol dehydration versus ethanol dehydrogenation by the MPV step. Thus, high selectivity toward propene is obtained in a


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Page 32 of 52

narrow temperature range. This suggests that MPV might be the slower step at temperatures higher than 400oC Adding more AgCeO2 (PM13-100/25- Figure 3) or t-ZrO2 (PM12-50/50-Figure 3) to the physical mixture increases the conversions. However, the selectivities to ethene, acetaldehyde, acetone and propene almost do not change. On the other hand, increasing the residence time, i.e., by employing higher amounts of PM11-50/25 (increasing the mass of both components, Figure 4), brings about an increase in both propene conversion and selectivity. Hence, the propene generation mechanism involves both oxides at a certain mass ratio. This reaction is not a simple cascade system. It can be proposed that the aldehyde generated by the MPV hydrogenation can be desorbed and readsorbed on AgCeO2. This means that the path of the reaction returns to AgCeO2. Acetaldehyde undergoes oxidation and condensation to yield acetone, which migrates to t-ZrO2 and then generates more propene (see scheme 4). Considering the ZrO2 catalytic behavior (Figure 2) one can propose that isopropanol might be also dehydrogenated to acetone. However, should this reaction occur, the intensity thereof would be very low (see Figure 4). Indeed, some authors also propose the generation of acetate species from ethyl acetate. Figures 2 and 4 show the ratio between propene and ethene selectivities reaching a value as high as 3.4. Thus, these catalysts do not seem to promote the decomposition of ethyl acetate (Scheme 2), since it would lead to a ratio propene / ethene near 0.75. The acetate species are also observed on t-ZrO2 (Figure 6A). As this catalyst does not show redox properties, it can be suggested that these acetates are associated with the hydrolysis or decomposition of ethyl acetate generated by the condensation of acetaldehyde and ethoxide species. These acetate species can condensate forming acetone; also, such acetates may react


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with ethanol via the MPV reaction, with further dehydration to synthesize propene. It is also worth stressing that ethyl acetate has neither been observed in the ethanol-TPD nor under reaction conditions. However, one can suggest that the hydrolysis or decomposition can be very fast. Indeed, Figure 2 shows that t-ZrO2 is able to generate propene at very low selectivity. Figure 5 exhibits that the ratio propene / ethene decreases as the temperature increases, reaching values much lower than 0.75. Thus, ethanol dehydration is relevant at this point. Therefore, is not possible to disregard the potential contribution of the acetate generation from ethyl acetate, especially under higher temperatures. The results previously exhibited suggest that the major contribution for acetate synthesis is the acetaldehyde oxidation promoted by AgCeO2, (see Figure 2 acetone selectivity). Figure 2 shows that the binary physical mixture composed by AgCeO2 and t-ZrO2 is the only one able to generate propene. This occurs because this mixture shows not only redox (AgCeO2), acid (t-ZrO2) and basic (t-ZrO2 and AgCeO2) properties, but is also active in the MPV reduction (t-ZrO2) under the reaction conditions.

Scheme 6. Proposed mechanism for the propene production from ethanol by the physical mixture of AgCeO2 and t-ZrO2.


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Scheme 6 depicts the mechanism of the propene synthesis from ethanol. Firstly, ethoxide species are generated on the surface of AgCeO2 and t-ZrO2 (Figure 6B and 7B). The synthesis of this species occurs on Lewis acid-basic pair sites. The acid sites adsorb ethanol and the strong basic sites abstract hydrogen34. After that, α-hydrogen moves from the alkoxide species to the superficial oxygen of AgCeO2 and acetaldehyde is synthesized. Water is also generated, desorbed while oxygen vacancies emerge. The oxygen vacancy is reoxidized by water dissociation, which generates hydrogen1 (Figure 8B and C). Acetaldehyde can also be synthesized by dehydrogenation of ethanol on t-ZrO2 (Figure 2). This reaction occurs on Lewis acid and strong basic sites. The former adsorbs the ethoxide and the latter abstracts the αhydrogen34. Next, acetaldehyde is oxidized to acetate species employing, once again, the oxygen of the CeO2 lattice. The aldehyde generated on t-ZrO2 may migrate to AgCeO2 (Figure 3). Ceria is reduced and reoxidized by the dissociation of water producing hydrogen1,18. Both oxidations described above follow the MVK mechanism, associated with the oxygen mobility of AgCeO2. The acetate species condensate and generate acetone. This step is promoted by strong basic sites and both oxides can carry out this reaction. Then acetone and ethanol react on t-ZrO2 (MPV step) and generate acetaldehyde and isopropanol. Table 2 indicates that AgCeO2 is not able to perform this reaction. Finally, isopropanol is dehydrated (strong acid sites and basic sites, Table 1 and 2) on t-ZrO2, generating propene. Acetaldehyde migrates to AgCeO2, oxidizes to acetate species, which undergoes ketonization to form acetone and propene.

4. Conclusions The use of physical mixtures represents an excellent tool to describe the mechanism of a rather complex reaction process, such as the generation of propene from ethanol. Acid and strong


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basic sites, redox properties and the capacity to conduct both water dissociation and MPV reduction are important characteristics for this reaction. Water is the oxidant agent, but it also interferes with the acid-basic sites of the oxides, as perceived with MgO. A physical mixture of tZrO2 and AgCeO2 is capable of conducting the oxidative dehydrogenation of ethanol, with formation of acetaldehyde, which is oxidized to acetate species. These species undergo ketonization to acetone, which undergoes MPV reduction with ethanol, generating isopropanol and acetaldehyde. The first is dehydrated to propene, whereas the second goes back to oxidation, ketonization and so on. This second way to produce acetaldehyde and its participation in the principal reaction path makes propene synthesis a special cascade system. At low temperatures (~300 ºC), acetone generation seems to be the slow step of the reaction, while at higher ones (>450 ºC), MPV seems to be the slower one. At these temperatures ethanol dehydration predominates, what indicates an optimum temperature range for operating. Eventually, this paper presents new perspectives for the propene generation from ethanol, shedding light on the mechanism of this rather complex process.

5. AuthorInformation Corresponding author * mail: [email protected] Author Contributions All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest. 6. Acknowledgment


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The authors acknowledge the DICAP/INT team for their assistance in the experimental work. The authors are indebted to prof. Fátima M. Z. Zotin, prof. Fabio B. Passos and Dr Priscila C. Zonetti, for the ethanol-TPD. Additionally, we are grateful to Saint-Gobain NorPro for supplying the zirconia samples. The financial support of CAPES is also acknowledged. 7. Abbreviations MPV, Meerwein-Pondorf-Verley; XRD, X-ray diffraction; TPD, temperature programmed desorption; DRIFTS, diffuse reflectance infrared Fourier transform spectroscopy; MS, mass spectroscopy; WHSV, Weight hourly space velocity.

Supporting Information Available: XRD results, thermodynamic constants for the conversion of acetaldehyde in acetic acid and the equations used for the thermodynamic analysis. This material is available free of charge via the internet at http://pubs.acs.org.

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Physical mixtures and catalysts selectivities at isoconversion (37 %). The gas mixture molar composition, temperature and flow rate are N2:H2O:C2H5OH = 91:8:1, 400oC and 50 mLmin-1, respectively. 209x160mm (150 x 150 DPI)


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Selectivities and ethanol conversion of the physical mixtures with different AgCeO2/t-ZrO2 mass ratios. The gas mixture composition, temperature and flow rate are: N2:H2O:C2H5OH = 91:8:1 mol%, 400oC, 50 mLmin1 , respectively. The catalysts PM11-50/25, PM12-50/50 and PM13-100/25 were employed using the following total masses: 75, 100 and 125 mg, respectively. 104x80mm (300 x 300 DPI)


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Ethanol conversions and selectivities of PM11-50/25 versus the ratio of catalyst mass/ gas flow rate. The gas mixture composition and temperature are N2:H2O:C2H5OH = 91:8:1 mol%, 400 °C, respectively. 104x80mm (300 x 300 DPI)


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Ethanol conversions and selectivities of PM11-50/25 versus the temperature. The gas mixture composition, catalyst mass and flow rate are: N2:H2O:C2H5OH = 91:8:1 mol%, 75mg, 50 mLmin-1, respectively. 104x80mm (300 x 300 DPI)


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Figure 6A and B. Spectra of the TPD of ethanol on t-ZrO2, 50 °C (a), 100°C (b), 200°C (c), 300°C (d) and 400°C (e). Underlined, black, white and italic numbers are related to acetate, carbonate, ethoxide species and ethanol adsorbed, respectively. 104x80mm (300 x 300 DPI)


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Figure 6A and B. Spectra of the TPD of ethanol on t-ZrO2, 50 °C (a), 100°C (b), 200°C (c), 300°C (d) and 400°C (e). Underlined, black, white and italic numbers are related to acetate, carbonate, ethoxide species and ethanol adsorbed, respectively. 104x80mm (300 x 300 DPI)


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Figure 7A and B. Spectra of the TPD of ethanol on PM11-50/25, 50 °C (a), 100°C (b), 200°C (c), 300°C (d) and 400°C (e). Underlined, black, white and italic numbers are related to acetate, carbonate, ethoxide species and ethanol adsorbed, respectively. 104x80mm (300 x 300 DPI)


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ACS Catalysis

Figure 7A and B. Spectra of the TPD of ethanol on PM11-50/25, 50 °C (a), 100°C (b), 200°C (c), 300°C (d) and 400°C (e). Underlined, black, white and italic numbers are related to acetate, carbonate, ethoxide species and ethanol adsorbed, respectively. 104x80mm (300 x 300 DPI)


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Figure 8. Spectra of the TPD-MS of ethanol on t-ZrO2 (A), AgCeO2 (B) and PM11-50/25 (C). 104x80mm (300 x 300 DPI)


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ACS Catalysis

Figure 8. Spectra of the TPD-MS of ethanol on t-ZrO2 (A), AgCeO2 (B) and PM11-50/25 (C). 104x80mm (300 x 300 DPI)


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