Synthesis of 1-Butanol from Ethanol over Calcium Ethoxide

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Synthesis of 1-Butanol from Ethanol over Calcium Ethoxide: Experimental and DFT Simulation Dong Wang, Zhenyu Liu, and Qingya Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04993 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

Synthesis of 1-Butanol from Ethanol over Calcium Ethoxide: Experimental and DFT Simulation Dong Wang, Zhenyu Liu and Qingya Liu State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China. ABSTRACT: Conversion of bioethanol to 1-butanol has attracted much interest in the field of renewable resource conversion. Development of efficient catalysts is a hot topic for this reaction. In this work, calcium ethoxide was evaluated for ethanol condensation in a batch reactor. Results show that ethanol conversion is about 25% at 275-300 ºC in 8-10 h, with 1-butanol and total alcohols carbon yields of 11% and 22%, respectively. Carbon balance shows that acetaldehyde is an intermediate of the reaction. Density functional theory (DFT) simulation confirms the intermediate and indicates that the promoting role of calcium ethoxide originates from the adsorption of O atom in ethanol on the Ca atom of calcium ethoxide, which activates the ethanol to form the adsorbed acetaldehyde and activated H. The adsorbed acetaldehyde reacts with ethanol to form 2-butenol via the aldol condensation while the 2-butenol is hydrogenated by the activated H to form 1-butanol. For the aldol-condensation reaction, ethanol and acetaldehyde prefer adsorbing on the same calcium ethoxide molecule to on adjacent two calcium ethoxide molecules. The highest energy barrier of the former route for ethanol dehydrogenation is 288 kJ mol-1 while that of the latter route for 2-butenol formation is 319 kJ mol-1.



Corresponding Author: [email protected].

(Qingya Liu) 1

ACS Paragon Plus Environment

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1. INTRODUCTION 1-Butanol is a bulk platform chemical for the synthesis of butyl esters plasticizer, general extraction agent of perfume and solvent of paints.1 It is also a good additive of gasoline because of a similar energy density to gasoline and low solubility in water. Furthermore, 1-butanol gasoline has better shock resistance than ethanol gasoline.2,3 1-Butanol has been produced in industry by carbonylation of propylene with carbonyl cobalt, rhodium or ruthenium as the catalysts.4 However, the technology suffers from a low atomic utilization efficiency and unsustainability in the long term due to the use of fossil fuels to produce propene. Fermentation of starch, saccharide or lignin was also developed to produce 1-butanol,5,6 but its economics on industrial scales was still a challenge.7 With the fast developments in fermentation of lignocellulose to ethanol in the past decade, efficient conversion of ethanol to 1-butanol is receiving extensive attention. The catalysts reported for ethanol condensation include homogeneous Ru and Ir complexes8-14 and heterogeneous solids. The homogeneous catalysts showed high activities and excellent selectivity to 1-butanol (up to 99%) under relatively mild reaction conditions9 but suffered from difficulties in separation from the products. The heterogeneous catalysts are easy for separation and have been extensively studied, such as hydroxyapatites15-18 and Mg-Al mixed oxides (hydrotalcites),19-22 as well as alkaline metal oxide,23-25 Co powder,26 Ni/Al2O3,27 Cu/CeO228 and Na/ZrO229 as comprehensively reviewed by Gabriels et al.30 Exploration of novel heterogeneous catalysts is still underway to improve ethanol conversion and 1-butanol selectivity under mild conditions. It is recognized that balance of acidic and basic sites on catalysts is of significance for development of a novel efficient catalyst. The acid sites adsorb ethanol and intermediate, while the base sites activate the β-H of ethanol and catalyze hydrogenation of intermediate.31,32 Our 2


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recent work on the reaction of ethanol with calcium carbide indicated that calcium ethoxide was the main solid product at low temperatures or in a short reaction time. The positively charged Ca atom and negatively charged O atom enable calcium ethoxide to accelerate adsorption and reaction of polar organic matters.33 Calcium ethoxide has been reported to catalyze the ring-opening polymerization of cyclic compounds34 and the ester conversion reactions35 by the active lattice oxygen sites (base sites). These information suggest that calcium ethoxide may be active to promote activation and transfer of β-H of ethanol as reported for sodium methoxide in a two-phase catalyst system36,37 and therefore is evaluated for ethanol condensation to 1-butanol. Reaction mechanism of ethanol condensation is always a hot topic. Two main pathways were reported in literatures. One is a tandem synthesis route known as Guerbet reaction, involving dehydrogenation of ethanol to form acetaldehyde, condensation of acetaldehyde to form crotonaldehyde and hydrogenation of crotonaldehyde to yield 1-butanol. The other is selfcondensation of two ethanol molecules, which includes activation and bonding of the β-H in an ethanol molecule with the hydroxyl in another ethanol molecule to form H2O, thereby to promote the formation of a C-C bond between the β-C of the former ethanol molecule to the α-C of the latter ethanol molecule. The dominant reaction route varies with the catalysts and the ethanol state (solution or gas phase), but divergence exists even for the same catalyst. For the hydroxyapatite catalysts, Scalbert et al. reported that at 350-410 °C the self-condensation of ethanol was the main route to generate 1-butanol while the aldol condensation of ethanol with acetaldehyde was the secondary route according to kinetics and thermodynamics analysis.17 However, Ho et al. concluded that the aldol condensation of two acetaldehyde molecules was the route for 1-butanol formation at 300-340 °C according to kinetic analysis.18 For the MgO catalysts, Nodu et al. reported that 1-butanol was from the self-condensation of ethanol at 450 °C based on a comparative 3



experimental study,23 while Birky et al. reported that the aldol condensation of acetaldehyde was the main pathway at 400 °C based on an isotope transient analysis.24 Obviously, the methods used in the literatures are not able to fully reveal the ethanol condensation pathway and catalytic mechanism. Quantum chemical calculations have been widely used to explore the reaction routes of various reactions during the past decade. The optimized configurations of various substances (reactants, transition states, intermediates and products) in a reaction system and the potential energy curves of the relevant reactions can be obtained by this method, which are powerful to distinguish the priority of different reaction mechanisms and rate-determining steps.38,39 Based on the above analysis, this work studies the ethanol condensation over calcium ethoxide of different loading. The effects of reaction temperature and reaction time on the ethanol conversion and the 1-butanol yield were obtained. The reaction mechanism and the evolution of calcium ethoxide during the reaction are examined based on the quantitative analysis of intermediates and verified by the quantum chemical calculation using density functional theory (DFT). 2. EXPERIMENTAL 2.1 Reaction experiment. The calcium ethoxide used is a commercial reagent from Gelest Inc. with a purity of greater than 97%. The ethanol used is from Beijing Chemical Works with a purity of 99.5% and was dehydrated with 3A molecular sieves before the experiments. In a typical experiment, 10.0 g ethanol (217 mmol) and 3.0 g calcium ethoxide (23 mmol) were introduced to a quartz liner of a stirred autoclave reactor (Parr 4597). The reactor was then sealed, purged with Ar to replace the air and heated to a designated temperature under stirring of 200 rpm. After a specified time, the reactor was immersed in an ice-water bath to quench the reaction. The 4


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gas product was collected in a gas bag while the solid and liquid products were transferred to a centrifuge tube for separation. The composition of gas product was analyzed by gas chromatography (Agilent GC-7890B). The average density of gas product was estimated according to its composition, and the mass of gas product was estimated according to the average density and the volume of gas product at the atmospheric pressure (Gas analysis conditions and detailed calculation can be found in Supporting Information). The solid residual was quantified after drying at 120 °C under a vacuum for 12 h. X-ray diffraction (XRD) analysis of the solid product was performed on a D8FOCUS Powder diffractometer. The organic carbon content of the solid residual was analyzed by a total organic carbon analyzer (SSM-5000A, Shimadzu, Japan). The liquid product was quantified by the mass difference between the total reactants and the gas and solid products. The composition of liquid product was analyzed on an Agilent GC-mass spectrometry (MS), and the quantitative analysis of some liquid components was determined on the Agilent GC-7890B (Liquid analysis conditions can be found in Supporting Information). Based on the product analysis results, ethanol conversion, carbon yields of liquid products, gas products and solid residual, as well as carbon balance (%C) were determined (For more details, see formulas (S6)-(S10) in Supporting Information). 2.2 Computational method. DFT quantum chemical calculations were performed using the Gaussian 09 software.40 The initial structures of all the involved molecules, such as the reactants, transition states, intermediates and products, were constructed using Gauss View 5.0, and the full geometries were optimized by DFT calculations using the B3LYP correlation functional with the 6-311G (d,p) basis set at 275 °C and 101.325 kPa. The intrinsic reaction coordinate (IRC) approach was employed to verify whether real transition states were obtained. The energy of each 5



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configuration was calculated at the same theory level. The energy barrier corresponding to a given elementary reaction was calculated using Eq. (1), where △E, ETS and Er denote the energy barrier, energy of the transition state and the reactant, respectively.  E = ETS - Er

Eq. (1)

3. RESULTS AND DISCUSSION 3.1 Effects of reaction conditions on ethanol conversion and yields of alcohol products. The GC-MS results, shown in Figure S1 and Table S1 in the Supporting Information, indicate that the main alcohol products are 1-butanol, 2-butanol, 2-pentanol, 2-methyl-3-hexanol and 2-butenol, the other products are 2-pentanone, ethyl acetate, ethyl butyrate and ethyl 2-ethylbutyrate. All the products except 2-pentanol and 2-pentanone have been reported in literatures on ethanol-upgrading reaction over metal oxide and hydroxyapatites catalysts.22,27,41 The ethyl butyrate, 2-methyl-3hexanol and ethyl 2-ethylbutyrate are downstream products of 1-butanol.

Figure 1. The effect of temperature on ethanol conversion and the carbon yields of 1-butanol and other alcohols in 8 h. 6


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Figure 1 shows the effect of reaction temperature on ethanol conversion and the carbon yields of alcohol products in 8 h. The carbon yield of 1-butanol is listed separately to compare with the literature data. The carbon balance of all the runs are higher than 94% (see Table S2 in Supporting Information), indicating good reliability of the data. It is seen in Figure 1 that the ethanol conversion increases with increasing reaction temperature, from 8.0% at 235 °C to 27.4% at 315 °C, which is accompanied by an increase of total alcohols carbon yield, from 5.8% to 22.8%. It is also seen in Figure 1 that the 1-butanol carbon yield increases gradually from 3.0% at 235 °C to a maximum of 10.8% at 300 °C and then slightly decreases at 315 °C. Since the carbon yields of other alcohols and esters (Table S2) increase rapidly over the reaction temperature, the selectivity of 1-butanol decreases from 37.1% at 255 °C to 26.5% at 315 °C (Table S2).

Figure 2. The effect of reaction time on ethanol conversion and the carbon yields of 1-butanol and other alcohols at 275 °C.

The effect of reaction time on ethanol conversion and the carbon yields of alcohols were investigated at 275 °C. Figure 2 shows that the ethanol conversion and the carbon yield of total 7



alcohols gradually increase with time, from 6.7% and 4.3% in 1 h to 22.1% and 17.8% in 12 h, respectively. The carbon yield of 1-butanol increases also with time in 10 h, from 1.8% to 9.4%, but decreases beyond 10 h. Correspondingly, the selectivity of 1-butanol increases from 18.6% in 1 h to 31.5% in 10 h, which is higher than that of esters in 10 h, 20.8% in Table S2, suggesting the main reaction being relatively faster than the side reaction. The effect of calcium ethoxide loading on ethanol conversion, the carbon yields of alcohols and the space-time yield (STY) of 1-butanol at the reaction temperature of 300 oC in 8 h is shown in Figure 3. It should be pointed out that Ca(OEt)2 alone shows little reaction at 235-300oC. Ethanol reaction without calcium ethoxide indicates that its conversion is about 6.9 % at 300 oC for 8 h and little alcohols is formed. With increasing calcium ethoxide loading from 10 to 30 mmol, ethanol conversion, the carbon yields of 1-butanol and total alcohols increased from 16.0% to 30.4%, 6.6% to 12.7% and 13.3% to 23.9%, respectively. These phenomena indicate the promoting effect of calcium ethoxide on ethanol coupling to alcohols. However, the STY of 1-butanol decreased with increasing calcium ethoxide loading, from 55.4 to 41.6 gpro kg Ca(OEt)2-1 h-1.

Figure 3. The effect of calcium ethoxide loading on ethanol conversion, the carbon yields of 1-butanol and other alcohols, and space time yield of 1-butanol at 300 °C in 8 h. 8


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It is worth noting that the ethanol conversions are lower than the total carbon yields of the liquid products under all the reaction conditions. For instance, at 275 °C in 8 h the ethanol conversion is 17.1% while the total carbon yield of the liquid product based on the ethanol loading is 22.6%. This phenomenon suggests that the carbon in calcium ethoxide may contribute to the liquid product. It was reported that calcium ethoxide is reactive with many chemical reagents, especially those containing hydroxyl.42 In this regard, one may suggest reaction of calcium ethoxide with ethanol to form 1-butanol as shown in Re. (1). This hypothesis however cannot be confirmed by subsequent DFT simulation because the calculation does not converge. The other possibility is hydrolysis of calcium ethoxide by H2O generated from the ethanol condensation, which yields ethanol and calcium hydroxide as shown in Re. (2). This ethanol provides additional feedstock to the reaction system and participates in coupling reaction to form liquid product. The pathway about ethanol conversion to 1-butanol over calcium ethoxide will be discussed in Section 3.4. Ca  OCH 2 CH 3 2  2CH 3CH 2 OH  2CH 3CH 2 CH 2 CH 2 OH  Ca  OH 2 Ca  OCH 2 CH 3 2  2H 2 O  2CH 3CH 2 OH  Ca  OH 2

Re. (1) Re. (2)

3.2 Evolution of calcium ethoxide during ethanol condensation. XRD patterns of the solid product under various reaction conditions is shown in Figure 4. It shows strong calcium ethoxide peaks, weak calcium hydroxide peaks and weak calcite-type calcium carbonate peaks after 1 h reaction at 275 °C. The calcium hydroxide might result from the hydrolysis of calcium ethoxide as discussed above. The calcium carbonate may indicate decomposition of calcium ethoxide with the participation of extra O atoms as reported in the literature.43 With increasing reaction time, the calcium ethoxide peaks decrease, the calcium hydroxide and calcite-type calcium carbonate peaks 9



increase, while the vaterite-type calcium carbonate peaks appear and increase.44 At the reaction temperature of 315 °C, the peaks observed are only those of calcium hydroxide and two kinds of calcium carbonate. In a word, calcium ethoxide is gradually converted to calcium hydroxide and calcium carbonate in the reaction, and this evolution is intensified at a higher reaction temperature. This observation agrees with the TOC analysis which shows decreasing organic carbon content in the solid product over time (Table S2). Since calcium ethoxide gradually evolves during the ethanol condensation, it is not a conventional catalyst but does take the catalytic effect.

Figure 4. XRD patterns of calcium ethoxide and the solid products at various reaction conditions.

10


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3.3 Reaction network of ethanol conversion. As discussed earlier, 2-butenol was observed in the liquid product. It is further found that its yield increases in the first 1 h but then decreases over time (Table S2), indicating 2-butenol being an intermediate in ethanol condensation. The 2-butenol is supposed to be the reaction product of acetaldehyde and ethanol and the ethanol reaction over calcium ethoxide may follow aldol condensation route as reported for hydroxyapatite catalyst.17 The important intermediate acetaldehyde was observed in many literatures reporting the aldol condensation route15,18,45 but not in this work. This difference suggests that the reaction rate of acetaldehyde is fast over calcium ethoxide, which is supported by the quick observation of downstream products of acetaldehyde (2-butenol, ethyl acetate and ethyl butyrate). This suggestion is further confirmed by the gas products in Table 1, where the dominant product H2 is attributed to the dehydrogenation of alcohol to aldehyde,31,32 and the minor products CH4 and CO are attributed to the decomposition of acetaldehyde.30 The additional minor product C2H4 is attributed to the dehydration of ethanol.24,32

Table 1. The amounts of gas products produced in different reaction time at 275 °C. Entry

Time (h)

H2 (mmol)

CH4 (mmol)

CO (mmol)

C2H4 (mmol)

1

1

3.9

0.4

0.2

0.0

2

3

6.6

0.7

0.5

0.2

3

6

10.1

1.0

0.9

0.3

4

8

13.6

1.4

1.2

0.5

5

10

16.1

1.7

1.5

0.6

6

12

19.4

2.0

1.8

0.8

11



Above analysis indicates that the acetaldehyde from dehydrogenation of ethanol reacts in four reactions as shown in Scheme 1. These reactions are (1) coupling of acetaldehyde and ethanol to form 2-butenol, followed by hydrogenation of 2-butenol to form 1-butanol; (2) disproportionation of acetaldehyde to form ethyl acetate; (3) disproportionation of acetaldehyde and 1-butyraldehyde to form ethyl butyrate; (4) decomposition of acetaldehyde to form CH4 and CO. According to the carbon yields of various products, the acetaldehyde consumption (i.e., the production of acetaldehyde) was calculated and the results of each route are shown in Table 2. It is worth noting that the acetaldehyde consumption of Route (1) includes also the production of downstream products of 1-butanol, i.e. 2-pentanol, ethyl butyrate, 2-methyl-3-hexanol and ethyl 2-ethylbutyrate. It is known from Scheme 1 that the H2 is derived from the dehydrogenation of ethanol or 1butanol and consumed by the hydrogenation of 2-butenol. The amount of H2 from dehydrogenation of 1-butanol can be estimated according to the amount of 1-butyraldehyde which is obtained from the carbon yield of ethyl butyrate. The amount of H2 from the dehydrogenation of ethanol is obtained according to the amount of acetaldehyde as shown in Table 2. Consumption of H2 is obtained according to the production of 1-butanol and its downstream products. Based on the production and consumption of H2, the theoretical amount of H2 is determined and shown in Table 2. Obviously, the theoretical value is close to the experimental value, indicating that the reaction network shown in Scheme 1 is reliable.

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Scheme 1. Ethanol reaction network speculated according to liquid and gas products. Table 2. Amounts of acetaldehyde produced in various reactions and the overall H2 balance. Amount of acetaldehyde (mmol)

Amount of H2 (mmol)

Temp.

Time

(oC)

(h)

Route (1)

Route (2)

Route (3)

Route (4)

Theoretically

Actual

1

275

6

18.2

1.4

3.5

1.0

11.2

10.1

2

275

8

22.8

1.6

4.5

1.4

14.2

13.6

3

275

10

25.7

1.9

5.1

1.7

15.5

16.1

4

275

12

25.5

2.5

6.1

2.0

17.7

19.4

5

255

8

17.8

1.0

2.7

0.3

9.0

8.1

6

300

8

31.1

2.0

5.9

2.2

18.2

17.1

7

315

8

30.0

1.9

6.6

3.6

20.4

21.0

Entry

3.4 Reaction mechanism of ethanol on calcium ethoxide. To understand the catalytic effect of calcium ethoxide in the ethanol condensation to 1-butanol, adsorption and reaction of ethanol on calcium ethoxide are studied by the quantum chemical calculation based on DFT. Firstly, the ethanol adsorption on calcium ethoxide was calculated and optimized. The optimized configuration shown in Figure 5(a) indicates that the calcium ethoxide adsorbs ethanol through the interaction of Ca atom with the O atom of ethanol, which is similar to the interaction of ethanol with the Ca atom (Lewis acid sites) in hydroxyapatite.15 The changes in charges and configuration of ethanol before and after the adsorption are shown in Table 3. The charge difference and bond

13



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length between 3O and 1H as well as the bond angle of 1H-3O-5C all increase notably by the adsorption, indicating that 1H in the hydroxyl becomes more active and is easy to migrate. The energy difference of ethanol before and after its adsorption on calcium ethoxide indicates that the adsorption energy is -120 kJ mol-1. The transition state (TS1) of ethanol dehydrogenation is shown in Figure 5(b). It is seen that the H of hydroxyl (1H) and the α-H of ethanol (3H) are adsorbed on 1O and Ca in calcium ethoxide, respectively, to yield two activated H, during which ethanol is converted into acetaldehyde. The optimized configuration of the product is shown in Figure 5(c) with the energy barrier of ethanol dehydrogenation of 288 kJ mol-1. For comparison, the reaction of H in hydroxyl (1H) and α-H in ethanol (3H) to form H2 is also calculated. The transition state is shown in Figure S2 and the energy barrier is 324 kJ mol-1. These results indicate that the adsorbed H is easier to form in ethanol dehydrogenation.

Table 3. Changes of configuration and charge in ethanol before and after adsorption on calcium ethoxide Bond length (10-10 m)

Charge

Ethanol

Bond angle (o)

3O

1H

6C

4H

3O-1H

3O-5C

1H-3O-5C

3O-5C-6C

5C-6C-4H

Before adsorption

-0.613

0.389

-0.442

0.140

0.969

1.425

107.886

107.796

110.640

After adsorption

-0.708

0.442

-0.463

0.155

1.021

1.428

112.824

109.324

110.898

14


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Figure 5. The route of ethanol dehydrogenate to acetaldehyde on calcium ethoxide.

The acetaldehyde generated by ethanol dehydrogenation then reacts with ethanol to form 2butenol via aldol condensation. There are two possible pathways for this reaction. Pathway (1) is the reaction of the adsorbed acetaldehyde in Figure 5(c) with the adsorbed ethanol in Figure 5(a); Pathway (2) is the reaction of the adsorbed acetaldehyde in Figure 5(c) with an ethanol molecule on one calcium ethoxide. These two pathways will be evaluated separately. The optimized configurations of Pathway (1) are shown in Figure 6. The atoms in the reaction system are renumbered to facilitate discussion. It is seen that adjacent two calcium ethoxide molecules associate with each other through the interaction between the Ca and O atoms, which makes the adsorbed ethanol close to the adsorbed acetaldehyde. The 2H in 2C of ethanol gradually moves from 2C to 6O of acetaldehyde, and the C=O bond between 3C and 6O in acetaldehyde becomes a single bond to form the transition state TS2. Subsequently, 2H in ethanol bonds with 6O in acetaldehyde to form hydroxyl, and 2C bonds with 3C to generate 1,3-butanediol intermediate (IM1). Then, 4H on 2C gradually migrates to 2O of calcium ethoxide to form the

15



transition state TS3. Finally, 2-butenol is generated by the formation of 2C=3C double bond as well as the breakage of 3C-6O in acetaldehyde. In short, one hydrogen (4H) and one hydroxyl (6O-2H) are removed in Pathway (1).

Figure 6. Pathway of aldol condensation of ethanol and acetaldehyde adsorbed on adjacent two calcium ethoxide molecules to form 2-butenol.

The hydrogenation process of 2-butenol by two activated H (5H and 6H in Figure 6(e)) is shown in Figure 7. In which, the activated 6H and 5H migrate to 2C and 3C, respectively, to generate 1butanol via the transition state TS4 with an energy barrier of 182 kJ mol-1. This process is compared with the hydrogenation of 2-butenol by H2 because of the presence of H2 in the gas product, which has the transition state shown in Figure S3 and an energy barrier of 323 kJ mol-1. The energy barrier 16


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data indicate that 2-butenol hydrogenation by H2 is not in favor which is consistent with the high H-H bond energy of H2, and the activated H in adsorbed state is mainly responsible for the hydrogenation.

Figure 7. Pathway of 2-butenol hydrogenation by the activated H in adsorbed state.

The potential energy curve of the aldol condensation reaction in Pathway (1) is obtained by comprehensively analyzing the energy barrier of each step and shown in Figure 8. For comparison, the potential energy curve of ethanol dehydrogenation to form acetaldehyde and activated H is also shown in Figure 8. It is seen that the energy barrier of the aldol condensation between ethanol and acetaldehyde is the largest (319 kJ mol-1), indicating that it is the rate-determining step in the ethanol conversion to 1-butanol. This result seems inconsistent with the above experimental phenomenon (the reaction rate of acetaldehyde is fast over calcium ethoxide).

17



Figure 8. Potential energy curve of ethanol condensation to 1-butanol on adjacent two calcium ethoxide molecules.

The optimized configurations of reactants, intermediates, transition states and products of Pathway (2) are shown in Figure 9. The ethanol adsorbs on the calcium ethoxide in Figure 5(c) by the interaction of O in ethanol (4O) with Ca atom to yield Figure 9(a). The 7H in 8C of ethanol moves to 3O of acetaldehyde, and the C=O bond between 5C and 3O in acetaldehyde becomes a single bond to form the transition state TS2. Subsequently, 7H in ethanol bonds with 3O in acetaldehyde to form hydroxyl, and 5C bonds with 8C to form 1,3-butanediol intermediate (IM1). 8H on 8C migrates to 2O of calcium ethoxide to form the transition state TS3. Then, 2-butenol is generated by the formation of 5C=8C double bond as well as the breakage of 5C-3O. The activated 1H and 3H migrate to 5C and 8C, respectively, to form the transition state TS4. Finally, 1H and 3H are bonded with 5C and 8C, respectively, to generate 1-butanol.

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Figure 9. Pathway of aldol condensation of ethanol and acetaldehyde to 1-butanol on one calcium ethoxide molecule.

The potential energy curves of Pathway (2) is shown in Figure 10 along with that of ethanol dehydrogenation to form acetaldehyde for comparison. It is seen that the energy barrier of aldol condensation is 193 kJ mol-1, much lower than that in Pathway (1) (319 kJ mol-1), probably due to the small steric hindrance in Pathway (2). The energy barrier of ethanol dehydrogenation to acetaldehyde is the largest in Pathway (2), indicating that it is the rate-determining step. This result is consistent with the earlier experimental observation. The discussion so far indicates that aldol condensation of ethanol and acetaldehyde on one calcium ethoxide molecule (Pathway (2)) is the 19



dominant route for 1-butanol formation and that on adjacent two calcium ethoxide molecules (Pathway (1)) is minor.

Figure 10. Potential energy curve of ethanol condensation to 1-butanol on one calcium ethoxide molecule.

4. CONCLUSIONS 1-Butanol can be synthesized from the widely available bioethanol in the presence of calcium ethoxide. The reaction follows aldol condensation route where ethanol reacts with acetaldehyde (from dehydrogenation of ethanol) to form the intermediate 2-butenol. The process starts from the adsorption of ethanol on calcium ethoxide via the interaction of Ca atom with O atom in ethanol with the adsorption energy of -120 kJ mol-1. The adsorbed ethanol is converted into acetaldehyde during which two activated H are generated. Aldol condensation of ethanol and acetaldehyde to form 2-butenol prefers on one calcium ethoxide molecule (Pathway 2) to on adjacent two calcium ethoxide molecules (Pathway 1). The 2-butenol is finally hydrogenated by the activated H to form 1-butanol. The rate-determining step of Pathway (1) is aldol condensation with the energy barrier 20


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of 319 kJ mol-1 and that of Pathway (2) is ethanol dehydrogenation with the energy barrier of 288 kJ mol-1. During the process, calcium ethoxide may be transformed to ethanol and calcium hydroxide, which provides additional feedstock ethanol to the reaction system. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Qualitative and quantification analysis of the gas product and liquid product, total ion chromatogram of the liquid product and assignment of the main peaks, carbon balance and carbon yield of various products generated under different reaction conditions, the route of ethanol dehydrogenate to acetaldehyde and H2, the route of 2-butenol hydrogenation by H2.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. 21



ACKNOWLEDGMENT This work was supported by the Major State Basic Research Project (No. 2011CB201306). REFERENCES (1) Uyttebroek, M.; Hecke, W. V.; Vanbroekhoven, K. Sustainability Metrics of 1-Butanol. Catal. Today 2015, 239, 7-10. (2) Xu, Y.; Avedisian, C. T. Combustion of n-Butanol, Gasoline and n-Butanol/gasoline Mixture Droplets. Energy Fuels 2015, 29, 3467-3475. (3) Harvey, B. G.; Meylemans, H. A. The Role of Butanol in the Development of Sustainable Fuel Technologies. J. Chem. Technol. Biotechnol. 2011, 86, 2-9. (4) Lang, Y. D.; Biegler, L. T.; Maier, E. E.; Majewski, R. A. An Optimal Catalyst Management Strategy for Oxo Processes. Comput. Chem. Eng. 2000, 24, 1549-1554. (5) Dhamole, P. B.; Wang, Z.; Liu, Y.; Wang, B.; Feng, H. Extractive Fermentation with Nonionic Surfactants to Enhance Butanol Production. Bio. Bioenerg. 2012, 40, 112-119 (6) Nanda, S.; Golemi-Kotra, D.; McDermott, J. C.; Dalai, A. K.; Gökalp, I.; Kozinski, J. A. Fermentative Production of Butanol: Perspectives on Synthetic Biology. New Biotechnol. 2017, 37, 210-221. (7) Hazelwood, L. A.; Daran, J. M.; Maris, A. J. V.; Pronk, J. T.; Dickinson, J. R. The Ehrlich Pathway for Fuel Alcohol Production: A Century of Research on Saccharomyces Cerevisiae Metabolism. Appl. Environ. Microb. 2008, 74, 2259-2266. (8) Tseng, K. N. T.; Lin, S.; Kampf, J. W.; Szymczak, N. K. Upgrading Ethanol to 1-Butanol with a Homogeneous Air-stable Ruthenium Catalyst. Chem. Commun. 2016, 52, 2901-2904.

22


Page 22 of 28

Page 23 of 28 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


(9) Chakraborty, S.; Piszel, P. E.; Hayes, C. E.; Baker, R. T.; Jones, W. D. Highly Selective Formation of n-Butanol from Ethanol through the Guerbet Process: A Tandem Catalytic Approach. J. Am. Chem. Soc. 2015, 137, 14264-14267. (10) Dowson, G. R. M.; Haddow, M. F.; Lee, J.; Wingad, R. L.; Wass, D. F. Catalytic Conversion of Ethanol into an Advanced Biofuel: Unprecedented Selectivity for n-Butanol. Angew. Chem. Int. Ed. 2013, 52, 9005-9013. (11) Aitchison, H.; Wingad, R. L.; Wass, D. F. Homogeneous Ethanol to Butanol CatalysisGuerbet Renewed. ACS Catal. 2016, 6, 7125-7132. (12) Xie, Y.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D. Highly Efficient Process for Production of Biofuel from Ethanol Catalyzed by Ruthenium Pincer Complexes. J. Am. Chem. Soc. 2016, 138, 9077-9080. (13) Mazzoni, R.; Cesari, C.; Zanotti, V.; Lucarelli, C.; Tabanelli, T.; Puzzo, F.; Passarini, F.; Neri, E.; Marani, G.; Prati, R.; et al. Catalytic Biorefining of Ethanol from Wine Waste to Butanol and Higher Alcohols: Modeling the Life Cycle Assessment and Process Design. ACS Sustain. Chem. Eng., 2019, 7, 224-237. (14) Matsu-Ura, T.; Sakaguchi, S.; Obora, Y.; Ishii, Y. Guerbet Reaction of Primary Alcohols Leading to beta-Alkylated Dimer Alcohols Catalyzed by Iridium Complexes. J. Org. Chem. 2006, 71, 8306-8314. (15) Ogo, S.; Onda, A.; Iwasa, Y.; Hara, K.; Fukuoka, A.; Yanagisawa, K. 1-Butanol Synthesis from Ethanol over Strontium Phosphate Hydroxyapatite Catalysts with Various Sr/P Ratios. J. Catal. 2012, 296, 24-30. (16) Tsuchida, T.; Sakuma, S.; Takeguchi, T.; Ueda, W. Direct Synthesis of n-Butanol from Ethanol over Nonstoichiometric Hydroxyapatite. Ind. Eng. Chem. Res. 2006, 45, 8634-8642. 23



(17) Scalbert, J.; Thibault-Starzyk, F.; Jacquot, R.; Morvan, D.; Meunier, F. Ethanol Condensation to Butanol at High Temperatures over a Basic Heterogeneous Catalyst: How Relevant is Acetaldehyde Self-aldolization. J. Catal. 2014, 311, 28-32. (18) Ho, C. R.; Shylesh, S.; Bell, A. T. Mechanism and Kinetics of Ethanol Coupling to Butanol over Hydroxyapatite. ACS Catal. 2016, 6, 939-948. (19) León, M.; Díaz, E.; Vega, A.; Ordónez, S.; Auroux, A. Consequences of the Iron-aluminium Exchange on the Performance of Hydrotalcite-derived Mixed Oxides for Ethanol Condensation. Appl. Catal. B 2011, 102, 590-599. (20) Perrone, O. M.; Lobefaro, F.; Aresta, M.; Nocito, F.; Boscolo, M.; Dibenedetto, A. Butanol Synthesis from Ethanol over CuMgAl Mixed Oxides Modified with Palladium (II) and Indium (III). Fuel Process. Technol. 2018, 177, 353-357. (21) Marcu, I. C.; Tichit, D.; Fajula, F.; Tanchoux, N. Catalytic Valorization of Bioethanol over Cu-Mg-Al Mixed Oxide Catalysts. Catal. Today 2009, 147, 231-238. (22) Sun, Z.; Vasconcelos, A. C.; Bottari, G.; Stuart, M. C. A.; Bonura, G.; Cannilla, C.; Frusteri, F.; Barta, K. Efficient Catalytic Conversion of Ethanol to 1-Butanol via the Guerbet Reaction over Copper- and Nickel-Doped Porous. ACS Sustainable Chem. Eng. 2017, 5, 1738-1746. (23) Ndou, A. S.; Plint, N.; Coville, N. J. Dimerisation of Ethanol to Butanol over Solid-base Catalysts. Appl. Catal. A 2003, 251, 337-345. (24) Birky, T. W.; Kozlowski, J. T.; Davis, R. J. Isotopic Transient Analysis of the Ethanol Coupling Reaction over Magnesia. J. Catal. 2013, 298, 130-137. (25) Chieregato, A.; Ochoa, J. V.; Bandinelli, C.; Fornasari, G.; Cavani, F.; Mella, M. On the Chemistry of Ethanol on Basic Oxides: Revising Mechanisms and Intermediates in the Lebedev and Guerbet Reactions. ChemSusChem 2015, 8, 377-388. 24


Page 24 of 28

Page 25 of 28 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


(26) Zhang, X.; Liu, Z.; Xu, X.; Yue, H.; Tian, G.; Feng, S. Hydrothermal Synthesis of 1-Butanol from Ethanol Catalyzed with Commercial Cobalt Powder. ACS Sustainable Chem. Eng. 2013, 1, 1493-1497. (27) Jordison, T. L.; Lira, C. T.; Miller, D. J. Condensed-Phase Ethanol Conversion to Higher Alcohols. Ind. Eng. Chem. Res. 2015, 54, 10991-11000. (28) Earley, J. H.; Bourne, R. A.; Watson, M. J.; Poliakoff, M. Continuous Catalytic Upgrading of Ethanol to n-Butanol and >C4 Products over Cu/CeO2 Catalysts in Supercritical CO2. Green Chem. 2015, 17, 3018-3025. (29) Kozlowski, J. T.; Davis, R. J. Sodium Modification of Zirconia Catalysts for Ethanol Coupling to 1-Butanol. J. Energ. Chem. 2013, 22, 58-64. (30) Gabriëls, D.; Hernández, W. Y.; Sels, B.; Voort, P. V. D.; Verberckmoes, A. Review of Catalytic Systems and Thermodynamics for the Guerbet Condensation Reaction and Challenges for Biomass Valorization. Catal. Sci. Technol. 2015, 5, 3876-3902. (31) Wu, X.; Fang, G.; Tong, Y.; Jiang, D.; Liang, Z.; Leng, W.; Liu, L.; Tu, P.; Wang, H.; Ni, J.; et al. Catalytic Upgrading of Ethanol to n-Butanol: Progress in Catalyst Development. ChemSusChem 2018, 11, 71-85. (32) Carvalho, D. L.; Avillez, R. R. D.; Rodrigues, M. T.; Borges, L. E. P.; Appel, L. G. Mg and Al Mixed Oxides and the Synthesis of n-Butanol from Ethanol. Appl. Catal. A 2012, 415416, 96-100. (33) Masood, H.; Yunus, R.; Choong, T. S. Y.; Rashid, U.; Taufiq Yap, Y. H. Synthesis and Characterization of Calcium Methoxide as Heterogeneous Catalyst for Trimethylolpropane Esters Conversion Reaction. Appl. Catal. A 2012, 425-426, 184-190.

25



(34) Dias, M. L.; Palermo, L. C.; Silvino, A. C. Bulk Ring Opening Polymerization of L-lactide with Calcium Methoxide. Macromol. Symp. 2011, 299-300, 156-163. (35) Liu, X.; Piao, X.; Wang, Y.; Zhu, S.; He, H. Calcium Methoxide as a Solid Base Catalyst for the Transesterification of Soybean Oil to Biodiesel with Methanol. Fuel 2008, 87, 1076-1082. (36) Carlini, C.; Girolamo, M. D.; Macinai, A.; Marchionna, M.; Noviello, M.; Galletti, A. M. R.; Sbrana, G. Selective Synthesis of Isobutanol by means of the Guerbet Reaction: Part 2. Reaction of Methanol/ethanol and Methanol/ethanol/n-propanol Mixtures over Copper Based/MeONa Catalytic Systems. J. Mol. Catal. A 2003, 200, 137-146. (37) Carlini, C.; Macinai, A.; Marchionna, M.; Noviello, M.; Galletti, A. M. R.; Sbrana, G. Selective Synthesis of Isobutanol by Guerbet Reaction Part 3. Methanol/n-propanol Condensation by using Bifunctional Catalytic Systems Based on Nickel, Rhodium and Ruthenium Species with Basic Components. J. Mol. Catal. A 2003, 206, 409-418. (38) Fang, Z.; Wang, Y.; Dixon, D. A. Computational Study of Ethanol Conversion on Al8O12 as a Model for γ-Al2O3. J. Phys. Chem. C 2015, 119, 23413-23421. (39) Li, M.; Chen, J.; Wang, G. Reaction Mechanism of Ethanol on Model Cobalt Catalysts: DFT Calculations. J. Phys. Chem. C 2016, 120, 14198-14208. (40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian, Inc., Wallingford CT, 2010.1 (41) Silvester, L.; Lamonier, J. F.; Faye, J.; Capron, M.; Vannier, R. N.; Lamonier, C.; Dubois, J. L.; Couturier, J. L.; Calais, C.; Dumeignil, F. Reactivity of Ethanol over Hydroxyapatitebased Ca-enriched Catalysts with Various Carbonate Contents. Catal. Sci. Technol. 2015, 5, 2994-3006. 26


Page 26 of 28

Page 27 of 28 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


(42) Anastopoulos, G.; Dodos, G. S.; Kalligeros, S.; Zannikos, F. Biodiesel Production by Ethanolysis of Various Vegetable Oils Using Calcium Ethoxide as a Solid Base Catalyst. Int. J. Green Energy 2013, 10, 468-481. (43) Liu, X.; Piao, X.; Wang, Y.; Zhu, S. Calcium Ethoxide as a Solid Base Catalyst for the Transesterification of Soybean Oil to Biodiesel. Energy Fuels 2008, 22, 1313-1317. (44) Grasby, S. E. Naturally Precipitating Vaterite (mu-CaCO3) Spheres: Unusual Carbonates Formed in an Extreme Environment. Geochim. Cosmochim. Ac. 2003, 67, 1659-1666. (45) Hanspal, S.; Young, Z. D.; Shou, H.; Davis, R. J. Multiproduct Steady-State Isotopic Transient Kinetic Analysis of the Ethanol Coupling Reaction over Hydroxyapatite and Magnesia. ACS Catal. 2015, 5, 1737-1746.

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Synthesis of 1-Butanol from Ethanol over Calcium Ethoxide

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