Conversion of Ethanol and Acetaldehyde to Butadiene over MgO

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Conversion of Ethanol and Acetaldehyde to Butadiene over MgO-SiO2 Catalysts: Effect of Reaction Parameters and Interaction between MgO and SiO2 on Catalytic Performance Qiangqiang Zhu, Bin Wang, and Tianwei Tan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02060 • Publication Date (Web): 24 Nov 2016 Downloaded from http://pubs.acs.org on November 26, 2016

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ACS Sustainable Chemistry & Engineering

1

Conversion of Ethanol and Acetaldehyde to Butadiene over MgO−SiO2 Catalysts:

2

Effect of Reaction Parameters and Interaction between MgO and SiO2 on

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Catalytic Performance

4 5

Qiangqiang Zhu, Bin Wang*, Tianwei Tan*

6 7

Beijing Key Laboratory of Bioprocess, National Energy R&D Center for Biorefinery, College of

8

Life Science and Technology, Beijing University of Chemical Technology, No. 15 of North

9

Three-ring East Road, Chaoyang District, Beijing 100029, PR China

10 11 12 13

*Corresponding authors

14

E−mail: [email protected], [email protected]

15 16 17 18 19 20 21 22

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ACS Paragon Plus Environment


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ABSTRACT：：In order to investigate the effect of structural features on the catalytic

2

performance for the conversion of ethanol and acetaldehyde to butadiene, a series of

3

MgO−SiO2 catalysts with different structural properties were synthesized by tuning

4

calcination temperature, investigated and characterized. The best butadiene selectivity

5

of 80.7% appears for MgO−SiO2 catalyst calcined at 500°C using the mixture of

6

acetaldehyde/ethanol/water (22.5 wt%:67.5 wt%:10 wt%) as feed. Addition of

7

appropriate amount of water (10 wt%) improved butadiene selectivity by inhibiting

8

the formation of 1-butanol and C6 compounds. Results from XRD, FT-IR, and

9

MAS NMR indicate the generation of a great amount of amorphous magnesium

10

silicates along with few crystalline magnesium silicates for the catalyst calcined at

11

500 °C. XPS results indicate that it contains the lowest binding energies of both Si−O

12

and Mg−O from Si−O−Mg bonds. For the catalysts calcined at low temperature

13

(350 °C and 400 °C), more 1-butanol and C6 compounds formed, which is considered

14

to be related to residual Mg(NO3)2. Additionally, more ethylene, diethyl ether and

15

butylene isomers were produced over the MgO−SiO2 catalyst calcined at 700 °C with

16

the formation of forsterite Mg2SiO4. Further results from Py-IR, and CO2-TPD show

17

that the high catalytic performance is related to the presence of Lewis acidic sites and

18

intermediate amount of basic sites.

19

KEYWORDS: ethanol; butadiene; Si−O−Mg bonds; Two-step process; amorphous

20

magnesium silicates

21 22

2


29

Si

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INTRODUCTION

2

Butadiene is an important building block for the production of a wide variety of

3

synthetic rubbers, elastomers and resins upon self-polymerization or in conjunction

4

with other polymerizable monomers. Currently, butadiene is mainly obtained via

5

extractive distillation of the C4 fraction from naphtha steam crackers.1 Additionally,

6

with increased availability and decreased cost, ethanol, especially bioethanol from

7

nonfood biomass feedstocks, is potentially a promising platform molecule for the

8

production of a variety of value-added chemicals.2 Thus, the conversion of ethanol

9

into butadiene has aroused a new increasing attention over the past years due to both

10

environmental concerns connected with petrochemical processes and new economic

11

opportunities seen in bio-based feedstocks.

12

The conversion of ethanol into butadiene is divided into two processes: the

13

one-step process developed by Sergey Lebedev3 and the two-step process.4 The

14

two-step process includes partial ethanol dehydrogenation to acetaldehyde as the first

15

step, followed by the transformation of the mixture of ethanol and acetaldehyde into

16

butadiene as the second step. The mechanism of the ethanol conversion to butadiene

17

is complicated and has not yet been fully elucidated. Nevertheless, there is a

18

consensus on several key steps5−7 (Scheme 1): (1) the dehydrogenation of ethanol to

19

acetaldehyde, (2) aldol-crotonic condensation which is the integration reaction of

20

acetaldehyde

21

Meerwein−Ponndorf−Verley (MPV) type reduction by ethanol resulting in crotyl

22

alcohol, and (4) a final dehydration step to butadiene.

aldol

condensation

and

the

followed

3


dehydration,

(3)

a


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The most studied catalysts in ethanol to butadiene conversion are MgO−SiO28−18

2

and ZrO2−SiO219−22 based materials in one-step process, and Ta-SiO223−25 catalysts in

3

two-step process. Besides, Hf−SiO226 and ZrO2−SiO227 based catalysts were also

4

reported in recent literatures in one-step and two-step processes, respectively. Among

5

these catalysts, MgO−SiO2 containing materials are the most widely studied and are

6

considered as a potential basis for the development of industrial catalysts of the

7

process. A lot of research works on the effect of catalyst components, catalyst

8

preparation

9

mechanochemical) and reaction conditions have been carried out. Recent researches

10

on MgO−SiO2 catalyst have focused on the nature of active sites, especially the

11

interaction between MgO and SiO2, in the conversion of ethanol to butadiene process.

12

Kvisle et al. conducted studies on individual oxides of MgO and SiO2, MgO/SiO2

13

catalyst prepared by wet kneading, and MgO−SiO2 prepared by mechanical mixing in

14

this process.28 They concluded the synergic effect in catalytic performance is unlikely

15

due to the solely presence of MgO in the SiO2 or vice versa. Conversely, they

16

mentioned the possibility that this result is related to new but unclear Mg−O−Si

17

interactions. Natta et al. reported the requirement of a high dispersion of magnesia on

18

silica and the presence of a limited amount of amorphous magnesia hydrosilicate

19

phase shown by XRD results for high performance, resulting from the interaction of

20

dissolved Mg2+ with the silanols of the silica surface.29 Additionally, Cavani et al.

21

studied MgO−SiO2 catalysts with a high Mg/Si ratio (9−15) and mentioned the

22

importance of Mg−O−Si pairs characterized by a Lewis-type acidity.13 Notably,

methods

(wet

kneading,

mechanical

4


mixing,

sol-gel

and

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recently Sels et al. observed the presence of amorphous magnesium silicates phase

2

demonstrated by 29Si NMR and the OH stretch region FT−IR analysis.11 Larina et al.

3

also show that the formation of the Mg−O−Si structural fragments detected by

4

MAS NMR analysis leads to increase of its activity and selectivity in the conversion

5

of ethanol into butadiene.17 Above researches point out the importance of SiO2−MgO

6

interaction in ethanol conversion to butadiene, however, the accurate active phase

7

between SiO2 and MgO is remaining unclear.

29

Si

8

Above mentioned difficulty, at least partly, is related to the difficulty on the

9

characterization of amorphous magnesium silicates structures. The most used catalyst

10

characterization methods have inherently drawbacks in such characterization, for

11

example, XRD analysis is blind to the amorphous silicates, and IR spectra of the OH

12

stretch region, though useful to detect the presence of magnesium silicates, is still

13

difficult to accurately distinguish different magnesium silicates. Recently,

14

NMR analysis has been used to detect the presence of amorphous magnesium silicates,

15

providing meaningful information.11,17 Hence a comprehensive characterization of

16

catalysts with different characterization methods is necessary to locate the active sites

17

and understand SiO2 and MgO interaction.

29

Si MAS

18

Up to date, the MgO/SiO2 catalyst system is mostly used in one-step ethanol to

19

butadiene process. It is thus of highly interest to test such a catalyst in two-step

20

process, which has a potential possibility to improve butadiene selectivity. Hence in

21

this paper, MgO/SiO2 catalysts with different structure features were synthesized and

22

tested in two-step ethanol to butadiene process. The effect of reaction parameters and

5



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interaction between MgO and SiO2 on catalytic performance are reported after

2

detailed evaluation and comprehensive characterization.

3

EXPERIMENTAL SECTION

4

Materials and chemicals

5

All chemicals were used as received without further purification. Silica gel

6

（50−100 µm） and Mg(NO3)2·6H2O were supplied by Aladdin and Sigma-Aldrich,

7

respectively. Ethanol, acetaldehyde (99.5%) and magnesium oxide (50nm, 99.9%

8

metals basis) were purchased from Cleman Chemical, Adamas Reagent Co. and

9

Aladdin, respectively. Deionized water was used in all reactions.

10

Catalyst preparation

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The MgO−SiO2 catalysts were prepared by impregnation using ethanol as solvent.

12

The preparation of MgO−SiO2 catalyst (the molar ratio of MgO to SiO2 is 1:1) was

13

shown as an example. 25.6 g of Mg(NO3)2.6H2O was dissolved in 100 g of ethanol

14

under stirring, and then 6.0 g of silica gel was added to the resultant solution. The

15

mixture was stirred at 60 °C until ethanol was totally evaporated, and a gel was

16

formed. Afterwards, the gel was calcined at selected temperature in air for 3 h with a

17

heating ramp of 5 °C min−1. The same procedure was used to prepare the MgO−SiO2

18

catalysts with different MgO/SiO2 ratio.

19

Catalyst Characterization

20

Nitrogen

adsorption−desorption

measurements

were

performed

on

a

21

Micromeritics ASAP 2020 HD 88 surface area and porosity analyzer. The catalysts

22

were degassed at 180 °C in a vacuum of 1.33 × 10-3 Pa for 10 h and then switched to

6


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the analysis station for adsorption−desorption analysis.

2

The morphology of the obtained solids was observed via scanning electron

3

microscope (SEM) (HITACHI S-4800, HITACHI Ltd., Japan). Transmission electron

4

microscope (TEM) was recorded with a JEOL JEM-2010 high-resolution transmission

5

electron microscope equipped with an energy−dispersive X-ray (EDX) detector.

6

X-ray diffraction (XRD) measurements were carried out using a Bruker

7

diffracttometer with Cu radiation (40 kV, 120 mA), data were recorded in the 2Theta

8

range of 5−90° with an angular step size of 0.2° and a counting time of 8 s per step.

9

29

Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra

10

were recorded on a Bruker AV300 spectrometer (7.05 T). At this field, the resonance

11

frequency of 29Si is 59.6 MHz, the recycle time 30 s, and the pulse length 5.0 µs. The

12

spinning frequency of the rotor is 5 kHz. The catalysts were packed in 4 mm Zirconia

13

rotors. Tetramethylsilane was used as chemical shift reference.

14

X ray-Photoelectron spectroscopy (XPS) were acquired on a Thermo Scientific

15

K-Alpha spectrometer using an Al Kα (hν = 1486.6 eV) monochromatic small-spot

16

X-ray source. All binding energies were referenced to the C1s line at 284.8 eV, and

17

the XPS spectra were fitted by a Lorentzian−Gaussian function assuming a Shirley

18

background by XPSPEAK Version4.1 software package.

19

Fourier transform infrared resonance (FT-IR) of MgO−SiO2 catalysts in the OH

20

region of 3800−3600 cm−1 were recorded on a thermo Nicolet 6700 FTIR

21

spectrometer equipped with a MCT detector and a high temperature environmental

22

cell fitted with KBr window. In each IR tests, approximately 0.020 g catalyst was

7



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1

pressed into a pellet and placed into the cell. The catalyst was first dried in situ at

2

350 °C under vacuum for 30 min. Then a spectrum was collected at this temperature

3

to investigate the OH signals of the various MgO−SiO2 catalysts. FT-IR spectra in the

4

wavenumber range of 4000−400 cm−1 were also recorded with a BRUKER TENSOR

5

27 spectrometer at room temperature by diluting samples in KBr.

6

The acidic properties of the samples were evaluated by temperature-programmed

7

desorption of CO2 (CO2-TPD). The TPD profiles were normalized by sample weight.

8

The sample (0.1 g) was preheated in a glass U-tube at 350 °C for 1 h in He flow (20

9

mL min−1), and then the catalyst was cooled from 350 °C to room temperature. CO2

10

(20 kPa) was introduced into the glass tube at room temperature for 30 min. And then,

11

the catalyst was evacuated at 100 °C for 30 min. The TPD measurements were

12

conducted from 100 to 400 °C at a heating rate of 10 °C min−1 and the signals were

13

recorded.

14

Catalyst evaluation

15

The conversion of ethanol (EtOH) and acetaldehyde (AA) to butadiene was

16

performed in a downstream fixed-bed quartz reactor with an internal diameter of 8

17

mm and a length of 300 mm under atmospheric pressure. Nitrogen was carrier gas and

18

the flow rate was fixed at 5 mL min−1. Before the introduction of feedstock, 1 g

19

catalyst was charged into a quartz reactor, heated under nitrogen flow at 5 °C min−1 to

20

reaction temperature, and kept for 30 min. The catalytic reactions were carried out for

21

4 h time on stream. The products, kept at 180 °C to prevent condensation of certain

22

products, were analyzed on line using a Shimadzu 2014 GC equipped with a HP Plot

8


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1

Q column (0.53 mm id × 40 µm thickness × 30 m length) and an FID detector. It was

2

capable of detecting ethylene, propylene, butylene isomers, butadiene, diethyl ether,

3

ethyl acetate and C4 compounds. A cold trap at 4 °C was used to collect heavier

4

hydrocarbons, mainly C6 compounds. C6 compounds was detected in part of reactions

5

and identified by Agilent 5975 GC-MS, but not quantified. EtOH/AA Conversion, selectivity of products (Si), butadiene productivity and

6 7

carbon balance were calculated by following equations:

8

EtOH / AA conversion（%）=

9

S（ i %）=

(nEtOH + n AA ) - (nunreacted EtOH + nunreacted AA ) ×100 , (1) (nEtOH + n AA )

ni (nEtOH + nAA ) - (nunreacted EtOH + nunreacted AA )

× 100 ,

m butadiene , mcatalyst × time

(2)

10

Butadiene productivity（g g -1 h -1）=

11

Carbon balance（%）=

12

where i represents ethylene, propylene, butylene isomers, butadiene, diethyl ether,

13

ethyl acetate, crotonaldehyde, and 1-butanol, n is the amount of C moles of product i

14

in the stream of the reaction products, mcatalyst is mass of catalyst used, and mbutadiene is

15

mass of butadiene produced.

16

17

Effect of reaction conditions and catalyst synthesis parameters on catalytic

18

performance

∑S

i

,

(3)

(4)

RESULTS AND DISCUSSION

19

The MgO−SiO2 catalysts were synthesized by impregnation method using

20

ethanol as solvent, and investigated for the conversion of EtOH and AA to butadiene.

9



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1

The effect of reaction conditions and catalyst synthesis parameters (especially

2

calcination temperature) on catalytic performance was investigated in detail. In order

3

to compare product selectivity, a similar EtOH/AA conversion level of ≈ 30% were

4

obtained by tuning the weight hourly space velocity (WHSV) for each catalyst.

5

Effect of reaction conditions on catalytic performance

6

Shown in Table 1 and Table S1 is the effect of AA content on catalytic

7

performance of MgO−SiO2 (Mg/Si=1:1) catalysts. The introduction of AA into EtOH

8

greatly improved the catalytic activity and butadiene selectivity. An increase of AA

9

content from 0 to 30 wt% resulted in a constant increase of reaction rates (indicated

10

by the lower WHSV) of EtOH/AA, while the butadiene selectivity presents a volcano

11

shape. The highest butadiene selectivity (75.9%) was obtained at AA:EtOH (25

12

wt%:75 wt%) ratio with the 1-butanol selectivity of 9.2%. Lower AA concentration

13

leads to more formation of ethylene and diethyl ether selectivity, whereas higher AA

14

concentration more that of 1-butanol and others (mainly C6 compounds). Similar

15

result has also been reported in other literatures in two-step process.23−26

16

The effect of water content in feed on catalytic performance was further

17

investigated (Table 2 and Table S2). During this investigation, optimized AA/EtOH

18

(25 wt%:75 wt%) ratio was used. It is found that the addition of appropriate amount

19

of water (10 wt%) inhibited the formation of 1-butanol and C6 compounds without

20

increasing ethylene and ethyl ether selectivity. Further increasing water content to 50

21

wt% led to a reaction rate decrease of EtOH/AA with little changes in product

22

selectivity. As a result, a highest butadiene selectivity of 80.7% was obtained using

10


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1

the mixture of EtOH/AA /water (22.5 wt%:67.5 wt%:10 wt%) as feed, and the feed

2

was selected to investigate the effect of reaction temperature on catalytic performance.

3

This result clearly demonstrates that the beneficial effect of appropriate water content

4

on catalytic performance in this process.

5

The effect of water addition in feed on catalytic performance for the one-step

6

ethanol conversion to butadiene was also studied by Cavani et al. over MgO−SiO2

7

(Mg:Si=4:1) catalyst.13 These authors found that the addition of water (≈ 9 wt%) in

8

ethanol led to a decrease in ethanol conversion and a selectivity decrease of butylene

9

from 1-butanol dehydration and heaviest products, which is in line with our results. At

10

the same time, the addition of water also resulted in an apparent increase of ethylene

11

selectivity due to the formation of Bronsted acidity, which is different from the result

12

in the present study. The different effect of water on catalytic performance especially

13

ethylene selectivity can be understand by the different mechanism and

14

rate-determining step in one-step and two-step process for butadiene production using

15

MgO−SiO2 catalyst. The conversion of EtOH to AA is reported to be the

16

rate-determining step in one-step process for MgO−SiO2 catalysts, which also

17

confirmed by present study.11,16 However, the aldol condensation is the

18

rate-determining step in two-step process since there is no presence of intermediate

19

products (crotonaldehyde and crotyl alcohol) in products. It can be understood that the

20

competitive adsorption of added water on the active sites has different impacts on the

21

reaction rates of different steps in the multi−steps reaction, thus giving diverse

22

products distribution. In Cavani et al.’ experiments, the decrease of ethylene yield

11



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1

from 16.7% to 11.8% was observed after addition of ≈ 9 wt% water in ethanol

2

although the ethylene selectivity increased, meaning that the formation rate of

3

ethylene decreased after water addition in ethanol. Therefore the increase of ethylene

4

selectivity in one-step process should be attributed to different inhibition levels on

5

EtOH conversion to AA, affecting the formation of butadiene and ethylene. However,

6

addition of AA in feed of two-step process can avoid the inhibition effect on EtOH to

7

AA in a two-step process, while the formation of butadiene is less affected. Besides,

8

the production of 1-butanol and C6 compounds were inhibited by adding appropriate

9

amount of water. As a result, butadiene selectivity increased after water addition of ≈

10

10% in feed using MgO−SiO2 catalysts in two-step process, as observed in present

11

study.

12

Table 3 and Table S3 display the effect of reaction temperature on catalytic

13

performance. The reaction rate and butadiene selectivity are critically influenced by

14

reaction temperature. The reaction rate of EtOH/AA constantly increases with the

15

reaction temperature increases. To be noted, butadiene selectivity changes from 70.1%

16

to 80.7% with the reaction temperature increases from 310 °C to 350 °C. When the

17

reaction temperature further increases, butadiene selectivity declines, while the

18

selectivity for ethylene, butylene isomers as well as diethyl ether by dehydration

19

reaction increases obviously.

20

The effect of WHSV on catalytic performance was also carried out. The results

21

shown in Table S4 suggest that low WHSV (0.18 g g-1 h-1) would lead to high

22

selectivities of ethylene and diethyl ether, thus decreasing the butadiene selectivity.

12


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1

When WHSV was over 0.24 g g-1 h-1, a decrease of EtOH/AA conversion was

2

observed with WHSV increasing, while little change in product distribution was

3

found.

4

Effect of catalyst synthesis parameters on catalytic performance

5

The catalytic performance of MgO−SiO2 catalysts with different Mg/Si ratio is

6

shown in Table 4 and Table S5. The results reveal that an increase of Mg/Si ratio

7

from 1:95 to 1:1 leads to an increase of butadiene selectivity from 53.2% to 80.7%

8

with the selectivity of ethylene and diethyl ether decreasing. However, further

9

increase in Mg/Si ratio to 65:35 results in a drop of butadiene selectivity to 75.8%.

10

Catalytic performance of the MgO−SiO2 catalysts obtained at different

11

calcination temperature indicates that a simple tuning of calcination temperature

12

would obviously affect the reaction rate and product selectivity (Table 5 and Table

13

S6). The conversion of EtOH/AA constantly increases with increasing the calcination

14

temperature. With regard to by-products, it can be seen that high selectivities of

15

1-butanol and others (mainly C6 compounds) were achieved for the catalysts calcined

16

at low temperature (350 °C and 400 °C), while high selectivities of ethylene and

17

diethyl ether at high calcination temperature (700 °C). The highest butadiene

18

selectivity (80.7%) was obtained at an intermediate calcination temperature (500 °C).

19

The butadiene productivity increases as the calcination temperature increase from 350

20

to 600 °C, and then show a slight drop with increasing calcination temperature to

21

700 °C.

22

The stability of MgO−SiO2 catalyst calcined at 500 °C was studied at 350 °C

13



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1

using AA/EtOH/Water (22.5 wt%:67.5 wt%:10 wt%) mixture as feed with a WHSV

2

of 0.24 g g−1 h−1. The results were shown in Figure 1. EtOH/AA conversion declines

3

noticeably within 2 h, and then shows a slow decrease till 12 h. Butadiene selectivity

4

decrease slightly with 12 h time on stream, while 1-butanol selectivity increase

5

slowly.

6

In recent years, MgO−SiO2 based materials were extensively investigated in the

7

conversion of ethanol to butadiene.8−18 The catalytic performance of MgO−SiO2 based

8

catalysts was summarized in Table S7.8,10−14,16−18 Generally, MgO−SiO2 catalysts

9

without modification give low butadiene selectivity, where the highest butadiene

10

selectivity (57%) is obtained by Larina et al. using the MgO−SiO2 composition for

11

over 1 h time on stream.17 And the main by−products are ethylene and diethyl ether

12

due to low catalytic activity for EtOH conversion to AA.8,30 The MgO−SiO2 catalysts

13

with dehydrogenation promoters (such as Ag, Cu and Zn) exhibit apparently improved

14

catalytic performance: a higher butadiene selectivity and lower selectivity to ethylene

15

and diethyl ether for over 1 h time on stream, which mainly results from enhanced AA

16

formation from EtOH.10−13 The highest selectivity to butadiene (69% at 30% ethanol

17

conversion) was obtained using the MgO−SiO2 catalyst modified by a bimetallic

18

combination of Zr(IV) and Zn(II) (1.5 and 0.5 wt % each).12 Other catalysts systems,

19

such as ZrO2−SiO2 and Ta−SiO2 based materials, were also investigated. Catalytic

20

results of these catalysts in this reaction are summarized in Table S8.19−21,23−27

21

Therefore, the catalysts prepared in this work show a high butadiene selectivity of

22

80.7%, which is higher than those of other MgO−SiO2 based catalysts reported in

14


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1

literatures. Moreover, more optimal product distribution is presented in the two-step

2

process: quite low selectivity of ethylene and diethyl ether (each approximately 3%)

3

were obtained in this work. More importantly, the MgO−SiO2 catalysts obtained via a

4

simple control of calcination temperature clearly gave different catalytic activities and

5

products distribution, which provides opportunities to explore the structure and

6

performance relationship of MgO−SiO2 materials in this reaction.

7

Catalyst characterization

8 9

The MgO−SiO2 catalysts obtained by different calcination temperature and original

silica

gel

support

were

extensively

characterized

with

N2

10

adsorption−desorption, SEM, TEM, XRD, 29Si MAS NMR，XPS, FT-IR, Py-IR, and

11

CO2-TPD. The results are reported as follows.

12

Textural and morphology analysis.

13

The BET surface area and pore volume of the catalysts were summarized in

14

Table 6. The silica gel shows the largest surface area (161 m2 g−1) and pore volume

15

(0.96 cm3 g−1). Compared with silica gel support, the prepared MgO−SiO2 catalysts

16

exhibit a clear decrease in BET surface area and pore volume, which may arise from

17

the blocking of the support pores by MgO. In addition, the BET surface area and pore

18

volume increase with increasing calcination temperature.

19

SEM and TEM characterizations were conducted to investigate the morphology

20

of the catalysts. The silica gel shows large and unregular particles with size between

21

50 and 100 µm (Figure 2). For the MgO−SiO2 catalysts, particles of different sizes

22

are observed and there is no obvious difference between the catalysts obtained under

15



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

1

different calcination temperature. It should be noted that some SiO2 particles seem to

2

be broken during the catalyst preparation, resulting in lower particle sizes. Also, small

3

particles on the surface of particles are also found in the SEM image of MgO−SiO2

4

catalyst. TEM investigation (Figure 3) provided further detail information, and it

5

seems the catalyst was constructed by the aggregation of smaller particles. EDS

6

analysis was carried out along with the TEM study to investigate the element

7

composition of different area. The results show that the central part of catalyst has a

8

Si/Mg ratio of 1.15, indicating an interaction of MgO and SiO2 during catalyst

9

synthesis. However, the separate particles in the outer surface are mainly MgO

10

according to EDS analysis, in which the Si/Mg atomic ratios of point 2 are 0.01.

11

Above results indicate the particle is generally homogenous in elemental composition

12

with some particles rich in MgO in the outer surface.

13

XRD.

14

The XRD patterns of the MgO−SiO2 catalysts prepared at different calcination

15

temperature are shown in Figure 4. The broad and low intensity band (2θ = 20−30°)

16

is due to the presence of amorphous silica. Strong characteristic diffraction peaks that

17

match with crystalline periclase MgO (2θ = 36.9°, 42.9°, 62.3°, 78.6°), common in all

18

catalysts, indicate the formation of MgO. For the catalysts calcined at lower than

19

600 °C, with the exception of the peaks attributing to MgO and silica gel, no clear

20

diffraction peaks can be observed. As the calcination temperature further increases to

21

700 °C, new reflections (2θ = 32.4°, 35.7°, 36.5°, 39.7°) that match well with

22

forsterite Mg2SiO4 was observed, which is also reported by Cavani et al.13 Though no

16


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1

diffraction peaks belonging to magnesium silicates are observed for the catalyst

2

calcined at 500 and 600°C, the existence of magnesium silicate needs further analysis

3

since XRD is blind to amorphous phase. The presence of Mg(NO3)2 has a high

4

possibility, although it was not detected as the decomposition temperature of

5

Mg(NO3)2 is between 360°C and 500°C.

6

29

Si MAS NMR.

7

In order to explore the interaction between MgO and SiO2, the MgO−SiO2

8

catalysts were characterized by 29Si MAS NMR, allowing detection of the presence of

9

amorphous magnesium silicates phase.

10

The

29

Si MAS NMR spectra of silica gel support and MgO−SiO2 catalysts are

11

depicted in Figure 5. 29Si MAS NMR spectrum of silica gel support show two signals

12

with chemical shifts typical for amorphous silica materials: −110 ppm for a Si atom

13

with four siloxane bonds (Si*(OSi)4); −100 ppm for a Si atom with three siloxane

14

bonds and a hydroxyl group (Si*(OH)(OSi)3).31,32 However, significant changes are

15

presented in the spectra of the MgO−SiO2 catalysts. The signals at −110 and −100

16

ppm drastically decrease, and new signals appear at −62, −70, −75, −84, −92 and −97

17

ppm. Taken the MgO−SiO2 catalyst calcined at 500 °C as an example, the NMR

18

signals centered at −70, −75, −92 and −97 ppm appear but cannot be identified with

19

any crystalline phase in the XRD patterns, so they must be present in amorphous

20

compounds. According to literature, the new bands at −70, −75, −92 ppm can be

21

assigned to isolated [SiO44−] monomers, (Si*(OMg)2(OSi)2), and (Si*(OMg)(OSi)3),

22

respectively.11,33,34 This phase at −97 ppm is likely to be talc (Mg3[Si4O10](OH)2) in

17



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

1

which Mg sites may be distorted according to be report by MacKenzie et al.35 The

2

distortion of Mg is likely the reason that this phase cannot be detected in XRD

3

analysis.35 Interestingly, it was reported that talc based catalysts has a high

4

performance in the ethanol conversion to butadiene.14 For MgO−SiO2 catalyst

5

calcined at 350 °C, the signals at −93 and −98 ppm should be intermediate phases in

6

the formation process of (Si*(OH)(OSi)3) and talc-like phase, respectively. Compared

7

with that of MgO−SiO2 catalyst calcined at 500 °C, the spectrum of catalyst calcined

8

at 700 °C presents a decrease of the signal at −70 ppm and an apparent increase of the

9

signal at −62 ppm ascribable to forsterite Mg2SiO4,36 indicating that an increasing

10

calcination temperature leads to the transformation of isolated [SiO44−] monomers into

11

Mg2SiO4. These new signals from −70 to −97 ppm except −84 ppm demonstrate the

12

use of the ethanol solution during the impregnation with subsequent calcination leads

13

to the formation of amorphous phase magnesium silicates. Moreover, with increasing

14

calcination temperature, these resonances assigned to silica become weaker, and those

15

attributed to magnesium silicates become more intense, confirming that higher

16

calcination temperature results in the generation of more Mg−O−Si chemical bonds.

17

However, the further increase from 500°C to 700°C leads to the formation of more

18

forsterite Mg2SiO4.

19

XPS.

20 21 22

The MgO−SiO2 materials were further characterized by XPS to distinguish Si and Mg atoms in different chemical environments. The Si 2p XPS spectra of all catalysts are shown in Figure 6a. Only one peak

18


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1

centered at 103.2 eV appears for the silica gel, which agrees well with the literature of

2

silica.37 Compared with that of the silica gel, there is an apparent red shift of Si 2p for

3

all the MgO−SiO2 catalysts, indicating that the new species is created. All of the Si 2p

4

spectra can be divided into two bands (Table 7): one at the higher binding energy

5

(103.2 eV) attributed to Si−O−Si bonds from silica, and the other at lower binding

6

energy in the range of 101.0 to 101.7 eV, which can be assigned to the Si−O−Mg

7

chemical bonds from the mixture of various magnesium silicates taking into account

8

XRD and

9

band into different Si−O−Mg linkages in view of the complexity and uncertain

10

binding energy of different Si−O−Mg linkages from amorphous magnesium silicates.

11

It is worth noting that the binding energies of these Si−O−Mg bands are lower than

12

that of crystalline magnesium silicates, such as forsterite (Mg2SiO4), enstatite

13

(MgSiO3), and talc (Mg3Si4O10(OH)2), which show binding energies at 101.9, 102.3,

14

and 102.6 eV respectively.39 The low binding energy may be related to disordered

15

configurations for the formation of amorphous magnesium silicates. It is interestingly

16

observed that a red shift of the Si 2p from Si−O−Mg bonds occurs with increasing the

17

calcination temperature from 350 to 500 °C likely because of the formation of more

18

amorphous magnesium silicates. And then a blue shift occurs with the further increase

19

of calcination temperature from 600 to 700 °C, which can be elucidated by the

20

presence of more crystalline forsterite Mg2SiO4. Moreover, the high relative intensity

21

of magnesium silicate with low relative intensity of silica signals implies the

22

formation of a great amount of magnesium silicates.

29

Si NMR results.38 However, it is difficult to accurately divide this broad

19



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1

The Mg 2p XPS spectra of the catalysts are shown in Figure 6b, and nano MgO,

2

taken as blank catalyst, presents a peak at 49.2 eV. The MgO−SiO2 catalysts exhibited

3

two bands: one band at lower binding energy between 49.0 eV and 49.7 eV, and the

4

other at higher binding energy corresponding to isolated Mg2+ ions.40 Combining with

5

the XRD and 29Si MAS-NMR results, for the MgO−SiO2 catalysts, the broad peak at

6

lower binding energy can be assigned to the mixture of Mg−O from MgO and

7

Si−O−Mg linkages from different magnesium silicates. Each Mg 2p spectrum was

8

fitted with the sum of two Voigt functions for all MgO−SiO2 catalysts. Similar with Si

9

2p XPS spectra, the lowest Mg 2p binding energy at 49.0 eV appears for the

10

MgO−SiO2 catalyst calcined at 500 °C.

11

The O 1s spectra of the catalysts show a similar result with that of Si 2p and Mg

12

2p spectra (Figure S1), and the lowest peak at O 1s binding energy is presented by the

13

MgO−SiO2 catalyst calcined at 500 °C.

14

Together with these results, it can be concluded that a great amount of amorphous

15

magnesium silicate formed, and the low binding energies of both Si−O and Mg−O

16

bonds from Si−O−Mg linkages was obtained for the catalyst calcined at 500 °C.

17

FT-IR analysis.

18

In order to gain more information about magnesium silicates, the FT-IR spectra of

19

MgO−SiO2 catalysts were collected after desorption of water at 350 °C in vacuum.

20

The Figure 7 shows the development of different kinds of OH species in the OH

21

region of 3800−3600 cm−1.

22

It has been reported that the bands in the range of 3750−3700 cm−1 can be

20


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1

observed in both silica and MgO catalysts.16 Recently, Bruijnincx et al. studied the

2

FT−IR spectra of OH region on MgO−SiO2 catalysts and observed the similar signals,

3

concluding that the signals at ~3745, ~3730 and ~3700 cm−1 can be ascribed to

4

isolated silanols, hydroxyl groups on MgO, and the stretching of silanol groups,

5

respectively.16 The vibrations at ~3743, ~3734 and ~3699 cm−1 could thus be assigned

6

to similar species. Notably, the bands in the range of 3677 to 3671 cm−1 can be

7

attributed to an interaction between MgO and SiO2. Antigorite (Mg3Si2O5(OH)4) and

8

talc (Mg3[Si4O10](OH)2), two different types of magnesium silicates, show the peaks

9

at 3670 cm−1 and 3674 cm−1, respectively.41,42 The results indicate that the presence of

10

magnesium silicates for MgO−SiO2 catalysts, whereas the types of these magnesium

11

silicates cannot be distinguished due to the complexity of magnesium silicates

12

confirmed by 29Si MAS NMR analysis.

13

The FT-IR spectra of the MgO−SiO2 catalysts were also collected at room

14

temperature by diluting catalysts in KBr. Figure 8 and Figure S2 present the FT-IR

15

patterns in the wavenumber range of 1500−400 cm−1 and 4000−400 cm−1, respectively.

16

The bands at 3439 and 1644 cm−1, observed in all catalysts, are attributed to Si−OH

17

and adsorption H2O, respectively.43,44 Bands with maxima at 1099, 809, and 467 cm−1

18

for silica gel are assigned to asymmetric stretching vibrations, symmetric stretching,

19

and the bending vibrations of Si−O−Si bonds.45−47 The spectra of the MgO−SiO2

20

catalysts are significantly modified. For the catalysts calcined at 350 and 400 °C, the

21

peaks at 1088, 801 and 456 cm−1 attributed to asymmetric stretching, symmetric

22

stretching, bending vibration of the Si−O−Si bonds, respectively, shift to lower

21



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

1

wavenumbers, indicating the linkage between the silica gel and MgO may happen. In

2

addition, the peak at 1384 cm−1 is assigned to nitrates, suggesting nitrate cannot be

3

totally removed at such calcination temperatures.49,50 This result demonstrated the

4

presence of Mg(NO3)2 for catalysts calcined at 350 and 400 °C. However, the

5

MgO−SiO2 catalyst calcined at 700 °C displays peaks at 432, 543, 624, 892, and 1013

6

cm−1 assigned to MgO6 octahedral, Si−O (asymmetric deformation vibrations), Si−O

7

(bending), Si−O (asymmetric stretching), and Si−O, respectively, which is due to the

8

formation of forsterite Mg2SiO4.48,50−52 Additionally, the development of the bands at

9

1056 and 1028 cm−1 appears to reflect the evolution of Si environment from isolated

10

Si to Si in a silica-like environment.53

11

Acid−base properties of the MgO−SiO2 catalysts.

12 13

To probe the acid−base properties of the MgO−SiO2 catalysts, Py-IR, and CO2-TPD analysis were carried out.

14

The acid properties of the catalysts were analyzed by Py-IR. Results depicted in

15

Figure 9 reveal that only Lewis acid sites are present in these catalysts, which is in

16

line with the results reported by Weckhuysen et al.9 The intensity of the observed

17

FT-IR bands was used to calculate the amount of acidic sites (Table 6). A gradual

18

increase is observed in the amount of acidic sites when the calcination temperature is

19

raised from 350 °C to 600 °C. Further increase to 700 °C would lead to a slight

20

decrease of the amount of acidic sites. Since the acidic properties have been

21

considered to be an important parameter of MgO−SiO2 catalyst and it has been

22

extensively studied in literature, it is important to compare our results with those in

22


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1

literature.10−11,13,16,17

2

CO2-TPD results shown in Figure 10 and Table 6 indicate that the intensity and

3

number of basic sites gradually increase with an increase of calcination temperature.

4

Though they were extensively studied, the quantitative result of basic properties for

5

the MgO−SiO2 catalysts is still limited in the literature. The result reported by

6

Bruijnincx et al (0.015 mmol g−1) is comparable to our results.10

7

The relationship of the interaction between MgO and SiO2, acid−base properties

8

and catalytic performance of the MgO−SiO2 catalysts

9 10 11

The interaction between MgO and SiO2 plays a significant role in determining the acid−base property and catalytic performance of the as-prepared catalysts. Above comprehensive characterization especially

29

Si MAS NMR provided

12

detailed structural information of the MgO−SiO2 catalysts obtained under different

13

calcination temperature. XRD technique detected the presence of crystalline forsterite

14

Mg2SiO4 for MgO−SiO2 catalysts calcined at 700 °C. Subsequently, 29Si MAS NMR

15

results confirmed the formation of amorphous magnesium silicates for all MgO−SiO2

16

catalysts. Also, the presence of crystalline forsterite Mg2SiO4 for MgO−SiO2 catalysts

17

was demonstrated further by

18

variation of bands for silica gel and magnesium silicates indicates that the content of

19

amorphous magnesium silicates increase with the increase of temperature calcination

20

from 350 to 600 °C. Detailed XPS analysis suggests that, among these MgO−SiO2

21

catalysts calcined at different temperature, MgO−SiO2 catalyst calcined at 500 °C has

22

the lowest binding energy of both Mg−O and Si−O from Mg−O−Si linkages, which is

29

Si MAS NMR technique. More importantly, the area

23



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Page 24 of 54

1

likely to the most percentage of Mg−O−Si linkages from amorphous magnesium

2

silicates. Relative intensity of Si2p signals of silica and magnesium silicates shows

3

that most of silica gel on the surface of all MgO−SiO2 catalysts has been transformed

4

into magnesium silicates. For catalysts calcined at low temperatures (350 and 400°C),

5

the presence of Mg(NO3)2 was demonstrated through IR spectra.

6

A highest butadiene selectivity of 80.7% was obtained for the MgO−SiO2

7

catalysts at 500 °C, which may be connected with the presence of large amount of

8

amorphous magnesium silicates and few crystalline magnesium silicates. Meanwhile,

9

the catalyst also has the lowest binding energies of both Mg−O and Si−O bonds from

10

Mg−O−Si linkages, while the relationship between low binding energy and high

11

butadiene is necessarily to be identified in further work. The presence of amorphous

12

magnesium silicates was also observed by Sels et al. and Larina et al. via

13

NMR.11,17 The results both in literature and in this study indicate the importance of the

14

presence of amorphous magnesium silicates of MgO−SiO2 catalysts on catalytic

15

performance for EtOH or EtOH/AA conversion to butadiene. Moreover, the content of

16

amorphous magnesium silicates and butadiene productivity have similar trend with

17

increasing calcination temperature, indicating the formation of butadiene is closely

18

related to the presence of amorphous magnesium silicates. The formation of ethylene

19

and diethyl ether would be correlated with presence of forsterite Mg2SiO4 for

20

MgO−SiO2 catalyst calcined at 700 °C. A weak peak attributed to forsterite Mg2SiO4

21

is observed for the catalyst calcined at 500 and 600 °C, showing low ethylene

22

selectivity (2.5 and 4.3%, respectively), while a strong peak attributed to forsterite

24


29

Si MAS

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1

Mg2SiO4 for the catalyst calcined at 700 °C exhibited a high ethylene selectivity

2

(12.0%). A similar result was also reported by Cavani et al.13 The MgO−SiO2 catalyst

3

calcined at 350 °C with the presence of residual Mg(NO3)2 gave more 1-butanol and

4

C6 compounds.

5

In one-step ethanol to butadiene conversion process, many efforts have been paid

6

to the development of correlation between acid/basic properties and catalytic

7

performance. Cavani et al. showed that MgO−SiO2 catalysts with a high Mg/Si

8

atomic ratio in the range between 9 and 15 gave better butadiene yields due to the

9

proper combination of strong basic sites, required for ethanol activation, and a

10

moderate number of medium-strength acid sites, needed for the dehydration of

11

intermediately formed alkenols to butadiene.13 Weckhuysen et. al studied the

12

acid−base properties of the equimolar MgO−SiO2 catalysts prepared by different

13

methods, and proposed that the wet kneaded ones were performing better because of

14

the appropriate balance among a small amount of strong basic sites, combined with an

15

the intermediate amount and proximity of acidic and basic sites of moderate strength

16

which perform the aldol condensation step most efficiently.16 Generally the results

17

call for a right balance between the acid/base amount and strength. In this work,

18

butadiene productivity and number of Lewis acidic sites showed similar trends: they

19

first increased with the calcination temperature increased from 350 to 600 °C, then

20

decreased with further increasing to 700 °C. Combining with characterization results,

21

it is proposed that the formation of amorphous magnesium results in Lewis acidic

22

sites, which influences catalytic performance. Nonetheless, the cooperation of basic

25



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

1

sites is significant and necessary. It is noteworthy that a great amount of stronger

2

basic sites together with least acid sites for MgO−SiO2 sample calcined at 700 °C

3

leads to highest ethylene and diethyl ether selectivity. Generally, dehydration of

4

alcohols occurs over acid sites，but Díez et al. reported that MgO catalysts modified

5

with alkali were also capable for dehydration via a cooperative interaction between

6

acidic and basic sites.54 In this case, a surface alkoxide is formed first, after which a

7

strong basic site abstracts a proton in β-position to the O atom; O elimination finally

8

yields the alkene. Also, ether formation involves cooperative interaction of acid and

9

basic sites and is thought to proceed via the same mechanism proposed for ethylene

10

formation. Therefore the mechanism on the formation of ethylene and diethyl ether

11

can be attributed the cooperative interaction between acidic and basic sites, which is

12

also observed by Weckhuysen et al.10 And accordingly, it seems suitable balance

13

between acid/base properties is also critical for the two-step process.

14

CONCLUSIONS

15

MgO−SiO2 catalysts with different surface properties were synthesized by a

16

simple tuning of calcination temperature, and investigated for the conversion of EtOH

17

and AA to butadiene. The effect of water content in EtOH and AA on catalytic

18

performance indicates that an appropriate addition of water (10 wt%) would improve

19

butadiene selectivity by reducing the formation of 1-butanol and C6 compounds. The

20

best MgO−SiO2 catalyst, showing highest butadiene selectivity (80.7%), was that with

21

a great amount of amorphous magnesium silicates and few crystalline magnesium

22

silicates. In the reaction, the mixture of AA/EtOH/water (22.5 wt%: 67.5 wt%: 10

26


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29

1

wt%) was used as feed. According to detailed

Si MAS NMR and XPS study, it is

2

found that the amorphous magnesium silicates contain lowest binding energies of

3

both Si−O and Mg−O from Si−O−Mg bonds. Residual Mg(NO3)2 or the formation of

4

crystalline forsterite Mg2SiO4 has a negative effect and is inclined to the formation of

5

more byproducts. Furthermore, the presence of Lewis acidic sites and intermediate

6

amount of basic sites is significant for the butadiene productivity in the two-step

7

process.

8

9

Supporting information

ASSOCIATED CONTENT

10

Table S1−S6, the effect of AA content and water content in feed, reaction temperature,

11

WHSV, Mg/Si ratio, calcination temperature on catalytic performance; Table S7 and

12

Table S8, catalytic performance in literatures for the conversion of ethanol into

13

butadiene in 2011-2016; Figure S1, O 1s XPS spectra of the MgO–SiO2 samples;

14

Figure S2, FT-IR spectra of the MgO–SiO2 samples in the region 4000–400 cm−1.

15

16

Corresponding Authors

17

*E−mail: [email protected], [email protected]

18

Notes

19

The authors declare no competing financial interest.

20

21

We gratefully acknowledge the support of the National Basic Research Foundation of

22

China (973 program) (2013CB733600). This work was supported by the China

AUTHOR INFORMATION

ACKNOWLEDGEMENTS

27



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1

Petrochemical Corporation Project (214086).

2

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3

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1 2

Scheme 1. Main Reaction Pathway of Ethanol Conversion to Butadiene

3

35



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

1

Page 36 of 54

Table 1. The effect of AA content in feed on catalytic performancea AA content

Carbon selectivity (C mol%)

WHSV

EtOH/AA

−1

conversion

（g g

Butylene

balance (%)

29.4

3.6

96.4

62.6

9.4

5.9

97.1

4.1

73.2

3.7

8.3

95.8

2.6

2.9

75.9

1.7

9.2

95.5

2.5

2.8

71.8

1.6

10.7

92.3

Butadiene

(%)

0

0.06

27.7

35.6

3.7

1.3

22.8

10

0.20

28.6

14.8

2.5

1.9

20

0.28

32.0

4.1

2.4

25

0.36

31.5

3.2

30

0.38

30.2

2.9

(wt%)

Diethyl

1-Butanol

Ethylene Propylene

h−1）

b

Carbon

isomers

ether

2

a

3

temperature: 350 °C. bAA content (wt%)= MassAA /MassAA+EtOH × 100

Conditions: MgO−SiO2 (Mg/Si=1) catalyst calcined at 500 °C: 1 g, reaction

4

36


Page 37 of 54

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1

Table 2. The effect of water content in feed on catalytic performancea

Water content WHSV （g g−1

Carbon selectivity (C mol%)

EtOH/AA conversion

Butylene

balance (%)

1.7

9.2

95.5

80.7

2.0

6.5

96.8

3.2

80.7

1.0

5.2

96.9

2.6

3.0

81.5

1.3

6.4

97.1

3.4

3.2

80.4

1.2

5.7

97.2

Butadiene

(%)

Conversion of Ethanol and Acetaldehyde to Butadiene over MgO

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