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Iron-Based Bimetallic Nanocatalysts for Highly Selective Hydrogenation of Acetylene in N,N-Dimethylformamide at Room Temperature Binbin Huang, Tao Wang, Zhan Yang, Wentao Qian, Jimei Long, Guangming Zeng, and Chao Lei ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02413 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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ACS Sustainable Chemistry & Engineering

Iron-Based Bimetallic Nanocatalysts for Highly Selective Hydrogenation of Acetylene in N,N-Dimethylformamide at Room Temperature Binbin Huang,†,‡,* Tao Wang,†,‡ Zhan Yang,†,‡ Wentao Qian,†,‡ Jimei Long,†,‡ Guangming Zeng,†,‡ Chao Lei§,* †

College of Environmental Science and Engineering, Hunan University, Lushan Road, Changsha

410082, China. E-mail: [email protected] ‡

Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of

Education, Lushan Road, Changsha 410082, China. §

School of Hydraulic Engineering, Changsha University of Science & Technology, Changsha

410114, China. Email: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: Selective hydrogenations of alkynes are a class of essential reactions in organic synthesis chemistry. Particularly, the selective hydrogenation of acetylene to ethylene is a key step in the production of polymers. Here we have successfully performed

selective

hydrogenation

of

acetylene

to

ethylene

in

N,N-dimethylformamide by iron-based nanoparticles (NPs), especially by Pd-Fe bimetallic NPs. NaBH4 as a hydrogen source can significantly increase the catalytic performances of nanocatalysts for acetylene hydrogenation. More importantly, the reaction is carried out at exceptionally mild temperature and under additive-free conditions with high ethylene selectivity (>90%) as well as excellent catalyst reactivity and stability. By this strategy, we could attain a catalytic activity higher by a factor of 2.2 orders of magnitude than that of the currently used industrial method. This approach may open a new way to perform selective acetylene and other alkynes hydrogenation under mild conditions, and offer another promising application for zero-valent iron reduction method.

KEYWORDS: iron-based bimetallic nanoparticles; Pd; acetylene; selective hydrogenation; N,N-dimethylformamide; isolation; room temperature.

2


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1

INTRODUCTION

2

Ethylene, known as a building block in the production of polymers, is one

3

of the most produced chemicals in the world. The global annual demand for

4

ethylene is over 150 million tons and still increases with an annual rate of

5

4.5%.1 Ethylene is industrially produced by the steam cracking method and

6

always contains traces of acetylene, which acts not only as an impurity for the

7

ethylene feedstock, but also as a poison for the downstream polymerization

8

catalyst.2 Therefore, the removal of acetylene from ethylene streams is a pivotal

9

step in the petrochemical process. Catalytic selective hydrogenation, with the

10

conversion of contaminant to valuable reactant, is regarded as the most efficient

11

way for acetylene removal and widely applied in the industrial process. 3,4

12

Herein, the heterogeneous catalyst plays a key role in this reaction. Palladium

13

(Pd) has long been recognized to possess the highest activity towards acetylene

14

conversion, but yet with limited ethylene selectivity and long-term stability,

15

resulting in overhydrogenation to ethane and the formation of oligomer (green

16

oil) simultaneously.2,5-6 It has been reported that the modification of Pd with a

17

second less active metal or organic ligand, at the expense of impairing Pd

18

activity, could result in a significant increase in ethylene selectivity.7-10

19

Particularly, intermetallic compounds (IMC) like, Pd-Ga,4,11-13 Ni-Zn,2,14 Fe or

20

Co with Al (Al13Fe4, Al13Co4),15,16 have gained increasing attention in the field

21

of catalysis research recently. Owing to the site-isolation concept and the

22

distinct structural and electronic modification for IMC, it leads to a high 3



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1

ethylene selectivity and an excellent stability towards acetylene hydrogenation

2

process. However, catalytically selective hydrogenation does have drawbacks,

3

including rigorous working conditions (i.e. high temperature (200°C) and high

4

pressure (typically 25-30 bar) with external CO addition), deactivation

5

problems associated with coking and green oil accumulation, lower ethylene

6

selectivity as a result of overhydrogenation and high processing cost due to the

7

use of expensive materials.

8

Acetylene isolation from ethylene streams is another important method

9

towards acetylene removal.17-21 Due to the chemical interaction differences

10

between carbon-carbon triple and double bonds, solid porous materials, 17-19

11

including zeolites, activated carbons, and metal-organic frameworks (MOFs)

12

exhibit promising separation characteristics toward acetylene removal. Notably,

13

with particularly high surface areas, adjustable pore dimensions, chemical

14

tenability and other excellent surface properties, MOFs are attracting

15

considerable attention as adsorbents in both gas storage and separation

16

applications.17-19 In addition, solvent-based absorbents like acetone and

17

N,N-dimethylformamide (DMF), owing to their excellent solubility for

18

acetylene but very poor solubility for ethylene, have been widely used in the

19

removal of acetylene from ethylene streams in the petrochemical industry. 20-21

20

Despite their remarkable capacity in acetylene removal, isolation methods do

21

not convert the separated acetylene to valuable ethylene. In our recent work, 22

22

acetylene

dissolved

in

DMF

can

undergo

selective

4


electrochemical

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1

hydrogenation to ethylene with high ethylene selectivity under mild conditions.

2

However, the selective hydrogenation of acetylene in DMF by a heterogeneous

3

catalyst has never been truly achieved.

4

Zero-valent metal nanoparticles (NPs), involving additional bimetals have

5

attracted considerable research interest due to their potential applications in a variety

6

of heterogeneous catalytic hydrogenation procesesses,22-31 especially in the

7

hydrodechlorination and Suzuki-Miyaura cross-coupling reactions.28-31 However,

8

there is no report on the potential liquid-phase hydrogenation of acetylene by

9

employing this method. Such an approach, if successful, would provide a means to

10

perform simultaneous isolation and in-situ hydrogenation of acetylene, and offer

11

another promising application for zero-valent metal reduction method. Herein, we

12

used nano-sized zero-valent iron (ZVI, Fe0) as electron donor and water as a hydrogen

13

source for providing available hydrogen as a result of iron corrosion, in order to

14

determine the catalytic property of Fe NPs toward acetylene hydrogenation in DMF. A

15

series of transition metals, including noble (Pd, Pt, Au and Ag) and non-noble ones

16

(Cu and Ni), were chosen and reductively deposited onto the surface of nano-sized

17

Fe0 to form the bimetallic nanocatalysts (0.2%, n/n), and their catalytic activities for

18

acetylene hydrogenation were subsequently evaluated. The choice of these metallic

19

catalysts is mainly based on their known catalytic properties for hydrogenation

20

processes. Our experiments show that Pd-Fe bimetallic NPs have promising

21

hydrogenation ability toward acetylene, while the others (Cu-Fe, Ag-Fe, Pt-Fe and

22

Au-Fe nanocatalysts) exhibit a high ethylene selectivity. An alternative hydrogen 5



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1

source (NaBH4) was used for acetylene hydrogenation in the end and importantly,

2

results show that it can significantly increase the catalytic performances of bimetallic

3

nanocatalysts. Particularly, the Pd-Fe NPs exhibit excellent catalytic activity and

4

recyclability, by which can attain a degree of acetylene conversion over 90% and high

5

ethylene selectivity (ca. 90%) even after 5 reaction cycles. Most importantly, this

6

process is performed at ambient temperature under additive-free conditions. This is

7

the first report of Pd-based nanocatalysts that make use of NaBH4 as a hydrogen

8

source for selective acetylene hydrogenation.

9

EXPERIMENTAL SECTION

10

Nanoparticle preparation and characterization. Nano-sized Fe0 and

11

bimetallic NPs (Pd-Fe, Cu-Fe, Ni-Fe, Pt-Fe, Au-Fe and Ag-Fe) were

12

synthesized by using chemical reduction method in a stepwise manner. The

13

prepared NPs were then characterized by scanning electron microscopy (SEM,

14

JSM-6700F) coupled with energy dispersive X-ray spectroscopy (EDS,

15

OXFORD, INCA EDS) and dynamic light scattering (DLS, Nano-ZS90)

16

methods, in order to obtain the particle size as well as the surface morphology

17

and elemental composition information. The Pd-Fe bimetallic NPs were

18

characterized by transmission electron microscopy (TEM, JEM-2010) coupled

19

with EDS before and after catalysis test, in order to obtain the variations of

20

nanocatalyst morphology and elemental composition. The detailed synthesis

21

procedures for iron and iron-based bimetallic NPs and their characterization

6


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1

methods were provided in the supporting information. The prepared NPs were

2

preserved in ethanol under the protection of nitrogen gas before use.

3

Catalysis tests. This study was conducted by carrying out the hydrogenation reactions

4

in two different reactors. The first was a vial reactor. Catalysis tests on 0.2 g iron or

5

iron-based bimetallic NPs were typically carried out in a 20 ml screw top headspace

6

vial that contained 10 ml H2O-DMF solution at a shaker (300 rpm and 25°C). Before

7

catalyst testing, the iron or iron-based NPs were dried at 100°C by purging nitrogen

8

gas flow until completely ethanol removal, and subsequently added into the vial that

9

contained 10 ml solvent under the protection of nitrogen gas. A pre-determined

10

amount of acetylene 0.0715 mmol (35.0% acetylene in argon) was then injected into

11

the vial, which were then immediately placed at the shaker to embark the catalytic

12

reaction. Three parallel experiments were conducted for each heterogeneous catalysis

13

conditions in order to ensure the repeatability of experiment. One control experiment

14

in the absence of catalyst was carried out simultaneously.

15

The second reactor was a 250 ml flask, which was mainly utilized to magnify the

16

reaction scale in order to further evaluate the efficacy of the proposed catalytic

17

reduction approach based on the iron-based bimetallic NPs for acetylene

18

hydrogenation. The pre-treatment of bimetallic NPs was as analogous as the vial

19

experiment, while the reaction conditions were carefully described in the main text.

20

Briefly, a determined amount of acetylene (1.43 mmol) was injected into the 250 ml

21

flask reactor containing a 100 ml DMF-H2O solution (7:3, V:V) and 0.4 g Pd-Fe NPs

22

or Cu-Fe NPs, afterwards, the reaction mixtures were stirred under 3000 rpm and the 7



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1

acetylene hydrogenation reaction was initiated. When NaBH4 was used as the

2

hydrogen source, after introduction of acetylene, a stoichiometric amount of aqueous

3

NaBH4 (1.0 equiv) was added to trigger the catalytic hydrogenation reaction.

4

Similarly, three parallel experiments and one control experiment in the absence of

5

catalyst were conducted simultaneously.

6

The gas samples were withdrawn at fixed time intervals by a tight gas

7

syringe

8

chromatograph (GC) equipped with a mass spectrometer (MS). The MS was

9

scanned from 12 to 100 m/z every 80 ms, where the sum of these ions is

10

referred to as the total ion count (TIC). Quantification of the gas components

11

was performed by integrating the TIC and comparing the peak areas with the

12

calibration curves prepared by using the mixed gas standards (31.7% acetylene,

13

31.9% ethylene, 14.9% ethane, 5.2% methane, 2.03% 1-butene, 1.99% n-butane,

14

1.99% 1,3-butadiene and 2.00% propane; argon was used as the balance gas).

15

Acetylene was handled as a gas containing 35.0% acetylene and 65.0% argon.

16

The acetylene and mixed gas standards were prepared using gasses (the purities

17

of these gas are beyond 99.99%) purchased from Airichem Specialty Gases &

18

Chemicals Co., Ltd. (Dalian, China). DMF (from Sigma Aldrich, >99.8%,

19

HPLC grade) and all other chemicals (analytical grade) were used as received.

20

The aqueous solutions were prepared with Millipore-Q water (18.2 MΩ).

21

RESULTS AND DISCUSSION

22

for

the

analysis

by

a

Shimadzu

GCMS-QP2010

ultra

gas

The synthesized zero-valent iron particles, prepared through the chemical 8


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1

reduction method, were found generally in homogeneous spherical shape with

2

diameter between 40 and 100 nm (shown in Figure S1 and Figure S3(a)). No obvious

3

changes in both particle size and shape were observed after the deposition of the

4

second transition metal catalyst (Pd-Fe, Cu-Fe, Pt-Fe, Au-Fe, Ag-Fe and Ni-Fe). The

5

XPS spectra of Pd-Fe bimetallic NPs (Figure S5) showed the presence of Fe(III)

6

oxides and oxyhydrides, as well as the presence of zero-valent Fe, while zero-valent

7

Pd was found to bound on the ZVI particle surface. These results suggest a typical

8

structure comprised of a thin iron oxides layer with metallic iron core and the second

9

catalyst was reductively deposited on the shell, which is in good agreement with the

10

previous publications.32-34 No evident aggregation occurred in the preparation and

11

characterization processes for nano-sized Fe0 and iron-based bimetallic NPs. The

12

detailed characterization results of these NPs were provided in the supporting

13

information.

14

Catalysis tests on 0.2 g naked nano-sized Fe0 and iron-based bimetallic NPs

15

were typically carried out in a 20 ml screw top headspace vial, which contained

16

a fixed amount of acetylene (0.0715 mmol) dissolved in 10 ml DMF-water

17

solution, placed at a shaker (300 rpm and 25°C). Meanwhile, three parallel

18

experiments and one blank experiment in the absence of catalyst were

19

conducted for each heterogeneous catalyst. The catalytic hydrogenation process

20

is significantly influenced by the DMF/water ratio, as the availability of

21

hydrogen is determined by the corrosion of iron with water and the content of

22

DMF in solution determines the solubility of acetylene. As expected, no 9



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Page 10 of 27

1

acetylene conversion occurred in pure DMF for all the tested nanocatalysts due

2

to the lack of hydrogen. Surprisingly, no acetylene conversion took place on the

3

naked Fe0 NPs for 24 h regardless of the DMF/water ratios, indicating that Fe

4

as a catalyst is not active enough in enabling acetylene hydrogenation. Catalysis

5

tests of Pd-Fe bimetallic NPs indicated that the optimal DMF/water ratio was

6

7:3 (V:V) for the hydrogenation of acetylene; in addition, at this ratio the

7

repartition of acetylene between gas and liquid phases was found to be 6:94 at

8

distribution balance, therefore, this ratio, for comparison purpose, was set and

9

used in the following experiments, in order to evaluate the ethylene selectivity

10

and catalytic activity of all the tested nanocatalysts toward acetylene

11

hydrogenation.

12 13

Figure 1. Catalytic hydrogenation of acetylene in DMF/H2O solvent (7:3, V:V) on (a)

14

Cu-Fe; (b) Ag-Fe; (c) Au-Fe; (d) Pd-Fe; (e) Pt-Fe; (f) Ni-Fe bimetallic NPs. ()

15

acetylene; () ethylene; (▲) ethane; (▼) 1,3-butandiene; ( ◆) 1-butene; (

16

n-butane. For the sake of better comparison, the amounts of C4 compounds (n) were 10


)

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1

doubled in view of the coupling reaction of C2 compounds. Reaction conditions: 0.2 g

2

bimetallic NPs; 0.0715 mmol of acetylene dissolved in 10 ml DMF-water solvent (7:3,

3

V:V); 25 °C, 300 rpm, 24 h.

4

The product concentrations as a function of acetylene conversion on different

5

bimetallic NPs are illustrated in Figure 1, which clearly shows that the selective

6

hydrogenation of acetylene indeed occurred in the bimetallic catalysis system. The

7

acetylene conversion was accompanied with a fast transformation to ethylene,

8

particularly in the first 6 hours. For instance, on the Pd-Fe NPs, ca. 43%, 77% and

9

89% of acetylene were converted at 2 h, 4 h and 6 h, respectively, with the ethylene

10

selectivity at 78%, 65% and 48% correspondingly. Ethane started to generate and

11

accumulate from 1 h gradually, along with a sharp increase of ethane selectivity from

12

4% at 2 h to 24% and 43% at 4 h and 6 h, respectively. The acetylene hydrogenation

13

on Pd-Fe bimetallic NPs was found to display an evident hydrogenation sequence,

14

namely the first and the second hydrogenation with ethylene and ethane formation

15

correspondingly. This is in keeping with the previous study, concerning the catalytic

16

gaseous acetylene hydrogenation, in which acetylene hydrogenation over Pd follows a

17

sequential series of hydrogen addition reactions.35 Furthermore, as shown in Figure 1

18

(d), a clear rate difference between the first and the second hydrogenation was

19

observed. Compared to the Pd-Fe nanocatalysts, the acetylene conversion became a

20

little slower on the Cu-Fe NPs, as illustrated in Figure 1(a), where ca. 22%, 44% and

21

60% of acetylene were converted at 2 h, 4 h and 6 h, respectively. However, it should

11



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1

be noted that the ethylene selectivity always kept consistent at beyond 90% without

2

any ethane generation throughout the whole process, suggesting that Cu has a

3

remarkably high ethylene selectivity despite of a hydrogenation activity that is

4

slightly lower than Pd.

5

Further close inspection of Figure 1 indicates that the nature of

6

heterogeneous catalyst plays a significant role in the hydrogenation process. In

7

marked contrast to the nanocatalysts of Ag-Fe, Au-Fe and Pt-Fe, the bimetallic

8

NPs of Pd-Fe, Cu-Fe, and Ni-Fe seem to have more remarkable catalytically

9

activity toward acetylene hydrogenation. Moreover, the product selectivity also

10

displays a strong catalyst dependent property, where ethylene obviously was

11

not the sole hydrogenation product of the reaction between acetylene and

12

reactant(s), ethane and C4 compounds as a result of overhydrogenation and C-C

13

coupling reactions, respectively, were inevitably formed along with the

14

conversion of acetylene. However, an ideal heterogeneous catalyst should be a

15

combination of both high catalytic activity and remarkable ethylene selectivity,

16

and could convert all the acetylene to ethylene, without undesired products like

17

ethane and oligomer generations, such that there is a net increase in the amount

18

of ethylene. The ethylene selectivity over the rates of acetylene hydrogenation

19

on bimetallic nanocatalysts is therefore calculated and illustrated in Figure 2.

20

We compared the industrial catalyst (Pd20Ag80) as a reference12 with respect to

21

the currently established catalytic reduction system of iron-based bimetallic

12


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1

NPs (please note the different reaction conditions in Figure 2). Based on the

2

result of industrial catalyst, four regions are classified: (I) low activity and low

3

selectivity; (II) low activity but high selectivity; (III) high activity but low

4

selectivity; (IV) high activity and high selectivity. Figure 2 clearly shows that

5

high ethylene selectivity was obtained over Ag-Fe (99%), Pt-Fe (93%) and

6

Au-Fe (80%) nanocatalysts, but with relatively weaker hydrogenation activities.

7

Despite remarkable acetylene conversion rates achieved on Ni-Fe and Pd-Fe

8

nanocatalysts, the ethylene selectivity over them were a little lower, particularly

9

the latter (48%) after 6 h. Results indicate that both Pd-Fe and Ni-Fe bimetallic

10

NPs exhibit strong hydrogenation ability, which trigger reaction product not

11

limited to ethylene, but could enable further hydrogenation until ethane

12

formation in the presence of excessive hydrogen. In fact, the excellent

13

hydrogenation capacities of both Pd and Ni catalysts have long been recognized

14

in the catalytic hydrogenation of gaseous acetylene, 2,5-6,14,36 while their

15

hydrogenation abilities in liquid phase were further confirmed in this research.

16

In addition to high ethylene selectivity (>91%), Cu has a comparative catalysis

17

activity with respect to the industrial catalyst. Indeed, the remarkable catalytic

18

activity of Cu for acetylene hydrogenation has been confirmed in previous

19

studies.22,37 Therefore, according to the catalytic performances of these

20

bimetallic NPs for acetylene hydrogenation, two groups based on the ethylene

21

selectivity and catalytic activity are generally classified: Ag, Pt, Au and Cu

22

belong to one group with high ethylene selectivity, while Ni and Pd are 13



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1

classified to another group with relative lower ethylene selectivity but high

2

catalytic activity. We thus singled out Pd-Fe and Cu-Fe bimetallic

3

nanocatalysts for the subsequent experiments.

4

5

Figure 2. Selectivity in ethylene (percentage) of the bimetallic NPs and the Pd20Ag80

6

catalyst as a function of the activity (per hour). Acetylene hydrogenation reactions

7

were performed in DMF/H2O (7:3, V:V) containing a fixed amount of acetylene

8

(0.0715 mmol) and 0.2 g bimetallic NPs, in a 20 ml vial, at 25 °C and 300 rpm. The

9

results refer to a 6 h reaction time. When NaBH4 was used as a hydrogen source, the

10

hydrogenation reactions were performed in DMF/H2O (7:3, V:V) containing 1.43

11

mmol acetylene, 0.2 g bimetallic NPs and 1.0 equiv NaBH4, in a 250 ml flask, at

12

25 °C and 3000 rpm. The data is obtained after 1 h. Pd20Ag80 is a commonly used

13

industrial catalyst: in this case, the conditions refer to a gas stream (flow rate of 30

14

ml/min) containing 0.5% C2H2, 5% H2, and 50% C2H4 in He at 200 °C. The data

15

pertain to a 20 h reaction time.

14


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1

In order to further test the efficacy of the iron-based bimetallic NPs

2

catalysis system for acetylene hydrogenation, the reaction was carried out on a

3

larger scale and the recyclability experiments were conducted successively. A

4

250 ml flask was chosen as a reactor, which contained 100 ml DMF-water

5

solution (7:3, V:V) and 0.4 g Pd-Fe NPs. The acetylene hydrogenation reaction

6

was initiated immediately after a determined amount of acetylene introduction

7

(1.43 mmol) under magnetic stirring conditions. Figure S6(a) demonstrates that

8

ca. 42%, 72% and 84% of acetylene were converted in 2 h, 4 h and 6 h,

9

respectively, with a fast transformation to ethylene simultaneously, which was

10

in excellent agreement with the previous vial experiment. However, in contrast

11

to the vial experiment, the ethylene selectivity was still beyond 90% in 6 h only

12

with trace of ethane generation. We speculate that it is the more efficient

13

mixing conditions of substrates with Pd-Fe bimetallic NPs (3000 rpm) that

14

results in the high ethylene selectivity, mainly due to the fact that as ethylene

15

forms it is immediately released to the headspace as a result of its poor

16

solubility and weak adsorption affinity with Pd catalyst surface. As a matter of

17

fact, we verified that when ethylene was injected into a DMF-water solution

18

(7:3, V:V), ethylene was then found in the gaseous phase over the solution but

19

not in the latter. However, it is worth noting that the acetylene conversion

20

became notably reduced (ca. 10% in 6 h, as shown in Figure S3(b)) when Pd-Fe

21

nanocatalysts were replaced by the Cu-Fe NPs, indicating that Pd as a catalyst

22

exhibits much stronger hydrogenation activity than that of Cu. These results 15



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1

further confirm that the selective hydrogenation of acetylene can take place

2

efficiently by using this novel approach and suggest that the reaction conditions,

3

such as stirring speed and headspace volume, are important in designing a

4

pilot-test reactor. Another major concern of this approach is the potentiality to

5

recycle the whole system. Once the acetylene hydrogenation was complete, a

6

nitrogen flow was inserted into the flask to discharge the reacted and un-reacted

7

acetylene, and then the system was sealed, followed by the introduction of a

8

determined amount of acetylene (1.43 mmol) to trigger the hydrogenation

9

reaction again. Three recycle tests for the iron-based bimetallic NPs catalysis

10

system displayed an evident decline trend toward acetylene conversion over

11

Pd-Fe nanocatalysts, indicating some potential changes of catalyst surface

12

during reaction. These could be explained by the deactivation of Pd catalyst due

13

to the partly covered of its active sites by iron-corrosion byproducts. A further

14

TEM characterization for the recovered Pd-Fe bimetallic NPs after reactions

15

supports this assumption, as shown in Figure S3(c), where a thicker layer and

16

more iron oxides appeared on the particle surface, and obvious aggregation

17

phenomena were clearly observed. In general, despite a high ethylene

18

selectivity, it is evident that the limited hydrogen production rate as a result of

19

iron corrosion and the potential deactivation of catalyst are two important

20

determinant factors for acetylene hydrogenation by using the iron-based

21

bimetallic NPs catalysis approach.

16


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1

2

Figure 3. Catalytic results of the hydrogenation of acetylene. (a) Reaction conditions:

3

1.0 equiv NaBH4 as hydrogen source; 0.2 g Pd-Fe NPs; 1.43 mmol acetylene

4

dissolved in 100 ml DMF-water solution (7:3, V:V); 25 °C, 3000 rpm, 2 h; (b) recycle

5

tests of Pd-Fe NPs. Reaction conditions: 0.2 g Pd-Fe NPs; 1.43 mmol acetylene

6

dissolved in 100 ml DMF-water solution (7:3, V:V) and 1.0 equiv NaBH4 for each

7

cycle; 25 °C, 3000 rpm, 1 h.

8

Alternatively, we introduced NaBH4 (1.0 equiv) in place of water as a hydrogen

9

source for acetylene hydrogenation, in order to find a more efficient hydrogenation

10

manner. Except that 0.2 g Pd-Fe bimetallic NPs were used as catalyst, all other

11

reaction conditions were kept unaltered. We used a 250 ml flask containing 100 ml

12

DMF-water solution (7:3, V:V) and 0.2 g Pd-Fe NPs as the catalyst and introduced

13

1.43 mmol acetylene. Hydrogenation reaction was initiated upon addition of 1.0 equiv

14

NaBH4 under the vigorous magnetic stirring conditions (3000 rpm). As shown in

15

Figure 3 (a), the acetylene conversion in the presence of NaBH4 becomes remarkably

16

enhanced with ca. 56% and 87% in 30 min and 1 h, respectively, which is far more

17

efficient than using water as a hydrogen source. It should be noted that no ethane was 17



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1

generated even after 2 h of reaction, indicating that overhydrogenation can be

2

effectively prevented in this process. As shown in Figure 2, compared to the industrial

3

catalysis system, when NaBH4 was used as a hydrogen source, our approach has a

4

much better hydrogenation activity (higher by a factor of 2.2 orders of magnitude) and

5

a higher selectivity (>87% vs. ca. 50%) performed in the liquid phase under mild

6

conditions. This suggests the enormous potential of this novel approach. It is worth

7

stressing that unlike the benchmark catalyst Pd20Ag80, which has been highly

8

optimized in industry for application to gaseous acetylene hydrogenation, the catalysts

9

we have described are just raw materials and no attempts were made to modify or

10

engineer them in view of practical applications. In fact, we expect that further

11

optimization of this catalytic strategy may lead to an even better performance.

12

Moreover, another major characteristic of this process is the possibility to recycle the

13

whole contents of reaction mixtures. Recycle tests (Figure 3(b)) show a very good

14

stability of Pd-Fe bimetallic NPs, which attain a degree of acetylene conversion over

15

90% and high ethylene selectivity of ca. 90% even after 5 times recycle. Noteworthy,

16

whereas the selectivity towards C4 compounds amounted to 10%-15% in each recycle

17

experiment, a little higher than the system using water as a hydrogen source (ca. 10%),

18

the outstanding recycle results exclude the deactivation of Pd-Fe nanocatalysts due to

19

the coupling reactions, as the formation of C4 compounds was consistently observed.

20

Also, the TEM image of the reacted Pd-Fe bimetallic NPs, as shown in Figure S3(d),

21

demonstrates that essentially no significant changes on the particle morphology and

22

no obvious aggregation occurred compared to those of freshly prepared particles 18


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1

(Figure S3(b)), suggesting a rather good stability of these nanocatalysts in the

2

presence of NaBH4.

3

The acetylene hydrogenation reactions on other bimetallic NPs (Cu-Fe, Ni-Fe,

4

Au-Fe, Ag-Fe and Pt-Fe) were also carried out in the presence of NaBH4, in order to

5

evaluate the catalytic performances of these nanocatalysts using NaBH4 as a hydrogen

6

source, and the results are shown in Figure 2. Similarly, the hydrogenation activity of

7

these bimetallic NPs shows a strong catalyst dependence, in which Pd-Fe NPs exhibit

8

far more efficiency than the others. Despite the variations of acetylene hydrogenation

9

activity, as shown in Figure 2, the presence of NaBH4 can significantly enhance the

10

catalytic performances of all the bimetallic nanocatalysts. Moreover, remarkably high

11

ethylene selectivity was attained (>97%) on the other bimetallic NPs, while only

12

traces of ethane and C4 compounds, as a result of overhydrogenation and C-C

13

coupling reactions, respectively, were formed on the Ni-Fe, Pt-Fe and Cu-Fe NPs.

14

These results are very exciting and quite significant by considering the potential

15

application of this approach. Particularly, Pd-Fe nanocatalysts display remarkably

16

high activity, selectivity and stability for the selective hydrogenation of acetylene and

17

most importantly, this process was carried out in liquid phase under very mild

18

conditions, although obviously the optimal conditions were found at high-temperature

19

and pressure for gaseous acetylene hydrogenation. This is the first report that NaBH4

20

is used as a hydrogen source by catalytic hydrogen transfer for selective acetylene

21

hydrogenation. Whereas the reduction of other alkynes by NaBH4 has already been

22

reported, it subjects to either poor alkenes selectivity or requirement of surfactants as 19



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1

additive.38-40 The results here demonstrated may thus conceivably provide a new way

2

on the selective hydrogenation of other alkynes to alkenes, reactants of paramount

3

importance in manufacturing petrochemicals and fine chemicals.

4

CONCLUSIONS

5

The high solubility of acetylene in DMF and its efficient selective

6

hydrogenation to ethylene by Pd-Fe bimetallic NPs have led to develop a viable

7

strategy that combines both advantages of isolation and heterogeneous catalytic

8

approaches. By this strategy, it can achieve the goal of simultaneous isolation

9

and in-situ conversion of acetylene and, attain a remarkable catalytic activity

10

higher by a factor of 2.2 orders of magnitude than that of the currently used

11

industrial method. For the first time, catalytic hydrogen transfer by NaBH4 is

12

reported for acetylene hydrogenation, by which it can significantly enable the

13

catalytic performances of Pd-Fe and other bimetallic nanocatalysts with

14

remarkably high hydrogenation activity and ethylene selectivity (>90%). More

15

importantly, this process is performed at ambient temperature under

16

additive-free conditions. Recycle tests indicate a very good reactivity and

17

stability for Pd-Fe nanocatalysts. Considering that the acetylene hydrogenation

18

is a paradigmatic reaction catalyzed in a gaseous phase under high-temperature

19

and pressure conditions, the present report may provide a new direction for

20

selective alkynes hydrogenation under mild conditions and, offer another

21

promising application for zero-valent iron reduction method. Moreover, due to

22

the impressively increased demand of ethylene and the oil crisis of worldwide, 20


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1

selective hydrogenation of highly concentrated or pure acetylene can provide a

2

promising alternative route for production of ethylene.

3

ASSOCIATED CONTENT

4

Experiment section; characterization results; Figures S1-S6.

5

AUTHOR INFORMATION

6

Corresponding Authors

7

*(B.H.) E-mail: [email protected]

8

*(C.L.) E-mail: [email protected]

9

ACKNOWLEDGEMENTS

10

This work was financially supported by the National Natural Science

11

Foundation of China (No. 51408209, 51509021 and Project 51521006) and the

12

Fundamental Research Funds for the Central Universities of China (No.

13

531107040689).

14

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For Table of Contents use only

Table of Contents (TOC) graphic

TITLE: Iron-Based Bimetallic Nanocatalysts for Highly Selective Hydrogenation of Acetylene in N,N-Dimethylformamide at Room Temperature AUTHORS: Binbin Huang, Tao Wang, Zhan Yang, Wentao Qian, Jimei Long, Guangming Zeng, Chao Lei

The high solubility of acetylene in DMF and its efficient selective hydrogenation to ethylene by Pd-Fe bimetallic NPs have led to develop a viable strategy that combines both advantages of isolation and heterogeneous catalytic approaches.


Iron-Based Bimetallic Nanocatalysts for Highly ... - ACS Publications

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