zeolites and deactivation

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Understanding the active sites of Ag/zeolites and deactivation mechanism of ethylene catalytic oxidation at room-temperature Hongling Yang, Chunyan Ma, Xin Zhang, Yang Li, Jie Cheng, and Zhengping Hao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02410 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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ACS Catalysis

1

Understanding the active sites of Ag/zeolites and deactivation

2

mechanism of ethylene catalytic oxidation at room-temperature

3

Hongling Yang,1,2 Chunyan Ma,*,1,2 Xin Zhang,1,2 Yang Li,1 Jie Cheng,1,2

4

Zhengping Hao*,1,2

5

1. Department of Environmental Nano-materials and Technologies, Research

6

Center for Eco-Environmental Sciences, Beijing 100085, China

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2. National Engineering Laboratory for VOCs Pollution Control Material &

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Technology, University of Chinese Academy of Sciences, Beijing 100049,

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China

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*E-mail: [email protected]; [email protected]

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ABSTRACT

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Microporous Ag/zeolite (ZSM-5, Beta, Y and Mordenite) catalysts were

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found to be potential good catalytic materials for ethylene oxidation at

4

room temperature. These catalysts were evaluated in both dry and humid

5

atmosphere, achieving 100% of 100 ppm of ethylene mineralized into

6

CO2 at 25 oC. Moreover, the zeolite framework type and relative humidity

7

had a significant effect on catalytic stability. Pyridine fourier transform

8

infrared spectra (FTIR) and solid-state hydrogen-1 (1H) magic angle

9

spinning nuclear magnetic resonance (MAS NMR) studies revealed that

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Brønsted acid sites were the active sites in Ag/zeolites, the deactivated

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Ag/zeolites had no available Brønsted acid site. When the number of

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Brønsted acid sites of Ag/ZSM-5-humid and Ag/Beta-humid reduced, the

13

catalytic activities of Ag/ZSM-5-humid and Ag/Beta-humid decreased.

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Temperature-programmed oxidation-mass spectrometry (TPO-MS) and

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water adsorption/desorption results indicated that H2O adsorbed on

16

Brønsted acid sites led to the disappearance of available Brønsted acid

17

sites, and thus resulted in Ag/zeolite catalyst deactivation. The

18

understanding of active sites and deactivation mechanism in ethylene

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oxidation on zeolite catalysts is helpful to synthesize a better ethylene

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oxidation catalyst and develop an effective technology of eliminating

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trace ethylene.

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Keywords: ethylene, low-temperature oxidation, Ag, zeolite, deactivation ACS Paragon Plus Environment

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ACS Catalysis

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1. INTRODUCTION

2

Ethylene (C2H4), a low-molecular-weight volatile organic compound

3

(VOC), is a plant hormone, which enhances ripening and decaying of the

4

fruits and vegetables.1,2 Even low concentrations of ethylene can cause a

5

faster deterioration of fresh fruits and vegetables, so it is important to

6

remove ethylene from storage facilities. Low-temperature catalytic

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oxidation is the most promising cost-effective method for removing

8

ethylene.3-5

9

To the best of our knowledge, the complete oxidation of ethylene at

10

low temperatures are achieved mainly on mesoporous Co3O4 supported

11

Au catalysts and Pt/MCM-41 catalyst.6,7 Their catalytic stabilities need to

12

be further improved, moreover, complicated preparation process of

13

mesoporous Co3O4 and low hydrothermal stability of MCM-41 support

14

extremely restricted their application in ethylene oxidation. In contrast

15

with mesoporous support of Co3O4 and MCM-41, the microporous

16

aluminosilicate zeolites are easily obtained, can act as a support to load

17

and disperse metal nanoparticles. Microporous aluminosilicate zeolites as

18

catalysts are used in different reactions, the acidic properties of zeolites

19

determine the catalytic performances in the acid-catalyzed reactions.8-14

20

Thus, it is desirable that zeolites acting as catalysts or catalyst supports

21

are used for ethylene oxidation at room temperature and obtain good

22

catalytic performances, specially the catalytic stabilities.


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Except for acidity of microporous zeolite, framework type of

2

microporous zeolite is of critical importance for the given catalytic

3

reaction. Sultana et al. investigated the conversion of levulinic acid to

4

aromatics over small pore FER, medium pore ZSM-5, and large pore Y,

5

BEA and MOR, the highest catalytic activity of ZSM-5 was attributed to

6

its three-dimensional pore geometry but not the acidity.12 Bokhoven et al.

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reported that the number of Brønsted acid sites varied with framework

8

topologies of zeolites. ZSM-5, MOR, Beta and Y showed different

9

catalytic activities in propane cracking and dehydrogenation.13 Hunger et

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al. compared the catalytic activity of ethylbenzene disproportionation

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over medium-pore HZSM-5 and the large-pore HY, HZSM-5 showed

12

higher reactivity due to its larger numbers of Brønsted acid sites and

13

smaller pores.14 In this case, it is essential to study the catalytic

14

performances of zeolites with different framework types for a given

15

reaction, which will help us to understand the effect of framework type or

16

acidity of zeolites on the chemical reaction.

17

In the study of catalytic stability of mesoporous Pt/MCM-41 in

18

ethylene oxidation, Fukuoka et al. reported the gradual deactivation of

19

Pt/MCM-41 was ascribed to the adsorption of H2O on the catalyst surface

20

for ethylene oxidation,7 but no unequivocal identification of the active

21

sites has been proposed. Moreover, there is very limited understanding of

22

mechanistic pathways over the mesoporous zeolite deactivation by


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adsorbing H2O in ethylene oxidation.

2

Herein, we report the catalytic performances of ethylene oxidation

3

over microporous zeolites (ZSM-5, Beta, Y and Mordenite) and the

4

corresponding Ag/zeolites in dry and humid atmosphere. To the best of

5

our knowledge, Ag/zeolites have not been studied so far for ethylene

6

oxidation reaction. The catalytic active sites and the deactivation reason

7

of Ag/zeolites in ethylene oxidation reaction are explored through

8

characterizing the fresh and deactivated catalysts. The reaction

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mechanism of ethylene oxidation over Ag/zeolites is proposed to

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understand Ag/zeolite catalysts deactivation in ethylene oxidation. These

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are very important to develop the meaningful environmental catalytic

12

materials and technology for elimination of ethylene.

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2. EXPERIMENTAL SECTION

14

2.1. Catalyst preparation. Commercial ZSM-5, Beta and

15

Mordenite zeolites with SiO2/Al2O3 ratio of 25, and Y zeolite with

16

SiO2/Al2O3 ratio of 5.2 were supplied by the Nankai Catalyst Plant. All

17

the Ag/zeolite catalysts were prepared by using the vacuum-assisted

18

impregnation method. The zeolite support material (1.0 g) was

19

impregnated with 3.2 mL of 0.15 mol·L-1 AgNO3 in a vacuum

20

environment (P = –0.1 MPa gauge pressure) for 1 h, and impregnated at

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atmospheric pressure for 12 h. The resulting solid was dried at 100 °C in

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air overnight, followed by calcination under air flow at 600 °C for 2 h. ACS Paragon Plus Environment

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2.2. Catalyst characterization. X-ray photoelectron spectroscopy

2

(XPS) measurement was carried to determine the elemental composition

3

and chemical states of the elements on the surface of catalysts. This data

4

was accumulated on an Axis Ultra ESCA system with a monochromatic

5

Al Kα standard radiation source. The binding energies were calibrated by

6

referencing the C1s at 284.8 eV.

7

Solid-state aluminum-27 (27Al) magic angle spinning nuclear

8

magnetic resonance (MAS NMR) and 1H MAS NMR spectra were

9

collected in a Bruker Advance 400 spectrometer. Prior to the MAS NMR

10

experiment, 0.2 g of sample was packed into a glass tube, which was later

11

connected to a vacuum line. The temperature was gradually increased at a

12

rate of 1.5 °C·min-1, and was held at 300 °C at a pressure below 1.3 × 10-6

13

kPa for 12 h to dehydrate the sample. After the sample was cooled to

14

room temperature, the glass tube was sealed. Next, the sealed sample was

15

transferred into a 4 mm NMR rotor. The

16

obtained at 104.23 MHz, and the chemical shifts referred to Al(H2O)63+.

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The chemical shifts of 1H MAS NMR were referenced to TMS at 0 ppm.

27

Al MAS NMR spectra were

18

Fourier Transform-Infrared Spectra (FT-IR) were collected using a

19

Bruker Tensor 27 FTIR spectrometer. Self-supporting thin wafers, used

20

in pyridine adsorption studies, were placed in a quartz holder with an

21

attached heating filament. This was mounted in an in situ cell connected

22

to a vacuum system. Prior to the measurements, each sample was


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degassed in vacuum (< 10-4 Pa) by heating to 500 °C at a rate of

2

10 °C·min-1 and held at this temperature for 4 h to desorb any possible

3

physisorbed species. After the wafer was cooled down to room

4

temperature, the adsorption of pyridine was carried out for 1 h.

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Desorption was performed in vacuum environment (< 10-4 Pa) at elevated

6

temperatures (150, 250, 350 and 450 °C). Relative determination of

7

Brønsted and Lewis acidity was derived from the areas of the IR bands at

8

ca. 1540 and 1450 cm−1, respectively, using the extinction coefficients

9

given by Emeis.15

10

X-ray diffraction (XRD) patterns were recorded on an X’Pert PRO

11

(PANalytical) diffractometer equipped with a Cu Kα radiation source (λ

12

= 0.15418 nm). A continuous mode was used for collecting data in the 2θ

13

range from 5° to 90° at a scanning speed of 5 °·min-1.

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Static adsorption equilibrium measurements of H2O on Ag/zeolite

15

catalysts were investigated using an intelligent gravimetric analyzer

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(Model IGA-002; Hiden Isochema Instrument, Warrington, UK) with a

17

sensitivity of 0.1 µg. The apparatus has an ultrahigh vacuum system with

18

a fully computerized microbalance, allowing adsorption isotherms to be

19

determined by setting pressure steps. Before the measurement, each

20

sample was degassed at 300 °C (< 10-9 kPa) for 4 h.

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The H2O-temperature programmed desorption (H2O-TPD) was

22

performed in the aforementioned IGA apparatus, each sample (60 mg)


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was degassed at 300 °C (< 10-9 kPa) for 4 h. Before H2O-TPD

2

measurement, the adsorption process was performed by allowing the

3

sample to adsorb saturated water vapor at 25 °C (P = 2.86 kPa). Then, the

4

sample was heated to 405 °C at a rate of 0.1 °C·min-1 at 2.86 kPa. The

5

desorbed water was measured by using the weight loss of the sample. The

6

weight loss curves were recorded, which were called the TPD curves of

7

H2O.

8

In-situ infrared spectra of the samples were recorded on a Bruker

9

Tensor27 using the diffuse reflectance infrared fourier transform (DRIFT)

10

spectroscopy scanned from 4000 to 850 cm-1 at a resolution of 4 cm-1. For

11

Ag/ZSM-5 in dry atmosphere, before the infrared spectra were recorded,

12

the sample was swept with He gas at 500 °C for 1 h, and then the catalyst

13

bed temperature was lowered to 25 °C. The mixed gas (100ppm C2H4/21%

14

O2/He balance) with a total gas flow of 25 mL·min-1 passed through 5A

15

molecule sieve to be dried, and then passed through the sample cell, after

16

30 min one spectrum was recorded. Next 21% O2/He balance gas passed

17

through 5A molecule sieve, and then passed through the sample cell, the

18

spectra were recorded after 5, 15, 25 and 35 min, respectively. For

19

Ag/ZSM-5 in humid atmosphere, the mixed gas (100ppm C2H4/21%

20

O2/He balance) with a total gas flow of 25 mL·min-1 passed through the

21

sample cell, after 30 min one spectrum was recorded. Then 21% O2/He

22

balance gas passed through the sample cell, the spectra were recorded


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ACS Catalysis

1 2

after 5, 15, 25 and 35 min, respectively. 2.3.

Ethylene

oxidation

activity

measurements.

Ethylene

3

oxidation experiments were performed in a U-shape quartz tubular

4

reactor loading with 0.2 g of catalyst (40-60 mesh), placed in a water bath

5

at 25 °C. The catalytic performance was evaluated under dry and humid

6

atmosphere. The reaction gas of 100 ppm C2H4/21% O2/79% He,

7

followed by drying or wetting, was flowed through the catalyst bed at a

8

flow rate of 25 mL·min-1, which corresponded to a space velocity (SV) of

9

7500 mL·h-1·g-1. The drying reaction gas is that 100 ppm C2H4/21%

10

O2/79% He passed through 5A molecule sieve to be dried. The relative

11

humidity of 50% (water vapor in air) was adjusted by varying the 100

12

ppm C2H4/21% O2/79% He gas flow rate through a bubbler placed in a

13

water bath at 25 °C. The water vapor concentration was measured online

14

using a hygrometer (Apresys, USA), which was expressed as relative

15

humidity (RH) at 25 °C.

16

The reactants and products were analyzed by using an Agilent

17

7890B gas chromatograph equipped with a hydrogen flame ionization

18

detector (Agilent HP-PLOT U column), and a thermal conductivity

19

detector (TCD) (Porapak-Q column and 5A molecular sieve column). The

20

conversion of ethylene into CO2 was calculated based on the effluent

21

content of CO2, which was based on the formula [CCO2out/(2CC2H4inlet)] ×

22

100%, where CCO2out and CC2H4inlet were the concentrations of the


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production of CO2 and C2H4 introduced into the reactor, respectively. The

2

carbonaceous and hydrous species deposited on the catalyst during

3

ethylene

4

temperature-programmed oxidation-mass spectrometry (TPO-MS). In a

5

typical TPO-MS experiment, 20 mg of deactivated Ag/zeolite catalyst in

6

dry atmosphere (21%O2/He) was loaded in a continuous flow quartz

7

reactor that was then coupled to a mass spectrometer (Omnistar TM).

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3. RESULTS AND DISCUSSION

oxidation

reaction

were

characterized

by

9

3.1. Catalytic performances for ethylene oxidation. The catalytic

10

performances of zeolite and Ag/zeolite catalysts for ethylene oxidation

11

were investigated at 25 °C under 100 ppm C2H4/21% O2/He in dry

12

atmosphere. We defined the value after the first 5 minute reaction as the

13

“initial conversion”. As shown in Figure 1a, in dry atmosphere, the initial

14

conversion of ethylene over ZSM-5, Beta, Y and Mordenite was 38%,

15

35%, 25% and 13%, respectively. Thus, it is obvious that there are active

16

sites in microporous aluminosilicate zeolites for ethylene oxidation. The

17

initial conversion of ethylene over all the Ag/zeolite catalysts was nearly

18

100% in dry atmosphere (Figure 1b). The variation in the ethylene

19

conversion with time on stream strongly depended on the zeolite type.

20

Ag/ZSM-5 and Ag/Beta catalysts provided the highest stability, showing

21

no considerable change in the first 6 h, although the conversion decreased

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slowly to zero in the following 18 h. While Ag/Y and Ag/Mordenite ACS Paragon Plus Environment

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showed more obvious deactivation, the initial conversion of ethylene over

2

Ag/Y was only kept for 3 h, and then the conversion decreased to zero in

3

the following 5 h. For Ag/Mordenite, the initial conversion was kept for

4

0.6 h, and then the conversion decreased to zero in the following 4.5 h.

5

An initial gas of 100 ppm C2H4/21% O2/He with relative humidity of

6

50% was also used to investigate catalytic performances over Ag/zeolite

7

catalysts, as also shown in Figure 1b. Compared with a dry atmosphere

8

stream, water vapor had almost no effect on the initial activity of ethylene

9

oxidation for all these Ag/zeolite catalysts. The initial conversion of

10

ethylene over all the Ag/zeolite catalysts under humid atmosphere was

11

still nearly 100%. However, in the case of humid atmosphere, the initial

12

conversion of ethylene over Ag/ZSM-5 was only kept for the first 1.5 h,

13

and then subsequently decreased slightly from 100% to 65% in the

14

following 1 h. There was no extra decrease of ethylene conversion during

15

the following test and the conversion was remained at 65% for Ag/ZSM-5

16

in the following 21.5 h. In contrast, the initial conversion over Ag/Beta

17

was sustained for the first 2.5 h and decreased rapidly to 30% in the next

18

0.5 h, and then the ethylene conversion was not changed during the

19

subsequent test of 21 h under humid atmosphere. Meanwhile, the initial

20

conversion of ethylene over Ag/Y and Ag/Mordenite catalysts was only

21

kept for 1.5 and 0.5 h, respectively, and then lost their activities

22

completely in the following 5.5 h.


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These results clearly showed that the initial ethylene conversion of

2

100% could be achieved on all Ag/zeolite catalysts in both dry and humid

3

atmosphere. In humid atmosphere, Ag/ZSM-5 and Ag/Beta catalysts kept

4

ethylene conversions of 65% and 30% after reacting for 3 h, respectively.

5

Until reacting for 24 h, the ethylene conversion over Ag/ZSM-5 and

6

Ag/Beta was still kept 65% and 30%, respectively. In dry atmosphere, the

7

Ag/ZSM-5 and Ag/Beta lost their activities completely and there was no

8

ethylene conversion after reacting for 24 h. However, the initial ethylene

9

conversion of 100% over Ag/ZSM-5 and Ag/Beta was kept for much

10

longer time in dry atmosphere than in humid atmosphere. It is suggested

11

that, at the beginning of the reaction, H2O has a negative effect on the

12

catalytic activities of ethylene oxidation over Ag/ZSM-5 and Ag/Beta,

13

which make Ag/ZSM-5 and Ag/Beta deactivate more quickly. However,

14

as the reaction goes on, H2O has a positive effect on the catalytic activity

15

of ethylene oxidation over Ag/ZSM-5 and Ag/Beta, which make

16

Ag/ZSM-5 and Ag/Beta show a certain activity of ethylene oxidation and

17

not completely deactivate. Furthermore, H2O only has a negative effect

18

on catalytic activity and stability of ethylene oxidation over Ag/Y and

19

Ag/Mordenite. Compared with the reaction in dry atmosphere, poor

20

catalytic activities and stabilities were shown in humid atmosphere for

21

both Ag/Y and Ag/Mordenite.

22

3.2. Physical parameters of Ag/zeolites. The textural properties of


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zeolites and the corresponding Ag/zeolite catalysts are presented in Table

2

S1. It could be seen that the surface areas and pore volumes of

3

Ag/zeolites were nearly identical to those of the zeolites, indicating the

4

structure of zeolites was largely maintained after the incorporation of

5

silver. All Ag/zeolites exhibited type I isotherms (Figure S1), implying

6

zeolites were dominated by a microporous structure. The present

7

hysteresis loops was associated with pore condensation into mesopores.11

8

Although the framework type and ring number for all the Ag/zeolites

9

were different, the micropore diameters of all Ag/zeolites were mainly

10

distributed around 0.6~0.9 nm, besides, there were a ~2.0 nm pore

11

distribution in Ag/ZSM-5 and a 1.1 nm pore distribution in Ag/Mordenite,

12

respectively (Figure S1a inset). The average diameter of Ag nanoparticles

13

(NPs) on Ag/zeolite catalysts before and after reaction (in dry atmosphere)

14

obtained from TEM images was about 2.3 nm, and Ag NPs dispersed

15

uniformly on all Ag/zeolite catalysts (Figure 2, Figure S2), indicating Ag

16

nanoparticles did not aggregate during the reaction. And it is difficult for

17

Ag particles to enter into the micropores of the zeolite, and the

18

micropores may not be blocked by the Ag particles. Considering the

19

molecule size of ethylene is 0.4 nm,16 the diffusion of ethylene into the

20

pores of these Ag/zeolite catalysts is relatively easy.

21

XPS analyses of Ag/zeolite catalysts before and after reaction (in dry

22

atmosphere) were performed to confirm the valence state of Ag in


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1

catalysts, the results being shown in Figure 3. The typical binding

2

energies Ag 3d5/2 at 368.6 and 367.8 eV on the Ag/zeolite catalysts were

3

assigned to Ag0 and Ag+, respectively.17 All these Ag/zeolites suggested

4

that there are two valence states corresponding to metallic Ag0 and ionic

5

Ag+. It can be seen from Table S2 that the relative concentration of

6

Ag0/Ag+ of Ag/zeolite catalysts after being used in dry atmosphere

7

decreases 77%–93% compared with that of Ag/zeolite before reaction. It

8

is indicated that Ag0 species were oxidized to Ag+ in ethylene oxidation

9

reaction.

10

3.3. Surface acidity of zeolite and Ag/zeolite catalysts. The acidic

11

properties of fresh zeolites and Ag/zeolites were examined by pyridine

12

FT-IR spectra in the region of 1580-1400 cm-1 with the desorption

13

temperature of 150, 250, 350, 450 °C, as shown in Figure 4. The typical

14

IR bands for pyridine adsorbed on Brønsted and Lewis acid sites, which

15

were observed at 1540 cm-1 and 1450 cm-1, respectively.18-20 Both the

16

Brønsted and Lewis acid sites existed in all the fresh zeolites and

17

Ag/zeolites. Due to sorption of pyridine allowed differentiating between

18

Brønsted acid sites and Lewis acid sites, the concentrations of the

19

Brønsted and Lewis acid sites are summarized in Table 1. There are less

20

Brønsted acid sites and Lewis acid sites in zeolites than in the

21

corresponding Ag/zeolites. The number of acid sites of zeolites can be

22

modulated by Ag interaction with framework Al and non-framework Al in


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zeolites.21,22

2

Both the Brønsted acid site and Lewis acid site concentrations of

3

Ag/zeolites determined by desorbing pyridine at 150 °C decreased as the

4

following order:

5

Ag/ZSM-5 > Ag/Beta > Ag/Y > Ag/Mordenite.

6

Thus, it can be seen that the total acidity decreased as the above

7

order. Furthermore, the distribution of very weak, weak, medium and

8

strong Brønsted acid sites were different. All the Ag/zeolites displayed

9

mainly in very weak and weak Brønsted acid sites. Ag/Y showed an

10

increase in the number of strong Brønsted acid sites compared with other

11

Ag/zeolite catalysts. Moreover, it can be observed that C(B)/C(L) of

12

Ag/ZSM-5 and Ag/Beta were greater than 1, indicating that Brønsted acid

13

sites in Ag/ZSM-5 and Ag/Beta accounted for more than half of the total

14

acidity (Table 1). The C(B)/C(L) of Ag/Y and Ag/Mordenite were 0.57

15

and 0.11, respectively. The results showed that the number of Brønsted

16

acid sites was much less than Lewis acid sites, especially for

17

Ag/Mordenite. As it was presumed that the majority of the Brønsted acid

18

sites and a minor portion of Lewis acid sites were located in the zeolite

19

channels, the decrease of the C(B)/C(L) ratio could be explained by the

20

less tetra-coordinated aluminum or decreasing accessibility of pyridine

21

molecules inside the zeolite channels.23 These results showed that there

22

were more Brønsted acid sites occurred than Lewis acid sites in both


ACS Catalysis 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

Ag/ZSM-5 and Ag/Beta, which showed better catalytic stabilities than

2

Ag/Y and Ag/Mordenite, in which there were more Lewis acid sites than

3

Brønsted acid sites. 3.4. Surface acidity changes of Ag/zeolites during reaction. The

4 5

27

Al MAS NMR spectra of fresh Ag/zeolites and used Ag/zeolites in dry

6

or humid atmosphere for 24 h are shown in Figure 5a. We named the

7

Ag/zeolites used in dry atmosphere for 24 h as Ag/zeolites-dry; and

8

named the Ag/zeolites used in humid atmosphere for 24 h as

9

Ag/zeolites-humid. It can be seen that Ag/zeolites-dry, Ag/Y-humid and

10

Ag/Mordenite-humid deactivated completely, but Ag/ZSM-5-humid and

11

Ag/Beta-humid showed 65% and 30% activity, respectively (Figure 1b).

12

In general, the chemical shifts at 55 ppm and 0 ppm are attributed to

13

tetra-coordinated and hexa-coordinated aluminum, respectively.24 Table 2

14

summarizes the relative integrated areas of various aluminum species in

15

fresh Ag/zeolites, Ag/zeolites-dry, and Ag/zeolites-humid. It was found

16

that all the fresh Ag/zeolites had tetra-coordinated aluminum and

17

hexa-coordinated aluminum. The relative amount of tetra-coordinated

18

aluminum for fresh Ag/zeolites ranked as follows:

19


20

The tetra-coordinated aluminum results were in accord with pyridine

21

FT-IR findings. Compared with fresh Ag/zeolites, the integrated area of

22

tetra-coordinated aluminum decreased for the Ag/zeolite-dry. However,


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ACS Catalysis

1

the integrated area of hexa-coordinated aluminum increased for the

2

Ag/ZSM-5-dry, Ag/Beta-dry and Ag/Y-dry. The integrated area of

3

hexa-coordinated aluminum decreased only for Ag/Mordenite-dry. There

4

were less tetra-coordinated aluminum in Ag/ZSM-5-humid and

5

Ag/Beta-humid than the corresponding fresh Ag/ZSM-5 and Ag/Beta, but

6

more than the corresponding Ag/ZSM-5-dry and Ag/Beta-dry. For

7

Ag/Y-humid

8

tetra-coordinated aluminum was similar to that of Ag/Y-dry and

9

Ag/Mordenite-dry. Thus, it is evident that more tetra-coordinated

10

and

Ag/Mordenite-humid,

the

integrated

area

of

aluminum in the Ag/zeolites contributes to a better catalytic activity.

11

The 1H MAS NMR spectra of fresh Ag/zeolites, Ag/zeolites-dry, and

12

Ag/zeolites-humid are exhibited in Figure 5b and are used to probe acid

13

sites. Two groups of resonances were seen for all the fresh Ag/zeolites at

14

4.2 and 1.7 ppm, which were attributed to Brønsted acid sites and silanol

15

groups, respectively.24-26 The resonance at 2.9 ppm was assigned to

16

extra-framework AlOH species. The relative peak areas of the fresh

17

Ag/zeolites, Ag/zeolites-dry, and Ag/zeolites-humid were listed in Table 3.

18

It could be observed that the percentage of Brønsted acid sites for

19

Ag/ZSM-5 was the highest among all the fresh Ag/zeolites. The

20

percentage of Brønsted acid sites decreased in the following order:

21


22

The absence of the 4.2 ppm feature in the spectra of Ag/zeolites-dry,


ACS Catalysis 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

Ag/Y-humid, and Ag/Mordenite-humid confirmed that Brønsted acid site

2

disappeared as Ag/zeolites deactivated. However, although the number of

3

Brønsted acid sites of Ag/ZSM-5-humid and Ag/Beta-humid was less

4

than the corresponding fresh Ag/ZSM-5 and Ag/Beta, there were a certain

5

number of Brønsted acid sites in Ag/ZSM-5-humid and Ag/Beta-humid. A

6

new broad peak at 5–9 ppm appeared for all the deactivated Ag/zeolites.

7

Fuchs et al. assigned this 5–9 ppm peak to residual water associated with

8

a Brønsted acid site, and this residual “water” was either due to water

9

molecules H-bonds to Brønsted acid sites, or to H3O+ ions, or both.27

10

White et al. also attributed the broad 5–9 ppm peak to residual water

11

feature.28 Thus, it revealed that no available Brønsted acid site for

12

ethylene oxidation led to the deactivation of Ag/zeolite catalysts. And the

13

dominant interaction that Brønsted acid sites were covered by residual

14

water which generated from ethylene oxidation resulted in the Ag/zeolite

15

deactivation.

16

3.5. TPO-MS analysis and adsorption-desorption properties.

17

TPO-MS is one of the most commonly used techniques to characterize

18

species deposition. Representative TPO-MS data of deactivated

19

Ag/zeolites are shown in Figure 6, which illustrate the evolution of CO2

20

and H2O, as well as O2 consumption. There was no O2 consumption and

21

no formation of CO2, indicating no deposited carbonaceous species was

22

removed from the deactivated Ag/zeolites. The H2O production profile


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ACS Catalysis

1

exhibited a shape centered at around 150°C. From the H2O production

2

profiles, it was seen that the features of the desorption peaks, in terms of

3

peak area, indicated the amount as well as the nature of the H2O

4

deposition produced during ethylene oxidation reaction. It can be

5

observed that the amount of H2O production changed for different

6

Ag/zeolites and decreased as the following order:

7

Ag/Y > Ag/Beta > Ag/ZSM-5 > Ag/Mordenite.

8

It was indicated that much more H2O deposited on the Ag/Y than

9

other Ag/zeolite catalysts. The H2O production profile comprised only

10

one distinct feature, a sharp peak from 50-250 °C, centered at 150 °C.

11

This indicated that the H2O deposition was of simple structure desorption

12

at only one temperature range.

13

Figure 7a exhibits the water adsorption isotherms on Ag/zeolites at

14

25 °C. The adsorption of water vapor on Ag/zeolites mainly occurred at

15

low relative pressures, which was related to the number of hydrophilic

16

sites in the Ag/zeolites.27,29 Due to the different SiO2/Al2O3 ratio of Y or

17

much more the strong Brønsted acid sites, Ag/Y had a dramatically larger

18

capacity of adsorbing water compared with other Ag/zeolites. The other

19

three Ag/zeolites with the same SiO2/Al2O3 ratio also had different

20

amounts of water adsorption. The decreased order was as follows:

21

Ag/Beta > Ag/ZSM-5 > Ag/Mordenite, which was in line with the H2O

22

deposition amounts calculated from TPO-MS results. Figure 7b shows


ACS Catalysis 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

H2O-TPD results of Ag/zeolites. It can be seen that the total desorbed

2

amount of H2O was equal to the adsorbed amount of H2O on the

3

Ag/zeolite catalysts. Thus, the adsorbed H2O could be desorbed

4

completely from the catalyst, indicating no chemical change of the

5

adsorbed H2O.

6

C2H4-temperature programmed desorption (C2H4-TPD) of Ag/zeolite

7

and H2O-Ag/zeolite was performed to investigate the effect of water

8

pre-adsorption on ethylene adsorption. H2O-Ag/zeolite refers Ag/zeolite

9

catalyst which adsorbs water to reach saturation. C2H4-TPD results gave

10

the C2H4 desorption amounts from fresh Ag/zeolite and H2O-Ag/zeolite,

11

as shown in Figure S3. There are two kinds of desorption peaks in the

12

C2H4-TPD curves of fresh Ag/zeolite catalysts. The peak at low

13

temperature may be ascribed to the presence of physical adsorption C2H4

14

species; and the other at high temperature is attributed to stronger

15

chemical adsorption C2H4 species.30,31 However, C2H4 desorption peak at

16

low temperature of H2O-Ag/zeolite can be negligible, indicating that the

17

weak adsorption sites were firstly occupied by H2O. In addition, the

18

intensity of C2H4 desorption peak at high temperature of H2O-Ag/zeolites

19

is much weaker than that of the fresh Ag/zeolites. It is indicated that the

20

total adsorption capacity of C2H4 on the H2O-Ag/zeolite is considerably

21

decreased by the first adsorption of H2O compared with fresh Ag/zeolite.

22

Therefore, H2O functioned as a competitive adsorption species to inhibit


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ACS Catalysis

1

the adsorption of C2H4 on the surface of Ag/zeolites.

2

CO2-TPD profiles from Ag/zeolite catalysts were shown in Figure

3

S4. CO2 is easily desorbed from Ag/ZSM-5 surface because the primary

4

desorption peak is located at relatively low temperature of 48 °C. CO2, as

5

the products of ethylene oxidation, its desorption capability may be

6

correlated to the catalytic performance of the Ag/zeolites.

7

H2O is the product of a C-H oxidation reaction and H2O vapor

8

cannot be always removed from the reactants. From the TPO-MS and

9

H2O adsorption-desorption data, we can conclude that H2O adsorbing on

10

Ag/zeolites existed in the form of H2O molecule. The desorption amount

11

of H2O from the deactivated Ag/zeolites was in accord with the H2O

12

adsorption capacity of the fresh Ag/zeolites. That is to say, the adsorbed

13

H2O saturated the Ag/zeolite catalysts, and resulted in the deactivation of

14

the catalysts. Thus, the Ag/zeolite catalysts could be regenerated after

15

being treated in Helium flow for 2 h at 200 °C, which was attributed to

16

the adsorbed H2O being removed from the Ag/zeolite catalysts (Figure

17

S5).

18

3.6. Crystalline states of Ag/zeolites during deactivation. Figure 8

19

shows the XRD patterns of zeolites, fresh Ag/zeolites, the deactivated

20

Ag/zeolites in dry atmosphere, and the standard XRD patterns of zeolites.

21

As seen from the XRD patterns, all the zeolites presented their typical

22

diffraction patterns, which are in good agreement with the standard cards.


ACS Catalysis 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

Page 22 of 44

1

After Ag loading, neither the crystal phase nor the particle size of zeolites

2

changed. No obvious characteristic diffraction peak of Ag was detected,

3

presumably due to the small crystalline size and good dispersion of Ag,

4

which was also confirmed by TEM images.

5

Compared with fresh catalysts, the intensity of all the peaks

6

increased for the deactivated Ag/ZSM-5 and Ag/Beta, indicating that the

7

crystallinity improved for the deactivated Ag/ZSM-5 and Ag/Beta. Acid

8

sites strength in zeolites will increase with the improved crystallinity,

9

because the strong interaction Al-O results in a weak bond O-H and

10

increases the strength of the proton in a well crystalline zeolite. The

11

increase of acid sites strength will deduce the weak proton-denoting

12

ability, thus results in a poor catalytic performance in an acidic catalysis

13

reaction. The difference in the relative intensity of (111) and (533) planes

14

between the fresh and the deactivated Ag/Y might be related to

15

preferential

16

happened to Ag/Mordenite, the relative intensity of (200) and (202)

17

planes for the deactivated Ag/Mordenite changed when comparing with

18

fresh Ag/Mordenite. It is indicated that the crystalline structure of Ag/Y

19

and Ag/Mordenite was not so stable and there was a change in the

20

orientation along a preferred direction for the deactivated Ag/Y and

21

Ag/Mordenite.

22

crystallographic

orientation.

The

same

phenomenon

3.7. Reaction and deactivation mechanism. The results of reaction


Page 23 of 44 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

ACS Catalysis

1

investigation and characterizations indicated that the Brønsted acid sites

2

but not the Ag sites were the active sites. First, zeolites without loading

3

Ag nanoparticles did show the catalytic activities for ethylene oxidation.

4

Second, the Ag/zeolites with more Brønsted acid sites showed a better

5

catalytic stability. Most of the important, the deactivated Ag/zeolites had

6

no available Brønsted acid site. When the number of Brønsted acid sites

7

of Ag/ZSM-5-humid and Ag/Beta-humid reduced, the catalytic activities

8

of

9

measurements were carried out on zeolite and Ag/zeolite catalysts to

10

study their oxygen species desorption behaviors, as shown in Figure S6.

11

Generally speaking, active oxygen species such as O- and O2- desorbed

12

below 300 °C, and lattice oxygen will desorb above 300 °C.32 The

13

desorption peaks of Ag/zeolites centered at the range 200-300 °C shifted

14

to lower temperature compared with the same peak of the corresponding

15

zeolites. Moreover, there is a new peak centered below 150 °C for

16

Ag/zeolites. It can be concluded that the desorption peak of surface active

17

oxygen species generated much easier on Ag/zeolites than zeolites, Thus,

18

Ag can promote the generation of surface active oxygen species on

19

Ag/zeolites, which may offer higher catalytic activities and stabilities of

20

Ag/zeolites compared with zeolites. Furthermore, the lower temperature

21

the corresponding desorption peak centered below 150 °C indicated that

22

it is easier to generate surface active oxygen species. The desorption

Ag/ZSM-5-humid

and

Ag/Beta-humid


decreased.

O2-TPD

ACS Catalysis 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

temperature of surface active oxygen species obeys the following order:

2

Ag/ZSM-5 > Ag/Beta > Ag/Y > Ag/Mordenite, which is consistent with

3

the stability sequence for ethylene oxidation.

4

DRIFT spectra of Ag/ZSM-5 and Ag/Mordenite in dry and humid

5

atmosphere are presented in Figure 9. The bands appearing at 2363 and

6

2339 cm-1 are attributed to the CO2 asymmetric stretch and 1630 cm-1 is

7

attributed to the H2O stretch.33 Both in dry and humid atmosphere, after

8

the 100 ppm C2H4 + 21% O2 + He mixed gas flowing through Ag/zeolites

9

for 30 min, formaldehyde species and carbonic acid species (1730 cm-1

10

for ν(CO) ) were formed, and CO2 and H2O was produced. CO2, H2O and

11

formaldehyde species were dominant on Ag/zeolites after the catalysts

12

were exposed to a flow of O2 + He for 35 min. In dry atmosphere, a

13

narrow peak at 3645 cm-1 was attributed to free OH groups. In humid

14

atmosphere, a broad band at 3200-3600 cm-1 was attributed to

15

hydrogen-bonded OH groups and a peak at 3645 cm-1 overlapped with

16

hydrogen-bonded OH groups was also attributed to free OH groups.34

17

Based on these results, we conclude that ethylene oxidation over

18

Ag/zeolites follows a pathway, that is C2H4 → HCHO* → CO2 + H2O.

19

The reaction mechanism of ethylene oxidation in dry atmosphere is

20

different from the mechanism in humid atmosphere. In dry atmosphere,

21

H2O adsorbed on Brønsted acid sites generates free OH groups and the

22

disappearance of Brønsted acid sites deduces the deactivation of


Page 24 of 44

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ACS Catalysis

1

Ag/zeolites. However, in humid atmosphere, except for a part of free OH

2

groups, hydrogen-bonded OH groups generate, which can keep a certain

3

of Brønsted acid sites.

4

A generalized mechanism was proposed to describe the conversion

5

of ethylene to CO2 and H2O and the deactivation process. The first step

6

involves the adsorption and activation of ethylene on a Brønsted acid site

7

of Ag/zeolite catalyst, forming adsorbed ethylene species, which is in

8

accord with reported ethylene activation on a Brønsted acid site.35-38 The

9

second step is the attack of active oxygen species on the adsorbed

10

ethylene species that are formed on the Brønsted acid sites of zeolite

11

catalyst. Because Ag can promote the generation of surface active oxygen

12

species from O2-TPD results, an ethylene oxidation catalyst with

13

improved catalytic performance requires the joint participation of

14

Brønsted acid sites and metallic ions sites. The third step is that the

15

carbon-carbon bond of adsorbed ethylene species is broken, and forms

16

formaldehyde species. Fukuoka et al. also reported HCHO adsorbed on Pt

17

is the intermediate species for ethylene oxidation over Pt/MCM-41.7 The

18

fourth step, formaldehyde species are oxidized into carbonic acid species.

19

The last step, carbonic acid species decompose into CO2 and H2O. In dry

20

atmosphere, because the generated H2O adsorbed onto the Brønsted acid

21

site, the Brønsted acid site was consumed and resulted in catalyst

22

deactivation, which has been confirmed by 1H MAS NMR of fresh and


ACS Catalysis 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

deactivated Ag/zeolite catalysts. However, when Ag/ZSM-5 and Ag/Beta

2

were used in humid atmosphere, more H2O molecules existed in the

3

reaction gases, some Brønsted acid sites were covered by H2O and lost,

4

the other Brønsted acid sites around by H2O molecules might form

5

hydrogen bonds with H2O molecules. That is the reason for not

6

deactivation of Ag/ZSM-5 and Ag/Beta when reacting in humid

7

atmosphere for 24 h. The detailed reaction and deactivation mechanism

8

of ethylene oxidation over Ag/zeolites are shown in Figure 10.

9

4. CONCLUSIONS

10

In summary, microporous Ag/zeolite (ZSM-5, Beta, Y and Mordenite)

11

catalysts have been found to exhibit high catalytic performances for

12

ethylene oxidation at room temperature. The initial activity of ethylene

13

oxidation over all the Ag/zeolite catalysts is 100% at 25 °C both in dry

14

and humid atmosphere, although the maintenance time of the initial

15

activity over the Ag/zeolites is different. Based on characterization results

16

of fresh and deactivated Ag/zeolites, we can conclude that Brønsted acid

17

site is the catalytic active site for ethylene oxidation. Water vapor in the

18

reactant atmosphere has a significant effect on the catalytic performances

19

of the Ag/zeolites. The main reason for catalyst deactivation is that H2O

20

molecules adsorbed onto Brønsted acid sites result in the disappearance

21

of available Brønsted acid sites in Ag/zeolites. It is desirable that zeolite


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ACS Catalysis

1

catalysts may have potential application in ethylene oxidation in the

2

solution of Brønsted acid sites stability under water vapor.

3

Acknowledgement

4

This work is financially supported by the National Key R&D Program of

5

China (2016YFC0204203) and National Natural Science Foundation of

6

China (21337003, 21477148 21477149 and 21777175), and Key

7

Research Program of Frontier Sciences, CAS (QYZDB-SSW-DQC018).

8

Supporting information

9

The Supporting Information is available free of charge on the ACS

10

Publications website.

11

BET surface area, pore volume; relative concentration change of

12

Ag0/Ag+1 before and after reaction; HRTEM images and particle diameter

13

distributions; C2H4-TPD; CO2-TPD; regeneration test; O2-TPD and gas

14

chromatography results.

15

References

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(30) Bolis, V.; Vedrin, J. J. C. S. Faraday I, 1980, 76, 1606–1616.

5

(31) Zhou, J. X.; Zhang, Y. C.; Guo, X. W.; Zhang, A. F.; Fei, X. M. Ind.

6

Eng. Chem. Res. 2006, 45, 6236–6242.

7

(32) Khodakov, A. Y.; Griboval-Constant, A.; Bechara, R.; Zholobenko, V.

8

L. J. Catal. 2002, 206, 230–241.

9

(33) Crowley, T. A.; Ziegler, K. J.; Lyons, D. M.; Erts, D.; Olin, H.;

10

Morris, M. A.; Holmes, J. D. Chem. Mater. 2003, 15, 3518–3522.

11

(34) Hsieh, C.-S.; Campen, R. K.; Okuno, M.; Backus, E. H. G.; Nagata,

12

Y.; Bonn, M. PNAS, 2013, 110, 18780–18785.

13

(35) Boronat, M.; Corma, A. Catal. Lett. 2015, 145, 162–172.

14

(36) Hsieh, M.-F.; Zhou, Y. W.; Thirumalai, H.; Grabow, L. C.; Rimer, J.

15

D. ChemCatChem 2017, 9, 1675–1682.

16

(37) Bolist, V.; Vedrine, J. C. J.C.S. Faraday I, 1980, 76, 1606–1616.

17

(38) Senchenya, I. N.; Kazansky, V. B. Catal. Lett. 1991, 8, 317–326.

18


Page 30 of 44

Page 31 of 44 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

ACS Catalysis

Table 1. Acidic properties of Ag/zeolite catalysts measured by pyridine FT-IR.

1

Catalyst

C(L)a

C(B) (mmol/g)

C(B)b

very (mmol/g) (mmol/g) weak

weak

medium strong

Total acidity

C(B) /C(L)

(mmol/g)

Ag/ZSM-5

1.03

1.22

1.22

0.52

0.21

0.12

2.25

1.18

Ag/Beta

0.98

1.09

1.09

0.49

0.18

0.11

2.07

1.11

Ag/Y

0.92

0.52

0.52

0.43

0.21

0.14

1.44

0.57

Ag/Mordenite

0.84

0.09

0.09

0.21

0.14

0.05

0.93

0.11

ZSM-5

0.98

1.11

1.11

0.47

0.19

0.11

2.09

1.13

Beta

0.91

0.97

0.97

0.45

0.16

0.10

1.88

1.06

Y

0.85

0.46

0.46

0.39

0.19

0.13

1.31

0.54

Mordenite

0.75

0.07

0.07

0.19

0.13

0.05

0.82

0.09

2

a

C(pyridine on L acid sites) = 1.42 * IA(L) *R^2 / W.

3

b

C(pyridine on B acid sites) = 1.88 * IA(B) *R^2 / W.

4

IA(B,L) = integrated absorbance of B or L band.

5

R = radius of catalyst disk (cm).

6

W = weight of disk (mg).

7


ACS Catalysis 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 32 of 44

Table 2. Relative peak areas determined from the 27Al MAS NMR.a Catalyst

55 ppm (%) fresh

30 ppm (%)

dry

humid

at.b

at.c

fresh

0 ppm (%)

dry

humid

at.b

at.c

fresh

dry

humid

at.b

at.c

Ag/ZSM-5

79.2

72.2

74.3

0

0

0

20.8

27.8

25.7

Ag/Beta

76.8

70.9

74.2

0

0

0

23.2

29.1

25.8

Ag/Y

70.5

65.8

65.6

0

0

0

29.5

34.2

34.4

Ag/Mordenite

62.8

59.5

59.8

0.4

4.8

4.8

36.8

35.7

35.4

2

a

3

b

4

atmosphere.

5

c

6

short for atmosphere.

The peak areas are obtained by integration method. indicates the deactivated Ag/zeolite catalysts in dry atmosphere, at. is short for

indicates the Ag/zeolite catalysts being used in humid atmosphere for 24 hours, at. is

7


Page 33 of 44 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

ACS Catalysis

1

Table 3. Relative peak areas determined from the 1H MAS NMR.a Catalyst

1.7 ppm (%)

fresh dry

humid

at.b

at.c

4.2 ppm (%) fresh

dry

humid

at.b

at.c

5.5-8 ppm (%) fresh

dry

humid

at.b

at.c

Ag/ZSM-5

2.8

2.7

2.7

97.2

0

63.1

0

97.3

34.2

Ag/Beta

9.9

9.6

9.6

90.1

0

26.9

0

90.4

63.5

Ag/Y

13.5

19.9

19.8

86.5

0

0

0

80.1

80.2

Ag/Mordenite 17.1

20.7

20.9

82.9

0

0

0

79.1

79.1

2

a

The peak areas are obtained by integration method.

3

b

indicates the deactivated Ag/zeolite catalysts in dry atmosphere.

4

c

indicates the Ag/zeolite catalysts being used in humid atmosphere for 24 hours.

5


ACS Catalysis

b)

40 ZSM-5 Beta Y Mordenite

30

20

10

0 20

30

40

50

60

C2H4 Conversion to CO2 (%)

a)

C2H4 Conversion to CO2 (%)

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

Page 34 of 44

Ag/ZSM-5 Ag/Beta Ag/Y Ag/Mordenite

100 80 60 40 20 0 0

4

Time-on-stream (min)

1

8

12

16

20

24

Time-on-stream (h)

2

Figure 1. Conversion of ethylene to CO2 with time-on-stream over (a) zeolites and (b)

3

Ag/zeolites, filled symbols:RH = 0, open symbols:RH = 50%. 100 ppm C2H4/21%

4

O2/H2O (RH = 0 or 50%)/He balance; Space velocity: 7500 mL·h-1·g-1; catalyst: 0.20

5

g; T=25 °C.

6


Page 35 of 44

120

average size 2.3 nm

Frequency (%)

100 80 60 40 20 0 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Particle diameter (nm)

1

100

average size 2.4 nm

Frequency (%)

80

60

40

20

0 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


2 140

average size 2.3 nm

Frequency (%)

120 100 80 60 40 20 0 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


3

average size 2.4 nm

100

80

Frequency (%)

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

ACS Catalysis

60

40

20

0 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


4 5

Figure 2. HRTEM images and particle diameter distributions of (a) Ag/ZSM-5 (b)

6

Ag/Beta (c) Ag/Y (d) Ag/Mordenite catalysts.


ACS Catalysis

a) 0

(Ag ) Ag/ZSM-5

Ag 3d5/2

+

0

Ag 3d3/2

(Ag ) (Ag )

b) +

0

(Ag )

(Ag ) Ag/ZSM-5

Intensity (a.u.)

Intensity (a.u.)

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

Ag/Beta

Ag/Y

380

376

Ag 3d5/2

Ag 3d3/2 +

0

+

(Ag ) (Ag )

(Ag )

Ag/Beta

Ag/Y

Ag/Mordenite

Ag/Mordenite

1

Page 36 of 44

372

368

Binding energy (eV)

364

380

376

372

Binding energy (eV)

368

364

2

Figure 3. Ag3d XPS profiles of (a) fresh Ag/zeolite and (b) deactivated Ag/zeolite

3

catalysts in dry atmosphere.

4


150 oC

1440

-1

1560

1520

250 oC 150 oC

1520

1480

-1

1440

-1

1400

1440

1400

Ag/Mordenite Mordenite 1450

0.2

450 oC 350 oC

1560

1480

d) 1540

Ag/Y Y

Absorbance (a.u.)

1450

1540

1490

0.2

1450

150 oC

Wavenmuber (cm )

c)

2

1400

250 oC

1490

1480

450 oC 350 oC

Wavenmuber (cm )

1

Ag/Beta Beta

Absorbance (a.u.)

250 oC

1520

0.2

450 oC 350 oC

1560

b) 1490

1450

1540

Ag/ZSM-5 ZSM-5

Absorbance (a.u.)

0.2

1490

a)

Absorbance (a.u.)

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

ACS Catalysis

1540

Page 37 of 44

450 oC 350 oC 250 oC 150 oC

1560

1520

1480

-1

1440

1400

Wavenmuber (cm )

Wavenmuber (cm )

3

Figure 4. Pyridine FT-IR of fresh zeolite and fresh Ag/zeolite catalysts after

4

desorption at 150, 250, 350 and 450 oC.

5


ACS Catalysis

a) fresh dry atmosphere humid atmosphere

27

Al NMR

55

b)

fresh dry atmosphere humid atmosphere

1

H NMR 4.2

7.6 0

2.9 1.7

Intensity (a.u.)

Intensity (a.u.)

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

Page 38 of 44

Ag/ZSM-5 Ag/Beta Ag/Y

Ag/ZSM-5 Ag/Beta Ag/Y

Ag/Mordenite

150

100

50

1 27

ppm

0

-50

-100

Ag/Mordenite

14

12

10

8

6

4

ppm

2

0

-2

-4

Al MAS NMR and (b) 1H MAS NMR spectra of fresh Ag/zeolites,

2

Figure 5. (a)

3

deactivated Ag/zeolites in dry atmosphere, and Ag/zeolites being used in humid

4

atmosphere for 24 hours.

5


Page 39 of 44

b)

a) 0.01

H2O O2

H2O O2

CO2

0

100

1

200

300

400

Temperature (oC)

CO2

500

0

600

100

200

300

400

500

600

Temperature (oC)

d)

c) 0.01

0.01

100

200

300

400

500

m/z=18 m/z=32 m/z=44

MS signal (a.u.)

m/z=18 m/z=32 m/z=44

H2O O2 CO2

0

2

m/z=18 m/z=32 m/z=44

0.01

MS signal (a.u.)

MS signal (a.u.)

m/z=18 m/z=32 m/z=44

MS signal (a.u.)

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

ACS Catalysis

H2O O2 CO2

600

0

100

Temperature (oC)

200

300

400

500

600

Temperature (oC)

3

Figure 6. TPO-MS profiles for deactivated Ag/zeolite catalysts in dry atmosphere: (a)

4

Ag/ZSM-5 (b) Ag/Beta (c) Ag/Y and (d) Ag/Mordenite.

5


ACS Catalysis

20

b)

Ag/Y Ag/Beta Ag/ZSM-5 Ag/Mordenite

15

Amount adsorbed (mmol·g-1)

a) Amount adsorbed (mmol·g-1)

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

10

5

0 0.0

0.2

0.4

0.6

0.8

Relative pressure (p/po)

1

1.0

Page 40 of 44

20

Ag/Y Ag/Beta Ag/ZSM-5 Ag/Mordenite

15

10

5

0 50

100

150

200

250

300

350

400

Temperature (oC)

2

Figure 7. (a) Adsorption isotherms and (b) TPD curves of water vapor of fresh

3

Ag/zeolites.

4


Page 41 of 44

b)

Ag/ZSM-5-deactivated

Ag/ZSM-5-fresh

(205)

(304) (008)

(302)

(101) (210)

Intensity (a.u.)

Intensity (a.u.)

(151) (303) (133)

(101) (111)

(051)

a)

Ag/Beta-deactivated

Ag/Beta-fresh

Beta

ZSM-5

10

15

20

25

30

35

40

45

50

5

10

15

20

(150) (241)

(110)

Ag/Y-fresh

25

30

2 Theta (o)

35

40

45

50

Ag/Mordenite-deactivated

Intensity (a.u.)

(751)

(642)

(331)

(511)

(111) (220)

Ag/Y-deactivated

(130)

d)

(533)

c)

(200)

2 Theta (o)

(350)(202) (530)

5

1

Intensity (a.u.)

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

ACS Catalysis

Ag/Mordenite-fresh

Mordenite

Y

5

2

10

15

20

25

30

35

40

45

50

5

10

15

2 Theta (o)

20

25

30

35

40

45

50

2 Theta (o)

3

Figure 8. XRD patterns of zeolites, fresh Ag/zeolites, deactivated Ag/zeolites in dry

4

atmosphere, and standard XRD patterns of zeolites.

5


ACS Catalysis

b)

25min 15min 5min O2+He

in humid atmosphere 2363 2339

2.0 35min

Absorbance (a.u.)

1730 1630

3645

35min

2363 2339

in dry atmosphere

0.5

25min 15min 5min O2+He

C2H4+O2+He

4000

1

3500

3000

2500

Wavenumber (cm-1)

1730 1630

a)

Absorbance (a.u.)

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

Page 42 of 44

C2H4+O2+H2O+He

2000

1500

4000

3500

3000

2500

2000

1500

Wavenumber (cm-1)

2

Figure 9. In-situ DRIFT over a) Ag/ZSM-5 in dry atmosphere after a flow of 100

3

ppm C2H4+ 21% O2+He for 30 min followed by 2) O2+He purging for 5, 15, 25 and

4

35 min, respectively; and b) Ag/ZSM-5 in humid atmosphere after a flow of 100 ppm

5

C2H4+21% O2+H2O+He for 30 min followed by 2) O2+He purging for 5, 15, 25 and

6

35 min, respectively.

7


Page 43 of 44 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

ACS Catalysis

1

2 3

Figure 10. Proposed mechanism of ethylene oxidation over Brønsted acid sites in

4

Ag/zeolite catalysts, including five distinct steps that are illustrated below: (i)

5

ethylene binding to the Brønsted acid sites; (ii) active oxygen species attacked the

6

adsorbed ethylene species; (iii) the carbon-carbon bond of the adsorbed ethylene

7

species was broken, and formed formaldehyde species; (iv) the formation of carbonic

8

acid species; (v) In dry atmosphere, the generation of CO2 and H2O; H2O molecules

9

bound to the Si-O-Al sites to consume Brønsted acid sites; Ag/ZSM-5 and Ag/Beta in

10

humid atmosphere, more H2O molecules around Brønsted acid sites formed hydrogen

11

bond with Brønsted acid sites.

12


ACS Catalysis 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

Table of Content

2

This work presents Ag/zeolite (ZSM-5, Beta, Y and Mordenite) as a

3

potential catalytic material for ethylene oxidation at room temperature

4

and systematically investigates the active sites and deactivation

5

mechanism.

6


Page 44 of 44

zeolites and deactivation

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