Transient BrÃ¸nsted Acid Sites in Propene Aromatization over Zn

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Transient Brønsted Acid Sites in Propene Aromatization over Zn-Modified HZSM-5 Detected by Operando Dual Beam FTIR Jiaxu Liu, Long Lin, Jilei Wang, Wei Zhou, Cuilan Miao, Chunyan Liu, Ning He, Qin Xin, and Hongchen Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01415 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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The Journal of Physical Chemistry

1

Transient Brønsted Acid Sites in Propene Aromatization over

2

Zn-Modified HZSM-5 detected by Operando Dual Beam FTIR

3

Jiaxu Liua, Long Lina, Jilei Wanga, Wei Zhoua, Cuilan Miaoa, Chunyan Liua, Ning

4

Hea, Qin Xinb, Hongchen Guoa*

5 State Key Laboratory of Fine Chemicals and School of Chemical Engineering, Dalian

6

a

7

University of Technology, No. 2 Linggong Road, Dalian 116024, PR China

8

b State

9

Sciences, Dalian 116023, PR China

Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

10

ABSTRACT: An operando dual-beam Fourier-transform infrared (DB-FTIR) spectrometer was

11

developed to identify the transient Brønsted acid sites (BAS) in propene aromatization over

12

acidic HZSM-5 and Zn-modified HZSM-5 catalysts under real reaction conditions. The

13

eliminated signals include gas-phase molecular vibrations and heat irradiation at reaction

14

temperatures. We directly observed that the initial activation of propene over Zn2+ of Zn

15

modified HZSM-5 generated a substantial number of transient BAS, which serve as active sites

16

for the subsequent aromatization reactions. Moreover, during propene aromatization process,

17

the desorption of aromatic precursors over the Zn2+ of Zn/HZSM-5 is easier than over the H+ of

18

HZSM-5 resulting in enhanced aromatics productivity. A DFT calculation certified the priority of

19

the metallic Zn2+ sites over BAS in the competitive activation of propene molecules. The

20

generation of transient BAS is energetically favorable.

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1

INTRODUCTION

2

Short-chain hydrocarbon aromatization over HZSM-5 is an important BAS-catalyzed reaction.

3

The incorporation of Zn into HZSM-5 is a well-accepted method to improve the aromatization

4

performance following bi-functional catalysis mechanism.1-3 Propene aromatization over metal

5

modified HZSM-5 zeolites occurs at solid-gas interfaces. The inhomogeneity of these interfaces

6

and the complex reactions between surface and reactants under real reaction conditions have

7

hampered the accurate determination of the exact nature of active sites. 4

8

In recent years, operando FTIR spectroscopy has been reported aiming to obtain dynamic

9

surface information of a working catalyst when the heterogeneous reaction is proceeding under

10

real conditions.5-7 However, the FTIR investigations using commercial single beam FTIR (SB-

11

FTIR) spectrometers are difficult to simultaneously deduct the background signals originating

12

from gas phase molecules and heat irradiation. Most recently, we reported a practical dual-

13

beam Fourier-transform infrared spectrometer (DB-FTIR) which was developed with two

14

identical interferometer infrared optical platforms via integration and coupling strategy.8

15

IR spectroscopy is one of the most powerful tools for identifying the strength and quantity of

16

acid sites on zeolites. In the pioneering work by Uytterhoeven et al.,9 the OH groups (Brønsted

17

acid sites, BAS) of faujasite-type zeolite Y were investigated by transmission IR spectrometer.

18

It is well-accepted that, in BAS catalyzed reactions, the BAS take part in the reaction and their

19

population will constantly decrease during reaction due to contamination or a decrease in

20

accessibility. However, the SB-FTIR results would be influenced by the gas-phase molecular

21

vibrations and heat irradiation under real reaction conditions caused by continuous reactant

22

flow at atmospheric pressure and high temperature.

23

In this paper, with the assistance of operando DB-FTIR and online analysis of effluent gas by

24

a mass spectrometer (MS), we directly observed the initial activation, oligomerization, and

25

cyclization of propene over the BAS of HZSM-5 under real reaction conditions. Coke was

26

formed during reaction, resulting in a persistent decrease of the BAS. Interestingly, different

27

from HZSM-5, the initial activation of propene over Zn/HZSM-5 generated a substantial amount

28

of transient BAS. DFT calculations suggested that the transient BAS was generated by C-H

29

bond heterolysis of methyl group of propene over Zn2+ of Zn/HZSM-5. Thus, the formation of

30

transient BAS in zeolite catalyzed aromatization were firstly observed under real reaction 2


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1

conditions. The DB-FTIR spectrometer proved to be powerful in clarifying the mechanisms of

2

real heterogeneous catalysis processes.

3 4

2. EXPERIMENTAL

5

2.1 CATALYST PREPARATION

6

Nano-sized NaZSM-5 zeolite (20-50 nm) with SiO2/Al2O3 molar ratio of 26 was manufactured

7

by Dalian Ligong Qiwangda Chemical Technology (Dalian, China). HZSM-5 was obtained by

8

exchanging the NaZSM-5 twice for 1 h at 80 ℃ with a 1 M solution of NH4NO3 with a liquid to

9

solid weight ratio of 5. The sample was filtered and washed with deionized water after each

10

exchange, and dried at 110 ℃ for 12 h and then calcined at 540 ℃ in flowing dry air for 6 h.

11

Zn-modified samples were obtained by incipient wet impregnation (IWI). The impregnation was

12

conducted at 80 ℃ for 1 h with HZSM-5 catalysts by using different solutions of zinc nitrate

13

(0.5, 1.0, and 1.5 wt%) and dried at 110 ℃ for 12 h and then calcined at 540 ℃ in flowing dry

14

air for 6 h. The real loadings of Zn on the modified samples were determined by ICP.

15

2.2 CATALYST CHARACTERIZATION

16

XRD patterns were obtained with a Rigaku D/max-2004 diffractometer with Cu Kα radiation

17

(40 kV, 100 mA) and 0.02° min-1 (2θ) scanning speed. XRF measurements were performed

18

with a Bruker SRS3400 spectrometer to determine the bulk Si/Al ratio. HR-TEM images of the

19

zeolites were recorded on a JEOL JEM-2100 (200 kV) microscope. Nitrogen physisorption was

20

conducted on a Micromeritics ASAP 2020 instrument at -196℃ to obtain textural information.

21

Prior to the measurement, the samples (380-830 μm sieve fraction) were degassed at 400 ℃

22

for 6 h. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method using

23

the adsorption branch in the p/po range from 0.10 to 0.15, the pore volumes were estimated at

24

p/po = 0.99, while the micro- and mesoporosity was discriminated by the t-plot method.

25

NH3 adsorption TPD measurements were employed to investigate the overall acidity of the

26

catalysts. Profiles were obtained with a Quantachrome ChemBet 3000 chemisorb instrument.

27

Samples (150 mg, 380-830 μm sieve fraction) were pretreated in He (99 wt% purity, Dalian

3



1

Special Gases CO. LTD) at 600 ℃ for 1 h and then cooled to 100 ℃ for ammonia adsorption.

2

The ammonia (99 wt% purity, Dalian Special Gases CO. LTD) adsorption was carried out at

3

100 ℃ for 30 minutes with a mixture of 5% NH3 in He. After adsorption the cell was purged in

4

50 mL/min He flow for 30 min to remove all physisorbed NH3. Then the NH3-TPD profiles were

5

recorded in a 50 mL/min He flow by ramping the temperature from 100 to 600 ℃ at a rate of

6

about 16 ℃/min. Obtained results were normalized by the sample weights. The quantitative

7

analysis of the amount of coke on the used catalyst was performed with a thermogravimetric

8

analyzer (TGA, Netzsch TG209F3).

9

FTIR was also used to characterize the acidity of the samples. The spectra of the surface

10

hydroxyl (OH) vibrations and pyridine (99 wt% purity, Dalian Special Gases CO. LTD)

11

adsorption were obtained with a Nicolet is10 FTIR spectrometer. The zeolitic samples were

12

pressed into self-supporting thin wafers (approximately 15 mg) and decontaminated at 400 ℃

13

under vacuum (10-3 pa) for 4 h in a quartz IR cell equipped with CaF2 windows. After the

14

pretreatment, the cell was cooled down to room temperature for sample measurement. Spectra

15

were recorded from 4000 to 400 cm -1 with a resolution of 4 cm -1. The hydroxyl vibration spectra

16

were obtained by subtracting the background spectrum (recorded with empty IR cell in the

17

absence of sample) from the measured sample spectra. The spectra of pyridine were obtained

18

as follows: First, pyridine adsorption was carried out at room temperature at saturated pressure

19

for 30 minutes. Then the samples were evacuated (10-3 Pa) for 30 minutes at 150 ℃. The

20

spectra were obtained by subtracting the background (obtained with decontaminated wafers

21

before pyridine adsorption) from the measured sample spectra.

22

A DB-FTIR spectrometer was employed to study the aromatization of propene (99 wt% purity,

23

Dalian Special Gases CO. LTD) over pristine and Zn-modified HZSM-5 catalysts under real

24

conditions.8 The catalysts were pressed into self-supporting thin wafers (1 cm 2) and placed in

25

the sample beam, and the reference beam was vacant. The sample was activated in the dual

26

beam IR cell reactor at 400 °C for 4 h under vacuum (10−3 Pa), and the spectra were recorded

27

at a resolution of 4 cm −1 with 8 scans in the region of ṽ =4000～1000 cm−1. The intensities of

28

the reference and sample beams were adjusted to the same level. The effluent from the IR cell 4


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1

reactor was analyzed by a quadrupole mass spectrometer (Omnistar, 1-200 amu, QMS 200).

2

The spectra of propene were obtained as follows: Propene aromatization was carried out at

3

different temperatures (170 and 240°C) in continuous propene flow (6% propene balanced with

4

nitrogen) at the speed of 3 ml/min for 1 h. The reaction pressure is 1 atm, the gas hourly space

5

velocity (GHSV) is 1080 h-1, and the interval of each two spectra was 8 seconds.

6

2.3 DFT CALCULATION

7

A 268T cluster model, including the whole intersection of straight and zigzag channels, was

8

obtained by cutting a periodic structure, as shown in Figure S1. The dangling silicon atoms

9

were terminated by hydrogen along the bond direction of the next lattice oxygen atoms with the

10

distance of 1.46 Å. To improve the energetic properties and take into account the effect of the

11

entire zeolite framework on the reaction mechanism, a two-layer ONIOM scheme was

12

employed. The region in an intimate relationship to the catalytic reactions was treated with the

13

high-level functional ωB97XD along with the 6-31+G(d,p) basis set for accuracy, which is a

14

long-range corrected functional. The regions away from the active center were treated at lower-

15

level with the semi-empirical calculation method (AM1) for efficiency. During all calculations,

16

the positions of the terminal SiH3 atoms were fixed, whereas the positions of the remaining

17

atoms and guest molecules were optimized. The transition state structures were characterized

18

by means of frequency calculations with only one imaginary frequency. The intrinsic reaction

19

coordinate (IRC) method was used when necessary to identify the two minima connected by a

20

transition state.

21 22

3. RESULTS AND DISCUSSION

23

3.1 Acidic Property of HZSM-5 and Zn/HZSM-5

24

Nano-sized H-ZSM-5 (HZ) and Zn-modified HZ with Zn loading of 0.72 wt. % (Zn0.72/HZ),

25

1.10 wt. % (Zn1.10/HZ), and 1.37 wt. % (Zn1.37/HZ) were used as aromatization catalysts. XRD

26

patterns (Figure S2), TEM image (Figure S3) and nitrogen physisorption results (Table S1)

27

indicated that all the catalysts maintained their MFI structure and that the Zn species were

28

highly dispersed on the HZ zeolite. Figure 1a shows the IR spectra of the hydroxyl groups on

29

the Zn-modified HZ zeolites. Three adsorption bands can be identified in the 3500-3750 cm-1

30

region.10 All four catalysts have the absorbance at 3604 cm-1 corresponding to BAS. The 5



1

intensity of this absorbance decreased with increasing Zn loading suggesting that the Zn

2

species are located at the BAS. The IR spectra of pyridine adsorption on the HZ and Zn-

3

modified HZ catalysts were employed to determine the number of BAS and Lewis acid sites

4

(LAS). As shown in Figure 1b, for HZ, the BAS dominates with the pyridine adsorption at 1544

5

cm-1. For the Zn-modified catalysts, the absorbance at 1454 cm -1 corresponding to metal-LAS

6

was observed.11 The Zn-modified catalysts with higher Zn loadings had less BAS and more Zn-

7

LAS. The BAS and LAS over HZSM-5 are 125 and 17μmolPy/g, respectively (Table S1). While,

8

for Zn1.37/HZ, the BAS and LAS are 75 and 101 μmolPy/g, respectively. This is because the

9

modification of H-ZSM-5 zeolite with zinc nitrate aqueous solution mainly proceeds via the ion-

10

exchange of H+ of BAS (Zeo-H+) with single Zn2+. This means, in the case of single Zn2+, that

11

each metal ion will consume two BAS, forming Zn-LAS in the form of ca. Zeo- Zn2+ Zeo-.12 NH3-

12

TPD is shown in Figure 1c. The Zn modification transformed strong acid sites (corresponding

13

to the high-temperature desorption peak) into weak ones (corresponding to the desorption at

14

lower temperature). This is due to the replacement of the BAS (such as the strongly acidic

15

bridge hydroxyls) by Zn, thus the BAS is transformed into LAS simultaneously, which leads to

16

partial conversion of strong acid sites to weak ones. The acid site density slightly decreased

17

after Zn modification (Table S1). 0.1 3740 3723

3604

(b)

21

Zn1.37/HZ

1540

19 20

1454

0.1

Zn1.37/HZ

Zn1.10/HZ

Zn1.10/HZ

Zn0.72/HZ

(c)

Zn1.37/HZ

Intensity / A.U.

(a)

Absorbance / A.U.

18

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

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Zn1.10/HZ

Zn0.72/HZ

Zn0.72/HZ

22

HZ

23 3800

3700

3600

3500 -1

24

HZ

HZ 3400

1600

1550

1500

1450

1400

-1

Wavenumber / cm

Wavenumber / cm

100

200

300

400

500

600

T / oC

25 26 27 28 29 30

Figure 1. Acidity of HZ and Zn modified HZ characterized by (a) OH-FTIR (The spectra of different catalysts were

31

Propene aromatization over HZ and Zn-modified HZ was systematically studied by operando

32

DB-FTIR-MS. All spectra of propene aromatization over HZ and Zn x/HZ are shown in Figures

normalized to the intensity of absorption band at 1880 cm -1 (the vibrations of the zeolite Si−O−Si fragments), respectively), (b) pyridine adsorption FTIR (the vibrations of the zeolite Si−O−Si fragments) and (c) NH3-TPD (obtained results were normalized by the sample weights).

3.2 Operando DB-FTIR-MS Study of Propene Aromatization

6


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1

2 and 3. High-quality and time-resolved spectra of the catalyst surface were recorded by the

2

DB-FTIR under real reaction conditions, which provided abundant information about surface

3

adsorbed species and their evolutions. As Figure 2a and e indicate, upon contact of propene

4

with HZ, the BAS absorbance at 3604 cm -1 immediately and continuously decreased in intensity.

5

A new absorbance at 3400-3500 cm-1 related to hydrogen-bonding appeared and increased

6

with reaction time.13,14 The change of the BAS absorbance was accompanied with intensity

7

increase at 2954, 2933, 2871, and 2860 cm-1 in the C-H stretching vibration region and the

8

development of two more bands at 1506 and 1463 cm -1 in the C-H bending vibration region

9

(Figure 3a). The bands at 2954, 2933, 2871, and 2860 cm-1 are attributed to C-H stretching

10

vibrations. As mentioned above, the bands at 2954 and 2871 cm -1 are attributed to asymmetric

11

and symmetric C-H stretching of the -CH3 group, respectively, while those at 2933 and 2860

12

cm-1 are assigned to the asymmetric and symmetric C-H stretching vibrations of the -(CH2)-

13

group, respectively. The band at 1463 cm -1 is attributed to the C-H bending vibration of the -

14

(CH2)- group. The band at 1560 cm -1 is absence over HZSM-5 while could be clearly seen over

15

all the Zn modified HZSM-5 catalysts. We attributed this absorbance to Zn-alkyl specie

16

generated by the dissociative adsorption of propene over Zn2+.

17

With an increase in the adsorption time from 0 to 25 minutes, the intensities of these

18

absorption bands increased linearly. The intensity of the absorbance at 1506 cm-1 related to the

19

aromatic ring also increased monotonically with reaction time. The effluent gas from the DB-

20

FTIR cell reactor was analyzed by an on-line mass spectrometer. As the results in Figure 4

21

show, the production of benzene increased with reaction time. Thus, the time-resolved surface

22

dynamic IR spectra of HZ display the whole process from propene adsorption activation to

23

aromatics formation catalyzed by BAS of H-ZSM-5. It is well-known that the aromatization of

24

short-chain hydrocarbons on H-ZSM-5 is dictated by the carbenium ion mechanism, which

25

means that the activation of propene on HZ will occur on BAS forming surface alkoxide

26

intermediates (Scheme 1).15 The BAS are engaged in all steps, from the formation of the

27

alkoxides

28

oligomerization/cracking and cyclization, which accounts for the attenuation of the HZ bridging

29

hydroxyl band.

30

and

their

subsequent

transformation

to

aromatic

products

via

In comparison to the foregoing HZ spectra, the spectra of the Zn-modified HZ catalysts (see 7



1

Figure 2e) showed different evolution tendencies of the relative intensity of BAS. For Zn-

2

modified HZ, the relative intensity of BAS always increased at the beginning and thereafter

3

decreased slightly. The higher the Zn loading, the more pronounced the increase of BAS

4

relative intensity was. The absorbance at 3400-3500 cm-1 related to hydrogen-bonding showed

5

up with reaction progress but increased slower in intensity than that over HZSM-5. As the

6

results in Figure 4 show, the production of benzene increased with reaction time. It was

7

surprisingly found that compared with Zn1.10/HZ and Zn1.37/HZ (Figure S4), the absorbance

8

band at 1506 cm-1 of HZ presented earlier and the intensity of this band increased significantly

9

as reaction time increased, while for MS spectra the production of benzene was firstly observed

10

on Zn1.37/HZ and the amount of benzene was increased as the loading of Zn increased. These

11

results suggest that desorption of aromatic precursors from the surface of HZ is more difficult

12

than that from Zn modified HZ. Moreover, the H2 production was also detected through MS

13

spectra and similar results was found that the production of H2 was firstly observed on Zn1.37/HZ

14

and the amount of H2 was increased as the loading of Zn increased (Figure 4).

15

Abovementioned results suggest that the incorporation of Zn into HZSM-5 could simultaneously

16

accelerate dehydrogenation reactions and the desorption of aromatic precursors leading to

17

enhanced hydrogen and aromatics productivity. Propene aromatization over HZSM-5 and Zn

18

modified HZSM-5 was also studied by operando SB-FTIR, as shown in Figure S5, the results

19

are strongly influenced by gas-phase molecular vibrations and heat irradiation at reaction

20

temperatures.

21

(a) 0.05

TOS: 0-25 min

26

3500

3400

3300

-1

3737

3700

3600

3500

3400

3300

-1

Wavenumber / cm

TOS: 0-25 min

0.05

3800

(d) 0.05

TOS: 0-25 min

3737

3604

3604

Absorbance/A.U.

28

3600

Wavenumber / cm (c)

27

3700

-1 Relative Intensisity of the absorbance at 3604 cm / %

3800

HZ Zn0.72 /HZ

120

24 25

(e) 130

3604

Absorbance/A.U.

23

(b) 0.05 3737

Absorbance/A.U.

22

TOS: 0-25 min

3604

3737

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 8 of 15

Zn1.10/HZ Zn1.37/HZ

110

100

90

80

70

60

29 50

30

3800

3700

3600

3500

Wavenumber / cm

3400

-1

3300

3800

3700

3600

3500

Wavenumber / cm

-1

3400

3300

0

8


1

2

3

TOS / min

4

5

Page 9 of 15

1 2

Figure 2. Selected spectra (wavenumber: 3800-3300 cm-1) of propene aromatization over HZ (a), Zn0.72/HZ (b),

3 4 5 6 7

Zn1.10/HZ (c) and Zn1.37/HZ (d) at 170℃, 1 atm, continuously flowing mixture of propene and nitrogen gas (6 % propene

weights). 0.1

(c)

TOS: 0-25 min

2933

0.1

2958 2871

2860

2960

1463

1506

2861 2957 2959

2964

(b)

3000

13

0.1

1376 1463 1506

2967

2900

2800

Wavenumber / cm -1

12

TOS: 0-25 min

0.1

2962

2967

3100

TOS: 0-25 min

2933 2954

1376

Absorbance/A.U.

2954 2956

10 11

TOS: 0-25 min

0.1

Absorbance/A.U.

(a)

Absorbance/A.U.

9

absorbance band at 3604 cm-1 over samples at different reaction time (obtained results were normalized by the sample

Absorbance/A.U.

8

- 94 % nitrogen), GHSV=1080 h-1, the interval of each two spectra is 8 seconds. (2) the relative intensity of the

Absorbance/A.U.

1600

1550

1500

1450

Wavenumber / cm

TOS: 0-25 min

-1

1400

1350

3100

(d)

TOS: 0-25 min

0.1

3000

2900

-1

Wavenumber / cm

2933

2954

2800

1600

2954

1500

1450

Wavenumber / cm

TOS: 0-25 min

0.1

1376

1550

1400

1350

-1

TOS: 0-25 min

0.1

2933

2864

2960

1506

Absorbance/A.U.

1463

Absorbance/A.U.

14

2958

Absorbance/A.U.

1376

2862

2957 2958 2962

1463 1506

2967

2964 2967

15 3100

21 22 23

2800

1600

1550

1500

1450

1400

1350

3100

Wavenumber / cm-1

3000

2900

-1

2800

1600

1550

1500

1450

Wavenumber / cm

Wavenumber / cm

-1

1400

1350

Figure 3. Selected spectra of propene aromatization over HZ (a), Zn0.72/HZ (b), Zn1.10/HZ (c) and Zn1.37/HZ (d) at 170℃, 1 atm, continuously flowing mixture of propene and nitrogen gas (6 % propene - 94 % nitrogen), GHSV=1080 h-1, the interval of each two spectra is 8 seconds. (a) (a)

HZ HZ Zn0.72Zn/HZ 0.72 /HZ

(b) (b)Benzene Benzene

Zn1.10Zn/HZ 1.10 /HZ

H2 H2

Zn1.37Zn/HZ 1.37 /HZ

Ion current / A.U.

20

2900

Ion current / A.U.

17 18 19

3000

Wavenumber / cm -1

Ion current / A.U.

16

Ion current / 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


24 25 26 27 28

0

0 10

10 20

20 30

30 40

40 50

50 60

60

0

0 10

10 20

20 30

mixture of propene and nitrogen gas (6 % propene - 94 % nitrogen), GHSV=1080 h-1.

Scheme 1

31 32 33

40 50

50 60

60

Figure 4. Online mass spectra of propene aromatization over HZ and Znx/HZ at 170℃, 1 atm, continuously flowing

29 30

30 40

TOSTOS / min/ min

/ min TOSTOS / min

Scheme 2

9


2 3 4 5 6 7 8 9 10 11 12

Page 10 of 15

105

105

105

105

100

100

100

100

95

95

95

95

90

0

5

TOS / min

Zn1.10/HZ

Zn0.72/HZ

HZ 10

90

0

5

90

10

0

5

Zn1.37/HZ 90

10

TOS / min

TOS / min

0

5

10

TOS / min

Figure 5. The relative intensity of the absorbance band at 3604 cm -1 over HZ and Znx/HZ at 240℃, 1 atm, continuously flowing mixture of propene and nitrogen gas (6 % propene - 94 % nitrogen), GHSV=1080 h-1, the interval of each two spectra is 8 seconds (obtained results were normalized by the sample weights).

100

13

100

HZ Zn0.72/HZ Zn1.10/HZ Zn1.37/HZ

14 15

HZ HZ HZ

Zn0.72/HZ /HZ Zn 0.72 0.72/HZ

16

Weight Loss / wt %

90 80

99

70

Zn /HZ ZnZn /HZ/HZ 60 1.10 Zn 98 1.371.37 1.10/HZ

Zn1.37/HZ

50

Ion current / A.U.

1

Weight Loss / wt %

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

Relative Intensisity of the absorbance at 3604 cm-1 / %


40

97

17 18

HZ

30

HZ HZ

Zn0.72 /HZ Zn /HZ 0.72

Zn1.10/HZ Zn /HZ Zn 1.10/HZ 1.10

20

Zn1.37/HZ Zn /HZ Zn 1.37/HZ 1.37

96100

200200300

400 400 500

T / oC o

T/ C

600 600 700

800 800

100

200

300

400

T / oC

19

Figure 6. Image and TG spectra of HZSM-5 and Zn modified HZSM-5 catalysts after propene aromatization at 240℃

20

in DB-FTIR study.

21

Further increasing reaction temperature to 240 ℃, upon contact of propene with HZ, the

22

BAS absorbance at 3604 cm-1 attenuated immediately and then kept decreasing monotonously

23

(Figure 5). However, for Zn-modified HZ, the relative intensity of BAS always remarkably

24

increased initially and then mildly decreased in intensity. The higher the Zn loading was, the

25

more pronounced the BAS relative intensity was, similar to the phenomenon observed at

26

170 ℃. The used catalysts were further analyzed by TG. As the images of the used catalysts

27

shown in Figure 6 show, the darkness of the used catalysts follows the sequence

28

HZ>Zn0.72/HZ>Zn1.10/HZ>Zn1.37/HZ. The TG results suggest that a higher amount of coke was

29

generated over HZSM-5 than over Zn-modified HZSM-5. The coke amount decreased with the

30

increase of Zn loading. In association with the DB-FTIR spectra (Figure 3), the intensities of 10


500

600

700

800

Page 11 of 15

1

the benzene-related absorbance at 3400-3500 cm-1 and 1506 cm-1 over HZ are much stronger

2

than those of Znx/HZ. Moreover, the attenuation of the absorbance at 3604 cm -1 over HZ was

3

also faster than that of Znx/HZ. According to literature,13 the decrease of BAS may be caused

4

by the formation of coke. The aromatization of propene over BAS of HZ follows a carbocation

5

mechanism and the desorption of formed aromatics is difficult. These adsorbed aromatics will

6

polymerize to coke, leading to the linear attenuation of BAS.

7

For different active sites BAS (H+) and Zn2+ in Zn/HZSM-5, the activation mechanisms of

8

propene studied by DFT calculation are different. For H+, after the physical adsorption the

9

proton inserted into C=C bond of propene to form a carbonium ion CH3-CH+-CH3 connected to

10

framework oxygen of zeolite next to one of Al (as shown in Scheme 1). While, for Zn2+, after

11

the physical adsorption the C-H bond of methyl group of propene went through heterolytic

12

splitting over of Zn/HZSM-5 (as shown in Scheme 2). As seen in Figure 7, for two active sites

13

the big difference is the much lower physical adsorption energy of propene on Zn site than that

14

on proton site. The heat of adsorption is much larger for Zn site, which lowers the adsorption

15

state by more than 16 kcal/mol for Zn site relative to that for proton site. Owing to the strong

16

heat of adsorption on Zn site, the energy of transition state is pulled down substantially and

17

even ends up below zero. Compared with activation of propene over H+, the activation of

18

propene over Zn2+ is no energy barrier, indicating the activation over Zn2+ is thermodynamically

19

and kinetically favorable.

20 12.39

21

TS

22 Energy (kcal/mol)

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


23 24 25

Med

8T_2H+C3H6 0.00

2.73 Ads -5.98

8T_Zn+C3H6 0.00

-4.83

26

TS

27

Ads -22.20

28 29 30

Med -6.65

Reaction Coordinate Figure 7. DFT calculation on the competitive adsorption activation of propene on BAS and Zn2+ of Zn modified H-ZSM5 catalyst.

11



1

Both the experimental and theoretical calculation results suggest that transient BAS was

2

generated during propene aromatization over Zn-modified HZSM-5. Currently, the dissociative

3

adsorption behavior of Zn-modified zeolites has been reported in several studies with molecules

4

of hydrogen, methane, ethane, and propane.16-22 In the same studies, the formation of transient

5

BAS was shown with in situ diffuse reflectance FTIR spectra (DRIFT), as the consequence of

6

dissociative adsorption of molecular hydrogen, methane, ethane, and propane. However, to the

7

best of our knowledge, the dissociative adsorption of propene on Zn-modified zeolite has

8

seldom been mentioned in literature so far. In addition, these DRIFT studies were performed at

9

room temperature after evacuation, which is different from real reaction conditions. The catalytic

10

function of these transient BAS is also unknown.

11

The reasons why the BAS change in Zn-modified catalysts, may be as follows. First, the

12

increase of the relative intensity of BAS is ascribed to the contribution of propene dissociative

13

adsorbed on single or isolated Zn2+ catalyst sites (Scheme 2). Thus, each activated propene

14

molecule will produce one BAS. Second, the decrease of the relative intensity of BAS is due to

15

the participation of transient BAS in the activation of propene (proton addition reaction) and

16

aromatization like it is in the hydrogen-form zeolite. Third, in view of the certainty of Scheme 1

17

and 2 co-existence in Zn-modified catalysts, the initial increase of the relative intensity of BAS

18

in Zn-modified catalysts means that propene molecules react over the metallic Zn2+ sites rather

19

than over BAS.

20 21

4. CONCLUSIONS

22

Formation of transient BAS on Zn-modified H-ZSM-5 was evidenced by DB-FTIR-MS, which

23

suggest that the activation of propene on Zn/H-ZSM-5 zeolite takes place preferentially on

24

metal sites rather than BAS. Based on the DB-FTIR results, we propose that the activation of

25

propene adsorbed on Zn2+ sites of Zn/H-ZSM-5 occurs by C-H bond heterolysis of methyl group.

26

Besides the property of accelerating dehydrogenation reactions in propene transformation, the

27

Zn2+ sites were also found to be capable of accelerating the desorption of aromatic precursors

28

leading to enhanced aromatics production. This work demonstrates that the application of

29

operando DB-FTIR-MS has a bright future in the area of heterogeneous catalysis.

30 12


Page 12 of 15

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


1

DECLARATION OF INTERESTS

2

The authors declare no competing interests.

3 4

Supporting Information

5

Additional computational details, additional characterization results (XRD and TEM), operando

6

single beam Fourier transform infrared (SB-FTIR) spectra of propene aromatization over

7

samples and photograph of the operando dual‐beam Fourier transform infrared (DB-FTIR)

8

spectrometer and IR reactor cell.

9

Corresponding Author

10

*E-mail: [email protected]

11

ACKNOWLEDGEMENTS

12

We acknowledge financial support from the National Natural Science Foundation of China

13

(21603023) and the Joint Fund Project of NSFC-Liaoning Province (U1508205).

14

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13C

Pasynkova, E.M.; Pirogov,

MAS NMR studies of light alkanes

38 39 40 14


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TOC Graphic

1 2 3 4 5 6 7 8 9 10 11 12 13

130

HZ Zn0.72 /HZ

120 -1 Relative Intensisity of the absorbance at 3604 cm / %

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


Zn1.10/HZ Zn1.37/HZ

110

100

90

80

70

60

Transient Brønsted Acid Sites 50

0

1

2

3

4

5

TOS / min

15


Transient BrÃ¸nsted Acid Sites in Propene Aromatization over Zn

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