23/4/18 1

Effect of water in direct methane dehydroaromatization over

3%Mo/HZSM-5 under supersonic jet expansion condition

B.S. Liua, L. Lia, C.T. Auc, A.S.-C. Cheunga*

a The University of Hong Kong, Pokfulam Road, Hong Kong, China c Hong Kong Baptist University, Kowloon Tong, Hong Kong, China

23/4/18 2

Acknowledgements:

The work was supported by a grant from the Research

Grants Council of the Hong Kong Special Administrative

Region, China (Project No. HKU 7015/07P).

Prof. Bingsi Liu, Tianjin University, China

Drs. Joanne W.H. Leung and Lynn L. Li, HKU

Prof. C.T. Au, Hong Kong Baptist University

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I. IntroductionEnergy

Petroleum Coal

SyngasChemicals

Aromatics

Hydrogen

Fuel cell

NH3

23/4/18 4

CH4

Mo/HZSM-5

700oC

C6H6

C7H8

C10H8

+ H2

The catalyst Mo/HZSM-5 needs to be activated during the induction period:

H2O (< 2%)+

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2. Problems ?(1) Activation of catalyst in induction period.

(2) Carbon deposition over catalyst

Non-traditional procedure

CH4 only ?

Membrane reactor

Supersonic jet expansion

CH4 + H2O ?

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II. Experiment

CH4 dehydro-aromatization apparatus is coupled to a

Time-of-flight (TOF) mass spectrometer.

CH4 dehydroaromatization apparatus – catalytic reaction

TOF – detect and monitor reagents (CH4, H2O) and

reacted species (eg. C10H8) etc..

Ionization of species by a pulsed laser (266 nm; 13 mJ)

CH4 dehydro-aromatization apparatus is coupled to a

Time-of-flight (TOF) mass spectrometer.

CH4 dehydroaromatization apparatus – catalytic reaction

TOF – detect and monitor reagents (CH4, H2O) and

reacted species (eg. C10H8) etc..

Ionization of species by a pulsed laser (266 nm; 13 mJ)

23/4/18 7Fig.1 Apparatus for CH4 dehydroaromatizationFig.1 Apparatus for CH4 dehydroaromatization

II. Experiment

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III. Results and discussion

Fig.2 Product distribution at different time on stream Fig.2 Product distribution at different time on stream

1. Investigation on catalyst activation – CH4

m/z

0 25 50 75 100 125 150

Inte

nsi

ty (

au

)

5 min

10 min

55 min

125 min

155 min

185 min

After air oxidation

C1

0H8

+

H2+

C+

290 min

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2. Effect of water during CH4 dehydroaromatization (after the induction period)

m/z

0 20 40 60 80 100 120 140

Inte

nsity (

au

)

(a) 17 min (b) 40 min (c) 75 min (d) 109 min (e) 165 min

a

bcd

e

H+

H2+

C+

CH

+

H2O

+

C1

0H8

+

C2+

(A)

m/z

0 20 40 60 80 100 120 140

Inte

nsity

(au

)

(a) 10 min (b) 20 min(c) 75 min (d) 90 min(e) 145 min

a

bc

d

e

H+

H2+

C+

CH

+

H2O

+

C2+

C10

H8+

(B)

Time on stream (min)

0 40 80 120 160

Re

lative

na

ph

tha

len

e f

orm

ation

(%

)

0

10

20

30

0% H2O

Plot 1 Regr1.0%H2O

Plot 2 Regr1.4%H2O

Plot 3 Regr2.0%H2O

Plot 4 Regr

(c)

Fig.4 Effect of water on Mo/ZSM-5 catalyst; (A) 1.4%H2O; (B) 2.0%H2O; (C) Relative stability of catalyst.

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3. Investigation on catalyst activation – CH4 + H2O

Figure 5 Direct addition of water in induction period of CH4 dehydroaromatic reaction NO C10H8 formation

Figure 5 Direct addition of water in induction period of CH4 dehydroaromatic reaction NO C10H8 formation

m/z

0 20 40 60 80 100 120 140

In

te

nsity (a

u)

a

b

c

CH

O+

CO+

H+

H2+

H2O

+

OH+

C+ C

H+C

H 2+

30 min

55 min

165 min

Mo2C/ZSM-5MoO3/ZSM-5CH4

H2O

CH4 CHx

CHO

H2O

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4. Properties of coke under condition of H2O/CH4 co-feed

Temperaure (oC)

300 400 500 600 700 800

Pure CH4

1.0%H2O

1.4%H2O

2.0%H2O

a

b

c

d

Carbon removal (%)

10.1%

8.4%

4.8%

3.3%

584

602

608

591

466

474

484

462

Fig.6 TPO profiles of used catalyst

Fig.7 Amorphous carbon deposition over Mo2C particle. (TEM).

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Binding energy (eV)

279282285288291294

Pure CH4

2.0%H2O/CH4

Excess waterFresh catalyst

CH

O

C (

Mo 2C

)

CH 2=

CH 2

28

4.6

C 1s

a

b

c

d

Fig.8 Carbon 1s XPS spectra used 3%Mo/HZSM-5 at different condition.

-H2

CO + H

+ O*

CHO

- H2 -H2C (Carbon)

-H2CHxCH4

23/4/18 13Fig.11 Mo 3d XPS spectra of 3%Mo/HZSM-5 samples

6. Properties of Mo2C at different H2O/CH4 co-feed by XPS

Binding energy (eV)

220224228232236240244

New catalyst Pure CH4

2%H2O/CH4

Excess water

3d3/2

3d5/2

a

b

c

d

Mo 3d

Mo 3d5/2 (Mo2C)

MoO2

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VI. Conclusion:

1. Direct dehydroaromatization of methane over 3%Mo/HZSM-5 forms naphthalene under the supersonic jet expansion condition. In this work, the gas mixture exited from reaction zone was analyzed directly using TOF-MS.

2. An appropriate amount of steam co-addition to methane feed over Mo/HZSM-5 can improve the stability of catalyst. Carbon deposition was reduced with increasing steam concentration.

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3. The results of TPO, XPS and HRTEM image demonstrate that the carbon deposition is basically amorphous in structure during the H2O/CH4 reaction but

the formation of graphite carbon when CH4 was used only.

4. Addition of excess water (> 2%) led to the deactivation of Mo2C/HZSM-5 catalyst and the

destruction of Mo2C.

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