Molybdenum catalyst dynamics in methane aromatization · Schematic presentation of lab-scale test 0 facility for methane aromatization . 1 2 3 0 200 400 600 Benzene (μmol/min) Time

Molybdenum catalyst dynamics in methane aromatizationKae S. Wong

Laboratory for Chemical Technology, Ghent University, Krijgslaan 281 (S5), 9000 Ghent, Belgium

http://www.lct.UGent.be *E-mail: Kaeshin.wong@UGent.be

Introduction• The Gas-To-Liquids (GTL) processes allow the conversion of natural gas (80-95%

methane) into more valuable liquid products.

• The direct upgrading of methane to aromatic hydrocarbons, under non-oxidativeconditions,yields BTX as main aromaticproductsand hydrogenas a valuableby-

Objectives

Experimental• Catalyst: 0.5 g of Mo-containing (5.3 wt.%)

MCM-22 (Si/Al=15.5) bifunctional catalyst(210-300µm).

• Reactor: Continuousflow reactor(10 mm i.d.)

973K, Mo/HZSM-5

CH4

N2

Air

Furnace

Catalyst

• Quantitative assessment in terms of elementary steps for methane aromatizationkinetics at various catalytic stages.

conditions,yields BTX as main aromaticproductsand hydrogenas a valuableby-product.

• Methane aromatization over Mo/HMCM-22 experiences distinct catalyticstages1.

• Reactor: Continuousflow reactor(10 mm i.d.)at atmospheric pressure.

• Reaction conditions:1. Space time: 35, 40, 54, 81, 161 kgcat s

mol-1.2. Temperature: 873, 898, 923, 948, 973 K.3. Methane inlet partial pressure: 20, 40, 60

98 kPa.

Kinetic model development

Schematic presentation of lab-scale testfacility for methane aromatization .0

1

2

3

0 200 400 600

Be

nze

ne

(μ

mo

l/m

in)

Time on stream, min

Extrinsic relaxation period

Optimum catalyst performance

Catalyst deactivation

1Appl. Catal. A 253 (2003) P. 271 -282

C6H11+

C6H12

+ +

+

C4H8 C4H7+

+

+ +

CH3+

+

C6H13+ C4H9

+ C2H5+

CH4 CH4* CH2* C2H4* C2H4

+H+

+C2H4+C2H4

-H+

-H+ +H+

+H++C4H7

+H+

+H+ -H+

-H+

-H+

-H+

-H+

-H+

+H+

-H+

+

+Mo2C+CH2

* -Mo2C-H2

-H+

Acid function

Metal function

3 4 5 6

9

101112

13

14 15 16

17

1819

26

27

2021

22

23 24 25

28

29

30

MoMoMoMo

CH4 H2

Reduction of MoO2→ Mo2C

COC

CH4 CH4

MoO3

zeolite

CH4

zeolite

Reduction of MoO3→ MoO2

C2/C3

GC

Catalyst

Quartz reactor

Heating lines

Stage 1: Active Mo/HMCM-22 formation Stage 2: Optimum methane aromatization Stage 3: Catalyst deactivation

Coke formation on metal and acid sites

MoMoMoMoC

zeolite

CH4CH4

MoMoMoMoC

zeolite

CC C

C

H2

CO2

O2

MoO2

zeolite

H2

MoO2H2 MoO2

zeolite

H2

Steps Reaction Time scale (s)

1 9.78 102

2 6.92 101

3for 1.10 10-4

3rev 1.53 10-4

4for 1.98 10-3

4rev 1.10 10-3

5for 4.05 101

5rev 7.09 102

6for 3.33 10-3

6rev 7.37 10-3

7 1.56 10-1

8 2.24 104

M2dcR2 Advisory board meeting, Gent, 19th June 2012.

Results

0.00

0.20

0.40

0.60

0.80

1.00

0 5000 10000 15000

Yie

ld (

mo

l.%

)

TOS (s)

Figure 1. Methane conversion and product yields as a function of time on stream for methane aromatization over Mo/HMCM-22, at space time of 54 kgcat s/mol, 973 K and methane inlet partial pressure of 98 kPa.

0

2

4

6

8

10

0

2

4

6

8

10

12

0 5000 10000 15000

Yie

ld (

mo

l.%

)

Co

nv

ers

ion

(%

)

TOS (s)

COC2H4

Conclusions

The research leading to this result has received funding from the European Union Seventh FrameworkProgram FP7/2007-2013 under grant agreement n° 229183.

Acknowledgement

CH4 + 2MoO3→ 2MoO2H2 + CO2

3CH4 + 2MoO2H2→ Mo2C+ 2CO + 8 H2 + O2

2CH4 + H+→ CokeH+ + 4H2

CH4 + Mo2C → CokeMo2C + 2H2

CH4 + Mo2C ↔ CH4Mo2C

CH4Mo2C ↔ CH2Mo2C + H2

2CH2Mo2C ↔ C2H4Mo2C + Mo2C

C2H4Mo2C ↔ C2H4 + Mo2C

• The formation of active Mo2C proceeds in 2 consecutive steps: of Mo(VI)→ Mo(IV) →Mo2C, and is relatively fast.

• Adsorption of methane on Mo2C (step 3) and dissociation of adsorbed methane (step 4) takeplace readily once active Mo2C is formed.

• Surface reaction of adsorbed CH2 is fast. The desorption of adsorbed C2H4 (step 6) happensinstantaneously with the coupling of CH2 into C2H4 on Mo2C surface (step 5).

• Ethene, formed via methane dimerization on Mo2C, migrates to acid sites and undergoes fastoligomerization steps into benzene.

• The catalytic stages of methane aromatization over Mo-based catalystexhibits 3 stages.�Stage 1: development of active Mo2C�Stage 2: optimum methane aromatization�Stage 3: catalyst deactivation

• The reaction rate of Mo2C formation (steps 1 and 2) is 10 times faster in stage 1 than instage 2.

• The concentration of Mo2C peaks at the stage of optimum methane aromatization,leading to higher rate of methane dimerization (steps 5-8) at stage 2.

• The slow rate of coke formation on acid and metal sites causes steady catalystdeactivation.

Conversion

Figure 1 shows that methane conversion as well as product yields are described adequatelyby the dynamic model.

10 15 20 25

Step 1

Step 2

Step 3f

Step 3r

Step 4f

Step 4r

Step 5f

Step 5r

Step 6f

Step 6r

Step 7

Step 8

log (Reaction Rate [mol g-1 s-1])

Stage 1: Activation

Stage 2: Optimum

Stage 3: Deactivation