Molybdenum catalyst dynamics in methane aromatization · Schematic presentation of lab-scale test 0...
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Molybdenum catalyst dynamics in methane aromatization Kae S. Wong Laboratory for Chemical Technology, Ghent University, Krijgslaan 281 (S5), 9000 Ghent, Belgium http://www.lct.UGent.be * E-mail: [email protected] 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-oxidative conditions, yields BTX as main aromatic products and hydrogen as a valuable by- Objectives Experimental • Catalyst: 0.5 g of Mo-containing (5.3 wt.%) MCM-22 (Si/Al=15.5) bifunctional catalyst (210-300 μm). • Reactor: Continuous flow reactor (10 mm i.d.) 973K, Mo/HZSM-5 CH 4 N 2 Air Furnace Catalyst • Quantitative assessment in terms of elementary steps for methane aromatization kinetics at various catalytic stages. conditions, yields BTX as main aromatic products and hydrogen as a valuable by- product. • Methane aromatization over Mo/HMCM-22 experiences distinct catalytic stages 1 . • Reactor: Continuous flow reactor (10 mm i.d.) at atmospheric pressure. • Reaction conditions: 1. Space time: 35, 40, 54, 81, 161 kg cat 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 test facility for methane aromatization . 0 1 2 3 0 200 400 600 Benzene (μmol/min) Time on stream, min Extrinsic relaxation period Optimum catalyst performance Catalyst deactivation 1 Appl. Catal. A 253 (2003) P. 271 -282 C 6 H 11 + C 6 H 12 + + + C 4 H 8 C 4 H 7 + + + + CH 3 + + C 6 H 13 + C 4 H 9 + C 2 H 5 + CH 4 CH 4 * CH 2 * C 2 H 4 * C 2 H 4 +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 10 11 12 13 14 15 16 17 18 19 26 27 20 21 22 23 24 25 28 29 30 Mo Mo Mo Mo CH 4 H 2 Reduction of MoO 2 → Mo 2 C CO C CH 4 CH 4 MoO 3 zeolite CH 4 zeolite Reduction of MoO 3 → MoO 2 C 2 /C 3 GC 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 Mo Mo Mo Mo C zeolite CH 4 CH 4 Mo Mo Mo Mo C zeolite C C C C H 2 CO 2 O 2 MoO 2 zeolite H 2 MoO 2 H 2 MoO 2 zeolite H 2 Steps Reaction Time scale (s) 1 9.78 10 2 2 6.92 10 1 3 for 1.10 10 -4 3 rev 1.53 10 -4 4 for 1.98 10 -3 4 rev 1.10 10 -3 5 for 4.05 10 1 5 rev 7.09 10 2 6 for 3.33 10 -3 6 rev 7.37 10 -3 7 1.56 10 -1 8 2.24 10 4 M2dcR2 Advisory board meeting, Gent, 19 th June 2012. Results 0.00 0.20 0.40 0.60 0.80 1.00 0 5000 10000 15000 Yield (mol.%) 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 kg cat 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 Yield (mol.%) Conversion (%) TOS (s) CO C 2 H 4 Conclusions The research leading to this result has received funding from the European Union Seventh Framework Program FP7/2007-2013 under grant agreement n° 229183. Acknowledgement CH 4 + 2MoO 3 → 2MoO 2 H 2 + CO 2 3CH 4 + 2MoO 2 H 2 → Mo 2 C+ 2CO + 8 H 2 + O 2 2CH 4 + H + → CokeH + + 4H 2 CH 4 + Mo 2 C → CokeMo 2 C + 2H 2 CH 4 + Mo 2 C ↔ CH 4 Mo 2 C CH 4 Mo 2 C ↔ CH 2 Mo 2 C + H 2 2CH 2 Mo 2 C ↔ C 2 H 4 Mo 2 C + Mo 2 C C 2 H 4 Mo 2 C ↔ C 2 H 4 + Mo 2 C • The formation of active Mo 2 C proceeds in 2 consecutive steps: of Mo(VI) → Mo(IV) → Mo 2 C, and is relatively fast. • Adsorption of methane on Mo 2 C (step 3) and dissociation of adsorbed methane (step 4) take place readily once active Mo 2 C is formed. • Surface reaction of adsorbed CH 2 is fast. The desorption of adsorbed C 2 H 4 (step 6) happens instantaneously with the coupling of CH 2 into C 2 H 4 on Mo 2 C surface (step 5). • Ethene, formed via methane dimerization on Mo 2 C, migrates to acid sites and undergoes fast oligomerization steps into benzene. • The catalytic stages of methane aromatization over Mo-based catalyst exhibits 3 stages. Stage 1: development of active Mo 2 C Stage 2: optimum methane aromatization Stage 3: catalyst deactivation • The reaction rate of Mo 2 C formation (steps 1 and 2) is 10 times faster in stage 1 than in stage 2. • The concentration of Mo 2 C 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 catalyst deactivation. Conversion Figure 1 shows that methane conversion as well as product yields are described adequately by 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
Transcript of Molybdenum catalyst dynamics in methane aromatization · Schematic presentation of lab-scale test 0...
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: [email protected]
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
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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
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4
6
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0
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6
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0 5000 10000 15000
Yie
ld (
mo
l.%
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Co
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(%
)
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
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