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