Methane dehydroaromatization on Mo/ZSM-5 catalysts: structure of active sites and carbonaceous deposits during catalytic cycle

Transcription

1 EUROPACAT Vll «CATALYSIS: A KEY TO REACHER AND CLEANER SOCIETY» Methane dehydroaromatization on Mo/ZSM-5 catalysts: structure of active sites and carbonaceous deposits during catalytic cycle E.V. Matus 1, Z.R. Ismagilov 1, V.I. Zaikovskii 1, L.T. Tsikoza 1, N.T. Vasenin 1, O.B. Sukhova 1, I.Z. Ismagilov 1,2, M.A. Kerzhentsev 1, V. Keller 2, N. Keller 2, F. Garin 2 1 Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russia 2 LMSPC, Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse, Strasbourg, France 28 August 1 September, 2005, Sofia Bulgaria

2 Methane dehydroaromatization (DHA) over Mo/ZSM-5 catalysts - a new perspective environmentally-friendly way to obtain both valuable aromatics and hydrogen 6СН 4 С 6 Н 6 + 9Н 2 bifunctional catalyst CH 4 MoO x => Mo 2 C (or MoO y C z ) 2CH 4 => C 2 H 6 => C 2 H 4 on formed Mo 2 C (or MoO y C z ), 3C 2 H 4 => C 6 H 6, etc. on HZSM-5 equilibrium CH 4 conversion to benzene under non-oxidative conditions at 720 C is about 12%.

3 Methane dehydroaromatization (DHA) over Mo/ZSM-5 catalysts: main questions - What is the nanostructure and localization of the active molybdenum species? - What is the nature of unwanted carbonaceous deposits formed during the reaction and how to minimize their formation? - Reaction mechanism - How to make the DHA of methane a commercially applicable process?

4 Approach - systematic study of the state of Mo at all stages of Mo/ZSM-5 preparation and catalytic cycle: - selection of molybdenum precursor - preparation of impregnation solutions - catalyst after treatment in air at 110 o C, 500 o C and 700 o C - catalyst after pretreatment in Ar at 720 o C - catalyst after the CH 4 DHA reaction at 720 o C - past reaction catalyst after treatment in O 2 at 600 o C depending on Si/Al ratio in the parent H-ZSM-5 and on Mo content

5 Preparation of Мо/ZSM-5 catalysts Method of incipient wetness impregnation of zeolite H-ZSM-5 by solution of ammonium heptamolybdate ((NH 4 ) 6 Mo 7 O 24 *4H 2 O, AHM) at controlled value of solution ph impregnation VARIATION of CONDITIONS concentration of AHM solution ph of AHM solution Si/Al atomic ratio of H-ZSM-5 zeolite? thermal treatment temperature composition of medium (air, Ar) PHYSICO-CHEMICAL PROPERTIES and ACTIVITY of Мo/ZSM-5 CATALYSTS

6 Investigation of Mo/ZSM-5 catalysts by group of methods Preparation ph, С, Si/Al Drying, Calcination o С, - O 2, Ar Reaction DHA of СН 4 (t h, %СН 4, T o C) Мо/ZSM-5 catalyst N 2 physisorption XRD ESR HRTEM, EDX DTA XPS TPO, TPH Oxygen treatment after reaction TPR

7 fume hood Laboratory flow setup for CH 4 DHA 14b 23 to FID to reference 17a 17b to TCD Chromaton N NaX GC (room T) GC (165 C) SKT FID TCD compressed air fume hood Test conditions: load = 1.0 cm 3 (0.6 g); fraction = mm flow = 13.5 cm 3 /min; GHSV = 810 h -1

8 Catalytic activity of Мо/ZSM-5 catalysts (Si/Al=17) in DHA of methane: dependence on Mo content Methane Conversion to Benzene, vol. % % Mo Reaction Time, min 2% Mo 5% Mo 1% Mo 0.6 % Mo Methane Conversion to Benzene, vol. % Molybdenum Content, wt. % The maximum catalytic activity (CH 4 conversion ~ 14%) and benzene formation selectivity (70%) were observed for the samples with 2-5% Mo.

9 Composition of gaseous reaction products (2% Мо/ZSM-5 catalyst (Si/Al=17)) concentration, vol. % H 2 C 6 H 6 CO concentration, vol. % CO 0,25 C 10 H 8 0,20 0,15 0,10 0,05 0,00 C 2 H 4 C C 7 H 8 C 10 H 2 H 6 8 C 2 H 4 C 7 H 8 C 2 H 6 The benzene and hydrogen are main products.

10 Catalytic activity of 2% Мо/ZSM-5 catalysts in DHA of methane: dependence on Si/Al ratio Conversion, vol.% Si/Al ratio in the parent H-ZSM-5. Si/Al total conversion - conversion to benzene Reaction Time, min. Benzene Selectivity, % 100 The activity and selectivity increase with Si/Al change from 45 to Si/Al=17 Si/Al=30 Si/Al= Reaction Time, min.

11 Methane Conversion, vol. % Catalytic activity of 2% Мо/ZSM-5 catalyst (Si/Al=17) in DHA of methane: stability - total convercion - conversion to benzene Reaction Time, h The catalyst performance is decreasing with time on methane stream. Methane Conversion to Benzene, vol. % O 2, 600 o C/2 h Reaction Time, h The oxygen treatment with following methane re-admission leads to recovery of catalyst performance.

12 XRD 10% Mo/ZSM-5 Si/Al=17 initial compounds halo at o halo at o halo at o * * intencity of main zeolite lines after O 2 treatment is decreased halo at o O 2 /520 o C/2h CH 4 /720 o C/6h Ar/720 o C/1h O 2 /500 o C/4h O 2 /110 o C/4h * MoO 3 (NH 4 ) 6 Mo 7 O 24 *4H 2 O* H/ZSM θ О 2 /500 о С Ar/720 o C (NH 4 ) 6 Mo 7 O 24 *2H 2 O/ZSM-5 MoO 3 /ZSM-5 Мо 9 О 26 or Мо 4 О 11 /ZSM-5 degrees О 2 /520 о С CH 4 /720 о С Мо 9 О 26 or Мо 4 О 11 /ZSM-5 η-moc or η-mo 3 C 2 /ZSM-5

13 HRTEM (2% Мо/ZSM-5) Ar/720 o C/1ч Surface Мо-O-clusters 1-5 nm CН 4 /720 o C/6h С, graphite β-мо 2 С 50 During the reaction, particles of β-mo 2 C with sizes 2-15 nm are forming on the ZSM-5 surface. The surface of Mo 2 C particle is covered by graphite layers with thickness of ~ 2 nm. EDX relative number, % size of particle, nm

14 HRTEM (10% Мо/ZSM-5, CН 4 /720 o C/6h) The particle number density on a flat projection of zeolite crystal grows with an increase of crystal thickness, and these particles are absent on the side projections of zeolite crystal. During the reaction, Mo-containing clusters with sizes ~ 1 nm are forming in the ZSM-5 channels.

15 HRTEM (2% Мо/ZSM-5, CН 4 /720 o C) During the reaction, carbonaceous deposits are forming on the ZSM-5 surface in the form of friable disordered layer with thickness up to 3 nm. after 6 h of reaction The content of friable carbonaceous deposits is increasing with time on stream. after 20 h of reaction 5 nm

16 HRTEM (2% Мо/ZSM-5, О 2 /520 o C/2h) The treatment of the Мо/ZSM-5 catalyst in oxygen stream at 520 o C for 2 h leads to formation of Mo-O-contained clusters on zeolite surface similar to those in parent Mo/ZSM-5 catalysts.

17 DTA С x H y +(x+y/4)o 2 = xco 2 + y/2h 2 O (weight loss) Mo 2 C (204 g/mol) + 4 O 2 = 2 MoO 3 (288 g/mol) + CO 2 (weight gain) 500 o C 445 o C 6h -3% 6h 0.5h -1% 450 o C Temperature, o C Temperature, o C For the 2 and 10% Mo/ZSM-5 catalysts and reaction duration between 0.5 and 6 h, carbonaceous deposits in the catalysts with Si/Al = 17 are characterized by a single maximum of the exothermal burn-out effect. The content of carbonaceous deposits and extent of their condensation degree (C/H ratio) are increasing with time-on-stream. 0.5h 290 o C -1.5% 2% Mo 10% Mo -1% +2% +3.5%

18 С, graphite β-мо 2 С The Mo-containing clusters localized in zeolite channels can be the active centers for DHA of CH 4.

19 Conclusions: 1. The maximum values of Mo/ZSM-5 catalytic activity (total CH 4 conversion (14%) and C 6 H 6 formation selectivity (70%) are achieved at Mo content 2-5% and are increasing upon the lowering of zeolite Si/Al ratio from 45 to Using the HRTEM it was shown, that during the reaction molybdenum carbide β- Mo 2 C is formed on the ZSM-5 surface, and it is characterized by the lattice parameters d 002 = nm, d 400 = 0.26 nm and the particle size of 2-15 nm. It was also demonstrated, that during the reaction Mo-containing clusters with sizes ~ 1 nm are forming in the ZSM-5 channels. 3. In the course of reaction, the carbonaceous deposits are formed both on the surface of Mo 2 C particles (in the form of graphite layers with lattice parameter d 002 = 0.35 nm and thickness of approximately 2 nm) and on the zeolite surface (in the form of friable disordered layer with thickness up to 3 nm). 4. The content of carbonaceous deposits and extent of their condensation (C/H ratio) are increasing with time-on-stream. 5. The oxygen treatment with following methane re-admission leads to recovery of catalyst performance.

20 Acknowledgements: The authors are grateful to Dr. V.A. Ushakov for the XRD measurements and discussion, Dr. G.S. Litvak for the DTA measurements.