Zeolitter Mekanismestudier som nøkkel til nye materialer Morten Bjørgen University of Oslo NIS Centre of Excellence Turin
Reaction The catalysis group at UiO Research vision Catalyst Reaction mechanism MTO Chabazite (110 surface) New Catalyst
Zeolites --Highly porous, high surface area, crystalline aluminosilicates --Framework based on SiO 4 and AlO 4 tetrahedra --Sharply defined channels/pores of molecular dimensions --Stable over a wide temperature range --Regenerable --Fast deactivation --Widely used as catalysts
O Si Framework charge balanced Brönsted by mobile cations acidity Si Al
In-situ FTIR studies of carbenium ions in zeolites have led to a new definition of zeolite acidity --Until the early 90 s, zeolites were believed to possess superacidity --Carbocation stability is inherently linked to the acid strength of the zeolites --Carbenium ions are likely reaction intermediates --Recently, we provided the first evidence of proton transfer from a zeolite to a benzene ring (hexamethylbenzene) M. Bjørgen et al. J. Am. Chem. Soc. 2003, 125, 15863-15868. M. Bjørgen et al. ChemPhysChem. In press 2004.
Spectroscopic evidence for the tetramethylbenzenium cation in zeolite H-betaH Conclusive evidence for proton transfer and cation formation H TetraMB adsorbed on H-beta Weakly acidic sites (defects) Outgassing + Strongly acidic, active sites H-beta Pure tetramb 3800 3600 3400 3200 3000 2800 1700 1600 1500 1400 Bjørgen et al. ChemPhysChem In press Wavenumber (cm -1 )
--Complementary DRUV/VIS experiments gave support to the FTIR results --From being classified as superacids, it now appears clear that zeolites have an acidic strength slightly lower than that of concentrated sulfuric acid π π M. Bjørgen et al. J. Am. Chem. Soc. 2003, 125, 15863-15868.
Conversion of methanol to hydrocarbons/olefins The methanol-to-hydrocarbons (MTH) technology represents a route for formation of olefins or gasoline from natural gas/coal Coal Gasification Natural Gas Steam Reforming Syn Gas CO+H 2 MTH MTG Methanol Synthesis Methanol Gasoline Olefins Natural Gas Liquids Dehydrogenation MTO
MTH/MTO chemistry How can two or more C 1 -entities react so that C-C bonds are formed? Which reactions lead to catalyst deactivation? The main catalytic cycle for olefin formation from methanol is based on a so-called hydrocarbon pool Methanol Hydrocarbon pool Alkenes H 2 O H-zeolite
Hydrocarbons retained within the zeolite pores Analysis (GC-MS, HRMS, NMR) Analyzed ex-situ by: Quenching the reaction (at a predetermined time) Dissolving the zeolite (15% HF) Extracting the organic material from the water phase Trapped organic species will be liberated and made available for analysis
Hydrocarbons retained in the zeolite pores when methanol is reacted over the H-beta H zeolite (GC-MS) Stability Are Hexamethylbenzene of these the retained compounds hydrocarbons is just a dominant inert was spectator retained probed molecules? by species stopping the feed and flushing the catalyst with carrier gas for 1 minute Tetramethylbenzene Pentamethylbenzene Hexamethylbenzene Among the trapped hydrocarbons, hexamb shows the fastest decomposition Hexamethylnaphthalene 8 10 12 14 16 18 20 Retention time (minutes)
Hexamethylbenzene is not an inert spectator molecule When fed alone over the beta zeolite, hexametylbenzene gives the same products as methanol How can these observations be rationalized? 13 CH 3 In-situ synthesis of isotopically labeled hexamethylbenzene inside the zeolite pores H 3 13 C H 3 13 C 13 CH 3 13 CH 3 13 CH 3 Bjørgen, M.; Olsbye, U.; Kolboe, S. J. Catal. 2003, 215, 30-44. Bjørgen, M.; Olsbye, U.; Petersen, D.; Kolboe, S. J. Catal. 2004, 221, 1-10
Co-reaction of 12 C-benzene and 13 C-methanol: 13 CH 3 H 3 13 C 13 CH 3 6 13 CH 3 OH H 3 13 C 13 CH 3 13 CH 3 OH H 3 13 C + 13 CH 3 H-Zeolite H 3 13 C 13 CH 3 H-Zeolite H 3 13 C 13 CH 3 6H 2 O 13 CH 3 H 2 O 13 CH 3 Zeolite - Hexamethylbenzene: Six labeled atoms Heptamethylbenzenium: Seven labeled atoms M. Bjørgen, U. Olsbye, D. Petersen and S. Kolboe, J. Catal. (2004), 221, 1-10.
The heptamethylbenzenium cation was found to be the reaction intermediate (i.e.( the hydrocarbon pool) of the MTH/MTO reaction H 3 13 C 13 CH 3 H 3 13 C + 13 CH 3 H 3 13 C 13 CH 3 13 CH 3
The catalytic cycle of the MTO/MTH reaction Methanol Reactant Zeolite + + Iso-butene + + Propene +
The hydrocarbon pool may also lead to deactivation + + - H + MeOH Deactivation Coke + A less steric demanding hydrocarbon pool is formed in zeolites with smaller channel dimensions (e.g. ZSM-5)
We have obtained a detailed insight into the mechanism of the MTH/MTO reaction A deeper insight into the catalyst itself is also crucial for understanding product selectivities and catalyst deactivation
MTO catalysts based on the CHA topology
1) The cage 2) Active sites, acidic protons in this example Four different positions for theacidicsites
H 2 for probing the local acidity in zeolites H 2 is a very sensitive probe molecule (single bond perturbation) The weak basic character requires low temperatures when studying interactions with zeolites
ν(o-h) region a) 0.2 0.25 FTIR: H 2 on low Al chabazite (H- SSZ-13) at 20 K ν(h-h) region b) The strongest Hinteraction 2 liquid ever phase Increasing H 2 pressures observed between H 2 and a zeolite Absorbance Defects Strongly acidic sites H-SSZ-13 3700 3600 3500 4160 4080 Wavenumber (cm -1 ) S. Bordiga, J. G. Vitillo, G. Ricchiardi, C. Lamberti, G. Spoto, A. Zecchina, M. Bjørgen, K. P. Lillerud, Submitted to Science (2004)
and where does this knowledge lead us?
Reaction The catalysis group at UiO Research vision Catalyst Reaction mechanism MTO Chabazite (110 surface) New Catalyst
Isotopic labelling studies indicate that the heptamethylbenzenium ion is the main intermediate for olefins AND coke formation over H-Beta zeolite. ZSM-5: Little deactivation. gem-pentamethylbenzenium ion probably main reaction intermediate. Would a smaller SAPO-34 cage lead to less coking, at similar olefin formation rates? And would the olefin selectivity change?
Could the acid strength be key to the MTH selectivity? An obstacle is the difficulty of preparing the exact same pore structure and acid site density with different elements A low-al Si/Al chabazite was recently prepared and will be tested for the MTH reaction.
24 23 UiO-12 AEN 22 21 UiO-20 DFT OSO UiO-6 UiO-7 ZON UiO-4 CHA OSO OsloSantaBarbara-1 OBW OsloSantaBarbara-2 FD FD Si 3 Si /1000Å 3 20 19 19 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 11 10 10 9 9 3 3+ 4 4+ 5 5+ 6 3 3+ 4 4+ 5 5+ 6 Size of Smallest Ring Size of Smallest Ring
OSO the only 3-ring only topology 0.3 nm
Crystallization of SAPO-34 We are slowly moving towards understanding the crystallization Ø.B. Vistad, D.E. Akporiaye, F. Taulelle, and K.P. Lillerud Chem. Mater. 2003, 15, 1639-1649
FTIR of interactions between CO and the Brönsted sites of H-SSZ-13 Increasing coverage of CO consumes the Brönsted sites O-H stretching vibrations of Brönsted sites Two families of Brönsted sites showing different accessibillity 3800 3700 3600 3500 Wavenumber (cm -1 )
Product distribution methanol is zeolite (GC-FID) distribution (400 C) when is reacted over the H-beta Aliphatics Aromatics Methane Ethane/ethene Propane/propene Dimethylether 1/i-butene Isobutane Methanol n-butane trans-2-butene cis-2-butene 2-methyl-2-butane Pentamethyl benzene Hexamethyl benzene 4 5 6 7 8 26 28 Retention time (minutes) Hexamethylbenzene is a dominant gas phase product
Questions about the mechanism still remain unanswered: Can two methanol molecules combine and form ethene? How are the light olefins formed? More than 20 proposed mechanisms (Involving intermediates as radicals, carbenes, oxonium ions, carbocations)
The beta zeolite is a wide pore zeolite (12-MR) allowing direct introduction of rather large molecules Conversion of methanol to hydrocarbons.. Zeolite H-beta H as a model system Zeolite beta: 7.7x6.6 Å
-H + + The catalytic cycle of the MTO/MTH reaction 13 CH 3 OH + Methanol + + 13 CH 3 OH -H + +