Luisa Palumbo Dipartimento di Chimica IFM - NIS Centre of Excellence Università di Torino

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1 Conversion of Methanol to Hydrocarbons: spectroscopic characterization of carbonaceous species formed over H-ZSMH ZSM-5 Luisa Palumbo Dipartimento di Chimica IFM - NIS Centre of Excellence Università di Torino MTH process Long term goal: upgrading the value of gas. How to make gasoline from natural gas (or Coal): Natural gas Coal Steam reform Gasification SynGas H 2 +C Methanol syntheis MTG Methanol MT Gasoline lefins

2 MT process: how does it work? More than 20 proposed mechanisms during the past 30 years (Involving intermediates such as radicals, carbenes, oxonium ions, carbocations). Reaction scheme: H 2 H 2 2 CH 3 H CH 3 CH 3 Alkenes Alkanes Aromatics It is known that the reaction needs acid catalysts and that the first step is the formation of CH 3 CH 3 The obscure point is the formation of the first C-C bond Building up a zeolite framework (Si 4 ) units chains cages rings Molecular sieve 2

3 Acidic centres Brønsted site Si Al Si H silanol + Lewis Site Al Lewis H Si Al Brønsted Zeolites employed in MT process The most critical properties of the catalysts are : methanol conversion product selectivity (ethylene propilene) MFI framework topology strength and density of acid sites zeolite morphology CHA 3

4 H-ZSM-5 5 in MT process ne of the major advantages of H-ZSM-5 when compared with other zeolites is its high resistance to deactivation by coke formation. Deactivation is thus explained by coke formation on the external surface of the zeolite crystallites only. H-ZSM-5 showed also a lower activity and selectivity to light olefine Conversion of methanol (%) SAP-34 H-ZSM Time on stream (hours) H-SAP-34 has shown better properties than H-ZSM-5 in yield % and selectivity for ethylene and propylene. L.-T. Yuen et al. Micropor. Mater. 2 (994) 05 Hydrocarbon pool C 2 H 4 CH 3 H (CH 2 ) n C 3 H 6 C 4 H 8 coke Big cages Small cavity CH 3 H C 3 H 6 C 4 H 8 C 3 H 6 CH 3 H C 6 H 2 C 5 H 0 M. Stöcker, Microporous Mesoporous Mater 999,. 29, CH 3 H Svelle, S.; Joensen, F.; Nerlov, J.; lsbye, U.; Lillerud, K.-P.; Kolboe, S.; Bjørgen, M. J. Am. Chem. Soc. 2006, 28,

5 DRUV-Vis Vis results: : CH 3 H in fixed bed reactor Treatment: He stream up to 370 C methanol flux (4-2 h) at 370 C nm range: multi-component band At increasing times, the bands at lower wavenumber become completely mixed Kubelka Munk Wavelength (nm) DRUV-Vis Vis results: : CH 3 H in static condiction methanol dosage: 70 torr heating in C range growth of components at about 400, 500 and 600 nm. 0.0 effect of NH 3 dosage: presence of carbocationic species Bands at 400 nm and at 500 nm decrease in intensity and shift Kubelka Munk 6 Kubelka Munk NH 3 effect Wavelength (nm) Wavelength (nm) 5

6 Raman results: : CH 3 H in fixed bed reactor Zeolite results very fluorescent By passing from the sample treated in methanol for 4h to the one treated for 2h, the fluorescence decreases The samples are characterized by hot spots where the reaction goes on Raman counts (a. u.) h Carbon peaks 2h Raman shift (cm - ) Fluorescence results: samples treated in reactor From DRSUV-Vis results: absorption at about 400cm - Excitation at 400cm - sample in air is more fluorescent than sample outgassed sample treated in methanol for more time (2h) is less fluorescent S / R 3.5x x x x0 6.5x0 6.0x x air λ ex =400nm flux MeH 4h flux MeH 2h degox500 Ch Wavelength (nm) 6

7 FTIR results: methanol dosage IR spectra of H-ZSM-5 sample: - outgassed at 500 C (solid line) - contacted by methanol 70 torr (dashed line) Silanols 3745 cm - Brønsted sites 365 cm - Absorbance (u.a.) 0.5 u.a Wavenumber (cm - ) FTIR results: effect of NH 3 heated for 5h at 370 C in methanol atmophere new species formation by dosing NH 3 this peak desappears, showing its carbocationic character Absorbance (u.a.) Wavenumber (cm - ) Absorbance (u.a.) 4 increasing the reaction time more structured peaks by dosing NH 3 these peaks remain in the same position Wavenumber (cm - ) 7

8 Conclusions DRUV-VIs VIs spectroscopy These preliminary results point out that the H-ZSM-5 sample, after methanol conversion, gives rise to species absorbing at 400 nm, which have a carbocationic character and that could be the coke precursors as they are present not only after a treatment in static conditions, but also upon a treatment in flux. Raman spectroscopy Due to fluorescence, Raman features, typical of carbon species, are visible only in sample treated for longer time and only in hot spots. Fluorescence spectroscopy Sample treated in methanol for longer time (2h) is not so fluorescence FTIR spectroscopy From these results, it is possible to conclude that in the MT process we observe carbocationic species formation just during the first steps of the reaction. Acknowledgements Prof Adriano Zecchina Prof Silvia Bordiga Dr Francesca Bonino Dr Laura Regli Dr Morten Bjørgen Dr Pablo Beato Dr Chiara Bertolino 8