Conversion of Methanol to Hydrocarbons:
ydrocarbons:
spectroscopic characterization of carbonaceous species
formed over HH-ZSMZSM-5
Luisa Palumbo
Dipartimento di Chimica IFM - NIS Centre of Excellence
Università di Torino
Perugia, 24 Febbraio 2007
MTH process
Long term goal: upgrading the value of gas.
How to make gasoline from natural gas (or Coal):
Natural gas
Steam
reform
MTG
SynGas
H2+CO
Coal
Methanol
syntheis
Gasoline
Methanol
Gasification
MTO
Olefins
Perugia, 24 Febbraio 2007
1
MTO process:
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:
H2O
H2O
Alkanes
2 CH3OH
CH3OCH3
Alkenes
Aromatics
It is known that the reaction needs acid catalysts and that the first step
is the formation of CH3OCH3
The obscure point is the formation of the first C-C bond…
Perugia, 24 Febbraio 2007
Building up a zeolite framework
(SiO4) units
chains
cages
rings
Molecular sieve
Perugia, 24 Febbraio 2007
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Acidic centres
Brø
Brønsted site
O
Si
Al
O
Si
H
Lewis Site
Al
Si
O
silanol
H
O
O
O
Al
Brønsted
+ Lewis
Perugia, 24 Febbraio 2007
Zeolites employed in MTO process
The most critical properties of the catalysts are :
methanol conversion
product selectivity
(ethylene – propilene)
framework topology
strength and density of acid sites
zeolite morphology
MFI
CHA
Perugia, 24 Febbraio 2007
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H-ZSMZSM-5 in MTO process
One 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.
Conversion of methanol (%)
H-ZSM-5 showed also a lower activity and selectivity to light olefine
100
SAPO-34
80
H-SAPO-34 has shown better
properties than H-ZSM-5 in
yield % and selectivity for
ethylene and propylene.
60
40
H-ZSM-5
20
0
0
5
10
15
20
Time on stream (hours)
L.-T. Yuen et al. Micropor. Mater. 2 (1994) 105
Perugia, 24 Febbraio 2007
Hydrocarbon pool
C2H4
CH3OH
(CH2)n
C4H8
C3H6
coke
Big cages
Small cavity
CH3OH
C3H6
C4H8
CH3OH
C3H6
C6H12
M. Stöcker, Microporous Mesoporous Mater 1999,. 29, 3-48.
C5H10
CH3OH
Svelle, S.; Joensen, F.; Nerlov, J.; Olsbye, U.; Lillerud, K.-P.; Kolboe, S.; Bjørgen, M. J.
Am. Chem. Soc. 2006, 128, 14770-14771.
Perugia, 24 Febbraio 2007
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DRUVDRUV-Vis results:
results: CH3OH in fixed bed reactor
Treatment:
He stream up to 370°C
methanol flux (4-112 h) at 370°C.
Kubelka Munk
0.1
400-700 nm range:
multi-component band
400 600
4
At increasing times, the bands
at lower wavenumber become
completely mixed
1
400
600
800
Wavelength (nm)
Perugia, 24 Febbraio 2007
DRUVDRUV-Vis results:
results: CH3OH in static condiction
methanol dosage: 70 torr
heating in 300-400 °C range
growth of components at
about 400, 500 and 600 nm.
effect of NH3 dosage:
presence of carbocationic
species
Bands at 400 nm and at 500 nm
decrease in intensity and shift
0.01
Kubelka Munk
Kubelka Munk
0.005
6
NH3 effect
1
3
1
2
400
600
800
Wavelength (nm)
400
600
800
Wavelength (nm)
Perugia, 24 Febbraio 2007
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Raman results:
results: CH3OH in fixed bed reactor
Zeolite results very fluorescent
The samples are characterized by
hot spots where the reaction goes
on
80000
Raman counts (a. u.)
By passing from the sample
treated in methanol for 4h to
the one treated for 112h, the
fluorescence decreases
60000
4h
40000
Carbon peaks
20000
112h
0
2500
2000
1500
1000
500
-1
Raman shift (cm )
Perugia, 24 Febbraio 2007
Fluorescence results:
results: samples treated in reactor
From DRSUV-Vis results: absorption at about 400cm-1
Excitation at 400cm-1
6
3.5x10
λex=400nm
6
3.0x10
sample in air is more fluorescent
than sample outgassed
6
2.0x10
S1 / R1
sample treated in
methanol for more time
(112h) is less fluorescent
6
2.5x10
flux MeOH 4h
air
6
1.5x10
6
1.0x10
flux MeOH 112h
5
5.0x10
degox500°C1h
0.0
450
500
550
600
650
700
750
Wavelength (nm)
Perugia, 24 Febbraio 2007
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FTIR results:
results: methanol dosage
IR spectra of H-ZSM-5 sample:
- outgassed at 500°C (solid line)
- contacted by methanol 70 torr (dashed line)
3745 cm-1
Brønsted sites
3615 cm-1
Absorbance (u.a.)
Silanols
0.5 u.a
3500
3000
2500
2000
-1
Wavenumber (cm )
1500
Perugia, 24 Febbraio 2007
FTIR results:
results: effect of NH3
0.2
Absorbance (u.a.)
heated for 5h at 370°C in methanol atmophere
• new species formation
• by dosing NH3 this peak desappears,
showing its carbocationic character
0.2
1500
1400
-1
Absorbance (u.a.)
Wavenumber (cm )
increasing the reaction time
• more structured peaks
• by dosing NH3 these
remain in the same position
4
peaks
1
1500
1400
-1
Wavenumber (cm )
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Conclusions
DRUVDRUV-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 (112h) is not so fluorescence
FTIR spectroscopy
From these results, it is possible to conclude that in the MTO process we observe
carbocationic species formation just during the first steps of the reaction.
Perugia, 24 Febbraio 2007
Acknowledgements
Prof Adriano Zecchina
Prof Silvia Bordiga
Dr Francesca Bonino
Dr Laura Regli
Dr Morten Bjørgen
Dr Pablo Beato
Dr Chiara Bertolino
Perugia, 24 Febbraio 2007
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