Implications for Catalysis
Institute for
Integrated Catalysis
Department of Chemistry
Catalysis Research Center
Lighter FeedstocksImplications and chances for catalysis
Johannes A. Lercher
Institute for Integrated Catalysis, Pacific Northwest National Laboratory
Catalysis Research Center, Technische Universität München
Washington D.C., March 2016
Department of Chemistry
Catalysis Research Center
Institute for
Integrated Catalysis
Availability of methane and light alkanes offers a transition
to lower the carbon footprint
Disproportionation
Synthesis gas generation
Metathesis
Aromatization
Direct selective oxidation
Oxidative coupling
CH4
Ethane steam cracking
Dehydrogenation
C 3H 8
Oxidative dehydrogenation
Selective oxidation to
acrolein/acrylic acid/acrylonitrile
C 2H 6
Oxidative dehydrogenation
Selective oxidation
ethanol/acetaldehyde/acetic acid
Selective oxidation
ethylene oxide
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Department of Chemistry
Catalysis Research Center
Methane conversion
from high-temperature catalysis to
bio-inspired selective oxidation
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Department of Chemistry
Catalysis Research Center
All reforming reactions share similar catalytic chemistry
H2O + CH4
CO + 3H2
+ 226 kJ/mol
½ O 2 + CH4
CO + 2H2
- 44 kJ/mol
CO2 + CH4
2 CO + 2H2
+ 261 kJ/mol
CH4 ⇌ CH3 + H ΔH = 421 kJ/mol
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Department of Chemistry
Catalysis Research Center
The key problem is to understand carbon formation
T = 900°C, 5 bar
H/C ratio
• Carbon deposition may occur
under many operating
conditions
• Challenge for local mass
transport limitations
• More severe at higher
pressures
O/C ratio
Department of Chemistry
Catalysis Research Center
Institute for
Integrated Catalysis
Kinetics point to a common set of reaction pathways
H
CH3
Kinetically relevant
C-H bond breaking
H*
H*
H*
C*
k1
O*
+ CH4
CO*
- H2
H*
H*
H*
K5
- H2O
OH*
K4
Quasi-equilibrated
adsorption-desorption steps
J. Wei, E. Iglesia, J. Catal. 224 (2004) 370.
Department of Chemistry
Catalysis Research Center
Institute for
Integrated Catalysis
r=
kO*Co -*Ni
[CO 2 ]
[CH 4 ]
KCO
[CO]
2
[CO 2 ]
1 + [CO]
KCO2
k1st
[mol (g-atom Msurface-s-kPa)-1]
Reforming rate depends on the oxygen coverage
1
Methane reforming on Ni-Co
CO2 provides O* at
the catalyst surfaces
0.8
O* contents are
equilibrated with gas
phase oxidant
0.6
0.4
0.2
Ni-Co clusters
873 K
0
0
5
10
15
20
25
CO2-to-CO ratio
30
[O*] KCO 2 [CO 2 ]
=
[*]
KCO [CO]
Coverages of CH4* and O* on catalysts during reforming at 15 kPa CH4 and 30 kPa CO2, 873 K
Catalyst
12Co
3Fe-9Ni
9Co-3Ni
6Co-6Ni
3Co-9Ni
12 Ni
3Cu-9Ni
O*
0.86
0.69
0.39
0.24
0.07
0.05
0
Chin et al. J. Am. Chem. Soc, 135 (2013) 15425
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Department of Chemistry
Catalysis Research Center
Carbon accumulation depends on the particle size
• High concentration of active
oxygen maintains low
carbon concentration.
• Surface remains clean even
under condition of severe
conversions.
• High rate of carbon
formation leads to
recrystallization.
• Particle size below 2 nm
begins to destabilize the
carbon fibers.
• Coke accumulation leads
then to carbon deposits,
which are easier to oxidize.
Department of Chemistry
Catalysis Research Center
Institute for
Integrated Catalysis
Alternative pathways may help stabilizing catalysts
50
1.0
Ea = 85 ± 4 kJ/mol
0.5
1% Ni/ZrO2
20
10
1% CO2/Ni/ZrO2
0.0
-0.5
lnk / -
CO2 regeneration
TOF / s
-1
30
CO2 regeneration
CO2 regeneration
40
-1.0
-1.5
1% CO2/Ni/ZrO2
1% Ni/ZrO2
-2.0
-2.5
-3.0
0.90
0.95
1.00
1.05
1000 / T /-1K
1.10
1% Ni/Al2O3
1% Ni/Al2O3
0
0
200
400
600
800
1000
Time / min
•
•
If activity only depends on surface oxygen level and carbon present, the support
should not influence the rates.
Identical activation energies point to identical mechanism, independent of the
treatment.
Reaction conditions: 800°C , 1 bar, CH4: 25 ml/min, CO2: 25 ml/min, N2: 50 ml/min, 20 mg cats, 480 mg SiC
1.15
Department of Chemistry
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CH4 oxidative coupling links surface and gas phase reactions
•
•
•
•
Generation of CH3·and HO2· involves steps
in the MgO surface.
Stabilization appears to be very
challenging.
Formation of O2- cations in other oxide with
easier stabilization and the ability to
polarize C-H bonds appears to be crucial.
– CaO with Mn2+ dopant cations
– La2O3, etc.
Why are high temperatures mandatory?
P. Schwach et al., J. Catal. 329 (2015) 560.
Mohammad-Sadegh Salehiet al., Ind. Eng. Chem. Res. 2016, 55, 1149.
Department of Chemistry
Catalysis Research Center
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Aromatization combines dehydrogenation, C-C bond
formation and acid-base catalyzed ring closure
3 wt% Mo/HZSM-5 at 973 K
CH4 conversion
Selectivity
Selectivity HC
Rates
• Dehydrogenation on MoC to carbene
like species
• Dimerization of carbenes
• Ring closure and aromatization on the
zeolite
• Carbon management is critical.
• New approach with Fe@SiO2 at 1360K
leads to 50 % stable conversion.
X.Gua, et al., Science 344 (2014) 616.
S.Liu, et al., J. Catal. 181 (1999) 175.
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Incorporation of methane via metathesis
CH4 + C3H8 ⇌ 2 C2H4
• Combination of dehydrogenation of
propane with olefin metathesis leads
to incorporation of CH4 into larger
alkanes.
• Problems lie in
– the tight specifications for
operating the catalyst,
– the low rates achievable,
– the thermodynamic limitations
D. Soulivong et al., Angew. Chem. Int. Ed. 43 (2004) 5366.
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Complex combined cycles require H2SO4/SO3 as oxidant
R.A. Periana et al., Science 280 (1998)
560
R.A. Periana et al., Science 301 (2003) 817
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Nature found a way to oxidize methane in monooxygenases
• Methane monooxygenases (MMOs) convert methane to methanol in the
presence of O2 with high efficiency under ambient conditions.
• Membrane bound MMO (pMMO) converts methane at ambient temperature
via oxygen insertion with a TOF of nearly 1s-1.
• The active site is either a Cu dimer or a Cu trimer.
9-10 Cu+
1 type 2 Cu center
1 binuclear Cu site
A site
B site
C site
D site
Tricopper cluster
Lieberman, R. L.; Rosenzweig, A. C., Nature 2005, 434, 177-182.
Chan, S. I.; Yu, S. S.-F. Acc. Chem. Res. 2008, 41, 969-979.
1 tri-Cu-oxo cluster
Department of Chemistry
Catalysis Research Center
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Combining site activation, methane activation, and
methanol removal
(CuO)x
• Direct formation of methanol over CuZSM-5 has been reported by
Groothaert et al. in 2005, but it had to
be extracted ex situ from the zeolite.
2
1
Groothaert, JACS, 2005, 127, 1395
OH
OCH3
OH
OH
(CuO)x
(CuO)x
3
Cu/zeolite
SiO2/Al2O3
Cu/Al
CH3OH
(µmol/g)
MOR
11
0.48
32.6
ZSM-5
25
0.47
26.7
SSZ-13
19.2
0.3
12.6
• Three-step procedure at TUM allows
quantitative desorption of products.
Department of Chemistry
Catalysis Research Center
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Three Cu cations are needed to convert CH4
2:3
1:3
1 MeOH per 3 Cu
Accumulation of Al as paired sites in the side pockets allows formation
of multinuclear active site even at low loadings and high Si/Al ratios.
S. Grunder, Nature Communications, 6 (2015) 7546.
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Trimeric clusters form only in the side pockets of MOR
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The process is repeatable but requires interim activation
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Cycle for reoxidation of the sites involves deeper reduction
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Methane can be oxidized with H2O2 on Cu/FeZSM5
CH3OH
CH3OOH
• Clusters similar to those active for
CH4 oxidation with O2
H2O2
H2O2
CH4
• Free radical processes initiated
• Fe seems to be indispensable
• H2O2 more costly than reactants
C. Hammond et al., Angew. Chem. Int. Ed. 51 (2012) 5129
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Ethane from steam cracking to oxidative dehydrogenation
Department of Chemistry
Catalysis Research Center
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Oxidative dehydrogenation of ethane is challenging
• Lewis acid sites bind olefins and together with anionic nucleophilic oxygen
and catalyze oxygen insertion as well as subsequent decarbonylation or
decarboxylation leading eventually to total oxidation.
Catalytic sites should favor C-H cleavage and minimize readsorption of
olefins.
Oxide surface minimizing the
concentration of accessible
Lewis acid sites
Surfaces that dynamically
rearrange prevent exposure of
cations
Mechanistic understanding on a molecular scale is mandatory to tailor
catalysts with a high olefin selectivity.
Requirement for economic competitivity: X(C2H6) > 50%, S(C2H4) ≥ 95%
V/Mo oxides
• Highly active
• Varying olefin yield
Supported alkali chlorides
• Formation of molten overlayer
• High stability
Selectivities of
95% achievable
C.A. Gärtner, A.C. van Veen, J.A. Lercher, ChemCatChem, 2013, 5, 2-24
Department of Chemistry
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60
40
20
1
MgO
MgO/Dy2O3
80
BET area / m²
LiKCl
LiKCl
LiKCl
BET spec. surf. area/ m2·g-
Supported alkali chloride catalysts
0
0
Solid oxide core
Overlayer (mp: 353°C)
10
20
30
40
mol% overlayer
Overlayer
/ mol%
Dense
chloride
overlayer
Department of Chemistry
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Influence of overlayer on selectivity
1,4
90
1,2
80
1,0
S(C2H4) / %
ethene production / µmol g-1 s-1
100
0,8
70
60
0,6
50
0,4
40
0
10
20
30
40
50
Overlayer
mol%/ mol%
overlayer
0
10
20
30
40
50
mol% overlayer
Overlayer
/ mol%
Support pore system accessible
Reaction on less selective support
Pore system covered, islands of
molten chloride formed
Reaction via new active species
Dense molten chloride layer established
LiKCl/MgDyO catalysts, WHSV = 0.8 h-1, pges = 1 bar , pEthane = pO2 = 70 mbar
Department of Chemistry
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Transients demonstrate reservoir of oxidizing species
O2 C2H6
O2
C2H6
C2H6 O2
O2
C2H6
• O2 uptake, but no C2H6
uptake
• O2 is soluble in polar melt,
however not the non-polar
paraffin.
• Catalyst stores reactive
intermediate species.
CO2
C2H4
CO2
C2H4
• Reactive intermediate
causes no CO2
production
➜ Ideal olefin selectivity
in absence of O2
Notable ethene production over 20 min, no CO2 production
T = 625°C, Flow = 40 ml/min
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Temperature programmed isotopic exchange
Formation of mixed isotope
16O18O is a measure for O
2
dissociation.
• Clear dependence of
O2 dissociation on
support
• Correlation between
O2 dissociation and
steady state activity
• MgO is rich in oxygen
defect sites being able
to dissociate O2.
Marginal diffusion limitations for O2 in overlayer.
Oxygen dissociation takes place at the
interface between support and melt.
Flows: 18O2: 0,25 ml/min, 16O2: 0,25 ml/min, He: 9,5 ml/min
Chloride overlayer does
not affect O2 dissociation
markedly.
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Dual interface helps maintaining high selectivity
Overlayer
(LiKCl)
Solid-liquid interface
(oxygen defect sites):
Oxygen dissociation,
oxidation of the
intermediate
O2
Gas-liquid interface:
C-H activation,
reduction of the
intermediate
C2H6
[O2]
[O*]
C2H4
H2O
Solid support
(MgO, MgO/Dy2O3, ZnO, ZrO2)
Mars-van Krevelen
mechanism: Both
interfaces mediate
different reaction steps
Rate determining step is
related to O2 activation
No diffusion limitation
for O2
Department of Chemistry
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Activation of alkanes on MoVTeNbO
Crystalline phases - M1 and M2
{001} plane of M1 phase
Only M1 is able to activate alkanes
• MO6 octahedra with Mo and V cations
• Pentagonal channels occupied by Nb5+
• Hexagonal and heptagonal channels partially
occupied by Te4+
• Vanadium preferentially in linking positions
between tetrahedra
b
Active site in M1 phase
• Activity is attributed to
V5+=O ⇌
V5+
4+V•-O•
a
species
Able to abstract first H of alkane
• Combination of V5+/V4+ and Mo5+/Mo6+ sites provide redox functionality and
selectivity (isolation of sites)
• Other hypothesis discussed involve heptagonal channels, amorphous overlayers
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Activity in ethane ODH
Selectivity > 95 % at conv. > 50%
k1 / mmol*s-1
C2H4
k3
C2H6
COx
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
10 20 30 40 50 60 70 80 90
M1 phase / wt. %
-6.00
370 ºC, X ≈ 1 %
-7.00
ODH reaction rate is directly related to
content of M1 phase
• Energy of activation is similar for all
catalysts (85-90 kJ/mol)
• Preexponential factors differ
➜ Varying site concentrations?
ln k
-8.00
-9.00
-10.00
-11.00
-12.00
1.35E-03
1.45E-03
1.55E-03
T-1 /K-1
1.65E-03
1.75E-03
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Intrinsic activity of M1 phase
If k1 is normalized to M1content, still
large dispersion of k1 values
300
k1 / µmol*gM1-1*s-1
250
200
150
100
50
0
0
20
40
60
M1 phase / %
80
100
200 nm
Factors affecting the surface concentration of active sites
•
•
•
Vanadium concentration and location in the lattice
Morphology of the crystals
All of them related to the concentration
Crystallinity of external layer
of proposed active site ensemble
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{010}
A
D
B
C
{010} does not expose any active site
B
C
D
It is the most frequent termination for
large flattened M1 particles
The termination of {010} planes is concluded to be inactive
➜ It supports the idea of {001} plane being the most active
200 nm
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A
D
B
However, other frequent lateral facets like
{120} and {210} expose fractions of the 5
octahedra group identified as active site.
{120}
C
B
C
D
{210}
Morphologies that expose the facets {120}
and {210} will potentially have a significant
density of active sites
Density {120} = 0,498 nm-2
Density {210} = 0,434 nm-2
Density {001} = 0,355 nm-2 (ab plane)
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Surface density of active sites for M1 particles
250
A
200
150
100
{001}
{010}
50
{120}
{210}
0
0
5E+18
1E+19
1.5E+19
2E+19
Exposed surface / nm² gM1-1
M1-normalized ODH activity k1 / µmol gM1-1 s-1
M1-normalized ODH activity k1 / µmol gM1-1 s-1
250
B
200
150
100
{001}
50
{001}+{120}+{210
}
0
0
1E+19
2E+19
3E+19
Exposed surface / nm² gM1-1
Rod
morphology
high lateral
activity
Flattened
morphology low
lateral activity
200 nm
4E+19
5E+19
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Ethane can be directly oxidized with H2O2 on Fe/Cu ZSM5
• Catalysis is initiated by Fe and Cu complexes supported in ZSM-5
• High conversion of ethane (ca. 56%) to acetic acid (ca. 70% selectivity).
• Complex reaction network involves carbon-based radicals leading to a range of C2
oxygenates, with sequential C–C bond cleavage.
• Ethene is formed in a parallel pathway and can be subsequently oxidized.
• Ethanol can be directly produced from ethane, and does not originate from the
decomposition of its corresponding alkylperoxy species, ethyl hydroperoxide.
• The mechanism of ethane oxidation, which lead to the high conversions we observe.
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Propane oxofunctionalization
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Aromatization on Ga-ZSM5
Rate [mmol·g-1·h-1]
15
Overall rate
10
Dehyd. rate
Crack. rate
5
0
0
0.5
1
Ga/Al ratio
1.5
• Aromatization is influenced by synergy between Ga+ and Brønsted acid sites
• Dehydrogenation rate followed same trend as overall rate.
– Optimum concentration of Ga+ for dehydrogenation
• Cracking rate decreased with increasing Ga/Al ratio.
– Lower concentration of BAS available
T=823K; p=1 bar ; 10ml/min propane + 15 ml/min N2; m(cat.) = 180mg; pretreatment: 1h H2 at 600°C
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Brønsted acid sites and Ga+ concentrations change inversely
Ga
c(BAS)/c(BASparent) · 100% [%]
100
•
•
80
60
Ga+
40
20
BAS
0
0
1
c(Ga)added 0.5
Ga/BASparent [mol/mol]
1.5
Cationic, monovalent Ga+ exchanges for BAS.
Isolated Ga species form when Ga+ is added beyond a Ga/Al ratio of 1.
Reduction: 873 K, 20 mL/min H2, 1h, Outgassing: 823 K, 1h, vacuum
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Activity is dominated by synergistic interactions
rdehyd = cBAS·TOFBAS + cGa+·TOFGa+ + cGa+- BAS·TOFGa+- BAS+ cGa ·TOFGa
Ga+ - BAS
Ga+
Ga
BAS
TOF550°C [h-1]
68.4
4.4
2.7
2.4
ΔH‡app. [kJ·mol-1]
109
134
76
153
ΔS‡app. [J·mol-1·K-1]
-66
-95
-133
-76
• Transition state is enthalpically favored and has a high transition entropy.
T=823 - 873 K; p=1 bar, Feed: 10ml/min propane + 15 ml/min N2; m(cat.) = 180mg; Pretreatment: 873 K, 1h, H2
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Process with potential
Propane to acrylic acid – selectivity and catalyst stability
Propane dehydrogenation – UOP and Star process
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Availability of methane and light alkanes offers a transition
to lower the carbon footprint
Disproportionation
Synthesis gas generation
Metathesis
Aromatization
Direct selective oxidation
Oxidative coupling
CH4
Ethane steam cracking
Dehydrogenation
C 3H 8
Oxidative dehydrogenation
Selective oxidation to
acrolein/acrylic acid/acrylonitrile
C 2H 6
Oxidative dehydrogenation
Selective oxidation
ethanol/acetaldehyde/acetic acid
Selective oxidation
ethylene oxide
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Conclusions and outlook
• Conversion of light alkanes may be challenging but also holds significant
untapped potential.
• A joint approach combining kinetics, spectroscopy and theory should be
used to
– Understand the catalytic chemistry on an atomistic/molecular level
– Translate this into a catalytic material with precisely tailored properties
– Maintain the nature and integrity of these sites under operating conditions
– Design the optimum reactor together with the catalyst
• To achieve this we require
– transformative developments of analytical capabilities; characterizing the
catalyst structurally, chemically, and in a time and spatially resolved way
– links between material science and catalysis to synthesize robust
single site catalysts
– Links between reactor engineering and catalysis to realize processes,
which allow variable scale development.
Integrated Catalysis
Department of Chemistry
Catalysis Research Center
Lighter FeedstocksImplications and chances for catalysis
Johannes A. Lercher
Institute for Integrated Catalysis, Pacific Northwest National Laboratory
Catalysis Research Center, Technische Universität München
Washington D.C., March 2016
Department of Chemistry
Catalysis Research Center
Institute for
Integrated Catalysis
Availability of methane and light alkanes offers a transition
to lower the carbon footprint
Disproportionation
Synthesis gas generation
Metathesis
Aromatization
Direct selective oxidation
Oxidative coupling
CH4
Ethane steam cracking
Dehydrogenation
C 3H 8
Oxidative dehydrogenation
Selective oxidation to
acrolein/acrylic acid/acrylonitrile
C 2H 6
Oxidative dehydrogenation
Selective oxidation
ethanol/acetaldehyde/acetic acid
Selective oxidation
ethylene oxide
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Methane conversion
from high-temperature catalysis to
bio-inspired selective oxidation
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All reforming reactions share similar catalytic chemistry
H2O + CH4
CO + 3H2
+ 226 kJ/mol
½ O 2 + CH4
CO + 2H2
- 44 kJ/mol
CO2 + CH4
2 CO + 2H2
+ 261 kJ/mol
CH4 ⇌ CH3 + H ΔH = 421 kJ/mol
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The key problem is to understand carbon formation
T = 900°C, 5 bar
H/C ratio
• Carbon deposition may occur
under many operating
conditions
• Challenge for local mass
transport limitations
• More severe at higher
pressures
O/C ratio
Department of Chemistry
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Kinetics point to a common set of reaction pathways
H
CH3
Kinetically relevant
C-H bond breaking
H*
H*
H*
C*
k1
O*
+ CH4
CO*
- H2
H*
H*
H*
K5
- H2O
OH*
K4
Quasi-equilibrated
adsorption-desorption steps
J. Wei, E. Iglesia, J. Catal. 224 (2004) 370.
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r=
kO*Co -*Ni
[CO 2 ]
[CH 4 ]
KCO
[CO]
2
[CO 2 ]
1 + [CO]
KCO2
k1st
[mol (g-atom Msurface-s-kPa)-1]
Reforming rate depends on the oxygen coverage
1
Methane reforming on Ni-Co
CO2 provides O* at
the catalyst surfaces
0.8
O* contents are
equilibrated with gas
phase oxidant
0.6
0.4
0.2
Ni-Co clusters
873 K
0
0
5
10
15
20
25
CO2-to-CO ratio
30
[O*] KCO 2 [CO 2 ]
=
[*]
KCO [CO]
Coverages of CH4* and O* on catalysts during reforming at 15 kPa CH4 and 30 kPa CO2, 873 K
Catalyst
12Co
3Fe-9Ni
9Co-3Ni
6Co-6Ni
3Co-9Ni
12 Ni
3Cu-9Ni
O*
0.86
0.69
0.39
0.24
0.07
0.05
0
Chin et al. J. Am. Chem. Soc, 135 (2013) 15425
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Carbon accumulation depends on the particle size
• High concentration of active
oxygen maintains low
carbon concentration.
• Surface remains clean even
under condition of severe
conversions.
• High rate of carbon
formation leads to
recrystallization.
• Particle size below 2 nm
begins to destabilize the
carbon fibers.
• Coke accumulation leads
then to carbon deposits,
which are easier to oxidize.
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Alternative pathways may help stabilizing catalysts
50
1.0
Ea = 85 ± 4 kJ/mol
0.5
1% Ni/ZrO2
20
10
1% CO2/Ni/ZrO2
0.0
-0.5
lnk / -
CO2 regeneration
TOF / s
-1
30
CO2 regeneration
CO2 regeneration
40
-1.0
-1.5
1% CO2/Ni/ZrO2
1% Ni/ZrO2
-2.0
-2.5
-3.0
0.90
0.95
1.00
1.05
1000 / T /-1K
1.10
1% Ni/Al2O3
1% Ni/Al2O3
0
0
200
400
600
800
1000
Time / min
•
•
If activity only depends on surface oxygen level and carbon present, the support
should not influence the rates.
Identical activation energies point to identical mechanism, independent of the
treatment.
Reaction conditions: 800°C , 1 bar, CH4: 25 ml/min, CO2: 25 ml/min, N2: 50 ml/min, 20 mg cats, 480 mg SiC
1.15
Department of Chemistry
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CH4 oxidative coupling links surface and gas phase reactions
•
•
•
•
Generation of CH3·and HO2· involves steps
in the MgO surface.
Stabilization appears to be very
challenging.
Formation of O2- cations in other oxide with
easier stabilization and the ability to
polarize C-H bonds appears to be crucial.
– CaO with Mn2+ dopant cations
– La2O3, etc.
Why are high temperatures mandatory?
P. Schwach et al., J. Catal. 329 (2015) 560.
Mohammad-Sadegh Salehiet al., Ind. Eng. Chem. Res. 2016, 55, 1149.
Department of Chemistry
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Aromatization combines dehydrogenation, C-C bond
formation and acid-base catalyzed ring closure
3 wt% Mo/HZSM-5 at 973 K
CH4 conversion
Selectivity
Selectivity HC
Rates
• Dehydrogenation on MoC to carbene
like species
• Dimerization of carbenes
• Ring closure and aromatization on the
zeolite
• Carbon management is critical.
• New approach with Fe@SiO2 at 1360K
leads to 50 % stable conversion.
X.Gua, et al., Science 344 (2014) 616.
S.Liu, et al., J. Catal. 181 (1999) 175.
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Incorporation of methane via metathesis
CH4 + C3H8 ⇌ 2 C2H4
• Combination of dehydrogenation of
propane with olefin metathesis leads
to incorporation of CH4 into larger
alkanes.
• Problems lie in
– the tight specifications for
operating the catalyst,
– the low rates achievable,
– the thermodynamic limitations
D. Soulivong et al., Angew. Chem. Int. Ed. 43 (2004) 5366.
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Complex combined cycles require H2SO4/SO3 as oxidant
R.A. Periana et al., Science 280 (1998)
560
R.A. Periana et al., Science 301 (2003) 817
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Nature found a way to oxidize methane in monooxygenases
• Methane monooxygenases (MMOs) convert methane to methanol in the
presence of O2 with high efficiency under ambient conditions.
• Membrane bound MMO (pMMO) converts methane at ambient temperature
via oxygen insertion with a TOF of nearly 1s-1.
• The active site is either a Cu dimer or a Cu trimer.
9-10 Cu+
1 type 2 Cu center
1 binuclear Cu site
A site
B site
C site
D site
Tricopper cluster
Lieberman, R. L.; Rosenzweig, A. C., Nature 2005, 434, 177-182.
Chan, S. I.; Yu, S. S.-F. Acc. Chem. Res. 2008, 41, 969-979.
1 tri-Cu-oxo cluster
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Combining site activation, methane activation, and
methanol removal
(CuO)x
• Direct formation of methanol over CuZSM-5 has been reported by
Groothaert et al. in 2005, but it had to
be extracted ex situ from the zeolite.
2
1
Groothaert, JACS, 2005, 127, 1395
OH
OCH3
OH
OH
(CuO)x
(CuO)x
3
Cu/zeolite
SiO2/Al2O3
Cu/Al
CH3OH
(µmol/g)
MOR
11
0.48
32.6
ZSM-5
25
0.47
26.7
SSZ-13
19.2
0.3
12.6
• Three-step procedure at TUM allows
quantitative desorption of products.
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Three Cu cations are needed to convert CH4
2:3
1:3
1 MeOH per 3 Cu
Accumulation of Al as paired sites in the side pockets allows formation
of multinuclear active site even at low loadings and high Si/Al ratios.
S. Grunder, Nature Communications, 6 (2015) 7546.
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Trimeric clusters form only in the side pockets of MOR
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The process is repeatable but requires interim activation
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Cycle for reoxidation of the sites involves deeper reduction
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Methane can be oxidized with H2O2 on Cu/FeZSM5
CH3OH
CH3OOH
• Clusters similar to those active for
CH4 oxidation with O2
H2O2
H2O2
CH4
• Free radical processes initiated
• Fe seems to be indispensable
• H2O2 more costly than reactants
C. Hammond et al., Angew. Chem. Int. Ed. 51 (2012) 5129
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Ethane from steam cracking to oxidative dehydrogenation
Department of Chemistry
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Oxidative dehydrogenation of ethane is challenging
• Lewis acid sites bind olefins and together with anionic nucleophilic oxygen
and catalyze oxygen insertion as well as subsequent decarbonylation or
decarboxylation leading eventually to total oxidation.
Catalytic sites should favor C-H cleavage and minimize readsorption of
olefins.
Oxide surface minimizing the
concentration of accessible
Lewis acid sites
Surfaces that dynamically
rearrange prevent exposure of
cations
Mechanistic understanding on a molecular scale is mandatory to tailor
catalysts with a high olefin selectivity.
Requirement for economic competitivity: X(C2H6) > 50%, S(C2H4) ≥ 95%
V/Mo oxides
• Highly active
• Varying olefin yield
Supported alkali chlorides
• Formation of molten overlayer
• High stability
Selectivities of
95% achievable
C.A. Gärtner, A.C. van Veen, J.A. Lercher, ChemCatChem, 2013, 5, 2-24
Department of Chemistry
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60
40
20
1
MgO
MgO/Dy2O3
80
BET area / m²
LiKCl
LiKCl
LiKCl
BET spec. surf. area/ m2·g-
Supported alkali chloride catalysts
0
0
Solid oxide core
Overlayer (mp: 353°C)
10
20
30
40
mol% overlayer
Overlayer
/ mol%
Dense
chloride
overlayer
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Influence of overlayer on selectivity
1,4
90
1,2
80
1,0
S(C2H4) / %
ethene production / µmol g-1 s-1
100
0,8
70
60
0,6
50
0,4
40
0
10
20
30
40
50
Overlayer
mol%/ mol%
overlayer
0
10
20
30
40
50
mol% overlayer
Overlayer
/ mol%
Support pore system accessible
Reaction on less selective support
Pore system covered, islands of
molten chloride formed
Reaction via new active species
Dense molten chloride layer established
LiKCl/MgDyO catalysts, WHSV = 0.8 h-1, pges = 1 bar , pEthane = pO2 = 70 mbar
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Transients demonstrate reservoir of oxidizing species
O2 C2H6
O2
C2H6
C2H6 O2
O2
C2H6
• O2 uptake, but no C2H6
uptake
• O2 is soluble in polar melt,
however not the non-polar
paraffin.
• Catalyst stores reactive
intermediate species.
CO2
C2H4
CO2
C2H4
• Reactive intermediate
causes no CO2
production
➜ Ideal olefin selectivity
in absence of O2
Notable ethene production over 20 min, no CO2 production
T = 625°C, Flow = 40 ml/min
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Temperature programmed isotopic exchange
Formation of mixed isotope
16O18O is a measure for O
2
dissociation.
• Clear dependence of
O2 dissociation on
support
• Correlation between
O2 dissociation and
steady state activity
• MgO is rich in oxygen
defect sites being able
to dissociate O2.
Marginal diffusion limitations for O2 in overlayer.
Oxygen dissociation takes place at the
interface between support and melt.
Flows: 18O2: 0,25 ml/min, 16O2: 0,25 ml/min, He: 9,5 ml/min
Chloride overlayer does
not affect O2 dissociation
markedly.
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Dual interface helps maintaining high selectivity
Overlayer
(LiKCl)
Solid-liquid interface
(oxygen defect sites):
Oxygen dissociation,
oxidation of the
intermediate
O2
Gas-liquid interface:
C-H activation,
reduction of the
intermediate
C2H6
[O2]
[O*]
C2H4
H2O
Solid support
(MgO, MgO/Dy2O3, ZnO, ZrO2)
Mars-van Krevelen
mechanism: Both
interfaces mediate
different reaction steps
Rate determining step is
related to O2 activation
No diffusion limitation
for O2
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Activation of alkanes on MoVTeNbO
Crystalline phases - M1 and M2
{001} plane of M1 phase
Only M1 is able to activate alkanes
• MO6 octahedra with Mo and V cations
• Pentagonal channels occupied by Nb5+
• Hexagonal and heptagonal channels partially
occupied by Te4+
• Vanadium preferentially in linking positions
between tetrahedra
b
Active site in M1 phase
• Activity is attributed to
V5+=O ⇌
V5+
4+V•-O•
a
species
Able to abstract first H of alkane
• Combination of V5+/V4+ and Mo5+/Mo6+ sites provide redox functionality and
selectivity (isolation of sites)
• Other hypothesis discussed involve heptagonal channels, amorphous overlayers
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Activity in ethane ODH
Selectivity > 95 % at conv. > 50%
k1 / mmol*s-1
C2H4
k3
C2H6
COx
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
10 20 30 40 50 60 70 80 90
M1 phase / wt. %
-6.00
370 ºC, X ≈ 1 %
-7.00
ODH reaction rate is directly related to
content of M1 phase
• Energy of activation is similar for all
catalysts (85-90 kJ/mol)
• Preexponential factors differ
➜ Varying site concentrations?
ln k
-8.00
-9.00
-10.00
-11.00
-12.00
1.35E-03
1.45E-03
1.55E-03
T-1 /K-1
1.65E-03
1.75E-03
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Intrinsic activity of M1 phase
If k1 is normalized to M1content, still
large dispersion of k1 values
300
k1 / µmol*gM1-1*s-1
250
200
150
100
50
0
0
20
40
60
M1 phase / %
80
100
200 nm
Factors affecting the surface concentration of active sites
•
•
•
Vanadium concentration and location in the lattice
Morphology of the crystals
All of them related to the concentration
Crystallinity of external layer
of proposed active site ensemble
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{010}
A
D
B
C
{010} does not expose any active site
B
C
D
It is the most frequent termination for
large flattened M1 particles
The termination of {010} planes is concluded to be inactive
➜ It supports the idea of {001} plane being the most active
200 nm
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A
D
B
However, other frequent lateral facets like
{120} and {210} expose fractions of the 5
octahedra group identified as active site.
{120}
C
B
C
D
{210}
Morphologies that expose the facets {120}
and {210} will potentially have a significant
density of active sites
Density {120} = 0,498 nm-2
Density {210} = 0,434 nm-2
Density {001} = 0,355 nm-2 (ab plane)
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Surface density of active sites for M1 particles
250
A
200
150
100
{001}
{010}
50
{120}
{210}
0
0
5E+18
1E+19
1.5E+19
2E+19
Exposed surface / nm² gM1-1
M1-normalized ODH activity k1 / µmol gM1-1 s-1
M1-normalized ODH activity k1 / µmol gM1-1 s-1
250
B
200
150
100
{001}
50
{001}+{120}+{210
}
0
0
1E+19
2E+19
3E+19
Exposed surface / nm² gM1-1
Rod
morphology
high lateral
activity
Flattened
morphology low
lateral activity
200 nm
4E+19
5E+19
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Ethane can be directly oxidized with H2O2 on Fe/Cu ZSM5
• Catalysis is initiated by Fe and Cu complexes supported in ZSM-5
• High conversion of ethane (ca. 56%) to acetic acid (ca. 70% selectivity).
• Complex reaction network involves carbon-based radicals leading to a range of C2
oxygenates, with sequential C–C bond cleavage.
• Ethene is formed in a parallel pathway and can be subsequently oxidized.
• Ethanol can be directly produced from ethane, and does not originate from the
decomposition of its corresponding alkylperoxy species, ethyl hydroperoxide.
• The mechanism of ethane oxidation, which lead to the high conversions we observe.
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Propane oxofunctionalization
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Aromatization on Ga-ZSM5
Rate [mmol·g-1·h-1]
15
Overall rate
10
Dehyd. rate
Crack. rate
5
0
0
0.5
1
Ga/Al ratio
1.5
• Aromatization is influenced by synergy between Ga+ and Brønsted acid sites
• Dehydrogenation rate followed same trend as overall rate.
– Optimum concentration of Ga+ for dehydrogenation
• Cracking rate decreased with increasing Ga/Al ratio.
– Lower concentration of BAS available
T=823K; p=1 bar ; 10ml/min propane + 15 ml/min N2; m(cat.) = 180mg; pretreatment: 1h H2 at 600°C
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Brønsted acid sites and Ga+ concentrations change inversely
Ga
c(BAS)/c(BASparent) · 100% [%]
100
•
•
80
60
Ga+
40
20
BAS
0
0
1
c(Ga)added 0.5
Ga/BASparent [mol/mol]
1.5
Cationic, monovalent Ga+ exchanges for BAS.
Isolated Ga species form when Ga+ is added beyond a Ga/Al ratio of 1.
Reduction: 873 K, 20 mL/min H2, 1h, Outgassing: 823 K, 1h, vacuum
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Activity is dominated by synergistic interactions
rdehyd = cBAS·TOFBAS + cGa+·TOFGa+ + cGa+- BAS·TOFGa+- BAS+ cGa ·TOFGa
Ga+ - BAS
Ga+
Ga
BAS
TOF550°C [h-1]
68.4
4.4
2.7
2.4
ΔH‡app. [kJ·mol-1]
109
134
76
153
ΔS‡app. [J·mol-1·K-1]
-66
-95
-133
-76
• Transition state is enthalpically favored and has a high transition entropy.
T=823 - 873 K; p=1 bar, Feed: 10ml/min propane + 15 ml/min N2; m(cat.) = 180mg; Pretreatment: 873 K, 1h, H2
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Process with potential
Propane to acrylic acid – selectivity and catalyst stability
Propane dehydrogenation – UOP and Star process
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Availability of methane and light alkanes offers a transition
to lower the carbon footprint
Disproportionation
Synthesis gas generation
Metathesis
Aromatization
Direct selective oxidation
Oxidative coupling
CH4
Ethane steam cracking
Dehydrogenation
C 3H 8
Oxidative dehydrogenation
Selective oxidation to
acrolein/acrylic acid/acrylonitrile
C 2H 6
Oxidative dehydrogenation
Selective oxidation
ethanol/acetaldehyde/acetic acid
Selective oxidation
ethylene oxide
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Conclusions and outlook
• Conversion of light alkanes may be challenging but also holds significant
untapped potential.
• A joint approach combining kinetics, spectroscopy and theory should be
used to
– Understand the catalytic chemistry on an atomistic/molecular level
– Translate this into a catalytic material with precisely tailored properties
– Maintain the nature and integrity of these sites under operating conditions
– Design the optimum reactor together with the catalyst
• To achieve this we require
– transformative developments of analytical capabilities; characterizing the
catalyst structurally, chemically, and in a time and spatially resolved way
– links between material science and catalysis to synthesize robust
single site catalysts
– Links between reactor engineering and catalysis to realize processes,
which allow variable scale development.