Implications for Catalysis

Lighter Feedstocks-

Implications and chances for catalysis

Johannes A. Lercher

Institute for Integrated Catalysis, Pacific Northwest National Laboratory

Catalysis Research Center, Technische Universität München

Availability of methane and light alkanes offers a transition

to lower the carbon footprint

CH

₄

C

₂

H

₆

C

₃

H

₈

Ethane steam cracking

Oxidative dehydrogenation

Selective oxidation

ethanol/acetaldehyde/acetic acid

Selective oxidation ethylene oxide Dehydrogenation

Oxidative dehydrogenation

Selective oxidation to

acrolein/acrylic acid/acrylonitrile

Synthesis gas generation Direct selective oxidation

Oxidative coupling Aromatization Disproportionation

Methane conversion

All reforming reactions share similar catalytic chemistry

CO

₂

+ CH

₄

2 CO + 2H

₂

+ 261 kJ/mol

H

₂

O + CH

₄

CO + 3H

₂

½ O

₂

+ CH

₄

CO + 2H

₂

+ 226 kJ/mol

- 44 kJ/mol

CH₄ ⇌ CH

The key problem is to understand carbon formation

• Carbon deposition may occur under many operating

conditions

• Challenge for local mass transport limitations

• More severe at higher pressures

H/

C r

a

tio

T = 900°C, 5 bar

Kinetics point to a common set of reaction pathways

Kinetically relevant C-H bond breaking

k₁ adsorption-desorption steps

Reforming rate depends on the oxygen coverage

the catalyst surfaces

O* contents are

equilibrated with gas phase oxidant

2

Ni-Co clusters

Methane reforming on Ni-Co

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,

Alternative pathways may help stabilizing catalysts

• 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

CH

₄

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.

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.

Incorporation of methane via metathesis

• 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 CH₄ + C₃H₈ ⇌ 2 C₂H₄

Complex combined cycles require H

₂

SO

₄

/SO

₃

as oxidant

R.A. Periana et al., Science 280 (1998) 560

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.

1 type 2 Cu center 1 binuclear Cu site

9-10 Cu+

1 tri-Cu-oxo cluster

A site

B site

C site

D site

Tricopper cluster

• Direct formation of methanol over Cu-ZSM-5 has been reported by

Groothaert et al. in 2005, but it had to be extracted ex situ from the zeolite.

• Three-step procedure at TUM allows quantitative desorption of products.

Groothaert, JACS, 2005, 127, 1395

1

Combining site activation, methane activation, and

methanol removal

Cu/zeolite SiO₂/Al₂O₃ Cu/Al CH3OH (µmol/g)

MOR 11 0.48 32.6

ZSM-5 25 0.47 26.7

Three Cu cations are needed to convert CH

₄

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.

1 MeOH per 3 Cu

1:3 2:3

Methane can be oxidized with H

₂

O

₂

on Cu/FeZSM5

• Clusters similar to those active for CH₄ oxidation with O₂

H₂O₂

CH₄ H₂O₂

CH₃OOH CH₃OH

• Free radical processes initiated • Fe seems to be indispensable • H₂O₂ more costly than reactants

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

C.A. Gärtner, A.C. van Veen, J.A. Lercher, ChemCatChem, 2013, 5, 2-24

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%

Selectivities of 95% achievable

V/Mo oxides

• Highly active

• Varying olefin yield

Supported alkali chlorides

Supported alkali chloride catalysts

MgO MgO/Dy₂O₃

Solid oxide core _{Overlayer (mp: 353°C)} LiKCl

mol% overlayer

B

Influence of overlayer on selectivity

LiKCl/MgDyO catalysts, WHSV = 0.8 h-1_{, p}

ges = 1 bar , pEthane = p_O2 = 70 mbar

Support pore system accessible

Pore system covered, islands of molten chloride formed

Dense molten chloride layer established

Reaction via new active species Reaction on less selective support

Overlayer / mol%

0 10 20 30 40 50

Transients demonstrate reservoir of oxidizing species

O₂  C₂H₆

Notable ethene production over 20 min, no CO₂ production C₂H₆  O₂

• O₂ uptake, but no C₂H₆

uptake

• O₂ is soluble in polar melt,

however not the non-polar paraffin.

• Catalyst stores reactive intermediate species.

• Reactive intermediate

causes no CO₂

production

Temperature programmed isotopic exchange

Formation of mixed isotope

16_O18_{O is a measure for O} 2

dissociation.

Marginal diffusion limitations for O₂ in overlayer.

Oxygen dissociation takes place at the interface between support and melt.

• 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.

Flows: 18_O

2: 0,25 ml/min, 16O2: 0,25 ml/min, He: 9,5 ml/min

Dual interface helps maintaining high selectivity

Solid-liquid interface (oxygen defect sites):

Oxygen dissociation, oxidation of the

intermediate

Solid support

(MgO, MgO/Dy₂O_3, ZnO, ZrO₂) Overlayer

(LiKCl)

Gas-liquid interface: C-H activation, reduction of the

intermediate Mars-van Krevelen

mechanism: Both interfaces mediate

different reaction steps

Rate determining step is related to O₂ activation

Activation of alkanes on MoVTeNbO

Active site in M1 phase

V5+_=O

⇌ 4+V•-O•

Able to abstract first H of alkane • Activity is attributed to V5+ _species

• Combination of V5+_/V4+ _{and Mo}5+_/Mo6+ _{sites provide redox functionality and}

selectivity (isolation of sites)

• Other hypothesis discussed involve heptagonal channels, amorphous overlayers

Crystalline phases - M1 and M2

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

{001} plane of M1 phase

a

Activity in ethane ODH

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?

370 ºC, X ≈ 1 %

1.35E-03 1.45E-03 1.55E-03 1.65E-03 1.75E-03

ln

k

T-1 _/K-1

0

Intrinsic activity of M1 phase

If k₁ is normalized to M1content, still large dispersion of k1 values

Factors affecting the surface concentration of active sites

• Vanadium concentration and location in the lattice • Morphology of the crystals

• Crystallinity of external layer All of them related to the concentration of proposed active site ensemble

B D

B

C

C

D

A

{010} does not expose any active site

It is the most frequent termination for large flattened M1 particles

{010}

The termination of {010} planes is concluded to be inactive

B D

A However, other frequent lateral facets like

{120} and {210} expose fractions of the 5 octahedra group identified as active site.

{120}

{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

Surface density of active sites for M1 particles

0 1E+19 2E+19 3E+19 4E+19 5E+19

M1

0 5E+18 1E+19 1.5E+19 2E+19

M1

morphology low

lateral activity

Rod

morphology

Ethane can be directly oxidized with H

₂

O

₂

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.

0 0.5 1 1.5

Aromatization on Ga-ZSM5

• 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

0

Ga/BAS_parent[mol/mol] c(Ga)_added

Ga

BAS

Ga+

Brønsted acid sites and Ga

+

_{concentrations change inversely}

• Cationic, monovalent Ga+ _{exchanges for BAS.}

• Isolated Ga species form when Ga+ _{is added beyond a Ga/Al ratio of 1.}

Activity is dominated by synergistic interactions

T=823 - 873 K; p=1 bar, Feed: 10ml/min propane + 15 ml/min N2; m(cat.) = 180mg; Pretreatment: 873 K, 1h, H2

r_dehyd = c_BAS·TOF_BAS + c_Ga+·TOF_Ga++ c_Ga+_{- BAS}·TOF_Ga+_{- BAS}+ c_Ga ·TOF_Ga

Ga+ _{- BAS Ga}+ _{Ga BAS}

TOF_550°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

Process with potential

Propane to acrylic acid – selectivity and catalyst stability

Availability of methane and light alkanes offers a transition

to lower the carbon footprint

CH

₄

C

₂

H

₆

C

₃

H

₈

Ethane steam cracking

Oxidative dehydrogenation

Selective oxidation

ethanol/acetaldehyde/acetic acid

Selective oxidation ethylene oxide Dehydrogenation

Oxidative dehydrogenation

Selective oxidation to

acrolein/acrylic acid/acrylonitrile

Synthesis gas generation Direct selective oxidation

Oxidative coupling Aromatization Disproportionation

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