WYJAZDY I WYSTĄPIENIAWAKACJE 2008
Kanada, Vancuver, lipiec 2008Australia, Sydney, wrzesień 2008Słowacja, Strbske Pleso, wrzesień 2008
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WATOC 2008Sydney, September 14-19 2008
The World Association of Theoretical and Computational Chemists (WATOC) former: The World Association of
Theoretically Oriented Chemists.
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IX. PANNONIAN INTERNATIONAL SYMPOSIUM ON CATALYSIS
Strbske Pleso, Slovakia
8 - 12 Sept. 2008
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THEORETICAL MODELING OF SMART CATALYTIC ACTIVITY OF COPPER
CENTRES IN ZEOLITES: FROM ELECTRON DENSITY FLOW
TOWARDS ACTIVATION
Ewa BroclawikPawel Rejmak
Institute of Catalysis Polish Academy of Sciences
Pawel KozyraJoanna ZaluckaMariusz Mitoraj
Faculty of ChemistryJagiellonian University
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„Smart catalysis” – very selective, fine chemistry supported by transition metal
active centres in homogeneous, heterogeneous and enzymatic catalysis
Enzymatic activity: heme and nonheme iron centres (cytochromes, oxygenases etc.)
Zeolitic activity: exchanged transtion metal (Ag, Cu, Co) sites in zeolites (MFI, FAU, FER etc)
+
Al
H
Si
O
O
O
O
O
O O
–
Cu+, Ag+
FeIVO
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Methodologies for theoretical (QM) studies:
DFT modeling of stable structures, reaction intermediates and their physical properties for cluster models of zeolitic sites:
relatively cheap method, reliable results but neglect of long-range interactions, discrimination between different types of lattices, Si/Al ratio or specific metal siting hardly possible
QMPOT (Combined QM – Interatomic Potential Functions Method) –
entire framowk considered, main drawabcks of cluster modeling
eliminated but framework treated approximately (classical Force
Fields), extra parametrization required
Periodic models – in principle accurate for entire framowk but computationally demandingvarying Si/Al ratio or metal loading breaks strict periodicity
New theoretical concepts for interpretation – NOCV
(Natural Orbitals for Chemical Valence): analyzis of electron density flow accompanying catalytic event
SPECIATION OF COPPER SITES IN FAUJASITE (by interaction with CO probe)
Small cluster models are not useful to mimic various envirinments in different zeolites (e.g. FAU, mordenite, ferrierite, ZSM-5)
Local environments may be very similar but framework and site properties will differ QMPOT
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1) Periodic framework (O) - MM
2) Cluster (C) - QM
3) Link atoms (L)
Etot = EMM(O) + EQM(C+L) + E(O,C,L) = EMM(O) + EQM(C+L) – EMM(C+L) + ΔQM/MM(O,C,L) ≈ ≈ EMM(O) + EQM(C+L) – EMM(C+L)
(Eichler, U., Koelmel, C. M., Sauer, J. J. Comp. Chem. 1997, 18(4), 463)Sierka, M., Sauer, J. In: Yip., S. (Ed.), The Handbook of Materials Modelling, Part A Springer, Dordrecht, 2005, 241-258)
COMBINED QUANTUM MECHANIC – INTERATOMIC POTENTIAL FUNCTIONS METHOD (QMPot)
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Possible cationic (black) and crystallographic O positions (gray)(Smith., J. V. Adv. Chem. Ser. 1971, 101, 171)
Cu(I)-Y XRD and ND experiments indicate Cu(I) only in I’, II or II’ positions(Palomino, G. T., Bordiga, S., Zecchina, A., Marra, G. L., Lamberti, C. J. Phys. Chem. B 2000, 104, 8641)
theoretical positions (red) – stability: II≈I’ >> III(Rejmak, P., Sierka, M., Sauer, J. – w przygotowaniu)
Site I is not accessible for adsorbed molecules
FAUJAZITE (FAU)
Regular cell F-d3m, 192 atoms T and 384 O
FAU – high alumina contentX zeolite – Si/Al close to 1Y zeolite – Si/Al about 2-3
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The nearest environment of Cu+ ions in site II (2 Al atoms in para positions)
(A) free cluster(B) Force Field only(C) QMPot modelenvironment importance!!
2) The impact of embedding
4Al
Interaction of benzene with
Cu+, Ag+ i Na+ in ZSM-5 zeolite
Experiment: J. Załucka, P. Kozyra, J. Datka
Theory: M. Mitoraj, A. Michalak
environment importance accounted for
no microscopic insight into electronic processes/ charge flow
(need for new theoretical development)
Summary for QMPOT modeling:
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M7-Me-benzene (Me=Cu(I), Ag(I), Na(I))
Adsorption by 2 carbon atomsC=C variation, aromaticity diminished for Cu(I) and Ag(I)
6,5 kcal/mol 11,9 kcal/molEint = 19,2 kcal/mol
activation
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ΔQben Δcalc /cm-1 Δexp /cm-1
Cu+-M7-C6H6 0.012 -22 -13
Ag+-M7-C6H6 0.041 -11 -8
Na+-M7-C6H6 0.056 -2 -2
Mechanism of C=C activation??? Donation or backdonation???
Cu+-M7 Ag+-M7 Na+-M7before/after ΔQ
before/after ΔQ before
/after ΔQ
Benzene
0
+0.0120
+0.0410
+0.0560.012 0.041 0.0565
M+0.364
+0.0490.480
-0.0430.453
-0.1010.413 0.437 0.352
M7-0.364
-0.059
-0.48
+0.003
-0.453
+0.044-0.423-0.477 -0.409
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NOCV – Natural Orbitals for Chemical Valence(Mitoraj M., Michalak A., Organometallics, 26 (2007) 6576-6580)
NOCV are the eigenfunctions of a valence operator:
V i(ri) = i i(ri)
where:
V = 1/2(P – P0)
P – charge and bond order matrix for the molecule
P0 – charge and bond order matrix for promolecule– fragments
in the geometry of common system but noninteracting
i(ri) describes spatial distribution and character of elementary electron
density flows („channels”) i the share of a given “channel” in total density flow
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occupied „promolecular” orbitals
occupied molecular orbitals in composed system
“pairing property” of NOCV (rigorous proof by M. Radon):
relation between eigenvalues of V and expectation values of the molecular density
operator, P
•total electronic transfer may be decomposed into sum of separated transfers within pairs of coupled NOCV
•for each k, the transfer of νk electrons from ϕ−k to ϕ+k orbital is observed
•during this transfer the total number of electrons in (ϕ−k, ϕ+k ) pair remains equal exactly to two electrons
Now:
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NOCV – Natural Orbitals for Chemical Valence(Mitoraj M., Michalak A., Organometallics, 26 (2007) 6576-6580)
-i-i(ri)2+ii(ri)2 describes spatial distribution and character of elementary electron density flows
(„channels”) i the share of a given “channel” in total density flow
Decomposition of differential density flow:
ρ(r)molekule/cluster - ρ (r)fragment1 – ρ(r)fragment2
into elementary flow channels i(ri)
The method to discriminate between donation and backdonation
(even σ or π)!!
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Example: heme + CO
CO
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NOCV [M7-Cu+]-C6H6
-0.484*
-0.356*
-0.218*
-0.105*
yellow – electron outflow
green– electron inflow
donation
backdonation
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[Cu+-zeo]-C6H6
[Cu+-C6H6]-zeobond Cu-zeo
[Cu+]-[C6H6]
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NOCV [M7-Ag+]-C6H6
-0.160*
-0.266*
-0.217*
donation
backdonation
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[Ag+-zeo]-C6H6
[Ag+-C6H6]-zeo
[Ag+]-[C6H6]
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Vd Vbd Vd + Vbd Δcalc /cm-1 Δexp /cm-1
Cu+-M7-C6H6 0.46 0.70 1.16 -22 -13
Ag+-M7-C6H6 0.27 0.38 0.65 -11 -8
Na+-M7-C6H6 < 0.1 < 0.1 < 0.2 -2 -2
Both donation and backdonation correlate with C=C activation measured by IR frequency shifts
Total charge change does not correlate positively with C=C activation
Na+ - only electrostatic interaction, no activation
ΔQben
0.012
0.041
0.056
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CONCLUSIONS
Cluster DFT modeling:
open path to advanced calculations reasonable reproduction of experiment (for
justified model) rough understanding of physical nature of
the process
Combined QM/MM modeling:
environment importance accounted for detailed speciation/discrimination of sites statistical interpretation of experimental
results
New interpretative theoretical tools
microscopic insight into electronic processes/
charge flow
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Thank you for kind attention!
Acknowledgments
Polish State Committee for Scientific ResearchIR studies supported by grant NN204198733. Computational studies supported by: EU project Transfer of Knowledge TOK-CATA, contract MTKD-CT-2004-509832
grant N20418031/3999. grant PZB-KBN-116/T09/2004
Thanks to prof. J. Sauer and dr. M. Sierka (Humboldt University) for QMPOT code.