Rate-enhancing roles of water molecules in methyltrioxorhenium-catalyzed olefin epoxidation by H2O2

Bryan R. Goldsmith, Taeho Hwang, Stefan Seritan, Baron Peters and Susannah L. Scott

“Greening” the Ethylene Oxide Process

M. Ghanta et al. Ind. Eng. Chem. Res., 52 (2013)

C2H4 + 12 O2 → C2H4O (+ 2 CO2, 15 %)

C2H4 + H2O2 → C2H4O + H2O

replace

by

CsRe-12 wt.% Ag/α-Al2O3

heterogeneous catalyst 10 – 30 bar, 200 – 260 °C

CH3ReO3 (methyltrioxorhenium, MTO)

C2H4O selectivity ≈100 % No H2O2 decomposition Works with higher olefins (e.g., C3H6) 20 – 40 °C, 50 – 60 bar

A. Ayame et al. Appl. Catal. A 244 (2003)

ⱡ ⱡ

k1, H2O2

k-1, H2O

CH3ReO2(η2-O2)A

CH3ReO(η2-O2)2(H2O)B

k2, H2O2

k-2

Epoxide OlefinEpoxide Olefin

CH3ReO3MTO

k4k3

J. H. Espenson, Chem. Comm. 479 (1999) W. A. Herrmann, R. W. Fischer, J. D. G. Correia, J. Mol. Catal. 94 (1994)

+

H2O

Methyltrioxorhenium activates H2O2 for oxygen atom transfer

Re O

C H H H

O

The ‘clean’ spectra of MTO make it amenable to kinetic studies

1H NMR spectra recorded at 3 minute intervals, 23.0 °C

fast slowk t k ttA A e e− −

∞= + α + β

1 fast slowk t k ttA A ( e ) e− −

∞= + α − + β

0.0

0.2

0.4

0.6

0.8

0 20 40 60 80 100ab

sorb

ance

time / min

320 nm

360 nm

kslow = 5.01(1) x 10-4 s-1kfast = 9.77(2) x 10-3 s-1

kfast = 9.16(3) x 10-3 s-1

kslow = 5.036(2) x 10-4 s-1

UV-vis spectra recorded at 1 minute intervals, 25.0 °C λmax(A) = 320 nm λmax(B) = 360 nm

T. Hwang*, B. R. Goldsmith*, B. Peters, S. L. Scott, Inorg. Chem. 52 (2013)

MTO ⇌ A ⇌ B MTO

Thermodynamics Calculated Experimental ∆H1 > 0 ∆H1 < 0 ∆S1 < 0 ∆S1 > 0 ∆G1 > 0 ∆G1 < 0 ∆G1 > ∆G2 ∆G1 < ∆G2 calc. ∆H2 = exp. ∆G2 claimed “excellent agreement”

Although MTO is a highly studied system, many discrepancies remain between experiment and theory

*JACS, 2015, 137, 9604 *Inorg. Chem., 2013, 52, 13904 Chem. Eur. J. 2009, 15, 1862 (in aq. CH3CN) Inorg. Chem. 2008, 48, 307 (in H2O) JACS, 2007, 129, 15794 (in aq. THF) JACS, 2001, 123, 2365 (in gas phase) Angew. Chem. Int. Ed. 1998, 37, 2211 (gas phase)

Kinetics ∆H1

‡ > 100 kJ mol-1 (exp. 24.5 kJ mol-1)

∆H1, ∆S1, ∆G1

∆H1‡, ∆S1

‡, ∆G1‡

∆H2, ∆S2, ∆G2

∆H2‡, ∆S2

‡, ∆G2‡

+ H2O2 + H2O CH3ReO3

15 °C, CD3CN(aq)

X = 1 – exp(-kobst)

Strong solvent effect on the observed rate

Besides the thermodynamic and kinetic discrepancies, the remarkable water-dependence has yet to be explained

kobs = -k[H2O]

Benchmark the thermodynamics and kinetics of MTO-catalyzed cyclohexene epoxidation: The key role of H2O

B. R. Goldsmith, T. Hwang, S. Seritan, B. Peters, S. L. Scott, J. Am. Chem. Soc. 137 (2015) T. Hwang, B. R. Goldsmith, B. Peters, S. L. Scott, Inorg. Chem. 52 (2013)

Density-functional theory Microkinetic modeling Experimental kinetic measurements UV-vis, 1H NMR, proton inventory

In acetonitrile solvent with 1−4 M [H2O]

cyclohexene cyclohexene

First-principles computational approach Range separated exchange-correlation functional ωB97X-D

J.-D. Chai and M. Head-Gordon, Phys. Chem. Chem. Phys. 10 (2008)

Def2-TZVP basis set, with 2 optimized f-orbitals for Re D. Andrae, U. Haeussermann, M. Dolg, H. Stoll, H. Preuss, Theor. Chim. Acta, 77 (1990) Include implicit acetonitrile solvation effects Geometries optimized in a polarizable continuum model (CPCM) Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 105 (2005) Ideal gas-phase entropies corrected for solvation D. H. Wertz J. Am. Chem. Soc. 102 (1980) Semi-classical proton tunneling corrections Γi and experimental H2O concentration included in reported thermodynamic and activation parameters B. R. Goldsmith, T. Hwang, S. Seritan, B. Peters, S. L. Scott, J. Am. Chem. Soc. 137 (2015)

Cyclohexene epoxidation: Experiments and theory indicate that water only weakly accelerates the kinetics

Cyclohexene epoxidation by B

Cyclohexene epoxidation by A

∆Gⱡ = 118 kJ mol-1 ∆Gⱡ = 115 kJ mol-1

∆Gⱡ = 96 kJ mol-1 ∆Gⱡ = 94 kJ mol-1

UV-vis experiments suggest water dramatically accelerates the formation of peroxo complexes A and B

0.2

0.4

0.6

0.8

0 1000 2000 3000 4000 5000

abso

rban

ce a

t 320

nm

time / s

a

0.20 M H2O●0.50 M H2O●1.0 M H2O●1.5 M H2O●

2.0 M H2O●3.0 M H2O●4.0 M H2O●

CH3ReO3 + H2O2 + H2O UV-vis, 25.0 °C, CH3CN

free

ener

gy, k

J m

ol-1

reaction coordinate

-14

-12

75

82

CH3ReO3 + 2 H2O2

A + H2O2 + H2O

experimental

110

121

reaction coordinate

-9 -5

CH3ReO3 + 2 H2O2

A + H2O2 + H2O B + H2O

calculated

0H2O

0H2O

91 97

1H2O

1H2O

96 94

2H2O

2H2O

Initial rate predictions too slow by 8 orders of magnitude!

Free energy profiles for the formation of A and B

B + H2O

(A→ B)

(MTO→A)

∆∆G2ⱡ= 13 kJ mol-1

∆∆G1ⱡ= 15 kJ mol-1

Activation free energy for the formation of A and B vs. number of water in the transition states

R. Gandour Tetrahedron. Lett. 15, 3 (1974)

“linearity of H-bonds in the transition state balances the entropic penalty of bringing four molecules together”

Proton inventory suggests two waters participating on average during peroxo complex formation

Measure solvent isotope effect on reaction kinetics Keep [H2O + D2O] constant

10 1 */ ( )m

nk k n nϕ += − +

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500 600

abso

rban

ce a

t 360

nm

time / sec

n = 0.9

n = 0

MTO 1.0 mM245.4 mM H2O23.1 M H2O

Vary mole fraction of D2O Experimental proton inventory

m = # of water molecules in transition state

1 0 56calc = *, .φ

2 0 63calc = *, .φ

The full catalytic cycle and the roles of water

B. R. Goldsmith, T. Hwang, S. Seritan, B. Peters, S. Scott, J. Am. Chem. Soc. 137 (2015) T. Hwang*, B. R. Goldsmith*, B. Peters, S. L. Scott, Inorg. Chem. 52 (2013)

B cycle

A cycle

experimental kinetic profiles

computed kinetic profiles

Strong water dependence is anticipated for other MTO-catalyzed oxidations e.g., oxidation of alkenes, alkynes, aldehydes, ketones, amines, sulfides, phosphines, and arenes

Careful benchmarking between experiment and theory can lead to new insight that is both explanatory and predictive

calc. ∆G1 ≈ exp. ∆G1 calc. ∆G2 ≈ exp. ∆G2 calc. ∆G1 < ∆G2 (exp. ∆G1 < ∆G2)

On average two H2O participate in the rate-determining transition states

calc. ∆G1ⱡ ≈ exp. ∆G1

ⱡ calc. ∆G2

ⱡ ≈ exp. ∆G2ⱡ

Similarly for ∆Hⱡi and ∆Sⱡi

Thermodynamic and activation parameters for all key steps fully characterized via both theory & experiment

Acknowledgements

Baron Peters Susannah L. Scott

Thanks to all of my colleagues at UC Santa Barbara and the Fritz

Fritz Haber Institute, Theory Department

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