A Laser Flash Photolysis Study of CO2 Reduction: Kinetics Leading to the Design of a Renewable Reducing Agent 7th International Conference on Chemical.
Download ReportTranscript A Laser Flash Photolysis Study of CO2 Reduction: Kinetics Leading to the Design of a Renewable Reducing Agent 7th International Conference on Chemical.
A Laser Flash Photolysis Study of CO Kinetics Leading to the Design of a Renewable Reducing Agent
2
Reduction:
7 th International Conference on Chemical Kinetics, MIT, 2011
Outline of the Talk • Computational and experimental study of photochemical reduction of CO 2 by Et 3 N.
• Use of the lessons learned in the design of a renewable amine.
• Future directions: Is an all-organic, renewable, visible light photoreductant for CO 2 possible?
h < 470 nm Photochemical CO 2 Reduction H 2 O (
l
) + CO 2 (
g
) 1/2 O 2 (
g
) + HCO 2 H (
l
)
H
° = +60.8 kcal/mol H 2 O PC –H • + HO –
The Key Idea
h Photochemical CO 2 Reduction Matsuoka, S.; Kohzuki, T.; Pac, C.; Ishida, A.; Takamuku, S.; Kusaba, M.; Nakashima, N.; Yanagida, S.,
J. Phys. Chem.
1992
,
96
, 4437 PC = PTP HCO 2 H
PTP •– Photochemical CO 2 Reduction Fujiwara, H.; Kitamura, T.; Wada, Y.; Yanagida, S.; Kamat, P. V.
J. Phys. Chem.
1999
,
103
, 4874.
Effect of Ionization on C–H Reactivity H • + Figures are
H
° in kcal/mol (exptl. + CBS– QB3)
Computational Results Hazardous system for common DFT functionals such as B3LYP, because of self interaction error in radical ions and long-range exchange error in CT states.
PCM model for CH 3 CN These results from empirically corrected UB3LYP, calibrated against UMP2 and UCCSD for smaller systems
J. Phys. Chem. A
,
2007
,
111
, 3719 Later results use UCAM-B3LYP Self-Interaction Error in DFT: Bally, T.; Sastry, G. N.
J. Phys. Chem. A J. Phys. Chem. A
D.
J. Chem. Phys
. , ,
1997 1998 2004
, ,
101 , 102 120
, 7923 Braieda, B.; Hiberty, P. C.; Savin, A.
, 7872 Graefenstein, J.; Kraka, E.; Cremer, , 524 CAM-B3LYP: Yanai, T.; Tew, D. P.; Handy, N. C.
Chem. Phys. Lett.
2004
,
393
, 51.
Reality Bites D 3 C H 2 C C H 2 N CD 3 CH 2 CD 3 + CO 2 h H –CO 2 – + D –CO 2 – [1] : 2.0
H 3 C D 2 C C D 2 N CH 3 CD 2 CH 3 0.3
M
CO 2
,
0.25
M
amine in CH 3 CN + CO 2 h H –CO 2 – + D –CO 2 – [1] : 0.35
A Radical New Mechanism Kanoufi, F.; Zu, Y.; Bard, A. J.
J. Phys. Chem. B
2001
,
105
, 210.
Dimers of this radical detected in photochemical CO 2 reduction
Blocking C–H Reactivity X Transient stability, at best.
Radical cation would presumably be worse.
CBS-QB3 Isodesmic Reactions Proton transfer H-atom transfer
Blocking C–H Reactivity X Transient stability, at best.
Radical cation would presumably be worse.
Stable to prolonged photolysis; affords no CO 2 reduction.
Generation of “PTP •– ” with the New Amine + PTP
Decay of “PTP •– ” from the New Amine + PTP 440 nm • • • • Appearance quite different from that with Et 3 N Amine radical cation should have no band from 400 – 500nm Decay of “PTP•–” is much faster than with Et 3 N Everything returns to baseline, whereas with Et 3 N it does not 470 nm 285 nm 0 1 2 3 time / s 4 5
The Ion-Pair Hypothesis Ion pair(s) Deprotonation blocks BET “Long-lived” PTP •– The dilemma: This radical seems to be necessary for CO 2 reduction, but: Ion pair(s)
Picosecond Infrared Studies PTP •– Spectra taken after 500 ps .
10 -4
M
PTP, 1
M
NEt 3 CO 2 •–
Picosecond Infrared Studies 12 CO 2 •– 13 CO 2 •–
Picosecond Infrared Studies Prompt CO 2 •– formed by direct Et 3 N photo ionization with 266 nm pump PTP •– Spectra taken after 500 ps .
10 -4
M
PTP, 1
M
NEt 3 CO 2 •–
e – solv
+ Picosecond Infrared Studies
k
1
k
2
Nanosecond Infrared Studies
Re-evaluation of the First Steps •– [0] kcal/mol –10 kcal/mol •–
•+ •– Re-evaluation of the First Steps CO 2 PTP + Et 3 N + CO 2 PTP + Et 3 N •+ + CO 2 •–
Formate Production as
f
(PTP, ) 254 nm, no PTP 1
M
Et 3 N in CH 3 CN 254 nm, sat. PTP >290 nm, sat. PTP >290 nm, no PTP
What Have we Learned?
• Electron addition to CO 2 is difficult, and probably doesn’t occur from PTP •– except by “inner-sphere” carboxylation mechanism.
• BET to Et 3 N •+ can occur from both PTP •– and carboxylated PTP •– in ion pairs • Deprotonation of Et 3 N •+ blocks BET and generates –amino radical • –Amino radical seems to be necessary for CO 2 reduction, but...
• –Amino radical is also responsible for several of the byproducts
An Idea for the New Amine
R R N H R
IP (amine)
R N H
+ e –
R
H
°
trans
R N H
+ e –
R R N
–BDE (C–H) +
H R
–IP (H)
R N
~PA (amine) +
H
+ + e – Δ
H
°
trans
= 414.6
– IP(amine) – PA(amine) (in kcal/mol) .
J. Am. Chem. Soc.
2008
,
130
, 3169
An Idea for the New Amine
Sweet spot
Aliphatic amines ArNMe 2 NH 3 ArNH 2
An Idea for the New Amine •• Janovsky, I.; Knolle, W.; Naumov, S.; Williams, F.
5524.
Chem. Eur. J. 2004 ,
10
, e – Beam Freon + • ‡
Adamantane-like TS for H transfer An Idea for the New Amine Replaces –H of –amino radical Bridgehead blocks –amino radical formation H transfer blocks BET hole Simple alkene should be easily hydrogenated
Synthesis and Testing H H h PTP ~ 2 x Et 3 N
A Lot More Synthesis
How it Works in Practice PTP PTP 250 –300 nm
Nature Chem.
2011
,
3
, 301.
It Also Works with Visible Light Re(Bipy)(CO) 3 (EtO) 3 PRe(Bipy)(CO) 3 + > 400 nm c.f. Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O.
J. Am. Chem. Soc.
2008
,
130
, 2023–2031.
One Long Term Plan...
N. Itoh, W. C. Xu, S. Hara, K. Sakaki, Catal. Today 2000 ,
56
, 307
Outline of the Talk • Computational and experimental study of photochemical reduction of CO 2 by Et 3 N.
• Use of the lessons learned in the design of a renewable amine.
• Future directions: Is an all-organic, renewable, visible light photoreductant for CO 2 possible?
Computational Results < 390 nm
J. Phys. Chem. A
,
2007
,
111
, 3719
Some Useful Information Reichardt, R.; Vogt, R. A.; Crespo-Hernández, C. E.
J. Chem. Phys.
2009
, 224518.
Görner, H.; Döpp, D.
J. Chem. Soc., Perkin Trans. 2
,
2002
, 120.
Predicted pH-dependent rotational profile about red C-C bond
PE rel (kcal/mol) B3LYP/6-31+G(d,p) PE Profile + Dihedral Angle
Putting the Pieces Together ~73 kcal/mol ~63 kcal/mol CAM-B3LYP/6-31+G(d,p) G ° (298 K, 1
M
standard state) PCM model for CH 3 CN Barrier ~4 kcal/mol 56 kcal/mol 43 kcal/mol Barrier 12 kcal/mol [0] kcal/mol 33 kcal/mol
An Unexpected Outcome…
Acknowledgments Rob Richardson Ed Holland Chris Stanley Claire Minton The Leverhulme Trust Mike George Sun Xue-Zhong James Calladine Charlotte Clark Royal Society/Wolfson Foundation