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Natural Gas: An Alternative to Petroleum?
Crabtree, R. H. Chem. Rev. 1995, 95, 987-1007American Methanol Institute, 2000
Natural gas reserves: ~ 60 years Petroleum reserves: ~ 40 years
Combustion of natural gas releases more energy per gram than that of petroleum
Combustion of natural gas releases more energy per CO2 molecule than that of petroleum
Approximately twice the amount of natural gas produced for consumption is vented or burned at its source
Pressurization and refrigeration required for liquefaction (bp -164 °C)
Largest reserves located in remote regions of the world
Natural Gas is a Source of Methane
H
CH H
H
Limitations for the Practical Use of Methane
Crabtree, R. H. Chem. Rev. 1995, 95, 987-1007
Physical
pressurization and refrigeration required for liquification
boiling point = -164 °C
Chemical
strong carbon-hydrogen bond CH4 CH3 + H
CH4 CH3- + H+
CH4 CH4+ + e-
CH4 + H+ CH5+
high ionization potential
low proton affinity
very weakly acidic439 kJ/molepKa = 481255 kJ/mole
443 kJ/mole
Methanol: a Fuel and a Chemical Feedstock
1995 U.S. Production2.2 billion gallons
41% methyl t-butylether oxygenated fuels
fuel cells25% formaldehyde
resins, urethane plastics, Spandex
10% acetic acidpolyethylene terephthalate (PET)
27% othercleaning fluid, solvents,
refrigerants,chlorine-free bleaches
I K E A
www.methanex.com
Direct Conversion of Methane to Methanol
CH3OH50 atm
oC450 8 % conversion81 % selectivityCH4 + O2
1 : 20
CH4 + O2 + NAD(P)H + H+ CH3OH + NAD(P)+ + H2O
thermodynamically favored but the high temperature required to activate the strong C-H bond (439 kJ/mol) leads to overoxidation, i.e. CO2 and H2O
Crabtree, R. H. Chem. Rev. 1995, 95, 987-1007Periana, R. A. et al. Science 1993, 259, 340-343
MethaneMonooxygenase
CH4(g) + 1/2 O2(g) CH3OH(l) HO = -130 kJ
Conversion of Methane to Methanol via Heterogeneous Catalysis
CH4(g) + H2O(g) CO(g) + 3H2(g)
NickelCatalyst
700-1000 oC
synthesis gas
H° = + 205 kJ
CO(g) + 2H2(g)
ZnO, Cu, AluminaH° = - 90 kJCH3OH(g)
10-20 atm
250 oC50-100 atm
Steam Reforming
Crabtree, R. H. Chem. Rev. 1995, 95, 987-1007
Substantial capital investment required to implement
Industrial Hydrogen Production
CH4(g) + H2O(g) CO(g) + 3 H2(g) H = 206 kJ
CO(g) + H2O(g) CO2(g) + H2(g) H = -41 kJ
water gas shift reaction
CH4(g) + 3/2 O2(g) CO(g) + 2 H2O(g) H = -519 kJ
2CH4(g) + 3/2 O2(g) CO2(g) + CO(g) + 4 H2(g) H = -354 kJ
Methane to Methanol Catalyzed by Soluble Pt(II) Salts
CH4 + PtCl62- + H2O CH3OH + CH3Cl + PtCl4
2- PtCl4
2-
120 °C
PtII PtII
CH3
PtIV
CH3
+ CH4 + H+
H2O
Cl-
PtIV
CH3Cl
CH3OH
PtII
Gol'dshleger, N. F.; Es'kova, V. V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. (Engl. Transl.) 1972, 46, 785-786
Alkane C-H Bond Activation Using Electron Rich Transition Metal Complexes
IrMe3P
HH
h
- H2Ir
Me3P
RHIr
Me3PR
HRT
IrMe3P
RH
IrMe3P
LIr
Me3P L
- RH
Oxidative Addition
Reductive Elimination
Ir(III) Ir(I) Ir(III)
Ir(III) Ir(I) Ir(I)
Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104, 352-354
C-H Bond Activation by an Electron Rich Metal Center
Mn + RHoxidative addition
reductive elimination
Mn+2
HR
R = alkyl or aryl
M = Rh, Ir, Pt
C-H Bond Activation by an Electron Rich Metal Center
HH
M M
Oxidative Addition has occurred
C-H Bond Activation Selectivity
H
Me
Me Me H
HMeMe Me
H
HH
tertiary secondary primary
> >
CH4H2C CH2 >> > > > H
Radical Process
Oxidative Additionby Late Transition Metal
Complexes
the stronger C-H bond is favored
A Remarkably Stable Pt(IV) Methyl Hydride
Pt
HCH3
CH3N
NNN
NN
BH
PtCH3
CH3
N
NNN
N
NB
H
HClK
THFRT
O'Reilly, S. A.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1996, 118, 5684
Tp’PtMe2H in the solid state begins to decompose at 140 °C
Lewis Acid Generates a Vacant Site at Pt(II)
Pt CH3CH3N
N
tbu
tbu
Pt CH3N
N
tbu
tbu
Pt CH3LN
N
tbu
tbu
+CH3B(C6F5)3-CH3B(C6F5)3-
+
B(C6F5)3 L
Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1996, 1809
PtCH3
CH3
N
NNN
N
NB
H
K
Would react similarly?
C-H Activation at Pt(II)
+ K[CH3B(C6F5)3]Pt
HCH3
RN
NNN
NN
BH
PtCH3
CH3
N
NNN
N
NB
H
KB(C6F5)3
RH25-60 oC
R = Ph, C5H9, C6H11
Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235
the first stable Pt(IV) alkyl hydride formed by alkane oxidative addition to Pt(II)
Proposed Mechanism of C-H Activation
Pt
HCH3
RN
NNN
NN
BH
PtCH3
CH3
N
NNN
N
NB
H
PtCH3
N
NNN
N
NB
H
PtCH3
N
NNN
N
NB
H
H
RPt
CH3
N
NNN
N
NB
H
H
R
RH
B(C6F5)3K
-K[CH3B(C6F5)3]
C-H Bond Activation by an Electron Rich Metal Center
HH
M M
Arrested StateAn Alkane Complex
Oxidative Addition has occurred
Mechanism of Reductive Elimination Involves Alkane Complexes
RhMe3P
HCH2CH3
WH
CH3
ReH
CH3
+
Rh
N N N
CH3
Me3P H
+
WH
CH3
IrMe3P
H PtH2N
Cl
HCH3
CH3
H2N
(0.7)*
(0.5)*
(0.75)*
(0.8)* (0.77)*
(0.74)
(0.29)*
BH
H
N
N
N
N
Rh
NC CH3
N
N
(0.62)*
[M]H
CH2H
[M] + CH4[M]H
CH3
Pt(IV) Dimethyl Hydride Reacts with Oxygen
Pt
O
CH3
CH3N
NNN
NN
BH
Pt
H
CH3
CH3N
NNN
NN
BH
C6D6O2+
1 atm2 days
OH
RT
Tp'PtMe2D Tp'PtMe2(OOD)C6D6
O286% D
Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1999, 121, 11900
A Pt(IV) Dialkyl Hydroxide
Pt
O
CH3
CH3N
NNN
NN
BH
Pt
OH
CH3
CH3N
NNN
NN
BH
C6D6
OH
heat
Hydroxide is thermally stable
Catalytic Functionalization of Methane by Pt(II)
CH4 + 2H2SO4 CH3OSO3H + 2H2O + SO2
(bpym)PtCl2
220 °C
PtIIN
N X PtIIN CH3
N X
PtIVN CH3
N XX
X
SO3 + 2HX
CH3X
SO2 + H2O
+X-
CH4 HX
PtIIN X
N X
X = OSO3H
N N =N
N
N
N
Periana, R. A. et al. Science 1998, 280, 560-564
Acknowledgements
University of WashingtonThe Goldberg Research Group
FundingThe National Science Foundation
The Union Carbide Innovation ProgramThe Dupont Educational Aid Program
The University of Washington
7 6 5 4 3 2 1 ppm
Synthesis of Dichloride Precursor
80 % yield
1H-NMR
BH
Cl
N
N
N
N
Rh
N
N
ClNC
RhTp'(Cl)2CH3CN CNCH2CMe3
C6H6, reflux
Structures of Isopropyl and Cyclopropyl Complexes
Distribution of SpeciesDistribution of Species
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200 250 300
tim e (m in)
% d
istr
ibut
ion
[Rh] Cl [Rh] H[Rh] H[Zr]H2
k1 k2
k3k4
[Rh] Dd5
k1 = 1.0 X 10-3 s-1
k2 = 3.8 X 10-4 s-1
k3 = 1.8 X 10-4 s-1
k4 = 5.7 X 10-4 s-1
Methyl Hydride Rearrangement
1.30 1.28 1.26 1.24 1.22 1.20 1.18 ppm
1.28 1.26 1.24 1.22 1.20 ppm
1.28 1.26 1.24 1.22 1.20 ppm
1H{2H}-NMR
BH
D
N
N
N
N
Rh
NC CH3
N
N
BH
H
N
N
N
N
Rh
NC CH2D
N
NC6H6
Keq = 6(1)
22oC
t = 0t = 1 h
t = 3 h
d, 1.236 ppmJRhH = 2 Hz
d, 1.225 ppmJRhH = 2 Hz
Reductive Elimination of Methane
0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 ppm
t, 0.134 ppmd, -14.818 ppm
JRhH = 24 Hz
1H -NMR
*
-14.0 -14.2 -14.4 -14.6 -14.8 -15.0 -15.2 -15.4 ppm
5
BH
N
N
N
N
Rh
NC CH2D(H)
N
N
BH
D
N
N
N
N
Rh
NC C6D5
N
N
H(D)
+ CH3DC6D6
22 oC16 h
Loss of Methane Shows Isotope Effects[Rh](CD3)(D)
C6D6[Rh](C6D5)(D) + CD4 kobs = 2.48(17) × 10-4 s-1
[Rh](C6H5)(H) + CH4 kobs = 1.63(4) × 10-4 s-1C6H6[Rh](CH3)(H)
[Rh](C6D5)(D) + CH4 kobs = 1.52(4) × 10-4 s-1C6D6[Rh](CH3)(H)
-2.5
-2
-1.5
-1
-0.5
0
0 2000 4000 6000 8000 10000 12000 14000 16000
time (sec)
ln(m
ethy
l hyd
ride
inte
grat
ion/
tota
l hyd
ride
inte
grat
ion) [Rh](CH3)(H) in C6D6
[Rh](CH3)(H) in C6H6
[Rh](CD3)(D) in C6D6
Solvent kH/kD = 1.07(6)
kH/kD = 0.62(7)
Loss of Methane is Dependent on Benzene Concentration
[C6D6] kobs (× 10-4 s-1)
2.82 0.661(2)5.64 1.04(3)8.47 1.34(4)
11.29 1.52(5)
[Rh](CH3)(H) [Rh](C6D5)(D) + CH4C6D6 / C6F6
-2.4
-1.9
-1.4
-0.9
-0.4
0.1
0 5000 10000 15000 20000 25000
time (sec)
ln(m
ethy
l hyd
ride
inte
grat
ion/
tota
l hyd
ride
inte
grat
ion)
11.29 8.47 5.64 2.82
[C6D6]
Double Reciprocal Plot
0
0.00005
0.0001
0.00015
0.0002
0.00025
0.0003
0 10 20 30 40 50
benzene concentration ([C6D6]) (M)
kobs
(sec
-1)
asymptote = 2.73 e-4
0
2000
4000
6000
8000
10000
12000
14000
16000
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
1/benzene concentration (1/[C6D6]) (1/M)1/
kobs
(sec
)
Plot is consistent with saturation behavior,i.e. a reversible Keq followed by the rate
determining step
Plot of 1/kobs vs. 1/[C6D6] is linear
Kinetic Data are Consistent with an Alkane Complex
RhN
NN
CNR
H
CH3
BH
k1
k-1
H
CH3
RhN
CNRN
N
BH
[C6D6]k2
RhN
NN
CNR
D
Ph-d5
BH
A B
RhN
CNRN
N
BH
d6d6
H
CH3
RhN
CNRN
N
BH
fast
fast
Kinetic Scheme
Reductive Elimination from Pt(IV)
PtII
N CH3
N CH3PtIV
N CH3
N CH3
H
Cl
PtII
N Cl
N CH3
N
N
HCl
CD2Cl2-78 °C
below RT
= tmeda, tbu2bpy
PtIV
N CH3
N CH3
H +Cl-
- CH4
Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 5961Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics 1995, 14, 4966
a 5-coordinate intermediate is required for both reductive elimination and oxidative addition
Mechanism of Shilov Type C-H Bond Activation
PtII PtIIR
PtIVR
H
+ RH + BHB-
PtII + RH PtII H
R
B-
PtIIR
+ BH
Oxidative Addition followed by Deprotonation of a Pt(IV) Alkyl Hydride
Deprotonation of a Pt(II) Alkane Complex
PtIIN CH3
N+
+ 13CH4 PtIVN CH3
N13CH3
+H
PtIIN
N+
13CH3
- CH4
C-H Activation at Pt(II)
Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1997, 119, 848
PtII
N CH3
N NC5F5 PtII
N 13CH3
N NC5F5[BArf]- + CH4
30 atm 13CH4
85°C
+ +
[BArf]-NC5F5
N
N = tmeda
PtIIN CH3
N NC5F5+
H313C
H
sigma bond metathesis
oxidative addition
Effect of Radical Initiator/Inhibitor
ReactionConditions
Time (hr) % Conversion ofPtTp'Me2H
% Yield ofPtTp'Me2(OOH)
50 CDark
1 4 100
50 C17 mole % AIBN
Dark1 31 100
AmbientTemperature and
Light48 100 98
AmbientTemperature and
Light40 mole % 1,4-cyclohexadiene
48 46 94
Tp’PtMe2H Tp’PtMe2(OOH)O2, 1 atm
C6D6
Reaction of Pt(IV) Dialkyl Hydride with Oxygen is Promoted by Light
Tp’PtMe2H Tp’PtMe2(OOH)O2, 1 atm
C6D6/RT
ReactionConditions
Time (hr) % Conversion ofPtTp'Me2H
% Yield ofPtTp'Me2(OOH)
Ambient Light 48 100 98
Dark 48 14 100
High IntensityLight
> 345 nm1 75 90
High IntensityLight
> 345 nmNo O2
1 NR NR
Proposed Radical Mechanism
Initiation
Propagation
Termination
[Pt]-H [Pt]
In-In 2 In
In
[Pt] O2 [Pt-OO]
[Pt]-OOH
H-In++
+
[Pt]-OO [Pt]-H+
[Pt] + [Pt]-OO [Pt]-OO-[Pt]
[Pt]+