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]+