Aluminum Hydride: the organometallic approach

Project ID # ST 034

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Chengbao Ni, W.M. Zhou and J. Wegrzyn Brookhaven National Laboratory

05/14/2013

Overview Timeline • Project start date: FY10 • Project end date: Continuing

Budget • Funding received in FY12 – $250K (DOE)

• Planned Funding for FY13 – TBD (DOE)

Barriers MYPP Section 3.3.4.2.1 On-Board Storage Barriers:

- Weight & Volume - Efficiency - Durability/Operability - Charge/Discharge Rates

Target Material development for meeting the packaging, safety, cost and driving range (greater than 300 miles) DOE performance targets for the PEM hydrogen fuel vehicle.

Hydrogen storage materials:

Chem. Soc. Rev. 2009, 38, 73; Angew. Chem. Int. Ed. 2012, 51, 6074.

physical adsorption/weak interaction; low H2 weight percentage; (high mass of the MOF) low temperature (such as 77 K).

Metal-Organic Frameworks (MOF):

Amine-Borane Compounds:

high H2 percentage (19.6% vs. 14.2%); dehydrogenation well-studied (catalysis); rehydrogenation remains challenging.

LiH, MgH2, AlH3, LiAlH4, and others; most of the hydrides are quite stable; rehydrogenation is challenging.

Metal-Hydrides:

H3N-BH3 polyborazylene {BNH}x

BH2

NH2

BN

H

H

Ti lowers the activation energy of the decomposition reaction;

AlH3 is completely unstable at Ti concentrations ≥ 0.1 mol%.

Aluminum hydride (alane, AlH3):

Al + H2AlH332

J. Alloys Compd. 2007, 446, 271.

Synthesis of AlH3:

J. Alloys Compd. 2007, 446, 271.

3 LiAlH4 + AlCl3

Et2O

3 LiCl + 4 AlH3(Et2O)n

AlH3(Et2O)n

filtration

AlH3 polymorph

desolvation/crystallization

Crystallization

Al + H232 AlH3?Direct hydrogenation:

Conventional synthesis:

Too expensive!

Morphology

The organometallic approach:

The 3-step regeneration process:

The “Parodox” and challenges:

Phys. Rev. B. 2006, 74, 214114; Chem. Soc. Rev. 2009, 38, 73.

Al + 2 amine1 + H2

+ amine2

(1)

(2)

(3)

catalyst

+ 2 amine1vacuumheat

vacuumheat

AlH3 + amine2

⋅AlH3(amine1)2 ⋅AlH3amine2

⋅AlH3amine2

⋅AlH3(amine1)232Hydrogenation:

Transamination:

Decomposition:

N N

N

N

NTMA DMEA TEDA quinuclidine

NN

N

N

hexamineAmine1:

Strong amines, type I structure; Facilitate hydrogenation; Decomposition to Al metal

directly.

Weak amines, type II or III structures; Decomposes to AlH3 (with Al contamination; Does not facilitate hydrogenation.

H

Al

HH

NR3Al

H

H H

R3N NR3H

Al AlH

NR3

R3N H

HHH

(I) (II) (III)

NR3 is an amine.

Common amine·alane structural types:

N

TEAAmine2:

NN

N

TEA

N

DMEA

N

DEMA

increasing basicity

NO

NO

N

N

piperidine morpholine pyrrolidine

N

TMA

Approaches to the project: Tune the Lewis Basicity of amines:

Objectives: 1. Search for an amine that facilitate both hydrogenation and decomposition:

• Reduce energy input and chemical costs • Increase the efficiency of the process

2. Optimize the transamination and thermal decomposition steps:

Al + 2 amine1 + H2catalyst ⋅AlH3(amine1)2

32

vacuumheat

AlH3 + 2 amine1

⋅AlH3TEAvacuum

heatAlH3 + TEA

• Suppress Al formation and alane evaporation • Increase the yield and purity of AlH3

vacuumheat

vacuumheat

Al + TEA + H2

Evaporation unpublished results ROI filed/manuscripts manuscript accepted

Synthesis of alane adducts:

Al

H

H H

NN

Nex.

Nex.

Hydrogenation:

Al*

Al* No Reaction

• Reactor set-up: • H2 pressure: ~ 1000 psi; • Temperature: 0 ~ 25Á

C;• Chemicals: Et2O (80 mL) and amine (20 mL); • Al*: Ti doped Al metal, ~ 2.0 g.

Al

H

H HH

Al

HH

NNNN

γ-AlH3

N1 2

Al

H

H HH

Al

HH

NNN

N

γ-AlH3

N1 2

Direct synthesis:N

N

N-methylpyrrolidine (NMPy)

N-ethylpyrrolidine (NEPy)

J. Phys. Chem. C. Just accepted; DOI: 10.1021/jp310848u

Characterization (NMR, IR, and XRD):

alane adduct

Calculation Experiment Type I Type II Type III

NMPy 1700 1789 1598, 1793 1709 (2:1), 1773 (1:1) NEPy 1700 1789 1598, 1793 1773 (1:1)

Al-H stretching frequencies (cm-1):

(NMPy)2∙AlH3

H

Al

HH

NR3

Al

H

H H

R3N NR3

HAl Al

H

NR3

R3N H

HHH

(I)

(II)

(III)

Theoretical calculations:

alane adduct Binding Free Energy in Et2O with BSSE Correction (kcal∙mol-1)

Type I Type II Type III NMPy -19.91 -22.11 -18.57 NEPy -14.72 -19.49 -15.94 Et2O -13.24 -15.92 -13.77

H

Al

HH

NR3

Al

H

H H

R3N NR3

HAl Al

H

NR3

R3N H

HHH

(I)

(II)

(III)

Reversible formation of (NMPy)2·AlH3:

Hydrogenation Decomposition

2 NMPy + Al* + H2Et2O

(NMPy)2⋅ΑlH3heat32

Reversible formation of (NMPy)2∙AlH3 in the reactor (~ 60% yield); Low temperature enhances the reaction rate, but not the final yields;

Thermal decomposition Studies:

Hydrogenation coupled with decomposition with LiH:

XRD IR in KBr pellets

Al* + 2 NMPyEt2O

(NMPy)2⋅ΑlH3 vacuum, 50°C LiAlH4 + 2 NMPy1.0 eq LiH1000 psi H2

Al + 2 NMPy + H2(NMPy)2⋅ΑlH3322 NMPy + AlH3

Role of LiH: (NMPy)2⋅ΑlH3 + LiHdecomposition ?

Summary:

Department of Energy Office of Basic Energy Sciences Contract No. DE-AC02-98CH10886

Department of Energy Office of Fuel Cell Technologies Contract No. DE-AC02-98CH10886

Acknowledgements Liu Yang, James Muckerman, Jason Graetz; Yusuf Celebi, John Johnson, James Reilly; Christine Brakel, Kimberley Elcess; Pat Looney, Sabrina Parrish, Lori Happich; All other sustainable energy technologies personnel; The Chemistry department (the NMR instrument).

N

N

NMPy

NEPy

Steric effects:

LiAlH4 regeneration: Al* AlH

H H

NNhydrogenationH2, NMPy, Et2O

vacuum, 50°C

+ LiH

LiAlH4- NMPy

Al

H

H H

NN γ-AlH3

N2Al

H

H H

NNN2

Al

H

H H

NNex.

Al*N Nex.

No Reaction