Chemical Hydrides: Amine BoranesChemical Hydrides: Amine BoranesChemical Hydrides: Amine Boranes

International Symposium on Materials Issues in Hydrogen Production and

Storage August 2006

Acknowledgements Department of Energy, Office of Basic Energy Sciences and Energy Efficiency and Renewable Energy.

2

NH BH (3)

Hydrogen-rich ammonia borane (AB)HydrogenHydrogen--rich ammonia borane (AB)rich ammonia borane (AB)

George Thomas

Hydrogen Densities of Materials

0

50

100

150

200

0 5 10 15 20 25 30Hydrogen mass density (mass %)

Hyd

roge

n vo

lum

e de

nsity

(kgH

2m

-3)

100

liquidhydrogen

700 bar

350 bar

CH4 (liq)

C2H5OH

C8H18

C3H8

C2H6NH3

CH3OH

Mg2NH4

LaNi5H6

FeTiH1.7

MgH2

KBH4

NaAlH4

NaBH4

LiAlH4

LiBH4

AlH3TiH2

CaH2

NaH

2015 system targets

2010 system targets

3 3

NH3BH3(2)

NH3BH3(1)

Mg(OMe)2.H2O

11M aq NaBH4

hexahydrotriazine

decaborane

LiNH2(2)

LiNH2(1)

NH4BH4(4)

3

NHxBHx Store hydrogen (>6 wt%/step)NHxBHx Store hydrogen (>6 wt%/step)

Wt% H2 T (K)NH4BH4 NH3BH3 + H2 6.1 <300NH3BH3 (NH2BH2)n + H2 6.5 <375(NH2BH2)n (NHBH)n + H2 6.9 >375(NHBH)n BN + H2 7.3 >>800

Two sequential steps > 12 wt% hydrogen!

Solid state chemistry: thermodynamics and kinetics of hydrogen release?

4

Reactants Products ∆E (kJ/mol)

NH4BH4(s) → NH3BH3(s) + H2 -10NH3BH3(s) → (NH2BH2)n + H2 +37(NH2BH2)n → (NHBH)n + nH2 -13(NHBH)n → BN(s) + nH2 -38

AB+ H2

PIB+ H2

PAB+ H2ABH2

ABH2 = NH4BH4AB = NH3BH3

PAB = (NH2BH2)n

PIB = (NHBH)n

Thermodynamics

BH4 + 4H2O B(OH)4 + 4H2∆H = -250 kJ/mol

5

ApproachesApproachesApproaches

Scanning calorimetry combined with mass detection and gravimetric analysis

Thermodynamics of hydrogen lossKinetics and insight into mechanism of H-release

NMR and Raman spectroscopy Solid state to study phase transitions and molecular dynamicsVariable temperature for in-situ kinetic investigations

Neutron spectroscopy QENS (for dynamics of H motion)INS (structural properties)

Combination of experiment & theory to gain fundamental understanding of chemical & physical properties of AB

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Ammonia borane isoelectronicisoelectronic with ethane

NH3BH3 CH3CH3

MW 30.81 30.07Mp[˚C] 124 -183M[D] 5.2 0bonding N->B dative C-C covalentR[A] 1.58 (1.62) 1.53kg H2/kg 191 2.1kg H2/liter 143 1.3

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Background crystalline NH3BH3Background crystalline NHBackground crystalline NH33BHBH33

• Synthesis (Shore and Parry in 1955)• AB in equilibrium with diammoniate of diborane

2NH3BH3 ↔ [NH3BH2NH3][ BH4]

• NH3BH3 H2 + polyaminoborane at < 370 K.∆Hrxn = -20 kJ/mol (Wolf et al. 2000)

•First order-disorder phase transition at 225 K, the low T structure is orthorhombic, the high T tetragonal (Reynhardt & Hoon)•Electrostatic bonding between (N)H+ and (B)H-

(dihydrogen bond) (Crabtree et al. 1999)

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0 500 1000 1500 2000 2500 3000

Rel

ativ

e H

eat R

elea

sed

(a. u

.)

Time (min)

85 °C

80 °C

75 °C70 °C

Neat AB

Thermal decomposition of solid AB

•Rates increase with temperature

•Activated process

•Sigmoidal kinetics

•Induction period

•Thermodynamics

•∆Hrxn = -21±3 kJ/mol

NHNH33BHBH33(s) (s) (NH(NH22BHBH22)(s) + H)(s) + H22

Solid state kinetics analysis

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Stabilizing polymerizationStabilizing polymerizationStabilizing polymerization

+

Large dipole moment → instability

Relieving instability•Coiling of oligomers → cyclization•Branching of oligomers

Simulated annealing reveals “coiled” or “cyclic” structure of oligomers, stabilized by dihydrogen bonds – precursors to

observed cyclic products

Ammonia borane decompositionAmmonia borane decompositionAmmonia borane decomposition

-(NH2BH2NH2)-

NH3BH3

(NH3)2BH2BH4

-(NH2BH(NH2)2- -(NH2BH3

11B NMR 800 MHz11B NMR 800 MHz

Complex mixture of cyclic, branched polymeric products

11

Nano-phase ammonia borane

Use mesoporous silica (SBA-15) as a scaffold. The 6-7 nm wide channels will hold Ammonia Borane (NH3BH3) in the nano-phase. Should also preserve nanophase.

Ammonia borane infiltrated

NH3BH3

Add saturated solution of NH3BH3 to

SBA-15

12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500 600

Rel

ativ

e H

eat R

elea

se Extent of Reaction

Time /min

Neat Amonnia Borane80 °C

Amonnia Borane on SBA Scaffold 50 °C

50 100 150 200 250

Rel

ativ

e Yi

eld

(arb

. uni

ts)

Temperature (°C)

Hydrogenm/e = 2

Borazinem/e = 80

Neat ABAB:SBA-15

Borazine (BHNH)3

Ammonia borane in SBA-15

Change in Reactivity and Selectivity

Angew. Chem. Int. Ed. 2005, 44, 3578.

Scaffolds: enhance rates of H2 release enhance purity of H2(little borazine) change in thermodynamics

x20

13

Fundamental Science QuestionsFundamental Science Questions

NH3BH3 (NH2BH2)n + H2 + ? <100 ˚C

Is the mechanism intra or intermolecular?What is the activation barrier?Can we change it with catalysis, (other)Can we control the decomposition pathways?

How is H2 formed from solid AB?How is H2 formed from solid AB?

Need approaches to study solid state kinetics

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Phase transformation kinetics

r (t)

Avrami Equation XCrystal = 1 - exp (-(kt)n)

15

Avrami rate as function of T (70-85 C)AvramiAvrami rate as function of T (70rate as function of T (70--85 C)85 C)

Xreaction = 1 - exp (-(kt)3)

16

Arrhenius analysis of k(Avrami) vs. temperature Arrhenius analysis of Arrhenius analysis of k(Avramik(Avrami) vs. temperature ) vs. temperature

Activation barrier for H2 formation from solid state ammonia borane is ca. 150 kJ/mol.

Ea ~ 150 kJ/mol

1

Kinetic model for hydrogen formationKinetic model for hydrogen formationKinetic model for hydrogen formation

Sigmoidal kinetic behaviorInduction, Nucleation & Growth

Induction: Rate limiting. How to decrease (scaffold)?

Nucleation:Physical or Chemical Change

Growth: hydrogen formationHow do you modify pathways, change thermodynamics?

2

Optical microscope studiesOptical microscope studiesOptical microscope studies

Time 0 75C 100C

7 min 10 min 13 min 14 min

Crystal of AB (~0.1 mm)

Induction, nucleation, growthInduction, nucleation, growthInduction, nucleation, growth

3

5

43

21

1

2

3 45

Raman microscopyRaman microscopyRaman microscopy

“top” of crystal (1) is crystalline AB, bottom (5) is amorphous AB + ??. Beginning of nucleation?

4

Growth?Growth?Growth?

2.02 Å

Literature: Reaction Pathways to H2Intermolecular

2 NH3BH3 NH3BH2-NH2BH3 + H2

or

IntramolecularNH3BH3 NH2=BH2 + H2

n NH2=BH2 (NH2BH2-NH2BH2)n

BH ----- HN Dihydrogen bondδ- δ+

Hydride atoms act as Proton acceptors

Intermolecular vs. IntramolecularIntermolecular vs. IntramolecularIntermolecular vs. Intramolecular

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Ionic pathway2 NH3BH3 [NH3BH2NH3]+[BH4]- (isomerization, no H2 loss)

AB DADB

NH3BH3 + [NH3BH2NH3 ]+[BH4]-[NH3BH2NH2-BH2NH3][BH4]- + H2

Neutral pathway2 NH3BH3 NH3BH2-NH2BH3 + H2

If [NH3BH2NH3]+[BH4]- is an intermediate, should see it by in-situ solid state 11B NMR

Nucleation? Nucleation? Do we have dehydrocoupling of ionic or neutral intermediates?

11B NMR 800 MHz Ammonia Borane1111B NMR 800 MHz Ammonia BoraneB NMR 800 MHz Ammonia Borane

Time from top to bottom: mins.Red: 0Blue: 10Green: 15Yellow: 20Aqua: 25Black: 30Pink: 40Bright Green: 120

T = ~90 °C

-(NH2BH2NH2)-

NH3BH3

(NH3)2BH2BH4

-(NH2BH(NH2)2--(NH2BH3

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Nucleation seedingNucleation seedingNucleation seeding

BH3NH3 doped with seeded material

-0.2

0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000

Time(min)

q(m

W)

AB Isothemal 70C

AB doped with AB seeded tostart of exotherm

Nucleation seeding of solid AB enhances rate of H2 release.

b) Mix seeds with fresh AB

a) Heat AB for 1000 minutes to synthesize ‘seeds’

ac) Heat AB mixture decreased induction time

c

What is the nucleation step? We must know this to better control rates of H2 evolution!

∆Hrxn ~ -21 kJ/mol

Ea = 155 kJ/mol

Ener

gy

AB (NH2BH2)n + H2

Intermolecular reaction of AB leads to H2 in the solid stateDiammoniate is important intermediateRates of H2 release can be controlled in the solid state

Enhance kinetics with nucleation seeding or scaffold

Solid state AB energeticsSolid state AB Solid state AB energeticsenergetics

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Role of dihydrogen bonds in H2 release:Role of Role of dihydrogendihydrogen bonds in Hbonds in H22 release:release:

BH ----- HN Dihydrogen bondδ- δ+

Hydride atoms act as Proton acceptor*

~2Å

*Klooster, et. al. JACS, 1999, 121, 6337

Use neutron scattering (INS and QENS) methods to study properties of H-rich materials.

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Inelastic neutron scatteringInelastic neutron scatteringInelastic neutron scattering

0

50

100

0 500 1000 1500 2000cm-1

inte

nsity

10 K75 K230 K

11BH3NH3 Vibrational spectra

FDS provides approach to measure low frequency vibrational modes due to intermolecular interactions

Experimental data to bench mark theory

To facilitate identification of modes use selective D/H labeling to ‘hide’ transition.

B-N stretch

B-N torsion

Intermolecular modes

11

Vibrational studies of dihydrogen bondingVibrationalVibrational studies of studies of dihydrogendihydrogen bondingbonding

BH ----- HN δ- δ+

linear

cyclic

12

Computational analysis of dihydrogen bondingComputational analysis of Computational analysis of dihydrogendihydrogen bondingbonding

BH ----- HN δ- δ+

linear cyclic

13

NH3BH3 SummaryNH3BH3 SummarySummary

•12 wt% hydrogen at relatively low temperatures (<400 K)

•exothermic ~ 20 kJ/mol – need to regenerate by chemical pathway

•Mechanism of H2 formation – Nucleation and Growth

•Induction period can be enhanced with scaffold or seeding

•INS provides low frequency vibrational modes to yield insight into di-hydrogen bonds – benchmark theory

•Computational approaches to understand dynamics

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Future DirectionsFuture DirectionsFuture Directions

MH ----- HY Dihydrogen bondδ- δ+

M is electropositive (Li, Mg, Ca, Na, Al, B) Y is electronegative (N, O, P, etc.)

AB will react with itself or with other hydridic and/or protic hydrogen. E.g., MgH2 ---H3NBH3 ---H2NLi

Are thermodynamics of alternative reactions more favorable?

Understanding Understanding dihydrogendihydrogen bonding interactions. How bonding interactions. How universal are these interactions in H storage materials?universal are these interactions in H storage materials?

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ContributorsContributorsNancy Hess

Anna GutowskaYongsoon Shin

Liyu Li Chun Lu

Shari Li Scott Smith Bruce Kay

Ashley Stowe Abhi KarkamkarDave Heldebrant

FSUNaresh Dalal

Ozge Gunaydin-SenR Achey

NCNRCraig Brown

Terry UdovicTeresa Jacques

Eugene Mamontov

VisitorsBenjamin Schmid (UO)

Mark Bowden (IRL) Rene Schellenberg (TUBF)

Rebecca Lowton (Oxford)

LANSCELuke DaemenThomas Proffen

Monika Hartl

Maciej GutowskiDon CamaioniGreg Schenter

Jun Li Venci Parvanov

Shawn KathmannJohn LinehanWendy Shaw

Andrew Lipton Paul Ellis

Jesse Sears

PNNL