ChemicalChemical Hydrides:Hydrides: AmineAmine BoranesBoranes

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

International Symposium on Materials Issues in Hydrogen Production and Storage August 2006 HydrogenHydrogenHydrogen-rich--richrich ammoniaammonia boraneborane (AB)(AB)

Hydrogen Densities of Materials 200

) NH BH (4) -3 4 4 Mg(OMe) .H2O m 2 2 TiH NH BH (3) 2 AlH3 33 150 Mg2NH4 LiNH2(2) MgH2 NH BH (2) 3 3 LiBH LaNi H 4 5 6 NaBH C H 4 decaborane 8 18 CH4 (liq) FeTiH1.7 NH3 C2H6 100 KBH C2H5OH 11M aq NaBH4 4 LiAlH4 CH3OH C3H8 CaH2 2015 system targets LiNH2(1) NaAlH4 liquid NH3BH3(1) hydrogen NaH hexahydrotriazine 50 2010 system targets 700 bar 350 bar Hydrogen volume density (kgH density Hydrogen volume

0 0 5 10 15 20 25100 30 Hydrogen mass density (mass %) George Thomas

2 NHNHxxBHBHxx StoreStore hydrogenhydrogen (>6(>6 wt%/step)wt%/step)

Wt% H2 T (K)

NH4BH4 Æ NH3BH3 + H2 6.1 <300

NH3BH3 Æ (NH2BH2)n + H2 6.5 <375

(NH2BH2)n Æ (NHBH)n + H2 6.9 >375

(NHBH)n Æ BN + H2 7.3 >>800

Two sequential steps > 12 wt% hydrogen!

Solid state chemistry: thermodynamics and kinetics of hydrogen release?

3 Thermodynamics

Reactants Products ∆E (kJ/mol)

NH4BH4(s) → NH3BH3(s) + H2 -10

NH3BH3(s) → (NH2BH2)n + H2 +37

(NH2BH2)n → (NHBH)n + nH2 -13

(NHBH)n → BN(s) + nH2 -38

ABH2 PAB+ H2 ABH2 = NH4BH4 AB = NH3BH3 AB+ H2 PAB = (NH2BH2)n

PIB = (NHBH)n PIB+ H2

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

Scanning calorimetry combined with mass detection and gravimetric analysis z Thermodynamics of hydrogen loss z Kinetics and insight into mechanism of H-release

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

Neutron spectroscopy z QENS (for dynamics of H motion) z INS (structural properties) Combination of experiment & theory to gain fundamental understanding of chemical & physical properties of AB

5 Ammonia borane isoelectronicisoelectronic with ethane

NH3BH3 CH3CH3

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

6 BackgroundBackground crystallinecrystalline NHNH33BHBH33

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

2NH3BH3 ↔ [NH3BH2NH3][ BH4]

• NH3BH3 Æ 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)

7 Thermal decomposition of solid AB

NHNH3BHBH3(s)(s) ÆÆ (NH(NH2BHBH2)(s))(s) ++ HH2

•Rates increase with temperatureNeat AB 85 °C •Activated process •Sigmoidal kinetics •Induction period •Thermodynamics 80 °C •∆Hrxn = -21±3 kJ/mol 75 °C Relative HeatRelative Released (a. u.) 70 °C

0 500 1000 1500 2000 2500 3000 Time (min) Solid state kinetics analysis 8 StabilizingStabilizing polymerizationpolymerization

+

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

9 AmmoniaAmmonia boraneborane decompositiondecomposition

-(NH2BH2NH2)-

NH3BH3 -(NH2BH(NH2)2- (NH ) BH -(NH2BH3 3 2 2 BH4

11B NMR 800 MHz

Complex mixture of cyclic, branched polymeric products 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.

NH3BH3

Add saturated solution of

NH3BH3 to SBA-15 Ammonia borane infiltrated

11 Change in Reactivity and Selectivity

Hydrogen Neat AB m/e = 2 AB:SBA-15

Borazine m/e = 80 Borazine

(BHNH)3 Relative Yield (arb. units)

50 100 150 200 250 Temperature (°C) Ammonia borane in SBA-15 1.4 1.0

1.2 Amonnia Borane on SBA Scaffold

50 °C 0.8 Extent ofReaction Scaffolds: 1.0

0.8 0.6 enhance rates of H2 release

0.6 x20 enhance purity of H2 0.4 Neat Amonnia Borane 0.4 80 °C (little borazine) Relative Heat Release 0.2 0.2 change in thermodynamics

0.0 0.0 0 100 200 300 400 500 600 Time /min Angew. Chem. Int. Ed. 2005, 44, 3578. 12 FundamentalFundamental ScienceScience QuestionsQuestions

NH3BH3 Æ (NH2BH2)n + H2 + ? <100 ˚C

HowHow isis HH22 formedformed fromfrom solidsolid AB?AB? Is the mechanism intra or intermolecular? What is the activation barrier? Can we change it with catalysis, (other) Can we control the decomposition pathways?

Need approaches to study solid state kinetics

13 Phase transformation kinetics

r (t)

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

14 AvramiAvrami raterate asas functionfunction ofof TT (70(70(70-85--8585 C)C)

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

15 ArrheniusArrhenius analysisanalysis ofof k(Avramik(Avramik(Avrami))) vs.vs. temperaturetemperature

Ea ~ 150 kJ/mol

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

16 KineticKinetic modelmodel forfor hydrogenhydrogen formationformation

Sigmoidal kinetic behavior Induction, Nucleation & Growth

Induction: z Rate limiting. How to decrease (scaffold)? Nucleation: z Physical or Chemical Change Growth: z hydrogen formation z How do you modify pathways, change thermodynamics?

1 Induction,Induction, nucleation,nucleation, growthgrowth

OpticalOptical microscopemicroscope studiesstudies

Time 0 7 min 10 min 13 min 14 min 75C Æ 100C

Crystal of AB (~0.1 mm)

2 RamanRaman microscopymicroscopy

1 2

3 4 1

2 3 4 5 5

“top” of crystal (1) is crystalline AB, bottom (5) is amorphous AB + ??. Beginning of nucleation? 3 Growth?Growth? IntermolecularIntermolecular vs.vs. IntramolecularIntramolecular

Literature: Reaction Pathways to H2 Intermolecular

2 NH3BH3 Æ NH3BH2-NH2BH3 + H2

2.02 Å or

Intramolecular

NH3BH3 Æ NH2=BH2 + H2

n NH =BH Æ (NH2BH -NH BH )n δ- δ+ 2 2 2 2 2 BH ----- HN Dihydrogen bond Hydride atoms act as Proton acceptors

4 Nucleation?Nucleation?

Do we have dehydrocoupling of ionic or neutral intermediates?

Ionic pathway + - 2 NH3BH3 ' [NH3BH2NH3] [BH4] (isomerization, no H2 loss) AB DADB

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

Neutral pathway

2 NH3BH3 Æ NH3BH2-NH2BH3 + H2

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

5 1111BB NMRNMR 800800 MHzMHz AmmoniaAmmonia BoraneBorane

T = ~90 °C Time from top to bottom: mins. NH3BH3 Red: 0 Blue: 10 Green: 15 Yellow: 20 Aqua: 25 Black: 30 Pink: 40 Bright Green: 120

-(NH BH (NH3)2BH2 -(NH BH(NH ) - 2 3 2 2 2 BH4

-(NH2BH2NH2)- NucleationNucleation seedingseeding

BH3NH3 doped with seeded material Nucleation seeding of 1 AB Isothemal 70C solid AB enhances

0.8 AB doped with AB seeded to rate of H2 release. start of exotherm

0.6 a) Heat AB for 1000 minutes 0.4 to synthesize ‘seeds’ q(mW)

0.2 b) Mix seeds with fresh AB c a 0 0 1000 2000 3000 4000 c) Heat AB mixture decreased induction time -0.2 Time(min)

What is the nucleation step?

We must know this to better control rates of H2 evolution!

7 SolidSolid statestate ABAB energeticsenergetics

Intermolecular reaction of AB leads to H2 in the solid state Ea = 155 kJ/mol Diammoniate is important intermediate

Energy Rates of H2 release can be controlled in the solid state ∆H ~ -21 kJ/mol z Enhance kinetics with rxn nucleation seeding or scaffold

AB Æ (NH2BH2)n + H2 RoleRole ofof dihydrogendihydrogen bondsbonds inin HH22 release:release:

~2Å

δ- δ+ BH ----- HN Dihydrogen bond Hydride atoms act as Proton acceptor* Use neutron scattering (INS and QENS) methods to study properties of H-rich materials.

*Klooster, et. al. JACS, 1999, 121, 6337 9 InelasticInelastic neutronneutron scatteringscattering

11 BH3NH3 Vibrational spectra 100 10 K 75 K 230 K FDS provides approach to measure low frequency Intermolecular modes vibrational modes due to intermolecular interactions 50 B-N torsion intensity Experimental data to bench B-N stretch mark theory To facilitate identification of

0 modes use selective D/H 0 500 1000 1500 2000 labeling to ‘hide’ transition. cm-1

10 VibrationalVibrational studiesstudies ofof dihydrogendihydrogen bondingbonding δ- δ+ BH ----- HN

linear

cyclic

11 ComputationalComputational analysisanalysis ofof dihydrogendihydrogen bondingbonding

δ- δ+ BH ----- HN

linear cyclic 12 NHNH33BHBH33 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

13 FutureFuture DirectionsDirections

Understanding dihydrogen bonding interactions. How universal are these interactions in H storage materials? δ- δ+ MH ----- HY Dihydrogen bond 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?

14 ContributorsContributors PNNL LANSCE Nancy Hess Maciej Gutowski Anna Gutowska Don Camaioni Luke Daemen Yongsoon Shin Greg Schenter Thomas Proffen Liyu Li Jun Li Monika Hartl Chun Lu Venci Parvanov Shari Li Shawn Kathmann NCNR Scott Smith John Linehan Craig Brown Bruce Kay Wendy Shaw Terry Udovic Ashley Stowe Andrew Lipton Teresa Jacques Abhi Karkamkar Paul Ellis Eugene Mamontov Dave Heldebrant Jesse Sears

Visitors FSU Benjamin Schmid (UO) Naresh Dalal Mark Bowden (IRL) Ozge Gunaydin-Sen Rene Schellenberg (TUBF) R Achey Rebecca Lowton (Oxford) 15