Chemical Hydrides: Amine Boranes

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1 Chemical Hydrides: Amine Boranes 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

2 Hydrogen-rich rich ammonia borane (AB) 200 Hydrogen Densities of Materials Hydrogen volume density (kgh 2 m -3 ) NH 4 BH 4 (4) Mg(OMe) 2.H2O TiH 2 AlH NH BH (3) Mg 2 NH LiNH MgH 2 (2) 2 NH 3 BH 3 (2) LiBH LaNi 5 H 4 6 NaBH 4 C 8 H 18 FeTiH CH 4 (liq) 1.7 decaborane NH 3 C 2 H 6 11M aq NaBH KBH 4 LiAlH C 2 H 5 OH 4 4 CH 3 OH C 3 H 8 CaH 2 NaAlH 2015 system targets LiNH 2 (1) 4 NH 3 BH 3 (1) NaH hexahydrotriazine 2010 system targets liquid hydrogen 700 bar 350 bar Hydrogen mass density (mass %) George Thomas 2

3 NH x BH x Store hydrogen (>6 wt%/step) Wt% H T (K) 2 NH 4 BH 4 NH 3 BH 3 + H <300 NH 3 BH 3 (NH 2 BH 2 ) n + H <375 (NH 2 BH 2 ) n (NHBH) n + H >375 (NHBH) n BN + H >>800 Two sequential steps > 12 wt% hydrogen! Solid state chemistry: thermodynamics and kinetics of hydrogen release? 3

4 Thermodynamics Reactants Products E (kj/mol) NH 4 BH 4 (s) NH 3 BH 3 (s) + H 2-10 NH 3 BH 3 (s) (NH 2 BH 2 ) n + H (NH 2 BH 2 ) n (NHBH) n + nh 2-13 (NHBH) n BN(s) + nh 2-38 ABH 2 ABH 2 = NH 4 BH 4 AB = NH 3 BH 3 PAB = (NH 2 BH 2 ) n PIB = (NHBH) n AB+ H 2 PAB+ H 2 PIB+ H 2 BH 4 + 4H 2 O B(OH) 4 + 4H 2 H = -250 kj/mol 4

5 Approaches Scanning calorimetry combined with mass detection and gravimetric analysis Thermodynamics of hydrogen loss Kinetics and insight into mechanism of H-release NMR and Raman spectroscopy Solid state to study phase transitions and molecular dynamics Variable 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 5

6 Ammonia borane isoelectronic with ethane NH 3 BH 3 CH 3 CH 3 MW Mp[ C] M[D] bonding N->B dative C-C covalent R[A] 1.58 (1.62) 1.53 kg H 2 /kg kg H 2 /liter

7 Background crystalline NH 3 BH 3 Synthesis (Shore and Parry in 1955) AB in equilibrium with diammoniate of diborane 2NH 3 BH 3 [NH 3 BH 2 NH 3 ][ BH 4 ] NH 3 BH 3 H 2 + polyaminoborane at < 370 K. H rxn = -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

8 Thermal decomposition of solid AB NH 3 BH 3 (s) (NH 2 BH 2 )(s) + H 2 Relative Heat Released (a. u.) 85 C 80 C Rates increase with temperature Activated process Sigmoidal kinetics Induction period Thermodynamics H rxn = -21±3 kj/mol Neat AB 75 C 70 C Time (min) Solid state kinetics analysis 8

9 Stabilizing 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 9

10 Ammonia borane decomposition -(NH 2 BH(NH 2 ) 2 - -(NH 2 BH 2 NH 2 )- -(NH 2 BH 3 NH 3 BH 3 (NH 3 ) 2 BH 2 BH 4 11B 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 (NH 3 BH 3 ) in the nano-phase. Should also preserve nanophase. NH 3 BH 3 Add saturated solution of NH 3 BH 3 to SBA-15 Ammonia borane infiltrated 11

12 Change in Reactivity and Selectivity Relative Yield (arb. units) Hydrogen m/e = 2 Borazine m/e = 80 Neat AB AB:SBA-15 Borazine (BHNH) 3 Relative Heat Release Temperature ( C) Amonnia Borane on SBA Scaffold 50 C x20 Neat Amonnia Borane 80 C Extent of Reaction Ammonia borane in SBA-15 Scaffolds: enhance rates of H 2 release enhance purity of H 2 (little borazine) change in thermodynamics Time /min 0.0 Angew. Chem. Int. Ed. 2005, 44,

13 Fundamental Science Questions NH 3 BH 3 (NH 2 BH 2 ) n + H 2 +? <100 C How is H 2 formed from solid 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

14 Phase transformation kinetics r (t) Avrami Equation X Crystal = 1 - exp (-(kt) n ) 14

15 Avrami rate as function of T (70-85 C) X reaction = 1 - exp (-(kt) 3 ) 15

16 Arrhenius analysis of k(avrami) ) vs. temperature Ea ~ 150 kj/mol Activation barrier for H 2 formation from solid state ammonia borane is ca. 150 kj/mol. 16

17 Kinetic model for hydrogen formation Sigmoidal kinetic behavior Induction, Nucleation & Growth Induction: Rate limiting. How to decrease (scaffold)? Nucleation: Physical or Chemical Change Growth: hydrogen formation How do you modify pathways, change thermodynamics? 1

18 Induction, nucleation, growth Optical microscope studies Time 0 75C 100C 7 min 10 min 13 min 14 min Crystal of AB (~0.1 mm) 2

19 Raman microscopy top of crystal (1) is crystalline AB, bottom (5) is amorphous AB +??. Beginning of nucleation? 3

20 Growth? Intermolecular vs. Intramolecular Literature: Reaction Pathways to H 2 Intermolecular 2 NH 3 BH 3 NH 3 BH 2 -NH 2 BH 3 + H Å or δ- δ+ BH HN Dihydrogen bond Intramolecular NH 3 BH 3 NH 2 =BH 2 + H 2 n NH 2 =BH 2 (NH 2 BH 2 -NH 2 BH 2 ) n Hydride atoms act as Proton acceptors 4

21 Nucleation? Do we have dehydrocoupling of ionic or neutral intermediates? Ionic pathway 2 NH 3 BH 3 [NH 3 BH 2 NH 3 ] + [BH 4 ] - (isomerization, no H 2 loss) AB DADB NH 3 BH 3 + [NH 3 BH 2 NH 3 ] + [BH 4 ] - [NH 3 BH 2 NH 2 -BH 2 NH 3 ][BH 4 ] - + H 2 Neutral pathway 2 NH 3 BH 3 NH 3 BH 2 -NH 2 BH 3 + H 2 If [NH 3 BH 2 NH 3 ] + [BH 4 ] - is an intermediate, should see it by in-situ solid state 11 B NMR 5

22 11 B NMR 800 MHz Ammonia Borane Time from top to bottom: mins. Red: 0 Blue: 10 Green: 15 Yellow: 20 Aqua: 25 Black: 30 Pink: 40 Bright Green: 120 NH 3 BH 3 T = ~90 C -(NH 2 BH(NH 2 ) 2 - -(NH 2 BH 3 (NH 3 ) 2 BH 2 BH 4 -(NH 2 BH 2 NH 2 )-

23 Nucleation seeding BH3NH3 doped with seeded material AB Isothemal 70C AB doped with AB seeded to start of exotherm Nucleation seeding of solid AB enhances rate of H 2 release. q(mw) a) Heat AB for 1000 minutes to synthesize seeds c a Time(min) b) Mix seeds with fresh AB c) Heat AB mixture decreased induction time What is the nucleation step? We must know this to better control rates of H 2 evolution! 7

24 Solid state AB energetics Energy E a = 155 kj/mol H rxn ~ -21 kj/mol Intermolecular reaction of AB leads to H 2 in the solid state Diammoniate is important intermediate Rates of H 2 release can be controlled in the solid state Enhance kinetics with nucleation seeding or scaffold AB (NH 2 BH 2 ) n + H 2

25 Role of dihydrogen bonds in H 2 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,

26 Inelastic neutron scattering intensity K 75 K 230 K Intermolecular modes B-N torsion B-N stretch cm-1 11 BH 3 NH 3 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. 10

27 Vibrational studies of dihydrogen bonding δ - δ + BH HN linear cyclic 11

28 Computational analysis of dihydrogen bonding δ - δ + BH HN linear cyclic 12

29 NH 3 BH 3 Summary 12 wt% hydrogen at relatively low temperatures (<400 K) exothermic ~ 20 kj/mol need to regenerate by chemical pathway Mechanism of H 2 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

30 Future Directions 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., MgH 2 ---H 3 NBH 3 ---H 2 NLi Are thermodynamics of alternative reactions more favorable? 14

31 Nancy Hess Anna Gutowska Yongsoon Shin Liyu Li Chun Lu Shari Li Scott Smith Bruce Kay Ashley Stowe Abhi Karkamkar Dave Heldebrant PNNL Contributors Maciej Gutowski Don Camaioni Greg Schenter Jun Li Venci Parvanov Shawn Kathmann John Linehan Wendy Shaw Andrew Lipton Paul Ellis Jesse Sears LANSCE Luke Daemen Thomas Proffen Monika Hartl NCNR Craig Brown Terry Udovic Teresa Jacques Eugene Mamontov Visitors Benjamin Schmid (UO) Mark Bowden (IRL) Rene Schellenberg (TUBF) Rebecca Lowton (Oxford) FSU Naresh Dalal Ozge Gunaydin-Sen R Achey 15