Toward Realization of ideal H2 Society~ What Can We 'Organic Chemists' Make a Contribution? ~Lit.seminar
10/07/07
M2. Yutaka SAGA

0. Introduction(1) Features of H2• Reported as independent substantial by H. Cavendish in 1766.
• Named as 'Hydrogen' by Lavoiser.
• The lightest element ! the heaviest element 'U' as nuclear energy.
• Harber-Bosch process: N2 + 3H2 ! 2NH3 • Trans fat synthesis• Expected as a promising energy carrier in future energy system.
<advantages><disadvantages>• Genuine clean energy ! CO2• Clean but higly cost for storage or deliver• The most abundant fuel• Abundant but not easy to extract from sources• Produced by extensive sources• The highest energy per kg• Not so high energy per L• Availability• Dangerous (take care for fire or reeking )(2) Future images of H2 societycf) Is H2 society really ideal energy system???

For example: In the case of 'Hydrogen Cars'
Officially : • the biggest reduction of greenhouse gas• economical and energy-saving device
Current condition: Almost all the aspects are inferior to electric vehicle.

•Many techinical obstacles• Political reasons???
(Oil-coal system should be favorable.)<Essence of infrastructure for H2 society>(2)Storage(1)Production(4)Conversion(3)Delivery
Technically possible

Technically impossible

Technically possible(5)Applications

(1) production
Fossil resource-derived: CO2 is the deepest energy sink.(Oil, coal, natural gas, MeOH...)H2O electrolysis-derived: Novel technology is continuously developed.
Biomass-derived: Recyclable and carbon-neutral

Inevitable CO2 emmision: Highly avoided method
Energy source should be clean one.
: Solar energy, wind energy etcRef)
review: Angew. Chem.Int. Ed. 2008, 47, 9200

Catalytic process for the selective de- andrefunctionalization is highly demanded.
Can we contribute???
(2) Storage (3) Delivery :Today's main topic(4) Convesion : Mostly relies on fuell cell
Fascinating precedent of artificial H2O electrolysis
Milstein et al Science. 2009, 324, 74> Ru-pincer ligand complex(No change in the metal oxidation)
See: Mr.Kimura's lit.seminar(10/04/21)
Milstein et al Science. 2007, 317, 790
Milstein et al J. Am. Chem. Soc. 2005, 127, 10840
Pioneering work:
Brookhart et al J. Am. Chem. Soc. 1998, 120, 7143
> Stepwise generation:
H2 thermally and O2 photochemically> Future model of artificial fuel cell - Can we contribute???

<Today's contents>

0. Introduction
1. Properties of H2 Storage
2. Methodologies for H2 Storage
3. What Can We Contribute to This Field ?
~ Organic Chemistry Approach ~4. Perspectives

1. Properties for H2 Storage
1-1. Difficulities of H2 Storage

Review: Angew. Chem.Int. Ed. 2009, 48, 2Chem. commun. 2008, 681

1. Most difficult challenge: H2 Storage must be simulated as real applications in vehicles(infrastructures)> Portable applications, then stationary applications, finally transportation purposes
2. Weight and volume> H2 has lowest energy density (< 1/ 3000 of gasoline)> Naturally harsh conditions are required (High pressure, law temperature)> Desired as compact as possible for application in vehicles

3. Efficiency = reversibility> Energy efficiency is required in order not to waste away the efficiency of fuel cell
> How to process byproducts> Recycable devices = with reversible ability to uptake/release H2
4. Applicability> Durability:
Devices are needed that allow H2 storage systems with a lifetime of 1500 cycles.
! semipermanent for gasolines> Cost " low boiling point> Safety = How to handle with dangerous H2 (fire, reeking)when accidents (also for shock, change of temperature etc)
5. Analysis> There is a lack of analyses of the life-cycle cost and efficiency for H2 storage systems.

1-2. Current situations for H2 Storage
DOE set goals for each point

However...

> Despite many developed methods,
none has yet reached the goals set by DOE.
> Goals should be simultaneously achieved.
> All have several advantages but also disadvantages.

1-3. Classifications of available methodologies for H2 Storage

1. CGH2(Compressed Gaseous Hydrogen)Physical Storage:No strong chemical bonds

2. LH (Liquid Hydrogen)1. Cryoadsorption

3. Material-based Storage

2. Carbon Materials
3. MOFs
1. Hydrolytic systems

1. Inorganic approach

2. Metal Hydrides
3. Complex Hydrides

Chemical Storage

1. FLP: Covalent bonds to metals

2. Organic approach
We ' Organic chemists' can enter these fieldsto exploit our aquired skills!!!

2. N-B containingmaterials
3. Combination
or New Entry

or New Entry

2. Methodologies for H2 Storage

2-1. Physical Storage : No strong chemical bonds between hydrogen and a host compound are involved.
2-1-1. CGH2 (Compressed Gaseous Hydrogen)> Compressed H2 greater than 70MPa> To store about 5-6 Kg to achieve vehicles with a range of 500 km.
> The best overall technical performance to date.
<Advantages>
J. Power. Sources. 2007, 165, 833• Highest energy density• Feasible to refill H2 completely within 3 min<Disadvantages>• Special vessel design to work with > 70MPa> Cylindrical design is essential.
> Three-vessel carbon composite unit
(135 kg! 600kg with steel system)
J. Power. Sources. 2007, 165, 833

2-1-2.LH2 (Liquid Hydrogen)> –253 °C, 0.1 MPa<Advantages>• Highest energy density<Disadvantages>• Significant hydrogen loses(on-board + at infrastructure)> heat flowing# increased pressure#H2 vented• Special vessel design to work with < –253°C

2-1-3.Material-based Storage> Relatively weak interactions with H2(van der Waals interactions etc )> H2 is molecularly adsorbed, not dissociated> H2 uptake is controlled by SSA(Supecific Surface Area), pore size
> Zeolites, carbon matrials, MOF(Metal-Organic Frameworks)<Disadvantages>• Low energy density@ MOF(Metal-Organic Framework): porous solids where inorganic building units are joined by organic links> provide shape/size selectivity with functionalized pores> gas purification, gas separation, H2 storage

Review: Nature. 2003,423, 725
Chem. Soc. Rev. 2009, 38, 1284

In 1965, birth year of MOFs

> 6-membered chelate> with Zn, Ni, Fe> Only thermal stabilities were tested.

> 7-membered chelate> with Th, U> 8-membered chelate> with Th

J. Appl. Polym. Sci 1965, 9, 3745

Zn4(O)O12C6 clustersROROOZn
Zn OOORZn
Zn OOONature. 1999, 402, 279

Pioneering work by O.M. Yaghi
OH

HO(BDC)OO
NEt3

ZnX2+CO2R+ H2O2

PhCl/DMF

R> Stable without solvent in pores> Without loss of framework at 300°C> Higher surface area and pore volume> Reversible sorption of gases (N2, CHCl3 etc)
! Univesal design for gas storage application
Ralated work by O.M.Yaghi

Science. 2003, 300, 1127

H2 binding site: Zn cluster> With the same MOF or derivatives

O4H2Zn8H224H2> H2 4.5 wt% at 77K, 0.8 bar> H2 1.0 wt% at r.t., 20 bar

O

H2 binding site: Ligand> Determination of H2 binding site by INS# INS :
> Clear surface/uptake relationship> Two binding sites aredetermined.

Gated absorption
Thomas et al. Science. 2004, 306, 1012
Ni(NO3)2+NNO2NNNONNi OO NO

MOF structure
J. Am. Chem. Soc. 2002, 124, 9574

> Cavities and connected windows(cavities >> windows)> Dynamical opening of windows<Adsorption-desorption isotherms>
AC: Activated Carbon
C: Control MOF
M, E: Gated absorption MOF> Hysteresis: kinetic phenomena where a sample is loded at high pressure and becomes metastable> Absorbed at high pressure and stored at lower pressure given the flexibility of frameworks

<Disdvantages><Advantages>• Significant lack of H2 capacity• Youngest in the class of porous materials• Still at the basic research phase• Fully reversibility by pressure or other control• Large pore volume to enhance H2 storage capacity• Recharging can proceed within minutesvs decrease in the strength of the interaction• Organic chemistry can enter the field???
> We can approach through complex ligand design,compared to feasible ligands(BP, DC) generally utilized.
cf) Dr.Oisaki's work: J. Am. Chem. Soc. ASAP: Covalently linked MOFs by NHCs

2-2. Chemical Storage:2-2-1.
Inorganic Approach
2-2-2-1. Hydrolytic systems: Upon reaction with metal and water , hydrogen is released with metal hydroxide or oxide.
> Any metals can be considered as H2 storage system(any metals with a redox potential below that of H+/H2 system)> Best = NaBH4
Ru cat.
NaBH4 + 4H2O
NaOH + H3BO4 +<Advantages>• Relatively high energy density<Disdvantages>• Reasonably stable to be stored• NaBO2 cannot be removed from vehicles• Liquid fuel = conventional gasoline• Irreversible

2-2-2-2. Metal hydrides

4H2 (7.3 wt%)

> 2.0 wt % H2
> Due to high atomic weight@ classical metal hydrides : K2[ReH9]
Inorg. Chem. 1964, 3, 555
Inorg. Chem. 1964, 3, 558<Ball-milling>@ Alloys : AB5 (LaNi5) (expensive) ; AB2 (A= Ti, Zr, Mg; B = V, Fe)@ Best metal hydrides : MgH2 with ball-milling# Ball-milling : Grinder used to grind materials into extremely fine powder!!  ! #  
!!!!!!!!!!!!"!#
Critical factor = Metal surface> Increased surface area and creation of defects on the surface> Faster hydrogen desorption kinetics: Milled (white) vs Unmilled (black)<Advantages>• High storage capacity : 7.7 wt%• Low material cost
• Good reversibility<Disdvantages>• High-temperature metal hydride : > 300°C with 0.1 MPa• High reactivity toward air and O2
Int. J. Hydrogen. Energy. 2007, 32, 1121

2-2-2-3. Complex hydrides: A complex anion = H covalentry bound to a metal> BH4-, AlH4-, NH2+
A metal cation> Li, Na, Mg@ Pioneering work in 1997: Doping system
J. Alloys. Compd. 1997, 253, 1
3NaAlH4 ! Na3AlH6 + 2Al +3H2 (3.7 wt%)
Na3AlH6 ! 3NaH + Al + 1.5H2 (1.9 wt%)
3NaH ! 3Na + 1.5H2> Irreversible: Rehydrogenation required too harsh condition.

Synthesis of LiBH4@ First report in 1940
H. C. Brown et al. J. Am. Chem. Soc. 1940, 62, 3429
> Not considered for reversible H2 storage

Ti dopants
3NaAlH4

Na3AlH6 + 2Al +3H2

Ti = TiCl3 or Ti(OtBu)> can be used as a reversible H2 storage material•••• : Undoped––– : Ti-doped> About the ability to supply H2> At undoped system, even at 200 °C, it takes 24h to supply H2> At Ti-doped system, it takes only 1h.

> About the ability to resupply H2(rehydrogenation)> Dramatic improvement on the rate
and the extent of rehydrogenation

J. Alloys. Compd. 2004, 370, 104> Ti with different oxidation states gives different amounts of H2(TiCl4!8 mol/Ti, TiCl3!6 mol/Ti, TiCl2!4 mol/Ti )> Formal Ti valence is zero-valent> Alloy of TiAl3?
J. Am. Chem. Soc. 2009, 131, 5032

> LiAlH4 contains > 40% more H2! Irreversibility> Ti-doped LiAlH4> Reversible H2 storage> 7.5 wt% release and uptake
at remarkably mild condition> Due to solvent Me2O effect(Me2O and Li+ interaction?)> Cat.loading of Ti
is relevant to the stability> Not new materials,
but new processes@ Summery - qualitive evaluation
Energy density(wt %) Refilling/reversibility Cost Safty/mildcondition
1. CGH2
2. LH
3. MOF
1. Hydrolytic systems
2. Metal hydrides
3. Complex hydrides
1. FLP
2. N-B> Prototype fuel-cell-powered cars(smaller-scale storage) are based on CGH2.
> For larger volume storage, LH is the most promising option.
> Other promising materials may be discovered ! no materials has yet reached a satisfactory level.
> Combination?

3.What Can We Contribute to This Field?
~ Organic Chemistry Approach~3-1. FLP(Frustrated Lewis Pairs)

Review: Angew. Chem. Int. Ed. 2010, 49, 46
Org. Biomol. Chem. 2008, 6, 1535

: Unquenched Lewis acids and Lewis bases
by steric and electronical frustration

Frustrated Lewis Pairs

Classical Lewis Adducts
B(C6F5)3 + PMe3

[Me3P B(C6F5)3]

BF(C6F5)2Cy3P

B(C6F5)3 + PCy3

J. Organometal. Chem. 1964, 2, 245@ First report in 1964

[B(C6F5)3][PPh3] or [B(C6F5)3][Py] or [B(C6F5)3][NEt3]

B(C6F5)3 + PPh3 or Py or NEt3

> Adducts were only synthesized> Afterward several reports utilizing for protection of silyl group or hydrosilylationcf) J. Org. Chem. 1999, 64, 4887 ; Org. Lett. 2000, 24, 3921

@ Pioneering work by Stephan et alFScience. 2006, 314, 1124

FF
H

H
(Me3C6H2)2P

B(C6F5)2

HFFF(Me3C6H2)2PB(C6F5)2AHBAFBF> First report with no-metal system for reversible H2 activation> P(Mes)3 is a key : too bulky to cooedinate B(C6F5)3Mechanism> First order process= Intramolecular process@ Studies for LA-LB combination
J. Am. Chem. Soc. 2007, 129, 1880
P(Mes)3 + BPh3
P(C6F5)3 + B(C6F5)3PtBu+ BMes3

N.R

PPh3 + B(C6F5)3PMe3 + B(C6F5)3

LA-LBadduct> FLP' s key = Adequate combination of LA and LB for steric or electric properties@ Application for metal-free hydrogenation
Angew. Chem. Int. Ed. 2007, 119, 8196
PG
5 mol% catNPh

H

Under 1-5 atm H2

FFPG(Me3C6H2)2PHN

B(C6F5)2HPhHCatalyst

y. 57-99 %FF@ Related studies
Angew. Chem. Int. Ed, 2008, 47, 7433

<FLP with carbene>
t

Bu

N
HNt

Bu

H

H

1 atm
BH(C6F5)3

H B(C6F5)3t

BuN

NHt

Bu

> Appropriate bulkiness= tBu substituents

@ FLP with phosphine Chem. Commn, 2008, 5966
H
PPh2 PPh2
B(C6F5)3H

PPh2 PHPh2H

H B(C6F5)3+2 bar@ FLP with amine

J. Am. Chem. Soc, 2008, 130, 14117> Second reported system> Uptake at 2 bar, 25 C
> Release at 60 C
> Utilized forhydrogenation ofenolsilylether

C-H +ÅdÅd - F-C interaction
HB(C6F5)2HN

B(C6F5)2H

H
N> Adequate sterical property Unquenched generation of FLP> C-F H-C interaction + N-H H-B interaction Stability> H2 splitting pocket
N-H +ÅdÅd - H-B interaction<Disdvantages>
Low energy density
Stability
LA only B(C6F5)3 for acceptor???

<Advantages>
Low cost, safty, easy handling
Reversibility at mild condition
Several combinations of LA/LB
Steric and electric tuning = Diversity
We can appoach through ligand design??
Combination of metals???
J. Am. Chem. Soc, ASAP

> No Lewis base was employed> Driving force=
Strong Lewis acidity + antiaromaticity> New entry

3-2. N(Nitrogen)-B(Borane) Containing Material Review : Dalton. T rans, 2007, 2613

Angew. Chem. Int. Ed, 2007, 46,
8116> Hydridic B-H bond and protic N-H bond<Advantages>
Highest energy density (19.6 wt%)
Thermally stable at ambient temperature
Chemical conversion to uptake/release H2@ H2 release
1. Solvolytic H2 release<Disdvantages>
Actual density in solvent is significantly lowered
Irreversiblity : Regeneration is rather difficult> Dehydrogenetion= enthalpically and entropically favorable
Volatile by-products (borazine etc)

NH3BH3 + H+ + 3H2O

Catalyzed byLewis/Bronsted acid
(HCl, B(C6F5)3)> Toward practical use in vehicles = solid and gaseous acid

B(OH)3 + NH4+ + 3H2

J. Power sources, 2006, 159, 855(a) Dowex (sulfonic acid + functional groups)(b) Amberlyst (sulfonic acid + functional groups)(c) Control(d) Nafion (sulfonic acid + functional groups)>With solid acid the rate of H2 release is greatly enhanced> The reaction rate depends on the type of solid acids.
> Controlled flow system???
H2 is generated only when AB is in contact with the acids
Drawbacks:> Solubility is problematic : 19.6 wt %> Volatile NH3 poisons fuel cells
2. Direct dehydrogenation by metal complexes<7 wt %

Rh = [Rh(1,5-cod)(í-Cl)]2 or RhCl3 (0.5 mol%)> First report by transition metal
Chem. commun, 2001, 962
J. Am. Chem. Soc, 2003, 125, 9424
> Ir-pincer type ligand complex with higher activity
J. Am. Chem. Soc, 2006, 128, 12048> Utilized for alkane dehydrogenation
J. Am. Chem. Soc, 2004, 126, 1804
Science, 2006, 312, 257> Theoritical analysis
Angew. Chem. Int. Ed, 2007, 46, 8153

> Higher catalyst activity than Rh cat
0.5 mol %, 14 min vs 0.6 mol % 2-4 days
> Concerted H2 transfer> Stable 5-membered ring TS> Concerted mechanism is due todissimilar natures of Hcf) ethane(protone donor + hydride donor)> Other examples> Initial N-H cleavage

Organometallics, 2007, 26, 3597
@ Regenaeration> Enthalpically and entropically unfavorable> Stronger B-O bond or B-N bond
> Requiring stronger reductant> NHC ligand> Intial B-H cleavage

J. Am. Chem. Soc, 2007,129, 1844

> Novel approach

Angew. Chem. Int. Ed, 2009, 48, 6812
<1. Digestion of PBs with thiols><2. Reduction with tins>> Thiols as first reducing reagent = weaker B-S bond> Sn reagent as second reductant> Other hydride sources failed> All reagents and byproducts are recycled> Fascinating example of C-B-N heterocycle compound

J. Am. Chem. Soc, 2010,132, 3289

ƒ¢G ~ 0 kcal/ mol
Exothermic dehydration of B-N+Endothermic dehydration of C-C> Reductant = H2 itself

Reversible H2 uptake/release

4. Future Prospects> So far comprehensively most reliable way = Liquid N2 and CGH2: We are unlikely to make a contribution...
> What is the goal? : Practical H2 storage in vehicles or new reaction/ligand design ?
Organic approach involves many methodologies/materials.
But large gaps between practical use and experimental successcf) Industrial supply of complex molecules> Low cost: Not precious metals (Pt, Ru, Rh)
Reversibility : Chemical conversion at mild condition
New catalyst system> Our strong point =We can synthesize compounds freely as we expect