The Interconversion of Ammonia with Its Elements
2018 Annual Meeting of the Andlinger Center
Paul J. Chirik and Robert R. Knowles Department of Chemistry Princeton University According to Nature, who are the most influential scientists of the 20th century? According to Nature, who are the most influential scientists of the 20th century?
Fritz Haber Carl Bosch 1918 - Nobel Prize 1931 - Nobel Prize According to Nature, who are the most influential scientists of the 20th century?
Fritz Haber Carl Bosch 1918 - Nobel Prize 1931 - Nobel Prize
“England and all civilized nations stand in deadly peril.” “As mouths multiply, food sources dwindle. Humans will begin to die of hunger in large numbers by 1930.”
Sir William Crookes (1898) According to Nature, who are the most influential scientists of the 20th century?
Fritz Haber Carl Bosch 1918 - Nobel Prize 1931 - Nobel Prize
“England and all civilized nations stand in deadly peril.” “As mouths multiply, food sources dwindle. Humans will begin to die of hunger in large numbers by 1930.”
Sir William Crookes (1898) Population Explosion
Population (in Billions) 9-11 Billion 10 (2050)
8 6.7 Billion
6 (2008)
Haber-Bosch 4 (1913)
2
Chemistry eradicated world hunger, starvation is mostly political, economic… Ammonia: A Most Important Molecule
Calibration: One Haber-Bosch plant makes 1 ton NH3 per minute! Ammonia: A Most Important Molecule
Calibration: One Haber-Bosch plant makes 1 ton NH3 per minute!
2-3% of the world’s energy supply Ammonia: A Most Important Molecule
Calibration: One Haber-Bosch plant makes 1 ton NH3 per minute!
2-3% of the world’s energy supply
Transportation: 30%, lighting: 7%
Distillation: 10-13% Ammonia: A Most Important Molecule
Calibration: One Haber-Bosch plant makes 1 ton NH3 per minute!
2-3% of the world’s energy supply
Transportation: 30%, lighting: 7%
Distillation: 10-13%
Supports 50% of the world’s population Ammonia: A Most Important Molecule
Calibration: One Haber-Bosch plant makes 1 ton NH3 per minute!
2-3% of the world’s energy supply
Transportation: 30%, lighting: 7%
Distillation: 10-13%
Supports 50% of the world’s population 60% of the N atoms in the human body Industry versus Nature
What is more energy efficient? Nitrogenase Enzymes enzyme - + N2 + 8e + 8 H 2 NH3 + H2 + 16 Pi
16 ATP 16 ADP 23 ºC, 1 atm Industry versus Nature
What is more energy efficient? Nitrogenase Enzymes enzyme - + N2 + 8e + 8 H 2 NH3 + H2 + 16 Pi
16 ATP 16 ADP 23 ºC, 1 atm
Chemical Overpotential = 1.2 V! Industry versus Nature
What is more energy efficient? Nitrogenase Enzymes enzyme - + N2 + 8e + 8 H 2 NH3 + H2 + 16 Pi
16 ATP 16 ADP 23 ºC, 1 atm
Chemical Overpotential = 1.2 V!
Haber-Bosch Process
Fe/Ru on Al2O3
N2 + 3 H2 2 NH3 400 ºC, 400 atm
Chemical Overpotential = 0.55 V
BASF, Ludwigshafen A Closer Look at Haber-Bosch
Bosch (1932): “Improvements to the catalyst will have minimal impact” (still true today). A Closer Look at Haber-Bosch
Bosch (1932): “Improvements to the catalyst will have minimal impact” (still true today).
H-B Catalyst is only ~8% of the energy! Most in the H2 synthesis.
Pfromm, P. H. J. Renew. Sus. Energy 2017, 9, 034702. Why Study Ammonia Synthesis? Why Study Ammonia Synthesis?
It Has a Carbon Footprint!
Fe/Ru on Al2O3
N2 + 3 H2 2 NH3 400 ºC, 400 atm
600 kg of CO2 per 1000 kg NH3.
7% of global CO2 emissions. Why Study Ammonia Synthesis?
It Has a Carbon Footprint! Coal
Fe/Ru on Al O 2 3 Natural Gas 27 % Gas: 2.1 t CO2 / t NH3
N2 + 3 H2 2 NH3 Coal: 3.3 t CO2 / t NH3 400 ºC, 400 atm 60 %
600 kg of CO2 per 1000 kg NH3.
7% of global CO2 emissions. Why Study Ammonia Synthesis?
It Has a Carbon Footprint! Coal
Fe/Ru on Al O 2 3 Natural Gas 27 % Gas: 2.1 t CO2 / t NH3
N2 + 3 H2 2 NH3 Coal: 3.3 t CO2 / t NH3 400 ºC, 400 atm 60 %
600 kg of CO2 per 1000 kg NH3.
7% of global CO2 emissions.
H-B Is Capital Intensive
Capital Investment: ~$1B + infrastructure
Break even: 7 years; Total lifetime: 15-30 years Why Study Ammonia Synthesis?
It Has a Carbon Footprint! Coal
Fe/Ru on Al O 2 3 Natural Gas 27 % Gas: 2.1 t CO2 / t NH3
N2 + 3 H2 2 NH3 Coal: 3.3 t CO2 / t NH3 400 ºC, 400 atm 60 %
600 kg of CO2 per 1000 kg NH3.
7% of global CO2 emissions.
H-B Is Capital Intensive H-B for the Developing World
Batch (not flow) synthesis.
Intermittent (renewable) H2.
New catalysts are needed!
Capital Investment: ~$1B + infrastructure
Break even: 7 years; Total lifetime: 15-30 years The Case for Ammonia as a Fuel
NH3 as a Hydrogen Carrier 40! 35! gasoline ! 2 NH3 N2 + 3 H2 30!
Low ED/V 25! ethanol 20! propane (14 bar)
15! ammonia (10 bar) methanol 10! methane (250 bar)
EnergyDensity (MJ/L) hydrogen (700 bar) 5! batteries 0! 0! 20! 40! 60! 80! 100! 120! 140! 160! Specific Energy (MJ/kg)! The Case for Ammonia as a Fuel
NH3 as a Hydrogen Carrier 40! 35! gasoline ! 2 NH3 N2 + 3 H2 30!
Low ED/V 25! ethanol 20! propane (14 bar)
15! ammonia (10 bar) methanol 10! methane (250 bar)
EnergyDensity (MJ/L) hydrogen (700 bar) 5! NH3 as a Fuel batteries 0! 0! 20! 40! 60! 80! 100! 120! 140! 160! Specific Energy (MJ/kg)! 4 NH3 + 3 O2 2 N2 + 6 H2O
X-15 Aircraft (Supersonic) Belgian Buses (WWII) Overview of Today’s Talk: PCET for N-H Formation and Breaking
Ammonia Synthesis
H H + (H+ + e-) + (H+ + e-) [M] N N [M] N N [M] N N H What are N-H bond strengths?
What are the optimal reagents for promoting efficient N-H bond formation? Overview of Today’s Talk: PCET for N-H Formation and Breaking
Ammonia Synthesis
H H + (H+ + e-) + (H+ + e-) [M] N N [M] N N [M] N N H What are N-H bond strengths?
What are the optimal reagents for promoting efficient N-H bond formation?
Hydrogen Evolution
H
[M] N H [M] NH2 + H H H
What does this value need to be for H2 loss?
How do we enable “bond weakening by coordination”? A Fundamental Chemical Challenge
Many N-H bonds are <49 kcal/mol (weak!). How do we overcome thermodynamic limitations?
iPr H H N N iPr N t Mo Ph H iPr Bu t Ph2 2 N BuAr P N N P HIPT iPr NtBuAr Mo N P P N Ph HIPT Mo N iPr iPr Ph2 N 2 N N N
BDFE = 41 kcal/mol BDFE = 23 kcal/mol BDFE = 43 kcal/mol
Blue light: ~ 55 kcal/mol Proton Coupled Electron Transfer
e– transfer
PCET Protons and electrons are exchanged n n–1 M R H B M R H B together in a concerted elementary step
H+ transfer Proton Coupled Electron Transfer
e– transfer
PCET Protons and electrons are exchanged n n–1 M R H B M R H B together in a concerted elementary step
H+ transfer
Improved Kinetics
mechanism: H+ e– PT/ET ET/PT e– PCET H+
+ –
Energy (H /e )
Reaction Coordinate Proton Coupled Electron Transfer
e– transfer
PCET Protons and electrons are exchanged n n–1 M R H B M R H B together in a concerted elementary step
H+ transfer
Improved Kinetics Prevalent in Biology (PSII, RNR, lipoxygenases, etc)
histdine 190
+ mechanism: H HN His HN His e– PT/ET N N ET/PT H e– PT H Tyr Tyr PCET O O H+
tyrosine Z 7 -1 tyrosyl radical kPCET ~ 1 x10 s ET proton-coupled + – electron transfer
Energy (H /e )
Me Me Me Me R R N N O N N O Mg Mg N N N N CO Me Me 2 Me CO2Me
Me Me Me Me chlorophyll Reaction Coordinate P680+
• H-bonding lowers the potential of the phenol by ~ 450 mV
• Removal of His190 renders PSII completely inactive Light Driven, Multi-Site Reduction PCET
Hydrogen Atom Donor Pairs
H+ Reductant Acid E1/2 (V) pKa ‘BDFE’ H B B H Cp Co PhCO H -1.34 21.5 54 N N 2 2 PCET N N Cp*2Co lutidinium -1.47 14.1 40 R2N R2N Mo NR2 Mo NR2 I R2N Ru (bpy)3 pyridinium -1.71 12.5 33 R2N Mn Mn+1 – e I Ru (bpy)3 pTSA -1.71 8.6 27 ΔG° = N-H BDFE – PCET 'BDFE' *Ir(ppy)3 (PhO)2PO2H -2.11 13 24
Challenging N-H bonds can be achieved by choice of reductant and acid.
We will rely on photocatalyst to convert light energy to redox potential.
Bordwell, F. G.; Zhang, X.-M. Acc. Chem. Res. 1993, 26, 510-517. Warren, J. J. Tronic, T. A. Mayer, J. M. Chem. Rev. 2010, 110, 6961-7701.
Wang, Knowles, Chirik unpublished Targeting a More Robust Complex
First N-H bond formation transfer is the most challenging in N-H formations step.
Wang, Knowles, Chirik unpublished Successful Ammonia Synthesis
tBu N tBu t t 5% Ru(bpy)3(PF6)2 Bu O Mn O Bu + - 10% NMe3H BF4 red N N + acrH2 NH3 + [Mn] + acr blue LED THF, 23 ºC
Change to Reaction Condition NH3%
1 - 44
N 2 dark 0 H acrH2 3 no Ru, no NMe3H BF4 3
4 no NMe3H BF4 5
N 5 no Ru 15 acr 6 Hantzsch ester instead of acrH2 10
7 CH2Cl2 as solvent 57
Light, acid, reductant and photocatalyst are all required for NH3 synthesis.
Wang, Knowles, Chirik unpublished Why 57% Yield and Where is the Manganese Going?
CO t t Bu N Bu NH3 (30%) tBu O O tBu Mn [H], hν N N [Mn]red – NH3 (57%) TMEDA NNHH33 (42%)(42%)
Total NH3: 57% (free) + 42% (bound) = 99% Why 57% Yield and Where is the Manganese Going?
CO t t Bu N Bu NH3 (30%) tBu O O tBu Mn [H], hν N N [Mn]red – NH3 (57%) TMEDA NNHH33 (42%)(42%)
Total NH3: 57% (free) + 42% (bound) = 99%
Fate of the Mn:
tBu N tBu tBu NH3 tBu C2Cl6 tBu Cl tBu [H], hν or NCS t t t t t t Bu O Mn O Bu Bu O Mn O Bu Bu O Mn O Bu - 57% NH N N 3 N N N N
(UV-Vis)
aq. NH3, bleach 87-92% (from starting MnN)
Mass balance: NH3 (99%) + Mn (~90%) Comparison of Catalyst Pairs with Varied BDFEs
tBu N tBu 5% redox cat. t O O t Bu Mn Bu 10% acid cat. red N N + acrH2 NH3 + [Mn] + acr blue LED CH2Cl2, 23 ºC
redox cat. acid cat. ‘BDFE’ NH3%
+ - 1 Ir(ppy)2(dtbpy)(PF6) NMe3H BF4 35 50
N 2 Ru(bpy) (PF ) NMe H+ BF - 40 57 H 3 6 2 3 4 acrH2 3 Ru(bpy)3(PF6)2 PhCO2H 45 55
t 4 Ir(ppy)2(dtbpy)(PF6) BuC6H4SO2NH2 46 50 N 5 Ru(bpy) (PF ) HOAc 48 20 acr 3 6 2
6 Ru(bpm)3(PF6)2 PhCO2H 55 19
t 7 Ru(bpm)3(PF6)2 BuC6H4SO2NH2 59 24
8 - - - 15
Wang, Knowles, Chirik unpublished Proposed Mechanism for Ammonia Synthesis
hv RuII + NMe3H acr
proton transfer RuII* RuII acrH+ NMe H+ 3 NMe3 PCET (Ru + H+) III II electron H-atom Mn NH2 Mn NH3 acrH or HAT 2 transfer transfer (acrH2) MnIVNH
I II Ru acrH2 Ru acrH2 + NMe3H proton-coupled NMe3 electron transfer
MnVN MnIVNH
Wang, Knowles, Chirik unpublished Team Ammonia
Dian Wang
Máté Bezdek Sangmin Kim Dr. Florian Loose (w/ Rob Knowles) Comparison of Catalyst Pairs with Varied BDFEs (in THF)
tBu N tBu 5% redox cat. t O O t Bu Mn Bu 10% acid cat. red N N + acrH2 NH3 + [Mn] + acr blue LED THF, 23 ºC
redox cat. acid cat. ‘BDFE’ NH3%
1 Ir(ppy)2(dtbpy)(PF6) NEt3HBF4 41 69
2 Ru(bpy) (PF ) NEt HBF 45 44 N 3 6 2 3 4 H acrH2 3 Ir(ppy)2(dtbpy)(PF6) pCNC6H4CO2H 52 30
+ - 4 Ru(bpy)3(PF6)2 TBDH Cl 53 13
N 5 Ru(bpy)3(PF6)2 mNO2C6H4CO2H 56 3 acr
6 Ru(bpy)3(PF6)2 PhCO2H 61 8
+ - 7 Ru(bpm)3(PF6)2 TBDH Cl 63 5
t 8 Ru(bpy)3(PF6)2 BuC6H4SO2NH2 68 2
Wang, Knowles, Chirik unpublished