Electrosynthesis - sustainable and disruptive Prof. Dr. Siegfried R. Waldvogel [email protected] Bingen, November 15 th , 2017
Electrosynthesis - sustainable and
disruptive
Prof. Dr. Siegfried R. Waldvogel
Bingen, November 15th, 2017
Dark side of the green world
•
• Current discussion: electrosyntheses as potential component
in smart grids
Regenerative:
• wind power
• photovoltaics
• solarthermal
• hydro power
• bio mass
Customers
„Power-to-
chemistry®" Conventional:
• Coal / gas
• nuclear power
time
supply
electricity
grid
Power sources
time
demand Energy storage,
„buffer systems“
Pros for electrosynthesis
•
• Inherently safe
• Saving metals and rare elements/resources
• No reagent waste
• Reactive power adjustable
• New synthetic approaches (short cut of many steps and IP space!)
• Power to chemicals
• Green aspects
Disruptive technology / game changer
C&EN 2017, 23-25 (March 13th)
Why higher organic molecules
•
• Reduction of CO2 only to CO cost efficient
• Electro-conversion to larger molecules (value-added)
Electroorganic synthesis
Folie Nr. 7
electroorganic
synthesis
reagents:
e.g. oxidizer
classical synthesis
no reagent waste
0.06 € per mole
(2F@3V in Germany)
Electroorganic synthesis
Folie Nr. 8
oxidation
reduction
radical reactions
radical sequences
generated bases
…
Challenges:
accumulation of product
control following up reactions
reversibility
Electroorganic synthesis
ANOD E
+
substrate
intermediate intermediate‘ intermediate‘‘
product
Electrode material: inert/electrocatalysis potential over-potential
Electrolyte: supporting electrolyte ionic strength solvation temperature convection, flow …
Current density
Org. Process Res. Dev. 2016, 20, 26-32.
Screening with product-driven criteria
• Electrode materials (10 different usually, >50 in stock)
• Electrolyte combinations (set of 5-8 tested, >200 possible)
• Product portfolio: GC/GC-MS or LC/LC-MS
• Divided/undivided cells
• Optimization:
- current density
- applied charge
- temperature
- separator material
(> 20 available)
- mediators
Org. Process Res. Dev. 2016, 20, 26-32.
Electrolysis cells (selection)
Screening: 1-4 mL
Small scale: 30-200 mL
Up-scaling to 1700 mL
Challenge: anodic cross-coupling reaction
Electrochemical potential as strong selector
Oxidation potential and nucleophilicity correlated
Potential solutions:
T. Morofuji, A. Shimizu, J. Yoshida, Angew. Chem. Int. Ed. 2012, 51, 7259.
● Separation of oxidation and coupling event: Cation-pool method
● Over stoichiometric amount of reagents: Hypervalent iodine reagents
→ Mostly not compatible with phenols.
→ Novel concept by electrolytes required!
HFIP as electrolyte
A. Berkessel et al. J. Am. Chem. Soc. 2006, 128, 8421.
L. Eberson et al. Angew. Chem. Int. Ed. 1995, 34, 2268; J. Chem. Soc., Perkin Trans. II 1995, 1735.
Electrochim. Acta, 2012, 62, 372.
ACS Catal. 2017, 7, 1846.
• Large electrochemical window
• Strong H-bonding donor
• Phase separation on nano-scale
• H-bonding network in solid state
• Increases half-life of spin centers dramatically
Decoupling of nucleophilicity and oxidation potential
electron rich • easy to oxidize
• less nucleophilic
electrophilic
solvate based
explains cross-coupling
14
A
A+•
B
B
less electron rich
nucleophilic coupling
Next level
Product Selectivity in HFIP/MeOH
Yield HFIP/MeOH
>100:1 50%
>100:1 36%
>100:1 61%
>100:1 63%
Angew. Chem. Int. Ed. 2014, 53, 5210-5213.
Merit of electrosynthesis (application)
Angew. Chem. Int. Ed. 2014, 53, 5210-5213.
Shortcut of 5-6 steps!
Phenol-phenol cross-coupling
Metal-free and reagent-free cross-coupling!
Angew. Chem. Int. Ed. 2014, 53, 5210-5213.
(VIP manuscript)
Highlighted in C&EN April 7, 2014, 34.
Highlighted in ChemCatChem 2014, 6, 2792-2795.
Partially protected biphenols
Partially protected non-symmetric biphenols:
• Selective modification of hydroxy groups
• Control of reactivity and selectivity of
desired ligands / catalysts
• Non-symmetric catalysis products via non-
symmetric ligands
Challenge for conventional synthesis:
Selective protection of chemically similar
hydroxy groups
Partially protected biphenols: Conventional synthesis
R. Francke, R. Reingruber, D. Schollmeyer, S. R. Waldvogel, J. Agric. Food. Chem. 2013, 61, 4709-4714.
Partial demethylation of protected biphenols with strong Lewis acids:
Partial protection of non-protected biphenols:
N. Nishimura, K. Yoza, K. Kobayashi, J. Am. Chem. Soc. 2010, 132, 777-790.
Protection of phenols as silyl ethers
• First direct synthesis of ABPG not viable by conventional chemical methods
• No deblocking while coupling reaction, high yields, and selectivity!
• Broad scope (synthetic scale ~ 2 g product)
Selection out of >40 screened substrate combinations
Up-scaling of anodic cross-coupling
Scale-up: 25 mL 200 mL beaker-type cell
8-fold reaction size
• No drop in yield: 84%
• Improved ratio of A:BTIPS: 1:3 1:2
• Selective coupling-reaction:
Improved work-up via short-path distillation
1,5 g ABTIPS 12,5 g ABTIPS
• Protecting group leads to torsion angle >60°, aryl moiety turns in EWG
• Improved yields and selectivity for partially protected biphenols
• Broadened substrate scope when using protection groups
• Very good yields up to 92%
• High selectivity: ≥100:1
Rationale
22 Angew. Chem. Int. Ed. 2016, 55, 11801–11805. (VIP and front cover)
Motivation – Using waste streams of pulping
cellulose
Kraft process
waste
lignin hemicellusose tall oil turpentine fraction
wood
Lignin
• Wood as superior renewable (no competition
with nutrition purposes)
• „Waste“ mostly serve for energy production
• Lignin is a side product of the Kraft process and
the most abundant, renewable source for
aromatic compounds (1 mio t/a // 70-100 mio t/a)
Lignin
• Partial use of waste stream as a feedstock
• Work-up concept should be cost efficient
Anodic treatment
Most scientist ignore down stream processing!
Electrochemical degradation of lignin
Goals:
- Selective formation of aromatic compounds
- Mild reaction conditions
- Highly selective formation of phenol derivatives
Challenges:
- Selective conversion → low yield but value-added
- Non-selective degradation → very complex mixture
26
Anodic cleavage
Vanillin
Acetovanillone
Guaiacol
Electrochemical degradation of lignin
Known conversions:[1]
27
*Based on used Kraft lignin
[1] C. Z. Smith, J. H. P. Utley, J. Hammond, J. Appl. Electrochem. 2011, 41, 363-375.
• Direct electrochemical oxidation
Yields up to 6 wt%*
• Classic oxidants
Toxic by-products
Problem: Very drastic conditions
T >150°C
p ~ 5 bar (autoclave!)
Strongly basic media
Our approach:
T <100°C
Ambient pressure
Weakly alkaline media
Challenge: anode corrosion!
Chemical background
Lignin conversion:
- inexpensive media
- electrocatalysis
- simple anodes
Initial conditions (2010):
• 3M NaOH, 80 °C
• current density: j = 0.68 mA/cm2
Nickel anodes
• Aromatic compounds already
in lignin
• Strong enrichment of vanillin
After electrolysis at Ni/Ni
Lignin upon dissolving in NaOH
Silver versus Nickel
• Ag and Ni as anodic materials
represent the key
• selective formation of vanillin
• minor components only in
traces
Electrolysis at Ag ║Ni
Electrolysis at Ni ║Ni
The anode will be the key
• up to 2.84% vanillin for electrolysis run
• highly selective formation of vanillin
• silver prone to corrosion
Yield
Nr. Anode Cathode Electrolyte Q [C g-1] V [%]* AV [%]* VS [%]*
1 Ag Ni 3 M NaOH 2703 1.20 0.66 0.21
2 Ni Ni 1 M NaOH 2662 1.03 0.11 -
3 Ni Ni 2 M KOH 2688 1.36 0.16 -
4 NiOOH NiOOH 3 M NaOH 2688 1.24 0.20 -
5 Ag/Ni Ni 1 M NaOH 2692 2.84 0.04 -
6 Monel Monel 3 M NaOH 2688 2.15 0.25 -
Nickel basis
Cobalt basis
0
0,5
1
1,5
2
2,5
3
3,5
Yie
ld o
f va
nill
in /
wt%
*
Influence of the base concentration on the yield of vanillin
Vanillinausbeuten (1 M NaOH)
Vanillinausbeuten (2 M NaOH)
Vanillinausbeuten (3 M NaOH)
Electrochemical degradation of lignin
• Important influence of the anode material
Materials with high stability against corrosion
* Based on used Kraft lignin
Yield >3 wt%* for electrolysis run
Yield (1 M NaOH)
Yield (2 M NaOH)
Yield (3 M NaOH)
Beil. J. Org. Chem. 2015, 11, 473-480.
Electrochemical degradation of lignin
- Current density strongly enhanced (only 5% of electrolysis time)
- 3D electrode / cell design
- Inexpensive electrode (Ni/P alloy)
- Stable in black liquor
Nickel foam electrodes:
Beil. J. Org. Chem. 2015, 11, 473-480.
Electrochemical degradation of lignin/black liquor
34
• Highly selective reaction
• Vanillin and acetovanillone are the predominant products which can be observed by
gaschromatic methods
time / min
time / min
Inte
nsi
ty
Inte
nsi
ty
Gaschromatogram of the lignin/BL
components prior electrolysis
Highly selective enrichment of
vanillin due to electrochemical
treatment
Electrochemical degradation of lignin
• Optimization of the anode material allows:
– Yields up to 3 wt% under mild conditions
– High stability against corrosion
– Highly selective formation of vanillin
Large amount of unreacted lignin remains!
Challenges:
• Enhanced degradation of lignin
• Recovery of the reaction products without complete
neutralization of the solution
Work-up concept
electrolysis
adsorption
next cycle
Solution: Solid phase extraction of „endangered“ products!
• allows multiple cycles of electrolysis and extraction
• new application for strongly basic anion exchange resins
• The aim is a partial degradation of the applied lignin
Work-up concept
• direct adsorption from
electrolyte/alkaline solution
• no loss
• easy regeneration of ion exchange
resin
• lignin particles remain unaffected
(size exclusion)
Adsorption via ion exchange resin
Beil. J. Org. Chem. 2015, 11, 473-480.
Ion exchange resins
Ion exchange resins work on a broad range of alkaline media:
interactions: • Coulomb • van der Waals • p,p interactions
Ion exchange resins
Folie Nr. 39
Dowex Monosphere 550a OH
quatenary ammonium functionalities stable polystyrene backbone up to 90% vanillin adsorption from alkaline solution but: low adsorbate-adsorbent ratio < 0.05 (ratio for technical purposes ~ 0.20)
Raw material black liquor
• lignin-containing liquor from Kraft process
• black liquor contains aromatic compounds: vanillin, guaiacol, acetovanillone
• 3 mg/mL aromatic compounds in black liquor
• possible loss due to over-oxidation during electrolysis
• project: adsorption of aromatic compounds before and after electrolytic
lignin degradation
• application of Dowex Monosphere 550a OH to black liquor:
adsorption of up to 74% aromatic phenols
Black liquor
D. Schmitt, N. Beiser, C. Regenbrecht, M. Zirbes, S. R. Waldvogel, Sep. Purif. Technol. 2017, 181, 8–17.
Adsorption from black liquor
• Adsorption enables easy access to four phenol derivatives
• Controlled release of adsorbed phenols by acidic treatment
• Complete depletion of black liquor regarding its content of low molecular phenols
• Anion exchange resin can be regenerated and reused
Time / min
Int.
Vanillin
Acetovanillone
Guaiacol
4,4‘-Dihydroxy-3,3‘-dimethoxy
stilbene
Total Yield of phenols up to 1.6 mg∙mL−1 black liquor
D. Schmitt, N. Beiser, C. Regenbrecht, M. Zirbes, S. R. Waldvogel, Sep. Purif. Technol. 2017, 181, 8–17.
Combined process of adsorption and anodic oxidation
• Electrochemical degradation of lignin in completely depleted black liquor?
Int.
ISTD
ISTD
Int.
time [min]
vanillin
acetovanillone
Successful anodic degradation of lignin in depleted black liquor!
Composition of completely
depleted black liquor
Composition of completely
depleted black liquor after
anodic oxidation
D. Schmitt, C. Regenbrecht, M. Schubert, D. Schollmeyer, S. R. Waldvogel, Holzforschung 2017, 71, 35–41.
Combined process of adsorption and anodic oxidation
• Combined process enables a drastic increase of the vanillin yield
Maximization of the total yield of phenols from 1.6 mg∙mL−1 up to 1.9
mg∙mL−1 by combination of adsorption and anodic degradation!
Yield
[mg∙mL−1]
adsorption anodic
oxidation
combined
yield
process step
guaiacol
vanillin
acetovanillone
4,4‘-dihydroxy-3,3‘-dimethoxy stilbene
D. Schmitt, N. Beiser, C. Regenbrecht, M. Zirbes, S. R. Waldvogel, Sep. Purif. Technol. 2017, 181, 8–17.
D. Schmitt, C. Regenbrecht, M. Schubert, D. Schollmeyer, S. R. Waldvogel, Holzforschung 2017, 71, 35–41.
Summary
Folie Nr. 44
• Electroorganic synthesis is a useful and versatile tool
• Reagent and metal-free coupling
• Lignin degradation at Ni based alloys
• Compatible with lack liquor
• Utility - workup is crucial
Acknowledgement
Waldvogel group, August 2017
Prof. Dr. Robert Francke
(Evonik)