Electrosynthesis - sustainable and

disruptive

Prof. Dr. Siegfried R. Waldvogel

waldvogel@uni-mainz.de

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)