Xu DuRenewable Bioproducts Institute

School of Chemical & Biomolecular Engineering

Georgia Institute of Technology

Low Energy Catalytic Electrolysis for

Simultaneously Hydrogen Evolution and

Lignin Depolymerization

2

Background

Aromatic ChemicalsPetroleum Sustainable

Biomass

Hydrogen

Steam Reforming

Natural Gas

Catalytic Cracking

Lignin:

Aromatic Rich Structure

Aromatic

ChemicalsGreat Potential

Cellulose Lignin Hemicellulose

Background

1. Weinstock, I.A., et al., Equilibrating metal-oxide cluster ensembles for oxidation reactions using oxygen in water. Nature, 2001. 414(6860): p. 191-195.2. Voitl, T. and P.R. von Rohr, Oxidation of Lignin Using Aqueous Polyoxometalates in the Presence of Alcohols.Chemsuschem, 2008. 1(8-9): p. 763-769.3. Chen, Y.X., et al., Nanotechnology makes biomass electrolysis more energy efficient than water electrolysis. Nature Communications, 2014. 5.

PEM (Proton Exchange Membrane) Electrolysis

Lignin depolymerization by POMs (Polyoxometalates) or Fe3+

Combination?E (Applied potential) >

1.23 V (Standard potential)

3

POM

Pt/

1. Lignin depolymerization 2. Electrolysis to hydrogen

E (lignin) < E (POMOx) or E (Fe3+)

0.6-1.2 V0.6-0.8 V

Thermodynamic Conditions

Electrolysis

POMOx(Fe3+)+H2O+lignin → H-POMRed(Fe2+)+Oxidized products (or CO2)

>0.77 V

Oxidation

E (Applied potential) > E (POMRed) or E (Fe2+)

Anode: used for electron transfer; Graphite replaced Pt

Reduction of applied potential (energy)

Lignin depolymerization

Schematic illustration of the lignin depolymerization and electrolysis

4

Results-Lignin depolymerization

5

POM: H3PMo12O40, noted as PMo12

0 5 10 15 20 25 300.0

0.5

1.0

1.5

2.0

2.5a

PM

o1

2 R

ed

uction

De

gre

e

Reaction Time (hour)

PMo12

-Kraft Lignin

PMo12

-Alkali Lignin

PMo12

-Sulfonated Lignin

0 5 10 15 20 25 300.0

0.1

0.2

0.3

0.4

0.5b

Fe

2+ c

on

ce

ntr

atio

n (

mo

l L

-1)

Reaction Time (hour)

FeCl3-Kraft Lignin

FeCl3-Alkali Lignin

FeCl3-Sulfonated Lignin

Fe3+: FeCl3

Reduced PMo12 / Fe2+

• Reduction Degree: the average number of electrons (mole) that were transferred from

the substrate to one mole of the PMo12 anion

• Fe2+ concentration

100 oC with N2 protection

PMo12: 0.1 mol/L

Lignin: 10 g/L

100 oC with N2 protection

Fe3+: 1 mol/L

Lignin: 10 g/L

6 h/cycle

6 h/cycle

6

The reduction of the catalyst (PMo12 or Fe3+) is accompanied by the oxidation of lignin

2D HSQC NMR 31P NMR FTIR XPS

TOC

GC-MS

GC

-functional groups -OH groups -functional groups

-amount of depolymerized products

-products in liquid phase

GPC-dissolved products weight

distribution

-gas products: CO2 and a little

methane (CH4)

-C/O ratio, C-O bonds

Results-Lignin depolymerization

PMo12: 0.1 mol/L100 oC with N2 protection; Lignin: 10 g/L; 6 h/cycle * 3 cycles = 18 h

Fe3+: 1 mol/L

7

Results-Lignin depolymerization

Solvent: Ethyl ether

GC-MS

Liquid

The PMo12 or FeCl3 can depolymerize the lignin to small aromatics.

100 oC with N2 protection;

Lignin: 10 g/L;

6 h/cycle * 3 cycles = 18 h

8

PMo12+co-catalyst, 150oC, 160 psi O2 80 min Products in liquid (conversion) ~ 90%

Results-Lignin depolymerization

9

Results-Lignin depolymerization

Solid Ether bond

2D HSQC

KL-PMo12: most of the ether linkages were broken with only a tiny amount of resinol (β-β) detected.

KL-FeCl3: no ether linkages observed

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Results-Lignin depolymerization

Solid31P NMR

Aliphatic OH

FTIR

-OH

Intensity of 3375 (–OH)

decrease, 1705 (C=O)

carbonyl increase and

wavenumber shift.

All the aliphatic –OH were

consumed and only a small

amount of –OH in guaiacyl

and catechol structures

(138.18-140.20) left after the

reactions.

PMo12 and FeCl3 both can oxidize the –OH group of lignin and cleave most ether linkages.

Results-PEM Electrolysis to Hydrogen

Electrolysis Performance

0.0 0.2 0.4 0.6 0.8 1.0 1.20.0

0.1

0.2

0.3

0.4

0.5b

Curr

ent

density (

A c

m-2)

Potential (V vs RHE)

1M H3PO

4

1M H3PO

4-KL

Fe3+

-KL-6h

Fe3+

-AL-6h

Fe3+

-SL-6h

0.0 0.2 0.4 0.6 0.8 1.0 1.20.0

0.1

0.2

0.3

0.4

0.5c

Cu

rre

nt

de

nsity (

A c

m-2)

Potential (V vs RHE)

Fe3+

-KL 1h

Fe3+

-KL 6h

Fe3+

-KL 10h

Fe3+

-KL 18h

Fe3+

-KL 28h

• the lignin alone cannot be directly used as feedstock

• the source of lignin did not make a significant difference on the electrolysis performance.

• the concentration of Fe2+ is the key factor

• PMo12-electrolysis system: similar conclusion

• Current density (Fe3+>PMo12 system when applied potential > 0.8V): high concentration

Without catalyst

0.0 0.2 0.4 0.6 0.8 1.0 1.20.0

0.1

0.2

0.3

0.4e

Cu

rre

nt

de

nsity (

A c

m-2)

Potential (V vs RHE)

0.1M PMo12

-KL-6h

1M Fe3+

-KL-6h

0.1M Fe3+

-KL-6h

11

Energy saved: 40.9%

Energy saved: 46.5%

12

Results-PEM Electrolysis to Hydrogen

Faraday (Current) Efficiency & Energy Consumption

Current density:100 mA cm-2 Average Faraday Efficiency

PMo12-KL: 24 ml/26.52ml (Theoretical)= 90.48%

FeCl3-KL: 24 ml/25.89ml (Theoretical)= 92.71%

Energy Consumption

PMo12-KL: 2.54 kW h Nm-3

Alkaline water electrolysis: 4.3 kW h Nm-3

FeCl3-KL: 2.30 kW h Nm-3

Faraday efficiency: efficiency of electrons utilization

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Conclusion

1. Combined lignin depolymerization and hydrogen production by a novel proton

exchange membrane (PEM) electrolysis process

2. Oxidation process: About 14% of Kraft lignin was converted to aromatic chemicals

in three cycles reaction; most of ether linkages were cleaved; most of –OH groups

were reacted

3. Electrolysis process: Noble metal catalyst at anode was replaced by cheap carbon

based material – Graphite; Faraday efficiency was higher than 90%; Saved more

than 40% electric energy in our electrolysis process comparing with the best

alkaline water electrolysis

Thank You14

Acknowledgement

Dr. Yulin Deng @GT, RBI/ChBE

Wei Liu @GT, RBI/ChBE

RBI PSE Fellowship

xudu@gatech.edu