Properties of cell walls prepared using supercritical fluids

Paul Callaghan (VUW), Robert Franich, Stefan J. Hill, and Roger Newman (Scion)

Presentation overview:

•Wood cell wallspolymerswater

•Fibre-saturation point

•Supercritical CO2

•Water extraction

•Wood material properties

•Summary

Structure of wood cell wall

H2O ?

Fibre-saturation point

Stamm et al, 1935-1971: cell water as gas, liquid, ‘solid solution’ phases

Cells from green wood –full lumens & fully-water-swollencell walls – xylem sap

FSP –empty lumens & fully-water-swollencell walls

-a chemical phenomenon at molecular/supramolecular scales

•FSP- independent of scale –log/fibres•Discrete water binding sites - OH•Exothermic wood cell wall hydration – a molecular chemical interaction

Partial specific volume of water at %mc below FSP

Stamm & Seborg, 1935; Stamm, 1967

‘Compression’ of water molecules at binding sites

200 bar

1000 bar

Wood material dynamics

Wood at variable moisture content – dimensional, conformational & MoE change

Water an integral part of the cell wall supramolecular cellulose-hemicellulose-lignin nanocomposite dynamics-

- ‘Velcro’ mechanics

Where is water located and how structured?

Difficulty in preparing wood specimens for study of FSP &material dynamics – heating, solvents, azeotropes, critical-point, high-pressure

Supercritical CO2 water extraction

Theory: Physical-chemical interaction of cell water and carbon dioxide according to Henry’s Law, the Le Chatelier Principle and the Phase Rule.

Variables:P, T, phase (gas, liquid, supercritical)

[CO2] P, 1/T

CO2 + H2O H2CO3 H+ + HCO3- H+ + CO3

2-

F = C – P + 2

The reactant’s properties

80.36 ε0 at 20 °C

40.68

108.9

1.85

Rel static permittivity

Dipole moment D

Enthalpy evap kJ/mol

Entropy evap J/(mol·K)

1.60 ε0 at 0 °C, 50 B

0

15.33 at –57.5°C

70.8

Supercritical Fluids

Phase-changedriving chemicalchange

Supercritical CO2 wood dewatering

Fibre SaturationPoint

Comparison of oven-dried (105C) and

SCCO2 dewatered radiata pine wood

Dewatering & air-drying

0

50

100

150

200

250

1 3 5 7 9 11 13 15 17 19 21 23 25

y = -33.837x + 233.2

y = -1.4143x + 36.781

FSP=28%

Dewatering & drying sequence

Wo

od

sp

ecim

en m

c (%

)

SCCO2 chemi-mechanical dewatering

Diffusion-evaporation drying

Practical specimen preparation

Wo

od

sp

ecim

en m

c (%

)

Preparation time (min)

0

50

100

150

200

250

0 2 4 6 8 10 12

5 SCCO2 – gaseous CO2 sequences

1H NMR imaging of SCCO2 dewatering

Cycle

105% 96% 94% 91% 91% mean0 168% 216% 221% 231% 223% 159%

192% 199% 207% 218% 208%165% 173% 167% 172% 123%116% 117% 117% 123% 144%

107% 146% 151% 161% 165% mean1 103% 101% 99% 91% 89% 142%

155% 167% 165% 162% 157%185% 200% 207% 209% 199%105% 116% 113% 103% 94%

91% 96% 94% 86% 83% mean2 143% 151% 157% 152% 138% 113%

82% 86% 103% 122% 84%161% 111% 75% 73% 89%124% 142% 141% 131% 105%

44% 43% 43% 43% 48% mean3 69% 72% 75% 81% 70% 59%

72% 81% 91% 95% 76%46% 50% 52% 54% 48%43% 42% 43% 44% 49%

45% 38% 36% 40% 46% mean4 50% 46% 37% 36% 38% 43%

44% 53% 56% 40% 38%39% 39% 47% 54% 48%45% 38% 38% 40% 46%

39% 39% 39% 39% 41% mean5 43% 41% 40% 40% 42% 40%

41% 45% 43% 40% 41%37% 37% 36% 37% 37%39% 39% 38% 38% 40%

SCCO2 dewatering

Preparation of woodspecimen with uniform mc distributionapproaching FSP

Re-wetting?

Further drying?

15968

402

Solid State 13C NMR Spectrum

O

HO

HO

OH

OH

OH

C1

C2

C3

C4

C5

C6

Pinus radiata wood at 12% mc

180 160 140 120 100 80 60 40 20 ppm

acetyl acetyl

methoxylignin

C1c

C1h

C4i/sC6i/s

C3,5

C2

Cellulose LigninHemicellulose

T1(H) Nutation-Diffusion NMR

Lignin

Cellulose

Hemicellulose

Lignin

T1

(H)

(ms)

Water in cell wall at FSP

NMR evidence of a water layer between the cellulose aggregate -phase and the matrix

in the supramolecular nanocomposite of green wood.

Stamm’s ‘solid-solution’ ?Cellulose

Hemicellulose

Lignin

Water layer

SCCO2 dewatering process - summary

Reaction of lumen water with SCCO2

with reversal of chemistry in gas phase

Bound water bond strength higher than enthalpy of SCCO2 reaction – no change

Wood material derived approaching FSP from green

Material suitable for wood-water dynamics & cell wall studies – Green to FSP & below

Acknowledgements

Hank Kroese, Suzanne Gallagher,

Bernard Dawson & Meeta Patel

FRST – ‘Wood Products for the Future’

Contract C04X0205