Properties of cell walls prepared using supercritical fluids Paul Callaghan (VUW), Robert Franich, Stefan J. Hill, and Roger Newman (Scion)

Presentation overview: Wood cell walls polymers water Fibre-saturation point Supercritical CO 2 Water extraction Wood material properties Summary

Structure of wood cell wall H 2 O ?

Fibre-saturation point Stamm et al, : cell water as gas, liquid, ‘solid solution’ phases Cells from green wood – full lumens & fully-water-swollen cell walls – xylem sap FSP – empty lumens & fully-water-swollen cell 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 CO 2 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) [CO 2 ]  P,  1/T CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - H + + CO 3 2- F = C – P + 2

The reactant’s properties ε 0 at 20 °C Rel static permittivity Dipole moment D Enthalpy evap kJ/mol Entropy evap J/(mol·K) 1.60 ε 0 at 0 °C, 50 B at –57.5°C 70.8

Supercritical Fluids Phase-change driving chemical change

Supercritical CO 2 wood dewatering Fibre Saturation Point Comparison of oven-dried (105  C) and SCCO 2 dewatered radiata pine wood

Dewatering & air-drying FSP=28% Dewatering & drying sequence Wood specimen mc (%) SCCO 2 chemi-mechanical dewatering Diffusion-evaporation drying

Practical specimen preparation Wood specimen mc (%) Preparation time (min) 5 SCCO 2 – gaseous CO 2 sequences

1 H NMR imaging of SCCO 2 dewatering

SCCO 2 dewatering Preparation of wood specimen with uniform mc distribution approaching FSP Re-wetting? Further drying? 159   2

Solid State 13 C NMR Spectrum Pinus radiata wood at 12% mc Cellulose Lignin Hemicellulose

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’ ?

SCCO 2 dewatering process - summary  Reaction of lumen water with SCCO 2 with reversal of chemistry in gas phase  Bound water bond strength higher than enthalpy of SCCO 2 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