Biorefining of Aucoumea klaineana wood: Impact of steam explosion on the composition and ultrastructure the cell wall

Aucoumea klaineana Pierre (okoume) is a tropical hardwood used in the timber industry in the Congo area to produce veneers and plywood. This paper described the utilization of sapwood co-products from this industry according to a biorefinery approach to generate both bioethanol and lignin. A batch Steam Explosion (SE) was used as a pretreatment after acid impregnation (H2SO4 0.25% or 0.50% w/w) at various pressure (8-19 MPa) and residence times (2.5-7.5 min). The impact of the process was examined at the molecular and parietal scales. The pulp composition was examined in terms of lignin, glucose and xylose content, cellulose crystallinity (Crl %) and degree of polymerization (DP). The conditions of high pretreatment severity led to an almost total xylans removal (residual xylans < 2%), to a strong decrease of cellulose DP and to a lignin fragmentation and relocation increasing its extractability with ethanol as solvent. At the parietal scale, the effect of the pretreatment on wood cell wall was analyzed by optical microscopy after dyeing, fluorescence microscopy and scanning electron microscopy. Enzymatic hydrolysis of the cellulosic pulps was experimented using commercial cellulase from T. Reesei. A SE pretreated wood produced at 210 degrees C during 5 min with an impregnation with 0.25 w/w% acid resulted in a glucose yield > 80% with an excellent fermentability into ethanol by S. Cerevisiae. Using these operating conditions, one ton of okoume sapwood could produce 112 kg of ethanol and 82 kg of high purity lignin.

Automated summary

This study focused on the utilization of sapwood co-products from Aucoumea klaineana Pierre in the timber industry to generate bioethanol and lignin through a biorefinery approach. Steam explosion pretreatment was used to produce ethanol and high purity lignin from the co-products.