2-19: Enhanced enzymatic hydrolysis of olive tree biomass by FeCl3 pretreatment

Olive tree biomass obtained by pruning constitutes one of the main agricultural residues in Mediterranean countries. Pruning generates a huge amount of a cheap, renewable lignocellulosic residue, lacking of alternative uses, which must be eliminated to prevent propagation of vegetal diseases. As an alternative, their conversion into ethanol and other chemicals has been proposed. For this conversion to be attained, a key step is the pretreatment, by which sugars are released from the lignocellulosic matrix.

Olive tree residue was pretreated with FeCl₃ to remove almost all of the hemicellulose present and then hydrolyzed with cellulase and β-glucosidase to produce glucose. The objective of this work is to determine the optimum pretreatment conditions, based on sugar release both from cellulose and hemicellulose fractions. Temperature (in the range 120-180ºC), FeCl₃ concentration (0.05-0.275 M) and process time (0-30 min) were selected as operation variables. Glucose and xylose composition were monitored and adjusted to a quadratic model. Non-sugar components of the hydrolysates were also determined. Enzymatic hydrolysis yields were used for assessing pretreatment performance.

Optimum FeCl₃ pretreatment conditions, based on the mathematical model, were found where the olive tree residue was pretreated at 180ºC and 0.094 M FeCl₃ for 16.9 min. Under such conditions, 100% of hemicellulose was removed, and enzymatic hydrolysis of pretreated solids in these conditions resulted in 36.9 g glucose/100 g glucose raw material; at the same time, hemicellulosic sugar recovery in the hydrolysate was 62.7%.