Chemical and physicochemical pretreatment of lignocellulosic biomass: a review

Abstract

Overcoming the recalcitrance (resistance of plant cell walls to deconstruction) of lignocellulosic biomass is a key step in the production of fuels and chemicals. The recalcitrance is due to the highly crystalline structure of cellulose which is embedded in a matrix of polymers-lignin and hemicellulose. The main goal of pretreatment is to overcome this recalcitrance, to separate the cellulose from the matrix polymers, and to make it more accessible for enzymatic hydrolysis. Reports have shown that pretreatment can improve sugar yields to higher than 90% theoretical yield for biomass such as wood, grasses, and corn. This paper reviews different leading pretreatment technologies along with their latest developments and highlights their advantages and disadvantages with respect to subsequent hydrolysis and fermentation. The effects of different technologies on the components of biomass (cellulose, hemicellulose, and lignin) are also reviewed with a focus on how the treatment greatly enhances enzymatic cellulose digestibility.

Figure 1

Schematic representation (adapted from [1, 12]) of the matrix of polymers in which cellulose exists. Pretreatment of biomass by different methods removes hemicellulose and lignin from this matrix before hydrolysis.

Figure 2

(a) [89]: Yield of reducing sugars as a function of time for four different samples—cellulose (dissolving pulp of degree of polymerization 1160) dissolved in NMMO (blue circle), regenerated cellulose suspended in DI water (red square), and untreated cellulose suspended in DI water (green triangle). The initial cellulose concentration is 7.22 mg/mL, pH is 5.7 and enzyme loading is 122 FPU/g. The lines are drawn to guide the eye. The cellulase used is Accellerase 1000 obtained from Genencor. (b) Yield of sugars as a function of time for in situ hydrolysis of cellulose (the same substrate as in Figure 2(a)) in NMMO and two ionic liquids—[Emim]Ac and [Emim]DEP. The initial cellulose concentration is 7.22 mg/mL, pH is 5.7, and enzyme loading is 122 FPU/g. The lines are drawn to guide the eye.