Page 50 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 29

nitric acid have been used for dilute acid pretreatment, while sulfuric acid
is studied the most because it is highly effective and inexpensive [55].
Phosphoric acid has also been widely used since it is relatively cheap and
can hydrolyze biomass efficiently. Moreover, using NaOH or ammonia to
neutralize the phosphoric acid, hydrolyzate produces sodium or ammonia
phosphate, which could be used as a nutrient for microorganisms in fer-
mentation step [56].
For dilute acid pretreatment of rice straw, a maximal sugar yield of
83% could be achieved by 1% (w/w) sulfuric acid for a reaction time of
1 5 min at 160°C or 180°C, followed by enzymatic hydrolysis [57].
Pretreatment of olive tree with 1.4% sulfuric acid at 210°C resulted in a
maximum enzymatic hydrolysis yield (76.5%), while the maximum sugar
yield (36.3 g sugar/100 g raw material) was achieved with 1% sulfuric acid
at 180°C, representing 75% of all sugars in the raw material [58].
Compared to dilute sulfuric acid pretreatment of rye straw, dilute nitric
acid pretreatment was found to give higher glucose concentration [59].
Nevertheless, it is difficult to remove the by-products from nitric acid pre-
treatment by washing the pretreated substrates [60]. The applications of
diluted phosphoric acid for corn stover achieved 85% glucose yield with
0.5% (v/v) H 3 PO 4 and 10% solid loading at 180°C for 15 min [61].
Although dilute acid pretreatment presents the advantage of solubiliz-
ing hemicelluloses, the acid must be neutralized for the downstream
enzymatic hydrolysis or fermentation processes. In addition, the formation
of some sugar degradation compounds, for example, furfural and HMF,
inhibits the microorganism metabolism [62]. Corrosion caused by dilute
acid pretreatment also mandates expensive construction material.

2.3.2.2 Acid catalyzed organosolv pretreatment
Organosolv pretreatment refers to the biomass pretreatment process with
organic solvents with or without the addition of catalyst. When no exter-
nal catalyst is used, organosolv pretreatment usually requires high organic
solvent concentration ($60%) and high pretreatment temperature
(160°C 220°C) [63], which is also termed auto-catalyzed organosolv pre-
treatment. In this process the formed organic acid, such as acetic acid, can
play as a catalyst. However, to promote the pretreatment efficiency, exter-
nal catalysts, such as mineral or strong organic acids, are usually used.
Acid-catalyzed organosolv pretreatment is a composite process involving
lignin and hemicellulose degradation, solvation, and solubilization of lig-
nin fragments. The supplementary of acid catalyst could increase the