First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 211
HO HO
HO
HO
HO
HO
HO Endocellulase
Cellobiase
(β-glucosidase)
Cellulose
Cellulose (crystal)
Glucose
Cellobiose or Cellotetrose
Exocellulase
HO
HO
HO HO
HO
HO
HO
HO
HO HO HO
HO
HO
O
O O
O O
O
O
O O
O
O O O
O OO
O
O
O
O
O
O
OH OH
n
n
n
OH
OH OH OH
OH
OH
OH
OH
OH
OH OH
OH
OH OH OH
OH
OH
OH
Figure 6.14The mechanism of enzymatic hydrolysis of cellulose to glucose.
Cellulase enzymes consist of 1,4-β-d-glucanohydrolase, 4-β-d-glucanocello-
biohydrolase, andβ-glucosidase.
These enzymes belonged to three groups: endoglucanase, exoglucanase, and cellobi-
ase, respectively. Endoglucanases break down cellulose chains randomly and produce
cellubiose, glucose, and cellotriose. Exoglucanases attack the nonreducing end of
cellulose and release cellobiose units at the end. Cellubiase enzyme converts cellubiose
units to glucose (Figure 6.14).
6.7.12 Advantages of Biological Pretreatment
- Low chemical and lower energy usage
- Biocompatibility
- Formation of inhibitors in this method is lower than with other methods.
6.7.13 Defects of Biological Pretreatment
- The process is slow and complete hydrolysis is time consuming.
- There is enzymatic inhibition.
- Some of the sugars produced during hydrolysis can inhibit cellulose enzyme activity.
- Enzymes are not cost-effective.
- To maintian optimal conditions, more care must be taken.
6.8 Summary
An increased use of biofuels would contribute to sustainable development by reducing
greenhouse gas emissions and the use of nonrenewable resources. Lignocellulosic
biomass, including agricultural and forestry residues instead of traditional feedstocks
