Front Matter

 

208 Introduction to Renewable Biomaterials

OH

OH

OH

OH OH OH

O+ O O+

O

OHCH (^2) H
CH 2 OH
OH
OH
OH
OH OH OH
OO
O
CH 2 OH H+
CH 2 OH
OH
OH
OH
OH OH OH
OO
O
CH 2 OH H
CH 2 OH
OH
OH
OH + H+
OH OH OH
OH
O
OH
O
CH 2 OH
CH 2 OH
O
C+
OH
OH
OH
OH OH OH
O
O
OHCH 2 H
CH 2 OH

• H 2 O + H 2 O
  OH
  OH
  OH
  OH OH OH
  OH
  O
  C+ H O
  CH 2 OH
  CH 2 OH
  Figure 6.12The major mechanism of acid hydrolysis of glycosidic bonds. Adapted from Fengel and
  Wegener (1984).
  use of the Scholler method dates back to World War II in Germany. American forest
  products laboratory improved Scholler method and increased the production yield of
  ethanol to 64 Gal per ton of wastes in a mere 3 h hydrolysis time. This method is named
  Madison. This procedure was never truly established commercially because it could
  not compete effectively with ethanol derived from petroleum sources.

6.7.5 Mechanism of Acid Hydrolysis

At first, we should say, acid hydrolysis is a relatively efficient method to break cellulose

component. Principal catalyst in this method is hydrogen aqueous ions (4 A∘). Bigger

particles (to 51 A∘) can penetrate into pores of microfibrils; therefore hydrogen ions can

penetrate more easily and not face the problem of accessibility compared to cellulase

enzymes. The main mechanism of acid hydrolysis is relatively simple (Figure 6.12). It

is similar to hydrolysis of other glycosidic bonds such as starch (α 1 →4linkedglucose

chains, withα 1 →6 branches).

Step 3 (Figure 6.12) is the rate-limiting step of the process because of the formation

of the high-energy half-chair configuration by the cyclic carbonium ion (Fengel and

Wegener, 1984, Goldstein, 1983).

Initial hydrolysis rates are typically very rapid (Goldstein, 1983). Grethlein (1991)

performed experiments to show that in the initial steps of the hydrolysis reaction,

larger pore volumes do correspond to faster reaction rates. However, after limited

hydrolysis, the reaction rate slows down considerably (Goldstein, 1983). The glycosidic

bonds that are most susceptible to hydrolysis are those either at the surfaces or in the

amorphous regions of cellulose. Rapid hydrolysis rates reflect hydrolysis activity in

these regions and can be seen as a decrease in the DP from several thousand to about

200 (Ladisch, 1989). This point is referred to as the leveling of degree of polymerization

(LODP). Further hydrolysis is much more difficult beyond the LODP because of the

high crystallinity of the remaining cellulose molecules.

6.7.6 Alkaline Pretreatment

In the alkaline pretreatment of lignocellulosic biomass, different chemicals such

as NaOH, Ca (OH) 2 , and KOH are used (Figure 6.13). This procedure is used for

delignification, and it is more effective on agricultural wastes. A large portion of lignin