Lignocellulosic Biomass Processing - Food and Industrial Bioproducts and Bioprocessing - page 291

The basic conditions for liquefaction are temperature and solvent, and sometimes
catalyst. The conventional liquefaction of lignocellulosics requires one to several hours of
treatments at 300-400 °C with or without catalysts. More recently developed liquefaction
processes can be conducted in organic solvents at temperatures of 240-270°C without
catalysts or at temperatures from 80 to 150 °C with acidic catalysts. The newer processes
can result in a yield around 90-95% compared with only 40-60% for the conventional
processes. Much research has been centered on the selection of liquefying reagents and
catalysts for particular biomass.
Pu and Shiraishi (1993a, 1993b) studied the liquefaction of air-dried wood chips or meals
in the presence of phenols at about 250 °C. They found that the biomass could be converted
to dioxane-soluble products within a few hours. Heating temperature greatly affected the
liquefaction rate; for example, it took less than 30 min at 280 °C and more than 1.5 days at
200 °C for complete liquefaction. The presence of water accelerated the liquefaction, and
even the existence of water content equivalent to that in green wood (100-150% by dry
mass) facilitated the liquefaction (Pu and Shiraish, 1993b). They also found that stronger
acids were more effective than weak acids in accelerating the liquefaction process while
alkali or alkaline salt retards the reaction (Pu and Shiraish, 1994). The converted wood-
phenol liquid was fairly fluid even at room temperature. In the initial stage of the liquefaction
(about 10min at 250°C), the majority of lignin was liquefied rapidly if the amounts of
phenol were enough. The cellulosic component was most resistant to liquefaction.
Yamada and Ono (1999) used ethylene carbonate (EC) or propylene carbonate (PC) in
the presence of acid catalyst at elevated temperature (120-150°C) in their liquefaction
process converting wood and cellulose waste into chemicals. It was found that the rate of the
EC liquefaction of cellulose was approximately 10 times faster than that of polyhydric
alcohol liquefaction. However, not all biomass responds to liquefying reagent the same way.
Satisfactory liquefaction seems to be dependent on the type of lignin, that is hardwood
lignin or softwood lignin. For example, when applied to softwood (Japanese cedar and
Japanese cypress), liquefaction could not be accomplished. Yamada and Ono (1999) solved
this problem by blending ethyleneglycol (EG) with EC. 13 C-NMR revealed that the EC
liquefaction products from cellulose include levulinic acid compounds, which also result
from EG liquefaction of cellulose. Beldman and co-workers (1982) found that lignin effects
liquefaction as well.
Other research revealed that liquefying reagent blends are more effective than a single
reagent. Yao and co-workers (1994) concluded after studying liquefaction of biomass in
dilute solvents that, in most cases, no single solvent could dissolve all of the liquefied
components completely. The most effective diluent solvents were binary systems composed
of solvents with different polarities, one an electron donor moderately polar solvent, such as
dioxane, tetrahydrofuran or acetone, and the other an hydroxyl-containing electron donor-
acceptor, highly polar solvent, such as methanol, ethylene glycol or water. The binary of
dioxane and water was found to be suitable for a wide range of liquefaction solvents, and a
ratio of 4:1 dioxane:water was recommended as a universal diluent for liquefied biomass.
Co-liquefaction is another approach to the effective use of liquefying reagents. Yao and
co-workers (1993) investigated the co-liquefaction of a wood and starch system. It is
generally very difficult to obtain a liquid with a large wood concentration. On the other
hand, starch is very easy to liquefy in the PEG/glycerine system, even at a very small liquid
ratio and catalyst concentration. In their study, a combined liquefaction procedure for wood
meal ( Betula sp.) and starch was designed for preparing large biomass content liquids.
However, it was found that the extent of the wood liquefaction in a simultaneous liquefaction