Catalysts are important in hydrothermal liquefaction processes, and a range of catalysts has been proposed for the subcritical processing of biomass to tailor the reaction to a specific product and enhance the reaction rates of the proceeding reactions. These catalysts comprise homogeneous catalysts such as mineral acids, organic acids, and bases as well as heteroge­neous catalysts such as zirconium dioxide, anatase titania, and other materials (Moller et al., 2011).

11.1.2 Homogeneous

The addition of alkali salts has a positive influence on hydrothermal processes. It improves gasification, accelerates the water gas shift, and increases liquid yields (Watanabe et al., 2005; Yang and Montgomery, 1996; Mok et al., 1992). In addition, the catalysts raise the pH, thereby inhibiting dehydration of the biomass monomers. A high degree of oxygen removal in the form of dehydration instead of decarboxylization might result in unsaturated compounds that easily polymerize to char and tar. Indeed, alkali is also known to suppress char and tar formation (Toor et al., 2011).

Song et al. (Song et al., 2004) investigated the effect of the addition of 1.0 wt% of Na2CO3 on the liquefaction of corn stalk and concluded that the use of a catalyst increased the yield of

biocrude (from 33.4% to 47.2%); however, no elaboration on the action of the catalyst was made. Similarly, alkali in the form of K2CO3 was shown to have a positive effect on hydro­thermal treatment of wood biomass at 280°C for 15 min (Karagoz et al., 2006). In a similar study performed with the same equipment and utilizing wood biomass, the authors observed that potassium salts were more effective than sodium salts, and they ranked the salts in order of catalytic activity as follows: K2CO3 > KOH > Na2CO3 > NaOH. The catalysts improved liq­uid yields and decreased the amount of solid residue. Minowa et al. (Minowa et al., 1998) tested the catalytic action of Na2CO3 during hydrothermal conversion of cellulose. Above 300° C the catalyst decreased secondary tar formation from the oil product and catalyzed the gasification of the aqueous organics. The study shows how nicely cellulose is converted at different temperatures.

One important catalytic action of alkali during hydrothermal liquefaction is the accelera­tion of the so-called water gas shift, and thus it favors H2 and CO2 formation at the expense of CO. The produced hydrogen gas may act as a reducing agent, increasing the heat value and quality of the oil product. The mechanism proceeds via formation of a formate salt (Schmieder et al., 2000; Sinag et al., 2004) and is more thoroughly described next.

A formate salt (HCOO~K+) is formed when the alkali salt reacts with CO from the gasification:

K2CO3 + H2O! KHCO3 + KOH (11.1)

KOH + CO! HCOOK (11.2)

Hydrogen is obtained when formate reacts with water:

HCOOK + H2O! KHCO3 + H2 (11.3)

In the next step, CO2 is produced from KHCO3:

2KHCO3 ! H2O + K2CO3 + CO2 (11.4)

The overall reaction can be written as:

H2O + CO $ HCOOH $ H2 + CO2 (11.5)

There are also other positive effects of homogeneous catalysts, such as enhanced decarbox­ylation of fatty acids. For example, Watanabe et al. (Watanabe et al., 2006) improved the con­version of C17-acid (fatty acid) decomposition from 2% to 32% by addition of a KOH catalyst.