In water, both ionic and radical reactions are developed and these are important for biomass conversion. By controlling these reactions, biomass component can be transformed into intended products selectively. Here basic matters for biomass reactions (such as kind of ionic reactions that have to be headed) are roughly mentioned and expected biomass conversion process is briefly reviewed.
11.3.1 Ionic Reactions
Typically, acid–base reactions are ionic reactions. Unlike in the case of petroleum (hydrocarbons), biomass has various function groups such as hydroxyl, carboxyl, carbonyl, ether, ester, and so on. In oil refinery for petroleum, attachment of functional group in hydrocarbons is the key reaction, on the other hand, detach- ment of functional group is selectively controlled in biomass refinery because of richness of function groups.
For the detachment, acid and base reactions are quite important. For example, it is well known that detachment of hydroxyl group, namely hydration, is promoted by proton, that is, acid reaction. Some kinds of carboxyl groups are detached by alkali catalyst (depending on substituted group). Ether and ester groups are hydrolyzed and promoted by both acid and base catalysts. Carbohydrates have ether and hydroxyl groups. Lipids have ester and carboxyl groups. Proteins have peptide groups, which are decomposed by acid and base catalysts. Lignin has ether and hydroxyl groups as well as carbohydrates, but the reactivity of these function groups are different because substituted group around the function group is different.
It is also well known that some types of reaction can be controlled by acid–base catalysts; for example, isomerization of glucose, Cannizzaro reaction, retro-aldol condensation, water–gas shift reaction are promoted by base catalyst, while alkylation such as combination between alcohol and phenols promoted by acid catalyst.
To enhance controllability of the ionic reactions, addition of catalyst is quite useful with control of temperature and pressure. Typical homogenous catalyst (mineral acids and alkali catalyst, in particular, H2SO4as acid catalyst, and NaOH and KOH as alkali catalysts are often used) can also work in high pressure high temperature water (HHW). As mentioned just before, the number of the reports for the acid–base reactions in the presence of heterogeneous catalysts has increased recently. As well as typical acid catalyst such as zeolite (silica–alumina), ion exchange resin, sulfated zirconia, and so on, it was reported that MoO3[12], TiO2 [13,14], and Nb2O5[15] worked as Bronsted and Lewis acid–base catalyst in high pressure high temperature water (HHW). Still useful catalytic system for the high pressure water media has been being looked for.
11.3.2 Radical Reactions
Thermal decomposition (pyrolysis) of organic molecules including biomass compounds is radical chain reaction. The contribution of pyrolysis is favored over 300C because thermal energy for homolytic decomposition of chemical bond in biomass can be gained at that temperature. If pyrolysis of biomass consists of the same elementary steps as that of hydrocarbons described above, the overall rate
constant depends on the concentration. In this case, water acts as promoter or inhibitor by adjusting the concentration of biomass (so it is quite important to know phase behavior of biomass compound with water, but still lack of the knowledge about it). At higher temperature, main reaction pathway of biomass gasification is pyrolysis and the concentration of biomass in water is one of the key factors of the gasification. To enhance the radical decomposition, metal catalysts such as nickel, ruthenium, and so on, have been employed.
Both radical and ionic reactions for the same reactants are developed in parallel in high pressure high temperature water (HHW). One of the examples is glycerol conversion. As Bühler et al. [16] pointed out, glycerol conversion into acrolein was favored at high pressure (namely high water density) compared with that at low pressure (lower water density). To consider the reaction behavior, it was suggested that the rate of ionic reaction (acrolein formation) was enhanced with increasing water density (proton concentration), while the rate of radical reactions such as allyl alcohol formation was insensitive to water density. As shown in this example, in high pressure high temperature water (HHW), radical reaction can be also controlled by changing the contribution of ionic reaction with water density and temperature.
The other important radical reaction on biomass conversion is oxidation. For biomass conversion into useful compounds, partial oxidation is sometimes useful.
There are several reports about hydrogen formation from biomass through partial oxidation [17,18]. The oxidants are oxygen and water. To improve the selectivity of hydrogen formation, some catalysts were selected, for example, ruthenium oxide, zinc oxide, zirconium oxide, alkali hydroxide, and so on.
11.3.3 Small Review of Biomass Conversion Process
Based on the huge number of basic researches, biomass conversion in high pressure high temperature water (HHW) has been tried to industrialize. Here it is roughly categorized into several process, both from raw material-oriented point of view and from product-oriented point of view.
As repeatedly mentioned, biomass (plant biomass) typically consists of protein, carbohydrate (sugar, cellulose, and hemicellulose), lipid, polyphenol (including lignin), and mineral. The solubility and reactivity of the component are organized by temperature. Figure11.2shows the schematic diagram of biomass modification in high pressure high temperature water (HHW). Protein is hydrolyzed into amino acid and solubilized into water up to 200C. Sugar is dissolved in water and starts to decompose over 100C; isomerization and dehydration of sugar proceed up to 200C, retro-aldol condensation is favored over 200C, and gasification of sugar meaningfully occurs over 400C. Hemicellulose is hydrolyzed into sugars at around 200C, cellulose is dissolved in water at around 350C, and cellulose is significantly hydrolyzed over 350C. By using these two-step reactions (hemi- cellulose hydrolysis at 200–300C and cellulose hydrolysis at 350C or higher),
a venture company starts business of sugars production from plant biomass [19].
Hydrolysis of lipid happens and lignin decomposition begins at 300C. Almost all water soluble minerals (Na, K, Ca, and so on) are dissolved in water up to 300C, while dissolution of water insoluble minerals such as Si requires high temperatures (over 400C at high water density).
Target material and compound from biomass are various but the appropriate condition for the target can be roughly categorized. Hot water has been used for extraction of ingredients of biomass and the reactive separation method (such as hydrolysis-sugar separation, as mentioned just above) by use of high pressure high temperature water (HHW) has been checked out [20]. Below 200C, separation of protein via hydrolysis would also be achieved.
Figure11.3 shows three demonstrated large-scale processes for biomass conversion in high pressure high temperature water (HHW). Dehydration of carbohydrate is favored around 200C, cellulose is not hydrolysed at this tem- perature range, and thus carbonization of biomass is suited at around 200C.
Recently, hydrothermal carbonization (HTC), which was invented by Dr. Bergius, the winner of Novel Prize in Chemistry, is rediscovered. Carbonaceous materials are widely applied for artificial coal, solid adsorbents, catalysts, templates of hollow metal oxides, battery materials, medical application, and so on. HTC is an energy efficient method for producing hydrochar from biomass because the process consists in the physicochemical conversion of biomass confined with water typi- cally below critical point of water, 374C, under the self-generated pressure [21,22]. Literature outlines a series of effect of raw materials on the characteristics of the hydrochars mainly. There have been many studies concerning carbonization of monosaccharides [23–27], cyclodextrins [28], cellulose [29], and some real biomass mainly consisted of carbohydrates [30–34]. The produced hydrochar in hydrothermal conditions can be used as a fuel [35], a catalyst after functionalization Fig. 11.2 Biomass modification in high pressure high temperature water (HHW)
[36–38], and template (or support) of hybrid materials [39–44]. In the methods of preparing these materials, hydrothermal carbonization is gaining increasing interest since the drying step can be eliminated and the process is energetically favorable compared with burning biomass. There have been huge numbers of basic researches and several venture companies that have begun business of the process [45–47].
In biomass refinery, chemical block such as furan, aldehyde, alcohol, carboxylic acid, phenols, and so on, which are useful for various industries, has to be provided from biomass. Sugar, which is a center compound in biomass refinery, can be converted into these useful chemicals in high pressure high temperature water (HHW) from 200 to 400C. Many researchers have studied to produce furan compound, mainly 5-hydroxylmethylfurfural (HMF), which is also considered to be precursor of hydrothermal carbonization, from sugars (typically glucose and fructose). It is well known that acid catalyst is effective to produce HMF from fructose [13, 14, 48]. It is found that microwave irradiation is useful for rapid production of HMF [14]. From glucose, HMF yield is not high because of low efficient in isomerization of glucose into fructose. In our laboratory, to overcome this issue, some base catalysts have been tested for glucose isomerization and some results will be introduced in the next section. Production of carboxylic acid (organic acid) from biomass has to be investigated widely. In the case of acid production, the starting material is not only sugars but also glycerol, which is byproduct of biodiesel production. Lactic acid can be produced from glucose with high yield in alkaline hydrothermal condition [49]. Lactic acid production from glycerol is much higher than that from glucose even at the similar reaction con- dition [50, 51]. Formic acid can be selectively formed from glucose via partial Fig. 11.3 Category of biomass conversion process in high pressure high temperature water (HHW)
oxidation in the presence of base catalyst [52]. For organic acid production, we have also studied conversion of alginic acid, which is main component of sea algae and is carbohydrate having carboxylic group in constituent sugar unit (uronic acid) [53, 54]. Aldehyde such as acrolein is important chemical block for producing polymer and could be selectively obtained from glycerol [55]. Phenols can be obtained from lignin in principle because it consists of phenyl–propane units, however, due to polymerization of phenol unit, the yield of single-ring phenol is quite low. To improve the yield of phenols, high yield of double-ring phenol can be obtained by adding single-ring phenol compounds (such as cresol) as radical capping agency [56–58].
Liquid fuels can be produced from biomass through hydrothermal reaction in the presence of alkali at around 350C, which is known as hydrothermal upgrading (HTU), as shown in Fig.11.3. Therefore, liquefaction of biomass (that is, production of water soluble compounds or liquid fuels) can be operated at around 200–400C, depending on the target compounds. Homogeneous and heterogeneous catalysts are employed to improve selectivity and the selection of suitable catalyst has been studied by many researchers [59].
To produce fuel gases from biomass, higher temperature (over 400C) is chosen and it is called supercritical water gasification (SCWG), as shown in Fig.11.3. The appropriate temperature depends on the kind of biomass and the selected catalyst. Carbohydrate-rich biomass can be gasified at around 400C in the presence of noble metal catalysts and alkali catalyst [59,60]. High content of lignin, which is resist compound for gasification, requires severe condition (over 300C) and active catalysts such as Ru metal. By employing partial oxidation, the condition for gasification can be moderated and glucose can be converted into hydrogen gas via formic acid formation by partial oxidation in the presence of base catalyst [61]. Still, there are quite a lot of basic researches for gasification and also many bench/pilot plants of biomass gasification in supercritical water have been tested around the world.