**Abstract**

Liquid fuels from biomass and up-conversion of biomass in advanced supercritical fluid are reviewed in this chapter. Lignin can be converted into heavy hydrocarbons in subcritical water extraction. Lipid, which is triglyceride, is catalytically converted into straight-chain hydrocarbons of free fatty acid (decarboxylation) formed by hydrolysis. Carbohydrate is also hydrothermally converted into furan ring compound and fatty acids. Protein is converted into amino acids in hydrothermal water and depolymerization of protein is favored with rapid heating and denaturation agency such as alkaline earth metals. Free amino acids are further decomposed into carboxylic acid through deamination and into amine through decarboxylation. To inhibit Maillard reactions, which result in polymerization, the deamination of amino acid at low temperature was favored and a solid catalyst was quite active for deamination of free amino acids at quite low temperature hydrothermal water. Cellulose was dissolved in some ionic liquids with high mass percentages (5–20 wt%) and converted into monomers and useful components such as furan ring compounds and supercritical fluid cosolvent such as hydrothermal water in ionic liquids supported improvement of reaction efficiency. For hydrogenation of biomass, it was confirmed that hydrogen solubility was enhanced with supercritical carbon dioxide and it must be helpful for hydrogen reaction with biomass molecule.

**Keywords:** liquid fuels, biomass, iso-conversion, ionic liquid, hydrogen solubility

## **1. Introduction**

Renewable carbon resource is only biomass on the earth and its utilization must be enhanced to replace fossil carbon resources into biomass. In particular, among a various energy demands, transport occupies 30% of total carbon footprint in a personal daily life of an American [1]. For short-distance travel at near a house and a town, electrical vehicle such as EV (electric vehicle), HV (hybrid vehicle), PHEV (plug-in hybrid electric vehicle), and so on (so-called xEV) should be spread, but still a long-distance travel with an airplane and a large passenger ship requires liquid fuels such as diesel and heavy oils even though secondary batteries would be further compacted and lightened. For managing of the demands, liquid fuels must be obtained from renewable and sustainable carbon resources, and the only answer is biomass. For transformation of biomass into liquid fuels, one of the useful techniques should be sub and supercritical fluids technique with catalyst and cosolvent, which is called advanced supercritical fluid technology here.

Carbohydrate is also hydrothermally converted into furan ring compound [8–13], which is used as liquid fuel, and fatty acids, which are the similar compounds produced from lipid. The furan ring compound such as hydroxymethyl furfural (HMF) was obtained with high yield in hydrothermal water in the presence of appropriate catalyst such as TiO2 under microwave irradiation [12]. Microwave irradiation affected HMF formation from fructose, and the selectivity of HMF was

*Resource Upgrading in Advanced Supercritical Fluid (Supercritical Fluid with Catalyst…*

*DOI: http://dx.doi.org/10.5772/intechopen.89793*

Protein is converted into amino acids in hydrothermal water, and depolymerization of protein is favored with rapid heating and denaturation agency such as alkaline earth metals [14]. Free amino acids are further decomposed into carboxylic acid through deamination and amine through decarboxylation. However, in high temperature reactions, carbohydrates and protein (and these monomers) are easily combined together to form melanoidins, which inhibit fragmentations of both of the components (such as carbohydrates and proteins), through so-called Maillard reaction, which is a group of amino-carbonyl reactions. The deamination of amino acid at low temperature is favorable because Maillard reaction is not developed. However, deamination requires high temperature in subcritical water. For example, alanine deamination is meaningfully developed over 513 K [15]. To know the favorable acid or basic condition for enhancing amino acid deamination, pH dependence on alanine reaction was investigated. As a result, as introduced in Section 3, lower temperature than 513 K, the amino acid degradation is remarkably slow. By screening of several types of solid acid additives, it was surprisingly found that a type of solid catalyst was quite active for deamination of free amino acids not only alanine but also glycine, leucine, and serine at quite low temperature hydro-

Further decomposition of small free fatty acid into alkene and CH4 is favored for iso-conversion because these can be acted as alkane source or hydrogen source for long chain hydrocarbon formation. By integration of each unit reactions/operation (hydrolysis, deamination, decarboxylation, hydrogenation, and distillation), liquid fuels such as straight-chain and aromatic-ring compounds are obtained from woody/protein-rich biomass. One of the images for process integration (process

In Section 3 of this chapter, we briefly introduce protein conversion into small fragments with catalyst in hydrothermal water. Then the effect of acid and alkali on subcritical water conversion of alanine, which is the smallest chiral amino acid, is also roughly explained as the basic knowledge of alanine conversion in advanced supercritical fluid technologies. With a solid acid catalyst, alanine conversion is enhanced and the discussion why the solid acid catalyst can promote alanine con-

The other advanced supercritical technology is combined utilization of supercritical or hydrothermal water with cosolvent such as ionic liquid in the absence and presence of additives (homogeneous or heterogeneous). To improve reaction efficiency (increase of rate, yield, and selectivity of a target component), catalyst is also used in the ionic liquid-supercritical (hydrothermal) fluid mixture. Cellulose, which is the most abundant biomass on the earth, is dissolved in some ionic liquids with high mass percentages (5–20 wt%) and converted into monomers and further conversion into useful components such as furan ring compounds. In our research group, the combinational utilization of ionic liquid with hydrothermal water (or other solvents with and without catalyst) for cellulose (and its monomer, glucose, and the derivative) conversion into small fragments and useful chemicals were reported [16–25]. There are several merits for use of supercritical fluid technology (not only supercritical region but also subcritical and hydrothermal water) with

version in low temperature hydrothermal water is provided.

enhanced under microwave heating in hydrothermal conversion [13].

thermal water.

**91**

intensification) shows in **Figure 1**.

In this chapter, we review our previous studies concerning liquid fuels from biomass and up-conversion of biomass (mainly carbohydrates) in advanced supercritical fluid technology (supercritical fluid with catalyst such as solid aid-base catalyst or cosolvent such as ionic liquid) are introduced.

In a previous study, lignin, which is a major component of woody biomass, can be converted into heavy hydrocarbons in subcritical water (or hydrothermal) extraction [2]. The dimensionless severity number (*k*<sup>r</sup>*<sup>R</sup>*o: *<sup>k</sup>*<sup>r</sup> [time<sup>1</sup> ] is kinetic constant at reference condition and *R*<sup>o</sup> [time] is severity factor) can be used to predict conversion of lignin into liquid fuel. The mean molecular weight of the recovered lignin (so-called "hydrothermal lignin" because "lignin" is usually named with the name of solvent, which is used for recovery, such as "alkali-lignin" and "organosolv-lignin") was about 1 kDa, which is 6-mer of syringyl structure. The hydrothermal lignin should be used as heavy oil or source of aromatic structure after post-treatment such as a conventional catalytic cracking and hydrogenation. In this review, the lignin recovery and dissolution into hydrothermal water (obtained lignin is "hydrothermal lignin") is briefly introduced in Section 2, and the importance of dimensionless severity number is emphasized.

Protein-rich biomass can be converted into liquid hydrocarbons through catalytic hydrothermal cracking (CHTC) and conventional hydrogenation/distillation process (total process is so-called "iso-conversion") [3–5]. That is, the first step of "iso-conversion" is CHTC, and the main reaction of CHTC should be catalytic decarboxylation because hydrolyzed lipid is mainly to form free fatty acid, and carboxyl group in the free fatty acid must be detached to form fuel component, such as hydrocarbons. In our previous study, it was found that lipid, which is triglyceride, is catalytically converted into straight-chain hydrocarbons of free fatty acid (decarboxylation) formed by hydrolysis and one of the active materials of catalytic decarboxylation is zirconia [6]. It was also found that zirconia is also active for decarboxylation of small fatty acid such as acetic acid in supercritical water [7]. In "iso-conversion," the remaining components after extraction of lipid from protein-rich biomass, which are carbohydrate and protein, are proposed to be fermented to form small organic acids or olefinic molecules to combine together with unsaturated hydrocarbon (olefin) produced from lipid decarboxylation. Anaerobic fermentation is suggested to get small molecules in "iso-conversion" process, but it requires large space and long time [3]. Then, additional hydrothermal process for carbohydrate and protein conversion into small molecules such as furan ring compounds and fatty acids are proposed by our research group, and a small review of the proposed process is described in Section 3 of this chapter.

### *Resource Upgrading in Advanced Supercritical Fluid (Supercritical Fluid with Catalyst… DOI: http://dx.doi.org/10.5772/intechopen.89793*

Carbohydrate is also hydrothermally converted into furan ring compound [8–13], which is used as liquid fuel, and fatty acids, which are the similar compounds produced from lipid. The furan ring compound such as hydroxymethyl furfural (HMF) was obtained with high yield in hydrothermal water in the presence of appropriate catalyst such as TiO2 under microwave irradiation [12]. Microwave irradiation affected HMF formation from fructose, and the selectivity of HMF was enhanced under microwave heating in hydrothermal conversion [13].

Protein is converted into amino acids in hydrothermal water, and depolymerization of protein is favored with rapid heating and denaturation agency such as alkaline earth metals [14]. Free amino acids are further decomposed into carboxylic acid through deamination and amine through decarboxylation. However, in high temperature reactions, carbohydrates and protein (and these monomers) are easily combined together to form melanoidins, which inhibit fragmentations of both of the components (such as carbohydrates and proteins), through so-called Maillard reaction, which is a group of amino-carbonyl reactions. The deamination of amino acid at low temperature is favorable because Maillard reaction is not developed. However, deamination requires high temperature in subcritical water. For example, alanine deamination is meaningfully developed over 513 K [15]. To know the favorable acid or basic condition for enhancing amino acid deamination, pH dependence on alanine reaction was investigated. As a result, as introduced in Section 3, lower temperature than 513 K, the amino acid degradation is remarkably slow. By screening of several types of solid acid additives, it was surprisingly found that a type of solid catalyst was quite active for deamination of free amino acids not only alanine but also glycine, leucine, and serine at quite low temperature hydrothermal water.

Further decomposition of small free fatty acid into alkene and CH4 is favored for iso-conversion because these can be acted as alkane source or hydrogen source for long chain hydrocarbon formation. By integration of each unit reactions/operation (hydrolysis, deamination, decarboxylation, hydrogenation, and distillation), liquid fuels such as straight-chain and aromatic-ring compounds are obtained from woody/protein-rich biomass. One of the images for process integration (process intensification) shows in **Figure 1**.

In Section 3 of this chapter, we briefly introduce protein conversion into small fragments with catalyst in hydrothermal water. Then the effect of acid and alkali on subcritical water conversion of alanine, which is the smallest chiral amino acid, is also roughly explained as the basic knowledge of alanine conversion in advanced supercritical fluid technologies. With a solid acid catalyst, alanine conversion is enhanced and the discussion why the solid acid catalyst can promote alanine conversion in low temperature hydrothermal water is provided.

The other advanced supercritical technology is combined utilization of supercritical or hydrothermal water with cosolvent such as ionic liquid in the absence and presence of additives (homogeneous or heterogeneous). To improve reaction efficiency (increase of rate, yield, and selectivity of a target component), catalyst is also used in the ionic liquid-supercritical (hydrothermal) fluid mixture. Cellulose, which is the most abundant biomass on the earth, is dissolved in some ionic liquids with high mass percentages (5–20 wt%) and converted into monomers and further conversion into useful components such as furan ring compounds. In our research group, the combinational utilization of ionic liquid with hydrothermal water (or other solvents with and without catalyst) for cellulose (and its monomer, glucose, and the derivative) conversion into small fragments and useful chemicals were reported [16–25]. There are several merits for use of supercritical fluid technology (not only supercritical region but also subcritical and hydrothermal water) with

**1. Introduction**

*Advanced Supercritical Fluids Technologies*

**90**

Renewable carbon resource is only biomass on the earth and its utilization must be enhanced to replace fossil carbon resources into biomass. In particular, among a various energy demands, transport occupies 30% of total carbon footprint in a personal daily life of an American [1]. For short-distance travel at near a house and a town, electrical vehicle such as EV (electric vehicle), HV (hybrid vehicle), PHEV (plug-in hybrid electric vehicle), and so on (so-called xEV) should be spread, but still a long-distance travel with an airplane and a large passenger ship requires liquid fuels such as diesel and heavy oils even though secondary batteries would be further compacted and lightened. For managing of the demands, liquid fuels must be obtained from renewable and sustainable carbon resources, and the only answer is biomass. For transformation of biomass into liquid fuels, one of the useful techniques should be sub and supercritical fluids technique with catalyst and cosolvent,

In this chapter, we review our previous studies concerning liquid fuels from biomass and up-conversion of biomass (mainly carbohydrates) in advanced supercritical fluid technology (supercritical fluid with catalyst such as solid aid-base

In a previous study, lignin, which is a major component of woody biomass, can

] is kinetic

be converted into heavy hydrocarbons in subcritical water (or hydrothermal)

constant at reference condition and *R*<sup>o</sup> [time] is severity factor) can be used to predict conversion of lignin into liquid fuel. The mean molecular weight of the recovered lignin (so-called "hydrothermal lignin" because "lignin" is usually named with the name of solvent, which is used for recovery, such as "alkali-lignin" and "organosolv-lignin") was about 1 kDa, which is 6-mer of syringyl structure. The hydrothermal lignin should be used as heavy oil or source of aromatic structure after post-treatment such as a conventional catalytic cracking and hydrogenation. In this review, the lignin recovery and dissolution into hydrothermal water

(obtained lignin is "hydrothermal lignin") is briefly introduced in Section 2, and the

Protein-rich biomass can be converted into liquid hydrocarbons through catalytic hydrothermal cracking (CHTC) and conventional hydrogenation/distillation process (total process is so-called "iso-conversion") [3–5]. That is, the first step of "iso-conversion" is CHTC, and the main reaction of CHTC should be catalytic decarboxylation because hydrolyzed lipid is mainly to form free fatty acid, and carboxyl group in the free fatty acid must be detached to form fuel component, such as hydrocarbons. In our previous study, it was found that lipid, which is triglyceride, is catalytically converted into straight-chain hydrocarbons of free fatty acid (decarboxylation) formed by hydrolysis and one of the active materials of catalytic decarboxylation is zirconia [6]. It was also found that zirconia is also active for decarboxylation of small fatty acid such as acetic acid in supercritical water [7]. In "iso-conversion," the remaining components after extraction of lipid from protein-rich biomass, which are carbohydrate and protein, are proposed to be fermented to form small organic acids or olefinic molecules to combine together with unsaturated hydrocarbon (olefin) produced from lipid decarboxylation. Anaerobic fermentation is suggested to get small molecules in "iso-conversion" process, but it requires large space and long time [3]. Then, additional hydrothermal process for carbohydrate and protein conversion into small molecules such as furan ring compounds and fatty acids are proposed by our research group, and a small review of the proposed process is described in Section 3 of this chapter.

extraction [2]. The dimensionless severity number (*k*<sup>r</sup>*<sup>R</sup>*o: *<sup>k</sup>*<sup>r</sup> [time<sup>1</sup>

importance of dimensionless severity number is emphasized.

which is called advanced supercritical fluid technology here.

catalyst or cosolvent such as ionic liquid) are introduced.

**Figure 1.** *Proposed process for liquid fuel production from protein-rich biomass by hydrothermal conversion together with "iso-conversion."*

ionic liquid as cosolvent: reduction of viscosity of ionic liquid solution [26], control of acidity and basicity of ionic liquid, solubility control of material in ionic liquid, separation of target molecule from ionic liquid into supercritical fluids, and so on. Our research group also progress the study of the separation of some components (biomass molecules or biomass refinery are targeted) from ionic liquid solution with supercritical carbon dioxide [27–30]. Here, we only briefly introduced some research results concerning biomass conversion into useful molecule in ionic liquidhydrothermal water mixture with additives (Section 4).

was 49, 5.9, 0.1, 1, and 42.2 wt% for C, H, N, S, and O content, respectively. Ash

*Resource Upgrading in Advanced Supercritical Fluid (Supercritical Fluid with Catalyst…*

molecular weight) with syringyl-rich structure [2].

**2.3 Dimensionless severity number**

as below:

**93**

A coupled system of two batch reactors was used (the photograph and schematic diagram of the reactor is shown in **Figure 2**, and its detail can be seen elsewhere [2]), and the sample was loaded into the upper reactor. The lower reactor was reservoir and used to recover hydrothermal extracts. The reservoir was maintained at a temperature of 473 K to inhibit the aggregation of lignin because lignin typically is melted into liquid phase at temperatures greater than 423 K in the presence of water [31]. Typically, a 7.5 g sample of bark (covered with stainless mesh) was loaded in the upper reactor along with 150 mL of pure water. After purging air inside the reactor, N2 gas was loaded. The upper reactor was heated up to 473–573 K. The pressure in the upper reactor (sample-loaded reactor) was close to the saturated pressure of water at the reaction temperature. After the desired treatment time, the valve between the two reactors was opened and the fractionated components in the reactor were recovered in the reservoir. The reservoir was then rapidly cooled with a water bath (ca. 10 min) and was then detached from the sample-loaded reactor. The residue in the upper (sample-loaded) reactor with the stainless mesh was weighed after drying at 333 K overnight. Connection lines between the two reactors were rinsed. The water-insoluble fraction among the recovered solutions was hydrothermal-soluble lignin (namely "hydrothermal lignin"). The soluble Klason lignin (hydrothermal lignin) was quantified, and the molecular characteristics (molecular weight with GPC and molecular structure with NMR) were evaluated. As a result of the molecular characteristics, the hydrothermal lignin obtained from bark of Japanese cedar by hydrothermal extraction at 523 K under the saturated pressure of water, 3.98 MPa, about was mainly 6-mer (about 1 kDa of mean

The kinetic study for lignin recovery (how much percentage of Klason lignin can be recovered based on the Klason lignin in the raw material, the bark of Japanese cedar) was performed, and severity factor was used for consideration of the effect of reaction temperature and reaction time. The severity factor, *R*<sup>o</sup> [min], is defined

content was 2 wt%.

*Coupled system of two batch reactors.*

*DOI: http://dx.doi.org/10.5772/intechopen.89793*

**Figure 2.**

In the conclusion of this chapter, we will conclude this chapter and mention outlook in this field.
