Water at high temperatures becomes a good solvent for hydrocarbons that are typically nonpolar hydrophobic under standard environmental conditions. Ionic reactions of organics should be favored by increased solubility in water. The enhancement of this solubility of hydrocarbons in water will further enhance the possibilities of contact of dissociated H+ with hydrocarbons and hence accelerates the activities of hydrolysis. Water has the ability to carry out condensation, cleavage, and hydrolysis reactions and to affect selective ionic chemistry. This is largely due to changes in its chemical and physical properties, which become more compatible with the reactions of organics as the temperature is increased.

Hot water as a reactant and catalyst likely creates a second pathway for the cascade of molecular transformations that leads to oil. In this pathway, water causes organic material to disintegrate and reform (by adding H+ to an open carbon bond) into fragments, which then transform into hydrocarbons. This implies that hot water becomes a catalyst for a series of ionic reactions. The acidic and basic nature of hot water—rather than heat—drives this cas­cade. For example, water may function first as a base, nibbling away at certain linkages in the organic material. As new molecular fragments build up and modify the reaction environ­ment, water can change its catalytic nature. It can then act as an acid, accelerating different reactions. The resulting products attack parts of the remaining molecules, further speeding the breakdown (Siskin and Katritzky, 1991).

The exact pathways of HTU to produce crude oil from biomass remain unclear, and addi­tional research is needed. The following examples may give some hints of possible pathways of HTU of waste biomass feedstock. The basic reaction mechanism can be described as depoly­merization of the biomass; decomposition of biomass monomers by cleavage, dehydration, de­carboxylation; and deammination and recombination of reactive fragments (Toor et al., 2011).

In a study by Appell et al. (Appell et al., 1975), one of the mechanisms for the conversion of carbohydrates into oil that was consistent with the results is as follows.

Sodium carbonate reacts with carbon monoxide and water to yield sodium formate:

Na2CO3 + 2CO + H2O! 2HCO2Na + CO2

Vicinal hydroxy groups in the carbohydrates undergo dehydration to form an enol followed by isomerization to a ketone. The following reaction will be initiated with the attack of H + on the compound with vicinal hydroxyl groups; the water molecule will be eliminated to form carbocation, and further rearrangement is:

H H H H H H

—— C—C——— ► ———- C—— C C^=C——— ——— ► ——— C— C——

-H2O + I — H+

OH OH OH OH HO

The hydroxyl ion then reacts with additional carbon monoxide to regenerate the formate ion:

OH~ + CO! HCO-

A variety of side reactions may occur, and the final product is a complex mixture of com­pounds. One of the beneficial side reactions occurring in alkaline conditions is that the car­bonyl groups tend to migrate along the carbon backbone. When two carbonyl groups become vicinal, a benzylic type of rearrangement occurs, yielding a hydroxy acid. The hydroxy acid readily decarboxylates, causing a net effect of reducing the remainder of the carbohydrate — derived molecule.

This type of reaction is beneficial to HTU because it leads to the formation of paraffin-type structures, which have less oxygen than the original compounds. In addition, the reaction happens by disproportionation and does not require any additional reducing agent. Aldol condensation may also be part of the reaction process. Aldol condensation occurs between a carbonyl group on one molecule and two hydrogens on another molecule with the elimi­nation of water. The condensation product is a high-molecular-weight compound, typically with high viscosity. Condensation reactions become a major pathway in the absence of reduc­ing agents such as carbon monoxide and hydrogen. Reducing agents keep the carbonyl con­tent of the reactant system sufficiently low so that liquid instead of solid products are formed.

In a study by Appell et al. (Appell et al., 1980), the authors believed that the free hydrogen radical (H*), not the hydrogen molecule (H2), participates in the chemical conversion reac­tions. Thus, they concluded that the addition of carbon monoxide (CO) to the process was more efficient than the addition of hydrogen gas. Based on the water-gas shift reaction, carbon monoxide reacts with water to form carbon dioxide and two hydrogen radicals:

C=O + H-O-H O=C=O + 2H-

In the presence of the hydrogen radicals, the oxygen is removed from the compounds containing carbonyl and hydroxyl groups, then forms paraffin and water. A possible pathway is described in the following four reactions (He, 2000):

O

I II II

— C — C + 2H’ —- ► —C = C— + H2O

Keto group

Hydroxyl group