Biomass To Transportation Fuels

There is a growing interest in processes that convert biomass to (green) transportation fuels or to fuel boosters. Some options are given in Figure 2.1. On a large and rapidly increasing scale, hydrolysis of the polysaccharide starch and the disaccharide sucrose to the monosaccharides is carried out, followed by fermentation to ethanol. Two ethanol and two CO2 molecules are formed from one C6-monosaccharide (glucose). The largest ethanol producers are Brazil [7, 8] and the United States. [9]. Zeolites play a role (adsorption [KA] or membrane techniques) in the dewatering of ethanol, which is required for mixing with gasoline. Forthcoming are processes in which cellulosics, as present in agricultural waste streams (wheat straw, corn stalks, bagasse), are also hydro-lyzed (enzymatically) and converted to ethanol [10] by a yeast.

When the ethanol is not blended but used as such as fuel, hydrous ethanol (e.g., 95%) can be applied. In several other chemical conversions of ethanol, there is no need to remove all the water. Two examples follow.

2.3.1 Example 1

In a cascade-type continuous setup [11], a partial evaporation of the fermentation liquid was carried out, and the ethanol-water mixture was passed over an H-ZSM5 zeolite catalyst (350°C, 1 atm). As in the MTG (methanol to gasoline) [12] process, a mixture of alkanes and aromatics was obtained. In this approach, the ethanol-water

Methane

Bio synth. gas

Diesel oil

Gasoline

Bio oil

Biomass HTU

Biocrude

H2 Hexane /

Monosacch. -> Ethanol-1> ETBE Booster

HMF H

COOH

HCOOH

Gasoline

Ethene

Booster

COOR Booster

FIGURE 2.1 Carbohydrates to transportation fuels.

separation is avoided, as the hydrocarbons and water are nonmiscible and separate by gravity. Though its octane number is good, there is no future in MTG-gasoline because of the trend to lower the aromatics content.

When passing the ethanol-water mixture over H-ZSM5 at lower temperature (200°C) or over zeolite H-Y, only dehydration occurs and ethene is obtained [13]. The track to gasoline is then oligomerization to C6-C8 alkene and hydrogena-tion/isomerization, e.g., over the TIP-catalyst Pt-H-mordenite.

Recently, Dumesic [14] disclosed a route from glucose to n-hexane (together with some pentane and butane) consisting of hydrogenation to sorbitol followed by stepwise hydrogenolysis over a Pt-catalyst in acid medium (pH 2). The hydrogen required is obtained [15] by aqueous-phase reforming of sorbitol or glycerol over a relatively inexpensive Raney Ni-Sn catalyst. Roughly 1.6 mol of glucose are required per mol hexane.

By gasification of biomass, and with supplementation of hydrogen, the Fischer-Tropsch synthesis can be used to produce clean diesel oil.

2.3.2 Example 2

Figure 2.1 also shows routes via hydroxymethylfurfural (HMF), a compound that can be selectively made from fructose using a dealuminated H-mordenite or a niobium-based catalyst. HMF might well develop to become a new key chemical; its chemistry (and that of furfural) has recently been reviewed by Moreau et al. [16]. When preparing HMF, it is advantageous to apply a cascade reaction by using a fructose precursor. Thus the hydrolysis of the fructan inulin (glucose [fructose]n) or of the glucose-fructose combination sucrose is coupled with the dehydration to HMF. In the case of sucrose as a starting compound, HMF and the remaining glucose can be easily separated.

HMF can be hydrogenated over a Pd-catalyst [17] to 2,5-dimethylfuran, a compound with the very high blending research octane number (BRON) of 215 [18].

Another interesting HMF-derived compound is levulinic acid formed together with formic acid by solid acid catalysis. A one-pot cascade route from, for example, inulin seems feasible. Manzer et al. [19] give examples in which an esterification step with an alkene is coupled as well. The levulinic esters exhibit good octane numbers. The by-product formic acid might also be esterified or used as a hydrogen source.

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