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The conversion of biomass into high quality fuels in high yield via industrially applicable processes has been the holy grail of fuel sciences for some time. Global fuel demand increases as geological crude oil deposits approach their inevitable depletion. Therefore, it is critical for research to find an acceptable means of producing fuels from renewable sources. Conventionally handled and easily portable liquid fuels are particularly desirable.

The twin cities of Odessa and Midland, and smaller towns in the region, hold significant stock in the oil empire. About 25% of all the oil and gas in the continental US comes from the Permian Basin (this is how this cluster of cities came to be known as the 'Oil Patch'). It is perhaps ironic that future refinery feedstock from biomass is being researched in this arid West Texas region, where little biomass now grows...

Throughout the history of this endeavor, the preferred starting material for chemical conversion of biomass has usually been wood, largely because of the higher carbohydrate content. The scientific question is whether the carbohydrate polymers, cellulose and hemicellulose, in biomass can be chemically transformed into a suitable gasoline, or multiple purpose fuel. Such a conversion must be done inexpensively.

O v e r v i e w

Our initial goal was to develop an efficient multistep process for the conversion of cellulose and hemicellulose into hydrocarbon fuels. Separation of these components and/or the use of selective reactions might allow for 100% carbon conversion by keeping the carbon chain intact. Furthermore, if initial reactions could be conducted in an aqueous medium, then the use of wet feedstocks would be possible. Overall, a six carbon sugar polymer, cellulose, would afford a single pure hydrocarbon product such as hexene. This is precisely what we have developed, a novel chemical process.

R e s e a r c h

Step 1 is a reductive depolymerization of carbohydrate biopolymers. Cellulose is simultaneously hydrolysed in dilute acid and catalytically hydrogenated to glucitol (commonly named sorbitol) in near quantitative yields. Hemicellulose is similarly converted into zylitol and sorbitol. Lignin, if present, is simply removed after the reaction by filtration from the aqueous solution of polyols. Uniquely this reaction is driven to completion by the irreversible hydrogenation but  provides for this simple mechanical separation of these principal components of biomass. Research Area and Selected Publications

Step 2 of the process is also a key reaction: the chemical conversion of polyhydric alcohols to liquid hydrocarbons. '96 Patent The major part of all the reduction requirements occurs during this conversion. Polyols such as sorbitol are reduced essentially quantitatively to a mixture of halocarbon and hydrocarbon compounds by reaction with hydriodic acid and a phosphorous type co-reducing agent, either phosphorous acid or hypophosphorous acid. The reaction occurs in boiling aqueous solution at atmospheric pressure for 1-2 hours. Reaction conditions were varied to give on one extreme about 99% 2-iodohexane, and on the other extreme about 86% hydrocarbons with the remainder being halocarbons. The immiscible products are simply removed as a separate phase from the water solution. So Step 2 not only provides a highly reduced C-6 compound but also C-12, C-18, and C-24 hydrocarbons. These groups represent fuels in the range of gasoline, kerosene, diesel, and fuel oil, respectively.

Step 3 might be considered a cleanup reaction in that all of the remaining halocarbons in mixtures from Step 2 are subsequently converted to alkenes by an elimination reaction with sodium hydroxide in boiling alcohol. Vast differences in boiling points of hexene from the other higher mass hydrocarbons, 200 & 300 °C, allow facile separation by distillation of the final mixture.

Step 4 (several choices): There are several optional steps to other chemicals and fuels. For example, catalytic hydrogenation of hexene furnishes hexane, an important industrial solvent. Thermally, hexene can be reformed to a higher branched alkene and finally to a higher octane value after reduction. Hydrolysis of 2-iodohexane to 2-hexanol is another optional reaction to a value added product. The C12 fraction can be used in diesel or jet fuels, or after hydrogenation has an 83 octane.

Summary

This multi-step chemical process for reduction of biomass to liquid hydrocarbon fuels is the first of its kind. It stands in sharp contrast to other research areas that follow classical lines of bio- (fermentation) and thermal (pyrolysis) conversion. In fact, uncoupling the reduction process to a series of mild selective chemical reactions was the key to the problem. As a result, simple separations and many economic advantages abound. 
Current Efforts (Fall 2002): A bench scale pilot plant using electrochemical regeneration of the incipient iodine, rather than phosphorous acid co-reducing agent, is being completed. This promises to establish a lower cost, commercially viable process producing liquid hydrocarbon fuels from biomass. (Patent Applied)

Updated February, 2003

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