A study of selectivity during acid-catalyzed dehydration of cellulosic sugars
Heltzel, Jacob M.
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Recent research in renewable fuels has been placing emphasis on the production of alternatives to petroleum that will not disrupt current fuel infrastructure. Of particular interest is cellulose-derived renewable fuel because the diversity in platform chemicals obtained from cellulose allows for the production of a wide variety of highly valuable materials: specialty chemicals; polymers; and fuels including gasoline, diesel, and jet fuel. However, the drawback to the development of platform chemicals from cellulose lies in the formation of humins, undesired carbonaceous waste products, which ultimately lower the production yield of platform chemicals. More specifically, upon formation of 5-(hydroxymethyl)furfural (HMF), a platform chemical, 2,5-dioxo-6-hydroxyhexanal (DHH) forms by hydrolysis of HMF and leads to the formation of humin byproducts. Herein we initially characterized the formation and primary composition of humins, and described techniques by which the morphology of humins can be altered and functional groups be added. Humins dispersed into two fractions, soluble and non-soluble, when treated with various basic, acidic, and organic solvents. Thermal gravimetric analysis, infrared spectroscopy, and electron microscopy revealed that the composition of the two fractions was fairly similar in carbon and oxygen ratios while the size of humins formed differed significantly. More specifically, the dispersed fraction of acetone-treated humins formed 2-3nm carbon spheres. In contrast, the non-dispersed fraction of acetone-treated humins formed much larger carbon spheres. Such carbon spheres averaged over 500 nm, which is comparable in size to untreated humins. Next, we attempted to eliminate or reduce humin formation using glucose as the carbohydrate, formic acid (FA), and γ-valerolactone (GVL), a renewable non-aqueous green solvent that is produced from cellulose. At all conditions used, humin formation in non-aqueous systems occurred when HMF concentrations exceeded 0.1 M. Formates formed upon addition of FA and hindered glucose dehydration. Glucose solubility in GVL was dependent on glucose esterification. GVL-FA systems primarily produced glucose oligomers during reaction. Water in GVL-FA systems shifted the esterification equilibrium and allowed for glucose dehydration. HPLC analysis of a 0.1 M glucose GVL-FA solution to which acetonitrile and water were added showed that 47% selectivity of HMF at an observed 67% conversion of glucose was achieved in the absence of humin formation. The actual selectivity of HMF approaches 90% if the unaccounted glucose is regenerable. In summary, this work demonstrates that GVL-FA systems increases reaction rate and HMF selectivity in comparison to traditional aqueous systems, making evident that GVL-FA systems can vastly improve the process of converting cellulose into platform chemicals at high yields.