Part one. Factors influencing furan formation from monosaccharides using maleic acid Part two. Silyl group migration studies
Abstract
The objective of my research is to study what factors affect the formation of useful platform chemicals (furfural and hydroxymethylfurfural) from monosaccharides (xylose and glucose) that can be obtained from plant biomass sources. I utilized methods that used fewer mineral acids and tried to replace them with an organic acid (maleic acid). We hypothesized that the mineral acids could be replaced in the conversion processes. It was also very important to also utilize actual plant materials to gauge the real world applications of these replacement techniques. The effect of different ions on the conversion of xylose to furfural was investigated. The effect of different halides on the conversion of xylose was studied through the use of different hydrohalic acids (HCl, HBr, HI). The selectivity (productive conversion/nonproductive conversion) increases as halide size increases (Cl- < Br- < I-). The effect of the different metals was also investigated. The LiCl was compared to KCl and NH4Cl. There was no noticeable difference between the alkali metals; the ammonium had a more pronounced effect in comparison. The nonproductive conversion increases and the productive conversion decreases. This change in selectivity can be contributed to possible Schiff base/Maillard reactions. The ionic strength of the solution between different metals was compared. Solutions with the same ionic strength have similar rates of xylose conversion. Weaker bases with limited H-bonding ability (e.g. I-) decrease nonproductive conversion of xylose to furfural. Maleic acid was used as an acid in xylose conversion to furfural. H 2MA was chosen due to maleate being a weak base with limited H-bonding due to intramolecular H-bonding. Maleic acid has been demonstrated to be a very selective in the hydrolysis of hemicellulose (xylans) to form monosaccharide xylose in very high yields (≥ 85%) from actual plant biomass material. The product maleic acid solution is high in xylose monomers. The maleic acid was an effective catalyst in furfural formation. The maleate anion decreases nonproductive xylose conversion furfural degradation. Maleic acid is a selective catalyst in the formation of furfural from both pure and plant-derived xylose. The formation of hydroxymethylfurfural from glucose and glucans using maleic acid as a catalyst was studied. The mechanism of HMF formation involves the isomerization of glucose to fructose before undergoing the triple dehydration to form the product HMF. Maleic acid is a very selective catalyst in the formation of HMF from fructose. The maleic acid does not effectively catalyze glucose; an isomerization catalyst is needed. A Lewis acid AlCl3·6H2O was used to isomerize glucose to fructose. A mixed Lewis/Brønsted acid system (AlCl3/H2MA) was utilized to form HMF using MTHF as an extraction solvent. MTHF was used to extract the product HMF to prevent the degradation of the HMF to levulinic and formic acid. The original process utilized THF as an extraction solvent and required saturated NaCl in the aqueous phase. MTHF enabled the conversion reaction to proceed without the need of the large amount of salt. This was very selective method in the formation of HMF from pure glucose, mannose, and cellobiose. This method yielded moderate yields of HMF with respect to biomass variants. The mixed Lewis/Brønsted system also has very good concurrent yields of furfural when using actual plant biomass (corn stover, switchgrass, lodgepole pine wood, and poplar wood). Finally, migration of a silyl group in an oxorhenium (V) complex was investigated. This is an interesting reaction that provides insights on the mechanism of hydrosilylation involving oxorhenium catalysts. The mechanism could occur via intramolecular rearrangement of the oxo groups followed by a silyl group migration or by silyl group dissociation followed by re-association to oxorhenium anion (Re(O)2hoz2- ).
Degree
Ph.D.
Advisors
Abu-Omar, Purdue University.
Subject Area
Chemistry
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