William Shu Lai Mok

Mechanism of formation of 5-(hydroxymethyl)-2-furaldehyde from D-fructose and sucrose.

The literature contains two alternative hypotheses for the mechanism of dehydration of fructose to 5-(hydroxymethyl)-2-furaldehyde (HMF), namely (1) a sequence of reactions commencing with and retaining the fructofuranose ring intact, and (2) a succession of reactions proceeding mainly via open-chain intermediates. The existing evidence for hypotheses (1) and (2) is reviewed and found to favor (1). The major products from fructose in water at 250 degrees, (with and without acid catalysis) have been investigated on a time-resolved basis and analysis of the results was found to confirm the first hypothesis. A necessary fructofuranosyl-cation intermediate in this hypothesis is produced directly by the hydrolysis of sucrose, and reacts to produce HMF in high yields.

Mechanism of formation of 2-furaldehyde from D-xylose

The literature records two alternative hypotheses for the mechanism of dehydration of xylose to 2-furaldehyde (furfural), namely (1) a succession of reactions proceeding mainly via open-chain intermediates, and (2) an acid-catalyzed sequence proceeding through a 2,5-anhydride intermediate. The existing evidence for hypotheses (1) and (2) is reviewed and found to favor (2). The major products from xylose in water at 250° (with and without acid catalysis) have been investigated on a time-resolved basis. A kinetic model based on the second hypothesis is found to be consistent with the experimental data; whereas kinetic models based on the first hypothesis do not fit the data.

Hydrogen production by steam reforming glucose in supercritical water

Results of the steam reforming of glucose in supercritical water at 34.5 MPa and temperatures between 450° and 650°C are presented. At 600°C and above glucose is completely gasified (no observable char or tar product) in less than one minute. The gas consists primarily of hydrogen and carbon dioxide, and a small amount of carbon monoxide and methane. Phase separation of the products exiting the reactor makes the gas available at very high pressure.

Kinetics of the thermal decomposition of cellulose in sealed vessels at elevated pressures. Effects of the presence of water on the reaction mechanism

Abstract The thermal decomposition of Avicel cellulose was studied by non-isothermal differential scanning calorimetry in hermetically sealed sample holders. The experimental results, which were published earlier, showed a marked catalytic effect of the water on the cellulose decomposition. Here we propose a reaction scheme containing two rate determining reactions to describe the result. The corresponding model resulted in a good fit between the experimental and the calculated data. The results indicate that the reaction starts with cellulose hydrolysis which may be followed immediately by decomposition reactions to intermediate products. The intermediates undergo further water-catalyzed decomposition reactions giving char, water and gases. The results may help in the understanding of biomass pyrolysis under experimental or industrial conditions where the thickness of the layer, the size of particles or the enclosure of the reactor keeps part of the water vapor formed during the reaction in the pores or between the particles of the decomposing material.

Biomass Fractionation by Hot Compressed Liquid Water

Samples of six woody and four herbaceous biomass species were washed with compressed, liquid water for 0 to 15 minutes at 200°–230°C. Between 40 and 60% of the sample mass was solubilized. In all cases, 100% of the hemicellulose was solubilized, of which 90% (on average) was recoverable as monomeric sugar. Concurrently, between 4–22% of the cellulose and 35–60% of the lignin were also solubilized. The solvolysis process is so facile that the results are practically independent of temperature and time within the range studied. The observed variations occurred primarily as a function of species. High lignin solubilization apparently reduced the ability to recover hemicellulose sugar.

A Study of the Acid Catalyzed Dehydration of Fructose in Near-Critical Water

Products of the acid catalyzed dehydration of fructose in near-critical water at 34.5 MPa and 250 oC are described as a function of residence time (2–32s), reactant concentration (0.01 – 0.1 M), and catalyst H2SO4 concentration (0 – 5 mM). The combined yields of 5-hydroxymethyl furfural and 2-furaldehyde reach 60% of the fructose feedstock.

Direct observation by molecular beam mass spectrometry of the photolytic enhancement of hydrocarbon reactions by concentrated sunlight

Photolytically induced free radical reactions at 150 °C in simulated concentrated sunlight have been studied using a molecular beam sampling mass spectrometer and a 1-kW xenon arc image furnace. The 1000-sun light source was focused on the entrance of a fused silica reactor maintained at 150 °C, where hydrocarbons and free radical initiators (such as acetone) were introduced. Reactions of the photolytically produced methyl radicals resulted in the formation of higher hydrocarbons from the initial hydrocarbon reactant. When the initial hydrocarbon reactant was an unsaturated species such as ethylene, propylene, or butylene, the observed product slate contained both unsaturated and saturated species. When the initial reactant was an alkane, only higher alkanes were observed. Similar results were obtained using acetaldehyde as the initiator. The observed effect is an example of the specific use of concentrated sunlight to influence chemical reactions of interest to the hydrocarbon-processing industry.