Geoffrey N. Richards

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.

Glycolaldehyde from pyrolysis of cellulose

Abstract Glycolaldehyde (hydroxyacetaldehyde) is found in yields up to 9.2% from vacuum pyrolysis of cellulose at 350°C. This observation confirms a recent report of formation of high yields of glycolaldehyde from fast pyrolysis of cellulose and suggests that glycolaldehyde is normally a major pyrolysis product of cellulose which has often escaped collection or detection in earlier studies.

Influence of metal ions and of salts on products from pyrolysis of wood : applications to thermochemical processing of newsprint and biomass

Abstract Several different individual metal ions have been incorporated into cottonwood by ion exchange and the products of vacuum pyrolysis have been investigated. K, Li and Ca induced formation of high char and low tar yields, the latter being very low in levoglucosan (LG, 1). In contrast, all of the other ions investigated (especially the transition metals) gave increased yields of LG, up to 15.8% from wood exchanged with ferrous ions, corresponding to 32% conversion of cellulose to LG. When salts were sorbed into the wood (instead of ion exchanged) smaller, but still major increases in LG yield were again obtained, especially when indigenous salts in the wood were first removed by acid washing. With sorbed salts the effect of the anion was investigated. Wood sorbed with acetate salts behaved similarly to ion-exchanged wood, and chlorides (especially of Cu and Fe) were also beneficial in terms of LG yield. With low levels of sorbed FeSO 4 in wood the formation of both LG and levoglucosenone (LGO, 2) was catalyzed. Similar advantageous effects were observed with newsprint sorbed with FeSO 4 , which yielded 17.2% of LG and 3.7% of LGO as well as other significant products. The possible mechanisms of these effects are discussed. A coordinated approach to the chemical utilization of lignocellulosic biomass is foreshadowed, whereby ions such as Fe or Cu are incorporated into the biomass to catalyze pyrolytic production of LG and/or LGO, while at the same time generating an increased yield of charcoal. The latter also has the added potential benefit that it now contains an efficient gasification catalyst.

Gasification of biomass chars in carbon dioxide: dependence of gasification rate on the indigenous metal content

Chars were prepared under standard conditions (750 °C, N2) from ten different types of biomass belonging to a wide range of plant origins and with a wide range of natural metal ion contents. When these chars were gasified with carbon dioxide at 700 °C the rates of gasification were found to show linear dependence on the combined molar concentration of the dominant metals (potassium and calcium), with correlation coefficient r = 0.970, provided two samples were omitted. The samples omitted were from wheat straw and coir dust, which had the highest silicon content of the samples studied and showed abnormally low rates of gasification. It was concluded that the catalytic effect of K was reduced by reaction with silica to form silicate during pyrolysis. This was confirmed by addition of silica to a high K, low Si sample (potato pulp) and by observation of the reduced gasification rate in the derived char. Catalysis of gasification by Ca (in lignite) does not appear to be significantly reduced by silica under the above conditions. These results indicate that the gasification reactivities of chars from any type of biomass can be predicted with reasonable certainty from knowledge of the metal ion content, provided the biomass has low Si content. It also appears that the chemical and physical properties of the chars are remarkably similar, despite the wide range of plant species and morphologies from which they are derived.

Influence of metal ions on volatile products of pyrolysis of wood

Abstract Volatile products from the pyrolysis of wood have been determined by gas-phase Fourier transform infrared spectrometry, coupled to a thermal balance. Progressive yields of water, carbon dioxide, carbon monoxide, acetic and formic acids and methanol have been determined isothermally at 523 K and at 373–823 K. The influence of the removal of inorganic salts and of ion-exchanged cations on these yields has been studied, as also has the effect of adding back potassium ions and calcium ions by ion exchange. Potassium, but not calcium, acts as a catalyst in pyrolytic reactions resulting in formation of carbon dioxide and carbon monoxide (especially from polysaccharides), acetic acid (from hemicelluloses), formic acid (from polysaccharides) and methanol (from lignin).

Influence of linkage position and orientation in pyrolysis of polysaccharides: A study of several glucans☆

Abstract Six naturally occurring and three synthetic glucans have been studied. The natural glucans contain 1,3-, 1,4- and 1,6- linkages with α- and β-orientations, while the synthetic glucans contain all possible linkage positions and orientations and also varying proportions of pyranose and furanose forms. The volatile products of pyrolysis are very dependent on the presence of trace amounts of inorganic-contaminants, which were detected as metal ions in all of the natural glucans. For most glucans these contaminants were effectively removed by washing with dilute acid. This removal of contaminants was, however, incomplete in the cases of amylose and of laminaran. The glucans, after purification in this way, all gave very similar pyrolytic yields of levoglucosan (LG), showing that LG formation is not dependent on polysaccharide linkage position or orientation, and thus confirming a recent hypothesis for mechanism of LG formation. The presence during pyrolysis of inorganic compounds (either indigenous or added), always led to decrease in LG and increase in yield of formic and acetic acids, glycolaldehyde, hydroxyacetone and phenols, thus indicating that the mechanism of formation of such compounds is also not dependent on polysaccharide linkage position or orientation.

First chemical events in pyrolysis of wood

Wood has been heated at 250°C on a thermal balance and the evolved gases have been analyzed by Fourier transform infrared spectroscopy (FTIR). The heated wood has been analyzed for glycoses, uronic acids and by nitrobenzene oxidation to vanillin and syringaldehyde. About 60% of the weight loss is accounted for by five compounds, which are the only products detected in the gases by FTIR. These products are water, carbon dioxide, methanol, acetic acid and formic acid. By relating the rates of formation of these compounds to weight loss, the following major conclusions are reached regarding the first chemical events in pyrolysis of wood. Uronic acids in the hemicelluloses and pectic substances decompose very readily to yield carbon dioxide, water, char (or char precursors) and perhaps some methanol. This decomposition may lead to further pyrolysis of the xylose units to which the uronic acids are attached in the hemicelluloses. Acetyl ester groups in the hemicelluloses are much more resistant to pyrolysis, but are released slowly as acetic acid. A small proportion of the potential methanol product is released very readily and at least part of this product is derived from lignin. Formic acid is released at a slow and continuing rate at 250°C by unknown mechanisms and is probably derived from degradation of hemicelluloses.

Thermal synthesis and pyrolysis of a xylan

Abstract A xylan has been synthesized in yields up to 64% by thermal polymerization of methyl β- d -xylopyranoside, catalyzed by phosphoric acid. The polymer is highly branched, with d.p. ∼20. The xylose units in the polymer are predominantly pyranose, with some furanose, and 14% of the nonreducing end groups are in the furanose form. The “reducing” end groups are at least partly in the form of methyl xylosides. Vacuum pyrolysis of the pure synthetic xylan gave a high char yield (48%, compare, for example 5% from pure cellulose), 1,4-anhydro-α- d -xylopyranose (5.7%), and 1,5-anhydro-4-deoxypent-1-en-3-ulose (2.4%). The structure of the latter compound was verified by degradation experiments and it was shown to be the compound previously isolated from pyrolysis of natural xylans and incorrectly assigned as 3-hydroxy-2-penteno-1,5-lactone. This glyculose has considerable importance as a marker compound for xylans in pyrolysis of biomass materials.

Measurement and modeling of air toxins from smoldering combustion of biomass.

Oxygenated organic compounds in condensed (-45 °C) smoke of 29 bench-scale fires of ponderosa pine sapwood, needles, bark, litter, duff, and humus have been identified and quantified under three types of conditions (smoldering, self-sustained smoldering, and flaming). The analyses were performed by gas chromatography/mass spectrometry and gas chromatography/flame ionization detection. The major condensible emissions were acetic acid, 2-furaldehyde, vinyl acetate, acetol, and methanol. The oxygenated organic emissions have been shown to be dependent primarily on fuel chemistry and secondarily on combustion efficiency. Molar ratios of individual compound emissions to CO emissions have been calculated, and exposure levels to these compounds for wildland firefighters have been estimated based on the ratios. Of the compounds measured, none was projected to exceed a toxic level, except for 2-furaldehyde and vinyl acetate, which are suspected carcinogens.