1.3. The chemical pathway

The interaction between xylose and bases, either Bransted or Lewis, is rather less reported in the literature when compared to the acid conversion of xylose to furfural indicated in the previous section. Many very different reactions have been reported as in the case of Popoff and Theander [50] that have quantified the cyclic compounds produced after a base-cata­lyzed reaction of pure D-xylose at 96 °C for 4 hours. The produced compounds are rather peculiars in comparison to other work made on the subject (see Figure 5) since most of the reported compounds are aromatics. The presence of aromatics may be a result that the reac-

tion time was long and the isomerisation that was required in order to induce such reaction was efficient. Johansson and Samuelson [51] tested the effect of alkali treatments (NaOH) on birch xylan and contrarily to the previous research; they found that the treatment led to the production of a variety of organic acids. Testing on untreated xylene showed that most of the organic acids were already obtained from xylans and the most distinctive impact was observed after a 2 day test at 40 °C where the concentrations of L-galactonic and altronic acids increased significantly which could be related to a less severe treatment of xylans that also include C6 sugars.

El Khadem et al. [52] studied the effect of xylose conversion in an alkali medium at low tem­peratures (room) and for long periods (1-4 weeks) and one of the interesting features of his work was that the process did lead to the epimerization of sugars, but furthermore, it leads to the production of C6 sugars most probably from a reverse aldol reaction. Among the sug­ars that were formed during the reaction, conversion of xylose was shown to be more effi­cient to lyxose (18 %) and arabinose (15 %) with a decrease observed for most of the compounds between 1 and 4 weeks (see Figure 6). A vast majority (more than 50 %) of xy­lose remains on its original form and the reaction leads to the production of 1 % glucose and 2.5 % of sorbose, both are C6 sugars.

Xylose, as the other carbohydrates, is converted to smaller organic acids when reacted with a strong alkali medium. As an example, Jackson et al. [53] have demonstrated that the con­version of xylose to lactic acid could reach 64 % (molar) accompanied by glyceric acid. Al­though they did not used xylose but rather ribose and arabinose, they were able to reach conversions between 35-43 % into lactic acid using potassium hydroxide as catalyst under microwave irradiation [54]. Rahubadda et al. [55] have provided a mechanism for the con­version of xylose to lactic acid under a base catalyst. The simplified pathway is depicted in Figure 7 below.

They mentioned in this report that methylglyoxal is most probably derived from glyceralde — hyde as depicted in Figure 8 below. The possible reaction leading to methylglyoxal may in­volve an E2 reaction on C2 leading to removal of the hydroxyl group on C3 then a keto-enol rearrangement to methylglyoxal.

Onda et al. [56] achieved a conversion rate of more than 20 % when using xylose as a feed­stock with a carbon-supported platinum catalyst in alkali solution. In a recent report by Ma et al. [57], it was shown that using model compounds, different carbohydrates tend to con­vert into lactic acid at different levels. Fructose was shown to be more effectively converted to lactic acid than glucose and finally than xylose. The work also showed a correlation be­tween the amount of catalyst (varying from 1-3 % wt.) of NaOH, KOH and Ca(OH)2 respec­tively. Part of the work by Aspinall et al. [58] was aimed at the non-oxidative treatment of xylans from different substrates using sodium hydroxide as solvent. The reaction was per­formed at room temperature for 25 days and amongst the products that emerged from this reaction, a majority was acidic and lactic acid as well as formic acid were the two major products. Other work by Yang et al. [59] showed that higher temperature treatments of xy­lose (200 °C) in a Ca(OH)2 solution produced about 57 % (mol.) of lactic acid with 2,4-dihy — droxybutanoic acid in second with 10 % (mol.). The same conversion patterns were observed by Raharja et al. [60] with production rates for lactic acid above 50 %.