Xylose, dehydration

The removal of water from initially formed biomass sugars is an important process for the production of primary biorefinery building blocks. Of particular interest are 5-hydrox-ymethylfurfural and levulinic acid (from the dehydration of glucose or other C6 sugars) and furfural (from xylose dehydration). Recent research has led to new catalytic processes for the production of each of these materials. 

Figure 3. Xylose dehydration rate constant as a function of catalyst cation ionization potential...

In this context, partially hydroxylated MgFj as well and modified by grafting fluorosulfonic Br0nsted sites on their surface were applied in the xylose dehydration with the aim to obtain FUR as main product [148]. 

You SJ, Park ED. Effects of dealumination and desUication of H-ZSM-5 on xylose dehydration. Microporous Mesoporous Mater 2014 186 121-9. 

Glucose/galactose malabsotption (GGM) is an intestinal monosaccharide (glucose and galactose) transport deficiency. The disorder manifests itself within the first weeks of life. The severe diarrhea and dehydration are usually fatal unless glucose and galactose are eliminated from the diet. Fiuctose and xylose are absorbed normally. Occurrence in both males and females, familial incidence, in particular in parental consanguinity, indicate autosomal recessive inheritance of... 

SCHEME 29. Dehydration of fructose and xylose to HMF and furfural, respectively. 

A delaminated zeolite with an Si/Al ratio of 29, derived from the layered zeolite Nu-6(1), was employed as catalyst for dehydration of xylose at 170 °C, using a water-toluene biphasic reactor-system.140 This material, designated del-Nu-6(l), proved to be efficient for this transformation, giving 47% selectivity to furfural at 90% xylose conversion. 

A new synthetic route for functionalized polyhydroxyalkyl-pyrimidines starting from unprotected aldoses and based on montmorillonite K-10 catalysis and solvent-free microwave irradiation conditions, has been reported by Yadav et al,m Thus, reaction of D-glucose and D-xylose with semicarbazide or thiosemicarbazide (186) in the presence of montmorillonite K-10, under microwave irradiation, proceeded via domino cycloisomerization, dehydrazination, and dehydration of the intermediate semi- or thiosemicarbazones (187) to afford l,3-oxazin-2-ones or l,3-oxazine-2-thiones (188) in one single step and in yields between 79% and 85% (Scheme 34). Other mineral catalysts tested, such as silica gel and basic alumina, were far less effective for this transformation and only silica gel was active at all, giving low yields (15-28%) of compounds 188a-d. The l,3-oxazin-2-ones(thiones) thus synthesized were subsequently converted into the target pyrimidines by reaction with aromatic... 

S. Lima, M. Pillinger, and A. A. Valente, Dehydration of D-xylose into furfural catalysed by solid acids derived from the layered zeolite Nu-6(1), Catal. Com-mun., 9 (2008) 2144-2148. 

The mechanism of pentose dehydration has been a matter of study for several years. The accepted pathway (see Scheme 1) to 2-furaldehyde from a pentose, in this case D-xylose (1), involves the reversible formation of a 1,2-enediol (2) followed by dehydration to the enolic form (3) of a 3-deoxypentosulose, which is further dehydrated to the 3,4-dideoxypent-3-enos-2-ulose (4) prior to cyclization to afford 2-furaldehyde 5. This mechanism, initially suggested by Isbell,has been substantiated by later work. This confirmation required incorporation of deuterium or tri-tium into the furaldehyde at various ring positions. However, when... 

D-xylose was converted into 2-furaldehyde in acidified, tritiated water, no carbon-bound isotope was detected. This suggested that the 1,2-enediol (2) reacted immediately, as otherwise, tritium would have been detected at the aldehydic carbon atom of 2-furaldehyde, as a result of aldose-ketose interconversion.An acidic dehydration performed with d-[2- H]xylose showed that an intramolecular C-2-C-1 hydrogen transfer had actually occurred. Thus, these data indicated that an intramolecular hydride shift is more probable than the previously accepted step involving a 1,2-enediol intermediate. 

Although small proportions of other products are formed when D-xylose is exposed to rather high acid concentrations, arabinose, lyxose, and ribose form considerably more of alternative products (generally reductic acid) than of 2-furaldehyde under these conditions. Reductic acid (2,3-dihydroxy-2-cyclopenten-l-one, 47) has been detected as a product after acid exposure of D-xylose or its major dehydration product, 2-furalde-hyde. Further work performed with D-[l- C]xylose and [a- C]2-fural-dehyde showed that reductic acid having identical label distribution was obtained from both starting materials. This indicated that a common primary source was involved, probably 2-furaldehyde, as it is readily formed from D-xylose under acidic conditions. 

Hydrolysis is the process by which a compound is broken down by reaction with water, thus it can be thought of as the opposite reaction of dehydration, where water is of course removed. Hydrolysis is a key reaction type in biomass chemistry, for it is central in the depolymerisation of polysaccharides to simpler monosaccharide building blocks, such as fructose, glucose, and xylose. 

Xylose. Catalytic dehydration of xylose, which is the most abundantly available pentose monomer in hemicellulose, has been known for a long time (Scheme 5). In fact, as early as 1922, an industrial process involving sulfuric acid catalyzed dehydration of xylose to produce furfural was developed by the Quaker Oats Co. 

Scheme 5 Xylose can be dehydrated to produce furfural. The reaction has been reported using several different catalysts including zeolites, sulfonic acid functionalized MCM-41 and immobilized heteropolyacids. The best selectivity towards furfural was achieved using zeolite H-mordenite, although at low conversion of xylose.Overall, the best yield of furfural was obtained using sulfonic acid functionalized MCM-41.

The major, monomeric dehydration-product, namely, 2-furaldehyde (27), formed during the reaction results from D-galacturonic acid in yields in the vicinity of 30% of the theoretical.110 This result is in contrast to its production from D-xylose,111 L-ascorbic acid,54 and D-arafotno-hexulosonic acid54 when treated under comparable conditions (3.3 M hydrochloric acid under reflux), these give almost theoretical yields of 27. 

Nuclear magnetic resonance (n.m.r.) studies have revealed that 4-amino-4-deoxy-L-xylose,35,350 4-amino-4,5-dideoxy-L-xylose,356 and 4-amino-4-deoxy-D-glucose34 (40) exist as equilibrium mixtures for example, of the pyrrolidine 40, the pyrroline 39 formed by its dehydration, and a dimer (41), the equilibrium lying strongly towards the last. Similar studies have shown that the hydrochlorides of 4-amino-... 

Hemicelluloses (cellulosans) is the family name of polysaccharides that includes pentosans (C3H804) , made up of the pentose units and hexosans (C6H,0O5) made up of hexose units. The pentosans include such substances as xylan and araban which are hydrolysed to xylose and arabinose respectively. On dehydration, furfuraldehyde is formed ... 

PENTOSAN. A complex carbohydrate (hemicellulose) present with the cellulose in many woody plant tissues, particularly cereal straws and brans, characterized by hydrolysis to give five-carbon-atom sugars (pentoses). Thus the pentosan xylan yields the sugar xylose (HOH,C CHOH CHOH CHOH CHO) that is dehydrated with sulfuric acid to yield furfural (C5H4O2). 

Furoic acid (furan-2-carboxylic acid, or pyromucic acid) is used as a bactericide, and the furoate esters are used as flavoring agents, as antibiotic and corticosteroid intermediates. It is obtained by the enzymatic or chemical/catalytic aerobial oxidation of furfural (2-furalaldehyde) the latter is the only unsaturated large-volume organic chemical prepared from carbohydrates today. D-Xylose and L-ara-binose, the pentoses contained in the xylan-rich portion of hemicelluloses from agricultural and forestry wastes, under the conditions used for hydrolysis undergo dehydration to furfural. 

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