Furfural from pentosans

Volumetric methods for determination of furfural from pentosans in-... 

This compound is one of the more important furan derivatives, and is commercially available from pentosans (polysaccharides) which are present in rice husks, oats and corn residues (furfur is the Latin name for bran ). When treated with sulfuric acid, pentosans decompose into pentoses, which then undergo dehydration to the aldehyde (Scheme 6.31). 

Furanose and pyranose, origin and definition of terms, III, 18 Furanose and pyranose rings, method of distinction in some cases, III, 103 Furfural, commercial production from pentosans, V, 288 from bagasse, IV, 296 from wood saccharification, IV, 178 —, 5-hydroxymethyl-, IV, 5, 307, 314, 336... 

The researches have dealt with each phase of the pentosan determination. Many modifications have been introduced in the distillation procedure, in attempts to obtain ma.ximal yields of furfural. For e.xample, some workers prefer to distil with 23 % hydrobromic acid, rather than with the conventional 12% hydrochloric acid. Others distil in the presence of added sodium chloride, to avoid changes in acid concentration. Steam distillation has been used by a number of workers, who claim theoretical yields of furfural from pentoses, but Launer and Wilson found no advantage either in salts or in steam in the analysis of pulps and papers. Interfering substances are of two types materials other than pentosans which form furfural in the pentosan analysis, and substances which yield products which may be determined as furfural. TJronic acids and polyuronides yield furfural, although not quantitatively, and, in the case of materials containing appreciable quantities of these substances, it is usual to make a correction. The value of the correction to be applied has been determined experimentally by several workers, with somewhat differing results. 

Norris and Resch determined furfural yields from uronic acids, under the conditions normally used for pentosan determination, and obtained about 37 to 44% of the theoretical yield of furfural. In later work, the effect of various furfural-yielding substances on one another was ascertained, and a standardized procedure was developed for analysis of the types of mixtures commonly met with in cell-wall chemistry. Apparently, the yield of furfural from uronic acids and polyuronides varies with the proportion of these substances present, and with their ratio to other sugars. From oxycellulose, TJnruh and Kenyon obtained furfural yields of 18 to 20 % of the theoretical on the basis of carboxyl content. Wise and Ratliff, however, in summative analysis of wood, used a correction based on the assumption that the uronic acids present evolve 35% of the theoretical amount of furfural. 

The rate of pentose formation from pentosan is proportional to the pentosan concentration, but is diminished by the sequence reaction of pentose to furfural, so that... 

With the sealed ampoule process used for their kinetic studies, Root, Saeman, Harris, and Neill [20] achieved fiirfiiral yields well in excess of 70 % at temperatures above 220 °C, whereas industrial furfural processes, operating at lower temperatures and featuring a continuous removal of the furfural by steam stripping, have typical yields below 60 %. By contrast, in analytical chemistry, at a proven yield of 100 % [21], the formation of furfural from xylose or pentosan is routinely used for the quantitative determination of these substances. It is of great importance to elaborate the reasons for this yield paradox . 

Furfural and thence furan, by vapour-phase decarbonylation, are available in bulk and represent the starting points for many furan syntheses. The aldehyde is manufactured from xylose, obtained in turn from pentosans, which are polysaccharides extracted from many plants, e.g. com cobs and rice husks. Acid catalyses the overall loss of three mole equivalents of water in very good yield. The precise order of events in the multi-step process is not known for certain, however a reasonable sequence " is shown below. 

Furfural. 2-Fttraucarboxaldehyde 2-furaldehyde pyromucic aldehyde artificial oil of ants "furfuro] . C5-H402 mol wt 96.08. C 62-50%, H 4.19%, O 33.30%. Occurs in some essentia] oils. Prepd industrially from pentosans which are contained in cereal straws and brans. Laboratory prepn from corncobs It. Adams, V. Voorhees, Org. Syn. 

Furfural is prepared industrially from pentosans which are contained in cereal strains and brans. It is obtained in the laboratory from corncobs. ... 

The two most important natural pentoses, 1 -arabinose and 1 -xylose, occur in nature as polymeric anhydrides, the so-called pentosans, viz. araban, the chief constituent of many vegetable gums (cherry gum, gum arabic, bran gum), and xylan, in wood. From these pentapolyoses there are produced by hydrolysis first the simple pentoses which are then converted by sufficiently strong acids into furfural. This aldehyde is thus also produced as a by-product in the saccharification of wood (cellulose) by dilute acids. Furfural, being a tertiary aldehyde, is very similar to benzaldehyde, and like the latter undergoes the acyloin reaction (furoin) and takes part in the Perkin synthesis. It also resembles benzaldehyde in its reaction with ammonia (p. 215). 

Decarbonylation. Furfural is easily obtained from biomass waste such as oat and rice hulls that are rich in pentosans. Further valorisation of furfural can be done by decarbonylation to produce furan, which can be further converted into tetrahydrofuran by catalytic hydrogenation. 

Furan derivatives remained commercially insignificant until about 1920, when furfural was prepared in bulk from acid digestion of the pentosans of oat hulls and corn cobs for use as an industrial solvent and in the preparation of synthetic resins. How this change was wrought is one of the epics of industrial chemistry (B-56MI31000). 

Pentosans. These compounds are polysaccharides which may he considered as anhydrides of pentose sugars, after the manner of the hexosans. sucrose, starch, from glucose, fructose. When pentosans or pentoses are heated with hydrochloric ot sulfuric acid, furfural C fljO CHO is formed, and addition of aniline produces a red color. Pentosans are present in gummy carbohydrates, in bran of wheat seed, and in woods. 

Furfural is obtained commercially by mealing pentosan-rich ugricullural residues (corncobs, oat hulls, cottonseed hulls, hagasse. rice hulls) with a dilute acid and removing the furfural by steam distillation. Major industrial uses of furfuraldehyde include (1) the production of t urnns and tetrahydrofurans where the compound is an intermediate (2) Ihc solvent refining of petroleum and rosin products (3) the solvent binding of bonded phenolic products and (4) the extractive distillation of butadiene from other C4 hydrocarbons. 

Cross and Bevan Cellulose- An impure cellulose isolated from the complex it forms with lignin, in materials such as wood straw. The method of isolation is described by Doree (Ref 1) Ott (Ref 2). Cross Bevan cellulose contains both furfural-yielding substances (pentosans), and hexosans of the mannan type. It retains, in the case of coniferous woods, 40-60% of the total furfural-yielding substances in the original wood, and about 55 66% iq the case of hard woods. 

When an acid solution of phloroglucinol (Pt. 11) is added to furfural a black precipitate, known as a phloroglucid, is formed. From the weight of this phloroglucid we. may calculate, by empirical methods, the amount of furfural and, from this, the amount of pentose sugar with which we started, or also, the amount of pentosan.. This is the basis of the analytical method for the determination of pentosan compounds in plant materials. As the pentosans possess considerable nutritive... 

Pentosan Reagent.—With furfural (p. 853) it forms an insoluble greenish black phloroglucid compound. As pentosans (p. 338) yield furfural on boiling with hydrochloric acid, phloroglucinol is used as a reagent for the determination of the furfural obtained and from this the amount of pentosan is calculated empirically. This is the official method for determination of pentosans in food stuffs. 

Furfural is universally made from agricultural raw materials rich in pentosan. By aqueous acid catalysis, the pentosan is hydrolyzed to pentose, and this pentose is dehydrated to furfural in a unified process. 

The production of furfural requires raw materials rich in pentosan. The pentosan content of some materials is given in Table 1. From these figures, it is readily understood why most furfural plants use corncobs. Bagasse, employed widely in hot climates, has not only less pentosan but also a very low bulk density, so that plants using this inferior raw material must accept the significant disadvantage of operating with less mass per unit of reactor volume. 

In the SUPRAYIELD process, the delayed decompression is a degenerated flash process slowed down to such an extent that the period of time for going from a high primary pressure to a lower secondary pressure corresponds to the reaction time needed for the desired conversion of pentosan to furfural. To make this a practical proposition, the primary temperature must be high, say 240 °C, and the secondary temperature should not be below 180 °C as in this range the reaction rate would be too slow. 

According to Hagglund, for woods the ratio of methyl pentosan to pentosan ranges from 0.04251 to 0.73256. It is believed that for bagasse, com cobs, and other raw materials of furfural manufacture this ratio is smaller than the low value of this range for wood, so that levels in the order of 1 percent are considered as reasonable. 

As discussed in chapter 11, the methyl pentosan content of the raw material is of considerable interest as it determines the level of 5-methyl furfural and furyl methyl ketone in the final furfural. Both 5-methyl furfural and furyl methyl ketone are unwanted by-products requiring a sizable expense if they are to be removed from the furfural, and if they exceed the critical level of 1 %, the furfural thus loaded loses the attribute of being a fine chemical , which is an important aspect for pricing the product. 

Calculating W minus W2 gives the weight of methyl furfural phloroglucide, from which the amount of methyl pentosan in the original sample can be computed, with 1 mg of methyl furfural phloroglucide corresponding to 0.61867 mg of methyl pentosan. 

After dilute acid treatment most of the hemicellulose is removed from the cellulosic materials. The other functions of dilute acid prehydrolysis include the increase in surface area and pore volume. However, the crystallinity of the cellulose changes little [16]. Dilute acid treatment is carried out at 110-220°C and 0.3-1.1% acid concentration. If the acid treatment is performed at elevated temperature,pentosans maybe converted into furfural,which is an inhibitor for most microorganisms. 

Wood contains from 8 to 10 per cent of a gum which is a mixture of complex carbohydrates. Wood gum yields xylose on hydrolysis and, like other substances which contain pentosans, is. converted into furfural on boiling with hydrochloric acid. The formation of furfural, it will be recalled, is a test for a pentose (365). 

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