Furfural Loss Reactions

As pointed out already in the preceding chapter, not all of the pentose consumed will necessarily end up as furfural, the reason being that in addition to the dehydration of the pentose two sequence reactions, both involving the furfural product, can take place  

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Inasmuch as the furfural loss reactions lead to larger molecules, they are analogous to polymerization, with its buildup of oligomer and polymer molecules from monomer molecules. In representing an increase in order , this buildup of larger molecules causes a decrease in entropy, so that the entropy change of the reaction ASr is a negative quantity. 

In other words, as the temperature approaches Tc, the furfural loss reactions diminish, so that the yield increases, and eventually, at a sufficiently high temperature, these loss reactions become insignificant, so that the furfural yield approaches 100 percent. 

Furfural is produced by hydrolysing pentoses of several natural products. As a by-product, furfural is also formed during decomposition of wood in paper mills. It must be separated from the aqueous effluent rapidly or secondary reactions will cause an increasing furfural loss by polymerisation or polycondensation. Distillative separation methods lead to a diminished recovery of furfural of only 35%. 

I omucic Acid.—When mucic acid is heated three molecules of water and one of carbon dioxide are lost and a monobasic acid is obtained known as pyromucic acid, and this acid by loss of CO2 goes to furfuran thus proving it to be the acid derived from furfuran. The acid is also obtained by oxidizing furfural. The reactions and relationships are as follows ... 

The reactions (1) and (2) may or may not take place, depending on whether or not the furfural formed by the dehydration of pentose is permitted to stay dissolved in the liquid phase. Reactions (1) and (2) represent loss reactions in that they consume furfural and lead to products other than furfural. Hence, when the reactions (1) and (2) are avoided, by measures to be discussed later, then all of the disappearing pentose is converted to furfural. In this case, furfural is obtained at theoretical yield. By contrast, when the reactions (1) and (2) do take place, then the quantity actually produced will be smaller than the theoretical yield, and the extent of the losses will depend on how long the furfural is permitted to stay and react in the liquid reaction medium. 

The loss reactions are possible only in the liquid phase, whereas they cannot take place in the vapor phase as the latter is devoid of catalytically active species. Thus, if furfural is instantly vaporized as it is formed, no loss reactions occur, and the yield will be 100 percent. 

In view of this situation, it may seem surprising that in the ampoule process , without any removal of furfural, the losses are hardly greater than in the industrial processes with their huge expense for steam stripping. The explanation lies in the simple facts that at any time the loss reactions are slower than the furfural formation, and that the principal loss, which is furfural condensation, diminishes as the xylose concentration diminishes, so that it comes to a halt when all of the xylose is consumed. 

The first process to make furfural from sulfite liquor was offered by VOEST-ALPINE of Austria in 1988. In this process, shown schematically in Figure 32, the sulfite liquor is first thickened to a dry solids content of 30 %. After heating the concentrate to 180 °C, and after holding it at this temperature in a tube reactor for a period of time sufficient to convert some pentose to furfural, the reaction mixture is passed into a distillation column where the furfural is stripped by steam. The treatment of providing residence time at 180 C in a tube reactor to convert more pentose to furfural, followed by removal of the furfural in a stripping column, is repeated two times. In this fashion, the furfural is removed stepwise soon after its formation, to reduce losses by furfural reacting with itself, with intermediates of the pentose-to-furfural conversion, and with other constituents of the liquor. 

In 27 runs at various conditions, the operators of this pilot plant found to their distress that the furfural yields were only in the order of 30 percent, in harsh contrast to their expectations of 85 percent, derived from calculations based on the known kinetics in water. The designers had made a fundamental mistake They had measured the rate of pentose disappearance in the sulfite liquor, but not the rate of furfural formation, and as the rate of pentose disappearance was far greater than what had to be expected when the liquor s hydrogen ion concentration was used in the known kinetics of xylose disappearance in acid water, they had concluded that the lignosulfonate of the liquor had a special catalytic effect on the pentose-to-furfural conversion. They had overlooked that the fast disappearance of the pentose in the liquor was not due to a mysterious catalysis but caused by loss reactions of the pentose with lignosulfonate and other ingredients of the liquor. 

In In the chapters 6 and 7, it was discussed in detail that in furfural reactors any furfural in the vapor phase is safe , i.e. incapable of undergoing loss reactions, but that any furfural in solution can react with itself and with intermediates of the xylose-to-furfural conversion as the solution contains hydrogen ions capable of specific acid catalysis as well as neutral molecules capable of general acid catalysis . 

The same reasoning applies to the loss reactions in furfural reactors Above a critical temperature Tc, there will be no significant loss reactions, and on the way to this... 

It is a tragedy that the first industrial furfural process, described in the introduction, had to be carried out in a total absence of these facts, and that its temperature was limited by the low pressure capacity of old reactors from an abandoned cereal process, so that severe loss reactions and correspondingly low yields became a trademark of the furfural industry from the very start. 

In acidic solution, the degradation results in the formation of furfural, furfuryl alcohol, 2-furoic acid, 3-hydroxyfurfural, furoin, 2-methyl-3,8-dihydroxychroman, ethylglyoxal, and several condensation products (36). Many metals, especially copper, cataly2e the oxidation of L-ascorbic acid. Oxalic acid and copper form a chelate complex which prevents the ascorbic acid-copper-complex formation and therefore oxalic acid inhibits effectively the oxidation of L-ascorbic acid. L-Ascorbic acid can also be stabilized with metaphosphoric acid, amino acids, 8-hydroxyquinoline, glycols, sugars, and trichloracetic acid (38). Another catalytic reaction which accounts for loss of L-ascorbic acid occurs with enzymes, eg, L-ascorbic acid oxidase, a copper protein-containing enzyme. 

The Cannizzaro reaction of heterocyclic aldehydes has been examined in a few cases only. Furfural,83 a-thiophenealdehyde,84 and a-pyridylal-dehyde 86 undergo the reaction normally to give the expected products. 3-Formyl-l,2,5,6-tetrahydro-l-ethylpyridine resinifies upon treatment with potassium hydroxide,86 a behavior consistent with the observation that it is structurally similar to an a,/3-unsaturated alicyclic aldehyde. 3,4-Dibromothiophene-2,5-dialdehyde undergoes a complex series of reactions, involving both cleavage (loss of —CHO) and dismutation, when treated with alkali.87 Of particular interest in this connection is the fact that under certain conditions the ester composed of the usual... 

The stability of the furan ring in the environment of acid-catalyzed mixtures was investigated by an experiment where 1 mole of furfural was added to a mixture containing 0.67 mole 1,1-dimethylurea (DMU), 1.33 moles 2,4-dimethylphenol, and acid catalyst. The hindered urea and phenol were chosen to limit products to simple, small compounds. To limit losses of the somewhat volatile furfural (bp 162 °C) over the 3-hour reaction time, the reaction flask was heated by a steam bath, using vapor temperatures (36 to 69 °C) much lower than in previous experiments. 

Although [fNT] can be taken to be proportional to the xylose concentration, there is no known experimental way to determine ka and kb explicitly. What is possible is to measure the actual yield as a function of time, xylose concentration, acidity, and temperature, for the experimental setup chosen, and to use these yield curves, together with the known pentose disappearance rate and the known furfural reslnification rate, as a graphical interpolation basis for determining the losses by the condensation reactions. Such a procedure, reported by Root, Saeman, Harris, and Neill [18], is given in an appendix chapter, but it is usually considered too complicated and too unreliable to be used for yield prognoses. 

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