Furfural resinification

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. 

This chapter will deal with the case when loss reactions are permitted to proceed. 

With their ampoule process described in the preceding chapter. Root, Saeman, Harris, and Neill [18] determined the resinification loss by starting with aqueous furfural solutions of various acidities. According to -d[FU]/dt = kz [FU] 

Combining this expression with equation (3) of the preceding chapter leads to k2/ki = 0.181 X 10 X exp(+5786 °K/T) 

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A reaction of furfural with itself, commonly called furfural resinification . 

Several early interpretations of the polymerization mechanism have been proposed (1,17,29—31). Because of the complexity of this polymerization and insoluble character of the products, key intermediates have not ordinarily been isolated, nor have the products been characterized. Later work, however, on the resinification of furfural (32,33) has provided a new insight on the polymerization mechanism, particularly with respect to thermal reaction at 100—250°C in the absence of air. Based on the isolation and characterization of two intermediate products (9) and (10), stmcture (11) was proposed for the final resin. This work also explains the color produced during resinification, which always is a characteristic of the final polymer (33). The resinification chemistry is discussed in a recent review (5). 

Comparatively little is known of the chemistry of resinification of either furfuryl alcohol or furfural. 

The photopolymerization of furfural by UV radiation has not received much attention. Although the products of heat polymerization of furfural are branched polycondensates with highly conjugated structures, the photopolymer of furfural is a linear polyaddition product (8,9). The gas-phase photolysis of furfural in the n — 7r and ir — 7r transitions (10) proceeds with fragmentation to carbon monoxide, furan, and C3 hydrocarbons, but a certain amount of resinification has also been noted (about 5% quantum yield with excitation of the n — 7r transition). Vacuum liquid-phase photolysis by UV radiation at room temperature has produced linear polymers (4) with a degree of polymerization of about... 

The resinification loss due to furfural reacting with itself can be determined directly, easily, and accurately by studying acidified aqueous furfural solutions. Thus, the rate constant of the resinification reaction is known precisely as formulated in subchapter 6.1. 

With resinification alone, without condensation, the furfural to be expected from the disappearance of xylose must be diminished by the term k2[FU] to give... 

Figure 130. The Yield of a Hypothetical Furfural Process involving Resinification as the Only Loss. Graphical representation of equation (7) in dimensionless form.

Left) Xylulose and furfural from isomerization and dehydration reaction together with by-products derived from the condensation and the resinification reactions. (Right) Low-molecular-mass organic acids, giycolaidehyde or pyruvaldehyde, from different retro-aldol reactions [148]. 

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