Furfuryl resinification

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

Kinetic observations of the homogeneous part of the reaction in water12,13 do not provide any substantially new element to the knowledge of this system. The obvious observations that the rate of resinification increases with increasing temperature and decreasing pH of the mixture only provide technically useful correlation parameters and the zero-order of reactions carried out to small conversion of 2-furfuryl alcohol13 does not indicate anything except an elementary kinetic approximation (the use of colour build-up as a criterion for the extent of alcohol consumed is also questionable since no firm relationship has ever been established between these two quantities). 

The resinification induced by 7-alumina11 seems to proceed by a somewhat different mechanism, probably because of the higher temperatures involved. Side reactions are more prominent from the beginning and it has been suggested, but not proved, that the C-3 and C-4 positions of the ring are vulnerable under these conditions to substitution reactions. A new compound, viz. 4-(2-furfuryl)-2-pentenoic acid-7-lactone, which is an isomer of the condensed dimers, was identified among... 

In conclusion, the self-condensation of 2-furaldehyde promoted by heat occurs with the formation of di- and trifurylic intermediates. The functionality of the growing chain increases after each oligomerization step until gelation and precipitation of the resin occurs. Thus, the process is non-linear from the onset since the condensation product 4 possesses three sites for further attack, namely the free C-5 position and the two formyl groups. It is interestering to note that while the polycondensation of 2-furfuryl alcohol is essentially linear and cross-linking is due to side reactions, the thermal resinification of 2-furaldehyde is intrinsically non-linear and gel formation occurs at earlier conversions. 

Compounds which are rather unstable. Typical members of this class are 2-furaidehyde, 2-furfuryl alcohol and 2-alkyl furans, the latter being more resistant than the former. The action of acids or oxygen on these derivatives produces appreciable resinification, but, if properly purified and stored in vacuo, they are indefinitely stable25 16s. ... 

To top off the appeal, the polymerization of furfuryl alcohol can be made to run with almost any partner containing reactive hydrogen. Thus, furfuryl alcohol can be reacted with formaldehyde to give a series of thermosetting resins featuring more methylene bridges than are obtained by the resinification of furfuryl alcohol alone. Such resins are commonly used for the bonding of wood. 

Hydroxyethylation of furfuryl alcohol with aqueous acetaldehyde was performed at 318 K in the presence of FAU, MOR and MFI zeolites in their protonic form. Besides the hydroxyethylation reaction, two other parallel reactions may compete, i.e. oligomerisation of acetaldehyde and resinification of furfuryl alcohol. The former reaction is easily controlled over a MFI catalyst with a Si/Al ratio of 25 but the latter is not, and linear furanic polymers, capable of deactivating the catalyst, were often formed. A selectivity of about 55 % in the carbinol intermediate was initially achieved in the presence of the MFI (25) catalyst for a furfuryl alcohol conversion up to 65 %. However, after optimization of the kinetic parameters, the selectivity is increased to 95 %, but to the detriment of furfuryl alcohol conversion. 

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