The Formation Mechanism Of Humins

In this study,HMF and furfural,the degradation products of glucose and xylose,were used as raw materials.We’ve lowered the reaction temperature and increased the concentration of reactants to capture the key intermediates in the formation of humins.By means of Py-GC-MS and other analytical methods,the structure evolution of humins at different temperatures was determined,and its structure models at different temperatures were obtained.Therefore,the formation of humins was deeply understood.The elementary reaction types involved in the formation of HMF humins include etherification,acetalization,esterification of HMF with LA and FA,and aldol condensation between HMF and LA.The infrared analysis of HMF humins at different reaction temperature and time reveals the priority of different reaction types in the formation process of humins.The-OH on HMF molecule reacts first,and it is the main reaction at 4℃.The consumption of-CHO is less than –OH,which results in the 1674cm-1 peak on the infrared spectrum remains almost the same in the whole process.At 20℃,the aldol condensation reaction increased gradually,which was reflected in the FTIR spectrum,that is,the peak 1674cm-1 decreased with time,while the peaks 1710cm-1 and 1620cm-1 increased.When the temperature rises to 170℃,the reaction speed is accelerated,and the aldol condensation reaction is further promoted.And the change of the key peaks are similar to the above.PyGC/MS confirmed that the benzene substances were produced due to pyrolysis.The content of benzene substances increased with the increase amount of C = C in the feed.The elementary reaction types involved in the formation of furfural humins include oxidation,electrophilic addition,esterification and acetalization.The infrared spectra of furfural humins at different temperature and time showed a similar change pattern compared to HMF humins.With the increase of reaction time,the absorption of 1670 cm-1 decreased,while the absorption of 1697 cm-1 and 1610 cm-1 increased.The absorption at 1670 cm-1 ascribed to the aldehyde group of furfural.1610 cm-1 corresponds to the carbon carbon double bond of C4 compound formed participated in humins formation,while 1697 cm-1 corresponds to the carbonyl and ester.The electron microscope analysis of furfural humins reveals the double-layer structure which has not been found before.By adding different amount of water into the reaction system,it is confirmed that the increase of water quantity can make the loose layer of double-layer structure more dense or dense layer thicker.The addition of LA can make the humins structure porous.With the increase of reaction time,the pore size decreased and the structure became more dense..