| As one of the important components in biomass,lignin is the most abundant carriers of the natural aromatic biopolymer,which is featured by extensive and renewable.However,the application of lignin is still used as low value-added products such as combustion and water reducer due to the higher cost of lignin extraction,low purity and uniformity of the extracted lignin.In order to explore the extraction technology for higher quality lignin which should meet the requirement as feedstocks for producing high value-added products,the present study was carried out on walnut shell in the following aspects:(1)Liquid hot water(LHW)coupled with high boiling solvent extraction was adopted to improve the quality of the extracted lignin.The traditional high boiling solvent extraction on lignin is characterized with lower extraction ratio,lower purity caused by containing residual carbohydrates,etc.To remove hemicellulose in walnut shell,liquid hot water was firstly used prior to lignin extraction.It was found that 83.45%of hemicellulose was removed and thus increased the lignin content by 14.91%.Subsequently,1,4-butanediol was used to extract lignin from the LHW-pretreated walnut shell.The lignin extraction ratio reached 44.66%which was almost 14.66%higher than that from single 1,4-butanediol extraction.Moreover,the carbohydrates left in the extracted lignin were greatly reduced to 0.15%-3.70%,while it was 4.10%-5.47in direct 1,4-butanediol extracted lignin.After the coupled extraction process,the high boiling solvent was easily recovered and reused due to its lower remained carbohydrates.This would increase its industrial feasibility.(2)Alkali extraction on lignin was optimized.After investigation on the effect of several factors including NaOH concentration,temperature,and extraction time on lignin extraction,the optimal condition was obtained with 12.5%NaOH(W/V),190℃,3.0 hrs.The maximum lignin dissolution ratio of 82.29%was achieved under this condition.To further improve the lignin extraction ratio,organic solvent combined with alkali was tested due to the better lignin dissolution property of organic solvent and alkali.The results showed that about 98.98%of the lignin was dissolved at 180℃ for 3.0 hrs in 15%ethanol(W/V)containing 15.0%NaOH(W/V).Via stepwise precipitation with pH adjusting,it was found that the purity of the lignin obtained was increasing.Besides,SEM images indicate that the submicron particles of lignin gradually bonded with each other and finally formed the porous structure.(3)The application of lignin in substituting phenol to produce phenol-formaldehyde resin foam was carried out.When 10%of industry lignin(w/w)was added,the compression strength of the synthesized foam were 0.10 MPa,38.88%higher than that without lignin addition.To obtain the most suitable lignin used in foams synthesis,different lignin including alkaline,alkaline-ethanol,and high boiling solvent lignin was tested.It was observed that addition of alkaline lignin brought better performance on foam with apparent density at 58.8 Kg/m3 and compression strength at 0.47 MPa,which was 571.42%higher than that of the control.The SEM images of alkaline lignin-based phenolic foams show that it had smaller,uniform and complete pores.According to the above studies,higher lignin dissolution ratio and fine lignin quality has been achieved,which proved the feasibility of lignin extraction by high boiling solvent combining with LHW or alkali coupling with ethanol.Meanwhile,the possibility of lignin to substitute phenol in phenol-formaldehyde resin foam synthesis was confirmed.All of these conclusions would provide both the theoretical and experimental basis on lignin extraction and application. |
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