A Study On Production Of Carboxylic Acids From Lignite By Two-stage Alkali-oxygen Oxidation And The Structure Of The Lignite

Lignite is a kind of low rank coal,which has a high oxygen content,high moisture content and high volatility,so its application is limited.It has been reported that lignite can be oxidized into carboxylic acids in alkali aqueous solutions with O2 as oxidant at high temperatures.The carboxylic acids mainly contain benzene carboxylic acids(BCAs)and small-molecular fatty acids(SMFAs),and they are valuable chemicals widely used in the industries of pharmacy,dyes,rubber,and so on.The traditional alkali-oxygen oxidation of lignite does not consider the complex structures of lignite,and the structures with different reactivities are treated under identical and harsh conditions,resulting in a low yield of carboxylic acids.In this work,we proposed a two-stage oxidation process of lignite to increase the yield and mitigate the harsh conditions.First,lignite was extracted using a NaOH aqueous solution to yield two parts of lignite(extract(Ex)and residue(Re))with different reactivities.Ex and Re then were oxidized separately with alkali-oxygen oxidation under different conditions.In this way,BCAs(12 types)and SMFAs could be obtained with high yields.The extraction conditions and the alkali-oxygen oxidation conditions of the Ex and the Re were investigated.In this study,the yield of carboxylic acids(based on carbon content,the same below)from the Ex was 35.32%,including 5.60%formic acid,6.15%acetic acid,11.95%oxalic acid,0.85%succinic acid,and 11.13%BCAs,when the temperature,initial O2 pressure,reaction time,and NaOH content were 230 ℃,4.5 MPa,40 min,and 1 M,respectively.The yield of carboxylic acids from the Re was 36.69%,including 0.35%formic acid,5.05%acetic acid,11.81%oxalic acid,and 19.48%BCAs when the temperature,initial O2 pressure,reaction time,and NaOH content were 260 ℃,5.5 MPa,70 min,and 3 M,respectively.The result indicates that the total yield of carboxylic acids,based on lignite,is 36.28%,including 19.02%SMFAs and 17.31%BCAs.Compared with the traditional one-stage alkali-oxygen oxidation method,the two-stage alkali-oxygen oxidation method can significantly increase the yields of BCAs and SMFAs and decrease the usage of alkali.We have also found that the oxidation of Re needs higher reaction temperature,higher initial O2 pressure,higher alkali concentration,and longer reaction time than the oxidation of Ex,which further demonstrates the expectation that the Ex has higher reactivity than the Re.The structures of the Ex and Re were characterized by 13C nuclear magnetic resonance(’3C NMR)and Fourier transform infrared(FT-IR)spectroscopy.The results indicate that most of aromatic structures in the Ex are single aromatic rings with side chains containing many oxygen-containing functional groups.While,aromatic structures in the Re are mainly condensed aromatic rings with side chains containing alkyl chains.The distribution of BCAs could give some information of the aromatic clusters of the Ex and Re.At last,based on the distributions of BCAs and combined with the structural parameters of carbon in Ex and Re from 13C NMR and FT-IR,we proposed organic structure models of Ex and Re.The models are consistent with not only the characterization data,but also the distributions of BCAs from Ex and Re via oxidation.It shows that the models can reflect the structural characteristics of Ex and Re from the lignite.