Catalytic Conversion Of Lignin Extracted From Straw To Aromatic Aldehyde

Resource and environmental problems are becoming more and more serious as the increase in the dependence on fossil fuels.Therefore,much attention has been paid to find alternative renewable resources.About 900 million tons of straw are annually produced in China.Howeve,most of the straw has not been fully utilized.Lignin is the second most abundant natural plant material of biomass,and it is the only renewable aromatic compound in nature.It is believed that the conversion and utilization of lignin could effectively reduce the dependence on fossil energy and environmental pollution,meeting the requirements of sustainable development strategy.In this paper,straw was used as raw material to extract lignin and further conveted to aromatic aldehyde via degradation.The main research contents are as follows:Toluene-ethanol mixture was first employed to remove waxy and soluble impurity from straws.Straw lignin was then fractionated by organic solvent/water mixed solvent.The influence of extraction time,acid concentration,organic solvent type and volume were investigated.The results showed that the extraction rate and lignin content of rice straw lignin rwere 56.1%and 82.1%with 1.5 wt%concentrated sulfuric acid,ethanol:water=2:1?volumeric ratio?at 180°C for 2.5 h,Extraction rate and lignin content of wheat straw lignin reached 67.2%and 90.8%with 1.5 wt%concentrated sulfuric acid,1,4-dioxane:water=2:1?volumeric ratio?at 180°C for 2.5h.The extracted straw lignin was characterized by infrared spectroscopy,ultraviolet spectroscopy,thermogravimetry,elemental analysis,gel chromatography and nuclear magnetic resonance.By means of elemental analysis and methoxyl determination,the phenylpropane structure of rice straw lignin was given.The phenylpropane structural formula of rice lignin and molecular weight are C₉H_7.31O_2.80?OCH₃?_0.99 and 191.08,rspectively;and wheat straw lignin are C₉H_6.97O_2.88?OCH₃?_1.11 and 195.91,rspectively.The results of infrared spectroscopy,nuclear magnetic resonance spectroscopy and nuclear magnetic resonance spectroscopy indicated that GHS lignin was given by organic solvent method,and the lignin structure extracted kept well.Howerver,the H-form structure was relatively low.The degradation and conversion of lignin were further studied under alkaline conditions,and the products derived from lignin were quantitatively analyzed by high performance liquid chromatography.The results showed that 8.1%yield of monophenolic compounds was obtained in NaOH solution at 170°C for 2.5 h using wheat lignin as substrate and nitrobenzene as oxidant.Howevre,only 4.9%yield of monophenol compounds was obtained using alkaline lignin as substrate under the identical reaction conditions.Gas chromatography-mass spectrometry and liquid chromatography-high resolution mass spectrometry were employed to determine the composition and distribution of oxidative product.The results showed that the products derived from wheat straw lignin were mainly composed of vanillin and syringaldehyde with a small amount of p-hydroxybenzaldehyde,vanillic acid,syringic acid and acetyl clove ketone,acetyl vanillin,3,4-dihydroxybenzaldehyde and ferulic acid.A possible pathway for the degradation of straw lignin into aromatic aldehyde was also proposed based on quantitative and qualitative analysis.