Liquefaction Of Crop Stalks In Sub-and Supercritical Alcohols

In this study, liquefaction of crop stalks, including rice straw, wheat straw, corn skin and corn core, with methanol and ethanol under sub/supercritical conditions without catalyst or with different catalysts such as KOH/γ-Al₂O₃, CaO/γ-Al₂O₃ and CaO/γ-Al₂O₃/Fe₃O₄ was investigated.Soxhlet, ultrasonic-, and microwave-assisted extractions for dewaxing from rice, wheat, corn skin and corn core stalks with petroleum ether were compared. The compositions of the waxes extracted by different methods were determined by gas chromatography/mass spectrometry (GC/MS) analysis. The results show that microwave-assisted extraction is the most effective method for dewaxing from the stalks. Besides chain alkanes, alcohols, aldehydes, ketones, and esters, a large number of sterides, palmitic acid, (E)-9-octadecenoic acid, vitamin E, and squalene are found, which are more value-added chemicals in the waxes from the four stalks mentioned. Sterols, (E)-9-octadecenoic acid, vitamin E, and squalene in waxes were first detected from crop stalks.The effect of temperature, time and the ratio of stalk to solvent on the liquefaction of corn skin in sub- and supercritical methanol was investigated. And the effect of temperature on the liquefactions of rice, wheat and corn core stalks in sub- and supercritical methanol as well as corn skin stalk in sub- and supercritical ethanol was also researched. The liquid products (LPs) from liquefaction of stalks were analyzed by GC/MS. The change of composition distribution of LPs with temperature was studied. The results show that the optimum conditions of corn skin liquefaction in methanol are that reaction temperature, reaction time and the ratio of stalk to methanol are 300 oC, 30 min and 1: 30 g/mL, respectively. The effect of temperature on liquefaction of stalk is the most notable. The liquefaction has better effect when ethanol was used as solvent comparing methanol as solvent. Under the optimum liquefaction condition, the conversion yields of corn skin, rice, wheat, and corn core in methanol as well as corn skin in ethanol are 87%, 75%, 85%, 94% and 87%,respectively, and LPs yields are 36%, 30%, 40%, 43% and 43%. The main compositions of LPs include shorter chain carboxylic acid derivatives, furan derivatives, cyclopentanone, phenol derivatives, benzene derivatives, cinnamic acid derivatives, longer chain carboxylic esters and sterides. Cinnamic acid derivatives and 2,3-dihydrobenzofuran from the cleavage of lignin branch chain are the dominant compositions of LPs from stalk liquefaction in subcritical alcohol. When stalks are liquefied, three main components of stalks are all depolymerized, and the depolymerization is intensified with the temperature increase. The depolymerization of lignin from stalks forms cinnamic acid derivatives at lower temperature and phenol derivatives at higher temperature. KOH/γ-Al₂O₃, CaO/γ-Al₂O₃ and CaO/γ-Al₂O₃/Fe₃O₄ were prepared and characterized, and used to catalyze wheat straw liquefaction in supercritical ethanol. The effect of catalyst on LP yields was investigated. LPs were analyzed by GC/MS. The change of composition distribution of LPs with temperature was studied. The yields of LPs from wheat straw liquefaction with KOH/γ-Al₂O₃, CaO/γ-Al₂O₃ and CaO/γ-Al₂O₃/Fe₃O₄ reach 86.1%,57.8% and 104.6%, respectively. Shorter chain carboxylic acid ethyl esters, furan derivatives, cyclopentanone, phenol derivatives, benzene derivatives, aromatic acid derivatives, longer chain carboxylic ethyl esters and sterides are main compositions of LPs with the above three catalysts. The three catalysts all enhance the yields of shorter chain carboxylic acid ethyl esters, cyclopentanone and phenol derivatives, and CaO/γ-Al₂O₃/Fe₃O₄ shows a better effect. KOH/γ-Al₂O₃ and CaO/γ-Al₂O₃/Fe₃O₄ improve the yield of furan derivatives, and KOH/γ-Al₂O₃ raises the yield of benzene derivatives.Mechanisms for stalks liquefaction in sub- and supercritical alcohol were proposed. Fuel properties of LP from wheat straw liquefaction in supercritical ethanol were tested. The composition of LP was analyzed by GC/MS. The main compositions of LP are shorter chain carboxylic acid and their ethyl esters, furan derivatives, cyclopentanone, phenol derivatives, benzene derivatives, aromatic ketone, aromatic acid derivatives, longer chain carboxylic ethyl esters and sterides. After upgrading, the main indexes reach the standard of fuel oil.