Hydrothermal Preparation Of The Novel Corncob-derived Solid Acid And Its Catalytic Performance

The energy crisis and environment pollution have become more and more serious nowadays.The application of carbon based solid acid in the conversion of biomass and the development of bioenergy has attracted much attention,especially its high activity in the deacidification of high acid value oil,which is of great significance for the production of biodiesel from high value acid oil.The novel solid acid makes it a promising candidate for liquid acid catalyst due to its high catalytic activity,easy recycling,renewable and so on.However,the preparation of the solid acid is a wasteful,energy-inefficient and environment-polluted process,and the catalytic activity of the carbon based solid acid really can not meet the requirements of industrial production.What’s more,the reported raw materials of the novel solid acid focused on the natural carbohydrates,such as glucose,starch and cellulose.Little attention has been paid to the ultilization of biomass towards the novel carbon based solid acid.This paper focused on the utilization of agriculture biomass and development of a low-cost and green approach for the preparation of the novel carbon based solid acid.The novel corncob-derived solid aid catalyst was synthesized for the first time by hydrothermal carbonization of corncob,which is the starting material,and followed by sulfonation of concentrated sulfuric acid.We selected the reaction of oleic acid and methanol to simulate the deacidification process of high acid value oil,and systematically investigated the effect of various preparation conditions on the catalytic activity of the prepared solid acid.In addition,we discussed the structure-function relationship of the solid acid catalysts on the basis of the analysis of structure and composition.In order to devise a more efficient reaction system,we optimized the reaction parameters of oleic acid and methanol.The effect of molar ratio between oleic acid and methanol,catalyst loading,reaction temperature and reaction time was analyzed through the model reaction of oleic acid and methanol.Expect the mentioned above,we also compared the catalytic activity of the prepared solid acid catalyst with concentrated sulfuric acid,niobic acid and Amberlyst-15.Moreover,the decreased recycle performance of the solid acid was studied through using methanol and n-hexane as the washing solvent.The relevant results are as follows:Within the tested range,the optimal preparation conditions for the most effective catalyst are found to be hydrothermal carbonization at 453 K for 10 h,followed by hydrothermal sulfonation at 363 K for 10 h.Under these conditions,a relatively highlyyield of methyl oleate（89.2%）was obtained by the catalysis with the corncob-derived solid acid after only 2 h of esterification.The physical and chemical properties of the corncob-derived solid catalyst was also characterized in detail,and its structure-function relationship was investigated with scanning electron microscopy（SEM）,Fourier transform infrared spectroscopy（FT-IR）,X-ray diffraction（XRD）,elemental analysis（EA）,and 13^C solid-state nuclear magnetic resonance（NMR）.The solid acid sample prepared under the optimized conditions is composed of polycyclic aromatic carbon sheet with acid function groups（-OH,-COOH,-SO₃H）.These three acid groups cooperated to catalyze the esterification reaction between oleic acid and methanol,and the-SO₃H content of the optimized catalyst is 1.54mmol/g.The results of the solid-state NMR indicated that the polycyclic aromatic carbon sheet started to form at 453 K,and the carbon sheet became less flexible under high carbonization and sulfonation temperature,which is not convenient for the introduction of-SO₃H groups.Taking both the energy consumption and the efficiency of the corncob-derived solid acid into consideration,10（molar ratio of methanol to oleic acid）,2 h,353 K and 10%catalyst loading（based on the mass of oleic acid）were selected as the optimized reaction conditions for oleic acid and methanol.As for the comparison of the catalytic activity among concentrated sulfuric acid,the prepared corncob-derived solid acid,niobic acid and Amberlyst-15,the corncob-derived catalyst（89.2%）exhibited higher activity than niobic acid（29.6%）and Amberlyst-15（68.8%）,even not higher than concentrated sulfuric acid（95.4%）.We found that the recycled solid acid washed by methanol and n-hexane,the washing solvents,showed really different recycle performance.The catalytic activity of the corncob-derived solid acid maintained with a 65% conversion for methanol washing and a 75% conversion with n-hexane washing after 8 cycles of reuse.The leaching of polycyclic aromatic hydrocarbons containing-SO3 H groups in the washing solvents caused the deactivation of the solid acid.In this study,we successfully developed a novel,effective and environmental friendly corncob-derived solid acid using corncob as the starting material,and studied the low-cost and green approach for its preparation.This work is of great significance for the eco-utilization of biomass in the fields of functional carbon material and convention of bioenergy.