Preparation Of Carbon-based Solid Acid Catalysts For Catalytic Esterification Of Bio-oil Model Compounds

As the renewable carbon-based fuel,biomass will an important role in the future world energy consumption.Fast pyrolysis of biomass to produce bio-oil is one of the important technical routes for efficient utilization of biomass.However,due to the complex structure of biomass,the components of the bio-oil are very complex,with some drawbacks such as high water content,high oxygen content,and low p H value,resulting in high viscosity,poor stability,strong corrosiveness,and relatively low calorific value.low.Therefore,bio-oil needs to be upgraded before it can be used.Catalytic esterification can effectively reduce the content of carboxylic acid and some sugars in the bio-oil,and convert those into useful esters.In this work,a water-resistant,highly recyclable corn stover-derived solid acid catalyst is developed to catalyze the esterification of carboxylic acids in bio-oil and improve the performance of bio-oil.In this study,the waste corn stalk is used as the carbon source to prepare the carbon-based solid acid catalyst by pyrolysis,carbonization and sulfonation.The effect of some key factors for catalyst preparation（i.e.,carbonization temperature,carbonization time,sulfonation temperature,and sulfonation time）are investigated to optimize the catalyst preparation conditions.The catalyst prepared under the optimized conditions was characterized using X-Ray Diffractomer,Fourier Transform Infrared Spectrometer,X-ray Photoelectron Spectroscopy,thermogravimetric,surface area analysis and other test methods,in order to elucidate the catalytic esterification of the carbon-based solid acid catalyst.The results show that the solid acid catalyst prepared under the conditions of carbonization temperature 350℃,carbonization time 0.5 h,sulfonation temperature 90℃,and sulfonation time 1h has high catalytic esterification activity,good water resistance,strong catalytic esterification performance,and good recyclability.After the catalyst is recycled for 5 times,the acetic acid conversion rate still riches>78%.The current study demonstrates that the corn stalk-derived solid acid catalyst can be easierly prepared with low cost,high esterification activity,good water resistance,and good recyclability,thus is suitable for bio-oil catalytic esterification upgrading.Secondly,acetic acid（a typical organic acid in bio-oil）,is used as a model compound to investigate the catalytic esterification of acetic acid and methanol,and the esterification conditions are optimized.The effects of catalyst loading,alcohol-acid molar ratio,esterification temperature and time on the conversion of acetic acid are studied.The results show that the prepared carbon-based solid acid catalyst achieves good catalytic esterification activity and high conversion（up to 89.44%）for the esterification of acetic acid and methanol under the conditions of 4%catalyst dosage,3:1 molar ratio of alkyd to acid,esterification temperature of 80℃,and esterification time of 3 h.Finally,under the experimental conditions of minimized internal and external diffusion,the prepared carbon-based solid acid catalyst is also used to study the reaction kinetics during the esterification of the key organic acids in bio-oil,i.e.,acetic acid,propionic acid,and levulinic acid.The esterification reactions of these acids are treated as a pseudo-homogeneous two-stage reversible reaction,and a reaction kinetic model is established.The activation energy and the the pre-exponential are determined to be 38.28 k J·mol^-1 and2.56×10³ L·（mol·min）^-1 for acetic acid,41.37 k J·mol^-1 and 8.9×10³ L·（mol·min）^-1 for propionic acid,and 29.22 k J·mol^-1 and 1.39×10² L·（mol·min）^-1 for levulinic acid.