Study On Optimized Preparation And Catalytic Performance For Biodiesel Synthesis Of Coal-based Solid Acid Catalyst

Biodiesel is renewable,biodegradable and environmental benign.It is of great significance to promote the biodiesel industry to alleviate energy pressure,reduce environmental pollution and increase energy security.At present,the main restricting factor for biodiesel commercialization is raw material cost.In order to reduce the production cost,the refined oil can be replaced by some low-quality feedstock,including waste cooking oils and animal fats.On the one hand,the use of solid-phase acid catalysts can avoid the toxic effect of free fatty acids in low-quality oil on alkaline catalysts.On the other hand,the drawbacks of equipment corrosion,environmental pollution and difficult recyclability caused by traditional liquid acids are prerequisite to be overcome.Among them,carbon-based solid acid can realize the liquid acids to be supported on the carbon-based carrier,which not only has strong catalytic performance,good thermal stability,but also perform high recycling rate and friendly environment.It has become a research hotspot in recent years.Coal is mainly composed of carbonaceous organic compounds.The condensation aromatic ring connected by bridge bond is the core part of the basic structural unit,and the periphery is connected with alkyl side chain and functional groups.Theoretically,the solid acid catalyst prepared after carbonization and sulfonation,would has abundant oxygen-containing functional groups to guarantee the high catalytic activity.Therefore,its carbonaceous organic compound structure makes it has the potential to be used as a solid acid carrier for catalyst research.However,the composition,phase and density of coal are quite different from those of conventional biomass carbon-based raw materials,which determines that the catalytic mechanism,catalytic process and performance of coal-based solid acid are quite different.However,the research and development of coal and its application in the field of biodiesel are still very weak.Hence,in this paper,the research ideas of coal-based solid acid used in catalyzing esterification and transesterification to synthesis biodiesel is proposed.The research contents including the catalyst preparation conditions,catalyst characterization analysis,reaction factors for esterification and transesterification,reaction kinetics and thermal degradation kinetics and thermodynamics are studied systematically,to reveal the mechanism of the coal-based solid acid in catalyzing the acidification oil esterification and transesterification to synthesis biodiesel.The effects of different sulfonation methods to prepare the coal-based solid acid on the catalytic performance in esterification are studied.Two kinds of coal-based solid acid are prepared through the incomplete carbonization-liquid phase sulfonation method and incomplete carbonation-hydrothermal sulfonation method,using Zhundong coal as the carbon-based carrier and concentrated sulfuric acid as sulfonating agent.By means of characterization approaches such as N2 adsorption and desorption,XRD,SEM,EA,XPS,TG,ATR-FTIR,acid density and Hammett acid strength,it can be found that the prepared solid acid is composed of the randomly arranged amorphous carbon structure with low degree of graphitization,with acid strength of 0.8<H0<3.3.And it can provide favorable thermal stability high to 220 ℃.Stretching vibration absorption peaks of 1165 cm-1 and 1032 cm-1 seen in the ATR-FTIR spectrum are identified as the O=S=O and SO3-in the-SO3H groups,respectively,confirming the successful introduction of-SO3H groups on carbonized product after sulfonation.The preparation parameters of two sulfonation methods are optimized as carbonization temperature of 350 ℃,sulfonation temperature of 135 ℃and 105 ℃,respectively.High pressure and temperature system environment supplied by hydrothermal reactor in the hydrothermal sulfonation method could reduce sulfonation temperature by 30 ℃,when compared with liquid phase sulfonation.Meanwhile,the acid density could increase to 1.15 mmol/g from 1.09 mmol/g.Under conditions of catalyst dosage of 10 wt.%,methanol to oleic acid molar ratio of 10,esterification duration of 4 h,two kinds of solid acid could gain over 97%oleic acid conversion.The corresponding optimal esterification temperature is 67 ℃ and 64 ℃,respectively.The leaching of sulfonic acid groups and the attachment of esterification product in the solid acid surface are the important reasons that affect the reuse performance.Hot deionized water washing would lead to serious deactivation of solid acid during recovery process.However,the reusability is significantly improved when the alcohol and n-hexane used,and the conversion could remain around 78%after the fifth run.The strengthening mechanism of biodiesel synthesis catalyzed by coal-based solid acid via external fields(ultrasonic and microwave)is studied,and the performance of solid acid in catalyzing raw oil transesterification is also investigated.C400(1)-S 105(2)is prepared using Yulin coal via incomplete carbonization-liquid phase sulfonation method.Compared with the carbonation and sulfonation duration,temperature performs more significant influence on the catalytic activity.The synthesis conditions are optimized as carbonization at 400 ℃ for 1 h and sulfonation at 105 ℃ for 2 h,where acid density of 1.57 mmol/g is measured.The cavitation phenomenon of ultrasonic radiation could enhance the mixture of reactants on the micro scale,weaken the mass transfer resistance between solid acid and liquid reactants.After passing 1 h,91.4%of conversion is obtained under ultrasonic power of 270 W,catalyst dosage of 6 wt.%,methanol to oleic acid molar ratio of 10 and esterification temperature of 67 ℃.Only 87.9%is achieved under the same operating conditions without ultrasonic irradiation.Compared with Amberlyst-15.C400(1)-S 105(2)has higher catalytic activity and reusability.After the fourth cycles,their catalytic activities are decreased from 97.6%and 68.l%to 91.0%and 51.3%.respectively.Phosphoric acid impregnation preprocessing and microwave-assisted sulfonation are employed into the solid acid synthesis.The acid density and specific surface area of Yulin coal-based solid acid exhibit opposite trend with the increase of carbonization temperature when phosphoric acid impregnation preprocessing is used.C250(120)-S90(120)possesses the negligible SBET but the largest acid density of 1.70 mmol/g.and exhibits the largest catalytic activity of 95.2%.So.the acid density may play a more important role in determining the catalytic activity of coal-based solid acid in this study.The introduction of microwave radiation into the catalyst preparation process could obviously shorten the sulfonation duration to 5 min.Compared with the traditional heating method,the sulfonation assisted by microwave radiation is a rapid and efficient method.C400(1)-S 105(2)performs excellent activity in catalyzing transesterification of palm oil with methanol.97.8%of biodiesel yield could be achieved with catalyst dosage of 6 wt.%,molar ratio methanol to oil of 18,reaction after 8 h under 160 ℃.The pore structure of coal-based solid acid is optimized and the kinetics and mechanism of esterification catalyzed by coal-based solid acid are studied.Coal-based activated carbon(AC)is prepared from Yulin coal with KOH as activator,and solid acid with high specific area is synthesized through the arylation diazonium salt reduction approach.Under the activation temperature of 900 ℃,mass ratio of 4-aminobenzenesulfonic acid to AC of 6 and arylation temperature of 45 ℃.the synthesized solid acid gains the surface area and pore volume of 629.3 m2/g and 0.29 cm3/g.respectively.The 4-sulfophenyl group acid density is tested to be 0.97 mmol/g.Favourable catalytic activity(95.1%of conversion)is mainly due to the high specific surface area,pore volume,4-sulfophenyl group acid density and acid strength.Coal-based AC supported phosphotungstic acid(PTA)as solid acid is prepared under optimal conditions as follows:activation temperature of 700 ℃,mass ratio of KOH to coal of 3 and PTA additive amount of 40%.The catalyst possesses high surface area of 1142 m2/g,pore volume of 0.16 cm3/g and n-Butyl amine acidity of 1.76 mmol/g.The excellent catalytic activity(97.2%of conversion)relates with the high acidity and the good dispersion of PTA on the AC.The activation energy and former factor of oleic acid and methanol esterification catalyzed by C350-S105 are calculated to be 31.05 kJ/mol and 9.45×102 min-1,respectively.The lower reaction activation energy indicates that the catalytic rate is fast and the reaction is easy to occur.The thermal degradation properties of stearic acid and its esterification derivants of methyl stearate and ethyl stearate catalyzed by solid acid are studied systematically by thermogravimetry.With the increase of heating rate,the whole process of volatilization and pyrolysis moves to the high temperature zone,and the weightlessness rate shows a increasing trend gradually.The values of thermal degradation activation energy of stearic acid,methyl stearate and ethyl stearate calculated by KAS and FWO methods are 75.05-90.66 kJ/mol and 79.57-95.31 kJ/mol,40.60-57.29 kJ/mol and 45.89-63.03 kJ/mol,33.67-50.91 kJ/mol and 39.33-56.92 kJ/mol,respectively.Meanwhile,the reaction orders of different thermal degradation stages calculated by Avrami theory are 1.27-1.84,1.26-1.94 and 1.62-2.03,respectively.Stearic acid has higher thermal degradation activation energy than its esterification derivates,indicating that its thermal activity is distinctly heightened after esterification and the derived derivants are more easily activated.Thermodynamic calculation results show that the thermal degradation processes are ascribed to endothermic property,which are non-spontaneous and require additional energy supply.