Study On The Preparation Of Liquid Biofuels By Catalytic Cracking Waste Oils

Environment-friendly clean energy and renewable energy to replace traditional fossil energy has attracted much attention,and the catalytic cracking of waste oils to prepare liquid biofuels is the hot research topic of biomass energy conversion.At present,the catalysts used for catalytic cracking of waste oils have some defects,such as low yield of target light hydrocarbon biofuels,high yield of overcracking gas components,easy coking and inactivation of catalysts,and poor recycling performance.Metal oxide catalysts are easy to obtain and have high activity,showing outstanding performance in deoxidation of waste oils and inhibiting coking.The multi-functional core-shell composite molecular sieve catalysts with high specific surface area and stability have obvious advantages in improving the content of light hydrocarbon biofuels and recycling performance.Therefore,in this paper,ZnO-MgO nanospheres and micro-mesoporous core-shell composite molecular sieves were studied for catalytic cracking of waste oils to prepare liquid biofuels.The catalysts of ZnO-MgO nanospheres,micro-mesoporous core-shell composite molecular sieves and ZnO/MgO modified micro-mesoporous core-shell composite molecular sieves were prepared.The structure of the catalysts was characterized by X-ray diffraction（XRD）,N₂adsorption-desorption,transmission electron microscopy（TEM）,scanning electron microscopy（SEM）,Fourier transform infrared spectroscopy（FT-IR）,chemisorption（NH₃-TPD,CO₂-TPD）,pyridine infrared（Py-IR）and other characterization means.And the synthesized catalysts were used in catalytic cracking of waste oils to prepare liquid biofuels.The performance of the catalysts was evaluated based on the yield of liquid biofuels,gas products,coke production,light hydrocarbon components(C₅-C₁₂),fatty acids(C₈,C₉,C₁₆and C₁₈)and esters(C₁₆and C₁₈).At the same time,the recycling performance of the synthetic catalysts was investigated,and the main results were as follows:（1）Nanospheres catalysts x ZnO-MgO（x is molar content of ZnO,x=0,0.25,0.5,0.75 and 1）were synthesized by microwave-assisted static hydrothermal synthesis method with PEG-2000 inducer.The results showed that:The morphology of the synthesized 0.5 ZnO-MgO is an independent nanoparticle similar to hexagonal prism（300 nm）,with uniform particle size and good dispersion.The effects of catalyst types and amounts,reaction temperatures and times on catalytic cracking reaction were investigated.The results showed that:Nanosphere 0.5 ZnO-MgO with ZnO molar content of 0.5 had the best catalytic performance,and under the optimal reaction conditions of waste oil 10 g,catalyst 0.3 g,reaction temperature 460 ^oC and reaction time 80 min,the yield of liquid biofuels was 77.3%,and the content of light hydrocarbon components was 36.5%.The cycling performance of 0.5 ZnO-MgO showed that the yield of liquid biofuels was still 70.1%after it was recycled for three times,indicating that 0.5 ZnO-MgO still maintained good recycling performance.（2）Micro-mesoporous core-shell composite molecular sieves catalysts Z5（x）@M41（x is Si/Al,x=80,100,120,140 and 160）with the microporous mononuclear sphere ZSM-5 as a core-coated mesoporous MCM-41 were synthesized.The results showed that:The synthesized ZSM-5@MCM-41 has good crystal shape,regular pore channel,large specific surface area(880-1080 m~2g^-1),and single and regular core-shell morphology.The catalytic performance of ZSM-5 and ZSM-5@MCM-41 with different Si/Al,particle size and shell thickness were investigated.The results showed that:Z5（100）@M41 with Si/Al of 100,core diameter of 176 nm and shell thickness of 56 nm had the best catalytic performance,and the yield of liquid biofuels was 83.8%and the content of light hydrocarbon components was 45.1%.With linoleic acid,linoleic acid isopropyl ester and 14 alkyl ester of formic acid as examples,the catalytic cracking reaction path was studied.The results showed that:The reaction of C=C fracture,β-elimination,decarboxylation,decarbonization,ester hydrolysis,isomerization and aromatization occurred on acid active center of catalyst Z5（100）@M41,and the yield of liquid biofuels and the content of light hydrocarbon components were improved.The mesoporous shell MCM-41 made the long chain macromolecules complete the initial catalytic cracking,and then the cracking molecules enter the core micropore ZSM-5 along the pore channel for the secondary deep cracking,and finally the liquid biofuels with less carbon content were generated.The cycling performance of Z5（100）@M41 showed that the yield of liquid biofuels was still 80.2%after it was recycled for three times,indicating that Z5（100）@M41 still maintained good recycling performance.（3）ZnO/MgO modified micro-mesoporous core-shell composite molecular sieves a ZnO/b MgO-Z5（100）@M41（a or b is 5%,10%and 15%of the concentration of impregnating solution Zn（Ac）₂or Mg（Ac）₂,respectively）were synthesized by incipient-wetness impregnation method.The results showed that:The introduction of modified metal oxides does not change the morphology of core-shell structure.ZnO/MgO-Z5（100）@M41 still has good crystal shape,regular channel and large specific surface area(725-789 m~2g^-1).And the impregnated metal oxide was evenly distributed in the pores of the shell,and there is no obvious agglomeration.The catalytic properties of ZnO/MgO-Z5（100）@M41 were investigated,and the results showed that:The best catalytic performance was 10%ZnO/10%MgO-Z5（100）@M41,and under the optimal reaction conditions of waste oil 10 g,catalyst 0.3 g,reaction temperature 460 ^oC and reaction time 80 min,the yield of liquid biofuels was 84.3%,the highest content of light hydrocarbon components was 48.9%,and the lowest content of fatty acids and esters was 13.8%,indicating that10%ZnO/10%MgO-Z5（100）@M41 has excellent decarboxylation and decarbonylation properties.And the more sites strong acid of the catalyst,the higher the content of the light hydrocarbon components.The cycling performance of catalyst10%ZnO/10%MgO-Z5（100）@M41 showed that the yield of liquid biofuels was still78.7%after it was recycled for three times,indicating that10%ZnO/10%MgO-Z5（100）@M41 still maintained good recycling performance.In conclusion,the synthesized micro-mesoporous core-shell composite molecular sieves not only have the characteristics of high crystallinity and stability of microporous molecular sieve ZSM-5,but also have the regular,orderly and developed pore structure of the mesoporous molecular sieve MCM-41.After modification of ZnO/MgO,the alkaline active site is introduced to realize the acid-base synergy,which makes the catalyst have the multi-function.In the catalytic cracking of waste oils to prepare liquid biofuels,efficient conversion of substrate and high yield of target product are achieved,and coupling of reaction-separation process and recycling of catalysts are completed.This paper provides an important basis for studying the catalytic cracking of metal oxide modified micro-mesoporous core-shell molecular sieves to prepare liquid biofuels.