Study On Catalytic Cracking And Aromatization Of Fatty Acid Esters For The Production Of Hydrocarbon-Rich Biofuels

Energy sustainability and environmental concerns are the essential driving forces in developing renewable biofuels as alternatives for traditional fossil fuels.Enormous attention has been focused on triglycerides,such as non-edible vegetable oils and waste oils as abundant biomass resources for the generation of biofuels.The key advantages of triglyceride feedstocks involve ready fluidity,high energy density and similar molecular structure with fossil fuels.However,straight vegetable oil was treated as an undesirable fuel for automotive applications as it has the high kinematic viscosity.Pyrolysis and transesterification are two fundamental strategies for decreasing the viscosity of triglycerides.The typical resulting products are known as bio-oil and biodiesel,respectively.However,these biofuels with oxygenated compounds suffered from their poor compatibility with fossil fuels.Catalytic cracking is considered as the most promising approach to convert triglycerides and their derivatives into oxygen-free biofuels,namely,hydrocarbon-rich biofuels,which are fully miscible with the traditional fossil fuels.In the present dissertation,we investigated catalytic aromatization of fatty acid methyl esters for the production of renewable aromatics and catalytic cracking of triglycerides for the production of hydrocarbon-rich biofuels over a series of solid acid catalysts including their preparation,characterizations,performance test and corresponding reaction mechanisms.Some conclusions drawn from this work are provided as follows:（1）Preparation and catalytic performance of Zn-modified HZSM-5 zeolite catalysts.The Zn/HZSM-5（25）catalysts were prepared using a wet impregnation method.The introduction of Zn species shows less effect on the MFI framework and pore structure of zeolite support.However,many Br（?）nsted acid sites are preferentially consumed while a new kind of Zn-Lewis acid site is generated,thereby reducing the B/L ratio.The existing state of Zn species on HZSM-5 zeolites is heavy depended on the amount of Zn loading.At a lower Zn loading,the Zn species are present as[ZnOH]⁺and Zn²⁺species,which are Zn-Lewis acid sites;At a higher Zn loading,the ZnO species would be formed including ZnO nanoclusters and large ZnO particles.Oleic acid methyl ester（OAME）is selected as a model of fatty acid methyl ester（FAME）and used to study the mechanism of the aromatization.It was found that the catalysts with lower Zn loading give high yield of aromatics and high selectivity of monocyclic aromatics,and the stability of catalysts is improved.The Zn-Lewis acid sites show great promotion on the generation of aromatics.They not only enhanced the dehydrogenation activation of alkanes to form more alkenes but also shifted the further dehydrogenation process from hydrogen transfer reactions to dehydrogenation reactions during aromatization of alkenes,therefore improving the production of aromatics.（2）Catalytic performance of 0.5%Zn/HZSM-5（25）catalyst on aromatization of FAMEs.During catalytic cracking of FAMEs at 515℃,3.0 L/h of carry gas flow rate,2.5 h^-1 of weight hourly space velocity（WHSV）,and 2.0 g of catalyst loading,the 0.5%Zn/HZSM-5（25）catalyst exhibits good adaptability to different biodiesel samples and shows a good ability to convert biodiesel produced from pyrolysis（PBD）and commercial biodiesel（CBD）into green aromatics.The distribution of aromatic hydrocarbons was mostly benzene,toluene and xylene（BTX）.The deactivation of the 0.5%Zn/HZSM-5（25）catalyst can impute to coking and the spent catalyst can be regenerated by calcining in the presence of oxygen.CBD with higher degree of unsaturation shows an excellent performance for the production of green aromatics,which represents a longer service lifetime and better regeneration property.The yield of total aromatic hydrocarbons（TAH）is 42.1 wt.%,which is comparable with the high yield of aromatics（42.6 wt.%）obtained from current process of FAME aromatization.（3）Preparation of SO₄^2-/TiO₂-ZrO₂ catalyst and its catalytic performance on catalytic cracking of non-edible oils.The binary oxide TiO₂-ZrO₂ was prepared using a coprecipitation method,which results in the formation of orthorhombic Ti ZrO₄ structure.The acid sites on the binary oxide are mainly Lewis acid sites.The modification of sulfuric acid has less effect on crystal shape and pore structure.However,the concentration of acid sites changes significantly.New Br（?）nsted acid sites appear while the Lewis acid sites reduce,which leads to a decrease in B/L ratio.Five non-edible oils,tung oil（TO）,rubber seed oil（RSO）,Jatropha curcas oil（JCO）,waste cooking oil（WCO）and waste acidified oil（WAO）were selected as triglyceride feedstocks.They show little differences in density,saponification value,and oxygen content（12–13%）,while they vary significantly in terms of acid number,the degree of unsaturation,and composition of fatty acids.Direct pyrolysis of triglycerides without a catalyst gives a high yield ofOLP（85 wt.%）,which contains a large sum of free fatty acids.In case of catalytic cracking of triglycerides,SO₄^2-/TiO₂-ZrO₂ catalyst exhibits good feedstock adaptability.The acid number shows less effect on the catalytic cracking process,while the unsaturated double bonds tend to favor the formation of lighter alkenes and more aromatics.The hydrocarbon-rich biofuels have low acid number（<6.4 mg KOH/g）and low oxygen content（<1.5%）,which are mostly consist of diesel fraction（49.0–57.6%）and gasoline fraction（32.0–42.5%）.The hydrocarbons are in the range of C6–C24,including alkenes（44–58%）,monocyclic aromatics（17–24%）and alkanes（14–17%）.（4）Catalytic cracking of waste acidified oil and upgrading of biofuels by catalytic hydrogenation.Waste acidified oil is selected for investigating the effects of reaction parameters on catalytic cracking process,and subsequently the biofuels produced is further upgraded by catalytic hydrogenation using a Mo/TiO₂-ZrO₂ catalyst.The optimized catalytic cracking process is carried out at 450℃,2.5 L/h of N₂ flow rate,1.0 h^-1 of WHSV,and 5.0 g of catalyst loading,which results in a high yield ofOLP（66 wt.%）with a low acid value（3.8 mg KOH/g）,low oxygen content（1.5 wt.%）and low kinematic viscosity（5.3 m Pa·s）.The reaction temperature plays an important role in catalytic cracking process.The efficient deoxygenation occurs at temperatures above 400℃and is accompanied by cracking and oligomerization.The high temperature（≥500℃）generally initiates secondary cracking and subsequently tends to favor dehydrogenation and aromatization.Catalytic hydrogenation of biofuel over the Mo/TiO₂-ZrO₂catalyst has little change on the fraction distribution and aromatics content,but the content of olefins was sharply reduced by 34%.The upgraded hydrocarbon-rich biofuel was more chemically near-identical to petroleum-based fuels and can be fractionated and used in different formulations depending on the type of desired fuels,such as bio-gasoline,bio-jet fuel and green diesel.