Facile Design And Performance Of Catalysts For Carbon-Carbon Double Bond Epoxidation In Liquid Phase

Epoxides are important organic compounds obtained from a bridging method of carbon-oxygen-carbon bonds in the presence of catalysts.The epoxides with epoxy groups are easy to react with chemicals,such as ammonia,amines,phenols,alcohols,and carboxylic acids etc.The products of these reactions are highly valuable and important industrial intermediates or final products.The epoxidized vegetable oil-based products can be used as environment-friendly plasticizers to replace the current o-benzene-based plasticizers,and have important significance for producing high-quality and safe plastic products.As one of the best choices for epoxidation catalysts,heterogeneous catalysts can effectively catalyze epoxidation and can be recycled many times,as well as overcome the shortcomings of using formic acid,acetic acid and concentrated sulfuric acid as epoxidation catalysts in industry at present.Porous silica materials are cheap and abundant,as well as have many excellent properties,such as high specific surface area,vivid porous structure and easy modification etc.,which are potential carriers for heterogeneous catalysts.During the catalytic process,the porous silica materials can fully develop the activity of catalytic sites and result in excellent activity.In this thesis,the porous silica was used as carriers,transition metal molybdenum was selected as active site,the carriers were successfully modified by the active sites and the obtained catalysts were applied in epoxidation of alkenes.In addition,the epoxidation of vegetable oil with carbon-carbon unsaturated double bonds was also studied and the solid acid catalysts have been successfully applied in vegetable oil-based epoxidation reaction.Moreover,the epoxidation mechanism was also discussed.The main contents and results of this thesis are listed as follows:?1?Molybdenum incorporated silica nanoparticles were synthesized by a general and simple method.The nanoparticles were prepared in mild conditions with cetyltrimethyl ammonium bromide as template,tetraethyl orthosilicate as silica source and triethanolamine as mineralizing agent.Molybdenum precursor was added during the sol?gel process,because of the ion pairing effects between C₁₆TAB and molybdenum precursor,cation CTA⁺would adsorb the molybdenum precursor anion and transferred to silica structure during the sol?gel process,active sites MoO₃ were obtained after calcination.To determine the reactivity of catalysts with different Si:Mo molar ratios,epoxidation of cyclooctene was carried out as a model reaction.After 12 h reaction with H₂O₂?50 wt%?,above 92%conversion and 95%selectivity were reached by the optimal catalyst?Mo-MSN-50?.Furthermore,the catalyst still had high conversion?74%?and selectivity?97%?for 4 h epoxidation of cyclooctene even after being recycled for 6 runs.Remarkably,the shape of silica nanoparticles gradually changed from raspberry-like to rose-like with the decreasing Si:Mo molar ratio.This result was assumed to the ion pairing effects between molybdenum precursor and cationic CTA⁺.?2?Highly dispersed Molybdenum?VI?incorporated hollow mesoporous silica catalysts were facilely synthesized with a selective etching strategy and then followed with a modified immersion method.Firstly,hollow mesoporous silica spheres were obtained from solid silica spheres resulting from the interaction of cetyltrimethyl ammonium bromide(C₁₆TAB)and Na₂CO₃.Then the catalytic active sites were loaded by modified immersion methods.Compared with traditional immersion method,the modified immersion method used here was carried out by removing excessive?NH₄?₆Mo₇O₂₄·4H₂O after immersion.The absorbed?NH₄?₆Mo₇O₂₄·4H₂O in porous structure was transferred to MoO₃ after calcination.Characterization methods such as SEM-EDS and TEM etc.were used to compare the distribution of active sites between traditional and modified immersion methods.As a result,catalysts prepared from modified method exhibited highly dispersed active sites and better textural properties,the BET surface specific area of Mo/HMSS-X was 484 m²/g.Furthermore,the active sites were stable and not easy to aggregate.Compared the reactivities of different catalysts,referring to cyclohexene epoxidation as a model reaction,at the reaction time of 6 h,Mo/HMSS-X displayed the best turnover number of 3873,and the TON value of Mo/HMSS-0.4 with similar molybdenum loading was 1915,and 268 with Mo/HMSS-5 prepared from the same Si:Mo molar ratio.?3?In order to overcome the metal leaching problem,a general thermal decomposition strategy was used to fabricate MoO₃@SiO₂ nanoreactors with a mesoporous silica shell and embedded MoO₃ nanoparticles.The novel preparation procedure involves mixing of certain mass ratio of?NH₄?₆Mo₇O₂₄·4H₂O?AMM?and hollow mesoporous silica spheres?HMSS?by grinding,fusion and thermal decomposition of?NH₄?₆Mo₇O₂₄·4H₂O under calcination and removing the residual via filtration.The as-prepared MoO₃@SiO₂ nanoreactors were utilized in epoxidation of alkenes and displayed high catalytic activity and stability.Using epoxidation of cyclooctene as a model reaction to study the reactivity of MoO₃@SiO₂ nanoreactors.The optimal mass ratio of AMM:HMSS has been confirmed as 1/2.After reacting for 12 h with H₂O₂?50?wt%?as oxidant,conversion and selectivity of optimal MoO₃@SiO₂-400-1/2 almost reached up to 98%and 99%,respectively.Furthermore,the catalyst still had high conversion?78%?and selectivity?95%?at 4?h epoxidation of cyclooctene after recycling for 6 runs.Kinetics study was also carried out and demonstrated the epoxidation of alkenes followed the first order model,which revealed that the reaction rate is relevant to the concentration of reactants.?4?In order to further improve the catalytic activity of nanoreactors,a facile“ship in bottle”strategy was developed to functionalize the preformed hollow mesoporous silica spheres by encapsulating the molybdenum dioxide?MoO₂?nanoparticles inside the interior cavity.Hollow mesoporous silica spheres were prepared and employed as carriers,and the encapsulation of MoO₂ nanoparticles was achieved through a one-pot hydrothermal protocol.In details,hollow mesoporous silica spheres were dispersed in a mixture of?NH₄?₆Mo₇O₂₄·4H₂O and ethylene glycol aqueous solution,during the hydrothermal process at 180 ^oC,?NH₄?₆Mo₇O₂₄·4H₂O would enter into the cavity of hollow mesoporous silica spheres through the porous structure in the shell,and transferred to MoO₂ nanoparticles.Using the cyclooctene epoxidation as a model reaction,after reacting for 10?h,the optimal catalyst MoO₂@SiO₂-1 achieved a conversion above 95%and selectivity above 95%,respectively.By using this strategy,the encapsulated MoO₂ nanoparticles were certified to be ultrafine and highly dispersed,which greatly promoted the catalytic activity.Moreover,the shell became coarser and thinner as well as the pore distribution became larger,which promoted the transportation of reactants and products and efficiently improved the catalytic activity.?5?In order to expand the epoxidation substrates to long carbon chain vegetable oil,the solid acid catalysts were successfully applied in vegetable oil-based epoxidation.During the catalytic system of methyl soybean oil/acetic acid/50 wt%H₂O₂,after reacting for 16 h,the conversion,selectivity and epoxidation of C=C were 23%,65%and 15%,respectively.Based on this reaction system,after adding the solid acid catalysts?1 wt%of methyl soybean oil?,the conversion,selectivity and epoxidation of C=C were 66%,63%and 42%,respectively.Obviously,the reactivity of methyl soybean oil/solid acid catalysts/acetic acid/50 wt%H₂O₂system was greatly improved.This catalytic system is environmental-friendly and safe without solvent.Moreover,the solid acid catalysts can also be recovered easily and recycled many times,which have excellent application in industrial production.