Fabrication Of Pickering Emulsion And Its Enhancement For Isobutene Oligomerization

Oligomerization reaction is one of the approaches to make high-value use of isobutene,and liquid acids,such as sulfuric acid,ionic liquids,are effective catalysts for isobutene oligomerization.Due to the low solubility of isobutene in the catalyst phase,the reaction mainly occurs on the interface between acid and hydrocarbon,and mass transfer of the system is limited.Pickering emulsions refer to emulsions stabilized with solid particles instead of traditional surfactants,which have been widely used to improve activity of multiphasic reaction systems.Based on the unique properties of Pickering emulsion,this thesis proposed a research idea to fabricate Pickering emulsion with strong acid resistant emulsifier to strengthen the isobutene oligomerization.By grafting functional groups on the surface of silica,strong acid resistant emulsifiers were obtained,and Pickering emulsions were constructed with the ionic liquid catalyt as the aqueous phase.The effects of the fabrication conditions on the physicochemical properties of Pickering emulsions were systematically studied,and the Pickering emulsion was then used as the medium to strengthen isobutene oligomerization.The major conclusions of the study were summarized as follows:?1?The strategy to modify the silica nanoparticles with alkyl chain and sulfonic acid groups was proposed,and emulsifiers that were hydrophobic and resistant to strong acids were obtained.The different alkyl chains(i.e.,C₁-,C₈-and C₁₈-)and alkyl sulfonic acid groups were grafted onto the silica surface.The composition,morphology and contact angle of these silica nanoparticles were characterized by FT-IR,TGA,XPS,SEM,EDS,etc.The results showed that the designed groups were successfully grafted onto the surface of the nanoparticles,making the contact angle increase from 22.8°for raw silica to 110°-130°for the modified silica,which would facilitate the construction of water-in-oil emulsions.The acid resistance test of modified silica showed that the groups of Si O₂ nanoparticles modified by dimethyldichlorosilane were unstable in strong acid,however,the nanoparticles modified with C₈-(or C₁₈-)and alkyl sulfonic acid groups were very stable in strong acids,which laid foundation for the fabrication of Pickering emulsions containing strong acid.?2?Pickering emulsions were constructed with strong acid resistant modified silica nanoparticles as the emulsifiers,ionic liquid?IL?/sulfuric acid mixture as the water phase,and cyclohexane as the oil phase.The regulation laws of the emulsions were revealed and its enhancement in the product distribution of isobutene oligomerization was studied.The effects of nanoparticle structure,ionic liquid concentration,emulsifier content,storage time and temperature on the physical and chemical properties of the emulsion were studied.The results showed that as the concentration of ionic liquid increased,the average droplet size gradually increased due to the increase in viscosity of the IL phase.As the alkyl chain of the oil phase increased,the average droplet size decreased.The emulsion fabricated by C₈-Si O₂-SO₃H?octyl and propylsulfonic acid groups modified silica?had the best stability,and when the emulsion was stored at room temperature for 14 days or when the temperature increased to 80°C,no obvious demulsification was observed.A method of demulsifying the constructed emulsion to separate the emulsifier by centrifugation was proposed.It was found that after centrifuging at 5000 r/min for 5 min,the emulsion can be completely demulsified and the solid particles can be easily recovered.The emulsion composed of C₈-Si O₂-SO₃H nanoparticles and 60 wt%IL/H₂SO₄ was used to improve the product distribution of isobutene oligomerization under mild conditions.It was found that compared with the system without emulsion,the weight percentage of trimers(C⁼₁₂)in the product could be enhanced by 4.8%at 40°C in 60 min,and that of C⁼₁₂ and C⁼₁₆ was increased by10.5%,while isobutene conversion remained unchanged.?3?Different types of neat ionic liquids and composite ionic liquids were used as the water phase to fabricate Pickering emulsions.It was found that 1-sulfopropyl-3-methylimidazolium trifluoromethanesulfonic cannot fabricate emulsions efficiently due to its high viscosity,while the composite ionic liquid 1-sulfopropyl-3-methylimidazole trifluoromethanesulfonic acid/1-butyl-3-methylimidazolium bis?trifluoromethylsulfonyl?imide can successfully construct water-in-oil emulsion,which had the characteristics of strong acidity and large interface area.The constructed emulsion system was used for isobutene oligomerization,and the results show that the weight percentage of dimers?C₈⁼?in the product could be increased by 6.7%and isobutene conversion can be enhanced by 26.4%within 60 min at 60°C.Therefore,the product distribution of dimers and trimers of isobutene oligomerization in Pickering emulsion system can be well controlled by changing the acidity of the catalyst phase.