| Metallocene catalysts and late transition metal catalysts for olefin polymerization exhibit excellent catalytic activity and controllability of polymer structure.Polyolefin materials synthesized with these catalysts greatly promote the development of national defense,medical care,energy,and life science.Alkylaluminoxane is the most important co-catalyst in the catalytic system and plays a vital role in the catalyst activation process.Alkylaluminoxane is synthesized conventionally via a controlled partial hydrolytic reaction in which the product contains a distribution of various oligomers.At present,the free-water hydrolytic route remains the primary method to obtain a universal alkylaluminoxane with high co-catalytic activity,without introducing impurities.The hydrolysis reaction of alkylaluminum is a complex series-parallel reaction network,and alkylaluminoxane is an intermediate in the hydrolysis of alkylaluminum to aluminum hydroxides.When the local water concentration is too high,the alkylaluminoxane would ultimately convert to an insoluble precipitate.Moreover,the alkylaluminum reagent is chemically active and its hydrolysis reaction is fast and highly exothermic.Therefore,the synthesis process of alkylaluminoxane must be carried out under safe and controllable conditions.The technical difficulties faced in the current synthesis process,such as low reaction temperature,low yield,long reaction time,and the use of unusual or complicated reactors,need to be solved urgently.The application of flow chemistry makes possible the goal of high efficiency,safety,and controllability in the hydrolysis of alkylaluminum.The micro-dispersion technology breaks through the limitation of heterogeneous micromixing to rapid reaction,and solves the problems of low selectivity of series-parallel reactions.However,the methane and solid by-products form a complex gas-liquid-solid multiphase system,which deteriorates the reaction by hindering mass transfer in a small space.Starting with the synthesis of alkylaluminoxane,we combine 3D printing with flow chemistry techniques to strengthen the multiphase reaction,transforming batch reactions into continuous reactions.Finally,flow chemistry platforms based on micromixing and microdispersion intensification technology are developed to realize the synthesis of methylaluminoxane(MAO)and isobutylaluminoxane(IBAO)and isobutyl modified MAO(MMAO)in a safe manner.The main conclusions are as follows:(1)The hydrolysis reaction characteristics of TMA and TIBA are explored with the help of visualization techniques such as high-speed camera and infrared thermal camera.The consumption of microdroplets and the formation of by-products are observed,and the exothermic heat of the hydrolysis reaction is measured.For the hydrolysis reaction of TMA,micrometer-sized water droplets are reacted instantaneously by TMA,accompanied by the release of a large amount of methane gas and the formation of small amounts of solids.The molar ratio of gas to TMA is greater than 1,and the solid content is less than 2 wt%.The apparent heat of TMA hydrolysis reaction is-164.63 kJ/mol.However,the reaction rate of the TIBA hydrolysis reaction is lower,and the release of isobutane gas is greatly reduced.The molar ratio of gas to TIBA is less than 0.4,and no aluminum is lost as solids.The apparent heat of TIBA hydrolysis reaction is-143.13 kJ/mol.The hydrolysis reactions of TMA and TIBA are extremely fast processes.Due to its higher reactivity,TMA is more likely to be excessively hydrolyzed to produce solids.(2)In order to improve the yield of alkylaluminoxane and realize the continuous operation of the gas-liquid-solid multiphase system in the microchannel,we establish flow chemistry platforms based on microdispersion and micromixing intensification technology to achieve the synthesis of alkylaluminoxane,including four functional modules of mixing,reaction,separation and inline characterization.In the mixing module,a microdroplet generator with capillary insertion is designed to generate uniform-sized microdroplets.The regulation of the dispersion size of the microdroplets shows that the microdroplet generator produces microdroplets with a diameter of 160μm using toluene at a flowrate of 2 mL/min,which increases the contact area of the liquid-liquid reaction.In the reaction module,a miniature continuous stirred tank reactor in series is designed to breaks the enlarged gas barrier in the microchannel and increases solid handling capacity by active mixing.In the inline characterization module,based on the coupling design of CFD calculation and 3D printing,a split-andrecombination(SAR)type micromixer is developed to blend two streams in the laminar flow regime.The mixing performance is confirmed by simulations and VillermauxDushman reaction.The whole process of the hydrolysis reaction of TMA or TIBA can be realized end-to-end through the rearrangement and reduplication of the modules.(3)The mixing,reaction and separation modules are selected to establish a flow chemical platform for the efficient synthesis of MAO.A production rate of 1.1 ton/yr is obtained in the form of 10 wt%MAO in toluene solution.According to the characteristics of TMA hydrolysis reaction,the gas-liquid-solid multiphase reaction is realized stably in the flow chemistry platform by the reastaonable configuration of key micro-devices such as microdroplet generator,miniature continuous stirred tank reactor in series,and gas-liquid-solid separator.We study the effects of microdroplet size,molar ratio of water and TMA,residence time,and TMA concentration on the reaction characteristics.The results show that the intensification of microinterfacial mass transfer is an effective way to improve the characteristics of hydrolysis reaction.The initial molar ratio of water and TMA is a key factor,and the preferred range is 0.7-0.8.The single-pass yield of MAO reaches 45%.The small effect of TMA concentration and residence time on the reaction performance provides a guarantee for increasing productivity.Cocatalytic activities of 4880 kg(mol Fe)-1 h-1 and 820 kg(mol Zr)-1 h-1 are obtained from ethylene polymerization experiments at atmospheric pressure using an iron-based catalyst system and metallocene as the catalyst.Compared with Albemarle’s MAO products,the synthesized MAO has a similar product structure.The cocatalytic activity of synthesized MAO in the ethylene oligomerization is increased by 30%,and the cocatalytic activity with metallocene as the catalyst is equivalent.The flow chemistry platform has good long-term stability and inherent safety,and provides guidance for handling other highly exothermic reactions involving the formation of solids and gases.(4)A flow chemistry platform is proposed to synthesize MMAO,including TIBA hydrolysis reaction,followed by a co-hydrolysis process with TMA.IBAO and MMAO are synthesized as an alternative catalyst to MAO,enhaning the storage stability of MAO and its solubility in aliphatic solvents.Firstly,we propose a new process based on the flow chemistry platform to realize the hydrolysis of TIBA to prepare IBAO,integrating mixing,reaction,separation and inline characterization modules.The effects of reaction temperature,molar ratio of water and TIB A,and residence time on the reaction characteristics and product performance are systematically studied.Under the optimized reaction conditions,the single-pass yield of IB AO is as high as 98%with no solids observed,thereby omitting the process of solid filtration and product concentration compared to the MAO synthesis process.The cocatalytic activity of IBAO in the ethylene oligomerization is 5000 kg(mol Fe)-1h-1.Secondly,we synthesize MMAO products by mixing IBAO and TMA and co-hydrolyzing them.The same mixing and reaction modules are connected in series in the flow chemistry platform.The production rate of pure MMAO is about 0.23 ton/yr.In the co-hydrolysis stage,we explore the influence of the molar ratio of water to TMA and the molar ratio of TMA and TIB A on the reaction performance and product properties.Under the cohydrolysis reaction conditions where the molar ratio of water to TMA is 0.75 and the molar ratio of TMA and TIBA is 2.33,the overall yield of MMAO is 51%,and the product structure and cocatalytic activity in ethylene oligomerization and zirconiumbased metallocene polymerization are equivalent to those of commercially available MMAO-3A products.Cocatalytic activities of 3430 kg(mol Fe)-1h-1 and 650 kg(mol Zr)-1 h-1 are obtained from ethylene polymerization experiments at atmospheric pressure using an iron-based catalyst system and metallocene as the catalyst.Compared with the MAO synthesis process,the partial exchange between methyl and isobutyl improves the yield of the hydrolysis reaction and the stability of the product. |
