Adsorptive Separation Of Olefins With Pillar-layered Microporous Materials Via Equilibrium-kinetic Synergetic Echanism

Separation and purification of olefins presents one of the most important yet the most energy-intensive separation processes in petrochemical industry.Adsorptive separation is an energy-efficient technique,which recently has shown great promise and become the hot topic for light hydrocarbon separation.The adsorptive separation process mostly follows either equilibrium or kinetic mechanism,but usually meets with limited success,because the analogous molecular structure,size and physical properties of light hydrocarbons exert great challenges on their splitting.In this dissertation,we reveal the high-efficient purification of C₂H₄ and C₃H₆ with pillar-layered microporous materials by exploiting synergetic effect of equilibrium and kinetics.We hope our research would provide some clues to the development of innovative separation mechanism and novel adsorbents.High-efficient adsorption separation of C₂H₄/C₂H₆ and C₃H₆/C3H8 mixtures were realized by exploiting the synergetic effect of equilibrium and kinetics within a phosphate anion-pillared microporous material ZnAtzPO4(Atz=3-amino-1,2,4-triazole).Adsorption isotherms and time-dependent adsorption curves were collected for C2 and C3 hydrocarbons,the heat of adsorption and separation selectivity were calculated,and breakthrough experiments were conducted to testify the separation performance of ZnAtzPO4 under dynamic conditions.The results indicated that the equilibrium uptake capacities of C₂H₄ and C₃H₆ were both higher than C₂H₆ and C3H8.In the meantime,C₂H₄ and C₃H₆ exhibited much faster adsorption kinetics than the corresponding paraffins,endowing ZnAtzPO4 with excellent separation capability.Under 298 K,the equilibrium-kinetic combined selectivities for C₂H₄/C₂H₆ and C₃H₆/C3H8 can be 12.4 and 8.5.respectively,setting a new benchmark.Moreover,ZnAtzPO4 adsorbed C₂H₄ and C₃H₆ via moderate physisorption interactions with the heat of adsorption being 17.3 and 27.5 kJ·mol-1 at zero loading,which allowed the material to be easily regenerated under mild conditions.Breakthrough experiments confirmed ZnAtzPO4 had excellent separation performance for olefin/paraffin mixtures under dynamic conditions,and the separation factor for equimolar C₂H₄/C₂H₆ mixture was determined to be 6.7.Additionally,ZnAtzPO4 showed good stability and could be repeatedly used at least for 5 cycles.The olefin/paraffin separation mechanism of ZnAtzPO4 was unveiled by computational studies.Monte Carlo simulation and density functional theory(DFT)calculations were conducted to give insights into their interaction modes,and molecular dynamics simulations were also conducted to reveal the gas diffusional behaviors.According to the results,the pore surface of ZnAtzPO4 decorated by high-density phosphate anions and amino groups mainly interacts with C₂H₄ and C₃H₆ via hydrogen bonding interactions,and the material shows stronger affinity for olefin molecules over the corresponding paraffins.Moreover,the flexible pore window of ZnAtzPO4 undergoes less significant structural transformation when C₂H₄ and C₃H₆ move through,which may be the important cause to the faster diffusional kinetics of the olefins.Highly efficient kinetic separation of C₃H₆ and C3H8 was realized with an interlayered pillared microporous material Zn-ATA(ATA=5-aminotetrazole)by enhancing the contribution of pore-network diffusion via control over the physical form of crystal aggregates.Zn-ATA crystals in different sizes were prepared,and their separation performance for C₃H₆ and C3H8 were evaluated by adsorption isotherms,time-dependent gas uptake profiles and breakthrough experiments.Additionally,the separation mechanism was explored by DFT calculations.The results indicated that millimetric-sized Zn-ATA crystals(Milli-Zn-ATA)exhibited prominent kinetic effect for C₃H₆/C3H8 separation with a selectivity of 60 under 298 K,attributed to the fact the adsorption process was mainly dominated by pore-network diffusion.After crystal downsizing,the effect of surface permeation became stronger and the separation performance of the crystals got debased.In experiments,the purity of C₃H₆ isolated from equimolar C₃H₆/C3H8 gas mixture by Milli-Zn-ATA reached 93%in one adsorption-desorption cycle,higher than other state-of-the-art materials.Simultaneous removal of C₂H₂ and C₂H₆ from a ternary C₂H₂/C₂H₄/C₂H₆ mixture was also achieved with Zn-ATA by molecular size matching and shape recognition.The adsorption isotherms and time-dependent uptake profiles of C2 hydrocarbons were collected for Zn-ATA,and the adsorption heat and separation selectivity were further calculated.The separation performance of Zn-ATA for ternary C₂H₂/C₂H₄/C₂H₆ mixture was confirmed by breakthrough experiments,and the molecular recognition mechanism was also explored by in situ X-ray powder diffraction analysis and molecular simulations.The results indicated that,the ideal adsorbed solution theory(IAST)selectivities of Zn-ATA for C₂H₂/C₂H₄(1/99)and C₂H₄/C₂H₆(10/90)gas mixtures were 1.81 and 1.84,respectively.In breakthrough experiments,C₂H₄ in high purity of 99.9%can be directly produced from a ternary C₂H₂/C₂H₄/C₂H₆ mixture with the productivity being 0.72 mmol·g-1.