Research On Preparation And Performance Of Crystalline Microporous Materials Based On The Separation Of Light Hydrocarbons

The light hydrocarbon industry with ethylene and propylene as the core component is the cornerstone of the petrochemical industry.From the production of raw materials to the application of downstream derivatives,the olefin industry affects all aspects of human society.With the high-speed growth of the global economy,annual olefins consumption is increasing rapidly,and the scale of production is also expanding.In the current olefin production technology,whether it is traditional steam cracking and catalytic cracking of naphtha,or the gradually developed new technologies such as alkanes dehydrogenation and coal to olefins,the all-product compositions are mixtures,which must be purified to obtain high-purity raw materials to meet the requirements of downstream derivative production.Among them,the removal of ethane(C₂H₆)from ethylene(C₂H₄),and propane(C₃H₈)and propyne(C₃H₄)from propylene(C₃H₆)are the key separation steps to obtain high purity olefin.The separation method used in the industry is based on the heat-driven cryogenic distillation technology with extremely high investment costs and energy consumption and does not meet the current requirements of energy conservation and emission reduction.The adsorption separation technology based on adsorbent is expected to reduce energy consumption in the petrochemical industry.The preparation of high-performance adsorbent is the key to the application of this separation process.Alkane selective adsorbent can preferentially capture alkanes in the separation process of alkane/olefin mixture,and high purity olefin can be directly obtained through one-step separation,which greatly improves the work efficiency of adsorption separation technology and helps to further reduce separation energy consumption.Similarly,the selective adsorption of trace amounts of C₃H₄ with C₃H₄ selective adsorbent can efficiently separate C₃H₄/C₃H₆ mixtures.Based on the above gas separation requirements,the preparation of adsorbent selective for alkanes and alkynes in gas mixtures is the research objective of this paper.In this paper,crystalline microporous materials such as molecular sieves,metal-organic framework(MOF),and covalent organic framework(COF)were used as adsorbents fabrication platforms.By functionalizing unsaturated metal sites,adjusting the adsorption environment of the pores,and fine-tuning the pore size,a series of target adsorbents were prepared for the separation of C₂H₆/C₂H₄,C₃H₈/C₃H₆ and C₃H₄/C₃H₆ mixtures.The main research contents and conclusions involved are as follows:1.In the one-step synthesis process,water molecules were used to directly functionalize the open metal sites in a series of rare earth-based metal-organic framework[Ln-BTC(Ln(BTC)(H₂O),Ln=Y,Sm,Eu,Dy)]to prepare C₂H₆ selective adsorbents.The single-component gas adsorption experiments and IAST calculations showed that the C₂H₆selective adsorbent was successfully prepared by this modification strategy.Among them,Y-BTC exhibits the highest C₂H₆ adsorption capacity and adsorption selectivity,showing the potential of separating C₂H₆/C₂H₄ mixtures.Grand Canonical Monte Carlo(GCMC)and density functional theory(DFT)theoretical calculations showed that the oxygen atoms on organic ligands can form stronger interactions with C₂H₆.Under ambient conditions,this series of MOF can be used to selectively remove low concentrations of C₂H₆ from C₂H₆/C₂H₄mixtures to directly obtain polymer-grade C₂H₄,showing excellent separation ability.Moreover,the synthesis method of Y-BTC is simple and has moderate hydrothermal stability.The above results show that Y-BTC has the potential for industrial separation.2.A series of functional ligands were introduced into Ui O-type MOF to regulate the suitable micropore environment for C₃H₈ adsorption to improve the separation performance of C₃H₈/C₃H₆.The single-component gas adsorption isotherms showed that BUT-10 has a stronger adsorption affinity for C₃H₈ over C₃H₆ in the temperature range of 298-338 K.At298 K and 0.1 bar,the C₃H₈ adsorption capacity of BUT-10 reached 105 cm³/g,which was the highest C₃H₈ adsorption uptake among the reported C₃H₈ selective adsorbents to date.Combined with GCMC and DFT theoretical simulation demonstrate that the favorable adsorption environment formed by the non-polar benzene ring and the carbonyl group is conducive to the formation of multiple host-guest interactions between the C₃H₈ molecule and the framework,which provides a stronger binding affinity for C₃H₈.Breakthrough experiments show that BUT-10 can selectively remove C₃H₈ from C₃H₈/C₃H₆(1/1 and 1/15,v/v)mixtures to directly obtain high purity of C₃H₆ under environmental conditions.In addition,BUT-10 showed stronger adsorption affinity for C₂H₆ over C₂H₄,and its adsorption capacity and selectivity were effectively improved compared with the fundamental materials.BUT-10 can be used as a C₂H₆ selective adsorbent to achieve the effective separation of the C₂H₆/C₂H₄ mixture.3.The construction of alkane selective adsorbent in the metal-organic framework usually needs to consider the interaction between the metal sites and the olefin.Unsaturated metal sites usually form?-complexation with olefins,resulting in a stronger affinity for olefins than alkanes.Therefore,some complicated and challenging modification process are required to block the open metal sites.The covalent organic framework structure is composed of light elements without metal elements,which can completely remove the adsorption interaction of metal sites with olefins,and provide an ideal research platform for constructing alkane selective adsorbents.Eight COF materials with different structural dimensions,bond types,and structure configurations were prepared and the separation of C₂H₆/C₂H₄ was studied in detail.Single-component gas adsorption test,adsorption selectivity calculation,DFT simulation,and breakthrough experiment proved that COF-1 has the best C₂H₆/C₂H₄ mixture separation performance among the COF materials studied.Under the conditions of 298 K and1 bar,the adsorption selectivity of COF-1 for the equimolar C₂H₆/C₂H₄ mixture is 1.92,which is higher than other COF materials.Due to the abundant weak polar surface and suitable pore cavities in the porous structure of COF-1,multiple hydrogen bonds and C-H···?interactions with C₂H₆ molecules can be formed and thus,C₂H₆ can be preferentially adsorbed during the separation process.Breakthrough experiments proved that COF-1 can be used as an efficient C₂H₆ selective adsorbent for the separation of C₂H₆/C₂H₄[1/9,1/15(v/v)]mixtures.4.The commercially available type-A zeolite with high stability and low cost was selected to fine-tune pore size to improve the separation performance of the C₃H₄/C₃H₆mixture.At room temperature,the resulting ion-exchanged 5A(y Na⁺)zeolite selectively blocks C₃H₆ while maintaining the strong adsorption affinity for C₃H₄ at low-pressure region.The adsorption capacity of 5A(y Na⁺)zeolite for C₃H₄ reaches 2.3 mmol/g under 1 mbar,which effectively improves the adsorption selectivity for C₃H₄/C₃H₆ mixture.Breakthrough experiments under environmental conditions further confirmed that 5A(y Na⁺)zeolite can directly separate high-purity C₃H₆ from C₃H₄/C₃H₆(1/99 and 0.1/99.9,v/v)mixtures.