Multifunctional Structure Tuning Of Two-Dimensional Layered MOF Materials And Gas Adsorption Separation

Gas energy is widely used in industry and daily life,and the energy consumption of the separation and purification process accounts for more than 40% of the total chemical energy consumption.The development of economical and efficient porous materials for gas separation is of fundamental industrial significance and is also one of the important directions of energy chemistry and materials research.As a novel kind of porous material,metal-organic framework materials(MOFs)have unique pore structures,such as high porosity,high designability and easy modification.Based on these characteristics,MOFs with different functions can be customized for different adsorption aims to promote different interactions between hosts and guests,and achieve the purpose of efficient and energy-saving separation.This paper is based on a series of new two-dimensional layered materials with different central metal ions and different inorganic angular anions M’(4-DPDS)2MO4(M’=Ni,Co,Ni0.5Co0.5;M=Cr,Mo,W),which are researched for the four representative gas separation applications:single atom separation(noble gas capture and purification),mixed multi-component separation(natural gas purification),molecular separation of similar size and properties(ethylene and propylene purification),and molecular separation of the same size and shape(acetylene and carbon dioxide separation).By reasonable designing MOF building and functional strategies,including(1)substitution of inorganic angular anions;(2)substitution of central metal ions;(3)mixed metal(hybrid)strategy.The adsorption separation performance and potential applications of industrialization are comprehensively studied,and the multi-functional regulation mechanism of single dominant and cooperative control within and between intralayer and interlayer channels in the process of guest and host recognition mechanism is deeply discussed,which provides certain guidance for the development and design of new adsorbents.By studying the intralayer channel,rationally utilizing the shape and size of the shorter tetrahedral MO42-inorganic angular anions,which provide a cooperative effect conferred by the pore confinement and multiple specific interactions,and a "shell-like" xenon nano-trap was constructed to improve its affinity.The perfect permanent pore channel(4-5 A)of Ni(4-DPDS)2MO4(M=Cr,Mo,W)can host Xe atoms efficiently.The Xe/Kr adsorption selectivity of Ni(4-DPDS)2CrO4,Ni(4-DPDS)2 MoO4 and Ni(4-DPDS)2WO4 were 23.7,20.3 and 30.2,respectively,showing the second-highest Xe/Kr selectivity among the reported MOF.At the same time,it has excellent dynamic separation performance and high capture ability of Xe under ppm concentration conditions,breaking the bottleneck of adsorption capacity and selectivity.Single crystal X-ray diffraction and DFT-D calculations show that tetrahedral Mn42-angluar inorganic anions not only provide polar groups such as oxygen atoms,but also make guest-host interactions in close proximity and hence create a perfect pore environment for capturing Xe atoms efficiently.And three materials have good air stability,water stability and chemical stability,and excellent recyclability.To achieve the separation of intermediate size components,the interlayer dynamics effect was enhanced by the multi-diffusion channels,combined with thermodynamic equilibrium,molecular dynamics and molecular sieve separation mechanism.Ni(4-DPDS)2CrO4 has the advantage of suitable pore size and structure,and intralayer channels are the main adsorption sites.Although the affinity of C3H8 is higher than that of C2H6,the additional interlayer channels prevent the diffusion of C3H8 and increase the discrepancy of molecular dynamics diffusion rate even the sieving effect.Thus,the remarkable inverse equilibrium-dynamic equilibrium selectivity for C2H6/C3H8 is achieved.This is the first example of preferential capture of ethane in C1~C3 separations.CBMC and MD simulations,DFT-D calculations and C1~C3loaded single crystal X-ray diffraction results support the mechanism of gas molecular separation in Ni(4-DPDS)2CrO4 well.This work not only demonstrates the potential of two-dimensional layered MOFs with two different types of micropores to effectively capture intermediate-sized molecules by controlling intra-and interlayer space,but also elucidated the feasibility of optimized cooperative control of pore properties and the significance of synergetic multiple separation mechanisms.The separation of molecules of different sizes,such as acetylene/ethylene and propyne/propylene,can be achieved.by simultaneous controlling of intralayer and interlayer space of two-dimensional layered materials.For the separation of acetylene and carbon dioxide with similar molecular sizes,the importance of symmetry in pore space is proposed by using the opposite quadrupole moment and the different positions of interaction in pores.In Ni(4-DPDS)2CrO4,the rigid and homogeneous pore structure is more favorable for selective adsorption of small guest molecules such as acetylene.Ni(4-DPDS)2CrO4 exhibits saturation adsorption for acetylene and propyne,and has an extremely high acetylene adsorption capacity of 2.04 mmol·g-1 at 0.01 bar,which exceeds many benchmark materials reported so far.Dynamic breakthrough experiments show that Ni(4-DPDS)2CrO4 can achieve high polymer-grade ethylene and propylene production capacity and C2H2/CO2 separation potential.Through DFT calculation and single crystal loaded experiments,confirmed the ability of the adsorption cage composed of MO42-and eight pyridyl rings rich in electronegative S atoms to identify the adsorption materials was determined.GCMC simulation results combined with single crystal X-ray diffraction experiments revealed that the adsorption behavior of the adsorbent first occurred in the intralayer,and then interlayer adsorption occurred.The dynamic breakthrough cycle and scale-up shaping experiments verified the excellent stability and recycling capacity of Ni(4-DPDS)2CrO4 and the low energy consumption and low-cost regeneration method.A series of two-dimensional layered MOF materials Co(4-DPDS)2MO4(M=Cr,Mo,W)with flexible structure characteristics were synthesized by using Co(Ⅱ)with weaker coordination ability through the central metal substitution strategy.The materials are isomorphic to the previous Ni(4-DPDS)2MO4(M=Cr,Mo,W).Using the flexibility of structure,static adsorption can effectively achieve sieving among acetylene/ethylene,propyne/propylene and acetylene/carbon dioxide in Co(4-DPDS)2MoO4 and Co(4-DPDS)2WO4.Through the strategy of modulating frameworks interactions to control flexibility,the shorter CrO42-enhances the interaction within the Co(4-DPDS)2CrO4 framework,solving the problem of the adsorption pressure threshold for propyne at low pressure and achieving acetylene and propyne high adsorption capacity at the same time negligible olefins and carbon dioxide adsorption at 0.01 bar.The mechanism and regulation method of gate-opening on flexible materials were revealed from a molecular level.The adsorption sites of acetylene and propyne were determined by quantitative DFT calculation,and the mechanism of anion identification was revealed.The actual sites of acetylene and propyne in channels were directly visualized by adsorbate-loaded single crystal X-ray diffraction.The isomorphic bicenter metal Ni0.5Co0.5(4-DPDS)2MO4(M=Cr,Mo,W)was synthesized by the mixed metal strategy,which not only keeps a certain rigidity of the material,but also significantly reduces the adsorption capacity of weak components.ICP-OES,SEM and EDS characterization proved that the materials were synthesized successfully.The bicenter metal Ni0.5Co0.5(4-DPDS)2CrO4 was similar to that of a single metal.Ni0.5Co0.5(4-DPDS)2MoO4 and Ni0.5Co0.5(4-DPDS)2WO4 exhibited considerable acetylene and propyne adsorption capacities under low pressure,which decrease threshold pressure on Co(4-DPDS)2MoO4 and Co(4-DPDS)2WO4.At the same time,the adsorption capacity of ethylene,propylene and carbon dioxide is almost half of Ni(4-DPDS)2MO4,which follows the mixing law.The mixture dynamic experiment shows that there is almost no "leakage" of acetylene in Ni0.5Co0.5(4-DPDS)2MO4 material,and the separation effect of C2H2/C2H4 and C2H2/CO2 is obviously improved.But the separation of the mixed strategy of propyne and propylene were not significantly improving the performance.The results indicate that the mixed metal strategy needs to be customized for the target separation system,and its sensitivity to different systems is significantly higher than that of single metal MOF,which may be related to the disorder of the internal structure.