| The storage and separation of light hydrocarbons are of vital importance to energy,environmental protection,and the chemical industry.At present,distillation technology is used to separate mixed gases in industrial gas purification,but this technology has high energy consumption and high cost,which is not conducive to large-scale use.Metal-organic frameworks(MOFs),as an emerging class of crystalline porous materials,have been widely developed and extensively used for many fields due to their special inorganic and organic components,modification of pore structures and high surface areas.In the past twenty years,MOFs have been developed by leaps and bounds.Thanks to the permanent and ordered porous properties,MOFs play an important role in gas adsorption and separation.An ideal adsorbent material should have a good combination of high storage capacity and high separation ability.A moderate pore size is an important factor in the design of an ideal adsorbent.Pore space partition(PSP)strategy is an effective method proposed by our group to precisely adjust pore size in MOFs.Adjustable pore size and pore environment based on super metal compatibility of [M3(O/OH)(COO)6] and multiple ligands can achieve precise control of adsorption and separation performance for pacs-MOFs(pacs = partitioned acs).In this thesis,based on 9-c pacs-MOF platform,a series of MOFs with different pore sizes,pore partitioning modes and functionalization were designed and synthesized by selecting different metal ions,linear ditopic ligands and triangular ligands.MOFs with high selectivity of C2Hx/CH4,CO2/CH4,C2H2/CO2 and high C2H2 capacity were successfully constructed.The thesis mainly consists of following four parts:1.A structure–performance relationship screening for sixty-two high-performance MOF adsorbents reveals that a moderate pore size distribution around 5.0~7.5 (?) is critical to fulfill this task.With subtle modulation of the ditopic linker lengths of the MIL-88-type framework,the PSP strategy precisely partitions 1D channels into small finite segments with the pore sizes varying from 4.5 (?)(SNNU-26)to 6.4 (?)(SNNU-27),7.1 (?)(SNNU-28),and 8.1 (?)(SNNU-29).Notably,the introduction of tetrazole groups provides high-density bare N sites that serve as hydrogen-bonding acceptors(HBAs)to preferentially bind C2H2 molecules.Suitable pore sizes,high density HBAs,and robust frameworks confer all PSP-MOFs with excellent C2H2 uptake capacity and high C2H2/CO2 selectivity.Optimized SNNU-27-Fe demonstrates an extraordinary C2H2 uptake capacity(182.4 cm3·g-1)as well as the longest C2H2/CO2 breakthrough time(91 min·g-1)under ambient conditions,which surpasses the performance of all reported MOF adsorbents to date.2.The exploration of new pore space partition mode is demonstrated in this chaper.SNNU-51–56 are constructed by metal-organic linker(MOL)strategy with [M(PDC)2]mononuclear units as MOLs.The trans-MOLs act as linear ditopic ligands in SNNU-51–52linking [M3(OH)(COO)6] subunits to give an acs net,which is further partitioned by C3-tripyridine ligands to give a(3,4,9)-c framework.But cis-MOLs link trinuclear clusters to form a 1D metal-organic chain containing trigonal bipyramidal cages,which further connects six nearby chains through C3-tripyridine linkers to give another(3,4,9)-c net of SNNU-53–56.Different from pacs-MOFs,tripyridine ligands in SNNU-51–52 partition the channels along the a and b-axis directions of the acs net and thus offer a new type of pore space partition of MOFs.As expected from highly connected architectures and the absence of open metal sites,all these MOFs show high thermal and moisture stability.They can even retain their frameworks after being treated at different p H values for 24 h.Furthermore,SNNU-51–56 not only inherit the excellent gas uptake performance of robust 9-c MOFs,but also exhibit extremely high CH4 separation from CO2,C2H2 and C2H4.Especially SNNU-51 demonstrates an extraordinary IAST selectivity for CO2/CH4(257.8)、C2H2/CH4(236.7)and C2H4/CH4(454.5)at 273 K and 1 bar.3.Based on the specific π-complexation between Cu+ and the C≡C bond or C(28)C bond of unsaturated hydrocarbons,the introduction of Cu+ into the MOF structure is expected to improve the adsorption and separation capacity of C2H2 or C2H4.Herein,one Cu atom coordinated with three N atoms from three coplanar pyz ligands to form a plane triangular Cu(pyz)3 unit.Five novel Ni-pacs-MOFs(SNNU-33s)were synthesized by pore partitioning agents Cu(pyz)3 and linear dicarboxylic acid ligands with different functional groups.The Brunauer-Emmett-Teller(BET)surface areas and pore volume of SNNU-33 s with shorter Cu(pyz)3 are lower than CPM-33 s with TPT ligands.Therefore,the adsorption values of SNNU-33 s for small molecular gases are not as good as CPM-33 s,but the values are still higher than some MOF materials with the same BET surface areas.Specially,the C2H2/CH4selectivity(6591)and C2H4/CH4 selectivity(890.3)of SNNU-33 b at 298 K and 1 bar are the highest one among all MOFs materials reported so far.The high separation performance of SNNU-33 b was further confirmed by the actual breakthrough experiments on C2H2/CH4(50/50,v/v)and C2H4/CH4(50/50,v/v).4.Based on previous researches,the introduction of Lewis base sites in MOFs with moderate pore size can further enhance the force of MOF framework on acetylene.In order to obtain more high performance acetylene adsorbent,different Lewis base sites are regulated based on Mg-pacs-MOFs.There are 12 permulations by combining four dicarboxylate ligands and three pore partitioning agents we are able to establish general trends in acetylene-selective C2H2/CO2 separation properties.The experimental results show that for the same pore partitioning agents,the hydroxyl functional group has a certain promotion effect on C2H2 adsorption and C2H2/CO2 separation.And the framework of MOFs based on 2,5-dihydroxyterephthalic acid(2,5-DHBDC)ligand has the strongest interaction with acetylene.With 2,5-DHBDC as the best choice for C2H2 adsorption and C2H2/CO2 separation,the effects of pore partitioning agents with different Lewis N sites on the separation potential were subsequently evaluated.Unfortunately,due to the different steric hindrance of the three ligands with different Lewis base sites,their performance did not improve with the increase in the number of Lewis N sites.Optimized SNNU-78-2,5DHBDC can achieve the best combination of high C2H2 adsorption capacity(170.8 cm3·g-1)and high C2H2/CO2 selectivity(21.2).Dynamic breakthrough studies show a C2H2/CO2 breakthrough time up to 82 min·g-1,among top-performing MOFs. |
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