| Metal-organic frameworks(MOFs)are a kind of microporous crystalline materials which are assembled by metals or metal clusters and organic linkers.Owning to the merits of ultrahigh porosity and specific surface areas,structural diversity and designability,MOFs are promising materials in adsorption separation fields.Currently,the focus of research has already been transformed from the preparation and designment of MOFs to development of industrial valuable MOF.However,most MOFs have the problems of poor stability,high cost of raw materials,complex and harsh synthetic process,and so on,greatly limiting their industrial application potential.Hence,in this thesis,focusing on dye-based metal-organic frameworks with good stability,cheap raw materials,green and mild synthetic process,we designed and synthesized a series of Prussian blue analogues and gallate-based metal-organic frameworks.The adsorption separation performance and interaction mechanism were carefully investigated,which provided theoretical basis and data support for the industrial application of MOFs for light hydrocarbon adsorption separation.We explored the adsorption separation thermodynamics and interaction mechanism of Prussian blue analogue-Zn3[Co(CN)6]2 for hexane isomers.Single-component adsorption isotherms showed that Zn3[Co(CN)6]2 exhibited high affinity for linear n-hexane(nHEX)and mono-branched 3-methyl-pentane(3MP),while nearly no adsorption uptake for di-branched 2,2-dimethyl-butane(22DMB)at temperature range of 303-423 K,realizing molecular sieving of di-branched component from hexane isomers mixture.Specifically,Zn3[Co(CN)6]2 displayed extremely high uptake ratios of nHEX/22DMB and 3MP/22DMB(20.1 and 19.3),second only to those of the best-performing Al-bttobt(42.8 and 26.0)at 303 K and 100 mbar.To evaluate the separation performance attainable of multicomponent conditions,column breakthrough experiments were performed on Zn3[Co(CN)6]2.The results showed that 22DMB eluted almost immediately with merely no retention,while 3 MP and nHEX broke out after a long retention time,further demonstrating the complete isolation of 22DMB from hexane isomers mixtures.Variable temperature powder X-ray diffraction indicated that Zn3[Co(CN)6]2 transformed from cubic phase to hexagonal phase under high temperature,along with the pore size changing from 8.5 A to 5.7 A,exactly realizing the sieving of di-branched isomer(kinetic diameter of 6.0 A)from alkane mixture.Adsorption kinetics of hexane isomers on Prussian blue analogue-Zn3[Co(CN)6]2 was further investigated.On the one hand,based on pulse gas chromatography,moment analysis was adopted to determine the adsorption equilibrium constants and microporous diffusion coefficients of hexane isomers on Zn3[Co(CN)6]2 as the gas chromatography stationary phase.The results showed that di-branched alkane had a retention time similar to that of the non-adsorbed component,almost no adsorption,consistent with the thermodynamics results,while linear and mono-branched alkanes eluted after quite long retention time,with eluting sequence accorded with the thermodynamics interaction strength.Moreover,the influence of temperature and adsorbates structure on microporous diffusion coefficients were investigated.The results demonstrated that with the same adsorbate,microporous difusion coefficients increased along with the rising of temperature;under the same temperature,microporous diffusion coefficients of mono-branched 3MP(0.76 × 10-2 s-1 at 393 K)were lower than that of linear nHEX(1.91×10-2 s-1 at 393 K).On the other hand,based on the kinetic module of vapor adsorption analyzer,volumetric method was adopted to determine the microporous diffusion coefficients.The results indicated that microporous diffusion coefficients calculated from volumetric method were in good agreement with that acquired from pulse gas chromatography,confirming the reliability and accuracy of these two methods.Furthermore,we explored the feasibility of adsorption separation of olefins/paraffins through Prussian blue analogue material-Cu3[Co(CN)6]2.Pure-component static adsorption indicated that this material exhibited IAST selectivities of ethylene/ethane(50:50)and propylene/propane(50:50)up to 78.5 and 16.1 respectively,far superior to those by most reported star MOFs.Besides,the adsorption heats of this material for ethylene and propylene are 37 and 27 kJ·mol-1 respectively,much lower than that of the typical M2(dobdc)(M=Mg,Mn,Fe,Co,Ni,Zn)(>40 kJ-mol-1)with coordination unsaturated metal sites(CUSs).Fixed-bed breakthrough tests further confirmed the excellent separation performance of Cu3[Co(CN)6]2 for equimolar C2H4/C2H6 and C3H6/C3H8 gas mixtures.Adsorption mechanism was explored by adopting FT-IR spectra loaded with adsorbed CD3CN as the probe molecule,X-ray photoelectron spectroscopy(XPS)and electrostatic potential energy calculation of adsorbates.The results exhibited that Cu3[Co(CN)6]2,which contained large amounts of coordination unsaturated cooper sites,was likely to interact with ethylene and propylene with higher electron cloud density,while interacted weakly with ethane and propane,contributing to the selective adsorption separation of ethylene/ethane and propylene/propane.The thermodynamics and interaction mechanism of gallate-based metal-organic frameworks,M-gallate(M=Mg,Co,Ni),were studied for the adsorption separation of trans/cis isomers.These materials featuring elliptical windows with suitable aperture sizes were perfect matching with the minimum cross-section size of trans-piperylene,while excluding the slightly larger cis-piperylene,realizing the efficient separation of trans/cis-piperylene via shape-size sieving.Pure-component adsorption isotherms indicated that Mg-gallate exhibited a high affinity for trans-piperylene,contributing to uptake ratio of trans/cis-piperylene up to 4.22 at 298 K and 100 mbar.The fixed-bed breakthrough tests further confirmed that M-gallate not only displayed prominent separation performance of trans-and cis-piperylene,but also could segregate the ternary mixture of trans-piperylene/cis-piperylene/isoprene into individuals combined with zeolite 5A.The strong adsorbate-framework interactions,including hydrogen bonding,supramolecular interactions and van der Waals interactions,were attested by single-crystal X-ray diffraction of Ni-gallate loaded with traps-piperylene molecules,illustrating the efficient adsorption of M-gallate fortrans-piperylene. |
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