Design And Functionalization Of Microporous Metal-organic Frameworks For CO₂ Capture And Light Hydrocarbon Purification

Separation and purification of gas mixtures is critically important for our daily life and modern chemical industry.Traditional gas separation and purification techniques based on distillation or liquid absorbent are highly energy-and capital-intensive.In contrast,it is estimated that the adsorptive separation based on porous materials as non-thermal separation agents is regarded as a suitable alternate to save the tremendous energy of distillation-based separation.The separation efficiency of this gas separation technique generally depends on the internal porosity and surface properties of the solid adsorbent.Compared with classical porous materials such as zeolite silicates and carbon-based materials,metal-organic framework(MOFs)are novel customizable porous materials affording precise tuning and functionalization of pore structure.Such unique features endow MOFs great potential in gas storage and separation.In this paper,we focus on giving consideration of high uptake capacity and high gas selectivity,keeping an appropriate balance between high gas selectivity and low regeneration cost,completely separating the gas mixtures with very similar physical properties.In this thesis,a series of metal-organic framework(MOF)materials were successfully prepared.Firstly,a highly stable MOF material ZJU-198,ZnL DMF(L?(2E,2E')-3,3'-(5-amino-1,3-phenylene)diacrylic acid;DMF = N,N-dimethylformamide)was successfully designed and synthesized.Gas adsorption studies showed that the activated ZJU-198a showed negligible adsorption of N2 due to the complete block-off of its unique window size,but exhibited relatively high CO2 adsorption uptake,featuring the ideal molecular sieving of CO2 from N2.Furthermore,its isothermal adsorption heat of CO2 is much lower than that of other representative CO2-capturing materials.As a result,ZJU-198 represnts a satisfactory adsorbent model,giving an efficient consideration of both selectivity and regeneration cost,affording it as one of the best-performing materials for CO2/N2 separation at application conditions.Secondly,a double-interpenetrated HKUST-like tbo-MOF(termed as ZJU-199,[Cu2L(DMF)(H2O)]·DMF.H2O,L= benzene-1,3,5-triacrytic acid)was successfully constructed.ZJU-199 features a high density of open metal sites and suitable pore sizes,thus showing a much higher C2H2 adsorption than those of CO2 and CH4,exhibiting highly selective separation of C2H2/CO2 and C2H2/CH4 gas mixtures at room temperature.Due to the double interpenetration,a significant decrease in the porosity of ZJU-199 has been observed,thus reducing the adsorption capacity of acetylene.It is quite siginificant to improve the surface area of MOF materials and thus to enhance high gas adsorption capacity.In this regard,a non-interpenetrated MOF material with m-phenylamine groups(named ZJU-195,[Cu2L(H2O)2].(DMF)0.5·(H2O)7),L = 5-amino-3,3,5,5-tetracarboxylic acid)with higher porosity was successfully constructed.Thus,the activated ZJU-195a shows one of the highest acetylene uptakes for MOF materials ever reported.The performance of C2H2/CO2 separation in ZJU-199 and ZJU-195 should be significantly improved due to the relatively low selectivity.However,separation of acetylene(C2H2)from carbon dioxide(CO2)is very difficult and challenging because of their similar molecular sizes and associated physical properties.Thus,realization of low-cost and high-performance porous materials is of importance to facilitate the implementation of energy-efficient adsorptive separation into practical gas separation applications.Here,we utilized a cheap and commercially available formic acid ligand to successfully construct a robust MOF material[Ni3(HCOO)6·DMF](1·DMF),offering high chemical stability,low cost,and high selectivity toward C2H2 over CO2.The exceptional separation performance of the activated 1 is mainly attributed to the small pore size(4.3 A)and functional O donor sites on the pore walls that provide strong binding affinity toward C2H2,as revealed by the detailed computational studies.This material thus exhibits ultrahigh low-pressure C2H2 uptake(38.2 cm3 cm-3 at 0.01 bar and 298 K)and possesses a high C2H2/CO2 selectivity(22.0 at ambient conditions),comparable to other leading porous materials.The high separation performance of 1 was further confirmed by the actual breakthrough experiments on a 50/50 C2H2/CO2 mixture.We further designed and synthesized a flexible Zn-based MOF material(ZJU-196,ZnL G,L = 3-amino-4-hydroxy benzoic acid,G = guest molecules)with one-dimensional flexible diamond channels.After the solvent molecules within the channels completely removed,the state of pores in ZJU-196 can be changed from gate-opening to gate-close.The activated ZJU-196a still keeps the gate-close state for carbon dioxide molecule under environmental conditions,but presents the gate-opening state for acetylene molecule,thus selectively adsorbing C2H2 over CO2.At 298 K,the adsorption capacity ratio of C2H2/CO2 in ZJU-196a can be as high as 25.0.This ratio is slightly higher than the benchmark UTSA-300(21.0),and significantly better than other representative MOF materials.In addition,we found that ZJU-196 shows unique acid-base resistance,which can maintain its framework under extremely harsh acid-base conditions.The breakthrough experiment of C2H2/CO2 mixture shows that ZJU-196a can effectively separate and purify C2H2 gas from C2H2/CO2 mixture,which further meets the various requirements of industrial separation and purification of acetylene.For boosting ethylene/ethane separation,we here designed and synthesized copper(I)-chelated metal-organic frameworks with tailor-made aperture sizes and specific ?-complexation.The larger 1,2,4-tricarboxylic acid and 1,2,4,5-tetracylic acid were selected as 2-coordination organic ligands with 12-coordination[Zr6(3_oh)8(O2C-)i2]to successfully construct UiO-66 series MOF materials.After the chelatation of copper(?)ions,the effective aperature of Cu?@Ui0-66-(COOH)2 is just 4.1 A,similar to the size of C2H4,but slightly smaller than that of C2H6 molecule.Gas sorption studies showed that the chelated Cu' did not only contract the aperture of material and thus reduce the adsorption of ethane,but significantly enhanced the affinity toward ethylene through the formation the specific ?-complex complexation.Particularly,the IAST selectivity of C2H4/C2H6 at roomtemperature in Cu?@UiO-66-(COOH)2 is so high up to 80.8 at 1.0 bar,which is orders of magnitude higher than that of other UiO-66 series MOF materials.Morever,the C2H4/C2H6 separation of Cu?@UiO-66-(COOHO2 is only lower than UTSA-280 with molecule-sieving efect,but ignificantly higher than that of other promising porous materials.