| The wastewater treatment and CO2 capture are of great importance from both environmental and economic points of view. Due to their advantages of large surface areas, various functional groups, and designable structures,metal-organic frameworks (MOFs) and porous organic frameworks (POFs)have fascinating performance in many areas including wastewater treatment,gas adsorption, and separation. However, attributed to the weak coordination bonds, MOFs generally have poor chemical stability, which seriously limits their industrial application. Therefore, synthesizing highly stable MOFs is of great importance. In addition, structure and composition of porous materials have great effect on their performance. Therefore, this work mainly focuses on chemical stability, pore size, and composition of porous materials, synthesizes some highly stable MOFs and POFs, and investigates their performance for adsorption and separation. The main contents and findings are given as follows.1. UiO-66 with defects was successfully prepared by a synthesis strategy of using benzoic acid as a modulator and postsynthetic acid treatment to remove the coordinated benzoate ligands. The resulting defective UiO-66 shows high BET surface area, large pore size, high total pore volume, and excellent chemical stability towards H20 and 1 M HCl solution. In addition,the defective UiO-66 prepared by this strategy has the highest BET surface area (1890 m2·g-1) among all the reported UiO-66 so far. The adsorption performance of the defective UiO-66 towards dyes with different sizes in aqueous solution was studied. The adsorption capacities of Safranine T and Crystal Violet in defective UiO-66 are 366 and 18 mg·g-1, respectively. The distinct difference in adsorption capacity makes the defective UiO-66 to separate Safranine T and Crystal Violet mixture. However, defect-free UiO-66 shows low adsorption capacities for both dyes and poor selectivity.2. We synthesized two trifluoromethyl-functionalized 4,4'-biphenyl-dicarboxylic acid (H2BPDC) — 3,3'-bis(trifluoromethyl)-4,4'-biphenyl-dicarboxylic acid (H2BPDC-o-2CF3) and 2,2'-bis(trifluoromethyl)-4,4'-biphenyldicarboxylic acid (H2BPDC-m-2CF3), and prepared two MOFs with UiO-67 structure by using these two linkers, named UiO-67-o-2CF3 and UiO-67-m-2CF3, respectively. Due to hydrophobicity of trifluoromethyl group,UiO-67-o-2CF3 and UiO-67-m-2CF3 are more stable than UiO-67.UiO-67-o-2CF3 and UiO-67-m-2CF3 can remain intact after treatment with water, 1 M HCl, and pH = 12 NaOH aqueous solutions for 1 day. While structure of UiO-67 collapses after these treatment. Moreover, after treatment with water for 60 days, UiO-67-o-2CF3 and UiO-67-m-2CF3 still remain intact.In addition, the effect of position of trifluoromethyl group on the stability of MOFs was investigated. UiO-67-o-2CF3 is more stable than UiO-67-m-2CF3.This is attributed to that steric hindrance effect of ortho-trifluoromethyl group could protect Zr-O bond from attacking. Moreover, CO2/CH4 selectivities of UiO-67-o-2CF3 and UiO-67-m-2CF3 are 6.5 and 6.4, respectively.3. We designed and synthesized a Zr-MOF (named Zr-DTDC) with UiO-66 topology structure by using 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTDC) and ZrCl4. Due to the steric hindrance and electronic effect of methyl group as well as strong Zr-O bond, Zr-DTDC has excellent chemical stability. Zr-DTDC remains intact after treatment with water, boiling water, and pH = 0-11 aqueous solutions for 7 days. At 298 K and 1 bar, the C2 adsorption capacity of Zr-DTDC is 49.4 cc·g-1. In addition,Zr-DTDC possesses excellent CO2/N2 and CO2/CH4 selectivities. Due to the presence of thiophene ring, Zr-DTDC exhibits good adsorption ability for Hg0 vapor (60 mg·g-1). XPS characterization shows that the adsorption of Hg0 in Zr-DTDC is a chemisorption process. Moreover, after the Hg0 vapor adsorption, about 70% of the photoluminescence intensity of Zr-DTDC was suppressed. Therefore, Zr-DTDC could be used to sense Hg0.4. By using fumaronitrile (FUM) and 1,4-dicyanonaphthalene (DCN), we synthesized two covalent triazine-based frameworks (CTFs) (named CTF-FUM and CTF-DCN, respectively) for capturing CO2. Due to strong C-N bond in triazine rings and the carbonization of surfaces of materials,CTF-FUM and CTF-DCN have excellent chemical stability. They remain intact after treatment with boiling water, 4 M HCl, and 0.1 M NaOH aqueous solutions. CTF-FUM and CTF-DCN have ultramicropores of 5.2 and 5.4 A,respectively. Therefore, they exhibit excellent C02 adsorption capacity and selectivity. After exposure to atmosphere of 70% relative humidity, CTF-FUM and CTF-DCN show 35% and 25% drops in CO2 capacity, respectively. Due to having the highest nitrogen content, CTF-FUM-350 possesses the highest CO2 adsorption capacity (57.2 cc·g-1 at 298 K) and selectivities for C02 over N2 and CH4 (102.4 and 20.5 at 298 K, respectively) among all CTF-FUM and CTF-DCN. Dynamic breakthrough curves indicate that both CTFs could indeed separate gas mixtures of CO2/N2 and CO2/CH4 completely.5. By using 1,3-dicyanobenzo[c]thiophene (DCBT), we synthesized CTF-DCBT, which shows excellent chemical stability. CTF-DCBT remains intact after treatment with boiling water, 4 M HCl, and 1 M NaOH aqueous solutions. CTF-DCBT possesses ultramicropore (6.5 A), high heteroatom contents (S and N), and high sorption capacity for C02 (37.8 cc·g-1 at 1 bar and 298 K). There is no obvious loss of uptake observed after six adsorption-desorption cycles. The CO2/N2 and CO2/CH4 selectivities of CTF-DCBT are 112.5 and 10.3, respectively. Dynamic breakthrough curves indicate that CTF-DCBT could separate gas mixtures of CO2/N2 and CO2/CH4 completely. |
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