Low-valent transition metal ions (Zn²⁺, Co²⁺, Ni²⁺, Fe²⁺, and Ag⁺, etc.) are soft acids, which can coordinate with N-heterocyclic ligands (imidazole, pyrazole, triazole, tetrazole, etc.) of soft bases to construct stable MOFs^[11]. The pK_a of N-heterocyclic ligands is generally higher than that of carboxylic acid ligands, and the affinity of N-heterocyclic ligands to low-valent transition metal ions is stronger, so the synthesized MOFs can exist stably in water/alkaline solution^[14,23]. In 2006, Yaghi et al. Reported 12 MOFs with zeolite topology formed by imidazole ligands modified by different functional groups and Zeolitic Imidazolate Framework s or Co²⁺, named ZIFs (Zeolitic Imidazolate Framework s), in which ZIF-8 can exist in boiling water for 7 days, and the structure can also be maintained in 8 M NaOH solution at 100 ℃ for 1 day. ZIF-68, ZIF-69 and ZIF-70 reported in 2008 also have similar stability, immersed in boiling benzene, methanol and water^[19][24]. In 2019, Huang et al., using a ligand containing imidazole and pyridine groups, 2- (1H-2-imidazolyl) -3H-imidazo [4,5-c] pyridine (Metal Azolate Framework), synthesized a new MOF: Zn (imPim) (named MAF-stu-1, MAF = Metal Azolate Framework),The thermal stability of Zn (imPim) is as high as 680 ℃ in nitrogen atmosphere, and its structure remains unchanged after soaking in boiling water for 9 days, and its PXRD pattern also remains unchanged after soaking in aqueous solution with pH = 2 ~ 13 and common organic solvents for 3 days at room temperature, which proves that Zn (imPim) has excellent chemical stability^[25].

In addition to imidazole ligands, MOFs formed by pyrazole ligands and low-valent transition metal ions also have good stability. A representative example is that in 2011, Long et al.5-Tris (4-pyrazolyl) benzene (H₃BTP) as a ligand and transition metal ions Ni²⁺, Cu²⁺, Zn²⁺, Co²⁺ were synthesized as four MOFs,Among them, the structure of Ni₃(BTP)₂ was not destroyed after soaking in 100 ℃ aqueous solution and pH = 2 ~ 14 (100 ℃) solution for 2 weeks^[26]. In 2020, Li et al. Successfully constructed two chemically stable Ni (II) -pyrazoline MOFs: BUT-32 and BUT-33 (BUT = Beijing University of Technology) with conformationally matched elongated pyrazole ligands through isosteric expansion. The two MOFs have the same sodalite-type network, and BUT-33 contains 2.6 nm mesopores.Thermogravimetric analysis (TGA) shows that the thermal stability of BUT-32 and BUT-33 can reach 398 ℃ and 380 ℃, respectively. The PXRD of the samples treated in HCl aqueous solution (pH = 3), 4 M NaOH aqueous solution and boiling water for 24 H at room temperature is unchanged, showing high acid/alkali resistance^[27]. In 2022, Li et al. Reported a class of double-layer metal-dipyrazolate framework materials, BUT-53 ~ 58, in which BUT-53 ~ 57 remained structurally intact after exposure to air for one year^[28]. The framework of BUT-53 ~ 56 and BUT-58 remained stable after soaking in HCl (pH = 5) and NaOH (pH = 14) solutions for 24 H (Fig. 2).

Low-valence transition metal ions and triazole ligands can also form highly stable MOFs. In 2020, Zhang et al. Synthesized a series of MOFs: ZnF (a/da/dmTZ) (TZ = 1,2,4-triazolium salt) and its isostructural MOFs by introducing amino groups on the triazole ring and Zn²⁺. The structure of ZnF (daTZ) (daTZ = 3,5-diamino-1,2,4-triazolium salt) remained unchanged after immersed in boiling water for 5 days, which proved that the MOFs^[29] The crystallinity and porosity of the MOFs remained unchanged after soaking in solutions with pH values from 1 to 12 for more than 20 H (Fig. 3), indicating that the MOFs have high chemical stability. In addition, thermogravimetric analysis shows that this kind of MOFs maintains the same framework structure in the range of 340 ~ 440 ℃, which proves that it has high thermal stability. In 2021, Shimizu et al. Reported a material with high capacity and selective adsorption for CO₂, CALF-20: (CALF = Calgary Framework), in which the 2D layer composed of 1,2,4-triazole (TZ) -bridged Zn²⁺ was pillared by oxalate ions to form a 3D structure, and the PXRD characterization and N₂ adsorption test of CALF-20 after exposure to 150 ° C vapor for one week showed that its porous structure remained unchanged, indicating that the CALF-20 material has high water stability^[30].

According to the HSAB theory,High-valence metal ions (Fe³⁺, Al³⁺, Cr³⁺,In³⁺, Ln³⁺, Ti⁴⁺ and Zr⁴⁺, etc.) Is a hard acid,Stable MOFs can be constructed by coordination with O-containing ligands of hard bases. The F FÉrey group was the first to use this strategy to synthesize stable MOFs. In 2002, F F Érey et al. Synthesized a series of 3D MIL-53 frameworks (MIL = Mat Mat Érial Institut Lavoisier) with 1 D diamond-shaped channels using the reaction of trivalent metal ion Cr³⁺ with H₂BDC ligand^[31]. In 2004 and 2005, the research group reported two Cr-MOFs with MTN topology: MIL-100 and MIL-101, which have very high water stability and acid/base stability. The high stability of Cr (Ⅲ) -MOFs is mainly due to the high charge/radius ratio of Cr (Ⅲ), the kinetic inertness of Cr (Ⅱ) -O bond and the large ligand field stabilization energy^{[17][18][32][12,13,21]}. In addition to the above MIL series, PCN-426-Cr (Ⅲ) (PCN = Porous Coordination Network) (2014) and PCN-333-Cr (Ⅲ) (2015) MOFs reported by Zhou et al. Can maintain the stability of the framework structure in the pH range of 0 to 11^[33][34]. In 2018, Eddaoudi et al. Reported that Cr-soc-MOF-1 has very high water stability, and after up to 100 water vapor adsorption cycles, Cr-soc-MOF-1 can still maintain its structural integrity (Fig. 4)^[35]. However, it is difficult to synthesize Cr-MOFs directly, so there are few reports on the de novo synthesis of Cr-MOFs so far, and most of Cr-MOFs are prepared by metal ion exchange after synthesis.

In recent years, researchers have also designed and synthesized some stable Al-MOFs. For example, in 2020, Zhang et Al. Reported a 2D tetrakis (4-carboxyphenyl) porphyrin aluminum material (Al-MOF), and the PXRD pattern of the Al-MOF did not change after soaking in water for one month, indicating that it had good water stability^[37]. In 2022, Manos et Al. Synthesized an Al-MOF-1: [Al (OH) (PATP)] · solvent by solvothermal reaction of AlCl₃·6H₂O and 2- (2-picolyl) amino terephthalic acid (H₂PATP), and the structure of the material did not change after heat treatment at 200 ° C under vacuum, and the PXRD pattern of Al-MOF-1 did not change after treatment in aqueous solution with pH = 2 – 12 (Fig. 5), indicating that the material had high chemical stability^[38]. In the same year, Wang et Al. Synthesized a stable Al-MOF: MIL-160 (Al) using 2,5-furandicarboxylic acid (FDCA), and its thermal stability reached 400 ° C^[39]. MIL-160 (Al) material was immersed in water, organic solvents (MeOH, DMF, MeCN, acetone), acidic (pH = 2) and alkaline (pH = 12) aqueous solutions for 24 H, and its PXRD pattern did not change, indicating its high chemical stability. Otherwise, MIL-160 (Al) has an ultrahigh volumetric capacity (227 cm³(STP)/cm³) for acetylene adsorption.

Zr⁴⁺ is a typical hard acid with high positive charge density and small radius, which has strong affinity with carboxyl oxygen and strong coordination bond, and is easy to form chemically stable MOFs^[40]. The most representative example is reported by Lillerud et al. In 2008 that the Zr-MOF:[Zr₆(μ₃-O)₄(μ₃-OH)₄(BDC)₁₂](UiO-66,UiO=University of Oslo),UiO-66 constructed by 12-linked [Zr₆(μ₃-O)₄(μ₃-OH)₄(CO₂)₁₂] clusters and H₂BDC ligands can maintain the structural integrity after soaking in aqueous solution at pH = 1 – 14 for 2 H, indicating its ultrahigh chemical stability^[20]. In 2020, Huang et al. Constructed a Zr-MOF: PCN-226 with a new topological (ztt) structure by connecting a zigzag ZrO₇ chain composed of seven-coordinated Zr⁴⁺ and copper (II) tetrakis (4-carboxyphenyl) porphyrin (CuTCPP). The PXRD pattern of the Zr-MOF did not change after soaking in aqueous solution with pH = 1 ~ 13 for 7 days (Fig. 6), and thermogravimetric analysis showed that it was stable before 400 ℃, indicating that PCN-226 had excellent chemical stability and thermal stability^[41]. It is worth mentioning that PCN-226 is highly active for catalyzing the oxygen reduction reaction (ORR).

In 2021, Wang et al. Used the hydrothermal reaction of 4,4 ′, 4 ″, 4- (1,4-bis (2,4,5-imidazolyl) phenyl) tetrabenzoic acid (BBI) and ZrCl₄ to synthesize a new luminescent Zr-MOF: Zr-BBI^[42]. Its thermal stability is up to 375 ℃, and its PXRD pattern remains unchanged after immersion in acidic aqueous solutions of pH = 1 and 6 M HCl for 24 H and alkaline aqueous solutions of pH = 10 ~ 12 for 12 H, showing high chemical stability. In the same year, Huang et al. Used bis [5-di (4-carboxyphenyl) phenyl] porphyrin (BBCPPP) ligand to coordinate with Zr₆ cluster to synthesize a new Zr-MOF with csq topology: PCN-625, whose topological network is similar to the well-known PCN-222 and NU-1000 (NU = Northwestern University)^[43][44][45]. It was found that the structure of PCN-625 was unchanged after soaking in water, boiling water, n-hexane, acetone, methanol, 6 M HCl and concentrated hydrochloric acid for 72 H, showing high chemical stability (Fig. 7). However, the intensity of the PXRD peak decreased significantly after immersion in 2 M NaOH aqueous solution for 72 H. In addition, PCN-625 can be used as a heterogeneous catalyst for size-selective hetero-Diels-Alder [4 + 2] cycloaddition.

In 2021, Gao et al. Selected 2,2 ′, 6,6 ′ -tetrakis (4-carboxyphenyl) -4,4 ′ -bipyridine (H₄TCPBP) ligand and 12-linked [Zr₆(μ₃-O)₄(μ₃-OH)₄(CO₂)₁₂] cluster self-assembly to synthesize a new Zr-MOF with positive cooperative protonation: Zr-TCPBP^[46]. The thermal stability of Zr-TCPBP is as high as 350 ℃ as shown by temperature-dependent PXRD. Moreover, the framework of Zr-TCPBP remains unchanged after immersion in 6 M HCl aqueous solution at pH = 0 – 11 for 12 H, which proves that the Zr-MOF has excellent chemical stability. Meanwhile, Zr-TCPBP also has pH-sensitive fluorescence and proton-conducting properties. In 2021, Cui et al. Used optically pure 1,1 '-biphenyl-20-crown-6-derived tetracarboxylic acid as a ligand (H₄L^1-3)^[47],

Three chiral porous Zr-MOFs:[Zr₆O₄(OH)₈(H₂O)₄(L^1-3)₂](CE-1~3) were synthesized by solvothermal reaction with ZrOCl₁₂·8H₂O, respectively. Their thermal stability is up to 400 ℃, and the PXRD peaks of these Zr-MOFs do not change after treatment in boiling water, concentrated HCl and NaOH (pH = 12) solutions for 1 week (Fig. 8), indicating that these materials have high chemical stability and can be used as chiral stationary phases for chiral resolution.

In 2022, Serre et al. Used 3,3 ′, 5,5 ′ -tetracarboxylic acid diphenylmethane (H₄mdip) ligand to construct the first 3D Zr-MOF:[Zr₆(μ₃-O)₄(μ₃-OH)₄(OAc)_0.24(OH)_5.76(H₂O)_5.76(mdip)_1.5](MIP-201,MIP=Materials of the Institute of Porous Materials from Paris) with soc-topology network by hydrothermal reaction with ZrCl₄ in acetic acid^[48]. Temperature-dependent PXRD and thermogravimetric analysis showed that MIP-201 was stable up to 375 ° C. The crystal structure of MIP-201 was not significantly altered after prolonged exposure to boiling water, nicotinic acid, aqua regia, high concentrations of H₃PO₄, pH = 10 buffer, and NH₄OH vapor (Fig. 9). At 77 K, the nitrogen adsorption test was carried out on the sample treated under extremely harsh conditions, and the results showed that the pore channels were still retained, indicating that MIP-201 had extremely high chemical stability. In addition, MIP-201 has excellent heterogeneous catalytic activity and recyclability for the hydrolysis of peptide bonds over a wide pH range.

In addition to Zr⁴⁺, the same family of Ti⁴⁺, Hf⁴⁺ can also prepare stable MOFs. In 2021, Horcajada et al. Constructed a new 3D porous Ti-MOF:[Ti₂O₃(C₄O₄)](IEF-11,IEF=IMDEA Energy Framework) using solvothermal reaction of squaric acid (H₂C₄O₄) ligand and Ti(OBu)₄ in isopropanol/acetic acid^[49]. Thermogravimetric analysis showed that IEF-11 was stable up to 300 ° C. After IEF-11 was immersed in a solution of pH = 1 ~ 10.5 for 16 H, its PXRD pattern showed that the structure was unchanged (Fig. 10), which proved that IEF-11 had very high chemical stability. It is worth noting that IEF-11 can be used as an effective photocatalyst for water photolysis under simulated sunlight, and the yield of H₂ reached 672μmol/g_catalyst within 22 H, which is a new record among MOFs materials, and its catalytic activity for water photolysis did not decrease significantly for at least 10 days. In 2020, Yang et al. Used 4,4 ′, 4 ″, 4- (4,4 ′- (1,4-phenyl) bis (pyridine-2,4,6-yl)) tetracarboxylic acid (H₄PBTA) to construct a new 3D Hf-MOF with sqc-a topology: Hf-PBTA via solvothermal reaction with Hf^4+[50]. When Hf-PBTA was immersed in pH = 1 ~ 10 solution at room temperature for 48 H, the PXRD pattern showed that the framework did not change, indicating that the material had high chemical stability. In addition, the Hf-PBTA can be used as a self-calibrating dual-emission fluorescence sensor, and shows sensitive detection capability and good selectivity for aromatic sulfonic acid and sulfite.

Compared with common main-group and transition-metal MOFs, Ln-MOFs constructed by lanthanide (Ln³⁺) metal ions have the advantages of more diverse structures and unique optoelectronic properties. In 2016, Zhu et al. Synthesized a microporous 3D Eu-MOF:(Me₂NH₂)[Eu(ox)₂(H₂O)]·3H₂O with dia topology in high yield by one-step reaction of inexpensive oxalic acid (H₂ox) with Eu³⁺ metal ions in water at room temperature^[51]. The thermal stability of the material is 335 ℃, and the PXRD patterns of the samples do not change when they are immersed in water at room temperature for 3 days, in boiling water for 1 day, and in aqueous solution at pH = 4 for 4 H, respectively, indicating that the Eu-MOF has high water stability and acid resistance. The MOF has the characteristic red fluorescence of Eu³⁺ and can be used as a sensitive sensor for fluorescence detection of Cu²⁺ ions in aqueous solution. Otherwise, this Eu-MOF exhibited high proton conductivity (2.73×10^-3S·cm^-1) at 55 ° C and 95% relative humidity (RH). In 2017, Zhu et al. Used 2,2 ′ -disulfo-4,4 ′ -biphenyl dicarboxylic acid as a ligand (H₄L⁴) to react with a series of Ln³⁺ metal ions solvothermally to synthesize an isostructural 3D LnMOFs:(Me₂NH₂)[LnL⁴(H₂O)](Ln=Eu(1),Gd(2),Tb(3),Dy(4)) with a bnn topological network^[52]. These LnMOFs materials are stable up to 350 ° C. When the samples are immersed in water at room temperature for three months and in aqueous solutions with pH = 4 to 12 for 1 H, their PXRD patterns show that the structure is maintained (Fig. 11), indicating that the LnMOFs have extremely high water stability and good chemical stability. Among them, 1 and 3 show the characteristic fluorescence of Eu³⁺ and Tb³⁺, respectively, while 2 and 4 show relatively weak antiferromagnetic coupling. The proton concentration and porosity of this LnMOFs are reduced due to the coordination of sulfonic acid groups with Ln³⁺ ions, resulting in only a very low proton conductivity (4.14×10^-8S·cm^-1/95%RH) of this material at room temperature. In order to prevent the coordination of sulfonic acid groups and improve the porosity of MOFs, Zhu et al. Designed and synthesized a monosulfonic acid substituted 4,4 '-biphenyl dicarboxylic acid ligand: 2-sulfonic-4,4' -biphenyl dicarboxylic acid (H₃L⁵), using the thermal reaction of H₃L⁵ ligand and Ln³⁺ ion solvent^[53].

A series of isomorphic 3D LnMOFs:(Me₂NH₂)₂(H₃O)[Ln(L⁵)₂]·8H₂O(Ln=Eu(1),Gd(2),Tb(3)) with two-fold interpenetrating dia topology networks were synthesized (Fig. 12). The thermal stability of these LnMOFs is up to 300 ℃, and the PXRD pattern of the samples remains unchanged after soaking in water for 1 day, indicating that the LnMOFs have good water stability. Because the sulfonic acid group in Ln-MOFs is not coordinated and the pore is large, the material exhibits high proton conductivity (8.83×10^-3S·cm^-1) at 95 ° C and 60% RH, which is about 210,000 times higher than that of Ln-MOFs constructed by 2,2 ′ -disulfonic acid-4,4 ′ -biphenyl dicarboxylic acid^[52]. More importantly, this high proton conductivity was maintained for 3 days.

Rare earth clusters can also be used to construct chemically stable Ln-MOFs. In 2021, Li et al. Synthesized nine new 3D microporous Ln-MOFs: PCN-50X (X = 1 – 9) using solvothermal reaction of eight-linked Ln₆ clusters (Ln=Eu³⁺,Y³⁺,Yb³⁺) and a series of aromatic tetracarboxylic acid ligands (H₄L1~H₄L9) and analyzed their topologies in detail, and found three (4,8) -linked new topologies (scu, lxl, jun)^[54]. The PXRD patterns of these Ln-MOFs remained unchanged after soaking in water and common organic solvents (acetone, dichloromethane, acetonitrile and methanol) for 24 H, which proved that PCN-50X had good chemical stability. In the same year, Li et al. Synthesized a series of 3D mesoporous Ln-MOFs: PCN-2020 by solvothermal reaction of a 12-linked Secondary Building Unit cluster (Ln=Eu³⁺,Y³⁺,Yb³⁺,Tb³⁺,Ce³⁺) and an organic Secondary Building Unit (SBU = Secondary Building Unit) containing pyrene core. The thermal stability of the material was as high as 450 ℃, and the PXRD pattern of the sample was unchanged after soaking in aqueous solution with pH = 1 – 9 for 24 H, which proved that PCN-2020 had excellent chemical stability^[55]. Further investigation revealed that PCN-2020 (Ce) could catalyze the cycloaddition of CO₂ with epoxides, and its catalytic activity remained unchanged after five cycles.

Compared with MOFs based on carboxylic acids, MOFs based on hydroxamic acid ligands have higher stability, mainly due to the five-membered ring chelation coordination mode of hydroxamic acid^[56]. In 2019, Mart Martí-Gastaldo et al. Reported the first 3D microporous MOF:[Ti₂(Hbdha)₂(H₂bdha)](DMF)_0.5(H₂O)_3.3(MUV-11,MUV=Materials of Universidad de Valencia) constructed by direct solvothermal reaction of p-phenylenedihydroxamic acid (H₄bdha) with various Ti⁴⁺ salts^[57]. The PXRD pattern of the MOF remained unchanged after soaking in the solution of pH = 2 ~ 11 for 24 H, indicating that MUV-11 has good chemical stability. In 2020, Tezcan et al. Synthesized a 3D MOF with tbo topology: Fe-HAF-1 (HAF = Hydroxamate Framework) using 3,3 ′, 5,5 ′ -biphenyl tetrahydroxamic acid (H₄BPTH) ligand and FeCl₃ solvothermal reaction^[58]. PXRD measurements showed that the crystalline structure of Fe-HAF-1 remained unchanged after treatment in polar organic solvents (acetone, acetonitrile, methanol, acetic acid, pyridine, triethylamine), aqueous solutions with pH = 1 – 14, and 5 M NaOH solution for one week, proving the superior chemical stability of Fe-HAF-1 (Fig. 13). Zeta potential measurements showed that Fe-HAF-1 crystals were electronegative above pH = 4. Therefore, Fe-HAF-1 crystals can be used to selectively separate positively charged organic dyes such as methylene blue (MB⁺), thionine (LV⁺), and rhodamine B(RB⁺) in aqueous solution with size selectivity. In 2022, Liu et al. Synthesized a 2D MOF with kag topology: SUM-1 (SUM = Sichuan University Materials) by rapid solvothermal reaction of p-phenylenedihydroxamic acid (Sichuan University Materials) with ZrCl₄^[59]. PXRD measurement showed that the crystalline structure of SUM-1 remained unchanged after it was treated in aqueous solution with pH = 2 ~ 11 for 24 H, indicating that SUM-1 has high chemical stability.

In recent years, ligands containing phosphonic acid groups have also been used to construct chemically stable MOFs. In 2023, Zhang et al. Used 3,5-dicarboxyphenylphosphonic acid (H₄L⁶) as the main ligand and 1,2-bis (4-pyridyl) ethane (BPE) as the pore partitioning ligand to react with Cu²⁺ to construct a new 3D Cu-MOF:[Cu(μ₂-H₂O)(L⁶)_0.5(BPE)_0.5]·DMF·2H₂O(FJU-112) with sp² topology^[60]. Among them, the phosphonic acid group connects four Cu²⁺ in a bridging/capping manner to form a Cu₄ cluster [Cu₄(PO₃)₂(μ₂-H₂O)₂(CO₂)₄] as a 10-linked node. The thermal stability of FJU-112 can reach 250 ℃, and after soaking in aqueous solution with pH = 2 ~ 12 for 24 H, its PXRD pattern shows that the framework remains unchanged (Fig. 14), which proves that the material has high chemical stability. Since the 3D pore channel in the structure of FJU-112 is divided into several smaller cages and smaller windows by the pore-partitioning ligand BPE, it shows high selectivity for C₂H₂/CO₂ separation and is a promising solid material for C₂H₂/CO₂ separation.

An early example is that in 2014, Zhou et al. Developed a post-synthetic replacement and oxidation strategy of metal nodes to gradually transform unstable 3D MOF: PCN-426-Mg into stable PCN-426-Fe (Ⅲ) and PCN-426- Cr (Ⅲ), which could not be synthesized by direct method^[33]. PCN-426-Fe (Ⅲ) can maintain its structural integrity after soaking in water for one day and in aqueous solution with pH = 4 ~ 10, while PCN-426-Cr (Ⅲ) can not change its structure after soaking in 4 M HCl to pH = 12 for at least 12 H, which indicates that the two post-synthesized MOFs have good chemical stability, and the chemical stability of PCN-426- Cr (Ⅲ) is better than that of PCN-.

In 2021, Li et al. Successfully converted a 3D Ni-MOF: BUT-33 (Ni) into a 3D Pd-MOF: BUT-33 (Pd) in deuterated chloroform by post-synthesis metal ion replacement method, while this Pd-MOF material could not be prepared by direct synthesis method^[69]. PXRD measurement showed that BUT-33 (Pd) was stable in boiling water, HCl (pH = 3) and 8 M NaOH solution for 1 day (Fig. 17), proving the high chemical stability of this MOF. Because BUT-33 (Pd) retains the mesopores of BUT-33 (Ni) and has open Pd²⁺ active sites, it not only shows good heterogeneous catalytic performance in organic cross-coupling (such as Suzuki and Heck) reactions, BUT also can photocatalyze the reduction of CO₂ to CH₄, which is a multifunctional palladium catalyst.

Compared with the post-synthetic metal ion replacement method, there are relatively few studies on the construction of chemically stable MOFs using the post-synthetic ligand exchange method. In 2018, Tu et al. Used 4-sulfonic acid-2,6-naphthalene dicarboxylic acid (H₃SNDC) as a ligand to construct a 3D Zr-MOF:[Zr₆O₇(OH)(H₂O)₆(SNDC)_2.2(HSNDC)_1.8(HCO₂)]·2.2C₂H₈N·12.5H₂O(reo-MOF-1) with reo topology with a Zr₆O₈(H₂O)₈(CO₂)₈ cluster.2,6-Naphthalene dicarboxylic acid (H₂NDC) was ligand-exchanged with reo-MOF-1 to obtain a new 3D Zr-MOF:[Zr₆O₇(OH)(H₂O)₆(NDC)_4.38(HSNDC)_0.62]·3.27H₂O(reo-MOF-1_A),reo-MOF-1_A with not only high BET surface area (2104 m²·g^-1)Moreover, the cuboctahedral cage (23 23 Å) and part of the sulfonic acid group of the original MOF were retained, so reo-MOF-1_A showed higher heterogeneous catalytic activity than reo-MOF-1 in the Brnsted acid-catalyzed reaction, and its catalytic activity did not decrease significantly after five times of reuse, indicating that reo-MOF-1_A has good chemical stability^[70]. In the same year, Liu et al. Used 2-methyl-4,4 ′ -terphenyl dicarboxylic acid (H₂TPDC-Me) as a ligand to construct a 3D Zr-MOF: UiO-68-Me by solvothermal reaction with Zr₆O₄(OH)₄(CO₂)₁₂ cluster, and then ligand-exchanged different Chiral M (salen) ligands H₂L^M with UiO-68-Me to synthesize a series of UiO-68 type chiral MOFs:UiO-68-M(M=Cu²⁺,Fe³⁺,Cr³⁺,Mn³⁺,V⁴⁺), which could not be prepared by direct synthesis method (CMOFs = Chiral MOFs)^[71]. The thermal stability of these CMOFs was up to 400 ℃, and the PXRD patterns of these CMOFs remained unchanged when they were treated in boiling water, 3 M HCl, and NaOH solution (pH = 12) for 24 H, respectively (Fig. 18), proving that these CMOFs also had high chemical stability. In addition, UiO-68-M showed excellent heterogeneous asymmetric catalytic activity and enantioselectivity in the cyanosilylation of aldehydes, the ring opening of epoxides, and the aminolysis of stilbene oxide, and did not significantly lose its asymmetric catalytic activity and enantioselectivity after 10 cycles. In 2019, Zhou et al. Developed a sequential ligand-reloading strategy and successfully constructed a series of isostructural MOFs with enlarged and reduced lattices^[72]. The authors first synthesized a parent MOF: UiO-67.5 using 4,4 '-azophthalic acid (L¹⁰=13.24Å) and a 12-linked Zr₆ cluster, and then used an equal-length ligand containing a labile imine bond, 4- (N-4-carboxybenzylidene) iminobenzoic acid (L¹⁰'=13.12Å), to react with UiO-67.5 by post-synthetic ligand exchange.An unstable non-interpenetrating Zr-MOF: PCN-161 was prepared, and then a longer ligand 4,4 ′ ′ -terphthalic acid (L¹¹=15.70Å) was exchanged with PCN-161 by post-synthetic ligand exchange, and a lattice enlarged Zr-MOF: UiO-68 was synthesized by SC-SC transformation. Similarly, if a shorter ligand, 4,4 ′ -biphenyl dicarboxylic acid (L⁰=11.0Å), was subjected to post-synthetic ligand exchange with PCN-161, a lattice reduced Zr-MOF: UiO-67 could be obtained. Importantly, the Zr-MOFs (UiO-67 and 68) synthesized based on this strategy have high water resistance and chemical stability. In 2022, Cao et al. First synthesized a 3D porous Cd-MOF:[Cd₃L^a(bpy)]·5DMA·15H₂O(1) with 2- (2-hydroxypropionamido) terephthalic acid (H₂L^a) and 4,4 '-bipyridine (bpy) as ligands by solvothermal reaction with Cd²⁺.Subsequently, a series of MOFs isostructural to Cd-MOF 1 were successfully prepared by solvent-assisted ligand exchange with four terephthalic acid ligands containing different substituents: 2- (propionamido) terephthalic acid (H₂L^b), 2- (acetamido) terephthalic acid (H₂L^c), 2-aminoterephthalic acid, and terephthalic acids (H₂L^e), respectively: 2 – 5^[73]. As the length of the side chain substituent becomes smaller, the pore channels of these ligand-exchanged MOFs materials become larger, and the adsorption performance of CO₂ becomes better.