MOFs with different morphologies can be formed by different metal sources and organic ligands, and these morphologies can be completely mapped into DMOs by pyrolysis. Ma et al. Synthesized MOF-74 with rod-like morphology and MOF-39 with spherical morphology from 2,5-dihydroxyterephthalic acid and 1,3,5-tris (4-carboxyphenyl) benzene using Co as a metal source, and then pyrolyzed them to form Co₃O₄-R with rod-like morphology and Co₃O₄-S（ with spherical morphology (Fig. 1A)^[35]. It is found that Co₃O₄-R mainly exposes the (220) crystal plane, and compared with the (311) crystal plane with three-coordination lattice oxygen exposed by Co₃O₄-S, the (220) crystal plane with two-coordination lattice oxygen has a lower oxygen vacancy formation energy and a higher oxygen adsorption energy, which is conducive to the formation of oxygen vacancies and the supplement of gaseous oxygen, so that Co₃O₄-R can convert 98% of o-xylene at 270 ℃. In addition, many studies have shown that specific crystal faces can effectively promote the adsorption of gaseous pollutants or oxygen, thereby improving catalytic activity^[38,39]. Lei et al. Also prepared three morphologies of Co-MOFs using Co as metal source and 2-methylimidazole, 2,5-dihydroxyterephthalic acid, imidazole-4,5-dicarboxylic acid as organic ligands^[40]. The study shows that the activity of Co₃O₄ with three different ligands and morphologies on toluene decreases according to the following rule: ：ZSA-1-Co₃O₄（ octahedral ）>MOF-74-Co₃O₄（ rod-like ）>ZIF-67-Co₃O₄（ dodecahedral shape) (Figure 1B),This is positively correlated with the specific surface area of the catalyst and the surface elemental state (more Co³⁺ and O_ads）). Larger specific surface area can provide more adsorption sites, and specific surface element States (such as Co³⁺ and Mn⁴⁺）) can be used as active sites for catalytic reactions to upgrade the catalytic activity of VOCs^[46⇓~48].

Zhang et al. Used Mn as the metal source and different organic ligands (terephthalic acid and trimesic acid) to form MOFs precursors with different morphologies, and then prepared octahedral Mn-100-Ar-O₂, spherical Mn-74-Ar-O₂ and rod-like Mn-BTC-Ar-O₂ by heat treatment^[49]. The results showed that these catalysts were all Mn₂O₃ with similar specific surface area, but the octahedral Mn-100-Ar-O₂ had better reducibility at low temperature and larger pore size, which was easier to reduce the migration resistance of VOCs, thus showing the best catalytic activity for toluene. Similarly, Chen et al. Also prepared two Mn-MOFs through 2,5-dihydroxyterephthalic acid and trimesic acid, and they found that different morphologies and structures would also lead to different phases of oxide formation, which was related to the combustion of organic ligand carbon.When the spherical Mn-MOFs-74 was also calcined at 400 ℃, the Mn₃O₄structure was obtained, while the Mn₂O₃ structure was obtained for Mn-BDC, which made the two catalysts have different toluene catalytic activities^[50]. Sun et al. Used Ce as the metal source to prepare Ce-MOF with three different morphological structures through terephthalic acid and trimesic acid, and they found that the morphology would seriously affect the temperature at which the framework was converted into carbon skeleton and the decomposition temperature of carbon skeleton during the pyrolysis of MOF^[41]. As shown in Figure 1C, Ce-MOF-808 begins to decompose the organic framework into carbon skeleton at 220 ℃ and recrystallize into CeO₂ at 300 ℃, while Ce-UIO-66 and Ce-BTC begin to transform into carbon skeleton at 320 ℃ and obtain crystalline CeO₂ at 330 and 350 ℃, respectively.At the same time, the temperature range of carbon skeleton preservation decreases according to the rule of Ce-MOF-808 > Ce-BTC > Ce-UiO-66. The existence of a large number of carbon skeletons can limit the grain growth and obtain a high concentration of defects in the combustion, which is consistent with its catalytic activity for the complete oxidation of toluene.

Of course, in addition to the regulation of the metal/organic ligand species, the ratio can also be regulated, and a smaller metal/organic ligand ratio will reduce the size of the crystal^[51]. Mei et al. Prepared a series of CeO₂ catalysts by varying the molar ratio of cerium nitrate and trimesic acid^[52]. It was found that with the increase of the molar ratio of cerium nitrate to trimesic acid, the crystal size of the oxide increased, while the pore volume, oxygen vacancy content and lattice oxygen mobility decreased.Therefore, the CeO₂-1 produced at a molar ratio of cerium nitrate to trimesic acid of 1:1 showed the best catalytic oxidation performance, which could degrade 90% of o-xylene at 198 ℃.

Nucleation and growth are the important steps of crystal crystallization, and temperature and solvent can control the nucleation rate and growth rate of crystal, especially the coordination of some solvents to metal ions, which leads to the selective growth of crystal faces^[53⇓~55]. Therefore, the morphology and structure of MOFs can also be changed by adjusting the synthesis temperature and solvent of MOFs. Han et al. Obtained one-dimensional nanoparticles (100 ℃), two-dimensional hexagonal sheets (25 ℃) and three-dimensional nanoflowers (200 ℃) by changing the synthesis temperature of Ce[Co(CN)₆]^[56]. Because the three-dimensional nanoflowers have more oxygen vacancies, Ce³⁺ and Co³⁺, the complete oxidation of toluene can be realized at 200 deg C. In addition, water, alcohol and N, N-dimethylformamide (DMF) are good solvents for the synthesis of MOFs precursors, and different ratios of alcohol/water, DMF/water, DMF/alcohol/water can affect the nucleation and growth rate of MOF crystals, thus regulating the morphology, structure, size and pore size of MOFs^{[54,57⇓~59]}. For example, Huang et al. Found that in the DMF/water two-solvent system, the proportion of water in Co-MOF-74 can regulate the deprotonation rate of organic ligands, affect the nucleation efficiency of crystals, and thus regulate the morphology and structure (Fig. 1D)^[42]. Zhao et al. Also prepared ZIF-67 with different sizes (400, 800, 1700 nm) by changing the synthesis temperature and solvent conditions (Figure 1E)^[43]. The results show that the precursor with small size also has smaller grain size after pyrolysis, which has higher atom utilization and exposes more Co³⁺, so that the removal rate of toluene can reach 90% at 259 ℃.

In addition to this, acid and CTAB have also been used as capping agents to control the morphology through selective coordination of metal ions^[60,61]. However, the results show that CTAB can not only change the morphology of the precursor, but also reduce the energy of recrystallization during pyrolysis, inhibit the agglomeration of particles, reduce the grain size, increase the oxygen vacancy content, and greatly increase the catalytic complete oxidation activity of o-xylene^[62]. In addition, considering that MOFs materials are composed of secondary building units (SBUs), Dong et al. Obtained cubic, net-like nanosheets, and rod-like Co-MOFs (Figure 1 f) by adding acid to modulate the SBUs and reconstructing the framework, in which the net-like nanosheet-derived cobalt-based oxides showed excellent toluene complete oxidation performance^[44].

In addition to the conventional hydrothermal/solvothermal synthesis of MOFs precursors to achieve morphology and structure control, other methods can also be used, such as microwave method, spinning method, mechanochemical method, etc., and the MOFs prepared by these methods often have more special structures^[63⇓-65]. Our research group prepared Mn/Co-BTC with different Mn-Co molar ratios by mechanical ball milling, and the synthesis route is shown in Fig. 1G. The morphology of the MnCoO_x prepared by the method is completely different from that of the catalyst prepared by the conventional hydrothermal method, which shows a porous flocculent morphology, rich defects and a large specific surface area, and can complete 90% of propane elimination at 255 deg C^[45]. In addition, Sun et al. Also prepared spherical Co_yCu_3−y-MOF-74 by mechanical ball milling, which showed excellent catalytic complete oxidation performance of toluene^[66].

The primary synthesis is often heavily doped to obtain binary complex oxides with uniform dispersion. Mn-MOF, Co-MOF derived oxides are currently the most studied and the most effective catalysts for catalyzing the complete oxidation of VOCs^[73,74]. Based on the similar size and physical and chemical characteristics of the two ions, it is easy to form MOFs with similar structures, so MnCo-MOF with uniform structure can be directly obtained by one-step synthesis, and then MnCo binary oxides with uniform dispersion can be formed by pyrolysis, and the physical and chemical properties of MOFs-derived oxides can be effectively controlled by the proportion of metal ions. Luo et al. Prepared different Mn₃[Co(CN)₆]2·nH₂O precursors by changing the ratio of Mn and Co (1:1, 1:2, 2:1), and pyrolyzed to form the corresponding cubic MOF-Mn_xCo_y^[75]. The results show that the ratio of Mn and Co can control the morphology and size of the oxide, and there is a strong interaction between Mn and Co, which can form (Co,Mn)(Co,Mn)₂O₄ after pyrolysis, and a large number of surface adsorbed oxygen, surface Mn⁴⁺, Co³⁺ concentration and good reducibility make the MOF-Mn₁Co₁ have the best toluene catalytic activity. Tu et al. Also found that the Mn₁Co₁ derived from M₃(HCOO)₆（M=Mn, Co) prepared when the ratio of Mn and Co was 1:1 had the best catalytic activity for formaldehyde oxidation^[76].

The synthesis of mixed metal oxides by Ce-MOF is also widely studied, especially the precursor of MOFs can achieve the mixing of bimetallic elements at the molecular level, which can enhance the degree of aliovalent metal substitution to CeO₂ in the subsequent pyrolysis process. Guo et al. First examined the similarities and differences between CeCu-BTC derived Ce-Cu-O_x and CeO₂-CuO obtained by first preparing Ce-BTC derived CeO₂ and then impregnating Cu and calcining^[77]. The results showed that the two catalysts were rod-like, and there was no significant difference in specific surface area and crystal structure, but the catalyst synthesized by MOFs had a higher dispersion scale, a strong interaction between Ce and Cu, and more defects. Wang et al. Also prepared diamond-shaped multilayer CeCuBDC by a one-step synthesis method, and then made a high degree of heterovalent substitution of Cu into the CeO₂ lattice by pyrolysis, which brought abundant defect structures while forming a solid solution (Figure 2A), so that the formed CeCuO_x had better catalytic performance for methanol, acetone and o-xylene^[68]. Other metals, such as Mn and Co, can also achieve a high degree of substitution for the CeO₂ lattice through MOF, thus showing excellent catalytic performance for VOCs^[67,78,79]. Li et al. Compared the different catalytic properties of CoCeBDC-derived CoCeO_x and common supported prepared Co₃O₄/CeO₂^[67]. As shown in Figure 2B, Co₃O₄ in Co₃O₄/CeO₂ exists on the surface of CeO₂, while Co in CoCeO_x is heterovalently substituted into the lattice of CeO₂, forming defects while effectively restricting grain growth and forming a larger specific surface area, thus showing superior catalytic performance in a series of VOCs (methanol, acetone, toluene, o-xylene). By comparing the MnO_x-CeO_x-MOF obtained by pyrolysis of MOF-74 with the MnO_x-CeO_x-CP prepared by precipitation method, Sun et al. Found that the MnO_x-CeO_x-MOF has higher specific surface area, pore volume, defect level and surface Mn⁴⁺, and is easier to adsorb, activate and migrate oxygen species. At the same time, they also found that Ce doped with Mn can limit grain growth, improve low-temperature reducibility and oxygen storage capacity, so that the MnO_x-CeO_x-MOF can convert 90% of toluene at 208 ℃^[78]. In addition, Wang et al. Prepared CuMn₂O_x with a three-phase (CuO, Mn₃O₄, CuMn₂O₄）) interface by pyrolyzing Cu/Mn-BTC, and found that the abundant defects on the interface were beneficial to the activation of oxygen adsorption, which could complete 90% of acetone catalytic oxidation at 144 ° C^[80]. Similarly, CuMn-ptcda derived flower-like CuMnO_x also showed excellent acetone complete oxidation performance^[81].

The secondary synthesis is mainly for modification on the basis of the first phase metal. As shown in Figure 2C, after the formation of monometallic MOFs, the second phase metal enters the MOFs structure through the connection of SBUs, SBUs ion replacement, pore intercalation, and organic ligand connection, respectively, to obtain MMOFs^[33]. The infiltration amount of the second phase metal can be adjusted by modifying the synthesis parameters of substitution (such as concentration, reaction time, etc.), and then MMOs with different doping amounts can be formed by pyrolysis^[82]. Jiang et al. First studied the difference between the MnO_x-CeO₂-h of Mn ion directly soaking doped CeO₂ and the formation of MnO_x-CeO₂-s by Mn doping on Ce-BTC and then pyrolysis^[83]. The results show that the Mn and Ce elements in the MnO_x-CeO₂-s are more dispersed, and the doping of Mn produces a large number of F-OVs (Frankel-type oxygen vacancies), which is conducive to the production of Frankel oxygen atoms for the reaction with ethyl acetate and improves the catalytic activity. Xu et al. Also found that Cu-doped ZIF-67-derived CuO/Co₃O₄ has a higher defect level than physically mixed CuO-Co₃O₄, making it easier for lattice oxygen for toluene oxidation to be replenished^[84]. The doping concentration of the second phase metal in MMOFs is often controlled by changing the concentration of the soaking solution. Lei et al. Obtained different Mn-doped ZSA-1 by soaking different amounts of ZSA-1 in manganese nitrate solution (50%). When the doping ratio was 1:1, the obtained M-Co₁Mn₁O_x showed the best toluene catalytic activity, which was the same as the ratio of MnCoO_x with the best complete oxidation performance prepared by one synthesis^[85]. Chen et al. Obtained a series of nickel nitrate-doped ZIF-67 by changing the concentration of nickel nitrate solution (10%, 20%, 35%, 50%), followed by pyrolysis to obtain hollow NiO_x/Co₃O₄（ Fig. 2D)^[69]. It was found that with the increase of nickel doping, nickel oxide entered the lattice of Co₃O₄ and the defects increased, and the nickel ions in the pore would restrict the collapse of the pore during pyrolysis.So as to obtain larger specific surface area and more surface adsorbed oxygen, but too much doping will lead to the destruction of the hollow structure, resulting in a decrease in activity, so the 35-NiO_x/Co₃O₄ shows the best toluene catalytic activity. They also studied the doping of different amounts of Mn on MIL-101-Cr, and found that the introduction of Mn reduced the strength of Cr-O bond, and the lattice oxygen was easier to dissociate to form active oxygen and oxygen vacancies.However, excessive Mn doping will lead to the coating of MnO_x on the surface of Cr₂O₃, and the isolated manganese oxide phase will be formed in the part exceeding the uniform distribution, which will reduce the surface active oxygen and the low temperature reduction ability^[86]. Wang et al. Studied the doping of different amounts (1%, 5%, 10%) of Co on spherical Ce-MOF, and the study showed that a small amount of Co could be highly dispersed into the CeO₂ lattice, but excessive Co agglomerated, thus forming a Co₃O₄ phase separated from the CeO₂ phase, so the CoCe-5 prepared with a doping amount of 5% had the best catalytic activity for toluene^[87]. Similar findings were found in other studies, for example, when Ni was doped into ZSA-1 and derived oxides, it was found that Ni was not doped into the lattice of Co₃O₄, but dispersed on the surface of Co₃O₄ in the form of NiO, and the active sites of Co₃O₄ decreased, which reduced the catalytic activity of toluene^[88]. However, it does not mean that excessive doping will lead to a decrease in the activity of catalytic VOCs, which is related to the monolayer dispersion threshold and the intrinsic activity of doped oxides^[89].

In addition to directly soaking metal ions with a specific content, the amount of doping can also be controlled by changing the soaking time. Hu et al. Studied the core-shell MnO_x/Co₃O₄ derived from Mn doped ZIF-67 with different soaking time, and found that with the increase of soaking time, the amount of Mn loaded on the surface of Co increased, and there was a strong interaction between Mn and Co. When the Mn/Co molar ratio was 0. 13 (soaking for 4 H), it showed the lowest reduction temperature and the most oxygen desorption, so it could oxidize 90% of chlorobenzene at 342 ℃^[90].

There are also some interesting strategies for repyrolysis of secondarily synthesized MMOFs (or their derivatives) to obtain DMMOs. Han et al. Proposed to prepare ZIF-67 carbonitride first, impregnate Mn on the adsorption sites of nitrogen-doped carbon matrix, and then cooperate with pyrolysis-oxidation-adsorption to prepare MnO_x/Co₃O₄（ with different ratios of Co to Mn (1:1, 1:5, 1:10) (Fig. 2e)^[70]. Mao et al. Prepared flower-like ZIF-67-on-CuCoBDC metal-organic framework by a simple exchange extended growth method, and derived a Co₃O₄-CuCo₂O₄ with a heterostructure for low-temperature catalytic oxidation^[91]. Zhao et al. Proposed a strategy of assembling CoMnO_x by oxidation etching^[92]. In this method, ZIF-67 was first prepared, then treated by different concentrations of potassium permanganate (1, 6, 11 G/L), and finally calcined to obtain the corresponding CoMnO_x. The addition of potassium permanganate makes the material etched and reassembled, and the smooth surface becomes stacked sheets, resulting in abundant low crystallization areas, dislocation areas and phase interfaces, which effectively reduces the complete oxidation temperature of toluene. In addition, combining MOF with oxides and then pyrolyzing to prepare bimetallic oxides is also a potential strategy. Some researchers first synthesized CeO₂ nanowires, then added 2-methylimidazole to synthesize CeO₂-ZIF-67, and finally obtained dodecahedral CeO₂-Co₃O₄ nanoframes with nanowire insertion by pyrolysis at 400 ℃^[93]. While Wen et al. Proposed to embed and anchor Co-MOFs in the ordered mesoporous CeO₂structure to prepare the Co-MOFs@CeO₂ composite precursor with ordered mesoporous channel structure (Fig. 2 f), and finally pyrolyze to obtain the CeCoO_x catalyst with mesoporous structure, which can completely oxidize 90% of toluene at 220 ° C^[71]. Through the Kirkendall effect, solid MOFs precursors can also be prepared into oxide catalysts with special hollow structures. Luo et al. And Chen et al. Respectively prepared MnCo₂O₄ and NiO_x/Co₃O₄（ with a cubic hollow structure by using the Kirkendall effect (Fig. 2g), and this three-dimensional hollow structure showed a higher defect level, which enabled the catalyst to effectively oxidize toluene deeply at low temperature^[72][69].

Lei et al. Investigated the transformation of ZSA-1 from MOFs to metal oxides by pyrolyzing Co-MOF (ZSA-1) at 250, 350, 450 ° C^[95]. It is found that when the temperature rises to 250 ℃, the MOF organic linker decomposes, the pores collapse and the material becomes amorphous, the specific surface area decreases from 1382 m²·g⁻¹ to 2.5 m²·g⁻¹, and the pore volume decreases from 0.67 to 0.003 cm³·g⁻¹. When the temperature continues to rise to 350 ℃, the material recrystallizes to form Co₃O₄.The specific surface area and pore volume increased, and the { 110 } crystal plane family with more Co³⁺ was exposed. With the temperature rising to 450 ℃, the material collapsed further, the grain growth became larger, the specific surface area decreased, and the { 111 } crystal plane family with less Co³⁺ was exposed, which led to the decrease of catalytic activity for toluene. Similarly, in our previous study, it was also found that with the increase of the temperature of pyrolyzed MOFs, the crystal agglomeration growth led to the increase of grain size, the decrease of specific surface area and the collapse of pore structure, while the column-layered Co-BTC pyrolyzed at 350 ℃ showed the best propane catalytic activity due to the smallest grain size, the largest pore volume and the most defect concentration^[96]. Chen et al. Also found that as the pyrolysis temperature of rod-like Ce-BTC increased from 350 ℃ to 500 ℃, the crystallite size became larger, the specific surface area decreased, and the Ce³⁺/Ce⁴⁺, O_sur/O_lat, defect concentration and oxygen storage capacity decreased. Therefore, the CeO₂-MOF-350 obtained by pyrolysis at 350 ℃ showed the best toluene catalytic activity, and 90% of toluene could be converted at 223 ℃^[97]. However, Zhang et al. Found that flower-like Ce-BTC could not be completely pyrolyzed to form CeO₂ when calcined at 400 ℃, so the specific surface area was smaller and the grain size was larger, while when the pyrolysis temperature rose to 500 ℃, it had the largest specific surface area and the smallest grain size, thus showing the best oxidation catalytic activity^[98]. It is worth noting that in their later work (Fig. 3A), by increasing the amount of oxygen for calcination （30%O₂/Ar）, the flower-like Ce-BTC was successfully pyrolyzed at 300 ℃ and formed CeO₂-300 with good crystallinity.At the same time, the DMF in the pore of Ce-BTC will form a large number of pore structures during pyrolysis (250 ℃), showing an ultra-high specific surface area （648 m²·g⁻¹）, but this pore structure will collapse during subsequent pyrolysis (300 ℃), and the specific surface area will be greatly reduced by （77 m²·g⁻¹）^[99].

Cui et al. Prepared Fe-MIL-100 using Fe as the metal source, and explored the physicochemical properties of Fe₂O₃-X（X=400, 430, 450, 500 ℃) derived from different pyrolysis temperatures^[100]. The results show that when the organic ligand is not completely pyrolyzed (400 ℃), the specific surface area and reduction ability increase with the increase of temperature, while after complete pyrolysis (430 ℃), the specific surface area and reduction ability decrease with the increase of temperature, so the Fe₂O₃-430 shows the best catalytic activity. Zhang et al. Pyrolyzed Mn-MIL-100 at different temperatures to obtain Mn₂O₃ with different physicochemical properties, among which Mn₂O₃-700℃ obtained at 700 ° C had the best catalytic oxidation performance due to the smallest particle size, the lowest reduction temperature, the highest Mn³⁺/Mn⁴⁺ and O_ads/O_latt^[101]. Many other studies have also shown that pyrolysis temperature can greatly affect the grain size, specific surface area, surface composition and redox properties^[102]. The calcination temperature also has a decisive effect on the physicochemical properties of bimetallic organic framework-derived composite metal oxides. Zhang et al. Prepared rod-like MnCe-BTC and pyrolyzed at 200, 300, 400 and 500 ℃ to prepare the corresponding MnCeO_x^[103]. The results show that the pyrolysis process of bimetallic MOFs is similar to that of monometallic MOFs, and too high calcination temperature may lead to grain agglomeration and reduced interaction and redox. It is worth noting that for the Co_yCu_3−y-MOF-74 prepared by mechanical ball milling, the calcination temperature has no significant effect on the catalytic performance of its derived M-Co₂Cu₁O_x for toluene^[66].

In addition, different heating rates during pyrolysis can also affect the catalytic activity. Zhao et al. Respectively prepared HW-Mn_xCo_3−xO₄ and BIB-Mn_xCo_3−xO₄ with pyrolysis heating rates of 1 and 10℃·min⁻¹, as shown in Fig. 2b, a higher heating rate （10℃·min⁻¹） would lead to the structural collapse of hollow dodecahedron,The specific surface area decreased sharply from 59. 7 to 7.3 m²·g⁻¹, and the lower heating rate （10℃·min⁻¹） would make the crystal have smaller particle size and more defect concentration, and improve the catalytic performance of toluene^[104]. Other researchers have also found that for UiO-66 (Ce), high heating rates lead to the formation of CeO₂ with larger crystal size, fewer defects and active sites, which leads to lower oxidation and reduced catalytic activity^[107]. This is mainly due to the fact that at low calcination rates, the organic ligands of MOFs can fully form carbon skeletons, which will restrict crystal growth during subsequent calcination and form defects during subsequent calcination^[105]. At higher calcination rates, the organic ligands are directly and completely oxidized, the carbon skeleton is not effectively formed, the pores collapse rapidly, a large number of metal ions agglomerate, the grain size increases, and the defect concentration and pore size decrease compared with those at lower calcination rates, so the ability to catalyze the complete oxidation of VOCs decreases.

In fact, the two-stage calcination method, in which MOFs are pretreated with inert gas, has the same principle as the lower calcination heating rate, which is to form a carbon skeleton. However, the catalyst obtained by the two-stage calcination method has a greater improvement in the catalytic performance of VOCs, which may be related to the degree of carbon skeleton formation by organic ligands. For example, when Zhang et al. Prepared metal oxides with different ligands, they found that the Mn₂O₃ obtained by argon treatment followed by air pyrolysis had better toluene catalytic activity, which was attributed to the formation of carbon skeleton by organic ligands in MOFs materials under inert atmosphere, thus limiting the growth of Mn metal particles^[49]. Zheng et al. systematically studied the differences in structure and catalytic performance of catalysts produced by nitrogen pretreatment and direct air pyrolysis (Fig. 3C)^[105]. The results show that nitrogen pretreatment can form amorphous carbon, which can limit the grain growth in the subsequent calcination process, while the carbon can form oxygen vacancies in the subsequent combustion process to enhance the catalytic activity. Therefore, compared with the MnO_x-A produced by direct air pyrolysis, the MnO_x-NA produced by nitrogen pretreatment and re-pyrolysis has smaller crystal size, larger specific surface area, pore volume, pore size and oxygen vacancy concentration, so that the mixture of acetone, toluene and acetone can be degraded by 90% at 167 and 180 ℃, respectively. On this basis, Li et al. used the carbon formed by nitrogen pretreatment as a molecular fence, and doped Mn on the surface of MOFs to form a double restriction, as shown in Figure 2D. The molecular fence can restrict the collapse of pores in the subsequent pyrolysis process, and restrict the growth of Mn doped on MOF-808, thereby improving the catalytic performance of toluene^[106].

There are also researchers who adopt other atmospheres to treat MOFs, such as typical oxidizing gas O₂, reducing gas H₂, inert gas Ar^[108]. Of course, it is also feasible to pyrolyze MOFs directly under nitrogen conditions, but this requires the presence of in-situ oxygen in the organic ligand itself. In addition, too high temperature will also lead to the destruction of the structure, the reduction of specific surface area, and the reduction of catalytic VOCs activity^[109].