**3. MOF's structure using catalytic reaction**

The active sites of MOFs can be designed depending on the type of catalytic process. The Rosseinsky group reported the methanolysis of rac-propylene oxide and expected to yield 2 methoxy-1-propanol and 1-methoxy-2-propanol reaction. They used the postsynthesis modification of a porous homochiral Ni(L-asp)bipy 0.5, 1 (L-asp = L-aspartate, bipy = 4,4 dipyridyl), leading to a functional Brønsted acidic material. These compounds are amino acids (L- or D-aspartate) together with dipyridyls as struts. The coordination chemistry is such that the amine group of the aspartate cannot be protonated by added HCl, but one of the aspartate carboxylates can. Thus, the framework-incorporated amino acid can exist in a form that is not accessible for the free amino acid. While the nickel-based compounds are marginally porous, on account of tiny channel dimensions, the copper versions are clearly porous [83].

The results showed that the carboxylic acids behave as Brønsted acidic catalysts, facilitating (in the copper cases) the ring-opening methanolysis of a small, cavity-accessible epoxide at up to 65% yield. These researchers pointed out that the superior homogeneous catalysts existed, but emphasized that the catalyst formed here is unique to the MOF environment, thus representing an interesting proof of concept [84].

Lewis acid solids are commonly used in selective oxidation. An example of this type of catalysts is trinuclear networks containing Cu2+, which have shown a high activity and selectivity for the peroxidative oxidation process of cyclohexane into the corresponding alcohols and ketones (MeCN/H2O/HNO3 media) [85]. The structure of such MOFs is composed of the secondary building unit of {Cu3(μ3-OH)(μ-pyrazole)} with tetracoordinate metal centers in axial positions of easy access.

Other structures with these types of catalytic sites on the Cu3(BTC)2 coordinated network are made of copper links. It is feasible to prepare this MOF with modulated amounts of physisor‐ bed (molecules placed into the channels) or chemisorbed (molecules occupying CuX coordi‐ nation sites) water molecules with high surface area straight from the reaction vessel without any postsynthetic steps [8]. Different reaction models have been tested in this MOF including: citronellal cyclization [86], benzaldehydecyanosilylation [87], rearrangement of ethylene acetal of 2-bromopropiophenone, isomerization of alpha-pinene oxide [86], among others [28].

Another example of MOF with high concentration of Lewis acidic sites is Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2; H3BTT = 1,3,5-benzene-tristetrazol-5-yl. Mn2+ ions that are exposed on the surface of the framework might serve as potent Lewis acids, and catalyze the cyanosilylation of aromatic aldehydes and ketones, as well as the more demanding Mukaiya‐ ma-aldol reaction. Moreover, in each case, a pronounced size-selectivity effect consistent with the pore dimensions is observed [88].

*Raman spectroscopy is* widely used in the characterization of noncrystalline or low-crystalline catalysts. Comparing the spectra before and after the reaction provides information about the

*Temperature-programmed reduction (TPR)* is used in determining redox reaction parameters. The

The active sites of MOFs can be designed depending on the type of catalytic process. The Rosseinsky group reported the methanolysis of rac-propylene oxide and expected to yield 2 methoxy-1-propanol and 1-methoxy-2-propanol reaction. They used the postsynthesis modification of a porous homochiral Ni(L-asp)bipy 0.5, 1 (L-asp = L-aspartate, bipy = 4,4 dipyridyl), leading to a functional Brønsted acidic material. These compounds are amino acids (L- or D-aspartate) together with dipyridyls as struts. The coordination chemistry is such that the amine group of the aspartate cannot be protonated by added HCl, but one of the aspartate carboxylates can. Thus, the framework-incorporated amino acid can exist in a form that is not accessible for the free amino acid. While the nickel-based compounds are marginally porous,

on account of tiny channel dimensions, the copper versions are clearly porous [83].

The results showed that the carboxylic acids behave as Brønsted acidic catalysts, facilitating (in the copper cases) the ring-opening methanolysis of a small, cavity-accessible epoxide at up to 65% yield. These researchers pointed out that the superior homogeneous catalysts existed, but emphasized that the catalyst formed here is unique to the MOF environment, thus

Lewis acid solids are commonly used in selective oxidation. An example of this type of catalysts is trinuclear networks containing Cu2+, which have shown a high activity and selectivity for the peroxidative oxidation process of cyclohexane into the corresponding alcohols and ketones (MeCN/H2O/HNO3 media) [85]. The structure of such MOFs is composed of the secondary building unit of {Cu3(μ3-OH)(μ-pyrazole)} with tetracoordinate metal centers in axial positions

Other structures with these types of catalytic sites on the Cu3(BTC)2 coordinated network are made of copper links. It is feasible to prepare this MOF with modulated amounts of physisor‐ bed (molecules placed into the channels) or chemisorbed (molecules occupying CuX coordi‐ nation sites) water molecules with high surface area straight from the reaction vessel without any postsynthetic steps [8]. Different reaction models have been tested in this MOF including: citronellal cyclization [86], benzaldehydecyanosilylation [87], rearrangement of ethylene acetal of 2-bromopropiophenone, isomerization of alpha-pinene oxide [86], among others [28].

Another example of MOF with high concentration of Lewis acidic sites is Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2; H3BTT = 1,3,5-benzene-tristetrazol-5-yl. Mn2+ ions that are exposed on the surface of the framework might serve as potent Lewis acids, and catalyze the

incorporation of new components into the MOF network.

104 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**3. MOF's structure using catalytic reaction**

representing an interesting proof of concept [84].

of easy access.

catalytic activity in redox conditions can be determined by this technique.

Different types of MOF have been used in catalytic process as base catalysis, Brønsted acid catalysis, Lewis acid catalysis, C–C bond formation and polymerization, enantio selective catalysis, and catalysis by organometallic complex supported on MOFs, among others. Table 3 summarizes the MOF structures used in some catalytic processes reported so far. The most common ions catalysis are: Ag+ , Al3+, Bi3+, Ce4+, Cr3+, Co2+, Cu2+, Fe3+, Mn2+, Mg2+, Pd2+, Sc3+, V4+, Zn2+, and Zr4+.




Ac = acetyl; bdc = 1,4-benzenedicarboxylate; BPB = 1,4-bis(4´-pyrazolyl)benzene; bpdb = 1,4-bis(3,5-dimethyl-1Hpyrazol-4-yl)benzene; bpdc = biphenyldicarboxylate; bpe = trans-1,2-bis(4-pyridyl)ethylene); bpy = 4,4´-bipyridine; btc = benzene-1,3,5-tricarboxylate; btapa = 1,3,5-benzene tricarboxylic acid tris[*N*-(4-pyridyl)amide]; btb = 5 -(4-carboxyphen‐ yl)-[1,1′:3′,1″-terphenyl]-4,4′-dicarboxylate; btt = 1,3,5-benzenetris(tetrazol-5-yl); bttp4 = benzene-1,3,5-triyl triisonicoti‐ nate; ChirBTB-1 = 5′-(4-carboxy-3-((S)-4-isopropyl-2-oxooxazolidin-3-yl)phenyl)-3-((S)-4-isopropyl-2-oxooxazolidin-3 yl)-3′-(3-isopropyl-5-oxooxazolidin-4-yl)-[1,1′:3′,1″-terphenyl]-4,4″-dicarboxylate;ChirBTB-2 = 3,3″-bis((S)-4-benzyl-2 oxooxazolidin-3-yl)-5′-(3-(3-benzyl-5-oxooxazolidin-4-yl)-4-carboxyphenyl)-[1,1′:3′,1″-terphenyl]-4,4″-icarboxylic; Dasp = D-aspartate; bdc = benzene-1,4-dicarboxylate; dhbdc = 2,5-dihydroxyisophthalic; ImDC = 4,5-imidazole dicarboxylate; ippb = 4,4′-(hexafluoroisopropyl-idene)bis(benzoate); L1 = (R)-6,6′-dichloro-2,2-dihydroxy-1,1′-binaphth‐ yl-4,4′-bipyridine; L2 = (4-formylphenoxy)acetic acid; L3 = 2-[2-[[(2-aminoethyl)imino]methyl]phenoxy]acetic acid; L4 = 4,5-imidazoledicarboxylic acid; L5 = (*R,R*)-(-)-1,2-cyclohexanediamino-*N,N*-bis(3-tert-butyl-5-(4-pyridyl)salicyli‐ dene)MnIIICl; L-lac = L-lactic acid; mdip = 5,50 -methylenediisophthalic; meim = 2-methyl-1H-imidazole;nds = naphtha‐ lenedisulfonic acid; papa = (S)-3-hydroxy-2-((pyridin-4-ylmethyl)amino)propanoic;pbbm = 1,1′-(1,5-pentanediyl)bis(1Hbenzimidazole); pdc-1 = pyrazole-3,5-dicarboxylate; pdc-2= pyridine-2,5-dicarboxylate; PhIM = phenyl imidazolate; ptdc = pyridine-2,3,5,6-tetracarboxylic; pymo = 2-hydroxypyrimidinolate; Py2(PhF5)2Por = 5,15-dipyridyl-1′,2′-bis(pentafluor‐ ophenyl)porphyrin; pvia= (E)-5-(2-(pyridin-4yl)vinyl)isophthalic; pvba = (E)-4-(2-(pyridin-4-yl)vinyl)benzoic; sal = salicylidene moiety; tcba = 4,′4″,′4″″-nitrilotris([1,10-biphenyl]-4-carboxylic); TCPB = 1,2,4,5-tetrakis(4-carboxyphen‐ yl)benzene; TpCPP = tetra-(*p*-carboxyphenyl)porphyrin; tpha = tris(4-((*E*)-1-(2-(pyridin-2-yl)hydrazono)ethyl)phe‐ nyl)amine.

**Table 3.** MOF structure used for catalytic reaction.

**MOF Catalysed reaction Reference**

Cu3(pdtc)(pvba)2(H2O)3 Henry reaction [115]

Cu(tcba)(DMA) Epoxidation of olefins [118]

Cu2(bpdc)2(bpy) Cross-dehydrogenative coupling

Cu2I2(bttp4) Three-component coupling of azides,

Cu-MOF-SiF6and Cu-MOF-NO3 Oxidation of benzylic compounds [122] CuPhos-Br and CuPhos-Cl and CuPhos-PF6 Ketalization reaction [123]

In(OH)(hippb) Acetalization of aldehyde [130]

Mg3(pdc1)(OH)3(H2O)2 Aldol condensation reactions [132] Mg(pdc2)( H2O) Aldol condensation reactions [133]

Mn2(pvia)2(H2O)2 Alcohol oxidation [137]

(Na20(Ni8L412)(H2O)28)( H2O)13(CH3OH)2 Oxidation to CO2 [138]

Mn(porphyrin)@[In48(HImDC)96] Oxidation of alkane [134] Ln(OH)(1,5-NDS) H2O Epoxidation of olefin [135]

(Mn(TpCPP)Mn1.5)(C3H7NO)5 C3H7NO Epoxidation of olefin; oxidation of

Isomerization; cyclization; rearrangement Oxidation of polyphenol Cyanosilylation of aldehyde

Biginelli reaction; 1,2-additionof a,b-

Click reaction Three-component couplings of amines, aldehydes and alkynes

Friedel–Crafts benzylation Oxidation of hydrocarbons Ring-opening of epoxides Claisen Schmidt condensation Oxidation of thiophenol to diphenyldisulfide Isomerization of a-pinene oxide

Reduction of nitroaromatic; oxidation

Cyanosilylation of aldehyde;

Intermolecular carbonyl ene reaction; Michael addition reaction; ketimine and aldimine formation

unsaturated ketones [115]

reaction [119]

alkynes, and amines [120, 121]

of sulfide [131]

alkane [136]

Mukaiyama-aldol [88]

[86, 87, 109–113]

[116, 117]

[124–129

[139]

Cu3(btc)2

106 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Cu2(papa)2Cl2

Cu(2-pymo)2

Fe3F(H2O)2O(btc)2 (MIL-100(Fe))

In2(OH)3(bdc)1.5

[Mn3((Mn4Cl)3BTT8(CH3OH)10)]2

Sc3(OH)( H2O)2O(btc)2 (MIL-100(Sc))

## **4. Limitation of MOF structures**

MOFs are excellent candidates for certain catalytic processes because: (1) they can be designed on a rational basis according to specific requirements; (2) their well-defined structure allows the assessment of structure–activity relationships; (3) the uniform catalyt‐ ic sites; and (4) the intrinsic nature of their pores.

However, the synthesis of MOF requires a series of steps to allow an activation network free of solvent molecules and to expose the active sites, which can often require coordinat‐ ing solvents that may be toxic, carcinogenic, and/or dangerous to the environment [59]. In some processes, the structure collapses, and activation prevents further use in catalysis.

Furthermore, when washed with solvent, they typically require energy, which increases the time synthesis process and drastically affects the efficiency. For example, with MOF-5 synthesized using solvothermal processes, a yield of 60% was achieved after 7 days [72].

The use of coordinating solvents during the synthesis of MOF such as dimethylforma‐ mide (DMF) or diethylformamide (DEF) may interfere with the availability of molecules to interact with the active sites. DMF and DEF decompose when heated at high tempera‐ tures for long periods and therefore cannot be reused [60]. The study of local defects is also crucial since catalytic processes can be favoured with the appearance of the same or conversely the process is catalysed not by excess thereof.

The MOF's purity can be affected by the formation of other crystalline compounds or the presence of reagents in the network. However, characterization of MOFs' purity and homogeneity can seldom be found in scientific papers about catalysis.

The thermal and chemical stability of MOFs is also a limitation for use in some catalytic process. The zirconium MOF reported by Hafizovic Cavka et al. [157], which has a thermal resistance above 500°C, resistance to most chemicals, and they remain crystalline even after exposure to 10 tons/cm2 of external pressure, whereas other MOFs have a lower thermal and chemical stability.
