Bifunctional MOFs in Heterogeneous Catalysis

Abstract

The ever-increasing landscape of heterogeneous catalysis, pure and applied, utilizes many different catalysts. Academic insights along with many industrial adaptations paved the way for the growth. In designing a catalyst, it is desirable to have a priori knowledge of what structure needs to be targeted to help in achieving the goal. When focusing on catalysis, one needs to cope with a vast corpus of knowledge and information. The overwhelming desire to exploit catalysis toward commercial ends is irresistible. In today’s world, one of the requirements of developing a new catalyst is to address the environmental concerns. The well-established heterogeneous catalysts have microporous structures (<25 Å), which find use in many industrial processes. The metal–organic framework (MOF) compounds, being pursued vigorously during the last two decades, have similar microporosity with well-defined pores and channels. The MOFs possess large surface area and assemble to delicate structural and compositional variations either during the preparation or through postsynthetic modifications (PSMs). The MOFs, in fact, offer excellent scope as simple Lewis acidic, Brönsted acidic, Lewis basic, and more importantly bifunctional (acidic as well as basic) agents for carrying out catalysis. The many advances that happened over the years in biology helped in the design of many good biocatalysts. The tools and techniques (advanced preparative approaches coupled with computational insights), on the other hand, have helped in generating interesting and good inorganic catalysts. In this review, the recent advances in bifunctional catalysis employing MOFs are presented. In doing so, we have concentrated on the developments that happened during the past decade or so.

1. Introduction

Catalysis has been so widely woven into the fabric of life that the practice and pursuit of the subject transcends across different disciplines of science and engineering. The canvas of catalysis has been large and continuously growing over the years. It may be noted that there has been considerable success in discovering newer catalysts compared to obtaining greater insights over the known ones.¹⁻³

For carrying out efficient catalysis and understanding the catalytic process, two different strategies exist toward the design of solid catalysts. In one approach, the individual steps for the overall reaction are considered and optimized, and the second approach focuses on the location of the active site within the structure. For one of the goals of preparing new compounds that could be potential catalysts, one needs to look for maximum surface area, precise catalytic reaction centers, and good activity, selectivity, and durability. The desiderata of designing a new catalyst need to encompass the following: (i) to be able to operate under mild conditions that are environmentally benign; (ii) reasonable freedom from restrictions imposed by diffusional considerations of molecules (reactants and products); (iii) possession of well-defined and specially separated reaction centers; and (iv) the scope to probe the mechanistic understanding of the catalytic action.

In the area of catalysis, the idea that the catalyst possesses bifunctional or multifunctional character is a desired property. There have been studies toward the multifunctional behavior of catalysts, especially those involving zeolites and mesoporous compounds.⁴ It is becoming clear that in seeking to create compounds with good surface areas and catalytic activity one needs to have solids that have pore diameters in the microporous region (∼20 Å).

Aluminosilicate zeolites have been the backbone of the study of catalysts and the science of catalysis over many decades. These compounds possess porosity in the microporous range of 4–20 Å. The discovery of metal–organic frameworks (MOFs) in the late 90s and its subsequent developments appear have caught the imagination of the catalyst community.⁵⁻¹⁰ The attractive features of many MOF compounds toward catalysis are as follows: (i) they permit free flow of reactants and products through the channels (pores) and cages; (ii) they have spatially distributed distinct catalytic sites; and (iii) they create the possibility for performing shape-selective, enantioselective, and regioselective reactions. The MOFs offer ease of synthesis coupled with a diverse range of compositional variations, which would be desirable toward heterogeneous catalysis. The MOFs offer the following advantages: (i) The MOF structures are flexible (breathable) and expand and contract by external stimuli;¹¹⁻¹⁴ (ii) the metal as well as the ligand that forms the structures can be replaced by postsynthetic modifications to render them attractive toward many physical and chemical properties;¹⁵⁻²¹ (iii) the organic ligands are amenable for manipulations to create specific functionality in the compounds, which would be desirable toward organocatalysis;²²⁻³⁰ and (iv) MOFs can provide hydrophilic and hydrophobic environments which can be exploited toward specific catalysis.³¹⁻³⁵

There have been many catalytic studies that were carried out employing MOFs. In most of the studies, the metal sites were always exploited for their Lewis-acidic behavior.^24,36−38 Most of the earlier attempts toward catalysis employing MOFs were predominantly Lewis acid catalysis only.³⁹⁻⁴² Brönsted acid catalysis was also attempted employing MOFs.^43,44 Over the years it has been shown that the organic ligands in MOFs also catalyze many reactions, especially acting as a Lewis base.⁴⁵⁻⁴⁸ As the MOFs possess both the Lewis acidic as well as basic centers, bifunctional catalysis was attempted.⁴⁹⁻⁵²

The bifunctional MOFs offer advantages toward studying the cascade reactions. The cascade/domino/tandem reactions^53,54 utilize at least two consecutive reactions and involve different chemical functionality available within the compound. The cascade process involves a set of reactions where the product(s) of the reaction(s) is consumed in a subsequent reaction. In these reactions, the isolation of intermediates is not required and the reaction(s) proceeds in a stepwise manner. The cascade reaction requires different catalytically active sites distributed over a large surface area and in a periodic manner. The MOFs with their large surface area with good pore size distribution and availability of functional groups would be ideally suited for such reactions.⁵⁵⁻⁵⁷

In this review, we focus on the recent developments on MOFs that offer multifunctionality toward heterogeneous catalysis. We have specifically given closer attention to the developments toward catalytic reactions that have been performed employing MOFs during the past decade or so. In this task, there may be a few oversights, which are not intentional.

2. Generation of Functionality in MOFs

Most of the MOF compounds possess Lewis acidity due to the metal centers—the strength of the acidity depends on the size of the metal ions, the oxidation state, and its coordination preferences. In addition to this, it may be possible to create acidity in MOFs by suitable postsynthetic modifications. Here, we outline a few such scenarios:

2.1. Lewis Acid Functionality

The acidity can be classified as Brönsted acid or Lewis acid—the former is the stronger acid compared to the latter. The MOFs that were explored toward acid catalysis are tabulated in Table 1. Lewis acidity in MOFs generally refers to an accessible metal site, with a low coordination number, which is also known as a coordinatively unsaturated metal site (CU). The coordinatively unsaturated metal sites can be achieved in situ by the removal of labile ligands bonded to the metal, which usually are the solvent molecules. A Zn MOF, {[Zn(BPBN)Cl]·5H₂O}_n (BPBN = 3,5-bis(4-oxo-4H-pyridin-1-yl)-benzonitrile) (Figure 1a) has been synthesized and has been utilized for the synthesis of naphthimidazole from 2,3-diaminonaphthalene and DMF at 120 °C with good yield (Figure 1b). The tetrahedrally coordinated Zn center acts as Lewis acid site and facilitates this reaction.⁵⁸ UiO-66 (UiO = University of Oslo) is one of the MOFs which was studied toward the Lewis acid catalytic reaction. Zr-MOFs provide unsaturated metal sites, which could be exploited as Lewis acid centers in catalytic reactions. The open metal sites generally act as electron pair acceptors and accelerate the reaction process.⁵⁹ Most of the earlier studies exploiting the Lewis acid functionality concentrated on cyanosilylation of imines.⁶⁰ These are typically low temperature reactions, and in most cases the yield is >95%. Recently, lanthanide centers were explored as possible Lewis acid catalytic centers in the compound, [La_2/3(qptca)_1/2] (qptca = 1,1′:4′,1′′:4″,1‴:4′′′,1⁗-quinquephenyl]-2,2′′,2′′′′,5′′-tetracarboxylic acid), toward the Friedel–Crafts reaction. The alkylation of indole and pyrrole with β-nitrostyrene with a wide substrate scope gave the desired product with high yield and recyclability.⁶¹

Table 1. MOFs with Different Lewis Acidic Functionalities and Their Utilities toward Catalysis^a.

Sr. No.	MOF Compound	Lewis acidic metal site	Labile solvent	Catalytic reaction	Ref
1	Zr6-fBDC and Zr6-fBPDC	Zr4+	H2O	Arene C–H Iodination	(258)
2	Zr6OTf-BTB	Zr4+	-	Povarov Reactions	(259)
3	[Zn2(TBIB)2(HTCPB)2]·9DMF·19H2O	Zn2+	-	Cycloaddition of CO2	(260)
				Friedländer Reaction
4	MOF-525, PCN-222 and PCN-224	Zr2+, Mn2+, Zn2+	-	Cycloaddition of CO2	(261)
5	M-NU-1008	M = Zr4+, Hf, Th, Ce3+	H2O	Cycloaddition of CO2	(262)
6	ZIF-8	Zn2+	-	Epoxide Hydroxylation	(263)
7	Tb(BTC)(H2O)3(DMF)1.1	Tb3+	H2O, DMF	Synthesis of β-Aminoalcohols	(264)
8	Er(BTC)(H2O)·(DMF)1.1	Er3+	H2O, DMF	Hantzsch Coupling and Tetrahydro-4H-Chromene Synthesis	(265)
9	[Ba2(BDPO)(H2O)]·DMA	Ba2+	H2O	Cycloaddition of CO2	(266)
10	NH2-MIL-101/PAN	Cr3+	-	Friedel–Crafts Acylation of Anisole	(267)
				Esterification reaction
11	Cu(II)-MOF	Cu2+	H2O	Cycloaddition of CO2	(268)
12	Zr6O4(OH)4(OAc)2.4[M(PNNNP)X]2.4 [M = Pd, Pt]	Zr4+, Pd(I), Pt(I)	-	Hydroamination of o-Alkynyl Aniline	(269)
13	[Zn2(iso)2(bpy)2]	Zn2+	DMF	Cycloaddition of CO2	(270)
14	[Zn(BPBA)Cl]·5H2O	Zn2+	H2O	Cyclization of ortho-Substituted Diaminonaphthalene to Naphthimidazole	(58)
15	Ce-doping MIL-88A(Fe)	Ce3+, Fe3+	-	Catalytic Ozonation	(271)
16	Pd(II)/UIO-66 (Zr), Pd(II)/MIL-101 (Cr) and Pd(II)/MOF-5 (Zn)	Pd2+, Cr3+, Zn2+	H2O	CO Esterification to Dimethyl Carbonate	(272)
17	UiO-66	Zr4+	H2O	Aldose Sugars to Polyhydroxyalkyl and C-Glycosyl Furans	(273)
18	[Cd3(BDC)3(OPP)(DMF)2]·2DMA	Cd2+	DMF	Hantzsch Reaction	(274)
19	Cu-BTC(MOF-199)	Cu2+	-	Aerobic Oxidative Synthesis of Imines	(275)
20	MIL-101(Fe,Sc)	Fe3+, Sc3+	-	Glucose to 5-Hydroxymethyl Furfural	(276)
21	Al-ITQ-Br, Al-ITQ-NO2, L-MOF-EB L-MOF-AB	Fe3+, Al3+	-	Oxidation of Thiophenol to Diphenyldisulfide	(277)
22	[Eu(tctb)(H2O)]	Eu3+	H2O	Diamines to Benzimidazoles	(278)
23	[M6(TATAB)4(DABCO)3(H2O)3]·12DMF·9H2O	M = Co2+, Ni2+	H2O	Chemical Fixation of CO2	(279)
24	MixUMCM-1-NH2.	Zn2+	-	Aldol–Tishchenko Reaction	(280)
25	[In3(NIPH)3(HNIPH)(OH)2]·4H2O	In3+	-H2O	Multicomponent Strecker Reactions	(281)
26	[Mn2(TDP)(H2O)2]·3H2O·3DMF	Mn2+	-H2O	Chemical Fixation of CO2	(282)
27	[Cu6(TADIPA)3(DABCO)(H2O)2(DMF)2]·13H2O [Cu6(TADIPA)3(H2O)6]·16H2O·8DMF	Cu2+	-H2O, DMF, DMA	Chemical Fixation of CO2	(283)
	[H3O][Cu6(TPTA)3(DMA)4(COO)]·12H2O·7DMA
	[Cu6(C17O9N2H8)3(C6H12N2)(H2O)2(DMF)2]·3DMF·8H2O
28	UiO-66-TA	Zr4+	-H2O	Hydrogenation of Cinnamaldehyde	(284)
29	MIL-101(Cr)-LP	Cr3+	-	Reduction of Imine	(285)
30	MIL-101(Cr) MOF	Cr3+	-	glucose to fructose	(286)
31	[Zn3(Hbtc)2(atz)2]·CH3CN·2CH3OH [Co3(Hbtc)2(atz)2]·H2O·2DMF	Zn2+ Co2+	-	Coupling of CO2 and Epoxides	(287)
32	[Zn(bix)]{V2O6}(V-Zn-MOF	Zn2+	-	Cyanosilylation Reaction of Aldehydes	(288)
33	[Dy3(data)3·2DMF]·DMF	Dy3+	DMF	Chemical Fixation of CO2	(155)
34	Cu-MOF	Cu2+	-	Catalytic CO2 Fixation	(289)
35	[Cd(bpp)(L)(H2O)]·DMF	Cd2+	-H2O	Strecker Reaction	(290)
36	Cr-UiO-66-CAT	Zr4+	-	Oxidation of Alcohols to Ketones	(85)

^aH₂fBDC = 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid; H₂fBPDC = 2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-biphenyldicarboxylic acid; TBIB = 1,3,5-tri(1H-benzo[d]imidazol-1-yl)benzene); H₃TCPB = 1,3,5-tris(4′-carboxyphenyl-)benzene; BTC = 1,3,5-benzenetricarboxylate; DMF = N,N-dimethylformamide; PAN = polyacrylonitrile; P^NN^NP = 2,6-(HNPAr₂)₂C₅H₃N; Ar = p-C₆H₄CO₂^–; X = Cl^–, I^–; iso = isophthalic acid; bpy = 4,4-dipyridyl; BPBA = 3,5-bis(4-oxo-4H-pyridin-1-yl)-benzoate; OPP = N,N′-(oxybis(4,1-phenylene))bis(1-(pyridin-4-yl)methanimine); H₂BDC = terephthalic acid; H₃tctb = tris(p-carboxylic acid)tridurylborane; H₃TATAB = 4,4′,4″-s-triazine-1,3,5-triyl-tri-p-aminobenzoic acid; DABCO = 1,4-diazabicyclo[2.2.2]octane; BDC = 1,4-benzenedicarboxylate; ABDC = 2-amino-1,4-benzenedicarboxylate, btb = 4,4′,4″,-benzene-1,3,5-triyl-trisbenzoic acid; H₂NIPH = 5-nitroisophthalic acid; H₄TPTA = 1,1′,3′,1″-terphenyl-3,3′′,5,5′-tetracarboxylic acid; H₄CBDA = 5,5′-(carbonylbis(azanediyl)) diisophthalic acid; TA = terephthalic acid; LP = Lewis pair; bix = 1,4-bis(imidazole-1-ylmethyl)benzene; 2,5-data = 2,5-diamino-terephthalate; H₂L = 5-(1-oxo-2,3-dihydro-1H-inden-2-yl)isophthalic acid; H₂L = 4,4′-(dimethylsilanediyl)bis-benzoic acid; bpp = 1,3-bis(4-pyridyl)propane.

Figure 1.

(a) 3D structure of the [Zn(BPBN)Cl]·5H₂O MOF. (b) Synthesis of naphthimidazole in the presence of heterogeneous catalyst. Reproduced with permission from ref (58). Copyright 2020 Elsevier.

As an example of unusual Lewis acidity in MOFs, the example of easily modifiable nature of the ligand was exploited in preparing Pd-mono(thiocatecholato) units inside the MOF UiO-66.⁸⁵ This modified MOF exhibited excellent regioselectivity toward the sp² carbon, oxidation of alcohols to ketones, etc.^85,86 A more interesting approach is to replace the ligand, which expands the MOF, allowing for enhanced catalytic activity.⁸³ In this work, C₂ symmetry ligands were exchanged for C₃ symmetric, which causes defects in the overall structure, paving the way for better Lewis acid activity.⁸⁷

2.2. Brönsted Acid Functionality

In the traditional framework compounds of aluminosilicate zeolites, metal phosphates, etc. Brönsted acidity was generated by manipulating the structure by having elements of different valencies.⁸⁸⁻⁹⁰ The charge compensating protons have been found to have strong Brönsted acid character. In MOFs, such a possibility is difficult to achieve, as the framework contains metal centers with fixed valences and organic ligands. There are examples of MOF compounds where elements of mixed valency exist as part of the structure.^91,92 The Brönsted acidity in many MOFs, however, appears to arise out of the nonbonded acidic groups of the ligands (Scheme 1).^43,72 The MOFs, thus, provide the versatility of having both the Lewis acidic as well as Brönsted acidic functionality within the same MOF.^68,93−95 The many catalytic reactions that have been carried out employing Brönsted acid functionality in MOFs are listed in Table 2. In MIL-(Cr)-101-SO₃H, the Cr³⁺ ions act as a Lewis acidic and the -SO₃H groups act as the Brönsted acidic centers for the catalytic conversion of glucose.⁸³

Scheme 1. Schematic Showing How the Additional Functionality Is Used in Generating Brönsted Acidity in MOFs: (a) Direct Synthesis; (b) via Postsynthetic Modification.

Table 2. MOFs with Brönsted Acidity and the Associated Catalysis^a.

Sr. No.	MOF	Brönsted acidic site	Catalytic reaction	Ref
1	PTA⊂MIL-101(Al)-NH2	H3[PW12O40]·nH2O	Glucose Dehydration to 5-Hydroxymethylfurfural	(62)
2	H3PW12O40@Zr-MOF	-SO3H	Levulinic Acid to γ-Valerolactone	(63)
3	[(CH2COOH)2IM]HSO4@H-UiO-66	[(CH2COOH)2IM]HSO4	Biodiesel Synthesis	(64)
4	PO4/NU(eq) and PO4/NU(half)	-PO4	Glucose to 5-Hydroxymethylfurfural	(65)
5	MIL-101(Cr)-SO3H	-SO3H	Cross-Dehydrogenative Coupling of C–H Bonds	(66)
6	CoFe2O4/MIL-88B(Fe)-NH2/(Py-Ps)PMo	-SO3H	Transesterification	(67)
7	MIL-101(Cr)-NH-CO-Pr-COOH	-COOH	Synthesis of Quinazolin-(4H)-1-one	(68)
8	MOF-808-SO4	-SO4	Dimerization of Isobutene (2-Methyl-1-propene)	(69)
9	[Zr6O4(OH)5.6(C9H3O6)2(HCOO)0.18(SO4)2.1](H2O)2	-OH	Isobutene Dimerization	(70)
10	Hf-MOF-808	μ3-OH	Meerwein–Ponndorf–Verley Reduction	(71)
			Styrene Oxide Ring-Opening
			α-Pinene Oxide Isomerization
11	MIL-100(Cr) and MIL-100(Fe)	-H2O-CO	Acetalization of Benzaldehyde with Methanol	(72)
12	PCN-222(Ni)-SO4	SO42–	Tandem Semisynthesis of Artemisinin	(73)
13	Zr-MOF-808-S	OH–/H2O	Glycerol Dehydration	(74)
14	Al-MIL-53-RSO3HAl-MIL-53-ArSO3H	-SO3H	[4 + 2] Cycloaddition Reaction	(75)
15	UiO-66 (Zr, Hf), Zr-BTC	μ3-OH	Cycloaddition Reaction	(76)
16	MIL-100(Fe) (Lys-PM2)	Lys-PM2	Conversion of Glucose to Levulinic Acid	(77)
17	[BSO3HMIm][HSO4](IRMOF-3)	[BSO3HMIm][HSO4]	Bligh–Dyer Method for Biodiesel Production	(78)
18	MIL-101(Cr)-SO3H	-SO3H	Methanolysis of Styrene Oxide	(79)
19	Cr3(μ3-O)(H2O)3(NDC(SO3H5/6)2)3(BUT-8(Cr)-SO3H	-SO3H	Esterification Reaction	(80)
20	MIL-IMAc-Br–	Br–	Cycloaddition of CO2	(81)
21	(H4SiW12O40) (POM@MOF)	H4SiW12O40	Glucose into 5-Hydroxymethylfurfural	(82)
22	MIL-(Cr)-101-SO3H	SO3H	Catalytic Conversion of Glucose	(83)
23	UiO-66(SO3H)2	SO3H	Synthesis of Dihydro-2-oxypyrrole Derivatives	(84)

^aPTA = phosphotungstic acid; Py-Ps = pyridine with 1,3-propanesultone; H₄TADIPA = 5-5′-(1H-1,2,4-triazole-3,5-diyl) diisophthalic acid; Lys = lysine functionalized phosphotungstic acid; [BSO₃HMIm][HSO₄] = 1-butylsulfonate-3-methylimidazolium bisulfate; NDC(SO₃H)₂^2– = 4,8-disulfonaphthalene-2,6-dicarboxylatlate); IMAc = 1H-imidazole-1-acetic acid.

The acidic stability of the MOFs was exploited in generating new Brönsted acidity through postsynthetic modifications in UiO-66(SH)₂.⁹⁶ In this study, UiO-66(SH)₂ was modified by treatment with H₂O₂ and H₂SO₄ to form UiO-66(SO₃H)₂, which was later found to be a good catalyst for synthesis of dihydro-2-oxypyrrole derivatives.⁸⁴

2.3. Basic Functionalities in MOFs

It has been well established that Lewis acidities are much easier to generate in MOFs. In many of the traditional framework compounds, the basic functionality is generated when the extra-framework protons are the Brönsted acid sites and the framework oxygens and their conjugate form the basic sites.^97,98 In addition, alkali and alkaline earth exchanged zeolites are also designated as basic.

An easier approach toward the generation of Lewis basicity is feasible in MOFs compared to the traditional framework of zeolites and aluminophosphates. In this approach, the functionality of organic ligands plays an important role in creating the basicity to the structure. More importantly the Lewis basic sites are spatially separated and, in a sense, can be considered as a single basic site, similar to the acid site, which many researchers have exploited.^99,100

Thus, in MOFs the -NH₂ group plays a crucial role as the Lewis basic site. In addition, researchers have also used nonbonded carboxyl (-COO^–) and similar units as Lewis bases.¹⁰¹ The Lewis basic sites in MOFs and their utility toward catalysis are listed in Table 3. For example, IRMOF-1 (commonly known as MOF-5) is constructed from the Zn²⁺ ions and ligand BDC, while replacement of BDC with NH₂-BDC yields IRMOF-3, which exhibits Lewis basic behavior toward the Knoevenagel condensation reaction.^102,103 A range of metals viz., Zn,⁴⁷ Zr,¹⁰⁴ Al,¹⁰⁵ Ti,¹⁰⁶ and Cu¹⁰⁷ were employed for the preparation of MOFs with NH₂–BDC, which were found to be useful in base catalyzed Knoevenagel condensation reactions. ZIFs (zeolite imidazolate frameworks), which were formed by assembling Zn metal and imidazole linker, provide high stability, high surface area as well as sufficient basicity, arising out of the N atoms of the imidazole have been exploited toward base catalysis.¹⁰⁸ Such ZIF compounds were also found to be good catalysts toward Knoevenagel condensation reaction at room temperature with ∼99% yield.¹⁰⁹

Table 3. MOFs with Different Basic Functionalities and Their Utility toward Catalysis^a.

Sr. No.	MOF Compound	Ligand responsible for basicity	Catalytic reaction	Ref
1	[Cd(C16H10N2O8S)(H2O)]	-NH	Knoevenagel Condensation	(110)
2	[(Nd2(TATMA)2·4DMF·4H2O]n	-N	Knoevenagel Condensation	(111)
3	[H2N(CH3)2]·[Zn4(L)1.5(ad)3(H2O)2)]·4DMF	-NH2	Knoevenagel Condensation	(112)
4	{[Zn2(D-CAM)2(L)]·MeOH·2H2O}n	-NH	Ring-Opening of Spiro-Epoxyoxindoles	(113)
5	Co2(bdda)1.5(OAc)1·5H2O	-NH	Henry Reactions	(114)
6	[Zn(OBA)(BPDB)0.5]n·2DMF	-N	Knoevenagel Condensation	(115)
7	[Y3(μ3-O)2(μ3OH)(H2O)2(BTCTBA)2]·2[(CH3)2NH2]·5DMF·C6H5Cl·4H2O	C=O–N–H	Knoevenagel Condensation	(116)
8	[Zn2(hfipbb)2(4-bpdh)]·0.5DMF and [Zn2(hfipbb)2(4-bpdb)]·2DMF	-N=N-	Knoevenagel Condensation	(117)
9	UiO-67-BPY@UiO-66	-Nbpy	Knoevenagel Condensation	(118)
10	[Zn2(3-tpom)(L)2]·2H2O	-O	Strecker Reaction	(119)
11	[Cu2(L)(H2O)2]·(3DMF)(4H2O)	-N	Henry Reactions	(120)
12	[(CH3)2NH2+]2[Zn3((μ3-O))(L)2(H2O)]·4DMF·2H2O	-NH2	Chemical Fixation of CO2	(121)
			Biginelli Reactions
13	[Cu2(L)(H2O)2]·(5DMF)(4H2O)	-NH2	Biginelli Reactions	(122)
14	[CoL(H2O)3]·2NO3	-N	Knoevenagel Condensation	(123)
15	UiO-67@Fe	-Nbpy	Morita–Baylis–Hillman Reaction	(124)
16	ZIF-8, ZIF-67	-Nim	Knoevenagel Condensation	(125)
17	MIL-125-NH2	-NH2	Plasticizers Syntheses	(126)
18	SB-Cu1	-N	N-Arylation	(127)
19	NH2cCo-PYI1 and NH2cCo-PYI2	-NH2	Aldol and Knoevenagel Condensations	(128)

^aNic = nicotinamide; pic = picrate; H₃dcp = 3,5-pyrazoledicarboxylic acid; DMA = N,N-dimethylacetamide; ad = adenine; H₄L = 5,5′-(1,3,6,8-tetraoxobenzo phenanthroline-2,7-diyl)bis-1,3-benzenedicarboxylic acid; H₃TATMA = 4,4,4″-s-triazine-1,3,5-triyltri-m-aminobenzoate; L = N′-(pyridin-4-ylmethylene)isonicotinohydrazide; bdda: 4,4′-[benzene-1,4-diylbis(methylidenenitrilo)] dibenzoic acid); H₃BTCTBA = 4,4′,4″-[1,3,5-benzenetriyltris(carbonylimino)]trisbenzoic acid; 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene; 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene; H₂L = dicarboxylic acid 4,4′-(dimethylsilanediyl)bis-benzoic acid; 3-tpom = tetrakis(3-pyridyloxymethylene)methane; H₄L = 5,5′-(piperazine-1,4-diyl)diisophthalic acid; L = 4-(trifluoromethyl)aniline, 1-bromo-3,5-dimethylbenzene; L = tris(4-(4H-1,2,4-triazol-4-yl)phenyl)amine; PYI = pyrrolidine-2-yl-imidazole.

3. Multifunctional Catalytic Centers in MOFs

From the above descriptions, it is clear that forming bifunctional catalytic sites in MOFs is facile. The metal sites can provide the Lewis acidity, and the Lewis basic sites can be generated from the carboxylate units, specific functional groups in the ligand. The acid–base pair can be spatially well separated and in the precise location to facilitate bifunctional catalysis.^45,129−131 The bifunctionality available in MOFs can be utilized in multiple ways: (i) in organic reactions involving multiple steps without separation and purification of the intermediates in each step;¹³² (ii) in catalyzing multiple reactions simultaneously involving more than one reaction pathway; (iii) in promoting tandem catalysis involving reactions that proceed sequentially; and (iv) in catalyzing acid–base reactions. The bifunctionality in MOFs toward catalysis can be achieved by the following: (i) coordinatively unsaturated metal sites for Lewis acid functionality along with a suitable ligand that offers Lewis basic functionality; (ii) use of multifunctional ligands either during the formation of the MOFs or through postsynthetic modifications; and (iii) incorporation of metal nanoparticles within the MOF structure (Scheme S1). These techniques are popular in generating bifunctional MOF compounds, and there are other techniques that may also be utilized to generate bifunctionality in MOFs. The number of MOF compounds with bifunctionality has grown rapidly in recent years. This is not surprising, as the initial efforts during the development of MOFs were toward establishing newer framework compounds and exploring their physical properties.^133−136 The emphasis on the utility of MOFs toward catalysis involving both the acidic as well as basic functionalities is summarized in Table 4. In addition to preparing MOFs with bifunctionality by careful choice of the ligands, it is also possible to generate bifunctionality by carrying out postsynthetic modifications. Examples of bi- or multifunctionality through postsynthetic modifications (PSMs) have been known.^137,138 The PSM allows better control on the structure and fine-tuning of the functionality toward a particular catalytic activity.

Table 4. MOFs with Bi-/Multifunctionality and Their Use in Catalysis^a.

Sr. No.	MOF Compound	Reactive site		Catalytic reaction	Ref
1	MIL-101(Cr)-N(CH2PO3H2)2	Cr(III) Phosphonates, -NH2		Synthesis of N-Amino-2-Pyridone and Pyrano [2,3-c]Pyrazole Derivatives	(139)
2	[Zn2(TCA)(BIB)2.5]·(NO3)	Zn (II), NTCA		Cycloaddition of CO2	(140)
3	[Mn2(DPP)(H2O)3]·6H2O	Mn (II), -Npyridine		Cycloaddition of CO2	(141)
4	[Zn(1,4-NDCA)(3-BPDB)0.5]·(DMF)(MeOH) [Cd4(1,4-NDCA)4(3-BPDB)4]·2(DMF)	Zn(II)Cd(II), -N=N-		Friedländer Reaction	(142)
				Michael Addition
5	[M3(5-CFIA)2(8H2O)]·H2O	Cd(II), Mn(II), -NH		Aldol Condensation	(143)
				β-Enamination Reactions
6	[Zn(HL)2]	Zn (II), -Ntriazole		Knoevenagel Condensation	(50)
7	[Zn15(L-NH2)6(HL-NH2)6(LNA)4(HLNA)2(μ3–OH)2]	Zn(II), -NH2		Knoevenagel Condensation	(144)
8	SulP1/SulP2-MOF-808(Hf)	Hf(IV), Phosphonate		Reductive Amination and Hydroaminomthylation Reactions	(145)
9	ED/MIL-101(Cr)	Cr (III), -NH2		Hantzsch Condensation Reaction	(146)
10	Zn-Bp-BTC MOF	Zn(II), -Nbipyridine		Knoevenagel Condensation	(147)
				Multicomponent Reaction
				Benzimidazole Synthesis
11	Lysine - (Zr)MOF-808	Zr(IV),- NH2		Henry Condensation and Friedel/Crafts Type Alkylation	(148)
12	C32H40Fe2N2S4Zn	Zn(II), -NH2		One Pot Synthesis of Chromene and Imidazopyrimidine Derivatives	(149)
13	Co/Ni2(BTC)(OH)(4-TPT)2(H2O)·(DMA)0.5(H2O)2	Co(II)Ni(II), Npyridyl		Oxidation–Knoevenagel Cascade Reaction	(150)
14	Cu3TATAT	Cu(II), -NH, N		Aerobic Oxidation/Knoevenagel Condensation	(151)
15	Hf/Zr MOF-808	Hf(IV)Zr(IV) Defective -OH		Tandem N-Alkylation of Amines with Benzyl Alcohol	(152)
16	PMoV2@DETA-MIL-101	Cr(III)PMoV2, -NH2		Aerobic Oxidation-Knoevenagel One-Pot Reaction	(153)
17	CuI@UiO-67-IM	CuI		One-Pot Azide–Alkyne Cycloaddition	(154)
18	[Dy3(data)3·2DMF]·DMF, NH2-TMU-73	Dy(III), -NH2		Solvent-Free Conversion of CO2 to Cyclic Carbonates	(155)
19	Co-NDTz and Co-NDPhTz	Co(II), -Ntetrazole		Tandem Oxidation and CO2 Conversion Reactions	(156)
20	[Cu2Br2(pypz)]n·nH2O	Cu(II), Br–		Homocoupling of Arylboronic Acids and Epoxidation of Olefins	(157)
21	[Cd(PBA)(DMF)]·DMF	Cd(II), -NH		Cyanosilylation and Hydroboration	(158)
22	Ni-DDIA	Ni(II)-COOH		Biginelli Reaction	(94)
23	MIL-101(Cr)-NH–CO-Pr-COOH	Cr(III)-COOH		Synthesis of Quinazolin-(4H)-1-one Derivatives	(68)
24	MIL-101(Cr)-SO3H	Cr(III)-SO3H		Hydrogenation of Imines	(159)
25	Arg2PTA/ZIF-8	Zn2+, -Nimidazole		Production of Biodiesel from Insect Lipid	(160)

^aAbbreviations: data = 2,5-data = 2,5-diamino-terephthalate; MA = melamine; ED = ethylene diamine; DPP = 2,6-di(2,4-dicarboxyphenyl)-4-(pyridine-4-yl)pyridine; H₈L = tetraphenylsilane tetrakis-4-phosphonic acid; H₃TCA = tricarboxy triphenyl amine; BIB = 1,3-bis(imidazol-1-ylmethyl)benzene; H₄L = 2,6-di(2,4-dicarboxyphenyl)-4-(pyridine-4-yl)pyridine); Bp = 4,4′-bipyridine; BTC = 1,3,5-benzene tricarboxylate; H₂L_NA = 2,6-naphthalenedicarboxylic acid; H₂L-NH₂ = 2,2′-diamino[1,1′-biphenyl]-4,4′-dicarboxylic acid; CSMCRI-10 = Central Salt & Marine Chemicals Research Institute; BuPh₃P = (4-bromobutyl)triphenylphosphonium bromide; SIPA = 5-sulfoisopthalic acid; ABPY = 4,4′-azopyridine; 5-CFIA = 5-(carboxyformamido)isophthalic acid; BDC = 1,4-benzenedicarboxylate; A = acid; B = base; (BINDIH₄) = N,N′-bis(5-isophthalic acid)naphthalenediimide; DATRZ = 3,5-diamino-1,2,4-triazole; pypz = bis[3,5-dimethyl-4-(4′-pyridyl)pyrazol-1-yl] methane; PBA = 5-(4-pyridin-3-yl-benzoylamino)isophthalic acid; H2NDTz = 2,6-naphthaleneditetrazole; H2NDPhTz = 2,6-bis(4-(1H-tetrazol-5-yl)phenyl)naphthalene; Arg = Arginine.

3.1. Examples of Catalysis Involving Acidic and Basic Centers

As listed in Table 4, there have been a number of studies involving bifunctionality in MOFs. In this section, we focus on a few select examples that characterize the bifunctionality in a MOF compound. The examples utilize both the acidic as well as basic functionality toward catalysis, but these reactions are carried out independently and not as tandem catalytic reactions. The tandem catalytic studies are dealt with separately.

The MOF [M₃(C₁₀H₄O₇N₁)₂(8H₂O)]·H₂O (M = Cd, Mn), was explored toward the formation of the β-enaminoester (Lewis acid catalyzed), and the -NH moiety present in the ligand was exploited toward the Claisen–Schmidt reactions (Lewis base catalyzed) (Figure 2a).¹⁴³ The Claisen–Schmidt reaction is a classic base catalyzed aldol condensation reaction involving an aromatic aldehyde and a ketone forming conjugated β-hydroxy carbonyl compounds. The uncoordinated -NH group acts as the basic center and catalyzes the Claisen–Schmidt reaction (Figure 2b). The formation of β-enaminoesters is acid catalyzed, and the Cd²⁺ center acts as the Lewis acidic center and catalyzes the reaction between ethyl acetoacetate and aniline (Figure 2c).¹⁴³ A two-dimensional (2D) MOF, [Cd(PBA)(DMF)]·DMF (Cd-PBA), (H₂PBA = 5-(4-pyridin-3-yl-benzoylamino)-isophthalic acid), was found to be a catalyst for the base catalyzed (the N from the pyridine acts as the base center) Knoevenagel condensation reaction and the Lewis acid catalyzed (Cd centers) cyanosilylation of various aldehydes with trimethylsilyl cyanide.¹⁵⁸

Figure 2.

(a) View of the structure of the MOF [Cd₃(C₁₀H₄O₇N₁)₂(8H₂O)]·H₂O with Cd centers (Lewis acidic) and an -NH moiety (Lewis basic). (b) Schematic of the possible reaction pathway for the base catalyzed aldol condensation reaction. (c) Schematic of the acid catalyzed enamine formation involving the Cd center. Reproduced with permission from ref (143). Copyright 2023 American Chemical Society.

There are many reports in the literature where only one of the functional groups (usually the Lewis basic functionality) was investigated toward catalytic studies. As mentioned earlier, in all the MOFs, the metal centers always act as Lewis acid centers. In most of the cases, it appears that the reaction of choice for the Lewis base catalyzed reaction is the Knoevenagel condensation (Table 3).

3.2. Tandem/Cascade Reaction

The chemical industries, generally, look to eliminate the number of steps in a chemical reaction process. Another impediment in many reactions is the need to isolate intermediates for further processing. Tandem/cascade reactions provide a possible alternative to reduce the need to isolate the intermediates and in that way also reduce the number of steps.^161,162 To carry out such tandem reactions, it is necessary to have different catalytically active centers, preferably distributed uniformly, across the surface of the compound. It is even more important if the reaction requires both the acidic as well as basic functionalities. There is a need to replace multistep as well as salt forming chemical reactions. The approach that provides some success is the tandem reaction, where multiple reactions are combined in a sequential way to yield a single product. This approach is also known as “one-pot”, “domino”, and cascade reactions.⁵³ There are many advantages in employing “tandem” reactions: (i) good atom economy; (ii) reduction of the formation of chemical wastes; (iii) reduction in the consumption of energy; (iv) no need to isolate any intermediates; etc. The tandem catalyses are known as concurrent tandem catalysis (CTC)^53,163 and auto tandem catalysis (ATC).¹⁶⁴ One of the important criteria in carrying out the tandem catalytic reactions is the compatibility of the catalyst toward the reactants, intermediates, and solvents. In addition, it is preferable to have the catalytically active centers separated spatially and in a periodic manner. It would be an added advantage if the catalyst can also host both acidic as well as basic catalytic centers. From the arguments as well as the descriptions above, the MOFs have positioned themselves to be an excellent candidate to investigate the multistep tandem/cascade reactions. The many cascade reactions that have been carried out using MOFs are summarized in Table 5. In this section, we provide select examples of such reactions. The reaction that was most studied as tandem reactions in MOFs is the deacetalization (acid catalyzed) followed by Knoevenagel condensation (base catalyzed).

Table 5. Summary of Tandem Deacetalization–Knoevenagel Reaction Employing Bifunctional MOFs^a.

Sr. No.	MOF compound	Reactive site	Reactants	Products	Reaction conditions	Ref
1	Yb-BDC-NH2Dy-BDC-NH2Sm-BDC-NH2	Yb3+Dy3+Sm3+, -NH2	BDA and MN	2-benzylidenemalononitrile	DMSO-d6 (2 mL), catalyst (100 mg), 50 °C, 24 h	(291)
2	3.1% Ru/UiO-66	Ru2+, Zn2+	BA and MN	2-benzylidenemalononitrile	toluene (1.5 mL), O2 1 atm,100 °C	(292)
3	HNUST-8	Cu2+, acylamide	BDA and MN	2-benzylidenemalononitrile	DMSO, 50 °C, 48 h, 0.5 mol % HNUST-8	(293)
4	HNUST-6	Cu2+, acylamide	BDA and MN	2-benzylidenemalononitrile	0.5 mol % catalyst, DMSO, 50 °C, 48 h	(294)
5	[Zn5(L)4(OH)2(H2O)4]n·8n(DMF)·4n(H2O)	Zn2+, – NH	BDA and MN	2-benzylidenemalononitrile	1 mol % of catalyst 1 or 2, DMF (0.5 mL), 80 °C, 3 h	(295)
6	Cz-MOF-253-800	Al3+, N	BD and MN	2-benzylidenemalononitrile	2 mL of toluene, and 5 mg of Pd/Cz-MOF-253–800, 80 °C, 17 h, 150 psi H2	(296)
7	ZIF-8,UiO-66(Zr)-NH2,MIL-101(Cr)-NH2)	Zn2+, O2–, N–, -OH, -NH2	BDA and MN	2-benzylidenemalononitrile	100 °C, 3 h in the Pickering emulsions consisting of water (3 mL)–toluene (2 mL) MOFs (50 mg)	(297)
8	CSMCRI-15	Cd2+, -N=N–NH2	BDA and MN	2-benzylidenemalononitrile	2 mol % catalyst, 4 h, 60 °C, solvent free	(298)
9	H-ZIF-8/Au@mSiO2	Au, Zn2+, -N-	p-nitro BD and MN	2-(4-nitrobenzylidene)malononitrile	catalyst (30 mg) and 2.0 mL tetrahydrofuran at 30 °C for 0.5 h	(299)
10	Hf-UiO-66-N2H3	Hf, N2H3	BD, MN	2-benzylidenemalononitrile	ethanol (0.3 mL), catalyst (20 mg), RT, 4 h	(300)
11	NUC-29	Cd2+, Npyridine	BDA, MN	2-benzylidenemalononitrile	1.0 mol %, based on the {Cd} center, DMSO 50 mL, 5 h, 70 °C	(301)
12	NUC – 53	Zn2+, Npyridine	BDA, MN	2-benzylidenemalononitrile	0.3 mol %, DMSO 3 mL, 6h, 70 °C	(302)
13	UiO-67-(NH2)2	Zr4+, -NH2	BDA, MN	2-benzylidenemalononitrile	ethanol (0.3 mL) and catalyst (15 mg) 10 h, 60 °C	(303)
14	IRA900(xOH)-MIL-101(Al)-NH2	Al3+, -NH2	BDA, MN	2-benzylidenemalononitrile	solvent free, catalyst 0.3 g, 5 h at 110 °C	(304)
15	Cu(ABDC)(DMF)	Cu2+, -NH2	BDA, MN	2-benzylidenemalononitrile	d3-acetonitrile, 0.1 mol % catalyst, 24 h, 60 °C	(305)
16	Zr12BDC-NH2	Zr4+, -NH2	BDA, MN	2-benzylidenemalononitrile	CDCl3 (1.5 mL), catalyst, 55 °C, 24 h, 100 mg	(306)
17	MIL-101(Cr)@PMF	Cr3+, -NH-NH2	BDA, MN	2-benzylidenemalononitrile	30 mg catalyst, 12 h, 4 mL ethanol, 65 °C	(307)
18	[(CH3)2NH2]2[BaZn(TDP)(H2O)]·DMF·5H2O, (NUC-51)	Zn2+, Ba2+, -COOH, Npyridine	BDA, MN	2-benzylidenemalononitrile	1.0 mol % catalyst, 5 mL (DMSO) 4 h 70 °C	(308)
19	(Me2NH2)[InZn(TDP)(OH2)]·4DMF·4H2O (NUC-42)	Zn2+, In3+, −COOH, Npyridine	BDA, MN	2-benzylidenemalononitrile	DMSO, 5 mL. 8h, 60 °C, 1 mol % catalyst	(309)
20	MIL-101(Cr)@MOF-867	Cr3+, Zn2+, -C=N	BDA, MN	2-benzylidenemalononitrile	20 mg, 70 °C, 12 h, DMSO (4 mL)	(310)

^aAbbreviations: BA = benzyl alcohol, MN = malononitrile; BDC-NH₂ = 2-aminobenzenedicarboxylate; BDA = benzaldehyde dimethyl acetal; PDBAD = 4′,4‴-((pyridine-3,5-dicarbonyl)bis(azanediyl))bis[1,1′-biphenyl]-3,5-dicarboxylic acid; MOPBB = (5-methoxy-isophthaloyl)-bis(azanediyl)]diisophthalic acid; 1,4-BDC = 1,4-benzene dicarboxylate; BD = benzaldehyde; H₃TCA = 4,4′,4′-tri- carboxytriphenylamine; DPA = (E)-1,2-di(pyridin-4-yl)diazene; 2-MeIm = 2-methylimidazole; 2- H₂BDC-N₂H₃ = hydrazinyl-1,4-benzenedicarboxylic acid; H₆TDP - 2,4,6-tris(2,4-dicarboxyphenyl)pyridine; ABDC = 2,2′-diamino-[1,1′-biphenyl]-4,4′-dicarboxylic acid; PMF = polymelamine formaldehyde.

3.2.1. Deacetalization–Knoevenagel Tandem Reaction

The functionalization of Cr-MIL-101, with amino and sulfo groups, through postsynthetic modifications, allowed the one-pot deacetalization–Knoevenagel tandem reaction.¹⁶⁵ The general mechanistic pathway involves the acidic (Cr or SO₃H) center to polarize the oxygen atom of the benzaldehyde dimethyl acetal to form benzaldehyde, which also increases the electrophilicity of the C center. The basic site helps in the nucleophile attack on the -C=O carbon, aiding the formation of the final product. The main outcome of this study appears to be the formation of the ammonium functionality, through proton transfer from the sulfonic acid to the amino group, which acts as the catalytic site. This approach of forming a zwitterionic form in a MOF could be an important development and would pave the way forward to carry out tandem reactions.¹⁶⁵ The use of 5-sulfoisopthalic acid as the primary linker and 4,4′-azopyridine (I) and 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (II) as the secondary linkers along with Cd²⁺ ions resulted in two different MOFs.¹⁶⁶ Both the compounds were explored toward the one-pot tandem deacetalization–Knoevenagel condensation reactions due to the presence of Lewis acidic (Cd metal centers) as well as basic (azine, free pyridine, and uncoordinated sulfo oxygens) sites. This highlight of the study is in identifying the formation of aldehyde (Lewis acidic) as slower compared to the Knoevenagel reaction (Lewis basic). In the tandem reaction, the aldehyde formed by the Lewis acid catalysis was immediately consumed in the subsequent Knoevenagel reaction, which facilitates the forward reaction due to Le Chatelier’s principle.¹⁶⁶ A detailed time dependent study clearly outlines the relative merits of the Lewis acidic as well as basic sites in the two-component stepwise cascade reaction (Figure 3).

Figure 3.

Kinetic study for the one-pot tandem deacetalization–-Knoevenagel reaction with I (a) and II (b) as a heterogeneous catalyst (solvent-free condition). Adapted with permission from ref (166). Copyright 2018 American Chemical Society.

The direct synthesis route was adapted in the preparation of PCN-700, where the Brönsted acidity was achieved by introducing H₂TPDC-(COOH)₂ [(1,1′,4′,1″-terphenyl)-2,2″,4,4″-tetracarboxylic acid] and the basicity by introducing H₂BDC-NH₂ (2-aminoterephthalic acid) in the framework (Figure 4).¹⁶⁷ The modified compound, PCN-700-AB, was found to be a good catalyst toward the one-pot tandem reaction of benzaldehyde dimethyl acetal into benzylidene malononitrile. The spatial distribution of the acidic and basic sites in PCN-700-AB was found to effectively catalyze this cascade reaction. When the acidic sites were blocked by making an ester-CH₃ group in the same MOF, PCN-MB (Figure S1), the tandem reaction yields were poorer.¹⁶⁷ This clearly establishes the need to have both the acidic as well as the basic functionalities in the same compound for this reaction. A general scheme for this tandem reaction is given in Scheme 2.

Figure 4.

Structures of PCN-700, PCN-700-B, and PCN-700-AB. Hydrogen atoms are omitted for clarity. Reproduced with permission under a Creative Commons CC-BY 3.0 from ref (167). Copyright 2019 CCS Chemistry.

Scheme 2. Mechanism of the Deacetalization–Knoevenagel Condensation Reaction.

LA = Lewis acidic site; LB = Lewis basic site.

3.2.2. Other Tandem Reactions

Though the deacetalization–Knoevenagel reaction was the dominant cascade reaction investigated over many MOFs, there are other tandem reactions that have been explored as well (Table 6). For example, Cr-MOF (MIL-101-Cr) (Cr₃(F)(H₂O)₂O[(O₂C)C₆H₄(CO₂)]₃) was modified by PSM to generate MIL-101-SO₃H-NH₂, which was found to be a good catalyst toward the one-pot tandem catalytic reaction.^168,169 The modified MOF was prepared through a postsynthetic route by sulfonation of the framework with chlorosulfonic acid in dichloromethane.¹⁷⁰ This compound was found to exhibit catalytic activity toward a three-component condensation reaction between aromatic aldehydes, resorcinol, and malononitrile in aqueous medium, forming 2-amino-4H chromene by the Knoevenagel condensation reaction followed by the Michael reaction (Figure S2).¹⁷⁰ The unmodified MOF, MIL-101-Cr, was found to give a lower yield of the desired product, and increased acidity by grafting -SO₃H to the framework gave improved catalytic activity for the overall reaction (Scheme S2).

Table 6. Summary of Other Cascade Reactions Employing Bifunctional MOFs^a.

Reaction name	Sr. No.	MOF compound	Reactive site	Reactants	Product	Conditions	Ref
Multicomponent Hanstch Reaction	1	OMS-MIL-101(Cr)	Cr, NH2,	aromatic aldehydes, dimedone, β-ketoesters ammonium acetate	methyl 2,7,7-trimethyl-5-oxo-4-phenyl-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate	4 mol % of cat., EtOH	(146)
	2	MIL-101(Cr)-N(CH2PO3H2)2	Cr -NH2, -PO3, -OH, O–	ethyl cyanoacetate or ethyl acetoacetate, hydrazine hydrate, malononitrile, aldehydes	N-amino-2-pyridone and pyrano [2,3-c]pyrazole	5–10 mol % cat.	(139)
Chromene Synthesis	3	Zn-Bp-BTC MOF	N, O–, -OH	benzaldehyde, malononitrile, and active methylene dimedone	2-amino-4H-chromene	0.06 mmol cat., ethanol	(147)
	4	zinc(II)-L1/L2/L3/L4	Zn, -NH2	aldehyde malononitrile 1,3-diketone	chromene	EtOH (2 mL), RT, 2 h, air	(149)
Henry Condensation–Fridel Craft Alkylation	5	(Zr)MOF-808	Zr, lysine	benzaldehyde, nitromethane, indole	1-(2-nitro-1-phenylethyl)-1H-indene	25 mg cat., 30 °C	(148)
Oxidation–Knoevenagel Cascade Reaction	6	M2(BTC)(OH)(4-TPT)2(H2O)·(DMA)0.5(H2O)	Co, Ni, N	benzyl alcohol malononitrile	benzylidene	0.25 mmol % cat., air, 400 μL CH3CN, n-dodecane, 12 h, 353 K	(150)
		Ni3(BTC)2(4-TPT)2(H2O)6·1.5H2O
	7	Cu3TATAT	Cu, N	benzyl alcohol malononitrile	benzylidene	8 mol % cat., TEMPO (0.5 equiv), 5 mL CH3CN, 75 °C, 1 atm O2, 12 h	(151)
	8	H5PMo10V2O40@ MIL-101	Mo, V, Cr	benzyl alcohol malononitrile	benzylidene	0.5 mol % cat., toluene (1 mL) O2, sealed, 120 °C, 24 h	(153)
Oxidation–imine Formation	9	SulP1MOF-808(Hf)-Ir/Rh	Rh/IrPO3, Hf	ketone, benzyl amine	N-(cyclopentylmethyl)aniline	1.5 mol % catalyst in 5 mL toluene under 50 bar of H2 at 90 °C for 24 h	(145)
	10	Zr/Hf-MOF-808	Zr/Hf, defect -OH	aniline, benzyl alcohols	N,1-diphenylmethanimine	T = 120 °C, 0.6 mmol of catalyst and o-xylene as solvent, 2 h	(152)
Knoevenagel Condensation–hydrogenation	11	IY-SO3H/Rh@S-ZIF-8	-SO3H, Zn, N	p-nitrobenzaldehyde, malononitrile	2-(4-aminobenzylidene) malononitrile	toluene (5 mL), 30 °C, 2 h; 2 MPa H2, 80 °C, 12 h	(221)

^aPDEAEMA = poly[(2-diethylamino)ethyl methacrylate; IY = Integrated yolk; H₆TATAT = 5,5′,5″-(1,3,5-triazine-2,4,6-triyl)tris(azanediyl)triisophthalate; AP = 2-aminopyridine; BTC = 1,3,5-benzene tricarboxylate; Bp = 4,4′-bipyridine; N-ferrocenylmethyl-N-butyl dithiocarbamate (L1); N-ferrocenylmethyl-N-ethylmorpholine dithiocarbamate (L2); N-ferrocenylmethyl-N-2-(diethylamino)ethylamine dithiocarbamate (L3); N-4-methoxybenzyl-N-3-methylpyridyl dithiocarbamate (L4).

The MOF Hf₆(μ₃-O)₄(μ₃-OH)₄(HCO₂)₆((O₂C)₃C₆H₃)_6/3, [MOF-808(Hf)], (Figure 5a) was modified postsynthetically to give rise to a bifunctional catalyst for the reductive amination of ketones. The compound MOF-808(Hf) was exchanged to incorporate sulfonated phenylphosphines without oxidation to give a ligand attached with the MOF (Figure 5b). This ligand (SulP1/-MOF-808(Hf), Figure 5b) complexes with Ir and Rh to give a bifunctional catalyst (SulP1/-MOF-808(Hf–Ir)), containing both the metal–phosphine complexes and the Lewis acidic framework (Hf metal sites). The metalated MOF is a good example of having possible a homogeneous catalyst anchored over a heterogeneous host, which helps in the tandem reductive amination and hydro aminomethylation reactions. The catalytic tandem reaction of functionalized acetophenones with benzyl amine derivatives under 50 bar of H₂ at 90 °C gave the product N-(cyclopentylmethyl)aniline with good yield (>90%). The formation of the product involves Lewis acid catalysis of the aldehyde addition to the amines, forming the corresponding imines, which are reduced further by the Ir/Rh centers (Figure 5c).¹⁴⁵ This work is reminiscent of the anchoring of chiral homogeneous noble metal catalysts within mesoporous MCM-41 toward superior performance in allylic amination and other reactions.²

Figure 5.

(a) Structure of MOF-808. (b) Schematic of the postsynthetic exchange of sulfonated phosphines for formate groups on MOF-808(Hf). (c) Scheme for the reductive amination reaction. Reproduced with permission from ref (145). Copyright 2018 John Wiley and Sons.

A chromium MOF, OMS-MIL-101(Cr) (OMS = open metal site), was reacted with ethylene diamine, which creates basic centers in addition to the Cr acidic centers (Figure 6a).¹⁴⁶ This catalyst was useful toward multicomponent Hantzsch reactions. The condensation between aromatic aldehydes, dimedone, β-ketoesters, and ammonium acetate gave the product polyhydroquinoline with good yield (>98%) (Figure 6b).¹⁴⁶ The mechanism proceeds via the activation of the aryl halides over the acidic Cr³⁺ sites, and the -NH₂ group helps in activating the β-ketoesters.¹⁴⁶ The grafting of lysine (2,6-diamino-hexanoic acid) to (Zr)MOF-808 allowed exploration of the multifunctionality by carrying out two sequential reactions: (i) Henry condensation (base catalyzed) and (ii) Friedel/Crafts type alkylation (acid catalyzed) in one-pot solvent-free conditions.¹⁴⁸ In this cascade reaction, both grafted basic sites (aliphatic amino groups) and framework acid sites (coordinatively unsaturated Zr sites) were employed.

Figure 6.

(a) Schematic illustration for the preparation of ED/MIL-101(Cr). (b) Plausible mechanism for MIL-101(Cr)-NH₂ catalysis of the Hantzsch reaction. Reproduced with permission from ref (146). Copyright 2018 John Wiley and Sons.

A detailed kinetic study indicated that the former step is slower than the later (Figure S3). The combination of naphthalene dicarboxylic acid as the primary ligand and 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (3-BPDB) as the secondary ligand gave three-dimensional structures of Zn and Cd. The Lewis acid character (Zn/Cd center) was employed toward the condensation of amino benzaldehyde and ketones (Friedländer reaction). The Lewis basic character (-N=N-) was used for the three-component condensation that involves Knoevenagel and Michael reactions.¹⁴² In this study, a cascade reaction of two distinct base catalyzed mechanisms has been investigated. Anchoring CuI over modified UiO-67 was found to be a good catalyst for the azide–alkyne cycloaddition reaction.¹⁵⁴ The mechanism involves mediated alkyne interactions followed by the azide reaction forming the triazole derivatives. It is a different approach, as metal salts have been anchored over the MOFs instead of simple metal nanoparticles.

In a recent work, for the first time, a 4-step cascade reaction was carried out using the same strategy of spatially separated Lewis acid and base functionality in [Zn₂(SDBA)(3-ATZ)] (SDBA = 4,4′-sulfonyldibenzoic acid; 3-ATZ = 3-amino 1,2,4-triazole) (Figure 7a).¹⁷¹ The different steps that were studied involve deacetalization (Lewis acid catalyzed), Henry, and Michael reactions (Lewis base catalyzed) (Scheme 3). This reaction was possible due to the presence of additional basic sites in the MOFs (primary amine, -NH₂, and sulfonyl oxygen atoms). The four-step reaction consists of the following: the first reaction is the formation of an aldehyde (B) from the benzaldehyde dimethyl acetal (A) (Lewis acid catalyzed); the second reaction is between the nitroalkane and the aldehyde, forming (Henry reaction) 2-nitro-1-phenyl ethanol (C) (Lewis base catalyzed); the third reaction is the dehydration of (C) to give trans β-nitrostyrene (D) (Lewis base catalyzed); and finally reaction between nitromethane and (D) gives (1,3-dinitropropan-2-yl)benzene (E) (Scheme 3). The time dependent study clearly indicates that the dehydration of 2-nitro-1-phenyl ethanol to trans-β-nitrostyrene was the rate limiting step and that the Le Chatlier’s principle was in action for the reaction to proceed in the forward reaction. It is clearly a good example where the bifunctionality of the MOF was exploited toward a cascade reaction that involved multiple steps. The number of steps (3) in the base catalyzed reactions is more compared to that of the acid catalyzed one in this cascade reaction. The presence of the primary -NH₂ group, which is a strong base, helped in this reaction¹⁷¹ (Figure 7b).

Figure 7.

(a, b) Connectivity between the Zn-ATZ layers through the acid ligand (SDBA) in I and II. (c, d) Time dependent study of the one-pot tandem four-step deacetalization–Henry–Michael reactions for I and II. Reprinted with permission from ref (171). Copyright 2023 American Chemical Society.

Scheme 3. Summary of the Four-Step Cascade Reaction Involving the Deacetalization–Henry–Michael Reaction.

Reproduced with permission from ref (171). Copyright 2023 American Chemical Society.

3.2.3. Oxidation–Knoevenagel/Amination Cascade Reaction

The compounds M(BTC)(OH)(4-TPT)₂(H₂O)·(DMA)_0.5(H₂O)₂ (H₃BTC = 1,3,5-tribenzoic acid; 4-TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine; M = Co, Ni) and Ni₃(BTC)₂(4-TPT)₂(H₂O)₆·1.5H₂O have terminal H₂O molecules coordinated to the metal centers (Co/Ni) which can be removed by heating (Figure 8a).¹⁵⁰ This creates an open coordination site at the metal center, which can act as Lewis acidic center. The secondary ligand 4-TPT can be the Lewis basic center. The compounds were found to be good catalysts for the oxidation of benzyl alcohol to benzaldehyde (acid catalyzed) followed by reaction with malononitrile to give benzilidine malononitrile (base catalyzed) (Figure 8b).¹⁵⁰ A similar reaction was also carried out over the Cu₃TATAT MOF (H₆TATAT = 5,5′,5″-(1,3,5-triazine-2,4,6-triyl) tris(azanediyl)triisophthalate) compound.¹⁵¹

Figure 8.

(a) Structure of MOF highlighting the acidic and basic centers. (b) Schematic of the oxidation–Knoevenagel condensation reaction. Reproduced with permission from ref (150). Copyright 2021 Royal Society of Chemistry.

The use of Zr/Hf-MOF-808 toward the synthesis of secondary amines was established by reacting anilines and benzyl alcohols. This reaction does not require any additional base and/or external H₂.¹⁵² The reaction proceeds through the deprotonation of the alcohol by the metal center followed by the dehydrogenation to form benzaldehyde, which reacts with the amino group of the aniline, forming the final benzylaniline product (Scheme 4). There are some important observations in this reaction. The presence of a defective -OH group in the Hf-cluster metal center enhances the acidity and helps in the formation of benzaldehyde in the first step. The captured proton at the defective -OH group site also aids in the formation of the final product by reacting with the imine nitrogen formed through the condensation of the aldehyde and amine (Scheme 4). This is an interesting strategy where the proton is captured initially and later released during the final step to give the desired product.

Scheme 4. Possible Mechanism for the N-Alkylation Reaction of Aniline with Benzyl Alcohol to Form the N-Benzylaniline Product.

Adapted with permission under a Creative Commons (CC BY 4.0) from ref (152). Copyright 2021 American Chemical Society.

The encapsulation of polyoxometalate H₅PMo₁₀V₂O₄₀ (PMoV₂) into the cages of an alkylamine-modified MIL-101 was employed for the aerobic oxidation–Knoevenagel one-pot tandem reaction (Scheme S3). The main observation in this reaction is that it does not use any noble metals for the aerobic oxidation of the alcohols.¹⁵³

3.3. Anchoring Metal Nanoparticles toward Catalysis

The development of metal nanoparticles during the 90s and the associated advancements in nanoscience provide an important opportunity to explore multifunctionality in MOFs.^172−174 It may be noted that the use of well distributed supported metal catalysis (e.g., Pd or Pt in Al₂O₃, SiO₂, zeolites, and mesoporous silica) is one of the earlier examples of bifunctional catalysts.^175−177 Many of the supported catalysts are useful toward the “spillover” reactions involving hydrogen.¹⁷⁸ The noble metal and the support influence the electronic state of the metal along with its precise morphology. Today, it has been possible to prepare noble metal nanoparticles with controlled sizes and shapes,^179−182 which may be of importance in heterogeneous catalysis. The noble metal nanoparticles can be stabilized by anchoring the particles through suitable functional groups—most notably the thiol (-SH) functionality. The usefulness of noble metal centers for diverse catalytic reactions has been known over many decades.^183−187 Many of these earlier reactions are homogeneous catalytic reactions, where the recyclability of the catalyst would be difficult. In recent years, it has been possible to prepare atom precise metal nanoclusters and assemble them into extended structures.^188−192 In addition, the ease of functionalizing the ligands that can be used in the preparation of MOFs opens up an interesting possibility toward anchoring metal nanoparticles at precise locations within the MOFs. The MOFs provide reasonable thermal and chemical stability and pore and channel sizes which can be gainfully employed to explore the usefulness of metal nanoparticles toward catalytic reactions. This approach would be similar to carrying out the homogeneous reactions within heterogeneous surroundings. Catalytic reactions of this nature were attempted by anchoring organometallic complexes directly into the mesoporous compounds, notably on MCM-41 and related ones.^193−195

Most of the MOFs possess Lewis acidity, and the functional ligand provides the necessary additional reactive center (either Bronsted acidic or basic). These MOFs are already bifunctional and the anchoring of metal nanoparticles adds to the existing functionality. The main advantages of anchoring the metal nanoparticles are their small (nano) size and the availability of periodically placed metals over the surface of the MOFs, which would be useful toward many heterogeneous catalytic reactions. Much of the earlier work on such type of nanoparticle anchored MOFs was termed “ship in a bottle” catalysts.^196,197 The usefulness of this approach was shown by anchoring Au nanoparticles in a MOF, Au@Cu(II)-MOF, toward a tandem oxidation–Knoevenagel condensation reaction that involves the conversion of benzyl alcohol to benzylidene malononitrile with good yield, conversion, and selectivity.¹⁹⁸ Similarly, Pd@Ni-MOF was shown to be a good catalyst toward Suzuki coupling of aryl chlorides.¹⁹⁹ It is notable that many reactions involving Suzuki coupling are carried out by employing aryl bromides and iodides and that the presence of Pd nanoparticles uniformly distributed in a heterogeneous environment (Pd@Ni-MOF) converts the aryl chlorides with good yield as well as recyclability.¹⁹⁹ Many studies of this nature have been known in the literature.^200−205 The important nanoparticle loaded MOFs and their utility in heterogeneous catalytic studies are listed in Table 7.

Table 7. Nanoparticle Loaded MOFs and Their Utility in Catalysis^a.

Sr. No.	MOF compound	Reactive site	Nanoparticle	Catalytic reaction	Ref
1	Au@ZIF-8	Zn2+/-Nim	Au	hydrogenation of n-hexene	(311)
2	nFe3O4@Pd/ZIF-8@ZIF-8	Zn2+/-Nim	Fe3O4/Pd	hydrogenation of styrene	(312)
3	UiO-66-biguanidine/Pd	Zr4+/-NH2	Pd	Suzuki–Miyaura coupling	(313)
4	Ag@UiO-66-SH	Zr4+/-NH2	Ag	three-component A3 coupling	(314)
5	AgPd@MIL-125-NH2-PDA	Cr3+/-NH2	Ag-Pd	Suzuki coupling reaction	(315)
				hydrogenation of aldehyde
6	MOF-Pd NPs	Cu2+/-NH2	Pd	aerobic oxidation of benzyl alcohol	(316)
7	Pt@UiO-66-NH2	Zr4+/-NH2	Pt	synthesis of nitrones	(173)
8	Au-NP/Ni-Cu MOF	Ni2+/Cu2+	Au	chemical degradation of Rhodamine B	(317)
9	Au@Cu(II)-MOF	Cu2+/-N	Au	oxidation–condensation reactions	(198)
10	Co–MOF-74@Cu–MOF-74	Co2+/Cu2+	CoCu	1, 4-diphenyl-1,3-butadiene from phenylacetylene	(209)
11	Pd(0)@UiO-68-AP	Zr4+/-N	Pd	oxidation–Knoevenagel condensation	(215)
12	Au/NH2–UiO-66	Zr4+/-NH2	Au	tandem reaction	(216)
13	Pd-Au@Mn(II)-MOF	Mn(II)/N	Pd-Au	alcohol to imines	(217)
14	Al-ITQ-SO3H/Pd	Al(III), SO3H	Pd	oxidation–acetalization	(219)
15	IY-SO3H/Rh@S-ZIF-8	Zn(II)SO3H	Rh	Knoevenagel condensation–hydrogenation reaction	(221)
16	Pd@UiO-66(Hf)	Hf(IV)	Pd	Hantzsch reaction	(318)
17	Pd@NH2-UiO-66	Zr(IV)/-NH2	Pd	Suzuki coupling/asymmetric aldol condensation	(224)
18	Ni@ZrOF	Zr(IV)/ -NH2	Ni	chemical fixation of CO2	(319)
19	Au@[Zn14(L)6(O)2(H2O)3]	Zn(II)	Au	chemical fixation of CO2	(249)
20	Ag(I)@MOF-NHC	Zn(II)/-N	Ag	chemical fixation of CO2	(250)
21	Pt@MOF-5, Pt@UiO-66, Pt@UiO-66-NH2	Zr(IV)/-NH2	Pt	biomass valorization	(320)
22	[Zn(4-bpdh)]3DMF	-N	Pd	Sonogashira coupling reaction	(321)
23	Ag/UiO-66	Zr4+	Ag	3,4-dihydropirimidin-2(1H)-one synthesis	(322)
24	Au/MOF-199	Cu2+	Au	A3-coupling reaction	(323)
25	metal/UiO-66	Zr4+	Pt, Pd, Ru	oxidation of volatile organic compound	(324)
26	Pd(0)@UiO-68-AP	Zr, -NH	Pd	oxidation–Knoevenagel cascade reaction	(215)
27	Au@NH2-UiO-66	Zr, -NH2	Au	oxidation–Knoevenagel cascade reaction	(216)
28	Pd-Au@Mn(II)-MOF	Mn, Npyr	Pd, Au	oxidation–imine/acetal formation cascade reaction	(217)
29	Pd@MIL-101	Cr, N	Pd	oxidation–imine/acetal formation cascade reaction	(218)
30	Pd@Al-MOF	Al, −SO3H	Pd	oxidation–imine/acetal formation cascade reaction	(219)
31	Pd@PDEAEMA-g-UiO-66	Zr, -NH2	Pd	Knoevenagel condensation–hydrogenation	(220)
32	Pd/MIL-101-SO3H	-SO3H	Pd	hydrogenation esterification cascade reaction	(222)
33	Pd@UiO-66(Hf)	Zr	Pd	hydrogenation esterification cascade reaction	(223)

^aOPNs = organic polymer networks; PDA = polydopamine; AP = aminopyridine; NHC = N-heterocyclic carbene; 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene.

As discussed above, anchoring of noble metal particles supported over the MOFs was found to be dominant in many catalytic reactions. There have been attempts at synthesizing 3d metal nanoparticles, at the expense of the MOF framework, by anchoring them over carbonized MOFs.^206−208 It has been shown that MOFs when heated in an inert atmosphere (Ar atmosphere) or under vacuum at elevated temperatures form carbon with the metals distributed over the carbon. This carbonization process, in general, results in the formation of different forms of carbon such as amorphous carbon, graphitized carbon, or a mixture of both. Depending on the composition of the MOFs, the resulting metal nanoparticles can be either single nanoparticles or bimetallic nanoparticles supported over the amorphous carbon. It has been known that amorphous carbon can act as a good support toward many catalytic reactions.^209−214 The bimetals supported over the carbonized MOFs can be exploited toward catalytic reactions where the differences in the catalytic activity of the different metals would be important.

This strategy was employed toward the preparation of Co-Cu bimetallic nanoparticles supported on carbon (Co-C@Cu-C) by pyrolyzing Co/Cu-MOF-74.²⁰⁹ This bimetallic catalyst was found to be a good catalyst for the conversion of phenylacetylene to 1,4-diphenyl-1,3-butadiene. This reaction involves C–C coupling as well as hydrogenation reactions, and the reaction proceeds over the Co centers. The possible mechanism for this cascade reaction is given in Scheme 5. Phenylacetylene is initially activated by metallic Cu NPs, forming Cu⁺-phenylacetylide complex on the surface of Cu NPs. The metallic Co NPs in Co-C@Cu-C help in the dissociation of NaBH₄ to form Co-BH₃ and CoH, which act as the active species for the hydrogenation of the phenylacetylene. The proposed mechanism also involves the migration the hydrides on Co nanoparticles to diffuse to interact with the adsorbed Cu⁺-phenylacetylide complex. The remaining hydrides in BH₃(i-PrO)⁻ dissociate over the metallic Co NPs, which helps in the spillover of hydrogen to react with the adsorbed Cu⁺-phenylacetylide complex. The in situ formed complex between CuI-phenylacetylide dimerizes to form 1, 4-diphenyl-1,3-butadiene.

Scheme 5. Co-MOF-74@Cu-MOF-74 Derived Bifunctional Co-C@Cu-C for One-Pot Production of 1,4-Diphenyl-1,3-butadiene from Phenylacetylene.

Reprinted with permission from ref (209). Copyright 2020 John Wiley and Sons.

The pyrolysis of a cobalt MOF, [Co(C₁₄H₈O₆)(C₁₀H₈N₂)2H₂O)]·(C₃H₇NO), results in Co supported on amorphous/graphitized carbon. The Co nanoparticles were shown to be a good green catalyst for the selective reduction of nitroarenes to amines in the presence of the hydrazine as a hydrogen source.¹⁹⁹ The Co metal particles supported on the carbon matrix help in the decomposition of hydrazine to produce the hydrogen in situ. The hydrogen spillover helps in the reduction of nitroaromatics to the aniline derivatives. The examples clearly support the spillover effect in employing metal nanoparticles toward hydrogenation reactions. A mixed precursor containing Cu, Co, and Ni introduced into MIL-101 was reduced in situ with NH₃BH₃ to give Cu@Co@Ni NPs inside MIL-101 pores. The compound Cu@Co@Ni/MOF catalyzes nitroarene hydrogenation in the presence of NH₃BH₃, which supplies the necessary hydrogen for the reaction.²⁰⁴

3.3.1. Cascade Reactions Involving Anchored Nanoparticles

A bifunctional Pd(0)@UiO-68-AP catalyst, prepared using the postsynthetic approach, catalyzes aerobic oxidation of benzyl alcohol by the Pd NPs followed by reaction with malononitrile via the Knoevenagel condensation, forming benzilidine malononitrile.²¹⁵ The oxidation of the benzyl alcohol is promoted by the Pd catalyst whereas the Knoevenagel condensation is facilitated by the basic nitrogen center.²¹⁵ A similar reaction was also carried out employing Au nanoparticles anchored over NH₂-UiO-66,²¹⁶ for the selective oxidation of primary alcohols in tandem with Knoevenagel condensation reactions. A Mn(II) MOF, (MnL₂)·2CH₃OH (L = 4,4,4-trifluoro-1-(4-(pyridin-4-yl)phenyl)butane-1,3-dione), with Pd-Au bimetallic alloy nanoparticles (Pd-Au@Mn(II)-MOF) was employed as a bifunctional heterogeneous catalyst for the one-pot tandem synthesis of imines from benzyl alcohols and aniline (Figure S4). The oxidation reaction was catalyzed by nanoparticles, and the imine formation was due to the N-containing ligand.²¹⁷ Similar reactions have also been carried out with Pd nanoparticles (NPs) encapsulated in MIL-101.²¹⁸ An aluminum MOF, Al-ITQ-SH, having thiol units was used to anchor Pd nanoparticles, and the thiol moieties were converted into sulfonic groups (Bro̷nsted acid).²¹⁹ This compound catalyzes a one-pot, two-step oxidation–acetalization reaction where the oxidation of benzyl alcohol into benzaldehyde under an O₂ atmosphere was catalyzed by Pd nanoparticles, followed by the acetalization of aldehydes employing the Brönsted acid functionality. A Pd NP loaded and pH-switchable polymer-grafted UiO-66-MOF, Pd@PDEAEMA-g-UiO-66, (PDEAEMA = poly[(2-diethylamino)ethyl methacrylate]), was employed toward a biphasic Knoevenagel condensation–hydrogenation cascade reaction using different atmospheres (Figure 9a). The Knoevenagel condensation step was carried out under air atmosphere, whereas the hydrogenation reaction was carried out in hydrogen atmosphere (Figure 9b).²²⁰

Figure 9.

(a) Schematic of synthesis of Pd@PDEAEMA-g-UiO-66 through PSM. Yellow balls: Pd nanoparticles. (b) Knoevenagel condensation followed by hydrogenation cascade reactions over the Pd@PDEAEMA-g-UiO-66 catalyst. Reproduced with permission from ref (220). Copyright 2017 American Chemical Society.

A bifunctional composite compound with a macro-/microporous ZIF-8 shell and rhodium nanoparticles anchored over sulfonated cross-linked polystyrene was prepared to carry out the Knoevenagel–hydrogenation reaction.²²¹ The reactants containing the aldehyde and malononitrile in toluene solvent under a pressure of hydrogen (2 MPa) gave the benzilidene malononitrile product in near ∼100% yield. The composite catalyst was found to be robust with negligible leaching of the anchored Rh nanoparticles.²²¹ Another reaction that utilizes nanoparticle supported over MOFs is the hydrogenation–esterification reaction. The hydrogenation is catalyzed by the noble metal nanoparticles, and the esterification reaction is catalyzed by the acidic sites. The compound MIL-101-SO₃H was appended with Pd nanoparticles to give Pd/MIL-101-SO₃H.²²² This compound was active to convert furoic acid (FA) (biomass derived) into value chemicals such as ethyl tetrahydro-2-furoate (ETF). The FA was esterified with ethanol, forming ethyl furoate (EF), which was hydrogenated to give ETF (yield = 100%, selectivity >99%) (Scheme 6). A similar reaction has also been attempted over Pd@UiO-66(Hf).²²³

Scheme 6. Scheme of the Cascade Reaction Process from FA to ETF.

Reproduced with permission from ref (222). Copyright 2019 Royal Society of Chemistry.

The use of bifunctionality was shown elegantly by adding an l-proline functionality to -NH₂-UiO-66, which also had Pd nanoparticles through PSM. This modified MOF was shown to be a chiral catalyst toward Suzuki coupling (Pd centered) followed by asymmetric aldol condensation (N centered). The reaction between 1,4-bromobenzaldehyde and phenylboronic acid in EtOH–H₂O was catalyzed by Pd@NH₂-UiO-66(pro) and K₂CO₃ at 80 °C.²²⁴

As described above, the overall activity of the MOFs has been enhanced by having the nanoparticles. Though most of the studies concentrated on noble metals, there have been efforts toward other transition metals as well. As can be noted, in most of the cases one finds a good synergistic and cooperative effect between the metal nanoparticles and the overall functionality of the MOFs. The reactions, in a way, replicate many of the well-established and known organometallic catalyses in these compounds. This effort paves the way for carrying out homogeneous catalysis under a heterogeneous environment. It is likely that many other reactions would also be attempted in the future to reap the benefits of having well dispersed nanomaterials in bifunctional MOFs. This approach really makes the MOF compounds truly multifunctional.

3.4. Fixation of Atmospheric CO₂

One of the important issues of concern is the control of CO₂ in the atmosphere. CO₂ has been designated as an important greenhouse gas, and it is desirable to convert it to other useful chemicals.^225−230 There have been attempts to convert the CO₂ under atmospheric conditions, both through the catalytic route^231−233 as well as electrochemically.^234−236 The intense research, over the years, has involved conversion of CO₂ into cyclic carbonate, carboxylation, reductive N-functionalization of various amines with CO₂ to furnish N-formyl compounds, and carboxylative cyclization of propargyl amines and alcohols.^237−241 In many of these attempts, organometallic compounds, nanoparticle grafted compounds, and others have been explored.²⁴² There have been considerable efforts toward fixing CO₂ at atmospheric pressures employing MOFs, notably bifunctional ones. Some of the important studies toward the fixation of CO₂ employing MOFs are listed in Table 8.

Table 8. Summary of Atmospheric CO₂ Fixation over MOFs^a.

Sr. No.	MOF	Reactive site	Reactant	Product	Reaction conditions	Ref
1	(I–)Meim-UiO-66	Zr4+, I–	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	0.1 MPa, 120 °C, 24 h	(248)
2	MIL-IL(A)	Cr3+, Br–	SO	4-phenyl-1,3-dioxolan-2-one	2 MPa, 110 °C, 2 h	(325)
	MIL-IL(B)
3	MIL-IMAc-Br–	Cu2+-COOH, Br–	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	0.5 MPa, 60 °C, 24 h	(81)
4	Mg-MOF-74	Mg2+, Co2+, OH	SO	4-phenyl-1,3-dioxolan-2-one	2 MPa, 100 °C, 4 h	(326, 327)
	Co-MOF-74
5	UiO-66-NH2	Zr4+, NH2	SO	4-phenyl-1,3-dioxolan-2-one	2.0 MPa, 100 °C, 4 h	(328)
6	BIT-101	Zn2+, COO	PO	4-methyl-1,3-dioxolan-2-one	3 MPa, 160 °C, 24 h	(329)
	BIT-102
	BIT-103
7	1-NH2	CONH, NH2	ECH, PO	4-(chloromethyl)-1,3-dioxolna-2-one-4-methyl-1,3-dioxolan-2-one	0.1 MPa, 90 °C, 50 h, 3 MPa, 100 °C, 6 h	(330)
8	UMCM-1-NH2	Zn4O, NH2	PO	4-methyl-1,3-dioxolan-2-one	1.2 MPa, 120 °C, 24 h	(331)
9	Zn-TATAB	Zn2+, NH	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	1 atm, 100 °C, 16 h	(332)
10	Co/Ni-TATAB	Co2+, Ni2+, NH	ECHECH	4-(chloromethyl)-1,3-dioxolan-2-one	1 bar, 80 °C, 15 h	(279)
11	Zn-BTC-2MeIm	Zn2+, 2MeIm	ECH, PO	4-(chloromethyl)-1,3-dioxolan-2-one	3.0 MPa, 100 °C, 6 h	(333)
12	Co(tp)(bpy) MOF-508a	Co2+, Zn2+, free pyridine N	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	1 MPa, 100 °C, 8 h	(334)
13	(Me2NH2)2·[Zn8(Ad)4(DABA)6O]·7DMF}n	Zn2+, Ad, and NH2	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	1 bar, 100 °C, 24 h	(335)
14	ZIF-8	Zn2+, Im	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	7 bar, 100 °C, 4 h	(336)
15	ZIF-90	Zn2+, CHO	PO	4-methyl-1,3-dioxolan-2-one	2 MPa, 120 °C, 8 h	(337)
16	CZ-ZIF	Zn2+/Co2+, HmIm	ECHPO	4-(chloromethyl)-1,3-dioxolan-2-one	7 bar, 100 °C, 4 h	(338)
17	ZIF-67	Co2+, HmIm	ECHPO	4-(chloromethyl)-1,3-dioxolan-2-one	10 bar, 120 °C, 6 h	(339)
18	Ti-ZIF	Ti4+, Im	SO	4-phenyl-1,3-dioxolan-2-one	1.72 bar, 100 °C, 8 h	(340)
19	MIL-101-P(n-Bu)3BrMIL-101-N(n Bu)3Br	Cr3+, Br–	POPO	4-methyl-1,3-dioxolan-2-one	2.0 MPa, 80 °C, 8 h	(341)
20	F-ZIF-90	Zn2+, I–	AGE	4-((allyloxy)methyl)-1,3-dioxolan-2-one	1.17 MPa, 120 °C, 6 h	(342)
21	IL-ZIF-90	Zn2+, I–	PO	4-methyl-1,3-dioxolan-2-one	1.0 MPa, 120 °C, 3 h	(343)
22	ZnTCPP⊂(Br)Etim-UiO-66	Zn2+, Br–	AGE	4-((allyloxy)methyl)-1,3-dioxolan-2-one	1 bar, 140 °C, 14 h	(344)
23	Salen-Co(23%)⊂(Br–)Etim-UiO- 66	Co3+, Br–	SO	4-phenyl-1,3-dioxolan-2-one	0.1 MPa, 120 °C,12 h	(345)
24	UiO-67-IL	Zr4+, Br–	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	1 atm, 90 °C, 12 h	(346)
25	FJI-C10	Cr3+, Br–	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	1 bar, 80 °C, 12 h	(347)
26	IL@MIL101-SO3H(4)	Cr3+, Br–	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	1 atm, 90 °C, 24 h	(348)
27	MIL-101-IMBr-6	Cr3+, Br–	PO	4-methyl-1,3-dioxolan-2-one	0.8 MPa, 80 °C, 4 h	(349)
28	IL/MIL-101-NH2	-COOH, NH2	PO	4-methyl-1,3-dioxolan-2-one	1.3 MPa, 120 °C, 1 h	(350)
29	polyILs@MIL-101	Cr3+, Br–	SO	4-phenyl-1,3-dioxolan-2-one	1 bar, 70 °C, 48 h	(351)
30	MIL-101-IP	Cr3+, Br–	PO	4-methyl-1,3-dioxolan-2-one	1 atm, 25 °C, 48 h	(352)
31	IL@ZIF-8(Zn/Co)	Zn2+, Co2+, Br–	SO	4-phenyl-1,3-dioxolan-2-one	1 atm, 100 °C, 24 h	(353)
32	F-IRMOF-3-2d	Zn4O, -NH2, I–	POPOPO	4-methyl-1,3-dioxolan-2-one	2 MPa, 140 °C, 1.5 h	(354)
	F-IRMOF-3-4d
	F-IRMOF-3-6d
33	F-IRMOF-3(MeI)	Zn-OH, I–	AGE	4-((allyloxy)methyl)-1,3-dioxolan-2-one	1.2 MPa, 120 °C, 6 h	(355)
	F-IRMOF-3(BuI)
34	MIL-101-tmzOH-Br	Cr3+, OH–, Br–	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	1 MPa, 80 °C, 2 h	(356)
35	2MeIm@Co-BTC-x	Co2+, MeIm	ECHECH	4-(chloromethyl)-1,3-dioxolan-2-one	3.0 MPa, 90 °C, 5 h, 3.0 MPa, 90 °C, 7 h	(357)
36	Cr-MIL-101-[BuPh3P]Br	Cr3+, Br–	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	1 MPa, 80 °C, 2 h	(358)
37	[TMPyPMn(I)]4+(I–)4@ZIF-8	Zn2+, Mn2+, I–	ECH	4-(chloromethyl)-1,3-dioxolan-2-one	1 bar, 100 °C, 36 h	(359)
38	[Zn(II)NMeTPyP]4+[I–]4@PCN- 224	Zn2+, I–	ECHPO	4-(chloromethyl)-1,3-dioxolan-2-one-4-methyl-1,3-dioxolan-2-one	0.8 MPa, 90 °C, 24 h	(360)

^aAbbreviations: ECH = epichlorohydrin; PO = propylene oxide; SO = styrene oxide; AGE = allyl glycidyl ether; dhtp = 2,5-dihydroxyterephthalate; NH₂-BDC = 2-aminoterephthalate; aip = 5-aminoisophthalic acid; NIP = 5-nitroisophthalic acid; L = N4,N4′-di(pyridine-4-yl) biphenyl-4,4′-dicarboxamide; Im = imidazole; BTC = 1,3,5-benzene tricarboxylate; HmIm = 2-methylimidazole; ICA = imidazole 2-carboxaldehyde; TATAB = 4,4′,4″-s-triazine-1,3,5-triyl-tri-p-aminobenzoic acid; DABCO = 1,4-diazabicyclo[2.2.2]-octane; MIL = Materials Institute Lavoisier; UiO = University of Oslo; BIT = Beijing Institute of Technology; Hip = 5-hydroxyisophthalic acid; Bpy = 4,4′-bipyridine; BTB = 1,3,5-tris(4-carboxyphenyl)benzene; DMF = N,N′-dimethylformamide; Ad = adeninate; DABA = 2,2′-dimethyl- 4,4′-azobenzoate; 2-MeIm = 2-methylimidazole; ZIF = zeolitic imidazolate framework; 2-F-BIM = 2-(furan-2-yl)-1H-benzo[d]imidazole; TCPP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin; IL = ionic liquid; ICA = imidazole-2-carboxyaldehyde; Etim = ethyl imidazolium; polyILs = poly(ionic liquids); IP = ionic polymer; MeI = methyl iodide; BuI = butyl iodide; IRMOF = isoreticular metal–organic framework; IMAc = 1H-imidazole-1-acetic acid; PCN = Porous coordination network; Zn(II)NMeTPyP = 5,10,15,20-tetrakis(1-methylpyridinium-4-yl) zinc(II) porphyrin; [(Etim-H₂BDC)⁺(Br)] = 2-(3-ethyl-imidazol-1-yl)-terephthalic acid; ImBDC = 2-(imidazole-1-yl)terephthalate; salen-Co(III) - N,N′-bis(3-carboxylsalicylidene)-1,2-cyclohexanediamino cobalt(III) acetate; [BuPh3P]Br = 4-(bromobutyl)triphenylphosphonium bromide; MPImBr = 1-methyl-3-propylimidazolium bromide; HmIm = 2-methylimidazole; 2-Br-BDC = 2-bromoterephthalate; L = (Br) allylium-2-bp functionalized biphenyl dicarboxylic acid.

Tetra(4-pyridyl)porphyrin (H₂TPyP) was employed along with 4,4′-oxybis(benzoic acid) (H₂OBA) to form a 2D compound, [Zn₂(C₄₀H₂₄N₈)_0.5(C₁₄H₈O₅)(DMA)](DMA)(H₂O)₆.²⁴³ The Zn²⁺ ions were ion-exchanged with the Cu²⁺ in a postsynthetic modification, and the Cu compound was employed for the cyclic carbonate formation reaction with tetra-n-tert-butylammonium bromide (TBAB) as a cocatalyst, which gave cyclic carbonates with high yield.²⁴³

A plausible reaction mechanism for the cycloaddition of CO₂ to cyclic carbonates using bifunctional MOFs is given in Scheme 7. The zirconium-phosphonate MOF Zr(H₄L) (H₈L = tetraphenylsilane tetrakis-4-phosphonic acid) was shown to be a good catalyst toward the CO₂/epoxide coupling reaction. The presence of a Bro̷nsted acid (protonated phosphonate) along with the Lewis acidic (Zr metal) functionality helped in catalyzing the reaction forming cyclic carbonates (Figure S5).²⁴⁴ A copper MOF, [Cu(2,5-BPTA)(bpy)(H₂O)] (2,5-BPTA = 2,5-bis(prop-2-yn-1-yloxy)terephthalic acid and bpy = 4,4′-bipyridine, Figure 10a), was shown to catalyze CO₂ to cyclic carbonates in the presence of TBAB as a cocatalyst.²⁴⁵ The Cu²⁺ ions acts as the Lewis acid centers to polarize the epoxide oxygen and increase the electrophilicity of the carbon. The Br^– from the TBAB helps in the ring opening of the epoxides. The Cu²⁺ acts as the center for the subsequent binding of CO₂ with the ring opened epoxide along with the elimination of the Br^– by ring closing, forming the cyclic carbonates (Figure 10b).²⁴⁵ As noted in Table 8, the number of examples of the conversion of CO₂ to cyclic carbonates are many. It appears that the presence of stronger acidic sites along with basic sites helps toward catalyzing this reaction. Lanthanide MOFs (Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb; NTB = 4,4′,4″-nitrilotribenzoic acid; NMP = N-methylpyrrolidone) have also been shown to effectively catalyze the CO₂ conversion.²⁴⁶ The compounds catalyze the reaction between CO₂ and epichlorohydrin in the presence of n-Bu₄NBr as the cocatalyst with a yield of 97%.²⁴⁶

Scheme 7. Plausible Reaction Mechanism for Cycloaddition of CO₂ and Epoxides Using Bifunctional MOFs.

LA = Lewis acid; LB = Lewis base; X^® = halide ions.

Figure 10.

(a) 2D layer of the [Cu(2,5-BPTA)(bpy)(H₂O)] MOF. (b) Possible mechanism for the formation of cyclic carbonate. Reproduced with permission from ref (245). Copyright 2023 American Chemical Society.

In many of the cycloadditions of CO₂ to epoxide, a cocatalyst is generally employed. The role of the cocatalysts such as TBAB, in the cycloaddition of CO₂, is to enhance the catalytic activity by participating during the ring opening step. A zeolitic imidazole framework-78 (ZIF-78) was found to be a good catalyst (Figure 11) toward the CO₂/propylene oxide (PO) cycloaddition, forming propylene carbonate (PC) in the absence of any cocatalyst. The compound ZIF-78 exhibits a strong basic framework due to the presence of imidazolate (-N) linkers with the Zn centers providing the Lewis acidity. This reaction, however, was carried out at an elevated temperature and pressure.²⁴⁷ The mechanism involves the formation of an adduct with propylene oxide and the Lewis acid centers, which is followed by the ring opening. The Lewis basic -NH groups attack on the O^– ion, and CO₂ forms the cyclic carbonates (Scheme 8)

Figure 11.

(a) Structure of ZIF-78, (b) Lewis acidic Zn center and Lewis basic N (Im) center. Reproduced with permission from ref (247). Copyright 2017 Elsevier.

Scheme 8. Proposed Catalytic Mechanism for the CO₂/PO Cycloaddition Reaction Using the ZIF-78 Heterogeneous Catalyst.

Reproduced with permission from ref (247). Copyright 2017 Elsevier.

To date, several examples of heterogeneous bifunctional MOF catalysts have been established. The quaternary ammonium, phosphonium, imidazolium, triazolium, or pyridinium functionalized linkers can be successfully introduced as pendant groups on organic linkers, while preparing the MOFs. This would eliminate the need to use the cocatalyst during the cycloaddition of CO₂ to carbonates. A N-containing ligand was introduced by the postsynthetic modification on UiO-66 to result in UiO-66-Py (pyridine) and UiO-67-Bpy (bipyridine) (Figure S6). N-Alkylation of the bipyridine and pyridine centers converts the neutral framework into an ionic one (R = Me, Et, nPr, iPr; X = I, Br, Cl) and gives rise to bifunctional behavior. This modified MOF catalyzes the cycloaddition of propylene oxide to CO₂, with yields of ∼99% (Figure S6).⁵¹ A halide functionalized bifunctional imidazolium (Im) zirconium metal–organic framework (Zr-MOF), (I^–)Meim-UiO-66, was shown to be an efficient and recyclable heterogeneous catalyst for the cycloaddition of CO₂ with epoxides without the use of cocatalysts (Figure S7). The presence of ionic I^– in the structure acts as the Lewis base, and the Bro̷nsted-acid (Zr-OH/Zr-OH₂) helps in the formation of cyclic carbonate with 100% conversion of styrene oxide.²⁴⁸

The use of 2,6-di(1H-tetrazol-5-yl)naphthalene (H₂NDTz) and 2,6-bis(4-(1H-tetrazol-5-yl)phenyl)naphthalene (H₂NDPhTz) with cobalt gave two cobalt MOFs, which catalyze the epoxidation–carboxylation of styrene with CO₂ in tandem fashion. The control experiments revealed the synergic effect between the oxidant and the MOF, as ∼4% of styrene oxide was obtained in the absence of catalyst. This work highlights the utility of nitrogen-rich heterocycles in MOFs for efficient development of bifunctional catalysts.¹⁵⁶

In addition to utilizing the Lewis acidic and basic sites of MOFs toward CO₂ fixation, it has been shown that the use of metal nanoparticles also catalyzes such reactions. Gold nanoparticles anchored over the MOF [Zn₁₄(L)₆(O)₂(H₂O)₃] (L = 2,6-bis(2′,5′-dicarboxylphenyl)pyridine) were employed toward the cycloaddition of CO₂ with epoxides with TBAB as the cocatalyst (Scheme 9).²⁴⁹ A bifunctional catalyst having Ag(I) ions grafted onto N-heterocyclic carbene (NHC) sites in a MOF was found to be good catalyst for the efficient fixation of CO₂ with propargyl alcohols. The availability of catalytically active alkynophilic Ag(I) and CO₂-philic NHC sites in the 1D pores of the MOF helped in the fixation of CO₂ with propargyl alcohols (Scheme 10).²⁵⁰ Silver nanoparticles immobilized on MOFs were shown to effectively catalyze the carboxylation of terminal alkyne with CO₂ to the corresponding carboxylic ester employing a porphyrin-based compound, [Zn₃(C₄₀H₂₄N₈)(C₂₀H₈N₂O₄)₂(DEF)₂](DEF)₃ (DEF = N,N-diethylformamide).²⁴² In this reaction Cs₂CO₃ acts as the cocatalyst. CO₂ fixation was achieved in a modified ZIF-90 compound where the carboxylate functionality provided the necessary catalytic reactive center. The catalytic activity of the modified ZIF-90-C was found to be ∼100% at 50 °C for the fixation of CO₂ in the presence of PhSiH₃ as the H-donor and morpholine.^238,251

Scheme 9. Tentative Mechanism for CO₂ Cycloaddition with Epoxides.

TBAB = Bu₄NBr. Reproduced with permission from ref (249). Copyright 2019 American Chemical Society.

Scheme 10. Proposed Mechanism for the Coupling of CO₂ with Propargylic Alcohol Catalyzed by Ag(1)@MOF-NHC.

Reproduced with permission from ref (250). Copyright 2022 Royal Society of Chemistry.

The number of catalytic reactions described and discussed here is indicative of the potential that the MOFs provide as a catalyst. The examples given in this review are to highlight the utility of MOFs toward a particular reaction. For more detailed discussion about the catalytic reactions and their possible mechanism, readers may need to look at the original references.

4. Concluding Remarks

The modern world is driven by the need to have clean technology where wastages are minimized. In the area of applied catalysis, the emergence of zeolites with precise and quantifiable Brönsted acidity more or less replaced the need to have corrosive ligand acids (such as HF, H₂SO₄, etc.) toward alkylations and isomerization of hydrocarbons.²⁵² The strategy to design new optimal catalyst, from the available knowledge of framework structures, with uniform pore/channel size having well dispersed catalytically active sites may require insights from computational design. Such an approach is likely to gain pace, and designer catalysts for a particular reaction would be feasible in the future. The flexible and ever ready synthetic chemist may be adept toward this new reality to reap the benefits. For the brighter side, as it is known in enzyme catalysis, “the placement of appropriate reactive groups in the right environment” may be enough for a designed catalyst.²⁵³

The many examples presented in this article already provide the needed enthusiasm to pursue the design strategy actively. The dual functionality that provides both the acidic as well as basic sites is unique to MOFs, as these can be achieved with reasonable ease. In addition, the “ship in a bottle” type catalysts are another facet of MOF compounds, where the electronic behavior of the participating moieties can be fine-tuned to achieve better catalytic behavior. MOF catalysis can be considered to be another variant of inorganic enzyme catalysis.²⁵⁴ Normally enzymes have a well-defined catalytic site (cavity) and molecular selectivity (shape and regio- or enantioselectivity). In MOFs, one finds the same features as exemplified in this article.

As highlighted in this article, many MOFs are useful in the synthesis of fine chemicals. The lack of stability at elevated temperatures and pressures would hinder the usage of MOFs in oil refining, petrochemical synthesis, etc. The development of MOFs as industrial catalysts needs to overcome the following requirements: (1) reproducibility and large-scale synthesis; (2) processability as extruded pellets and membranes; (3) quantifications of relative acidities; and (4) lack of quantifiable hydrophobicity and hydrophilicity. The number of MOFs that are stable and amenable to structural manipulations are limited to ZIFs, UiO, and the MIL family of compounds.^255,256 There is considerable scope to expand this for catalysis.

The areas where MOFs can be useful could be in converting biomass as feedstocks in the synthesis of sugars and related products.²⁵⁷ Exploration of MOFs as supports and as composite materials also requires more attention. The structural defects that may be inherent in many MOFs need to be studied carefully, as some of the structural defects could be important for the overall catalytic activity. These are just a few of the aspects that may be worth investigating in the future so as to arrive at more robust and functional catalysts that are based on MOFs.

In addition, the use of computational approaches, especially in making newer and designer MOFs, would be beneficial. The various studies with respect to the MOF chemistry and structures indicate that they are on firm and sturdy foundations. This would be a boon for further growth in heterogeneous catalysis.

Data Availability Statement

The data underlying this study are available in the published articles and its Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsorginorgau.3c00033.

• Generation of different functionality in the MOF (Scheme 1), bifunctional modification of MOF-101 (Scheme S2), synthetic process of PMoV₂@DETA-MIL-101 (Scheme S3), structure of PCN-700-MB and the steps in the synthetic procedure for the conversion of PCN-700 to PCN-700-MB (Figure S1), MIL-101(Cr)-SO₃H catalyzed one-pot three component condensation reaction (Figure S2), simplified model of two consecutive reactions A → B → C for the calculation of kinetic rate constants (Figure S3), synthesis of Mn(II) MOF and Pd-Au@Mn(II) MOF (Figure S4), coordination environment in Zr(H₄L) with thermal ellipsoids set at 50% probability (Figure S5), syntheses of ionic UiO-67-Bpy (bipyridine) and UiO-66-Py (pyridine) (Figure S6), syntheses of Meim-UiO-66 and (I^–)Meim-UiO-66 via reticular chemistry and by a PSM method, respectively (Figure S7) (PDF)

Author Contributions

CRediT: Srinivasan Natarajan conceptualization, writing-review & editing; Krishna Manna writing-original draft.

The authors declare no competing financial interest.