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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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.

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.

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.