Transition metal catalysts are important industrial catalysts. The combination of CDs with metal catalysts can simultaneously exert the catalytic performance of metals and the molecular recognition and phase transfer functions of CDs. The co-catalytic systems formed by CDs and transition metals can participate in a variety of catalytic reactions. The following describes the application of catalyst systems formed by CDs and transition metals of different valence states.
CD/Cu(I) Catalysis
Some metal catalysts for C-C coupling reactions and click reactions have shortcomings such as product contamination and high cost. CD can be combined with monovalent copper Cu(I) cations to synthesize a new catalyst, which not only improves the reaction yield but also is recycled without causing environmental pollution.
Cravotto et al. synthesized a new solid cross-linked β-CD-based catalyst by tightly embedding Cu(I) cations into the cavity of β-CD and physically activating it through microwave radiation. This β-CD/Cu(I) catalyst has a unique polar structure, is particularly suitable for microwave-assisted reactions, and can reach the best yield in a short time. This CD-based catalyst has been successfully used in the most common click reaction, namely the alkyne/azide [3+2] cycloaddition with benzyl azide and phenylacetylene (Fig. 1). When the reaction temperature is 70 °C and the reaction condition is 100 W microwave radiation, the reaction can be completed in only 10 minutes [1].
Fig. 1 Alkyne/azide [3+2] cycloaddition of benzyl azide and phenylacetylene [1].
CD/Cu(II) Catalysis
There are many studies on the catalytic systems of CD and Cu(II). It is worth noting that homogeneous catalysts from Cu sources have shortcomings such as being difficult to separate and recover, cytotoxic, and easily destroyed during the reaction process. In contrast, heterogeneous catalysts are easier to separate and recover and have low toxicity in nature. As a compatible material for the formation of many metal ion complexes, CD was used to form a copper-CD complex supported on insoluble supports, which is a heterogeneous catalyst.
Nie et al. immobilized a copper-γ-CD complex on hexagonal boron nitride (h-BN) to prepare a heterogeneous catalyst h-BN@γ-CD@-Cu(OAc)2 for the first time (Fig. 2). It is a novel and green heterogeneous catalytic system that can be used for the multicomponent 1,3-dipolar cycloaddition reaction to synthesize 1,2,3-triazole derivatives. The catalyst has the advantages of being highly active, mild, recycled, and reused. The h-BN@γ-CD@Cu(OAc)2 catalyst also has good functional group tolerance and can catalyze reactions in the aqueous phase with high yield [2].
Fig. 2 Structure of h-BN@γ-CD@Cu(OAc)2 catalyst [2]
Other Catalyst Systems
In addition, CD can also be used to synthesize complexes with transition metals in other valence states and serve as catalytic systems. Trivalent Fe(III) ions and tetravalent titanium Ti(IV) ions were used to prepare catalytic systems with CDs, respectively, as described below:
- Instance of CD/Fe(III) Catalysis
Wang et al. prepared a magnetic Fe3O4@β-CD nanocomposite via Fe(III) ions and β-CD (Fig. 3a) and used it as a heterogeneous Fenton-like catalyst to catalyze the degradation of 4-chlorophenol (4-CP) and chlorobenzene (CB). The results showed that Fe3O4@β-CD exhibited a higher catalytic activity than Fe3O4 toward 4-CP and CB degradation. In addition, Fe3O4@β-CD also exhibits stable mechanical strength and good reusability [3].
- Instance of CD/Ti(IV) Catalysis
Sharavath et al. synthesized TiO2-β-CD-graphene (TiO2-CD@GNS) nanocomposite via Ti(IV) ions and β-CD by simple and low temperature in situ method (Fig. 3b). β-CD served as a stabilizing agent to fix graphene in the water phase, preventing it from aggregating. This catalyst system has high photocatalytic activity. Using TiO2-CD@GNS under visible light, 100% photodegradation of methylene blue can be achieved within 25 minutes, which shows its efficient photocatalytic activity [4].
Fig. 3 (a) Schematic diagram of the preparation of the Fe3O4@β-CD catalyst [3]; (b) Preparation of TiO2-CD@GNS nanocomposite [4].
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References
- Cravotto, G.; et al. A green approach to heterogeneous catalysis using ligand-free, metal-loaded cross-linked cyclodextrins. Green Proc. Synth. 2012, 1: 269.
- Nie, R.; et al. Copper-c-cyclodextrin complexes immobilized on hexagonal boronnitride as an efficient catalyst in the multicomponent synthesis of 1,2,3-triazoles. J. Catal. 2016, 344: 286-292.
- Wang, M. L.; et al. Fe3O4@β-CD nanocomposite as heterogeneous Fenton-like catalyst for enhanced degradation of 4-chlorophenol (4-CP). Appl. Catal., B 2016, 188: 113-122.
- Sharavath, V.; et al. Low temperature synthesis of TiO2-β-cyclodextrin–graphene nanocomposite for energy storage and photocatalytic applications. Electrochim. Acta. 2016, 210: 385-394.
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