Fluorinated Catalysts

Fluorinated Catalysts

Introduction

Back in the 1970s, it has been recognized for some time that the incorporation of fluorine in oxide catalysts (for example, alumina, silica-alumina, or zeolites) enhances their activity for acid-catalyzed reactions such as cracking, isomerization, alkylation, polymerization, and disproportionation tonic sites. Because the incorporation of fluorine increases the activity by enhancing the acidic properties of the catalyst. Since that time, interest in fluorinated catalysts has increased greatly. The incorporation of fluorine has a significant effect on the structural transformation, surface engineering, electronic state tuning, and the easy formation and exposure of the active phase of the catalyst, thus achieving a good effect. Therefore, in recent years, a large number of fluorinated catalysts have been synthesized and reported.

Types of Fluorinated Catalysts

Today, there are many types of fluorinated catalysts, and they include fluorinated oxide catalyst, metal fluoride catalyst, fluorinated organocatalyst, fluorinated metal complex catalyst, fluorinated metal-organic frameworks (F-MOFs) catalyst, etc. The first two types of fluorinated catalysts have been extensively studied in the early years, while recent research has focused on the latter three.

• Fluorinated organocatalyst: This type catalyst is made by incorporation of the appropriate size and number of fluorine or fluoro-containing groups (usually perfluoroalkyl groups) to the organic catalyst, which is mainly produced to solve the problem that non-fluorinated organocatalysts cannot be recovered in large quantities. Fluorinated organocatalyst includes fluorinated amine catalysts, fluorinated cinchona base catalyst, fluorinated thiourea catalyst, fluorinated 1, 2-diamine derivative catalyst, etc. The fluorinated organocatalyst does not contain metal, and the catalyst can be recycled, which has the advantages of green, environmental protection and low cost, making the development of such catalysts is attractive.

Fig.1. Two fluorinated cinchona base catalysts.

• Fluorinated metal complex catalyst: This type catalyst is formed by introducing fluorine atoms or fluoro-containing groups to the ligand of the complex catalyst. The introduction of fluorine atoms in the ligand can increase the space volume of the ligand, and can enhance the electron absorption performance of the ligand, which will reduce the electron cloud density in the metal center. These two changes can significantly change the activity and selectivity of the metal complex catalyst. There are many types of fluorinated metal complex catalysts, and common ones include fluorinated ruthenium complex catalysts, fluorinated cobalt complex catalysts, fluorinated nickel(II) complex catalysts, fluorinated iridium complex catalysts, fluorinated titanium complex catalysts, fluorinated iron complex catalysts and so on. These fluorinated metal complex catalysts can be used to catalyze a variety of reactions.

Fig.2 Two fluorinated cobalt(II) complexes catalysts ^[1].

• Fluorinated metal-organic frameworks (F-MOFs) catalyst: This type catalyst is synthesized by fluorine-containing ligands and metal ions. As we all know, MOFs as catalyst have a good catalytic activity with a broad application. However, compared to MOFs, F-MOFs with fluorinated channels or cavities are expected to have increased catalytic activity. In addition, F-MOFs have increased thermal stability, higher gas affinity, selectivity, etc. Therefore, F-MOFs as catalyst have been favored by many scholars in recent years. For example, Joharian et al ^[2]. synthesized two fluorinated metal–organic frameworks [Zn₂(hfipbb)₂(4-bpdh)]·0.5DMF (TMU-55) and [Zn₂(hfipbb)₂(4-bpdb)]·2DMF (HTMU-55) with different pillars and hydrophobic structures by solvothermal method. The two F-MOFs catalyst exhibit good catalytic activity with excellent conversions even in water at room temperature using a low amount of catalyst (0.5 mol%) and a short reaction time (5 min).

Fig.3 Synthetic details of the two F-MOFs [Zn₂(hfipbb)₂(4-bpdh)]·0.5DMF (TMU-55)
and [Zn₂(hfipbb)₂(4-bpdb)]·2DMF (HTMU-55).

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References

1. Zhang Q., et al. Fluorinated cobalt catalysts and their use in forming narrowly dispersed polyethylene waxes of high linearity and incorporating vinyl functionality[J]. Catalysis Science & Technology, 2021, 11(2): 656-670.
2. Joharian M., et al. Water-stable fluorinated metal–organic frameworks (F-MOFs) with hydrophobic properties as efficient and highly active heterogeneous catalysts in aqueous solution[J]. Green chemistry, 2018, 20(23): 5336-5345.