Abstract
The development of an efficient solid catalyst for Friedel–Crafts (FC) reactions is of great importance to organic synthetic chemistry. Herein, we reported the hafnium-doped mesoporous silica catalyst Hf/SBA-15 and its first use for Friedel–Crafts alkylation reactions. Catalysts with different Si/Hf ratios were prepared and characterized, among which Hf/SBA-15(20) (Si/Hf = 20:1) was the most active catalyst, offering up to 99.1% benzylated product under mild reaction conditions. The influences of reaction conditions on the product were systematically investigated and compared. Pyridine-IR characterization of the catalyst showed that Lewis acid formed the primary active sites for the Friedel–Crafts alkylation reaction. X-ray photoelectron spectroscopy (XPS) characterization revealed that the electron shift from the Hf center to the silica framework resulted in a more active Lewis metal center for FC reactions. Moreover, the catalyst was successfully applied to the alkylation reaction with different alcohols and aromatic compounds. Finally, the Hf/SBA-15(20) catalyst also showed good recyclability in the recycling runs, demonstrating its high potential of being used for large scale FC reactions in the industry.
Highlights
Friedel–Crafts (FC) alkylation is one of the most important organic chemistry reactions that allow the production of substituted aromatic compounds through the formation of new C–C bonds [1,2]
To further explore the potential of mesoporous materials supporting Group IVB metals in the alkylation reaction, we extended our research by using Zr and Hf as the center metals for the FC reaction with benzyl alcohols as the benzylating agents
We found that only a moderate yield could be achieved over Amberlyst-15 and H-β, with even less yield achieved over H-ZSM-5, implying that the catalytic efficiency of Hf/SBA-15(20) in the FC alkylation of benzyl alcohol and toluene was very high
Summary
Friedel–Crafts (FC) alkylation is one of the most important organic chemistry reactions that allow the production of substituted aromatic compounds through the formation of new C–C bonds [1,2]. Kaper et al [15] reported the use of mixed oxide SiO2-ZrO2 for the FC alkylation of anisole and benzylic or allylic alcohols and they claimed the importance of creating a mesoporous structure of the catalysts, which enabled better mass diffusion during the reaction. Of all these solid catalysts, mesoporous Lewis type catalysts are more promising and practical for FC reaction in terms of their high reaction efficiency, better thermal stability, and recyclability
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