- Catalytic coupling of epoxides and CO2 to cyclic carbonates by carbon nanotube-supported quaternary ammonium salts
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Quaternary ammonium chlorides bound to multi-walled carbon nanotubes as a catalyst for coupling of CO2 and epoxides to produce cyclic carbonates were explored. Reaction variables such as the epoxide structure, the length of alkyl substituents in the quaternary ammonium salts and the spacer chain on the catalytic performance were discussed. The yield of the cyclic carbonates varied between 7 and 89% after 6 h at 110 °C under low pressure (2 MPa of CO2). The epoxide:catalyst mass ratio was 20-30, while 1 mmol g-1 of the quaternary salt was grafted on the carbon nanotubes. A synergy between carboxyl moiety and ammonium moiety grafted on carbon nanotubes was found, and a strong impact of the length of the spacer group used for grafting of the quaternary ammonium salt on nanotubes was observed. The best performance was achieved with short (2 carbon atoms) and long (10 atoms) spacer groups, while a middle-sized spacer group (6 atoms) was not suitable. The length of the alkyl chain of the substituents of the ammonium salt (head group) had a low impact where ethyl and methyl groups performed better than butyl. The reactivity of epoxides was as follows: epichlorohydrin > propylene oxide > styrene oxide. Observations were rationalized by a mechanism where Br?nsted's sites on the surface of nanotubes play an important role during carboxylation of epoxides. The catalyst can easily be separated by filtration recycled without a significant decrease in the catalytic activity if dried properly between the runs.
- Baj, Stefan,Krawczyk, Tomasz,Jasiak, Katarzyna,Siewniak, Agnieszka,Pawlyta, Miros?awa
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- Bifunctional one-component catalysts for the addition of carbon dioxide to epoxides
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Several bifunctional ammonium salts were synthesized and employed as one-component catalysts for the conversion of CO2 and epoxides to produce cyclic carbonates. These catalysts show superior activities compared to their monofunctional analogs. A turnover number of up to 693 and a turnover frequency of up 392h-1 could be achieved for the best catalyst. Moreover, the effect of various solvents has been studied. All employed solvents and the product formed had a negative influence on substrate conversion. The scope and limitation of the reaction has been studied carefully for two general reaction protocols at 45 and 90-°C. In over 20 examples, the isolated yields after filtration were 90-%. In addition, we present the first organocatalyzed synthesis of a cyclohexene-based naturally occurring cyclic carbonate, and its molecular structure was determined by XRD. Furthermore, we demonstrate that the reaction can be performed even on a multigram scale and can be monitored by insitu FTIR spectroscopy. Couple up: Recyclable bifunctional ammonium salts are identified as one-component catalysts for the 100-% atom-economic coupling reaction of CO2 and epoxides even at 45-°C. The alcohol moiety that donates a hydrogen bond accelerates the catalytic reaction remarkably. This metal and solvent-free process can be performed on a multigram scale and is applied to the synthesis of a naturally occurring cyclic carbonate.
- Büttner, Hendrik,Lau, Kornelia,Spannenberg, Anke,Werner, Thomas
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p. 459 - 467
(2015/03/04)
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