Catalytic coupling of epoxides and CO2 to cyclic carbonates by carbon nanotube-supported quaternary ammonium salts

•Immobilized quaternary ammonium salts as a catalyst for cyclic carbonates synthesis.•Strong synergistic effect between quaternary ammonium and carboxylic moieties found.•Length of alkyl chain and anhydrous conditions crucial for activity.•Retention of catalyst's activity after at least five cycles.

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 Brønsted'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.

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