Heterogenously catalyzed one-step alkylation and carboxylation of N -methylimidazole with dimethyl carbonate in continuous flow

Abstract The alkylation of N -methylimidazole with dimethyl carbonate yields the respective solid zwitterion N,N ′-dimethylimidazolium-2-carboxylate, a versatile precursor for manufacturing non-halide-containing ionic liquids (ILs). This reaction requires high p,T-conditions, i.e., up to 200°C and 70 bar pressure, and even then requires reaction times up to several days under autoclave batch conditions. To perform these reactions continuously in stainless steel tubes with inner diameters in the millimeter range, precipitation of the salts must be suppressed to avoid any blockage. The achievable yield of the zwitterion is remarkably increased when a packed Al 2 O 3 catalyst is used instead of an empty tube. For example, with a catalyst-packed column, a nearly 100% yield of N,N ′-dimethylimidazolium-2-carboxylate can be achieved within 20 min at 200°C, whereas only a 67% yield in 50 min can be obtained without a catalyst at the same temperature. The transfer of the autoclave process into continuous flow is advantageous and allows a more economical scale-up.