Capture and Catalytic Reduction of SO2 with H2 to Elemental Sulfur by Novel Guanidinium-Based Deep Eutectic Solvents

In this work, a series of novel functionalized deep eutectic solvents (DESs) using 1,1,3,3-tetramethylguanidinium tetrazolate ([TMG][Tetz]) as a hydrogen-bond acceptor (HBA) and azole derivatives as hydrogen-bond donors (HBDs) were synthesized for the capture and catalytic reduction of SO2. [TMG][Tetz]:imidazole (Im) (1:2) DES with favorable stability and reversibility could effectively capture 2000 ppm of SO2. The absorption capacity of [TMG][Tetz]:Im (1:2) DES for 2000 ppm of SO2 was high up to 2.0 mol SO2/mol DES (0.40 g SO2/g DES) at 293.15 K. The results of the UV–vis and FTIR spectra revealed that two kinds of combination existed between SO2 and the DES: charge transfer interaction and chemical interaction. Surprisingly, the absorbed SO2, which was found to form charge transfer complexes with the DES, can be reduced by H2 to elemental sulfur under mild conditions without byproduct and additional catalysts. The yield of elemental sulfur could reach up to 37% at 373.15 K.