3. Results and discussion
Styrene oxide, as a typical representative, was selected as a probe for the addition reaction (Scheme 2 ). To acquire more superior conditions for the conversion of H2S, the effect of PILs types, temperature, and loading amount were systematically studied (Table 1 ). Firstly, we examined the reaction process in the absence of PILs as a blank experiment. Trace conversion was achieved even at 50 ºC for 4.0 h (Entry 1 ), which can be further supported by NMR results (Figures S1 ~ S2 ). Subsequently, [BDMAEEH][MeOAc] was fed into the H2S/styrene oxide system to explore the role of PILs. To our delight, the conversion of styrene oxide can reach 99% at 50 ºC and 4.0 h, and the selectivity of 1b was 70% (Figure 1 ). The two products can be separated easily by TLC. The structure and molecular weight of 1b and 1c were verified by NMR spectra and ESI-MS (Figures S3 ~ S5 , andS6 ~ S8 , respectively).
Scheme 2. Styrene oxide as the probe with various PILs catalyst.
To screen the reaction conditions, the reaction time, temperature, and catalyst loading were further optimized. It is found that the conversion of styrene oxide could reach 99% (Entry 2) , even with the catalyst loading of 10 mol% and within 2.0 h at 30 ºC, indicating the high catalytic performance of tertiary amine-functionalized PILs. In order to evaluate the influence of PILs with different cations on the conversion of H2S, [TMEDAH][MeOAc], [TMPDAH][MeOAc], and [TMHDAH][MeOAc] were investigated for the H2S addition reaction (Entries 3 ~ 5 ). The conversion of H2S mediated in the PILs except [TMEDAH][MeOAc] surpasses 90%, indicating the excellent catalytic performance of these tertiary amines functionalized PILs. [TMEDAH][MeOAc] and [TMHDAH][MeOAc] exhibits the lowest and highest catalytic performance probably due to the weakest and strongest alkalinity2.
It is reported that the primary amine group can react with epoxide under certain conditions 34-37 . To investigate whether the primary amine affects the addition of H2S and epoxide, three different cations, including [DMAEAH], [DMAPAH], and [DEAPAH] were furtherly explored. From the 13C NMR (Figure S9 ), there is no amino alcohol compound generated despite the almost quantitative conversion of the substrate (Entries 6 ~ 8 ).
Figure 1 . Dept135 and13C NMR of styrene oxide (1a ) after the reaction with H2S in [BDMAEEH][MeOAc] at 50 ºC for 4.0 h.
The influences of PILs with different anions have also been investigated, including [BDMAEEH][4-F-PhO], [BDMAEEH][NIA], [BDMAEEH][Br], and [BDMAEEH][I] (Entries 9 ~ 12 ). The conversion of styrene oxide can also reach 93% ~ 99%, which reflects the high catalytic activity of the tertiary amine group. Aprotic ILs, [EMIM][Ac] and [BDMAEE-C1][I], were also investigated for the addition reaction (Entries 13 and 14 ). High conversion of the substrate (96%) is still achieved, indicating the tertiary amine probably plays a key role in the reaction condition. It is well known that moisture accompanies the H2S in almost all industrial gases. To investigate the water effect on H2S conversion, [BDMAEEH][MeOAc] with 10 wt.% H2O was employed as a reaction medium (Entry 15 ). It is found that the conversion ratio of styrene oxide is as high as 99% within 2.0 h, almost the same with anhydrous conditions. ESI-MS and NMR confirmed that only the target products were present, indicating that the presence of water did not lead to the formation of glycols (Figure S10 ~ S11 ). This is because -SH has a stronger nucleophilic ability than -OH, which results from the electronegativity of the oxygen atom is greater than that of sulfur. It should be noted that the selectivity ratio of 1b to 1cis around 7/3 except for [BDMAEEH][I]-medium system. As is well known, iodide ion is a good nucleophilic group. However, ESI-MS confirmed that no organic iodine compound was generated, suggesting that the nucleophilic ability of iodine was weaker than that of -SH, and the related NMR details can be seen in Figure S12 .
Table 1 . Optimization of reaction conditions on the conversion of H2S with styrene oxide (substrate) at 30 ºC.