Pyrrolidinium Imides: Promising Ionic Liquids for Direct Capture of Elemental Mercury from Flue Gas

Abstract

A new approach to vapor phase elemental mercury capture has been explored; this approach exploits an ionic liquid coating layer to oxidize elemental mercury for subsequent immobilization by chelating ligands. The room temperature ionic liquid 1-butyl-1-methyl pyrrolidinium bis(trifluoromethane sulfonyl)imide (P 14 ) was selected for study based on its oxidation potential window, thermal stability, and low vapor pressure. Tests were also completed in which KMnO 4 was added to P 14 to form a new ionic liquid, P 14 -KMnO 4 , with a higher oxidation potential. In room-temperature bulk liquid phase capture experiments, 59% of the elemental mercury in the inlet gas was captured using P 14 alone; mercury capture using P 14 -KMnO 4 was quantitative. P 14 and P 14 -KMnO 4 coatings were successfully applied to mesoporous silica substrates and to silica substrates functionalized with mercury chelating ligands. The coating layers were found to be thermally stable up to 300°C. Fixedbed tests of nonfunctionalized silica coated with P 14 showed an elemental mercury uptake of 2.7 mg/g adsorbent at 160°C; at the same temperature, functionalized silica coated with P 14 -KMnO 4 showed an elemental mercury capacity of at least 7.2 mg/g adsorbent, several times higher than that of activated carbon. The empty bed gas residence time in these tests was 0.04 s. A chelating adsorbent incorporating P 14 in the coating layer, may be capable of simultaneous removal of elemental and oxidized mercury from coal combustion flue gases.

Integrated Removal of NOx with Carbon Monoxide as Reductant, and Capture of Mercury in a Low Temperature Selective Catalytic and Adsorptive Reactor

“…The authors previously observed that 25 wt% [bpyrro][N(Tf) 2 ] coated silica has an elemental mercury saturation capacity of 2.7 mg/g. [Ji et al, 2008] To investigate the effect of cation and anion on elemental mercury removal, five more RTILs were synthesized and coated on silica as described above. Results of Hg 0 dynamic adsorption experiments performed using these materials are shown in Table IV Cl showed comparable Hg 0 capacity (2.2 mg/g).…”

“…The [bpyrro][N(Tf) 2 ] coating layer was previously reported to be thermally stable up to approximately 300°C. [Ji et al, 2008] Figure IV. It has been reported that halide anions dramatically reduce the thermally stability of the RTILs, reducing decomposition temperatures by at least 100°C below those of the corresponding ILs with non-halide anions. [Huddleston et al, 2001] The results presented here are consistent with Huddleston's results.…”

“…This salt was selected because of its stable liquid range, oxidation potential window, and negligibe volatility. Fixed-bed adsorption trials showed that mesoporous silica coated with P14 had an elemental mercury capacity of ~ 3 mg Hg 0 /mg [Ji et al, 2008].…”

“…The coating procedure is described in detail elsewhere. [Ji et al, 2008] In all cases, 25 wt% ionic liquid was coated onto a silica substrate, Grace Davison silica gel grade #62, and onto 3mercaptopropyltrimethoxy-silane-silica (MPTS-silica), a chelating adsorbent. Detailed particle characteristics of the silica gel substrate have been published previously.…”

“…As originally conceived, nanostructured chelating adsorbents rely on chelation of oxidized mercury; hence, the possibility of using an oxidizing room-temperature ionic liquid (RTIL) to capture elemental mercury has been tested by the authors. [Ji et al, 2008] The RTIL 1-butyl-1-methyl pyrrolidinium bis(trifluoromethane sulfonyl) imide (P 14 ), was found to be able to capture Hg 0 directly from nitrogen but showed no capacity for Hg 2+ .…”