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Analytical Chemistry and Electrochemistry H 0 in Various ILs CATION EFFECT ANION EFFECT ANION EFFECT The cation effect is investigated in NTf 2 - based.

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Presentation on theme: "Analytical Chemistry and Electrochemistry H 0 in Various ILs CATION EFFECT ANION EFFECT ANION EFFECT The cation effect is investigated in NTf 2 - based."— Presentation transcript:

1 Analytical Chemistry and Electrochemistry H 0 in Various ILs CATION EFFECT ANION EFFECT ANION EFFECT The cation effect is investigated in NTf 2 - based ILs with HNTf 2 as spiking acid. No significant difference is observed for a same CI ( ○,□,∆): BMIm + = BMImH + = HNEt 3 +  The acidity increases when acids are added in ILs and high acidity levels can be reached. The Hammett acidity function allows the evaluation of the acidity level in our ILs which ranges from -2 to -8. Nevertheless, because the solvent affects the dissociation equilibrium, the Hammett function must be understood as an apparent function, underestimating the real acidity.  Changing the IL cation does not affect the accessible acidity level: BMIm = BHIm = HNEt 3. On the contrary, clear variations of solvating power towards the proton are observed between several IL anions. Accessible acidity levels can be classified with the following anion order:  Differences of acidity are observed between HOTf and HNTf 2 in BMImNTf 2 and much more in BMImBF 4. This is in agreement with high acidity levels and the proposed acidity order: the higher is the acidity level, the more strong acids can be differentiated.  All these conclusions are confirmed by our electrochemical measurements with the hydrogen electrode. 2 Hammett Acidity Scale in Ionic Liquids : An Indication of Their Weak Dissociating Character ? Robert T.*, Magna L.**, Olivier-Bourbigou H.**, Gilbert B*. *University of Liège, Sart-Tilman, B6c, B-4000 Liège, Belgium ** Institut Français du Pétrole, BP Solaize, France References: References: (1) T. Robert, L. Magna, H. Olivier-Bourbigou and B. Gilbert, J. Electrochem. Soc., 156, 115 (2009) (2) C. Malherbe, T. Robert, L. Magna, H. Olivier-Bourbigou and B. Gilbert, ECS Trans., 16, 3 (2009) Use of 2 Indicators … For a same concentration of a same added acid; a difference of H 0 is observed when two different CI are used in a given IL ( ▼, ○, △ ). This is typical of a non totally dissociating media. Then, the Hammett acidity function becomes an apparent function underestimating the real acidity level. formation constant of IH + X - unitary slope Hammett Acidity Function Introduction Solvating power, for instance towards the proton, is a fundamental property of solvents. Therefore, we are interested to investigate the acid-base properties in ionic liquids (ILs) in order to ultimately establish a correlation with the acidic catalysis activity. To determine the accessible acidity levels in these new “green” ionic solvents, we suggest the colorimetric Hammett acidity function H 0. 1 Following the Hammett proposition, this UV-Vis spectroscopic method is based on the protonation equilibrium for a family of coloured indicator (CI) with pKa’s assumed as solvent independent,. The comparison of two common acids, HOTf and HNTf 2, is investigated in BMImOTf, BMImNTf 2 and BMImBF 4. In a given IL, the comparison is made with the same CI. IL autoprotolysis: BMIm + A - ⇌ BMIm + HABMImOTf no difference of H 0 (CI -3,32: ▼,▽ )  HNTf 2 is strong HNTf 2 + BMImOTf → HOTf + BMImNTf 2 (leveling effect) BMImNTf 2 HOTf (●) is weaker than HNTf 2 ( ○ ) (CI – 4.53) HOTf + BMImNTF 2 HNTf 2 + BMImOTf BMImBF 4 HNTf 2 ( ◇ ) is stronger than HOTf ( ◆ ) but both are weak acids BMImBF 4 + HNTf 2 HBF 4 + BMImNTf 2 BMImBF 4 + HOTf HBF 4 + BMImOTf HNTf 2 ≠ HOTF high acidity level Results and Discussion HOTf vs. HNTf 2 Experimental  Because the ILs and the acids (HOTf and HNTf 2 ) are very hygroscopic and/or moisture sensitive, storage, preparation of all solutions and UV-cells filling are performed in a glove box.  Spectroscopic measurements are made with a Perkin Elmer Lamda 14P spectrometer in quartz cuvettes. References are the same IL with the same acid concentration but without indicator. Acid concentration increases Acidic batch solution of indicator: IL + acid + HI + Batch solution of indicator: IL + I Intermediate solutions: IL + acid + HI + /I Figure 2: Comparison of the apparent Hammett acidity with addition of (filled symbol) HOTf or (empty symbols) HNTf 2 in ( ▼▽ ) BMImOTf, (●○) BMImNTf 2, ( ▲△ ) HNEt 3 NTf 2, ( ∎ ) BHImNTf 2, ( ◆◇ ) BMImBF 4 and (  )BMImPF 6. Basic form, I Acidic form, HI + In practice Theory with X, the ratio of the unprotonated form of the indicator determined by computer The Hammett acidity function is evalu- ated in several ILs to study the effect of the ILs nature towards the proton acidity. Very acidic level, -2 to -8, can be reached in common ILs. Conclusions In this case, the IL anion nature plays a dominant role on the accessible acidity. The accessible acidity level follows : OTf - < NTf 2 - < BF 4 - < PF 6 - A higher acidity corresponds to a lower solvating power of the IL anion towards the proton. the proton linked on the IL cation has no influence on the accessible acidity because its acidity is masked by HNTf 2 + acid Hammett assumption H 2 O OTf - NTf 2 - BF 4 - PF 6 - Figure 1: Evolution of the 2,4-dichloro - 6-nitroaniline spectrum in BMImNTf 2 with the addition of HNTf 2


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