(544d) Effects of Physical Structure of Alkylammonium Hydrogen Sulfate Ionic Liquids on Acidity

Alkylammonium [HSO₄] protic ionic liquids (ILs) are a promising class of ILs, shown to be effective in a range of applications such as biomass pretreatment, catalysis, chemical synthesis, and protein stabilisation [1]–[5]. Their main advantage is their extremely low cost ($0.78/kg [6]), due to a simple one-step synthesis of two cheap reagents. Previous studies on ILs have found that the chemical activity of the IL is dominated by the anion. Therefore, while a number of physical properties of hydrogen sulfate ILs have been investigated, they have tended to involve a small number of cations, with relatively little investigation into the effect of cation structure on chemical behaviour. Additionally, most applications have focused on the use of tertiary alkylammonium cations. Changing the physical structure of the cation could potentially further reduce costs, as well as widen the range of potential applications by modifying chemical behaviour. Four alkylammonium cations were screened in this study, with three different degrees of substitution (shown in Fig. 1). The acidity of the resulting IL solutions (containing 20% wt. water) was quantified using the Hammett acidity function.

Decreasing degree of alkylammonium substitution was found to significantly enhance solution acidity. The acidity of [MBA][HSO₄] was found to be equivalent to that of [DMBA][HSO₄] with 11% excess sulfuric acid, while that of [HA][HSO₄] and [BA][HSO₄] was found to be equivalent to that of [DMBA][HSO₄] with 31% and 32% excess acid respectively. Addition of up to 20% excess base to [BA][HSO₄] expectedly made the IL solution less acidic, although its Hammett acidity was still substantially lower than that of [DMBA][HSO₄], as well as lower than that of [MBA][HSO₄]. Such a strong trend in acidity was unexpected from aqueous acid-base theory, due to the narrow range of constituent amine pKa’s and small differences in pH of heavily diluted IL solutions. This was attributed to increased solvation of primary and secondary alkylammonium cations, decreasing the solvation of the [HSO₄^-] anion and thus increasing its proton activity. The effect of solution density was found not to play a significant role. When adding excess sulfuric acid to [DMBA][HSO₄], the IL solution displayed a buffering effect at low excesses (1-5% wt.), showing a higher Hammett acidity than purely aqueous sulfuric acid.

Biomass pretreatment was identified as an application which would benefit from enhancement of alkylammonium [HSO₄] solution acidity. Pretreatments using primary and secondary alkylammonium cations were carried out and compared to those using [DMBA][HSO₄], a current gold-standard protic IL. Pretreatment with primary alkylammonium [HSO₄] ILs led to overly severe pretreatments with very accelerated kinetics, while pretreatment with [MBA] [HSO₄] ILs showed equally high performance to [DMBA][HSO₄] but with accelerated kinetics. Addition of up to 20% excess base to primary alkylammonium ILs had relatively little effect on pretreatment severity. Additional pretreatments were performed using [DMBA][HSO₄] with excess acid, matching the Hammett acidities of [HA][HSO₄] and [MBA][HSO₄]. Despite equal Hammett acidities, pretreatments using excess acid were significantly more severe than those using the primary and secondary alkylammonium HSO₄ ILs. The observed differences in pretreatment severity suggest different sources of acidity between the two types of solutions, which cannot be distinguished using the Hammett acidity parameter. Pretreatment findings add credence to the theory that the acidity of the primary and secondary alkylammonium ILs is due to the increased activity of protons from lower solvation of the [HSO₄^-] anion, rather than increased proton dissociation in solution. In addition, the development of [MBA][HSO₄] as a promising pretreatment solvent highlights the possible improvements from screening of ion structure in ILs.

[1] F. J. V. Gschwend, C. L. Chambon, M. Biedka, A. Brandt-Talbot, P. S. Fennell, and J. P. Hallett, “Quantitative glucose release from softwood after pretreatment with low-cost ionic liquids,” Green Chem., vol. 21, no. 3, pp. 692–703, 2019.

[2] F. J. V. Gschwend, F. Malaret, S. Shinde, A. Brandt-Talbot, and J. P. Hallett, “Rapid pretreatment of Miscanthus using the low-cost ionic liquid triethylammonium hydrogen sulfate at elevated temperatures,” Green Chem., pp. 1–3, 2018.

[3] P. Attri and P. Venkatesu, “Exploring the thermal stability of α-chymotrypsin in protic ionic liquids,” Process Biochem., vol. 48, no. 3, pp. 462–470, Mar. 2013.

[4] E. T. Kermani, H. Khabazzadeh, and T. Jazinizadeh, “Friedländer synthesis of poly-substituted quinolines in the presence of triethylammonium hydrogen sulfate [Et3NH][HSO4] as a highly efficient, and cost effective acidic ionic liquid catalyst,” J. Heterocycl. Chem., vol. 48, no. 5, pp. 1192–1196, Sep. 2011.

[5] M. R. M. Shafiee, B. H. Najafabadi, and M. Ghashang, “Triethylammonium Hydrogen Sulfate as a Catalyst for the Preparation of 1,5-diaryl-3-(arylamino)-1 H -pyrrol-2(5 H )-one derivatives,” J. Chem. Res., vol. 35, no. 11, pp. 634–636, Nov. 2011.

[6] Baaqel et al, Green Chemistry, 2020 (in press)