The High Performance of Choline Arginate for Biomass Pretreatment Is Due to Remarkably Strong Hydrogen Bonding by the Anion

Abstract

The ionic liquid choline arginate [Ch][Arg] is more effective for biomass pretreatment than other choline based amino acid ILs, but the underlying mechanism has been unclear. In the present work we use the highlevel CCSD(T)/CBS(MP2) and G4(MP2) thermochemical protocols to probe the H-bonding interactions of [Ch][Arg] with water, and organic functional groups commonly found in biomass. We show that the [Ch][Arg] IL forms unusually strong H-bonding interactions with prototypical H-bond donors. For example, we obtain H-bonding interactions of 76.6, 80.0, and 103.6 kJ mol −1 with water, methanol, and phenol, respectively. Our theoretical results shed light on the capacity of [Ch][Arg] to dissolve biomass, and they demonstrate the importance of ion conformation, in addition to speciation, for IL performance more generally. As a point of reference, we compare the H-bonding interactions of [Ch][Arg] with those of a related IL, choline glycinate ([Ch][Gly]), which does not dissolve biomass as effectively as [Ch][Arg].

Liquid Nanostructure of Cholinium Argininate Biomass Solvents

“…The strength of the H-bonds varied considerably with the donor group, but H-bonds with [Arg] were always considerably stronger than those with [Gly]. 26 These high-level calculations could only be performed on small model systems composed of one cation, one anion, and a simple hydrogen bond donor. Thus, it is open to question whether a cation charge group is located in position to polarize hydrogen bonds with the guanidine nitrogen in the bulk bio-IL.…”

“…Our previous ab initio calculations revealed that strong hydrogen bonds between the guanidine group and model HBDs were due to bond polarization by the nearby choline charge center. 26 Figure 5A shows that 2.3 ± 1.2 (1:3 [Ch][Arg]/water) and 1.3 ± 1.0 (1:10 [Ch][Arg]/water) cation charge centers are closely correlated with the guanidine group (cyan data) at average separations of 5.1 and 5.8 Å, respectively. While 0.5−1.2 Å larger than the ab initio calculated distances, these data show that correlations between the guanidine group and cation charge center do persist in the bulk liquid.…”

“…One consequence of the bond polarization found in our previous ab initio calculations for [Ch][Arg] ion pairs was that it predicted the stabilization of a closed ring argininate conformer. 26 Here, we consider an argininate ion to be in the ring conformation if the distance between the carboxylate and guanidine carbons is less than 4.6 Å (Figure 7). The intramolecular RDFs in Figure 7 show that 15.6% of the anion is in the ring conformation in 1:3 [Ch][Arg]/water, but this increases to 37.7% in 1:10 [Ch][Arg]; the two maxima seen in 1:10 [Ch][Arg]/water at ∼2.9 and ∼3.9 Å denote two separate conformers.…”

“…This leads to the charge on partially negative atoms becoming more negative and to an increase in the partial positive charge of the proton, leading to strong H-bonds. The strength of the H-bonds varied considerably with the donor group, but H-bonds with [Arg] were always considerably stronger than those with [Gly] . These high-level calculations could only be performed on small model systems composed of one cation, one anion, and a simple hydrogen bond donor.…”

Nanostructured ionic liquids and their solutions: Recent advances and emerging challenges

“…[54] Some correlation with e.g. anion basicity, charge density, or local structure have been identified, [55] but a fuller understanding awaits systematic studies that correlate performance with structural variation. The diversity of naturally-occurring (and synthetic amino acid) structures includes potentially amphiphilically nanostructured ILs and their solutions, but how this may correlate to process efficiency remains an open question.…”