Understanding Acidity of Molten Salt Hydrate Media for Cellulose systems, which indicate that the main effect of a salt on the rate stems from the effect on the Hydrolysis by Coupling Kinetic Studies, Electrolyte Solution Modeling, acidity rather than catalytic properties of the ions. Prior investigations suggest that in addition to 13 the nature of the salt, the mineral acid used to acidify the MSH plays a key role in cellulose and C-NMR Experiments dissolution and reactivity.[1,6] It has been proposed that a synergistic effect between LiBr and H2SO4 renders this system more reactive than HCl acidified MSH. Similar to the effect of the Natalia Rodriguez Quiroz, Dionisios G. Vlachos* salt, we found that the effect of different acids on the reactivity could be attributed to their effect Chemical and Biomolecular Engineering Department, University of Delaware, on the overall acidity. However, as seen in Figure 2b, different acid-salt pairs will result in a 150 Academy St., Newark, DE 19716 (USA) different acidity based on (1) the strength of the acid that will contribute to the initial proton *[email protected] concentration (2) the interactions between the salt and the acid that can affect the speciation of the acid in solution and therefore the proton concentration. Lastly, we assessed the calculated pH values in concentrated LiBr solutions experimentally by extending the 13C-NMR method Introduction developed by Farcasiu[7] for the calculation of Hammett acidity of super acids, to concentrated Homogeneous chemo-catalytic systems play an important role in biomass upgrading. inorganic salt solutions. Figure 2c shows good agreement between the OLI calculated pH and Recently, the depolymerization of lignocellulosic biomass in concentrated metal salts and more the experimentally measured pH in different salt solutions. specifically in acidified LiBr molten salt hydrate (AMSH) has been shown to result in high -4 10 0.1 KCl/H SO [1] a) H SO b) c) 2 4 glucose yields at low acid concentrations, low temperatures, and very short times. MSH are 2 4 KCl/H SO 10M LiBr/H SO 2 4 2 4 11M LiBr/H SO concentrated inorganic salt solutions with a molar water to salt ratio close to the coordination KBr/H SO H PO 2 4 2 4 0.08 3 4 -2 MgCl /H SO 8M LiBr/H SO [2] 2 2 4 HCl 2 4 number of the strongest hydrated cation. The effectiveness of MSH for cellulose hydrolysis LiBr/H SO 2 4 H SO 9M LiBr/H SO 2 4 2 4 LiBr/HCl 0.06 LiBr/8M H SO has been attributed to the ability of the MSH to solubilize cellulose and to increase the acidity of -5 2 4 10 LiBr/H PO the reaction mixture.[3,4] However, quantification of these effects has not been systematically 3 4 0.04 -3 explored. In this work we coupled (1) differential kinetic measurements of cellobiose, a surrogate Modeled pH Concentration [M] compound of cellulose, with (2) thermodynamic modeling, using the OLI software, in various 0.02 Initial Rate [M/min] -6 + acidic media, and (3) ex situ NMR measurements to elucidate the effect of concentrated, non- 10 H 0 -4 ideal brines in the de-etherification of beta glycosidic bonds. -1.5 -1 -0.5 0 0.5 0 5 10 -4 -3 -2 pH LiBr Concentration [M] Experimental pH Materials and Methods Figure 2. a) Initial rate of cellobiose hydrolysis in different salt/acid systems vs. OLI De-etherification reactions were conducted 0.1 + 500

H + H Concentration calculated pH. b) H concentration in LiBr MSH with HCl, H3PO4 or H2SO4. c) Parity plot H+ Activity + in glass vials placed on a preheated aluminum block Activity Coefficient 13 Coefficient 400 showing the agreement between the OLI calculated pH and the C-NMR measured pH. (45 °C) filled with mineral oil. The composition of the 0.09 mixture after reaction was determined using High 300 Significance Performance Liquid Chromatography (HPLC). We 0.08 Coupling modeling, reaction kinetics and spectroscopy provided a fundamental studied the speciation of the different systems using 200 understanding of the effect of metal salts on the acidity and speciation of dilute mineral acid the OLI software (OLI, 2018).[5] Finally, we used 13C 0.07 solutions. The understanding of these highly non-ideal solutions transcends beyond cellulose and 1H-NMR spectroscopy (acquired on an Avance III Concentration [M] 100 hydrolysis to general Brønsted acid catalyzed reactions in metal salt media. Furthermore, the 400 MHz NMR spectrometer (Bruker)) to evaluate the 0.06 0 developed methodology can be applied to better select catalytic systems for cellulose hydrolysis effect of the salt on the pH and on the shielding of the 0 5 10 and help alleviate this bottle neck step for biomass upgrading into chemicals. protons in solution. LiBr Concentration [M]

+ Figure 1. OLI calculated H activity References Results and Discussion coefficient and concentration with [1] W. Deng. et al. Ind. Eng. Chem. Res. 2015, 5226–5236. Using thermodynamic modeling we found increasing LiBr for a 50 mM H2SO4 [2] H.-H. Emons, Electrochim. Acta 1988, 33, 1243–1250. that dilute solutions of mineral acids in LiBr MSH solution. [3] J. A. Duffy, M. D. Ingram, Inorganic Chemistry, 1978, 17, 2798–2802. become super acidic (50 mM H2SO4 has a pH of 1.2 [4] C. Geun Yoo, S. Zhang, X. Pan, RSC Adv. 2017, 7, 300–308. while 11.5 M LiBr/50 mM H2SO4 has a pH of ~ -1.3). Furthermore, Figure 1 shows that the [5] P. Wang. et al. Fluid Phase Equilibria 2002, 203, 141–176. increase in acidity in LiBr MSH can be attributed primarily to an increase in the proton activity [6] S. Sadula. et al. Green Chem. 2017, 19, 3888–3898. coefficient and secondary to the complete de-protonation of the sulfuric acid. Figure 2a shows [7] D. Fărcaşiu, A. Ghenciu, Prog. Nucl. Magn. Reson. Spectrosc. 1996, 29, 129–168. the de-etherification rate of cellobiose as a function of the calculated pH. The rate of hydrolysis is first order in proton activity in both media and independent of the origin of acidity (mineral acid or LiBr induced acidity). Our results strongly indicate that LiBr increases the rate of hydrolysis by enhancing the overall acidity. Similar behavior is observed in other salt-acid