Understanding Acidity of Molten Salt Hydrate Media for

Subscriber access provided by Karolinska Institutet, University Library

Article

Understanding Acidity of Molten Salt Hydrate Media for Cellulose Hydrolysis by Combining Kinetic Studies, Electrolyte Solution Modeling, Molecular Dynamics Simulations and C-NMR Experiments 13

Natalia Rodriguez Quiroz, Arul Mozhi Devan Padmanathan, Samir Hemant Mushrif, and Dionisios G. Vlachos ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b03301 • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Understanding Acidity of Molten Salt Hydrate Media for Cellulose Hydrolysis by Combining Kinetic Studies, Electrolyte Solution Modeling, Molecular Dynamics Simulations and 13C-NMR Experiments Natalia Rodriguez Quiroz1, Arul M. D. Padmanathan2, Samir H. Mushrif2 and Dionisios G. Vlachos1* 1Catalysis

Center for Energy Innovation and Department of Chemical and Biomolecular Engineering University of Delaware, 221 Academy St., Newark, DE 19716

2Department

of Chemical and Materials Engineering, University of Alberta, 9211-116 Street Northwest, Edmonton, Alberta T6G 1H9, Canada

*Corresponding author: [email protected] Abstract Depolymerization of lignocellulosic biomass in concentrated metal salts and more specifically in acidified LiBr molten salt hydrate (AMSH) results in high glucose yields at low acid concentrations, low temperatures, and very short times with potentially considerable economic benefits. However, our understanding of this promising medium is limited. Here, we study the effect of different LiBr concentrations on acidity and hydrolysis of cellobiose, a cellulose surrogate molecule, in dilute H2SO4 solutions. We use thermodynamic modeling to predict the H+ (hydron) activity and the speciation and correlate these with the experimentally measured reaction rates. We find that the main contribution of the salt to the reactivity stems from the dramatic increase in H+ activity and secondary to an interaction of salt with the acid species that effectively renders the inorganic acid very strong. We perform molecular dynamics simulations and reveal that the increased hydron activity can be attributed to the decrease in the number of water molecules in the hydron solvation shell upon salt addition. Additionally, we extend the analysis to other salts and acids concluding that the effects of different cations, anions, and acids in cellobiose hydrolysis can likewise be primarily attributed to changes in acidity. A key physicochemical descriptor of various salts is their enthalpy of dissolution. Finally, we explore the use of 13C-NMR spectroscopy to estimate the pH of AMSH solutions. Keywords: Acid catalysis, molten salt hydrates, thermodynamic modeling, metal salts, cellulose hydrolysis, 13C-NMR, super acids, molecular dynamics.

ACS Paragon Plus Environment

1

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

Introduction Cellulose dissolution and de-polymerization into glucose is one of the most challenging and economically important transformations in cellulose utilization.1 The low reactivity of cellulose arises from its recalcitrant nature, due to strong hydrophobic interactions and inter and intramolecular hydrogen bonds across chains.2–4 This structure limits dissolution in common solvents and accessibility to the glycosidic bonds.1–9 Current techniques for the dissolution of cellulose involve use of expensive enzymes over long reaction times,10–12 corrosive solutions of concentrated mineral acids at low temperature 8,13,14 and dilute mineral acids at high temperature 13–15

the latter two resulting in the formation of undesirable byproducts.13,16 Alternatively,

concentrated inorganic salt solutions and more specifically molten salt hydrates (MSH) give high glucose yields in one-step, at short reaction times, low reaction temperatures, and with low byproduct formation.17–23 Among the different MSHs investigated, LiBr acidified MSH (AMSH) in dilute mineral acids, e.g., H2SO4, gives the highest reported glucose yields from cellulose or lignocellulose.17,19 MSH are concentrated inorganic salt solutions with a molar water to salt ratio close to the coordination number of the strongest hydrated cation.21,22,24 In an ideal MSH, all the water molecules are located in the inner coordination sphere of the cation while the anion is free in solution.24 As a result, the cation and anion are shielded from each other through one hydration sphere. The concentration of the salt to form MSH depends on the coordination number of that salt’s cation (Figure 1).24 In LiBr MSH, for example, the coordination number varies with Li+ concentration and depends slightly on the experimental and computational technique used to estimate it (for Li+, it is 3-6 ).25–27 It is generally accepted that the molar water to salt ratio for the LiBr MSH system is 3-4:1.17,28,29

Figure 1. Graphical depiction of ion-water and ion-ion interactions in the inner hydration sphere of a lithium cation with changing salt concentration. In MSHs, there is only one hydration layer around the cation.


2

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

The effectiveness of MSH for cellulose hydrolysis has been attributed to the ability of the MSH to solubilize cellulose and to increase the acidity of the reaction mixture.19,29–31 The former results from electrostatic interactions of the cations with the oxygens and of the anions with the hydrogens of the cellulose’s hydroxyl groups (Figure 2a).21,29,30,32,33 The metal ion coordinates with the hydroxyl group replacing a water molecule in the inner coordination sphere of the metal-water coordination complex.21,22,33,34 These ions-cellulose interactions enable breaking of hydrogen bonds holding the cellulose chains together. The latter has been attributed to the polarization of the water molecules in the inner coordination sphere of the cation (Figure 2b).17,19,31,35 a)

b)

Figure 2. a) Lithium and bromine interactions with the hydroxyl groups of the cellulose glucose units. The solvating capability arises from the cellulose hydroxyl groups replacing water molecules in the hydration sphere of the hydrated cation. b) Inner hydration sphere of Li+ in LiBr MSH and graphical representation of water polarization in the Li+water adduct.

Here we investigate the effect of salt addition on the acidity through thermodynamic modeling using the Optimum Logic Inc. Systems' Stream Analyzer software (OLI, 2018)36 and elucidate the mechanism for increased Brønsted acidity, with main focus on LiBr. We correlate the calculated concentration of hydronium ions with the experimentally estimated rate of de-etherification of beta (1-4) glycosidic bonds in cellobiose, a cellulose surrogate molecule, to understand the effect of AMSH on cellulose hydrolysis. We perform classical molecular dynamics to rationalize experimental results. We estimate the pH of LiBr MSH by modifying Farcasiu’s37

13C-NMR

method and applying it to concentrated inorganic salt solutions. Finally, we extend the study to other acidified salt media to understand the effect of the interactions between the salt and the acid on the acidity, and the impact of the different cations and anions on the reactivity.

Materials and Methods Chemicals


3

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

D-(+)-cellobiose (98+%), and D- (+)-glucose, all reagent grade, were purchased from SigmaAldrich. Inorganic salts and acids including LiBr, LiCl, Li2SO4, KBr, KCl, NaBr, MgCl2, 95-98 wt% H2SO4, 85 wt% H3PO4 and 37 wt% HCl were purchased from Sigma Aldrich and used as received. All salt and acid solutions were prepared using deionized water from a Millipore water purification system (model: Direct-Q3 UV R). Reaction Conditions and Setup De-etherification reactions were conducted in 5 mL glass vials (VWR) sealed with aluminum – silicon septa caps (ChemGlass) and placed on a preheated aluminum block consisting of 17 wells filled with mineral oil. After reaction, the vials were removed from the oil bath and placed in an ice bath to quench the reaction. Reaction mixtures of 2 mL in volume consisted of 1.5 wt% cellobiose in varying salt and acid solutions prepared in deionized nanopure water. Analytical Techniques The reaction mixture was analyzed at various times to determine the concentration of the reactants and products. The samples were first prepared by diluting 100 μL of the reaction mixture in 900 μL of nanopure water and filtered with a 0.2 μm nylon filter (Tisch Scientific). The composition of the mixture was determined using high performance liquid chromatography (HPLC) on a Waters Alliance Instruments e2692 HPLC with photo diode array (Waters 2998) and reflective index (Waters 2414) detectors. Separation was done on a Bio-Rad HPX-87H column maintained at 55 °C. The mobile phase was 0.005 M H2SO4 flowing at 0.5 mL/min. The analysis time for each sample was 50 minutes. Glucose and cellobiose were identified by the time of elution, 9.6 and 7.9 minutes, respectively. Conversion and Initial Rate The conversion of cellobiose and the glucose yield were calculated as follows: 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) =

𝑌𝑖𝑒𝑙𝑑 (%) =

=0 𝑐𝑡𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 ― 𝑐𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 =0 𝑐𝑡𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡

𝑐𝐺𝑙𝑢𝑐𝑜𝑠𝑒 =0 2 ∗ 𝑐𝑡𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡

× 100%

× 100%

(1)

(2)

The calculated yield and conversion were the same at all time points due to negligible carbon loss for all studied reactions consistent with HPLC spectra that did not show any unknown peaks. The initial rates were calculated by taking the slope of the change of glucose concentration over time for the first five time points before achieving 15% conversion.


4

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

pH Measurements The pH of all acid and salt solutions was measured at room temperature using a Fisher Scientific Accumet Basic AB15 pH meter. The pH meter was calibrated using low pH buffers (FSCI pH 1, 3, 6) obtained from Fisher Scientific. Acidity Measurements via 13C-NMR Probe Method Carbon NMR spectra were acquired on an Avance III 400 MHz NMR spectrometer (Bruker). Spectra were measured at room temperature and 104.27 MHz, for 254 scans. The samples were prepared using 430 μL of the acidic solution with 20 μL of Mesityl oxide (Sigma Aldrich) and 50 μL of D2O (ACROS Organics) in NewEra quartz NMR tubes. The NMR data was processed using the Mestrelab Research Software (mNOVA). Thermodynamic Model (OLI Systems) Activity coefficients, pH and species concentrations were calculated using the OLI software (OLI, 2018)36. This thermodynamic software combines an excess Gibbs energy model consisting of (1) long-range interactions represented by the Pitzer-Debye-Hückel expression, (2) a middlerange term for specific ionic interactions, and finally (3) a short-range term from the UNIQUAC model. All calculations were performed for isothermal conditions at reaction temperature. Molecular Dynamics Simulations Molecular dynamics (MD) simulations on the LiBr and LiCl systems were carried out using GROMACS v4.6.738–41 (see SI for details of computational methodology and simulation systems). Dynamic Light Scattering DLS experiments were conducted on a MP-PALS (massively parallel phase analysis light scattering) by Wyatt Technology. The solutions were prepared using nanopure filtered DI water and upon salt and acid addition, the mixtures were allowed to equilibriate for 24 hours before the measurments. The autocorrelation functions were analyzed using DYNAMICS. Ion Chromatography Ion chromatography experiments were carried out in a 850 Professional IC by Metrohm. The samples were prepared by disolving both the reference and filtered solids (from the LiBr/H2SO4 mixture) in nanopure filtered DI water following by filtration through a 0.45 micron filter prior to analysis. Each of the samples was run for 20 minutes for cation and anion detection.

Results and Discussion Effect of LiBr Concentration and pH on Cellobiose De-etherification


5

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

To isolate the effect of the salt on the kinetics, cellobiose, a reducing sugar consisting of two β-glucose molecules linked by a β (1-4) bond, was used as a cellulose surrogate. Unlike cellulose, cellobiose is completely soluble at the studied conditions (1.5 wt%), eliminating complications arising from dissolution. The accepted reaction mechanism for the Brønsted acid-catalyzed cellobiose hydrolysis is shown in Figure 3.42 The hydrolysis of the ether bond is limited by the slow unimolecular heterolysis of the conjugated acid (Figure 3b) to form a glucose molecule and a carbonium-oxonium ion. This slow step is followed by a rapid nucleophilic water addition. The overall reaction rate is zero order with respect to water and first order with respect to the H+ activity and to the cellobiose concentration (Eqn (3)) (Figure S1).

Figure 3. Reaction mechanism for Brønsted acid catalyzed de-etherification of glucose disaccharides. (a) Fast equilibrium-controlled protonation of the glycosidic oxygen. (b) Slow unimolecular heterolysis of the conjugated acid forming a glucose molecule and carbonium-oxonium ion. (c) Rapid water addition to the resonance stabilized carbonium ion. (d,e) Regeneration of H+ to form a second glucose molecule.

𝑟 = 𝑘 ∗ 𝑎𝑐𝑒𝑙𝑙 ∗ 𝑎𝐻 +

(3)

The rate-enhancing effect of LiBr was studied by varying the LiBr concentration (0, 10, 20, 40, 50 and 58 wt% corresponding to 0, 1.24, 2.68, 6.40, 8.93, and 11.2 M, respectively) in a 50 mM sulfuric acid solution (Figure 4). The experimental results in Figure 4 show that the conversion of cellobiose increases monotonically with increasing LiBr concentration, up to the molten salt hydrate concentration (58 wt%) at which it reaches 100% conversion. Considering that cellobiose is completely soluble in the reaction mixture, the observed behavior stems from salt-induced enhanced reactivity, consistent with previous studies.17,19,29


6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Glucose Percent Yield (%)

Page 7 of 26

T= 100 °C T= 85 °C

100 80 60 40 20 0

0

10

20

40

50

58

LiBr Concentration ( wt %) Figure 4. Experimental evaluation of the effect of LiBr concentration on the glucose yield. All solutions had a constant sulfuric acid concentration of 50 mM and were run at 85 °C and 100 °C at constant stirring (500 rpm) for 40 minutes.

To assess whether the increase in the rate of hydrolysis with increasing LiBr is solely due to enhanced acidity or some other catalytic effect, the rate of de-etherification of cellobiose in acidified LiBr solutions (0 M – 4.5M LiBr in 0.2 M H2SO4) was compared to that in sulfuric acid (0.05 – 0.6 M H2SO4). Figure 5a shows the initial rate of cellobiose hydrolysis vs. measured pH. The rate in acidified LiBr solutions appears lower than that in sulfuric acid. We refer to non-isotopic specific hydrogen cations (xH+) as hydrons. In solution these cationic species are hydrated forming H3O+ (oxonium cation). Since the level of hydration can vary drastically in different solutions, we refer to them as H+. We evaluate the acidity of the solution based on its pH as defined in equation 4, where 𝑎𝐻 + refers to the activity of H+ in solution. A decrease in the pH corresponds to an increase in the proton activity and thus an increase in the acidity (Eqn 4). The proton activity can increase via an increase in the proton concentration (𝑥𝐻 + , the molar fraction of H+ in solution) or an increase in the activity coefficient (𝛾𝐻 + ). pH = ―log (𝑎𝐻 + )

(4)

𝑎𝐻 + = 𝑥 𝐻 + ∗ 𝛾 𝐻 +

(5)

Previous studies have reported that potentiometric measurements of pH in electrolyte solutions can result in errors. Some of these include the formation of a potential at the junction,43–46 the cation interference in the glass membrane,47,48 and the acid error.46 To evaluate these effects on our acidified LiBr system, we computed the pH of different H2SO4/LiBr solutions using the OLI


7

ACS Catalysis

thermodynamic software and compared it to the measured pH. As shown in Figure S2, the difference between the measured and the predicted pH values increases with increasing LiBr concentration. Assuming that OLI estimates the pH accurately, this difference indicates that the measured pH leads to an overestimation of the solution’s acidity. To understand the sources of error for the acidic LiBr system, we compared the calculated and the experimentally obtained pH of various salt solutions at room temperature (see SI-Error in pH Measurements for more details). We concluded that the discrepancy between the modeled and measured pH in concentrated LiBr solutions likely arises from an induced potential at the liquid junction (between the inner electrolyte of the probe and the test solution) and from the interference of lithium cations with the glass membrane (commonly referred as alkaline or sodium error). By comparison, a good correlation between the modeled and measured pH values was found for sulfuric acid solutions and for salts with low junction potential and lower reported “alkaline error”. We evaluated the Hammett acidity as an alternative to potentiometric pH measurements; however, interactions of the lithium cations with the weak base in the Hammett indicator resulted in inaccurate estimation of the acidity for high concentrations of LiBr. Figure 5b shows the de-etherification rate of cellobiose as a function of the calculated pH. The rate of hydrolysis is first order in hydron 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. While pH measurements in acidified salt solutions are inaccurate, the OLI software is reliable to simulate these systems.

a)

b)

4.5 M LiBr

0.5 M H2SO4

10 -4

0.3 M H2SO4

4.4 M LiBr 2.7 M LiBr

0.1 M H2SO4 1.2 M LiBr

LiBr/0.2 M H2SO LiBr + 0.05 MH SO4 4 2

10

0.6 M LiBr

SO HH SO 2 2 4 4

-5

0

0.5 Measured pH pH

1

Initial Rate [M/min]

0.6 M H2SO4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

10 -4

0.69 M H2SO4

3.5 M LiBr

0.4 M H2SO4

1 M LiBr 0.2 M H2SO4

LiBr/0.2 M HM2SO LiBr/0.05 H 24SO4

10 -5

SO HH SO 2 2 4 4

0

0.5 OLI Calculated pH pH

1

Figure 5. a) Experimental initial rate of glucose formation vs. ex-situ, measured pH in H2SO4 solutions and LiBr/0.2 M H2SO4 solutions. Reactions were carried out at 80 °C. b) Experimental initial rate of glucose formation vs. 8 calculated pH at 80 °C. The pH was calculated at reaction temperature to account for temperature effects on the pH. ACS Paragon Plus Environment Salt and acid concentrations are presented in Table S1.

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Hydroxo-Aqua Species and Hydrogen Cations in Concentrated LiBr Solutions Next, we investigate the mechanism of hydron formation in an acidified LiBr solution of 50 mM sulfuric acid when increasing the concentration of LiBr. Figure S4 shows that the calculated pH decreases from 1.2 (0 M LiBr) to -1.3 (11.5 M LiBr). Prior studies observed an increase of acidity in concentrated LiCl and LiBr salts using potentiometry and the Hammett acidity19,49,50; however, the origin of the super acidic nature was not elucidated. For low concentrations of metal cations, the increase in acidity has been attributed to the deprotonation of the metal-aquo-complex31,51 [𝑀(𝑂𝐻2)𝑥]𝑛 + :

[𝑀(𝑂𝐻2)𝑥]𝑛 + + 𝐻2O⇌[𝑀(𝑂𝐻2)𝑥 ― 1(𝑂𝐻)](𝑛 ― 1) + 𝐻3𝑂 +

(6)

For salts with transition metal cations, the interaction between the metal and the coordinated water molecules results in a decrease in the electron density around the hydrogen atom forming a Brønsted-Lowry acid species (Eqn (4)).51,52 This effect has been extensively studied for transition metals, such as Cr3+ and Fe3+, which possess Lewis and Brønsted acid centers.51 However, alkali and alkaline metal cations lack the strong electric charge of transition metals. Nevertheless, it is generally accepted that the acidity of LiBr MSH stems from the deprotonation of the coordinated water molecule,15,17,29 leading to the formation of lithium hydroxide as the conjugate base (Eqn (5)):

[𝐿𝑖(𝑂𝐻2)4] + + 𝐻2O ⇌[𝐿𝑖(𝑂𝐻2)3(𝑂𝐻)] + 𝐻3𝑂 +

(7)

Using OLI, we calculate the concentration of the hydroxide species and hydronium ions from the water molecules inside the inner coordination sphere of the Li+ cation. Figure S5 shows that the concentration of LiOH peaks with increasing LiBr at 4 M LiBr at a very low concentration of 6.7x10-13 M. Since the concentrations of LiOH and H+ are equal, the Brønsted acidity in MSH is most likely not stemming from the de-protonation of the metal-aqua species since the formation of LiOH is thermodynamically unfavorable. To elucidate the mechanism of hydron formation in acidified LiBr solutions, key reactions of H2SO4 dissociation and coupling with HBr were included (Figure 6a). The order in which the reactions are presented is not indicative of the order these reactions occur in solution. At 0 M LiBr and 50 mM sulfuric acid, the equilibrium favors the formation of hydrogen bisulfate and hydronium ions (first pKa of -3) (Eqn (8)). The dissociation of the second hydrogen cation is less


9

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

favorable (pKa of 1.99). Approximately 30% of the hydrogen bisulfate is deprotonated, resulting in a concentration of circa 65 mM H+ (Eqn (9)). 𝐻2𝑆𝑂4 + 𝐻2𝑂⇋𝐻𝑆𝑂4― + 𝐻3𝑂 +

(8)

𝐻𝑆𝑂4― + 𝐻2𝑂⇋𝑆𝑂24 ― + 𝐻3𝑂 +

(9)

LiBr completely dissociates in water (Eqn (10)) and upon increasing the concentration, hydrogen bisulfate reacts with LiBr forming LiSO4- and HBr (Eqn (11)). Following the wellknown theory of Hard and Soft Acids and Bases, the equilibrium in Eqn (11) favors the formation of LiSO4- over LiBr since both Li+ and SO4- are harder (less polarizable) ions than Br- which is a borderline hard Lewis base.53 HBr readily dissociates (pKa -9) to form bromine and hydronium ions (Eqn (12)). Lithium bisulfate reacts with lithium cations to form lithium sulfate (Eqn (13)), which precipitates out of solution above 5.7 M LiBr. 𝐿𝑖𝐵𝑟⇋𝐿𝑖 + + 𝐵𝑟 ―

(10)

𝐻𝑆𝑂4― + 𝐿𝑖𝐵𝑟⇋𝐿𝑖𝑆𝑂4 ― + 𝐻𝐵𝑟

(11)

𝐻𝐵𝑟 + 𝐻2O⇋𝐻3𝑂 + + 𝐵𝑟 ―

(12)

𝐿𝑖 + + 𝐿𝑖𝑆𝑂4 ― ⇋ 𝐿𝑖2𝑆𝑂4 (𝑠)

(13)

The formation of the solid lithium sulfate further drives the equilibrium towards LiSO4and HBr, which in MSH results in complete deprotonation of sulfuric acid. The formation of solid lithium sulfate was assessed using dynamic light scattering (DLS) and ion chromatography (IC). DLS of 6 and 7 M LiBr samples before and after the addition of sulfuric acid showed the formation of solids upon acid addition (Figure S6). Furthermore, the nature of the solids was confirmed using ion chromatography by comparing the chromatographs of the purified solids from an 8 M LiBr/ 0.05 M H2SO4 solution and that of a solution of Sigma Aldrich purchased Li2SO4 (Figure S7). All contributions added result in hydronium ions concentration of 0.1 M, i.e., a decrease in pH from 1.2 in 50 mM sulfuric acid to 1 in AMSH. This small increase in H+ concentration cannot explain the drastic decrease in pH of AMSH (Figure S4).


10

Page 11 of 26

To understand the sharp increase in acidity, the activity coefficient of the hydronium ions was calculated using the mix-solvent thermodynamic framework of OLI. The hydron activity coefficient increases 400-fold (at 11.2M LiBr (Figure 6b)) owing to the deshielding effect of the salt on the hydronium ions. As more cations are added to solution, the water molecules surrounding the hydronium ions are displaced to preferentially coordinate with the salt cations, resulting in increased activity. The combined effects of LiBr on deprotonating HSO4-, precipitating Li2SO4 and increasing profoundly the activity coefficient of the protons explain the super acidity of LiBr AMSH.

Concentration [M]

0.04

LiSO-4

HSO-4

SO24

Li 2SO4(solid)

Concentration [M]

b)

a) 0.05

0.03 0.02 0.01 0

0

5

10

LiBr Concentration [M]

0.1

500

H + Concentration H + Activity Coefficient

0.09

400 300

0.08 200 0.07 0.06

100 0

5

10

H+ Activity Coefficient

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0

LiBr Concentration [M]

Figure 6. Effect of LiBr on (a) the concentration of various species and (b) the H+ concentration and activity coefficient, calculated using OLI software. Solution of 50 mM H2SO4 at 22 ºC. Calculations were carried out for solutions at room temperature to isolate the effects of temperature from the effects of the salt on the speciation.

Cellobiose Reactivity in Concentrated LiBr Solutions The above analysis was conducted at relatively low concentrations of LiBr (0 – 4.5 M in 0.2 M H2SO4). Here we turn onto concentrated LiBr solutions. The reactions were carried out at a lower temperature (45 °C) to allow accurate measurement of the fast initial-rates. The initial rates in concentrated brine and in sulfuric acid solutions were compared to determine the acid contribution to the rate.


11

ACS Catalysis

Figure 7 indicates that at these low acid/high salt concentrations, the main effect of the salt on the rate is due to the increased hydron activity. However, at higher LiBr concentrations, the rate seems higher in concentrated salt solutions (by up to (14 ± 1)%) that in sulfuric acid, maybe due to the stabilization of the charged transition state complex by the high concentration of ions. When normalized by the acidity, the rate increases monotonically with increasing ionic strength (Figure S8) independently of the salt (LiBr, LiCl, KCl, and MgCl2 were tested).

10 -4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

11 M 9M

10

-5

3.8 M 8M 2.5 M 1.8 M

10 -6

LiBr/0.05 MM H2HSOSO LiBr/0.05 2 4 4 SO H2HSO 2 4 4

-1.5

-1

5.5 M 0.6 M

-0.5 0 0.5 pH OLI Calculated pH

1

Figure 7. Experimental initial rate of glucose formation in cellobiose de-etherification at 45 °C for sulfuric acid (0.6 – 5 M) and concentrated LiBr solutions (4 – 11.5 M LiBr in 50 mM H2SO4) against calculated pH. The pH was calculated using the OLI aqueous model at reaction temperature. Salt and acid concentrations presented in Table S1.

Effect of Salt on Acidity and Reactivity of Cellobiose De-etherification Previous studies have explored cellulose dissolution and hydrolysis in the presence of different MSH capable of swelling and dissolving cellulose.13,17,19,21,54 It has been suggested that differences in the reactivity in various MSH stem from differences in cellulose conformation in the solvent or in catalytic and stabilization properties of the salt’s ions in solution17. For example, much lower cellulose conversions have been reported in LiCl and LiNO3 compared to LiBr17,29. However, the mechanisms for the disparity in reactivity have not been elucidated.


12

Page 13 of 26

To provide insights into the effect of the anions, we conducted a comparative study with a 4 LiCl solution in 50 mM H2SO4. The results in Figure S9 suggest that high concentrations (> 5 M) of LiCl are less effective in increasing the hydron activity than LiBr (the differences at small concentrations are less discerned) resulting in lower acidity and lower reactivity (Figure S9d). We propose that the main effect of a salt on the rate stems from the increase in the hydron activity rather than different catalytic properties of the ions. To evaluate this hypothesis, we extended the study to other salts including KBr, KCl, and MgCl2. Figure 8 supports this hypothesis.

10 -4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

H2SO4 KCl/H2SO4 KBr/H2SO4 MgCl2/H2SO4

10 -5

10 -6

LiBr/H2SO4 LiBr/HCl LiBr/H3PO4

-1.5

-1 -0.5 0 OLI Calculated pH pH

0.5

Figure 8. Experimental initial rate of glucose formation in various mineral acids and inorganic salt solutions (Table S2) against calculated pH. Reactions were carried out at 45 °C and the pH was calculated using the OLI software at reaction temperature. Salt and acid concentrations presented in Table S3.

As seen for LiCl (Figure S9), the ionic strength (Figure S8a) is not a good descriptor of the effect of the salt on the H+ activity. Although all the studied salts increase the H+ activity (Figure S10a), and thereby increase the acidity of the solution, their effect is limited by their solubility. Previous studies have reported that hard cations with soft polarizable anions are normally good cellulose solvents.17,30 Interestingly, these properties also result in more acidic solutions. Hard cations tend to be strongly hydrated resulting in the deshielding of protons in solution and consequently higher activity coefficients. Soft anion – hard cation salts are highly soluble and can reach molten salt hydrate concentrations. To better identify salt systems that can result in super acidic media, we propose the use of the salt’s enthalpy of dissolution as a descriptor of acidity. Figure S10b shows that salts with negative enthalpies of dissolution result in higher hydron activity coefficients, because they promote hydration of the cation with a concomitant loss of water in the first hydration sphere of


13

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

the H+. In contrast, salts with positive enthalpies of dissolution, such as NaBr, NaCl, KBr, and KCl, behave similarly at low concentrations but have little impact on the activity coefficient at high concentrations due to their limited solubility (Figure S10a and Figure S10b). Molecular Dynamics (MD) Simulations of the LiBr and LiCl Systems Since experimental evidence regarding the molecular interactions leading to the increase in the hydronium activity coefficient and change in the hydration structure upon salt addition is challenging to obtain, we carried out molecular dynamics simulations to gain insights. MD simulations revealed that the total number of water molecules in the first solvation shell (N1mol) of Li+ ions increases with increase in the salt concentration, leading to decrease in N1mol of the H3O+ (hydronium) ion (Figure 9a). This results in less water being available for the solvation of hydronium ion and thus, a less stable hydronium ion in the bulk. The relatively less stabilized hydronium ions can explain the high hydronium activity coefficient (Figure 6) in AMSH at high salt concentrations (>10 M). However, experiments reveal higher hydronium activity coefficient in LiBr AMSH, as compared to LiCl AMSH (Figure S9a). To further understand the effect of anion (Cl vs. Br), the possible solvation/stabilization of hydronium by the anions Cl- and Br- was also investigated. The radial distribution function (RDF) of hydronium–anion shows a significantly higher intensity peak (Figure 9b) in LiCl AMSH, as compared to LiBr AMSH and at a slightly shorter distance. Since the RDF is related to the the potential of mean force, a higher intensity peak for Cl- suggests stronger interaction of Cl- with the hydronium ion.


14

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

a)

b)

c)

d)

Figure 9. Analysis of local solvation structures of Li and hydronium (H3O+) ions using molecular dynamics simulations. (a) Effect of LiBr and LiCl concentration [M] on the number of water molecules in the first solvation shell (N1mol) of H3O+ (black and green lines) and Li+ (blue and red lines) ions in LiBr and LiCl AMSH systems, respectively. (b) Oxygen-anion radial distribution functions (RDFs) of H3O+ with bromide (black) and chloride (red) ions. The dotted lines indicate the peak heights. (c) Schematic of hydronium complexes: Zundel-like structures formed in LiCl AMSH and Eigen structures formed in LiBr AMSH. (d) Comparison of the forcefield calculated potential energy of isolated Zundel (+ two H2O molecules) and Eigen structures (+ a chloride ion).

The 12 M AMSH equilibrated simulation systems were also visually inspected using VMD 1.9.355 to understand the difference in solvation of the hydronium ion in LiCl and LiBr systems. This revealed two different hydronium complexes (Figure 9c) – a Zundel-like56 structure in LiCl AMSH and a Eigen57 structure in LiBr AMSH. Br- ions are not involved in the stabilization of the hydronium, thus letting the hydronium being stabilized by water molecules and only forming the Eigen structure. However, in the case of LiCl, Cl- ions are dominant in the first solvation shell of


15

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

hydronium, providing the stability forming Zundel structures. This can also be attributed to the lower polarizability and higher hardness of Cl- ion compared to Br-. Subsequently, the stability of Eigen and Zundel structures was analyzed by comparing their potential energies. Two non interacting H2O molecules and a non - interacting Cl - ion was included to the Eigen and Zundel structures, respectively, to maintain the same composition of the systems being compared. The Zundel structure (in LiCl AMSH) was found to be significantly more stable than the Eigen structure (in LiBr AMSH) (Figure 9d). Therefore, in LiCl AMSH, despite realtively less water molecules in the solvation shell of hydronium, the Cl- ions provide a greater stabilization effect (Zundel structure). In contrast, in LiBr AMSH, only the water molecules are involved in stabilizing the hydronium (Eigen structure). Given the higher stability of Zundel structure, hydroniums would be more stable in LiCl AMSH because of the anionic effect. This explains the greater proton activity coefficient in LiBr AMSH compared to LiCl AMSH seen in the experiments (Figure S9a). Effect of the Salt/Acid Synergy on Acidity and Reactivity of Cellobiose De-etherification Prior investigations suggest that in addition to the nature of the salt, the mineral acid used to acidify the MSH plays a key role in cellulose dissolution and reactivity.17,19 For example, it has been proposed that a synergistic effect between LiBr and sulfuric acid renders this system superior in comparison to HCl acidified MSH; however, the reasons remain unclear. We evaluated the synergistic effect between the salt and the acid by carrying out cellobiose hydrolysis in monoprotic and polyprotic acids (HCl and H3PO4) at various LiBr concentrations and comparing results to those in H2SO4. Similar to the effect of the salt, we found that the effect of different acids on the reactivity could be attributed to their effect on the overall acidity (Figure 8). To rationalize the effect of different acids on the acidity of the AMSH, we calculated the pH, H+ activity coefficient and concentration for increasing LiBr in 50 mM solutions of HCl and H3PO4 (Figure S18 and Figure S19). We found that although the salt-induced increase in the H+ activity coefficient decreases the pH, the interactions between the salt and the acid result in changes in speciation and therefore changes in the H+ concentration, as discussed next. In contrast to the enhancement of the deprotonation of sulfuric acid in LiBr MSH, the H+ concentration of strong monoprotic acids, such as HCl, remains constant irrespective of the salt concentration (Figure S18a). Effectively, both H2SO4 and HCl behave as strong acids in LiBr MSH. The difference in the number of protons released by each molecule of an acid rationalizes almost quantitatively why the H2SO4 AMSH is more acidic than the HCl AMSH (pH -1.19 vs. -


16

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0.89) and explains the difference in reactivity at the same acid concentration. For phosphoric acid, there are again complex interactions of species with LiBr. The addition of LiBr results in a slight increase at first and eventually a decrease in proton concentration owing to the protonation of H2PO4- and the condensation of phosphoric acid to form pyrophosphoric acid (Figure S20). Furthermore, the protonation of dihydrogen phosphate, H2PO4-, is driven by the increase in acidity while the condensation is mainly promoted by the salt-induced increase in the activity of the charged species (H(3-x)PO4-x) that condense to form pyrophosphoric acid (Figure S21). Additionally, the increase in the salt concentration would result in a decrease in the number of water molecules affecting the equilibrium reactions controlling the deprotonation and condensation of the H(3-x)PO4-x species (Figure S20). Clearly, the interaction of speciation arising from different acids with the MSH differs substantially; yet, the difference in pH among all these dilute acid solutions is not as dramatic and can be thought of as a second order effect when quantitative comparisons are made. In summary, although the effect of the salt on the activity coefficients is the most relevant factor that drives the overall decrease in the pH, the interactions between the acid and the salt affect the acidity by means of the proton concentration, increasing in the case of sulfuric acid and decreasing in the case of phosphoric acid. Ultimately, chemical speciation, elucidated through thermodynamic modeling, can help in identifying good acid-salt combinations resulting in super acidic media for cellulose hydrolysis. 13C

NMR Method for Strong Acidity Measurements Lastly, we assessed the calculated pH values in concentrated LiBr solutions experimentally by

extending the 13C-NMR method developed by Farcasiu37 for the calculation of Hammett acidity of super acids, to concentrated inorganic salt solutions. Farcasiu’s technique uses the 13C-NMR spectrum of probe molecule mesityl oxide (MO) to estimate the Hammett acidity of super acidic solutions. In the presence of Brønsted acids, the probe molecule MO gets rapidly protonated. Upon protonation of the conjugated carbonyl system (Figure 10a), the acquired charge density is mostly localized at the beta carbon while the charge density changes minimally around the alpha carbon. This uneven distribution of the charge allows to correlate the level of protonation of the probe molecule to the difference in the chemical shift between the alpha and beta carbons (Figure 10b), while using the alpha carbon as an internal reference to account for the magnetic properties of the acid as a solvent. Consequently, the difference in the chemical shift between the alpha and beta


17

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

carbons (Figure 10c) increases with increasing acidity and can be used to quantitatively describe the acidity of the system.

Figure 10. a) Protonation of mesityl oxide in the presence of a Brønsted acid and the effects on the beta and alpha carbons. b) Example of 13C NMR spectra of protonated mesityl oxide. c) Definition of the change in chemical shift (∆𝛿) as a function of the chemical shifts of the beta and alpha carbons as described by Farçasiu and coworkers.

Unlike acid solutions, brines cause a change in the chemical shift of the other carbons in the probe molecule. The effect of the salt on the shift differs at the neutral and the charged carbons. To account for these effects each of the chemical shifts for the alpha and beta carbons were subtracted from their respective chemical shifts in chloroform as seen in Eqn (14): 𝑁𝑀𝑅 𝑆ℎ𝑖𝑓𝑡 (∆𝛿) = 𝛿(𝛼 ― 𝛼𝐶𝐷𝐶𝑙3) ― 𝛿(𝛽 ― 𝛽𝐶𝐷𝐶𝑙3)

(14)

Given the low natural abundance of 13C and the detrimental effect of the ionic strength on the NMR signal, solutions with higher salt concentration decrease the signal to noise ratio making the beta peak indistinguishable from noise. To overcome this problem, manual shimming, locking, and tuning were done for every sample. Additionally, the NMR acquisition parameters were adjusted for each sample to obtain the best signal to noise ratio at the lowest number of scans. Unlike Farcasiu’s method, we constructed a sulfuric acid calibration curve that correlates the NMR shift with the OLI calculated pH (Figure 11a) rather than the Hammett acidity, which was shown to be inaccurate for concentrated LiBr solutions. Subsequently, we calculated the pH based on the carbon NMR Shift (Eqn (14) and compared them to the OLI calculated pH (Figure 11b) for various concentrations of LiBr and KCl in sulfuric acid (Table S8).


18

Page 19 of 26

The pH obtained by applying the calibration curve to the measured NMR shift is in good agreement with the OLI calculated pH, as seen in the parity plot in Figure 11b, supporting both the accuracy of the OLI calculations for the LiBr and KCl solutions and the applicability of the proposed method. We propose that one could use this method (the sulfuric acid calibration curve and measured NMR shifts) to estimate the acidity down to very low pH values in highly concentrated brines. b)

a) -1 H 2SO4

-2

Modeled pH

-2

pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

-3 -4 -5

10

15

20

25

30

KCl/H2SO4 11 M LiBr/H2SO4 10 M LiBr/H2SO4 9 M LiBr/H2SO4 8 M LiBr/H2SO4 LiBr/8 M H2SO4

-3

-4 -4

NMR Shift ppm

-3

-2

Experimental pH

Figure 11. NMR shift calculated with respect to the chemical shift of mesityl oxide in deuterated chloroform ( 𝑁𝑀𝑅 𝑆ℎ𝑖𝑓𝑡 = (𝛼 ― 𝛼𝐶𝐷𝐶𝑙3) ―(𝛽 ― 𝛽𝐶𝐷𝐶𝑙3) ) to account for solvent effects and salt-induced shifts in the spectra. a) Calibration curve relating the OLI calculated pH and the NMR shift. b) Parity plot showing the agreement between the calculated pH and the experimentally measured pH obtained by applying the calibration curve to the measured chemical shift (sample NMR spectra of mesityl oxide in chloroform, sulfuric acid, and LiBr AMSH are shown in Figure S22, all concentrations are tabulated in Table S9).

Conclusions In this paper we analyzed the super acidic properties of concentrated salts and molten salt hydrates (MSH), with main focus on acidified LiBr/H2SO4 solutions and their effect on hydrolysis kinetics of glycosidic bonds. We coupled (1) differential kinetic measurements of cellobiose, a surrogate compound of cellulose, with (2) thermodynamic modeling, using the OLI software, in various acidic media, (3) molecular dynamics simulations of the LiBr and LiCl AMSH systems, and (4) ex situ NMR measurements to elucidate the effect of concentrated, non-ideal brines in the de-etherification of beta glycosidic bonds. The main effect of LiBr on increasing the rate of de-etherification of cellobiose is the increase of acidity. OLI modeling suggests that the drastic increase in the acidity (pH decrease) stems


19

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

primarily from an increase in the activity coefficient of the protons and secondary from the complete de-protonation of the sulfuric acid due to complexation with the LiBr speciation. MD simulations revealed that increase in the salt concentration results in decrease in the number of water molecules in the solvation shell of proton, thus, destabilizing the proton and enhancing its activity coefficient. The simulations also explained the role of the anion in altering the stability, and hence, the activity coefficient of the proton. Furthermore, the effects of other salts and acids on the rate of cellobiose de-etherification can likewise be attributed to their effect on the Brønsted acidity. Different acid-salt pairs result in a different acidity based on (1) the potential of the salt to increase the proton activity coefficient, which we found to be strongly correlated to the salt’s enthalpy of dissolution, (2) the strength of the acid itself in water and the number of protons per molecule that contribute to the proton concentration, and (3) the complex speciation interactions between the salt and the acid that affect the speciation of the acid and therefore control the degree of deprotonation. Finally, we developed a method to estimate H+ activity for super acidic concentrated salt systems with pH down to -2.0 and lower. Extension of this work to cellulose hydrolysis is interesting and important to carry out in future work. In contrast to small oligomers whose rate of de-etherification depends mostly on the effective acidity, the rate of dissolution and exfoliation of cellulose highly depends on both the acidity and the total concentration of ions in solution, i.e., the presence of cations and anions facilitate interaction with the hydroxyl groups in the cellulose and the braking of hydrogen bonds promoting dissolution. Comparison across solutions with different total ion concentration is challenging since both the rate of dissolution and hydrolysis would be affected. Finding a way to couple the rate of de-etherification (addressed in the current manuscript) from the rate of dissolution in cellulose hydrolysis in AMSH is an important future topic. Beyond acquiring a fundamental understanding of the LiBr/H2SO4 AMSH system, the combined computational and experimental method can be employed for (a) screening of salt solutions for tailored acidity, guided by the enthalpy of dissolution of salts, (b) identification of the effects of metal salts on the acidity of Brønsted acid catalyzed reactions, such as fructose or xylose dehydration, commonly conducted in saturated alkali metal halide solutions.


20

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Supporting Information Reaction order of cellobiose de-etherification, sources of error in pH measurements, effect of LiBr concentration on the pH of a 50 mM solution of sulfuric acid, effect of LiBr on the concentration of Li(OH), experimental evidence for the formation of lithium sulfate in LiBr/H2SO4 systems, effect of ionic strength on the rate of reaction, effect of salt on acidity and reactivity of cellobiose de-etherification, correlation between enthalpy of dissolution and activity coefficient, details of computational methodology, systems and supplementary results for MD simulations, effect of the acid on acidity and speciation of LiBr MSH and 13C NMR experimental data

Acknowledgements This work was supported as part of the Catalysis Center for Energy Innovation, funded by the US Dept. of Energy, Office of Science, Office of Basic Energy Sciences under award number DESC0001004. SHM and AP acknowledge financial support for the computations from NSERC Canada Scholarship and Future Energy Systems Fellowship of the University of Alberta. Computations were performed using Compute Canada resources. The authors would like to thank Dr. Elizabeth McCord (UD) and Dr. Shi Bai (UD) for their assistance with the

13C

NMR

experiments, Dr. Chin Chen Kuo and Lily Cheng for their help with the IC and DLS experiments and Professors Raul Lobo and Yushan Yan for valuable discussions.

References (1)

Li, G.; Liu, W.; Ye, C.; Li, X.; Si, C. Chemocatalytic Conversion of Cellulose into Key Platform Chemicals. Int. J. Polym. Sci. 2018, 1–22.

(2)

Gross, A. S.; Chu, J.-W. On the Molecular Origins of Biomass Recalcitrance: The Interaction Network and Solvation Structures of Cellulose Microfibrils. J. Phys. Chem. B 2010, 114, 13333–13341.

(3)

Nishiyama, Y.; Langan, P.; Chanzy, H. Crystal Structure and Hydrogen-Bonding System in Cellulose I from Synchrotron X-Ray and Neutron Fiber Diffraction. J. Am. Chem. Soc. 2002, 124, 9074–9082.

(4)

Medronho, B.; Romano, A.; Graça Miguel, M.; Stigsson, L.; Lindman, B. Rationalizing Cellulose (in)Solubility: Reviewing Basic Physicochemical Aspects and Role of Hydrophobic Interactions. Cellulose 2012, 19, 581–587.

(5)

Matthews, J. F.; Skopec, C. E.; Mason, P. E.; Zuccato, P.; Torget, R. W.; Sugiyama, J.;


21

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

Himmel, M. E.; Brady, J. W. Computer Simulation Studies of Microcrystalline Cellulose Ib. Carbohydr. Res. 2006, 341, 138–152. (6)

Beckham, G. T.; Matthews, J. F.; Peters, B.; Bomble, Y. J.; Himmel, M. E.; Crowley, M. F. Molecular-Level Origins of Biomass Recalcitrance: Decrystallization Free Energies for Four Common Cellulose Polymorphs. J. Phys. Chem. B 2011, 115, 4118–4127.

(7)

Heinze, T.; Koschella, A. Solvents Applied in the Field of Cellulose Chemistry: A Mini Review. Polímeros Ciência e Tecnol. 2005, 15, 84–90.

(8)

Bhaumik, P.; Dhepe, P. L. Conversion of Biomass into Sugars. RSC Green Chem. 2016, 44, 1–53.

(9)

Medronho, B.; Lindman, B. Competing Forces during Cellulose Dissolution: From Solvents to Mechanisms. Curr. Opin. Colloid Interface Sci. 2014, 19, 32–40.

(10)

Walker, L. P.; Wilson, D. B. Enzymatic Hydrolysis of Cellulose: An Overview. Bioresour. Technol. 1991, 36, 3–14.

(11)

Shastri, Y.; F, F. Optimal Control of Enzymatic Hydrolysis of Lignocellulosic Biomass. Resour. Technol. 2016, 2, S96–S204.

(12)

Sun, Y.; Cheng, J. Hydrolysis of Lignocellulosic Materials for Ethanol Production: A Review. Bioresour. Technol. 2002, 83, 1–11.

(13)

Ragg, P. L.; Fields, P. R.; Agg, P. L. R.; Ields, P. R. F.; Ragg, P. L.; Fields, P. R. The Development of a Process for the Hydrolysis of Lignocellulosic Waste. Philos. Trans. R. Soc. London A 1987, 321, 537–547.

(14)

Mäki-Arvela, P.; Salmi, T.; Holmbom, B.; Willför, S.; Murzin, D. Y. Synthesis of Sugars by Hydrolysis of Hemicelluloses-A Review. Chem. Rev. 2011, 111, 5638–5666.

(15)

Novosel’tsev, P. P.; Tyuganova, M. A.; Krichevskii, G. E.; Buyanova, M. V. Some Special Features in the Thermolysis of Cellulose Which Has Been Modified with Linear Oligoaminophosphazenes. Fibre Chem. 1993, 24, 205–208.

(16)

Tian, J.; Wang, J.; Zhao, S.; Jiang, C.; Zhang, X.; Wang, X. Hydrolysis of Cellulose by the Heteropoly Acid H3PW12O40. Cellulose 2010, 17, 587–594.

(17)

Deng, W.; Kennedy, J. R.; Tsilomelekis, G.; Zheng, W.; Nikolakis, V. Cellulose Hydrolysis in Acidified LiBr Molten Salt Hydrate Media. Ind. Eng. Chem. Res. 2015, No. 54, 5226– 5236.

(18)

Van Den Bergh, J.; Babich, I. V; O’Connor, P.; Moulijn, J. A. Production of Monosugars


22

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

from Lignocellulosic Biomass in Molten Salt Hydrates: Process Design and TechnoEconomic Analysis. Ind. Eng. Chem. Res. 2017, 56, 13423–13433. (19)

Sadula, S.; Oesterling, O.; Nardone, A.; Dinkelacker, B.; Saha, B. One-Pot Integrated Processing of Biopolymers to Furfurals in Molten Salt Hydrate: Understanding Synergy in Acidity. Green Chem. 2017, 19, 3888–3898.

(20)

Bhaumik, P.; Chou, H.-J.; Lee, L.-C.; Chung, P.-W. Chemical Transformation for 5Hydroxymethylfurfural Production from Saccharides Using Molten Salt System. ACS Sustain. Chem. Eng. 2018, 6, 5712–5717.

(21)

Sen, S.; Martin, J. D.; Argyropoulos, D. S. Review of Cellulose Non-Derivatizing Solvent Interactions with Emphasis on Activity in Inorganic Molten Salt Hydrates. ACS Sustain. Chem. Eng. Eng. 2013, 1, 858–870.

(22)

Leipner, H.; Fischer, S.; Brendler, E.; Voigt, W. Structural Changes of Cellulose Dissolved in Molten Salt Hydrates. Macromol. Chem. Phys. 2000, 201, 2041–2049.

(23)

Yang, X.; Li, N.; Lin, X.; Pan, X.; Zhou, Y. Selective Cleavage of the Aryl Ether Bonds in Lignin for Depolymerization by Acidic Lithium Bromide Molten Salt Hydrate under Mild Conditions. J. Agric. Food Chem. 2016, 64, 8379–8387.

(24)

Emons, H.-H. Structure and Properties of Molten Salt Hydrates. Electrochim. Acta 1988, 33, 1243–1250.

(25)

Rempe, S. B.; Pratt, L. R.; Hummer, G.; Kress, J. D.; Martin, R. L.; Redondo, A. The Hydrtation Number of Li+ in Liquid Water. J. Am. Chem. Soc. 2000, 122, 966–967.

(26)

Newsome, J. R.; Neilson, G. W.; Enderby, J. E. Lithium Ions in Aqueous Solution. J. Phys. C Solid State Physiscs 1980, 13, L923–L926.

(27)

Olsher, U.; Izatt, R. M.; Bradshaw, J. S.; Dalley, N. K. Coordination Chemistry of Lithium Ion: A Crystal and Molecular Structure Review. Chem. Rev. 1991, 91, 137–164.

(28)

Yoo, C. G.; Li, N.; Swannell, M.; Pan, X. Isomerization of Glucose to Fructose Catalyzed by Lithium Bromide in Water. Green Chem. 2017, 19, 4402–4411.

(29)

Geun Yoo, C.; Zhang, S.; Pan, X. Effective Conversion of Biomass into Bromomethylfurfural, Furfural, and Depolymerized Lignin in Lithium Bromide Molten Salt Hydrate of a Biphasic System. RSC Adv. 2017, 7, 300–308.

(30)

Fischer, S.; Leipner, H.; Thümmler, K.; Brendler, E.; Peters, J. Inorganic Molten Salts as Solvents for Cellulose. Cellulose 2003, 10, 227–236.


23

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(31)

Page 24 of 26

Duffy, J. A.; Ingram, M. D. Acidic Nature of Metal Aquo Complexes: Proton-Transfer Equilibria in Concentrated Aqueous Media. Inorg. Chem. 1978, 17, 2798–2802.

(32)

Morgenstern, B.; Kammer, H. W.; Berger, W.; Skrabal, P. 7Li- NMR Study on Cellulose/ LiCl/ N. N-Dimethylacetamide Solutions. Acta, Polym. 1992, 43, 356–357.

(33)

Brendler, E.; Fischer, S.; Leipner, H. 7 Li NMR as Probe for Solvent-Cellulose Interactions in Cellulose Dissolution. Cellulose 2002, 00, 1–6.

(34)

Richards, N. J.; Williams, D. G. COMPLEX FORMATION BETWEEN-AQUEOUS ZINC CHLORIDE AND CELLULOSE-RELATED D-GLUCOPYRANOSIDES. Carbohydr. Res. 1970, 12, 409–420.

(35)

Sare, E. J.; Moynihan, C. T.; Angelí, C. A. Proton Magnetic Resonance Chemical Shifts and the Hydrogen Bond in Concentrated Aqueous Electrolyte Solutions. J. Phys. Chem. 1973, 77, 32.

(36)

Wang, P.; Anderko, A.; Young, R. D. A Speciation-Based Model for Mixed-Solvent Electrolyte Systems. Fluid Phase Equilib. 2002, 203, 141–176.

(37)

Fărcaşiu, D.; Ghenciu, A. Determination of Acidity Functions and Acid Strengths by 13C NMR. Prog. Nucl. Magn. Reson. Spectrosc. 1996, 29, 129–168.

(38)

Berendsen, H. J. C.; van der Spoel, D.; van Drunen, R. GROMACS: A Message-Passing Parallel Molecular Dynamics Implementation. Comput. Phys. Commun. 1995, 91, 43–56.

(39)

Hess, B.; Kutzner, C.; Van Der Spoel, D.; Lindahl, E. GRGMACS 4: Algorithms for Highly Efficient, Load-Balanced, and Scalable Molecular Simulation. J. Chem. Theory Comput. 2008, 4, 435–447.

(40)

Lindahl, E.; Hess, B.; van der Spoel, D. GROMACS 3.0: A Package for Molecular Simulation and Trajectory Analysis. J. Mol. Model. 2001, 7, 306–317.

(41)

Van Der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. C. GROMACS: Fast, Flexible, and Free. J. Comput. Chem. 2005, 26, 1701–1718.

(42)

Timell, T. E. The Acid Hydrolysis of Glycosides. Can. J. Chem. 1964, 42, 1456–1472.

(43)

Barry, P. H.; Lynch, J. W. Liquid Junction Potentials and Small Cell Effects in Patch-Clamp Analysis. J. Membr. Biol. 1991, 121, 101–117.

(44)

Illingworth, J. A. A Common Source of Error in PH Measurements. Biochem. J 1981, 195, 259–262.

(45)

Marvin, E. PH Measurement in High Ionic Strength Brines, Helsinki Metropolia University


24

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

of Applied Sciences, 2013. (46)

Nordstrom, D. K.; Alpers, C. N.; Ptacek, C. J.; Blowes, D. W. Negative PH and Extremely Acidic Mine Waters from Iron Mountain, California. Environ. Sci. Technol. 2000, 34, 254– 258.

(47)

Doles, M.; Wiener, B. Z. The Theory of the Glass Electrode. IV. Temperature Studies of the Glass Electrode Error. J. Electrochem. Soc. 1937, 72, 107–127.

(48)

Licht, S. PH Measurement in Concentrated Alkaline Solutions. Anal. Chem. 1985, 57, 514– 519.

(49)

Hogfeldt, E.; Staples, P. J. Hammett Acidity Function in Concentrated Aqueous Solutions of Hydrochloric Acid-Lithium Chloride. J. Chem. Soc. A 1971, 0, 2074–2077.

(50)

Ojeda, M.; Wyatt, P. A. H. The Effects of Neutral Salts on the Hammett Acidity Function. J. Phys. Chem. 1964, 68, 1857–1862.

(51)

Grzybkowski, W. Nature and Properties of Metal Cations in Aqueous Solutions. Polish J. Environ. Stud 2006, 15, 655–663.

(52)

Franzyshen, S. K.; Schiavelli, M. D.; Stocker, K. .; Ingram, M. D. Proton Acidity and Chemical Reactivity in Molten Salt Hydrates. J. Phys. Chem. 1990, 94, 2684–2688.

(53)

Pearson, R. G. Hard and Soft Acids and Bases HSAB, Part I. J. Chem. Educ. 1968, 45, 581– 587.

(54)

de Almeida, R. M.; Li, J.; Nederlof, C.; O’Connor, P.; Makkee, M.; Moulijn, J. A. Cellulose Conversion to Isosorbide in Molten Salt Hydrate Media. ChemSusChem 2010, 3, 325–328.

(55)

Humphrey, W.; Dalke, A.; Schulten, K. VMD: Visual Molecular Dynamics. J. Mol. Graph. 1996, 14, 33–38.

(56)

Fulton, J. L.; Balasubramanian, M. Structure of Hydronium (H3o+)/Chloride (Cl -) Contact Ion Pairs in Aqueous Hydrochloric Acid Solution: A Zundel-like Local Configuration. J. Am. Chem. Soc. 2010, 132, 12597–12604.

(57)

Eigen, M. Proton Transfer, Acid-Base Catalysis, and Enzymatic Hydrolysis. Angew. Chemie Int. Ed. 1964, 3, 1–19.


25

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

For Table of Contents Only


26

Understanding Acidity of Molten Salt Hydrate Media for

Recommend Documents