Abstract
The solution-phase stability of the hydronium ion catalyst significantly affects the rates of acid-catalyzed reactions, which are ubiquitously utilized to convert biomass to valuable chemicals. In this work, classical molecular dynamics simulations were performed to quantify the stability of hydronium and chloride ions by measuring their solvation free energies in water, 1,4-dioxane (DIOX), tetrahydrofuran (THF), γ-valerolactone (GVL), N-methyl-2-pyrrolidone (NMP), acetone (ACE), and dimethyl sulfoxide (DMSO). By measuring the free energy for transferring a hydronium ion from pure water to pure organic solvent, we found that the hydronium ion is destabilized in DIOX, THF, and GVL and stabilized in NMP, ACE, and DMSO relative to water. The distinction between these organic solvents can be used to predict the preference of the hydronium ion for specific regions in aqueous mixtures of organic solvents. We then incorporated the stability of the hydronium ion into a correlative model for the acid-catalyzed conversion of 1,2-propanediol to propanal. The revised model is able to predict experimental reaction rates across solvent systems with different organic solvents. These results demonstrate the ability of classical molecular dynamics simulations to screen solvent systems for improved acid-catalyzed reaction performance.
Highlights
The catalytic upgrading of biomass is a promising strategy to obtain valuable chemicals from renewable resources while limiting waste products (Huber et al, 2006; Stöcker, 2008; Tock et al, 2010; Shuai and Luterbacher, 2016; Nguyen et al, 2017; Walker et al, 2019)
We find that the free energy for transferring a hydronium ion from pure water to organic solvent can distinguish between solvents that favorably (NMP, ACE, dimethyl sulfoxide (DMSO)) and unfavorably (DIOX, THF, GVL) solvate the acid catalyst
In our previous study of solution-phase acid-catalyzed reactions (Walker et al, 2019), we hypothesized that the transition state is lower in free energy relative to the initial reactant state in mixed-solvent environments due to two reasons: (1) the catalyst is destabilized in bulk solvent relative to a water-enriched local domain near the reactant, leading to a thermodynamic driving force for the transfer of catalytic protons to the local domain, and (2) the transition state is stabilized by water confined within this domain
Summary
The catalytic upgrading of biomass (e.g., wood, crops, etc.) is a promising strategy to obtain valuable chemicals from renewable resources while limiting waste products (Huber et al, 2006; Stöcker, 2008; Tock et al, 2010; Shuai and Luterbacher, 2016; Nguyen et al, 2017; Walker et al, 2019). In these reactions, the hydronium ion catalyst (H3O+) protonates the reactant (R) to form a reactant/proton complex (RH+). The relative stabilities of the reactant, transition state, and catalyst in solution are critical for determining reaction kinetics (Shuai and Luterbacher, 2016) Understanding how these solvent effects influence reaction kinetics is necessary to guide the optimization of solvent compositions and reactor conditions and maximize the productivity of biomass conversion reactions
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call