Semester

Spring

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Chemical and Biomedical Engineering

Committee Chair

John W. Zondlo

Committee Co-Chair

Richard Turton

Committee Member

Christopher R. Iacovella

Committee Member

Charter D. Stinespring

Committee Member

Berend C. Rinderspacher

Committee Member

Konstantinos A. Sierros

Committee Member

Shuo Wang

Abstract

The structural, transport, and thermodynamic properties related to cellulose dissolution by tetrabutylphosphonium chloride (TBPCl) and tetrabutylphosphonium hydroxide (TBPH)-water mixtures have been calculated via molecular dynamics simulations. For both ionic liquid (IL)-water solutions, water veins begin to form between the TBPs interlocking arms at 80 mol % water, opening a pathway for the diffusion of the anions, cations, and water. The water veins allow for a diffusion regime shift in the concentration region from 80 to 92.5 mol % water, providing a higher probability of solvent interaction with the dissolving cellulose strand. The hydrogen bonding was compared between small and large cellulose bundles, being 18 and 88 strands respectively. The dissolution of an 18 strand cellulose bundle was simulated in the TBPCl-water solution at various water concentrations. The Cl, TBP, and water enable cellulose dissolution by working together to form a cooperative mechanism for separating the cellulose strands from the bundle. The anions initially break the intra-strand hydrogen bonding and water helps delay the strand reformation. The TBP cation then can more permanently cleave the cellulose strand from the bundle. The TBP-peeling cellulose strand pairwise energy is net negative during the peeling process, indicating an energetically favorable process at moderate temperatures. The cellulose dissolution rapidly decays with increasing water concentration as the hydrogen bonding lifetimes for the chloride-cellulose hydroxyl hydrogens begin to fall below the lifetime of the largest cellulose intra-strand hydrogen bonds. The change in the diffusion regime occurs near the rapidly decaying dissolution region and may play a role in delaying the declining cellulose dissolution with increasing water concentrations.

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