Author ORCID Identifier

https://orcid.org/0000-0002-6793-5658

Date Available

8-13-2023

Year of Publication

2023

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Chemical and Materials Engineering

Advisor

Dr. Barbara L. Knutson

Co-Director of Graduate Studies

Dr. Stephen E. Rankin

Abstract

Confinement of ionic liquids (ILs) and deep eutectic solvents (DESs) within mesoporous materials such as silica helps to control the local environment within the pores for applications such as catalysis, electrochemistry, and absorption. Silica thin films with 2.5 and 8 nm pores and micron-sized silica particles with pore diameters of 5.4 and 9 nm were synthesized to study the effect of nanoconfinement on ILs 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]), and DESs reline and ethaline (choline chloride and urea or ethylene glycol). Silica thin films with vertically aligned, well ordered, and accessible pores were synthesized via the evaporation-induced self-assembly (EISA) method with cetyltrimethylammonium bromide (CTAB) templated EISA and titania doping giving 2.5 nm pores and Pluronic P123 (P123) templated films with a neutrally charged substrate giving 8 nm pores. Grazing incidence small angle x-ray scattering (GISAXS) and transmission electron microscopy (TEM) characterization gave evidence that the pores were aligned perpendicular to the substrate. Micron-sized mesoporous silica nanoparticles were synthesized through a sol-gel hydrothermal aging method where CTAB and P123 are used for templating and the pore diameter may be controlled through temperature, with higher temperatures giving larger pore sizes (2-12 nm).

Diffusion properties of confined ILs is relevant to understanding how solute molecules will behave when entering and moving through the confined IL. The diffusivity of a probe molecule to serve as an example solute are two fluorescent dyes rhodamine 6G (R6G), a cationic dye, and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), a neutral dye. The dyes were mixed with [BMIM][PF6] and confined within mesoporous silica microparticles with 5.4 and 9 nm pores. The 9 nm diameter particles were functionalized (tethered) with an IL-like molecule, 3-methyl-1-[3-(trimethoxysilyl)propyl]-1-imidazolium chloride [TMS-MIM][Cl]. The fluorescence recovery after photobleaching (FRAP) technique in confocal microscopy was used to estimate the diffusivity of the probes within the confined IL. Water saturated particles and pore diameter differences did not significantly impact the diffusion. The cationic dye had a lower diffusivity than the neutral dye from interactions between the dye and pore wall and anion of IL, while tethering increased the diffusivity of the dye by making the interior of the pore more hydrophobic. These results are important for diffusion considerations of species to catalytic sites in nanoconfined IL.

To further understand the interactions between ILs and the silica substrate, mesoporous silica thin films were synthesized with 3.5 and 8.5 nm pores and loaded with [BMIM][Cl]. The 8.5 nm pores were also functionalized with [TMS-MIM][Cl]. The films were characterized with x-ray photoelectron spectroscopy to measure the electronic environment of the elements within the sample. Bulk IL only has one peak in the nitrogen spectra due to the shared positive charge on the imidazolium ring. After confinement, a second peak at lower binding energy (BE) appeared from interfacial interactions between the imidazolium group in the IL with the pore wall of the silica. The reduction in positive charge on the imidazolium from this interaction causes a reduction in BE. This work gives insight into the ordering and location of the IL within the pores and is an important consideration since catalysis occurs on the surface of a material.

DESs are an emerging area of alternative solvents; often considered analogous to ILs, they are made of natural components that are customizable (choline chloride and urea being the classical example) but are still usually highly viscous so confinement is still advantageous. They can be used as a solvent for catalysis with metal additives, so the diffusion of FeCl3 was studied using cyclic voltammetry. Mesoporous silica thin films with 3 and 8 nm pores were synthesized as above on Fluorine-doped Tin Oxide glass slides and used as a working electrode. The diffusion was calculated using the Randles-Sevcik equation where the diffusion can be calculated by varying the voltage scan rate and measuring the peak current output for either the oxidation or reduction event of the probe molecule. The diffusion of FeCl4- in reline was hindered significantly in both pore diameters compared to the bulk value, while ethaline had pore size dependent accessibility. This result is beneficial to catalysis and electrochemical research with redox active species.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2023.388

Funding Information

This work was financially supported by the United States National Science Foundation (NSF) under grant no. CBET-1604491

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