Iron-Based Nanomaterials for Environmental Applications: Waste Glycerol Valorization and Trace Sulfur Dioxide Capture

Abstract

Nanomaterials and nanoparticles have been largely investigated for various applications due to their unique properties. Among these, iron-based nanoparticles have become of increasing interest with various applications due to their non-noble composition, innocuous nature, cost-effectiveness, and magnetic properties. In the first part of this thesis research project, Fe-based nanomaterials were designed and synthesized using traditional coprecipitation methods for revalorization of waste glycerol. Biodiesel production has surged over the past decade, generating large amounts of glycerol, 10% (w/w) of output, which has become a waste stream, creating a strong need for glycerol valorization applications. Catalytic glycerol oxidation to value-added products, like dihydroxyacetone (DHA), is promising, although challenging as DHA is a 3-carbon product, which can be easily further oxidized. Accordingly, this project aims to develop cost-effective Fe3O4 nanoparticle-based heterogenous catalysts for controlled glycerol oxidation, using photo-enhanced Fenton-like reactions. Fe3O4 nanoparticles were synthesized by coprecipitation and compared to a traditional Fenton catalytic system, employing FeCl3. Further, the impact of Cu and Ag doping on Fe3O4 nanoparticles was investigated in the Fenton-like photocatalytic system, analyzing the impact of metal type, loading and hydride equivalents (eq.) reducing agent used. Cu and Ag doped Fe3O4 nanoparticles showed a reduced band gap, as confirmed by the Tauc method. Further, catalysts were characterized by transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and inductively coupled plasma optical emission spectroscopy (ICP-OES). Glycerol oxidation was initially tested using Fe3O4 nanoparticles and compared to a traditional Fenton system. While there was a decrease in glycerol conversion when employing Fe3O4 nanoparticles, as expected due to mass transfer limitations, an increase in the DHA: Formic acid (FA) ratio was observed going from 0.14 in the traditional system to 0.29 when using Fe3O4 nanoparticles under comparable conditions. Further, when comparing the performance of Fe3O4 nanoparticles under Fenton-like (H2O2), Fenton-like photocatalytic (light and H2O2) and photocatalytic (light) reaction conditions, it was found that a photocatalytic enhanced Fenton-like system exhibited higher glycerol conversion of 19%, indicating the synergistic effects of light and H2O2. Thus, the effects of doping with Cu and Ag were investigated in this system, with doped Fe3O4 nanoparticles further increasing glycerol conversion from 19% to 30% and 21% when using 10 wt.% CuO-doped and 1 wt.% Ag-doped Fe3O4 nanoparticles respectively. Additionally, it was observed that the formation of CuO and Cu2O had an impact on reactivity, with the Cu2O form maximizing DHA yield. With 10 wt.% Cu2O-doped Fe3O4 exhibiting a DHA yield of 11% as opposed to 5% when in its CuO oxide form. Alternatively, Ag-doped Fe3O4 nanoparticles demonstrated strong catalytic ability, favoring complete oxidation to FA, with 1 wt.% Ag-doped Fe3O4 (28 eq. NaBH4) having highest FA yield of 28%. Overall, Cu-doped Fe3O4 nanoparticles promoted selective oxidation, favoring higher DHA yields. In the second part of this thesis research project, Fe-based nanomaterials were designed, synthesized and investigated for trace SO2 capture in flue gas. Trace SO2 capture remains challenging, as even low levels cause environmental and health risks and disrupt downstream processes like carbon capture. Conventional desulfurization works for high concentrations, however, is less effective at low levels, highlighting the need for alternative approaches. Microcrystalline cellulose (MCC)-supported Fe, Mg and Cu nanoparticles were synthesized by a green, one-pot method, utilizing green tea as a reducing and stabilizing agent and tested for SO2 adsorption. Adsorbents were characterized by ICP-OES, scanning electron microscopy (SEM) and EDS, which confirmed metal deposition, notably Fe had highest weight concentration and appeared to deposit preferentially in joint metal adsorbents. SEM and EDS also indicated that Fe and Cu tend to form clusters upon deposition, while this was not observed for Mg. Breakthrough experiments were performed at room temperature and pressure with an Ar gas stream containing 18 ppm SO2, both in the absence and presence of CO2. Metal functionalized MCC-based adsorbents (ads) showed substantial improvements for SO2 adsorption from Ar containing gas streams relative to pristine MCC. At 1 ppm SO2, 50 wt.% Mg , Cu , and Fe MCC achieved capacities of 0.14, 0.64, and 1.77 mgSO2/gads, representing a 7 , 32 , and 88 fold increase over pristine MCC. Notably, the joint 25 wt.% - 25 wt.% Fe-Mg MCC adsorbent (ads) exhibited an additive effect, outperforming its individual metal counterparts with a capacity of 3.53 mgSO2/gads, 155 times higher than pristine MCC. Further, it was found that when capturing SO2 from gas streams containing CO2 and Ar that adsorption capacities decreased for all adsorbents, likely due to competitive adsorption, however metal functionalized MCC still outperformed pristine MCC. Under these conditions, 37.5 wt.% Fe MCC showed highest capacity, reaching 0.74 mgSO2/gads at 1 ppm SO₂. Moreover, for SO2 capture in both gas streams tested it was found that adsorption seemed to be primarily governed by the actual amount of deposited metal up to an optimum, rather than by the specific metal type.