Shilpa Agarwal

Ceria Nanocatalysts: Shape Dependent Reactivity and Formation of OH

Ceria nanoshapes (octahedra, wires, and cubes) were investigated for CO adsorption and subsequent reaction with water. Surprisingly, the reactivity of specific OH groups was explicitly determined by the ceria nanoshape. The nanoshapes showed different levels of carbonates and formates after exposure to CO, the amount of carbonates increasing from octahedral≪wires

Catalytic Hydrotreatment of Humins in Mixtures of Formic Acid/2-Propanol with Supported Ruthenium Catalysts

The catalytic hydrotreatment of humins, which are the solid byproducts from the conversion of C6 sugars (glucose, fructose) into 5-hydroxymethylfurfural (HMF) and levulinic acid (LA), by using supported ruthenium catalysts has been investigated. Reactions were carried out in a batch setup at elevated temperatures (400 °C) by using a hydrogen donor (formic acid (FA) in isopropanol (IPA) or hydrogen gas), with humins obtained from d-glucose. Humin conversions of up to 69 % were achieved with Ru/C and FA, whereas the performance for Ru on alumina was slightly poorer (59 % humin conversion). Humin oils were characterized by using a range of analytical techniques (GC, GC-MS, GCxGC, gel permeation chromatography) and were shown to consist of monomers, mainly alkyl phenolics (>45 % based on compounds detectable by GC) and higher oligomers. A reaction network for the reaction is proposed based on structural proposals for humins and the main reaction products.

Experimental Studies on the Hydrotreatment of Kraft Lignin to Aromatics and Alkylphenolics Using Economically Viable Fe-Based Catalysts

Limonite, a low-cost iron ore, was investigated as a potential hydrotreatment catalyst for kraft lignin without the use of an external solvent (batch reactor, initial H2 pressure of 100 bar, 4 h). The best results were obtained at 450 °C resulting in 34 wt % of liquefied kraft lignin (lignin oil) on lignin intake. The composition of the lignin oil was determined in detail (elemental composition, GC-MS, GC×GC-FID, and GPC). The total GC-detectable monomeric species amounts up to 31 wt % on lignin intake, indicating that 92 wt % of the products in the lignin oil are volatile and thus of low molecular weight. The lignin oil was rich in low-molecular-weight alkylphenolics (17 wt % on lignin) and aromatics (8 wt % on lignin). Performance of the limonite catalyst was compared to other Fe-based catalysts (goethite and iron disulfide) and limonite was shown to give the highest yields of alkylphenolics and aromatics. The limonite catalyst before and after reaction was characterized using XRD, TEM, and nitrogen physisorption to determine changes in structure during reaction. Catalyst recycling tests were performed and show that the catalyst is active after reuse, despite the fact that the morphology changed and that the surface area of the catalyst particles was decreased. Our results clearly reveal that cheap limonite catalysts have the potential to be used for the depolymerization/hydrodeoxygenation of kraft lignin for the production of valuable biobased phenolics and aromatics.

Exploratory catalyst screening studies on the liquefaction of model humins from C6 sugars

A catalyst screening study is reported on the liquefaction of humins, the solid byproducts from C6 sugar biorefineries for levulinic acid and 5-hydroxymethylfurfural production. Experiments were carried out in a batch reactor using an artificial model of humin derived from glucose with isopropanol (IPA) as the solvent at 400 °C for a 3 h batchtime. Initial studies using noble metal catalysts (Rh, Pt, Pd, Ru) on a carbon support revealed that Pt was the best catalyst in terms of humin conversion (77%) and amounts of alkylphenolics and aromatics in the product oil (GCxGC-FID). Subsequent support screening studies (TiO2, ZrO2, CeO2) were performed using Pt as the active metal and the results were compared with Pt/C. Detailed liquid product analysis (GPC, GC-MS, GCxGC) including blank reactions in the absence of humins revealed that the humins are mainly converted to monomeric alkylphenolics and aromatics oligomers (GPC) and (GC). IPA was shown not to be inert and is converted to acetone and hydrogen, and the latter is the hydrogen source for the various metal catalysed hydrogenolysis and hydro(deoxy)genation reactions. In addition, acetone is converted to aldolcondensation products (like methylisobutylketone, MIBK) and hydrogenation products derived thereof. The best results were obtained with Pt/C when considering humin conversion. However, Pt/CeO2 was shown to be more attractive when considering the amounts of alkylphenolics in the product oils (20.4 wt% based on humin intake).

Chapter 2 - Ceria Nanoshapes-Structural and Catalytic Properties

This chapter describes the synthesis of ceria nanoshapes (cubes, rods, and octahedra) and detailed characterization of their structure and exposed surface facets. We show how the surface facets influence catalytic performance for the low-temperature water gas shift reaction. The use of Fourier transform infrared spectroscopy provides insights into the mechanistic steps involved in this industrially important reaction. Ceria is a versatile material used in diverse areas such as medicine, fuel cells, sensors, and water treatment, but the primary focus of this chapter is on heterogeneous catalysis by ceria.

Surface chemistry of tailored ceria nanoparticles: interaction with CO and H2O

Steam reforming of bio-oil combined with the gasification of coke deposits in the presence of water is a conceptually promising alternative to generate hydrogen gas. H2O can be activated in the gasification stage to form hydroxyl groups (OH) on oxide-supported (like ceria) metal catalysts, which increases both the H2 yield and the catalyst’s lifetime. The reactivity for the water dissociation as well as the reactivity of resulting hydroxyl groups can be further improved by altering the shape and size of ceria support. Based on the recent studies, ceria nanoshapes exhibit excellent redox properties and high specific activity/selectivity in comparison to the bulk ceria particles. However, the knowledge related to the surface species actually responsible for enhanced catalytic activity of ceria nanocatalysts so far remain lacking. The work presented in this thesis highlights the fundamental aspects of ceria nanoshapes, with emphasis on the effects of surface planes on overall catalytic performance. The main objectives of this work are to investigate the true exposed facets, as well as to understand the reactivity of hydroxyl species and the role of defects on the ceria nanoshapes.