Amorphous Silica-Alumina Catalysts for the Conversion of Renewable Feedstocks
Ferdy Coumans defended his thesis at the Department of Chemical Engineering and Chemistry on September 6th.
In the coming years, climate change will be the primary driving force for changes in the chemical industry. To meet these sustainability goals, mitigating CO2 emissions by closing the carbon cycle will be imperative. In recent years, plastic recycling has gained much interest due to the potential to reduce both CO2 emissions and waste build-up. Currently, only a tiny percentage of the used plastics are recycled, and repurposing these discarded products into novel materials or converting them into chemicals has significant potential. Biomass, on the other hand, is an alternative carbon source for chemical production and is often considered a suitable short- and medium-term solution. In his Ph.D. research, Ferdy Coumans investigated solid acid materials for their catalytic activity in the Diels-Alder cycloaddition (DAC) and pyrolysis of ultra-high molecular weight polyethylene. Furthermore, the use of organic protection strategies in electrochemical reactions was investigated.
The first step was the project focused on identifying the type of acid sites needed (i.e., Lewis or Brønsted) for producing aromatic p-xylene via the DAC between bioderived 2,5-dimethylfuran and ethylene. This was achieved by grafting different metals, like Al3+, Ga3+, and In3+, on silica supports.
Various IR and NMR spectroscopic techniques extensively characterized the materials for their acidic properties. It was found that materials containing Brønsted acidity only led to the formation of the desired product (p-xylene). In the following stages of the project, the synthesis of amorphous silica-alumina (ASA) materials was studied in greater detail.
Effect of aluminum concentration
First, the effect of Al concentration on the catalytic activity was investigated. The activity was found to increase initially with aluminum loading. After observing that inactive alumina formed at higher aluminum concentrations, thereby limiting the acid site concentration, methods to increase the acid content were investigated.
Materials derived from mesoporous supports were able to generate more acid sites at higher aluminum loadings. These materials were able to match the performance of microporous zeolite benchmarks. Having identified that exchanging of the support can increase the acid site concentration.
Two new sets of ASA were synthesized to study the relation between aluminum concentration and the surface area of the support. The loading in these samples was based on the OH concentration of commercial fumed silicas with varying surface areas (i.e., particle size).
NMR characterization revealed that isolated flexible aluminum sites were obtained. Quantifying the acidity with IR showed that only a small percentage of the aluminum atoms contribute to forming acid sites, and their concentration depends on the metal content and surface area.
Evaluating these materials in the pyrolysis of ultra-high molecular weight polyethylene in a TGA machine revealed that the pyrolysis temperature depends on the acid site concentration and surface area. The ASA could approach the performance of zeolite while also forming less coke than the microporous materials.
Highly alkaline and acidic solutions, which are vital for many (electro-) chemical processes, are detrimental to the stability of highly reactive bioderived furanic compounds.
The application of synthetic protection strategies in the electrochemical oxidation of 5-HMF was investigated using acetals derived from mono-ols and diols. It was found that the acetals remained stable over time, whereas most 5-HMF decomposed after 24 h in a basic solution.
The electrooxidation using a Ni-foam anode revealed that 5-HMF mainly yields 2,5-FDCA, whereas the furanic acetals selectively produce the more reactive 5-FFCA. This highlights the effective passivation of a functional group by employing protection strategies.
Furthermore, changes in the reaction pathways were observed, as some reaction steps were blocked due to the presence of acetals. Lowering the base concentration to potentially promote the deprotection did not enhance the 2,5-FDCA formation but resulted in higher conversion and higher yields of the 2,5-DFF intermediate.
This study shows that smart material design can produce catalysts that rival complex benchmark materials, despite often being considered unattractive. Additionally, this work demonstrates that research can lead to unexpected findings, such as investigating plastic pyrolysis and electrochemical reactions.
Title of PhD thesis: “Amorphous silica-alumina catalysts for the conversion of renewable feedstocks”
Supervisor: Emiel Hensen