- Title
- Synthesis of highly ordered N-rich mesoporous carbon nitride and its catalytic properties for base-catalysed transformations
- Creator
- Maria Ruban, Sujanya
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2023
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Base catalysis is a primary organic reaction for several chemical transformations in fine chemical and petrochemical industries. Traditionally, the base catalyses are performed using homogeneous Brönsted and Lewis bases, such as piperidine, pyridine transition metal halides, and tetraalkyl ammonium salts. However, a crucial disadvantage lies in the separation and recovery of catalysts and recovery due to their homogeneity. Alternatively, the heterogeneous solid base and metal oxide catalysts, such as alkali metal hydroxide and alkaline earth metal oxides, such as Mg-Al-O, MgO, and Al2O3, give good catalyst recyclability. However, they suffer from the leaching of active metal sites in the reaction system owing to the poor stability and a lower number of active sites, leading to reduced catalytic activity. Therefore, it is imperative to design and develop low-cost, sustainable, and efficient heterogeneous catalysts from a commercial point of view. Graphitic carbon nitride (g-CN) possesses unique properties including abundant basic sites, and polymeric conjugation between C and N atoms and has emerged as one of the promising materials for basic catalysis. Moreover, the catalytic activity of g-CN can be improved by adjusting the basicity and introducing structural porosity using various strategies. Generally, the carbonisation of C and N containing precursors such as cyanamide, melamine, and dicyanamide at high temperatures (550-600 oC) yields g-CN having a typical structure of g-C3N4 with tectonic units linked by s-triazine rings or tri-s-triazine by N bridges. The difference in electronegativity between C and N implies that the valence electrons will transfer to N atoms. Using mesoporous silica KIT-6 or SBA15 templates, ordered mesoporous structures can be introduced in carbon nitride which also results in large surface areas (>100 m2 g-1). Their stability and easily accessible pores for many types of molecules are promising for upgrading the base catalysis in performance and recycling. However, such a mesoporous CN requires high-temperature carbonisation and suffers from the drawback of low catalytic performance due to low nitrogen content which is linked to low basicity. Recently, it was revealed that it is possible to increase the nitrogen content to higher levels in CN by altering the carbonisation temperature to a lower range (350-450 °C). This has led to reports on newly developed CNs such as C3N7, C3N6, and C3N5. The higher nitrogen content of these compounds and good surface area present an attractive proposition for base-catalysed reactions such as Knoevenagel condensation. Knoevenagel condensation is one of the highly sought-after base-catalysed reactions in the pharmaceuticals industry to synthesise α, β unsaturated carbonyl compounds using an aldehyde and an active methylene compound as starting precursors. A base catalyst is necessary to eliminate a proton from the active methylene compound and activate it for the reaction. However, as mentioned previously, most base catalysts are homogenous, and their separation, recovery, and reuse are dominant issues. The heterogeneous catalysts are also confronted with problems such as reduced active sites and leaching. The research on nitrogen-rich carbon nitrides has recently come to light with their discovery, and since then, efforts have been made to realise their usefulness for various applications. However, there are several hurdles in their synthesis and characterisation of high N-containing CN including the lack of literature on their application in the field of catalysis. Therefore, this PhD thesis is focused on the development of new nitrogen-rich carbon nitride-based materials through the exploration of various starting CN precursors and the experimental conditions and their use thereof in base catalysis using the Knoevenagel condensation.
- Subject
- base catalysis; nitrogen-rich carbon nitrides; knoevenagel condensation; synthesis
- Identifier
- http://hdl.handle.net/1959.13/1514239
- Identifier
- uon:56831
- Rights
- Copyright 2023 Sujanya Maria Ruban
- Language
- eng
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