Synthesis and characterisation of sulfonated Starbons®, bio-based catalysts

The overall objective of this thesis was to develop an enhanced understanding of the sulfonation of Starbons® and the properties of the sulfonated materials, using conventional and microwave heating. Starbons® are materials prepared from expanded starch. Due to their renewable nature, these materials are great candidates to be explored in our search for sustainable development. The sulfonated Starbons® (S-Starbons®) were tested as solid-acid catalysts in esterification reactions using microwave irradiation.

A green chemistry analysis of some current routes to sulfonated carbonaceous materials is presented in Chapter 1. Here, an in-depth description of Starbons® is also given, as a brief review of some techniques used in their characterisation. Synthesis and characterisation of S-Starbons® by conventional heating and microwave irradiation are presented in Chapter 2 and Chapter 3, respectively. Elemental composition of Starbons® depends on carbonization temperature, changing from high-oxygenated (300 °C) to more carbon-like (800 °C) materials. After sulfonation, the sulfur content varies with carbonization temperature, being particular high for the most-oxygenated samples. All S-Starbons® present a characteristic IR band ca. 1030 cm-1 independently of sulfonation method. 13C solid-state NMR gives structural information from S-Starbons®, however, due to overlapping resonances it was difficult to identify the C–S resonance. XPS showed that sulfur (VI) is the only one observed in S-Starbons® 300, but sulfur (II) appears in higher temperature carbonized Starbons®. The main difference between microwave and conventional sulfonated Starbons® 300, was the appearance of another sulfur (VI) species in the microwaved samples. Chapter 4 relates to the stability of the sulfur-bonding in S-Starbons®,through TG-FTIR analysis. This study suggests that stability of the sulfur bonding is higher in S-Starbon® 300 than in S-Starbon® 800. This observation was correlated with the catalytic performance of these materials (Chapter 5), where S-Starbon® 800 loses its activity more rapidly than S-Starbon® 300.