Sugars from formaldehyde: a study on the formose reaction

Abstract

The FR is the oligomerization of FA to give monosaccharides. This fascinating mechanism is thought to be the source of prebiotic sugars that might have originated the life in our planet. Since its discovery in 1861, scientists have studied the reaction from different points of view and, nowadays, even if its main features are known, some aspects are still under investigation. The oligomerization of FA is base-catalyzed and follows the mechanism of the aldolic addition, although the concurrence of side reactions, recombination, and cleavages makes the FR a very complex system from which a mixture of products containing linear and branched monosaccharides ranging from C2 to C6 or higher, polyols, and sugar acids. The FR suffers an important drawback, which is the unselective nature of its mechanism that makes it impossible to harness the potential of the reaction for practical applications. In this Thesis, some challenging aspects of the FR are addressed. A robust analysis method is developed that combines a double derivatization approach and the use of HPLC and GC-MS and permits the quantification of FA and the identification of the formose products. This method is used throughout the Thesis and is the backbone that supports all the Chapters. Since almost any kind of base can catalyze the FR, along the Thesis, a profound investigation is carried out to find out the most innovative and less studied catalysts with interesting properties that can be favorable for increasing the FR selectivity. Special emphasis is given to heterogeneous porous materials, such as MOFs and zeolites, that are known for their versatility and the wide application they find in organic catalysis. Furthermore, inspired by the theories that attribute to the FR the origin of life on our planet, a plasma-induced FR is presented in this Thesis for the very first time. Remarkable results are obtained when MOFs and zeolites are used as catalysts, as an effect of size selection is observed, attributable to their porous structure, which determines an increased selectivity compared to traditional catalysts such as Ca(OH)2. In the past, many attempts to increase the selectivity of the FR have been carried out, with only modest results. The achievements presented in this Thesis are thus especially valuable since they are connected to the use of two particularly versatile classes of materials, MOFs, and zeolites, whose basic strength, porosity, hydrophilicity, and stability can be tailored and adapted to the specific application. Therefore, this Thesis represents a solid point of start for future research, where the enormous potential of MOFs and zeolites is explored and consolidated and may lead to the ultimate solution to FR's lack of selectivity. Moreover, it is here demonstrated that plasma can induce the reaction without the addition of any further initiator. The effects of heating and UV light are also separated, proving that plasma is the only responsible for the activation of the mechanism at the used conditions. As the plasma-induced FR is presented in this Thesis for the very first time, this work opens the path for future investigations in this new and exciting field of study.