S. Renjitha, Nakul S. and Dr. Naveen V. Kulkarni
Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri 690525, Kerala, India
We are experiencing increased population and human demands in the 21stcentury.Our needs are growingconstantly forthe fossil products, especially the gasoline and other petrochemicals. Recent reports state that our petroleum resources will end in another 50 years. Thus, these important petrochemicals will not be available in the future for human needs in the same means we currently use them. Therenewable energy sources which can be replace the fossil fuels aremost essentialfor the sustainable future. Scientists worldwide are constantly looking for alternative energy options with the primary aim to use biomass to make valuable things[1].
Biomass contains varieties of organic molecules with a high number of oxygen functionalities. The high oxygen content can pose challenges in the practical use of biomass products as especially as fuels. Removing oxygen is a challenging task, various methods including dehydrogenation, decarboxylation, and deoxy dehydrogenation(DODH) have been successfully employed in this direction so far.The DODH reaction is mainly used to convertvicinal diols into olefins (Figure 1). This reaction can be done with the help of a metal catalyst or without, the reactive conditions,temperature, solvent, and the presence of a reducing agent are found to play key role[2]. This reaction was first developed in 1921 by O. Kamm and C.S. Marvelto synthesise allyl alcohol from the glycerol[3].
Figure 1: DODH reaction of vicinal diols
Metal Catalyzed DODH Reaction
Metal catalyzed DODH reaction was first reported in 1996 by G. Cook and M. Andrews, where they used a ReO3 catalyst along with the PPh3 reductant[4]. The trioxo-rhenium complexes have been much explored in this reaction. Numerous studies were carried out using the trioxo-rhenium complexes and the detailed analysis of the influence of solvents, reductants,temperatures on the reaction mechanism have been well established [5]. Despite of their best performance in the DODH reaction, due to the low abundance in the earth crust and the high cost involved, the rhenium-based catalysts cannot be used in the bulk-industrial processes. This has led to the development of cheaper, earth abundant, transition-metal based catalysts for the DODH reaction. Currently, vanadium and molybdenum-based systems have found to exhibit competitive performance in this reaction, however, they require higher temperatures and high catalyst loading as compared to the rhenium-based catalysts [6].A general reaction mechanism, as depicted with a dioxo-molybdenum catalyst involvesthree main steps (Figure 2). First, the catalyst gets reduced by the reducing agent producing a reductant oxide species and then the condensation between the catalyst and diol takes place to form a metal diolate. Then the deoxygenated product is released to regenerate the oxo-catalyst [7].
Scheme 2: DODH reaction mechanism
Metal-free DODH Catalyst
Metal-free methods typically have much lower efficiency than the homogeneous or heterogeneous metal-based methods. Metal-free processes rely on stoichiometric protocols, which use chemical reagents in excess to drive the reaction to completion.Metal-free DODH processes are becoming more popular due to their potential advantages, suchas avoiding hazardous metals and lowering costs. Many metal-free catalysts have been developed for the DODH reaction, including solid acids, zeolites, and other photocatalysts. However,the challenge of high selectivity and avoiding unwanted side effectsremains the same[2].Organiccatalysts, including eosin Y, Rose Bengal, and riboflavin, are used in metal-free DODH processes because they can produce reactive intermediates when exposed to visible light. These intermediates can then aid the formation of alkenes from the deoxy dehydration of diols. Mild reaction conditions, great selectivity, and minimal environmental effect are some of the benefits associated with metal-free processes [8]. Using acid-catalysed dehydration processes is another recent development in metal-free DODH reactions. This method selectively dehydrates diols; diols are selectively dehydrated to alkenes utilising strong acids like para-toluenesulfonic acid (PTSA)[9].
Overall, both metal-catalyzed and metal-free DODH reaction have their own drawbacks and advantages. However, it should be noted that, in order to develop the reactions in industrialscale the overall efficiencies of these reactions should be improved and the reaction conditions should be optimized to make it more nature friendly.
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