Cellulose Photocatalysis for Renewable Energy Production

Abstract

Renewable energy, especially biofuels, has the potential to supplement a part of the existing fossil-based transportation fuels. Biofuels such as bioethanol can be produced from established glucose fermentation processes; however, cheap and efficient glucose production is a hindering factor. Cellulose, the world’s most abundant organic material, is a polymer of glucose and is locked within the lignocellulosic framework of plants. Pre-treatment is required to facilitate effective cellulose saccharification. Semiconductor photocatalysis, an advanced oxidation process, is a potential method for cellulose breakdown and the focus of this chapter will be on harnessing photocatalysis for cellulose-based renewable energy production.We reviewed over 100 publications along the lines of structural properties of cellulose polymorphs, current pre-treatment strategies, conversion methods from native cellulose I to an easily degradable polymorph cellulose II, and how semiconductor photocatalysis, an advanced oxidation process, can facilitate cellulose breakdown for saccharification and energy generation. Early reports in the 1980s utilized cellulose as sacrificial electron donor for the photocatalytic H2 production over TiO2 and noble metal co-catalysts; however, recently visible light activated photocatalysts have been used. An often-overlooked area with photocatalysis is cellulose saccharification. Only a handful of reports talk about saccharification over P25 TiO2, which is predominantly due to the lack of control over process conditions leading to undesired products. Addressing the recalcitrance of cellulose, by converting native cellulose I to cellulose II using a simple dissolution and regeneration technique, can be advantageous for enhancing valorization via semiconductor photocatalysis. For example, ~1.5-fold increase in photocatalytic cellulose saccharification was observed with cellulose II compared to cellulose I over P25 TiO2. A similar increase in photocatalytic H2 production rate by over twofolds was also observed when cellulose II was used in suspension with Pt/TiO2. With advances in the development of semiconductor photocatalytic materials, especially in visible light activated materials such as CdS, reactor design, light emitting diode technology, and a novel “simple to produce” feedstock such as cellulose II, vast possibilities have opened up in this novel area of photocatalytic cellulose saccharification and H2 production. Overall, the chapter aims to instil interest in the readers that combining cellulose II and semiconductor photocatalysis can be a feasible way forward to contribute toward renewable energy production.