Nanocatalyst for Biofuel Production: A Review

Abstract

The demand for renewable and alternative types of energy has taken a new dimension; the primary reason is traceable to the climate change effects that fossil-based fuels have in the earth atmosphere. Bioenergy is one of the many arrays of renewable forms of energy that have taken centre stage in replacing the con- ventional fossil-based energy sources. Biofuel is the liquid or gaseous fuel derived from biological processes such as agriculture (biomass) or anaerobic digestion (solid or liquid wastes) or a combination of both, rather than geological processes that are known with the traditional fossil-based counterparts. Biomass energy is readily available and environmentally friendly, because it does not lead to a net increase in carbon dioxide levels and produces low amounts of sulphur. An effective implementation of biomass in the current energy scheme would involve the development of new technologies for the large-scale production of biofuel. The two primary methods for converting biomass to biofuels are thermochemical and biochemical processes. Thermochemical conversion is a major path for producing products such as bioethanol, biodiesel, bio-oil, bio-syngas and biohydrogen. It includes fast pyrolysis, liquefaction, combustion and gasification. Liquefaction of biomass to bio-oils involves two main routes which are hydrothermal and catalytic liquefaction. Catalytic liquefaction is very similar to hydrothermal liquefaction; however, a catalyst is used to reduce the residence time, operating temperature and pressure thereby increasing the quality of liquid products. Thermochemical biofuels are getting much more attention lately as these biofuels offer several technical and strategic advantages, such as highly developed industrial infrastructure and the biofuels can be produced from virtually all sorts of available biomass in a rea- sonable timeframe without significant modification in the overall process. At lower reaction temperatures, thermal processing of biomass with catalytic methods offers the possibility of selectively yielding a narrow range of products and reducing the energy requirements of the transformations. In terms of catalysts used, for biodiesel production, heterogeneous catalysts in comparison to homogeneous catalysts provide more effective separation steps for products and catalysts, eliminate quenching process, and offer conditions for the continuous production process. The objective of this review is to discuss the trends, recent advances in heterogeneous catalysts and expected contribution to knowledge, specifically in nanocatalysts for biofuel production, such as metal oxide catalysts (e.g. ZnO), metal supported by metal oxide catalysts (e.g. Au–ZnO), Alloy (e.g. Cu–Co), Metal oxide supported by metal oxide (e.g. KF–CaO–Fe3O4). Our focus will be on heterogeneous catalysts.