The field of nanocatalysis (the use of nanoparticles to catalyze reactions) has undergone an explosive growth during the past decade, both in homogeneous and heterogeneous catalysts. Since nanoparticles have a large surface-to-volume ratio compared to bulk materials, they are attractive candidates for use as catalysts. Nanoparticles of metals, semiconductors, oxides and other compounds have been widely used for important chemical reactions.

In recent years, nanomaterials have attracted extensive interest for their unique properties in various fields (such as catalytic, electronic and magnetic properties) in comparison with their bulk counterparts. In view of biomass conversion, nanocatalysts come into view as one of the most promising additives to make fuel combustion complete and fast, decrease ignition time, and therefore produce little or non-toxic by-products. In fact, the large surface areas of nanoscale catalysts as well as reports on novel chemical reactivity of particles with nanometer dimen­sions make these materials highly interesting.

Only limited studies are available in the open literature for the application of nano metal oxides in biomass pyrolysis/gasification [34, 35]. Regarding increased relative surface area of the nanomaterials, it is highly expected that nanocatalysts would have a better catalytic activity in enhancing the performance of biomass gasification/pyrolysis. Gokdai et al. found that variation in pyrolysis temperature had a distinct effect on gas evolution in the presence of nano SnO2 particles [36]. The maximum gas yield in this study was obtained by nano SnO2—hazelnut shell interaction at 700°C, while the pyrolytic oil yield obtained by nano SnO2 at 700°C reached its minimum value compared to the other catalysts used. This behavior of nano SnO2 can be explained by accelerated primary and secondary decomposition reactions of hazelnut shell in the presence of nano SnO2 due to the size (3-4 nm) and larger external surface area of the nanoparticles as given by Li et al. [34]. This behavior of nano SnO2 can also be seen by the comparison of the yields obtained by bulk SnO2. In view of the gaseous products generated, nano SnO2 showed better performance at higher temperatures among the catalysts used.

Li et al. prepared nano NiO and tested its activity during biomass pyrolysis using a thermogravimetric analyzer [34]. Lu et al. investigated that nano TiO2 and its modified catalysts were used for experiments and confirmed to have some good catalytic activities [37]. In this study, six nano metal oxides were used as catalysts to test whether they had the capability to upgrade the fuel properties of bio-oil or maximize the formation of some valuable chemicals. The experiments were performed using an analytical Py-GC/MS instrument which allows direct analysis of the pyrolytic products. The catalytic and non-catalytic products were compared to reveal the catalytic capabilities of these catalysts.

Among the six nano metal oxides, CaO was the most effective catalyst in altering the pyrolytic products. It reduced most of the heavy products (anhyd — rosugars and phenols), and eliminated the acids, while it increased the formation of hydrocarbons and cyclopentanones. Moreover, it increased four light products (acetaldehyde, acetone, 2-butanone and methanol) greatly, which made the catalytic bio-oil a possible raw material for the recovery of these products. ZnO was a mild catalyst because it only slightly altered the distribution of the pyrolytic products. With regard to the other catalysts, they all reduced the linear aldehydes, while they increased the methanol, linear ketones, phenols and cyclopentanones levels. They also reduced the anhydrosugars remarkably, except for NiO. Moreover, the catalysis by Fe2O3 was capable of forming various hydrocarbons, but with several PAHs. These catalytic effects suggested a potential for bio-oil quality improvement, due to the enhanced stability promotion due to the reduced aldehyde levels and increased methanol, and the heating value increase by the formation of cyclopentanones and hydrocarbons. In addition, the increased phenol content after catalysis enabled the recovery of the valuable phenols from the catalytic bio-oils. However, none of these catalysts except CaO were able to greatly reduce the acids, which could be a problem for the use of catalytic bio-oils as liquid fuels.

5.2 Conclusion

The sharp increase in the worldwide oil prices will play an important role in the realization of alternative, renewable energy systems such as bio-oil production, syngas generation from biomass in which the types of catalysts play an important role. Although catalytic behaviors of catalysts differing in acid/base properties, metal (Ni, Pt, etc.) content and porous structure on thermal biomass conversion are widely known, it is needed to develop new types of catalysis for biomass conversion in order to improve the quality of products. Nanoparticles with increased surface area are attractive candidates for such applications.