Pyrolysis is the thermal heating of materials in the absence of oxygen, which results in the production of three categories: gases, pyrolytic oil and char [22, 23].

Fructose ♦ Furans —- ♦ Biofuels

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Dimcthymiran

—- ► Biochemials

Lcvulinic Acid

♦ Biomaterials

Fig. 5.2 Effect of catalysts on biomass conversion

Pyrolytic oil, also known as ‘‘tar or bio-oil’’, cannot be used as transportation fuels directly due to the high oxygen (40-50 wt%) and water contents (15-30 wt%) and also low H/C ratios. However, pyrolytic oil is viscous, corrosive, relatively unstable and chemically very complex [1, 24-26]. To use bio-oil as a conventional liquid transportation fuel, it must be catalytically upgraded [31]. Catalytic pyro­lysis (Fig. 5.2) is an acceptable method for improving the quality of pyrolytic oil such as removal of oxygen, increasing calorific value, lowering the viscosity and improving stability. Many researches have been carried out on upgrading pyro­lytic oil in the presence of different catalysts such as HZSM-5, MCM41, Al2O3, Al2O3/B2O3, Na2CO3, NaOH, NaCl, Na2SiO3, TiO2, Fe/Cr, etc. [27-30]. Upgrading of the gaseous products from pyrolysis can also be achieved by reacting the vapors directly with a catalyst (in situ pyrolysis).

ZSM-5 is an aluminosilicate zeolite with a high silica and low aluminum content. Its structure is based on channels with intersecting tunnels (Fig. 5.3). The aluminum sites are very acidic. The substitution of Al3+ in place of the tetrahedral Si4+ silica requires the presence of an added postive charge. When this is H+, the acidity of the zeolite is very high. The reaction and catalysis chemistry of the ZSM-5 is due to this acidity.

Zeolite catalysts added into the pyrolysis process can convert oxygenated compounds generated by pyrolysis of the biomass into gasoline-range aromatics. Using zeolite catalysts in pyrolysis, Carlson et al. [31] reported that gasoline-range aromatics can be produced from solid biomass feedstock in a single reactor at short residence times (less than 2 minutes) and at intermediate temperatures (400- 600°C). In fact, acidity of an ideal catalyst for biomass pyrolysis should be manupilated by various methods such as ion exchange with alkalis. Silica-alumina containing catalysts (weak acids) might also be given as an example.

Mobile crystalline material (MCM-41) is one of the most used catalysts for the conversion of biomass to value-added products during pyrolysis (Fig. 5.4).

Fig. 5.4 The hexagonal pore structure of molecular sieve MCM-41 (red oxygen, blue silicon, light blue hydrogen, brown carbon)

Pore size of MCM-41 is relatively narrow and this catalyst has a large surface area (>1000 m2 g-1). MCM-41 type mesoporous catalysts converted the pyrolysis vapors into lower molecular weight products, and hence, more desired bio-oil properties could be achieved. The catalytic properties of MCM-41 materials can be significantly improved when specific transition metal cations or metal complexes are introduced into the structure. Pore enlargement allows the processing of larger molecules. Different pore sizes were obtained by altering the chain length of the

template and by applying a spacer. Due to the activity of the catalysts, the product distribution of pyrolysis vapors changed significantly. In accordance with pub­lished reports, higher coke and water formation was observed during the reaction in the presence of the catalysts. The various catalysts showed different influences on the product distribution, and the greatest difference was achieved by using the unmodified Al-MCM-41 catalyst [32].

Stefanidis et al. [33] recently investigated the catalytic activity of Silicalite, ZSM-5, MCM41 and Al2O3 for the pyrolysis of beech wood. The results are given in Table 5.1. They found that the use of strongly acidic zeolite H-ZSM-5 leads to a decrease in the total liquid yield (bio-oil) while decreasing the organic phase of bio-oil and increasing its water content, accompanied by an increase of gases and formation of coke on the catalyst.

According to this study, it was found that zeolite silicalite with very low number of acid sites and the mildly acidic Al-MCM-41 induced similar effects with those of H-ZSM-5 but to a less extent, except of the significantly higher coke that was deposited on Al-MCM-41. With regard to the composition of bio-oil, all the catalysts and mostly the strongly acidic H-ZSM-5 zeolite reduced the oxygen content of the organic fraction, mainly by decreasing the concentration of acids, ketones and phenols.