Catalytic upgrading of bio-oil produced from fast pyrolysis of Pinewood sawdust

Catalytic biomass pyrolysis (CBP) is considered an effective approach to convert the oxygenated compounds into various hydrocarbons and improve bio-oil quality. The introduction of a catalyst generally decreases the temperature of the pyrolysis process and removes the oxygen, and converts the oxygenated compounds like phenols, ketones, alcohols, esters into various hydrocarbons through a variety of catalytic reactions such as dehydration (removing oxygen as H2O), decarboxylation (removing oxygen as CO2) and decarbonylation (removing oxygen as CO), hydrogenation, condensation, aromatization and polymerization. There are different modes of CBP in a fixed-bed reactor, primarily used in-situ and ex-situ, and less studied combined in-situ and ex-situ mode. In-situ pyrolysis involves the addition of a catalyst mixed with biomass. In contrast, in ex-situ pyrolysis, the catalyst is separately placed downstream to the biomass, and the produced pyrolytic vapours are passed through the catalyst bed. The combined in-situ and ex-situ mode utilizes a catalyst mixed with biomass and a similar or different catalyst placed downstream to convert the unreacted pyrolytic vapours. This thesis examines the application of microporous and mesoporous solid acid catalysts like zeolites, Al2O3 and basic catalysts such as CaO in three modes of CBP for bio-oil upgrading. Radiata pine sawdust was selected as the feedstock for pyrolysis to produce bio-oil. 

The first key chapter of the thesis examines the comparative catalytic activity of zeolite catalysts (Zeolite, Cu/zeolite and Ni/zeolite) on bio-oil upgrading in three modes of pyrolysis: in-situ, ex-situ, and combined in-situ and ex-situ. Though noticeable bio-oil deoxygenation was achieved in ex-situ and combined pyrolysis mode, the study concludes that ex-situ pyrolysis mode is economically beneficial compared to either in-situ or combined since the catalyst can be easily retrieved from the process, oxidized to remove the coke and reused in the pyrolysis. Therefore, considering the importance of ex-situ pyrolysis mode, bio-oil upgrading was further investigated using various catalysts. 

In the next chapters, the catalytic activity of monometallic catalysts Cu/zeolite and Ni/zeolite was compared with a bimetallic catalyst NiCu/zeolite in one-stage ex-situ pyrolysis. The catalysts were used in ex-situ pyrolysis with three different C/B ratios of 1, 2, and 3. CuNi/zeolite showed better deoxygenation efficiency than monometallic catalysts and produced a comparatively higher percentage of aromatic hydrocarbons at 14.3% and aliphatic hydrocarbons at 39.9%. The main deoxygenation pathway during monometallic catalytic pyrolysis was found to be dehydration and decarboxylation because a higher CO2 yield was observed during the reaction. The CuNi/zeolite converted the oxygenated compounds into hydrocarbons via dehydration, decarboxylation, and decarbonylation because higher yields of both CO2 and CO were observed. Overall, CuNi/zeolite catalytic pyrolysis of biomass resulted in improved bio-oil quality when compared to the monometallic counterparts. The activity of CuNi/zeolite was further compared with combined mono-metallic catalysts in two-stage ex-situ pyrolysis mode. The results demonstrated that in comparison to the combined mono-metallic catalysts, the sole bi-metallic catalyst showed better deoxygenation for all the oxygenated compounds and favoured the production of aliphatic hydrocarbons, whereas the combination of monometallic catalysts generated higher proportion of aromatic hydrocarbons in the bio-oil. Considering the significance of bimetallic catalysts over monometallic catalysts, more bimetallic catalysts with combinations of Ni, Cu, Fe, and Mo on ZSM-5 support were prepared and examined for bio-oil upgrading. It was observed that the synergistic effect of Ni-Cu and Ni-Fe showed higher hydrocarbon production compared to Cu-Fe, Ni-Mo, Cu-Mo, and Fe-Mo, which can be attributed to their higher surface area that probably resulted in better metal dispersion on ZSM-5 surface and synergistic catalytic sites that favoured selective deoxygenation reactions. Ex-situ CBP could be either one-stage (a single catalyst is used) or two-stage (two catalysts are used). Though one-stage ex-situ CBP has been widely explored with different types of catalysts, the effect of catalyst type on bio-oil upgrading during two-stage ex-situ CBP was not investigated. Thus, to understand the impact of the nature of catalyst support in two-stage ex-situ CBP, different catalytic supports, from strongly acidic to basic such as ZSM-5, Al2O3, Al2O3/CaO/MgO, and CaO with and without Ni loading were demonstrated in both modes of ex-situ CBP. It was found that in one-stage ex-situ CBP, microporous acidic catalysts like ZSM-5 and Ni/ZSM-5 promoted the formation of naphthalenes and other polycyclic aromatics, mesoporous Al2O3, and Al2O3/CaO/MgO or Ni-modified counterparts also favoured the formation of benzene derivatives and cycloalkanes, while CaO or Ni/CaO generated aliphatic hydrocarbons. It was further noticed that the combination of mesoporous and microporous catalysts in two-stage ex-situ CBP provided varying catalytic properties and improved mass transfer kinetics which was advantageous to produce a variety of hydrocarbons.