|Fig. 1: The Hydrodeoxygenation Process.  (Source: J. Adams)|
Bio-fuels are currently popular alternative sources of energy to petroleum-based fuels. They are especially attractive as they can be produced within relatively short cycles and negligibly contribute to environmental pollution.  A high bio-oil yield from biomass can be achieved using fast-pyrolysis, a technique that involves rapidly heating the biomass feedstock in a fluidized bed reactor in the absence of oxygen to approximately 500°C in less than one second. The bio-oil vapor exits the reactor and bulk solid particulates, and char are removed. 
The product is then condensed to produce a raw, liquid bio-oil, as illustrated in Fig. 1. The resultant bio-oil has a high oxygen content and low stability over time and a low heating value. Upgrading is then needed to remove the oxygen and make it resemble crude oil.  Among the oxygen removal techniques, catalysis is believed to be one of the most efficient. There are two major routes of upgrading bio-oil to industrial grade level: high pressure hydrodeoxygenation (HDO), the focus of this report, and catalytic fast pyrolysis with zeolites. 
With zeolites, used as catalysts for the deoxygenation reaction, the process can occur at atmospheric pressure because hydrogen is not required. But low hydrogen-content results in a low H/C ratio making the oil from zeolite catalysis a low grade with heating values approximately a fourth lower than that of crude oil.  HDO is the preferred method as it is the frontrunner in producing a high-grade oil equivalent to crude oil in price and heating value.
HDO is a high pressure operation through which hydrogen is used to extract oxygen from the bio-oil, giving a high-grade oil product. Furthermore, high pressure hydrogenation can prevent carbon deposition on the catalyst surface, which would ease reactor operation. The reaction utilizes traditional hydro-desulphurization (HDS) catalysts, such as Cobalt MoS2/Al2O3 or metal catalysts such as Pd/C.  Catalytic upgrading of bio-oil is a complex reaction network due to the high diversity of compounds in the feed. The overall reaction is exothermic and overall heat of reaction is in the order of 2.4 MJ/kg when using bio-oil. 
Cobalt and Nickel donate electrons to the molybdenum atoms, weakening the bond between molybdenum and sulphur, creating a vacancy in the sulfur that becomes the active site. The oxygen of the bio-oil comes in and a neighboring sulphur donates a proton to the attached molecule, forming a carbocation, which subsequently undergoes carbon and oxygen bond cleavage. Oxygen exits through the forming of water. 
For the mechanism to work, the oxygen group formed on the metal site needs to be eliminated as water. After a prolonged operation, the catalyst will transform from sulphide to an oxide form, diminishing catalytic activity. In order to avoid this, adding H2S to the system will regenerate the sulphide sites.  Yet the influence of sulphur on catalyst stability is currently unknown and needs to be further evaluated.
Noble metal catalysts Ru, Rh, Pd are all high-performing catalysts for the HDO synthesis, but the high price of the metals make them unfeasible for extended use. Cheaper alternatives including Pd/C and Raney Nickel can be effective catalysts when combined with Nafion/SiO2, however efficacy has been quantified only in batch experiments with low phenol-containing compounds. 
A myriad of issues must first be addressed before the HDO process can be fully commercialized. Catalyst development and refinement, understanding of the coke-formation, the effect of impurities on bioactivity and performance, and analyzing performance of different catalysts are needed to optimize the process and bring it closer to industrial utilization. Currently, many resources and efforts are allocated to the Co-MoS2 system or metal catalysts, but due to carbon-formation tendencies and the high prices of raw material for the noble metals, alternatives are needed.
© Jennifer Adams.The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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