Edinburgh Research Explorer Selective Deoxygenation of Biomass-Derived Bio-oils within Hydrogen-Modest Environments: A Review and New Insights Citation for published version: Rogers, K & Zheng, Y 2016, 'Selective Deoxygenation of Biomass-Derived Bio-oils within Hydrogen-Modest Environments: A Review and New Insights', Chemsuschem, vol. 9, DOI:10.1002/cssc.201600144, pp. 1750- 1772. <http://onlinelibrary.wiley.com/doi/10.1002/cssc.201600144/abstract> Link: Link to publication record in Edinburgh Research Explorer Document Version: Peer reviewed version Published In: Chemsuschem General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact [email protected] providing details, and we will remove access to the work immediately and investigate your claim. Download date: 07. Oct. 2021 DOI:10.1002/cssc.201600144 Reviews Selective Deoxygenation of Biomass-Derived Bio-oils within Hydrogen-Modest Environments:AReview and New Insights Kyle A. Rogers[a] and Ying Zheng*[a, b] ChemSusChem 2016, 9,1750 –1772 1750 2016 The Authors. PublishedbyWiley-VCH Verlag GmbH &Co. KGaA, Weinheim Reviews Research development of processes for refiningbio-oils is be- ing acombination of oxophilicity and an active metal phase comingincreasingly popular.One issue that these processes appear to be the most beneficial for selectivedeoxygenation possess is their high requirement for H2 gas. In response, re- processes in aH2-modest environment. It is importantthat cat- searchers must develop catalysts that perform deoxygenation alysts have asupply of disassociated hydrogen, because with- while minimizingH2 consumption—selectivedeoxygenation. out such, activity and stability will suffer.The authors recom- Unlike traditionaldeoxygenation processes, selective deoxyge- mend to maximize the use of internally availablehydrogen in nation reactions and catalysts represent an information gap bio-fuel, which may be the only viable approach for deoxyge- that, prior to this publication,has yet to be reviewed. This nation if externalH2 gas is limited. This would be possible review addresses the gap by providing both asummary of through the development of catalysts that promote both the recent research developments and insightinto future develop- water–gas-shift and deoxygenation reactions. ments of new catalytic materials. Bifunctionalcatalysts contain- 1. Introduction Compared to typical hydrocarbon fuels, both vegetable oil and bio-oil are poor selections as direct fuels owing to their high Within recent years, there hasbeen increasing interestinthe oxygen content,which leads to high viscosity,low volatility, development of biofuels. The reason forsuch has been argued corrosiveness, poor solubility in other hydrocarbons, and low to be due to the threat of globalclimate change and ashort- energy content.[3b,4] These oils tend to be hydrophilic, which age of oil reserves.[1] It is in the best interestofthe current so- can lead to high water contentsand/or polymerization. Bio-oil, ciety that engineers and scientists endeavor to develop new, in particular,typically hasanoxygen content within the 10– sustainable energy resources whilst reducing greenhouse-gas 40 wt%range or even as high as 50 wt%and awater content emissions. Withinthe transportationindustry,biofuelsare the of 15 to 30 %.[5] One solution to these oxygen-related problems only carbon-neutral alternatives, and they are becoming in- is to perform deoxygenation with aheterogeneous catalyst creasingly important,asthey possess the potentialtobeincor- similar to how petroleum oil undergoes desulfurization and de- porated into the existing infrastructure.[2] nitrification by ahydrotreatment process.The main objective Some biofuelshavealready begun to merge into the trans- is to effectivelyremove oxygen in the form of CO2,CO, or H2O. portation industry,including bioethanol and biodiesel com- Therein lies an issue—the use of H2,which is typically posed of fatty acid methyl esters (FAMEs). Thereare, however, amajor requirement for the deoxygenation of vegetable oils [1c,5b, 6] concerns about these first-generation biofuels. First, they and bio-oils. However, there is adesire to reduceH2 con- cannottotally replacefossil fuels—gasoline and diesel—owing sumption and use systemsthat are either H2-modest (low H2 [1b,c,6] to differences in properties such as reduced energy densities pressures/flowrates) or use an inert atmosphere (no H2). and viscosities at low temperatures.[1b,3] Second, especially for The reason for this is because of the costs associated with the first-generation bioethanol, biofuels shouldideally not be pro- use of H2 and the fact that the majority of the world’sH2 pro- duced from food. Second-generation biofuels aim to resolve ductioncomes from fossil-fuel reforming.Ideally,biofuels, the issues of first-generation fuels by producing fully compati- which are supposed to be considered sustainable and renewa- ble fuels from sourcesthat cannotbeused for food such as ble, should not be heavily dependentonnonrenewable sour- nonedible vegetable oils, lignocellulosic material, and wastes. ces. Second-generation bio-fuels include fuels derived from non- Various reviews have been published in recent years with edible vegetable oils or bio-oil derived from lignocellulosic ma- regard to the deoxygenation of biomass-derived bio-oils with terial. Thisreview concernssecond-generation bio-fuels pro- focus on deoxygenation reaction pathways and hydrodeoxyge- duced by thermal processes such as pyrolysis and liquefaction. nation processes in the presence of high-pressure H2,and therefore, they are out of the scope of this review.Gosselink [a] K. A. Rogers, Prof. Y. Zheng et al.[1b] review the deoxygenation of vegetable oils, fattyacid Department of Chemical Engineering esters, and free fatty acids with amajor focus on reactionpath- University of New Brunswick Fredericton, NB E3B 5A3 (Canada) ways, especially in the presence of H2.Santillan-Jimenez and [1c] E-mail:[email protected] Crocker focus primarilyonreaction pathways for the deoxy- [1a] [b] Prof. Y. Zheng genationoffatty acids under inert atmospheres. De et al. SchoolofEngineering provideanoverview of recent work in the hydrodeoxygena- University of Edinburgh tion of bio-oil compounds derived from thermalprocesses. In The King’s Buildings,Edinburgh EH93DW (UK) E-mail:[email protected] their review of the generalcatalytic upgrading of bio-oil, Mor- [5b] The ORCID identification number(s) for the author(s) of this article can tensenetal. briefly cover hydrodeoxygenation processes of be found under http://dx.doi.org/10.1002/cssc.201600144. bio-oil. Reviewsonthe individual constituents that comprise 2016 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA. bio-oil are also available, including one by Nakagawa et al.,[7] This is an openaccessarticleunder the termsofthe Creative Commons who cover the upgradingofholocellulose-derived furanic com- Attribution-NonCommercial-NoDerivs License, which permits useand pounds with afocus on general reactionpathways, and one distribution in any medium, provided the original work is properly cited, [8] the use is non-commercial and no modifications or adaptations are by Bu et al., who cover lignin-derived phenolic compounds. made. ChemSusChem 2016, 9,1750 –1772 www.chemsuschem.org 1751 2016 The Authors. PublishedbyWiley-VCH Verlag GmbH &Co. KGaA, Weinheim Reviews Despite the plethora of research that has been done within 2. Vegetable Oils/Triglycerides/Fatty Acids the past decade, there appears to be alack of acknowledge- ment towards the development of catalysts for selectively de- The general deoxygenation of fatty acids and triglycerides oxygenating bio-oils in H2-modest environments. Therefore, from both edible and nonedible vegetable oils is widely dis- the purpose of this review is to summarize work that has been cussedinthe literature.[1b,c,6,10] Reaction conditions that have done in the development of the deoxygenation of vegetable typicallybeen applied are temperatures of 230 to 3758Cand oils and major bio-oil compounds (phenolic and furaniccom- H2 pressures of 10 to 110bar (1 bar= 0.1 MPa);however,some pounds)within H2-modestenvironments. researchers have beguntostudy the application of atmospher- Herein, H2-modestenvironments are defined as having ase- ic or even inert atmospheres. Researchers have evaluated the verely reduced externalsupply of H2 gas with pressures near processes by lookingatvarious compounds and model oils atmosphericpressures, well below the typical minimum pres- such as the vegetable oils themselves, methyl and ethyl esters, sures required for hydrodeoxygenation processes. Emphasis and fatty acids. Vegetableoils are composed mostly of trigly- and insight are provided for the developmentofcatalysts that cerides and some free fatty acids. Triglycerides contain three promote major reaction pathways that require low amountsof fatty acids bound to asingle propaneunit through ester H2.Ithas been shown that traditional catalysts such as sulfide bonds(see Figure1). Notably,all fatty acids have even-num- catalysts provideunfavorable