Catalytic routes for the conversion of lignocellulosic biomass to aviation fuel range hydrocarbons

Catalytic routes for the conversion of lignocellulosic biomass to aviation fuel range hydrocarbons

Renewable and Sustainable Energy Reviews xxx (xxxx) xxx Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal h...

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Renewable and Sustainable Energy Reviews xxx (xxxx) xxx

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: http://www.elsevier.com/locate/rser

Catalytic routes for the conversion of lignocellulosic biomass to aviation fuel range hydrocarbons Hongliang Wang a, *, Bin Yang b, Qian Zhang c, Wanbin Zhu a, ** a

College of Biomass Sciences and Engineering /College of Agronomy and Biotechnology, China Agricultural University, Beijing, 100193, China Department of Biological Systems Engineering, Washington State University, Richland, WA, 99354, USA c Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Biomass Aviation fuel Hydrocarbons Hydrodeoxygenation Lignin Alternative jet fuels

The catalytic conversion of lignocellulosic biomass to aviation fuel is identified as a key strategy to alleviate high operating costs and serious environmental pollution caused by using petroleum-derived fuels. Aviation fuel with stringent end-use requirements consists of several specific hydrocarbon compositions, and the conversion of lignocellulose to aviation fuel is more challenging than that to other fuels. In this study, the latest cutting-edge innovations on the catalytic conversion of lignocellulose to aviation fuel was summarized. Promising routes for the catalytic conversion of cellulose, hemicellulose, lignin, and their derivatives were elaborated, with emphasis on those catalytic approaches including depolymerization of C–O bonds, formation/rearrangement of C–C bonds, and hydrodeoxygenation (HDO) removal of oxygen-containing functional groups. Innovations on reaction mechanism exploration, catalyst development, solvent screening, and reaction condition optimization were introduced. It revealed that a 100% biomass-derived aviation fuel could be produced by catalytic methods with the full utilization of all lignocellulosic compositions. Straight and branched paraffins in aviation-fuel range could be generated from cellulose and hemicellulose via various intermediates including 5-hydroxymethylfurfu­ ral (HMF), furfural, levulinic acid, and γ valerolactone. The degradation and HDO conversion of lignin could yield aromatics and cycloparaffins in aviation range. The development of hydrothermal stable catalysts for the controllable formation of C–C bonds among platform chemicals from carbohydrates as well as for the efficient HDO conversion of fuel precursors is particularly important.

1. Introduction The exploration of biomass as a renewable organic carbon resource for the production of chemicals and fuels has become a global effort in response to numerous challenges, including fossil resources depletion, energy demands growth, and greenhouse gas (GHG) emission [1,2]. In this respect, lignocellulosic biomass, as compared with other types of biomass feedstocks (e.g. starch, chitin, triglycerides, pectin), has received much more attentions from R&D society and markets due to it is inexpensive, abundant, easily available, and security to food. The annual production of lignocellulosic biomass around the world is over 170 billion metric tons, but currently with no more than 5% being uti­ lized by humans [3]. With the purpose to alleviate diverse crisis and fully unlock the potential of lignocellulosic biomass, increasing eco­ nomic giants have passed legislations mandating that a necessary

proportion of energy and chemicals must be produced from biomass, especially from lignocellulose. The U.S. Department of Energy and U.S. Department of Agriculture set a mandatory target to produce 20% of transportation fuels and 25% of bulk chemicals from biomass by 2030 [4]. Similarly, the European Union set goals to generate 27% of energy from renewable sources, mainly from lignocellulosic biomass, and reduce 40% GHG emissions by 2030 [5]. China, as the world’s largest developing country, also set an ambitious goal to cut GHG emissions per unit of GDP by 60%–65% by 2030 from the 2005 level, which will be achieved by using biomass as the raw material instead of coal to generate fuels and chemicals [6]. The driver from government policies has already contributed to the development of biorefinery. Biomass has become the largest source of renewable energy both in the United States and European Union. The development of biorefinery for biofuels, especially in term of bioethanol and biodiesel production, has made significant contributions

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Wang), [email protected] (W. Zhu). https://doi.org/10.1016/j.rser.2019.109612 Received 25 March 2019; Received in revised form 8 November 2019; Accepted 17 November 2019 1364-0321/© 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Hongliang Wang, Renewable and Sustainable Energy Reviews, https://doi.org/10.1016/j.rser.2019.109612

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Abbreviations ATJ AL ASTM BDE DDO DME DMO FFAs FTS GHG GVL HAA

HDN HDS HDO HPA HYD HMTHFA 5-HMF LA 2-MF PA PEAs R&D THFA

Alcohol-to-jet Angelica lactone American society for testing material Bond dissociation energy Direct deoxygenation Demethylation Demethoxylation Free fatty acids Fischer–Tropsch synthesis Greenhouse gas γ-Valerolactone Hydroxyalkylation/alkylation

to our society’s transportation sector [7,8]. While, as to aviation fuel, a specific type of fuel generally with higher standards (stricter quality requirements) than fuels used in less critical applications (e.g. heating, land or marine transportation), is almost exclusively extracted from the kerosene fraction of crude oil [9], which leads to growing concerns on energy security supply and global environmental issues [9,10]. The consumption of aviation fuel in the U.S was about 17.9 billion gallons in 2018, with a predicted growth of 5% per a year in the further. Moreover, the rapid development of aviation industry in emerging economies, such as China and India, will aggravate the issue of aviation fuel sustainable supply. At the same time, the supply of aviation fuel is predicted to decrease each year by 2026 [9]. Increasing inconsistency between aviation fuel supply and demand will deteriorate the development of whole aviation industry in the near future. Moreover, unlike other transportation vehicles which can use electricity, hydrogen, wind, or solar energy as power, airplanes are much less flexible in energy source. Fuels used for airplanes must meet certain criteria [11,12], including 1) high energy density, 2) high specific heat capacity, 3) rapid evaporation,

Hydrodenitrogenation Hydrodesulfurization Hydrodeoxygenation 4-hydroxypentonioic acid Hydrogenation followed by deoxygenation Hydroxymethyltetrahydrofurfural 5-Hydroxymethylfurfural Levulinic acid 2-methylfuran Pentanoic acid Pentenoic acids Research & development Tetrahydrofufural

4) low viscosity and high lubricity, 5) low freezing point, 6) low contaminant, 7) compatible to sealing materials, 8) minimum char for­ mation, 9) good stability, and 10) good turbine combustion character­ istics, which means the aviation industry will rely on specific carbon based compounds for a long term. On the other hand, air traffic is responsible for a large portion of GHG emissions around the world. It is reported U.S aviation alone contributes approximately 11% of global transportation GHG emissions [13], and it will continue to increase in the next decades though the engine efficiency of aircraft keeps improving [13]. One efficient way to achieve the goal of reducing CO2 emission is to improve the use of biomass derived fuels. It was reported GHG emission could be reduced up to 98% (in an open pond algal oil case) when biomass instead of petroleum was used for aviation fuel production [14]. There are several routes available for converting biomass-based materials into aviation fuels, including oils-to-jet, alcohols-to-jet, syngas-to-jet, and sugars-jet, as depicted in Fig. 1 [12]. Some of them, e. g. oils-to-jet and syngas-to-jet have been developed at commercial or

Fig. 1. Current routs for the conversion of biomass to aviation fuels. 2

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pre-commercial scale, and fuels from these processes have been approved by ASTM international method D7566 for blending with current aviation fuels at levels up to 50%, as listed in Table 1 [15]. Other processes, such like fermentation or catalytic conversion of sugars to hydrocarbons, are still at research stage. The commercial feasibility of an aviation fuel is determined by its production cost. The production cost of petroleum-derived aviation fuels is directly affected by the price of crude oil. However, according to a whole techno-economic analysis (TEA) of different jet fuel pathways conducted by NREL, factors that can influence the production cost of biomass-derived aviation fuels are diverse and complex, mainly including the cost of feedstock and catalyst, the consumption of energy, the efficiency of conversion process, and the co-production of value-added products [12]. Lignocellulose is the most abundant and widespread form of biomass on earth, and can be found in almost all plants, including soft woods, hard woods, and herbaceous plants [31]. As an easily available energy source, lignocellulose was employed as a solid fuel for humans throughout recorded history. However, till the late 20th, with the emerging crisis of fossil resource depletion and environment deteriora­ tion, lignocellulose was considered as the feedstock for the large-scale industrial production of liquid fuels. In recent years, the direct cata­ lytic conversion of lignocellulose to aviation fuels has received consid­ erable attentions due to advantages including low cost of raw materials, mild reaction conditions, and high selectivity of desired products [1,32, 33]. In this strategy, lignocellulose or its components are first catalyti­ cally depolymerized into small pieces through thermolysis, hydrolysis, solvolysis, or hydrogenolysis [1], and then these low-molecular pieces undergo controlled re-oligomerization reactions (e.g. aldol condensa­ tion, hydroxyalkylation, ketonization) to properly increase the carbon-chain length [34], and finally are converted into aviation fuel ranged hydrocarbons via hydrodeoxygenation (HDO) [35], as shown in Fig. 2. The mild reaction conditions of this route ensure the high controllability of key processes involved, and result in low energy con­ sumption and high product selectivity. The minimum selling price (MSP) of aviation fuel from the catalytic conversion pathway is competitive to those from other pathways [30], as shown in Table 1. Tremendous efforts have been paid to fully unlock the potential of this strategy in converting lignocellulose to aviation fuel. Many excellent reviews have been done on alterative aviation fuel. Wang et al. summarized the conventional pathways for bio-jet fuel production, e.g. alcohol-to-jet fuel, oil-to-jet fuel, gas-to-jet fuel, and sugar-to-jet fuel, including process description, economic analysis, and life-cycle assessment [12]. Blakey et al. [36] and Hui et al. [11] reviewed the fuel production pathways of alternative jet fuels with emphasis on the combustion characteristics, including life-cycle anal­ ysis, economic availability, flight tests, and engines tests. However, most of these works have focused on well-developed technologies, e.g. oil-to-jet, gas-to-jet, with few available for the catalytic conversion of lignocellulose or its components to aviation fuel though numerous studies recently have been carried out and published on this aspect.

The aim of this paper is to provide a holistic overview of the latest cutting-edge innovations on the catalytic conversion of lignocellulose to aviation fuels, with emphases on several key routes for cellulose, hemicellulose, and lignin conversion to aviation fuel. The scientific and technological advances related to catalyst development, process design, and mechanism exploration involving in the conversion of several key platform compounds, e.g. furfural, levulinic acid, γ valerolactone, and phenols from cellulose, hemicellulose, or lignin are elaborated. The separation, purification and preliminary depolymerization (e.g. hydro­ lysis) or transformation (e.g. dehydration) of lignocellulose are not included, although they are important related with the process of lignocellulose to aviation fuel, mainly due to these contents can be found in related reviews by other researchers [1,3,37–39]. This paper mainly focuses on the catalytic carbon-chain elongation, carbon-skeleton isomerization, and hydrodeoxygenation reactions to convert lignocel­ lulose compositions to qualified hydrocarbons with the required carbon-chain length, molecular structure, and energy density for avia­ tion usage, and it reveals that 100% biomass-derived aviation fuel can be produced, and all the three compositions of lignocellulose can be converted into aviation fuel. 2. Catalytic conversion of furan-based compounds to aviation fuels Carbohydrates (cellulose and hemicellulose) account for 65–85 wt% of dry lignocellulose [3,40]. The successful conversion of lignocellulose to aviation fuel must be capable of transforming carbohydrates. How­ ever, the direct catalytic conversion of cellulose and hemicellulose to aviation fuels is a great challenge due to the basic units of the carbo­ hydrates are C5~C6 sugars from which can only yield pentanes and hexanes once directly hydrodeoxygenated [41]. Therefore, new C–C bonds must be built among the depolymerized intermediates to generate aviation fuel precursors with suitable carbon numbers. While, the for­ mation of C–C bonds among sugars is quite difficult, and thus it is necessary to initially convert sugars to other compounds, e.g. furfural [42,43], 5- hydroxymethylfurfural (5-HMF) [32,44,45], levulinic acid (LA) [46], angelica lactone (AL) [47], γ-valerolactone (GVL) [48], etc. These compounds are usually regarded as “platform chemicals” which have active functional groups for various transformations including C–C bond formation [49–51]. The conversion of carbohydrates to platform chemicals has been well studied and reviewed, and thus this paper will just concentrate on the methods for the conversion of carbohydrate based platform chemicals to aviation fuel ranged hydrocarbons, mainly including C–C bond formation reactions and HDO reactions. Furan based chemicals, especially 5-HMF and furfural, are very important platform compounds, and they can be produced from cellu­ lose and hemicellulose, respectively. Compared with sugars, furan-based chemicals have higher carbon to oxygen ratio, and their properties are much closer to those of fuel compounds [52]. Moreover, both 5-HMF and furfural contain unsaturated C–C bonds and carbonyl groups

Table 1 Current status of various technologies for conversion biomass to aviation fuels. Pathways

Feedstock

Certification (By ASTM)

Fuel yield (GGE/BDT)

MSP ($/gal)

Projects & Companies

Refs

Oil-to-aviation fuels

Plant oils, waste oils, algal oils, and pyrolysis oils CO and H2 from all biomass including woody feedstocks and municipal solid waste C2–C5 alcohols derived from any biomass source including sugars and lignocellulose Sugars from any feedstock and those derived from lignocellulose Sugars and sugar derivates, e.g. furfural, HMF, levulinic acid, GVL, etc.

Approved by D7566 Annex A2 Approved by D7566 Annex A1 Approved by D7566 Annex A5 Approved by D7566 Annex A3 Not Approved

19–120

2.6–34.7

[16–19]

9–89

4.8–16.2

11–81

4.1–14.4

24–45

4.3–25.4

25–92

1.0–6.31

Dynamic Fuels; Neste Oil; UOP; SG Preston Solena Group; Syntroleum; NETL LanzaTech; PNNL; celorMittal; Renewable Energy Group; Amyris; LS9 Virent; Shell; GIEC; Total

Syngas-to-aviation fuels Alcohols to-aviation fuels Sugars to-aviation fuels (biological conversion) Sugars to-aviation fuels (catalytic conversion)

[20–22] [23–26] [12,17,21, 23,27] [2,23, 28–30]

GGE: Gallon of Gasoline Equivalent; BDT: Biomass Dry Ton; MSP: Minimum Selling Price; GIEC:Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences. 3

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Fig. 2. Catalytic conversion of lignocellulosic biomass to aviation fuels.

which are vitally important for the formation of new C–C bonds to extend their carbon chains [53]. The production of aviation-fuel ranged hydrocarbons from carbohydrates via furanic intermediates involves a series of reactions, including the first step of acid catalyzed hydrolysis of polysaccharides to C5~C6 monosaccharides, followed by dehydration to generate furanic intermediates (e.g. 5-HMF and furfural) [54–56]. Subsequently, 5-HMF and furfural are condensed into C8~C16 oxy-compounds through reactions such like aldol condensation, keto­ nization, hydroxyalkylation, or self-coupling [31,43,57]. Finally, these long-chain oxy-compounds are converted into aviation fuels via HDO reactions.

hydrogenation of furan ring without effect of carbonyl groups in 5-HMF or furfural can generate compounds containing α-H atoms that are capable for aldol self-condensation. Generally, in the hydrogenation of unsaturated aldehydes, both thermodynamic and reaction kinetics – C bond hydrogenation over the C– – O bond considerations favor of C– – C bonds in hydrogenation [58,61]. However, the hydrogenation of C– –O 5-HMF and furfural is more difficult than the hydrogenation of C– bonds, mainly due to the steric effects of furan ring, making the pro­ duction of THFA and MHTFA challenging. There are many factors that can influence the performance of aldolcondensation reaction. Catalyst is one of the most important one. Aldolcondensation is generally promoted by base catalysts [62,63]. Homo­ geneous catalysts, e.g. NaOH, KOH, ammonium hydroxide, are effective and widely applicated in aqueous phase reactions. However, these cat­ alysts can be hardly recycled, and have problems in equipment corrosion and environment pollution. Thus, heterogeneous catalysts including alkali and alkaline earth oxides [62,64], basic zeolites [65], phosphates [66], and hydrotalcites [67,68] are employed. Besides eliminating the aforementioned disadvantages of homogeneous catalysts, the use of heterogenous catalysts for aldol-condensation at least have the following two advantages. One is that the strength and quantity of base sites in heterogenous catalysts can be finely tuned, which offers a way to improve their catalytic behavior. The other is that solid bases can be combined with metals (e.g. Pd, Pt, Ru, Ni) to form solid bifunctional catalysts which can be used for both aldol-condensation and the sub­ sequent hydrodeoxygenation reactions [69]. It should be noted that most of the solid bases for aldol-condensation are used in organic sol­ vents or in the vapor phase. The poor hydrothermal stability of solid bases leads to the leaching of catalyst components and prevents them from using in aqueous phase [70]. Thus, the development of hydro­ thermal stable solid bases is important. Besides catalyst, reaction temperature and the ratio of furan-based compounds to other carbonyl compounds also have important effects on aldol-condensation [71,72]. Aldol-condensation is usually carried out at mild temperatures, from room temperature to 373 K [58,63]. Generally, high selectivity can be achieved at low temperatures, but high loading of catalyst or long reaction time are needed to guarantee

2.1. Catalytic conversion of furan-based compounds to aviation fuels via aldol-condensation The controllable formation of new C–C bonds is essential for insuring the successful conversion of furan-based chemicals to long-chain hy­ drocarbons. Aldol-condensation is the widely-used method to build C–C bonds between two carbonyl-containing compounds with a reactive α-H on at least one of the carbonyls [58–60]. It is noteworthy that 5-HMF and furfural cannot undergo aldol self-condensation due to both are lack of α-H atom. Monosaccharides contain carbonyl groups, but form ring structures in aqueous solution leading to a very low reactivity for aldol-condensation reaction. Moreover, the abundant of –OH groups in sugars results in the formation of various by-products in alkali reaction system. For furan-based compounds, there are two strategies to extend their carbon-chain length via aldol-condensation. One involves the aldol-condensation of aldehyde groups in 5-HMF, furfural, or methyl­ furan with other molecules that contain carbonyl group with a α-H atom, e.g. acetone, dihydroxyacetone, and glyceraldehyde [59]. The other method is selective hydrogenation of furan ring to generate carbonyl-containing molecules that have α-H atoms, e.g. tetrahy­ drofufural (THFA), hydroxymethyltetrahydrofurfural (HMTHFA), and then undergo aldol self-condensation to produce long-chain in­ termediates [59]. The solubility of aldol-adducts is poor in aqueous phase, and a sub­ sequent hydrogenation step is usually required. In addition, the selective 4

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satisfied conversion rates. The ratio of reaction substrates determines the product distribution. Low proportion of furan based compounds to symmetric carbonyl compounds benefits for the formation of mono-adding products, while high ratio tends to generate di-adding products [59]. Lots of work has been done on the conversion of carbohydrates to long-chain hydrocarbons via furanic intermediates by using aldolcondensation to extend carbon chains. The pioneering work was con­ ducted by James A. Dumesic and George W. Huber [59]. Firstly, they selectively produced C9~C15 large organic compounds by sequent re­ actions, i.e. acid catalyzed dehydration of sugars to 5-HMF and base catalyzed aldol-condensation of 5-HMF with acetone to long-chain oxy-compounds. Then, the oxy-compounds were subjected to hydro­ deoxygenation reactions over an acid-metal bifunctional catalyst in a four-phase reactor to remove oxygens and add hydrogens to produce long-chain hydrocarbons. The distribution of products with different carbon numbers could be tuned by varying the ratio of 5-HMF to acetone. A demonstration unit that produces 100 tons of jet fuel ranged hydrocarbons a year from corn stalk has been built by Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (GIEC) led by Longlong Ma and Chenguang Wang via the aldol-condensation of furfural and levulinic acid [30,73]. It is planned to be scaled up to a 1000 tons/year plant before 2023. A number of other research groups, including groups of Alexis T. Bell [34], Ning Li and Tao Zhang [71], Andrew D. Sutton and John C. Gordon [52], and Yanqin Wang [74] were also devoted to unlock the potential of aldol-condensation pathway for the conversion of carbohydrates to long-chain hydrocarbons. They contributed to the understanding of reaction mechanisms, the devel­ opment of effective and stable catalysts, as well as the extension and/or improvement of products.

2.2. Catalytic conversion of furan-based compounds to aviation fuels via hydroxyalkylation/alkylation Hydroxyalkylation/alkylation (HAA) is another frequently-used method for the condensation of furan-based compounds, especially furan and 2-methylfuran (2-MF), to aviation-fuel ranged precursors [75]. George W. Huber and co-authors reported the use of furan as a nucleophile to condensed with electrophiles, including furfural, 5-HMF or their derivates, via HAA reactions under the catalysis of H2SO4 to produce large oxy-compounds with suitable carbon numbers for avia­ tion fuel production [76], as shown in Scheme 1. The mechanism of C–C bond formation between furan and furfural involves the protonation of the carbonyl group in furfural followed by the addition of furan. The formed difuryl methane can further react with another furan to yield trifuryl methane, enabling the production of C9 and C13 hydrocarbons. Similarly, 5-HMF can afford C10, C14, C16 carbon unites after HAA reacting with 1–3 furan molecules as described in Scheme 1. Besides furan, 2-MF can also undergo nucleophilic addition via HAA reactions to make different sets of precursors for linear and branched hydrocarbons as shown in Scheme 2. 2-MF is a by-product in the in­ dustrial production of furfuryl alcohol from furfural, and its selectivity can be increased to 93% by simply raising the reaction temperature from 408 K (for furfuryl alcohol production) to 523 K [77]. It is promising to use 2-MF as a starting material due to its high reactivity and selectivity in HAA reactions as well as its hydrophobic nature which allows it facile separation from aqueous phase at room temperature. Moreover, since one of the reactive α-positions in 2-MF is protected by the unreactive methyl group, the condensation of 2-MF with aldehydes or ketones is controllable, meaning the formation of undesired polymers can be inhibited [78]. Avelino Corma and co-authors did excellent work on using 2-MF in the production of long-chain hydrocarbons via HAA

Scheme 1. Production aviation fuel ranged hydrocarbons from carbohydrates via furanic intermediates by hydroxyalkylation/alkylation and hydro­ deoxygenation reactions. 5

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Scheme 2. Production aviation fuel ranged hydrocarbons from 2-MF by hydroxyalkylation/alkylation and hydrodeoxygenation reactions.

reactions [78–81]. Research groups, such as Tao Zhang and Ning Li [57, 60,82,83], Alexis T. Bell [34,53], Longlong Ma and Tiejun Wang [84], also contributed a lot on this aspect. HAA of 2-MF can be conducted with a variety of carbonyl com­ pounds, including furfural, methylfurfural, 5-HMF, and aldehydes and ketones containing 1–5 carbon atoms, allowing the capable production of diverse hydrocarbons with different carbon numbers and chemical structures as shown in Scheme 2. 2-MF can undergo hydrolysis to its ring-opening product named 4-oxopentanol by the addition of water under acid catalysis such as H2SO4 [78]. 4-oxopentanol can further react with two 2-MF via HAA reaction to generate a C15 intermediate from which a branched C15 hydrocarbon can be produced after HDO reaction. 2-MF is also able to react with its precursor, i.e. furfural or 5-HMF to form large organic compounds (e.g. compound 1 in Scheme 2) with suitable carbon numbers and superior structures for aviation fuel pro­ duction [53,82]. Besides production of hydrocarbons, the selective hy­ drogenation of furan ring in the condensed intermediates can generate cyclic ethers, e.g. C16 cyclic ether, and these ethers can not only improve the combustion and lubricity properties of aviation fuel but also have high cetane number and energy density that fully meet or exceed most of the specifications for aviation fuels [34]. Moreover, the HAA of 2-MF with C1~C5 aldehydes or ketones can form mono- or di-added oxy-­ intermediates which can result in linear or branched C8~C15 hydro­ carbons after hydrodeoxygenation [50,83,85,86]. HAA reactions are usually promoted by acid catalysts. Both homo­ geneous acids (e.g. H2SO4, HCl) [78,79] and heterogeneous acids [87] are used. The commonly used heterogeneous catalysts include acidic resins (e.g. Nafion-212, Nafion-115, Amberlyst-15, Amberlyst-36) [57], acidic zeolites (e.g. H–Y, H-ZMS-5, H-USY) [82], acidic phosphates (e.g. zirconium phosphate) [82], and activated carbon modified with acidic groups (e.g. AC-SO3H) [87]. Among all the investigated catalysts, Nafion-212 exhibits the best catalytic performance with good stability. The catalytic activity of these solid acids decrease as following: Nafion-212>Nafion-115>Amberlyst-15>Amberlyst-36>AC-SO3H > H-USY > H-ZMS-5>H–Y [79,82], which indicates HAA reaction is sen­ sitive to acid strength, and strong acids were more active than weak

acids for this reaction. The reaction temperature of hydroxyalkylation is as mild as that of aldol-condensation, generally ranging from 298 K to 403 K. 2.3. Catalytic conversion of furan-based compounds to aviation fuels via pinacolic coupling Apart from aldol-condensation and HAA reactions, the direct con­ structing C–C bonds among furanic compounds with formyl group on the a-position (e.g. furfural, 5-HMF, 5-methylfurfural) by self-coupling can be achieved. These furanic compounds are aryl aldehydes which can undergo pinacolic coupling through the radical reaction pathway by using reductants under mild conditions (at room temperature in atmo­ sphere) [88,89]. Yaobing Huang and Yao Fu firstly reported the use of pinacolic coupling for the direct formation of C–C bonds between furanic alde­ hydes to generate C10~C14 fuel precursors in 2012, as shown in Scheme 3 [88]. Metallic powders including Al, Mg, Zn were used as reductants for the pinacolic coupling of furfurals in various mediums including H2O, 5%–10% NaOH, 10% KOH, 0.1 M NH4Cl aqueous solutions, and the optimized reaction (99% furfural conversion with 96% target product selectivity) was obtained by using Zn as the reductant in 10% NaOH solution. However, when this reaction condition was applied to 5-HMF, polymers with the molecular weight ranging from 200 to 800 were obtained. The authors considered that this result was ascribed to the low stability of 5-HMF under the reaction conditions. To solve this problem, the author turned 5-HMF to 5-methylfurfural (5-MF) with higher stability, and 85% yield of C12 fuel precursors was obtained. This route provides an interesting reaction of C–C bond formation for the producing aviation fuel ranged hydrocarbons. However, the use of metal powders as reductants is a limitation, since the reuse of these metals is complicated and cost. Thus, the development of more economic and green electron donor systems is needed.

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Scheme 3. Production of aviation-fuel ranged hydrocarbons via pinacolic coupling and hydrodeoxygenation [88].

2.4. Catalytic hydrodeoxygenation of furanic condensed intermediates to aviation fuels

when Pd/H-zeolites (zeolites ¼ ZMS-5 or β with high Si/Al ratio) were used as catalysts for HDO of furanic condensed compounds at 503 K–533 K [60]. By the synergistic catalysis of metal and acid, oxygens are mainly removed in the form of water. Inexpensive metal based catalysts, e.g. Ni–Mo2C/SiO2 and Ni–W2C/SiO2 exhibited catalytic behaviors of noble metals in HDO conversion of furanic condensed compounds at a high temperature of 623 K [83]. The further development of inexpensive metal catalysts with high catalytic activities at low temperatures for the replacement of noble mental catalysts in HDO conversion of biomass will be an interesting and important research subject.

After formation of new C–C bonds, the carbon-chain length of the organic compounds falls in the scope of aviation fuel range. However, these compounds have excess oxygens and unstable groups, e.g. –OH – O bonds, and C– – C bonds, leading to low thermal stability groups, C– and energy density. Therefore, hydrodeoxygenation (HDO) is a neces­ sary step for the upgrading of these large oxy-compounds to “drop-in” fuels. The HDO of furanic condensed intermediates usually consists of hydrogenation, ring-opening, and deoxygenation reactions, and both one-step and two-step reactions were applied [31,71]. Two-step reaction is usually used for the upgrading of oxy-intermediates with poor thermal stability and inferior flow properties, involving two pathways, i.e. 1) hydrogenation followed by ring-opening and deoxygenation reactions, and 2) ring-opening followed by hydrogenation and deoxygenation re­ actions. The hydrogenation reaction is usually promoted by metal cat­ – C and alysts, such as Pd [90], Pt [71], Ru [86], Ni [91], to saturate C– – O bonds, and improve the stability of the oxy-intermediates. How­ C– ever, after fully hydrogenated, it will be more difficult to break the ring structure. Thus, the selective hydrogenation of exocyclic unsaturated bonds is a good choice before ring-opening. The ring-opening is usually catalyzed by acid or base catalysts, including HCl, acetic acid, and metal triflates. Metal catalyzed hydrogenolysis reaction can also break the etheric ring structure, but needs harsh conditions. The substituted groups on furan ring have a great effect on ring-opening reaction [34,43, – C bonds can obviously prevent 85]. Substituted groups containing C– ring-opening reaction. The deep deoxygenation reaction is usually in­ tegrated in the second step, and catalyzed by acid-metal bifunctional catalysts. Diels-Alder and esterification reactions occasionally accom­ pany with HDO reactions, to form larger fuel precursors which can be upgraded into biodiesel [34,63,92]. For the oxy-intermediates with good thermal stability and flow properties, one-step HDO reactions are often applied. One-step processes are also promoted by metals or metal-acid catalysts, but generally under severer conditions with high temperatures (>473 K) as compared with two-step reactions. Metal catalyzed process usually used noble metals supported on inert solids, e.g. Pd, Pt, or Ru supported on SiO2 or acti­ vated carbon, as catalysts [83,93]. While, for metal-acid catalyzed process, catalysts are metals, e.g. Pd, Pt, Ru, Ni, Co, Mo, or W supported on acidic solids, including Al2O3, WO3, NOPO4, TaOPO4, poly­ oxometalates, and acidic zeolites [70,79]. The metal catalyzed one-step HDO reaction is often carried out at temperatures higher than 573 K, and decarbonylation reactions are usually occurred at such high tempera­ tures resulting in the loss of carbons [94]. Oxygens are removed in the form of CO and water by metal catalysis. The combination of acids with metals can improve oxygen removal efficiency and reduce the reaction temperature, e.g. high yields (>70%) of hydrocarbons could be obtained

3. Catalytic conversion of levulinic acid and its derivatives to aviation fuels 3.1. Catalytic conversion of levulinic acid to aviation fuels Levulinic acid (LA, 4-oxopentanoic acid) is another important biomass based building block, which has been identified by the United States Department of Energy as one of the 12 promising biomass de­ rivatives with the hope to produce a wide range of chemicals [95]. LA can be obtained inexpensively in high yields through the pathway of acid catalyzed hydrolysis of cellulosic materials, as depicted in Scheme 4. There are two important functional groups in LA, i.e. ketone and carboxylic groups, which endow LA with multiple transformation reactivities. LA contains 5 carbon atoms, and therefore, two or more of LA mol­ ecules have to connect together if aviation fuel ranged hydrocarbons are desired. Acid ketonization, which involves the connection of two car­ boxylic acids to a ketone via decarboxylation reaction, is a welldeveloped reaction. Acid ketonization was very popular before World War I for the synthesis of small ketones (e.g. acetone synthesis from acetic acid) [96]. The emergence of cost-effective petrochemical pro­ cesses for acetone production, e.g. the cumene process, made ketoni­ zation process declining. However, acid ketonization revives recently as biorefinery receives increasing attention, since it can not only remove the highly reactive carboxylic functional groups in biomass but also can increase the carbon-chain length of small molecules [97]. The ketoni­ zation of 2 LA can yield 2,5,8-nonatrione which can be further converted into 5-nonanone or nonene by hydrodeoxygenation reactions [98]. This route opens a pathway to directly convert LA to C9 hydrocarbons with only releasing CO2 and H2O as byproducts, as shown in Scheme5. A variety of catalysts, including metal oxides and zeolites, have been used for the promotion of ketonization reaction [97,99,100]. Glinski et al. screened 20 different metal oxides supported on silica, including basic, acidic, and amphoteric oxides, for the ketonization of acetic acid, and found amphoteric oxides (e.g. CeO2, MnO2, La2O3) are better than pure acidic or basic oxides [101]. Prereduction or doping can modify the catalyst properties, including the change of oxygen vacancies, acid or 7

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Scheme 4. Production of levulinic acid (LA) from biomass.

Scheme 5. Production of nonene from LA via ketonization and hydrodeoxygenation reactions.

base sites, coordination status, to tune the catalytic performance of ox­ ides. Zeolites with strong acidic sites also have good catalytic activity for ketonization [97]. Extensive efforts have been paid to reveal the mechanism of ketonization reaction but still with some debates [97]. Bulk ketonization involving the formation of bulk carboxylates with low-lattice-energy oxides, and surface ketonization involving the for­ mation of surface intermediates (e.g. ketene, β-ketoacids) with high-lattice-energy oxides, are the two typical ketonization approaches. For the first approach, acids interact strongly with alkali and alkali earth oxides, including CaO, MgO, BaO, to form bulk carboxylates salts which can subsequently decompose to ketone, H2O and CO2 by thermal treatment [97]. In contrast, on high-lattice-energy oxides (e.g. CeO2, ZrO2, TiO2), the reaction proceeds on the surface of the oxides [102]. The general pathway and possible reaction mechanism of surface keto­ nization via the formation of ketene intermediates are depicted in Scheme 6. It involves the initial dehydration of one acid to the corre­ sponding ketene on the surface of oxides. The ketene, as a highly reac­ tive intermediate, reacts with the other acid to produce the ketone via formation of a six-membered transition state followed by generating the enol as the initial product accompanying with the loss of a CO2 [98]. Most of acid ketonization reactions were carried out in gas phase under relative high temperatures (473K–673K) [97]. The product (2, 5,

8-nonatrione) from direct ketonization of LA is highly reactive under such conditions, and high selectivity to nonene can be hardly achieved. Thus, the development of low-temperature ketonization process in aqueous phase is crucial, especially for LA which has a low vapor pressure and a high solubility in water. To achieve that goal, it is important to synthesize effective catalysts with good hydrothermal stability and resistance to inhibition and deactivation by other com­ pounds (e.g. furanics). Besides ketonization, LA can undergo self-aldol condensation to form aviation-fuel precursors [103,104]. While, carbonyl group in LA neu­ tralizes/deactivates base catalysts, leading to a very low efficiency [104]. Recently, Xianglin Hou and Tiansheng Deng reported a process that using Brønsted (trichloroacetic acid) and Lewis acids (ZnCl2) to promote the selective C–C bond formation between two LA molecules via self-aldol condensation, which resulted in the production of two C10 lactone compounds [103]. This catalytic system can eliminate the negative effect of carboxylic acid in LA leading to high yields of condensed products. The proposed mechanism of this process is depicted in Scheme 7.

Scheme 6. C–C bond formation between two acids via ketonization reaction. 8

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Scheme 7. Pathway of LA to aviation fuels via self-dimerization of LA promoted by Brønsted and Lewis acids [103].

3.2. Catalytic conversion of angelica lactone to aviation fuels

also couple with other compounds that containing carbonyl groups, e.g. furfural, benzaldehyde, butyraldehyde, via aldol-condensation reactions to produce C10 to C12 hydrocarbons, as shown in Scheme 9 [90]. A high carbon yield (about 96%) of C10 oxygenates was produced from aldol condensation of AL with furfural by the catalysis of Mn2O3 under 353 K for 4 h. Nearly quantitative branched alkanes could be obtained after a following hydrodeoxygenation reaction [90].

By the catalysis of acids, LA can undergo intramolecular dehydration reaction to produce an unsaturated lactone named α-angelica lactone – C bond and a carbonyl group is more reactive (AL) [47]. AL with a C– than LA, and can be dimerized or trimerized under relatively mild conditions to yield C10 or C15 oxy-intermediates which can be subse­ quently converted to C8–C15 hydrocarbons [105–107]. The pathway of LA to AL and then to C8–C15 hydrocarbons is depicted in Scheme 8. It should be noted that the generated products from these routes are all branched alkanes with high octane numbers that are perfect for aviation fuel usage. The conversion of LA to AL is usually catalyzed by strong acids under negative pressures with the generated AL being continuously separated from the mixture of LA and catalysts by distillation [80,108]. Though this method gives a good yield of AL, the high polymerization reactivity of AL always results in the formation of tremendous solid byproducts. The development of novel catalysts and processes can alleviate this issue. Mark Mascal and co-authors used montmorillonite clay (K10) as a heterogeneous acid catalyst for this reaction, which not only facilitated product separation and catalyst recycling but also reduced the yield of polymeric byproducts, resulting in >90% yields of isolated AL [35]. AL self-condensation is a conjugated addition reaction which occurred be­ tween the double bond isomers of AL, and mainly promoted by alkali catalysts, including hydroxide or alkoxide salts [105,109], active metals [109], and carbonates such as K2CO3. Quantitative amounts of dimers and trimers could be obtained from AL condensation under mild con­ ditions. For example, 100% AL was converted to its dimers (66% yield) and trimers (32% yield) by the catalysis of K2CO3 under 353 K for 30 min [105]. The generated dimers and trimers could be subsequently hydrodeoxygenated into long-chain hydrocarbons by catalysts such as Ir-ReOx/SiO2 [35], Pt-ReOx/C [35], Pd/Al2O3 [106], Pd/C [107], Pd/SiO2 [90], and Pd-FeOx/SiO2 [90]. Besides self-condensation, AL can

3.3. Catalytic conversion of γ valerolactone to aviation fuels – C bond in AL ring can result in another The hydrogenation of the C– important platform compound named γ-valerolactone (GVL) [51, 110–112]. Generally, GVL is produced directly from selective hydroge­ nation of LA via two possible pathways. One is through AL intermediate, and the other is through 4-hydroxypentonioic acid (HPA) intermediate. AL intermediate is generally formed in gas-phase hydrogenation route under elevated-temperatures via enolization [113,114], while HPA in­ termediate is usually generated in liquid-phase hydrogenation route via the selective hydrogenation of the carbonyl group in LA at C4 position [114,115]. Molecule H2, formic acid, and alcohols can be used as hydrogen sources for the reduction reaction [114]. Since LA can be generated from C5 or C6 sugars, thus both hemicellulose and cellulose can be employed as raw materials for GVL production. GVL is stable in water and air, and does not form measurable amount of peroxides. GVL is also a kind of widely used food additive with acceptable smell. All these characteristics make GVL a safe building block for the synthesis of various value-added products including advanced fuels [116,117]. GVL itself is a good fuel additive due to its high-energy density and low vapor pressure. GVL keeps 97% energy of glucose, and its energy density is obviously higher than that of ethanol [116]. Moreover, GVL does not form an azeotrope with water, which can save extensive energy in its separation and distillation process. However, the direct and widespread use of GVL as fuel suffers from several limitations including

Scheme 8. C–C bond formation between two acids via ketonization reaction. 9

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Scheme 9. Aldol condensation of AL with compounds containing carbonyl groups for the production of branched alkanes.

high water solubility, corrosiveness in storage, and relative lower energy density as compared with hydrocarbon fuels. The conversion of GVL to 2-methyltetrahydrofuran (MTHF) or valerate esters can partially alle­ viate these issues, but these compounds can hardly meet the re­ quirements of aviation fuels. The upgrading of GVL to long-chain hydrocarbons is feasible and has been demonstrated by several research groups [115,118,119]. Gener­ ally, there are two typical routes for the conversion of GVL to liquid hydrocarbons. One involves the ring-opening of GVL to a mixture of unsaturated pentenoic acids (PEAs), followed by decarboxylation to isomeric butenes, as depicted in Scheme 10. GVL can also directly un­ dergo decarboxylation to butenes, but the apparent activation barrier (about 175 kJ mol 1) is higher than that for decarboxylation of PEAs (about 142 kJ mol 1) [120]. The generated butenes can be converted into long-chain hydrocarbons via controlled oligomerization [117,120, 121]. Jesse Q. Bond and co-authors in 2010 reported an integrated catalytic process using a dual reactor system for the conversion of GVL to liquid hydrocarbons via butene routes [117]. In the first reactor, GVL underwent ring-opening and decarboxylation reaction to produce equimolar amounts of butenes and CO2 over the catalysis of SiO2/Al2O3 at 648 K under 3.6 Mpa H2. The yield of butene could reach as high as 96% [117]. The produced gas stream was subsequently fed directly into the second rector containing an acid catalyst (e.g. Amberlyst-70, H ZSM-5) to produce C8~C16 hydrocarbons via oligomerization at 443–532 K under 1.7–3.6 Mpa H2. Over 75% yields of liquid hydro­ carbons (C8þ) could be obtained. This process only needs one simple pumping system for the delivery of feeds, which minimizes processing and equipment cost. Moreover, noble metal catalysts and external hydrogen source are not necessary. The economic analysis of this route

revealed that this approach was superior to that of lignocellulosic ethanol production [117]. Suojiang Zhang and co-authors in 2014 re­ ported another interesting process for conversion of GVL to aviation fuel ranged hydrocarbons via butene route [119]. The first step was quite similar to that reported by Bond and co-authors, which involved the decarboxylation of GVL to butene isomers at elevated temperatures (623 K) over SiO2/Al2O3 catalysis. The generated butenes was then subjected to controlled oligomerization reaction over catalysis of an acidic ionic liquid [CF3CH2OH2][CF3CH2OBF3] under mild conditions (283 K for 10 min).73% yield of trimethylpentane with a high research octane number of 95.4 was obtained. The other route, as depicted in Scheme 11, for converting GVL to hydrocarbon fuels is by way of pentanoic acid (PA) intermediate, which mainly includes the following several steps: 1) reductive ring-opening of GVL to PA; 2) ketonization of PA to 5-nonanone; 3) hydrogenation of 5nonanone to 5-nonanol; 4) dehydration of 5-nonanol to C9 alkenes; 5) hydrogenation of C9 alkenes to C9 alkanes, or condensation of C9 alkenes followed by hydrogenation to generate C9þ alkanes. The first two steps, i.e. PA and 5-nonanone production, are crucial for achieving a high yield of final alkane products. The production of PA from GVL is usually promoted by bifunctional catalysts containing acid sites (for ringopening reaction) and metal sites (for hydrogenation reactions). Water stable bifunctional catalysts, e.g. Pd/Nb2O5, are especially promising for this reaction [122,123]. James A. Dumesic’s group developed a catalytic system containing two continuous flow reactors for the conversion of GVL to 5-nonanone [122], in which 100% GVL conversion with 92% carbon selectivity to PA was obtained in the first reactor charged with Pd (0.1%)/Nb2O5 catalyst at 598 K. The produced PA was subsequently converted into 5-nonanone with 84% carbon selectivity in the second

Scheme 10. Conversion of GVL to jet-fuel ranged hydrocarbons via butenes. 10

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Scheme 11. Conversion of GVL to aviation-fuel ranged hydrocarbons via pentanoic acid.

reactor containing Ce0.5Zr0.5O2 catalyst at 698 K. A single-reactor aqueous catalytic system by using a dual catalyst bed containing Pd/Nb2O5 and ceria-zirconia was also developed by the same research group for the conversion of GVL to 5-nonanone, and 90% yield of 5-non­ anone was obtained [124]. Due to its hydrophobicity, the generated 5-nonanone can be spontaneously separated from water, and then be facilely hydrogenated to 5-nonanol at mild conditions (e.g. 423 K, 5 Mpa H2) by the catalysis of Ru/C. After dehydration over acidic catalysts (e.g. acidic zeolites or acidic resins), 5-nonanol can be converted into C9 olefins which can be subsequently hydrogenated to nonane or oligo­ merized to C9þ alkanes. Though this route has been demonstrated to be feasible for conversion of GVL to aviation fuel-ranged hydrocarbons, lots of efforts including development multifunctional catalysts to reduce reaction steps, optimization of reaction parameters to increase the yield of final alkane products should be done to make this route competitive in industrial production.

aviation fuel production [128,129]. Compared to cellulose and hemi­ cellulose, lignin has higher carbon to oxygen ratio and energy density, which makes it a promising candidate for fuel production. More spe­ cifically, the phenylpropane based structure characteristics of lignin endows it can serve as a unique renewable resource for cycloalkanes and aromatics production [37,130]. In addition, polysaccharides’ monomers (e.g. glucose, fructose, and xylose) or their dehydrated derivatives (e.g. furfural, 5-hdroxymethylfurfural, levulinic acid) have relatively short carbon-chain lengths (C5~C6). Without additional C–C bond formation, once hydrodeoxygenated, the generated products are in general too short for aviation fuel purposes. While, it is feasible to directly obtain long chain hydrocarbons from lignin due to that the depolymerized products of lignin (monomers and dimers) usually contains 7–18 carbon atoms which are perfect for aviation fuel production [72,130]. Several challenges must be addressed before lignin being converted into aviation fuels [125]. Lignin is designed to be a robust polymer in nature, acting as the vital glue that provides structural stability and integrity to plants. One of the difficulties that hampers lignin selective conversion is caused by its stable and heterogeneous structure which is also referred as lignin recalcitrance [131]. The structure of lignin varied with feedstock, pretreatment methods, and pretreatment severities. Various kinds of C–O and C–C linkages with broad bond dissociation energy (BDE) brings intractable difficulties in lignin selective depoly­ merization. Moreover, functional groups, like methoxyl groups, phenolic hydroxyl groups, and terminal aldehyde groups in lignin, also add variables on its reactivity, increasing the difficulty of lignin selective conversion. The second problem, principally caused by its relatively high molecular weight and amorphous structure, which leads to limited solubility of the biopolymer in most common solvents at ambient tem­ perature. Thirdly, the high reactivity of lignin degraded intermediates which are prone to side reactions (e.g. forming C–C bonds to generate chars), makes it very difficult to produce high yield of desired products. Thus, joint efforts including the development of suitable pretreatment technologies and highly active catalysts should be paid to fully unlock the potential of lignin. It is very difficult to obtain detail information from lignin conversion due to its complexity and variability [132–134]. Thus, the use of simpler model compounds with low molecular weight for the study of lignin conversion is necessary. These model compounds, which are frequently found in lignin degradation streams, contain basic structures (linkages) of lignin, and thus their reactivity can provide insights into lignin

4. Catalytic conversion of lignin to aviation fuels As mentioned above, n-paraffins and iso-paraffins in aviation fuel range can be produced from carbohydrates in lignocellulosic biomass through various routes. However, another two main compositions (cycloalkanes and aromatic hydrocarbons) which are also essential for aviation fuels can be hardly generated from cellulose or hemicellulose. Currently, bio-jet fuels are blended with traditional petroleum-based fuels to address this problem. To produce 100% bio-jet fuels, a suit­ able biomass feedstock which can be facilely transformed into cyclo­ alkanes and aromatics is required. Lignin is the nature’s dominant aromatic polymer and constitutes 15 to 35 wt% of lignocellulosic biomass [33,37,72,125]. Lignin is poly­ merized from three monolignols (p-coumaryl, coniferyl, and sinapyl alcohols) by random radical coupling reactions during biosynthesis, resulting in three dimensional amorphous structures without regularity. Lignin monomers are connected by various C–C and C–O–C linkages including β-O-4, β-5, β-1, β-β, 5-5, 4-O-5, with the β-O-4 linkage to be the dominant [126,127]. The amount of lignin and its structure differ from plant to plant. The highly irregular and stable polymeric structure of lignin makes it can prevent the plant cells from microbial attack and chemical destroying, and also brings daunting challenges in its selective conversion. While, the aromatic based chemical structure of lignin provides great opportunities to improve other biorefinery processes for 11

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depolymerization, deoxygenation, hydrogenation, and other types of conversions [127,135]. Moreover, the use of model compounds signifi­ cantly reduces the analytical difficulties caused by complicated products in real lignin conversion. Phenolic hydroxyl group and methoxy group are the two common functionalities on lignin aromatic ring, and thus phenol, anisole, guaiacol, syringol, and their derivatives with C1~C3 alkyl groups (on the para-position of phenolic hydroxyl group) are often used as lignin monomers for the exploration of dehydroxylation, demethoxylation, alkylation, transalkylation, and hydrogenation re­ actions in lignin conversions. Lignin dimers with different C–O–C and C–C linkages are also synthesized to mimic real lignin coversion [136–138]. Lignin monomers or dimers which usually contain 7–18 carbon atoms once fully deoxygenated they can directly turn into jet fuel range hydrocarbons. Thus, the removal of excessive oxygen in lignin becomes the main objective. HDO is widely used for transformation of lignin and its derivatives to hydrocarbons [72,128,130,139]. During HDO, in the presence of hydrogen and catalysts at elevated temperatures (473 K–773 K), oxygen in lignin is removed in forms of H2O, CO, CO2, and methanol by hydrogenolysis, dehydration, decarboxylation, or deme­ thoxylation reactions. As depicted in Scheme 12, there are two typical routes for deoxygenation of hydroxyl or methoxyl on the aromatic ring of lignin: 1) Direct deoxygenation to generate aromatics, and subsequent hydrogenation to produce saturated hydrocarbons (DDO); 2) hydroge­ nation of aromatic rings to form cyclohexyl oxy-compounds, followed by deoxygenation to yield hydrocarbons (HYD). DDO route is more chal­ lenging than HYD route due to that the cleavage of aromatic C–O bonds is more difficult than that of aliphatic C–O bonds [140,141]. Never­ theless, HDO route can be regulated by adjusting two kinds of compet­ itive reactions, i.e. hydrogenolysis and hydrogenation. High activity of hydrogenolysis with low activity of hydrogenation benefits for DDO route, otherwise in favor of HYD route. The competitiveness of the two reactions is strongly depended on catalysts and reaction conditions [139,142]. Noble metals, e.g. Ru, Pt, Pd based catalysts, with high hy­ drogenation catalytic activity, prefer HYD to DDO. While, catalysts contain species with high catalytic activity on C–O bond cleavage, e.g. NbOx in Ru/Nb2O5 or Ru/Nb2O5–SiO2, benefit for DDO route [143,144]. Reaction conditions, including hydrogen pressure and temperature, are

also essential for regulating HDO route. Low hydrogen pressures and high reaction temperatures are in favor of DDO route since at such conditions the generated H� species prefer to adsorb on the metal surface and react with the nearby oxygen atoms [140,145–147]. Methoxy group is abundant in lignin, of which transformation is very important for lignin conversion. Methoxy group in lignin can be removed in forms of methanol and methane via demethoxylation and demethylation, respectively. Moreover, the transfer of methyl groups from methoxy groups to aromatic rings through transalkylation or disproportionation is also commonly found in lignin conversion, resulting in the production of alkylphenols or alkylbenzenes [135]. As compared with demethoxylation or demethylation, transalkylation re­ actions can build up the carbon chain and reduce carbon loss and hydrogen consumption during lignin HDO conversion. Alkylphenols and alkylbenzenes produced from transalkylation reactions have high octane numbers that are perfect for fuel blend usage. Besides deoxygenation and hydrogenation reactions, coupling re­ actions which involve the formation of C–C bonds among aromatic rings are also widely found during the HDO conversion of lignin degraded compounds [44,129,135,138]. Coupling reactions, leading to the yields of long-chain products, are beneficial for the conversion of small lignin fractions to jet fuel ranged hydrocarbons, as depicted in Scheme 13. It should be noted that coupling reactions must be strictly controlled to avoid the formation of oversize molecules that are not suitable for fuel usage. The control could be achieved by the adjustment of catalysts [135,138]. Acid catalysts that promote alkylation, transalkylation, and isomerization reactions are in favor of coupling reactions. Metal cata­ lysts that have strong hydrogenation catalytic activity for destroying aromaticity can restrain coupling reactions [135]. As compared with lignin model compounds, real lignin is much complicated and its selective conversion is a great challenge. Several catalytic routes, especially HDO [128,130,131,148], have been exten­ sively studied with the aim of producing fuel ranged hydrocarbons. Since lignin is a biopolymer with relatively high molecular weight, the first step of lignin conversion to aviation fuels usually involves the rupture of lignin macromolecular into small fractions. The predominant linkage between lignin monomers are C–O–C bonds, which constitute around two-thirds to three-quarters of the total linkages. Generally, C–O–C bonds have lower bond dissociation energy (~218–314 kJ mol 1) than that of C–C single bonds (~384 kJ mol 1) [149]. Therefore, it is conceptually possible to selectively cleave C–O–C bonds in lignin without disrupting C–C bonds [150]. It was calculated that 30%–69% of lignin would be released as dimers (containing 12–18 carbon atoms) if C–O–C linkages in lignin were selectively cleaved [151,152]. HDO of these lignin substructure-based dimers can directly generate hydrocar­ bons in jet fuel range. This route is promising, since it can be conducted at relatively mild conditions and does not need additional C–C bond formation. Long chain hydrocarbons (C7~C20) could be also generated via coupling reactions of lignin-degraded monomers [138,152–155]. Most of products from lignin primary degradation are phenols which have high alkylation reaction activity, and they can be further trans­ formed to alkylbenzenes over solid acid catalysts especially when al­ cohols are used as reaction media [156–159]. Alkylbenzenes is an important components usually used as high octane number enhancers in aviation fuel. It should be noted that most of lignin used for HDO conversion is not pure. Impurities, containing sugars, furans, and their derivatives, are coexistence with lignin. These impurities could competitively adsorb on the metal surface of catalyst and form humins-like polymers which would hinder the adsorption of lignin and its degraded intermediates to catalytic sites, and prevent lignin from hydrogenolysis and hydrogena­ tion conversion [160]. Thus, the design and synthesis of robust catalysts that could eliminate the negative influences of sugar impurities on lignin HDO conversion is important and necessary.

Scheme 12. Pathways of DDO and HYD in the HDO conversion of lignin model compounds. 12

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Scheme 13. The production of jet fuel ranged hydrocarbons via coupling reactions of lignin fractions.

5. Trends and perspectives

(3) All the main compositions in lignocellulose should be converted to aviation fuel to achieve a high atom economy. Lignocellulose contains mainly cellulose, hemicellulose, and lignin, and the current catalytic technologies can realize the full utilization of lignocellulosic compositions, transforming them into aviation fuel components. As elucidated in this paper in sections 2-4, cellulose and hemicellulose could be catalytically converted into a plenty of n- and iso-paraffins, while lignin could be transformed into cycloparaffins and aromatics. The type of biomass, produced fuels, catalysts, and fuel yields were summarized in Table 2. (4) Fuels for special purpose, e.g. aviation fuel with high energy density for military use, are also expected to be generated from biomass, and all the alternative fuels must meet one or several specifications before their large-scale commercial application. Currently, there are Jet A (used in the United States) and Jet A-1 (used in other countries) for commercial use, and also JP-5 and JP-8 for military use. The main difference between Jet A and Jet A-1 lies in their freeze point, of which Jet A-1 (226 K) is lower than Jet A (233 K). The difference between commercial-use jet fuels and military-use jet fuels is that the military-use jet fuel contains higher ratio of cycloalkanes which can endow them higher energy densities and lower freeze points, and capable being used in more critical conditions. The United Kingdom is­ sued the first fuel specification in 1943, followed by the US in 1944 [2,36]. Many modifications on fuel specifications have been made since then to satisfy the improving standards on fuel safety, quality, and/or security of supply. Currently, there are mainly three standards for certifying aviation fuel: ASTM D1655 and D7566 (issued by American Society for Testing and Materials), IATA GM kerosene type (issued by International Air Transport Association), and Def Stan 91–91(issued by United Kingdom Ministry of Defense) [12].

With the intensive research activities being carried out recently, various catalytic routes are available for the transformation of ligno­ cellulose to aviation fuels. However, to achieve an industrially viable application, the following issues should be considered. (1) Fuel properties of aviation fuels derived from biomass via cata­ lytic routes should be much similar to those of conventional aviation fuel from kerosene, and only that can they be used directly or as safe “drop in” fuels in aviation industry. This is attributed to the following several reasons: 1) The combustion of alternative aviation fuel under extreme conditions (e.g. high altitude) must be very safe and reliable due to the consequences of engine failure of aircrafts are much more severe than those of cars and ships; 2) Alternative fuels must be compatible with related materials in aircraft engine to insure a long life of expensive jets; 3) Alternative aviation fuels must be fully inter­ changeable with the current aviation fuel to eliminate commer­ cial limitations and avoid the problem of handling multiple fuels with varying qualities in airports. (2) All compositions in aviation fuel should be produced from lignocellulose to achieve a complete replacement of the conven­ tional fuel. Conventional aviation fuel is a mixture of hydrocar­ bons with a specific carbon range of C8~C16 between the gasoline fraction and the diesel fraction, typically consisting of 20 vol% nparaffins, 40 vol% iso-paraffins, 20 vol% cycloparaffins, and 20 vol% aromatic hydrocarbons, with small amounts of olefins (<5 vol%) and sulfur (<3000 ppm) [156]. The exact ratio of com­ ponents in aviation fuel varies a little with the crude oil and refinement process. Each kinds of components play an indis­ pensable role in aviation fuel. n- and iso-paraffins with high hydrogen to carbon ratio ensure aviation fuel a high energy density and a clean burn [36]. Cycloalkanes also have a high energy density, and more importantly, they can help reduce the fuel freeze point, which is very important considering the fuel is used in high altitudes. Aromatics with one or more ring structures have a maximum limitation (25 vol%) in aviation fuel, since they are toxic to environment and deficient in hydrogen, which leads to a low heat content per unit mass and a high particle emission during burn. However, aromatics are essential to aviation fuel, due to that they play a vital role in fuel’s elastomeric swelling, material compatibility, and lubricity characteristics [36]. To produce a 100% biomass-derived aviation fuel, n-, iso-, and cy­ clo-paraffins together with aromatics should be produced from biomass.

Though lignocellulose holds great potential in aviation fuel pro­ duction, current technologies are still in their infancy. Several chal­ lenges must be addressed to fully unlock lignocellulose’s potential. The first challenge is the large-scale production of platform chemicals from cellulose and hemicellulose in an inexpensive way. The second chal­ lenge is the selective formation of C–C bonds among carbohydrate derived platform chemicals to produce fuel precursors with desired carbon skeletons. The third one is the activation and degradation of lignin to its monomers and dimers in high yields. Last challenge but not least, is the efficient HDO conversion of oxy-compounds to targeted hydrocarbons with minimum consumption of hydrogen. To overcome these challenges and pave the way for the successful transformation of lignocellulose to aviation fuel, new technologies must involve several necessary chemical transformations, including but not limited to the 13

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Renewable and Sustainable Energy Reviews xxx (xxxx) xxx

Table 2 Catalytic production of various hydrocarbons for aviation fuels from lignocellulosic feedstock a. Biomass Feedstock

Intermediate

Carbohydrates, including cellulose, hemicellulose, and their units

Furanic compounds, e.g. ,

,

,

,

,

, etc.

Levulinic acid

a-Angelica lactone

,

γ-Valerolactone

Lignin

Lignin degraded monomer and dimers ,

,

,

C–C bond formation strategy (Catalyst)

HDO Catalyst

Aldol-condensation (NaOH, KOH, alkaline earth oxides, zeolites, hydrotalcites); Hydroxyalkylation (H2SO4, HCl, acidic resins, acidic phosphates); Pinacolic coupling (Zn, Mg, or Al with NaOH, or KOH).

Pd/MgO–ZrO2, Pd/WO3–ZrO2, Pd-FeOx/SiO2, Pd/H-beta, Pt/NbOPO4, Pt/Al2O3, Ru/Al2O3, Ni/H-β, Cu/MgAl2O4, etc.

Ketonization (metal oxides, e.g. CeO2, MnO2, La2O3, acidic zeolites); Self-aldol condensation (trichloroacetic acid with ZnCl2).

Ir-ReOx/SiO2, Pt-ReOx/C, Pd/Al2O3, Pd/C, Pd/SiO2, Pd-FeOx/SiO2, etc.

Self-conjugated addition (hydroxide or alkoxide salts, active metals, carbonates); Aldol-condensation (Alkaline earth oxides, hydrotalcites, zeolites).

Ir-ReOx/SiO2, Pt-ReOx/C, Pd/Al2O3, Pd/C, Pd/SiO2, Pd-FeOx/SiO2, etc.

,

Ce0.5Zr0.5O2, Pd/Nb2O5, Pd/Nb2O5, Ru/C, etc.

Catalysts for direct deoxygenation to generate aromatics, including, CoMo/Al2O3, NiMo/Al2O3, Ru/Nb2O5, Ru/ Nb2O5–SiO2 etc.

Catalysts for hydrodeoxygenation to generate saturated hydrocarbons, including Ru/C, Ru/HY, Pd/C, Pd/ Al2O3, Pt/Al2O3, etc.

,

Fuel yield

Refs

45–85% (from one step HDO reactions); 65–96% (from two step HDO reactions).

[58,62, 63,67]

25–70% (from ketonization); 18–32% (from self-aldol condensation).

[97–99, 103]

60–66% (from dimerization); 29–32% (from trimerization).

[35,47, 85,90]

60–75% (from butenes routes); 75–80% (from pentanoic acid route).

[48, 115, 117, 119]

25–40% (from hardwood lignin); 20–30% (from softwood lignin); 20–35% (from herbaceous lignin). 40–60% (From hardwood lignin); 20–30% (From softwood lignin); 30–40% (From herbaceous lignin).

[33, 129, 140, 161]

, etc.

,

Oligomerization via butenes (SiO2/Al2O3 with Amberlyst-70, H ZSM-5, or acidic ionic liquid); Ketonization via pentanoic acid (metal oxides, metalacid bifunctional catalysts). With no need for C–C bond formation.

Fuel structure

etc.

,

etc.

, etc. ,

, etc. R ¼ Alkyl groups

, etc. R ¼ Alkyl groups

[33, 128, 130, 156]

a Fuel yields from carbohydrates are given on the basis of carbon in products divided by carbon in Intermediates (platform compounds). Fuel yields from lignin are given on the basis of carbon in products divided by carbon in lignin.

following aspects:

carbon-chain length of their monomers is short. The cleavage of glucosidic bonds in cellulose and hemicellulose will depolymerize them into C5~C6 sugars without sufficient carbons for aviation fuel (C8~C16) production. Thus, the formation of a certain amount of new C–C bonds is required for the generation of aviation fuel precursors with suitable carbon-chain length. 3) Removal of oxygen-containing functional groups and addition of hydrogen. High oxygen content is one of the distinctive features of biomass that differentiates them from fossil resources. Typically, deoxygenation or hydrodeoxygenation is a necessary operation for converting biomass or their derivatives to high quality liquid fuels. Oxygen-containing functional groups including hydroxyl, methoxyl, carbonyl, must be removed, and simultaneously hydrogen should be

1) Cleavage of C–O–C bonds. The cleavage of C–O–C bonds in ligno­ cellulose can effectively depolymerize cellulose, hemicellulose, and lignin to low-molecular-weight intermediates (e.g. their monomer and dimer units) with suitable carbon numbers that can be processed more easily to aviation fuels. C–O–C bonds are the dominated link­ ages in all the three lignocellulosic components (e.g. glucosidic bonds in cellulose and hemicellulose, β-O-4 bonds in lignin), and they have lower bond dissociation energies than those of C–C bonds. The selective cleavage of C–O–C bonds without effect of C–C bonds is possible and can benefit for the generation of fuel precursors. 2) Formation of necessary C–C bonds. This is crucial especially when cellulose and hemicellulose are used as raw materials, because the 14

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added to increase the fuel energy density, stability, and compatibility. 4) Isomerization, cyclization, and aromatization of fuel precursors to formation iso-paraffins, cycloalkanes, and aromatics. The production of iso-paraffins, cycloalkanes, and aromatics from cellulose and hemicellulose is not straightforward, while these kinds of hydro­ carbons are essential for aviation fuels. To ensure that lignocellulosic aviation fuel meets quality standards, isomerization, cyclization, and aromatization are required in lignocellulose transformation.

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Moreover, processes for the catalytic conversion of biomass to aviation fuel should be economically competitive and environmentally sustainable. To achieve these goals, the development of highly inte­ grated systems that allow the conversion of entire lignocellulose in a “one-pot” reaction, which negates the high-cost biomass pretreatment step, is essential. The design and synthesis of robust and recyclable catalysts with high catalytic activity is required. Catalysts that can selectively break C–O–C bonds, or build C–C bonds, or have tunable catalytic activities in hydrogenolysis and hydrogenation reactions are urgently needed. Multifunctional catalysts that can integrate several conversion steps in one process are promising. In addition to the syn­ thesis of novel catalysts, the screening of green and reusable solvents is necessary. Moreover, the co-production of high value chemicals or materials during aviation fuel synthesis should be also considered. 6. Conclusions Aviation fuel from the catalytic conversion of lignocellulose could completely replace petroleum-derived fuels. Plenty of n- and iso-paraf­ fins with varied structures in aviation fuel range could be produced by using cellulose, hemicellulose, or their derivatives as feedstocks. How­ ever, the production of aviation fuel from cellulose and hemicellulose is not straightforward. The transformation of monosaccharides to platform chemicals (e.g. 5-HMF, furfural, LA, GVL, angelica lactone, polyols) which have functional groups that enable them to form C–C bonds is required. C–C bonds could be built among platform chemicals via re­ actions including aldol-condensation, hydroxyalkylation/alkylation, pinacolic coupling, and ketonization, and aviation fuel range hydro­ carbons with linear or branched structures could be produced when hydrodeoxygenation reactions are combined. Different from cellulose and hemicellulose, lignin is an aromatic biopolymer consisting of methoxylated phenylpropane structures. The selective cleavage C–O–C linkages without disrupture of C–C linkages could degrade lignin into C8~C16 monomers and dimers. Once hydrodeoxygenated, these frac­ tions could be directly converted into aviation fuel ranged hydrocarbons without the need of new C–C bonds formation. Moreover, cycloparaffins and aromatics which are indispensable to aviation fuel could be straightly produced from lignin HDO conversion. Thus, all the compo­ sitions in aviation fuels, including linear and branched alkanes, cyclo­ paraffins, and aromatics could be generated from lignocellulosic biomass through catalytic conversion. Declaration of competing interest None. Acknowledgements This work was supported by the National Key R&D Program of China (2018YFB1501500) and the National Natural Science Foundation of China (No. 21706277). References � Catalytic conversion of carbohydrates to initial [1] Mika LT, Cs� efalvay E, N�emeth A. platform chemicals: chemistry and sustainability. Chem Rev 2018;118:505–613.

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