catalysts

Review An Overview on Catalytic Hydrodeoxygenation of Pyrolysis Oil and Its Model Compounds

Zhan Si 1,2,3, Xinghua Zhang 2, Chenguang Wang 2,*, Longlong Ma 2 and Renjie Dong 1,3,4

1 Bioenergy and Environment Science & Technology Laboratory, College of Engineering, China Agricultural University, No. 17 Qinghua Donglu, Haidian District, Beijing 100083, China; [email protected] (Z.S.); [email protected] (R.D.) 2 Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; [email protected] (X.Z.); [email protected] (L.M.) 3 Key Laboratory of Clean Production and Utilization Technology for Renewable Energy, Ministry of Agriculture, College of Engineering, China Agricultural University, Beijing 100083, China 4 National Center for International Research of BioEnergy Science and Technology, Ministry of Science and Technology, China, Beijing 100083, China * Correspondence: [email protected]; Tel.: +86-20-37029721

Academic Editor: Yu-Chuan Lin Received: 9 April 2017; Accepted: 15 May 2017; Published: 1 June 2017

Abstract: Pyrolysis is considered the most promising way to convert biomass to fuels. Upgrading biomass pyrolysis oil is essential to produce high quality hydrocarbon fuels. Upgrading technologies have been developed for decades, and this review focuses on the hydrodeoxygenation (HDO). In order to declare the need for upgrading, properties of pyrolysis oil are firstly analyzed, and potential analysis methods including some novel methods are proposed. The high oxygen content of bio-oil leads to its undesirable properties, such as chemical instability and a strong tendency to re-polymerize. Acidity, low heating value, high viscosity and water content are not conductive to making bio-oils useful as fuels. Therefore, fast pyrolysis oils should be refined before producing deoxygenated products. After the analysis of pyrolysis oil, the HDO process is reviewed in detail. The HDO of model compounds including phenolics monomers, dimers, furans, carboxylic acids and carbohydrates is summarized to obtain sufficient information in understanding HDO reaction networks and mechanisms. Meanwhile, investigations of model compounds also make sense for screening and designing HDO catalysts. Then, we review the HDO of actual pyrolysis oil with different methods including two-stage treatment, co-feeding solvents and in-situ hydrogenation. The relative merits of each method are also expounded. Finally, HDO catalysts are reviewed in order of time. After the summarization of petroleum derived sulfured catalysts and noble metal catalysts, transitional metal carbide, nitride and phosphide materials are summarized as the new trend for their low cost and high stability. After major progress is reviewed, main problems are summarized and possible solutions are raised.

Keywords: biomass; pyrolysis oil; hydrodeoxygenation; catalysts

1. Introduction Rapid economic development remarkably increases the energy demand, especially in transportation fuels. Environmental concerns and uneven distribution of fossil fuel resources have aroused interest in developing renewable sources. Progress in technology of converting renewable sources into energy and fuels has become a research highlight. Biomass, as the only renewable organic carbon source feedstock, is a suitable renewable feedstock that can be converted into chemicals and transportation fuels [1,2]. Fast pyrolysis is a promising way to convert solid biomass to liquid products

Catalysts 2017, 7, 169; doi:10.3390/catal7060169 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 169 2 of 22 due to its high economic efficiency [3]. Different lignocellulosic biomass resources, including energy crops, forest waste residues, herbaceous and woody biomass, were studied to produce bio-oil through fast pyrolysis. Bio-oil generally contains 50–65 wt % organic components, involving acids, aldehydes, ketones, furans, phenolics, guaiacols, syringols and sugars; 15–30 wt % moisture; and 20 wt % colloidal fraction [4]. Oxygens contained in these compounds cause undesirable properties such as low energy density, instability, high viscosity and corrosion. Therefore, removing redundant oxygen atoms is required for bio-oil upgrading. In the past decades, oxygen removal attracted researchers’ attention all over the world. Since the 1990s, enormous papers in biomass pyrolysis oil upgrading were published and many great review papers inspired researchers all over the world. Hydrotreating is an effective way to upgrade bio-oil, which removes oxygen through HDO, generally at 400–773 K and high H2 partial pressure. Different catalysts were used in HDO to remove the oxygen atom in the molecules. In this paper, after a brief introduction about the properties of bio-oil, we mainly focus on the recent progress in catalytic HDO of bio-oil. With the rapid development of catalysts and upgrading technology, we hope this review can provide useful information and inspire ideas in producing high quality bio-oil.

2. Characteristics of Pyrolysis Oils

2.1. Properties of Bio-Oil The elemental compositions of bio-oil and petroleum derived fuel are quite different, and the basic data are shown in Table1. Normally, there is 15–30 wt % water in bio-oil, derived from the moisture in the feedstock and dehydration process during the pyrolysis. The existence of water lowers the heating value and flame temperature of bio-oil. On the other hand, water reduces the viscosity and enhances the fluidity. Due to relatively high amounts of carboxylic acids mainly formed from hemicellulose pyrolysis, bio-oil generally shows acidity with pH value between 2 and 4, resulting in fatal problems in downstream utilization [5]. As shown in Table1, the oxygen content of bio-oil can reach as high as 50%. The presence of oxygen is the main difference between bio-oils and hydrocarbon fuels. Since the oxygen content is quite high, the heating value of bio-oil is much lower than that of conventional fuel (50%). Bio-oil has a heating value of about 20 MJ/kg using lignocellulosic biomass as feedstock, while that of fossil fuel is 45.5 MJ/kg. In addition, immiscibility with hydrocarbon fuels was also caused by the high oxygen content.

Table 1. Comparison between biomass derived bio-oil and diesel.

Bio-Oil Property Diesel [6] Pine Saw Dust [7] Eucalyptus [7] Rice Husk [8] Density (kg/L) 1.206 1.229 1.14 0.84 C (wt %) 40.6 42.3 39.92 86.58 H (wt %) 7.6 7.5 8.15 13.29 O (wt %) 51.7 50.1 51.11 0.01 N (wt %) <0.1 0.1 0.61 65 ppm Ash 0.03 0.03 0.25 - Viscosity (c St) 17 (313 K) 23 (313 K) 13.2 (313 K) 2.1 (323 K) Moisture content (wt %) 23.9 20.6 28 - Higher heating value (MJ/kg) 16.9 17.3 16.5 45.5 PH 2.7 2.2 3.2 - Flash point (K) 326 374 341 327

As can be seen in Table1, the viscosity of bio-oil is much higher than the conventional fuel. Depending on the types of biomass, pyrolysis conditions and storage time, a wide range of the bio-oil viscosities are obtained. Sipilaè et al. found that the viscosities of bio-oils were reduced with higher water content and less water insoluble components [5]. Boucher et al. added methanol into bio-oil, finding that methanol reduced the density and viscosity and increased the stability. Moreover, the ash in bio-oil can cause problems in engines, such as corrosion, abrasion and deposition. Alkali and alkali Catalysts 2017, 7, 169 3 of 22 earth metals are problematic components in the ash. The concentration of alkali and alkali earth metals in bio-oil are listed in Table2. The concentrations are distinct when the bio-oils are produced from different feedstocks and conditions. The inorganic impurities in bio-oil could cause the catalyst poisoning during upgrading process. Yildiz et al. reported that an ash concentration of 3 wt % could have effect on the zeolite catalysts [9]. Kubiˇckaand Horáˇcekfound that phosphorus, alkali and alkali earth metal could cause deactivation of CoMo/γ-Al2O3 [10]. Hot gas filtering is an efficient method to remove metallic heteroatoms in bio-oil. Scahill et al. employed hot gas filtration to decrease the alkali metal concentration below 10 ppm [11]. The ash content of cassava stalk bio-oil decreased from 0.2% to 0.01% after using gas filter [12]. Baldwin and Feik reduced the concentration of Ca < 10 ppm, Mg < 1 ppm, K < 5 ppm and Na < 5 ppm by hot gas filtration with a ceramic filter element [13]. In order to understand where the ash come from and how it affect the bio-oil’s properties, the existence form of alkali and alkaline metals in both biomass and pyrolysis vapor maybe more important.

Table 2. The concentration of alkali and alkali earth metals in bio-oil.

Alkali and Alkali Earth Bio-Oil Metals (ppm) Rice Straw [14] Bamboo [14] Japanese Larch [15] Na 46 16 4.2 K 25 6 <0.1 Ca 32 9 2.8 Mg 9 2 0.1

2.2. Compositions of Bio-Oil Insights in the composition of bio-oils are essential to determine its potential uses and the type of process required for upgrading methods [16]. Wang et al. used gas chromatography-mass spectrometer (GC/MS) to analyze the composition of bio-oils and illustrate that most of the identified components were phenolic compounds attached with ketones or aldehydes groups [17]. Therefore, bio-oils were highly hydrated, resulting in a difficult elimination of water. Conventional GC, especially when coupled with MS, is commonly used to analyze the chemical components for decades. However, GC-MS is not the most favorable technique for bio-oil characterization, as the poor elution properties of various compounds resulted in strong tail and coelution peaks. Comprehensive two-dimensional gas chromatography (GC × GC) coupled with different detectors is a new method in bio-oil characterization. Coelutions in the 1 DGC could be resolved by GC × GC, thus the number of compounds identified using GC × GC was approximately 4–7 times greater than that using conventional GC [18]. However, only 25–40% of the analytes are detected because carbohydrates and lignin-derived oligomers are hard to evaporate which makes them difficult to enter the GC column [16]. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is able to characterize the nonvolatile compounds in bio-oils [19]. For pine pellets and peanut hull bio-oils characterized by FT-ICR MS, much lower oxygens per molecule was obtained in the oil phase than aqueous phase [20]. Red pine bio-oil was analyzed by FT-ICR MS coupled with negative ion electrospray ionization (ESI) source. The predominant compounds in bio-oil are O2–O17 class species with 1–22 double-bond equivalent (DBE) values and 4–39 carbon numbers. The N1Ox was also identified, which involved 1–16 DBE and 6–30 carbon numbers [21]. NMR spectroscopy is also employed for the characterization of bio-oils, which can provide the concentrations of chemical functionalities and indications of highly substituted aromatic groups. For instance, the 13 C and DEPT analyses showed that corn stover bio-oil is highly branched owing to the high percentage of CH1 groups and the aromatic region generally has CH0:CH1 ratios of >2:1, suggesting that the average aromatic ring has at least four substituents. Additionally, high content of intact carbohydrate in corn stover bio-oil is demonstrated by 1H NMR analysis, because a relatively high concentration of protons was observed at ~5 ppm [22]. Catalysts 2017, 7, 169 4 of 22

Thus, integrated techniques are generally employed to obtain a comprehensive characterization of bio-oil. Currently, GC × GC combined with HRMS (using different ionization methods) seem to be most promising to obtain a specific chemical characterization of bio-oils. Spectroscopic method is also crucial as it enables to provide the bio-oils composition profile assessment [23].

3. HDO As discussed earlier, some undesirable properties of bio-oil, such as chemical instability and a strong tendency to re-polymerize are caused by the high oxygen content. High viscosities, acidity, water content and low heating value are all major defects making it very hard to use bio-oils as fuels. Therefore, fast pyrolysis oils must be refined to produce deoxygenated products that are compatible with existing transportation fuels. The general methods for bio-oil upgrading include physical, chemical and catalytic process. Physical upgrading includes filtration, solvent addition and emulsion. Liquid filtration is extremely difficult because of the physicochemical properties of bio-oil, while hot-vapor filtration can lower the average molecular weight of the liquid product and improve its stability during accelerated aging studies [13]. For solvent addition and emulsion, solvents and surfactants are added into bio-oil to improve stability. Chemical upgrading mainly involves esterification, aqueous phase processing, mild cracking, gasification, etc. Catalytic upgrading pathways include HDO, zeolite cracking and steam reforming, of which the last one is a route to produce H2, not transportation fuel. Zeolite cracking is correlated with fluid catalytic cracking (FCC) of petroleum fractions, where oxygen atoms are removed by simultaneous dehydration-decarboxylation reactions over cracking catalysts (generally acidic zeolite) at atmospheric pressure without external H2. However, a major defect of this method is the low hydrocarbon yield due to severe carbonaceous deposits and dealumination of zeolite. Hydrotreating is also a conventional process for petroleum reforming, including hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and HDO to remove sulfur, nitrogen, and oxygen heteroatoms respectively. Unlike petroleum oil, the content of sulfur and nitrogen in bio-oils is negligible whereas O content is extremely high. Hence, HDO becomes a crucial research topic. During HDO, reactions may occur including deoxygenation (C-O cleavage), hydrogenation (saturation of C=O and aromatic rings), hydrogenolysis, hydrocracking, etc.

3.1. HDO of Model Compounds The reaction pathway of bio-oil HDO is still not clear due to its complicated nature resulting in numerous concurrent reactions during the upgrading process. Therefore, rather than employing pyrolysis oil at lab-scale, most studies use model compounds to obtain sufficient information in order to understand HDO reaction networks and mechanisms. Meanwhile, model compounds experiments contribute to screening and designing HDO catalysts [24]. Currently, model compounds are selected among the most active compounds which will cause the instability of bio-oil. These compounds with different functional groups allow one to investigate the relative activities and selectivity of different reactions, such as dehydration, decarboxylation, hydrogenation, hydrogenolysis and hydrocracking. Furthermore, dimeric compounds were also selected as model compounds to provide an insight into the cleavage of some major types of linkage.

3.1.1. HDO of Aromatic Monomers Since bio-oil contains almost 30 wt % of aromatic compound which has high energy density [25], a large amount of researches focus on HDO of these compounds. Additionally, the existence of phenolic compounds is considered as the primary cause for coke formation and catalyst deactivation [26]. Thus, conversion of these oxygen-contained aromatic compounds is important in bio-oil upgrading. Phenol is widely investigated as a model compound in HDO process because of its simple structure. In accordance with numerous investigations, HDO of phenol undergoes two main parallel routes. The first route is direct deoxygenation (DDO) that leads to formation of benzene by the Catalysts 2017, 7, 169 5 of 22 Catalysts 2017, 7, 169 5 of 22 cleavagehydrogenated of C-O to bond. cyclohexanol. The second isSubsequent hydrogenation hydrogenation (HYD), in which of thebenzene aromatic or ring deoxygenation is hydrogenated of tocyclohexanol cyclohexanol. occurs, Subsequent leading hydrogenation to the formation of benzene of cyclohexane. or deoxygenation Meanwhile, of cyclohexanol alkylation occurs, and leadingetherification to the formationtake place ofto cyclohexane. form bicyclic Meanwhile, compounds. alkylation Hong et and al. etherification also reported take similar place reaction to form bicyclicpathways compounds. of phenol Hongto bicyclics et al. also over reported Pt/HY similarcatalyst reaction [27]. Studies pathways on ofboth phenol aqueous-phase to bicyclics overand Pt/HYvapor-phase catalyst HDO [27 ].of Studiesphenol indicated on both aqueous-phase that non-sulfide and catalysts vapor-phase favor HYD HDO route of phenol to saturate indicated the thataromatic non-sulfide ring. For catalysts example, favor no aromatic HYD route hydrocarbo to saturatens thewere aromatic produced ring. in HDO For example, of phenol no over aromatic Pd/C and H3PO4 at 523 K [25]. Therefore, HDO catalysts properties can significantly affect the reaction hydrocarbons were produced in HDO of phenol over Pd/C and H3PO4 at 523 K [25]. Therefore, HDO catalystspathways. properties can significantly affect the reaction pathways. Anisole with a methoxyl group is another model compound used for the study of bio-oil HDO. Figure1 1 shows shows aa general general reaction reaction scheme scheme of of anisole anisole HDO HDO studied studied by by Sankaranarayanan Sankaranarayanan etet al.al. overover supported Ni andand CoCo catalyst,catalyst, involvinginvolving fourfour reactionreaction pathways:pathways: (1) demethylationdemethylation of anisoleanisole toto phenolphenol andand methane; methane; (2) (2) direct direct deoxygenation deoxygenation to benzene to benzene and methanol;and methanol; (3) hydrogenation (3) hydrogenation to methyl to cyclohexylmethyl cyclohexyl ether; and ether; (4) isomerizationand (4) isomerization to methylphenol to methylphenol and dimethylphenol and dimethylphenol [28]. These [28]. primary These primary products can be further converted to cyclohexane, cyclohexanol, methylcyclopentane, etc. products can be further converted to cyclohexane, cyclohexanol, methylcyclopentane, etc. Pichaikaran et al. Pichaikaran et al. studied the vapor-phase HDO of anisole over supported Ni and Ru catalysts studied the vapor-phase HDO of anisole over supported Ni and Ru catalysts atatmospheric pressure [29]. atatmospheric pressure [29]. The major products obtained are phenol, benzene and toluene, The major products obtained are phenol, benzene and toluene, indicating that the conversion of anisole indicating that the conversion of anisole mainly proceeds via hydrodeoxygenation, hydrogenolysis mainly proceeds via hydrodeoxygenation, hydrogenolysis and methyl group transfer. and methyl group transfer.

Figure 1.1. ReactionReaction network network for for anisole HDO: (1) dealkylationdealkylation and demethylation;demethylation; (2) direct deoxygenation; (3) hydrogenation; (4) isomerization; (5) dehydration; (6) alkylation; and (7) ring opening reaction [[28].28].

Guaiacol with two kinds of C-O bond (hydroxyl group (-OH) and methoxy group (-OCH3)) Guaiacol with two kinds of C-O bond (hydroxyl group (-OH) and methoxy group (-OCH3)) appears to be be an an attractive attractive compound compound for for bio-oil bio-oil HDO. HDO. The The HDO HDO scheme scheme of ofguaiacol guaiacol proposed proposed by byNguyen Nguyen et al. et al.is shown is shown in inFigure Figure 2,2 ,indicating indicating that that the the reactions proceededproceeded viavia demethylation,demethylation, demethoxylation, hydrogenation and methyl transfer [[30].30]. The results of numerous researches for guaiacol HDO with different catalyst indicated that the catalyst composition plays a significantsignificant role during the reaction. Zhao et al. studied vapor phasephase HDO of guaiacol catalyzed by supported metal phosphide.phosphide. The formation of anisole demonstrated that dehydroxylation occurs as a new reactionreaction pathwaypathway of guaiacolguaiacol HDO.HDO. Meanwhile,Meanwhile, the hydrogenation reaction did not take placeplace underunder thisthis condition sincesince nono saturatedsaturated productsproducts werewere detecteddetected [[31].31].

3.1.2. HDO of Aromatic Dimers The water insoluble fraction, often known as pyrolytic lignin (PL), can account for 25–30% of thethe bio-oilbio-oil [[32].32]. The oligomeric PL portion is consideredconsidered to be responsible for poorpoor propertiesproperties likelike viscosityviscosity and instability.instability. BayerbachBayerbach etet al.al. proposedproposed severalseveral oligomericoligomeric structures of pyrolyticpyrolytic ligninlignin thatthat areare basedbased onon combinationscombinations ofof phenolicphenolic dimersdimers with different linkers [[33].33]. Therefore, oligomers, especially phenolic dimers, were selected as model compounds to investigate the cleavage of typical linkages (e.g., β-O-4, α-O-4, and β-5) during HDO process. Jongerius et al. studied HDO of

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Theover[34].a 5-5Hthe sulfidecommercialThe2 ′in Thelinkage5-54-O-5Zhanga a 4-O-5commercial ′batch linkage4-O-5CoMo/Al and et[34]. andreactor. sulfideal. and α [34]. Zhangtested -O-4α 2Osulfide-O-4 α 3 ZhangAsCoMo/Al-O-4catalyst dimers HDO et showndimers CoMo/Alal. dimers et testedof at 2al.were O in a 5733 were testedseriesTablecatalyst were2HDO O Kquantitative3 quantitativeandcatalyst 3,ofHDO quantitative of conversionsatphenolic 50 a573 ofseries barat aK ly573seriesH and dimes lyof2converted pressure,K ly ofconvertedphenolic 50 andofallconverted with barphenolic the 50 H bartodimes2 to pressure,cyclohexane H dimestocyclohexane2 withcyclohexanepressure, with Ru/HZSM-5revealingphenoliccondition, dimers exceptthatbut revealingcondition,phenolicRu/HZSM-5at not473ether condition,over phenolicrevealing2,2 revealingtheK ′and -biphenoldimerslinkages a5-5thatbut commercial at′50 linkage dimersnotbut473ether thatbar thatover theincludingKnot of [35]. etherandlinkages etheraover[34]. H5-5the commercialsulfide2 ′in50The linkage 5-5linkagesZhang alinkagesa bar commercialβbatch ′ 4-O-5-O-4 includinglinkageCoMo/Al of et[34].H reactor. andincludingsulfide 2al.andincluding in [34]. Zhangtested a 2phenylcoumaran Oβsulfide αbatch-O-4 3-O-4AsZhangCoMo/Al catalyst HDOet shownβ βreactor.-O-4 and al.dimersCoMo/Al-O-4 et tested of at2phenylcoumaranal. Oandin and a3 573 AstestedTable seriescatalystwere 2 HDOphenylcoumaran O canshownphenylcoumaranK3 catalystand 3, HDOofquantitativebe ofconversions atphenolic in50 brokena573 seriesofTable bar at canaK 573series H and dimes3,underof 2be ly can pressure,K ofconversionscanphenolic 50 brokenconvertedandofall bewith bar bethisphenolicthe 50 broken broken H dimesbarunder2 pressure,of toHdimes all under 2 cyclohexane withunderpressure,this the with thisthis Ru/HZSM-5andandandexceptand C6-C10 atRu/HZSM-5 C6-C10C6-C10 473C6-C10Ru/HZSM-5 2,2 K alkanes, and′ -biphenol alkanes,alkanes, alkanes, at50 473 bar atrespectively. K respectively.473of respectively.[35]. and respectively.H K2 in 50andThe a bar batch50 4-O-5 ofThe bar TheHThe reactor. The 2of formation andin formationHformation a 2formation batchαin As-O-4 a batchshown reactor. ofdimers of of C8–C10 reactor. of C8–C10inC8–C10 C8–C10AsTable were shown Asalkanes 3, alkanes quantitativealkanesshown conversionsalkanes in Table during in duringduring Tableduring 3, lyofconversions HDO 3,convertedall HDOHDO conversions theHDO of of ofα of-O-4 α αto -O-4allα-O-4 cyclohexane-O-4of thedimers all dimersdimers thedimers withcondition,revealingRu/HZSM-5model acid compounds catalyzed andthatbutandand condition,Ru/HZSM-5revealingat not 473 C6-C10etherC6-C10C6-C10Ru/HZSM-5revealingcondition, thecondition,K almost and linkagestransalkylation 5-5butthat alkanes, alkanes,alkanes, at′50 linkage notachieved473 thatetherbutbar butat the including K 473ofnot ethernot and respectively.linkages respectively.[34].respectively.5-5H Kthe 2the100% ′inand 50 linkagelinkages Zhang 5-5 a bar5-5of βbatch 50′-O-4and including′ linkage C6–C7oflinkagebar et [34].H thereactor. TheandincludingTheal.of2The in HZhangtested[34].overall a [34]. formation2phenylcoumaranproducts,formation formationβbatchin-O-4 As Zhanga Zhang HDOet batchβshownselectivity -O-4 reactor.andal. ettested of reactor. etofofof phenylcoumaranandfollowed inal. a al. C8–C10 AsC8–C10C8–C10 seriesTabletested testedtoHDOphenylcoumaran canshown Asalkenes 3,of be shownHDO ofHDO conversionsalkanes byalkanesphenolicalkanes inabroken series Tablethewereof ofincan a aTable isomerization duringseries duringdimesduring of3,aboveunderseries becan ofconversionsphenolic 3,broken allof bewithof conversions96% thisHDO theHDOHDOphenolic phenolicbroken dimesunder ofofof of αalldimes αunder dimes with-O-4of-O-4-O-4 substitutedthisthe all with dimersthethisdimerswithdimers modelrevealingRu/HZSM-5 andcompoundsand andthat andC6-C10 Ru/HZSM-5revealingmodelat C6-C10 473C6-C10etherC6-C10Ru/HZSM-5revealing K compoundsalmostalkanes, and linkagesalkanes,that alkanes, alkanes,at 50 achieved473ether thatbar at respectively. including K 473almostrespectively.of etherand respectively.linkages respectively.H K 2100% 50andin linkagesachieved a bar β batch50and-O-4including of Thebar TheH thereactor. Theincludingand 2ofThe100% formationin Hoverallformation a 2formationphenylcoumaran β formationbatchin -O-4 andAs a batchβshownselectivity thereactor.-O-4 andof of overallC8–C10 reactor. of phenylcoumaranof andC8–C10in C8–C10As C8–C10Table tophenylcoumaran canshownselectivity As alkenesalkanes 3, alkanesshownbe alkanesconversionsalkanes inbroken Tablewere duringtoincan during alkenesTableduring duringabove3,under becan conversionsof HDO broken3,all HDO be wereconversions96% HDOthistheHDO broken of aboveunderof α ofof of-O-4α allα-O-4underα -O-4of96%-O-4 this thedimers all dimers dimersthethisdimers modelcondition,except started compounds2,2startedstarted′ startedand-biphenolbutstartedstartedmodelcondition, notwithC6-C10 modelwithwith thewithwithalmostcompoundswith [35].acid 5-5 acid but acid compoundsalkanes, acidTheacid′ acid catalyzedlinkageachievednot catalyzed catalyzed 4-O-5 catalyzedcatalyzedthe catalyzedalmost respectively. [34].5-5 and almost100% ′transalkylation linkage Zhangachievedtransalkylationαtransalkylation -O-4 transalkylation transalkylationtransalkylationand achieved dimerset [34].the Theal.100% overallZhangtested formationwere100% ofand of of HDOC6–C7 et quantitativeselectivityof ofandofthe C6–C7 C6–C7al. C6–C7C6–C7 C6–C7 overall testedtheofof products,a C8–C10 overallproducts,series products, to products, lyHDOproducts,products,selectivity alkenesconverted of selectivity of alkanesphenolicfollowed a followed followed wereseries followedfollowedtofollowed to alkenes cyclohexane during abovedimestoof by alkenesphenolic byby theby by bywith96%were theHDOthe thetheisomerization the were isomerizationdimesaboveisomerization isomerizationofisomerizationisomerization aboveα with-O-496% 96% dimers of of of of ofof cyclohexanesexceptRu/HZSM-5modelcondition, 2,2compoundsstarted′ -biphenolbut Ru/HZSM-5modelcondition,exceptat on not473 withcondition,Ru/HZSM-5 Brønsted Ru/HZSM-5the K2,2compounds almost[35]. and ′ 5-5-biphenolbutacid at The′50 linkagenot473achieved butbar catalyzed4-O-5 acidat theKat almostnotof473[35]. and473 [34].5-5H sites.and the K2 100% TheK′50in and linkage Zhangachievedαand 5-5transalkylationabar-O-4 4-O-5batch Ethylcyclohexane, ′50and linkage50of bardimers et[34].Hbar andthereactor. 2al.100% ofin of Zhangoverall[34]. testedαHa Hwere-O-4 2batch 2 inand As in Zhang ofa dimers HDOetquantitativea shown selectivitybatch thereactor. batchC6–C7al. et overalltestedof the reactor. were al.inreactor. a As Tableseriesproducts,tested main toly HDOquantitativeshownselectivity convertedalkenesAs As 3, ofHDO product shownof conversionsshown phenolicin a followedTable serieswere ofto ly to in ain alkenesconverted cyclohexaneseriesTable for 3,dimesaboveTableof conversions ofphenolic HDOby all3,of 3,withwere96% conversionsthe phenolic conversionsto of cyclohexane dimesisomerizationabove βof-5 alldimes with dimers,96% ofthe of all allwith the the of was except started2,2′-biphenolstartedstartedexcept withexceptmodel with2,2 with[35]. acid′-biphenol 2,2 compounds Theacidacid ′catalyzed-biphenol 4-O-5 catalyzedcatalyzed [35]. and almost [35]. Thetransalkylation α-O-4 4-O-5Thetransalkylationtransalkylation achieved dimers4-O-5 and αand were-O-4100% of α -O-4 dimersquantitativeC6–C7 ofandof C6–C7dimersC6–C7 the were products, overall werely products,quantitativeproducts, converted quantitativeselectivity followed followed followedlyto convertedcyclohexane toly by convertedalkenes bytheby to the theisomerization werecyclohexane to isomerizationisomerization cyclohexane above 96% of ofof Ru/HZSM-5exceptand C6-C10substituted 2,2substituted′-biphenolsubstituted exceptRu/HZSM-5atalkanes, 473exceptRu/HZSM-5 K2,2cyclohexanes [35]. andcyclohexanesrespectively.′ -biphenol cyclohexanes2,2 atThe50′ -biphenol473 bar 4-O-5 at K of473[35]. and TheHon and K2on [35]. The in 50 Brønstedandonformation α aBrønstedbar-O-4 4-O-5Thebatch Brønsted50 of bardimers4-O-5 H andreactor. 2acidof inacid αandH C8–C10a acidwere-O-4 2batch sites.in Asαsites. a -O-4 dimers sites. quantitativeshownbatch reactor.alkanesEthylcyclohexane, dimersEthylcyclohexane, Ethylcyclohexane, reactor. werein AsduringTable were ly quantitativeshown convertedAs 3, quantitative HDO shownconversions in Table theof ly tothein α converted themaincyclohexane-O-4Table 3,lymain ofconversionsconvertedmain dimers all3,product product conversionsthe toproduct cyclohexane to offor cyclohexane forall HDOfor of theHDO allHDO theof of of andmodelRu/HZSM-5 C6-C10 compoundssubstitutedsubstitutedsubstitutedsubstituted modelRu/HZSM-5alkanes,andat 473 C6-C10Ru/HZSM-5model model Kcompoundsalmost respectively.andcyclohexanes cyclohexanes cyclohexanescyclohexanes compounds atalkanes,50compounds achieved473 bar at K almostof473 and respectively. TheH K2100%almoston 50in almost onandformationononachieved abarBrønsted batch BrønstedBrønsted50andBrønsted ofachieved achievedbar The H thereactor. 2 100%of ofin formation acidoverallC8–C10 H a acidacid acid2 batch100% andin 100%As sites. a sites. sites.sites.shownselectivitybatch thereactor.alkanesand andof Ethylcyclohexane, overall C8–C10 Ethylcyclohexane,the Ethylcyclohexane,reactor.Ethylcyclohexane, thein As duringTableoverall overallto shownselectivity alkanes alkenesAs 3, HDO shown selectivity conversionsselectivity in duringTable were ofto in the α alkenes the the-O-4 theTable above3, toHDOmainto conversions of mainmain mainalkenes dimers alkenes all3, were 96% of conversionsproductthe productproductproductα -O-4were abovewere of dimers allforabove above for96% for forofthe HDO all HDO HDOHDO96% 96%the of ofofof obtainedandexcept C6-C10 substituted2,2substituted by′-biphenolsubstituted alkanes,andexcept hydrogenation andC6-C10except 2,2cyclohexanes [35]. respectively. C6-C10cyclohexanes′-biphenol 2,2 cyclohexanes alkanes,The′-biphenol alkanes,4-O-5 of[35]. respectively.The onaromaticand [35]. onThe respectively.formation Brønsted onα Brønsted -O-44-O-5The Brønsted 4-O-5dimersThering and ofacid Theformation acidC8–C10αandfollowed -O-4wereacid sites.formation αsites. -O-4dimers quantitative sites.alkanes Ethylcyclohexane,of dimersEthylcyclohexane, C8–C10by Ethylcyclohexane,wereof during dehydrationC8–C10 were lyquantitative alkanes converted quantitativeHDO alkanes during ofthely tothe αand during converted -O-4cyclohexanemainthely HDOmain converted hydrogenationmaindimers HDOproduct ofproduct to αproduct cyclohexane-O-4of to α for cyclohexane-O-4dimers for HDOfor dimersHDO reactions. HDO of of of exceptmodelandstarted C6-C10β 2,2 compounds-5withββ-5′ -biphenolsubstituted-5βdimers, -5 andmodelalkanes,dimers,aciddimers, dimers, andC6-C10 modelexceptcatalyzed compoundsalmost[35]. wasrespectively.C6-C10 wascyclohexaneswas 2,2compoundsalkanes,Thewas obtained ′achieved -biphenol transalkylationobtained alkanes,4-O-5obtained obtained almost respectively.The and 100% almost[35]. byrespectively. onformation αachievedbyby -O-4 by hydrogenationBrønsted Theand hydrogenationofhydrogenationachieved dimers hydrogenation The 4-O-5C6–C7the of100% Theformation overallC8–C10 andacidwere 100% products,andformation α sites.-O-4quantitative selectivity ofthe alkanesand of of aromaticdimersof overallC8–C10 followedEthylcyclohexane,the aromaticaromatic ofaromatic duringC8–C10overall to lywere selectivity alkanes convertedalkenes byring HDO quantitativeringselectivity alkanesringthering followedduring were isomerizationoffollowedto followed to followed αduring alkenesthecyclohexane-O-4 above tolyHDO main alkenesconverteddimersby HDO by were96%by of bydehydrationof product αdehydration dehydration -O-4ofwereabovedehydration toα-O-4 cyclohexanedimers above for96% dimers HDO and 96% andand and of startedmodeland C6-C10 β compoundswith-5β -5ββdimers,-5 -5 exceptandmodel-5startedalkanes,acid dimers, dimers,dimers,dimers, andC6-C10 modelcatalyzed except2,2 compounds almostwith wasC6-C10respectively.′ -biphenol was compoundsalkanes,waswas2,2 acidwas obtainedachieved ′transalkylation -biphenol obtainedalkanes, catalyzedobtained obtainedobtained almost[35]. respectively. The 100% Thealmostrespectively. by[35]. formationachieved by transalkylation 4-O-5by byhydrogenationby andThe of hydrogenationachieved hydrogenationhydrogenationThehydrogenation C6–C7 and4-O-5the 100%of Theformation αoverallC8–C10-O-4 andproducts, 100% andformation of dimersα C6–C7-O-4selectivity theofalkanesand of of ofaromatic overallofC8–C10 ofdimersfollowed the were aromaticof products,0aromatic aromaticaromatic duringC8–C10overall to quantitative selectivity were alkanesalkenes byring HDO selectivityringalkanesfollowedquantitative the ringringring duringfollowed wereisomerizationoftoly followed followed αfollowedfollowedduringconvertedalkenes -O-4 by above toHDOly the alkenes dimersconverted by HDO were96% isomerizationbyof bytoofby dehydrationby α cyclohexane dehydration-O-4of were abovedehydrationdehydrationdehydration αto -O-4dimers cyclohexaneabove 96% dimersof and96% and andand and Onestartedexceptsubstituted reason 2,2with′-biphenolβ for-5 cyclohexanesstartedstartedexceptacid dimers, thestarted catalyzed 2,2 relativelywithwith[35].′-biphenol with was Theacidacidon transalkylation Brønsted 4-O-5 acidcatalyzedcatalyzedobtained low [35]. catalyzedandhydrogenation The acid α transalkylationtransalkylation-O-4 4-O-5by sites.of transalkylation dimershydrogenationC6–C7 and Ethylcyclohexane, α were-O-4products, rate ofof dimers quantitativeC6–C7C6–C7 ofof 5-5 C6–C7offollowed were products,products, aromaticthedimer lyproducts, quantitativemain convertedby might followed followedtheproductring followedisomerization lyto befollowed converted cyclohexaneforbyby caused the HDOtheby isomerizationisomerization the byofto byof isomerization cyclohexane itsdehydration intramolecular ofof of and substitutedandstartedexcept C6-C10hydrogenation 2,2withhydrogenationhydrogenation′hydrogenation-biphenolhydrogenation cyclohexanesand startedexceptsubstitutedalkanes,acid C6-C10started exceptandcatalyzedand 2,2 with[35]. respectively. ′C6-C10 -biphenolC6-C10reactions. cyclohexanes2,2with alkanes, reactions.Theacidonreactions. reactions.′ reactions.-biphenol transalkylation Brønsted 4-O-5 acidalkanes, catalyzedalkanes, [35]. respectively. TheOne catalyzedand One One The[35]. onOneacidrespectively.formationOne respectively.reasonα transalkylation -O-4Brønsted 4-O-5reason reasonThe sites. reasonreasonof transalkylation dimers The4-O-5 C6–C7andfor Ethylcyclohexane,of for for formationacid fortheTheαC8–C10for andThe -O-4werethe theproducts, therelativelysites.theformationof αformation relativelydimers relatively-O-4quantitative C6–C7 relatively relativelyalkanesrelatively ofofEthylcyclohexane, dimersC6–C7 C8–C10followed were products,oflow theof during lowlowC8–C10 C8–C10 ly werelow quantitativeproducts,lowlow mainhydrogenationalkanes converted hydrogenationbyhydrogenation hydrogenationHDO quantitativehydrogenationhydrogenation followedproductthealkanes alkanes the during followedisomerizationofly to mainα converted during-O-4cyclohexane forbyduring lyHDOrate HDOtheproduct convertedratebydimersrate rate rate rateHDO ofisomerizationofHDOthe ofto ofof5-5α of of -O-4cyclohexaneof isomerization5-5for 5-5of ′ of 5-5to5-5dimer5-5 ′αHDO ′ dimerαdimerscyclohexane-O-4dimer′′′ -O-4′ dimerdimerdimerdimer ofmightdimers dimers mightmight mightmightofmightmight hydrogensubstitutedandβ-5 C6-C10dimers,hydrogenationhydrogenation bonds hydrogenationcyclohexanes substitutedsubstitutedandalkanes,was substitutedC6-C10 thatobtained respectively. reactions.couldcyclohexanes cyclohexanes alkanes,onreactions. reactions.cyclohexanesBrønstedby hinderhydrogenation respectively. TheOne One on onacidformationOne reasonthe BrønstedBrønsted reasonon sites. reason attackBrønsted The for of Ethylcyclohexane,of for acidacid formation aromatictheC8–C10for ofthe acid sites.sites.relatively the relatively sites. alkanesrelatively catalyst. Ethylcyclohexane,ringEthylcyclohexane,of C8–C10 Ethylcyclohexane, followedlowthe during low mainlow Parsellhydrogenationalkanes hydrogenation HDO hydrogenationbyproduct the the etduringdehydration of al. mainmaintheα for-O-4 used HDOmain rate HDOproductproduct ratedimers rate of Zn/Pd/Cproductandof ofof 5-5α offor-O-4for5-5′ 5-5 dimer HDOHDO′ fordimersdimer′ dimerHDOcatalyst ofofmight might ofmight for βstartedsubstitutedand-5 dimers,C6-C10be withbe becausedbehydrogenationbe caused cyclohexanescausedstarted substitutedandβalkanes, acid wascaused-5caused substitutedC6-C10dimers, andstartedbycatalyzedobtainedstarted bywithby its respectively.by C6-C10by its its cyclohexanes intramolecularalkanes, its withacidits wasonwith intramolecularintramolecular reactions. intramoleculartransalkylationbycyclohexanesintramolecularBrønsted alkanes,catalyzedacid obtainedacidhydrogenation respectively. The catalyzed catalyzed on Onerespectively.acidformation transalkylationbyhyBrønsted on hy sites.reasonhydrogenof hydrogenation hy hyBrønsted transalkylationdrogen Thedrogen transalkylationC6–C7drogenof drogen Ethylcyclohexane,Ethylcyclohexane,of acidformation aromaticforThe C8–C10bonds products,bonds bonds acidthe sites.formation ofbondsbonds C6–C7 relativelythat sites.alkanesof ringEthylcyclohexane,of thatthat of thataromatic C8–C10thatC6–C7 followedcould C6–C7 Ethylcyclohexane,products,offollowedcould could thethe during couldcouldC8–C10 low mainhinder mainproducts, alkanesproducts, hinderringbyhinder hinderhydrogenation HDOhinderfollowedby productthealkanesproduct followed th thedehydrationduring thisomerizationeoffollowed thfollowed th attackemain thetheα attackduringe-O-4foreforattackbye attack HDOattackmainattack bytheproductHDOHDO ofdimers by rateby ofdehydrationHDO ofisomerizationtheandof product of the ofof theofthe the α of the catalyst. -O-4thefor theisomerization of catalyst.isomerization5-5catalyst. catalyst. αHDOcatalyst. catalyst.fordimers-O-4′ dimer HDOand Parsell ofdimers Parsell Parsell Parsell ParsellParsellmight ofof of βhydrogenation-5 dimers,bebe causedbe causedβwas -5caused reactions.βdimers, -5byobtained by dimers,its by its intramolecular wasOneits intramolecularby intramolecular wasreason obtainedhydrogenation obtained for by thehy hy drogenhydrogenationrelativelyby hy drogen ofdrogenhydrogenation aromatic bonds lowbonds bonds hydrogenation thatofring that aromatic thatof couldfollowed couldaromatic could hinder ring hinderrate hinderby ring of followedthdehydration 5-5 the followedattackthe′ dimerattacke attackby of might dehydrationof byandthe of thedehydration catalyst.the catalyst. catalyst. and Parsell Parsell and Parsell HDOhydrogenationsubstitutedstartedβ-5 ofdimers, guaiacylglycerol-b-guaiacylwithbe cyclohexanes substituted startedhydrogenationβ acidwascaused-5 reactions. βdimers, startedsubstituted-5catalyzedsubstitutedobtained with dimers,by cyclohexanes itswithOneacidonwas reactions. intramoleculartransalkylationby Brønstedcyclohexanes wasreasoncyclohexanescatalyzedacidobtained hydrogenation obtainedcatalyzed forOne onacid transalkylationthebyBrønsted ether reason on sites. ofon relativelyhydrogenation hyby transalkylation Brønsted C6–C7Brønsted underdrogenof hydrogenationEthylcyclohexane,for acidaromatic the products,low sites. relativelyacid ofbondsrelativelyacid hydrogenation C6–C7 ofsites. Ethylcyclohexane,ringofsites. aromaticthatC6–C7followedof products,Ethylcyclohexane,followed low mildEthylcyclohexane,the aromaticcould hydrogenationmainproducts, ring condition,rateby hinder byfollowedproductthe ringof followed the dehydration 5-5isomerizationfollowed followedthmain′ the dimer eforthebyrate 423 attack mainby productHDOthemain of Kby might dehydrationby5-5isomerization andofproductthe ofproduct ′ dehydration thedimer forisomerization 20.7 HDOcatalyst. for barmightfor and HDO ofHDOof hydrogen andParsell of of hydrogenationβ-5 dimers,etet etal. etet etal.al. hydrogenation β usedal.was-5al. al. usedreactions.used hydrogenationβuseddimers,used used-5 obtainedZn/Pd/C Zn/Pd/Cdimers,Zn/Pd/C Zn/Pd/CZn/Pd/CZn/Pd/C Onewas reactions. by catalystreasonwas obtained catalystreactions.catalyst hydrogenation catalyst catalystcatalystobtained forOne for thebyfor for One reason forHDO forfor relativelyhydrogenation HDObyHDO reason HDOHDO HDOof hydrogenationforof aromaticof of theguaiacyl offor lowofof guaiacylguaiacyl relativelyguaiacyltheguaiacylguaiacyl hydrogenation of relativelyring glycerol-b-guaiacyl aromaticglycerol-b-guaiacylofglycerol-b-guaiacyl glycerol-b-guaiacylglycerol-b-guaiacyllow followedaromatic hydrogenationlow ringrate hydrogenation by ringoffollowed dehydration5-5 followed′ether dimer etherrateether etheretherby rateofunder might underdehydrationunder5-5by and underofunder ′ dehydration dimer5-5 relatively relatively′relatively dimerrelativelyrelatively might and might mild and mildmild mildmild beβsubstitutedhydrogenation-5 caused dimers,et etal. by et al. cyclohexanes βhydrogenationsubstituted usedwasits-5al. reactions. used intramolecular dimers,hydrogenation substitutedβused obtainedβ-5Zn/Pd/C-5 Zn/Pd/C dimers, dimers, Zn/Pd/C cyclohexanes Onewason reactions. by Brønstedcyclohexanes catalystreason obtainedwas hywasreactions.catalysthydrogenation drogencatalyst obtained obtained forOne on acidfor bythe for Brønstedbonds One reasonon HDOsites.for hydrogenation relativelyHDO by Brønsted reasonby HDOofthat Ethylcyclohexane, forhydrogenation ofhydrogenationacid aromatic of couldthe guaiacyl for lowof guaiacylsites.acid relativelytheguaiacyl hinder hydrogenation of sites.relativelyringEthylcyclohexane, glycerol-b-guaiacylaromatic glycerol-b-guaiacyl of th Ethylcyclohexane,offollowed lowglycerol-b-guaiacyl thee aromatic attackaromatic lowhydrogenationmain ringrate hydrogenation of byproduct oftheringfollowed ringthe dehydration5-5 catalyst. mainfollowed′ ether thefollowed dimer forrateether by ethermainHDOproduct rateofParsellunder might dehydration under5-5 andby product byofunderof′ dimer 5-5dehydrationforrelativelydehydration ′relatively dimer HDO relativelyfor might and HDO ofmight mild andmild andof mild pressure,substitutedhydrogenationcondition,condition,condition, gotetcondition,condition, cyclohexanes behydrogenationsubstitutedal. complete causedreactions. hydrogenationused 423 423423 423 byK423Zn/Pd/C cyclohexanes K KandOneitson conversion K andKreactions.and intramolecularBrønsted and andreason20.7 reactions. 20.7 20.7 catalyst 20.7 20.7bar barforOne onbar acid barhydrogenbar andthe Brønsted hydrogenOnereasonhydrogen for sites.hy hydrogen hydrogenrelativelyyielded drogenreasonHDO Ethylcyclohexane,for acidpressure, thepressure, bondsofpressure, for low pressure,sites.two pressure, relativelyguaiacylthe hydrogenation that mainrelativelyEthylcyclohexane,got got gotcould gotglycerol-b-guaiacylcompletelowgot the aromaticcompletecomplete complete hinderhydrogenationlowcompletemain rate hydrogenation conversion product th of conversion productsconversionthee conversion5-5 attackconversion main′ dimer forrate etherof productHDO and rateofthe [andmight36and 5-5 and catalyst.underand yieldedof].of ′ yielded dimeryielded The5-5 for yieldedyielded′ HDOdimerrelatively βParsell mighttwo-O-4 twotwo oftwo mighttwomain main linkagemain mainmildmain beβbeet-5 causedal.caused dimers, usedcondition, bycondition,bycondition, Zn/Pd/C beβ itswasits-5 caused intramolecularintramolecularbedimers, obtained caused 423catalyst 423 423by K was its K byK and by intramolecular and forandits obtainedhyhy hydrogenation 20.7intramolecularHDO drogen drogen20.720.7 bar bar ofbar by bonds bondshydrogenguaiacyl hyhydrogenhydrogenhydrogenationdrogen thathyofthat drogenglycerol-b-guaiacylaromatic couldcould pressure,bonds pressure,pressure, bonds hinderhinder ofthatring aromatic gotthat could th thgotgotfollowedee complete couldattack attack ethercomplete completehinder ring hinderofofunderby ththethe followed conversione dehydration conversion conversion attackcatalyst. catalyst.threlativelye attack ofby ParsellParsellthe and ofdehydration mild and and catalyst.the yielded yieldedcatalyst.yielded Parsell andtwo Parselltwotwo main mainmain ethydrogenationβbe-5 al.caused dimers, usedcondition, condition,by condition,Zn/Pd/C hydrogenationbeβet was-5its al.reactions.caused intramolecularbedimers, βhydrogenation used423hydrogenationobtained-5 caused catalyst dimers,423by423K Zn/Pd/C Onewasandits Kby Kreactions. byintramolecularand for andreasonits20.7wasobtained hy hydrogenation reactions. HDOintramolecularcatalyst reactions. drogen20.7 20.7barobtained forOne ofbar barhydrogenbythe forbonds guaiacylreasonOne hy Onehydrogen hydrogenationrelativelyhydrogenHDObydrogen reason hyof thatreason forhydrogenation glycerol-b-guaiacyldrogen aromaticpressure,of couldthe bonds lowguaiacylfor pressure,for pressure,relatively thebonds hydrogenation hinderthe ofthatring relatively gotglycerol-b-guaiacylrelativelyaromatic thatcould of gotthlowgotfollowedcomplete earomatic could attackether completehydrogenationhindercomplete low low ringrate hinderunderofhydrogenation by hydrogenation conversion th ofringfollowedthe e dehydration 5-5conversion attack conversionethercatalyst.th relativelyfollowed′ edimer rateattack ofunderby and of the Parsell rate might rate dehydrationofmild 5-5 andby catalyst.and relativelyyieldedtheof′ ofdimer dehydration 5-5 yieldedcatalyst.5-5yielded′ ′ dimer Parsell dimermighttwo mildand Parsell two twomightmain mightand mainmain wasetcondition, al. effectively usedaromaticaromaticaromatic aromatic aromatic423aromaticZn/Pd/Cet al.K cleaved etproducts and used productsproductsal. catalystproducts productsproducts 20.7used Zn/Pd/C to bar [36]. Zn/Pd/Caromatic for[36]. [36]. hydrogen [36].[36]. [36]. HDOThecatalyst TheThe The ThecatalystThe βof fragments,-O-4 β βpressure, -O-4for guaiacylβ-O-4β-O-4-O-4-O-4 HDOlinkagefor linkagelinkage linkagelinkageHDO linkageglycerol-b-guaiacylgot of and wasguaiacylcomplete ofwaswas was its wasguaiacylwaseffectively effectively alcoholeffectively glycerol-b-guaiacyl effectivelyeffectivelyeffectivelyconversionglycerol-b-guaiacyl ether oxygencleaved cleavedcleaved cleavedandundercleavedcleaved yielded onto ether relativelyto to aromaticalkyl to toto aromaticaromaticether aromatictwounderaromaticaromatic chains undermildmain fragments,relatively fragments,fragments, fragments, fragments, wasfragments,relatively subsequentlymild and andandmild and andandits its its itsits its condition,behydrogenationet al.caused usedaromaticaromatic by423 aromaticZn/Pd/Cbehydrogenationcondition,et its Kcausedal.reactions. intramolecularetproductsandhydrogenation be usedbe productsal. caused catalystproductscaused20.7 byused 423Zn/Pd/C One itsbar K reactions.[36].by Zn/Pd/C intramolecular byfor and[36].reason hydrogen its hy its[36]. reactions.HDO catalystTheintramoleculardrogen20.7 intramolecularThe for OnecatalystThe βbarof -O-4 βpressure, the bonds for -O-4guaiacylreasonOnehydrogenβ hy -O-4 relatively HDOlinkagefordrogen reason linkagethat hy forHDOhylinkagegotglycerol-b-guaiacyl drogenof drogencouldpressure,the bonds completelowwasguaiacylfor of wasrelatively the hinderwashydrogenationguaiacylbonds effectivelybondsthat relativelyeffectively gotglycerol-b-guaiacyl conversioneffectively could that th completethatlowglycerol-b-guaiacyle attackcouldether hinder hydrogenationcould lowcleaved ratecleaved and underofhydrogenationhindercleavedconversion hinder th ofthe yieldede 5-5to attack catalyst.ether relatively toth ′′aromaticth dimeredimer to rateearomatic ether attack andtwoattackofunderaromatic oftheParsell rate mightmight yieldedmain under mild5-5of catalyst.offragments, relatively of the′′ fragments, thedimerdimer 5-5 fragments, relatively catalyst. twocatalyst.′ dimerParsell mightmight mainmild and Parsell mightandParsellmild andits its its condition,et al. usedalcohol aromatic423 Zn/Pd/Ccondition,et Kal.oxygen condition,etand used al. productscatalyst20.7 used423 Zn/Pd/C onbar K423 Zn/Pd/C andforalkylhydrogen [36].K catalystHDOand20.7 chainsThe catalyst 20.7bar of pressure, for β guaiacylhydrogenbar-O-4 HDOwasfor hydrogen linkage HDOgotglycerol-b-guaiacyl subsequentof pressure, completeguaiacyl of pressure, wasguaiacyl gotglycerol-b-guaiacyl effectivelyconversionly completegotglycerol-b-guaiacylremoved ethercomplete and cleavedconversionunder withyielded conversion etherrelatively toloss ether andtwoaromaticunder of andyielded mainundermild relativelyaromatic yielded fragments, relativelytwo two mainmildgroups. mainmild and its removedbecondition,aromatic causedalcohol alcohol withproducts alcoholbyalcohol423alcoholcondition,be its Kcaused lossintramolecular condition,andbeetoxygen oxygen [36]. oxygen oxygenal.causedoxygen 20.7 of by423 usedThe aromatic its bar Kon423 byon intramolecular βZn/Pd/Conand onon -O-4hydrogen itshyKalkyl alkyl alkylintramolecularanddrogen20.7alkylalkyl linkage groups. 20.7catalystbarchains chains pressure,chains chainschainsbonds hydrogenbar washy drogenforhydrogen was waseffectivelythat Strassbergerhywas waswasgotHDOdrogen couldpressure,subsequent bondssubsequent complete subsequent subsequentsubsequent ofpressure, hinderbondscleavedguaiacyl that got conversion etcould thatly th ly completegottoly lyglycerol-b-guaiacyle al.ly ly removedattack couldremoved aromatic completehinderremovedremovedremovedremoved obtained andofhinderconversion th the fragments, yieldedeconversionwith with attackcatalyst. withwiththwithwith phenole attack lossether and losstwo of losslossloss loss andParsellthe and yielded main ofofunder of and catalyst.of ofitsofof theyieldedaromatic aromatic aromatic aromatic aromatic catalyst.aromatic ethylbenzenerelativelytwo Parsell twomain groups.Parsell groups. main groups.groups.mildgroups.groups. as aromaticetbecondition, al.caused usedalcohol alcoholproducts alcoholby 423alcoholZn/Pd/Cetcondition,bearomatic its al.Kcaused oxygen intramolecularcondition, andbe usedoxygenet [36].oxygen oxygencausedcatalyst al.20.7products by 423 Zn/Pd/C Theused onits bar K 423 onby intramolecular β onforand -O-4onZn/Pd/Calkyl hydrogen its [36].Khy alkyl catalystHDO alkylintramolecularand20.7drogen alkyllinkage Thechains 20.7bar chainsofcatalyst pressure,chains βforchainsbonds guaiacyl -O-4hydrogenbar washy HDOwas drogen hydrogenlinkage forwaseffectively thathywas wasglycerol-b-guaiacyl gotdrogenofHDOsubsequent couldpressure, bondssubsequentguaiacyl complete subsequentwassubsequent pressure,of cleavedhinderbonds effectivelythatguaiacyl glycerol-b-guaiacylgot lyconversion could that th lycompletegottolyremovede lyglycerol-b-guaiacyl attack aromaticremovedcouldether completehindercleavedremovedremoved andunderconversionhinderof th thewith fragments,to yieldedeconversion with attack etheraromaticcatalyst. withrelativelythwith eloss attack and lossundertwoof etherloss loss and theParsell offragments, andyieldedmain mildofof undercatalyst.relativelyitsofaromatic oftheyielded aromatic aromatic catalyst.aromatictwo relatively andParsell twomain mildgroups. its Parsell groups.main groups. groups.mild aromaticetalcohol al. usedStrassberger Strassberger oxygenStrassbergerproductsStrassbergeralcoholStrassberger Zn/Pd/Caromaticet al. aromaticon used[36]. oxygen catalystalkyl productset etThe Zn/Pd/C et etproductsal. et chains al. βal. foron-O-4al. al.obtained [36]. obtained HDOcatalystobtainedalkyl linkageobtainedobtained was[36].The of The subsequentchainsβ for -O-4guaiacyl -O-4phenolwas phenol phenolβHDO -O-4phenol phenollinkagelinkageeffectively was glycerol-b-guaiacyl oflinkagely and and guaiacyl andremovedwassubsequentwasand and cleaved ethylbenzene waseffectivelyeffectively ethylbenzene ethylbenzeneglycerol-b-guaiacylethylbenzeneethylbenzene effectivelywith toly aromatic ether cleavedcleavedremovedloss cleavedas underof as as fragments,totoas asmainaromatic mainaromatic aromaticethermain withrelativelytomainmain aromatic products under loss products groups.andproducts fragments,fragments,products productsmild its relativelyoffragments, aromatic in in inandandin inHDO mild HDO HDOanditsitsHDO HDO groups. its of of ofof of mainalcoholcondition,aromaticet al. products usedStrassberger oxygen Strassbergerproducts 423 StrassbergerZn/Pd/Ccondition,aromaticetalcohol Kal. in aromaticandon etcondition, usedcondition,HDO[36]. al. oxygencatalystalkyl20.7products et 423 usedZn/Pd/C Theet productsbar ofal.etK chains 423 al.β Zn/Pd/C423andfor 2-phenoxy-1-phenylethanoneon-O-4hydrogen al. obtained[36]. K catalystHDOK obtainedalkyl 20.7 and linkagewas obtainedand[36].The catalyst bar20.7 of chains 20.7pressure, subsequentTheβ for -O-4guaiacyl hydrogen phenolwasbar bar phenolHDOβ -O-4for phenollinkagewashydrogen effectivelyhydrogen gotHDOglycerol-b-guaiacyl of lylinkage pressure, andsubsequent completeguaiacyl andremovedwas ofand pressure, cleaved pressure,ethylbenzene guaiacylwaseffectivelyethylbenzene gotglycerol-b-guaiacyl ethylbenzene conversionlyeffectivelywith complete and toremovedgotglycerol-b-guaiacyl got aromatic ether cleavedlosscomplete itscomplete andcleaved alcoholasconversionunder ofwith as tofragments,yieldedaromaticas main conversion aromatic etherconversion main relativelylosstomain analog aromatic etherandtwo productsunder of groups. productsand fragments, yielded aromatic mainproductsand mildunderand on relatively itsfragments, yielded supportedyielded twoinrelatively ingroups.and inHDOmain mildtwo two HDOanditsHDO main mildmain copperits of of of alcoholStrassberger oxygenStrassberger alcoholet alcoholonal. oxygenalkylobtained oxygen etchains onal. phenolalkyl onobtainedwas alkyl chains subsequentand chains phenolwasethylbenzene lywassubsequent removed andsubsequent asethylbenzene ly with mainremovedly loss removedproducts of with asaromatic with lossmainin loss HDOof groups. productsaromaticof ofaromatic groups.in groups.HDO of Strassbergeraromaticcondition,alcohol2-phenoxy-1-phenylethanone 2-phenoxy-1-phenylethanone oxygen2-phenoxy-1-phenylethanoneproducts 2-phenoxy-1-phenylethanone2-phenoxy-1-phenylethanone4232-phenoxy-1-phenylethanone aromaticcondition,Strassbergeralcohol etK alcoholandoncondition,aromatical. aromatic[36]. oxygenalkylproducts20.7 obtained423 The oxygen baretproducts K chains products423 βand on-O-4al. hydrogen [36]. K phenol alkyl on20.7 andobtainedlinkagewas The [36].[36]. alkyl barand20.7 chains pressure,subsequent andβandTheand -O-4 hydrogen Theand was andanditsbarchains phenol itsβits alcohol βlinkage -O-4ethylbenzeneits hydrogenwaseffectivelyits its-O-4 alcoholalcohol got alcohol alcoholalcohol lywas linkage pressure,subsequent linkage andcomplete removedwasanalog analogsubsequentanalogpressure, cleaved analog analogethylbenzeneeffectivelyanalog was was asgot on conversionly oneffectivelywith on effectivelycompletemain supported toon removedgotonon ly supported supported aromatic supported cleavedsupportedsupportedloss completeremoved products as andconversion cleaved of cleavedwith coppermain tofragments, yieldedaromatic copper copperconversion aromaticwith copper coppercopper lossinto toproducts catalysts. aromatic andloss twoHDOaromatic catalysts.ofcatalysts. groups. andcatalysts. catalysts. fragments,catalysts. yieldedaromaticmainandof itsof inaromaticfragments, yieldedThe fragments, The Thetwo HDO The The Themechanismgroups.and mechanism mechanismmain two mechanism mechanismgroups.mechanism itsofand and main its its catalysts.Strassbergeralcohol2-phenoxy-1-phenylethanone 2-phenoxy-1-phenylethanoneoxygen The2-phenoxy-1-phenylethanone Strassbergeralcoholet mechanism Strassbergeralcoholonal. oxygenalkylobtained oxygen etchains foronal. et phenol HDOalkylon obtainedal.was alkyl obtainedand chains of subsequentand alcoholanditschainsphenol its ethylbenzenealcohol wasits alcoholphenol alcohol lywas dimersubsequent and removedanalog subsequent analogand ethylbenzeneanalog was as only ethylbenzene withon proposedmain supported removedonly supported supportedlossremoved productsas of with as maincopperasaromatic copper thewith maincopperloss inproducts hydrogenolysis catalysts. HDOloss ofcatalysts.productsgroups. catalysts. aromatic of ofinaromatic The TheHDO in Thegroups.mechanism mechanismHDO of groups.mechanismof C-O of(aryl) alcoholaromaticStrassberger2-phenoxy-1-phenylethanonefor foroxygen for products2-phenoxy-1-phenylethanoneforHDO HDOStrassbergeraromaticHDO etHDO Strassbergeronofaromaticalcoholal. [36].of of alcohol alkyl ofproducts obtainedalcoholalcohol The alcohol oxygenetproductschains βdimerand-O-4al. et dimer[36].dimer phenol dimer its al.obtainedonlinkagewas [36].Thealcohol was alkylwasobtained was subsequentβwas Theand -O-4proposedand was proposedanalogproposedchainsphenol βproposed -O-4 linkageethylbenzeneitseffectivelyphenol alcohol on ly linkage was as and supportedremoved wasas as theas subsequentand cleavedtheanalog the ethylbenzene effectively wasthehy as hy hyethylbenzenedrogenolysis effectivelycopperwithhydrogenolysis maindrogenolysistoondrogenolysis ly aromatic supported cleavedloss removedcatalysts. productsas cleavedof mainasoffragments,toaromatic of of Thearomatic copperC-Owithmainof inC-OtoC-O products mechanismC-Oaromatic (aryl) HDO loss (aryl)groups. products(aryl)andcatalysts. fragments,(aryl) of bonditsof infragments,bondaromatic bond bond HDOinTheoccurred and occurredoccurred HDO occurred mechanism groups.itsofand oftoits to to to 2-phenoxy-1-phenylethanonearomaticStrassbergerforfor products forforHDOfor alcohol Strassbergeraromatic2-phenoxy-1-phenylethanoneHDO etHDOHDOHDO Strassbergeraromaticofal. alcohol [36].of oxygenalcohol ofproductsof ofobtainedalcohol Thealcoholalcoholalcohol etproductsoxygen βonanddimeral.-O-4 et [36]. dimer phenolalkyldimeritsdimerdimer obtainedal.linkage on The[36].alcohol was obtainedwasalkylchains wasβ wasThe wasand-O-4 proposed was analog proposedphenol chainsitsβproposed proposedproposed linkagewas-O-4 ethylbenzeneeffectively alcoholphenol on linkagesubsequent was andassupported was as analog the asas andas cleavedsubsequent theethylbenzene effectively was thethethehy as onlyhyethylbenzene drogenolysis coppereffectivelyhy hyhy maindrogenolysissupportedremovedtodrogenolysisdrogenolysis aromaticly cleaved catalysts. removedproductsas cleaved withcopper mainastofragments,of of Thearomatic C-Omainofloss ofwith intocatalysts.C-O products mechanism C-OC-O aromatic (aryl)HDOof loss products(aryl) and fragments,(aryl)aromatic(aryl) The ofbonditsof infragments, bond aromatic mechanismbond bondHDO inoccurred groups.and occurred HDO occurred occurred itsofandgroups. ofitsto to toto bond2-phenoxy-1-phenylethanonealcoholfor HDO occurred oxygen forof 2-phenoxy-1-phenylethanonealcoholalcohol HDO to2-phenoxy-1-phenylethanoneon phenol oxygendimeralkylof alcohol chainswas and on proposed itsdimeralkyl 1-phenylethanol,was alcohol chainssubsequentwas and as analog itstheand proposedwas alcohol hyits on lydrogenolysis subsequentalcohol supportedremoved analog followedas analogthe on ly copper withhyof supported removed ondrogenolysisC-O by supportedloss catalysts. hydrodeoxygenation(aryl) of copperwith aromaticbond Thecopper ofloss catalysts. mechanismoccurredC-O ofcatalysts.groups. (aryl)aromatic The to mechanismThebond to groups.mechanism ethylbenzene. occurred to forStrassberger2-phenoxy-1-phenylethanonealcohol HDOphenol phenoloxygenphenol ofphenolphenol Strassberger2-phenoxy-1-phenylethanonealcoholforalcohol etand HDO and2-phenoxy-1-phenylethanoneandonalcoholStrassberger al. andStrassberger and1-phenylethanol, oxygendimer1-phenylethanol,alkyl 1-phenylethanol,obtainedof 1-phenylethanol,1-phenylethanol, alcoholoxygenet waschains andon al. et etproposedphenol dimeralkylits obtainedonal.was al.alcohol followed alkyl obtainedchainswas followedobtainedfollowedsubsequent and followed followedas analog chainsproposedphenol theits andethylbenzenewas by alcohol hyphenolits by by phenol on hydrodeoly drogenolysisbywas subsequentby alcoholandhydrodeo hydrodeosupported asremovedhydrodeo hydrodeoanalog subsequent theandethylbenzeneand analog hyasxygenation only xygenation drogenolysis ethylbenzenexygenationcopperwithof ethylbenzenexygenationmainsupportedxygenationremoved onC-Oly supportedloss removed catalysts.products(aryl)as to of to copperwithto ethylbenzene.ofmain to tobond asethylbenzene.aromaticethylbenzene.asC-O copperethylbenzene. Theethylbenzene.with loss main incatalysts. mainproducts occurred(aryl) mechanism HDO lossof catalysts. groups. products productsaromaticbond TheofThe toofThe Thein aromaticThe Theoccurred alcoholmechanismThe HDOalcohol alcohol in groups.alcoholinalcoholmechanism HDO HDO dimersgroups.ofto dimers dimers dimers dimers ofof TheforStrassbergerphenol alcoholHDOphenol andphenol ofphenol 1-phenylethanol, dimersforforStrassbergeralcohol etand HDOHDO forandal. and 1-phenylethanol, HDOdimerwere 1-phenylethanol,ofobtainedof 1-phenylethanol, alcoholalcohol etofwas also followedalcoholal. proposed phenoldimerdimer catalyzedobtained dimer byfollowed was wasfollowed hydrodeo and asfollowed was proposedproposedphenolthe byethylbenzene byproposedhy alumina xygenationby drogenolysis hydrodeo by and hydrodeoasas hydrodeo thethe asethylbenzene to hyhytotheasxygenation form drogenolysis ethylbenzene.xygenation ofhymainxygenation C-Odrogenolysis oligomers (aryl)productsas to to ofethylbenzene. mainThe bond to ethylbenzene.C-O of ethylbenzene. asalcohol in C-Ooccurred(aryl)products by-productsHDO (aryl) dimers bond The to ofThebondin occurredThe alcohol HDOalcohol [ occurred37alcohol]. dimerstoof dimers dimersto phenol2-phenoxy-1-phenylethanoneforStrassberger HDOwere andwerewere ofphenolwere 1-phenylethanol, 2-phenoxy-1-phenylethanonealsoforStrassbergerphenolalcohol also etalso HDOalso forcatalyzedStrassberger2-phenoxy-1-phenylethanone al.and 2-phenoxy-1-phenylethanonecatalyzed catalyzedand HDOdimercatalyzed ofobtained 1-phenylethanol, 1-phenylethanol, alcohol etof wasby followed andalcoholby al.by aluminaet by proposedphenol dimeraluminaitsaluminaobtained al.alumina alcohol dimerby obtained was tohydrodeofollowedand and followed asto toform analog wasproposed tophenol form itstheform and ethylbenzeneformand alcohol proposedoligomershy phenolby xygenation itsonoligomers oligomersitsdrogenolysisby hydrodeooligomersandalcohol supportedalcoholas hydrodeoanalog the andasethylbenzene as analoghy totheasanalog xygenation onasby-products drogenolysisethylbenzene.ethylbenzene ascopper by-productsof by-productsxygenationhysupportedmain by-products C-Oondrogenolysis on supported catalysts. supportedproducts(aryl) toas ethylbenzene. copper[37]. of Themain to[37]. as bond[37].C-O [37]. ethylbenzene.The ofcopper alcoholcopper main incatalysts. C-O products(aryl)occurredmechanism HDO catalysts. (aryl) dimersproductscatalysts.The bond The alcoholto ofinbond The occurredmechanism The TheHDOin alcohol occurreddimers mechanism mechanismHDO toof dimers toof phenol wereandwerewerewere 1-phenylethanol, phenolalso also alsoalsophenol catalyzed catalyzedand catalyzedcatalyzed 1-phenylethanol,and 1-phenylethanol,byfollowed by alumina byby alumina aluminaalumina by hydrodeofollowedto to form totofollowed form formform oligomersbyxygenation oligomers hydrodeo oligomersoligomers by hydrodeo asto xygenationas by-products ethylbenzene. asas by-products xygenation by-productsby-products to ethylbenzene. [37]. Theto [37]. ethylbenzene. [37].[37]. alcohol dimers The alcoholThe alcohol dimers dimers werefor2-phenoxy-1-phenylethanonephenol HDO also and catalyzedofwere 1-phenylethanol, forphenol2-phenoxy-1-phenylethanonealcohol HDOalsophenol2-phenoxy-1-phenylethanoneforfor byand dimercatalyzed HDO ofHDOalumina and1-phenylethanol,alcohol of was 1-phenylethanol,of followed andalcohol toalcohol by proposeddimer formits alumina alcohol bydimer oligomersdimerwas hydrodeofollowedand as analogproposed towas followedtheits wasand form alcohol ashyproposed byxygenation proposedits onby-productsdrogenolysis hydrodeooligomers alcoholsupportedbyas analog hydrodeothe as as analoghyto xygenation the on the[37].drogenolysis ethylbenzene. ascopperof supported hyxygenation by-products hy C-Oondrogenolysisdrogenolysis supported catalysts. (aryl) to ethylbenzene.copperof toThe bond C-O ethylbenzene. [37]. The copperalcoholof of catalysts. (aryl)occurredC-O mechanismC-O catalysts. dimers (aryl)Thebond(aryl) The alcoholTheto bond occurred bondmechanism Thealcohol dimers occurred mechanismoccurred dimersto to to 2-phenoxy-1-phenylethanonephenol and 1-phenylethanol,werephenol2-phenoxy-1-phenylethanonephenol also and catalyzed 1-phenylethanol,and 1-phenylethanol, followedand by its alumina alcohol by followedhydrodeoand to analog followedformits alcohol by xygenation oligomerson hydrodeo supportedby analog hydrodeo as toxygenation onby-products ethylbenzene.copper supportedxygenation catalysts. to [37]. ethylbenzene.copper toThe ethylbenzene. The alcohol catalysts. mechanism dimersThe The alcoholThe mechanism alcohol dimers dimers wereforwere HDO alsoalso catalyzedcatalyzedof wereforalcohol HDOwere foralso byby dimerHDO also catalyzedaluminaofalumina alcohol catalyzed ofwas alcohol toto by proposed dimerformform alumina by dimer oligomersoligomers aluminawas as to proposedwas theform to asas hyproposedform oligomersby-productsby-productsdrogenolysis asoligomers the as as hythe [37].by-products[37].drogenolysis ofas hy by-products C-Odrogenolysis (aryl) [37]. of bond [37]. C-O of occurred(aryl)C-O (aryl) bond to bond occurred occurred to to phenolforwere HDO also and catalyzedof 1-phenylethanol, phenolwereforalcohol HDOwere forphenolalsoTablephenol andby dimerHDO catalyzedalso ofalumina 1-phenylethanol, andTablealcohol 3.and catalyzedTable ofHydrodeoxygenationwas followedTable1-phenylethanol, 1-phenylethanol,alcohol 3. to by proposed dimer3. Hydrodeoxygenationform alumina3.Hydrodeoxygenation by Hydrodeoxygenationbydimer oligomersaluminawas followedhydrodeo asto proposed was theformfollowed followed to as hyproposedbyformxygenation oligomers by-products ofdrogenolysis hydrodeo Phenolicofasbyoligomers byof Phenolic the hydrodeo of Phenolichydrodeoas asPhenolichyto xygenationthe by-products[37].drogenolysisethylbenzene. Dimers ofas Dimershy xygenationDimersby-products C-Oxygenationdrogenolysis Dimers over(aryl)overto over[37].ethylbenzene. of over The Ru/HZSM-5 to bond to C-O[37].Ru/HZSM-5 ethylbenzene. alcoholethylbenzene.Ru/HZSM-5of (aryl)occurredC-O dimersThe (aryl)[35].bond [35]. [ 35 alcohol[35].to The Thebond]. occurred alcohol alcohol dimersoccurred todimers dimers to Table 3. HydrodeoxygenationTableTableTable 3. 3. 3.Hydrodeoxygenation Hydrodeoxygenation HydrodeoxygenationHydrodeoxygenation of Phenolic Dimers of of of ofPhenolic overPhenolic PhenolicPhenolic Ru/HZSM-5 Dimers Dimers DimersDimers over[35]. over overover Ru/HZSM-5 Ru/HZSM-5 Ru/HZSM-5Ru/HZSM-5 [35]. [35]. [35].[35]. phenol and 1-phenylethanol,phenolTablephenolwere and 3.also 1-phenylethanol,HydrodeoxygenationandTable catalyzedTable followed1-phenylethanol,Table 3. 3.Hydrodeoxygenation Hydrodeoxygenation3. by byHydrodeoxygenation alumina hydrodeofollowed of Phenolicfollowed to by xygenationform hydrodeo Dimers ofby oligomers ofPhenolic hydrodeo ofPhenolic over Phenolictoxygenation ethylbenzene. Ru/HZSM-5 asDimersxygenation Dimersby-products Dimers overto over[35].ethylbenzene. Theover Ru/HZSM-5to Ru/HZSM-5[37].ethylbenzene. alcohol Ru/HZSM-5 dimersThe [35]. [35]. alcohol The[35]. alcohol dimers dimers werephenol also and catalyzed 1-phenylethanol,werephenol phenolalsowere byand catalyzed alumina also1-phenylethanol, and catalyzed Tablefollowed1-phenylethanol,Table toby form alumina3.3. Hydrodeoxygenation HydrodeoxygenationHydrodeoxygenation by oligomers aluminafollowedhydrodeo to formfollowed asto by xygenationoligomers by-productsform hydrodeo by ofof oligomers Phenolic Phenolichydrodeoof asPhenolic toxygenation by-products [37].ethylbenzene. DimersDimers asxygenation by-productsDimers overover to [37].ethylbenzene. Ru/HZSM-5Ru/HZSM-5over The to ethylbenzene. [37].Ru/HZSM-5alcohol [35].[35]. dimersThe alcohol[35]. The alcohol dimers dimers Table 3. HydrodeoxygenationConversioTableTable 3. Hydrodeoxygenation 3. Hydrodeoxygenation of Phenolic Dimers of Phenolic ofover Phenolic Ru/HZSM-5 Dimers Dimers over [35]. overRu/HZSM-5 Ru/HZSM-5 [35]. [35]. were also catalyzedwereTable werealsoConversio by catalyzed3.aluminaalso HydrodeoxygenationHydrodeoxygenation catalyzedConversioConversioTableConversioConversio to by form 3.alumina Hydrodeoxygenationby oligomers alumina toofof formPhenolicPhenolic to as form oligomersby-products DimersDimers ofoligomers Phenolic overoveras by-products[37]. Ru/HZSM-5Ru/HZSM-5 Dimersas by-products over [37].[35].[35]. Ru/HZSM-5 [37]. [35]. wereSubstratesSubstrates also catalyzedSubstratesSubstrates wereSubstratesSubstratesSubstratesSubstrates werealsoConversio Conversionby catalyzed aluminaalso Conversio catalyzedConversioConversioConversio to (%)by Tableform alumina by oligomers3. alumina Hydrodeoxygenation to form to as formoligomers by-products oligomersSelectivity of as Phenolic by-products[37]. (%)as by-productsSelectivity Dimers SelectivitySelectivitySelectivitySelectivitySelectivitySelectivity [37]. over (%) (%)[37]. (%)Ru/HZSM-5 (%) (%)(%) [35]. SubstratesSubstrates SubstratesSubstratesSubstratesConversio n (%) n (%)Conversionn (%) n(%)nn (%) (%)Conversio(%) Selectivity (%) SelectivitySelectivitySelectivitySelectivity (%) (%) (%) (%) Substrates SubstratesSubstratesTableConversionSubstrates (%) 3. Hydrodeoxygenation nTable (%)Conversion n(%)n (%) TableConversio(%)3.Table Hydrodeoxygenation 3. 3. Hydrodeoxygenation Hydrodeoxygenation of Phenolic Dimers ofSelectivity Phenolic overof of Phenolic Phenolic Ru/HZSM-5(%) Dimers Selectivity Dimers Dimers overSelectivity [35]. Ru/HZSM-5 (%)over over (%)Ru/HZSM-5 (%)Ru/HZSM-5 [35]. [35]. [35]. Substrates SubstratesConversionSubstrates (%) Conversionn (%)(%)Conversio n (%) Selectivity (%) SelectivitySelectivity (%) (%) Substrates SubstratesTablenSubstrates (%) 3. Hydrodeoxygenation Tablen (%)Table 3. nHydrodeoxygenation (%) 3. Hydrodeoxygenation of Phenolic Dimers ofSelectivity Phenolic overof Phenolic Ru/HZSM-5 (%)Dimers Selectivity Dimers overSelectivity [35]. Ru/HZSM-5(%)over (%)Ru/HZSM-5 [35]. [35]. TableConversion (%) 3. HydrodeoxygenationTableConversion (%)Table 3.ConversioConversio nHydrodeoxygenation (%) 3. Hydrodeoxygenation of Phenolic Dimers of Phenolic overof Phenolic Ru/HZSM-5 Dimers Dimers over [35]. Ru/HZSM-5 over Ru/HZSM-5 [35]. [35]. Substrates SubstratesSubstratesSubstrates Selectivity (%) Selectivity SelectivitySelectivity (%) (%) (%) O O 99.8 99.899.8 OO ConversioOOOO n (%) Conversio99.899.8n99.8 99.8(%)99.8 nn (%) (%) O O99.8 99.8 Conversio Substrates O O ConversioO 99.8Conversio99.8 99.8 Conversio Selectivity (%) O SubstratesO99.8Substrates O 99.8 99.8 Selectivity Selectivity (%) (%) Substrates(4-O-5) SubstratesO nSubstrates (%) 96.8 99.8 99.8 0.3 Selectivity (%) SelectivitySelectivity (%) (%) (4-O-5)(4-O-5)O (4-O-5)(4-O-5)(4-O-5) (4-O-5)(4-O-5)(4-O-5)(4-O-5) O 99.8 O 96.8 n (%) n (%)96.80.3 96.8 96.896.8 96.896.896.896.8 0.30.3 0.30.30.3 0.3 0.3 O (4-O-5)On99.8 (%) n99.8 (%) 99.8n (%) 96.8 0.3 (4-O-5) (4-O-5)(4-O-5) (4-O-5)(4-O-5) (4-O-5) O 96.8 96.80.396.896.8 96.8 96.8 0.30.3 0.3 0.3 0.3 (4-O-5) (4-O-5)(4-O-5)(4-O-5) 96.8 0.396.8 96.8 96.8 0.3 0.30.3 O O 99.8 O O 99.8 99.899.8 OH (4-O-5) (4-O-5)(4-O-5) 96.8 0.396.8 96.8 0.3 OH0.3 OH OHOHOHOHOHOHOH HOO OH 99.8 OH OHOH OH OHOH O O 99.8 O 99.8 99.8 HO(4-O-5)HOO HOHOHO HOHO OHO O(4-O-5)O OO O (4-O-5) OH(4-O-5) OH OH OH OH 96.8 0.396.8 96.896.8 0.3 OH0.30.3 OH O HOHOOOO 99.899.8 O 99.8 OH OH 99.899.899.8 99.899.8 99.8 OH OH HOO HO OH HOO O OH OH 99.899.899.8 OH OH HOHOHOO O O 99.8 OH OH OH 99.8 99.8 OH HO(4-O-5)(4-O-5)O HO OH HOO O OH 99.896.897.4 OH99.8 99.8 0.30.1 HO (4-O-5)O 99.8(4-O-5) OH 99.8 99.8 96.8 96.8 0.3 0.3 HO(4-O-5)O OH 97.4 0.1 (4-O-5)(4-O-5) (4-O-5)HO(4-O-5)(4-O-5) HO(4-O-5) O (4-O-5) O OH 96.8 OH 99.8 97.40.397.4 97.496.897.4 97.4 96.8 0.10.30.10.1 0.1 0.1 0.3 (4-O-5) (4-O-5)(4-O-5)(4-O-5)(4-O-5)(4-O-5) 99.8(4-O-5) 97.499.8 99.8 0.197.4 97.497.497.4 OH 97.4 0.1 0.1 0.10.10.1 0.1OH OHOH (4-O-5) (4-O-5)(4-O-5)(4-O-5)(4-O-5) (4-O-5) 97.4 97.40.197.497.4 97.4 97.4 0.10.1 0.1 0.1 0.1 HOO HO OH HOHOO O O OH OH OH (4-O-5) (4-O-5) (4-O-5)99.8(4-O-5) 97.499.8 99.899.8 0.1 97.4 97.4 OH 97.4 0.1 0.10.1OH OH O O O OH OH OH O OO OOOOO O HOO HO OH HOO O OH OH (4-O-5)O O O(4-O-5)HOOO 99.8(4-O-5) (4-O-5) O O 97.4 OH99.8 99.8 0.197.4 97.497.4 0.1 0.10.1 HOO HO OH O 99.999.8 OH 99.8 99.8 O O O 99.9 O 99.9 (4-O-5) (4-O-5)99.9(4-O-5) 99.9 99.997.499.999.9 99.999.999.999.9 99.9 0.197.4 97.4 0.1 0.1 (4-O-5)O (4-O-5)(4-O-5) O 97.4 99.9 0.197.4 97.4 0.1 0.1 (α-O-4) O 99.9 O 99.934.599.9 99.999.9 48.8 13.1 (α-O-4) (α-O-4)99.9 34.599.999.9 99.9 48.834.5 13.148.8 13.1 ((α-O-4)-O-4)O (α-O-4)(α -O-4)O (α -O-4)O 34.5 34.548.8 34.534.5 34.5 48.813.148.8 48.848.8 13.1 13.1 13.1 O (α(α-O-4)(((-O-4)α(α-O-4)-O-4)-O-4)-O-4) O 34.534.534.534.534.534.5 48.848.848.848.848.848.8 13.113.1 13.113.113.113.1 (α-O-4)O (α-O-4)O 99.9(α -O-4) 34.599.9 99.999.948.8 34.5 34.5 13.1 48.8 48.8 13.1 13.1 O (α-O-4) (α-O-4)(α-O-4)(α(α-O-4)O -O-4) 34.5 34.548.834.534.5 34.5 48.813.148.848.8 48.8 13.113.1 13.1 OH (α-O-4) 34.5 48.8 13.1 O O (α-O-4) O 34.5 48.8 13.1 OH (α-O-4) 34.5 48.8 13.1 OH O 99.9 O O 99.9 99.9 O O OOO O 99.9O (α-O-4) OH (α-O-4)99.9( α( α-O-4)-O-4)OH OH34.599.9 99.948.8 34.5 34.534.5 13.148.8 48.848.8 13.1 13.113.1 O O O O O 99.9O OH 99.9 OH OHOH OHOHOHOH OH O O 99.9 99.9 OH 99.9 (α-O-4) OH OH38.6 50.4 6.7 (α-O-4) (α-O-4)OH(α -O-4)OH OH 34.5 99.9 99.9 48.834.5 34.5 13.148.8 48.8 13.1 13.1 (αO -O-4) O 99.9 O O 99.9 99.938.699.999.9 99.999.999.9 50.4 6.7 (α-O-4)( α-O-4)OH 34.538.6 34.5 48.850.4 48.8 13.16.7 13.1 (α-O-4)HO 99.9 34.5 99.9 99.9 48.8 13.1 (α-O-4) OH (α-O-4)(α -O-4) 99.938.699.9OH 99.9 50.438.6 38.6 6.750.4 50.4 6.7 6.7 HO HO OH OH (ααO -O-4) (αO -O-4)(α O-O-4) 38.699.9 50.438.6 38.6 6.750.4 50.4 6.7 6.7 ( -O-4)HO (α(-O-4)α(α-O-4)(((-O-4)α(α-O-4)-O-4)-O-4) -O-4) HO HO 38.638.6 38.638.6 38.638.638.638.6 50.450.450.4 50.450.450.4 50.4 6.76.7 6.7 6.76.7 6.76.7 6.7 (αO -O-4) (αO -O-4)99.9(α -O-4)O 38.6 99.9 99.999.9 50.4 38.6 38.6 50.46.7 50.4 6.7 6.7 HO HO OH HO OH (α-O-4)(α-O-4)(α-O-4) OH 38.638.6 38.6 50.450.4 50.4 6.76.7 6.7 O HO HOHO HO HO HOHOHO OH OH (α-O-4) HO OHHO 38.6 50.4 6.7 O O99.9 (α-O-4) (HOα-O-4)HO99.9HO( α( α-O-4)-O-4) 38.6 99.9 99.950.4 38.6 38.638.6 50.46.7 50.450.4 6.7 6.76.7 O O99.9HO O 99.9 HO 99.9HO HOHO 99.9 (α-O-4)O (α-O-4)O (α -O-4)O 38.6 50.438.6 38.6 6.750.4 50.4 6.7 6.7 O O 99.9O 99.9 99.9 (α-O-4) O(Oα-O-4)OOOO ( α-O-4)O 38.6 50.438.6 38.6 50.46.7 50.4 6.7 6.7 HO 99.9HO 99.9 99.9 (α-O-4) O O O HO 25.8 33.5 2.1 15.1 20.5 HO 99.9HO HO 99.999.999.9 99.999.999.9 99.9 (α-O-4)O (α-O-4)O O O99.9O 25.899.9 33.525.8 2.133.5 2.115.1 15.120.5 20.5 OH OH (α-O-4) 99.999.9 99.9 25.8 33.5 2.1OH 15.1OH 20.5 (OHα-O-4) (α-O-4) 25.8 33.5 25.8 OH 2.1 33.5 15.12.1 20.515.1 20.5 OH 99.9 99.999.9 99.999.9 OH OH OH OH OH O (α-O-4)O (α -O-4) 25.8 25.8 33.5 33.5 2.1 2.1 15.1 15.1 20.5 20.5 (α-O-4) O 25.8 33.5 2.1 15.1 20.5 OH OH OH OH OH OH OH OH (α-O-4)O (α-O-4)O (α -O-4) O 25.8 33.525.8 25.8 33.52.1 33.5 2.1OH 15.12.1 15.1OH 20.5OH 15.1 20.5 OH 20.5 OH OH (OHα-O-4)(α(-O-4)α(α -O-4)(((-O-4)α(αOH-O-4)-O-4)-O-4) -O-4) 99.9 99.9 99.925.8 25.8 25.825.8 25.825.825.825.8 OH33.533.533.5 33.533.533.5 33.5 OH 2.1OH 2.1 2.12.12.1 2.12.1 2.1 OH OH15.1OH15.1 15.115.1 15.115.115.115.1 OH 20.5OH 20.520.520.5 20.520.520.520.5 OH OH OH 99.9 OH OH OH OH OH OH OH OH(α-O-4)(α-O-4)(α-O-4) 99.9 99.9 25.825.8 25.8 33.533.5 33.5 2.12.1 2.1 15.1OH15.1 15.1 OH20.520.5 20.5 (α-O-4) OHOH OHOHOHOH(α-O-4)99.8( α( α-O-4)-O-4) 25.8 33.525.8 25.825.8 33.52.1 33.533.5 OH OH2.1OH OHOH15.1 OH 2.12.1 OH15.120.5OH OH 15.115.1 20.5 OH OH 20.5OH20.5 (α-O-4)99.8 25.8 33.5 OH 2.1 OH OHOHOH15.1 OH OHOH OH20.5 OHOH OH OH 99.8 OH OH OH OH OH OH OH OHOH OH OH OH OHOH OH OHOH OHOHOH OH OH OHOH OH OH OH 99.8 99.8 99.8 (α-O-4) OH (α-O-4)(αOH -O-4) 25.899.8 99.8 33.525.8 25.8 2.133.5 33.5 2.1OH15.1 2.1 15.120.5 OH 15.1 20.5 20.5OH (α-O-4)OH (α-O-4)99.8(OH α -O-4) OH 25.8 33.525.8 25.8 33.52.1 33.5 2.115.1 2.1 15.120.5 15.1 20.5 20.5 OH (5-5′) OH OH 50.2 2.0 OH 10.8 OH 22.7 9.3 0.8 OH 99.8OH OH 99.8 99.8 OH OH OH OH OH OH OH OH(5-5′) OH OH 50.2 2.0 OH 10.8 OH 22.7OH 9.3OH 0.8 OH O OH (5-5′) OH 50.2 2.0 10.8OH OH 22.7OH OH 9.3 0.8 (5-5′) OH 99.899.899.8 99.899.899.8 50.2 2.0 10.8 22.7 9.3 0.8 O (5-5′) O 99.8(5-5 ′) 50.299.8 99.899.82.0 50.2 10.8 2.0 22.710.8 9.3 22.7 0.8 9.3 0.8 OH(5-5OHOH′)OH (5-5′) 99.899.8 99.8 50.2 50.2 2.0 2.0 10.8 10.8 22.7 22.7 9.3 9.3 0.8 0.8 O (5-5′) O OOHOHOH 99.8 50.2 2.0 10.8 22.7 9.3 0.8 (5-5′) OH (5-5 ′) (5-5OH ′) OHOH 50.299.8 2.050.2 50.2 10.82.0 2.0 10.822.7 10.8 22.79.3 22.7 0.89.3 9.3 0.8 0.8 O O OH OHO OH 99.8OH 99.8 99.8 O (5-5(5-5(5-5′)(5-5(5-5O (5-5(5-5′)′) ′′)′)′99.3)99.8 ) O OH 99.8 99.850.2 50.250.2 50.250.250.250.2 2.02.0 2.02.02.0 2.02.0 10.810.810.8 10.810.8 10.810.8 22.7 22.722.7 22.722.722.722.7 9.39.3 9.39.39.3 9.39.3 0.80.8 0.80.80.8 0.80.8 (5-5′0) OH (5-5′) OH(5-5 (5-5′) ′)OH 50.2 2.050.2 50.250.2 10.82.0 2.0 2.0 10.822.7 10.8 10.8 22.7 9.3 22.722.7 0.89.3 9.39.3 0.8 0.80.8 99.3 OH 99.3 OHO (5-5OO (5-5O′) (5-5′) ′) OH 50.250.2 50.2 2.02.0 2.0 10.810.8 10.8 22.722.7 22.7 9.39.3 9.3 0.80.8 0.8 O(5-5 ) OOO(5-5O ′99.3) O O 99.3 99.3 50.250.2 2.0 2.0 10.8 10.8 22.722.7 9.3 9.3 0.80.8 (5-5(β-5)′) O O O(5-5(5-5′)99.3 ′) (5-5 ′) 50.25.499.3 99.3 28.62.050.2 50.2 50.2 10.861.42.0 2.0 2.0 10.822.7 10.8 10.8 22.7 9.3 22.722.7 0.8 9.3 9.3 9.3 0.8 0.8 0.8 (β-5) O (5-5 ′) 5.4 28.6 50.2 61.4 2.0 10.8 22.7 9.3 0.8 O (5-5′) O O (5-5(β-5)99.3′) 50.299.3 99.3 2.050.25.4 10.828.62.0 10.861.422.7 22.79.3 0.8 9.3 0.8 O (β-5) (β-5) O 5.4 28.65.4 61.428.6 61.4 O O O(β-5) 5.4 28.6 61.4 O (β-5) O (β-5)99.3 (β -5) 99.399.35.499.3 99.399.3 99.399.328.6 5.4 5.4 61.4 28.6 28.6 61.4 61.4 O (β-5) O (β-5) O (β -5) 5.499.399.3 99.3 28.65.4 5.4 61.428.6 28.6 61.4 61.4 O O 99.399.3 99.3 O 99.3 O (β-5) (β-5)O (β-5)99.9O ( β( β-5)99.3-5) 5.499.3 99.35.428.6 5.4 5.45.4 28.6 61.428.6 28.6 28.6 61.4 61.4 61.4 (β(β-5)((-5)(β(ββ-5)-5) -5)-5) 99.999.3 99.399.9 5.45.45.45.4 5.45.4 28.628.628.628.628.628.6 61.461.4 61.461.4 61.461.4 O (β-5)(β-5) (β-5) O 5.45.4 5.4 28.628.6 28.6 61.461.4 61.4 (ββ-5) O O (β-5)99.9 O( β-5) 5.499.9 99.9 28.65.4 5.4 61.428.6 28.6 61.4 61.4 ((β-5) OO OOO (β(-5)β-5)99.9 ( β -5) 3.85.499.9 99.9 22.628.6 5.45.4 5.4 5.4 70.461.428.628.6 28.628.6 61.4 61.4 61.461.4 (β-5) O O O (β-5)99.9 3.899.9 22.63.8 70.422.6 70.4 O O O 99.9 O (β-5) O O (β-5)O (β-5) 3.8 22.63.8 3.8 70.422.6 22.6 70.4 70.4 (β-5) (β-5)99.9 (β -5) 3.8 99.922.6 3.8 3.8 70.422.6 22.6 70.4 70.4 (β-5) (β-5) (β-5) 3.899.9 99.922.6 3.8 3.8 70.422.6 22.6 70.4 70.4 99.999.999.9 99.999.999.9 99.9 99.999.9 99.999.9 99.9 (β-5) (β-5) 99.9 ( β( β-5) 99.9-5) 3.899.999.9 99.922.6 3.8 3.83.8 70.422.6 22.6 22.6 70.4 70.4 70.4 ( β-5)(β(β-5)(( -5)(β(ββ-5)-5) -5)-5) 3.83.8 3.83.83.8 3.83.8 22.622.622.6 22.622.622.622.6 70.470.470.4 70.470.470.470.4 (β-5) (β-5)(β-5) (β(β-5) -5) (β-5) 3.8 22.63.83.83.8 3.8 3.8 22.670.422.622.6 22.6 22.6 70.470.4 70.470.4 ((ββ-5) (β(-5)β-5) 3.8 22.6 3.83.8 3.8 70.422.622.6 22.6 70.4 70.4 70.4

Catalysts 2017, 7, 169 7 of 22 Catalysts 2017, 7, 169 7 of 22

Catalysts 2017, 7, 169 7 of 22

FigureFigure 2. 2.Possible Possible pathwayspathways of of guaiacol guaiacol HDO HDO [30]. [30 ].

3.1.3. HDO of Other Oxygenates 3.1.3. HDO of Other OxygenatesFigure 2. Possible pathways of guaiacol HDO [30]. In addition to lignin-based aromatic oxygenates, furans, carboxylic acids, alcohols and In addition to lignin-based aromatic oxygenates, furans, carboxylic acids, alcohols and carbohydrates carbohydrates3.1.3. HDO of areOther also Oxygenates common products in biomass pyrolysis. A number of investigations focused are alsoon HDO common of these products cellulosic in biomassor hemicell pyrolysis.ulosic fractionsA number of biomass of investigations pyrolysis. focused on HDO of these In addition to lignin-based aromatic oxygenates, furans, carboxylic acids, alcohols and cellulosicA or hypothetical hemicellulosic mechanism fractions for of furan biomass HDO pyrolysis. was proposed including two reaction routes. For carbohydrates are also common products in biomass pyrolysis. A number of investigations focused Aone hypothetical route, furan first mechanism formed a for transition furan HDOstate, followed was proposed by the includingformation of two butadiene reaction which routes. could For one on HDO of these cellulosic or hemicellulosic fractions of biomass pyrolysis. further undergo hydrogenation to butane. Moreover, partial hydrogenation of the ring also route, furanA hypothetical first formed mechanism a transition for furan state, HDO followed was proposed by the formationincluding two of butadienereaction routes. which For could occurred followed by ring opening to a new transition state, which subsequently decomposed to furtherone undergo route, furan hydrogenation first formed toa transition butane. Moreover,state, followed partial by the hydrogenation formation of butadiene of the ring which also could occurred propylene or butylene [38]. Zheng et al. proposed a proper pathway for vapor-phase HDO of followedfurther by ringundergo opening hydrogenation to a new transition to butane. state, Moreov whicher, subsequentlypartial hydrogenation decomposed of the toring propylene also or furfural over Cu-based catalyst, as shown in Figure 3 [39]. The conversion of furfural yields both butyleneoccurred [38]. Zhengfollowed et al.by proposedring opening a proper to a new pathway transition for state, vapor-phase which subsequently HDO of furfural decomposed over Cu-based to furfuryl alcohol and furan via hydrogenation and C-C bond cleavage, followed by sequential catalyst,propylene as shown or inbutylene Figure 3[38].[ 39 ].Zheng The conversionet al. proposed of furfural a proper yields pathway both furfurylfor vapor-phase alcohol HDO and furan of via hydrogenation to tetrahydrofurfuryl alcohol and tetrahydrofuran. In addition to tetrahydrofuran, hydrogenationfurfural over and Cu-based C-C bond catalyst, cleavage, as shown followed in Figure by sequential 3 [39]. The hydrogenationconversion of furfural to tetrahydrofurfuryl yields both n-butanol,furfuryl alcoholn-butanal, and ethanolfuran via an dhydrogenation hydrocarbon alsoand C-Cderive bond from cleavage, furan conversion. followed by Furthermore, sequential alcohol and tetrahydrofuran. In addition to tetrahydrofuran, n-butanol, n-butanal, ethanol and 2-methyltetrahydrofuran,hydrogenation to tetrahydrofurfuryl 2-pentanone, alcohol 2-pentanol and te trahydrofuran.and 1-pentanol In additionare all tothe tetrahydrofuran, hydrogenation hydrocarbonproductsn-butanol, alsoof 2-methylfurann-butanal, derive from ethanol obtained furan and conversion. hydrocarbonfrom a cleavage Furthermore, also of derive C-O bond. from 2-methyltetrahydrofuran, Sitthisafuran conversion. et al. carried Furthermore, out 2-pentanone,furfural 2-pentanolHDO2-methyltetrahydrofuran, over and supported 1-pentanol metal are 2-pentanone, allcatalysts the hydrogenation under 2-pentanol atmosp andheric products 1-pentanol pressure, of 2-methylfuranindicatingare all the that hydrogenation obtainedthe product from a cleavagedistributionproducts of C-O of was bond.2-methylfuran strongly Sitthisa depended obtained et al. carried onfrom the a out metalcleavage furfural used of inC-O HDO the bond. catalyst. over Sitthisa supported Buta etnal, al. butanolcarried metal out and catalysts furfural butane under atmosphericwereHDO obtained over pressure, supported by ring indicating opening metal catalysts thatreaction the onunder product Ni/SiO atmosp distribution2 catalyst,heric pressure,while was Pd/SiO strongly indicating2 catalyzed depended that thethe formation product on the metal of furan by decarbonylation [40]. used indistribution the catalyst. was Butanal, strongly butanoldepended and on butanethe metal were used obtained in the catalyst. by ring Buta openingnal, butanol reaction and butane on Ni/SiO 2 were obtained by ring opening reaction on Ni/SiO2 catalyst, while Pd/SiO2 catalyzed the formation catalyst, while Pd/SiO2 catalyzed the formation of furan by decarbonylation [40]. of furan by decarbonylation [40].

Figure 3. Proper reaction pathway proposed for furfural hydrogenation [39]. VL, δ -valerolactone.

FigureFigure 3. Proper 3. Proper reaction reaction pathway pathway proposed proposed forfor furfural hydrogenation hydrogenation [39]. [39 VL,]. VL,δ-valerolactone.δ-valerolactone.

Conversion of carboxylic acids is crucial since they are considered as typical component that contribute to the corrosive property of bio-oil. Figure4 shows the reaction network of aqueous phase Catalysts 2017, 7, 169 8 of 22 Catalysts 2017, 7, 169 8 of 22 Conversion of carboxylic acids is crucial since they are considered as typical component that contributehydrogenation to the of corrosive acetic acid property proposed of by bio-oil. Wan and Figure co-workers 4 shows [41 the]. Primaryreaction reactionnetwork of of acetic aqueous acid phaseincludes hydrogenation dehydrogenation of acetic to acetate acid species proposed or dehydroxylation by Wan and co-workers to acetyl species. [41]. AcetatePrimary reacts reaction further of acetic acid includes dehydrogenation to acetate species or dehydroxylation to acetyl species. via decarboxylation or reforming to form CH4 or H2, respectively. Acetyls can either decompose via Acetate reacts further via decarboxylation or reforming to form CH4 or H2, respectively. Acetyls can the cleavage of C-C bond to CH4 and CO or be converted to ethanol. The formed ethanol is further either decompose via the cleavage of C-C bond to CH4 and CO or be converted to ethanol. The converted to ethane via hydrodeoxygenation, CH4 via C-C bond cleavage, and ethyl acetate via formedesterification. ethanol Moreover, is further HDO converted of acetic to acidethane mixed via withhydrodeoxygenation,p-cresol was studied CH on4 via Ru/C C-C catalystbond cleavage, at 573 K and 48ethyl bar acetate hydrogen via pressure.esterification. In the Moreover, mixed feed HDO system, of acetic acetic acid acid mixed hydrogenation with p-cresol was was suppressed studied ondue Ru/C to the catalyst competitive at 573 adsorption K and 48 on bar catalyst hydrogen surface. pressure. On the contrary, In the mixed the HDO feed of psystem,-cresol isacetic promoted, acid hydrogenationresulting in high was selectivity suppressed to methyl-cyclohexane. due to the competitive adsorption on catalyst surface. On the contrary, the HDO of p-cresol is promoted, resulting in high selectivity to methyl-cyclohexane.

FigureFigure 4. 4. ProposedProposed reaction reaction network network for aqueous ph phasease hydrogenation of acetic acid [41]. [41].

Wildschut et al. selected D-glucose and D-cellobiose as model compounds for carbohydrates in Wildschut et al. selected D-glucose and D-cellobiose as model compounds for carbohydrates in bio-oil, and investigated the HDO of these monomeric sugars over Ru/C catalyst at 523 K and 100 bio-oil, and investigated the HDO of these monomeric sugars over Ru/C catalyst at 523 K and 100 bar bar H2 pressure. The main reaction pathway was D-sorbitol hydrogenation, followed by H2 pressure. The main reaction pathway was D-sorbitol hydrogenation, followed by hydrogenolysis of hydrogenolysis of smaller polyols (e.g., glycerol and propanediol) and gaseous hydrocarbon (e.g., smaller polyols (e.g., glycerol and propanediol) and gaseous hydrocarbon (e.g., methane and ethane). methane and ethane). In addition to hydrogenation, a thermal pathway proceeded to form In addition to hydrogenation, a thermal pathway proceeded to form hydromethylfurfural, levulinic hydromethylfurfural, levulinic acid and humins. Furthermore, the study demonstrated that the acid and humins. Furthermore, the study demonstrated that the acetic acid added in this system acetic acid added in this system increased solids formation by promoting the thermal route [42]. increased solids formation by promoting the thermal route [42].

3.2. HDO HDO of of Actual Pyrolysis Oil Nemours studies focused on bio-oil hydrotreatin hydrotreatingg to achieve liquid fuels compatible with petroleum fraction. Elliot Elliot et et al. al. initially initially carried carried out out HDO HDO of of poplar poplar wood wood bio-oil bio-oil over over a sulfide sulfide CoMo catalyst in 1983. This This direct direct hydrodeoxygenation hydrodeoxygenation yielded yielded only only 23 23 wt wt % % upgraded oil and resulted inin severesevere coke coke deposition deposition and and bed bed plugging plugging [43 ].[43]. Xiong Xiong et al. et also al. testedalso tested direct direct HDO ofHDO bio-oil of bio-oilusing ausing Raney a NiRaney catalyst Ni incatalyst a batch in reactor, a batch leading reactor, to lowleading yield to of low organic yield liquid of organic [44]. Therefore, liquid [44]. in Therefore,order to overcome in order the to obstaclesovercome mentioned the obstacles above, mentioned alternatives above, such alternatives as employing such a two-stage as employing process a two-stageand co-feeding process with and solvents co-feeding were with investigated solvents forwere bio-oil investigated hydrotreatment. for bio-oil hydrotreatment.

3.2.1. Two-Stage Two-Stage Hydrotreatment The two-stage process was initially carried out by researchers from Pacific Pacific Northwest National Laboratory (PNNL) (PNNL) in in 1989 1989 [45]. [45 ].The The first first stage stage was wasoperated operated below below 553 K 553 over K Ni over or sulfide Ni or sulfideCoMo catalystCoMo catalyst to produce to produce stabilized stabilized oil by hydrotreating oil by hydrotreating the most the reactive most reactive compound compound such assuch furans as aldehydes,furans aldehydes, and ketones. and ketones. The second The secondstep was step performed was performed at severe at severeconditions conditions in a second in a second stage usingstage conventional using conventional petroleum petroleum hydrotreating hydrotreating catalyst. catalyst. With the Withtwo-stage the two-stage hydrotreatment, hydrotreatment, the yield ofthe upgraded yield of upgraded oil could oil be could increased be increased to 30–55% to 30–55% with withthe thecomparison comparison of ofdirect direct HDO, HDO, and and the deoxygenation degrees degrees could be up to 99% [[46].46]. No Nobleble metal metal catalysts have have also been applied to two-stage hydrotreatment by Elliot et al.al. It waswas demonstrateddemonstrated thatthat Pd/CPd/C was was an an effective effective catalyst to

Catalysts 2017, 7, 169 9 of 22 hydrogenate various bio-oils to stabilized oils suitable for more severe hydrocracking step. In the first hydrotreating step, the gas yield improved while the oil yield decreased at higher temperature, and the product oxygen contents reached the minimum level at 613 K. The procedure could successfully run for 10 to 100 h depending on the operating conditions. Subsequent step were performed at about 673 K under 103 bar using a conventional sulfide catalyst, and the oxygen content of final products were all below 1%. In addition to the configuration mentioned above, a non-isothermal system incorporating the two steps was further explored at 523–683 K under 138 bar. It was indicated that the non-isothermal configuration could effectively minimize the carbon loss leading to the higher oil yield (50%) compared to the sequential one [47]. Gholizadeh et al. investigated the hydrotreament of mallee wood bio-oil in a non-isothermal configuration using NiMo/γ-Al2O3 as main catalyst. The authors observed that the reaction temperature had a role in coke formation and quality of upgraded oil. Although Pd/C catalyst laid in upstream had some effects on stabilizing bio-oil, it was ineffective in keeping off coke formation in long-term operation [48]. In order to increase catalyst lifetimes, PNNL partnered with Battelle developed a new process for bio-oil hydrotreatment based on a two-stage system. A cleanup step and ion exchange were performed before the first stabilization step, obtained the cleaned up bio-oil of which the contents of metallic impurities were obviously decreased. For the first reaction zone, non-carbon supported catalysts were employed and catalyst regeneration processes were developed. Therefore, the Battelle-PNNL bio-oil upgrading process is stable for over 1000 h time on stream [49].

3.2.2. Co-Feeding Solvents Hydrogen donor solvents were widely used in the hydrogenation of coal liquids to remove the heteroatoms and decrease molecular weight. In order to facilitate the transformation of active hydrogen and reduce coke formation during HDO process, hydrogen donor solvents, such as tetralin and decalin, were used for bio-oil upgrading. Churin et al. investigated the influence of the presence of tetralin on hydrotreating of bio-oil over NiMo catalyst, indicating that the hydrogen donor resulted in an additional reduction in the oxygen content of 15% [50]. The oil phase of sawdust bio-oil was upgraded by Zhang et al. over sulfide Co-Mo-P, using tetralin and tar oil as solvent, respectively. The authors considered that tetralin transferred hydrogen from the gas phase with high activity to the radical fragments in oil phase, leading to higher liquid yield and less gas, char and water [51]. Zhang et al. used decalin as hydrogen donor in the hydrotreatment of bio-oil and its model compounds over Ni/TiO2-ZrO2 at 573 K. Higher reaction conversion was obtained when the initial H2 pressure increased, because the solubility of H2 in decalin was improved with the increased pressure [52]. Employing hydrogen donor solvent in bio-oil HDO significantly reduce the operating pressures and lighten charring and coking phenomena. Moreover, alkanes have also been used during bio-oil HDO. Zhao et al. investigated the HDO of n-hexane-extracted bio-oil over Ni/HZSM-5 under mild reaction conditions. The obtained liquid products were almost hydrocarbons including C5–C9 alkanes, cycloalkanes, and aromatics molecules [53]. However, the hydrocarbon solvents cannot completely blend with crude bio-oil due to their hydrophobicity, which hinders the HDO reaction to some extent. Hence, supercritical solvents were used for bio-oil hydrotreating. Xu et al. selected 1-butanol as solvent in the supercritical upgrading of bio-oil over a Ru/C catalyst, indicating that the features of upgraded bio-oil were significantly improved. The authors further illustrated that the solvent acted as both reaction medium and reactant during the reaction [54]. Oh et al. studied the HDO of bio-oil over Pt/C catalyst in the presence of three supercritical solvents with different polarities. The polar protic solvent (ethanol) efficiently improved the degree of deoxygenation and typical bio-oil properties, such as acidity, viscosity, HHV and water content, and the polar aprotic solvent (acetone) resulted in the highest yield of heavy oil. The non-polar ether hardly affected the organics, but improved its thermal stability [55].

3.2.3. In-Situ Hydrogenation Recently, in-situ hydrogenation as a novel method using liquid hydrogenation donors instead of hydrogen gas attracted lots of interesting. Acids and alcohols are common hydrogenation Catalysts 2017, 7, 169 10 of 22 donors, which can generate hydrogen via aqueous-phase reforming. Xiong et al. conducted an in-situ hydrogenation of rice husk bio-oil in the presence of formic acid over various metal catalysts. During the upgrading process, formic acid decomposed to produce hydrogen while by-product CO2 dissolved in solvents improving the rate of hydrogenation. High yield of liquid phase (>86%) and slight coke formation (<5%) were achieved with partial hydrogenation that alkenyl and aldehyde groups were almost completely reduced [44]. Xu et al. used methanol as hydrogen donor instead of hydrogen gas in the hydrogenation of bio-oil at 493 K and 30 bar. Over Raney Ni catalyst, ketones and aldehydes were converted to alcohols via in-situ hydrogenation, while esterification took place transforming acids to esters [56]. Furthermore, the authors carried out the upgrading process of bio-oil in different hydrogen donors (methanol, ethanol and formic acid) over Ni based catalysts to investigate the effect of hydrogen donors on the in-situ hydrogenation. The results showed that compositions of the upgraded bio-oils were different according to different hydrogen donors. When alcohols were used as hydrogen donors, the acetic acid in bio-oil could be converted to esters through esterification reaction. The phenol conversion was promoted in the presence of formic acid, along with restraining the conversion of acetic acid [57]. Currently, in-situ hydrogenation is not common for bio-oil upgrading due to its relative low hydrogenation degree leading to the intermediate hydrogenation products instead of hydrocarbons.

4. HDO Catalysts

4.1. Transition Metal Sulfide Catalysts Conventional HDS catalysts for petroleum refining, using Mo as active component, Co or Ni as promoter, are first employed for the hydrogenation of pyrolysis oil. The HDO reactions catalyzed by metal sulfides are summarized in Table4. These HDS catalysts need to be kept in sulfide form, so a sulfur source like H2S is commonly added during the reaction due to the negligible sulfur content of bio-oil. However, severe carbon deposition caused by sulfur leaching prohibits further application of petroleum hydrotreating catalysts. A number of suitable catalysts were investigated by researchers. Recently, Pawelec et al. contributed a review on the HDO of biomass derived liquids over transition metal sulfide catalysts [58]. This section focuses on the use of sulfide catalysts in HDO process.

Table 4. HDO catalyzed by metal sulfides.

Reaction Conditions Catalyst Reactor Reactant Con./% Major Product Ref. Pressure/bar Temp/◦C WHSV/h−1 Benzene, cyclohexane, MoS Batch 28 350 - Phenol 71 [59] Cyclohexene Benzene, cyclohexane, CoMoS Batch 28 350 - Phenol 98 [59] Cyclohexene Cyclohexane, NiMoS Batch 28 350 - Phenol 96 [60] Benzene, Cyclohexene Cyclohexane, NiS Batch 28 350 - Phenol 35 [60] cyclohexene, Benzene Toluene, MoS Batch 28 350 - 4-Methylphenol 52 [61] 2 2,4-dimethylphenol Phenol, cyclohexane, MoS2 Fixed-bed 40 300 - Guaiacol 100 benzene, [62] methylcyclopentane CoMoS Fixed-bed 40 300 - Guaiacol ≈95 Phenol, benzene [62] 2-Ethylphenol, NiMoS/Al O Fixed-bed 21 280 - 2,3-Dihydrobenzofuran ≈50 [63] 2 3 ethylcyclohexane 2-Cyclohexylphenol, CoMoS/MgO Batch 50 350 - Phenol 17 cyclohexylbenzene, [64] cyclohexanol Phenol, catechol, MoS /AC Batch 50 300 - Guaiacol ≈50 [65] 2 cyclohexene CoMoWS/SBA-15 Fixed-bed 30 310 24.5 Anisole 38 Phenol, cresol, xylenol [66]

ReS2/AC Batch 50 300 - Guaiacol ≈40 Phenol, catechol [67] Catalysts 2017, 7, 169 11 of 22

In an early investigation, sulfide CoMo and NiMo supported on γ-A12O3 were employed for HDO of carbonyl, carboxyl and guaiacyl groups. It is found that NiMoS/γ-A12O3 showed higher decarboxylating activity compared to CoMoS/γ-A12O3 and increased the content of heavy products in HDO process of guaiacol [68]. Massoth et al. carried out phenol HDO catalyzed by sulfide CoMoS/A12O3 at 573 K and 28.5 bar hydrogen pressure and obtained benzene, cyclohexene, cyclohexane and H2O as major products [69]. The mechanism of guaiacol HDO promoted by sulfide CoMo and NiMo/A12O3 involved demethylation, demethoxylation and deoxygenation, followed by benzene ring saturation [70]. Compared with sulfide catalyst supported on A12O3, Nava et al. found that the activity of all the studied sulfide CoMo catalysts supported on mesoporous silicates were much higher. The selected mesoporous silicates included disordered mesoporous silica (DMS-1), hexagonal mesoporous silica (HMS) and cubic mesoporous silica (SBA-15, SBA-16). Among these four catalysts, CoMo/SBA-16 was considered as the most effective catalyst, indicating that the support morphology could strongly influence the catalytic response of catalysts [71]. Bui et al. investigated the effect of cobalt on MoS2 catalyst during the conversion of guaiacol. With addition of cobalt, the direct deoxygenation pathway involved in guaiacol conversion was obviously enhanced compared to the non-promoted MoS2, resulting in higher selectivity of aromatic hydrocarbons [62]. Yoosuk et al. compared the HDO activities of MoS2, NiMoS2 and NiS2 using phenol as a model substrate. They observed that NiMoS2 were more active than either of the other two catalysts and the maximum synergy was observed at a Ni/(Mo + Ni) molar ratio of 0.3 [60]. Several unsupported MoS2 with different morphology were used by Yang et al for HDO of phenolic compounds. They found that was favored over MoS2 with a lower degree of stacking (MoS2 derived from AHM) facilitated the C–OH bond hydrogenolysis, whereas MoS2 with a higher degree of stacking (exfoliated MoS2) facilitated aromatic ring hydrogenation [61].

4.2. Noble Metal Catalysts Noble metal catalysts such as Pt, Pd, Rh and Ru have high catalytic activities for hydrotreating and can obviously improve the H/C ratio of pyrolysis oil. A large study showed that noble metal catalysts had high activities for HDO of pyrolysis oil. The effect of Pt on oxygen removal was better than NiMo and CoMo catalysts under the same conditions. However, high costs and difficult recovery limit the industrial application of noble metal catalysts. Recent studies about HDO catalyzed by noble metal catalysts are listed in Table5. For this series of zirconia-supported catalysts (Pd, Pt, Rh/ZrO2) studied by Arditanti et al., Pd/ZrO2 gave the highest activity and Rh/ZrO2 results least carbon deposition [72]. All these noble metal catalysts showed higher activities than conventional CoMo/Al2O3. Zanuttini et al. carried out cresol deoxygenation with Pt-Al2O3 catalysts at atmospheric pressure, observing that methylcyclohexane was the major product at low temperature, while toluene achieved its highest yield at 573 K [73]. The vapor-phase HDO of anisole over a bifunctional Pt/Hβ catalyst was conducted by Zhu et al. at 673 K and atmospheric pressure showed that the major products were benzene, toluene and xylene. They found that the acidic function catalyzed transalkylation, meanwhile the metal function promoted both demethylation and hydrodeoxygenation. Thus, compared with H-Beta and Pt/SiO2 catalysts, less phenolic compounds and saturated hydrocarbons were obtained on the bifunctional catalyst [74]. In addition, a similar result was carried out from the HDO of m-cresol over the bifunctional Pt/HBeta catalyst [75]. Dwiatmoko et al. performed HDO of phenolic monomers over Ru catalysts supported on five different carbon materials. Among these catalysts, Ru supported on multi-walled carbon nanotubes (Ru/MWCNT) exhibited the highest HDO activity, owing to high surface area of Ru and high external surface area of the MWCNT [76]. Lu et al. synthesized TiO2-modified Pd/SiO2 catalyst (TixPd/SiO2) for the conversion of guaiacol to investigate synergistic effect between Ti and Pd. They reported that addition of TiO2 suppressed the sintering of Pd particles resulting in high conversion of guaiacol, and TixPd/SiO2 showed deoxygenation activity while Pd/SiO2 only exhibits hydrogenation activity [77]. Echeandia et al. compared the catalytic activities of Catalysts 2017, 7, 169 12 of 22

Pd catalysts supported on different support (zeolite HY, Al2O3 and HY-Al2O3) for the HDO of phenol. The activity of Pd/20%HY-Al was the largest, which could be attributed to H-spillover phenomenon and a large Pd dispersion [78].

Table 5. HDO catalyzed by noble metal catalysts.

Reaction Conditions Catalyst Reactor Reactant Con./% Major Product Ref. Pressure/bar Temp./◦C WHSV/h−1 methoxycyclohexanol, TixPd/SiO Fixed-bed 20 300 25 guaiacol 100 [77] 2 cyclohexanol, cyclohexane Cyclohexanol,3-methyl, Pt/TiO2 Fixed-bed 20 350 200 Cresol ≈82 Cyclohexanone,3-methyl, [79] Cyclohexane, methyl

Pd-FeOx/SiO2 Fixed-bed 60 450 ≈1.3 Furans - 2-Methyl-decane [80] Cyclohexanol, Ru/ZrO2-La(OH)3 Batch 10 170 - Guaiacol 100 2-methoxycyclohexanol, [81] methane Methylcyclohexane, PtMo/Al O Fixed-bed 5 250 - Cresol ≈95 [82] 2 3 3-methylcyclohexanol Pd/HZSM-5 Batch 20 200 - Cresol 100 Methylcyclohexane [83] Pt/Hβ Fixed-bed 1 350 2 Cresol 100 Toluene [84] Cyclohexanol, Ru/CARF a Batch 40 250 - Guaiacol 97 [76] 2-methoxycyclohexanol, 4-Methylcyclohexanol, Ru/MWCNT b Batch 40 270 - Vanillin 100 4-propylcyclohexanol, [76] methylcyclohexane

Ru/ZrO2 Fixed-bed 64 200 1 Propanoic acid 94 Ethane, propane, methane [85]

Ru/Al2O3 Fixed-bed 64 200 1 Propanoic acid 58 Propanol, propane, ethane [85] Ru/C Fixed-bed 64 190 1 Propanoic acid 94 Ethane, methane, propane [85] Phenol, catechol, Pt/MgO Fixed-bed 1 300 11 Guaiacol 6 [86] cyclopentanone Bicyclohexyl, Pt/MZ-5 c Fixed-bed 40 200 6 (LHSV) Dibenzofuran 98 [87] Cyclopentylmethyl-cyclohexane Pt-Sn/CNF/Inconel d Fixed-bed 1 400 0.3 Guaiacol 100 Phenol, benzene [88] Catechol, phenol, Pt/γ-Al O Fixed-bed 1 300 20 Guaiacol ≈6 [89] 2 3 3-methylcatechol Zn/Pd/C Batch 21 150 - Vanillylalcohol >99 2-Methoxy-4-methylphenol [36] Ethanol, 1,2-propanediol, Pt/Al O Fixed-bed 29 225 - Glycerol 90 [90] 2 3 carbon dioxide

Rh/SiO2-Al2O3 Batch 40 250 - Guaiacol 100 Cyclohexane, [91]

Ru/SiO2-Al2O3 Batch 40 250 - Guaiacol 100 Cyclohexane, cyclohexanol [91] 1-Methyl-1,2-cyclohexanediol, RhPt/ZrO Batch 80 100 - Guaiacol ≈100 [92] 2 cyclohexanol a CARF means carbon aerogel; b MWCNT means multi-walled carbon nanotubes; c MZ-5 means mesoporous ZSM-5; d CNF means carbon nanofiber.

4.3. Non-Noble Metal Catalysts Recently, non-noble metal catalysts including base metals, metal oxides, phosphides, carbides and nitrides are widely used for upgrading of bio-oil. Due to the low price of biomass feedstock, using noble catalysts in upgrading process is uneconomical. Therefore, HDO catalyzed by non-noble metal catalysts has drawn mounting attention. The HDO reactions catalyzed by non-noble metals are summarized in Table6. Mortensen et al. screened in total 23 catalysts for phenol HDO at 548 K and 100 bar, observing that Ni had the best performance among tested non-noble metal catalysts [93]. Compared with the high activity of oxide supported nickel catalysts, Ni/AC showed very low activity for phenol HDO, proving that hydrogenation occurred on the oxidic support while deoxygenation took place on the crystallites. Interestingly, reverse result was obtained by Dongil et al. for the conversion of guaiacol over Ni/CNT catalysts at 50 bar and 573 K in a batch reactor [94]. The guaiacol conversion could up to 100% yielding cyclohexanol, cyclohexane and methoxycyclohexanol as major products. These results means the property of support may also dramatically affect the HDO performances. Shafaghat and co-workers investigated the HDO of a phenolic mixture catalyzed by Ni/HBeta, Fe/HBeta and NiFe/HBeta [95]. They observed that hydrogenation was the dominant pathway in Catalysts 2017, 7, 169 13 of 22 producing cycloalkanes over Ni/HBeta, while aromatic hydrocarbons were mostly produced though hydrogenolysis over Fe/HBeta. The phenolic compounds catalyzed by NiFe/HBeta could be converted to both cycloalkanes and aromatic hydrocarbons due to the synergistic effect between Ni and Fe. Olcese et al. compared the activity of Fe/SiO2 and Co/Kieselguhr for the gas phase HDO of guaiacol at 623–723 K [96]. They found that Fe/SiO2 showed good selectivity to form aromatic hydrocarbons but did not exhibit activity for the hydrogenation of the aromatic ring. The n-electrons of the oxygen atoms are more basic than the π-electrons of C=C aromatic system, so the O atoms in hydroxyl or methoxyl groups were adsorpted on the weak acidic OH sites. Subsequently, C-O cleavage was catalyzed by the active H-species coming from the dissociation of hydrogen molecules on the iron particles. Metallic oxide also been investigated as catalyst for bio-oil upgrading. Selvaraj et al. considered HDO of guaiacol over MoO3-NiO/mesoporous silicates at atmospheric pressure, observing the following order of HDO activities: MoO3-NiO/MAS (mesoporous aluminosilicate) > MoO3-NiO/Ti-SBA-15 > MoO3-NiO/SBA-15 [97]. However, naphthenic hydrocarbon was the major product over MoO3-NiO/MAS compared to other catalysts, where and aromatic hydrocarbon was the major product. The effect of support on the catalytic activity was also investigated by Loricera and co-workers. ZnNi/Ti (TiO2), ZnNi/2Ti-Si (hybrid 2TiO2-SiO2), ZnNi/S15 (SBA-15) and ZnNi/Ti-S15 (SBA-15 decorated with TiO2) were used for the HDO of phenol [98]. It was found that large BET surface area and acidity of supports were conducive to improving the catalytic efficiency. Ghampson et al. investigated ReOx supported on oxidized carbon nanofiber (ReOx-CNFox) to catalyze phenol HDO at 573 K, finding that phenol was completely converted at 10% ReOx loading [99]. The coordinatively unsaturated Re4+, which acts as a Lewis acid, interacts with the oxygen lone pair on phenol. The C-1 of phenol accepts H+ from surface hydroxyl groups resulting in an adsorbed phenoxide ion. After that, benzene was produced though C-O bond cleavage and the recovery of the oxygen vacancy were accomplished by the elimination of water. Metal carbides have attracted great attention as hydroprocessing catalysts since tungsten carbide (WC) was proved to have Pt-like property [100]. Frühberger and Chen also reported that surface modified by carbon converts the reactivity of Mo to that of Pt-group metals [101]. In particular, the catalytic activity of metal carbides can approach or surpass that of Pt-group metals in reaction like hydrogenation and dehydrogenation [102]. Lee et al. investigated the vapor-phase HDO of anisole catalyzed by Mo2C at 420–520 K under ambient pressure, in which benzene selectivity was up to 90% and cyclohexane selectivity was lower than 9% [103]. Chen et al. further obtained high yields of benzene and toluene from HDO of phenolic mixtures over Mo2C catalyst at 533–553 K and atmospheric hydrogen pressure. It is demonstrated that Ar-OCH3 in guaiacol was cleaved prior to the Ar-OH, and the low selectivity for saturated hydrocarbons (e.g., cyclohexane and methylcyclohexane) was due to in-situ modification of Mo2C surface by oxygenates [104]. Recently, Lu et al. synthesized two ordered mesoporous metal carbides, Mo2C and W2C, to investigated the conversion of anisole in gas-phase at relatively low temperatures (423–443 K) [105]. Mesoporous W2C catalyst showed a highest benzene selectivity reported to date (~96%) in anisole HDO, ascribed to the stronger oxygen affinity of tungsten and stronger W−O bond in comparison to the Mo−O bond. Metal nitrides have also drawn interest as candidate catalysts for hydrotreating bio-oil because of the coexistence of acidic and basic sites on nitrides caused by electronegativity differences between the metal and nitrogen atoms [106]. Currently, few researchers work on metal nitrides catalysts for bio-oil upgrading. Ghampson et al. considered the effect of Mo2N on the conversion of guaiacol at 573 K and 50 bar hydrogen pressure in a batch reactor, finding that Mo2N was the most active compared to Mo2N0.78 and other molybdenum compounds [107]. Moreover, the addition of cobalt produced higher yields of deoxygenated products than Mo2N due to the presence of Co3Mo3N particles. HDO of guaiacol catalyzed by Mo2N catalysts supported on activated carbons with different textural and chemical properties was studied by Sepúlveda and co-workers [108]. The Mo2N/Norit catalyst exhibited highest activity relative to Mo2N/Cudu and Mo2N/Pica, because the high mesoporosity of Norit carbon facilitated reactant diffusion to the internal surfaces. Mo2N/Norit also displayed Catalysts 2017, 7, 169 14 of 22 higher phenol/catechol ratio than other catalysts. Metal carbides, nitrides and sulfides have similar preparation method. Generally, the starting materials of Mo-based catalysts are MoO3 or its precursor (e.g., ammonium heptamolybdate). Mo2C, Mo2N, MoS2 could be prepared with pretreatment in CH4/CO/C3H8, NH3 and H2S flow, respectively [109–111]. Saito and Anderson compared the activities of these three catalysts with different pretreatments, indicating that Mo2C and Mo2N had similar adsorbability of hydrogen, which was three times higher than that of MoS2 [111]. Due to the ensemble and/or ligand effects of P, transition-metal phosphides exhibit high activity as hydrotreating catalysts and have drawn great attention for bio-oil upgrading [112]. Bui et al. synthesized a series of metal phosphides supported on SiO2 to investigate the catalytic activity on conversions of 2-methyltetrahydrofuran at 573 K and 1 bar. The HDO activities decreased in the sequence of Ni2P > WP > MoP > CoP > FeP and the selectivity toward HDO products followed the order of MoP > WP > Ni2P > FeP > CoP (phosphite method) and MoP ~WP > FeP > Ni2P > CoP (phosphate method) [113]. An investigation of the anisole HDO catalyzed by Ni2P/SiO2, MoP/SiO2, and NiMoP/SiO2 with different Ni/Mo ratios led to the conclusion that Ni2P exhibit superior activity to that of MoP and the coexistent Ni and Mo could facilitate the phosphides dispersion. Three major products, phenol, benzene and cyclohexane, were obtained via demethylation, hydrogenolysis, and hydrogenation, and both metal sites and PO-H groups were active for these reactions [112].

4.4. Catalyst Deactivation Catalyst deactivation is an obvious problem in HDO, which means loss of catalyst activity as the reaction progress. Catalyst deactivation can be caused by coking, sintering, poisoning, and metal deposition. Catalysts properties are influenced with the degree of these phenomena. Especially, carbon deposition is known as the major cause of HDO catalysts deactivation. Polymerization and polycondensation are considered as main reactions leading to coke formation due to the obvious difference between the average molecular weight of coke and those present in the feeds. Therefore, the adsorbed reactants have a significant effect on the degree of carbon deposition. Unsaturated oxygenates in bio-oil, especially phenols and furans, are regarded as the predominant coke precursors since they can strongly interact with the catalytic surface [114]. Moreover, it has been reported that double-oxygen compounds were much more prone to coke formation than single-oxygen compounds [115]. The HDO of anisole and guaiacol conducted by González-Borja and Resasco also suggested that more severe catalyst deactivation was observed with guaiacol relative to anisole [88]. In addition to reactant structures, catalyst properties such as acidity play an important role in coke formation. In the study for HDO of guaiacol over CoMoS catalysts, Bui et al. reported the conventional alumina support obviously induced the formation of heavier products resulting in catalyst deactivation, while supports with less acidity (ZrO2 and TiO2) formed less heavy by-products [116]. Meanwhile, it is proposed that Lewis acid sites bind reactant species adsorb to catalyst surface, and Brønsted acid sites supply protons to form carbocations as coke precursors [114]. Zanuttini et al. confirmed that Pt/SiO2 had higher carbon deposition in the HDO of m-cresol compared to Pt/Al2O3 due to the presence of Brønsted acid sites on silica supported catalyst, while alumina catalyst only had Lewis acid sites [117]. Therefore, the contribution of the Lewis and Brønsted acidic sites to coke formation is still debatable. Operating conditions such as temperature, H2 pressure and contact time are also relevant to coke formation during hydroprocessing. Low H2 pressure and reaction temperature tend to promote coke formation reactions rather than hydrogenation reaction. Li et al. studied the effect of reaction temperature on coke formation, finding that increased temperature caused severe coke formation despite the improvement of HDO extent [118]. Catalysts 2017, 7, 169 15 of 22

Table 6. HDO catalyzed by non-noble metal catalysts.

Reaction Condition Catalyst Reactor Reactant Con./% Major Product Ref. Pressure/bar Temp./K WHSV/h−1 Fe/Ni/Hβ Fixed-bed 1 573–723 0.25~0.4 Guaiacol 100 Phenol [119] ReOx/CNF a Batch 30 573 - Phenol 100 Cyclohexane, benzene [99] Cyclohexane, Ni/CNT b Batch 50 573 - Guaiacol 100 cyclohexanol, [94] Methoxycyclohexanol Ethylcyclohexane, Ni2P/Al2O3@TiO2 Fixed-bed 30 - 4 Benzofuran 95 ethylbenzene, [120] methylcyclohexane Cyclohexanol, NiCu/CNT b Batch 50 573 - Guaiacol 100 Cyclohexane, [121] Methoxycyclohexanol

Ni2P/Al-SBA-15 Batch 40 493 - Phenol ≈100 Cyclohexane [122] Cyclohexanol, Co P/Al-SBA-15 Batch 40 493 - Phenol 83 [122] 2 Cyclohexane Cyclohexanone, MoP/Al-SBA-15 Batch 40 493 - Phenol ≈20 [122] benzene, Cyclohexane Ni/Al-MCM-41 Fixed-bed 1 673 0.6 Guaiacol ≈100 Methane [123] Benzene, phenol, Co/Al-MCM-41 Fixed-bed 1 673 0.6 Guaiacol ≈100 [123] methane a Mo2C/CNF Batch 55 623 - Guaiacol >99 Phenol, cresol [124]

Meso-W2C Fixed-bed 1 343 - Anisole ≈2 Benzene [105] a W2C/CNF Batch 55 623 - Guaiacol 66 Phenol, cresol [124] Benzene, phenol, MoO Fixed-bed 1 593 ≈0.2 Guaiacol 98 [125] 3 anisole, toluene Cyclohexane, benzene, NiCu/ZrO -SiO Batch 50 613 - Guaiacol 91 [126] 2 2 catechol, Cyclohexane, Ni/SiO -ZrO Batch 50 573 - Guaiacol 100 [127] 2 2 Cyclohexene Methane, phenol, Fe/SiO Fixed-bed 1 673 0.67 Guaiacol 100 [96] 2 benzene

Ni/CeO2 Batch 100 548 - Phenol 100 Cyclohexanol [93] Cyclohexanol, Ni/MgAl O Batch 100 548 - Phenol 100 [93] 2 4 cyclohexane 2-Ethylphenol, NiMoP/Al O Fixed-bed 70 613 - Benzofuran 48 [128] 2 3 2,3-dihydrobenzofuran

Ni/Cr2O3 Fixed-bed 10 573 6 (LHSV) Anisole 90.2 cyclohexane [129]

Ni–Cu/CeO2 Fixed-bed 10 523 1 (LHSV) Anisole 100 cyclohexane [129] Ni-W(Si) c Fixed-bed 15 523 0.5 Phenol 100 cyclohexane [130] Ni-W(P) d Fixed-bed 15 523 0.5 Phenol 100 cyclohexane [130] a CNF means carbon nanofiber; b CNT means carbon nanotube; c tungsten precursor was silicotungstic acid (HSiW); d tungsten precursor was phosphotungstic acid (HPW).

5. Conclusions Although the hydrogenation technology applied for pyrolysis oil has been developed for decades, some problems still exist that clog the development of this technology. (1) High cost and low hydrogen efficiency: HDO process can improve the pyrolysis oil quality, but it generally has low hydrogen utilization efficiency. Now, a better way to optimize the process is the combination of different hydrogen production to supply operations such as two-stage hydrodeoxygenation and in situ hydrogenation. The combination of these operations lowers the operation cost and produces more high value chemicals. (2) Complexity of operation process: In order to make the system work continuously, solid particles in pyrolysis oil have to be removed. Downstream treatment after hydrodeoxygenation is required to remove water to lower the water content in the meantime. Another vital operation unit is hydrogen regeneration unit, which can decrease the hydrogen consumption. Due to the process in whole system is complex, effectively simplify the operation system can dramatically lower the cost and make it easy to enlarge the system. (3) Proper catalyst and catalytic system: One of the key research Catalysts 2017, 7, 169 16 of 22 subjects is to find out suitable catalysts. Noble metals are considered as a kind of efficient catalysts if its high cost is not taken into consideration. These problems we summarized have hindered further development of biomass pyrolysis oil for a long time. However, with the development of modern analysis instruments and new generation technology, we believe there will be some breakthrough in this area. (1) Understanding the composition of biomass pyrolysis oil: Since biomass pyrolysis oil is a mixture of organic compounds, the properties of these components are complex, and molecular weight distribution is also very wide. FT-ICR, NMR and GC × GC have been proven as powerful analysis methods to identify different groups, of which conventional GC/MS can only provide limited information about these components. MS is the most common way to identify chemicals in biomass research. Considering the conditions of pyrolysis oil, a soft ionization mass spectrum is a better choice to identify mixtures. With the rapid development of new mass spectrum, it gives us a better understanding about both organic and inorganic components. (2) Understanding pyrolysis chemistry: Compared to other solid feedstock, biomass pyrolysis is a more complex process since it contains many O atoms. Thus, it is a big challenge to understand its pyrolysis pathways. Researchers designed different experiments to study the pyrolysis chemistry. Generally speaking, biomass pyrolysis contains two stage reactions. Thus, understanding the two stage reactions and their interactions are extremely of great value for pyrolyzer design and downstream processes. (3) Catalyst and catalytic process design: A high efficient catalyst and catalytic process is the core of pyrolysis oil upgrading. Transitional metal carbides were proved to act like noble metals, which may give us a series of low cost catalyst systems which can work at low hydrogen pressure. A good design of catalytic process should be based on both catalytic reactions and interaction between reactions. Since different pyrolysis compositions have dramatically different properties and transform pathway, synergy effect of different components should be considered during the designation. In this review, we summarized recent progress in biomass pyrolysis oil upgrading to hydrocarbon fuels, mainly focusing on hydrodeoxygenation process. Although biomass pyrolysis oil upgrading research has been developing for decades, problems still exist as the bottleneck for further industrialization. Herein, we proposed our solutions: with the usage of modern analysis technology to understand the pyrolysis intermediates and product distribution, a more precise reaction network should be developed which is extremely useful to understand what will be converted. With the help of more precisely understanding on upstream composition, high activity and selectivity catalysts can be designed. In addition, source of coke formation and effect of alkaline salt are profitable to extend the life-span of catalyst. Based on selected catalyst, a better catalytic process will be designed for large scale industrialization.

Acknowledgments: This work was supported by NSFC (Natural Science Foundation of China) project (51476175 and 51606205), national basic research program of china (2013CB228105), Chinese Academy of Sciences “one hundred talented plan”, and Beijing Municipal Key Discipline of Biomass Engineering. Author Contributions: Zhan Si carried out the literature collection and the manuscript writing; Chenguang Wang supervised the study and partially wrote the introduction and conclusion; Xinghua Zhang and Renjie Dong proofread the paper; and Longlong Ma proposed the review topics. Conflicts of Interest: The authors declare no conflict of interest.

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