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Hydroprocessing in Aqueous Phase Edward Furimsky* IMAF Group 184 Marlborough Avenue Ottawa, Ontario, Canada K1N 8G4 ABSTRACT: A large consumption of H 2 aects the overall economy of conventional hydroprocessing. The costs can be decreased by using water as the source of active hydrogen. This can be achieved under subcritical and supercritical water conditions providing that an active and stable catalyst is developed. Hydroprocessing in aqueous phase has been studied for potential applications in upgrading of high oxygen content feeds and heavy petroleum feeds to liquid hydrocarbons. The feeds were tested at temperatures ranging from less than 200 to 500 °C and total pressure from 1 to 30 MPa. These conditions cover subcritical and supercritical regions of water. Water takes part in hydroprocessing reactions as a free radical scavenger and a hydrogen donor. Hydrogen generated in situ via partial reforming and watergas shift reactions is more reactive than external hydrogen. Catalyst development for hydroprocessing in aqueous phase has been receiving much attention. High performance was observed over the catalysts containing noble metals (Pt, Pd, Ru, and Rh) supported on various supports; however, the information on a long-term stability of these catalysts is limited. 1. INTRODUCTION Hydroprocessing (HPR) is the most important route for upgrading petroleum and nonpetroleum feeds to commercial fuels. It involves conversion of compounds containing contaminants such as sulfur, nitrogen, and metals to hydro- carbons via reactions with hydrogen. In some liquids, a nal polishing step is required to attain specication of trans- portation fuels and lubricants. For example, aromatics must be removed by hydrogenation while straight chain hydrocarbons by hydroisomerization. The cost of conventional HPR is aected by a large consumption of hydrogen. Water has been identied as an alternative source of hydrogen. Both subcritical and supercritical water (SCW) conditions have been attracting attention. Water is an important constituent of the feeds produced via hydrothermal liquefaction and pyrolysis of biomass. In this case, operating conditions and/or type of biomass dictate that liquid products are obtained in an aqueous medium. Separation of the water-soluble components from the aqueous phase may be dicult and inecient. Therefore, conversion of polar compounds to hydrocarbons directly in the same environment may be more advantageous. 1 In such applications, HPR is the method of interest. Once polar components in the feed are converted to hydrocarbons, the separation of hydrophobic phase from the aqueous phase is simple. A similar approach may be applied for upgrading of the aqueous phase separated from the primary products obtained during FischerTropsch synthesis (FTS). This byproduct may contain up to 10 wt % of dissolved oxygenates. Direct removal of these oxygenates from aqueous phase via catalytic route has also been attracting attention. 3,4 Depending on the method of production, petroleum crudes may be obtained in the mixture with large quantities of water. This may be the case of heavy crudes produced during the enhanced-oil-recovery using steam ooding method and via hot water separation process employed during the bitumen production from tar sands. 5 A direct conversion of such crudes (without dewatering) via HPR may be a potential route for primary upgrading. A unique case of an aqueous phase may be slurry bed hydrocracking (HCR) of heavy feeds. In this case, a catalyst dissolved in water is coslurried with feed before entering the reactor. The information on conventional HPR methods has been extensively reviewed elsewhere. 6 All reactions occurring in parallel during the HPR of conventional feeds, that is, hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodeoxygenation (HDO), hydrocracking (HCR), hydro- genation (HYD), hydroisomerization (HIS), hydrodemetalliza- tion (HDM), and hydrodeasphaltization (HDAs), have been discussed in details. In addition, the most important HPR reactions were reviewed separately (i.e., HDS, 68 HDN, 911 HDO, 12,13 HYD, 14 HIS, 2 HCR, 2,15 HDM 15,16 and HDAs 15,16 ). Some similarities in the mechanisms of these reactions in the presence of water may be anticipated. However, rather than repeat this information here, the main focus of this review is on potential role of water in modifying the mechanism of HPR. Also, the eect of water on operating parameters under aqueous conditions requires attention. Of particular signicance are the eects of water on catalyst activity and stability. In this regard, the advances in catalyst development for applications in aqueous phase are one of the objectives of this review. 2. PROPERTIES OF WATER During the HPR in aqueous phase, water plays an important role as both solvent and reactant. Chemical and physical properties of water as well as their change with temperature and pressure were described in details elsewhere. 17 This included the properties in subcritical and supercritical regions. Thus, under mild conditions, a direct involvement of water in HPR reactions may be much less evident. For HPR, the miscibility and/or solubility of various feeds (including H 2 ) in water as Received: October 15, 2013 Revised: October 30, 2013 Accepted: November 21, 2013 Published: November 21, 2013 Review pubs.acs.org/IECR © 2013 American Chemical Society 17695 dx.doi.org/10.1021/ie4034768 | Ind. Eng. Chem. Res. 2013, 52, 1769517713

Hydroprocessing in Aqueous Phase

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Hydroprocessing in Aqueous PhaseEdward Furimsky*

IMAF Group 184 Marlborough Avenue Ottawa, Ontario, Canada K1N 8G4

ABSTRACT: A large consumption of H2 affects the overall economy of conventional hydroprocessing. The costs can bedecreased by using water as the source of active hydrogen. This can be achieved under subcritical and supercritical waterconditions providing that an active and stable catalyst is developed. Hydroprocessing in aqueous phase has been studied forpotential applications in upgrading of high oxygen content feeds and heavy petroleum feeds to liquid hydrocarbons. The feedswere tested at temperatures ranging from less than 200 to 500 °C and total pressure from 1 to 30 MPa. These conditions coversubcritical and supercritical regions of water. Water takes part in hydroprocessing reactions as a free radical scavenger and ahydrogen donor. Hydrogen generated in situ via partial reforming and water−gas shift reactions is more reactive than externalhydrogen. Catalyst development for hydroprocessing in aqueous phase has been receiving much attention. High performance wasobserved over the catalysts containing noble metals (Pt, Pd, Ru, and Rh) supported on various supports; however, theinformation on a long-term stability of these catalysts is limited.


Hydroprocessing (HPR) is the most important route forupgrading petroleum and nonpetroleum feeds to commercialfuels. It involves conversion of compounds containingcontaminants such as sulfur, nitrogen, and metals to hydro-carbons via reactions with hydrogen. In some liquids, a finalpolishing step is required to attain specification of trans-portation fuels and lubricants. For example, aromatics must beremoved by hydrogenation while straight chain hydrocarbonsby hydroisomerization. The cost of conventional HPR isaffected by a large consumption of hydrogen. Water has beenidentified as an alternative source of hydrogen. Both subcriticaland supercritical water (SCW) conditions have been attractingattention.Water is an important constituent of the feeds produced via

hydrothermal liquefaction and pyrolysis of biomass. In this case,operating conditions and/or type of biomass dictate that liquidproducts are obtained in an aqueous medium. Separation of thewater-soluble components from the aqueous phase may bedifficult and inefficient. Therefore, conversion of polarcompounds to hydrocarbons directly in the same environmentmay be more advantageous.1 In such applications, HPR is themethod of interest. Once polar components in the feed areconverted to hydrocarbons, the separation of hydrophobicphase from the aqueous phase is simple. A similar approachmay be applied for upgrading of the aqueous phase separatedfrom the primary products obtained during Fischer−Tropschsynthesis (FTS). This byproduct may contain up to 10 wt % ofdissolved oxygenates. Direct removal of these oxygenates fromaqueous phase via catalytic route has also been attractingattention.3,4

Depending on the method of production, petroleum crudesmay be obtained in the mixture with large quantities of water.This may be the case of heavy crudes produced during theenhanced-oil-recovery using steam flooding method and via hotwater separation process employed during the bitumenproduction from tar sands.5 A direct conversion of such crudes(without dewatering) via HPR may be a potential route for

primary upgrading. A unique case of an aqueous phase may beslurry bed hydrocracking (HCR) of heavy feeds. In this case, acatalyst dissolved in water is coslurried with feed beforeentering the reactor.The information on conventional HPR methods has been

extensively reviewed elsewhere.6 All reactions occurring inparallel during the HPR of conventional feeds, that is,hydrodesulfurization (HDS), hydrodenitrogenation (HDN),hydrodeoxygenation (HDO), hydrocracking (HCR), hydro-genation (HYD), hydroisomerization (HIS), hydrodemetalliza-tion (HDM), and hydrodeasphaltization (HDAs), have beendiscussed in details. In addition, the most important HPRreactions were reviewed separately (i.e., HDS,6−8 HDN,9−11

HDO,12,13 HYD,14 HIS,2 HCR,2,15 HDM15,16 and HDAs15,16).Some similarities in the mechanisms of these reactions in thepresence of water may be anticipated. However, rather thanrepeat this information here, the main focus of this review is onpotential role of water in modifying the mechanism of HPR.Also, the effect of water on operating parameters underaqueous conditions requires attention. Of particular significanceare the effects of water on catalyst activity and stability. In thisregard, the advances in catalyst development for applications inaqueous phase are one of the objectives of this review.


During the HPR in aqueous phase, water plays an importantrole as both solvent and reactant. Chemical and physicalproperties of water as well as their change with temperature andpressure were described in details elsewhere.17 This includedthe properties in subcritical and supercritical regions. Thus,under mild conditions, a direct involvement of water in HPRreactions may be much less evident. For HPR, the miscibilityand/or solubility of various feeds (including H2) in water as

Received: October 15, 2013Revised: October 30, 2013Accepted: November 21, 2013Published: November 21, 2013



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well as diffusivity and reactivity of water are of prime interests.For the purpose of this review, only a brief account of theseproperties is given in the following text. Thus, rather extensiveinformation on these and other aspects of water may be readilyaccessed in several books published elsewhere.17−19

The original structure of liquid water, dominated byhydrogen bonds, is changing with increasing temperature.While critical temperature is being approached, an almostcomplete collapse of the hydrogen bond network occurs. As theresult of this change, polarity of water is significantlydiminished. This was confirmed by a dramatic decrease indielectric constant.17−19 Above the critical point, water behavesas a nonpolar medium, capable of dissolving organic substrates.In this regard, water is approaching properties of solvents suchas acetone, methanol, ethanol, etc. Then, the solubility ofvarious feeds (e.g., bio oils, petroleum residues, coal derivedliquids, etc.) in SCW is significantly enhanced.19 From the HPRpoint of view, it is important that under super criticalconditions, gaseous H2 is completely miscible with SCW. Ahigh homogeneity of reaction streams attained in SCW isfavorable for the efficient transfer of hydrogen to reactantmolecules.Above the critical point, water behaves as a dense gas while

still retaining some characteristics (e.g., density) of liquid water.This behavior is the reason for a high diffusivity and uniquetransportation properties of SCW. While increasing temper-ature from subcritical region toward critical temperature at 22MPa, the density of water abruptly decreases, for example, fromabout 0.6 g/mL at ∼350 °C to less than 0.2 g/mL at ∼374 °C.Although to a lesser extent, the SCW density further decreasedwith temperature increase above critical point temperature.This density decrease may be offset by increasing pressure.19

Some effect of density of subcritical water and SCW on HPRreactions may be anticipated. Then, if necessary, an optimalcombination of density with temperature and pressure may beestablished.It is obvious that the reactivity of water may change

dramatically as the consequence of hydrogen bonds networkcollapse. For example, while approaching 374 °C, the pKw ofthe water dissociation equilibrium almost doubled.17−19 Amuch higher concentration of H3O

+ and HO− ions in SCWthan that in liquid water increases the chances for theinvolvement of these ions during HPR. For example, H3O


ions tend to add readily to heteroatoms such as S, N, and O.6,9

An interaction of HO− ions with carbons, particularly thoseattached to heteroatoms, may be anticipated.20 These factsincrease the probability of an ionic mechanism as part of theoverall mechanism during the HPR in aqueous phase. Ionicreactions are favored by high density of water. This may beachieved under subcritical conditions; however, under super-critical conditions, high pressures are needed to get densitiessuitable for ionic chemistry.


In this section, attention is being paid to those feeds that havebeen included in the studies on HPR in aqueous phase. In thisregard, model compounds alone and/or mixtures of variousmodel compounds used to study HPR under conventionalconditions have been also studied under aqueous phaseconditions. This is illustrated on several examples presentedin this review. In the case of real feeds, the focus is on thosefeeds which are being produced in an aqueous environment.

The objective of research in this field was a direct upgrading ofsuch complex mixtures without pretreatment.The most typical source of high water content feeds is the

conversion of biomass (both by pyrolysis and hydrothermaltreatment) always to a high water content biocrude. Detailedaccounts of the conversion of an aquatic biomass viahydrothermal liquefaction and gasification, both in the presenceand absence of catalysts was given by Yeh et al.21 and Savage.22

An example of the biocrude (primary liquids) from pyrolysisand liquefaction of lignocellulosic biomass is shown in Table1.23 At least two stages may be needed to upgrade such feeds to

hydrocarbons via HPR route.24 Chemical compositions of thesebiocrudes and corresponding products were discussed in detailselsewhere.13 Unless an extensive dewatering was conducted, ahigh water content in the biocrude obtained from algae biomassand municipal solid wastes using similar methods may beanticipated.12 Interests in the catalytic conversion of sorbitol toa great variety of products have been noted. The HPR ofsorbitol in an aqueous phase to produce hydrocarbons has beenone of the evaluated routes.25

A unique case of the feed for potential HPR in the presenceof water may be the reaction water produced in FTS process.Such aqueous phase contains a mixture of water-solubleoxygenates, among which alcohols, ketones, aldehydes, andcarboxylic acids are far predominant structures. The totalamount of the oxygenates in the aqueous phase may approach10 wt %. The most abundant oxygenates dissolved in theaqueous phase from FTS are shown in Table 2.2−4

Besides the high oxygen containing feeds, heavy petroleumfeeds have been focus of attention for potential upgrading viaHPR in an aqueous phase. Table 35 indicates a significantdifference between the properties of the latter feeds and thehigh oxygen content feeds shown in Tables 1 and 2. A strongemulsifying potential of some asphaltenic and resinouscomponents suggests that heavy feeds produced via enhancedoil recovery using steam flooding method may contain a largeamount of water in the form of stable emulsions.5 Similarly,relatively high water content bitumen may be produced fromtar sands using a hot water separation process.15 If necessary,

Table 1. Property Ranges of Bio-crude Obtained byLiquefaction and Pyrolysis of Biomass23

liquefaction pyrolysis

carbon, wt % 68−81 56−66sulfur + nitrogen, wt % 0.1 0.1oxygen,a wt % 9−25 27−38water in crude, wt % 6−25 24−52density, g/cm3 1.10−1.14 1.11−1.23

aDry basis.

Table 2. Most Abundant Oxygenates in Aqueous Productfrom High Temperature FTS2

rank oxygenate yield, mass %a

1 ethanol 3.42 propanone 2.53 butanone 1.24 1-propanol 1.05 acetic acid 0.9

aOn total syncrude basis.

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stable emulsions involving heavy feeds and water may beprepared with the aid of surface agents using mechanical means.An atomization of heavy feeds is achieved by water evaporationduring rapid temperature increase, that is, during theintroduction of water-heavy feed emulsion into HPR reactor.At temperatures employed during HCR of heavy feeds (e.g.,>400 °C), water molecules can stabilize free radicals andtransfer hydrogen to products and, as such, offset highhydrogen consumption. A water-soluble catalyst may beadded before the emulsion preparation. A catalytic processreferred to as the slurry bed HCR has been approachingcommercial stage.5 Properties of the heavy feeds, which can beupgraded by this method as well as the operating parameters ofslurry bed reactor, were discussed elsewhere.15

In practical situation, a number of other high water contentfeeds can be identified. For example, such feeds may bedisposed from various industrial operations and municipalities(e.g., refinery sludge, waste from pulp and paper industry,municipal solid waste, etc.). Little information on the upgradingof such materials under aqueous conditions could be found inthe literature, so far. Apparently, HPR in an aqueous mediummay be an attractive option, although the homogeneity of thesefeeds for processing may require attention. Other feeds that canbe upgraded under aqueous phase conditions include coalderived pitch, waste plastics, etc.With respect to HPR, the miscibility and/or solubility of

organic phase in water phase is of a primary importance. Highoxygen content feeds are usually in the form of a homogeneousmixture of water with an organic phase. However, significantproblems may be encountered during the preparation of thewater−heavy feed mixtures for HPR under aqueous conditions.For such feeds, a temperature exceeding that of SCW isrequired to achieve desirable conversion. Also, an optimalcombination of pressure and temperature has to be identified toensure homogeneity of the system. The phase diagram inFigure 126 developed for heavy feed containing ∼37 wt % ofasphaltenes shows three miscibility regions, that is, nonmiscibletwo phase region, partially miscible two phase region, andpseudo-single phase region. It was evident that the regions areinfluenced by the water/feed ratio. The dashed region, which issuitable for upgrading of the heavy feed, is, in fact, the SCWregion. Mechanistic aspects of the conversion of various feedsunder subcritical and supercritical conditions are discussed inmore details later in the review.

4. UPGRADING UNDER AQUEOUS CONDITIONSThe information in literature suggests that, in most cases,temperature and pressure employed during the upgradingunder aqueous conditions of various feeds range from less than

200 to almost 500 °C and 4−40 MPa, respectively.26−36

Processing under such conditions may have some energeticadvantages. Thus, avoiding the liquid water-steam phasetransformation results in substantial energy savings the extentof which is influenced by severity. For the purpose of thisreview, the severity of conditions is referred to temperatureranges employed, i.e., mild, subcritical and supercritical (e.g.,below 300 °C, 300−374 °C, and above 374 °C, respectively).Under subcritical conditions, a high pressure is required toensure that most of the water in the system is in a liquid phase.Besides energy savings, this improves the interaction of watermolecules with reactants thus ensuring a higher conversion ofthe latter. Rather unique properties are exhibited by water inthe supercritical region (above 374 °C and 22 MPa).26 Theproducts from upgrading in an aqueous phase include H2,synthesis gas (H2+CO), gaseous and liquid hydrocarbons. Theconditions employed during the upgrading may be optimized tomaximize the yield of products of interest.36 The production ofliquid hydrocarbons via HPR in an aqueous phase is theprimary focus of this review.The presence of large quantities of water in the system

indicates on the occurrence of reactions (Figure 2), which

under conditions of conventional HPR are either absent or playa minor role. The reforming of hydrocarbons with the aid ofsteam (reaction {1}) may be one of the hydrogen sourcesgenerated in situ. The high yield of CO2, combined with thelow yield of CO confirmed the involvement of the water−gasshift (WGS) reaction {2}, which may always be present as thenext step of reforming reactions. A higher reactivity of the in

Table 3. Properties of Heavy Feedsa for Hydroprocessing inAqueous Phase5

heavy feed

Maya Cold Lake Arab heavy

density, kg/L 0.93 1.0 0.89sulfur, wt % 3.8 4.9 2.9nitrogen, wt % 0.3 0.6 0.2vanadium, ppm 273 160 50nickle, ppm 50 80 16CCRb, wt % 15 19 7

aVacuum residues. bConradson carbon residue.

Figure 1. Phase structure of petroleum residue in subcritical andsupercritical water; water/residue (1) 1/4; (2) 1/2.

Figure 2. Tentative reactions during biomass reforming.

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situ generated hydrogen than that of the external gaseous H2may be anticipated. As it was pointed out, production of liquidhydrocarbons from various feeds under aqueous conditions isthe primary objective of this review. In the case of glucose, thismay be indicated by reaction {3}. In addition, reaction {4}(methanation) is also involved. The hydrogen required for thereactions {3} and {4} would be supplied in situ via reactions{1} and {2} as well as using an external source (gaseous H2).During the HPR in an aqueous phase, the reactions {1} to

{4} occur in parallel. The extent of these reactions depends onthe experimental conditions (e.g., temperature, total pressure,type of experimental system, origin of feed, etc.) and type ofcatalyst. The success of HPR in aqueous phase for theproduction of liquid fuels depends on the optimization ofexperimental conditions to ensure the maximization of reaction{3} and minimization of reaction {4}. A large hydrogenconsumption in reaction {3} should be noted. This may beoffset by the in situ hydrogen production via reactions {1} and{2}. Therefore, an efficient process for conversion of variousfeeds to liquid fuels using a concept based on the HPR inaqueous phase may require a delicate balance involving anumber of operating parameters. These issues were discussedextensively in the study published by Davda et al.37 In thisregard, significant advancements in the understanding ofreactions occurring under aqueous phase conditions weremade by Dumesic and co-workers.37−43 For example, at 483and 498 K, Pt and Pd supported on silica were selective for

generation of H2, while Rh, Ru, and Ni supported on silicaexhibited a low selectivity for H2 and a high selectivity foralkane production.42 The selectivity for H2 production was alsoinfluenced by the structure of substrate; for example, itincreased from sugars toward ethylene glycol and methanol.43

For ethanol, ethylene glycol, and sorbitol (below 500 K, ∼3MPa, Pt/Al2O3 and a tin-promoted Raney-Ni catalysts), almostcomplete suppression of reaction {4} in favor or reactions {1}and {2} could be achieved.38,39 On the other hand,methanation of ethanol (7.5 wt % in SCW) was dominantreaction over Ru/C catalyst at 400 °C and 24.5 MPa.44,45

Lercher and co-workers29,33−36 made an important con-tribution to the understanding of the HPR under aqueousconditions using model compounds typical of those present inbiofeeds. In this case, hydrocarbons were the targeted products.Therefore, the conditions favoring reaction {3} were the focusof their attention. More detailed accounts of these studies aregiven in the latter sections of this review.Huber et al.40 conducted extensive evaluations of various

catalysts in a wide range of experimental conditions. This studyis introduced here to illustrate the attempt for identifyingoptimal conditions ensuring high yields of liquid hydrocarbons(reaction {3}). Using Pt(4%)/SiO2−Al2O3 catalyst (498 K; ∼4MPa; continuous system) and 5 wt % sorbitol in water, thecombined yield of pentane and hexane approached 60 wt %without external H2 being present. The yield was furtherincreased to about 80% in the presence of external H2.

Figure 3. Structure of asphaltenes derived from (a) Athabasca bitumen; (b) Maya crude. Reprinted with permision from ref 50. Copyright 2005,Elsevier.

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For most part, the discussions on aqueous phase upgradingwas focusing on high oxygen feeds. To various extents,reactions {1} to {4} are also present during the upgrading ofpetroleum residues, coal tar pitch, waste plastics, etc. For suchfeeds, thermal cracking of large molecules to light fractions mayplay a dominant role during the overall conversion to liquidhydrocarbons.Apparently, HPR is not the only option for liquid fuels

production under aqueous conditions. For example, theconcept based on the conversion of polysacharides viadehydration, aldol-condensation, and hydrogenation canproduce liquid alkanes under mild conditions.46 In addition, ahigh yield of synthesis gas (CO+H2) may be generated ifreaction {1} is carried out under controlled conditions. Liquidfuels can then be produced via FTS using synthesis gas as thefeed.47 It should be noted that, for this alternative, all stages ofthe FTS process and subsequent upgrading of products havebeen used on a commercial scale.2−4 There might be anotherpotential non-HPR routes for production of liquid hydro-carbons under aqueous conditions.48 These routes are not inthe scope of the present review. However, they should bealways considered as an alternative to HPR while evaluating theviability of upgrading under aqueous phase conditions. In somespecific cases (e.g., for refinery applications), the production ofH2 (reaction {1}) may be attractive. In other cases, theproduction of synthetic natural gas via methanation (reaction{4}) may also be of an interest.


For the purpose of this review, HPR reactions are those inwhich both an in situ produced hydrogen and an external

gaseous H2 are involved. Participation of the former wasindicated by the reactions {1} to {4}. In addition, H2Omolecules may be directly involved in supplying hydrogen.During the HPR under aqueous conditions, both noncatalyticand catalytic reactions occur simultaneously. For the overallmechanism of HPR, decoupling the noncatalytic reactions fromcatalytic reactions may be of an interest. The extent of theformer reactions increases during the temperature increasefrom mild conditions to subcritical and finally to supercriticalregion. Because of the unique physical state of SCW, theoccurrence of entirely new chemical reactions (e.g., ionicreactions) between SCW and substrate in the latter region maybe anticipated. Additional noncatalytic reactions occur underSCW + H2 conditions. It is believed that, under mildconditions, rather low conversion of feed involving watershould be observed unless an active catalyst is present. Once acatalyst is present, a new set of reactions becomes evident evenunder mild conditions. Although a primary focus is on catalyticHPR, a brief account of noncatalytic reactions occurring inparallel with catalytic reactions may be useful for bettercomprehending the overall mechanism of HPR under aqueousconditions.As it was pointed out earlier, nonpetroleum high oxygen

content feeds and petroleum residues are two main types of thefeeds that have been used for the HPR in an aqueous phase.Figures 349 and 450 respectively show tentative structures oflignin and asphaltenes. Lignin containing feeds may be used forthe HPR in aqueous phase either directly or as the source of abio-oil after upgrading (e.g., via pyrolysis and liquefaction). Ahigh oxygen content ensures a good miscibility/solubility in theaqueous phase. On the other hand, vigorous mixing of heavyfeeds of petroleum origin with water may be required to obtain

Figure 4. Tentative structure of soft wood lignin. Reprinted with permission from ref 49. Copyright 2012, Elsevier.

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a homogeneous mixture suitable for HPR. The structures inFigures 3 and 4 clearly indicate a significant difference betweenthe overall mechanism of HPR of these feeds in aqueous phase.5.1. Noncatalytic Reactions. Because of higher temper-

atures involved, thermal cleavage of chemical bonds in variousreactants in SCW (above 374 °C) is more extensive than undersubcritical conditions.51 The thermal cleavage of the weak C−Cbonds begins at bout 300 °C. It was reported that free radicalsproduced by the cleavage of organic bonds were rapidlystabilized in the presence of water via the set of tentativereactions shown in Figure 5.52,53 Based on the models in

Figures 3 and 4, the radical (R•) may involve complexstructures comprising aromatic and aliphatic entities as well asheteroatoms. Once generated, such radicals may decompose tolighter products. This suggests that in the presence of a freeradicals generating agent, the conversion of large reactantmolecules could be enhanced. This was indeed confirmed inthe study of Zhu et al.54 who added ditert-butyl peroxide to aheavy feed for pyrolysis in SCW (653 K; water density 0.30 kg/L). Consequently, the overall conversion of aspahltenes andresins was increased. However, decomposition of large radicalsinvolves a parallel formation of lighter products and smallerradicals. The latter may lead to coke formation unless they arestabilized. This was confirmed by a significant decrease in cokeformation during copyrolysis of the heavy feed with poly-ethylene conducted at 683 K under otherwise similarconditions.54 Polyethylene is the source of paraffinic hydrogen,an excellent stabilizer of free radicals.The reactions {5} and {8} (Figure 5) suggest that hydrogen

from H2O may be transferred to the feed and correspondingproducts. This was indeed confirmed by Dutta et al.57 using themixture of H2O + D2O during thermal cracking of bitumenbetween 350 to 530 °C. In this case, deuterium was transferredboth to liquid products and coke. In the absence of water, theconversion of radicals would proceed via reaction {9} whichrepresents the formation of coke. According to this mechanism,water acts as both radical scavenger and hydrogen donor.In recent study, Xu et al.58 concluded that SCW cannot

donate hydrogen to reactants via the dissociation of H2O toHC and HOC radicals, followed by the reaction of radicals witha reactant. This reaction is energetically unfavorable and shouldbe distinguished from the reaction in which hydrogen isabstracted from H2O by radical via reaction {5}. Ability of theradicals generating reactants to transfer hydrogen from H2Oobserved in their study may be attributed to reactions {7} and{8} (Figure 5). Similar observations were made during thedecomposition of several S-containing compounds in SCW.59

In this case, the experimental results could be interpreted interms of free radicals mechanism.At sufficiently high temperatures, the reforming of hydro-

carbons initiated by free radicals, according to the tentativereaction {1} may take place. For example, at about 600 °C, thetransfer of oxygen from water to carbon of a highlydisintegrated organic molecules was a dominant reactionproducing high yields of CO2 and H2.

51 At such temperatureof SCW, the total pressure may be in the range of 30 MPa. Thehigh yields of CO2 and H2 combined with low yields of COsuggested that H2O reacted with CO via WGS reaction(reaction {2}). Because of the equilibrium effects, the excess ofH2O in the system was favorable for WGS reaction while thepresence of H2 had an opposing effect. Therefore, theconsumption of H2 in HPR reactions, that is, its removalfrom equilibrium mixture, drives WGS reaction forward. It isbelieved that the extent of reforming and WGS reactions(reactions {1} and {2}) is being gradually diminished bydecreasing temperature from about 600 to 374 °C and below.However, a high yield of CO2 obtained between 300 to 350 °Cduring hydrothermal liquefaction of lignocellulosic biomass inwater as reported by Goudriaan and Peferoen60 confirmed thatin this temperature range the presence of these reactions wasstill evident. Between 100 to 200 °C, little conversion ofrelatively reactive compounds such as alky-aryl ethers and aryl-ether in water even in the presence of H2 was observed.

36 A lowconversion of the most reactive oxygenates such as alcoholsunder mild conditions may be anticipated as well. Even themost probable reaction such as dehydration would be inhibitedconsiderably because of the excess of water in the systemfavoring the shift of the equilibrium in reaction {10} to the leftwhere R″ represents corresponding olefin. Of course, the shiftof this equilibrium to the right would be maintained by rapidlyremoving R″ from the system via HYD, that is, in the presenceof catalyst with a high HYD activity (reaction {11}).In the study of Liu et al.26 residual feeds were converted in

the aqueous phase (sub- and supercritical) in an autoclaveunder typical cracking conditions, that is, absence of H2. It wasproposed that thermal conversion of asphaltenes proceeded viaradical mechanism rather than via an ionic hydrolysismechanism. In the presence of water, the formation ofunwanted coke was significantly suppressed. It is believedthat water molecules behaved as free radicals scavenger andhydrogen donor involving reactions {5} to {8}, thus slowingdown reaction {9}. In the case of a heavy feed, the radical (R•)may involve complex structures comprising aromatic andaliphatic entities as well as heteroatoms. For example, accordingto the model of asphaltenes shown in Figure 3,50,61 the −C−S−C− entity (site 1) in model A represents the weakest bonds inthe molecule which after rupture yield two large free radicals.Such radicals must be stabilized (e.g., via reaction {5} to {8})before being converted to coke (reaction {9}).An optimal temperature and pressure, ensuring the highest

level of upgrading in SCW (determined by high yield of liquidproducts and a low yield of coke) may be identified. Suchtemperature depends on the origin of feed. For example, for avacuum residue, Cheng et al.62 reported maximum of the yieldof liquids and the lowest coke at 420 °C while for coal derivedasphaltenes Han et al.52 observed an optimal temperature of460 °C. In the latter study, the yield of maltenes in SCW wassignificantly greater compared with the experiment conductedin N2. The H/C ratio of the former was higher as well. In thestudy conducted by Zhao et al.,63 a VR was upgraded in SCW

Figure 5. Tentative reactions during hydroprocessing in aqueousphase.

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as indicated by decrease in the content of asphaltenes, resins,and aromatics and a significant increase in the content ofsaturates. The experiments were carried out in an autoclavefrom 380 to 460 °C at 25.0 MPa. In addition, a significantreduction in viscosity of products, as well as the content ofsulfur, nitrogen, and metals was observed.Bitumen upgrading in SCW alone was compared with that in

the presence of 10% HCOOH (semibatch reactor from 633 to693 K) by Sato et al.64,65 The decomposition of HCOOHyielded the mixture of SCW + H2 + CO2. In the case of themixture, the involvement of H2 was confirmed by the higher H/C ratio of the unconverted asphaltenes compared with theasphaltenes in bitumen and those obtained in SCW alone. Thecoke formation in the SCW + H2 + CO2 mixture wassuppressed as well. These observations suggest that H2successfully competed with water (reactions {5} to {8}) insuppressing coke formation (reaction {9}). In this case,reaction {12} was involved.Hydrogen donor mechanism involving saturated hydro-

carbons, particularly naphthenic structures, may also beinvolved. Thus, it has been generally observed that naphthenicstructures can readily donate hydrogen.14 In the presence ofhydrogen, this may be depicted by the hydrogen transfer cycleshown in Figure 6. The occurrence of such reactions may be

anticipated during the upgrading of distillation residues derivedfrom naphthenic crude. The recent study published byZachariah et al.66 provides a direct experimental evidence forthe occurrence of such reactions.There are contradictory reports on the role of water during

the bitumen upgrading in SCW. For example, Morimoto et al.67

observed little difference between the yield and composition ofgaseous products obtained during the treatment of bitumen(autoclave, at 420−450 °C and 20−30 MPa for up to 120 min)in SCW and nitrogen. This confirmed that only very smallamount of water was involved in upgrading. Moreover, theresidue produced in SCW had lower molecular weightdistribution, lower H/C ratio, and higher aromaticity. Inother studies, beneficial role of SCW was observed by anincreased yields of liquid products and decreased yield ofcoke.52,53,57,60−63,68,69 It should be noted that all these resultswere obtained in autoclave. This suggests that the distributionof products was changing with time.70 For example, littleparticipation of water may be anticipated during the early stagesbecause of the naphthenic structures present effectively trappedfree radicals formed (Figure 6). However, once this source ofhydrogen donors was exhausted, the involvement of water asradical scavenger appears to be plausible. Therefore, exper-imental system and conditions used for bitumen upgrading inSCW are another factor to be considered in designing theoverall mechanism under noncatalytic conditions.For heavy petroleum feeds, the involvement of noncatalytic

reactions during overall conversion was reported by Marafi etal.71 under typical HPR conditions. A higher probability forsuch and additional reactions in an aqueous environment may

be anticipated. For example, an interaction of H2O with metals(V and Ni) leading to disintegration of the porphyrin skeletonmay be the initial step of HDM during an aqueous phase HPR,as it was indicated by Kokubo et al.69

The effect of sub- and supercritical conditions on conversionof coal tar was also investigated.72,73 It was observed that at thesame temperature, liquefaction of tar (autoclave, 623 and 673K, 25−40 MPa) increased with increasing water density at thesame reaction temperature. It was proposed that under sub- andsupercritical conditions, hydrolysis was involved in theconversion of macromolecular structure of tar to lighterproducts such as phenol, biphenyl, diphenylether and diphenyl-methane.

5.2. Ionic Reactions. The involvement of an ionicmechanism as part of the overall conversion of different feedsin aqueous phase may be anticipated, although this issue hasbeen receiving little attention. Yet, it was indicated earlier that,compared with liquid water, the dissociation constant of H2Oincreased with increasing temperature from 100 °C towardsubcritical and supercritical conditions.17−19 Therefore, in anaqueous medium, the involvement of H+ and HO− ions inparallel with the free radicals as well as conventional HPRreactions, as part of the overall conversion in the presence ofcatalysts and H2, is highly probable. By adding to a heteroatom(e.g., S, O and N), H+ may enhance the rate of hydrogenolysisof the corresponding heterobonds. Autocatalysis by fatty acidproducts formed during the hydrolysis of triglycerides (soybeanoil) in subcritical water (250−300 °C) supports theinvolvement of an ionic mechanism as well.74 The H+ ionsrequired for such mechanism were generated by the partialdissociation of the fatty acids produced by triglycerideshydrolysis.The nitrogen content of biomass of an algae origin may

exceed 10 wt %. A strong tendency of N-heterorings tocombine with H+ ions has been well documented.9,12 Thissuggests that the ionic mechanism plays key role during theconversion of algae biomass to hydrocarbons and ammoniaunder sub- and supercritical water conditions without a catalystbeing present. The HPR of biocrude obtained from an algaebiomass via hydrothermal route may be affected unless most ofnitrogen ends up in aqueous phase rather than in biocrude.This issue was the focus of attention of the study published byValdez et al.75 Additional efforts may be needed to clarifymechanistic aspects of the conversion of N-heterorings underhydrothermal conditions.During the noncatalytic hydrothermal conversion of lignin in

subcritical (300−370 °C) and supercritical (390−450 °C)regions, Yong and Y. Matsumura76−78 obtained results whichsupport both radical and ionic mechanisms occurring inparallel. A rapid change in pKw on approaching supercriticalregion from subcritical region enhanced the involvement ofionic reactions. However, increased temperature required forthis change favored radicals formation as supported by theincreased yield of coke in supercritical region. The kineticnetwork for the noncatalytic conversion of guaiacol in sub- andsupercritical regions proposed by Yong and Matsumura78

considered both ionic and radical reactions. Thus, the rateconstants for the overall conversion of guaiacol obeyedArrhenius law in subcritical region, but they deviated insupercritical region unless both radical and ionic reactions wereconsidered.Under typical HPR conditions (e.g., with H2 and catalyst

being present), the involvement of H+ ions during the

Figure 6. Free radicals scavenging cycle using naphthenic hydrogen.

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elimination of H2O and NH3 from alcohols and amins, as thefinal intermediates of HDO and HDN reactions, respectively,has been well documented.9,12,13 This may be illustrated usingreactions {13} and {14} in Figure 7. In these reactions,

carbenium ion intermediate may be converted to either anolefin (e.g., RCHCH2) or isomerized to different iso-olefinsbefore it is hydrogenated to the final saturated hydrocarbons.Rates of the H2O and NH3 elimination reactions may be furtherenhanced over acidic catalysts, which can be the additionalsource of protons.2

Montgomery et al.79 reported that a disintegration ofasphaltenes in water to light fractions began already at 250°C and ∼4 MPa total pressure. This was significantly enhancedby increasing temperature and pressure to 350 °C and ∼11MPa, respectively. These observations are consistent with theparticipation of H+ and HO− ions in the overall conversion.Thus, thermal effects could not account for all reactionsobserved.The results published by Li and Egiebor80 may be interpreted

in terms of ionic reactions being present during the extractionof oxygen from coal in SCW (360 to 400 °C) and their absenceduring the extraction in supercritical toluene. Thus, asignificantly greater amount of oxygen removed under theformer conditions can be attributed to the enhanced hydro-genolysis aided by H+ ions. The involvement of H+ ions duringthe dehydration of alcohols is indicated by reaction {12}. Theobservations made during the HDO of phenol by Massoth etal.81 could be attributed to the participation of H+ during theoverall HDO as well. Thus, even the hydrogenolysis of phenolto benzene could be aided by H+ ions.The promotional effect of acetic acid on the aqueous phase

HDO of p-cresol over Ru/C (300 °C; 4.8 MPa) observed byWan et al.82 could only be attributed to the participation of H+

ions. Methyl-cyclohexanol was the main product in the absenceof acetic acid. The appearance of methyl-cyclohexane as themain product in the presence of acetic acid can be attributed tothe increased rate of dehydration of methyl-cyclohexanol(reaction {12}) caused by the presence of H+ ions. It should

be noted that the content of acetic acid in some bio-oils frompyrolysis of biomass may exceed 10 wt %.12,13 This suggeststhat ionic reactions play an important role during the HPR ofthese feeds in aqueous phase.The HO− ions formed via partial dissociation of H2O may

also participate in some reactions. The involvement of HO−

ions in the overall conversion of anisole to phenol andmethanol was proposed by Wu et al.83 For heavy feeds, HO−

ions may interact with Me-C bonds present in porphyrins,where Me = Ni and/or V metals.50 In the absence of anyexperimental data on the ionic mechanism, only a speculativeargument on the involvement oh HO− ions may beforwarded.57

In the presence of solid catalysts, HO− ions may behave asLewis bases suggesting that they may adsorb on Lewis acids ofsolid supports. For the (-Al2O3 support, this may be thebeginning of a gradual transformation toward boehmite. Someinteraction of HO− ions with carbon supports may beanticipated as well. It is believed that such an interaction willincrease with the increasing irregularities of carbon surface.

5.3. Catalytic Reactions. The above discussions indicateda distinct difference between the mechanisms in an aqueousphase involving high oxygen content feeds (biofeeds, FTSaqueous phase, etc.) and that involving distillation residuesderived from petroleum, although some common reactions maybe evident. This is illustrated on several examples fromliterature which are based on experimental observations. Inthis regard, a significant difference in the structure of thesefeeds and their miscibility/solubility in water may play animportant role. Of course, in the presence of catalyst, the role ofexternal H2 during the overall HPR mechanism can bedominant. In this case, hydrogen activation, that is, theconversion of dihydrogen to an active surface hydrogen withthe aid of catalyst, must precede HPR reactions. The hydrogengenerated in situ must be activated as well. The tentativescheme shown in Figure 8 accounts for the potential sources ofhydrogen. In an aqueous phase under HPR conditions, sources1 and 2 represent steam reforming of hydrocarbons and WGS(i.e., reactions {1} and {2} in Figure 1, respectively). Potentialof H2O as free radicals scavenger in noncatalytic reactions(reactions {5} to {8} in Figure 5) was indicated earlier. Theoverwhelming evidence for the presence of reforming and WGSreactions was clearly confirmed in several studies.84−93

Apparently, active surface hydrogen formed on catalystsurface according to Figure 8 can be abstracted by freeradicals.94 Consequently, the involvement of noncatalyticreactions (e.g., {5} to {8} and {11}) in radicals stabilizationis diminished. At the same time, radical stabilization viahydrogen transfer cycle in Figure 6 may be sustained in thepresence of active hydrogen. It is believed that, in the presenceof HPR catalysts, the reactions shown in Figures 6 and 8 may

Figure 7. Dehydration {13} and ammonia elimination {14} with aid ofH+ ions.

Figure 8. Active hydrogen sources during hydroprocessing in aqueous phase.

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play much more important role in the overall mechanism ofHPR of petroleum feeds than that of a high oxygen contentfeeds.5.3.1. HPR Mechanism for High Oxygen Feeds. For high

oxygen content feeds (e.g., biofeeds and aqueous phase fromFTS), the dehydration of alcohols (ROH) to alkenes (R″) maybe the rate determining step during the overall HDO, althougha direct hydrogenolysis (e.g., phenol to benzene) may beinvolved. The corresponding dehydration equilibrium reaction(reaction {10} in Figure 5) is affected by the excess of water inthe system which tends to shift the equilibrium from alkenes toalcohol. An efficient removal of oxygen from the system can stillbe accomplished providing that a rapid removal of alkenes (R″)from the system can be ensured. This may be achieved by arapid HYD of alkenes to alkanes (reaction {11}. Therefore,catalysts with a high HYD activity and at the same time stablein aqueous environment may be suitable for these applications.The HDO mechanism applicable to conventional HPRconditions was discussed extensively elsewhere.2,12,13

The rate of dehydration of alcohols, as the final stage of theHDO of many oxygenates, may be influenced by reactionconditions. For example, over Pd/C catalyst at ∼200 °C,phenol was converted predominantly to cyclohexanol whilehydrogenolysis to benzene was not observed.34,35 However,after acidifying the solution, cyclohexanol was quantitativelydehydrated to cyclohexene followed by HYD to cyclohexane.95

This confirmed that hydronium ions aided dehydration ofcyclohexanol (e.g., reaction {13} in Figure 7). At the same time,noble metals facilitated the HYD function. This concept ofbifunctional catalysis was confirmed using more complexreactants (e.g., guaiacols and syringols).Zhang et al.96 reported results on the HDO of phenol over

Ni catalysts supported on HZSM-5 (Si/Al = 38 and 50) and on(-Al2O3 (autoclave; 160−240 °C; 2.0 g phenol; 1.5 g catalyst;40 mL water; 4.0 MPa of H2). For all three catalysts, thehydrogenolysis to benzene and the HYD to cyclohexanol werethe main routes. Dehydration of the latter to cyclohexene wasenhanced over the catalysts supported on HZSM-5 comparedwith that on Al2O3. Moreover, over the former catalysts, smallamounts of methyl-cyclopentane were observed. This clearlyconfirmed the involvement of H+ in dehydration of alcohols toalkenes (reaction {13}).2 Isomerization of the carbocation isanother potential reaction to occur as indicated by the presenceof methyl-cyclopentane among the products over Ni/HZSM-5catalysts. For Ni/Al2O3 catalyst, an insufficient strength ofBronsted sites may be the reason for the absence of isomerizedproduct.The study published by Peng et al.29 contributes to the

fundamental understanding of the HPR of oxygenates in anaqueous environment under mild conditions. The reactantssuch as 1-propanol, 2-propanol, 1,2-propanediol, 1,3-propane-diol, and glycerol (10 wt % in water) were studied in a batchreactor at 473 K, 4 MPa of H2, and 0.3 g of Pt(3 wt %)/Al2O3.Under these conditions, the direct cleavage of C−C and C−Obonds was not observed. For 2-propanol and 1,2-propanol, thedeHYD to ketone was the main reaction while for 1,3-propanoland glycerol the C−O bond was cleaved by dehydration. Foralcohols with the terminal hydroxyl group, the C−C bondcleavage occurred in steps via deHYD to aldehyde followed byeither disproportionation and subsequent decarboxylation ordecarbonylation. Reactivity of the alcohols increased withincreasing number of hydroxyl groups. Thus, over Pt/Al2O3,the following overall reactivity order was established: glycerol ∼

1,3-propanol > 1,2-propanol > 1-propanol ∼ 2-propanol.However, a significant effect of catalyst type on the HDOmechanism was indicated in the study conducted by Chen etal.30

Diphenyl ether, 2-phenylethyl phenyl ether and benzylphenylether, as representatives of lignin, were studied over Ni/SiO2catalyst at 120 °C and 0.6 MPa of H2 in an autoclave in anexcess of water.36 For diphenyl ether, the initial reactions(Figure 9) were dominated by HYD (65% cyclohexyl phenyl

ether), followed by hydrolysis (25% cyclohexanol) andhydrogenolysis (10% benzene). With progress of reaction, theyield of cyclohexyl phenyl ether reached a maximum and thendecline to zero while that of cyclohexanol and benzeneincreased. The importance of hydrolysis was confirmed bymuch higher yield of cyclohexanol compared with that ofbenzene. It is postulated that hydrolysis was result of theelectron deficiency on carbon of the C−O bond which wasoffset by the interaction with unpaired electrons on oxygen ofH2O molecule. In this case, catalyst surface could facilitate a flatadsorption of diphenyl ether which improved the access of H2Omolecules to the C−O bond. The HYD of phenyl ring, as theinitial step was only minor reaction. Initially, hydrogenolysiswas a dominant route for the other two reactants. Under thesame conditions, no reactions took place over SiO2, thusconfirming the role of Ni in the overall conversion.The study of Duan and Savage86 showed how the

observations made during the HPR of a real feed in aqueousphase can be explained in terms of mechanistic aspectsdiscussed above. In this case, a microalgae paste and a seriesof catalysts were mixed with deionized water in batch reactorand tested under subcritical conditions (∼350 °C). The reactorwas pressurized with either He or 3.5 MPa of H2. In theabsence of catalyst, the external H2 had beneficial effect on theyield of bio-oil. However, except for Pt/C catalyst, the yield ofbio-oil in H2 over other catalysts (e.g., Pd/C, Ru/C, CoMo/Al2O3, Ni/SiO2−Al2O3, and zeolite) was lower than that in theinert environment. The analysis of gaseous products confirmedCO2 to be dominant product. This indicated the involvementof WGS reaction. Thus, it was shown that decarbonylationgenerating CO was an important route during decompositionof the algae biomass.12 Therefore, in the presence of a catalyst,

Figure 9. Mechanism of conversion of diphenyl ether in water (Ni/SiO2; 120 °C; 0.6 MPa).

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the reaction pathways shown in Figure 8 may be part of theoverall HPR mechanism even under subcritical conditions.5.3.2. HPR Mechanism for Petroleum Residues. In the case

of residual feeds, the temperature of at least 400 °C is requiredto achieve desirable conversion. This results from the presenceof high molecular weight components (e.g., resins, asphaltenes,and porphyrins) in the feed. Such temperatures dictate that formost part, the HPR in aqueous phase must be carried out undersupercritical conditions. This suggests that thermal cracking oforganic bonds, leading to the formation of free radicals is animportant part of the overall mechanism. Several weak bonds aspotential cracking sites can be identified in the models ofasphaltenes shown in Figure 3. The reactions occurring underthe conditions of conventional HPR are also occurring duringthe HPR in an aqueous phase. The extent of the HPR in thelatter phase depends on the origin of residues and type ofcatalyst. Generally, catalysts comprising noble metals (Pt, Pd,Ru, Rh, etc.) exhibit a high HYD activity. This facilitates a highrate of the HYD of heterorings accompanied by the change ofthe CAR-S(N,O) bonds to corresponding CAL-S(N,O) bonds.Consequently, strength of the latter bonds is significantlydecreased. The stabilization of free radicals involving the cyclein Figure 6 is expected to be much more important for residualfeeds than that for a high oxygen content feeds because of amuch higher content of naphthenic structures in the formerfeeds.14−16 The involvement of ionic reactions, particularly overan acidic catalyst should be anticipated as well.Model compounds used to study the mechanism of HPR

reactions under conditions of conventional HPR have been alsoevaluated under aqueous conditions. This may simulatesecondary upgrading of reactants produced initially during theconversion of heavy components (e.g., asphaltenes). Forexample, the conversion of DBT over CoMo/Al2O3 catalystin SCW was observed in the presence of CO suggesting thatthe required hydrogen was produced via WGS reaction. Similarobservation was made by Arai et al.53,87,88 over sulfided NiMo/Al2O3 catalyst, while little conversion was observed over thecorresponding oxidic catalysts. In fact, in CO+SCW system, thereaction rate was higher than in H2+SCW suggesting that thehydrogen which originated from the source 1 and 2 (Figure 8)was more reactive than from the source 3. In the studyconducted by Ng and Milad89 on the HDS of BT, the hydrogenproduced via WGS reaction was about seven times morereactive than the external H2. The transfer of hydrogen fromSCW to reactants was also confirmed using D2O.

90 No WGSreaction was observed without catalyst. Even in the CO2 + H2 +SCW system, the rate of reaction was higher than in H2 + SCW.When the feed was partially oxidized in situ in SCW to generateCO, the conversion of several reactants (e.g., DBT, carbazole,quinolin and naphthalene) involving the HYD route was higherthan in H2 + SCW.83−89,91−93 The involvement of H+ ionsduring the HPR of residues in SCW, as part of ionic reactions(e.g., reactions {13} and {14}) was anticipated above. Underthe conditions of conventional HPR, the final stages ofHDO12,13 and HDN,9−11 that is, elimination of oxygen andnitrogen from the last intermediates, respectively wereinterpreted in terms of a proton transfer from catalyst to theintermediate. Such reactions are more likely to take place inSCW than in a liquid water.Heavy Arabian heavy crude containing 3 wt % sulfur was

treated in an autoclave (without external H2) in SCW.97 Underthese conditions, only about 7% of sulfur in the feed wereremoved in the absence of catalyst. It is believed that most of

this sulfur was removed thermally. In the presence of MoS2 theremoval of sulfur approached 12%. Again, thermal reactionsaccounted for most of the sulfur removed. Rather low sulfurremoval can be attributed to the absence of reforming andWGS reactions, which are the source of active hydrogen, as itwas demonstrated by Ng and Milad89 who carried out similarexperiments in the presence of CO.In the absence of any experimental data, only a speculative

mechanism of hydrodemetallization (HDM) in SCW may beproposed. A high reactivity of H2O molecules in SCW wouldfavor a direct interaction with the porphyrinic form of V and Nimetals in the residue. Consequently, the rate of disintegrationof porphyrin skeleton as the final stage of HDM, would beenhanced. The presence of V in a vanadyl form (O = V)suggests a more direct interaction with Ni than that with the V= O group. The HO− ions can interact with the metals as well.However, this would have to be confirmed by experimental dataon the HDM of model porphyrin compounds obtained in SCWover a catalyst.Based on the results obtained during the pyrolysis of bitumen

in SCW (at 723 K) in the presence of cubic (8 nm) andoctahedral (50 nm) CeO2 nanoparticles, Dejhosseini et al.98

observed a higher activity of the former than that of hexagonalCeO2. This was attributed to a much higher oxygen storagecapacity of the cubic CeO2. It is speculated that the HO− ionsgenerated from H2O were involved in red-ox reactionsproducing surface hydrogen and carbon oxide.

5.3.3. HPR Mechanism for Other Feeds. The HPR in anaqueous phase involving other feeds (e.g., coal derived liquids,oil shale liquids, etc.) has been receiving little attention.However, the database of experimental results established forbiofeeds and petroleum feeds may serve as a basis for proposingthe mechanism of the HPR of other feeds. Thus, after a detailedcharacterization of any feed and corresponding products, a setof reactions, particularly those involving H2O may be proposed.Obviously, experimental results are always needed tostrengthen arguments one way or the other.


Attempts have been made to develop catalysts with desirableactivity and selectivity as well as a high stability in the presenceof large quantities of water. In this regard, mild, subcritical andsupercritical conditions have been receiving attention. Appa-rently, the conventional HPR catalysts consisting of theCo(Ni)-Mo(W)-S active phases supported on γ-Al2O3 maynot be suitable for a direct use unless they are modified toenhance stability.6−9,11−16,94 As it was indicated above, both thesupports and active metals have to be carefully selected forcatalyst design for the applications under aqueous phaseconditions. In this regard, besides conventional materials,combinations of various nonconventional metals and supportshave been explored. However, a limited number of articlespublished in scientific literature indicates that the developmentof HPR catalysts for applications in aqueous phase is still in anearly stage and may be rather challenging.The adverse effects of H2O on performance of the

conventional catalysts were discussed in detail elsewhere.12,13

It is believed that in the presence of large amounts of H2O as inthe case of sub- and supercritical conditions employed duringthe HPR in an aqueous phase, a detrimental effect of H2Owould be much more evident than that under the conditions ofconventional HPR. The requirement of a high HYD activity to

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minimize catalyst deactivation during the HPR in aqueousphase was indicated above. It appears that for theseapplications, the catalysts comprising noble metals (e.g., Pt,Pd, Ru, and Rh) supported on various supports may be moresuitable than conventional HPR catalysts. The summary ofstudies on catalyst development is given in Table 4.6.1. Methods for Catalysts Testing. There is little

difference in the methodology for catalysts preparation for theapplications under aqueous conditions and those used for theconventional HPR. An extensive information on preparation ofthe latter catalysts can be found in the scientific literature.2,6−12

The validity of experimental data on catalyst performance isinfluenced by the testing protocol employed.99,100 The catalystevaluations in both batch and continuous systems have beengenerally observed. Different sizes of batch reactors, starting

with micro autoclave up to several liters volume size systems,have been available for testing. They are useful screening tools,however catalyst activity may be affected by some products(e.g., NH3, H2S, etc.), which accumulate in the system ratherthan being carried out with reaction streams as it is in the caseof continuous systems. Moreover, some inherent limitations ofbatch systems (i.e., an unsteady-state operation, continuouslyvarying catalyst/oil ratio, a lengthy heat-up periods betweenpreheat and reaction, etc.) should not be overlooked. On theother hand, in batch systems, the mixture/slurry (water +catalyst + feed) for testing in an aqueous phase can be readilyprepared and tested.High-throughput techniques have been used for rapidly

prescreening a large number of catalysts in batch reactors.However, to obtain a more comprehensive information, more

Table 4. Summary of Catalysts Tested in Aqueous Phase under Mild, Subcritical, and Supercritical Conditions

catalyst feed conditions ref.

Mild ConditionsPd/C, RANEY Ni, Ni/SiO2, Ni/ASA, Nafion suspensions, Nafion/SiO2,zeolites

propyl-phenol batch.; 200−300 °C; 4 MPa 35

Pt/SiO2−Al2O3, Pd/SiO2−Al2O3 sorbitol cont.; 225 and 265 °C; co-fed H2 39Ni-MgO sorbitol batch; 200 °C; 4 MPa; H2 119Ru/C, Ru/Al2O3, Pt/Al2O3, Pt/C, Pd/Al2O3, Pd/C, sulfided CoMo/Al2O3,NiMo/Al2O3, and NiW/Al2O3

acetic acid, p-cresol batch; 150 and 300 °C; 4.8 MPaof H2

82, 110

Ru/C and Pd/C furfural, guaiacol, acetic acid batch; 150−300 °C; 6.9 MPatotal


Ru/ZrO2 and RuMo/ZrO2 propanoic acid cont.; 150−230 °C; 6.4 MPatotal


Pt (3−5 nm) protected by polyethyleneimine glucose and fructose batch; 130−279 °C; 6−11 MPatotal


Pt on HY, H$, HZSM-5, (-Al2O3 and SiO2 phenol batch.; 200 and 250 °C; 4 MPaH2


Pd/C phenol batch; 180 and 200 °C 33, 34Pt on AC, MWCN and CB; Pt on ZrO2, TiO2, and CeO2 4-propylphenol 280 °C; 4 MPa; H2 112Ru(5%)/H-β lignin batch; 200−300 °C; 4−12 MPa 114Pt(1%)/H-β zeolite cellulose batch; 150−190 °C 115Ni−W/SiO2 cellulose batch; 245 °C; 6 MPa; H2 120Pt/ZrO2 with H4SiW12O40, H3PW12O40, H3PMo12O40 glycerol cont.; 160−240 °C; flow of H2 116Ni/SiO2 aryl-ethers batch; 160 °C; 0.6 MPa; H2 36Ni/HZM-5, Ni/Al2O3 phenol batch; 16−240 °C; 4 MPa; H2 101Cu/ZrO2 varying Cu/Zr ratio glycerol cont.; 200 °C; 4 MPa; H2 113Ru + transition metal (e.g., Zn, Cr, Mn, Co, Fe, and Ni) benzene batch; 150 °C; 5 MPa 118

Subcritical ConditionsPd/C, Pt/C, Ru/C, Ni/SiO2−Al2O3, zeolite, sulf. CoMo/Al2O3 microalga batch; 350 °C; 3.5 MPa H2 86sulf. NiMo/Al2O3 pyrolysis oil batch; 350 °C 24Pt(5%)/C fatty acids (stearic, palmitic, lauric,

oleic, and linoleic)batch; 330 °C 122

Pt, Pd and Ni all supported on carbon jathropa biofeed batch reactor; 350 °C; 0.5−5 h;without H2


sulf. Pt/C hydrothermal liquef. biofeed batch; 330−370 °C; 4 h; eitherCO or H2


Supercritical ConditionsPt(5%)/C benzofuran batch; 380 °C; 6−29 MPa 125Pt/C, Pd/C, Ru/C, Rh/C, Pt/Al2O3, Mo2C, MoS2, PtO2, Al2O3,sulf. CoMo/Al2O3

pyridine batch; 380−420 °C; 0−6.9 MPaH2


Pt/C and Pd/C palmitic acid batch; 380 °C; 28 MPa; no H2 127,128

Pd(5%)/C, Pt(5%)/C microalgae, algae bio-oil batch; 400 °C; 3.4 MPa; H2 127,128

NiMo/Al2O3, CoMo/Al2O3 DBT batch; 400 °C; 30 MPa total 53, 87,88

Pt/C, Pd/C, Ru/C, Rh/C, Pt/Al2O3, Mo2C, MoS2, PtO2, Al2O3, CoMo/Al2O3, carbon

algae biomass batch; 380−420 °C; 6.9 MPa; H2 128

NiMo/Al2O3 DBT, naphthalene, tetraline, carbazole batch, 400 °C; 2.5−4.0 MPa; H2 92, 93cubic and octahedral CeO2 nanoparticles bitumen batch; 723 K 93

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detailed studies may be required on selected catalysts. Lee etal.101 used this method for determining the initial activity of thesupported monometallic catalysts such as Pd, Pt, Ru, Rh, Ni,and Co for the HYD of various carbonyl group containingreactants in an aqueous phase. They used reactor whichconsisted of 24 wells machined into a cylindrical stainless steelhigh pressure chamber. Using this system, the database wasgenerated much faster than in a typical batch reactor.Patwardhan et al.59 described the operation of a continuous

stirring tank reactor (CSTR) rated for more than 30 MPa andup to 650 °C during the conversion of S-containing reactants inSCW. In this case, water and model reactants were fedseparately to the top, without premixing. It might be the onlystudy, in which CSTR system was used to study conversion inSCW.Figure 10 shows the continuous system,102 which may be

adapted for catalyst testing in the presence of water. The list ofmajor components and modes of operation are evident fromthe schematics. The continuous system is one of the few foundin literature, being used for catalyst testing under SCWconditions. Of a particular importance is the separate unitrequired for the generation of SCW and subsequent mixingwith reaction streams. Absence of the source of external H2 inthe schematic of continuous system should be noted. However,because of the catalytic upgrading of an algae feed beingstudied,102 hydrogen required for HPR reactions was generatedin situ via WGS reaction involving CO produced duringdecarbonylation of the feed.Obviously, for the HPR of liquid feeds (e.g., biofeeds, FTS

reaction water, etc.) in an aqueous phase, the continuoussystem in Figure 10102 requires modifications. First of all, thepreheater−reactor unit would have to be replaced by a catalyticreactor. It is believed that such a change could be made withoutany difficulties. For example, in the study conducted by Zohreret al.,103 biofeed (glycerol) was injected directly into SCWentering catalytic reactor. Little problems during the feedinginto continuous reactors are anticipated for high water contentliquid feeds such as biofeeds and reaction water from FTS

contrary to that for heavy petroleum feeds. For the latter feeds,a special feeding system would have to be designed.Nevertheless, it is believed that a special continuous systemneeds to be designed to bring catalyst testing for theapplications in an aqueous phase to the next level.

6.2. Selection of Supports. The choice of support forpreparation of the catalysts to be used in aqueous media iscrucial because of a potential reaction with water. Thus, in thecase of conventional catalysts, the most frequently used γ-Al2O3tends to undergo the H2O aided transformation toboehmite,29,104−106 as it is shown by reaction {15} in Figure 11.

Ravenelle et al.107 observed that the conversion of γ-Al2O3 toboehmite was complete within 10 h at 200 °C. However, forthe Ni/γ-Al2O3 and Pt/γ-Al2O3, the transformation wassignificantly slowed down. On the other hand, α-Al2O3exhibited a high stability at 350 °C during dehydration of the1:1 mixture of heavy alcohols in water during more than 30days.3 Under similar conditions, SiO2−Al2O3 exhibited a highstability as part of the Ni/SiO2−Al2O3 catalyst used for thepartial HYD of the aqueous phase obtained during FTS.3 It isbelieved that other metal oxides (e.g., zeolites, TiO2, ZrO2,mixed oxides, etc.) may be more suitable supports thantraditionally used γ-Al2O3.It should be emphasized that for the catalysts to be used for

the aqueous phase HPR of biofeeds and reaction water fromFTS, a proper acidity of supports may be required to maintain ahigh rate of alcohol dehydration as the last step of HDO(reaction {13} in Figure 7). In this case, some zeolites mayexhibit a desirable acidity. Therefore, the interests in zeolites as

Figure 10. Continuous reactor system for hydrothermal conversion of biomass. Reprinted with permission from ref 102. Copyright 2010, AmericanChemical Society.

Figure 11. Potential reactions of H2O with γ-Al2O3 and carbon.

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the supports for catalysts have been steadily growing. Ravenelleet al.108 investigated the hydrothermal stability of zeolites Y andZSM-5 with the varying Si/Al ratios in liquid water at 150 and200 °C. Under these conditions, ZSM-5 zeolite was notmodified. However, the zeolite Y with the Si/Al ratio of 14 orhigher was transformed into an amorphous solid. The zeolitedegradation was caused by hydrolysis of Si−O−Si bonds ratherthan by dealumination. This resulted in the loss of microporevolume and that of accessible acidic sites.The stability of several ZrO2 and TiO2 samples in SCW was

evaluated by Zohrer et al.103 with the aim to select the moststable support for Ru catalysts. The stability of supports andcorresponding Ru (2 wt %) catalysts were tested in anautoclave at 400 °C, 28.5 MPa, for 20 h. The Ru/ZrO2 catalystsupported on tetragonal ZrO2 exhibited the highest activity andstability during the conversion of glycerol in a continuoussystem operating under SCW conditions.Carbons are neutral and hydrophobic supports; therefore, in

an aqueous medium, they exhibit high stability. Various formsof carbons (i.e., activated carbons, carbon blacks, carboncomposites, carbon nanotubes, etc.) may be suitable support.20

For reactive carbon solids, the potential reaction of carbon withH2O (reaction {16}) becomes evident at above 700 °C. A lowreactivity for this reaction is ensured by a high severity used forcarbon supports preparation.20,109 For example, activatedcarbon is prepared by steaming and/or partial oxidation(diluted air) of various carbonaceous solids at about 850 °C.Oxygen containing entities (carbon centered peroxides andperoxy radicals, etheric groups, hydroxyl groups, etc.) on thesurface of activated carbons left behind may play an importantrole during the impregnation with active metals. Therefore, adesirable stability of the carbon supported catalysts in anaqueous environment is anticipated, although some exper-imental data obtained under subcritical and particularly undersupercritical conditions are still needed.6.3. Catalyst Testing. As Table 4 shows, the catalysts

testing was conducted under mild conditions (less than 300°C) as well as in subcritical (300−370 °C) water and SCW(above 374 °C) using both the batch and continuous systems.Most of the testing has been carried out under mild conditions.Studies were dominated by model feeds while to a lesser extentreal feeds were used as well.6.3.1. Mild Conditions. The catalyst development for HPR

in aqueous media under mild conditions (less than 300 °C) hasbeen focusing on two groups of metals, that is, noble metals(e.g., Pt, Pd, Ru, and Rh) and other metals (Ni, Co, Cu, etc). Awide range of supports were tested; however, carbons were thesupports of choice. γ-Al2O3 has also been used, although somestability problems during a long-term performance of catalystsmay be anticipated, as it was indicated above. Noble Metals Containing Catalysts. As the most

abundant product in biomass pyrolysis liquids, acetic acid aloneand in the mixture with p-cresol was investigated at 150 and300 °C over a series of catalysts (i.e., Ru/C, Ru/Al2O3, Pt/Al2O3, Pt/C, Pd/Al2O3 and Pd/C).82 The experiments wereconducted in a batch reactor at 4.8 MPa of H2. Forexperiments, 0.05 mol of substrate and 0.2 g of catalyst wereadded to 40 mL of water. With respect to the conversion ofacetic acid, the following activity order was established at 300°C: Ru/C > Ru/Al2O3 > Pt/C > Pt/Al2O3 > Pd/Al2O3 > Pd/C.However, ethane as the minor product was formed only overPt/Al2O3 and trace amounts of ethane over Ru/C and Pt/Ccatalysts. The Ru/C was the only catalyst used for the HDO of

p-cresol and the mixture of p-cresol with acetic acid. Theinhibiting effect of p-cresol on conversion of acetic acid and abeneficial effect of acetic acid on the HDO of p-cresol wasdiscussed earlier.82 Thus, the beneficial effect resulted from thecontribution of ionic mechanism to the overall conversion.The conventional sulfided CoMo/Al2O3, NiMo/Al2O3, and

NiW/Al2O3 catalysts were compared with the Pt, Pd, and Rucatalysts supported either on carbon or on γ-Al2O3 by Wan etal.110 under identical conditions as above82 at 300 °C. Underthese conditions, conventional catalysts were inactive. Thehighest activity for hydrocarbons formation was exhibited bythe Pt catalysts.Elliott and Hart111 used the mixture containing 200 g water

and 5 wt % of substrate (furfural, guaiacol, and acetic acid each)in a batch reactor to study reactivity of the substrates between150 to 300 °C and 6.9 total pressure of H2 over Ru/C and Pd/C. With respect to HYD, the former catalyst was more activethan Pd/C catalyst; however, above 250 °C, reforming andmethanation reactions (i.e., reactions {1}, {2} and{4} in Figure1) became quite evident over Ru/C catalyst. A lower activity ofPd/C could be increased by increasing temperature. At thesame time, the production of methane and CO2 was kept at aminimum.The aqueous phase HDO of propanoic acid was used by

Chen et al.30 to compare Ru/ZrO2 and RuMo/ZrO2 catalysts ina trickle-bed reactor after the catalysts (6 g of 20−40 mesh)were activated in situ in the flow of H2 at 300 EC for 3 h. In thetemperature range 150−230 °C, the total pressure of 6.4 MPaensured an aqueous-phase system of 0.83 mol/L of thereactant. For the RuMo/ZrO2 catalyst, the effect of Mo/Ruratio (0−1.5) on the activity and selectivity was investigated.The catalyst with the Mo/Ru ratio of 0.2 exhibited the highestactivity for the conversion of propanoic acid. The activitydecreased with further increase in the Mo/Ru ratio. Theproducts included propanol, methane, ethane, propane, andtrace amounts of ethanol and acetic acid. Over Ru/ZrO2catalyst, C−C bond cleavage to methane and ethane wasdominant reaction compared with the HYD of CO bond,while the RuMo/ZrO2 catalyst favored the HYD of CObond in propanoic acid.The Pt nanoparticles (3 to 5 nm) protected by

polyethyleneimine (Pt-PEI) were used as catalyst for theconversion of glucose and fructose at 403−543 K in subcriticalwater and H2 in a batch reactor.31 For the experiments, H2 wasintroduced into reactor at ambient temperature until thepressure reached 5.0 MPa. In the temperature range used, thetotal pressure increased from 6 to 11 MPa. Typically, 0.48 g ofglucose, 1 g of aqueous dispersion Pt-PEI (∼ 5 mg Pt) and 60 gof ion-exchanged water, were used. Compared with inertatmosphere (Ar), conversion was significantly enhanced underH2. At 403 K, glucose could be readily isomerized to fructose.In the temperature range 483−543 K, glucose produced 1,2-propanediol, 1,2-hexanediol, and ethylene glycol, while fructoseyielded 1,2-propanediol, 1,2-hexanediol, and glycerol. Othercatalysts tested included the Pt protected by polyvinylpyrroli-done, Pt/SiO2, and Pt/Al2O3; however, Pt-PEI exhibitedsuperior activity.Hong et al.32 studied the effect of support on the activity of

Pt (1 wt %) catalysts during the conversion of phenol. In thiscase, zeolites such as HY, H$, and HZSM-5 as well as γ-Al2O3and SiO2, were compared (473 and 523 K; H2 pressure of 4MPa; WHSV of 20 h−1; 10 wt % H2O). For all catalysts, theoverall phenol conversion reached almost 100%. However,

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significant difference in product distribution, was observed.Thus, cyclohexane accounted for more than 90% of all productsover Pt/zeolite catalysts compared with less than 3 wt % overPt/Al2O3 and Pt/SiO2 catalysts. For the latter catalysts,cyclohexanol accounted for almost 95% of the convertedphenol. This may be attributed to a lack of surface acidity whichwould enhance dehydration of cyclohexanol (e.g., via reaction{10} and {13}). Small amounts of bicyclics and tricyclicproducts were observed over the Pt/zeolite catalyst.During the HDO of phenol in aqueous medium (below 453

K, Pd/C catalyst, batch reactor), Zhao et al.33,34 observed anincreased yield of cyclohexcanol and decreased yield cyclo-hexanone with time on stream. This suggests that the latter wasan intermediate for the formation of cyclohexenol. At 453 K,small amount of cyclohexane was formed if the solution wasacidified with H3PO4. However, temperature increase to 473 Kresulted in a complete dehydration of cyclohexanol tocyclohexene (reaction {10}) followed by the HYD of latter(reaction {11) to cyclohexane. There was little evidence of thehydrogenolysis of phenol to benzene. Similarly, a highselectivity to cyclohexanol in a neutral aqueous solution wasreported over Pt-, Ru-, and Rh-based catalysts.74 However, withtemperature increase to 473 K of the acidified solution,cyclohexanol was quantitatively dehydrated to cyclohexenefollowed by HYD to cyclohexane. Similarly, 4-n-propylguaiacol,4-allylguaiacol, and 4-acetonylguaiacol were converted tocycloalkanes (∼80%), methanol (7−8%) 12−18% intermediatecycloalcohols or cycloketones (12−18%). In this case, thereaction mixture comprised 5 wt % Pd/C (0.040 g), reactant(0.0106 mol), 0.5 wt % H3PO4 in 80 mL of water.Ohta et al.109 prepared series of Pt catalysts (2 wt % Pt)

supported on activated carbon (Norit and Wako), mesoporouscarbon, multiwalled carbon nanotube, and carbon black viaimpregnation with aqueous solution of H2PtCl6. The catalystswere used for the HDO of 4-propylphenol in water at 280 ECunder 4 MPa H2. The Pt/ACN exhibited high activity with 97%yield of propylcyclohexane, similarly to the Pt catalystsupported on mesoporous carbon and carbon nanotube. ThePt supported on carbon black was less active. The Pt catalystssupported on ZrO2, TiO2, and CeO2 were moderately active,but besides propylcyclohexane, propylbenzene in 3−10% yieldwas also present. These results are consistent with the HDOmechanism observed during conventional HPR. Contrary tothese observations, Pt/Al2O3 catalyst was inactive because ofthe structural transformation of (-Al2O3 into boehmite. Theactivity of the Rh, Ru, and Pd catalysts supported on activatedcarbon was much lower than that of Pt/C.The mixture of 0.5 g lignin in 10 mL of water was used to

study conversion at 300 °C and 5 MPa of H2 in an autoclave inthe presence of the Ru(5%)/H-β catalyst.113 A dozen ofoxygenates were identified in liquid products. However, ratherlow overall conversion of lignin (less than 20%) should benoted. Patil et al.114 used Ru(5%)/H-β catalyst for theconversion of lignin to hydrocarbons in an autoclave from200 to 300 °C and total pressure of 4 to 12 MPa. Under theseconditions, less than 20% conversion of lignin was observed.After adding a basic solution (1 M NaOH) to the mixture, theconversion increased to almost 33%. This supports theinvolvement of HO− ions during the overall lignin conversion.The Pt(1%)/H-β zeolite catalyst tested by Kato and

Sekine115 exhibited high selectivity for C3 and C4 hydrocarbonsduring the conversion of cellulose in distilled water under mildconditions (423−463 K) in batch reactor. Under the same

conditions, high yields of gaseous products were formed overthe catalysts supported on H-β and H-Y zeolites with three-dimensional structure and large pores. Apparently, among thecatalysts tested, a catalyst which can maximize the yield of theproduct of interest can be selected.Pt/ZrO2 catalysts modified with heteropolyacids such as

H4SiW12O40, H3PW12O40, and H3PMo12O40 were used for thehydrogenolysis of aqueous glycerol (10% glycerol) to 1,3-propanediol.116 The unmodified Pt/ZrO2 was used forcomparison. The modified Pt/ZrO2 catalysts were more activebecause of a higher acidity. Thus, the catalyst modified withH4SiW12O40 was the most active because of suitable Brønstedacid sites. At the same time, 1,2-propanediol yield increasedwith increasing concentration of Lewis acid sites. In this study,experiments were conducted in a continuous system in the flowof H2 between 160 to 240 °C.The catalyst consisting of Ru nanoparticles having average

diameter of ∼3 nm dispersed on carbon spheres (∼500 nm)was used by Yang et al.117 for the HYD of ethyl lactate to 1, 2-propanediol in water in an autoclave (423 K; 5 MPa H2; 8 h).The catalyst exhibited a high activity and selectivity even afterbeing recycled six times.The bimetallic catalysts containing Ru and a transition metal

(e.g., Zn, Cr, Mn, Co, Fe, and Ni) were used by Sun et al.118 forthe HYD of benzene at 150 °C and 5 MPa in an autoclave.Under these conditions, transition metals were present in anoxidic form, while Ru in a reduced form. The reaction mixturecomprised 49.2 g catalyst, 280 mL H2O, and 140 mL benzene.The catalyst preparation involved adding NaOH solution to themixture of RuCl3·H2O and sulfate of the correspondingtransition metal under continuous stirring at 353 K. Theblack precipitate obtained was dispersed in distilled water andtransferred to autoclave to be treated at 423 K under 5 MPa ofH2. The isolated powder was washed and vacuum-dried beforebeing used for activity tests and extensive spectroscopicevaluations. The bimetallic catalysts such as Ru−Mn(0.23),Ru−Fe(0.47), and Ru−Zn(0.27) exhibited a high selectivity forcyclohexene. Other Metals Containing Catalysts. Series of theNi/HZSM-5 catalysts containing variable amount of Ni (e.g., 6,10, 14, and 17 wt %) and the Ni(10%)/Al2O3 catalyst werecompared during the HDO of phenol between 160 to 240 °Cand 4 MPa of H2 in an autoclave (2 g phenol, 40 mL water).101

The highest activity was exhibited by the Ni/HZSM-5 (Si/Al =38) containing 10 wt % of Ni. Benzene and cyclohexane weredominant products. Small amounts of methyl-cyclopentanewere formed as well. The Ni/SiO2 catalyst exhibited a highactivity for the HDO of aryl-ethers in water at 160 °C and 0.6MPa of H2 in an autoclave.36

Zhao et al.35 expanded their study with the aim to develop asolid acids as the source of hydroniom ions, which are stable inan aqueous medium at high temperatures. For example, theNafion polymer used as one of the solid acids was hardlyionized as confirmed by little change in pH with time onstream. To test the concept, a series of liquid and solid acids inthe presence of Pd/C and RANEY Ni catalysts were used forthe aqueous HDO of 4-propylphenol. The results aresummarized in Table 5.35 A combination of aqueous solutionsof either H3PO4 or CH3COOH with Pd/C yielded 84 and 74%of propylcyclohexane, respectively, while 98% yield ofpropylcyclohexane was obtained with both Nafion suspensionin water and the Nafion supported on SiO2 (13 wt % ofNafion). A combination of either Nafion water suspensions or

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the Nafion/SiO2 composite with freshly prepared Ni catalysts(RANEY Ni) resulted in 100% n-propylcyclohexane yieldcompared with 51 and 96% yields for commercial RANEYNi2400 and RANEY Ni4200, respectively. Compared withNafion, zeolites were poor source of hydronium ions asindicated by rather low yields of propylcyclohexane. Thecombination of RANEY Ni with Nafion/SiO2 was used to studythe HDO of 2-methoxy-4-n-propylphenol.The conversion of glycerol to propanediols has been

attracting attention as the source of the latter. In the presenceof a catalyst and H2, this may be achieved under mildconditions. This is illustrated using two studies publishedrecently. In one study, an aqueous solution of glycerol (40%glycerol) was used as the feed to produce 1,2-propanediol overa series of Cu/ZrO2 catalysts with different Cu contents.114

Experiments were carried out in batch reactor at 200 °C and 4.0MPa H2. The selectivity increased with the increasing Cu/Zrratio while the overall glycerol conversion exhibited littlechange.Chen et al.119 used an aqueous solution of sorbitol (20%) to

study its hydrogenolysis to glycols and glycerol (473 K; 4 MPaH2; batch) over a series of Ni-MgO catalysts with varying Ni/Mg ratio. In terms of conversion and selectivity, the bestperformance was exhibited by the catalyst with the Ni/Mg ratioof 3/7. However, catalyst deactivation was noted under moresevere conditions. This was indicated by the more complexmixture of products, although the sorbitol conversion increasedwith increasing severity.The bimetallic Ni/W/SiO2 catalysts containing 5 wt % Ni

and 25 wt % W was used for conversion of cellulose into lowmolecular weight polyols (sorbitol, mannitol, erythritol, ethyl-ene glycol, and 1,2-propanediol) in an aqueous solution.120 Forthe experiments, 500 mg cellulose and 50 mg catalysts werecoslurried with 30 mL deionized water in an autoclave whichwas subsequently pressurized with H2 to 6 MPa at ambienttemperature. The experiments were conducted at 518 K. Thebimetallic catalyst was much more active than either Ni/SiO2 +W or Ni/SiO2 + WO3 mixtures. The reduced form of thebimetallic catalyst was more active than the oxidic form.The nano-metallic carbides of the NiWMoC formulations

were prepared by mechanical alloying and used as catalysts forupgrading a residual feed in an autoclave at 200 °C, 3 MPa, and24 h.121 The size of catalyst particle decreased with increasing

length of milling, that is, from 126 to 10 nm from 0 to 240 hmilling period. For the experiments, 50 g of residue were mixedwith 50 g of seawater and 1 g of catalyst. A significant viscosityreduction was achieved with catalyst prepared with milling timeexceeding 200 h. This coincided with a marked decrease in thecontent of resins and asphaltenes in the feed. Rather superioractivity of the was evident considering the extent of viscosityreduction under rather mild conditions.

6.3.2. Subcritical Conditions. Fu et al.122 observed a highactivity of the Pt(5%)/C catalyst for the conversion of fattyacids (stearic, palmitic, lauric, oleic, and linoleic). Oleic andlinoleic acids had one and two double bonds, respectively, whilethe remaining acids were saturated. The experiments wereconducted under subcritical water conditions at 330 °C in anautoclave. For the unsaturated acids, the conversion involvedthe HYD of double bond first, followed by decarboxylation.The latter reaction dominated the overall conversion of theacids to saturated n-alkanes. In line with the mechanismdiscussed earlier, the HYD of double bonds involved hydrogengenerated in situ. Thus, no external H2 was present.Hayashi et al.123 studied the conversion of Jathropa biofeed

in subcritical water (batch reactor; 350 °C; 0.5−5 h; withoutH2) over the carbon supported Pt, Pd, and Ni catalysts. Theresults in Table 6 confirmed a high activity of the Pt and Ni

catalysts as indicated by low content of free fatty acids in theproduct mixture. However, while the former catalyst had a highselectivity for C15 and C17 hydrocarbons, methane was adominant product over the Ni catalyst. At the end ofexperiments, the spent Pt and Ni catalysts were isolated fromthe reaction mixture for reuse. A high activity was maintainedduring the three subsequent test cycles conducted underidentical conditions. The results support the involvement of theactive hydrogen generated in situ via reactions {1} and {2}(Figure 1), as well as the tentative mechanism in Figure 8.The pyrolysis oil containing 33 wt % of water was upgraded

in two stages. After the first stage carried out under mildconditions, partially upgraded feed was further upgraded overthe sulfided NiMo/Al2O3 catalyst at 350 °C in a batchreactor.24 Under such conditions, stability of the catalyst wasaffected due to hydrolysis of the (-Al2O3 support.Duan et al.124 studied the biofeed from hydrothernal

liquefaction of the duckweed biomass under subcriticalconditions (330−370 °C; 2 and 4 h; autoclave) in either H2or CO using the sulfided Pt/C catalyst. The yield and quality ofliquids (e.g., lower viscosity, higher hydrogen content, etc.)were better under CO than under H2. This confirmed a higherreactivity of the in situ generated hydrogen as discussed earlier(Figure 8). Rather viscous and tarry product was obtained

Table 5. Aqueous-Phase HDO of 4-n-propylphenol over Pdand Ni Based Catalysts and Acids (473 K, 4 MPa H2, and 0.5h)35

catalyst acid conv., % cycloalkane select. %

Pd/C H3PO4 100 84Pd/C CH3COOH 100 74RANEY Ni H3PO4 0RANEY Ni CH3COOH 0Pd/C zeolite (H-β) 100 1.5Pd/C zeolite (H-Y) 100 5.2Pd/C Nafion solution 100 98Pd/C Nafion/SiO2 100 98RANEY Ni Nafion/SiO2 100 99RANEY Ni Nafion solution 100 98RANEY Ni 2400 Nafion/SiO2 51 36RANEY Ni 4200 Nafion/SiO2 96 64Ni/SiO2 Nafion/SiO2 9 43Ni/ASA Nafion/SiO2 37 50

Table 6. Yields of Products (% of C) from Conversion ofFatty Acids at 350 °C (1 h) in Subcritical Water123

catalysts C1−4 C7−14,16,18 C15 C17 fatty acids

no catalyst 0 0.2 <0.1 <0.1 80.9Pt/C 1.2 1.4 5.5 40.8 7.6Pd/C 0.3 0.4 0.7 6.2 66.7NiC 54.5 2.0 0.4 1.0 0.7Pt/NiC 19.0 3.6 1.6 5.6 21.6PtC-Pa 0.2 0.3 0.4 3.8 48.3PdC-Pa 0 <0.1 <0.1 <0.1 83.3

aDuring preparation, strongly basic anion-exchange resin was usedafter treatment with aqueous NaOH.

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during the parallel experiments conducted in the absence ofcatalyst.The catalysts such as Pd/C, Pt/C, Ru/C, Ni/SiO2−Al2O3,

sulfided CoMo/Al2O3 and zeolite with the variable SiO2/Al2O3ratio were used for the HPR of a microalga Nanochloropsisfeed.86 The experiments in a batch reactor (350 °C; 60 min)were conducted with the mixture containing 0.384 g of catalyst,4.27 g of microalgae paste and 13.5 mL of deionized water. Allcatalysts produced higher yields in N2 than in H2. The higheryields under N2 can be attributed to a highly reactive hydrogengenerated in situ via WGS of CO released via decarbonylationof microalgae (i.e., reactions {1} and {2}).12 With respect tothe yield, the most active catalysts under N2 and H2 (3.5 MPa)were Pd/C and Pt/C catalysts, respectively.6.3.3. Supercritical Conditions. Dickenson et al.125 studied

the HDO of benzofuran in di-ionized water over the Pt(5%)/Ccatalyst in a batch reactor at 380 °C. The effects of the waterdensity, amount of water, and corresponding total pressure (6to 29 MPa) as well as the H2/reactant ratio on the conversionand products distribution were investigated. With respect tohydrocarbons production, the Pt/C catalyst exhibited a goodactivity and stability.Duan and Savage126 studied hydrothermal transformations of

pyridine in an aqueous medium with the aim to simulate theHPR of an algae biofeed under SCW conditions. Nitrogenremoval from the algae biofeed above critical temperature ofwater may have some advantages. For example, a large portionof ammonia produced from nitrogen compounds ends up inaqueous phase rather than in oil phase. The efficiency ofupgrading can be enhanced in the presence of catalyst. In thisregard, a series of commercially available catalysts (i.e., 5% Pt/C, 5% Pd/C, 5% Ru/C, 5% Rh/C, 5% Pt/C-sulfided, 5% Pt/Al2O3, Mo2C, MoS2, PtO2, Al2O3, CoMo/Al2O3-sulfided andactivated carbon) were evaluated for potential applications(380, 400, 420 °C, 10−150 min, catalyst loading of 50−200 wt%, H2 pressure of 0−6.9 MPa). The 5% Pt/Al2O3 catalystexhibited the highest activity and stability.The conversion of palmitic acid was studied in batch reactor

using Pt (5%)/C and Pd(5%)/C catalysts under a near criticaland SCW (380 °C; 28 MPa) conditions without H2 beingpresent.127 The Pt/C catalyst was more active than Pd/Ccatalyst giving more than 90% selectivity to pentadecane. At theend of run, the catalysts retained most of their activity for reuse.Under similar conditions, activated carbon alone was used forconversion palmitic and oleic acids. In this case, n-C8−C15 andn-C12−C17 were the major products.128 The absence of alkenesamong the products confirmed an in situ formation of activehydrogen which was consumed in HYD reactions. Both waterand fatty acid molecules could be the source of active hydrogen.The SCW hydrothermal process, operating in the presence of

Pt/C catalyst under a high H2 pressure was developed by Duanand Savage127,128 for upgrading biofeed from liquefaction of amicroalgae. The upgraded oil was a freely flowing liquidcompared with a tarry consistency biofeed. The characterizationof upgraded bio oil identified 72 compounds which accountedfor almost 70% of the product mixture A series of n-alkanesstarting at C9 were dominant species in the products. Thisconfirmed a high level of the HYD of alkenes present in thefeed. For example, phytenes present in the biofeed wereconverted to phytane (2,6,10,14-tetramethylhexadecane). Asignificant amount of alkyl substituted benzenes was formed aswell. The derivatives of piperidine, indole and O-methyloxime,which were present in the crude material, were not detected in

the products after the supercritical upgrading process. Thissuggests that the catalyst and reaction conditions used causedan extensive denitrogenation. The overall content of cholester-ol, cholestane, and cholestene decreased in the treated oils. Theonly sulfur-containing compound, such as 1-methyl-2-piper-idinethione, detected in the biofeed, was removed duringupgrading. The experimental conditions had a pronouncedeffect on the products distribution.128 Without Pt/C catalyst,the content of fatty acid in the produced liquids was ratherhigh. Basic conditions favored the conversion of fatty acids. Atthe same time, the relative amount of pentadecane in theproduct oil was much higher than that in the crude feed. Inanother study, the biofeed from the hydrothermal liquefactionof a microalgae was upgraded over the Pd(5 wt.%)/carboncatalyst in a SCW at 400 EC and a partial pressure of 3.4 MPaof H2.

127 The longer reaction times and a lower feed/catalystratio increased the amount of gas and coke. At the same time,the quality of liquid products improved, although their yielddecreased.The cubic and octahedral CeO2 nanoparticles of 8 and 50 nm

size, respectively, were used for the conversion of bitumen inSCW at 723 K.97 In this case, the solution of 10 wt % ofbitumen in 1-methylnaphthalene was used to reduce viscosityof the former. The batch reactor was filled with the solution (1g) and water (1 g) and slurried with 10−20 mg of catalyst.Significant reduction in the asphaltenes content and an increasein the yield of maltenes (toluene solubles-pentane insolubles)was achieved, particularly in the presence of the cubic CeO2.The temperature employed during these experiments indicatescomplex mechanism of the overall conversion involving thermaldecomposition of asphaltenes initially, followed by stabilizationof radicals via reactions {5} to {8} and scavenging cycle inFigure 6.


For high oxygen and water contents feeds, the HPR in aqueousphase for production of liquid hydrocarbons is an alternative toconventional HPR. Potential for generating in situ hydrogen viapartial reforming and WGS during the HPR in aqueous phasemay be an answer to large hydrogen consumption observedduring upgrading high oxygen content feeds using conventionalHPR.Advancements toward commercialization of the aqueous

phase HPR of high oxygen content feeds may be accelerated ifan active and selective catalysts exhibiting desired performancein the presence of large quantities of water, can be developed.Depending on feed composition, a multifunctional catalystpossessing high activities for HYD, HDO, HCR, hydrolysis,hydrogenolysis, WGS, and dehydration may be required.Published information suggests that this stage of catalysis maybe approached by an optimal selection of active metals andsupport as well as by performing overall HPR in stages. Goodperformance was exhibited by noble metals (e.g., Pt, Pd, Ru,and Rh) catalysts supported on supports with suitable surfaceacidity. It should be, however, noted that most of theexperimental results were obtained in batch reactors. A paralleltesting in continuous fixed bed reactors is needed to obtain datafor scale-up consideration. In this case, the kinetics of HPRreactions in aqueous phase require attention. Thus, traditionalexpressions used to study kinetics of HPR under conventionalconditions must be modified to account for potentialinvolvement of water.

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Among other factors, high consumption of hydrogen duringthe conventional HPR of heavy petroleum feeds may be thereason for interests in HPR in aqueous phase. A higherreactivity of the in situ generated hydrogen (via WGS andpartial reforming) than that of the external H2 was clearlydemonstrated. Consequently, formation of coke was dimin-ished and the yields of liquid products increased. In this regard,both active hydrogen and water molecules played a beneficialrole. Moreover, the transfer of hydrogen from water toproducts was confirmed. In spite of the potential benefits, theinterests in aqueous phase HPR of heavy feeds have faded.Apparently, catalysts and reactor design for such applicationsare much more challenging than those for liquid feeds. Theresults obtained predominantly in batch reactors need to becompared with those from continuous systems. Among reactorstype, fixed bed, ebullated bed, and slurry bed reactors have tobe carefully evaluated for aqueous HPR. It is believed thatebullated and slurry bed reactors may be more advantageouscompared with fixed bed reactors. For these purposes, anextensive database of experimental parameters has still to beestablished.In published studies on the HPR in aqueous phase, material

problems caused by enhanced corrosivity by water are rarelyaddressed. As expected, corrosivity increases with increasingtemperature. Therefore, it may require special attention duringthe design of catalytic reactors as well as upstream anddownstream units, particularly for SCW.


Corresponding Author*E-mail: [email protected].

NotesThe authors declare no competing financial interest.

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