Hydroprocessing in Aqueous Phase - Industrial & Engineering

Review pubs.acs.org/IECR

Hydroprocessing in Aqueous Phase Edward 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 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 water−gas 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 hydrocarbons via reactions with hydrogen. In some liquids, a final polishing step is required to attain specification of transportation fuels and lubricants. For example, aromatics must be removed by hydrogenation while straight chain hydrocarbons by hydroisomerization. The cost of conventional HPR is affected by a large consumption of hydrogen. Water has been identified 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 difficult and inefficient. 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 Fischer−Tropsch 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 flooding 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 © 2013 American Chemical Society

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), hydrogenation (HYD), hydroisomerization (HIS), hydrodemetallization (HDM), and hydrodeasphaltization (HDAs), have been discussed in details. In addition, the most important HPR reactions 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 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 effect of water on operating parameters under aqueous conditions requires attention. Of particular significance are the effects 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 H2) in water as Received: Revised: Accepted: Published: 17695

October 15, 2013 October 30, 2013 November 21, 2013 November 21, 2013 dx.doi.org/10.1021/ie4034768 | Ind. Eng. Chem. Res. 2013, 52, 17695−17713

Industrial & Engineering Chemistry Research

Review

The objective of research in this field was a direct upgrading of such complex mixtures without pretreatment. The most typical source of high water content feeds is the conversion of biomass (both by pyrolysis and hydrothermal treatment) always to a high water content biocrude. Detailed accounts of the conversion of an aquatic biomass via hydrothermal liquefaction and gasification, both in the presence and absence of catalysts was given by Yeh et al.21 and Savage.22 An example of the biocrude (primary liquids) from pyrolysis and liquefaction of lignocellulosic biomass is shown in Table 1.23 At least two stages may be needed to upgrade such feeds to

well as diffusivity and reactivity of water are of prime interests. For the purpose of this review, only a brief account of these properties is given in the following text. Thus, rather extensive information on these and other aspects of water may be readily accessed in several books published elsewhere.17−19 The original structure of liquid water, dominated by hydrogen bonds, is changing with increasing temperature. While critical temperature is being approached, an almost complete collapse of the hydrogen bond network occurs. As the result of this change, polarity of water is significantly diminished. This was confirmed by a dramatic decrease in dielectric constant.17−19 Above the critical point, water behaves as a nonpolar medium, capable of dissolving organic substrates. In this regard, water is approaching properties of solvents such as acetone, methanol, ethanol, etc. Then, the solubility of various feeds (e.g., bio oils, petroleum residues, coal derived liquids, etc.) in SCW is significantly enhanced.19 From the HPR point of view, it is important that under super critical conditions, gaseous H2 is completely miscible with SCW. A high homogeneity of reaction streams attained in SCW is favorable for the efficient transfer of hydrogen to reactant molecules. 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 unique transportation properties of SCW. While increasing temperature from subcritical region toward critical temperature at 22 MPa, the density of water abruptly decreases, for example, from about 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 decreased with 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 HPR reactions may be anticipated. Then, if necessary, an optimal combination of density with temperature and pressure may be established. It is obvious that the reactivity of water may change dramatically as the consequence of hydrogen bonds network collapse. For example, while approaching 374 °C, the pKw of the water dissociation equilibrium almost doubled.17−19 A much higher concentration of H3O+ and HO− ions in SCW than that in liquid water increases the chances for the involvement 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 those attached to heteroatoms, may be anticipated.20 These facts increase the probability of an ionic mechanism as part of the overall mechanism during the HPR in aqueous phase. Ionic reactions are favored by high density of water. This may be achieved under subcritical conditions; however, under supercritical conditions, high pressures are needed to get densities suitable for ionic chemistry.

Table 1. Property Ranges of Bio-crude Obtained by Liquefaction and Pyrolysis of Biomass23 carbon, wt % sulfur + nitrogen, wt % oxygen,a wt % water in crude, wt % density, g/cm3 a

liquefaction

pyrolysis

68−81 0.1 9−25 6−25 1.10−1.14

56−66 0.1 27−38 24−52 1.11−1.23

Dry basis.

hydrocarbons via HPR route.24 Chemical compositions of these biocrudes and corresponding products were discussed in details elsewhere.13 Unless an extensive dewatering was conducted, a high water content in the biocrude obtained from algae biomass and municipal solid wastes using similar methods may be anticipated.12 Interests in the catalytic conversion of sorbitol to a great variety of products have been noted. The HPR of sorbitol in an aqueous phase to produce hydrocarbons has been one of the evaluated routes.25 A unique case of the feed for potential HPR in the presence of water may be the reaction water produced in FTS process. Such aqueous phase contains a mixture of water-soluble oxygenates, among which alcohols, ketones, aldehydes, and carboxylic acids are far predominant structures. The total amount of the oxygenates in the aqueous phase may approach 10 wt %. The most abundant oxygenates dissolved in the aqueous phase from FTS are shown in Table 2.2−4 Table 2. Most Abundant Oxygenates in Aqueous Product from High Temperature FTS2

a

rank

oxygenate

yield, mass %a

1 2 3 4 5

ethanol propanone butanone 1-propanol acetic acid

3.4 2.5 1.2 1.0 0.9

On total syncrude basis.

Besides the high oxygen containing feeds, heavy petroleum feeds have been focus of attention for potential upgrading via HPR in an aqueous phase. Table 35 indicates a significant difference between the properties of the latter feeds and the high oxygen content feeds shown in Tables 1 and 2. A strong emulsifying potential of some asphaltenic and resinous components suggests that heavy feeds produced via enhanced oil recovery using steam flooding method may contain a large amount of water in the form of stable emulsions.5 Similarly, relatively high water content bitumen may be produced from tar sands using a hot water separation process.15 If necessary,

3. PROPERTIES OF FEEDS In this section, attention is being paid to those feeds that have been included in the studies on HPR in aqueous phase. In this regard, model compounds alone and/or mixtures of various model compounds used to study HPR under conventional conditions have been also studied under aqueous phase conditions. This is illustrated on several examples presented in this review. In the case of real feeds, the focus is on those feeds which are being produced in an aqueous environment. 17696

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Table 3. Properties of Heavy Feedsa for Hydroprocessing in Aqueous Phase5 heavy feed density, kg/L sulfur, wt % nitrogen, wt % vanadium, ppm nickle, ppm CCRb, wt % a

Maya

Cold Lake

Arab heavy

0.93 3.8 0.3 273 50 15

1.0 4.9 0.6 160 80 19

0.89 2.9 0.2 50 16 7

Vacuum residues. bConradson carbon residue.

stable emulsions involving heavy feeds and water may be prepared with the aid of surface agents using mechanical means. An atomization of heavy feeds is achieved by water evaporation during rapid temperature increase, that is, during the introduction 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 and transfer hydrogen to products and, as such, offset high hydrogen consumption. A water-soluble catalyst may be added before the emulsion preparation. A catalytic process referred to as the slurry bed HCR has been approaching commercial stage.5 Properties of the heavy feeds, which can be upgraded by this method as well as the operating parameters of slurry bed reactor, were discussed elsewhere.15 In practical situation, a number of other high water content feeds can be identified. For example, such feeds may be disposed from various industrial operations and municipalities (e.g., refinery sludge, waste from pulp and paper industry, municipal solid waste, etc.). Little information on the upgrading of such materials under aqueous conditions could be found in the literature, so far. Apparently, HPR in an aqueous medium may be an attractive option, although the homogeneity of these feeds for processing may require attention. Other feeds that can be upgraded under aqueous phase conditions include coal derived pitch, waste plastics, etc. With respect to HPR, the miscibility and/or solubility of organic phase in water phase is of a primary importance. High oxygen content feeds are usually in the form of a homogeneous mixture of water with an organic phase. However, significant problems may be encountered during the preparation of the water−heavy feed mixtures for HPR under aqueous conditions. For such feeds, a temperature exceeding that of SCW is required to achieve desirable conversion. Also, an optimal combination of pressure and temperature has to be identified to ensure homogeneity of the system. The phase diagram in Figure 126 developed for heavy feed containing ∼37 wt % of asphaltenes shows three miscibility regions, that is, nonmiscible two phase region, partially miscible two phase region, and pseudo-single phase region. It was evident that the regions are influenced by the water/feed ratio. The dashed region, which is suitable for upgrading of the heavy feed, is, in fact, the SCW region. Mechanistic aspects of the conversion of various feeds under subcritical and supercritical conditions are discussed in more details later in the review.

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

200 to almost 500 °C and 4−40 MPa, respectively.26−36 Processing under such conditions may have some energetic advantages. Thus, avoiding the liquid water-steam phase transformation results in substantial energy savings the extent of which is influenced by severity. For the purpose of this review, the severity of conditions is referred to temperature ranges 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 to ensure that most of the water in the system is in a liquid phase. Besides energy savings, this improves the interaction of water molecules with reactants thus ensuring a higher conversion of the latter. Rather unique properties are exhibited by water in the supercritical region (above 374 °C and 22 MPa).26 The products from upgrading in an aqueous phase include H2, synthesis gas (H2+CO), gaseous and liquid hydrocarbons. The conditions employed during the upgrading may be optimized to maximize the yield of products of interest.36 The production of liquid hydrocarbons via HPR in an aqueous phase is the primary focus of this review. The presence of large quantities of water in the system indicates on the occurrence of reactions (Figure 2), which

Figure 2. Tentative reactions during biomass reforming.

under conditions of conventional HPR are either absent or play a minor role. The reforming of hydrocarbons with the aid of steam (reaction {1}) may be one of the hydrogen sources generated in situ. The high yield of CO2, combined with the low yield of CO confirmed the involvement of the water−gas shift (WGS) reaction {2}, which may always be present as the next step of reforming reactions. A higher reactivity of the in

4. UPGRADING UNDER AQUEOUS CONDITIONS The information in literature suggests that, in most cases, temperature and pressure employed during the upgrading under aqueous conditions of various feeds range from less than 17697



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

generation of H2, while Rh, Ru, and Ni supported on silica exhibited a low selectivity for H2 and a high selectivity for alkane production.42 The selectivity for H2 production was also influenced by the structure of substrate; for example, it increased from sugars toward ethylene glycol and methanol.43 For ethanol, ethylene glycol, and sorbitol (below 500 K, ∼3 MPa, Pt/Al2O3 and a tin-promoted Raney-Ni catalysts), almost complete 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 dominant reaction over Ru/C catalyst at 400 °C and 24.5 MPa.44,45 Lercher and co-workers29,33−36 made an important contribution to the understanding of the HPR under aqueous conditions using model compounds typical of those present in biofeeds. In this case, hydrocarbons were the targeted products. Therefore, the conditions favoring reaction {3} were the focus of their attention. More detailed accounts of these studies are given 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 study is introduced here to illustrate the attempt for identifying optimal conditions ensuring high yields of liquid hydrocarbons (reaction {3}). Using Pt(4%)/SiO2−Al2O3 catalyst (498 K; ∼4 MPa; continuous system) and 5 wt % sorbitol in water, the combined yield of pentane and hexane approached 60 wt % without external H2 being present. The yield was further increased to about 80% in the presence of external H2.

situ generated hydrogen than that of the external gaseous H2 may be anticipated. As it was pointed out, production of liquid hydrocarbons from various feeds under aqueous conditions is the primary objective of this review. In the case of glucose, this may be indicated by reaction {3}. In addition, reaction {4} (methanation) is also involved. The hydrogen required for the reactions {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 on the experimental conditions (e.g., temperature, total pressure, type of experimental system, origin of feed, etc.) and type of catalyst. The success of HPR in aqueous phase for the production of liquid fuels depends on the optimization of experimental conditions to ensure the maximization of reaction {3} and minimization of reaction {4}. A large hydrogen consumption in reaction {3} should be noted. This may be offset by the in situ hydrogen production via reactions {1} and {2}. Therefore, an efficient process for conversion of various feeds to liquid fuels using a concept based on the HPR in aqueous phase may require a delicate balance involving a number of operating parameters. These issues were discussed extensively in the study published by Davda et al.37 In this regard, significant advancements in the understanding of reactions occurring under aqueous phase conditions were made by Dumesic and co-workers.37−43 For example, at 483 and 498 K, Pt and Pd supported on silica were selective for 17698



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

gaseous H2 are involved. Participation of the former was indicated by the reactions {1} to {4}. In addition, H2O molecules may be directly involved in supplying hydrogen. During the HPR under aqueous conditions, both noncatalytic and catalytic reactions occur simultaneously. For the overall mechanism of HPR, decoupling the noncatalytic reactions from catalytic reactions may be of an interest. The extent of the former reactions increases during the temperature increase from mild conditions to subcritical and finally to supercritical region. Because of the unique physical state of SCW, the occurrence of entirely new chemical reactions (e.g., ionic reactions) between SCW and substrate in the latter region may be anticipated. Additional noncatalytic reactions occur under SCW + H2 conditions. It is believed that, under mild conditions, rather low conversion of feed involving water should be observed unless an active catalyst is present. Once a catalyst is present, a new set of reactions becomes evident even under mild conditions. Although a primary focus is on catalytic HPR, a brief account of noncatalytic reactions occurring in parallel with catalytic reactions may be useful for better comprehending the overall mechanism of HPR under aqueous conditions. As it was pointed out earlier, nonpetroleum high oxygen content feeds and petroleum residues are two main types of the feeds that have been used for the HPR in an aqueous phase. Figures 349 and 450 respectively show tentative structures of lignin and asphaltenes. Lignin containing feeds may be used for the HPR in aqueous phase either directly or as the source of a bio-oil after upgrading (e.g., via pyrolysis and liquefaction). A high oxygen content ensures a good miscibility/solubility in the aqueous phase. On the other hand, vigorous mixing of heavy feeds of petroleum origin with water may be required to obtain

For most part, the discussions on aqueous phase upgrading was focusing on high oxygen feeds. To various extents, reactions {1} to {4} are also present during the upgrading of petroleum residues, coal tar pitch, waste plastics, etc. For such feeds, thermal cracking of large molecules to light fractions may play a dominant role during the overall conversion to liquid hydrocarbons. Apparently, HPR is not the only option for liquid fuels production under aqueous conditions. For example, the concept based on the conversion of polysacharides via dehydration, aldol-condensation, and hydrogenation can produce liquid alkanes under mild conditions.46 In addition, a high yield of synthesis gas (CO+H2) may be generated if reaction {1} is carried out under controlled conditions. Liquid fuels can then be produced via FTS using synthesis gas as the feed.47 It should be noted that, for this alternative, all stages of the FTS process and subsequent upgrading of products have been used on a commercial scale.2−4 There might be another potential non-HPR routes for production of liquid hydrocarbons under aqueous conditions.48 These routes are not in the scope of the present review. However, they should be always considered as an alternative to HPR while evaluating the viability of upgrading under aqueous phase conditions. In some specific cases (e.g., for refinery applications), the production of H2 (reaction {1}) may be attractive. In other cases, the production of synthetic natural gas via methanation (reaction {4}) may also be of an interest.

5. HYDROPROCESSING MECHANISM IN AQUEOUS PHASE For the purpose of this review, HPR reactions are those in which both an in situ produced hydrogen and an external 17699



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a homogeneous mixture suitable for HPR. The structures in Figures 3 and 4 clearly indicate a significant difference between the overall mechanism of HPR of these feeds in aqueous phase. 5.1. Noncatalytic Reactions. Because of higher temperatures involved, thermal cleavage of chemical bonds in various reactants in SCW (above 374 °C) is more extensive than under subcritical conditions.51 The thermal cleavage of the weak C−C bonds begins at bout 300 °C. It was reported that free radicals produced by the cleavage of organic bonds were rapidly stabilized in the presence of water via the set of tentative reactions shown in Figure 5.52,53 Based on the models in

In this case, the experimental results could be interpreted in terms of free radicals mechanism. At sufficiently high temperatures, the reforming of hydrocarbons initiated by free radicals, according to the tentative reaction {1} may take place. For example, at about 600 °C, the transfer of oxygen from water to carbon of a highly disintegrated organic molecules was a dominant reaction producing high yields of CO2 and H2.51 At such temperature of SCW, the total pressure may be in the range of 30 MPa. The high yields of CO2 and H2 combined with low yields of CO suggested that H2O reacted with CO via WGS reaction (reaction {2}). Because of the equilibrium effects, the excess of H2O in the system was favorable for WGS reaction while the presence of H2 had an opposing effect. Therefore, the consumption of H2 in HPR reactions, that is, its removal from equilibrium mixture, drives WGS reaction forward. It is believed that the extent of reforming and WGS reactions (reactions {1} and {2}) is being gradually diminished by decreasing temperature from about 600 to 374 °C and below. However, a high yield of CO2 obtained between 300 to 350 °C during hydrothermal liquefaction of lignocellulosic biomass in water as reported by Goudriaan and Peferoen60 confirmed that in this temperature range the presence of these reactions was still evident. Between 100 to 200 °C, little conversion of relatively reactive compounds such as alky-aryl ethers and arylether in water even in the presence of H2 was observed.36 A low conversion of the most reactive oxygenates such as alcohols under mild conditions may be anticipated as well. Even the most probable reaction such as dehydration would be inhibited considerably because of the excess of water in the system favoring the shift of the equilibrium in reaction {10} to the left where R″ represents corresponding olefin. Of course, the shift of this equilibrium to the right would be maintained by rapidly removing R″ from the system via HYD, that is, in the presence of 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 autoclave under typical cracking conditions, that is, absence of H2. It was proposed that thermal conversion of asphaltenes proceeded via radical mechanism rather than via an ionic hydrolysis mechanism. In the presence of water, the formation of unwanted coke was significantly suppressed. It is believed that water molecules behaved as free radicals scavenger and hydrogen donor involving reactions {5} to {8}, thus slowing down reaction {9}. In the case of a heavy feed, the radical (R•) may involve complex structures comprising aromatic and aliphatic entities as well as heteroatoms. For example, according to the model of asphaltenes shown in Figure 3,50,61 the −C−S− C− entity (site 1) in model A represents the weakest bonds in the 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 liquid products and a low yield of coke) may be identified. Such temperature depends on the origin of feed. For example, for a vacuum residue, Cheng et al.62 reported maximum of the yield of liquids and the lowest coke at 420 °C while for coal derived asphaltenes Han et al.52 observed an optimal temperature of 460 °C. In the latter study, the yield of maltenes in SCW was significantly greater compared with the experiment conducted in N2. The H/C ratio of the former was higher as well. In the study conducted by Zhao et al.,63 a VR was upgraded in SCW

Figure 5. Tentative reactions during hydroprocessing in aqueous phase.

Figures 3 and 4, the radical (R•) may involve complex structures comprising aromatic and aliphatic entities as well as heteroatoms. Once generated, such radicals may decompose to lighter products. This suggests that in the presence of a free radicals generating agent, the conversion of large reactant molecules could be enhanced. This was indeed confirmed in the study of Zhu et al.54 who added ditert-butyl peroxide to a heavy feed for pyrolysis in SCW (653 K; water density 0.30 kg/ L). Consequently, the overall conversion of aspahltenes and resins was increased. However, decomposition of large radicals involves a parallel formation of lighter products and smaller radicals. The latter may lead to coke formation unless they are stabilized. This was confirmed by a significant decrease in coke formation during copyrolysis of the heavy feed with polyethylene conducted at 683 K under otherwise similar conditions.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 corresponding products. This was indeed confirmed by Dutta et al.57 using the mixture of H2O + D2O during thermal cracking of bitumen between 350 to 530 °C. In this case, deuterium was transferred both to liquid products and coke. In the absence of water, the conversion of radicals would proceed via reaction {9} which represents 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 to HC and HOC radicals, followed by the reaction of radicals with a reactant. This reaction is energetically unfavorable and should be distinguished from the reaction in which hydrogen is abstracted from H2O by radical via reaction {5}. Ability of the radicals generating reactants to transfer hydrogen from H2O observed in their study may be attributed to reactions {7} and {8} (Figure 5). Similar observations were made during the decomposition of several S-containing compounds in SCW.59 17700



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be anticipated. For example, an interaction of H2O with metals (V and Ni) leading to disintegration of the porphyrin skeleton may 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 conversion of coal tar was also investigated.72,73 It was observed that at the same temperature, liquefaction of tar (autoclave, 623 and 673 K, 25−40 MPa) increased with increasing water density at the same reaction temperature. It was proposed that under sub- and supercritical conditions, hydrolysis was involved in the conversion of macromolecular structure of tar to lighter products such as phenol, biphenyl, diphenylether and diphenylmethane. 5.2. Ionic Reactions. The involvement of an ionic mechanism as part of the overall conversion of different feeds in aqueous phase may be anticipated, although this issue has been receiving little attention. Yet, it was indicated earlier that, compared with liquid water, the dissociation constant of H2O increased with increasing temperature from 100 °C toward subcritical and supercritical conditions.17−19 Therefore, in an aqueous medium, the involvement of H+ and HO− ions in parallel with the free radicals as well as conventional HPR reactions, as part of the overall conversion in the presence of catalysts and H2, is highly probable. By adding to a heteroatom (e.g., S, O and N), H+ may enhance the rate of hydrogenolysis of the corresponding heterobonds. Autocatalysis by fatty acid products formed during the hydrolysis of triglycerides (soybean oil) in subcritical water (250−300 °C) supports the involvement of an ionic mechanism as well.74 The H+ ions required for such mechanism were generated by the partial dissociation of the fatty acids produced by triglycerides hydrolysis. The nitrogen content of biomass of an algae origin may exceed 10 wt %. A strong tendency of N-heterorings to combine with H+ ions has been well documented.9,12 This suggests that the ionic mechanism plays key role during the conversion of algae biomass to hydrocarbons and ammonia under sub- and supercritical water conditions without a catalyst being present. The HPR of biocrude obtained from an algae biomass via hydrothermal route may be affected unless most of nitrogen ends up in aqueous phase rather than in biocrude. This issue was the focus of attention of the study published by Valdez et al.75 Additional efforts may be needed to clarify mechanistic aspects of the conversion of N-heterorings under hydrothermal 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 which support both radical and ionic mechanisms occurring in parallel. A rapid change in pKw on approaching supercritical region from subcritical region enhanced the involvement of ionic reactions. However, increased temperature required for this change favored radicals formation as supported by the increased yield of coke in supercritical region. The kinetic network for the noncatalytic conversion of guaiacol in sub- and supercritical regions proposed by Yong and Matsumura78 considered both ionic and radical reactions. Thus, the rate constants for the overall conversion of guaiacol obeyed Arrhenius law in subcritical region, but they deviated in supercritical region unless both radical and ionic reactions were considered. Under typical HPR conditions (e.g., with H2 and catalyst being present), the involvement of H+ ions during the

as indicated by decrease in the content of asphaltenes, resins, and aromatics and a significant increase in the content of saturates. The experiments were carried out in an autoclave from 380 to 460 °C at 25.0 MPa. In addition, a significant reduction in viscosity of products, as well as the content of sulfur, nitrogen, and metals was observed. Bitumen upgrading in SCW alone was compared with that in the presence of 10% HCOOH (semibatch reactor from 633 to 693 K) by Sato et al.64,65 The decomposition of HCOOH yielded the mixture of SCW + H2 + CO2. In the case of the mixture, the involvement of H2 was confirmed by the higher H/ C ratio of the unconverted asphaltenes compared with the asphaltenes in bitumen and those obtained in SCW alone. The coke formation in the SCW + H2 + CO2 mixture was suppressed as well. These observations suggest that H2 successfully competed with water (reactions {5} to {8}) in suppressing coke formation (reaction {9}). In this case, reaction {12} was involved. Hydrogen donor mechanism involving saturated hydrocarbons, particularly naphthenic structures, may also be involved. Thus, it has been generally observed that naphthenic structures can readily donate hydrogen.14 In the presence of hydrogen, this may be depicted by the hydrogen transfer cycle shown in Figure 6. The occurrence of such reactions may be

Figure 6. Free radicals scavenging cycle using naphthenic hydrogen.

anticipated during the upgrading of distillation residues derived from naphthenic crude. The recent study published by Zachariah et al.66 provides a direct experimental evidence for the 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 of gaseous 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 small amount of water was involved in upgrading. Moreover, the residue produced in SCW had lower molecular weight distribution, lower H/C ratio, and higher aromaticity. In other studies, beneficial role of SCW was observed by an increased yields of liquid products and decreased yield of coke.52,53,57,60−63,68,69 It should be noted that all these results were obtained in autoclave. This suggests that the distribution of products was changing with time.70 For example, little participation of water may be anticipated during the early stages because of the naphthenic structures present effectively trapped free radicals formed (Figure 6). However, once this source of hydrogen donors was exhausted, the involvement of water as radical scavenger appears to be plausible. Therefore, experimental system and conditions used for bitumen upgrading in SCW are another factor to be considered in designing the overall mechanism under noncatalytic conditions. For heavy petroleum feeds, the involvement of noncatalytic reactions during overall conversion was reported by Marafi et al.71 under typical HPR conditions. A higher probability for such and additional reactions in an aqueous environment may 17701



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be noted that the content of acetic acid in some bio-oils from pyrolysis of biomass may exceed 10 wt %.12,13 This suggests that ionic reactions play an important role during the HPR of these 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 and methanol 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 any experimental data on the ionic mechanism, only a speculative argument on the involvement oh HO− ions may be forwarded.57 In the presence of solid catalysts, HO− ions may behave as Lewis bases suggesting that they may adsorb on Lewis acids of solid supports. For the (-Al2O3 support, this may be the beginning of a gradual transformation toward boehmite. Some interaction of HO− ions with carbon supports may be anticipated as well. It is believed that such an interaction will increase with the increasing irregularities of carbon surface. 5.3. Catalytic Reactions. The above discussions indicated a distinct difference between the mechanisms in an aqueous phase involving high oxygen content feeds (biofeeds, FTS aqueous phase, etc.) and that involving distillation residues derived from petroleum, although some common reactions may be evident. This is illustrated on several examples from literature which are based on experimental observations. In this regard, a significant difference in the structure of these feeds and their miscibility/solubility in water may play an important role. Of course, in the presence of catalyst, the role of external H2 during the overall HPR mechanism can be dominant. In this case, hydrogen activation, that is, the conversion of dihydrogen to an active surface hydrogen with the aid of catalyst, must precede HPR reactions. The hydrogen generated in situ must be activated as well. The tentative scheme shown in Figure 8 accounts for the potential sources of hydrogen. In an aqueous phase under HPR conditions, sources 1 and 2 represent steam reforming of hydrocarbons and WGS (i.e., reactions {1} and {2} in Figure 1, respectively). Potential of H2O as free radicals scavenger in noncatalytic reactions (reactions {5} to {8} in Figure 5) was indicated earlier. The overwhelming evidence for the presence of reforming and WGS reactions was clearly confirmed in several studies.84−93 Apparently, active surface hydrogen formed on catalyst surface according to Figure 8 can be abstracted by free radicals.94 Consequently, the involvement of noncatalytic reactions (e.g., {5} to {8} and {11}) in radicals stabilization is diminished. At the same time, radical stabilization via hydrogen transfer cycle in Figure 6 may be sustained in the presence of active hydrogen. It is believed that, in the presence of HPR catalysts, the reactions shown in Figures 6 and 8 may

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

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

carbenium ion intermediate may be converted to either an olefin (e.g., RCHCH2) or isomerized to different iso-olefins before it is hydrogenated to the final saturated hydrocarbons. Rates of the H2O and NH3 elimination reactions may be further enhanced over acidic catalysts, which can be the additional source of protons.2 Montgomery et al.79 reported that a disintegration of asphaltenes in water to light fractions began already at 250 °C and ∼4 MPa total pressure. This was significantly enhanced by increasing temperature and pressure to 350 °C and ∼11 MPa, respectively. These observations are consistent with the participation of H+ and HO− ions in the overall conversion. Thus, thermal effects could not account for all reactions observed. The results published by Li and Egiebor80 may be interpreted in terms of ionic reactions being present during the extraction of oxygen from coal in SCW (360 to 400 °C) and their absence during the extraction in supercritical toluene. Thus, a significantly greater amount of oxygen removed under the former conditions can be attributed to the enhanced hydrogenolysis aided by H+ ions. The involvement of H+ ions during the dehydration of alcohols is indicated by reaction {12}. The observations made during the HDO of phenol by Massoth et al.81 could be attributed to the participation of H+ during the overall HDO as well. Thus, even the hydrogenolysis of phenol to 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 by Wan et al.82 could only be attributed to the participation of H+ ions. Methyl-cyclohexanol was the main product in the absence of acetic acid. The appearance of methyl-cyclohexane as the main product in the presence of acetic acid can be attributed to the increased rate of dehydration of methyl-cyclohexanol (reaction {12}) caused by the presence of H+ ions. It should

Figure 8. Active hydrogen sources during hydroprocessing in aqueous phase. 17702



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1,3-propanol > 1,2-propanol > 1-propanol ∼ 2-propanol. However, a significant effect of catalyst type on the HDO mechanism was indicated in the study conducted by Chen et al.30 Diphenyl ether, 2-phenylethyl phenyl ether and benzylphenyl ether, as representatives of lignin, were studied over Ni/SiO2 catalyst at 120 °C and 0.6 MPa of H2 in an autoclave in an excess of water.36 For diphenyl ether, the initial reactions (Figure 9) were dominated by HYD (65% cyclohexyl phenyl

play much more important role in the overall mechanism of HPR of petroleum feeds than that of a high oxygen content feeds. 5.3.1. HPR Mechanism for High Oxygen Feeds. For high oxygen content feeds (e.g., biofeeds and aqueous phase from FTS), the dehydration of alcohols (ROH) to alkenes (R″) may be the rate determining step during the overall HDO, although a direct hydrogenolysis (e.g., phenol to benzene) may be involved. The corresponding dehydration equilibrium reaction (reaction {10} in Figure 5) is affected by the excess of water in the system which tends to shift the equilibrium from alkenes to alcohol. An efficient removal of oxygen from the system can still be accomplished providing that a rapid removal of alkenes (R″) from the system can be ensured. This may be achieved by a rapid HYD of alkenes to alkanes (reaction {11}. Therefore, catalysts with a high HYD activity and at the same time stable in aqueous environment may be suitable for these applications. The HDO mechanism applicable to conventional HPR conditions was discussed extensively elsewhere.2,12,13 The rate of dehydration of alcohols, as the final stage of the HDO of many oxygenates, may be influenced by reaction conditions. For example, over Pd/C catalyst at ∼200 °C, phenol was converted predominantly to cyclohexanol while hydrogenolysis to benzene was not observed.34,35 However, after acidifying the solution, cyclohexanol was quantitatively dehydrated to cyclohexene followed by HYD to cyclohexane.95 This confirmed that hydronium ions aided dehydration of cyclohexanol (e.g., reaction {13} in Figure 7). At the same time, noble metals facilitated the HYD function. This concept of bifunctional catalysis was confirmed using more complex reactants (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, the hydrogenolysis to benzene and the HYD to cyclohexanol were the main routes. Dehydration of the latter to cyclohexene was enhanced over the catalysts supported on HZSM-5 compared with that on Al2O3. Moreover, over the former catalysts, small amounts of methyl-cyclopentane were observed. This clearly confirmed the involvement of H+ in dehydration of alcohols to alkenes (reaction {13}).2 Isomerization of the carbocation is another potential reaction to occur as indicated by the presence of methyl-cyclopentane among the products over Ni/HZSM-5 catalysts. For Ni/Al2O3 catalyst, an insufficient strength of Bronsted sites may be the reason for the absence of isomerized product. The study published by Peng et al.29 contributes to the fundamental understanding of the HPR of oxygenates in an aqueous environment under mild conditions. The reactants such as 1-propanol, 2-propanol, 1,2-propanediol, 1,3-propanediol, and glycerol (10 wt % in water) were studied in a batch reactor 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−O bonds was not observed. For 2-propanol and 1,2-propanol, the deHYD to ketone was the main reaction while for 1,3-propanol and glycerol the C−O bond was cleaved by dehydration. For alcohols with the terminal hydroxyl group, the C−C bond cleavage occurred in steps via deHYD to aldehyde followed by either disproportionation and subsequent decarboxylation or decarbonylation. Reactivity of the alcohols increased with increasing number of hydroxyl groups. Thus, over Pt/Al2O3, the following overall reactivity order was established: glycerol ∼

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

ether), followed by hydrolysis (25% cyclohexanol) and hydrogenolysis (10% benzene). With progress of reaction, the yield of cyclohexyl phenyl ether reached a maximum and then decline to zero while that of cyclohexanol and benzene increased. The importance of hydrolysis was confirmed by much higher yield of cyclohexanol compared with that of benzene. It is postulated that hydrolysis was result of the electron deficiency on carbon of the C−O bond which was offset by the interaction with unpaired electrons on oxygen of H2O molecule. In this case, catalyst surface could facilitate a flat adsorption of diphenyl ether which improved the access of H2O molecules to the C−O bond. The HYD of phenyl ring, as the initial step was only minor reaction. Initially, hydrogenolysis was a dominant route for the other two reactants. Under the same conditions, no reactions took place over SiO2, thus confirming the role of Ni in the overall conversion. The study of Duan and Savage 86 showed how the observations made during the HPR of a real feed in aqueous phase can be explained in terms of mechanistic aspects discussed above. In this case, a microalgae paste and a series of catalysts were mixed with deionized water in batch reactor and tested under subcritical conditions (∼350 °C). The reactor was pressurized with either He or 3.5 MPa of H2. In the absence of catalyst, the external H2 had beneficial effect on the yield of bio-oil. However, except for Pt/C catalyst, the yield of bio-oil in H2 over other catalysts (e.g., Pd/C, Ru/C, CoMo/ Al2O3, Ni/SiO2−Al2O3, and zeolite) was lower than that in the inert environment. The analysis of gaseous products confirmed CO2 to be dominant product. This indicated the involvement of WGS reaction. Thus, it was shown that decarbonylation generating CO was an important route during decomposition of the algae biomass.12 Therefore, in the presence of a catalyst, 17703



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this sulfur was removed thermally. In the presence of MoS2 the removal of sulfur approached 12%. Again, thermal reactions accounted for most of the sulfur removed. Rather low sulfur removal can be attributed to the absence of reforming and WGS reactions, which are the source of active hydrogen, as it was demonstrated by Ng and Milad89 who carried out similar experiments in the presence of CO. In the absence of any experimental data, only a speculative mechanism of hydrodemetallization (HDM) in SCW may be proposed. A high reactivity of H2O molecules in SCW would favor a direct interaction with the porphyrinic form of V and Ni metals in the residue. Consequently, the rate of disintegration of porphyrin skeleton as the final stage of HDM, would be enhanced. 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 data on the HDM of model porphyrin compounds obtained in SCW over a catalyst. Based on the results obtained during the pyrolysis of bitumen in SCW (at 723 K) in the presence of cubic (8 nm) and octahedral (50 nm) CeO2 nanoparticles, Dejhosseini et al.98 observed a higher activity of the former than that of hexagonal CeO2. This was attributed to a much higher oxygen storage capacity of the cubic CeO2. It is speculated that the HO− ions generated from H2O were involved in red-ox reactions producing surface hydrogen and carbon oxide. 5.3.3. HPR Mechanism for Other Feeds. The HPR in an aqueous 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 for biofeeds and petroleum feeds may serve as a basis for proposing the mechanism of the HPR of other feeds. Thus, after a detailed characterization of any feed and corresponding products, a set of reactions, particularly those involving H2O may be proposed. Obviously, experimental results are always needed to strengthen arguments one way or the other.

the reaction pathways shown in Figure 8 may be part of the overall 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 required to achieve desirable conversion. This results from the presence of high molecular weight components (e.g., resins, asphaltenes, and porphyrins) in the feed. Such temperatures dictate that for most part, the HPR in aqueous phase must be carried out under supercritical conditions. This suggests that thermal cracking of organic bonds, leading to the formation of free radicals is an important part of the overall mechanism. Several weak bonds as potential cracking sites can be identified in the models of asphaltenes shown in Figure 3. The reactions occurring under the conditions of conventional HPR are also occurring during the HPR in an aqueous phase. The extent of the HPR in the latter phase depends on the origin of residues and type of catalyst. Generally, catalysts comprising noble metals (Pt, Pd, Ru, Rh, etc.) exhibit a high HYD activity. This facilitates a high rate of the HYD of heterorings accompanied by the change of the CAR-S(N,O) bonds to corresponding CAL-S(N,O) bonds. Consequently, strength of the latter bonds is significantly decreased. The stabilization of free radicals involving the cycle in Figure 6 is expected to be much more important for residual feeds than that for a high oxygen content feeds because of a much higher content of naphthenic structures in the former feeds.14−16 The involvement of ionic reactions, particularly over an acidic catalyst should be anticipated as well. Model compounds used to study the mechanism of HPR reactions under conditions of conventional HPR have been also evaluated under aqueous conditions. This may simulate secondary upgrading of reactants produced initially during the conversion of heavy components (e.g., asphaltenes). For example, the conversion of DBT over CoMo/Al2O3 catalyst in SCW was observed in the presence of CO suggesting that the required hydrogen was produced via WGS reaction. Similar observation was made by Arai et al.53,87,88 over sulfided NiMo/ Al2O3 catalyst, while little conversion was observed over the corresponding oxidic catalysts. In fact, in CO+SCW system, the reaction rate was higher than in H2+SCW suggesting that the hydrogen which originated from the source 1 and 2 (Figure 8) was more reactive than from the source 3. In the study conducted by Ng and Milad89 on the HDS of BT, the hydrogen produced via WGS reaction was about seven times more reactive than the external H2. The transfer of hydrogen from SCW to reactants was also confirmed using D2O.90 No WGS reaction 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 generate CO, the conversion of several reactants (e.g., DBT, carbazole, quinolin and naphthalene) involving the HYD route was higher than in H2 + SCW.83−89,91−93 The involvement of H+ ions during the HPR of residues in SCW, as part of ionic reactions (e.g., reactions {13} and {14}) was anticipated above. Under the conditions of conventional HPR, the final stages of HDO12,13 and HDN,9−11 that is, elimination of oxygen and nitrogen from the last intermediates, respectively were interpreted in terms of a proton transfer from catalyst to the intermediate. Such reactions are more likely to take place in SCW than in a liquid water. Heavy Arabian heavy crude containing 3 wt % sulfur was treated in an autoclave (without external H2) in SCW.97 Under these conditions, only about 7% of sulfur in the feed were removed in the absence of catalyst. It is believed that most of

6. CATALYST DEVELOPMENT FOR HYDROPROCESSING IN AQUEOUS PHASE Attempts have been made to develop catalysts with desirable activity and selectivity as well as a high stability in the presence of large quantities of water. In this regard, mild, subcritical and supercritical conditions have been receiving attention. Apparently, the conventional HPR catalysts consisting of the Co(Ni)-Mo(W)-S active phases supported on γ-Al2O3 may not be suitable for a direct use unless they are modified to enhance stability.6−9,11−16,94 As it was indicated above, both the supports and active metals have to be carefully selected for catalyst design for the applications under aqueous phase conditions. In this regard, besides conventional materials, combinations of various nonconventional metals and supports have been explored. However, a limited number of articles published in scientific literature indicates that the development of HPR catalysts for applications in aqueous phase is still in an early 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 in the case of sub- and supercritical conditions employed during the HPR in an aqueous phase, a detrimental effect of H2O would be much more evident than that under the conditions of conventional HPR. The requirement of a high HYD activity to 17704



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

feed

conditions

Mild Conditions Pd/C, RANEY Ni, Ni/SiO2, Ni/ASA, Nafion suspensions, Nafion/SiO2, propyl-phenol zeolites Pt/SiO2−Al2O3, Pd/SiO2−Al2O3 sorbitol Ni-MgO sorbitol Ru/C, Ru/Al2O3, Pt/Al2O3, Pt/C, Pd/Al2O3, Pd/C, sulfided CoMo/Al2O3, acetic acid, p-cresol NiMo/Al2O3, and NiW/Al2O3 Ru/C and Pd/C furfural, guaiacol, acetic acid

ref.

batch.; 200−300 °C; 4 MPa

35

cont.; 225 and 265 °C; co-fed H2 batch; 200 °C; 4 MPa; H2 batch; 150 and 300 °C; 4.8 MPa of H2 batch; 150−300 °C; 6.9 MPa total cont.; 150−230 °C; 6.4 MPa total batch; 130−279 °C; 6−11 MPa total batch.; 200 and 250 °C; 4 MPa H2 batch; 180 and 200 °C 280 °C; 4 MPa; H2 batch; 200−300 °C; 4−12 MPa batch; 150−190 °C batch; 245 °C; 6 MPa; H2 cont.; 160−240 °C; flow of H2 batch; 160 °C; 0.6 MPa; H2 batch; 16−240 °C; 4 MPa; H2 cont.; 200 °C; 4 MPa; H2 batch; 150 °C; 5 MPa

39 119 82, 110

33, 34 112 114 115 120 116 36 101 113 118

batch; 350 °C; 3.5 MPa H2 batch; 350 °C batch; 330 °C

86 24 122

batch reactor; 350 °C; 0.5−5 h; without H2 batch; 330−370 °C; 4 h; either CO or H2

123

125 126

palmitic acid

batch; 380 °C; 6−29 MPa batch; 380−420 °C; 0−6.9 MPa H2 batch; 380 °C; 28 MPa; no H2

Pd(5%)/C, Pt(5%)/C

microalgae, algae bio-oil

batch; 400 °C; 3.4 MPa; H2

NiMo/Al2O3, CoMo/Al2O3

DBT

batch; 400 °C; 30 MPa total

Pt/C, Pd/C, Ru/C, Rh/C, Pt/Al2O3, Mo2C, MoS2, PtO2, Al2O3, CoMo/ Al2O3, carbon NiMo/Al2O3 cubic and octahedral CeO2 nanoparticles

algae biomass

batch; 380−420 °C; 6.9 MPa; H2

127, 128 127, 128 53, 87, 88 128

DBT, naphthalene, tetraline, carbazole bitumen

batch, 400 °C; 2.5−4.0 MPa; H2 batch; 723 K

92, 93 93

Ru/ZrO2 and RuMo/ZrO2

propanoic acid

Pt (3−5 nm) protected by polyethyleneimine

glucose and fructose

Pt on HY, H$, HZSM-5, (-Al2O3 and SiO2

phenol

Pd/C Pt on AC, MWCN and CB; Pt on ZrO2, TiO2, and CeO2 Ru(5%)/H-β Pt(1%)/H-β zeolite Ni−W/SiO2 Pt/ZrO2 with H4SiW12O40, H3PW12O40, H3PMo12O40 Ni/SiO2 Ni/HZM-5, Ni/Al2O3 Cu/ZrO2 varying Cu/Zr ratio Ru + transition metal (e.g., Zn, Cr, Mn, Co, Fe, and Ni) Pd/C, Pt/C, Ru/C, Ni/SiO2−Al2O3, zeolite, sulf. CoMo/Al2O3 sulf. NiMo/Al2O3 Pt(5%)/C Pt, Pd and Ni all supported on carbon

phenol 4-propylphenol lignin cellulose cellulose glycerol aryl-ethers phenol glycerol benzene Subcritical Conditions microalga pyrolysis oil fatty acids (stearic, palmitic, lauric, oleic, and linoleic) jathropa biofeed

sulf. Pt/C

Pt(5%)/C Pt/C, Pd/C, Ru/C, Rh/C, Pt/Al2O3, Mo2C, MoS2, PtO2, Al2O3, sulf. CoMo/Al2O3 Pt/C and Pd/C

hydrothermal liquef. biofeed Supercritical Conditions benzofuran pyridine

111 30 31 32

124

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 rather than being carried out with reaction streams as it is in the case of continuous systems. Moreover, some inherent limitations of batch systems (i.e., an unsteady-state operation, continuously varying catalyst/oil ratio, a lengthy heat-up periods between preheat and reaction, etc.) should not be overlooked. On the other hand, in batch systems, the mixture/slurry (water + catalyst + feed) for testing in an aqueous phase can be readily prepared 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

minimize catalyst deactivation during the HPR in aqueous phase was indicated above. It appears that for these applications, the catalysts comprising noble metals (e.g., Pt, Pd, Ru, and Rh) supported on various supports may be more suitable than conventional HPR catalysts. The summary of studies 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 the applications under aqueous conditions and those used for the conventional HPR. An extensive information on preparation of the latter catalysts can be found in the scientific literature.2,6−12 The validity of experimental data on catalyst performance is influenced by the testing protocol employed.99,100 The catalyst evaluations in both batch and continuous systems have been generally observed. Different sizes of batch reactors, starting 17705



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

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 system needs to be designed to bring catalyst testing for the applications in an aqueous phase to the next level. 6.2. Selection of Supports. The choice of support for preparation of the catalysts to be used in aqueous media is crucial because of a potential reaction with water. Thus, in the case of conventional catalysts, the most frequently used γ-Al2O3 tends to undergo the H 2 O aided transformation to boehmite,29,104−106 as it is shown by reaction {15} in Figure 11.

detailed studies may be required on selected catalysts. Lee et al.101 used this method for determining the initial activity of the supported monometallic catalysts such as Pd, Pt, Ru, Rh, Ni, and Co for the HYD of various carbonyl group containing reactants in an aqueous phase. They used reactor which consisted of 24 wells machined into a cylindrical stainless steel high pressure chamber. Using this system, the database was generated 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 and up to 650 °C during the conversion of S-containing reactants in SCW. In this case, water and model reactants were fed separately to the top, without premixing. It might be the only study, in which CSTR system was used to study conversion in SCW. Figure 10 shows the continuous system,102 which may be adapted for catalyst testing in the presence of water. The list of major components and modes of operation are evident from the schematics. The continuous system is one of the few found in literature, being used for catalyst testing under SCW conditions. Of a particular importance is the separate unit required for the generation of SCW and subsequent mixing with reaction streams. Absence of the source of external H2 in the schematic of continuous system should be noted. However, because of the catalytic upgrading of an algae feed being studied,102 hydrogen required for HPR reactions was generated in situ via WGS reaction involving CO produced during decarbonylation of the feed. Obviously, for the HPR of liquid feeds (e.g., biofeeds, FTS reaction water, etc.) in an aqueous phase, the continuous system in Figure 10102 requires modifications. First of all, the preheater−reactor unit would have to be replaced by a catalytic reactor. It is believed that such a change could be made without any difficulties. For example, in the study conducted by Zohrer et al.,103 biofeed (glycerol) was injected directly into SCW entering catalytic reactor. Little problems during the feeding into continuous reactors are anticipated for high water content liquid feeds such as biofeeds and reaction water from FTS

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

Ravenelle et al.107 observed that the conversion of γ-Al2O3 to boehmite was complete within 10 h at 200 °C. However, for the Ni/γ-Al2O3 and Pt/γ-Al2O3, the transformation was significantly slowed down. On the other hand, α-Al2O3 exhibited a high stability at 350 °C during dehydration of the 1:1 mixture of heavy alcohols in water during more than 30 days.3 Under similar conditions, SiO2−Al2O3 exhibited a high stability as part of the Ni/SiO2−Al2O3 catalyst used for the partial HYD of the aqueous phase obtained during FTS.3 It is believed that other metal oxides (e.g., zeolites, TiO2, ZrO2, mixed oxides, etc.) may be more suitable supports than traditionally used γ-Al2O3. It should be emphasized that for the catalysts to be used for the aqueous phase HPR of biofeeds and reaction water from FTS, a proper acidity of supports may be required to maintain a high rate of alcohol dehydration as the last step of HDO (reaction {13} in Figure 7). In this case, some zeolites may exhibit a desirable acidity. Therefore, the interests in zeolites as 17706



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p-cresol and the mixture of p-cresol with acetic acid. The inhibiting effect of p-cresol on conversion of acetic acid and a beneficial effect of acetic acid on the HDO of p-cresol was discussed earlier.82 Thus, the beneficial effect resulted from the contribution 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 Ru catalysts supported either on carbon or on γ-Al2O3 by Wan et al.110 under identical conditions as above82 at 300 °C. Under these conditions, conventional catalysts were inactive. The highest activity for hydrocarbons formation was exhibited by the 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 between 150 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 active than Pd/C catalyst; however, above 250 °C, reforming and methanation reactions (i.e., reactions {1}, {2} and{4} in Figure 1) became quite evident over Ru/C catalyst. A lower activity of Pd/C could be increased by increasing temperature. At the same time, the production of methane and CO2 was kept at a minimum. The aqueous phase HDO of propanoic acid was used by Chen et al.30 to compare Ru/ZrO2 and RuMo/ZrO2 catalysts in a 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 the temperature range 150−230 °C, the total pressure of 6.4 MPa ensured an aqueous-phase system of 0.83 mol/L of the reactant. For the RuMo/ZrO2 catalyst, the effect of Mo/Ru ratio (0−1.5) on the activity and selectivity was investigated. The catalyst with the Mo/Ru ratio of 0.2 exhibited the highest activity for the conversion of propanoic acid. The activity decreased with further increase in the Mo/Ru ratio. The products included propanol, methane, ethane, propane, and trace amounts of ethanol and acetic acid. Over Ru/ZrO2 catalyst, C−C bond cleavage to methane and ethane was dominant reaction compared with the HYD of CO bond, while the RuMo/ZrO2 catalyst favored the HYD of CO bond in propanoic acid. The Pt nanoparticles (3 to 5 nm) protected by polyethyleneimine (Pt-PEI) were used as catalyst for the conversion of glucose and fructose at 403−543 K in subcritical water and H2 in a batch reactor.31 For the experiments, H2 was introduced into reactor at ambient temperature until the pressure reached 5.0 MPa. In the temperature range used, the total pressure increased from 6 to 11 MPa. Typically, 0.48 g of glucose, 1 g of aqueous dispersion Pt-PEI (∼ 5 mg Pt) and 60 g of ion-exchanged water, were used. Compared with inert atmosphere (Ar), conversion was significantly enhanced under H2. At 403 K, glucose could be readily isomerized to fructose. In the temperature range 483−543 K, glucose produced 1,2propanediol, 1,2-hexanediol, and ethylene glycol, while fructose yielded 1,2-propanediol, 1,2-hexanediol, and glycerol. Other catalysts tested included the Pt protected by polyvinylpyrrolidone, Pt/SiO2, and Pt/Al2O3; however, Pt-PEI exhibited superior activity. Hong et al.32 studied the effect of support on the activity of Pt (1 wt %) catalysts during the conversion of phenol. In this case, zeolites such as HY, H$, and HZSM-5 as well as γ-Al2O3 and SiO2, were compared (473 and 523 K; H2 pressure of 4 MPa; WHSV of 20 h−1; 10 wt % H2O). For all catalysts, the overall phenol conversion reached almost 100%. However,

the supports for catalysts have been steadily growing. Ravenelle et al.108 investigated the hydrothermal stability of zeolites Y and ZSM-5 with the varying Si/Al ratios in liquid water at 150 and 200 °C. Under these conditions, ZSM-5 zeolite was not modified. However, the zeolite Y with the Si/Al ratio of 14 or higher was transformed into an amorphous solid. The zeolite degradation was caused by hydrolysis of Si−O−Si bonds rather than by dealumination. This resulted in the loss of micropore volume 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 most stable support for Ru catalysts. The stability of supports and corresponding Ru (2 wt %) catalysts were tested in an autoclave at 400 °C, 28.5 MPa, for 20 h. The Ru/ZrO2 catalyst supported on tetragonal ZrO2 exhibited the highest activity and stability during the conversion of glycerol in a continuous system operating under SCW conditions. Carbons are neutral and hydrophobic supports; therefore, in an aqueous medium, they exhibit high stability. Various forms of carbons (i.e., activated carbons, carbon blacks, carbon composites, carbon nanotubes, etc.) may be suitable support.20 For reactive carbon solids, the potential reaction of carbon with H2O (reaction {16}) becomes evident at above 700 °C. A low reactivity for this reaction is ensured by a high severity used for carbon supports preparation.20,109 For example, activated carbon is prepared by steaming and/or partial oxidation (diluted air) of various carbonaceous solids at about 850 °C. Oxygen containing entities (carbon centered peroxides and peroxy radicals, etheric groups, hydroxyl groups, etc.) on the surface of activated carbons left behind may play an important role during the impregnation with active metals. Therefore, a desirable stability of the carbon supported catalysts in an aqueous environment is anticipated, although some experimental data obtained under subcritical and particularly under supercritical 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 extent real 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) has been focusing on two groups of metals, that is, noble metals (e.g., Pt, Pd, Ru, and Rh) and other metals (Ni, Co, Cu, etc). A wide range of supports were tested; however, carbons were the supports of choice. γ-Al2O3 has also been used, although some stability problems during a long-term performance of catalysts may be anticipated, as it was indicated above. 6.3.1.1. Noble Metals Containing Catalysts. As the most abundant product in biomass pyrolysis liquids, acetic acid alone and in the mixture with p-cresol was investigated at 150 and 300 °C over a series of catalysts (i.e., Ru/C, Ru/Al2O3, Pt/ Al2O3, Pt/C, Pd/Al2O3 and Pd/C).82 The experiments were conducted in a batch reactor at 4.8 MPa of H2. For experiments, 0.05 mol of substrate and 0.2 g of catalyst were added to 40 mL of water. With respect to the conversion of acetic 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 over Pt/Al2O3 and trace amounts of ethane over Ru/C and Pt/C catalysts. The Ru/C was the only catalyst used for the HDO of 17707



Review

conditions, high yields of gaseous products were formed over the catalysts supported on H-β and H-Y zeolites with threedimensional structure and large pores. Apparently, among the catalysts tested, a catalyst which can maximize the yield of the product of interest can be selected. Pt/ZrO2 catalysts modified with heteropolyacids such as H4SiW12O40, H3PW12O40, and H3PMo12O40 were used for the hydrogenolysis of aqueous glycerol (10% glycerol) to 1,3propanediol.116 The unmodified Pt/ZrO2 was used for comparison. The modified Pt/ZrO2 catalysts were more active because of a higher acidity. Thus, the catalyst modified with H4SiW12O40 was the most active because of suitable Brønsted acid sites. At the same time, 1,2-propanediol yield increased with increasing concentration of Lewis acid sites. In this study, experiments were conducted in a continuous system in the flow of 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, 2propanediol in water in an autoclave (423 K; 5 MPa H2; 8 h). The catalyst exhibited a high activity and selectivity even after being 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 for the HYD of benzene at 150 °C and 5 MPa in an autoclave. Under these conditions, transition metals were present in an oxidic form, while Ru in a reduced form. The reaction mixture comprised 49.2 g catalyst, 280 mL H2O, and 140 mL benzene. The catalyst preparation involved adding NaOH solution to the mixture of RuCl3·H2O and sulfate of the corresponding transition metal under continuous stirring at 353 K. The black precipitate obtained was dispersed in distilled water and transferred to autoclave to be treated at 423 K under 5 MPa of H2. The isolated powder was washed and vacuum-dried before being used for activity tests and extensive spectroscopic evaluations. The bimetallic catalysts such as Ru−Mn(0.23), Ru−Fe(0.47), and Ru−Zn(0.27) exhibited a high selectivity for cyclohexene. 6.3.1.2. Other Metals Containing Catalysts. Series of the Ni/HZSM-5 catalysts containing variable amount of Ni (e.g., 6, 10, 14, and 17 wt %) and the Ni(10%)/Al2O3 catalyst were compared during the HDO of phenol between 160 to 240 °C and 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 were dominant products. Small amounts of methyl-cyclopentane were formed as well. The Ni/SiO2 catalyst exhibited a high activity for the HDO of aryl-ethers in water at 160 °C and 0.6 MPa of H2 in an autoclave.36 Zhao et al.35 expanded their study with the aim to develop a solid acids as the source of hydroniom ions, which are stable in an aqueous medium at high temperatures. For example, the Nafion polymer used as one of the solid acids was hardly ionized as confirmed by little change in pH with time on stream. To test the concept, a series of liquid and solid acids in the presence of Pd/C and RANEY Ni catalysts were used for the aqueous HDO of 4-propylphenol. The results are summarized in Table 5.35 A combination of aqueous solutions of either H3PO4 or CH3COOH with Pd/C yielded 84 and 74% of propylcyclohexane, respectively, while 98% yield of propylcyclohexane was obtained with both Nafion suspension in water and the Nafion supported on SiO2 (13 wt % of Nafion). A combination of either Nafion water suspensions or

significant difference in product distribution, was observed. Thus, cyclohexane accounted for more than 90% of all products over Pt/zeolite catalysts compared with less than 3 wt % over Pt/Al2O3 and Pt/SiO2 catalysts. For the latter catalysts, cyclohexanol accounted for almost 95% of the converted phenol. This may be attributed to a lack of surface acidity which would enhance dehydration of cyclohexanol (e.g., via reaction {10} and {13}). Small amounts of bicyclics and tricyclic products 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 an increased yield of cyclohexcanol and decreased yield cyclohexanone with time on stream. This suggests that the latter was an intermediate for the formation of cyclohexenol. At 453 K, small amount of cyclohexane was formed if the solution was acidified with H3PO4. However, temperature increase to 473 K resulted in a complete dehydration of cyclohexanol to cyclohexene (reaction {10}) followed by the HYD of latter (reaction {11) to cyclohexane. There was little evidence of the hydrogenolysis of phenol to benzene. Similarly, a high selectivity to cyclohexanol in a neutral aqueous solution was reported over Pt-, Ru-, and Rh-based catalysts.74 However, with temperature increase to 473 K of the acidified solution, cyclohexanol was quantitatively dehydrated to cyclohexene followed by HYD to cyclohexane. Similarly, 4-n-propylguaiacol, 4-allylguaiacol, and 4-acetonylguaiacol were converted to cycloalkanes (∼80%), methanol (7−8%) 12−18% intermediate cycloalcohols or cycloketones (12−18%). In this case, the reaction 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), mesoporous carbon, multiwalled carbon nanotube, and carbon black via impregnation with aqueous solution of H2PtCl6. The catalysts were used for the HDO of 4-propylphenol in water at 280 EC under 4 MPa H2. The Pt/ACN exhibited high activity with 97% yield of propylcyclohexane, similarly to the Pt catalyst supported on mesoporous carbon and carbon nanotube. The Pt supported on carbon black was less active. The Pt catalysts supported on ZrO2, TiO2, and CeO2 were moderately active, but besides propylcyclohexane, propylbenzene in 3−10% yield was also present. These results are consistent with the HDO mechanism observed during conventional HPR. Contrary to these observations, Pt/Al2O3 catalyst was inactive because of the structural transformation of (-Al2O3 into boehmite. The activity of the Rh, Ru, and Pd catalysts supported on activated carbon 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 in the presence of the Ru(5%)/H-β catalyst.113 A dozen of oxygenates were identified in liquid products. However, rather low overall conversion of lignin (less than 20%) should be noted. Patil et al.114 used Ru(5%)/H-β catalyst for the conversion of lignin to hydrocarbons in an autoclave from 200 to 300 °C and total pressure of 4 to 12 MPa. Under these conditions, less than 20% conversion of lignin was observed. After adding a basic solution (1 M NaOH) to the mixture, the conversion increased to almost 33%. This supports the involvement 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 hydrocarbons during the conversion of cellulose in distilled water under mild conditions (423−463 K) in batch reactor. Under the same 17708



Review

length of milling, that is, from 126 to 10 nm from 0 to 240 h milling period. For the experiments, 50 g of residue were mixed with 50 g of seawater and 1 g of catalyst. A significant viscosity reduction was achieved with catalyst prepared with milling time exceeding 200 h. This coincided with a marked decrease in the content of resins and asphaltenes in the feed. Rather superior activity of the was evident considering the extent of viscosity reduction under rather mild conditions. 6.3.2. Subcritical Conditions. Fu et al.122 observed a high activity of the Pt(5%)/C catalyst for the conversion of fatty acids (stearic, palmitic, lauric, oleic, and linoleic). Oleic and linoleic acids had one and two double bonds, respectively, while the remaining acids were saturated. The experiments were conducted under subcritical water conditions at 330 °C in an autoclave. For the unsaturated acids, the conversion involved the HYD of double bond first, followed by decarboxylation. The latter reaction dominated the overall conversion of the acids to saturated n-alkanes. In line with the mechanism discussed earlier, the HYD of double bonds involved hydrogen generated 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; without H2) over the carbon supported Pt, Pd, and Ni catalysts. The results in Table 6 confirmed a high activity of the Pt and Ni

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

acid

conv., %

Pd/C Pd/C RANEY Ni RANEY Ni Pd/C Pd/C Pd/C Pd/C RANEY Ni RANEY Ni RANEY Ni 2400 RANEY Ni 4200 Ni/SiO2 Ni/ASA

H3PO4 CH3COOH H3PO4 CH3COOH zeolite (H-β) zeolite (H-Y) Nafion solution Nafion/SiO2 Nafion/SiO2 Nafion solution Nafion/SiO2 Nafion/SiO2 Nafion/SiO2 Nafion/SiO2

100 100 0 0 100 100 100 100 100 100 51 96 9 37

cycloalkane select. % 84 74

1.5 5.2 98 98 99 98 36 64 43 50

the Nafion/SiO2 composite with freshly prepared Ni catalysts (RANEY Ni) resulted in 100% n-propylcyclohexane yield compared with 51 and 96% yields for commercial RANEY Ni2400 and RANEY Ni4200, respectively. Compared with Nafion, zeolites were poor source of hydronium ions as indicated by rather low yields of propylcyclohexane. The combination of RANEY Ni with Nafion/SiO2 was used to study the 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 presence of a catalyst and H2, this may be achieved under mild conditions. This is illustrated using two studies published recently. In one study, an aqueous solution of glycerol (40% glycerol) was used as the feed to produce 1,2-propanediol over a series of Cu/ZrO2 catalysts with different Cu contents.114 Experiments were carried out in batch reactor at 200 °C and 4.0 MPa H2. The selectivity increased with the increasing Cu/Zr ratio while the overall glycerol conversion exhibited little change. Chen et al.119 used an aqueous solution of sorbitol (20%) to study its hydrogenolysis to glycols and glycerol (473 K; 4 MPa H2; batch) over a series of Ni-MgO catalysts with varying Ni/ Mg ratio. In terms of conversion and selectivity, the best performance was exhibited by the catalyst with the Ni/Mg ratio of 3/7. However, catalyst deactivation was noted under more severe conditions. This was indicated by the more complex mixture of products, although the sorbitol conversion increased with increasing severity. The bimetallic Ni/W/SiO2 catalysts containing 5 wt % Ni and 25 wt % W was used for conversion of cellulose into low molecular weight polyols (sorbitol, mannitol, erythritol, ethylene glycol, and 1,2-propanediol) in an aqueous solution.120 For the experiments, 500 mg cellulose and 50 mg catalysts were coslurried with 30 mL deionized water in an autoclave which was subsequently pressurized with H2 to 6 MPa at ambient temperature. The experiments were conducted at 518 K. The bimetallic catalyst was much more active than either Ni/SiO2 + W or Ni/SiO2 + WO3 mixtures. The reduced form of the bimetallic 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 for upgrading a residual feed in an autoclave at 200 °C, 3 MPa, and 24 h.121 The size of catalyst particle decreased with increasing

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

C1−4

C7−14,16,18

C15

C17

fatty acids

no catalyst Pt/C Pd/C NiC Pt/NiC PtC-Pa PdC-Pa

0 1.2 0.3 54.5 19.0 0.2 0

0.2 1.4 0.4 2.0 3.6 0.3

Hydroprocessing in Aqueous Phase - Industrial & Engineering

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