Review of CC Coupling Reactions in Biomass

0 downloads 0 Views 218KB Size Report
these oxygenated molecules can take place by aldol condensation, alkylation, or ketonization as will .... essing approach does not require use of additional H2.
Current Organic Synthesis, 2010, 7, 00-00

1

Review of C-C Coupling Reactions in Biomass Exploitation Processes Eleni F. Iliopoulou* Center for Research and Technology Hellas, Chemical Process Engineering Research Institute, Laboratory of Environmental Fuels and Hydrocarbons, 6th Kilometer Harilaou-Thermi Road, P.O. Box 60361 – Thermi, GR-57001 Thessaloniki, Greece Abstract: Biomass represents the type of renewable energy source that will play a substantial role in the future global energy balance, as it is the only renewable source with the potential to be converted to liquid, solid and gaseous fuels. Today lignocellulosic biomass appears to be the cheapest and most abundant biomass feedstock, yet no economically and technologically feasible process is developed for its conversion to fuels, at least competitive with the currently used petroleum fuels. Biomass pyrolysis is an emerging thermochemical conversion process that leads to the production of bio-oil. Bio-oil however, presents several undesired properties that do not permit its direct exploitation. On the other hand, sugars derived from lignocellulosic biomass make up the predominant class of compounds, also exhibiting undesired properties. In both cases presence of oxygen in biomass derived copounds seems the major problem. Recently the targeted pathways for the conversion of lignocellulose-derived carbohydrates to liquid transportation fuels involve the partial removal of oxygen, CC bond formation of functional molecules, and removal of the remaining oxygen. The formation of C-C bonds between these oxygenated molecules can take place by aldol condensation, alkylation, or ketonization as will be described in the present review study.

Key Words: C-C coupling, ketonization, aldol-condensation, biomass conversion. 1. INTRODUCTION

An approximately 50% increase above 2002 level is expected in world energy demand, which is nowadays mainly fulfilled from the conventional energy resources like coal, petroleum and natural gas. A review of energy consumption data reveal that currently global energy systems rely heavily on coal for electrical power generation and oil for transportation energy, the latter being one of the largest and fastest growing global energy sectors [1]. However, the current energy resources are in the verge of getting extinct, estimating tentative depletion of oil sources till 2050. Global energy crisis including depleting deposits and consequent increasing prices of petroleum oil, combined with environmental problems and concerns strongly motivated our society to search for alternative, renewable energy sources and especially for liquid transportation fuels. In this respect, exponentially increasing attention has been focused on biomass, which is considered worldwide as a major renewable energy source and is expected to play a substantial role in the future global energy balance [2]. Despite its lower energy content, biomass exhibits several advantages, as compared to fossil fuels. It is the only renewable, organic and the most abundant resource, of low cost, it is considered carbon dioxide neutral (as CO2 produced during biomass conversion and utilization has been already adsorbed during photosynthesis) [3, 4], while due to lower S and N contents, it is related with less environmental pollution concerns regarding toxic emissions (NOx and SOx)

*Address correspondence to the author at the Center for Research and Technology Hellas, Chemical Process Engineering Research Institute, Laboratory of Environmental Fuels and Hydrocarbons, 6th Kilometer HarilaouThermi Road, P.O. Box 60361 – Thermi, GR-57001 Thessaloniki, Greece; Tel: +30-2310-498312; Fax: +30-2310-498380, E-mail: [email protected] 1570-1794/10 $55.00+.00

[5-10]. Biomass is the only energy source with the potential to be converted to liquid, solid and gaseous fuels [11, 12], while using energy crops and agricultural residues for energy production can provide an alternative solution in the problem of waste disposal [13]. In view of all above and the fact that the practical potential of biomass residues in the world is 114 EJ/year, (equivalent to one-third of commercial energy consumption in the world in 1990 [14], the use of biomass to partially substitute fossil fuels is becoming of growing importance [6, 8, 9, 15-17]. Today, biomass accounts for the 14–15% of the world’s primary energy consumption and about 38–43% of the primary energy consumption in developing countries [18, 19]. Biomass feedstocks can be most accurately classified into three groups: lignocellulosics biomass (the non-edible portion of biomass, e.g., bagasse, wood, etc.), amorphous sugars (e.g., starch, glucose, etc.), and triglycerides (e.g., vegetable oils). Among them, lignocellulosic biomass appears to be the cheapest and most abundant, composed of three primary building blocks: cellulose, hemicellulose and lignin [8] However, while lignocellulosic biomass is a desirable feedstock, it is not yet economically viable to convert it into liquid transportation fuels and especially not competitive with the currently used fossil-based fuels [7]. The three basic categories of biomass conversion technologies include: direct-combustion processes, biochemical processes and thermochemical processes. Biochemical processes include fermentation and anaerobic digestion, while thermochemical conversion processes can be subdivided into gasification, pyrolysis, direct liquefaction and carbonization. Thermochemical conversion techniques are considered the most common and convenient methods of harnessing energy from biomass, while detailed description of them can be found elsewhere [12, 20, 21]. It should be noted here that © 2010 Bentham Science Publishers Ltd.

2 Current Organic Synthesis, 2010, Vol. 7, No. 6

Eleni F. Iliopoulou

Fig. (1). Schematic of lignocellulosic biomass conversion (adapted from reference [28]).

since the composition of the biomass feedstock and the degree of its contamination vary widely, all conversion processes needs to be flexible [22], facilitating conversion of biomass to valuable, liquid, gaseous and solid fuels [23, 24]. Renewable carbohydrate feedstocks exhibit high functionality and corresponding reactivity (i.e., high oxygen to carbon stoichiometry) compared to petroleum feedstocks. Thus, alternative catalytic processing techniques are required to convert these resources into transportation fuels (e.g., gasoline, diesel, and jet fuels) [25]. Transportation sector requires fuels that burn cleanly, have high energy densities and properties allowing their efficient storage at ambient conditions, criteria that are currently best fulfilled by liquid hydrocarbons derived from petroleum oil [1]. Thus the major challenge with biomass conversion strategies is the efficient oxygen removal as CO, CO2, or H2O in order to form products with the previously described appropriate properties (e.g. high energy density and good combustion properties). Today technologies for the production of first-generation biofuels, such as biodiesel and bioethanol, are progressively improved; however, current interest is focused on the second generation of biofuels [26]. As mentioned earlier, current technologies for lignocellulosic conversion involve gasification to synthesis gas (syngas), hydrolysis (either using acids or enzymes) into sugar (cellulose and hemicellulose) and lignin fractions, and selective thermal processes such as pyrolysis and liquefaction. However, a combination of different processes must be utilized to maximize yields, leading to the development of the biorefinery concept (a facility that integrates the various steps involved with biomass conversion). In general, research has focused on the conversion of lignocellulosic biomass resources into simpler fractions and processes that convert these lignocellulosic fractions into heat, electricity, various fuels and platform chemicals [26-28]. A schematic description of processes associated with lignocellulosic biomass conversion in a biorefinery is given in Fig. (1) [28]. 2. C-C COUPLING REACTIONS

It is well known that sugars derived from the cellulose and hemicellulose fractions of lignocellulosic biomass make

up the predominant class of compounds in this biomass feed. These compounds have undesirable properties for use as transportation fuels, such as a C/O stoichiometry of 1:1, low volatility, high water solubility etc. One strategy to convert these sugars into liquid transportation fuels is to remove most of the oxygen atoms, producing such mono-oxygenated fuels as ethanol, butanol, or dimethylfuran. Another attractive approach is to produce higher-molecular- weight hydrocarbons that more closely resemble current transportation fuels derived from oil (e.g., C5–C12 for gasoline, C9–C16 for jet fuel, and C10–C20 for diesel applications). These hydrocarbons could be processed and distributed by existing petrochemical technologies and infrastructure, respectively, and could be used as fuels in existing transportation vehicles. Conversion of sugars into liquid hydrocarbons requires reduction steps to remove oxygen, combined with C-C bondformation steps to increase the molecular weight of the derived compounds [28]. A schematic representation of various strategies to convert carbohydrates and polyols into liquid transportation fuels is given in Fig. (2). As described in Fig. (2), the biomass-derived reactants are converted into functional intermediates by the combination of reforming and reduction reactions and/or by selective dehydration processes. An advantage of the combination of reforming and reduction reactions is that the H2 required is produced in situ, leading to improved energy efficiency and potentially decreased capital costs. However, it is possible that intermediates produced from dehydration reactions may exhibit desirable properties for C-C coupling reactions, thereby justifying the need to supply H2 in a separate processing step. Two are the alternative pathways: The functional intermediates produced from carbohydrates and/or polyols can be converted directly into liquid transportation fuels by dehydration, alkylation, and aromatization reactions to produce alkanes, olefins, and aromatic compounds. This processing approach does not require use of additional H2. Alternatively, the functional intermediates can undergo C-C coupling reactions, e.g. by aldol condensation and/or ketonization processes, followed by hydrogenation and hydrodeoxygenation reactions to produce alkanes with targeted molecular weights. The H2 required for these hydrogenation and

Review of C-C Coupling Reactions in Biomass Exploitation Processes

Current Organic Synthesis, 2010, Vol. 7, No. 6

3

Fig. (2). Strategies for converting carbohydrates and polyols into liquid transportation fuels, involving the combination of various processes including catalytic reforming, reduction, dehydration, alkylation, aromatization, C-C coupling (aldol condensation, ketonization), hydrogenation, and hydrodeoxygenation (adapted from reference [1]).

hydrodeoxygenation steps would be produced from the carbohydrate and polyol reactants by reforming reactions. These C-C reactions are the subject of this paper and will be reviewed in the following sections. In general, coupling at the molecular level has led to the development of inorganic–organic hybrid materials that may be used as heterogeneous catalysts in these reactions [1]. Such catalysts have been recently reviewed [1] and exhibit a multifunctional nature, which provides opportunities for combining reduction and C-C bond-formation reactions. Furthermore, the use of bifunctional catalysts (metals supported on basic metal oxides) to carry out condensation and hydrogenation cycles in a single reactor highlights the use of active sites to combine C-C bond formation with oxygen removal [1]. Among them zeolites (ZSM-5) can be used to form C-C bonds through alkylation reactions between carbohydrate-derived olefins [1]. Furthermore supported rare-earth oxides (CeO2 /ZrO2) can be used for C-C bond formation through ketonization reactions between carbohydrate-derived organic acids [1]. In addition, long-chain alkanes can be produced by a recently developed process that combines polyol reforming catalysis with Fischer–Tropsch catalysis in a single reactor and thus allows for the direct conversion of glycerol into liquid hydrocarbons that are suitable for diesel applications [29, 30]. In this system, the heat necessary for the conversion of glycerol into synthesis gas is supplied by the Fischer– Tropsch reaction, thereby offering a less energyintensive alternative to biomass gasification/Fischer–Tropsch process and requiring fewer reactors and separations [29, 30]. A review of the application of C-C coupling reactions developed and used to convert biomass-derived carbohydrates to a variety of fuel-grade products is the subject of the current communication, examining ketonization, aldolcondensation, as well as their combination towards a more integrated process.

2.1. Ketonization Reactions The synthesis of ketones from carboxylic acids in the vapour phase is called ketonization. Two molecules of carboxylic acids are required to form a ketone via ketonization, producing carbon dioxide and water [31], while the general reaction scheme is expressed as follows: R

OH

R'

OH

R

R' H2O

O

O

CO2

O

In the above reaction, RCOOH reacts with RCOOH via cross-ketonization; on the other hand, two symmetric ketones of RCOR and RCOR can be formed via homoketonization, namely, two different carboxylic acids produce three different ketones [32]. Ketonization has been investigated in the literature using a variety of basic, acidic and amphoteric oxide catalysts, and occurs at temperatures near or higher than 350oC [33]. A number of articles on the ketonization of carboxylic acids are available using several heterogeneous catalysts: oxides, such as Cr2O3, Al2O3, PbO2, TiO2, ZrO2, CeO2, iron oxide, and manganese oxide, as well as Mg/Al hydrotalcites. Many other catalysts have been explored since the 19th century: BaCO3, MgO, ThO2, UO2, CdO, ZnO, NiO, Bi2O3, and SnO 2 are reported in the corresponding literature [32]. Alcohols, aldehydes, and esters are also reported to convert into ketones through dimerization in the presence of CeO2 and ZrO2 [32]. In this respect, the carboxylic acids and esters can be converted into heavier ketones via ketonization reactions, involving the condensation of two molecules of acid (or ester) to produce a linear ketone with 2n - 1 carbon atoms, CO2, and water [34-37]. Use of this reaction as a means of reducing organic acid is of particular importance, because biomass-derived organic effluents can possess a high content of carboxylic acids. For example acid concentration, as high as 30 wt% is reported, when glucose is used

4 Current Organic Synthesis, 2010, Vol. 7, No. 6

Eleni F. Iliopoulou

as reactant [38]. In addition, during thermochemical conversion of biomass, where the lignocellulosic feedstock is converted to a liquid-oil, named bio-oil, which is a mixture of hundreds of organic compounds, organic acids being a significant portion among them.

was studied [25]. Under the condition of this study [25] (fixed-bed, upflow reactor, used for reaction kinetic studies, mixture of hexanoic acid in several solvents, varying reaction temperatures (177–350oC), total pressure of 1 atm), two primary reactions can proceed in parallel:

Both ketonization of propanoic acid and cross ketonization of different carboxylic acids were investigated over CeO2-based catalysts at 300–425oC. CeO2 catalysts modified with Mg, Al, Mn, Fe, Ni, Cu, and Zr are reported as effective for the ketonization of propanoic acid to form 3-pentanone. Among them, CeO2–Mn2O3 is reported as the most efficient. Examining the effect of the synthesis method used for preparing the catalyst it seems that the citrate process is more appropriate than co-precipitation for preparing a solid solution of CeO2–Mn2O3. The degree of mixing in the solid solution was also suggested to affect the catalytic activity for the ketonization [32].

• the reversible esterification of hexanoic acid and 1pentanol to generate 1-pentylhexanoate, and

The same study reports that during the reaction of propanoic acid with another linear carboxylic acid over CeO2 – Mn2O3 at 375 C, the reactivity of the carboxylic acid is slightly decreased, as its chain length is increased. Two kinds of symmetric ketones are formed via homoketonization, and one asymmetric ketone is formed via cross-ketonization. In contrast to linear carboxylic acids, branched acids are less reactive. This is explained as follows: Methyl group substituents at - and -positions in the acids, i.e., decreasing the number of -hydrogen and increasing steric hindrance, decrease the reactivity of the acid for both homo- and cross-ketonization [32]. Ketonization of carbohydrate-derived carboxylic acids in the presence of other monofunctional oxygenated species was investigated over a Ce0.5Zr0.5O2 catalyst [38]. In this study, a catalytic process recently developed [38], can be tuned to convert biomass-derived carbohydrates to a variety of fuel-grade products, such as light alkenes (C1–C4), benzene, substituted aromatics, branched alkenes (C6–C12) and linear or singly branched alkanes (C7–C12). This process consists of an initial de-oxygenation step that converts concentrated aqueous solutions of sugars and polyols (40–60%) over a carbon-supported Pt–Re catalyst into hydrophobic mixtures of mono-functional C4–C6 alcohols, ketones, carboxylic acids and heterocyclic compounds [38]. However, the carbon chain length of these monofunctional species is limited to six by the chain length of the parent carbohydrate, i.e., glucose and sorbitol. For this reason the mixture of monofunctional species is subsequently subjected to catalytic C–C coupling processes (ketonization and aldol condensation/hydrogenation) in order to produce higher molecular weight species required for gasoline, diesel and jet fuel applications. The combination of these C–C coupling processes yields C7–C12 linear and branched ketones, which can be converted into fuel-grade alkanes via dehydration/hydrogenation over e.g. solid acid supported noble metal catalysts, such as Pt/NbOPO4 [39]. As Ce0.5Zr0.5O2 catalyst exhibited desirable catalytic properties for ketonization of carbohydrate-derived carboxylic acids [38], it was selected for further study. As a model for monofunctional organic effluents obtained in the previous correlating work of the same group [38], ketonization of hexanoic acid in mixtures with 1-pentanol and 2-butanone

• the ketonization of hexanoic acid to produce 6undecanone, carbon dioxide, and water. Effect of several parameters on the rates of ketonization and esterification reactions was investigated (e.g. effects of solvents, partial pressures of the reactants and products, and reaction temperature). Tentative inhibiting effects of carbon dioxide and water (produced by ketonization) on the kinetics of esterification and ketonization were additionally studied. Co-presence of CO2 is reported to decrease the ketonization rate, while removing CO2 did not regain initial ketonization catalytic activity  In contrast to the ketonization reaction, the esterification reaction rate was not significantly affected by the presence of CO2. The water presence also exhibited a significant inhibiting effect on the ketonization reaction [25]. Using this ceria–zirconia catalyst it was suggested that esterification and ketonization reactions take place simultaneously. Whereas basic sites are needed for ketonization to take place [25], esterification reactions are commonly carried out over acidic catalysts [25]. It was consequently assumed that ketonization (basic sites) and esterification (acid sites) are catalyzed by different types of sites over ceria–zirconia. This assumption is justified by the fact that the esterification rates increase, when zirconia oxide, which has more acidic sites, is used as a catalyst. In this respect, a ceria–zirconia catalyst was employed to ketonize fully the carboxylic acids and esters present in liquid organic streams derived from the processing of glucose over a Pt–Re/C catalyst [25, 38]. The ketonization of esters, which are an essential intermediate product in many biomass-conversion processes, is a more difficult problem compared to the conversion of carboxylic acids, and the literature does not agree about the details of the reaction mechanism [40, 41]. One suggested pathway is the intermediate formation of an acid by hydrolysis. Thus, one approach to convert a large amount of esters to ketones is to provide water with the reaction mixture fed to the reactor. Water, however, decreases the activity of the ceria–zirconia catalyst, so this approach is not the optimal solution. On the other hand, it is possible that biomass-derived intermediate feeds will contain acids and esters, as observed in the conversion of glucose over Pt–Re/C [38]. Because of the stronger adsorption of acids on the catalyst surface compared to esters, direct ketonization of esters will not take place as long as acids are present. Thus, ketonization of acids takes place preferentially compared to ketonization of esters. Importantly, the water formed by the ketonization of carboxylic acids leads to the subsequent hydrolysis of esters to form carboxylic acids and alcohols, which is followed by the ketonization of these newly formed carboxylic acids. In this way, the coupling between ketonization and hydrolysis reactions (the latter being the reverse of esterification) provides an efficient pathway for the condensation of esters to larger ketones [32].

Review of C-C Coupling Reactions in Biomass Exploitation Processes

Acid adsorption on the catalyst surface is an important step in the reaction, and the rate of ketonization is reported to shift from second order to zero order as the partial pressure of e.g. hexanoic acid increases. Product inhibition takes place through strong binding of CO2, and to a lesser extent water, on the basic sites [25]. Undoubtedly, esterification reactions, which consume acids, are important side reactions, when performed under the same conditions used for ketonization, because alcohols are also present in the hydrophobic mixture produced from glucose and sorbitol [25]. Accordingly, the interaction between esterification and ketonization reactions for mixtures of acids and alcohols was extensively investigated over ceria– zirconia. Secondary alcohols were included in this study, because these compounds are more abundant than primary alcohols in the organic streams produced by conversion of glucose and sorbitol over Pt–Re catalysts [42]. Steric hindrance due to the presence of a bulky group near the reaction center is proposed to explain the different reactivities of primary and secondary alcohols in esterification reactions [43]. Consequently, esterification as a side reaction during the ketonization upgrading of acids over ceria–zirconia becomes less dominant for those oils, rich in secondary alcohols. However, despite the slow esterification rate of secondary alcohols in this mixture of intermediates, the formation of secondary esters cannot be avoided completely. As already mentioned, esters are commonly present along with acids in typical effluent streams from biomass conversion processes [44], and new strategies are thus required for the upgrading of these compounds. An indirect route, involving hydrolysis of the ester and subsequent ketonization of the acid, has been previously proposed to upgrade esters [44]. This approach involves the cofeeding of water along with the reaction mixture to the reactor, which negatively affects the activity of ceria–zirconia for ketonization. Esters can undergo direct ketonization, however, over metal oxides such as CeO2, MnO2, and ZrO2 at temperatures ranging from 350–400oC [40, 41]. Even though the mechanism of this reaction is a debatable topic, it is known that the acid moieties of two ester molecules condense to produce a larger ketone, with the production of CO2 and a mixture of olefin and alcohol having the same number of carbon atoms as the alcoholic moiety of the ester. In addition, esters do not adsorb as strongly as acids on the ceria–zirconia surface. Furthermore, esters of secondary alcohols undergo ketonization at higher rates compared to esters of primary alcohols at the same temperature and partial pressure. Ceria–zirconia, used for acid ketonization, also catalyzes the esterification of mixtures of acids and alcohols, as well as the direct ketonization of esters. Consequently, to investigate the competition of these processes over the surface of the catalyst, feed mixtures containing e.g. hexanoic acid combined with primary and secondary pentanols (using 2butanone as a solvent) were reacted over a ceria–zirconia at various temperatures. Products of the reaction were 6undecanone (as a result of acid and/or ester ketonization), the primary and secondary esters, CO2, and pentene. Ketonization was the main reaction taking place over the ceria– zirconia for all the ranges of partial pressures at the tempera-

Current Organic Synthesis, 2010, Vol. 7, No. 6

5

tures of this study, which are typical for biomass-upgrading processes [38]. In summary, ceria–zirconia is reported as an effective catalyst for the upgrading of acids in bio-oils to larger ketones through ketonization reactions. Esterification is an important side reaction that competes with ketonization, when alcohols are present in the hydrophobic mixture, while secondary alcohols are less reactive for this side reaction. Since this reaction is difficult to avoid over ceria–zirconia, the direct ketonization of esters was investigated as a route for the conversion of these side products to the desired ketones on the same catalyst, without the need to add water to the feed. Compared to acids, the direct ketonization of esters is slower and takes place once the carboxylic acids have been largely converted. Consequently, this ceria–zirconia catalyst can also be applied for the direct conversion of mixtures rich in esters, which are common intermediates in biomassconversion processes [44]. 2.2. Condensation Reactions Aldol condensation/hydrogenation is the reaction network in which two ketone or alcohol molecules react to form a heavier branched ketone [32]. Aldol-condensation is generally carried out in the presence of base catalysts to form a carbon–carbon bond between two carbonyl-containing compounds. The reaction is carried out in polar solvents such as water or water–methanol, in the presence of base catalysts at low temperatures. The aldol-products have limited solubility in water and they precipitate out of the aqueous phase [45]. Many aldol-condensation reactions are practiced industrially, using homogenous base catalysts such as sodium and calcium hydroxide. However, these processes generate significant wastewater streams that must be neutralized, which leads to additional disposal cost. Developing a new active and stable solid base catalyst can simplify the process, promote clean manufacturing, and decrease the production cost. Among the solid base catalysts, reported as active for aldolcondensation, are alkali and alkaline earth oxides, anionexchange resins, phosphates, MCM41, and hydrotalcites [46]. Most of the applications with solid base catalysts for aldol-condensation have been realised either in liquid phase, using organic solvent or in vapor phase. Magnesia–zirconia and magnesia–titania have been tested as base catalysts for acetone condensation in the vapor phase. Few studies have been carried out using solid base catalysts for aldolcondensation in pure water as the solvent. Aldolcondensation in an aqueous environment has been reported for carbohydrate-derived molecules, using heterogeneous catalysts such as weak anion exchange resins, organocatalysts like cyclic secondary amines, and homogeneous catalysts like NaOH. Cross-condensation of furfural with acetone has been conducted using heterogeneous aminofunctionalized mesoporous base catalysts in organic solvents [46]. The subsequent step after aldol-condensation is hydrogenation of aldol-adducts to increase their solubility in the aqueous phase [45]. The combined aldol condensation/hydrogenation of alcohols and ketones has been studied in the literature on metals (Cu and Pd) supported on mixed oxides such as MgAlOx [30, 47] and anion-exchange resins [48] in the temperature range from 150 to 200oC in the presence of hydrogen. This reaction consists of metal-catalyzed

6 Current Organic Synthesis, 2010, Vol. 7, No. 6

dehydrogenation of alcohol groups, followed by basecatalyzed aldol condensation, acid-catalyzed dehydration and subsequent metal catalyzed hydrogenation of the a–b unsaturated aldol adduct. Hydrogenation of the C=C bond in the dehydrated aldol adduct improves the overall thermodynamics of the process, allowing a greater extent of C–C coupling, than aldol condensation/dehydration alone [30, 47]. Several investigators report that the presence of intermediate strength, acidic and basic sites, in proximity results in optimal catalytic performance [33, 47], and the presence of strong acid or basic sites leads to undesirable side reactions that may also lead to coking [49]. Most studies of aldol condensation/hydrogenation have focused on self-condensation of acetone or isopropanol to form methyl-isobutyl-ketone (MIBK), and few reports have been published regarding self-condensation of higher ketones (such as the ones produced from the conversion of carbohydrates on Pt–Re/C) [31, 33]. In contrast to acetone, C4+ methyl ketones possess a single set of reactive a-hydrogen atoms, resulting in a substantial decline in reactivity, when compared to acetone. As a result, higher reaction temperatures are required to achieve appreciable conversion, and appropriate catalysts must be used; catalysts that are resistant to coking and sintering at these conditions. In this respect, West et al. [42] have reported the self-condensation/hydrogenation of 2-hexanone on a CuMg10Al7Ox catalyst at 300oC and have shown that even small amounts of carboxylic acids are detrimental to the self-condensation reaction. For this reason, the ketonization reaction provides an efficient means of removing carboxylic acids from the feed prior to aldol condensation/hydrogenation. It has been reported that a catalyst consisting of Pd-supported on CeZrOx is effective at 350oC for the condensation of ketones present in the ketonized mixture of monofunctional species derived from glucose [38]. The CeZrOx support was selected, because it possesses high lattice oxygen mobility and the ability to interact strongly with supported metals – properties that have been associated with its resistance to the formation of carbonaceous species that lead to deactivation [33]. Additionally, CeZrOx contains a combination of acidic and basic functionalities for aldol condensation. Thus, ketonization and aldol condensation/hydrogenation is reported to be feasibly carried out at comparable operating conditions, and can potentially be integrated into a single reactor system, containing a catalytic bed consisting of CeZrOx to perform ketonization, followed by a downstream bed of Pd/CeZrOx to perform aldol condensation/hydrogenation [33], as it will be discussed in a following section. One strategy to achieve C–C coupling between ketones in the presence of hydrogen at 247oC is to employ a bifunctional catalyst containing basic (or acid) sites and metal sites, where the basic (or acid) sites facilitate aldolcondensation and the metal sites facilitate hydrogenation and dehydrogenation reactions. Because the hydrogenation of ketones to alcohols is not favorable at 247oC, the concentration of ketones in the reactor remains high, even in the presence of H2, allowing bi-molecular aldol-condensation reactions to take place over basic (or acidic) sites. Subsequently, dehydration of the aldol adducts produces a molecule with an unsaturated carbon-carbon bond that is readily hydrogenated at these reaction conditions [42].

Eleni F. Iliopoulou

The reduction/dehydration of a branched ketone to the corresponding alkane is thermodynamically favorable. Thus, alkanes with longer carbon chains than the initial polyol or sugar reactant can be formed by first forming ketones, then aldol condensing these ketones, and finished by combining hydrogenation and dehydration steps [42]. As depicted in Fig. (3), the production of heavier liquid phase alkanes from carbohydrates involves a series of reaction steps starting with acid hydrolysis of polysaccharides to produce monosaccharides, followed by acid-catalyzed dehydration to form carbonyl-containing furan compounds such as 5-hydroxymethylfurfural (HMF) and furfural. Subsequently, these compounds can be condensed via aldol reaction to produce larger organic molecules (>C6) by forming C–Cbonds, and these aldol-products can be hydrogenated to form large water-soluble organic compounds. These molecules are then converted to liquid alkanes (ranging from C7 to C15) by aqueous-phase dehydration/hydrogenation (APD/H) over a bifunctional catalyst containing acid and metal sites in a flow reactor. Thus, dehydration of carbohydrates to produce furanic compounds and the aldolcondensation reaction play important roles in multi-step catalytic production for liquid alkanes starting from biomass feedstocks [45]. In detail, a schematic diagram for conversion of biomass into liquid alkane fuels (Fig. 3) is based on aldolcondensation followed by dehydration/hydrogenation. In the first step, polysaccharides such as cellulose, hemicellulose, starch, inulin, etc., are converted to monosaccharides such as xylose, glucose and fructose. Pentoses and hexoses thus formed are converted to carbonyl-containing compounds such as furfural and HMF, respectively, using acid-catalyzed dehydration. The xylose and glucose can also be converted to H2 for use in subsequent steps by aqueous-phase reforming (APR) [50]. Similarly, sugars can be fermented to produce acetone that can be cross-condensed with HMF or furfural to form large organic molecules ranging from C7 to C15. The cross-condensed molecules are then hydrogenated to increase their solubility in the aqueous phase, and these compounds are then fed to a four-phase dehydration/hydrogenation reactor to produce liquid alkanes. The four-phase dehydration/hydrogenation reactor system consists of (i) an aqueous inlet stream containing large watersoluble molecules, (ii) a hexadecane alkane sweep inlet stream, (iii) a H2 inlet gas stream, and (iv) a solid catalyst (e.g. Pt/SiO2–Al2O3) [51]. In a recent work Huber and Dumesic overviewed aqueous-phase processing for conversion of biomass-derived oxygenates into hydrogen and alkanes (ranging from C1 to C15) [52]. Hydrogen can be produced by aqueous-phase reforming (APR) of biomass-derived oxygenated hydrocarbons at low temperatures (150oC-265oC) in a single reactor over supported metal catalysts. Alkanes, ranging from C1 to C6 can be produced by aqueous-phase dehydration/hydrogenation (APD/H). This APD/H process involves a bifunctional pathway in which sorbitol (hydrogenated glucose) is repeatedly dehydrated by a solid acid (SiO2–Al2O3) or a mineral acid (HCl) catalyst and then hydrogenated on a metal catalyst (Pt or Pd). Liquid alkanes ranging from C7 to C15 can be produced from carbohydrates by combining the

Review of C-C Coupling Reactions in Biomass Exploitation Processes

Current Organic Synthesis, 2010, Vol. 7, No. 6

7

Fig. (3). Schematic diagram for production of liquid alkanes from biomass resources in a biorefinery (adapted from reference [45]).

dehydration/hydrogenation process with an upstream aldol condensation step to form C–C bonds. In this case, the dehydration/hydrogenation step takes place over a bi-functional catalyst (4 wt.% Pt/SiO2–Al2O3) containing acid and metal sites in a specially designed four-phase reactor (previously described) employing an aqueous inlet stream containing the large water-soluble organic reactant, a hexadecane alkane sweep stream, and a H2 inlet gas stream. The aqueous organic reactant become more hydrophobic during dehydration/ hydrogenation, and the hexadecane sweep stream removes these species from the catalyst as valuable products before they go on further to form coke. These aqueous phase processes could be used in an integrated biorefinery to produce a range of fuels, as shown in Fig. (4). The first step in the biorefining process is conversion of biomass into an aqueous sugar solution. Production of hydrogen for biorefining processes is accomplished by aqueous-phase reforming (APR) [36, 52]. The biorefinery can also produce light alkanes ranging from C1 to C6 by aqueous-phase dehydration/hydrogenation (APD/H) [53]. The light alkanes could be used as synthetic natural gas, liquefied petroleum gas, and a light naptha stream. Aqueous-phase processing can also produce larger alkanes ranging from C7 to C15 by combining the dehydration/hydrogenation reactions with an aldol condensation step prior to the APD/H step [53]. These larger alkanes could be used as premium, sulfur-free diesel fuel components. The APD/H process described above to convert sorbitol to hexane cannot be used to produce alkanes from large watersoluble organic compounds, because extensive amounts of coke form on the catalyst surface (e.g., 20–50% of the reactant is converted to coke). Accordingly, to produce liquid alkanes the reactor system employed to carry out dehydration/hydrogenation reactions must be modified to the four-

phase reactor system previously described. As dehydration/hydrogenation takes place, the aqueous organic reactants become more hydrophobic, and the hexadecane alkane stream serves to remove hydrophobic species from the catalyst before they react further to form coke. These results show that it is possible to make heavier liquid alkanes by crossed aldol-condensation of furfural and HMF with dihydroxyacetone, hydroxyacetone, or glyceraldehydes; however, it will be necessary to improve the selectivities of these processes. An effective catalyst for the production of H2 from APR should be active for C–C bond cleavage and water-gas shift. Importantly, the catalyst must not facilitate the subsequent methanation and/or Fischer–Tropsch synthesis reactions to produce alkanes, which consume H2. Catalysts for production of H2 should also have low rates of C–O bond cleavage to form alkane products. Reaction kinetics studies, suggest that Pt is the best monometallic catalyst in terms of activity and selectivity for APR. Several bimetallic catalysts were identified, including PtNi, PtCo, PtFe and PdFe, which have higher activities than monometallic catalysts. While Ni catalysts are active for APR, they produce a large amount of methane, and these catalyst exhibit deactivation. The amount of methane can be decreased by alloying the Ni with Sn. The stability of Ni-based catalysts can be improved by using a Raney-Ni catalyst. Accordingly, a Raney-NiSn catalyst can be used to achieve good activity, selectivity, and stability for production of H2 by APR of biomass-derived oxygenated hydrocarbons [52]. A stream of alkanes ranging from C1 to C6 is reported to be formed by aqueous-phase dehydration/hydrogenation (APD/H) of sorbitol over bi-functional catalyst systems in which sorbitol is repeatedly dehydrated by an acid catalyst

8 Current Organic Synthesis, 2010, Vol. 7, No. 6

Eleni F. Iliopoulou

Fig. (4). An integrated biorefinery for conversion of carbohydrates to fuels by aqueous-phase processing. (adapted from reference [52]).

(e.g., a solid acid or an aqueous mineral acid) and then hydrogenated on a metal catalyst (e.g., Pt or Pd). Hydrogen, which is needed for the hydrogenation reaction, can be produced in-situ by APR of sorbitol over a catalyst (such as Pt) that facilitates C–C bond cleavage and water-gas shift reactions, or it can be co-fed to the reactor with the aqueous sorbitol reactant. Liquid alkanes ranging from C7 to C15 can be produced from carbohydrates by combining the dehydration/hydrogenation process with an aldol condensation step to form C–C bonds. In another study, biomass-derived compounds, such as furfural, HMF and acetone, that contain carbonyl groups, were coupled by aldol condensation at room temperature using a mixed Mg–Al-oxide base catalyst to form large organic molecules. The reaction mixtures were then hydrogenated, using a 5% Pd/Al2O3 catalyst, to increase the aqueous solubility of these molecules and to minimize possible coking reactions that may take place from unsaturated molecules in the following step. The hydrogenated molecules were subsequently converted to alkanes by dehydration/hydrogenation over a bi-functional catalyst (4 wt.% Pt/SiO2 – Al2O3) containing acid and metal sites in a specially designed four-phase reactor system. This reactor system consists of an aqueous inlet stream containing the large watersoluble organic reactant, a hexadecane alkane sweep stream, a H2 inlet gas stream, and a solid catalyst. During dehydration/hydrogenation processing, organic species are transferred from the aqueous phase reactant stream to the hexadecane sweep stream, as these organic species become more hydrophobic, removing these species from the catalyst as valuable products before they go on further to form coke [52]. In an additional research study [46] the same group has shown that Pd/MgO-ZrO2 is an active, selective, and hydrothermally stable catalyst for aldol-condensation over basic sites (MgO-ZrO2) followed by sequential hydrogenation over

metal sites (Pd). This bifunctional catalyst thus allows carbohydrate-derived compounds, like furfural and HMF, to be converted in a single reactor to large water-soluble intermediates for further aqueous phase processing to produce liquid alkanes (C8-C15). The selectivity and overall yield of the process can be controlled by the reaction temperature and the molar ratio of the aldol-condensation reactants. This aqueous phase process is still in its infancy. However it is likely that this process can be applied to other biomass-derived compounds containing carbonyl compounds, such as replacing acetone with glyceraldehyde or dihydroxyacetone. Thus, the development of this new metal-base catalyst will be of particular interest also for other base-catalyzed reactions in aqueous phase solutions to develop environmentally benign processes [46]. The same research group has recently reported another catalytic approach to selectively guide the conversion of lactic acid (an important low-cost biomass derived commodity chemical, with an expected growing market) toward a spontaneously separating organic layer that can serve as source of valuable products (propanoic acid and C4–C7 ketones) and/or can be used to produce high energy-density liquid fuels (C4–C7 alcohols) using a low metal-content Pt(0.1%)/Nb2O5 catalyst [54]. The novel aspect of this approach that by using a bifunctional catalyst containing metal and acid sites, the series of required reactions leading to the desired products (e.g., dehydration–hydrogenation and C–C coupling) can be carried out in a single flow reactor, thus reducing capital and operating costs for the process. The niobia support plays a crucial role in directing the synthesis to valuable compounds by catalyzing C–C coupling reactions such as aldol-condensation and ketonization, instead of C–C cleavage reactions that take place over monofunctional Pt/Vulcan and lead to loss of carbon in the gas phase. This conversion of lactic acid over Pt(0.1%)/ Nb2O5 maintains approximately 50% of the feed carbon in an organic effluent phase (rich in valuable products) that spontaneously sepa-

Review of C-C Coupling Reactions in Biomass Exploitation Processes

rates from the aqueous layer, thus eliminating the need to remove these products from water. This oil could be used as a direct source of valuable chemicals and/or can be potentially upgraded to liquid fuels [54]. Finally, this approach of oxygen removal and subsequent upgrading of intermediates in a single reactor is flexible in that it could be applied not only to lactic acid, but also to other over-functionalized biomass-derived molecules. One especially interesting example would be the catalytic processing of levulinic acid, which can be derived from waste biomass sources (e.g., paper mill sludge, urban waste paper, agricultural residues) by the Biofine process [55]. 3. INTEGRATED APPROACHES As described above, Dumesic and co-workers, among other research groups, have recently explored reaction strategies in which biomass derived sugars and polyols can be converted to hydrocarbons that are identical to those obtained from petroleum [38]. As a result these bio-fuels can be used directly in current combustion engines. In particular, sugars and polyols can be converted first to mono-functional intermediates such as ketones, alcohols, heterocycles and carboxylic acids over a Pt–Re bi-metallic catalyst supported on carbon, followed by catalytic upgrading of these species (containing between 4 and 6 carbon atoms) to longer-chain alkanes appropriate for fuel applications. A similar process has been described by Blommel et al. [56] in which the oxygen content of biomass derived carbohydrates is reduced by the use of H2 generated in situ from aqueous-phase reforming to obtain mono-oxygenates such as alcohols, ketones and aldehydes. As described in previous sections a promising strategy for catalytic upgrading of monofunctional intermediates to produce liquid transportation fuels is to carry out C–C coupling of carboxylic acids by ketonization processes, forming higher molecular weight linear ketones plus CO2 and H2O, [32] followed by additional C–C coupling reactions of ketones and alcohols by aldol condensation/hydrogenation to produce heavier branched ketones [31]. As previously described, aldol condensation seems an effective means to produce large organic molecules by C–C bond coupling between carbonyl-containing furan compounds, such as 5- hydroxymethylfurfural and furfural. These molecules can then be converted to liquid alkanes by dehydration/hydrogenation over a bifunctional catalyst containing acid and metal sites [45]. The ketonization processing must be carried out upstream of aldol-condensation/hydro-genation reactions, because this latter processing step requires basic catalyst sites, and these sites are poisoned by carboxylic acids [33, 42]. Ketonization and aldol condensation/hydrogenation reactions could be integrated in a single reactor with a double bed system, because the conditions for both reactions are similar. However, an important issue that must be addressed is the extent to which the catalyst employed for aldolcondensation/hydrogenation is inhibited by the CO2 and water, by-products of ketonization reactions. Especially the CO2 produced in the ketonization reaction is a known poison for basic sites active in aldol condensation [59,60]. In an earlier work of Kunkes etal. [33] the presence of 5% CO2 in the feed is presented detrimental to aldol conden-

Current Organic Synthesis, 2010, Vol. 7, No. 6

9

sation activity (~90% decrease in activity) over Pd/Ce1Zr1Ox. Accordingly, it was we concluded as beneficial to remove CO2 prior to the aldol condensation/ hydrogenation step over Pd/Ce1Zr1Ox [35]. In this work, the possibility of the aforementioned integration between ketonization and aldol condensation/hydrogenation is explored by elucidating the reaction pathways for aldol condensation of a representative ketone (2-hexanone) on Pd/CeZrOx by varying reaction conditions, such as temperature, pressure and space velocity. Monofunctional oxygenated additives such as primary alcohols and carboxylic acids were found to reversibly inhibit the self-coupling activity of 2-hexanone. The detrimental effects of these species could be overcome by increasing the overall reaction residence time. Water (the product of both aldol condensation and ketonization) and carbon dioxide (a product of ketonization) were also observed to reversibly inhibit 2-hexanone self-coupling. These observations lead us to conclude that it may be difficult to integrate efficiently the ketonization of carboxylic acids and the aldol condensation/hydrogenation of ketones in a single reactor system. However, because water and CO2 can easily be separated from the organic reaction mixture, the operation of a cascade system consisting of separate ketonization and aldol condensation reactors appears to be preferred. On the other hand, it is interestingly shown in literature that the nature of the interaction of CO2 with the oxide surface can be modified by changing the composition of mixed oxides [59]. Thus, the possibility of modifying the composition of ceria-zirconia mixed oxide to formulate an aldol condensation catalyst that is not severely inhibited by CO2 was attempted [59], so as to permit the integration of ketonization and aldol condensation reactions in a single reactor. In addition to CO2 inhibition, inhibition by water also needs to be investigated, because water is a by-product of ketonization as well as aldol condensation. In general ceria-containing catalysts display significant inhibition by CO2, showing yields of less than 20% to C–C coupling products. The decrease in activity for these catalysts can be correlated with the abundance of isolated O2 sites that strongly bind CO2, as shown during CO2 TPD studies. On the other hand, a Pd/ZrO2 catalyst is reported to exhibit a significant resistance to CO2 poisoning (20% decrease in condensation activity), and this catalyst does not contain strong CO2 binding sites. Inhibition by water is an additional obstacle to integrating ketonization and aldol condensation/hydrogenation steps. Pd/ZrO2 has high activity and it is resistant to inhibition by CO2 and water. Thus a double bed system was suggested by adding a downstream Pd/ZrO 2 catalyst to achieve aldol condensation/hydrogenation following the ketonization step. It seems that production of liquid transportation fuels by catalytic upgrading of hydrophobic mixtures of monofunctional intermediates obtained from carbohydrates over Pt-Re/C can be achieved by the integration in a single reactor of ketonization and aldol condensation/hydrogenation steps. With this integration of steps, the energy consumption as well as reactor infrastructure associated with cooling and reheating the products obtained from ketonization prior to the aldol condensation/hydrogenation step can be eliminated. For this integration to be realized, the catalyst for aldol con-

10 Current Organic Synthesis, 2010, Vol. 7, No. 6

densation must be resistant to inhibition by the CO2 and water produced in the ketonization step. It has been found that Pd/Ce2Zr5Ox and Pd/ZrO2 show high activity for aldol condensation of 2-hexanone, a representative ketone produced from carbohydrates over Pt-Re/C. Formation of C12 products was predominant on both catalysts, and secondary reaction products, such as C18 ketones and aromatic species, were produced with higher selectivities over Pd/ZrO2, likely due to the high surface acidity of this catalyst. The inhibiting effect of CO2 on the rate of aldol condensation was significant over all compositions of ceria-zirconia mixed-oxide catalysts, as well as Pd/CeOx. However, Pd/ZrO2 was found to be substantially more resistant to CO2 inhibition. The effect of water co-feeding was investigated over Pd/ZrO2, in comparison with Pd/Ce1Zr1Ox, and it was found that Pd/ZrO 2 was also more resistant to water inhibition compared to Pd/Ce1Zr1Ox. With these promising catalytic properties of Pd/ZrO2, a double bed system consisting of Ce1Zr1Ox (for ketonization) followed by Pd/ZrO2 (for aldol condensation) was successfully implemented to achieve the combined C–C coupling of carboxylic acids and ketones, as illustrated with a simulated mixture of 20 mol% butanoic acid in 2-hexanone [59]. Pd/ZrO2 catalysts are effective [59, 60] for aldolcondensation/hydrogenation in the presence of CO2 and water, allowing for the integration of ketonization and aldolcondensation/hydrogenation processes for achieving C–C coupling of mono-functional intermediates in a single reactor, dual-bed catalyst system, thereby streamlining the overall catalytic upgrading process. The novelty of this process lies in the integration of two separate C–C coupling processes, such that an alkane stream suitable for Diesel fuel can be obtained in a two stage process, the latter being a dehydration/hydrogenation step to convert long-carbon chain ketones to alkanes, starting from biomass-derived monooxygenated streams. This combined process can be used for any oxygenate stream that contains carboxylic acids, carbonyl and alcohol species, and has potential for application in the emerging bio-refinery industry [60]. A detailed comparison of catalytic upgrading processes for a mixture of mono-functional intermediates obtained by conversion of an aqueous solution of 60 wt% sorbitol over a Pt-Re/C catalyst is discussed [60] in the recent literature. Specifically, the performance of two separate reactors is compared for ketonization over Ce1Zr1Ox followed by aldol condensation/hydrogenation over 0.25 wt% Pd/ZrO2, with removal of CO2 and water between reactors, versus the performance of a dual-bed of these two catalysts in a single reactor. In the latter case this Pd/ZrO2 catalyst importantly operates effectively in the presence of CO2 and water formed during upstream ketonization reactions.The performance of the two reactor systems is essentially the same. The dualbed, single reactor system is however the preferred mode of catalytic upgrading, because the energy consumption as well as the reactor infrastructure associated with cooling and reheating the products obtained from ketonization prior to the aldol condensation/hydrogenation step can be eliminated. More generally, this single-reactor upgrading strategy can be applied to other biomass-derived oils that contain carboxylic acids, ketones and alcohols. The major difference of this work compared to the work of Blommel et al. [56] is the

Eleni F. Iliopoulou

integrated C–C coupling route to utilize both acids and ketones/alcohols in the biomass-derived mono-functional stream to increase the amount of fuel-grade components in the final effluent. Undoubtedly, this strategy for achieving C–C coupling, combined with a dehydration/hydrogenation step, produces a mixture of linear and singly branched alkanes that is desirable for Diesel fuel applications. Last but not least a recent novel strategy is reported for the efficient synthesis of renewable fuels [61]. This approach converts aqueous solutions of gamma-valerolactone (GVL), produced from biomass-derived carbohydrates, to liquid alkenes in the molecular weight range appropriate for transportation fuels by an integrated catalytic system that does not require an external source of hydrogen. The GVL feed undergoes decarboxylation at elevated pressures (e. g., 36 bar) over a silica/alumina catalyst to produce a gas stream composed of equimolar amounts of butene and carbon dioxide. This stream is fed directly to an oligomerization reactor containing an acid catalyst (e. g., H ZSM-5), which couples butene monomers to form condensable alkenes with molecular weights that can be targeted for gasoline and/or jet fuel applications. In addition the effluent gaseous stream of CO 2 at elevated pressure can potentially be captured and then treated or sequestered to mitigate greenhouse gas emissions from the process [61]. 4. CONCLUSIONS As most interestingly reviewed above, ketonization and aldol condensation reactions are unique routes to achieve C– C coupling, leading to reaction products that are valuable for use as liquid transportation fuels comparable to the currently used fossil fuels. It seems that these routes are very promisisng for fuel production, as the extent of branching in the high molecular weight alkanes is minimized and controlled, in contrast to acid-catalyzed coupling processes such as oligomerization of olefins. In this way mono-functional intermediates produced by catalytic conversion of sugars and polyols can be upgraded to fuel-grade compounds, while it has been recently proposed that this can be achieved using two catalytic reactors operated in a cascade mode. The first reactor achieves C–C coupling of monofunctional intermediates using a dual-bed catalyst system, where the upstream catalyst bed is employed to carry out ketonization of carboxylic acids, and the downstream catalyst bed is used to achieve aldol condensation/hydrogenation of alcohols and ketones. Most importantly this second bed is reported to be not significantly inhibited by CO2 and H2O produced during ketonization. The high molecular weight ketones produced by C–C coupling reactions in the dual bed catalyst system are subsequently converted to alkanes by hydrodeoxygenation (i.e., dehydration/hydrogenation) over e.g. a Pt/SiO2–Al2O3 catalyst. Summarising, using the aforementioned approach, an aqueous sorbitol feed can be converted to a liquid stream of alkanes, consisting manly of C7+ alkanes with minimal branching, desirable for Diesel fuel. REFERENCES [1]

Simonetti, D.A.; Dumesic, J.A..; Catalytic strategies for changing the energy content and achieving C-C coupling in biomass-derived oxygenated hydrocarbons. ChemSusChem. 2008, 1, 725 – 733.

Review of C-C Coupling Reactions in Biomass Exploitation Processes [2]

[3] [4] [5]

[6] [7]

[8] [9]

[10] [11] [12] [13]

[14] [15]

[16] [17] [18]

[19]

[20] [21]

[22] [23] [24]

[25] [26]

[27] [28]

Ozcimen, D.; Karaosmanoglu, F.; Production and characterization of the bio-oil and biochar from rapeseed cake. Renew. Energy, 2004, 29(5), 779-787. Demirbas, A.; Çalar, A.; Akdeniz, F.; Güllü, D.; Conversion of olive husk to liquid fuel by pyrolysis and catalytic liquefaction, Energy Sources, 2000, 22, 631-639. McKendry, P.; Energy production from biomass (part 1): overview of biomass. Bioresour. Technol., 2002, 83, 37-46. Tsai, W.T.; Lee, M.K.; Chang, Y.M.; Fast pyrolysis of rice husk: product yields and compositions. Bioresour. Technol., 2007, 98, 22-28. Goyal, H.B.; Seal, D.; Saxena, R.C.; Bio-fuels from thermochemical conversion of renewable resources: A review. Renewable and Sustainable Energy Reviews, 2008, 12, 504-517. Lin, Y.C.; Huber, G.W.; The critical role of heterogeneous catalysis in lignocellulosic biomass conversion. Energy Environ. Sci., 2009, 2, 68-80. Huber, G.W.; Iborra, S.; Corma, A.; Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. Chem. Rev., 2006, 106, 4044-4098. Mohan, D.; Pittman, C.U.; Steele, P.H.; Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review. Energy Fuel, 2006, 20, 848-889. Corma, A.; Iborra, S.; Velty, A.; Chemical routes for the transformation of biomass into chemicals. Chem. Rev., 2007, 107, 24112502. Yaman, S.; Pyrolysis of biomass to produce fuels and chemical feedstocks. Energy Convers Manage, 2004, 45, 651-671. Bridgwater, A.V.; Peacocke, G.V.C.; Fast pyrolysis processes for biomass. Renew. Sust. Energy Rev, 2000, 4 (1), 1-73. Zabaniotou, A.A.; Kalogiannis, G.; Kappas, E.; Karabelas, A.J.; Olive residues (cuttings and kernels) rapid pyrolysis product yields and kinetics. Biomass Bioenergy, 2000, 18, 411-420. Yamamoto, H.; Fujino, J.; Yamaji, K.; Evaluation of bioenergy potential with a multi-regional global-land-use-and-energy model. Biomass Bioenergy, 2001, 21,185-203. Huber, G.W.; Corma, A.; Synergies between bio- and oil refineries for the production of fuels from biomass. Angew. Chem. Int. Ed., 2007, 46, 7184-7201. Zhang, S.; Yan, Y.; Li, T.; Ren, Z.; Upgrading of liquid fuel from the pyrolysis of biomass. Bioresour. Technol., 2005, 25, 235-255. Qi, Z.; Jie, C.; Tiejun, W.; Ying, X.; Review of biomass pyrolysis oil properties and upgrading research. Energy Convers. Manage., 2007, 48, 87-92. Scurlock, J.M.O.; Hall, D.O.; House, J.I.; Utilizing biomass crops as an energy sources:A European perspective. Water Air Soil Poll., 1993, 70, 499-518. Hall, D.O.; Barnard, G.W.; Moss, P.A.; Biomass for Energy in Developing Countries, Permagon Press, Oxford, 1982., Demiral, I.; Sensoz S.; The effects of different catalysts on the pyrolysis of industrial wastes (olive and hazelnut bagasse). Bioresour. Technol., 2008, 99, 8002–8007. Bridgwater, A.V.; Renewable fuels and chemicals by thermal processing of biomass.Chem. Eng. J., 2003, 91(2–3), 87-102. Carlson, T.R.; Tompsett, G.A.; Conner, W.C.; Huber, G.W.; Aromatic Production from Catalytic Fast Pyrolysis of Biomass-Derived Feedstocks. Top Catal, 2009, 52, 241-252. Taralas, G.; Kontominas, M.G. Science in Thermal and Chemical Biomass Conversion, In Bridgwater, A. V.; Ed. Victoria, Vancouver Island, BC, Canada, 2004. Mc Kendry, P.; Energy production from biomass (part 2): conversion technologies Bioresour. Technol., 2002, 83 (1), 47-54. Sensoz, S.; Slow pyrolysis of wood barks from Pinus brutia Ten. and product compositions Bioresour. Technol., 2003, 89 (3), 307311. Gaertner, C.A.; Serrano-Ruiz, J.C.; Braden, D.J.; Dumesic J.A.; Catalytic coupling of carboxylic acids by ketonization as a processing step in biomass conversion , J.Catal, 2009, 266, 71-78. Stocker, M. Biofuels and biomass-to-liquid fuels in the biorefinery: catalytic conversion of lignocellulosic biomass using porous materials. Angew. Chem. Int. Ed., 2008, 47, 9200-9211. Biorefineries—Industrial Processes and Products: Status Quo and Future Directions . Kamm, B.; Gruber, P.R.; Kamm, M.; Eds. Wiley-VCH, Weinheim, 2006. Breaking the Chemical and Engineering Barriers to Lignocellulosic Biofuels: Next Generation Hydrocarbon Biorefineries, National

Current Organic Synthesis, 2010, Vol. 7, No. 6

[29]

[30]

[31]

[32]

[33] [34] [35]

[36]

[37] [38]

[39]

[40] [41]

[42]

[43] [44]

[45]

[46]

[47]

[48] [49]

[50]

11

Science Foundation: Chemical, Bioengineering, Environmental, and Transport Systems Division, 2008. Simonetti, D.A.; Rass-Hansen, J.; Kunkes, E.L.; Soares, R.R.; Dumesic, J.A.; Coupling of glycerol processing with FischerTropsch synthesis for production of liquid fuels. Green Chem. 2007, 9, 1073-1083. Soares, R.R.; Simonetti, D.A.; Dumesic, J.A.; Glycerol as a source for fuels and chemicals by low-temperature catalytic processing. Angew. Chem. 2006, 118, 4086-4089; Angew. Chem. Int. Ed. 2006, 45, 3982-3985. Nikolopoulos, A.A.; Jang B.W.L.; Spivey, J.J.; Acetone condensation and selective hydrogenation to MIBK on Pd and Pt hydrotalcite-derived Mg–Al mixed oxide catalysts. Appl. Catal., A, 2005, 296, 128–136. Nagashima, O.; Sato, S.; Takahashi R.; Sodesawa, T.; Ketonization of carboxylic acids over CeO2-based composite oxides . J. Mol. Catal.A: Chem., 2005, 227, 231-239. Kunkes, E.L.; Gurbuz E.I.; Dumesic, J.A.; Vapour-phase C–C coupling reactions of biomass-derived oxygenates over Pd/CeZrOx catalysts. J. Catal., 2009, 206, 236-249. Renz, M. Ketonization of carboxylic acids by decarboxylation: mechanism and scope. Eur. J. Org. Chem. 2005, 979-988. Corma, A.; Renz, M.; Schaverien, C. Coupling fatty acids by ketonic decarboxylation using solid catalysts for the direct production of diesel, lubricants, and chemicals. Chem. Sus. Chem. 2008, 1, 739-741. Dooley, K.M.; Bhat, A.K.; Plaisance, C.P.; Roy, A.D. Ketones from acid condensation using supported CeO2 catalysts: Effect of additives. Appl. Catal. A, 2007, 320, 122-133. Martinez, R.; Huff, M.C.; Barteau, M.A. Ketonization of acetic acid on titania-functionalized silica monoliths. J. Catal., 2004, 222, 404-409. Kunkes, E.L.; Simonetti, D.A.; West, R.M.; Serrano-Ruiz, J.C.; Gaertner, C.A.; Dumesic, J.A. Hydrocarbons and targeted liquidfuel classes catalytic conversion of biomass to monofunctional. Science, 2008, 322, 417-421. West, R.M.; Liu, Z.Y.; Peter, M.; Dumesic, J.A. Liquid Alkanes with targeted molecular weights from biomass-derived carbohydrates. ChemSusChem, 2008, 1, 417-424. Klimkiewicz, R.; Grabowska, H.; Syper, L. Vapor-phase conversion of esters into ketones in the presence of an Sn-, Ce-, and Rhcontaining oxide catalyst. Kinet. Catal., 2003, 44, 283-286. Glinski, M.; Szymanski, W.; Lomot, D. Catalytic ketonization over oxide catalysts X. Transformations of various alkyl heptanoates. Appl. Catal. A, 2005, 281, 107-113. West, R.M.; Kunkes, E.L.; Simonetti D.A.; Dumesic, J.A. Catalytic conversion of biomass-derived carbohydrates to fuels and chemicals by formation and upgrading of mono-functional hydrocarbon intermediates. Catal.Today, 2009, 147, 115-125. Gärtner, C.A.; Serrano-Ruiz, J.C.; Braden, D.J.; Dumesic, J.A. Catalytic upgrading of bio-oils by ketonization. ChemSusChem, 2009, 2, 112-1124. Rennard, D.C.; Dauenhauer, P.J.; Tupy, S.A.; Schmidt, L.D. Autothermal catalytic partial oxidation of bio-oil functional groups: Esters and acids. Energy Fuels, 2008, 22, 1318-1327. Chheda J. N.; Dumesic, J.A. An overview of dehydration, aldolcondensation and hydrogenation processes for production of liquid alkanes from biomass-derived carbohydrates. Catal. Today, 2007, 123, 59-70. Barrett, C.J.; Chheda, J.N.; Huber, G.W.; Dumesic, J.A.; Singlereactor process for sequential aldol-condensation and hydrogenation of biomass-derived compounds in water. Appl.Catal. B, 2006, 66, 111-118. Torres, G.; Apesteguia, C.R.; DiCosimo, J.I. One-step methyl isobutyl ketone (MIBK) synthesis from 2-propanol: Catalyst and reaction condition optimization. Appl. Catal. A, 2007, 317, 161170. Lin, K.H.; Ko, A.N. Na promotion of Pd/MgO catalysts for lowpressure one-step synthesis of MIBK from acetone plus H-2. Appl. Catal. A, 1996, 147, L259-L265. Diez, V.K.; Apesteguia, C.R.; DiCosimo, J.I. Deactivation of MgyAlOx mixed oxides during a1dol condensation reactions of ketones. Stud. Surf. Sci. Catal., 2001, 139, 303-310. Cortright, R.D.; Davda, R.R.; Dumesic, J.A. Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water. Nature, 2002, 418, 964-967.

12 Current Organic Synthesis, 2010, Vol. 7, No. 6 [51]

[52] [53]

[54] [55]

[56]

Huber, G.W.; Chheda, J.N.; Barrett, C.J.; Dumesic, J.A. Production of liquid alkanes by aqueous-phase processing of biomass-derived carbohydrates. Science, 2005, 308, 1446–1450. Huber, G.W.; Dumesic, J.A. An overview of aqueous-phase catalytic processes for production of hydrogen and alkanes in a biorefinery. Catal. Today, 2006, 111, 119-132. Huber, G.W.; Cortright, R.D.; Dumesic, J.A. Renewable alkanes by aqueous-phase reforming of biomass-derived oxygenates. Angewandte Chemie Int Ed., 2004, 43 (12), 1549–1551. Serrano-Ruiz, J.C.; Dumesic, J.A. Catalytic upgrading of lactic acid to fuels and chemicals by dehydration/hydrogenation and C–C coupling reactions. Green Chem., 2009, 11, 1101-1104. Fitzpatrick, S.W. Production of levulinic acid by the hydrolysis of carbohydrate-containing materials. World Pat. 9640609, 1997 to Biofine Incorporated. Blommel, P.G.; Keenan, G.R.; Rozmiarek R.T.; Cortright, R.D.; Catalytic conversion of sugar into conventional gasoline, diesel, jet fuel, and other hydrocarbons. Int. Sugar J., 2008, 110, 672-679.

Eleni F. Iliopoulou [57]

[58] [59]

[60] [61]

Abello, S.; Vijaya-Shankar, D.; Perez-Ramirez, J. Stability, reutilization, and scalability of activated hydrotalcites in aldol condensation. Appl. Catal. A, 2008, 342, 119-125. Kamimura, Y. Sato, S. Takahashi, R. Sodesawa, T. Akashi, T. Synthesis of 3-pentanone from 1-propanol over CeO2-Fe 2O3 catalysts. Appl. Catal. A, 2003, 252, 399-410. Gürbüz, E.I.; Kunkes E.L.; Dumesic, J.A. Integration of C–C coupling reactions of biomass-derived oxygenates to fuel-grade compounds. Appl. Catal. B., 2010, 94, 134-141. Gürbüz, E.I.; Kunkes E.L.; Dumesic, J.A. Dual-bed catalyst system for C–C coupling of biomass-derived oxygenated hydrocarbons to fuel-grade compounds. Green Chem., 2010, 12, 223-227. Bond, J.Q.; Alonso, D.M.; Wang, D.; West, R.M.; Dumesic, J.A. integrated catalytic conversion of gamma-valerolactone to liquid alkenes for transportation fuels. Science, 2010, 327, 1110-1114.