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Transformation of biomass via the selective hydrogenolysis of CO bonds by nanoscale metal catalysts
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Transformation of biomass via the selective hydrogenolysis of C–O bonds by nanoscale metal catalysts Ning Yan1 and Paul J Dyson2 Selective hydrogenolysis of C–O bonds is a key transformation in the depolymerization of biomass and deoxygenated biomass-derived platform compounds for the production of renewable chemical feedstocks and fuels. Recently, many highly efficient homogeneous and heterogeneous catalysts have been developed for these reactions and herein we highlight the use of dispersed and immobilized nanoscale metal catalysts for the hydrogenolysis of C–O bonds. Bifunctional systems comprising a nanoscale metal catalyst and an acid catalyst, which significantly improves the reaction efficiency, are also described. Potential limitations that must be overcome to enable the widespread use of nanoscale metal catalysts in the selective hydrogenolysis of C–O bonds in biomass utilization are also identified. Addresses 1 Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117576, Singapore 2 Institut des Sciences et Ingenierie Chimiques, Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Lausanne CH 1025, Switzerland Corresponding authors: Yan, Ning ([email protected]) and Dyson, Paul J ([email protected])
and their derivatives. In principle, deoxygenation can be achieved via C–O bond hydrogenolysis, dehydration, decarbonylation and decarboxylation reactions. Among them, C–O bond hydrogenolysis is of particular significance as it is the only one that may be used to depolymerize biomass. Hydrogenolysis is also essential in the production of certain high value chemicals from biomass derived intermediate compounds (e.g. furfural to 1,5pentadiol). Unfortunately, C–O bond hydrogenolysis is not an easy reaction to achieve. The average energy of C–O bonds is as high as 358 kJ/mol, well above that of most other C–X single bonds (only C–H and C–F bonds are higher) making the C–O bond tolerant (and hence inert) to many reagents and catalysts. For example, diethyl ether, tetrahydrofuran and dioxane, are comparatively stable and consequently are used as solvents in many reactions. In terms of classical organic chemistry, the C–O bond in ethers may be cleaved in the presence of concentrated HBr and BBr3, but is stable in catalytic hydrogenolysis reactions, unless activated (e.g. as in allylic and benzylic ethers) or strained (e.g. as in ethylene oxide) [1].
Current Opinion in Chemical Engineering 2013, 2:178–183 This review comes from a themed issue on Nanotechnology Edited by Hong Yang and Hua Chun Zeng For a complete overview see the Issue and the Editorial Available online 26th February 2013 2211-3398/$ – see front matter, # 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.coche.2012.12.004
Introduction Mankind’s dependence on fossil fuels and fossil fuel derived chemical feedstocks has naturally influenced the priorities of modern chemistry/chemical engineering. Fossil fuels are composed of largely hydrocarbons and as a result, the overwhelming research efforts have focused on activating/functionalizing the various hydrocarbons available, and their downstream transformations. In contrast, the chemistry of biomass, and specifically that of catalysts to transform biomass, was left behind, but recent years has witnessed a resurgence in both academia and industry. Compared to fossil fuel components which are mostly unfunctionalized, biomass feedstocks are over-functionalized since they contain a large amount of oxygen. The central challenge in biomass utilization is the catalytic, selective deoxygenation of polysaccharides, lignin, lipids, Current Opinion in Chemical Engineering 2013, 2:178–183
In the past decade, however, well-defined nanoscale catalysts that achieve C–O bond hydrogenolysis have been developed. Figure 1 summarizes the nanoscale metal catalysts discussed herein that have been used for C–O hydrogenolysis in biomass transformations. The catalysts can be categorized into two groups, first, catalysts based on pure metals or metal alloys, and second, bifunctional catalysts combining nanometallic sites with acid catalysis. Selected examples of both types of catalytic systems are described that emphasize the key design strategies, and more comprehensive reviews of the literature may be found elsewhere [2,3,4,5,6,7].
Zero valent metal NP catalysts Cellobiose is often selected as a simplified model of cellulose and was used to evaluate the efficacy of water dispersed PVP-stabilized Pd, Pt, Rh, and Ru NPs catalysts [8]. Interestingly, only Ru NPs were found to be effective. At pH 2 the first step involves acid catalyzed hydrolysis affording glucose, which is hydrogenated by the Ru NPs to afford sorbitol in >99% yield. At neutral pH 88% conversion could be reached with a mixture of bD-glucopyranosyl-D-sorbitol, sorbitol, dideoxyhexitols, and a small amount of glucose obtained. The presence of a deoxygenated hexitol is indicative of direct hydrogenolysis of the C–O bond by the Ru NPs. A subsequent www.sciencedirect.com
Transformation of biomass via the selective hydrogenolysis of C–O bonds by nanoscale metal catalysts Yan and Dyson 179
Figure 1
n-Pr lignin
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ethylene glycol via glucose
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1,5-pentadiol
5-hydroxylmethyl furfural (5-HMF) OH
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2,5-dimethylfuran (DMF)
Ru NPs
OH
O
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sorbitol + deoxygenated polyol
β-D-glucopyranosyl-D-sorbitol Current Opinion in Chemical Engineering
Selected examples of C–O bond hydrogenolysis of biomass related chemicals (including model substrates) using NP catalysts.
study on Ru NP catalyzed cellobiose conversion in water confirmed that the cellobiose was first converted to b-Dglucopyranosyl-D-sorbitol via hydrogenolysis, and then sorbitol was formed through the cleavage of b-1,4-glycosidic bond in 3-b-D-glucopyranosyl-D-glucitol. Smaller Ru NPs favored the first step but disfavored the second step — the smaller Ru NPs also accelerated the degradation of sorbitol [9]. In fact, Ru NPs exhibit exceptional hydrogenolysis activity, and are limited not only to cellulose depolymerization, but also to the hydrogenolysis of glycerol [10], 5hydroxylmethyl furfural (5-HMF) [11] and cyclic ethers [12,13]. Other metal NPs that are particularly active for hydrogenolysis of C–O bonds include Cu [14], Ni [15], and Pt systems [16]. It is noteworthy that Ni and Pt metals are widely used as hydrocracking catalyst in petroleum refineries, which means they are also able to cleave C–C bonds. This additional feature raises the www.sciencedirect.com
issue of selectivity control, but it also allows catalysts to be designed that promote C–O and C–C hydrogenolysis simultaneously, for example, enabling a one-pot synthesis of ethylene glycol from cellulose [17]. Both noble metal [18] and Ni NPs [19] catalyze the selective hydrogenolysis of lignin into a variety of monomeric and dimeric phenolic compounds. The active sites for the C–O hydrogenolysis are likely to be under-coordinated metal atoms on the NP surface and therefore stabilizing these sites is an important challenge toward catalyst design. The incorporation of a second metal with superior stability is commonly applied to achieve this goal. For example, a Cu nanocatalyst was found to be very active in converting fructose derived 5HMF into 2,5-dimethylfuran (DMF), the latter of which is considered as superior biofuel compared to ethanol. However, trace chloride present in 5-HMF, due to the use of saturated NaCl aqueous solutions to improve the Current Opinion in Chemical Engineering 2013, 2:178–183
180 Nanotechnology
extraction efficiency of 5-HMF from water into organic phases, deactivates the Cu NPs due to chloride-induced sintering [20]. In contrast, Ru NPs loaded on carbon are poison resistant, but the major product is (tetrahydrofuran-2,5-diyl)dimethanol and not DMF. As such, a bimetallic Ru–Cu/C catalyst was developed. As Cu has a lower surface energy than Ru a core shell structure in which the Cu coats the surface of the Ru core results. This core– shell bimetallic catalyst exhibits Cu-like hydrogenolysis behavior combined with Ru-like chloride resistance, leading to the highly efficient transformation of 5-HMF into DMF, although it is not entirely clear why a Ru core should lead to this improvement [21]. Ru NPs immobilized on carbon nanotubes (CNTs) were evaluated in the selective hydrogenolysis of glycerol to glycol [22]. As is usual, smaller Ru NPs (2.8 nm) are more active than larger ones (6.9 nm), however, the smaller Ru NPs are also significantly more active for C–C bond cleavage, giving a considerable amount of methane in the hydrogenolysis product. Incorporation of Fe into the Ru NPs using a co-impregnation method afforded NPs that are significantly more selective to glycols, and more stable compared with monometallic Ru NPs loaded on CNTs. The Ru–Fe NP/CNT catalyst has a high selectivity toward C–O bonds relative to C–C bonds, probably due to the decreased number of active sites for C–C hydrogenolysis after incorporation of Fe (note that C–C hydrogenolysis is a structure sensitive reaction [23]). In addition to the formation of a Ru–Fe alloy, iron oxides are also present on the NP surface, which appears to be responsible for the enhanced stability of the NPs. A thorough understanding of the origin of the benefits induced by Fe is needed; nevertheless, this study elegantly demonstrates how bimetallic NPs exhibit high stabilities and selectivities. Despite these advances a comprehensive understanding of the reactivity of metal NP catalysts with respect to C– O bond hydrogenolysis remains unavailable. Nevertheless, a computational study concerned with C–O bond scission of methanol on a cuboctahedron Pd79 cluster provides some clues on how C–O bond cleavage takes place on a NPs surface:[24] methanol is weakly absorbed on a top site of the Pd cluster via the oxygen, with a rather long Pd–O bond (246 pm) and a binding energy of 23 kJ/ mol (Figure 2, left). C–O bond scission occurs at a C–O distance 197 pm for CH3OH and in the transitional state the carbon and the oxygen are coordinated to two adjacent Pd atoms (240 and 219 pm, respectively, Figure 2, right). The energy barrier for C–O scission is 158 kJ/mol and the overall reaction is slightly endothermic by 15 kJ/ mol. Coordination of both the O and C atoms to two adjacent metals (m2–h2-bonding) helps to induce C–O bond scission. Extrapolating these studies it is not unreasonable to assume that the polarity of two different metals on a NP surface (assuming one metal is more Current Opinion in Chemical Engineering 2013, 2:178–183
Figure 2
Current Opinion in Chemical Engineering
Adsorption complex (left) and transition states for C–O bond scission (right) of methanol on a (1 1 1) facet of a Pd79 cluster. Modified and reproduced [24] by permission from the American Chemical Society.
electronegative than the other) results in well-defined orientations with polar C–O bonds in a m2–h2-fashion, that is even more activated, and consequently more readily cleaved.
Bifunctional systems — NPs combined with acid catalysis Following the observation that cellulose can be transformed into C6-polyols by Pt NPs in hot water [25], it was shown that the low pH in near critical-water catalyzes cellulose hydrolysis into glucose, which is subsequently hydrogenated by a nanocatalyst (in this case based on Ru supported on carbon) to afford C6-polyols [26]. A series of cellulose transformations using metal catalysts in near critical water have since been reported [17,27,28,29], and this approach has been further refined with a number of metal NP catalysts being used in combination with mineral acids [18,30], acidic ionic liquids [31], or solid acids [32,33]. These bifunctional catalytic systems enable tandem dehydration–hydrogenation reactions for upgrading bio-oil components into fuels. Efficient hydrodeoxygenation of biomass-derived ketones using a bifunctional catalyst composed of Pt NPs immobilized on an acidic polyoxometalate scaffold has recently been reported [34]. Again, the system takes advantage of both the acid catalyst (for hydrolysis and dehydration) and the metal NP catalyst (for hydrogenation), enabling the removal of oxygen without direct C–O hydrogenolysis. Bifunctional catalysts in which the acid and metal components work synergistically to facilitate direct hydrogenolysis of C–O bonds under mild conditions have been described—the most prominent example using Re. Indeed, Re containing bimetallic catalysts have a long history with the majority of commercial reformer catalysts using Re promoted Pt on alumina [35]. Platinum is the active hydrogenating element, and rhenium is used to improve catalyst stability, lifetime and selectivity. In recent years, Re promoted Pt [36], Ru [12,13], Ir [37,38], and Rh [39,40] nanoscale catalysts were found to be highly active and selective for C–O bond scission of www.sciencedirect.com
Transformation of biomass via the selective hydrogenolysis of C–O bonds by nanoscale metal catalysts Yan and Dyson 181
11.8
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Figure 3
edge atoms corner atoms facet (111) atoms
-4.0 13.2 Rh
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Re Current Opinion in Chemical Engineering
DFT-calculated deprotonation energies and oxygen binding energy for unmodified and Re or Mo modified Rh nanoparticles. Data abstracted from Ref. [41].
a variety of substrates, including glycerol, 2-(hydroxymethyl) tetrahydropyran, furfural derived tetrahydrofurfuryl alcohol (THFA), and others. Mechanistic investigations of the active site and reaction pathway of C–O bond cleavage over bimetallic Rh– Re [41] and Ir–Re nanocatalysts [42] have been conducted. DFT calculations on a model 201 atom Rh–Re cluster (Figure 3) revealed the Brønsted acidity of hydroxylated Re centers on Rh–Re NPs, which arises from the strong Re–O bonds, resulting in a weak O–H bond as well as a high electron affinity for the conjugate base [41]. The deprotonation energy of the hydroxylated Re species on Rh–Re NPs is 1140 kJ mol 1, irrespective of the surface site Re occupies. This value is significantly higher than that of pure Rh NPs and is comparable to that of solid acids [43]. DFT-predicted acidities of hydroxylated molybdenum species on Rh– Mo NPs are markedly lower than those of Rh–Re NPs, especially when the Mo atom occupies corner site, consistent with the lower hydrogenolysis rates observed with Rh–Mo NPs. Based on calculations, NH3 temperature-programmed desorption (TPD) profiles, and the experimentally observed reactivity trends, a mechanism has been proposed that commences with the protonation of THFA by the hydroxyl group on Re atoms and subsequent hydrogenation at Rh. Subsequent studies on Ir–Re NPs showed that C–O hydrogenolysis potentially proceeds via different mechanisms depending on the conditions, recalling the complexities of C–O hydrogenolysis reactions [42]. NPs have been combined with other catalysts, an interesting example being Ln(OTf)3/Pd NPs for selective www.sciencedirect.com
etheric C–O bond hydrogenolysis in ionic liquids [44]. C–O fusion via intramolecular alkene hydroalkoxylation over lanthanide triflates in ionic liquids is a well-documented process [45], and from microscopic reversibility, this catalytic system should, in principle, be ideally competent for the reverse C–O scission reaction, despite the reverse step being endothermic (DH 14 kcal/mol). To overcome the thermodynamic limitations Pd NPs (deposited on Al2O3 via atomic layer deposition) were used to catalyze an exothermic C C hydrogenation reaction (DH 25 kcal/mol), so that hydrogenolysis of the cyclic ether becomes energetically favorable.
Conclusions A growing series of studies demonstrate the utility of nanocatalysts in direct C–O bond hydrogenolysis and mechanistic insights are also available. However, systematic studies and further quantification of reaction parameters, with respect to NP shape, size and composition are required for a fuller picture that will ultimately facilitate rational design of catalysts that convert biomass feedstocks into value added chemicals and liquid fuels. Ideally, these next generation catalysts should be designed according to current mechanistic understanding and constructed via the precise synthetic methods now available for nanoscale systems. Stabilizer and solvent effects have scarcely been touched on here but must also be considered in the design process [46]. The use of earth abundant metals is also highly desirable — recently a homogeneous Ni hydrogenolysis catalyst was reported [47], and NP variants would be welcome even if the NP proves to be a precursor to an active homogeneous catalyst as is the case for many Pd NPs used in cross coupling reactions [48,49]. Current Opinion in Chemical Engineering 2013, 2:178–183
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NPs combined with acid catalysts, irrespective of being a solid or liquid acid, are also very promising for biomass transformations. The nature of active sites in these bifunctional catalysts, and the reaction pathway of the C–O bond scission, is reasonably well understood, providing guidance for their use in the selective deoxygenation of biomass related chemicals. The majority of these bifunctional systems can only be applied to a limited substrates and therefore developing a broader range of catalytic systems for the selective hydrogenolysis of C–O bonds would increase substrate scope and potential applications. In parallel to research on C–O bond hydrogenolysis of biomass related compounds, renewable production routes for hydrogen must be developed. Solar energy based water splitting [50] is probably the most promising technology for sustainable hydrogen, and abundant, renewable hydrogen would greatly facilitate biomass processing as well as other key technologies for the future.
Acknowledgements
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We thank the Swiss National Science Foundation (NRP 66, No. 136630) and the National University of Singapore (R-279-000-368-133, R-279-000387-112) for financial support.
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Current Opinion in Chemical Engineering 2013, 2:178–183
Transformation of biomass via the selective hydrogenolysis of C–O bonds by nanoscale metal catalysts Ning Yan1 and Paul J Dyson2 Selective hydrogenolysis of C–O bonds is a key transformation in the depolymerization of biomass and deoxygenated biomass-derived platform compounds for the production of renewable chemical feedstocks and fuels. Recently, many highly efficient homogeneous and heterogeneous catalysts have been developed for these reactions and herein we highlight the use of dispersed and immobilized nanoscale metal catalysts for the hydrogenolysis of C–O bonds. Bifunctional systems comprising a nanoscale metal catalyst and an acid catalyst, which significantly improves the reaction efficiency, are also described. Potential limitations that must be overcome to enable the widespread use of nanoscale metal catalysts in the selective hydrogenolysis of C–O bonds in biomass utilization are also identified. Addresses 1 Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117576, Singapore 2 Institut des Sciences et Ingenierie Chimiques, Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Lausanne CH 1025, Switzerland Corresponding authors: Yan, Ning ([email protected]) and Dyson, Paul J ([email protected])
and their derivatives. In principle, deoxygenation can be achieved via C–O bond hydrogenolysis, dehydration, decarbonylation and decarboxylation reactions. Among them, C–O bond hydrogenolysis is of particular significance as it is the only one that may be used to depolymerize biomass. Hydrogenolysis is also essential in the production of certain high value chemicals from biomass derived intermediate compounds (e.g. furfural to 1,5pentadiol). Unfortunately, C–O bond hydrogenolysis is not an easy reaction to achieve. The average energy of C–O bonds is as high as 358 kJ/mol, well above that of most other C–X single bonds (only C–H and C–F bonds are higher) making the C–O bond tolerant (and hence inert) to many reagents and catalysts. For example, diethyl ether, tetrahydrofuran and dioxane, are comparatively stable and consequently are used as solvents in many reactions. In terms of classical organic chemistry, the C–O bond in ethers may be cleaved in the presence of concentrated HBr and BBr3, but is stable in catalytic hydrogenolysis reactions, unless activated (e.g. as in allylic and benzylic ethers) or strained (e.g. as in ethylene oxide) [1].
Current Opinion in Chemical Engineering 2013, 2:178–183 This review comes from a themed issue on Nanotechnology Edited by Hong Yang and Hua Chun Zeng For a complete overview see the Issue and the Editorial Available online 26th February 2013 2211-3398/$ – see front matter, # 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.coche.2012.12.004
Introduction Mankind’s dependence on fossil fuels and fossil fuel derived chemical feedstocks has naturally influenced the priorities of modern chemistry/chemical engineering. Fossil fuels are composed of largely hydrocarbons and as a result, the overwhelming research efforts have focused on activating/functionalizing the various hydrocarbons available, and their downstream transformations. In contrast, the chemistry of biomass, and specifically that of catalysts to transform biomass, was left behind, but recent years has witnessed a resurgence in both academia and industry. Compared to fossil fuel components which are mostly unfunctionalized, biomass feedstocks are over-functionalized since they contain a large amount of oxygen. The central challenge in biomass utilization is the catalytic, selective deoxygenation of polysaccharides, lignin, lipids, Current Opinion in Chemical Engineering 2013, 2:178–183
In the past decade, however, well-defined nanoscale catalysts that achieve C–O bond hydrogenolysis have been developed. Figure 1 summarizes the nanoscale metal catalysts discussed herein that have been used for C–O hydrogenolysis in biomass transformations. The catalysts can be categorized into two groups, first, catalysts based on pure metals or metal alloys, and second, bifunctional catalysts combining nanometallic sites with acid catalysis. Selected examples of both types of catalytic systems are described that emphasize the key design strategies, and more comprehensive reviews of the literature may be found elsewhere [2,3,4,5,6,7].
Zero valent metal NP catalysts Cellobiose is often selected as a simplified model of cellulose and was used to evaluate the efficacy of water dispersed PVP-stabilized Pd, Pt, Rh, and Ru NPs catalysts [8]. Interestingly, only Ru NPs were found to be effective. At pH 2 the first step involves acid catalyzed hydrolysis affording glucose, which is hydrogenated by the Ru NPs to afford sorbitol in >99% yield. At neutral pH 88% conversion could be reached with a mixture of bD-glucopyranosyl-D-sorbitol, sorbitol, dideoxyhexitols, and a small amount of glucose obtained. The presence of a deoxygenated hexitol is indicative of direct hydrogenolysis of the C–O bond by the Ru NPs. A subsequent www.sciencedirect.com
Transformation of biomass via the selective hydrogenolysis of C–O bonds by nanoscale metal catalysts Yan and Dyson 179
Figure 1
n-Pr lignin
Rh, Pt, Ni NPs
+ others
metal NPs + acid
OMe
bifunctional
hydrocarbon fuels
OH Pd NPs + Yb(OTf)3
OH OH + HO 1,3-propandiol 1,2-propandiol
Ru, Cu, Ru-Fe NPs OH
HO
glycerol cellulose
+ octane
bifunctional
OH HO
OH
O
lignin model compound
Ru, Pt NPs
sorbitol + other C6-polyols + others
hemicellulose Ni NPs + WC2
via xylose and furfural
OH
CH2OH
tetrahydrofurfuryl alcohol (THFA)
O
CHO
O
Ru-Cu NPs
O
O OH
HOH2C
OH
O
HO
bifunctional Re-Ir or Re-Rh NPs HO
+ others
ethylene glycol via glucose
O
OH
HO
OH
OH OH
OH
Ru NPs cellobiose (cellulose model compound)
OH
1,5-pentadiol
5-hydroxylmethyl furfural (5-HMF) OH
OH O
HO
OH OH
2,5-dimethylfuran (DMF)
Ru NPs
OH
O
OH
OH
OH
sorbitol + deoxygenated polyol
β-D-glucopyranosyl-D-sorbitol Current Opinion in Chemical Engineering
Selected examples of C–O bond hydrogenolysis of biomass related chemicals (including model substrates) using NP catalysts.
study on Ru NP catalyzed cellobiose conversion in water confirmed that the cellobiose was first converted to b-Dglucopyranosyl-D-sorbitol via hydrogenolysis, and then sorbitol was formed through the cleavage of b-1,4-glycosidic bond in 3-b-D-glucopyranosyl-D-glucitol. Smaller Ru NPs favored the first step but disfavored the second step — the smaller Ru NPs also accelerated the degradation of sorbitol [9]. In fact, Ru NPs exhibit exceptional hydrogenolysis activity, and are limited not only to cellulose depolymerization, but also to the hydrogenolysis of glycerol [10], 5hydroxylmethyl furfural (5-HMF) [11] and cyclic ethers [12,13]. Other metal NPs that are particularly active for hydrogenolysis of C–O bonds include Cu [14], Ni [15], and Pt systems [16]. It is noteworthy that Ni and Pt metals are widely used as hydrocracking catalyst in petroleum refineries, which means they are also able to cleave C–C bonds. This additional feature raises the www.sciencedirect.com
issue of selectivity control, but it also allows catalysts to be designed that promote C–O and C–C hydrogenolysis simultaneously, for example, enabling a one-pot synthesis of ethylene glycol from cellulose [17]. Both noble metal [18] and Ni NPs [19] catalyze the selective hydrogenolysis of lignin into a variety of monomeric and dimeric phenolic compounds. The active sites for the C–O hydrogenolysis are likely to be under-coordinated metal atoms on the NP surface and therefore stabilizing these sites is an important challenge toward catalyst design. The incorporation of a second metal with superior stability is commonly applied to achieve this goal. For example, a Cu nanocatalyst was found to be very active in converting fructose derived 5HMF into 2,5-dimethylfuran (DMF), the latter of which is considered as superior biofuel compared to ethanol. However, trace chloride present in 5-HMF, due to the use of saturated NaCl aqueous solutions to improve the Current Opinion in Chemical Engineering 2013, 2:178–183
180 Nanotechnology
extraction efficiency of 5-HMF from water into organic phases, deactivates the Cu NPs due to chloride-induced sintering [20]. In contrast, Ru NPs loaded on carbon are poison resistant, but the major product is (tetrahydrofuran-2,5-diyl)dimethanol and not DMF. As such, a bimetallic Ru–Cu/C catalyst was developed. As Cu has a lower surface energy than Ru a core shell structure in which the Cu coats the surface of the Ru core results. This core– shell bimetallic catalyst exhibits Cu-like hydrogenolysis behavior combined with Ru-like chloride resistance, leading to the highly efficient transformation of 5-HMF into DMF, although it is not entirely clear why a Ru core should lead to this improvement [21]. Ru NPs immobilized on carbon nanotubes (CNTs) were evaluated in the selective hydrogenolysis of glycerol to glycol [22]. As is usual, smaller Ru NPs (2.8 nm) are more active than larger ones (6.9 nm), however, the smaller Ru NPs are also significantly more active for C–C bond cleavage, giving a considerable amount of methane in the hydrogenolysis product. Incorporation of Fe into the Ru NPs using a co-impregnation method afforded NPs that are significantly more selective to glycols, and more stable compared with monometallic Ru NPs loaded on CNTs. The Ru–Fe NP/CNT catalyst has a high selectivity toward C–O bonds relative to C–C bonds, probably due to the decreased number of active sites for C–C hydrogenolysis after incorporation of Fe (note that C–C hydrogenolysis is a structure sensitive reaction [23]). In addition to the formation of a Ru–Fe alloy, iron oxides are also present on the NP surface, which appears to be responsible for the enhanced stability of the NPs. A thorough understanding of the origin of the benefits induced by Fe is needed; nevertheless, this study elegantly demonstrates how bimetallic NPs exhibit high stabilities and selectivities. Despite these advances a comprehensive understanding of the reactivity of metal NP catalysts with respect to C– O bond hydrogenolysis remains unavailable. Nevertheless, a computational study concerned with C–O bond scission of methanol on a cuboctahedron Pd79 cluster provides some clues on how C–O bond cleavage takes place on a NPs surface:[24] methanol is weakly absorbed on a top site of the Pd cluster via the oxygen, with a rather long Pd–O bond (246 pm) and a binding energy of 23 kJ/ mol (Figure 2, left). C–O bond scission occurs at a C–O distance 197 pm for CH3OH and in the transitional state the carbon and the oxygen are coordinated to two adjacent Pd atoms (240 and 219 pm, respectively, Figure 2, right). The energy barrier for C–O scission is 158 kJ/mol and the overall reaction is slightly endothermic by 15 kJ/ mol. Coordination of both the O and C atoms to two adjacent metals (m2–h2-bonding) helps to induce C–O bond scission. Extrapolating these studies it is not unreasonable to assume that the polarity of two different metals on a NP surface (assuming one metal is more Current Opinion in Chemical Engineering 2013, 2:178–183
Figure 2
Current Opinion in Chemical Engineering
Adsorption complex (left) and transition states for C–O bond scission (right) of methanol on a (1 1 1) facet of a Pd79 cluster. Modified and reproduced [24] by permission from the American Chemical Society.
electronegative than the other) results in well-defined orientations with polar C–O bonds in a m2–h2-fashion, that is even more activated, and consequently more readily cleaved.
Bifunctional systems — NPs combined with acid catalysis Following the observation that cellulose can be transformed into C6-polyols by Pt NPs in hot water [25], it was shown that the low pH in near critical-water catalyzes cellulose hydrolysis into glucose, which is subsequently hydrogenated by a nanocatalyst (in this case based on Ru supported on carbon) to afford C6-polyols [26]. A series of cellulose transformations using metal catalysts in near critical water have since been reported [17,27,28,29], and this approach has been further refined with a number of metal NP catalysts being used in combination with mineral acids [18,30], acidic ionic liquids [31], or solid acids [32,33]. These bifunctional catalytic systems enable tandem dehydration–hydrogenation reactions for upgrading bio-oil components into fuels. Efficient hydrodeoxygenation of biomass-derived ketones using a bifunctional catalyst composed of Pt NPs immobilized on an acidic polyoxometalate scaffold has recently been reported [34]. Again, the system takes advantage of both the acid catalyst (for hydrolysis and dehydration) and the metal NP catalyst (for hydrogenation), enabling the removal of oxygen without direct C–O hydrogenolysis. Bifunctional catalysts in which the acid and metal components work synergistically to facilitate direct hydrogenolysis of C–O bonds under mild conditions have been described—the most prominent example using Re. Indeed, Re containing bimetallic catalysts have a long history with the majority of commercial reformer catalysts using Re promoted Pt on alumina [35]. Platinum is the active hydrogenating element, and rhenium is used to improve catalyst stability, lifetime and selectivity. In recent years, Re promoted Pt [36], Ru [12,13], Ir [37,38], and Rh [39,40] nanoscale catalysts were found to be highly active and selective for C–O bond scission of www.sciencedirect.com
Transformation of biomass via the selective hydrogenolysis of C–O bonds by nanoscale metal catalysts Yan and Dyson 181
11.8
-7.0
12.0
-6.5
12.2
-6.0
12.4
-5.5
12.6
-5.0
12.8
-4.5
13.0
Oxygen Binding Energy, eV
Deprotonation Energy (DPE), eV
Figure 3
edge atoms corner atoms facet (111) atoms
-4.0 13.2 Rh
Rh
Mo
Mo
Re
Re
Re Current Opinion in Chemical Engineering
DFT-calculated deprotonation energies and oxygen binding energy for unmodified and Re or Mo modified Rh nanoparticles. Data abstracted from Ref. [41].
a variety of substrates, including glycerol, 2-(hydroxymethyl) tetrahydropyran, furfural derived tetrahydrofurfuryl alcohol (THFA), and others. Mechanistic investigations of the active site and reaction pathway of C–O bond cleavage over bimetallic Rh– Re [41] and Ir–Re nanocatalysts [42] have been conducted. DFT calculations on a model 201 atom Rh–Re cluster (Figure 3) revealed the Brønsted acidity of hydroxylated Re centers on Rh–Re NPs, which arises from the strong Re–O bonds, resulting in a weak O–H bond as well as a high electron affinity for the conjugate base [41]. The deprotonation energy of the hydroxylated Re species on Rh–Re NPs is 1140 kJ mol 1, irrespective of the surface site Re occupies. This value is significantly higher than that of pure Rh NPs and is comparable to that of solid acids [43]. DFT-predicted acidities of hydroxylated molybdenum species on Rh– Mo NPs are markedly lower than those of Rh–Re NPs, especially when the Mo atom occupies corner site, consistent with the lower hydrogenolysis rates observed with Rh–Mo NPs. Based on calculations, NH3 temperature-programmed desorption (TPD) profiles, and the experimentally observed reactivity trends, a mechanism has been proposed that commences with the protonation of THFA by the hydroxyl group on Re atoms and subsequent hydrogenation at Rh. Subsequent studies on Ir–Re NPs showed that C–O hydrogenolysis potentially proceeds via different mechanisms depending on the conditions, recalling the complexities of C–O hydrogenolysis reactions [42]. NPs have been combined with other catalysts, an interesting example being Ln(OTf)3/Pd NPs for selective www.sciencedirect.com
etheric C–O bond hydrogenolysis in ionic liquids [44]. C–O fusion via intramolecular alkene hydroalkoxylation over lanthanide triflates in ionic liquids is a well-documented process [45], and from microscopic reversibility, this catalytic system should, in principle, be ideally competent for the reverse C–O scission reaction, despite the reverse step being endothermic (DH 14 kcal/mol). To overcome the thermodynamic limitations Pd NPs (deposited on Al2O3 via atomic layer deposition) were used to catalyze an exothermic C C hydrogenation reaction (DH 25 kcal/mol), so that hydrogenolysis of the cyclic ether becomes energetically favorable.
Conclusions A growing series of studies demonstrate the utility of nanocatalysts in direct C–O bond hydrogenolysis and mechanistic insights are also available. However, systematic studies and further quantification of reaction parameters, with respect to NP shape, size and composition are required for a fuller picture that will ultimately facilitate rational design of catalysts that convert biomass feedstocks into value added chemicals and liquid fuels. Ideally, these next generation catalysts should be designed according to current mechanistic understanding and constructed via the precise synthetic methods now available for nanoscale systems. Stabilizer and solvent effects have scarcely been touched on here but must also be considered in the design process [46]. The use of earth abundant metals is also highly desirable — recently a homogeneous Ni hydrogenolysis catalyst was reported [47], and NP variants would be welcome even if the NP proves to be a precursor to an active homogeneous catalyst as is the case for many Pd NPs used in cross coupling reactions [48,49]. Current Opinion in Chemical Engineering 2013, 2:178–183
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NPs combined with acid catalysts, irrespective of being a solid or liquid acid, are also very promising for biomass transformations. The nature of active sites in these bifunctional catalysts, and the reaction pathway of the C–O bond scission, is reasonably well understood, providing guidance for their use in the selective deoxygenation of biomass related chemicals. The majority of these bifunctional systems can only be applied to a limited substrates and therefore developing a broader range of catalytic systems for the selective hydrogenolysis of C–O bonds would increase substrate scope and potential applications. In parallel to research on C–O bond hydrogenolysis of biomass related compounds, renewable production routes for hydrogen must be developed. Solar energy based water splitting [50] is probably the most promising technology for sustainable hydrogen, and abundant, renewable hydrogen would greatly facilitate biomass processing as well as other key technologies for the future.
Acknowledgements
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We thank the Swiss National Science Foundation (NRP 66, No. 136630) and the National University of Singapore (R-279-000-368-133, R-279-000387-112) for financial support.
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