Heterogeneous Catalysts for Levulinic Acid Conversion

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Development of Heterogeneous Catalysts for the Conversion of Levulinic Acid to g -Valerolactone

William R. H. Wright and Regina Palkovits*

^[a]

Introduction

Worldwide dependence on fossil reserves for the production of both fuels and chemicals is alarming, as these resources will ultimately be depleted.^[1] The urgent need to identify more sustainable resources has provoked interest in utilization of renewable biomass, a topic that is

now the focus of substantial research effort.^[2] Of particular appeal is the utilization of ligno- cellulosic materials, as exploitation of these inedible materials would not diminish food production.^[3] A major constituent of lignocellulose is cellulose

(35–50 %), a renewable biopolymer that could serve as a feedstock to the chemical industry, as it is produced naturally in vast quantities.^[4]Cellulose consists of covalently linked chains of d-glucose monomers, which through extensive hydrogen bonding form highly stable networks.^[5] Thus, the braking down of cellulose into reducible glucose requires energy inputs.^[6] Furthermore, the highly functionalized structure of glucose requires extensive modification to engender less functionalized derivatives more suitable for application.^[7]

Despite the current challenges associated with cellulose processing, many carbohydrate-derived compounds are highly applicable and could find widespread utilization. One such compound isg-valerolactone (GVL), which is suited for use as a fuel additive,^[8]food ingredient,^[9]an intermediate for the production of chemicals^[10]and high-grade alkene fuels,^[11]a renewable solvent,^[12] nylon intermediate,^{[2e, 13]}or in a wide range of niche applications (e.g., cutting oils and brake fluids).^[14] The wide ranging applicability of GVL partially results from the high stability and low toxicity of this compound, factors that enhance its general usability.^[8]

Targeted GVL production can be achieved by hydrogenation of levulinic acid (LA), a precursor that can be obtained directly from raw biomass. The cost-efficient manufacturing of LA via the acid-catalyzed hydrolysis of waste cellulosic materials using

H₂SO₄has already been demonstrated on a pilot plant scale.^[15]

This processing renders aqueous product streams that contain LA, formic acid (FA), and trace H₂SO₄ (Scheme 1), motivating the design of systems that can utilize such feedstocks.

The applicability of GVL has provoked numerous studies, which have assessed the suitability of a wide range of hydrogenation catalysts for GVL production. Homogeneous catalysts have been developed that facilitate the hydrogenation of LA to a range of promising derivatives, including GVL and ethers suitable for use as biofuels.^[16]However, homogeneous systems are arguably not suited to targeted GVL production, as the high boiling point of GVL (207–2088C) makes product/catalyst separation by means of distillation uneconomical.^[8] Recently, homogeneous Ru catalysts have been confined within an aqueous phase, enabling product separation and catalyst recycling.^[17] However, only a single catalyst recycle was demonstrated, which resulted in a considerable decrease in LA conversion (81 vs. 55 %). Realistically, large-scale manufacturing of GVL will almost certainly rely on solid catalysts, which can more easily be separated from non-volatile GVL. With this reality in mind, this short Minireview seeks to outline the development of heterogeneous catalysts for the targeted conversion of LA to GVL. Emphasis has been placed on discussing specific Scheme 1.Conversion of cellulose to LA and FA via acid-catalyzed hydration followed by hydrogenation to GVL.

g-Valerolactone (GVL) has been identified as a potential intermediate for the production of fuels and chemicals based on renewable feedstocks. Numerous heterogeneous catalysts have been used for GVL production, alongside a range of reaction setups. This Minireview seeks to outline the development of heterogeneous catalysts for the targeted conversion of levulin-

ic acid (LA) to GVL. Emphasis has been placed on discussing specific systems, including heterogeneous noble and base metal catalysts, transfer hydrogenation, and application of scCO₂as reaction medium, with the aim of critically highlighting both the achievements and remaining challenges associated with this field.

[a]Dr. W. R. H. Wright, Prof. Dr. R. Palkovits

Institut fr Technische und Makromolekulare Chemie RWTH Aachen University

Worringerweg 1, 52074 Aachen (Germany) E-mail: [email protected]

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systems in detail, with the aim of critically highlighting both the achievements and remaining challenges associated with this field.

General Properties of LA and Implications for Hydrogenation

While LA can be utilized for GVL production, it should be noted that this versatile feedstock can be used in numerous other applications, including the production of polymers, food additives, resins, herbicides, pharmaceuticals, and antifreeze agents.^[18] Furthermore, levulinate esters are now actively considered as fuel additives.^[19]Overall, both the viability and ver- satility of LA has resulted in its inclusion in the US Department of Energy’s list of “key platform chemicals”, the exploitation of which could lead to more effective biomass processing.^[20]The chemical properties of LA (a keto acid) have also been thor- oughly investigated, and it has long been known that cyclization of LA renders pseudo-LA, the reversible dehydration of which gives angelica lactone (Scheme 2).^[21]

The capacity of LA to undergo condensation has implications regarding hydrogenation, with GVL production possible through hydrogenation of either unsaturated carbon–carbon or carbon–oxygen bonds. Furthermore, when treated with alcohols LA readily undergoes esterification (Scheme 2),^[22]

a transformation crucial to many of the processing methodologies highlighted in this Minireview.

Production of GVL using H

₂

Vapor-phase hydrogenation

One method considered for the commercial-scale manufacturing of GVL is vapor-phase LA hydrogenation (Table 1). This approach was explored by the Quaker Oats Company in the 1950s, with the aim of developing continuous processes.^[14]

By passing mixtures of vaporized LA and H₂ at atmospheric pressure over a catalyst prepared from CuO and Cr₂O₃ quantitative synthesis of GVL was achieved at 2008C. Higher reaction temperatures were observed to lower reaction yields due to the conversion of GVL ton-valeric acid.

More recently, Chang et al. have investigated the use of precious metal catalysts using a similar vapor-phase reaction setup.^[23]Screening of Ru/C, Pd/C, and Pt/C demonstrated that Ru/C offers the highest activities, enabling qualitative conversion to GVL with no loss of activity observed after 10 days.

Again atmospheric pressure was utilized, demonstrating that vapor-phase hydrogenation can be achieved without the costly requirements of high pressures. While Pd/C and Pt/C also facilitated full LA conversion, the selectivity for GVL in each system was 90 % and 30 % respectively, with significant amounts of the intermediate angelica lactone persisting in the reaction mixtures. Analysis of the Ru/C catalyst by X-ray Diffrac- tion (XRD) failed to identify diffraction peaks that could be as- signed to metallic Ru particles. Chang et al. suggest that this indicates a high degree of Ru dispersion, which could explain the higher performance of the Ru catalyst.^[23]

The principle advantage associated with a vapor-phase approach is that such systems could be especially suited to continuous processing. However, such methodologies are likely to be energy intensive, with vaporization of LA (boiling point 245–2468C) requiring a high energy input. Furthermore, a vapor-phase reactor could prove to be incompatible with the LA product mixtures obtained from carbohydrate feedstocks, which will almost certainly contain strong acids such as H₂SO₄ (Scheme 1), the partial vaporization of which would be undesirable. Overall, closer process integration could be facilitated by the development of liquid-phase hydrogenation systems that are capable of directly converting aqueous LA product streams into GVL.

Liquid-phase hydrogenation

Of the numerous liquid-phase systems used to convert LA to GVL, arguably the simplest utilize hydrogen in conjunction with a transition metal heterogeneous catalysts (Table 1). Such an approach has long been utilized for GVL synthesis, with Schuette and Thomas using PtO₂in 1930.^[24]Stirring a solution of LA dissolved in diethyl ether, ethanol and acetic acid alongside PtO₂ enabled a maximum conversion of 87 % after 48 h according to elemental analysis. In the 1940s, procedures based on Raney Ni were developed, which entailed heating mixtures of LA and Raney Ni to temperatures of 2008C under pressures of 50–60 bar (1 bar=10⁵Pa) of H₂.^[25] Under such conditions, GVL yields of up to 94 % were obtained after a reaction time of 3 h. Later in the 1950 s, reduced Re (termed Re

“black”) was used for the hydrogenation of LA at 1068C using solvent free conditions and a H₂pressure of 148 bar.^[26]Heating at 1068C for 18 h resulted in a 71 % yield of GVL and the production of “polymeric esters” (29 %), the structures of which were not specified.

More recently, supported metal catalysts have been exten- sively investigated for GVL synthesis, an approach which is ac- Scheme 2.Alternate reaction pathways for the hydrogenation of LA to GVL.

Due to the propensity of LA to undergo cyclization both unsaturated carbon–carbon or carbon–oxygen bonds can be reduced. Furthermore, when alcohols are utilized as reaction solvents, esterification occurs.

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tively considered for practical applications (Table 1).^[27] The different capacity of alternate catalysts to facilitate GVL synthesis was explored by Manzer, who screened the catalytic activity of

5 wt % Ir, Rh, Pd, Ru, Pt, Re, and Ni supported on activated car-

bon.^{[10a, 28]}These catalysts were used alongside the solvent 1,4-

dioxane, a reaction temperature of 1508C, and a H₂pressure of Table 1.Contrasting alternative heterogeneous catalysts used in GVL synthesis.

Catalyst Reaction conditions^[a] ReactionT

[8C]

LA conv.

[%]

GVL selec.

[%]

GVL yield^[b]

[%]

Productivity^[c]

[mol_GVLg_metal ¹h¹] Ref.

CuO/Cr₂O₃ (red.)^[d]

vapor phase 200 100 100 100 – [14]

5 wt % Ru/C vapor phase, H2(1 bar) 265 100 98.6 98.6 0.09 [23]

5 wt % Pd/C vapor phase, H2(1 bar) 265 100 90 90 0.08 [23]

5 wt % Pt/C vapor phase, H2(1 bar) 265 100 30 30 0.03 [23]

PtO₂ batch reaction, diethyl ether, H₂(2–3 bar) 25 – – 87 0.028 [24]

Raney Ni batch reaction, no solvent, H2(50 bar) 220 – – 94 0.10 [25b]

Re “black” batch reaction, no solvent, H2(148 bar) 106 100 71 71 0.04 [26]

5 wt % Ir/C batch reaction, 1,4-dioxane, H₂(55 bar) 150 49 97 47 – [10a]

5 wt % Rh/C batch reaction, 1,4-dioxane, H2(55 bar) 150 30 95 28 – [10a]

5 wt % Pd/C batch reaction, 1,4-dioxane, H2(55 bar) 150 30 90 27 – [10a]

5 wt % Ru/C batch reaction, 1,4-dioxane, H2(55 bar) 150 80 90 72 – [10a]

5 wt % Pt/C batch reaction, 1,4-dioxane, H2(55 bar) 150 13 80 10 – [10a]

5 wt % Re/C batch reaction, 1,4-dioxane, H2(55 bar) 150 7 80 6 – [10a]

5 wt % Ni/C batch reaction, 1,4-dioxane, H2(55 bar) 150 2 20 0.4 – [10a]

5 wt % Ru/C batch reaction, methanol, H2(12 bar) 130 90 95 86 2.92 [29]

5 wt % Pd/C batch reaction, methanol, H2(12 bar) 130 17 38 6 – [29]

Raney Ni batch reaction, methanol, H2(12 bar) 130 18 30 5 – [29]

Urushibara Ni batch reaction, methanol, H₂(12 bar) 130 45 5 2 – [29]

5 wt % Ru/TiO2

(Tronox)

batch reaction, ethanol/water, H2(12 bar) 130 0 0 0 – [30]

5 wt % Ru/TiO2

(Degussa P25)

batch reaction, ethanol/water, H2(12 bar) 130 81 87 71 0.90 [30]

5 wt % Ru/C batch reaction, ethanol/water, H₂(12 bar) 130 99 89 89 1.12 [30]

5 wt % Ru/

Al2O3

batch reaction, ethanol/water, H2(12 bar) 130 94 80 76 0.98 [30]

5 wt % Ru/SiO2 batch reaction, ethanol/water, H2(12 bar) 130 98 76 75 0.98 [30]

5 wt % Ru/

Al₂O₃

batch reaction, scCO2, total pressure 250 bar (105 bar CO2, 145 bar H₂)

150 99 99 99 1.69 [35]

5 wt % Ru/SiO2 reaction setup utilized, scCO2 200 100 100 100 – [36]

1.4 wt % Ru-P/

SiO2and 5 wt % Ru/C

FA used as H2source alongside a 6 h reaction time 150 – – – 1.75 [38b]

5 wt % Ru/C mixtures of FA and LA passed over a dual bed reactor;

FA decomposed by 1.4 wt % Ru-P/SiO2into H2and CO2(40 bar);

Ru/C utilized in second stage

150 – – 67 2.64 [38b]

RuSn (3.6:1)/C reaction streams comprised of LA and FA in alkyl-phenol solvents 220 98 95 93 0.36 [44a]

5 wt % Ru/C reaction stream comprised of LA and FA (2.2 mol L ¹) and H2SO4(0.5 mol L¹)

150 >80 – – 0.0034 [44b]

15 wt % RuRe (3:4)/C

reaction stream comprised of LA and FA (2.2 mol L ¹) and H2SO4(0.5 mol L¹)

150 >80 – – 0.014 [44b]

1 mol % Au/

ZrO2

aqueous reaction mixture containing equimolar amounts of LA and FA

150 >99 >99 99 1.68 [45]

1 mol % Au/

ZrO2

GVL synthesized from 1:1 molar mixtures of butyl levulinate and butyl-formate in H2O

170 98 96 95 0.80 [47]

10 wt % Pd/C and 5 wt % Ru/

C

dual bed reactor alongside a reaction feed comprising of 1-butanol containing 4.43mbutyl levulinate and 1.55mbutyl formate ; reaction mixture passed over the catalyst (WHSV^[e]=0.9 h¹)

170 – – 95 0.11 [49]

5 wt % Ru/C dehydration and hydrogenation ofd-fructose using TFA^[f]and FA 180 – – 52 0.04 [51]

5 wt % Cu/SiO2 vapor phase reaction, WHSV=0.513 h ¹, H2(10 bar) 265 100 99.9 99.9 0.088 [55]

ZrO2 batch reactor, solutions comprised of 5 wt % butyl levulinate in 2- butanol; utilization of an inert gas (He, 20.6 bar)

150 99.9 84 84 0.001 [56]

Ru/C batch reactor, solvent-free conditions, H2(12 bar), 50 h 25 100 97.5 97.5 0.067 [30]

Ru/C batch reactor, solvent-free conditions, H2(12 bar), 45 min 190 100 100 100 5.33 [30]

5 wt % Ru/C and Amberlyst- 70

batch reactor, acid co-catalyst alongside H2O solvent and H2

(30 bar)

70 100 100 100 5.63 [57]

[a] Summarized from reported protocols. [b] (mol_GVLmol_LA ¹) 100. [c] Calculated from literature data, using the mass of metal in a given catalyst. [d] 1:1 mixture of CuO and Cr2O3reduced. [e] Weight hourly space velocity. [f] Trifluoroacetic acid.

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55 bar. Low LA conversions were rendered by the Pt, Re, and Ni systems (%15 %), and moderate activity was obtained from supported Ir, Rh, and Pd (30–40 %). However, of the catalysts screened, Ru/C was noticeably the most active, giving the highest conversions (80 %). Further optimization allowed for complete LA conversion and a GVL selectivity that exceeded 95 %. Systems using Ru/C have been further optimized by Liu and co-workers, who also compared Ru/C with Pd/C, Raney Ni, and Urushibara Ni.^[29] Under the reaction conditions used (1308C, 12 bar H₂, 5 wt % LA in methanol), Ru/C again facilitated the highest GVL conversion (92 %) with a GVL selectivity of 99 %. Both forms of Ni were found to catalyze LA hydrogenation, although Urushibara Ni gave low selectivity (%5 %) and Raney Ni gave poor LA conversion (%20 %). These results demonstrate that for LA hydrogenation base metals often display lower activities than those offered by precious metals. Analysis of the Ru/C reaction solution using GC–MS confirmed the presence ofg-hydroxyvaleric acid,g-valerolactone andpseudolevu- linic acid implicating these species as reaction intermediates as proposed in previous investigations (Scheme 2).^{[15, 23]}Moreover, methyl levulinate was identified in the reaction mixture, resulting from esterification of LA (Scheme 2).^[22]Systematic variation of the H₂partial pressure established that for the Ru/C utilized, the optimal H₂pressure is 12 bar. Using higher H₂pressures resulted in a marked decrease in LA conversion presumably due to saturation of the active catalyst sites by hydrogen. A similar trend was observed when investigating the influence of reaction temperature. The highest GVL yields occurred at 1308C, with higher reaction temperatures resulting in decreased GVL yields.

While variation of reaction parameters can influence GVL yields, it should also be noted that alteration of the catalyst support can have a profound influence on reaction outcomes.

For instance, Palkovits et al. have found that Ru/TiO₂ (Tronox) was unable to catalyze LA hydrogenation, whereas under the same reaction conditions Ru/TiO₂(Degussa P25) facilitated GVL yields of 71 %.^[30]Furthermore, it was observed that Ru/C gave rise to a marginally higher yield (89 %) than Ru/Al₂O₃ (76.0 %) or Ru/SiO₂ (75.0 %). Overall, further research is needed to rationalize the origin of these discrepancies, as currently few investigations have systematically correlated support structure and chemical composition with catalyst reactivity. Another pa- rameter that can drastically influence catalyst activity is the extent of metal dispersion on the catalyst surface. Separate investigations that focused on using Ru/TiO₂ for the hydrogenation of carboxylic acids (including LA) suggest that superior catalytic activity can be rendered by catalysts comprised of small Ru nanoparticles (average crystal size 2.00.1 nm).^[31]

This conclusion is consistent with the claim that highly dispersed Ru/C is more active for LA vapor-phase hydrogenation.^[23]

Although high conversions can be facilitated using supported Ru catalysts, recycling experiments conducted by Liu and co-workers indicate that the stability of such catalysts is often low. For instance, Liu have found that the 99 % LA conversion initially rendered in their system by Ru/C dropped to 42 % by the 4^threuse.^[29]A similar trend has been observed by Dumesic

and co-workers, who have used Ru/C for the hydrogenation of 50 wt % aqueous LA as part of an overall process for the production of alkene transportation fuels.^[32]The Ru/C catalyst was initially able to facilitate quantitate GVL synthesis, but when used in a continuous setup significant catalyst deactivation occurred (90 to 68 % conversion after 106 h). Similar loss of activity has also been observed for catalyst systems based on other precious metals, including processes developed by Shell in which pentanoic acid is synthesized from LA through GVL.^[33]

In this investigation, 0.8 wt % Pt/SiO₂ was packed into a flow reactor, and a LA mixture was passed over the catalyst bed at 2008C under a constantly maintained pressure of H₂. The flow of LA over the catalyst bed was also kept constant at a weight hourly space velocity of 2.0 g_LAg_Catalyst ¹h ¹. Under these conditions, LA was predominantly converted to GVL, with only traces of 2-methyltetrahydrofuran and pentanoic acid observed (%1.0 mol %). However, the effluent composition drastically changed with time, comprising of 86 % GVL after 15 h, but 26 % GVL after 460 h. Regeneration of the catalyst by calcining in air was sufficient to restore its original activity, confirming that the diminished GVL yields are due to catalyst deactivation.

Furthermore, utilization of a feedstock that contained 87 wt % GVL and 13 wt % LA seemingly facilitated a more stable system with no reduction in hydrogenation observed after 140 h. The problem of catalyst stability has also been addressed by Lange et al. while exploring the development of new transportation fuels from cellulose.^[11c] Screening of 50 catalysts using a flow reactor indicated that the best performance was rendered from Pt on TiO₂ or ZrO₂, with constant GVL yields obtained over a 100 h period (>95 mol % at 2008C and 40 bar H₂).

Evidently, stable systems can be obtained by combining appropriate supported catalyst with optimized reaction conditions.

However, arguably further research is required to rationalize catalyst deactivation, knowledge that could enable the development of a wider range of stable hydrogenation systems.

Hydrogenation in super critical CO₂

Super critical CO₂(scCO₂) is widely considered to be a potential

“green” solvent, as its utilization could enable milder reaction conditions and a lower net environmental impact than that of traditional solvents.^[34] With these factors in mind, Manzer and Hutchenson have screened the activities of numerous catalysts in scCO₂, using both batch and continuous reactors.^[35]A high yielding system resulted from using 5 wt % Ru/Al₂O₃ in scCO₂ at 1508C for 2 h alongside a cosolvent, 1,4-dioxane (Table 1).

A total pressure of 250 bar was maintained throughout the reaction, which produced GVL quantitatively, establishing that GVL can be efficiently synthesized in scCO₂. It was also demonstrated that scCO₂is compatible with continuous flow reactors, which in combination with 1 wt % Ru/Al₂O₃enabled near quantitative conversion of LA to GVL.

The observation that scCO₂can be used in GVL synthesis inspired Poliakoff et al. to investigate a new GVL separation approach based on manipulation of the CO₂-phase behavior (Table 1).^[36]Their setup utilized a feedstock comprised of 75 % LA and 25 % H₂O. A near quantitative yield of GVL was ob-

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tained in scCO₂(total pressure of 10 bar) by using an automat- ed reactor alongside 5 wt % Ru/SiO₂. The crude product mixture was comprised of both H₂O and GVL, components that are normally miscible. Next, the reactor’s pressure was reduced until the vessel contained a sub-critical pressure of CO₂. This induced a liquid–liquid separation resulting from the immisci- bility of the distinct H₂O and CO₂/GVL phases. Through the use of an appropriate reactor design, this phenomenon can be used for the isolation of GVL, which contained only trace H₂O (0.4 %). Furthermore, traces of unreacted LA remain in the aqueous phase enabling recycling. This novel approach could provide an alternative to distillation, although the high reactor pressures required also have an energy cost. Moreover, CO₂sa- turated in the isolated GVL product would inevitably be lost from the reaction system. However, the authors argue that in this system isolation of GVL does not require a full release of CO₂ reactor pressure. Furthermore, integrating separation steps into a supercritical reaction could enable higher process efficiencies.

Production of GVL using LA/FA Product Streams

Currently, worldwide H₂production is reliant on the steam re- forming of fossil carbon, a notoriously energetically intensive procedure.^[37] Hence, processes that utilize petroleum-derived H₂ for GVL synthesis will arguably be less sustainable and could have low net environmental benefits. With this factor in mind, the synthesis of GVL using alternative hydrogen sources, such as FA (formic acid), has become the focus of substantial research efforts. FA is an especially attractive hydrogen source for GVL synthesis, as this compound is also produced in the acid-catalyzed dehydration of carbohydrates (Scheme 1). Theo- retically, this process is predicted to produce LA and FA in a 1:1 stoichiometry. However, in practice FA is produced in a slightly higher molar ratio than LA, a selectivity that is be- lieved to result from side reactions involving dehydration intermediates.^[38]Thus, product streams resulting from carbohydrate dehydration will contain sufficient FA to enable full hydrogenation of the LA component, an attractive prospect, as such an approach would avoid costly purification of LA and allow for more atom-efficient processes requiring reduced H₂inputs.

Numerous strategies have been used to convert LA/FA feeds into GVL (Table 1). Recently, an innovative investigation used inorganic salts alongside hydrothermal conditions;^[39]however, most studies have focused on heterogeneous and homogeneous systems based on transition metals.^[40] A pioneering study conducted by Fu and Guo et al. explored homogeneous catalysts formed in situ from coordination of PPh₃ and RuCl₃·H₂O.

The authors demonstrated that in the presence of a base (NEt₃ or pyridine) at 1508C such catalysts can convert LA and FA to GVL in high yields (94 %).^[38a]Detailed examination demonstrated that GVL formation is slow at 1008C. However, at these temperatures a rapid rise in the pressure of the autoclave was observed, which resulted from the Ru-catalyzed decomposition of FA into H₂and CO₂, a pathway that has previously been examined for H₂ storage.^[41] Thus, the authors conclude that in

this system transfer hydrogenation does not occur. Instead, FA decomposition liberates H₂, which is then consumed in a classical hydrogenation process. With this knowledge in mind, Fu and Guo et al. next investigated heterogeneous systems based upon initial selective decomposition of FA.^[38b] To realize this methodology, RuCl₃was immobilized on a range of functionalized silicas, and these materials were used as catalysts for the decomposition of 4m aqueous FA at 1208C. Higher turnover frequencies were observed (e.g., 7357 h ¹ for Ru-S/SO₂) with no undesirable CO detected by means of GC. While catalysts such as Ru-S/SiO₂ or Ru-P/SiO₂ gave rise to superior activities for FA decomposition, these systems displayed lower activities for LA hydrogenation than that given by Ru/C. In an attempt to derive a system with the advantages of both types of catalysts, Ru-P/SiO₂and Ru/C were combined in a single reaction mixture comprised of LA, FA and water. This produced GVL in low yields, which was attributed to poisoning of Ru/C by FA.

Due to the poor performance of this system, an alternative two-stage process was developed. Mixtures of LA and FA were heated alongside Ru-P/SiO₂at 1708C for 1 h, which caused decomposition of FA. This reaction mixture was then recovered and mixed with Ru/TiO₂ at 1708C and a H₂pressure of 45 bar (the same H₂ pressure generated by FA decomposition). The GVL yield obtained was 88 % with the catalyst found to be highly recyclable giving yields of 88 % to 93 % over eight recycle runs. The authors suggest that this approach could facilitate continuous synthesis of GVL through a dual bed tubular reactor, potentially enabling a fully continuous process.

Although removal of FA prior to contact with the hydrogenation catalyst can diminish catalyst deactivation, a simpler process could be obtained by designing catalysts with sufficient stability to withstand acidic conditions. Mixed-metal catalysts have previously been utilized for GVL synthesis,^[42] and twinning hydrogenation metals such as Ru with oxophilic metals has been demonstrated to confer advantages to various conversion processes.^[43]This factor has inspired Dumesic and co-workers to utilize bimetallic catalysts in integrated processes designed to convert cellulose first to an LA/FA mixture, which is then used to produce GVL (Scheme 3).^[44]In such systems efficient separation of LA and GVL from the aqueous product streams is problematic, although efficient extraction can be achieved using alkylphenol solvents.^[44a] However, such processing places demands on the hydrogenation catalyst, as selective hydrogenation of LA must be achieved without modification of the unsaturated solvent. To meet this challenge, Dumesic et al. used RuSn (3.6:1)/C as the hydrogenation catalyst.^[44a] The presence of Sn on the catalyst surface advanta- geously increased the selectivity of FA decomposition by di- minishing the formation of CO and CH₄. Furthermore, the selectivity of LA hydrogenation was increased, with GVL obtained without hydrogenation of the alkylphenol solvent. Finally, the RuSn (3.6:1)/C catalyst was found to be highly stable, with stable activity observed over a 230 h TOS (time on stream).

Dumesic and co-workers have also utilized 15 wt % RuRe (3:4)/C as part of detailed investigations focused around designing and evaluating a complete process intended to convert cellulose into alkene oligomers.^{[44b, c]}The catalyst 15 wt %

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RuRe (3:4)/C was utilized in a fixed bed reactor alongside aqueous solutions of LA and FA (2.2 mol L ¹ of each acid).

Under these conditions, stable catalysis was achieved enabling a GVL selectivity of95 %. Furthermore, the RuRe (3:4)/C catalyst was not impeded by addition of H₂SO₄, a significant find- ing as conversion of carbohydrates into LA/FA requires strong acid catalysts such as H₂SO₄ that will invariably be present in the resulting product streams. Notably, Ru/C was rapidly deactivated by H₂SO₄, making this catalyst unsuitable for this application. Overall, the drastic differences displayed by Ru/C and RuRe (3:4)/C demonstrate that catalyst design can have a profound influence on catalyst activity and thus net process viability. While significant advantages are associated with the utilization of RuSn/C or RuRe (3:4)/C as part of integrated processes, systems based on these catalysts tend to rely on elevated H₂ pressures (35 bar) to enhance the rate of hydrogenation. Such pressures could be achieved by compression of the H₂ obtained from FA dehydrogenation, although this would require careful hydrogen management, may entail energy costs, and would require robust reactor design. Thus, the development of processes that utilize low H₂pressures could enable more economical GVL production.

Due to the propensity of Ru/C to be deactivated by acids, there is a need to develop more robust catalyst systems.

In this regard, Cao et al. have developed materials based on Au nanoparticles, which were then utilized to convert FA and LA to GVL.^[45] These investigations were partially inspired by the observations that Au dispersed on alumina can selectively decompose FA in the gas phase and that supported Au can catalyze the transfer reduction of carbonyl compounds by formate salts.^[46] Initial work utilized catalysts based on Au nanoparticles (1.2–2.5 nm) supported on acid-tolerant ZrO₂ to reduce LA with FA (both 0.43m) at 1508C. This resulted in qualitative conversion to GVL, with the catalyst found to be highly recyclable. Moreover, it was demonstrated that variation of Au particle size can impact on catalyst performance, with larger Au particles (approx. 3.0 nm) having diminished activity.

Comparison of Au/ZrO₂, with related Pd-, Pt-, Ru-, and Pd-containing materials showed that when ZrO₂ was utilized alongside the conditions of this study, Au was the most active noble metal. However, immobilization of Au on TiO₂ reduced GVL yields and only trace conversion was observed for Au/C and Au/SiO₂, indicating that effective Au catalysis requires alternative supports to those utilized in Ru systems. Finally, systems based on Au/ZrO₂were utilized to modify a number of alternative product streams obtained from a range of carbohydrates.

Under optimized conditions LA derived in situ from fructose, glucose, starch, and cellulose was converted into GVL, with the overall GVL respective yields being 60, 51, 50, and 33 % based on the starting mass of carbohydrates. In each instance, the LA generated was fully converted to GVL, strongly indicating that the efficiency of carbohydrate conversion dictates the overall process yield. These results demonstrate that the Au/ZrO₂ catalyst could feasibly modify the product streams obtained from crude biomass.

The potential of Au/ZrO₂has inspired Cao et al. to utilize this catalyst as part of a strategy designed to separate LA and FA from aqueous product streams.^[47]This approach is based upon using alcohols to produce hydrophobic esters, enabling their separation from aqueous H₂SO₄, which can then be reused in cellulose dehydration (Scheme 4). This builds upon previous work by Ayoub, who has demonstrated the separation of LA and FA using 1-pentanol.^[19b]Notably, development of systems focused on utilization of alkyl levulinates could prove to be particularly advantageous as cellulose can be converted to alkyl levulinates in higher yields than LA.^[48]

In Cao’s system, esterification of LA and FA usingn-butanol resulted in products that auto separated from the aqueous H₂SO₄ solution, which could then be recycled.^[47] The organic stream was found to contain 98 and 95 % of LA and FA (present as butyl levulinate and butyl formate) that existed in the aqueous phase, demonstrating that this methodology enables efficient product separation. Next, the decomposition of butyl formate at 1708C in the presence of Au/ZrO₂was demonstrated, which produced exclusively H₂, CO₂, and n-butanol. As decomposition of butyl formate produced H₂, this hydrogen source was used for the hydrogenation of butyl levulinate using Au/ZrO₂. Smooth conversion to GVL proceeded at 1708C, with near quantitative yields obtained after 6 h and reaction rates enhanced by the addition of water. The catalyst was found to be recyclable, with XPS and TEM analysis of Au/

ZrO₂after reaction indicating that no observable change in the catalyst occurred. Notably, Au/ZrO₂ was demonstrated to initiate hydrogenation without an overpressure of H₂, a factor that could lead to simpler process design than that required for similar systems based on current Ru catalysts.

An alternative approach to the separation of LA and FA from aqueous product streams has been developed by Dumesic and co-workers and involves reaction of LA and FA with butene.^[49] This methodology is argued to be particularly advantageous, as butene can theoretically be manufactured from GVL and the volatility of butene could enable its convenient removal from the reaction products. Treatment of LA and FA with butene generated sec-butyl formate andsec-butyl levulinate at moderate temperatures (758C) after a short contact time of 120 min. Again, these hydrophobic esters spontane- ously separate from the aqueous solution, enabling recycling of the aqueous H₂SO₄catalyst. Next, a dual bed reactor (com- Scheme 3.An integrated biorefinery approach based upon extracting GVL after hydrogenation.

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prised of 10 wt % Pd/C and 5 wt % Ru/C) enabled quantitative synthesis of GVL, generating both CO₂ and 2-butene as reaction by-products. The inclusion of Pd/C is required to ensure efficient conversion of FA and formate esters into H₂and CO₂, a transformation for which Ru/C has a lower activity and selectivity, generating CO and CH₄. Notably, reaction rates were again enhanced by inclusion of water in the product stream, presumably as decomposition of formate esters proceeds via FA. The dual bed catalyst is stable, with no loss of activity observed after over 400 h on stream. This arguably demonstrates that, as in similar systems, stable catalysis is enabled by removal of H₂SO₄ prior to hydrogenation, pre- venting catalyst deactivation.

The examples highlighted herein demonstrate that utilization of FA and formic esters as a hydrogen source can enable quantitative GVL synthesis via elimination of CO₂. When twin- ned with the conversion of LA and FA to hydrophobic esters, sophisticated biorefinery approaches can be realized that can enable efficient separation and reuse of H₂SO₄. Such approaches require appropriate catalyst design and selection, emphasizing the importance of catalyst development to the achievement of large-scale GVL production.

In situ generation of LA and subsequent hy- drogenation to GVL

Conversion of carbohydrates directly into GVL without isolation of the intermediate LA could provide a means of producing GVL with minimal processing steps, although this methodology requires the combination of both acid and hydrogenation catalysts. For efficient GVL production, the relative reaction rates of each step must be balanced, as slow rates of acid-catalyzed sugar dehydration can lead to the formation of sugar alcohols such as sorbitol (Scheme 5). It should also be noted that acid-catalyzed dehydrogenation of carbohydrates can lead to the formation of insoluble humins via the acid-catalyzed polymerization of sugars or the reaction intermediate 2,5-bis- (hydroxymethyl)furan, a pathway that must be avoided to obtain GVL in high yields (Scheme 5). Investigations have reported the combination of homogeneous catalysts for this application,^{[40c, 50]}and recently Heeres et al. have combined homo-

geneous acid dehydration and heterogeneous hydrogenation catalysts.^[51] To enable produc- tive tandem catalysis, the Brønsted acid was carefully se- lected, with catalysts advantageous for carbohydrate dehydration (such as H₂SO₄) found to poison the hydrogenation catalyst (Ru/C). Instead, TFA was found to be capable of generating LA from carbohydrates without impeding the catalysts.

With optimization, combinations of Ru/C and TFA were able to convert fructose into GVL with a yield of 52 % at 1808C; however, residual LA (11 %) persisted in the reaction mixture. Glu- cose was also used as a feedstock alongside H₂, although lower optimized yields were obtained, with the highest GVL yields reported as 46 mol %. Control reactions using acid catalysts in the absence of H₂indicated that this lower yield is due to less efficient conversion of glucose to LA, presumably be- cause of the higher stability of glucose relative to fructose.

Similarly, conversion of cellulose also resulted in a lower yield (38 mol %), which correlates well with the yields of LA from cellulose reported in the literature.^[52] Overall, this investigation demonstrates that it is possible to balance both hydrogenation and dehydration steps using combinations of heterogeneous and homogeneous catalysts.

Development of New Base Metal Catalysts for GVL Synthesis

One factor that could make implementation of processes based on the conversion of LA to GVL less advantageous is the financial cost associated with precious metal hydrogenation catalysts. Dumesic and co-workers have evaluated the costs associated with large-scale conversion of cellulose materials to liquid alkenes via the conversion of LA to GVL using a RuRe (3:4)/C hydrogenation catalyst.^[44b] This demonstrated that the costs of Ru and Re are likely to be significant for a large chemi- Scheme 4.An integrated biorefinery approach utilizing reactive extraction to remove H2SO4prior to

hydrogenation.

Scheme 5.Possible reaction pathways resulting from combined carbohydrate dehydration and hydrogenation.

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cal plant,^[44c] with the purchasing outlay alone estimated at

$ 33 million. Furthermore, reserves of precious metals are finite and unevenly distributed, factors that could lead to disrupted supplies,^[53]threatening processes based on precious metal catalysts. Evidently, such difficulties and costs could be mitigated by the utilization of less expensive base metal catalysts.

Raney Ni has already been widely used for GVL synthesis alongside H₂, facilitating quantitative GVL conversion, albeit after extensive heating (^2008C).^[25] In contrast, at 1308C Raney Ni gives substantially lower LA conversions (%20 %), demonstrating that this catalyst is ineffective at low temperatures.^[29]

More recent attempts to utilize base metals for GVL synthesis have focused on using metal particles leached from the reaction vessel’s wall as catalysts.

This approach is inspired by the reality that LA/FA

product streams (which may contain trace H₂SO₄) are corrosive and will inevitably cause the leaching of metals into reaction solutions, a process that will be heightened at elevated reaction temperatures. Dumesic et al. have investigated the corrosion of metals form reactors made of stainless steel and Hastel- loy-C276.^[44b] Unsurprisingly, a higher extent of corrosion was observed from stainless steel, although control reactions indicate such leached metals are unable to facilitate GVL synthesis.

This approach has been expanded by Schlaf and co-workers, who deliberately exposed stainless steel reactors to trifluoro- methanesulfonic acid under an atmosphere of H₂.^[54]This treatment removed the surface-protecting chromium oxide layer, generating a surface able to initiate LA hydrogenation, without the presence of any additional catalyst. Quantitative yields of GVL were obtained by heating (758C or 1008C) aqueous solutions of LA with H₂(55 bar) for 24 h. Analysis of the chromium oxide precipitates and reactor body, using plasma optical emis- sion spectroscopy and mass spectrometry, demonstrated that hydrogenation was not initiated by the presence of trace Ru, Re, Rh, Ir, Pd, or Pt. Arguably, utilization of such a novel approach could drastically simplify catalyst design and reduce catalyst costs. However, the deliberate corrosion of a reaction vessel requires caution as continuous corrosion could ultimately compromise reactor integrity.

More conventional strategies for LA hydrogenation that use base metal catalysts have been developed. Hwang and Chang et al. have investigated the capacity of nanocomposite Cu/SiO₂ catalysts to convert LA into a range of hydrogenation derivatives including GVL.^[55] The Cu/SiO₂catalysts were prepared by classical precipitation–deposition methods, which enabled a range of Cu loadings (5–80 wt %). These catalysts were then utilized in a vapor-phase reactor, with mixtures of H₂/LA (80:1) passing over the catalyst bed at 2658C. When a 5 wt % Cu/SiO₂ catalyst was utilized in this setup, quantitative conversion of LA to GVL was obtained. However, catalysts with higher Cu loadings (30–80 wt %) had a low selectivity for GVL, which was hydrogenated further to produce methyl-THF, 1-pentanol, and 1,4-pentanediol.

With the aim of developing inexpensive methods for GVL synthesis, Chia and Dumesic have developed new transfer-hy-

drogenation methodologies based upon inexpensive ZrO₂catalysts.^[56] In this investigation, a secondary alcohol was utilized as the hydrogen source instead of FA (Scheme 6). Such an approach was proposed by Chai and Dumesic to be an alternative more suited to base metal catalysts, as decomposition of FA often requires precious metals or forcing reaction condi-

tions. After GVL synthesis, it would be possible to convert the aldehyde product back to an alcohol by using base metal hydrogenation catalysts (Scheme 6). Recycling of the alcohol could then enable a sustainable system that does not require precious metals. With these factors in mind, Chai and Dumesic undertook an in initial screening investigation focused on converting butyl levulinate to GVL. Of the catalysts screened, ZrO₂ was the most active, giving rise to notably higher conversions than MgO/Al₂O₃, MgO/ZrO₂, CeZrO_x, or g-Al2O₃. The reaction was found to proceed at 1008C, with increases in temperature or molar ratio of hydrogen donor found to increase GVL yields.

Notably, LA was found to undergo transfer hydrogenation less readily than its esters. However, LA was hydrogenated by combining ZrO₂ with 2-butanol, which facilitated a maximum GVL yield of 92 % at 1508C. The only other reaction product identified was butyl levulinate, which was proposed to originate from esterification on the catalyst surface. It is proposed that a slower LA hydrogenation is a consequence of coordination of this acid to the active basic sites of ZrO₂, blocking these sites and inhibiting catalysis. This hypothesis is supported by the observation that addition of basic MgO to LA reactions resulted in a marked increase in the rate of GVL formation. Final- ly, Chia and Dumesic tested catalyst stability using an on- stream continuous flow reactor. Rapid deactivation of the catalyst was observed during a 100 h period, although after this time activity stabilized. Catalyst deactivation could be reversed by calcination of the catalyst in air, which resulted in restora- tion of its activity.

The investigations highlighted herein have successfully identified promising base metal catalysts; however, the number of systems developed is limited. Arguably, more studies are required to develop a wider range of base metal catalysts, a research effort that could potentially facilitate a more economical GVL production.

Utilization of High and Low Reaction Tempera- tures for GVL Synthesis

Systems tailored to GVL production should arguably be optimized around either high or low reaction temperatures, with Scheme 6.Utilization of alcohols as a hydrogen source for the synthesis of GVL from alkyl levulinates.

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each alternative regime offering different advantages. High reaction temperatures can facilitate rapid GVL synthesis, and subsequently higher space time yields. For example, Palkovits et al. have demonstrated that Ru/C can quantitatively convert neat LA to GVL within 40 min at 1908C under a pressure of 12 bar.^[30] Such rapid conversion is required for viable applications, although high temperatures inevitably incur an energy cost. With this in mind, researchers have begun to re-examine the possibility of synthesizing GVL at low or ambient temperatures.

It has long been established that GVL synthesis can be catalyzed at 258C using PtO₂ alongside 2–3 bar H₂.^[24] In this system, reaction rates were slow, with over 40 h required to obtain a 87 % GVL yield. Recently, Palkovits and co-workers have re-examined the room temperature synthesis of GVL, using Ru/C and neat LA alongside a H₂ pressure of 12 bar.^[30]

Under such conditions, most LA was converted to GVL within 24 h (72 %). However, significant amounts of g-hydroxyvaleric acid persisted in solution, with a 50 h reaction time required to obtain GVL in a 100 % yield. Control reactions indicate thatg- hydroxyvaleric acid does not readily undergo dehydration to GVL at 258C and that a catalyst is required to initiate dehydration at low temperatures.

With the aim of identifying protocols for the synthesis of GVL under mild conditions (50–708C, 30–5 bar H₂), Galletti et al. have explored combinations of Ru-supported catalysts and acidic co-catalysts (Amberlyst-70 and -15, niobium phos- phate, and niobium oxide).^[57]Under these reaction conditions, both Ru/Al₂O₃ and Ru/C facilitated limited LA conversion (24 and 48 %, respectively). Addition of acidic materials increased catalyst performance, with Amberlyst-70 found to be the most effective co-catalyst. Combining Amberlyst-70 with Ru/Al₂O₃ and Ru/C enabled respective LA conversions of 57 and 100 %, with both reactions having selectivities of over 98 %. Control reactions using substituents such as butan-2-one indicated that acids drastically enhance the capacity of supported Ru to initiate hydrogenation, although the mechanism by which this occurs requires clarification. However, it is clear that the presence of an acid co-catalyst will enhance the intermolecular esterification ofg-hydroxyvaleric acid, facilitating high reaction selectivity. Notably, under optimized conditions (708C) and in the presence of Amberlyst-70, within 30 min, an LA conversion of 90 % was achieved alongside 100 % selectivity for GVL. This result suggests that further investigations could identify systems that enable the rapid synthesis of GVL at mild reaction temperatures, an attractive combination for commercial applications.

Outlook and Conclusions

The targeted production of GVL has been the focus of considerable academic and industrial research efforts due to both the potential usability of GVL and the viability of LA as a feedstock.

Heterogeneous catalysts have been widely utilized for this application, and it has been demonstrated that they are compatible with either H₂ or FA and liquid- or vapor-phase setups.

A substantial number of heterogeneous catalysts have been

used, with the most common being supported transition metals. In this regard, catalysts containing Ru are arguably the most prevalent, with characterization indicating that highly dispersed Ru enables more efficient catalysis. Although catalysts containing Ru are arguably the most common, screening studies have suggested that Pt/TiO₂and Pt/ZrO₂can offer superior stability, and promising catalysts containing Au nanoparticles have been developed. While numerous catalysts have been screened, more examination is required to rationalize the influence of a given support on catalyst activity. Furthermore, more research is necessary to examine the mode of catalyst deactivation as such investigations could enable the identification of a wider range of stable systems.

Innovation in catalyst design has enabled the development of a range of creative integrated bioprocessing methodologies, focused upon producing GVL from aqueous solutions containing H₂SO₄, LA, and FA. Acid-resistant catalysts have enabled the direct conversion of such feedstocks, with later extraction allowing for both isolation of GVL and reuse of H₂SO₄. Alterna- tively, conversion of LA and FA to hydrophobic esters enables auto separation, allowing hydrogenation of the alkyl levulinates in the absence of H₂SO₄. While such approaches could enable the widespread manufacture of GVL, it should be noted that current systems rely exclusively on precious metals, with the use of base metals in general remaining underexplored.

This is problematic, as the identification of alternate less expensive base metal catalysts, capable of initiating rapid LA hydrogenation in high yields at low temperatures (%1008C), could facilitate a more economical GVL production.

A second means by which more economical GVL could be realized is by operating at low reaction temperatures (%1008C). This is a viable target as GVL synthesis has been demonstrated at low temperatures (258C). However, such systems invariably have poor space–time yields. This has partially been overcome by using an acid co-catalyst alongside low reaction temperatures (50–708C), suggesting that further catalyst innovation could enable systems that facilitate rapid GVL synthesis at low temperatures.

In summary, numerous heterogeneous catalysts have been used for GVL production alongside a range of reaction setups.

However, there is still scope for catalyst improvement and innovation. This could enable the development of more economical systems, potentially enabling the wider and sustainable utilization of renewable GVL.

Acknowledgements

This work has been funded by the Robert Bosch Junior professor- ship for sustainable utilization of renewable natural resources.

It was performed as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass” funded by the Excellence Initiative by the German federal and state governments to promote science and research at German universities.

Keywords: biomass·heterogeneous catalysis·levulinic acid· sustainable chemistry·g-valerolactone