Advances in the Catalytic Production of ... - Wiley Online Library

DOI: 10.1002/cctc.201200113

Advances in the Catalytic Production of Valuable Levulinic Acid Derivatives Jun Zhang, ShuBin Wu,* Bo Li, and HongDan Zhang[a]

1. Introduction Currently, the conversion of biomass to levulinic acid and its derivatives has been one of the hottest topics in the field of energy and resources, because it opens up a new avenue for achieving sustainable energy supply and chemicals production. Levulinic acid can be used as a platform chemical for the production of a wide range of value-added compounds, such as levulinate esters and g-valerolactone. This review mainly discusses the catalytic routes for synthesis of valuable levulinate esters and g-valerolactone under different efficient reaction systems. Meanwhile, some promising and valuable researching directions and effective catalysts are suggested based on the major challenges emerged in recent studies. With the gradual depletion of fossil resources and further deterioration of the environment and its ecosystem, abundant renewable biomass are regarded as a promising alternative to non-renewable natural resources for the sustainable production of biofuels and biochemicals in the future (Figure 1).[1–8] Recently, extensive research has been performed worldwide to identify and study chemical or biological transformations for converting biomass into fuels and raw chemicals.[9–11] Among these explorations, one attractive approach is the preparation and conversion of levulinic acid (LA), owing to its importance as one of the 12 important target chemicals that US-DOE selected in their biomass program.[12] It is widely used in food, agriculture, drugs, cosmetics, spice industries, as well as its derivatives. With regard to sources of LA, the LA preparation method can be classified into two categories according to the raw material resources, based on previous studies, namely, furfuralco-

Figure 1. Concept of a bio-refinery.

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hol hydrolysis and biomass conversion.[13] Both of these two methods were conducted under acidic conditions or with acid catalysts, while the materials including starch and cellulose were used as the reactant for method of biomass conversion. Besides, high purity, LA can be obtained by furfuralcohol hydrolysis with a suitable solvent and catalyst. Considerable research over the past years has focused on the catalytic conversion of biomass into LA by both batch processes and continuous technology.[14–16] As for transformation pathways, if agricultural and forest wastes can be rationally utilized, many valuable chemicals can be created at the same time. Most importantly of all, it opens a new way for the production of biochemicals in an economical manner. Many kinds of raw materials such as lignocellulose, corn starch, and discarded cellulose are used extensively in this route.[14–18] However, there still are some clear shortcomings, especially as the apparent LA yield is not high and the final reaction products are complicated. In subsequent research, glucose has also been adopted as a raw material to yield LA, and a desired yield of LA can be obtained with solid acids as catalyst.[19–21] As the studies develop in depth, some valuable levulinic acid derivatives have been discovered by researchers in recent years, especially levulinate esters and g-valerolactone. As these compounds are widely applied in food and chemical industry, it is of great significance for us to do more research on the preparation of the above high value-added chemicals by means of heterogeneous or homogeneous reaction. The levulinate esters were produced by the esterification of LA between alcohols, such as methanol, ethanol, and butanol under acidic conditions. Meanwhile, g-valerolactone (GVL) is a selective hydrogenation and dehydration product of LA catalyzed by metal catalysts. Clearly, it is a research direction with great significance and many researchers are now working on this new aspect. Currently, there is no review of the catalytic preparation of levulinate esters and GVL to date. Given that the rapid progress on the catalytic conversion of biomass, this minireview mainly concentrates on describing the work reported over the last few years. Basic information about levulinic acid is given, [a] Dr. J. Zhang, Prof. S. Wu, Prof. B. Li, Dr. H. Zhang State Key Laboratory of Pulp and Paper Engineering South China University of Technology 510640, Guangzhou, Guangdong (China) Fax: (+ 86) 020 22236808 E-mail: [email protected]

2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Valuable Levulinic Acid Derivatives followed by the improvement in the catalytic preparation of levulinate esters and g-valerolactone with suitable catalyst systems, as well as their applications in agriculture, food, and chemistry.

2. Essential Information Regarding Levulinic Acid 2.1. The structure and properties of levulinic acid Levulinic acid, also known as 3-acetylpropionic acid, 4-oxovaleric acid, or 4-oxopentanoic acid, was first described in 1870. It is a widely used industrial chemical with one carbonyl, one carboxyl and a-H in its inner structure (Figure 2), which belongs to short chain and non-volatile fatty acid. The detailed information for physical properties of levulinic acid are shown in Table 1.[22]

Figure 2. The molecular structure of LA.

Table 1. Physical properties of LA. Molecular Weight

Refractive Index[a]

Density [kg m 3][a]

pKa

Melting point [K]

Boiling point [K]

116.2

1.4796

1140

4.5

306–308

518–519

[a] Value given for 293 K

By analyzing the special structure of LA, the carbon–oxygen double bond from carbonyl group has strong polarity, while oxygen atom has stronger attracting electron ability compared to the carbon atom, so the p electron will transfer into the greater electronegative oxygen, thus leading to the formation of positive charge center in carbon atom. The electrophilic center of the carbon atom plays an important role when the carbonyl group performs chemical reactions. Owing to the relatively strong electron receptor effect of the oxygen atom of the carbonyl group, LA has higher dissociation constants than a common saturated acid, so that its corresponding acidity is stronger. Furthermore, LA can be isomerized into the enolisomer, owing to the existence of carbonyl group. LA belongs to a group of compounds, which have several highly active sites and can also be used as a platform chemical for preparing many other high value-added products. 2.2. Applications of levulinic acid Based on the special structure of LA, various kinds of products can be obtained by esterification, halogenation, hydrogenaChemCatChem 2012, 4, 1230 – 1237

tion, oxydehydrogenation, condensation, and other chemical reactions of LA. For example, in the pharmaceutical industry, calcium levulinate is a new calcium supplement that can be made into pills, capsules, or injections.[23] In addition, it serves as the food nutrition enhancer that enhances bone formation and muscular excitability. Remarkably, a recent study revealed that calcium levulinate could be subjected to high temperature to form significantly deoxygenated and dense energy products, which have great research value in the study of biomass conversion.[24] Moreover, nonsteroidal anti-inflammatory drugs and medical levulinate also can be made from LA. As for applications in agriculture, d-aminolevulinic acid (DALA) is a new photoactivation weedicide of high environmental compatibility, selectivity, and biodegradability, and it is harmless to crops and human health.[25, 26] DALA can be used as defoliant for the treatment of fallen leaves before picking cotton and apples. The levulinate potash has the advantage of cold resistance, drought resistance, and insect resistance, so it is a highly effective fertilizer. 2-Methyl-3-indoleacetic acid, produced from levulinic acid, is a common plant growth hormone that can promote the growth of root and stem.[13] When levulinic acid is applied to the spice and food industry, its derivatives such as levulinates, a-angelica lactone, and GVL can all be used as perfume material and food additives. Levulinates are mainly used for removing nicotine and keeping fruits fresh;[27] a-angelica lactone is a flavor and cigarette additive,[28] which gives off a sweet smell by mixing smoke incense, caramel aroma, and chocolate aroma. GVL is widely used as an edible essence and in tobacco flavors, for its soft and lingering fresh fruit aroma, medical aroma, and sweet smell.[29] As a new high polymer material, the diphenolic acid (DPA), produced from levulinic acid, has wide practical use and extensive application value in polymeride and other materials.[30] It can serve as feedstock for the production of water soluble resins that are used in industrial millipore filter and oil filter paper.[31] Besides, LA and its derivatives also have many other applications such as plasticizer, surfactant, softener, cleanser and emulsifier.[32] 2.3. The mechanism of LA formation 5-HMF can be easily degraded into LA and formic acid under acidic condition, and many other chemicals are created at the same time. Horvat et al. proposed the specific reaction mechanism of LA formation by analyzing intermediate product with 13 C NMR technique,[33] as shown in Scheme 1. First, 2,5-dioxo-3hexenal is produced from 5-HMF via a series of dehydration and rearrangement reactions, then one molecule of formic acid is removed from 2,5-dioxo-3-hexenal, finally LA could be formed by rearrangement reaction.

3. Synthesis of Levulinate Esters As far as the esterification products of LA are concerned, levulinate esters (Figure 3) including methyl, ethyl, and n-butyl levulinate belong to short-chain fatty esters, which are also versatile chemical feedstocks with numerous potential industrial ap-


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Scheme 1. The mechanism of LA formation.

Figure 3. Structure of levulinate esters.

lyacid catalysts were explored for the synthesis of ethyl levulinate, analysis of the catalyst recycling experiments revealed that the 40WD-S catalyst exhibited excellent catalytic activity, although the product yield declined from 72 to 68 % during three reaction cycles.[46] The above results indicated that these solid acids are promising catalysts for the esterification reaction of levulinic acid and alcohols. Strikingly, it was found that LA could be transformed to levulinate esters in good to excellent yields with solid acids as the catalyst. As is well known, carbohydrate compounds can be easily converted into LA via HMF through dehydration and hydration reactions. The catalysts used should be acidic, like that those used for the esterification reaction. For this reason, researchers introduced a new idea for levulinate ester production, that is, direct conversion of biomass into levulinate esters by alcoholysis under acidic conditions. Recently, Mascal and Nikitin reported a new and efficient procedure for the conversion of cellulosic biomass into levulinate esters in overall yields exceeding 80 % through two reaction steps: biomass reacted with hydrochloric acid followed by the alcoholysis of resulting product.[47] Notably, the intermediates obtained from biomass were isolated for the subsequent process, and the isolation procedure did use some energy consuming techniques and environmentally harmful solvents. Tominaga and coworkers developed mixed-acid systems consisting of both Lewis and Brønsted acids for the catalytic synthesis of methyl levulinate from cellulose, the specific reaction proceeds in two steps; cellulose was solvolyzed to sugars, which were readily converted to methyl levulinate.[48] They found that the former step was mainly catalyzed by sulfonic acids, and the latter one was metal triflates. In recent study, to address issues concerning cellulose dissolution, supercritical MeOH and MeOH-H2O mixtures were used as reaction media for one-pot dissolution-conversion of microcrystalline cellulose.[49] Solid-acid catalysts, such as Cs2.5H0.5PW12O40 and sulfated zirconia, gave remarkable high yields up to 20 % in methyl levulinate. For the production of levulinate esters from other biomassbased sugars, Peng et al. found that glucose can be directly converted to ethyl levulinate over sulfated zirconia catalysts with a moderate yield up to 30 % at 473 K.[50] In their subse-

plications either in the flavoring and fragrance industry or as a blending component in biodiesel,[34–36] and their physicochemical properties are similar to the biodiesel fatty acid methyl esters (FAME).[37, 38] These esters are suitable for additives of gasoline and diesel transportation fuels with manifold excellent properties, such as low toxicity, high lubricity, flashpoint stability, and moderate flow properties under low temperature conditions.[39] In other cases, levulinate esters can be used as substrates for various kinds of condensation and addition reactions in organic chemistry.[40] In earlier studies, levulinate esters were mainly produced from LA in the presence of mineral acids, which leads to a high yield of corresponding products.[41–44] Some drawbacks were exposed during this research, however, such as catalyst recycling, product separation, environmental problems, and reaction conditions, which, in turn led to the development of green, efficient, and recyclable solid-acid catalysts. The commonly used solid acids include heteropolyacid, sulfated metal oxides (such as SO42 /TiO2 and SO42 /ZrO2), zeolite molecular sieves, hydrotalcite-like compounds, and so on. The merits of these catalysts are easy recovery, high activity, and easy activation, which provide a basis for wide application in acid-catalyzed reactions. Listed in Table 2 is the main data on the most recent advances in the bifuncTable 2. Catalytic preparation of levulinate esters with various catalysts. tional catalytic preparation of various levulinate esters. Catalyst Solvent Substrate Conc. [wt %] t [h] T [K] Product yield Ref. For example, Dharne and 2 [a] SO4 /TiO2 methanol sucrose 5.0 2 473 ML, 43 mol % [51] Bokade studied esterification of methanol glucose 5.0 2 473 ML, 33 mol % [51] SO42 /TiO2 methanol fructose 5.0 2 473 ML, 59 mol % [51] SO42 /TiO2 levulinic acid to n-butyl levuliOH/H O levoglucosan 5.58 3 443 ML, 80 mol % [52] Amberlyst 70 4.5 CH 3 2 nate over a heteropolyacid supmethanol cellulose 2.81 5 453 ML, 75 mol % [48] In(OTf)3-2NSA ported on acid-treated clay. An 9 CH3OH/H2O[b] cellulose 1.4 1/60 563-573 ML, 20 % [49] Cs2.5H0.5PW12O40 optimized n-butyl levulinate ethanol glucose 5.0 3 473 EL[c] , 30 mol % [50] SO42 /ZrO2 ethanol fructose 6.76 24 413 EL, 57 % [53] SO3H-SBA-15 yield of 97 % was achieved by 40-WD-S ethanol LA 3.95 10 351 EL, 76 % [46] using a catalyst of 20 % dodeca[45] DTPA/K10 n-butanol LA 26.11 4 393 BL[d] , 97 % tungestophosphoric acid (DTPA) [a] ML = methyl levulinate; [b] supercritical CH3OH/H2O (9:1) mixture was used as reaction media; [c] EL = ethyl supported on K10.[45] Silica-inlevulinate; [d] BL = n-butyl levulinate. cluded Wells–Dawson heteropo-

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Valuable Levulinic Acid Derivatives quent research, with SO42 /TiO2 as catalyst, methyl levulinate in approximately 43, 33, and 59 mol % yields could be obtained from sucrose, glucose and fructose, respectively.[51] They reported that their heterogeneous catalyst was easily recoverable by filtration and exhibited good catalytic activity after calcination over five cycles. Hu and Li were able to generate methyl levulinate by the acid-catalyzed reactions of sugars and alcohols. Methyl levulinate was formed from levoglucosan via series of reaction processes in a methanol–water media system.[52] Recently Saravanamurugan and Riisager developed an efficient solid catalyst for production of ethyl levulinate from monoand disaccharides, catalyst SO3H-SBA-15-D gave a high yield of ethyl levulinate (56 %) with 99 % fructose conversion, compared to zeolites and sulfated zirconia catalysts, which could be reused without any significant loss of activity for at least three consecutive runs.[53] However, the ethyl levulinate yield was a little low when cellobiose was used as the raw material with SO42 -SGN as catalyst. Therefore, with the development of green chemistry, studies on exploring various kinds of solidacid catalysts will be a meaningful research direction. On the whole, considerable research remains for the production of levulinate esters. High efficiency catalysts are in demand, and other biomass materials are also available to yielding high value added chemicals, such as straw, corn stalk, forestry waste, secondary fiber, etc. Even though cellulose is the main component in most of these natural materials, cellulose dissolution remains a significant hurdle in the utilization of the above recyclable resources. Consequently, new reaction systems must be explored to increase the solubility and alcoholysis level of cellulose, and proper product selectivity is absolutely necessary for practical application. More importantly, high reactant concentration and better levulinate ester yield are the premise of industrialization.

4. Synthesis of g-Valerolactone The extensive properties of GVL described above and the specific physical properties of GVL are shown in Table 3.[54] Recent studies have revealed that some fuels of high energy density, such as alkene, paraffin, and valerate ester, can be formed through catalytic conversion of GVL with metal catalysts. For instance, first GVL could be converted into butene and CO2 with SiO2/Al2O3 as catalyst, subsequently a qualified transportation and aircraft fuel (polyolefin) can be produced by polymerization of butene.[55] Alternatively, Lange et al. used metal catalysts to catalyze the production of valeric acid from GVL, then valerate esters were prepared by the esterification of obtained valeric acid with alcohols, such as methanol and en-

Table 3. Physical properties of GVL. Molecular Weight

Refractive Index[a]

Density [kg m 3][a]

Flash point [K]

Melting point [K]

Boiling point [K]

100.12

1.4301

1052

369

242

478.0–479.5

[a] Value given for 293 K

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thanol.[56] Horvth et al. noted that GVL exhibited very attractive physical and chemical properties, such as low toxicity, and could be considered as a sustainable liquid for global storage and transportation.[57] In industry, GVL is produced by hydrogenation and lactonization of LA in a H2 atmosphere with a heterogeneous noble metal as catalyst. The pathway for the hydrogenation of LA into GVL, with the minor products angelica lactone and 2methyltetrahydrofuran (MTHF), through series of reactions of reduction and hydrogenation is shown in Scheme 2.[58]

Scheme 2. The schematic representation for reaction pathway of LA hydrogenation to GVL and 2-MTHF.

4.1. Production of g-valerolactone under a hydrogen atmosphere In recent years, the study of the preparation of GVL has become a research hotspot. However, GVL is often produced from levulinic acid with a noble metal catalyst under hydrogen atmosphere, and the reaction is usually performed in the neat liquid phase of a batch reactor system. In 1930 Sehuette and Thomas investigated the hydrogenation of LA with a platinum oxide catalyst in organic solvents to give a GVL yield of 87 % after 44 h under 3 bar hydrogen.[59] An improved yield of 94 % for GVL was reported by Christian et al who used a Raney nickel catalyst in the liquid phase with copper chromite.[60] The LA hydrogenation was performed at 523 K under 202 bar hydrogen, which resulted in a complex mixtures composed of GVL, 1,4-pentanediol, and MTHF. With the rapid development of research, precious metals are gradually being applied for LA hydrogenation, owing to their superior catalytic activity, especially Ru and Pt catalysts.[58, 61] Mehdi and coworkers found that LA could be hydrogenated into GVL through the dehydration and lactonization of HPA using Pd, Ni, and Pt catalysts.[62] Serrano-Ruiz et al. noted that nearly quantitative GVL yields was achieved for 50 wt % LA at hydrogen pressure of 35 bar.[63] Then they also used the hydrolysis products of cellulose as raw materials for the production of GVL with Pd/Nb2O5 catalyst.[64] In further investigations they found that levulinic acid and its esters could be converted to GVL over metal oxide catalysts by catalytic transfer hydrogenation via the Meerwein–Ponndorf–Verley reaction, and a GVL


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S. Wu et al. yield of 92 % was achieved at 423 K over 16 h with a ZrO2/LA mass ratio of 2:1.[65] A developed RuRe/C catalyst also was used for the catalytic conversion of LA to GVL.[66] Manzer et al. patented the process of producing GVL from LA in supercritical CO2 with a noble metal/carrier as catalyst.[67] They found that LA could be converted into GVL completely by using a Ru/Al2O3 catalyst; conditions of 473 K and 200 atm H2. With Ru/C as the catalyst, the GVL yield was almost up to 100 % at 423 K and 30 atm H2. Later, Poliakoff et al. used supercritical CO2 for the hydrogenation of LA to GVL over 5 % Ru/SiO2 at 473 K with 100 bar hydrogen, their studies further proved the superior properties of supercritical CO2, which could facilitated the separation of final products and reaction rate.[68] Another study also indicated that the hydrogenation rate greatly improved as the hydrogen pressure was increased.[69] Although researchers have begun to focus on the field of homogeneous catalytic reduction, noble-metal salts still are mainly used for these catalytic processes. Furthermore, Ru and Rh complexes are commonly used for LA reduction in aqueous solution. Joo et al. demonstrated the use of water-soluble homogeneous ruthenium catalysts for the hydrogenation of oxoand keto acids.[70, 71] Osakada et al. reported that 99 % yield of GVL was obtained with [RuCl2(PPh3)3] as catalyst.[72] The recent study showed that biphasic catalysis in a DCM/water mixture using homogeneous Ru-catalyst made in situ from RuCl3·3 H2O and Na3TPPTS allowed the synthesis of GVL in near quantitative yields at mild conditions.[73] Ethyl levulinate was converted into GVL using a Ru-BINAP complex obtained in situ from [Ru(acetate)2(BINAP)] with 2 equiv of HCl in ethanol, and a chemical yield of 96 % was reached under 100 bar hydrogen with only 0.1 mol % of catalyst.[74] However, with sodium borohydride as reductant, LA also could be converted into GVL in the presence of hydrogen and methanol.[75] Based on the above work, we come to the conclusion that noble metals, including Re, Ru, Pd, and Pt, exhibit excellent catalytic activity in GVL production, particularly for reduction reactions. As we know that the prices of these precious metals are very high, in consideration of production cost, other cheap catalysts such as noble-metal-based alloy catalysts and hydrotalcite-like materials should be further explored. For instance, part nickel or copper can be added into noble metals to form alloy catalysts, which is a good way cut costs and improve catalytic efficiency. Also, we can increase the profitability of the catalysts so as to cut down the production cost. With regard to the application of supercritical systems, it may not be an ideal manner to improve GVL yield in actual production, because the corresponding requirements for all reaction equipment are very strict. However, a proposed replacement for the supercritical CO2 system is to add into the reaction system some chemicals that could generate H2 or CO2. Despite the high efficiency of homogeneous catalysts, the need for separating catalysts from reaction products may be a big existing problem that limits their application to large- or industrialscale production. One thing still needs to be pointed out; because of the excess H2 used during the reaction a little 2-methyltetrahydro-

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furan is inevitably produced. As is well known, peroxide can be formed in the storage of 2-methyltetrahydrofuran, so there is an explosion hazard. To solve this problem, over excess of hydrogen should be avoided and other hydrogen sources, such as formic acid, should be considered. 4.2. Synthesis of g-valerolactone with formic acid as the hydrogen source Researchers are now starting to explore some other hydrogen sources to replace hydrogen for catalytic synthesis of GVL. In many earlier studies, formic acid had been used as the hydrogen source rather than hydrogen gas.[76–80] Fortunately, noble metal catalysts showed great catalytic ability in the conversion of formic acid into hydrogen, thus making it suitable for the reaction system in yielding GVL. Additionally, with formic acid as the hydrogen source, the requirements for reaction equipment were lowered significantly. Recently Deng et al. reported a new route for converting various biomass-derived oxygenates (cellulose, starch, and sugars) into GVL without using an external H2 supply, and an inexpensive RuCl3/PPh3/pyridine catalyst was used to convert a 1:1 aqueous mixture of levulinic acid and formic acid into GVL. Interestingly, it was found that the addition of CO2 favored GVL production, and formic acid could prevent LA hydrogenation.[81] Kopetzki and Antonietti demonstrated that transfer hydrogenation of LA under hydrothermal conditions could be catalyzed by bases, as well as simple sodium sulfate.[82] Then Horvth and coworkers used [(h6-C6Me6)Ru(bpy)(H2O)][SO4] in water for the transfer hydrogenation of LA with formic acid as hydrogen donor.[62] Method of combined dehydration/(transfer)-hydrogenation was developed for converting C6-sugars to GVL, the use of a pre-formed homogeneous water soluble ruthenium catalyst from RuCl3 and TPPTS in combination with TFA gave quantitative C6-sugar conversions with lower GVL yield (23 mol %) compared to heterogeneous Ru catalyst.[83] Two-stage conversion of biomass-derived carbohydrate into GVL was further studied, it was found that the reaction system involving HCOOH-mediated gold catalysis was one of the most simple, efficient, ecologically friendly, and robust catalytic systems developed to-date for the selective reductive transformation of biomass-derived compounds.[84] When excess sodium formate was used as hydrogen source, the GVL selectivity markedly decreased with a corresponding yield of 50 %.[62] By taking into account the above research, it is easily concluded that homogeneous catalysts still play an important part in formic acid reduction for GVL production, especially noble metal complexes. Formic acid is commonly adopted as an effective hydrogen source to produce GVL for its low price and easy availability. Furthermore, one attraction is that the produced hydrogen will be consumed immediately as the experiments progress with CO2 produced at concurrently. Therefore, the pressure requirement for the equipment is lower, and the GVL yield increased significantly in the presence of CO2. In the following work, other practicable hydrogen sources and efficient homogeneous or heterogeneous catalysts must be ade-


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Valuable Levulinic Acid Derivatives quately exploited, and the invention of novel and efficient catalytic systems is highly desirable to convert biomass carbohydrates into GVL. Significant attention should be paid to heterogeneous catalysts, with regards to the separation of products, catalyst reuse, and solving the problem of the inhibition of formic acid. According to the above analysis and summary, herein we put forward a feasible route for conversion of biomass resources into high value-added chemicals in practical production (Figure 4). For the initial conversion process, biomass materials are quickly hydrolyzed into reducing sugar in acidic conditions under high temperature. With further acid-catalysis of the obtained monosaccharide mixtures, sugars like fructose and glucose undergo a series isomerization and dehydration reactions for LA production. During this step the LA in reactor 2 either could be hydrogenated to GVL or esterified to levulinate esters with different catalysts in reactor 3 and 4, respectively. Meanwhile, the obtained LA could also be purified for commercial sale through separation. The used acidic catalysts and metal catalysts in the whole process should be activated for the next use so that the actual cost in mass production can be Figure 4. Route for continuous conversion of biomass into levulinic acid derivatives. cut down. It is concluded that the whole technical process described here provides a practical route for mass production of system was applied to cellulose conversion so as to enhance levulinate esters and GVL from various materials. the solubility of cellulose; 2) for GVL production, the major routes included direct hydrogenation of LA and the two-stage hydrolytic hydrogenation of sugars with multi-functional cata5. Summary and Outlook lysts and hydrogen sources, in particular, both heterogeneous and homogeneous Ru catalysts exhibited high catalytic activity The production of levulinate esters and g-valerolactone hold in the formation of GVL. great potential, owing to the rapid development of food inAlthough great progress has been achieved for the catalytic dustry and chemical industry, and especially in the fields of preparation of levulinate esters and GVL, further improvement energy and fuel. In this review, we have focused on the study in productivity and selectivity are still necessary in many cases of catalytic methods for the synthesis of valuable levulinic acid for achieving the goal of industry production. With the introderivatives. In summary: 1) levulinate esters including methyl, duction of the concept of green chemistry, efforts towards bioethyl, and n-butyl levulinate have can be obtained by the mass conversion should be devoted to the economical, rapid, esterification of materials and alcohols catalyzed by mineral and eco-friendly production of valuable levulinic acid derivaacids and various solid-acid catalysts, sometimes a supercritical ChemCatChem 2012, 4, 1230 – 1237


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S. Wu et al. tives. Furthermore, multi-functional catalysts, which allow several reaction steps to be completed in one reactor and avoid the complex intermediate separation process, deserve the priority in catalytic chemistry. Above all, activation and reuse of the catalyst and effective separation of the target products are always the hottest topics for catalytic processing research. The challenges include the following: 1) Catalyst development and efficient reaction systems; 2) The structure-property relationships of catalysts; 3) Mass production.

Acknowledgements This work was supported by the National High Technology Research and Development Program of China (863 Program), and the Science and Technology Department of Guangdong Province, China (No. 2010y1-C071), the National Natural Science Foundation of China (No. 21176095), and the Major Research Projects of Guangdong Province, China (No. 2011A090200006). Keywords: biomass · fructose · levulinate esters · levulinic acid · valerolactone [1] A. J. Ragauskas, C. K. Williams, B. H. Davison, G. Britovsek, J. Cairney, C. A. Eckert, W. J. Frederick, Jr., J. P. Hallett, D. J. Leak, C. L. Liotta, J. R. Mielenz, R. Murphy, R. Templer, T. Tschaplinski, Science 2006, 311, 484 – 489. [2] S. N. Naik, V. V. Goud, P. K. Rout, A. K. Dalai, Renewable Sustainable Energy Rev. 2010, 14, 578 – 597. [3] B. Girisuta, L. P. B. M. Janssen, H. J. Heeres, Chem. Eng. Res. Des. 2006, 84, 339 – 349. [4] C. H. Christensen, J. Rass-Hansen, C. C. Marsden, E. Taarning, K. Egeblad, ChemSusChem 2008, 1, 283 – 289. [5] P. Gallezot, ChemSusChem 2008, 1, 734 – 737. [6] A. Takagaki, C. Tagusagawa, K. Domen, Chem. Commun. 2008, 5363 – 5365. [7] R. M. de Almeida, J. R. Li, C. Nederlof, P. O’Connor, M. Makkee, J. A. Moulijn, ChemSusChem 2010, 3, 325 – 328. [8] M. Balat, H. Balat, Appl. Energy 2010, 87, 1815 – 1835. [9] S. Govindaswamy, L. M. Vane, Bioresour. Technol. 2010, 101, 1277 – 1284. [10] H. B. Zhao, J. E. Holladay, H. Brown, Z. C. Zhang, Science 2007, 316, 1597 – 1600. [11] A. Corma, S. Iborra, A. Velty, Chem. Rev. 2007, 107, 2411 – 2502. [12] T. Werpy, G. Petersen, A. Aden, J. Bozell, J. Holladay, A. Manheim, D. Eliot, L. Lasure, S. Jones, Top Value Added Chemicals from Biomass, Vol. 1, U.S. Department of Energy, Oak Ridge, TN, 2004. [13] C. Chang, X. J. Ma, P. L. Cen, Chem. Ind. Eng. Prog. 2005, 24, 350 – 356. [14] W. A. Farone, P. C. Irvine, J. E. Cuzenss, WO 9819986, 1998. [15] W. A. Farone, J. E. Cuzenss, US Patent 5892107, 1999. [16] W. A. Farone, J. E. Cuzenss, US Patent 6054611, 2000. [17] Q. Fang, M. A. Hanna, Bioresour. Technol. 2002, 81, 187 – 192. [18] R. E. Rodriguez, US Patent 3701789, 1972. [19] W. Zeng, D. G. Cheng, H. H. Zhang, F. Q. Chen, X. L. Zhan, Reac. Kinet. Mech. Cat. 2010, 100, 377 – 384. [20] W. Zeng, D. G. Cheng, F. Q. Chen, X. L. Zhan, Catal. Lett. 2009, 133, 221 – 226. [21] C. Chang, X. J. Ma, P. L. Cen, Chin. J. Chem. Eng. 2006, 14, 708 – 712. [22] H. Peng, Y. H. Liu, J. S. Zhang, R. S. Ruan, Chem. Ind. Eng. Prog. 2009, 28, 2237 – 2241. [23] B. Q. Xue, S. Q. Wang, China Patent 1304923, 2001. [24] T. J. Schwartz, A. R. P. van Heiningen, M. C. Wheeler, Green Chem. 2010, 12, 1353 – 1356. [25] Y. B. Zhang, Y. Y. Zhou, World Pesticides 2000, 22, 8 – 14. [26] Y. X. Yang, Y. Wang, F. L. Xu, Shaanxi J. Agric. Sci. 1997, 2, 47 – 48. [27] Z. S. He, L. F. Zhao, Chem. Res. Appl. 2001, 13, 537 – 539.

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