Acido Lactico

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  Lactic acid as a platform chemical in the biobasedeconomy: the role of chemocatalysis Michiel Dusselier,* Pieter Van Wouwe, Annelies Dewaele, Ekaterina Makshinaand Bert F. Sels* Upcoming bio-re 󿬁 neries will be at the heart of themanufacture of future transportationfuels, chemicals andmaterials.Anarrownumberofplatformmoleculesareenvisionedtobridgenature'sabundantpolysaccharidefeedstock to the production of added-value chemicals and intermediate building blocks. Such platformmolecules are well-chosen to lie at the base of a large product assortment, while their formation should bestraightforward from the re 󿬁 ned biomass, practical and energy e ffi cient, without unnecessary loss ofcarbon atoms. Lactic acid has been identi 󿬁 ed as one such high potential platform. Despite its establishedfermentation route, sustainability issues  –  like gypsum waste and cost factors due to multi-step puri 󿬁 cationand separation requirements  –  will arise as soon as the necessary orders of magnitude larger volumes areneeded. Innovative production routes to lactic acid and its esters are therefore under development,converting sugars and glycerol in the presence of chemocatalysts. Moreover, catalysis is one of thefundamental routes to convert lactic acid into a range of useful chemicals in a platform approach. Thiscontribution attempts a critical overview of all advances in the  󿬁 eld of homogeneous and heterogeneouscatalysis and recognises a great potential of some of these chemocatalytic approaches to produce andtransform lactic acid as well as some other promising  a -hydroxy acids. Broader context  Lacticacidis oneof the top biomass derived platform chemicals, with apromising roleinfuturebio-re  neries.A wide rangeof catalytic transformationsoflacticacid are feasible leading to the selective production of green solvents,   ne chemicals, commodity chemicals and fuel precursors. Even more appealing is its roleas the precursor for biodegradable PLA (polylactic acid) polymers. These polyesters have the potential to replace fossil derived plastics in particular applications,and based on life cycle analyses, they have a more positive impact on the environment. PLA can also be used  in vivo  and in biomedical applications. The demandfor PLA and green solvents is growing and stresses the current fermentative production of lactic acid, which su ff  ers from up-scaling and environmental issuesdue to concerning waste co-generation and puri  cation steps. Novel chemocatalytic routes are under development to obtain pure lactic acid or esters directly from sugar feedstock. These mainly heterogeneous catalytic processes have potential and could lie at the heart of an evolution in lactic acid research. Once lacticacid is available at competitive prices, its use as a feedstock in a platform approach could become commercially viable, thereby providing renewable and CO 2 -neutral alternatives for fossil derived chemicals. 1 Introduction There is a growing interest in deriving platform molecules frombiomass resources to produce fuels, chemicals and materials.This is mainly due to the rise of fossil feedstock prices together with the concerning incline in greenhouse gases linked withglobal warming. Biomass resources, renewable and abundant inNature, harvest energy from the sun and capture CO 2  from theatmosphere. If grown sustainably and processed with a criticalcradle-to-cradle approach, biomass has an enormous potentialas a feedstock for chemical intermediates as well as fuels andmaterials. 1 – 9 Carbohydrates represent the largest fraction of theannual biomass production and they are therefore envisioned tobe the major starting input in the upcoming bio-re  neries that  will fuel the biobased chemical industry. 10 – 13 Starting from thisfeedstock, a number of high-potential platform molecules arecreated asprecursorsfor hundredsoffunctionalderivativessuchas fuels, chemicals and materials. 14 – 21 The use of heterogeneouscatalysis herein is proving to be imperative. 22,23  A molecule of great appeal in this respect is 2-hydroxy-propanoic acid –  betterknown aslactic acid(LA) –  seencentrally in Fig. 1. It was discovered in 1780 by the Swedish chemist Scheele in acid milk. 24,25 Despite its late discovery, this moleculeoccurs in almost every living organism and estimations predict that it has been around since the dawn of life. Next to itsessential role in the anaerobic energy metabolism of billions of life forms, this molecule is industrially produced from sugarson a medium-size scale. Its production capacity is over 500 000 Center for Surface Chemistry and Catalysis - KU Leuven, Kasteelpark Arenberg 23,3001 Leuven, Belgium. E-mail:;; Fax: +32 16 321 998; Tel: +32 16 3 21 59 Cite this: DOI: 10.1039/c3ee00069a Received 8th January 2013Accepted 6th March 2013DOI: 10.1039/c3ee00069a This journal is  ª  The Royal Society of Chemistry 2013  Energy Environ. Sci. Energy & Environmental Science REVIEW    D  o  w  n   l  o  a   d  e   d   b  y   C  a  p  e   B  r  e   t  o  n   U  n   i  v  e  r  s   i   t  y  o  n   1   6   /   0   4   /   2   0   1   3   1   9  :   0   8  :   3   8 .   P  u   b   l   i  s   h  e   d  o  n   0   6   M  a  r  c   h   2   0   1   3  o  n   h   t   t  p  :   /   /  p  u   b  s .  r  s  c .  o  r  g   |   d  o   i  :   1   0 .   1   0   3   9   /   C   3   E   E   0   0   0   6   9   A View Article Online View Journal  tons while the actual production is nearing 260 000 tonsper year. 26,27 The gap between production capacity and the actual output is partially explained by sustainability problems associated withthe up-scaling of the current fermentation route and its wastedisposal. Di ff  erent groups currently focus on alternative che-mocatalytic routes for the synthesis of lactic acid from di ff  erent kinds of sugar feedstock. The main approach here is the use of  Fig.1  Roleofcatalysisinnovel synthesisroutestoLAandinitsuseasaplatformchemical.  Michiel Dusselier obtained amaster of engineering in Cata-lytic Technology at the CatholicUniversity of Leuven in 2009. He performed a part of this study at the Technische Universit ¨at  M ¨unchen, Germany. His master thesis was performed under  guidance of Prof. Sels and Prof. P.A. Jacobs and dealt with theconversion of cellulose withheterogeneous catalysts. Now heis in the process of    nishing his PhD under guidance of Prof. Sels, dealing with the tailoring of catalytic routes towards and from lactic acid, as well as examining novel renewable monomers and their polymers. He has authored 7  papers and a patent. Pieter Van Wouwe obtained hismaster degree in Catalytic Tech-nology (Bioscience Engineering) at the Catholic University of  Leuven in 2011. He did hismaster thesis under the supervi-sion of Prof. Sels at the Center for Surface Chemistry and Catalysis,where he explored new catalyticroutes with lactic acid as a feed-stock. He has recently started his PhD in the same lab, where he focuses on lactic acid conversionsby both biocatalysis and hetero- geneous catalysis. Annelies Dewaele obtained her  Master of Catalytic Technology in 2012 at the faculty of Bioscience Engineering (Catholic UniversityofLeuven). Inher master thesis at the Center for Surface Chemistryand Catalysis, she studied thesynthesis of polylactic acid and other biopolymers, under super-vision of Prof. Sels. She hasrecently started her PhD onmetathesis reactions for theconversion of biomass. Dr. Ekaterina V. Makshinaobtained her PhD degree in 2007 in the frame of a cooperationagreement (Co-tutelle) of MoscowState University (Laboratory of  Kinetics and Catalysis) and  Littoral University (Laboratory of Catalysis and Environment), Dunkerque. Her PhD thesis dealt with the oxidation of methanol and VOCs using supported oxidecatalysts. Since 2010 she hasbeen working as a post-doc fellowat the Center for Surface Chemistry and Catalysis (University of  Leuven) in the group of Prof. Sels on biomass transformation intocommodity chemicals. She has published 16 papers. Prof. Bert F. Sels obtained his PhD in 2000 at the CatholicUniversity of Leuven on oxidationchemistry, a    er which he did a post-doc with BASF until 2002. Another 3 year post-doc for the National Science Foundation wasdedicated to the  ‘  activation of nitrous oxide ’   and the  ‘  micro-scopic imaging of catalytic eventsin single particles ’  . He became Assistant Professor in 2003,teaching courses on analytical organic chemistry, sustainability and heterogeneous catalysis. Hehas been a Full Professor since 2006 in the Faculty of Bioscience Engineering, Leuven. He has published about 160 scienti    c papersand11patents,andisarecipientofnumerousawardsincluding theinternational DSM chemistry award. His current research exploresheterogeneous catalysis for converting renewables such as ligno-(hemi)cellulose and small molecule activation. Energy Environ. Sci.  This journal is  ª  The Royal Society of Chemistry 2013 Energy & Environmental Science Review    D  o  w  n   l  o  a   d  e   d   b  y   C  a  p  e   B  r  e   t  o  n   U  n   i  v  e  r  s   i   t  y  o  n   1   6   /   0   4   /   2   0   1   3   1   9  :   0   8  :   3   8 .   P  u   b   l   i  s   h  e   d  o  n   0   6   M  a  r  c   h   2   0   1   3  o  n   h   t   t  p  :   /   /  p  u   b  s .  r  s  c .  o  r  g   |   d  o   i  :   1   0 .   1   0   3   9   /   C   3   E   E   0   0   0   6   9   A View Article Online  heterogeneous catalysts in water or alcoholic solvents toproduce lactic acid or its corresponding esters. The latter are very useful as such for di ff  erent applications,  e.g.  ethyl lactate asa green solvent. Moreover lactic acid esters are intermediates inthe current puri  cation process of lactic acid. Alternative routesfrom glycerol to lactic acid also harness potential.Lactic acid and its salts have many long known applicationsineverydayliferangingfromfood additivestoprocessing   uids.For instance, in the food industry it is used as an acidulant, apreservative and an emulgator. Next to these existing applica-tions, lactic acid has a major potential for the synthesis of thebiopolymer PLA (polylactic acid or polylactide). Already 187 000tons of this thermoplastic polyester were produced in 2011, 28 mainly by NatureWorks LLC which has a large production plant of 140 000 tons installed in Nebraska. 29 Smaller-volume playerssuch as Purac, Futerro (Galactic-Total) and Uhde Inventa-Fischer are becoming increasingly active. 30,31 The production volume of PLA is mostly excluded from the estimated 260 000tons demand of lactic acid each year. The global productioncapacity for PLA grows at a rate somewhere between 10 and 24%per annum 28,32 and an up-to-date survey among producers hasshown that a capacity of 800 000 tons should be established in2020. 32  With this in mind, the lactic acid demand is projected toshow an annual growth of 5 – 8% per year. 33 – 35  A recent survey foresees a market of 329 000 tons by 2015 (ref. 34) and wetentatively estimate the lactic acid demand in 2020 to be wellover 600 000 tons. It is however di ffi cult to obtain exact measures, even now, because the larger PLA producing companies produce LA themselves, which is usually excludedfrom the LA market. Taking the rise of fossil fuel and eventually all fossil derived plastic prices into account, this growth is not likely to stop. Therefore, the lactic acid price is forecast to drop, while the production will increase. 36 In this respect and becauseof its reactive structure, the role of lactic acid as a platformchemical to synthesize a variety of intermediates is very attrac-tive. 37 Given the right conditions and catalytic functionalities,lactic acid may be converted into a wide range of useful inter-mediates such as acrylic acid, propylene glycol, 2,3-pentane-dione, acetaldehyde, pyruvic acid, and lactide (the monomer inPLA synthesis). Besides the latter cyclic ester, a wide range of linear esters (alkyl lactates) are easily produced as well andthese possess unique solvation properties.This review tries to outline the intriguing    eld of lactic acidand foresees a unique role for heterogeneous catalysis in thenear future. Both in the synthesis of lactic acid from sugars andglycerol as well as in the conversion of lactic acid to added-valuechemical intermediates, (heterogeneous) chemocatalysts couldplay a decisive role. This review, summarized in Fig. 1, isorganised as follows. The   rst part brie   y describes the prop-erties, current fermentative production and long standing commercial applications of lactic acid. The second part concentrates in length on the catalytic conversion of lactic acidas a platform chemical while the third part describes the cata-lytic developments in new synthesis routes towards lactic acid.Lastly, a remark is made on the enantiomeric resolution of lactic acid and on the synthesis of other highly interesting novelbio-derived  a -hydroxy acids. 2 Properties, current production andapplications 2.1 Properties Lactic acid (LA) is an  a -hydroxy carboxylic acid with a chiralcentre at its second carbon. Due to the presence of two func-tional groups in a three carbon molecule, it incorporates a lot of chemical reactivity. The carboxylic acid group is mildly acidicand the stereochemistry of the second carbon is of utmost importance in the polylactic acid chemistry. 38 – 40 The most relevant properties of lactic acid are summarized in Table 1. LA commercially occurs in aqueous solutions of 20 to 90 wt%. AsLA is prone to esteri  cation, commercial solutions at equilib-rium always contain a fraction of oligomers, and their amount andaverage lengthdepends onthetotal concentrationoflactoylunits. The utmost example of such an intermolecular esteri  -cation is the formation of lactoyl lactate  –  the linear dimer of lactic acid further denoted as L 2  A   –  as presented in Scheme 1.Because of the reversibility of the esteri  cation, L 2  A is readily hydrolysed into lactic acid. Depending on the water content, thedimer L 2  A may also condense with another LA molecule form-ing a linear trimer (L 3  A) and water, and so on. As an example,due to the esteri  cation tendency, a 90 wt% solution in equi-librium only contains 65.9% free LA, while 25.0% is encoun-tered in the L 2  A form. Sometimes, the presence of sucholigomers has been overlooked while studyingthe conversion of aqueous lactic acid solutions. 45 The group of Miller has accu-rately measured (with HPLC) and modelled the oligomerdistributions in concentrated lactic acid solutions. 46 Thesecompositions can also be evidenced from  1 H-NMR spectra as well, preferably in DMSO-d 6 . 47,48 Pure solid lactic acid is di ffi cult to obtain on a large scale because of condensation and its highhygroscopicity, and its production is only feasible  via  laborious Table 1  Chemical and physical properties of lactic acid a Property Unit (conditions)Isomer orconcentrationReportedrangeMelting point    C  L  or  D  52.7 – 53.0racemic 16.4 – 18.0Boiling point    C (at 1.87 kPa)  L  or  D  103racemic 122Solid density g mL  1 (at 20   C)  —  1.33Liquid density of aq. solutiong mL  1 (at 25   C) 20 wt% 1.05788.6 wt% 1.201p  K  a  n/a  L  or  D  3.79 – 3.86racemic 3.73 a Compiled from ref. 31, 41 – 44. Scheme 1  (i) Reversible formation of linear dimer (L 2 A) from LA. (ii) Formationor hydrolysis of oligomers. Usually,  n ¼ 1, adding LA to a growing chain. This journal is  ª  The Royal Society of Chemistry 2013  Energy Environ. Sci. Review Energy & Environmental Science    D  o  w  n   l  o  a   d  e   d   b  y   C  a  p  e   B  r  e   t  o  n   U  n   i  v  e  r  s   i   t  y  o  n   1   6   /   0   4   /   2   0   1   3   1   9  :   0   8  :   3   8 .   P  u   b   l   i  s   h  e   d  o  n   0   6   M  a  r  c   h   2   0   1   3  o  n   h   t   t  p  :   /   /  p  u   b  s .  r  s  c .  o  r  g   |   d  o   i  :   1   0 .   1   0   3   9   /   C   3   E   E   0   0   0   6   9   A View Article Online  crystallisation. 49 The condensation process to oligomers andpolymers, when deliberately executed under vacuum condi-tions, forms an important step in processing LA to PLA. A cyclicester of two lactic acid molecules, called lactide, also exists andthis cyclic dimer is the foremost important building block inproducing PLA (see Section 3.9). As lactide is less stable in water, it is found only in trace amounts in aqueous lactic acidsolutions. 47 2.2 Current fermentative production Over 90% of the current commercial production of LA is per-formed  via  fermentation. Generally, a suitable carbohydratesource is transformed into lactic acid by micro-organisms.Possible feedstocks are hexose sugars and easily hydrolysablepoly- or disaccharides derived from corn syrups, molasses, beet extracts, whey, and all kinds of starches. Today, glucose andsucrose are the main starting resources. 6,26,29 – 31,44,50,51 Hugepotential lies in the usage of non-edible cellulose, as it isconsidered to be a key substrate in the future chemical industry as soon as its hydrolysis becomes economically feasible. 10 – 12,52 – 58 The fermentative production of lactic acid is not the scope of this review. The reader is referred to other recent work for anextensive discussion on the fermentation. 42,50,59 – 62  An introduc-tion with focus on some interesting developments will be brie   y summarized for the sake of completeness and a general block scheme for this process can be seen in Fig. 2. The fermentationof sugar to lactic acid is carried out under anaerobic conditionsto direct the conversion towards LA instead of CO 2  and water. 42 Besides fungi such as  Aspergillus niger  , mainly homo-fermen-tative bacteria like  Lactobacillus delbrueckii   or  L. amylophilus  areused,whichproducetwoLAmoleculesfromonehexose. 51,63 Thefermentation is mainly a batch process and takes around 2 – 4days to complete, providing a lactate yield of up to 90% withdextrose. 31,59 However, fed-batch, repeated batch and contin-uous systems are also reported. 36,62 The fermentation process has two major drawbacks,encountered when following the block scheme in Fig. 2. To start  with, lactic acid bacteria have their optimum productivity in thepH range of 5 to 7. As lactic acid builds up in the batch, the pHof the fermentation broth lowers causing inhibition of themicrobial culture. Alkali bases such asCa(OH) 2 , CaCO 3 , NH 4 OHor NaOH are therefore added continuously to the broth. Thethus formed lactate salts reach a  nal concentration of about 10 wt%. A    er fermentation, the broth is   ltered for biomass (cell)removal and sulphuric acid is added to the aqueous calciumlactate solution. In this way, free LA and low-value insolublesalts as by-products (o   en CaSO 4  or gypsum) are formed. Thesesalts are un  t for use in construction or other industries andtherefore have to bedeposited. About 1 ton of gypsum perton of lactic acid is formed. A    er removal of gypsum, the   ltrate isfurther puri  ed with ion exchange columns and concentrated.This process produces technical grade LA of low purity as seenin the block scheme of Fig. 2. For food, solvent and polymerapplications, impure lactic acid is esteri  ed with (m)ethanol 51,59 and subjected to puri  cation  via  distillation. In this way, a   erhydrolysis with water, highly pure aqueous solutions of LA areobtained, while the alcohol is recycled. Besides the gypsum waste, the entire multistep puri  cation work-up highlights thesecond critical issue hampering the current up-scale of thelactic acid production units.Taking into account the productivity of the fermentationprocess with the commercially applied micro-organisms, typi-cally between 0.3 and 5 g L  1 h  1 , 36,50,51,62 a lot of research e ff  ort has been dedicated to improve the fermentative LA productionprocess with mixed success. One approach deals withincreasing the acid tolerance of modi  ed bacteria by means of genetic engineering or working with modi  ed yeasts. 64,65  A considerably more successful approach is the application of electrodialysis membranes to the process to split the salts without sulphuric acid. 36,41,59,66 – 68 Despite the recent progress, fermentative production of LA isa capital intensive business nowadays. Mainly due to thecomplex workup with diverse puri  cation steps and the enor-mous gypsum waste, further up-scaling is complicated. In view of the rising   ‘ number one ’  application of LA,  viz.  synthesis of polymers, the demand for very pure LA will be further fuelledand this evolution will even more put emphasis on this bottle-neck. Searching for alternative production routes, many groupsinvestigated chemocatalytic ways to produce LA from renewableresources, which are discussed in full in Section 4 andcompared to the fermentation at the end of Section 4.3. More-over, through fermentation, mainly   L -lactic acid is produced.Due to the excellent properties of stereocomplexed PLA, 69 discovered by the group of Tsuji, 70  which combines stereopure L -PLA and  D -PLA in a 1 : 1 ratio, in the near future, a cheapsource of abundant   D -LA will be required. Although certainbacteria, sometimes modi  ed  via  genetic engineering, arecapable of producing LA with an excess of the  D -enantiomer, Fig. 2  Simpli 󿬁 ed block scheme of the fermentative production of LA. Energy Environ. Sci.  This journal is  ª  The Royal Society of Chemistry 2013 Energy & Environmental Science Review    D  o  w  n   l  o  a   d  e   d   b  y   C  a  p  e   B  r  e   t  o  n   U  n   i  v  e  r  s   i   t  y  o  n   1   6   /   0   4   /   2   0   1   3   1   9  :   0   8  :   3   8 .   P  u   b   l   i  s   h  e   d  o  n   0   6   M  a  r  c   h   2   0   1   3  o  n   h   t   t  p  :   /   /  p  u   b  s .  r  s  c .  o  r  g   |   d  o   i  :   1   0 .   1   0   3   9   /   C   3   E   E   0   0   0   6   9   A View Article Online
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