Governing Chemistry of Cellulose Hydrolysis in Supercritical Water

1 On he Go e ning Chemis y o Cellulose 1 Hyd olysis in Supe c i ical Wa e 2 Danilo A. Can e o, M. Dolo es Be mejo and M. José Coce o* 3 High P essu e P ocesses G oup, Depa men o Chemical Enginee ing and En i onmen al 4 Technology, Uni e si y o Valladolid, P ado de la Magdalena s/n, 47011 Valladolid, SPAIN. 5 FAX: +34-983423013. *E-mail: [email p o ec ed] 6 7 Keywo ds: Biomass • Ionic P oduc • Kine ic • Suga s • Wa e Chemis y 8 2 Abs ac 9 This a icle summa izes he ecen e o s in he High P essu e P ocesses G oup labs a UVa 10 ega ding he undamen als o biomass hyd olysis in p essu ized wa e medium. A ex emely 11 low eac ion imes (0.02 s), cellulose was hyd olyzed in supe c i ical wa e (400ºC and 25 12 MPa) ob aining a suga s yield highe han 95% w·w-1 while 5-HMF yield was lowe han 13 0.01% w·w-1. When he eac ion imes was inc eased up o 1 s, he main p oduc was 14 glycolaldehyde (60% w·w-1). Independen ly o he eac ion ime, he yield o 5-HMF was 15 always lowe han 0.01% w·w-1. In o de o e alua e he eac ion pa hway and mechanism o 16 plan biomass in p essu ized wa e , se e al pa ame e s ( empe a u e, p essu e, eac ion ime 17 and eac ion medium) we e s udied o di e en biomasses (cellulose, glucose, uc ose and 18 whea b an). I was conside ed ha he eac ions o glucose isome iza ion o uc ose as well 19 as uc ose dehyd a ion o 5-HMF ake place ia p o on o hyd oxide anion associa ion. So, 20 hei concen a ion was aken in o accoun as eagen concen a ion in he eac ion 21 e alua ions. I was ound ha he p o on and hyd oxide anion concen a ion in he medium 22 due wa e dissocia ion is he de e mining ac o in he selec i i y o he p ocess. The eac ion 23 o glucose isome iza ion o uc ose and i s u he dehyd a ion o p oduce 5-HMF a e highly 24 dependen on ions concen a ion. By inc easing pOH/pH, hese eac ions we e minimized 25 allowing he con ol o 5-HMF p oduc ion. A his condi ion, he e o-aldol condensa ion 26 pa hway was enhanced ins ead o isome iza ion/dehyd a ion pa hway. 27 3 INTRODUCTION 28 The biomass exploi a ion as aw ma e ial is g owing as an al e na i e o he sus ainable 29 p oduc ion o uels and chemicals1. Cellulose is one o he main compounds o biomass, 30 ep esen ing he mos abundan biopolyme 2. An impo an challenge in he p ocessing o 31 cellulosic biomass is o hyd olyze he β1-4 glucose-glucose bond p oducing a s eam o 32 suga s wi h low concen a ion o byp oduc s, by using an e icien p ocess3-5. This suga s 33 s eams could be u he ans o med in aluable chemical like py u aldehyde, 34 glycolaldehyde6-9, 5-hyd oxyme yl u u al (5-HMF)10, 11, o ganic acids o poly-alcohols12, 13. 35 Acid and enzyma ic hyd olysis o cellulose a e wo con en ional me hods ha need long 36 ea men imes (>3 h) o ob ain a poo -selec i e p oduc (<60% w/w)14, 15. The use o ionic 37 liquids as sol en and eac ion medium has been in ensi ely s udied due o he possibili y o 38 dissol e cellulose making i mo e ‘accessible’ o he hyd olysis eac ion16, 17. Howe e , his 39 kind o p ocess ake a leas 3 h o hyd olysis o ob ain a selec i i y nea o 30% w/w o 40 educing suga s16. These p ocessing me hods equi e la ge eac ion imes (hou s), which will 41 demand big eac o s a he scaling up ime. The use o p essu ized wa e is an al e na i e as 42 eac ion medium o he p ocessing o cellulosic biomass in a one-s ep as p ocess. To al 43 hyd olysis o cellulose can be achie ed in 0.02 s o eac ion ime in a supe c i ical wa e 44 medium p oducing a s eam o wa e soluble suga s wi h low concen a ion o de i ed 45 p oduc s (<2% w/w)18, 19. This kind o p ocess ep esen s an ad an ageous in ensi ica ion ha 46 will educe he ene ge ic and equipmen equi emen s in he scaling up. 47 Cellulose depolyme iza ion in ho p essu ized wa e ha e been done in di e en kind o 48 eac o s (ba ch, semi-ba ch and con inuous) a di e en empe a u es and p essu e, wi h o 49 wi hou ca alys s20. The yield o suga s a e biomass hyd olysis is enhanced by using 50 supe c i ical wa e eac o s ope a ed in a con inuous mode a high empe a u e and low 51 eac ion imes19, 21. The combina ion o hese wo pa ame e s is c ucial o ob aining high 52 4 yields in suga s. A long eac ion imes, he suga s a e de i ed and; a low eac ion 53 empe a u es se e al side eac ions ake place p oducing many compounds. In ac , i was 54 obse ed ha some eac ions a e a oided a supe c i ical condi ions. Especially a en ion 55 should be played o he o ma ion o 5-HMF. The p oduc ion o 5-HMF om cellulose in 56 p essu ized wa e is highly dependen on eac ion empe a u e. In Figu e 1 i is shown se e al 57 expe imen al esul s o cellulose hyd olysis in p essu ized wa e om 300ºC o 400ºC a 58 di e en p essu es along eac ion imes. 59 60 Figu e 1. 5-HMF yield om cellulose hyd olysis in p essu ized wa e along eac ion ime. 61 Expe imen empe a u e: ed: 400ºC; yellow: 350ºC and; blue: 300ºC. Expe imen p essu e: 62 (diamonds) 27 MPa; (squa es) 25 / 23 MPa and; ( iangles) 23 / 18 MPa. 63 I can be obse ed ha 5-HMF p oduc ion was as e , bu he yield lowe , when he 64 eac ion empe a u e was inc eased om 300ºC o 350ºC. The eac ion ime was educed 65 om 40 s o 10 s by inc easing he eac ion empe a u e. This beha io was expec ed and i 66 ollows he A henius law. Howe e , an expec ed beha io was de ec ed by inc easing he 67 eac ion empe a u e o e he c i ical poin o wa e , he p oduc ion o 5-HMF was highly 68 a oided. Al hough his beha io was p e iously de ec ed in bibliog aphy6, 7, 18, 22-35, a clea 69 and quan i a i e explana ion has no been de eloped ye . The di e en disco e ed beha io s 70 0% 5% 10% 15% 20% 25% 30% 0.001 0.01 0.1 1 10 100 Yield, w·w-1 , s 400-27 400-25 400-23 350-27 350-23 350-18 300-27 300-23 300-18 5 can be classi ied in h ee main g oups. (1) The maximum amoun o 5-HMF om cellulose in 71 p essu ized wa e wi hou ca alys is p oduced a empe a u es lowe han 300ºC 18, 23, 29, 31, 33. 72 An inc ease in empe a u e bene i s he e o aldol condensa ion eac ions o uc ose33. (2) 73 The p oduc ion o 5-HMF is enhanced inc easing he a ailabili y o p o ons (H+) in he 74 eac ion medium by adding acids25-28, 30. (3) The p oduc ion o 5-HMF is enhanced in a 75 p essu ized wa e medium when p essu e is inc eased a a cons an empe a u e6, 7. 76 The aim o his wo k was o s udy he eac ions o cellulose hyd olysis, ocusing in he 5- 77 HMF p oduc ion om suga s. The yields we e analyzed om a chemical poin o iew o he 78 eac ion pa hway. Se e al eac ions we e un in o de o ob ain accu a e da a. Cellulose 79 hyd olysis was expe imen ed a 300ºC, 325ºC, 350ºC, 375ºC and 400ºC a 25 MPa o 80 p essu e. Also, he p essu e e ec was es ed a 300ºC, 350ºC and 400ºC be ween 18 and 27 81 MPa. The s udies we e also conduc ed analyzing glucose and uc ose hyd olysis in 82 p essu ized wa e be ween 300ºC and 400ºC a 25 MPa. Finally, he esul s we e con as ed 83 wi h he p oduc s ob ained om whea b an hyd olysis in supe c i ical wa e . A eac ion 84 pa hway was de eloped and a no el kine ic model was es ed o unde s anding he beha io 85 o glucose and uc ose eac ion in supe c i ical wa e . 86 METHODS 87 Ma e ials 88 The cellulose (99%) used in he expe imen s was pu chased om VWR. Glucose (99%) 89 and uc ose (99%) used as s a ing biomass in he expe imen s we e pu chased om Sigma. 90 Whea b an was supplied by a local supplie . Dis illed wa e was used as eac ion medium in 91 he expe imen s. The s anda ds used in HPLC (High Pe o mance Liquid Ch oma og aphy) 92 analysis we e: cellobiose (+98%), glucose (+99%), uc ose (+99%), glyce aldehyde (95%), 93 6 py u aldehyde (40%), glycolaldehyde dime (99%), le ulinic acid (+99%), 5-HMF (99%) 94 pu chased om Sigma. 95 Analysis 96 The ca bon con en o he liquid p oduc s was de e mined by o al o ganic ca bon (TOC) 97 analysis wi h Shimadzu TOC-VCSH equipmen . The composi ion o he liquid p oduc s was 98 de e mined by High Pe o mance Liquid Ch oma og aphy (HPLC) analysis. The HPLC 99 column used o he sepa a ion o he compounds was Suga SH-1011 Shodex a 50ºC using 100 H2SO4 (0.01 N) as mobile phase wi h a low a e o 0.8mL/min. A Wa e s IR de ec o 2414 101 was used o iden i y and quan i y he suga s and hei de i a i es. An UV-Vis de ec o was 102 used o de e mine he 5-hid oxy-me hyl- u u al (5-HMF) concen a ion a a wa eleng h o 103 254nm. The selec i i y o each compound (Si) was calcula ed as he a io o : compound 104 ca bon composi ion (Xc) mul iplied by compound concen a ion (Ci) and o al ca bon a he 105 eac o inle (TC). Si=Ci Xc/TC. 106 Expe imen al Facili y 107 The expe imen s we e ca ied ou in a con inuous pilo plan able o wo k a empe a u es 108 up o 425ºC and p essu es up o 30 MPa. A schema ic diag am o he p ocess is shown in 109 Figu e 2. Two s eams con inuously ed a mic o eac o : a cellulose s eam and supe c i ical 110 wa e s eam. S ic con ol o he eac ion imes was achie ed by a combina ion o h ee 111 ac o s: (1) apid hea ing by supe c i ical wa e injec ion o he cellulose suspension s eam, 112 (2) apid cooling by sudden dep essu iza ion down o a mosphe ic p essu e and ~100ºC using 113 a mic o me e ing al e able o s and empe a u es up o 425 ºC, and (3) selec ion o a se ies 114 o ubula eac o s o di e en olumes accu a ely de e mined. The olume o he used 115 eac o s a ied om 0.12 ml o 64.5 ml, which in combina ion wi h lows be ween 1 g·s-1 and 116 2 g·s-1, and ha ing in o accoun he densi y o wa e a he expe imen ed condi ions, gi es 117 7 eac ion imes o 0.004 s o 40 s. A de ailed desc ip ion o he expe imen al se up was 118 de eloped in Suppo ing In o ma ion. Al hough he eac o is ed by wo s eams (biomass 119 and wa e ), no ex a wa e is needed in he p ocess when he s eady s a e is achie ed. As i 120 can be seen in Figu e 2, a e he eac o a lash chambe sepa a o p oduces wo s eams: 121 apo (wa e ) and liquid (suga s dissol ed). The apo is almos pu e wa e ha can be 122 eci cula ed. 123 124 Figu e 2. Schema ic diag am o he supe c i ical wa e hyd olysis acili y. 125 Reac ion medium and eac ion pa hway 126 Supe c i ical wa e (SCW) is wa e a empe a u e and p essu e alues abo e i s c i ical 127 poin (Tc=374ºC and Pc=22.1 MPa). In he su oundings o he c i ical poin , he p ope ies 128 o wa e can be highly in luenced by changing p essu e and empe a u e. So, he iden i y o 129 he medium can be modi ied wi hou changing he sol en . The medium densi y ep esen s 130 he quan i y o wa e pe olume uni (kg·m-3); his is a measu emen o wa e concen a ion, 131 an impo an ac o o ake in o accoun in he eac ions whe e wa e pa icipa es as eagen o 132 o ming in e media e s a es36. Ano he impo an p ope y o wa e as eac ion medium is he 133 ion p oduc (mol2·kg-2), which ep esen s how dissocia ed is wa e molecule (ion 134 Reac o Flash Suga s Supe c i ical Wa e Hyd olysis Biomass Hea e Wa e Ou Wa e 8 concen a ion). I he molal concen a ion o OH- (squa e oo o ionic p oduc ) is mul iplied 135 by densi y, he mola concen a ion o p o ons o hyd oxide anions in he medium is ob ained. 136 This concen a ion pa ame e includes bo h, he a ia ions in wa e olume and i s 137 dissocia ion. The concen a ion o OH- (which is he same o H+) in he su oundings o he 138 c i ical poin o wa e is plo ed in Figu e 3. 139 140 Figu e 3. Hyd oxyl concen a ion (mol·L-1) along empe a u e and p essu e. pOH=-log(|OH- 141 |)=pH 37, 38. Wa e densi y was calcula ed acco ding he IAPWS indus ial o mula ion37, 142 while he molal ionic p oduc o wa e was calcula ed ollowing ‘In e na ional Fo mula ion o 143 Ionic P oduc o Wa e Subs ance’38. 144 Impo an changes in he iden i y o he medium can be ob ained i empe a u e and 145 p essu e a e changed a he same ime. Fo example, he densi y o wa e a 300ºC and 27 146 MPa is a ound 750 kg·m-3; his alue can be dec eased o 130 kg·m-3 i he condi ions a e 147 modi ied o 400ºC and 23 MPa. The H+/OH- concen a ion a ies six o de s o magni ude in 148 he neighbo hood o he c i ical poin allowing he possibili y o wo king wi h ma kedly 149 di e en eac ion mediums. The H+/OH- concen a ion a 300ºC and 23 MPa is a ound 2·10-6 150 mol·L-1 which means ha he medium has high concen a ion o ions ([H+] and [OH-]) 151 a o ing he ionic eac ions6, 39-41. The H+/OH- concen a ion will ake a alue o 5.5·10-12 152 4 6 8 10 12 14 16 18 320 340 360 380 150 200 250 300 pOH Tempe a u e / ºC P essu e / ba Ionic eac ions Non Ionic eac ions 9 mol·L-1 i he empe a u e and p essu e a e changed o 400ºC and 23 MPa; his eac ion 153 medium would a o adical eac ions 42. 154 The eac ions we e assumed o ollow he eac ion pa hway shown in Schema 1. This 155 eac ion pa hway was buil ollowing he schemas de eloped in li e a u e23. The eac ion o 156 glucose isome iza ion occu s ia ing-opening and ke o-enol au ome ism. These eac ions 157 ake place o ming ansi ion s a es wi h OH- o H+. Also, uc ose dehyd a ion akes place 158 o ming ansi ion s a es inco po a ing H+ (one pe H2O molecule los ) 43. In o de o iden i y 159 hese eac ions in Schema 1, he symbols OH-/H+ we e added abo e he eac ion a ow. The 160 p oduc ion o glycolaldehyde was enhanced a supe c i ical condi ions because he 161 hyd oxide/p o on concen a ion is highly dec eased (pH=pOH=13) and so is he 162 concen a ion o uc ose and i s de i ed p oduc s. Al hough he eac ion o glucose 163 isome iza ion is a oided a low concen a ion o hyd oxide anions, uc ose yield nea o 10% 164 w·w-1 was ob ained a supe c i ical condi ions. 165 166 Schema 1. Main eac ion pa hway o cellulose hyd olysis in p essu ized wa e . 167 As i is shown in Schema 1, uc ose can ollow wo main eac ion pa hways: uc ose 168 dehyd a ion o e o aldol condensa ion. The second eac ion was mo e bene i ed compa ed 169 o he i s one ob aining, in his way, glyce aldehyde as main p oduc om uc ose. The 170 OOH H H H OH OH H OH H OH Glucose F uc ose O O OH 5 - HMF O OH OH Glyce aldehyde CH3 O O Py u aldehyde Cellulose Oligosaccha ides OH O Glycolaldehyde C C C O C CH2 OH H OH H OH CH2 H OH OH k kog OH-/H+ kg kgg k g OH-/H+ khm kglyp 16 kine ics sys em. Al hough he esul s shown in Figu e 6-A co espond o 27 MPa se ies, he 286 esul s o Model 3 ollow he A henius law o all he expe imen ed p essu es, as i is shown 287 in Figu e 7-B. I should be aken in o accoun ha he expe imen s we e always done in 288 p essu ized liquid o supe c i ical phase. This is because he se ies a 18 MPa was only 289 expe imen ed a 300ºC and 350ºC. 290 291 Figu e 7. (A) Kine ic cons an o uc ose dehyd a ion conside ing OH- concen a ion 292 ( iangles), conside ing wa e concen a ion (squa es) and non-conside ing OH- no wa e 293 concen a ion (ci cles). P essu e= 27MPa. (B) Kine ic cons an o uc ose dehyd a ion 294 conside ing OH- concen a ion a : 27 MPa (ci cles); 25 MPa (squa es); 23 MPa ( iangles) 295 and 18 MPa (diamonds). E o ba s ep esen he expe imen al and i ing e o s. 296 Reac ion model e alua ion: glucose hyd olysis a 25 MPa o di e en empe a u es 297 The eac ion pa hway and he kine ic model de eloped in sec ion 3, 4 and 5 we e also 298 es ed analyzing he glucose eac ions in p essu ized wa e . Glucose hyd olysis eac ions 299 we e expe imen ed a 25 MPa o p essu e a empe a u e a ound he c i ical poin o wa e 300 (350, 385 and 400ºC), whe e he change in he ionic p oduc o wa e is he highes . As i was 301 expec ed, he ob ained kine ic cons an s ollow he A henius law when he concen a ion o 302 ions was conside ed as eagen concen a ion (model 3). 303 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 -40 -30 -20 -10 0 10 20 30 0.0014 0.0016 0.0018 ln k h, s-1 ln k h, L.mol-1 .s-1 T -1 , K-1 Wi h H/OH Wi h Cw Wi hou H/OH no Cw A 0 5 10 15 20 25 30 0.0014 0.0016 0.0018 ln khm , L.mol-1 .s-1 T-1 ,K-1 18 MPa 23 MPa 25 MPa 27 MPa B 17 The eac ion mechanism p oposed in his wo k was es ed in h ee di e en ways: cellulose 304 hyd olysis a cons an p essu e changing empe a u e, cellulose hyd olysis changing p essu e 305 and empe a u e, and glucose hyd olysis a cons an p essu e changing empe a u e nea he 306 c i ical poin o wa e . Fo he h ee si ua ions, he kine ic cons an s o glucose hyd olysis 307 eac ions ollow he A henius pa ame e s when he ions concen a ion o he medium was 308 aken in o accoun . In addi ion, he kine ics cons an s o glucose isome iza ion and uc ose 309 dehyd a ion ook simila o de o magni ude o he di e en analyzed si ua ions. 310 311 312 Figu e 8. Glucose hyd olysis kine ic cons an a 350ºC, 385 ºC and 400ºC. P essu e = 25 313 MPa. E o ba s ep esen he expe imen al and i ing e o s. (A) Kine ic cons an o glucose 314 isome iza ion o uc ose ( iangles) and uc ose dehyd a ion (squa es) conside ing OH- 315 concen a ion as eagen . (B) Kine ic cons an o glucose e o aldol condensa ion (squa es), 316 uc ose e o aldol condensa ion (diamonds). 317 Tes ing he concep wi h modi ied eac ion mediums and na u al biomass 318 Finally, in o de o es he de elopmen s in he kine ics mechanism, uc ose was 319 hyd olyzed in modi ied eac ions mediums (wi h empo and oxalic acid). On he o he hand, 320 whea b an was hyd olyzed in supe c i ical wa e o es ing he p oduc ion o 5-HMF om a 321 na u al biomass in supe c i ical wa e . 322 0 5 10 15 20 25 30 35 0.0014 0.0015 0.0016 0.0017 ln k, L·mol-1 ·s-1 T-1 , K-1 kg khm A -3 -2 -1 0 1 2 3 4 5 6 0.0014 0.0015 0.0016 0.0017 ln k, s-1 T-1 , K-1 kgg kgp k g B 18 The yields o he main p oduc s ob ained a e uc ose hyd olysis a e shown in Figu e 9. 323 The expe imen s we e ca ied ou in he expe imen al se up explained abo e. Howe e , he 324 eac ion medium was modi ied by pumping empo o oxalic acid. Tempo (2,2,6,6- 325 Te ame hylpipe idin-1-oxyl) is a ee adical kidnappe usually employed o con ol adical 326 eac ions in he o ganic syn hesis and polyme iza ion47. Oxalic acid was used o inc ease he 327 concen a ion o ions in he eac ion medium. As i was expec ed, he p oduc ion o 5-HMF 328 om uc ose a supe c i ical condi ions (400ºC and 23 MPa) was negligible, being 329 py u aldehyde he main p oduc a e 0.9 s o eac ion ime. The addi ion o oxalic acid o he 330 eac ion medium inc eased he a ailabili y o p o ons in he medium, which would p omo e 331 he uc ose dehyd a ion eac ion. In ac , when he eac ion medium was acidi ied, he 332 p oduc ion o 5-HMF in supe c i ical wa e was enhanced o 15 % w·w-1. The same beha io 333 was obse ed using empo as eac ion medium modi ie . This ee adical kidnape has an 334 acid ole due o he dissocia ion o he OH g oup bonded o he ni ogen a om. Once again, an 335 acid medium p omo ed he p oduc ion o 5-HMF co obo a ing ha 5-HMF p oduc ion is 336 highly dependen on he p o ons a ailabili y in he medium. 337 338 Figu e 9. Yields uc ose hyd olysis a 400ºC, 23 MPa and 0.9 s o a eac ion ime. The 339 eac ion medium was modi ied wi h empo o oxalic acid. 340 0 0.2 0.4 0.6 0.8 1 Tempo Oxalic No Addi i e Yield, w·w-1 Py u aldehyde 5-HMF 19 Whea b an was also hyd olyzed in he expe imen al se up a o emen ioned. The eac ion 341 empe a u e was se a 400ºC wi h a eac o p essu e o 25 MPa. The eac ion ime was a ied 342 om 0.19 s o 0.69 s. Fo una ely, as i can be seen in Figu e 10, he yields o 5-HMF we e 343 lowe han 0.05% w·w-1. A de ailed desc ip ion o whea b an hyd olysis o suga s and 344 lignin p oduc ion can be ound in a p e ious wo k48. 345 346 Figu e 10. Yields whea b an hyd olysis a 400ºC and 25 MPa be ween 0.19 s and 0.69 s o 347 eac ion ime. 348 CONCLUSION 349 The p ocess p esen ed in his wo k shows an e icien al e na i e o hyd olyze cellulose 350 selec i ely. The con ol o he eac ion ime is he key o ob ain yields highe han 95% w·w-1 351 o soluble suga s o 60% w·w-1 o glycolaldehyde. F om he iewpoin o chemis y, he 352 selec i i y o he p ocess is go e ned by he ions concen a ion in he eac ion medium. A 353 eac ion mechanism model was buil and in ensi ely es ed o demons a e i s eliabili y. The 354 eac ions o glucose and uc ose e o aldol condensa ion a e low demanding o ions. In ac , 355 his eac ions a e highly imp o ed when he wa e molecules ( eac ion medium and eagen ) 356 a e highly associa ed. On he o he hand, he isome iza ion eac ion o glucose- uc ose as 357 well as dehyd a ion eac ions o hese suga s a e ex emely diminished when he wa e 358 molecules a e associa ed. Fo i s ime, i was quan i a i ely explained and demons a ed he 359 0 0.02 0.04 0.06 0.08 0.1 0.19 0.22 0.30 0.69 5-HMF yield, w·w-1 , s 20 easons why he p oduc ion o 5-HMF is highly a oided a supe c i ical wa e condi ions. I 360 was succeeded by adding he concen a ion o p o ons o hyd oxide ions due o wa e 361 dissocia ion as eagen in he kine ic modelling o he eac ions. 362 The ex ao dina y changes in he chemical and physical p ope ies o supe c i ical wa e 363 allows he biomass hyd olysis choosing he desi ed p oduc s by simply selec ing he co ec 364 eac ion empe a u e and p essu e. 365 366 ACKNOWLEDGEMENTS 367 The au ho s hank he Spanish Minis y o Economy and Compe i i eness o P ojec 368 CTQ2011-23293, CTQ2011-27347, CQT2013-44143-R and ENE2012-33613. M.D.B 369 hanks he Spanish Minis y o Economy and Compe i i eness o Ramón y Cajal esea ch 370 ellowship RYC-2013-13976. 371 21 REFERENCES 372 373 1. A. J. Ragauskas, C. K. Williams, B. H. Da ison, G. B i o sek, J. Cai ney, C. A. 374 Ecke , W. J. F ede ick, J. P. Halle , D. J. Leak, C. L. Lio a, J. R. Mielenz, R. 375 Mu phy, R. Temple and T. Tschaplinski, Science, 2006, 311, 484 -489. 376 2. D. Klemm, B. Heublein, H. P. Fink and A. 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