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On he Go e ning Chemis y o Cellulose
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Hyd olysis in Supe c i ical Wa e
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Danilo A. Can e o, M. Dolo es Be mejo and M. José Coce o*
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High P essu e P ocesses G oup, Depa men o Chemical Enginee ing and En i onmen al
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Technology, Uni e si y o Valladolid, P ado de la Magdalena s/n, 47011 Valladolid, SPAIN.
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FAX: +34-983423013. *E-mail: [email p o ec ed]
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Keywo ds: Biomass • Ionic P oduc • Kine ic • Suga s • Wa e Chemis y
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Abs ac
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This a icle summa izes he ecen e o s in he High P essu e P ocesses G oup labs a UVa
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ega ding he undamen als o biomass hyd olysis in p essu ized wa e medium. A ex emely
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low eac ion imes (0.02 s), cellulose was hyd olyzed in supe c i ical wa e (400ºC and 25
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MPa) ob aining a suga s yield highe han 95% w·w-1 while 5-HMF yield was lowe han
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0.01% w·w-1. When he eac ion imes was inc eased up o 1 s, he main p oduc was
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glycolaldehyde (60% w·w-1). Independen ly o he eac ion ime, he yield o 5-HMF was
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always lowe han 0.01% w·w-1. In o de o e alua e he eac ion pa hway and mechanism o
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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
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and eac ion medium) we e s udied o di e en biomasses (cellulose, glucose, uc ose and
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whea b an). I was conside ed ha he eac ions o glucose isome iza ion o uc ose as well
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as uc ose dehyd a ion o 5-HMF ake place ia p o on o hyd oxide anion associa ion. So,
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hei concen a ion was aken in o accoun as eagen concen a ion in he eac ion
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e alua ions. I was ound ha he p o on and hyd oxide anion concen a ion in he medium
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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
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o glucose isome iza ion o uc ose and i s u he dehyd a ion o p oduce 5-HMF a e highly
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dependen on ions concen a ion. By inc easing pOH/pH, hese eac ions we e minimized
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allowing he con ol o 5-HMF p oduc ion. A his condi ion, he e o-aldol condensa ion
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pa hway was enhanced ins ead o isome iza ion/dehyd a ion pa hway.
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3
INTRODUCTION
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The biomass exploi a ion as aw ma e ial is g owing as an al e na i e o he sus ainable
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p oduc ion o uels and chemicals1. Cellulose is one o he main compounds o biomass,
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ep esen ing he mos abundan biopolyme 2. An impo an challenge in he p ocessing o
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cellulosic biomass is o hyd olyze he β1-4 glucose-glucose bond p oducing a s eam o
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suga s wi h low concen a ion o byp oduc s, by using an e icien p ocess3-5. This suga s
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s eams could be u he ans o med in aluable chemical like py u aldehyde,
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glycolaldehyde6-9, 5-hyd oxyme yl u u al (5-HMF)10, 11, o ganic acids o poly-alcohols12, 13.
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Acid and enzyma ic hyd olysis o cellulose a e wo con en ional me hods ha need long
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ea men imes (>3 h) o ob ain a poo -selec i e p oduc (<60% w/w)14, 15. The use o ionic
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liquids as sol en and eac ion medium has been in ensi ely s udied due o he possibili y o
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dissol e cellulose making i mo e ‘accessible’ o he hyd olysis eac ion16, 17. Howe e , his
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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
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educing suga s16. These p ocessing me hods equi e la ge eac ion imes (hou s), which will
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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
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eac ion medium o he p ocessing o cellulosic biomass in a one-s ep as p ocess. To al
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hyd olysis o cellulose can be achie ed in 0.02 s o eac ion ime in a supe c i ical wa e
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medium p oducing a s eam o wa e soluble suga s wi h low concen a ion o de i ed
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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
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will educe he ene ge ic and equipmen equi emen s in he scaling up.
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Cellulose depolyme iza ion in ho p essu ized wa e ha e been done in di e en kind o
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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
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wi hou ca alys s20. The yield o suga s a e biomass hyd olysis is enhanced by using
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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
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eac ion imes19, 21. The combina ion o hese wo pa ame e s is c ucial o ob aining high
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yields in suga s. A long eac ion imes, he suga s a e de i ed and; a low eac ion
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empe a u es se e al side eac ions ake place p oducing many compounds. In ac , i was
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obse ed ha some eac ions a e a oided a supe c i ical condi ions. Especially a en ion
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should be played o he o ma ion o 5-HMF. The p oduc ion o 5-HMF om cellulose in
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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
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expe imen al esul s o cellulose hyd olysis in p essu ized wa e om 300ºC o 400ºC a
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di e en p essu es along eac ion imes.
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Figu e 1. 5-HMF yield om cellulose hyd olysis in p essu ized wa e along eac ion ime.
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Expe imen empe a u e: ed: 400ºC; yellow: 350ºC and; blue: 300ºC. Expe imen p essu e:
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(diamonds) 27 MPa; (squa es) 25 / 23 MPa and; ( iangles) 23 / 18 MPa.
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I can be obse ed ha 5-HMF p oduc ion was as e , bu he yield lowe , when he
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eac ion empe a u e was inc eased om 300ºC o 350ºC. The eac ion ime was educed
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om 40 s o 10 s by inc easing he eac ion empe a u e. This beha io was expec ed and i
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ollows he A henius law. Howe e , an expec ed beha io was de ec ed by inc easing he
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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
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a oided. Al hough his beha io was p e iously de ec ed in bibliog aphy6, 7, 18, 22-35, a clea
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and quan i a i e explana ion has no been de eloped ye . The di e en disco e ed beha io s
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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
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can be classi ied in h ee main g oups. (1) The maximum amoun o 5-HMF om cellulose in
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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.
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An inc ease in empe a u e bene i s he e o aldol condensa ion eac ions o uc ose33. (2)
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The p oduc ion o 5-HMF is enhanced inc easing he a ailabili y o p o ons (H+) in he
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eac ion medium by adding acids25-28, 30. (3) The p oduc ion o 5-HMF is enhanced in a
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p essu ized wa e medium when p essu e is inc eased a a cons an empe a u e6, 7.
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The aim o his wo k was o s udy he eac ions o cellulose hyd olysis, ocusing in he 5-
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HMF p oduc ion om suga s. The yields we e analyzed om a chemical poin o iew o he
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eac ion pa hway. Se e al eac ions we e un in o de o ob ain accu a e da a. Cellulose
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hyd olysis was expe imen ed a 300ºC, 325ºC, 350ºC, 375ºC and 400ºC a 25 MPa o
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p essu e. Also, he p essu e e ec was es ed a 300ºC, 350ºC and 400ºC be ween 18 and 27
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MPa. The s udies we e also conduc ed analyzing glucose and uc ose hyd olysis in
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p essu ized wa e be ween 300ºC and 400ºC a 25 MPa. Finally, he esul s we e con as ed
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wi h he p oduc s ob ained om whea b an hyd olysis in supe c i ical wa e . A eac ion
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pa hway was de eloped and a no el kine ic model was es ed o unde s anding he beha io
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o glucose and uc ose eac ion in supe c i ical wa e .
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METHODS
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Ma e ials
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The cellulose (99%) used in he expe imen s was pu chased om VWR. Glucose (99%)
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and uc ose (99%) used as s a ing biomass in he expe imen s we e pu chased om Sigma.
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Whea b an was supplied by a local supplie . Dis illed wa e was used as eac ion medium in
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he expe imen s. The s anda ds used in HPLC (High Pe o mance Liquid Ch oma og aphy)
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analysis we e: cellobiose (+98%), glucose (+99%), uc ose (+99%), glyce aldehyde (95%),
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py u aldehyde (40%), glycolaldehyde dime (99%), le ulinic acid (+99%), 5-HMF (99%)
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pu chased om Sigma.
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Analysis
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The ca bon con en o he liquid p oduc s was de e mined by o al o ganic ca bon (TOC)
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analysis wi h Shimadzu TOC-VCSH equipmen . The composi ion o he liquid p oduc s was
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de e mined by High Pe o mance Liquid Ch oma og aphy (HPLC) analysis. The HPLC
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column used o he sepa a ion o he compounds was Suga SH-1011 Shodex a 50ºC using
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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
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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
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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
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254nm. The selec i i y o each compound (Si) was calcula ed as he a io o : compound
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ca bon composi ion (Xc) mul iplied by compound concen a ion (Ci) and o al ca bon a he
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eac o inle (TC). Si=Ci Xc/TC.
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Expe imen al Facili y
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The expe imen s we e ca ied ou in a con inuous pilo plan able o wo k a empe a u es
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up o 425ºC and p essu es up o 30 MPa. A schema ic diag am o he p ocess is shown in
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Figu e 2. Two s eams con inuously ed a mic o eac o : a cellulose s eam and supe c i ical
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wa e s eam. S ic con ol o he eac ion imes was achie ed by a combina ion o h ee
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ac o s: (1) apid hea ing by supe c i ical wa e injec ion o he cellulose suspension s eam,
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(2) apid cooling by sudden dep essu iza ion down o a mosphe ic p essu e and ~100ºC using
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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
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o ubula eac o s o di e en olumes accu a ely de e mined. The olume o he used
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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
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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
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eac ion imes o 0.004 s o 40 s. A de ailed desc ip ion o he expe imen al se up was
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de eloped in Suppo ing In o ma ion. Al hough he eac o is ed by wo s eams (biomass
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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
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can be seen in Figu e 2, a e he eac o a lash chambe sepa a o p oduces wo s eams:
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apo (wa e ) and liquid (suga s dissol ed). The apo is almos pu e wa e ha can be
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eci cula ed.
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Figu e 2. Schema ic diag am o he supe c i ical wa e hyd olysis acili y.
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Reac ion medium and eac ion pa hway
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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
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poin (Tc=374ºC and Pc=22.1 MPa). In he su oundings o he c i ical poin , he p ope ies
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o wa e can be highly in luenced by changing p essu e and empe a u e. So, he iden i y o
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he medium can be modi ied wi hou changing he sol en . The medium densi y ep esen s
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he quan i y o wa e pe olume uni (kg·m-3); his is a measu emen o wa e concen a ion,
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an impo an ac o o ake in o accoun in he eac ions whe e wa e pa icipa es as eagen o
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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
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ion p oduc (mol2·kg-2), which ep esen s how dissocia ed is wa e molecule (ion
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Reac o Flash
Suga s
Supe c i ical Wa e Hyd olysis
Biomass
Hea e
Wa e Ou
Wa e
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concen a ion). I he molal concen a ion o OH- (squa e oo o ionic p oduc ) is mul iplied
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by densi y, he mola concen a ion o p o ons o hyd oxide anions in he medium is ob ained.
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This concen a ion pa ame e includes bo h, he a ia ions in wa e olume and i s
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dissocia ion. The concen a ion o OH- (which is he same o H+) in he su oundings o he
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c i ical poin o wa e is plo ed in Figu e 3.
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Figu e 3. Hyd oxyl concen a ion (mol·L-1) along empe a u e and p essu e. pOH=-log(|OH-
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|)=pH 37, 38. Wa e densi y was calcula ed acco ding he IAPWS indus ial o mula ion37,
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while he molal ionic p oduc o wa e was calcula ed ollowing ‘In e na ional Fo mula ion o
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Ionic P oduc o Wa e Subs ance’38.
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Impo an changes in he iden i y o he medium can be ob ained i empe a u e and
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p essu e a e changed a he same ime. Fo example, he densi y o wa e a 300ºC and 27
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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
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modi ied o 400ºC and 23 MPa. The H+/OH- concen a ion a ies six o de s o magni ude in
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he neighbo hood o he c i ical poin allowing he possibili y o wo king wi h ma kedly
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di e en eac ion mediums. The H+/OH- concen a ion a 300ºC and 23 MPa is a ound 2·10-6
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mol·L-1 which means ha he medium has high concen a ion o ions ([H+] and [OH-])
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a o ing he ionic eac ions6, 39-41. The H+/OH- concen a ion will ake a alue o 5.5·10-12
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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
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medium would a o adical eac ions 42.
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The eac ions we e assumed o ollow he eac ion pa hway shown in Schema 1. This
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eac ion pa hway was buil ollowing he schemas de eloped in li e a u e23. The eac ion o
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glucose isome iza ion occu s ia ing-opening and ke o-enol au ome ism. These eac ions
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ake place o ming ansi ion s a es wi h OH- o H+. Also, uc ose dehyd a ion akes place
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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
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hese eac ions in Schema 1, he symbols OH-/H+ we e added abo e he eac ion a ow. The
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p oduc ion o glycolaldehyde was enhanced a supe c i ical condi ions because he
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hyd oxide/p o on concen a ion is highly dec eased (pH=pOH=13) and so is he
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concen a ion o uc ose and i s de i ed p oduc s. Al hough he eac ion o glucose
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isome iza ion is a oided a low concen a ion o hyd oxide anions, uc ose yield nea o 10%
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w·w-1 was ob ained a supe c i ical condi ions.
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Schema 1. Main eac ion pa hway o cellulose hyd olysis in p essu ized wa e .
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As i is shown in Schema 1, uc ose can ollow wo main eac ion pa hways: uc ose
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dehyd a ion o e o aldol condensa ion. The second eac ion was mo e bene i ed compa ed
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o he i s one ob aining, in his way, glyce aldehyde as main p oduc om uc ose. The
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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
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kine ics sys em. Al hough he esul s shown in Figu e 6-A co espond o 27 MPa se ies, he
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esul s o Model 3 ollow he A henius law o all he expe imen ed p essu es, as i is shown
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in Figu e 7-B. I should be aken in o accoun ha he expe imen s we e always done in
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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.
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Figu e 7. (A) Kine ic cons an o uc ose dehyd a ion conside ing OH- concen a ion
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( iangles), conside ing wa e concen a ion (squa es) and non-conside ing OH- no wa e
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concen a ion (ci cles). P essu e= 27MPa. (B) Kine ic cons an o uc ose dehyd a ion
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conside ing OH- concen a ion a : 27 MPa (ci cles); 25 MPa (squa es); 23 MPa ( iangles)
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and 18 MPa (diamonds). E o ba s ep esen he expe imen al and i ing e o s.
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Reac ion model e alua ion: glucose hyd olysis a 25 MPa o di e en empe a u es
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The eac ion pa hway and he kine ic model de eloped in sec ion 3, 4 and 5 we e also
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es ed analyzing he glucose eac ions in p essu ized wa e . Glucose hyd olysis eac ions
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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).
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-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
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eac ions ollow he A henius pa ame e s when he ions concen a ion o he medium was
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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
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