Comparison of thermal behavior of natural and hot-washed sisal fibers based on their main components: Cellulose, xylan and lignin. TG-FTIR analysis of volatile products

1 Compa ison o he mal beha io o na u al and ho -washed sisal ibe s based on hei main componen s: cellulose, xylan and lignin. TG-FTIR analysis o ola ile p oduc s Mónica Bení ez-Gue e o 1*, Jo ge López-Becei o 2, Ped o E. Sánchez-Jiménez3 and José Pascual-Cosp 1. 1 Depa amen o de Ingenie ía Ci il, Ma e iales y Fab icación, Uni e sidad de Málaga, Escuela de Ingenie ías. C/ D . O iz Ramos s/n. Campus Tea inos. 29071 Málaga. Spain. 2 Depa amen o de Ingenie ía Indus ial II. Escola Poli écnica Supe io . Uni e sidade da Co uña, A da. Mendizábal, 15403 Fe ol, Spain. 3 Ins i u o de Ciencia de Ma e iales de Se illa, CSIC-Uni e sidad de Se illa, C. Amé ico Vespucio 49, 41092 Se illa, Spain *Co esponding au ho . Email: monica_beni ez_gue e [email protected] Tel: +34 951952594 Fax: +34 951952601 Abs ac : This pape p esen s in a comp ehensi e way he he mal beha io o na u al and ho -.washed sisal ibe s, based on he undamen al componen s o lignocellulosic ma e ials: cellulose, xylan and lignin. The esea ch highligh s he in luence exe ed on he he mal s abili y o sisal ibe s by o he cons i uen s such as non-cellulosic polysaccha ides (NCP) and mine al ma e . The mal changes we e in es iga ed by he mal X- ay di ac ion (TXRD), analyzing he c ys allini y index (%Ic) o cellulosic samples, and by simul aneous he mog a ime ic and di e en ial he mal analysis coupled wi h Fou ie - ans o med in a ed spec ome y (TG/DTA-FTIR), which allowed o examine he e olu ion o he main ola ile compounds e ol ed du ing he deg ada ion unde ine and oxidizing a mosphe es. The wo k demons a es he po en ial o his echnique o elucida e di e en s eps du ing he he mal decomposi ion o sisal, p o iding ex ensible esul s o o he lignocellulosic ibe s, h ough he analysis o he e olu ion o CO2, CO, H2O, CH4, ace ic acid, o mic acid, me hanol, o maldehyde and 2-bu anone, and compa ing i wi h he ola ile p oduc s om py olysis o he biomass componen s. The hyd oxyace aldehyde de ec ed du ing py olysis o sisal is indica i e o an al e na i e ou e o ha o le oglucosan, gene a ed du ing cellulose py olysis. Ho -washing a 75 ºC mos ly ex ac s non-cellulosic componen s o low decomposi ion empe a u e, and educes he ange o empe a u e in which sisal decomposi ion occu s, causing a e a d in he py olysis s age and inc easing TbNCP and TbCEL, empe a u es a he maximum mass loss a e o non-cellulosic polysaccha ides and cellulose decomposi ions, espec i ely. Howe e , en iching sisal ibe s in cellulose p oduces a dec ease o TbCEL unde an oxidizing a mosphe e, and u he mo e, a delay o he combus ion p ocess, displacing TbCOM o highe empe a u es. The esul s and indings o he pape would help u he unde s anding o he mal p ocesses whe e aga e ibe s a e in ol ed, as he decomposi ion o hei composi es. Keywo ds: Sisal ibe ; Ho -wa e ea men ; Biomass componen s; TGA-FTIR; Gas e olu ion; Py olysis; Combus ion. 2 1. In oduc ion The la ge numbe and di e si y o lignocellulosic p oduc s as well as plu ali y o hei applica ions in many indus ial sec o s, implies he need o comp ehensi ely know hei he mal beha io , s abili y and deg ada ion p ocesses a ele a ed empe a u es. Py olysis s udies o syn he ic biomass, ob ained by mixing i s undamen al cons i uen s, e ealed ha , in gene al, he o e all deg ada ion o biomass ollows he p inciple o addi i i y [1,2]. Howe e , di e en au ho s [3,4] show ha in e ac ion among he cons i uen s exi s, whe eby he p opo ion o he py olysis p oduc s di e s om ha o na u al biomass. I has been demons a ed ha he addi ion o ex ac i es and mine al compounds o he undamen al biomass componen s [5-7], and he deg ee o c ys allini y o cellulose [8], a ec he py olysis p ocess. The in luence o ex ac i es and mine al ma e on he mal decomposi ion o di e en lignocellulosic ma e ials has been s udied ex ensi ely [9-11]. Wa e washing is a simple and economical p e ea men , which has been used o emo e alkali me als om biomass, o a oid he oxici y o mis s gene a ed by he combus ion o biomass, so p e en ing accumula ion p oblems (deposi ion, co osion, e osion, e c.) in combus ion and gasi ica ion plan s, and also o imp o e p ocess yields [12-14]. Ho washing p ocess educes he p opo ion o subs ances adhe ed o he cell walls, emo ing o ganic compounds, as a hemicellulose ac ion and o he low molecula weigh ca bohyd a es, and mine als such as sal s o alkali, alkaline ea h and ansi ion me al ca ions. In addi ion o he adi ional pu poses, he use o Aga e species ibe s has sp ead o a a ie y o p ocesses o in e es in many indus ial sec o s, due o hei ease o cul i a ion, wi h high p oduc i i y and low cos associa ed, as well as hei enewable and biodeg adable cha ac e . Fu he mo e, hei biomass componen p opo ion makes hem sui able o p ocessabili y and bio uel p oduc ion [15,16]. In pa icula , Aga e sisalana ibe o sisal ibe is one o he mos widely used due o i s good mechanical p ope ies, used in a g ea a ie y o adi ional applica ions due o i s ha dness, coa seness and esis ance o wea , which has gained g ea in e es du ing he las decades in he manu ac u ing o composi e ma e ials, pa icula ly as ein o cemen o mo a s, conc e e and polyme ic ma ices [17-19], as polyp opylene [20-22]. Sisal ibe s a e mainly composed o cellulose and non-cellulosic polysaccha ides (NCP): hemicellulose O-ace yl-(4-O-me hylglucu ono)xylan [23] and pec ins composed o hamnose, a abinose and u onic acid a a 2.3:1:3.3 a io [24], along wi h a smalle amoun o lignin, whose mola a io o cons i u ional uni s, p-hyd oxyl phenol (H), guaiacyl (G) and sy ingyl (S), has been es ablished in 2% H, 22% G and 76% S [25]. Fu he mo e, he ibe is cons i u ed by a lowe p opo ion o ex ac i es, such as lipophilic componen s [26], simple phenolic de i a i es and mine al ma e , mainly ep esen ed by calcium oxala e deposi s loca ed be ween he elemen a y ibe s comp ising he ibe bundle [27]. The composi ion o sisal, as any na u al ibe , a ies depending on he o igin, age and ex ac ion p ocess o he componen s, among o he ac o s. The composi ional a iabili y in e ms o cons i uen s is e y la ge as can be deduced om di e en e iews [15,28,29]. The e is a lo o in o ma ion sca e ed in he li e a u e abou he he mal beha io o sisal ibe s, bu ew s udies ha e analyzed i in dep h [30-32]. The ex ensi e wo k ecen ly published by Ma in e al. 3 [33] ocuses on he he mal decomposi ion o sisal and he main biocomponen s ex ac ed om he ibe i sel . Howe e , gi en he complexi y o he mal p ocesses occu ing in lignocellulosic ma e ials, he compa ison o he mog a ime ic s ages wi h hei associa ed ene gy e ec s equi es a de ailed analysis, in e p e ed h ough he analysis o he decomposi ion o pu e biomass componen s, adding a new pe spec i e ha would allow o a be e de ini ion o he phenomena occu ing. Fu he mo e, he e olu ion o he he mal deg ada ion p oduc s o sisal ibe s is poo ly desc ibed. Only he liquid ac ion eleased du ing as py olysis has been cha ac e ized [34], he e being a lack o in o ma ion abou he gaseous p oduc s emi ed. The he mog a ime y coupled wi h Fou ie - ans o med in a ed spec oscopy (TG-FTIR) echnique has been p o ed o be a aluable and simple ool, no only o analyze he gas phase du ing he he molysis o lignocellulosic compounds [35-37], bu also o cla i y di e en s eps du ing ibe decomposi ion. This pape aims o analyze and p esen in a comp ehensi e way he he mal beha io o na u al and ho washed sisal ibe s, discussing hei he mog a ime ic and di e en ial he mal analysis (TG- DTA) on he basis o he deg ada ion o undamen al componen s, cellulose, lignin and xylan, and compa ing he e olu ion o he gas phase decomposi ion p oduc s by TG-FTIR analysis. 2. Ma e ials and Me hods The sisal ibe s used in his wo k we e p o ided by Caye ano Ga cía del Mo al S. L. (Cab a de San o C is o, Jaén, Spain). They we e cu o 2 - 4 mm leng h and hen subjec ed o a washing p ocess wi h s i ing in ho wa e a 75 °C o 2 hou s. He eina e he samples will be e e ed o as Na u al Sisal and Washed Sisal. Mic oc ys alline cellulose powde (Ald ich), alkali low sul ona e con en lignin (Sigma Ald ich) and bi ch xylan (xylose ≥ 90%, Ald ich) we e used as ep esen a i es o he main biomass componen s. X- ay di ac ion (XRD) measu emen s we e pe o med in a Philips X Pe PRO MPD, wi h a X Cele a o eal ime mul iple s ip RTMS sys em. The modi ac ion was pe o med in Helium a mosphe e a empe a u es up o 400 °C, using a TTK450N chambe o Ald ich cellulose and a one HTK1200N o Na u al Sisal. Spec a we e acqui ed e e y 20 °C, each one was eco ded o 50 minu es in he 2θ ange om 5 o 45°. Tempe a u e was linea ly changed be ween iso he ms a 5 ºC/min, and he s abiliza ion ime a each iso he m was 5 minu e. Da a we e analyzed employing X'Pe HighSco e so wa e. The c ys allini y index, %Ic(XRD), was calcula ed a se e al empe a u es using he equa ion p oposed by Segal [38]: 1001)( max  I I =XRD%I am c whe e Imax is he maximum in ensi y o he (002) la ice e lec ion o cellulose I ype, loca ed a 2θ be ween 22 and 23º, and Iam is he in ensi y a ibu ed o amo phous pa , gi en a 2θ be ween 18º and 19º o I cellulose. 4 The mog a ime ic s udy was pe o med in a TA Ins umen s STD 2960 simul aneous he mal analyze . The expe imen al se up consis ed o a 10 °C/min linea hea ing p og am om 20 o 750 °C, wi h low a es o 50 and 120 mL/min, bo h in ai and in ni ogen. The gases eleased we e immedia ely analyzed in a B uke VECTOR 22 in a ed spec ome e , coupled o he ou le po o he SDT. The ans e line and he cell we e kep o a empe a u e o 200 °C. The spec um o he e ol ed gas was eco ded e e y 61 seconds, and consis s o an a e age o 64 scans wi h a esolu ion o 4 cm-1 in he ange om 4000 o 500 cm-1. Spec a we e analyzed using B uke OPUS 5.5 so wa e, being baseline-co ec ed by a conca e ubbe band algo i hm compu ed wi h 10 i e a ions on 64 poin s. Ashes om he combus ion o na u al and washed sisal ibe s we e semi quan i a i ely analyzed by X- ay luo escence, using a PANaly ical AXIOS spec ome e . 3. Resul s and Discussion 3.1. E olu ion o he c ys allini y index o cellulose and sisal The di ac ion pa e ns a di e en empe a u es o mic oc ys alline cellulose and Na u al Sisal a e shown in Fig. 1. A oom empe a u e, cellulose clea ly p esen s e lec ions a ound 2θ = 15, 16.5, 20.5, 22.5 and 34.5º, co esponding espec i ely o he planes (101), (101), (021), (002) and (040) o I ype allomo ph, while sisal ibe only exhibi s he main (002) peak, and a con olu ion o (101)-(10 1) e lec ions, which a e supe imposed o calcium oxala e hyd a e Ca(COO)2·H2O ones (pa e n 20-0231 o he Join Commi ee on Powde Di ac ion S anda ds - In e na ional Cen e o Di ac ion Da a, JCPDS – ICDD). Fo bo h, i is obse ed ha he in ensi y o e lec ions dec eases wi h inc easing empe a u e, shi ing he di ac ion peaks owa ds a g ea e spacing. I can be explained due o he p og essi e de e io a ion o cellulose c ys al la ice, un il he s uc u e becomes comple ely amo phous, obse ed be o e 310 ºC. Fu he mo e, he cons i u i e mine als o sisal ibe expe imen ans o ma ions wi h inc easing empe a u e, gene a ing s able c ys alline compounds unde ine a mosphe e. Figu e 1. X- ay di ac og ams a di e en empe a u es, a o mic oc ys alline cellulose and b o Na u al sisal, whe e can be obse ed e lexions om Ca(COO)2·H2O (■). 5 The e olu ion o %Ic(XRD) wi h empe a u e o bo h cellulose and sisal is displayed in Fig. 2, being obse ed a much mo e p onounced a ia ion o he na u al ibe . A apid dec ease occu s o sisal index a empe a u es below han 100 °C, emaining s able a a ound 42% un il 210 °C. In con as , i diminishes slowly o 76% o cellulose. Nea 230 ºC, he c ys allini y index o bo h sisal and cellulose alls sha ply, p esen ing almos he same slope in bo h cases. A 270 ºC i eaches he smalles alue o 17% o sisal, and 65% o he s udied cellulose, which is o ally amo phized abo e 310 ºC. This beha io o sisal ibe is close o ha epo ed by Saikia [32], which desc ibes a educ ion o he deg ee o c ys allini y om 51 o 41% when sisal is hea ed in ai a 177 °C, becoming comple ely amo phous a a ound 260 °C. Figu e 2. Va ia ion o he c ys allini y index, %Ic(XRD), wi h empe a u e o mic oc ys alline cellulose (▲) and Na u al Sisal (●). 3.2. TG-DTG/DTA analysis o he biomass componen s P e iously o he analysis o he he mal decomposi ion o sisal ibe s, i is impo an o unde s and he beha io o he main lignocellulosic componen s. Figu e 3 exhibi s TG, DTG and DTA plo s o he s udied cellulose, xylan, and lignin, unde bo h ai and ni ogen a mosphe es. Tempe a u e anges, empe a u e o maximum mass loss a e (Tb) and he mass loss on a d y basis a e summa ized in Table 1. The weigh on d y basis is calcula ed om each he mog am excluding he mass loss below 180ºC. 6 Figu e 3. TG/DTG-DTA he mog ams o he main biomass componen s, unde ni ogen (a-b) and ai pu ges (c-d) wi h low a es o 120 mL/min. The inne inse in Fig. 3b no es h ee supe imposed s ages on DTG cu e o lignin. Table 1: The moanaly ical da a TG/DTG-DTA o biomass componen s, unde ai and ni ogen a mosphe es (120 mL/min low a e). Tb a(ºC) 21 – 185 65 52 endo 185 – 329 325 61.9 325 endo 329 – 354 333 17.9 350 exo Cellulose Ash (737 ºC) 0.3 20 – 165 88 90 endo 165 –298 247 58.9 268 endo 298 –423 344 31.2 390 exo 423 – 502 435 1.3 438 exo 502 –653 562 0.7 565 exo Xylan 7.3 % w bTpeak (ºC) DTA Ai Ash (737ºC) Ni ogen 200 – 447 338 83.8 335 endo 10.8 23 – 161 85 80 endo 161 –242 228 7.2 242 –362 286 56.0 291 exo 362 – 548 460 8.8 458 exo Tb a(ºC) % w bTpeak (ºC) DTA 23 – 200 66 56 endo a DTG peak empe a u e b mass loss pe cen age and ash/ esidue pe cen age in d y base 354 – 737 473 19.9 475, 600 exo 20.8 Lignin 26 – 180 86 98 endo 180 – 344 305 22.8 319 exo 344 – 426 380 6.6 403 exo Ash (737 ºC) 62.5 426 – 737 456 8.1 474 exo 26 – 176 88 90 endo 176 – 414 (278) 315 (358) 28.2 (292 exo) 312 endo (353 exo) 414 – 486 453 4.2 478 exo 486 – 737 678 9.9 647 exo 57.7 400 exo Residue (737 ºC) Residue (737ºC) Residue (737 ºC) 7 I can be dis inguished undamen ally h ee s ages, desc ibed unde nea h. i) In he low empe a u e ange, below 180 °C, undamen ally physically adso bed wa e e apo a es. The mass losses accoun o abou 4% o cellulose, 7% o xylan and 9% o lignin. The p ocess implies a la ge endo he mic e ec wi h DTA peaks unde ni ogen a mosphe e a 56, 80 and 90 °C, espec i ely ii) Be ween 200 and 400 °C p ocesses o depolyme iza ion, decomposi ion and ea angemen occu , in ol ing a conside able mass loss. Chemical bonds o he s a ing biomass b eak (p ima y eac ion), gene a ing a eac i e esidue ( eac i e cha ) plus liquid ( a ) and gaseous compounds, aking place unde ine and ai a mosphe e by di e en mechanisms, py olysis and oxida i e py olysis espec i ely. The las one explains an accele a ed decomposi ion in he p esence o oxygen, ela ed o oxida i e ype eac ions desc ibed o cellulose [39,40] and o lignin [41], which in u n esul s in he ad ancemen o he maximum a e o decomposi ion. A his s age, endo he mal ola iliza ion and exo he mal cha ing p ocesses a e esponsible o he esul ing ene gy balance [42,43]. Compa ing biomass componen s, xylan decomposes a lowe empe a u es, ollowed by cellulose, while lignin does i o e a wide ange o empe a u es, due o i s he e ogeneous s uc u e, comp ising a oma ic ings wi h mul iple b anches and unc ional g oups, whose bonds clea e along a wide ange o empe a u e. Two sub-s ages o exo he mic na u e can be obse ed in hemicellulose decomposi ion. The i s one, e y sligh wi h Tb a a ound 228 ºC, can be associa ed o he decomposi ion o esidual side chains, s ill p esen in he comme cial xylan, assigned o he decomposi ion o 4-O-me hyl glucu onic acid and ace yl g oups o xylan by Shen e al. [44], while he second sub-s age, loca ed a a ound 286 °C, is due o up u e o he xylan main chain. These sub-s ages a e ha dly dis inguishable in ai , whe e he decomposi ion p ocess occu s a much highe speed han in ine a mosphe e. Cellulose is he biomass componen which p esen s he maximum mass loss and maximum mass loss a e. Py olysis o cellulose can be desc ibed in a simpli ied o m as a mechanism o chain depolyme iza ion ia ansglycosyla ion eac ions [45]. One impo an endo he mic p ocess is obse ed in ni ogen a mosphe e a Tb = 338 °C, associa ed o he hea o e apo a ion o ola ile p oduc s gene a ed by he py olysis o he glycosyl uni s [40], and i ma ches wi h he collapse o he s uc u e and he comple e loss o he c ys alline s uc u e, as shown in Fig. 1A. In con as , a sha pe endo he m a Tb = 325 °C is obse ed in ai . This shi ing o lowe empe a u e is due o he oxida i e deg ada ion o cellulose which s a s a lowe empe a u es as a esul o adical au oxida ion mechanism ha occu s in he p esence o oxygen [39]. In ai , immedia ely a e he endo he mic p ocess, i can be dis inguished a la ge exo he mic one wi h Tb a 333 °C and maximum DTA peak a 350 °C, associa ed o he oxida ion o ola iles gene a ed in he p eceding endo he mic p ocess. Lignin is he one o he biomass componen s which expe iences he lowe mass loss and a he lowes a e. I s he mal decomposi ion, in which a leas h ee supe imposed p ocesses can be dis inguished a a ound 278, 315 and 358 ºC in ni ogen a mosphe e (as shown he inse o he Fig. 3B), p esen an o e all exo he mic p ocess o g ea e in ensi y in ai han in ni ogen. Fu he mo e, a e py olysis s age, lignin is he componen which p esen s he highes cha con en , abou a 60%, gene a ed 8 by condensa ion o he phenolic g oups du ing i s he mal deg ada ion unde ine a mosphe e. The G- ype lignin used in his wo k p obably con ibu es o a highe cha gene a ion due o he own na u e o he biomass polyme ha , in one hand, allows a high deg ee o c osslinking acili a ed by a educed con en in me hoxyl g oups, which in u n inc eases i s he mal s abili y, and ha , on he o he hand, due o he own cons i uen guaiacyl uni s, is p one o expe ience he mal condensa ion eac ions [46]. iii) A empe a u es highe han 350-400 ºC, he he mal beha io depends on he a mosphe e. Unde ine gas, he s uc u e o he p oduc s gene a ed in he p e ious s age s a s o change h ough clea age o bonds and ola iliza ion o he e oa oms, in ol ed in he condensa ion and eo ganiza ion p ocesses leading o he o ma ion o p oduc s wi h high ca bon con en . This ca boniza ion s age is accompanied by a small and g adual mass loss and by sligh ene gy changes, as i can be espec i ely obse ed on he TG and DTA plo s. In addi ion, he ola ilized compounds unde go seconda y he moly hic eac ions in gas phase, which do no a ec he mass o he emaining sample. The e ec on he DTA signal is weak since he he mocouples o he ins umen a e loca ed a he bo om o he sample and e e ence c ucibles, ela i ely dis an om whe e he e ol ed gases may eac . Fishe e al. [47] main ain ha seconda y eac ions can be he mally neu al, depending on he condi ions unde which he cha is gene a ed. In he case o cellulose, a small exo he mic e ec is obse ed nex o he 338°C endo he m, a a ound 400 °C, which is associa ed o he cha ing p ocess. Milosa lje ic e al. [48] claim ha he cha ing p ocess o he cellulose can be exo he mic (-2.6 kJ/g e e ed o esidual ca bon), endo he mic (0.86 kJ/g) o combina ion o bo h, depending on he p oduc s gene a ed. Simila ly, exo he mic e ec s a a ound 460 °C in he case o xylan, and a 480 and 650 °C in he case o lignin, a e associa ed o ca bonizing p ocesses. In he p esence o oxygen, he oxida ion o ola ile p oduc s (homogeneous eac ion) and combus ion o he cha (he e ogeneous eac ion) ake place in he ange be ween 350 and 500 °C, depending on he na u e o he lignocellulosic ma e ial. These exo he mic phenomena appea a lowe empe a u e and wi h highe in ensi y in xylan (390 ºC) han in cellulose (475 ºC). In addi ion, small exo he mic e ec s a e obse ed a highe empe a u es, which can be associa ed o esidual combus ion p ocesses, accompanied by a e y small mass loss, which will be discussed a e wa ds. Howe e , in he case o lignin, he combus ion s age does no cons i u e a no iceable phenomenon. I appea s o pass h ough se e al consecu i e mild s ages, wi h small mass loss a Tb = 380 and 456 °C. Mo eo e , unlike wha may be expec ed, he pe cen age o he inal esidue o lignin is sligh ly g ea e in ai han in ni ogen (63% > 58%). This can also be explained by se e al easons: a) ha a pa ial oxida ion has occu ed, gene a ing compounds o highe molecula weigh ; b) ha a s able cha has been o med due o he ele a ed con en in a oma ics [49]; c) ha he me als p esen in lignin oxidize, hus inc easing hei mass. Di Blasi [50] desc ibes many ac o s ha a ec he eac i i y o he cha which could explain he ele a ed pe cen age ha emain a e he mal decomposi ion o lignin unde ai . The da a p esen ed he e con i m ha he he mal s abili y o he biomass componen s dec eases in he ollowing o de : lignin, cellulose and hemicellulose. These di e en beha io s a e due o hei di e en chemical composi ion and s uc u e [51]. Hemicellulose has an amo phous s uc u e, o med by 9 a ious polysaccha ides wi h b anches o di e en composi ions, while cellulose is a polyme composed o linea chains o glucose, wi h a longe ange o de and a deg ee o polyme iza ion o en o one hund ed imes highe han ha o hemicellulose. Howe e , lignin is a highly c oss-linked polyme consis ing o highly b anched uni s o phenylp opane, which con e s a g ea he mal s abili y. 3.3. TG-DTG/DTA analysis o na u al and ho -washed sisal ibe s The mal deg ada ion o sisal and washed sisal has been s udied by means o simul aneous TG- DTA using pu ges o ai and ni ogen, bo h a 50 and 120 mL/min. Figu e 4 shows a compa ison o hei he mog a ime ic cu es in ai and ni ogen. Table 2 shows he mass loss on a d y basis, anges o empe a u e and maximum empe a u e o he mass loss a e (Tb) o each s age. Figu e 4. Compa ison o TG/DTG-DTA he mog ams o Na u al and Washed Sisal, unde 50 mL/min low a e. 16 polysaccha ides and cellulose. As in he case o mic oc ys alline cellulose, bu o a lesse ex en , i is obse ed a b oad band a abo e 2500 cm-1, which a e co ec ion o he spec um emains a a ound 3600-3000 cm-1, and a cha ac e is ic band a a ound 1047 cm-1, as indica ed by he a ows in Fig. 6. These bands a e a ibu ed o a glycol ype condensed phase, which was swep by he pu ge gas, as i will be discussed in he sec ion o gases e ol ed om cellulose. Fu he mo e, di e en ly han in he case o he pu e biopolyme s, a iple band cen e ed a 858 cm-1 is obse ed du ing he decomposi ion o sisal, deno ed by an as e isk in Fig. 6, which is assigned o hyd oxyace aldehyde, (OH)CH2C(O)H) [60]. Figu e 6. FTIR spec a o gases eleased ou du ing decomposi ion o Washed Sisal unde ni ogen a mosphe e, a TbPNC (295 ºC) and TbCEL (350 ºC). A ows deno e cha ac e is ic bands o he emi ed glycol phase, while he as e isk shows he ib a ion assigned o hyd oxyace aldehyde. I is known ha he abso bance a a speci ic wa enumbe is linea ly dependen on he concen a ion o gas, so he changes in abso bance e lec he endency o concen a ion a ia ion o he di e en gas species, p o ided ha he FTIR p ocedu es a e he same [35,42]. The mog ams o Fig. 7, 8, 9 and 10 show he e olu ion o he main in a ed bands associa ed wi h gaseous p oduc s o pu e biomassic componen s and o washed sisal, which will be discussed in mo e de ail in he ollowing sec ions. Figu e 11 shows he compa ison o he e olu ion o hese bands o he di e en componen s and he ho -washed sisal, unde wo low a es o ni ogen, 50 and 120 mL/min. Da a de i ed om he men ioned he mog ams a e collec ed in Tables 4 and 5, which show he ela i e in ensi ies o di e en ib a ions wi h espec o he maximum abso bance o CO2 emi ed in he py olysis s age o each biocomponen and washed sisal. 17 Table 4 Maximum in ensi ies o he main in a ed bands o ola iles emi ed by biomass componen s, unde 120 mL/min o ni ogen o ai pu ges, exp essed as ela i e pe cen age espec o he maximum abso bance o CO2 e ol ed in he espec i e py olysis s ages. Table 5 Maximun in ensi ies o he main in a ed bands o ola iles emi ed by Washed Sisal, unde di e en a mosphe e and low a e, exp essed as ela i e pe cen age espec o he maximum abso bance o CO2 e ol ed in he espec i e py olysis s ages. 18 3.4.1. Analysis o gases e ol ed om he pu e lignocellulosic componen s The analysis o gases e ol ed in he decomposi ion o pu e componen s in ai and ni ogen is desc ibed, p e iously o s udy he mo e complex case o sisal ibe . i) Xylan: In ni ogen a mosphe e, he ollowing species a e de ec ed a empe a u es below 350°C: CO2, CO, H2O, ace ic acid, o mic acid, me hanol, o maldehyde and a ke one, wi h ib a ions nea 1178 and 1734 cm-1 (Fig. 7). Figu e 7. E olu ion o he main in a ed bands o he ola iles emi ed du ing xylan decomposi ion, unde 120 mL/min ni ogen (a-c) and ai (d- ) pu ges, wi h he co esponding DTG cu e. Among he h ee biomass componen s conside ed, xylan is he one which p esen s he highes abso bance o CO2 and he highes CO/CO2 a io (Table 4). CO2 comes om he b eakdown o C=O and COOH g oups [42]. I would mainly come om deca boxyla ion eac ions o O-ace yl uni s acco ding o 19 Shen e al. [61]. In he p esen case ha con ibu ion mus be less impo an , since he s a ing ma e ial is a hemicellulose mos ly composed o xylose uni s. The CO o ma ion is a ibu ed o deca bonyla ion eac ions, speci ically o he b eakdown o COC and C=O [42], due o he decomposi ion o he ing- opened in e media e p oduc s [61]. The H2O emission du ing he py olysis s age is mainly de i ed om he hyd oxyl g oups o xylopy anose uni s. The emission o ace ic acid and me hanol is a ibu ed o he b eakdown o he O-ace yl and O-me hyl g oups o he esidual side chains, which a e e en p esen a e he ex ac ion and pu i ica ion o comme cial xylan. The p esence o a ious ke ones in he p oduc s esul ing om xylan py olysis was iden i ied using Py-GC/MS by Y. Wu e al. [62], who ound a 260 °C mainly 1-hyd oxy-2-bu anone, 1-hyd oxy-p opanone, and, o a lesse ex en , 4-hyd oxy-3-hexanone. In ni ogen a empe a u es abo e 400 °C ce ain gases gene a ed by seconda y eac ions o decomposi ion o anhyd ous suga s a e dis inguished. Thus, he CO2 emission con inues, bu sligh ly, and CH4 is de ec ed wi h a maximum nea 470 °C, close o he exo he mic e ec obse ed in he DTA o xylan (Table 1). CH4 is o med by b eaking R-CH3 bonds, which equi es ele a ed empe a u es due o i s high s abili y [61]. The CO2 emission s a s o inc ease a 550 °C, eaching a maximum a e a abou 700 °C, empe a u e a which a new CO emission begins. Du ing he py oly ic decomposi ion s age in ai he same gases a e de ec ed han when using ine a mosphe e. Thei p opo ions wi h espec o he CO2 emi ed a e qui e simila o hose ob ained unde ni ogen a mosphe e, wi h he excep ion o CO p incipally, whose emission diminishes in ai (Table 4). Fu he mo e, mass losses in ai and ni ogen a e simila , 59 and 63% espec i ely (Table 1). I can be explained h ough he phenomena ha a ec he ola iles emi ed in he decomposi ion s ep. In his case, he decomposi ion empe a u e o xylan (abou 290 °C) is su icien ly low so ha , i s ly, mos o he ola iles gene a ed a e no oxidized o CO o CO2 in ai , and on he o he hand, he sel -gene a ed CO would oxidize o CO2 in con ac wi h ai h ough he he modynamically a o ed eac ion: 2CO + O2  2CO2, which explains he lowe p opo ion o CO. The CO2 emission du ing he combus ion p ocess, be ween 300 – 420 °C, is in ense and p esen s an asymme ical shape which indica es ha he p ocess is igge ed by he empe a u e once he ene gy ba ie o he combus ion p ocess is eached. Then, he combus ion a e dec eases as he emaining amoun o sample dec eases. The co esponding DTG plo e lec s he same p ocess, bu he DTG peak is mo e esol ed due o he delay and mixing o componen s ha may occu du ing he gas low om he TGA u nace o he FTIR de ec o [63]. Addi ionally, small emissions o CO2, H2O and CO, associa ed wi h mild exo he mic e ec s a e obse ed a a ound 440 ºC and a 570 °C (Fig. 7D and Table 4). The exo he m a 440 ºC co esponds o he comple ion o he 300 – 420 ºC combus ion p ocess, which p oduced an o e hea ing on he p og ammed amp. Then, he p ocess was hal ed as he empe a u e dec eased o eco e he p og ammed alues, as i can be obse ed on he DTG plo . The exo he m a 570 ºC co esponds o combus ion o a mo e s able ca bon esidue, which esul ed om deg ada ion o he mos s able s uc u es. ii) Cellulose: In ni ogen, a empe a u es below 400 °C, CO2, CO, H2O, o maldehyde, o mic acid, and 2- bu anone a e de ec ed, besides a glycol condensa e (Fig. 8). This condensa e is assigned o 1,2- 20 p opanediol, in e media e eac ion p oduc , and o e hylene glycol, which was obse ed in py oly ic decomposi ion o cellulose [64,65]. I should be no ed ha he signal o o mic acid (1105 cm-1) is inc eased due o he glycol (1047 cm-1, Fig. 8B and 11C), which also hides me hanol (1032 cm-1). Thus, i is no possible o de e mine he p opo ion o each componen (Fig. 8A and 11F, and Table 4). Figu e 8. E olu ion o he main in a ed bands o he ola iles emi ed du ing mic oc ys alline cellulose decomposi ion, unde 120mL/min ni ogen (a-c) and ai (d- ) pu ges, wi h he co esponding DTG cu e. In con as o xylan mos o he main gaseous p oduc s o cellulose a e e ol ed oge he , in a na owe ange o empe a u es, wi h maximum close o 330 - 340 ºC, excep o CH4, which is emi ed a highe empe a u e. Cellulose is he componen ha p oduces he highes emissions o H2O and o ganic p oduc s wi h espec o he CO2 emi ed (Table 4). The b eaking o he glycosidic linkages and ing opening gene a e liquid and gaseous in e media es, whose iden i y and p opo ion a e dependen on he py olysis condi ions. Sha izadeh [66] and subsequen ly Shen e al. [67] collec ed and p oposed di e en ou es o o ma ion o CO2, CO, CH4, o maldehyde, me hanol and ace ic acid, among o he p oduc s o 21 decomposi ion o cellulose. In ou s udy ace ic acid has no been de ec ed, so he bands loca ed a 1178, 1772 and 1795 cm-1, shown in Fig. 8B, 8C, 11G, 11H and 11L a e associa ed o ace one, o maldehyde and o mic acid. Shen e al. [67] indica e ha he o ma ion o CO is a o ed by high empe a u es and long esidence imes, which explain ha CO o ma ion is mainly a ec ed by he seconda y eac ions, especially o aldehyde- ype low molecula weigh compounds. Ne e heless, CO2 is mainly o med in he ea ly s ages o he py olysis o cellulose. Li e al. [60] ound ha o maldehyde o ma ion is a o ed by a lowe pu ge low and by longe esidence imes, which would explain i s o ma ion h ough seconda y eac ions. They explain ha a high empe a u es, nea 550 °C, o maldehyde decomposes o gene a e CO and H2, which is consis en wi h he obse a ion o Shen e al. Va ious ke ones ha e been p oposed and de ec ed a he p ima y decomposi ion and py olysis o cellulose. Thus, 2,3-bu anedione is among he p oduc s o py olysis o le oglucosan, a majo decomposi ion p oduc o cellulose [68]. 2-bu anone, de ec ed he e a empe a u es below 400 ºC, has also been de ec ed by Py-GC/MS analysis [62]. Acco ding o o he epo s, i may come om deca boxyla ion o le ulinic acid [66]. Radlein e al. [69] ound o mic acid in he py olysis p oduc s o di e en celluloses, which would be o igina ed om he decomposi ion o le oglucosan acco ding o Hosoya e al. [70]. In ni ogen, a empe a u es abo e 450 °C, CO2, CO and CH4 a e de ec ed, esul ing om side eac ions. In his s age, maximum emission o CO2 and CH4 is obse ed a abou 530 ºC, while he maxima o CO2 and CO appea a 650°C, as shown in Fig. 8A, 11A, 11E and 11I. The p oduc s de ec ed du ing he decomposi ion in ai o cellulose a e he same han unde ine a mosphe e. Howe e , compa ed o ha o CO2, he emission o o ganic compounds and H2O has dec eased signi ican ly in ai , while he p opo ion o CO is main ained in almos he same p opo ion as in ni ogen (Table 4). Since he mass loss a e py olysis unde ai and ni ogen a e simila , he explana ion lies p ima ily in he changes su e ed by he emi ed gaseous compound. In an oxidizing a mosphe e, when he empe a u e is high enough, i could happen ha a he same ins an in which he decomposi ion/py olysis o he cellulose occu s, he ola ile o ganics gene a ed oxidize, which would be a o ed by he p esence o s eam. Thus, a gasi ica ion eac ion explains he lowe p opo ion o H2O de ec ed. On he o he hand, he consump ion o CO acco ding o he eac ion 2CO + O2  2CO2 is less he modynamically a o ed, which explains i s highe ela i e p opo ion unde ai a mosphe e. The e olu ion o CO2 in he combus ion p ocess comp ises a wide in e al o empe a u es, om 370 o 700 ºC and maximum a 470 ºC, whe e CO and H2O a e de ec ed. Exo he mic e ec s, a ound 540 and 600 ºC, de ec ed in he DTA o he cellulose, co espond mainly o he emission o CO2, as obse ed in Fig. 8B, associa ed o addi ional combus ion phenomena. iii) Lignin: Unlike cellulose and hemicellulose, deg ada ion o lignin unde ine a mosphe e co e s a e y wide ange o empe a u e, om 130 o 500 °C (Fig. 9). The gases de ec ed in his in e al a e CO2, CO, H2O, CH4 and CH3OH. No o he o ganic compounds we e de ec ed. 22 Figu e 9. E olu ion o he main in a ed bands o he ola iles emi ed du ing lignin decomposi ion unde 120 mL/min ni ogen pu ge, wi h he co esponding DTG cu e. O he h ee biomass componen s, lignin is he one which p oduces he highes p opo ion o CH4 and CH3OH and he lowes o CO wi h espec o he maximum abso bance o CO2 emi ed a he py olysis s age (Table 4). A empe a u es below 500 °C, he emission o H2O can be associa ed o alipha ic and a oma ic hyd oxyl g oups, and he emission o CO2 and CO o c acking and e o ming o he ca boxyl, ca bonyl and es e g oups, p esen in he side chains o he phenylp opane uni s [41]. Thei e olu ion abo e 500 °C is associa ed wi h he seconda y py olysis o ola iles. In pa icula , se e al au ho s claim ha CO is o igina ed a he b eaking o wo di e en ypes o e he bonds: a low empe a u e he e he bonds be ween di e en lignin subuni s, whils a high empe a u e dia yl e he g oups a e in ol ed [71,72]. Below 400°C, he e olu ion o CH3OH and CH4 is mainly associa ed wi h he clea age and deme hyla ion o me hoxyl g oups (-O-CH3) p esen in lignin [42,73]. Me hanol 23 o ma ion may also be con ibu ed by he -CH2OH g oups loca ed on he ca bon o he alkyl la e al chain o he phenylp opane uni s [41,71]. Abo e 400°C me hane emission con inues, eaching a second maximum a 525 °C, mo e in ense han he i s one. I is p oduced by side eac ions, and also by p ima y eac ions o condensa ion and eo ganiza ion o he cha . A hese high empe a u es he up u e o he a oma ic ings s a s, gene a ing me hyl adicals and me hylene g oups, and inally o ming me hane [41] [71]. F om 600 °C me hane signal begins o disappea , because i b eaks down in o H2 and C [72]. The composi ion and e olu ion o gases emi ed by lignin in his s udy u n ou o be i ually iden ical o hose o alkaline lignin s udied by Yang e al. [42]. O he s udies desc ibe he emission o o he o ganic compounds, no de ec ed in he p esen wo k, such as o maldehyde, o mic acid and phenols o igina ed by he up u e o phenylp opane uni s. The ype o lignin and he ex ac ion me hod would explain he di e ences in he composi ion o he emi ed gases. The basic eason why a smalle a ie y o ola ile o ganic compounds is de ec ed is ha cha o ma ion eac ions a e a o ed agains he gene a ion o ola ile compounds. G ype lignins, as he one used in his s udy a e p one o unde go condensa ion and coupling eac ions be ween phenylp opane uni s, causing hem o gene a e mo e amoun o cha and emi less amoun o phenolic de i a i es [41], which would be cha ac e ized by in ense ib a ions a ound 1500, 1250 and 1100 cm-1 [73]. On he o he hand, lignins con aining li le sul u , as he one s udied, a e mo e a ec ed by his ype o condensa ion eac ions. Fenne and Lepha d [73] indica e ha he inco po a ion o sul u acili a es he agmen a ion o phenylp opane uni s and dec eases i s condensa ion. 3.4.2. Analysis o he gases emi ed by ho -washed sisal ibe The e olu ion wi h empe a u e o he main in a ed bands o he gases emi ed du ing he decomposi ion unde ine and oxidan a mosphe e o washed sisal ibe s is shown in Fig. 10. The gaseous componen s obse ed a di e en s ages a e summa ized below: i) A empe a u es below 150 °C, H2O emission is mainly om o he mois u e in he ibe s. ii) In he i s sub-s ep, below 300 °C, he gases CO, CO2 and H2O come om he decomposi ion o sisal lignin and mainly om hemicelluloses and o he NCP s ill p esen in he washed sisal. The emission o ace ic acid, o mic acid, me hanol, o maldehyde, and some ke one, which may be en a i ely assigned o 2-bu anone, is mainly due o decomposi ion o hemicellulose. Sisal lignin should also con ibu e o he emission o me hanol and ace ic acid as sugges ed by o he s udies wi h HGS ype lignins [41]. Ne e heless, in s udies wi h wood, he emission o o mic acid a his s age has been associa ed only wi h he hemicellulose and no wi h lignin [74]. No ably, he p opo ion o ace ic acid emi ed (Table 5) is p opo ionally much highe han ha obse ed in he pu e xylan (Table 4). This is because sisal hemicelluloses con ain mo e side chains wi h ace yl g oups han he xylan s udied. 24 Figu e 10. E olu ion o he main in a ed bands o he ola iles emi ed du ing mic oc ys alline cellulose decomposi ion, unde 120 mL/min ni ogen (a-c) and ai (d- ) pu ges, wi h he co esponding DTG cu e. In he second sub-s age, be ween 300 and 400 °C, he emi ed compounds a e mainly due o he py olysis o sisal cellulose: CO, CO2, H2O, glycol, o maldehyde, me hanol, o mic acid and ke one, assigned en a i ely o 2-bu anone, as well as hyd oxyace aldehyde which s a s o be gene a ed in he p e ious s age. The la e compound may appea as a p oduc o decomposi ion o cellulose by a compe i i e ou e o he o ma ion o le oglucosan, he main componen o cellulose a [60,67]. The low amoun o hyd oxyace aldehyde emi ed du ing he py olysis o sisal cellulose sugges s a ou e o decomposi ion di e en om ha o he mic oc ys alline cellulose s udied in his wo k, in which his compound was no obse ed. The maximum emissions o all hese compounds ma ch wi h he maximum mass loss a e, as shown in Fig. 10. Ace ic acid bands dec ease e y quickly a he end o he i s sub- s age. Thus, he ib a ions o he ca bonyl g oups ha emain in he second sub-s ep can be only assigned o o maldehyde, o mic and 2-bu anone (Fig. 10C). To a lesse ex en sisal lignin con ibu e o he 25 emission o CO, CO2 and H2O, and p obably also o me hanol, o mic acid and ace ic acid. The emission o CH4 in ni ogen s a s a a ound 360°C (Fig. 10A and 11I). I is mainly associa ed wi h he up u e o he me hoxyl g oups o lignin, bu i s signal is weak, being obscu ed by he C-H s e ching ib a ions o o he o ganic compounds. Addi ionally, a he end o his sub-s age in ai , he combus ion o sisal hemicellulose and o he NCP occu s, emi ing CO2 and H2O (Fig. 10D). Figu e 11. Compa ison o he e olu ion o he main in a ed bands o gaseous compounds e ol ed unde ni ogen a mosphe e, du ing decomposi ion o biomass componen s unde 120 mL/min pu ges, and decomposi ion o Washed Sisal unde pu ges o 50 and 120 mL/min. The as e isk o Fig. 11i py og am on he i s s age o CH4 e olu ion co esponds o backg ound signal and no o me hane ib a ion. iii) Bo h in ni ogen and in ai , emission o mos o ganic compounds disappea s a empe a u es abo e 400 °C. Unde ine a mosphe e, he e olu ion o gases is simila o ha o pu e cellulose, as i is he majo i y componen . Emission o CO2 and CO s a again a 400 ºC due o side eac ions, being he g ow h o CO in ensi y much slowe . Emission o CH4 is also obse ed o inc ease a 400 ºC, whose signal emains a a nea ly cons an alue up o 580 °C, when i began o dec ease and a new inc ease o