Hydrothermal CO2 reduction using biomass derivatives as reductants

Ti le: Hyd o he mal CO2 educ ion using biomass de i a i es as educ an s Au o s: M. Andé ez-Fe nándeza, E. Pé ezb, A. Ma ína, M.D. Be mejo *a A ilia ions: aHigh P essu e P ocess G oup, Depa men o Chemical Enginee ing and En i onmen al Technology, Uni e si y o Valladolid (Spain) bTERMOCAL Resea ch G oup, The modynamics and Calib a ion, Uni e si y o Valladolid (Spain) Hyd o he mal CO2 educ ion using biomass de i a i es as educ an s M. Andé ez-Fe nándeza, E. Pé ez, A. Ma ína, M.D. Be mejo *a aHigh P essu e P ocess G oup, Depa men o Chemical Enginee ing and En i onmen al Technology, Uni e si y o Valladolid (Spain) bTERMOCALResea ch G oup, The modynamics and Calib a ion, Uni e si y o Valladolid (Spain) *Email: [email p o ec ed]a.es Abs ac A wide ange o o ganic subs ances, po en ially de i ed om biomass, we e es ed as educ an s o CO2 (added in o m o sodium bica bona e) in hyd o he mal media. The eac ions we e ca ied ou in ba ch eac o s a 300 ºC and 3h. All he subs ances educed CO2 o o mic acid in yields up o 6065%. Fo e hanol and e hylenglycol, addi ional condi ions we e es ed o s udy he dependence o he eac ion wi h ime and empe a u e. These esul s ag ee o he mechanisms p oposed in li e a u e ha sugges ed ha educ ion is ca ied ou by a p ima y o a seconda y alcohol. Howe e , some subs ances no con aining hese g oups ga e signi ican yields o o mic acid so new mechanisms we e p oposed o explain hem. Ou o all he compounds es ed, glucose ga e he highes yield o o mic acid, p obably due o i s pa icula eac ion pa hways a he s udied condi ions. Keywo ds: NaHCO3; glucose; HTW; o mic acid; hyd ogena ion 1. In oduc ion Ca bon dioxide is one o he mos conce ning g eenhouse gasses because o i s inc easing a mosphe ic le el due o an h opogenic ac i i y, e.g. ossil combus ion and indus ial p ocesses. Di e en me hods ha e been p oposed o diminish he a mosphe ic CO2 by cap u ing a i s sou ce and s o ing i . As his echnology de elops, a g ea a ailabili y o CO2 is expec ed, hen new oppo uni ies a ise o alo ise i by ans o ming i in o use ul chemicals. This a ac i e app oach is highly desi able since, apa om he economic bene i s, i would b ing a posi i e en i onmen al impac because o he einco po a ion o CO2 o he ca bon cycle [1, 2]. Among he di e en p ocesses o i s con e sion in o chemicals, pho ochemical educ ion, elec ochemical educ ion o hyd ogena ion o CO2 a e he mos p omising s a egies [3]. Howe e , CO2 owns a g ea he modynamic s abili y, which equi es o i s ans o ma ion high ex e nal ene gy subs ances, e. g. hyd ogen, unsa u a ed compounds, o ganome allic compounds and small-membe ed ing compounds [4]. Ano he op ion o o e coming he high s abili y o CO2 is using eac ion in hyd o he mal media [4-7] i.e. using liquid wa e as sol en a high empe a u es.. Ano he ad an age o hyd o he mal educ ion o CO2 in hyd o he mal media is a oiding he use o gaseous H2 as educ an . H2, is s ill a de i a i e o ossil uels so he en i onmen al bene i would be los . Mo eo e , H2 u iliza ion comp ises sa e y issues due o i s lammabili y and eac i i y. The main p oduc ob ained om hyd o he mal educ ion o CO2 is o mic acid [4-7], ha is an impo an chemical eeds ock wi h a wide ange o indus ial applica ions such as s o ing hyd ogen and o elease i o powe uel cells and ob ain ene gy. [8-10] Indus ial o mic acid p oduc ion by CO2 educ ion would be an a ac i e app oach o alo ise his gas a he same ime ha i s emissions a e educed. Ze o- alen me als ha e p oposed as educ an s in hyd o he mal media [7, 11-14]. Howe e , despi e o ob aining accep able yields, he me als should be educed again in o de o ecycle hem. The e o e, he e is an inc easing in e es in ob aining an al e na i e CO2 educ an . Biomass is a wo ld-wide sp ead, sus ainable and inexpensi e eeds ock and, some imes, conside ed as a esidue [15]. Due o he deple ion o ossil uel ese es and he en i onmen al p oblems a ached o hei consump ion, se e al echnologies a e being de eloped in o de o ob ain alue-added chemicals and bio uels [15-17]. Biomass is mainly composed o cellulose, hemicellulose and lignin. These biopolyme s can be isola ed and depolyme ized in o i s monome ic uni s such as monosaccha ides and phenols. Hyd o he mal ou es ha e been in ensi ely s udied o p ocess biomass due o he ou s anding p ope ies ha ho comp essed wa e exhibi s such as lowe dielec ic cons an and highe ion p oduc han ambien liquid wa e [18-21]. Wa e can ac as acidic and basic ca alys as well as en i onmen ally benign sol en . Following his app oach, biomass has been con e ed in o a wide ange o in e media es and/o aluable p oduc s, such as lac ic acid, ace ic acid, 5-hyd ozyme hyl u u al (5-HMF), phenol and anillin, among o he s [22-27], and many o hese eac ions in ol e oxida ions. The e o e, he combina ion o bo h hyd o he mal p ocesses, biomass con e sion and CO2 educ ion in one-po eac ion, would p o ide an a ac i e and sus ainable app oach o he alo isa ion o lignocellulosic esidues and he dec ease o CO2 a mosphe ic emissions by in eg a ing his p ocess in he main CO2 p oduce s, such as powe and indus ial ac o ies. Despi e he undeniable ad an ages o his app oach, he e a e no many epo s in li e a u e abou i . Mos o hem a e ocused on educ ion o bica bona e wi h isop opanol and glyce ol [4, 6, 28, 29]. Acco ding o hese epo s, p ima y o seconda y -OH g oups ac s as CO2 educ an s and a e oxidised o he co esponding aldehydes o ke ones. Howe e , addi ional mechanisms a e p esen because he oxida ion o glyce ine yields lac ic acid. Jin e al also demons a ed ha glucose can ac as educing agen o con e bica bona e as sou ce o CO2 in o o mic acid [30] bu he mechanisms could no be esol ed. Su e al. [31] es ed some o he hyd oxylic compounds a 240 ºC wi h he e ogeneous Pd-based ca alys s. They also ob ained ha along wi h CO2 educ ion, p ima y alcohols a e oxidised o he co esponding ca boxylic acid, seconda y alcohols o he ke one, e ia y alcohols did no eac and polyols yielded lac ic acid. Howe e , he e a e s ill a lo o model compounds de i ed om lignocellulosic biomass ha ha e no been es ed. In his wo k, he abili y o di e se compounds o educe sodium bica bona e (NaHCO3) as sou ce o CO2 in hyd o he mal medium is in es iga ed. Pa icula ly monosaccha ides, disaccha ides and lignin-de i ed phenols, bu simple C2 and C3 alcohols and ke ones a e also ied o be e unde s and he eac ions pa hways unde lying. The main aim is o iden i y a po en ial compounds ha makes possible o in eg a e bo h CO2 educ ion and biomass con e sion by hyd o he mal p ocesses. 2. Ma e ials and me hods 2.1. Ma e ials NaHCO3 (100%) was acqui ed om COFARCAS (Spain). Glyce ol (99.5%), n- p opanol (>99.7%), glyce aldehyde (90%), lac ic acid (≥85%),py u aldehyde (40%), 5- HMF (99%), u u al (99%), uc ose (99%), D-(+)-glucose (100%),D-(+)-cellobiose (≥98%), eso cinol (99%), ca echol (99%), guaiacol (>99%) and anillin (99%) we e pu chased om Sigma Ald ich (Spain). E hanol (E OH, 99.5%), ace one (99.5%), isop opanol (iP OH, 99.9%), suc ose (100%) and sul u ic acid (H2SO4, 96%) we e ob ained om Pan eac (Spain), while p opanaldehyde (99%) and phenol (99%) we e acqui ed om Ac os O ganics. E hylenglycol was pu chased om Fluka. The eac an s we e used wi hou u he ea men o pu i ica ion. 2.2. Expe imen al p ocedu e Solu ions in ul apu e (MilliQ) wa e o each o ganic compound we e p epa ed wi h a concen a ion o 0.05M and a mola a io o o ganic/NaHCO3 equal o 1:10 (NaHCO3 concen a ion equal o 0.50M).Expe imen s we e ca ied ou in ba ch eac o s (leng h: 12 cm; o.d.: ½”, wi h 1 mm o hickness) made o SS 316 s ainless s eel wi h an in e nal olume o 15.6 mL. The NaHCO3 and educ an solu ions we e loaded in he eac o , illing he 50% o i s o al olume. The eac o was placed hen in an elec ic o en p e iously hea ed o he desi ed eac ion empe a u e300ºC. P essu e gene a ed should be 85.9 ba aking as e e ence he p ope ies o pu e wa e .[32] A e he equi ed180 min eac ion ime, he eac o was apidly quenched in a cold wa e /e hylene glycol ba h and liquid samples we e collec ed. In he cases o E OH and e hylene glycol, di e en eac ion imes (30, 90 and 180 minu es) and eac ion empe a u es (250ºC and 300ºC) we e es ed, as well. Reac ions we e pe o med a leas wice o assu e ep oducibili y. In o de o check he co ec closu e o he eac o and he no exis ence o leaks, he eac o s we e weigh ed be o e and a e he eac ion. 2.3. P oduc analysis A e being il e ed h ough a 0.45mm il e , liquid samples we e analysed by HPLC (Wa e s, Alliance sepa a ion module e2695) using an Aminex 87H (Bio-Rad) column and wo de ec o s: RI (Wa e s, 2414 module) and UV (210 nm, Wa e s, 2998 module). The mobile phase was 5 mM H2SO4 wi h a low a e o 0.6 mL/min. The empe a u es o he column and he de ec o we e 60ºC and 30ºC, espec i ely. The yield o o mic acid was calcula ed as shown below: 𝑌 𝐹𝐹 = 𝐶𝐹𝐹,𝑓 𝐶𝑜𝑜𝑜,𝑖 × 100 (1) Whe e CFA, is he mola concen a ion o o mic acid ob ained a he end o he eac ion and Co g,i is he ini ial mola concen a ion o he co esponding solu ion o he o ganic compound. 3. Resul s and discussion The po en ial o educ ion o di e en o ganic subs ances de i ed om lignocellulosic biomass was es ed. The educ an s we e classi ied in h ee ca ego ies: saccha ides; phenolic de i a i es, which a e model compounds om lignin depolyme iza ion and simple molecules such C2, C3 alcohols and aldehydes ha may be ob ained om hyd o he mal decomposi ion o he o me compounds [18, 21, 26]. The yields o o mic acid ob ained om he di e en solu ions o o ganic compounds a e shown in Figu e 1. Figu e 1. Yield o o mic acid (%) om di e en o ganic compounds. In o de o e i y ha o mic acid p oduc ion was whe he due o he CO2 hyd o he mal educ ion, con ol eac ions a 300ºC o 180 min we e ca ied ou wi hou he addi ion o NaHCO3 bu in p esence o NaOH, as he o me can pa ially decompose in o he la e and his can ca alyse o mic acid o ma ion om o ganic ma e . In all hese es s, NaOH 0.15 M was added. No o mic acid o negligible amoun was de ec ed. Addi ional es s we e ca ied ou using only NaHCO3 and wa e a 300ºC du ing 180 min o check whe he o mic acid is p oduced wi hou o an o ganic educ an . In hese es s, again no o mic acid was de ec ed. Thus, o mic acid was con i med o be p oduced due o NaHCO3 educ ion in hyd o he mal p ocesses a 300ºC. In mos o he cases, oxidized by-p oduc s ob ained om he s a ing o ganic molecule we e iden i ied. The p esence o hese molecules gi es clues o de e mine possible eac ion mechanisms. Table 1 ga he s he main by-p oduc s ob ained o he di e en eac ions along wi h p oposed eac ion pa hways. 0 10 20 30 40 50 60 70 80 Yield FA (%) C2 and C3 molecules Saccha ides Phenolic de i a es The di e ences obse ed can hen be ela ed o he ac ha glucose spli s in o h ee C2 molecules (glycolaldehyde) whe eas uc ose spli s in o wo C3 molecules (glyce aldehyde and py u aldehyde) [26, 35-38], o dehyd a es o one 5-HMF molecule. I can be no iced ha h ee imes he yield ob ained o glycolaldehyde is signi ican ly highe han he sum o he yields o glyce aldehyde and py u aldehyde (Figu e 1). Mo e su p ising a e he yields o o mic acid when using he disaccha ides suc ose (glucose – uc ose) and cellobiose (glucose – glucose), 60% and 35% espec i ely, since hey eadily hyd olyze in HTW o hei cons i uen monosaccha ides. The explana ion may be a di e en he mal s abili y o ei he suc ose o cellobiose. In he case o saccha ides de i a i es u u al and 5-HMF, bo h compounds eached a yield o o mic acid o 15%. The main by-p oduc ob ained om monosaccha ides and disaccha ides we e ace ic and lac ic acids, in di e en p opo ions. Tha is in ag eemen o he mechanisms p e iously desc ibed [26, 35, 40]. Howe e , he simila i ies in he composi ion o he by-p oduc s do no allow disce ning di e ences in he eac ion pa hway. 3.3.- Lignin de i a i es OCH 3 Guaiacol OH OH Vanillin OCH 3 O Phenolic de i a es OH Ca echol OH OH Reso cinol OH OH Phenol Figu e 5. Molecula s uc u e o phenolic de i a i es. When using phenolic lignin model compounds, a wide ange o yields o o mic acid was ob ained. The molecula s uc u es o he di e en es ed compounds a e shown in Figu e 5. The lowes yield was ob ained om phenol, achie ing only 2%. Highe yields o o mic acid we e ob ained by using eso cinol and ca echol solu ions (19% and 9%, espec i ely). The highes yield o o mic acid was 51% when using anillin solu ion, ollowed by guaiacol (24%). Su e al. [31] also epo ed low eac i i y o phenol, which is expec able since he educ ion o CO2 is pe o med by p ima y o seconda y -OH g oups [4, 6]. Howe e , he ela i ely high yields o o mic acid achie ed by he es o he a oma ic compounds es ed a e unexpec ed. Figu e 6. P oposed mechanism o he educ ion o bica bona e o o ma e by ca echol. By-p oduc s om eso cinol we e p opanaldehyde and/o ace one and ace ic acid whe eas ha om ca echol was lac ic acid. These p oduc s can only be o med om a ing-opening mechanism. I should also be men ioned ha he solu ions ob ained om a oma ic compounds we e eddish-b own colou ed. In HTW, he a oma ic ings can be oxidized o quinone g oups, which a e highly colou ed and end o unde go ing opening in wa e a high empe a u es [41]. A p oposed mechanism o CO2 educ ion by ca echol is depic ed in Figu e 6. Oxida ion akes place h ough a cyclic ansi ion s a e simila o ha epo ed in he li e a u e wi h he excep ion ha he H- o be ans e ed is on he hyd oxyl g oup a o ho posi ion. The main by-p oduc s om guaiacol a e me hanol and ca echol mos likely p oduced om a hyd olysis o he me hoxy g oup. Howe e , yield o o mic acid om guaiacol is highe han om ca echol, indica ing ha me hanol has also a educing ole, and i may p oduce ex a amoun o o mic acid. On he o he hand, oxida ion o anillin o anillic acid can occu in he p esence o oxidan s. Addi ionally, a high empe a u es (>280 ºC), he la e eadily deca boxyla es o guaiacol [41-43]. Tha is he eason why he same by-p oduc s a e obse ed when s a ing om anillin o guaiacol. Howe e , yields o o mic acid a e highe han o he o me , which sugges s again ha he oxida ion o he ca bonyl accompanies he educ ion o CO2. Tha can be possible conside ing ha anillin can unde go a Cannizza o disp opo iona ion o anillic acid and anillic alcohol [44] and he la e educes CO2. OH OH OMe Cannizza o . Vanillin CH 2 OH OH OMe COOH OH OMe + HCO 3 HCOO Vanillic alcohol Vanillic acid Deca - boxyla ion OH OMe Guaiacol Hyd olysis OH OH Ca echol HCO 3 - HCOO - O O Quinone Ring opening +MeOH HCO 3 - HCOO - CH 2 O Figu e 7. P oposed sequen ial mechanism o HCO3- educ ion wi h anillin, guaiacol and ca echol. Figu e 7 shows a p oposed sequen ial mechanism o oxida ion eac ion. Each o hese oxida ions can be accompanied o a HCOO- educ ion, which explains ha he YFA also ollow a dec easing end. Mo eo e , de ec ed in e media e compounds (me hanol and ca hecol) ag ee wi h he p oposed pa hway. Código de campo cambiado 3.4.- Dependence o he eac ion ime and empe a u e The dependence o o mic acid yield wi h eac ion ime and empe a u e o e hanol and e hylene glycol is shown in Figu e 8. The e olu ion o he eac ion is posi i e wi h ime o e e y condi ion es ed. A 300 ºC he eac ion akes place mainly du ing he i s 90 minu es while he a e dec eases signi ican ly a longe imes. Tempe a u e plays a mo e de e minan ole in he eac ion a e as yields a e much lowe a 250 ºC. This beha iou has been obse ed in p e ious epo s o CO2 educ ion by glyce ol [4] and isop opanol [6] indica ing hey p oceed h ough simila eac ion mechanisms. Reac ions o hese compounds a highe eac ion empe a u e has no been es ed since inc easing he eac ion empe a u e u he han 300ºC would no p o ide be e yields o o mic acid. P e ious epo s showed he deg ada ion o o mic acid a highe empe a u e han 300ºC a sho eac ion imes (<1m) [39] and, he e o e, highe empe a u e would lead o a as e disappea ance o i in he medium. Figu e 8. Dependence o o mic acid yield wi h ime and empe a u e o he educ ion o NaHCO3 using e hanol and e hylene glycol as educ an s. (  ): E hylene glycol, 300 ºC; (  ) E hylene glycol, 250 ºC; (  ): E hanol, 300 ºC; (  ): E hanol, 250 ºC. 3.5.- In luence o NaOH concen a ion 4. Conclusions In his wo k, he hyd o he mal educ ion o NaHCO3 o o mic acid was achie ed by he con e sion o di e en lignocellulosic biomass model compounds. Yields o o mic acid up o 65% we e ob ained by using he di e en o ganic solu ions as educ an s a e 180 min a 300ºC, being he yield o o mic acid om glucose he highes yield achie ed. Al hough he de ailed eac ion mechanism is s ill unknown and he ope a ional pa ame e s should be op imized, his s udy demons a es he possibili y o combine he CO2 hyd o he mal educ ion wi h he lignocellulosic biomass con e sion o ob ain alue-added chemicals and ene gy om enewable sou ces. 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