Applications of supercritical technologies to CO2 reduction: Catalyst development and process intensification

1 Applica ions o supe c i ical echnologies o CO2 cap u e and u iliza ion Ángel Ma ín*, Alexande Na a e e, Ma ía Dolo es Be mejo High P essu e P ocesses G oup, Depa men o Chemical Enginee ing and En i onmen al Technology, Uni e si y o Valladolid C/D . Me gelina, s/n, 47011 Valladolid, Spain Tel:+34 983184077. e-mail: [email p o ec ed] (Á. Ma ín) Abs ac Keywo ds: Ca bon dioxide cap u e and u iliza ion, supe c i ical wa e , pho oca alysis, nanoma e ial, ae ogel, p ocess in ensi ica ion 1. In oduc ion The e is an inc easing global conce n abou he nega i e e ec s o clima e change, which a e ela ed o he inc easing concen a ion o g eenhouse gases in he a mosphe e, and in pa icula CO2, due o he use o ossil uels. The ecen Pa is Ag eemen , se in 2015, has as i s main objec i e he educ ion o CO2 emissions in o de o a oid a global empe a u e inc ease highe han 2ºC. To a ain his objec i e, i is necessa y o g adually eplace ossil uels by enewable ene gies wi h ze o CO2 emissions, bu i can be expec ed ha ossil uels will emain as he main ene gy sou ce o many yea s, pa icula ly o p oduc ion o elec ici y and as uels o ehicles. The e o e, o he solu ions such as ca bon cap u e and s o age (CCS) echnologies mus be conside ed, because hey would enable a signi ican educ ion o he CO2 emissions o he mal powe plan s o chemical indus ies such as he indus ies o p oduc ion o ammonia, hyd ogen and cemen [1]. Di e en CO2 cap u e echnologies ha e been esea ched. Among hem, he mos de eloped echnology a indus ial le el is he abso p ion using aqueous solu ions o alkanolamines, a me hod ha has been used since he 30s o emo e CO2 and o he acid gases om na u al gas [2]. Alkanolamines such as monoe hanolamine (MEA), die hanolamine (DEA) amd 2 me hyldie hanolamine (MDEA) selec i ely and e e sibly eac wi h CO2 o ming ca bama es, and can cap u e 0.5-0.7 mol CO2/mol amine [3,4]. Howe e , he hea o abso p ion is high, implying ha he amine egene a ion s ep, which is ca ied ou by deso p ion a empe a u es o 100 – 150ºC, has high ene gy consump ion. Al e na i e abso p ion p ocesses o CO2 cap u e p ocesses ha e been de eloped, such as he “Chilled Ammonia” p ocess p esen ed by he F ench company Als om [5]. This me hod uses an aqueous ammonia solu ion a low empe a u e (2-10ºC), whe e CO2 is abso bed as ammonia ca bona e, bica bona e and ca bama e. Typically, 0.33-0.67 molCO2/molNH3 can be abso bed. As in he case o amines, egene a ion is ca ied ou by deso p ion a empe a u es o 100-150ºC and p essu es o 2-136 ba . Compa ed o he case o amines, he hea o deso p ion om he ammonia solu ion is much lowe , which implies ha he ene gy cos s o he egene a ion also a e lowe . The disad an age o his me hod is he need o big amoun s o e ige a ion wa e . Fo his eason, in gene al his me hod is in only conside ed o acili ies loca ed nea he sea. Besides hese abso p ion me hods, o he CO2 cap u e echnologies a e being de eloped, such as adso p ion o memb ane sepa a ion [6]. In gene al, all hese p ocesses a e cos ly, because in addi ion o he cos o he CO2 sepa a ion p ocess, he cos o comp essing and anspo ing CO2 o he s o age si e is also high. In ac , i has been es ima ed ha e en employing a o able echnologies o CO2 sepa a ion such as oxy- uel combus ion, he inco po a ion o a CO2 cap u e and s o age uni in a he mal powe plan implies a signi ican penal y on he global e iciency o he plan , which is es ima ed as 2 – 3 % o he global e iciency o he powe plan [7], a penal y ha is assumed es ima ing ha he bene i s o educed CO2 emissions ou weigh i . The e o e, i can be concluded ha CO2 cap u e is a complex and cos ly p ocess, which jus i ies he gene al app ecia ion o CO2 as a p oblema ic esidue ha causes conside able economic p oblems and en i onmen al ha ms. In con as , looking in o na u e, ca bon dioxide is he basic chemical esou ce used by plan s. By ans o ming i in o chemical uels, hey s o e 3 su plus sola ene gy o pe iods o need. By con e ing i in o complex o ganic compounds, hey ob ain he building blocks ha hey need o g ow and li e. A simila app oach can be ollowed in he indus y, con e ing CO2 in aluable compounds ha can gene a e a p o i ins ead o simply s o ing i [8]. Fu he mo e, i mus be conside ed ha ossil uels no only p o ide mos o he ene gy we use, bu also a e he main aw ma e ial used by he chemical indus y. The e o e, i is necessa y o ind an al e na i e o base compounds o he chemical indus y ha can g adually eplace he compounds ob ained om ossil uels as hey un ou . CO2 u iliza ion echnologies can also con ibu e o his pu pose, because hey can p oduce impo an pla o m chemicals such as o mic acid o me hanol, and CO2 is a widesp ead esou ce ha can con ibu e o he c ea ion o a no el decen alized indus ial s uc u e ha p oduces “jus enough” [9]. Following his app oach, di e en Ca bon Cap u e and U iliza ion (CCU) echnologies a e being esea ched [10]. Among o he op ions, ou key CO2 con e sion echnologies a e being in ensi ely s udied: (1) ca aly ic copolyme iza ion o CO2, pa icula ly wi h highly eac i e epoxides o he syn hesis o polyca bona es [11]; (2) he mo-ca aly ic CO2 con e sion p ocesses [12, 13]; (3) CO2 ixa ion in pho o-bio eac o s [14]; and (4) pho oca aly ic CO2 con e sion p ocesses, also known as “a i icial pho osyn hesis” me hods [15]. The applica ion o supe c i ical luids can con ibu e o he de elopmen o hese echnologies in se e al aspec s such as ma e ials design, p ocess in ensi ica ion and eac ion enginee ing. The objec i e o his a icle is o p esen an o e iew o he ecen p og ess and he pe spec i es in he applica ion o supe c i ical luids in ca bon dioxide u iliza ion echnologies. Since he copolyme iza ion o CO2 is discussed in ano he a icle o his special issue, his applica ion will no be conside ed in his a icle. 2. Ca alys s o A i icial Pho osyn hesis The o igin o “a i icial pho osyn hesis” sys ems is usually acked back o he seminal wo k o o Fujishima and Honda, who in 1972 demons a ed he pho oelec ochemical spli ing o 4 wa e using a single-c ys al TiO2 pho oanode and a P ca hode wi h an ex e nal elec ical bias [16]. Following his wo k, a conside able wo k has been ca ied ou in he de elopmen o pho oelec ochemical and pho oca aly ic CO2 con e sion sys ems, and pa icula ly in he de elopmen o sui able pho oca alys s, which a e an essen ial elemen o hese sys ems. The pho oca alys s employed a e ma e ials ha a e able o abso b ligh ene gy, gene a ing ee elec ons (e-) and holes (h’), as shown in Figu e 1 [17]. Some o he mos employed ma e ials a e TiO2 and o he semiconduc o s. The pho o-induced elec on-hole pai s p oduced in hese ma e ials can ini ia e he edox eac ions equi ed o con e CO2. The e o e, a undamen al aspec de e mining he pho oca aly ic ac i i y o he ma e ial is he e iciency o s abiliza ion o he cha ge sepa a ion induced by ligh exci a ion. Cha ge sepa a ion can be enhanced adding me al co-ca alys s o he ma e ial, such as P , ha ac ua e as sinks o he ee elec ons p oduced. Such semiconduc o -coca alys sys ems a e some o he mos p omising pho oca aly ic ma e ials o wa e spli ing and CO2 educ ion a ailable oday [18]. Cha ge sepa a ion can also be s abilized by geome ical ac o s, p oducing ca alys s wi h app op ia e and con olled (nano) sizes and shapes [19]. Following his app oach, di e en (and, in some cases, pa en ed) designs desc ibe nano-s uc u ed ma e ials wi h a high aspec a io (e.g. ibe s o ods), including: TiO2 nano ibe s [20], TiO2-coa ed ZnO nano ods deposi ed o e a silicon subs a e [21], o a double-laye s uc u e, o med by Si nanowi es e ically disposed o e he wo sides o a PEM memb ane [22] (Figu e 2). Mo eo e , he ange o ligh wa eleng hs in which he pho oca aly ic ma e ial is ac i e is an ex emely impo an p ope y. Ideally, he ma e ial should ha e a maximum ac i i y in he isible ligh ange, bu commonly used semiconduc o pho oca alys s, such as comme cial TiO2, show hei maximum ac i i y in he UV ange. The de elopmen o pho oca alys s sui able o ligh abso p ion in a wide wa eleng h ange is ex emely challenging, because ligh abso p ion depends on many ac o s, including he chemical composi ion, he c ys alline s uc u e and ype and densi y o de ec s, he pa icle size, o he in e ac ions a he in e ace be ween he pho oca alys and he co- 5 ca alys . I is he e o e necessa y o de elop a me hod o p oduc ion o he ca alys ha can con ol all hese aspec s. Conside ing he possibili ies o supe c i ical luid echnologies o he de elopmen o such ma e ials wi h ailo ed p ope ies, se e al esea che s ha e s udied he p oduc ion o pho oca alys s in supe c i ical media. Figu e 1: Schema ic ep esen a ion o he eac ion mechanism o he pho oca aly ic CO2 educ ion o e a semiconduc o pa icle. Rep oduced om [17] wi h pe mission. As p e iously desc ibed, semiconduc o -coca alys pho oca aly ic sys ems a e equen ly used as pa icles o app op ia e size, gene ally in he nanome e ange. Di e en supe c i ical luid p ocesses can be used o p oduce such me al nanopa icles wi h con olled and na ow pa icle size dis ibu ion [23]. A i s al e na i e is he syn hesis in supe c i ical CO2 media by he molysis o an o ganome allic p ecu so . In o de o apply his me hod, a sui able p ecu so wi h a high solubili y in supe c i ical CO2 mus be a ailable. The i s s ep o he p ocess is he dissolu ion o his p ecu so in CO2. I he deposi ion o he p oduced pa icles o e a sca old is desi ed, he supe c i ical dissolu ion is pu in o con ac wi h he sca old du ing his s ep, wi h he objec i e o adding he p ecu so o he sca old by adso p ion o physical deposi ion. A e his, he supe c i ical dissolu ion is hea ed up, causing he decomposi ion o he o ganic pa o he o ganome allic p ecu so . Me al pa icles a e hus p oduced, gene ally as me al oxides. Employing his me hod, Alonso e al. [24] esea ched he syn hesis o TiO2 nanopa icles in supe c i ical CO2 and e alua ed he in luence o syn hesis condi ions on he pho oca aly ic ac i i y o he ma e ial. In his wo k, TiO2 ana ase nanopa icles we e ob ained 6 by he mohyd olysis o DIPBAT (diisop opoxi i anium bis ace ylace ona e) in supe c i ical ca bon dioxide wi h di e en alcohols (e hanol and isop opylalcohol). Mo eo e , he pho oca aly ic ac i i y o he ma e ial was es ed conside ing he deg ada ion o aqueous solu ions o me hyl o ange as model eac ion. I was ound ha he c ys allini y was a undamen al pa ame e on he pho oca aly ic ac i i y o he ma e ial. The c ys allini y could be ailo ed modi ying he ope a ion empe a u e. The ope a ion ange was 200ºC – 300ºC, and highe c ys allini y was ob ained a highe empe a u es, leading o a highe pho oca aly ic ac i i y. In con as , o he ope a ing pa ame e s such as he ope a ing p essu e did no ha e a signi ican e ec on he pho oca aly ic ac i i y. The ac i i y also showed some a ia ion when he alcohol employed in he he mohyd olysis was changed, wi h highe ac i i y in he case o pa icles ob ained wi h isop opanol. Acco ding o au ho s, his esul could be ela ed o he densi y o su ace hyd oxyl g oups in he pa icles. Figu e 2 p esen s a SEM image o he TiO2 pa icles p oduced. Figu e 2: TiO2 mic opa icles syn he ized in supe c i ical CO2. Adap ed om [24] wi h pe mission. Cama illo e al. [25] applied TiO2 nanopa icles syn he ized in supe c i ical CO2 wi h a simila p ocedu e o he pho oca aly ic educ ion o CO2, compa ing hem wi h comme cial Degussa 7 P-25 TiO2 pa icles (E onik). They cha ac e ized he ligh abso p ion p ope ies o he TiO2 pa icles, inding ha pa icles syn he ized om supe c i ical CO2 exhibi ed a highe ligh abso p ion in he isible ange han he comme cial pa icles. This is a majo imp o emen o he p ope ies o he ca alys , since TiO2 ca alys s a e gene ally limi ed o ope a ion in he UV ange and show poo ligh abso p ion in he isible ange. In connec ion wi h his esul , pa icles syn he ized in supe c i ical media also showed lowe band gap ene gy han comme cial ca alys s, a esul ha also indica es a mo e e icien ligh abso p ion. Au ho s sugges ed ha his esul could be due o he exis ence o su ace oxygen acancies in he ma e ials syn he ized in supe c i ical luids, which e ec i ely ha es isible ligh and c ea e a colo cen e [26]. Indeed, as p esen ed in Figu e 3, i could be obse ed by a simple isual inspec ion ha while comme cial pa icles showed a b igh whi e colo , pa icles syn he ized in supe c i ical luids p esen ed da ke g ey o g een colo s, a clea indica ion o ligh abso p ion in he isible ange. Cama illo e al. [25] also assessed he s abiliza ion o he cha ge sepa a ion a ained wi h he di e en ma e ials. X- ay di ac ion (XRD) assays showed ha he pa icles p oduced in supe c i ical luid had smalle c ys alli e sizes han comme cial pa icles, a p ope y ha is a o able o he s abiliza ion o cha ge sepa a ion, because he c ys alline ac u es and he con ac su aces be ween c ys alli es slow down he ecombina ion o ee elec ons and holes. Howe e , while comme cial pa icles consis ed o a mix u e o ana ase and u ile c ys alline phases, CO2-syn e ized pa icles consis ed exclusi ely o ana ase, as also obse ed by Alonso e al. [24], which in his case is an un a o able p ope y because he p esence o di e en c ys alline phases can also slow down cha ge ecombina ion. As a esul o he combina ion o hese p ope ies, pa icles syn he ized in supe c i ical CO2 showed a highe pho oca aly ic ac i i y in he educ ion o CO2, enabling he p oduc ion o CH4 and CO wi h eac ion a es ha we e 22 and 1.7 imes highe , espec i ely, han he a es ob ained wi h comme cial TiO2 pa icles. 8 Figu e 3: TiO2 pa icles syn he ized by Cama illo e al. [25] a e pho oca aly ic expe imen s, p epa ed om di e en Ti p ecu so s and sol en s: (a) TTIP-isop opyl alcohol; (b) TTIP-e hanol; (c) DIPBAT-isop opyl alcohol; (d) DIPBAT-e hanol. Rep oduced om [25] wi h pe mission The syn hesis p ocedu e in supe c i ical CO2 can also be used o ob ain composi e TiO2 – me al coca alys pa icles. In u he wo ks, S. Tos ón e al. [27] and Cama illo e al. [28] in es iga ed he applica ion o TiO2 pa icles doped wi h Cu o P . TiO2/P ca alys s, ob ained by simul aneous he mohyd olysis o TiO2 and P p ecu so s ( i anium e aisop opoxide o diisop opoxi i anium bis ace ylace ona e, and palladium ace yl ace ona e, espec i ely). As in p e ious cases, XRD assays showed a high c ys allini y o he TiO2 pa icles p oduced, composed exclusi ely o ana ase phase, bu in his case di ac ion peaks ha could co espond o PdO phases could also be obse ed, indica ing ha a ac ion o Pd was no well dispe sed in he TiO2 ma ix. As in he p e ious wo k, a shi in he ligh abso p ion ange o he isible ange was obse ed in UV-VIS di use e lec ance assays, which is a o able o he applica ion o he ca alys s wi h na u al sunligh . CO2 educ ion expe imen s showed he p oduc ion o CO, me hane and e hane, and he a es o o ma ion we e 3-22 imes highe 9 han he a es obse ed wi h comme cial, undoped TiO2 pa icles. Compa ing wi h he esul s o hei p e ious wo ks, au ho s obse ed ha he addi ion o Pd enhanced he selec i i y owa d me hane. The bes esul s we e ob ained wi h he ca alys s wi h he highes Pd load (3 w %), indica ing ha he p esence o seg ega ed PdO phases obse ed in XRD assays did no ha e a de imen al e ec on he pho oca aly ic ac i i y. Ano he me hod o he syn hesis o me al and semiconduc o nanopa icles is he hyd o he mal syn hesis unde supe c i ical condi ions [29]. The p ocess consis s o mixing an aqueous solu ion o a me al p ecu so wi h a s eam o wa e a nea -c i ical o supe c i ical condi ions, which causes he deg ada ion o he o ganic pa o he p ecu so and he o ma ion o me al oxide nanopa icles. The mixing kine ics be ween he p ecu so solu ion and he supe c i ical wa e a e a c ucial pa ame e o he success ul syn hesis o he nanopa icles. Wi h an app op ia e design o his mixe , he p ocess can be ca ied ou con inuously. Chowdhu y e al. [30] es ed he pho oca aly ic ac i i y o TiO2 pa icles syn he ized wi h his me hod in he isible ligh ange. The syn hesis was ca ied ou using wa e -e hanol sol en mix u es, a ope a ing empe a u es anging om 200ºC o 400ºC. Pa icles wi h ana ase c ys alline s uc u e we e ob ained, wi h sizes ha anged om 10 nm a 250ºC o 100 nm a 400ºC. The mos in e es ing esul o he wo k was he nano- winned s uc u e o he pa icles, which is a a o able p ope y o he pho oca aly ic ac i i y o he ma e ial because he changes o o ien a ion o he c ys al a win c ys al su aces enhance cha ge sepa a ion. In his wo k, he pho oca aly ic ac i i y o he ma e ial was es ed conside ing he deg ada ion o me hylene blue as model eac ion. A high pho oca aly ic ac i i y was obse ed e en wo king unde isible ligh , which is a p omising esul o he applica ion o his ma e ial in CO2 educ ion eac ions. The esul s o hese wo ks show ha TiO2 mic o/nano pa icles p oduced wi h supe c i ical luids show p omising pho oca aly ic p ope ies. Howe e , he handling o mic o/nano pa icles in eal applica ions can be p oblema ic, as in low eac o s hey a e p one o 16 chemical compounds and uels [44]. The con e sion o se e al biomass o o ganic esidues in hyd o he mal media has al eady been demons a ed, by hyd olysis wi h o wi hou ca alys s [45, 46] o by gasi ica ion [47]. These p ocesses ha e e en achie ed an incipien comme cializa ion. Renma ix [48] has comme cialized he p oduc ion o suga s in hyd o he mal condi ions, while Hyd ome han AG [49] applies he ca aly ic gasi ica ion o o ganic esidues in supe c i ical wa e o he p oduc ion o me hane. Following hese esul s, he educ ion o CO2 in hyd o he mal media has aised a conside able in e es because i can o e come one o he main di icul ies o o he CO2 con e sion me hods: he p oduc ion o H2 by a cheap and a simple me hod. Indeed, i has been p o en ha in hyd o he mal eac o s, hyd ogen can be p oduced in-si u by oxida ion o ze o- alen me als. The e o e, in his me hod wa e ac s simul aneously as en i onmen ally iendly sol en and as hyd ogen sou ce. Mo eo e , he in-si u p oduc ion o hyd ogen also enhances he a e o he educ ion eac ion o CO2, p obably due o he pa icipa ion o adical in e media es o o adso bed species in he eac ion [50]. Figu e 7 p esen s a schema ic ep esen a ion o he eac ions in ol ed in he hyd o he mal con e sion o CO2 wi h me al educ an s. Figu e 7: Schema ic diag am o he mechanism o con e sion o CO2 by hyd o he mal educ ion wi h me al educ an s CO 2 +H 2 -H 2 HCOOH CO+H 2 O +H 2 -H 2 O +H 2 O -H 2 CH 2 O +H 2 -H 2 CH 3 OH +H 2 -H 2 O +H 2 O -H 2 CH 4 educ ion CH 4 +H 2 Ni Me(0)+ H 2 O+CO 2 oxida ion Me x O y + H 2 +CO 2 (Basic media) (High T) MeCO 3 + H 2 Me x O y + R-OH R=O+ H 2 O+ M(0) Reacciones de educción de CO 2 Reacciones de oxidación de me ales pa a gene a H 2 Reacciones de educción de los óxidos me álicos con alcoholes CO 2 educ ion eac ions Reduc ion o me al oxides wi h alcohols Oxida ion o me als o gene a e H 2 17 The educ ion o CO2 in hyd o he mal media has been p o ed employing di e en me als as educ an s and/o ca alys s, o ming o mic acid. The mos equen ly s udied combina ion is Fe as educ an , wi h Ni [50] o Cu [51] as ca alys . Wi h hese ma e ials, yields o con e sion o o mic acid highe han 75% ha e been achie ed ope a ing a empe a u es o 200ºC – 350 ºC wi h esidence imes o 1 – 2 h. O he au ho s ha e p o ed ha he educ ion can be ca ied ou employing only Fe, wi hou ca alys s, bu in his case eac ion imes inc eased up o 75 h [51, 52, 53]. Besides Fe, o he me als ha e been used as educ an s, including Mg, Mn, Zn and Al. The mos p omising esul s ha e been ob anied using Zn o Al, ob aining yields up o 60% wi hou using ano he me al as ca alys [50]. In he case o Zn, con e sion o o mic acid wi h a yield o 60% has been achie ed ope a ing a 325ºC wi h a esidence ime o only 10 min, while a longe ope a ing imes me hane aces a e o med [54]. In his p ocess, Zn ac s as educ an , and he ZnO hus o med ac s as ca alys . In u he s udies o he same au ho s i has been epo ed ha employing Cu as ca alys and Zn as educ an , he o mic acid ini ially ob ained by hyd o he mal con e sion o CO2 is u he con e ed o me hanol [55]. Usually, o mic acid is he main p oduc ob ained wi h his p ocess [50, 51], al hough some au ho s ha e ob ained phenol [53], me hane [52] o me hanol [55] by a simila p ocess. Al hough he p ecise mechanism o he eac ion is unknown, he esul s so a ob ained sugges ha o mic acid is he i s p oduc o CO2 educ ion, which is hen con e ed o o maldehyde, me hanol and inally me hane [43]. In ac , hese p oduc s a e obse ed when eac ion imes a e inc eased [52, 53, 55]. Expe imen al esul s indica e ha he selec i i y owa d o mic acid is a o ed by ope a ion in basic pH condi ions, which s abilize o mia e as a ca ion in aqueous solu ion. Fo his eason, expe imen s a e usually ca ied ou using bica bona e as CO2 sou ce, o adding NaOH o he medium i CO2 is supplied as a gas in o de o dissol e i as bica bona e [56]. Mo eo e , changes in he ca alys employed o i s concen a ion can lead o he p oduc ion o di e en compounds. Fo example, when Ni is 18 used as ca alys , me hanol p oduc ion is a o ed a high Ni concen a ions, while he use o Fe a o s he o ma ion o o mic acid, and he use o Fe3O4 inc eases he o ma ion o me hanol [56]. As i can be obse ed, a d awback o his me hod is ha he me al used as educ an is consumed du ing he p ocess, o ming a me al oxide. In o de o eco e he me al, he pho oca aly ic educ ion o he me al has been p oposed [54]. Ano he al e na i e is adding ano he educ an o he hyd o he mal medium sui able o educing he me al oxide, hus enabling he ope a ion in a closed cycle wi h espec o he me al. In pa icula , biomass esidues, glyce in [50] o esidues o Sulphu -con ainin polyme s ha e been used o educe he me al and eu ilize i in he p ocess [56]. Addi ionally, some au ho s ha e p o ed ha CO2 educ ion can be ca ied ou in hyd o he mal media by oxida ion o an alcohol, wi hou addi ion o any me als [57]. 5. Conclusions and pe spec i es 19 Re e ences [1] E.J. Maginn. Wha o Do wi h CO2. 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