Tuned Pd/SiO 2 aerogel catalyst prepared by different synthesis techniques

1 Tuned Pd/SiO2 ae ogel ca alys p epa ed by di e en syn hesis echniques L.M. Sanz-Mo al1, A. Rome o1, F.Holz2, M. Rueda1, A.Na a e e1, A. Ma ín1*. -Línea en blanco, 12 1High 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, Doc o Me gelina s/n,47011 Valladolid, Spain 2Ruh Uni e si ä Bochum, Uni e si ä ss . 150, 44801 Bochum, Ge many Tel: +34 983184077 e-mail: [email p o ec ed] (Á. Ma ín) 2 Tuned Pd/SiO2 ae ogel ca alys p epa ed by di e en syn hesis echniques L.M. Sanz-Mo al1, A. Rome o1, F.Holz2, M. Rueda1, A.Na a e e1, A. Ma ín1*. -Línea en blanco, 12 1High 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, Doc o Me gelina s/n,47011 Valladolid, Spain 2Ruh Uni e si ä Bochum, Uni e si ä ss . 150, 44801 Bochum, Ge many Tel: +34 983184077 e-mail: mam[email p o ec ed] (Á. Ma ín) Abs ac Pd nanopa icles ha e been embedded on silica ae ogel by using h ee di e en echniques. In each o hem he me al was loaded in he ma ix a di e en s eps o he p oduc ion: he di ec syn hesis, he we imp egna ion and he supe c i ical imp egna ion o he p e iously d ied ae ogels. The esul an ma e ials ha e been cha ac e ized o analyze he di e ences depending on he applied echnique o i s imp egna ion. A omic abso p ion, ni ogen physiso p ion, X- ay di ac ion, in a ed spec oscopy and ansmission elec on mic oscopy whe e pe o med. In all he echniques he concen a ion o me al has been a ied ( om 0.13 o 1.61 % w .) by modi ying he concen a ion o he suspension (Pd-poly inylpy olidone nanopa icles used in he di ec syn hesis) o o he solu ion o he me allic p ecu so (palladium ace ylace ona e), bo h in he o ganic sol en and he supe c i ical media. The cha ac e iza ion had gene ally shown a good dis ibu ion o he me allic pa icles in he ma ix, and he negligible e ec o he me al on he ex u al p ope ies. Finally, conside able a ia ions whe e obse ed on he silanol g oups on he su ace o he ca alys s. These ma e ials we e es ed in D-glucose hyd ogena ion, obse ing signi ican 3 di e ences on he pe o mance o he ca alys depending on he syn hesis echnique employed. Keywo ds ae ogel; silanol; uned selec i i y; palladium ca alys ; nanopa icles; supe c i ical CO2 1. INTRODUCTION E ec i e ou es o ob ain mo e aluable p oduc s equi e he design o e icien ca alys s. No el ca aly ic s uc u es a e he e o e needed o o e come he p esen challenges. These no el s uc u es equi e he in eg a ion o suppo ac i e si es in a way ha p ese es hei ad an ages and capabili ies. The e o e he de elopmen o no el ca aly ic s uc u es achie ed by he in eg a ion o me allic nanopa icles e enly dis ibu ed in a mesopo ous and high-su ace ae ogel appea s as a p omising al e na i e. Silica ae ogels p esen ema kable p ope ies which make hem sui able ma e ials o o e come hese new challenges: high po e olumes, a o able anspo p ope ies, s abili y and su ace ac i i y. Wha is mo e, hei p ope ies can be easily uned: hei ex u al p ope ies can be ailo ed by changing he a ios o p ecu so [1]; he chemis y o hei su ace can be con olled by using di e en alkyl-alkoxy/chlo o silanes allowing o go e n hei g ade o hyd ophobici y [2]; in addi ion, he op ion o c ea ing hyb id ae ogels make almos all p ope ies equi emen s achie able [3]. By cons as , due o i s b eakabili y special echniques mus be applied o he me al imp egna ion be o e o a e he d ying in o de o a oid he capilla i y o ces which could damage he s uc u e o he ma ix. Cogelled ae ogel and Imp egna ed Ae ogel Ca alys s we e al eady p oduced [4], concluding ha he cogelled ones showed be e esis ance o sin e ing. Also Ni and Pd nanopa icles we e embedded on ae ogels by imp egna ion o he gels ollowed by supe c i ical d ying [5]. Ionic liquids ha e also been conside ed as a possible ou e [6]. Finally supe c i ical CO2 has been used as 4 imp egna ion media and 1,1,1,5,5,5-Hexa luo o-2,4-pen anedione-palladium (2:1) as me al p ecu so [7]. Fu he mo e, he solubili y o Palladium(II) ace ylace ona e in supe c i ical CO2 has been al eady s udied, p o iding ano he Pd p ecu so which could be used in he supe c i ical imp egna ion (SCI) echnique[8]. The e o e, di e en p epa a ion echniques ha e been p oposed o inco po a ing Pd ca aly ic nanopa icles in a silica ma ix, showing di e en esul s depending on he echnique employed. Bu o ou knowledge, a sys ema ic s udy o he a ia ion o he inal p ope ies o he ma e ials depending on he echniques and sol en s used o he me al imp egna ion has no been done. Wha is mo e, his a ia ion o p ope ies could be ansla ed in o di e ences on he unc ionali y o he inal ca alys s. Ca aly ic hyd ogena ion o D-glucose in o so bi ol seems o be a simple eac ion, bu in ac D-glucose can ollow di e en eac ion pa hways ins ead o being con e ed in o so bi ol. In essence, D-glucose can isome ize in o D- uc ose by Lob y de B uyn Albe da – Van Ekens ein eac ion [9] and i s subsequen hyd ogena ion allows o ob ain manni ol / so bi ol mix u e[10]. In addi ion, byp oduc s such as glycolaldehyde and glyce aldehyde could appea as a esul o e o aldol condensa ion eac ion [11], which a e hyd ogena ed in o smalle suga alcohols like e hylene glycol and glyce ol espec i ely. Ru-based ca alys s demons a ed o be he mos e ec i e o ca aly ic hyd ogena ion in o so bi ol. Howe e , me als such as Ni, P , Pd and Rh ha e been used o simila pu poses due o hei lowe p ice in compa ison wi h Ru [12]. Bizhano e al. epo ed ha he combina ion o Pd and Ni in he hyd ogena ion o D-glucose was e y e ec i e in compa ison wi h o he bime allic ca alys s [13]. In his wo k, Pd nanopa icles ha e been embedded on silica ae ogel by using h ee di e en echniques. In each o hem he me al was loaded in he ma ix a di e en s eps o he p oduc ion: he di ec syn hesis (DS), he we imp egna ion (WI), and he 5 SCI o he p e iously d ied ae ogels. Kine ic es s o D-glucose hyd ogena ion in o suga alcohols we e ca ied ou in o de o check he ca aly ic beha io o he ca alys s. The in luence o he p epa a ion echnique in he ac i i y o each ca alys was epo ed. 2. EXPERIMENTAL 2.1 Reagen s Te ame hoxysilane (TMOS, 98%), Palladium(II) ace ylace ona e (Pd(acac)2, 99%), Poly inylpy olidone (PVP) a e age mol w 10,000, Bo ane-ammonia complex (97%) and D-(+)-glucose (≥99.5%) we e pu chased om Sigma–Ald ich. Me hanol (99.8%) and ammonium hyd oxide (25%) we e ob ained om Pan eac. CO2 (>99.95 mol%) and echnical H2 we e supplied by Ca bu os Me álicos S.A. Deionized wa e was used in all expe imen s. 2.2 Ae ogels syn hesis: Hyd ophilic silica alcogels we e p oduced ollowing he single s ep sol–gel p ocess [14]. The mola a io was TMOS:CH3OH:H2O:NH4OH, 1:2.3:3.8:1.2 × 10−2.Then he alcogels we e d ied by using supe c i ical CO2. The d ying ook place in a closed ci cui whe e he CO2 a 10.5 MPa and 45ºC was eci cula ed ill he sol en was comple ely emo ed. Th ee loads o esh CO2 whe e needed. A de ailed desc ip ion o he se up can be ound elsewhe e [15]. 2.2.1 Palladium imp egna ion Th ee di e en echniques we e used o imp egna e Pd in o he silica ma ix by using he same me al p ecu so . The i s one was he adi ional WI me hod, which consis ed on adding Pd(acac)2 in o he aging sol en . Tha was ollowed by he supe c i ical d ying wi h CO2. Two sol en s we e used: me hanol and ace one. Ace one one was chosen because o he highe solubili y o he p ecu so . The solu ions we e sa u a ed a 20, 40 and 50ºC. 6 The second one was he SCI. I is based on he dissolu ion-p ecipi a ion p inciple. A e he supe c i ical d ying o ae ogels, he supe c i ical CO2 was also used as sol en media o he Pd(acac)2. The p ecu so was placed in excess in a ba ch eac o whe e he samples s ayed a 25 MPa and 60ºC o a long ime o secu e he solubiliza ion ill sa u a ion condi ions and di usion o he me al p ecu so . Then he solubili y o he p ecu so was dec eased o o ce i s p ecipi a ion in o he ae ogels po es by educing empe a u e ill oom empe a u e. Finally he sys em was decomp essed a a a e o 0.3 MPa/min. The hi d one, he DS, was made by suspending me allic Pd nanopa icles in he me hanol o he alcogels syn hesis. These nanopa icles we e p oduced by aking as e e ence he me hodology desc ibed by o he au ho s, which educes he me al p ecu so wi h ammonia bo ane in a me hanol solu ion [16]. Then he samples we e calcina ed a 400°C du ing 3 hou s in o de o elimina e he su ac an . Finally, in all he cases, he ae ogels we e milled du ing 60 minu es a 100 pm wi h a Plane a y Ball Mill PM 100 (Re sch) and he powde was ea ed wi h a low o 2 Nl/min o pu e H2 a 150°C du ing 30 minu es o ac i a e he ca alys . 2.2.2 D-glucose hyd ogena ion: The eac ion was pe o med in ba ch in an expe imen al se -up wi h a comme cial s ainless s eel high p essu e eac o (Be gho BR-25) wi h an in e nal olume o 25 cm3, agi a ed wi h a magne ic s i ing ba 1400 pm and i ed up wi h a p opo ional– in eg al–de i a i e sys em o empe a u e con ol. The hyd ogena ion eac ion was pe o med by pumping 5 mL o glucose solu ion (10g/L) and cha ging 150 mg o he hyd ophilic ae ogels wi h di e en loads o Pd. All he ca aly ic es s we e pe o med a 120ºC and 2.5 MPa o pu e hyd ogen du ing 360 min. A mo e de ailed desc ip ion o he se -up can be ound elsewhe e [17]. Ac i i y o he di e en ca alys (A, molcon e ed 7 glucose·molme al-1·min-1) and selec i i ies o he p oduc s (S, %) we e calcula ed using Eq. (1) and Eq. (2). (2) 2.3 Cha ac e iza ion: The ae ogel s uc u e was s udied by Fou ie ans o m in a ed spec a (FT-IR model TENSOR omBRUKER, Spain) Me al loading was de e mined by a omic abso p ion (AA) using a VARIAN SPECTRA 220FS analyze . Diges ion o he samples was pe o med wi h HCl, H2O2 and HF using mic owa e a 250 ºC. The c ys allini y o he imp egna ed Pd pa icles we e analyzed by X- ay di ac ion (Disco e D8-B uke ) A JEOL ield emission mic oscope, model JEM-FS2200 HRP, ope a ing a 200 kV was used o HR TEM (High Resolu ion T ansmission Elec on Mic oscopy) and EDX (Ene gy-Dispe si e X- ay spec oscopy). The ex u al p ope ies o he ca alys s we e de e mined by ni ogen iso he mal adso p ion-deso p ion. A Su ace A ea and Po osi y Analyze (ASAP2020- Mic ime ics) was used. The speci ic su ace a ea was calcula ed by he BET (B unaue –Emme –Telle ) me hod. The speci ic po e olume is de e mined by he single poin adso p ion me hod. The shown a e age po e diame e is based on he deso p ion iso he m o he Ba e -Joynes-Halenda (BJH) me hod. 8 Hyd ogena ion p oduc s we e analyzed by liquid ch oma og aphy (HPLC). The HPLC column used was SUGAR SC-1011 om Shodex a 80 °C and a low o 0.8 cm3·min-1 using wa e Milli-Q as he mobile phase. A Wa e s IR de ec o 2414 was used o iden i y suga s, polyols and hei de i a i es. 3. Resul s 3.1 In a ed spec a s udies Fig. 1 shows he in a ed spec a o he silica ae ogel and he di e en syn hesized ca alys s. The un ea ed suppo shows some me hyl g oups on he su ace (815, 2862, 2936 and 2974cm-1); hese g oups a e expec ed o co espond o esidual non- hyd olyzed alkoxy g oups on he su ace o he silica ae ogels [18]. The ee me al suppo was also educed unde he same condi ions which we e used o educe he ca alys s and no impo an luc ua ions whe e no iced on he spec a. By con as a signi ican dec ease on he me hyl g oups ollowed by an inc emen o he silanol peak (960 cm-1) was no iced in he samples p oduced by WI. Wha is mo e, compa ing he sample p epa ed a 20ºC and he one a 50ºC, he hyd oxyla ion seems o be ela ed wi h he empe a u e o he imp egna ion. A simila bu sligh e e ec is obse ed on he sample p epa ed by SCI. Al hough he empe a u e o SCI is 60ºC, he educ ion in he me hyl g oups and he inc emen a 960cm-1 is less p onounced in supe c i ical CO2 han in ace one. A di e en phenomena is obse ed in he ca alys made by di ec syn hesis which has comple ely los hese me hyl g oups bu does no show he highe in ensi y on he silanol egion. This could be explained by he empe a u es eached du ing he calcina ion o he PVP which allows he loose o he me hyl g oups bu also he ini ializa ion o he dihyd oxyla ion [19]. 9 Fig. 1. In a ed spec a o he aw ae ogel (1), educed ae ogel (2), educed WI wi h ace one a 20ºC (3), educed WI wi h ace one a 50ºC(4), educed SCI (5) and educed DS (6). Conce ning he OH s e ching egion, h ee di e en bands can be dis inguished. The one a 3735cm-1 co esponds o he ee silanol g oups on he silica su ace. The b oad band a ~3502cm-1 belongs o he s e ching ib a ions o he hyd oxyl g oups o wa e physically adso bed on SiO2 su ace and he su ace silanol g oups en e ing in o a hyd ogen bond. The band a ~3660cm-1 can be assigned o he hyd oxyls ha ha e o med weak hyd ogen bonds [20]. The di e en ca alys s show di e en in ensi ies in hese peaks, which is ansla ed in o di e en chemical p ope ies o he su aces o he ma e ials. 3.2 Ni ogen physiso p ion s udies The N2-adso p ion expe imen wi h he aw ae ogel and he ac i a ed ca alys led o he iso he ms illus a ed in Fig. 2. The iso he ms belongs o “ ype IV” which is ypical o mesopo ous ma e ials. The hys e esis loop is wide, and he deso p ion cu e is mo e p ecipi ous han he adso p ion cu e. This si ua ion is classi ied as H2 ype loops and usually occu s when he dis ibu ions o po e size adius a e wide [21]. Like he aw 16 be aken in o accoun . Fi s o all, he di e en concen a ion and na u e o he hyd oxyl g oups on he su ace seem o be playing a ole. Secondly, he o ma ion o di e en ypes o ca bide on he su ace due o he di ec educ ion unde hyd ogen a mosphe e [26] canno be igno ed. On he o he hand, selec i i y o so bi ol esul s p esen ed in Table 3 demons a ed he in luence o he p epa a ion me hod and me al loading in he composi ion o he inal p oduc . Compa ing bo h selec i i ies o so bi ol o WI 20ºC and WI 50ºC, i was obse ed ha an inc ease in Pd loading om 0.54 o 1.03 % enhanced selec i i y o so bi ol om 54 o 76 %. Low amoun s o uc ose we e de ec ed sugges ing e o aldol condensa ion eac ion o p oduce glyce aldehyde ha is subsequen ly hyd ogena ed in o glyce ol which is he main byp oduc in bo h cases. In addi ion, glyce ol was he main compound in he liquid p oduc when hyd ogena ion o D-glucose was ca ied ou o e he ca alys p epa ed by SCI achie ing a selec i i y o glyce ol a ound 89 %. In he case o SCI so bi ol was no de ec ed in he inal p oduc . As i was explained abo e, hyd oxyl g oups and ca bide on he su ace o he suppo could dec ease he ac i i y o he ca alys and a o glyce ol p oduc ion. Finally, D-glucose is hyd ogena ed wi h a selec i i y a ound 47 % o so bi ol o e he ca alys p epa ed by DS and no o he p oduc s could be iden i ied by HPLC in his case. Table 3 Ca aly ic ac i i y, (moles o con e ed glucose pe moles o me al and pe minu e) and selec i i y o so bi ol o he syn hesized ca alys s a 120 ºC, 2.5 MPa H2 and 360 min WI 20ºC WI 50ºC SCI DS Ac i i y 1.42·10-2 1.07·10-2 4.45·10-3 2.27·10-3 SSORBITOL (%) 54.23 75.85 0 47.26 SGLYCEROL (%) 34.71 22.43 88.45 0 4. CONCLUSIONS 17 Th ee di e en ou es ha e been used o imp egna e silica ae ogels wi h Pd. The load o he me al could be uned by con olling he concen a ion o Pd in he di e en imp egna ion media bu a he same ime he achie able concen a ion is limi ed by he solubili y o he p ecu so . Well dis ibu ed pa icles we e ob ained wi h WI and SCI bu agglome a ion was obse ed on he DS ca alys . The es o he ca alys s in he hyd ogena ion o D-glucose ha e p o ed ha he in luence o he concen a ion o me al and he size o he me allic pa icles a e impo an . In addi ion, he chemis y o he suppo , which is modi ied depending on he way in which he suppo is imp egna ed, and he p esence o ca bides, seem o play an impo an ole on ac i i y and selec i i y. Acknowledgmen s This esea ch has been inanced by he Spanish Minis y o Economy and Compe i i eness h ough p ojec ENE2014-53459-R. Á. Ma ín hanks he Spanish Minis y o Economy and Compe i i eness o a Ramón y Cajal esea ch ellowship. L.M. Sanz-Mo al hanks he Spanish Minis y o Economy and Compe i i eness o a FPI p edoc o al g an . M. Rueda hanks he Uni e si y o Valladolid o a FPI p edoc o al g an . A. Na a e e hanks he kind suppo o he FP7 Shyman Eu opean p ojec (P ojec e e ence: 280983). A. Rome o hanks o he p og am o p edoc o al schola ships om Jun a o Cas illa y Leon o his g an (E-47-2015-0062773). 18 Re e ences [1] Rao AV, Pajonk GM, Pa a hy NN. In luence o Mola Ra ios o P ecu so , Ca alys , Sol en and Wa e on Monoli hici y and Physical P ope ies o TMOS Silica Ae ogels. J. Sol-Gel Sci. 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