Conversion of sugars to methyl lactate with exfoliated layered stannosilicate UZAR-S4

Con en s lis s a ailable a ScienceDi ec Ca alysis Today jou nal homepage: www.else ie .com/loca e/ca od Con e sion o suga s o me hyl lac a e wi h ex olia ed laye ed s annosilica e UZAR-S4 Bea iz Mu illo a , Ósca de la Iglesia a,b , Césa Rubio a , Joaquín Co onas a , Ca los Téllez a, * a Ins i u o de Nanociencia de A agón (INA), Ins i u o de Ciencia de Ma e iales de A agón (ICMA) and Depa men o Chemical and En i onmen al Enginee ing, Uni e sidad de Za agoza-CSIC, 50018, Za agoza, Spain b Cen o Uni e si a io de la De ensa Za agoza, Academia Gene al Mili a , 50090, Za agoza, Spain ARTICLE INFO Keywo ds: He e ogeneous ca alysis Suga con e sion Lac ic acid Laye ed s annosilica e Ex olia ed ma e ial ABSTRACT Biomass has been shown as an al e na i e o ossil uels o ob aining chemicals. In his wo k, he ans o ma ion o suga s in o me hyl lac a e (ML) a 160°C was ca ied ou using he laye ed s annosilica e UZAR-S3 (Uni e si y o Za agoza-solid numbe 3) and he delamina ed ma e ial UZAR-S4 (Uni e si y o Za agoza-solid numbe 4) ob ained om i s ex olia ion. The ex olia ion o UZAR-S3 o UZAR-S4 inc eased he accessibili y o he com- pounds o he ca aly ic si es and he medium-s eng h acidi y. Thus, he yield o ML o suc ose ans o ma ion inc eased om 8% o UZAR-S3 o 49.9 % o UZAR-S4. In he eusabili y es s, he UZAR-S4 ca alys was cha ac e ized be o e and a e eac ion by se e al echniques such as X- ay diff ac ion, he mog a ime y analysis, scanning elec onic mic oscopy, ene gy dispe si e X- ay spec oscopy and ni ogen adso p ion. A de- ac i a ion o he ca alys was obse ed, which was ela ed o ca bonaceous deposi s ha dec eased he specific su ace a ea and he po e olume o he ca alys . 1. In oduc ion In he las decades he ans o ma ion o enewable biomass o gene a e ene gy o added alue p oduc s has become a field o g owing in e es o many esea che s because o unce ain y abou ossil e- sou ces and en i onmen al conce ns. Among he a ie y o p oduc s ha can be ob ained om biomass, lac ic acid is o significan im- po ance due o i s abili y o se e as a pla o m chemical o a wide ange o applica ions in he chemical, pha maceu ical, cosme ics and ood indus ies [1]. Beyond i s adi ional uses, cu en ly he e is an inc eased demand o lac ic acid as a eeds ock o he p oduc ion o biopolyme poly-lac ic acid (PLA), which is a p omising biodeg adable, biocompa ible, and en i onmen ally iendly al e na i e o plas ics de i ed om pe ochemicals [2]. PLA can be used o p oduce com- me cial p oduc s ela ed o packaging, ag icul u al uses, anspo a ion, elec onics and housewa e, among o he s [3]. The main way o he p oduc ion o lac ic acid is by means o he e men a ion o ca bohyd a es (usually pen oses o hexoses) in aqueous solu ions. Ne e heless, nu ien cos s, gypsum was e p oduced in he neu aliza ion s ep and low olume ic p oduc i i ies a e majo d aw- backs o his p ocess [4]. Fu he mo e, he pu ifica ion o lac ic acid om aqueous solu ions is a complex p ocess since i in ol es he es- e ifica ion o lac ic acid wi h an alcohol o ob ain he co esponding alkyl lac a e, dis illa ion and subsequen hyd olysis, which suppose a high ene gy cos [5]. As an al e na i e o e men a ion, ca aly ic con e sion o ca bohy- d a es o he p oduc ion o lac ic acid a ose as p ocess wo h s udying. In ac , fi s wi h homogeneous ca alys s, Hayashi e al. epo ed he pionee ing ca aly ic con e sion o ioses o lac ic acid wi h a Sn halide [6]. Since hen, he use o o he homogeneous ca alys s ha e been e- po ed such as SnCl 4 [7]o Sn 4+ -based o ganome allic complexes [8]. On he o he hand, he use o a he e ogeneous ca aly ic p ocess acili a es he eco e y o he ca alys a e eac ion and i s la e euse. In his con ex , Janssen e al. [9], used a ious FAU- ype zeoli es in e hanol o he con e sion o ioses eaching a yield o e hyl lac a e up o 65 % a e 6 h a 90 °C. O he Sn-based ca alys s we e applied o he con e sion o ioses o lac ic acid o alkyl lac a es, o ins ance, Sn-MFI [10], Sn-MCM-41 [10–12], Sn-SBA-15 [10], Sn-MWW [13], Sn-Si mixed oxides [14] o ecen ly Sn-Nb mixed oxides [15], he las ca alys s wi h me hyl lac a e yield o 98 % a e 2 h a 140 °C. Fu he , o he me al oxide wi hou in (e.g. γ-Al 2 O 3 [16]o Nb 2 O 5 [17]) o me al phospha es (e.g. wi h Sn [18]o Nb[19]) ha e been es ed o he con e sion o dihyd oxyace one o lac ic acid de i a es. In las ca alys s using in phospha es, Wang e al. [18] ha e eached yield o 96 % o lac ic acid a e 4 h a 140 °C. While he con e sion o ioses o lac ic acid o alkyl lac a es h ps://doi.o g/10.1016/j.ca od.2020.03.064 Recei ed 18 No embe 2019; Recei ed in e ised o m 4 Ma ch 2020; Accep ed 27 Ma ch 2020 ⁎ Co esponding au ho . E-mail add ess: [email p o ec ed] (C. Téllez). Ca alysis Today xxx (xxxx) xxx–xxx 0920-5861/ © 2020 Else ie B.V. All igh s ese ed. Please ci e his a icle as: Bea iz Mu illo, e al., Ca alysis Today, h ps://doi.o g/10.1016/j.ca od.2020.03.064 p o ides a solid es o benchma king ca alys s, he use o abundan hexoses (glucose o uc ose) o e en di- o polysaccha ides (suc ose) is indus ially p e e ed [20]. To ca y ou hese eac ions, highe eac ion empe a u es a e equi ed (≥150 °C) in compa ison wi h hose applied o ioses (usually ± 90 °C). This esul s in mo e side eac ions, and consequen ly lowe yields o lac ic acid o alkyl lac a es a e ob ained [4]. Holm e al. [21] epo ed he con e sion o suc ose o alkyl lac a es using zeoli es Ti-Be a, Z -Be a and Sn-Be a wi h a maximum yield o ML o 64 % a 160 °C o 20 h. Recen ly, o he au ho s ha e epo ed he u iliza ion o a ca bon-silica composi e [11], Sn-MWW- ype zeoli e [13], hie a chical zeoli e Sn-Be a [22], Fe-doped SnO 2 [23], Mg-MOF- 74 [24]o γ-NiOOH [25], among o he s. In p e ious wo ks, we also applied se e al me al o ganic amewo ks: Zn imidazola e ZIF-8 [26] and Sn-based ca boxyla es UZAR-S10 and MIP-177-LT (Ti/Sn) [27] o his eac ion wi h good esul s. Laye ed silica es ep esen a e y a ac i e field o esea ch because o he ad an ages o laye ed ma e ials as ac i e ca alys s [28]. A cha ac e is ic ea u e o some laye ed ma e ials is hei abili y o swelling and delamina ion, p ocesses ha in ol e he in e cala ion o gues molecules o inc ease he in e laye space and subsequen ex- olia ion. In a p e ious wo k, we epo ed he use o delamina ed ma e ials UZAR-S1 and UZAR-S2 in he ca aly ic con e sion o glucose wi h an inc ease o me hyl lac a e yield wi h espec o hei p ecu so ma e- ials, mic opo ous i anosilica es JDF-L1 and AM-4, espec i ely, since hey p o ided a be e access o he eac an s o ca aly ic si es, hus inc easing he ca aly ic ac i i y o he ma e ials bu wi h low me hyl lac a e yields, below 6% [29]. In his wo k he delamina ed s annosilica e UZAR-S4 was applied as ca alys o he con e sion o glucose and suc ose o me hyl lac a e. Tin con aining ca alys s ha e been shown in he li e a u e as ac i e and selec i e o his eac ion [10,27]. USAR-S4, ha was p e iously syn- hesized in ou g oup [30], is ob ained om he ex olia ion o USAR-S3 which consis s o SiO 4 e ahed al uni s obse ed by 29 Si NMR and oc ahed a SnO 6 obse ed by 119 Sn NMR, he e o e hese ma e ials be- long o he so-called OPT amily (Oc ahed al-Pen ahed al-Te ahed al (OPT) amewo k silica es) [31]. 2. Ma e ials and me hods 2.1. Syn hesis o ca alys s UZAR-S4 was p epa ed as epo ed elsewhe e [30] by delamina ion o s annosilica e UZAR-S3. Fo he syn hesis o UZAR-S3 a sodium si- lica e solu ion (27 w % SiO 2 , 8 w % Na 2 O, Me ck) was mixed wi h deionized wa e and NaOH (98 w %, Sigma-Ald ich), a e wa ds in(II) chlo ide dihyd a e ( eagen g ade 98 w %, Sigma-Ald ich) was added. The mola composi ion o his gel was: 4.2 Na 2 O:1 SnCl 2 :2.9 SiO 2 :101 H 2 O. The gel was s i ed o 1.5 h a oom empe a u e and degassed o a ew minu es in an ul asound ba h. The c ys alliza ion was ca ied ou in a 40 mL Teflon-lined au ocla e unde hyd o he mal condi ions a 230 °C o 96 h. The ob ained powde was fil e ed, washed epea edly wi h deionized wa e and d ied a 100 °C o e nigh . UZAR-S3 was p o on exchanged and subsequen ly swollen by he in e cala ion o nonylamine molecules. Fo he p o on exchange, he powde was imme sed in a solu ion o wa e and ace ic acid (glacial ace ic acid in deionized wa e , pH = 2.7) o 10 min, he pH eaching a alue o 3.9. Then, a solu ion o nonylamine was added and he eac ion was ca ied ou unde eflux a 60 °C o 14 h a a cons an pH 9.6. Swollen UZAR-S3 was eco e ed by cen i uga ion and d ied a 70 °C o 3 h. Finally, UZAR-S4 was ob ained by ex ac ing he amine wi h an HCl/H 2 O/e hanol solu ion (5:15:870, mola a io) unde eflux a 55°C o 8 h. A e his ime, he solid was eco e ed, washed 5 imes wi h deionized wa e and d ied a 100 °C o 12 h. Mo e de ails o he syn hesis o ca alys s can be ound in a p e ious publica ion [30]. 2.2. Ca alys cha ac e iza ion Phase iden ifica ion was ca ied ou by a D-Max 2500 Rigaku X- ay diff ac ome e wi h a coppe anode and a g aphi e monoch oma o using Cu-Kα 1 adia ion (λ=1.5418 Å), aking da a om 2θ= 5° o 40° a a scan a e o 0.03°s −1 and ope a ing pa ame e s o 40 kV and 80 mA. Ni ogen adso p ion–deso p ion iso he ms we e ob ained wi h a Mic ome i ics T is a 3000 a -196 °C. Be o e hese measu emen s, he samples we e degassed a 200 °C, du ing 8 h unde acuum. The specific su ace a ea was calcula ed using he B unaue -Emme -Telle (BET) equa ion. The mal beha io was de e mined by he mog a ime ic analysis (TGA) which was ca ied ou using a Me le Toledo TGA/SDTA 851 e sys em. Samples (abou 5 mg) we e placed in 70 mL alumina pans and hen hea ed unde ai flow up o 800 °C a a hea ing a e o 10 °C·min −1 . Finally, scanning elec onic mic oscopy (SEM) and ene gy dispe si e X- ay spec oscopy (EDX) we e pe o med o e P -coa ed specimens using an Inspec F50 model scanning elec on mic oscope (FEI). Py idine FTIR was used o e idence he p esence o B øns ed and Lewis acidic si es. UZAR-S3 and UZAR-S4, p e iously d ied a 100 °C o e nigh , we e exposed o adso p ion o he py idine apo o 24 h a ambien empe a u e. A e his adso p ion s ep, he ca alys samples we e exposed o a ni ogen s eam o 50 m L(STP)· min −1 a 80 °C o 30 min o emo e he physiso bed py idine. FTIR spec a we e achie ed wi h a Ve ex 70 o B uke and a Specac’s Golden Ga e ATR. Tempe a u e-p og ammed deso p ion s udies o NH 3 (NH 3 -TPD) we e pe o med o UZAR-S3 and UZAR-S4. These analyzes ha e been ca ied ou in he Mic ome i ics AUTOCHEM II 2920 uni wi h he mal conduc i i y de ec o . The es condi ions ha e been as ollows. Fi s , samples we e d ied in a He s eam o 50 m L(STP)· min −1 up o 200 °C a 10 °C min −1 and an iso he mal ime o 60 min. A e cooling he samples o 100 °C, in He 50 m L(STP)·min −1 , he adso p ion s ep was ca ied ou wi h a s eam o 50 m L(STP)· min −1 o 0.5 % NH 3 /He a 100 °C o 1 h. Fo he co ec deso p ion o he physiso bed adso ba e, a He s eam o 50 m L(STP)· min −1 was passed h ough a 100 °C o 1 h. Finally, deso p ion o he adso ba e was analyzed wi h a He s eam o 50 m L(STP)· min −1 upon hea ing up o 700 °C a 10 °C·min −1 . 2.3. Ca aly ic es s Con e sion o suga s o me hyl lac a e was pe o med in 35 m L- Teflon-lined au ocla e. Fo he ca aly ic es s 225 mg o D-(+)-glucose (99 w %, Al a Aesa ) o suc ose (99 w %, Sigma-Ald ich), 8.0 go me- hanol (Mul isol en HPLC g ade, Scha lau), 160 mg o ca alys and 30 mg o naph halene (99 w %, Sigma-Ald ich), as in e nal s anda d, we e added o he au ocla e. Then, he mix u e was hea ed o 160 °C o 20 h in a o a y o en a 15 pm. Finished he eac ion ime, he ca alys was eco e ed by cen i uga ion and d ied in an o en a 70 °C o e nigh p io o i s eu iliza ion. The de e mina ion o p oduc s in he eac ion liquid was ca ied ou in he Gas Ch oma og aph (Agilen 6850) equipped wi h a capilla y column HP-5MS (30 m×0.250 mm × 0.25 μm). The equipmen is coupled wi h an Agilen 5975C mass spec ome y de ec o . The amoun o diffe en compounds was de e mined based on he in e nal s anda d o naph halene using he calib a ion cu es made wi h he comme cial compounds me hyl-S-(-)-lac a e (ML, 98 w %, Sigma- Ald ich), me hyl glycola e (MG, 98 w %, Al a Aesa ), py u aldehyde dime hyl ace al (PADA, 97 w %, Sigma-Ald ich), and 1,1,2,2- e a- me hoxyp opane (TMP, 99 w %, Sigma-Ald ich). Uniden ified com- pounds de ec able by GCeMS we e calcula ed om he esponse ac o o he me hyl lac a e, while he eac ion yields we e calcula ed on a ca bon basis. On he o he hand, suga s we e de e mined using a comme cial analy ical me hod (suc ose/ uc ose/D-glucose Assay Ki , Megazyme). The concen a ion o suga is ela ed wi h ha o educed nico inamide adenine dinucleo ide phospha e (NADPH), which has a B. Mu illo, e al. Ca alysis Today xxx (xxxx) xxx–xxx 2 specific abso bance band a 340 nm. The a ia ion o abso bance was measu ed using a V-670 Jasco UV– is spec opho ome e . 3. Resul s and discussion 3.1. Cha ac e iza ion o ca alys The p epa a ion and cha ac e iza ion o ma e ials UZAR-S3 and UZAR-S4 we e p e iously epo ed [30]. In his wo k, he same p e- pa a ion p ocedu e was ep oduced: fi s he syn hesis o he laye ed ma e ial UZAR-S3, a e i s swelling wi h nonylamine o ob ain swollen UZAR-S3 and finally he delamina ion o he la e by ex ac ion wi h hyd ochlo ic acid in a mix u e o sol en s (e hanol and wa e ) o ob ain UZAR-S4. Only he concen a ion o e hanol in he ex ac ion solu ion ( om HCl/H 2 O/e hanol 5:17:870 o 5:15:870 he e) has been sligh ly a ied o a o he nonylamine ex ac ion and he e o e he p ocess o delamina ion. Wi h his sligh a ia ion, he a e age specific su ace alue o he h ee p epa ed samples o UZAR-S4 is 352 ± 38 m 2 ·g −1 , sligh ly highe han he highes alue epo ed in [30] o 269 m 2 ·g −1 . In o de o unde s and he acidi y o UZAR-S3 and UZAR-S4, py - idine FTIR and NH 3 -TPD analyzes we e pe o med. Fig. 1 shows he FTIR spec a be o e and a e py idine adso p ion om UZAR-S3 and UZAR-S4. In he case o UZAR-S4, se e al peaks a e app ecia ed a e py idine adso p ion. The peak a 1618 cm −1 is due o s ong Lewis acid si es (SL) [23] while peaks a 1577 cm −1 and 1559 cm −1 co espond o weak Lewis acid si es (WL). The band cen e ed a 1490 cm −1 indica es ha py idine may be coo dina ed wi h Lewis acid si es, while p o ona ed py idine would be in e ac ing wi h B øns ed acid si es (B + L) [32]. A 1598 cm −1 he e is a peak due o he hyd ogen bond in e ac ion be- ween py idine and he ca alys su ace (H) [23]. In addi ion, he e a e wo mo e peaks ha co espond o he B øns ed acid si es (B) a 1637 cm −1 and 1541 cm −1 [32]. The p esence o bo h B øns ed and Lewis acid si es, coming, espec i ely, om silanol g oups and me al acan o bi als, would make UZAR-S4 sui able o ob aining me hyl lac a e as obse ed by o he au ho s in hei ca alys s [23]. On he o he hand, no significan bands o adso bed py idine a e obse ed in UZAR-S3. This may be due o he ac ha : i) py idine, wi h a kine ic diame e o 5.7 Å [33], canno pene a e in o he UZAR-S3 po es (ni ogen canno go inside he po es) because i is no delami- na ed and does no ha e access, ii) he delamina ed ma e ial UZAR-S4 is ich in high aspec a io pa icles wi h a highe concen a ion o silanol g oups. This occu s in he s udy o some laye ed zeoli es and silica es, whe e he limi a ions o allow py idine access [34] a e sa ed upon delamina ion, in ag eemen wi h he change o ex u al p ope ies (inc ease o ex e nal su ace a ea) an inc ease o silanols (as ypically obse ed by solid s a e NMR [30]). The e o e, he TPD analysis wi h ammonia has been pe o med due o he ac ha ammonia is ano he ela i ely s ong basic molecule commonly used o p obe acid si es in ca alys s like zeoli es. Ammonia, wi h a smalle kine ic diame e , 3.0Å, han py idine, has a g ea e accessibili y o he po es. The o al acidi y o UZAR-S3 and UZAR-S4 ca alys s, as well as he s eng h o hese acid si es, can be compa ed using he ammonia TPD (Fig. 2). Like o he au ho s [23], i has been conside ed ha peaks below 200 °C, be ween 200 and 400 °C and abo e 400 °C co espond o weak, medium and s ong acid si es, espec i ely. The o al alue o ammonia in UZAR-S3 is 1.45 mmol· g −1 while in UZAR-S4 i is 2.58 mmol· g −1 . These alues a e consis en wi h wha was p e iously seen wi h py idine FTIR and UZAR-S4 show a g ea e amoun o ac- cessible acid si es han UZAR-S3. Mo eo e , he deso p ion empe a- u es obse ed (Fig. 2) o UZAR-S4 a e 160 °C (weak), 283 °C (medium), 424 °C (s ong) and 496 °C (s ong), whe eas UZAR-S3 p e- sen s hem a 227 °C (medium), 375 °C (medium) and 464 °C (s ong). In Fig. 2 he inc ease in acidi y o UZAR-S4 is obse ed, which is ela ed o he appea ance o weak acidi y and abo e all he inc ease in medium acidi y. 3.2. Ca aly ic esul s The esul s ob ained in he con e sion o suga s (glucose and su- c ose) using UZAR-S3 and UZAR-S4 as ca alys s o 20 h a 160 °C a e shown in Table 1 and Fig. 3. In he case o glucose (Fig. 3), he yield o me hyl lac a e inc eased when he ca alys used in he eac ion was swollen and la e delamina ed: 3.2 % wi h UZAR-S3, 12.2 % wi h swollen UZAR-S3 and 37.4 % wi h UZAR-S4. This sugges s ha he delamina ion o UZAR-S3 s a ed wi h he in e cala ion o he amine, ad ancing pa o he ca aly ic p ope ies o he ully delamina ed UZAR-S4. In addi ion, glucose con e sion also inc eased om 89 % o 98 % (Table 1). This is due o he ac ha delamina ion p o ides a good access o he eac an s and in e media e p oduc s o ca aly ic si es as well as a as deso p ion o p oduc s, bo h effec s esul in an inc ease o he ca aly ic ac i i y o he ma e ial. In addi ion, as seen abo e, he ca alys UZAR-S4 has a highe p opo ion o acid si es, (especially medium-s eng h acid si es) han UZAR-S3 and hese acid si es become, h ough delamina ion, accessible o la ge molecules. This is in ag eemen wi h he li e a u e whe e he combi- na ion o Lewis and B øns ed acids along wi h he medium-s eng h Fig. 1. FTIR spec a o UZAR-S3 and UZAR-S4 as made, and UZAR-S3 and UZAR-S4 a e py idine adso p ion and hea ing a 353 K. Fig. 2. NH 3 TPD o UZAR-S3 and UZAR-S4. B. Mu illo, e al. Ca alysis Today xxx (xxxx) xxx–xxx 3 acid si es a o s he con e sion o me hyl lac a e [23]. Then, excep o MG, he yield o o he p oduc s sligh ly inc eased wi h he delamina ed ca alys s (Table 1). I should be aken in o accoun ha glucose has se e al eac ion pa hways [29], one o hem p oduces du ing he e o- aldol condensa ion e y h ose and glycolaldehyde. Then, glycolaldehyde can e ol e in o he iden ified MG, and e y h ose can e ol e in o me hyl inyl glycola e [11,35]. This las compound could be ela ed o some o he uniden ified peaks. The ano he pa hway o ms compounds wi h h ee ca bon a oms (glyce aldehyde and dihyd oxyace one) ha finally can be ans o med in o ML, PADA and TMP. The e o e, he delami- na ed ca alys seems o a o his second pa hway bu no he fi s . I should also be commen ed ha since he s e ic hind ances in UZAR-S4 a e lowe han in UZAR-S3, he o ma ion o se e al bigge compounds h ough he kind o eac ions s udied he e is plausible. In ac , i has been obse ed ha he po e size in mesopo ous ma e ials (e.g. MCM-41) in compa ison wi h ma e ials wi h smalle po es (e.g. zeoli e be a) can do shape-selec i e con ol [36]. Howe e , hese compounds was no de ec ed he e, al hough he e was an inc ease in uniden ified compounds, which could be ela ed o hem. The same inc ease in ML yield was epo ed in he con e sion o glucose wi h delamina ed po ous i anosilica es as ca alys s [29]: 3.9 % wi h UZAR-S1 and 1.8 % wi h i s p ecu so JDF-L1; and 5.2 % wi h UZAR-S2 and 1.2 % wi h i s p ecu so AM-4. The much be e esul s o UZAR-S4 can be a ibu ed mainly o he p esence o in in he s uc- u e, ha has a highe ca aly ic ac i i y o his eac ion han i anium [29]. Rega ding he ca aly ic es s ca ied ou wi h suc ose. In all cases, he yields o me hyl lac a e a e be e using suc ose ins ead o glucose (Fig. 3). In he case o UZAR-S3 i inc eases om 3.2% o 8.0%, i suc ose was used ins ead o glucose. The swollen ma e ial shows a g ea e inc ease, om 12.2% o 33.0%, due o he abo e men ioned incipien delamina ion. And delamina ed ma e ial UZAR-S4 inc eased he yield om 37.4% o 49.9%. This is because suc ose is a non- edu- cing suga , which has an o-glycosidic bond ha p o ec s ca bonyl g oups. In his way, he elease o hexoses om suc ose in solu ion is slow, a oiding he occu ence o unwan ed side eac ions wi h he mo e eac i e glucose and uc ose, and a less amoun o by-p oduc s is gene a ed [26,37]. Rega ding he suga con e sion, his was simila o glucose and suc ose, being almos o al wi h delamina ed UZAR-S4. The amoun o ca alys used in he eac ion was s udied, keeping cons an he amoun o he o he eagen s (225 mg o suc ose and 8 g o me hanol). Diffe en expe imen s we e pe o med wi h 50, 80, 100 and 160 mg o ca alys . A e age me hyl lac a e yields and s anda d de ia- ions a e shown in Fig. 4. I can be seen ha he diffe ence in ML yield is negligible using 100 mg ins ead o 160 mg o ca alys . Ne e heless, he ML yield dec eased i he amoun o ca alys was educed o 80 o 50 mg. The e o e, 100 mg is conside ed o be he op imum amoun o ca alys , because he p epa a ion o he ma e ial is expensi e and i is desi able o use he smalles amoun o i , while main aining a high le el o ca aly ic ac i i y. Wi h a educ ion in he amoun o ca alys nea 40 % he ML yield was educed only a 3.3 %. 3.3. Compa ison wi h li e a u e The yield o me hyl lac a e achie ed using UZAR-S4 as ca alys was 37.4 % wi h glucose and 49.9 % wi h suc ose. These alues a e in he o de o hose ob ained by Holm e al. [21] wi h Sn-be a zeoli e as Table 1 Ca aly ic esul s ob ained o he con e sion o suga s (160 °C, 20 h, 160 mg o ca alys and 225 mg o suga ) wi h diffe en ca alys s. Me hyl lac a e (ML), me hyl glycola e (MG), py u aldehyde dime hyl ace al (PADA), 1,1,2,2- e ame hoxyp opane (TMP) and non-iden ified p oduc s (n.i.p.). Mean alues and he co esponding s anda d de ia ions we e calcula ed om 4 eplica ed expe imen s in he same ope a ion condi ions. Ca alys Suga Yield (%) To al yield (%) Suga con . (%) ML MG PADA TMP n.i.p. Blank Glucose 1.1 ± 0.7 –2.5 ± 1.2 –6.0 ± 1.7 9.6 87 UZAR-S3 Glucose 3.2 ± 1.3 4.3 ± 1.2 1.1 ± 0.5 0.5 ± 0.2 5.6 ± 1.2 14.7 89 Swollen UZAR-S3 Glucose 12.2 ± 2.5 2.9 ± 0.9 3.3 ± 0.9 1.5 ± 0.6 9.2 ± 1.0 29.1 89.6 UZAR-S4 Glucose 37.4 ± 3.6 1.5 ± 0.6 6.2 ± 1.3 5.6 ± 1.8 7.8 ± 1.5 58.5 98 Blank Suc ose 1.4 ± 0.9 –2.5 ± 1.4 0.1 ± 0.04 4.3 ± 1.3 8.3 88.6 UZAR-S3 Suc ose 8.0 ± 0.6 1.2 ± 0.2 0.5 ± 0.1 0.2 ± 0.07 3.8 ± 0.3 13.7 88.8 Swollen UZAR-S3 Suc ose 33.0 ± 3.1 0.4 ± 0.2 1.7 ± 0.1 0.6 ± 0.2 6.0 ± 1.6 41.7 89.5 UZAR-S4 Suc ose 49.9 ± 9.6 0.1 ± 0.01 5.3 ± 1.1 1.5 ± 0.4 6.7 ± 1.6 63.5 99.5 Fig. 3. ML yield ob ained o he con e sion o suga s (160 °C, 20 h, 160 mg o ca alys and 225 mg o suga ) wi h diffe en ca alys s. Fig. 4. ML yield in he con e sion o suc ose wi h diffe en amoun s o UZAR- S4 as ca alys . Expe imen al condi ions: 160 °C, 20 h and 225 mg o suc ose. E o ba s co espond o s anda d de ia ions om a leas 3 eplica ed uns. B. Mu illo, e al. Ca alysis Today xxx (xxxx) xxx–xxx 4 ca alys : 44 % o glucose and 64 % o suc ose and he yield alue eached wi h Sn-MCM-41 43 % o glucose [29]. Table 2 shows a e- iew o he e ogeneous ca alys s wi h ea u ed esul s in he con e sion o suc ose o me hyl lac a e. This able includes esul s wi h eac ion condi ions simila o hose es ed he e.. The pe o mance o his wo k is among he bes in he li e a u e, he zeoli e Sn-be a based ca alys s being hose ha ha e achie ed he bes esul s o da e. Specifically, he highes yield co esponds o a Sn-be a zeoli e wi h hie a chical po e s uc u e ha a o s he access and deso p ion o compounds in he ca aly ic si es, as occu s as a esul o he ex olia ion s a egy ollowed he e. 3.4. Reusabili y o UZAR-S4 The in e es o a he e ogeneous ca alys is in i s eusabili y, he e- o e he UZAR-S4 solid was used in h ee consecu i e ca aly ic cycles o suc ose ans o ma ion using 100 mg o ca alys s. Be ween he ca aly ic cycles, he ca alys was d ied in he o en a 150 °C o 12 h in o de o emo e weakly adso bed compounds. The yield o diffe en p oduc s du ing 3 ca aly ic cycles is shown in Fig. 5. I can be seen ha when eusing UZAR-S4 ca alys in a second ca aly ic cycle he ML yield sligh ly dec eased by only 4% ( om 46.6% o 42.6%). Howe e , a hi d ca aly ic cycle implied a loss o ca aly ic ac i i y, he yield o ML dec easing by up o 30.5 %. The e o e, he physical-chemical changes ha occu ed in he ca alys du ing i s use we e s udied o find ou he easons o his deac i a ion. The o he p oduc s emained p ac ically cons an du ing he successi e ca aly ic cycles pe o med. 3.5. Cha ac e iza ion o ca alys a e ca aly ic cycles The X- ay diff ac og ams o UZAR-S4 be o e and a e one ca aly ic es wi h suc ose a e shown in Fig. 6a. As can be obse ed, UZAR-S4 has he XRD ea u es co esponding o an amo phous ma e ial and no ap- p eciable changes occu ed du ing he eac ion. I should be no ed ha o he po ous delamina ed ma e ials also show an amo phous X- ay diff ac og am [40], bu in he specific case o he UZAR-S4 ma e ial he elec on diff ac ion pe o med om some c ys alli es sugges s ha he s uc u e would be main ained a some ex ension espec o he UZAR- S3 ma e ial [30]. In he he mog a ime ic analysis pe o med and shown in Fig. 6b, i can be seen ha he weigh loss cu e o esh UZAR-S4 is clea ly diffe en om ha o he eco e ed solid a e eac ion. UZAR-S4 be o e he eac ion has a main loss below 100 °C due o adso bed sol en s (wa e and e hanol), while in he case o UZAR-S4 a e he eac ion he cu e p esen se e al weigh losses abo e 100 °C due o adso p ion o ca bonaceous compounds on he su ace o UZAR-S4 du ing he eac- ion. Fig. 7 shows he SEM images o UZAR-S4 be o e and a e eac ion wi h suc ose. The ex e nal mo phology o UZAR-S4 is e iden in Fig. 7a o shee s wi h a hickness o ca. 100 nm. A e eac ion, in gene al, he e we e no subs an ial changes in he appea ance o he pa icles al hough some pa icles seem o ha e been agmen ed. In addi ion o SEM, ene gy dispe si e X- ay spec oscopy (EDX) was pe o med o he same samples. Si/Sn a omic a io o esh UZAR-S4 was 39 ± 17 and Si/Sn a io o he solid eco e ed a e eac ion wi h suc ose was 42 ± 16. Va ia ions may indica e leaching o in in he eac ion solu ion bu he a ia ion is in he e o and canno be affi med empha ically. This leacha e could be he cause o he loss o ca aly ic ac i i y bu as will be seen below he e we e mo e ele an ex u al changes in he ca alys du ing eac ion. Gi en he high me hyl lac a e yield, he con ibu ion o ca aly ic ac i i y in homogeneous phase o his possible low amoun o in in he solu ion would ha e low influence on he ca aly ic esul s. Table 3 shows he ex u al p ope ies o UZAR-S4 ma e ial be o e and a e he eac ion wi h suc ose o each ca aly ic cycle. The e is a Table 2 Li e a u e e iew o me hyl lac a e yield om suc ose unde expe imen al condi ions analogous o hose applied in his wo k using he e ogeneous ca alys s. Ca alys s T (ºC) (h) MeOH (g) Suc ose (mg) Ca alys s (mg) ML Yield Re Ti-Be a 160 20 8 225 160 44 [21] Z -Be a 160 20 8 225 160 40 [21] Sn-Be a 160 20 8 225 160 64 [21] Sn-Be a 160 16 15 450 150 59 [10] CSM (1) 155 20 6 225 160 45 [11] Sn‐MWW 160 20 4 112.5 80 55 [13] Li-Sn-Be a 170 16 15 450 150 59 [38] Na-Sn-Be a 170 16 15 450 150 52 [38] K-Sn-Be a 170 16 15 450 150 66 [38] Rb-Sn-Be a 170 16 15 450 150 67 [38] Cs-Sn-Be a 170 16 15 450 150 62 [38] Sn-Be a-WO 3 160 5 12 320 200 60 [39] ZIF-8 160 24 8 225 160 34.8 [26] Hie a chical Sn-Be a 160 20 8 225 160 72.1 [22] Fe-doped SnO 2 160 20 16 220 160 44 [23] Mg-MOF-74 220 6 7 60 20 47 [24] MIP‐177‐LT (Ti/Sn) 160 24 8 225 160 39.4 [27] γ-NiOOH 200 12 40 400 200 38.7 [25] UZAR-S4 160 20 8 225 160 49.9 This wo k (1) CSM: Ca bon silica composi e ma e ial. Fig. 5. Yield o diffe en p oduc s in he con e sion o suc ose wi h UZAR-S4 up o 3 ca aly ic cycles. Expe imen condi ions: 160 °C, 20 h, 100 mg o ca alys and 225 mg o suga . B. Mu illo, e al. Ca alysis Today xxx (xxxx) xxx–xxx 5 sligh dec ease in he BET a ea a e he fi s cycle ( om 352 o 315 m 2 ·g −1 ) and also in he po e olume ( om 0.25 o 0.22 cm 3 ·g −1 ). This ac could be a ibu ed o he compounds adso bed in he ca alys (in line wi h he TGA cu es in Fig. 6b), a de e io a ion o ca alys s uc u e and a change in i s chemical composi ion due o leaching. Degasifica ion p io o he measu e o ni ogen iso he ms was ca ied ou a 200 °C, ne e heless in he TGA cu e o he eco e ed solid a e eac ion wi h suc ose he e is a weigh loss o 13.5 % om 200 °C. This sugges s ha ce ain adso bed compounds ( eac ion p oduc s) could dec ease bo h su ace a ea and po e olume. A e he second and hi d cycles, he dec ease in he specific su ace a ea and po e olume be- came g ea e , indica ing a u he deg ada ion o he ma e ial UZAR-S4 wi h he con inued use. This loss o ex u al p ope ies a e he second cycle is in ag eemen wi h he dec ease in ML yield (see Fig. 5). 4. Conclusions S annosilica e UZAR-S4 was success ully syn hesized by delamina- ion o UZAR-S3. This ma e ial can be applied as ca alys o he con- e sion o glucose and suc ose wi h high ac i i y and selec i i y o me hyl lac a e. The highe ML yield was 49.9 % in he con e sion o suc ose a 160 °C o 20 h wi h 160 mg o ca alys wi h a con e sion o suga nea 100 %. Fu he mo e, he delamina ion o he ca alys in- c eases i s ac i i y (ML yield inc eased om 8.0 % o UZAR-S3 o 49.9 % wi h UZAR-S4 in he con e sion o suc ose) because i imp o es he access o eac an s and in e media e p oduc s o ca aly ic si es. In addi ion, he ca aly ic acid si es ha e he app op ia e combina ion o Lewis and B øns ed acid si es and in UZAR-S4 he medium-s eng h acid si es a e g ea e compa ed o hose in UZAR-S3. Rega ding i s s abili y, UZAR-S4 was eused up o h ee imes, wi h a educ ion o me hyl lac a e yield ha can be explained by he deg ada ion o he ma e ial in he eac ion medium in ag eemen wi h he dec ease o specific su ace a ea. UZAR-S4 can be conside ed an in e es ing ma e ial in he con- e sion o suga s since he esul s ob ained a e in he o de o e en be e han hose epo ed be o e wi h o he he e ogeneous ca alys s. CRediT au ho ship con ibu ion s a emen Bea iz Mu illo: In es iga ion, Me hodology, Fo mal analysis, W i ing - o iginal d a . Ósca de la Iglesia: In es iga ion, Me hodology, Fo mal analysis, W i ing - o iginal d a , W i ing - e iew & edi ing, Visualiza ion. Césa Rubio: In es iga ion, Me hodology, Fig. 6. UZAR-S4 be o e and a e i s use in one ca aly ic es wi h suc ose (160 °C, 20 h, 100 mg o ca alys and 225 mg o suga ): a) X- ay diff ac og ams. b) TGA cu es. Fig. 7. SEM images o UZAR-S4: a) be o e eac ion; b) a e eac ion wi h suc ose (160 °C, 20 h, 100 mg o ca alys and 225 mg o suga ). Table 3 Tex u al p ope ies o UZAR-S4 esh and a e each ca aly ic cycle. Po e o- lume measu ed a P/P 0 = 0.97. Sample BET specific su ace a ea [m 2 g −1 ] Po e olume [cm 3 g −1 ] F esh UZAR-S4 352 ± 38 0.35 A e cycle 1 315 ± 15 0.29 A e cycle 2 174 ± 16 0.22 A e cycle 3 89 ± 5 0.12 B. Mu illo, e al. Ca alysis Today xxx (xxxx) xxx–xxx 6 Fo mal analysis, W i ing - o iginal d a , W i ing - e iew & edi ing. Joaquín Co onas: Concep ualiza ion, Me hodology, Supe ision, P ojec adminis a ion, Funding acquisi ion, W i ing - o iginal d a , W i ing - e iew & edi ing. 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