1
Gold ca alys ecycling s udy in base- ee glucose oxida ion eac ion
C. Megías-Sayago*, L. Bobadilla, S. I ano a, A. Penko a, M. A. Cen eno, J. A.
Od iozola
Depa amen o de Química Ino gánica, Uni e sidad de Se illa e Ins i u o de Ciencia de
Ma e iales de Se illa, US-CSIC, Amé ico Vespucio 49, 41092, Se illa, Spain
co esponding au o : c is ina.meg[email p o ec ed]
Abs ac
This wo k is de o ed o he s udy o iabili y o immobilized gold colloids on ca bon as
ca alys s o he base- ee glucose oxida ion eac ion wi h a special emphasis made on
ca alys s’ ecycling, ope a ional li e and possible ou es o deac i a ion/ eac i a ion
unde ba ch condi ions. The obse ed ca aly ic beha io is ela ed o all possible manne s
o deac i a ion, like gold me al s a e changes (pa icle size agglome a ion o leaching),
suppo modi ica ions o ac i e si es blocking by in e media es. In an a emp o eco e
he ini ial ca aly ic ac i i y, he samples a e subjec ed o di e en ea men s such as H2O
and NaOH washings and calcina ion. The ailu e o he egene a ion p ocedu es o
eco e he ini ial ac i i y and a e de ailed ca alys ’ cha ac e iza ion allows us o ind
ou he main cause o deac i a ion.
Keywo ds: base- ee glucose oxida ion, gold, ac i a ed ca bon, deac i a ion, ecycling
2
In oduc ion
In he las decades, an inc easing necessi y o eadd essing he in e es om ossil
aw ma e ials o enewable eeds ocks a ises due o he con inuous deple ion o he
o me s. A eo ien a ion o he scien i ic in e es owa ds ca bohyd a es as eeds ock o
ine chemis y is cu en ly o ac uali y [1]. The “g een ca alysis” app oach including
o ien a ion owa d aqueous media low empe a u e ca aly ic ans o ma ions and
ca bohyd a es highly unc ionalized ype molecules inc eases by a i s po en ial as ine
chemical eeds ock [2, 3]. I is hen o c ucial impo ance o explo e his po en ial owa d
e icien , en i onmen ally iendly and economically iable echnologies o la ge-scale
con e sion o ca bohyd a es in o indus ially iable bulk o in e media e compounds o
chemical, pha maceu ical o polyme ic use [4]. Among he exis ing eac ions o
ca bohyd a es con e sion, he selec i e oxida ion o plan -biomass monome s (hexoses
o pen oses) o co esponding ca boxylic acids p esen s a possibili y o easy in eg a ion
in al eady exis ing echnologies and he e o e an impo an economic in e es . The
he e ogeneously ca alyzed ans o ma ion o glucose o gluconic acid ecei ed impo an
a en ion [5-8]. Ne e heless, he p oposed p ocesses mus show an impo an
ac i i y/selec i i y balance and especially long e m s abili y in o de o be compe i i e
o he exis ing bio echnological p ocess o gluconic acid p oduc ion. The slow o se e e
ca alys deac i a ion and side eac ions p oduc s a e conside ed as he main challenges o
unde ake o u u e indus ializa ion o he e ogeneous p ocess [9].
Among he me al ca alys s discussed o selec i e oxida ion o glucose o gluconic
acid, he noble me al based ones a e he mos equen ly epo ed. Impo an p oduc i i y
was epo ed o P and Pd based ca alys s [10-12]. Especially imp o ed ac i i ies and
p oduc selec i i y o he P based ca alys s we e ob ained a e doping wi h Bi [12,13].
Howe e , du ing he eac ion, bismu h leaching was obse ed which con e s his
3
ca aly ic sys em inadequa e o pu e chemicals p oduc ion and may cause deac i a ion.
In addi ion, alkaline condi ions a e equi ed o inc ease he eac ion a e and o a oid
deac i a ion by eac ion in e media es. Al hough bene icial o he eac ion a e, he use
o base condi ions a ec s nega i ely he economics o he p ocess as glucona e sal
ins ead o pu e gluconic acid is ob ained a he end o eac ion. O en, a dec ease o
selec i i y due o he a o ed glucose o uc ose isome iza ion eac ion a high pH is
obse ed [6, 14]. Conside ing ha he ca alys s ope a e con inuously unde oxygen ich
a mosphe es P and Pd based ca alys could su e also deac i a ion by “o e -oxida ion”
esponsible o ac i e si es los by oxida ion. The majo i y o hese incon enien s could
be esol ed by using gold based ca alys s ins ead o pla inum and palladium based ones.
High selec i i y and ac i i y we e epo ed o suppo ed gold ca alys s and con e hem
in he mos p omising candida es o u u e indus ial implan a ion [8, 15-19]. Gold
ca alys s success ully ope a e unde oxygen. The ine na u e o he me al a oids loss o
ac i i y by oxida ion and p esen an impo an ac i i y o aldehyde g oup oxida ion and
ine ness owa ds seconda y alcoholic g oup oxida ion [6]. P obably he mos impo an
indings conce ning gold based sys ems is ha hey could ope a e unde a wide ange o
pH wi h he same ac i i y, which allows a oiding he glucose o uc ose isome iza ion
unde base condi ions.
Recen ly, he g ea po en ial o gold ca alys s o di ec glucose oxida ion in base ee
condi ions was demons a ed [20-23]. I was con i med ha ca alys p oduc i i y is ba ely
in luenced by he pH o he eac ion and ha he p ope choice o suppo is e y
impo an o ca alys pe o mance. Mine al oxides, as pu e ce ia o mixed ce ium oxides,
a e no sui able as suppo s, since he ca alys s su e slow deac i a ion by me al leaching
[21, 23]. On he o he hand, basic suppo s a e epo ed o be mos sui able in acidic
condi ions [20], al hough hei s abili y owa ds hyd oxyla ion has o be imp o ed.
4
Poin ing indus ial applica ion, he ca alys s should wi hs and con inuous ope a ion and
epea ed ecycling o a long ime, which implies low deac i a ion a e and/o easy
ac i i y eco e y. Ca bon based ca alys s p esen excellen s abili y in acidic media and
low ac i e me al leaching which con e s hem in po en ial candida es o s udy hei
ope a ion unde base ee glucose oxida ion eac ion.
In his con ex , he aim o his wo k is o s udy he ca aly ic pe o mance o a se ies o
Au/C ca alys s p epa ed by gold colloids immobiliza ion and hei ecyclabili y unde
epea ed ope a ion. All possible deac i a ion causes as gold leaching, sin e ing and
chemical poisoning (ac i e si es blocking by eac ion in e media es) a e con empla ed
and ela ed o he a ia ion o he ini ial gold pa icle size. The iabili y o some ca alys ’
ea men s be ween he ope a ion cycles aiming o eco e he ini ial deac i a ion was
also in es iga ed.
Expe imen al
Ca alys p epa a ion
Gold was deposi ed om p e- o med colloids p epa ed by educing o he PVA s abilized
gold p ecu so wi h NaBH4. 5.10-4 M aqueous solu ion o HAuCl4 (2w % nominal alue)
was mixed wi h he co esponding quan i y o PVA (1 w . % aqueous solu ion) and s i ed
o 20 min. A e , app op ia e amoun o 0.1 M eshly p epa ed NaBH4 solu ion was
quickly added o o he 20 min and he solu ion was pu in con ac wi h comme cially
a ailable ac i a ed ca bon powde DARCO® (Sigma Ald ich, 100 mesh pa icle size).
A e aging, he inal mix u e was cen i uged a 15000 pm o 20 minu es and esul ed
solids sepa a ed by il a ion. Samples we e inally calcined in s a ic ai a 300 ºC o 2
hou s.
5
Following his p ocedu e, ou samples we e p epa ed keeping cons an PVA:Au weigh
a io o 0.85 and a ying he NaBH4 :Au mola a io om 3 o 10. The la e was chosen
in o de o ge di e en inal gold pa icle size, as epo ed in p e ious s udies [24]. Table
1 summa izes he syn hesis pa ame e s and labelling o he ca alys s.
Table 1. Syn hesis pa ame e s o he p epa ed samples.
Sample
NaBH4 : Au
mola a io
AuC_I
3
AuC_II
5
AuC_III
10
Cha ac e iza ion
The gold con en s we e es ima ed h ough ICP analysis by using Ho iba Jobin Y on
spec ome e .
T ansmission elec on mic oscopy (TEM) s udy on pa icle size and dispe sion o he
ca alys s was pe o med on PHILIPS CM-200. The a e age gold pa icle size was
es ima ed based on su ace dis ibu ion calcula ions as shown in equa ion 1
D [3,2]= ∑D𝑖
3𝑣𝑖
n
1
∑D𝑖
2𝑣𝑖
n
1 eq. 1
whe e Di is he geome ic diame e o he i h pa icle, and i he numbe o pa icles wi h
his diame e . Fo pa icle size dis ibu ion, he o al numbe o measu ed pa icles
o e comes 200 o e e y sample.
The DRIFTS spec a we e eco ded a oom empe a u e wi hou sample dilu ion using a
The mo Nicole Nexus FT-IR spec ome e equipped wi h a liquid ni ogen cooled MCT

6
de ec o a 4 cm-1 esolu ion and a e age o 128 scans. The whole op ical pa h was pu ged
wi h CO2- and H2O- ee ni ogen. Abou 50 mg o esh o used ca alys inely g ounded
was loaded in he P aying Man isTM cell o each measu emen .
XPS measu emen s we e ca ied ou on Leybold-He eus LHS-1020 ins umen coupled
wi h EA200 de ec o and using non ch oma ic Mg K (220W, 11kV, 1253,6 eV). P io
o use he sample we e p essed in o a hin disk. The XPS spec a o all sample we e
eco ded a oom empe a u e and he binding ene gy was calib a ed on C1s a 284,6 eV
wi h an unce ain y ±0.2 eV. The spec a we e eco ded wi h cons an pass ene gy o 44
eV and 0.1 eV esolu ion o he s udied zones.
Base ee ae obic oxida ion o glucose
The oxida ion o glucose was pe o med in a glass ba ch eac o (50 mL) a cons an
empe a u e and s i ing a e a sa u a ed oxygen a mosphe e (app oxima e PO2 o 0.1
MPa). In a ypical expe imen , he eac o was cha ged wi h 0.2M glucose aqueous
solu ion, ca alys in Glucose/Au mola a io o 100 and oxygen sa u a ed by bubbling 20
mL/min o pu e oxygen low o ew minu es. The eac o was hen closed and eac ion
mix u e s i ed a 600 pm, a 40ºC du ing 18 hou s in base- ee condi ions.
The ecycling s udy was ca ied ou in a simila manne sepa a ing he ca alys om he
p oduc s by il a ion be ween he uns and e-using i main aining Glucose/Au mola
a io o 100 a e e y un. In some cases eac i a ion ea men s we e pe o med. The
samples a e il a ion we e ea ed ei he wi h dis illed wa e , 0.1 M NaOH solu ion o
he mally ea ed a 300ºC o 2h be o e e e y un.
7
The eac ion p oduc s we e iden i ied and quan i ied by HPLC equipped wi h e ac i e
index de ec o (Va ian 360-LC) and Hi-Plex H column (300 × 7,7 mm) se o 40ºC using
MilliQ wa e as mobile phase.
Glucose con e sion and p oduc selec i i y o gluconic acid (GA) we e calcula ed as
desc ibed by equa ion (1) and (2).
𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%)= [𝐺𝑙𝑢𝑐𝑜𝑠𝑒]𝑖𝑛−[𝐺𝑙𝑢𝑐𝑜𝑠𝑒]𝑜𝑢𝑡
[𝐺𝑙𝑢𝑐𝑜𝑠𝑒]𝑖𝑛 ∗100 eq (1)
𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝐺𝐴 (%)= 𝑚𝑜𝑙𝑒 𝐺𝐴
𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠*100 eq (2)
Resul s and discussion
The eal gold loading measu ed by ICP and he mean pa icle size deduced om TEM is
p esen ed in Table 2.
Table 2. Gold me al loadings and pa icle size o esh and spen ca alys s.
Sample
Au w .%
( esh),
Au, w .%
(spen 4 h cycle)
Au pa icle
size, nm
( esh)
Au pa icle
size, nm
(spen 1s cycle)
Au pa icle
size, nm
(spen 4 h cycle)
AuC_I
2.2
1.9
16.1
n.m.
20.4
AuC_II
2.4
1.9
8
17
16.6
AuC_III
2.3
1.9
4.8
8.9
7.7
n.m. no measu ed
All esh samples p esen simila gold loadings close o he a ge ed alue. The a e age
gold pa icle size a ies wi hin he se ies a ibu ed o he modi ica ion o he syn hesis
pa ame e s, chosen in a way o ha e a eal dispa i y o he ini ial pa icle size. Highe he
NaBH4:Au a io a a cons an PVA:Au a io, lowe he a e age gold pa icles size. The
8
syn hesis pa ame e s in luence s ongly he a e age pa icle size bu no he ac ual me al
loadings. The ep esen a i e mic og aphs o AuC_II sample and i s pa icle size
dis ibu ion a e p esen ed in Figu e 1.
Figu e 1. Rep esen a i e TEM mic og aphs and gold size dis ibu ions o esh and spen (4 h
cycle) AuC_II samples.
Ca aly ic ac i i y and ecycling s udy
The ca alys ac i i y o all samples and in e e y cycle, exp essed as glucose con e sion,
is shown in Figu e 2a. The calcula ed ca bon balance anges om 86 o 95 % ( i s s.
las cycle) indica ing possible adso p ion o eac i e o in e media es a e he i s cycle
bu no a e all successi e ones. The gluconic acid selec i i y on he base o liquid non-
adso bed p oduc s was ound 100% and will no be discussed om now on.
No ma e he gold pa icles size simila glucose con e sions a e ob ained in he i s
cycles neighbo ing 80%. Sys ema ically he second cycle gi es always less con e sion
han he i s one being he di e ence in he ange o 20-25 %. The pe o med blank es
wi h ac i a ed ca bon shows ha a his empe a u e only 8% o glucose could be
adso bed on he ca bon su ace and alse he ini ially calcula ed glucose con e sion.
Anyway he deac i a ion o he ca alys be ween he 1s and he 2nd cycle occu s
9
unequi ocally. I is also obse ed e y simila 2nd and 3 d cycles and sligh ly di e en 4 h
cycle. The sample AuC_III shows con inuous dec ease o glucose con e sion, while he
AuC_I and AuC_II s abilized a e he ini ial loss o ac i i y. Ne e heless, om
s a is ical poin o iew, wi hin he se ies o samples only AuC_II beha e di e en ly
(Figu e 2a).
Figu e 2. a) Glucose con e sion on esh and spen ca alys s unde 4 eac ion cycles b)
TOF (ba s) and pa icle size (poin s) compa ison in 1s and 4 h cycles on esh and spen
ca alys s.
As epo ed abo e he easons o deac i a ion could be assembled in wo g oups, hose
al e ing di ec ly he ac i e si es, as sin e ing o me al loss, and hose esponsible o
ca alys ’ blocking o suppo modi ica ions. The leaching o gold was checked by ICP
analysis o he emaining gold on ca alys a e accomplished ecycling s udy (Table 2)
and in he il a ion wa e be o e e e y cycle. No ma e he s a ing ca alys and/o
pa icle size, he same gold loss is de ec ed a e aging 15% o he ini ial gold loading and
always he gold loss be ween he 1s and he 2nd cycle ep esen s almos 80% o he o al
leaching.
16
indica ing ha no he adso bed in e media es no he modi ica ion o he suppo play
impo an ole in ca alys deac i a ion. The simila 2nd, 3 d and 4 h cycles demons a e also
ha he p esence o adso bed in e media es o modi ica ions o he suppo does no
inhibi he eac ion. I seems ha gold me al s a e, i.e. gold pa icle size a ia ion and
me al leaching, is he mos impo an pa ame e inducing loss o ac i i y. On one hand
he pa icles agglome a ion could esul in a bene icial op imal pa icle size and inc ease
in he speci ic ac i i y (AuC_II). And on he o he , in ou p e ious s udy [23] i was
epo ed ha he leached gold could pa icipa e in he eac ion wi h a ound 10%
con e sion inc ease be ween he 4 h and he 18 h hou o eac ion which oge he wi h he
8% o glucose adso p ion on ac i a ed ca bon accoun s o he 20% o glucose con e sion
dec ease be ween he 1s and he 2nd cycle. All his leads o he conclusion ha he mos
impo an p oblem o sol e is gold me al leaching.
This s a emen leads o he assump ion ha he ca bon suppo is no a key pa ame e in
his eac ion and ha he glucose oxida ion is occu ing only on he low de ec su ace
gold pa icles wi h p e e able a e age size o 15-20 nm. Ishida e al. [8] also sugges ed
ha he glucose oxida ion eac ion occu s p e e ably on gold pa icles su ace and no on
he gold/suppo in e ace o suppo . Ou obse a ions also suppo his idea, as he mos
impo an deac i a ion is p oduced by he gold s a e change.
The mechanism o glucose oxida ion in alkaline condi ions is al eady known and includes
p e e able glucose adso p ion on hyd oxyla ed Au su ace on which he glucose CHO
g oup is i s ly ac i a ed. A e wa e is eleased by o myl g oup H emo al and gluconic
acid is o med by OH ans e om he Au su ace [35]. Howe e , in base ee condi ions
he glucose oxida ion should p oceed h ough ca bonyl conjuga ed adical [36] and
p obably will include oxida ion h ough hyd ogen pe oxide o ma ion, as he ac i a ion
o molecula oxygen is imp obable on gold. Como i e al. [37] p oposed as a key eac ion

17
s ep he p esence o elec on- ich gold species, o med by hyd a ed glucose anions on
gold su ace a oms, which ac i a e he molecula oxygen by nucleophilic a ack and lead
o he o ma ion o hyd oxype oxides, ep esen ed schema ically in Figu e 6.
Figu e 6. Rep esen a ion o possible mechanism o glucose oxida ion in base ee
condi ions.
This s ep could be conside ed as one o he o Baeye –Villige eac ion s eps, including
addi ion o pe oxide o o myl g oup wi h hyd oxype oxide o ma ion and i s ea anging
wi h clea age o he C-C bond and p oduc s o ma ion [38]. The p oposed eac ion s eps
a e only a en a i e o p edic ing he eac ion mechanism in base ee condi ions.
Howe e a de ailed s udy is needed o con i m he iabili y o he p oposed eac ion
pa hway.
Conclusions
Se e al sample wi h a ying gold pa icle size a e p epa ed and es ed in base ee
oxida ion o glucose. The samples show simila beha io a ound 80% o glucose
con e sion owa d 100% selec i e gluconic acid p oduc ion in he i s cycle o eac ion.
The ecycling o he samples e eals ha deac i a ion occu s a e he 1s cycle
independen ly on he ini ial gold pa icle size. Gold leaching is con i med o be he
18
p ima y easons o ca alys deac i a ion. Also a gold agglome a ion occu s and esul s
bene icial o he speci ic ac i i y imp o emen when op imal gold a e age size is
achie ed. On he o he hand, he suppo na u e, i s chemical s a e o adso bed species do
no seem o be e y impo an . All eac i a ion ea men s esul in he same ac i i y end
indica ing ha he ca alys ope a ion is no a ec ed by in e media es adso p ion, emo al
o suppo modi ica ion. The e o e, e e y loss o ac i i y is due o gold me al leaching
a e he 1s cycle and when he leaching s ops he ac i i y emains cons an (2nd, 3 d, 4 h
cycle).
Acknowledgemen s
Financial suppo o his wo k has been ob ained om he Spanish Minis e io de
Economía y Compe i i idad (MINECO) (ENE2013-47880-C3-2-R) co- inanced by
FEDER unds om he Eu opean Union.
19
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