Jou nal o
Ca bon Resea ch
C
A icle
A Compa a i e S udy o A oma iza ion Ca alys s: The
Ad an age o Hyb id Oxy/Ca bides and Pla inum-Ca alys s
Based on Ca bon Gels
Luisa M. Pas ana-Ma ínez * , Se gio Mo ales-To es and F ancisco J. Maldonado-Hóda


Ci a ion: Pas ana-Ma ínez, L.M.;
Mo ales-To es, S.;
Maldonado-Hóda , F.J. A
Compa a i e S udy o A oma iza ion
Ca alys s: The Ad an age o Hyb id
Oxy/Ca bides and
Pla inum-Ca alys s Based on Ca bon
Gels. C2021,7, 21. h ps://doi.o g/
10.3390/c7010021
Recei ed: 22 Janua y 2021
Accep ed: 9 Feb ua y 2021
Published: 12 Feb ua y 2021
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2021 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
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dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
Depa men o Ino ganic Chemis y, Facul y o Sciences, Uni e si y o G anada, A enida Fuen e Nue a s/n,
ES-18071 G anada, Spain; semo o@ug .es (S.M.-T.); jmaldon@ug .es (F.J.M.-H.)
*Co espondence: lpas ana@ug .es; Tel.: +34-(958)-240-443
Abs ac :
This manusc ip is ocused on he ela ionship be ween sol-gel syn hesis p ocesses and
he de elopmen o new ac i e phases wi h i ed mo phology, po osi y and su ace chemis y. The
in luence o he abo e pa ame e s on he ca aly ic pe o mance o he p epa ed ma e ials o he
a oma iza ion o n-hexane o benzene is also e alua ed. Di e en se ies o ca alys s we e p epa ed,
ei he using noble me als (i.e., P ) o me al oxides (i.e., Mo, W), as ac i e phases. In bo h cases, he
ca aly ic pe o mance and s abili y o classical a oma iza ion ca alys s was signi ican ly imp o ed.
In e es ing one-po ca bo educ ion p ocess o he me al oxide du ing ca boniza ion is sugges ed as
a eal al e na i e o he p epa a ion o high-pe o mance a oma iza ion ca alys s, leading o he
o ma ion o less acidic and non-s oichiome ic oxides and ca bides.
Keywo ds: n-hexane; benzene; ca bon nanoma e ials; me al ac i e-phases; a oma iza ion
1. In oduc ion
A oma ic hyd oca bons a e in high demand because hey a e he main eeds ock o
he p oduc ion o a wide ange o p oduc s including pha maceu icals, cosme ics, dyes
o polyme s. The a oma iza ion o cheap and abundan ligh alkanes in o a oma ic ones
has been an impo an and al e na i e eac ion o oil he mal c acking o he las 50 yea s,
allowing he p epa a ion o high-oc ane gasoline blending componen s om naph ha,
among o he s [1–4].
The de elopmen o new ca alys s o he alo iza ion o ligh alkanes by ans o -
ma ion in o a oma ic p oduc s is an impo an challenge om an economic and scien i ic
poin o iew. T adi ional ca alys s such as P /Cl–Al
2
O
3
ha e p o ided low pe o mances
o ligh alkane (C
≤
7) a oma iza ion and, mo eo e , hey can be deac i a ed by loss o
chlo ine which is o med by decomposi ion along a eac ion o he Cl
–
g oups p esen
in he solid ca alys s. In ac , chlo ine is added o he eed causing co osion and sa e y
haza ds [
5
]. Zeoli es ha e been used as al e na i e non-chlo ina ed ca alys s. The bes
pe o mance o C
6
e o ming o benzene was desc ibed using P suppo ed on zeoli e L
exchanged wi h K (P /KL). These ca alys s showed di e en cha ac e is ics including weak
acidi y, low coke o ma ion, shape selec i i y and esis ance o sin e ing [
6
,
7
]. In p e ious
wo ks, we ha e also iden i ied simila ac o s o P -suppo ed ca alys s on exchanged
β
-zeoli es [
8
–
10
]. The exchange o zeoli e wi h alkali o alkali-ea h me als, namely Cs,
induces ex u al changes, educing he pa a in-adso p ion capaci y and he su ace acidi y,
minimizing he coke o ma ion and a o ing he o ma ion o small P clus e s wi h speci ic
P -suppo in e ac ions, hus enhancing he a oma iza ion yield.
Nowadays, he op imiza ion o a oma iza ion ca alys s con inues o be o g ea in e es
o he scien i ic communi y, as demons a ed by he high numbe o ecen publica ions in
his ield [
5
,
11
–
14
]. Re o ming o ligh alkanes is ca ied ou using zeoli es, mainly medium
po e-size ZSM-5 zeoli e which p esen s molecula shape selec i i y owa ds benzene,
C2021,7, 21. h ps://doi.o g/10.3390/c7010021 h ps://www.mdpi.com/jou nal/ca bon
C2021,7, 21 2 o 11
oluene and xylene (BTX). Di e en p omo e s (mainly Ga, Zn and B) a e used o enhance
he a oma ic yield by modi ying he acidi y o zeoli e, a o ing he alkane dehyd ogena ion
and minimizing coke deposi ion.
In gene al, ca bon ma e ials p esen a basic cha ac e and a de eloped su ace a ea
and po osi y, a oiding coke deposi ion and a o ing he dispe sion o suppo ed me allic
phases [
15
–
19
]. These p ope ies could inc ease he pe o mance o ca bon-based ca alys s
in a oma iza ion eac ions. I was epo ed ha high BTX yields can be ob ained using
Zn/HZSM5 ca alys s o me hanol a oma iza ion, al hough deac i a ion by coking is
s ong. Howe e , a e egene a ion, he esidual o coke emaining on he zeoli e su ace
can play an ac i e ole in he con ol o acidi y, dec easing he acid si es’ s eng h and
dis ibu ion, in such a way ha egene a ed ca alys s show good BTX yields and imp o ed
s abili y [20].
The numbe o e e ences using ca bon ma e ials o de elop a oma iza ion ca alys s
is qui e limi ed. Rod íguez-Ramos [
21
] sugges ed ha he a oma iza ion o n-hep ane on
Pd/ac i a ed ca bon ca alys s is a s uc u e-insensi i e eac ion. As he P pa icle size
dec eases, hyd ogenolysis is a o ed, bu some isome iza ion and a oma iza ion emain
unchanged e en a e doping wi h MgO o inc ease he numbe o basic si es. When
Ru/g aphi e is used as ca alys , [
22
] he elec onic densi y o Ru-nanopa icles is enhanced
by elec on ans e om g aphi e. This esul s in an inc ease o he ac i i y o he n-
hexane con e sion, bu accompanied only by a hyd ogenolysis inc ease (a oma iza ion
le els only eached alues a ound 10%). On he o he hand, T unschke e al. [
23
] poin ed
ou ha he in e ac ions o he ca bon phase wi h me al oxides du ing he mal ea men s
emo e he acidi y o he oxides, a oiding undesi able and pa allel eac ions (c acking).
The ca bon phase also a o s dehyd ogena ion o alkanes o alkenes, which in u n leads
o an inc eased yield o C
8
a oma iza ion in o e hylbenzene (EB) and o-xylene (OX) [
23
].
Simila conclusions we e ob ained by Hoang e al. [
24
]. They p epa ed Z O
2
/C composi es
(77.2% w . Z O
2
) p e ea ed a di e en empe a u es (be ween 700 and 1600
◦
C) o bo h
C
6
and C
8
a oma iza ion. In his case, he ca bon phase a oids he Z O
2
sin e ing and
induces he des uc ion o acid si es du ing he mal p e ea men s. The mos selec i e
ca alys is ob ained a e p e ea men a 1000 ◦C, p o iding a benzene selec i i y o 67%.
Toge he wi h classical ac i a ed ca bons, oday he e is a ailable a la ge amoun
o ad anced ca bon ma e ials ( om 1D o 3D nanoma e ials) wi h imp o ed and i ed
p ope ies o be used in ca alysis [
15
,
17
,
18
,
25
]. Among hem, ca bon gels (ae ogels o
xe ogels) p esen addi ional ad an ages such as homogenei y and pu i y; mo eo e , i s
chemical and po ous cha ac e is ics can be speci ically designed and con olled du ing
he sol-gel p ocess, hus showing la ge possibili ies in de eloping new ca alys s [
26
].
Di e en a oma iza ion ca alys s con aining se e al ac i e phases such as me al oxides
o C , Mo o W, o noble me als like P ha e been p epa ed by modi ying hei syn hesis
p ocedu e [
27
–
29
]. In his manusc ip , he ad an age o using ca bon ma e ials in he design
o new a oma iza ion ca alys s is b ie ly summa ized. The esul s ob ained in he n-hexane
a oma iza ion show ha hese easily p epa ed ca alys s a e a easible al e na i e o me al-
exchanged zeoli ic ma e ials, imp o ing aspec s such as s abili y o selec i i y. Co ela ions
be ween he a iables o p epa a ion (e.g., su ac an s, d ying, ca boniza ion empe a u e),
he mo phology and ac i e phases o med (non-s oichiome ic oxides/ca bides) and he
ca aly ic pe o mances a e analyzed.
2. Resul s and Discussion
Today, one o he mos deeply s udied sys ems o he a oma iza ion o n-hexane is
he me al-exchanged ZSM5 zeoli e. Tshabalala e al. [
30
] poin ed ou he highe ac i i y
and s abili y o acid HZSM5 ega ding Ga, Mo o Zn-exchanged ZSM5, whe e mainly
Ga/ZSM5 deac i a ed quickly. The highes a oma iza ion selec i i y was obse ed o
Zn/ZSM5 (a ound 43%) while Mo/ZSM5 exhibi s he lowes selec i i y alues (be ween 20
and 25%), e en wo se han hose obse ed o acid o ms. Mo e ecen ly, Huyen [
31
] e al.
p epa ed Zn/ZSM-5 and B-Zn/ZSM5 ca alys s. Zn/ZSM5 is mo e ac i e and selec i e o
C2021,7, 21 3 o 11
benzene, bu deac i a ed as e . The selec i i y eached 52% a 500
◦
C, bu also dec eased
wi h deac i a ion a e 3 h on s eam.
We ha e p epa ed di e en ca alys se ies. Figu e 1summa izes he pe o mance o
Mo and W-ca bon xe ogels (p epa ed ollowing he me hodology b in he Sec ion 3) a e
di ec ca boniza ion a 500
◦
C. The highe ac i i y o Mo ca alys s compa ed o he W-ones
is no ewo hy, and i is also sligh ly mo e selec i e o benzene. Con e sion inc eased
mo e o less linea ly in bo h cases wi h inc easing empe a u e, bu in his sense undesi ed
c acking inc eased a he expense o a oma iza ion.
C 2021, 7, 21 3 o 12
Ga/ZSM5 deac i a ed quickly. The highes a oma iza ion selec i i y was obse ed o
Zn/ZSM5 (a ound 43%) while Mo/ZSM5 exhibi s he lowes selec i i y alues (be ween
20 and 25%), e en wo se han hose obse ed o acid o ms. Mo e ecen ly, Huyen [31]
e al. p epa ed Zn/ZSM-5 and B-Zn/ZSM5 ca alys s. Zn/ZSM5 is mo e ac i e and selec i e
o benzene, bu deac i a ed as e . The selec i i y eached 52% a 500 °C, bu also de-
c eased wi h deac i a ion a e 3 h on s eam.
We ha e p epa ed di e en ca alys se ies. Figu e 1 summa izes he pe o mance o
Mo and W-ca bon xe ogels (p epa ed ollowing he me hodology b in he Ma e ials and
Me hods sec ion) a e di ec ca boniza ion a 500 °C. The highe ac i i y o Mo ca alys s
compa ed o he W-ones is no ewo hy, and i is also sligh ly mo e selec i e o benzene.
Con e sion inc eased mo e o less linea ly in bo h cases wi h inc easing empe a u e, bu
in his sense undesi ed c acking inc eased a he expense o a oma iza ion.
Figu e 1. Con e sion and p oduc s dis ibu ion ob ained wi h W (opened symbols) and Mo
(closed symbols) p epa ed as doped ca bon xe ogels a 500 °C (adap ed om e . [27]).
The physicochemical p ope ies o hese samples a e summa ized in Table 1. Bo h
ca bon ae ogels showed a simila po e size dis ibu ion, being samples wi h high
mac opo e olumes and SBET a ound 500 m2 g−1, bu wi hou any app eciable mesopo os-
i y. The di e ence be ween he samples is ela ed o he chemical composi ion o hei
su aces. In spi e o he ac ha bo h samples we e p epa ed wi h a simila me al con en
in he s a ing solu ion, he o al me al con en de ec ed by TG is highe in he case o he
Mo sample (i.e., 1.9% and 1.4% w . o Mo500 and W500, espec i ely). This implies a
di e en M–C in e ac ion, in luencing p ocesses such as lixi ia ion du ing he sol en ex-
change p ocess. I is also no ewo hy ha he educ ion o he Mo species du ing he ca -
boniza ion e en a 500 °C is a o ed, ega ding he W-ones. Ca bon e ol ed as COx and
Mo (VI) educed in a signi ican p opo ion o Mo (V). This deepe gasi ica ion du ing he
ca boniza ion p ocess also inc eased he o al me al con en in his sample ega ding he
W500, whe e ungs en emains as W(VI). Mo eo e , he su ace me al con en de ec ed by
XPS o Mo-500 is also highe han o W500, indica ing a bes dispe sion (smalle sin e ing)
o he Mo phase.
Figu e 1.
Con e sion and p oduc s dis ibu ion ob ained wi h W (opened symbols) and Mo (closed
symbols) p epa ed as doped ca bon xe ogels a 500 ◦C (adap ed om e . [27]).
The physicochemical p ope ies o hese samples a e summa ized in Table 1. Bo h
ca bon ae ogels showed a simila po e size dis ibu ion, being samples wi h high mac opo e
olumes and S
BET
a ound 500 m
2
g
−1
, bu wi hou any app eciable mesopo osi y. The
di e ence be ween he samples is ela ed o he chemical composi ion o hei su aces. In
spi e o he ac ha bo h samples we e p epa ed wi h a simila me al con en in he s a ing
solu ion, he o al me al con en de ec ed by TG is highe in he case o he Mo sample
(i.e., 1.9% and 1.4% w . o Mo500 and W500, espec i ely). This implies a di e en M–C
in e ac ion, in luencing p ocesses such as lixi ia ion du ing he sol en exchange p ocess.
I is also no ewo hy ha he educ ion o he Mo species du ing he ca boniza ion e en a
500
◦
C is a o ed, ega ding he W-ones. Ca bon e ol ed as CO
x
and Mo (VI) educed in a
signi ican p opo ion o Mo (V). This deepe gasi ica ion du ing he ca boniza ion p ocess
also inc eased he o al me al con en in his sample ega ding he W500, whe e ungs en
emains as W(VI). Mo eo e , he su ace me al con en de ec ed by XPS o Mo-500 is also
highe han o W500, indica ing a bes dispe sion (smalle sin e ing) o he Mo phase.
Table 1. Physicochemical p ope ies o me al-doped ca bon ca alys s.
Sample Me al
(%w .)
XPS
M/C
Ac i e
Species
SBET
(m2g−1)
Vmac o
(cm3g−1)
Vmeso
(cm3g−1)
Mo500 1.9 7.6 Mo (V) 82%
Mo (VI) 18% 481 1.40 0.00
W500 1.4 1.4 W (VI) 100% 528 1.32 0.05
SBET = appa en su ace a ea; Vmac o = mac opo e olume; Vmeso = mesopo e olume.
C2021,7, 21 4 o 11
Di e en na u e and dis ibu ions o ac i e si es a e ob ained depending on he expe -
imen al condi ions used o he p epa a ion o me al–ca bon doped gels. By inc easing he
ca boniza ion empe a u e up o 1000
◦
C, i o ced he p og essi e educ ion o he me al
oxides. In his sense, a mix u e o Mo (III) and Mo (V) in a a io o 45/55 was obse ed
o he sample Mo-1000 ( esul s no shown) [
27
], howe e , al hough W emains mainly
(93%) as W (VI), he e is obse ed a ce ain ca bidiza ion (7% o WC). Compa ing he
esul s ob ained wi h W-doped ca bon ae ogels ca bonized a 500
◦
C and 1000
◦
C [
27
],
i was poin ed ou ha he o al acidi y de e mined by isop opanol decomposi ion is
main ained, bu s ongly dec eased he p opo ion o he p opene/ace one o med in his
eac ion, which deno es a s ong inc ease o basic cen e s ega ding hose acid ones wi h
inc easing ca boniza ion empe a u e. This has led o a s ong inc ease in he selec i i y o
benzene using W1000 ega ding W500, which is enhanced om 17 o 38% a 550
◦
C. These
esul s a e in ag eemen wi h hose p e iously published [
23
,
24
], in which he ca bon phase
de e mines he dispe sion and acid cha ac e o me al oxides.
In spi e o he la ge educ ion o Mo phases, he ca bidiza ion o hese non-s oichiome ic
oxides du ing ca boniza ion is no obse ed. I was p e iously sugges ed ha Mo-
ca bidiza ion equi es he p esence o H
2
and empe a u es anging 700
◦
C [
32
]. Ne -
e heless, because a oma iza ion is ca ied ou in he p esence o H
2
, he o ma ion o
ca bides along he eac ion canno be uled ou . In ac , Delpo e e al. [
33
] obse ed
he o ma ion o ca bide and oxyca bide species (MoC
x
O
y
) using MoO
3
ca alys o he
isome iza ion o n-hexane a 350
◦
C. They epo ed ha he Mo phases o med a e highly
dependen o he H2/hexane a io, de e mining he ca alys ’s pe o mance.
In ou case, when he e olu ion o con e sion and selec i i y wi h he ime on s eam
we e analyzed, a e y s able pe o mance (Figu e 2) o Mo-doped samples was obse ed,
p obably also indica ing a negligible ans o ma ion o he ca alys ’s su ace. I is no e-
wo hy ha his s abili y is obse ed in spi e o he same sample being used a di e en
empe a u es (Figu e 1) o a ound 3h. MoO
x
a e no mally used as isome iza ion ca alys s
o ob ain b anched alkanes, howe e , i is obse ed ha in ou case he isome iza ion is
negligible independen o he eac ion empe a u e o ime on s eam. C acking is he main
p ocess, a o ed by he acidi y o he ca alys s and he inc easing eac ion empe a u e.
C 2021, 7, 21 5 o 12
Figu e 2. Con e sion and p oduc s dis ibu ion ob ained wi h Mo500 ca bon ae ogel a 550 °C, as
a unc ion o ime on s eam.
Molybdenum ca bide ca alys s ha e a ac ed he a en ion o di e en esea ch
g oups [34,35] as a oma iza ion ca alys s o ligh alkanes, being p oposed as al e na i es
o noble me al (P ) ca alys s. The sol-gel p ocedu e o ob ain Mo-doped ca bon gels was
modi ied and samples we e p epa ed ollowing he me hodology (c), in he Ma e ials and
Me hods sec ion. This implies ha he i s RF polyme iza ion s ep is ca ied ou in agi-
a ed ba ch eac o s wi h he p esence/absence o su ac an s, hus de ining he mo phol-
ogy o ma e ials ( ibe s, sphe es, e c. can be o med). Gels a e pa ially cu ed in suspen-
sion be o e doping wi h he co esponding ac i e phase. This p ocedu e is as e han p o-
cedu e (b), a oiding la ge cu e pe iods inside he glass molds, bu also a oids he o -
ma ion o me al nanopa icles apped inside he o ganic s uc u e o he RF o ganic gels.
A e p e-gela ion, nanopa icles a e o ced o be o med on he su ace o p e iously
gelled RF-s uc u es. Addi ionally, su ac an molecules a e inco po a ed in o he RF
chemical s uc u e o ming RFS composi es ha o e imp o ed ancho ing si es o me al
adso p ion du ing he imp egna ion p ocess [29], allowing he ob aining o a high co e -
age on he ca bon su ace o me al nanopa icles (Figu e 3) e en a e he mal ca boniza-
ion a high empe a u e (900 °C).
Figu e 2.
Con e sion and p oduc s dis ibu ion ob ained wi h Mo500 ca bon ae ogel a 550
◦
C, as a
unc ion o ime on s eam.
Molybdenum ca bide ca alys s ha e a ac ed he a en ion o di e en esea ch
g oups [
34
,
35
] as a oma iza ion ca alys s o ligh alkanes, being p oposed as al e na i es
C2021,7, 21 5 o 11
o noble me al (P ) ca alys s. The sol-gel p ocedu e o ob ain Mo-doped ca bon gels was
modi ied and samples we e p epa ed ollowing he me hodology (c), in he Sec ion 3. This
implies ha he i s RF polyme iza ion s ep is ca ied ou in agi a ed ba ch eac o s wi h
he p esence/absence o su ac an s, hus de ining he mo phology o ma e ials ( ibe s,
sphe es, e c. can be o med). Gels a e pa ially cu ed in suspension be o e doping wi h he
co esponding ac i e phase. This p ocedu e is as e han p ocedu e (b), a oiding la ge
cu e pe iods inside he glass molds, bu also a oids he o ma ion o me al nanopa icles
apped inside he o ganic s uc u e o he RF o ganic gels. A e p e-gela ion, nanopa icles
a e o ced o be o med on he su ace o p e iously gelled RF-s uc u es. Addi ionally,
su ac an molecules a e inco po a ed in o he RF chemical s uc u e o ming RFS com-
posi es ha o e imp o ed ancho ing si es o me al adso p ion du ing he imp egna ion
p ocess [
29
], allowing he ob aining o a high co e age on he ca bon su ace o me al
nanopa icles (Figu e 3) e en a e he mal ca boniza ion a high empe a u e (900 ◦C).
C 2021, 7, 21 6 o 12
Figu e 3. Highly dispe sed Mo nanopa icles on ca bon xe ogel S10 (RF + CTAB ca ionic su ac an
+ -bu anol) a e ca boniza ion a 900 °C.
The in e ac ion o he RFS composi es wi h he me al p ecu so is mainly de e mined
by he na u e o he su ac an . The s onge he RFS–Mo links, he deepe he educ ion
o he Mo phase will be obse ed du ing ca boniza ion [28]. The XPS analysis o samples
p epa ed by his p ocedu e indica es ha up o 60% o he Mo de ec ed in su ace is o m-
ing like β-Mo2C; his phase is also de ec ed by XRD in some cases [36]. Thus, highly dis-
pe sed Mo nanopa icles can be easily o med and ca bu ized when suppo ed on
nanos uc u ed RFS gels ollowing an easy “one po ” p ocedu e, mainly when using a
ca ionic su ac an ha in e ac s a ac i ely wi h he anionic RF mac omolecule.
Figu e 4 shows he ca aly ic pe o mance o S7 ca alys s, as an example, showing he
ypical e olu ion o his ca aly ic se ies. This sample is syn hesized in he p esence o ca -
ionic CTAB, TMB and -bu anol as co-su ac an s, and a e ca boniza ion he a io
MoOx/MoCx = 42/58. As in he p e ious Mo se ies, con e sion inc eased mo e o less lin-
ea ly wi h empe a u e, isome iza ion is negligible and c acking is a o ed wi h inc eas-
ing empe a u e. Due o he s ong deg ee o educ ion o he Mo-oxide phase, he smalle
ac i i y o S7 ega ding Mo-500 is no ewo hy, which showed a o al n-hexane con e sion
a 580 °C. Howe e , selec i i y o benzene eached highe alues (o e 60% a 500 °C). In
hese expe imen al condi ions (500 °C; Figu e 4b), bo h con e sion and selec i i y slowly
dec eased wi h ime on s eam. Thus, less ac i e bu mo e selec i e phases a e ob ained
by he p og essi e educ ion o he Mo phase and he o ma ion o Mo oxyca bides. E en
when inc easing empe a u e, he selec i i y a 550 °C shows alues a a ound 50%, in-
c easing signi ican ly om alues anging 25% o Mo-500 ca alys (Figu e 1) a his em-
pe a u e.
Figu e 3.
Highly dispe sed Mo nanopa icles on ca bon xe ogel S10 (RF + CTAB ca ionic su ac an +
-bu anol) a e ca boniza ion a 900 ◦C.
The in e ac ion o he RFS composi es wi h he me al p ecu so is mainly de e mined
by he na u e o he su ac an . The s onge he RFS–Mo links, he deepe he educ ion
o he Mo phase will be obse ed du ing ca boniza ion [
28
]. The XPS analysis o samples
p epa ed by his p ocedu e indica es ha up o 60% o he Mo de ec ed in su ace is o ming
like
β
-Mo
2
C; his phase is also de ec ed by XRD in some cases [
36
]. Thus, highly dispe sed
Mo nanopa icles can be easily o med and ca bu ized when suppo ed on nanos uc u ed
RFS gels ollowing an easy “one po ” p ocedu e, mainly when using a ca ionic su ac an
ha in e ac s a ac i ely wi h he anionic RF mac omolecule.
Figu e 4shows he ca aly ic pe o mance o S7 ca alys s, as an example, showing
he ypical e olu ion o his ca aly ic se ies. This sample is syn hesized in he p esence
o ca ionic CTAB, TMB and -bu anol as co-su ac an s, and a e ca boniza ion he a io
MoO
x
/MoC
x
= 42/58. As in he p e ious Mo se ies, con e sion inc eased mo e o less
linea ly wi h empe a u e, isome iza ion is negligible and c acking is a o ed wi h inc eas-
ing empe a u e. Due o he s ong deg ee o educ ion o he Mo-oxide phase, he smalle
ac i i y o S7 ega ding Mo-500 is no ewo hy, which showed a o al n-hexane con e sion a
580
◦
C. Howe e , selec i i y o benzene eached highe alues (o e 60% a 500
◦
C). In hese
expe imen al condi ions (500
◦
C; Figu e 4b), bo h con e sion and selec i i y slowly de-
c eased wi h ime on s eam. Thus, less ac i e bu mo e selec i e phases a e ob ained by he
p og essi e educ ion o he Mo phase and he o ma ion o Mo oxyca bides. E en when

C2021,7, 21 6 o 11
inc easing empe a u e, he selec i i y a 550
◦
C shows alues a a ound 50%, inc easing
signi ican ly om alues anging 25% o Mo-500 ca alys (Figu e 1) a his empe a u e.
C 2021, 7, 21 7 o 12
Figu e 4. E olu ion o con e sion and p oduc s dis ibu ion o sample S7: (a) as a unc ion o empe a u e and (b) as a
unc ion o ime on s eam a 500 °C.
P ca alys s [28] we e p epa ed by classical incipien we ness imp egna ion using as
suppo a mesopo ous ca bon ae ogel p e iously syn hesized (p ocedu e a). A e p e-
ea men s in He low a di e en empe a u es, only me allic P 0 species we e de ec ed.
The ca aly ic pe o mance is ela ed he e o e only wi h he dispe sion o me allic nano-
pa icles. Figu e 5 summa izes he pe o mance o C900 2P ca alys , which con ains a 2%
o P (%w .). The mean P pa icle size inc eased om 2.8 o 7.3 nm a e p e ea men a
600 o 900 °C in He low, espec i ely. A a glance, signi ican di e ences a e obse ed
ega ding he p e ious se ies using oxide/oxyca bide ca alys s such as: (i) P ca alys s a e
ac i e a signi ican ly lowe empe a u es; (ii) in gene al, benzene is he main eac ion
p oduc ob ained; (iii) signi ican isome iza ion p ocess akes place a low empe a u e;
and (i ) a highe empe a u es, con e sion inc eases a o ing a oma iza ion ega ding
isome iza ion and c acking.
Figu e 5. E olu ion o con e sion and p oduc s dis ibu ion o sample C900 2P as a unc ion o empe a u e a e p e-
ea men in He a (a) 600 °C and (b) 900 °C.
Small P nanopa icles ob ained a low p e ea men empe a u e a e highly ac i e
and n-hexane con e sion inc eased om 35 o 90% in a sho empe a u e ange (be ween
400 and 465 °C); in his case, a oma iza ion emains mo e o less cons an and c acking
(a) (b)
(a) (b)
Figu e 4.
E olu ion o con e sion and p oduc s dis ibu ion o sample S7: (
a
) as a unc ion o empe a u e and (
b
) as a
unc ion o ime on s eam a 500 ◦C.
P ca alys s [
28
] we e p epa ed by classical incipien we ness imp egna ion using as
suppo a mesopo ous ca bon ae ogel p e iously syn hesized (p ocedu e a). A e p e ea -
men s in He low a di e en empe a u es, only me allic P
0
species we e de ec ed. The
ca aly ic pe o mance is ela ed he e o e only wi h he dispe sion o me allic nanopa icles.
Figu e 5summa izes he pe o mance o C900 2P ca alys , which con ains a 2% o P
(%w .). The mean P pa icle size inc eased om 2.8 o 7.3 nm a e p e ea men a 600 o
900
◦
C in He low, espec i ely. A a glance, signi ican di e ences a e obse ed ega ding
he p e ious se ies using oxide/oxyca bide ca alys s such as: (i) P ca alys s a e ac i e
a signi ican ly lowe empe a u es; (ii) in gene al, benzene is he main eac ion p oduc
ob ained; (iii) signi ican isome iza ion p ocess akes place a low empe a u e; and (i ) a
highe empe a u es, con e sion inc eases a o ing a oma iza ion ega ding isome iza ion
and c acking.
C 2021, 7, 21 7 o 12
Figu e 4. E olu ion o con e sion and p oduc s dis ibu ion o sample S7: (a) as a unc ion o empe a u e and (b) as a
unc ion o ime on s eam a 500 °C.
P ca alys s [28] we e p epa ed by classical incipien we ness imp egna ion using as
suppo a mesopo ous ca bon ae ogel p e iously syn hesized (p ocedu e a). A e p e-
ea men s in He low a di e en empe a u es, only me allic P 0 species we e de ec ed.
The ca aly ic pe o mance is ela ed he e o e only wi h he dispe sion o me allic nano-
pa icles. Figu e 5 summa izes he pe o mance o C900 2P ca alys , which con ains a 2%
o P (%w .). The mean P pa icle size inc eased om 2.8 o 7.3 nm a e p e ea men a
600 o 900 °C in He low, espec i ely. A a glance, signi ican di e ences a e obse ed
ega ding he p e ious se ies using oxide/oxyca bide ca alys s such as: (i) P ca alys s a e
ac i e a signi ican ly lowe empe a u es; (ii) in gene al, benzene is he main eac ion
p oduc ob ained; (iii) signi ican isome iza ion p ocess akes place a low empe a u e;
and (i ) a highe empe a u es, con e sion inc eases a o ing a oma iza ion ega ding
isome iza ion and c acking.
Figu e 5. E olu ion o con e sion and p oduc s dis ibu ion o sample C900 2P as a unc ion o empe a u e a e p e-
ea men in He a (a) 600 °C and (b) 900 °C.
Small P nanopa icles ob ained a low p e ea men empe a u e a e highly ac i e
and n-hexane con e sion inc eased om 35 o 90% in a sho empe a u e ange (be ween
400 and 465 °C); in his case, a oma iza ion emains mo e o less cons an and c acking
(a) (b)
(a) (b)
Figu e 5.
E olu ion o con e sion and p oduc s dis ibu ion o sample C900 2P as a unc ion o empe a u e a e
p e ea men in He a (a) 600 ◦C and (b) 900 ◦C.
C2021,7, 21 7 o 11
Small P nanopa icles ob ained a low p e ea men empe a u e a e highly ac i e
and n-hexane con e sion inc eased om 35 o 90% in a sho empe a u e ange (be ween
400 and 465
◦
C); in his case, a oma iza ion emains mo e o less cons an and c acking
inc eased a he expense o isome iza ion p ocesses. Wi h inc eases in he P pa icle size
he e is a s ong educ ion o he ca alys ac i i y, bu a oma iza ion is s ongly a o ed wi h
inc easing eac ion empe a u e ( eaching alues o 78% a 500
◦
C), he e o e dec easing in
his sense bo h isome iza ion and c acking. These changes clea ly poin ou he s uc u e-
sensi i e cha ac e o a oma iza ion on P -suppo ed ca alys s.
A simila e ec was obse ed when inc easing he P con en om 2 o 5%
(Table 2).
Con e sion dec eased in spi e o he highe P con en , bu benzene becomes he main
p oduc ob ained, wi h a selec i i y alue o a ound 65%. Mo eo e , i is also no ewo hy
he high s abili y o P ca alys s when suppo ed on ca bon ae ogels (Figu e 6). These
esul s poin ou he impo ance o he ca bon gel ma e ials used o de elop highly selec i e
and s able ca alys s o he a oma iza ion o ligh alkanes (n-hexane). In gene al, he main
a oma iza ion p oduc de ec ed was benzene, while oluene was no de ec ed. C acking
leads o he o ma ion o he C1–C5 compounds and me hylcyclopen ane (MCP) was ound
as he main isome iza ion p oduc . Con e sion was calcula ed aking in o accoun he
amoun o hexane ans o med and selec i i y as he amoun o each de ec ed p oduc o e
he o al hexane consumed. Thus, he alues o c acking selec i i y p esen ed include he
amoun s o he C1–C5 compounds [27,28].
Table 2.
In luence o he P con en on he ca aly ic pe o mance o suppo ed ca alys s p e ea ed a
600 ◦C. Con e sion and p oduc s dis ibu ion ob ained a 475 ◦C.
Sample d P C S C ack S Iso S A om Yield A om
(nm) (%) (%) (%) (%) (%)
C900 2P 2.8 86.8 50.7 4.7 44.7 38.8
C900 5P 9.9 60.5 32.7 2.4 64.9 39.3
d P = P pa icle size; C = Con e sion; S C ack = Selec i i y o c acking; S Iso = Selec i i y o isome iza ion;
S A om = Selec i i y o a oma iza ion.
C 2021, 7, 21 8 o 12
inc eased a he expense o isome iza ion p ocesses. Wi h inc eases in he P pa icle size
he e is a s ong educ ion o he ca alys ac i i y, bu a oma iza ion is s ongly a o ed
wi h inc easing eac ion empe a u e ( eaching alues o 78% a 500 °C), he e o e de-
c easing in his sense bo h isome iza ion and c acking. These changes clea ly poin ou
he s uc u e-sensi i e cha ac e o a oma iza ion on P -suppo ed ca alys s.
A simila e ec was obse ed when inc easing he P con en om 2 o 5% (Table 2).
Con e sion dec eased in spi e o he highe P con en , bu benzene becomes he main
p oduc ob ained, wi h a selec i i y alue o a ound 65%. Mo eo e , i is also no ewo hy
he high s abili y o P ca alys s when suppo ed on ca bon ae ogels (Figu e 6). These e-
sul s poin ou he impo ance o he ca bon gel ma e ials used o de elop highly selec i e
and s able ca alys s o he a oma iza ion o ligh alkanes (n-hexane). In gene al, he main
a oma iza ion p oduc de ec ed was benzene, while oluene was no de ec ed. C acking
leads o he o ma ion o he C1–C5 compounds and me hylcyclopen ane (MCP) was
ound as he main isome iza ion p oduc . Con e sion was calcula ed aking in o accoun
he amoun o hexane ans o med and selec i i y as he amoun o each de ec ed p oduc
o e he o al hexane consumed. Thus, he alues o c acking selec i i y p esen ed include
he amoun s o he C1–C5 compounds [27,28].
Table 2. In luence o he P con en on he ca aly ic pe o mance o suppo ed ca alys s p e ea ed a 600 °C. Con e sion
and p oduc s dis ibu ion ob ained a 475 °C.
Sample d P C S C ack S Iso S A om Yield A om
(nm) (%) (%) (%) (%) (%)
C900 2P 2.8 86.8 50.7 4.7 44.7 38.8
C900 5P 9.9 60.5 32.7 2.4 64.9 39.3
d P = P pa icle size; C = Con e sion; S C ack = Selec i i y o c acking; S Iso = Selec i i y o isome iza ion; S A om =
Selec i i y o a oma iza ion.
Figu e 6. E olu ion o con e sion and benzene selec i i y as a unc ion o ime on s eam a 475
°C, using C900 5P ca alys s p e ea ed a 600 °C.
3. Ma e ials and Me hods
Ca bon gel suppo s and he pos e io unc ionaliza ion wi h di e en ac i e phases
we e syn hesized ollowing he me hod desc ibed by Pekala [37] wi h di e en modi ica-
ions:
0
20
40
60
80
0
25
50
75
100
0 50 100 150 200 250 300
S
b
e
n
c
e
n
e
(%)
Con e sion (%)
Time on s eam (min)
Con e sion
S-A oma iza ion
Figu e 6.
E olu ion o con e sion and benzene selec i i y as a unc ion o ime on s eam a 475
◦
C,
using C900 5P ca alys s p e ea ed a 600 ◦C.
3. Ma e ials and Me hods
Ca bon gel suppo s and he pos e io unc ionaliza ion wi h di e en ac i e phases
we e syn hesized ollowing he me hod desc ibed by Pekala [
37
] wi h di e en modi ica ions:
C2021,7, 21 8 o 11
(a) P epa a ion o ca bon-gel suppo s (Figu e 7): Reso cinol (R) and o maldehyde (F) in
a mola a io o R/F=
1
/2
and Na
2
CO
3
, as polyme iza ion ca alys (C) a e dissol ed in
he app op ia e amoun s o dis illed wa e (W). Solu ions a e cas in o glass molds,
sealed and cu ed o 1 day a oom empe a u e, 2 days a 50
◦
C and 5 days a
80 ◦C.
A e ha , gel ods a e cu in pelle s and in oduced in ace one o emo e he wa e
inside he po es. Finally, gels we e supe c i ically d ied wi h ca bon dioxide o ob ain
he co esponding o ganic RF ae ogels o d ied in he o en a 120
◦
C ob aining he
co esponding xe ogels. Py olysis o o ganic gels o ob ain he de i a i e ca bon
gels is ca ied ou in a ubula u nace unde N
2
low by hea ing up o di e en
empe a u es (500 o 1000 ◦C) wi h a hea ing a e be ween 0.5 and 1.5 ◦C min−1.
C 2021, 7, 21 9 o 12
a) P epa a ion o ca bon-gel suppo s (Figu e 7): Reso cinol (R) and o maldehyde
(F) in a mola a io o R/F= ½ and Na2CO3, as polyme iza ion ca alys (C) a e dis-
sol ed in he app op ia e amoun s o dis illed wa e (W). Solu ions a e cas in o
glass molds, sealed and cu ed o 1 day a oom empe a u e, 2 days a 50 °C and
5 days a 80 °C. A e ha , gel ods a e cu in pelle s and in oduced in ace one o
emo e he wa e inside he po es. Finally, gels we e supe c i ically d ied wi h
ca bon dioxide o ob ain he co esponding o ganic RF ae ogels o d ied in he
o en a 120 °C ob aining he co esponding xe ogels. Py olysis o o ganic gels o
ob ain he de i a i e ca bon gels is ca ied ou in a ubula u nace unde N2 low
by hea ing up o di e en empe a u es (500 o 1000 °C) wi h a hea ing a e be-
ween 0.5 and 1.5 °C min−1.
Figu e 7. Sequence ollowed o he p epa a ion o ca bon-gel suppo s.
A e ob aining he co esponding ca bon gels, suppo ed ca alys s we e p epa ed by
classical incipien we imp egna ion wi h he co esponding me al sal p ecu so s ([P
(NH3)4]Cl2) and he mally p e ea ed in si u in ine (He) a empe a u es 600–900 °C.
a) Me al-doped ca bon gels we e ob ained di ec ly by eplacing Na2CO3 by he co e-
sponding me al p ecu so in he s a ing solu ion [30,38]. In his case, ammonium mo-
lybda e o ungs a e was used. The es o he syn hesis p ocedu e was main ained as
in p ocedu e (a). In such a way, simul aneous ca boniza ion o he RF o ganic ac ion
and decomposi ion o he p ecu so sal s occu s du ing he same he mal p ocess,
leading di ec ly o ma e ials sui able o be used in ca alysis.
b) Me al-suppo ed ca bon xe ogels we e ob ained in powde ollowing a p ocedu e in
wo s eps [29]; i s ly, he polyme iza ion o RF in he p esence/absence o su ac an s
is ca ied ou in a ba ch eac o a 50 °C o 2 h; a e he mo phology o he o ganic
RF is de ined, he solid in suspension is doped wi h me al p ecu so s, hen he em-
pe a u e o he ba h is inc eased up o 90 °C o an addi ional 24 h, allowing he com-
ple e polyme iza ion o he o ganic ac ion. Finally, solids a e eco e ed by il a ion,
p ope ly washed, d ied in an o en and inally ca bonized a 900 °C. The p ocedu e is
summa ized in Figu e 8.
Figu e 7. Sequence ollowed o he p epa a ion o ca bon-gel suppo s.
A e ob aining he co esponding ca bon gels, suppo ed ca alys s we e p epa ed
by classical incipien we imp egna ion wi h he co esponding me al sal p ecu so s ([P
(NH3)4]Cl2) and he mally p e ea ed in si u in ine (He) a empe a u es 600–900 ◦C.
(a)
Me al-doped ca bon gels we e ob ained di ec ly by eplacing Na
2
CO
3
by he co e-
sponding me al p ecu so in he s a ing solu ion [
30
,
38
]. In his case, ammonium
molybda e o ungs a e was used. The es o he syn hesis p ocedu e was main ained
as in p ocedu e (a). In such a way, simul aneous ca boniza ion o he RF o ganic
ac ion and decomposi ion o he p ecu so sal s occu s du ing he same he mal
p ocess, leading di ec ly o ma e ials sui able o be used in ca alysis.
(b)
Me al-suppo ed ca bon xe ogels we e ob ained in powde ollowing a p ocedu e in
wo s eps [
29
]; i s ly, he polyme iza ion o RF in he p esence/absence o su ac an s
is ca ied ou in a ba ch eac o a 50
◦
C o 2 h; a e he mo phology o he o ganic
RF is de ined, he solid in suspension is doped wi h me al p ecu so s, hen he
empe a u e o he ba h is inc eased up o 90
◦
C o an addi ional 24 h, allowing
he comple e polyme iza ion o he o ganic ac ion. Finally, solids a e eco e ed by
il a ion, p ope ly washed, d ied in an o en and inally ca bonized a 900
◦
C. The
p ocedu e is summa ized in Figu e 8.
C2021,7, 21 9 o 11
C 2021, 7, 21 10 o 12
Figu e 8. P epa a ion o me al-doped ca bon xe ogel powde .
Ca alys s we e p ope ly ex u ally and chemically cha ac e ized using complemen-
a y echniques, including me cu y po osime y and gases adso p ion (N2 a −196 °C and
CO2 a 0 °C), XRD, SEM, HRTEM, XPS, TG, e c.
The a oma iza ion o n-hexane expe imen s was ca ied ou a a mosphe ic p essu e
using a qua z mic o- eac o wi h 0.15 g ca alys o ming a ixed bed. The eac o was ed
wi h a con inuous low, 60 cm3 min−1, o a mix u e 50%He/50%H2 bubbled in a hexane
sa u a o cooled o ix he hexane concen a ion a 2.5% / . P oduc s we e analyzed by
gas ch oma og aphy, using a lame ioniza ion de ec o and a Ch omoso b column. Con-
e sion was de ined on he basis o he n-hexane disappea ance and selec i i y as he ac-
ion o hexane molecules ans o med in o each de ec ed p oduc .
4. Conclusions
The main d awback o a oma iza ion ca alys s is ela ed o he loss o ac i i y, selec-
i i y and s abili y, mainly due o a as deac i a ion by coke. Because hese eac ions a e
ca ied ou in he absence o oxygen, gua an eeing he ca bon s abili y, he use o ca bon
gel-based ca alys s esul s in e iden ad an ages based on a de eloped po ous ex u e
and basic cha ac e .
The sol-gel echnology applied o he de elopmen o an a oma iza ion ca alys p e-
sen s a di e en al e na i e o op imize he pe o mance o ac i e phases, de ining he
nanos uc u e, na u e and dis ibu ion o he co esponding ac i e phases. Using non-no-
ble ac i e phases (Mo, W), he p og essi e educ ion o he me al oxide phases akes place
simul aneously o he ca boniza ion o he o ganic phase. These p ocesses minimize he
acidi y o he ca alys s, inc easing he a oma iza ion selec i i y. When hese samples a e
syn hesized in he p esence o ca ionic su ac an (CTAB), new ancho ing si es o me als
a e p oduced a o ing a s ong dispe sion o he ac i e phase and a deepe deg ee o e-
duc ion/ca bidiza ion. The o ma ion o Mo2C again inc eases he a oma iza ion selec i -
i y (up o 60% in some cases) ega ding samples whe e he ac i e phase emains as me al
oxide. In his ca alys se ies, he s eng h and dis ibu ion o acid si es de e mines he
p oduc dis ibu ion, inc easing acidi y and eac ion empe a u e, a o ing c acking p o-
cesses.
Using noble me als (P ) as ac i e phase, he a oma iza ion p ocess akes place on he
me allic su ace (P 0) and he eac ion s ongly depends on he P pa icle size. The C6
a oma iza ion is a s uc u e-sensi i e eac ion. Small nanopa icles a e mo e ac i e, bu
Figu e 8. P epa a ion o me al-doped ca bon xe ogel powde .
Ca alys s we e p ope ly ex u ally and chemically cha ac e ized using complemen a y
echniques, including me cu y po osime y and gases adso p ion (N
2
a
−
196
◦
C and CO
2
a 0 ◦C), XRD, SEM, HRTEM, XPS, TG, e c.
The a oma iza ion o n-hexane expe imen s was ca ied ou a a mosphe ic p essu e
using a qua z mic o- eac o wi h 0.15 g ca alys o ming a ixed bed. The eac o was ed
wi h a con inuous low, 60 cm
3
min
−1
, o a mix u e 50%He/50%H
2
bubbled in a hexane
sa u a o cooled o ix he hexane concen a ion a 2.5% / . P oduc s we e analyzed by gas
ch oma og aphy, using a lame ioniza ion de ec o and a Ch omoso b column. Con e sion
was de ined on he basis o he n-hexane disappea ance and selec i i y as he ac ion o
hexane molecules ans o med in o each de ec ed p oduc .
4. Conclusions
The main d awback o a oma iza ion ca alys s is ela ed o he loss o ac i i y, selec-
i i y and s abili y, mainly due o a as deac i a ion by coke. Because hese eac ions a e
ca ied ou in he absence o oxygen, gua an eeing he ca bon s abili y, he use o ca bon
gel-based ca alys s esul s in e iden ad an ages based on a de eloped po ous ex u e and
basic cha ac e .
The sol-gel echnology applied o he de elopmen o an a oma iza ion ca alys
p esen s a di e en al e na i e o op imize he pe o mance o ac i e phases, de ining
he nanos uc u e, na u e and dis ibu ion o he co esponding ac i e phases. Using
non-noble ac i e phases (Mo, W), he p og essi e educ ion o he me al oxide phases akes
place simul aneously o he ca boniza ion o he o ganic phase. These p ocesses minimize
he acidi y o he ca alys s, inc easing he a oma iza ion selec i i y. When hese samples a e
syn hesized in he p esence o ca ionic su ac an (CTAB), new ancho ing si es o me als
a e p oduced a o ing a s ong dispe sion o he ac i e phase and a deepe deg ee o educ-
ion/ca bidiza ion. The o ma ion o Mo
2
C again inc eases he a oma iza ion selec i i y
(up o 60% in some cases) ega ding samples whe e he ac i e phase emains as me al oxide.
In his ca alys se ies, he s eng h and dis ibu ion o acid si es de e mines he p oduc
dis ibu ion, inc easing acidi y and eac ion empe a u e, a o ing c acking p ocesses.
Using noble me als (P ) as ac i e phase, he a oma iza ion p ocess akes place on he
me allic su ace (P
0
) and he eac ion s ongly depends on he P pa icle size. The C
6
a oma iza ion is a s uc u e-sensi i e eac ion. Small nanopa icles a e mo e ac i e, bu
con e sion inc eases wi h empe a u e, a o ing c acking. Wi h inc easing P pa icle size,
con e sion dec eased bu he selec i i y o benzene s ongly inc eased, eaching alues