Ar
ti
cle
0103 - 5 05 3 $ 6 .00+ 0.00
* e-m ail: sh irley @ q uim ica.uf p r.b r
Preparation of Catalysts based on Iron(III) Porphyrins Heterogenized on Silica
obtained by the Sol-Gel Process for Hydroxylation and Epoxidation Reactions
Kelly A. D. F. Castro,a Matilte Halma,a Guilherme S. Machado,a Gustavo P. Ricci,b
Geani M. U cosk i,a Katia J . Ciuf ib and Shirley N ak ag ak i*,a
aUniversidade Federal do Paraná, Departamento de Química - L ab oratório de Química
Bioinorg ânica e Catálise, CP 1 9 0 8 1 , 8 1 5 3 1 - 9 9 0 Curitib a- PR , Brazil
bUniversidade de Franca, Departamento de Química, CP 8 2 , 1 4 4 0 4 - 6 0 0 , Franca- SP, Brazil
C atalisadores sólidos f oram p rep arados p ela interação de f errop oririnas com sílica ob tida p elo p rocesso sol-g el. A p resença da f errop oririna na m atriz de sílica e a m orf olog ia das p artículas ob tidas f oram estudadas p or esp ectroscop ia eletrônica na reg ião do ultrav ioleta e v isív el,
dif ratom etria de raios X de p ó, esp ectroscop ia v ib racional na reg ião do inf rav erm elh o, m icroscop ia eletrônica de transm issão, ressonância p aram ag nética eletrônica e análises term og rav im étricas. A ativ idade catalítica dos sólidos ob tidos f oi inv estig ada f rente a ox idação dos sub stratos (Z)
-cicloocteno, cicloex eno e cicloex ano p or iodosilb enzeno em m eio de diclorom etano e acetonitrila. O s resultados catalíticos ob tidos f oram com p aráv eis àq ueles ob serv ados q uando as f errop oririnas em solução ( catálise h om og ênea) f oram utilizadas.
Solid cataly sts h av e b een p rep ared b y ch em ical interaction of iron( I I I ) p orp h y rins w ith
th e surf ace of th e p ores of a silica m atrix ob tained b y th e sol-g el m eth od. T h e p resence of th e com p lex es in th e silica m atrix and th e m orp h olog y of th e ob tained p articles w ere studied b y U V-Vis sp ectroscop y , p ow der X -ray dif f ractom etry , inf rared sp ectroscop y , transm ission electron m icroscop y , electron p aram ag netic resonance and th erm og rav im etric analy sis. T h e cataly tic
activ ity of th e im m ob ilized iron( I I I ) p orp h y rins in th e ox idation of (Z) -cy clooctene, cy cloh ex ene
and cy cloh ex ane w as ev aluated in dich lorom eth ane/ acetonitrile 1: 1 solv ent m ix ture ( v / v ) using iodosy lb enzene as ox idant. R esults w ere com p ared w ith th ose ach iev ed w ith th e h om og eneous counterp arts.
K eyw ords: iron p orp h y rin, silica, sol-g el, ox idation, cy toch rom e P-45 0 m im etic sy stem s
Introdu ction
Porp h y rins and related m acrocy cles p rov ide an ex trem ely v ersatile sy nth etic b ase f or a v ariety of ap p lications in th e area of new m aterials.1-3 M etallop orp h y rins are
im p ortant ex am p les of m acrocy clic com p lex es4 th at h av e
dem onstrated cataly tic ab ility in a w ide v ariety of reactions ov er th e last 30 y ears. Such reactions include ox idativ e p rocesses, esp ecially in h om og eneous m edia.
M ore recently , p orous m etallop orp h y rin netw ork s
h av e p rov en to b e p otentially ap p licab le as ef f icient h eterog eneous cataly sts5 af ter im m ob ilization in org anic
am orp h ous p oly m ers, am orp h ous inorg anic m atrices, or cry stalline inorg anic m aterials such as silica,6 ,7 zeolites,8 ,9
clay f rom th e sm ectite g roup ( m ontm orillonite) ,10-12 lay ered
doub le h y drox ides,13,14 tub ular and f ib rous m atrices,15
silica m atrix ob tained b y th e sol-g el p rocess, am ong oth ers.16 -23 T h is v ersatile class of m olecules is ef icient and
selectiv e f or th e h eterog eneous cataly tic h y drox y lation and ep ox idation of a w ide v ariety of org anic sub strates.
C ataly st im m ob ilization enab les site-isolation of th e m etal center and is one of th e ap p roach es to reducing
m etallop orp h y rin deg radation b ecause it m ay p rev ent m olecular ag g reg ation or b im olecular self -destruction
reactions,20 w h ich all lead to deactiv ation of th e cataly tically
activ e m etallop orp h y rin sp ecies.19,21 I ndeed, cataly st
stab ility is essential f or recy cling p rocedures and recov ery of v aluab le m etallop orp h y rins.23,24
T o ob tain an ideal h eterog eneous cataly tic sy stem consisting of p orp h y rin-m atrix , th e p orp h y rin activ e site m ust b e readily accessib le f or b oth ch arg e transf er and m ass
to b uilding h eterog eneous cataly sts b ased on p orp h y rins is crucial to ob tain econom ically v iab le ap p lications f or th ese sy stem s.
A lth oug h v arious w ork s in th e literature h av e rep orted th e h eterog enization of m etallop orp h y rin cataly sts, w e h av e
v eriied th at m uch h as to b e done in term s of op tim ization and m atrix p rep aration. T h e inv estig ation of f actors such as p article sh ap e and size, as w ell as activ e site distrib ution on th e m atrices, m ay f urnish cataly sts w ith p otentially ap p licab le f eatures in a w ide v ariety of ields in b oth th e scientiic and industrial areas.
Stöb er et al.25 h av e describ ed a m eth od f or th e
p rep aration of m onodisp ersed sp h erical silica p articles of nanom etric dim ensions th roug h tetraeth y lorth osilicate ( T E O S) h y droly sis in eth anol, em p loy ing b asic conditions and am b ient tem p erature. T h is m eth od is w idely ap p lied now aday s b ecause its sim p licity enab les th e p roduction of sp h erical p articles of dif f erent sizes.21 T h us, th is can b e an
alternativ e p rocess in th e search f or m ore ef icient m atrices f or cataly st im m ob ilization.
O ur research team18 ,19 and oth ers20,21 h av e ob serv ed
th at th e addition of p orp h y rins to silica m atrices ob tained b y th e sol-g el m eth odolog y can af f ect p article p orosity and m orp h olog y . A s already rep orted,21 th e p resence of
p orp h y rins can contrib ute to th e p oly m erization of th e silica netw ork .
I n th e sol-g el m eth odolog y , control of th e p rep aration conditions can easily alter th e structure and p rop erties of th e
ob tained m aterials. T h e use of an acid cataly st can also b e an attractiv e alternativ e ap p roach in th e p rep aration of th e h eterog eneous iron p orp h y rin cataly sts. I n th e case of silica,
th e utilization of acidic cataly sts leads to transp arent g els of low p orosity , w h ereas b asic cataly sts f urnish translucent, h ig h ly p orous g els.21
T h is w ork rep orts th e im m ob ilization of th ree structurally
dif f erent iron p orp h y rins, nam ely : [ F e( T M5] , Py P) ] C l [ 5 ,10,15 ,20-tetrak is( 4-N-m eth y lp y ridy l) p orp h y rin iron( I I I )
ch loride] ; [ F e( T D F PP) ] C l] , [ 5 ,10,15 ,20-tetrak is( 2,6 -dif luorop h eny l) p orp h y rin iron( I I I ) ch loride] and [ F e( T D C PP) ] C l, [ 5 ,10,15 ,20-tetrak is( 2,6 -dich lorop h eny l) p orp h y rin iron( I I I ) ch loride] ( F ig ure 1) in th e silica
structure ob tained b y th e h y droly tic sol-g el p rocess.25 A f ter
ch aracterization, th e cataly tic activ ity of th e ob tained solid com p ounds in ox idation reactions w as inv estig ated.
Experim ental
M aterials
A ll ch em icals used in th is study w ere p urch ased f rom A ldrich , Sig m a or M erck and w ere of analy tical g rade. I n p articular, tetraeth y lorth osilicate ( T E O S) w as acq uired
f rom A ldrich ; eth anol ( E tO H ) , dich lorom eth ane ( dcm ), dim eth y lf orm am ide ( dm f ) , m eth anol ( M eO H ) , isop rop y l alcoh ol ( PriO H ) , and tetrah y drof uran ( th f ) w ere ob tained
f rom M erck . I odosy lb enzene ( Ph I O ) w as sy nth esized b y h y droly sis of iodosy lb enzenediacetate,26 and th e ob tained
solid w as caref ully dried under reduced p ressure and k ep t at 5 °C . Ph I O p urity w as p eriodically controlled b y iodometric titration.27 D eionized w ater w as used in all ex p erim ental
p rocedures.
Po r p h y r i n s : T h e f r e e b a s e p o r p h y r i n s
5 ,10,15 ,20-tetrak is( 1-m eth y lp y ridinium -4-y l) p orp h y rin ([ H2( T M Py P) ]4+) , 5 ,10,15 ,20-tetrak is( 2,6 -luorop h eny l)
p orp h y rin ( [ H2( T D F PP) ] ) ] and 5 ,10,15 ,20tetrak is( 2,6 -dich lorop h eny l) p orp h y rin ( [ H2( T D C PP) ] ) w ere sy nth esized, p urif ied, and ch aracterized f ollow ing th e p rev iously describ ed m eth odolog y .28 -30
I ron( I I I ) porphy rins: I ron( I I I ) p orp h y rins ( F ePors) w ere ob tained b y m etallation of th e f ree b ase w ith f errous ch loride tetrah y drate in dm f f ollow ing th e m eth od describ ed b y A dler et al.31,32 T h e F ePors w ere
p urif ied b y colum n ch rom atog rap h y on silica or alum ina. T h e p roducts w ere ch aracterized b y U V-Visib le and E PR sp ectroscop y , and th e data w ere consistent w ith th e ex p ected com p ound af ter th e m etallation
reaction. [ F e( T M Py P) ] ( deionized w ater) : 390 nm
(ε = 13×103 L m ol-1 cm-1) , [ F e( T D F PP) ] ( dich lorom etane)
412 nm (ε = 45 ×103 L m ol-1 m-1) , [ F e( T D C PP) ]
( dich lorom etane) 412 nm (ε = 47×103 L m ol-1 cm-1) , w h ich
w ere denoted F ePor 1, F ePor 2, and F ePor 3, resp ectiv ely .
T h e p ositiv e ch arg es and th e ch lorine counter ion w ill b e om itted in th e tex t f or sim p liication p urp oses.
I mmob ilization of 1, 2, and 3 in silica prepared b y the
sol-g el process ( FePor- SG B and FePor- SG A solids)
Basic cataly sis
T h e F ePor-SG B cataly sts w ere p rep ared b y th e Stöb er m eth odolog y ,25 th roug h h y droly sis of T E O S in eth anol/
m eth anol, em p loy ing NH4O H as cataly st, 1×10-5 m ol of
th e desired F ePor, and th e f ollow ing m olar ratios: H2O / Si ( T E O S) = 5 0, NH3/ E tO H = 10, and H2O / NH3 = 200. A n alternativ e m eth odolog y utilizing isop rop y l alcoh ol ( PriO H ) instead of eth anol ( NH3/ PriO H = 10) in th e p resence of 7×10-6 m ol of F ePor 1, 2 or 3 w as also em p loy ed. T h e
reaction m ix ture w as k ep t at 40 °C ; th e solution w as stirred f or 30 m in and k ep t at ab out 6 0 °C f or com p lete dry ness. T h e inal m aterials, g elatinized af ter ab out 14
day s, consisted in transp arent b row n-em erald g reen stab le
g els. T h e w et g els w ere ex tensiv ely w ash ed w ith M eO H , E tO H , H2O and dcm b y Sox h let p rocedure, f ollow ed b y
dry ing at 70 °C f or 24 h . T h e F ePor loading in th e silica w as q uantiied b y U V-Vis sp ectroscop y , b y m easuring th e am ount of non-im m ob ilized F ePor recov ered during th e w ash ing of th e solid. A control sam p le containing no F ePor w as also sy nth esized in th e sam e w ay as describ ed ab ov e, f urnish ing a transp arent colorless g el th at w as also w ash ed w ith M eO H , E tO H , H2O and dcm b y th e sam e Sox h let p rocedure, and th en dried in an ov en at 70 °C f or 24 h .
A cid cataly sis
T h e F ePor-SG A cataly st w as p rep ared b y th e sol-g el p rocess th roug h h y droly sis of T E O S in eth anol/ m eth anol, em p loy ing H C l as cataly st, 1×10-5 m ol of th e desired
F ePor and th e f ollow ing m olar ratios: H2O / Si ( T E O S) = 4, th f / H C l = 308 , and H2O / H C l = 92.21 T h e reaction m ix ture
w as stirred f or 1 h at room tem p erature, and k ep t at ab out
70 °C f or com p lete dry ness. T h e inal m aterials, g elatinized af ter ab out 7 day s, consisted in transp arent b row n-em erald g reen stab le g els sim ilar to th e F ePor-SG B solids. T h e w et g els w ere also ex tensiv ely w ash ed w ith M eO H , E tO H , H2O and dcm b y Sox h let p rocedure, f ollow ed b y dry ing f or 24 h
at 70 °C . I n th e case of th e acid sol-g el p rocess, U V-Vis sp ectroscop y detected no F ePor in th e solv ent recov ered at th e end of th e w ash ing p rocess. A control sam p le containing no added F ePor w as also sy nth esized in th e sam e conditions describ ed ab ov e.
Cataly tic ox idation reaction
C ataly tic ox idation reactions w ere carried out in a 2 m L
th erm ostatic g lass reactor eq uip p ed w ith a m ag netic stirrer p laced inside a dark ch am b er. A lk ene and alk ane reactions w ere accom p lish ed using F ePor-SG B or F ePor-SG A as cataly sts ( ab out 0.01 g ) , and (Z) -cy clooctene, cy cloh ex ene
or cy cloh ex ane as sub strates. T h e alk enes h ad b een p rev iously p uriied on an alum ina colum n. I n a standard ex p erim ent, th e solid cataly st and iodosy lb enzene ( F ePor/ Ph I O m olar ratio = 1: 5 0) w ere susp ended in 0.400 m L of th e solv ent ( dich lorom eth ane/ acetonitrile 1: 1 m ix ture v / v ) inside a 2 m L v ial and deg assed w ith arg on f or 15 min. T h e
sub strate ( F ePor/ sub strate m olar ratio = 1: 5 000) w as added, and th e ox idation reaction w as carried out f or 1 to 24 h , under m ag netic stirring . Sodium sulite w as added at th e end
of th e reaction, to elim inate th e ex cess of iodosy lb enzene. T h e solub le reaction p roducts w ere sep arated f rom th e solid cataly st b y centrif ug ation and transf erred to a v olum etric lask . T h en, th e solid cataly st w as w ash ed sev eral tim es w ith dich lorom eth ane and acetonitrile, in order to ex tract any rem aining p roduct. T h e solution containing th e inal reaction solution p lus th e solv ents f rom th e w ash ing s of th e solid cataly st w as analy zed b y g as ch rom atog rap h y . Product y ields w ere q uantiied on th e b asis of Ph I O , using n-octanol
( acetonitrile solution, 1.0×10-2 m ol L-1) of h ig h p urity
deg ree ( 99.9 % ) as internal standard. C ontrol reactions w ere
carried out using th e sam e p rocedure in th e case of ( a) th e sub strate alone, ( b ) sub strate + Ph I O , and ( c) sub strate + Ph I O + silica ( w ith out F ePor) . T h e F ePor in solution w as also tested as a cataly st ( h om og eneous cataly sis) , and th e ex p erim ental p rocedure in th is case w as sim ilar to th at used f or th e h eterog eneous cataly sis.
A f ter th e irst use, th e solid cataly st w as th oroug h ly w ash ed and dried f or reuse in anoth er reaction, under th e sam e ex p erim ental conditions em p loy ed in its irst use.
Characterization and apparatus
rang e. Sp ectra of th e solid sam p les w ere recorded in a 0.1 cm p ath leng th q uartz cell ( H ellm a) in nuj ol m ull.
F T I R sp ectra w ere reg istered on a Biorad 35 00 G X sp ectrop h otom eter in th e 400 to 4000 cm-1 rang e, using K Br
p ellets. K Br w as crush ed w ith a sm all am ount of th e solid sam p les, and th e sp ectra w ere collected w ith a resolution of 4 cm-1 and accum ulation of 32 scans.
F or th e X ray dif f raction ( X R D ) m easurem ents, self -oriented ilm s w ere p laced on neutral g lass sam p le holders.
T h e m easurem ents w ere p erf orm ed in th e relection m ode using a Sh im adzu X R D -6 000 dif f ractom eter op erating at 40 k V and 40 m A ( C u-Kα radiation, λ = 1.5 418 Å) w ith a
dw ell tim e of 1θ/ m in.
E lectron p aram ag netic resonance ( E PR ) m easurem ents of th e p ow der m aterials w ere carried out on an E PR BR U K E R E SP 300E sp ectrom eter ( standard concav ity : 4102-SP, f req uency X b and 9.5 G H z) , at 293 or 77 K , using liq uid N2.
T h erm og rav im etric ( T G A ) m easurem ents w ere p erf orm ed on a T h erm al A naly st T A I nstrum ent SD T 6 00 Sim ultaneous D T A -T G A -D SC , in nitrog en, w ith a h eating rate of 20 ºC m in-1, f rom 25 to 1200 ºC .
T ransm ission electron m icroscop y ( T E M ) w as
accom p lish ed on an 8 0 k V J E M 1200E X I I E lectron
M icroscop e. A drop of a susp ension of th e cataly st p ow der w as dep osited on a 300 m esh cop p er g rid ( f orm v ar/ carb on) .
Products f rom th e cataly tic ox idation reactions w ere identiied using a Sh im adzu G C -14B g as ch rom atog rap h ( lam e ionization detector) eq uip p ed w ith a D B-W A X cap illary colum n, 30 m ( J & W Scientiic) .
Sp eciic surf ace areas w ere determ ined b y ap p ly ing th e BE T m eth od33 to th e corresp onding nitrog en adsorp tion
isoth erm s, ob tained b y using a p h y sical adsorp tion analy zer
M icrom etrics A SA P 2020. T h e sam p les w ere p rev iously deg assed b y treatm ent at 18 0-200 ºC until th e sy stem
p ressure reach ed 10 μm H g . T h e nitrog en adsorp tion data w ere ob tained using 0.2 g of th e sam p le.
Resu lts and D iscu ssion
T h e F ePor-SG A and F ePor-SG B solids w ere p rep ared th roug h h y droly sis and condensation of th e alk ox y silane ( T E O S) in alcoh ol and w ater, using am m onia or H C l as cataly sts in th e p resence of one of th e F ePors (1, 2 or 3) .
T h e h y droly sis and p oly condensation reactions initiate at num erous sites as soon as T E O S and H2O are m ix ed. U nder norm al conditions, b oth h y droly sis and condensation m ay occur b y acid or b ase-cataly zed b im olecular nucleop h ilic sub stitution reactions. T h e acid-cataly zed m ech anism s p roceed via rap id dep rotonation of th e −O R or −O H sub stituents b ound to Si,21 w h ereas under b asic conditions
h y drox y l or silanolate anions attack Si directly . Hy droly sis of alk y lsilicates f ollow ed b y condensation in ap p rop riate p H f urnish es h ig h p urity silica p articles.25 T h e ch oice b etw een
acid or b asic cataly sis f or silica sy nth esis inluences th e nature of th e p rep ared cataly st. F or th is reason, we decided
to study th e role p lay ed b y p H in th e cataly tic activ ity of th e m aterials p rep ared h erein.
T h e control silica sam p le, containing no F ePor, is w h ite, w h ereas th e solids ob tained af ter F ePor im m ob ilization are
of a lig h t b row n-g reen color th at g radually deep ens w ith increasing F ePor concentration.
A f ter th e solids w ere dried, F ePor loading in th e silica w as determ ined b y m easuring th e dif f erence b etw een th e ab sorb ance of th e com p lex in th e solution p rep ared b ef ore th e im m ob ilization p rocess and th e ab sorb ance of th e solution and com b ined w ash ing s f rom th e Sox h let p rocedure ob tained af ter F ePor im m ob ilization ( T ab le 1) .
T ab le 1 rev eals h ig h F ePor im m ob ilization ( > 95 % ) in th e silica sup p ort, and F ePor im m ob ilization is sup erior
in th e m aterials p rep ared b y acid cataly sis. T h e structure of th e inal silica m aterials ex p lains th e F ePor p ercentag e of im m ob ilization. A n op en silica netw ork arises during acid cataly sis, w h ich p rov ides a larg er num b er of av ailab le sites and f acilitates F ePor entrap m ent. I n contrast,
m aterials ach iev ed b y m eans of b asic cataly sis consist in interconnected silica sp h erical p articles, th ereb y leading to
low er F ePor p ercentag e of im m ob ilization.
A cid cataly sis and low H2O / Si ratios f urnish w eak ly
b ranch ed p oly m eric sols and g els, w h ich results in h ig h ly m icrop orous x erog els w ith a v ery f ine tex ture. Basic cataly sis and h ig h H2O / Si ratios, on th e oth er h and, p roduce colloidal p articles th at g iv e rise to m eso- or m acrop orous x erog els com p osed b y translucent g els of h ig h p orosity .21,34
T h e m ech anism th roug h w h ich F ePor im m ob ilization occurs in neutral m edium is not com p letely understood.
I nteractions b etw een th e π-conj ug ated electron cloud
T able 1. I m m ob ilization of F ePors in silicas ob tained b y th e sol-g el p rocess
Solid Percentag e of im m ob ilization ( % )a
L oading ( concentration of F ePor in th e silica)
( m ol g-1) % ( m / m )
F ePor 1-SG A 100 8 .5 × 10-6 0.8 3
F ePor 1-SG B 98 .0 2.2 × 10-5 2.1
F ePor 2-SG A 99.6 × 9.010-6 0.75
F ePor 2-SG B 95 .4 2.3 × 10-5 1.9
F ePor 3-SG A 100 × 8 .410-6 0.8 1
F ePor 3-SG B 96 .7 1.8 × 10-5 1.7
of th e m acrocy clic ring s and th e structure of th e h ig h ly h y drox y lated silica are p ossib le. F urth erm ore, th e F ePor can insert into th e th ree-dim ensional netw ork of th e silica resulting f rom Si−O−Si link s. T h is leads to ag g lom eration of th e m olecules, th ereb y f orm ing larg er p articles and ag g lom erates, or a colloidal sol ( F ig ure 2) .
I n th e case of F ePor 1, in addition to th e p ossib le interactions m entioned ab ov e, electrostatic interactions b etw een F ePor 1 and th e silica surf ace could also tak e
p lace, g iv ing rise to F ePor im m ob ilization on th e surf ace of th e sup p ort.
F ig ure 3 p resents th e transm ission electron m icrog rap h s of th e F ePor-SG cataly sts and th e control m aterials ( containing no F ePor) . A s ob serv ed p rev iously b y us,19
addition of p orp h y rins to silica m atrices ob tained b y th e sol-g el m eth odolog y can af f ect p article m orp h olog y and ag g lom eration. C om p arison of th e m icrog rap h s ob tained
f or th e control silica SG B and F ePor-SG B sh ow s th at th e silica p articles are m ore sp h erical in th e p resence of F ePors.
H ow ev er, p article m orp h olog y is irreg ular in th e cases of th e F ePor 2-SG B and F ePor 3-SG B h y b rid m aterials, b ut sp h erical f or F ePor 1-SG B.
T h e p ositiv e ch aracter of th e ch arg ed F ePor 1 m ay f av or ag g lom eration of th e F ePor 1-SG B p articles, w h ich w ould account f or th e p roduction of b eads only in th e p resence of th is F ePor, under th e ex p erim ental conditions of th is w ork .
W ang et al.23 ob serv ed th at silica p rep ared b y th e
Stöb er25 m eth odolog y tends to ab sorb cationic p orp h y rins
strong ly . T h is is b ecause th is m eth odolog y can p roduce neg ativ ely ch arg ed sp h erical p articles dep ending on th e p H
conditions em p loy ed during silica p rep aration. T h e silanol g roup s p resent on th e m aterial ach iev ed at h ig h p H can dissociate and p roduce neg ativ e ch arg es, th us f acilitating F ePor adsorp tion. T h is is an ex cellent ap p roach f or th e p rep aration of m aterials consisting of sp h eres.23
T h e solids ob tained b y im m ob ilization of F ePors 1, 2 and 3 in th e silica ach iev ed b y th e sol-g el p rocess ( acid and b asic cataly sis) as w ell as th e control silica ( w ith out FePor) w ere
analy zed b y X R D ( F ig ure not sh ow n) . T h e X R D p atterns
of all p rep ared solids rev ealed only a h alo in th e reg ion of
20 and 30º f or 2θ, w h ich m eans th at th ey are am orp h ous. T h e p resence of F ePor in th e h y b rid F ePor-SG solids w as conirm ed b y E PR analy sis. F ig ure 4 dep icts th e E PR sp ectra of SG A ( a) , F ePor 1-SG A ( b ) , F ePor 1-SG B ( c) ,
F ePor 2-SG A ( d) , F ePor 2-SG B ( e) , F ePor 3-SG A ( f ) , and F ePor 3-SG B ( g ) at 77 K . T h e control silicas SG A ( F ig ure 4a) and SG B ( not sh ow n) disp lay E PR -silent sp ectra. T h e ab sence of E PR sig nals indicates th at th ese solids are f ree of contam inating p aram ag netic sp ecies th at
F igu re 2. Sch em atic rep resentation of th e F ePor-silica solid f orm ed b y th e sol-g el p rocess.
could h av e b een inserted into th e silica during th e sy nth etic p rocess. A ll th e p rep ared F ePor-SG solids disp lay EPR sig nals at g = 6 .0 ( 1000-1200 g auss) , ch aracteristic of h ig h -sp in iron( I I I ) in ax ial sy m m etry , 24,35 -37 and a sm all
sig nal at g = 4.3, ty p ical of h ig h -sp in iron( I I I ) in rh om b ic
sy m m etry .37,38 T h e latter E PR sig nal sug g ests th at, up on
im m ob ilization, som e or all F ePor m olecules m ay underg o a rh om b ic distortion b ecause of th eir interaction w ith th e structure of th e sup p ort. Nev erth eless, th e ax ial sy m m etry sig nal at g = 6 .0 is still p resent in h ig h intensity .
T h e p resence of th e ty p ical E PR iron( I I I ) sig nal in th e silica solids conirm s th at during th e im m ob ilization p rocess no sig niicant dem etallation of th e F ePors took
p lace. D em etallation could h av e b een caused b y b inding of th e m etal ion to th e coordinating g roup s in th e solid deriv ativ e, in th e p resence of th e h y drox y l g roup s.37
F ig ure 5 sh ow s th e F T I R sp ectra of th e solids F ePor-SG p rep ared in th is w ork ( F ig ure 5 a-5 e) .
T h e m ain ch aracteristic of th e F T I R sp ectrum of p ure silica is an intense, w ide b and in th e reg ion of 3420 cm-1,
due to th e ty p ical ax ial def orm ation of sup ericial SiO−H g roup s. A lso, th ere is a larg e b and at 16 30 cm-1, ascrib ed
to w ater m olecules adsorb ed/ ab sorb ed in th e silica. T h e b and in th e reg ion of 1100 cm-1 is ch aracteristic of th e
stretch ing of th e 18 0-deg ree ang le of th e Si−O−Si g roup s of th e f our tetrah edral SiO2. T h e b and at 96 0 cm-1 is assig ned
to Si−O H v ib ration, and th e b and at 798 cm-1 is associated
w ith th e distorted structure of SiO4.36 T h e F T I R b ands of
th e F ePors are not ob serv ed, p rob ab ly b ecause of th e low concentration of th e adsorb ate com p ared w ith th e sup p ort ( T ab le 1) . Nev erth eless, som e dif f erences are noted w h en th e sp ectrum of th e control solid is com p ared w ith th ose
of th e F ePor-SG m aterials. F or instance, th e ch aracteristic Si−O stretch ing b and sh if ts to low er w av enum b er af ter F ePor im m ob ilization. T h is sh if t is m ore p ronounced f or th e F ePor 1-SG A solid, indicating th at th is F ePor interacts w ith th e silica silox ane g roup s, and th at F ePor 1 m ig h t b e located inside th e m atrix p ores.39
T h e p rep ared solids w ere also analy zed b y U V-Vis sp ectroscop y in nuj ol m ull ( F ig ure 6 ) . T h e sp ectra of all F ePor-SG solids disp lay ed a ty p ical p eak enlarg em ent40
and p resented th e Soret b and ch aracteristic of F ePors in th e reg ion of 400 nm , th ereb y conirm ing th e p resence of F ePor 1, 2, or 3 in th e p rep ared silicas. H ow ev er, th e Soret b and of all th e F ePor-SG m aterials w as sh if ted to low er energ y com p ared w ith th e sp ectrum of th e p arent F ePor in solution ( sp ectrum collected in Nuj ol oil em ulsion -
ig ure not sh ow n) . T h is dem onstrates th at no dem etallation ( ch aracterized b y a b lue sh if t of th e Soret b and typ ical of th e p resence of a sig niicant am ount of f ree-b ase p orp h yrin)18 or
sig niicant m etal ion ex ch ang e b etw een th e m etallocom p lex and th e sup p ort occurs during th e im m ob ilization p rocess. O n th e oth er h and, th e red-sh if ted Soret b and ob tained in th e case of th e im m ob ilized F ePors g iv es ev idence th at F ePor im m ob ilization in th e solid causes no im p ortant steric constraints th at w ould sub stantially m odif y th e m etallop orp h y rin structure in th e sup p orted cataly st.36 I n
f act, th e E PR sp ectrum ( F ig ure 4) disp lay s sig nals b oth at g = 6 .0 and g = 4.3, th us conirm ing th at only som e
distortion of th e F ePor tak es p lace af ter th e im m obilization
p rocess. T h e U V-Vis sp ectrum of th e solid does not allow f or a m ore detailed analy sis of th e v isib le reg ion. F or
m etallop orp h y rins in solution, th is reg ion of ten ex h ib its b ands th at aid understanding of th e ax ial coordination of
F igu re 4. E PR sp ectra of solid sam p les ob tained at 77K . ( a) SG A ; ( b ) F ePor 1-SG A ; ( c) F ePor 1-SG B; ( d) F ePor 2-SG A ; ( e) F ePor 2-SG B; ( f )
F ePor 3-SG A ; ( g ) F ePor 3-SG B.
th e m etal and h elp determ ine w h eth er dim eric sp ecies are f orm ed during th e im m ob ilization p rocess.41
F ePor dim eric sp ecies (e.g ., µ-ox o dim er sp ecies) are of ten ob serv ed w h en F ePors are ob tained in alk aline m edia, p ref erab ly in th e p resence of w ater traces. I n th is case, th e F ePor solution usually acq uires a g reen color, and so does th e solid ob tained b y im m ob ilization of th e dim eric sp ecies. T h e U V-Vis sp ectrum of th e dim eric sp ecies in solution ty p ically p resents tw o b ands, nam ely one in th e 5 70-5 8 0 nm reg ion ( m ore intense) and anoth er b etw een 6 10-6 20 nm ( sh oulder) . F urth erm ore, th e Soret b and sh if ts to low er energ y , com p ared w ith th e corresp onding b and f or th e m onom eric sp ecies.41 D ue to th e dif icult analy sis
of th e v isib le reg ion of th e sp ectrum in th e case of F ePor im m ob ilization in solid sup p orts lik e silica, th e presence of
som e dim eric sp ecies in th e sup p ort cannot b e discarded, m ainly in th e silica solids ob tained b y th e b ase-cataly zed sol-g el p rocess. H ow ev er, if th is dim erization p rocess took p lace, it occurred in m inor p rop ortion, since th e E PR sp ectra ( F ig ure 4) indicate th e p resence of m onom eric iron( I I I ) p orp h y rin sp ecies. T h e μ-ox o dim eric sp ecies of F ePors are E PR silent, due to th e antif errom ag netic coup ling of th e iron ions via μ-ox o b ridg e.41
F rom th ese results, w e can assum e th at th e F ePor m olecules w ere really introduced into th e silica g els.21
T h erm og rav im etric and dif f erential th erm al analy ses ( T G A and D T A ) ( ig ure not sh ow n) p rov ide inf orm ation on th e th erm al stab ility of th e solids ob tained af ter F ePor
im m ob ilization. Such analy ses g iv e ev idence of tw o ev ents. F irstly , th ere is a m ass loss of around 20% , w h ich is associated w ith an endoth erm ic transition centered
at 15 0 °C and corresp onds to loss of adsorb ed H2O m olecules. T h ere is anoth er m ass loss b etw een 200 and
6 00 ºC , attrib uted to F ePor decom p osition, f ollow ed b y condensation and collap se of th e inorg anic structure,37
resulting in SiO2. F ePor loss occurs ap p rox im ately b etw een 400 and 6 00 ºC , and th e residual g roup s are lost f rom 200 to 6 5 0 ºC . T h ere is no ex oth erm ic ef f ect. Sim ilar results w ere ob tained f or all th e F ePor-SG solids.
T h e surf ace area and p orosity of th e m aterials, determ ined f rom nitrog en adsorp tion, are sh ow n in Tab le 2. C om p arison of th e m aterials ob tained b y acid or b asic cataly sis rev eals th at th e SG B m aterials h av e th e larg est surf ace areas. I ndeed, low surf ace area v alues are f ound f or m aterials ob tained b y acid cataly sis.
A s already m entioned, th e p H em p loy ed in th e p rep aration of th e silica sup p ort inluences th e ch aracteristics
of th e ob tained m aterial. A h ig h ly b ranch ed m aterial is ach iev ed b y acid cataly sis ( SG A ) , w h ich usually contains a larg e num b er of m icrop ores. I f th e F ePor is located on th e surf ace of SG A , it h inders nitrog en g as adsorp tion
b y th e F ePor-SG A cataly st, th ereb y m ak ing th is ty p e of analy sis im p ossib le.
Cataly tic ox idation reactions
T h e cataly tic activ ities of F ePors 1, 2 and 3 ( h om og eneous cataly sis) and th ose of th e corresp onding sup p orted cataly sts ( h eterog eneous cataly sis) in th e ox idation of th ree sub strates, nam ely tw o alk enes ( (Z) -cy clooctene
and cy cloh ex ene) and one alk ane ( cy cloh ex ane) , w ere inv estig ated ( F ig ure 7) . T h e results are disp lay ed in T ab les 3 to 5 . T h e cataly tic y ields dep icted in th ese tab les can b e attrib uted to all th e solids resulting f rom im m ob ilization of th e F ePs in th e silicas ( SG A or SG B) , since control reactions carried out w ith th e p ure m atrices ( containing no F ePor) did not f urnish any ox idation p roducts.
Since th e irst rep ort of G rov es et al.,42 iodosy lb enzene
( Ph I O ) h as b een f req uently em p loy ed as ox y g en transf er ag ent in ox idation reactions using m etallop orp h y rins.
T h ose auth ors dem onstrated th at th is reactant is relativ ely inert in th e ab sence of th e cataly st and reacts w ith F ePors to p roduce th e interm ediate activ e sp ecies ox oiron(I V)
F igu re 6. U V-Vis sp ectra ( a) F ePor 1-SG A ; ( b ) F ePor 1-SG B; ( c) F ePor 2-SG A ; ( d) F ePor 2-SG B; ( e) F ePor 3-SG A ; ( f ) F ePor 3-SG B.
T able 2. Ph y sicoch em ical p rop erties of F ePor-SG A and F ePor-SG B m aterials
Sam p le SBE T ( m
2 cm-1) a V p ( cm
3 g-1) a D
p ( Å) a
F ePor 1-SG B 26 6 0.6 5 98 .0
F ePor 3-SG B 25 5 0.41 6 4.5
F ePor 2-SG A 35 0.02 24.0
F ePor 3-SG A 0.01 --
--aBE T surf ace area (S
BE T) , av erag e p ore v olum e (Vp) and av erag e p ore
p orp h y rin cation radical com p lex ( O = F eI Vp orp h y rin•+) .
F urth erm ore, th e p oly m eric nature of th is ox idant is ef icient to sup p ress m etal-m ediated f ree radical p rop ag ation reactions of ten ob serv ed in th e case of cy toch rom e P-45 0 m im etic sy stem s.43
A U V-Vis sp ectrum of th e sup ernatant reaction solution w as recorded af ter all th e cataly tic reactions. T h e ty p ical Soret b and of F ePors w as not detected in any of th e cases, p rov ing th at no cataly st leach ing f orm th e sup p ort occurred in any of th e reaction conditions inv estig ated in th is study . I n oth er w ords, th e cataly sis is truly h eterog eneous.
( Z ) - Cy clooctene
I t is w ell estab lish ed th at th e ox idation of (Z)
-cy clooctene b y m etallop orp h y rin/ Ph I O sy stem s p roduces ep ox ide as th e sole ox idation p roduct, w ith no traces of
ally lic alcoh ol or k etone.44 F or th is reason, th is alk ene is
f req uently em p loy ed as a diag nostic sub strate in b iom im etic cataly tic sy stem s inv olv ing m etallop orp h y rins. I n th is w ork , w e used th is sub strate to inv estig ate th e ef iciency and
stab ility of th e im m ob ilized F ePors as cataly sts f or alk ene ox idation b y Ph I O . T h is study also p rov ided inf orm ation ab out th e accessib ility of th e sub strate and th e ox idant to th e iron( I I I ) sites in th e im m ob ilized cataly st.
T ab le 3 sh ow s th e results f rom (Z) -cy clooctene
ox idation w ith dif f erent cataly sts. T h e reactions were accom p lish ed in b oth h om og eneous and h eterog eneous env ironm ents.
T h e h om og eneous F ePors 2 and 3 f urnish h ig h ep ox ide y ields ( T ab le 3, runs 4 and 7) . F ePor 1 is not com p letely solub ilized in th e reaction solv ent, accounting f or th e low er y ields ( run 1) . Both F ePors 2 and 3 are ex p ected to b e h ig h ly ef icient cataly sts f or ox idation reactions. T h is is b ecause th e electroneg ativ e sub stituents on th eir p h eny l ring s contrib ute to a reduction of th e electronic
density on th e p orp h y rin ring , th ereb y stab ilizing th e cataly st ag ainst ox idativ e deg radation.45 F urth erm ore, th e
relativ e solub ility ob serv ed f or th e th ree F ePors in th e reaction solv ent ( dich lorom eth ane/ acetonitrile m ix ture) is F ePor 3 > F ePor 2 > > F ePor 1, and th is relativ e solub ility can also ex p lain th e cataly tic y ields dep icted in T ab le 3.
A ll th e im m ob ilized F ePors disp lay cataly tic activ ity f or (Z) -cy clooctene ox idation, th us conirm ing th at th e im m ob ilization p rocess and th e ex p erim ental conditions did not destroy or transf orm th e F ePors into inactiv e sp ecies ( T ab le 3) .
I n reactions p erf orm ed w ith th e solid ob tained b y th e sol-g el p rocess ( h eterog eneous cataly sis, T ab le 3, runs 2, 3, 5 , 6 , 8 and 9) th e ov erall results ob tained at one h our of reaction are g enerally low er th an th ose ob tained at th e sam e reaction tim e w ith th e corresp onding h om og eneous cataly sts. I t is f req uently rep orted th at th e cataly tic activ ity
of m etallop orp h y rins is reduced up on im m ob ilization, b ecause th e access of b oth th e sub strate and th e ox idant to th e activ e site is lim ited b y th e structure of th e sup p ort.46
T h is seem s to b e th e case w ith th e h eterog eneous cataly sts inv estig ated h ere, ex cep t f or th e F ePor 3-SG A solid
( T ab le 3, run 5 ) , w h ich f urnish ed h ig h er y ields af ter F ePor im m ob ilization in SG A . T ab le 3 sh ow s th at b etter cataly tic results are ach iev ed at 24 h . T h is sug g ests th at access of th e
reactants to th e m etallic cataly tic center is f acilitated af ter long er contact tim e. F ePor 1-SG A is an ex cep tion ( T ab le 3, run 2) , since long er reaction tim e actually decreased
F igu re 7. Sch em atic rep resentation of th e cataly tic ox idation reactions using F ePor-SG as h eterog eneous cataly sts.
T able 3. (Z) -cy clooctene ox idation b y Ph I O cataly zed b y F ePors in h om og eneous and h eterog eneous m ediaa
C ataly stb R un E p ox ide y ield ( % )
1 hc 24 hd
F ePor 1 1 6 0 6 6
F ePor 1-SG A 2 42 28
F ePor 1-SG B 3 27 44
F ePor 2 4 6 5 79
F ePor 2-SG A 5 8 6 97
F ePor 2-SG B 6 6 0 6 9
F ePor 3 7 6 8 8 3
F ePor 3-SG A 8 6 3 75
F ePor 3-SG B 9 6 5 8 3
SG A + Ph I O ( control) 10 5
-SG B + Ph I O ( control) 11 5
-Ph I O ( control) 12 5
-aT h e y ield of cy clooctene ox ide w as calculated on the b asis of th e am ount
of Ph I O used in th e reaction. T h e results rep resent an av erag e of at least dup licate reactions. F ePor/ Ph I O / (Z) -cy clooctene m olar ratio = 1: 5 0: 5 000.
bT h e p iece of g lass ob tained f rom th e sol-g el p rocess w as h ardly g rounded,
p roduct y ield. T h is m ig h t b e related to th e F ePor structure, w h ich is not rob ust enoug h to sup p ort long reaction tim es. T h us, som e F ePor destruction m ay h av e occurred.
G enerally , th e solids p rep ared b y acid cataly sis ( F ePor-SG A ) led to b etter cataly tic y ields ( T ab le 3, runs 2 and 5 ) th an th ose sy nth esized b y th e b ase-cataly zed sol-g el route ( F ePor-SG B) ( T ab le 3, runs 3 and 6 ) . T h ese results ag ree w ith th e ch aracterization data ob tained f or th e solids b y E PR and U V-Vis sp ectroscop y , sug g esting th at som e dim eric F ePor sp ecies (e.g., µ-ox o dim er sp ecies) could
b e p resent tog eth er w ith th e m onom er in SG B, p rep ared b y th e b ase-cataly zed sol-g el p rocess. T h e dim erization of F ePor 1 and F ePor 2 can tak e p lace b ecause b oth p orp h y rin lig ands do not h av e b ulk y sub stituents lik e F ePor 3, w h ich contains ch lorine atom s th at p rotect th e p orp h y rin ring f rom
dim erization. I n f act, som e dim eric and oth er ag g reg ated sp ecies h av e b een w ell ch aracterized f or F ePors 147-49 and
2. M oreov er, dim eric F ePor 1 p resents low cataly tic y ield com p ared w ith th e m onom eric sp ecies.5 0
Cy clohex ane
I n g eneral, th e ox idation of cy cloh ex ane w ith iodosy lb enzene in th e p resence of m etallop orp h y rins com m only y ields cy cloh ex anol and cy cloh ex anone as m aj or p roducts, so th is k ind of cataly st is f req uently
considered cy toch rom e P-45 0 b iom im etic m odels. H ig h selectiv ity f or th e alcoh ol p roduct is also ob serv ed. A lcoh ol
f orm ation occurs via p roton ab straction b y th e activ e
interm ediate sp ecies ox oiron( I V) p orp h y rin π-cation radical
com p lex , resp onsib le f or th e h y drox y lation reaction.5 1
C y cloh ex ane is a v ery usef ul sub strate to inv estig ate th e ef iciency of F ePors as cataly sts f or alk ane h y drox y lation b y iodosy lb enzene. T h e activ ation of inert C−H b onds
in alk anes calls f or m ore drastic conditions th an th ose necessary f or alk ene f unctionalization, th us allow ing f or a b etter dif f erentiation b etw een th e p erf orm ances of a m etallop orp h y rin cataly st in solution and im m ob ilized in a solid sup p ort.10,42,48 ,5 2,5 3
T ab le 4 sh ow s th at F ePors 1, 2 and 3 w ere cataly tically activ e f or cy cloh ex ane ox idation ( h om og eneous cataly sis) . H ow ev er, w e h ad ex p ected th at b oth F ePors 2 and 3 w ould p resent h ig h er ef iciency th an F ePor 1 ( T ab le 4, run 1) , since th e f orm er contain rob ust, electron-deicient p orp h yrin structures. I n f act, h ig h p roduct y ields ( 5 9% ) and h ig h selectiv ity f or cy cloh ex anol w ere ob serv ed f or F ePor 2 ( T ab le 4, run 4) , b ut not so h ig h f or F ePor 3 ( T ab le 4, run 7, 37% ) . T h e relativ e solub ility and electron-deiciency caused b y th e electron-w ith draw ing sub stituents on th e p orp h y rin ring of b oth F ePors 2 and 3 could ex p lain th ese results. Nam et al. ob serv ed th at th e ox idizing p ow er
of th e cataly tic interm ediate activ e sp ecies ox oiron( I V) p orp h y rin π-cation radical com p lex is directly related to th e electronic nature of th e p orp h y rin lig and.5 4 Prob ab ly ,
th e electron-deiciency of th e p orp h y rin lig ands containing ch lorine and luorine sub stituents is v ery sim ilar, b ut any dif f erence in th eir ox idizing cap acity is m ore ev ident w h en th e sub strate is h ard to ox idize, as in th e case of cy cloh ex ane and oth er alk anes.5 5
I n line w ith th e results f rom (Z) -cy clooctene ox idation
( T ab le 3) , im m ob ilization of all th ree F ePors rendered solids w ith low cataly tic cap acity f or selectiv e cycloh ex ane
h y drox y lation as w ell as low er alcoh ol y ield, com p ared w ith th e h om og eneous counterp arts. F or th e less reactiv e cy cloh ex ane, access to th e cataly tic centre could b e ev en m ore critical th an in th e case of h ig h ly reactiv e sub strates lik e (Z) -cy clooctene.38
I n contrast w ith h om og eneous m edia, reactions 6 , 8 and 9 ( T ab le 4) rev eal larg e k etone p roduction. T h is b eh av ior is a conseq uence of th e p rop osed reaction m ech anism .5 1 F or
h om og eneous cataly tic sy stem s using F ePors, L indsay-Sm ith
et al.5 6 ob serv ed th at, dep ending on th e p orp h y rin structure and
cataly sis conditions, k etone w as f orm ed f rom f urth er alcoh ol ox idation. A s f or th e h eterog eneous cataly sis inv estig ated in th e p resent w ork , th e p ores of th e silica structure can delay dif f usion of th e alcoh ol into th e solution, and th e p resence of th is p roduct near th e activ e m etal centre can result in its f urth er ox idation, in a com p etitiv e p rocess w ith cycloh ex ane. T h is situation is m ore dram atic w h en th e structure of th e
T able 4. C y cloh ex ane ox idation b y Ph I O cataly zed b y F ePors in h om og eneous and h eterog eneous m ediaa
C ataly stb R un A lcoh ol y ield
( % )c
K etone y ield ( % )d
F ePor 1 1 6 1
F ePor 1-SG A 2 7
-F ePor 1-SG B 3 2 1
F ePor 2 4 5 5 1
F ePor 2-SG A 5 4
-F ePor 2-SG B 6 3 14
F ePor 3 7 37 2
F ePor 3-SG A 8 2 20
F ePor 3-SG B 9 7 7
SG A + Ph I O ( control) 10 trace
-SG B + Ph I O ( control) 11 trace
-Ph I O ( control) 12 trace
-aT h e y ields of th e reactions w ere calculated on th e b asis of th e am ount
of Ph I O used in th e reaction. T h e results rep resent an av erag e of at least dup licate reactions. F ePor/ Ph I O / cy cloh ex ane m olar ratio = 1: 5 0: 5 000.
bT h e p iece of g lass ob tained f rom th e sol-g el p rocess w as h ardly
g rounded, and th e resulting p ow der w as used as cataly st. cC y cloh ex anol
p orp h y rin ring contains b ulk ier sub stituents, as in th e case
of F ePor 3 ( T ab le 4, run 2) . I n f act, w e recently ob serv ed th e sam e cataly tic b eh av ior w h en m etallop orp h y rins im m ob ilized in L D H ( lay ered doub le h y drox ide)5 5 and
silica19 w ere em p loy ed as cataly sts f or alk ane ox idation.
C ataly tic ex p erim ents at dif f erent reaction tim es corrob orated k etone p roduction f rom alcoh ol re-ox idation.
Cy clohex ene
T h e p roducts g enerated f rom cy cloh ex ene ox idation m ediated b y Ph I O / F ePor sy stem s result f rom a com p etition b etw een th e C = C and ally lic C−H g roup s on th e alk ene
f or th e electrop h ilic activ e sp ecies ox oiron( I V) p orp h y rin
π-cation radical com p lex5 7,5 8 f orm ed in th e reaction
b etw een F ePor and iodosy lb enzene. O x idation of th ese g roup s sh ould lead to cy cloh ex ene ox ide and/ or allylic alcoh ol ( 1-cy cloh ex ol) and k etone ( 1-cy cloh ex en-3-one) , resp ectiv ely .5 9,6 0 H om og eneous Ph I O / F ePor sy stem s
f urnish ally lic p roducts in m inor y ields com p ared w ith th e ep ox ide, and th e ef iciency and selectiv ity of th e cataly tic reaction tow ard th e latter com p ound is controlled b y th e reaction conditions ( solv ent, tem p erature, inert atm osp h ere and reactants m olar ratio) ,6 1 th e structure of th e p orp h y rin
ring , and th e p resence of ax ial lig ands to th e iron center.35 ,36
I n a p relim inary ex am ination ( T ab le 5 ) , th e use of th e th ree inv estig ated F ePors in h om og eneous and h eterog eneous sy stem s f or cy cloh ex ene ox idation g ave ep ox ide and sig niicant y ields of ally lic ox idation p roducts
( T ab le 5 ) . H ow ev er, th e control reactions indicate th at th e silica ( SG A and SG B) sup p ort alone larg ely contrib utes to th e total p roduct y ields ( T ab le 5 , runs 10 and 11) , and th e reaction p erf orm ed w ith Ph I O and sub strate only ( T ab le 5 , run 12) also leads to sig niicant am ounts of ally lic p roducts.
T h eref ore, th e results concerning th e inal p ercentag e of p roduct y ields ob tained af ter deduction of th e y ields ach iev ed in th e control reactions sh ould p rov ide a m ore realistic p icture of th e cataly tic results ob tained w ith b oth h om og eneous and h eterog eneous sy stem s.
T h e p roduct distrib utions and y ields ach iev ed in th e ox idation reactions cataly zed b y th e F ePors are consistent w ith th e p resence of diox y g en in th e reaction m edium , esp ecially in th e case of th e h eterog eneous sy stem s. C ontrolling th e p resence of diox y g en is a p rob lem inh erent to h eterog eneous cataly sis since th is g as m ig h t b e p resent in th e v oids of th e sm all p ores and cav ities of th e silica, m ak ing it dif icult to ensure its ab sence. T h e p resence of
diox y g en in th e sup p ort w as conirm ed b y th e h ig h y ields of ally lic p roducts ob tained w h en only th e SG A or SG B sup p ort ( w ith no im m ob ilized F ePor) w as used as cataly st ( T ab le 5 , runs 10 and 11) .
I n f act, w e recently rep orted th at th e ox idation of cy cloh ex ene in th e p resence of im m ob ilized F ePor and air, under m ag netic stirring , in cataly tic reaction conditions
sim ilar to th ose em p loy ed h ere, resulted in th e p ref erab le conv ersion of cy cloh ex ene to ally lic p roducts.35 T h is result
w as consistent w ith a f ree-radical autoox idation m ech anism
m ediated b y th e F ePor in solution or im m ob ilized into Z nnA l-L D H .6 2 M ost of th e ally lic p roducts f rom
cy cloh ex ene conv ersion are p roduced w h en th e sub strate and solv ent are stirred m ag netically in air, f or th e sam e tim e p eriod and under th e sam e tem p erature conditions as th e cataly tic reaction.
I n sp ite of th e f act th at th e ally lic p roducts y ields w ere sup erior to 100% b ecause of th e uncontrolled p resence of ox y g en f rom air, th e cataly st F ePor 2-SG B ( T ab le 5 , run 9)
f urnish ed h ig h ep ox ide y ield, sh ow ing th at th is sy stem can b e v ery ef icient f or cy cloh ex ene ox idation.
T h e recy clab ility of th e solids resulting f rom im m ob ilization of th e th ree F ePors w as inv estig ated in th e ox idation of cy cloh ex ene. I t w as done in th e sam e ex p erim ental conditions em p loy ed during th e irst use of each cataly st, af ter th e solids h ad b een recov ered and w ash ed f or rem ov al of traces of p roducts ( or iodob enzene and iodosy lb enzene) th at could h av e rem ained inside th e silica structure. U V-Vis analy sis of th e solv ents used in th e
w ash ing p rocess p rov ed th at th ere w as no F ePor leach ing f rom th e sup p ort. T h e cataly st solids w ere reused in tw o f urth er cataly tic reactions and, in sp ite of th e h ig h q uantity
T able 5. C y cloh ex ene ox idation b y Ph I O cataly zed b y F ePors in h om og eneous and h eterog eneous m ediaa
C ataly st R un E p ox ide y ield
( % )b
K etone + A lcoh ol ( % )c
F ePor 1 1 36 40
F ePor 1-SG A 2 16 > 100
F ePor 1-SG B 3 26 > 100
F ePor 2 4 13 36
F ePor 2-SG A 5 34 22
F ePor 2-SG B 6 5 7 > 100
F ePor 3 7 35 42
F ePor 3-SG A 8 21 > 100
F ePor 3-SG B 9 72 > 100
SG A + Ph I O ( control) 10 6 > 100
SG B + Ph I O ( control) 11 6 > 100
Ph I O ( control) 12 - 25
aR eaction tim e: 1 h . F ePor/ Ph I O / cy cloh ex ene m olar ratio = 1: 5 0: 5 000.
T h e y ield of th e reaction w as calculated on b asis of th e am ount of Ph I O em p loy ed used in th e reaction. T h e results rep resent an av erag e of at least dup licate reactions. A ll th e cataly st solids used w ere m acerated.
bE p ox ide = cy cloh ex ene ox ide. cK etone + A lcoh ol = 2-cy cloh ex en-1-one +
of ally lic p roducts, th e ep ox ide y ields w ere sim ilar to th ose ach iev ed during th e irst use of th e cataly st.
Surp rising ly , desp ite th e low am ount of F ePor in th e silica, th ere is a non-linear dep endence of cataly tic
ef iciency on surf ace area and loading . A dditionally , th ere is no direct relationsh ip b etw een cataly tic ef iciency and surf ace area or F ePor loading on th e sup p ort f or any of th e studied sub strates. Surf ace area dep ends solely on th e silica
p rep aration m eth od ( acid or b asic cataly sis) . Based on th e cataly tic results and ch aracterization of th e solids, w e arg ue th at cataly st ef iciency is m ore related to F ePor disp ersion on th e sup p ort th an to th e inal surf ace area of th e cataly st. T h is disp ersion can b e af f ected b y such
p aram eters as F ePor interaction w ith th e sup p ort, w h ich in turn is g ov erned b y th e p H used during silica p rep aration. Strong interactions b etw een th e cataly st and th e sup p ort m ay lead to b etter F ePor disp ersion in th e m atrix .
F inally , it is notew orth y th at th e cataly sts p rep ared h ere
h av e low er F ePor loading s com p ared w ith oth er sy stem s rep orted in th e literature.Noneth eless, our cataly sts led to
sim ilar (Z) -cy clooctene conv ersion results. F or ex am p le,
Battioni et al.6 3 rep orted th at th e F ePor-sup p orted silica
sy stem cataly zed th e ep ox idation of cy clooctene b y Ph I O
w ith h ig h ep ox ide y ields ( > 90% ) , b y em p loy ing m aterials w ith h ig h surf ace area (ca. 700 m2 g-1) and cataly st loading
of 15 0 m g g-1 sup p ort. T h e cataly tic y ields are sim ilar to
th ose ach iev ed w ith th e SG A -F ePor or SG B-F ePor cataly sts p rep ared b y us, b ut it m ay b e considered th at th e cataly st p rep ared b y Battioni et al.6 3 h as an iron p orp h y rin content
of 15 g g-1 ( 15 % ) , w h ich is 10 tim es h ig h er th an th e content
in our SG -F ePor cataly sts ( T ab le 1, 0.8 to 2.1 g g-1) .
Conclu sions
T h ree structurally dif f erent F ePors h av e b een successf ully im m ob ilized in silica p rep ared b y m eans of an inex p ensiv e, p ractical ex p erim ental p rocedure using th e
sol-g el p rocess. F actors such as th e nature of th e F ePor and th e ty p e of cataly st used during silica sy nth esis inluence
th e m orp h olog y of th e ob tained p articles. C h arg ed F ePors such as [ F e( T M Py P) ] f urnish h ollow sp h erical p articles. A lternativ ely , th e F ePor m ay direct th e construction of
h ollow sp h eres th roug h a f acile sy nth esis carried out in m ild conditions, w ith out th e need f or tem p lates or th e use of em ulsif y ing m eth ods. I n g eneral, th e im m ob ilization p rocess does not seem to f av or th e cataly tic activ ity of th e F ePors inv estig ated h ere. Prob ab ly , th e access of th e ox idant
and sub strate to th e activ e site of th e im m ob ilized F ePor is dif icult, and th e cataly tic y ield f or th e h eterog eneous cataly st is low er th an th at ach iev ed w ith th e p arent h om og eneous sy stem . [ F e( T D F PP) ] is an ex cep tion, since
it led to h ig h er y ields f or (Z) -cy clooctene and cy cloh ex ene
ox idation com p ared w ith th e corresp onding h om og eneous sy stem . Nev erth eless, it m ust b e b orne in m ind th at F ePor h eterog enization is adv antag eous since it enab les cataly st recy cling , th ereb y resulting in h ig h total turnov ers.
A ck now ledgm ents
T h e auth ors are g ratef ul to C onselh o Nacional de D esenv olv im ento C ientíf ico e T ecnológ ico ( C NPq ) , C oordenação de A p erf eiçoam ento de Pessoal de Nív el Sup erior ( C A PE S) , F undação A raucária, F undação da U niv ersidade F ederal do Paraná ( F U NPA R ) and U niv ersidade F ederal do Paraná ( U F PR ) f or inancial sup p ort. T h ey also th ank th e C entro de M icroscop ia E letrônica da U F PR f or th e T E M analy ses.
References
1. K osal, M . E .; Suslick , K . S.; J. Solid. State Chem. 2 0 0 0, 1 5 2, 8 7.
2. C osta, C . O .; Souza, A . A .; L uz, R . C . S.; L em os, T . L . G .; Pessoa, O . D . L .; K ub ota L . T .; G oulart M . O . F .; J. Braz. Chem.
Soc. 2 0 0 8, 1 9, 6 97.
3. Silv a, A . R .; Peleg rino, A . C .; T edesco, A . C .; J org e, R . A .; J. Braz. Chem. Soc. 2 0 0 8, 1 9, 491.
4. Nak ag ak i, S.; C astro, K . A . D . F .; M ach ado, G . S.; H alm a, M .; D rech sel, S. M .; W y p y ch , F .; J. Braz. Chem. Soc. 2 0 0 6, 1 7,
16 72.
5 . Suslick , K . S.; Bh y rap p a, P.; C h ou, J .-H .; K osal, M . E .; Nak ag ak i, S.; Sm ith enry , D . W .; WA cc. Chem. R es.ilson, S. R .; 2 0 0 5, 3 8, 28 3.
6 . Benedito, F . L .; Nak ag ak i, S.; Saczk , A . A .; Peralta-Z am ora, P. G .; C osta, M . C . M .; A ppl. Catal., A 2 0 0 3, 2 5 0, 1.
7. I am am oto, Y .; I dem ori, Y . M .; Nak ag ak i, S.; J. M ol. Catal. A : Chem. 1 9 8 5, 9 9, 18 7.
8 . Nak ag ak i, S.; X av ier, C . R .; W osniak , A . J .; M ang rich , A . S.; W y p y ch , F .; C antão, M . P.; D enicoló, I .; K ub ota, L . T .; Colloids
Surf ., A 2 0 0 0, 1 6 8, 26 1.
9. H erron, N.; J. Coord. Chem. R ev. 1 9 9 8, 1 9, 25 .
10. Barloy , L .; L allier, J . P.; Battioni, P.; M ansuy , D .; Pitf ard, Y .; T ournoux , M .; Valim , J . B.; J ones, W .; N ew J. Chem. 1 9 9 2, 1 6,
71.
11. M ach ado, A . M .; W y p y ch , F .; D rech sel, S. M .; Nak ag ak i, S.; J. Colloid I nterf ace Sci. 2 0 0 2, 2 5 4, 15 8 .
12. F aria, A . L .; M ac L eod, T . O . C .; Barros, V. P.; A ssis, M . D .; J. Braz. Chem. Soc. 2 0 0 9, 2 0, 8 95 .
13. Nak ag ak i, S.; H alm a, M .; Bail, A .; A rízag a, G . G . C .; W y p y ch , F .; J. Colloid I nterf ace Sci. 2 0 0 5, 2 8 1, 417.
15 . Nak ag ak i, S.; W y p y ch , F .; J. Colloid I nterf ace Sci. 2 0 0 7, 3 1 5, 145 .
16 . Vidoto, E . A .; M oreira, M . S. M .; Vinh ado, F . S.; C iuf i, K . J .; Nascim ento, O . R .; I am am oto, Y .; J. N on- Cry st. Solids 2 0 0 2,
3 0 4, 15 1.
17. Sacco, H . C .; C iuf i, K . J ; Biazzotto, J . C .; Z ucck i, M . R .;
L eite, C . A . M .; Nascim ento, O . R .; Serra, O . A .; I am am oto, Y .; J. N on- Cry st. Solids 2 0 0 0, 2 7 3,15 0.
18 . O liv eira, D . C .; Sacco, H . C .; Nascim ento, O . R .; I am am oto, Y .; C iuf i, K . J .; J. N on- Cry st. Solids 2 0 0 1, 2 8 4, 28 .
19. Pap acídero, A . T .; R och a, L . A .; C aetano, B. L .; M olina, E .; Sacco, H . C .; Nassar, E . J .; M artinelli, Y .; M ello, C .; Nak ag ak i, S.; C iuf i, K . J .; Colloids Surf ., A 2 0 0 6, 2 7 5, 27.
20. Podb ielsk a, H .; U latow sk a-J arza, A .; M üller, G .; H olow acz, I .; Bauer, J .; Bindig , U .; Biom. E ng . 2 0 0 7, 2 4, 425 .
21. T anak a, H .; Y am ada, T .; Sug iy am a, S.; Sh iratori, H .; H ino, R .; J. Colloid I nterf ace Sci. 2 0 0 5, 2 8 6, 8 12.
22. D ror, I ; Baram , D .; Berk ow itz, B.; E nviron. Sci. T echnol. 2 0 0 5; 3 9, 1 2 8 3.
23. W ang , H .; Song , Y .; W ang , Z .; M edf orth , C . J .; M iller, J . E .; E v ans, L .; L i, P.; Sh elnutt, J . A .; Chem. M at. 2 0 0 8, 2 0, 7434.
24. H alm a, M .; W y p y ch , F .; D resch el, S. M .; Nak ag ak i, S.; J. Porph. Phth. 2 0 0 2, 6, 5 02.
25 . Stöb er, W .; F ink , A .; J. Colloid I nterf ace Sci. 1 9 6 8, 6 2, 26 . 26 . Sh aref k in, J . G .; Saltzm ann, H .; O rg . Sy nth. 1 9 6 3, 4 3, 6 2. 27. L ucas, J .; K ennedy , E . R .; F orno, M . W .; O rg . Sy nth. 1 9 6 3, 4 3, 48 3. 28 . R och a-G onsalv es, A . M .; J. H eterocy cl. Chem. 1 9 9 1, 28 . 29. Sm ith , K . M .; Porphy rins and M etalloporphy rins, E lsev ier:
A m sterdam , 1975 , p . 317.
30. A dler, A .; L ong o, F . R .; J. A m. Chem. Soc. 1 9 6 4, 8 6, 3145 . 31. A dler, A .; L ong o, F . R .; K am p as, F .; K im , J .; I norg . N ucl. Chem.
1 9 7 9, 3 2, 2443.
32. Buck ley , A . M .; G reenb latt, M .; J. Chem. E duc. 1 9 9 4, 7 1, 5 99. 33. Brunauer, S.; E m m ett, P. H .; T eller, E .; J. A m. Chem. Soc. 1 9 3 8,
6 0, 309.
34. Brink er, C . J .; Sch erer, G . W .; Sol- G el Science. T he Phy sics and Chemistry of Sol- G el Processing, A cadem ic Press: San D ieg o, 1990, ch . 3.
35 . H alm a, M .; C astro, K . A . D . F .; T av iot-G ueh o, C .; Prév ot, V.; F orano, C .; W y p y ch , F .; Nak ag ak i, S.; J. Catal. 2 0 0 8, 2 5, 233.
36 . Nak ag ak i, S.; Benedito, F . L .; W y p y ch , F .; J. M ol. Catal. A : Chem. 2 0 0 4, 2 1 7, 121.
37. H alm a, M .; Bail, A .; W y p y ch , F .; Nak ag ak i, S.; J. M ol. Catal. A : Chem. 2 0 0 6, 2 4 3, 44.
38 . Piov ezan, C .; C astro, K . A . D . F .; D rech sel, S. M .; Nak ag ak i, S.; A ppl. Catal., A 2 0 0 5, 2 9 3, 97.
39. T ry tek , M .; F iedurek , J .; L ip k e, A .; R adzk i, S.; J. Sol- G el Sci. T echnol. 2 0 0 9, 5 1, 272.
40. R ab elo, S. L . H .; C onçalv es, A . R .; Pereira, M . M .; Sim ões, M . M . Q.; Nev es, M . G . P. M . S.; C av aleiro, J . A . S.; J. M ol. Catal.
A : Chem. 2 0 0 6, 2 5 6, 221.
41. M anso, C . M . C . P.; Neri, C . R .; Vidoto, E . A .; Sacco, H . C .; C iuf i, K . J .; I w am oto, L . S.; I am am oto, Y .; Nascim ento, O . R .; Serra, O . A .; J. I norg . Biochem. 1 9 9 9, 7 3, 8 5 .
42. G rov es, J . T .; Nem o, T . E .; M y ers, R . S.; J. A m. Chem. Soc. 1 9 7 9, 1 0 1, 1032.
43. G rov es, J . T .; J. I norg . Biochem. 2 0 0 6, 1 0 0, 434.
44. A p p leton, A . J .; E v ans, S.; L indsay -Sm ith , J . R .; J. Chem. Soc., Perk in T rans. 2 . 1 9 9 6, 28 1.
45 . D olp h in, D .; T ray lor, T . G .; X ie, L . Y .; A cc. Chem. R es. 1 9 9 7, 3 0, 25 1.
46 . Nak ag ak i, S.; M ach ado, G . S.; H alm a, M .; M arang on, A . A . S.; C astro, K . A . D . F .; M attoso, N.; W y p y ch , F .; J. Catal. 2 0 0 6,
2 4 2, 110.
47. H arris, F . L .; T op p en, D . L .; I norg . Chem. 1 9 7 8, 1 7, 71.
48 . Y am am oto, Y .; I dem ori, I . M .; Nak ag ak i, S.; J. M ol. Catal. A : Chem, 1 9 9 5, 9 9, 18 7.
49. M isk elly , G . M .; W eb ley , W . S.; C lark , C . R .; Buck ing h am , D . A .; I norg . Chem. 1 9 8 8, 2 7, 3773.
5 0. Nak ag ak i, S.; I am am oto, Y .; Baf f a, O .; Nascim ento, O . R .; I norg . Chim. A cta 1 9 9 1, 1 8 6, 39.
5 1. G rov es, J . T .; Nem o, T . E .; J. A m. Chem. Soc. 1 9 8 3, 1 0 5, 6 243.
5 2. G rov es, J . T .; H aush alter, R . C .; Nak am ura, M .; Nem o, T . E .; E v ans, B. J .; J. A m. Chem. Soc. 1 9 8 1, 1 0 3, 28 8 4.
5 3. Berk ow itz, J .; E llison, G . B.; G utm an, D .; J. Phy s. Chem. 1 9 9 4, 9 8, 2744.
5 4. G oh , Y . M .; Nam , W .; I norg . Chem. 1 9 9 9, 3 8, 914.
5 5 . H alm a, M .; C astro, K . A . D . F .; Prév ot, V.; F orano, C .; W y p y ch , F .; Nak ag ak i, S.; J. M ol. Catal. A : Chem. 2 0 0 9, 3 1 0, 42.
5 6 . I nch ley , P.; L indsay -Sm ith , J . R .; L ow er, R . J .; N ew J. Chem. 1 9 8 9, 1 3, 6 6 9.
5 7. G rov es, J . T .; Sub ram anian, D . R .; J. A m. Chem. Soc. 1 9 8 4, 1 0 6, 2177.
5 8 . L indsay -Sm ith , J . R .; Sleath , P. R .; J. Chem. Soc., Perk in T rans. 2 1 9 8 2, 1009.
5 9. G rov es, J . T .; Nem o, T . E .; J. A m. Chem. Soc. 1 9 8 3, 1 0 5, 5 78 6 . 6 0. A p p leton, A . J .; E v ans, S.; L indsay -Sm ith , J . R .; J. Chem. Soc.,
Perk in T rans. 2 1 9 9 5, 28 .
6 1. Song , W . J .; R y u, Y . O .; Song , R .; Nam , W .; J. Biol. I norg . Chem. 2 0 0 5, 1 0, 294.
6 2. Paulson, D .; U llm an, R .; Sloane, R . B.; J. Chem. Soc., Chem. Commun. 1 9 8 4, 18 6 .
6 3. Battioni, P.; C ardin, E .; L ouloudi, M .; Sch ollh orn, B.; Sp y roulias, G . A .; M ansuy , D .; T ray I or, T . G .; Chem. Commun. 1 9 9 6, 2037.
R eceived: Septemb er 2 8 , 2 0 0 9
W eb R elease Date: A pril 2 2 , 2 0 1 0
