Deactivation of Polymer-Supported Palladium Catalystsin the Hydrogenation of 4Nitrotoluene

Deact ivat ion of Polymer-Supported Pal ladium Catalysts 1763

DEACTIVATION OF POLYMER-SUPPORTED PAI,I,ADIUM CATAIJSTS

IN THE HYDROGENATION OF 4-NITROTOLUENE

Roman FISERA"i. Milan KnAltr"i *, Julius ANNUS"/. Vieroslav KRArKv"/, Murco ZEccRi'

and Mi lan HRONEC"2'1 Depurtment ol ()rgunic Technologt, Slovuk Tet ltttitul Universitt', 812 -17 Bmtislut'u,

Slovuk Republ ic : e mui l : t kro l [email protected] f ' .snhu.sk.2 hnnet '@t het t lek. t 'h t f .s tubu.skt' Diytrtirnento tli ChiniLu Irutrguttit u, Metullorgunicu a Anulitit u, r'iu Murtilo l, -l5l3l Padovtt,

I t ul y : e - rnu i I : m :.et t u @) c lt i tn02. c h i n. un i pd. it

Reccived June -5, 1997

Accepted August 12. 1997

The deact ivat ion of pal ladium catalysts supported on comrnercia l ly a i ,a i lablc sul fonated polystyrcnc

div inylbenzene rcsins (DOWEX) in the hyclnrgcnat ion of '1-n i t roto luenc was studied. Catalysts were

prepared l iorn z[ and 8 rnole %. cr<iss l inked polymers, wi th part ic le s ize bctwccn 0.04 and 0.{10 rnnt

and pal ladium contents ranging l iom 0.2-5 to I wt .c/c (dry state) . The catalyt ic tcsts were carr ied out

in a batch rcactor v igorously shaken, at 30--50'C, and 0.25 and 0.5 MPa. The analysis of the con-

vers ion curves showed that three factors are mainly responsible fbr the re lat ively h igh deact ivat ion

rates: ( i ) thc concentrat ion of hydrogcn in the neighbourhood ol 'pal ladium crystal l i tcs. ( i i ) tempera-

ture, ( i i i ) the nanomorphology o1' the support . Some pract ical advice concerning the employnrcnt o l

polynier ic hydrogenat ion catalysts and rc lcvant oper. l t ional condi t ions can be achieved f rorn the re-

sul ts presentcd hcrc in.

Key words: Hydrogcnat ion; ; l -Ni t roto luene; Ion e xchange polymers; Polymcr-supported pal ladium

catalvsts: Catalvst deact ivat ion.

Catalytic hydrogenation of nitro compounds to amines has been long practised in indus-

try for the prclduction of a number of aromatic amines which are used as intermediates

and specialty productsl. Palladium-, platinum- and nickel-based catalysts are mainly

used for these processctl. Therrnodynamics. kinetics and reaction conditions allow a

practically quantitative conversion of nitro compounds to the corresponding amines.

Burge et o/.r disclosed a complicated reaction pathway fbr the hydrogenation of nitro-

benzene. Emmet and Yuol found that azoxy- und azobenzene are formed as stable inter-

mediates on nickel catalysts. Burge zt a1.r ulso showed that catalytic hydrogenation of

nitrobenzene follows the same pathway of the electrochemical reduction, which was

suggested by Habers in I 898.

x The author to whom correspondence should bc addressed.

Col lect . Czech. Chem. Commun. (Vol . 62) (1997)

1764 Fi iera, Krz i l ik , Annus, Krdtky. Zecca, Hronec:

Bird and Thomsono stated that the reduction of any nitro group attached to an aro-mal ic r ing wi l l fo l low the same route when a supported pal ladium cata lyst is used.Moreover, the samc authors ref'erred that up to 10Vc of the pallacliurn ntay be disscllveddur ing the hydrogenat ion of a s ingle ba(ch of 2,4-d in i t roto luene. ' l 'hese

authors a lsostated that pal ladium bccornes insolublc i f i t is conver tec l to pal ladiurn hydr ide bymeans of dihydrogen, either tiorn thc gas phase or fbrrned iz .slla upon decornpositionof a suitable compouncl. This can only occur if the rate of the hydrogen flow to themetal sur face is greater than the rate of consumpt ion. r .e . , the react ion rs contro l led bvsurf'ace kinetics and not by mass transport.

Janssen et u l .2 ' t observecl a rapid deact ivat ion of pal ladium on carbon in thehydrogcnatit ln ol ' 2,ul-dinitrotoluene and found that the activity of the catalyst clropsgradually to about 25-50Vc ofthe starting lcvel, depending on the reaction tcmperaturc.On the othcr hnnd. it was also reportecl that the deactivation of palladiurn-bascd cata-lysts d id not occurE.e.

Polymer-supported catalysts are a class of materials that play an important role in thesynthcsis technologies l0. In our prev ious work, we observcd two extremc s i tuat ions. Inone case. pal ladiurn cata lysts supported on microporous poly(N,N-dimethylacry l -amide)-r ' r . r - (sodium 2-rnethacry loy loxyethy l -2-sul fonate)-c.o-(N,N,-methylenebisacry l_a rn ide ) l l ( ca ta l ys t 1 ) unde rwen t rap id dcac t i va t i on , whe reas a pa l l ad ium ca ta l ys tprepared from poly(N,N-dirnethylacrylamidc)-r'o-[bis(3-isocyanopropylacrylato)dichlo-ropal ladiurn l - r ' r . , - (N.N' -nrethy lenebisacry larn ic le)12 (cata lyst 2) was apparent ly verystable. Thesc polyrncr materials had comparable crosslinking degrces (47r, mol/mgl),but they rernarkably d i f tered in e last ic i ty . Cata lyst I was e last ic and behaved l ike a gelin the swollcn state. Catalyst 2 was very rigid. as derlonstrated by its glassy appearancein the swollen statc and by a relatively long tirnc required fbr grinding it to fi le par-t i c l es .

our t lbscrvat ions and thc possib i l i ty of a very easy contro l o f the d is t r ibut ion ofpal ladiurn nanopart ic lcs throughout the polyrner support l l . l r prompted us to invest igatcmore systematically thc problem of pall itdiurn deactivation in the hyclrogenation of aro-mat ic n i t ro cornpounds, thc solut ion of which has not appeared in the ment ioned l i te-r i l ture. The cata lysts wcre prepared f rom commercia l ly avai lable ion-exchangepolymers - DOWEX. which are used in inc lust ry fbr ac id-cata lyzed react ions. In th ispapcr. we report on the charactelization clf the catalysts by means clf scveral techniques(swel labi l i ty , AAS. X-ray r r icroprobe analys is (XRMA)) and the resul ts 6f mgdelreact lons. cata ly t ic hydrogenat ion o l -4-n i t roto luene (c leact ivat ion tests) and cata ly t ichydrogenat ion of cyc lohexene (b lank tests) . The a im was to s tudy the in f luence ofdiff 'erent particle size of the catalyst. crosslinking degree of the polymer supporr, con_centration clf palladiurn in the particles and reaction temperature on the cleactivation ofpal ladium cata lysts in the hydrogenat ion of 4-n i t roto luene.


l ) e i r r i r u t i , r r r o l Po l ) r ne r -Sunpp r t cd Pu l l ad i u r r r C r r l i r l r s l s 1765

EXPERIMENTAI ,

Ma te r i a l s

' l -Ni t roto luene. ' l -aminotoluenc, ethanol (EIOH). mcthanol (MeOH). to lucne ( to l ) . acetone (Mc,CO)ancl cvclohexenc were suppl icd f rom I-achcrna. Brno. DOWEX 50W x. l (part ic le s izes 0.04 0.98.0 . 0 8 - 0 . 1 6 . 0 . l 6 0 . 3 2 . 0 . 3 2 0 . t 1 0 m m ) , D O W b , X 5 0 W x 8 ( p a r t i c l e s i z e 0 . 1 6 0 . 3 2 l r m ) a n d p a l l a -d i un l ace ta t c we re l r om F luka . The ac i d capac i t v o f a l l po l y rnc r s was : 1 .9 -5 .0 n teq . pe r d r y g ran t .A l l t hc p roduc t s wc re emp loycd as recc i vec l . w i t h t he exccp t i on o f 4 -n i t r o to l uene anc l c yc l 6 -hcxene . wh i ch we re pu r i f i cd p r i o r t o usc by sub l i r na t i on unc l c r r educcc l p ressu re anc l by c l i s t i l l a -t r on . r espcc t i ve l y .

Apparatus

Atomic absorpt ion nteasurentcnts ( f i r r Pd deterrn inat ion) $ 'ere pert i r r r led o1 a Zeiss Jena AAS 3atonl ic absorpt ion spectro lnetcr . Scanning elcctron rn icroscopy (SEM) and XRMA analvses wcre car-r ied out orr a Canrbr idgc Stercoscan 250 EDX pW 9g(X) apparalus.

Clatalvst Prcparat ions

Al l the pal ladium cxtalysts supportecl on I )owcx polynrers (sul t i rnatccl polystyrene c l iv inylbenzencres lns ) we rc p repa red s ta r t i ng l i rm t hc ac i d f o r r r o f t he res i n by cha rg i n - r1 w i t h an app rop r i a teI t t t t t l t tnt of pal ladiunr acetate in I : I acctone water. The product was washed wi th water and reclucedp r i o r t o use w i t h hyd rogcn i n me thano l , a t - 50 .C and 1 .5 Mpa l i r r I h .

Elcnrental Analyscs

A weighed amount o1' resin (about 0. 1 g) rvas digcsted in 2 rn l o l conccntratecl hvclrochlor ic acid. I r r lo l concentrated ni t r ic acid lnd -5 rn l of 32 r , " ' t .% hyt l rogen peror ide in watcr ancl thc solut ion wassubsequcnt ly s lowly cvaporated under ln IR lamp. This proccclure vr 'as repcated l i rur t i r lcs wi thoutni t r ic acid Thc obtainet l product was c l i iu tet l to a conccntrat ion sui t lb le f i r r atorr ic absorpt ion mcasu-re l nen t s .

Solvcnt Cornpat ib i l i ty

The solvent cornpat ib i l i ty of thc resins wrs er . 'a luatcd f rorr t l re i r bulk crpandccl volume (BEV'1 i1di f lbrent solvents l l . Thc measurenrcnts $ 'erc per lormecl ut i l iz in! the pr-oce<lurc t lescr ibed in ref . ls .BEV values wcre measure( l a lso l i r r the rccluccd pal ladiurn loaded rcsins, in orc ler to evaluate thee f l c c t o l ' t he i nco rpo ra t i on o l ' pa l l ad i un r .

Catalyt ic Tests

The catalyt ic tests were pcrt i r r rncd in a -50 ml g lass- l ined sta in lcss stecl reactor cc lnnectecl wi th al - lex ib lc rnetal capi l lary to an apparatus l i r r rneasur ing the hydrogen consunrpt ion ar conslant prcssures in t i l a r t o t ha t desc r i bed i n r e f . l 6 . Typ i ca l l y . 6 r n l o f a I v so l u t i on o f , l - n i t r o t o l uene i n mc thano lwere cnrployed' wi th thc arnount of catalyst (catalyst was t reated before the re i ic t ion wi th 6 rn l of aI n solut ion of 4 to lu id ine lbr 30 min) rn thc range 0.25 | mrnol Pd pcr l i t re of ' the solut ion. c lepencl-ing on the type of catalyst . The reactor was charged wi th reactants. catalyst ancl hyclrogen, put in athermostat ted oi l bath and v ig0rously agi tatccl using a shaker (at a f requency ofabout l2 Hz, t .

Col lect . Czech. Chem. Commun. (Vot . 62) (1997)

1766 Fi iera, Krz i l ik , Annus, Kr i i tky, Zecca. Hronec

Stabi l i ty Tcsts

Stabi l i ty cxpcr imcnts wcre carr ied out in the same reactor under the same condi t ions as in the cata-

ly t ic tests. A weighed anrount of catalyst (0.064 g1 was t reated wi th 6 ml o l 'methanol at 0.25 MPa,

30'C and 0.5 MPa, 50 "C tbr 6 h. Then. the catalysts were tested in react ion wi th 6 ml of a I r , t

solut ion of cyclohexene in methanol . at the temperature 25 'C and pressure 0.5 MPa. The resul ts

obtainecl were conrpared with the results from hydrogenation tests carried out with untreated catalysts.

RIISTJLTS AND DISCUSSION

Exp I rt ru tr t rt E.Y pe r i m e nt s

Atornic absorption measurement of rnineralized samples of thc catalysts showed thatpractically all palladium (about 967o) available in solution during the ion-exchangeprocess was taken up by the polyrneric resins. The metal crystall i tes, produced uponreduction of the polymer-bound Pdl* ions, were unifbrmly distributcd throughout thesupport (Fig. l), probably due to the low metal loading and to the high cornpatibil i ty of

the supports with the l iquid medium (methanol).

The solve nt cornpatibil i ty of the ernployed polymeric resins was quantitatively evalu-ated from the bulk expanded volume (BEV) values given in Fig. 2. lt appears that the

Dowex resins exhibit a rnarked hydrophil ic character and water, methanol and ethanolare suitable media fi lr their explclitation in chenrical applications. As expected, thepolymer with the highest crosslinking degree exhibited the lowest BEV valuc. The

same polymer exhibited a tendency to decreasing BE,V values with increasing particle

s izes (F ig.2, co lumns A D; par t ic les of 0.16 0.32 mm show the except ion to th is tend-ency). This observation indicates that the surface layer of a particle can swell to a

higher cxtcnt than thc corc. This circurnstancc can be attributed either to a decreasingcxtcnt of su l lbnat ion or to increasing e last ic res is tance against swel l ing wi th increasing

distance frorn the particle surface. The hornogeneous distribution of sulfur throughoutthe particles, as detected by means of XRMA, rules out thc non-uniform sulfbnation as

Frc. I

D i s t r i bu t i on o f me ta l l i c pa l l ad i um i n t he

catalyst part ic le (0. l6-0.32 mrn, 0.5 wt.c/o

Pd, dry state l 47c crossl ink ing, mol /mol)


Deact ivat ion of Polyrner-Supported Pal ladiurn Catalysts 1767

the possible cause of the observed phenomenon. The polymeric resins supporting re-duced palladium exhibited the same values of BEV as the starting ion-exchange resins.The BEV values of pal ladium-bear ing polymers swol len wi th e i ther a I l t so lut ion of4-nitrotolucne (substrate) or a I M solution of 4-toluidine (product) are practically thesame as in the case of swel l ing wi th neat methanol (2.93, 2.99, 3.08 rn l /g for ncatmethanol, I H,t 4-nitrotctluene and I n,l 4-toluidine, respectively).

Ctttult ' t i t 'Te.sts

We propose that the deuctivation process proceeds through the fbllowing three steps: (i)ox idat ion of Pd(O) to Pd( l l ) wi th n i t roto luene (Eq. (A)) , ( i i ) srabi l izat ion of pal la-d ium(I l ) in so lut ion as amine complexes (Eq. (B)) , and ( i i i ) d i f fus ion of rhe aminecomplexes f rom cata lyst par t ic les to the bulk l iqu id phase (Eq. (C)) .

pdO , pd2+ (A)

. ' [Pd(ArNH ),,)2* (8)Pdr* + ArNH, + ' [Pd(ArNH,) ]2n + ArNH,

IPdtArNH, ) , , ] ;1 , - [Pd(ArNH:),,li jr

In the course of deactivation. the role of water seems to be important,chiometric reaction .rmong rnetall ic pirl ladium, 4-nitrotoluene and waterbe assumcd:

P d o + A r N O , + H . O ' P d 2 + + A r N O + 2 O H

(c)

e.9. , the sto i -(Eq. (D)) may

; ^ ]

I i b

J.r , l cI l oI .

(D)

F t< ; . f

Bu l k -expandcc l vo l umes o l ' po l ymer i c

res i ns (BL ,V i n n r l / - g ) i r r vu r i ous so l -vents. A 0.0; l -0.08 r lm. .1% crossl inki n g . n r o l / r r o l : B 0 . 0 8 0 . 1 6 n r n t . . { . z lcrossl ink ing. nto l / rnol l C 0. 16--0.32 mnr.'1% cxrsslinking. rnrl/nrol: D 0.32{).t30 ntrl.'l% crosslinking. mol/nrol: E 0. |(r 1).32 nrnr.l l ? i c ross l i nk i ng . n ro l /mo l . H ,O wa te r :McOH r . nc thano l ; E tOH c thano l :

Me ,CO ace tonc ; t o l t o l uenc ; d r1 'r r eans spcc i f i c vo l un te o l d r y san rp l e

3

BEV

2

1

0


t768 F i i e ra , K l r r l i k . Annus . Kn i t k r . Zc r l r ' l r . l l r o t t ec :

The parameter se lec ted t i r r thc quant i ta t i ve assessmont o f ca ta lys t ac t i v i t y i s the ha l f ' -

t imc o l ' rcac t ionrT i t , , ) . r .e . . the t imc a t wh ich 5( )% o f the ovcra l l d ihydrogen up take is

reached. In our c i tse , the ha l l ' - t in rc o l ' i cac t ion cor responds approx in ta tc ly to -50% o1 ': l-nitrotoluene conversion anci theretirre thc amounts of ,{-nitrotolucnc. wate r and 4-to-

lu id ine in so lu t ion are la rqe cnough fo r the poss ib le ox ida t ion o f pa l lad iu r l (0 ) and

complcxa t ion o f pa l lad iunr ( l l ) . Accord ing to thc p roposcd dcac t iva t ion scheme. a t the

ha l l - t i rnc o f reac t ion the cond i t ions shou ld be par t i cu la r lv f t rvo lab lc fo r the deac t iva t ion

t t f lhe ca ta lys ts . ' l -he d i l ' f ' e rence rn ha l l ' - t i rnes in the lo l low inq runs w i th the santc ca ta-

lys t sarnp le in thc ha tch reac t ( ) r rhou lc l sc rve i l s A measurc ( ) l ca ta lys t dcac t iva t ion . For

the same reason. wc used the sunr o f reac t ron ha l f ' - t imes in a se t o f runs w i th rccyc led

cata lvs t (11 , . , ) as the quant i t y wh ich charac tc r ises the in t ' lnence o l ' the t i rnc on thc ca ta-

l l rs t c leac t iva t ion .- fhc

va lucs o l t , , in b lank hyc l rogcnat ion fes ts ( | \4 cyc lohcxcne so lu t ion in rne thano l :

P = 0 . -5 MPa. 7 '= l -5 "C) u 'c rc 73 :1 ancr -5 .10 s whcn t l rc ca ta lys t had been t rea ted pr io r

to use w i th c l ihyd logcn rn n rc thano l l i r r 6 h . rcspec t ive ly . a t 0 . - i IV IPa and 50 'C and a t

0 .25 MPa anc l 30 ' ' (1 . Thcse v l lues , cach ob ta incd lbnr r i , " 'o para l le l fL lns . a rc to be

c t r r t tparcc l i l i th tha t obsc lvcd when thc ca ta lys t was usec l \ , \ " r thout p re l i ( rL ls t leu t t r )L ' l l t .

-5 .50 s . ' ihese I ' i gu lcs c le rnons t ra te tha t thc ca ta lys t t rea tec l a t 50 'C was lcss ac t ivc o r

u 'as c lcac t iva tcd nrore qLr ick ly than thc o the l two, wh ich ga i ,c p rac t ica l l y the sanre per -

f i t r t t tancc . S i t t cc r to l cach in .u o l 'pa l lad ium dur ing the c t ta lv t i c runs wAS obscrvcc i . thc

i rpparc l r t loss o l 'uc t iV i ty upo l r l rc i r tn lcn t a t rc la t iVe ly h igh te rnpcr i r tu t ' c c iu t bc a t t r ibu tcd

io s ( )n rc n rorpho lo-u ic l l chun- l l cs in thc suppor t .

In o t ' c lc t ' to es t in ta tc thc in l ' l ucncc o f par t i c lc s izc on the ca la ly 's t l leac t i r . 'a t ion . r ,vc

cur r icc l ou t consccL l t i vc luns o f 'hydroeen l t ion o1 ' - l -n i t ro to luene a l 0 . -5 MPa anc l -50 'C .

Thc- ca ta lvs t c rnpkrycd conta incd ( ) .1 . -5 wt .7 o f 'pa l lad iu r .n anc l the c ross l ink ing dcg lcc o l '

t h e s u p p o r t w a s - 1 7 ( n r o l / n r o l ) . F o r - c a t a l y s t r e c y c l i n - u . t l r c l i q u i d p h a s c r v a s I ' i r s t

rcn to 'n 'cd l l ' r tn t thc rcac tor anc l thcn thc ca ta lys t w ls washec l th rcc t in rcs w i th l0 n r l o1 '

r t t c thano l in thc rcac t io r t vcssc l . F ina l l y ' . the lcac tor was char ' -ucd w i th ( r rn l o l ' l t r . t

so l r - r t ion o l J -n i t ro to lL rcne i r r n rc thano l and thc hvdrogcnat io l t s ta r tcd agu in . - lhc

i t r -

c r c a s i r t g c x t c n t o l ' d c a c t i v a t i o n o l t h e p a l l a d i u n i s u p p o r t c d c a t a l v s t w i t h i n c r e a s i n g p a f -

t i c l c s i z c i s i l l L r s t r i r t c d i n F i g . 3 .

I t i s sccn tha t thc ovcra l l hyc ln rgcnat ion ra tc decreases \ \ , i th inc reas in -u s izc o l ' thc

ca ta lvs t par t i c lcs . T I r i s can bc cxp la incc l by thc inc rcase o f the in f luence o l ' l l tass t rans

por . t r i i th inc rcus ing s ize o l ' thc ca ta lys t p l f t i c les . LJndcr conc l i t ions o f c l i f fus ion feg i lne .

thc cscapc o l 'p loc luc ts l i l rn the in tc l io l o f the ca ta lys ts i s d i l ' l - i cu l t and. thc rc lb rc . thc

co t lccn t ra t io l r o f 'watc r (o r tc poss ib lc conrponent fo r t l . rc rcac l ion w i th Pd(0) ) anc l :+ - to -

lL r ic l inc ( the cornp lex in -u a tcn t l i r r Pc l ( l l ) ) c lose 1o thc rnc ta l su l face are re la t i ve ly h igh .

Thesc conc l i l i ons arc luvo lab lc f r l r pa l lad iLun so lub i l i za t i c ln and. as a consequence,

s t ron-ger r leac l i la t io r t i s cxh ib i ted by la rger par t i c lcs in wh ich d i f fus iona l res is tance is


Deact ivat ion ol Polvmer-Supported Pal ladiunr Catalysts 1769

greater. Leaching of palladium is also fairlt ' dernonstrated by lowering the content o1'pal ladium in the cata lyst a l ier the t reatment as is seen in entr ies l - ,1 of Table I .

Figure 4 shows the cclnversir)n curves l irr catalysts with differenl crosslinking dc-

grees. The experiments were carried out a1 pressure 0.5 MPa and temperaturc -50 oC

with cata lysts in which the Pd concentrat ion in the par t ic les was 0.25 wt .c / r , . Due to the

decreased swel labi l i ty of the cata lyst wi th a h igher crossl ink ing degrce (87c, rnol /mol) ,

the react ion ratc achieved wi th th is cata lyst was l 'emark.rb ly lower. However, i t is ev i -

clent from the sarne figure that the overall loss of activit l, ' afier rcpcated catalytic runs.

referred to the surr i o f the hal l - t imcs of rcact ion. \ \as n() i as cxtensive as in the case of

the catalysl with 4cit, (rnol/nrol ) of'crosslinker. Runs 5 ancl 6 in Table I show the values

T,rer.r, IDcactrvat ion ol 'vanous pal ladium catal l -s ts in thc hydlogenat ion of ,1-ni t roto luene l t 0. -5 MPa

Entr l ' '

I

2

3

1

5

6

1

8

t 0

l l

IPd ]x

4

4

1

4

4

d

1

1

1

4

1

0.04-0.08

0.0u 0 . 16

0. r6 0 .32

0.32 0 .80

0 t 6 0 3 2

0. t6 0 .32().12-0.80

0.32-0.t30

0. l6 -0 .32

0. l6 0 .32

0. I6 0 .32

0.25 0.2-5

0.25 0.2.5

0 .25 0 .25

0 2.s 0.25

0.2.s I

0 .25 I

0 .25 I

l l

0 .5 |

0.-5 |

0 .5 I

n, Tt,.r APt l A l )d / t , , . , . l ( l l

988 0.3 3.04

I 532 0 .5 3 .26

IJ 906 4.3 .1 tt3

22 260 12 5.39

2 t307 t.3 4.63

9 105 3 .4 3 .73

-5 -5'18 3.0 5.41

l0 06.1 lJ. I t'i.05

2 7 2 6 1 3 4 . 1 1

2 809 t4 49 .8

1 213 16 37.1

50505050-50505050504030

" For desigrrat ion see Syrnbols

F r r ; . 3

BfTect o1' the part ic le s ize: conversion

.r r ' , r t ime t in repeated runs of hy-

drogcnat ion of I l t 4-ni t roto lucne in

methanol at 50 "Cl and 0.5 MPa and

lPcl l 0.25 mol /mr. Catalyst : 0.25 wt.9Z

Pd. 4%, crosslrnking. mol /mol l part ic le

sizes: 0.0 ' l - -0.08 rnm (7) : 0.08-0.16 rnm( 2 ) : 0 . 1 6 0 . 3 2 m m ( 3 ) ; 0 . 3 2 0 . 8 0 r n m(4 \ The so l i d and do t t ed cu rves

correspond to the f i rs t cycle and thc

f i rs t rccycle. rcspcct ively

100

x, "h

50

u ^ ?' 1 0 ' t , s "


1770 FiSera. Krhl ik . Annus. Krhtky. Zecca. Hronec

of tota l lcaching referred to the sum of the hal f ' - t imes of leact ion in eonsecutrve runs(APd / r r , . r ) , wh i ch a re equa l t o 4 .63 . l 0 as l and 3 .73 . l 0 4s l f o r l andgo /c , (mo l /mo l )crossl inked supports , rcspect ive ly .

This f ind ing is in accordancc wi th the assurnpt ion that the deact ivat ion process pro-ceeds l ia thc format ion of amine complexcs of pal ladiurr ( l l ) , as a resul t o f the rcact ionwi th the ncwly lbrmed 4- to lu id ine. In fact . both the therr rodynamics and k inct ics ofconip lex lbrmat ion ins ide thc cala lyst probably depcnd on the stcr ic in f luencc of thepolynrer ic network. Thc stabi l izat ion of pal ladium( l l ) is cxpectcd to increase as thenumber of coordinated ani l ine nro lecules increases, unt i l e lcct ronic and coordinat ivesaturat ion o l - the Pdr* ions is achieved ( format ion of thc te l raarn inc cornplcxlPd(ArNH,) . r l r * ) . Of course. thc h igher thc number of coordinated i l - to lu id ine nro le-cules is . the bulk icr is thc rne(a l cotrp ler . Hence i ts format ion is increasingly h inc leret lby the ster ic const t 'a ints imposed by the polyrne r chains. Moreclver . the larger thc mole-cular dirncnsions ttf t lrc cotnplex are, thc lower is its clif l 'usicln ri i te froln inside thccata lyst to the bt r lk of the l iqu id phase. Thus. ins ic le a c^ata lyst based on a re lat ive lyhigh-crossl inkccl sr - rppor t or on a r ig id suppol t , the concl i t i t )ns are not cxpectcc l to bc assuitable for the fbrntation oi highly coordinatccl spccies ancl for their cscapc frorr thcintcr ior of the cata lyst as ins ide a cata lyst based on e last ic supports . Accorc l ing ly , bothlcaching of 'pa l ladium and deact ivat ion of the cata lyst are expectec l to occLl r nrgrc reacl -i ly whcn thc cr t lss l ink ing de-9rce is thc lowcst . This explanal ion is a lso supportec l by thebehavior o l ' the vcry r ig id cata lyst 2 descr ibed abclvc. which undcrwenl a r -e lat ive lvs low deact i r , 'a t ion.

In orc lcr to est imalc the in f luence c l1 ' the n ic ta l concentrat ion in the cata lys l on boththe cata ly t ic act iv i ty and the deact ivat ion process of the 4-n i t roto lucne hydrogenat ion.the erpcr imcnts wcre pcr fc l r rncd at -50 'C and hycl rogen prcssurc 0.5 Mpa. in thepresence of'two catalysts bascd on a 1c/c (nolh'nol)crosslinkccl supprtrt with c.l i l- l 'crentpal ladium concentrat ion in thc cata lyst . The convers ion curves in repeatcd r -ur . rs ob,

I100 f

x, ",L

5 0 -

l

0 -0

-z

F l r ; . . 1

E1 '1cc t o f t hc c ross l i nk i ng c i cg ree o l 'l l t c : t t p p t r r l : \ ' o t l \ e r \ i r ) t ) . r r . r l i l t t c t i r rrepcated runs of hvdrogenat ion of I r r.1-ni tnr to luenc in ntcthanol at -50 "C

ancl 0.-5 M Pa and I Pd I I rnol /nr ' r .C ia ta l ys t s : 0 . 25 w t . 7 , Pd . ' 1 o r l i%c ross l i nk i n - r , r no l / n t o l : pa r t i c l c s i ze :0. l6 0.32 ntrn. The l ' i rs t ancl secondf i gu res i nc l i ca te t hc c ross l i nk i ng de -g r cc and t he recyc le nu r rbe r ( 0 i sf i rs t cycle) . respect ively

*/t^)t:'at"

//t ';//

/

1o-3 I s

Col lect . Czech. Chem. Commun. (Vol . 62) (1 997)

Deact ivat ion of ' Polymer-Supportcd Pal ladium Catalysts t77 l

tairred with catalysts with 0.2-5 and lLh of palladium are shown in Fig. 5. Thc quantita-t ive evaluat ion of leaching o l pal ladium is g iven in entr ics 7 and 8 in Table I .

The rate is rc lat ive ly low wi th cata lysts having a h igh metal concentrat ion in thepart ic les, which is not surpr is ing. In fact , a proport ional ly lowcr amount of h ighlyloaded catalyst is necessary fbr achieving the same amount of palladium in the reactionmixture in compar ison wi th a cata lyst wi th a smal lcr metal content . As a consequcnce.nrass transfer and difTusional hindrances play a morc important role in the formcr case.The deact ivat ion of the cala lyst in repeated runs is faster in the cata lyst wi th h ighcrpal ladiurn concentr i r t ion, which is i l lust rated in F ig.5. ln th is casc, the h igher concen-t rat ion of pal ladium accclcrates the reduct ion of n i t ro compound and, as a consequence.tl.tc concentration of hydrogen is expected to rapidly decrease as the depth from thesur f ice of par t ic lc increases. Al though the concentrat ion o l - 4-n i t roto luene is lowerc lose to the crysta l l i tes than in thc bulk solut ion. i t is h ighcr than the concentra l ion ofhydrogen, duc ttt the lclw solubil ity of the gas. Moreovcr, the concentration of watcrincrcases as the react ion proceeds and thc hydrophi l ic nature of ' thc supports helps inreta in ing i t ins ide the polyrncr network. Therelbrc,4-n i t roto luene as thc possib le ox i -clant and water are presen( in such amounts to ensurc the oxidation of thc rnctal. In thiscase. thc deact ivat ion of cata lyst should bc faster than in the casc o l cata lysts chargcdwi th k lwer amount of pal ladium.

Thc cffect of tempcrature on the deactivation process in the hydrogcnation of . l-nitro-

tolucnc has been asst:sscd by perforrning cxpcrirnents at threc different temperatures:30. :10 and 50 "C, at the prcssurc of hydrogen 0.5 MPa. The catalyst was charged with0.-5 wt.o/r, of Pd ancl the crosslinking degree of the polymcr suppoft was 47c (rnol/mol,F ig. 6) . Thc cxtent of dcact ivat ion of pal ladium cata lysts was appafcnt ly larger ath igher tcmperatures, a l though pal ladium leaching was more extcnsive at lower ternperatures. Entr ies 9 I I in Table I i l lust ratc that the h ighest rc lat ive leaching occurs at40 'C. The s igni f icant decrease in react ion rates and the low cxtcnt of leaching of

Fr r ; . - 5

Eftcct of the rnctal loading: convcr-s ion . r r ' , r t ime / in repeatcd rurrs of

hydro-tenat ion of I u ; l -n i t roto lucnc

in rncthanol at 50 "C and 0.-5 MPa

and lPc l l I mo l /mr . Ca ta l ys t s : 0 . 25(so l i d cu r ves ) o r I ( do t t ed cu rves )wt.7, Pd. 47c crossl ink ing, rnol / r ro l ;part ic le s ize: 0.32 0.80 mrn. The f ig,

ur t : indicates the recycle nuntber (0 isf i rs t cyclc)


1oo [ -

x . ' k 1i

1772 FiSera. Krdl ik , Annus, Kr i i tky, Zecca, Hronec

palladium observed in runs with recycled cataiysts at -50 'C indicate that there must besome changes in the morphology of the support. which result in an increase ol- thediff lsional resistance to the transport of reaction species, including the amine comple xof palladium formed in situ. This behaviour is in agreement with the results obtaincdliom the stabil ity tests described above, in which cyclohexene was use6 as the sub-strate. The issue of the change of the nanoscopic morphology of polymer supports isunder investigation by inverse steric exclusion chromatographyls and wil l be addressedin the fbllowing part of our work.

Table Il i l lustrates lhe extent of palladium leaching fiom the catalyst to the reactionmixture and washing solut ion. As in our prev ious exper iments, h igher leaching hasbeen observed at lower concentrations of hydrogen and lower temperatures.

In order to evaluate the catalytic activit), of the palladium complex leached tiom thecatalyst particles, catalytic tests were perfbrmed. Thus, an aliquot (3 ml) o1, a fresh I v

TrBLe I lRclat ivc leaching o1'pal ladium LPd (va) into the react ion mixture (R) ant l in to the washing mecl iurn(M) dur ing thc hydrogenat ion of 4-ni t roto luene on 0.5 wt.9 i Pd catalysts (4 mole %,crossl ink ing.

i"": l: sze 0.16-0.32 mrn, lPdl I mol/rnr)

APd" APdi,Run

8.21

9.6ti

1 2 . 5 3

6.46

5.99

7.68

0.5 r0 . l 80.200. l-5

M

0.300.3 l0 . 3 10.28

M

21

4

" 0 .2 -5 MPa , 30 'C , t he avc rasc ha l l , t ime o f t he reae t i onhal l - t ime of the re lct ion 2 120 s.

100

x, "k

50

- |

tO-3 I s

4 290 s; " 0.5 Mpa. 50 .C, the averasc

Frc; . 6El l 'ect of the temperature: convcrs ion. t ls t ime / in repeated runs ol ' hy-drogenat ion of I v z l -n i t roto luene innrethanol at -50 ( 7), 40 (4 and 30 (O .C;

0.5 MPa and [P<11 I mol / rn] . Cata-lyst : 0.5 wt.7o Pd, .1% crossl ink ing.mol /mol ; part ic le s izc: 0. l6 0.32 nrrn.The so l i d and do t t ed cu rves co r re -spond to the f i rs t cycle and the f ' i rs trecycle, respect ively0


[ )eact ivat ion of Polymer-Supported Pal ladium Catalysls \773

solution of 4-nitrotoluene in methanoi was added to a portion (3 ml) of the hydrogen-

ated reaction mixture. f iom which thc catalyst had been removed. In the range of palla-

d ium amount pcr uni t vo lume of the solut ion (0. ( ) -10 mol /nrr i r t the maximum). asrcsulted from the leaching data and experiments given in Table Il, no consumption ofhydrogen was recorded in 2 h at 0.75 MPa and 50 "C. Measurements of XRMA line

prof i les of pal ladium ins ide the employcd cata lyst showed a homogeneous leaching ofthe rnetal f iom the bulk of thc catalvsts oarticles.

()ortc Iusions

E,xpcr . imenta l invest igat ions of :+-n i t roto luene hydrogenat ion over pal ladium cata lystssupportcd on micropor 'ous sui fonated poly(styrene-div iny lhcnzene) proved that thc

dcsi rcd cata ly t ic rcact ion was accompanied by the cata lyst dcact ivat ion. The rate of th is

i lcact ivat ion incrcascd wi th c lecreasing concentrat ion of hydrogcn, which was derron-st ra led indi rect ly us ing cata lysts wi th d i f ferent s ize of par t ic les and d i rect ly in erper i -rnenls carried out under difTerent total prcssurcs ol' hydrogen. We supposc that the

r le i lcr ivat ion proceeds in th lcc stcps: ( i ) ox idat ion ot 'Pd(O) to Pd( l l ) , ( i i ) lbr rnat ion ofpal ladiurn arn ine complexes. and t i i i ; t ransport o l pal ladium amine complexcs f rom

the inter ior of the cata lyst to the bulk of l ic lu id. As an addi t ional ef fect . changes in the

nrorphology of ' the support are l ike ly to occur . in pal t icu la l a t re lat ive ly h igh tempera-lurcs and hydro-een pressures. The extent of the stcps ( i i ) and ( i i i ) depends on thc stcr ic

in l ' lucncc of ' thc polyrncr nctwork. as i t was demonstrated by the h igh re lat ive deact iva-t ion o[ 'a cata lyst wi th a z17c (mol / rnol ) crossl inked support wi th rcspect to a cata lystbascd on a 8% (mol / r lo l ) crossl inkccl support .

Thc re la t ionship bctwccn thc conccntr i l t ion of hydrogen and the rate of dcact ivat ionvu'as fa i r ly demonstratec l by the hehaviur o1 ' thc cata lysts wi th re lat ive ly low and h ighpal ladiurn loads. In the cata lysts wi th h igh amounts of pal ladium. the protcct ion o l - thcnrcta l sur l 'acc by hydrogen was less ef l 'ect ive due to i ts low conccntrat ion in the neigh-borhood o l 'pa l lad iurn crysta l l i tes. This c i rcurnstance causcd a rc lat ivc ly h igh deact i r ,a-t i t ln la tc .

Thc tcrnpcraturc incrcascs the rate o l ' the cata lyst deact ivat ion s igni l ' icant ly . Thisphcnorncnon can bc at t r ibutcd to c i thcr one or both of the fo l lowing c l - lccts : ( i ) re lat ivc ly low hydrogcn conccntrat ion at h igher ternperatures due to a h ighcr hydrogen

consunrpt ion; ( i i ) h igher rnobi l i ty of polymer chains. which rnakcs casicr both the tor-nrat ion of h ighly coordinated pal ladiurn( l l ) -ar .n inc cornplexes and thei r t ransport lnrn ' t

the cata lysts to the bulk of l iqu id.

Frorn thc practical point of view, the tbllclwing considerations should be taken intir

account to opt imizc thc propert ies of a polymer cata lyst and the exper imcnta l condi-t ions of thc hydrogcni i t ion of aromat ic n i t ro compounds: ( i ) e i ther low-pal ladiurn-

charged catalysts or very slnall catalyst particles and vigorous rnixing of the reacticlnrn ix ture should be appl ied to rnakc thc d i l ' fus ivc res is tance as low as possib le; ( i i ) thc


t774 Fi iera, Krdl ik , Annus, Kr i i tky, Zecca, Hroncc:

pressure of hydrogen must be set to an optirnum value, corresponding to the best com-promise between the stabil ity of palladium crystall i tes and the stabil ity of the support;(i i i) i f porous supports are cmployed, the pref-erence should be given to rigid ones; (iv)the temperature of hydrogenation must be set up to an optimum, as much as possiblelow, value. For other than polymer catalysts, e.g., palladium on charcoal or on sil ica,similar recommendations are valid. However, for these supports optirnum values oftemperature and pressure can be significantly higher in comparison to the polymer-based catalysts, due to a greater chemical resistance of the supports to the hyclrogena-t ion condi t ions.

SYMBOI,S

X crossl ink ing degree. mole % of d iv inylbenzened lverage part ic le s ize, mml|pct perccntage of pal ladiurn in thc dry catalyst , wt .7c

IPdl pal ladium arnount per uni t volume of the solut ion. mol /mlT Icntperature. oC

nr number of runs

Th.t sum of react ion hal f - t intes. sAPd overal l re lat ive leaching of pal ladium af ter r r , . runs, %\ ( r )nvefs lon- ' ,1

I react ion t inte. s

We.strrtrtglr ockrtttwletl.qe Mr C. Furlart ol C.U.G.A.S., (Jniversitt,ol Patlova,.fitr skiltful SEM antlXRMA tnea.surerttent.s. We urc grutelhl to Mr L. Rurtalos, Deportm?nI ol Anult'tital Chemistrt, Slovuk7'ctltnitul Uttiyer,si^'in Rruti.sktvtt, lir aknit ult.srtrptiort n1(usur(ments.

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1 t r ) tX)) .

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l J . Nc r i f i . . Muso l i no N ' I . ( i . . M i l one c . . S i gno r i no G . : I nd . Eng . Chem. Res . - 14 .2226 i l 99 -5 ) .9 . Ne l i C . . Muso l i no M . C . . M i l onc (1 . , V i sco A . M . . D i Ma r i o A . : J . Mo l . Ca ta l . g -5 .21 -5 ( 1995 ) .

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/ ( r / . l . t 3 ( 1995 ) .

12 . P r - i n rave ru A . . Zecc l M . . Co ra in B . : . 1 . Mo l . Ca ta l . / 0 , 9 . l 3 l ( 1996 ) .


Deact ivat ion of Polyrre r Supported [ 'a l ladiunr Catr lysts t775

l l . B i l T i s A . . Co la i n B . . ( i r eng ro i ov r i 2 . . l l r onec M . . . l c i i r bck K . . Kn ' r l i k M . : App l . Ca t r l . / 2 f . 355

( I 9t )5 ) .l - 1 . A r shadv R . : Ac l v . Ma te r . - i . I l 3 l ( l t ) 9 l ) .

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( 199.s ) .16 . H r -oncc M . . I l a vsk_v - J . : Che tn . Papc rs - i 9 . 705 (198 -5 ) .

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Documents

Deactivation of Polymer-Supported Palladium Catalystsin the Hydrogenation of 4Nitrotoluene