Chapter 11 Catalytic Hydrogenation and Dehydrogenation 1995 Studies in Surface Science and Catalysis

8/10/2019 Chapter 11 Catalytic Hydrogenation and Dehydrogenation 1995 Studies in Surface Science and Catalysis

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4 7 7

Chapte r 11

C A T A L Y T IC H Y D R O G E N A T I O N A N D D E H Y D R O G E N A T I O N

1 1 .1 H y d r o g e n a t i o n o f a l k e n e s

11.1 .1 Ge nera l p r inc ip les

Th e a d d i t io n o f h y d ro g e n to th e c a rb o n -c a rb o n d o u b le b o n d i s o n e o f th e s imp le s t

ca ta lysed reac t ions in which the produc ts a re p la in ly d i f fe ren t f rom the reac tan ts , and the

hydrogena t ion of e thene , the smal les t a lkene , i s the re fore the a rch type of a g rea t family of

re a c t io n s , s o me o f wh ic h h a v e g re a t in d u s t r i a l imp o r ta n c e . Fo r th i s r e a s o n i t h a s b e e n

mu c h u s e d (p e rh a p s to o mu c h ? ) b y th o s e wis h in g to d e mo n s t ra te th e e f fe c t s o f s o me

var iab le of the type of ca ta lys ts under s tudy on an easy ye t s ign if ican t reac t ion . L ike many

other ca ta lysed reac t ions , i t was f i rs t observed by the grandfa ther o f ca ta lys is , Paul

Saba t ie r , and i t has been the cont inued objec t o f sys temat ic and quant i ta t ive s tudy for

three-quar te rs o f th is cen tury [1-4] . There is no sc ien t is t o f any s ta ture , work ing in the

f i e ld o f c a ta ly s i s b y me ta l s , wh o h a s n o t a t s o me t ime h y d ro g e n a te d e th e n e .

Par t o f i t s a t t rac t iveness l ie s in i t s apparen t bu t decept ive s impl ic i ty . I t appears

c a p a b le o f y ie ld in g o n ly a s in g le p ro d u c t - e th a n e - a n d th u s o f b e in g s tu d ie d fo r e x a mp le

by change of p ressure in a cons tan t vo lume reac tor : and to a ce r ta in leve l o f sophis t ica t ion

th is i s t rue . But as wi th so many o ther ca ta ly t ic sys tems , the more one de lves in to i t , and

th e mo re re f in e d a n d re v e a l in g th e a n a ly t i c a l me th o d s e mp lo y e d , th e mo re o n e c o me s to

rea l ise tha t th is s impl ic i ty is in fac t , l ike beauty , on ly sk in-deep , and tha t the re a re

c o u n t l e s s c o mp l ic a t io n s a n d ra mi f i c a t io n s to th i s n o min a l ly s t r a ig h t fo rwa rd re a c t io n .

Le t u s n o w t ry to s e t d o wn in a s y s te ma t ic a n d lo g ic a l ma n n e r wh a t s o me o f th e s e

are . We a re concerned in th is sec t ion on ly wi th molecules tha t conta in a s ing le , non-

c o n ju g a te d c a rb o n -c a rb o n d o u b le b o n d , w i th i ts a n d rt c o mp o n e n t s : m o le c u le s c o n ta in in g

two o r mo re s u c h b o n d s c o n ju g a te d wi th e a c h o th e r o r w i th s o me o th e r u n s a tu ra te d

fu n c t io n wi l l b e d e a l t w i th l a t e r . We d o h o we v e r in c lu d e h e re l in e a r , b ra n c h e d a n d c y c l i c

a lkenes of any s ize .

The f i rs t and major complica t ion , to which we sha l l re turn , i s the ques t ion of

' c a rb o n d e p o s i t io n ' , i . e . th e fo rma t io n o n th e s u r fa c e o f ' h y d ro c a rb o n a c e o u s re s id u e s ' th a t ,

in the case of gas -phase reac t ions a t leas t , may rap id ly cover a subs tan t ia l f rac t ion of the

ac t ive surface . Of such importance is th is e ffec t tha t i t wi l l be t rea ted in some de ta i l in a

la te r sec t ion (11 .1 .2) . Secondly , in addi t ion to th is pa ras i t ic reac t ion , the re a re two o ther


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478 chap ter 11

processes observable wi th e thene: these are (a) homologation and (b) isotopic exchange.

These may be desc r i bed fo rma l ly a s

2C 2H 4 + H 2 - -- )

an d C 2H 4 + D 2 ----)

C 4 H 1

C2H3D + HD,

the la t te r reac t ion being in par t respons ible for the format ion of a range of deutera ted

ethanes [3 ,4] . Thi rdly , when four or more carbon a toms are present in the molecule ,

pos i t i ona l and geomet r i c i somer s o f t he monoa l kene become poss i b l e , and t hese can

interconver t dur ing hydrogenat ion ( see scheme I , be low) . Of course , the processes of

double-bond migra t ion and of c i s - t rans i somer iza t ion can be fol lowed as the a lkene reac ts

wi th deuter ium, and there i s cons iderable mechanis t ic informat ion in compar ing, for

example , the deuter ium dis t r ibut ion in the three n-butenes formed when 1-butene reac ts

wi th deuter ium [5] . Four thly , a fur ther s tereochemical face t of the reac t ion c lass becomes

vis ible wi th cycl ic a lkenes and other appropr ia te ly subs t i tu ted molecules : for example ,

hydrogenat ion of 1 ,2-dimethylcyclohexene [6] may in pr inc iple g ive e i ther c i s - or t rans-

d ime thy l cyc lohexane depend ing on whe the r t he hydrogen a toms a r e added t o t he s ame o r

to di f ferent s ides of the molecule ( see a l so scheme I ) . In the fol lowing paragraphs these

addi t ional processes wi l l be cons idered fur ther .

The obse rva t i on o f t he e thene -deu t e r i um exchange r eac t i on [7 ] was one o f t he ea r l y

mechanis t ic t r iumphs of the use of deuter ium as an i sotopic label : h i ther to i t had been

assumed t ha t hydrogena t i on by t he s imul t aneous add i t i on o f two hydrogen a toms was a l l

tha t was enta i led . Hor iut i and Polanyi [8] therefore proposed the mechanism (scheme I I ,

be low) , which bea r s t he i r name , i n whi ch t he a t oms added consecu t i ve l y , an adsorbed

ethyl radica l be ing the in termedia te ; for the addi t ion-exchange reac t ion:

D 2 + 2* ---) 2D

C 2 H 4 + 2 *

---) C2H4

C 2 H 4 + D

~ C 2 H 4 D

----)

C 2 H 3 D +

H

C 2 H 4 D + D - - - ) C 2 H 4 D 2

In t h i s ea r l y work , t he exchange was de t ec t ed by HD in t he gas phase fo rmed by

desorpt ion of the hydrogen a tom re leased in the exchange, together wi th a deuter ium a tom:

th i s was fo l l owed by t he change i n t he rma l conduc t i v i t y o f t he non-condens ib l e gaseous

f rac t ion, th i s be ing the only analyt ica l t echnique avai lable a t the t ime. Inc identa l ly , th i s

work was ca r r i ed ou t w i th ' hydrogen ' t ha t was on ly abou t 30% enr i ched i n deu t e r i um. The


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Cataly t ic hydrogen at ion and dehydrogenat ion 479

role of ethyl radicals as the ha l f -hyd rogena ted s ta te in the mercury-photosensi t i sed ethene-

hydrogen react ion had in fact been es tab l i shed by H.S.Taylor some years before . Hydroge-

nat ion of h igher a lkenes i s accompanied by o ther react ions which are presented by scheme

I.

S c h e m e I

React ions accom panying the hydrogenat ion of. h igher l inear and cycl ic a lkenes

CH 3 CH 3 CH 3 H

C = C ~ .m , . - C = C c is - t rans

/ ~ - ' ~ ~ ~ isomerization

H H H CH 3

cis- 2- butene trans - 2- butene

C H ~ - C H 2 - -- C H = C H 2 ~ C H ~ - - C H - - C H - - C H 3 d ou ble -b an d

migration

1 - butene 2- butene

H

( > ( o H 3 _ .

CH 3

1 ,2 - dimethylcyclohexene cis - 1, 2 - dimethyl -

cyclohexane

CH3 H

CH 3

CH3 + ~ N CH3

trans- 1, 2 - dimethyl -

cyclohexane

The Horiu t i -Polanyi mechanism, of which the above s teps are only an incomplete

s tatement for the ethene-deuter ium react ion , i s capable of considerable ex tension and

refinement [9], but is st i l l regarded as containing the essential t ruth [3,4]. What remain

under d iscussion are quest ions such as the precise form of the react ive adsorbed ethene

molecule and the source of the hydrogen (deuter ium) atoms involved in the addi t ion s teps .

I t requi red the advent of mass-spect roscopy to reveal the t rue sp lendour of the

ethene-deuterium reaction. I t emerged that , with nickel wire as catalyst , al l possible

deuterated ethenes and ethanes ( including

C2H6,

i .e. ethane-do) were formed by rei terat ion

of the basic steps, as well as dihyd rogen and hydro gen deuteride [10]. Indeed , etha ne-d o

was the major in i t ia l p roduct , due to the h igh concent rat ion of hydrogen atoms that

accumulated on the surface through the react ions shown above. Only one hydrogen atom

in ethene was substi tuted for deuterium in a single residence. Similar studies using a series

of p lat inum catalysts [11 ,12] revealed an ear ly example of a suppor t ef fect , and showed

that a lkene exchange was less prominent wi th p lat inum than wi th n ickel . Later systemat ic

work by Kembal l [13] was rat ional i sed in terms of a quant i f ied descr ip t ion of the Horiu t i -

Polanyi mechanism in which numerical probabi l i t ies were ass igned to four basic s teps

(scheme I I ) .


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480 chapter 11

Scheme 1-I

Quantitative description of the Horiuti- Polanyi mechanism [13]

P

C 2 x 4 ' q 1 -2 -- C 2 x 4 ~ ~ C 2 x 5

+ H + D

C 2 X 5 H ~ ~ 1 C 2 X 5 ~ m , . -s C 2 X 5 D

- X + X

C 2 X 4 ~ ~ C X ~ ~ , - - C X

r 2 5 1 2 6

+ H + D

C 2 X 4 H ' ~ ~ 1 C 2 X 4 ~ m , .- q C 2 X 4 D

I t i s then possib le to wri te down equat ions for e ighteen s imul taneous react ions , s ince there

are s ix possib le e thenes and twelve ethyls , including posi t ional i somers . Calculated

amounts of the product e thanes and ethenes are then obtained for any set of probabi l i ty

parameters p ,q , r and s , by so lv ing the equat ions (most easi ly by matr ix invers ion) . I t i s

in teres t ing to note that more informat ion i s avai lab le by calcu lat ion than by exper imentat i -

on (e.g. the isomers

CH3-CD 3

and CH D-C HD 2 are calcu lated indiv idual ly , bu t c annot be

separated by mass-spect rometry) . Mechanis t ic work performed before 1965 has been

reveiwed [5 ,9 ] ; the reader i s refer red to the outs tanding work of Dumesic and h is

associates [3,4] for the most recent statements on this reaction.

With th is las t except ion , i t i s remarkable how l i t t le fur ther research has been

performed on alkene-deuter ium react ions s ince that date; desp i te thei r g reat po ten t ia l for

sensing the surface of metal l ic (or a l loy) catalysts , and the avai lab i l i ty of analy t ical

methods even more ref ined than mass-spect rometry [14-16] , the focus of in teres t has

sh i f ted to o ther mat ters ; and your au thors too , l ike so many o thers , have succumbed to the

temptat ion to move on to more t imely pro jects . The work of Francois Gaul t and h is

col leagues [17 ,18] revealed new and unsuspected mec hanis t ic r ichness in the excha nge of

alkenes over i ron and n ickel catalysts . Another development of major impor tance was the

appl icat ion of microwave spect roscopy by Hiro ta and h is associates [15 ,16 ,19 ,20] (see also

[21]) ; th is has been subsequent ly cont inued by Nai to and Nanimoto [22 ,23] . The technique

permits analysis of posi t ional i somers of deuterated molecules , and i t s scope and power

are i l lus t ra ted by the fo l lowing examples .

(1) O ver n ickel , pal lad ium and rhodium catalysts , p rop ene-3d 1 (CH2D-CH=C H2)

underg oes sel f -exchange, forming propene -d o and propene-d2; use of m icrow ave spect ro-

scopy es tab l i shed that on al l metals deuter ium atoms were l iberated as 7 t -al ly l complexes


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Cataly t ic hydrogenat ion and dehydrogenat ion

481

were formed (see scheme I I I ) , and that these subsequent ly reacted wi th n-adsorbed

propene to g ive cy-propyl rad icals , whence the exchanged propene molecule was recon-

structed [22,23].

Scheme

Mechanism of self-exc ha ng e of propene --3d 1

CH

2 =

CH -CH2D

. - -~ - I , ,- - H 2 C - C H - C H 2 + D

. . ) .

C H 2 ~ C H - C H 2 D + D

- - x -

f

C H - c H D - C H 2 D

- . x -

C H 2 D - C H - C H 2 D

C H 2 _ C H D _ C H 2 D - -- -- D-- C H 2 = C H - C H 2 D + D

C H 2 D - C H - C H 2 D ~ C H D = C H - C H 2D + D

(2) Over rhodium, n ickel and i r id ium catalysts , i sobutene (2-methylpropene) reacts

wi th deuter ium to form the monodeutero-products (CH3)3CD and (CH3)2C=CHD; th is

requires different butyl radicals to be involved in exchange and in hydrogenation [20]. 1,2-

Dideutero-2-methylpropane i s the major product ob tained by react ing i sobutene wi th

deuter ium over n ickel [24] .

(3) Analysis of monodeutero-products f rom the react ion of propene wi th deuter ium

on nickel , plat inum, palladium and copper catalysts is given in table 1 [22]. These results

counteract the impression that may have been created by the ear l ier examples that a l l

metals (a t leas t those of Groups 8-10) behave s imi lar ly , and form related i f no t ident ical

in termediates . The analyses shown in Table 1 are in i tia l ones , ob tained when the mean

number of deuter ium atoms in the propene was only 0 .01-0 .03: in the cases of pal lad ium

and p lat inum, relat ive amounts of the four products changed as the react ion proceeded due

to ' in t ramolecular i somerizat ion ' [23] . These observat ions are not readi ly expl icable in

mechanis t ic terms, a l though i t i s c lear that over n ickel exchange proceeds by d issociat ive

chemisorp t ion of the C2-H bond, ra ther than v ia a n-al ly l rad ical as in sel f -exchange.


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482 chap ter 11

table 1

R e l a t i ve amount s o f p ropene -d l , i somer s fo rmed i n i t i a l l y by t he r eac t i on o f p ropene w i th

deu ter ium [22]. d H a

H 3 C N C = C /

i f /

\ H

C ata lys t T /K a /% b /% c /% d /%

Ni 200 2 3 95 0

Pd 210 35 33 16 16

Pt 235 9 57 28 6

Cu 263 14 14 72 0

N i-Cu 180 11 12 78 0

Pd-C u 225 20 17 58 5

Pt-C u 248 13 9 68 10

Note:

the a l loys w ere prepared f rom mixed ox ide s and con ta ined 60% copper

(surface concentrat ions 70-80% copper)

The la t te r work [22] provides one of the very few examples of a s tudy of an

alkene-deuter ium react ion on a l loys . Wi th th i s sys tem a lso the se l f -exchange - scheme I I I -

can t ake p l ace [22] . A l loys o f coppe r w i t h n i cke l , pa l l ad i um and p l a t i num showed p roduc t

dis t r ibut ions having dis t inc t s imi lar i t i es to those of

copper,

al though ra tes were fas ter than

that of copper as seen by the tempera tures used in table 1 . I t may be tha t , whi le copper as

expected occupies most of the sur face due to i t s smal ler sur face energy, a few a toms of

t he Group 10 me ta l p rov ide a channe l by whi ch more hydrogen a t oms a t t a i n t he coppe r

sur face than would otherwise be poss ible . A s tudy of th i s reac t ion on Pt -Au/SiO2 has

recent ly been publ i shed [251.

One other observat ion mer i t s a br ief ment ion. The in terac t ion of e thene and

deuter ium on nickel -copper f i lms conta ining low concent ra t ions of n ickel resul t s only in

exchange and not addi t ion ( i . e . hydrogenat ion) [26] . I t was sugges ted tha t i sola ted nickel

cou ld e f f ec t exchange .

We r e tu rn now to t he ma t t e r o f a l kene i somer i za t i on ( see s cheme I ) . A moment ' s

t hough t shows t ha t bo th c i s - t r ans i somer i za t i on and doub l e -bond mi gra t i on may p roceed

by addi t ion of a hydrogen a tom, fol lowed by abs t rac t ion of a d i f ferent a tom (scheme IV) .


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483

Alkyl reversal

C is - t rans isomerization

S c h e m e I V

Mechanisms of a lkene i somerizat ion

CH 3 CH 3

C = C

H H

Double - band migration

CH 3 CH 3 H

+ H ~ f ~ / - H ~ /

C H ~ C m H ~ C = C

/ I CH 3 I H

. .

CH 3

_ - H

CH3- CH2 CH - - CH2 ~ CH3- CH 2- C H -C H 3 ~ CH3- CH - - CH - CH

3

I / I

Via ~'- allyl intermeditate

H

- H C C H 3 + H

1 - Bu ten e ~ . /.5 . -- N, / ~. m ,. - Cis - and t rans - 2 - butene

H2C ' ' CH

Alternatively a r t-al lyl ic intermediate could also be effective (see also scheme IV). A

detai led s tudy of the in teract ion of the butene i somers wi th deuter ium over pal lad ium,

plat inum and i r id ium catalysts [5 ,21] suggested that the addi t ion-abst ract ion route was

operat ive, bu t there i s c lear ev idence of i somerizat ion of 1-butene to 2-butenes over i ron

[17] and nickel [18] f i lms without deuterium incorporation, which is only explicable by an

in t ra-molecular hydrogen atom t ransfer , involv ing hydrogen abst ract ion at a cer ta in s tage.

On the base metals i ron , cobal t and n ickel , and on pal lad ium and rhodium the format ion

of rc-al lyl ic species is always a strong possibil i ty.

Unfor tunately , and inexpl icably , there are a lmost no s tudies of a lkene i somerizat ion

on al loy catalysts to report [27,28].

Whi le the processes involved in i so topic exchange of a lkenes are of purely

academic in teres t , the corresponding i somerizat ions do have pract ical s ign i f icance, most

importan t ly in

fat hardening.

The select ive hydrogenat ion of mul t ip ly-unsaturated natural

o i l s to a s tab le product contain ing on average about one carbon-carbon double bond per

hydrocarbon chain requi res f i rs t the i somerizat ion of non-conjugated double-bonds in to

conjugation [29]: that is why nickel is an appropriate catalyst , and why both palladium

[30] and copper have been considered . The former has fa i led on cost grounds and the

lat ter because t races of copper ion d isso lved f rom the catalyst catalyse au toxidat ion of the

product . Unfortunately nickel contains the seeds of i ts own destruction, so to speak,

because i t a l so catalyses c is- t rans i somerizat ion of the exclusively cis-conformers

originally present to the less-desirable trans-isomers (e.g. oleic to elaidic acid groups).

What i s needed i s a catalyst that wi l l hydrogenate and g ive double-bond migrat ion but no t

cis- t rans i somerizat ion: i t has not yet been d iscovered , a l though pal lad ium-based catalysts

hold out some promise.


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484 chapter 11

I t i s impor tan t to unders tand that a metal act ive in the react ions of a lkenes wi th

hydrogen shows essent ia l ly the same character is t ics i r respect ive of i t s physical form, type

of suppor t and method of preparat ion . There may be minor d i f ferences [12] , bu t in broad

out l ine the pr incipal metals exhib i t a character is t ic ' react ion f ingerpr in t ' in processes

occurr ing in both gas and l iqu id phases , wi th a var ie ty of so lvents [5] and in a great range

of re la ted molecules . The d i f ference in character between pal lad ium and p lat inum is

par t icu lar ly c lear ly marked: wi th the former , i somerised and/or exchanged alkenes appear

abundant ly as products , thei r ra tes of format ion being typical ly comparable wi th or even

exceeding that of hydrogen addi t ion . With p lat inum on the o ther hand, the appearance of

al tered alkenes i s usual ly s low and subsid iary to hydrogenat ion . The consis tency of the

avai lab le ev idence i s qu i te remarkable and we shal l encounter another manifes tat ion of

what i s p robably the same effect when we come to consider the select ive hydrogenat ion of

alkynes and alkadienes . I t appears that i ron , cobal t and n ickel , and probably rhodium,

share the character of pal lad ium, and i r id ium resembles p lat inum. Not much work has

been carr ied out wi th ru thenium and osmium as hydrogenat ion catalysts [31] . The

dif ference can be unders tood in terms of two factors , which may themselves be related : ( i )

a s t rong tendency (al ready ment ioned) of metals in the f i rs t two rows to form rc-al ly l ic

in termediates , and ( i i ) ready desorp t ion of the adsorbed al tered alkene in to the f lu id phase

in the case of these metals . Analogies have been drawn wi th the s tab i l i t ies of organometal -

l ic com plexes contained r t -coord inated alkenes as ligands [ 5] . W e su ggest a usefu l

def in i t ion of a s t ructure- insensi t ive react ion or fami ly of react ions i s that i t shows the

same broad react ion character is t ics under a var ie ty of c i rcumstances . By such a def in i t ion

the react ions of a lkenes wi th hydrogen or deuter ium fal l in to th is category .

11 .1 .2 Carbon deposi t ion

I t i s a wel l -known fact that as soon as any act ive metal catalyst i s exposed to an

alkene, or indeed any o ther unsaturated hydrocarbon or carbon-contain ing molecule , i t s

surface becom es at leas t par t ia lly covered by ' acety len ic res idues ' [32 ] or ' carb onac eou s

species ' or , as we shall cal l i t for the sake of brevity, car b on . The effect has been widely

noted and studied [33-35] and i ts general occurrence is not in dispute: what is uncertain

however i s what ro le i f any i t p lays in the mechanisms of a lkene react ions .

I t i s no t easy to summarise what i s known, but cer ta in t rends emerge clear ly f rom

the l i terature . (1) At low to moderate temperatures , the ' carbon ' takes the form of a

somewhat dehydrogenated der ivat ive of the react ing alkene: for example the (111) surfaces

of p lat inum and rhodium are on exposure to e thene quickly covered by ethyl idyne (CH 3-

C-) rad icals which have low react iv i ty [36] . (2) Format ion of e thyl idyne i s suppressed by

the presence of hydrogen . At h igher temperatures by fur ther dehydrogenat ion these species

polym erize in to a two-dime nsional laye r [37 ] which m ay u l t imately graphi t i se [35]. (3)


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486 chap te r 11

1 1 .1 .3 Kin e t i c s a n d me c h a n i s m o f h y d ro g e n a t io n

I t was necessa ry to suffe r the d iscurs ion of the las t sec t ion , so tha t the poss ib le ro le

o f th e c a rb o n d e p o s i t in th e re a c t io n me c h a n i s m c o u ld re c e iv e d u e a t t e n t io n . Th e p o p u la r i -

ty o f e th e n e h y d ro g e n a t io n a s a me a n s o f e v a lu a t in g c a taly t ic a c t iv i ty o f me ta l s a n d a l lo y s

is a t tes ted by a cursory g lance a t the l i te ra ture , and i ts a t t rac t iveness does no t fade wi th

t ime : in d e e d o n e mig h t s a y o f th i s r e a c t io n , a s d id Sh a k e s p e a re o f C le o p a t ra - 'Ag e c a n n o t

wi th e r n o r c u s to m s ta le h e r in f in i ty v a r i e ty ' . Ho we v e r , a s wa s s a id in th e o p e n in g

paragraphs of th is chapte r , i t s fo rmal s impl ic i ty is a snare and a de lus ion , fo r smal l

mo le c u le s c a n s o me t ime s a c t in wa y s d e n ie d to h ig h e r h o mo lo g u e s . I t i s wo r th re c a l l in g in

th is c o n te x t th e wo rd s o f P ierc e W .Se lw o o d [43 ] wr i t te n in 1 9 6 2 : 'No p ro b le m s in s u r fac e

c h e mis t ry h a v e b e e n mo re h o t ly d e b a te d th a n th e a d s o rp t io n a n d h y d ro g e n a t io n me c h a -

n i s m fo r e th y le n e ; a n d fe w d e b a te s h a v e re s u l t e d in s u c h me a g re c o n c lu s io n s ' . Th e

p a s s a g e o f mo re th a n th re e d e c a d e s h a s n o t in v a l id a te d th e s e v ie ws . As n o te d e l s e wh e re in

th is book , chemis ts have an in te res t amount ing to an obsess ion in f ind ing a

mech nism

for

the reac t ion under s tudy . To a ce r ta in ex ten t th is i s o f course f i t t ing and proper , bu t i t on ly

b e c o me s a v a l id e x e rc i s e wh e n s u f f i c i e n t r e l i a b le in fo rma t io n i s a v ia la b le . Wo u ld th a t we

a l l h a d th e h u mi l i ty o f S i r E r ic R id e a l , wh o s p e a k in g o f e th e n e h y d ro g e n a t io n s a id in h i s

p lenary lec ture to the F irs t In te rna t iona l Congress on Ca ta lys is [2 ] :

'A g re a t n u mb e r o f wo rk e rs in th e f i e ld o f c a ta ly s i s f ro m Sa b a t i e r o n wa rd h a v e

g iv e n e x p la n a t io n s o f th e me c h a n i s m o f th e re a c t io n ; I my s e l f h a v e a d v a n c e d th re e .

At l e a s t two m u s t b e e r ro n e o u s a n d ju d g in g b y th e fa c t th a t n o l e ss th a n th re e

communica t ions a re to be made on th is sub jec t dur ing th is week , i t i s qu i te l ike ly

th a t a l l th re e o f th e m a re wro n g ' .

Th e fo rma l k in e t i c s o f th e e th e n e h y d ro g e n a t io n a re q u i t e s t r a ig h t fo rwa rd , a l th o u g h

showing some var ia t ion with ca ta ly t ic meta l , i t s phys ica l fo rm and tempera ture [3] .

Bro a d ly s p e a k in g th e re a c t io n h a s a p o s i t iv e o rd e r in h y d ro g e n , wh ic h a t l e a s t o v e r

p la t inum increases wi th tempera ture f rom 0 .5 to about 1 .0 , due to a change in ra te -

de te rmin ing s tep : the order in e thene is e i the r ze ro or nega t ive . S imila r va lues a re recorded

for h igher a lkenes [5 ,9 ] . Ac t iva t ion energ ies a re remarkably cons is ten t a t about 40-50 kJ

to o l -1, a l th o u g h wo rk p e r fo rme d b e fo re 1 9 6 0, wh ic h h a s b e e n re v ie w e d [9 ,44 ] , l e d to s o m e

very low va lues : d i f fus ion l imita t ion is o f ten encounte red and is no t eas i ly avoided . The

p ro b le m i s th a t a n u mb e r o f fo rma l me c h a n i s ms b a s e d o n th e Ho r iu t i -Po la n y i p ro p o s a l

(sec t ion 11 .1 .1 and scheme II ) can genera te the same reac t ion orders , and i t i s we l l

u n d e rs to o d th a t k in e t i c me a s u re me n ts c a n n e v e r o f th e ms e lv e s l e a d to th e e s ta b l i s h me n t o f

a s u re me c h a n i s m: e q u a l ly h o we v e r a n y me c h a n i s t i c p ro p o s a l n e e d s to b e c o mp a t ib le w i th

the observed k ine t ics .

Ment ion was made in the las t sec t ion of the sugges t ion [42] tha t a ca rbonaceous

o v e r la y e r o r c a rb o n d e p o s i t wa s th e v e h ic le wh e re b y h y d ro g e n wa s a d d e d to th e a lk e n e


11/63

C ata ly t i c hydrogena t i on and dehydrogena t i on 487

molecule [36,45] . One of the pr inc ipal bases for the idea was the s imi lar i ty in k ine t ic

pa rame te r s shown by a va r i e ty o f d i f f e r en t me t a l s ; wha t was no t made c l ea r however was

how th i s mode l migh t accoun t quan t i t a t i ve l y fo r t he p roduc t s o f a l kene -deu t e r i um

react ions , or more genera l ly for the sys temat ic d i f ferences in reac t ion charac ter tha t

d i f f e r en t me t a l s show. The i dea r ema ins

sub judice:

many obv i ous and s t r a igh t fo rward

expe r imen t s ( e . g . deu t e r i um exchange w i th t he ca rbon depos i t ) t ha t wou l d t e s t t he concep t

have not ye t been per formed. I t was a l so a t one t ime bel ieved tha t the e thyl idyne radica l

C H3-C - , f o r which much ev idence acc rued f rom s tud i e s o f e t hene chemisorp t i on on s i ng l e

crys ta l sur faces [36,45] , was the reac t ive in termedia te in hydrogenat ion. Once again the

proponen t s o f t he concep t omi t t ed t o exp lo re whe t he r o r how t he i dea mi gh t exp l a i n t he

broader sweep of exper imenta l observat ions , and i t was no longer defens ible af ter i t had

been show n tha t the species was qui te unreact ive [36] . I t has now been re leg ated to the

role of a spectator .

Qui t e t he mos t p ro found i nves t i ga t i on o f e t hene hydrogena t i on has been t ha t

conduc t ed by Dum es i c and h i s co -w orke r s us ing a P t / S iO 2 ca t a lys t ; i t has a l r eady bee n

refer red to severa l t imes [3 ,4] . Thei r conclus ions res t on a qui te comple te analys i s of the

products of the reac t ion wi th deuter ium a t var ious tempera tures and reac tant ra t ios , of

o rde r s o f r eac t i on a t va r i ous t empera tu r e s and on TPD measurement s made w i t h p l a t i num

single crys ta l s . A most impress ive body of informat ion i s avai lable , f rom which they

conc lude t ha t t he r e a r e two modes o f deu t e r i um chemi sorp t i on , and t ha t a t oms fo rmed i n

e i ther way need to be 'ac t iva ted ' before they can reac t : one of these routes i s compet i t ive

wi th e thene and the other i s not . Whi le th i s analys i s might be taken to provide suppor t for

a ro l e fo r t he ca rbon depos i t , we mus t r emember Poppe r ' s d i c t um t ha t ' I t i s on l y poss i b l e

to

disprove

a hypothes i s , not to

prove

i t ' . O t he r mechan i s t i c f o rmula t i ons may be dev i sed

and may be more economical in reac t ion s teps . The fol lowing thoughts are of fered. (1) I t

i s assumed tha t the f rac t ion of f ree sur face remains cons tant , and unaf fec ted by reac tant

ra t io and tempera ture . (2) React ion orders are only expressed in power ra te l aw language

and no t i n Langmui r -H inshe lwood t e rms . ( 3 ) No a l l owance i s made fo r a r eac t i on

involving dideuter ium wi th adsorbed e thene or e thyl : i f i t has a h igher ac t iva t ion energy

than reac t ion of deuter ium a toms, th i s might expla in why the order in deuter ium increases

wi th tempera ture f rom 0.5 to uni ty . (4) Al though there i s a

prima facie

case fo r accep t i ng

thei r mechanism as a t l eas t a bas i s for d i scuss ion, one cannot be cer ta in tha t i t would

apply to o ther a lkenes and other meta l s . Never the less the in teres ted reader i s s t rongly

recommended to s tudy these papers [3 ,4] , and those c i ted there in , wi th the grea tes t care .

Tw o m ore t echn i ques need t o be no t ed be fo re we p roceed t o s ee wha t is know n

abou t a l kene hydrogena t i on on a l l oys . A mos t f a sc ina t i ng and nove l t echn ique has been

deve loped by R ober t L . Augus t i ne and h i s s t uden t s , t e rmed t he

single turnover

(STO)

method

[46,47] , where in a hydrogen-covered sur face i s subjec ted to a pulse of an a lkene

such as 1-butene and the products of the one-s tep in terac t ion analysed. These would


12/63

488 cha pter 11

consist of ( i) n-butane, ( i i ) isomerised 2-butenes, and (i i i ) butyl radicals which remain

aff ixed to the surface and which can be removed only by a fur ther pulse of hydrogen . In

th is way, the number of s i tes responsib le for each type of react ion can be quant i ta t ively

assessed . The technique has the advantage (or d isadvantage ?) of descr ib ing the condi t ion

of the v i rg in surface, and should provide an invaluable source of in format ion on surface

st ructure . Unfor tunately ( i ) i t does not appear to ass is t unders tanding of select iv i ty in

alkane react ions at h igher temperatures [48 ] nor ( i i) i t does ref lect the s i tuation on the

surface dur ing cont inuous hydrogenat ion . I t i s however an in teres t ing development of the

earl ier lkene titr tion techniques devised by Se rmon [49 ] and Leclercq [50]. I t does not

yet appear to have been appl ied to a l loys .

Final ly i t i s wor th ment ioning the ro le of theoret ical analysis in the d iscussion of

react ion mechanisms. Fol lowing ear ly fumbl ing and amateur ish at tempts to draw analogies

be tw een chemiso rp t ion and coo rd ina t ion complexes , and be tw een mechan i sms in homoge-

neous and heterogeneous systems [51] , there have been many more sk i l led appl icat ions of

theoret ical const ructs to the s t i l l unreso lved quest ions of hydrogenat ion mechanisms [46] .

The types o f methodo logy used have been he lp fu l ly descr ibed by Romanow sk i [52 ] . I t

would be an exaggerat ion to say that th is work has revolu t ionised our percept ion of the

si tuat ion but i t i s r igh t and proper that theory and pract ice should walk hand in hand ( i f

possible).

11 .1 .4 Hydrogenat ion of a lkenes by al loys

Once again i t i s convenient to c lass i fy the avai lab le informat ion by the type of

al loy system used , and to consider f i rs t the ef fect of adding an inact ive Group 11 or o ther

metal to an act ive metal of Groups 8-10 . The pr ize for the most popular system is won by

nickel -copper : there have been a number of s tudies of e thene hydrogenat ion using powders

[52-56], foi ls [44,57,58] and fi lms [59-63], part icularly during the period 1950-1970. Some

very ear ly work on th is system was performed by Schwab and Brennecke [64] . The resu l t s

are not unexpected ly somewhat d iscordant , and wi th benef i t o f h indsight we can recognize

the fo l lowing factors as cont r ibutory . (1) The surface concent rat ions are not in general the

same as the bulk concent rat ions , due to the occurrence of surface segregat ion of copper

and of a miscib i l i ty gap , as explained in chapter 4 . This accounts for the d i f feren t resu l t s

obtained w i th f i lms s in tered at d i f feren t temperatures [62 ] and whe n f i lms a nd fo i ls are

compared wi th powders [57] . (2) The almost inevi tab le rap id format ion of carbon deposi t s ,

to an ex ten t that wi l l depend on surface composi t ion , defect concent rat ion par t ic le s ize e tc .

In those cases where addit ion of copper to nickel leads to an increase in activi ty [53,54,-

59] the cause is very l ikely a lowering in the extent of self-poisoning. (3) There is

however another possib le explanat ion for the scat ter of resu l t s , and th is i s the promot ional

ro le p layed by s t rongly retained hydrogen , a phenomenon invest igated by Kei th Hal l , Paul


13/63

C at a ly t i c hydrogena t i on and dehydrogena t i on

4 8 9

Emmet t and the i r associa tes [55,59 ,62] . This wi l l c lear ly be absent in i t i a l ly f rom f i lms

prepared in high vacuum, but i t i s a fea ture of copper -nickel powders (not the pure meta l s

[55] ) , and i s a t t r ibuted to the presence of d i ssolved oxygen a toms to which the hydrogen

atoms become a t tached. I t s modus operandi remains something of a mystery , but i t can

exer t a powerful inf luence on ca ta lyt ic ac t iv i ty .

The pape r by Tuu l and Fa rnswor th [44] dese rves a spec i a l commenda t i on :

H.E.Famswor th was one of the ear l ies t , but forgot ten , p ioneers of the s tudy of adsorpt ion

and ca ta lys i s on meta l sur faces in UHV condi t ions , and his work [44,65] (and other

references c i ted there in) represented a quantu m jum p in sur face c leanl iness achieved. I t

requi red grea t s t rength of charac ter and exper imenta l ski l l to cons t ruc t and opera te a UHV

system in the ear ly 1950 's .

I f one a t tempts to d i scern a pa t tern in the way tha t the ra te of e thene hydrogenat ion

var ies wi th co pp er content , one can only say tha t n ickel i s mu ch m ore ac t ive than copp er ,

and tha t the manner of var ia t ions i s inf luenced by the fac tors ment ioned above: somet imes

there i s a smooth decrease [53] , of ten a maximum [44,54,59 ,62] and on occas ion a per iod

of a lmost cons tant ac t iv i ty before a ca tas t rophic fa l l [57,60] . Act iva t ion energies are very

var iable , and are a t t imes suspic ious ly low, especia l ly on contaminated sur faces (compare

[52,53] wi th [44] ) . Ear ly s tudies were conducted on hydrogenat ions in solut ion ( s tyrene

[65] , benzene [66] , c innamic ac id [67] ) , and there are a few repor t s of the behaviour of

other n ickel a l loys (wi th gold [62] and t in [68] ) in e thene hydrogenat ion.

I t i s very surpr i s ing tha t there are so few and such super f ic ia l inves t iga t ions of

a lkene hydrogenat ion and re la ted reac t ions over a l loys of the noble meta l s of Groups 8-10

wi th an inact ive meta l . What there i s to repor t inc ludes some qui te o ld work by Rien/ icker

and his col leagues on foi l s (Pt -Cu and Pd-Cu [69] ) where ac t iv i ty for e thene hydrogenat i -

on a t f i r s t fa l l s s lowly as the copper concent ra t ion i s increased, and then very quickly a t a

composi t ion c lose to tha t of pure copper [9] . In these cases the low act iv i ty points l i e on a

separa te compensat ion l ine [9] , which may betoken the opera t ion of a d i f ferent type of

ac t ive cent re in th i s conce nt ra t ion range [70] . W i th pa l ladium -gold micro spheres [71], and

less obvious ly wi th pa l ladium-s i lver a l loys [9 ,72] , ra tes increase as the Group 11 e lement ' s

concent ra t ion i s ra i sed, in the former case by as much as 60%. Some informat ion i s

avai lable on suppor ted pal ladium- and pla t inum-t in ca ta lys t s [68,73] and there i s recent

br ief repor t on the ruthenium-copper sys tem [74] . Apar t f rom what i s sa id in the las t

sec t ion concerning the propene-deuter ium react ion ( see table 1) , there i s l i t t l e more to te l l .

I t seems incredible tha t no one has thought to look a t , for example , the e thene-deuter ium

react ion on pal ladium-gold a l loys .

There are one or two s tudies only of a l loys formed by e lements wi thin Groups 8-

10 . N icke l -pa l l ad ium f i lms show a max imum ra t e o f e t hene hydrogena t i on t owards t he

cent re of the composi t ion range [75] , th i s be ing ass igned to a d iminut ion of the toxic

ef fec t of hydrogen dissolved in the pa l ladium; Ni -Pd/SiO2 ca ta lys t s have a l so been s tudied


14/63

490 chapte r 11

[76] . Addi t ion of i ron decreases the ac t iv i ty of p la t inum ca ta lys ts fo r p ropene hydrogena t i -

on [77] , whi le i ron-rhodium ca ta lys ts supported on graphi te a f te r reduc t ion a t h igh

tempera ture g ive se lec t ive isomeriza t ion of 1 -butene [28] . This behaviour , a sc r ibed to a

t rue a l loy , i s remin iscen t o f the behaviour of d i lu te copper-n icke l ca ta lys ts in e thene

deute ra t ion [26] . There have a lso been repor ts o f a lkene (and a lkyne) hydrogena t ion on

in te rmeta l l ic compounds [78-80] , amorphous meta ls [81] and co l lo ida l a l loys [82] .

11 .1 .5 H ydro gena t ion of the cyc lop ropan e r ing

Cyclopropane i tse l f i s a fasc ina t ing molecule , pa r tak ing as i t does of the proper t ies

of bo th a lkenes and a lkanes ; i t i s , a s one says , 'Ne i ther f ish , f lesh , fowl nor good red

herr ing . ' Theore t ica l chemis ts have had a f ie ld day seek ing the bes t way to descr ibe

bond ing in th is m olecule [83-90] . The s t rain induced by the 60 ~ angle be tw een the ca rbon

a toms means tha t addi t ion reac t ions can proceed with ease and the r ing is read i ly hydroge-

n a te d a t a mb ie n t t e mp e ra tu re o r e v e n b e lo w [9 ,9 1 ,9 2 ] . F ro m s o me p o in t s o f v ie w th e

molecule appears to reac t as i f i t comprised a methylene rad ica l (CH2:) in te rac t ing with

the r t -o rb i ta ls o f e thene . I ts hom ologu es a re even more use fu l : m ethy lcyc lopr opa ne reac ts

[93 ,94] to g ive e i the r n - o r i so-butane (scheme V), and th is reac t ion has somet imes been

used as a means of charac te r is ing the surface of smal l meta l pa r t ic les ( [94] , and re fe rences

therein).

S c h e m e V

Pro d u c t s o f h y d ro g e n a t io n o f me th y lc y c lo p ro p a n e

CH CH

+H 2

CH ~

>

3

3

\

CH

/

2

H CH

methy lcyc lopropane

CH 3

\

C H 2 ~ CH 3 + CH ....... CH 3

/ CH /

3

n - butane isobutane

Ea r ly k in e t i c wo rk re v e a le d th a t h y d ro g e n a t io n o f c y c lo p ro p a n e o n s u p p o r te d

Group 10 meta ls was f i rs t o rder in the hydrocarbon and ze ro in hydrogen [9 ] , sugges t ing

tha t the surface was main ly covered by the la t te r and tha t the concentra t ion of hydrocar-

bon rad ica ls was low. La te r i t was shown tha t the reac t ion with deute r ium gave deute ra ted

propanes , the d is t r ibu t ions of which resembled those ob ta ined in propane exchange , wi th

p ro p a n e -d 8 b e in g a p r in c ipa l p ro d u ct ; th e re w a s s ma l l fo rma t io n o f e x c h a n g e d c y c lo p ro p a -

ne . Mor e de ta i led ana lys is , e spec ia l ly on the produc ts o f the reac t ion w ith hydrog en o ver

n ic k e l [9 2 ,9 5 -9 7] a n d ru th e n iu m [9 8 -1 0 0 ] s h o w e d th a t h y d ro g e n o ly s i s o f C-C b o n d s c o u ld


15/63

C ata ly t i c hydrogena t i on and dehydrogena t i on

491

occur , g iv ing me thane and e thane ( scheme VI ) . Somet i mes t he me thane : e t hane r a t i o

ba re ly exceeds un i t y , showing t ha t on ly t wo C -C bonds a r e b roken , bu t on l a rge ru then i um

par t ic les a t h igh tempera ture there i s cons iderable excess format ion of methane [98] . I t

seems l ike ly tha t these fur ther bond-breaking s teps occur through a propyl radica l formed

by add ing a hydrogen a tom to cyc lopropane : because t hey occur a t t empera t u r e s be l ow

that a t which desorbed propane could reac t fur ther ( scheme VI) .

S c h e m e V I

Hydrogena t i on and hydrogeno l ys i s o f cyc l opropane

C H

2

/ ~ + H t~ > CH 2 - CH 2 - CH

CH 2 CH 2 . , .~

+ H

C 3 H 8

+ 3 H C 2 H 6

+ C H 4

In the expecta t ion tha t the process of hydrogenolys i s might requi re a l a rger

ensemble of ac t ive a toms than hydrogenat ion, there have been severa l s tudies of the

ef fec t s of adding copper or gold to a meta l of Groups 8-10 (Ni -Cu [95-97] ; Fe , Co-Cu

[97] ; Ru-Au [101]) . Addi t ion of copper to n ickel l eads to a se lec t ive suppress ion of

hydrogenolys i s , and when the resul t s a re cor rec ted for the t rue sur face concent ra t ion of

copper i t appears tha t , qui te prec i se ly , hydrogenat ion requi res two nickel a toms and

hydrogenolys i s three [96] . Addi t ion of gold to ruthenium inhibi t s excess methane format i -

on, but has l i t t l e e f fec t on the hydrogenat ion/hydrogenolys i s ra t io , so tha t the two reac t ions

probab ly i nvo lve t he s ame i n t e rmedia t e . Wi t h R h- I r / S i O2 ca t a l ys t s , r hod i um i s much more

ac t i ve t han i r i d ium, bu t max ima i n r a t e s bo t h o f hydrogena t i on and o f hydrogeno l ys i s a r e

seen a t about 40% i r id ium [102] ( see a l so chapter 13) .

1 1 .2 H y d r o g e n a t i o n o f a l k y n e s a n d a l k a d i e n e s

11.2 .1 Gen era l pr inc iples

In t he hydrogena t i on o f mul t i p ly -unsa tu r a t ed hydroca rbons we encoun t e r a new

phenomenon, or a t l eas t one which has only fea tures tangent ia l ly in what has gone before .

This i s the concept of

degree of selectivity,

or

selectivity

for shor t . Molecules tha t conta in

two or more unsatura ted funct ions can usual ly be hydrogenated in such a way tha t

i n t e rmed ia t e p roduc t s can be ob t a ined i n some degree o f s e l ec t i v i t y , wh i ch somet imes

approaches 100%. Scheme VII l i s t s some of the more impor tant reac t ions tha t show thi s


16/63

492 chapte r 11

S c h e m e V l l

.Examples of se lec t iv i ty in hydrogena t ion of mul t ip ly unsa tura ted hydrocarbons

Rea ctant Intermediate product ( s ) Final product

Ethyne Ethene

HC --- CH H2C = CH 2

P r o p y n e P r o p e n e

C H 3 - C = C H C H 3 - C H = C H

P r o p a d i e n e ( a i l e n e )

C H 2 = C H = C H 2

Ethane

C H - C H

3 3

P r o p a n e

2 CH3- CH2- CH3

2 - Butyne

C H 3 - C = C - C H 3

1 - Butyne

C H 3 - C H 2 - C - C H

1,3 - Butadiene

H 2C = C H - C H = C H

1,2 - Butadiene

H 2 C = C = C H - C H 3

m

1 - Butene

C H 3 - C H 2 - C H = C H 2

2 - Butenes

C H 3 - C H = C H - C H 3

n - Butane

But -1 - yne - 3 - ene

(v iny lacety lene)

C H = C - C H = C H 2

1, 3- Butadiene Butenes

charac te r is t ic : each reac tan t i s t rans formed in to the produc t shown in the next co lumn to

th e r ig h t b y a d d i t io n o f o n e mo le o f h y d ro g e n . Ca rb o n -c a rb o n u n s a tu ra t io n ma y a c c o mp a -

ny a var ie ty of o ther unsa tura ted func t ions (e .g . the a romat ic r ing , as in s ty rene (phenyl-

e thene) or phenyle thyne , e tc . , o r the ca rbonyl g roup as in c ro tona ldehyde) . Se lec t ive

reduc t ion of some of these combina t ions wi l l be cons idered la te r .

Processes based on some of these reac t ions have cons iderab le indus tr ia l importance

and th is has occas ioned much research . Perhaps the mos t importan t family of reac t ions is

th a t e mp lo y e d b y th e p e t ro c h e mic a l in d u s t ry to r e mo v e s ma l l c o n c e n t ra t io n s o f mu l t ip ly -

unsa tur a ted mo lecules f rom s t reams com pris ing ch ie f ly a lkenes , e i the r C 2, C 3 or C4. Th ese

s t re a ms a r i s e b y f ra c t io n a t io n o f s t e a m-c ra c k e d n a p h th a , a n d v i r tu a l ly c o mp le te r e mo v a l o f

a lkynes and a lkadienes is essen t ia l be fore the a lkenes can be fur ther p rocessed by

polymerisa t ion , se lec t ive ox ida t ion e tc . [103] . In a succes fu l p rocess to ach ieve th is , the

ca ta lys t mus t ( i ) no t hydrogena te any of the a lkene , and ( i i ) des i rab ly reduce the a lkyne /a l -

kadiene to a lkene ra ther than a lkane . The f i rs t o f these requirements in t roduces a second

face t o f se lec t iv i ty : in a mix ture of A + B, i t may be poss ib le to ob ta in a se lec t ive reac t ion


17/63

Ca ta ly t i c h y d ro g e n a t io n a n d d e h y d ro g e n a t io n 4 9 3

o f c o mp o n e n t A wi th o u t a f fe c tin g B . Th is w i l l h a p p e n i f fo r e x a mp le A i s mu c h m o re

s t rongly adsorbed on the surface of the ca ta lys t than B: we then speak of

thermodynami-

cally controlled selectivity [104] . When the in te rmedia te produc t i s des i red , i t i s necessa ry

for i t to be ab le to desorb more qu ick ly than i t i s fu r ther hydrogena ted ; th is i s mechanisti-

cally-controlled selectivity:

X Y

a s a s

A - - - - - P - - X ~ Y

The two fac tors come toge ther because to ob ta in X in h igh se lec t iv i ty i t mus t no t re -

adsorb once i t has vaca ted the surface .

Fo r tu n a te ly , a lk y n e s a n d a lk a d ie n e s a re b o th mu c h mo re s t ro n g ly a d s o rb e d th a n th e

corresponding a lkenes , so the re is a ve ry favourab le the rmodynamic fac tor in th is sys tem.

Thus in a ba tch reac tor no a lkene wil l reac t un t i l near ly a l l the a lkyne or a lkad iene has

b e e n re mo v e d . Eq u a l ly fo r tu n a te ly th e re i s o n e me ta l -p a l l a d iu m- th a t i s o u t s t a n d in g ly

ac t ive and se lec t ive for the i r hydrogena t ion ; indeed , se lec t iv i t ie s to a lkene approaching

100% can of ten be ob ta ined . The o ther meta ls o f Groups 8-10 a re ac t ive in some degree

but a re normal ly much less se lec t ive . Alkadienes a re a lmos t as s t rongly chemisorbed as

a lkynes , and the i r se lec t ive reduc t ion to a lkenes is the re fore a lso poss ib le [5 ,103] .

Before proceeding to de ta i l , the re a re two o ther fea tures of these reac t ions to no te :

f i rs t , s te reochemis t ry . Reduc t ion of a d isubs t i tu ted a lkyne can g ive , as p r imary produc ts ,

e i the r the c is o r the t rans -a lkene : pa l lad ium ca ta lys ts o f ten show a h igh degree of

s te reochemica l spec if ic i ty to the c is - isomer , showing tha t bo th the hydrogen a toms a re

added to the same s ide of the molecu le (schem e VIII) . M uch resea rch has been d irec ted to

obta in ing the h ighes t poss ib le y ie lds of c is - isomers , a s these a re normal ly the des i red

produc t (e .g . in syn thes is o f v i tamin A); h igh s te reospec if i ty usua l ly accompanies h igh

se lec t iv i ty . Secondly , dur ing i ts hydrogena t ion , e thyne can unfor tuna te ly undergo a

hydropolymerisation to o l igomers conta in ing C4, C6, e tc . molecules . This aspec t o f the

reac t ion has a lso been inves t iga ted , and some par t ia l so lu t ions have been ob ta ined [9 ] .

In the fo l lowing sec t ions we sha l l b r ie f ly rev iew the k ine t ics and mechanism of

a lk y n e h y d ro g e n a t io n wi th e mp h a s i s o n e th y n e , a n d i t s r e a c t io n wi th d e u te r iu m a n d i t s

po lymerisa t ion . We sha l l cons ider s te reochemica l aspec ts , and the e ffec ts o f a l loy ing and

of o ther modif ie rs on the reac t ion charac te r is t ics . We sha l l then g ive some a t ten t ion to the

corresponding reac t ions of a lkad ienes .


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494 cha pter 11

Scheme V I I I

Hydrogenat ion o f 2-butyne

C H - C - C - C H 3

H H

L ~> CH 3 CH 3

C -'--~. C

J

H H

Cis- 2- butene

11.2 .2 Kinet ics and mechanism of a lkyne hydrogenat ion

When ethyne i s hydrogenated in a batch reactor over a catalyst contain ing a metal

of Groups 8-10, the product rat io remains constant unti l almost al l the ethyne has reacted:

then a much fas ter hydrogenat ion of the ethene ensues [5 ,9 ,21 ,103,105,106] . This

demonst rates c lear ly the absence of hydrogenat ion s i tes that wi l l chemisorb ethene non-

competi t ively, at least at the part ial pressures involved. This is the case even when ethyne

hydrogenat ion proceeds wi th somewhat low select iv i ty ( th is being def ined as the percenta-

ge o f ethene in the C 2 products) as i t does w ith plat inum [107,108]. On Pd/A1203 catalys ts,

the thermodynamic factor favour ing ethyne i s greater than 2000 [21 ,105] , bu t when ethene

is added in sufficient excess, in order to simulate industrial condit ions [103,105,109] i t can

adsorb and be hydrogenated on s i tes not b locked by ethyne. By sophis t icated appl icat ion

of isotopic labell ing, using for example 13C [105] or 14C [21] labelled ethene and ethyne,

and double-labell ing techniques [109] i t has proved possible to identify three types of si te

[21,108]. ( i) A type I (or type X) si te at which ethyne is hydrogenated

on ly to e thane .

(ii)

A type II si te at which ethyne is reduced to ethene. ( i i i ) A type III (or type Y) si te at

which added ethene may be reduced by ethane (scheme IX) . There have been no ser ious

attempts to al locate these functions to any part icular geometric features of the metal

part icles, for example by systematic variat ion of part icle sizes, al though as we shall see

short ly this approach has provided very useful insights into the behaviour of C4 hydrocar-

bons. A possib le reason for th is omission i s the fact that the work performed wi th

pal lad ium has of ten employed a commercial catalyst which has a very low metal content

(e.g. 0.04%). There has been a hint [109] that the Type III si te may in fact be on the

suppor t and be fed wi th hydrogen by sp i l lover f rom the metal . Another very p lausib le

suggest ion i s that non-select ive hydrogenat ion occurs only on the 8-PdH phase and not on

the t~-PdH phase, which because of the ef fect of par t ic le s ize on the thermodynamics of

hydr ide format ion [110] i s probably absent f rom catalysts in which the pal lad ium is h ighly


19/63

C ata ly t i c hydrogena t i on and dehydrogena t i on

495

dispersed [21] . Tende ncy to form the 13-PdH phase i s a l so su ppressed by a l loying wi th

pla t inum [111] and presumably other meta l s a l so . I t i s cur ious tha t no one has thought to

apply the s ingle turnover method to a lkyne hydrogenat ion.

S c h e m e I X

r-- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |!

' Ty p e I S i t e s

H\ /tt

I

!

I H

1 H-C =C - -H -+

I

C2H2 (g)

Z H H, / \ /

C = C - - - * C = C ~ - - - *

/ i - - \

+ H / \ + H

** H * H

T y p e I I S i t e s

H H ;

9 * H H 9 H{

_ _ \ . . . . a r - ' - I

,

. i C2H4(ads) i

H

! \ ',

H\ /

C--C--H --*I C2H6 g) I ',

/ I \ +21 I', I , Type l l I '1

9 H ', ,, S i t e s i

,, i

{ . . . . . . . . . . . .. i

A react ion scheme por t raying poss ible routes i s shown in scheme IX [108] . I t i s

in teres t ing to note tha t e thyl idyne or v inyl and e thyl idene species fea ture as in termedia tes ;

these were recognised in the last sect ion as being

un re a c t i v e ,

and perhaps the pr inc ipal

components of 'carbon depos i t s ' formed f rom alkenes . This sugges t s a poss ible genera l

principle:

s p e c i e s f o r m e d p a r a s i t i c a l ly in a n e a s y r e a c ti o n m a y b e e s s e n t i a l i n t e r m e d i a t e s

in a more d i f f i cu l t reac t ion . We noted above tha t a lkyne (and a lkadienes) a re usual ly less

reac t ive than a lkenes .

Orders of reac t ion are typica l ly f i r s t or grea ter in hydrogen and zero or somewhat

negat ive in a lkyne [9 ,103] ; the la t te r be tokens compet i t ive adsorpt ion of the two reac tants ,

but orders in hydrogen grea ter than uni ty are only expla ined wi th di f f icul ty wi thout

invoking the involvement of d ihydrogen in reac t ions wi th adsorbed hydrocarbon species .

An al ternat ive suggest ion [112] that the area of react ive surface is a posi t ive funct ion of

hydrogen pressure , through i t s removal of over -s t rongly adsorbed hydrocarbon moi t ies ,

depends for i ts c redibil i ty on the ease wi th which the i r format ion m ay be reverse d w hen

hydrogen pressure i s ra i sed. Selec t iv i t i es on a l l meta l s not unexpectedly decrease wi th

increasing hydrogen pressure [5] . Activat ion energies are variable, but frequently in the

region of 60 kJmo1-1, high er than for alkenes [9]. Select ivi t ies increa se w ith increas ing

tempera ture , except for pa l ladium [5] .

In teres t ing cor re la t ions have been developed by workers a t the Ins t i tu t Francais du

P6trole between the inhibi t ing effect of alkyne concentrat ion on the rate ( i .e . negat ive

order of react ion) and the extent to which the rate is a funct ion of part icle size (st ructure

sensi t ivi ty) [107]. Over rhodium [107], pal ladium [113], and plat inum [107] catalysts , rates

of hydrogenat ion of 1-butyne in the l iquid phase decrease wi th decreas ing par t ic le s ize ;


20/63

496 chapter 11

1 ,3-butadiene behaves s imi lar ly over pal lad ium and rhodium, but over p lat inum there i s no

var iat ion . There i s then a good correlat ion between negat ive orders and the appearance of

par t ic le s ize sensi t iv i ty ; on ly for butad iene over pal lad ium is the expected negat ive order

not observed . The explanat ion advanced [107] i s that a lkynes and alkadienes chemisorb

ext remely s t rongly on surface atoms of low coordinat ion number (e .g . edge atoms) , wi th

perhaps two molecules forming a k ind of organometal l ic complex , which i s unreact ive. In

the l imi t , the metal may d isso lve in to the l iqu id phase when a s t rongly complexing

molecule such as v inylacety lene (but - l -yne-3-ene) i s present [103] . Thus act iv i ty per uni t

area d iminishes wi th decreasing par t ic le s ize , i . e . as the f ract ion of surface atoms wi th low

coordinat ion number increases , and the occurrence of negat ive orders i s neat ly explained .

I t would be in teres t ing to s tudy the proper t ies of a lkynes chemisorbed on s tepped s ingle

crystal surfaces. In the references quoted, small metal part icles are also thought to be

elect ron-def icien t because thei r act iv i ty i s improved by addi t ion of an elect ron donor such

as p iper id ine [107] . Par t ic le s ize ef fects having an elect ronic or ig in have also been claimed

in 1 ,3-butadiene hydrogenat ion catalysed by pal lad ium par t ic les formed by an atomic beam

m etho d [ 114].

Hydrogenat ion of 1-butyne leads exclusively to 1-butene [107 ,113] and of 2-butyne

to predominant ly c is-2-butene [5 ,115] . Product select iv i t ies f rom hydrogenat ion of 1 ,3-

butadiene and of 1-butene are independent of d ispers ion over pal lad ium [113] , suggest ing

that wi th the form er i t is jus t the nu mbe r of act ive s i tes , and not thei r comp osi t ion , that

changes with part icle size. Clearly in these cases there is no facil i ty for the adsorbed

alkene to i somerise before i t i s forced f rom the surface.

A s t r ange phenomenon w as observed many year s ago concern ing the gas -phase

hydro gena t ion of e thyn e ove r n ickel in a constan t vo lume b atch reactor [ 116 ,117] . W hen

the hydrogen; ethyne rat io exceeds unity, the pressure-t ime curve is str ict ly l inear ( i .e. the

rate remains constant) unti l al l the ethyne has reacted, after which the rate accelerates.

This by i tself is not easi ly explained, because the rate is in fact proport ional to the

in i t ia l

hydrogen pressure , bu t i t does not respond to changes in hydrogen pressure as the react ion

proceeds. Then i f the hydrogen pressure i s suddenly changed by adding more, the ra te

increases in propor t ion to the quant i ty added . I t would appear that some k ind of surface

chain reaction is set up in the ini t ial instants and that i t is self-sustaining unti l a step

change in pressure i s made. These observat ions have never been repeated or fo l lowed up .

The react ion of e thyne wi th deuter ium has been s tudied on seven of the n ine

metals o f Groups 8-10 (not i ron or cobalt ) [5] . Deuterated ethynes are not re turned to the

gas phase over rhodium, i r id ium, p lat inum and pal lad ium, nei ther i s hydrogen deuter ide

over n ickel and p lat inum. Novel in format ion comes f rom the analysis of the d ideuteroethe-

ne (C2H2D2), bec ause this exists as three isom ers (cis, t rans and as ym me tric) , the prop ort i-

ons of which can be es t imated by infrared spect roscopy. The cis- i somer usual ly predomi-

nates but 30-40% of the t rans- isomer i s formed over the metals of Groups 8 and 9 ; the


21/63


49 7

asym-isomer i s a lways a minor component . Scheme X i l lus t rates possib le routes to the

format ion of these products : the ethyl idyne rad ical postu lated as the in termediate for

forming the asymmetr ic i somer i s a lso impl icated in the non-select ive hydrogenat ion route

(see scheme X) .

S c h e m e X

Routes to the format ion of d ideuteroethenes in the react ion of e thyne wi th deuter ium

H

\

H C ~ D H H

+ D ~ / / + D \ /

H - - C - - C - - H ~ C - -- ,- - C - - H

/ \

I I D D

H

\

H C-- D H H

~ / / \ / + D

C ~ C - - C ~ c i s - o r t r a n s - d i d e u t e r o e t h e n e

I I I \

D

H - - C ~ C - - H

I

H H H H

\ / + D \ / + D

C ~ C ~ a s y m - d i d e u t e r o e t h e n e

I I I I

C C - - D

I I

The las t feature of a lkyne hydrogenat ion needing at ten t ion i s

hydropoly-

merisation. On al l metals the hydrogenat ion of e thyne leads to the format ion of considera-

b le amounts of o l igomers , main ly s t ra ight -chain hydrocarbons contain ing even numbers of

carbon atoms, in parallel with the C2 products: with nickel , the oligom ers can be the ma jor

product [5] . They are a lso formed by alkadienes , bu t no t by alkenes . In the indust r ia l

processes for removing t races of mul t ip ly-unsaturated molecules f rom alkene s t reams

[103], the Pd/A1203 catalyst over a period of t ime produces a 'Green Oil ' , which fouls the

plant and deact ivates the catalyst : th is i s a complex mixture of hydrocarbons formed by

hydropolymerisat ion of e thyne. I t i s an unmit igated nuisance, and many expedients have

been examined to decrease or e l iminate i t s format ion . Sher idan [118] proposed that the

free-rad ical form of the v inyl rad ical (scheme X) in i t ia ted a surface chain react ion (scheme

XI):

this seems to be an explanation that sat isfactori ly explains the observations. Higher

homologues (propyne [119 ,120] , bu tynes [115 ,121] , e tc . ) g ive progress ively smal ler

amounts of o l igomers , due to s ter ic in ter ference wi th the C-C bond forming s tep . The

increase in ethene selectivi ty with conversion in a stat ic reactor, and with increasing

par t ic le s ize using Pd/SiO 2 catalysts , has been correlated w i th the reluctance of smal l


22/63

498 cha pter 11

par t ic les to form carbonaceous res idues , the format ion of which may be impor tan t in

achieving h igh select iv i ty .

S c h e m e X I

Hyd ropolym erizat ion of e thyne [ 118]

H

\ - /

H - - C - - - - C - - H C - - C

I I I x

* * . . H

9 H H /

C C C C

I I I

I\

There does not appear to have been any comprehensive at tempt to model the

hydrogenat ion of a lkynes in a quant i ta t ive manner , incorporat ing al l the avai lab le

informat ion on temperature and pressure ef fects , the resu l t s of i so topic label l ing wi th

deute rium , ~3C and ~4C etc. Certain un derly ing cause s of the obse rved be hav iour the refore

remain obscure: the outs tanding act iv i ty and select iv i ty shown by pal lad ium seems to be

related to i t s ab i l i ty to absorb hydrogen , and indeed i t s excel len t select iv i ty may be a

consequence of there being an abnormal ly low concent rat ion of hydrogen atoms at the

surface: react ion may occur as d isso lved hydrogen atoms emerge f rom beneath the surface,

at tacking adsorbed hydrocarbon species f rom below. Another possib i l i ty i s that d isso lved

hydrogen changes the elect ronic s t ructure of pal lad ium and i t s propensi ty to a t t ract the r~-

elect rons of e thene i s thereby d iminished .

11 .2 .3 Hy droge nat ion of a lkynes by al loys

The main purpose of the ex tensive work that has been carr ied out wi th al loys , and

with addi t ion of surface modif iers (apar t f rom the sat i sfact ion of cur iosi ty) , has been to

improve even fur ther the select iv i ty and s tereospeci f ic i ty shown by pal lad ium. One might

th ink wi th Shakespeare that 'To g i ld ref ined gold , to pain t the l i ly , to add another hue

unto the rainbow, were wastefu l and r id icu lous excess ' ; bu t pal lad ium is no t qu i te perfect

and the polymeric 'Green Oi l ' p roblem is a major one. Modif iers (or select iv i ty promoters

or select ive poisons, as they are somet imes cal led) such as carbon monoxide or n i t rogen

bases or metal cat ions achieve the same end by d i f feren t means, and because thei r modus

oper ndi

can somet imes be descr ibed as the format ion of a two-dimensional surface al loy

they must receive some at ten t ion . Only a l i t t le work has been done wi th al loys of metals

that normal ly show only low select iv i ty [122 ,123] .

The addi t ion of gold to pal lad ium in the form of powder resu l t s in an increase in

activi ty [124], and of si lver to Pd/o:-AleO 3 in som e im prov em ent in se lectivi ty [125].


23/63


49 9

Fol lowing th is ear ly work there have numerous paten ts granted that c la im amel iorat ion of

pal lad ium's shor tcomings by addi t ion of smal l amounts of o ther e lements (see [126] for an

example): the trouble with patents is that they do not invariably achieve the standard of

scientif ic r igour that publications in refere ed journa ls ha ve to meet, and i t is therefo re not

easy to assess the mer i t s of the claims. Moreover , o f course, in paten ts one does not have

to explain one ' s d iscovery . In an informat ive s tudy [115] i t was shown that the var ia t ion

of ra te of 2-butyne hydrogenat ion brought about by changing the composi t ion of pal lad i -

um-gold al loy wires was chiefly at tr ibutable to al terat ions in the pre-exponential factor,

which ref lects the number of act ive s i tes , thus categor izing alkyne hydrogenat ion as an

ensemble-s ize insensi t ive react ion which might requi re only a s ingle pal lad ium atom as i t s

active centre. Addit ion of copper to dilute Pd/AI203 leads to increased stabil i ty and lower

ethane formation [109].

A s tudy of n ickel -copper and cobal t -n ickel a l loy powders [127] revealed the

interest ing fact that the rel tive act iv i t ies of the former ser ies were temperature dependent

because act ivat ion energ ies were not a l l the same. Whi le a t 323K nickel was much more

act ive than any al loy , a t 423 and 473K the maximum act iv i ty was in the cent re of the

ser ies because here the act ivat ion energ ies were h igher . I t i s a sober ing thought that much

ingenui ty has been exercised to explain a manner of var ia t ion of act iv i ty wi th al loy

composi t ion that may be an ar tefact of the par t icu lar temperature chosen for the compar i -

son . Ranveer S .Mann and h is associates have ex tended th is work to cover propyne [120]

and 2-bu tyne [ 121 ] .

The best known and longest es tab l i shed improved vers ion of a pal lad ium catalyst

for l iqu id-phase hydrogenat ion i s Pd/CaCO3 select ively poisoned by Pb ions , usual ly in the

form of Pb(OAc)4, and the nitrogen base quinoline [128]: i t is generally referred to as the

Lindlar catalyst . I t s success has s t imulated considerable research in to the ro le of lead [129-

131] which may be present ei ther as an oxide or as the ordered al loy Pd3Pb [131]. The

effect of vacuum-deposi ted lead on the surface of a pal lad ium single crystal has a lso been

examined [130] , as has the pal lad ium-boron system [132] . One cannot to tal ly ignore the

possibil i ty that one of the factors responsible for the egregious propert ies of palladium is

the format ion of an in ters t i t ia l pal lad ium-carbon al loy dur ing ethyne hydrogenat ion

[38,133].

The addit ion of ei ther gold or copper to ir idium [122] or of plat inum to ir idium or

rhenium to p lat inum [123] a l l lead to increased ethene select iv i ty in e thyne hydrogenat ion ,

al though usual ly a t the sacr i f ice of some act iv i ty . I t seems that in these cases , ensemble

size determine the adsorption modes and by that the result ing selectivi ty.

The act iv i t ies of a number of in termetal l ic compounds for the hydrogenat ion of 1-

butyne (Pd-RE [134]) and of 1-octyne (Zr-Rh-Pd and Zr-Rh-Ru [135] , MRh3_xPdx where

M=Ce or Zr [136]) have been invest igated .


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500 cha pter 11

11.2 .4 Hydrogenat ion of a lkadienes

As noted in the in t roduct ion to th is sect ion , a lkadienes (both conjugated , and non-

conjugated , provided there i s no t more than one methylene group between the double

bonds) are adsorbed wi th s t rengths comparable to those of a lkynes and therefore a lso show

the phenomenon of select ive reduct ion to in termediate a lkenes . By and large the select iv i -

t ies that var ious metals show in the hydrogenat ion of a l lene (propadiene) , 1 ,3-butadiene

and 1 ,4-pentadiene [5] are s imi lar to those g iven by alkynes: pal lad ium is again outs tan-

d ing in th is respect . The process as exempl i f ied by butadiene i s however in some respects

more complex than that say of 2-butyne, in that the re la t ive amounts of the three i someric

butenes vary considerably f rom one metal to another , and wi th react ion condi t ions [5 ,21] ,

due to var ious am ounts o f 1 ,2- and 1 ,4- addi t ion , and great ingenui ty has been exe rcised in

conver t ing the observat ions , including those ar i s ing f rom the use of deuter ium, in to

convincing mechanisms [21] . A select ion of the resu l t s ob tained i s g iven in tab le 2 .

I t i s unnecessary to en ter in great detai l in to the somewhat complex mechanis t ic

schemes that have been devised . The pr incipal ind icator of mechanism is the t rans:cis ra t io

of the 2-butene: on pal lad ium and somet imes on cobal t th is i s very h igh (8-14) and th is

betokens a mechanism (cal led B) based on syn- and an t i -~-al ly l in termediates which

cannot in terconver t on the surface, the propor t ions which s imply ref lect the amount of the

two conformers in the gas phase. In mechanism A, which descr ibes the behaviour of o ther

metals , showing a t rans:cis ra t io of about uni ty , in termediates are e i ther ~-alkenes or ~-

alkyls that may in terconver t more f reely , a l though the in tervent ion of ~-al ly l ic species a t

some poin ts in the mechanism is to lerated . Abbreviated representat ions of these two

mechanisms, bo th of which may operate s ide-by-s ide, are shown in scheme XII .

The select ive reduct ion of 1 ,2- and 1 ,3-d ienes i s a lso impor tan t in the t reatment of

s team-cracked naphtha f ract ions to prepare them for fur ther pet rochemical processing

[103] , and recent s tudies of bu tad iene hydrogenat ion in the l iqu id phase have shown the

same k ind of par t ic le s ize sensi t iv i ty as g iven by 1-butyne on pal lad ium [113] and

rhodium [107] catalysts , bu t a lack of par t ic le s ize dependence in the case of p lat inum

[107] . Once again the not ion of s t rong complexat ion of d iene to a toms of low coordinat ion

number i s advanced to explain th is type of s t ructure sensi t iv i ty where i t occurs . This ef fect

i s a lso at work in the s tudy of pal lad ium par t ic les of var ious s izes formed by atomic-beam

vacuum deposi t ion onto carbon or graphi te [137] . Large par t ic les (> 2 .8 nm) behaved l ike

bulk pal lad ium, but smal l ones (< 1 .4 nm) were quickly deact ivated probably by com-

plexat ion; a marked s ize dependence of ra te was shown in between. Butene select iv i t ies

were 100% but thei r d is t r ibu t ion was not g iven . Related work [114] has a l ready been

noted. In the l iquid phase [113] palladium shows the expected high trans:cis rat io (12),

independent of par t ic le s ize . Suppor ted gold catalysts are a lso act ive and very select ive for

butadiene hydrogenat ion ( [21] ; see tab le 2) .


25/63

C a t a l y t ic h y d r o g e n a t i o n a n d d e h y d r o g e n a t i o n

5 0 1

S c h e m e X I I

P o s s i b l e m e c h a n i s m s f o r t h e h y d r o g e n a t i o n o f 1 , 3 - b u t a d i e n e [ 2 1 ]

C 4 H 6

H 2 C - '- - C H

I \

* C H ~ C H

I

H 2 C ~ C H ~ C H ~ C H

I

H 2 C - - C H

I \

* C H

H2CJ/\

m e c h a n i s m A

H 2 C - - - C H

I \

* C H ~ C H 3

I

C H 3 ~ C H ~ C H - - C H 2

I

H 2 C ~ C H

\

C H

/ \

C H 3 *

C H 3 ~ C H

~ ' C H ~ C H

C H 3 ~ C H 2 ~ C H - - - C H 2

I

C H 3 ~ C H

~ c H

* /

CH

a

t r a n s

- 2 - butene

1 - b u t e n e

c i s

- 2 - butene

H 2 C ~ C H

I cH - - cH

I

H 2 C - - C H

I C H C H 3

C 4 H 6

H 2C _._--- --,C %

I ',' cH

..~ ,'

CH 2

I

L

H 2 C ~ C H

CH 3

m e c h a n i s m B

CH 3

\

C H - - - C H

I \

CH 3

H 2 C ~ C H ~ C H 2 ~ C H 3

I

CH 3

\

CH

/

C H - - - C H

I

t r a n s - 2 - butene 1 - butene cis - 2 - butene

N o t e. Th e h yd r o g e n a t om s a d d e d a n d r emo ve d a r e n o t sh own a n d some o f t h e p o s s i b l e

i n t e rm ed i a t e s a r e om i t t e d o r t h e sa ke o f c l a r i t y .


26/63

502 cha pter 11

table 2

Select iv i t ies and butene i somer d is t r ibu t ions for the hydrogenat ion of 1 ,3-butadiene on

alumina-suppor ted metals [5]

Me tal T/K S 1-b t -2-b c-2-b

Fe 471 0 .980 23 45 32

Co 348 1.000 28 64 8

Ni 373 >0.99 27 63 10

Cu 333 1.000 87 6 7

Ru 273 0.736 69 19 12

Rh 289 0.943 51 32 17

Pd 273 1.000 65 33 2

Os 297 0.431 65 19 16

Ir 297 0.251 59 19 22

Pt 273 0.501 72 18 10

Au 443 1.000 58 13 28

Notes: Select iv i ty = ( total butenes) / ( total butenes + butane)

The d i s t r ibu t i ons shown f o r n i cke l and cobal t are o f t he 'Type B ' k ind , probab ly r epresen-

tat ive o f sul fur-con tam inated sur faces.

There have been comparat ively few systemat ic s tudies of a lkadiene hydrogenat ion

on al loys , a l though there are several c la ims of improved al loy catalysts in the paten t

l i terature . Rates shown by 1 ,3-butadiene on n ickel -copper f i lms at 328K are independent

of composi t ion in the range 3-97% copper , as expected for the ' cherry ' model (see chapter

4) , and th is ra te i s a hundred-fo ld less than that shown by n ickel bu t ten t imes larger than

that given by copper [138]. The al loys show high trans:cis rat ios, as does nickel , but the 1-

butene:2-butene rat io decreases wi th increasing copper content because copper i s loath to

form r t -a l ly l ic in termediates . Pumice-suppor ted pal lad ium-gold al loys show simi lar product

d is t r ibu t ions throughout the composi t ion range, and the ef fect of the t ransi t ion f rom the ~-

to the 6-Pd H phase at 353-403K on products was noted [139] . S tudies of bu tad iene

hydrogenat ion on Pd-Cu/Nb205 [140] have been repor ted , and there have been several

studies on s imp le crystal al loys [141a-c]. With PtsoNis0(111), whic h has a plat inu m o uter

layer, the rate at 300K is a l i t t le faster than on Pt(111), but the selectivi ty to butenes is

m uch h ighe r (>80% ) [141a]. In the case of PtsoNis0(110) [141b], equil ibrat ion at 80 0-9 00K

leads to p lat inum enr ichment in the surface, whi le h igher temperatures ( l100-1200K)

result in a higher nickel surface concentrat ion (see chapter 4). In the lat ter state the surface

is both active and selective for butad iene hydro gena tion. Studies of Pt80Fe20(111), Pt75Ni25-


27/63


503

(111) have also been reported [141c]. I t is however unfortunate that so much effort is

sp en t in characteriz ation and so l it t le in condu cting a full and prope r catalytic investiga ti-

on: no detai led in terpretat ion of the observat ion has been advanced . Amorphous in ters t i t ia l

a l loys of phosphorous and boron wi th n ickel have also been employed as catalysts for

butadiene hydroge nat ion [81 ] .

Isoprene (2-methyl - l ,3 -butadiene) i s an in teres t ing molecule , s ince the two double-

bonds are rendered non-equivalen t by the subst i tu t ion of the methyl group, and the three

isopentenes (2-methyl - l -butene, 3-methyl - l -butene and 2-methyl -2-butene) are a l l formed

[9 ,142] . On pal lad ium-gold and -s i lver a l loys , however , thei r re la t ive amounts are only

sl ight ly af fected by composi t ion as expected for an ensemble s ize- insensi t ive react ion

[143].

As noted above, the hydrogenat ion of an imal , f i sh and vegetab le o i l s to s tab le

products f i t for human consumpt ion involves reduct ion of non-conjugated double bonds to

a product contain ing pr incipal ly a s ingle double-bond. Nickel catalysts are universal ly

emp loyed , bu t a benef icial ef fect of a l loy ing wi th copper has been repor ted [ 144].

We conclude th is sect ion wi th a rev iew of some of the pr incipal unanswered

quest ions surrounding the hydrogenat ion of a lkynes and alkadienes . A cent ral quest ion i s

whether in these systems al loying simulates the behaviour of small metal part icles, i .e.

whether the proper t ies of act ive atoms in smal l ensembles on an al loy surface are the same

as or s imi lar to those of smal l ensembles on smal l par t ic les of a pure metal , where the

mean coord inat ion number i s low. Do reactan ts b ind

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Chapter 11 Catalytic Hydrogenation and Dehydrogenation 1995 Studies in Surface Science and Catalysis

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