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CHROMATOGRAPHY BEYOND ANALYSIS

C.S.G. PHILLIPS Ino rgan ic Chemistry Labora tory , Oxford U n i v e r s i t y , South Parks Road, Oxford OX1 3QR ( U . K . )

SUMMARY

The a p p l i c a t i o n o f chromatography t o n o n - a n a l y t i c a l problems i s i l l u s t r a t e d

by p a r t i c u l a r re fe rence t o s tud ies o f heterogeneous c a t a l y s i s . Emphasis i s

l a i d on t h e unusual v a r i a t i o n s o f t h e chromatographic method which may be use-

f u l l y employed and t h e advantages which can a r i s e f rom t h e a b i l i t y t o probe

sur faces w i t h a v a r i e t y o f molecules. Some p o s s i b l e p r e p a r a t i v e , s y n t h e t i c

and pedagogic a p p l i c a t i o n s o f chromatography a r e a l s o discussed.

1 . INTRODUCTION

Chromatography i s now v e r y f i rm ly e s t a b l i s h e d as t h e most power fu l o f ana-

l y t i c a l methods. It i s thus t h e va lued se rvan t of a lmost eve ry sc ience. Each

yea r we see t h e p u b l i c a t i o n o f a hos t o f papers which i l l u s t r a t e i t s use t o

so l ve new and even more v a r i e d a n a l y t i c a l problems. I n many l a b o r a t o i r e s i t

has become as commonplace as t h e t e s t tube, t h e b u r e t t e and t h e balance. Other

papers i n t h i s Symposium w i l l i l l u s t r a t e t h i s a n a l y t i c a l d i v e r s i t y , and show

how even f u r t h e r improvements and ex tens ions o f t h e a n a l y t i c a l power o f ch ro -

matography may be expected.

I w i l l be concerned, however, w i t h what I see as t h e w ide r and p o s s i b l y a

more dominant r o l e f o r chromatography i n sc ience. My pr ime i n t e r e s t w i l l be

the use o f chromatographic methods t o i n v e s t i g a t e a whole range o f phys ico-

chemical phenomena. I propose t o i l l u s t r a t e t h i s by s p e c i f i c r e f e r e n c e t o an

area i n which I have been p e r s o n a l l y most i n v o l v e d i n r e c e n t years , and y e t i n

which, so i t seems t o me, we have o n l y begun as i t were " t o s c r a t c h t h e s u r f a -

ce". Th is i s t h e s tudy of heterogeneous c a t a l y s i s by gas-chromatographic me-

thods. However t h e general p r i n c i p l e s a r e o f much w ide r a p p l i c a t i o n , and o f

course embrace bo th gas and l i q u i d chromatography.

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I n such a s tudy we a r e e s s e n t i a l l y s e t t i n g o u t t o f e e l o r probe c a t a l y s t

sur faces w i t h molecules r a t h e r than say w i t h e l e c t r o n s or photons, and thus

under c o n d i t i o n s much c l o s e r t o those met i n t h e i r p r a c t i c a l a p p l i c a t i o n . A

p a r t i c u l a r f e a t u r e o f such chromatographic i n v e s t i g a t i o n s i s t h e i n f o r m a t i o n

which i s p rov ided by t h e ready exchange o f one mo lecu la r probe f o r another :

t h i s w i l l be a r e c u r r i n g theme th roughout t h i s paper. I t i s a l s o p o s s i b l e t o

f o l l o w t h e r e a c t i o n s themselves w i t h i n va r ious chromatographic systems, and t o

make use o f s u b t l e combinat ions o f r e a c t i o n and separa t i on t o p r o v i d e an e x t r a

dimension t o t h e s tudy o f c a t a l y t i c processes.

I n such work, v a r i a t i o n s o f chromatcgraphy a r e commonly employed which a r e

n o t u s u a l l y f a m i l i a r t o those concerned e s s e n t i a l l y w i t h i t s a n a l y t i c a l a p p l i -

ca t i ons . Some o f these v a r i a t i o n s w i l l be r e f e r r e d t o i n va r ious p a r t s o f t h i s

paper. These a r e l i s t e d i n Table I t oge the r w i t h re fe rences i n which they a r e

descr ibed i n more d e t a i l .

Table 1 Some v a r i a t i o n s o f Chromatographic Method

Dusted Column E l u t i o n on a P la teau F r o n t a l Ana lys i s and E l u t i on by C'harac t e r i s t i c Po i n t Heater Displacement I so tope Exchange (Tracer Pul se) Peak Shape Ana lys i s Pulse T i t r a t i o n Reversed Flow Sample Vacancy Stopped Flow Thermal Desorp t ion

1,2 2

2 9 3 495

6,7,8 2 Y 9 l o y l l ,12 13 14 2,15 16,17

I n t h e l a s t p a r t o f my paper, I wish a l s o t o draw a t t e n t i o n t o some

p o s s i b l e i m p l i c a t i o n s f o r Chromatography i n p r e p a r a t i v e and s y n t h e t i c chemis-

t r y , and t o suggest f u r t h e r t h a t chromatography cou ld p l a y a q u i t e fundamental

r o l e i n t h e teach ing o f chemis t ry and p a r t i c u l a r l y i n i t s most f o r m a t i v e

stages.

2. THERMODYNAMIC ASPECTS OF ADSORPTION

Chromatographic da ta p r o v i d e v e r y p r e c i s e and d e t a i l e d i n f o r m a t i o n on t h e

335

d i s t r i b u t i o n o f molecules between a gas and a s t a t i o n a r y phase. I n t h e case

o f sur faces they l ead thus d i r e c t l y t o f r e e energ ies , heats and e n t r o p i e s of

adso rp t i on a t a v a r i e t y o f su r face coverages, t o adso rp t i on iso therms and t o

su r face areas ( r e f . 2 ) . It i s wor th s t r e s s i n g moreover t h a t q u i t e smal l ener -

g i e s o f i n t e r a c t i o n a r e ch romatog raph ica l l y s i g n i f i c a n t . Thus i n t h e s tudy o f

complex fo rma t ion (e.g., between a vapour l i g a n d and a metal su r face atom)

tens o f j o u l e s have r e a l chromatographic impact, w h i l e i n t h e normal prepara-

t i o n o f s t a b l e complexes one would be concerned w i t h tens o f k i l o j o u l e s . I t

i s thus p o s s i b l e t o i n v e s t i g a t e complexec, such as those f o r example between

o l e f i n s and Cd2+ o r Zn2+ ions , which have never been i s o l a t e d by t h e prepara-

t i v e chemis t ( re f .18 , 19) . Furthermore these i n t e r a c t i o n s a r e o f t e n enhanced

when t h e metal i o n i s exposed on a su r face r a t n t r than embedded i n a s o l u t i o n .

Thus

s e l e c t i v e r e t a r d a t i o n o f o l e f i n s over p a r a f f i n s cor respond ing t o a few carbon

atoms, b u t w i t h AgNO, adsorbed on A120, t h i s s e l e c t i v i t y i s extended t o some

f i f t y carbon atoms.

t h e use o f s o l u t i o n s o f AgNO, f o r g a s - l i q u i d chromatography can g i v e a

Two examples w i l l i l l u s t r a t e t h e use o f s imp le e l u t i o n chromatography t o

i l l u m i n a t e c a t a l y t i c s tud ies . I n t h e f i r s t ( r e f . 20) i t was observed t h a t

bo th t h e a c t i v i t y and t h e s e l e c t i v i t y (e.g., bu tad iene from butene, o r acro-

l e i n f rom propene) o f a Bi2MoO6 s e l e c t i v e - o x i d a t i o n c a t a l y s t were d r a m a t i c a l -

l y improved as t h e b u l k c a t a l y s t compos i t ion changed from one t h a t was s l i -

g h t l y B i - r i c h t o one t h a t was s l i g h t l y Mo- r ich . A t t h e same t ime t h e chroma-

tog raph ic p r o p e r t i e s o f t h e su r face a l s o changed comple te ly ( f o r example t h e

r e l a t i v e r e t e n t i o n o f o f benzene as a g a i n s t cyclohexane increased by a f a c t o r

o f 16 ) , and i n a manner ve ry s i m i l a r t o t h a t found on pass ing f rom a B i203 t o

an Moo3 su r face . Pho toe lec t ron spectroscopy then conf i rmed t h a t smal l b u l k

dev ia t i ons f rom s t o i c h i o m e t r y wert accompanied by d r a s t i c changes i n t h e com-

p o s i t i o n o f t h e sur face . The smal l excess o f Bi20, or Moo3 was thus concent ra -

t e d on the r e l a t i v e l y smal l area (2m2g-l) o f t h e su r face and gave r i s e t o t h e

p r e c i p i t a t e change i n c a t a l y t i c p r o p e r t i e s .

The second example ( r e f . 15) concerns t h e mechanism o f wa te r e l i m i n a t i o n

f rom a l coho ls on t h e su r faces o f m o d i f i e d aluminas. I n t h i s case mo lecu la r

models o f t h e a l t e r n a t i v e cis- and t m n s - e l i m i n a t i o n mechanisms showed t h a t

t h e r e should be d i f f e r e n c e s i n t h e number o f CH2-groups which migh t be expe-

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c ted t o be a t tached t o t h e c a t a l y s t su r face i n t h e t r a n s i t i o n s t a t e s . From t h e

d i f f e r e n c e i n t h e r e t e n t i o n t imes o f two ad jacen t alkanes when chromatographed

on t h e c a t a l y s t su r face i t was then s imp le t o compute t h e d i f f e r e n c e s i n t h e

f r e e energ ies of these t r a n s i t i o n s t a t e s and hence t h e expected r a t i o s o f t h e

var ious produc t o l e f i n s . The exper imenta l r e s u l t s , o f which examples a r e g i ven

i n Table 2, f i t t e d q u i t e remarkably w e l l w i t h t h e t r a n s - e l i m i n a t i o n mechanism

and n o t t h e cis.

Table 2 P red ic ted and Exper imental Ra t ios o f Produc t O le f i ns

P red ic ted p roduc t r a t i o s c i s - E l i m i n a t i o n traans-El i m i n a t i o n Products observed Alcohol

n-Heptan-4-01 ____ 0.6 trans c i s

___ 1.8 trans cis

__ 1.86 trans cis

n- Bu t an- 2- o 1 Large ( 5 0 ) % n-but-1-ene

__ __ 0 . 6 trans t rans

cis 1.8 CiS

1-ene t t runs +-ene+truns 1 - e n e + t r m s i .e., c i s 3.2 cis 1.4 cis 1.20

n-Pentan-2-01 Small % n-pent-1-ene trans traans lrans __ cis 1 .8 cis 0.6 c i s + l - e n e 0.83

n- Hexan- 2- o 1 No n-hex-1 -ene trans trans trans c1.s 1.8 c i s 0.6 c i s + l -ene 0.58 - -

n-Octan-2-01 No n-oct-1 -ene No n-oct-1-ene

2- Methyl-n-pentan-2-01

trans c i s 1.8 __ trans

~ 0.6 c i s

a1 k-1 -ene a1 k-2-ene 1.5

- 0.60 trans cis

11 k-1 -ene a1 k - l -ene 1.53

By employing d i f f e r e n t sample molecules i t i s o f course p o s s i b l e t o probe

s e l e c t i v e l y d i f f e r e n t p a r t s o f t h e sur face . Thus, f o r example, a z e o l i t e - s u p -

po r ted metal cou ld be probed w i t h N, t o determine i t s t o t a l s u r f a c e area, w i t h

molecules o f d i f f e r e n t mo lecu la r geometr ies t o i n v e s t i g a t e i t s pore s t r u c t u r e ,

and w i t h H, and CO t o de termine t h e na tu re and amount o f f r e e meta l exposed

( r e f . 10 ) . A f u r t h e r ex tens ion o f t h i s concept i s p rov ided by a s tudy o f a

N i / S i O , hydrocrack ing c a t a l y s t u s i n g isotope-exchange chromatography ( r e f . 7 ) .

The c a t a l y s t was packed i n t o a chromatographic column and a s t ream o f hy-

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drogen used as c a r r i e r gas w i t h a t h e r m a l - c o n d u c t i v i t y d e t e c t o r . Deuter ium gas

was now i n j e c t e d a t t h e column i n l e t . I t s c o r r e c t e d r e t e n t i o n t ime ( i . e . , a f t e r

t h e dead t ime of t h e column) p rov ided then a d i r e c t measure o f t h e exchangea-

b l e hydrogen on t h e su r face o f t h e c a t a l y s t . A t temperatures below about 120°C

t h i s corresponded t o hydrogen chemisorbed on t h e N i - su r face : t h i s was measured

independent ly by p u l s e - t i t r a t i o n chromatography. Above 120°C k i n e t i c a l l y - c o n -

t r o l l e d exchange w i t h adsorbed water began t o be s i g n i f i c a n t reach ing a thermo-

dynamica l l y - con t ro l l ed p la teau a t about 260°C: these observa t ions were checked

q u a n t i t a t i v e l y by i n j e c t i o n s o f known amounts o f water . F i n a l l y , a t s t i l l h i g h e r

temperatures, f u r t h e r k i n e t i c a l l y - c o n t r o l l e d exchange took p lace which appeared

t o i n v o l v e t h e H i n S i O H groups: t h i s was supported by i n f r a - r e d spectroscopy.

I n t h e case o f t h e k i n e t i c a l l y - c o n t r o l l e d exchanges t h e k i n e t i c s were i n v e s t i -

gated i n more d e t a i l by s topped- f low chromatography. I n j e c t i o n o f hydrocarbons

i n t o t h e system then made p o s s i b l e t h e i n v e s t i g a t i o n o f t h e f u r t h e r exchange o f

hydrogen w i t h adsorbed hydrocarbon.

C l e a r l y as w e l l as p rob ing d i f f e r e n t types o f su r face s i t e , e.g., a c i d i c

and bas i c s i t e s w i t h bas i c and a c i d i c vapours r e s p e c t i v e l y , s t r o n g l y adsorbed

molecules may a l s o be used t o s e l e c t i v e l y po ison such s i t e s . Th is i s o f t e n i m -

p o r t a n t and r e v e a l i n g i n t h e general s tudy o f a r e a c t i o n mechanism. I t can ho-

wever become a v i t a l t e s t when t h e r e i s a r i s k t h a t t h e exper imenta l r e s u l t s

observed cou ld be l a r g e l y caused by r e l a t i v e l y smal l t r a c e s o f i m p u r i t y i n t h e

c a t a l y s t , e.g., smal l amounts o f f i n e l y - d i v i d e d metal i n a supposedly a c t i v e

o rganometa l l i c c a t a l y s t .

3 . ADSORPTION K I N E T I C S

I n t h e preced ing s e c t i o n we cons idered t h e way i n which su r faces may be

probed thermodynamical ly by gas-chromatographic methods. It i s , o f course, a l s o

p o s s i b l e t o e x p l o r e ch romatog raph ica l l y t h e k i n e t i c s o f adso rp t i on ( r e f . 2 ) . I n

t h e s imp les t cases, w i t h a l l a d s o r p t i o n s i t e s hav ing s i m i l a r adso rp t i on k i n e -

t i c s , i n c r e a s i n g slowness o f t h e adso rp t i on -deso rp t i on process i s man i fes ted i n

the broadening o f t h e e l u t i o n peak, f o l l o w i n g f o r example t h e c l a s s i c a l t r e a t -

ment o f t h e van Deemter equa t ion ( r e f . 21) . Thus w i t h mo lecu la r s ieve 5A as s t a -

t i o n a r y phase t h e C t e rm o f t h i s equa t ion i s markedly inc reased on pass ing f rom

the non-penet ra t ing branched t o t h e s t r a i g h t cha in hydrocarbons which a r e a b l e

t o pene t ra te i n t o t h e narrow pores o f t h e z e o l i t e s t r u c t u r e .

338

As Giddings ( r e f . 22) has shown (see a l s o r e f . 23) adso rp t i on on t o two o r

more k i n e t i c a l l y - d i s t i n g u i s h a b l e s i t e s w i l l l e a d t o skewed peaks. These peaks

may be regarded as t h e s u p e r p o s i t i o n o f a symmetr ical peak (produced by those

molecules which adsorb o n l y on t h e f a s t s i t e s d u r i n g t h e i r passage th rough t h e

chromatographic column) and a broad t r a i l i n g peak (produced by those molecules

which have been absorbed a t l e a s t once on s lower s i t e s ) . We have come across

what appears t o be a good example ( r e f . 24) o f t h i s phenomenon i n s t u d i e s made

w i t h a s t a t i o n a r y phase c o n s i s t i n g o f anatase coated w i t h a carbonaceous mate-

r i a l (which had been produced by t h e d i s p r o p o r t i o n a t i o n r e a c t i o n o f 1,3-buta-

d iene ) . The phenomenon i's here brought o u t p a r t i c u l a r l y c l e a r l y as i t s e f f e c t s

a re o n l y apparent w i t h c e r t a i n molecules. Thus n-hexane produces normal symme-

t r i c a l peaks a t a l l temperatures i nves t i ga ted , w h i l e around 200°C, f o r example,

i t s isomer 2,2-dimethyl butane produces a peak which i s much broader than t h a t

o f n-hexane and which t a i l s s i g n i f i c a n t l y . The skewed n a t u r e o f t h i s peak va-

r i e s w i t h bo th temperature and w i t h f l o w - r a t e o f c a r r i e r gas. It can be d e s c r i -

bed ve ry adequatedly i n terms o f t h r e e types o f s i t e i n v o l v i n g f a s t , medium and

slow adsorp t i on -deso rp t i on k i n e t i c s , t h e l a s t be ing o f r e a l s i g n i f i c a n c e o n l y

a t t h e s lowest gas f l ow- ra tes employed. I n p a r t i c u l a r t h e p o s i t i o n o f t h e peak

maximum ( r e l a t i v e t o t h a t o f n-hexane) inc reases w i t h decreas ing f l o w - r a t e as

adso rp t i on on t h e slower s i t e s p lays an i n c r e a s i n g l y impor tan t r o l e . I n s tudy-

i n g a wide range o f molecules t h e skewing and broadening phenomenon was found

t o i nc rease i n general w i t h i nc rease i n cha in b ranch ing and w i t h hydrogenat ion

o f a benzene r i n g .

I n many circumstances t h e d i s t i n c t i o n between f a s t and s low adsorp t ion-de-

s o r p t i o n k i n e t i c s inc reases even f u r t h e r so t h a t t h e chromatographic e l u t i o n

peak c o n s i s t s o f an apparen t l y normal symmetr ical peak toge the r w i t h a l ong

and very low t a i l . Th is t a i l i s produced by those molecules which have been

invo lved i n adso rp t i on -deso rp t i on processes which a r e now ve ry s low

r i s o n w i t h those i n which t h e symmetr ical peak molecules have been invo lved . It

i s a l l t o o easy then t o be unaware o f t h e ve ry ex i s tence o f such a t a i l , a l t h o -

ugh i t should be suspected i n v iew o f t h e l a c k o f q u a n t i t a t i v e sample recovery .

Indeed such a l o s s would appear t o occur a l l t o o f r e q u e n t l y i n some o f t h e no r -

mal a n a l y t i c a l a p p l i c a t i o n s o f gas chromatography. Th is l o n g t a i l may be revea-

l e d by t h e use o f s topped- f low chromatography i n which molecules s l o w l y desor-

b i n g f rom t h e surface a r e accumi la ted by s topp ing t h e gas f l o w f o r a p e r i o d o f

i n compa-

339

t ime. An example i s p rov ided i n some s t u d i e s w i t h ion-exchange r e s i n s as gas-

-chromatographic s t a t i o n a r y phases . ( re f . 25 ) .

4. CATALYTIC REACTIONS

The s tudy o f c a t a l y s i s i s , o f course, commonly a s s i s t e d by t h e use o f gas

chromatography as a p u r e l y a n a l y t i c a l dev ice . Such s t u d i e s may o f t e n be sim-

p l i f i e d cons ide rab ly by a more i n t i m a t e connect ion between t h e c a t a l y t i c reac -

t o r and t h e a n a l y t i c a l chromatographic column as i n t h e now c l a s s i c a l m ic ro -

r e a c t o r techn ique ( r e f . 2 6 ) . The i n j e c t i o n system o f t h e chromatographic column

can sometimes be s imp ly adapted t o f u n c t i o n as t h e m ic ro reac to r , and t h e t i m e

spent on t h e c a t a l y s t i n t h e m i c r o r e a c t o r v a r i e d by s topp ing t h e gas f l o w i m -

med ia te l y a f t e r i n j e c t i o n ( r e f . 27, 28) . By t h e use o f sample-vacancy chromato-

graphy ( r e f . 14 ) , i n which a sample o f t h e r e a c t a n t f eed i s i n j e c t e d between

t h e coup led r e a c t o r and chromatographic column w h i l e r e a c t a n t i s f e d cont inuou-

s l y i n t o t h e r e a c t o r , d i f f e r e n t i a l r e a c t i o n chromatograms may be generated. I n

these nega t i ve peaks correspond t o r e a c t i o n produc ts , a p o s i t i v e peak measures

the amount o f r e a c t a n t which has reac ted , w h i l e non - reac t i ng i m p u r i t i e s a r e

e l i m i n a t e d f rom t h e chromatogram and n o n - v o l a t i l e r e a c t i o n produc ts may be e s t i -

mated f rom t h e d i f f e r e n c e between p o s i t i v e and nega t i ve peaks.

However much a l s o may be l ea rned by combining i n t h e same column, o r even

i n t h e same pack ing m a t e r i a l , bo th a n a l y t i c a l and chromatographic c h a r a c t e r i -

s t i c s : see f o r example t h e rev iew a r t i c l e s by van Swaay ( r e f . 29) on "The Stu-

dy of React ion K i n e t i c s by t h e D i s t o r t i o n o f Chromatographic E l u t i o n Peaks" and

by Langer and Pa t ton ( r e f . 30) on "Chemical Reactor A p p l i c a t i o n s o f t h e Gas

Chromatographic Column", and Chapter 13 on "On-Column React ions" i n Conder and

Young ( r e f . 2 ) . I t i s a p i t y t h a t t h e te rm " r e a c t i o n chromatography" cannot now

be a p p l i e d t o such s t u d i e s , s i n c e i t has become t h e p r a c t i c e t o employ i t t o

cover almost e x c l u s i v e l y r e a c t i o n s o c c u r r i n g b e f o r e o r a f t e r b u t n o t w i t h i n a

chromatographic column (e.g. , r e f . 31 ) . Th is i s a f u r t h e r i l l u s t r a t i o n o f t h e

heavy a n a l y t i c a l b i a s o f gas chromatography.

One immediate advantage o f u s i n g t h e same m a t e r i a l as bo th c a t a l y s t and

chromatographic s t a t i o n a r y phase i s t h a t thermodynamic and a d s o r p t i o n - k i n e t i c

s tud ies o f t h e sur face , such as those o u t l i n e d i n t h e two p rev ious sec t i ons ,

may be made w i t h e s s e n t i a l l y t h e same exper imenta l system. Moreover a l l t hese

s tud ies a r e then c a r r i e d o u t under c o n d i t i o n s very s i m i l a r t o those i n which

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t h e c a t a l y t i c r e a c t i o n s m igh t be o c c u r r i n g i n say i n d u s t r i a l p r a c t i c e , r a t h e r

than t h e more abs t rac ted c o n d i t i o n s (e.g., under h i g h vacuum o r s u b j e c t t o

e l e c t r o n bombardment) o f t e n used i n modern phys i ca l s tud ies o f c a t a l y s i s and

c a t a l y t i c sur faces . There a r e a l s o many o t h e r advantages i n c l u d i n g r a p i d i t y

and p r e c i s i o n o f measurement, and o f course, once aga in , t h e ready a b i l i t y t o

probe t h e su r face w i t h a whole range o f mo lecu la r species.

The i m p o s i t i o n o f a chemical r e a c t i o n on t o t h e normal chromatographic

process n a t u r a l l y leads t o a more complex chromatogram. I n p r i n c i p l e t h i s com-

p l e x i t y can be t rans formed t o g i v e the k i n e t i c s o f t h e r e a c t i o n (see espec ia l -

l y r e f . 30) . Thus i n t h e case o f a s imple r e v e r s i b l e r e a c t i o n A B, i n j e c t i o n

o f A w i l l l ead t o a chromatogram spanning t h e normal e l u t i o n peaks o f A and

o f B f rom t h e a n a l y s i s o f which ( r e f . 30, 32) t h e fo rward and reve rse r a t e

cons tan ts (and hence t h e e q u i l i b r i u m cons tan t ) may be determined. Exper imental

examples i n c l u d e the i s o m e r i s a t i o n o f oyn- and an t i -ace ta ldox ime ( r e f . 30), t h e

enan t iomer i za t i on o f l -ch lo r0-2 ,2-d imethy l a z i n i d i n e on a r e s o l v i n g s t a t i o n a -

ry phase c o n t a i n i n g n i c k e l (11) b i s 3-(trifluoroacetyl)-l-R-.camphorate ( r e f .

33), and t h e i n t e r c o n v e r s i o n o f ortho and para hydrogen ( r e f . 34) . For a de-

compos i t ion r e a c t i o n (p roduc ing r e a c t i o n produc ts l e s s s t r o n g l y r e t a r d e d chro-

ma tog raph ica l l y than t h e r e a c t a n t ) e.g., d i cyc lopen tad iene -

( r e f . 30) , t h e chromatogram c o n s i s t s o f a normal peak o f unreac ted r e a c t a n t

preceded by a l o n g t r a i l i n g peak o f p roduc ts . T h i s l a t t e r peak s t a r t s s tepwise

( i n t h e case o f more than one p roduc t ) a t t h e r e t e n t i o n t imes o f t h e normal

e l u t i o n peaks o f t h e produc ts ( i . e . , cor respond ing t o p roduc ts produced a t t h e

very beg inn ing o f bo th t h e r e a c t i o n and t h e column) and t r a i l s r i g h t back t o

t h e r e a c t a n t peak. Once aga in t h i s chromatogram r e f l e c t s and can be t r a n s f o r -

med t o g i v e r e a c t i o n - k i n e t i c i n fo rma t ion .

pentad iene

However, whenever t h e r e i s a r e l a t i v e l y l a r g e chromatographic separa t i on

( b u t see a l s o r e f . 13) between r e a c t a n t and produc ts (e.g., decomposi t ion,

c rack ing , e l i m i n a t i o n , Fischer-Tropsch r e a c t i o n s ) , t h e a n a l y s i s o f t h e reac-

t i o n may be cons ide rab ly extended and s i m p l i f i e d by t h e use o f s topped- f low

chromatography ( r e f . 15) . Here t h e gas f l o w i s p e r i o d i c a l l y stopped d u r i n g

the passage o f t h e r e a c t a n t th rough the column, so t h a t t h e r e a c t i o n and t h e

chromatographic processes a r e e f f e c t i v e l y uncoupled. The fo rmer cont inues wh i -

l e t h e l a t t e r s tops d u r i n g t h e f l o w i n t e r r u p t i o n s . Each t i m e t h e gas f l o w i s

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stopped, r e a c t i o n produc ts accumulate a t t h a t p o i n t i n t h e column where t h e

r e a c t a n t was s i t t i n g d u r i n g t h e s top . On r e s t a r t i n g t h e f l o w , these accumula-

t i o n s o f p roduc t then behave as though they had been i n j e c t e d a t t h i s p a r t i c u -

l a r p o i n t and produce sharp chromatographic peaks superimposed on t h e broad

r e a c t i o n chromatogram. I n favourab le cases i t i s p o s s i b l e t o genera te more

than two hundred s e t s o f such peaks d u r i n g one passage o f r e a c t a n t th rough t h e

column. F u r t h e r i n f o r m a t i o n can be ob ta ined e.g., by v a r y i n g t h e s t o p t i m e

( t o i d e n t i f y a u t o c a t a l y t i c and success ive r e a c t i o n s ) , by i n t r o d u c i n g p o t e n t i a l

i n h i b i t o r s and c o c a t a l y s t s i n t o the column ( t h e i r p o s i t i o n s i f they a r e V O l a t i -

l e be ing r e a d i l y determined a t any t i m e from t h e i r chromatographic behav iou r ) ,

and by c a r r y i n g o u t s imu l taneous ly more than one r e a c t i o n a t d i f f e r e n t p a r t s

o r a t t h e same p a r t o f t h e column. There a r e o t h e r ways i n which advantage may

be taken o f t h e v a r i e t y o f mo lecu la r probes. Thus t h e hydrogenat ion o f an ad-

sorbed C, spec ies on a N i / S i O , su r face ( r e f . 3 5 ) can be i n v e s t i g a t e d by produ-

c i n g C, spec ies f rom methane, f rom C O Y o r v i a t h e i d r o c r a c k i n g o f p a r a f f i n

hydrocarbons. So a l s o t h e d e t a i l s o f a s e r i e s o f success ive r e a c t i o n s ( r e f . 36)

may be u n r a v e l l e d by i n j e c t i o n o f t h e produc ts .

Even t h e r e a c t i o n s themselves may be used t o s tudy t h e n a t u r e o f t h e s u r -

face. Thus on c e r t a i n sur faces some e l i m i n a t i o n r e a c t i o n s (e.g., water f rom

a l coho ls o r hydrogen c h l o r i d e f rom a l k y l c h l o r i d e s t o g i v e o l e f i n s i n each ca-

se) t a k e p lace w i t h o u t any p e r c e p t i b l e movement o f r e a c t a n t on t h e column ( t h e

r e t e n t i o n t imes o f t h e p roduc t o l e f i r l s remain cons tan t f o r a whole s e r i e s o f

successive s topped- f low chromatograms). The a n a l y s i s o f t h e k i n e t i c s then

leads t o a cho ice between two mechanisms, one i n v o l v i n g f i r s t - o r d e r r e a c t i o n s

on a s e r i e s o f s i t e s o f d i f f e r e n t a c t i v i t y and t h e o t h e r a h ighe r -o rde r rea -

c t i o n ( r e f . 37) . The second mechanism can be e f f e c t i v e l y d isproved by demon-

s t r a t i n g t h a t t h e e l i m i n a t i o n r e a c t i o n o f one mo lecu la r species i s u n a f f e c t e d

by t h a t o f another s i m i l a r mo lecu la r species and, more s u b t l y , by p a r t i a l l y

r e a c t i n g one mo lecu la r species (and thus on t h e a l t e r n a t i v e hypo thes i s f r e e i n g

t h e more r e a c t i v e s i t e s ) and then i n j e c t i n g and f o l l o w i n g t h e r e a c t i o n o f ano-

t h e r . The d e t a i l e d computer a n a l y s i s o f t h e k i n e t i c s pe rm i t s a c a l c u l a t i o n o f

t h e d i s t r i b u t i o n on t h e su r face o f s i t e s o f v a r y i n g c a t a l y t i c a c t i v i t y , i n es-

s e n t i a l l y t h e same manner as i n d i v i d u a l r a d i o a c t i v e i so topes may be i d e n t i f i e d

by d e t a i l e d a n a l y s i s o f t h e decay r a t e s o f a m i x t u r e o f i so topes measured ove r

a p e r i o d o f t ime.

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5. PREPARATIVE AND SYNTHETIC CHEMISTRY

I t u r n now t o a d i f f e r e n t n o n - a n a l y t i c a l aspec t o f chromatography: i t s w i -

der a p p l i c a t i o n i n p r e p a r a t i v e and s y n t h e t i c chemis t ry . Chromatography has o f

course been used f o r a ve ry l o n g t i m e as a p r e p a r a t i v e t o o l . However i t s poten-

t i a l i t i e s seem n o t even y e t t o have been f u l l y developed. I propose t o comment

i n p a r t i c u l a r on two mat te rs namely (A) t h e a b i l i t y t o d r i v e a chemical r e a c t i o n

beyond i t s normal thermodynamic l i m i t s and ( B ) t h e p o s s i b i l i t y o f c o n t r o l l i n g

the n a t u r e o f t h e s y n t h e t i c i n v e s t i g a t i o n s themselves.

(A) As has been i n d i c a t e d above i n t h e d i s c u s s i o n o f heterogeneous c a t a l y -

s i s , chemical r e a c t i o n s ( c a t a l y s e d o r uncata lysed) may be c a r r i e d o u t i n a ch ro -

matographic column w h i l e a t t h e same t ime t h e column i s separa t i ng r e a c t a n t s

and t h e va r ious produc ts . L e t us take , f o r example, a r e v e r s i b l e r e a c t i o n such

as t h a t i n v o l v i n g t h e i n t e r c o n v e r s i o n o f t h e va r ious hexane isomers ove r a

Pt/A1,0, c a t a l y s t . Then by u s i n g heater d i sp lacemen t chromatography , n-hexane

may n o t on l y te conver ted i n t o i t s isomers and t h e va r ious isomers then separa-

ted, b u t one can ar range t h a t t h e lass-branched isomers ( s t a r t i n g w i t h n-hexa-

ne i t s e l f ) a r e success i ve l y and s e l e c t i v e l y re isomer ised. I n t h i s way t h e reac-

t i o n i s d r i v e n t o produce a lmost comp le te l y t h e most-branched isomer, 2,2-dime-

thy1 butane ( r e f . 4 ) . I t i s thus poss ib le , i n p r i n c i p l e a t l e a s t , t o c o n t r o l

chemical r e a c t i o n s so as t o produce almost 100 % o f a p roduc t wh ich m igh t o n l y

be formed a t r a t h e r low y i e l d s by convent iona l means.

( B ) Th i s leads on t o a b roader aspect o f t h e r o l e o f chromatography i n syn-

t h e s i s . H i t h e r t o chromatography has been used a lmost e n t i r e l y as t h e handmaiden

of t h e s y n t h e t i c chemist . He dec ides what r e a c t i o n s and what t a r g e t m o l v u l e s

t o s tudy and mere ly c a l l s i n chromatographic a n a l y s i s t o t e l l him how w e l l he

has achieved h i s goa ls . I t seems t o me, however, t h a t chromatography shou ld a t

t imes take a more commanding p o s i t i o n a t t h e i n i t i a l des ign o f t h e s y n t h e t i c

exper iments. Thus r e a c t i o n s c o u l d be u s e f u l l y s t u d i e d which were s e l e c t e d t o

g i v e manv new produc ts i n s t e a d o f one t a r g e t molecule. As a s imp le examDle one

may quote some work i n which my group was i n v o l v e d many years ago i n t h e prepa-

r a t i o n of s i l i con-german ium hydr ides , boraz ines and Group I V a l k y l s . Here a

hos t o f h i t h e r t o unknown molecu les were produced, separated and r a p i d l y i d e n t i -

f i e d by a v a r i e t y o f gas-chromatographic methods. Thus gas-chromatographic co-

lumns, va r ious de tec to rs , and a n c i l l i a r y apparatus were hooked up t o p r o v i d e t h e

e q u i v a l e n t o f t h e t r a d i t i o n a l high-vacuum p r e p a r a t i v e l i n e , b u t w i t h f l o w i n g

343

gas streams r e p l a c i n g vacuum.(ref . 38).

I t cou ld w e l l be o f cons ide rab le va lue t o s tudy a l l t h e produc ts o f a syn-

t h e t i c process. Th is c o u l d n o t o n l y p r o v i d e a much f u l l e r i n s i g h t i n t o t h e na-

t u r e of t h e s y n t h e t i c r e a c t i o n s b u t c o u l d l e a d t o new and more d i r e c t s y n t h e t i c

rou tes . Thus q u i t e minor p roduc ts o f t h e r e a c t i o n under one s e t o f c i rcumstan-

ces may become s i g n i f i c a n t s y n t h e t i c p roduc ts o f t h e r e a c t i o n when t h e c i rcum-

stances a r e a l t e r e d i n a manner suggested by t h e d e t a i l s o f chromatographic

analyses. What I am t h u s advocat ing i s a sys temat ic s tudy o f what a r e a t p r e -

sent o n l y s i d e r e a c t i o n s . To p u t i t another way, we should s u r e l y s tudy t h e

mu1 t i f a r i o u s produc ts o f a r t i f i c i a l s y n t h e t i c r e a c t i o n s j u s t as we have s t u d i e d

t h e m u l t i f a r i o u s produc ts o f r e a c t i o n s o c c u r r i n g i n na tu re . How many f a s c i n a -

t i n g species a r e be ing thrown down t h e l a b o r a t o r y s ink , as h i g h po ly r ie rs once

were b e f o r e a whole i n d u s t r y became founded upon them?

6. CHROMATOGRAPHY I N EDUCATION

My f i n a l peep i n t o t h e f u t u r e i s concerned w i t h t h e use o f chromatography

i n educat ion. I f one looks , f o r example, th rough t h e "Journa l o f Chemical Edu-

c a t i o n " one f i n d s each yea r some dozen o r so re fe rences t o chromatographic ex-

per iments . However t h e y a r e v i r t u a l l y a l l concerned w i t h demonst ra t ing how chro-

matography works o r w i t h i t s use f o r s p e c i f i c ana lyses . Now I see chromatogra-

phy hav ing a much more fundamental pedagogic r o l e , p a r t i c u l a r l y as chromato-

graphs become w i d e l y a v a i l a b l e and as t h e computer and t h e VDU became f a m i l i a r

i n our schools. What b e t t e r t o o l i s t h e r e than chromatography f o r demonst ra t ing

so many o f t h e bas i c p r i n c i p l e s o f mo lecu la r chemis t r y? It i s much more e f f e c t i -

ve than t h e t e s t t ube and s u r e l y more i n f o r m a t i v e , i f l e s s spec tacu la r , than

t h e t r a d i t i o n a l bangs and sme l l s w i t h which t h e average s tuden t i s i n i t i a t e d

i n t o t h e mys te r ies o f chemis t ry .

L e t me b r i e f l y o u t l i n e a p o s s i b l e s e r i e s o f i n t r o d u c t o r y exper iments. A

m i x t u r e o f t h e lower p a r a f f i n hydrocarbons i s f i r s t separated r a p i d l y , e.g., by

c a p i l l a r y gas chromatography. The s t r a i g h t - c h a i n molecules a r e a t once i d e n t i -

f i e d by t h e r e g u l a r p a t t e r n o f t h e i r r e t e n t i o n times, and t h e v a r i o u s isomers

then p i cked o u t and t h e i r number and behav iour r a t i o n a l i s e d . The d i s t i n c t i o n s

between t h e d i f f e r e n t mo lecu les can then be f u r t h e r i n v e s t i g a t e d by t h e use of

mo lecu la r s ieves , s e l e c t i v e v o l a t i l i t y and s e l e c t i v e d i f f u s i o n . Thus i n a v e r y

s h o r t space o f t i m e t h e s tuden t becomes f a m i l i a r w i th t h e n a t u r e and p r o p e r t i e s

344

o f a whole range o f s i g n i f i c a n t and na tu ra l l y -abundan t species. From then on i t

i s a s t r a i g h t f o r w a r d m a t t e r t o develop i n t o mo lecu les w i t h d i f f e r e n t a c t i v e

groups and t o a v a r i e t y o f chemical reac t i ons , a l l o f which may be s t u d i e d r a -

p i d l y and conv inc ing l y w i t h a r e l a t i v e l y s imp le gas chromatograph. What one

needs f o r such a programme i s j u s t a simple, w ide ly -used and r e l a t i v e l y inexpen-

s i v e chromatographic des ign toge the r w i t h t h e d e d i c a t i o n o f an i n s p i r e d teacher

t o dev i se a s e t o f w e l l - t e s t e d and graded exper iments t o i n t r o d u c e s tudents e f -

f i c i e n t l y and c o n v i n c i n g l y t o t h e r i c h e s o f mo lecu la r chemis t ry .

7. WHY CHROMATOGRAPHY AND WHY NOT?

F i n a l l y one may pose two r a t h e r bas i c ques t ions . ( A ) Why shou ld one t r y t o

persuade people t o use chromatographic methods i n o t h e r than t h e i r normal ana-

l y t i c a l r o l e ? ( B ) Why a r e chromatographic methods as y e t so l i t t l e accepted

o u t s i d e ana lys i s?

(A) To t h e f i r s t ques t ion , t h e r e a re o f course many answers wel l -known t o

the p a r t i c i p a n t s a t t h i s meet ing. I would w ish t o h i g h l i g h t p a r t i c u l a r l y t h e

f a c t t h a t "it i s there" , t h a t i s t h a t chromatographic apparatus i s now v e r y

w ide ly d i s t r i b u t e d in a l l s o r t s o f s c i e n t i f i c l a b o r a t o r i e s , and indeed i n com-

pa r i son w i t h so many o t h e r techn iques i s v e r y easy t o s e t up and t o employ.

Chromatographic exper iments a r e u s u a l l y remarkab ly rap id . They r e q u i r e o n l y

smal l amounts o f m a t e r i a l and i n many cases these need n o t even be pure, s ince

the chromatographic method con t inuous ly separates and d i s t i n g u i s h e s t h e va r ious

components o f a m ix tu re . Yet t h e methods, e s p e c i a l l y w i t h a l i t t l e i n g e n u i t y

can y i e l d a wea l th o f r a t h e r p r e c i s e i n fo rma t ion .

Moreover t h e r e a r e now two q u i t e e x c e l l e r t books ( r e f . 2, 39) wh ich s p e l l

o u t ex t remely c l e a r l y a l l t h a t one needs t o know t o app ly gas-chromatographic

methods t o physicochemical measurements. The ex tens ion o f these ideas t o t h e

use o f l i q u i d chromatography i s e s s e n t i a l l y s t r a i g h t f o r w a r d .

(B) Yet, and here I come t o t h e second ques t ion , i t must be admi t ted t h a t

chromatographic methods a r e s u r p r i s i n g l y l i t t l e used o u t s i d e ana lys i s , and then

o n l y f o r t h e most p a r t by those whose background and c e n t r a l i n t e r e s t l i e i n

a n a l y t i c a l chromatography (see e.g., r e f . 40). Why i s t h i s ? I n p a r t I t h i n k i t

stems from t h e cu r ious i n t e l l e c t u a l c loud t h a t somehow hangs over a n a l y t i c a l

procedures. There seems t o be something o f magic and w i t c h c r a f t about them, a

v iew perhaps a s s i s t e d by t h e e x o t i c chemicals t h e y o f t e n employ, and i n t h e ca-

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se o f chromatography the m u l t i p l i c i t y o f phys i ca l processes which a r e i n v o l v e d

and which need t o be un rave l l ed . Now i t may appear t o some t h a t t h i s must a l -

ways be so, b u t I b e l i e v e t h a t sc ience i s s t i l l ve ry o f t e n a ma t te r o f f ash ion .

Fashions change and i n t h e end wisdom may y e t p r e v a i l . A f t e r a l l gas chromato-

graphy was f i r s t exemp l i f i ed i n 1512 ( r e f . 41), was c l e a r l y expounded i n 1941

( r e f . 42), b u t o n l y came i n t o i t s own a f t e r 1952 ( r e f . 43). For t h e c r u c i a l

c o n t r i b u t i o n s on t h e l a s t two o f these occasions we a r e o f course indebted t o

the man i n whose honour t h i s meet ing i s he ld.

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n

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W.K. Al-Thamir, J.H. Purne l l and R.L. Laub, J. Chromatogr., 176 (1979)

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