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Infra-Red Absorption Spectra of Adsorbed Molecules
Yaroslavskii, N. G.; Terenin, A. N.
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hATIO,W RESlEAEPCH COUNCIL
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CANADAD I V I S I O N OF I N F O R U T I O N SERVICES
Technf e a l T r a n s l a t i o n No T.T+:!.13
IWRA-RED ABSOIIYTZOW SPECTRA
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ADSORBED MOLECULES( I n f r a - k r a s n y e C ' e k t r y Pogloshchenga Adsorbiro- vamykh Niolekul
7
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Go Y a r o s l a v s k i i and A. Ns T3reninThis
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may n o t be publishedi n
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part w i t h o u t the w r i t t e n consent of Natf o n a l Research Council .;Ottawa, Canada
TECIIJICHL T ; - ~ ~ ~ ~ ~ S U T I O N NO L-L 3
D i v i s i o n of I n f a r m t i o n S e r v i c e s
T i t l e : I n f ra-Red Absorption S p e c t r a of Adsorbed Mole c ~ l e a
BY:
No
G O Y a r o s l a v s k i i and A. N o T e r e n i nReference : Doklady Akademii Nauk, U o S c S s H o , V o l o 6G9
NO"
50
by
N o
G. YarosXavskfi and A. N o TereninTihe problem of d e t e c t i n g t h e changes, which a r e ex- perienced by t h e valency bonds o f a gaseous molecule d u r i n g ad-
s o r p t i o n on t;he s u r f a c e of a c a t a l y t i c a l l y a c t i v e s o l i d 'body, can most d i r e c t l y be solved w i t h t h e a i d of i n f r a - r e d s p e c t r a by e s t a b l i s h i n g t h o s e changes which t a k e p l a c e i n t h e charac- t e r i s t i c v i b r a t i o n f r e q u e n c i e s of t h e molecule (1) The experimental d i f f i c u l t i e s of o b t a i n i n g a s p e c t r a f o r a mono- m o l e c u l a r l y adsorbed l a y e r a r e i n c r e a s e d i n t h e i n f r a - r e d
r e g i o n of t h e s p e c t r a due t o t h e f a c t t h a t t t ~ e value fos. L h e a d s o r p t i o n c o e i f i c i e u ~ ; is t e n s of t h o u s a n d s times srn~'.:! ~7 th-9-
t h e value f o r t h e v f s i ' b l e and u l t r a . v i o l e t , r e g i o n s , T h e ex- periment, t h e r e f o r e , demands t h e u t i l i z a t i o n of comparatl.vely
t h i c k l a y e r s of" a h i g h l y d i s p e r s i n g a d s o r b e n t , w h i c h must be t r a n s p a r e n t in t h e s e c t i o n of t h e spectrum where t h e v i b r a t 1 on f r e q u e n c i e s of t h e adsorbed molecule a r e f oundo The i n i t i a l a t t e m p t , c a r r i e d out w i t h a n a e r o s o l of s i l i c a g e l on which ammonia was adsorbed, d i d n o t g i v e convincing r e s u l t s , p a r t i ~ l l:r
due t o t h e f a c t t h a t t h e s p e c t r a l i n s t r u m n t was one of s m a i l
2.
a d s o r b e n t miorocrys t a . l l i n o l a y e r s of CaFE a r ~ d ' T l C 1 , d e p o s i t e d by e v a p o r a t i o ~ ? i x i a va@uum, T h i s was ~ S ( w s u c c e s s f ~ l l . J dua t o an i n s u f f i c i e n t t h i c k n e s s o f t h e layer, 112 % h i c k l a y e r s a
s h a r p d e c r e a s e of i n f r a - r e d r a d i a t i o n r e s u l t s due t o s c a t t e r i n g . . Wave l e n g t h s from 2 t o 8,p were used in t h e s e e x p e r i m e n t s ,
L a t e l y we have changed o v e r t o t h e u s e of a n a d s o r b e n t , s u c h a s g l a s s - l i k e s i l i c a g e l , which can be p r e p a r e d i n the form o f a n o p a l e s c e n t microporous p l a t e , the p h y s i c a l and c h e m i c a l p r o p e r t i e s of which s r e w e ~ l known (2)- We have a l s o changed o v e r t o t h e r e g i o n o f l o n g e r wave l e n g t h s f r o m !, t o 2J(
,
i n which a r e found t h e f i r s t o v e r t o n e s o f t h e b a s i c v i b r a t i o n f r e q u e n c i e s , and have u t i l i z e d t h e e a r l i e r worked o u t d i f - f e r e n t i a l method u s i n g a n a u t o m a t i c n o t a t i o n of t h e s p e c t r a on a f i v e - p r i s m g l a s s i n s t r 7 m e n t of l a r g e d i s p e r s i o n ( 3 )
,*
The b e h a v i o u ~ of t h e mf c r o - porous g l a s s - l i k e s i l i c a g e l i n r e l a t - i o n t o w a t e r vapours has been i n v e s t i g a t e d i n d e - t a i l by H O G u 'faroclavskii., who w i t 1 1 t h e a l d ~7f t h e same s p e c -
t r a l a p p a r a t u s , gi-aplnically demonstra.tzd she p r e s e n c e i n s z i ~ c n
g e l u n d e r v a r i o u s c o n d i t i o n s of w a t e r i n t h r e e s t a t e s :
c a p i l l a r y - c o n d e n s e d , a d s o r b e d and s t r u c t u r a l l y - connected on
- -
W h e work i n t h i s r e g i o n o f t h e i n f r a - r e d spect-um h a s a n ad- d i t i o n a l a d v a n t a g e , t h a t it, i s p o s s i b l e t o t r e a t t h e samples i n a vacuum and c a r r y o u t the a d s o r p t i o n c f v u p a w s i n g l a s s v e s s e l s , which a r e t s a n s p a ~ e n t to t h e above n e n t i o n e d r e g i o r , o f wave l e n g t h s ,
the s u r f a c e , The last c o n d i t i o n c o c s i s z s i n t h e f a c t , t h a t on t h e s u r f a c e of a n l c r a - p o r o u s g l a s s - l i l : ~ s i l - i c a gel. sub- j e c t e d , f o r e x a x p l e , t o a 3-,hour d e h y d r a t i o n i n a hl.gh
vacuiun a t 550°, t h e r e remain hydroxyl g r o u p s OH, s t r u c t u r a l l y connected w i t h t h e s u r f a c e . . b u t n o t e x c i t e d by t h e p r e s e n c e of f o r e i g n p a r t i c l e s on 'the s u r f a c e , These ' f r e e " g r o u p s a r e s p e c t r a l l y d e t e c t e d a s a s h a r p peak l y i n g n e a r ';= c a w - 1 1 . 3 6 5 ~ (7326
1 0
We have s p e c t r a l l y i n v e s t i g a t e d t h e a d s o r p t i o n of' v a p o u r s of b e n z o l , t o l u e n e , a n i l i n e , p y r i d i n e , p h e n o l and o t h e r s u s i n g t h e above d e s c r i b e d a d s o r b e n t , The f i r s t two compounds were s e l e c t e d because t h e y a r e c o r ~ p a r a t i v e l y i n e r t i n r e l a t i o n t o s i l i c a g e l ; t h e two c o n s e c u t i v e , because t h e y p o s s e s s b a s i c p r o p e r t i e s which would n o t s t r o n g l y i n t e r a c t w i t h t h e " a c i d i c n h y d r o x y l s of s i l i c a g e l , The p h e n o l , c a ne n t e r o n l y i n t o hydrogen bonds by n e a n s of i t s h y d r o x y l group w i t h t h e h y d r o x y l g r o u p s o r oxygen b r i d g e s of t h e s u r f a c e , F o r a c o n p a r i s o n we have a l s o o b t a i n e d t h e s p e c t r a o f t h e
s a n e m o l e c u l e s i n I n e r t s o l . v e n t s (CCl4 and CSz), and t h e spec- t r a of t h e l i q u i d s ,
The a d s o r p t i o n of v a p o u r s of t h e above mentioned corn- pounds was c a r r i e d o u t by b r e a k i n g a n a n p u l e c o n t a i n i n g t h e l i q u i d , f r e e from a i r and n o i s t u r e , i n s i d e of a n u n s o l d e r e d e v a c u a t e d v e s s e l , i n which was f o ~ i d t h e i n i t i a l l y t r e a t e d s a m p l e ' o f t h e a d s o r b e n t i n t h e form of a p l a t e a p p r o x i m a t e l y
4 mn i n t h i c k n e s s . The spectrum of t h e adsorbent was noted before t h e ampule was broken, and a f t e r t h a t during d e f i n i t e i n t e r v a l s of time. The adsorbent was k e p t a t room temperature, and t h e l i q u i d a t a s 1 , i g h t l y lower temperature,
Benzol and Toluene
During t h e course. of t h e a d s o r p t i o n of t h e vapours of benzol ( a t 200 and
75
mtr p r e s s u r e ) and t o l u e n e ( 2 2mm)
i n t h e spectrum,along w i t h t h e narrow peaks due t o t h e s u r f a c e groups OH of t h e a d s o r b e n t , g r a d u a l l y appear frequency peaks of t h e C-H r i n g and of t h e group CHJ ( i n t h e case of t o l u e n e )
.
We have not d e t e c t e d any n o t i c e a b l e displacements i n t h e p o s i t i o n of t h e s e f r e q u e n c i e s by comparing w i t h t h e same molecules d i s - solved i n C C l 4 .A s t h e a d s o r p t i o n i n c r e a s e s t h e appearance of t h e spectrum becomes more s i m i l a r t o t h e spectrum of t h e l i q u i d ' benzol o r t o l u e n e , which i n d i c a t e s t h e beginning of c a p i l l a r y
condensation of t h e vapours. An i n t e r e s t i n g f a c t i s s i m u l t s - neously observed, which c o n s i s t s i n t h e g r a d u a l weakening of t h e narrow peak due t o the hydroxyl groups of t h e s u r f a c e of t h e adsorbent. This phenomena a l s o t a k e s p l a c e f o r a d i f f e r e n t type of adsorbent ( p h e n o l ) , when approaching t h e r e g i o n of t h e c a p i l l a r y condensation of vapours,
A n i l i n e
proceed very slowly due t o t h e very small vapour p r e s s u r e ( 0 . 1
am
a t 2 0 0 ) , a c h a r a c t e r i s t i c phenoaaena i s t h a t of t h e g r a d u a l a i s a p p e a r a n c e of t h e peak1 . 3 6 5 ~
d u e t o t h e s u r f a c e groups OH, long before t h e moment, when t h e peaks of t h e bands C-H and N-H of t h e adsorbed a n i l i n e molecules appear (Figo 1),
This d i f f e r e n c e i n t h e s p e c t r a l s e n s i t i v i t y of t h e bands must be assigned t o t h e d i f f e r e n c e i n t h e i r a d s o r p t i o n c o e f f i c i e n t s , The adsorbed a n l l i n e molecules ( a l s o t h e p y r i d i n e ) a r e
f i r s t
d e t e c t e d by an i n d i r e c t method, due t o t h e a c t i o n which t h e y e x e r t on t h e s u r f a c e groups OH, a p p a r e n t l y by e n t e r i n g w i t h them i n t o hydrogen bonds 0-Hoe , N o I n f a c t , the e n t r a n c e i n t o a hydrogen bond w i t h a s t r o n g acceptance of a p r o t o n b r i n g s about i n t h e r e g i o n of overtones t h e disappearance of t h e f r e -
quency OH of t h e d o n a t i n q group ( 4 )
I n
o r d e r .o d e t e c t s p e c t r a l l y t h e adsorbed a n i l i n e n o l e c u l e s t h e n s e l v e s , it i s n e c e s s a r y t o have much h i g h e r s u r - f a c e c o n c e n t r a t i o n s o From F i g , 1 it f o l l o w s t h a t a t t h i s s t a g e of a c o n s i d e r a b l y f i l l e d i n s u r f a c e , t h e peak a t 1.52,~~ c h a r a c t e r i s t i c f o r t le frequency of t h e bond N-H, i s broadened and i s n e a r i n i t s p o s i t i o n t o t h e peak, c h a r a c t e r i s t i c of t h e a s s o c i a t e d groups hH i n l i q u i d a n i l i n e , shown on F'go 1 by a t u n c t u a t e d l i n e , Ilowever, a p e c i f i c f a c t f o r t h e adsorbed n o l e c u l e s of a n i l i n e appears t o be t l e displacement of t h e peak lu-H by 0 . 0 1 p i n r e l a t i o n t o he p ak of t h e a s s o c i a t e d groupsI\w, and by 0 . 0 2 5 ~ i n r e l a t i o n t o t h e peak of t h e i s o l a t e d g r o u p s lvII ( f o r a s o l u t i o n of a n i l i n e i n C C 1 4 ) i n t h e d i r e c t i o n of t h e l o n g e r wave l e n g t h s , which p r e s e n t s p r o o f on t h e weaken- i n g of t h e v a l e n c y bond a t a d s o r p t i o n o
The band peak f o r t h e v i b r a t i o n s of t h e bond C-H, t h e a r o n a t i c r i n g i n a n i l i n e , a s i n t h e o t h e r c a s e s , h a s n o t d i s - p l a y e d a n y n o t i c e a b l e changes, A t t e n t i o n must be drawn t o t h e f a c t , t h a t t h e r a t i o of t h e peak h e i g h t s 1 - H and C-H d u r i n g a d s o r p t i o n c h a n g e s i n t h e d i r e c t i o n of d e c r e a s i n g h e i g h t s i n t h e peak N-H. The a c - c e p t a n c e of a p r o t o n by t h e amino-group w i t h a t r a n s f o r m a t i o n
+
of it i n t o a p o s i t i v e ammonium group NH3 b r i n g s a b o u t t h e d i s - a p p e a r a n c e of a b s o r p t i o n of t h e f r e q u e n c y N-H ' 4 ) , Hence, t h e change i n t h e h e i g h t of t h e peak N-H may be a s s i q n e d t o s u c h a p a r t i a l r e a c t i o n of a n i l i n e w i t h t h e s u r f a c e a c i d i c g r o u p s OH* P y r i d i n e The l a s t mentioned r e a c t i o n w i l l n o r e d e f i n i t e l y t a k e p l a c e w i t h a s t r o n g e r b a s e , s u c h a s p y r i d i n e , I n f a c t t h e a d s o r p t i o n of p y r i d i n e v a p o u r s ( a t 20° and 1 0 mm p r e s s u r e ) b r i n g s a b o u t a f a s t , corzplete d i s a p p e a r a n c e of t h e narrow peak 1 . 3 6 5 ~ of t h e s u r f a c e g r o u p s OH, and l o n g b e f o r e t h e a p p e a r a n c e i n t h e s p e c t r u m of t h e peak C-H a t 1 . 6 6 ~ of t h e p y r i d i n e r i n gPheno 1
-
During t h e a d s o r p t i o n of t h e phenol vapours (at 20° and 0 , l mm p r e s s u r e ) , t h e peak o f t h e s u r f a c e groups OH remains narrow
and does n o t d i s a p p e a r a s f a s t , a s i n t h e p r e v i o u s two c a s e s . Moreover, t h e peak a t 1.418/Y b e l o n g i n g t o t h e OH groups of t h e phenol, e x p e r i e n c e s a c o n s i d e r a b l e d e c r e a s e i n i n t e n s i t y and a b r o a d e n i n g i n t h e d i r e c t i o n of l o n g e r wave l e n g t h s ( F i g . 3 ) . The f r e q u e n c y of t h e C-H r i n g h e r e a l s o r e m a i n s unchanged.
From t h i s we c a n draw t h e c o n c l u s i o n s t h a t t h e ad-, s o r b e d m o l e c u l e s of p h e n o l do n o t e x c i t e t h e s u r f a c e g r o u p s
OH (up t o l a r g e f i l l i n g s i n ) , b u t a p p a r e n t l y , form hydrogen bonds by means of i t s a c i d i c OH groups w i t h t h e oxygen atoms o f t h e s i l i c a g e l s u r f a c e . I n t h e r e g i o n o f c a p i l l a r y conden-
s a t i o n o f vapours broad maximums a p p e a r , c h a r a c t o r i s t i c o f t h e a s s o c i a t e d between t h e m s e l v e s OH groups o f p h e n o l ( 4 ) The above c i t e d e x p e r i m e n t a l r e s u l t s i n d i c a t e a p o s s i b i l i t y o f a d a p t i n g t h e i n f r a - * r e d s p e c t r a f o r the i n v e s t i - g a t i o n of p a r t i c u l a r c a s e s of a d s o r p t i o n 4 The a d a p t i o n o f t h i s method f o r c a t a l y t i c r e a c t i o n s i s n e x t i n l i n e , S e n t t o t h e E d i t o r A p r i l 19, 1949
'53kbZi~
1. A- T e r e n i n Zhurnal. ~ ' i z i c l ~ k o i KMimli, !J?urnal of Physic81 Chemistry) k 4 , 1 3 6 2 : [ i kc0
1,
2. 0 0 C. hiolchanova and E - V e Greberitschikau, Zhurml Obshchei Khimii (Journal of General Chemistry), 1 2 , 587, (19421 ; Q e C c
Molchanova, Priroda (Nature )
,
4 , 4 7 , ( 1 9 4 7 ) .3 0 A - lio Terenic and N o G o Y a r o s l a v s k i i , Acta Physicochfm.
URSS, 1 7 , 260, (19421,
Fig. 1. The abeorption spectrograme f o r a p l a t s o f g l a e s - l i k e s i l i c a g e l : 1
-
before breaking the t u a ~ ~ l s with a n i l i n e ; 11-
one hour a f t e r the ampule was broken; 111-
three houra a f t e r ; 1 V-
four hours a f t e r ; V-
thlrfy-pig. 2. 1
-
The absorption of a pyridine eolution i n CC14 a t a volume concentration 1:6; 11-
t h e absorption of a p l a t e of g l a s s - l i k ee i l i c a g e l before the ampule with pyridine was broken; 111
-
f i v e minutes a f t e r the ampule wae broken;1V
-
f i f t e e n minutes a f t e r : V-
one hour a f t e r ; V l-
t h r e e hours a f t e r ; V 1 1-
ten hours ~ f t e r .Big.
3.
1-
The absorption of a phenol eolution i n C C l 4 a t aconcentration 0.1 W / 1 ; 11
