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21 An NMR Study of Surface Complexes under the

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0034.ch021

Conditions of Fast Exchange between Physisorbed and Chemisorbed Molecules

V. YU. BOROVKOV, G. M. ZHIDOMIROV, and V. B. KAZANSKY Zelinsky Institute of Organic Chemistry of Academy of Sciences USSR, Moscow, U.S.S.R.

The phenomenon of fast exchange between bound and free solvent molecules i s widely used for the investigation of complexformation i n solutions by high resolution NMR. It was determined / 1 , 2 / , that on the surfaces of solids such as s i l i c a , silica-alumina and alumina, a fast exchange can take place between different kinds of adsorbed species. This exchange leads to an effective averaging of the NMR spectra of physically adsorbed molecules and those molecules coordinated by active surface sites. Hence, the parameters of the observed spectra contain information about the i n t e r action of the adsorbate with the surface sites. The existence of such exchange leads also to the considerable increase i n the NMR sensitivity for detection of the complex formation. This fact w i l l be made obvious by the experimental results discussed i n this paper. The use of isolated transition metal ions as the active surface sites allows one to considerably i n crease the efficacy of NMR for the study of surface phenomena. In addition to line broadening, large paramagnetic shifts appear i n the NMR spectra of the adsorbates, and the l a t t e r especially gives information concerning the interaction between coordinated 233

In Magnetic Resonance in Colloid and Interface Science; Resing, Henry A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

234

M A G N E T I C

RESONANCE

molecules and t r a n s i t i o n metal ions*


Experimental In our work we used samples o f A e r o s i l , which contained on the surface c o o r d i n a t i v e l y unsaturated i o n s of Co " and Ni " ", and I - z e o l i t e s i n which the Na+ ions were p a r t i a l l y substituted with b i v a l e n t cob a l t and n i c k e l ions* The method o f sample preparat i o n and the c o n d i t i o n s o f t h e i r pretreatment were described i n d e t a i l i n / 3 / * The NMR measurements were performed u s i n g a Soviet-made NMR spectrometer having an operating frequency of 60 MHz* The main t e c h n i c a l data and the process of simultaneous measurements of NMR spectra and adsorbtion isothermes f o r the adsorbate s were a l s o described i n / 3 / * Before measurements the i n v e s t i g a t e d substances were p u r i f i e d by the freeze-pump-thaw method. Molec u l a r hydrogen was p u r i f i e d by d i f f u s i o n through a glowing palladium c a p i l l a r y . 24

2

1

Experimental R e s u l t s The c r i t e r i o n f o r the d e t e c t i o n of i n t e r a c t i o n of adsorbed molecules with paramagnetic surface ions i s the existence of large paramagnetic s h i f t s i n the NMR spectra o f the adsorbates and the dependence of these s h i f t s on the surface coverage /1, 3 / · F i g * 1 shows the PMR spectra o f propylene adsorbed on n i c k e l - c o n t a i n i n g A e r o s i l * Line s h i f t s * on the order of several tens of p*p*m* and t h e i r dependence on the e q u i l i b r i u m pressure i n the system grea t l y t e s t i f y t o the complex formation of the propylene molecule with N i ion* We have i n v e s t i g a t e d the adsorption of many d i f f e r e n t molecules - benzene, unsaturated hydrocarbons (ethylene, propylene, butene, cyclohexene;, a l c o h o l s (methanol, ethanol), saturated hydrocarbons (methane, isopentane, n-hexane* cyclohexane, c i s and trans-1,4-dimethylcyclohexane) and molecular hydrogen - on adsorbents which have contained cobalt and n i c k e l ions* The r e s u l t s o f these i n v e s t i g a t i o n s are given i n table 1, i n which the p l u s or minus sign c o r r e s ponds t o the existance or absence of large paramagnet i c s h i f t s i n the NMR spectra of adsorbed species* 9

2 +



BOROVKOV

E T AL.

Surface Complexes under Fast Exchange

Figure 1. NMR spectra of propylene adsorbed on Ni * containing Aerosil at equilibrium pressures of 3, 6, 25, and 150 Torr, respectively. All spectra were recorded at +10°C. 2


236

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Table 1.


* adsorbent adsorbate^

2

C o * on Aerosil

Benzene

Φ

Unsaturated hydrocarbons

4*

Alcohols Saturated hydrocarbons Hydrogen

2

N i * on Aerosil

-

CoNal NiNaT

Φ

Φ

Φ Φ

Φ

Φ

The PMR spectra of each k i n d of adsorbed sub stance has c h a r a c t e r i s t i c features* F o r example, PMR spectra of unsaturated hydrocarbons /V> as a r u l e , c o n s i s t of several l i n e s . The l i n e s corresponding to the v i n y l protons are s h i f t e d t o higher f i e l d s r e l a t i v e t o the p o s i t i o n of these l i n e s i n the spectra o f physisorbed molecules. On the other hand, the protons on a methyl o r methylene group adjacent t o a double bond are s h i f t e d t o lower f i e l d s . Protons a t greater distance from the double bond show no change· Some times i n PMR spectra one can detect the nonequivalen cy of the protons l o c a t e d near a double bond. T h i s i s the reason f o r the appearance of four l i n e s i n the spectra of propylene adsorbed on n i c k e l - c o n t a i n i n g A e r o s i l (see f i g . 1 d ) . PMR spectra o f a l c o h o l s a l s o c o n s i s t o f s e v e r a l l i n e s . L i n e s of CH2 and CH3 groups are s h i f t e d t o lower f i e l d , and the p o s i t i o n of the l i n e s f o r the hydroxyl group changes very l i t t l e . Adsorption of a l l saturated hydrocarbons on co b a l t supported A e r o s i l gave a single symmetrical l i n e s h i f t e d t o higher f i e l d s . The observed s h i f t s ranged over s e v e r a l tens of p.p.m. Hydrogen adsorption on A e r o s i l c o n t a i n i n g cobalt or n i c k e l was studied a t l i q u i d n i t r o g e n temperature. I t i s i n t e r e s t i n g t o note that f o r the cobalt-con t a i n i n g samples, the l i n e f o r adsorbed hydrogen was s h i f t e d t o higher f i e l d s , but f o r n i c k e l saaç>les the s h i f t was i n the opposite d i r e c t i o n . Upon adsorption of hydrogen on CoNaT and NiNaY z e o l i t e s a t temperat u r e s below 0°C, one observes a l i n e s h i f t e d t o lower f i e l d f o r both adsorbents. The s h i f t values are weakl y dependent on surface coverage and show an exponent i a l increase with a decrease i n temperature. Using the dependence l i n e s h i f t s on coverage, a l l complexes which we detected by means of PMR, can



21.

BOROVKOV E T A L .

Surface Complexes

under Fast

Exchange

237

"be d i v i d e d i n t o three d i f f e r e n t groups. The f i r s t one includes the complexes of benzene, unsaturated hydrocarbons and a l c o h o l s , f o r which i s observed a l i n e a r increase of s h i f t on decrease i n coverage ( F i g . 2, 1 ) . The complexes of saturated hydrocarbons with Co2+ ions belong to the second group. For a l l saturated species i n v e s t i g a t e d by us the dependence of the s h i f t s on coverage was s i m i l a r to that represented on f i g . 2, curve 2 . In the t h i r d group we place the complexes of hydrogen with Co2+ and Ni2+ ions i n Y z e o l i t e , f o r which there i s almost no dependence of the s h i f t s on the amount of the hydrogen adsorption. A study of 13c NMR spectra of these same adsorbed molecules would allow us to obtain importent i n formation about the adsorbate-adsorbent i n t e r a c t i o n . Unfortunately, the small n a t u r a l abundence of 13c, and the small magnetic moment of t h i s nucleus, hamper the a p p l i c a t i o n of 13c NMR f o r i n v e s t i g a t i n g surface phenomena. From our point of view, the resonance of the 1 T F nucleus i s more convenient f o r t h i s purpose. The scale o f s h i f t s f o r t h i s nucleus i s l a r g e r than f o r **C and i t s large magnetic moment provides large s e n s i t i v i t y . I t allows one to obtain w e l l resolved NMR spectra of adsorbed species without any d i f f i c u l ties. At t h i s time we are begining the systematic i n v e s t i g a t i o n of adsorption u s i n g 1?F NMR. F o r exanqple, f i g . 3 shows the NMR spectra of pentafluorobenzene adsorbed on pure A e r o s i l and A e r o s i l containing C o * i o n s . One can see that the l i n e s f o r the d i f f e r e n t kinds of F atoms i n the molecule have d i f f e r e n t paramagnetic s h i f t s . We have already i n v e s t i g a t e d the set of f l u o r i n e s u b s t i t u t e d benzenes CgFg, CgFi^H and C^FHc on c o b a l t - c o n t a i n i n g A e r o s i l . The main feature o r t n e spectra of these compounds i s a s h i f t of the 1-?F l i n e s t o lower f i e l d s . T h i s i s i n contrast to the d i r e c t i o n of the s h i f t i n the 1H NMR spectra of C H adsorbed on the same system. 2

g

6

Theory The t h e o r e t i c a l d e s c r i p t i o n of the NMR spectra f o r the molecules undergoing exchange between the s i t e s of p h y s i s o r p t i o n and chemisorption deals with the problem of the averaging of the NMR spectra by dynemic motion of the molecules. As i t w i l l be shown below the methods developed i n general theory / 8 / can be a p p l i e d to the d e s c r i p t i o n of the NMR spectra of adsorbed molecules.


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RESONANCE


238

Figure 3. NMR spectra of pentafluorobenzene adsorbed on (a) pure Aerosil and (b) Co *-containing Aerosil. The spectra were recorded at room temperature. 2


21.

BOROVKOV E T A L .

In

Surface Complexes under Fast Exchange

239

the case of f a s t exchange the observed

cP-

shift

O)

Pc-cTc

where Y Ρ = «. the mole f r a c t i o n o f che mi sorted mole cules. V ss Τ 4the f u l l nutf)er of adsorbed molecuΛ c ρ les, σ - the l i n e s h i f t i n NMR spectra of ohemisorbed molecule relative t o i t * s p o s i t i o n f o r physisorbed molecule. The amount of chemisorbed molecules i s given by the equelibrium K*


Λ

0

M • O c Z£jBc

(2)

here Ν* i s the number of empty s i t e s f o r chemisorp tion. V For the case Ρ *

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