Magnetic Resonance in Colloid and Interface Science

13 ESR Studies of Radicals Adsorbed on Zeolite J. SOHMA and M. SHIOTANI

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0034.ch013

Faculty of Engineering, Hokkaido University, Sapporo 060, Japan

Introduction It has been established that ESR studies on radicals adsorbed on a zeolite provide important information on interactions between the adsorbed radicals and the adsorbent.(1-6) Interesting finding of an extra-coupling was reported on the ethyl radical adsorbed on the zeolite.(7) Furthermore, observed changes in anisotropy of hyperfine couplings and g factors may be helpful to elucidate motion of the radicals trapped on the zeolite. Thus it seems interesting to study the details of ESR spectra from simple radicals, such as methyl or amino, trapped on zeolite and to discuss natures of the trapping sites of the zeolite in relation to behaviors of the trapped radicals. Experimental The zeolites used in the experiments were Linde molecular sieves, 4-A supplied by the Union-Carbide Corp in U.S.A. and Nishio Industry in Japan. The zeolites were heat-treated under vacuum (ca. 10-4 Torr) for five hours usually at different tempe ratures from room temperature to 550°C after initial pre-heat -treatment in air. Gases were introduced through either a break seal or a cock to the zeolite contained in a spectrosil ESR sample tube at -196°C and allowed to adsorb on the zeolite up to room temperature. The samples used were: normal methane, 99.7% purity, obtained from Gaschro Industry; deuterated methane, CD4, 99 atomic %, from Stobler Isotope Chemicals, ethane and normal ammonia, 99.5% purity, from Takachiho Chemical Co. and isotopic ammonias, 15NH3 and ND4, 97% purity, from Merk Sharp Dohome. The ammount of gas adsorbed was controlled and checked by measuring the pressure drop in a known volume. Radicals were produced at -196°C by γ-irradiation from 60Co source to the gas adsorbed zeolites. Total dose per sample was 0.2-0.5 Mrad. ESR spectra were recorded in the temperature range from -196°C to +50°C by JEOL JES-PE-1 spectrometer operating at X-band with 100 KHz 141

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



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modulation. Temperature o f a sample i n the ESR c a v i t y was con t r o l l e d by an attached v a r i a b l e temperature u n i t . Types o f Methyl R a d i c a l s Adsorbed on the Heat-Treated Z e o l i t e s


Methane on Very Weakly Heat-Treated Z e o l i t e Below 50°C).

(Heat-Treatment

The ESR spectrum observed a t -196°C a f t e r γ-irradiation c o n s i s t s o f a broad c e n t r a l band and a doublet w i t h the s e p a r a t i o n of 508G from a hydrogen atom. T h i s broad spectrum i s completely d i f f e r e n t from any spectrum o f the methyl r a d i c a l s and i d e n t i c a l to that observed from a γ-irradiated z e o l i t e adsorbing no methane. Apparently no methyl r a d i c a l i s s t a b i l i z e d a t -196°C i n the z e o l i t e h e a t - t r e a t e d below 50°C. Methane Adsorbed on M i l d l y Heat-Treated Z e o l i t e . (Heat-Treat ment a t ca. 80°C).

The ESR spectrum observed a t -196°C i n t h i s case i s r e p r o duced as (B) i n F i g . 1. I t appears to be n e a r l y a quartet w i t h s e v e r a l s a t e l l i t e s . Apparently the spectrum i s d i f f e r e n t from that o f the normal m e t h y l , but the s e p a r a t i o n of the main q u a r t e t , 22.3G, i s c l o s e t o the normal one. I t was found that the spectrum changed i t s shape w i t h r i s i n g temperatures and the spectrum observed a t -55°C was i d e n t i c a l w i t h normal one, as shown i n F i g . 2. The temperature v a r i a t i o n of the l i n e shape was completely r e v e r s i b l e except f o r the s m a l l decrease i n the t o t a l i n t e n s i t y . The spectrum shown i n F i g . 3 ( A ) , which was observed a t -196°C a f t e r warming t o -55°C, may be analysed as a quartet m o d i f i e d w i t h the a n i s o t r o p i c c o u p l i n g o r i g i n a t i n g from an a d d i t i o n a l proton. The spectrum d e r i v e d from t h i s assumption i s represented by the s t i c k diagram i n F i g . 3 (B). I n t h i s spectrum parameters used are f o l l o w i n g : 18.9G and 8.4G f o r A and Αχ o f the e x t r a proton c o u p l i n g and 22.3G f o r the methyl p r o t o n s , r e s p e c t i v e l y . The correspondence o f the main peaks of the observed spectrum w i t h the s t i c k diagram seems s a t i s f a c t o r y . These experimental r e s u l t s i n d i c a t e s t r o n g l y that the r a d i c a l species r e s p o n s i b l e f o r the spectrum i s the methyl r a d i c a l having c o u p l i n g w i t h an a d d i t i o n a l p r o t o n , namely ΗβΟ· ··· Η ", i n which the unpaired e l e c t r o n i n t e r a c t s w i t h the e x t r a proton m a i n l y by the d i p o l a r c o u p l i n g . T h i s species i s c a l l e d an abnormal methyl r a d i c a l ( I ) . B

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Methane Adsorbed on S t r o n g l y Heat Treated Z e o l i t e (Heat-Treat ment a t ca. 250°C).

The ESR spectrum observed from i r r a d i a t e d methane adsorbed on



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s t r o n g l y h e a t - t r e a t e d z e o l i t e i s shown as (D) i n F i g . 1. The p a t t e r n i s d i f f e r e n t from e i t h e r one of the normal methyl r a d i c a l and the abnormal methyl r a d i c a l ( I ) . Although t h e r e a r e s e v e r a l d i f f u s e peaks i n the spectrum, i t i s p r i n c i p a l l y a double t r i p l e t . T h i s simulated p a t t e r n , which i s d e r i v e d from the assumption t h a t the methyl r a d i c a l has an a n i s o t r o p i c g f a c t o r w i t h two e q u i v a l e n t protons and one non-equivalent one, i s shown i n as (C) i n F i g . 4 together w i t h the observed spectrum (A). The used parameters a r e g = 2.0023, gx = 2.0032, A , = 21.8, A j. = 23.2 f o r e q u i v a l e n t two p r o t o n s , k\ = 37.8G, A^L= 35.6G f o r the other proton and Δ % ] _ ( L o r e n t z i a n ) = 1.3G. The agreement between the observed double t r i p l e t and the simulated one (B) i s s a t i s f a c t o r y , i f one n e g l e c t s the d i f f u s e peaks. The methyl r a d i c a l g i v i n g t h i s spectrum i s c a l l e d an abnormal methyl r a d i c a l ( I I ) . T h i s r a d i c a l i s u n s t a b l e on warming and b e g i n t o decay above -160°C near b o i l i n g p o i n t o f CHt, (-161.7°C). The ESR s p e c t r a observed f o r the methyl r a d i c a l trapped i n the z e o l i t e s h e a t - t r e a t e d i n the temperature range between 80°C and 250°C are m i x t u r e o f the two s p e c t r a o f the abnormal methyl r a d i c a l s ( I ) and ( I I ) . An example o f t h i s spectrum i s shown as (C) i n F i g . 1. No ESR spectrum was observed f o r γ - i r r a d i a t e d methane, which had been trapped i n the z e o l i t e s h e a t - t r e a t e d a t temperature above 500°C. The d e t a i l e d i d e n t i f i c a t i o n s of these s p e c t r a was published i n the other paper. ( 9 ) 2


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E t h y l R a d i c a l Adsorbed on Z e o l i t e (7) The z e o l i t e used i n t h i s experiments was h e a t - t r e a t e d f o r three hours a t 150°C. ESR spectrum observed a t -125°C from γ i r r a d i a t e d z e o l i t e s adsorbing ethane showed c l e a r m u l t i p l e t s as shown as A i n F i g . 5. The spectrum i s d e f i n i t e l y d i f f e r e n t from t h a t o f the e t h y l r a d i c a l e i t h e r i n l i q u i d phase, (8)or i n adsorbed s t a t e on s i l i c a g e l . (10) The main f e a t u r e s o f t h e spectrum agree w i t h that o b t a i n e d by assuming t h a t each l i n e o f the spectrum (8) ( F i g . 5-C) o f e t h y l r a d i c a l i s s p l i t i n t o a d o u b l e t , as shown i n F i g . 5-B. A spectrum simulated from t h e above-mentioned assumption i s shown as "D" i n the same f i g u r e . I n t h i s s i m u l a t i o n the couplings of 28.7G and 21.4G were taken f o r the t h r e e β protons and the two α p r o t o n s , r e s p e c t i v e l y , i n t h e e t h y l r a d i c a l , and 8.0G f o r the s e p a r a t i o n o f the e x t r a doublet and 7.0G f o r the l i n e - w i d t h . The above assumed v a l u e s f o r t h e c o u p l i n g s o f both β and α protons i n the e t h y l r a d i c a l are n e a r l y equal t o 26.9G and 22.4G, r e s p e c t i v e l y , which a r e the r e p o r t e d v a l u e s (8) f o r the e t h y l r a d i c a l . From the c l o s e s i m i l a r i t y between the observed spectrum and the simulated one i t i s con cluded that the observed spectrum i s the spectrum o f the abnormal type of e t h y l r a d i c a l having the c o u p l i n g w i t h an a d d i t i o n a l p r o t o n , namely H3C-CH2· ··· Η* . The spectrum was observed from the ethane adsorbed on the z e o l i t e s , which had been h e a t - t r e a t e d at h i g h e r temperature, such as 550°C, although no spectrum from


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the γ - i r r a d i a t e d methane was adsorbed on such a v e r y s t r o n g l y heac-treated z e o l i t e . The spectrum from t h e trapped e t h y l r a d i c a l obtained under t h i s c o n d i t i o n was i n t e r p r e t e d s i m i l a r l y as that from the abnormal methyl r a d i c a l ( I ) mentioned. Types of t h e Adsorbed Amino R a d i c a l s


Ammonia Adsorbed on M i l d l y Heat-Treated Z e o l i t e . -2 a) High coverage w i t h ammonia (ca. 1.7x10 m o l / g r . ) : N e a r l y 1 . 7 x l 0 " mol. of ammonia was adsorbed per one gram of the z e o l i t e . I n such a h i g h e r coverage each c a v i t y i n z e o l i t e i s almost f u l l w i t h ammonia molecules. An example of t h e ESR spec trum observed a t -196°C from the i r r a d i a t e d ammonia h i g h l y adsorbed on t h e m i l d l y h e a t - t r e a t e d z e o l i t e i s shown as A i n F i g . 6, which i s a broad q u i n t e t . T h i s i s a t y p i c a l p a t t e r n observed f o r t h e r a d i c a l possessing a n i s o t r o p i c h y p e r f i n e and g t e n s o r s which i s trapped i n e i t h e r amorphous or p o l y c r y s t a l l i n e m a t r i x . T h i s spectrum i s q u i t e c l o s e to t h a t (11) observed from t h e i r r a d i a t e d aqueous s o l u t i o n of ammonia i n g l a s s y s t a t e . From the c l o s e s i m i l a r i t y between t h e observed s p e c t r a one may taken NH2 * r a d i c a l surrounded w i t h water molecules as the r a d i c a l r e s p o n s i ble t o the spectrum. I t was found that t h i s r a d i c a l was u n s t a b l e and almost decayed out a t -80°C. T h i s u n s t a b l e , water-surrounded NH · r a d i c a l i s c a l l e d NH · ( I ) . The w e l l - r e s o l v e d spectrum, shown as Β i n F i g . 6, was observed a t -75°C a f t e r the decay of the r a d i c a l NH · ( I ) although the s p e c t r a l i n t e n s i t y decreased t o n e a r l y one t e n t h of t h e i n i t i a l one. T h i s spectrum i s almost i s o t r o p i c and a p p a r e n t l y a t r i p l e - t r i p l e ( F i g . 6-C), i n which the Η c o u p l i n g constant a, of t h e two protons i s 23.5G and t h a t of 14N ° the n i t r o g e n a i s 11.7G. T h i s i d e n t i f i c a t i o n was reconfirmed by u s i n g t h e deuterated ammonia ND3. The s p e c t r a observed a t -30°C i s reproduced i n F i g . 7. As shown i n same f i g u r e the observed p a t t e r n i s e x p l a i n e d by a s u p e r p o s i t i o n of t h e two s p e c t r a from ND · and »NDH. T h i s s t a b l e r a d i c a l s u r v i v e d above -80°C i s c a l l e d amino r a d i c a l ( I I ) . _ b) Low coverage of ammonia (ca. 8.5x10 m o l / g r . ) : Appro x i m a t e l y 8 . 5 x 1 ο " " mol of ammonia was adsorbed on one gram of t h e z e o l i t e . I n such a low coverage o n l y one ammonia molecule o r l e s s e x i s t s , on average, i n a c a v i t y i n the z e o l i t e . The ESR spectrum observed a t -196°C under these c o n d i t i o n e d looked s i m i l a r t o A i n F i g . 6. A f t e r decaying t h e u n s t a b l e NH r a d i c a l (I) by annealing t h e sample a t room temperature, t h e l i n e - s h a p e observed a t -196 C was a l i t t l e but c l e a r l y changed t o t h a t shown as A i n F i g . 8. The r a d i c a l , which g i v e s the spectrum and i s s u r v i v e d a f t e r a n n e a l i n g , i s so s t a b l e that one could f o l l o w the temperature v a r i a t i o n of t h e spectrum up t o 50°C, as shown i n F i g . 8. One can i d e n t i f y t h e r a d i c a l as NH · by comparing t h e 2

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w e l l r e s o l v e d and completely i s o t r o p i c spectrum D w i t h the s t i c k diagram i n the f i g u r e . T h i s s t a b l e amino r a d i c a l which i s i d e n t i c a l t o the ΝΗ · ( I I ) . The r e l a t i v e r a t i o of the s t a b l e NH r a d i c a l ( I I ) to the u n s t a b l e NH r a d i c a l (I) was increased w i t h decreasing i n the amount of coverage of ammonia. The spectrum observed a t -65°C i s shown as A i n F i g . 9. The major part of t h i s spectrum i s almost same t o the spectrum C i n F i g . 8 but there i s apparently other component c o n s i s t i n g of the t h i r t e e n l i n e s w i t h a s e p a r a t i o n o f 14.6G» which were not c l e a r l y apparent a t the low g a i n of the spectrometer. I t was found that the same 1 3 - l i n e component was observed even when the i s o t o p i c NU3 was adsorbed. T h i s f a c t i n d i c a t e s that t h i s s p e c t r a l compo nent does not o r i g i n a t e from the adsorbed ammonia. The observed r e l a t i v e i n t e n s i t i e s of outer most peaks of the 1 3 - l i n e component were e x p e r i m e n t a l l y determined t o be 1:3:8:15:?:?:?:?:20:10:4:1. I f one assume a r a d i c a l N a | (I( Na) = 3/2), the r e l a t i v e i n t e n s i t i e s of the expected 1 3 - l i n e spectrum i s 1:4:10:20:31:40:44:40: 31:20:10:4:1. Agreement between these r e l a t i v e i n t e n s i t i e s i s r a t h e r s a t i s f a c t o r y i f one take account of the f a c t that t h e main p a r t of t h i s spectrum i s obscured w i t h the other component. Thus, i t seems p l a u s i b l e t o a t t r i b u t e t h i s 1 3 - l i n e spectrum t o the r a d i c a l Na$ . 2

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Ammonia Adsorbed on Very S t r o n g l y Heat-Treated Z e o l i t e Treatment a t ca. 350°C).

(Heat-

a) High coverage with ammonia (ca. 1.7x10"^ mol/gr.): ESR spectrum observed at -196°C in the high coverage of ammonia on this heat-treated zeolite appeared as a broad quintet, which i s similar i n the main character to that shown i n Fig. 6-A. However, the maximum separation i n this case was found as 121G, which i s a l i t t l e smaller than that, 126G, i n the case of the mildly heattreated zeolite. And the radical was so unstable as to decay out at lower temperature, such as -150 C. One could not observe a well-resolved spectrum, which i s merely obtained at higher temper atures. It was hard to identify clearly the responsible radical on the basis of such unresolved spectrum. However, similarity between the patterns observed at -196°C, as mentioned above, leads one to presume that the responsible radical i s NH *(I), character i s t i c behaviors of which are the smaller maximum separation and rapid decay at lower temperatures. ^ b) Low coverage of ammonia (ca. 8.5x10 mol/gr.): The ESR spectrum observed at -196°C from the irradiated zeolite covered thinly with ammonia was again a broad quintet similar to those observed at the same temperature i n the various cases. Resolution of the spectrum increased gradually with the raised temperatures up to -100°C, from which the signal intensity was found to decrease. The well-resolved spectrum observed at -120°C showed additional four line structure on each hyperfine line of the amino e

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Magnetic Resonance in Colloid and Interface Science

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