Catalytic Materials - American Chemical Society

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Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 13, 2014 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch013

Combined Physical Techniques in the Characterization of Zeolite ZSM-5 and ZSM-11 Acidity and Basicity JACQUES C. VEDRINE, ALINE AUROUX, and GISÈLE COUDURIER Institut de Recherches sur la Catalyse, C.N.R.S., 2, avenue Albert Einstein, 69626— Villeurbanne, France

Acidic pentasil zeolite ZSM-11 and ZSM-5 present fascinating properties related to the size of the channels and to the acidic characteristics of the material. Moreover the Al distribution along the material particles was shown to depend on the synthesis procedure and to be more or less regular resulting in differences in the strength and the distribution of the acid sites. Samples differing by their Al content, particle size, acidification or chemical treatment procedures were prepared and characterized by X-ray diffraction, TEM, EDX-STEM, XPS and n-hexane adsorption capacity. Their acidic properties were determined using ir spectroscopy (OH groups, NH adsorption), microcalorimetric measu rements of the differential heat of NH adsorption at different temperatures vs pulses of NH and ESR. By ESR, electron acceptor (acid) or donor (basic) properties were characterized by means of the charge transfer complexes formed with organic molecules able to give or accept one electron resulting in parama gnetic radical ions such as C H + and SO - or C6H6(NO ) respectively. It is found that the combination of several techniques is necessary to obtain information about the nature, strength and concentration of sites. Acid sites were found to be very strong for ZSM-5 and to a lesser extent for ZSM-11 samples. No electron donor (basic) sites could be evidenced. 3

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0097-6156/84/0248-0253$06.25/0 © 1984 American Chemical Society

In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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A c i d i c p r o p e r t i e s o f z e o l i t i c m a t e r i a l s have w i d e l y been i n v e s t i g a t e d i n the past f i f t e e n years (1 ), p a r t i c u l a r l y f o r Y-type z e o l i t e s . The presence of s t r o n g acid s i t e s , as i t i s u s u a l l y the case f o r acid z e o l i t e s , r e s u l t s i n the presence of o n l y weak b a s i c s i t e s , i f any. T h e r e f o r e , up to now, m a j o r i t y of the s t u d i e s has d e a l t with the c h a r a c t e r i z a t i o n o f a c i d r a t h e r t h a n b a s i c p r o p e r t i e s . ^ T h e a c i d s i t e s ( B r b n s t e d : Η , L e w i s : A l ) and b a s i c s i t e s (0 , OH ) may be c h a r a c t e r i z e d d i r e c t l y by u s i n g p h y s i c a l techniques or i n d i r e c t l y by the a d s o r p t i o n o f a b a s i c (ammonia, p y r i d i n e , n - b u t y l a m i n e e t c ) or an a c i d i c (00^, a c e t i c a c i d , SO^, h y d r o c h l o r i c a c i d , e t c ) probe. In the case of small pore z e o l i t e , as i n the present work, the methods i n v o l v i n g l a r g e probe molecu l e s as Hammett's i n d i c a t o r s (2,3) have o b v i o u s l y to be r e j e c t e d . The main techniques a v a i l a b l e can be summarized as f o l l o w s : i. *- fj * spectroscopy (3,4) of hydroxyl groups (3500-3800 cm r e g i o n ) , o f adsorbed m o l e c u l e probes and o f t h e i r f u r t h e r t h e r m a l d e s o r p t i o n . The t e c h n i q u e i s the most w i d e l y used s i n c e i t i s v e r y f r u i t f u l and a l l o w s t o d i f f e r e n t i a t e the Bronsted and the Lewis s i t e s . ii. o p t i c a l UV technique (5) of adsorbed appropriate molecules The t e c h n i q u e i s v e r y s e n s i t i v e but o f n a r r o w e r a p p l i c a t i o n s than i r s p e c t r o s c o p y p a r t i c u l a r l y b e c a u s e quan t i t a t i v e determination i s very l i m i t e d and o v e r l a p p i n g of broad peaks occurs p r e c l u d i n g any p r e c i s e c h a r a c t e r i z a t i o n of d i f f e r e n t s i t e s . iii. n u c l e a r m a g n e t i c r e s o n a n c e t e c h n i q u e (6^) a p p l i e d to the measurement o f the p r o t o n jump f r e q u e n c y , even i n the absence o f a d s o r b a t e . T h i s f r e q u e n c y i n c r e a s e s w i t h a h i g h e r p r o t o n m o b i l i t y and c h a r a c t e r i z e s the occupancy f a c t o r of the proton at the various oxygen atoms. iv. e l e c t r o n s p i n r e s o n a n c e o f r a d i c a l i o n s ( a n i o n s or c a t i o n s ) (7) , as d i n i t r o b e n z e n e , t e t r a c y a n o e t h y l e n e or perylene, anthracene, benzene, r e s p e c t i v e l y which probe the e l e c t r o n donor ( b a s i c ) and e l e c t r o n a c c e p t o r ( a c i d ) p r o p e r t i e s of a s u r f a c e . v. c a l o r i m e t r i c determination of the d i f f e r e n t i a l heat (8) of adsorption of a probe molecule (NH^, pyridine, benzene, η-butyl amine, e t c ) at a g i v e n t e m p e r a t u r e . T h i s heat i s r e l a t e d to the a b i l i t y of the s i t e s to react with the probe molecule, i . e . to i t s b a s i c or a c i d i c c h a r a c t e r . Note, t h a t the c o n c e n t r a t i o n and strength of s i t e s but not t h e i r nature may be obtained. vi. t e m p e r a t u r e programmed d e s o r p t i o n (DTA, TPD or TG) (9) of the adsorbed probe molecule. Information about the strength of the s i t e s and about the number o f s i t e s , assuming one adsorbed m o l e c u l e per s i t e , may be obtained. However, the exact nature of the s i t e s remains unknown while the n a t u r e o f the d e s o r b e d m o l e c u l e s i s assumed t o be i d e n t i c a l to


+

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that o f the s t a r t i n g probe, which i s not always t r u e . c a t a l y t i c t e s t r e a c t i o n s which are postulated to occur only f o r a g i v e n a c i d o r b a s i c s t r e n g t h o f s i t e s depending on the r e a c t i o n and on the reactants chosen ( 1 0 ) . viii. c h e m i c a l t i t r a t i o n o f p r o t o n s t r a n s f e r e d t o aqueous s o l u t i o n where the m a t e r i a l i s introduced. In t h e p r e s e n t work i r , e s r and m i c r o c a l o r i m e t r y techniques were used t o c h a r a c t e r i z e the acid-base p r o p e r t i e s o f a c i d ZSM-5 and ZSM-11 s a m p l e s . Complementary s t u d i e s by TEM, EDX-STEM and XPS were also c a r r i e d out t o determine the s i z e and shape o f z e o l i t e p a r t i c l e s and t h e A l d i s t r i b u t i o n w i t h i n a p a r t i c l e . C a t a l y t i c p r o p e r t i e s f o r methanol conversion were a l s o determined.


vii.

EXPERIMENTAL PART ZSM-5 and ZSM-11 samples were p r e p a r e d as p r e v i o u s l y described (11) u s i n g t et r apr ο pyl ammonium hydroxide and t e t r a b u t y l ammonium b r o m i d e , r e s p e c t i v e l y . The n a t u r e and c r y s t a l l i n i t y o f t h e m a t e r i a l s were v e r i f i e d by X r a y d i f f r a c t i o n , i r s p e c t r o s c o p y o f l a t t i c e v i b r a t i o n a l bands (1^) , n-hexane adsorption c a p a c i t y at room t e m p e r a t u r e and c o n s t r a i n t i n d e x (J^3) measurements. A l l samples c o r r e s p o n d t o h i g h l y c r y s t a l l i n e ZSM-5 or ZSM-11 mate r i a l s . The chemical compositions of the samples as determined from chemical a n a l y s i s o f A l and Na contents, are given i n t a b l e 1. The i r measurements were c a r r i e d out w i t h a P e r k i n Elmer 580 s p e c t r o m e t e r and f u s e d s i l i c a c e l l with KBr windows a l l o w i n g t o o u t g a s s t h e z e o l i t i c wafer at a d e s i r e d t e m p e r a t u r e and t o i n t r o d u c e and f u r t h e r o u t g a s s a probe molecule without contact with a i r . The e s r experiments were performed i n usual s i l i c a 4 mm i . d . tubes u s i n g a Varian E9 spectrometer monitored i n X-band mode. Q u a n t i t a t i v e d e t e r m i n a t i o n o f r a d i c a l i o n c o n c e n t r a t i o n was obtained by comparison with a Varian s t r o n g P i t c h standard sample. A h e a t - f l o w c a l o r i m e t e r o f Tian-Calvet type from Setaram m a i n t a i ned at a d e s i r e d temperature, from room t e m p e r a t u r e up t o 400°C., was used i n connection with a volumetric apparatus equipped with a Mc Leod gauge. Sample weights were t y p i c a l l y 100 mg and ammonia doses 0.1 cm NTP. T r a n s m i s s i o n e l e c t r o n microscopy p i c t u r e s were taken u s i n g a JEOL 100 CX microscope. For some samples l a t e r a l m i c r o - a n a l y s i s o f t h i n s e c t i o n s o f z e o l i t e was c a r r i e d out u s i n g a HB-5 VG microscope equipped with EDX accessory at IFP ( 1 1 ) . ESCA experiments were c a r r i e d out with a HP 5950A spectrometer o f the "Centre Commun ESCA de l'Université de Lyon". The A l s i g n a l b e i n g s m a l l because o f t h e low A l content, i t s accumulation was n e c e s s a r y f o r one h o u r . A12p peak was t h e o n l y A l peak t o be a n a l y z e d s i n c e o v e r l a p p i n g o f S i phonon peak with A12s peak takes place. Smoothing o f the peaks, s u b s t r a c t i o n of the background and d e t e r m i n a t i o n o f t h e s u r f a c e , A, o f t h e peaks were c a r r i e d out with the computer. The r e l a t i v e c o n c e n t r a t i o n η f o r element 1 and 2 ( S i and A l f o r i n s t a n c e ) was c a l c u l a t e d u s i n g the approximate relation :


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n

n

l

A

A

l

σ

σ

E

2

J/2 k2

E


2 2 1 kl where σ i s the e l e c t r o n cross s e c t i o n tabulated by S c o f i e l d and Ε the k i n e t i c e n e r g y v a l u e o f the peaks a n a l y z e d elements 1 and 2.

(14) for

EXPERIMENTAL RESULTS I n f r a r e d experiments 2 The s e l f s u p p o r t i n g wafers of z e o l i t e s (3 to 4 mg.cm ) presented an i . r . absorption continuum depending on the p a r t i c l e s i z e . The most i n t e n s e a b s o r p t i o n s were o b t a i n e d f o r the samples 5 and 6 with p a r t i c l e s i z e of ca. 6 urn and a s p h e r u l i t i c shape. For a l l the samples ou£gassed at 400°C two i r bands were observed i n the 3750-3600 cm r e g i o n , c o r r e s p o n d i n g t o two t y p e s o| h y d r o x y l groups. The higher frequency band was found at 3720 cm ( h a l f band width = 55 cm ) f o r the ZSM-5 samples. I t was r e s t o r e d a f t e r NH^ adsorption by outgassing at room temperature. This band has been assigned to n o n - a c i d i c t e r m i n a l s i l a n o l s and t h e r e f o r e i t s i n t e n s i t y may be r e l a t e d t o the c r y s t a l s i z e and/or the presence of s t r u c t u r a l d e f e c t s . Indeed f o r h i g h l y c r y s t a l l i n e m a t e r i a l s w i t h l a r g e c r y s t a l s i z e t h i s band i s absent (15) while f o r w e l l c r y s t a l l i z e d samples 1 and 2 with p a r t i c l e s i z e of 1 um, i t i s weak ( f i g . 1). For a l l our ZSM-11 s a m p l e s , an i n t e n s e and narrow band was o b s e r v e d at 3740 cm" ( h a l f band width = 20 - 30 cm" ) ( f i g . 1 ) . I t c o u l d be t h o u g h t at f i r s t g l a n c e t h a t i t c o r r e s p o n d e d t o amorphous s i l i c a m a t e r i a l . However a d e t a i l e d high r e s o l u t i o n and m i c r o d i f f r a c t i o n a n a l y s i s (JJ) c o n j o i n t l y w i t h i . r . framework s p e c t r a and n-hexane adsorption c a p a c i t y s t u d i e s showed that the m a t e r i a l s were z e o l i t i c i n nature, but mainly composed of aggrega t e s o f t i n y p a r t i c l e s (5 to 10 nm i n diameter). Sample 5 was even more pecular s i n c e i t presented such an aggregate as a c o r e w i t h e m e r g i n g n e e d l e s (11) g i v i n g a s p h e r u l i t e - t y p e shape ( o v e r a l l d i a m e t e r = 6 um). M i c r o d i f f r a c t i o n s t u d y of the needles showed that they are s i n g l e - c r y s t a l i n nature with a very n i c e p a t t e r n o f d o t s ( 1 1 ) . The o v e r a l l p a r t i c l e s l o o k e d as g o l f b a l l s i n TEM p i c t u r e s . Moreover a NaOH 6N treatment at 80°C r e s u l t i n g i n sample 6 was shown by TEM t o have d i s s o l v e d the above n e e d l e s and o b v i o u s l y any amorphous s i l i c a m a t e r i a l l e a v i n g the aggregated core (diameter = um). The i . r . spectrum s t i l l e x h i b i t e d a narrow band at 3740 cm more intense than f o r sample 5. I t may thus be c o n c l u d e d | h a t the band at 3740 cm i n ZSM-11 samples as the one at 3720 cm i n ZSM-5 was due to t e r m i n a l s i l a n o l groups, obvious l y i n large amount because of the p r e s e n c e o f t i n y p a r t i c l e s i n the aggregated core, and not due to extrazeoliçic m a t e r i a l (15). The low f r e q u e n c y band was found at 3602 cm ( h a l f band width = 30pi ) f o r ZSM-5 samples and 3612 cm ( h a l f band width : 32-40 cm ) f o r ZSM-11 samples. I t s i n t e n s i t y depended on the A l content o f the z e o l i t e s as a l r e a d y o b s e r v e d (15^). I t was a s s i g n e d t o



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F i g u r e 1 : i r s p e c t r a hydroxyl groups o f the d i f f e r e n t samp l e s : 1 (3.6 mg cm~2) 3 (3.2 mg cm~2), 4 (3.6 mg cm~2) 5 (4.3 mg cm"~2) and 6 (3.7 mg cm~2). 9


y


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a c i d i c hydroxyl groups of the z e o l i t e s . NH^ adsorption and f u r t h e r d e s o r p t i o n at i n c r e a s i n g temperatures allowed t o f o l l o w the a c i d s t r e n g t h o f the h y d r o x y l groups (16). In f i g . 2 the i n t e n s i t y of the vNH v i b r a t i o n (3390 cm ) i s reported against the outgassing t e m p e r a t u r e s . C o n s i d e r i n g the a b s o l u t e v a l u e s o f the o p t i c a l d e n s i t y (o.d.) and of t h e i r v a r i a t i o n s with outgassing temperature i t can be concluded t h a t ZSM-11 sample 3 has l e s s e r and weaker a c i d s i t e s t h a n the ZSM-5 samples 1 and 2. The number of a c i d i c s i t e s i s a l s o r e l a t e d t o the i n t e n s i t y o f the low f r e q u e n c y OH band (o.d. = 0.081 and 0.050 r e s p e c t i v e l y f o r 4 mg cm of samples 1 and 3) and to the A l c o n t e n t . The a c i d s t r e n g t h o f the s i t e s also decreased with the increase i n frequency of t h i s OH v i b r a t i o n : 3602 cm f o r ZSM-5, 3612 cm f o r ZSM-11. This r e s u l t , that ZSM-5 i s more a c i d i c t h a n ZSM-11, i s i n agreement w i t h o t h e r f i n d i n g s by J a c o b s et a l (17) u s i n g OH group band s h i f t upon benzene a d s o r p t i o n . By h e a t i n g the samples at high temperatures (> 600°C) i r r e v e r s i b l e m o d i f i c a t i o n o f the m a t e r i a l was o b s e r v e d t o o c c u r (18). As a m a t t e r o f f a c t i t was o b s e r v e d t h a t the low frequency OH band disappeared upon outgassing at 800°C whereas the i n t e n s i t y o f the 3740 cm band c o n s i d e r a b l y d e c r e a s e d . In the mean time, a new band appeared at 1380 cm which may be assigned to an aluminic a c i d ty£| s p e c i e s ( J ^ 9 ) . F u r t h e r r e h y d r a t i o n d i ^ not r e s t o r e the 3612 cm band but generated a band at 3700 cm that may be due to hydroxyl groups bound to the aluminic s p e c i e s . One has suggested p r e v i o u s l y (18) that such h i g h temperature dehydration r e s u l t s i n a p a r t i a l dealumination of the z e o l i t i c framework and t h e r e f o r e to a lower a c i d i t y . C a l o r i m e t r y experiments D i f f e r e n t i a l heats of NH^ adsorption were measured f o r the samples outgassed at d i f f e r e n t t e m p e r a t u r e s r a n g i n g from 400 t o 800°C. Ammonia was chosen as a b a s i c probe because i t s s i z e i s s m a l l , which may l i m i t a t e d i f f u s i o n e f f e c t s i n s m a l l pore z e o l i t e m a t e r i a l s . The v a r i a t i o n s of the d i f f e r e n t i a l heats of adsorption are p l o t t e d i n f i g . 3 as a f u n c t i o n o f the s u c c e s s i v e p u l s e s o f NH^ i n t r o d u c e d , i . e . of NH~ coverage. The adsorption temperature was changed from 150 up to 400°C. The main e x p e r i m e n t a l f e a t u r e s can be summarized as f o l l o w s : i. some samples, p a r t i c u l a r l y those a c i d i f i e d by NH^Cl and with l a r g e r p a r t i c l e s i z e , gave r i s e t o a c u r v e w i t h a maximum, whose shape g r e a t l y depended on the adsorption temperature ( f i g . 3). ii. when the NH^ a d s o r p t i o n t e m p e r a t u r e i n c r e a s e d t h e r e was competition between the f o r m a t i o n o f ammonium i o n s between NH^ and a B r o n s t e d s i t e and t h e i r decomposition ( f i g . 3 ) . Upon NH^ a d s o r p t i o n a d r o p o f p r e s s u r e o c c u r e d almost i n s t a n t a n e o u s l y due t o s t r o n g c h e m i s o r p t i o n . However f o r weaker c h e m i s o r p t i o n the p r e s s u r e d i d not drop t o z e r o i n s t a n t a n e o u s l y . Such a change i n b e h a v i o u r , which i s p r e s e n t l y c o n s i d e r e d as a change i n chemisorpt ion s t r e n g t h t y p e , i s shown by an arrow i n t h e f i g u r e s d e a l i n g w i t h


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o.d. t

F i g u r e 2 : V a r i a t i o n s of o p t i c a l d e n s i t y of vNH band (3390 cm" ) f o r ZSM-5 (samples 1 and 2) and ZSM-11 (sample 3) vs the outgassing temperature. Wafers of 4.0 mg. cm~2. The dashed curve 3 corresponds t o sample 3 outgassed at 800°C rehydrated at room temperature and dehydrated again a t 400°C. 1



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AMOUNT

NH

3

1

0+

5 AMOUNT NH~

ADSORBED,per U.C. 2

3

10 ADSORBED, cm , g 3

15 1

F i g u r e 3 : V a r i a t i o n s w i t h coverage of the d i f f e r e n t i a l heats of a d s o r p t i o n of ammonia on H-ZSM-5 (sample 1) measu red at 150°C., (A), 200°C., (•), 250°C., 300°C (0) and 400°C (φ). The sample was outgassed at 400°C p r i o r NH3 adsorp t i o n . The meaning of the arrows i s explained i n the t e x t .



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c a l o r i m e t r y . For instance f o r samples outgassed at 400°C one t h e n g e t s . 2.4, 2.2, 1.9, 1.4 and 0.6 NH~ molecules per u n i t c e l l f o r a d s o r p t i o n at 150, 200, 250, 300 and 400°C r e s p e c t i v e l y . Tljese numbers have to be compared with the number o f p r o t o n s H e q u a l t o 3.2 per u.c.and are always s m a l l e r . This presumably a r i s e s from the presence o f "weak" B r o n s t e d a c i d s i t e s i n the sample. T h e r e f o r e i t appears t h a t the c a l o r i m e t r y t e c h n i q u e a l l o w s t o d e t e r m i n e the number o f " s t r o n g " B r o n s t e d s i t e s assuming t h a t o u t g a s s i n g a£ 400°C b e f o r e NH~ a d s o r p t i o n g i v e s r i s e s to a maximum i n H i . e . , that no d e n y d r o x y l a t i o n had o c c u r e d . T h i s i s a c r u d e ap p r o x i m a t i o n (2Ό) but can be considered as v a l i d i n a rough approximation and f o r comparison of d i f f e r e n t samples. M o r e o v e r , t e m p e r a t u r e o f NH~ a d s o r p t i o n as low as 150°C seemed t o be r e l i a b l e f o r c a l o r i m e t r i c e x p e r i m e n t s . Room temperature a d s o r p t i o n d i d not a l l o w to d i f f e r e n t i a t e strong and weak a c i d s i t e s . I t a l s o a p p e a r e d , as p r e v i o u s l y ob s e r v e d , t h a t the number of s t r o n g a c i d s i t e s was lower that the number o f p r o t o n s c a l c u l a t e d from c h e m i c a l a n a l y s i s , w h i c h may be i n t e r p r e t e d as due to a weak a c i d i t y of part of these protons. i i i . i f the samples were c a l c i n e d at 800°C before NH^ adsorption, a curve with a maximum was s t i l l o b s e r v e d ; t h e r e are l e s s a c i d s i t e s but t h e y are s t r o n g e r . I f such samples were f u r t h e r rehydrated at room temperature and outgassed again at 400°C., the s t a r t i n g curve was not obtained ( f i g . 2 ) . I t can then be concluded that such a heat treatment has i r r e v e r s i b l y m o d i f i e d the m a t e r i a l . From XPS d a t a o f S i : A l r a t i o measurement and from i . r . experiment of p y r i d i n e a d s o r p t i o n , i t was p r e v i o u s l y (18) suggested that A l from the l a t t i c e was e x t r a c t e d upon c a l c i n a t i o n r e s u l t i n g i n an a l u m i n i c a c i d compound w i t h i n the c a v i t i e s and c o n s e q u e n t l y i n a d d i t i o n a l s t e r i c c o n s t r a i n t s . I r study, as described above, a l s o showed that dehydroxylation was not r e v e r s i b l e f o r ZSM-11 sample. i v . the shape o f the c a l o r i m e t r i c c u r v e s w i t h a maximum was p r e v i o u s l y (20-21) a s s s i g n e d t o an u n u s u a l c o n j u n c t i o n of three phenomena : ( i ) immobile adsorption, ( i i ) mass t r a n s f e r l i m i t a t i o n and ( i i i ) p r e f e r e n t i a l l o c a t i o n o f the most e n e r g e t i c a c i d s i t e s i n the i n t e r n a l pores of the z e o l i t e . The l a t t e r p o i n t c o u l d c o r r e s p o n d t o an h e t e r o g e n e o u s d i s t r i b u t i o n o f A l w i t h i n the z e o l i t e g r a i n as i t was o b s e r v e d l a t e r on ( 1 1 , 22 - 2 4 ) . Mass t r a n s f e r l i m i t a t i o n o b v i o u s l y p l a y e d a r o l e s i n c e the shape w i t h a maximum d i s a p p e a r e d when the NH^ a d s o r p t i o n temperature increased ( f i g . 3), i . e . when d i f f u s i o n r a t e increased. This c o n c l u s i o n was a l s o s u p p o r t e d by the experiment f o r which c a l o r i m e t r i c c e l l was d i s c o n n e c t e d from the l i n e , h e a t e d o u t s i d e the c a l o r i m e t e r f o r 2 h at 250°C between each NH^ p u l s e t o f a c i l i t a t e the d i f f u s i o n of adsorbed NH«. The maximum i n the curve was not observed anymore as d e s c r i b e d i n r e f . 21 when the samples were p a r t l y d e a c t i v a t e d by methanol conversion r e a c t i o n or by i n t r o d u c t i o n o f a phosphorous c o m p o u n d ;



CATALYTIC MATERIALS

s t r o n g e r b u t i n n e r a c i d s i t e s were then a c c e s s i b l e t o the f i r s t NH^ m o l e c u l e s i n t r o d u c e d . The maximum i n t h e curve disappeared whereas h i g h e r heat o f a d s o r p t i o n v a l u e s were o b t a i n e d as i t c a n be seen i n f i g s . 3 and 5 i n r e f . 24. I t followed that f o r curves with a maximum the h i g h e s t v a l u e o f t h e NH^ a d s o r p t i o n heat d i d n o t c h a r a c t e r i z e r e a l l y t h e strongest acid s i t e s as i t could be thougt at f i r s t g l a n c e . As a m a t t e r o f f a c t t h e r e was a competition f o r ammonia t o n e u t r a l i z e the strongest acid s i t e s which are l o c a t e d i n s i d e the z e o l i t e p a r t i c l e s and t o f r e e l y d i f f u s e w i t h i n t h e channel to reach t h e s e s i t e s . A k i n d o f average v a l u e was t h e n o b t a i n e d which depended on adsorption temperature, s i z e of the p a r t i c l e s and channels, t h e time between s u c c e s s i v e p u l s e s i f slow d i f f u s i o n occured. Nevertheless the number o f strong acid s i t e s , d e t e r m i n e d as d i s c u s s e d above, remains r e l i a b l e d a t a from t h e e x p e r i m e n t s . When a maximum i n the c a l o r i m e t r y curve was not obtained i t can be e i t h e r b e c a u s e the p a r t i c l e s i z e o f t h e z e o l i t e was s m a l l o r more o f t e n because the strongest a c i d s i t e s were l o c a t e d i n t h e o u t e r l a y e r s o f t h e g r a i n s . Such a phenomenon i s c l e a r l y seen i n f i g . 4 f o r t h e d i f f e r e n t ZSM-11 samples. Note a l s o t h a t h y d r o c h l o r i c acid treatment p a r t l y dealuminated surface l a y e r s of the z e o l i t e p a r t i c l e s as evidenced by XPS ( t a b l e 1) and r e s u l t e d i n s t r o n g e r s u r f a c e a c i d e s i t e s , i . e . i n no maximum i n the c a l o r i m e t r y curve. another s t r i k i n g feature o f c a l o r i m e t r y measurements i s worth while m e n t i o n i n g . Sample 1 was used r e s p e c t i v e l y f o r met h a n o l , C^H. and C^H^ + CHÔH c o n v e r s i o n r e a c t i o n s f o r 20 min. The c a l o r i m e t r y r e s u l t s a r e p l o t t e d i n f i g . 5 f o r samples outgassed a f t e r r e a c t i o n . I t c l e a r l y appeared that no s t r o n g a c i d i c s i t e s were t h e n a c c e s s i b l e t o NH^ i f C^H^ c o n v e r s i o n was p e r f o r m e d w h i l e a c i d s i t e s were s t i l l a c c e s s i b l e a f t e r CH-OH o r C^H^ + CHÔH r e a c t i o n s . T h i s was presumably due t o tne formation o f l i n e a r polymeric r e s i d u e s when C^H^ was used alone which then f i l l e d the channels. This a l s o e x p l a i n e d t h e l o w a c t i v i t y o f ZSM-5 z e o l i t e f o r C 4 c o n v e r s i o n r e a c t i o n ( 2 5 , 2 6 ) . Such a l o w a c t i v i t y was c o n s i d e r e d b y some authors to r u l e out the p o s s i b i l i t y that C^H, was an intermediate species i n CHÔH conversion r e a c t i o n (27). However, the c a l o r i m e t r i c curve u s i n g a mixture o f C«H^ and CHÔH showed c l e a r l y i n connection with c a t a l y t i c r e s u l t s that methanol acted as an a l k y l a t i n g agent and t h a t s t r o n g a c i d s i t e s were s t i l l a v a i l a b l e a f t e r r e a c t i o n . Therefore i n CHÔH c o n v e r s i o n C^H^ y H be intermediate compounds, the presence o f CH«0H i n the feed l e d the r e a c t i o n t o proceed to higher hydrocarbons r a t h e r than to d e a c t i v a t e the m a t e r i a l by t h e f o r m a t i o n o f l i n e a r hydrocarbon residues which f i l l the channels. Methanol conversion s t u d i e s at low c o n v e r s i o n l e v e l (< 1 %) and low temperature (250°C) c l e a r l y showed that to o l e f i n s were a l l p r i m a r y p r o d u c t s . Such an e x periment i n d i c a t e d that the r a t t l e mechanism, as proposed b y Guisnet e t a l . (27) , was v a l i d on ZSM-5 samples as represented as f o l l o w s on p. 266 (28). H

2

m

a

w

e

a n



VEDRINE ET AL.


F i g u r e 4 : V a r i a t i o n s w i t h coverage of the d i f f e r e n t i a l heats of NH^ a d s o r p t i o n measured at 143°C on H-ZSM-11 samp l e s outgassed at 400°C. Samples 3 (0) 4 (•), 5 (.) and 6


In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984. +

23 20 - 30

30 20 - 50

4.8 0.3 4.5

platelets 0.5-2um single crys t a l type

ZSM-5

19

3.4 0.2 3.2

4

N H . M/2

no

2

27

v

platelets 0.5-2um single crys t a l type

ZSM-5

HC1, N/2

no

1

24 25 - 45

21

37

2.5 0.3 2.2

+

6

4

ZSM-11

N H . M/2

+

NaOH, 20 % wt

43

2.2 0.0 2.2

15

5.9 0.2 5.7

spherulite spheroids 6 um 6 urn aggregate core o f aggre gates + emer of t i n y par ticles ging needles (0.2 - 0.4 um) ( * 5-10nm)

ZSM-11

N H , M/2 4

5 no

70 16 core : 15-25 15 - 25 needles.-30-70 from the same o f the three batches r e s p e c t i v e l y .

53

1.8 0.1 1.7

spheroids spheroids 0.6 urn 0.6 urn aggregates aggregates of t i n y p a r t i of t i n y p a t i cles cles (ψ * 5-10nm) ( * 5-10nm)

ZSM-11

+

ZSM-11

4

no

4

N H , M/2

no

3

HC1, N/2

* Samples 1 and 2, 3 and 4, 5 and 6 o r i g i n a t e

Al Na H Si : A l ( atoms ratios) Chemical analysis XPS data EDX-STEM

Composi tion (per u.c.)

shape size structure

Samples treatment acidifica t i o n at 80° C zeolite type Particles

*

Table 1 : Main c h a r a c t e r i s t i c s o f the d i f f e r e n t ZSM-5 and ZSM-11 samples.



VEDRINE ET A L .


Figure 5 : V a r i a t i o n s w i t h coverage of the d i f f e r e n t i a l heats of NH3 adsorption a t 143°C on H-ZSM-5 sample not mentionned i n t a b l e 1 • : s t a r t i n g sample outgassed at 400°C Ο > 20 min. C2H4 conversion r e a c t i o n followed by outgassing at 400°C χ : 20 min. C2H4 + CH3OH r e a c t i o n f o l l o w e d by outgassing a t 150°C φ : 20 min. C-lfy + CH3OH r e a c t i o n followed by outgassing a t 400°C Λ : 20 min. CH~0H r e a c t i o n followed by outgassing a t 400°C


265

266

CATALYTIC MATERIALS

Gas phase :


Adsorbed intermediate

Me 0 or MeOH

U CH 0-Z 3

MeOEt

Î1 MeOEt

v i . the a c i d i f i c a t i o n p r o c e d u r e and c a l c i n a t i o n or c h e m i c a l t r e a t m e n t may a l s o be important i n the d i s t r i b u t i o n of acid s i t e s w i t h i n the channels. P a r t i c u l a r l y , i f protons stemming fro^m t e t r a p r o p y l ammonium i o n decomposition were exchanged by Na or n e u t r a l i z e d by NH^ gas b e f o r e a c i d i f i c a t i o n by exchange by ammonium s a l t or by HC1, t h e m a t e r i a l might behave d i f f e r e n t l y (^9). The d i f f e r e n c e i n c a l o r i m e t r i c c u r v e s when comparing the rwo a c i d i f i c a t i o n procedures, with or w i t h o u t n e u t r a l i z i n g Η from TPA i o n decomposition, i s c l e a r l y seen i n f i g . 6 w h i l e n o t i c e a b l e d i f f e r e n c e s i n c a t a l y t i c p r o p e r t i e s were observed (16,29). More and s l i g h t l y h e a v i e r a r o r a a t i c s were formed (see f o r instance t a b l e 2 i n r e f . 29) which c o u l d be i n t e r p r e t e d as due to an h i g h e r a c i d i t y . The c a l o r i m e t r i c c u r v e s d i d not s u p p o r t such a c o n c l u s i o n s i n c e e i t h e r t h e y were v e r y s i m i l a r or c o r r e s ponded to s l i g h t l y l e s s a c i d i c s i t e s . Jacobs et a l . (15) have s u g g e s t e d t h a t exchange by Na ions of protons formed by TPA i o n d e c o m p o s i t i o n may l i m i t the h y d r o l y s i s o f framework a l u m i n i u m under t r e a t m e n t at h i g h t e m p e r a t u r e . Such hy d r o l y s i s may t h e n e x p l a i n the c a l o r i m e t r y shape w i t h a maximum more marked i n f i g u r e 6 since d i f f u s i o n l i m i t a t i o n may be enhanced. However i t i s worth w h i l e n o t i n g t h a t the number o f s t r o n g a c i d s i t e s as c h a r a c t e r i z e d by c a l o r i m e t r y was not modified s i g n i f i c a n t l y . As c a l o r i m e t r y d i d not a l l o w t o d e t e r m i n e the n a t u r e o f the a c i d s i t e s , one cannot go f u r t h e r i n the i n t e r p r e t a t i o n a l t h o u g h the m a t e r i a l was o b v i o u s l y s 1 i g h t l y m o d i f i e d . However, i t turns out that the r o l e of Na exchange or NH^ adsorption p r i o r to a c i d i f i c a t i o n modifies the f u r t h e r exchange o f the s t a r t i n g Na i o n s , i n other words the l o c a t i o n of a c i d s i t e s . +

ESR experiments I t i s now w e l l e s t a b l i s h e d that when a surface presents e l e c t r o n donor o r e l e c t r o n a c c e p t o r s i t e s , i t i s p o s s i b l e t o i o n i z e m o l e c u l e s o f r e l a t i v e l y h i g h e l e c t r o n a f f i n i t y (> 2 eV) or low ionization potential v a l u e s , r e s u l t i n g i n paramagnetic r a d i c a l i o n s . For i n s t a n c e anthracene and perylene are e a s i l y p o s i t i v e l y i o n i z e d on a l u m i n a (7) ( I P = 7.2 and 6.8 eV r e s p e c t i v e l y ) . The a d s o r p t i o n at room temperature of benzenic s o l u t i o n of perylene, anthracene and napthalene on H-ZSM-5 and H-ZSM-11 samples h e a t e d up to 800°C p r i o r to adsorption d i d not give r i s e to the formation of the corresponding r a d i c a l c a t i o n . For samples outgassed at high



VEDRINE ET A L .

267


150H

ο Ε

\ \

100Η

\ \

Û.

ce Ο CO Ο

Catalytic Materials - American Chemical Society

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