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Characterization and Stability of Porous Structure of Titanium-Silicalite by Sorption Methods
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II
553
0 1991 Elsevier Science Publishers B.V., Amsterdam
CHARACTERI ZATI ON AND STAB1L I TY OF POROUS STRUCTURE OF T I TAN1 UM-
-SI L I CALI TE BY SORPTION METHODS G. A.
1 Genoni , M.
1 L e o f a n t i 3 , F. Zecchina .
1 Padovan ,
G.
Petrini
'Montedipe S . r . l . Units d i Ricerca d i Bollate, 20021 B o l l a t e ( M i l a n o ) , I t a l y .
1
,
G.
Trezza
2
,
Via San P i e t r o 50
'Montedipe S. r. 1. C e n t r o R i c e r c h e d i Marghera, Via d e l l a Chimica 5 30175 P o r t o Marghera ( V e n e z i a ) , I t a l y . 3 D i p a r t i m e n t o d i Chimica I n o r g a n i c a Chimica F i s i c a e d e i M a t e r i a l i Via P. G i u r i a 7, 10125 Torino, I t a l y .
SUMMARY S o r p t i o n measurements of s m a l l ( N 2 ) and l a r g e ( i .e. xylenes) p r o b e molecules t o g e t h e r w i t h s p e c t r o s c o p i c and d i f f r a c t o m e t r i c t e c h n i q u e s have been used t o c h a r a c t e r i z e t h e porous s t r u c t u r e of T i - s i l i c a l i t e b e f o r e and a f t e r t h e r m a l t r e a t m e n t s i n t h e r a n g e 823-1573K. I t i s n o t i c e a b l e t h a t N2 and p-xylene a d s o r p t i o n a r e c o r r e l a t e d t o t h e z e o l i t e framework w h i l e m-xylene u p t a k e depends on t h e c o n t e n t of T i i n t h e framework. The r e s u l t s a l s o i n d i c a t e t h a t t h e T i e x t r a c t i o n from t h e l a t t i c e d u r i n g t h e t h e r m a l t r e a t m e n t s b e g i n s b e f o r e t h e framework c o l l a p s e . INTRODUCTION During t h e l a s t p e r i o d c o n s i d e r a b l e a t t e n t i o n has been g i v e n t o Ti-silicalite,
a
zeolite
derived
from
S i with T i
s u b s t i t u t i o n o f framework
silicalite
(refs.
1-3).
by
partial
This
zeolite
e x h i b i t s v e r y v a \ u a b l e c a t a l y t i c p r o p e r t i e s f o r a v a r i e t y of r e a c t i o n s of
industrial
interest, i n
particular
for
cyclohexanone
ammoximation, phenol h y d r o x i l a t i o n and o l e f i n s e p o x i d a t i o n 4,
5).
Despite t h a t , t h e s t a b i l i t y
fundamental a s p e c t s
of i t s
of T i - s i l i c a l i t e ,
c h a r a c t e r i s t i c s and
(refs.
as w e l l
catalytic
as
beha-
v i o u r , has n o t been s t u d i e d . I n t h i s work s o r p t i o n measurements of and m-xylene) probe molecules w e r e
per-
formed t o c h a r a c t e r i z e t h e porous s t r u c t u r e of T i - s i l i c a l i t e ,
be-
s m a l l ( N 2 ) and l a r g e ( p -
f o r e and a f t e r v a r i o u s t h e r m a l t r e a t m e n t s .
The r e s u l t s ,
compared
t o t h o s e o b t a i n e d by s p e c t r o s c o p i c and d i f f r a c t o m e t r i c t e c h n i q u e s , provided information
on s t a b i l i t y
t i t a n i u m i n s e r t e d i n t h e framework.
of
both z e o l i t e
lattice
and
554 EXPERIMENTAL
Sample p r e p a r a t i o n The samples have been obtained following t h e p a t e n t l i t e r a t u r e ( r e f . 6 ) s t a r t i n g from a l k y l s i l i c a t e s and t i t a n a t e s and an aqueous s o l u t i o n of tetrapropylammonium hydroxide. A f t e r removal of t h e alcohols formed by t h e hydrolysis of S i and T i compounds w i t h t h e organic base t h e s o l u t i o n has been charged i n t o a vessel equipped with s t i r r e r and kept a t 443R f o r t h e required time. The so obtained s l u r r y has been centrifuged, t h e s o l i d c a r e f u l l y washed with water, d r i e d a t 3 9 3 R f o r 16 hours and f i n a l l y c a l c i n e d a t 023K f o r 4 hours. The r e s u l t i n g z e o l i t e sample has been t h e n part i t i o n e d i n t o equal p a r t s which were submitted to a f u r t h e r t r e a t ment i n t h e 973-1573K range f o r 4 hours. When needed, t h e samples
examined i n t h e p r e s e n t paper a r e i n d i c a t e d by TS followed by a number corresponding to t h e c a l c i n a t i o n temperature ( f o r i n s t a n c e TS1173 r e p r e s e n t s a sample calcined a t 1173K). Besides T i - s i l i c a l i t e , s i l i c a l i t e and s i l i c a samples have been used i n t h e pres e n t work as r e f e r e n c e materials. S i l i c a l i t e has been obtained following t h e above procedure, obviously a p a r t from a l k y l t i t a n a t e a d d i t i o n . Amorphous s i l i c a i s a commercial sample (Grace SG360).
-N2
adsorption N2 a d s o r p t i o n isotherms a t
77K have been
measured using a
C.
Erba SORPTOMATIC 1900 on samples outgassed a t 57313 ( 1 0 hours, final vacuum mbar). The z e o l i t i c channels volume was determined by t h e as method ( r e f . 7 ) by e x t r a p o l a t i n g t o as=O t h e l i n e a r The method has been checked by m u l t i l a y e r r e g i o n of t h e as p l o t . comparing t h e higher as region of t h e TS823 sample with t h e corresponding as region of TS393 sample s t i l l containing tetrapropylammonium hydroxide. These samples d i f f e r e x c l u s i v e l y i n t h e a v a i l a b i l i t y of z e o l i t i c channels which a r e made f r e e only under high temperature treatment. According t o t h a t , t h e corresponding as-plots show t h e same s l o p e i n t h e high as region and d i f f e r e n t o r - d i n a t e values when e x t r a p o l a t e d to a s = O ; i n p a r t i c u l a r the s t r a i g h t l i n e corresponding t o t h e sample containing t h e organic base passes through t h e o r i g i n of t h e axes. The e x t e r n a l s u r f a c e a r e a of t h e c r y s t a l s or, more exactly, of t h e primary p a r t i c l e s has been determined on t h e b a s i s of above considerations from t h e s l o p e of l i n e a r high as region, using as a r e f e r e n c e t h e d a t a of nonporous hydroxylated s i l i c a of r e f . 7.
555
Xyl enes a d s o r p t i o n P a r a and meta-Xylene a d s o r p t i o n has been performed i n a m i c r o b a l a n c e connected t o a vacuum manifold ( u l t i m a t e vacuum
Cahn lo-'
mbar). Before t h e a d s o r p t i o n measurement e a c h sample has been The t e m p e r a t u r e t r e a t e d i n s i t u a t 573K up t o c o n s t a n t weight. has been t h e n lowered t o 303K, t h e x y l e n e i s o m e r vapour dosed on t h e sample and t h e weight i n c r e a s e due t o t h e a d s o r p t i o n recorded. Xylenes TPD (Temperature programmed d e s o r p t i o n ) Before t h e measurement, each sample has been i n s e r t e d i n t o a h o l d e r connected t o a vacuum manifold ( u l t i m a t e vacuum mbar) and h e a t e d a t 573K f o r 2 hours. The h o l d e r has been t h e n c o o l e d t o r . t . , i s o l a t e d from vacuum, and a s l i g h t e x c e s s o f l i q u i d x y l e n e i n j e c t e d i n t o it. A f t e r 10 minutes t h e w e t sample has been removed, p u t i n t o a Mettler TA 3000 thermobalance and k e p t a t r . t . i n a H e f l o w ( 1 0 0 cm3 m i n - l ) f o r a w h i l e u n t i l removing t h e most x y l e n e e x c e s s . F i n a l l y t h e sample has been h e a t e d t o 673K a t 4 K min-l i n a H e f l o w and t h e weight loss r e c o r d e d . R e s u l t s have been e x p r e s s e d a s c o n v e n t i o n a l TG c u r v e s . XRD -
The X-ray
diffraction
patterns
have
been
recorded
with
a
P h i l i p s d i f f r a c t o m e t e r equipped w i t h a p r o p o r t i o n a l c o u n t e r by u s i n g a N i - f i l t e r e d CuKa r a d i a t i o n . The samples have been examined w i t h o u t any p r e v i o u s p r e t r e a t m e n t . The c r y s t a l l i n i t y d e g r e e has been d e t e r m i n e d by a p r o c e d u r e developed i n o u r l a b o r a t o r y ( r e f . 8 ) . The method i s based on a comparison of t h e i n t e g r a t e d i n t e n s i t i e s of two d i f f e r e n t s p e c t r a l ranges a f f e c t e d r e s p e c t i v e l y by t h e c r y s t a l l i n e and amorphous f r a c t i o n s of t h e s o l i d . I n t h i s mann e r t h e u s e of s t a n d a r d s w i t h a known c r y s t a l l i n i t y c a n be a v o i d The c r y s t a l l i t e s i z e has been determined from t h e h a l f h e i g h t ed. width using the Scherrer equation (ref. 8 ) a f t e r t h e c o r r e c t i o n s f o r t h e K a l , a 2 d o u b l e t and t h e i n s t r u m e n t a l broadening. I R-DRS
D i f f u s e r e f l e c t a n c e s p e c t r a have been measured between 400 and 4000 cm-' u s i n g a P e r k i n E l m e r F T - I R 1640 s p e c t r o p h o t o m e t e r equipped w i t h a d i f f u s e r e f l e c t a n c e attachment. Before e a c h measure t h e samples have been f i n e l y ground w i t h KBr. The r e s u l t s have been e x p r e s s e d i n terms of Kubelka-Munk f u n c t i o n ( r e f . 9 ) .
556 UV-Vis
DRS
D i f f u s e r e f l e c t a n c e s p e c t r a i n t h e 12500-50000 cm-' been o b t a i n e d
with a
Perkin
E l m e r LAMBDA
15
range
have
spectrophotometer
equipped w i t h a d i f f u s e r e f l e c t a n c e a t t a c h m e n t u s i n g MgO a s a r e f e r e n c e . A s p e c i a l l y d e s i g n e d quartz c e l l a l l o w e d t h e c o n n e c t i o n electric
furnace
(maximum
e l i m i n a t e t h e adsorbed 393R b e f o r e
temperature
1100K).
w a t e r , each sample
Rubelka-Munk f u n c t i o n has
In
to
order
has been o u t g a s s e d
i n t h e c a s e of been used t o e x p r e s s t h e
t h e measurement.
to
mbar) and i t s i n s e r t i o n i n a
a vacuum s y s t e m ( u l t i m a t e vacuum
Like
IR-DRS,
at the
experimental
data. RESULTS AND DISCUSSION
-N2
adsorption The channel volume of TS823 sample
was 0. 195 cm
3
g
-1
.
By
c r e a s i n g t h e t r e a t m e n t t e m p e r a t u r e up t o 1273K, o n l y a v e r y change i n N2 a d s o r p t i o n i s o t h e r m s
i s observed ( F i g . l a ,
insmall
lb).
For
higher treatment temperatures t h e z e o l i t e progressively l o s s e s i t s porosity;
at
1573K
i n i t i a l value.
A
t h e microporosity
similar
behaviour i s
is
less than shown
by
5% of the
the
external
s u r f a c e a r e a (Fig. l c ) .
y
mE 0 1 l00
.. '., .
5
O
V
z
I d, 1.0
2.0
973
1173
1373
K
Fig. 1. N2 a d s o r p t i o n a t 77K: a ) a d s o r p t i o n i s o t h e r m s (from t h e t o p t o t h e bottom: TS823, TS973, TS1173, TS1273, TS1373, TS1473, TS1573); b ) v a r i a t i o n of micropore volume w i t h t h e t r e a t m e n t t e m p e r a t u r e ; c ) v a r i a t i o n of e x t e r n a l s u r f a c e a r e a w i t h t h e treatment temperature. para-Xylene A t r. t . ,
a d s o r p t i o n and TPD t h e TS823
sample r e a d i l y
a d s o r b s p-xylene
vapours,
showing a n i s o t h e r m w e l l comparable t o t h o s e r e p o r t e d i n r e f s .
10,
11 f o r s i l i c a l i t e .
the
The adsorbed v a l u e
a t 1 mbar ( t h a t i s i n
557
l o w e r p r e s s u r e p a r t of t h e p l a t e a u r e g i o n i n o r d e r t o minimize t h e a d s o r p t i o n on t h e e x t e r n a l s u r f a c e of t h e c r y s t a l s ) c o r r e s p o n d s t o 8. 5 molecules/u. c.
As observed
for
s i l i c a l i t e (ref.
ll),
the
s o r b a t e c a n be e a s i l y removed under vacuum. The same sample p r e v i o u s l y impregnated under vacuum w i t h l i q u i d p-xylene,
g i v e s under h e a t i n g t h e
The c u r v e s show two
(complex) between 330 and 500K. l i q u i d p-xylene
TG c u r v e s r e p o r t e d i n Fig.
weight l o s s e s : one below
wetting t h e
f i r s t loss i s due t o
The
2a.
310R and t h e
external surface
of t h e
other excess
particles,
w h i l e t h e 330-500K weight loss corresponds t o p-xylene adsorbed i n the zeolite
channels.
following res ults:
These
i) in
attributions are
a TG
experiment on
microporous s i l i c a impregnated w i t h
confirmed by
the
a n amorphous,
non
l i q u i d p-xylene o n l y t h e
low
t e m p e r a t u r e weight l o s s i s observed ( t h e TG c u r v e i s c l o s e t o t h a t of t h e TS1573 sample shown i n Fig.
2 a ) ; i i ) t h e weight loss i n t h e
330-350K r a n g e (TS823 sample) corresponds t o 8. 2 molecules/u. c. of adsorbed p-xylene,
i n good agreement
d i r e c t a d s o r p t i o n measurement. techniques t h e
d a t a of
with t h e value obtained
Due t o t h e e q u i v a l e n c e of t h e
t h e adsorption
c a p a c i t y vs
the
thermal
t r e a t m e n t s have been o b t a i n e d by u s i n g t h e f a s t e r TG method 2b).
The t o t a l
amount of
so showing t h a t
c o r r e l a t e d each other, adsorption capacity capacity i s
adsorbed p-xylene and
up
found o n l y
t o 1273K. for
A
N2 a r e
(Fig. closely
t h e samples r e t a i n
their
of
this
higher
than
progressive loss
treatment temperatures
by two
1273K and goes t o z e r o a t 1 ' 3 K .
973
\
1173
1373
K
Fig. 2. p-xylene a d s o r p t i o n by TG a n a l y s i s : a ) s e l e c t e d TG c u r v e s ; - b ) v a r i a t i o n of 373 473 573 K adsorption capacity (calculated from t h e weight loss i n t h e 310-500R r a n g e and e x p r e s s e d a s volume of l i q u i d p - x y l e n e ) w i t h t h e treatment temperature. TS 1573
meta-Xylene a d s o r p t i o n and TG The b e h a v i o u r of m-xylene i s remarkably d i f f e r e n t from t h a t
of
558
t h e p-isomer. The a d s o r p t i o n i s v e r y slow ( t h e p r o c e s s i s n o t complete even a f t e r some t e n s of h o u r s ) so t h a t t h e e q u i l i b r i u m v a l u e s a r e d i f f i c u l t t o d e t e r m i n e (see a l s o r e f . 1 2 ) . D e s p i t e t h e low a c c u r a c y , t h e e q u i l i b r i u m v a l u e s seem t o be l o w e r t h a n t h o s e o b t a i n e d f o r t h e p-isomer i n agreement w i t h l i t e r a t u r e d a t a on ZSM-5 t y p e z e o l i t e s ( r e f . 13, 1 4 ) . Thermogravimetric a n a l y s i s on samples impregnated w i t h l i q u i d m-xylene g i v e s t h e TG c u r v e s shown i n Fig.
3a.
The TG c u r v e s
are
c h a r a c t e r i z e d , by a w e l l d e f i n e d weight l o s s i n t h e 370-500K i n t e r v a l ( b e s i d e s by a low t e m p e r a t u r e l o s s due t o l i q u i d l i k e m-xylene). According t o t h e p-xylene r e s u l t s , w e a t t r i b u t e t h e 370-500K w e i g h t loss t o m-xylene e n t r a p p e d i n t o T i - s i l i c a l i t e c h a n n e l s . On t h e examined samples, t h e 370-500K weight l o s s corresponds t o a value not exceeding 2 m o l e c u l e s / u . c . I f the above e x p e r i m e n t i s performed on T i f r e e s i l i c a l i t e and amorphous s i l i c a t h e r e s u l t s a r e c o m p l e t e l y d i f f e r e n t : t h e w e i g h t l o s s due t o d e s o r p t i o n of m-xylene o c c u r s c o m p l e t e l y below 320K ( t h e TG c u r v e s a r e c l o s e t o t h e c o r r e s p o n d i n g c u r v e of TS1573 sample r e p o r t e d i n Fig. 3 a ) . I n c o n c l u s i o n , under t h e adopted e x p e r i m e n t a l c o n d i t i o n s , T i - s i l i c a l i t e a d s o r b s m-xylene ( t h e adsorbed amount b e i n g however l o w e r t h a n t h a t found f o r t h e p-isomer) w h i l e s i l i c a l i t e does not. A second i m p o r t a n t o b s e r v a t i o n i s t h a t T i - s i l i c a l i t e shows, upon t h e r m a l t r e a t m e n t s , a loss of a d s o r p t i o n c a p a c i t y ( a s measured by T G ) s t a r t i n g below 1170K ( F i g . 3b). The TS1373 sample does n o t a d s o r b any more m-xylene, w h i l e it s t i l l r e t a i n s a b o u t 70% o f t h e a d s o r p t i o n c a p a c i t y towards N2 and p-xylene.
I 2ht
\
1
\ TS1573 & 373
473
573
n
973
1173
1373
K
Fig. 3. m-xylene a d s o r p t i o n by TG a n a l y s i s : a ) s e l e c t e d TG curves; b ) v a r i a t i o n of adsorption capacity (expressed a s volume of l i q u i d m-xylene) with t h e treatment temperature.
559 XRD -
The XRD p a t t e r n of t h e TS823 sample shows ( F i g . 4a) a c r i s t a l l i n i t y degree (99%). The s u r f a c e a r e a c a l c u l a t e d
high from
c r y s t a l l i t e s i z e a g r e e s w i t h t h e e x t e r n a l s u r f a c e a r e a a s measured by N2 a d s o r p t i o n ( 4 7 and 39 m2 g - l r e s p e c t i v e l y ) . Upon h e a t i n g up t o 1273K, t h e c r y s t a l l i n i t y does n o t change.
A
f u r t h e r r i s e of c a l c i n a t i o n t e m p e r a t u r e c a u s e s p r o g r e s s i v e loss i n z e o l i t e c o n t e n t ( w i t h o u t s i g n i f i c a n t change i n c r y s t a l s i z e ) a t 15733 no more r e s i d u a l z e o l i t e i s d e t e c t e d ( F i g . 4b, 4 c ) .
and The
spectrum of TS1473 sample i s a m i x t u r e of T i - s i l i c a l i t e and c r y s t o b a l i t e p a t t e r n s w h i l e i n t h e spectrum of TS1573K sample o n l y t h e c r y s t o b a l i t e i s detected. a
I
75
b
Fig. 4. XRD a n a l y s i s : a ) s e l e c ted patterns; b ) v a r i a t i o n of c r y s t a l l i n i t y degree with the treatment temperature; c ) v a r i a t i o n of c r y s t a l l i t e s i z e ( c a l c u l a t e d from two d i f f e r e n t p e a k s ) w i t h t h e t r e a t m e n t t e m p e r a t u r e . 10
IR
-
30
50
DRS
The I R s p e c t r a of T i - s i l i c a l i t e shows t w o s i g n i f i c a n t bands, t h e f i r s t a t a b o u t 550 cm-l and t h e l a t t e r a t a b o u t 970 cm-' ( F i g . 5 a ) . The 550 cm-' band i s t y p i c a l of a l l t h e ZSM-5 f a m i l y z e o l i t e s and can be r e l a t e d t o t h e five-membered r i n g system c h a r a c t e r i s t i c of t h e z e o l i t e framework ( r e f . 1 3 ) . I t s i n t e n s i t y depends on b o t h c r y s t a l l i n i t y and c r y s t a l l i t e s i z e and c a n be used t o e v a l u a t e t h e t h e r m a l s t a b i l i t y of t h e samples. I n Fig. 5b t h e r a t i o between t h e i n t e n s i t i e s of t h e band a t 550 -1 -1 cm and of t h a t a t 800 cm (common t o b o t h c r y s t a l l i n e and amorphous s a m p l e s ) i s r e p o r t e d a s a f u n c t i o n of t h e t r e a t m e n t temperature. I t c a n be n o t i c e d t h a t t h e dependance of 1550/1800
560
c
b
.-u) Y
c
U
3
1000
800
600
cr
I
913
1173
1373
K
Fig. 5. IR-DRS: a ) selected I R spectra; b) v a r i a t i o n I w i t h t h e t r e a t m e n t t e m p e r a t u r e ; c ) v a r i a t i o n of I 9 7 0 ” 800 treatment temperature. upon t e m p e r a t u r e a g r e e s w e l l w i t h t h e XRD r e s u l t s . The 9 7 0 cm-l band observed on T i - s i l i c a l i t e i s a s s o c i a t e d a s t r e t c h i n g mode of a [ T i 0 4 ] u n i t i n v o l v i n g a framework T i
with (ref.
3, 1 7 ) . This band, which i s p r e s e n t on amorphous S i 0 2 - T i 0 2 g l a s s e s 14, 15) g i v e s i n f o r m a t i o n on t h e and a b s e n t i n T i 0 2 ( r e f .
concentration
of
substitutional
Ti
(either
in
the
zeolite
framework o r i n a n amorphous p h a s e ) and t h e r e f o r e on d e t i t a n a t i o n .
In Fig. 5c t h e dependence of t h e 1970/1800 r a t i o upon t u r e treatment i s i l l u s t r a t e d .
tempera-
The
f o l l o w i n g can be n o t i c e d : i) i s n o t going t o z e r o a s t e m p e r a t u r e approaches 1573K: t h i s c a n be e x p l a i n e d w i t h t h e format i o n of a n amorphous p h a s e s t i l l c o n t a i n i n g s u b s t i t u t i o n a l t i t a nium. i i ) l i k e t h e m-xylene a d s o r p t i o n c a p a c i t y , t h e ‘970”550 r a t i o d e c r e a s e s a l r e a d y a t t e m p e r a t u r e lower t h a n 1 1 7 3 K ; this means t h a t s u b s t i t u t i o n a l T i is going i n t o extraframework p o s i t i o n b e f o r e t h e o c c u r r e n c e of d r a m a t i c changes i n sample c r y s t a l l i n i t y ratio). ( a s r e v e a l e d by XRD and 1550/1800 unlike t h e previous case, t h e r a t i o
UV-Vis
DRS
The framework t e t r a h e d r a l T i
i n pure T i - s i l i c a l i t e i s
a t e d w i t h a v e r y s t r o n g band a t 48000 cm-’
associ-
having a l i g a n d t o m e t -
a l c.t. character (ref. 3 ) . Bands a t l o w e r f r e q u e n c y ( a s found on Si02-TiOZ amorphous c o p r e c i p i t a t e ( r e f . 16, 1 7 ) ) a r e i n d i c a t i v e of non t e t r a h e d r a l T i .
As a consequence t h e DRS i s a s e n s i t i v e
tool
f o r b o t h framework and extraframework T i . However, due t o i t s high i n t e n s i t y , t h e 48000 cm-‘ band c a n n o t be used f o r q u a n t i t a t i v e de-
561
t e r m i n a t i o n of t h e framework T i . can n o t be
I n f a c t t h e Kubelka-Munk values as l o w
u t i l i s e d f o r reflectance
theory
as those
ob-
s e r v e d a t 48000 cm-'. The e v o l u t i o n of DRS spectrum of t r e a t m e n t t e m p e r a t u r e i s shown
a TS sample a s a f u n c t i o n
i n Fig.
6a.
, I n the starting
of ma-
most of t h e t i t a n i u m i s i n framework p o s i t i o n (band a t 46000 cm-') w h i l e a minor p a r t is p r e s e n t a s an a n a t a s e l i k e phase -1 (band a t 30500 cm ). By i n c r e a s i n g t h e c a l c i n a t i o n t e m p e r a t u r e
terial,
t w o new bands show up: one a t f r e q u e n c y lower t h a n t h e 30500
cm
-1
band and s t r o n g l y o v e r l a p p e d w i t h t h e a n a t a s e band and t h e o t h e r -1 a t a b o u t 40000 cm The f i r s t band c a n be a s s o c i a t e d w i t h a
.
r u t i l e l i k e phase
with i t s appearance
i n agreement
highest calcination
temperatures
(1473, 1573K)
only a t
(Fig.
the
6b).
The
second peak i s of more u n c e r t a i n a t t r i b u t i o n ; however t h e p r e s e n c e of
bands
at
similar
frequency
c o p r e c i p i t a t e s w i t h v e r y low t h a t we a r e dealing with
in
Si02-Ti02
T i concentration ( r e f .
amorphous
17)
suggests
amorphous domains c o n t a i n i n g v e r y
small
T i c l u s t e r s o r i s o l a t e d T i atoms i n o c t a h e d r a l c o o r d i n a t i o n .
The
its appearance a t moderately l o w t e m p e r a t u r e s , b e f o r e t h e c o l l a p s e of
a t t r i b u t i o n of
the
40000 cm-'
band
is also
t h e c r y s t a l l i n e s t r u c t u r e (1537K) ( F i g . 6 d ) .
confirmed
by
The p e r s i s t e n t h i g h
.15 -
.05-.
.
I
./
2.2
Y
Y
0 1 20000
40000
cm
1
Fig. 6. U V- V i s DRS: a ) U V - V i s 0 973 1173 1373 K s p e c t r a (from t h e t o p t o t h e bottom t h e d a t a of TS823, TS973, TS1173, TS1273, TS1373, TS1473, TS1573 a r e r e p o r t e d ( t h e c a p i t a l l e t t e r s i n d i c a t e t h e p o s i t i o n of t h e bands a t t r i b u t e d t o r u t i l e l i k e phase (A), a n a t a s e l i k e phase (B), isolated o r nearly isoleted o c t a h e d r a l T i (C), framework i s o l a t e d t e t r a h e d r a l T i (D)). b, c1 d ) v a r i a t i o n of i n t e n s i t i e s a t 25500, 30500 and 40000 cm r e s p e c t i v e l y with t h e treatment temperature.
562
i n t e n s i t y of t h e peak a t 48000 c m - l i n TS1573 s u g g e s t s t h a t a h i g h p e r c e n t a g e o f Ti remains i s o l a t e d and t e t r a h e d r a l l y c o o r d i n a t e d i n a S i 0 2 - T i 0 2 s o l i d s o l u t i o n even a f t e r t h e t o t a l c o l l a p s e of t h e zeolite structure. FINAL CONS1 DERATIONS
The c o l l e c t i o n of numerical t e c n i q u e s i s shown i n Tab. 1. TABLE 1
-
values
obtained
with
various
C h a r a c t e r i s t i c s of t h e s a n p l e s c a l c i n e d at various t e n p e r a t u r e s
U2 AOSOBPTIOU EXTERUAL SURFACE AREA
SAIPLE
XYLEM ADSORPTIOU PBOH FC B-XIL
XBD AUALYSIS CRYSTALLIUITY CBISTALLITB DEGBKE
n-XyL
64
IB-DRS
08-VIS DRS
Ig60 '800
'800
1.45
4.20
l'S823
,195
39
,168
,042
99
TS973
,192
37
1.354.05
34
,036 ,032
71
,188
-
97
TS1173
96
59
1.15
4.00
TS1273
,180
37
.156
,015
.953.85
TS1373
,156
34
,141
,002
81
60
TS1173
,078
16
,057
29
56
TS1573
,009
2
(.005
0
I 25500 I 30500 I 40000
.04
.17
-23
-04
.19
.26
.04
,20
.27
-04
.I7
,61
,631.60 -48 ,95
.04
.20
,08
-31
.74 .74
-
.16
-38
1.15
I n Fig. 7 t h e r e s u l t s o b t a i n e d by n o r m a l i z i n g t h e above d a t a a r e r e p o r t e d . On t h e o r d i n a t e w e have t h e RT f u n c t i o n e x p r e s s e d as: RT
=
T' v053
-
'1573 v1573
-
Fig. 7. Normalized d a t a from v a r i o u s t e c h n i q u e s vs t r e a t m e n t temperature:
RT .75
A c r y s t a l l i n i t y d e g r e e by XRD p-xylene adsorbed by TG 0 micropore volume by N2
0
adsorption
.50
/I by I R s p e c t r p s c o p y w ia?8ns!@ a t 40000 cm- by UV-Vis s p e c t r o s c o p y A I /I by I R s p e c t r o s c o p y 0 m9@le@0adsorbed by TG 0 I
.25
0
973
1173
1373
K
563
d a t a from a
where V i n d i c a t e s t h e
volume o r
t h e micropore
the
given technique ( f o r
XRD c r y s t a l l i n i t y ) ,
i n d i c a t e s t h e t e m p e r a t u r e t o which and V1573
are
the
reference
the
band
On t h e c o n t r a r y
with
latters
t h e m-xylene a d s o r p t i o n c o r r e l a t e s w e l l w i t h c m - 1 and t h e U V - V i s peak a t 40000 cm-' which T i e x t r a c t i o n from t h e framework i s
a r e s t r u c t u r a l T i parameters.
1173K and i s accompanied by
o c c u r r i n g a t t e m p e r a t u r e a s low a s small relaxation
is
data,
are s t r i c t l y correlated
This f a c t i n d i c a t e s t h a t t h e
2 a r e mainly dependent upon t h e o v e r a l l c r y s t a l l i n i t y .
t h e I R band a t 910
v~~~
it
figure
XRD and I R 550 cm-'
which a r e c r y s t a l l i n i t y p a r a m e t e r s , and p-xylene a d s o r p t i o n .
From
can be c o l l e c t e d i n t w o groups.
I t i s most remarkable t h a t t h e N
subscript
t h e d a t a i s r e f e r r e d and
values.
apparent t h a t a l l t h e parameters
instance
the
of t h e
framework
whith subsequent
change
a
the
c h a n n e l s dimensions ( w i t h o u t s i g n i f i c a n t m o d i f i c a t i o n of t h e t o t a l p o r e volume).
m-Xylene,
having
r e s p e c t t o p-xylene and N2 transformation:
a larger
k i n e t i c diameter
with
( r e f . 1 8 ) , i s a s e n s i t i v e p r o b e of t h i s
consequently
it
correlates very
well
with
the
s t r u c t u r a l T i parameters.
As a f i n a l remark w e p o i n t o u t t h a t some a s p e c t s r e l a t e d t o t h e framework/extraframework T i ( i . e . , t h e n a t u r e of t h e d e t i t a n a t i o n process
at
temperature
s t r u c t u r e of o c t a h e d r a l treatment), a r e
below
1513K
s p e c i e s of T i
not s u f f i c i e n t l y
(Fig.
7)
and
the
formed d u r i n g t h e
c l a r i f i e d and
require
exact thermal further
i n v e s t i g a t i o n s (work s t i l l i n p r o g r e s s ).
REFERENCES Perego, G. B e l l u s s i , C.Corno, M. Taramasso, F. Buonomo, A. E s p o s i t o , New Developments i n Z e o l i t e S c i e n c e and Technology, E l s e v i e r , Amsterdam, 1986, pp. 129-136. J . M . Popa, J . L . Guth, H. Kessler, Europ. Pat. 884011479, 1988. M.R. B o c c u t i , K. M. Rao, A. Zecchina, G. L e o f a n t i , G. P e t r i n i , S t r u c t u r e and r e a c t i v i t y of s u r f a c e s , E l s e v i e r , Amsterdam, 1989, pp 134-144. P. R o f f i a , G. L e o f a n t i , A. Cesana, M. Mantegazza, N. Fadovan, G. P e t r i n i , S. T o n t i , P. G e r v a s u t t i , New Developments i n S e l e c t i v e O x i d a t i o n , E l s e v i e r , Amsterdam, 1990, pp. 43-52. U. Romano, M. C l e r i c i , A. E s p o s i t o , F. Maspero, C. Neri, i b i d , pp. 33-42. M. Taramasso, B. Perego, B. N o t a r i , U S Pat. 4410501, 1983. S. J. Gregg, K.S. W. Sing, Adsorption, S u r f a c e Area and Por o s i t y , 2nd e d n . , Academic P r e s s , London, 1982. G. Trezza e t a l . , I n t e r n a l Report Montedipe 014/83, 1983; to be p r e s e n t e d of V Convegno Naz. s u "Applicazioni d e l l e Tecniche RX", Bressanone, I t a l y , 1991. G.
564
9
10
11 12
13 14 15 16
17
18
(Ed. ), C h a r a c t e r i s a t i o n of R. A. Schoonheydt i n F. Delannay 1984, pp. Heterogeneous C a t . , M. Dekker I n c . , New York, 125- 160. Proc. I n t . Syrnp. Z e o l i t e C a t a l y s i s , V.S. Nayak, L. R i e k e r t , S i o f o l k , Hungary, 1985, pp. 157-166. D. H. Olson, G. T. K o k o t a i l o , S. L. Lawton, J. Phys. Chem., 85, 1981, pp. 2238-2243. R. L e Van Mao, 0. P i l a t i , A. Marzi, G. L e o f a n t i , A. V i l l a , V. R a g a i n i , React. K i n e t . C a t a l . L e t t . , 15, 1980, pp. 293-297. P. J a c o b s , J. Valyon, H.K. Beyer, Zeolites, 1, 1981, pp. 1 6 1- 1 6 4 . P. Wu, A. Debebe, Y. Hvama, Z e o l i t e s , 3, 1983, pp. 118-122. M.F. B e s t , R.A. Condrate, J. M a t . S c i . L e t t e r s , 4, 1985, 994. A. Fernandez, J. L e y r e r , A. R. Gonzalez-Elipe, G. Manuera, Knozinger, 3. C a t a l . , 1 1 2 , 1988, pp. 489-494. A. Zecchina, S u b m i t t e d t o I n t . Syrnp. on Z e o l i t e C h e m i s t r y and C a t a l y s i s , Prague, Cz echos 1o v a k i a , 19 9 1. Y.H. M a , T.D. Tang, L . B . Sand, L . Y . HOU, New Developments in Zeolite S c i e n c e and Technology, E l s e v i e r , Amsterdam, 1986, pp. 531-538.
553
0 1991 Elsevier Science Publishers B.V., Amsterdam
CHARACTERI ZATI ON AND STAB1L I TY OF POROUS STRUCTURE OF T I TAN1 UM-
-SI L I CALI TE BY SORPTION METHODS G. A.
1 Genoni , M.
1 L e o f a n t i 3 , F. Zecchina .
1 Padovan ,
G.
Petrini
'Montedipe S . r . l . Units d i Ricerca d i Bollate, 20021 B o l l a t e ( M i l a n o ) , I t a l y .
1
,
G.
Trezza
2
,
Via San P i e t r o 50
'Montedipe S. r. 1. C e n t r o R i c e r c h e d i Marghera, Via d e l l a Chimica 5 30175 P o r t o Marghera ( V e n e z i a ) , I t a l y . 3 D i p a r t i m e n t o d i Chimica I n o r g a n i c a Chimica F i s i c a e d e i M a t e r i a l i Via P. G i u r i a 7, 10125 Torino, I t a l y .
SUMMARY S o r p t i o n measurements of s m a l l ( N 2 ) and l a r g e ( i .e. xylenes) p r o b e molecules t o g e t h e r w i t h s p e c t r o s c o p i c and d i f f r a c t o m e t r i c t e c h n i q u e s have been used t o c h a r a c t e r i z e t h e porous s t r u c t u r e of T i - s i l i c a l i t e b e f o r e and a f t e r t h e r m a l t r e a t m e n t s i n t h e r a n g e 823-1573K. I t i s n o t i c e a b l e t h a t N2 and p-xylene a d s o r p t i o n a r e c o r r e l a t e d t o t h e z e o l i t e framework w h i l e m-xylene u p t a k e depends on t h e c o n t e n t of T i i n t h e framework. The r e s u l t s a l s o i n d i c a t e t h a t t h e T i e x t r a c t i o n from t h e l a t t i c e d u r i n g t h e t h e r m a l t r e a t m e n t s b e g i n s b e f o r e t h e framework c o l l a p s e . INTRODUCTION During t h e l a s t p e r i o d c o n s i d e r a b l e a t t e n t i o n has been g i v e n t o Ti-silicalite,
a
zeolite
derived
from
S i with T i
s u b s t i t u t i o n o f framework
silicalite
(refs.
1-3).
by
partial
This
zeolite
e x h i b i t s v e r y v a \ u a b l e c a t a l y t i c p r o p e r t i e s f o r a v a r i e t y of r e a c t i o n s of
industrial
interest, i n
particular
for
cyclohexanone
ammoximation, phenol h y d r o x i l a t i o n and o l e f i n s e p o x i d a t i o n 4,
5).
Despite t h a t , t h e s t a b i l i t y
fundamental a s p e c t s
of i t s
of T i - s i l i c a l i t e ,
c h a r a c t e r i s t i c s and
(refs.
as w e l l
catalytic
as
beha-
v i o u r , has n o t been s t u d i e d . I n t h i s work s o r p t i o n measurements of and m-xylene) probe molecules w e r e
per-
formed t o c h a r a c t e r i z e t h e porous s t r u c t u r e of T i - s i l i c a l i t e ,
be-
s m a l l ( N 2 ) and l a r g e ( p -
f o r e and a f t e r v a r i o u s t h e r m a l t r e a t m e n t s .
The r e s u l t s ,
compared
t o t h o s e o b t a i n e d by s p e c t r o s c o p i c and d i f f r a c t o m e t r i c t e c h n i q u e s , provided information
on s t a b i l i t y
t i t a n i u m i n s e r t e d i n t h e framework.
of
both z e o l i t e
lattice
and
554 EXPERIMENTAL
Sample p r e p a r a t i o n The samples have been obtained following t h e p a t e n t l i t e r a t u r e ( r e f . 6 ) s t a r t i n g from a l k y l s i l i c a t e s and t i t a n a t e s and an aqueous s o l u t i o n of tetrapropylammonium hydroxide. A f t e r removal of t h e alcohols formed by t h e hydrolysis of S i and T i compounds w i t h t h e organic base t h e s o l u t i o n has been charged i n t o a vessel equipped with s t i r r e r and kept a t 443R f o r t h e required time. The so obtained s l u r r y has been centrifuged, t h e s o l i d c a r e f u l l y washed with water, d r i e d a t 3 9 3 R f o r 16 hours and f i n a l l y c a l c i n e d a t 023K f o r 4 hours. The r e s u l t i n g z e o l i t e sample has been t h e n part i t i o n e d i n t o equal p a r t s which were submitted to a f u r t h e r t r e a t ment i n t h e 973-1573K range f o r 4 hours. When needed, t h e samples
examined i n t h e p r e s e n t paper a r e i n d i c a t e d by TS followed by a number corresponding to t h e c a l c i n a t i o n temperature ( f o r i n s t a n c e TS1173 r e p r e s e n t s a sample calcined a t 1173K). Besides T i - s i l i c a l i t e , s i l i c a l i t e and s i l i c a samples have been used i n t h e pres e n t work as r e f e r e n c e materials. S i l i c a l i t e has been obtained following t h e above procedure, obviously a p a r t from a l k y l t i t a n a t e a d d i t i o n . Amorphous s i l i c a i s a commercial sample (Grace SG360).
-N2
adsorption N2 a d s o r p t i o n isotherms a t
77K have been
measured using a
C.
Erba SORPTOMATIC 1900 on samples outgassed a t 57313 ( 1 0 hours, final vacuum mbar). The z e o l i t i c channels volume was determined by t h e as method ( r e f . 7 ) by e x t r a p o l a t i n g t o as=O t h e l i n e a r The method has been checked by m u l t i l a y e r r e g i o n of t h e as p l o t . comparing t h e higher as region of t h e TS823 sample with t h e corresponding as region of TS393 sample s t i l l containing tetrapropylammonium hydroxide. These samples d i f f e r e x c l u s i v e l y i n t h e a v a i l a b i l i t y of z e o l i t i c channels which a r e made f r e e only under high temperature treatment. According t o t h a t , t h e corresponding as-plots show t h e same s l o p e i n t h e high as region and d i f f e r e n t o r - d i n a t e values when e x t r a p o l a t e d to a s = O ; i n p a r t i c u l a r the s t r a i g h t l i n e corresponding t o t h e sample containing t h e organic base passes through t h e o r i g i n of t h e axes. The e x t e r n a l s u r f a c e a r e a of t h e c r y s t a l s or, more exactly, of t h e primary p a r t i c l e s has been determined on t h e b a s i s of above considerations from t h e s l o p e of l i n e a r high as region, using as a r e f e r e n c e t h e d a t a of nonporous hydroxylated s i l i c a of r e f . 7.
555
Xyl enes a d s o r p t i o n P a r a and meta-Xylene a d s o r p t i o n has been performed i n a m i c r o b a l a n c e connected t o a vacuum manifold ( u l t i m a t e vacuum
Cahn lo-'
mbar). Before t h e a d s o r p t i o n measurement e a c h sample has been The t e m p e r a t u r e t r e a t e d i n s i t u a t 573K up t o c o n s t a n t weight. has been t h e n lowered t o 303K, t h e x y l e n e i s o m e r vapour dosed on t h e sample and t h e weight i n c r e a s e due t o t h e a d s o r p t i o n recorded. Xylenes TPD (Temperature programmed d e s o r p t i o n ) Before t h e measurement, each sample has been i n s e r t e d i n t o a h o l d e r connected t o a vacuum manifold ( u l t i m a t e vacuum mbar) and h e a t e d a t 573K f o r 2 hours. The h o l d e r has been t h e n c o o l e d t o r . t . , i s o l a t e d from vacuum, and a s l i g h t e x c e s s o f l i q u i d x y l e n e i n j e c t e d i n t o it. A f t e r 10 minutes t h e w e t sample has been removed, p u t i n t o a Mettler TA 3000 thermobalance and k e p t a t r . t . i n a H e f l o w ( 1 0 0 cm3 m i n - l ) f o r a w h i l e u n t i l removing t h e most x y l e n e e x c e s s . F i n a l l y t h e sample has been h e a t e d t o 673K a t 4 K min-l i n a H e f l o w and t h e weight loss r e c o r d e d . R e s u l t s have been e x p r e s s e d a s c o n v e n t i o n a l TG c u r v e s . XRD -
The X-ray
diffraction
patterns
have
been
recorded
with
a
P h i l i p s d i f f r a c t o m e t e r equipped w i t h a p r o p o r t i o n a l c o u n t e r by u s i n g a N i - f i l t e r e d CuKa r a d i a t i o n . The samples have been examined w i t h o u t any p r e v i o u s p r e t r e a t m e n t . The c r y s t a l l i n i t y d e g r e e has been d e t e r m i n e d by a p r o c e d u r e developed i n o u r l a b o r a t o r y ( r e f . 8 ) . The method i s based on a comparison of t h e i n t e g r a t e d i n t e n s i t i e s of two d i f f e r e n t s p e c t r a l ranges a f f e c t e d r e s p e c t i v e l y by t h e c r y s t a l l i n e and amorphous f r a c t i o n s of t h e s o l i d . I n t h i s mann e r t h e u s e of s t a n d a r d s w i t h a known c r y s t a l l i n i t y c a n be a v o i d The c r y s t a l l i t e s i z e has been determined from t h e h a l f h e i g h t ed. width using the Scherrer equation (ref. 8 ) a f t e r t h e c o r r e c t i o n s f o r t h e K a l , a 2 d o u b l e t and t h e i n s t r u m e n t a l broadening. I R-DRS
D i f f u s e r e f l e c t a n c e s p e c t r a have been measured between 400 and 4000 cm-' u s i n g a P e r k i n E l m e r F T - I R 1640 s p e c t r o p h o t o m e t e r equipped w i t h a d i f f u s e r e f l e c t a n c e attachment. Before e a c h measure t h e samples have been f i n e l y ground w i t h KBr. The r e s u l t s have been e x p r e s s e d i n terms of Kubelka-Munk f u n c t i o n ( r e f . 9 ) .
556 UV-Vis
DRS
D i f f u s e r e f l e c t a n c e s p e c t r a i n t h e 12500-50000 cm-' been o b t a i n e d
with a
Perkin
E l m e r LAMBDA
15
range
have
spectrophotometer
equipped w i t h a d i f f u s e r e f l e c t a n c e a t t a c h m e n t u s i n g MgO a s a r e f e r e n c e . A s p e c i a l l y d e s i g n e d quartz c e l l a l l o w e d t h e c o n n e c t i o n electric
furnace
(maximum
e l i m i n a t e t h e adsorbed 393R b e f o r e
temperature
1100K).
w a t e r , each sample
Rubelka-Munk f u n c t i o n has
In
to
order
has been o u t g a s s e d
i n t h e c a s e of been used t o e x p r e s s t h e
t h e measurement.
to
mbar) and i t s i n s e r t i o n i n a
a vacuum s y s t e m ( u l t i m a t e vacuum
Like
IR-DRS,
at the
experimental
data. RESULTS AND DISCUSSION
-N2
adsorption The channel volume of TS823 sample
was 0. 195 cm
3
g
-1
.
By
c r e a s i n g t h e t r e a t m e n t t e m p e r a t u r e up t o 1273K, o n l y a v e r y change i n N2 a d s o r p t i o n i s o t h e r m s
i s observed ( F i g . l a ,
insmall
lb).
For
higher treatment temperatures t h e z e o l i t e progressively l o s s e s i t s porosity;
at
1573K
i n i t i a l value.
A
t h e microporosity
similar
behaviour i s
is
less than shown
by
5% of the
the
external
s u r f a c e a r e a (Fig. l c ) .
y
mE 0 1 l00
.. '., .
5
O
V
z
I d, 1.0
2.0
973
1173
1373
K
Fig. 1. N2 a d s o r p t i o n a t 77K: a ) a d s o r p t i o n i s o t h e r m s (from t h e t o p t o t h e bottom: TS823, TS973, TS1173, TS1273, TS1373, TS1473, TS1573); b ) v a r i a t i o n of micropore volume w i t h t h e t r e a t m e n t t e m p e r a t u r e ; c ) v a r i a t i o n of e x t e r n a l s u r f a c e a r e a w i t h t h e treatment temperature. para-Xylene A t r. t . ,
a d s o r p t i o n and TPD t h e TS823
sample r e a d i l y
a d s o r b s p-xylene
vapours,
showing a n i s o t h e r m w e l l comparable t o t h o s e r e p o r t e d i n r e f s .
10,
11 f o r s i l i c a l i t e .
the
The adsorbed v a l u e
a t 1 mbar ( t h a t i s i n
557
l o w e r p r e s s u r e p a r t of t h e p l a t e a u r e g i o n i n o r d e r t o minimize t h e a d s o r p t i o n on t h e e x t e r n a l s u r f a c e of t h e c r y s t a l s ) c o r r e s p o n d s t o 8. 5 molecules/u. c.
As observed
for
s i l i c a l i t e (ref.
ll),
the
s o r b a t e c a n be e a s i l y removed under vacuum. The same sample p r e v i o u s l y impregnated under vacuum w i t h l i q u i d p-xylene,
g i v e s under h e a t i n g t h e
The c u r v e s show two
(complex) between 330 and 500K. l i q u i d p-xylene
TG c u r v e s r e p o r t e d i n Fig.
weight l o s s e s : one below
wetting t h e
f i r s t loss i s due t o
The
2a.
310R and t h e
external surface
of t h e
other excess
particles,
w h i l e t h e 330-500K weight loss corresponds t o p-xylene adsorbed i n the zeolite
channels.
following res ults:
These
i) in
attributions are
a TG
experiment on
microporous s i l i c a impregnated w i t h
confirmed by
the
a n amorphous,
non
l i q u i d p-xylene o n l y t h e
low
t e m p e r a t u r e weight l o s s i s observed ( t h e TG c u r v e i s c l o s e t o t h a t of t h e TS1573 sample shown i n Fig.
2 a ) ; i i ) t h e weight loss i n t h e
330-350K r a n g e (TS823 sample) corresponds t o 8. 2 molecules/u. c. of adsorbed p-xylene,
i n good agreement
d i r e c t a d s o r p t i o n measurement. techniques t h e
d a t a of
with t h e value obtained
Due t o t h e e q u i v a l e n c e of t h e
t h e adsorption
c a p a c i t y vs
the
thermal
t r e a t m e n t s have been o b t a i n e d by u s i n g t h e f a s t e r TG method 2b).
The t o t a l
amount of
so showing t h a t
c o r r e l a t e d each other, adsorption capacity capacity i s
adsorbed p-xylene and
up
found o n l y
t o 1273K. for
A
N2 a r e
(Fig. closely
t h e samples r e t a i n
their
of
this
higher
than
progressive loss
treatment temperatures
by two
1273K and goes t o z e r o a t 1 ' 3 K .
973
\
1173
1373
K
Fig. 2. p-xylene a d s o r p t i o n by TG a n a l y s i s : a ) s e l e c t e d TG c u r v e s ; - b ) v a r i a t i o n of 373 473 573 K adsorption capacity (calculated from t h e weight loss i n t h e 310-500R r a n g e and e x p r e s s e d a s volume of l i q u i d p - x y l e n e ) w i t h t h e treatment temperature. TS 1573
meta-Xylene a d s o r p t i o n and TG The b e h a v i o u r of m-xylene i s remarkably d i f f e r e n t from t h a t
of
558
t h e p-isomer. The a d s o r p t i o n i s v e r y slow ( t h e p r o c e s s i s n o t complete even a f t e r some t e n s of h o u r s ) so t h a t t h e e q u i l i b r i u m v a l u e s a r e d i f f i c u l t t o d e t e r m i n e (see a l s o r e f . 1 2 ) . D e s p i t e t h e low a c c u r a c y , t h e e q u i l i b r i u m v a l u e s seem t o be l o w e r t h a n t h o s e o b t a i n e d f o r t h e p-isomer i n agreement w i t h l i t e r a t u r e d a t a on ZSM-5 t y p e z e o l i t e s ( r e f . 13, 1 4 ) . Thermogravimetric a n a l y s i s on samples impregnated w i t h l i q u i d m-xylene g i v e s t h e TG c u r v e s shown i n Fig.
3a.
The TG c u r v e s
are
c h a r a c t e r i z e d , by a w e l l d e f i n e d weight l o s s i n t h e 370-500K i n t e r v a l ( b e s i d e s by a low t e m p e r a t u r e l o s s due t o l i q u i d l i k e m-xylene). According t o t h e p-xylene r e s u l t s , w e a t t r i b u t e t h e 370-500K w e i g h t loss t o m-xylene e n t r a p p e d i n t o T i - s i l i c a l i t e c h a n n e l s . On t h e examined samples, t h e 370-500K weight l o s s corresponds t o a value not exceeding 2 m o l e c u l e s / u . c . I f the above e x p e r i m e n t i s performed on T i f r e e s i l i c a l i t e and amorphous s i l i c a t h e r e s u l t s a r e c o m p l e t e l y d i f f e r e n t : t h e w e i g h t l o s s due t o d e s o r p t i o n of m-xylene o c c u r s c o m p l e t e l y below 320K ( t h e TG c u r v e s a r e c l o s e t o t h e c o r r e s p o n d i n g c u r v e of TS1573 sample r e p o r t e d i n Fig. 3 a ) . I n c o n c l u s i o n , under t h e adopted e x p e r i m e n t a l c o n d i t i o n s , T i - s i l i c a l i t e a d s o r b s m-xylene ( t h e adsorbed amount b e i n g however l o w e r t h a n t h a t found f o r t h e p-isomer) w h i l e s i l i c a l i t e does not. A second i m p o r t a n t o b s e r v a t i o n i s t h a t T i - s i l i c a l i t e shows, upon t h e r m a l t r e a t m e n t s , a loss of a d s o r p t i o n c a p a c i t y ( a s measured by T G ) s t a r t i n g below 1170K ( F i g . 3b). The TS1373 sample does n o t a d s o r b any more m-xylene, w h i l e it s t i l l r e t a i n s a b o u t 70% o f t h e a d s o r p t i o n c a p a c i t y towards N2 and p-xylene.
I 2ht
\
1
\ TS1573 & 373
473
573
n
973
1173
1373
K
Fig. 3. m-xylene a d s o r p t i o n by TG a n a l y s i s : a ) s e l e c t e d TG curves; b ) v a r i a t i o n of adsorption capacity (expressed a s volume of l i q u i d m-xylene) with t h e treatment temperature.
559 XRD -
The XRD p a t t e r n of t h e TS823 sample shows ( F i g . 4a) a c r i s t a l l i n i t y degree (99%). The s u r f a c e a r e a c a l c u l a t e d
high from
c r y s t a l l i t e s i z e a g r e e s w i t h t h e e x t e r n a l s u r f a c e a r e a a s measured by N2 a d s o r p t i o n ( 4 7 and 39 m2 g - l r e s p e c t i v e l y ) . Upon h e a t i n g up t o 1273K, t h e c r y s t a l l i n i t y does n o t change.
A
f u r t h e r r i s e of c a l c i n a t i o n t e m p e r a t u r e c a u s e s p r o g r e s s i v e loss i n z e o l i t e c o n t e n t ( w i t h o u t s i g n i f i c a n t change i n c r y s t a l s i z e ) a t 15733 no more r e s i d u a l z e o l i t e i s d e t e c t e d ( F i g . 4b, 4 c ) .
and The
spectrum of TS1473 sample i s a m i x t u r e of T i - s i l i c a l i t e and c r y s t o b a l i t e p a t t e r n s w h i l e i n t h e spectrum of TS1573K sample o n l y t h e c r y s t o b a l i t e i s detected. a
I
75
b
Fig. 4. XRD a n a l y s i s : a ) s e l e c ted patterns; b ) v a r i a t i o n of c r y s t a l l i n i t y degree with the treatment temperature; c ) v a r i a t i o n of c r y s t a l l i t e s i z e ( c a l c u l a t e d from two d i f f e r e n t p e a k s ) w i t h t h e t r e a t m e n t t e m p e r a t u r e . 10
IR
-
30
50
DRS
The I R s p e c t r a of T i - s i l i c a l i t e shows t w o s i g n i f i c a n t bands, t h e f i r s t a t a b o u t 550 cm-l and t h e l a t t e r a t a b o u t 970 cm-' ( F i g . 5 a ) . The 550 cm-' band i s t y p i c a l of a l l t h e ZSM-5 f a m i l y z e o l i t e s and can be r e l a t e d t o t h e five-membered r i n g system c h a r a c t e r i s t i c of t h e z e o l i t e framework ( r e f . 1 3 ) . I t s i n t e n s i t y depends on b o t h c r y s t a l l i n i t y and c r y s t a l l i t e s i z e and c a n be used t o e v a l u a t e t h e t h e r m a l s t a b i l i t y of t h e samples. I n Fig. 5b t h e r a t i o between t h e i n t e n s i t i e s of t h e band a t 550 -1 -1 cm and of t h a t a t 800 cm (common t o b o t h c r y s t a l l i n e and amorphous s a m p l e s ) i s r e p o r t e d a s a f u n c t i o n of t h e t r e a t m e n t temperature. I t c a n be n o t i c e d t h a t t h e dependance of 1550/1800
560
c
b
.-u) Y
c
U
3
1000
800
600
cr
I
913
1173
1373
K
Fig. 5. IR-DRS: a ) selected I R spectra; b) v a r i a t i o n I w i t h t h e t r e a t m e n t t e m p e r a t u r e ; c ) v a r i a t i o n of I 9 7 0 ” 800 treatment temperature. upon t e m p e r a t u r e a g r e e s w e l l w i t h t h e XRD r e s u l t s . The 9 7 0 cm-l band observed on T i - s i l i c a l i t e i s a s s o c i a t e d a s t r e t c h i n g mode of a [ T i 0 4 ] u n i t i n v o l v i n g a framework T i
with (ref.
3, 1 7 ) . This band, which i s p r e s e n t on amorphous S i 0 2 - T i 0 2 g l a s s e s 14, 15) g i v e s i n f o r m a t i o n on t h e and a b s e n t i n T i 0 2 ( r e f .
concentration
of
substitutional
Ti
(either
in
the
zeolite
framework o r i n a n amorphous p h a s e ) and t h e r e f o r e on d e t i t a n a t i o n .
In Fig. 5c t h e dependence of t h e 1970/1800 r a t i o upon t u r e treatment i s i l l u s t r a t e d .
tempera-
The
f o l l o w i n g can be n o t i c e d : i) i s n o t going t o z e r o a s t e m p e r a t u r e approaches 1573K: t h i s c a n be e x p l a i n e d w i t h t h e format i o n of a n amorphous p h a s e s t i l l c o n t a i n i n g s u b s t i t u t i o n a l t i t a nium. i i ) l i k e t h e m-xylene a d s o r p t i o n c a p a c i t y , t h e ‘970”550 r a t i o d e c r e a s e s a l r e a d y a t t e m p e r a t u r e lower t h a n 1 1 7 3 K ; this means t h a t s u b s t i t u t i o n a l T i is going i n t o extraframework p o s i t i o n b e f o r e t h e o c c u r r e n c e of d r a m a t i c changes i n sample c r y s t a l l i n i t y ratio). ( a s r e v e a l e d by XRD and 1550/1800 unlike t h e previous case, t h e r a t i o
UV-Vis
DRS
The framework t e t r a h e d r a l T i
i n pure T i - s i l i c a l i t e i s
a t e d w i t h a v e r y s t r o n g band a t 48000 cm-’
associ-
having a l i g a n d t o m e t -
a l c.t. character (ref. 3 ) . Bands a t l o w e r f r e q u e n c y ( a s found on Si02-TiOZ amorphous c o p r e c i p i t a t e ( r e f . 16, 1 7 ) ) a r e i n d i c a t i v e of non t e t r a h e d r a l T i .
As a consequence t h e DRS i s a s e n s i t i v e
tool
f o r b o t h framework and extraframework T i . However, due t o i t s high i n t e n s i t y , t h e 48000 cm-‘ band c a n n o t be used f o r q u a n t i t a t i v e de-
561
t e r m i n a t i o n of t h e framework T i . can n o t be
I n f a c t t h e Kubelka-Munk values as l o w
u t i l i s e d f o r reflectance
theory
as those
ob-
s e r v e d a t 48000 cm-'. The e v o l u t i o n of DRS spectrum of t r e a t m e n t t e m p e r a t u r e i s shown
a TS sample a s a f u n c t i o n
i n Fig.
6a.
, I n the starting
of ma-
most of t h e t i t a n i u m i s i n framework p o s i t i o n (band a t 46000 cm-') w h i l e a minor p a r t is p r e s e n t a s an a n a t a s e l i k e phase -1 (band a t 30500 cm ). By i n c r e a s i n g t h e c a l c i n a t i o n t e m p e r a t u r e
terial,
t w o new bands show up: one a t f r e q u e n c y lower t h a n t h e 30500
cm
-1
band and s t r o n g l y o v e r l a p p e d w i t h t h e a n a t a s e band and t h e o t h e r -1 a t a b o u t 40000 cm The f i r s t band c a n be a s s o c i a t e d w i t h a
.
r u t i l e l i k e phase
with i t s appearance
i n agreement
highest calcination
temperatures
(1473, 1573K)
only a t
(Fig.
the
6b).
The
second peak i s of more u n c e r t a i n a t t r i b u t i o n ; however t h e p r e s e n c e of
bands
at
similar
frequency
c o p r e c i p i t a t e s w i t h v e r y low t h a t we a r e dealing with
in
Si02-Ti02
T i concentration ( r e f .
amorphous
17)
suggests
amorphous domains c o n t a i n i n g v e r y
small
T i c l u s t e r s o r i s o l a t e d T i atoms i n o c t a h e d r a l c o o r d i n a t i o n .
The
its appearance a t moderately l o w t e m p e r a t u r e s , b e f o r e t h e c o l l a p s e of
a t t r i b u t i o n of
the
40000 cm-'
band
is also
t h e c r y s t a l l i n e s t r u c t u r e (1537K) ( F i g . 6 d ) .
confirmed
by
The p e r s i s t e n t h i g h
.15 -
.05-.
.
I
./
2.2
Y
Y
0 1 20000
40000
cm
1
Fig. 6. U V- V i s DRS: a ) U V - V i s 0 973 1173 1373 K s p e c t r a (from t h e t o p t o t h e bottom t h e d a t a of TS823, TS973, TS1173, TS1273, TS1373, TS1473, TS1573 a r e r e p o r t e d ( t h e c a p i t a l l e t t e r s i n d i c a t e t h e p o s i t i o n of t h e bands a t t r i b u t e d t o r u t i l e l i k e phase (A), a n a t a s e l i k e phase (B), isolated o r nearly isoleted o c t a h e d r a l T i (C), framework i s o l a t e d t e t r a h e d r a l T i (D)). b, c1 d ) v a r i a t i o n of i n t e n s i t i e s a t 25500, 30500 and 40000 cm r e s p e c t i v e l y with t h e treatment temperature.
562
i n t e n s i t y of t h e peak a t 48000 c m - l i n TS1573 s u g g e s t s t h a t a h i g h p e r c e n t a g e o f Ti remains i s o l a t e d and t e t r a h e d r a l l y c o o r d i n a t e d i n a S i 0 2 - T i 0 2 s o l i d s o l u t i o n even a f t e r t h e t o t a l c o l l a p s e of t h e zeolite structure. FINAL CONS1 DERATIONS
The c o l l e c t i o n of numerical t e c n i q u e s i s shown i n Tab. 1. TABLE 1
-
values
obtained
with
various
C h a r a c t e r i s t i c s of t h e s a n p l e s c a l c i n e d at various t e n p e r a t u r e s
U2 AOSOBPTIOU EXTERUAL SURFACE AREA
SAIPLE
XYLEM ADSORPTIOU PBOH FC B-XIL
XBD AUALYSIS CRYSTALLIUITY CBISTALLITB DEGBKE
n-XyL
64
IB-DRS
08-VIS DRS
Ig60 '800
'800
1.45
4.20
l'S823
,195
39
,168
,042
99
TS973
,192
37
1.354.05
34
,036 ,032
71
,188
-
97
TS1173
96
59
1.15
4.00
TS1273
,180
37
.156
,015
.953.85
TS1373
,156
34
,141
,002
81
60
TS1173
,078
16
,057
29
56
TS1573
,009
2
(.005
0
I 25500 I 30500 I 40000
.04
.17
-23
-04
.19
.26
.04
,20
.27
-04
.I7
,61
,631.60 -48 ,95
.04
.20
,08
-31
.74 .74
-
.16
-38
1.15
I n Fig. 7 t h e r e s u l t s o b t a i n e d by n o r m a l i z i n g t h e above d a t a a r e r e p o r t e d . On t h e o r d i n a t e w e have t h e RT f u n c t i o n e x p r e s s e d as: RT
=
T' v053
-
'1573 v1573
-
Fig. 7. Normalized d a t a from v a r i o u s t e c h n i q u e s vs t r e a t m e n t temperature:
RT .75
A c r y s t a l l i n i t y d e g r e e by XRD p-xylene adsorbed by TG 0 micropore volume by N2
0
adsorption
.50
/I by I R s p e c t r p s c o p y w ia?8ns!@ a t 40000 cm- by UV-Vis s p e c t r o s c o p y A I /I by I R s p e c t r o s c o p y 0 m9@le@0adsorbed by TG 0 I
.25
0
973
1173
1373
K
563
d a t a from a
where V i n d i c a t e s t h e
volume o r
t h e micropore
the
given technique ( f o r
XRD c r y s t a l l i n i t y ) ,
i n d i c a t e s t h e t e m p e r a t u r e t o which and V1573
are
the
reference
the
band
On t h e c o n t r a r y
with
latters
t h e m-xylene a d s o r p t i o n c o r r e l a t e s w e l l w i t h c m - 1 and t h e U V - V i s peak a t 40000 cm-' which T i e x t r a c t i o n from t h e framework i s
a r e s t r u c t u r a l T i parameters.
1173K and i s accompanied by
o c c u r r i n g a t t e m p e r a t u r e a s low a s small relaxation
is
data,
are s t r i c t l y correlated
This f a c t i n d i c a t e s t h a t t h e
2 a r e mainly dependent upon t h e o v e r a l l c r y s t a l l i n i t y .
t h e I R band a t 910
v~~~
it
figure
XRD and I R 550 cm-'
which a r e c r y s t a l l i n i t y p a r a m e t e r s , and p-xylene a d s o r p t i o n .
From
can be c o l l e c t e d i n t w o groups.
I t i s most remarkable t h a t t h e N
subscript
t h e d a t a i s r e f e r r e d and
values.
apparent t h a t a l l t h e parameters
instance
the
of t h e
framework
whith subsequent
change
a
the
c h a n n e l s dimensions ( w i t h o u t s i g n i f i c a n t m o d i f i c a t i o n of t h e t o t a l p o r e volume).
m-Xylene,
having
r e s p e c t t o p-xylene and N2 transformation:
a larger
k i n e t i c diameter
with
( r e f . 1 8 ) , i s a s e n s i t i v e p r o b e of t h i s
consequently
it
correlates very
well
with
the
s t r u c t u r a l T i parameters.
As a f i n a l remark w e p o i n t o u t t h a t some a s p e c t s r e l a t e d t o t h e framework/extraframework T i ( i . e . , t h e n a t u r e of t h e d e t i t a n a t i o n process
at
temperature
s t r u c t u r e of o c t a h e d r a l treatment), a r e
below
1513K
s p e c i e s of T i
not s u f f i c i e n t l y
(Fig.
7)
and
the
formed d u r i n g t h e
c l a r i f i e d and
require
exact thermal further
i n v e s t i g a t i o n s (work s t i l l i n p r o g r e s s ).
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