- Email: [email protected]
Formation of Carbocations from C6 Compounds in Zeolites
H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
FORMATION OF CARBOCATIONS FROM C,
COMPOUNDS I N ZEOLITES
Imre K i r i c s i ' , H o r s t F o r s t e r 2 and Gyula T a s i ' ' A p p l i e d Chemistry Department, J o z s e f A t t i l a U n i v e r s i t y , R e r r i c h B. t e r 1, H-6720 Szeged, Hungary ' I n s t i t u t e o f P h y s i c a l Chemistry, U n i v e r s i t y o f Hamburg, Bundesstr. 45, 0-2000 Hamburg 13, Federal R e p u b l i c o f Germany ABSTRACT Formation o f u n s a t u r a t e d carbenium i o n s f r o m 1-hexene, cyclohexane, c y c l o hexene, cyclohexadiene and benzene upon i n t e r a c t i o n w i t h t h e H-forms o f z e o l i t e s ZSM-5 and Y was p r o v e d by U V - V I S and I R spectroscopy. W i t h t h e e x c e p t i o n o f benzene, which forms d i e n y l i o n s a t t h e b e g i n n i n g , c a r b o c a t i o n f o r m a t i o n from t h e o t h e r C, compounds s t a r t s w i t h monoenylic species, which t r a n s f o r m i n t o o l i g o e n y l i c i o n s w i t h t i m e o f c o n t a c t . From the c y c l i c hydrocarbons c y c l i c i o n s as w e l l as open-chain a l k e n y l i o n s a r e formed. The i o n f o r m a t i o n c a p a b i l i t y decreases i n t h e sequence c y c l o h e x a d i e n e > c y c l o h e x ene > 1-hexene > cyclohexane > benzene. F o r t h e f i r s t stages o f carbenium i o n development t h e f o r m a t i o n o f a r o m a t i c s u r f a c e species may be excluded.
INTRODUCTION Formation o f a r o m a t i c s i n t h e MTG process i s supposed t o proceed v i a and
alkenyl
carbenium i o n s formed f r o m o l e f i n s as p r i m a r y
t h i s process ( r e f .
alkyl
intermediates
of
1 ) . R e c e n t l y some papers have been p u b l i s h e d d e a l i n g w i t h
g e n e r a t i o n and t r a n s f o r m a t i o n o f a l k y l carbenium i o n s f r o m ethene and
propene
i n zeolites (ref.
olefins
in
zeolites
2 ) . Formation o f a l k e n y l carbenium i o n s f r o m l o w e r
mordenite,
f a u j a s i t e and ZSM-5 has been p r o v e n a l s o
by
UV-VIS
spectroscopy ( r e f . 3 ) . The o b j e c t i v e o f t h i s paper was t o i n v e s t i g a t e i n d e t a i l t h e i d e n t i f i c a t i o n of
unsaturated
zeolites.
s u r f a c e i n t e r m e d i a t e s d e r i v e d f r o m some
cyclohexadiene
+
benzene,
assumed t o be one o f t h e
a r o m a t i c s i n t h e MTG process ( r e f . carbenium
C,
hydrocarbons
S p e c i a l a t t e n t i o n was d i r e c t e d t o t h e t r a n s f o r m a t i o n cyclohexene
ions
4),
pathways
for
in +
yielding
although the formation o f unsaturated
f r o m these c y c l i c compounds has n o t y e t
been
experimentally
proved. EXPERIMENTAL Starting
m a t e r i a l s were z e o l i t e NaY f r o m Union Carbide and z e o l i t e
synthesized i n the laboratory o f Prof.
ZSM-5,
L e c h e r t . D e t a i l s o f sample p r e p a r a t i o n
356 and f u r t h e r t r e a t m e n t a r e g i v e n by K i r i c s i and F o r s t e r ( s e e r e f . cell
compositions
determined
by
neutron
activation
5 ) . The u n i t
analysis
and
atomic
a b s o r p t i o n spectroscopy were HNaY: H, ,Na, ,A1 HNaZSM-5 : T r a n s f o r m a t i o n o f C, 1-Hexene, chased
,Si
.O Z N a O .
,03 0 6 A 1 1 . O ESi
9 4 . go, 9 2 .
hydrocarbons was more t h o r o u g h l y s t u d i e d w i t h t h e l a t t e r .
cyclohexane,
f r o m Merck,
cyclohexene,
Darmstadt,
c y c l o h e x a d i e n e - 1 . 4 and benzene, p u r -
i n a q u a l i t y b e t t e r t h a n 99%, were
used
as
adsorbates. For the spectroscopic i n v e s t i g a t i o n s s e l f - s u p p o r t i n g wafers o f 5-7
mg/cm2
pressed and outgassed a t 770 K o v e r n i g h t
vacuum
thickness
were
under
high
c o n d i t i o n s i n t h e o p t i c a l c e l l , made f r o m f u s e d q u a r t z f o r t h e U V - V I S and f r o m g l a s s w i t h S i l v a c a - g l u e d KBr windows f o r t h e I R e x p e r i m e n t s . The
UV-VIS
spectra
were r u n i n t r a n s m i s s i o n on a
c o n t r o l l e d by a B a s i s 108 computer. background
correction.
deconvolution
of
the
Cary
17
spectrometer
The d i g i t i z e d s p e c t r a were smoothed a f t e r
A n o n l i n e a r l e a s t squares p r o c e d u r e was used f o r e l e c t r o n i c spectra,
fitting
Gaussian
peaks
the
to
the
measured d a t a . I R s p e c t r a were r e c o r d e d on a P e r k i n - E l m e r 225 s p e c t r o m e t e r . The a c i d i t y o f t h e samples was determined a p p l y i n g p y r i d i n e as a p r o b e . The r a t i o o f B r o n s t e d t o Lewis a c i d s i t e s Bpy/Lpy was f o u n d t o be 0.19 and 2 . 2 1 i n case o f HNaY and HNaZSM-5, r e s p e c t i v e l y . RESULTS U n s a t u r a t e d carbenium i o n s have been formed f r o m a l l C 6 compounds gated.
Similarities
investi-
as w e l l as d i f f e r e n c e s were f o u n d i n t h e s p e c t r a o f
v a r i o u s hydrocarbons a f t e r a d s o r p t i o n .
the
While a f t e r cyclohexane admission o n l y
a weak band near 310 nm appeared a t room temperature,
two bands o f v e r y
low
i n t e n s i t i e s a t 373 and 495 nm were f o u n d a f t e r a d s o r p t i o n o f benzene. Generation and t r a n s f o r m a t i o n o f a l k e n y l carbenium i o n s f r o m 1-hexene v e r y s i m i l a r t o those o b t a i n e d f r o m propene i n b o t h z e o l i t e s HNaY and 5.
After
developed, ions,
a d s o r p t i o n and r e a c t i o n o f 1-hexene bands a t 325, which
can be assigned t o t h e mono-,
respectively.
Details
HNaZSM-
380 and
d i - and t r i e n y l i c
o f 1-hexene a d s o r p t i o n and c o n v e r s i o n
comparison o f these U V - V I S r e s u l t s w i t h those o f Maixner e t a l . ,
were
450
nm
carbenium and
the
obtained
by
NMR spectroscopy a r e g i v e n elsewhere ( r e f . 5 ) . Cyclohexene adsorbed i n HNaZSM-5 gave r i s e t o a b s o r p t i o n s a t 298, and 391 nm,
as can be seen i n F i g .
1.
310, 340
W i t h c o n t a c t t i m e t h e 310 and 340
nm
bands i n c r e a s e d and two new a b s o r p t i o n s near 420 and 530 nm became d e t e c t a b l e . Upon e v a c u a t i o n a t 370 K t h e i n t e n s i t y o f t h e 310 nm band was enhanced and additional
band a t 380 nm developed and became d o m i n a t i n g i n t h e whole
an
spec-
357
aI
U
n m
aJ U c
0
v)
n m
n 4
m 0
n
4
3 300
Fig.
400
X/nm
Transmission
1.
500
300
electronic
s p e c t r a o f z e o l i t e HNaZSM-5 loaded w i t h 666 P a cyclohexene. ( a ) A t room temperature, immediately a f t e r admission, ( b ) 1. ( c ) 2, ( d ) 3 h l a t e r . A f t e r evacuation f o r 1 h a t
room temperature, ( f ) 370 K, (9) 470 K and (h) 570 K.
(e)
trum.
Fig.
400
X/nm
500
U V - V I S spectra o f z e o l i t e exposed t o 133 Pa cyclohexadiene. ( a ) A t room temperature, s h o r t l y a f t e r admission, ( b ) 30 min, ( c ) 1 h l a t e r . A f t e r evacuat i o n f o r 1 h a t ( d ) room temperature, ( e ) 370 K, ( f ! 470 K and ( 9 ) 570 K.
2.
HNaZSM-5
A f t e r vacuum t r e a t m e n t a t 470 K a b s o r p t i o n s a t 270 ( s h o u l d e r ) , 295 ( t h e
most i n t e n s e band),
380, 420 and 530 nm were observed, w h i l e a t 570 K a
340,
broad a b s o r p t i o n o f o v e r l a p p i n g bands remained. a t 320
S h o r t l y a f t e r admission o f 133 Pa cyclohexadiene v e r y i n t e n s e bands
410, 460, 520 and 570 nm c o u l d be d i s t i n g u i s h e d even by t h e naked
(shoulder), eye (see F i g . at
2);
t h e i n t e n s i t y o f each i n c r e a s e d w i t h t i m e . Upon e v a c u a t i o n
room temperature t h e i n t e n s i t y o f t h e l o w frequency bands
spectrum
d),
while
at
370
K a new band arose a t 590
nm.
decreased
At
a b s o r p t i o n s a t l o n g e r wavelengths almost c o m p l e t e l y disappeared. after
(see
K
470
the
The spectrum
e v a c u a t i o n a t 570 K was s i m i l a r t o those o f cyclohexene under t h e
same
c o n d i t i o n s (compare s p e c t r a h and g i n F i g . 1 and 2 ) . I t was
a general observation t h a t w i t h exception o f
benzene
carbocation
f o r m a t i o n s t a r t s w i t h t h e monoenylic species f o l l o w e d by t h e o l i g o e n y l i c i o n s . In
t h e case o f benzene o n l y t h e development o f t h e d i - and t r i e n y l i c i o n s
is
observed.
As from
i n f o r m a t i o n about t h e s t r u c t u r e o f t h e s u r f a c e s p e c i e s can be the
i n f r a r e d region,
supplementary I R s p e c t r a o f t h e two
most
obtained impor-
t a n t compounds cyclohexene and cyclohexadiene were r u n upon a d s o r p t i o n i n
the
358 zeolites. because
The
IR
s t u d y o f benzene a d s o r p t i o n i n t h e z e o l i t e s
t h i s has a l r e a d y
was
been conducted i n d e t a i l b y Karge and
omitted,
Datka
(ref.
6). The 3.
s p e c t r a o f cyclohexene adsorbed i n z e o l i t e HNaZSM-5 a r e shown i n
The s p e c t r a l changes a r e s i m i l a r t o t h o s e observed by Haber e t
al.
Fig. (ref.
7 ) . Bands a t 1653 and 3025 CN’ a r e c h a r a c t e r i s t i c o f t h e C=C i n t e r n a l c y c l i c double bond and =C-H bond s t r e t c h i n g v i b r a t i o n s , b o t h d e c r e a s i n g w i t h t i m e . The
f a s t e v o l u t i o n o f an a b s o r p t i o n near 1510 cm-’ r e f l e c t s t h e
u n s a t u r a t e d carbenium i o n s . stretching
vibration
No band a t 1478 cm-I,
o f benzene was d e t e c t a b l e .
formation
of
c h a r a c t e r i s t i c f o r the r i n g A f t e r evacuation a t
470
K
to
be
almost a l l bands disappeared. I
al
U
c
m
Y
CI
.r
E
VI
C
a L
I-
1 3100
2900
2700
1700
1600
I 500
1400
Wavenumber/cm-’
F i g . 3 . IR s p e c t r a o f cyclohexene adsorbed i n z e o l i t e HNaZSM-5. ( a ) Z e o l i t e background spectrum. A t beam temperature ( b ) 2 min a f t e r admission o f 666 Pa o f cyclohexene, ( c ) 1 h, ( d ) 3 h l a t e r . A f t e r e v a c u a t i o n a t ( e ) beam temperature, ( f ) 370 K and ( 9 ) 470 K f o r 1 h. Spectra
obtained
from
c y c l o h e x a d i e n e i n z e o l i t e HNaZSM-5
proved
r a t h e r complex (see F i g . 4 ) . The band a t 3032 cm-’ due t o i n t a c t c y c l o h e x a d i e n e decreased
with
t i m e o f c o n t a c t a t beam t e m p e r a t u r e as a
t r a n s f o r m a t i o n o f t h i s compound. assigned
result
of
surface
Bands d e v e l o p i n g a t 1505 and 1535 cm-’ can be
t o u n s a t u r a t e d carbenium i o n s formed on t h e
zeolite
surface.
Here
again t h e band c h a r a c t e r i s t i c o f adsorbed benzene a t 1478 c d i s absent. S i n c e t h i s band i s u s u a l l y v e r y sharp and i n t e n s e and we c a n n o t f i n d any
indication
359
I
31 00
2900
..
2700
1600
1500
1400
Wavenumber/cm-’ F i g . 4 . IR s p e c t r a o f cyclohexadiene adsorbed i n z e o l i t e HNaZSM5. ( a ) Z e o l i t e background spectrum. ( b ) A t beam t e m p e r a t u r e , 3 min a f t e r admission o f 213 P a o f cyclohexadiene, ( c ) 1 h, ( d ) 2 h, ( e ) 4 h l a t e r . ( f ) A t 370 K a f t e r 1 h. ( 9 ) A f t e r 1 h e v a c u a t i o n a t beam temperature. of
it
in
our
spectra,
the
formation
of
benzene
from
cyclohexene
upon admission t o z e o l i t e s a t low temperatures does n o t
cyclohexadiene
and occur
t o any a p p r e c i a b l e e x t e n t .
DISCUSSION Electronic unresolved
spectra
usually
extremely d i f f i c u l t . t i o n program (see F i g .
as broad
absorptions
rendering t h e i r
due
complete
to
their
analysis
Consequently, a d j a c e n t e l e c t r o n i c t r a n s i t i o n s merge i n t o
broad o v e r l a p p i n g bands, analysis
appear
rovibrational f i n e structure,
which can be deconvoluted a p p l y i n g a d a t a
5),
d e t a i l s o f which and a p p l i c a t i o n t o
o f U V - V I S s p e c t r a w i l l be p u b l i s h e d elsewhere ( r e f .
manipula-
quantitative
8).
Using
the
bands r e s o l v e d by t h i s procedure q u a n t i t a t i v e c o n c l u s i o n s c o n c e r n i n g c a r b o c a t i o n f o r m a t i o n can be drawn. UV-VIS
bands
of
hydrocarbons adsorbed on s o l i d
surfaces
are
generally
360
compared
t o t h e s p e c t r a o f u n s a t u r a t e d carbenium i o n s observed
in
superacid
s o l u t i o n s . T h i s i s t h e most advantageous way f o r a t t r i b u t i n g bands t o d i s t i n c t i o n i c species, as c a r b o c a t i o n f o r m a t i o n has been v e r y e x t e n s i v e l y s t u d i e d superacids ( r e f . 9 ) .
in
a
10.2 Absorbance u n i t s
C
400
300
I n some cases, rate
of
and
environments not
e.g. f o r t h e appearance o f c e r t a i n carbenium i o n s and t h e i r
formation,
solution only
the
500
X/nm
F i g . 5. Deconvoluted s p e c t r a o f ( a ) benzene, ( b ) c y c l o h e x e n e and ( c ) c y c l o h e x a d i e n e adsorbed i n z e o l i t e s . The s p e c t r a were r e c o r d e d a f t e r e x p o s i n g t h e zeol i t e s t o t h e adsorbates f o r 1 h a t room t e m p e r a t u r e .
the
good
correlations
z e o l i t e surface could
or be
similarities
between
ascertained,
even
o f t h e c a r b o c a t i o n s i n b o t h systems a r e d i f f e r e n t and r a t e o f formation b u t also the s t a b i l i t y
of
the
superacid though
the
influence unsaturated
species ( r e f . l o ) . In
other
cases,
l a r g e r d i f f e r e n c e s i n t h e band
maxima
of
carbocations
formed i n s u p e r a c i d s and i n z e o l i t e s were found. T h i s i s demonstrated by Table
1,
in
which
the
wavelengths o f maximum absorbance o f
the
carbenium
i d e n t i f i e d i n a c i d i c s o l u t i o n s a r e compared t o t h o s e observed upon o f hydrocarbons i n z e o l it e s
.
ions
adsorption
I n t h e case o f benzene t h e two bands observed a t 373 and 495 nm do n o t agree w i t h t h e f i n d i n g s o f p r e v i o u s a u t h o r s ( r e f . 1 2 ) , w h i c h means t h a t f u r t h e r i n v e s t i g a t i o n s are required.
The absence o f monoenylic species, p r o v e d by us,
361
is
quite
understandable s i n c e p r o t o n a t i o n o f t h e a r o m a t i c
y i e l d s d i e n y l i c s p e c i e s and,furthermore,
ring
immediately
r i n g - o p e n i n g seems t o be u n f a v o u r a b l e
a t low temperatures. Comparing t h e s p e c t r a o f benzene,
TABLE 1 A b s o r p t i o n maxima superacids.
cyclohexene and c y c l o h e x a d i e n e ( s e e F i g .
o f c a r b o c a t i o n s formed
Carbocations i n s u p e r a c i d s *
CH(
zeolites,
maxi''
+
C-CH-CH, - - - - - - - CH,
0
305
315
CH, =CH-CH, -CH, -CH, -CH,
325 380 450 298 310 340 391
0
284 324 414 464 517 5 74
0
440, 550 373, 495
325
0
0
330
compared
Hydrocarbons adsorbed i n z e o l i t e s
max'nm CH,\
in
310
0 cb
470
to Ref
**
**
**
12
**
** 13
~
315
450 530 325 357 377 392 415
408
From r e f . 11;
**
T h i s work
12
-14
5 ) i t becomes e v i d e n t t h a t those o f t h e . l a t t e r two resemble each o t h e r . I t i s a l s o obvious t h a t t h e c a p a b i l i t y o f cyclohexadiene f o r i o n f o r m a t i o n exceeds that
o f cyclohexene.
B u t i n t h e i r s p e c t r a t h e r e i s no e v i d e n c e f o r bands
373 and 495 nm c h a r a c t e r i s t i c f o r adsorbed benzene. a
correct
and
unambiguous
assignment o f t h e
at
I t must be mentioned t h a t
UV-VIS
bands
obtained
upon
362
a d s o r p t i o n o f c y c l i c hydrocarbons has n o t y e t been p u b l i s h e d . As
far
as
the
f o r m a t i o n o f benzene
is
concerned,
Gibbs
i n t h e sequence o f t h e r e a c t i o n m e t h y l c y c l o p e n t a n e
formation cyclohexene
+
cyclohexadiene
+
energies cyclohexane
benzene were c a l c u l a t e d f o r d i f f e r e n t
+
of +
tempera-
t u r e s u s i n g t h e thermodynamic d a t a g i v e n i n r e f . 15 (see Table 2 ) . Herefrom i t becomes e v i d e n t t h a t f o r m a t i o n o f benzene and c y c l o h e x a d i e n e i s u n f a v o u r e d low
temperatures.
according
This
is
in
good
agreement
with
experimental
t o which t h e f o r m a t i o n o f a r o m a t i c s f r o m l o w e r o l e f i n s
on
at
results zeolite
HZSM-5 proceeds a t temperatures above 570 K ( r e f . 4 ) . From o u r s p e c t r o s c o p i c r e s u l t s t h e same c o n c l u s i o n s can be drawn. TABLE 2 Gibbs f r e e energy o f f o r m a t i o n o f m e t h y l c y c l o pentane, cyclohexane, cyclohexene, c y c l o h e x a diene, benzene and 1-hexene i n t h e t e m p e r a t u r e range 298-673 K (see r e f . 15).
I
T'K
As
AGf/kJ mol-'
-
MCP
CHAN
CHEN
CHOEN BENZ
298 323 373 473 573 673
35.6 36.6 72.4 123.8 176.8 231.1
31.6 44.7 71.6 127.2 184.4 242.8
106.7 116.3 135.7 176.1 217.8 260.3
186.5 193.0 206.4 234.6 264.0 294.2
1-HEXENE
129.6 133.6 141.8 159.3 177.6 196.5
87.4 98.3 120.6 166.8 214.5 263.2
t h e z e o l i t e s used i n t h i s s t u d y c o n t a i n e d b o t h B r o n s t e d und Lewis
acid
s i t e s , t h e f o r m a t i o n o f t h e f o l l o w i n g s u r f a c e s p e c i e s may be assumed:
b 0 0 0
uv-vis active
*
* ) means a b s o r p t i o n a t h>200 nm This
v e r y s i m p l i f i e d r e a c t i o n scheme shows t h a t f o r m a t i o n
carbenium i o n s can be e x p e c t e d f r o m each compound,
of
unsaturated
i n c l u d i n g cyclohexane,
in
363 t h i s case by secondary t r a n s f o r m a t i o n v i a t h e c y c l o h e x y l carbonium may
be
the
reason
f o r the observation
that
cyclohexane
is
ion.
only
This slowly
UV-VIS a c t i v e s u r f a c e compounds.
converted i n t o
Interconversion o f
carbenium
ions
can
also
take
place
in
zeolites.
T h e r e f o r e t h e bands observed w i t h cyclohexene and cyclohexadiene can be a t t r i buted
to
different
intermediates
formed
upon
transformation
of
primary
generated i o n s . CONLCUSIONS
-
From o u r i n v e s t i g a t i o n s t h e f o l l o w i n g c o n c l u s i o n s must be drawn: From a l l C,
compounds under i n v e s t i g a t i o n u n s a t u r a t e d carbenium i o n s
could
be generated. The c a r b o c a t i o n f o r m a t i o n s t a r t s w i t h t h e monoenyl species, i n t h e case o f benzene w i t h t h e d i e n y l i c carbenium i o n .
- Carbocations cyclohexyl depending sites.
formed and
on
from
cyclohexene and
cyclohexenyl
or
cyclohexadiene
cyclohexenyl
and
may
either
cyclohexadienyl
whether i n t e r a c t i o n t a k e s p l a c e w i t h B r o n s t e d o r
These
be
ions
Lewis
species undergo r i n g - o p e n i n g t o open-chain a l k e n y l
acid
carbenium
ions .
-
Cyclohexane
transforms
in
a b s o r b i n g i n t h e FUV range,
the f i r s t
step
into
saturated
carbocations,
y i e l d i n g e n y l i c carbenium i o n s o n l y a f t e r r i n g -
opening .
-
From
the
formation
spectroscopic capability
sequence cyclohexadiene
-
At
the
i n v e s t i g a t i o n s i t can be
of
>
inferred
t h e hydrocarbons i n v e s t i g a t e d cyclohexene
>
1-hexene
>
cyclohexane
f i r s t stages o f c a r b o c a t i o n development f r o m
C,
that
decreases
>
the
ion
in
the
benzene.
hydrocarbons
the
f o r m a t i o n o f a r o m a t i c s u r f a c e species may be excluded. ACKNOWLEDGEMENT One o f t h e a u t h o r s ( I . K . )
i s g r a t e f u l t o t h e Alexander von Humboldt Founda-
t i o n f o r a r e s e a r c h f e l l o w s h i p . We thank D r . J . t e n Pas f o r t h e s y n t h e s i s o f z e o l i t e ZSM-5. REFERENCES
1 S. Ceckiewicz, J.C.S. Faraday 1, 77 (1981) 269-280; R.L.V. Rao, P. Levesque and B. S j i a r i e l , Can. J . Chem. Eng., 64 (1986) 514516; J . Heering, M. K o t t e r and L. R i e k e r t , Chem. Eng. S c i . , 37 (1982) 581-584; J . Nowakova, L . Kubelkova and Z. D o l e j s e k , J . C a t a l . , 108 (1987) 208-213; T. Mole, G . B e t t and D.Seddon, J . C a t a l . , 84 (1983) 435-445; J.P. Wolthuizen, J.P. van den Berg and J.C.H. van H o o f f , i n B. I m e l i k e t a l . ( E d i t o r s ) , C a t a l y s i s by Z e o l i t e s , E l s e v i e r , Amsterdam, 1980, pp. 85-92.
364 H.G. Karge, i n J.R. Katzer ( E d i t o r ) , Molecular Sieves 11, ACS Symp. Ser. No. 40, Am. Chem. SOC., Washington D.C., 1977, pp. 584-595; V . B o l i s , J. Vedrine, J.P. van den Berg, J.P. Wolthuizen and E.G. Derouane, J.C.S. Faraday 1, 76 (1980) 1606-1616; J.P. van den Berg, J.P. Wolthuizen, A.D.H. Clague, G.R. Hays, R. Huis and J.C.H. van Hooff, J. Catal., 80 (1983) 130-138; M. Zardkoohi, J.F. Haw and J.H. Lunsford, J. Am. Chem. SOC., 109 (1987) 5278- 5280. 3 E.D. Garbowski and H. P r a l i a u d , J. Chim. Phys. Phys.-Chim. B i o l . , 76 (1979) 687-692; H. F o r s t e r , S. Franke and J. Seebode, J.C.S. Faraday 1, 79 (1983) 373-382; H. F o r s t e r , J. Seebode, P. F e j e s and I. K i r i c s i , J.C.S. Faraday 1, 83 (1987) 1109-1117; M. Laniecki and H.G. Karge, i n D. Shopov e t a l . ( E d i t o r s ) , Heterogeneous C a t a l y s i s , Proc. V I t h I n t . Symp., S o f i a , 1987, Vol. 2, pp. 129-134; H. F o r s t e r and I. K i r i c s i , Z e o l i t e s , 7 (1987) 508-510. Garbowski, i n B. I m e l i k e t a l . 4 J.C. Vedrine, D. D e j a i f v e and E.D. ( E d i t o r s ) , C a t a l y s i s by Z e o l i t e s , E l s e v i e r , Amsterdam, 1980, pp. 29-37. 5 I.K i r i c s i and H. F o r s t e r , J.C.S. Faraday 1, 84 (1988) 491-499; H. F o r s t e r , I. K i r i c s i and J. Seebode, i n P.J. Grobet e t a l . ( E d i t o r s ) , I n n o v a t i o n i n Z e o l i t e M a t e r i a l s Science, E l s e v i e r , Amsterdam, 1988, pp. 435-442; S. Maixner, C.Y. Chen, P.J. Grobet, P.A. Jacobs and J. Weitkamp, i n Y . Mukami e t a l . ( E d i t o r s ) , Proc. 7 t h I n t . Z e o l i t e Conf., Tokyo, 1986, pp. 693-700. 6 H.G. Karge, J. Ladebeck, Z.Sarback and K. Hatada, Z e o l i t e s , 2 (1982) 94-102 J. Datka, J.C.S. Faraday 1, 77 (1981) 511-517. 7 J. Haber, J . Komorek-Hlodzik and T. Romotowski, Z e o l i t e s , 2 (1982) 179-184; J. Haber, J. Komorek and T . Romotowski, Proc. o f Zeocat 85, I n t . Symp. on Z e o l i t e C a t a l y s i s , Siofok, 1985, pp. 671-679. 8 I. K i r i c s i and Gy. Tasi, i n p r e p a r a t i o n . 9 G.A. Olah and P. Schleyer ( E d i t o r s ) , Carbonium I o n s I - I V , WileyI n t e r s c i e n c e , New York, 1970. 10 P. Fejes, H. F o r s t e r , I. K i r i c s i and J. Seebode, i n P.A. Jacobs e t a l . (Editors), S t r u c t u r e and R e a c t i v i t y o f M o d i f i e d Z e o l i t e s , E l s e v i e r , Amsterdam, 1984, pp. 91-98. 11 G.A. Olah, C.U. Pittman and M.C.R. Symons, i n G.A. Olah and P . Schleyer ( E d i t o r s ) , Carbonium Ions, Wiley-Interscience, New York, Vol. 1, pp. 153222. 12 A . V . Kiselev, L.A. Kupcha, V . I . Lygin and V.G. S h a t s k i i , K i n e t i k a i Katal i z , 10 (1969) 449-451. 13 C . Bezuhanova, A . V . Kiselev, D.G. K i t i a s h v i l i and V . I . Lygin, K i n e t i k a i Katal i z , 13 (1972) 431-434. 14 H.G. Karge, W. Abke, E.P. Boldingh and M. L a n i e c k i , Actas Simp. Iberoam. C a t a l . 9th, Lisboa, 1984, 1, 582-583. 15 L. Seres, L. Z a l o t a i and F. M a r t a , Acta Phys. Chem. Szeged, 23 (1977), 433468.
2
FORMATION OF CARBOCATIONS FROM C,
COMPOUNDS I N ZEOLITES
Imre K i r i c s i ' , H o r s t F o r s t e r 2 and Gyula T a s i ' ' A p p l i e d Chemistry Department, J o z s e f A t t i l a U n i v e r s i t y , R e r r i c h B. t e r 1, H-6720 Szeged, Hungary ' I n s t i t u t e o f P h y s i c a l Chemistry, U n i v e r s i t y o f Hamburg, Bundesstr. 45, 0-2000 Hamburg 13, Federal R e p u b l i c o f Germany ABSTRACT Formation o f u n s a t u r a t e d carbenium i o n s f r o m 1-hexene, cyclohexane, c y c l o hexene, cyclohexadiene and benzene upon i n t e r a c t i o n w i t h t h e H-forms o f z e o l i t e s ZSM-5 and Y was p r o v e d by U V - V I S and I R spectroscopy. W i t h t h e e x c e p t i o n o f benzene, which forms d i e n y l i o n s a t t h e b e g i n n i n g , c a r b o c a t i o n f o r m a t i o n from t h e o t h e r C, compounds s t a r t s w i t h monoenylic species, which t r a n s f o r m i n t o o l i g o e n y l i c i o n s w i t h t i m e o f c o n t a c t . From the c y c l i c hydrocarbons c y c l i c i o n s as w e l l as open-chain a l k e n y l i o n s a r e formed. The i o n f o r m a t i o n c a p a b i l i t y decreases i n t h e sequence c y c l o h e x a d i e n e > c y c l o h e x ene > 1-hexene > cyclohexane > benzene. F o r t h e f i r s t stages o f carbenium i o n development t h e f o r m a t i o n o f a r o m a t i c s u r f a c e species may be excluded.
INTRODUCTION Formation o f a r o m a t i c s i n t h e MTG process i s supposed t o proceed v i a and
alkenyl
carbenium i o n s formed f r o m o l e f i n s as p r i m a r y
t h i s process ( r e f .
alkyl
intermediates
of
1 ) . R e c e n t l y some papers have been p u b l i s h e d d e a l i n g w i t h
g e n e r a t i o n and t r a n s f o r m a t i o n o f a l k y l carbenium i o n s f r o m ethene and
propene
i n zeolites (ref.
olefins
in
zeolites
2 ) . Formation o f a l k e n y l carbenium i o n s f r o m l o w e r
mordenite,
f a u j a s i t e and ZSM-5 has been p r o v e n a l s o
by
UV-VIS
spectroscopy ( r e f . 3 ) . The o b j e c t i v e o f t h i s paper was t o i n v e s t i g a t e i n d e t a i l t h e i d e n t i f i c a t i o n of
unsaturated
zeolites.
s u r f a c e i n t e r m e d i a t e s d e r i v e d f r o m some
cyclohexadiene
+
benzene,
assumed t o be one o f t h e
a r o m a t i c s i n t h e MTG process ( r e f . carbenium
C,
hydrocarbons
S p e c i a l a t t e n t i o n was d i r e c t e d t o t h e t r a n s f o r m a t i o n cyclohexene
ions
4),
pathways
for
in +
yielding
although the formation o f unsaturated
f r o m these c y c l i c compounds has n o t y e t
been
experimentally
proved. EXPERIMENTAL Starting
m a t e r i a l s were z e o l i t e NaY f r o m Union Carbide and z e o l i t e
synthesized i n the laboratory o f Prof.
ZSM-5,
L e c h e r t . D e t a i l s o f sample p r e p a r a t i o n
356 and f u r t h e r t r e a t m e n t a r e g i v e n by K i r i c s i and F o r s t e r ( s e e r e f . cell
compositions
determined
by
neutron
activation
5 ) . The u n i t
analysis
and
atomic
a b s o r p t i o n spectroscopy were HNaY: H, ,Na, ,A1 HNaZSM-5 : T r a n s f o r m a t i o n o f C, 1-Hexene, chased
,Si
.O Z N a O .
,03 0 6 A 1 1 . O ESi
9 4 . go, 9 2 .
hydrocarbons was more t h o r o u g h l y s t u d i e d w i t h t h e l a t t e r .
cyclohexane,
f r o m Merck,
cyclohexene,
Darmstadt,
c y c l o h e x a d i e n e - 1 . 4 and benzene, p u r -
i n a q u a l i t y b e t t e r t h a n 99%, were
used
as
adsorbates. For the spectroscopic i n v e s t i g a t i o n s s e l f - s u p p o r t i n g wafers o f 5-7
mg/cm2
pressed and outgassed a t 770 K o v e r n i g h t
vacuum
thickness
were
under
high
c o n d i t i o n s i n t h e o p t i c a l c e l l , made f r o m f u s e d q u a r t z f o r t h e U V - V I S and f r o m g l a s s w i t h S i l v a c a - g l u e d KBr windows f o r t h e I R e x p e r i m e n t s . The
UV-VIS
spectra
were r u n i n t r a n s m i s s i o n on a
c o n t r o l l e d by a B a s i s 108 computer. background
correction.
deconvolution
of
the
Cary
17
spectrometer
The d i g i t i z e d s p e c t r a were smoothed a f t e r
A n o n l i n e a r l e a s t squares p r o c e d u r e was used f o r e l e c t r o n i c spectra,
fitting
Gaussian
peaks
the
to
the
measured d a t a . I R s p e c t r a were r e c o r d e d on a P e r k i n - E l m e r 225 s p e c t r o m e t e r . The a c i d i t y o f t h e samples was determined a p p l y i n g p y r i d i n e as a p r o b e . The r a t i o o f B r o n s t e d t o Lewis a c i d s i t e s Bpy/Lpy was f o u n d t o be 0.19 and 2 . 2 1 i n case o f HNaY and HNaZSM-5, r e s p e c t i v e l y . RESULTS U n s a t u r a t e d carbenium i o n s have been formed f r o m a l l C 6 compounds gated.
Similarities
investi-
as w e l l as d i f f e r e n c e s were f o u n d i n t h e s p e c t r a o f
v a r i o u s hydrocarbons a f t e r a d s o r p t i o n .
the
While a f t e r cyclohexane admission o n l y
a weak band near 310 nm appeared a t room temperature,
two bands o f v e r y
low
i n t e n s i t i e s a t 373 and 495 nm were f o u n d a f t e r a d s o r p t i o n o f benzene. Generation and t r a n s f o r m a t i o n o f a l k e n y l carbenium i o n s f r o m 1-hexene v e r y s i m i l a r t o those o b t a i n e d f r o m propene i n b o t h z e o l i t e s HNaY and 5.
After
developed, ions,
a d s o r p t i o n and r e a c t i o n o f 1-hexene bands a t 325, which
can be assigned t o t h e mono-,
respectively.
Details
HNaZSM-
380 and
d i - and t r i e n y l i c
o f 1-hexene a d s o r p t i o n and c o n v e r s i o n
comparison o f these U V - V I S r e s u l t s w i t h those o f Maixner e t a l . ,
were
450
nm
carbenium and
the
obtained
by
NMR spectroscopy a r e g i v e n elsewhere ( r e f . 5 ) . Cyclohexene adsorbed i n HNaZSM-5 gave r i s e t o a b s o r p t i o n s a t 298, and 391 nm,
as can be seen i n F i g .
1.
310, 340
W i t h c o n t a c t t i m e t h e 310 and 340
nm
bands i n c r e a s e d and two new a b s o r p t i o n s near 420 and 530 nm became d e t e c t a b l e . Upon e v a c u a t i o n a t 370 K t h e i n t e n s i t y o f t h e 310 nm band was enhanced and additional
band a t 380 nm developed and became d o m i n a t i n g i n t h e whole
an
spec-
357
aI
U
n m
aJ U c
0
v)
n m
n 4
m 0
n
4
3 300
Fig.
400
X/nm
Transmission
1.
500
300
electronic
s p e c t r a o f z e o l i t e HNaZSM-5 loaded w i t h 666 P a cyclohexene. ( a ) A t room temperature, immediately a f t e r admission, ( b ) 1. ( c ) 2, ( d ) 3 h l a t e r . A f t e r evacuation f o r 1 h a t
room temperature, ( f ) 370 K, (9) 470 K and (h) 570 K.
(e)
trum.
Fig.
400
X/nm
500
U V - V I S spectra o f z e o l i t e exposed t o 133 Pa cyclohexadiene. ( a ) A t room temperature, s h o r t l y a f t e r admission, ( b ) 30 min, ( c ) 1 h l a t e r . A f t e r evacuat i o n f o r 1 h a t ( d ) room temperature, ( e ) 370 K, ( f ! 470 K and ( 9 ) 570 K.
2.
HNaZSM-5
A f t e r vacuum t r e a t m e n t a t 470 K a b s o r p t i o n s a t 270 ( s h o u l d e r ) , 295 ( t h e
most i n t e n s e band),
380, 420 and 530 nm were observed, w h i l e a t 570 K a
340,
broad a b s o r p t i o n o f o v e r l a p p i n g bands remained. a t 320
S h o r t l y a f t e r admission o f 133 Pa cyclohexadiene v e r y i n t e n s e bands
410, 460, 520 and 570 nm c o u l d be d i s t i n g u i s h e d even by t h e naked
(shoulder), eye (see F i g . at
2);
t h e i n t e n s i t y o f each i n c r e a s e d w i t h t i m e . Upon e v a c u a t i o n
room temperature t h e i n t e n s i t y o f t h e l o w frequency bands
spectrum
d),
while
at
370
K a new band arose a t 590
nm.
decreased
At
a b s o r p t i o n s a t l o n g e r wavelengths almost c o m p l e t e l y disappeared. after
(see
K
470
the
The spectrum
e v a c u a t i o n a t 570 K was s i m i l a r t o those o f cyclohexene under t h e
same
c o n d i t i o n s (compare s p e c t r a h and g i n F i g . 1 and 2 ) . I t was
a general observation t h a t w i t h exception o f
benzene
carbocation
f o r m a t i o n s t a r t s w i t h t h e monoenylic species f o l l o w e d by t h e o l i g o e n y l i c i o n s . In
t h e case o f benzene o n l y t h e development o f t h e d i - and t r i e n y l i c i o n s
is
observed.
As from
i n f o r m a t i o n about t h e s t r u c t u r e o f t h e s u r f a c e s p e c i e s can be the
i n f r a r e d region,
supplementary I R s p e c t r a o f t h e two
most
obtained impor-
t a n t compounds cyclohexene and cyclohexadiene were r u n upon a d s o r p t i o n i n
the
358 zeolites. because
The
IR
s t u d y o f benzene a d s o r p t i o n i n t h e z e o l i t e s
t h i s has a l r e a d y
was
been conducted i n d e t a i l b y Karge and
omitted,
Datka
(ref.
6). The 3.
s p e c t r a o f cyclohexene adsorbed i n z e o l i t e HNaZSM-5 a r e shown i n
The s p e c t r a l changes a r e s i m i l a r t o t h o s e observed by Haber e t
al.
Fig. (ref.
7 ) . Bands a t 1653 and 3025 CN’ a r e c h a r a c t e r i s t i c o f t h e C=C i n t e r n a l c y c l i c double bond and =C-H bond s t r e t c h i n g v i b r a t i o n s , b o t h d e c r e a s i n g w i t h t i m e . The
f a s t e v o l u t i o n o f an a b s o r p t i o n near 1510 cm-’ r e f l e c t s t h e
u n s a t u r a t e d carbenium i o n s . stretching
vibration
No band a t 1478 cm-I,
o f benzene was d e t e c t a b l e .
formation
of
c h a r a c t e r i s t i c f o r the r i n g A f t e r evacuation a t
470
K
to
be
almost a l l bands disappeared. I
al
U
c
m
Y
CI
.r
E
VI
C
a L
I-
1 3100
2900
2700
1700
1600
I 500
1400
Wavenumber/cm-’
F i g . 3 . IR s p e c t r a o f cyclohexene adsorbed i n z e o l i t e HNaZSM-5. ( a ) Z e o l i t e background spectrum. A t beam temperature ( b ) 2 min a f t e r admission o f 666 Pa o f cyclohexene, ( c ) 1 h, ( d ) 3 h l a t e r . A f t e r e v a c u a t i o n a t ( e ) beam temperature, ( f ) 370 K and ( 9 ) 470 K f o r 1 h. Spectra
obtained
from
c y c l o h e x a d i e n e i n z e o l i t e HNaZSM-5
proved
r a t h e r complex (see F i g . 4 ) . The band a t 3032 cm-’ due t o i n t a c t c y c l o h e x a d i e n e decreased
with
t i m e o f c o n t a c t a t beam t e m p e r a t u r e as a
t r a n s f o r m a t i o n o f t h i s compound. assigned
result
of
surface
Bands d e v e l o p i n g a t 1505 and 1535 cm-’ can be
t o u n s a t u r a t e d carbenium i o n s formed on t h e
zeolite
surface.
Here
again t h e band c h a r a c t e r i s t i c o f adsorbed benzene a t 1478 c d i s absent. S i n c e t h i s band i s u s u a l l y v e r y sharp and i n t e n s e and we c a n n o t f i n d any
indication
359
I
31 00
2900
..
2700
1600
1500
1400
Wavenumber/cm-’ F i g . 4 . IR s p e c t r a o f cyclohexadiene adsorbed i n z e o l i t e HNaZSM5. ( a ) Z e o l i t e background spectrum. ( b ) A t beam t e m p e r a t u r e , 3 min a f t e r admission o f 213 P a o f cyclohexadiene, ( c ) 1 h, ( d ) 2 h, ( e ) 4 h l a t e r . ( f ) A t 370 K a f t e r 1 h. ( 9 ) A f t e r 1 h e v a c u a t i o n a t beam temperature. of
it
in
our
spectra,
the
formation
of
benzene
from
cyclohexene
upon admission t o z e o l i t e s a t low temperatures does n o t
cyclohexadiene
and occur
t o any a p p r e c i a b l e e x t e n t .
DISCUSSION Electronic unresolved
spectra
usually
extremely d i f f i c u l t . t i o n program (see F i g .
as broad
absorptions
rendering t h e i r
due
complete
to
their
analysis
Consequently, a d j a c e n t e l e c t r o n i c t r a n s i t i o n s merge i n t o
broad o v e r l a p p i n g bands, analysis
appear
rovibrational f i n e structure,
which can be deconvoluted a p p l y i n g a d a t a
5),
d e t a i l s o f which and a p p l i c a t i o n t o
o f U V - V I S s p e c t r a w i l l be p u b l i s h e d elsewhere ( r e f .
manipula-
quantitative
8).
Using
the
bands r e s o l v e d by t h i s procedure q u a n t i t a t i v e c o n c l u s i o n s c o n c e r n i n g c a r b o c a t i o n f o r m a t i o n can be drawn. UV-VIS
bands
of
hydrocarbons adsorbed on s o l i d
surfaces
are
generally
360
compared
t o t h e s p e c t r a o f u n s a t u r a t e d carbenium i o n s observed
in
superacid
s o l u t i o n s . T h i s i s t h e most advantageous way f o r a t t r i b u t i n g bands t o d i s t i n c t i o n i c species, as c a r b o c a t i o n f o r m a t i o n has been v e r y e x t e n s i v e l y s t u d i e d superacids ( r e f . 9 ) .
in
a
10.2 Absorbance u n i t s
C
400
300
I n some cases, rate
of
and
environments not
e.g. f o r t h e appearance o f c e r t a i n carbenium i o n s and t h e i r
formation,
solution only
the
500
X/nm
F i g . 5. Deconvoluted s p e c t r a o f ( a ) benzene, ( b ) c y c l o h e x e n e and ( c ) c y c l o h e x a d i e n e adsorbed i n z e o l i t e s . The s p e c t r a were r e c o r d e d a f t e r e x p o s i n g t h e zeol i t e s t o t h e adsorbates f o r 1 h a t room t e m p e r a t u r e .
the
good
correlations
z e o l i t e surface could
or be
similarities
between
ascertained,
even
o f t h e c a r b o c a t i o n s i n b o t h systems a r e d i f f e r e n t and r a t e o f formation b u t also the s t a b i l i t y
of
the
superacid though
the
influence unsaturated
species ( r e f . l o ) . In
other
cases,
l a r g e r d i f f e r e n c e s i n t h e band
maxima
of
carbocations
formed i n s u p e r a c i d s and i n z e o l i t e s were found. T h i s i s demonstrated by Table
1,
in
which
the
wavelengths o f maximum absorbance o f
the
carbenium
i d e n t i f i e d i n a c i d i c s o l u t i o n s a r e compared t o t h o s e observed upon o f hydrocarbons i n z e o l it e s
.
ions
adsorption
I n t h e case o f benzene t h e two bands observed a t 373 and 495 nm do n o t agree w i t h t h e f i n d i n g s o f p r e v i o u s a u t h o r s ( r e f . 1 2 ) , w h i c h means t h a t f u r t h e r i n v e s t i g a t i o n s are required.
The absence o f monoenylic species, p r o v e d by us,
361
is
quite
understandable s i n c e p r o t o n a t i o n o f t h e a r o m a t i c
y i e l d s d i e n y l i c s p e c i e s and,furthermore,
ring
immediately
r i n g - o p e n i n g seems t o be u n f a v o u r a b l e
a t low temperatures. Comparing t h e s p e c t r a o f benzene,
TABLE 1 A b s o r p t i o n maxima superacids.
cyclohexene and c y c l o h e x a d i e n e ( s e e F i g .
o f c a r b o c a t i o n s formed
Carbocations i n s u p e r a c i d s *
CH(
zeolites,
maxi''
+
C-CH-CH, - - - - - - - CH,
0
305
315
CH, =CH-CH, -CH, -CH, -CH,
325 380 450 298 310 340 391
0
284 324 414 464 517 5 74
0
440, 550 373, 495
325
0
0
330
compared
Hydrocarbons adsorbed i n z e o l i t e s
max'nm CH,\
in
310
0 cb
470
to Ref
**
**
**
12
**
** 13
~
315
450 530 325 357 377 392 415
408
From r e f . 11;
**
T h i s work
12
-14
5 ) i t becomes e v i d e n t t h a t those o f t h e . l a t t e r two resemble each o t h e r . I t i s a l s o obvious t h a t t h e c a p a b i l i t y o f cyclohexadiene f o r i o n f o r m a t i o n exceeds that
o f cyclohexene.
B u t i n t h e i r s p e c t r a t h e r e i s no e v i d e n c e f o r bands
373 and 495 nm c h a r a c t e r i s t i c f o r adsorbed benzene. a
correct
and
unambiguous
assignment o f t h e
at
I t must be mentioned t h a t
UV-VIS
bands
obtained
upon
362
a d s o r p t i o n o f c y c l i c hydrocarbons has n o t y e t been p u b l i s h e d . As
far
as
the
f o r m a t i o n o f benzene
is
concerned,
Gibbs
i n t h e sequence o f t h e r e a c t i o n m e t h y l c y c l o p e n t a n e
formation cyclohexene
+
cyclohexadiene
+
energies cyclohexane
benzene were c a l c u l a t e d f o r d i f f e r e n t
+
of +
tempera-
t u r e s u s i n g t h e thermodynamic d a t a g i v e n i n r e f . 15 (see Table 2 ) . Herefrom i t becomes e v i d e n t t h a t f o r m a t i o n o f benzene and c y c l o h e x a d i e n e i s u n f a v o u r e d low
temperatures.
according
This
is
in
good
agreement
with
experimental
t o which t h e f o r m a t i o n o f a r o m a t i c s f r o m l o w e r o l e f i n s
on
at
results zeolite
HZSM-5 proceeds a t temperatures above 570 K ( r e f . 4 ) . From o u r s p e c t r o s c o p i c r e s u l t s t h e same c o n c l u s i o n s can be drawn. TABLE 2 Gibbs f r e e energy o f f o r m a t i o n o f m e t h y l c y c l o pentane, cyclohexane, cyclohexene, c y c l o h e x a diene, benzene and 1-hexene i n t h e t e m p e r a t u r e range 298-673 K (see r e f . 15).
I
T'K
As
AGf/kJ mol-'
-
MCP
CHAN
CHEN
CHOEN BENZ
298 323 373 473 573 673
35.6 36.6 72.4 123.8 176.8 231.1
31.6 44.7 71.6 127.2 184.4 242.8
106.7 116.3 135.7 176.1 217.8 260.3
186.5 193.0 206.4 234.6 264.0 294.2
1-HEXENE
129.6 133.6 141.8 159.3 177.6 196.5
87.4 98.3 120.6 166.8 214.5 263.2
t h e z e o l i t e s used i n t h i s s t u d y c o n t a i n e d b o t h B r o n s t e d und Lewis
acid
s i t e s , t h e f o r m a t i o n o f t h e f o l l o w i n g s u r f a c e s p e c i e s may be assumed:
b 0 0 0
uv-vis active
*
* ) means a b s o r p t i o n a t h>200 nm This
v e r y s i m p l i f i e d r e a c t i o n scheme shows t h a t f o r m a t i o n
carbenium i o n s can be e x p e c t e d f r o m each compound,
of
unsaturated
i n c l u d i n g cyclohexane,
in
363 t h i s case by secondary t r a n s f o r m a t i o n v i a t h e c y c l o h e x y l carbonium may
be
the
reason
f o r the observation
that
cyclohexane
is
ion.
only
This slowly
UV-VIS a c t i v e s u r f a c e compounds.
converted i n t o
Interconversion o f
carbenium
ions
can
also
take
place
in
zeolites.
T h e r e f o r e t h e bands observed w i t h cyclohexene and cyclohexadiene can be a t t r i buted
to
different
intermediates
formed
upon
transformation
of
primary
generated i o n s . CONLCUSIONS
-
From o u r i n v e s t i g a t i o n s t h e f o l l o w i n g c o n c l u s i o n s must be drawn: From a l l C,
compounds under i n v e s t i g a t i o n u n s a t u r a t e d carbenium i o n s
could
be generated. The c a r b o c a t i o n f o r m a t i o n s t a r t s w i t h t h e monoenyl species, i n t h e case o f benzene w i t h t h e d i e n y l i c carbenium i o n .
- Carbocations cyclohexyl depending sites.
formed and
on
from
cyclohexene and
cyclohexenyl
or
cyclohexadiene
cyclohexenyl
and
may
either
cyclohexadienyl
whether i n t e r a c t i o n t a k e s p l a c e w i t h B r o n s t e d o r
These
be
ions
Lewis
species undergo r i n g - o p e n i n g t o open-chain a l k e n y l
acid
carbenium
ions .
-
Cyclohexane
transforms
in
a b s o r b i n g i n t h e FUV range,
the f i r s t
step
into
saturated
carbocations,
y i e l d i n g e n y l i c carbenium i o n s o n l y a f t e r r i n g -
opening .
-
From
the
formation
spectroscopic capability
sequence cyclohexadiene
-
At
the
i n v e s t i g a t i o n s i t can be
of
>
inferred
t h e hydrocarbons i n v e s t i g a t e d cyclohexene
>
1-hexene
>
cyclohexane
f i r s t stages o f c a r b o c a t i o n development f r o m
C,
that
decreases
>
the
ion
in
the
benzene.
hydrocarbons
the
f o r m a t i o n o f a r o m a t i c s u r f a c e species may be excluded. ACKNOWLEDGEMENT One o f t h e a u t h o r s ( I . K . )
i s g r a t e f u l t o t h e Alexander von Humboldt Founda-
t i o n f o r a r e s e a r c h f e l l o w s h i p . We thank D r . J . t e n Pas f o r t h e s y n t h e s i s o f z e o l i t e ZSM-5. REFERENCES
1 S. Ceckiewicz, J.C.S. Faraday 1, 77 (1981) 269-280; R.L.V. Rao, P. Levesque and B. S j i a r i e l , Can. J . Chem. Eng., 64 (1986) 514516; J . Heering, M. K o t t e r and L. R i e k e r t , Chem. Eng. S c i . , 37 (1982) 581-584; J . Nowakova, L . Kubelkova and Z. D o l e j s e k , J . C a t a l . , 108 (1987) 208-213; T. Mole, G . B e t t and D.Seddon, J . C a t a l . , 84 (1983) 435-445; J.P. Wolthuizen, J.P. van den Berg and J.C.H. van H o o f f , i n B. I m e l i k e t a l . ( E d i t o r s ) , C a t a l y s i s by Z e o l i t e s , E l s e v i e r , Amsterdam, 1980, pp. 85-92.
364 H.G. Karge, i n J.R. Katzer ( E d i t o r ) , Molecular Sieves 11, ACS Symp. Ser. No. 40, Am. Chem. SOC., Washington D.C., 1977, pp. 584-595; V . B o l i s , J. Vedrine, J.P. van den Berg, J.P. Wolthuizen and E.G. Derouane, J.C.S. Faraday 1, 76 (1980) 1606-1616; J.P. van den Berg, J.P. Wolthuizen, A.D.H. Clague, G.R. Hays, R. Huis and J.C.H. van Hooff, J. Catal., 80 (1983) 130-138; M. Zardkoohi, J.F. Haw and J.H. Lunsford, J. Am. Chem. SOC., 109 (1987) 5278- 5280. 3 E.D. Garbowski and H. P r a l i a u d , J. Chim. Phys. Phys.-Chim. B i o l . , 76 (1979) 687-692; H. F o r s t e r , S. Franke and J. Seebode, J.C.S. Faraday 1, 79 (1983) 373-382; H. F o r s t e r , J. Seebode, P. F e j e s and I. K i r i c s i , J.C.S. Faraday 1, 83 (1987) 1109-1117; M. Laniecki and H.G. Karge, i n D. Shopov e t a l . ( E d i t o r s ) , Heterogeneous C a t a l y s i s , Proc. V I t h I n t . Symp., S o f i a , 1987, Vol. 2, pp. 129-134; H. F o r s t e r and I. K i r i c s i , Z e o l i t e s , 7 (1987) 508-510. Garbowski, i n B. I m e l i k e t a l . 4 J.C. Vedrine, D. D e j a i f v e and E.D. ( E d i t o r s ) , C a t a l y s i s by Z e o l i t e s , E l s e v i e r , Amsterdam, 1980, pp. 29-37. 5 I.K i r i c s i and H. F o r s t e r , J.C.S. Faraday 1, 84 (1988) 491-499; H. F o r s t e r , I. K i r i c s i and J. Seebode, i n P.J. Grobet e t a l . ( E d i t o r s ) , I n n o v a t i o n i n Z e o l i t e M a t e r i a l s Science, E l s e v i e r , Amsterdam, 1988, pp. 435-442; S. Maixner, C.Y. Chen, P.J. Grobet, P.A. Jacobs and J. Weitkamp, i n Y . Mukami e t a l . ( E d i t o r s ) , Proc. 7 t h I n t . Z e o l i t e Conf., Tokyo, 1986, pp. 693-700. 6 H.G. Karge, J. Ladebeck, Z.Sarback and K. Hatada, Z e o l i t e s , 2 (1982) 94-102 J. Datka, J.C.S. Faraday 1, 77 (1981) 511-517. 7 J. Haber, J . Komorek-Hlodzik and T. Romotowski, Z e o l i t e s , 2 (1982) 179-184; J. Haber, J. Komorek and T . Romotowski, Proc. o f Zeocat 85, I n t . Symp. on Z e o l i t e C a t a l y s i s , Siofok, 1985, pp. 671-679. 8 I. K i r i c s i and Gy. Tasi, i n p r e p a r a t i o n . 9 G.A. Olah and P. Schleyer ( E d i t o r s ) , Carbonium I o n s I - I V , WileyI n t e r s c i e n c e , New York, 1970. 10 P. Fejes, H. F o r s t e r , I. K i r i c s i and J. Seebode, i n P.A. Jacobs e t a l . (Editors), S t r u c t u r e and R e a c t i v i t y o f M o d i f i e d Z e o l i t e s , E l s e v i e r , Amsterdam, 1984, pp. 91-98. 11 G.A. Olah, C.U. Pittman and M.C.R. Symons, i n G.A. Olah and P . Schleyer ( E d i t o r s ) , Carbonium Ions, Wiley-Interscience, New York, Vol. 1, pp. 153222. 12 A . V . Kiselev, L.A. Kupcha, V . I . Lygin and V.G. S h a t s k i i , K i n e t i k a i Katal i z , 10 (1969) 449-451. 13 C . Bezuhanova, A . V . Kiselev, D.G. K i t i a s h v i l i and V . I . Lygin, K i n e t i k a i Katal i z , 13 (1972) 431-434. 14 H.G. Karge, W. Abke, E.P. Boldingh and M. L a n i e c k i , Actas Simp. Iberoam. C a t a l . 9th, Lisboa, 1984, 1, 582-583. 15 L. Seres, L. Z a l o t a i and F. M a r t a , Acta Phys. Chem. Szeged, 23 (1977), 433468.
2