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27 Zeolite Mediated Carbonylation
Downloaded by CALIFORNIA INST OF TECHNOLOGY on December 9, 2016 | http://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.ch027
PATRICK GELIN, F R É D É R I C LEFEBVRE, BOUBAKER ELLEUCH, C L A U D E NACCACHE, and YOUNÈS BEN TAARIT Institut de Recherches sur la Catalyse, C.N.R.S., 2 avenue Albert Einstein, 69626, Villeurbanne Cédex, France
T r a n s i t i o n metal i o n s , w i t h i n the zeolite framework, may undergo a r e d u c t i v e c a r b o n y l a t i o n to give mononuclear monovalent carbonyl coumpounds M ( I ) ( C O ) and u l t i m a t l y t o give zerovalent polynuclear carbonyl c l u s t e r s . The rhodium(I)and i r i d i u m(I)carbonyls were i d e n t i f i e d using s p e c t r o s c o p i c and volumetric methods, the zerovalent rhodium and i r i d i u m c l u s t e r s M (CO) were a l s o synthetized in the z e o l i t e matrix and t h e i r s t r u c t u r e i n v e s t i g a t e d using IR, NMR and s p i n labelling methods. Carbonylation o f organic substrates was i n v e s t i g a t e d u s i n g these w e l l defined complexes. These carbonyl compounds e x h i b i t e d c a t a l y t i c p r o p e r t i e s in the c a r b o n y l a t i o n o f organic s u b s t r a t e s . In p a r t i c u l a r methan o l c a r b o n y l a t i o n to methyl acetate in the gas phase was s u c c e s s f u l l y attempted. Mechanistic and k i n e t i c s t u d i e s of t h i s r e a c t i o n over rhodium and i r i d i u m z e o l i t e s showed the similarities between the homogeneous and the z e o l i t e mediated r e a c t i o n s . Aromatic nitro compounds were a l s o converted to aromatic i s o c y a nates using s i m i l a r c a t a l y t i c systems. The mechanistic aspect o f t h i s r e a c t i o n will be a l s o examined. n
4
12
In a r e l a t i v e l y few years z e o l i t e s were promoted from simple adsorption agents to c a t a l y s t s of wide spread use in a l l f i e l d s o f chemistry. Apart from t h e i r a c i d i c p r o p e r t i e s generated by exchanging t h e i r N a or K s t a r t i n g forms by ammonium ions and subsequent decomposition o f the l a t t e r , t h e i r unique p r o p e r t i e s as supports f o r various precious metals and t h e i r s o l u t i o n behaviour a t t r a c t e d much of the a t t e n t i o n devoted to c a t a l y s i s . In sustained e f f o r t s to l o c a l i z e t r a n s i t i o n metal cations w i t h i n the z e o l i t e framework, EPR, UV, IR and X-ray d i f f r a c t i o n studies were undertaken by s e v e r a l authors (1-5). This l o c a l i z a t i o n was thought to help c l a r i f y and p o s s i b l y account f o r v a r i a t i o n s o f c a t a l y t i c p r o p e r t i e s upon various parameters i n c l u d i n g the exchange l e v e l . The overwhelming conclusions o f these i n v e s t i gations was the high m o b i l i t y o f the exchanged t r a n s i t i o n metal +
+
0097-6156/83/0218-0455$06.00/0 © 1983 American Chemical Society Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
456
INTRAZEOLITE
CHEMISTRY
c a t i o n s induced by the presence o r removal o f p o t e n t i a l l i g a n d s be they organic o r i n o r g a n i c (1-5). Furthermore, it was shown t h a t these l i g a n d s when competing w i t h l a t t i c e oxide ions may w e l l form d e f i n e d c o o r d i n a t i o n complexes w i t h the t r a n s i t i o n metal c a t i o n (1-5). I n some complexes the l i g a n d s were l o o s e l y bound (almost r e v e r s i b l y ) t o the c a t i o n ' ' i n others the complexes may have the same s t r u c t u r e and s i m i l a r s t a b i l i t y as those formed i n s o l u t i o n (1, 4 ) . Then z e o l i t e s r a p i d l y appeared as most convenient matrices o r s o l i d s o l v e n t s capable o f accomodating and s o l v a t i n g c o o r d i n a t i o n complexes o f p o t e n t i a l c a t a l y t i c use. The v a l u a b l e advantages o f h e t e r o g e n i z i n g homogeneous c a t a l y s t s w i t h i n the z e o l i t e porous s t r u c t u r e was then e n v i s i o n e d . Some o f the complexes were introduced w i t h i n the z e o l i t e u s i n g simp l e s u b l i m a t i o n and a d s o r p t i o n techniques (6, 7). Others were d i r e c t l y synthesized i n s i t u , s t a r t i n g u s u a l l y from the appropriate t r a n s i t i o n metal c a t i o n introduced by conventional ion-exchange
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3
(1,
4,
8).
F a u j a s i t e type z e o l i t e s because o f the s i z e o f t h e i r c a v i t i e s and apertures were the most f r e q u e n t l y used i n t h i s purpose. T h e i r w e l l known s t r u c t u r e , acid-base and redox p r o p e r t i e s helped much i n s e l e c t i n g those z e o l i t e s . Many e f f o r t s were devoted to many aspects o f the i n t r a z e o l i t i c chemistry o f t r a n s i t i o n metal i o n s . Carbonylation r e a c t i o n s both o f the t r a n s i t i o n metal ions themselves and organic s u b s t r a tes were w i d e l y i n v e s t i g a t e d i n view o f t h e i r academic i n t e r e s t as w e l l as p r a c t i c a l use. These s t u d i e s were favoured by the r a p i d l y expanding chemistry o f carbon monoxide which grew t o a major research f i e l d . Carbon monoxide would appear as a p o t e n t i a l subst i t u t e t o the v a n i s h i n g o i l and perhaps more s u r e l y as a p r i v i l e ged reagent used t o f u n c t i o n a l i z e o r t o expand organic molecules (9).
Carbonylation o f methanol and n i t r o a r o m a t i c s , hydroformylation of o l e f i n s and a l c o h o l homologation were among the p r i n c i p a l react i o n s aimed a t producing high added value molecules. The v a r i o u s aspects o f c a r b o n y l a t i o n i n z e o l i t e media u s u a l l y i n c l u d e the c a r b o n y l a t i o n o f the i n o r g a n i c precursor t o form the a c t i v e c a r b o n y l a t i o n complex. The t r a n s f e r o f the a c t i v a t e d carbon monoxide molecule t o an organic one may subsequently take p l a c e . We s h a l l r e s t r i c t ourselves t o the a c t i v a t i o n o f carbon monoxide by the exchanged t r a n s i t i o n metal ions and the r e s u l t i n g formation of t r a n s i t i o n metal carbonyls. The mechanistic aspects o f the t r a n s f e r o f carbon monoxide t o other s u b s t r a t e s w i l l be examined i n the l i g h t o f s p e c t r o s c o p i c k i n e t i c and other data a v a i l a b l e i n the l i t e r a t u r e o r provided by recent r e s u l t s i n our l a b o r a t o r y . Synthesis and s t r u c t u r e o f t r a n s i t i o n metal carbonyl i n the z e o l i te medium E a r l y work showed the a f f i n i t y o f carbon monoxide f o r t r a n s i t i o n metal c a t i o n s introduced i n t o z e o l i t e s by conventional i o n
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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27.
GELIN E T A L .
Zeolite Mediated Carbonylation
457
exchange. E s s e n t i a l l y IR, g r a v i m e t r i c , UV, EPR and NMR techniques were used to probe the s t r u c t u r e of the complexes, t h e i r s t a b i l i t y and the nature of the bonding of the CO molecule to the t r a n s i t i o n metal c a t i o n (3, 10, 11, 12). In p a r t i c u l a r , the IR s p e c t r o s copy of such complexes has been r e c e n t l y reviewed and it appeared that the i n t e r a c t i o n was r a t h e r weak w i t h absorptions at high frequency u s u a l l y higher than f r e e carbon monoxide ( 3 ) . The general c o n c l u s i o n was that r a t h e r weak bonding was e s t a b l i s h e d between the c a t i o n and the carbon monoxide molecule. The scheme of the bonding was depicted as e s s e n t i a l l y a t r a n s f e r of the carbon lone p a i r to the c a t i o n empty o r b i t a l s with a v a r i a b l e extent back-donation from the c a t i o n f i l l e d o r b i t a l s to the a n t i bonding Π . o r b i t a l s of the CO molecule. T h i s bonding scheme gene r a l l y r e s u l t e d i n an e l e c t r o n d e f i c i e n t carbonyl carbon e s p e c i a l l y i n view of the weak back-donation to the Π . molecular o r b i t a l s of CO. This makes the carbon p a r t i c u l a r l y s u i t a b l e f o r n u c l e o p h i l i c attacks by e l e c t r o p h i l e s . However more s t a b l e and b e t t e r d e f i n e d complexes are a l s o formed w i t h rhodium, i r i d i u m and ruthenium exchanged z e o l i t e s . Upon r e a c t i o n w i t h C0;we s h a l l r e s t r i c t ourselves to the f i r s t two elements to give a d e t a i l e d p i c t u r e of t h e i r c a r b o n y l s . These may i n v o l v e one or more metal nucleus at a time. Therefore we s h a l l d i s t i n g u i s h mononuclear and p o l y n u c l e a r c a r b o n y l s . The Mononuclear Carbonyls When exchanged i n t o the z e o l i t e as | R h ( N H ) ^ C l t h e rhodium ammine complex could decompose i n an oxygen stream at temperatures ranging 150-350°C i n t o a r h o dium I I I - h y d r o x y - s p e c i e s with a p a r t i a l r e d u c t i o n i n t o diamagnet i c Rh(I) and paramagnetic ( l e s s than 10%) R h l l s p e c i e s . E q u i l i b r a t i o n with carbon monoxide at room temperature and low pressure (a few t o r r ) y i e l d e d the rhodium (I) - d i c a r b o n y l com pound (13) i n a d d i t i o n to the Rh(I)(C0) paramagnetic complexe (11). The s t r u c t u r e of t h i s complex was e l u c i d a t e d by ESCA and UV mea surements (13) which showed that the t r i v a l e n t rhodium was indeed reduced to the monovalent s t a t e and by i n f r a r e d spectroscopy which provided evidence f o r a gem d i c a r b o n y l (14). Use of 1:1 CO : 13co mixture confirmed the i d e n t i f i c a t i o n of the complex. Assuming a C 2 v . symmetry f o r the complex, p r e d i c t i o n of the adsorption f r e quencies depending on the degree of CO s u b s t i t u t i o n is p o s s i b l e (15) and it was confirmed that the observed frequencies agreed remarkably with the p r e d i c t e d ones thus confirming the assumed symmetry and therefore the number of CO ligands per rhodium (14). whether the rhodium d i c a r b o n y l was attached to the z e o l i t e l a t t i c e or to an extra-framework anion such as OH, 0^ or a l a b i l e i o n , could be a l s o decided upon u s i n g IR spectroscopy. Indeed l a t t i c e v i b r a t i o n between 1300 and 300 cnT. c h a r a c t e r i s t i c of an NaY z e o l i t e (16) are s e n s i t i v e to the i n t e r a c t i o n of l a t t i c e oxide ions with c a t i o n s . In p a r t i c u l a r , it was observed that an IR a b s o r p t i o n band at 877 cm~l grew simultaneously with the growth of CO absorptions at 2115-2048 c h a r a c t e r i s t i c of the d i c a r b o n y l ( 1 3 ) . T h i s 3
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
458
INTRAZEOLITE CHEMISTRY
former band was i n t e r p r e t e d as due to the lengthening o f T - 0 bonds as a r e s u l t of the i n t e r a c t i o n o f the concerned oxide i o n with the monovalent rhodium. This provided evidence f o r the bonding o f the rhodium I d i c a r b o n y l to a l a t t i c e oxide i o n . Moreover u s i n g C NMR spectroscopy only a broad f e a t u r e l e s s a b s o r p t i o n was observed even though the sample was h i g h l y enriched with . C 0 w h i l e more cumbersome molecules such as Mo(CO)^ (17), Cr(CO) 0 7 ) , Ni(CO) (18), F e ( C O ) and even F e ( C O ) sublimed i n t o NaY z e o l i t e s gave r a t h e r sharp C resonances. This is a c l e a r i n d i c a t i o n that while these l a t t e r compounds r e t a i n e d t h e i r degrees of freedom, the rhodium d i c a r b o n y l motion is p a r t i c u l a r l y r e s t r i c t e d thus g i v i n g r i s e to an important a n i s o t r o p y . Indeed f a s t s p i n n i n g (^ 3000 Hz) at the Magic Angle ( M . A . S . ) gave r i s e to a sharp a b s o r p t i o n at 183 ppm downfield w i t h respect to TMS c o n s i s tent w i t h a Z e o - O - R h ( I ) ( C 0 ) complex. In t a b l e 1 values of the chemical s h i f t of v a r i o u s X-Rh(I)(CO) are gathered. I t is apparent that the z e o l i t e l a t t i c e acted as l i g a n d f o r the c e n t r a l ion. 3
6
4
5
3
J 2
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1 3
2
Table 1 : Values of 6 ( C )
f o r XRh(C0)
1 3
Compound
compounds.
6( C)
Solvent
( -c H )Rh(co)
2
, 3
CH C1
3
190.9
(pyridine)Rh(CO) C1
CH C1
2
181.1,
(acac)Rh(C0)
C H
5
n
5
5
2
2
2
3
2
2
C 1
181.2
183.8
2
IR s t u d i e s showed that each band of the VCO doublet c h a r a c t e r i s t i c of the Rh(I) gem d i c a r b o n y l was s p l i t i n t o two components. Recent experiments (14) showed that the presence o f r e s i d u a l water i n v a r i a b l e c o n t e n t s T i g n i f i c a n t l y a l t e r e d the i n t e n s i t y r a t i o of the two components of each of the two bands. Only the low f r e q u e n cy components appeared i n the case o f the s t r i c t l y anhydrous zeo l i t e . As the r e s i d u a l water content i n c r e a s e d (as monitored by the vOH absorptions a t 3640 and 3550 c m " ) , the high frequency components grew s i m u l t a n e o u s l y . A f u l l y hydrated z e o l i t e was c h a r a c t e r i z e d by a c l e a n doublet at 2090-2030 c m " ' . Evacuation o f excess water r e s t o r e d the low frequency components o f the d o u b l e t . Thus the presence o f one o r more water molecules as l i g a n d s o f the monovalent rhodium s i g n i f i c a n t l y a l t e r e d i t s back-donating p o t e n t i a l towards the Π . of the CO molecule and modified the bond angle as w e l l ( t a b l e 2 ) . 1
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
27.
GELIN ET AL. Table 2 :
Zeolite Mediated Carbonylation Values of v
r n
f o r Rh(I)(CO)
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Compound
v
459 i n NaY z e o l i t e .
0
co
(
c
m
'
l
)
Rh(I)(CO)
2
i n NaY
2101
2022
Rh(I)(CO)
2
with water i n NaY
2090
2030
Hence the rhodium I I I solvated by l a t t i c e oxide ions and p r e sumably e x t r a framework oxide ions or hydroxo ligands (depending on the dehydration s t a t e ) could be carboxylated r e d u c t i v e l y to rhodium I d i c a r b o n y l according to one of the f o l l o w i n g r e a c t i o n scheme depending on the h y d r a t i o n s t a t e Rh(III) + 3 C A l ^ S i ' ) "
+ 3 CO
Rh(I)(C0)
+ ("Si'^Al'T
o
+ OAl'°X)~ ( A1 Si) V
when t o t a l l y dehydrated. " or / Rh(lII)(0H) + ( A l ^ S i ) " + 3 CO -.C0
'
x
o
V
-
+ H 0+
o
Rh(I)(C0)
/ ^ '
o
+ C0
o
+
s
^Al^Si')"
2
when hydrated. Rh(I) could complete i t s c o o r d i n a t i o n s h e l l by b i n d i n g to lat t i c e oxide ions or to free water molecules i n a d d i t i o n to at l e a s t one oxide i o n . The I r i d i u m Carbonyl I r i d i u m (III ) z e o l i t e could be obtained by conventional ion-exchange of sodium ions by I r ( N H ) c C l ^ com plex i n aqueous s o l u t i o n and subsequent a c t i v a t i o n i n flowing oxy gen at temperatures not exceeding 2 5 0 ° C . Then I r ( I I I ) - h y d r o x o species were obtained (8). This l a t t e r may undergo subsequent c a r b o n y l a t i o n at low or atmospheric pressure i n the temperature range 150-170°C. The r e s u l t i n g complex was analyzed u s i n g d i f f e r e n t volumetric spectroscopic and s p i n l a b e l i n g methods. Volumetric measurements showed t h a t , upon treatment with c a r bon monoxide at 170°C under about 5 t o r r , assuming all i r i d i u m has been i n v o l v e d , 4 CO molecules were consumed per c a t i o n . Hence a s suming r e d u c t i o n of the t r i v a l e n t c a t i o n to the monovalent s t a t e , as usual f o r i r i d i u m , the carbonyl compound should i n c l u d e three CO l i g a n d s . Indeed IR determinations showed two strong VQQ bands at 2086-2001 cm" which could be due to e i t h e r I r ( C 0 ) „ (O, symme t r y ) or Ir(C0)3 (0^ symmetry). Unambiguous assignment couYd be sought through a n a l y s i s of the IR s p e c t r a of the ^QOi ^ C O com pound. Indeed it was confirmed that the compound i n c l u d e d a M(C0)g moiety thus e q u a l l y demonstrating the r e d u c t i o n o f the t r i v a l e n t i r i d i u m to the monovalent s t a t e . S i m i l a r l y the s h i f t of the l a t t i c e +
3
1
ν
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
INTRAZEOLITE CHEMISTRY
460
1
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vibration upon formation of the iridium compound to about 877 cm" again provided evidence for the binding of the monovalent iridium tri-carbonyl to the zeolite l a t t i c e .
Polynuclear Carbonyls Zeolite attached Rh(I)(CO)^ complexes, though f a i r l y stable at room temperature under CO and water, slow ly transform to another compound when subjected to C0:H«0 mixture. The slow transformation was perceptible at room temperature but was more important around 50°C. The new species was again charac terized via i t s IR and NMR spectra. As the vCO absorption bands due to the dicarbonyl decreased an IR band at 2340 cm" due to C0« developed gradually together with a set of absorptions around 2100-2000 cm" due to new linear carbonyls and absorptions around 1800 cm" presumably due to brid ged carbonyls (14). C0« appearance was interpreted as an indica tion of the further reduction of the monovalent rhodium either by CO or via the water gas s h i f t reaction producing H~ which is reported to occur on monoculear monovalent carbonyls (19, 20). As rhodium I was reduced to the zerovalent state, the observed VCO bands were ascribed to Rh^(CO)j2 compound, i n view of the excel lent agreement between the observed frequencies and those repor ted for Rh^(CO)j« i n CH^Cl^ or nujol or when adsorbed on f u l l y dehydrated zeolite. Again ^C0 labeling gave rise to an IR spectrum i n the brid ged VCO region which confirmed the assignment to a M3 (CO)3 sys tem with a Όsymmetry thus confirming the structure of the com pound (14). Also C NMR measurements obtained under the (M.Â.S.) conditions gave two sharp absorptions corresponding to the linear and bridged carbonyls at the same chemical shifts reported i n solution (34) except that the resolution reached i n solid state NMR precluded observation of Rh-C coupling usually of previous help i n structure determinations. However the chemical shifts values being close to those observed i n solution indicate that the interactions between the zeolite and the carbonyl compound do not exceed Van der Waals interaction similar to that prevailing between Rh, (CO)j and such solvents as CHCl^, CH C1 , etc... a situation similar to that reported for mononuclear zerovalent carbonyls (17). Kinetics of the formation of Rh^(CO) i n zeolites showed that the rate of formation decreased with increasing temperature, an i n d i cation of a reaction order > 2 and of a rather complex mechanism of formation starting from zeolite bound Rh(l)(C0) . The s t a b i l i t y domain of Rh.(CO) i n zeolites does not expand over a few tens of degrees (30-/0°C) and at even mild temperatures yet a new carbonyl compound formed also readily obtainable using a C0:H mixture as reported by Mantovani et al.(21) who identified this compound characterized by an infrared spectrum showing vCO bands at 2095, 2080, 2060 and 1765 cm" which they assigned to the linear and bridged carbonyls of R h ^ ( C O ) . In agreement with Montovani et al. recent synthesis of Rh^CO) . u s i n g a C0:H mixture at atmospheric pressure, starting from Rn I (CO) Y was carried out at 100-150°C. Characterization of the structure was made by analogy of the IR spectra of sublimed Rh (C0)j£ into the 1
1
1
1
13
2
2
2
J2
2
J2
2
1
2
2
6
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
27.
GELIN E T
AL.
Zeolite Mediated Carbonylation
461
z e o l i t e . Further use of ^ C O i ' ^ C O i n a 1:1 r a t i o i n the synthesis mixture gave r i s e to CO bands due to the face b r i d g e d carbonyls which f i t t e d a Td symmetry c o n s i s t e n t w i t h an Rh (CO)^ model thus confirming the assignment of the IR spectrum to the Rh^(CO)]^ compound . The z e o l i t e i n t e r a c t i o n w i t h t h i s compound was evidenced by the low frequency s h i f t experienced by the CO v i b r a t i o n of the face b r i d g e d c a r b o n y l s . Competition between r e s i d u a l or added wat e r was witnessed suggesting that the s o l v a t i n g p r o p e r t i e s of the z e o l i t e and water were s i m i l a r and rather weak. Therefore the s t a b i l i z a t i o n of these zerovalent carbonyls w i t h i n the z e o l i t e porous s t r u c t u r e should be a t t r i b u t e d to a cage r a t h e r than to a chemical e f f e c t . When treated w i t h a C O ^ O mixture or a C O : H mixture I r ( I ) (C0)~ c o u l d be transformed i n t o a new compound showing a d i f f e r e n t IR a b s o r p t i o n p a t t e r n c h a r a c t e r i s t i c of I r ^ i C O ) ^ . Again p r o c e dures i d e n t i c a l to those d e s c r i b e d p r e v i o u s l y helped to determine the s t r u c t u r e of the new carbonyl compound : mass spectrometry and IR spectroscopy a s s o c i a t e d w i t h C 0 l a b e l i n g , magic Angle S p i n ning high r e s o l u t i o n NMR and f i n a l l y e x t r a c t i o n o f t h i s commonl y known as an i n e r t compound i n r e f l u x i n g toluene and f u r t h e r s p e c t r o s c o p i c measurements to confirm the " i n s i t u " i d e n t i f i c a t i o n . Higher n u c l e a r i t y i r i d i u m carbonyl c l u s t e r s were a l s o thought to form under more severe c o n d i t i o n s i . e . higher C O : H p r e s s u r e s . However the i n t e r e s t i n g feature concerning the I r / f C O ) . synthesis i n the z e o l i t e medium is c e r t a i n l y the r a t h e r m i l d c o n d i t i o n s used compared to those imposed i n s o l u t i o n chemistry where I r C l ^ ^ O y i e l d e d I r ^ ( C 0 ) j 2 provided a CO pressure of over 100 atmospheres is used. A l s o it is noteworthy that z e o l i t e s appear to be b e t t e r s o l v a t i n g m a t e r i a l than any other known s o l v e n t f o r I r ^ ( C 0 ) - . This is a p a r t i c u l a r l y i n t e r e s t i n g example of s o l v a t i n g a b i l i t i e s of z e o l i t e s towards i n e r t compounds.
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fi
2
, 3
2
2
2
Carbonylation of organic
substrates
Apart from the property of i n o r g a n i c species to add carbon monoxide, t h i s molecule could add to or be i n s e r t e d i n t o organic substrates. Carbonylation of Methanol Carbonylation of methanol provides an example of a r e a c t i o n c a t a l y z e d i n homogeneous and heterogeneous media u s i n g monovalent rhodium or i r i d i u m carbonyl complexes. Due to the tremendous importance of t h i s r e a c t i o n from the p r a c t i c a l p o i n t of view, numerous s t u d i e s were devoted to the i n v e s t i g a t i o n of i t s mechanism and k i n e t i c s . On the other hand t h i s reaction provides yet an i n t e r e s t i n g example of the f i r s t C-C bond format i o n which could be matched w i t h that o c c u r i n g d u r i n g conversion of methanol to hydrocarbons. G e n e r a l l y t h i s r e a c t i o n proceeds under m i l d c o n d i t i o n s i n the presence of an i o d i d e promotor (HI or CH^I). The homogeneous phase s t u d i e s (22, 23) e s t a b l i s h e d three major steps f o l l o w i n g the formation of the monovalent carbonyl :
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
INTRAZEOLITE CHEMISTRY
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462
these could be summarized as : ( i ) o x i d a t i v e a d d i t i o n of the p r o motor e s s e n t i a l l y the methyl h a l i d e (formed by the r e a c t i o n CH^OH + HI i f not added d i r e c t l y ) ( i i ) methyl m i g r a t i o n to i n s e r t the carbonyl between the metal and the a l k y l . The C-C bond is then formed ( i i i ) the r e d u c t i v e e l i m i n a t i o n of the a c e t y l h a l i d e and subsequent e s t e r i f i c a t i o n to methylacetate. The r e l a t i v e importance of these steps from the k i n e t i c p o i n t of view may vary depending on the nature of the c a t a l y s t . In the case of z e o l i t e s w i t h the d e c l a r e d aim of h e t e r o g e n i z i n g s o l u b l e c a t a l y s t s sustained e f f o r t was devoted to understand the r e a c t i o n mechanism, the r o l e o f the c e n t r a l m e t a l , the impor tance of the promotor, the importance of the medium, e t c . . . The K i n e t i c s of Methanol C a r b o n y l a t i o n Over RhX, RhY and IrY zeolites C a r b o n y l a t i o n o f methanol proceeds r e a d i l y at atmosphe r i c pressure under m i l d temperature c o n d i t i o n s 1 5 0 ° - 1 8 0 ° C . This r e a c t i o n 2CH OH + CO ->. CH^COOCH^ + H 0 produces mainly methyl acetate and water. A c e t i c a c i d was detected at h i g h conversions and high temperatures. Traces of dimethyl ether could a l s o form. In most cases the s e l e c t i v i t y to methyl acetate was at l e a s t 90% i n presence of the i o d i d e promotor. As i n homogeneous media c a r b o n y l a t i o n of methanol e x h i b i t e d a f i r s t order r a t e law w i t h respect to methyl i o d i d e and a zero o r der w i t h r e s p e c t to CO and CH^OH when R h - Z e o l i t e s were used. S i m i l a r l y when I r - z e o l i t e s were employed the r e a c t i o n r a t e was f i r s t order with respect to methanol and zero order with r e s p e c t to CO and C E L L Thus the k i n e t i c s of the z e o l i t e c a t a l y z e d r e a c t i o n s c l e a r l y p a r a l l e l e d c l o s e l y those of the homogeneous r e a c t i o n u s i n g the same t r a n s i t i o n metal as c a t a l y s t . Again z e o l i t e s acted as simple solvents f o r the c a r b o n y l a t i o n r e a c t i o n though perhaps a s p e c i f i c r o l e of the z e o l i t e should not perhaps be e n t i r e l y excluded. The k i n e t i c s i m i l a r i t i e s s t r o n g l y suggested n e a r l y i d e n t i c a l r e a c t i o n pathways i n the z e o l i t e medium and i n s o l u t i o n w i t h i d e n t i c a l r a t e l i m i t i n g step (24, 27). 3
2
Reaction pathways There is general agreement as to the n a ture of the c a t a l y s t precursor (26, 30). I t is w e l l admitted that even under CO low pressure R h ( I I I ) - Y was reduced to the monovalent rhodium d i c a r b o n y l attached to the z e o l i t e framework v i a one or more oxide ions i r r e s p e c t i v e of the rhodium i n t r o d u c t i o n procedure onto the z e o l i t e . S i m i l a r l y , it was shown, though few s t u d i e s were reported f o r i r i d i u m - z e o l i t e s , that I r ( I ) t r i c a r b o n y l was formed upon r e a c t i o n of CO w i t h I r ( I I I ) - Y z e o l i t e s at 170°C which is w i t h i n the methanol c a r b o n y l a t i o n temperature range. These compounds added methyl i o d i d e at room temperature e i t h e r slowly i n the case of R h ( I ) ( C 0 ) Z (27) ( Z e o l i t e = Z) or r e a d i l y i n the case of the I r ( I ) ( Ο Ο ) ^ Ζ {21J. It was suggested that t h i s was an o x i d a t i v e a d d i t i o n as might be concluded from the high f r e quency s h i f t of the VCO bands of the r e s u l t i n g complex. UV 2
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
27.
Zeolite Mediated Carbonylation
GELIN E T AL.
463
measurements performed i n the case of the a d d i t i o n of Mel to Rh(CO) Z showed that indeed rhodium(l)was r e o x i d i z e d to the t r i v a l e n t s t a t e (35). On the other hand the C H I a d d i t i o n to the r h o dium carbonyl gave r i s e to a species c h a r a c t e r i z e d by a vCO at 1725 cm" which was i d e n t i f i e d as an a c e t y l group coordinated to the t r i v a l e n t rhodium. In both cases NMR showed the carbonyls attached to the rhodium or i r i d i u m precursor e x h i b i t e d an important h i g h f i e l d s h i f t (31) upon CH I a d d i t i o n , i n d i c a t i v e of CO coordinated to high o x i d a t i o n s t a t e c a t i o n . Furthermore it was shown that such carbonyl carbons were h i g h l y e l e c t r o n d e f i c i e n t thus p a r t i c u l a r l y s u i t e d f o r a n u c l e o p h i l i c a t t a c k by species such as a l k y l s e t c . . . A c c o r d i n g l y an o v e r a l l r e a c t i o n scheme was p o s t u l a t e d 2
3
1
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3
6~ Ζ - Rh(I)(C0)
o
+ Mel
CO δ
+
I
C H - Rh(III) - I 0
short
lived
CO ζ CO t CH
3
-
CO - R h ( l l l ) - I
Ζ The low c o n c e n t r a t i o n of the adduct compound is probably due to a slow a d d i t i o n of the methyl i o d i d e and a r a p i d rearrangement of the adduct to form the a c e t y l . This is reasonable i n view of the f i r s t order r a t e found f o r methyl i o d i d e . The f o l l o w i n g step could be the r e d u c t i v e e l i m i n a t i o n o f the a c e t y l h a l i d e to r e a c t w i t h methanol. The growth of IR bands i n the 1710-1685 cm" domain might be i n t e r p r e t e d as due to CH C0I accumulation and p o s s i b l y f u r t h e r r e a c t i o n w i t h substrates present i n the medium. Nevertheless r e a d d i t i o n of CO r e s t o r e d the monova l e n t rhodium d i c a r b o n y l thus i n d i c a t i v e that somehow CH C0I was eliminated. In the case of i r i d i u m z e o l i t e the a c e t y l formation was not reported to occur spontaneously though the o x i d a t i v e a d d i t i o n was a r a p i d s t e p . The r e s u l t i n g complex was s t a b l e and d i d not r e a r range even at 1 7 0 ° C . Only p a r t i a l decarbonylation occurred.However under carbon monoxide pressure the adduct complex seemed to r e a r range i n t o the a c e t y l or perhaps give r i s e to yet another surface organic carbonyl s p e c i e s . Methanol reacted a l s o r a p i d l y w i t h the adduct to form what might be an alkoxy carbonyl l i g a n d to the t r i v a l e n t i r i d i u m by d i r e c t attack of the methoxy group on the e l e c t r o p h i l i c carbonyl carbon. Such an alkoxy group would account f o r the VCO band which appeared at 1705 cm" f o l l o w i n g methanol a d d i t i o n onto the adduct. However a methanol a s s i s t e d rearrangement of the adduct cannot be excluded. Subsequent i n t r o d u c t i o n of CO r e s u l t e d i n the formation of the methyl acetate presumably by a concerted e l i m i n a t i o n of the a l k y l and the alkoxy group and simultaneous c a r b o n y l a t i o n . T h i s might be an i n d i c a t i o n of a more covalent CH« - I r ( I ) bond which 1
3
3
1
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
464
INTRAZEOLITE CHEMISTRY
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then would prevent the eis nucleophilic attack on the carbonyl carbon of the addition complex. The differences displayed by rhodium and iridium zeolites when interacting with the reaction partners are consistent with their different kinetic behaviour. These differences seem to be independent upon the nature of the medium (solution or z e o l i t i c ) and appear to be essentially relevant to the chemistry of rhodium and iridium. The Role of Zeolites Where zeolites seem to play a more specific role appears probably when activities and selectivities are examined. Though few comparative studies were made possible upon examination of the littérature, it would appear that both rhodium and iridium exhibited lower a c t i v i t i e s than i n the homogeneous reaction systems but far larger a c t i v i t i e s than exhibited when supported over traditional carriers (Al^O^, Al^O^-SiO^, carbon, etc...). This is probably due to the better molecular dispersion of the rhodium and iridium catalytic precursors i n z e o l i te media than over any of these carriers and s t i l l more important diffusion limitations than i n solution. The catalytic influence seemed to be reflected by a number of features : - Rhodium Y zeolites appear to be efficient at significantly lower temperature than rhodium X zeolites. This might be due to the higher polarization a b i l i t y of the Y type zeolite which would favor the methyl addition onto the rhodium dicarbonyl which is the slow step i n the case of rhodium. - Compared to other carriers and particularly functionalized polymers the s t a b i l i t y of the rhodium catalyst in the zeolite medium is far superior presumably due to the stronger Rh-0 bond compared to P-Rh bond in phosphine bound rhodiumiI)dicarbonyl. Also the cage structure of the zeolite almost precluded inner interaction of the active species and therefore their association into inactive compounds. - In the case of ethanol and higher alcohols the zeolite mat r i x may well have a negative effect i n that it might favor dehydration or dehydrogenation reactions to form ethers and olefins. The Importance of the Nature of the Alcohol It was shown by Scurrel and Coworkers (32) that alcohols were not equally carbonylated. Under identical conditions the rates follow the sequence CH^ > C^H , isoCJH^ where only propene was produced. As the actually carbonylated species is the alkyl halide it is expected that the activity should depend on the polar or non polar character of the R-X bond. Therefore the size and the nucleop h i l i c character of the σ alkyl attached to the transition metal should be considered. Further the alkyl migration to effect the nucleophilic attack on the carbonyl should also depend on the M-R polarizability as - M° and indeed on the tendency of the radical to give rise to elimination processes.
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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27.
GELIN E T A L .
Zeolite Mediated Carbonylation
465
T h i s i n t r i c a t e behaviour was r e f l e c t e d i n the competitive c a r b o n y l a t i o n of the methyl and e t h y l r a d i c a l s when E t I + MeOH and Mel + EtOH mixtures were r e a c t e d . E t h y l acetate but a l s o t r a ce methyl acetate were produced. No e t h y l propionate was detec ted on the one hand and e s s e n t i a l l y methyl acetate and lower amounts of p r o p i o n i c a c i d , methyl propionate and e t h y l acetate were formed (25) using rhodium z e o l i t e s i n d i c a t i n g that various exchange r e a c t i o n s were proceeding with h i g h enough r a t e s so as to r e s u l t i n a d i f f e r e n t e f f e c t i v e feed composition. These s i d e r e a c t i o n s r e v e a l the importance of the p o l a r i t y of the medium as w e l l as the nature of the t r a n s i t i o n m e t a l . The p o l a r i t y of the medium and the complexing a b i l i t y of the t r a n s i t i o n metal may w e l l account f o r the a c t i v i t y d i f f e r e n c e s observed between the X and Y type z e o l i t e s . On the other hand the i n t r i n s i c a c t i v i t y of each of these elements decreased with i n c r e a s i n g the l o a d i n g . This could be due to a progressive washing of the a c t i v e metal (which has been reported to occur) or to a l e s s e r a c c e s s i b i l i t y of the reactants which may not reach the inner depth of i n d i v i d u a l z e o l i t e c r y s t a l s where the a c t i v e substance could yet r e s i d e . I t is not unreasonable to assume that the z e o l i t e - M i l ) (CO) i n t e r a c t i o n could be modified by higher exchange l e v e l s and therefore e f f e c t the e l e c t r o n d e n s i t y at the a c t i v e metal and therefore i t s c a t a l y t i c p r o p e r t i e s . This would be i n agreement with reports i n homogeneous c a r b o n y l a t i o n of the e f f e c t of the p o l a r i t y of the medium on the a c t i v i t y where an optimum e l e c t r i c constant e x i s t s (33). Indeed the ion-exchange l e v e l may modify d r a m a t i c a l l y be proton content of the z e o l i t e thus a f f e c t i n g i t s d i e l e c t r i c constant. Carbonylation of Nitroaromatics matics to isocyanates R--N0 + 3C0 2
-> 2C0
2
C a r b o n y l a t i o n of n i t r o a r o -
+ R - φ - NCO
was reported to proceed i n a homogeneous mixture of P d C l - p y r i d i n e i n various solvents at 1 8 0 - 2 4 0 ° C under CO pressures of 50-500 atmospheres (34). In view of the severe experimental c o n d i t i o n s used, l i t t l e is known as to the nature of the a c t i v e s p e c i e s . Yet two p o s s i b i l i t i e s could be c o n s i d e r e d . E i t h e r the palladium operates as a s o l u b l e complex or as metal p a r t i c l e s i n a heterogeneous r e a c t i o n . In support of the l a t t e r c l a i m formation of metal p a r t i c l e s was evidenced. However it could be argued that such p a r t i c l e s r e s u l t from the d e a c t i v a t i o n of the c a t a l y s t . Z e o l i t e s as e x c e l l e n t s t a b i l i z i n g agents f o r Pd(II) or w e l l dispersed Pd(0) were then used as a c a r r i e r f o r t h i s r e a c t i o n (35). I t was shown that the a c t i v i t y of the c a t a l y s t depended c h i e f l y on the a b i l i t y of the t r a n s i t i o n metal i o n to undergo e a s i l y redox c y c l e s . In t h i s r e s p e c t , t r a n s i t i o n metal with quasi-permanent c a t i o n i c s t a t e such as c o b a l t or with quasi permanent m e t a l l i c s t a t e such as platinum e x h i b i t e d the lowest a c t i v i t y . By contrast copper, rhodium, i r i d i u m and p a r t i c u l a r l y palladium showed the highest a c t i v i t y though with v a r y i n g s e l e c t i v i t i e s . 2
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
INTRAZEOLITE
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466
CHEMISTRY
A s s o c i a t i o n of oxygen t r a n s f e r agents such as F e ( I I I ) , Cu(II) MoO^ to p a l l a d i u m increased i t s a c t i v i t y . Furthermore both o x i d i zed and reduced forms of P d - z e o l i t e appeared to be e q u a l l y a c t i v e and s e l e c t i v e . P a r t i a l o x i d a t i o n of the reduced form and reduction of the o x i d i z e d form were observed. Hence a redox c y c l e during c a t a l y s i s seems h i g h l y probable i n view of these r e s u l t s which is not unreasonable c o n s i d e r i n g the redox nature of the r e a c t i o n where n i t r o g r o u p s have yet to be reduced by carbon monoxide before the isocyanate could form. Indeed IR s t u d i e s showed that a c t u a l l y nitrogroups were reduced to n i t r o s o before e l i m i n a t i o n of the l a t t e r on f u r t h e r h e a t i n g adsorbed nitrobenzene adsorbed on P d - z e o l i t e i n presence of carbon monoxide. The r e d u c t i o n could then lead u l t i m a t e l y to the n i t r e n e group e v e n t u a l l y coordinated to the t r a n s i t i o n metal i o n . The n i t r e n e l i g a n d could w e l l be part of carbonyl complex. As most carbonyls of p a l l a d i u m , i r i d i u m , e t c . . . have r a t h e r e l e c t r o n d e f i c i e n t carbons even i n zerovalent complexes (36), the n i t r e n e may w e l l e f f e c t a n u c l e o p h i l i c a t t a c k of these c a r b o n y l s . Such a rearrangement would be a s s i s t e d by such bases as p y r i d i n e when present i n the c o o r d i n a t i o n sphere o f the metal c e n t e r . Thus the key to a h i g h a c t i v i t y would be an easy redox c y c l e of the t r a n s i t i o n metal center so as to c a t a l y z e the r e d u c t i o n of the n i t r o g r o u p to a n i t r e n e group. However the s e l e c t i v i t y to the isocyanate would depend on the p r e f e r e n t i a l n u c l e o p h i l i c attack of the carbonyl carbon by the n i t r e n e by comparison to a s s o c i a t i o n of the n i t r e n e condensation w i t h other groups e t c . . . Such a r e a c t i o n path would be favoured by a pronounced p o s i t i v e c h a r a c t e r of the carbonyl carbon, which is s o l e l y dependent on the nature of t r a n s i t i o n metal center when the same coordinates are a v a i l a b l e , and by an increased e l e c t r o n d e n s i t y on the n i t r e n e group. Such a mechanism is q u i t e s i m i l a r to methanol c a r b o n y l a t i o n i n i t s essence. The i d e a l t r a n s i t i o n metal center would have to meet e s s e n t i a l l y two c o n t r a d i c t o r y c o n d i t i o n s : d r a m a t i c a l l y increase the e l e c t r o n d e n s i t y o f the n i t r e n e w h i l e d e p l e t i n g the carbonyl e l e c t r o n s . This is probably the reason f o r which very few t r a n s i t i o n metal elements e x h i b i t both high a c t i v i t y and s e l e c t i v i t y to i s o c y a n a t e s . The presence of the organic base would presumably h e l p the c a t i o n meet these extreme c o n d i t i o n s by a trans e f f e c t which would i n c r e a s e the e l e c t r o n d e n s i t y at the n i t r e n e without a f f e c t i n g to a large extent the e l e c t r o n d e f i c i e n t charac t e r o f the c a r b o n y l . Thus c l e a r l y c a r b o n y l a t i o n proceeds v i a two important f u n c t i o n s of z e o l i t e hosted t r a n s i t i o n metal ions and/or complexes i ) the a b i l i t y to undergo easy redox c y c l e i i ) the s t a b i l i z a t i o n of δ carbonyl carbons together w i t h a n u c l e o p h i l i c substrate which would then migrate to i n s e r t CO p r i o r to r e d u c t i v e elimination. +
The key feature of the adequate t r a n s i t i o n element seems to be i n t h i s a b i l i t y to s t a b i l i z e both e l e c t r o p h i l e s and n u c l e o p h i l e s w i t h i n the same complex. E x t r a l i g a n d s may or may not increase the r e a c t i v i t y of one of these two species and t h e r e f o r e makethe design of a t r a n s i t i o n metal z e o l i t e based c a t a l y s t w i t h i n the same ease or d i f f i c u l t y as t h e i r homogeneous analogues.
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
27. GELIN ET AL.
Zeolite Mediated Carbonylation
467
Litterature Cited 1. 2.
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R E C E I V E D November 16, 1982
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