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Reactions of Unsaturated Ethers on a Copper-Chromium Catalyst
M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
287
REACTIONS OF UNSATURATED ETHERS ON A COPPER-CHROMIUM CATALYST R. HUBAUT and J.P. BONNELLE L a b o r a t o i r e de C a t a l y s e Heterogene e t Homogene,
U.R.A.
C.N.R.S.
N’
04020,
U n i v e r s i t e des Sciences e t Techniques de L i l l e F l a n d r e s - A r t o i s F - 59655 V i l l e n e u v e d’Ascq Cedex (France).
ABSTRACT The copper-chromium o x i d e has two d i f f e r e n t a c t i v e s i t e s i n a reduced s t a t e . The cuprous i o n s a s s o c i a t e d w i t h a h y d r i d e and two a n i o n i c vacancies a r e t h e h y dro genat io n (HYD) s i t e s . The chromium i o n s i n t h e same environment a r e t h e s i t e s where o c c u r t h e i s o m e r i z a t i o n ( I ) and t h e hydrodeoxygenation (HDO) r e a c t i o n s . The use o f uns a t u r a t e d e t h e r s p e r m i t s t o c o n f i r m and t o p r e c i s e t h e n a t u r e and t h e r o l e o f t h e a c t i v e s i t e s . Wi t h t h e compounds which have t h e oxygen atom k e p t away o f t h e c a t a l y s t ’ s surface, t h e HYD a c t i v i t y i s v e r y l o w and t h e HDO/I r a t i o too, whereas, i n t h e o p p o s i t e case, t h e s e values increase. W i t h t h e v i n y l i c e t h e r s , t h e s a t u r a t e d compound i s t h e main p r o d u c t because t h e I and t h e HDO r e a c t i o n s proceed v i a a c o n c e r t e d mechanism w i t h a common p r e l i m i n a r s t e p and an a l l y l i c rearrangement which i s i m p o s s i b l e w i t h geminate f u n c t i o n s . INTRODUCTION I t has been r e p o r t e d r e c e n t l y , t h a t t h e copper chromium, a wellknown h y dro genat io n c a t a l y s t ( l ) , has two d i f f e r e n t a c t i v e s i t e s i n i t s reduced s t a t e ( 2 ) . The cuprous i o n s a s s o c i a t e d w i t h a h y d r i d e i o n and two a n i o n i c vacancies
- t h e s o - c a l l e d CH s i t e s f o l l o w i n g t h e SIEGEL’s
nomenclature ( 3 ) -
are the
hy dro genat io n s i t e s . The chromium i o n s i n t h e same environment a r e r e s p o n s i b l e f o r t h e o t h e r r e a c t i o n s which o c c u r w i t h u n s a t u r a t e d compounds. Because o f t h e weak a c i d i c c h a r a c t e r o f t h e f i r s t s i t e s , o n l y s t r o n g l y b a s i c s u b s t r a t e s a r e adsorbed and can be hydrogenated.
T h i s i s t h e case f o r conjugat ed dienes,
a , P - e t h y l e n i c c a r b o n y l compounds, a l l y 1 i c a l c o h o l s and carbonyl compounds, b u t i t i s n o t t h e case f o r monoenes. Th i s i s t h e reason why t h e monohydrogenation s e l e c t i v i t y i s v e r y h i g h w i t h t h e c o n j u g at ed dienes ( 4 ) , b u t l o w f o r t h e a , P - e t h y l e n i c c a r b o n y l compounds (5). On t h e c o n t r a r y , t h e chromium i o n s a r e more a c i d i c s i t e s and t h e y p e r m i t t h e a d s o r p t i o n o f l e s s b a s i c molecules. On t h e o t h e r hand, o n l y t h e r e a c t i o n s where t h e h y d r i d e i o n i s exchanged a g a i n s t an a n i o n i c p a r t o f t h e s u b s t r a t e a r e p o s s i b l e . Thus, we observe on t h e Cr3+ s i t e s : t h e c i s - t r a n s i s o m e r i z a t i o n o f e t h y l e n i c compounds (4) and t h e a l l y l i c a l c o h o l s l e a d t o , a t l e a s t , two d i f f e r e n t p r o d u c t s : a c a rbonyl compound f rom an i s o m e r i z a t i o n r e a c t i o n (I), some hydrocarbons f r o m a h y d r o d e h y d r o x y l a t i o n r e a c t i o n (HDOH) (6). The I and HDO r e a c t i o n s have a common i n i t i a l s t e p ( 7 ) , t hen, t h e a n i o n i c p a r t
288
which l e a v e s t h e s u b s t r a t e depends on i t s s p a t i a l c o n f o r m a t i o n and on i t s adsorbed s t a t e . I t i s u s u a l l y assumed t h a t t h e f r a c t i o n n e a r e s t t o t h e c a t a l y s t s u r f a c e i s p i c k e d up by t h e s o l i d . T h i s a b s t r a c t i o n t a k e s p l a c e a f t e r an a l l y l i c m i g r a t i o n o f t h e e t h y l e n i c d o u b l e bond i n a c o n c e r t e d mechanism. U n f o r t u n a t e l y , t h e p r e v i o u s models do n o t p e r m i t t o v e r i f y t h e s e assumptions and t o compare t h e a c t i v i t y o f each a c t i v e s i t e . The u n s a t u r a t e d e t h e r s can l e a d t o t h e same t y p e o f r e a c t i o n . Moreover, an a p p r o p r i a t e c h o i c e o f t h e s e m o lecules enables us t o p r e c i s e t h e mechanism o f t h e r e a c t i o n s because t h e y p r e s e n t some advantages : The s p a t i a l p o s i t i o n of t h e oxygen atom can be f r o z e n , i . e . t h e c y c l i c
i)
ethers ii)
The anchorage by t h e f r e e e l e c t r o n s o f t h e het eroat om i s s t r o n g e r because o f t h e donnor e f f e c t o f t h e v i c i n a l a l k y l group.
i i i ) The v i n y l i c p o s i t i o n o f t h e a l c o x y l group i s s t a b l e . i i i i ) T h e l o s s o f t h e a l c o x y l group i s n o t p o s s i b l e as a d e h y d r a t i o n r e a c t i o n . EXPERIMENTAL The copper chromium o x i d e (Cu/Cr = 1) has been prepared by c o p r e c i p i t a t i o n o f copper and chromium n i t r a t e s w i t h ammonium hydroxide, f o l l o w e d b y t hermal dec omp os it io n i n f l o w i n g n i t r o g e n up t o t h e f i n a l t emperat ure (370'C), according t o a p r e v i o u s l y d e s c r i b e d method ( 8 ) . The apparatus and t h e c a t a l y t i c procedure have a l s o been d e s c r i b e d elsewhere i n case o f gas phase r e a c t i o n s ( 5 ) and l i q u i d phase r e a c t i o n s (7). RESULTS Reac t io ns
of
a l l v l i c ethers
F i r s t o f a l l , we have t o n o t e t h e v e r y weak r e a c t i v i t y o f a l l y l i c e t h e r s i n comparison w i t h a l l y l i c a l c o h o l s ( Ta b l e 1).At a r e a c t i o n t emperat ure o f 60°C, 2 , 5 - d i h y d r o f u r a n (V) - t h e most r e a c t i v e e t h e r among t hose t e s t e d - i s about f i v e t i m e s l e s s r e a c t i v e t h a n 2 - m e t h y l - 2 propen-1-01 ( 1 1 ) .
"\ , SCHEME 1 /
2-CYCLOHEXEN-1-OL
I
289
i f t h e HYD/ItHDO and t h e HDO/I r a t i o s a r e v e r y weak f o r 2, 5 -d ihy dro f u ra n (V), 0.04 and 0.1 r e s p e c t i v e l y , t hese values i n c r e a s e w i t h e t h y l - 2 - p r o p e n y l e t h e r , (III), up t o 0.12 and 11. These r e s u l t s a r e s i m i l a r t o o t h os e obt a ined f o r 2-cyclohexenol, ( V I I I ) , which present s weak hydroge n a t i o n and HDO a c t i v i t i e s ( Ta b l e 2 ) . On t h e o t h e r hand,
TABLE 1 R e l a t i v e a c t i v i t i e s o f a l l y l i c and v i n y l i c oxygenated compounds
a) 0.34
a)
5x10-3
)
b) 16x10-3
70x10-3
) l l O ~ l O - ~b) 18x10-3
c ) 27x10-3 a) 0.3
a) 0 . 8 ~ 1 0 - 3 b) 1 . 7 ~ 1 0 - 3
) 1.6~10-3 ) 3 . 2 ~ 1 0 - 3 b) 14x10-3
c ) 23x10-3
t 3)
a) 0.45
I)
0.004
0.036
a) 4 . 2 ~ 1 0 - 3 b ) 1 2 . 8 ~ 1 0 ' 3 )) 0 . 3 1 ~ 1 0 ' 3 :) 0 . 5 6 ~ 1 0 - 3
=
Hydrogenation ; HDO
-
:)
=
) 9 . 2 ~ 1 0 - 3 b) 4 . 0 ~ 1 0 - : c ) 4.4x10-: ) 66x10-3
a) b) 1 . 5 ~ 1 0 - 3
gas phase : m c a t a l y s t = 100 mg ; P s u b s t r a t e a) T = 60'C b) T = 1OO'C HYD
) 0.85~10-3
-
) 9 8 ~ 1 0 - ~ b)
c)
-
10 t o r r s ; QH = 60 ml/min C ) T = 140'C
Hydrodeoxygenation ; I = I s o m e r i z a t i o n
Reactions of v i n v l i c e t h e r s These compounds a r e v e r y i n t e r e s t i n g because t h e corresponding a l c o h o l s a r e n o t s t a b l e . The most s t r i k i n g r e s u l t i s t h e extreme weakness o f t h e i r r e a c t i v i t y , even a t h i g h temperature ( Ta b l e 3) : a t 14OoC, e t h y l 1-propenyl e t h e r , ( I V ) , l e a d s t o 1% o f co n v e r s i o n and 2 , 3 - d i h y d r o f u r an ( V I ) i s o n l y j u s t more r e a c t i v e . I n b o t h cases, t h e hydrogenated compound i s t h e main p r o d u c t . The HYD/ItHDO r a t i o reaches 2.3 f o r t h e f i r s t r e a c t a n t and 5.2 f o r t h e second one (T able 2 ) . I t i s a l s o i n t e r e s t i n g t o n o t e t h a t these molecules a r e n o t isomerized.
290
TABLE 2 HYD/HDOtI and HDO/I r a t i o s f o r a l l y l i c alcohols conversions ; comparison w i t h
a l l y l i c and v i n y l i c ethers
coH ' 7 0"" (VIII)
c)
liYD 1iDO+1
7.5
1
0.38
E
0.11
0.11
0.05
0.01
2.3
5.2
~
I
b
IlDOfl IlYD
HDO I
0.12
0.04
11
0.1
a ) gas phase 60°C ; b) gas phase 140'C ; c ) l i q u i d phase 140°C
2.5-DIHYDRGFURAN
Tension i n the cycles SCHEME 2
W
29 1
TABLE 3 Conversion (%) and p r o d u c t d i s t r i b u t i o n (%) o f a l l y l i c and v i n y l i c e t h e r s a t v a r i o u s temperatures
DISCUSS I O N The hy dro g e n a t i o n (HYD) and t h e hydrodeoxygenation (HDO) r e a c t i o n s o f a - u n s a t u r a t e d compounds need a simultaneous a d s o r p t i o n o f t h e n e l e c t r o n s o f t h e e t h y l e n i c double bond and o f t h e f r e e e l e c t r o n s o f t h e heteroatom (6). When t h i s double anchorage i s hindered, b o t h r e a c t i v i t i e s must decrease. A t t h e same t ime, t h e HDO/I r a t i o s h o u l d decrease s i n c e t h e oxygenated group i s k e p t away f r o m t h e surface o f t h e c a t a l y s t and cannot r e p l a c e t h e h y d r i d e on t h e chromium s i t e . Indeed, i n t h e case o f a l l y l i c a l c o h o l s , t h e r e i s a d r a s t i c decrease o f b o t h HYD/ItHDO and HDO/I r a t i o s as t h e s t e r i c hindrance increases f rom 2-methyl -2 propen-1-01 (11) t o 2-cyclohexenol (VIII) t h rough 1-met hyl-2 propen-1-01 ( I ) and n e r o l ( V I I ) (T a b l e 2 ) . To t h i s purpose, 2 - c yclohexenol i s a good model because t h e hy dro x y l group t a k e s a pseudo a x i a l p o s i t i o n (9) (scheme l), and i n such a p o s i t i o n , t h e anchor e f f e c t as w e l l as t h e c a t a l y s t hindrance should be considered. On a m e t a l l i c c a t a l y s t , l i k e n i c k e l , t h e anchor e f f e c t seems more e f f e c t i v e t han
292
the c a t a l y s t hindrance ( l o ) , b u t the opposite e f f e c t i s observed w i t h mixed oxide c a t a l y s t s , probably because the s t r u c t u r e o f t h e a c t i v e s i t e i s more r e s t r i c t i n g towards adsorption. These assumptions are confirmed by comparing two a l l y l i c ethers : e t h y l 2-propenyl ether ( 1 1 1 ) and 2,5-dihydrofuran ( V ) . This l a s t molecule has some s i m i l a r i t i e s w i t h 2-cyclohexenol ( V I I I ) because t h e a l k o x y l group i s n o t e a s i l y d i r e c t e d towards t h e c a t a l y s t surface (scheme 2) ; t h i s i s the reason why hydrogenation and hydrodealkoxylation a c t i v i t i e s are weak. The recovering o f the free r o t a t i o n around the o bond i n e t h y l 2-propenyl e t h e r leads t o an increase of the HYD a c t i v i t y and o f t h e HDO/I r a t i o . Moroever, the strong donnor e f f e c t of the a l k y l group, which supplies a strong n u c l e o p h i l i c character t o t h e oxygen atom, enhances the HDO reaction. The HDO and isomerization r e a c t i o n s were p r e v i o u s l y described as bimolecular nucleophil i c s u b s t i t u t i o n s w i t h a l l y l i c m i g r a t i o n s - t h e s o - c a l l e d SN2' mechanism ( 7 ) . The f i r s t common step i s the f i x a t i o n o f t h e hydride on the carbon sp2 o f the substrate. The l o s s o f t h e hydroxyl group o f t h e alcohols could not be a simple dehydration -a p r e l i m i n a r e l i m i n a t i o n r e a c t i o n - as t h e 3-butene-1-01 leads t o n e i t h e r isomerization nor hydrodehydroxylation ( 6 ) . The r e s u l t s observed w i t h v i n y l i c ethers confirm t h a t o n l y a l l y l i c oxygenated compounds are able t o undergo e a s i l y isomerization and HDO r e a c t i o n s . Moreover, we can note t h a t f u r a n t e t r a h y d r o and furan do n o t r e a c t a t a l l even a t high temperature (200'C). A t l a s t , because o f the strong donnor e f f e c t o f t h e a l k y l group, the ethers
r e a c t d i f f e r e n t l y than the corresponding alcohols.
It i s wellknown t h a t the
hydrogenation s i t e s are poisoned by t h e oxygenated groups (11) ; the s t r e n g t h o f t h i s poison depends on the b a s i c i t y o f t h e group : a c e t i c a c i d poisons d e f i n i t e l y , whereas a l l y l i c alcohols only p a r t l y . Ethers are an average between these compounds, so the HYD a c t i v i t y i s very weak but never n i l assuming t h a t the f r e e e l e c t r o n s o f the heteroatom, a c t u a l l y ,
take p a r t i n t h e adsorption phenomenon. On the contrary, we can forecast t h a t the stronger t h e basic character o f t h e oxygenated
group i s , the easier the l o s s o f t h i s one i s . Ethers, e f f e c t i v e l y , c o n f i r m t h a t because the HDO r e a c t i o n becomes more important w i t h these a l l y l i c compounds. CONCLUSION
The reactions observed on a copper chromium oxide w i t h unsaturated ethers permit t o c o n f i r m and precise t h e nature and t h e r o l e o f t h e d i f f e r e n t a c t i v e s i t e s o f the c a t a l y s t . On the copper ions, only s t r o n g l y basic substrates are able t o adsorb and t o be hydrogenated. So, the monoenes and t h e unsaturated oxygenated molecules w i t h t h e oxygen atom kept away from the c a t a l y s t ' s surface are n o t very r e a c t i v e . On the contrary, when the anchoragewith the 0-group i s possible, the hydrogenation a c t i v i t y increase, but, i n t h e same time, t h e poisoning too. On the chromium ions, the r e a c t i o n s which occur need a concerted mechanism.
293
Thus, t h e y a r e e a s i e r w i t h t h e 0-group i n an a l l y l i c p o s i t i o n . I s o m e r i z a t i o n and hydrodeoxygenation r e a c t i o n s have a common p r e l i m i n a r s t e p and t h e p a r t which l e a v e s t h e s u b s t r a t e i s depending on t h e s p a t i a l p o s i t i o n o f t h e 0 - g r o u p by r e s p e c t t o t h e c a t a l y s t . When t h i s group i s f a r f r o m t h e c a t a l y s t ’ s s u r f a c e , t h e i s o m e r i z a t i o n r e a c t i o n i s preponderant. I n t h i s o p p o s i t e case, because o f t h e s t r o n g donnor e f f e c t o f t h e a1 k y l group, t h e hydrodeal k o x y l a t i o n becomes more important.
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H. Adkins and R. Connors, J. Am. Chem. SOC., 53 (1931), 1091. R. Bechara, G. Wrobel, M. Daage and J.P. Bonnelle, A p p l . C a t a l . 16 (1985) 15. S. S i e g e l , J. C a t a l . , 30 (1973) 139.
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Daage and J.P. Bonnelle, Appl. C a t a l . , 16 (1985 355. Hubaut, M. Daaqe and J.P. Bonnelle, Appl. C a t a l , 22 (1986) 231. Hubaut, M. Daage and J.P. Bonnelle, Appl. C a t a l 22 (1986) 243. Hubaut and J.P. B o n n e l l e , J. o f Mol. C a t a l . , 55 {l-3) (1989 170. G. Wrobel, P. W a l t e r and J.P. B e a u f i l s , C.R. Acad. S c i . C 283 i976) 335. K. Manaya; J. Chem. SOC. Japan, 91 (1970) 82 Y. Senda, S. Imatzumi, S. O c h i a i and K. F u j i t a , Tetrahedron, 30 (1974) 539. S. M i t s u i , M. I t o , A. Nanbo and Y. Senda, J. o f C a t a l . , 36 (1975) 119. R. Hubaut, M. Daage and J.P. Bonnelle, Proceedings du l l h e C o l l o q u e Franco-Polonais s u r l a Catalyse, septembre, Caen, 1985.
287
REACTIONS OF UNSATURATED ETHERS ON A COPPER-CHROMIUM CATALYST R. HUBAUT and J.P. BONNELLE L a b o r a t o i r e de C a t a l y s e Heterogene e t Homogene,
U.R.A.
C.N.R.S.
N’
04020,
U n i v e r s i t e des Sciences e t Techniques de L i l l e F l a n d r e s - A r t o i s F - 59655 V i l l e n e u v e d’Ascq Cedex (France).
ABSTRACT The copper-chromium o x i d e has two d i f f e r e n t a c t i v e s i t e s i n a reduced s t a t e . The cuprous i o n s a s s o c i a t e d w i t h a h y d r i d e and two a n i o n i c vacancies a r e t h e h y dro genat io n (HYD) s i t e s . The chromium i o n s i n t h e same environment a r e t h e s i t e s where o c c u r t h e i s o m e r i z a t i o n ( I ) and t h e hydrodeoxygenation (HDO) r e a c t i o n s . The use o f uns a t u r a t e d e t h e r s p e r m i t s t o c o n f i r m and t o p r e c i s e t h e n a t u r e and t h e r o l e o f t h e a c t i v e s i t e s . Wi t h t h e compounds which have t h e oxygen atom k e p t away o f t h e c a t a l y s t ’ s surface, t h e HYD a c t i v i t y i s v e r y l o w and t h e HDO/I r a t i o too, whereas, i n t h e o p p o s i t e case, t h e s e values increase. W i t h t h e v i n y l i c e t h e r s , t h e s a t u r a t e d compound i s t h e main p r o d u c t because t h e I and t h e HDO r e a c t i o n s proceed v i a a c o n c e r t e d mechanism w i t h a common p r e l i m i n a r s t e p and an a l l y l i c rearrangement which i s i m p o s s i b l e w i t h geminate f u n c t i o n s . INTRODUCTION I t has been r e p o r t e d r e c e n t l y , t h a t t h e copper chromium, a wellknown h y dro genat io n c a t a l y s t ( l ) , has two d i f f e r e n t a c t i v e s i t e s i n i t s reduced s t a t e ( 2 ) . The cuprous i o n s a s s o c i a t e d w i t h a h y d r i d e i o n and two a n i o n i c vacancies
- t h e s o - c a l l e d CH s i t e s f o l l o w i n g t h e SIEGEL’s
nomenclature ( 3 ) -
are the
hy dro genat io n s i t e s . The chromium i o n s i n t h e same environment a r e r e s p o n s i b l e f o r t h e o t h e r r e a c t i o n s which o c c u r w i t h u n s a t u r a t e d compounds. Because o f t h e weak a c i d i c c h a r a c t e r o f t h e f i r s t s i t e s , o n l y s t r o n g l y b a s i c s u b s t r a t e s a r e adsorbed and can be hydrogenated.
T h i s i s t h e case f o r conjugat ed dienes,
a , P - e t h y l e n i c c a r b o n y l compounds, a l l y 1 i c a l c o h o l s and carbonyl compounds, b u t i t i s n o t t h e case f o r monoenes. Th i s i s t h e reason why t h e monohydrogenation s e l e c t i v i t y i s v e r y h i g h w i t h t h e c o n j u g at ed dienes ( 4 ) , b u t l o w f o r t h e a , P - e t h y l e n i c c a r b o n y l compounds (5). On t h e c o n t r a r y , t h e chromium i o n s a r e more a c i d i c s i t e s and t h e y p e r m i t t h e a d s o r p t i o n o f l e s s b a s i c molecules. On t h e o t h e r hand, o n l y t h e r e a c t i o n s where t h e h y d r i d e i o n i s exchanged a g a i n s t an a n i o n i c p a r t o f t h e s u b s t r a t e a r e p o s s i b l e . Thus, we observe on t h e Cr3+ s i t e s : t h e c i s - t r a n s i s o m e r i z a t i o n o f e t h y l e n i c compounds (4) and t h e a l l y l i c a l c o h o l s l e a d t o , a t l e a s t , two d i f f e r e n t p r o d u c t s : a c a rbonyl compound f rom an i s o m e r i z a t i o n r e a c t i o n (I), some hydrocarbons f r o m a h y d r o d e h y d r o x y l a t i o n r e a c t i o n (HDOH) (6). The I and HDO r e a c t i o n s have a common i n i t i a l s t e p ( 7 ) , t hen, t h e a n i o n i c p a r t
288
which l e a v e s t h e s u b s t r a t e depends on i t s s p a t i a l c o n f o r m a t i o n and on i t s adsorbed s t a t e . I t i s u s u a l l y assumed t h a t t h e f r a c t i o n n e a r e s t t o t h e c a t a l y s t s u r f a c e i s p i c k e d up by t h e s o l i d . T h i s a b s t r a c t i o n t a k e s p l a c e a f t e r an a l l y l i c m i g r a t i o n o f t h e e t h y l e n i c d o u b l e bond i n a c o n c e r t e d mechanism. U n f o r t u n a t e l y , t h e p r e v i o u s models do n o t p e r m i t t o v e r i f y t h e s e assumptions and t o compare t h e a c t i v i t y o f each a c t i v e s i t e . The u n s a t u r a t e d e t h e r s can l e a d t o t h e same t y p e o f r e a c t i o n . Moreover, an a p p r o p r i a t e c h o i c e o f t h e s e m o lecules enables us t o p r e c i s e t h e mechanism o f t h e r e a c t i o n s because t h e y p r e s e n t some advantages : The s p a t i a l p o s i t i o n of t h e oxygen atom can be f r o z e n , i . e . t h e c y c l i c
i)
ethers ii)
The anchorage by t h e f r e e e l e c t r o n s o f t h e het eroat om i s s t r o n g e r because o f t h e donnor e f f e c t o f t h e v i c i n a l a l k y l group.
i i i ) The v i n y l i c p o s i t i o n o f t h e a l c o x y l group i s s t a b l e . i i i i ) T h e l o s s o f t h e a l c o x y l group i s n o t p o s s i b l e as a d e h y d r a t i o n r e a c t i o n . EXPERIMENTAL The copper chromium o x i d e (Cu/Cr = 1) has been prepared by c o p r e c i p i t a t i o n o f copper and chromium n i t r a t e s w i t h ammonium hydroxide, f o l l o w e d b y t hermal dec omp os it io n i n f l o w i n g n i t r o g e n up t o t h e f i n a l t emperat ure (370'C), according t o a p r e v i o u s l y d e s c r i b e d method ( 8 ) . The apparatus and t h e c a t a l y t i c procedure have a l s o been d e s c r i b e d elsewhere i n case o f gas phase r e a c t i o n s ( 5 ) and l i q u i d phase r e a c t i o n s (7). RESULTS Reac t io ns
of
a l l v l i c ethers
F i r s t o f a l l , we have t o n o t e t h e v e r y weak r e a c t i v i t y o f a l l y l i c e t h e r s i n comparison w i t h a l l y l i c a l c o h o l s ( Ta b l e 1).At a r e a c t i o n t emperat ure o f 60°C, 2 , 5 - d i h y d r o f u r a n (V) - t h e most r e a c t i v e e t h e r among t hose t e s t e d - i s about f i v e t i m e s l e s s r e a c t i v e t h a n 2 - m e t h y l - 2 propen-1-01 ( 1 1 ) .
"\ , SCHEME 1 /
2-CYCLOHEXEN-1-OL
I
289
i f t h e HYD/ItHDO and t h e HDO/I r a t i o s a r e v e r y weak f o r 2, 5 -d ihy dro f u ra n (V), 0.04 and 0.1 r e s p e c t i v e l y , t hese values i n c r e a s e w i t h e t h y l - 2 - p r o p e n y l e t h e r , (III), up t o 0.12 and 11. These r e s u l t s a r e s i m i l a r t o o t h os e obt a ined f o r 2-cyclohexenol, ( V I I I ) , which present s weak hydroge n a t i o n and HDO a c t i v i t i e s ( Ta b l e 2 ) . On t h e o t h e r hand,
TABLE 1 R e l a t i v e a c t i v i t i e s o f a l l y l i c and v i n y l i c oxygenated compounds
a) 0.34
a)
5x10-3
)
b) 16x10-3
70x10-3
) l l O ~ l O - ~b) 18x10-3
c ) 27x10-3 a) 0.3
a) 0 . 8 ~ 1 0 - 3 b) 1 . 7 ~ 1 0 - 3
) 1.6~10-3 ) 3 . 2 ~ 1 0 - 3 b) 14x10-3
c ) 23x10-3
t 3)
a) 0.45
I)
0.004
0.036
a) 4 . 2 ~ 1 0 - 3 b ) 1 2 . 8 ~ 1 0 ' 3 )) 0 . 3 1 ~ 1 0 ' 3 :) 0 . 5 6 ~ 1 0 - 3
=
Hydrogenation ; HDO
-
:)
=
) 9 . 2 ~ 1 0 - 3 b) 4 . 0 ~ 1 0 - : c ) 4.4x10-: ) 66x10-3
a) b) 1 . 5 ~ 1 0 - 3
gas phase : m c a t a l y s t = 100 mg ; P s u b s t r a t e a) T = 60'C b) T = 1OO'C HYD
) 0.85~10-3
-
) 9 8 ~ 1 0 - ~ b)
c)
-
10 t o r r s ; QH = 60 ml/min C ) T = 140'C
Hydrodeoxygenation ; I = I s o m e r i z a t i o n
Reactions of v i n v l i c e t h e r s These compounds a r e v e r y i n t e r e s t i n g because t h e corresponding a l c o h o l s a r e n o t s t a b l e . The most s t r i k i n g r e s u l t i s t h e extreme weakness o f t h e i r r e a c t i v i t y , even a t h i g h temperature ( Ta b l e 3) : a t 14OoC, e t h y l 1-propenyl e t h e r , ( I V ) , l e a d s t o 1% o f co n v e r s i o n and 2 , 3 - d i h y d r o f u r an ( V I ) i s o n l y j u s t more r e a c t i v e . I n b o t h cases, t h e hydrogenated compound i s t h e main p r o d u c t . The HYD/ItHDO r a t i o reaches 2.3 f o r t h e f i r s t r e a c t a n t and 5.2 f o r t h e second one (T able 2 ) . I t i s a l s o i n t e r e s t i n g t o n o t e t h a t these molecules a r e n o t isomerized.
290
TABLE 2 HYD/HDOtI and HDO/I r a t i o s f o r a l l y l i c alcohols conversions ; comparison w i t h
a l l y l i c and v i n y l i c ethers
coH ' 7 0"" (VIII)
c)
liYD 1iDO+1
7.5
1
0.38
E
0.11
0.11
0.05
0.01
2.3
5.2
~
I
b
IlDOfl IlYD
HDO I
0.12
0.04
11
0.1
a ) gas phase 60°C ; b) gas phase 140'C ; c ) l i q u i d phase 140°C
2.5-DIHYDRGFURAN
Tension i n the cycles SCHEME 2
W
29 1
TABLE 3 Conversion (%) and p r o d u c t d i s t r i b u t i o n (%) o f a l l y l i c and v i n y l i c e t h e r s a t v a r i o u s temperatures
DISCUSS I O N The hy dro g e n a t i o n (HYD) and t h e hydrodeoxygenation (HDO) r e a c t i o n s o f a - u n s a t u r a t e d compounds need a simultaneous a d s o r p t i o n o f t h e n e l e c t r o n s o f t h e e t h y l e n i c double bond and o f t h e f r e e e l e c t r o n s o f t h e heteroatom (6). When t h i s double anchorage i s hindered, b o t h r e a c t i v i t i e s must decrease. A t t h e same t ime, t h e HDO/I r a t i o s h o u l d decrease s i n c e t h e oxygenated group i s k e p t away f r o m t h e surface o f t h e c a t a l y s t and cannot r e p l a c e t h e h y d r i d e on t h e chromium s i t e . Indeed, i n t h e case o f a l l y l i c a l c o h o l s , t h e r e i s a d r a s t i c decrease o f b o t h HYD/ItHDO and HDO/I r a t i o s as t h e s t e r i c hindrance increases f rom 2-methyl -2 propen-1-01 (11) t o 2-cyclohexenol (VIII) t h rough 1-met hyl-2 propen-1-01 ( I ) and n e r o l ( V I I ) (T a b l e 2 ) . To t h i s purpose, 2 - c yclohexenol i s a good model because t h e hy dro x y l group t a k e s a pseudo a x i a l p o s i t i o n (9) (scheme l), and i n such a p o s i t i o n , t h e anchor e f f e c t as w e l l as t h e c a t a l y s t hindrance should be considered. On a m e t a l l i c c a t a l y s t , l i k e n i c k e l , t h e anchor e f f e c t seems more e f f e c t i v e t han
292
the c a t a l y s t hindrance ( l o ) , b u t the opposite e f f e c t i s observed w i t h mixed oxide c a t a l y s t s , probably because the s t r u c t u r e o f t h e a c t i v e s i t e i s more r e s t r i c t i n g towards adsorption. These assumptions are confirmed by comparing two a l l y l i c ethers : e t h y l 2-propenyl ether ( 1 1 1 ) and 2,5-dihydrofuran ( V ) . This l a s t molecule has some s i m i l a r i t i e s w i t h 2-cyclohexenol ( V I I I ) because t h e a l k o x y l group i s n o t e a s i l y d i r e c t e d towards t h e c a t a l y s t surface (scheme 2) ; t h i s i s the reason why hydrogenation and hydrodealkoxylation a c t i v i t i e s are weak. The recovering o f the free r o t a t i o n around the o bond i n e t h y l 2-propenyl e t h e r leads t o an increase of the HYD a c t i v i t y and o f t h e HDO/I r a t i o . Moroever, the strong donnor e f f e c t of the a l k y l group, which supplies a strong n u c l e o p h i l i c character t o t h e oxygen atom, enhances the HDO reaction. The HDO and isomerization r e a c t i o n s were p r e v i o u s l y described as bimolecular nucleophil i c s u b s t i t u t i o n s w i t h a l l y l i c m i g r a t i o n s - t h e s o - c a l l e d SN2' mechanism ( 7 ) . The f i r s t common step i s the f i x a t i o n o f t h e hydride on the carbon sp2 o f the substrate. The l o s s o f t h e hydroxyl group o f t h e alcohols could not be a simple dehydration -a p r e l i m i n a r e l i m i n a t i o n r e a c t i o n - as t h e 3-butene-1-01 leads t o n e i t h e r isomerization nor hydrodehydroxylation ( 6 ) . The r e s u l t s observed w i t h v i n y l i c ethers confirm t h a t o n l y a l l y l i c oxygenated compounds are able t o undergo e a s i l y isomerization and HDO r e a c t i o n s . Moreover, we can note t h a t f u r a n t e t r a h y d r o and furan do n o t r e a c t a t a l l even a t high temperature (200'C). A t l a s t , because o f the strong donnor e f f e c t o f t h e a l k y l group, the ethers
r e a c t d i f f e r e n t l y than the corresponding alcohols.
It i s wellknown t h a t the
hydrogenation s i t e s are poisoned by t h e oxygenated groups (11) ; the s t r e n g t h o f t h i s poison depends on the b a s i c i t y o f t h e group : a c e t i c a c i d poisons d e f i n i t e l y , whereas a l l y l i c alcohols only p a r t l y . Ethers are an average between these compounds, so the HYD a c t i v i t y i s very weak but never n i l assuming t h a t the f r e e e l e c t r o n s o f the heteroatom, a c t u a l l y ,
take p a r t i n t h e adsorption phenomenon. On the contrary, we can forecast t h a t the stronger t h e basic character o f t h e oxygenated
group i s , the easier the l o s s o f t h i s one i s . Ethers, e f f e c t i v e l y , c o n f i r m t h a t because the HDO r e a c t i o n becomes more important w i t h these a l l y l i c compounds. CONCLUSION
The reactions observed on a copper chromium oxide w i t h unsaturated ethers permit t o c o n f i r m and precise t h e nature and t h e r o l e o f t h e d i f f e r e n t a c t i v e s i t e s o f the c a t a l y s t . On the copper ions, only s t r o n g l y basic substrates are able t o adsorb and t o be hydrogenated. So, the monoenes and t h e unsaturated oxygenated molecules w i t h t h e oxygen atom kept away from the c a t a l y s t ' s surface are n o t very r e a c t i v e . On the contrary, when the anchoragewith the 0-group i s possible, the hydrogenation a c t i v i t y increase, but, i n t h e same time, t h e poisoning too. On the chromium ions, the r e a c t i o n s which occur need a concerted mechanism.
293
Thus, t h e y a r e e a s i e r w i t h t h e 0-group i n an a l l y l i c p o s i t i o n . I s o m e r i z a t i o n and hydrodeoxygenation r e a c t i o n s have a common p r e l i m i n a r s t e p and t h e p a r t which l e a v e s t h e s u b s t r a t e i s depending on t h e s p a t i a l p o s i t i o n o f t h e 0 - g r o u p by r e s p e c t t o t h e c a t a l y s t . When t h i s group i s f a r f r o m t h e c a t a l y s t ’ s s u r f a c e , t h e i s o m e r i z a t i o n r e a c t i o n i s preponderant. I n t h i s o p p o s i t e case, because o f t h e s t r o n g donnor e f f e c t o f t h e a1 k y l group, t h e hydrodeal k o x y l a t i o n becomes more important.
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