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Bifunctional zeolite catalysts for the selective synthesis in one step of various ketones
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
3011
Bifunctionai zeolite catalysts for the selective synthesis in one step of various ketones P. M agnoux', N. Lavaud', L. Melo b, G. Giarmetto b, A.I. Silvac, F. Alvarez~, M. Guisnet" "UMR 6503 - Catalyse en Chimie Organique, 40 av. du geeteur Pineau, 86022 Poitiers France- r f [email protected] b Facultad de Ingenieria, U.C.V. Ap. 47100, Caracas, Venezuela c Instituto Superior Teenieo, Av. Rovisco Pais, 1096 Lisboa, Portugal.
Abstract The gas phase transformation of acetone, cyclohexanone and acetophenone was investigated in a fixed bed reactor over series of bifunctional Pt and Pal/molecular sieves (HFAU, HBEA, HMFI, HMCM41). Ketones resulting from three successive steps : aldolisation and dehydration of the resulting ketoalcohols on the acid sites then hydrogenation on the metallic sites can be obtained in one apparent step. Various other bifunctional transformations of the reactant, of the ketone product alone or of both can also be observed. However, the ketone products can be obtained with a high selectivity by an adequate choice of the pore structure and the adjustment of the acid and hydrogenating properties. Thus the pores of the molecular sieves should be large enough to allow an easy desorption of the ketone products and narrow enough to limit the formation of very bulky secondary compounds and of coke. Whereas for acetone transformation into methylisobutylketone an average pore size zeolite (e.g. MFI) should be used, a large pore zeolite (e.g. FAU) is preferred for cyclohexanone transformation into cyclohexyleyclohexanone and a mesoporous MCM41 silicoaluminate for acetophenone transformation into 1,3-diphenyl-butan 1-one. 1-INTRODUCTION
Bifunctional metal acid catalysts are used in various processes of refining and petrochemicals : reforming, hydroisomerization of alkanes and of the C8 aromatic cut, hydroeracking etc. These catalysts can also allow to carry out in one operation the synthesis of specialty and fine chemicals which usually requires several successive steps. This limits the number of separation steps hence the pollution [1 ]. A typical example is the direct synthesis of methylisobutylketone from acetone, which, although requiring three successive steps : aldolisation, dehydration and hydrogenation, can be carried out over Pd-doped sulfonated resin [2], Pd HMFI [1] etc without apparent formation of diaeetone alcohol and mesityloxide. o 2 CH
o - CH3-----~ I-i+
CH
- CH
3 --'-"~ I-l*
CH
- CI-I~C- C H 3 L'-'H3
-~
~ Pd
C
C H 2 ~ . H - CH3 (~H3
3012 In this paper the reaction schemes of transformation of acetone, cyclohexanone and acetophenone [3-5] over Pt or Pd zeolite catalysts will be compared and the characteristics of optimal catalysts will be specified. It should be emphasized that the desired products and derivatives are used as solvents, fragrances or sunscreens. 2-EXPERIMENTAL The reactions were carried out in a fixed bed reactor under the following conditions : Acetone transformation : 160~ Pm = 0.25 bar, P k ~ = 0.75 bar, Cyclohexanone transformation : 200~ Pm = 0.75 bar, Pk~t~ = 0.25 bar, Acetophenone transformation : 250~ P m = 0.2 bar, Pk~to~c= 0.8 bar. Pt and Pd molecular sieves (HFAU, HMFI, HBEA, MCM41) were prepared by ion exchange with Pt~3)4C12 and Pd(NH3)4CI2 followed by calcination under dry air flow and reduction under hydrogen. The acidity of the catalysts was characterized by pyridine adsorption and the metal dispersion by CO adsorption both followed by IR spectroscopy. 3-RESULTS AND DISCUSSION 3-1- Reaction schemes The schemes of acetone, cyclohexanone and acetophenone transformations were established over various Pt and Pd zeolite catalysts with at least 0.2 wt % metal, under the operating conditions described above. Whatever the reactant, the desired products : methylisobutylketone (MIBK), cyclohexylcyclohexanone (CHCHO), 1,3-diphenyl-butanl-one (DPBO) can be directly obtained (apparent primary products) without apparent formation of the ketoalcohol. Furthermore only traces of olefinic ketone intermediates : mesityl oxide (MO), cyclohexenylcyclohexanone (CHCHO=) and diphenylbutenone (DPBO=), corresponding to the amounts expected at the thermodynamic equilibrium of the hydrogenation step are observed. Therefore it can be concluded that aldolisation is the limiting step of the bifunctional three step synthesis of ketones (Path 1). Whatever the reactant, saturated hydrocarbons with the same number of carbon atoms : propane (C3) from acetone, cyclohexane (CH) from cyclohexanone and ethylbenzene (EB) from acetophenone are formed in parallel to the desired products (Table 1). These hydrocarbons result also from a bifunctional three-step process (Path 2) e.g. in the case of acetone transformation : O ~ CH 3-- C - CH 3
~ Pt, Pd
OH ~ CH 3 - C H - CH 3
-H20 H+
CH 3 - C H = CH 2
H2 ~ Pt, Pd
CH 3-- CH 2- CH 3
Again no alcohol intermediate can be observed. No alkene is found in the reaction products (but from cydohexanone benzene is formed in addition to cyclohexane). The limiting step of this bifunctional scheme is therefore the hydrogenation of the C=O bond. No other primary products are formed from acetone and from cydohexanone. From acetophenone, there is a significant formation of other products through the acid cracking of the aldolisation products. This third path leads mainly to the formation of cumene (IPB), isopropenylbenzene (IPB =) and benzoic acid (BA). BA and IPB = result from acid cracking of 1,3 diphenyl but-2-ene-l-one (DPBO--), IPB from hydrogenation of IPB ="
3013
H+
OH +
H2
H20
Pt, Pd
(DPBO =)
(BA)
(IPB =)
(IPB)
This path is very favoured in the case of acetophenone transformation because of the high stability of the carbocations involved one tertiary carbenium ion and one acylium ion stabilized by mesomeric effect"
c~Ol
c-.OI
This stabilization does not exist with the acylium ions resulting from acetone and cyclohexanone transformation, which could explain that this third path does not occur from these reactants. Table 1 Main reaction products from acetone, cyclohexanone and acetophenone transformations over bifunctional Pt or Pd zeolite catalysts nt
Acetone
Cyclohexanone
Acetophenone
O MIBK
DPBO
CHCHO A
C3
CH
'~
~OH
~
C-C-C-C-C C 2MP
BCH
o c-~-c-c-c-~-c C
DIBK
C
~
BCHCHO
EB
BA
DPB
3014 Various other products are observed resulting from secondary transformations of the desired products MIBK, CHCHO and DPBO (Table l) i) Bifimctional transformation into saturated hydrocarbons with the same number of carbon atoms (Path 4) : 2 methylpentane (2MP) from MIBK, cyclohexylcyclohexane (BCH) from CHCHO and 1,3-diphenyl-butane (DPB) from DPBO (Table 1). This transformation involves the same steps than the direct transformation of the reactants into saturated hydrocarbons e.g. in the case of acetophenone transformation :
It, Pd
-I-I~0
Pt, Pd (DPB)
It should be emphasized that part of these products can also result from acid dimerization of alkenes followed by hydrogenation : e.g.
2
~ Pt, Pd
ii) Condensation with the reactant (aldolisation) followed by dehydration and hydrogenation (Path 5) i.e. a bifunctional scheme similar to the formation of the desired products. The reaction products are diisobutylketone (DIBK) from acetone and biscyclohexylcylclohexanone (BCHCHO) from cyclohexanone. The corresponding product is not observed in transformation of acetophenone probably because of difficulty in desorbing this very bulky compound. 3-2- Influence of the catalyst characteristics. The activity, selectivity and stability of the bifunctional Pt or Pd zeolite catalysts depend on the nature, amount and dispersion of the metal, on the acidity and on the pore structure. Palladium is always more selective than platinum for the formation of the desired products. This is shown in Figure 1 for acetone transformation over 0.5 wt % Pt and Pd HMFI catalysts with similar acidities and number of accessible metal atoms [6]. The much higher selectivity of the palladium catalyst can be related to the fact that this metal is much more active for hydrogenating C=C bonds than C=O bonds, which is not the case for platinum. The effect of the hydrogenating and acid functions, hence on the balance between these two functions on the activity, selectivity and stability of bifunctional Pd zeolite catalysts was determined over series of chosen samples. This balance was characterized by the ratio nPd/nA between the number of accessible Pd atoms and the number of protonic sites able to retain pyridine adsorbed at 150~ The effect of nPd/nA on the turnover frequency of the acid sites (TOF) for the formation of the desired products is that expected from a bifunctional scheme : firstly an increase for low values of nPd/nA indicating that the reaction was limited by the hydrogenating step then a plateau, the acid step being then the limiting one. Figure 2 shows as an example the effect of nPd/nA on TOF for the formation of methylisobutylketone from acetone.
3015 10o
2SO
8o
.~ 200
Pd MR
/~
~' 150 eo
~
40
:E 100 u_
PtMR 2o
SO
0
|
|
10
20
0
30
Conversion I%)
,~
i
i
0
0.0S
0.1
0.15
nPd/nA
Fig. 1. Weight percentage of MIBK in the products formed over Pt and Pd MFI as a function of acetone conversion.
Fig. 2. Effect of nPd/nA on the turnover frequency (TOF) for the formation of MIBK.
Whereas the catalytic properties are mainly determined by the balance between the hydrogenating and the acid functions, various results demonstrate an effect of the porosity. This effect was shown in particular for acetophenone transformation over a series of Pd HFAU samples differing by the framework Si/AI ratio (from 4 to 100) and for which the amount and dispersion of palladium was enough to have the formation of 1,3-diphenyl-butanl-one (DPBO) limited by the acid step. Figure 3 shows that the greater the Si/A1 ratio of the zeolite hence the lower nA the greater the catalyst activity for DPBO production (and also the higher the selectivity). This unexpected increase in activity when the acidity of the samples decreases is most likely due to the creation of secondary pores by dealumination. Thus, the higher the Si/AI ratio the greater the mesopore volume : HFAU 4 has 25% of pore volume constituted by pores larger than the structural micropores, HFAU 100 more than 55%. Therefore the high activity of Pd HFAU 100 is certainly due to the presence of a large number of mesopores which make easier the desorption of the bulky molecules of DPBO. The selectivity is also much better : at 10% conversion, 65 wt % of the products are constituted by DPBO over Pd HFAU 100 against only 10 % over Pd HFAU 4.
OD
E
Fig. 3. Activity of various Pd HFAU catalysts for acetone transformation into DPBO as a function of time on stream. 9 0.5 Pd HFAU 4, A 0.5 Pd HFAU 17, ll 0.5 Pd HFAU 47, 9 0.2 Pd HFAU 100
E 2o
-~
o
E
lOO 47
'~
~ir
17
o
20
40
60
Time on stream (min)
80
100
3016 This effect of the pore structure was confirmed by the high activity and selectivity of Pd MCM41 samples. Thus a 0.5 wt % Pd HMCM41 (30) sample was found to be 1.4 times more active for DPBO formation than the best Pd HFAU 100 sample and its selectivity was slightly higher (70% instead of 65%). These observations can be related to the large size of the pores of MCM41 which allow an easy desorption of the bulky reaction products such as DPBO. On the other hand because of their narrow pores, Pd MFI samples are much less active than Pd HFAU 100. An effect of the pore structure is also shown in cyclohexanone transformation : for identical values of nPd/nA, the activity of Pd HFAU catalysts is higher and their selectivity to cyclohexylcylclohexanone (CHCHO) are much better (75% instead 55%) than those of Pd HMFI catalysts. However these latter catalysts are preferred for acetone transformation into methylisobutylketone (MIBK) because in their narrow pores the formation of bulky reaction products such as DIBK and of coke is very limited. 4-CONCLUSIONS Pd bifunctional zeolite catalysts are able to catalyze in one apparent step the synthesis of ketones which requires three successive steps catalyzed by acid or metal sites : aldolisation, dehydration then hydrogenation. This synthesis can be made selective (_> 75%) by using a well balanced catalyst with a chosen pore structure. Indeed the porosity of the zeolite plays a determining role both in the activity and in the selectivity : too narrow pores limit the desorption of the desired products which are bulkier than the non desired products and too large pores favour the formation of very bulky secondary products and of coke. Therefore, whereas for acetone transformation, an average pore size zeolite (e.g. MFI) should be used, a large pore zeolite (e.g. FAU) is preferred for cyclohexanone transformation and a mesoporous MCM41 silicoaluminate for acetophenone transformation. ACKNOWLEDGEMENT Financial support by the EC within the International Scientific Cooperation ECALA/MED countries (Contract CII*-CT 94-0044) is gratefully acknowledged. REFERENCES 1. W.F. HOlderich, H. van Bekkum, Stud. Surf. Sci. Catal., 58 (1991) 631. 2. W. Reith, M. Dcttmer, H. Widdecke, B. Fleischer, Stud. Surf. Sci. Catal., 59 (1991) 487. 3. L. Mdo, P. Magnoux, G. Giannetto, F. Alvarez, M. Guisnet, J. Mol. Catal. 124 (1997) 155. 4. F. Alvarez, P. Magnoux F.R. Ribeiro, M. Guisnet, J. Mol. Catal., 92 (1994) 67 5. N. Lavaud, P. Magnoux, F. Alvarez, L. Melo, G. Giannetto, M. Guisnet, J. Mol. Catal., 142 (1999) 223.
6. L. Melo, Thesis, Poitiers, 1994.
3011
Bifunctionai zeolite catalysts for the selective synthesis in one step of various ketones P. M agnoux', N. Lavaud', L. Melo b, G. Giarmetto b, A.I. Silvac, F. Alvarez~, M. Guisnet" "UMR 6503 - Catalyse en Chimie Organique, 40 av. du geeteur Pineau, 86022 Poitiers France- r f [email protected] b Facultad de Ingenieria, U.C.V. Ap. 47100, Caracas, Venezuela c Instituto Superior Teenieo, Av. Rovisco Pais, 1096 Lisboa, Portugal.
Abstract The gas phase transformation of acetone, cyclohexanone and acetophenone was investigated in a fixed bed reactor over series of bifunctional Pt and Pal/molecular sieves (HFAU, HBEA, HMFI, HMCM41). Ketones resulting from three successive steps : aldolisation and dehydration of the resulting ketoalcohols on the acid sites then hydrogenation on the metallic sites can be obtained in one apparent step. Various other bifunctional transformations of the reactant, of the ketone product alone or of both can also be observed. However, the ketone products can be obtained with a high selectivity by an adequate choice of the pore structure and the adjustment of the acid and hydrogenating properties. Thus the pores of the molecular sieves should be large enough to allow an easy desorption of the ketone products and narrow enough to limit the formation of very bulky secondary compounds and of coke. Whereas for acetone transformation into methylisobutylketone an average pore size zeolite (e.g. MFI) should be used, a large pore zeolite (e.g. FAU) is preferred for cyclohexanone transformation into cyclohexyleyclohexanone and a mesoporous MCM41 silicoaluminate for acetophenone transformation into 1,3-diphenyl-butan 1-one. 1-INTRODUCTION
Bifunctional metal acid catalysts are used in various processes of refining and petrochemicals : reforming, hydroisomerization of alkanes and of the C8 aromatic cut, hydroeracking etc. These catalysts can also allow to carry out in one operation the synthesis of specialty and fine chemicals which usually requires several successive steps. This limits the number of separation steps hence the pollution [1 ]. A typical example is the direct synthesis of methylisobutylketone from acetone, which, although requiring three successive steps : aldolisation, dehydration and hydrogenation, can be carried out over Pd-doped sulfonated resin [2], Pd HMFI [1] etc without apparent formation of diaeetone alcohol and mesityloxide. o 2 CH
o - CH3-----~ I-i+
CH
- CH
3 --'-"~ I-l*
CH
- CI-I~C- C H 3 L'-'H3
-~
~ Pd
C
C H 2 ~ . H - CH3 (~H3
3012 In this paper the reaction schemes of transformation of acetone, cyclohexanone and acetophenone [3-5] over Pt or Pd zeolite catalysts will be compared and the characteristics of optimal catalysts will be specified. It should be emphasized that the desired products and derivatives are used as solvents, fragrances or sunscreens. 2-EXPERIMENTAL The reactions were carried out in a fixed bed reactor under the following conditions : Acetone transformation : 160~ Pm = 0.25 bar, P k ~ = 0.75 bar, Cyclohexanone transformation : 200~ Pm = 0.75 bar, Pk~t~ = 0.25 bar, Acetophenone transformation : 250~ P m = 0.2 bar, Pk~to~c= 0.8 bar. Pt and Pd molecular sieves (HFAU, HMFI, HBEA, MCM41) were prepared by ion exchange with Pt~3)4C12 and Pd(NH3)4CI2 followed by calcination under dry air flow and reduction under hydrogen. The acidity of the catalysts was characterized by pyridine adsorption and the metal dispersion by CO adsorption both followed by IR spectroscopy. 3-RESULTS AND DISCUSSION 3-1- Reaction schemes The schemes of acetone, cyclohexanone and acetophenone transformations were established over various Pt and Pd zeolite catalysts with at least 0.2 wt % metal, under the operating conditions described above. Whatever the reactant, the desired products : methylisobutylketone (MIBK), cyclohexylcyclohexanone (CHCHO), 1,3-diphenyl-butanl-one (DPBO) can be directly obtained (apparent primary products) without apparent formation of the ketoalcohol. Furthermore only traces of olefinic ketone intermediates : mesityl oxide (MO), cyclohexenylcyclohexanone (CHCHO=) and diphenylbutenone (DPBO=), corresponding to the amounts expected at the thermodynamic equilibrium of the hydrogenation step are observed. Therefore it can be concluded that aldolisation is the limiting step of the bifunctional three step synthesis of ketones (Path 1). Whatever the reactant, saturated hydrocarbons with the same number of carbon atoms : propane (C3) from acetone, cyclohexane (CH) from cyclohexanone and ethylbenzene (EB) from acetophenone are formed in parallel to the desired products (Table 1). These hydrocarbons result also from a bifunctional three-step process (Path 2) e.g. in the case of acetone transformation : O ~ CH 3-- C - CH 3
~ Pt, Pd
OH ~ CH 3 - C H - CH 3
-H20 H+
CH 3 - C H = CH 2
H2 ~ Pt, Pd
CH 3-- CH 2- CH 3
Again no alcohol intermediate can be observed. No alkene is found in the reaction products (but from cydohexanone benzene is formed in addition to cyclohexane). The limiting step of this bifunctional scheme is therefore the hydrogenation of the C=O bond. No other primary products are formed from acetone and from cydohexanone. From acetophenone, there is a significant formation of other products through the acid cracking of the aldolisation products. This third path leads mainly to the formation of cumene (IPB), isopropenylbenzene (IPB =) and benzoic acid (BA). BA and IPB = result from acid cracking of 1,3 diphenyl but-2-ene-l-one (DPBO--), IPB from hydrogenation of IPB ="
3013
H+
OH +
H2
H20
Pt, Pd
(DPBO =)
(BA)
(IPB =)
(IPB)
This path is very favoured in the case of acetophenone transformation because of the high stability of the carbocations involved one tertiary carbenium ion and one acylium ion stabilized by mesomeric effect"
c~Ol
c-.OI
This stabilization does not exist with the acylium ions resulting from acetone and cyclohexanone transformation, which could explain that this third path does not occur from these reactants. Table 1 Main reaction products from acetone, cyclohexanone and acetophenone transformations over bifunctional Pt or Pd zeolite catalysts nt
Acetone
Cyclohexanone
Acetophenone
O MIBK
DPBO
CHCHO A
C3
CH
'~
~OH
~
C-C-C-C-C C 2MP
BCH
o c-~-c-c-c-~-c C
DIBK
C
~
BCHCHO
EB
BA
DPB
3014 Various other products are observed resulting from secondary transformations of the desired products MIBK, CHCHO and DPBO (Table l) i) Bifimctional transformation into saturated hydrocarbons with the same number of carbon atoms (Path 4) : 2 methylpentane (2MP) from MIBK, cyclohexylcyclohexane (BCH) from CHCHO and 1,3-diphenyl-butane (DPB) from DPBO (Table 1). This transformation involves the same steps than the direct transformation of the reactants into saturated hydrocarbons e.g. in the case of acetophenone transformation :
It, Pd
-I-I~0
Pt, Pd (DPB)
It should be emphasized that part of these products can also result from acid dimerization of alkenes followed by hydrogenation : e.g.
2
~ Pt, Pd
ii) Condensation with the reactant (aldolisation) followed by dehydration and hydrogenation (Path 5) i.e. a bifunctional scheme similar to the formation of the desired products. The reaction products are diisobutylketone (DIBK) from acetone and biscyclohexylcylclohexanone (BCHCHO) from cyclohexanone. The corresponding product is not observed in transformation of acetophenone probably because of difficulty in desorbing this very bulky compound. 3-2- Influence of the catalyst characteristics. The activity, selectivity and stability of the bifunctional Pt or Pd zeolite catalysts depend on the nature, amount and dispersion of the metal, on the acidity and on the pore structure. Palladium is always more selective than platinum for the formation of the desired products. This is shown in Figure 1 for acetone transformation over 0.5 wt % Pt and Pd HMFI catalysts with similar acidities and number of accessible metal atoms [6]. The much higher selectivity of the palladium catalyst can be related to the fact that this metal is much more active for hydrogenating C=C bonds than C=O bonds, which is not the case for platinum. The effect of the hydrogenating and acid functions, hence on the balance between these two functions on the activity, selectivity and stability of bifunctional Pd zeolite catalysts was determined over series of chosen samples. This balance was characterized by the ratio nPd/nA between the number of accessible Pd atoms and the number of protonic sites able to retain pyridine adsorbed at 150~ The effect of nPd/nA on the turnover frequency of the acid sites (TOF) for the formation of the desired products is that expected from a bifunctional scheme : firstly an increase for low values of nPd/nA indicating that the reaction was limited by the hydrogenating step then a plateau, the acid step being then the limiting one. Figure 2 shows as an example the effect of nPd/nA on TOF for the formation of methylisobutylketone from acetone.
3015 10o
2SO
8o
.~ 200
Pd MR
/~
~' 150 eo
~
40
:E 100 u_
PtMR 2o
SO
0
|
|
10
20
0
30
Conversion I%)
,~
i
i
0
0.0S
0.1
0.15
nPd/nA
Fig. 1. Weight percentage of MIBK in the products formed over Pt and Pd MFI as a function of acetone conversion.
Fig. 2. Effect of nPd/nA on the turnover frequency (TOF) for the formation of MIBK.
Whereas the catalytic properties are mainly determined by the balance between the hydrogenating and the acid functions, various results demonstrate an effect of the porosity. This effect was shown in particular for acetophenone transformation over a series of Pd HFAU samples differing by the framework Si/AI ratio (from 4 to 100) and for which the amount and dispersion of palladium was enough to have the formation of 1,3-diphenyl-butanl-one (DPBO) limited by the acid step. Figure 3 shows that the greater the Si/A1 ratio of the zeolite hence the lower nA the greater the catalyst activity for DPBO production (and also the higher the selectivity). This unexpected increase in activity when the acidity of the samples decreases is most likely due to the creation of secondary pores by dealumination. Thus, the higher the Si/AI ratio the greater the mesopore volume : HFAU 4 has 25% of pore volume constituted by pores larger than the structural micropores, HFAU 100 more than 55%. Therefore the high activity of Pd HFAU 100 is certainly due to the presence of a large number of mesopores which make easier the desorption of the bulky molecules of DPBO. The selectivity is also much better : at 10% conversion, 65 wt % of the products are constituted by DPBO over Pd HFAU 100 against only 10 % over Pd HFAU 4.
OD
E
Fig. 3. Activity of various Pd HFAU catalysts for acetone transformation into DPBO as a function of time on stream. 9 0.5 Pd HFAU 4, A 0.5 Pd HFAU 17, ll 0.5 Pd HFAU 47, 9 0.2 Pd HFAU 100
E 2o
-~
o
E
lOO 47
'~
~ir
17
o
20
40
60
Time on stream (min)
80
100
3016 This effect of the pore structure was confirmed by the high activity and selectivity of Pd MCM41 samples. Thus a 0.5 wt % Pd HMCM41 (30) sample was found to be 1.4 times more active for DPBO formation than the best Pd HFAU 100 sample and its selectivity was slightly higher (70% instead of 65%). These observations can be related to the large size of the pores of MCM41 which allow an easy desorption of the bulky reaction products such as DPBO. On the other hand because of their narrow pores, Pd MFI samples are much less active than Pd HFAU 100. An effect of the pore structure is also shown in cyclohexanone transformation : for identical values of nPd/nA, the activity of Pd HFAU catalysts is higher and their selectivity to cyclohexylcylclohexanone (CHCHO) are much better (75% instead 55%) than those of Pd HMFI catalysts. However these latter catalysts are preferred for acetone transformation into methylisobutylketone (MIBK) because in their narrow pores the formation of bulky reaction products such as DIBK and of coke is very limited. 4-CONCLUSIONS Pd bifunctional zeolite catalysts are able to catalyze in one apparent step the synthesis of ketones which requires three successive steps catalyzed by acid or metal sites : aldolisation, dehydration then hydrogenation. This synthesis can be made selective (_> 75%) by using a well balanced catalyst with a chosen pore structure. Indeed the porosity of the zeolite plays a determining role both in the activity and in the selectivity : too narrow pores limit the desorption of the desired products which are bulkier than the non desired products and too large pores favour the formation of very bulky secondary products and of coke. Therefore, whereas for acetone transformation, an average pore size zeolite (e.g. MFI) should be used, a large pore zeolite (e.g. FAU) is preferred for cyclohexanone transformation and a mesoporous MCM41 silicoaluminate for acetophenone transformation. ACKNOWLEDGEMENT Financial support by the EC within the International Scientific Cooperation ECALA/MED countries (Contract CII*-CT 94-0044) is gratefully acknowledged. REFERENCES 1. W.F. HOlderich, H. van Bekkum, Stud. Surf. Sci. Catal., 58 (1991) 631. 2. W. Reith, M. Dcttmer, H. Widdecke, B. Fleischer, Stud. Surf. Sci. Catal., 59 (1991) 487. 3. L. Mdo, P. Magnoux, G. Giannetto, F. Alvarez, M. Guisnet, J. Mol. Catal. 124 (1997) 155. 4. F. Alvarez, P. Magnoux F.R. Ribeiro, M. Guisnet, J. Mol. Catal., 92 (1994) 67 5. N. Lavaud, P. Magnoux, F. Alvarez, L. Melo, G. Giannetto, M. Guisnet, J. Mol. Catal., 142 (1999) 223.
6. L. Melo, Thesis, Poitiers, 1994.