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Selective synthesis of cyclohexylcyclohexanone on bifunctional zeolite catalysts. Influence of the metal and of the pore structure
Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) © 1997 Elsevier Science B.V. All rights reserved.
609
Selective synthesis of cyclohexylcyclohexanone on bifunctional zeolite catalysts. Influence of the metal and of the pore structure F. Alvarez^, A.I. Silva«, F. Ramoa Ribeiro^, G. Giannetto^ and M. Guisnet^ ^D. E. Q., Institute Superior Tecnico, Av. Rovisco Pais, 1096 Lisboa, Portugal ^Facultad de Ingenieria, U. C. V., Ap. 47100, Caracas, Venezuela «URA CNRS 350, Catalyse en Chimie Organique, 40, av. recteur Pineau, 86022 Poitiers, France
Summary The transformation of cyclohexanone into cyclohexylcyclohexanone was carried out on various Pt or Pd zeohte catalysts with Si/Al (or Si/Ga) ratios around 40 under the following conditions : flow reactor, 473 K, pressures of cyclohexanone and hydrogen equal to 0.25 and 0.75 bar. The effect of the percentages of platinum or of palladium (from 0.1 to 0.5 wt%) was shown with series of Pt and PdHFAU catalysts. The initial activity first increased with metal content then remained constant for metal contents > 0.2 wt%, which is typical of bifunctional catalyzed processes. The initial activity did not depend on the metal but the selectivity to cyclohexylcyclohexanone was much higher with the PdHFAU sample (75% against 47% with PtHFAU, at a cyclohexanone conversion of 30%). The catalytic properties of various 0.2 wt% Pt or Pd zeohte catalysts with average or large pore size (HMFI alumino and gallosihcates, HBEA, HMOR and HFAU) were compared. The higher selectivity to cyclohexylcyclohexanone was found with all the Pd zeohtes while the initial activities of 0.2 wt% PtHMFI and PtHMOR were greater than those of the corresponding Pd samples. The initial activities and the selectivities of 0.2 wt% Pd zeohtes depended on the zeohte pore structure. The PdHFAU catalyst which has the largest pores was the more active and the more selective to cyclohexylcyclohexanone.
1. INTRODUCTION Bifunctional catalysts can allow to carry out in one operation the synthesis of compounds which usually requires several successive reactions catalyzed by monofunctional catalysts, which hmits the number of separation steps hence the pollution [1]. T5^ical examples are the sjnithesis over noble metal zeohte catalysts of methyhsobutylketone by transformation of acetone over PdMFI 12,3], that of cyclohexylcyclohexanone [4] which is a percusor of o-phenylphenol, an
610
important wide spectrum conservative [5]. A two step process for the synthesis of the title compound by condensation of cyclohexanone on an acidic ion exchanger, followed by the hydrogenation over Pd or Pt of the cyclohexenylcyclohexanone produced has already been reported [6]. Over bifunctional catalysts the synthesis in one operation of cyclohexylcyclohexanone involves the following successive steps catalyzed either by the acid sites (aldolisation and dehydration) or by the metallic sites (hydrogenation): OH
6^C:rts^CK)^CK> In a previous work [41, the reaction scheme of cyclohexanone transformation over PtMFI catalysts has been estabUshed. With these catalysts, the selectivity to cyclohexylcyclohexanone is very poor because of the rapid formation of Ce cycUc hydrocarbons. So as to develop more selective catalysts, the effect of noble metal (Pt or Pd) and of the zeoUte carrier (HMFI alumino and gallosilicates, HBEA, HMOR and HFAU) on the rate and the selectivity of cyclohexanone transformation was investigated.
2. RESULTS AND DISCUSSION PdHFAU and PtHFAU catalysts had metal contents ranging from 0.1 to 0.5 wt%. The metal percentages of the other catalysts were of about 0.2 wt%. Catalysts were named as xMZ (x - wt% metal; M - Pt or Pd; Z - zeoUte carrier). 2.1. Reaction Products With all the bifunctional Pt and Pd catalysts the reaction products were Ce cyclic hydrocarbons (family 1), C12 bicycUc hydrocarbons (family 2), cyclohexenylcyclohexanone isomers (familly 3), 2-cyclohexylcyclohexanone (4), insaturated C12 bicyclic alcohols and ketones (family 5) and tricycUc compounds (family 6). The reactions involved in the product formation on Pt and Pd zeolites are indicated in the following scheme:
oo
OH
o
6
Pt/Pd
6
-HoO
oVi'' O OH
OH
O
Pt/Pd ^ x ^
" ..^ + Pt/Pd • v , ^
611
The compounds of family 1 result from the hydrogenation of cyclohexanone on Pt or Pd sites, dehydration of cyclohexanol on the acid sites, hydrogenation and dehydrogenation of cyclohexene on the metallic sites. Cyclohexanol is not observed in the products, for dehydration steps on the acid sites are faster than ketone hydrogenation. It is for this reason that the alcohol intermediates in the formation of the compounds of families 2, 3, 4 and 6 are not observed in the products. The compounds of family 2 could result from cyclohexene dimerization or most likely from the transformation of 2-cyclohexylcyclohexanone (hydrogenation, dehydration, hydrogenation or dehydrogenation). Cyclohexenylcyclohexanone isomers and cyclohexylcyclohexanone can undergo dehydrogenation into the 5 compounds or aldohsation followed by dehydration then eventually hydrogenation or dehydrogenation leading to the 6 compounds. 2.2. Influence of the nature and of the percentage of noble metal The effect of these parameters on the rate and selectivity of cyclohexanone transformation was determined with two series of PtHFAU and PdHFAU catalysts with different metal contents. Figure la) shows the change with time on stream of the cyclohexanone conversion for a contact time of 1.2 s over three catalysts : 0.1 and 0.2PdHFAU and 0.2PtHFAU. The cyclohexanone conversion increases with the Pd content but is practically independent of the nature of the metal. The deactivation of all the catalysts is rapid, which makes difficult the determination of the initial activity. This is why several experiments (at least 5 for each catalyst) at different contact times were carried out to obtain accurate values of the initial activity (Figure lb). It should be emphasized that in the experiments the initial values of conversion are estimated by extrapolation of curves similar to those reported in Figure la) after elimination of the experimental value obtained at very short time on stream. Indeed this experimental value is at least for high values of contact time hence of the conversion abnormally high because of an initial temperature increase due to the exothermicity of the reaction [4]. X(%i
Xo(%) oO.lPdHFAU • 0.2PdHFAU A 0.2PtHFAU
b)
40 30 20
y
10 n
U n
100
200
300
Time on stream (min)
400
/
0
0.5
1
1.5
Contact time (s)
Figure 1. Transformation of cyclohexanone over Pd and PtHFAU catalysts. a) Conversion X(%) vs. time on stream. Xo : value of the initial conversion obtained by extrapolation at time on stream equal zero. b) Initial conversion, Xo, vs. contact time.
2
612
The influence of the metal content on the initial activity (Ao) for cyclohexanone transformation is shown in Figure 2 for the PtHFAU and PdHFAU catalysts. For both series of catalysts, Ao first increases with the metal content reaching a constant value for percentages of platinum or palladium equal to or greater than 0.2 wt%. This shape of curve is that expected from a bifunctional mechanism [7]. At low metal contents the cyclohexanone conversion is limited by hydrogenation steps hence the activity increases with the metal content. For metal contents > 0.2 wt% the cyclohexanone conversion is Umited by the acid steps hence the activity depends no more on the metal content. The stabilities of the catalysts were compared for identical values of the initial conversion. The nature and the percentage of the metal have no effect on the stability. Thus for an initial conversion of 30% the conversion after 3 hours' reaction is around 10% whatever the catalyst (residual activity equal to 0.35). The catalyst deactivation can be due to the retention inside the zeolite pores of heavy reaction products ("coke'' precursors) [8]. However a sintering of the metal particles can also occur owing to the presence of water resulting fi'om dehydration reactions. Indeed preUminary results indicate that, after removal of non desorbed products by oxidative treatment at 773 K, the acidity of the catalyst is completely recovered, which is not the case for the hydrogenating activity. Ao (mmol/h/g)
Selectivity to 4 (%)
50 -
• PdHFAU
> ? ^ 5 ^ = * — '\
40 -
r .
3020 i
10 -
/
A PtHFAU 0HFAU
0
^^—-•"
^•
•
60-
^ 40 -
r
n U
80 -
20 1
0.2
n U i
1
0.4
0.6
metal (wt,%)
Figure 2. Initial activity, Ao, of PdHFAU and PtHFAU catalysts vs. metal content.
0
I
0.2
1
0.4
0.
metal (wt,%)
Figure 3. Selectivity to cyclohexylcyclohexanone over PdHFAU and PtHFAU catalysts vs. metal content.
Figure 3 shows the change of the initial selectivity to cyclohexylcyclohexanone, calculated for an initial conversion of 30%, as a function of the metal content. For both series of catalysts a small quantity of metal (about 0.1 wt %) is enough to obtain the maximal selectivity value to the desired product. Nevertheless, contrary to what was found for the catalyst activity, the selectivity value strongly depends on the nature of the hydrogenating function (47% on PtHFAU against 75% on PdHFAU). The better selectivity of Pd catalysts was also observed with all the other bifunctional zeoUte catalysts. However, contrary to what was found
613
with HFAU catalysts, the initial activities of PtHMFI and PtHMOR were greater than those of the corresponding Pd catalysts. The lower selectivity of PtHFAU catalysts is due to the very rapid formation of Ce cycUc hydrocarbons (family 1). The same trend has been found in the case of acetone transformation [3]. This can be explained by the lower activity of the palladium relatively to the platinum to hydrogenate the C=0 bond. This lower activity which has been found in the case of cyclohexanone hydrogenation on platinum group metals was explained by a weaker adsorption of the ketone on Pd in comparison with Pt and Ru 19]. The lower activity of Pd for ketone hydrogenation is also responsible for the lower selectivity of PdHFAU catalysts for the compounds of family 2 whose formation involves cyclohexylcyclohexanone hydrogenation. The value of the ratio between cyclohexylcyclohexanone and cyclohexenylcyclohexanone (4/3) increases with the metal content (Pt or Pd) up to 0.2 wt%, then remains constant. Nevertheless this ratio is greater on PtHFAU than on PdHFAU catalysts (25 against 19 for a metal content of 0.2 wt%). This greater value found with PtHFAU catalysts is certainly due to their higher hydrogenating activity. 2.3. Influence of the zeolite pore structure Figure 4 compares the values of the initial activities of acid and bifunctional (0.2 wt% Pt or Pd) zeoUte catalysts. The most active acid catalyst is HBEA, HFAU is 1.3 times less active, HMOR and HMFI aluminosilicate about 3 times less active and HMFI gallosiUcate 30 times less active. The low activity of the gallosilicate was expected from the low strength of its acid sites [10]. However the difference in activity between the MFI gallo and aluminosUicate is more pronounced than in m-xylene isomerization. This suggests the existence of diffusion limitations during cyclohexanone transformation on the MFI zeoUte samples. These diffusion limitations are more pronounced with the gallosilicate sample for which the paralortho ratio found in m-xylene isomerization is greater than with the MFI aluminosiUcate sample [10]. Furthermore the greater activity of the HBEA sample could be due to the very small size of its crystallites hence to its large external surface area [11]. HMOR catalysts are generally less active than HFAU catalysts because of diffusion limitations in their monodirectional pores. The difference between these zeolites found in this work is very limited probably because mesopores created during the mordenite preparation by dealumination render the diffusion of organic molecules quasi tridirectional [12]. 0.2 wt% Pd exchanged catalysts are generally more active than the corresponding acid zeoUtes. An exception however: PdHBEA has the same activity as HBEA but the selectivities are totally different : as it could be expected the main reaction products observed on the acid zeolite are the cyclohexenylcyclohexanone isomers (selectivity equal to 74 % against 37% on PdHBEA, at 10% conversion) whereas only a selectivity value of 4% to cyclohexyl-cyclohexanone is observed. The most active bifunctional Pd catalyst is PdHFAU. This catalyst is about twice more active than PdHBEA, 3-4 times than PdHMOR and PdHMFI
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aluminosilicate and 30 times than PdHMFI gallosilicate. These differences in activity cannot be explained by differences in acidity only. Most likely, other catalyst characteristics such as their porosity and their hydrogenating activity play also a significant role. Ao(mmol/h/g) PtHFAU PdliFAU 40 PtHMFI 30 PdHBEAl HBEA 20 PdHMFI 10 +P(iH-[Ga].MFI HMFI
PtHMOR HFAU IPdHMOR
HMOR
|H-[Ga]-MFI/ 0
Catalyst Figure 4. Transformation of cyclohexanone. Initial activities of acid and bifunctional (0.2 wt% Pt or Pd) zeolite catalysts. Table 1 shows that the product distribution on Pd catalysts depends on the zeoUte. PdHFAU and PdHMOR are the most selective to cyclohexylcyclohexanone. This is also the case when all the products which can be transformed into o-phenylphenol (3+4+5) are considered. However the hydrogenating activity of PdHMOR is weaker than that of PdHFAU. Indeed the cyclohexylcyclohexanone/cyclohexenylcyclohexanone ratio (4/3) is lower (Table 1). The difference in selectivity to 3+4+5 between PdHFAU, PdHMOR on the one hand and PdHMFI, PdHBEA on the other is partly due to the formation of Ce or Ci2 hydrocarbons (products 1 and 2) even if some other differences exist between the catalysts. In particular only 0.3% of Cs hydrocarbons are found in the products on PdHMOR against around 2% on the other catalysts; 10% of C12 hydrocarbons are found on PdHBEA against around 6% on the other catalysts. Furthermore there is a more significant production of the heavy products 6 with PdHMFI and PdHBEA than on PdHFAU and PdHMOR. This faster production of 6 is probably due to Umitations in the desorption of the reaction products 3 and 4 which can therefore undergo secondary transformations into heavy products. This faster production could also be due to the weaker hydrogenating activity of PdHMFI and PdHBEA (see in Table 1 the low value of the 4/3 ratio) if it is admitted that aldoHsation occurs more rapidly from the 3 alkylenic compounds than from the 4 compound because of a stronger adsorption on the acid sites.
615 Table 1 Transformation of cyclohexanone. Selectivities of bifunctional zeolite catalysts Selectivity to products (%) Catalyst
1
2
3
4
5
6
4/3 ratio
0.2PtHMFI 0.2PdHMFI 0.2PtHMOR 0.2PdHMOR 0.2PtHFAU 0.2PclHFAU 0.2PdHBEA
61.2 2.6 34.4 0.3 30.1 2.0 1.8
7.7 6.5 21.5 6.0 12.1 6.0 10.2
2.0 17.0 3.8 7.0 1.9 4.0 18.3
23.2 52.9 34.5 73.2 47.2 75.0 47.7
1.8 6.0 1.0 2.5 1.5 3.5 5.0
4.1 15.0 4.8 11.0 7.2 9.5 17.0
11.6 3.1 9.1 10.5 24.8 18.7 2.6
3. EXPERIMENTAL HZSM5, HFAU, HMOR and HBEA zeoUtes had framework and total Si/Al ratios of about 40. They were commercial Valflor zeolites supplied by PQ, or obtained from them by dealumination by acid treatment. The H-[Ga]-ZSM5 had a Si/Ga ratio of 35 and its synthesis has already been described [10]. The Pt and Pd zeolites catalysts were prepared by ion exchange with [Pt(NH3)4]Cl2 and [Pd(NH3)4]Cl2, respectively, followed by calcination under dry air flow at 573K and reduction under hydrogen at 773 K. The reaction was carried out in a flow reactor at 473 K, atmospheric pressure and PH2^Pcyclohexanone~ 3- Reaction products were identified by a GS/MS system and analyzed by gas chromatography using a CPSil 5 CB capillary column with 50 m of length and 0.25 mm of interior diameter [4]. In order to study the influence of contact time on the catalytic properties, different catalyst weights (0.07 - 0.6 g) and different flows of Liquid cyclohexanone were used (1.9 - 4.25 cm^/h).
4. CONCLUSIONS Bifunctional Pt or Pd zeolite catalysts (with large or average pore sizes) can catalyze in one pot the transformation of cyclohexanone into cyclohexylcyclohexanone which requires three successive steps catalyzed by acid sites : aldolisation and dehydration or by metal sites : hydrogenation. Pd catalysts are more selective than Pt catalysts, for palladium catalyzes preferentially the hydrogenation of C=C double bonds (compared to the C=0 bonds). PdHFAU zeolites because of their large pores and of their tridirectional pore system are the most active and selective catalysts. With these catalysts the formation of
616
cyclohexylcyclohexanone is not limited by the desorption of this bulky product from the zeolite pores, which is the case with Pd deposited in zeolites with narrower pores or with unidirectional pore systems.
REFERENCES 1. W.F. Holderich and H. van Bekkum, in "Introduction to Zeolite Science and Practice", (H. van Bekkum et al., Eds.), Studies in Surface Science and Catalysis, vol. 58, Elsevier, Amsterdam, 1991, p. 631. 2. P.V. Chen, S.J. Chu, N.S. Chang, T.K. Chuang and L.Y. Chen, in "Zeolites as Catalysts, Sorbents and Detergent Builders", (H.G.Karge and J. Weitkamp, Eds.), Studies in Surface Science and Catalysis, Vol. 46, Elsevier, Amsterdam, 1989, p. 231. 3. L. Melo, Ph.D.Thesis, Universite de Poitiers, 1994. 4. F. Alvarez, P. Magnoux , F. R. Ribeiro and M. Guisnet, J. Mol. Cat., 92 (1994) 67. 5. A. Mitschker, R. Wagner and P.M. Lange, in "Heterogeneous Catalysis and Fine Chemicals", (M. Guisnet et al., Eds.), Studies in Surface Science and Catalysis, Vol. 41, Elsevier, Amsterdam, 1988, p. 61. 6. P. Thomissen and J. Hubertu (Stamicarbon B.V.), Eur. Pat. Appl. EP 87187 (1983). 7. M. Guisnet and G. Perot, in "Zeolites Science and Technology", (F. R. Ribeiro et al., Eds.), NATO ASl Series E, Vol. 80, Martinus Nijhoff Publishers, The Hague, 1984, p. 397. 8. A.I. Silva, F. Alvarez, P. Magnoux and M. Guisnet, unpubUshed results. 9. C. Sungbom and K. Tanaka, BuU. Chem. Soc. Jpn., 55 (1982) 2275. 10. F. Jayat, I. Neves, M. Guisnet, M. Goldwasser, G. Giannetto and J. Papa, in "Proc. XIV Simposio Iberoamericano de Catalisis", Chile, 1994, p. 573. l l . C . Coutanceau, J.M. Silva, F. Alvarez, F.R. Ribeiro and M. Guisnet, J. Chim. Phys., in press. 12.N.S. Gnep, P.Roger, P. Cartraud, M. Guisnet, B. Juguin and C. Hamon, C.R. Acad. Sci. Paris, 309 QI) (1989) 1743.
ACKNOWLEDGEMENT Financial support by the EC within the International Scientific Cooperation EC-ALA/MED countries (Contract CIl*-CT94-0044) is gratefully acknowledged.
609
Selective synthesis of cyclohexylcyclohexanone on bifunctional zeolite catalysts. Influence of the metal and of the pore structure F. Alvarez^, A.I. Silva«, F. Ramoa Ribeiro^, G. Giannetto^ and M. Guisnet^ ^D. E. Q., Institute Superior Tecnico, Av. Rovisco Pais, 1096 Lisboa, Portugal ^Facultad de Ingenieria, U. C. V., Ap. 47100, Caracas, Venezuela «URA CNRS 350, Catalyse en Chimie Organique, 40, av. recteur Pineau, 86022 Poitiers, France
Summary The transformation of cyclohexanone into cyclohexylcyclohexanone was carried out on various Pt or Pd zeohte catalysts with Si/Al (or Si/Ga) ratios around 40 under the following conditions : flow reactor, 473 K, pressures of cyclohexanone and hydrogen equal to 0.25 and 0.75 bar. The effect of the percentages of platinum or of palladium (from 0.1 to 0.5 wt%) was shown with series of Pt and PdHFAU catalysts. The initial activity first increased with metal content then remained constant for metal contents > 0.2 wt%, which is typical of bifunctional catalyzed processes. The initial activity did not depend on the metal but the selectivity to cyclohexylcyclohexanone was much higher with the PdHFAU sample (75% against 47% with PtHFAU, at a cyclohexanone conversion of 30%). The catalytic properties of various 0.2 wt% Pt or Pd zeohte catalysts with average or large pore size (HMFI alumino and gallosihcates, HBEA, HMOR and HFAU) were compared. The higher selectivity to cyclohexylcyclohexanone was found with all the Pd zeohtes while the initial activities of 0.2 wt% PtHMFI and PtHMOR were greater than those of the corresponding Pd samples. The initial activities and the selectivities of 0.2 wt% Pd zeohtes depended on the zeohte pore structure. The PdHFAU catalyst which has the largest pores was the more active and the more selective to cyclohexylcyclohexanone.
1. INTRODUCTION Bifunctional catalysts can allow to carry out in one operation the synthesis of compounds which usually requires several successive reactions catalyzed by monofunctional catalysts, which hmits the number of separation steps hence the pollution [1]. T5^ical examples are the sjnithesis over noble metal zeohte catalysts of methyhsobutylketone by transformation of acetone over PdMFI 12,3], that of cyclohexylcyclohexanone [4] which is a percusor of o-phenylphenol, an
610
important wide spectrum conservative [5]. A two step process for the synthesis of the title compound by condensation of cyclohexanone on an acidic ion exchanger, followed by the hydrogenation over Pd or Pt of the cyclohexenylcyclohexanone produced has already been reported [6]. Over bifunctional catalysts the synthesis in one operation of cyclohexylcyclohexanone involves the following successive steps catalyzed either by the acid sites (aldolisation and dehydration) or by the metallic sites (hydrogenation): OH
6^C:rts^CK)^CK> In a previous work [41, the reaction scheme of cyclohexanone transformation over PtMFI catalysts has been estabUshed. With these catalysts, the selectivity to cyclohexylcyclohexanone is very poor because of the rapid formation of Ce cycUc hydrocarbons. So as to develop more selective catalysts, the effect of noble metal (Pt or Pd) and of the zeoUte carrier (HMFI alumino and gallosilicates, HBEA, HMOR and HFAU) on the rate and the selectivity of cyclohexanone transformation was investigated.
2. RESULTS AND DISCUSSION PdHFAU and PtHFAU catalysts had metal contents ranging from 0.1 to 0.5 wt%. The metal percentages of the other catalysts were of about 0.2 wt%. Catalysts were named as xMZ (x - wt% metal; M - Pt or Pd; Z - zeoUte carrier). 2.1. Reaction Products With all the bifunctional Pt and Pd catalysts the reaction products were Ce cyclic hydrocarbons (family 1), C12 bicycUc hydrocarbons (family 2), cyclohexenylcyclohexanone isomers (familly 3), 2-cyclohexylcyclohexanone (4), insaturated C12 bicyclic alcohols and ketones (family 5) and tricycUc compounds (family 6). The reactions involved in the product formation on Pt and Pd zeolites are indicated in the following scheme:
oo
OH
o
6
Pt/Pd
6
-HoO
oVi'' O OH
OH
O
Pt/Pd ^ x ^
" ..^ + Pt/Pd • v , ^
611
The compounds of family 1 result from the hydrogenation of cyclohexanone on Pt or Pd sites, dehydration of cyclohexanol on the acid sites, hydrogenation and dehydrogenation of cyclohexene on the metallic sites. Cyclohexanol is not observed in the products, for dehydration steps on the acid sites are faster than ketone hydrogenation. It is for this reason that the alcohol intermediates in the formation of the compounds of families 2, 3, 4 and 6 are not observed in the products. The compounds of family 2 could result from cyclohexene dimerization or most likely from the transformation of 2-cyclohexylcyclohexanone (hydrogenation, dehydration, hydrogenation or dehydrogenation). Cyclohexenylcyclohexanone isomers and cyclohexylcyclohexanone can undergo dehydrogenation into the 5 compounds or aldohsation followed by dehydration then eventually hydrogenation or dehydrogenation leading to the 6 compounds. 2.2. Influence of the nature and of the percentage of noble metal The effect of these parameters on the rate and selectivity of cyclohexanone transformation was determined with two series of PtHFAU and PdHFAU catalysts with different metal contents. Figure la) shows the change with time on stream of the cyclohexanone conversion for a contact time of 1.2 s over three catalysts : 0.1 and 0.2PdHFAU and 0.2PtHFAU. The cyclohexanone conversion increases with the Pd content but is practically independent of the nature of the metal. The deactivation of all the catalysts is rapid, which makes difficult the determination of the initial activity. This is why several experiments (at least 5 for each catalyst) at different contact times were carried out to obtain accurate values of the initial activity (Figure lb). It should be emphasized that in the experiments the initial values of conversion are estimated by extrapolation of curves similar to those reported in Figure la) after elimination of the experimental value obtained at very short time on stream. Indeed this experimental value is at least for high values of contact time hence of the conversion abnormally high because of an initial temperature increase due to the exothermicity of the reaction [4]. X(%i
Xo(%) oO.lPdHFAU • 0.2PdHFAU A 0.2PtHFAU
b)
40 30 20
y
10 n
U n
100
200
300
Time on stream (min)
400
/
0
0.5
1
1.5
Contact time (s)
Figure 1. Transformation of cyclohexanone over Pd and PtHFAU catalysts. a) Conversion X(%) vs. time on stream. Xo : value of the initial conversion obtained by extrapolation at time on stream equal zero. b) Initial conversion, Xo, vs. contact time.
2
612
The influence of the metal content on the initial activity (Ao) for cyclohexanone transformation is shown in Figure 2 for the PtHFAU and PdHFAU catalysts. For both series of catalysts, Ao first increases with the metal content reaching a constant value for percentages of platinum or palladium equal to or greater than 0.2 wt%. This shape of curve is that expected from a bifunctional mechanism [7]. At low metal contents the cyclohexanone conversion is limited by hydrogenation steps hence the activity increases with the metal content. For metal contents > 0.2 wt% the cyclohexanone conversion is Umited by the acid steps hence the activity depends no more on the metal content. The stabilities of the catalysts were compared for identical values of the initial conversion. The nature and the percentage of the metal have no effect on the stability. Thus for an initial conversion of 30% the conversion after 3 hours' reaction is around 10% whatever the catalyst (residual activity equal to 0.35). The catalyst deactivation can be due to the retention inside the zeolite pores of heavy reaction products ("coke'' precursors) [8]. However a sintering of the metal particles can also occur owing to the presence of water resulting fi'om dehydration reactions. Indeed preUminary results indicate that, after removal of non desorbed products by oxidative treatment at 773 K, the acidity of the catalyst is completely recovered, which is not the case for the hydrogenating activity. Ao (mmol/h/g)
Selectivity to 4 (%)
50 -
• PdHFAU
> ? ^ 5 ^ = * — '\
40 -
r .
3020 i
10 -
/
A PtHFAU 0HFAU
0
^^—-•"
^•
•
60-
^ 40 -
r
n U
80 -
20 1
0.2
n U i
1
0.4
0.6
metal (wt,%)
Figure 2. Initial activity, Ao, of PdHFAU and PtHFAU catalysts vs. metal content.
0
I
0.2
1
0.4
0.
metal (wt,%)
Figure 3. Selectivity to cyclohexylcyclohexanone over PdHFAU and PtHFAU catalysts vs. metal content.
Figure 3 shows the change of the initial selectivity to cyclohexylcyclohexanone, calculated for an initial conversion of 30%, as a function of the metal content. For both series of catalysts a small quantity of metal (about 0.1 wt %) is enough to obtain the maximal selectivity value to the desired product. Nevertheless, contrary to what was found for the catalyst activity, the selectivity value strongly depends on the nature of the hydrogenating function (47% on PtHFAU against 75% on PdHFAU). The better selectivity of Pd catalysts was also observed with all the other bifunctional zeoUte catalysts. However, contrary to what was found
613
with HFAU catalysts, the initial activities of PtHMFI and PtHMOR were greater than those of the corresponding Pd catalysts. The lower selectivity of PtHFAU catalysts is due to the very rapid formation of Ce cycUc hydrocarbons (family 1). The same trend has been found in the case of acetone transformation [3]. This can be explained by the lower activity of the palladium relatively to the platinum to hydrogenate the C=0 bond. This lower activity which has been found in the case of cyclohexanone hydrogenation on platinum group metals was explained by a weaker adsorption of the ketone on Pd in comparison with Pt and Ru 19]. The lower activity of Pd for ketone hydrogenation is also responsible for the lower selectivity of PdHFAU catalysts for the compounds of family 2 whose formation involves cyclohexylcyclohexanone hydrogenation. The value of the ratio between cyclohexylcyclohexanone and cyclohexenylcyclohexanone (4/3) increases with the metal content (Pt or Pd) up to 0.2 wt%, then remains constant. Nevertheless this ratio is greater on PtHFAU than on PdHFAU catalysts (25 against 19 for a metal content of 0.2 wt%). This greater value found with PtHFAU catalysts is certainly due to their higher hydrogenating activity. 2.3. Influence of the zeolite pore structure Figure 4 compares the values of the initial activities of acid and bifunctional (0.2 wt% Pt or Pd) zeoUte catalysts. The most active acid catalyst is HBEA, HFAU is 1.3 times less active, HMOR and HMFI aluminosilicate about 3 times less active and HMFI gallosiUcate 30 times less active. The low activity of the gallosilicate was expected from the low strength of its acid sites [10]. However the difference in activity between the MFI gallo and aluminosUicate is more pronounced than in m-xylene isomerization. This suggests the existence of diffusion limitations during cyclohexanone transformation on the MFI zeoUte samples. These diffusion limitations are more pronounced with the gallosilicate sample for which the paralortho ratio found in m-xylene isomerization is greater than with the MFI aluminosiUcate sample [10]. Furthermore the greater activity of the HBEA sample could be due to the very small size of its crystallites hence to its large external surface area [11]. HMOR catalysts are generally less active than HFAU catalysts because of diffusion limitations in their monodirectional pores. The difference between these zeolites found in this work is very limited probably because mesopores created during the mordenite preparation by dealumination render the diffusion of organic molecules quasi tridirectional [12]. 0.2 wt% Pd exchanged catalysts are generally more active than the corresponding acid zeoUtes. An exception however: PdHBEA has the same activity as HBEA but the selectivities are totally different : as it could be expected the main reaction products observed on the acid zeolite are the cyclohexenylcyclohexanone isomers (selectivity equal to 74 % against 37% on PdHBEA, at 10% conversion) whereas only a selectivity value of 4% to cyclohexyl-cyclohexanone is observed. The most active bifunctional Pd catalyst is PdHFAU. This catalyst is about twice more active than PdHBEA, 3-4 times than PdHMOR and PdHMFI
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aluminosilicate and 30 times than PdHMFI gallosilicate. These differences in activity cannot be explained by differences in acidity only. Most likely, other catalyst characteristics such as their porosity and their hydrogenating activity play also a significant role. Ao(mmol/h/g) PtHFAU PdliFAU 40 PtHMFI 30 PdHBEAl HBEA 20 PdHMFI 10 +P(iH-[Ga].MFI HMFI
PtHMOR HFAU IPdHMOR
HMOR
|H-[Ga]-MFI/ 0
Catalyst Figure 4. Transformation of cyclohexanone. Initial activities of acid and bifunctional (0.2 wt% Pt or Pd) zeolite catalysts. Table 1 shows that the product distribution on Pd catalysts depends on the zeoUte. PdHFAU and PdHMOR are the most selective to cyclohexylcyclohexanone. This is also the case when all the products which can be transformed into o-phenylphenol (3+4+5) are considered. However the hydrogenating activity of PdHMOR is weaker than that of PdHFAU. Indeed the cyclohexylcyclohexanone/cyclohexenylcyclohexanone ratio (4/3) is lower (Table 1). The difference in selectivity to 3+4+5 between PdHFAU, PdHMOR on the one hand and PdHMFI, PdHBEA on the other is partly due to the formation of Ce or Ci2 hydrocarbons (products 1 and 2) even if some other differences exist between the catalysts. In particular only 0.3% of Cs hydrocarbons are found in the products on PdHMOR against around 2% on the other catalysts; 10% of C12 hydrocarbons are found on PdHBEA against around 6% on the other catalysts. Furthermore there is a more significant production of the heavy products 6 with PdHMFI and PdHBEA than on PdHFAU and PdHMOR. This faster production of 6 is probably due to Umitations in the desorption of the reaction products 3 and 4 which can therefore undergo secondary transformations into heavy products. This faster production could also be due to the weaker hydrogenating activity of PdHMFI and PdHBEA (see in Table 1 the low value of the 4/3 ratio) if it is admitted that aldoHsation occurs more rapidly from the 3 alkylenic compounds than from the 4 compound because of a stronger adsorption on the acid sites.
615 Table 1 Transformation of cyclohexanone. Selectivities of bifunctional zeolite catalysts Selectivity to products (%) Catalyst
1
2
3
4
5
6
4/3 ratio
0.2PtHMFI 0.2PdHMFI 0.2PtHMOR 0.2PdHMOR 0.2PtHFAU 0.2PclHFAU 0.2PdHBEA
61.2 2.6 34.4 0.3 30.1 2.0 1.8
7.7 6.5 21.5 6.0 12.1 6.0 10.2
2.0 17.0 3.8 7.0 1.9 4.0 18.3
23.2 52.9 34.5 73.2 47.2 75.0 47.7
1.8 6.0 1.0 2.5 1.5 3.5 5.0
4.1 15.0 4.8 11.0 7.2 9.5 17.0
11.6 3.1 9.1 10.5 24.8 18.7 2.6
3. EXPERIMENTAL HZSM5, HFAU, HMOR and HBEA zeoUtes had framework and total Si/Al ratios of about 40. They were commercial Valflor zeolites supplied by PQ, or obtained from them by dealumination by acid treatment. The H-[Ga]-ZSM5 had a Si/Ga ratio of 35 and its synthesis has already been described [10]. The Pt and Pd zeolites catalysts were prepared by ion exchange with [Pt(NH3)4]Cl2 and [Pd(NH3)4]Cl2, respectively, followed by calcination under dry air flow at 573K and reduction under hydrogen at 773 K. The reaction was carried out in a flow reactor at 473 K, atmospheric pressure and PH2^Pcyclohexanone~ 3- Reaction products were identified by a GS/MS system and analyzed by gas chromatography using a CPSil 5 CB capillary column with 50 m of length and 0.25 mm of interior diameter [4]. In order to study the influence of contact time on the catalytic properties, different catalyst weights (0.07 - 0.6 g) and different flows of Liquid cyclohexanone were used (1.9 - 4.25 cm^/h).
4. CONCLUSIONS Bifunctional Pt or Pd zeolite catalysts (with large or average pore sizes) can catalyze in one pot the transformation of cyclohexanone into cyclohexylcyclohexanone which requires three successive steps catalyzed by acid sites : aldolisation and dehydration or by metal sites : hydrogenation. Pd catalysts are more selective than Pt catalysts, for palladium catalyzes preferentially the hydrogenation of C=C double bonds (compared to the C=0 bonds). PdHFAU zeolites because of their large pores and of their tridirectional pore system are the most active and selective catalysts. With these catalysts the formation of
616
cyclohexylcyclohexanone is not limited by the desorption of this bulky product from the zeolite pores, which is the case with Pd deposited in zeolites with narrower pores or with unidirectional pore systems.
REFERENCES 1. W.F. Holderich and H. van Bekkum, in "Introduction to Zeolite Science and Practice", (H. van Bekkum et al., Eds.), Studies in Surface Science and Catalysis, vol. 58, Elsevier, Amsterdam, 1991, p. 631. 2. P.V. Chen, S.J. Chu, N.S. Chang, T.K. Chuang and L.Y. Chen, in "Zeolites as Catalysts, Sorbents and Detergent Builders", (H.G.Karge and J. Weitkamp, Eds.), Studies in Surface Science and Catalysis, Vol. 46, Elsevier, Amsterdam, 1989, p. 231. 3. L. Melo, Ph.D.Thesis, Universite de Poitiers, 1994. 4. F. Alvarez, P. Magnoux , F. R. Ribeiro and M. Guisnet, J. Mol. Cat., 92 (1994) 67. 5. A. Mitschker, R. Wagner and P.M. Lange, in "Heterogeneous Catalysis and Fine Chemicals", (M. Guisnet et al., Eds.), Studies in Surface Science and Catalysis, Vol. 41, Elsevier, Amsterdam, 1988, p. 61. 6. P. Thomissen and J. Hubertu (Stamicarbon B.V.), Eur. Pat. Appl. EP 87187 (1983). 7. M. Guisnet and G. Perot, in "Zeolites Science and Technology", (F. R. Ribeiro et al., Eds.), NATO ASl Series E, Vol. 80, Martinus Nijhoff Publishers, The Hague, 1984, p. 397. 8. A.I. Silva, F. Alvarez, P. Magnoux and M. Guisnet, unpubUshed results. 9. C. Sungbom and K. Tanaka, BuU. Chem. Soc. Jpn., 55 (1982) 2275. 10. F. Jayat, I. Neves, M. Guisnet, M. Goldwasser, G. Giannetto and J. Papa, in "Proc. XIV Simposio Iberoamericano de Catalisis", Chile, 1994, p. 573. l l . C . Coutanceau, J.M. Silva, F. Alvarez, F.R. Ribeiro and M. Guisnet, J. Chim. Phys., in press. 12.N.S. Gnep, P.Roger, P. Cartraud, M. Guisnet, B. Juguin and C. Hamon, C.R. Acad. Sci. Paris, 309 QI) (1989) 1743.
ACKNOWLEDGEMENT Financial support by the EC within the International Scientific Cooperation EC-ALA/MED countries (Contract CIl*-CT94-0044) is gratefully acknowledged.