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Use of carbon fabrics as support for hydrogenation catalysts usable in polyphasic reactors
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.
995
Use of carbon fabrics as support for hydrogenation catalysts usable in polyphasic reactors. J.P. Reymond and P. Fouilloux L.G.P.C.; E.S.C.P.E.; 43 Bd du 11 Novembre ; F-69616 Villeurbanne cedex; France. In order to improve the performances of fixed bed reactors or slurry reactors in polyphasic catalytic hydrogenation of organic molecules, routes allowing to deposit noble metals on bidimensionnal supports, which could transformed further in structured catalytic systems (monolithic reactors), have been studied in this paper. Carbon fabrics have been selected as bidimensionnal support and the obtained catalytic systems have been tested in two triphasic reactions 9hydrogenation of acetophenone and hydrogenation of 4-chlorophenol. 1. INTRODUCTION A great number of catalytic conversions of organic molecules take place in triphasic reactors (gas-liquid-solid). Compared to conventionnal fixed bed or slurry reactors, the monolithic reactor is an interesting alternative [1 ]. Due to its particular structure, constituted of a great number of small parallel channels (up to 100 per cm2), a monolith presents some advantages compared to the usual reactors : mass and heat transferts are more effective; pressure drops are smaller; as the active phase is anchored on the channel walls the liquidcatalyst particles separation is eliminated. Typically, the channel walls of a monolith are covered by a porous oxide layer (washcoat), over which the catalytic phase is deposited. It is also possible to deposit the metal rightly on the channel walls. We have studied two routes to deposit metal clusters on a bidimensionnal support, support which could be further shaped into monolith, making up the channel walls. Use of stainless steel grids, or sheets, has been described elsewhere [2], use of carbon clothes is described in this paper. 2. E X P E R I M E N T A L
2.1. Preparation of catalysts Two routes of deposition of active metal have been investigated (the amount of metal precursor is calculated to lead to catalysts containing 5 wt% of metal). - the well known impregnation method has been used to deposite ruthenium. Four fabrics pieces (each of 4x4 cm, total weight ~ 0.45g) are immersed in an aqueous solution of RuC12 for 15 hours, drained, dried (3 hours at 443 K under nitrogen flow). An activation treament, reduction under H2 flow, is necessary to reduce the deposited metal complex and generate the zerovalent active metal. - the autocatalytic deposition method [2], deriving from the electroless plating method [3] has been used to deposit palladium. This process comprises four steps 9 a- cleaning of carbon fabrics pieces (4 pieces; each of 4x4 cm; 0.45g), which is operated in a Kumagawa apparatus with acetone
996
b- preparation of an aqueous PdCI2 solution : adding of hydrochloric acid (0.5 to 1 g HCI 36 wt% in 25 cm 3 of water) is necessary to dissolve PdC12
c- deposition of the metal : the carbon clothes are immersed in the PdC12 solution. A given volume of a reductant solution (sodium hypophosphite) is then added in well controlled conditions (temperature, reactant concentrations and hypophosphite adding rate). d- drying : 2 hours in air at 298 K. This process is similar to a galvanic process (anodic and cathodic reactions take place in the mechanism) in which the electrons are provided by the chemical reductant [3]. It leads to the zerovalent metal which is catalytically active without activation treatment. 2.2. M a t e r i a l s 9
Carbon clothes (CECA and Actitex) are obtained from carbonization (hydrogen consumption) and activation (porosity formation) treatments of viscose fabrics (rayon). The diameter of carbon fibers is 20 ~tm and cloth thickness is ~ 0.5 mm. Some textural characteristics of three carbon clothes and of an active carbon in grains are given in table 1. Table 1 Characteristics of carbon supports. Support (type and manufacturer)
SBET
(m2/g)
Smicr~176 T~ Vp~ (m2/g) (cm3/g)
Vmicr~176 d micropore (cm3/g) (nm) 0.215 0.7 0.475 0.5
TE80 : Active carbon in grains (Degussa)
1098
495
0.550
IS : woven carbon cloth (CECA)
1250
948
0.505
RS 1301 : woven carbon cloth (Actitex)
1348
1004
0.731
0.462
0.5
FC 1201 : non woven carbon cloth (Actitex)
1000
889
0.444
0.410
0.6
1.3. C a t a l y t i c r e a c t i o n s :
The best way to evaluate the catalytic efficiency of a solid is to study its activity in a test reaction. Two reactions have been used : hydrogenation ofacetophenone (palladium and ruthenium catalysts) and hydrogenation of 4-chlorophenol (ruthenium). Catalytic tests take place in a 125 c m 3 stainless steel stirred reactor (semi-batch). In the case of acetophenone hydrogenation, catalyst samples are immersed in cyclohexane, hydrogen pressure is 2.5 MPa and reaction temperature is 393 K. Extent of reaction is evaluated from hydrogen and acetophenone consumption and product formation. Catalyst activity is characterized by initial activity and selectivity values. Acetophenone (AC), has an aromatic ring with a ketone function and reaction products depend on the nature of the active metal [4, 5]. On ruthenium catalysts the aromatic cycle is hydrogenated, leading to the formation of methylcyclohexylketone (MCC) and cyclohexylethanol (CE). On palladium the ketone function is hydrogenated in phenylethanol (PE) and ethylbenzene(EB). Hydrogenation of 4-chlorophenol is part of a two steps process of depolluting waste water which has been developped in our laboratory [6]. The first step is the adsorption of chlorophenol on the carbon support of the catalyst (Ru/carbon). The second step is the catalytic dechlorination by hydrogenation of the adsorbed chlorophenol, in an aqueous basic media (soda) and under mild operating conditions (313 to 353 K, and hydrogen pressure 0.3 to 0.5 MPa) [6]. The reaction products are cyclohexanol and hydrochloric acid (neutralized by soda).
3. RESULTS AND DISCUSSION
997
3.1. Catalysts based on ruthenium deposited on carbon clothes: 3.1.1. Hydrogenation of acetophenone :
The IS carbon cloth (see table 1) has been used as support to prepare ruthenium catalytic systems, following the impregnation method. Figure 1 depicts the conversion of reactants and products (Ru/carbon cloth; 393 K; 2.5 0,3MPa H2; A i is 7.29 molH2/g.h), as a function of time. Acetophenone is first hydrogenated in 0,2 methylcyclohexylketone (MCC) which, in g turn, is hydrogenated in cyclohexylethanol e(CE) which is the main (or only) product at 9 0,1 0 tthe end of the reaction (no more hydrogen O 0 consumption). Light formation of phenyl 0~ ethanol (PE) and ethyl benzene (EB) is 0 20 40 60 noted, indicating hydrogenation of the Time (mn) ketone function of acetophenone. Fig. 1. Product and reactant conversion during The influence of several operating acetophenone hydrogenation over Ru/C cloth. parameters of preparation on the activity of Ru/Carbon cloth have been evaluated. Table 2 depicts some results. For example, there is an optimal reduction temperature (773 K). Above 773 K the support reacts with hydrogen and sintering of ruthenium particles probably occurs, leading to a poor catalytic activity. ~nH2
---.Zk--AC
t'-
Table 2 Effect of preparation operating parameters of Ru/carbon cloth catalysts on initial activity. Reduction temperature (K) Reduction time (h) mass (g) Activity (moIH2.glRuh-l) 573 573 573
3 20 6
693
773 873
6
0.46 0.45 0.9 0.45 0.22 0.1 0.5 0.235 0.09 0.47 0.44
5-9 7-9 5.5 6.5 6.1 5.9 12.15 13.14 12.2 21-25 7
As it is observed in table 2 the initial activity does not depend on the catalyst mass. Moreover, the activity of Ru/carbon cloth is egal to that of the same catalyst finely grinded. It can be concluded from the experiments that there is no internal diffusion problem with the catalytic system, that the reactor is perfectly stirred and that the reaction occurs in chemical regime. Carbon cloth appears as a convenient support to prepare catalytic systems.
998 However, it has to note that in same conditions of activation and reaction, a commercial catalyst (Ru over carbon powder, from Degussa), is twice more active (up to 53 molm.gtRu hl) than our catalysts but presents similar selectivities. 3.1.2. Adsorption and hydrogenation of 4-chlorophenol on Ru/Carbone: The depolluting process, mentioned above, was first operated over a conventional catalyst : ruthenium deposited on active carbon in grains (Pica, TE 80). To improve this process, the properties of two carbon fabrics, RS1301 (woven cloth) and FC1201 (non woven), towards adsorption of 4-chlorophenol have been evaluated and compared to that of the carbon in grains. Adsorption isotherms of 4-chlorophenol on the supports, presented on figure 2, are similar (in fig.2, Q is the amount adsorbed at a given time and Qmaxis the maximal amount adsorbed). The adsorption rates, determinated in a stirred batch reactor, are also similar while adsorption rate of 4-chlorophenol is slightly greater on FC 1201.
1
1 0,8
0'8 t
f
o,6
.~ o,6 E0,4
E0,4--O--FC1201 ---i~--TE80 ---X-- RS1301
0,2
0,2
C
I
0~ o
750 1500 2250 Equilibrium concentration (ppm) Fig.2. Adsorption isotherms of chlorophenol on adsorbents (293 K).
- -
--~---TE 80 ~FC 1201 ---X'-- RS 1301
I
2000 4000 Time (s)
6000
Fig.3. Breakthrough curves of adsorbents (3g; 1l/h; initial concentration" 4000 ppm; 293 K)
Two important parameters, Q~ and t~, are determined from the so-called breakthrough curves which are presented on figure 3. These curves, obtained from in-line adsorption experiments (column), give the pollutant concentration in the outlet flow versus time. t~ is the critical time, time for which the pollutant concentration becomes higher than the maximun value allowed for pollutant rejection (0.1 ppm for 4-chlorophenol) and Qr is the corresponding critical amount of adsorbed pollutant. Table 3 Comparison of adsorptive properties of carbon supports. Qmax tc (s) Qc (g/g) FC1201 RS1301 TE80
0.489 0.577 0.455
0.5 1/h 2070 1710 840
1.5 1/h 614 175 37
0.5 1/h 1.43 1.04 0.48
1.5 1/h 0.43 0.26 0.06
999 The values of Qmax, tc and Qc are given for two flow rates in table 3. For FC1201 the outlet flow remains below the limit value for a longer time and then rises more stiffly than for the other adsorbents. With FC 1201 the in-line adsorption of 4-chlorophenol could be carried out with a higher flow rate. From these results it can be concluded that adsorption properties of carbon fabrics are slightly better than that of carbon grains. Compare to active carbon grains, carbon fabrics present improved characteristics : greater adsorption capacity and stiffness of the breakthrough curves. Moreover, the packing of an adsorption column with carbon cloth disks leads to an ordered structure presenting a lower pressure drop than it is in the case of carbon grains (the same result will be obtained in an inline reactor). Then, ruthenium has been deposited by the impregnation method on the RS 1301 cloth which presents the higher adsorption capacity. The resulting catalyst has been tested in the hydrodechlorination of 4-chlorophenol and its activity has been compared to that of the conventional catalyst (Ru/TE80). Catalytic activities (transformation rates of 4-chlorophenol in cyclohexanol) are very similar over the two tested catalysts : 0.4molH2gRu-lhI in each case. As evidenced by batch and in-line adsorption experiments carbon clothes show interesting performances for chlorophenol adsorption. They are also useful as support to prepare effective catalysts in the further hydrogenation of the adsorbed 4-chlorophenol.
3.2. Catalysts based on palladium: hydrogenation of acetophenone. The autocatalytic deposition method, usable on all kinds of support, has been used to deposit palladium on the IS carbon fabrics (see table 1) instead of the impregnation method. The influence of several preparation operating parameters on the activity of obtained catalytic systems has been studied in the hydrogenation of acetophenone. As evidenced from chemical analysis and X-ray emission analysis [2], the autocatalytic deposition process leads to the formation of a layer of palladium-phosphorus alloy coating the whole surface of the support and to the formation of discrete particles of the same alloy. If sodium borohydride is used as the reducer, instead of sodium hypophosphite, the deposit is an alloy of palladium and boron. Mechanism of phosphorus and palladium codeposition in electroless plating baths is not really elucidated, it could occur via interaction of hypophosphite ion with atomic hydrogen or with hydride ions or with palladium metallic surface [7]. Catalytic properties of Pd/carbon fabrics have been compared to that of industrial hydrogenation catalysts (Degussa and Doduco) in which palladium (5 wt%) is supported on powdered active carbon (SBET'~1000 m2g~). Results are presented in table 4. Table 4 Comparison of initial activity and selectivities of industrial and laboratory Pd/carbon catalysts. Catalyst Reduction Ai Phenylethanol Ethylbenzene treatment mo1H2g-lpd hl % % Pd/C (Degussa) N 52 99 1 Pd/C (Doduco) N 123 18 82 Pd/C cloth N 68 0 100 Pd/C (Degussa) Pd/C cloth
Y Y
219 45
76 15
24 85
1000 All catalysts have the behaviour of palladium : they catalyze the hydrogenation of the ketone function. However, our catalytic system appears quite different to industrial catalysts. When catalysts are not prereduced by hydrogen, Pd/C-fabrics and Pd/C-Degussa have quite similar initial activity (hydrogen consumption rate: ~ 60 molH2gpdl.h-~) while Pd/C-Doduco is twice more active (~120 molH2gpdl.h~). On the other hand, selectivities at the end of the reaction are more different. Pd/carbon cloth leads only to ethylbenzene while industrial catalysts have a more lighter hydrogenolysis activity, producing only phenylethanol (Degussa) or phenylethanol and ethylbenzenze (Doduco). A reducing pretreatment (H2, 2h, 493 K) decreases slightly the initial activity of Pd/C fabrics (45 molH2gpd~.h~), while initial activity of industrial catalyst (Pd/C Degussa) is strongly enhanced (220 molH2gpdl.hl). This is not surprising because the autocatalytic preparation method leads to the deposition of zerovalent palladium which is catalytically active (the influence of phosphorus is not elucidated). On the contrary, impregnation of a support from a metal salt solution leads to the formation of a surface metal complex which must be decomposed in order to generate the zerovalent active metal. Nevertheless, reduced Pd/C-Degussa has a great hydrogenation activity (formation of phenylethanol) and a weak hydrogenolysis action (conversion of phenylethanol in ethylbenzene). Selectivities observed with our catalyst are different, it leads mainly to the formation of ethylbenzenze, its hydrogenolysis action is greater, but phenylethanol is not fully conversed. Combined to the autocatalytic deposition process carbon clothes allow to prepare interesting bidimensionnal catalytic systems which could shaped as structured catalyst (monoliths and column packings are intended). 4. CONCLUSION Compare to carbon powders or granulates the implementation of carbon fabrics in polyphasic reactors present several improved caracteristics : good textural characteristics and adsorptive properties, possibility to prepare structured catalytic systems. Carbon clothes have been used as support to prepare efficient hydrogenation catalysts. Nevertheless, the activity of actual catalytic systems on carbon fabrics has to be improved to reach the activity level of industrial catalysts. REFERENCES 1. A. Cybulski and J.A. Moulijn, in "Structured Catalysts and Reactors" (A. Cybulski J.A. Moulijn Eds., M. Dekker, Amsterdam, 1995), 1. 2. J.P. Reymond, D. Dubois and P. Fouilloux, Studies in Surface Science and Catalysis, (1998), 63-72. 3. D. Dukes in "Electroless Plating : fundamentals and Applications" (G.O. Mallory, Hadju eds., A.E.S.F., Orlando, 1992), 511. 4. N.S. Barinov, D.V. Mushenko and E.G. Lebeva, Zh. Prikl. Khim., 39 (1966), 2599. 5. J. Mason, P. Cividino, J.M. Bonnier and P. Fouilloux in "Heterogeneous Catalysis Fine Chemicals II" (1991), 245. 6. V. Felis, C.de Bellefon, P.Fouilloux and D. Schweich, Appl. Catal.,20 (1999), 91. 7. G. Salvagno and P.L. Cavalotti, Plating, 59 (1972), 665.
and 118 J.B.
and
995
Use of carbon fabrics as support for hydrogenation catalysts usable in polyphasic reactors. J.P. Reymond and P. Fouilloux L.G.P.C.; E.S.C.P.E.; 43 Bd du 11 Novembre ; F-69616 Villeurbanne cedex; France. In order to improve the performances of fixed bed reactors or slurry reactors in polyphasic catalytic hydrogenation of organic molecules, routes allowing to deposit noble metals on bidimensionnal supports, which could transformed further in structured catalytic systems (monolithic reactors), have been studied in this paper. Carbon fabrics have been selected as bidimensionnal support and the obtained catalytic systems have been tested in two triphasic reactions 9hydrogenation of acetophenone and hydrogenation of 4-chlorophenol. 1. INTRODUCTION A great number of catalytic conversions of organic molecules take place in triphasic reactors (gas-liquid-solid). Compared to conventionnal fixed bed or slurry reactors, the monolithic reactor is an interesting alternative [1 ]. Due to its particular structure, constituted of a great number of small parallel channels (up to 100 per cm2), a monolith presents some advantages compared to the usual reactors : mass and heat transferts are more effective; pressure drops are smaller; as the active phase is anchored on the channel walls the liquidcatalyst particles separation is eliminated. Typically, the channel walls of a monolith are covered by a porous oxide layer (washcoat), over which the catalytic phase is deposited. It is also possible to deposit the metal rightly on the channel walls. We have studied two routes to deposit metal clusters on a bidimensionnal support, support which could be further shaped into monolith, making up the channel walls. Use of stainless steel grids, or sheets, has been described elsewhere [2], use of carbon clothes is described in this paper. 2. E X P E R I M E N T A L
2.1. Preparation of catalysts Two routes of deposition of active metal have been investigated (the amount of metal precursor is calculated to lead to catalysts containing 5 wt% of metal). - the well known impregnation method has been used to deposite ruthenium. Four fabrics pieces (each of 4x4 cm, total weight ~ 0.45g) are immersed in an aqueous solution of RuC12 for 15 hours, drained, dried (3 hours at 443 K under nitrogen flow). An activation treament, reduction under H2 flow, is necessary to reduce the deposited metal complex and generate the zerovalent active metal. - the autocatalytic deposition method [2], deriving from the electroless plating method [3] has been used to deposit palladium. This process comprises four steps 9 a- cleaning of carbon fabrics pieces (4 pieces; each of 4x4 cm; 0.45g), which is operated in a Kumagawa apparatus with acetone
996
b- preparation of an aqueous PdCI2 solution : adding of hydrochloric acid (0.5 to 1 g HCI 36 wt% in 25 cm 3 of water) is necessary to dissolve PdC12
c- deposition of the metal : the carbon clothes are immersed in the PdC12 solution. A given volume of a reductant solution (sodium hypophosphite) is then added in well controlled conditions (temperature, reactant concentrations and hypophosphite adding rate). d- drying : 2 hours in air at 298 K. This process is similar to a galvanic process (anodic and cathodic reactions take place in the mechanism) in which the electrons are provided by the chemical reductant [3]. It leads to the zerovalent metal which is catalytically active without activation treatment. 2.2. M a t e r i a l s 9
Carbon clothes (CECA and Actitex) are obtained from carbonization (hydrogen consumption) and activation (porosity formation) treatments of viscose fabrics (rayon). The diameter of carbon fibers is 20 ~tm and cloth thickness is ~ 0.5 mm. Some textural characteristics of three carbon clothes and of an active carbon in grains are given in table 1. Table 1 Characteristics of carbon supports. Support (type and manufacturer)
SBET
(m2/g)
Smicr~176 T~ Vp~ (m2/g) (cm3/g)
Vmicr~176 d micropore (cm3/g) (nm) 0.215 0.7 0.475 0.5
TE80 : Active carbon in grains (Degussa)
1098
495
0.550
IS : woven carbon cloth (CECA)
1250
948
0.505
RS 1301 : woven carbon cloth (Actitex)
1348
1004
0.731
0.462
0.5
FC 1201 : non woven carbon cloth (Actitex)
1000
889
0.444
0.410
0.6
1.3. C a t a l y t i c r e a c t i o n s :
The best way to evaluate the catalytic efficiency of a solid is to study its activity in a test reaction. Two reactions have been used : hydrogenation ofacetophenone (palladium and ruthenium catalysts) and hydrogenation of 4-chlorophenol (ruthenium). Catalytic tests take place in a 125 c m 3 stainless steel stirred reactor (semi-batch). In the case of acetophenone hydrogenation, catalyst samples are immersed in cyclohexane, hydrogen pressure is 2.5 MPa and reaction temperature is 393 K. Extent of reaction is evaluated from hydrogen and acetophenone consumption and product formation. Catalyst activity is characterized by initial activity and selectivity values. Acetophenone (AC), has an aromatic ring with a ketone function and reaction products depend on the nature of the active metal [4, 5]. On ruthenium catalysts the aromatic cycle is hydrogenated, leading to the formation of methylcyclohexylketone (MCC) and cyclohexylethanol (CE). On palladium the ketone function is hydrogenated in phenylethanol (PE) and ethylbenzene(EB). Hydrogenation of 4-chlorophenol is part of a two steps process of depolluting waste water which has been developped in our laboratory [6]. The first step is the adsorption of chlorophenol on the carbon support of the catalyst (Ru/carbon). The second step is the catalytic dechlorination by hydrogenation of the adsorbed chlorophenol, in an aqueous basic media (soda) and under mild operating conditions (313 to 353 K, and hydrogen pressure 0.3 to 0.5 MPa) [6]. The reaction products are cyclohexanol and hydrochloric acid (neutralized by soda).
3. RESULTS AND DISCUSSION
997
3.1. Catalysts based on ruthenium deposited on carbon clothes: 3.1.1. Hydrogenation of acetophenone :
The IS carbon cloth (see table 1) has been used as support to prepare ruthenium catalytic systems, following the impregnation method. Figure 1 depicts the conversion of reactants and products (Ru/carbon cloth; 393 K; 2.5 0,3MPa H2; A i is 7.29 molH2/g.h), as a function of time. Acetophenone is first hydrogenated in 0,2 methylcyclohexylketone (MCC) which, in g turn, is hydrogenated in cyclohexylethanol e(CE) which is the main (or only) product at 9 0,1 0 tthe end of the reaction (no more hydrogen O 0 consumption). Light formation of phenyl 0~ ethanol (PE) and ethyl benzene (EB) is 0 20 40 60 noted, indicating hydrogenation of the Time (mn) ketone function of acetophenone. Fig. 1. Product and reactant conversion during The influence of several operating acetophenone hydrogenation over Ru/C cloth. parameters of preparation on the activity of Ru/Carbon cloth have been evaluated. Table 2 depicts some results. For example, there is an optimal reduction temperature (773 K). Above 773 K the support reacts with hydrogen and sintering of ruthenium particles probably occurs, leading to a poor catalytic activity. ~nH2
---.Zk--AC
t'-
Table 2 Effect of preparation operating parameters of Ru/carbon cloth catalysts on initial activity. Reduction temperature (K) Reduction time (h) mass (g) Activity (moIH2.glRuh-l) 573 573 573
3 20 6
693
773 873
6
0.46 0.45 0.9 0.45 0.22 0.1 0.5 0.235 0.09 0.47 0.44
5-9 7-9 5.5 6.5 6.1 5.9 12.15 13.14 12.2 21-25 7
As it is observed in table 2 the initial activity does not depend on the catalyst mass. Moreover, the activity of Ru/carbon cloth is egal to that of the same catalyst finely grinded. It can be concluded from the experiments that there is no internal diffusion problem with the catalytic system, that the reactor is perfectly stirred and that the reaction occurs in chemical regime. Carbon cloth appears as a convenient support to prepare catalytic systems.
998 However, it has to note that in same conditions of activation and reaction, a commercial catalyst (Ru over carbon powder, from Degussa), is twice more active (up to 53 molm.gtRu hl) than our catalysts but presents similar selectivities. 3.1.2. Adsorption and hydrogenation of 4-chlorophenol on Ru/Carbone: The depolluting process, mentioned above, was first operated over a conventional catalyst : ruthenium deposited on active carbon in grains (Pica, TE 80). To improve this process, the properties of two carbon fabrics, RS1301 (woven cloth) and FC1201 (non woven), towards adsorption of 4-chlorophenol have been evaluated and compared to that of the carbon in grains. Adsorption isotherms of 4-chlorophenol on the supports, presented on figure 2, are similar (in fig.2, Q is the amount adsorbed at a given time and Qmaxis the maximal amount adsorbed). The adsorption rates, determinated in a stirred batch reactor, are also similar while adsorption rate of 4-chlorophenol is slightly greater on FC 1201.
1
1 0,8
0'8 t
f
o,6
.~ o,6 E0,4
E0,4--O--FC1201 ---i~--TE80 ---X-- RS1301
0,2
0,2
C
I
0~ o
750 1500 2250 Equilibrium concentration (ppm) Fig.2. Adsorption isotherms of chlorophenol on adsorbents (293 K).
- -
--~---TE 80 ~FC 1201 ---X'-- RS 1301
I
2000 4000 Time (s)
6000
Fig.3. Breakthrough curves of adsorbents (3g; 1l/h; initial concentration" 4000 ppm; 293 K)
Two important parameters, Q~ and t~, are determined from the so-called breakthrough curves which are presented on figure 3. These curves, obtained from in-line adsorption experiments (column), give the pollutant concentration in the outlet flow versus time. t~ is the critical time, time for which the pollutant concentration becomes higher than the maximun value allowed for pollutant rejection (0.1 ppm for 4-chlorophenol) and Qr is the corresponding critical amount of adsorbed pollutant. Table 3 Comparison of adsorptive properties of carbon supports. Qmax tc (s) Qc (g/g) FC1201 RS1301 TE80
0.489 0.577 0.455
0.5 1/h 2070 1710 840
1.5 1/h 614 175 37
0.5 1/h 1.43 1.04 0.48
1.5 1/h 0.43 0.26 0.06
999 The values of Qmax, tc and Qc are given for two flow rates in table 3. For FC1201 the outlet flow remains below the limit value for a longer time and then rises more stiffly than for the other adsorbents. With FC 1201 the in-line adsorption of 4-chlorophenol could be carried out with a higher flow rate. From these results it can be concluded that adsorption properties of carbon fabrics are slightly better than that of carbon grains. Compare to active carbon grains, carbon fabrics present improved characteristics : greater adsorption capacity and stiffness of the breakthrough curves. Moreover, the packing of an adsorption column with carbon cloth disks leads to an ordered structure presenting a lower pressure drop than it is in the case of carbon grains (the same result will be obtained in an inline reactor). Then, ruthenium has been deposited by the impregnation method on the RS 1301 cloth which presents the higher adsorption capacity. The resulting catalyst has been tested in the hydrodechlorination of 4-chlorophenol and its activity has been compared to that of the conventional catalyst (Ru/TE80). Catalytic activities (transformation rates of 4-chlorophenol in cyclohexanol) are very similar over the two tested catalysts : 0.4molH2gRu-lhI in each case. As evidenced by batch and in-line adsorption experiments carbon clothes show interesting performances for chlorophenol adsorption. They are also useful as support to prepare effective catalysts in the further hydrogenation of the adsorbed 4-chlorophenol.
3.2. Catalysts based on palladium: hydrogenation of acetophenone. The autocatalytic deposition method, usable on all kinds of support, has been used to deposit palladium on the IS carbon fabrics (see table 1) instead of the impregnation method. The influence of several preparation operating parameters on the activity of obtained catalytic systems has been studied in the hydrogenation of acetophenone. As evidenced from chemical analysis and X-ray emission analysis [2], the autocatalytic deposition process leads to the formation of a layer of palladium-phosphorus alloy coating the whole surface of the support and to the formation of discrete particles of the same alloy. If sodium borohydride is used as the reducer, instead of sodium hypophosphite, the deposit is an alloy of palladium and boron. Mechanism of phosphorus and palladium codeposition in electroless plating baths is not really elucidated, it could occur via interaction of hypophosphite ion with atomic hydrogen or with hydride ions or with palladium metallic surface [7]. Catalytic properties of Pd/carbon fabrics have been compared to that of industrial hydrogenation catalysts (Degussa and Doduco) in which palladium (5 wt%) is supported on powdered active carbon (SBET'~1000 m2g~). Results are presented in table 4. Table 4 Comparison of initial activity and selectivities of industrial and laboratory Pd/carbon catalysts. Catalyst Reduction Ai Phenylethanol Ethylbenzene treatment mo1H2g-lpd hl % % Pd/C (Degussa) N 52 99 1 Pd/C (Doduco) N 123 18 82 Pd/C cloth N 68 0 100 Pd/C (Degussa) Pd/C cloth
Y Y
219 45
76 15
24 85
1000 All catalysts have the behaviour of palladium : they catalyze the hydrogenation of the ketone function. However, our catalytic system appears quite different to industrial catalysts. When catalysts are not prereduced by hydrogen, Pd/C-fabrics and Pd/C-Degussa have quite similar initial activity (hydrogen consumption rate: ~ 60 molH2gpdl.h-~) while Pd/C-Doduco is twice more active (~120 molH2gpdl.h~). On the other hand, selectivities at the end of the reaction are more different. Pd/carbon cloth leads only to ethylbenzene while industrial catalysts have a more lighter hydrogenolysis activity, producing only phenylethanol (Degussa) or phenylethanol and ethylbenzenze (Doduco). A reducing pretreatment (H2, 2h, 493 K) decreases slightly the initial activity of Pd/C fabrics (45 molH2gpd~.h~), while initial activity of industrial catalyst (Pd/C Degussa) is strongly enhanced (220 molH2gpdl.hl). This is not surprising because the autocatalytic preparation method leads to the deposition of zerovalent palladium which is catalytically active (the influence of phosphorus is not elucidated). On the contrary, impregnation of a support from a metal salt solution leads to the formation of a surface metal complex which must be decomposed in order to generate the zerovalent active metal. Nevertheless, reduced Pd/C-Degussa has a great hydrogenation activity (formation of phenylethanol) and a weak hydrogenolysis action (conversion of phenylethanol in ethylbenzene). Selectivities observed with our catalyst are different, it leads mainly to the formation of ethylbenzenze, its hydrogenolysis action is greater, but phenylethanol is not fully conversed. Combined to the autocatalytic deposition process carbon clothes allow to prepare interesting bidimensionnal catalytic systems which could shaped as structured catalyst (monoliths and column packings are intended). 4. CONCLUSION Compare to carbon powders or granulates the implementation of carbon fabrics in polyphasic reactors present several improved caracteristics : good textural characteristics and adsorptive properties, possibility to prepare structured catalytic systems. Carbon clothes have been used as support to prepare efficient hydrogenation catalysts. Nevertheless, the activity of actual catalytic systems on carbon fabrics has to be improved to reach the activity level of industrial catalysts. REFERENCES 1. A. Cybulski and J.A. Moulijn, in "Structured Catalysts and Reactors" (A. Cybulski J.A. Moulijn Eds., M. Dekker, Amsterdam, 1995), 1. 2. J.P. Reymond, D. Dubois and P. Fouilloux, Studies in Surface Science and Catalysis, (1998), 63-72. 3. D. Dukes in "Electroless Plating : fundamentals and Applications" (G.O. Mallory, Hadju eds., A.E.S.F., Orlando, 1992), 511. 4. N.S. Barinov, D.V. Mushenko and E.G. Lebeva, Zh. Prikl. Khim., 39 (1966), 2599. 5. J. Mason, P. Cividino, J.M. Bonnier and P. Fouilloux in "Heterogeneous Catalysis Fine Chemicals II" (1991), 245. 6. V. Felis, C.de Bellefon, P.Fouilloux and D. Schweich, Appl. Catal.,20 (1999), 91. 7. G. Salvagno and P.L. Cavalotti, Plating, 59 (1972), 665.
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