Znd. E n g . Chem. Res. 1996,34,1031-1036
1031
Activity and Selectivity of Palladium Catalysts during the Liquid-Phase Hydrogenation of Phenol. Influence of Temperature and Pressure J. R. Gonzllez-Velasco,* M. P. Gonzdlez-Marcos, S. Amaiz, J. I. Gutibrrez-Ortiz, and M. k Gutibrrez-Ortiz Departamento de Ingenieria Quimica, Facultad de Ciencias, Universidad del Pais Vasco / EHU, E-48080 Bilbao, Spain
Two series of highly dispersed palladium catalysts supported on alumina have been prepared by adsorption from solution, with palladium contents varying from 0.25 to 2.0 wt %. The first series was calcined at 773 K for 4 h in air, whereas the second series was just heated at 423 K for 1 h in nitrogen, before reduction. Complete dispersion of the metal has been found for the calcined catalysts, and metal dispersion was favored with low palladium contents for the noncalcined catalysts. The kinetic behavior of the catalysts has been analyzed for the liquidphase hydrogenation of phenol in a stirred tank reactor, ensuring a chemically controlled regime for stirring speed above 750 rpm and catalyst particle below 0.08-0.16 mm in the studied conditions. Despite their higher metallic dispersion, the calcined catalysts presented lower activity than their corresponding noncalcined catalysts. The influence of hydrogen partial pressure on activity showed a reaction order of 2. The apparent activation energy resulted in 56.8 kJ mol-l. Selectivity to cyclohexanone was found to be very high for all experiments.
Introduction Catalytic hydrogenation processes are used in the chemical industry as a way to obtain many products and intermediates. As hydrogen is always in the gas phase, it is said that there is a gas-phase or a liquid-phase hydrogenation depending on the state in which the other reactants are fed. Usually, supported catalysts for hydrogenationconsist of nickel, palladium, or platinum dispersed on porous materials such as silica, alumina, or active carbon. Among these possibilities, palladium catalysts are known to present good activity and much higher selectivity when compared to nickel or platinum. When phenol undergoes hydrogenation, the products may be cyclohexanone, benzene, cyclohexene, and cyclohexanol, depending on the type of catalyst used and the conditions at which the reaction is carried out. Over nickel, the main product of the reaction is cyclohexanol, and to obtain cyclohexanone (in the manufacture of €-caprolactam) it is necessary to dehydrogenate cyclohexanol. There has been a tendency in recent years to replace this two-stage process for the production of cyclohexanone by the more selective and economical single-stage method using palladium catalysts. In any case, high activity and selectivity are required from the catalysts as cyclohexanone is difficult to separate from large amounts of phenol because of the formation of phenol-cyclohexanol and phenol-cyclohexanone azeotropes. In previous work (Gutikrrez-Ortiz, 1984; GonzalezVelasco et al., 1987) various Pd catalysts supported on different materials were prepared for use in the gasphase hydrogenation of phenol to cyclohexanone. Some important conclusions with respect to the effects of different stages of the preparation on the catalyst performance were drawn and some general considerations on catalyst preparation were established.
* Address correspondenceto this author. e-mail: IQPGOVEJ@ LG.EHU.ES.
However, carrying out the reaction in the liquid phase, in a slurry reactor, may present important advantages with respect t o the gas phase, e.g., better temperature control due to the higher heat capacity of the liquids, thus increasing catalyst life and quality and uniformity of the reaction products (Sharma, 19831, characteristics of great importance for the increasing production of fine chemicals. Moreover, liquid-phase processes allows a better selectivity for similar conversion than do gas-phase processes (Koopman et al., 1980). The activity and selectivity of these catalysts are strongly dependent on the type of support, the method of catalysts preparation, the activation conditions, and the operational conditions (pressure and temperature) at which the reaction is carried out. This work deals with the preparation of alumina-supported palladium catalysts using the adsorption from solution method, the influence of the activation procedure-calcination and/ or reduction-on the metal dispersion and subsequent effect on the activity and selectivity (to cyclohexanone) in the phenol liquid-phase hydrogenation. The influence of hydrogen partial pressure and temperature on activity and selectivity is also analyzed, allowing some conclusions on the kinetic reaction rate equations and the apparent activation energies of phenol to cyclohexanone and cyclohexanone to cyclohexanol.
Experimental Section Materials and Methods. Harshaw AL-3945 alumina, extruded y-alumina, essentially neutral, 99% pure with Na2O (100 ppm), Fez03 (100ppm), and Si02 (100 ppm) as the main impurities, was used as the support of the prepared catalysts. This material had been used previously as a support of palladium catalysts (Guti6rrez-Ortiz, 1984; Gonzalez-Velasco et al., 1986, 19871, providing good activity and selectivity in the gas-phase hydrogenation of phenol to cyclohexanone. To ensure that the textural characteristics of the alumina did not change during the various steps involved in the catalyst preparation, the supplied material was treated at 773 K for 4 h. The characteristics
0888-5885/95/2634-1031$09.00/0 0 1995 American Chemical Society
1032 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 Table 1. Textural Properties of the Alumina AL-3945 fresh calcined (773 K) S, (BET), m2 g-I 201 196 V,, cm3 8-l 0.47 0.54 rp (average),nm 3.7 3.7 r p(mode),nm 3.3 3.1 7.5 IEPS, pH
of the AL-3945 alumina are shown in Table 1, before and after calcination. The little variation between fresh and calcined support properties suggests that the supplied material was already calcined. The surface area (BET) and pore volume of the support and the resulting catalysts were determined from nitrogen adsorption-desorption isotherms at 77 K, in a Micromeritics Accusorb 2100E apparatus. The isoelectric point of the alumina was obtained in a conventional titration glassware following the neutralization method proposed by Jirtitova (1981). Characterization of the catalysts included evaluation of metal loading and dispersion. Elemental analysis for Pd was accomplished by atomic absorption spectrometry, in a llOOB Perkin-Elmer spectrometer. The measurements were realized with a palladium lamp at a wavelength of 247.6 nm, an intensity of 30 d, and in an oxidizing air-acetylene flame. For these standard conditions, the working range is linear up to concentrations around 15 ppm. To prevent interferences from the alumina, final solutions were made adding lanthanum up to a concentration of 5000 ppm. The palladium dispersion on the prepared catalysts was calculated from hydrogen chemisorption at a temperature of 343 K, in a Micromeritics Accusorb 2100E apparatus. The hydrogen to palladium surface atom stoichiometric ratio is supposed to be 1:2, as the hydrogen adsorption is considered to occur dissociatively on the palladium surface atoms (Scholten et al., 1985). The dispersion value is given as the surface to total platinum atoms ratio:
D
= NslNT
(1)
The procedure followed to clean the sample surface before the chemisorption measurements was evacuation at room temperature for 1h and 13.3 mPa, increase to 573 K in vacuum, prereduction at 573 K and a hydrogen pressure of 13.3 kPa for 1 h, and evacuation at 623 K under 0.13 mPa vacuum for 16 h. The chemisorption data were taken a t hydrogen equilibrium pressures up to 4 kPa (Martin et al., 19791, and the isotherms were represented as adsorbed hydrogen molecules per gram of catalyst, NM,versus hydrogen equilibrium pressure. The metallic dispersion was calculated by linearization of the data in the range 1-4 kPa and extrapolation to zero pressure in order to eliminate the support contribution. The extrapolated value is changed to units of dispersion using the following expression:
D
= 3.53 X 10-20(NM/cpd)
(2)
From the dispersion values thus obtained, palladium crystallite sizes were calculated assuming cubic shape with five faces accessible to the adsorbate (Scholten et al., 1985). Catalytic activity was determined for the liquid-phase hydrogenation of phenol in a cylindrical stirred tank reactor Parr Instruments Model 4562, 150-mm height and 63.5" internal diameter. The reactor disposes of a 45" pitched blade turbine with four blades connected
0
1
2
3
4
Time, h
Figure 1. Concentration profiles with time for noncalcined 1.0 wt % Paalumina catalyst. T = 453 K P = 3.0 MPa (0 phenol; 0 cyclohexanone; 0 cyclohexanol).
to an electric engine with variable stirring speed up t o 750 rpm, an automatic temperature controller up to 673 K, a pressure controller with a top operation pressure of 4 MPa, and a hydrogen flow controller. The schematics of the apparatus has been presented elsewhere (Gutihez-Ortiz et al., 1993). The reactor operated in a semicontinuous way, bubbling 50 cm3 min-' of pure hydrogen in 200 cm3 of the liquid feed. The general conditions of the kinetic experiments carried out were as follows: catalyst concentration, 10 g L-l; initial reaction mixture composition, 60140 wt % phenolkyclohexane (Cph = 5.94 mol L-l); catalyst palladium content, 0.25-1.0 wt %; catalyst particle size, 0.08-0.16 to 0.25-0.40 mm; stirring speed, 450-750 rpm; total pressure, 1.0-3.0 MPa; temperature, 433473 K. Samples for analysis, about 1 cm3 each to minimize the reaction volume variation, were taken discontinuously through the liquid outlet. Gas chromatography was used to analyze the sample composition, in a Perkin-Elmer Sigma 3B chromatograph with Sigma 10B integrator, with a 25-m-long, 0.22-mm-diameter, fused silica capillary column. The conditions were as follows: injector temperature, 673 K, oven temperature, 373 K, carrier (H2) pressure, 350 kPa; split ratio, 50:l. The obtained reaction data resulted in concentration versus time curves typical for a consecutive reaction scheme as shown in Figure 1. Catalyst Preparation. Taking into account the isoelectric point of the alumina, the catalysts were prepared by adsorption from an aqueous solution of palladium dichloride in hydrochloric acid (molar ratio, l:lO), mainly [PdCl$ as the active species in this acidic medium (Brunelle, 1979). Thus, precursors with nominal percentages 0.25, 0.50, and 1.0 wt % of palladium were prepared. Adsorption studies of the palladium species on the alumina were carried out a t 298 K and with a solution-support ratio of 40 cm3g-l. The percentage of adsorbed palladium is represented versus time in Figure 2. The isotherms show that the adsorption equilibrium is practically reached in short times. Because of that, an adsorption time of 3 h was used in the catalyst preparation. It was also observed that exchange extents close to 90% of the initial metal concentration were obtained for palladium contents below 1 w t %. To obtain the actual palladium charge in the catalyst as close to the nominal value as possible, an excess of the palladium complex of 15% was used. As only 40% of the initial concentration was reached for the 2 wt % Pd catalysts (i.e., about 1 wt % Pd was
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1033 Table 2. Characteristics of the Prepared Catalysts
*O
I
cat.
particle size, mm
Pd 'Ontent, nominal
actual
thermal treatment
D
dpd, nm
1 2 3 4 5 6 7 8
0.25-0.40 0.16-0.25 0.08-0.16 0.08-0.16 0.08-0.16 0.08-0.16 0.08-0.16 0.08-0.16
1.0 1.0 1.0 1.0 0.5 0.5 0.25 0.25
0.88 0.89 0.94 0.94 0.51 0.51 0.19 0.19
NC NC NC C NC C NC C
0.78 0.76 0.68 0.91 0.72 1.08 1.25 1.12
1.2 1.2 1.4 1.0 1.3 0.9 0.9 0.9
wt %
Table 3. Experimental Runs
oL'"''"'"'''"''"'"'''''"'"'~'~''' 0
30
60
150
90 120 Time, min
Figure 2. Adsorption of palladium with time on alumina AL3945 for different concentrations: A 2.0 wt %; 0 1.0wt %; 0 0.5 wt %; 0 0.25 w t %.
0
1
2
3
conditions
180
4
Equilibrium pressure, kPa Figure 3. Hydrogen chemisorption isotherms ( x support; 0 0.25 wt %; 0 0.5 wt %; 0 1.0 wt %). Empty and filled points correspond to noncalcined and calcined catalysts.
obtained in the final catalyst), it was decided not to prepare catalysts with palladium contents above 1 wt %.
The precursors were filtered and dried at 393 K for 16 h, and some calcined a t 773 K for 4 h in air after the temperature was gradually increased a t 7.6 K min-l; others just heated at 423 K for 1 h in nitrogen. We will refer to the former group as the calcined catalysts (C) and to the latter as the noncalcined catalysts (NC).All the catalysts became active after reduction in a hydrogen flow of 50 cm3 min-' at 573 for 3 h.
Results and Discussion Characterization of Catalysts. The hydrogen chemisorption isotherms for the different catalysts are represented in Figure 3. In the range 1-4 kPa of hydrogen pressure, a good fit of the data to the straight line can be observed. The characteristics of the prepared catalysts are shown in Table 2. Highly dispersed palladium catalysts have been obtained with this preparation method. Though a dispersion above unity is clearly not realistic as reported previously (Sermon and Bond, 1973; Scholten et al., 19851, the obtained relative values are useful to compare the different catalysts. For the calcined catalysts, a complete palladium dispersion on the sup-
P,
run
cat.
T.K
MPa
N, rDm
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
1 2 3 3 3 3 3 4 5 5 5 5 5 5 5 5 6 7 8
453 453 473 453 453 453 433 453 473 453 453 453 433 433 443 473 453 453 453
2.0 2.0 2.0 3.0 2.0 1.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.8 1.9 2.3 2.0 2.0 2.0
750 750 750 750 750 750 750 750 750 750 600 450 750 750 750 750 750 750 750
mol L-lh-1
TOF,
Sone, mol,,,
s-l
mOlprod-1
0.410 0.460 0.700 6.350 0.390 0.050 0.910 0.320 0.570 0.680 0.640 0.005 0.920 0.310 0.480 1.120 0.190 0.005 0.005
0.176 0.202 0.325 2.938 0.180 0.022 0.422 0.110 0.459 0.546 0.517 0.005 0.739 0.251 0.386 0.903 0.103 0.007 0.008
-rph,
0.935 0.900 0.965 0.950 0.950 0.960 0.960 0.965 0.952 0.950 0.960 0.916 0.951 0.966 0.880
port may be assumed. For the noncalcined catalysts, metal dispersion is favored with low palladium content and not very influenced by the catalyst particle size. The beneficial effect in metallic dispersion of calcination at high temperature deals with the fact that this thermal treatment avoids sintering of the metal when compared t o direct activation by reduction (Martin et al., 1984). Chou and Vannice (1987) concluded that catalyst activation temperature must be as low as possible provided that all the metal is reduced. In any case, this beneficial effect will be effective only if it produces a similar promotive effect on activity and selectivity of the catalysts as will be seen later. Activity of the Catalysts. A relation of the kinetic experiments carried out is shown in Table 3. The temperature and pressure were maintained constant during each experiment. From Figure 1 it can be deduced that there is no formation of cyclohexanol a t the beginning of the reaction, until some of the cyclohenanone was formed, Le., palladium hydrogenates phenol t o cyclohexanone but not to cyclohexanol, although cyclohexanone is also hydrogenated t o cyclohexanol in a consecutive reaction with the former. Thus we have to analyze both activity and selectivity of the catalysts to compare their kinetic behavior. At the very beginning of the reaction, the behavior of the system suffered from some disturbances until good control of temperature and pressure was attained and, as a consequence, irregular values for activity and selectivity could be found (Grau et al., 1987; Chaudhari et al., 1986). Thus, the activity and selectivity of the catalysts were determined a t that operation time in which the phenol concentration in the reaction medium was 5.0 mol L-' (16% conversion). Activity was obtained by derivation of the curves concentration versus time for phenol and has been
1034 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 0.25 0.6
0.5
1
t
I
0
I
/
/
0.15 -
/
i
/ /
400
I
/
500
8c
I
/
i. / t . 5 "
.
.
I
1
P /
F
1
'
'
.
.
600
.
' 700
.
.
I
.
0.10
-
0.05
-
t . . . ' . , . , . . , ) '
I
800
0
100
Stirring rate, rpm
Figure 4. Activity vs stirring speed. Runs 10, 11, and 12.
300
200
Catalyst particle size, pm
Figure 5. Activity vs catalyst particle size. Runs 1, 2, and 5.
expressed as reaction rate, -rph, and turnover frequency, TOF,h. Both variables are related taking into account the amount of active palladium present in the reaction medium:
35
. '
"
"
"
"
108 P (), -0.25
-0.5
-0.75
-1
"
"
"
"
0 "
'
0.25 "
"
"
0.5 "
''1
1
I 3.0 :
2.5 'w
where c p d and D are the percentage and dispersion of palladium in the catalyst, respectively. Selectivity was expressed as the ratio between the formed cyclohexanone moles and the total moles of cyclohexanol and cyclohexanone formed in the reaction. For the experiments carried out, the activity and selectivity are given in the last two columns in Table 3. The reaction system is rather complex, as there are three phases present and several mass-transfer steps involved. The mass-transport steps are (i) hydrogen diffusion within the gas bubble, (ii)hydrogen gas-liquid film diffusion around the gas bubble, (iii)hydrogen and benzene liquid-solid film diffusion around the catalyst particle, and (iv) hydrogen and phenol diffusion within the particle. Step i can be neglected in this case, because hydrogenations were carried out with pure gaseous hydrogen. Stirring of the reaction mixture keeps the catalyst suspended and disperses the hydrogen gas in the liquid phase. Increasing stirring speed is known to increase gas-liquid interfacial area, thereby increasing the rate of mass transfer in steps ii and iii. The influence of stirring speed was studied at 453 K and a total pressure of 2.0 MPa with the catalyst number 5 (experiments 1012 in Table 3). Figure 4 shows the TOF,h obtained versus stirring speed. The TOF,h is shown to be independent of the stirring speed above 600 rpm. The catalyst particle size affects step iv. The lower the particle size, the higher the rate of mass transfer. The influence of the catalyst particle size was studied a t 453 K, 2.0 MPa, and 750 rpm (Figure 5). The TOF,h is shown to be independent of the particle size for all the catalysts tested, i.e., the hydrogen and phenol diffusion within the particle can be considered negligible even with the bigger sizes analyzed. Then, most of the experiments were performed at stirring speed 750 rpm and catalyst particle size 0.080.16 mm. The effect of temperature on activity was analyzed (experiments 3, 5, and 7, Table 31, observing that a minimum in activity was obtained at the intermediate temperature. This fact led us to think that there was a significant variation in the vapor pressure of the liquid
2.0 : 1.5 :
1.0 : 0.5
1 0.0
/
0.5
1.o
1.5
2.0
2.5
Hydrogen partial pressure, MPa
Figure 6. Influence of hydrogen partial pressure on activity. Runs 4, 5, and 6.
mixture in the studied range of temperature and that hydrogen partial pressure had a strong influence on activity. Thus, the influence of hydrogen pressure on TOF,h was studied at 453 K (experiments 4,5, and 6, Table 3) and is shown in Figure 6 . As expected, increased hydrogen pressure increases the TOF,h with an exponent above unity. If we consider that the process can be described by a general Langmuir-HinshelwoodHougen-Watson kinetic equation, with control of the chemical reaction and dissociative adsorption of hydrogen molecules, and taking into account that activity measurements were obtained at constant c p h (5 mol L-l) and that the amount of cyclohexanone and cyclohexanol formed were very small by this time, the activity can be expressed as (4) where n is the number of active sites involved in the Kz, and K3 are a combination of limiting step, and KI, constants for these particular conditions. When working at relatively small hydrogen pressures, that is to say KZ>> K ~ P H this ~ , ~equation , can be reduced to
in which a second-order dependence of activity with hydrogen partial pressure is found. Experimental data were found to follow this correlation reasonably well. Taking into account the vapor pressure of the liquid mixture and varying total pressure in order to keep
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1035 0.98
0.96
0.94
0.92
0.90 210
220
2.15
2.25
230
420
2.35
lOOo/T, K"
0.8 I
I
I1
0
I
~
0
10
20
'
l
30
440
450
460
470
480
Temperature, K
Figure 7. Effect of temperature on consumption rate of phenol. Runs 10, 14, 15, and 16.
0.6
430
"
'
~
40 Surface palladium concentration, mz1.'
c
'
~
'
,
50
Figure 8. Influence of surface palladium concentration on consumption rate of phenol. 0 calcined catalysts, runs 8, 17, and 19; 0 noncalcined catalysts, runs 5, 10, and 18 (filled points correspond to runs 1 and 2).
hydrogen partial pressure constant in all the experiments, the effect of temperature on activity was again analyzed (experiments 10, 14, 15, and 16, Table 3). Figure 7 is a plot of ln(-rph) vs 1/T. The slope of the straight line was used t o determine the apparent activation energy for the liquid-phase phenol hydrogenation, resulting in a value of 56.8 kJ mol-l. The relation between activity, expressed as -Tph, and concentration of palladium surface in the reaction mixture is represented in Figure 8. As all experiments were carried out with a constant amount of catalyst (10 g L-l), the abscissa is proportional to the surface of palladium in the catalyst. If all palladium in the surface of the catalyst was equally active, as is the case of the calcined catalysts, the points should be in a straight line (squares in the figure). The fact that the line does not pass through the origin is related to the presence of poison in the liquid mixture initially fed to the reactor (Zwicky and Gut, 1978). Noncalcined catalysts, however, do not follow a straight line. Figure 8 shows that noncalcined catalysts present higher activity than their corresponding calcined and, even considering so, the 0.5 wt % Pd noncalcined catalyst activity is 4 times above the expected value. Such an unexpected value led us to try and confirm the following: first, that there was no intraparticle diffusion control of the reaction rate for the catalyst with the higher metallic content; and second, that the obtained values were repeatable. The absence of intra-
Figure 9. Influence of temperature on selectivity. Runs 10, 14, 15. and 16.
particle diffusion control was theoretically studied (Roberts, 1976; Koopman et al., 1980), obtaining effectiveness factor values of practically unity. Filled symbols in Figure 8 correspond to some experiments carried out a t the same operational conditions than those of the nearest empty point, with the only difference of the particle catalyst size, which confirm the absence of internal diffusional control in the catalyst. Selectivity to cyclohexanone, as can be deduced from Table 3, was very high, above 0.88 in all experiments and in the range 0.94-0.97 in most of them, as expected. Selectivity has not been calculated for the experiments with the lower reaction rates, as the dispersion of the activity data with time was rather high, and no confident selectivity results could be derived from them. Figure 9 shows the evolution of selectivity with temperature. An increase in selectivity can be observed as the temperature is increased, that is to say, the activation energy of cyclohexanone formation from phenol is higher than the activation energy of cyclohexanol formation from cyclohexanone.
Summary and Conclusions Two series of palladium on alumina catalysts were prepared by adsorption from an acidic solution of PdCld HC1, with nominal percentages of 0.25,0.5, and 1.0 w t % of palladium. After drying at 393 K, the first series was calcined a t 773 K for 4 h in air, while the second series was just heated at 423 K for 1 h in nitrogen, before reduction. Both series of catalysts presented highly dispersed palladium, determined by hydrogen chemisorption at 343 K. Calcination has been found to improve palladium dispersion on the alumina support, and a complete dispersion of the metal may be assumed for the calcined catalysts. Metal dispersion for the noncalcined catalysts was found to be favored with low palladium contents. Experimental conditions were chosen in order to avoid mass-transfer controls, using theoretical calculations as well as experimental data. Thus, a stirring rate of 750 rpm and a catalyst particle size of 0.08-0.16 mm were used for the kinetic experiments. Comparison of the two series of catalysts showed that calcined catalysts, despite their higher metallic dispersion, presented lower activity than their corresponding noncalcined catalysts. Among the latter series, catalysts with 0.5 wt % Pd presented 4 times the activity expected assuming a linear correlation activity-surface palladium content. Repeatability of the experiments
1036 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
was confirmed, as well as the absence of intraparticle diffusion control. The influence of hydrogen partial pressure on activity shows that experimental data can be expressed as a second-order function in the studied range. Such a dependence can be theoretically deduced from a Langmuir-Hinshelwood-Hougen- Watson kinetic equation, simplified for the particular conditions of.the experiments, considering that the experiments were carried out at relatively low hydrogen pressures, at which K2 >> K ~ P H ; , ~ . The effect of temperature on activity was also analyzed, and the apparent activation energy for the hydrogenation of phenol to cyclohexanone was determined, resulting in a value of 56.8 k J molw1. Selectivity to cyclohexanone was found to be very high for all experiments, as expected being palladium the active phase, and to increase as the temperature was increased. This means that apparent activation energy of cyclohexanone formation from phenol is higher than apparent activation energy of cyclohexanone hydrogenation to cyclohexanol.
Acknowledgment The authors wish to thank the Universidad del Pais Vasco/Euskal Herriko Unibertsitatea (Ref. EA082/92) and the Ministerio de Educaci6n y Ciencia (Ref. PB900645) for their economical support.
Nomenclature c p d = palladium content in the catalyst, wt % Cph = phenol concentration in the liquid mixture, mol L-' D = palladium dispersion on the catalysts as defined by eq 1,fractional IEPS = isoelectric point of the solid, pH KI, Kz, and K3 = constants in eq 5 K'1 = constant in eq 6 NM = molecules of hydrogen adsorbed per gram of catalyst, extrapolated to zero pressure Ns = atoms of palladium in the surface of the catalyst, as determined by hydrogen chemisorption NT = total number of palladium atoms in the catalyst, as determined by atomic absorption spectrometry P = total pressure, MPa P H= ~ hydrogen partial pressure, MPa rp = pore radius, nm -rph = reaction rate, molphL-l h-l S, = BET specific surface area, m2 g-' So,, = Selectivity to cyclohexanone, molonemolprod-l T = absolute temperature, K TOF,h = turnover frequency, s-l Vp = specific pore volume, cm3 g-'
Delmon, B., Grange, P., Jacobs, P., Poncelet, J., Eds.; Elsevier: Amsterdam, 1979. Chaudhari, R. V.; Jaganathan, R.; Kolhe, D. S.; Emig, G.; Hofmann, H. Effect of Catalyst Pretreatment on Activity and Selectivity of Hydrogenation of Phenylacetylene over PdiC Catalyst. Ind. Eng. Chem. Prod. Res. Dev. 1986,25,375. Chou, P.; Vannice, M. A. Benzene Hydrogenation over Supported and Unsupported Palladium. I. Kinetic Behavior. J . Catal. 1987,107,129. Gonzalez-Velasco, J. R.; Gutibrrez-Ortiz, J. I.; Gonzdlez-Marcos, J . A. Kinetics of the Selective Hydrogenation of Phenol to Cyclohexanone over a Pd-Alumina Catalyst. React. Kinet. Catal. Lett. 1986,32,505. Gonzalez-Velasco,J. R.; Gutibrrez-Ortiz, J. I.; Gutibrrez-Ortiz, M. A.; Martin, M. A.; Mendioroz, S.; Pajares, J. A.; Folgado, M. A. Palladium Catalysts for Selective Gas-Phase Hydrogenation of Phenol to Cyclohexanone. In Preparation of Catalysts N; Delmon, B., Grange, P., Jacobs, P. A,, Poncelet, G., Eds.; Elsevier: Amsterdam, 1987. Grau, R. J.; Cassano, A. E.; Baltanas, M. A. The Cup-and-Cap Reactor: A Device To Eliminate Induction Times in Mechanically Agitated Slurry Reactors Operated with Fine Catalyst Particles. Ind. Eng. Chem. Res. 1987,26,18. Gutierrez-Ortiz, J. I. Ph.D. Thesis, Universidad del Pais Vasco/ EHU, Bilbao, 1984. Gutibrrez-Ortiz, M. A,; Gonzalez-Marcos, J. A.; Gonzalez-Marcos, M. P.; Gonzalez-Velasco, J . R. Behavior of Highly Dispersed Platinum Catalysts in Liquid-Phase Hydrogenations. Ind. Eng. Chem. Res. 1993,32,1035. Jiratova, K. Isoelectric Point of Modified Alumina. Appl. Catal. 1981,1 , 165. Koopman, P. G. J.; Kieboom, A. P. G.; Van Bekkum, H. Mass Transport in Liquid-Phase Hydrogenation. Delft Prog. Rep. 1980,5,24. Martin, M. A,; Pajares, J. A,; Gonzalez-Tejuca, L. Characterization and Sintering of Alumina-Supported Palladium Catalysts. 2. Phys. Chem. N . F. 1984,140,107. Roberts, G. W. The Influence of Mass and Heat Transfer on the Performance of Heterogeneous Catalysts in GasLiquidSolid Systems. In Catalysis in Organic Synthesis; Rylander, P. N., Greenfield, H., Eds.; Academic Press: New York, 1976. Scholten, J . J. F.; Pijpers, A. P.; Hustings, A. M. L. Surface Characterization of Supported and Nonsupported Hydrogenation Catalysts. Catal. Rev.-Sci. Eng. 1985,27,151. Sermon, P. A,; Bond, G. C. Hydrogen Spillover. Catal. Rev. 1973, 8,211. Sharma, M. M. Perspectives in Gas-Liquid Reactions. Chen. Eng. Sei. 1983,38,21. Zwicky, J. J.; Gut, G. Kinetics, Poisoning, and Mass-Transfer Effects in Liquid-Phase Hydrogenations of Phenolic Compounds over a Palladium Catalyst. Chem. Eng. Sci. 1978,33,1363.
Received for review November 29, 1994 Accepted December 13, 1994@ IE940426K
Literature Cited Brunelle, J. P. Preparation of Catalysts by Adsorption of Metal Complexes on Mineral Oxides. In Preparation of Catalysts II;
Abstract published in Advance A C S Abstracts, February 15, 1995. @
