Ind. Eng. Chem. Res. 1993,32, 2457-2463
2457
Surface Features and Catalytic Performance of Platinum/Alumina Catalysts in Slurry-Phase Hydrogenation Miguel A. GutiBrrez-Ortiz,’ M. Pilar Gonzilez-Marcos, Sixto Arnaiz-Aguilar, Jose A. Gonzilez-Marcos, and Juan R. Gonzilez-Velasco Departamento de Ingenierh Quimica, Facultad de Ciencias, Uniuersidad del Pals VascolEuskal Herriko Unibertsitatea, Apartado 644,E-48080-Bilba0, Spain Several platinum catalysts supported on three commercial y-aluminas were prepared by impregnation and anionic exchange using aqueous solutions of HzPtCla. A number of methods were used to characterize the precursors as well as the final catalysts, including TGA, TPR, and hydrogen chemisorption a t 298 K. T P R measurements showed two reduction peaks for the catalysts: the first one corresponding to reduction of the metal precursor to platinum and the second one associated to formation of a surface complex, Pt-A1zOsz, with partial reduction of the support. The activity of the different catalysts was tested in the slurry-phase hydrogenation of benzene. The results obtained in the activity measurements have been correlated to the characteristics of the catalysts.
Introduction Catalytic hydrogenation processes are commonly encountered in the chemical industry, both in liquid and gas phase. The liquid-phase operation allows, in general, a better quality of the products, with higher conversions and selectivities, which makes interesting its application to the production of fine chemicals (Chauveland Lefebvre, 1989). These processes are mainly carried out with group VI11 metals as the active phase, supported on highly porous materialssuch as silica,alumina, or active coals. Numerous studies have been reported using Pd- or Pt-supported catalysts (Acres and Bond, 1966; Boitiaux et al., 1989; Chaudhari et al., 1986; Galvagno et al., 1986; Gonzo and Boudart, 1978; Konyukhov et al., 1987; Kut et al., 1987; Murzinet al.,1989;Oberender et al., 1973;Smedler, 1989; Smeykal et al., 1967). It is well known that the activity and selectivity of these catalysts are strongly dependent on the method of catalyst preparation, activation conditions, the type of support used, and metal dispersion characteristics. Many studies have been devoted to the characterization of catalysts (Baiker, 1985a; Delannay and Delmon, 1984; Lemaitre et al., 198413;Scholtenet al., 1985). In particular, the use of temperature-programmedmethods (TPR,TPO, TPD, etc.) has been attracting attention lately (Baiker, 1985b; De Miguel et al., 1990; Huitzinga et al., 1984; Lemaitre, 1984a; Subramanian and Schwarz, 1990,1991; Yao et al., 19791, as a powerful instrument to study the precursor composition, the strength with which the active phase joins the support, and their relationship with the final activity and selectivity of the catalysts. While some published literature is available on this subject for the gas-phase hydrogenation of different compounds, little information is available on hydrogenation in the liquid phase. In this work, several platinum catalysts supported on three commercial aluminas have been prepared by means of impregnation and anionic exchange. The behavior of the precursors toward calcination in air and reduction in hydrogen has been studied using thermogravimetry (TGA) and temperature-programmed reduction (TPR) analysis. After activation,the catalysts obtained were characterized by atomic absorption spectrometry (AAS),to determine the actual platinum content, and hydrogen chemisorption, to determine the diepersion of the platinum crystallites. The characteristics of both precursors and final catalysts
have been related to their activity in the slurry-phase hydrogenation of benzene.
Experimental Section Materials. Hydrogen, nitrogen, and helium gases used in the preparation and characterization of the catalysts were taken directly from cylinders, with a purity of 99.998%. Hydrogen gas as reagent for the activity measurementswas 99.990 % pure. All gases were supplied by Argon S.A. Benzene and cyclohexane, supplied by Panreac S.A., were 99 and 98 % pure, respectively. In the catalyst preparation, HzPtCb was usedas platinum source, supplied by Aldrich with 99.995 % purity. Several commercial y-aluminas were used as catalytic supports: Harshaw AL-3945, Girdler T-126, and Rh8nePoulenc SCS-79. The aluminas were supplied as pellets of different shapes and sizes and were subsequentlymilled and sieved to particle size of 0.08-0.16 mm. The particles thus obtained were stabilized toward subsequent thermal treatment by calcination at 773 K and 4 h. Experimental Methods. Characterizationof Supports. To determine the specific surface area and poresize distribution of the supports, as well as some of the catalysts, nitrogen adsorption-desorption isotherms were carried out in a Micromeritics AccuSorb 2100E apparatus. The surface of the samples was cleaned before the measurements by heating under a vacuum of 13.6 mPa and a temperature of 623 K for 16 h. The specific surface area was determined by linearization of the first points of the adsorption branch, up to relative pressures of 0.20, to the BET equation. The pore-size distribution was determined from the desorption branch of the isotherm, assuming cylindrical shape for the pores. Isoelectric point determination, as well as a study of the adsorption of HnPtCh, for the different supports were carried out as described elsewhere (Gutihrrez-Ortizet al., 1993). Incorporation of the Active Phase. The catalysts, platinum on alumina, were prepared using two different methods to incorporate the active phase onto the support surface: impregnation at incipient wetness and adsorption from solution. The impregnation at incipient wetness of the supports was carried out in a vacuum rotary evaporator, at 313 K and a pressure of 4 kPa.
o a a a - ~ ~ a ~ ~ ~ ~ ~ ~ 0s 1993 ~ ~ American - ~ ~ ~ ~Chemical ~ o ~ Society . o o / o
2458 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
The adsorption from solution consisted of an ionic exchange between the support surface and hexachloroplatinic acid, H2PtCk. The isoelectric point of the aluminas was found to range from 7.5 to 8.3. Anionic exchange was thus chosen to incorporate the platinum onto the alumina surface. The use of an acidic medium polarizes the alumina surface positively,favoringthe interchange with the anions which tend to surround it. Using HzPtCk both an acidic medium and platinum taking part of a complex anion are achieved at the same time. The adsorption was carried out at room temperature (293K) under continuous stirring, with 40 cm3of solution per gram of support. The contact was maintained for 2 h, in which time equilibrium was reached as deduced from the adsorption studies. Characterization of Catalyst Precursors. After incorporation of the platinum onto the support surface, both series of catalysts were dried at 393 K for 16 h. In order to determine the best conditions to activate the catalyst precursors, two different studies were carried out: thermogravimetric analysis (TGA) and temperature programmed reduction (TPR). The weight variations uersus temperature have been determined for the different catalytic precursors as well as the hexachloroplatinic acid. The analysis were carried out in a Perkin-Elmer thermogravimetric system, Model TGS-2, equipped with a data acquisition and analysis computer, in two different atmospheres: 30 cm3 min-l air/nitrogen and 30 cm3 min-l hydrogentnitrogen, to analyze both the calcination and the reduction steps in the activation process of the precursors. TPR measurements were carried out in an Altamira Instruments apparatus, Model AMI-1. The calcined precursor samples were first dried in 30 cm3min-l nitrogen for 30 min at 423 K. A volume of 30 cm3 min-l of a gas mixture with 10% Hz/N2 was fed through the precursor samples as the temperature was raised from 293 to 773 K at a rate of 25 K min-l. The amount of hydrogen consumed during reduction of the samples was measured by means of a thermal conductivity detector. Characterization of Catalysts. Activation of the catalyst precursors was carried out in two steps: calcination in air at 773 K for 4 h, followed by reduction in hydrogen at 573 K for 3 h. The actual metallic content of the different catalysts, NT, was calculated from atomic absorption spectroscopy measurements, as described elsewhere (Gutibrrez-Ortiz et ai., 1993). The platinum dispersion in the prepared catalysts was determined using hydrogen chemisorption in a Micromeritics AccuSorb 2100E apparatus. The samples were submitted to a conditioningpretreatment in order to clean their surface, which consisted of evacuation at room temperature and 13.6 mPa for 1h, reduction at 573 K and a hydrogen pressure of 13.6 kPa for 1 h, and evacuation at 673 K and a dynamic vacuum of 0.14 mPa for 16 h. Following this treatment, a complete elimination of the unreduced metal zones as well as the gases adsorbed on the catalytic surface is assumed, without significant sintering of the platinum. The hydrogen chemisorption isotherm was carried out at 298 K, a temperature which is reported to give dispersion values similar to those obtained with other techniques such as TEM (Kobayashi et al., 1980), measuring 7 or 8 points up to hydrogen equilibrium pressures of 7 kPa. The equilibration time was 1h for the first point of the isotherm and 30 min for the remaining points. Catalyst Activity. The activity of the catalysts was
Table I. Characteristics of the Calcined Aluminas scs-79 T-126 AL-3945 104 186 S, (BET),m2 g-l 196 0.50 0.37 V,, cm3 g-1 0.54 5.8 2.9 3.7 rp (average),nm 2.9 6.1 rp (mode), nm 3.1 8.0 8.3 7.5 IEPS,pH ads,, % 98.7 98.9 0.5% Pt 98.8 97.6 97.7 98.9 1.0% Pt 64.5 71.0 77.1 3.0% Pt ~~~
determined for the liquid-phase hydrogenation of benzene. All hydrogenation experiments were carried out in a stainless-steel stirred tank reactor, Parr Instrument Co. Model 4562, with 450 cm3 available capacity and 63.5-mm internal diameter. The reactor is designed to operate a t pressures up to 14 MPa and temperatures up to 623 K. It was provided with automatic pressure and temperature control, and the hydrogen flow was also controlled by a mass flow meter. The experimental setup has been previously described (Gutibrrez-Ortiz et al., 1993). All kinetic experiments were carried out at constant pressure and temperature. Liquid reactant (benzene), solvent (cyclohexane), and catalyst were loaded into the reactor at the beginning of the experimental run, while hydrogen was continuously bubbling in the liquid mixture. Samples for analysis were taken at intervals through the liquid outlet, using gas chromatography to determine their composition. The experimental conditions to test the catalysts were chosen after a certain number of experimental runs in order to avoid mass-transfer control (Gutibrrez-Ortiz et al., 1993)as follows: temperature, 413 K; pressure, 2 MPa; stirring speed, 10 rps; initial benzene concentration, 50 w t % ;catalyst concentration, 5 g L-I; catalyst particle size, 0.08-0.16 mm. The activity of the different catalysts was expressed, in order to make comparisons, as the initial benzene reaction rate, mo. These values were determined by derivation of the benzene concentration versus time curves obtained from the experimental data. An induction period was observed during the first 10 min in each run, time which was not considered in the calculation of the initial reaction rate. Results and Discussion Table I shows the characteristics determined for the three alumina supports after calcination at 773 K for 4 h, using the experimental methods previously described. The results make clear the differences in the textural properties of the supports. AL-3945 and T-126 aluminas have similar specific surface areas, around 200 m2g1, while for the SCS-79 its specific surface area is about one-half that value. The pore volume is very similar for the AL3945 and SCS-79 aluminas, about 0.5 cm3 g-1, and much lower for the T-126 support. With respect to the average pore size, the order from lower to higher values is T-126 < AL-3945 < SCS-79. This is clearly visualized in Figure 1, where the pore size distribution for the three supports is shown. The isoelectric point of the supports varies in a small range, from 7.5 to 8.3, with values typical for this kind of alumina, which normally range from 7 to 9.5. The studies of H2PtC16 adsorption onto the support surface have shown that the adsorption occurs quickly in the very first moments of contact between solution and support, reaching the equilibrium in less than 2 h. The
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2459 400
350 300
r - - - - - -
a 250 d
e
3.&200 2 150 100
/ / /
50
/
I , , ,
10
1
r , nm
0
10
20
, , , I , , , l , . /,
30
40
50
,
60
,
,
I
,
70
Time, min
Figure 1. Pore-size distribution for the three alumina supports: ( O ) , AL-3945; (M), T-126; (A),SCS-79.
Figure 2. TGA in air/Nz for the 3% Pt/SCS-79 precursor prepared by anionic exchange,as wellas the support (thick line). Discontinuous line corresponds to the temperature sequence.
higher the platinum concentration in the solution, the lower the adsorption yields. This effect is supposed to be due to saturation of the adsorption sites, OH surface groups, and becomes more evident for the SCS-79 alumina which has the smallest specific surface area (see Table I). After incorporation of the platinum by both methodsimpregnation at incipient wetness and anionic exchange-a study was accomplished to determine the most adequate way to carry out the activation of the precursors, due to the importance of this process in the characteristics of the final catalysts. A review of the papers published on this subject (Bournonville et al., 1983; Kobayashi et al., 1980; Vdter, 1986)showed the convenience of carrying out a calcination step previous to the reduction, for Pt/alumina catalysts, in order to obtain the highest metallic dispersion. Two main reasons are adduced to explain this: on the one hand, the decomposition of the platinum complex in the support surface proceeds more slowly when heated in air than in hydrogen. On the other hand, the calcination step eliminates the moisture present in the precursor previous to the reduction. The presence of moisture during the reduction step is known to cause an important decrease in the metallic dispersion. The most adequate temperature for the calcination treatment was found to be in the range 723-773 K. There are different theories on how H2PtCb is attached to the surface in the Pt/alumina catalyst precursors before calcination. Martens and Prins (1989) proposed a [=A1-0-PtCl~]-~ surface complex, while for Castro et al. (1983) it would be a [=A1-O-(PtC~)-O-Al=l-2 complex and for Santacesaria et al. (1977) a [-Al(O-AlPtCls)-O-Al(OH)-I complex. Subramanian and Schwarz (1991) assumed that at low pH values in the solution, [PtCld-2is directly the adsorbed species, which becomes PtCL with little interaction with the support after drying at 423 K. Together with this species, some of the H2PtC4 could be deposited by crystallization in the pores of the support, specially at high platinum content and for catalysts prepared by impregnation. Calcination of the precursors produces decomposition of the surface complex to obtain PtO,ClY/Al2O3(Liu et al.,1991),which is probably
a mixture of Pt-Al203-Cl and PtOz-Al203 complexes (Yao et al., 1979). TGA in an air/nitrogen mixture for the different precursors up to 773 K showed the maximum decomposition rate to be at around 573 K, very close to the temperature obtained for the decomposition of pure H2PtC4 to PtO2. This suggests that the adsorbed species is probably [PtCld-2, as proposed by Subramanian and Schwarz (1991),which partially decomposes to PtOz during calcination at 773 K. The presence of chlorine in the precursors will probably lead to PtO,ClY/A1203. Figure 2 shows, as an example, the weight loss uersus temperature observed for the 3.0% Pt/SCS-79 catalyst precursor prepared by anionic exchange, as well as the support. The reduction step plays a decisive role in the characteristics of the final catalyst. If too high a temperature is used, sintering of the active phase is feasible to occur as well as SMSI (strong metal support interaction) effects. On the contrary, if the chosen temperature is too low, the reduction of the active metal may not be complete. TGA on the calcined precursors was carried out in a hydrogen/ nitrogen stream in order to determine the temperature at which reduction of the platinum was complete. This method, however, was not sensitive enough, due to the little weight loss involved in the reduction process. TPR analysis were carried out for all samples in order to improve sensitivity. Figure 3 shows, as an example, how the reduction process takes place with temperature for the 3.0% Pt/SCS-79 catalyst precursor prepared by anionic exchange, as well as the support. Two different reduction peaks were found to appear in all analyses,except the ones carried out for the 3.0% PtlAL-3945 precursor prepared by anionic exchange. The reduction patterns obtained were very similar to those reported by Subramanian and Schwarz (1991). Table I1 shows the results of the analysis. The results obtained in TPR measurements have been analyzed considering two different aspects: the temperature at which the maximum of the peaks was found and also the total amount of hydrogen consumed in each peak, which was quantified by integration of the peak areas. The hydrogen consumption has been normalized both per
2460 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
0
.-
/
0
5
10
15
20
25
30
Time, min
Figure 3. TPR for the calcined 3 96 Pt/SCS-79 precursor prepared by anionicexchange,as well as the support (thickline).Discontinuous line corresponds to the temperature sequence.
platinum content and per total sample weight, as can be observed in Table 11. Analysis of the results obtained for the first peak, particularly those concerned with the precursors prepared by anionic exchange on T-126, showed the amount of hydrogen consumed is not significant when expressed as pmol H2/g precursor. When expressed as mol Hdmol Pt, on the contrary, the values obtained are quite close. This strongly suggests that hydrogen consumption in this peak can be directly related to the reduction of the platinum in the precursor. The hydrogen consumed per unit platinum in the first peak is then a measurement of the average oxidation state of the platinum in the precursors after calcination, which ranges from 2.48 to 4.02. The absence of second peak for the 3 % Pt/AL-3945 prepared by anionicexchange suggests that a superimposition of both peaks may be taking place. This could explain so high a value for the calculated oxidation state of the platinum in this sample: 6.09. The temperatures a t which the first peak maximum is found is related to the degree of interaction between metal and support: the higher the temperature, the stronger the interaction. Thus, the results show a higher interaction for the catalysts prepared on AL-3945 alumina, with respect to the other two supports. A higher metal-support interaction has also been found as the platinum content in the precursor decreases. This result was also reported by Yao et al. (1979). With respect to the preparation method, no significant difference has been found for the AL-3945 and SCS-79 supports, while for the T-126 the interaction was found to be higher when the precursors were prepared by impregnation. For the second peak, analysis of the amount of hydrogen consumed shows no particular relation when expressed per platinum content. However, when expressed per sample weight, the precursors prepared by anionic exchange on the same T-126by the same preparation method, but different platinum content, give similar hydrogen consumption. Thus, hydrogen consumption in this second peak seems to be related not to the platinum, but to the alumina. Besides, considering the total hydrogen consumption (both peaks) related to the reduction of the platinum would lead to average oxidation states above 4 in all the samples. All the previous results took us to consider that the first peak was associated to the reduction of platinum in the complex PtOXC1,/A120~,where it is present in a mixture of oxidation states, the average being in the range 2.484.02 for our samples. The second peak is considered to be associated to the formation of a Pt-A120sx surface complex, with partial reduction of the support surface.
TPR carried out for the calcined supports in the same conditions showed no peaks in this temperature range, as can be seen in Figure 3. This suggests that the reduction of the support surface is being catalyzed by the presence of the platinum and is supported by the fact that the lower the platinum content in the catalyst, the higher the temperature at which the second peak is obtained. In view of the results, all the precursors were activated in two steps: first a calcination in air at 773 K for 4 h, followed by reduction in a flow of 50 cm3min-1of hydrogen at 573 K and 3 h. In this way, the platinum is considered to be completelyreduced, without formation of Pt-A1203 The characterization of the catalysts was carried out using the methods previously described. Table I11 shows the characteristics measured for the different Pt/y-alumina catalysts prepared. Figure 4 shows, as an example, the hydrogen chemisorption isotherms obtained for the Pt/T-126 catalysts. The isotherms were found to be linear in the pressure range 1-6 kPa. Dispersion values were obtained by linear extrapolation to zero pressure, as described elsewhere (Gutibrrez-Ortiz et al., 1993). The crystallite particle diameter, dp, was determined assuming spherical shape according to:
The results show the high degree of platinum dispersion obtained for all the catalysts prepared, which supports the activation procedure followed. A priori, taking into account that the activation procedure was the same in all cases, anionic exchange was expected to produce catalysts with higher dispersion values than impregnation. An analysis of the results given in Table I11 shows that this is not always true. The corresponding Pt/AL-3945 catalysts prepared by both anionic exchange and impregnation were found to present similar dispersions, slightly lower for platinum contents above 0.5 % . An increase in the platinum content was found to produce a decrease in the dispersion values obtained, as expected. The Pt/T-126 catalysts were found to have dispersions above unity in all the platinum content range studied. The obtained dispersion values were slightly higher for the catalysts prepared by impregnation and the lower platinum content. The behavior of the Pt/SCS-79 catalysts with respect to dispersion is quite different. The impregnation Catalysts had much higher dispersion that the corresponding anionic-exchange catalysts, as much as 30-40 % higher, the latter with values above unity. In both series of catalysts, the dispersion was found to decrease with increasing platinum content. Several references (Yao et al., 1979) have been found in which similar catalysts present dispersions above unity, with different explanations: either a multiple hydrogen adsorption, adsorption in the interstices of the platinum atoms in the monolayer, a stoichiometric factor F other than 0.5 (which varies with temperature and crystallite size), or spillover of hydrogen to the support. Probably, the best explanation for the behavior of our catalysts is spillover of hydrogen from the platinum to the support. The variations observed with support and preparation method indicate a different metal-support interaction, caused by the different surface properties of the three aluminas. Spillover seems to be favored when impregnation is used to incorporate the active phase onto
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2461 Table 11. Temperature-Programmed Reduction Results for the Calcined Precursors Hz consumed, mol Hdmol Pt precursor sample 3% Pt/AL-3945 A.E. 3% Pt/AL-3945 Imp. 0.5% Pt/T-126 A.E. 1% Pt/T-126 A.E. 3% Pt/T-126 A.E. 3% Pt/T-126 Imp. 3% Pt/SCS-79 A.E. 3% Pt/SCS-79 Imp.
lpeak 3.05 2.01 1.24 1.50 1.38 1.69 1.70 1.40
2peak
1.53 2.54 2.25 0.70 1.29 1.93 1.18
total 3.05 3.54 3.78 3.75 2.08 2.98 3.63 2.58
Hz consumed, pmol Hz/g precursor 2 peak
1peak 469 257 31 68 204 221 190 184
total
-
469 453 94 169 308 389 405 340
196 63 101 104 168 215 156
maximum temperature, K 1peak 2 peak 545 543 660 543 704 554 676 493 646 520 664 509 663 654 502
Table 111. Characteristics of the Catalysts support AL-3945
T-126
SCS-79
prep method Imp. Imp. Imp. A.E. A.E. A.E. Imp. Imp Imp. A.E. A.E. A.E. Imp. Imp. Imp. A.E. A.E. A.E.
Pt content, w t % nominal actual 0.5 0.41 1.0 0.82 3.0 2.49 0.5 0.39 1.0 0.88 3.0 3.00 0.5 0.42 1.0 0.87 3.0 2.55 0.5 0.48 1.0 0.88 2.90 3.0 0.5 0.41 0.84 1.0 3.0 2.57 0.5 0.43 1.0 0.91 3.0 2.17
D 1.00 0.95 0.76 0.98 0.99 0.80 1.13 1.09 1.19 1.09 0.99 1.19 1.15 1.16 1.03 0.89 0.82 0.71
d , (spheres), nm 1.08 1.13 1.42 1.10 1.09 1.35 0.95 0.99 0.91 0.99 1.09 0.91 0.94 0.93 1.05
L
1.21 1.31 1.52
0
the support surface and with a higher isoelectric point of the support. The results obtained suggest that it would be useful to determine the catalyst dispersion using the double isotherm method and/or pulse chemisorption, so that only the hydrogen adsorbed irreversibly is taken into consideration. Finally, the catalysts were tested for their activity in the liquid-phase hydrogenation of benzene. The results of activity are shown in Table 111,and represented in Figure
0
2
4
6
Equilibrium pressure, kPa Figure 4. H2 chemisorption isotherms for the Pt/T-126 catalysts. Filled and empty symbola correspond to catalysts prepared by impregnation and anionic exchange,respectively. Nominal platinum content: circles, 0.5; squares, 1.0; triangles, 3.0%.
5.
Initial benzene reaction rate, n o , was used as a measure of catalyst activity. An induction period of 10 min was found at the beginning of each kinetic experiment, which was not considered in the calculation of TOFo. Induction periods are commonly found in this kind of system. Though several causes have been proposed to explain this behavior, the main reason is considered to be the partial oxidation of the platinum crystallite surface during storage previous to use. During the first moments of each kinetic experiment, the platinum is again reduced by the hydrogen reactant in parallel to the main reaction. The temperature at which experiments are carried out is much lower than the minimum catalyst reduction temperature, as determined previously; however, the metallic nucleus of the platinum crystallite has been found to catalyze this reactivation process (Huitzinga et al., 1984). As can be deduced from the results shown in Figure 5, activity was found to be slightly lower for the catalysts prepared by impregnation compared to the anionicexchange catalysts. An analysis of the activity obtained for the catalysts prepared by impregnation, shows that the dispersion value-calculated by volumetric hydrogen chemisorption-produces satisfactory results, as all of them could be represented by the same straight line (dotted line in Figure 5 ) . However, it would be interesting to study also
0
10
20 30 Surface platinum, m2 1"
40
Figure 5. Catalyst activity versus platinum surface in the reaction mixture. Filled and empty symbolscorrespond to catalysts prepared by impregnationand anionicexchange, respectively. Support: circles, AL-3945; squares and rhombs, T-126; triangles, SCS-79.
how the correlation activity-dispersion varies when only hydrogen adsorbed reversibly is taken into account in the calculation of metallic dispersion. The anionic-exchange catalysts supported on AL-3945 and SCS-79 show also a similar activity behavior (discontinuous line in Figure 5). Figure 5 showsthat catalysta with 3% Pt/T-126 prepared by both impregnation and anionic exchange present initial
2462 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
benzene hydrogenation rates far below the expected values. As this support presents the smallest average pore size (see Figure 1 and Table I), diffusion pore control was suspected in both experiments. In order to determine the actual behavior of the anionic-exchange catalysts supported on T-126, two additional experiments were carried out in the same experimental conditions but with 7.5 and 15 g L-' of anionic exchange 1% Pt/T-126 catalyst concentration, respectively (rhombs in Figure 5). No pore diffusion control was found in this case. The anionic exchange T-126 catalysts were found to have quite a different behavior (continuous line in Figure 5) compared to the ones supported on AL-3945 and SCS79: activity was found to be lower for small platinum surface in the reaction mixture and to increase with a higher slope as the platinum surface increases, presenting finally much higher activity. A certain amount of platinum has been found to remain inactive for the benzene hydrogenation, as had been previously described (GutiBrrez-Ortiz et al., 1993),which varies with catalyst preparation method and support, in the following order: A.E. AL-3945 and SCS-79 < Imp. < A.E. T-126. Inaccessibility of some of the active centers to the reactants is thought to be the most probable reason for this. Summary and Conclusions Several platinum catalysts were prepared supported on three commercial aluminas by two different methods: impregnation and anionic exchange. The supports, as well as the precursors and final catalysts were characterized by several procedures. TPR measurements, carried out for the eighteen precursors prepared, after calcination at 773 K, showed that the reduction of the platinum takes place at temperatures below 573 K. Thus, the precursor reduction in the activation process was accomplished at this temperature. A second peak was found in the analysis, which was attributed to the formation of a Pt-A120>, surface complex, with partial reduction of the support surface, catalyzed by the presence of platinum. The hydrogen chemisorption isotherms obtained (see Figure 4) were found to be linear in the pressure range 1-6 kPa. Extrapolation to zero pressure was thus used to determine the platinum dispersion in the catalysts. The support, as well as the method of preparation, influences the dispersion results obtained for the catalysts. In a general way, impregnation and a higher isoelectric point of the support seem to favor higher dispersions. Volumetric chemisorption measurements of hydrogen carried out at 298 K and with equilibrium pressures up to 7 kPa, though this gives dispersion values above unity for several of the samples, produces results reasonably in accordance with the activity as determined for the liquidphase hydrogenation of benzene. The catalyst activity was measured as the initial reaction rate in the slurry-phase hydrogenation of benzene. The activity was found to depend on the preparation method, as well as the support. For the catalysts prepared by impregnation, the activity was found not to depend on the support, as all the catalysts showed the same behavior when varying the platinum surface in the reaction mixture. The activity of the catalysts prepared by anionic exchange, however, varied with the support. It was found to be similar for the catalysts supported on AL-3945 and SCS-79, but much higher for the catalysts supported on T-126.
This support, however, was found to present porediffusion problems for the catalysts with the higher metallic contents, due to ita small average pore radius (2.9 nm) and sharp pore-size distribution. All the catalysts presented a certain amount of platinum which remained inactive for the benzene hydrogenation. This amount varies with catalyst preparation method and support and is supposed to be due to inaccessibility of the active centers to the reactants. Acknowledgment The authors wish to thank Ministerio de Educaci6n y Ciencia and Universidad del Pa& Vasco (UPV/EHUE115/90) for their economical support in the realization of this work. Nomenclature ads, = relative platinum adsorbed on the support when equilibrium is reached, % initial platinum in the solution c = crystallite shape constant, 6 for spheres dpt = platinum crystallite diameter, nm D = platinum dispersion, defined as Ns/NT f = factor of accessibility of the active phase, assumed 1 F = stoichiometric factor, given as adsorbed hydrogen molecules per atom of platinum, assumed 0.5 IEPS = isoelectric point of the solid, pH Mw = platinum molecular weight, 195.09 g mol-' N A = Avogadro's number, 6.O23.1Oz3 atom mol-' Ns =platinum atoms in the catalytic surface per gram of catalyst NT = total platinum atoms per gram of catalyst rBO = initial rate of hydrogenation of benzene, mol L-1 h-l rp = pore radius, nm S, = BET specific surface area, m2 g1 Vp = pore volume, cm3 g-1 p = platinum density, 21.45 g cm-3. Q = average surface occupied per atom of platinum, 8.41.10-2 nm2 atom-' Literature Cited Acres, G.J. K.; Bond, G.C. Evaluation of Supported Catalysts for Liquid Phaee Reactions.Effect of Experimental Variableson Rates of Reaction. Platinum Met. Reu. 1966, 10, 122. Baiker, A. Experimental Methods for the Characterization of Catalysts. I. Gas Adsorption Methods, Pycnometry and Pore simetry. Znt. Chem. Eng. 198Sa, 25,16. Baiker, A. Experimental Methods for the Characterization of Catalysts. 11. X-Ray Diffraction, Temperature Programmed Desorption and Reduction, Thermogravimetryand Differential Thermoanalysis. Znt. Chem. Eng. 198Sb, 25,30. Boitiaux,J. P.; Cosyns, J.; Robert, E. Additive Effects in the Selective Hydrogenation of Unsaturated Hydrocarbons on Platinum and Rhodium Catalysts. I Influence of Nitrogen-Containing Compounds. Appl. Catal. 1989, 49, 219. Bournonville,J. P.;Franck,J. P.; Martino, G. Influenceof the Various Activation Steps on the Dispersion and the Catalytic Properties of Platinum Supported on ChlorinatedAlumina. In Preparation of Catalysts ZZk Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier Science Publishers: Amsterdam, 1983. Castro, A. A.; Scelza, 0. A.; Benbanuto, E. R.;Baronetti, G. T.; De Miguel, S. R.;Parera, J. M. Competitive Adsorption of H2PtCb and HCl on A 1 2 0 3 In the Preparation of Naphtha Reforming Catalysts. In Preparation of Catalysts ZZfi Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier Science Publishers: Amsterdam, 1983. Chaudhari, R.V.; Jaganathan,R.;Kolhe, D. S.;Emig, G.;Hofmann, H. Effect of Catalyst Pretreatment on Activity and Selectivity of Hydrogenationof Phenylacetyleneover Pd/C Catalyst. Znd. Eng. Chem. Process Des. Deu. 1986,25, 375.
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Received for review April 2, 1993 Revised manuscript received July 19, 1993 Accepted July 26, 1993' e Abstract published in Advance ACS Abstracts, October 1, 1993.
