Ind. Eng. Chem. Res. 1993,32, 1035-1040
1036
Behavior of Highly Dispersed Platinum Catalysts in Liquid-Phase Hydrogenations Miguel A. Gutierrez-Ortiz,' Jose A. Gonzllez-Marcos, M. Pilar GonzBlez-Marcos, and Juan R. Gonzllez-Velasco Departamento de Ingenierk Qutmica, Facultad de Ciencias, Universidad del PaL VascolEuskal Herriko Unibertsitatea, Apartado 644, E-48080 Bilbao, Spain
Group VI11 metals are found to present good behavior as hydrogenation catalysts when supported on highly porous materials such as alumina. A series of highly dispersed platinum catalysts supported on alumina has been prepared by means of adsorption from solution, with platinum contents varying from 0.5 to 3.0 w t % The kinetic behavior of the catalysts has been analyzed for the liquid-phase hydrogenation of benzene in a stirred tank reactor, assuring a chemically-controlled regime for stirring speed above 600rpm and catalyst particle size below 0.08-0.16 mm in the studied conditions. For a constant dispersion, a certain amount of surface platinum has been found to remain inactive, either due to inaccessibility of the reagents or due to poisoning. As hydrogen pressure increases, the reaction order shifts from 1 to 0. The apparent activation energy resulted in 72 k J mol-l.
.
Table I. Characteristics of AL-3946Alumina
Introduction Studies on catalytic hydrogenationreactions have been carried out in the gas phase or in the liquid phase. Nowadays attention is being led toward some liquid-phase processes of increasing importance in the chemical industry (Boitiaux et al., 1985). Carryingout the reaction in the liquid phase, in a slurry reactor, presents important advantages with respect to 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, saving in devices and energy, higher rate of reaction per unit weight of catalyst because of the use of smaller particles, and higher conversions and selectivity attained. Nevertheless, derived from the fact of working simultaneously with three phases-a solid catalyst, liquid reactants and solvents, and gaseous hydrogen-several problems mainly of diffusional nature, arise (Kiperman, 1986; Odenbrand and Lundin, 1980), with the most important due to hydrogen access to the liquid phase (Koopman et al., 1980;Sharma, 1983;Acres and Bond, 1966). Studying these problems and limiting the working conditions in which their effects are negligible are essential before going any further. The catalysts employed in this kind of processes consist mainly of one or more metallic phases-usually nickel, palladium or platinum and less frequently ruthenium, rhodium, cobalt, or even iron-supported on highly porous materials such as metallic oxides or activated charcoal (Pajonk and Teichner, 1986). The objective of these supports is multiple: reducing the quantity of metal required, increasing ita thermal stability and mechanical strength, and, in some cases, affecting the activity and/or selectivity by metal-support interaction (Marcelin et al., 1983). The catalyst preparation method depends on both the metallic phase and the support characteristics, as well as the process in which the catalyst is being used (Geus, 1983). Impregnation (Yao et al., 1979) and adsorption from solution (Brunelle, 1979)are the methods more commonly used to prepare alumina-supported platinum Catalysts. This work deals with the preparation of aluminasupported platinum catalysta using the adsorption from solution method, and the effects of the stirring speed, particle size, and platinum percentage on their kinetic behavior in benzene hydrogenation carried out in the liquid
BET surface area, m2g1 pore volume, cms g1 average pore radius, i\ pore radius (mode),i\ isoelectric point, pH
fresh 201 0.47 31 33
calcined (773 K) 196 0.54 37 31 7.5
phase at different temperatures and pressures, the effects of which are also analyzed.
Experimental Section Materials and Methods. Harshaw AL-3945alumina, extruded y-alumina, essentially neutral, 993' % 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 (Gutibrrez-Ortiz,1984; Gonzhlez-Velascoet al., 19871,providing good activity and selectivity in the hydrogenation of phenol to cyclohexanone. In order 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 of the AL-3945alumina are shown in Table I, before and after calcination. The little variation between fresh and calcined support properties suggeststhat the supplied material was already calcined. The surface area (BET) and pore volume of the support and the resulting catalysts were determined from the nitrogen adsorption-desorption isotherms at 77 K, in a Micromeritics Accusorb 2100Eapparatus. The isoelectric point of the aluminawas obtained by microelectrophoresis using a Shimadzu-Kalnew 2235 optical microscope and a Northrop-Khitz electrophoretic cell. Elemental analysis for Pt was accomplished by atomic absorption spectrometry, in a ll00B Perkin-Elmer spectrometer. The measurements were realized with a platinum lamp at a wavelength of 265.9 nm, an intensity of 30 mA, and in an oxidizing air-acetylene flame. For these standard conditions, the working range is linear up to concentrations of around 75 ppm in aqueous solution. The platinum dispersion on the prepared catalysts was calculated from hydrogen chemisorption at a temperature
0888-588519312632-1035$04.00/0 0 1993 American Chemical Society
1036 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993
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Figure 1. Experimentalsetup. M, stirring motor; FIC, flow indicator and controller; PI, pressure indicator; PIC, pressure indicator and controller; TIC, temperature indicator and controller.
0' 0
,
30
80
90
120
Time, min
of 293-298 K, in a volumetric device. The hydrogen to platinum surface atom stoichiometric ratio is supposed to be 1:2, as the hydrogen adsorption is considered to occur dissociatively on the platinum surface atoms (Scholten et al., 1985). The dispersion value is given as the surface to total platinum atoms ratio:
D = N,/N,
(1)
The procedure followed to clean the sample surface before the chemisorption measurements was evacuation at room temperature, increase to 573 K in vacuum, prereduction at 573 K and a hydrogen pressure of 13.5 kPa for 1h, evacuation at 573 K and increase to 673 K in vacuum, evacuation a t 673 K for 12 h in dynamic vacuum, and descent to room temperature. The chemisorption data were taken a t hydrogen equilibrium pressures up to 7 kPa, 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 1-6 kPa and extrapolation to zero pressure in order to eliminate the support contribution (Berzins et al., 1984). The extrapolated value is changed to units of dispersion using the following expression:
and for platinum catalysts eq 2-after substitution of Mw, to
F, and NAby their corresponding values-reduces D = 6.48 X 10-20(NM/Cp,)
(3)
From the dispersion values thus obtained, platinum crystallite sizes were calculated assuming spherical shape. Catalytic activity was determined for the liquid-phase hydrogenation of benzene in a cylindrical stirred tank reactor Parr Instruments Model 4562,150-mm height and 63.5-mm internal diameter. A scheme of the experimental device is shown in Figure 1. The reactor disposes of a 45' pitched blade turbine with four blades connected to an electric engine with variable stirring speed up to 750 rpm, and automatic temperature controller up to 673 K, a pressure controller with a top operation pressure of 4 MPa, and a hydrogen flow controller. The benzene hydrogenation (Odenbrand and Lundin, 1980; Koopman, 1979) was carried out in a semicontinuous way: continuous for the hydrogen and discontinuous for the liquid reactants and the catalyst. The hydrogenation of benzene is generally considered as a "facile" reaction (Basset et al., 1975; Moss et al., 1979; Parmaliana et al., 1983). In any case, the catalysts have been prepared so that their crystallite size is almost constant (high dispersion, about
Figure 2. Adsorption isotherms of hexachloroplatinic acid on AL3945 alumina.
10 A), to assure that this parameter has no influence on the calculated reaction rates. The general conditionsof the kinetic experiments carried out were as follows: catalyst concentration,5 g dm4; initial reaction mixture composition, 50 wt % benzene-cyclohexane; catalyst platinum content, 0.5-3.0 wt 5% ;catalyst particle size,
