Ind. Eng. zyxwvuts Chem. Res. zyxwvu 1994,33, zyxwvu 2571-2577 2571 zyxwvutsrqp Influence of Operational Variables on the Catalytic Behavior of Pt/Alumina in the Slurry-Phase Hydrogenation of Phenol Miguel A. Gutibrrez-Ortiz,' Alfonso Castano, M. Pilar Gonzlez-Marcos, Jose I. GutiBrrez-Ortiz, and Juan R. Gonzilez-Velasco Departamento de Ingenieria Quimica, Facultad de Ciencias, Universidad del Pais Vasco zyxw I Euskal Herriko Unibertsitatea, Apartado 644, E-48080-Bilbao, Spain Hydrogenation of phenol in the liquid phase has been studied. The reaction system was a stirred tank reactor with the catalyst-highly dispersed platinudy-alumina prepared by adsorption from an aqueous solution of H2PtCls-suspended as a slurry. In order to prevent the mass transport steps to control the reaction rate, several experiments were carried out varying stirring rate, catalyst particle size, and catalyst weight. In the studied conditions, working with stirring rate above 9 Hz, catalyst particle size below 50 zyxw pm, and catalyst weight below 0.5 g assured the kinetic regime. The influence of pressure, temperature, and catalyst platinum content on both initial activity and selectivity was analyzed, for the chemically-controlled regime. Increasing total pressure was found to increase activity almost linearly and to decrease selectivity. The apparent activation energies for phenol consumption and cyclohexanone and cyclohexanol formation (from phenol) were determined. Introduction Slurry-phase catalytic hydrogenation processes are often preferred industrially to gas-phase processes in the production of chemicals, mainly when selectivity is an important factor to consider (Chauvel and Lefebvre, 1989). In the production of cyclohexanone-intermediate in the manufacture of nylon-6 via ecaprolactam-by selec- tive hydrogenation of phenol, though several gas-phase processes have been described (Grasshoff et al., 1981; Naumann et al., 1977; Oberender et al., 19731, the majority of the production is carried out in liquid phase (Areshide et al., 1971; Chauvel and Lefebvre, 1989; Inventa, 1968; Smeykal et al., 1967). The process requires both high activity and selectivity (Gutierrez- Ortiz, 19841, and is carried out using mainly group VI11 metal supported catalysts. This kind of processes, in which three phases are present-solid catalyst, liquid solvent, reactant (phenol) and products (cyclohexanone and cyclohexanol), and hydrogen gas-are rather complex, and though they are industrially used, not many studies have been devoted to them (Coussemant and Jungers, 1950; Kiperman, 1986; Kotova et al., 1988, 1991; Takagi et al., 1970; Zwicky and Gut, 1978). In this work, hydrogenation of phenol in the liquid phase carried out in a slurry-phase reactor with plati- nudalumina catalysts has been studied. The influence of variables such as stirring rate, catalyst weight, and catalyst particle size on reaction rate has been studied from a theoretical point of view and/or experimentally, in order to assure a chemically-controlledregime in the working conditions. The effect of hydrogen partial pressure, temperature, and catalyst metallic content on both initial activity and selectivity to cyclohexanone has been analyzed, for the chemically-controlled regime, and the apparent activa- tion energies for phenol consumption and cyclohexanone and cyclohexanol formation have been determined. Experimental Section Catalysts. A series of platinudalumina catalysts has been prepared by means of adsorption and anionic zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0888-5885194/2633-2571$04.50/0 Table 1. Textural Properties of T-126 y-Alumina fresh calcined S, (BET), m2 g-l zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ 151 186 Vp, cm3 g-l 0.30 0.37 rp (average), nm 3.0 2.9 rp (mode), nm 2.9 2.9 IEPS, pH 8.3 Table 2. Characteristics of Prepared Catalysts catal actual platinum catal particle no. content, zyxw wt % size, pm D 1 0.63 2 0.43 3 0.91 4 2.86 5 0.63 6 0.81 30 1.19 43 0.82 43 1.09 43 1.06 115 1.19 235 1.05 exchange in aqueous solution of hexachloroplatinic acid, HzPtCls, as described elsewhere (Gutierrez-Ortiz et al., 1993a,b). A commercial y-alumina, Girder Sud-Chemie T-126, milled, sieved and calcined a t 773 K for 4 h, has been used as catalytic support. The textural properties of the support have been determined by nitrogen adsorption-desorption at 77 K and neutralization at constant pH, and are listed in Table 1. Actual platinum content and dispersion of the pre- pared catalysts have been determined by atomic absorp- tion spectrometry (AAS) and pulse chemisorption, re- spectively, and are shown in Table 2. Chemisorption measurements were carried out at 298 K, using hydro- gen as adsorbate gas, in an AMI-1 apparatus. Disper- sion was calculated from the chemisorption results, assuming 0.5 Hz molecule adsorbed per surface plati- num atom, as the surface to total platinum atom ratio: D = NSINT (1) Kinetic Experiments. Hydrogenation of phenol in the liquid phase was carried out in a stirred tank reactor, described elsewhere (GutiBrrez-Ortiz et al., 1993a), with the catalyst suspended as a slurry. Temperature and pressure were kept constant during each zyxwv run, and initially 200 cm3 of 50 wt % phenol- methylcyclohexane was fed to the reactor together with zyxwvutsr 0 1994 American Chemical Society