Ind. Eng. Chem. Res. 1987,26, 403-410
403
Kinetics of Adsorption and Reaction for the Consecutive Hydrogenation of 2-Ethylhexenal on a Ni/Si02 Catalyst Claes Niklasson and Gudmund Smedler* Department of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96 Goteborg, Sweden
-
The kinetics of the heterogeneous hydrogenation reactions, 2-ethylhexenal - 2-ethylhexanal 2-ethylhexanol, was studied in the presence of a Ni/Si02 catalyst. The hydrogenation experiments were performed in the gas phase under steady-state conditions in a perfectly mixed continuous tank reactor of Sunderland type. Furthermore, the kinetics of adsorption and desorption, as well as the site surface density for the unsaturated species 2-ethylhexenal and 2-ethylhexanal, were studied in a continuous-flow microbalance system by means of step response analysis. For the hydrogenations, as well as for the adsorption experiments, a number of mechanistic rate equations, based on the Langmuir formalism, were tested by a nonlinear parameter estimation routine. This analysis yielded that the rate of hydrogenation could be satisfactorily described by a Langmuir-Hinshelwood model where the first reaction step takes place on a different kind of site than the second step. The adsorption experiments showed that only on a small fraction of the adsorption sites is the desorption fast enough to fulfill the equilibrium requirement for the Langmuir-Hinshelwood theory. It was therefore concluded that the main part of the catalyst surface is covered by almost irreversibly adsorbed aldehydes under the experimental conditions applied during hydrogenation. The catalytic hydrogenation of acroleinic compounds has been studied by many scientists for quite a long time. The main reaction paths may be vizualized by the following principal scheme: (81
R-CH-C-CH I1 (A)
5 HZ
1
16
R-CH%-FH2
y R - C H 2 - C H - IC H I 2 H2
R OH
In 1984, Noller and Lin (1984) published results concerning a NiO/CuO catalyst that was found to be selective with respect to the formation of the unsaturated alcohol (D) in the hydrogenation of crotonaldehyde. References were also given to previous work on that reaction. The hydrogenation of 2-ethylhexenalhas also been the subject of several investigations, although most of them have been focused on practical applications rather than mechanistic explanations of the catalytic behavior of the system. Liquid-phase hydrogenations have been reported on nickel catalysts (Sousa-Aguiar and Schmal, 1980; Collins et al., 1983; Marcelin et al., 1983, 1984), nickel-chromium (Ioffe et al., 1982), copper-chromium (Bel'chikova et al., 1978), and even with homogeneous Ru complexes (Strohmeier and Holke, 1980). An extremely high selectivity with respect to the formation of 2-ethylhexanal (B) is reported for palladium (Rylander, 1979; Macho and Polievka, 1969) and for nickel boride (Collins et al., 1983) catalysts. Filardo et al. (1976) made use of a copper anode and performed the hydrogenation simultaneously with electrolysis. Their reported products were 2-ethylhexand and 2-ethylhexenol; the latter is (as far as we know) earlier only reported when Pd or Ru complexes are the active species. Another catalyst that appeared to favor the complete hydrogenation was a fused iron catalyst that, when activated with VzOs, gave a very low yield of intermediates
* Author to whom
all correspondence should be addressed.
(Glebov et al., 1982). As may be seen, there are many different approaches to the problem, and the task of explaining the phenomena is quite challenging. Qualitative explanations of the different behavior of different catalysts have been suggested by many of the previously cited authors, but there are few papers that propose quantitative mechanistic rate models. For the industrial high-pressure 2-ethylhexanol manufacturing process, a simple numerical fit is proposed by Bel'chikova et al. (1978). They assumed all reactions to be first order with respect to the hydrocarbons and zero order with respect to hydrogen, an assumption that might be reasonable for very high hydrogen pressures. Such a model reveals however very little information concerning the detailed mechanism of the different elementary steps of the entire process. Palla Carreiro and Baerns (1983) applied a more ambitious approach in a study of the gas-phase hydrogenation of 2-ethylhexenal. They managed to describe their rate data by means of kinetic models of the Langmuir-Hinshelwood-Hougen-Watson (LHHW) type, but the reported statistical significance of the rate parameters was low, probably due to the high correlations between the different parameters. This last investigation is an example of the perpetual difficulty connected with kinetic model fitting: how to describe the observed rate data well without introducing so many parameters that the significance of each parameter gets lost. The aim of this paper is to develop a mechanistic model for the hydrogenation reactions mentioned in the title by making use of separately determined parameters for the kinetics of adsorption and desorption for the reaction aldehydes 2-ethylhexenal and 2-ethylhexanal. The idea of using fixed, separately determined, numerical values of the adsorption parameters (mainly adsorption equilibrium constants) in model fitting procedures has been fruitfully applied by Kabel and Johansson (1962), Raghavan and Doraiswamy (1977), and Magnusson (1983) in the hydrogenation of methyl esters of fatty acids in the presence of Ni and Cu catalysts. Magnusson used a pulse response analysis technique that made it possible to calculate the adsorption equilibrium constants by means of the Kubin-Kucera method. The main disadvantagesof this method are (1)the uncertainity in the determination of the bed and particle porosities, the
0888-5885/ 871 2626-0403$01.50/0 0 1987 American Chemical Society
404
Ind. Eng. Chem. Res., Vol. 26, No. 3, 1987
effective diffusion coefficient, the axial and radial dispersion coefficients, and, perhaps most important of all, the total concentration of active sites; (2) The assumption of linear adsorption, which is valid only when the fractional surface coverage is negligibly low everywhere in the bed; (3) the possibility that the surface properties may change dramatically when hydrogen is present. The difficulties connected with points 1 and 2 may be overcome if the adsorption could be measured directly instead of via the change of fluid-phase composition. This may be accomplished by means of a microbalance. As Satterfield (1980) has stressed, however, point 3 is much more difficult to circumvent. The present work may therefore partially be regarded as an investigation of to what degree separately performed adsorption measurements are useful for the modeling of the hydrogenation kinetics.
Experimental Section Analysis. The feed and the outlet gas streams were analyzed by means of a gas chromatograph (CARLE 311, FID) supplied with a Supelco (15% DEGS on 80/100 Chrom WHP) glass column (2.0 m). The sampling was accomplished by a computer-controlled valve (CARLE 4200). Chemicals. The starting materials, 2-ethylhexenal and 2-ethylhexanal, were delivered by a Swedish company (BEROXO AB). As analyzed on the gas chromatograph, the 2-ethylhexenal was found to be of 97% purity (about 1.5% each of 2-ethylhexanol and 2-ethylhexanal and traces of C4aldehydes). The 2-ethylhexanal was of 99.5% purity. The aldehydes were further purified by film evaporation in a Rota Vapor evaporator before they were used in the experimental systems. The gases used in the catalytic experiments (N2and H,)and the GC analysis (N2,H2, and air) were all of SR quality (purity better than 99.98%). For the catalyst preparation, the following chemicals were used Ni(N03),.6H20 from BDH Chemicals Ltd. (purity better than 98%, with Ca (
