Catalytic hydrogenation of cyclohexene: Part II. Liquid phase reaction on supported platinum in a gradientless slurry reactor

Abstract

The areal rate of liquid phase hydrogenation of cyclohexene on supported platinum catalysts does not depend on the nature of the support. Nor does it depend on particle size of the metal. The rate constant is independent of the nature of the solvent when the concentration of hydrogen in it is expressed as its measured or calculated solubility. All observations are compatible with the idea that the measured rate is that of chemisorption of dihydrogen on a metal surface covered with reactive hydrocarbon intermediates. SCOPEThe effect of the dimensions of metallic particles on the catalytic activity of supported metals is of theoretical and practical importance. The present work was undertaken to examine the effect of platinum particle size on the liquid phase hydrogenation of cyclohexene in a rocking slurry reactor. The reaction was studied in a static system at constant hydrogen pressure, normally atmospheric pressure, between 275O and 316OK. As diffusion coefficients in the liquid phase are often two or more orders of magnitude lower than in the vapor phase, diffusional influence can play a substantial role in slurry reactors. It is thus imperative that reactions studied in slurry reactors be examined for diffusional effects and proof be given that observed rates have not been disguised by either intraparticle or interphase mass transport. We set out to obtain true kinetic data devoid of either the influence of transport phenomena or catalyst deactivation during an experimental run.The present study also focused on two important aspects of liquid phase reactions, When heterogeneous catalytic reactions are carried out in the gas phase, ideal gas behavior is ordinarily assumed. However, for kinetics in the liquid phase, as in a slurry or trickle bed reactor, it is necessary first of all to ascertain whether concentrations or activities of dissolved reactants should be used in rate expressions. Second, it is also important to check whether different solvents affect the reaction rate or the rate constant. The literature of catalytic reaction engineering offers few experimental or theoretical guidelines bearing on these two important questions. CONCLUSIONS AND SIGNIFICANCEKinetic data have been obtained for the liquid phase hydrogenation of cyclohexene on supported platinum catalysts in a rocking slurry reactor. First of all, values of the turnover frequency N , defined as molecules reacted per site per second, were obtained on catalysts containing different amounts of platinum of the same dispersion, defined as the ratio of surface metal atoms to total metal atoms. Values of N were found to be identical. As this re- Page 904September, 1978 sult was obtained at two different temperatures, it offers sufficient proof that rates were not masked by artifacts such as the influence of interphase and intraparticle heat and mass transfer, or deactivation and poor contacting of the catalyst by reactants. The main result of this work is that values of N were identical on nine different supported platinum catalysts where the me...

Inhibition of Metal Hydrogenation Catalysts by Biogenic Impurities

“…As shown in Table 2, the TOF was observed to be invariant with changes in the site density for each metal, which indicates the absence of mass transport limitations [30]. The reaction rates were consistent with published rates that were collected at room temperature and scaled to 323 K [20][21][22]. Figure 2 shows the reactivity profiles obtained using the eight representative biogenic impurities.…”

“…This trend, combined with the observed adsorption stoichiometry, suggests that cysteine binds directly to the metal surface through its terminal sulfur atom but without decomposition to form atomic sulfur. Thiamine strongly inhibits all three metals and binds to a Rates determined by integrating the rate law [20][21][22] over the initial 20 % conversion. Reaction conditions: 323 K, 30 bar H 2 , 15 % cyclohexene in methanol [mol cyclohexene: mol surface metal = 3,700 (Ni), 145,000 (Pd), 120,000 (Pt)] a Dispersion and TOF based on surface metal atoms determined using a stoichiometry of 1 CO/Pt, 0.5 CO/Pd [24], and 0.5 CO/Ni [25].…”

“…The hydrogen consumption rate was determined by evaluating the rate constant using published rate expressions for cyclohexene hydrogenation. The reactions are known to be zero-order with respect to cyclohexene on all three metals, half-order with respect to hydrogen on Ni and Pd, and first-order with respect to hydrogen on Pt [20][21][22]. Cyclohexene consumption and cyclohexane production were verified for selected reactions by gas chromatography (Shimadzu GC2010 equipped with an FID and a DB-5MS capillary column).…”

“…Benefiting from the well-documented kinetics of cyclohexene hydrogenation using Pt [20], Pd [21], and Ni [22], we have measured the hydrogenation rate in the presence of four representative amino acids and four representative vitamins. These data were then used to quantify the effects of the impurities on Pt, Pd, and Ni, and we elucidate the inhibitory effects of the various functionalities present in these biogenic impurities.…”

A comparative study of the liquid-phase reduction of acrylamide and methacrylamide with cyclohexene and with molecular hydrogen over sepiolite-supported catalysts

“…This phenomenon cannot be attributed to catalyst poisoning and deactivation because a second introduction of reactant after the first sample had been completely converted gave the same reaction rate obtained for the first AC (or MAC) sample. This behavior is consistent with that found in the liquid-phase hydrogenation of 1-hexene, styrene, a-methylstyrene, acrolein, and methyl vinyl ketone on Ni/A1P04 catalysts (19)(20)(21)(22) and in the liquid-phase hydrogenation of cyclohexene on Pt/Alz03 and Pt/SiO, (23), where the reaction order in the olefin changes from zero to unity as a result of the catalyst "starvation" of olefm molecules at very low concentrations. Thus, our work was only concerned with the linear portion of the plot of the decrease in the hydrogen pressure as a function of the reaction time, where the reaction rate was zero order in AC, highest reaction rates.…”