Platinum toluene adsorption

We finish this section with an example of a dehydrogenation reaction on a metal, which is instructive as it does not rely on equilibrium adsorption, unlike most other examples given in this book. The dehydrogenation of methylcydohexane, CfiHiiCHs, to toluene, QH5CH3, on platinum is well described by a reaction mechanism consisting of consecutive steps in the forward direction only ... 

We then designed model studies by adsorbing cinchonidine from CCU solution onto a polycrystalline platinum disk, and then rinsing the platinum surface with a solvent. The fate of the adsorbed cinchonidine was monitored by reflection-absorption infrared spectroscopy (RAIRS) that probes the adsorbed cinchonidine on the surface. By trying 54 different solvents, we are able to identify two broad trends (Figure 17) [66]. For the first trend, the cinchonidine initially adsorbed at the CCR-Pt interface is not easily removed by the second solvent such as cyclohexane, n-pentane, n-hexane, carbon tetrachloride, carbon disulfide, toluene, benzene, ethyl ether, chlorobenzene, and formamide. For the second trend, the initially established adsorption-desorption equilibrium at the CCR-Pt interface is obviously perturbed by flushing the system with another solvent such as dichloromethane, ethyl acetate, methanol, ethanol, and acetic acid. These trends can already explain the above-mentioned observations made by catalysis researchers, in the sense that the perturbation of initially established adsorption-desorption equilibrium is related to the nature of the solvent. 

Thus it appears that an interpretation not involving adsorption equilibria is reasonable in accounting for the observed kinetics of dehydrogenation of methylcyclohexane to toluene. However, some additional information, such as data on the heat of adsorption of toluene on supported platinum, would be desirable in establishing the correctness of this interpretation. 

The remaining interesting feature of the picoline results is the direction of the compensation effect between the increased adsorption due to the nitrogen lone-pair interaction and the steric hindrance of the methyl group. The data from 3-picoline for the a exchange show that some deactivation of one position (presumably the 2-position, or ortho to the methyl) is observed, although the effect is not as pronounced as in the ortho positions of toluene. Thus, the compensation effect would appear to favor increased adsorption from the lone pair on the nitrogen and this is confirmed by the results of 4-picoline on borohydride-reduced platinum. 

Another kinetic method of determining relative adsorption constants does show that toluene is adsorbed more strongly than benzene on the platinum metals. The method determines the effect of the partial pressure of toluene on the rate of hydrogenation of benzene. With the assumption that adsorption is reversible, the ratio of adsorption coefficients, br/bB, is evaluated by the use of equation (37) where U°B and Ub(T) represent the rate of hydrogenation of benzene in the absence of toluene and in its presence at stated partial pressures of toluene (Ft) and benzene (Pb). This method avoids the determination of the relative individual rate constants that is required by the method of Wauquier and Jungers. Xylene inhibits the hydrogenation of alkenes on Pd more effectively than does benzene, which is consistent with the effect of alkyl groups on the basicity of benzene. ... 

Surface reactivity was measured on selected samples by Pt(acac)2 adsorption. In order to homogenize the surface of the samples, the tested solids were exposed 12 h to a water saturated air at room temperature before calcination 2 h at 500 °C in a diy air stream. The composite was cooled down to room temperature overnight in the same air stream. After the pre-treatment, the solid was impregnated during 8 h at room temperature with a 4 mM solution of platinum bis-acetylacetonate (R(acac)2) in toluene with a solvent to solid ratio of 5 mL g" The kinetics of Pt(acac)2 impregnation, i.e. the evolution of Pt precursor concentration in solution, was followed by UV-vis. Spectra were recorded in transmission mode on a PERKIN-ELMER Lambdall UV-vis spectrometer, using a 1 mm quartz cells. Pt(acac)2 adsorption principle and details on the data treatments can be found in [11]. 

The presence of a single alkyl substituent in the 1-position has a marked effect on which ring is reduced first with Pd/C as catalyst, the proportions of reduction of the ring carrying the substituent increase in the sequence methyl (34%) < ethyl (45%) < isopropyl (68%) < fert-butyl (97%). With platinum, however, the unsubstituted ring is the more reactive. " The size and position of the substituent also influences its rate of reduction. There are no kinetic studies to differentiate between effects on the adsorption coefficients and on the rates of reaction of the molecules once they have been adsorbed, of the kind conducted with benzene and toluene a combination of steric and electronic factors may be at work. 

---