Cumene adsorption

The temperature dependence of the rate constants is shown in Fig. 10. The rate constants calculated from these data are summarizedinTable4. The absolute values of the rate constants k, determined taking into account cumene adsorption data, are given in the last column of Table 4. 

The initial entropy of cumene adsorption on a commercial aluminosilicate was measured to be about —750JmoP K , whereas the gas-phase entropy of cumene at 298 K is 389 J moP K (709). This seemingly inconsistent result appears to be caused by the dissociative adsorption of cumene at low coverages on this catalyst. In this case, the measured heat corresponds to a combination of heats of adsorption and reaction. Higher coverages produced lower, nearly constant heats and entropies of adsorption. These entropies correspond to the loss of between two and three degrees of translational freedom. The adsorption of benzene on these samples did not show abnormally... 

Cumene adsorption experiments were carried out in a microbalance apparatus equipped with a quartz spring whose deflections were measured with a cathetometer. The unit also included a vacuum system capable of reducing the pressures to below 0.01 micron, and a Baratron continuous pressure-sensing device with a sensitivity of 1 micron. The amount adsorbed was measured by spring deflection, and checked by material balance on the gas phase based on an approximate chamber volume of 600 cc. The pressure measurements were noted at regular intervals until equilibrium was reached (less than 1 micron change in 5 minutes). These pressure vs. time data were used for diffusivity calculations. 

Cumene Adsorption. Cumene adsorption equilibrium data are presented in the form of isotherms at room temperature in Figure 1. 

Brei, V.V., et al., A study of the Bronsted site acidity of crystalline and amorphous aluminosilicates Desorption of ammonia, dehydration of 2-propanol and cracking of cumene. Adsorpt. Sci. Technol.. 14(6). 349-358 (1996). 

The specific surface area was measured by nitrogen adsorption at -195 C. The cumene cracking reaction was conducted by pulse technique under the following conditions O.IO g catalyst, H, flow rate 75 ml/nin, pulse volume 1 ul. 

As expected from the TPD results, Al-sapo was more active for the cracking of cumene on a per weight of catalyst basis than Al-mont. In order to compare the catalytic activity on a basis of active sites, we evaluated the number of active sites on these catalysts. TPD spectra were measured with varying the temperature of ammonia adsorption. Typical results on Al-mont are shown in Fig. 2. By integrating these spectra, the concentration of acid sites corresponding to different strength of acidity can be determined. 

Table 13 collects values of adsorption coefficients of some compounds determined by means of this equation from experiments on the cracking of cumene on an aluminosilicate catalyst. 

The purpose of the present work is to incorporate aluminum into the framework of SBA-15 during the synthesis in order to create acid sites on the surface of the material directly and to enhance its activity in acid-catalyzed reactions and to study the stability of SBA and AlSBA molecular sieves under various treatments. The influence of these treatments on the pore size, wall thickness and the environment of Al in these materials are investigated in detail. X-ray diffraction (XRD), Electron Microscopy (TEM) and N2 adsorption were used to characterize the structure, the porosity and the stability of these materials. 27Al MAS NMR was used to ascertain the nature and environment of Al, cumene cracking to test the catalytic activity of parent materials and ammonia chemisorption to probe their surface acidity. 

Adsorption geometry could cause the observed rate difference. Crawford and Kemball have found that deuterium exchange of the two methyl groups of the isopropyl side-chain in cumene over nickel films occurs in two steps (32). Both methyl groups cannot exchange at the same time. The cumene molecule probably must leave the surface and re-adsorb before the second... 

Special care has to be taken, however, that the quinoline titer truly represents the minimum amount of catalyst poison. In most cases this type of base is adsorbed by inactive as well as active sites. Demonstration of indiscriminate adsorption is furnished by the titration results of Roman-ovskii et al. (52). These authors (Fig. 13) showed that introduction of a given dose of quinoline at 430°C in a stream of carrier gas caused the activity of Y-zeolite catalyst (as measured by cumene conversion) to drop with time, reach a minimum value, then slowly rise as quinoline was desorbed. The decrease in catalytic activity with time is direct evidence for the redistribution of initially adsorbed quinoline from inactive to active centers. We have observed similar behavior in carrying out catalytic titrations of amorphous and crystalline aluminosilicates with pyridine, quinoline, and lutidine isomers. In most cases, we found that the poisoning effectiveness of a given amine can be increased either by lengthening the time interval between pulse additions or by raising the sample temperature for a few minutes after each pulse addition. 

The preferential adsorption behavior of poly(vinylpyrrolidone) (PVP) in binary solvent containing aromatic components has been also studied [110]. In this case, it was concerned with the influence of the chemical structure of different binary solvents in the preferential adsorption of this polymer. 2 - propanol - cumene, 2 -propanol - mesitylene, 2 - propanol - p-xylene, 2 - propanol - ethylbenzene and 2 - propanol - toluene. Figure 1.15 shows the variation of X with the solvent composition. In both cases aromatic components are adsorbed in the range 0 to 40%, but the amount of adsorbed molecules is rather different for the two isomers. This result could be explained in terms of steric hindrance due to the isopropyl groups of cumene, which would be reflected in the lower X value. 

Fig. 1.15 Dependence of the preferential adsorption parameter X on mixed solvent composition for the ternary system. (.) PVP-2-propanol-cumene and (-) ...

The adsorption of cumene and inhibitors on active cracking sites follows a Langmuir type of isotherm. This means that there is little or no interaction among the chemisorbed molecules on the surface. This might be expected to be the case as studies of the chemisorption of the inhibitor quinoline by similar catalyst (14) show that the surface is sparsely covered with active sites (<5% of internal surface area covered with chemisorbed quinoline at 315°C.). In addition, the active sites are homogeneous with respect to adsorption energies. 

Since G describes the adsorption characteristics of cumene, the value of GKi gives the adsorption strength of an inhibitor relative to cumene. The value of GKi for xylene at 360°C. is unity, as seen from the data in Fig. 12. This means that xylene and cumene compete on an equal basis for actual sites. 

---