Propylene adsorption

The spectrum of chemisorbed propylene in the CH and O—H stretching region is shown in Fig. 15. The band at 3593 cm-1 is clearly due to an OH frequency hence, dissociation accompanies propylene adsorption. Careful scrutiny of the region from 1500 to 2000 cm-1 reveals no band assignable to a ZnH band. Since the presence of adsorbed propylene has been found to block out the infrared active hydrogen chemisorption on the ZnO pair sites, we may assume that propylene adsorption occurs as follows ... 

The acidic character of 5A zeolite as a function of the calcium content has been explored by different techniques propylene adsorption experiments, ammonia thermodesorption followed by microgravimetry and FTIR spectroscopy. Propylene is chemisorbed and slowly transformed in carbonaceous compounds (coke) which remain trapped inside the zeolite pores. The coke quantities increase with the Ca2+ content. Olefin transformation results from an oligomerization catalytic process involving acidic adsorption sites. Ammonia thermodesorption studies as well as FTIR experiments have revealed the presence of acidic sites able to protonate NH3 molecules. This site number is also correlated to the Ca2+ ion content. As it has been observed for FAU zeolite exchanged with di- or trivalent metal cations, these sites are probably CaOH+ species whose vas(OH) mode have a spectral signature around 3567 cm"1. 

The first electron spectroscopic study of adsorbed hydrocarbons was that reported by Eastman and Demuth (78) who used He radiation to probe the valence electrons of benzene, acetylene, and ethylene. Figure 17 shows the difference spectrum of C2H4 adsorbed on Ni(lll) at 100 and 230 K compared with the results of Clarke et al. (79) for ethylene adsorption on Pt(lOO) at 290 K, propylene adsorption on Pt(lOO), and ethylene adsorption on Pt(lll). 

As mentioned above, it is reasonable to assume that this tetrahedral V species forms at defect sites (hydroxyl nests) in the zeolite framework, but is stabilized by this interaction in a well defined environment through V-O-Si bonds. As indicated by the characterization data, the local coordination of vanadium must be different from that found for well dispersed vanadium sites on silica. This stabilization probably limits the unselective metal-bonded propane or propylene adsorption, in agreement with the role of adsorbate bonding on the selection of partial and total oxidation pathways of ethane on vanadium supported on silica (76) and in agreement with IR evidence (Fig. 

Transition metal ion-exchanged zeolites are active catalysts for alkene oxidation but generally result in deep oxidation to carbon dioxide and water (43-45). In common with CO and alkane oxidation, the platinum metal ions are more active than the first-row transition metal ions. Mochida et al. (43) have been able to correlate the catalytic activity of ion-exchanged Y zeolites for propylene oxidation with a so-called Y parameter as shown in Fig. 9. This parameter was considered to express the tendency of the metal ion toward the formation of a dative re-bond with propylene. Further, it was shown that with increasing Y factor there was a decrease in reaction order, which was considered evidence of increased propylene adsorption. In a more recent study of CuX zeolites, Gentry et al. (45) found some evidence... 

Senchenya et al. (96) have treated the adsorption of ethanol on a structural hydroxyl group (Fig. 14) using a CTP scheme and the CNDO/BW method. The separation of a molecule and cluster with respect to the z axis was optimized, the optimal values being r = 1.19 A and R = 1.28 A The adsorption energy was 23.2 kcal/mol, which was close to the experimental value (97). Note that this was essentially the two-point adsorption involving both acid and base sites. This case is quite similar to the above propylene adsorption (90). There is also no definite trend toward proton transfer from the hydroxyl group of a zeolite to the alcohol molecule. The carbocation state is also predicted to be activated. This, in turn, increases relative efficiency of the synchronous mechanism (with the same recommendation for its experimental examination). The estimation (96) of the energetics of the intermediate structures of the synchronous mechanism showed that such a mechanism is quite realistic. 

To clarify the mechanism of propylene adsorption on Ru-Co clusters the quantum-chemical calculation of interaction between it and Ru-Co, Ru-Ru, and Co-Co clusters were carried out. During the calculation it was assumed that carbon atoms of C-C bond are situated parallel to metal-metal bond. The distance at which the cluster and absorbable molecule begin to interact is characterized by the nature of active center. Full optimization of C3H6 molecule geometry confirms that propylene adsorbs associatively on Co-Co cluster and forms Jt-type complex. In other cases the dissociate adsorption of propylene is occurred. The presence of Ru atom provides significant electron density transfer from olefin molecule orbitals to d-orbitals of ruthenium in bimetallic Ru-Co- or monometallic Ru-Ru-clasters (independently on either the tertiary carbon atom is located on ruthenium or cobalt atom.). At the same time the olefin C-C bond loosens substantially down to their break. 

Thus, analysis of experimental data and quantum-chemical calculations shows that the direction of interaction of propylene and CO2 or fragments of dissociation is determined by the mechanism of CO2 and propylene adsorption on clusters of different composition. 

Where xcs xc3, and xo2 are the mole fractions of propane, propylene, and oxygen in the gas phase, kc3°, kacr. and kcox are the rate constants of propylene, acrolein, and COx production, Kc3" is a propylene adsorption constant, and (O2) denotes the zero order oxygen dependence. 

The addition of potassium to AI2O3- [5] or Ti02-supported [17-19] vanadia catalysts increases the selectivity to olefins during the ODH of n-butane and propane, respectively. This variation of the selectivity to oxydehydrogenation products has been related to a decrease in the number of Lewis acid sites [5] or to a decrease in the heat of the propylene adsorption [17-19]. 

It has been suggested that the Incorporation of alkali metals on Ti02-vanadia catalysts decreases both the V=0 stretching frequencies and their polarizing power, while the incorporation of acid anions produces an opposite trend [20]. In addition, the presence of alkali ions decreases the heat of the propylene adsorption [17,18, 21]. Thus the different catalytic behavior of doped alumina supported vanadia catalysts, could be explained on the bases of the influence of the acid-base character of catalysts on the adsorption/desorption of propane and propene. In any case, the redox properties must be also considered. In this way, it will be interesting to study if, realy, a lower reducibility of the active sites could favor a lower rate of the consecutive reactions, as it has been observed in the case of K-doped catalysts. 

The initial rate data show a 4.17-fold increase in rate (75/18) as the B/A ratio goes from 37 to 5 and the mole fraction of propylene changes from 1/38 to 1/6, a 6.33-fold increase. Therefore a rate equation based on propylene adsorption is needed to fit the fractional-order behavior. The effect of benzene concentration cannot be determined from the data, since the mole fraction of benzene changes by only 14%. The kinetic data are analyzed using mole fractions instead of actually concentrations, since the reaction takes place in the liquid phase, and the small volume change can be neglected. 

Figure 7. Propylene adsorption and desorption on 13X zeolite and temperature history. Points are experimental data from Jarvelin [22]. Lines are model predictions.

At this point it is interesting to compare the evolution of propylene adsorption over catalysts with different surface acid characteristics, i.e. a MoVTeNbO catalyst (active and selective in the partial oxidation of propane to acrylic acid), an alumina-supported vanadium oxide (active in the ODH of propane to propylene), or a MoVNbO mixed oxide (active in the oxidative transformation of propane to propylene and acetic acid). The final products observed in each case were related to the characteristics of the adsorbed intermediates (Fig. 24.7) (i) a ir-allylic compound, interacting with a redox site intermediate in the selective oxidation of... 

Fig. 4. Propylene adsorption at 298 K on CMS ( ), CMS-H2O2 (A) and CMS-PINOs ( ), differential heat plot (a) and adsorption isotherm (b).

Microcalorimetric measurements of propylene adsorption in the oxidized carbons treated at 1073 K under He and under H2 are plotted in Fijg. 5. For the samples treated under He, the initial heats of adsorption are 64 and 58 kJ-mof for CMS-HNO3 and CMS-H2O2, respectively. For the samples treated under H2, the initial heats of adsorption are 53 kJ-mof for both carbons. The samples treated in H2 have a weaken adsorption strength of propylene, while samples treated in He give the same results than when out-gassed at 527 and 773 K under vacuum. 

Propylene oligomerization in the presence of alumina-silica gels and zeolites, for example, HNaY [657], are initiated on the surface of acidic Bronsted type sites after propylene adsorption. An increase in the amount of the acidic centers on the carrier surface and their acidic strength results in increasing of the propylene oligomerization rate and greater branching of the product ... 

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