Adsorption of propylene

The 02 ion on MgO does not react with CO or alkanes at 77 K but the EPR signal disappears slowly at room temperature (361). Similarly, on ZnO (390) it reacts only slowly with propylene at room temperature and not with CO, H2, or ethylene. A slow reaction with propylene is also observed for 02 on V2Os/MgO at room temperature (391). Yoshida et al. (392) have studied the reactivity of adsorbed oxygen with olefins on the V20j/Si02 system. Adsorption of propylene destroyed the signal from 02 slowly at room temperature and the reaction products, aldehydes with some acrolein, were desorbed as the temperature was raised to 150°C. More quantitative... 

Effect of Propylene Pressure on Selectivity - The partial pressure of propylene is also one of key factors for selective formation of 4,4 -DIPB.22"25 The high partial pressure of propylene effectively enhanced the isopropylation, but the selectivity for 4,4 -DIPB decreased at lower partial pressures over HM(220), as shown in Figure 6.24-25 However, the isomerization of 4-IPBP did not occur at any propylene pressures. Because 3,4 -DIPB is a more thermodynamically stable isomer than 4,4 -DIPB,43 this decrease of the selectivity was ascribed to the isomerization of 4,4 -DIPB to 3,4 -DIPB, not to the lower selectivity to 4,4 -DIPB. Figure 7 shows the effect of propylene pressure on the selectivities for 4,4 -DIPB in bulk and encapsulated products. The selectivity of 4,4 -DIPB inside the pores was almost constant at every pressure. These results indicate that the isomerization does not occur inside the pores but at the external acid sites. The effect of the pressure on the isomerization of 4,4 -DIPB was similar to that on the isopropylation of biphenyl. 4,4 -DIPB itself isomerizes significantly to 3,3 - and 3,4 -DIPB over the catalyst in the absence of propylene. However, no significant isomerization of 4,4 -DIPB occurred in the presence of sufficient propylene pressure. On the other hand, the selectivity of 4,4 -DIPB in encapsulated DIPB isomers was almost constant at any pressure. These differences support the hypothesis that the isomerization of 4,4 -DIPB to 3,4 -DIPB occurs on the external surfaces. The isomerization of 4,4 -DIPB under high pressures is considered to be retarded by the preferential adsorption of propylene on acid... 

However, the reduction with propylene shows no correlation with molybdenum coordination. This suggests that the bismuth centers are responsible for the adsorption of propylene. 

Further evidence supporting the bismuth center as a site of propylene activation comes from the analysis of the rates of formation and product distribution of propylene oxidation over bismuth oxide, bismuth molybdate, and molybdenum oxide. Bismuth molybdate is highly active and selective for the conversion of propylene to acrolein. However, the interaction of propylene with its component oxides yields very different results. Haber and Grzybowska (//. ), Swift et al. 114), and Solymosi and Bozso 115) showed that in the absence of oxygen, propylene is converted to 1,5-hexadiene over bismuth oxide with good selectivity and at a high rate, whereas molybdenum oxide is known to be a fairly selective but a nonactive catalyst for acrolein formation. The formation of 1,5-hexadiene over bismuth oxide can be explained if the adsorption of propylene on a bismuth site yields a ir-allylic species. Two of these allylic intermediates can then combine to give 1,5-hexadiene. 

The surface chemical reaction seems to follow the Eley-Rideal (ER) mechanism, in which the adsorption of propylene is predominant over benzene, as represented by the equation ... 

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. 

Some experiments were made with the adsorption of propylene at 100 torr but the data did not permit a clear-cut separation of physical adsorption and chemisorption. However, no substantial drift in weight occurred at 100 torr, most of the adsorbed propylene was liberated by helium flushing at 100° and all, by the original temperature of activation. 

Fig.6 shows PO yield over 8 wt % Au/TiOj/SiO, as a function of time. The catalytic activity of Au/TiOj/SiOj catalyst is not stable. Water is continuously formed during the oxidation of propylene and the oxygenated intermediates may block the active sites and depress the adsorption of propylene on the surface of the catalyst. MCM materials have hydrophobic character and Ti-MCM preferentially adsorbs less polar olefin molecules. This decreases the competition from water and probably avoid the accumulation of the oxygenated intermediates to lead to more stable catalytic activity. 

Example 3.9. Propylene (A) and propane (P), which are difficult to separate by distillation, have been separated on an industrial scale by preferential adsorption of propylene on silica gel (S), the equilibrium data at 25°C and 101 kPa being as shown in Fig. 3.21. 

The dependencies of adsorption of propylene, divinyl, allyl chloride, PCH, CDC and DCBD (Pig.5) on potential of platinum electrode vfere constructed on the base of chronopotentiometric curves. 

Figure 24.7. Selectivity to the main partial oxidation product achieved at low conversion (Xx = 2%) and high conversion (Xx = 20%) during propane oxidation at 400°C over (i) AI2O3-supported vanadium oxide (VOX/AI2O3) (ii) Mo-V-Nb mixed oxides calcined in air at 450 C, presenting an amorphous material (MoVNbO) and (iii) Mo-V-Te-Nb mixed oxides heat-treated at 600 C in N2 presenting Ml phase (MoVTeNbO) catalysts. For comparison, the main adsorbed species observed during the adsorption of propylene on each catalyst it is also presented. From Ref. 125.

A number of different mechanisms have been proposed for isotactic polymerization (stereoreguiar polymerization). Among these a mechanism found wide acceptance utilizing the stereo selective adsorption (ji-complex formation) of propylene over a vacant site on the surface of TiCla produced by the action of AlEts. Adsorption of propylene in a stereo selective manner and subsequent insertion to the alkyl group, as mentioned above, gives isotactic polymer. 

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