C3H6 formation

CjHe quantum yield on the light intensity clearly indicate that free radicals are involved in cyclopropane formation . The rapid increase of cyclo-C3H6 formation with increasing temperature between 120 and 180 °C suggests the oeeurrence of chain propagation steps - ... 

A sequence initiated through HO2 addition to isobutene could account for C3H6 formation. 

Neither hydrogen nor methane was detected for temperatures up to 533 °C. At 642 hydrogen but no methane is detected, indicating initial propene (C3H6) formation. At 693 °C both hydrogen and methane are found in the ejector off gas. Nevertheless, at this temperature the ejector operated continuously for about 50 minutes without increase of primary pressure or decrease of the propane flow rate. At 745 C the nozzle inlet pressure rises and the propane flow collapses during 5 minutes because of soot formation and plugging of the nozzle. As result, ejector operation with propane is limited to temperatures below 650 °C. 

In the cases of Cr03/Si02 and Cr(7r-C3H6)3/Si02 systems a considerable part of the chromium contained in the catalyst is involved in the propagation center formation. In these catalysts all the ions of the transition metals are on the surface and the active component seems to be the main type of compounds present on the catalyst surface. 

A few studies have been carried out on the parent four- and five-membered cyclic sulfones—for thietane 1,1-dioxide (30) by Scala and Colon65 and for thiolane 1,1-dioxide (sulfolane) (31) by Honda and coworkers66 and, later, by Schuchmann and von Sonntag67. In the former compound, the major photochemical process, in the vacuum UV range, is the initial production of a trimethylene (C3H6) biradical and S02 (equation 9). In both the solid- (77 K) and gas-phase photolyses, formation of a triplet biradical appears to be favored. As well as the expected cyclopropane and propylene, ethylene is also obtained during these photolyses, presumably by a cycloreversion process (equation 10). 

Figure 8.62, Effect of temperature on the catalytic rates of C02, N2 and N20 formation and on the corresponding N2 selectivity, for open (unpromoted) and closed (NEMCA) circuit conditions on Rh/YSZ during NO reduction by C3H6.67,68 Reprinted from ref. 68 with permission from Elsevier Science.

Relatively detailed study has been done for the reaction pathways over Au/Ti02 catalysts mainly because of simplicity in catalytic material components. The rate of PO formation at temperatures around 323 K does not depend on the partial pressure of C3H6 up to 20vol% and then decreases with an increase, while it increases monotonously with the partial pressure of O2 and H2 [57]. A kinetic isotope effect of H2 and D2 was also observed [63]. These rate dependencies indicate that active oxygen species are formed by the reaction of O2 and H2 and that this reaction is rate-determining [57,63,64]. 

Haneda et al. [134,135] studied the formation and reaction of adsorbed species in NO reduction by propene over Ga203-Al203. IR transient reaction technique was employed to examine the reactivity and dynamic behaviour of surface species. The catalyst was first exposed to either C3H6/02/Ar or NO/Oz/Ar at 623 K for a long time to form and accumulate the surface species. The catalyst was further purged with pure Ar and the reaction gas then switched to various gas mixtures. Changes in the intensity of IR bands were measured with time on stream. The main surface species detected by IR during... 

Figure 3 shows IR spectra taken during the step switch from steady state He/C3H6 to He/02/C3H6. The exposure of the catalyst to propylene prior to the 02 step switch produced the bands similar to those in Figure 1. Introduction of 02 into the reaction by switching from He/C3H6 to He/Oycyy flow led to immediate formation of a C02 band followed by the PO C-O-C band at 977 cm". The bands at 977, 1390, and 1640 cm"1 are consistent with those in adsorbed PO which is shown as the upper-most spectrum in Figure 3.

Figure 14. Variation with relative kinetic energy of cross sections for the formation of product ions resulting from the interaction of Co+ with isomeric pentenes. Symbols represent products corresponding to elimination of H2 (open circle), CH4 (solid square), C2H4 (open triangle), and C3H6(solid circle). Data from reference 50.

The proposed Re6 cluster (8) with terminal and bridged-oxygen atoms acts as a catalytic site for selective propene oxidation under a mixture of propene, Oz and NH3. When the Re6 catalyst is treated with propene and Oz at 673 K, the cluster is transformed back to the inactive [Re04] monomers (7), reversibly. This is the reason why the catalytic activity is lost in the absence of ammonia (Table 8.5). Note that NH3, which is not involved in the reaction equation for the acrolein formation (C3H6+02->CH2=CHCH0+H20) is a prerequisite for the catalytic reaction as it produces the active cluster structure under the catalytic reaction conditions. 

Recently, UV laser stimulation of catalyst samples has been developed to overcome the problem of interference by coke (carbon deposition) on catalysts.Fig. 9 shows a typical Raman data set that was obtained for carbon deposition as a function of temperature. To explore different coke formation behavior, the reaction of propene on a zeolite was performed. The spectra obtained were (A) C3H6/He flow at 773 K for 3 h (B) O2 flow at 773 K for 1 h and (C) O2 flow at 873 K for 1 h. This data shows that most of the carbon, identified as polyaromatic and pregraphite, can be removed at 773 K with oxygen. However there is still carbon present as identified by the broad band at 1610 cm suggesting that carbon is in a more inert form such as coke. Not until the temperate is taken to 873 K with oxygen is that carbon removed. 

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