Propylene active sites

The active site on the surface of selective propylene ammoxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an a-H abstraction component such as Sb ", or Te" " an olefin chemisorption and oxygen or nitrogen insertion component such as Mo " or and a redox couple such as Fe " /Fe " or Ce " /Ce" " to enhance transfer of lattice oxygen between the bulk and surface... 

The sterically unencumbered catalyst active site allows the copolymerization of a wide variety of olefins with ethylene. Conventional heterogeneous Ziegler/Natta catalysts as well as most metallocene catalysts are much more reactive to ethylene than higher olefins. With constrained geometry catalysts, a-olefins such as propylene, butene, hexene, and octene are readily incorporated in large amounts. The kinetic reactivity ratio, rl, is approximately... 

They correspond to the cross-metathesis of propylene with the neopentyli-dene fragment (Scheme 18), and their relative ratio corresponds to a photograph of the active site as they are formed. Depending on how propylene will approach the carbene, it will generate different metallacyclobutanes, whose stabilities can direct the relative amounts of cross-metathesis (and selfmetathesis) products. This model is based on the following the favoured cross-metathesis product arises from the reaction pathway, in which [1,2]-interactions are avoided and [1,3]-interactions are minimized (here shown with both substituents in equatorial positions) [83]. 

Cp Th(CH3)2] MgCl2.3oo (73) than for surface species 67. This proves the importance of cahonic surface species in polymerizahon reachons, since the number of active sites is >35% for the former. The reactivity of [Cp Th(CH3)2] MgCl2.3oo (73) was further examined toward propylene and/or 3,3 -dimethylbutene. This study rather suggests an aUyhc C-H bond activation/methane ehmination (Equation 12.1) followed by olefin inserhon than direct propylene insertion into the Th-R bond (Equahon 12.2). This observed reactivity is in agreement with that one described previously for organolanthanide complexes [CpJLnR] [142, 180, 181]. 

The organoactinide surface complexes exhibited catalytic activities comparable to Pt supported on sihca [at 100% propylene conversion at —63°C, >0.47s (U) and >0.40 s (Th)], despite there being only a few active sites (circa 4% for Th, as determined by CO poisoning experiments and NMR spectroscopy) [92]. Cationic organoactinide surface complexes [Cp An(CH3 ) ] were proposed as catalytic sites. This hypothesis could be corroborated by the use of alkoxo/hydrido instead of alkyl/hydrido surface ligands, which led to a marked decrease of the catalytic activity, owing to the oxophilic nature of the early actinides [203, 204]. Thermal activation of the immobihzed complexes, support effects, different metal/ligand environments and different olefins were also studied. The initial rate of propylene conversion was increased two-fold when the activation temperature of the surface complexes under H2 was raised from 0 to 150°C (for Th 0.58 0.92 s" ). 

Epoxidation of olefins over Mo containing Y zeolites was studied by Lunsford et al. [86-90]. Molybdenum introduced in ultrastable Y zeolite through reaction with Mo(C0)g or M0CI5, shows a high initial activity for epoxidation of propylene with t-butyl hydroperoxide as oxidant and 1,2-dichloroethane as solvent [88]. The reaction is proposed to proceed via the formation of a Mo +-t-butyl hydroperoxide complex and subsequent oxygen transfer from the complex to propylene. The catalyst suffers however from fast deactivation caused by intrazeolitic polymerization of propylene oxide and resulting blocking of the active sites. 

The nature of the titanium-containing active site has been investigated with different techniques, including theoretical calculations. The formation of a hydroperoxidic species or of a bidentate side-on titanium peroxo structure was suggested by many authors . Alternatively, some DFT calculations indicated an undissociated molecule of H2O2 weakly interacting with Ti centers or an active Ti-O-O-Si peroxo moiety as a reactive site . Recently, Lin and Frei reported the first direct detection, obtained using in situ FT-infrared spectroscopy, of a Ti-OOH moiety as active species in the oxidation of small olefins like ethylene or propylene . 

The rate of reaction of propylene over the MeReOs/HMDS/silica-alumina catalyst (1.4 wt% Re) is shown in Figure 2b. The profile is similar to that of the Sn-promoted perrhenate catalyst, with kobs = (1-78 + 0.09) x 10" s, and the activity responds similarly to subsequent additions of propylene. In fact, the pseudo-first-order rate constant for the organometallic catalyst lies on the same line as the rate constants for the Sn-promoted perrhenate catalyst. Figure 3. Therefore we infer that the same active site is generated in both organometallic and promoted inorganic catalyst systems. 

Takahata and his co-workers found an increase in the selectivity for 4,4 -DIPB with increasing propylene pressure over HM with a low Si02/Al203 ratio.45-46 Fellman suggested that the increase in selectivity was due to the accumulation of propylene at active sites.7 However, it is obvious from above discussion that the... 

The concept proposed by us is pictured simply in Fig. 23. The level of water represents the chemical potential of active oxygen involved in the oxidation of propylene, and the vessels connected on the tank are involved in two kinds of active sites that activate molecular oxygen to atomic species and oxidize propylene to acrolein. If active sites expressed by vessels are isolated from each other, each site must do everything by itself to convert propylene to acrolein. This situation is less convenient than the preparation of the active catalyst system. When active species of oxygen can migrate rapidly through the bulk diffusion of oxide ion as shown in Fig. 23, equal-... 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

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