Propene formation

Figure 6.13.Dependence of initial propene formation rate on Cs Mo atomic ratio for Cs-Mo/Zr catalyst samples (703K, 14 kPa, C3H8, 1.7 kPa 02, balance He).84 Reprinted with permission from Academic Press.

This work has been compared with analogous cyclobutane thermolytic decompositions. The siletanes were found to fragment more readily than the cyclobutanes. Although fragmentation via propene formation (most substituted C—C bond) was favored in both classes of compounds, it was more dominant with the siletanes. These effects are apparent from the kinetic data in Table IV.144-147... 

Table 2 Results from the decomposition reaction of isopropanol on (MPMol2)b and (MPMol2)b series at 150°C conversion, rate for propene formation (rp) and rate for acetone formation (ra).

The pulse experiments demonstrated that active sites for propane dehydrogenation are formed upon exposure of the oxide form of gallium modified ZSM-5 to propane itself. A constant 1 1 ratio of hydrogen produced to propane consumed is attained after a number of pulses with little propene formation, which suggests that, after propane dehydrogenation to propane, aromatization proceeds through hydrogen transfer reactions. 

Dahl, I. and Kolboe, S. (1993) On the reaction mechanism for propene formation in the MTO reaction over SAPO-34. Catal Lett., 20, 329-336. 

Such higher order prerequisites could be fulfilled by ensemble operation of several sites. For example, a dimeric cluster of cuprous ions on silica gel is very active for the oxidation of CO with NzO at room temperature, but isolated cuprous ions are entirely inactive for this reaction 60). More interesting selectivity may be found in the reaction of olefins with methylene complexes the reaction of olefins with mononuclear methylene undergoes an olefin metathesis reaction, but the reaction of ethylene with bridging methylene in /i-CH2Co2(CO)2(Cp)2 61), /<-CH2Fe2(CO)8 (62), and /<-CH2-/i-ClTi(Cp)2Al(Me)2 (65) (Cp = cyclopentadiene) leads to propene formation (homologation reaction). 

Ajmera S, Wu JC, Worth J, Rabow LE, Stubbe J, Kozarich JW (1986) DNA degradation by bleomycin evidence for 271-proton abstraction and for C-O bond cleavage accompanying base propenal formation. Biochemistry 25 6586-6592... 

McGall GFI, Rabow LE, Ashley GW, Wu SH, Kozarich JW, Stubbe J (1992) New insight into the mechanism of base propenal formation during bleomycin-mediated DNA degradation. J Am Chem Soc 114 4958-4967... 

McGee also found the cyclopropane/propene ratio to decrease with increasing pressure in the photolysis of cyclobutanone. However, his data indicated that the change is entirely due to an increase in propene formation, while cyclopropane formation is claimed to be independent of pressure. McGee suggested, on the basis of these results, that propene and cyclopropane were not formed from the same excited state. The deuterium content of the olefin, formed in the photolysis of cyclopentanone-2,2,5,5-rf4, also indicates that the hot cycloalkane is not the only source of the CH2 = CH(CH2) 4CH3 product - . 

Static system rate of propene formation (in isopropyl formate decomposition) was relatively insensitive to surface. 

Major yields of propene (ca. 10%) are found in the initial products from isobutene oxidation between 673 and 773 K, but effectively no propene is observed initially from the oxidations of butene-1, 2-methylbutene-2 and 2,3-dimethylbutene-2. Structurally, propene formation is possible via C3H7 radicals in all cases through the hydroxy adduct. 

The 1 1 Sb-V and 1 1 5 Nb-V-Si systems only were tested at the different space velocity of 100 ml min" g". Almost all the prepared systems exhibited propane conversions of about 30% and propene selectivities higher than 20%. The most selective catalysts with respect to propene formation were the 1 1 Nb-V prepared via the hydrolytic method and 1 1 Sb-V systems (Scshs ca 40%). The 1 1 Sb-V and 1 1 5 Nb-V-Si prepared via the non-hydrolytic method gave the best results in terms of propane conversion and yield in propene. In these two cases, while the higher conversion is in contradiction with... 

Catalysts based on transition metal molybdates, typically bismuth, cobalt and nickel molybdates [2-6], have received recent attention. Of the transition metal molybdates, those based on nickel, and in particular the stoichiometric NiMo04, have attracted the greatest interest. NiMo04 presents two polymorphic phases at atmospheric pressure a low temperature a phase, and a high temperature P phase [2,7]. Both phases are monoclinic with space group dim. These phases differ primarily in the coordination of molybdenum which is distorted octahedral in the a phase and distorted tetrahedral in the P phase. The P phase has been shown to be almost twice more selective in propene formation than the a phase for comparable conversion at the same temp>erature [2]. A similar effect has been noted for oxidative dehydrogenation of butane, with the P phase being approximately three times more selective in butene formation than the a phase [8]. The reason for the difference in selectivities is unknown, but the properties of the phases are known to be dependent on the precursors from which they are derived. Typically, nickel molybdates are prepared by calcination of precipitated precursors. 

The results of Table 6 showed that the addition of a very small amount of bismuth increases significantly the selectivity towards acrolein and acrylic acid with no change in the propane conversion. The propene formation and the acetic acid production decreased at the same time, which is quite an important result of the effect of bismuth on the reaction scheme. 

In both reactions, with n- or tjo-propanol, no C-alkylated products are formed. Propene formation was not detected. 

In the reduction proceeding in the presence of propene, formation of N2, H2O and CO2 was detected over the copper containing catalyst. 

Procedure (B) was performed based on 5 different redox metal oxides which had proven to be catalytically active towards propene formation as a result of procedure A and 3 additional metal oxides of strong metal oxygen bond strength i. e., MgO of basic nature, B2O3 of acidic nature and LajOs on which dissociative oxygen adsorption takes place. 

Recent data, published and unpublished, provide strong evidence that the common views on the reaction mechanism of the MTH reaction are not tenable. The data rather point to ethene and propene formation from an adsorbate hydrocarbon pool, probably of aromatic nature. There are strong indications that the catalytic cycle is based on arenes that are continually methylated by methanol/dimethyl ether, and dealkylations leading to ethene, propene and most likely also isobutene via molecular rearrangements. Penta- and hexamethylbenzene appear prone to undergo this reaction. However, there is also clear evidence that higher alkenes, if present in substantial amount, may take part in the classical homologation system. 

The acid-base properties of zeolites or oxides are often studied by measuring the selectivities to the different products in the decomposition of alcohols and particularly isopropanol. The rate of propene formation can very often be correlated to the number of acidic sites determined by ammonia adsorption. A relationship has been found between the strength of the acid sites of bulk oxides, as determined by ammonia adsorption microcalorimetry [95], and the activation energy of dehydration, while the activation energy of dehydrogenation was independent of the strength of the sites [149]. 

Propose a two-step mechanism similar to that proposed for the addition of HCI to propene. Formation of the carbocation intermediate is rate determining. 

The Walsh diagram in Rgure 6 shows the behavior of the frontier orbitals during the reaction for the case of propene formation. 

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