Propene disproportionation

The development of bifunctional catalysts for specific catalytic sequences of reactions in which the product of the first reaction can serve as substrate for the second is of great importance. There are many examples of such reactions. They are, for instance, the monomer-isomerizing polymerization of heptene-2, heptene-3 and 4-methyl-2-pentene and the combination of propene disproportionation with oligomerization, etc. Bifunctional catalysts are most widely used for ethylene copolymerization with a-butene in situ in the production of so-called low-density linear polyethylene (LDLPE). All general methods for LDLPE production are based on incorporation into a PE backbone of short-chain branches, which can be made by catalytic copolymerization of ethylene with a-olefins C3-C10. A macromolecnlar ligand offers wide possibilities of joining the different types of active site in the same matrix (see also Section 12.5.2). 

The main products of alliin transformation in polar media are sulfides. At room temperature or on heating, allicin is converted by water into diaUyl disulfide, diaUyl trisulfide and diallyl polysulfides, which are the principal components of garlic essential oil and aged garlic (Figure 8.62). Other reaction products are allyl alcohol, sulfur dioxide and propene. Disproportionation of thiosulfinates in aqueous (polar) solutions also leads to disulfides and thiosuhonates of general formula R-SO2-S-R. For example. 

Data on product composition, conversion, and selectivity of propene disproportionation by metathesis... 

FIG. 1 Schematic diagram of the Phillips triolefin process for propene disproportionation (a) disproportionation (b) fractionation (c) fractionation (d) distillation. (From Ref. 4.)... 

Polymerization grade 1-butene, used as comonomer in polyethylene production, is obtained by metathesis disproportionation of propene according to the triolefin process, followed by 2-butene isomerization on specific catalysts [14]. When propene is available, the method is an alternative to the above-mentioned process of ethene dimerization to 1-butene. 1-Butene and 2-butene produced by propene disproportionation constitute a valuable source for manufacturing high purity butadiene via dehydrogenation, since the product contains only trace amounts of branched hydrocarbons. 

The major hydrocarbon product is the parent alkane, propane, formed by loss of the carboxyl group. Smaller amounts of other fragments, such as propene, methane and hydrogen are also observed. These are most likely formed as a result of reactions of the propane radical for example propene can be formed by disproportionation of the propane radical ... 

Frey38 produced CH2 by CH2N2 photolysis in excess propane at 50°C. and found all the products (ethane, n-hexane, 2,3-dimethylbutane, 2-methylpentane, propene, and propane) which would be expected to arise from recombination and disproportionation reactions of the radicals CH3-, CH2CH2CH3, and CH3CHCH3. 

Dithiophenylbutane 66 is obtained by a radical-radical combination reaction, while 3-isopropylthiophene 67 and 3-(propen-2-yl)thiophene 68 form by a radical-radical disproportionation process. The solid-state reaction was shown to be highly temperature dependent. While irradiation of powdered samples at 20°C led to no observable product after 2 days, samples exposed to the same UV source for 24 h at ca. 45°C gave 66 as the only product in ca. 5-10% yield. Photochemical reactions were carried out with nanocrystalline suspensions of 65 in a Pyrex tube acting both as a container and a light filter (X > 290 nm). 

Selective trapping of alkyl radicals from the alkyl halide component during the course of the catalytic disproportionation is the same as the previous observation with silver, and it indicates that the prime source of radicals in the Kharasch reaction lies in the oxidative addition of alkyl halide to reduced iron in Equation 47. Separate pathways for reaction of i-propyl groups derived from the organic halide and the Grignard reagent are also supported by deuterium labelling studies which show that they are not completely equilibrated.(49) Furthermore, the observation of CIDNP (AE multiplet effect) In the labelled propane and propene... 

Scheme 10.17 shows an imusual disproportionation of thiiranes. These strained sulfides react, in the presence of catalytic amounts of 4, to afford 1,2,3-trithiolanes and 1,2,3,4-tetrathianes and alkenes [28]. Monosubstituted thiiranes such as styrene sulfide and propene sulfide react to form the corresponding olefin and the 4-substi-tuted 1,2,3-trithiolane in a 2 1 ratio in isolated yields in excess of 90% (Scheme 10.17). The reaction is thought to arise through initial thiirane coordination to the ruthenium center and subsequent nucleophilic attack of free thiirane on the carbon of coordinated thiirane.

CH3, C3H5, C4H7, C4H9 radical combination and disproportionation methane, ethane, allene, propene, -butane, butenes, butadiene, isopentane, pentenes, hexadienes, heptanes, heptenes, octanes, octenes and octadiene... 

Studies conducted in the presence of radical scavengers such as NO (refs. 383, 245, 409, 410), Oj (ref. 408) or H2S (ref. 246) have shown the importance of free-radical reactions in forming the products isobutane, 2,3-dimethylbutane, -butane, isopentane and others. The ethylene and propene yields are decreased by the presence of the scavengers owing to the disappearance of the fraction of these products that arises from disproportionation reactions. The products which are formed in the presence of inhibitors must arise from molecular or ion eliminations, ion-molecule reactions, excited molecule reactions or charge-neutralization reactions. Work on the inhibited radiolysis has led to a better understanding of the source of these products " . 

This interpretation was supported by Mol, Moulijn, and Boelhouwer in experiments with i C-Jabeled propene (28). Propene-2-i C underwent disproportionation yielding ethylene free of activity and butylene showing a specific radioactivity twice that of starting propylene. The authors proposed the four-membered ring intermediate. 

De-aluminated mordenites were claimedto give more active and stable catalysts for toluene disproportionation than conventional H-mordenite. Becker, Karge, and StreubeP studied the alkylation of benzene with ethene and propene over an H-mordenite catalyst. Shape-selective catalysis was found because only ethylbenzene, w-diethylbenzene, p-diethylbenzene, cumene, p-di-isopropylbenzene, and m-di-isopropylbenzene were detected in the products neither o-diethylbenzenes nor higher alkylated products were found. The results are in agreement with earlier transalkylations over H-mordenite. Catalyst aging was caused by olefin polymerization. The selectivity of Be-mordenite... 

The disproportionation (or alkyl exchange) and the alkylation reactions of alkylsilanes have been carried out in a closed recirculation reactor at 373 - 623 K and 373 - 473 K, respectively, by using 100 - 200 mg of catalysts. For the disproportionation reaction, 30 Torr of diethylsilane (E2), diethyidimethylsilane (E2M2), and triethylsilane (E3) were used. For the alkylation reaction, 30 Torr of E2 and 30 Torr of alkylating reagents (propene, 1- and c/s-2-butene, 2-methyl-1-butene, 1,3-butadiene, methylacetylene, ethylacetylene) were used. Cyclic olefins, nitriles, benzene and carbonyl compounds were also tested. 

The monocyclic terpenes are easily disproportionated to the dehydrogenated p-cymene and the hydrogenated /7-menthane. For a short time on stream TOS (< 2 h), cracking of /)-cymene to toluene and propane/propene on the strongly acidic sites is the dominant reaction. With increasing TOS, these strong acid sites of the catalysts deactivate and, thus, the formation of products obtained by cracking decreases. 

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