Thermal reactions of propylene

Thermal Reaction of Propylene. Thermal reaction of propylene has been studied extensively. Laidler and Wojciechowski (16) reported that main products were ethylene, methane, and hydrogen and that minor ones were ethane, propane, butenes, cyclopentadiene, cyclohexadiene, benzene, toluene, and diallyl at temperatures from 580° to 640°C pressures from 40 to 400 mm Hg in a static system. No allene was detected, which is in contrast to the results obtained at higher temperatures by Szwarc (21) and by Sakakibara (19). Reaction order was determined as 3/2, and the A-factor and activation energy were reported as 1013 34 ml1/2 mole"1/2 sec"1 and 56.7 kcal/mole, respectively. Kallend et ah (9) carried out a detailed analysis of the reaction product at 555° -— 640°C and pressures 7 300 mm Hg. The main C6 compounds present were 1,3-and 1,4-hexadiene. Methylcyclohexene and cyclohexadienes were not found. 

In this paper the kinetics of the thermal reaction of propylene is described at temperatures ranging from 703° to 854°C, at atmospheric pressure and at residence times from 0.078 to 3.3 sec, with and without nitrogen dilution. 

Figure 7. Selectivities of methane and ethylene in the thermal reaction of propylene...

These products are consumed consecutively, probably to form benzene and polycyclic compounds. Toluene may also react consecutively to benzene. The ratio of toluene or xylenes to benzene was about twice that obtained in the thermal reaction of ethylene, respectively, at temperatures from 703° to 854°C and at conversions up to 40 mole %. The ratio of styrene to benzene was about one-third as large as that obtained in the thermal reaction of ethylene. Addition of butadiene in the thermal reaction of propylene increased the selectivity of cyclic compound formation, although the increase was smaller than in the case of ethylene. These facts support the mechanism for the formation of monocyclic aromatic compounds proposed by Wheeler and Wood (24) this is discussed in detail later. 

Thermal Reactions of Olefins with Butadiene. The rate constant of the reaction of ethylene with butadiene was reported by Rowley and Steiner (18), whereas that of propylene or butenes with butadiene has not been reported. Tarasenkova (22) reported that the thermal reaction of propylene with butadiene at 600°C gave toluene, the yield of which was twice as large as the yield of benzene plus xylenes. Moreover, the thermal reactions of 1-butene with butadiene and 2-butene with butadiene at 500° — 550°C gave as main products ethylbenzene and o-xylene, respectively. The ratio of ethylbenzene to total xylenes was close to the ratio of 1-butene to 2-butene in the feed. 

Figure 18. Rate of formation of C6 cyclic compounds in thermal reaction of propylene at 753°C...

A similar mechanism has been proposed above for the formation of five-membered ring compounds in the thermal reaction of propylene. In fact, for propylene the ratios of the yields of toluene plus xylenes to benzene were about twice as large as those in thermal reaction of ethylene. 

As the yields of these initial products decrease with increased residence times, cyclic compounds such as cyclopentene, cyclopen tadiene, cyclohexene and benzene are produced. In the case of propylene (, 7 ), the reaction proceeds 2-4 times faster than that of ethylene and ethylene, methane, butadiene, butenes, acetylene, and methylcyclopentene are the main products during the initial step cyclopentadiene, cyclopentene, benzene, toluene and polycyclic compounds higher than or equal to naphthalene are products of secondary reactions. A remarkable fact for the thermal reaction of propylene is that the yields of five membered ring compounds are larger than those in the case of ethylene. 

Reactions other than those of the nucleophilic reactivity of alkyl sulfates iavolve reactions with hydrocarbons, thermal degradation, sulfonation, halogenation of the alkyl groups, and reduction of the sulfate groups. Aromatic hydrocarbons, eg, benzene and naphthalene, react with alkyl sulfates when cataly2ed by aluminum chloride to give Fhedel-Crafts-type alkylation product mixtures (59). Isobutane is readily alkylated by a dipropyl sulfate mixture from the reaction of propylene ia propane with sulfuric acid (60). 

This has been shown by Pines and Arrigo 40) to be the case in the thermal reaction of toluene with propylene. Schramm and Langlois 22) reported that some n-butylbenzene may be produced in the base-catalyzed reaction of toluene with propylene. The amount of n-butylbenzene apparently... 

Fig. 26. Infrared spectra following the reaction of propylene with OJ (a) MgO background (b) after the reaction at 175°C (c) after thermal treatment under vacuum at 300°C (d) 450°C (e) 550UC (f) difference spectrum between (b) and (a) (g) difference spectrum between (c) and (a) (393).

Figure 1. Selectivities of methane and propylene in thermal reaction of ethylene...

Thermal Reaction of Ethylene. Dahlgren and Douglas (5) reported that the primary products of the reaction were propylene, butenes, butadiene, and ethane at temperatures ranging from 480° to 582°C and at pressures from 9 to 137 mm Hg. However, analyses of reaction products were incomplete, and the primary products were not distinguished from the secondary ones. 

Selectivity of formation of methylcyclopentene decreases rapidly with conversion of propylene. In the thermal reaction of ethylene this compound was not identified. Formation of five-membered ring compounds—i.e., methylcyclopentene, cyclopentene, and cyclopentadiene— may be attributed to allyl type radicals (3,14). 

Thermal Reactions of Butenes. Among the C4 olefins, 1-butene has been studied most extensively. Bryce and Kebarle (2) pyrolyzed 1-butene at 490° 560°C in a static system, and the main gaseous products were methane, propylene, ethylene, and ethane. The main liquid products were cyclohexadiene, benzene, cyclopentene, cyclopentadiene, and toluene. The rates of formation of methane, propylene, ethylene, and ethane showed first-order dependence on the initial butene concentration. The activation energy for 1-butene disappearance was ca. 66 kcal/mole. 

Thermal reactions of olefins with butadiene were examined in this study at temperatures from 510° to 670°C and with short residence times. Thermal reaction of the mixture ethylene-propylene-butadiene gave cyclohexene (CH), 4-methylcyclohexene (MCH), and 4-vinylcyclohex-... 

Formation of Cyclic Compounds When we noted that the addition of small amounts of butadiene increased the yield of cyclics formed in the thermal reaction of ethylene and propylene, an effort was made to relate directly the formation of cyclics in thermal reaction of ethylene and propylene, respectively, to the Diels-Alder reaction between feed olefins and product butadiene. Reactions between product olefins and product butadiene were neglected owing to their small concentrations. Cyclics were deferred as the sum of C rings with and without alkyl or vinyl groups. 

To investigate the route to polycyclic compounds, the experiments were carried out on thermal reactions of benzene to diphenyl, of benzene with butadiene and of styrene with ethylene to Tetralin isomers, and of cyclohexene with butadiene to octalin. However, the rates of these reactions were too small to account for the formation of polycyclics in thermal reactions of both ethylene and propylene. The obtained result leads to the speculation that some reactive radicals should play an important role in polycyclic formation. 

Propylene Ammoxidation. The possibility of producing acrylonitrile by the reaction of propylene and ammonia was first revealed in 1949 in a patent assigned to Allied Chemical (1). However, the reaction when catalyzed by a V-Mo-P-0 mixed oxide produced acrylonitrile in less than 10% yield. The commercialization of a propylene-to-acrylonitrile process only became possible with the invention of catalysts that effectively change the mechanism from that dictated by gas-phase kinetics to one in which the surface reaction provides a pathway, giving a higher yield of acrylonitrile than is possible from the uncatalyzed thermal reaction. 

For the pyrolysis of paraffinic hydrocarbons at 700- 800 C, yields of olefins such as ethylene, propylene, butenes, butadiene and cycloolefins increase during the initial stage of the reaction, pass through their maxima, and later decrease yields of aromatics, hydrogen and methane however increase monotonically throughout the reaction course. Sakai et al. (1 ) reported previously the result of a kinetic study on thermal reactions of ethylene, propylene, butenes, butadiene and these respective olefins with butadiene at the conditions similar to those of paraffin pyrolysis, directing their attention on the rates of formation of cyclic compounds. Kinetic features of the thermal reactions of these olefins are sunnnarized in Table I combined with the results obtained in later investigations for thermal reactions of cycloolefins ( 2) and benzene O). 

Thermal reactions of ethylene (A>A) require higher temperatures (750- 800 C) than the other olefins. Initial reaction products are butadiene, 1-butene, propylene, ethane and acetylene. 

Different features were observed between the thermal reaction of 1-butene and those of cis- and trans-2-butenes at 640- 680 C (1 ). In the former case, the reaction proceeded mainly in three ways these were pyrolysis to methane and propylene, dehydrogenation to butadiene, and pyrolysis to two moles of ethylene the ratio of rates for these three reactions are 4 3 1, respectively. In the latter cases, the main reaction was isomerization between... 

Cyclization proceeded in nearly 100% selectivity in the case of thermal reaction of butadiene (1 ), yielding 4-vinylcyclohexene (VCH) for the first step and ethylene, cyclohexene, cyclohexa-diene, and benzene in the secondary steps. Similar highly selective cyclizations were observed for the reactions between butadiene and ethylene, propylene, 1-butene, cis-2-butene, trans-2-butene or isobutylene (1), yielding cyclohexene (HCH), 4-methyl-cyclohexene (MCH), 4-ethylcyclohexene, cis-4,5-dimethylcyclo-hexene, trans-4,5-dimethyIcyclohexene or 4,4-dimethylcyclohexene, respectively. Based on the above information, it can be said that butadiene plays an important role in the formation of cyclic compounds in pyrolysis conditions. 

From Figure 1, it is clear that the primary products of the thermal reaction of diallyl are ethylene, propylene, 1-butene, butadiene, 1-pentene, cyclopentene, cyclopentadiene, and 1,3,5-hexatriene, and the secondary products are 1,3-cyclohexadiene and benzene. Trace amounts of methane, propane, and 1,4-pentadiene were also found in some experiments. No hydrogen was detected by a nitrogen carrier gas chromatograph with MS 5A column. The formation of C 2 compounds was noticed at low temperatures. A small amount of liquid product was found in the separator tube after 50 or more experimental runs. The average molecular weight of the liquid product was 428 based on the method of Hill (15). 

Thermal Reaction of DAO in Excess Ethylene. To get better knowledge about the formation of cyclopentene and 1-pentene, DAO was employed as another source material for allyl radicals. In the case of the pyrolysis of DAO in excess ethylene, the reaction temperature was considerably lowered, and a large amount of diallyl was produced, accompanied with cyclopentene, 1-pentene, propylene, 1-butene, butadiene and cyclopentadiene. All of these except diallyl were the same main products as obtained in the pyrolysis of diallyl in excess ethylene. In other words, the same mechanism regulate both the reactions of DAO and those of diallyl in the presence of ethylene. 

The thermal decomposition of propylene involves a series of primary and secondary reactions leading to a complex mixture of products. Studies showed that the distribution of pyrolysis products varies considerably with the pyrolysis conditions and the type of reactor used. There is agreement among the studies on propylene pyrolysis that the three major products of pyrolysis are methane, ethylene, and hydrogen. However, there is disagreement on the types and amounts of minor or secondary product species. Ethane, butenes, acetylene, methylacetylene, allene, and heavier aromatic components are reported in different studies, Laidler and Wojciechowski (1960), Kallend, et al. (1967), Amano and Uchiyama (1963), Sakakibara (1964), Sims, et al. (1971), Kunugi, et al. (1970), Mellouttee, et al. (1969), conducted at different conversion and temperature levels. Carbon was also reported as a product in the early work of Hurd and Eilers (1943) and in the more recent work of Sims, et al. (1971). 

One feature of the pyrolysis reaction which is not fully understood is the role of reactor surface. Although the thermal decomposition of propylene has been at least implicitly assumed in some cases to be a completely homogeneous gas-phase reaction, the influence of reactor surfaces has been demonstrated frequently. Factors such as the material of construction, surface-volume ratio of the reactor, and chemical treatment of reactor surface often affect the rate of reaction and/or the product distribution. Many previous investigations are of limited value in determining the role of surface in the pyrolysis reaction. The literature on surface effects contains predominantly qualitative information and often contradictions and anomalies. Early studies... 

Allyl chloride is produced by the thermal chlorination of propylene at elevated temperatures and relatively low pressures. Along with the main reaction, several side reactions also take place. These are shown below. 

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