Initiation hydrocarbons

Similar chemical steps occur in the ambient air and in laboratory smog chamber simulations. Initially, hydrocarbons and nitric oxide are oxidized... 

The competition between dilution of NMHQ (here [NMHCJq represents the initial hydrocarbon concentration) and its reaction with HO to generate an oxidant molecule enters through the dimensionless parameter S fcd/k25[HO ] that compares the rate of the HO reaction to the rate of dilution. Also important is the relative reactivity of the oxidation product PROD to the parent hydrocarbon as defined by the dimensionless parameter If the oxidation products... 

Under the conditions where the chain oxidation process occurs, this reaction results in chain termination. In the presence of ROOH with which the ions react to form radicals, this reaction is disguised. However, in the systems where hydroperoxide is absent and the initiating function of the catalyst is not manifested, the latter has a retarding effect on the process. It was often observed that the introduction of cobalt, manganese, or copper salts into the initial hydrocarbon did not accelerate the process but on the contrary, resulted in the induction period and elongated it [4-6]. The induction period is caused by chain termination in the reaction of R02 with Mn"+, and cessation of retardation is due to the formation of ROOH, which interacts with the catalyst and thus transforms it from the inhibitor into the component of the initiating system. 

As alkylaromatic hydrocarbon (toluene, p-xylene, etc.) is oxidized, aldehydes appear radicals and peracids formed from them play an important role. First, aldehydes react rapidly with the Co3+ and Mn3+ ions, which intensifies oxidation. Second, acylperoxyl radicals formed from aldehydes are very reactive and rapidly react with the initial hydrocarbon. Third, aldehydes form an adduct with primary hydroperoxide, which decomposes to form aldehyde and acid. 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

A 10-yd3 soil sample was excavated from the site, blended, and characterized for initial hydrocarbon content and nutrient content. The reactor was filled with soil compacted to field density (Figure 12.10). The tank at the bottom was filled with water nutrients and surfactants. Water from this tank was sprayed over the top of the soil at a rate that maintained aerobic conditions. A significant amount of LNAPL was initially released from the soil, which required additional air to be pumped into the well points to maintain favorable growth conditions. After 105 days of operation, more than 87% of the total aliphatics and 89% of the total aromatics were removed. 

It is interesting to review a general pattern for oxidation of hydrocarbons in flames, as suggested very early by Fristrom and Westenberg [29], They suggested two essential thermal zones the primary zone, in which the initial hydrocarbons are attacked and reduced to products (CO, H2, H20) and radicals (H, O, OH), and the secondary zone, in which CO and H2 are completely oxidized. The intermediates are said to form in the primary zone. Initially, then,... 

The gaseous oxidation of n-alkanes can, in suitable circumstances, yield substantial amounts of O-heterocycles of the same carbon number as the initial hydrocarbon. A comparative study has been carried out of the formation of O-heterocyclic products during the combustion of n-butane, n-pentane, and n-hexane. The way in which the yields of such compounds vary with reaction conditions has been investigated. As a result of the optimization of the amounts of O-heterocycles it has been possible to obtain maximum yields of these compounds of up to 30% from n-pentane but only about 10% from n-butane and n-hexane. An attempt is made to account for the observed differences in the amounts and nature of the O-heterocyclic products formed from the three n-alkanes. 

The successful isolation of substantial amounts of O-heterocycles will depend to a considerable extent on the design of the reaction vessel. Thus, for example, high yields of such compounds are produced in the falling cloud reactor under conditions corresponding to cool-flame combustion (10, 11). Comparatively little information is available, however, as to the variation in yield with molecular structure of the initial hydrocarbon. 

At the end of reaction the initial hydrocarbon concentration is still important, but there are also many other hydrogen donors. Thus, in the absence of oxygen, reactions of alkoxy radicals will lead to aldehydes by pyrolysis and to alcohols by abstraction. Let us examine these two possible reactions for each alkoxy radical. 

However, for toluene and p-xylene, the rates are constant up to 70% conversion as the NaBr/cobalt ratio increases (Figure 5). The rate seems independent of hydrocarbon concentration during the oxidation, although the steady oxidation rate is exactly proportional to initial hydrocarbon concentration. Separate experiments showed that the rate increases if benzaldehyde is added but decreases as hydrocarbon and bromide ion are consumed or water is added. 

From the authors point of view, introduction of this elementary stage to mechanisms of saturated hydrocarbon gas-phase oxidation [26] makes clearer the mechanisms of such complex chemical processes, especially at olefin formation with the same number of carbon atoms as in the initial hydrocarbon. 

The concept provides for adiabatic steam catalytic one-stage conversion with carbon dioxide removal through short-cycle heat-free adsorption, followed by return of a part of product fraction to conversion. This option assumes maximum usage of initial hydrocarbon raw material to produce the HMM. 

The activity of granulated rhenium catalyst, obtained from copolymer carbonisate, has been investigated in reactions of cyclohexane or ethylbenzene dehydrogenation in bed -packed quartz tube reactor at the plug flow conditions at temperatures from 650 to 900 K, the reagents feed of 30 - 100 ml/min and initial hydrocarbons partial pressure of 0.5 kPa. 

The reaction mechanism for the conversion of methanol to hydrocarbons over molecular sieve catalysts has been extensively investigated over the past 25 years. It is widely accepted that methanol conversion initially proceeds through equilibration with DME. Early work with ZSM-5 showed that light olefins are then the initial hydrocarbon products, followed by heavier olefins, paraffins and aromatics (Figure 12.5) (2). 

Yur ev and Magdesieva41 found a very convenient procedure to obtain selenophene and its homologs by the reaction of butylenes with metallic selenium at 580°C. The selenophene ring closure is most favored when the four-carbon chain of the initial hydrocarbon has a central double bond and the doubly bonded carbons carry as many substituents as possible. 

By connecting the apparatus directly to a time of flight mass spectrometer. It was found that the Initial hydrocarbon product was Isobutylene, not Isobutane. After about 10 seconds, however, the gas was mainly Isobutane, and olefin was no longer detected. 

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