Steam Dealkylation

Petroleum-derived benzene is commercially produced by reforming and separation, thermal or catalytic dealkylation of toluene, and disproportionation. Benzene is also obtained from pyrolysis gasoline formed ia the steam cracking of olefins (35). 

Dealkylation also can be effected by steam. The reaction occurs at 600-800°C over Y, La, Ce, Pr, Nd, Sm, or Th compounds, Ni-Cr203 catalysts, and Ni-Al203 catalysts at temperatures between 320-630°C. Yields of about 90% are obtained. This process has the advantage of producing, rather than using, hydrogen. 

Examples 31-33 are for the steam dealkylation of toluene. The reaction is complicated, and there are several possibilities for a rate-determining step. For Example 31 Kochloefl 103) found that water adsorbs better than toluene, suggesting the possibility of Step 11 for H2O, for which log L= 14. Grenoble 104) reported several transition-metal catalyzed dealkylations of toluene one of his systems is described in Example 32. The orders in the various reactions reported vary in several the order in toluene is negative. But log L = 8 for Step 11 for toluene in Example 32 is rather low. It is just possible that in Examples 31-33 that a modification of Step 5 should be considered. The modification would take into account the fact that HjO and toluene compete for sites and the entropy of reaction is not zero, contrary to the usual entropy assumption made for Step 5. 

Di-S-ethers of 3,5-dimercapto-l,2,4-thiadiazole are generally very stable and can be steam-distilled. The weakly basic diethyl ether is dealkylated by alcoholic potassium hydrogen sulfide, but resists attack by alcoholic ammonia at 200°. Hydrogen chloride under pressure causes complete degradation.144... 

Steam Dealkylation. - The steam dealkylation of aromatic hydrocarbons, such as toluene ... 

It follows that most work reported over the last few years on steam reforming and methanation has been concerned with nickel catalysts. The following sections will therefore deal mostly with nickel-based catalysts, particularly those which have some importance in commercial practice. Particular stress will be laid on work, with which the author has been associated, concerned with steam reforming and methanation catalysts but mention will also be made of parallel studies from other laboratories. In sections on the catalysts for steam dealkylation and steam reforming of methanol, where catalyst selectivity is a prerequisite, other types of catalyst will also be discussed. 

The alternative process is steam dealkylation reaction [20] as shown below CgHjCHa + H2O CeHg + gaseous products... 

Table 1 Activities of precious metals for steam reforming and steam dealkylation...

Dealkylation and Steam Reforming. Precious metals and nickel are effective for the steam dealkylation of alkylaromatics ... 

Kochloefl compared Rh, Pt, and Pd for toluene steam dealkylation (Table 1) and studied Rh on different supports insulator oxides gave high activity while... 

C. Aromatic Hydrocarbons, Steam Dealkylation.—Research on steam dealkylation has thrown light on the mechanism of reforming. Rabinovich and other Russian workers, following their earlier research in this area which is summarized by Grenoble, concluded that the toluene reaction with steam over precious metal/alumina catalysts proceeds by simultaneous-consecutive reactions ... 

A. Reactivity of Hydrocarbons.—In their work on steam dealkylation over Rh catalysts Kikuchi, Mori, and their co-workers determined the relative... 

As a result, a high inlet catalyst temperature is required. However, high temperature also increases the rates of nonselective thermal reactions and dealkylation reactions, which form benzene and toluene by-products. In particular, as temperature is increased, the rate of benzene formation increases significantly relative to the rate of styrene formation. This means there is an effective upper limit to the inlet temperature if high styrene selectivity is a required criterion. Reaction temperature is generally adjusted by changing either the steam temperature or the steam-to-oil ratio. 

At the process level, steam cracking consists of the pyrolysis of saturated hydrocarbons from natural gas or petroleum fractions, in the presence of steam. Before going into the technological study of these units, it is first important to examine the physicochemical properties of the reactions involved (dehydrogenation, pyrolysis, dehydrocydization and dealkylation). 

Dealkylation processes are hig hydrogen consumers. Many investigations have been and are being conducted to replace it with steam according to the following main reaction, which would occur in the presence of a catalyst based on precious metals on alumina, aronnd 500 to 550 C, and at 5.10s Pa absolute ... 

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