Aromatics formation

Wang, H. and Frenklach, M., A detailed kinetic modeling study of aromatics formation in laminar premixed acetylene and ethylene flames. Combust. Flame, 110,173, 1997. 

Contributions of three types of Ga sites to propane conversion into aromatics were examined by using model catalysts, i.e., gallosilicate of MOR structure with deposited GaaOs particles. The rates of propane conversion and aromatics formation were correlated with the densities of three types of Ga sites determined by NH3-TPD, and it was shown that the propane conversion and the aromatics formation were limited by Ga sites on Ga20j surface. 

The propane aromatization was conducted under the differential condition by using Ga203/Ga-MOR catalysts thus characterized. The contributions of L, HI, and H2 sites to the propane conversion and the aromatics formation were estimated by assuming that the observed reaction rates are the sum of the reaction rate on each site which is equal to the product of the turnover frequency (TFij) and the amount of active sites per weight of catalyst (Aj) ... 

Table 1 Turnover frequencies of propane conversion and aromatics formation over L, HI and H2 sites of Ga203/Ga-M0R catalysts.

The incorporation of a ZSM-5 class zeolite into a ruthenium Fischer-Tropsch catalyst promotes aromatics formation and reduces the molecular weight of the hydrocarbons produced. These composite catalysts can produce a high octane aromatic gasoline in good yield in a single step directly from synthesis gas. 

C10-C14 long paraffin dehydrogenation is a key-step for linear alkyl benzene (LAB) production. However, this reaction, which requires monofunctional catalysis, is implemented on Pt-Sn catalysts deposited on controlled acidity alumina. It is generally associated with several secondary reactions, among which aromatic formation is extremely problematic it is catalyzed by a metallic phase (M) or by residual support (A) activity. Indeed, on the one hand, these arylaromatics are very good coke precursors and are consequently responsible for a large part of the... 

Under dehydrogenation conditions (385 °C ratio H2/HC = 4), an increase in the selectivity for aromatics with PtSn,(/Si02 catalyst has been observed. The increase in aromatic selectivity with tin content seems to be due to a geometric effect, favoring aromatic desorption. When the catalyst contains only small amounts of tin, an important poisoning by coke has been observed. As a consequence, it is possible that coke comes from adsorbed aromatic degradation. If aromatic formation starting from olefins had already and previously been proposed in the literature, their formation mechanism was still unknown. The coexistence of two possible dehydrocycHzation mechanisms has been proposed (Scheme 3.24). 

The data ln Table III show that the concentration of aromatics in the ZSM-5 gasoline fraction increases. As the data in Table V show, the increase is not due to aromatics formation, but rather due to the concentration of the aromatics in a smaller amount of gasoline. 

Padwa and co-workers (120-122) also utilized this carbonyl ylide cycloaddition strategy to advance to the aromatic pterosin family of compounds. The same intermediates used to approach the nonaromatic illudins and ptaqualosides are also useful for aromatic formation through cleavage and dehydration (Scheme 4.62). 

Examples of thermodynamic equilibria are shown in Table IV. The conversion of five- and six-member ring naphthenes to aromatics is quite favorable. Methylcyclopentane conversion is the least favorable. Equilibria for aromatic formation improves with carbon number. For five-member ring naphthenes, the largest improvement occurs between the six-carbon... 

Fries rearrangement of aromatic formate esters suggests that phenols are the major products (.24) obtained in the reaction. As poly(p-hydroxystyrene) is remarkably clear in the deep UV, it is likely that poly(p-formyloxystyrene) will not suffer from the same problem of photostabilization upon exposure as was the case with poly (p-acetoxystyrene). This expectation was confirmed by our study of the photo-Fries reaction of p-cresyl formate no ortho rearranged product was isolated after reaction while p-cresol and a small amount of starting material were obtained. 

The octane number improvement obtained by isomerization of paraffin hydrocarbons is not great since the amounts of the more highly branched paraffins formed at equilibrium are small at the temperatures employed in catalytic reforming (5). Naphthene isomerization, on the other hand, plays a more important role in reforming. In most naphthas about 50% of the naphthene hydrocarbons are of the cyclopentane type (4) so that in order to obtain the maximum aromatic formation, isomerization of these rings to cyclohexane rings must be promoted by the catalyst. 

If this catalyst were to be used in the temperature range of 850° to 950° F. where extensive aromatic formation may be obtained, one would be operating in a range far to the right of the peak isomer yield and in a region where very severe cracking of paraffin hydrocarbons occurs. In fact, it is questionable whether much of the paraffin hydrocarbons in a naphtha would survive such treatment. 

Catalytic reforming has become the most important process for the preparation of aromatics. The two major transformations that lead to aromatics are dehydrogenation of cyclohexanes and dehydrocyclization of alkanes. Additionally, isomerization of other cycloalkanes followed by dehydrogenation (dehydroisomerization) also contributes to aromatic formation. The catalysts that are able to perform these reactions are metal oxides (molybdena, chromia, alumina), noble metals, and zeolites. 

Experimental work using a pulse-quench catalytic reactor650 to probe transition between induction reactions and hydrocarbon synthesis on a working H-ZSM-5 catalyst has resulted in the suggestion that stable cyclopentenyl cations are formed during the induction period from small amounts of olefins formed in an induction reaction 647 One study reports a surprising observation, namely, enhanced aromatic formation over the physical mixture of Ga203 and H-ZSM-5 (1 1) (18.2% of benzene and methylbenzenes).651... 

H. Wang and M. Frenklach. A Detailed Kinetic Modeling Study of Aromatics Formation in Laminar Premixed Acetylene and Ethylene Flames. Combust. Flame, 110 173-221,1997. 

Effects of Tin and Thiophene on Bicyclic Aromatic Formation over Platinum Catalysf b... 

The analysis of product gasoline indicates an increased aromatic content with the ZSM-5 but this arises solely from a concentration effect [1 6]. The gain in aromatics with REHY, however, is greater than can be explained by concentration alone and suggests additional aromatic formation from more extensive hydrogen transfer reactions. 

Aromatics may be formed by a bifunctional pathway when an acidic alumina support is used. Furthermore, it has been reported that the bifunctional pathway leads to aromatics at least 20 times faster than the metal only pathway (84). In addition, the bifunctional pathway is more selective for aromatics formation than the metal cyclization pathway (84). 

The suggested formation of an olefinic compound as shown in equation 1 would be consistent with the low conversion-high space velocity experiments which reveal substantial initial olefin product formation. It is well established that olefins undergo reactions over zeolites such as ZSM-5 resulting in both paraffin and aromatic formation. There has been extensive discussion of the various reaction mechanisms which may be operative in the disproportionation reactions of olefins over ZSM-5. Thus, under reaction conditions of longer space times and/or higher reaction temperatures, it would be anticipated that any initially formed olefins would react further to yield paraffins and aromatics. Clearly, subsequent olefin reactions could also include catalyst promoted reactions with acetylene. 

Alcohol and aldehyde decarbonylation on Rh(l 11), activation of C-H, C-C, and C-0 bonds, 345-353 Alkane dehydroeyelization with Pt-Sn-alumina catalysts aromatic formation, 120 preparation condition effect, 119... 

The goal of the MTO process is to convert methanol to light olefins, in particular ethylene, propylene and butenes. The key by-products of the reaction include the co-product water, C5+ hydrocarbons such as aromatics and heavier olefins, coke that remains on the catalyst at process conditions and light paraffins that are the primary sink for hydrogen lost during aromatic formation. Small amounts of H2 and COx are also typically observed in the MTO product, although these by-products could arise from feed and product decomposition on the reactor walls and internals at the temperatures that are typically used. 

For instance the formation of aromatics in the gasoline fraction increases significantly with the reactor temperature, while the rate of aromatics formation remains relatively constant, see figure 7. 

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