Cracking of alkylaromatics

The published data give a clear picture of effect of structure on reactivity in the cracking of alkylaromatic compounds. The reactivity increases with the size of the alkyl group, and with its branching, as the studies of... 

It seems that other acidic sites are the most efficient for the alkylation of aromatic compounds than for the reverse reaction, the cracking of alkylaromatic compounds [361]. For the forward process, a linear correlation was observed between the activity of decationized Y zeolites and the number of acidic sites corresponding to H0 < + 3.3, whereas for the cracking, the sites corresponding to H0 < —3.0 correlated with the activity. 

The cylization of alkylaromatics over platinum catalysts is usually accompanied by isomerization, hydrogenation and dehydrogenation, fragmentation (i.e., hydrogenolysis and cracking), and other reactions. 

Main reactions in CR processes are dehydrogenation of cyclohexane and alkylcylohexanes, cyclization of alkanes, isomerization of n-parafines, alkylcyclopentanes and alkylaromatics, and hydrocracking. Secondary reactions are the demethylation and cracking of cyclic compounds. 

In 1960, Weisz, Frilette, and co-workers first reported molecular-shape selective cracking, alcohol dehydration, and hydration with small pore zeolites (6,7), and a comparison of sodium and calcium X zeolites in cracking of paraffins, olefins, and alkylaromatics (8). In 1961, Rabo and associates (9) presented data on the hydroisomerization of paraffins over various zeolites loaded with small amounts of noble metals. Since then, the field of zeolite catalysis has rapidly expanded,... 

The relative rates of C C Cf.C alkene cracking is 1 24 192 603. This means that to crack C -C olefins on the small-pore zeolite with significant diffusion constraints, the olefins should first interact with a coke precursor or aromatic precursor. The cracking and dealkylation of alkylaromatics is very fast and leads to ethylene and propylene. Taking into account that the methylation reactions are at least an order of magnitude faster than cracking. 

Other interesting products that can be obtained from waste plastics using combined thermal and catalytic processes are alkylaromatic compounds, which possess industrial applications as automatic transmission fluids (ATF), detergents (linear alkyl benzenes, LAB), and improvers of cetane number in diesel fuels [104]. The process uses as raw material the olefins generated in a previous step of thermal and catalytic cracking, which represent a cheaper source of olefins alternative to the currently existing ones. No special details about the conditions applied for the olefin production are indicated, the emphasis being focused on the alkylation step. Alkylation catalysts comprise conventional Lewis... 

Acidic zeolites are known for their excellent catalytic activity in cracking and isomerization of hydrocarbons (75). In the absence of metal, however, these catalysts rapidly deactivate due to the formation of carbonaceous products, usually referred to as coke. The carbonaceous residues are mainly formed via alkylaromatics and polyaromatics, which are the result of dehydrogenation, cyclization, and further alkylation processes. The coke deposits lower the catalytic activity by site poisoning and eventually also by pore blocking, which inhibits access of hydrocarbon molecules to the acid sites (286). 

Transformations of hydrocarbons promoted by solid metals and their oxides play very important roles in chemical industry [1], Heterogeneous metal-containing catalysts [2] are widely employed for cracking (see, e.g [3]) of oil fractions, oxidation, dehydrogenation, isomerization and many other processes of saturated as well as alkylaromatic hydrocarbons. 

In two papers by Walsh and Rollman [14-C]labelled hydrocarbons were used to study the origin of carbonaceous deposits on zeolites. With feeds composed of an aliphatic + an aromatic hydrocarbon, the initial reaction involved in the formation of coke was the alkylation of aromatics by the olefmic fragments of alkane cracking. Since ZSM-5 and mordenite have the same framework A1 content, it was possible to compare directly the coke yields of these zeolites. Under the same experimental conditions it was found that C deposition on mordenite was almost two orders of magnitude greater than on ZSM-5. The differences were explained in terms of pore size. In the smaller ZSM-5 pore, the alkylaromatics, once formed were prevented from reacting further to produce coke, because of the spacial constraints. 

At low temperature coking occurs rapidly from olefins and from polyaromatics and very slowly from monoaromatics. This is also true from alkylaromatics such as cumene since their cracking into olefins is slow [47]. However a rapid formation of coke occurred during benzene hydrogenation at 80 C on PtUSHY and PtHMOR catalysts. All the coke components resulted from condensation reactions through... 

The first examples of molecular shape-selective catalysis in zeolites were given by Weisz and Frilette in 1960 [1]. In those early days of zeolite catalysis, the applications were limited by the availability of 8-N and 12-MR zeolites only. An example of reactant selectivity on an 8-MR zeolite is the hydrocracking of a mixture of linear and branched alkanes on erionite [4]. n-Alkanes can diffuse through the 8-MR windows and are cracked inside the erionite cages, while isoalkanes have no access to the intracrystalline catalytic sites. A boom in molecular shape-selective catalysis occurred in the early eighties, with the application of medium-pore zeolites, especially of ZSM-5, in hydrocarbon conversion reactions involving alkylaromatics [5-7]. A typical example of product selectivity is found in the toluene all lation reaction with methanol on H-ZSM-5. Meta-, para- and ortho-xylene are made inside the ZSM-5 chaimels, but the product is enriched in para-xylene since this isomer has the smallest kinetic diameter and diffuses out most rapidly. Xylene isomerisation in H-ZSM-5 is an often cited example of tranSition-state shape selectivity. The diaryl type transition state complexes leading to trimethylbenzenes and coke cannot be accommodated in the pores of the ZSM-5 structure. 

Data from colloid analysis show that the concentration of the dispersion medium may be related to the distillable fraction AG400, whereas the concentration of asphaltenes, or the total of asphaltenes and petroleum resins, determines the quantity of coke residue after pyrolysis. That portion of the sample which can be cracked, CR, will usually be determined from die concentration of petroleum resins. The aliphatic side chains of the alkylaromatic system of the asphaltenes have a small influence. The coke residue can be related to the data from structural group analysis which describe the aromatic character of the samples. 

The calculated BDEs for reaction (23) and (24) are around 56 kcal/mol. As was suggested by many experimental studies (Freund 1992 Lewan 1998), most of the n-alkylaromatics and NSO functional groups are much more reactive (have much weaker bonds) and may enhance the over kerogen thermal cracking. Besides the much lower BDEs, the weak linkages may also generate radicals that are more reactive and hence will... 

So, the ethylene production does correlate with coke presence, in particular with aromatics formation as far as the diffusion limitations are not significant. However, it seems that the majority of ethylene is not always formed directly from MeOH [115]. The aromatics and other coke species could be the products of the conversion of primary carbenium ions, which are mobile and could equilibrate each other [28]. This may explain the isotopic distribution in products and retained coke molecules and the coexistence of aromatics and carbenium ions [28], In addition to the coproduction of ethylene with aromatics in olefins interconversion cycle, formation of ethylene via alkylation-dealkylation of methyl aromatics with heavy olefins or with the equivalent carbenium ions like ethyP, propyP, and butyP could be an option. The alkyl aromatics with the side chain length of two carbons or longer are not stable in the pore and dealkylates on the acid sites due to too long residence time and steric hindrances. This may lead to formation of ethylene, other olefins, and alkylaromatics with different structure, namely PMBs [129]. In other words, the ethylene is formed via interaction of the carbenium ions like ethyP, propyP, and butyP formed from MeOH or heavy olefins with aromatics and other coke precursors followed by cracking and in a less extent by a direct alkylation of PMBs with methanol. The speculation is based properly on analysis of the prior arts and is not contradictory with the concept of the aromatic cycle for ethylene formation. 

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