Xylenes dealkylation

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

Benzene, toluene and / -xylene are the most industrially important aromatic hydrocarbons. Their relative proportions must be adjusted according to the needs of the market, which presently requires more benzene and xylene. Dealkylation of toluene can be carried out by hydrogenolysis with H2 using Cr, Mo or Pt oxides as catalyst precursors (Equation 4). Methyl redistribution in toluene is best carried out around 420°C on ZSM-5 zeolite. Under these conditions, xylene is mainly present as the para isomer (90% selectivity. Equation 5) due to the shape-selectivity of the zeolite. 

The feedstock is usually extracted toluene, but some reformers are operated under sufftciendy severe conditions or with selected feedstocks to provide toluene pure enough to be fed directiy to the dealkylation unit without extraction. In addition to toluene, xylenes can also be fed to a dealkylation unit to produce benzene. Table 20 Hsts the producers and their capacities for manufacture of benzene by hydrodealkylation of toluene. Additional information on hydrodealkylation is available in References 50 and 52. 

A typical catalytic hydrodealkylation scheme is shown ia Figure 3 (49). The most common feedstock is toluene, but xylenes can also be used. Recent studies have demonstrated that and heavier monoaromatics produce benzene ia a conventional hydrodealkylation unit ia yields comparable to that of toluene (51). The use of feeds containing up to 100% of C —aromatics iacreases the flexibiUty of the hydrodealkylation procedure which is sensitive to the price differential of benzene and toluene. When toluene is ia demand, benzene suppHes can be maintained from dealkylation of heavy feedstocks. 

The molecular weight distribution of the feed affects the distribution of the product. If the naphtha is concentrated in the C -Cg range, more benzene and toluene are found in the product. If the feed is weighted to Cg—C q, more xylenes and higher aromatics are found. Some carbon number "shppage" occurs by dealkylation some C s form benzene by losing a methyl group, some CgS form toluene, etc. 

Consuming hydrogen is mainly a function of the number of benzene substituents. Dealkylation of polysubstituted benzene increases hydrogen consumption and gas production (methane). For example, dealkylating one mole xylene mixture produces two methane moles and one mole of benzene it consumes two moles of hydrogen. 

HDA [Hydrodealkylation] A proprietary dealkylation process for making benzene from toluene, xylenes, pyrolysis naphtha, and other petroleum refinery intermediates. The catalyst,... 

Xyloflning [Xylol refining] A process for isomerizing a petrochemical feedstock containing ethylbenzene and xylenes. The xylenes are mostly converted to the equilibrium mixture of xylenes the ethylbenzene is dealkylated to benzene and ethylene. This is a catalytic, vapor-phase process, operated at approximately 360°C. The catalyst (Encilite-1) is a ZSM-5-type zeolite in which some of the aluminum has been replaced by iron. The catalyst was developed in India in 1981, jointly by the National Chemical Laboratory and Associated Cement Companies. The process was piloted by Indian Petrochemicals Corporation in 1985 and commercialized by that company at Baroda in 1991. 

Production of p-xylene via p-xylene removal, i.e., by crystallization or adsorption, and re-equilibration of the para-depleted stream requires recycle operation. Ethylbenzene in the feed must therefore be converted to lower or higher boiling products during the xylene isomerization step, otherwise it would build up in the recycle stream. With dual-functional catalysts, ethylbenzene is converted partly to xylenes and is partly hydrocracked. With mono-functional acid ZSM-5, ethylbenzene is converted at low temperature via transalkylation, and at higher temperature via transalkylation and dealkylation. In both cases, benzene of nitration grade purity is produced as a valuable by-product. 

We have shown that the high selectivity of ZSM-5 in xylene isomerization relative to larger pore acid catalysts is a result of its pore size. It is large enough to admit the three xylenes and to allow their interconversion to an equilibrium mixture it also catalyzes the transalkylation and dealkylation of ethylbenzene (EB), a necessary requirement for commercial feed but it selectively retards transalkylation of xylenes, an undesired side reaction. 

Disproportionation. A chemical reaction in which a single compound serves as both an oxidizing and reducing agent such as the dealkylation of toluene to give benzene (the more reduced product) and xylene (the more oxidized product). 

Benzene and para-xylene are the most sought after components from reformate and pygas, followed by ortho-xylene and meta-xylene. While there is petrochemical demand for toluene and ethylbenzene, the consumption of these carmot be discussed in the same way as the other four. Toluene is used in such a large quantity in gasoline blending that its demand as a petrochemical pales in comparison. Fthylbenzene from reformate and pygas is typically dealkylated to make benzene or isomerized to make xylenes. On-purpose production of petrochemical ethylbenzene (via ethylene alkylation of benzene) is primarily for use as an intermediate in the production of another petrochemical, styrene monomer. Ethylbenzene plants are typically built close coupled with styrene plants. 

The rather low concentration of the desired p-xylene component in the Parex unit feed means a large fraction of the feed stock contains other A8 components that are competing for adsorption sites in the adsorbent zeoHte cages. Due to this typically lean feed, a significant hike in the Parex unit capacity can be obtained by even a small increase in the composition of the p-xylene. Techniques to increase the p-xylene feed concentration include greater dealkylation of the ethylbenzene in the Isomar unit by converting from an ethylbenzene isomerization catalyst to... 

There are two general types of C8A isomerization, defined by the method of converting EB. There is the EB dealkylation type, which converts EB into benzene and ethane, and the EB isomerizahon type, which converts EB into xylenes. The differences between these two categories are addressed in Section 14.4.1.3. 

Dealkylation of the ethyl group of EB is a side reaction that is undesirable if isomerization to xylene is desired. This is an acid-catalyzed reaction that is espe-... 

The presence of metal may catalyze demethylation and can occur to some extent in catalysts where the metal function is under-passivated, as by incomplete sulfiding. This would convert valuable xylenes to toluene. The demethylation reaction is usually a small contributor to xylene loss. Metal also catalyzes aromatics saturation reactions. While this is a major and necessary function to facilitate EB isomerization, any aromatics saturation is undesirable for the process in which xylene isomerization and EB dealkylation are combined. Naphthenes can also be ring-opened and cracked, leading to light gas by-products. The zeolitic portion of the catalyst participates in the naphthene cracking reactions. Cracked by-products can be more prevalent over smaller pore zeolite catalysts. 

The catalysts used for isomerization of Cg aromatics contain an acidic function to perform xylene isomerization and naphthene isomerization for EB conversion to xylenes. Relatively high metal activity is needed to maintain the naphthene/ aromatic equilibrium that allows isomerization of EB. For conversion of EB by dealkylation, an acidic function is required along with metal activity capable of capturing and hydrogenating the ethylene by-product before it can re-alkylate another aromatic ring. 

The catalysts for xylene isomerization with EB dealkylahon are dominated by MFI zeolite. The de-ethylation reaction is particularly facile over this zeolite. There have been several generations of catalyst technology developed by Mobil, now ExxonMobil [84]. The features in their patents include selectivation and two-catalyst systems in which the catalysts have been optimized separately for deethylation of EB and xylene isomerization [85-87]. The crystallite size used for de-ethylation is significantly larger than in the second catalyst used for xylene isomerization. Advanced MHAI is one example. The Isolene process is offered by Toray and their catalyst also appears to be MFI zeoUte-based, though some patents claim the use of mordenite [88, 89]. The metal function favored in their patents appears to be rhenium [90]. Bimetallic platinum catalysts have also been claimed on a variety of ZSM-type zeolites [91]. There are also EB dealkylation catalysts for the UOP Isomar process [92]. The zeolite claimed in UOP patents is MFI in combination with aluminophosphate binder [93]. 

Where the nucleophile is the conjugate base of a primary amine, NH2 can be a leaving group. The method has been used to prepare secondary amines.827 In another process, primary amines are converted to secondary amines in which both R groups are the same (2RNH2 — R2NH + NH3)828 by refluxing in xylene in the presence of Raney nickel.829 Quaternary salts can be dealkylated with ethanolamine.830... 

A cracking process, the dealkylation of alkylbenzenes, became an established industrial synthesis for aromatics production. Alkylbenzenes (toluene, xylenes, tri-methylbenzenes) and alkylnaphthalenes are converted to benzene and naphthalene, respectively, in this way. The hydrodealkylation of toluene to benzene is the most important reaction, but it is the most expensive of all benzene manufacturing processes. This is due to the use of expensive hydrogen rendering hydrodealkylation too highly dependent on economic conditions. 

Dealkylation and isomerization (disproportionation) processes used in the manufacture of benzene and xylenes are discussed in Sections 4.5.2 and 5.5.4. 

Transalkylation and Dealkylation. In addition to isomerizations (side-chain rearrangement and positional isomerization), transalkylation (disproportionation) [Eq. (5.56)] and dealkylation [Eq. (5.57)] are side reactions during Friedel-Crafts alkylation however, they can be brought about as significant selective hydrocarbon transformations under appropriate conditions. Transalkylation (disproportionation) is of great practical importance in the manufacture of benzene and xylenes (see Section 5.5.4) ... 

---