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Disproportionation of Toluene to Produce Benzene and P- Xylene - Areview

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Disproportionation of Toluene to Produce Benzene and P- Xylene - Areview 10urnal of Scientific & Industrial Research Vol. 60, April 200 I , pp 3 19-327 Disproportionation of Toluene to Produce Benzene and p- Xylene - AReview Abdal Kareem MA* , Shri Chand' and 1M Mishra Department of Chemical Engineering, University of Roorkee, Roorkee- 247 667 Fa x: +91-1 332-73560 An increasing demand for benzene and xylenes as raw materials for polyester fibres and sy nthetic res ins in the rece nt past has stimulated newer developments in converting toluene to benzene and xylenes. Among the xylenes, fI-xylene is or considerable importance as raw material for terephthalic acid and dimethyl tereph th alate manufacture, required for polyester fibre and for the synthesis of vitamins and other pharmaceuticals. The pape r reviews the ac ti vity and se lecti vity (towards fI- xylene production) of various catalysts viz., amorphous SiO/ Alp" X- and Y-zeolites , mordenite, faujasite, HZSM-S in its unmodified , modified, as well as ion exchange forms. The effecls of preparatiun method uf the catalys ts, surface ac idity, shape se lecti vity, and crystal size orthe zeo lites as well as temperature, pressure , and carri er gas on the activilY and selectivity for p-xylene is discussed here. The performance of perovski te based oxides as prospective ca tal ys t is also exp lored. Introduction also as important feed stock to the petrochemical in­ dustry. The large amounts of toluene produced Disproportionation reaction is one of the recent worl dover do not find much use as a raw material fo r techniques for the conversion of less valu able toluene petrochemical industry except for the productions of to more valuable benzene and xy lenes. The impor­ the TNT (trinitrotoluene, an explosive material) and tance of these compounds as feed stocks fo r petro­ benzoic acid which require relati vely smaller amounts. chemical industry need not be stressed. Bulk of the toluene produced is used as a reagent and The relevance of disproportionation process, espe­ as a solvent for paint industry. cially with reference to our country is enormous when This reaction to produce equimolar amounts of ben­ we look into the avai lable toluene as well as the short zene and xylene is a well known reaction. Equil ib­ supply of benzene and xylene for the petrochemical rium constants for this reaction are not changed sig­ industry. In the recent past, there has been a phenom­ nificantly by shifting from liquid to vapour phase or e nal growth in the use of benzene and xy le nes in or­ by large changes in temperature I.:! . The reaction is ganic synthesis. Benzene is the aromatic hydrocarbon normally carri ed out in the presence of hydrogen. Tol u­ having divergent uses fo r the production of petro­ ene and hydrogen are mi xed together and are passed chemicals, synthetic fibers, sy nthetic detergents, plas­ over a catalyst. Hydrogen is employed to suppress tics, elastomers, phenolic resins, lube oi l additi ves, crackin g and to preserve the activity of the catalyst. and in secticides. Since hydrogen is also a compone nt in the feed stream. Paraxylene finds its use in the production of dime­ toluene will have a tendency to combine with hydro­ thyl terephthalate and terephthalic acid which are in­ gen in the presence of a cata lyst to give benzene and termediates in the production of polyester fibres, and methane, i.e., dealkylation of toluene. Some amount of tri-and tetramethylbenzene may * Author to whom all the co rrespo nden ces shuu ld be add ressed ** AI-Balqa' Applied University, AI-Husn Pulytechnic. Irbid, Jor- also be formed during toluene disproportionation with dan certain catalysts. Tri-methyl benzene are formed by 320 J SCI INO RES VOL 60 APR IL 200 1 di sproportionation of xy lene formed and tetTameth yl dry air, dealuminated mordenite treated under we t air benzene are fo rmed by di sproporti onatio n of trimeth yl and a sa mpl e of fiu ori na ted al umin a have also been benzene thus fo rmed. Thu s, di sporporti onation of reported II. produ cts is al so envis aged. Apart from th ese side re­ ac ti ons, some amount of xyle nes also isomerize to give For all the mordenites, the reacti on ra te in creases eth yl benzene. In th e di sproporti onati on process, theo­ with increase in hydrogen pressure. However, with reticall y there is no loss of hydrocarbo ns as th ere is fiu orin ated alumina, the tolu ene disp roporti onation no methane fo rma ti o n, as seen in the case of ra te does not change with the hydrogen pressu re. hyd rodealkyl ati on process. 95-96 vo lume per cent of Effect of Zeolite Preparation Method liqui d products are obtain ed in co ntrast to th e Doubl e structure catalys ts comprisin g ZS M-S and hydrodealkylation process where the li qui d yield is boros ili cate cores were prepared 12 with a shell made limi ted to about 82 vo lume per cent, due to the ac­ of silicate with no acidic properties and havi ng a uni­ companyin g loss of th e meth yl group in th e fo rm of form po re size of 6A. Thi s catalyst showed hi gher 1)­ meth ane. selecti vit y in comparison to ZS M-5 Iy pe catalysts at The disproportionati on process is usuall y carried th e sa me conversion level. The resul ts suggest that jJ­ our in the vapour phase over solid acid catalysts such selec ti vity could be enh anced by 'electively in acti­ as sili ca-alumina and zeo li tes at elevated temperatures. vating th e ex ternal acid sites withou t chang ing the pore Mordenite, fa uj asite3 and va ri ous form s ofmo r de ni te~ mouth size. The catalysts were prepared by the kn own in cl uding dealuminized modi fica ti ons"' are know n to method". be superior in many respects to silica-alu mi na. Oth er Doubl e structure ca talys ts were prepared by mi x­ zeoli te ca talys ts which in clude mordeni te 6.7 , rare-earth in g Na-ZS M-5 and Na-borosili cate seeds wi th fumed exchanged X-zeolites 8,9 and catio n exchanged Y ­ sili ca, NaOH, distill ed water and TPAB r, an d sti rrin g zeolites 8, 10 possess high acti vity. Unfo rtu nately, these the mix ture until it became homogeneous (pH 10. 24h catalysts deacti va te qu ic kl y due to coke depos ition and crystalli sation done at l700 e for 48h). The fina l within a few hours on stream and also show low se­ double stru cture catalys ts of ZS M-5 type cores and lecti vity for di sporporti onati on. Us in g ZSM-5 zeolite sili calite shell (weight rati o I: I of shell to core), H­ as cataly st, the product distri buti on can be directed to ZSM-5/sili ca li te, and H-borosili cate/sili ca lite were the selective fo rm ation of p-xylene, the mos t va lu ­ prepared by a simil ar procedu re. abie isomer, modi fyi ng the shape selecti vity of the zeo­ lite. The di sproporti onati on of tolu ene catalyzed by zeolites of th e pentasi l type has already been in vesti­ Subsequentl y the efi'ect of various pa rameters such gated by many researc h e r sl~ -16. It was shown th at the as temperature, pressure, surface acidity, crys tallin­ product di stribution in thi s reacti on system is particu­ ity, SiO/AI 0 rati o, and carrier gas on the selective 2 J larl y sensiti ve to the interacti on of intracrystalline dif­ disporportionation of toluene to produce be nze ne an d fusion and reacti on. Pentas il zeo li tes were generally p-xylene is summarized. prepared by the crystall izati on of alumi nosi li cate gel in the presence of organi c template. The common templatin g agents used in the synthesis are nitrogen Effect of Pressure containing organic bases such as tetrapropyl-ammo­ nium hydroxide/halidel7 and 1-6-diaminohexanelx. 1- I9 The rate of disproportionation reaction is usually 3-diaminopropane . Pentasil zeolites can also be pre­ reported to increase with increasing pressure of th e pared in the absence of an organic templating agent 20· reaction. The effect of hydrogen pressure on toluene 21 d isproporti onation using different samples of mordenite viz., ammonia exchanged mordenite, The catalytic activity of pentasil zeolites in the mordenite dealuminated wi th HCI and treated under di sproportionation of toluene at near steady stale af- KAREEM el at.: DISPROPORTIONATION OFTOLUENE 321 ter initial partial deactivation is almost proportional activity is also related to its lower crystallinity in com­ to the AI-content, independent of the nature of the tem­ parison to another sample with SiO / AI 0 rati o of 15 2 2 3 plate used for zeolite synthesis. The ratio of benzene and crystallinity 100 per cent ., ... to xylenes in the products depends only on toluene conversion, independent of composition or synthesis With the decrease in silica-to-alumina ratio, the drop mode of zeolite and of reaction temperature. in selectivity to toluene disproportionation may be be­ cause of different secondary reactions favoured by a higher a lumina content (tolue ne and xy le ne Effect of Surface Acidity of Zeolites dealkylation).
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