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Gas Phase Toluene Isopropylation Over High Silica Mordenite

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Gas Phase Toluene Isopropylation Over High Silica Mordenite J. Chem. Sci., Vol. 122, No. 4, July 2010, pp. 613–619. © Indian Academy of Sciences. Gas phase toluene isopropylation over high silica mordenite SREEDEVI UPADHYAYULA Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110 016 e-mail: [email protected] MS received 12 March 2009; revised 11 June 2009; revised 27 July 2009 Abstract. Mordenite (HM) catalysts with three different Si/Al ratios were compared for their activity and selectivities in gas phase toluene isopropylation with isopropanol. Catalyst with Si/Al ratio 44⋅9 of- fered better cumene selectivity, hence, it was chosen for detailed kinetic investigations. The influence of various process parameters like temperature, time-on-stream, weight hourly space velocity (WHSV), reactant mole ratio on this catalyst activity are discussed. The cymene selectivity was found to increase with reaction temperature and passed through a maximum at 473 K. The deactivation with time-on- stream is almost negligible. Lower isopropyl alcohol concentration in the feed improved cymene selecti- vity. The conversion and selectivity to cymenes were compared with those of the large pore beta catalyst. The rate constant and activation energy were found to be 7⋅34 m3/kg h and 41⋅84 kJ/mol, respectively us- ing homogeneous kinetic model. Keywords. Mordenite; cymene; p-cymene; kinetics; isopropylation; toluene; silica. 1. Introduction HY produced only cymenes, HZSM-5 catalysts gave a very high proportion of n-propyl toluene. Parikh et In recent times, considerable attention is given on al4 studied the same reaction over zeolites varying in the selective synthesis of para-dialkylbenzenes, like pore system, crystal size after silylation and pro- xylene, ethyl toluene, diethyl benzene via alkylation posed a more realistic mechanism on ZSM-5. Guo et reactions catalysed by shape selective zeolites. Iso- al5 investigated the alkylation of toluene by propyl- propyl toluene (cymene), in particular, the para- ene over HZM-12 zeolite and obtained cymene iso- isomer of it, is one such commercially important mers with a near thermodynamic equilibrium dialkylbenzene. It is used as an important starting composition. Cejka et al6 investigated the factors material for the production of a range of intermedi- controlling iso-/n- and para-selectivity in the alkyla- ates and end products, such as cresols, isopropyl tion of toluene with isopropyl alcohol on molecular phenols, fragrances, pharmaceuticals, herbicides, sieves possessing different acidity (Al- and Fe- heat transfer media, etc. silicates) and structure type (Y, mordenite and MFI The alkylation of toluene over zeolites is normally structure). The n-propyl toluene is a result of prod- considered to follow the well-known Friedel–Crafts uct selectivity on the MFI type catalysts. It comes mechanism which evidently proceeds by direct out easily, whereas the formation of stable secon- ortho-para attack of the ring followed by positional dary carbocation is favoured, which should give iso- 1,2 isomerisation producing the meta isomer. Modifi- propyl toluene. However, due to the size constraints, cation of zeolites can alter the chemical properties n-propyl selectivity is increased. Toluene isopropy- of the catalytic center so that it ceases to partially or lation on beta zeolites has been reported by Reddy et completely facilitate isomerisation, thus increasing al.7 These zeolites were observed to be highly stable the selective yield of the important para-isomer. with no shape selectivity. Witchterlova et al8 also 3 Fraenkel and Levy studied toluene isopropylation studied the selective formation of p-cymene on Al reaction on medium pore HZSM-5 zeolites differing and Fe silicates. in crystal size and morphology and proposed a reac- From the above discussion, it is evident that tion mechanism. They also compared the product although the reaction has been studied over mor- distribution of the reaction on these medium pore denite catalysts for the purpose of comparing with zeolites with large pore HY and HM. While HM and other catalysts, a detailed study of this reaction over 613 614 Sreedevi Upadhyayula high silica mordenite has not been reported. Mor- Hence, pseudo first order kinetics were investigated denite, a large pore zeolite has shown promise in based on isopropyl alcohol. The term ‘conversion of acid catalysis of many hydrocarbon reactions in- IPA’ is defined as IPA conversion (mol%) = (moles volving large aromatic molecules. The pore size of of IPA consumed/moles of IPA fed per unit time) × mordenite is near to the kinetic diameters of toluene 100. and cymene molecules. However, it has not been in- The terms toluene conversion, cymene selectivity, vestigated thoroughly in this reaction by earlier p-cymene selectivity are defined as follows: workers, especially with varying Si content/acidity. Hence, this work was taken up to study the kinetics Toluene conversion (wt%) = [(toluene in product)/ of this reaction in detail over this zeolite and the toluene in feed] × 100. effect of various parameters on the yield and cy- Cymene selectivity (wt%) = [(cymenes in product)/ mene selectivity. total aromatics (excluding toluene) in product] × 100 p-cymene selectivity (wt%) = [(yield of p-cymene)/ 2. Experimental total yield of cymenes] × 100. 2.1 Materials Based on the product distribution pattern shown in table 1, the reaction scheme of toluene isopropyla- The various mordenite samples used in this investi- tion with IPA over the HM catalysts may be repre- gation were obtained from M/s Süd Chemie AG, sented as Germany. The beta zeolite used for comparative Main reaction: study was supplied by PQ Corporation, USA. The toluene + IPA → p-cymene + water. catalyst powder was pelletised, crushed and sieved Secondary reactions: to 30–50 mesh particles before loading in the reactor. p-cymene → m-cymene (isomerisation) All the samples used in this study have been p-cymene + IPA → 2,4-diisopropyl toluene found to be highly crystalline as revealed by XRD (DIPT) + water (cymene isopropylation) studies. The Si/Al ratios of the three mordenite cata- 2,4-diisopropyl toluene + toluene → 2p-cymene lysts are 9⋅9, 22⋅5 and 44⋅9 and these are designated (transalkylation of DIPT) as HM-1, HM-2 and HM-3 respectively. The Si/Al p-cymene → toluene + propylene ( cymene deal- ratio of the beta catalyst was 13. kylation) Toluene and isopropyl alcohol (IPA) used in the DIPT → p-cymene + propylene (dealkylation of present study were obtained from E Merck (India) DIPT) Ltd. and Qualigens Fine Chemicals, India respec- Toluene → benzene + xylene (toluene dispropor- tively and were of ‘Analytical Reagent’ grade. tionation) IPA → propylene + water (isopropyl alcohol de- 2.2 Procedure hydration) Propylene → aliphatics (oligomerisation) The experiments were conducted in a down flow fixed × –3 bed reactor (15 mm i.d.) with 2 10 kg of catalyst. 3.1 Effect of Si/Al ratio on activity of HM Nitrogen was used as the carrier gas. Toluene to isopropyl alcohol mole ratio in the feed was main- Variation of Si/Al ratio is expected to influence the tained at 6 : 1 during the runs. A feed flow rate of –3 acidity characteristics of mordenite catalysts, which 6⋅8 × 10 kg/h and more was used for the kinetic in turn relates to their activity in toluene alkylation study. The products of the reaction were analysed us- with isopropyl alcohol. Table 1 shows the effect of ing flame ionization detector (FID) on a SHIMADZU variation of Si/Al ratio on the product distribution. 15A Gas chromatograph fitted with a ‘CP-Xylenes’ The cymene selectivity, in general, increased with capillary (50 m, 0⋅32 mm i.d.) column. the increase in Si/Al ratio until an optimum ratio of 22⋅5, while a simultaneous reduction in aliphatics 3 Results and discussion content was observed. This shows that side reactions yielding aliphatics were suppressed, improving the In the present study, toluene was taken in excess and alkylation selectivity. Product distribution shows isopropyl alcohol (IPA) was the limiting reactant. that there are considerable amounts of benzene and Toluene isopropylation over high silica mordenite 615 Table 1. Comparison of activities of mordenite samples with varying Si/Al ratio.a Catalyst HM-1 HM-2 HM-3 Si/Al ratio 9⋅9 22⋅5 44⋅9 Product yields (wt%) Aliphatics Trace Trace Trace Benzene Trace Trace Trace Toluene 77 77 77 C8 aromatics Trace Trace bd* Cumene Trace Trace Trace p-Cymene 6 6 10 m-Cymene 15 15 11 o-Cymene bd* bd* bd* DIPTs# 1 Trace 2 Total cymenes (%) 21 21 21 Cymene sela (%) 91 93 92 p-Cymene selb (%) 30 30 48 aConditions: temperature = 453 K; IPA/toluene mole ratio = 1 : 6; WHSV = 3 h–1. *Below detectable limits. #Diisopropyl toluenes. cumene, indicating that the dealkylation/realkylation the catalyst HM-3. The yield and total cymene selec- reactions cannot be neglected. On the catalyst with tivity was observed to increase with increase in tem- Si/Al ratio of 44⋅9, the yield of cumene was negligi- perature up to an optimum value of 473 K. ble. From table 1, it is also observed that when the Thereafter, a slight decrease in the cymene yield and Si/Al ratio is increased from 22⋅5 to 44⋅9, there is a selectivity was noticed when the reaction tempera- slight drop in the yield and selectivity of cymenes ture was raised further. due to increase in the yield of diisopropyl toluene. At higher temperatures, dealkylation reactions On catalysts with lower Si content, dealkyla- become pronounced and hence yield and selectivity tion/transalkylation of DIPT with toluene may not of total cymenes are reduced. Also, as the tempera- be taking place to the required levels. As a result, ture is increased from 433 to 493 K, the selective DIPTs are retained in the product. Whereas, DIPTs yield of p-cymene reaches 30% which is nearer to are converted to cymenes over the catalysts with op- the equilibrium cymene distribution value of around timum Si content.
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