of Chemical Engineering SYNTHESIS OF CUMENE BY TRANSALKYLATION OVER MODIFIED BETA ZEOLITE : A KINETIC STUDY

In the present study, transalkylation of 1,4-diispropylbenzene (DIPB) with benzene in the presence of modified beta zeolite was performed to produce cumene in a fixed bed reactor. Beta zeolite was exchanged with cerium in order to modify its catalytic activity. Activity of the modified catalyst was evaluated in the range of temperature 493K–593K, space time 4.2 kg h/kmol–9.03 kg h/k mol and benzene/1,4-DIPB molar ratio 1–15 to maximize the reactant conversion and selectivity of cumene. The activity and selectivity of the modified catalyst was found to increase with increase in cerium loading. Maximum selectivity of cumene (83.82%) was achieved at 573 K, benzene/1,4-DIPB 5:1 at one atmosphere pressure. A suitable kinetic model for this reaction was proposed from the product distribution pattern following the Langmuir–Hinshelwood approach. Applying non-linear regression, the model parameters were estimated. The activation energy for the transalkylation reaction was found to be 116.53 kJ/mol.


INTRODUCTION
Cumene is a colorless liquid, also known as cumol or isopropyl benzene having the boiling-range motor fuel of high antiknock value.It is of industrial demand for the production of high molecular weight hydrocarbons such as cymene and polyalkylated benzene.The main end uses for cumene are for the production of phenolic resins, bisphenol A, and caprolactam.However, 5-10 wt% diisopropylbenzene (DIPB) isomers are produced as low value byproduct during the isopropylation of benzene to cumene (Leu et al., 1990;Sridevi et al., 2001;Reddy et al., 1993).The by-products, DIPB isomers, can be recycled for cumene production, making this process more economical.With the liquid catalysts, there are inherent problems of product separation, recycling and corrosiveness (Maity and Pradhan, 2006;Barman et al., 2005;Ercan et al., 1998).In that respect, zeolites can exhibit acidities close to those of traditional mineral acid solutions and hence proved to be better catalyst (Best and Wojciechowski, 1978;Slaugh, 1983;Bakas and Barger, 1989).Moreover, the number and strength of acid sites in zeolite can be changed to a great extent by exchanging its H + /Na + ions with rare earth cations in the zeolite framework.A comparative study was carried out on transalkylation of DIPB with benzene over Y, beta and mordenite with different Si/Al molar ratios in supercritical CO 2 and liquid phase (Sotelo et al., 2006).The influence of Si/Al ratio on the activity of catalyst was explained in terms of cumene selectivity and yield considering the competitive isomerization and by-product formation.The use of supercritical CO 2 did not show superior catalytic transalkylation activity for the Y zeolite.In Mobil Oil Corp., USA, production of cumene was carried R. Thakur, S. Barman and R. Kumar Gupta Brazilian Journal of Chemical Engineering out by introducing the feed to a transalkylating zone over beta zeolite/alumina and then feeding to an alkylating zone where MCM-22/alumina catalyst was used (Collins et al., 1999).Transalkylation of DIPB has also been carried out over large pore zeolites, which proved to be very active catalysts (Pradhan and Rao, 1993).In another process, DIPBs were recycled for transalkylation in the reactor containing a single catalyst bed of beta catalyst.The combined alkylation and transalkylation was performed for alkyl aromatic production to evaluate the performance of different catalysts like MCM-22 and beta zeolite based on their Si/Al ratio, selectivity, and pore size for liquid phase production of cumene (Perego and Ingallina, 2004).The catalysts such as zeolite X, MCM-22, MCM-49, PSH-3, SSZ-25, zeolite Y, beta zeolite (Yeh et al., 2008;Barger et al., 1989;Huang et al., 1997) were used in transalkylation reaction.These studies show that choice of catalyst, its Si/Al ratio and the acidity of the catalyst highly affect the process.
Kinetics of transalkylation of diisopropylbenzene were studied over Ca modified YH zeolite catalyst which proved to be a good active catalyst (Grigore et al., 2001).Cumene synthesis over beta zeolite has been reported in the literature (Bellussi et al., 1995;Perego et al., 1996;Smirnov et al., 1997;Halgeri and Das, 1999).Therefore, further investigation was necessary to carry out transalkylation of DIPB with benzene over the modified beta zeolite to obtain higher cumene selectivity and reactant conversion.Replacement of sodium ions in zeolites with polyvalent cations like rare earth metals (La, Ce, etc.) has been reported to produce materials of superior catalytic activity (Venuto et al., 1966;Rabo et al., 1968;Hunter and Scherzer, 1971).However, very scarce literature is available on the use of rare earth metal modified beta zeolite for cumene synthesis.It was, therefore, thought desirable to investigate the kinetics of this commercially important reaction over zeolite H-beta modified by exchanging H + ions with cerium ions.A further objective of this study was to develop a suitable kinetic model for the synthesis reactions.

Catalyst Preparation
The commercially available H-beta zeolite containing H + ions was modified with Ce 4+ ions.At first, the zeolite extrudates were calcined for 3 h at 623 K. Calcined zeolite was then refluxed with the required percentage of ceric ammonium nitrate solution at 363 K for 24 h, thereby modifying H-beta zeolite into the Ce-beta form.The catalyst particles were then filtered and washed several times with deionized water and then dried at 393 K for 14 h.Finally, they were calcined for 4 h at 723 K to remove the excess ions.The cerium-exchanged zeolite was characterized by TPD, XRD and FTIR.Beta zeolite treated with 4%, 6%, 8%, and 10% cerium ammonium nitrate solution (CeB 4 , CeB 6 , CeB 8 , and CeB 10 ) was used for the present study.

Determination of Cerium in the Exchanged Catalysts
The amount of cerium ions exchanged with the H + ions was calculated analytically (Krishnan et al., 2002).Freshly calcined cerium modified beta zeolite was taken in a flask and digested for 1 h in concentrated HCl.The digested catalyst was diluted with distilled water and filtered.The filtrate was transferred to a beaker and its volume was made up to about 250 ml by adding distilled water.50 ml of saturated oxalic acid solution was mixed with this solution, which produced a white precipitate of cerium oxalate.The precipitate was then filtered using a Whatman no.40 ashless filter paper and washed with distilled water.The filter paper was ignited in a previously weighed silica crucible at 1173±10 K to a constant weight.On heating, cerium oxalate was converted to cerium oxide.From the weight of cerium oxide the percentage of cerium was then calculated.CeB 4 , CeB 6 , CeB 8 and CeB 10 were found to have been loaded with 2.87%, 4.46 wt%, 6.64 wt% and 8.34 wt% of cerium respectively.

Experimental Setup for Transalkylation Reactions
Vapor phase transalkylation reaction was carried out in a fixed-bed, continuous down-flow, stainless steel (SS 316) reactor.The reaction conditions were maintained at atmospheric pressure.A preheater was fitted with the reactor in the upstream and a condenser Brazilian Journal of Chemical Engineering Vol. 33, No. 04, pp. 957 -967, October -December, 2016 in the downstream.A thermowell extending from the top of the reactor to the centre of the bed was used to measure the temperature of the reactor.Typically, 0.002 kg of the catalyst supported on a wire mesh was loaded into the reactor.Before conducting the experiments, catalyst activation was done at a temperature 100 K higher than the reaction temperature (maintained according to reaction conditions), for 3 h under the atmosphere of nitrogen.A dosing pump was used to introduce the reactant feed mixture into the reactor.Nitrogen gas was flown through the reactor at the rate of 0.565 L/h to activate the catalyst before experimental runs.However during all experimental runs, the nitrogen to feed flow rate ratio was kept constant at 0.2.The reactants were vaporized in the preheater, which is maintained at a temperature 30 K lower than the reaction temperature.The vaporised reactant feed mixture passes through the catalyst bed in the reactor at proper reaction conditions.The product vapors, along with the unreacted reactants, were condensed in the condenser (277 K-279 K).The samples were collected and analyzed in a gas chromatograph (Bruker, Model: 436 GC Scion) using a fused silica capillary column having 10 m × 0.53 mm × 1.5 µm dimensions.The sample was introduced through a micro syringe into the injector port of the GC.The temperature of the injector was set at 493 K during the analyses.The column temperature was initially set at 323K, and then increased to 523 K at a rate of 10 K/min.The flow rate of carrier gas (nitrogen) was maintained at 1.5 L/h.A Flame Ionisation Detector was used at 553 K to detect the products.Peaks were identified by retention time matching with known standards.Various products like aliphatics (propene), benzene, toluene, xylene (C 8 ), cumene, cymene (C 10 ), isomers of DIPB were found.The selectivity of cumene, 1,3 DIPB and conversion of 1,4 DIPB were calculated as: (1,4 DIPB in feed 1,4 DIPB in exit) 1,4 DIPB conversion 100 (1,4 DIPB in feed) (Cumene in product mixture) Cumene selectivity 100 (aromatics in product excluding 1,4 DIPB and benzene) = × ( ) ( ) 1,3 DIPB in product mixture 1,3 DIPB selectivity 100 aromatics in product excluding 1,4 DIPB and benzene = × The mechanism of transalkylation of 1,4 DIPB with benzene is shown in Figure 1.

Activity of Ca
The activit eta zeolite w tream at 573 hows that a mene was obt alue of 81.0 ue to deactiv which blocks he selectivity n cerium co

Effect of Be Product Sele
In the tran DIPB ratio wa erature of 57 Fig. 7

1,4-DIPB M
reaction, the m 1 to 15 at ace time of 9 ximum selec at a benzen ratio, the is ase and henc bserved.tion of 1,4 DIPB with benzene is a complex reaction which is followed by isomerization and disproportionation reactions.
i) 1,4-DIPB transalkylation: (1) ii) Isomerisation: iii) Dispropotionation: For the above reactions, the possible rate equations based on different mechanisms are presented below.k 4 is not considered while developing the model because the model is in terms of conversion of 1,4-DIPB and this reaction does not involves DIPB.k 3 is also not considered since only those reactions whose product yield is significant are taken into considerations.
Dual-site mechanism: where, Single-site mechanism: 1 where, The partial pressure of 1,4 DIPB and benzene are related to the fractional conversions and the total pressure (P) by these following equations: ( ) The optimum values of the parameters were obtained by minimizing the objective function given by the equation:

Model Selection
By using the values of the constants for Equation (5) for the dual site mechanism, as shown in Table 5, the standard error of estimate for the rate of disappearance of 1,4-DIPB was ±3.14 x 10 -4 .For Equation (6), with the values of the constants from Table 6, the standard error was ± 2.41 x 10 -3 .For Equation ( 7), with the values of the constants from Table 7, the standard error was ± 9.92 x 10 -3 .By comparing the standard errors, model Equation ( 5) was considered to be the best for representing the reaction system
Fig sel tem N 2 lys Figure 10: Ex

Table 2 : Prod
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