IJCPE Vol.10 No.1 (March 2009) Iraqi Journal of Chemical and Petroleum Engineering Vol.10 No.1 (March2009) 17-22 ISSN: 1997-4884 Kinetic Study of Catalytic Hexane Isomerization Abdul Halim A.K. Mohammed * ,Ameel M. Rahman and Maha Al-Hassani * * Chemical Engineering Department - College of Engineering - University of Baghdad – Iraq Abstract The isomerization of n-hexane on platinum loaded acidic zeolite was studied at atmospheric pressure, H2/nC6 molar ratios of 1-4 and temperature range of 240-270ºC. The measured kinetic data were fitted to an equation based on the bifunctional mechanism and by using independently obtained dehydrogenation and adsorption data. The activation energies of protonation (∆Hpro) and the elementary isomerization step (Eact,iso) and as well as the corresponding preexponential factor were simultaneously determined. The observed values of both ∆Hpro and Eact,iso are in agreement with the results of quantum-chemical calculations. Introduction More stringent limits on the amount of aromatics that may be included in gasoline have resulted in a renewed interest in the skeletal isomerization of n- alkanes with a view to use the branched isomers as octane-enhancing components. Branched isomers are more desirable because of their higher octane number. At equilibrium lower temperatures favor branched isomers; therefore there has been an effort to develop catalysts that are effective at 270ºC or less. Over the past several decades the progression has been from Pt/Al2O3, which is active for n-hexane isomerization in the 450-500ºC range to Pt/chlorinated Al2O3, which is active in the 350-400ºC range and more recently to Pt/H-zeolite catalysts, which are active in the 200-270ºC range. Additional research has been carried on Pt/sulfated ZrO2, Pt/WOx-ZrO2 and related materials (1). Based on the earlier studies of Mills et al. (2) a bifunctional mechanism for isomerization as follows; (i) dehydrogenation of alkane occurs at the metal centre (ii) the resulting alkene molecule is isomerized at an acid site and (iii) the isomerized alkene intermediate is hydrogenated at the metal centre to form the product (3). Molecular hydrogen is added to The system in order to enhance isomerization selectivity and prevent deactivation of the catalyst. The bifunctional mechanism is somewhat misleading since it suggests that protonation of the alkene generates carbenium ions in the same way as occur in liquid-phase super acids (4). However, quantum chemical calculations have shown that protonation of alkenes on solid acid sites renders so-called alkoxy species as stable reaction intermediates which are bonded to the lattice by strong covalent C-O bond(s). These calculations suggest that carbenium ion like species only exist as a transition state at the (de) protonation reaction. Due to the formation of strong covalent bonds, the enthalpy of protonation (∆Hpro) is expected to be high; the same holds for the activation energy of the isomsrization step (Eact,iso). To calculate Eact,iso from kinetic measurements, Eact,iso is expressed as a function of a number of parameters (5) including the apparent energy and, which is the actually measured quantity, and ∆Hpro, for which nearly only quantum-chemical calculations are available. Therefore values for Eact,iso determined in this way may also be unreliable. University of Baghdad College of Engineering Iraqi Journal of Chemical and Petroleum Engineering Kinetic study of catalytic hexane isomerizatin 18 IJCPE Vol.10 No.1 (March 2009) In conclusion, a meaningful kinetic analysis should involve an experimental determination of ∆Hpro. In the present study, the kinetic data are analyzed using a rate equation based on the bifunctional mechanism, which allows for the determination of Eact,iso and ∆Hpro. To distinguish between the effects of adsorption and the intrinsic kinetics on the reaction rate, independent adsorption data are used. Kinetic Analysis If it is assumed that the isomerization step is the rate determining and conversion is so low then the reverse reaction can be neglected. It can be assumed that the dehydrogenation steps are equilibrated as reported by Maha Al-Hassani (6) The dehydrogenation enthalpy (∆Hdh) and preexponential factor of dh K ( o dh K ), were calculated from standard enthalpy and entropy of formation of n-hexane and of all branched hexane isomers; Hdh ≈ Hof hexane - Hof hexane (1) R SS dh o o hexanef o hexenef ePK    (2) Where, o if H ,  and o if S ,  are respectively the standard enthalpy and entropy of formation of component (i) in the gas phase (the temperature dependence of o if H ,  and o if S ,  was neglected) and  P is the standard pressure (  P =101,325 Pa). The value of o if S ,  and o if H ,  of n-hexane and n-hexene were taken from the work of reference (7). Although dh k refers to adsorbed phase, it is assumed that the difference between the formation enthalpy/entropy in the gas phase and the adsorbed phase is equal for n- hexane and n-hexene, so that the enthalpy/entropy of dehydrogenation is equal in both phase (the calculation in eqns. [1] and [2] can be safely made, since the values for individual isomers vary by less than 10%). The adsorption parameters for zeolite were taken from studies published by Eder and Lercher (8, 9). The preexponential factors in units (Pa-1) were calculated from Henery’s constant KH (in units mmol/g.atm) measured at temperature Tm using equation [3].            m adso mH xads RT H QP TK TK exp )( )( m ax … (3) Where m ax Q denote the maximum concentration of n-hexane in the porous crystals (in unit’s mol/kg). The value of m ax Q was extracted from literature (8, 10). The net number of isomerized molecules produced per unit time per acid site is called the turnover frequency (TOF) and is given by Maha Al-Hassani (6): 1 22 2   H n dhproad H n dhadnad H n dhproads b iso P P KKK P P KKPK P P KKKk TOF (4) Where,Kads, Kdh and Kpro are the equilibrium constants of, respectively adsorption, dehydrogenation, and protonation for n-hexane; Kiso is the rate constant of conversion of the intermediate n- hexyl alkoxide into iso-hexyl alkoxide (no distinction is made between the various isomers) PH2 is the hydrogen pressure; and Pn is the n-hexane pressure. Equation (4) can be linearized by taking the reciprocal expression: nproads b iso H b isoproisodhpro b iso H PKKk P kKkKKk P TOF 22 111  (5) The plot of 1/TOF vs. 1/pn should be linear, and since Kads and Kdh are known or can be determined from independent experiments, Kiso and Kpro can be calculated from the slope(s) and intercept ( i ): dhadsHadsH H iso KKsPKsiP P k   22 2 (6) Abdul Halim A.K. Mohammed, Ameel M. Rahman and Maha Al-Hassani 19 IJCPE Vol.10 No.1 (March 2009) 122  dh H dhads H pro K P KKs Pi K (7) By determining Kiso and Kpro as a function of temperature and plotting ln Kiso and ln Kpro vs. the reciprocal temperature, respectively, Eact, iso and ∆Hpro can be obtained. Experimental and Materials n-Hexane supplied by BDH with 99% purity was used as a feedstock for isomerization experiments. HY- zeolite (CBV 600) catalyst powder was supplied from Zeolyst International and used as a support for catalyst preparation. 100 g of HY zeolite powder was mixed with 30% montmorillonite clay as binder as suggested by Murry (11). The resulting mixture was mixed with water to form a paste. An extrudates with 0.3 cm were formulated and dried over night at 100ºC, then 0.3 wt%Pt/HY-zeolite was prepared by impregnation method with a proper solution of hexachloroplatinic acid. The impregnated extrudates were dried at 110ºC then calcinated at 300ºC for 3 hours in furnaces with dry air (12). The calcinated catalyst was then reduced with hydrogen at 350ºC for 3 hours (13). Procedure and Equipments The catalytic unit performance tests were carried out in a continuous fixed bed reaction unit. The reactor was a carbon steel tube with an outside diameter of 1.9 cm, 2 mm thick and 80 cm length. 0.3wt%Pt/HY- zeolite catalyst was charged between two layers of inert materials (glass balls). The catalytic reactions were carried out in the temperature range of 240- 270ºC, LHSV of 1-4 h-1, H2/nC6 mole ratio of 1-3 and atmospheric pressure. n-Hexane partial pressure was kept at 0.28 bar while hydrogen pressure varied between 0.29-0.56 bar using nitrogen as a makeup gas. The nitrogen pressure varied between 0.16-0.45 bar to obtain the final atmospheric reaction pressure. Liquid products were trapped by condenser at -5ºC, collected periodically and analyzed by using gas chromatography. The gas chromatography model 438Aa-VSA supplied by Agilent technologies company was used for the analysis. This device equipped with column of 0.25mm diameter 100m length and FID detector. Results and Discussion The adsorption parameters were calculated using the available thermodynamics data (9-14) using equation [3]. These results are tabulated in Table (1). Table 1, Adsorption parameters. Tx(K) Kads(Tx)(Pa -1 ) 513 1.60x10 -5 523 1.31 x10 -5 533 1.08 x10 -5 543 9.08 x10 -6 Fig.1: Shows The Plot of lnKads vs. Reciprocal Temperature for ∆Hads Calculation ∆Hads of 44.2 kJ/mol was obtained from Figure 2. Since the heat of adsorption is exothermic, the adsorption-equilibrium constant Kads decreases with increasing temperature. The preexponential factor of adsorption equilibrium has a value of 5.08×10-10Pa-1 The dehydrogenation parameters were presented in Table (2) using equation [1] and equation [2]. Which ∆Hdh equals to 118kJ/mol (4). Since the dehydrogenation is endothermic process, the dehydrogenation-equilibrium constant Kdh increases with increasing temperature. The preexponential factor of dehydrogenation reaction has a values of 3.868×1010 Pa. This values are agreed well with the reported results (4, 15, 16). Table 2, Dehydrogenation parameters. TX (K) Kdh(Tx)(Pa) 513 0.3733 523 0.63 533 1.05 543 1.72 Kinetic study of catalytic hexane isomerizatin 20 IJCPE Vol.10 No.1 (March 2009) Catalyst Activity The conversion rates of n-hexane were calculated from the slops of conversion isotherms represents as a function of space time which expressed by gram catalyst per moles of n-hexane feed per second, (gcats/mol). Apparent activation energies were determined from temperature dependence of the rates of the total conversion, according to the Arrhenius equation (17). According to the differential method the derivative 6 6 nC nC wF dx evaluated from experimental data was used to obtain the reaction rate 6nC r . The obtained overall rate of reaction is normalized to the number of acid sites. The values of the 6nC r were used for TOF calculations is generally proportional to the catalyst concentration and surface area of the catalyst, so a turnover frequency (TOF) as number of moles reactant converted per mole of Brönsted acid sites per unit time must be calculated. Figure 2 is used for the simultaneous determination of Kiso and Kpro from slopes and intercepts of 1/TOF vs. 1/Pnc6 lines plots according to equation (5). Table (3) shows the values of Kiso and Kpro estimated at PH2=0.57bar and Pnc6 ranging from 0.14-0.42bar. Fig.2 Experimental reciprocal rate equation plots obtained for n-hexane isomerization. Table 3 Isomerization and Protonation Parameters. TX (k) Kiso (sec -1 ) Kpro (-) 513 1.82 x10 -2 9.84 x10 +5 523 2.19 x10 -2 6.41 x10 +5 533 2.67 x10 -2 5.75 x10 +5 543 9.67 x10 -2 3.24 x10 +5 A high value of Kpro indicates that the olefin protonation equilibrium entirely displaced toward the formation of carbenium ion as reported by Riberiro and Gauw et al. (4). The measured protonation energy -74kJ/mol from Fig.3 is agree well with those obtained by of quantum-chemical method of Kazansky et al. (19) and Viruela et al. (20). Fig.3, Plot of lnKpro vs. Reciprocal Temperature. The value of Eact,iso was 119.7kJ/mol obtained from Fig.4. The preexponential factor of protonation and isomerization reaction have a value 125.2 and 2.2×10 11 , respectively and within the experimental temperature range of 240-270ºC, the variation in the rate coefficient of branching rearrangement was relatively small, leading to the conclusion that observed differences in turnover frequencies for the various catalysts was predominantly caused by differences in the adsorption constants. Fig.4 Plot of lnkiso vs. Reciprocal Temperature. Abdul Halim A.K. Mohammed, Ameel M. Rahman and Maha Al-Hassani 21 IJCPE Vol.10 No.1 (March 2009) It was possible to simultaneously determined the energy of protonation and energy of branching rearrangement as well as correspondingly preexponentioal factor by fitting the measured kinetic data to an equation based on the bifunctional mechanism and using independency the obtained dehydrogenation and adsorption data as follows:  RTk iso /107.119exp102.2 311  … (5)  RTK ads /102.44exp1008.5 310   … (6)  RTK dh /100.118exp1086.3 310  … (7)  RTK pr /1079exp2.125 3  … (8) Consider these terms in equation (4), the overall rate equation per acid sites was obtained by equation 9:                           2H6nC 33 2H6nC 3 6nC 310 2H6nC 314 P/PRT/10x5exp10x5.2 P/PRT/10x8.73exp6.19 PRT/10x2.44exp5101 P/PRT/10x5.114exp10x4.5 TOF (9) In the case of a low pressure of n-hexane the overall rate equation per acid sites can be approximated by:       2H6nC 33 2H6nC 314 P/PRT/10x79exp10x5.21 p/PRT/10x5.114exp10x4.5 TOF    (10) Conclusions By measuring the rate of the n-hexane hydroisomerization reaction on Pt/HY-zeolite as a function of n-hexane pressure and fitting the results to a rate equation on the bifunctoinal isomerization scheme of Weisz, it was possible to simultaneously determine the equilibrium constant of n-hexane protonation and the rate of constant of isomerization of the resulting n-hexyl alkoxides. By repeating this procedure at different temperatures, the protonation energy ∆Hpro, the activation energy of isomerization Eact,iso and corresponding preexponential factor could be determined. The measured values of ∆Hpro equal to -79 kJ/mol whereas the value of Eact,iso equal to 119.7 kJ/mol. These values are agree well with the results of quantum- chemical calculations. References 1. Gates, B.C., Katzer, J.R and Schuit, G.CA., “Chemistry of Catalytic Process”, P.184. McGraw Hill, Chemical Engineering Series, New York, 1979. 2. Mills, G.A., Heinemann, H., Milliken, T.H. and Oblad, A.G., Ind. Eng. Chem. 45, 134, (1953). 3. Hon Yue Chu, Michaal, P. Roynek and Jack, H.L., “Selected Isomerization of Hexane Over Pt/HBeta Zeolite; Is The Classical Mechanism Correct?”, J. Cat., 178, 325-362, (1998). 4. Gauw, F.J.M.M., Grandell, J. and Santen R.A., “Intrinsic Kinetics of n-Hexane Hydroisomer Catalysed by Platinum Loaded Solid Acid Catalysts”, J. Catal., 206, 295-304, (2002). 5. Kazansky, V.B., Frash, M.V. and Van Santen, App. Cat., A. Gen., 146, 225, (1996). 6. Maha Al-Hasani, “Kinetic Study of n-Hexane Isomerization”, M.Sc. Thesis, Baghdad University, 2007. 7. Rossini, F.D., Pitzer, K.S., Arnett, R.L., Brawn, (1953) “Physical and Thermodynamics Properties of Hydrocarbons and Related Compounds”, API project No. 44, Carnegire Press, Pittsburgh. 8. Eder E., and Lercher J.A., J. Phys. Chem. B: 101, 1273-1278 (1997). 9. Eder F., Stockenhuber M., and Lercher J.A., J. Phys. Chem. B: 101, 5411-5419 (1997). 10. Denayer J.F., Gino V., Baron, J. Phys. Chem. B: 102, 3077-3081 (1998). 11. Murray Brendan, Process for “Isomerization Linear Olefins to Iso-olefins”, US patent, 5648584, (1997). 12. Riberiro, F., Marcilly, C., and Guisnet, M., Hydroisomerization of n-hexane on Platinum Zeolite”, J. Cat., 78, 267-273, (1982). Kinetic study of catalytic hexane isomerizatin 22 IJCPE Vol.10 No.1 (March 2009) 13. Masologites., Gates, “Method of Treating a Used Pt Group Alumina Catalyst with a Metal Promoter”, US patent, 4070306, 1987. 14. Denayer J.F., Baron V., Gina, Vanbustsel A., Pierre A., Jacobs Johan A., Martens, Chem. Eng. Science, 54, 3553-3561 (1999). 15. Bokhoven J.A., Tramp M., Koningsbarger J.T., Miller and Pieters J.A.Z., Lercher J.A., Williams and Kung, J. Catal., 202, 129-140 (2001). 16. Ribeiro F., Marcilly C., and Guisnet J. Catal. 78, 267- 274 (1982). 17. Fogler H. Scott, “Element of Chemical Reaction Engineering” 2 nd edition Englewood Cliffs, New Jersey, 1994. 18. Viruela- Martin P., Zicovich- Wilson C.M., Corma, "Ab Initio Molecular orbital calculation of the protonation reaction of propelene and isobutene by acidic OH group " J. Phys-Chem 97, 13713(1993). 19. Kazanasky V.B., Frash M.V. and Van Santen, R.A., " Quantum chemical study of isobutene cracking on zeolite", Appl. Catal.A:general 146,225 (1996). دراسة حركية تفاعل ازمرة الحفاز للهكسان االعتيادي ايٍم رحًٍ و يها انحسًُ, عثذ انحهٍى عثذ انكرٌى * انعراق-تغذاد- قسى انهُذسح انكًٍاوٌح – كهٍح انهُذسح –جايعح تغذاد * أجرٌد . تطرٌقح انرحًٍم انرطة واسرخذيد الزيرج انهكساٌ االعرٍادي 0.3wt%Pt/HY-zeolite ذى ذحضٍر انعايم انًساعذ -240انرجارب انًخرثرٌح تضغط جىي فً يُظىيح رٌادٌح ذحرىي عهى يفاعم رو انحشىج انثاترح و تذرجاخ حرارٌح ذراوحد تٍٍ 270 º سا3-1 و و سرع فراغٍح -1 . 4-1و انُسة انًىنٍح نههٍذروجٍٍ إنى انهكساٌ االعرٍادي اسرخذيد انثٍاَاخ انًرىفرج فً األدتٍاخ نحساب . ذى اشرقاق انًعادنح انعايح نسرعح انرفاعم تاالعرًاد عهى يٍكاٍَكٍح ثُائٍح انذانح . ثىاتد انرىازٌ نعًهٍح االيرصاص وإزانح انهٍذروجٍٍ يىل / كٍهى جىل79-يىل فً حٍٍ قًٍح اَثانثٍح ذفاعم ذىنٍذ انثروذىَاخ / كٍهى جىل119.7اٌ قًٍح طاقح ذُشٍط نرفاعم االزيرج هً : ًٌكٍ انرعثٍر عٍ انًعادنح انعايح نسرعح انرفاعم كانرانً.                           26 33 26 3 6 310 26 314 //105exp105.2 //108.73exp6.19 /102.44exp1051 //105.114exp104.5 HnC HnC nC HnC PPRT PPRT PRT PPRT TOF