Aromatics Production By Paraffin Dehydrocyclization: Mechanism And The Kinetics Of n-c 8 Dehydrocyclization Reaction On Acidic Pt/Al 2 O 3 Catalyst

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1 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) 4-5 Aomatics Poduction By Paaffin Dehydocyclization: Mechanism And The inetics Of n- 8 Dehydocyclization Reaction On Acidic Pt/Al O atalyst A. Jaiyeola The inetics of n-octane dehydocyclization eaction has been. Ao studied using 0.% acidic platinum-alumina catalyst in a mico catalytic eacto in a tempeatue ange of with hydogen as the caie gas at.8 atmospheic pessue. A Langmui-ishenwood-ougen-Watson type of ate model was developed fo the poposed mechanism involving the aomatization of adsobed species on the catalyst sites. Sixteen ate equations wee deived and the ate equations that gave the best fit of the expeimental data wee selected. onsequently, the ate and the themodynamic constants wee evaluated fom the model equations developed using a seach technique employing the elde and Mead modified simplex optimization outine. By using the least squaes egession analysis, the activation enegies and the heat of adsoption values wee evaluated to be cal/gmol and 9.90 cal/gmol espectively. The fequency facto was also estimated as x 0 8 gmol/ (g-catalyst) (h). Results obtained compaed excellently with that of Onuwuli (988) who obtained a value of.56 x 0 8 gmol/(g-catalyst) (h) fo the fequency facto and values anging between 8 8 cal/gmol fo the activation enegy of nomal octane convesion in the tempeatue ange 67 7 at a total pessue of atm, using vaious eactants and diluents patial pessues. These esults ae in excellent ageement with the nown and expected tends.. Intoduction Alane dehydocylization to poduce aomatics was fist discoveed in the late 90s (azansii and Plate, 96), and has been a subject of consideable theoetical and industial impotance since that time. In dehydocyclization, staight chain hydocabons, paticulaly alanes ae conveted to aomatics and hydogen (IUPA, 997). Much of the studies on this subject, involving the paaffins, have been diected towads elucidating the eaction mechanism and not much at quantifying its inetic paametes. Dehydogenation and cyclization of paaffins and cyclopaaffins of the coesponding cabon numbe leads diectly to the synthesis of Benzene, Toluene and Xylenes, (BTX) the most impotant aomatics. In spite of the intense desie to convet alanes to aomatics, the extent of the study unde conditions that appoximate commecial efoming conditions is spase. omal-octane had been suggested to Depatment of hemical Engineeing Faculty of Engineeing Univesity of Lagos, Lagos, igeia. leejaiyeola@yahoo.com, adeleejaiyeola@yahoo.com Depatment of hemical Engineeing Faculty of Engineeing Univesity of Lagos, Lagos, igeia chuchillao@yahoo.com 4

2 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) 4-5 dehydocyclize to aomatics en-oute the classical bifunctional mechanism involving both metal and acid sites, o on metal alone o via a eaction pathway involving dehydogenation on the metal and cyclization on the acidic suppot (Gates, atze and Schuit, 979). The most pobable mechanisms fo dehydocyclization eaction on metals ae those common to isomeization eactions on metals (Gate, atze and Schuit, 979). Attempts have been made to explain the ole of bifuntional catalyst on naphtha efoming and also to define the ate of convesion of n- heptanes to aomatics by dehydocyclization (Mills, einemann, Millien and Oblad, 95: ettinge, eith, Ging and Tete, 955). Some eseaches (Leste, 969) ae of the opinion that the platinum function alone could catalyze aomatics fomation by cyclization to a five-cabon ing followed by ing expansion to a six-cabon ing and subsequent dehydogenation to aomatics. Othes contended that diect six - cabon ing fomation was the only pathway by which platinum poduces significant amount of aomatics (Fogelbeg, Goe and Ronby, 967; Davis and Venuto, 969). By using a mechanical mixtue of a non-acidic platinum and acidic alumina (Shum, Butt and Sachtle, 984) concluded that the metal function contols dehydocyclization. Sivasane and Padala (988) investigating the dehydocyclization of hexanes though octanes using Platinum alumina catalyst concluded that the metal function was the citical paamete with n-hexane and n-heptane; howeve, both metal function and acid sites could be impotant fo n-octane. Ao and Susu (99) also investigated the dehydocyclization of n-octane on both monofunctional and bifunctional platinum-alumina catalysts and obseved that the pimay poducts of the dehydocyclization eaction wee ethylbezene and otho-xylene at tempeatues between 59 and 65. allende et al (97) expessed the view that platinum catalyzed cyclization though 5 ing closue and that ing expansion led to aomatics. They epoted that alumina had activity fo cyclization of the alanes poduced fom dehydogenation but that fo dispesed platinum, the metal catalyzed cyclization was moe apid. eningsen (97) epoted that the pedominant pathway fo the convesion of n-heptane to toluene is via a bifunctional mechanism, and that the acid catalyzed pathway was moe apid. Dantzenbeg and platteeuw (970) studied the convesion of n-hexane with platinum on a nonacidic alumina catalyst. They concluded that dehydocyclization of n- hexane to benzene poceeds along two diffeent eaction paths: (a) Platinum - catalyzed ing closue whose contibution to total ate of aomatization is popotional to total platinum suface egadless of metal paticle size and (b) Themal six-ing closue of hexatienes fomed by dehydogenation fom n- hexanes ove the platinum function. They agued fo a monofunctional eaction pathway fo platinum that involves both five and six-ing fomation. Davis (976) compaed the convesion of n- octane at atmosphees and 00 psig and found that the metal catalyzed dehydocyclizaton selectivity based upon the distibution of 8 aomatic isomes, was the same at both pessues. Thus the dominant metal catalyzed cyclization pathway to poduce aomatics was,6-ing closue povided the suppot did not have any activity to effect isomeization. Davis (98) late poposed that both,5 and,6-ing cyclization pathways occued with these catalysts, howeve at atmospheic pessue the,5-cyclization pathway led to the poduction of 8 -isoalanes though hydogenolysis of the cyclopentanes that wee fomed though,5- cyclization 5

3 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) 4-5 A bifunctional pathway may fom aomatics when acidic alumina suppot is used. Futhemoe, it has been epoted that the bifunctional pathway leads to aomatics at least twenty-times faste that the metal pathway only (Davis, 99). In addition, bifunctional pathway is moe selective fo aomatics fomation than the metal cyclization pathway (Davis, 99). Thee ae contovesial opinions egading the exact mechanism of aomatization of alanes (Ao and Susu, 986). While the confusions and the doubts as to whethe the convesion of alanes to aomatics on platinum alumina catalysts follow the classical bifunctional mechanism, involving both metal and acid sites anges on, moe wo must be done and diected at claifying and assuaging these doubts Spas et al (994). The objective of the pesent wo theefoe was as follows:. To study the intinsic inetics of the dehydocyclization eaction of n 8 using Pt/Al O catalyst in a mico catalytic eacto.. To deive and establish appopiate mechanistic ate equation fo the poposed eaction netwo.. To estimate the inetics and equilibium paametes fo the best models. 4. To elucidate futhe, both the mechanism and the oles of the metal and the acidic components of the catalyst. 5. To obtain the activation enegies of the dehydocyclization eaction. 6. To obtain the fequency factos. 5. onstuction of the Reaction etwo Reaction path fo the dehydocyclization of n- octane on acidic and non-acidic platinumalumina catalysts is a complex one. It compises of seveal consecutive and simultaneous elementay steps. FIGURE : n- 8 Dehydocyclization on Acidic Platinum Alumina Ao and Susu (99), using the pulse technique, epoted that, on the bifunctional platinumalumina catalyst and in hydogen caie gas at the tempeatue ange 56 to 67, n-octane was conveted to i-octane as the sole pimay poduct which in tun was conveted to ethylbenzene and otho-xylene as poducts thoughout the tempeatue ange investigated. The sequences of elementay steps fo the scheme above ae hee pesented. 6

4 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) 4-5 TABLE : Mechanism of n-c 8 Dehydocyclization Reaction on Acidic Platinum-Alumina. The symbols used have the usual meaning, and ae listed in page. S signifies an empty site while the alpha-bets with an S at the end efes to absobed species such that S and IS stand fo absobed n-octane and absobed i-octane espectively. 7

5 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) The Rate Model Development The fundamental appoach to inetic analysis of a heteogeneous catalytic eaction involves the postulation of a ate detemining step and the fomulation of an equation to expess the ate in tems of the concentation of the intemediates in that step. (ougen and Watson, 94: Foment and Bischoff, 990; Fogle, 99 and Sattefield, 99). The concentations of the intemediates must be elated to the fluid phase concentations of the eactants and poducts (adsoption isothems). The simplest theoetical expession fo an adsoption isothem is the Langmui isothem, on which the LW fomalism is based. The undelying assumptions of the Langmui isothem includes: i. Mono laye coveage ii. Unifomly enegetic adsoption sites and iii. o inteaction between adsobed molecules. LW fomalism has been employed in this study to deive the ate laws fo the poduct fomation models. In the scheme above, n- octane was fist adsobed on the catalyst site whee it was dehydo-isomeized to i-octane. The adsobed i-octane was then dehydogenated by aomatization to the two pimay poducts; ethyl-benzene and otho-xylenes, which wee then isomeized to othe xylenes. In conventional inetic analysis, S is consideed as an immeasuable quantity and must be elated to the measuable entities such as the concentations o the patial pessues of the eactants as the case may be. In pinciple, each of the sixteen elementay steps above could be the ate-detemining step. Fo example, if the adsoption of n-octane is the ate-detemining step (eaction of table ), the ate of the eaction is given by. S S () By assuming that othe steps ae in quasiequilibium, the suface coveages S, IS, S, S, S, S and S can be expessed as follows: S S () I S () I S S S (4) S S (5) S S (6) 4 S 5 6 S (7) S S (8) A total site balance S as a function of the fluids concentations of the eactants can be witten as S I 6 5 In this manne equation () then becomes I I (9) (0) A summay of the sixteen ate equations obtained in this way is shown in table. 8

6 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) 4-5 TABLE : Deived Rate Expessions. Adsoption of n- 8 is ate contolling. I I 6 5. Isomeization of adsobed n- 8 is ate contolling. I I. Aomatization of adsobed i- 8 to Ethyl benzene is ate contolling. I I Aomatization of adsobed i- 8 to Otho-xylene is ate contolling. 4 I 4 I Isomeization of adsobed adsobed Paaxylene is ate contolling. 5 I Isomeization of adsobed Paa-xylene to adsobed Meta-xylene contols. 6 I 6 7. Isomeization of adsobed M-xylene to adsobed O-xylene is ate contolling. 7 I Isomeization of adsobed O-xylene to adsobed is ate contolling. 8 I Isomeization of adsobed to adsobed M- xylene is ate contolling. 9 I 9 0. Isomeization of adsobed O-xylene to adsobed P-xylene is ate contolling. 0 I Desoption of adsobed i- 8 is ate contolling.. Desoption of adsobed is ate contolling. 6 I Desoption of adsobed P-xylene is ate contolling Desoption of adsobed M-xylene is ate contolling Desoption of adsobed O-xylene is ate contolling. 5 I 5 4 I Desoption of adsobed ydogen is ate contolling. 6 6 I

7 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) 4-5 Fo this pupose, sixteen ate equations wee deived and the ate equations that gave the best fit of the expeimental data wee selected. The lists of equations consideed ae as pesented in Table. 7. Paamete Estimation And Model Discimination The expeimental data of Ao and Susu (99) was analyzed, using the equation F R () j / Ai R Ai j Whee F is the objective function to be minimized and epesenting the sums of squaes of the diffeence between the obseved and pedicted ates and is the numbe of expeimental data. The paamete estimates wee obtained by minimizing the objective function F using the elde and Mead s modified simplex optimization outine. The values of the ate paametes and the esiduals (Φ min ) ae pesented in Tables and 4. Discimination among the models was based on the positive ness of the ate and the equilibium constants, the goodness of fit, and the incease of ate constants with tempeatue. 8. Discussion of Results It was discoveed that only six of the sixteen deived ate equations shown in Table, wee found to explain the expeimental data. These ae the ones based on the following elementay eactions as the ate detemining steps: (i) Adsoption of n 8 ( ). (ii) onvesion of adsobed n 8 to adsobed i- 8 ( ). (iii) onvesion of adsobed i- 8 to adsobed ethyl- benzene ( ). (iv) onvesion of adsobed i- 8 to adsobed otho-xylene ( 4 ). (v) Desoption of adsobed meta-xylene ( 4 ). (vi) Desoption of adsobed hydogen ( 6 ). Although the six steps enumeated above wee found to pedict the expeimental data, it is howeve nown that the dehydogenation of n- 8 to adsobed i- 8 and desoption steps ae athe fast elative to othe elementay steps in the sequence on the bifunctional catalyst. These models wee theefoe discaded. Tables and 4. Lists the pedicted inetics and equilibium paametes fo the two models etained. The tempeatue dependence of the ate constants is as shown in Figues,4 and 5 fom which the activation enegies wee calculated fo models and 4 as 5.465cal/gmol and.89cal/gmol espectively. The adsoption coefficients obtained fo the model showed the usually expected tend of decease in the values of adsoption coefficients with tempeatue while an unexpected tend was noted in model 4. Fo this eason model 4 was also sceened out along with othes and model etained as the ate detemining step and the best model fo epesenting the inetics of nomal octane dehydocyclization eaction on acidic platinum - alumina. The heats of adsoption as calculated fom the Ahenius plots of ln vs. T - wee found to be 9.90 cal/gmol and.07 cal/gmol fo models and 4 espectively. These esults ae in excellent ageement with that of Onuwuli (988) who has obtained a value of.56 x 0 8 gmol/(g-catalyst)(h) fo the fequency facto and values anging between 8 8 cal/gmol fo the activation enegy of nomal octane convesion in a Bety STR in the tempeatue ange 67 7 at a total pessue of atm, using vaious eactants and diluents patial pessues. 0

8 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) 4-5 Table : Model paametes fo ate equation () in table Φmin.99E E-6.6E-6.65E E-6.6E-6 A gmol/ (g-catalyst) (h) ΔE, Δ cal/gmol E E Table 4: Model paametes fo ate equation (4) in table Φmin.0E E-6.99E-7.09E-5 5.4E E-7 A gmol/ (g-atalyst) (h) ΔE, Δ cal/gmol E E

9 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) 4-5 FIGURE : Ahenius plot using ate constants fo model FIGURE : Ahenius plot using equilibium constants fo model

10 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) 4-5 FIGURE 4: Ahenius plot using ate constants fo model 4 FIGURE 5: Ahenius plot using equilibium constants fo model 4

11 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) onclusion The inetics of n-octane dehydocyclization eactions using 0.% commecial Platinum alumina catalyst has been studied in a mico catalytic eacto ove a tempeatue ange with hydogen as caie gas at.8 atmospheic pessue using the pulse technique. It was found that the ate of dehydocyclization of n-octane on acidic catalyst was coectly pedicted by the model equation involving the ate of aomatization of adsobed i-octane to ethyl benzene, model, which is the best model fo epesenting the inetics of nomal octane dehydocyclization eaction on acidic platinum - alumina. The heat of adsoption and the activation enegy fo the choice model, as calculated fom the Ahenius plots of ln vs. T - and ln vs. T - wee found to be 9.90 cal/gmol and cal/gmol espectively. The fequency facto was also estimated as x 0 8 gmol/(gcatalyst)(h). These esults ae in excellent ageement with that of Onuwuli (988) who obtained a value of.56 x 0 8 gmol/(g-catalyst)(h) fo the fequency facto and values anging between 8 8 cal/gmol fo the activation enegy of nomal octane convesion in a Bety STR in the tempeatue ange 67 7 at a total pessue of atm, using vaious eactants and diluents patial pessues. SYMBOLS USED S omal octane Total catalyst site Ethyl benzene S ΔE Δ ydogen Adsobed ydogen Activation Enegy, cal/gmol Adsoption Enegy, cal/gmol oncentation of nomal octane, mol /cm Adsoption equilibium constant, atm - A Fequency facto, gmol/(g-catalyst)(h) R Gas constant, cal/(gmol)( o ) T W F / R Ai Gee Lettes Rate constant, g. mol/(h) (g-catalyst) (atm) Absolute tempeatue, o atalyst weight, g Objective function to be minimized Pedicted expeimental ates Φ min Paamete defined by equation. Refeences [] Ao,.T. and Susu, A.A. (99) hem. Eng. Tech. 6,0. [] Ao,.T. and Susu, A.A. (986) J. hem. Tech. Biofunctional 6,59. [] allende, W. L, Bandenbege, S.G., and Meebott, W. (97) Poc. 5 th Int. ong. atal., oth-olland, Amstedam [4] histofel, E.. and Paal, Z. J. (98) J. atal.75 IS Paa-xylene Meta xylene Othoxylene Iso- octane [5] Dautzenbeg, F.M., and Platteeww, J.. (970) J. atal. 9, 4. [6] Davis, B.. (976). J. atal. 4, 76. [7] Davis, B.. (98). Pepint Am. hem.. Soc. Div. Pet. hem. 8, 40. 4

12 West Indian Jounal of Engineeing Vol. 0, o., (Januay 008) Technical Pape (Jaiyeola & Ao) 4-5 [8] Davis, B. (99) Selectivity In atalysis AS. Symp. Se. 57, 09. [9] Davis, B.., and Venuto, P.B. (969). J. atal. 5, 6. [0] Fogelbey, L.g., Goe, R. and Ronby, B. (967) Acta. hem. Scand.. 04, 050. [] Fogle,.S., (99), Elements of hemical Reaction Engineeing nd Edition. Pentice all. Englewood liffs..j. [] Foment, G.F and Bischoff,.B., (990). hemical Reacto Analysis and Design nd Edition. Wiley..Y. [] Gates, B.., atze J.R., and Schuit, G..A., (979) hemisty of catalytic pocesses McGaw-ill Boo ompany. [4] eingsen, J. (97) Discussion at the end of ef. [5] ethinge, W.P., eith,.d., Ging, J.L., and Tete, J.W (955) Ind. Eng. hem. 47, 79. [6] ougen, O.A and Watson,.M, (94) Ind. Eng. hem. 5, 59. [7] IUPA (997) ompendium of chemical Technology nd Edition. [0] Leste, G.R., (969). J. atal, 87. [] elde, J.A: Mead, R., ompute J. 7 (965) 08. [] Mills, G.A., einemann,., Millien, T.., and Oblad, A.G (95). Ind. Eng. hem [] Moldavsy, B.L., and amusche,. (96). Dol. Aad. au. SSSR, 55. [4] Onuwuli, O. D., (988) PhD Thesis, Univesity of Lagos. [5] Paal, Z. (980) Adv. atal, [6] Sattefield,.., (99) eteogeneous catalysis in industial pactice nd Edition. Mc Gaw-ill,.Y. [7] Shum, V.., Butt, J.B. and Sachtle, W..M. (984). Appl. atal. l, 5. [8] Sivasane, S. and Padala, S.R. (988). Appl. atal. 9,. [9] Spas, D.E., Sinivasan, R. and Davis, B. (994). Jounal of molecula catalysis, 88, 5. [0] Susu, A.A., (997) hemical inetics and eteogeneous catalysis J Pess Limited, igeia. [8] azansii, B.A. and Plate, A.F. (96). Beichte, 69, 86. [9] Langmui, I. (98), J. Am. hem. Soc. 40, 6. 5