Calero, S.

Dubbeldam, D.

Maesen, T. L. M.

Smit, B.

Infoscience - École polytechnique fédérale de Lausanne

AngewandteChemieCommunicationsWhen adsorbates are similar in size to zeolite channels then the surfacepores of zeolites only adsorb part of the adsorbate molecule. The notadsorbed section can be cleaved to react further or adsorb elsewhere.This window effect can be examined with the aid of Monte Carlosimulations. For more information see the following Communicationby D. Dubbeldam et al.3623Angew. Chem. Int. Ed. 2003, 42, 3623 DOI: 10.1002/anie.200351110  2003 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimCracking Mechanisms in ZeolitesUnderstanding the Window Effect in ZeoliteCatalysis**David Dubbeldam,* Sofa Calero, Theo L. M. Maesen,and Berend SmitThe “window effect” is one of the most controversial andintriguing phenomena in the zeolite literature. Chen et al.discovered in 1968 that ERI-type zeolites yield a bimodalproduct distribution of the products for the zeolite-catalyzedhydrocracking of n-C21 and longer n-alkanes, with maxima forthe alkanes n-C3–4 and n-C10–12 but no products in the C5–8range (the window).[1] Conventional zeolite-catalyzed(hydro)cracking yields a product distribution with only asingle maximum, which is consistent with the currentlyaccepted reaction mechanisms.[2,3] A comprehensive andfundamental understanding of the product selectivity associ-ated with these catalytic processes is of considerable practicalsignificance, as the results of Chen would open the possibilityof length-selective hydrocracking.[4] In the case in which themandatory addition of volatile ethanol to gasoline is immi-nent,[5–7] the more volatile components that are currently partof gasoline would have to be removed so as to maintaingasoline volatility. This revives the interest in a process thatselectively hydrocracks linear pentane and hexane so as toreduce volatility and increase the octane number. Such aprocess was commercialized in the 1960s, went by the name“selectoforming”,[8] and used ERI-type zeolite cages.Improvements on such a process would involve a betterunderstanding of the hydrocracking process of linear alkanesup to a certain chain length. The newly gained understandingaffords a prediction of length-selective hydrocracking.For a long time the window effect has been related almostexclusively to the diffusion rate of n-alkanes in ERI-typezeolites. Gorring showed that the product distribution and thediffusion coefficient as a function of n-alkane length correlateextremely well.[9] The low diffusion coefficients for n-C7 to n-C9 suggest that these molecules diffuse too slowly to leave thezeolite without cracking; the high diffusion coefficients for n-C10 to n-C12 suggest that these molecules diffuse rapidlyenough to escape. However, recent diffusion measurementsby Cavalcante, Jr. et al.[10] and Magalh¼es et al.[11] failed toreproduce the increase in diffusion coefficient for theappropriate n-alkane lengths. This controversy motivated usto simulate the window effect at the molecular level by usingadvanced molecular simulations.[12–14] In these Configura-tional-Bias Monte Carlo simulations molecules are grownatom by atom biasing the growth process towards energeti-cally favorable configurations and avoiding overlap with thezeolite. During the growth we calculate the Rosenbluthfactor, which is directly related to the excess chemicalpotential, the free energy, and the Henry coefficient.[15]Although our simulations of the diffusion[16] confirm theexperimental observations of Gorring, the results neverthe-less point to an alternative mechanism based on the anom-alously low adsorption of long molecules. At sufficiently lowpressures the amount of adsorbed molecules is directlyproportional to the pressure. The proportionality constant isknown as the Henry coefficient, and shown in Figure 1 as afunction of chain length for various zeolites. OFF-type sievesexhibit the usual monotonic increase of the Henry coefficientwith n-alkane length. Longer n-alkanes have more attractiveadsorbent–adsorbate interactions, and—therefore—a loweradsorption enthalpy. Longer n-alkanes also have fewerconformations in the adsorbed phase as compared to thegas phase, and—therefore—a lower adsorption entropy. Thedecrease in enthalpy offsets the decrease in entropy, so thatthe Gibbs free energy of adsorption decreases (and the Henrycoefficient increases) with the lengthening of the n-alkane.We found that pores with constrictions (windows) thatapproach the diameter of the adsorbate exhibit a dramaticallydifferent behavior. Instead of attractive adsorbate–adsorbentinteractions, these windows exert repulsive adsorbate–adsorbent interactions that increase the adsorption enthalpyof any n-alkane partially adsorbed inside such a window.Accordingly, the usual compensation between adsorptionenthalpy and adsorption entropy ceases as soon as n-alkanesbecome too long to fit comfortably inside the wider part ofthese pores (cages). For these longer n-alkanes the loss ofentropy with increasing length dominates their adsorptionproperties. For example, n-C13 in an ERI-type cage predom-Figure 1. Henry coefficients at 600 K for various types of zeolites as afunction of alkane chain length. At sufficiently low pressures theamount of adsorbed molecules is linearly related to the pressure withthe Henry coefficient as the proportionality constant.[*] Dr. D. Dubbeldam, S. Calero, B. SmitDepartment of Chemical EngineeringUniversity of AmsterdamNieuwe Achtergracht 166, 1018 WV Amsterdam (The Netherlands)Fax: (+31)20-525-5604E-mail: dubbelda@science.uva.nlT. L. M. MaesenChevron Texaco, Energy Research and Technology CompanyChevron Way 100, Richmond CA 94802-0627 (USA)[**] We would like to thank the Netherlands Research Council forChemical Sciences (CW), the European Commission for a MarieCurie Fellowship to S.C., ChevronTexaco for financial support, andC. Wilson, C. H. Roemkens, A. Kuperman, S. I. Zones, and R.Krishna for their comments on our manuscript.Communications3624  2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200351110 Angew. Chem. Int. Ed. 2003, 42, 3624 –3626inantly adsorbs in a limited number of curled conformations,whereas n-C14 has to stretch through a window into two cages.For even longer n-alkanes this pattern is repeated, thoughnow involving a second window. The Henry coefficients ofmolecular sieves with < 0.45 nm windows exhibit periodicbehavior (Figure 1). The local minima correspond to alkanesthat barely fit into x cages, so that alkanes one methylenegroup longer require x+ 1 cages. Our simulations indicatethat all molecular sieves with a window smaller than the0.45 nm across EUO-type window exhibit such a remarkabledecrease of the Henry coefficient with n-alkane length. To thebest of our knowledge there are no experimental dataavailable for comparison. In fact, anomalously low adsorptionhas never been cited when interpreting the catalytic ordiffusion data on the large number of zeolites that shouldexhibit this window or cage effect in adsorption. Below, weamend this situation for ERI- and AFX-type zeolites.Our simulations for ERI-type zeolite indicate that exactlyfor n-C9 and longer n-alkanes the amount adsorbed decreases.This nicely corresponds with the observed decline in activ-ity,[17] and if we assume that the hydroconversion in ERI issimilar to that of zeolites with windows nearly twice as wide(e.g. FAU-type zeolites), this decline may be seen as indirectevidence for our Henry coefficients. The selectivity asobserved by Chen[1] concerns n-alkanes as long as n-C36. Itis important to note that the window effect has becomeassociated only with a high n-C10–12 yield. However, a closerinspection of the raw data of Chen et al. published for n-C36reveals additional yield maxima at n-C24–26 and at n-C15–17 inthe complete chromatogram of the n-C36 cracking products.From the calculated variation of Henry coefficients with chainlength it is immediately apparent that the complete n-C36adsorption can be excluded, and yet ERI-type zeolites crackn-C36 shape selectively. The preferential yield of n-C24–26 andn-C15–17 is indicative of consecutive scissions of C10–12 frag-ments from n-C36. The maximum at n-C10–12 indicates that themost rapidly diffusing cracking product can escape completeconsumption into more refractory alkanes with six or fewercarbon atoms. This selective cracking process leaves minimaat n-C18–23 and at n-C13–14. A reasonable explanation for thepersistence of shape-selective cracking of very long alkanesby ERI-type zeolites is that these long molecules adsorbpartially in the surface pockets created by the ERI-type cagesat the outer crystal surface. Full adsorption by more than 12carbon atoms is unlikely. The adsorbed n-C10–12 part ischopped off, and the nonadsorbed part is released. Thereleased parts end up in the product slate or undergo furtherscission reactions. This process becomes less selective with anincreasing number of cracking steps because of concomitantisomerization reactions. AFX-type zeolites exhibit a hydro-cracking pattern similar to ERI-type zeolites. This is notsurprising, as these zeolites only differ in the size of the cages.Similar to ERI-type zeolites,[4] AFX-type zeolites preferen-tially hydrocrack shorter rather than longer n-alkanes[18] whengiven a choice. Thus, they consume exclusively n-C6 whenprocessing a mixture of n-C6 and n-C16, even though theyhydroconvert more than 50% of either n-C6 or n-C16 whenprocessing them separately. Inspection of our Henry coef-ficients shows that n-C6 adsorbs fully, whereas n-C16 can onlyadsorb partially. As partial adsorption is thermodynamicallyhighly unfavorable,[19] fully adsorbing molecules can effec-tively block adsorption and hydroconversion for partiallyadsorbing molecules. Naturally, windows approaching thediameter of the adsorbate are a barrier to both adsorption anddiffusion, and these two factors cannot be seen in isolation.Diffusion rates determine the impact of the very lowadsorption on catalysis. Its impact will be higher when thediffusion is slower, because the path through consecutivewindows is highly tortuous (as in ERI-, AFX-, and CHA-typezeolites). Its impact will be lower when the diffusion is fasterbecause the path through consecutive windows is straight (asin RHO- and KFI- type zeolites) (Figure 2). This newlygained understanding of length-selective hydrocrackingaffords a prediction of selectivity as a function of cage size.We would predict that the larger AFX-type cage shifts thecracking selectivity by two carbon atoms towards longer n-alkanes as compared to the smaller ERI-type cage, and thatFigure 2. The crystalline structures OFF-, ERI-, RHO-, CHA-, KFI-, andAFX-type zeolites. The size of the CHA-, ERI-, and AFX-type cages limitthe n-alkanes to 11, 13, and 16 carbon atoms, respectively. The largecages of RHO- and KFI-type zeolites accommodate molecules up to n-C24 and n-C26, respectively, in snake-in-a-basket conformations. The redballs represent CH3- or CH2-groups. Arrows at the origin are 0.3 nm inlength.AngewandteChemie3625Angew. Chem. Int. Ed. 2003, 42, 3624 –3626 www.angewandte.org  2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheimthe even smaller CHA-type cages shift the cracking selectivityfurther towards n-C5. We further predict that RHO- and KFI-type zeolites afford length-selective hydrocracking of n-alkanes longer than those removed by AFX-type zeolites.In summary, we refute some of the traditional crackingmechanisms found in zeolite literature based on full adsorp-tion. The simulation results indicate abnormally low adsorp-tion in ERI-, AFX-, CHA-, RHO-, and KFI-type zeolites forchain lengths close to or longer than the cage size. Hence, verylong molecules adsorb partially into a cage near the outersurface. After scission the nonadsorbed part can end up in theproduct or re-adsorb and undergo another scission. Thismechanism is characteristic for cage/window-type zeoliteswith small windows close to the diameter of the adsorbate.The newly gained understanding of length-selective hydro-cracking affords prediction of selectivity as a function of cagesize.Received: February 5, 2003 [Z51110].Keywords: alkanes · catalysis · computer chemistry · cracking ·zeolites[1] N. Y. Chen, S. J. Lucki, E. B. Mower, J. Catal. 1969, 13, 329 – 332.[2] J. Weitkamp, S. Ernst, L. Puppe in Catalysis and Zeolites (Eds.: J.Weitkamp, L. Puppe), Springer, Berlin, 1999, pp. 326 – 376.[3] J. A. Martens, P. A. Jacobs in Zeolite Microporous Solids:Synthesis, Structure, and Reactivity (Eds.: E. G. Derouane, F.Lemos, C. Naccache, F. Ram¼o-Ribeiro), Kluwer, Amsterdam,1992, pp. 511 – 529.[4] J. Wei, Ind. Eng. Chem. Res. 1994, 33, 2467 – 2472.[5] M. Radler, Oil Gas J. 2002, 100, 5 – 9.[6] K. Watkins, Chem. Eng. News 2001, 79(30), 21 – 21.[7] M. Lorenzetti, Oil Gas J. 2003, 101, 20 – 22.[8] N. Y. Chen, J. Maziuk, A. B. Schwartz, P. B. Weisz, Oil Gas J.1968, 66, 154 – 157.[9] L. R. Gorring, J. Catal. 1973, 31, 13 – 26.[10] C. L. Cavalcante, Jr., M. Eic´, D. M. Ruthven, M. L. Occelli,Zeolites 1995, 15, 293 – 307.[11] F. D. Magalh¼es, R. L. Laurence, W. C. Conner, AIChe. J. 1996,42, 68 – 86.[12] B. Smit, T. L. M. Maesen, Nature 1995, 374, 42 – 44.[13] T. J. H. Vlugt, R. Krishna, B. Smit, J. Phys. Chem. B 1999, 103,1102 – 1118.[14] T. L. M. Maesen, M. Schenk, T. J. H. Vlugt, J. P. de Jonge, B.Smit, J. Catal. 1999, 188, 403 – 412.[15] D. Frenkel, B. Smit in Understanding molecular simulation, 2nded., Academic Press, London, UK, 2002.[16] D. Dubbeldam, S. Calero, T. L. M. Maesen, B. Smit, Phys. Rev.Lett. 2003, 90, 245901.[17] N. Y. Chen, W. E. Garwood, Adv. Chem. 1973, 121, 575 – 582.[18] D. S. Santilli, S. I. Zones, Catal. Lett. 1990, 7, 383 – 387.[19] E. B.Webb III, G. S. Grest, J. Chem. Phys. 2002, 116, 6311 – 6321.Communications3626  2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org Angew. Chem. Int. Ed. 2003, 42, 3624 –3626

Understanding the window effect in zeolite catalysis

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Understanding the window effect in zeolite catalysis

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