Dubbeldam, D.

Maesen, T. L. M.

Smit, B.

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Reply to the Comment on “Computer Simulationof Incommensurate Diffusion in Zeolites:Understanding Window Effects”D. Dubbeldam,*,† T. L. M. Maesen,‡ and B. Smit†Department of Chemical Engineering,UniVersity of Amsterdam, Nieuwe Achtergracht 166,1018 WV Amsterdam, The Netherlands, and CheVronTexaco,Energy Technology Company, CheVron Way 100,Richmond, California 94802-0627ReceiVed: July 14, 2004We would like to thank Professor Ruthven for highlightingsome experimental difficulties in determining the diffusioncoefficients in zeolites. We would like to clarify the two pointsraised in the comment.(1) Diffusivities for Linear Alkanes in LTA-Type Zeolite.In the book of Ka¨rger and Ruthven1 and ref 2, it is stated thatthe lack of anomalies in 5A provides no evidence for theincommensurate diffusion (a window effect) in LTA-typezeolites. Our simulations show that in the cation free form of5A (LTA) the onset of incommensurate diffusion is not to beexpected until alkanes are as long as C20. In the C8-C20 range,the diffusion rate is nearly constant. Accordingly, simulationsindicate that the alleged lack of anomalies in the experimentaldata for alkanes shorter than C17 has no bearing on incom-mensurate diffusion in LTA-type zeolites.In our original publication, we used the experimental data ascompiled by Ka¨rger and Ruthven1 and extrapolated these to thetemperature of the simulations. In his comment, ProfessorRuthven states that only the experimental data on large crystalsare reliable. This data set shows a decrease of the diffusioncoefficient in 5A with chain length. Figure 1 shows the datawe have used previously together with recent data from Jobicet al.3 In addition, we have clearly indicated the large crystaldata points. The agreement between simulated and experimentaldata remains remarkable, especially if one considers that (1)we have obtained the parameters for our model from theadsorption of alkanes in MFI-type silica and (2) we simulateLTA-type silica, and not the mixed calcium and sodium formof the zeolite. More importantly, none of the experimental orsimulated data suggests any incommensurate diffusion, irrespec-tive of (the reliability of) the data points.In his comments, Professor Ruthven stresses that the variationof the diffusion rate with alkane chain length shows noanomalies for LTA-type zeolites. We are inclined to disagree.In our opinion, the nearly constant diffusion rate in the C8-C20 range is anomalous, for usually diffusion rates decrease withincreasing n-alkane chain length. In addition, a more detailedinspection of our simulation data suggests that there is a smallincrease in diffusion for C5-C10. This local maximum is relatedto a cage effect; that is, at C5-C10, the molecule starts to feelthe limitations of the cage, whereas the nearly constant diffusioncoefficient in the C8-C20 range is a consequence of the cage-window structure of LTA.Very recently, experimental data were published by Jobic etal.3 that support the two anomalies predicted by our simulations.These new data do not agree with the data mentioned in thecomment of Ruthven, but do support our conclusion that thediffusion coefficient shows a local maximum near C10 or C12(depending on the loading, temperature, and amount of cations)due to a cage effect.(2) The “Window Effect” in Zeolite T. One could say that“zeolite T” denotes a complex family of materials comprisinga myriad of possible intergrowths between OFF- and ERI-typezeolites. Gorring reported that the potassium form of one ofthe zeolite T family members exhibited incommensurate dif-fusion. Reproducing Gorring’s diffusion coefficients for “zeoliteT” by modern methods proved to be a challenge. SinceGorring’s diffusion measurements do not meet contemporarystandards, the existence of incommensurate diffusion in zeoliteshas become controversial. In our view, evaluating the relevanceto zeolites of incommensurate diffusion requires samples betterdefined than zeolite T. Our simulations predict that ERI-typesilica and, by extension, AlPO-17 exhibit incommensuratediffusion. Very recently, a paper was published that reports aperiodic dependence of the diffusion coefficient on the numberof carbon atoms in n-alkanes in MTW- and LTL-type zeolites.The paper relates the diffusion coefficient to the degree to whichthe shape of a molecule and the shape of the channels are(in)commensurate.5 To the best of our knowledge the diffusioncoefficients were obtained using contemporary experimentaltechniques and validation criteria. In addition to this recent reporton incommensurate diffusion, Jobic et al. found experimentalevidence of an increase in diffusion due to an innercage re-organization of a molecule as predicted by our work.4 Thisillustrates an important observation, namely that a diffusioncoefficient in a zeolite can increase as a function of chain length.References and Notes(1) Ka¨rger, J.; Ruthven, D. M. Diffusion in Zeolites and othermicroporous solids; John Wiley & Sons, Inc.: New York, 1992.(2) Eic, M.; Ruthven, D. M. Zeolites 1988, 8, 472-479.(3) Jobic, H.; Me´thivier, A.; Ehlers, G.; Farago, B.; Haeussler, W.Angew. Chem. Int. Ed. 2004, 43, 364-366.(4) Dubbeldam, D.; Smit, B. J. Phys. Chem. B 2003, 107, 12138-12152.(5) Yoo, K.; Tsekov, R.; Smirniotis, P. G. J. Phys. Chem. B 2003,107, 13593-13596.* To whom correspondence should be addressed. E-mail: dubbelda@science.uva.nl.† University of Amsterdam.‡ ChevronTexaco.Figure 1. Diffusivities for linear alkanes in LTA-type zeolite as afunction of carbon number. The simulations are compared to theexperimental results of Eic and Ruthven.1,2 For reference, the disputeddata of Figure 11b (ref 4) and the new experimental data of Jobic etal.3 are included.16330 J. Phys. Chem. B 2004, 108, 1633010.1021/jp040487o CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 09/17/2004COMMENTSComment on “Computer Simulation ofIncommensurate Diffusion in Zeolites:Understanding Window Effects”Douglas M. RuthvenDepartment of Chemical Engineering, UniVersity of Maine,Orono, Maine, 04469ReceiVed: January 7, 2004This paper1 presents an interesting example of the power ofmolecular simulations to predict the diffusivities of sorbatemolecules in zeolite adsorbents under sterically restrictedconditions. There are, however, two major points which shouldnot be allowed to pass without comment.(1) Diffusivities for Linear Alkanes in 5A Zeolites. Figure11b, which purports to show the experimental data of Eic andRuthven2 extrapolated to 600 K, indicates a clear maximum inthe plot of diffusivity vs carbon number at C10. This is inaccordance with the molecular simulation results which alsosuggest a maximum at C10, leading the authors to state: “Theagreement between simulated data on a cation-free LTA typesieve and the experimental data on a cation-loaded LTA typezeolite is remarkable.” In fact the experimental data of Eic andRuthven, which are reproduced in Figure 1, show a monotonicdecline in diffusivity with carbon number. There is no evidenceof a maximum in the diffusivity at C10. It should be noted thatthe ZLC measurements reported by Eic and Ruthven were madeusing several different samples of relatively large (7.3-55 ím)laboratory synthesized crystals of 5A zeolite. Earlier gravimetricdata for small (3-4 ím) commercial Linde 5A crystals obtainedby Loughlin, Derrah, Doetsch, and Vavlitis3-5 were includedfor comparison. As may be seen from Figure 1, the two datasets are quite distinct with the reported diffusivities for thecommercial material being substantially smaller and showing asteeper decrease with carbon number. We now suspect that thelow diffusivity values observed for the commercial materialreflect a surface resistance resulting from the severe conditionsused in the initial dehydration step, as suggested by Kondis andDranoff many years ago.6 However, neither data set shows anyevidence of a maximum in diffusivity at C10. The plot shownin Figure 11b of the paper by Dubbeldam and Smit is misleadingand could only have been obtained by a judicious selection ofdata points from the two different data sets.(2) The “Window Effect” in Zeolite T. The 1973 paper byGorring7 in which the idea of a “window effect” was firstintroduced is probably one of the most frequently citedreferences in zeolite science. Unfortunately, because the reportedeffect appears novel and interesting the validity of Gorring’sexperimental data has often been accepted without criticalreexamination. He used very large (5 gm) samples of small(micron sized) zeolite crystals and made integral gravimetricmeasurements between zero loading and one atmosphere ofsorbate pressure. Only adsorption measurements were made andneither the mass of the sample nor the pressure step was changedto check for system linearity. Anyone with practical experienceof this kind of measurement would realize that it would bevirtually impossible to obtain reliable intracrystalline diffusivitydata from such experiments. The results would almost certainlyhave been impacted by nonlinearity, extracrystalline diffusionand heat transfer effects. Such considerations led two inde-pendent research groups to attempt to reproduce these measure-ments using more modern experimental techniques.8,9 This isnot straightforward since zeolite T is a poorly defined materialbeing an intergrowth of erionite and offretite. Cavalcante et al.8therefore used two different materials containing differentproportions of offretite and erionite. Their results are shown,together with the data of Magalhaes et al.9 in Figure 2. Althoughthe data show considerable scatter the two data sets areremarkably consistent and suggest a monotonic decline indiffusivity with carbon number. There is no evidence to suggestthat the minimum and maximum reported by Gorring (at C8and C12 respectively) are genuine.Although, on the basis of this evidence, it would bepresumptuous to state that the “window effect” does not exist,Figure 1. Variation of corrected diffusivity (D0) with carbon numberfor linear alkanes in 5A zeolites at 600K. 9,2; gravimetric data ofYucel obtained with 3 different samples of large 5A crystals.10 b, ZLCdata of Eic and Ruthven2 for large laboratory synthesized 5A crystals.[, gravimetric data of Doetsch3 and Vavlitis5 for small (3-4 ím)commercial 5A crystals.Figure 2. Variation of diffusivity with carbon number for linear alkanesin Zeolite T at 573K. 9, Data of Magalhaes et al.;9 [, data ofCavalcante et al.8 (D0).16328 J. Phys. Chem. B 2004, 108, 16328-1632910.1021/jp040003o CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 09/17/2004I believe that it is reasonable to assert that there are, as yet, noreliable macroscopic measurements that show the existence ofsuch an effect.References and Notes(1) Dubbeldam, D.; Smit, B. J. Phys. Chem. B 2003, 107, 12138-12152.(2) Eic, M.; Ruthven, D. M. Zeolites 1988, 8, 472-479.(3) Doetsch, I. H.; Loughlin, K. F.; Ruthven, D. M. Can. J. Chem.1974, 52, 2717.(4) Ruthven, D. M.; Derrah, R. I.; Loughlin, K. F. Can. J. Chem. 1973,51, 3514.(5) Vavlitis, A. P.; Ruthven, D. M.; Loughlin, K. F. J. Colloid InterfaceSci. 1981, 84, 526.(6) Kondis, E. F.; Dranoff, J. S. Ind. Eng. Chem. Process Design DeV.1971, 10, 108.(7) Gorring, R. L. J. Catalysis 1973, 31, 13-26.(8) Cavalcante, C.; Eic, M.; Ruthven, D. M.; Occelli, M. L. Zeolites1995, 15, 293-307.(9) Magalhaes, F. D.; Lawrence, R. L.; Conner, W. C. AIChE J. 1995,42, 68-86.(10) Yucel, H.; Ruthven, D. M. J. C. S. Faraday Trans. 1 1980, 76,71-83.Comments J. Phys. Chem. B, Vol. 108, No. 41, 2004 16329

Reply to the Comment on “Computer Simulation of Incommensurate Diffusion in Zeolites: Understanding Window Effects”

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Reply to the Comment on “Computer Simulation of Incommensurate Diffusion in Zeolites: Understanding Window Effects”

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