Molecular Cage Nestling in the Liquid-Phase Adsorption ofn-Alkanes in 5A Zeolite

Inge Daems,† Gino V. Baron,† Sudeep Punnathanam,‡ Randall Q. Snurr,‡ andJoeri F. M. Denayer*,†

Department of Chemical Engineering, Vrije UniVersiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium, andDepartment of Chemical & Biological Engineering, 2145 Sheridan Road, Northwestern UniVersity,EVanston, Illinois 60208

ReceiVed: October 17, 2006; In Final Form: December 4, 2006

The liquid-phase adsorption of C5-C24 linear alkanes in zeolite 5A at room temperature was studied usingthe configurational bias Monte Carlo molecular modeling technique and the batch experimental method. Upto C16, a highly discontinuous variation of the saturation capacity with the carbon number occurred. Whereasthe shortest chains remain as a whole within the cage, longer chains are distributed in a more bent configurationover adjacent cages or even adopt highly coiled configurations within one cage. For alkanes greater than C16,a sharp, unexpected drop in adsorption capacity with a minimum at C18-C19 was observed experimentally.The almost complete exclusion of these alkanes was not observed in the molecular simulation under equilibriumconditions and is explained by an extremely slow diffusion of C18-C19 because of “cage nestling”, i.e., animperturbable adsorption of tightly fitting highly coiledn-alkanes in the first supercage they enter. Thecompetitive adsorption and counterdiffusion of alkane mixtures is strongly affected by the differences inadsorption mechanism for alkanes of different chain length.

Introduction

Because of their unique shape selectivity, i.e., the discrimina-tion between molecules on the basis of their molecular size orshape, zeolites possess outstanding properties for use asheterogeneous catalysts or adsorbents. An extreme case is foundin zeolite 5A, which adsorbs linear hydrocarbon chains insideits pore system but completely excludes branched hydrocarbonsfrom its internal voids, as the latter molecules are too large todiffuse through the narrow pore entrances of 5 Å. Recently,more subtle cases of size and shape selectivity, based onmolecular packing effects, were reported. “Molecular packing”can be defined as the arrangement of adsorbed molecules insideconfined pore systems, thereby optimizing the balance betweenenergetic and entropic contributions. Such packing effects areexpected to become important at a high degree of pore filling.

Unexpected size-dependent selectivity effects were, forexample, observed in the adsorption of binary C5-C22 n-alkanemixtures on ZSM-5, a zeolite with intersecting channel-likepores.1-3 Besides the expected preferential adsorption of longeralkanes over shorter ones, selectivity inversions (i.e., preferentialadsorption of the short chain) and even azeotropic behavior wereobserved. This was explained by the entropic advantage ofmolecules whose chain length matches the dimensions of thechannel segments between intersections. Those molecules areable to fill up the complete pore volume with a minimal entropiccost and are preferentially adsorbed compared to molecules thatare too long for this optimal fit. This phenomenon, theoreticallydefined for the first time for C6-C7 alkanes in silicalite as“commensurate freezing” by Smit and Maesen,4 was experi-mentally observed with other zeolites having intersecting poresystems.5 Chain-length-induced selectivity effects are not re-

stricted to channel-type zeolites but also occur in cage-typezeolites, as was, for example, shown for the liquid-phaseadsorption of C6-C12 alkenes on NaY.6

Remarkable chain-length dependencies of intracrystallinediffusion coefficients of hydrocarbon chains have been reported.Much interest has been shown in the so-called “window effect”,i.e., a periodic rise and fall of the diffusion coefficients withalkane chain length in zeolites with cages connected via narrowwindows. This effect was first reported by Gorring on ERI-typezeolites in 1973.7 Gorring’s data are now widely discredited, butseveral theoretical studies indicate a window effect should oc-cur,8 and this has sustained interest in this controversial topic,despite more recent experiments where no window effect wasseen.9-11

Only recently, a window effect was found in molecular simula-tions using configurational-bias Monte Carlo and transition-statetheory. Deviations from the expected monotonous variation indiffusion coefficient with carbon number on all-silica ERI-,AFX-, CHA-, RHO-, and KFI-type zeolites12-13 were observed.According to these simulations, short alkanes diffuse slowlybecause they tend to stay caught inside a cage and have to sur-mount a large energy barrier generated by the window whendiffusing between cages. The diffusion coefficient increases byorders of magnitude when the effective length of the alkanechain approaches the cage size. Because of the uncomfortableadsorption of such molecules, a maximum in diffusion coef-ficient is obtained for the largest alkane chain still fitting insidea cage. Once the effective chain length exceeds the cage dimen-sions, the molecules no longer adsorb in a curled conformationinside a single cage but stretch through the window across twocages. Now, stabilizing interactions between the alkane and thezeolite framework atoms increase again with chain length, lead-ing to a decrease in mobility. Also according to these simula-tions, the window effect can be observed on Linde Type A(LTA) zeolites for alkanes containing more than 23 C atoms.

* Corresponding author. E-mail: joeri.denayer@vub.ac.be.† Vrije Universiteit Brussel.‡ Northwestern University.

2191J. Phys. Chem. C2007,111,2191-2197

10.1021/jp0668145 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 01/17/2007

Unfortunately, the maximum chain length that has beenexperimentally studied is C16.14,15In this study a monotonic trendof the diffusivity with carbon number was found. On the otherhand, neutron spin-echo experiments performed in 5A, a zeolitewith the LTA topology, showed the transport diffusivity dropsto a minimum at C8, reaches a maximum at C12, and subse-quently decreases again for C14.16 This local maximum at C12

was not confirmed by Gunadi and Brandani,17 who measuredthe kinetics of C6-C14 by means of the zero-length columntechnique. They observed an initial monotonic decrease of thediffusivity until C11, followed by a slight increase between C11

and C13 which leveled of at C14. Gravimetric experiments withzeolites LTL and ZSM-12 demonstrated a periodic dependenceof diffusion coefficients on carbon number for C1-C11n-alkanes.18

Despite these efforts to measure and model transport proper-ties (diffusion coefficients) of alkanes in cage-type zeolites withnarrow windows, not much attention has been paid to chain-length effects in the adsorption and molecular packing of alkanesin such materials. Moreover, most studies are limited to gas-phase conditions, which yield a low degree of pore filling andrelatively short alkanes. Therefore, the aim of this work was tostudy chain-length effects in the liquid-phase adsorption of longchainn-alkanes on zeolite 5A. This zeolite is of major industrialimportance for the adsorptive separation ofn-alkanes used forsolvent production, octane boosting, and the production ofbiodegradable detergents and plasticizers.19 Zeolite 5A is a Na+-and Ca2+-containing LTA zeolite with pore openings (windows)of 5 Å. When passing through these windows, linear alkanesenter a large spherical supercage (R-cage) having an internaldiameter of 11.4 Å.20 Each supercage contains six windowsfacing each other perpendicularly. Each supercage is surroundedby eight sodalite cages (â-cages) that are inaccessible for linearalkanes.

To date, various techniques have been used to determine thesaturation adsorption capacities ofn-alkanes on zeolite 5A (seeTable 1).21-27 Most studies only involve a very limited number

of n-alkanes, and few results are available in the interestingcarbon-number region (C10-C24). Generally, the maximumnumber of molecules adsorbed per supercage is found todecrease with increasing carbon number,21,23,25,27but the avail-able data do not allow one to derive a clear relationship betweenadsorption capacity and chain length. The adsorption ofn-alkanemixtures on zeolite 5A is poorly investigated. A predominantadsorption of the longer alkane is observed for a limited numberof mixtures.22,26,28-30 This is in line with the generally observedmonotonic increase of interaction energy with chain length onzeolite 5A.22,27,31,32

In the present work, the liquid-phase adsorption of C5-C24

n-alkanes and binary mixtures of these components on zeolite5A is studied experimentally and by molecular modeling. Ahighly discontinuous dependency of the adsorption capacity onthe chain length is observed. The observations are discussed interms of molecular packing in the cages of the zeolite.

Computational Methods

Configurational-bias Monte Carlo simulations of linear al-kanes in LTA-5A were performed in the grand canonicalensemble. Configuration-bias Monte Carlo33-35 has become apowerful tool for studying adsorption in zeolites4,36 and hasrecently been applied to liquid-phase adsorption conditions.2,37

The alkanes were modeled as flexible chain molecules with rigidbond lengths. Methyl and methylene groups were modeled asunited atoms.2,36-38 The zeolite’s silicon, aluminum, and oxygenatoms were placed at the crystallographic positions determinedby Pluth et al.39 and were not allowed to move. The simulationcell consisted of eight supercages. Similar to the sample usedin experiments, the Si/Al ratio was kept equal to unity, and thesilicon and aluminum atoms were arranged to satisfy theLowenstein rule. The sodium and calcium ions were mobile. Atotal of 28 Na+ ions and 34 Ca2+ ions were present in thesimulation cell to closely match the Na+/Ca2+ ratio found inthe experimental sample while keeping the entire system neutral.The force field for the entire system was obtained from Garcia-Perez et al.38 and is summarized in Table 2. This is a Kiselev-type potential40,41 in which the sorbate molecules interact onlywith the oxygen atoms of the framework. The electrostaticinteractions between the cations and the framework atoms werecalculated using Ewald summation.

The simulations were carried out using the MUSIC code.40

We studied adsorption of linear alkanes from C5 to C24 underliquid-phase conditions at a temperature of 298 K using a similarstrategy as in our past work.2,37The fugacities needed as inputsto the simulations were calculated using the Peng-Robinsonequation of state. They corresponded to the liquid-phasefugacities of the mixture ofn-alkanes and isooctane at the bubblepoint pressures of the mixtures and a temperature of 298 K.The binary interaction parameters betweenn-alkanes andisooctane were taken from the commercial software packageHYSYS. Since isooctane is not adsorbed, only the adsorptionof then-alkanes was simulated. Each simulation run consistedof 10 million Monte Carlo moves starting from an empty zeolite.The thermodynamic averages were obtained from the final 7million moves. Monte Carlo moves for alkanes includedtranslation, insertion, deletion, and partial regrowth, as describedby Macedonia and Maginn.36 The Monte Carlo moves forcations included local translations and more aggressive reinser-tions at random locations. Prior to each simulation run, thecations were randomly placed in the simulation cell along withthe framework atoms, and the entire system was equilibratedin the canonical ensemble.

TABLE 1: Literature Data of C 5-C20 n-Alkane AdsorptionCapacities on Zeolite 5A

mmol/g zeolitecarbon

no.

ref 19a

molecules/supercage ref 20b ref 21c ref 22d ref 23e ref 24f ref 25g

5 2.03 1.956 0.34 1.62 1.637 1.43 1.258 3.1 0.35 1.349 2.7

10 2.7 0.29 0.07 0.63 0.9811 2.5 0.71 0.9712 2.3 0.28 0.73 0.77 0.64 0.9213 1.9 0.27 0.64 0.5814 2.0 0.57 0.69 0.511516 1.6 0.22 0.431718 1.3 0.51920 0.35

a Thermal desorption by thermogravimetry.b Pellets containing inertclay binder; inverse gas chromatography; 320-380 °C. c Pelletscontaining inert clay binder; liquid-phase batch experiments; 291 K.d Pellets containing inert clay binder; liquid-phase batch experiments;303 K. e Pellets containing inert clay binder; liquid-phase batchexperiments; 303 K.f Zeolite composition: Ca4.5Na3(AlO2)12(SiO2)12;liquid-phase batch experiments; 295 K.g Zeosorb 5A (60% Ca2+

exchange); gas-phase experiments using volumetric or gravimetricequipment at 295 K for C5-C6 and 457-595 K for C10-C18.

2192 J. Phys. Chem. C, Vol. 111, No. 5, 2007 Daems et al.

Experimental Methods

Zeolite 5A (Purmol 5, Zeochem) having the dehydrated unitcell composition 8[Ca4.2Na3.6(AlO2)12(SiO2)12] was used as anadsorbent in the physical form of crystals. The Dubininmicropore volume and surface area of the 5A sample determinedby N2-porosimetry (Sorptomatic 1990) were 0.26 mL/g and 724m2/g, respectively. On the basis of the molecular weight of adehydrated unit cell, the number of supercages per g of 5A wascalculated to be 3.5848× 1020.

For both equilibrium and kinetic experiments, isooctane(>99.5% purity, BioSolve) was used as a nonadsorbing solvent.Before its use, isooctane was dried and purified and added in100 mL portions to all∼25 g samples of zeolite 5A beadsactivated at 350°C (Z5-01, Zeochem). Alln-alkanes used inthis work were of analytical grade, and experiments wereperformed at room temperature (299 K) and atmosphericpressure.

Batch adsorption experiments were performed to (i) measuremaximum adsorption capacities of puren-alkanes and (ii) studythe competitive adsorption of binaryn-alkanes mixtures atequilibrium. Zeolite samples (∼0.2 g) were put into 10 mL glassvials and heated at 0.5°C/min in a ventilated oven until atemperature of 350°C was achieved. This final temperature wasmaintained overnight. For the capacity measurements,n-alkane/isooctane mixtures were prepared and added to the zeolite-containing vial. The concentration of then-alkane in the mixtureranged from 4.07 to 5.02 wt %. To verify whether the saturationcapacity was affected by the concentration of the adsorbate inthe mixture, the capacities of C10 and C18 were measured atthree different concentrations (2.5, 4.5, and 7 wt %). No effecton the adsorption capacity was observed. For the competitiveadsorption experiments, binary alkane mixtures in isooctane withvarying fractions of the adsorbing components were added to

the activated zeolite. The total amount of the adsorbingcomponents varied from 4 to 25 wt %.

To study adsorption and desorption kinetics, mixtures with atotal volume of about 250 mL were added to about 8 g ofzeolite5A. Such a large mixture volume is needed to keep the volumeof the external phase quasi constant during sampling. For thestudy of the adsorption kinetics of pure alkanes (C8 and C18),samples of the external liquid were withdrawn at regular timeintervals after addition of then-alkane/isooctane mixture to theadsorbent. In the counterdiffusion experiments, a second alkanewas added after 48 h, when the adsorption equilibrium of thefirst alkane was reached. Here periodic sampling only startedafter addition of the second alkane.

The composition of the starting liquid mixtures and thesamples withdrawn after contact with the zeolite were analyzedin a gas chromatograph with a flame ionization detector usingan Agilent HP-5 column (5% phenyl methyl siloxane, 30 m×320µm × 0.25µm). The amount adsorbed at equilibrium wasobtained by calculation of the mass balance.

Results and Discussion

In typical grand canonical Monte Carlo (GCMC) simulations,the number of molecules in the system fluctuates around anaverage value, and the average value is reported. For the systemsstudied here, however, we found that under maximum loadingconditions, there were virtually no fluctuations in the totalnumber of molecules adsorbed. The zeolite pores were full ofmolecules, and hence, insertions and deletions of molecules werehighly unfavorable. In addition, the movements of cations duringthe simulations were very limited. To obtain reliable averages,we performed four sets of simulations, each with different initialarrangement of cations. Figure 1 shows the maximum loadingof alkanes in zeolite 5A as a function of the carbon number asobtained from these simulations. The values for the maximumloading are consistent across the different cation arrangementsand starting configurations of the simulations. As expected, themaximum loading of molecules per supercage decreases withincreasing carbon number. However, this decrease is not uniformand plateaus in the number of molecules per supercage areobserved at 3 for C6-C7, 2 for C9-C12, 1.5 for C13-C14, and1 for C16- C23.

The number of methyl or methylene groups per supercagealso shows interesting behavior. The series of local maxima and

TABLE 2: Force Field Parameters, IncludingAlkane-Alkane, Alkane-Cation, Oxygen-Alkane, andOxygen-Cation Lennard-Jones Parameters, IntramolecularInteractions, and Partial Charges

Nonbonded

description ε/kB, K σ, Å

CH3-CH3 108.0 3.76CH3-CH2 77.7 3.86CH2-CH2 56.0 3.96CH3-O 93.0 3.48CH2-O 60.5 3.58CH3-Na+ 443.73 2.65CH2-Na+ 310 2.95CH3-Ca2+ 400.73 2.6CH2-Ca2+ 440 2.8Na+-O 23.0 3.4Ca2+-O 18.0 3.45

Bond-Bending

Ubending) 12

kθ(cosθ - cosθ0)2 kθ/kB ) 625 000K θ0 ) 1140

Bond-Torsion

Utorsion) ∑n)0

5

ηn cosn φ,

η/kB, K ) {1204.654, 1947.74,-357.845,

-1944.666, 715.690,-1565.572}Partial Charges, e

qSi ) 2.05 qAl ) 1.75 qo ) - 1.2 qNa+ ) 1.0 qCa2+ ) 2.0

Figure 1. Maximum loading ofn-alkanes in LTA-5A as determinedfrom simulations plotted against the carbon number of the alkane. Thehistograms give the average number of molecules adsorbed in onesupercage with the values shown on the lefty-axis vs carbon numberon thex-axis.

Liquid-Phase Adsorption ofn-Alkanes in 5A Zeolite J. Phys. Chem. C, Vol. 111, No. 5, 20072193

minima are due to the preference for integer (or half integer)numbers of molecules per supercage. For example, C9 to C12

all adsorb at 2 molecules per supercage, so the number of Catoms per supercage increases from C9 to C12. At C13, theloading drops to 1.5 molecules per supercage, causing a dropin the number of C atoms per supercage. It is also noteworthythat in the entire range of chain lengths studied the averagenumber of C atoms per supercage never exceeds 25.

The experimental adsorption capacity of zeolite 5A for thesame series ofn-alkanes is plotted as a function of carbonnumber in Figure 2. In these experiments, the composition ofthe external liquid in contact with the zeolite remained constantbetween 1 and 100 h, indicating that adsorption equilibrium wasreached. Similar to that of the simulations, the volume ofn-alkane adsorbed per unit mass of zeolite 5A varies in a highlynonmonotonic way with the molecular chain length (Figure 2A).None of the alkanes is able to occupy the entire N2-microporevolume of 0.26 mL/g in the experimental conditions. Of allalkanes, C12 occupies the largest volume in the micropores (0.21mL/g). Unexpectedly, a steep drop in volume adsorbed isobserved for molecules larger than C16. This sudden decreasein volume adsorbed gradually recovers for alkanes containingmore than 19 carbon atoms to eventually reach a capacity of0.19 mL/g 5A for C24, which is again comparable to the volumeoccupied by smaller alkanes. Although the simulations consis-tently predict a higher loading compared with that of experiment,the variation of loading with carbon number is similar up toC16 (parts B and C of Figure 2). From C5 to C9, the experimentalnumber of molecules adsorbed per supercage gradually de-creases with increasing carbon number from 2.75 to 1.5 (Figure2B). For linear alkanes with chain lengths between 9 and 13,about 1.5 molecules are adsorbed per supercage. For longeralkanes the number of adsorbed molecules per supercage

decreases again to reach an adsorption capacity of 1 moleculeper supercage for C16.

As discussed above, the number of carbon atoms adsorbedper supercage periodically increases and decreases with chainlength (Figure 4C). Between C5 and C16, both experiments andsimulations show local maxima at C7 and C12. The van derWaals length of C5 to C7 increases from 8.84 to 11.38 Å.37 Thismeans that heptane can still sit inside a supercage (diameter11.4 Å) in a stretched conformation although the simulationsshow very few heptane molecules in the all-trans, stretchedconformation. Octane and nonane, having lengths of 12.65 and13.91 Å, respectively, must bend in order to fit into a supercage,which causes the packing efficiency to decrease. From C9 toC12, about 1.5 molecules are experimentally adsorbed persupercage. One molecule is fully adsorbed in the supercage,whereas a second molecule is distributed between the same cageand a neighboring cage. From the simulations, it is possible tolook at the different conformations that the adsorbed alkanemolecules adopt in the zeolite cages (Figure 3). For alkanescontaining 12 or more C atoms, configurations are indeedobserved where a single molecule is adsorbed over two adjacentsupercages. Alkanes C12- C16 show configurations where one

Figure 2. Experimental results (closed symbols) for adsorptioncapacities of C5-C24 n-alkanes in LTA-5A. The average of thesimulations sets is added in graph B and C (open symbols) (SC)supercage).

Figure 3. Snapshots showing different conformations ofn-alkanesadsorbed in LTA-5A during molecular simulations: (A) stretched C5

molecules in a supercage, (B) two C14 molecules coiled in adjacentsupercages and one molecule distributed between the two cages, (C)highly coiled C16 molecule in a supercage. For clarity, each alkanemolecule has a different color. The cations are not shown.

2194 J. Phys. Chem. C, Vol. 111, No. 5, 2007 Daems et al.

molecule is fully adsorbed in a supercage and a second one isdistributed between the same cage and a neighboring cage, asshown in Figure 3B. According to the experimental results thisconfiguration leads to an optimal packing for C12, resulting inan adsorption capacity of 18-19 C atoms per supercage. Withincreasing chain length, the well organized adsorption of 1.5molecules per supercage becomes more difficult because ofsteric constraints, such that the number of adsorbed moleculesstarts to evolve toward 1 molecule per cage. Both in theexperiments and in simulations, this adsorption capacity isexactly reached for C16. Given its length in the all-trans state(22.79 Å), C16 is severely bent and coiled in order to fit into asingle supercage (Figure 3C). When the number of carbon atomsof the adsorbate becomes comparable to the experimentalmaximum number of C atoms that fit inside a supercage (about19 C atoms; Figure 2C), the experimental saturation capacitydrastically decreases. An absolute minimum in the adsorptioncapacity is reached for C18-C19, occupying about 2.5 C atoms/SC, corresponding to only a small fraction of the available porevolume (Figure 2A). The adsorption capacity again increasesfrom C20 on. Remarkably, this large variation in adsorptioncapacity in the C16-C24 range observed experimentally is notreproduced by the simulations, which predict a constant loadingof 1 molecule/supercage for C16-C24. Also, experiments includ-ing C10, C12, C13, C14, C16, and C18 performed in gas-phaseconditions at much higher temperatures than those of the presentstudy show no drop in the pore filling capacity for C18.27 Ourobservations indicate that C18 and C19 adsorb in a completelydifferent way compared to the other chains in liquid phase atroom temperature (vide supra).

The liquid-phase adsorption of alkanes in 5A was furtherevaluated by measuring the binary adsorption isotherms of C6/C7, C7/C8, C9/C10, C12/C13, C12/C14, C12/C15, and C16/C18. Theisotherms of C12/C13, C12/C15, and C16/C18 are illustrated inFigure 4. For C12/C13 and C12/C15, the isotherms show a classicalshape (Figure 4A,B). The adsorbed amounts of C13 for the C12/C13 mixture and of C15 for the C12/C15 mixture increase withtheir respective concentration in the external liquid phase. Atthe same time, the total amount of C atoms adsorbed persupercage decreases from the saturation capacity of C12 (18.8C atoms/SC) to that of respectively C13 (18.3 C atoms/SC) andC15 (16.4 C atoms/SC). For the C16/C18 mixture, however, theamount of C16 adsorbed, and herewith the total adsorptioncapacity, decreases drastically in the presence of trace amounts

of C18 (Figure 4C). It appears that the presence of C18 preventsC16 from adsorbing in the zeolite.

Figure 5 shows the selectivity diagrams of all studied binarymixtures except that of C16/C18. The latter mixture is excludedbecause of its unusual behavior. Selective adsorption of thelonger alkane was observed for C6/C7 and C9/C10. Selectiveadsorption of the shorter alkane occurred with C7/C8, C8/C12,C12/C13, C12/C14, and C12/C15. In all cases, the alkane with thehigher adsorption capacity is selectively adsorbed during thecompetitive adsorption of binary mixtures in the C5-C16 range.For instance, C7, having a higher adsorption capacity than C6

and C8 (Figure 2), is selectively adsorbed from its mixture withboth components. Thus, a better fitting ofn-alkanes inside thesupercages reflects not only on the maximum adsorptioncapacities but also on the adsorption selectivities.

The unusual behavior of C16-C22 alkanes, especially thealmost complete exclusion of C18 and C19 from the internal voidsof 5A, is in disagreement with molecular simulations of theadsorption equilibrium capacity, which predict a constantadsorption capacity of one molecule/supercage for C16-C24. Itcould be argued that adsorption equilibrium is not reached atroom temperature in the experiments with long alkane chainsas a result of a slower diffusion rate of those molecules.However, uptake experiments in which 5A is contacted withmixtures of (1) C8/isooctane and (2) C18/isooctane show thatboth C8 and C18 are adsorbed at about the same rate (Figure 6).In both cases, a plateau in the uptake curve is reached after 6min for both components. Even after 100 h, no further variationin the amount adsorbed occurred. The zeolite pores adsorb no

Figure 4. Binary adsorption isotherms of (A) C12/C13, (B) C12/C15,and (C) C16/C18 mixtures on zeolite 5A withXi the external mole fractionof the longer alkane.

Figure 5. Selectivity diagram for the adsorption of binary alkanemixtures on zeolite 5A.

Figure 6. Adsorption of pure C8 and C18 from iso-C8 on zeolite 5A asa function of time.

Liquid-Phase Adsorption ofn-Alkanes in 5A Zeolite J. Phys. Chem. C, Vol. 111, No. 5, 20072195

more than 0.03 mL of C18 per g of 5A, corresponding to 2.8 Catoms/SC. Thus theoretically, a large fraction of the internalpore volume should still be available for the adsorption of otheralkane molecules. When C16 is added to such a 5A zeolitealready exposed to a C18/isooctane mixture, no uptake at all ofC16 in the 5A pores is observed (Figure 7A). When 5A is firstexposed to a C16/isooctane mixture, an adsorption capacity ofabout one C16 molecule per cage is obtained (Figure 7B). WhenC18 is added to this system, C16 slowly desorbs from the zeolitepores and is replaced by C18 molecules. In these conditions,C18 is capable of filling a much larger fraction of the porevolume as compared with experiments in which C18 is addedto an “empty” zeolite. Both for C16 and C18, a capacity of about0.5 molecules per cage is reached, which might correspond toa filling of 1 out of 2 cages with each molecule. A similar effectis seen when 5A is first contacted with C7 prior to addition ofC18 (Figure 7C). In this case, it takes more than 100 h to reacha constant liquid composition.

The present experiments and simulations indicate that mo-lecular packing and size matching between the alkanes and thezeolite cages govern the adsorption and transport properties.As indicated in the work of Dubbeldam and Smit, a windoweffect occurs with zeolite 5A at higher chain lengths than wascommonly expected.13 This is because long alkane chains adoptcoiled configurations in the cages of 5A. Thus, the behavior ofstretched molecules should not be considered but rather thealkanes in their coiled configuration in the cages. The presentmolecular simulation allowed the determination of the percent-age of bonds in the trans configuration versus those in gaucheconfigurations for all the alkanes adsorbed in LTA-5A. Thisgives a measure of the extent to which a molecule is bent.During simulations it was observed that only molecules of C5-C7 and C13 showed some configurations where the entiremolecule was in the all-trans (linear) conformation. The shortlengths of C5-C7 make it possible for them to be in the all-trans conformation inside a supercage (Figure 3A). The C13

molecule that was found in the all-trans conformation wasstretched over two adjacent supercages. Figure 8 shows thepercentage of bonds in the trans configuration for all the alkanes.For shorter alkanes, this value is between 70 and 80%. Withincreasing chain length, the configurations of the alkanes becomemore and more coiled to allow fitting in the cage. From C16

on, only one alkane is adsorbed per supercage. In order to diffusethrough the 5A crystal, these molecules have to partially uncoiland surmount an energy barrier when moving through the smallwindow connecting the cages. According to the experiments(Figure 2C), the maximal number of carbon atoms adsorbedper cage is about 18-19 on average. Thus, molecules smallerthan C18 still leave some free space inside the cage, such thatthis reorganization still occurs relatively easily. However, C18

and C19 fit so tightly in a highly coiled configuration in thecage that uncoiling and diffusion to a neighboring cage becomevery unfavorable. During this “cage nestling”, each adsorbingC18 or C19 molecule is trapped inside the first cage it enters,leading to diffusion coefficients which are tremendously smallerthan those of less strongly adsorbed molecules. In this “frozen”system, only a limited number of cages of the 5A crystals arefilled and further access for other molecules is inhibited. Oncethe chain length exceeds this critical carbon number, alkaneshave more difficulty in fitting in one cage and are forced tostretch along different cages, resulting in a faster diffusion. Themutual exchange of C18 and preadsorbed alkanes reduces theactivation energy for diffusion, such that the real equilibriumcapacity is reached within the time scale of the experiment. Itis possible that the presence of other alkanes prevents C18 fromadopting its coiled configuration inside the supercage, leadingto a faster diffusion mechanism.

Conclusions

The adsorption and diffusion mechanisms ofn-alkanes inzeolite 5A depend strongly on chain length. Experiments andmolecular simulations demonstrate a high degree of organizationinside the pores. In liquid-phase conditions at a high degree ofpore filling, certain alkanes adopt highly coiled configurationsto fit inside the supercages, whereas other chains are extendedover more than one supercage. Alkanes having a chain lengthwhich approaches the limiting number of C atoms that can behosted in one supercage show an abnormally low experimentalsaturation capacity and are almost completely excluded fromthe internal voids of the zeolite. These alkanes prevent theadsorption of other linear alkanes present in the liquid mixture.Thus, the confinement between zeolite substructures (cages andwindows) andn-alkane molecules, in either a slightly bent or ahighly coiled state, leads to unexpected capacity and selectivityeffects. Obviously, the data shown above provide essentialinformation for the separation of hydrocarbon mixtures contain-ing linear C17-C20 alkanes.

Figure 7. Counterdiffusion of (A) C16 (adsorbing) and C18 (desorb-ing), (B) C18 (adsorbing) and C16 (desorbing), and (C) C18 (adsorbing)and C7 (desorbing) on zeolite 5A.

Figure 8. Percentage of bonds in trans configurations plotted againstthe carbon number of the alkane as observed from simulations.

2196 J. Phys. Chem. C, Vol. 111, No. 5, 2007 Daems et al.

Acknowledgment. J.D. is grateful to the F.W.O.-Vlaanderenfor a postdoctoral research fellowship. Acknowledgment is madeto the donors of the American Chemical Society PetroleumResearch Fund for partial support of this research. The authorsalso thank David Dubbeldam and Sofia Calero for making theirforce field parameters available to us prior to publication andfor helpful discussions.

References and Notes

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