J. Phys. Chem. C 2008, 112, 16593–16599
16593
Cage and Window Effects in the Adsorption of n-Alkanes on Chabazite and SAPO-34 Joeri F. M. Denayer,*,† Lisa I. Devriese,† Sarah Couck,† Johan Martens,‡ Ranjeet Singh,§ Paul A. Webley,§ and G. V. Baron† Dienst Chemische Ingenieurstechniek, Vrije UniVersiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium, Centrum Voor OpperVlaktechemie en Katalyse, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg 23, B-3001 LeuVen, Belgium, and Department of Chemical Engineering, Monash UniVersity, PO Box 36, Clayton Victoria, 3800, Australia. ReceiVed: May 16, 2008; ReVised Manuscript ReceiVed: August 07, 2008
The adsorption of C1-C14 n-alkanes on SAPO-34 and Na-CHA zeolites was studied at low surface coverage using the pulse chromatographic method. On both isostructural materials, containing cages interconnected via small windows, the Henry adsorption constant K′ varies in a nonmonotonous way with carbon number. K′ increases more or less exponentially with carbon number from methane to n-hexane. On Na-CHA, K′ decreases from n-hexane, reaches a minimum at n-octane, and then increases again. On SAPO-34, a minimum is attained at n-decane. From this point on, K′ increases till the longest alkane studied, i.e., n-tetradecane. On SAPO-34, the increase in zero coverage adsorption enthalpy per additional methylene group in the alkane chain becomes smaller for carbon numbers between 5 and 11. A local minimum in adsorption enthalpy occurs at n-undecane. A compensation plot between adsorption entropy and enthalpy shows a pronounced discontinuity at n-undecane for SAPO-34. The present observations provide additional proof for the existence of “window effects”. Given their molecular length, C1-C6 alkanes adsorb in a relatively unhindered way in the SAPO-34 cages. C7-C10 alkanes adopt a coiled configuration inside the cage, which strongly affects their adsorption enthalpy and entropy. From n-undecane onward, the alkanes are no longer fitting within the cage and protrude the small windows connecting the cages. The presence of cations on Na-CHA results in stronger energetic interactions compared to SAPO-34, which shifts the window protrusion to lower chain lengths. Introduction Adsorption, diffusion and reaction in the confinement of zeolitic nanopores often occur according to unconventional patterns. The window effect as first described by Gorring is probably the earliest experimental example of unusual behavior in the diffusion of n-alkanes inside micropores.1 It was found that the diffusion coefficient of n-alkanes reaches a maximum when the molecule is just too large to fit within one zeolite cage.2 Since then, more than a few experimental and theoretical studies have demonstrated or at least suggested the occurrence of specific and unexpected phenomena in various zeolitic systems, which can be exploited to achieve highly specific separation or catalysis.3-16 The higher the degree of confinement, i.e., the smaller the size of the pores or the windows the molecules have to diffuse through, the more pronounced the atypical behavior. The isostructural materials chabazite (CHA) and SAPO-34 are two pertinent examples of small pore or eight-membered ring zeolites, offering a highly selective and spatially constrained environment for adsorption, diffusion and reaction. Chabazite has a three-dimensional pore system with ellipsoidal shaped cages of 6.7 × 10 Å2 interconnected via 8-membered (8MR) ring windows with pore apertures of 3.8 × 3.8 Å2 for the dehydrated form of CHA. Each cage is connected to 6 neighboring cavities. SAPO-34 is a silicon-, aluminum-, and phosphorus-based molecular sieve with the CHA topology. * To whom correspondence should be addressed. E-mail: Joeri.Denayer@ vub.ac.be. Phone: +32 2 629 17 98. Fax: +32 2 629 32 48. † Vrije Universiteit Brussel. ‡ Katholieke Universiteit Leuven. § Monash University.
Because of the restricted pore size, mainly adsorption of small gaseous molecules has been studied on CHA.17-20 An overview of the application of natural CHA in the purification and separation of gases is given in the review by Ackley et al.21 CHA can be used for separation of nitrogen, oxygen, argon and methane and removing water from HCl gas streams.22-26 Adsorption of water, ethanol, and their mixture on a zeoliterich tuff containing philipsite and chabazite was studied by Caputo et al.27 Separation of water from propanol in vapor phase on Mordenite/ZSM-5/chabazite membranes was achieved by Salomon et al.28 Zhang et al.29 showed that NaCHA and CaCHA are promising materials for high-temperature CO2 separation. In a Grand Configurational-Bias Monte Carlo (GCMC) simulation study, the segregated nature of CO2/CH4 mixture adsorption in cage-type zeolites such as DDR, CHA, LTA, and ERI is demonstrated.30 The window regions of these zeolites contain a significantly higher rate of CO2 than the intracage regions. The presence of CO2 in the windows reduces intercage transport of partner molecules, further enhancing separation selectivity. Recent membrane permeation experiments confirmed the molecular simulation results for segregation in DDR zeolites.31 Both Si-CHA and AlPO-34 allow kinetic separation of propane and propylene.32-34 Self-diffusion of small hydrocarbons in high silica CHA, DDR, and LTA was studied by pulse-field gradient nuclear magnetic resonance spectroscopy.35 A strong relation between self-diffusion coefficients, window size, and kinetic diameter of the molecules is reported. As a result of its small pore size, chabazite is a highly selective catalyst. Bein and Brown reported preferential sizeselective reactivity of ammonia over tributylamine on chabazite/ silica films.36 Catalytic cracking of n-octane was studied on a
10.1021/jp804349v CCC: $40.75 2008 American Chemical Society Published on Web 09/27/2008
16594 J. Phys. Chem. C, Vol. 112, No. 42, 2008 series of small pore zeolites including Chabazite by Altwasser et al.37 Dahl et al.38 studied the effect of acid site density on selectivity and deactivation rate in the conversion of methanol to olefins on H-CHA and H-SAPO-34 at high space velocities. Although both materials show similar initial selectivities, deactivation rates differ as a result of different acid site density. The small-pore silicoaluminophosphate molecular sieve SAPO34 was found to be an excellent catalyst for the methanol-toolefins and methanol-to-gasoline catalytic processes.39-48 Shapeselective catalysis in the narrow pore system combined with mild acidity is at the basis of the high selectivity for the formation of C2-C4 olefins even at almost full conversion of methanol. With respect to adsorption, attention has been focused on small molecules, like with its structural homologue CHA. The separation of CO2 from CO2/N2 mixtures on metal incorporated SAPO-34 by pressure swing adsorption was reported by Takeguchi et al.49 Inverse gas chromatography experiments demonstrated separation of nitrogen and methane on SAPO-34.50 Many efforts have been delivered to develop SAPO-34 membranes for gas separations. Such membranes are capable of separating CO2 from CH4 or H2 with high selectivity.50-57 In contrast to the extended literature on adsorption of small gaseous molecules on CHA and SAPO-34, much less attention has been paid to the adsorption of larger molecules, although their pore diameter allows adsorption of, e.g., nalkanes. In recent work, we have studied adsorption of linear C1-C8 alcohols in liquid-phase conditions on chabazite zeolite.58 Only methanol, ethanol, and propanol are adsorbed in significant amounts; larger alcohols are almost fully excluded from adsorption in the internal voids. Henry constants of n-alkanes as calculated by configurational-bias Monte Carlo simulations show an unexpected decrease with chain length from a certain chain length on the pure silica forms of CHA, ERI, AFX, RHO, and KFI, all materials with cages interconnected via small windows.59 Krishna and van Baten studied adsorption and separation of linear alkanes and their mixtures in CHA, AFX, and ERI type zeolites by performing GCMC simulations.60 It was found that the longer alkane in the mixture is adsorbed preferentially at low fluid phase partial fugacity. However, at higher fugacities, selectivity reversal occurs, i.e., the shorter chain is adsorbed preferentially because it packs more easily in the zeolite cages. In view of the intriguing results from molecular simulations and liquid phase adsorption experiments concerning the chain length dependency of adsorption properties on CHA, low coverage adsorption properties of C1-C14 n-alkanes on CHA and its structural homologue SAPO-34 were determined using the pulse chromatographic method in the present work. Experimental Methods Chabazite was synthesized following the procedure reported by M. Bourgogne et al.61 with gel composition 0.17 Na2O:2.0 K2O:5.18 SiO2:Al2 O3:224 H2O. A typical procedure involved mixing of the required amounts of potassium hydroxide (45% solution) and water followed by addition of zeolite Y (H-form). The mixture was shaken and transferred into a polypropylene bottle (500 mL, FEP, Nalgene) and heated in an oven for 8 days at 95 °C. The polypropylene bottle was quenched with cold water; the product obtained was filtered, washed with water, and dried in an oven at 100 °C. The composition of the unit cell was Na9.5K0.9[Al10.4Si25.6O72] according to inductively coupled plasma (ICP) spectroscopy. SAPO-34 was synthesized according to the method described previously62 with gel composition Al2O3:1.06 P2O5:1.08 SiO2:
Denayer et al. 2.09 R:66 H2O (with R ) morpholine). In a first step water and phosphoric acid were mixed. To this solution pseudoboehmite was added slowly (time range of 2 h) under constant stirring. Finally another small amount of water was added. This mixture was kept overnight under vigorous stirring. In a second step a mixture of fumed silica, morpholine, and water was prepared. This mixture was added dropwise to the first solution while stirring. To this another amount of water was added. The obtained mixture was then stirred thoroughly for 7 h. The obtained gel was divided over 2 stainless steel autoclaves. After incubation for 24 h at 38 °C without agitation the temperature was raised to 200 °C and kept there for 24 h. After synthesis, the autoclaves were cooled down, and the obtained material was washed by filtration and dried at 60 °C. The unit cell formula of the SAPO-34 sample is (H2O)9.5R5.2[Al17.7 P12.0 Si771O72]. The zeolite powders were compacted into disks, broken into fragments, and sieved. The 500-630-µm fraction was filled into stainless steel columns (1/8-in. diameter) with a length of 0.30 m. The SAPO-34 sample was calcined in the column under flowing air by heating to 823 K at a rate of 1 K/min before transfer to the chromatograph. A HP-4890 gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) was used for the pulse chromatographic experiments. In situ activation of the adsorbent was performed under constant Helium flow by raising the temperature from ambient to 383 to 723 K at a rate of 1 K/min and maintaining this temperature overnight. Helium was used as the carrier gas. Henry adsorption constants were determined from the first moment of the response curve on the TCD after injection of an alkane trace. Adsorption enthalpy and pre-exponential factor of the van’t Hoff equation were obtained from the temperature dependence of the Henry adsorption constants
K ′ ) K′0e
-∆H0
(1)
RT
The pre-exponential factor K0′ of the van’t Hoff equation is related to the adsorption entropy. Although the exact form of the relationship between the pre-exponential factor and the adsorption entropy is the subject of debate,63-66 analysis of the chain length dependency of the pre-exponential factor yields information on the evolution of adsorption entropy with chain length, regardless of the form of the equation. One possible expression is given below67
[
K0 ) exp
( )]
Θ ∆S0,local nT + R 2pΘ
(2)
The subscript 0 refers the zero coverage limit, pθ to the standard state of the gas phase (chosen as 1 atm), and nT equals the number of adsorption sites. To test if the experiments were performed in the Henry domain of the adsorption isotherm, different volumes were injected. Figure 1 shows the first moment of the response curves of n-pentane and n-decane on SAPO-34 at 488 K. No change in first moment with injection volume was observed, pointing at isotherm linearity in the present experimental conditions. Similar results were obtained on Na-CHA. From Figure 2, a linear relationship between first moment and reciprocal flow rate can be observed, showing absence of diffusion limitations. In practice, a hydrocarbon injection volume of 0.02 µL and a helium carrier flow rate of 0.5 Nml/s was used in all experiments. The experimental temperature was limited by the retention time of the different components. Typically, retention time increases with chain length such that experiments with short alkanes have to be performed at lower temperatures than experiments with long alkanes. To obtain reliable data, the temperature
Adsorption of n-Alkanes on Chabazite and SAPO-34
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16595
Figure 1. First moment of the response curve obtained after injection of different amounts of n-pentane and n-decane on SAPO-34 at 488 K. Error bars give the standard deviation for 3 injections. The dotted lines represent the average first moment for all injection volumes.
Figure 3. Henry adsorption constants of C1-C10 n-alkanes on NaCHA. Open symbols represent experimental values; closed symbols correspond to extrapolated values.
Figure 2. Variation of the first moment of n-pentane and n-decane with reciprocal flow rate on SAPO-34 at 483 K.
Figure 4. Henry adsorption constants of C1-C14 n-alkanes on SAPO34. Open symbols represent experimental values; closed symbols correspond to extrapolated values.
was chosen between 320 and 580 K such that retention times varying between 30 s and 30 min were obtained for every component. As a result of this, experiments were limited to n-C14 for SAPO-34 and n-C10 for Na-CHA. For each component, 6-8 measurements at intervals of 15 K were performed. Results Figures 3 and 4 represent Henry adsorption constants as a function of carbon number Nc on Na-CHA and SAPO-34 at different temperatures. Because different temperature domains were used for the different alkanes, some data for the light alkanes in Figures 3 and 4 had to be calculated via extrapolation from data at lower temperature. K′ increases in a more or less exponential way with Nc from methane to n-C6 or n-C7 on NaCHA and SAPO-34, respectively. This increase is not as monotonous as is commonly observed, as can be perceived from Table 1, where Henry constants and ratios of Henry constants of n-alkanes differing one carbon atom are tabulated. For example, on Na-CHA, the Henry constant of ethane at 553 K is about 5.6 times larger than that of methane, while the Henry constant of n-C6 is only 2.8 times larger than that of n-C7. On Na-CHA, K′ decreases from n-C6 or n-C7 on (depending on the temperature) (see Figure 3), reaches a minimum at n-C8, and then increases again. On SAPO-34, an even much more
pronounced decrease of K′ with Nc occurs. A minimum is attained at n-C10. From this point on, K′ increases till the longest alkane studied, i.e., n-C14. At 553 K, the Henry constant of n-C10 is 1 order of magnitude lower than that of n-C7 on SAPO-34. The magnitude of this effect is more obvious from the insert in Figure 4, where the evolution of Henry constants with Nc is shown in a linear scale. A comparison in Henry constants between both materials is made in Figure 5, clearly showing that Na-CHA has the largest adsorption constants. Figures 6 and 7 show the temperature dependence of the Henry constants of selected alkanes. All van’t Hoff plots are linear but are not regularly distributed and tend to intersect with each other within the experimental temperature range. Zero coverage adsorption enthalpies calculated from the slope of the van’t Hoff plots by linear regression are shown as a function of Nc in Figure 8. Generally, more negative adsorption enthalpies are obtained for Na-CHA compared to SAPO-34, the difference becoming larger with increasing chain length (Figure 8). For methane, the difference in adsorption enthalpy equals 8.2 kJ/ mol, while for n-decane, the difference between both materials already augments to 31.7 kJ/mol (see Table 1). For comparison, adsorption enthalpies on the 10-membered ring zeolite ZSM22 are given in Table 1. ZSM-22 contains unidimensional pores with a free cross section of 5.5 × 4.5 Å2. Na-CHA exhibits
16596 J. Phys. Chem. C, Vol. 112, No. 42, 2008
Denayer et al.
TABLE 1: Zero Coverage Adsorption Enthalpies, Henry Adsorption Constants at 553 K and Ratio of Henry Constants (at 553 K) of Successive n-Alkanes on Na-CHA, SAPO-34, and ZSM-22 -∆H0 (kJ/mol) Na-CHA CH4 C2H6 C3H8 n-C4H10 n-C5H12 n-C6H14 n-C7H16 n-C8H18 n-C9H20 n-C10H22 n-C11H24 n-C12H26 a
21.6 34.1 46.8 58.4 71.7 84.6 93.6 100.1 108.9 122.8
SAPO-34 13.4 25.7 35.9 45.6 53.4 64.4 72.8 81.3 88.6 91.1 85.3 105.8
K′ 553 K (mol/kg/Pa) H-ZSM-22a
Na-CHA
6.15 × 1.26 × 10-6 2.02 × 10-6 5.93 × 10-6 1.56 × 10-5 3.04 × 10-5 3.09 × 10-5 1.97 × 10-5 9.39 × 10-6 2.95 × 10-6 3.89 × 10-6 7.14 × 10-6
5.14 × 2.87 × 10-6 1.30 × 10-5 5.14 × 10-5 2.05 × 10-4 5.65 × 10-4 6.27 × 10-4 4.59 × 10-4 7.96 × 10-4 3.03 × 10-3
10-7
63.3 77.1 89.4 100.6 112.5
K′ Cn+1/K′ Cn
SAPO-34
SAPO-34
Na-CHA
2.1 1.6 2.9 2.6 2.0 1.0 0.6 0.5 0.3 1.3 1.8
5.6 4.5 4.0 4.0 2.8 1.1 0.7 1.7 3.8
10-7
Reference 65.
Figure 5. Comparison of Henry constants on Na-CHA and SAPO-34 at 553 K closed symbols correspond to extrapolated values.
Figure 6. van’t Hoff plots for selected n-alkanes on Na-CHA.
comparable or even lower (more negative) adsorption enthalpies than ZSM-22, showing that the ellipsoidal shaped CHA cages offer at least the same degree of confinement as the narrow ZSM-22 pores. The adsorption enthalpy of alkanes confined in zeolites is mainly attributable to the dispersive interactions between hydrogen atoms of the alkane and oxygen atoms in the zeolite lattice. Because of the linear additively of such interactions, adsorption enthalpies of
Figure 7. van’t Hoff plots for selected n-alkanes on SAPO-34.
Figure 8. Zero coverage adsorption enthalpy of n-alkanes as a function of carbon number on Na-CHA and SAPO-34. The dashed line represents the linear regression of the data. Standard errors on the estimated adsorption enthalpy are indicated.
n-alkanes typically decrease linearly with Nc. This is clearly not the case for Na-CHA and SAPO-34, where large deviations from linearity are observed, as shown in Figure 8. Instead of a constant contribution to the adsorption enthalpy per additional -CH2group in the chain, differences in adsorption enthalpy ∆∆H0 (∆H0,Cx - ∆H0,Cx-1) varying from 6.5 to 13.9 kJ/mol are found between successive alkanes on Na-CHA (Table 1 and Figure 9).
Adsorption of n-Alkanes on Chabazite and SAPO-34
Figure 9. Difference in adsorption enthalpy ∆∆H0 between alkanes with carbon number Nx and Nx+1. In this graph, Nc represents the carbon number of the longest chain.
Figure 10. Pre-exponential factor of the van’t Hoff equation of n-alkanes as a function of carbon number on Na-CHA and SAPO-34. Standard errors on the estimated pre-exponential factors are indicated.
On SAPO-34, even an increase in adsorption enthalpy occurs between n-C10 and n-C11 (Figure 8). Also the pre-exponential factor, calculated from the intercept of the van’t Hoff plots, varies in a nonmonotonous way with Nc (Figure 10). Again, a remarkable discontinuity between n-C10 and n-C11 is visible on SAPO-34. A direct interpretation of the evolution of -ln K0′ or the adsorption entropy with Nc is rather difficult since the freedom of a molecule in the adsorbed state depends on the strength of the adsorptive interactions in the zeolite pores. Instead, the relationship between the pre-exponential factor and the adsorption enthalpy will be discussed below. Discussion The evolution of the different low coverage adsorptive thermodynamic properties (Henry equilibrium constants, adsorption enthalpy, and entropy) with carbon number on the isostructural materials Na-CHA and SAPO-34 occurs according to unconventional patterns; the most spectacular effect is the decrease in Henry constants from a carbon number of 6 or 7. The cages of Na-CHA and SAPO-34 have a restricted size (6.7 × 10 Å2) and can only contain a limited number of small molecules. In an earlier study of liquid phase adsorption of alcohols, alkenes, and alkanes on chabazite, it was found that a single cage can contain as much as
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16597
Figure 11. Compensation plot showing the natural logarithm of K0′ as a function of the zero coverage adsorption enthalpy on SAPO-34 and Na-CHA.
5.5 ethanol or 3.5 propane molecules, corresponding to, respectively, 11 and 10.5 C atoms.58 High-pressure gas phase adsorption experiments showed that, for propane, 3 molecules are adsorbed per cage, corresponding to 9 carbon atoms per cage. CBMC simulations showed a maximum adsorption capacity of 10 C-atoms/ cage for ethane and 12 C-atoms/cage for propane in a cation free chabazite. In the all-trans configuration, n-hexane has a length of 10.1 Å.68 This means that chains longer than n-hexane have to tend away from the optimal all-trans configuration to adopt a coiled or bent conformation to fit within the cage, as suggested by Dubbeldam et al.10 and Daems et al.,15 or to protrude part of their chain through the narrow 8MR windows connecting adjacent cages. Since the free diameter of this window (3.8 × 3.8 Å2) is smaller than the kinetic diameter of the n-alkanes (4.3 Å), local repulsion effects will occur. It is expected that both mechanisms (chain coiling or window protrusion) would affect adsorption enthalpy and/or entropy. From the data in Table 1 or the graphical representation of the evolution of adsorption enthalpy and pre-exponential factor (which is a measure for the adsorption entropy) with Nc in Figures 8 and 10, it is difficult to observe a clear-cut effect at carbon numbers of 6 or 7. Notwithstanding, the decrease of adsorption enthalpy per additional -CH2- group ∆∆H0 is less steep at carbon numbers between 6 and 8 for Na-CHA (Figure 9). From n-C8 on, ∆∆H0 increases again. On SAPO-34, ∆∆H0 continues decreasing and even becomes negative for n-C11. For n-C12, ∆∆H0 suddenly increases drastically, pointing at a change in adsorption mechanism in the C11-C12 region. The Henry adsorption constant is related to the adsorption enthalpy and entropy. Both properties are plotted as a function of each other in Figure 11. In this so-called compensation plot, the adsorption entropy is represented by the logarithm of the pre-exponential factor (eq 2). In many adsorbent-adsorbate systems, a linear relationship between adsorption enthalpy and entropy is observed for homologous series of molecules,67-74 indicating that the loss of freedom in the adsorbed state is directly proportional to the strength of interaction between the molecule and the adsorbent. To obtain a continuous exponential increase of the Henry constant with Nc, the data points for the n-alkanes in the compensation plot should fall on a straight line at equal distances, which is clearly not the case for Na-CHA and SAPO-34. Let us now first consider SAPO-34. The curvature of the compensation plot increases continuously between n-C6 and
16598 J. Phys. Chem. C, Vol. 112, No. 42, 2008 n-C10. This means that the adsorption entropy decreases more rapidly with chain length than the adsorption enthalpy. This effect is very pronounced between n-C9 and n-C10, where the adsorption entropy decreases drastically, whereas the adsorption enthalpy only decreases marginally. For n-C11, a discontinuity, i.e., sudden increase in ∆H0 and ln K0′ is observed, while for n-C12 ∆H0 and ln K0′ again decrease drastically. These observations could be explained as follows. Although molecular simulations indicate that some degree of coiling might occur with n-alkanes with a carbon number below 7,60 it can be stated that alkanes between methane and n-C6 are small enough to fit in a nearly undistorted configuration in the SAPO-34 cage. Therefore, adsorption enthalpy and entropy decrease in a regular way with Nc for these molecules; the longer the alkane, the larger its energetic interaction or the more negative its adsorption enthalpy and as a consequence, the larger the loss of freedom. For n-C7 and longer chains, there are two possibilities: deformation of the chain to fit within one cage or a distribution of the chain over adjacent cages implying protrusion of part of the chain through the window between two cages. If window protrusion would already occur for n-C7, a rather pronounced change in ∆∆H0 due to repulsive van der Waals interactions in the narrow window is expected between n-hexane and n-heptane, but such an effect only occurs from n-C10 (Figure 9). It is more probable that n-C7-n-C9 are sitting in a bent configuration in the cage. Such an effect was recently reported for the liquidphase adsorption of long-chain alkanes in 5A zeolite, where C16 is severely bent and coiled in order to fit into a single supercage.15 These “cage nestling” or “snake in a basket” configurations were observed for other zeolites with cages in GCMC simulations.10 It is expected that a bent molecule in such a densely packed cage cannot optimize its energetic interactions with the framework atoms to the same degree as a smaller molecule in a less-filled cage. The increment in adsorption enthalpy ∆∆H0 per additional methylene group will thus become less important with increasing chain length, as can be seen in Figure 9. Since the free space in the cage becomes minimal, the freedom of the molecules becomes severely limited, leading to a fast decrease of adsorption entropy with carbon number (Figure 10). According to capacity measurements, no more than 10-11 methylene groups can be packed in a single cage. n-C10 and n-C11 thus are at the limit of fitting in a highly coiled configuration in one cage or being forced to extend through the window to adsorb in 2 cages because of size restrictions. Molecular simulations indicate that n-C12 is the first alkane to stretch across two cages in the CHA structure.59 For SAPO-34, the experiments are not fully conclusive. Adsorption enthalpy and entropy reach a local maximum at n-C11, and also the compensation plot shows a discontinuity for n-C11. On the other hand, a minimum in K′ is reached at n-C10 rather than n-C11 (Figure 4). We speculate that n-C11 is no longer fitting in the cage. This component has a less negative adsorption enthalpy or lower interaction energy than n-C10 because its chain is protruding the window region where repulsion forces occur. As a result of this lower energetic interaction and the fact that, unlike n-C10, n-C11 is not restricted to the limited space inside 1 cage, it loses less freedom, explaining the local maximum in adsorption entropy. Because of this change in adsorption mechanism between n-C10 and n-C11, Henry constants again increase from n-C10 on. A different behavior is observed for Na-CHA. Henry constants already start to increase again from n-C8 on (Figure 3). This can be attributed to the different chemical nature of Na-CHA, the presence of extra-framework Na-cations in this
Denayer et al. structure and possibly the difference in effective cage size as a result of the presence of cations. The presence of these extraframework cations results in much more negative adsorption enthalpies compared to the cation-free SAPO-34 (see Figure 8 and Table 1). In fact, these adsorption enthalpies are also much lower than those calculated by GCMC on the pure silica form of CHA (almost 50 kJ/mol difference for n-C9).59 Figure 9 shows a decrease in ∆∆H0 between n-C6 and n-C8, followed by an increase to the value observed for C1-n-C6. No strong discontinuities in the compensation chart are observed for Na-CHA, apart from a change in slope in the n-C6-n-C9 region, which is much less pronounced compared to SAPO-34 (Figure 11). Therefore, it is proposed that, instead of coiling, molecules longer than n-C6 adopt a stretched configuration through the window between two cages. This has a negative effect on the adsorption enthalpy, but given the high absolute values of the adsorption enthalpy, this remains the preferred adsorption mechanism. This hypothesis was confirmed by a limited set of experiments on Ca-CHA (data not shown). With this material, which even showed more negative adsorption enthalpies than Na-CHA, no decrease at all in Henry constants could be observed at chain lengths between C1 and C9. Thus, while molecular simulations on the pure silica form of chabazite predict a decrease in K′ from n-C6 till n-C11 or even further, experiments show that the presence of cations in the structure moderates this effect.59 The present data should be contrasted to earlier observations regarding the adsorption of n-alkanes in liquid phase conditions at room temperature.58 Methane, ethane, and propane fill up the pore volume efficiently. The largest adsorption capacity is obtained for propane, corresponding to 3-4 molecules per cage. The adsorption capacity for n-C5 equals 1 molecule per cage. For longer chains, the adsorption capacity decreases drastically. In gas-phase conditions, K′ only starts to decrease from n-C6 or n-C7. The large difference in experimental temperature between gas-phase (>250 °C) and liquid-phase (room temperature) experiments implies a different enthalpy/entropy balance and molecular flexibility, which might affect the adsorption mechanism. Another remarkable difference is that, in liquid-phase conditions, on average less than 0.2 n-C8 molecules are adsorbed per cage. GCMC calculations on the all-silica form of chabazite predict an adsorption capacity of 1 molecule per cage for n-C6 to n-C8 at room temperature.60 Thus, although n-C8 should be able to adsorb in a coiled configuration inside the CHA cage, such that a capacity of 1 molecule per cage could be theoretically reached, this does not seem to happen. Possibly, much higher external concentrations are needed in the liquid phase experiments to force the n-C8 molecules inside the pores. An alternative explanation would be that diffusion of n-C8 is so slow that it cannot be observed within the experimental time scale (120 h) of the liquid phase experiments. Further research will be devoted to this unresolved issue. Conclusions Pulse chromatographic experiments reveal pronounced discontinuities in the chain length dependency of the low coverage adsorption properties of n-alkanes on Na-CHA and SAPO-34. When the chain length becomes larger than the cage size, Henry adsorption constants start to decrease. To fit within the cage, alkanes longer than n-hexane have to adopt a coiled configuration, which implies large entropic effects and a less optimal energetic interaction. When the chain becomes too long to fit within the cage, the alkane stretches between two cages,
Adsorption of n-Alkanes on Chabazite and SAPO-34 protruding the narrow cage connecting window. Repulsive forces in this narrow window affect the interaction energy. These cage and window effects depend on the chemical nature of the adsorbent; on SAPO-34, a material free of cations, Henry constants decrease from n-C6 to n-C10, while on Na-CHA, this effect is less pronounced and only occurs for n-C6-n-C8. The present observations validate tendencies found in recent molecular simulations and can help in explaining unusual selectivity patterns in catalytic cracking reactions as reported decades ago by Gorring. Preferentially adsorbing shorter chains over longer ones offers perspectives with respect to selective separation processes and chain length selective cracking reactions. Acknowledgment. J. Denayer is grateful to the F.W.O. for a fellowship as postdoctoral researcher. The involved teams are participating in the IAP-PAI programme (IUAP IV-11) on Supramolecular Chemistry and Catalysis, sponsored by the Belgian Federal Government. References and Notes (1) Gorring, L. R. J. Catal. 1973, 31, 13. (2) Chen, N. Y.; Lucki, S. J.; Mower, E. B. J. Catal. 1969, 13, 329. (3) Krishna, R.; Calero, S.; Smit, B. Chem. Eng. J. 2002, 88, 81. (4) van Well, W. J. M.; Cottin, X.; de Haan, J. W.; van Santen, R. A.; Smit, B. Angew. Chem., Int. Ed. 1998, 37, 1081. (5) Jobic, H.; Me´thivier, A.; Ehlers, G.; Farago, B.; Haeussler, W. Angew. Chem., Int. Ed. 2004, 43, 364. (6) Gunadi, A.; Brandani, S. Microporous Mesoporous Mater. 2006, 90, 278. (7) Yoo, K.; Tsekov, R.; Smirniotis, P. G. J. Phys. Chem. B 2003, 107, 13593. (8) Ruckenstein, E.; Lee, P. S. Phys. Lett. 1977, 56A, 423. (9) Nitsche, J. M.; Wei, J. AIChE J. 1991, 37, 661. (10) Dubbeldam, D.; Calero, S.; Maesen, L. M.; Smit, B. Angew. Chem., Int. Ed. 2003, 42, 3624. (11) Derouane, E. G.; Andre, J. M.; Lucas, A. A. J. Catal. 1988, 110, 58. (12) Denayer, J. F. M.; De Meyer, K.; Martens, J. A.; Baron, G. V. Angew. Chem. 2003, 42, 2774. (13) Denayer, J. F. M.; Ocakoglu, R. A.; Arik, I. C.; Kirschhock, C. E. A.; Martens, J. A.; Baron, G. V. Angew. Chem., Int. Ed. 2005, 44, 400. (14) Alaerts, L.; Kirschhock, C. E. A.; Maes, M.; van der Veen, M.; Finsy, V.; Depla, A.; Martens, J. A.; Baron, G. V.; Jacobs, P. A.; Denayer, J. F. M.; De Vos, D. E. Angew. Chem., Int. Ed. 2007, 46 (23), 4293. (15) Daems, I.; Baron, G. V.; Punnathanam, S.; Snurr, R. Q.; Denayer, J. F. M. J. Phys. Chem. C. 2007, 111, 2191. (16) Zhu, W.; Kapteijn, F.; Moulijn, J. A. Sep. Purif. Technol. 2003, 32, 223. (17) Barrer, R. M.; Davies, J. A. Proc. R. Soc. London, A 1970, 320, 289. (18) Dyer, A.; Zubair, M. Microporous Mesoporous Mater. 1998, 22, 135. (19) Grey, T. J.; Nicholson, D.; Gale, J. D.; Peterson, B. K. Appl. Surf. Sci. 2002, 196, 105. (20) Inui, T.; Okugawa, Y.; Yasuda, M. Ind. Eng. Chem. Res. 1988, 27, 1103. (21) Ackley, M. W.; Rege, S. U.; Saxena, H. Microporous Mesoporous Mater. 2003, 61, 25. (22) Maroulis, P. J.; Coe, C. G. Anal. Chem. 1989, 61 (10), 1112. (23) Soonmo, A.; Joye, S. B. Mar. Chem. 1997, 59, 63. (24) Coe, C. G.; Gaffney, T. R. US Patent No. 4,943,304, 1990. (25) Ackley, M. W.; Saxena, H.; Henzler, G. W.; Nowobilski, J. J. WO03053546, 2003. (26) Rege, S. U.; Yang, R. T.; Qian, K.; Buzanowski, M. A. Chem. Eng. Sci. 2001, 56, 2745. (27) Caputo, D.; Iucolano, F.; Pepe, F.; Colella, C. Microporous Mesoporous Mater. 2007, 105, 260. (28) Salomon, M. A.; Coronas, J.; Merendez, M.; Santamaria, J. Chem. Commun. 1998, 1, 125. (29) Zhang, J.; Singh, R.; Webley, P. A. Microporous Mesoporous Mater. 2008, 111, 478. (30) Krishna, R.; van Baten, J. M. Sep. Purif. Technol. 2008, 61, 414. (31) Krishna, R.; van Baten, J. M. Chem. Eng. Sci. 2008, 63, 3120.
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16599 (32) Ruthven, D. M.; Reyes, S. C. Microporous Mesoporous Mat. 2007, 104, 59. (33) Reyes, S. C.; Krishnan, V. V.; DeMartin, G. J.; Sinfelt, J. H.; Strohmaier, K. G.; Santiesban, J. G. US Patent No 6,730,142, 2004. (34) Olson, D. US Patent No. 6,488,741, 2002. (35) Hedin, N.; DeMartin, G. J.; Stromaier, K. G.; Reyes, S. C. Microporous Mesoporous Mater. 2007, 98, 182. (36) Bein, T.; Brown, K. J. Am. Chem. Soc. 1989, 111, 7640. (37) Altwasser, S.; Welker, C.; Traa, Y.; Weitkamp, J. Microporous Mesoporous Mater. 2005, 83, 345. (38) Dahl, I. M.; Mostad, H.; Akporiaye, D.; Wendelbo, R. Microporous Mesoporous Mater. 1999, 29, 185. (39) Travalloni, L.; Gomes, A. C. L.; Gaspar, A. B.; da Silva, M. A. P. Catal. Today 2008, 133-135, 406. (40) Chen, J. Q.; Bozzano, A.; Glover, B.; Fuglerud, T.; Kvisle, S. Catal. Today 2005, 106, 103. (41) Lee, Y.-J.; Baek, S.-C.; Jun, K.-W. Appl. Catal. A: Gen. 2007, 329, 130. (42) Alwahabi, S. A.; Froment, G. F. Ind. Eng. Chem. Res. 2004, 43, 5098. (43) Chen, D.; Rebo, H. P.; Moljord, K.; Holmen, A. Ind. Eng. Chem. Res. 1999, 38, 4241. (44) Galperin, L. B. Appl. Catal. A: Gen. 2001, 209, 257. (45) Wei, Y.; Zhang, D.; Xu, L.; Chang, F.; He, Y.; Meng, S.; Su, B.L.; Liu, Z. Catal. Today 2008, 131, 262. (46) Lesthaege, D.; De Sterck, B.; Van Speybroeck, V.; Marin, G. B.; Waroquier, M. Angew. Chem., Int. Ed. 2007, 46, 1311. (47) Dahl, I. M.; Wendelbo, R.; Andersen, A.; Akporiaye, D.; Mostad, H. B.; Fuglerud, T. Microporous Mesoporous Mater. 1999, 29, 159. (48) Marchi, A. J.; Froment, G. F. Appl. Catal. 1991, 71, 139. (49) Takeguchi, T.; Tanakulrungsank, W.; Inui, T. Gas Sep. Purif. 1993, 7, 3. (50) Maple, M. J.; Williams, C. D. Microporous Mesoporous Mater. 2008, 111, 627. (51) Hong, M.; Li, S.; Falconer, J. L.; Noble, R. D. J. Membr. Sci. 2008, 307, 277. (52) Li, S.; Falconer, J. L.; Noble, R. D. J. Membr. Sci. 2004, 241, 121. (53) Lixiong, Z.; Dong, J. M.; Enze, M. Stud. Surf. Sci. Catal. 1997, 105, 2211. (54) Posthusta, J. C.; Tuan, V. A.; Falconer, J. L.; Noble, R. D. Ind. Eng. Chem. Res. 1998, 37, 3924. (55) Li, S.; Falconer, J. L.; Noble, R. D. Microporous Mesoporous Mater. 2008, 110, 310. (56) Yu, M.; Li, S.; Falconer, J. L.; Noble, R. D. Microporous Mesoporous Mater. 2008, 110, 579. (57) Krishna, R.; Li, S.; van Baten, J. M.; Falconer, J. L.; Noble, R. D. Sep. Purif. Technol. 2008, 60, 230. (58) Daems, I.; Singh, R.; Baron, G. V.; Denayer, J. F. M. Chem. Commun. 2007, 13, 1316. (59) Dubbeldam, D.; Smit, B. J. Phys. Chem. B. 2003, 107, 12138. (60) Krishna, R.; van Baten, J. M. Sep. Purif. Technol. 2008, 60 (3), 315. (61) Bourgogne, M.; Guth, J.-L.; Wey, R. US Patent No. 4,503,024, 1985. (62) Lopez Nieto, J.; Tusar, N. Verified Syntheses of Zeolite Materials; Robson, H., Ed.; Elsevier Science B.V.; 2001. (63) Maesen, T. L. M.; Krishna, R.; van Baten, J. M.; Smit, B.; Calero, S.; Sanchez, J. M. C. J. Catal. 2008, 256 (1), 95. (64) Dremetsika, A. V.; Siskos, P. A.; Katsanos, N. A. J. Hazard. Mater. 2007, 149 (3), 603. (65) Katsanos, N. A.; Kapolos, J.; Gavril, D.; Bakaoukas, N.; Loukopoulos, V.; Koliadima, A.; Karaiskakis, G. J. Chromatogr. A 2006, 1127 (1-2), 221. (66) Myers, A. L. Colloids Surf. A 2004, 241 (1-3), 9. (67) Ocakoglu, A. R.; Denayer, J. F. M.; Marin, G. B.; Martens, J. A.; Baron, G. V. J. Phys. Chem. B 2003, 207, 398. (68) De Meyer, K.; Chempath, S.; Denayer, J. F. M.; Martens, J. A.; Snurr, R. Q.; Baron, G. V. J. Phys. Chem. B. 2003, 107, 10760. (69) Bond, C. G.; Keane, M. A.; Kral, H.; Lercher, J. A. Catal. ReV. 2000, 42 (3), 323. (70) Ruthven, D. M.; Kaul, B. K. Adsorption 1998, 4, 269. (71) Hampson, J. A.; Jasra, R. V.; Rees, L. V. C. Characterization of porous solids II; Rodriguez, F., Rouquerol, J., Singh, K., Eds.; Elsevier: 1991. (72) Eder, F.; Stockenhuber, M.; Lercher, J. A. J. Phys. Chem. B 1997, 101, 5414. (73) Denayer, J. F. M.; Baron, G. V.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1998, 102 (17), 3077. (74) Zhu, W.; Kapteijn, F.; Van der Linden, B.; Moulijn, J. A. Phys. Chem. Chem. Phys. 2001, 3, 1755.
JP804349V
