Article pubs.acs.org/JPCC
Quasi-Equilibrated Thermodesorption Combined with Molecular Simulation for Adsorption and Separation of Hexane Isomers in Zeolites MFI and MEL Andrzej Sławek,† José Manuel Vicent-Luna,‡ Bartosz Marszałek,† Wacław Makowski,*,† and Sofía Calero*,‡ †
Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Kraków, Poland Universidad Pablo de Olavide, Department of Physical, Chemical and Natural Systems, Ctra. Utrera Km. 1, Seville ES-41013, Spain
‡
S Supporting Information *
ABSTRACT: Adsorption of hexane isomers in high silica MFI and MEL zeolites was studied by means of quasiequilibrated temperature-programmed desorption and adsorption and Monte Carlo simulations. Configurational bias and continuous fractional component Monte Carlo were applied for these systems, and their efficiency and effectiveness were compared. All branched hexane isomers, i.e., 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,2-dimethylbutane, were used. The agreement between experimental and calculated adsorption isobars confirmed that quasi-equilibrated temperature-programmed desorption and adsorption is an effective method for studying adsorption of branched alkanes in zeolites. Detailed literature review on adsorption of branched paraffins in MFI revealed diffusion limitations which make reaching adsorption equilibria of these molecules difficult with isothermal approach at low temperatures. Temperature-dependent measurements conducted in this work largely allowed avoiding such limitations. Adsorption equilibria of branched alkanes in MEL zeolite have been studied experimentally for the first time. Detailed analysis of the adsorption behavior of these systems revealed the presence of two adsorption sites located in the different intersections of the straight channels characteristic of the MEL framework. Molecular simulations for adsorption of mixtures of branched hexane isomers showed potential application of the MFI and MEL zeolites in separation of mono- and dibranched isomers.
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INTRODUCTION Zeolites (i.e., microporous crystalline aluminosilicates) constitute a well-known and widely used group of porous materials. The main applications of these materials are based on their catalytic activity,1−3 ion exchange capacity,4 and unique adsorption properties.5−7 They are referred to as “molecular sieves” due to their capability of selective adsorption of small molecules fitting the channels and cavities present within their structure. Adsorptive properties of zeolites play a key role in their separation and catalytic applications. Due to higher octane rating, branched alkanes are preferred to straight-chain ones as components of gasoline. To enrich the hydrocarbon fuel mixture, catalytic isomerization of linear alkanes8 followed by separation of the obtained products is carried out. While isolation of n-alkanes can be easily performed with the use of adsorbents selective only toward linear molecules (e.g., Ca-LTA zeolite), separation of monobranched and dibranched paraffins is more demanding. In this work, we investigate differences between adsorption of hexane isomers in silicalite-1 and silicalite-2, which are pentasil-based pure-silica polymorphs of ZSM-5 (Zeolite Socony Mobil - five, MFI © XXXX American Chemical Society
topology) and ZSM-11 (Zeolite Socony Mobil - eleven, MEL topology) zeolites, respectively. Quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) has been found as an effective experimental technique of investigating porosity-related adsorptive properties of zeolites.9 QE-TPDA is based on temperature-dependent experiments during which cycles of desorption and adsorption of volatile hydrocarbons are measured. The results can be transformed into desorption and adsorption isobars, comparable with other experimental or simulated data. Some advantages of this technique are relatively short measurements (one desorption−adsorption cycle that may last less than 2 h), low amount of sample required (2−10 mg), cheap maintenance and parts of the experimental setup, and applicability of many different probe molecules. Experiments with the use of n-alkanes,9−11 cyclohexane,12 alcohols,13 and aromatic hydrocarbons14 were performed earlier in various Received: June 1, 2017 Revised: July 31, 2017 Published: August 9, 2017 A
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independent of the loading. In a subsequent isobaric study by Bellat et al.,23 an unexpected increase of the adsorbed amount of 22DMB during the heating of the sample from 343 to 443 K for the first time was observed. This phenomenon was ascribed to the phase transition from monoclinic to orthorhombic MFI structure. This explanation disagrees with the XRD study of Millot et al.24 showing that adsorption of branched paraffins leads to stabilization of the orthorhombic structure of silicalite1 at room temperature. Temperature-programmed equilibration, a technique like the QE-TPDA exploited in our study, has been found as a good tool for investigating adsorption of monobranched alkane molecules in silicalite-1 but also showed limitations for branched alkanes at low temperatures. Diffusion coefficients computed by Luna-Triguero et al.25 were over 10 times higher for nC6 than for 2MP and extremely low for 3MP, 22DMB, and 23DMB. Recent studies by Ferreira et al.26 on the influence of Si/Al ratio on diffusion and adsorption of hexane isomers in ZSM-5 indicated that the zeolite with Si/Al = 100 (in comparison to the one with Si/Al = 25 or silicalite-1) shows the highest saturation loading and diffusion rate at 293 K. Another recent study by Titze et al.27 on the synergy between adsorption and diffusion of the nC6/2MP mixture in MFI revealed that the configurational entropy effect tends to suppress the adsorption of 2MP. Vlugt et al.28 investigated the adsorption of various alkanes (including nC6 and 2MP) and their mixtures in silicalite-1 by means of MC molecular simulations. They reported an inflection in the adsorption isotherms of branched alkanes for four molecules per unit cell that was attributed to the preferential adsorption of these molecules in the intersections. Hence, higher pressure is demanded to force them into the channel interiors. Dubbeldam et al.29 used part of the above-mentioned experimental results in developing the united atoms force field for alkanes in nanoporous materials used in this work. All available experimental adsorption isotherms have been unified and shown together with the computed ones in the Adsorption Isotherms section. The results of previous works may seem coherent, but their quantitative comparison reveals some discrepancies. Even though adsorption of the branched hexane molecules in MFI zeolite has been thoroughly investigated, the qualitative characterization of the corresponding adsorption equilibria in this zeolite remains challenging. Unlike for MFI, the literature concerning adsorption of branched alkanes in MEL zeolite is scarce. The calculated adsorption isotherms of binary mixtures of n-heptane and 2methylhexane in MFI and MEL provided by Maesen et al.30 revealed the lack of selectivity for MFI and its presence toward the linear alkane for MEL at loadings higher than two molecules per unit cell. To the best of our knowledge, onecomponent experimental adsorption isotherms of hexane isomers in MEL have not been reported yet. Some molecular simulation data have been published in a paper dealing with the screening of nanoporous adsorbents for separation of alkane isomers.7 However, this work does not provide in-depth insight into the sorption properties of this system.
porous materials. Recently, we have successfully carried out an interdisciplinary study combining the Monte Carlo (MC) molecular simulations with the QE-TPDA measurements for adsorption of linear alkanes (C5−C10) in MFI and MEL zeolites.10 In this work, we widen the range of adsorbates by four hexane isomers: 2-methylpentane (2MP), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB), and 2,2-dimethylbutane (22DMB) (Figure 1).
Figure 1. Skeletal formulas of hexane isomer molecules and their kinetic diameters.15
MC molecular simulations are considered as an efficient computational tool being used for modeling equilibrium properties of adsorbent−adsorbate systems that have been successfully exploited for numerous zeolites.7,16 This technique allows one to mimic experimental results and helps to identify the adsorption mechanisms that occur at the molecular level or to predict the behavior of the systems under conditions difficult to be maintained experimentally. MC simulation is also matched to screen large numbers of structures in search of the best adsorption properties.7 One of the difficulties in the use of molecular simulations is the lack of good-quality experimental data serving as a base for validation of the simulation methods and models. For this reason, studies encompassing both experimental and calculated adsorption data are required. Adsorption of hexane isomers in MFI has been widely studied experimentally and with molecular simulations. Cavalcante et al.17 reported type I adsorption isotherms for all hexane isomers and noted that the steric hindrance strongly affects diffusion of guest molecules in MFI as well as adsorption equilibrium. Jolimaitre et al.18 found agreement between the experimental adsorption isotherms obtained with pulse chromatography, step chromatography, and uptake measurements for 2MP and 3MP. However, slow kinetics of 22DMB adsorption impeded reaching local equilibrium with the pulse chromatography technique. This agrees with studies of Gener et al.19 showing that adsorption of 22DMB is related to the steric hindrance of this molecule, not to the preferential adsorption sites, while 2MP, 3MP, and 23DMB exhibit similar adsorption behavior. Zhu et al.20 observed two-step adsorption of 2MP and 3MP which is in agreement with the results obtained by CBMC simulations,21 yet they were not able to obtain saturation for 23DMB under the investigated conditions. Lemaire et al.22 studied adsorption and diffusion of n-hexane (nC6) and 22DMB paraffins in MFI with isothermal and isobaric approaches. As they concluded, the nC6 molecules were adsorbed in most of the micropore volume of the MFI framework, while 22DMB molecules probed less than 40% of the porosity. The diffusivities obtained in this work were ca. 100 times greater for the linear molecule and almost
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EXPERIMENTAL SECTION To study adsorption properties of MFI and MEL, we used the commercial high silica ZSM-5 (Zeolyst CBV 28014, Si/Al = 140, H+ form) and pure silica ZSM-11 synthesized according to previous reports.10,31 Structures of these materials were confirmed with PXRD.10 To improve through-bed diffusion of the samples, the zeolites were pressed into pellets, crushed in B
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The Journal of Physical Chemistry C mortar, and sieved to a 400−500 μm fraction. The adsorbates utilized during experiments are commercially available: nC6 (analytical pure, Acros Organics), 2MP (≥99%, Sigma-Aldrich), 3MP (≥99%, Sigma-Aldrich), 23DMB (98%, Sigma-Aldrich), and 22DMB (99%, Sigma-Aldrich). The measurements of quasi-equilibrated temperature-programmed desorption and adsorption were performed with a homemade apparatusa modified flow setup for temperatureprogrammed desorption equipped with a chromatographic TCD detector (Micro Volume TCD, Valco), described in detail in earlier works.9,32 Before each experiment, a sample of 7−10 mg was heated up to 500 °C in a flow of pure helium (Air Products, purity 5.0) to remove preadsorbed water or other guest molecules. After activation and cooling the sample, sorption at room temperature was performed by replacing pure He used as a carrier gas with He containing a small admixture of hydrocarbon. To measure the QE-TPDA profiles, the samples were heated and cooled in a flow of the mixture, according to linear temperature programs with heating/cooling rates of 5 °C/min. Calibration constants, relating the TCD signal with the concentration of each hydrocarbon in helium flow, were determined by weighting mass losses of the glass vials containing the hydrocarbon before and after each experiment.
temperatures ranging from 173 to 573 K with 20 K intervals. Each point on the adsorption isobars was obtained by running cycles of equally probable trial moves: translations, rotations, and swap between the framework and reservoir. Additional Monte Carlo moves corresponding to different insertion techniques were also employed. The calculations for mixtures were conducted with the use of CBMC, adding MC moves for identity changes, i.e., making attempts to replace guest molecules present in the host for other components from the reservoir. The number of cycles varied for different systems, but in all cases, we used more than 2 × 105 after reaching insertion−deletion equilibrium. The average occupation profiles were calculated by averaging 103 snapshots of equilibrated adsorbent−adsorbate systems obtained with CFCMC. Though grand-canonical Monte Carlo is considered a fast and effective computational technique, it has limitations. Since the number of adsorbed molecules varies during the simulations, the desired property that is computed is the average number of adsorbed molecules per unit of volume at each condition (μVT ensemble). These systems suffer from a major drawback. When the structure is almost fully filled with the adsorbate molecules, the probability of MC insertion/ deletion move becomes vanishingly low which hinders achieving equilibrium. The configurational-bias Monte Carlo (CBMC) technique has been developed to increase the ratio of successful insertions.41 In this method, a new molecule is built segment by segment in the modeled framework space instead of inserting the whole molecule at once. The growth process is biased toward energetically favorable configurations reducing the number of attempts that overlap new segments with existing atoms of guest or host. This is particularly useful for long-chain molecules such as n-alkanes. Despite the evident benefits, the effectiveness of the CBMC method is limited at high loadings.16 To solve this problem, the continuous fractional component Monte Carlo (CFCMC) method was developed.42 This method inserts a “fractional” molecule with scaled interactions with other molecules and the host structure. The scaling parameter λ of fractional molecule ranges from 0 to 1, adding additional MC moves to change this value. The λ value equal to 0 means that the molecule does not interact with other molecules and the framework (i.e., is not present). When the value decreases below 0, the molecule is deleted from the system. On the other hand, if λ exceeds 1, the fractional molecule is built successfully in the system and a new fractional molecule is randomly inserted. To maintain continuity of the simulation, it is most desired to make all system λ states equally probable. Before each simulation, the λ range was divided into 21 adjacent bins. Biasing factors corresponding to each bin were obtained in the first 5 × 104 equilibration cycles with the Wand−Landau sampling method.43,44 The change of the λ value was being chosen uniformly between −Δλmax and +Δλmax and scaled to achieve ca. 50% of acceptance. More detailed information on the CFCMC technique can be found in the literature.16,42,45 The most noteworthy advantage of the CFCMC technique is forcing the molecules in or out of the structure, thus making the insertion/deletion process continuous. We used both CBMC and CFCMC Monte Carlo techniques to calculate the adsorption isobars of alkanes in MFI and MEL zeolites. The three most time-consuming simulation systems studied in this work were 3MP/MFI, 23DMB/MFI, and 2MP/ MEL. Figure 2 shows the trends of achieving sorption
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MOLECULAR SIMULATIONS For adsorbates, we used the TraPPE force field of Martin and Siepmann.33,34 The model unites alkyl groups (CH3, CH2, CH) into singular interaction centers (pseudoatoms) and successfully reproduces macroscopic thermodynamic properties of adsorbates such as critical points and phase diagrams. This approximation considerably shortens the calculation time without loss of accuracy of adsorption calculations compared to full-atom models.29 The molecules are represented as flexible chains internally linked through stretching, bending, torsion, and long-range van der Waals interactions (L-J potentials between pseudoatoms separated more than three bonds). The harmonic bonding potentials have been used to connect adjacent pseudoatoms. Bends between three neighboring groups have been described by harmonic bending potential. To model torsions, we used the TraPPE cosine series. The guest−guest and guest−host interactions were calculated using effective Lennard-Jones potentials (L-J),29 with the potential cut and shifted with the cutoff distance set to 12 Å. The Coulomb interactions were omitted because dipole moments along the alkane molecules as well as changes in electric field in pure silica zeolites are considered as marginal.35 A full set of parameters used in the calculations can be found in the Supporting Information (page S1). To better represent the crystal lattice of the zeolites, periodic boundary conditions36 were applied requiring the dimensions of the simulation box to be at least 2 times longer than the cutoff of L-J potentials (24 Å). This requires the use of two unit cells of MFI or MEL in every direction. Although the studied structures exhibit temperature-dependent phase transitions,37−39 only high-temperature rigid orthorhombic MFI37 and rigid I4̅m2 hexagonal MEL39 frameworks have been exploited. In our previous work,10 we proved that the other structures do not affect hydrocarbon adsorption, which is consistent with other works.23,24 We performed grand-canonical Monte Carlo simulations to calculate the adsorption isobars with RASPA software.40 We used values of pressures matching the experimental values and C
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Figure 3. Scheme of the two types of intersections between straight channels present in the MEL framework: type I (top) and type II (bottom). Their geometrical centers are marked with black dots.
representations of adsorption sites defined for MEL are presented in Figures S3 and S4 of the Supporting Information. The calculations were carried out using homemade code developed by the RASPA group, which will be published shortly.
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RESULTS AND DISCUSSION Adsorption Isotherms. The experimental and calculated adsorption isotherms of hexane isomers in MFI zeolite are compiled in Figure 4. The adsorption isotherms for 2MP and 3MP are similar. The experimental and calculated data agree at middle and high pressure ranges and at temperatures below 348 K. The kink of the adsorption isotherms at the loading of four molecules per unit cell is noticeable between 0.1 and 1 kPa. At temperatures between 373 and 423 K, the results obtained by Cavalcante et al.17 and Zhu et al.20 differ from one another. At high temperatures, the calculated isotherms agree with the results of Jolimaitre et al.18 for 3MP and those of Cavalcante et al.17 for 2MP. The single QE-TPDA points obtained from the adsorption isobars agree with part of the experimental and molecular simulation values reported for the monobranched hexane isomers. For adsorption of dibranched hexane isomers in MFI, experimental data at low temperatures are scarce. Molecular simulations agree with the 23DMB adsorption isotherm of Zhu et al.20 and differ from the isotherm provided by Gener et al.19 However, the similarity of the adsorption isotherms of 23DMB and nC6 reported in latter work19 makes them less reliable. Adsorption capacities obtained by Ferreira et al.26 for dibranched molecules are underestimated for all samples studied. Molecular simulations agree with the results reported for high temperatures. The QE-TPDA data points agree with the isotherms for 23DMB, but they are overestimated for 22DMB. Most likely, the diffusion limitations for dibranched molecules through MFI channels make reaching the equilibration difficult or even impossible at low temperatures in static measurements of adsorption isotherms. On the other hand, in the dynamic QE-TPDA measurements, the adsorptive-containing carrier gas flows through the sample, enforcing direct contact between the adsorbent and adsorptive that results in reaching adsorption equilibrium faster. Moreover, the adsorption process occurring during cooling of the sample from high temperatures facilitates diffusion. QE-TPDA Profiles. Figure 5 shows the experimental QETPDA profiles, represented as plots of temperature-dependent specific sorption rate (ssr). They consist of desorption maxima and adsorption minima, which should be construed as absolute
Figure 2. Effect of the number of cycles used in simulations on the rate of achieving Monte Carlo equilibrium for adsorption of selected alkanes in MFI and MEL at saturation. Black lines stand for CBMC and red for CFCMC techniques.
equilibrium at saturation (173 K). Even though the two methods provide the same results, CFCMC is faster than CBMC. It is worth noting that CBMC is more efficient to generate subsequent states. Namely, the average time needed to compute 106 cycles under saturation conditions on a single thread of a 2.4 GHz processor was equal to ca. 85 and 102 h for 3MP/MFI with the use of CBMC and CFCMC methods, respectively. However, the equilibrium is still reached in much shorter time using CFCMC. A distinction between different adsorptions sites within the MFI and MEL framework has been done to calculate distributions of the adsorbed molecules. We can distinguish three adsorption sites present in MFI: the straight channels, the zigzag channels, and the intersections between them. Spaces of the straight channels and intersections have been defined together by cylinders of 4 Å radius crossing the whole structure in the [010] direction with centers located in (1/2, 0, 0) and (0, 0, 1/2). Spaces of the intersections have been detached from these cylinders as spheres of 4 Å radius located in (1/2, 1/4, 0) and (0, 1/4, 1/2). Adsorption in other positions than the straight channels or the intersections has been ascribed to the zigzag channels. Graphical representations of the adsorption sites defined for MFI can be found in Figures S1 and S2 of the Supporting Information. The straight channels of the MEL framework intersect in two ways (Figure 3), creating preferential adsorption sites. The intersections located at (0, 0, 1/2) and (1/2, 1/2, 0) have been marked as adsorption sites of type I, and those at (0, 1/2, 3/4) and (1/2, 0, 1/4) as sites of type II. Spaces of sites I and II were defined by cylinders centered in the corresponding positions, oriented parallel to [001], with base radius equal to 2.25 or 2.5 Å and heights of 13 or 10 Å, respectively. The molecules with centers of mass located inside the above-described cylinders were counted as belonging to the corresponding sites. The rest of the molecules were assigned to the channels. Graphical D
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Figure 4. Adsorption isotherms of hexane isomers in MFI, from top to bottom: 2MP, 3MP, 23DMB, and 22DMB. For clarification, the isotherms are split in three temperature ranges: low (up to 348 K) (a), middle (373−448 K) (b), and high (over 473 K) (c). Experimental data17−20,22,23,26,27 were taken from the literature, while single QE-TPDA points were extracted from our adsorption isobars. The values obtained from molecular simulation (dashed lines) were either taken from the Dubbeldam work29 or calculated.
(desorption) or normal (adsorption) temperature derivatives of desorption/adsorption isobars. Mirror symmetry of the desorption and adsorption branches forming the QE-TPDA profiles indicates equilibrium control of sorption. A unit on the y-axis was chosen to enable comparison of the adsorption amount. For both materials, almost the same profiles were
obtained for the monobranched paraffins in the entire temperature range except for minor differences at low temperatures (up to 340 K). Disparities appear between linear, monobranched, and dibranched molecules. Desorption maxima and adsorption minima are separated (MFI) or partially separated (MEL) for n-hexane, which was ascribed to the E
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order−disorder transition of adsorbed phase.10,46 The shapes of the profiles of 23DMB and monobranched alkanes are similar for the two zeolites. However, desorption and adsorption profiles obtained for 22DMB differ from each other, especially for MFI zeolite. Moreover, while all recorded desorption− adsorption cycles were completely repeatable, for 22DMB/ MFI, the first desorption profile differs from the ones that follow. Literature reports indicate that achieving adsorption equilibrium for this molecule can be disturbed by diffusion limitations at low temperatures.22 Most likely heating during the first cycle (after adsorption at room temperature) causes thermal “opening” of the pore structure allowing adsorption of a higher amount of this alkane. This effect is not observed for the following cycles because adsorption occurs directly after desorption, when the sample is being cooled from high temperatures. A very similar increase on the adsorbed amount of 22DMB during the first heating between 343 and 433 K was reported by Bellat et al.23 Adsorption Isobars. MFISingle Components. Experimental adsorption isobars were calculated by integrating the QE-TPDA profiles according to the previously described procedures.10 In Figure 6, we compiled the experimental and calculated adsorption isobars of hexane isomers in MFI. The vertical dotted lines mark the experimental restrictions resulting from the vapor−liquid phase transition of the adsorbates. We observe that the adsorption equilibrium can be obtained in the simulations by applying the two computational methods. For the monobranched alkanes, there is agreement between the experimental and calculated isobars. Reaching saturation loadings of eight molecules per unit cell is impossible for these adsorbates under the applied conditions using the QETPDA method but seems feasible with other experimental techniques (Figure 4). The kinks on the computed isobars at
Figure 5. QE-TPDA profiles of adsorption hexane isomers in MFI (a) and MEL (b) zeolites. The orange dashed line corresponds to the first desorption cycle for 22DMB. Horizontal black dashed lines show the background level indicating the absence of adsorption or desorption. The values of the partial pressure of hydrocarbons are equal to 7.7 (nC6), 9.3 (2MP), 10.8 (3MP), 13.3 (23DMB), and 14 (22DMB) mbar for MFI and 8 (nC6), 9.3 (2MP), 10 (3MP), 10.5 (23DMB), and 14.5 (22DMB) mbar for MEL.
Figure 6. Adsorption isobars of hexane isomers in MFI: experimental (lines), calculated using CFCMC (dots), and CBMC (triangles). Values of pressure used in experiment/simulation: 7.7/8 for nC6, 9.4/9 for 2MP, 10.8/11 for 3MP, 13.3/13 for 23DMB, and 14/14 mbar for 22DMB. Adsorption isobars for nC6 are added for comparison.10 Vertical dotted lines indicate the temperature of condensation of pure adsorbates obtained from the Antoine equation.47 F
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Figure 7. Average occupation profiles of hexane isomers in MFI zeolite in the plane zx obtained for saturation conditions. The profiles represent 2 × 2 × 2 unit cells; i.e., unit cell vectors are half of the length of an appropriate edge of each picture. The color scale is indicated.
Figure 8. Contributions of adsorption sites in total calculated adsorption amount for hexane isomers in MFI zeolite. Adsorption in site intersections, straight channels, and zigzag channels is indicated in red, green, and blue dots, respectively. Black open dots stand for total adsorption.
but 23DMB molecules can be adsorbed in the zigzag channels as well (Figure 7). A more extensive collection of average occupation profiles for each studied alkane in MFI and MEL at different loadings (from saturation to the Henry regime) is presented in the Supporting Information (Figures S5−S10). Figure 8 shows a quantitative analysis of the adsorption of hexane isomers in MFI. Intersections are the most preferential positions for all molecules, since they are adsorbed there at the highest temperatures. At loadings higher than two molecules per unit cell, nC6 starts to adsorb in zigzag channels and finally displaces part of the molecules from the intersections to the straight channels. This effect could be responsible for two-step adsorption of n-hexane and n-heptane in MFI zeolite known in the literature as the “commensurate freezing” effect.46 Channels of this zeolite are energetically unfavorable for the branched molecules, so additional increase of the thermodynamic potential of the adsorptive is required to push the molecules inside,48 which can be achieved by lowering the temperature (or increasing the pressure in the standard approach). Unlike for nC6, the branched molecules do not leave preferential positions in the intersections of MFI channels at higher loadings. Only nC6 and 2MP can be adsorbed in straight channels.
four molecules per unit cell are more pronounced for 2MP than for 3MP. The agreement between experimental and simulated data is worse for dibranched molecules. On the one hand, we observe overlapping of adsorption isobars of 23DMB in MFI above 390 K. On the other hand, the experimental loading below this temperature is significantly higher. In the case of 22DMB, we observe a shift (ca. 30 K) of the calculated isobar to lower temperatures. The adsorption of this bulky molecule is probably affected by thermal vibrations of the MFI structure, resulting in a different adsorption mechanism than that for the other adsorbates. Since we model frameworks as rigid, this effect cannot be mimicked with molecular simulations. Nonetheless, we find quantitative agreement of the calculated and experimental sorption capacities at saturation. On the basis of the average occupation profiles, one can find that at the values of pressure under study the intersections are the only adsorption sites of 22DMB molecules. The average occupation profiles (Figure 7) indicate that adsorption of 2MP and 3MP at low temperatures occurs initially in the preferential positions located in the intersections and it is followed by immobilization of these molecules in the zigzag channels. Simulations indicate that intersections are preferential adsorption sites for 22DMB and 23DMB molecules G
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Figure 9. Adsorption isobars of hexane isomers in MEL: experimental (lines), calculated using CFCMC (dots), and CBMC (triangles). Values of pressure used in experiment/simulation: 8/8 for nC6, 9.3/9 for 2MP, 10/10 for 3MP, 10.5/11 for 23DMB, and 13/13 mbar for 22DMB. Adsorption isobars for nC6 are added for comparison.10 Vertical dotted lines indicate the temperature of condensation of pure adsorbates obtained from the Antoine equation.47
Figure 10. Average occupation profiles of hexane isomers in the MEL framework in the plane zx obtained for saturation conditions. The profiles represent 2 × 2 × 2 unit cells; i.e., unit cell vectors are half of the length of an appropriate edge of each picture.
MELSingle Components. Figure 9 shows the experimental and calculated isobars of hexane isomers in MEL. As for MFI, we find agreement between experimental and calculated isobars for 2MP, 3MP, and 23DMB, while the isobar of 22DMB in MEL obtained from molecular simulation is shifted to lower temperatures. The kink at four molecules per unit cell is also more clear-cut for 2MP than for 3MP. The positions of the pseudoatoms are marked in Figure 10. A comprehensive collection of average occupation profiles can be found in the Supporting Information (Figures S5−S10). One interesting finding extracted from our simulations is the lack of adsorption of 23DMB molecules in the straight channels of MEL. The average occupation profiles depicted in Figures 10 and 11 show that these molecules occupy the intersections only. The structures of MFI and MEL zeolites are similar considering their pore diameters, but apparently, the curvature of the zigzag MFI channels increases their effective size. Figure 12 shows the occupation analysis of the adsorption sites in MEL. As shown in Figure 3, the straight channels of MEL intersect each other in two ways, affecting the geometry of these adsorption centers. At high loadings, the molecules of nC6, 2MP, and 3MP are located in the straight channels outside the intersections. Differences in occupation of the
Figure 11. Average occupation profiles of hexane isomers in the MEL framework in the plane xy obtained for saturation conditions. The profiles represent 2 × 2 × 2 unit cells; i.e., unit cell vectors are half of the length of an appropriate edge of each picture.
intersection appear at low loadings (
