Adsorption of n-Alkanes in MFI and MEL: Quasi-Equilibrated

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Adsorption of n-Alkanes in MFI and MEL: Quasi-Equilibrated Thermodesorption Combined with Molecular Simulations Andrzej S#awek, Jose Manuel Vicent-Luna, Bartosz Marsza#ek, Salvador Rodríguez-Gómez Balestra, Wac#aw Makowski, and Sofia Calero J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06957 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 20, 2016

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The Journal of Physical Chemistry

Adsorption of n-Alkanes in MFI and MEL: Quasi-Equilibrated Thermodesorption Combined with Molecular Simulations

Andrzej Sławek†, José Manuel Vicent-Luna‡, Bartosz Marszałek†, Salvador R.G. Balestra‡, Wacław Makowski*,†, 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 ‡

* [email protected] tel: +48 12 663 22 45 * [email protected] tel: +34 954 977594 ABSTRACT Adsorption of n-alkanes on high silica MFI and MEL zeolites was studied by means of experimental quasi-equilibrated temperature programmed desorption and adsorption (QETPDA) and Monte Carlo simulations. An unusual, isobaric approach to adsorption measurements and simulations was applied. Good agreement between the experimental and calculated data observed for MFI indicate that the QE-TPDA is a reliable method for studying porosity-related adsorptive properties of molecular sieves. The calculated average occupation profiles confirmed limited mobility of hexane and heptane molecules adsorbed in the sinusoidal channels on the MFI, thus proving the concept of order-disorder phase transition postulated in explanation of the two-step desorption profiles of these alkanes observed for MFI zeolites. Partial agreement of the calculated isobars with the experimental data found for zeolite MEL indicated that adsorption of n-alkanes in this structure is more complex than assumed in the simulation model. However, two-step desorption profiles and immobilization of the molecules adsorbed in the straight channels of the MEL structure were also found for hexane.

1. Introduction

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Owing to extraordinary properties resulting from their unique structure, zeolites are extensively utilized in technology, mainly as shape selective catalysts and adsorbents. They are referred to as “molecular sieves” because of their ability of adsorbing molecules with size and shape fitting channels and cavities present within the framework. Despite the fact that crystal structure of zeolites controlling geometry of these voids is well known, experimental characterization of the zeolites based on their micropore-related adsorption properties remains demanding. Slow diffusion of the adsorbed molecules often results in prohibitively timeconsuming adsorption measurements1. In the case of hydrocarbons this is a serious problem, since detailed knowledge of their adsorption in zeolites is of considerable practical interest for a large number of technological applications2. Therefore, development and optimization of experimental and theoretical methods allowing better description and understanding of hydrocarbon adsorption in microporous molecular sieves is important. Monte Carlo (MC) simulations are a useful tool for investigating static properties related to adsorption at given conditions (T, p), such as average loading of adsorbate in the studied material, radial distribution functions, average distances and a variety of average energies of the systems including isosteric heat of adsorption. Force fields to reproduce these properties in zeolites have been extensively used and validated for a large number of adsorbates and systems3,4,5,6. One can relate these thermodynamic properties to the microscopic behavior of the systems. This fact makes MC simulations a very attractive computational method for studying adsorption phenomena. Adsorption of hydrocarbons in MFI-type molecular sieves was extensively studied experimentally7 and using molecular simulations8. For most linear alkanes the type I adsorption isotherm, characteristic for the microporous materials, is observed. However, for hexane and heptane a kink on the isotherm appears, indicating a two-step adsorption mechanism. Smit and Maesen9 proposed a “commensurate freezing” explanation of this effect. They postulated a kind of order-disorder phase transition during adsorption due to “freezing” of the adsorbed hexane and heptane molecules in the zigzag channels. Immobilization of the adsorbed molecules involves increase of the entropy-loss (-∆Sads) due to adsorption. Since this is thermodynamically

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unfavorable, additional increase of pressure or decrease of temperature is needed to fill the remaining pore volume. ZSM-11 zeolite (MEL topology) has similar structure to MFI but with straight channels instead of zigzag channels. These two structures also differ in the way of connecting channels since MFI has one type of intersection and MEL has two types, large and small ones. For adsorption of hydrocarbons in MEL-type molecular sieve less information can be found. Bibby et al.10 reported two-step desorption of n-alkanes with chain longer than C5 from silicalite-1 (puresilica ZSM-5) and silicalite-2 (pure-silica ZSM-11) due to self-blocking of the channels at intersection points. Earlier work of Makowski and Majda11 with the use of TG method also revealed the two-step desorption of hexane and heptane from ZSM-11 (Si/Al=45) and ZSM-5 (Si/Al=66) zeolites. However, two-step adsorption isotherms of hexane observed experimentally for Al containing and pure silica ZSM-11 by Marguta et al.12 were not reproduced using molecular simulations. The most popular experimental approach to adsorption is measuring the adsorption isotherms. However, the adsorption isobar measurements, where pressure is fixed and temperature changes, are also interesting. Several experimental techniques such as thermogravimetric analysis13,14, temperature programmed adsorption equilibrium15, large temperature jump16 or even infra-red spectroscopy17, can provide adsorption isobars. Most of these methods have low resolution, and they are treated as complementary to adsorption isotherms or other studies, not related to adsorption itself. A new experimental technique allowing determination of the adsorption isobars of hydrocarbons is quasi-equilibrated temperature programmed desorption and adsorption (QETPDA). This has been found an efficient method for studying adsorptive properties of zeolites18, metal-organic frameworks19 and mesoporous materials20. In QE-TPDA measurements the amount of sorbate desorbed or adsorbed by a sample is recorded as a function of temperature which is changing cyclically. A QE-TPDA profile consists of desorption maxima observed during heating, and adsorption minima observed while cooling the sample. For zeolites, desorption of volatile n-alkanes (C5-C9) from the micropores observed at temperatures 25-400 °C, depend on the sorbate molecular mass, the framework type and the extra-framework cations. The aim of

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this work is to combine this technique with Monte Carlo simulations for in-depth interpretation of the QE-TPDA based adsorption isobars of several n-alkanes obtained for high silica ZSM-5 and ZSM-11 zeolites. These zeolites were chosen for this study due to similarity of their frameworks. The main difference between them, i.e. presence of the 3D system of 10MR sinusoidal and straight channels in MFI and 3D system of 10MR straight channels only in MEL results from different symmetry in stacking of identical building units.

2. Experimental

The experimental part of the study was performed on commercial high silica ZSM-5 (Zeolyst, CBV 28014, Si/Al = 140, H+ form) and pure silica ZSM-11. The latter material was synthesized accordingly to procedure published by Lu et al.21: 20.02 g of tetrabuthylammonium hydroxide aqueous solution (TBAOH, 55%) and equal amount of distilled water were mixed in a propylene bottle. 24.30 g of Ludox® silica solution (30% aq.) was added dropwise to the TBAOH solution while stirring. Then 8.46 g of H2O was added to reach the molar ratio of 1:0.35:25 SiO2:TBAOH:H2O. Obtained gel underwent static ageing overnight at room temperature. Afterwards 60 g of gel solution was moved to the 100 ml teflon-lined autoclave and heated in oven at 170 °C for 48 hours. Product was recovered by filtration, dried in air and calcined for 6 h at 560°C. Structures of pure-silica ZSM-5 (MFI) and ZSM-11 (MEL) were confirmed by analysis of XRD patterns (Fig. S1 and S2, Supporting Information), which were recorded by Rigaku MiniFlex powder diffractometer with Cu Kα radiation at 10mA and 10 kV, 2θ step scans of 0.02° and a counting time of 1 s per step. The measurements of quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) of pentane (analytical pure, POCh), hexane (analytical pure, Acros Organics), heptane (analytical pure, Chempur), octane (99%, Sigma-Aldrich), nonane (99%, Acros Organics), decane (98%, Sigma-Aldrich) were performed using a modified temperatureprogrammed desorption setup equipped with a thermal conductivity detector (Micro Volume TCD, Valco) which was described in details in earlier works (Makowski and Ogorzałek18, Mańko et al.22). The studied zeolites were pressed into pellets, crushed and sieved (fraction of 400–500

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μm was used). The samples of zeolites (7-10 mg) were activated before each experiment by heating up to 500 °C in a flow of pure He (Air Products, purity 5.0). The actual QE-TPDA measurements were performed by heating and cooling samples with the preadsorbed alkane in a flow (6.5 cm3∙min-1) of He containing small admixture of the alkane (ca 0.1-1 mol%), according to different temperature programs up to 400 °C. Calibration constants, relating the detector signal with the concentration or partial pressure of each hydrocarbon, were determined by measurements of the mass loss of the vial containing liquid alkane during experiments. The detector signal, when adjusted to zero for pure He, is proportional to the partial pressure of the hydrocarbon in the carrier gas. However, when it is compensated to zero for the background level corresponding to the inlet He-alkane mixture, the signal is proportional to the overall desorption or adsorption rate.

3. Molecular simulations

We used united-atom models for the hydrocarbons and the force field reported by Dubbeldam et al.3 to describe guest-guest and host guest interactions. These force fields were parameterized to reproduce the complete isotherms of hydrocarbons in zeolites and have been validated widely over the years23,24,25,26,27. Models used in the simulations can always be described via bonded and non-bonded interactions. In the non-bonded interactions guest-guest, guest-host and long-range intramolecular (between C1 and C5, C6, C7 etc.) Lennard-Jones (L-J) potentials were taken into account, with the potential cut and shifted with the cutoff distance set to 12 Å. The bonded intramolecular interactions of the alkanes molecules were also included, i.e. bond-stretching, bond-bending and torsion. The CH2 and CH3 pseudoatoms were connected in a chain by the harmonic bonding potentials. The bond bending between three neighboring beads was modeled by the harmonic bending potential, while the torsions between four following alkyl pseudoatoms were defined by the TraPPE cosine series potential. The Coulomb contribution to total energy has been neglected, since the electric field in pure silica frameworks does not change much across the channels and intersections28. The complete set of

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the potential parameters and details were taken from the literature3 and can be also found in the Supporting Information (Table S1). The size of the simulation box was chosen in such a way that minimum length in each of the coordinate directions was larger than twice the cutoff distance. This corresponds to 2 x 2 x 2 MFI and MEL unit cells. Periodic boundary conditions29 were adopted in the three dimensions. For MFI material reversible polymorphic phase transition from monoclinic to orthorhombic structure takes place above 323 K30. Rigid orthorhombic31 as well as rigid monoclinic32 MFI framework has been used in the simulations, since flexibility of this particular zeolite during adsorption in the studied temperatures is considered as marginal33. For MEL material phase transition from lower-symmetry (I4) to higher-symmetry (I4m2) tetragonal structure in 313-323

K has been reported34. In our work the rigid high-temperature (I4m2)34 and rigid low-

temperature (I4)35 frameworks have been exploited. The host energies of all the structures were

thus assumed null. We integrated the above–descripted force fields and models to the RASPA code36,37. Grandcanonical Monte Carlo simulations using this code, were performed to obtain the adsorption isobars of n-alkanes from C5 to C10 on orthorhombic MFI and (I4m2) MEL structures, for temperature between 173 K and 673 K with 10 K interval and for values of pressure that matched to experimental values. To verify the choice of the model structures additional simulations with the use of monoclinic MFI and (I4) MEL frameworks for temperatures between 273 K and 573 K with 20 K interval were also performed. In the grand-canonical ensemble (μVT) chemical potential, volume and temperature are fixed. The fugacity coefficients, relating fugacity to pressure were assumed equal to 1. During the Monte Carlo simulations three trial moves of molecules were randomly attempted: translation, rotation and swap between the framework and the reservoir with equal probability of creation and deletion. Each point of the calculated isobars was obtained by running 106 cycles after equilibration (number of the equilibration cycles depended on n-alkane). For the average occupation profiles we carried out separate calculations of 5∙105 cycles (starting from the equilibrated state), in order to obtain the configurational distributions of molecules every 5∙102 cycles for each n-alkane for 3 different conditions: 1) for the saturation

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loading, at low temperatures, 2) for an intermediate loading of 4 molecules per unit cell, and 3) for the low coverage regime at temperatures corresponding to the loading of 2 molecules per unit cell. To calculate low coverage heats of adsorption for each alkane, we performed simulations in the NTV ensemble using the Widom particle-insertion method38. These simulations were performed for 3∙105 cycles at high temperatures (423, 453, 473, 493, 513 and 513 K for C5, C6, C7, C8, C9 and C10 respectively for MFI and 373, 413, 433 and 453 K for C5, C6, C7 and C8 respectively for MEL) that correspond with the low coverage regime given by the adsorption isobars in the studied conditions. For the purpose of comparison the accessible micropore volume, it was estimated for all the studied structures using helium with the Widom particle-insertion method38.

4. Results and Discussion

Adsorption isobars

The experimental QE-TPDA profiles for different heating/cooling rates are shown for MFI (Fig. 1) and MEL (Fig. 2). We can observe desorption maxima and adsorption minima. The shift between their positions is characteristic for the QE-TPDA method and increases with the heating/cooling rate. The occasional overlapping of the low-temperature parts of the adsorption minima results from ineffective cooling in this temperature range (293-353 K).

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Figure 1. QE-TPDA profiles for n-alkanes in MFI. TCD signal was compensated to zero for the inlet composition of hydrocarbon-containing carrier gas. HC partial pressures equal to 11.5 (C5), 7.7 (C6), 7.2 (C7), 4.5 (C8), 3.5 (C9), 1.4 (C10) mbar.

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Figure 2. QE-TPDA profiles for n-alkanes in MEL. TCD signal was compensated to zero for the inlet composition of hydrocarbon-containing carrier gas. HC partial pressures equal to 5.2 (C5), 7.6 (C6), 4.5 (C7), 2.9 (C8) mbar.

The QE-TPDA profiles may be converted to the adsorption isobars by integration, normalization and averaging (Eq. 1-3)18:

, | |

(1)

1

,

(2)

,

where i denotes desorption or adsorption, T0 and Tf are the initial and final temperatures (K) of the desorption maximum or adsorption minimum, Ades or Aads is the area (mV∙min) below desorption maximum or above adsorption minimum, respectively. Even though the integration is performed over time, its limits are given in temperatures because the time dependence of temperature is known. θdes or θads is the coverage (or adsorption degree) normalized to 0-1 range. After interpolation of the normalized desorption and adsorption profiles for an arbitrarily chosen set of θ values, the isobar θ(Tθ) is obtained by averaging of desorption and adsorption temperatures, corresponding to a given θ value: 1 2

(3)

An illustration of such transformations is presented in Fig. 3.

Figure 3. QE-TPDA profiles, measured as the TCD detector signal and sample temperature changing with time (b) are better represented as temperature dependent (a). Integration of the desorption maxima

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and adsorption minima, followed by normalization to 0-1 range and averaging, leads to the adsorption isobar (c). This example is shown for hexane/MFI, with partial pressure equal to 7.7 mbar.

In this work the isobars were calculated for a series of θ values from 0.02 to 0.98, with 0.02 step. Only the profiles recorded at 5°C/min for nC5–nC9 and 2°C/min for nC10 were used for this purpose. For faster temperature programs large shifts between desorption maxima and adsorption minima positions result in considerable differences between the integrated desorption and adsorption profiles (Fig. 3c), thus leading to inaccurate adsorption isobars. Molecular and mass saturation loadings of adsorbate in MFI were calculated as !! !. $.

, * -./ ∙ ∙ 100 ∙ '() +,

0$123$ 4 !. $. !! !. $.

(4)

-5 -./

(5)

where Ccal is the calibration constant (mV∙%mol-1), F is the flow rate of carrier gas (cm3∙min-1), Vm is the molar volume of ideal gas (cm3∙mol-1), ms is the mass of the sample (g), Mz and MHC are molar masses (g∙mol-1) of zeolite unit cell and hydrocarbon molecule, respectively. Momentary loadings, changing with the increasing temperature, were calculated by multiplying the maximum loadings (Eqs. 4 and 5) by the actual value of adsorption degree θ. A comparison of the experimental and calculated isobars of alkanes in MFI is presented in Fig. 4. The vertical dotted lines represent minimum temperatures that can be maintained for particular values of pressure. Additionally, the temperature derivatives of the experimental and calculated isobars, allowing easier spotting of differences in their behavior, are shown in Fig. S3 (Supporting Information). As we have found better conformity of the shapes and intensities of the calculated and experimental adsorption profiles for the orthorhombic MFI framework than for the monoclinic one only the results obtained for the former structure will be taken into further considerations. Lower saturation loading obtained for monoclinic structure also corresponds with lower accessible micropore volume (Tab. 1). A systematic shift of the high temperature parts of the experimental profiles to the higher temperatures does not exceed 20 K. Larger values of the experimental molecular loading observed at low temperatures for octane, nonane, and decane result most probably from phenomena not included in the

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simulation model, such as capillary condensation in the interparticle mesopores or adsorption on the external surface. Molecular simulations reproduce well subtle details of the experimental profiles, especially a two-step desorption observed for hexane, heptane, octane and nonane, with the intermediate loading of 4 molecules per unit cell. The results obtained for MFI are in agreement with earlier reports39,40,41.

Figure 4. Adsorption isobars of n-alkanes in MFI: experimental (lines), calculated for the orthorhombic (dots) and monoclinic (triangles) frameworks. Experimental values of partial pressure of hydrocarbons equal to 11.5 (C5), 7.7 (C6), 7.2 (C7), 4.5 (C8), 3.5 (C9), 1.4 (C10) mbar. Values of pressure used in simulations: 11.0 (C5), 8.0 (C6), 7.0 (C7), 4.0 (C8), 5.0 (C9), 1.4 (C10) mbar. Vertical lines represent temperature values corresponding to the vapor-liquid equilibrium obtained from the Antoine equation42.

Table 1. Calculated accessible micropore volume of the structures used for the molecular simulations. micropore volume / cm3∙g-1 MFI orthorhombic 0.380 MFI monoclinic 0.366 0.351 MEL (I4m2) 0.369 MEL (I4)

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In Fig. 5 a comparison of the experimental and calculated isobars of alkanes in MEL is presented, while in Fig. S4 (Supporting Information) the corresponding differential profiles are collated. We have obtained almost identical results for both MEL frameworks for pentane, hexane and heptane. However, the isobar of octane calculated for the (I4) framework exceeds that obtained for the other MEL framework in the low temperature range and is slightly closer to experimental data. This may result from slightly higher accessible micropore volume obtained for (I4) framework (Tab. 1). Differences between the simulation results obtained for different MEL frameworks do not seem meaningful, therefore in the next parts of this work only the results found for better known high-temperature (I4m2) MEL framework will be discussed. Unlike for MFI, for MEL only a partial agreement between the experimental and calculated adsorption profiles of n-alkanes was found. The high-temperature parts of the experimental isobars, corresponding to low molecular loading (below 2 molecules per unit cell), are quite close to the calculated values, and only slightly shifted to higher temperatures. However, the remaining parts of some experimental profiles differ considerably in their behavior from the calculated counterparts. These differences are best visible in Fig. S4 (Supporting Information), revealing that a two-step desorption patterns observed experimentally for hexane, heptane and octane were not reproduced in the simulations. On the other hand, while the experimental and calculated isobars obtained for pentane seem very close, comparison of the corresponding differential profiles shows that a single step experimental desorption is split into two steps in the simulation results. Literature reports concerning adsorption of n-alkanes in MEL type molecular sieves are scarce, so it is difficult to explain discrepancies between the experimental and calculated isobars obtained in this study. Similar differences regarding to the experimental and calculated adsorption isotherms of hexane in MEL at various temperatures were reported by Marguta et al.12. A two-step experimental isotherm and a single-step calculated isotherm (with maximum loadings of ca 8 and 6 molecules per unit cell, respectively) were found. However, the authors’ explanation referring the second step in the experimental isotherm to the multilayer adsorption on the external surfaces of crystallites seems unlikely, as it would require very large external surface area, typical for only nanoparticles. Other reasons of these discrepancies may be higher

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flexibility of the MEL framework or its phase transition upon adsorption. Such transitions were reported for adsorption of toluene43 and argon44,45. Therefore, the forcefield used for molecular simulations could not fully reflect adsorption phenomena for this particular zeolite. It is worth noticing that the level of agreement between the calculated and experimental isotherms obtained by Marguta et al. is very close to that found for adsorption isobars of C6-C8 in MEL in this work. However, in the case of hexane the calculated loading approaches the experimental value of ca. 8 molecules/u.c. at low temperature (below 223K) and it is the only adsorbate which clearly exhibits two-step adsorption in the simulation results for MEL.

Figure 5. Adsorption isobars of n-alkanes in MEL: experimental (lines), calculated for the (I4m2) (dots)

and (I4) (triangles) frameworks. Experimental values of partial pressure of hydrocarbons equal to 5.09

(C5), 7.7 (C6), 7.2 (C7), 4.5 (C8). Pressure values used in simulations: 6.0 (C5), 7.0 (C6), 6.0 (C7), 3.5 (C8) mbar. Vertical lines represent temperature values corresponding to the vapor-liquid equilibrium obtained from the Antoine equation42.

In Fig. 6 the calculated adsorption isobars, representing both molecular loading (a) and mass loading (b), are compared for MFI. These plots indicate that larger amount of shorter molecules can be packed in the MFI micropores and the interactions between the structure and the adsorbed molecules are higher for longer alkanes as they desorb at higher temperatures. 13



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Comparison of the saturation mass loadings of following n-alkanes (Fig. 6b inset) shows that the MFI framework exhibits noticeably higher sorption capacity for hexane and heptane than for the others n-alkanes. Analogous compilations of the calculated isobars for MEL are presented in Fig. 7. Lower desorption temperatures indicate that the interactions between the alkane molecules and the framework are weaker for MEL than for MFI. The highest mass saturation loading was found for hexane, and the lowest – for heptane (Fig. 7b inset).

Figure 6. Calculated adsorption isobars of n-alkanes in orthorhombic MFI in molecules per unit cell (a) and milligrams per grams (b). Saturation loading is also presented (b inset).

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Figure 7. Calculated adsorption isobars of n-alkanes for the (I4m2) MEL framework in molecules per unit cell (a) and milligrams per grams (b). Saturation loading is also presented (b inset).

Figs 8, 9 and 10 show the average occupation profiles of selected alkanes in MFI and MEL frameworks projected to three different crystallographic planes of the zeolites crystal lattice. The profiles correspond to the entire simulation box (8 unit cells for both zeolites) and the structure schemes are shown for one single unit cell. The color scale was chosen to allow comparison of the different profiles for each alkane and framework. In the xy or (001) plane (Fig. 8) the straight channels of MFI framework are visible vertically, and the zigzag channels – horizontally. In the yz or (100) plane (Fig. 9) the straight channels are horizontal and the zigzag channels perpendicular to the plane, alternately one on top of the other. In the xz or (010) plane (Fig. 10) the zigzag channels are in plane while the straight channels are perpendicular to the plane. Naturally, for the framework of MEL we observe straight channels in every direction. More extensive collection of average occupation profiles for each studied alkane at three different temperatures (at the saturation loading, intermediate one and corresponding to ca 2 molecules per unit cell.) is presented in Supporting Information (Figs. S5–S10).

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The fact that the molecules of heptane fit best the zigzag channels of MFI (Fig. 10b) and the molecules of hexane – the straight channels of MEL (Fig. 10e) results in the highest mass saturation loadings for these adsorbates (Figs 6b inset and 7b inset). The molecules located in the zigzag channels of MFI or in the straight channels of MEL do not block the intersections so that the remaining straight channels can be fully occupied as well. An interesting observation is that at saturation, the mentioned molecules do not have translational freedom inside the channels, what can be seen as cyan spots for every single CH3/CH2 pseudoatom of the alkane chain (Figs 10b and 10e). The average occupation profiles obtained at low adsorption extent (Figs S5-S10) differ slightly, depending on the framework. The position points scattered randomly along the length of the channels in MFI for all studied alkanes indicate that the adsorbed molecules have no preferential positions. On the other hand, the profiles found for MEL, with position points clustered in unbranched segments of the channels, indicate that that adsorption in the intersections is less favorable. The profiles obtained for the intermediate loadings are similar to the former ones. For both studied frameworks other interesting packing effects can be noticed. On the one hand, the molecules of pentane are remarkable too short to fit the zigzag channels of MFI so they can move inside and cannot fill the structure efficiently. On the other hand, the molecules of octane, with the chain length only one methylene group longer than of heptane, are just a little too long for the zigzag channels blocking the intersections and thus barring the access to some channels for other molecules. This effect may be seen on average occupation profiles as black or grey regions where molecules should be present. The straight channels of MEL are slightly shorter than the corresponding sinusoidal (or zigzag) channels of MFI. For this reason, the structure of MEL exhibits analogical adsorptive properties for different adsorbates i.e. pentane molecules are too short and heptane molecules are too long to fit unbranched sections of straight channels. Average occupation profiles reveal void spaces in a different parts of frameworks for the alkanes too long to fit the channels. Most likely these voids are created randomly due to described packing effect. Moreover, cyan spots similar as for heptane/MFI or hexane/MEL can

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be observed for long alkanes. Monte Carlo simulations sample entire structure of the studied systems with adsorbate but previously adsorbed molecules block changes in the system. Generated states are stable and it is difficult to perform deletion and reinsertion of any molecule, however, it does not exclude the fact that presented results are equilibrated. For example, even for a huge number of cycles, it was not possible to obtain higher loading than 44 octane molecules per MFI simulation box (i.e. 5.5 molecules per unit cell.).

Figure 8. Average occupation profiles of pentane (a), heptane (b) and octane (c) adsorption in the orthorhombic MFI framework and pentane (d), hexane (e) and heptane (f) adsorption in the (I4m2) MEL framework, in the plane xy for saturation loading. At intervals of 5∙102 cycles positions of all CH2 and CH3 pseudoatoms were drawn as dots. The total number of the snapshots was 103. The color scale is also presented.

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Figure 9. Average occupation profiles of pentane (a), heptane (b) and octane (c) adsorption in the orthorhombic MFI framework and pentane (d) hexane (e) and heptane (f) adsorption in the (I4m2) MEL framework, in the plane yz. At intervals of 5∙102 cycles positions of all CH2 and CH3 pseudoatoms were drawn as dots. The total number of the snapshots was 103.

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Figure 10. Average occupation profiles of pentane (a), heptane (b) and octane (c) adsorption in orthorhombic MFI framework and pentane (d), hexane (e) and heptane (f) adsorption in the (I4m2) MEL framework, in the plane zx. At intervals of 5∙102 cycles positions of all CH2 and CH3 pseudoatoms were drawn as dots. The total number of the snapshots was 103.

Isosteric heat of adsorption

The QE-TPDA profiles comprise quasi-equilibrium data points differing in temperature and pressure of the adsorptive. Such data may be used for constructing the adsorption isosters and 19



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determination of the isosteric heat of adsorption, provided that values of the adsorption degree (or coverage) are known. In order to construct the adsorption isosters, the QE-TPDA measurements were performed for different heating and cooling rates (10, 8, 6, 4 and 2°C/min). The resulting QE-TDPDA profiles were integrated and normalized to 1. For a series of arbitrarily chosen θ values (from 0.1 to 0.9, with 0.05 step) the corresponding values of the detector signal and temperature were found by linear interpolation. The corresponding values of molecular loading were determined similarly as for the adsorption isobars, by multiplying saturation loading by θ values. Examples of the experimental data with the isosteric data points indicated and the resulting isosters, plotted as ln(signal TCD) vs T-1, are shown in the Figure 11. The linearity of all transformed isosters obtained for hexane is reasonable. However, for other hydrocarbons several isosters corresponding to some θ values were incorrect, therefore they were excluded from further calculations. Values of the experimental isosteric heat of adsorption Qst.

exp.

were calculated

from the slope of the linearized isosters46 ln!89: $ '; 20:!

?. @

(6)

All the linearized isosters used to calculate the isosteric heat of adsorption are presented in Figs S11 and S12 in Supporting Information.

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Figure 11. a) The QE-TPDA profiles of n-hexane in MFI with marked data points used to construct the adsorption isosters. The detector signal was adjusted to zero for pure He. b) The linearized adsorption isosters.

In the case of molecular simulations the absolute values of isosteric heat of adsorption Qst. sim. were computed as: |

Adsorption of n-Alkanes in MFI and MEL: Quasi-Equilibrated

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