Ordering of n-Alkanes Adsorbed in the Micropores of AlPO4-5: A

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Ordering of n-Alkanes Adsorbed in the Micropores of AlPO-5: a Combined Molecular Simulations and Quasi-Equilibrated Thermodesorption Study Andrzej S#awek, Jose Manuel Vicent-Luna, Bartosz Marsza#ek, Wac#aw Makowski, and Sofia Calero J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08927 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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The Journal of Physical Chemistry

Ordering of n-Alkanes Adsorbed in the Micropores of AlPO4-5: a Combined Molecular Simulations and Quasi-Equilibrated Thermodesorption Study Andrzej Sławek†, José Manuel Vicent-Luna‡, Bartosz Marszałek†, Wacław Makowski*,†, Sofía Calero*,‡ †

Jagiellonian University, Faculty of Chemistry, Gronostajowa 2, 30-387 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

from

methane

to

decane

(C1-C10)

on

AlPO4-5

aluminophosphate was studied by means of Monte Carlo simulations. The additional experimental study with use of quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) yielded high quality adsorption isobars of C5-C10 n-alkanes. The QETPDA based isotherms were accurately reproduced in the simulation, thus validating the applied force field. The agreement between experimental and simulated data indicated that the QE-TPDA is a reliable method for studying porosity-related adsorptive properties. The simulations revealed differences in the adsorption mechanism of the n-alkane series, with unmistakable site-based adsorption for short molecules (C1-C5) at saturation. Kinetic properties of the studied systems determined with the use of molecular dynamics exposed chain length dependency of the self-diffusion coefficient, indicating probably a resonant diffusion mechanism.

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Introduction Aluminophosphate molecular sieves (AlPO4-n) constitute a family of crystalline porous materials similar to zeolites.1 The main difference between these groups of materials is the chemical composition. Zeolite-like frameworks of AlPOs are built of corner-sharing tetrahedra: negatively charged [AlO4]- and positively charged [PO4]+ positioned alternately. In fact, the net charge of the AlPO framework is equal to zero excluding the presence of extra-framework cations. Unlike zeolites, the Al atoms of AlPOs may be either four, five or six-fold coordinated.2 The isomorphous substitution of the framework Al or P with transition metal ions can lead to formation of redox or acid centers exhibiting high activity in various technologically important catalytic such e.g. selective oxidation or isomerization of hydrocarbons.3,4 Silicoaluminophosphate materials5 (SAPOs) are structurally similar to AlPOs, but Si atoms replacing P in the framework contribute to its overall negative charge that must be compensated by the extraframework cations. Brönsted acidity obtained by exchange of the cations with protons is relevant in acid-catalyzed reactions such as n-alkane cracking and hydrocracking,6 oxidative dehydrogenation of alkanes,7 and methanol-to-olefins (MTO) process.8 One of the best known aluminophosphate materials is AlPO4-5 (AFI framework) which comprises one-dimensional system of cylindrical channels arranged in a honeycomb pattern.9 The channels have an effective diameter of 7.3 Å10 but in addition to the narrow segments they contain also wide regions with diameter of 8.6 Å (Figure 1).11


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Figure 1. The structure of AlPO4-5 channels in the side (a) and cross-sectional (b) views. Oxygen, aluminum, and phosphorus atoms are in red, grey, and orange, respectively. Arrows point to the wide (green) and narrow (blue) regions of the channels.

Despite its apparently simple porous system, the adsorption properties of this molecular sieve are unexpectedly complex. Choudhary et al.12 reported sorption capacity for different adsorbates corresponding to the pore volume of 0.14 cm3/g. On the other hand, Martin et al.13 showed that for AlPO4-5 and SAPO4-5 it is not possible to determine a single value of micropore volume as the corresponding values calculated from the saturation sorption capacity for different adsorbates range from 0.094 cm3/g (for neopentane) up to 0.17 cm3/g for molecular deuterium (D2). Newalkar et al.14 reported higher adsorption of cyclopentane than for n-pentane and 2-methylbutane suggesting stacking arrangement of the cyclic molecules inside the AlPO4-5 channels. Based on the sorption capacities Chiang et al.15 assumed face-to-face packing for benzene and o-xylene, shoulder-to-shoulder for mand o-xylenes, and coiled for straight-chain alkanes. In another work on AlPO4-5 by Newalkar et al.16 an increase of sorption capacity with decrease of the molecule length (or increase of branching) has been reported for C6 isomers. However, the presence of mesopores in the material did not allow determining micropore volume directly from experimental isotherms and they had to be estimated by fitting the Langmuir model to low pressure data (p/psat < 3 ACS Paragon Plus Environment


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0.2). Sudden drop of the isosteric heat of adsorption above loading of 0.4–0.6 mmol/g reported for different hexane isomers seems to confirm this explanation.16,17,18 McCullen et al.19 revealed almost identical heats of adsorption of n-hexane in AlPO4-5 (68.8 kJ/mol) and its siliceous analog SSZ-24 (68.4 kJ/mol). Heats of adsorption of benzene were also similar – 101.3 kJ/mol for AlPO4-5 and 108.8 kJ/mol for SSZ-24. The aim of this work is to study the adsorption of a series n-alkanes (C1-C10) in the channels of AlPO4-5 with the use of Monte-Carlo molecular simulations, combined with the experimental quasi-equilibrated temperature programmed desorption and adsorption (QETPDA). We attempted to extend our understanding of the adsorption and diffusion mechanisms of similar molecules differing in size within the 1D system of cylindrical pores of the AFI framework. QE-TPDA is a novel experimental technique developed to investigate the adsorption in porous materials allowing to determine high quality adsorption isobars of volatile hydrocarbons in microporous materials.20,21 Recently we successfully combined the QE-TPDA technique with Monte Carlo molecular simulations for pure silica ZSM-5 (MFI) and ZSM-11 (MEL) zeolites with 3-dimensional pore systems.21,22 In this work we provide the first report concerning application of the QE-TPDA in characterization of the adsorption properties of AlPO materials. Monte Carlo (MC) molecular simulations is a tool often used for modelling the equilibrated adsorbent-adsorbate systems.23 In order to reproduce experimental data proper force fields were developed and validated for numerous materials.24,25,26,27 Calculations allow determining the average number of adsorbate molecules in microporous framework, their distribution in host structure, and different energies (such as heat of adsorption) at given conditions (T, p). MC can serve as supplementary method helpful in interpreting and understanding experimental results on the molecular level or predicting behavior of the adsorbent-adsorbate systems at conditions unobtainable experimentally. Adsorption and diffusion of some alkanes in AlPO4-5 was already investigated with the use of molecular simulations. Bhide and Yashonath11,28,29 reported variability of the guesthost interactions energy across the AlPO4-5 channels due to periodicity of the wide and narrow regions. They did not find such an effect for smooth-walled carbon nanotubes. A study by Maris et al.30 showed complex adsorption behavior of short-chain alkanes in AlPO45: at the saturation conditions methane, ethane, and propane molecules are “sticking” to 4 ACS Paragon Plus Environment

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the pore walls and prefer wide regions of the channels, while pentane molecules are too long to occupy specific adsorption sites. Fox et al.31 arranged saturation sorption capacities in series: cyclohexane > 2-methylpentane > n-hexane showing better compatibility of the cyclic and branched molecules with AFI framework than of the linear molecules. Lucena et al.32,33 investigating differences between the values obtained in simulations with the use of united atom (UA) and full atom (AA) force fields, showed that despite its larger computational cost the latter approach does not give any real advantage over the former one.

Experimental The AlPO4-5 material used in this work was synthesized according to the method reported by Newalkar et al.14 Pseudoboehmite (7.36 g, UOP VersalTM) was mixed with diluted orthophosphoric acid (20.14 g H2O, 14.14 g H3PO4, 85%, POCh). Then tripropylamine (13.20 g, 98%, Fluka) and water (19.88 g) were added to the reaction gel. The gel was transferred to the teflon-lined autoclave and heated at 150 °C for 40 h with rotation of 30 rpm. Product was recovered by Büchner filtration, rinsed several times with distilled water and left in air to dry. Dried material was calcined in air (ramp 1 °C/min then 6 h at 540 °C). To confirm the structure of the material the PXRD analysis was performed. The diffraction pattern was recorded with Rigaku Miniflex powder diffractometer (Cu Kα radiation at 10 mA and 10 kV, 2θ step scans of 0.02°, counting time of 1 s per step). Nitrogen adsorption isotherms were determined by the standard method at 77 K (liquid nitrogen temperature) using an ASAP 2025 (Micromeritics) static volumetric apparatus. Before adsorption, the sample was outgassed at 300 °C using a turbomolecular pump to remove adsorbed water. Morphology of the calcined material was confirmed with scanning electron microscopy (SEM) technique, using Tescan Vega3 LMU instrument with a LaB6 emitter (voltage of 20 kV). The samples were coated with gold before imaging to reduce charging of the crystals. The QE-TPDA measurements were performed using a home-made setup equipped with a thermal conductivity detector (Micro Volume TCD, Valco), which was described in detail in earlier works.34,35 This technique requires high-purity adsorbates and a carrier gas. Analytical pure pentane (POCh), hexane (Acros Organics), and heptane (Chempur), 99% octane (Sigma5 ACS Paragon Plus Environment


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Aldrich), nonane (Sigma-Aldrich), and decane (Sigma-Aldrich) as well as helium (purity 5.0, Air Products) have been used in the measurements. Prior each measurement a sample of 7-10 mg was activated by heating up to 500 °C in pure helium flow (6.5 cm3∙min-1). After cooling the sample to room temperature, the flow of carrier gas was switched to the mixture of helium containing a small admixture (0.6-0.8 mol%) of adequate hydrocarbon and preadsorption began. After stabilization of the detector signal, signifying the end of preadsorption, cyclic desorption-adsorption measurements were performed according to linear temperature programs (heating/cooling ramp of 5 and 10

°C/min, up to 400 °C). More details concerning the QE-TPDA methodology and formalism of data reduction can be found in the Supporting Information (Figure S1 and Eqs S1-S6).

Molecular simulations To describe guest molecules we used the TraPPE united atom force field of Martin and Siepmann.36 This model represents alkyl groups as single interaction centers reducing the number of degrees of freedom of adsorbing molecules and significantly shortening the calculation time. The full atom model was tested for adsorption of pentane and hexane in AFI by Lucena et al.33 but did not bring evident improvement of the values. The guest-host interactions have been described by Lennard-Jones (L-J) potentials between CH2, CH3, or CH4 pseudoatoms and oxygen atoms of AlPO4-5 framework with the cutoff distance set to 12 Å. Values of the L-J parameters used in this work were given by Dubbeldam et al.24 The structure of AlPO4-5 used in this work was reported by Qiu et al.37 and considered as a rigid framework. To apply periodic boundary conditions23 the size of the simulation box usually should be larger than twice of the cutoff distance of the L-J potential (24 Å) in every direction. We found that the values obtained for the saturation of long-chain molecules are sensitive to this choice. Figure 2 shows adsorption isobars of n-alkanes in AlPO4-5 calculated with the use of different sizes of the simulation box. Differences in saturation loadings observed for particular adsorbates are due to the fact that for dense adsorption states the average amount of the molecules adsorbed in a simulation box is a natural number, which means that the resolution of the saturation loading (expressed in molecules per unit cell) is equal to the inverse number of the unit cells in the box. We assumed that the system with 2 × 2 × 12 unit cells (ca 100 Å in z direction) is long enough to mimic infinity of the crystal 6 ACS Paragon Plus Environment

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lattice and also provides a good resolution. Saturation loadings were calculated with the use of this simulation box. However, computing whole adsorption isobars with this model would be very expensive in computational time. Therefore, adsorption isobars presented in the Results and discussion section (Figure 8) were selected for the systems best reflecting saturation loading obtained for a simulation box of 2 × 2 × 12 unit cells. These results should prompt one to be careful while choosing the dimensions of the adsorbent for these systems.

Figure 2. Comparison between experimental (black lines) and calculated adsorption isobars of nalkanes in AlPO4-5. Red diamonds, orange circles, green squares, and blue triangles correspond to simulations performed for simulation boxes of 2 × 2 × 3, 2 × 2 × 5, 2 x 2 x 7, and 2 x 2 x 9 unit cells, respectively. Vertical dotted lines correspond to temperature of vapour-liquid equilibrium and horizontal ones for saturation loading for a simulation box of 2 × 2 × 12 unit cells, respectively. The values of pressure were equal to 7.0, 8.0, 4.0, 3.0, 5.0, and 2.0 mbar for C5–C10, respectively. Periodic boundary conditions were applied for the adsorbates.

To compute adsorption isotherms and isobars grand-canonical Monte Carlo (GCMC) simulations were performed with the use of the above-descripted models implemented in the RASPA simulation code.38,39 In the grand-canonical ensemble (μVT) chemical potential, volume, and temperature are fixed. Chemical potential includes fugacity, which is related to pressure with fugacity coefficient assumed to be 1. Each point on adsorption isotherm/isobar, corresponding to a single molecular simulation with different values of pressure/temperature, was obtained by running cycles of random trial moves: 7 ACS Paragon Plus Environment


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insertion/deletion (with equal probability), translation, and rotation of guest molecules. The number of Monte Carlo cycles depended on the particular system but was not lower than 105 after equilibration. The average occupation profiles were obtained from 103 configurations of adsorption for optimal size of the simulation box. They were computed taking into account every pseudoatoms of the n-alkanes. For clarification, obtained graphics were cut to match a box of 2 × 2 × 3 unit cells. The radial distribution functions between terminal CH3 (or CH4 for methane) groups and framework oxygen atoms were obtained from GCMC calculations performed for saturation with the use of a simulation box of 2 × 2 × 12 unit cells. Heats of adsorption for each alkane were computed with the Widom particle-insertion method in the NTV ensemble.40 The above mentioned packing effect does not affect here since just one molecule is entering the host structure. Hence, we used relatively small simulation boxes – 2 × 2 × 3 unit cells for C1–C5 and 2 × 2 × 5 for C6–C10. The calculations were performed for 105 cycles at conditions corresponding to low loadings on the adsorption isobars – 393, 433, 453, 473, 533, and 553 K for C5-C10, respectively – or 10 Pa for C1-C4 on the adsorption isotherms. Diffusion was calculated by molecular dynamics (MD) simulations of a single molecule of hydrocarbon in a simulation box of 2 × 2 × 3 unit cells. The initial configurations for MD simulation were generated by randomly placing a single molecule inside the structure using MC. The MD simulation was performed in the NVT ensemble, where temperature was set to 473 K and fixed using Nose–Hoover thermostat.41,42 The production run consists of 109 cycles with a time step interval of 2 fs (2 µs total) for obtaining statistics of the properties. Selfdiffusion coefficients DS were obtained from the slope of the mean square displacements (MSDs) in the diffusive regime using the Einstein relationship:

〈∑‖ − 0‖ 〉 → 6

= lim

(1)

The term in pointed brackets refers to an ensemble average of the MSD of a chain, where r(0) is the reference position of a tagged atom and r(t) is its position at time t. For each adsorbent-adsorbate system five separate simulations were carried out to obtain the average self-diffusion coefficients. 8 ACS Paragon Plus Environment

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Results and discussion

Figure 3. Comparison of the experimental and modelled XRD pattern of the studied AlPO4-5 37 aluminophosphate based on the structure from Qiu et al.

Figure 3 shows the XRD pattern recorded for calcined material and calculated from Qiu et al.37 data. All peaks present in measured diffractogram have counterparts in modelled one and no additional phases can be found. This confirms the purity of the obtained material.



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Figure 4. Adsorption isotherm of nitrogen in AlPO4-5 at 77 K. Pressure axis is presented in normal (a) and logaritmic (b) scale. Open symbols stand for adsorption and closed symbols for desorption.

Figure 4 shows the adsorption isotherm of nitrogen in AlPO4-5. The low-pressure region of the isotherm is characteristic for microporous materials. Micropore volume and external surface area obtained from t-plot method are equal to 0.101 cm3/g and 64 m2/g, respectively. Small hysteresis loop at relative pressure higher than 0.4 and non-zero slope of the isotherm indicate the presence of interparticle mesopores and well developed external area. Closing the hysteresis loop at p/p0 equal to ca. 0.4 may result from cavitation effect.43 Presented adsorption isotherms are very similar to ones reported by Shutilov et al.44


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Figure 5. SEM images of the AlPO4-5 aluminophosphate under study.

The SEM images presented in Figure 5 show that AlPO4-5 crystalizes in hexagonal rods due to its honeycomb-like framework geometry (Figure 1b). Sizes of single crystals are substantial – over 20 μm in length – but usually they form even larger aggregates. The SEM values are consistent with the images reported by Gelsthorpe et al.45

Figure 6. Adsorption isotherms of methane, pentane and hexane in AlPO4-5. Experimental data of Marin et al.,13 Newalkar et al.,14,16 Stach et al.,17 Choudharry et al.,12 and single points taken from isobars presented in this work are included for comparison. Vertical lines represent pressure values corresponding to vapor-liquid equilibrium obtained from Antoine equation.46

To validate the force field used in this work we performed simulations aimed at reproducing available literature data. In Figure 6 previous experimental values are compared with our calculations with good agreement for methane and only partial for hexane. For the former alkane the molecular simulations reproduce the experimental data of Newalkar et al.16 at low pressure (

Ordering of n-Alkanes Adsorbed in the Micropores of AlPO4-5: A

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