Diffusion of Trimethylbenzenes and Xylenes in Zeolites with 12- and

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Diffusion of Trimethylbenzenes and Xylenes in Zeolites with 12 and 10Ring Channels as Catalyst for Toluene-trimethylbenzene Transalkylation Jordi Toda, Avelino Corma, and German Sastre J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03806 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 18, 2016

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The Journal of Physical Chemistry

Diffusion of Trimethylbenzenes and Xylenes in Zeolites with 12 and 10-Ring Channels as Catalyst for TolueneTrimethylbenzene Transalkylation

Jordi Toda, Avelino Corma, German Sastre*

Instituto de Tecnología Química U.P.V-C.S.I.C. Universitat Politecnica de Valencia-Consejo Superior de Investigaciones Cientificas, Avenida de los Naranjos s/n, 46022 Valencia, Spain. Email: [email protected] Tel.: +34 963879445

Abstract A molecular dynamics study of the diffusion of trimethylbenzene (TMB) and xylene molecules involved in toluene and TMB transalkylation reaction has been performed over 6 different puresilica zeolites, containing 10 and 12-ring channels: BOG, MSE, IWR, SFS, SOF and UWY. The shape selective properties of these six frameworks have been tested using two different loadings: one loading characteristic of the early stage and another of the late stage of the reaction. The collected data explains the diffusion behaviour of these molecules in the zeolite frameworks and allows to obtain trends and also rationalise their performance as candidates for the selective production of p-xylene during transalkylation of toluene and TMB. UWY appears a promising zeolite that allows the reaction of the TMBs in the 12-ring channels, is able to host the transition states, and favours the preferential diffusion of p-xylene in the 10-ring channels. 1 ACS Paragon Plus Environment


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1. Introduction In 1990, the EPA (U.S. Environmental Protection Agency) promoted a drastic reduction of the emissions of hazardous air pollutants from major sources, via the Clean Air Act (CAA)1. One type of those molecules involve aromatic molecules, such as trimethylbenzenes (TMB), obtained from the fractional distillation of the crude oil that belong to the called C9 fraction. As a direct consequence of the limitation in the use of TMBs in the gasoline the market price of these substances decreased dramatically. The oil companies, in the need to find a new use for the TMBs, focused their research in processes that could recycle these molecules into new ones with better conditions for the market. The heavy fraction of gasoline coming from the reformate are rich in aromatic compounds and they are the main source for the C7-C9 aromatic derivatives used in petrochemistry. Among those aromatics, the C8 fraction and, especially para- and ortho-xylenes are used to produce phtalic and terephtalic acid. To maximize xylene production, it is possible to react toluene and trimethylbenzenes present in the heavy gasoline fraction through a transalkylation reaction that yields xylenes as well as through disproportionation of toluene to give benzene and xylenes (Figure 1).

Figure 1. Reaction scheme of the transalkylation between toluene and TMBs to produce xylenes.

2

In the former transalkylation reaction, it is possible to transform low-price TMBs into the more desired products xylenes, being the p-xylene isomer the most valuable among them. The reaction is complex owing to parallel and secondary reactions that can take place apart from the main transalkylation reaction. Toluene disproportionation or TMB dealkylation enhance the p-xylene production, but isomerization of both TMBs and xylenes can also occur. In the case of TMBs, the isomerization to 1,2,4-TMB and further transalkylation should be maximised since 1,2,4-TMB can produce twice more p-xylene than the other isomers (1,2,3- and 1,3,5-) (Figure 2). Zeolites, crystalline microporous alumino-silicates, have been employed as catalysts for this reactions as we will see below. In most of the cases the active site is due to a Brønsted acid site formed by a bridging hydroxyl in the form Si(OH)Al.

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Figure 2. Distribution of xylenes obtained from transalkylation of toluene with trimethylbenzenes.

The methyl transfer process is the core of the transalkylation reaction, that may occur through two different mechanisms (Figure 3). In one of them, the mechanism takes place through the formation of a methyl carbocation (CH3+)3,4,5 between the two aromatic rings. Initially, a TMB molecule (A) is protonated in one of its aromatic carbons by the proton of the acid site of the zeolite (B). Then, the protonated TMB reacts with a toluene molecule (C) giving the dealkylation of the methyl group and forming the transition state of the reaction (D). The methyl carbocation attacks electrophilically into the -para, ortho or -meta position in the toluene ring (F) and, finally, the proton is returned to the zeolite, obtaining two xylene molecules (E). The other mechanism starts in a different way. An hydride abstraction over a methyl group of a TMB molecule (A) takes place, forming a benzilic carbocation (B) and H2. This hydrogen is generated when the hydride reacts with the proton of the zeolite. The next step is the electrophilic attack of the benzilic carbocation (-CH2+) to one of the para-, ortho- or meta- positions of toluene (C). As a result, a bi-phenyl methane6,7,8 intermediate is formed (D) with a methylene bridge (-CH2) between the two aromatic rings. Then, there is a proton transfer to the zeolite, generating the corresponding neutral by-phenyl 3 ACS Paragon Plus Environment


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intermediate (E), and then a similar reaction to the previous one (D←→E) gives another by-phenyl cationic intermediate (F) with the opposite ring protonated. Then, the scission of the C-CH2 bond yields the first xylene molecule (G) and a benzilic carbocation (H). This, from a hydride abstraction near a zeolite proton gives another xylene (I) and the charged TMB (B).

Figure 3. The two possible transalkylation mechanisms. Via methyl carbocation (top) and via byphenil methane (bottom). Adapted from Dumitru et. al.3 and Hong et. al.9

As we mentioned before, zeolites have been one of the key catalyst for the selective obtention of pxylene10. Industrial processes like Xylene-Plus11,12 (developed by ARCO-IFP), Tatoray13,14 (UOP), and TransPlus15 (Mobil-CPC) are used to obtain p-xylene by xylene isomerization and toluene disproportionation using zeolites. Zeolite ZSM-516 has been the most used17,18, due to its favourable structure of 10-ring channel systems, that introduces diffusion shape selectivity for separating pxylene from ortho- and meta- isomers. However, this zeolite is not appropriate for the transalkylation reaction of toluene with trimethylbenzenes. The larger size of reactant molecules (TMB), and the transition states, in the case of the bimolecular mechanism, requires a different framework architecture. In this way, the use of large-pore zeolites with 12-ring channels like Mordenite19, Beta20, Faujasite21 and Omega22, have been tested for transalkylation of toluene with 4 ACS Paragon Plus Environment

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trimethylbenzenes. Dumitru et. al.7 showed the benefits of employing Ultra Stable Y zeolite (USY) for the transalkylation reaction of toluene with trimethylbenzenes, being the degree of dealumination of the zeolite a key factor for a high xylene yield. Lee et. al.23 focused their research in H-Beta, HMordenite and H-Omega zeolites concluding that H-Mordenite was a good candidate for the transalkylation reaction of toluene with 1,2,4-TMB due to its high catalytic activity and stability after dealumination. Al-Khattaff et. al.24 employed USY as catalyst using the same reactants, and obtained a higher p-/o- ratio with lower amounts of 1,2,3- and 1,3,5-TMB. Recently, Krejci et. al.25 evaluated the catalytic activity of zeolites Beta, Mordenite and Y for the transalkylation reaction, concluding that Beta was the best choice, obtaining higher xylene yields when decreasing the Si/Al ratio. Li et. al.26 studied the effect of the pore size in Mordenite, Beta and USY frameworks in the transalkylation reaction of toluene and 1,2,4-TMB. They concluded that the large-pore zeolites are better than the medium-pore ones for the purpose of hosting that reaction due to the rapid formation and decomposition of the large intermediate species and the good diffusivities of the TMB molecules. Nevertheless, despite the above zeolites having large channels wide enough to allow the diffusion of the large TMB molecules, they do not have other channels for the shape selective diffusion of pxylene. So, in order to improve the para-selectivity, it could be of interest to explore the possibilities of zeolites with a dual pore system, with different (and intersecting) channel sizes in the same structure. Zeolites with both 12 and 10-ring channels will be the main target in this study in order to allow the diffusion of the large reactant molecules (TMB) and, at the same time, improving the selectivity for the resulting p-xylene molecules. Moreover, the structure should have large enough space to host the formation of the transition states between TMB and toluene molecules and, in the best scenario, be selective for the formation of those transition states (Figure 4) that lead to enhance the formation of p-xylene with respect to the other xylene isomers. In our computational study we will employ Molecular Dynamics (MD) in order to learn the diffusion properties of the reactants and products of the transalkylation between TMBs and toluene in zeolites containing medium and large pores. The results of MD have been shown to be in reasonable agreement with the self-diffusivity as measured from Pulse Field Gradient Nuclear Magnetic Resonance (PFG-NMR) technique27. Song et al.28 employed MD and Frequency Response to study and clarify discrepancies regarding the diffusion of benzene in silicalite, and although they were able to explain experimental results on the diffusivity of benzene at different loadings, the diffusion coefficients differed in three orders of magnitude. A long-standing discrepancy on diffusion coefficients obtained with different techniques seems to have come to an end recently with the advent of imaging techniques. When other methods are used, it is recommended that the self-diffusion coefficients obtained from MD should be compared to experimental results of self-diffusivity obtained with PFG-NMR. Fernandez et al.29 employed MD and PFG NMR to determine the self-diffusivities of n-butane and iso-butane in MFI and FAU zeolites. In both cases MD showed good qualitative agreement with the experimental data. In that study, for MFI zeolite, MD was able to explain the decrease of the diffusion coefficient of n-butane with the increasing loading. In a similar way, Ramanan and coworkers30, used molecular dynamics to investigate self-transport and cooperative transport of benzene in NaX, comparing their results with PFG NMR experiments. In their work, they 5 ACS Paragon Plus Environment


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concluded that despite the agreement showed between theoretical and experimental results, the membrane fluxes calculated with the MD diffusivities overestimate experiments by about one order of magnitude when support resistance effects are accounted for in the transport model, and by about two orders of magnitude when such resistances are neglected. The authors suggest that this discrepancy may arise from the polycrystalline nature of present-day NaX membranes. Chmelik and Kärger31 used Infra-red Microscopy (IRM) to study the adsorption and diffusion of methanol and ethanol in zeolites, showing the compatibility between transport diffusivities and self- diffusivities. A recent work in the group of Theodorou32 on diffusion of benzene in silicalite examines in depth the long standing discrepancy within computational results and shows the importance of obtaining a correct behavior of the benzene rotation in the channel intersections. This allows to correctly simulate motions (and their corresponding free energy) related to channel crossing, from the straight to the sinusoidal and viceversa, which strongly influence the diffusivity along directions y and x. Moreover, it is indispensable to have a detailed knowledge of structural information of the adsorbed molecules inside the pores of the zeolites at the molecular level. In this way, many studies have proved the efficiency of MD to predict the diffusivity of aromatic molecules in zeolites. In our group, we employed MD simulations to see the diffusion behaviour of para- and ortho-xylene as well as 1,2,4-TMB in NU-87, SSZ-33, β, and ZSM-5 zeolites in xylene isomerization33. Our approach will be to screen the database of known zeolites using specific descriptors aimed to select structures that should present optimum capabilities to maximise p-xylene yield in the transalkylation of TMBs with toluene. With the resulting selection of structures we will employ molecular dynamics in order to unveil the particularities of the diffusivity of reactants and products and will try to identify the requirements needed to enhance p-xylene production. This approach is similar to that used by Smit and Maesen34 trying to find optimum zeolites for hydrodewaxing processes. 2. Methodology 2.1. Zeolite screening. With the molecular sizes (Figure 4) and the channel diameters reported by IZA35,36, we screened the IZA database in order to find structures matching the following conditions, that apply sequentially, each one applies to the outcome of the previous: 1) Presence of 12 and 10-ring channels, 2) Allow the diffusion of TMBs in the 12-ring channel, 3) Allow the diffusion of p-xylene, preferentially to other xylenes, in the 10-ring channels. These preliminary conditions are only a first guess of the diffusional behaviour which will later be calculated more precisely. 16 structures are currently listed in the IZA: BOG, CON, DFO, ITG, ITN, IWR, IWW, MSE, PUN, SEW, SFS, SFV, SOF, UOV, USI and UWY that match condition (1). Regarding condition (2), 14 structures were selected by comparing the diameter of the 12-ring channel listed in the IZA with the size of the TMB described in Figure 4. The frameworks with a 12-ring window diameter larger than the height of 1,2,4-TMB were selected. The candidates are: BOG, CON, DFO, ITG, IWR, IWW, MSE, PUN, SEW, SFS, SFV, SOF, UOV and UWY. Hence, topologies ITN and USI, present in the previous selection, did not pass this condition. Applying condition (3) to the previous 14 topologies, only 7 structures pass the criterium. This condition has been applied by comparing the diameter of the 10-ring channel listed in the IZA with the molecular dimensions of the p-xylene described in Figure 4. The frameworks whose size of the 10-ring channel is smaller than m-xylene (5.8 Å) and larger than p-xylene (5.1 Å) were selected. 6 ACS Paragon Plus Environment

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The resulting structures were: BOG, ITN, IWR, MSE, SFS, SFV, SOF and UWY. Then, we refined our results with structures that match conditions 1-3 simultaneously, and the seven following structures: BOG, IWR, MSE, SFS, SFV, SOF and UWY were selected. From these candidates, SFV has been discarded due to its large unit cell (2352 atoms), whose corresponding supercell in a calculation would demand a too high computational cost. One unit cell can not be used for an adequate sampling because the behaviour of all cells would be coupled. Therefore, in our simulations we tend to use several unit cells not only to achieve a large number of atoms, but mainly to avoid a strong coupling of the results of diffusion. Therefore, the minimum unit cell that should be used is 2×2×2 and, for SFV, this would be 2352×8=18816 atoms. The other cells employed in the study are clearly smaller than this value (Table S1). Hence, the selected zeolites were BOG (Boggsite)37, IWR (ITQ-24)38, MSE (MCM-68)39, SFS (SSZ-56)40, SOF (SU-15)41 and UWY (IM-20)42. ITQ-24 has been used to increase the yield of aromatic hydrocarbons, during the pyrolysis of waste tyres43 and to study the reaction mechanism of cumene formation via benzene alkylation with propylene.44 MCM-68 zeolite has been widely used for many applications such as enhancing the formation of propylene in hexane cracking45, alkylation of naphthalene46 with propene, 1-butene, and 2methylpropene to give 2,6-dialkylnapthalene, and also has been employed in the alkylation of phenol47 with tert-butyl alcohol to give ortho-tert-butyl phenol (2-TBP), para-tert-butyl phenol (4TBP) and 2,4-di-tert-butyl phenol (2,4-DTBP), and in isopropylation of bi-phenyl to give shape selective isomers of diisopropylbiphenyl48. The shape-selectivity properties of the SSZ-56 (SFS) zeolite have been tested for hydroisomerization of n-hexane49. Boggsite (BOG), SU-15 (SOF) and IM-20 (UWY) have not been used -to the best of our knowledge- for any of these purposes yet.



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Figure 4. Approximate dimensions (Å) of organic molecules (1,2,3-, 1,2,4- and 1,3,5-TMB; o-, m- and p-xylene; toluene and benzene) and transition states involved in the transalkylation of toluene with trimethylbenzenes. The geometries of the transition states have been optimised using MOPAC50,51.


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2.2. Channel description. Table 1. Window size (Å) and channel descriptions of pure-silica zeolites used in this work. Zeolite BOG

IWR

10-ring window 5.5×5.8 [010] (1)

Channel characteristics

12-ring window

Channel characteristics

Straight

7.0×7.0 [100]

Straight

5.8×6.8 [001]

Intersected ┴ by cavities [100].

Both channels make a hexagonal shape

6.4×6.8 [001]

Intersected ┴ by 10-ring channels in [110] and [1-10].

Straight

5.8×8.4 [010]

Straight

4.4×9.7 [001]

Straight

6.1×7.6 [001]

Intersected ┴ by 10-ring channels in [010] and [100], giving a large space.

4.6×5.3 [110]

Sinusoidal ┴ [001]

4.6×5.3 [010]

Straight Both intersected by cavities [100]

5.2×5.8 [100] (2) MSE 5.2×5.2 [110] SFS

5.1×5.5 [001]

SOF

4.3×5.5 [100] 4.1×4.5 [101] (3) (9-R)

Sinusoidal

UWY

5.0×5.7 [010] ┴ 12-R 5.0×5.0 [010] ┴ 10-R 5.4×5.8 [100] ┴ 12-R 5.1×5.8 [001] ┴ 10-R

Straight Straight Straight Straight

(1)

This ring is not perpendicular to the channel, and therefore the channel size is smaller (Figure 5) This ring is not perpendicular to the channel, and therefore the channel size is smaller (Figure 8) (3) This ring is not reported in IZA database. Its size has been obtained from the CIF file of the IZA web. ┴ = Intersected perpendicularly (2)

BOG (Figure S1) contains 12-ring straight channels with circular windows of 7.0×7.0 Å along [100], and 10-ring channels with windows of 5.5×5.8 Å across [010]. IWR (Figure S2) is formed by 12-ring straight channels with elliptical windows of 5.8×6.8 Å in [001], perpendicular to a cavity in [100], and the 10-ring channel has a window size of 4.6×5.3 Å through [010] and they act as short segments connecting the cavities. MSE (Figure S3, S4) presents a straight 12-ring channel along [001] direction with window size of 6.4×6.8 Å intersected perpendicularly by 10-ring channels with windows of 5.2×5.8 Å in [110] and [1-10]. The two 10-ring channels across [100] and [110] are connected through a hexagonal channel containing intersections (Figure 7). SFS has straight 12-ring channels with elliptical windows of 5.8×8.4 Å along [010], intersected in the edges by straight 10-ring channels with windows of 5.1×5.5 Å in [001] (Figure S5). SOF framework (Figure S6) contains straight 12-ring channels with elliptical windows of 9.7×4.4 Å in [001], while the 10-ring channels have a sinusoidal shape in [101] with windows of 4.3×5.5 Å in [100]. A 9-ring window of 4.1×4.5 Å, parallel to the 10-ring window, limits the dimensions of the 10-ring channel. UWY zeolite (Figure S7) shows a very special channel system. It has a 12-ring channel in [001] with a window of 6.1×7.6 Å intersected perpendicularly by 10-ring channels in [010] (5.0×5.7 Å) and in [100] (5.4×5.8 Å). Another 10-ring channel is parallel to [001] with window of 5.1×5.8 Å, 9 ACS Paragon Plus Environment


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intersected perpendicularly by 10-ring channels in [100] and [010]. Another 10-ring channel goes through [010] with windows of 5.0×5.0 Å and is intersected perpendicularly by 10-ring channels in [100] and in [001]. Moreover, the intersection between the 12-ring channel in [001] and the two 10ring channels in [100] and [010] generates a very large space. 2.3. Molecular dynamics. Atomistic MD calculations have been performed to simulate the diffusion of all TMB isomers (1,2,3-, 1,2,4-, 1,3,5-) and xylene isomers (o-xylene, m-xylene and p-xylene) in purely siliceous BOG, IWR, MSE, SFS, SOF, and UWY. All the molecular dynamics simulations reported in this work have been carried out using the DL_POLY 2.20 code52,53 including full flexibility and periodic boundary conditions for all the atoms of the system. The temperature chosen is 573 K because this is the usual temperature for the industrial process of xylene isomerization54. We selected the NVT ensemble using the Verlet-leapfrog integration algorithm and the Hoover thermostat, with a timestep of 1×10-3 ps, and each run comprised an equilibration stage of 5×104 steps and a production stage of 4×106 steps (4 ns). For the simulated unit cells, two sets of loadings have been selected. One loading called “Reactants and Products”, characteristic of the early stage of the transalkylation reaction, and another loading, called “Products”, characteristic of a later stage of the transalkylation reaction. The first loading comprises 15 molecules of TMB, distributed as 5 molecules of 1,2,3-TMB, 5 molecules of 1,2,4-TMB, 5 molecules of 1,3,5-TMB and 6 molecules of xylenes, distributed as 2 molecules of p-xylene, 2 molecules of o-xylene and 2 molecules of mxylene. The loading, called “Products” contains 6 molecules of xylenes, distributed as 2 molecules of p-xylene, 2 molecules of o-xylene and 2 molecules of m-xylene. The idea here is that xylene molecules will diffuse without any possible blockage due to TMB molecules. The loadings per unit cell are further explained in Table S2. Four additional loading sets have been also considered for BOG and UWY with the purpose of testing the effect of the concentration in the diffusion pathways. In the case of BOG, the first loading contains 31 molecules of TMB (10 1,2,3-TMB, 11 1,2,4-TMB, and 10 1,3,5-TMB) and 12 molecules of xylenes (4 p-xylene, 3 o-xylene and 5 m-xylene). The second loading contains the 12 xylene molecules of the former case. For UWY, a loading of 21 xylene molecules (7 p-xylene, 7 oxylene and 7 m-xylene) has been chosen. A second additinal loading has been considered with 21 pxylene molecules. The characteristics of the units cells employed are defined in Tables S1 and S2. None of these loadings can be considered fully representative of the real system, due to the poor statistics that can be expected from such small number of diffusing molecules per unit cell: 21 molecules (15 TMB + 6 xylenes) in the case of the first loading and 6 molecules (6 xylenes) for the second one. In the case of the extra loadings of BOG and UWY, 43 molecules (31 TMB + 12 xylenes) and 12 molecules (12 xylenes) are considered for BOG, and 21 molecules (21 xylenes and 21 p-xylenes) have been considered for the UWY. All the details are included in Table S2. As a consequence, the diffusion coefficients obtained in this work will be far from quantitative and they should be seen only as qualitative approximations. In particular, the calculations will show some examples of molecules unable to diffuse, showing what we call extensive local motion (ELM), for which the numerical values of the diffusion coefficients are irrelevant, but whose small value should be taken as a clear indication that the given molecule will not diffuse. Our aim is that the qualitative assessment for the diffusion coefficients, together with the analysis of 10 ACS Paragon Plus Environment

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trajectories (Supporting Information Section 3, Figures S8-S31) will allow us to obtain an interpretation of the diffusional features of each molecule as well as rationalise the results in terms of the topology of the zeolite frameworks. Regarding the latter, a comparison of the window (10ring and 12-ring) diameters from molecular dynamics and from reported values has been performed (Supporting Information Section 5, Figures S33–S38) in order to assess the structural reliability of the forcefield employed. 2.4. Force field. For the description of the different zeolite frameworks, we employed the core-shell force field developed by Sanders, Leslie and Catlow55 that we used in our previous work56. This force field is a pseudo-non-bonding force field which employs formal ionic charges, Buckingham terms for the SiO and O-O interactions, and an added bonding term for the O-Si-O triads described by a harmonic term with 109.47º as equilibrium angle. The oxygen ions are split into core and shell linked by a harmonic spring and respective charges of +0.8482 and -2.8482. For TMB and xylene molecules we employed the force field by Oie et al.57. The force field by Kiselev et. al.58 has been used to describe the intermolecular interactions hydrocarbon-hydrocarbon and zeolite-hydrocarbon. This core-shell dynamics has been possible due to the relaxed shell model of Lindan et. al.59 In this model, the core and shell carry different electric charges, the sum of which equals the charge on the original atom. There is no electrostatic interaction (i.e. self interaction) between the core and shell of the same atom. Moreover, all the atomic mass is concentrated in the core, and a zero mass is assigned to the shell. This implies that the shell must be minimized to a zero force condition before starting the integration of the atomic motion, which is not needed in the rigid-ion algorithm. As a consequence, core-shell MD tends to be at least 10 times more demanding in computing time. A benchmark is included in the supplementary information (Supporting Information Section 6). Nevertheless, this core-shell force field has been demonstrated to be the most accurate to reproduce structural 56

properties of zeolites60 and has been successfully tested in a recent MD study . Despite the cost of each timestep using the Lindan algorithm, it is faster than other core-shell implementations which suffer from the caveats of a much larger integration timestep. 2.5 Pore size variation We have analysed the flexibility of the zeolite framework during the diffusion process, through the width and height of the 12 and 10-ring windows along all the simulation. This allows us to check the accuracy of the force field, by comparing the resulting values with the values reported from Xray diffraction. All the information and analysis related with the pore size distribution can be found in the supplementary information (Supporting Information Section 5).



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3. Results and Discussion 3.1 Diffusivity of TMB and xylenes in BOG, IWR, MSE, SFS, SOF and UWY. BOG. For the diffusion coefficients (Table 2) obtained in the simulation regarding the first loading (reactants and products), two large values were obtained for p-xylene and 1,2,4-TMB. The reason why 1,2,4-TMB is diffusing faster than o-xylene and m-xylene is that the latter are blocked by other TMBs. Removing the blockage leads to an enhanced diffusion of o-xylene and m-xylene, and this can be better appreciated in the second loading (“Products” in Table 2). Table 2. Diffusion coefficients (10-8 cm2·s-1) of TMB and xylenes in BOG zeolite at 573 K with two different loadings.

Hydrocarbon

Loadings Reactants and Productsa Productsb

135-TMB

33*

--

124-TMB

1135

--

123-TMB

169

--

p-xylene

2083

1159

o-xylene

48*

1149

m-xylene

127

976

a

5 1,2,3-TMB + 5 1,2,4-TMB + 5 1,3,5-TMB + 2 p-xylene + 2 o-xylene + 2 m-xylene 2 p-xylene + 2 o-xylene + 2 m-xylene * Small average displacements resulting in extensive local motion (ELM) and low diffusivity.

b

In the second loading (Products) the three xylenes have similar diffusion coefficients as it corresponds to the large 12-ring channel where no xylene selectivity appears, hence all molecules (xylenes and TMB) tend to use the 12-ring channel. The decrease of the diffusion coefficient of pxylene in the two loadings (from 2083 to 1159) is due to the larger mobility of o-xylene and mxylene in the second loading, leading to shorter hydrocarbon-hydrocarbon distances and more repulsions. In turn, the larger diffusivity of o-xylene and m-xylene in the second loading is due to the absence of TMB molecules whose location in the first loading was blocking all xylenes. Another issue is the lack of diffusion of p-xylene in the 10-ring channel whose dimensions are 5.5×5.8 Å. In our recent work56 we obtained a diffusion coefficient of 1160×10-8 cm2·s-1 (at 573 K using the same force field) for p-xylene in pure silica ZSM-5, with channels of 5.5×5.1 Å, and we have estimated that this is the critical size of a channel for p-xylene diffusion. In this case, with a 10-ring window of 5.5×5.8 Å we do not observe diffusion of p-xylene in the 10-ring channel, and the reason is the relative orientation of the 10-ring window with respect to the channel (Figure 5). In Boggsite the 10-ring window (5.5×5.8 Å) is not perpendicular to the channel direction and, as a consequence, the effective channel diameter is 5.5×5.0 Å, which limits the passage of the xylene molecules. The above values correspond to the IZA values, but the same conclusions are obtained using the molecular diameters obtained in the present simulation (Figure S33), which are very similar. This limitation is absolute for o-xylene and m-xylene, but it is possible for p-xylene to enter and diffuse into this channel, although little diffusion of p-xylene is observed in this 10-ring channel. More details are given in the Supporting Information (Section 4). 12 ACS Paragon Plus Environment

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Figure 5. BOG zeolite projections of 10-ring window in [010] (top) and in [100] (bottom) including lengths of the 10-ring window (D10w = 5.8 Å) and the 10-ring channel diameter (D10c = 5.0 Å).

IWR. The diffusion coefficients of the first loading (Table 3) show low diffusivity for all TMB molecules, with a slightly larger value for 1,2,4-TMB, and very similar behaviour is found for the xylenes. The low diffusivity is explained by the presence of cavities perpendicular to the 12-ring channels (Figure 6). The 1,3,5-TMB isomer is trapped in the cavity (Figure S15) being unable to diffuse through the 12-ring channel in [001], and the same happens to the 1,2,3-TMB, with less steric restrictions but also trapped in the cavities, therefore showing little diffusion in the 12-ring channel. Meanwhile 1,2,4-TMB behaves similarly to p-xylene, diffusing along the 12-ring channel without trapping in the cavities. However, the window dimensions (Table 1 and Figure S34) indicates that all TMB molecules (Figure 4) should diffuse significantly. The reason for the low diffusivity is the presence of large cavities (Figure 6) where molecules can be strongly adsorbed. Table 3. Diffusion coefficients (10-8 cm2·s-1) of TMB and xylenes in IWR zeolite at 573 K with two different loadings.

Hydrocarbon

Loadings Reactants and Productsa

Productsb

135-TMB

4*

--

124-TMB

134

--

123-TMB

25*

--

p-xylene

132

1375

o-xylene

45*

358

m-xylene

21*

696

a


b



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Figure S15 (bottom) shows that only 1,2,4-TMB diffuses through the 12-ring whilst the other isomers (top and middle) remain trapped in this cavity. The blockage of the 12-ring channels by TMB affects the xylenes, which can only diffuse moderately (Figure S16). In a simulation without the TMB molecules (second loading called 'Products'), an increase of the diffusion coefficients of the xylenes is noticed (Table 3) and the same enhancement is observed in the trajectories with an enhanced mobility along the 10-ring channel (Figure S17). As can also be seen in Figure 6, the 10ring channel is sinusoidal along [010] and well connected to the large cavities which trap the xylene molecules. This precludes diffusion along the 10-ring channel. In fact there is not really a 10-ring channel but rather the 10-ring windows connecting cavities to each other (Figure 6, top).

Figure 6. IWR along [001] (top) and [100] (bottom). The yellow ellipse shows the cavity, the green circles highlight the connections between cavities and the blue winding line represents the 10-ring channel (top). The 12-ring cavity section is highlighted (bottom).

MSE. The data obtained from the simulations over the MSE zeolite (Table 4) show low diffusion coefficients in the first loading, being the 1,2,4-TMB the one with the largest value, which is interesting for the selectivity required. The trajectories of the other TMBs (Figure S18) show no 14 ACS Paragon Plus Environment

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diffusion but rather a molecular mobility limited to the intersection of the channels, in what we call ELM (extensive local motion)61, where only some little diffusion is observed through the 12-ring channel along [001]. Molecules of 1,2,4-TMB show certain diffusion along the 12-ring channel, indicating some selectivity for the 1,2,4-TMB isomer. In spite of the larger reported (Table 1) size of the 12-rings in MSE (6.4×6.8 Å) than in IWR (5.8×6.8 Å), the diffusivity of 1,2,4-TMB in MSE and IWR shows similar values. This can be rationalized from the calculated values of the pore sizes of the 12-ring channels for IWR (6.3×6.8 Å, Figure S34) and MSE (6.3×6.5 Å, Figure S35), which are similar. Xylene molecules show diffusion restraints in the first loading (Table 4), but larger values are found for the second loading. The analysis of trajectories for both loadings (Figures S19 and S20) shows no diffusion in the 10-ring channel, due to its small size (Figure S35), 4.9×5.2 Å, and architecture (Figure 7). This 10-ring channel has a hexagonal distorted shape where diffusion requires to follow a very tortuous pathway with acute corners which reduce the diffusivity drastically. Moreover, the TMB molecules block the 12-ring channels for the xylene molecules, resulting in the low diffusion values reported in Table 4. In the second loading, where there are no TMB blocking in the 12-ring channels, the xylene molecules (initially located in the 10-rings) tend to migrate to the larger channel, where diffusion is faster. This fact is reflected in the trajectories (Figure S20) where all xylene molecules employ the 12-ring channel exclusively. Table 4. Diffusion coefficients (10-8 cm2·s-1) of TMB and xylenes in MSE zeolite at 573 K with two different loadings.

Hydrocarbon


Productsb

135-TMB

3*

--

124-TMB

194

--

123-TMB

10*

--

p-xylene

7*

479

o-xylene

8*

243

m-xylene

2*

188

a b

5 1,2,3-TMB + 5 1,2,4-TMB + 5 1,3,5-TMB + 2 p-xylene + 2 o-xylene + 2 m-xylene 2 p-xylene + 2 o-xylene + 2 m-xylene



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Figure 7. Hexagonal pathway of 10-ring channels of MSE, view across [001].

SFS. The diffusion coefficients in SFS zeolite (Table 5) show that, for the first loading, both TMB and xylene molecules have similarly low diffusion values and, in the particular case of m-xylene no diffusion is observed, with the trajectories showing that molecules are trapped. The trajectories (Figures S21 and S22) also show that all TMB molecules diffuse through the 12-ring channels in [010], as it corresponds to the pore size diameters of the 12-ring windows (8.45×5.95 Å) (Figure S36). Regarding xylenes, there is no diffusion along the 10-ring channel in [001]. The reason is, as well as in BOG, the different orientation between the 10-ring window respect to the channel (Figure 8). In this case, it makes a difference of 0.8 Å between the window size and the channel diameter. So, the effective diameter of the 10-ring channel in SFS is ca. 4.7 Å, which is not large enough to allow the diffusion of xylene molecules, not even p-xylene. Table 5. Diffusion coefficients (10-8 cm2·s-1) of TMB and xylenes in SFS zeolite at 573 K with two different loadings. Hydrocarbon


Productsb

135-TMB

46*

--

124-TMB

87*

--

123-TMB

78*

--

p-xylene

39*

205

o-xylene

14*

202

m-xylene

1*

456

a


b


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Figure 8. Projection of the 10-ring channel of SFS in [001] direction (top) and in [010] direction (bottom) including lengths of the 10-ring window (D10w) and the 10-ring channel diameter (D10c).

The simulation without TMB molecules (second loading in Table 5) shows an increase in the diffusion coefficients of the xylene molecules, which can be explained for the absence of the TMB molecules in the 12-ring channels, allowing the xylene molecules to diffuse freely. Especially interesting is the change in m-xylene, whose diffusion coefficient value is more than twice in comparison with the values of the other two isomers. The trajectories (Figure S23) show that both para and ortho xylenes spend some time in the intersections with the 10-ring channels, but not for the meta-xylene, which diffuse through the 12-ring channel without trapping in the intersections. SOF. In this zeolite, large diffusion coefficients are obtained for all TMB isomers, which diffuse along the 12-ring channels (Table 6 and Figure S24). The dimensions of the 12-ring window (9.7×4.4 Å) allow the diffusion of the TMB molecules, and the IZA dimensions are in close agreement with the calculated values (Figure S37) 9.38×4.50 Å. As can be seen in the trajectories (Figure S24), these dimensions do not restrict the diffusion of the TMB molecules. Xylene molecules show very low diffusion coefficients (first loading) and the trajectories (Figure S25) indicate that they are not using the 12-ring channel. This is due to the initial configuration, with xylenes initially located in the 10-ring channels, where they remain during all simulation time. 17 ACS Paragon Plus Environment


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Table 6. Diffusion coefficients (10-8 cm2·s-1) of TMB and xylenes in SOF at 573 K. Hydrocarbon


Productsb

135-TMB

401

--

124-TMB

498

--

123-TMB

360

--

p-xylene

26*

17681

o-xylene

6*

12666

m-xylene

10*

8659

a


b

Xylenes show very low diffusivity due to their location in the narrow 10-ring channels. Moreover, there are 9-ring windows (currently not described in the IZA Atlas) parallel to the 10-ring channel (Figure 9). This channel is indeed not a 10-ring channel but rather the channel is made of alternating 10-ring and 9-ring, with the diffusing bottleneck being the smaller channel (9-ring), whose dimensions are 4.2×4.5 Å. The second loading (Products) shows very large diffusion coefficients for all xylene molecules. This is due to the diffusion of xylene molecules in the 12-ring channel (Figure S26), whilst in the previous loading, xylene molecules were located in the 10/9-ring channel where they remained blocked (Figure S25). The large diffusivity is due to this 12-ring channel being the largest among all other structures discussed in this work.

Figure 9. Projections of 10-rings and 9-rings along [001] in SOF zeolite.


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UWY. The case of UWY zeolite is particularly interesting because it contains four different straight 10-ring channels, although one of them, along [010], is too small (5.0×5.0 Å) for diffusion of xylenes. The diffusion coefficients (Table 7) indicate small values for all TMB isomers, although the value for 1,2,4-TMB is slightly larger. The trajectories (Figure S27) show that all TMB isomers are diffusing in the 12-ring channel along [001]. Table 7. Diffusion coefficients (10-8 cm2·s-1) of TMB and xylenes in UWY zeolite at 573 K. Hydrocarbon


Productsb

135-TMB

60*

--

124-TMB

90*

--

123-TMB

51*

--

p-xylene

45*

275

o-xylene

105

158

m-xylene

110

269

a


b

Regarding the diffusion of xylenes in the first loading (“Reactants and Products”), it can be seen (Figure S28) that p-xylene diffuses both in the 10-ring channel along [100] (5.4×5.8 Å) as well as in the 12-ring channel along [001]. The diffusion of p-xylene in the 10-ring channel contributes to a decrease in the diffusion coefficient with respect to o-xylene and m-xylene which diffuse almost exclusively in the 12-ring channel. When TMB molecules are absent (second loading, “Products”), there is an increase of the diffusion coefficients of all xylene molecules. This is due to the larger availability of the 12-ring channels which are free from TMB molecules. It can be clearly appreciated (Figure S29) that p-xylene is equally using the 10-ring [100] and the 12-ring channels [001], whilst the other isomers are mostly using the 12-ring channels. The large size of the 10-ring channels along [100], with dimensions 5.8×5.4 Å (IZA) or 5.9×5.5 Å (our MD calculations, Figure S38) suggests that some diffusion of mxylene and o-xylene can not be excluded, but, importantly, p-xylene can diffuse with notably less constraints than the other isomers. In order to further clarify the diffusional features of xylenes in UWY, we performed another two simulations, whose full details are included as Supporting Information (Section 4), with a larger loading (21 molecules) of xylene molecules. The first simulation contained 7 molecules of each xylene isomer, and the second simulation contained 21 p-xylene molecules, with the aim of finding whether all the 10-ring channels are accessible. The results of the first simulation (Figure S30) show that ortho-xylene and meta-xylene are using mainly the 12-ring channels, whilst para-xylene is using all 10-ring channels except the smallest (5.0×5.0 Å), and using preferentially the 10-ring channel along [100]. The second simulation, with 21 p-xylene confirmed the preferential diffusion of p-xylene in the 10-ring channel along [100] while being the rest of the 10-ring channels available (Figure S31). 19 ACS Paragon Plus Environment


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The values of the diffusion coefficients reflect the use of the large (12-ring) and medium-pore (10ring) channels by decreasing values as the smaller (10-ring) channels become more populated. Only p-xylene shows significant diffusion along the 10-ring channels, and decreasing values of the diffusion coefficient (even below o-xylene and/or m-xylene) are observed as increasing use is made of the smaller (10-ring) channels. A trade-off in the diffusion coefficients of p-xylene appears with larger values than o-xylene and m-xylene if only the large (12-ring) channels are used, but with values proportionally decreasing to the extent of use of the smaller (10-rings) channels. Importantly, when the larger (12-ring) channels are fully loaded, p-xylene is the only molecule that shows the possibility of significant diffusion through the 10-ring channels. This is a unique feature which suggests that p-xylene could be preferentially obtained in UWY from transalkylation of TMBs with toluene. It must be noted that from the 10-ring channels in UWY (Table 1), p-xylene can diffuse through the channels of dimensions: 5.0×5.7 Å [010], 5.4×5.8 Å [100], and 5.1×5.8 Å [001], hence the only one too small for p-xylene is that whose dimensions are 5.0×5.0 Å [010]. 3.2 Accuracy of the force field. As noted in the literature, the accurate calculation of diffusion coefficients using molecular dynamics requires an accurate modeling of the zeolite window dimensions through which the sorbates diffuse62 63, and this critically depends on the quality of the force field used. A detailed analysis of the windows (10-ring and 12-ring) of all zeolites employed in this study has been included as Supporting Information (Section 5), where in most of the cases an excellent accuracy within ±0.2 Å was achieved. This accuracy has been calculated using as experimental 35,36

value(s) either the structures of maximum symmetry reported in the IZA-Atlas , or those reported by the authors of the synthesis when the CIF file is available (IZA-CIF). In only in a few cases, the discrepancies between calculated and experimental diameters become larger than 0.2 Å. For IWR, only one of the 12-ring dimensions (6.31 Å) shows values ~0.5 Å different from the IZA, while all the remaining values are within 0.2 Å. In the case of the 12-ring of MSE, the discrepancies are below 0.4 Å. The calculated value of the smaller 12-ring diameter is 6.31 Å, whilst that of the IZA-Atlas is 6.40 Å and the IZA-CIF is 6.65 Å, indicating discrepancies of 0.09 Å and 0.34 Å respectively. The calculated value of the larger 12-ring diameter of IWR is 6.54 Å, we have a difference of 0.26 Å with IZA-Atlas (6.80 Å) and 0.15 Å with IZA-CIF (6.69 Å). Regarding the 12ring in UWY zeolite, our second MD value (7.70 Å) shows a large difference (0.47 Å ) with the IZA-CIF (8.17 Å), but it is in excellent agreement (0.1 Å) with the IZA-Atlas value (7.60 Å). 4. Conclusions Molecular dynamics has been used to study the diffusional behaviour of aromatic molecules relevant to the industrial process of transalkylation of trimethylbenzenes (1,2,3-, 1,2,4- and 1,3,5-) with toluene to give xylenes, and in particular with the aim of finding a zeolite which can be used as catalyst in this process giving the maximum amount of p-xylene. The models have been selected with a considerable number of simplifications such as pure silica zeolites and only consider low loading of aromatics, so that the zeolite-hydrocarbon interactions are dominant over the hydrocarbon-hydrocarbon and hence the role of the zeolite topology is maximised. With the idea in mind that zeolites containing medium and large pores may be adequate for this 20 ACS Paragon Plus Environment

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process, a screening of the zeolites in the IZA database using specific descriptors suggested the six following zeolites as candidates: BOG, MSE, IWR, SFS, SOF and UWY. The condition of maximising the productivity of p-xylene means, on the one hand, to focus on zeolites which favour the diffusivity of 1,3,5-TMB among all other TMBs because this is the only TMB which gives two (instead of one) molecules of p-xylene upon transalkylation with toluene. On the other hand, zeolites whose 10-ring channel allows the preferential diffusion of p-xylene over the other xylenes will also result in an increased production of p-xylene. A number of factors make difficult to meet the above conditions and some candidate zeolites have to be discarded as follows. BOG and SFS contain 10-ring channels with the proper size to maximise p-xylene, but the 10-ring window is not perpendicular to the 10-ring channel direction, and hence the effective size of the 10ring channels is considerably smaller than the 10-ring windows. Although IWR also contains 10-ring windows of the proper size to maximise p-xylene, these 10rings do not make channels, but rather they are only connections between large cavities along the 12-ring channels. These cavities contribute to trap the bulkier TMB molecules (1,2,3-TMB and 1,3,5-TMB, whilst the smaller 1,2,4-TMB is not trapped), whose low diffusivity is also a negative effect that discards IWR from the list of candidates. MSE contains an hexagonal system of 10-rings channels which makes very difficult for xylene molecules, even p-xylene, to diffuse following this tortuous system. The larger length of p-xylene is an additional constraint to sneak following the corners of the hexagons. In the case of SOF framework, the 10-ring channel is made of alternating 10 and 9-ring windows which preclude the selective diffusion of p-xylene. The presence of 9-rings, currently not described in the IZA Atlas, makes the smaller channel in SOF as effectively being 9ring instead of 10-ring. Finally, UWY zeolite appears as a promising structure for selective production of p-xylene. This zeolite has four straight 10-ring channels with three of them having appropriate size and shape for the selective diffusion of p-xylene, without diffusional constraints such as tortuosity or trapping cavities. All the 10-ring windows are perpendicular to the channel direction, resulting in straight channels adequate for fast diffusion. P-xylene is shown to diffuse through the 10-ring channels unlike the other xylenes, resulting in the aimed selectivity. Further, all xylene isomers as well as the TMBs diffuse in the 12-ring channels and we suggest that the transition states can be formed in the channel intersections. The force field employed in the simulations has shown good accuracy in reproducing the channel dimensions compared to that reported in the literature, with most of the cases within only ±0.2 Å. Our screening has been able to suggest UWY zeolite for the selective production of p-xylene from TMBs and toluene. The presence of cavities that trap molecules precluding diffusion, and tortuosity in the channels, and also the presence of 10-ring channels smaller than 5.0×5.5 Å, are some of the possible inconvenients that have been unveiled from our screening process. For this reason, the synthesis of new zeolites containing interconnected 12-ring and 10-ring channels continues to be an important topic that can contribute to improve existing industrial processes.



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Acknowledgements We thank Ministerio de Economia y Competitividad (MINECO) of Spain for funding through project CTQ2015-70126-R, and the excellence programme Severo Ochoa (SEV-2012-0267). J.T. thanks MINECO of Spain for the SVP-2013-067662 PhD scholarship. We thank ASIC-UPV and Paco Rosich for computational facilities and computational support. Supporting Information The Supporting Information includes figures showing the channels of the zeolites, data on cell parameters and loadings, trajectories of all the molecular-dynamics runs, a more detailed analysis of the effect of the loading in BOG and UWY zeolites, data on mean-square displacements, full analysis of all 10-ring and 12-ring diameters in all the zeolites, computational benchmark, and a movie showing an extract of the molecular dynamics of p-xylene in UWY. Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.xxxxx


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Table of Contents Image

UWY zeolite with TMB and xylene molecules diffusing in their channel system.



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Diffusion of Trimethylbenzenes and Xylenes in Zeolites with 12- and

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