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C: Physical Processes in Nanomaterials and Nanostructures

Diffusion of Trimethylbenzenes, Toluene and Xylenes in UWY Zeolite as Catalyst for the Transalkylation of Trimethylbenzenes with Toluene Jordi Toda, and German Sastre J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10407 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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The Journal of Physical Chemistry

Diffusion of Trimethylbenzenes, Toluene and Xylenes in UWY Zeolite as Catalyst for the Transalkylation of Trimethylbenzenes with Toluene

Jordi Toda and 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 protected]

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Abstract A molecular dynamics study has been carried out on the diffusion of trimethylbenzene (TMB), toluene and xylene molecules in the transalkylation of TMB and toluene in UWY zeolite, containing crossing 10-ring and 12-ring channels. Two models of UWY have been employed, the pure silica and an acidic form of UWY including Al and Brønsted sites, using a recently parameterised general force field for zeotypes which reproduce pore diameters within ca. ± 0.2 Å. Molecular traffic has been found from the result of TMBs using almost exclusively the 12-ring channels and the preferential location of p-xylene and toluene in the 10-ring channels at high loading (reaction conditions). From the 3 different 10-ring channels, only p-xylene and toluene can fit in the two smallest, whilst also m-xylene and 1,2,4-TMB can diffuse in the largest 10-ring. An in-depth analysis of transition-state shape selectivity has been performed, showing that all transition states of the transalkylation of toluene and TMBs can be formed in the 12-ring channels. Although this is a disadvantage for the selective production of p-xylene, the previous factor of molecular traffic will contribute to a selectivity of p-xylene over the other xylene isomers. Overall, UWY is suggested as a promising catalyst for the transalkylation of toluene and TMBs.


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1. Introduction The production of xylenes through disproportionation of toluene as well as transalkylation of heavy reformates (containing >80% C9 aromatics), with or without toluene or benzene, allow refineries to increase the value of the heavy reformate 1 after the CAA (Clean Air Act) banned its use as additive to gasoline due to environmental regulations. The main applications of xylenes are in the production of synthetic fibers, plasticizers as well as solvents and gasoline blending. O-xylene is employed for the production of anhidrid phtalic, which in turn is used to produce PVC fibers and fiberglass. Mxylene is employed to produce isophtalic acid, wich plays a key role in the synthesis of many polyester compounds used in underwater pipes due to its resistance to sea corrosion, and also used as coating in cars, household appliances, and office furnitures. P-xylene is employed for the massive production of terephtalic acid, which is used for the synthesis of poliethylene terephtalate (PET), and also the production of dye-stuff, drugs and pesticides. Commercial aromatic transalkylation processes employ xylenes and/or toluene as feeds for the selective production of p-xylene, such as Xylene-Plus 2, Trans- Plus 3 and Tatoray 4, with p-xylene amounting ca. 80 % of the global demand for xylenes, with a total of ca. 40 million tons per year. Whilst for the latter processes, medium pore zeolites (ZSM-5 in particular) are mostly employed due to their selectivity to the diffusion of p-xylene with respect to the other xylene isomers, for the transalkylation of toluene and trimethylbenzene (TMB), zeolites containing large pores, such as NaX 5, BEA and MOR 6

7 8

seem more suitable based on the fact that TMBs are too large to diffuse

in medium pore zeolites and that the bimolecular transition states involved in the reaction are favored in large crossing pores. A thorough study by Serra et al. explored the transalkylation of TMB with toluene using a large number of different zeolite topologies, containing medium pores (MFI, IMF), large pores (BEA, FAU), small and large pores (MOR) and medium and large pores (CON, NES) 9. That study found that the lowest yield of xylenes was obtained with medium and large pore zeolites, whilst the highest xylene yield was obtained with zeolites containing a combination of 10-ring and 12-ring (CON, NES) as well as a combination of 12-ring and 8-ring (MOR). In the latter case of mordenite (MOR) it is clear that the presence of the 8-ring channel in itself is not important since aromatic molecules can not diffuse through it, however the presence of two pockets (8-ring size) at opposite sites of the 12-ring channel provides the extra-space needed for the formation of the transition states. Secondly, mordenite does not contain large cavities where coke formation results in catalyst deactivation. In summary, zeolites with large (but not too large) intersections, and in particular with crossing channels of different sizes seem good candidates to catalyze the transalkylation of TMB with toluene.



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In particular, the choice of a zeolite with crossing medium (10-ring) and large (12-ring) pores seems appropriate. The medium pores contribute to maximize the yield to xylenes while the large pores contribute to the reaction of TMB. At the time of the study by Serra et al.9 there were few zeolites containing medium and large pores that could be used in catalysis, and the authors opted by suggesting a mixed catalytic system combining both functionalities in two different catalysts, with topologies BEA and MFI. Since then, new zeolites containing 10-ring and 12-ring channels have been reported up to the number of 16: BOG, CON, DFO, ITG, ITN, IWR, IWW, MSE, OKO, PUN, SEW, SFS, SFV, UOV, USI, and UWY, which we have explored in a previous study 10, where SOF has been removed from the previous list by being reclassified by IZA has having 9-ring channels instead of 10-ring channels, and where OKO had not been considered previously. The interest of some of these zeolites has been reviewed recently 11, including the relevant structural features that play a significant role in reactions of aromatic transformations, and in particular the production of xylenes from TMB + toluene, which we consider in this study. In our previous study10, after screening all the zeolites currently synthesized, we analyzed six candidate zeolites (BOG, IWR, MSE, SFS, SOF, and UWY) and identified UWY as a promising catalyst for the transalkylation of TMB and toluene. In the present study we carry out a more detailed analysis of the diffusion of, not only TMB and xylenes, but also toluene and some transition states of the transalkylation. Two possible mechanisms have been proposed 12, 13,14 for the transalkylation. One of them (monomolecular) goes through an intermediate methoxy species 15,16,17 (from the first aromatic molecule) adsorbed in the zeolite. The methoxonium subsequently attacks the second aromatic molecule forming an aromatic carbenium ion as transition state. The second mechanism (bimolecular) 18,19

20

goes through a diphenyl methane transition state produced by the reaction

between a protonated aromatic and the second aromatic molecule. The latter is the most accepted 21 22,23,24,25

and its bulkier transition state has been employed in our simulations.

The role of loading and the diffusion of mixtures are also analyzed, and the UWY in acid form, as well as pure silica, were selected as models.

2. Models and methods. 2.1 Channel description. UWY contains an interconnected system of four 10-ring and one 12-ring channels, which is the most complex among all zeolites containing 10-ring and 12-ring channels, shown in Figure 1 and Table 1, and also as Supporting Information (Figures S2-S7). The 12-ring channel (6.1×7.9 Å) runs along [001] and is intersected perpendicularly by a 'short' 10-ring channel along [010] (5.1×6.2 Å) and by a 10-ring channel along [100] (5.2×5.9 Å), creating a triple channel 4 ACS Paragon Plus Environment

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intersection (Figure 1, Int-3). Two other 10-ring channels exist, which intersect each other (Int-2) and do not intersect the 12-ring channel, and they run through [001] (4.4×5.8 Å) and [010] (4.5×5.1 Å). Another intersection (Int-1) occurs between the 10-ring channels along [001] and [100]. As noted above, there are two different 10-ring channels running through [010], with dimensions 5.1×6.2 Å and 4.5×5.1 Å. The latter is too narrow to allow diffusion of aromatic molecules and will not be further considered in this work. Hence, all further content regarding the 10-ring along [010] will refer to the channel with dimensions 5.1×6.2 Å.

Figure 1: Unit cell (2x2x2) of UWY showing four 10-ring channels and one 12-ring channel, as well as the resulting intersections. The 10-rings along [010] are perpendicular to the Figure. The 10-ring channel along [010] (5.1×6.2 Å), has been above named 'short' because it contains many intersections with the 12-ring channel along [001]. The channel, therefore, could be described as large intersections (Int-3) connected by 10-ring short segments (Figures S3, S5).



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Table 1: Description of channels and intersections of UWY zeolite. See Figure 1. 4.4×5.8 [001] (10-ring)

5.2×5.9 [100] (10-ring)

5.1×6.2 [010] (10-ring)

4.5×5.1 [010] (10-ring)

6.1×7.9 [001] (12-ring)

4.4×5.8 [001] (10-ring)

×

Int-1

--

Int-2

--

5.2×5.9 [100] (10-ring)

Int-1

×

Int-3

--

Int-3

5.1×6.2 [010] (10-ring)

--

Int-3

×

--

Int-3

4.5×5.1 [010] (10-ring)

Int-2

--

--

×

--

6.1×7.9 [001] (12-ring)

--

Int-3

Int-3

--

×

2.2 Chemical composition of the UWY models. In order to build a realistic model of the UWY zeolite, as catalyst for TMB transalkylation, we have included Brønsted sites (≡SiO(H)Al≡), which are the active sites for the reaction. An Al content corresponding to Si/Al = 7.5 has been selected as this is within the typical range in zeolites in terms of a correct balance between number of sites (increasing with Al content) and acid site strength (decreasing with Al content) 26. The Al distribution has been created by a proprietary software derived from zeoTsites 27 which ensures that the Lowenstein rule (no Al-O-Al linkages present) is fulfilled. It also has been ensured that the Al distribution does not contain clustering of nearby Al atoms with regions without Al, and the Al atoms are distributed across all the crystal regions. A second aspect of the model, apart from the Al distribution is the OH distribution, resulting from the fact that the UWY zeolite contains 24 crystallographically different oxygens which we have labelled according to reference28. This leads to OH pointing to different microporous environments, and in particular accessible from different channels (Table S1), and this has been taken into account into the model. Three important oxygen positions were selected to locate the protons with the condition of being accessible for the reactant molecules: O9, O11, O21, which are located in the 12-ring channel along [001], near or at the intersections with the 10-ring channels along [100] (5.2×5.9 Å) and [010] (5.1×6.2 Å). This is a particularly important region of UWY since transition states will try to form in this location. Since it is not possible to reach the desired Si/Al ratio (7.5) using only these three oxygen positions, other oxygen locations for the Brønsted sites were selected so as to sample other environments where reactions are also likely to occur, such as the two types of intersections between 10-ring channels, as well as positions of all the channels not at intersections. Rather than 6 ACS Paragon Plus Environment

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giving an exhaustive description, we provide a CIF file (Supplementary Information) containing all the data. Finally, in order to evaluate the effect of the Brønsted sites in the diffusivity, an equivalent model with pure silica composition (this is the same model than that in our previous study10 was also employed in the simulations. 2.3 Molecular dynamics. Atomistic molecular dynamics (MD) calculations have been performed to simulate the diffusion of: toluene, xylenes (o-xylene, m-xylene, and p-xylene), TMBs (1,2,3TMB, 1,2,4-TMB, 1,3,5-TMB), transition states of the TMB+toluene transalkylation, as well as some binary component systems. Two different loadings of the above were included in a large model of either acidic UWY (Si/Al=7.5) or pure silica UWY. The unit cell of UWY has dimensions a = 25,16 Å; b = 12,70 Å; c = 11,60 Å 28, and a 3×3×2 supercell was considered, which is large enough so as to contain a representative part of the microporous environment capable to provide reasonably good statistics. The chemical composition of the acid UWY is Si953Al127O2160(OH)127, and Si1080O2160 for the pure silica, which gives an orthorhombic cell of dimensions a = 75.5 Å, b = 38.1 Å, c = 23.2 Å. Keeping the same cell parameters for the acid UWY and the pure silica UWY has been done for a better comparison of the effect of protons in the channel diameters. Secondly, the original parameters correspond to a cell with Si/Ge=2.4, with a similar effect on cell volume than our cell with Si/Al=7.5. The molecular dynamics simulations have been carried out using DL_POLY 2.20 29 including full flexibility and periodic boundary conditions for all the atoms of the system. The temperature chosen is 573 K, which is close to the temperatures chosen for the industrial processes of aromatic transalkylation reactions 30 and xylene isomerization in H-ZSM-5 zeolite 31

32

. Moreover, working at

higher temperatures may favor other reactions, such as disproportionation 33, rather than the main transalkylation reaction between TMB and toluene. We selected the NVT ensemble using the Velocity-Verlet 34 integration algorithm and the Evans thermostat 35, with a time step of 10−3 ps (1 fs). Each run comprised an equilibration stage of 105 steps and a production stage of 107 steps (10 ns). For the diffusion coefficients, the mean square displacements (MSD) were calculated, from which a least-square fitting was performed in order to obtain the self-diffusion coefficient, D, using the Einstein relation 36: 1

2

𝑀𝑀𝑀𝑀𝑀𝑀 = 〈∆𝑟𝑟⃗(𝑡𝑡)2 〉 = 𝑁𝑁 ∑𝑁𝑁 �⃗(𝑡𝑡) − �𝑟𝑟⃗(0)� = 6 ⋅ 𝐷𝐷 𝚤𝚤 𝚤𝚤 𝑖𝑖=1�𝑟𝑟

(1)

Taking into account that the three main channels preferentially used for diffusion are perpendicular to each other 10-ring along [010] (5.1×6.2 Å), 10-ring along [100] (5.2×5.9 Å), and 12-ring along [001] (6.1×7.9 Å); it follows that the analysis of the different contributions (x,y,z) of the diffusion 7 ACS Paragon Plus Environment


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coefficient will allow to guess the channel(s) where the dominant diffusion is happening. Instead of giving the absolute values of the diffusion coefficient along x,y,z we have found more significant to give the corresponding relative values in %, calculated as: 100×D(x)/3D, 100×D(y)/3D and 100×D(z)/3D; where the factor '3' comes from the equation: D(x) + D(y) + D(z) = 3×D.

(2)

In all the results, the diffusion coefficients presented, called D, refer to the self-diffusivity which is the diffusion at equilibrium, when no concentration gradient is present. However, the diffusion coefficients will not give sufficient information on how the diffusion is occurring since the values obtained will not be very accurate. In spite of the relatively long simulation time (10 ns), the complex nature of the channel system of UWY as well as the relatively small number of molecules diffusing makes very difficult to obtain statistically significant accurate values of the diffusion coefficients even if the simulations would be extended an order of magnitude longer. Instead of using the diffusion coefficient as an accurate value, they will be used on a comparative basis with caution. On this basis, it will be possible to obtain valuable information on the preferential channels that each molecule tends to use. In this sense, the analysis of the trajectories will be also of great help. Trajectories along XY, XZ and YZ will be plotted for those runs where specific features need to be illustrated. In all cases the conclusions will be obtained taking into account the analysis of trajectories which give the information on the channels employed for the diffusion. The diffusivity is expected to be highly dependent of the loading 37

38 39

. The most simple expected

behavior is that increasing loadings will lead to larger sorbate-sorbate repulsions and hence decreased diffusivity, but this will also be coupled with the extent to which each channel is occupied at different loadings. Hence given the importance of this aspect, two extreme loadings have been considered, corresponding to a total of either 10 (low) or 21 (high) molecules in the 3×3×2 cell of UWY. In each case, molecules corresponding to a mixture of reactants (toluene and TMBs) or desired products (xylenes) of the transalkylation reaction will be considered, and also the diffusion of pure component molecules will also be simulated. In the industrial process, a 1:1 TMB/toluene molar ratio is chosen in order to favor the transalkylation reaction in the active sites of the catalyst, and to minimise disproportionation and isomerisation33. 2.4 Force field. For the description of the UWY zeolite, we have employed the recently parameterised force field 40 which has been specifically designed for molecular dynamics and alumino-silicate zeolites as well as zeotypic silico-alumino-phosphates (SAPOs). This is a rigid-ion force field40 which is 5 times faster 41 than the shell-model force field employed previously10.


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Although the shell-model force field is structurally more accurate, the present rigid-ion force field simulates better the diffusional features by using a partial charge model for the zeolite (qSi = 2.1) as opposed to the shell-model force field which employs a formal charge scheme. The equilibrium structural properties of zeolites are sufficiently accurate with this rigid-ion force field, and it also has been tested recently in molecular dynamics calculations of the flexibility of ZSM-541. The force field by Kiselev et al. 42

43

has been used to describe the intermolecular

hydrocarbon−hydrocarbon interactions. The zeolite−hydrocarbon interactions are taken from a previous work43 which specifically differentiates aliphatic and aromatic carbons in C-Oz interactions, where Oz is the zeolite oxygen. For the intramolecular hydrocarbon interactions, the force field by Oie et al. 44 has been used due to its easy implementation. The all-atoms force fields for hydrocarbon-hydrocarbon and zeolite-hydrocarbon, as well as the intramolecular hydrocarbon force field have been used successfully by our group in previous work of diffusion in zeolites 45 46. 2.5 Pore size variation. We have analyzed the flexibility of the zeolite framework through the dynamic analysis of all the diameters of the 12- and 10-ring channel windows along all the 10 ns simulation. For this purpose, a new version of the zeoTsites software27 has been employed, which has been recently tested and benchmarked using experimental results of pure silica ZSM-541. From the analysis of all the diameters, it has been found convenient, for the sake of simplicity without sacrificing accuracy, to report only the smallest and the largest diameters. Note that this choice does not generally correspond to the channel dimensions plotted in the Atlas of Zeolite Framework Types 47. In this regard, for the sake of comparison, the smallest and largest diameters obtained from the CIF (crystallographic information file) corresponding to the structures named as 'Type Material' in the Atlas will be used. All pore size diameters obtained from the MD simulations are shown in Figures S15-S20 in the Supporting Information (Section 3). 2.6 Energies of adsorption. The values of the adsorption energies for each aromatic molecule are important in order to explain the results of diffusion. Whether a given molecule is more or less stabilised in each channel allows to justify the relative occupation of channels. Additionally to the energetic factor there will be an entropic factor, so that the resulting distribution of molecules in channels obtained from molecular dynamics will include both. Adsorption energy calculations have been performed using the General Utility Lattice Program (GULP) 48 by doing lattice energy minimisations including an infinite diluted loading of each given molecule. It is important to select an appropriate initial configuration, and to this effect the results of the molecular dynamics will provide a very large sampling in order to select the initial coordinates. 9 ACS Paragon Plus Environment


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3. Results and discussion. 3.1 Diffusivity of xylenes in acid UWY at large loading. 21 xylene molecules where considered in the 3×3×2 cell of acid UWY for the single component runs. The diffusion coefficients (Table 2) show significant differences between the three isomers, being o-xylene the isomer with the lowest diffusion coefficient and m-xylene showing the largest diffusivity. Although p-xylene was expected to show the largest diffusivity, UWY zeolite contains many different channels and the distribution of molecules in each channel will determine the diffusivity values. Table 2: Self-diffusion coefficients, D, (10−8 cm2·s−1) and x,y,z components (in %), of xylene isomers at large loading in acid UWY at 573 K. D (10−8 cm2 s−1)

Dx (%)

Dy (%)

Dz (%)

21 p-xylene

56.5

20

10

70

21 o-xylene

35.8

1

3

96

21 m-xylene

94.6

0

7

93

Molecule

The trajectories show (Figures S8-S10) that all isomers diffuse mainly through the 12-ring channel (6.1×7.9 Å) along [001] and also through the 10-ring channel (5.1×6.2 Å) along [010]. However, pxylene, being smaller, is the only isomer that is also diffusing through the 10-ring channel (5.2×5.9 Å) along [100] (Figures 2, S8). The diffusion in this narrow channel is more difficult, which justifies that p-xylene does not show the largest self-diffusion coefficient among the xylenes. The dimensions of ortho- and meta-xylene (5.8×6.3 and 5.8×7.5 Å, Table S5) are not sufficiently large so as to enter effectively the 10-ring channel along [100] (5.2×5.9 Å). Hence, the similar behaviour of the xylene isomers in the 12-ring channels is not the explanation for the lower diffusivity of p-xylene with respect to the other isomers. The explanation comes from the 20% amount (Table 2) of diffusion of p-xylene in the small 10-ring (5.2×5.9 Å) along [100], through which only p-xylene, and not the other xylene isomers, can diffuse. The strong adsorption of p-xylene in this channel (-0.61 eV, Table 3) results in low diffusivity, which contributes to the small value of the overall diffusivity of p-xylene. The trajectories of p-xylene (Figures 2, S8) indicate molecular rotation, with both effects (strong adsorption and rotation) hindering diffusivity. A similar effect has been recently reported by Varanasi and Yashonath 49. These authors explain how molecules matching the pore size experience a tight fitting which leads to supermobility, or the so called “levitation effect”, which maximizes translational and minimizes rotational motions. 10 ACS Paragon Plus Environment

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Figure 2: Trajectories of a single p-xylene molecule from the high loading (left) and the low loading (right) in the acid version of the UWY. Color code: Green arrow, 12-ring channel in [001]; blue arrow, 10-ring channel in [001]; yellow arrow, 10-ring channel in [100].

Table 3: Adsorption energies (eV) of xylenes, toluene, and TMBs in the channels of UWY and at the triple channel intersection (Int-3, Figure 1). Molecule

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

Adsorption Energy (eV) 10-ring [100] (5.2×5.9 Å)

10-ring [010] (5.1×6.2 Å)

10-ring [001] (4.4×5.8 Å)

12-ring [001] (6.1×7.9 Å)

Int-3 (Fig. 1)

-0.61 -0.04 -0.01 -0.60 -0.08 ---

-0.69 0.16 -0.13 -0.41 -0.58 ---

-0.59 -0.31 -0.36 -0.41 -0.22 ---

-0.42 -0.50 -0.45 -0.22 -0.58 -0.67 -0.48

-0.31 -0.30 -0.34 -0.29 -0.40 -0.42 -0.42

Table 2 also shows that m-xylene diffuses better than o-xylene. A possible interpretation for this comes from two results: the weak adsorption (-0.13 eV) in the 10-ring channel along [010] which enhances its mobility; and the absence of rotation (see Figure S9-right) facilitating that the kinetic energy is fully converted in translation. Meanwhile in Figure S8, rotation of p-xylene can be observed along these 10-ring channels, giving lower diffusivity. 11 ACS Paragon Plus Environment


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This indicates a tight fit of m-xylene in this channel which favours the diffusivity efficiently along the channel (with a moderately low adsorption strength, -0.13 eV, Table 3), which explains the larger diffusivity with respect to o-xylene where this phenomenon does not appear. In fact o-xylene hardly enters this channel due to the repulsive interaction (+0.16 eV in Table 3), and the trajectories show (Figure S10) that the diffusivity goes almost exclusively through the 12-ring channel along [001]. The large occupancy of molecules in the 12-ring channel means a larger number of xylenexylene interactions, hindering diffusivity, which then becomes smaller than that of m-xylene. So, being the TMB more strongly adsorbed than the toluene molecules in both 12-ring and intersection, is logic to think that they will tend to occupy the 12-ring channels and this will force some toluene molecules off the 12-rings to find another locations, namely the 10-rings. The results of Table 3 also suggest that TMBs do not diffuse through 10-rings, with the exception of 1,2,4-TMB which may diffuse albeit with constraints, as seen from the trajectories (Figure 3 right). The strong adsorption of 1,2,4-TMB in the 10-ring [010] channel (-0.58 eV) will result in slow diffusivity along this channel (Figure S11). The main diffusion, by far, will be through the 12-ring channels. Hence, these results suggest the existence of molecular traffic, with TMBs using preferentially the 12-ring channels and with p-xylene and toluene being partly displaced towards the 10-ring channels. The Monte Carlo results (section 3.7) are in agreement with this interpretation. 3.2 Diffusivity of xylenes in acid UWY at low loading. The low loading is modelled by 10 xylene molecules in the 3×3×2 cell of acid UWY. The self-diffusion coefficients (Table 4) indicate larger values than those at high loading (Table 2). A comparison with the values at large loading (Table 2) indicates that o-xylene undergoes a larger increase, from 35.8 to 87.0×10−8 cm2 s−1. Taking into account that o-xylene only diffuses through the 12-ring channel (and this agrees with the unfavorable adsorption energies in Table 3), the lower the number of molecules the smaller the number of interactions between them, hence the larger diffusivity. Table 4: Self-diffusion coefficients, D, (10−8 cm2·s−1) and x,y,z components (in %), of xylenes at low loading in acid UWY at 573 K.

Molecules 10 p-xylene 10 o-xylene 10 m-xylene

D (10−8 cm2 s−1)

Dx (%)

Dy (%)

Dz (%)

90.5 87.0 114.5

5 0 3

6 1 10

89 99 87

For p-xylene, the difference (high loading vs. low loading) is also noticeable (from 56.5 to 90.5×10−8 cm2 s−1). The explanation is different than that for o-xylene; in this case the occupation of


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channels at high and low loadings is different. At high loading there were p-xylene molecules diffusing in the 12-ring channel, as well as in the 10-ring channels along [100] and [010]. At low loading, a lower occupation of the 10-ring channels is observed, and therefore the larger occupation of the 12-ring channels, where p-xylene is less adsorbed (see Table 3), contributes to a larger diffusivity (Figures 2, S8). An analysis of the effect of concentration in the diffusion coefficients of the xylene isomers reveals that the normal trend is a decrease in the diffusion coefficient at higher concentration that we attribute to the more frequent intermolecular collisions at larger loading. This is true for p-xylene (90.5 and 56.5 at low and high loading) and for o-xylene (87.0 and 35.8 at low and high loading), but not for m-xylene for which similar diffusion coefficients have been found at low (114.5, Table 4) and high loadings (94.6, Table 2). Together with p-xylene, m-xylene (Figure S9) is the only xylene which can diffuse through the 10-ring channels along [010] (5.1×6.2 Å). However, instead of the resulting decreasing diffusivity of p-xylene along this 10-ring channel due to its strong adsorption (-0.69 eV, Table 3), for m-xylene the adsorption is much weaker (-0.13 eV, Table 3) this resulting in an increase (instead of decrease) of diffusivity. For o-xylene, with a positive value of adsorption (+0.16 eV, Table 3) this indicates repulsion and therefore will not enter this 10-ring channel. There is a subtle balance in the fitting of molecules whose dimension match the channel size. Along this study, in all cases except the present one of m-xylene in 10-ring channels along [010] (5.1×6.2 Å), tight fitting leads to strong adsorption and decrease in diffusivity. Molecules that match the channel size maximise the van der Waals interactions by an adequate intermolecular host-guest distance giving a strong adsorption energy. Smaller channels result in repulsion and larger channels in lower adsorption energy. However, for even tighter fitting of the molecules in the channel, if the adsorption energy approaches zero, the so called “levitation effect” 50

51 52 53 54 55

introduced by Eric

Derouane starts to appear and it contributes to an enhanced diffusion. This effect occurs when the distances between the sorbate molecule and the oxygens of the zeolite channel do result in neither attractive nor repulsive interactions, with sorbate molecule “floating” inside the channel, being able to diffuse with little constraints. The general trend is first to fill the 12-ring channels, where diffusivity is faster, and, at larger loading, some of the 10-ring channels, [100] and [010] for p-xylene, and [010] for m-xylene. 3.3 Diffusivity of toluene and trimethyl-benzenes (TMB). Two loadings (single and binary component) will be considered, 10 molecules in the 3×3×2 cell of acid UWY which will be alone (pure component) or accompanied by (binary component) another 10 molecules of one of the



Page 14 of 30

following three TMBs, 1,2,4-TMB, 1,2,3-TMB or 1,3,5-TMB. Mixture (TMB + toluene). The self-diffusivities of each separate component are shown in Table 5, where a striking feature is that in two cases TMB molecules (1,2,4-TMB and 1,3,5-TMB) diffuse better than toluene (76.3 vs 24.9 and 15.3 vs 105.5, in Table 5). This is entirely similar to the behavior of p-xylene vs. the other xylenes (Table 2), with p-xylene diffusing slower than the other isomers. Hence in both cases the counter-intuitive result that the smaller molecule diffuses slower is obtained. Again the analysis of trajectories provides the interpretation, showing (Figure 3) that TMBs (1,2,4-TMB and 1,3,5-TMB) occupy the 12-ring channel and force-out some molecules of toluene towards 10-ring in [100] and [010] (Figure 4) channel where diffusion is slower. Moreover, it is also observed that 1,2,4-TMB isomers can use (albeit to a small extent) the 10-ring channel along [010] (5.1×6.2 Å) to exchange between 12-ring channels (Figure 3, right). The other TMB isomers only diffuse through the 12-ring channel along [001], as expected.

Table 5: Self-diffusion coefficients, D, (10−8 cm2·s−1) and x,y,z components (in %), of toluene and TMBs (single component) or accompanied by TMB (binary component) in acid UWY at 573 K.

D (10−8 cm2 s−1)

Dx (%)

Dy (%)

Dz (%)

10 toluene + 10 1,2,4-TMB 24.9 (tol) 76.3 (TMB)

6 (tol) 0 (TMB)

27 (tol) 0 (TMB)

67 (tol) 100 (TMB)


6 (tol) 0 (TMB)

6 (tol) 0 (TMB)

88 (tol) 100 (TMB)


12 (tol) 0 (TMB)

38 (tol) 0 (TMB)

50 (tol) 100 (TMB)

0 0 0 5

2 0 0 5

98 100 100 90

Molecules Binary component

Single component 10 1,2,4-TMB 10 1,2,3-TMB 10 1,3,5-TMB 10 toluene

107.6 36.8 208.0 88.4


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Figure 3: Trajectories of the 1,3,5-TMB (left) and 1,2,4-TMB (right) in the acid version of the UWY zeolite in the mixture of toluene + TMB. Color code: Green arrow, 12-ring channel in [001]; purple arrow, 10-ring channel in [010] (5.1×6.2 Å); red arrow, 10-ring channel in [010] (4.5×5.1 Å).

Figure 4: Trajectories of the toluene in the acid version of the UWY zeolite in the mixture of toluene + TMB. Color code: yellow arrow, 10-ring channel in [100]; purple arrow, 10-ring channel in [010] (5.1×6.2 Å); red arrow, 10-ring channel in [010] (4.5×5.1 Å).



Page 16 of 30

Taking Dz as the diffusion through the 12-ring (parallel to [001]), Table 5 shows that the more important component of the toluene diffusivity through the 12-ring, in a way such that the larger is Dz (for toluene), the lower is the diffusion coefficient for the corresponding TMB in the mixture. This is explained by the fact that the larger is the mobility of toluene in the 12-ring, the more difficult becomes the diffusion of the bulkier TMB. For instance, for 1,2,3-TMB + toluene, the Dz for toluene is the largest (88) and the corresponding D for TMB is the smallest (14.5). The diffusional behaviour of toluene in the mixture is more complex but, as a general rule, equally to p-xylene, the larger the number of toluene molecules diffusing through 10-ring channels, the lower will be the diffusion coefficient (Figure S12). Single component (TMB). TMBs diffuse better as single component (Table 5). This has been justified above in terms of the larger mobility of toluene molecules in the 12-ring channel, which act as a larger constraint to the mobility of TMB in the mixture. But, more importantly, at half total loading in the single with respect to the binary component, there will be less intermolecular interactions and hence larger diffusivity. A particular case is 1,2,4-TMB, the smallest TMB isomer (Table S5), and the only one able to diffuse along the relatively large 10-ring channels along [010] (5.1×6.2 Å). In the pure component, some molecules of 1,2,4-TMB spend some time in the 10-ring channel through [010] (Figure S11), thus giving a smaller increase in the diffusion coefficient with respect to the mixture than the other TMB isomers (Table 5). Single component (toluene). Regarding toluene, an increase of the diffusion coefficient as pure component with respect to the mixture (from 24.9, 44.9, 15.3, ×10−8 cm2 s−1 in the mixture to 88.4×10−8 cm2 s−1, as pure component, Table 5) is observed. As well as the case of 1,2,4-TMB above, toluene can employ occasionally the 10-ring channels for diffusion although the 12-ring channels are preferred. A difference with 1,2,4-TMB is that the latter can also use the 10-ring channel along [010], but toluene can also use the smaller 10-ring channel along [100] (5.2×5.9 Å). The partial diffusion of toluene in the 10-ring channels makes the diffusion coefficient to be lower than it would be if the diffusion would proceed only through the 12-ring channel. More molecules of toluene diffuse in the 12-ring channel in the single component than in the mixture, and this is the reason of the larger diffusivity in single component. In the mixture, TMB molecules force-off some toluene molecules towards the 10-ring channels ([010] and [100]), decreasing the diffusivity of toluene. This is molecular traffic. 3.4 Diffusion of the transition states of transalkylation of TMB+toluene. From the suggested geometries of the transition states of the transalkylation of TMB+toluene [10], the largest (1,2,4TMB_toluene_para) and the smallest (1,2,3-TMB_toluene_meta) have been selected, being their 16 ACS Paragon Plus Environment

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dimensions 7.5×12.5 and 6.3×12.1 Å, respectively (Table S5). These sizes indicate that these species can not be formed in any of the 10-ring channels of UWY. However UWY contains large micropores and there is the 12-ring channels as well as the triple intersection between the 12-ring channel in [001] and the 10-ring channels in [010] (5.1×6.2 Å) and [100] (Int-3 in Figure 1), where these transition states can be formed (Figure 5).

Figure 5. Schematic view of UWY showing one of the transition states of TMB-toluene, located at the triple intersection between 12 ring channel in [001], and 10-ring channels in [100] and [010].

The self-diffusion coefficients obtained (Table 6) indicate that although this is slow, there is certain diffusivity. In this context, it is clear that the lifetime of a transition state is shorter than the 10 ns simulation time, but the reason of this calculation is to sample the locations of UWY where these transition states can be formed. In this sense, again, the trajectories (Figure S14) show that the transition states can diffuse (and hence be formed) in the triple channel intersection (Int-3, Figure 1) and through the 12-ring channels along [001]. Table 6: Self-diffusion coefficients, D, (10−8 cm2·s−1) and x,y,z components (in %), of two transition states (TS) of the transalkylation of TMB+toluene in acid UWY at 573 K. The name of the TS comes from the reactants (TMB and toluene) plus the position (meta,ortho,para) where one of the methyl groups of TMB is attacking toluene. The loading is 6 TS in the 3×3×2 cell of acid UWY. Transition States

D (10−8 cm2 s−1)

Dx (%)

Dy (%)

Dz (%)

6 1,2,4-TMB_toluene_para

11.2

0

0

100

6 1,2,3-TMB_toluene_meta

13.5

0

0

100

Taking into the account that the 1,2,4-TMB_toluene_para has the largest dimensions (7.4×12.5 Å) of all 9 possible transition states and the 1,2,3-TMB_toluene_meta has the smallest (6.3×12.1 Å) (Table S5), it means that all the remaining transition states can also be located at any place of the



Page 18 of 30

12-ring channel. This gives an important argument for the viability of the transalkylation reaction between TMB and toluene, since the large configurational feasibility along the 12-ring channel provides a larger number of acid centers available for transalkylation and this means a larger probability for the reactants to collide and react. 3.5 Effect of chemical composition on diffusivity: acid UWY vs. pure silica UWY. Molecular dynamics are usually performed with pure silica composition, and it is clear that the pore size becomes lower in protonic (acid) zeolites since the hydrogen atoms tend to point towards the inside of the n-rings. However the rings are flexible and the protons may reorient to make room to incoming molecules. These two scenarios can be guessed to lead to decreased diffusivity (protons pointing towards inner parts of n-rings) or equal diffusivity (proton reorientation within a flexible nring) of molecules in acid vs. pure silica forms of zeolites. As stated above, the lack of molecular dynamics studies using acid zeolites makes difficult to analyse the literature and suggest one of these two scenarios as the most plausible one, and then the calculations below will give some indication regarding this question for our particular case of study. The results obtained should not be taken as a general answer in zeolites since the multiplicity of topologies with different flexibilities may change the picture from case to case. The results of high loading in acid UWY of section 3.1 have been completed with equivalent calculations using the same loading of 21 xylene molecules in the 3×3×2 cell of pure silica UWY. The comparison of the diffusion coefficients in silica-UWY (Table 7) with respect to acid-UWY (Table 2), indicate little changes with respect to the chemical composition and hence the presence of Brønsted sites is not a constraint for diffusion. For m-xylene, the diffusion still goes mainly through the 12-ring channel along [001] and not even the molecules that diffuse through the more constrained 10-ring channel (along [010]) show significantly different diffusion paths with and without Brønsted sites, as shown by the similar diffusivities (94.6 and 100.9×10−8 cm2 s−1). Table 7: Self-diffusion coefficients, D (10−8 cm2·s−1), and x,y,z components (in %) of xylene isomers at high concentration in acid and pure silica UWY at 573 K.

System 21 p-xylene in acid UWY 21 p-xylene in pure silica UWY 21 o-xylene in acid UWY 21 o-xylene in pure silica UWY 21 m-xylene in acid UWY 21 m-xylene in pure silica UWY

D (10−8 cm2 s−1) Dx (%) 56.5 56.7 35.8 23.4 94.6 100.9

Dy (%)

Dz (%)

20

10

70

26

10

64

1

3

96

0

4

96

0

7

93

2

8

90


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Regarding p-xylene, the diffusion coefficient does not change (ca. 56×10−8 cm2 s−1) and the number of molecules diffusing through the 10-ring channels along [100] and [010] are almost the same regardless the presence of Brønsted sites. The same applies to o-xylene, where the diffusion is even slightly worse in pure silica UWY (23.4×10−8 cm2 s−1) than in UWY with acid sites (35.8×10−8 cm2 s−1), but this small difference is within the error expected during to insufficient sampling. For this particular case we have enlarged the simulations up to 20 ns but the results are still similar. The accuracy of the technique does not allow a numerical accuracy enough so as to interpret small differences. In any case, qualitatively the results show similar diffusivity without and with acid sites present. Hence, acid sites do not seem to pose a particular restraint for diffusivity. Conversely, at low loading, different diffusivities in acid and pure silica UWY are observed for toluene and 1,2,4-TMB (Table 8). For 1,2,4-TMB, the trajectories (Figure S11) show that the diffusion goes mainly through the 12-ring channels, with some diffusion along the 10-ring channel in [010]. From the relative dimensions of the molecule and the channel (Table 9), it can be seen that a small but significant difference in the dimension of the 10-ring channel along [010] appears in the pure silica (5.1×6.2 Å) and the acid UWY (5.2×6.3 Å). Table 8: Self-diffusion coefficients, D, (10−8 cm2·s−1) and x,y,z components (in %), of toluene and 1,2,4TMB at low concentration in acid and pure silica UWY at 573 K.

System

D (10−8 cm2 s−1)

Dx (%)

Dy (%)

Dz (%)

107.6 179.9 88.4 176.5

0

2

98

0

2

98

5

5

90

2

3

95

10 1,2,4-TMB in acid UWY 10 1,2,4-TMB in pure silica UWY 10 toluene in acid UWY 10 toluene in pure silica UWY

Table 9: Dimensions (Å) of the 10-ring and 12-ring channels in UWY (acid and pure silica) obtained from the molecular dynamics calculations, and critical dimensions (Å) of the molecules fitting in each channel. Channel type

Channel dimensions (Å)

Molecules fitting (Å)

acid-UWY

silica-UWY

10-ring [100]

5.2×5.9

5.2×5.9

toluene (5.1), p-xylene (5.1)

10-ring [010]

5.2×6.3

5.1×6.2

toluene (5.1), p-xylene (5.1), m-xylene (5.8), o-xylene (5.8), 1,2,4-TMB (6.2)

10-ring [001]

4.5×5.9

4.4×5.8

toluene (5.1), p-xylene (5.1)

6.1×7.9

toluene (5.1), p-xylene (5.1), m-xylene (5.8), o-xylene (5.8), 1,2,4-TMB (6.2), 1,2,3TMB (6.6), 1,3,5-TMB (6.6), all TS (6.3-12.5)

12-ring [001]

6.2×8.0



Page 20 of 30

Considering that the critical dimension of 1,2,4-TMB is 6.2 Å, it follows that it will be easier for 1,2,4-TMB to fit (diffuse) along the 10-ring [010] in acid UWY than in pure silica UWY, and this is supported by a careful analysis of the trajectories (Figure S11). As the diffusion through the 10-ring channel is slower than in the 12-ring, it follows that the diffusion coefficient is larger in pure silica UWY (179.9×10−8 cm2 s−1) than in acid UWY (107.6×10−8 cm2 s−1). For toluene, the difference in diffusion coefficients is more than twice in pure silica (176.5×10−8 cm2 s−1) than for the acid sites (88.4×10−8 cm2 s−1). The trajectories (Figure S12-S13), show that the 12-ring channel is still the main channel used for diffusion. The reason for such increase in the diffusion coefficient for pure silica UWY is the lower number of molecules in the 10-ring channels along [100] and [010], where toluene is less adsorbed (Table 3). Regarding the diffusion of toluene and p-xylene along the 10-ring channel in [001], despite the favorable dimensions (Table 9) and interaction energies (Table 3), no diffusion has occurred in the simulations. It is expected that at larger loadings these channels may also be filled. 3.6. Accuracy of the Force Field. In order to test if the force field employed [40] corresponds to a realistic model of UWY zeolite, a comparison with the minimum and maximum pore diameters of each 12-ring and 10-ring of the experimentally reported structure is needed. This comparison is shown in Table 10. The values obtained from the MD simulations in acid UWY are very similar to the experimental values, with an average error 56 of 0.12 Å. This fully justifies the choice made of fixing the experimentally reported cell parameters (from a Si/Ge=2.428) on the UWY-acid model with Si/Al=7.5. The average error of the UWY-silica, compared to the experimental structure is reasonably small, although larger (0.23 Å).

Table 10. Pore size diameters (minimum and maximum) of UWY corresponding to 12-ring and 10-ring channels. MD-acid: obtained from molecular dynamics using acid UWY. MD-silica: obtained from molecular dynamics using pure silica UWY. IZA: obtained from the DLS-minimised structure of the Atlas of the International Zeolite Association. Exp.: experimentally reported structure from XRD data. 12-ring [001] 10-ring [001] 10-ring [100] 10-ring [010]

10-ring [010]

MD-acid

6.2×8.0

4.5×5.9

5.2×5.9

5.2×6.3

4.7×5.3

MD-silica

6.1×7.9

4.4×5.8

5.2×5.9

5.1×6.1

4.5×5.1

IZA

6.2×8.4

5.1×6.1

5.0×6.2

5.7×6.5

4.4×5.4

Exp. [28]

6.2×8.0

4.7×6.1

5.3×6.1

5.3×6.4

4.8×5.5


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The diameters of the IZA model show an average error with respect to the experimental structure of 0.22 Å, and using the IZA cell parameters for our model leads to a larger error in the MD calculations than the error obtained with the currently employed cell parameters. This also justifies our choice of cell parameters. Overall, the force field employed for our UWY-acid model can reproduce the pore size diameters of the UWY channels.

3.7. Discussion 3.7.1. Catalytic aspects of UWY in the translakylation reaction. In order to determine the catalytic capacity of UWY zeolite for the TMB+toluene → xylenes reaction, some factors should be taken into account. This include the transition state shape selectivity, the reactivity of the organic species in the different channels of the UWY zeolite, the comparison with other catalyst and, the diffusional features of reactants and products in the 12-ring and 10-rings of UWY, which in case of channels selectivity will be an indication of the existence of molecular traffic. It has been shown that all 9 possible transition states can be formed in the 12-ring channels of UWY, mainly in the intersections with the 10-ring channels. This will not contribute to an enhanced selectivity to p-xylene and so it would have been more favourable to find that only the transition states leading preferentially to p-xylene can be formed. Previous work21, 22, 6, 57 shows, in zeolites such as H-TNU-10, H-ZSM-57, La-Na-Mor and H-ZSM-5, that the formation of some specific transition states enables a more precise control of the product output. On the other hand, in our previous work10 we analysed all zeolites containing 10-ring and 12-ring channels and suggested that UWY would be among the best candidates to be used as catalyst in the reaction TMB+toluene → xylenes. UWY contains two selective 10-ring channels for p-xylene diffusion and, moreover, these channels do not allow p-xylene to isomerize or react with toluene since the transition states do not fit. This will contribute to enhance the selectivity towards p-xylene. Our results indicate that only p-xylene and toluene can diffuse along some of the 10-ring channels in UWY, along [100] (5.2×5.9 Å) and [001] (4.4×5.8 Å). These dimensions imply that the transition states of any subsequent bimolecular reaction between toluene and p-xylene will not fit and hence this will contribute to maximise the selectivity towards p-xylene. This argument relies on the fact that, from the mechanistic viewpoint 58, the transition states (diphenylmethane, DPM) 59

60

are of

similar size than the resulting xylene. This has been shown for other medium-pore zeolites such as MFI58

61 62

, MEL61 and TUN61, whose pore dimensions are similar to some 10-ring channels of

UWY (Table 11). Section 4 of the Supporting Information contains more details.



Page 22 of 30

Table 11: Pore size dimensions of the 10-ring channels of ZSM-5 (MFI), ZSM-11 (MEL) and TNU-9 (TUN) compared to the 10-ring channels along [100] and [010] in UWY.

Zeolite

10-ring diameters (Å)

ZSM-5

5.1×5.5, 5.3×5.6

ZSM-11

5.3×5.4

TNU-9

5.5×6.0, 5.2×6.0, 5.4×5.5

UWY

5.2×5.9, 5.2×6.3

3.7.2. Comparison of UWY with NU-87 (NES). Another important factor to establish the catalytic performance of UWY is the comparison with a catalyst that has shown good results such as the HNU-87 (NES) zeolite reported in the work by Serra et al.9. The structure of NES has a straight 10ring channel with 12-ring cages intersected perpendicularly. The 10-ring dimension (4.8×5.7) is smaller than the main 10-rings in UWY (5.1×6.1; 5.0×6.2; and 5.7×6.5), which means that in NES the diffusivity of even the smallest TMB (1,2,4-TMB) will be strongly restricted. Molecular dynamics simulations on pure silica NU-87 have been performed in order to compare the results with those obtained in UWY. We employed the same number of molecules than in Table 7, with which the results below (Table 12) should be compared.

Table 12. Self-diffusion coefficients (10-8 cm2·s-1) of p-xylene and o-xylene and a mixture of toluene and 1,2,4-TMB at high concentration (with 3672 atoms in the 3×3×1 unit cell of NES and 3240 atoms in the 3×3×2 unit cell if UWY) in pure silica NU-87 (NES) at 573 K. Results for UWY are those of Table 7 which are showed again for the sake of comparison.

D (10−8 cm2 s−1) Loading (molec/SiO2)

NES (SiO2)

UWY (SiO2)

21 p-xylene

151

56

21 o-xylene

7

23

10 toluene + 10 1,2,4-TMB

67 (tol) 23 (TMB)

25 (tol) 76 (TMB)

The results show that 1,2,4-TMB will diffuse slowly in NES, and this is the isomer which gives a larger amount of p-xylene upon reaction with toluene (see Figure 2 of our previous study10). Further, the diffusional selectivity p-xylene/o-xylene is larger in NES than in UWY, and also pxylene diffuses better in NES than in UWY, as demonstrated by the diffusion coefficients (Table 12). In the work by Serra et al.9, it is argued that most of the transalkylation reaction occurs at the 22 ACS Paragon Plus Environment

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external surface of the NES catalyst. We can not base our results on these facts since we do not take into account the external surface, but, more importantly, we do not think that an optimum zeolite should be selected in terms of the behaviour of its external surface, unless the external surface shows a marked shape selectivity, which is unlikely, although possible. In summary, the comparison between NES and UWY shows: the enhanced diffusivity of p-xylene in NES, the high diffusional selectivity of p-xylene/o-xylene in NES with respect to UWY, and the low diffusivity of TMBs in NES with respect to UWY. This shows that NU-87 is a good candidate for this reaction (TMB+toluene → xylenes). However, a downside is the low diffusivity of 1,2,4TMB in NES, while in the case of UWY a larger diffusivity is observed, which is an advantage of UWY with respect to NES. 3.7.3. Molecular traffic in UWY. Molecular traffic can be defined, for a mixture of molecules in a multipore zeolite, as the preferential diffusion of different molecules in different channels 63. From the results of Table 10 it can be seen that the 10-ring channels along [100] and [001] are selective for the diffusion of p-xylene and toluene, while any other molecule of the transalkylation process will not fit. Toluene and p-xylene will preferentially use these channels at high loading since the preferred diffusion channel is (for all molecules, not only toluene and p-xylene) the 12ring along [001]. A Monte Carlo simulation using RASPA 64 (details in Section 7 of Supporting Information) has been performed using a TMB:tol 1:1 mixture that resembles the experimental conditions of the transalkylation reaction at the initial stage. The TMB mixture contains equal fractions of the three isomers. The Monte Carlo simulation gives as a result the equilibrium uptake of each molecule inside the zeolite micropores, although the diffusional aspects have to be also taken into account since exclusion zones have not been defined in the calculation. This is important for large molecules that can be strongly adsorbed in pore intersections but whose size would not allow to diffuse through the channels to reach those interactions which are more spacious locations. The results at 300 K (Table 13) indicate that at low pressure 1,2,4-TMB is the TMB isomer more adsorbed in the zeolite micropores. At low pressure (10-4 bar) 1,2,4-TMB is even more adsorbed than toluene due to its larger size that is expected to result in stronger adsorption. At higher pressure (1 bar) the opposite result is found and toluene is the most adsorbed molecule since it is the only one that fits in the 10-ring channels whilst the TMB isomers can fit only at the 12-ring/10-ring channel intersection as well as all through the 12-ring channels. Although the results at 1 bar indicate that the more adsorbed TMB isomer is 1,3,5-TMB, this result should be discarded since,



Page 24 of 30

from the two adsorption locations found, 10-ring/10-ring and 10-ring/12-ring intersections, with 6 and 10 molecules respectively, the former should be ruled out since this large molecule can not diffuse through the 10-ring channels. Hence, it can be assumed that, also at the larger pressure, 1,2,4-TMB will be the TMB isomer more adsorbed.

Table 13. Equilibrium loading (molecules per 2×3×3 unit cell) of reactants in UWY at 300 K and different pressures (bar) obtained from Monte Carlo simulation.

Molec.\Pressure

10-4

10-3

10-2

10-1

1

1,2,4-TMB

28

19

16

12

10

1,2,3-TMB

10

10

7

7

5

1,3,5-TMB

7

16

12

10

16

toluene

5

19

37

46

43

53

64

72

75

74

Total

This is a particularly important result since a preferential adsorption of 1,2,4-TMB (among the TMB isomers) will lead to a certain enhancement in the selectivity to p-xylene because 1,2,4-TMB is the isomer which gives more amount of p-xylene in the transalkylation with toluene (see Figure 2 of our previous study10). On the other hand, there are three different intersections in UWY (Figure 1), due to the different channels crossings. These intersections act as a traffic junction 65 because they allow the formation of larger molecules that can not be formed in some of the zeolite channels, and also react with another molecules to produce new molecules that can fit inside the channels. Due to the presence of a large amount of 10-ring channels, with many intersections, it is expected that p-xylene will be favoured due to its better diffusion in those 10-rings compared to the other xylene isomers. A study in the group of Lercher 66 shows that in toluene methylation over medium pore zeolites, the bulky TMB molecules become trapped in the zeolite pores, undergoing more favourably dealkylation reactions, giving products that fit favourably in the channels available. A study from Demuth et al. 67 reported this traffic junction effect due to the pore size on the xylene disproportionation in TON zeolite, showing that from the TMB molecules formed in the zeolite pores, the formation of 1,2,4TMB was the most favored, leading through dealkylation to the selective production of p-xylene. In the transalkylation of TMB+toluene to give xylenes, UWY zeolite shows promising features for several reasons; on the one hand due to the possibility of selective transport of either reactant in each channel system, TMB (12-ring channels) and toluene (10-ring channels); and also due to the 24 ACS Paragon Plus Environment

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large number of intersections that will allow to host the corresponding bulky transition states. From the results above, although all transition states can be formed, it is expected that a probable enhancement of the concentration of 1,2,4-TMB inside the micropores (with respect to the other TMB isomers) will contribute to some selectivity to the production of p-xylene. Also, as said above, the presence of many 10-rings, where p-xylene can diffuse better than the other xylene isomers is expected to contribute also to increase the selectivity to p-xylene. Finally, another positive aspect of UWY zeolite is the absence of large cavities, whose presence favours the production of coke.

4. Conclusions. Molecular dynamics has been used to study the diffusional behavior of aromatic molecules relevant to the industrial process of transalkylation of trimethylbenzenes (1,2,3-, 1,2,4-, and 1,3,5-TMB) with toluene to give xylenes using UWY zeolite, containing interconnected multipores of 10-ring and 12-ring channels. A recently parameterised rigid ion force field of general use and specifically designed for molecular dynamics has been found suitable for this task, as resulted from the pore sizes of the UWY channels being within ±0.23 Å with respect to the experimentally reported structure. The important trade-off between adsorption and diffusion has been studied by comparing diffusion features (trajectories and diffusion coefficients) and adsorption energies at infinite dilution as well as equilibrium uptake results. TMBs diffuse preferentially and almost exclusively through the 12-ring channels, except 1,2,4TMB which may diffuse, albeit with considerable constraints, through the largest 10-ring, along [010], with dimensions 5.1×6.2 Å. In that channel, m-xylene shows a unique behaviour resembling the so called “levitation effect”, first described by Derouane, which appears due to the very weak adsorption energy in this channel. In the other two, smaller, 10-ring channels along [100] (5.2×5.9 Å) and [001] (4.5×5.9 Å) only toluene and p-xylene can diffuse. Hence this leads to molecular traffic, with 12-ring channels where TMBs diffuse preferentially, and two small 10-ring channels ([100] and [001]) where only p-xylene and toluene can diffuse. In general, stronger adsorption, found for tight fitting zeolite-adsorbate, leads to slower diffusion, although for the particular case of m-xylene indicated above, an even tighter fitting leads to very weak adsorption and hence a diffusivity enhancement. At low loading, from the viewpoint of uptake inside the crystal by a 1:1 toluene:TMB mixture, 1,2,4-TMB is the reactant molecule that is more adsorbed inside the UWY micropores due to its strong adsorption in the 12-ring channels and its better diffusivity than the other TMB isomers. As 25 ACS Paragon Plus Environment


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the loading increases, toluene is the reactant molecule with larger concentration inside the UWY micropores due to the filling of the 10-ring channels, once the 12-ring channels have been filled up. This may favour transalkylation, with toluene (and eventually p-xylene as the reaction proceeds) preferentially located in the 10-ring channels and TMBs almost exclusively located in the 12-ring channels. An analysis based on transition-state selectivity shows that all 9 possible intermediates of the toluene-TMB transalkylation can be formed inside the 12-ring channels, not only at the intersections with the 10-ring channels. Initially this lack of transition-state shape selectivity is not useful to maximize the production of p-xylene and is a downside of UWY zeolite. This might be compensated by above referred probable larger concentration of 1,2,4-TMB inside the catalyst with respect to the other TMB isomers, with this isomer leading to a larger yield of p-xylene than the others. UWY zeolite has so far being synthesized only as silico-germanate. The present study suggests the interest of trying to extend its composition to the alumino-silicate in order to test the promising features of this material as catalyst for the transalkylation of toluene and TMBs.

Supporting Information The Supporting Information is available free of charge on the XXXX website at DOI: xxxxxxx, including figures with the channels of the zeolites, data on cell parameters and loadings, trajectories of all the molecular dynamics runs, data on mean-square displacements, analysis of all 10-ring and 12-ring diameters of acid-UWY and silica-UWY, schemes of the transalkylation reactions, molecular dimensions, details of the Monte Carlo simulations, and a CIF file with the model of acid-UWY.

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-2016-0683). J.T. thanks MINECO of Spain for the SVP-2013-067662 Ph.D. scholarship. We thank ASIC-UPV for computational facilities and computational support.


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UWY zeolite features crossing 10-ring and 12-ring channels where molecular traffic is observed for the transalkylation of trimethylbenzenes and toluene 82x44mm (72 x 72 DPI)

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