A first principle study on Fe incorporated MTW-type zeolite

Microporous and Mesoporous Materials 199 (2014) 83–92

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A first principle study on Fe incorporated MTW-type zeolite Gang Feng a, Zhang-Hui Lu b, Deqin Yang a, Dejin Kong a,⇑, Jianwen Liu c,⇑ a

Shanghai Research Institute of Petrochemical Technology SINOPEC, 201208 Shanghai, PR China College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR China c National Supercomputing Center in Shenzhen, 518055 Shenzhen, PR China b

a r t i c l e

i n f o

Article history: Received 14 January 2014 Received in revised form 19 July 2014 Accepted 2 August 2014 Available online 12 August 2014 Keywords: Substitution energy Acid property MTW-type zeolite Adsorption First principle

a b s t r a c t The distribution scheme of Fe3+ cation in MTW-type zeolite and its influence on the acid property were systematically studied using dispersion-corrected density functional theory. The calculated energy differences are small for different incorporated T sites, indicating that Fe atoms might distribute in all kinds of T sites. Substitution energy is firstly introduced to evaluate relative synthesis difficulty for metal incorporated zeolite. For MTW-type zeolites, the calculated substitution energies give the order of NaAl-MTW > NaFe-MTW > NH4Al-MTW > NH4Fe-MTW > HAl-MTW > HFe-MTW, which are in line with experimental observations, illustrating a real case that substitution energies can be used to pre-select the prescriptions for zeolite synthesis. Acidity studies show that the Lewis acidities of HFe-MTW zeolite are stronger than those of HAl-MTW zeolite, while the Brønsted acidities of HFe-MTW zeolite are weaker than those of HAl-MTW zeolite. Adsorption studies of NH3 and pyridine show that adsorption on the Lewis acid sites are less stable than on the Brønsted acid sites. These results are also in well agreement with the previous experimental observations and provide new insights for MTW-type zeolites. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Zeolites are crystalline microporous materials with tetrahedrally-coordinated framework structures incorporating welldefined channel systems and cavities. Because of their outstanding solid-acid and adsorption–desorption properties for small molecules, zeolites have been widely used as catalysts and adsorbents in industry [1–4]. The MTW-type zeolite, which is widely used in a wide variety of chemical reactions, was first synthesized by Rosinski and Rubin [5]. It was reported that the MTW-type zeolite in the monoclinic space C2/m with a = 25.552 Å, b = 5.256 Å, c = 12.117 Å and b = 109.312°, and each unit cell contains Si28O56. The pores of MTW-type zeolite are one-dimensional 12-membered ring channels along the b axis without intersection [6,7]. Usually, the MTW-type zeolites were synthesized in hydrothermal conditions [8–17]. The silicates sources (silica solution, sodium silicate and white carbon black), other kinds of framework cations (Al3+, Fe3+, Ga3+ and B3+), bases (NaOH, KOH, LiOH and NH4OH) and structure directing agents (tetraethylammonium bromide, methyltriethylammonium bromide, tetraethylammonium hydroxide, 4-cyclohexyl-1,1-dimethylpiperazinium hydroxide,

⇑ Corresponding authors. Tel.: +86 21 68462542; fax: +86 21 68462283 (D. Kong). Tel.: +86 755 86576112; fax: +86 755 86576000 (J. Liu). E-mail addresses: [email protected] (D. Kong), [email protected] (J. Liu). http://dx.doi.org/10.1016/j.micromeso.2014.08.009 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.

4,40 -trimethylenebis (dimethylpiperidinium) hydroxide and crystal seeds) were mixed with water in a certain order. The mixture was stirred till the gel became uniform. Then, the gel was transferred to a crystallization reactor. The crystallization could be carried at 80–180 °C for 6 h to 150 days. A more preferred temperature is 150–170 °C for 5–12 days. The produced MTW-type zeolite has both tetravalent Si and trivalent cation (Al3+, Fe3+, Ga3+, and B3+) in the framework T sites. In addition, a monovalent atom (K, Na, Li, H) stays near the trivalent cation to maintain the charge balance of the zeolite [6,9–11]. In order to acquire Brønsted acid of the zeolites in catalysis, the prepared K-, Na- and Li-form zeolites should be submitted to successive NH+4 ion exchange procedures and calcined to acquire the H-form zeolites [2]. The incorporated trivalent cation and the proton, which work as the Lewis and Brønsted acid sites, respectively, have important influences on the pore structure, acid properties and the applications of the zeolites [18–27]. For example, a series of isomorphously framework-substituted ZSM-5 zeolites have been characterized by temperature programmed NH3 desorption and the relative Brønsted acidity was found to increase according to order of SiOH < B(OH)Si  Fe(OH)Si < Ga(OH)Si < Al(OH)Si. These experimental results were also demonstrated by theoretical calculations [26,28]. For the MTW-type zeolite, the preparation methods [9,12,15–17,29], the acid properties [18], and the catalytic performance of zeolite were investigated in the previous experiments [30,31]. However, the detailed schemes for the distribution

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of trivalent cations and their influence to the acid properties of the MTW-type zeolite remain questions. For the Al-MTW zeolite, Shantz et al. investigated the Al distribution of Al-MTW using NMR techniques, and found that the distribution of Al is not random but controlled by electrostatic forces arising from the presence of template molecules during synthesis [29,32,33]. Smirniotis et al. also characterized the synthesized Al-MTW using 27Al NMR spectroscopy [14], and found that all Al atoms are incorporated into the zeolite framework in tetrahedral coordination. The NMR line-widths of Al signals of the calcined samples were significantly larger than those with template incorporated samples, since the creation of several different, non-equivalent, tetrahedral coordination environments due to the removal of the template molecules. In our previous work, the distribution of Al and the adsorption of NH3 and pyridine in both Na-form and Hform ZSM-12 were investigated using dispersion corrected density functional theory [19]. Except for Al incorporated zeolites, Fe incorporated zeolites also attracted great attentions due to the modified acid properties and pore structures of zeolites [9,24,34–36]. Compared to Al incorporated zeolites, Fe incorporated zeolites show different catalytic behavior with altered activity, selectivity and stability, offering the potential to design zeolites for new applications [28,34,37]. Both Al and Fe incorporated MTW-type zeolites were synthesized ˇ ejka [9]. in a broad range of Si/Al (Si/Fe) ratios by Košová and C The crystallization process, the type and concentration of acid sites in dependence on the Si/Al (Si/Fe) ratio and calcinations procedure were studied to discover the final concentration of the individual types of acid sites. They found that minimum Si/Al ratio achieved for Al-MTW is around 35, which is very close to the minimum Si/Fe ratio of 37 for Fe-MTW. Incorporation of Al into the zeolite framework was more easily compared to Fe. FTIR spectroscopy used to study the adsorption of probe molecules on Brønsted and Lewis acid sites revealed that both sites are present in significant concentrations in all calcined Al-MTW and Fe-MTW zeolites. While, experimental investigations have not given explanations on why Al could be more easily incorporated into the framework of the zeolite than Fe. In addition, the acidities of both the AlMTW and Fe-MTW zeolites were not described at molecular level in the previous works. In order to map out detailed structures and acid properties of both Al-MTW and Fe-MTW zeolite and provide the prescriptions for synthesis, this work studied Fe incorporated MTW-type zeolite using periodic density functional theory. The acid properties of the Fe-MTW zeolite were investigated via NH3 and pyridine adsorption. These results were analyzed and compared with our previous work on the Al-MTW zeolite [19].

2. Computation details Density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP) was applied [38,39]. The dispersive interactions were calculated using DFT-D2 method of Grimme [40,41]. Spin-polarized calculations were carried out with the PBE generalized-gradient exchange and correlation functional with a kinetic energy cutoff of 350 eV [42]. Core electrons are described by the projected augmented wave method (PAW pseudopotential) [43,44]. GGA + U calculations were performed for the strong on-site Coulomb repulsion of the Fe d electrons with the parameter: U = 4 eV and J = 1 eV, as tested in the previous works for the trivalent Fe atoms [45,46]. The p(1  2  1) cell is sampled with a 2  2  2 k-points mesh, generated by the Monkhorst–Pack algorithm. Various spin multiplicities for the Fe atom were investigated in our work. As previous work reported that the low spin state Fe3+ is usually less stable than the high spin state

on oxide surface [46]. For the p(1  2  1) MTW cells with one Fe3+, the low spin (doublet) state structures are about 1.8 eV less stable than the high spin (sixtet) state structures of Fe-MTW zeolites. For structure optimizations, both the cell parameters and the atoms of MTW were fully relaxed. The convergence criteria were 1.0  104 eV for the SCF energy, 1  103 eV for total energy and 0.05 eV/Å for atomic force, respectively. The DFT-D2 parameters were set as follows: cutoff radius for pair interactions is 30.0 Å, global scaling factor s6 is 0.75, the C6 and R0 parameters for H, C, N, O, Na, Al, Fe and Si atoms are 0.14, 1.75, 1.23, 0.70, 5.71, 10.79, 10.80 and 9.23 Jnm6mol1 and 1.001, 1.452, 1.397, 1.342, 1.144, 1.639, 1.562 and 1.716 Å, respectively [41]. The vibrational frequencies and normal modes were calculated by diagonalization of the mass-weighted force constant matrix, which was obtained using the method of finite differences of forces as implemented in VASP. The ions are displaced in the ± directions of each Cartesian coordinate by 0.02 Å. All reported minimum energy structures have only real frequencies. These computational methods and parameters have been tested and validated in the previous works [19,46–51]. As shown in Fig. 1, each unit cell (Si28O56) of the MTW-type framework contains 7 inequivalent Si and 11 inequivalent O sites [19]. The inequivalent Si sites are indexed with numbers (1–7), and the inequivalent O sites are indexed with letters (a–k). To minimize the interaction of adsorbates (NH3 and pyridine) of the neighboring cells, a p(1  2  1) cell (Si56O112) was used for all calculations. Our calculated cell parameters of the p(1  2  1) pure silica MTW cell are: a = 25.891 Å, b = 10.555 Å, c = 12.277 Å, a = c = 90° and b = 109.312°, which agree well with the reported experimental data [6,7]. In order to investigate the local structures of Fe in the zeolite, one Si atom was substituted by Fe atom in the p(1  2  1) cell to get the Si/Fe ratio of 55, since the reported Si/Fe and Si/Al ratios of the MTW-type zeolite in experiments were usually around 50 [9,13,30,31,52–54]. Both H-form and Na-form MTW-type zeolite structures with all possible Fe locations were systemically calculated. The adsorption of NH3 and pyridine on the Lewis and Brønsted acid sites were calculated. For the optimization of the H-form and Na-form MTW-type zeolite, 7 inequivalent T sites of the zeolite were investigated for the substitution of Si by Fe. For Fe in each T sites, we tried to bond the H and Na atoms to the four different O sites near the Fe atom, and calculated the H and Na atoms pointing to their nearby O atoms to form different hydrogen bonds in the input structures. The optimized local minima structures were used to investigate the adsorption of NH3 and pyridine. Both the Lewis and Brønsted acid sites were calculated for the adsorption of NH3 and pyridine. Furthermore, we also investigated the adsorption of NH3 and pyridine in both the main channel and the small cages of the zeolite. The adsorption energies (Eads) for the adsorption of NH3 and pyridine in the zeolite were calculated by

Eads ¼ Eðmolecule@MTWÞ  ½EðmoleculeÞ þ EðMTWÞ

ðaÞ

where E(molecule@MTW), E(molecule) and E(MTW) are the total energies of the MTW-type zeolite cell with adsorbed molecule in the pore, total energies of adsorpted molecules (molecule = NH3 or pyridine) and the total energies of MTW-type zeolite, respectively. The larger adsorption energy indicates the stronger adsorption on the acid site for the probe molecules. For the substitution of Si atoms of the MTW-type zeolite by Fe/Al, it can be described as following reaction

nðO-Si-OÞMTW þ nMðOHÞ3 þ nYOH ! nSiðOHÞ4 þ nðO-M-OYÞMTW

ðbÞ

where M = Fe, Al; Y = H, NH4, Na, and n = 1, 2, 3, . . . The corresponding substitution energy is defined as following:

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85

Fig. 1. The supercell of MTW-type zeolite. The Si and O atoms are shown in yellow (indexed with numbers 1–7) and red (indexed with letters a–k), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Esub ¼ nE½SiðOHÞ4 Þ þ nE½ðO-M-OYÞMTW   nE½ðO-Si-OÞMTW   nE½MðOHÞ3   nE½YOH

ðcÞ

Where E[Si(OH)4)], E[(O-M-OY)MTW], E[(O-Si-O)MTW], E[M(OH)3] and E[YOH] are the total energies of the free Si(OH)4, Y-M-form MTW-type zeolite cell, pure silica MTW-type zeolite cell, free M(OH)3 and YOH molecules, respectively. From the thermodynamical point of view, Esub could be used to evaluate and pre-select the experimental prescriptions for zeolite synthesis and structure determination. For example, for a given n, Esub is a function of M and Y, a larger Esub indicates a more preferred prescription for the zeolite synthesis. For a given M and Y, Esub is a function of n, a larger Esub indicates preferred Si/M ratio for the zeolite. In addition, a larger Esub indicates a more favored substitution T site for M and Y for n = 1 and a given M and Y. 3. Results and discussion In order to figure out the importance of the dispersion interaction, the relative total energies and van der Waals energies for the structures were calculated as shown in Table 1 (The original total energies and van der Waals energies of the structures are shown in Table S1 in Supplementary Materials). Similar to our previous published Al-MTW data [19], the van der Waals contribution of Fe-MTW are also very important, which significantly change the order of the relative stability. Table 1 shows the computed relative energies for Fe-MTW using DFT-D2 and DFT, respectively. It shows that the energies differences for Fe in different T sites of the MTWtype zeolite were less than 0.30 eV. However, the most stable substitution sites were different. The DFT-D2 indicates that T(3) site is the most favored substitution site for HFe-MTW zeolite. While the DFT show that T(1) is the most favored site. Both DFT-D2 and DFT indicate that T(6) is the most favored substitution site for NaFeMTW zeolite, but the local minima structures are different. The computed adsorption energies show that the DFT underestimates the adsorption energies by 0.23–0.71 and 0.34–1.05 eV for the adsorption of NH3 and pyridine, respectively (Table 2). Table 3 shows that DFT also underestimated the substitution energies by 1.23–1.70, 0.72–1.04, 1.18–1.48 and 0.691.01 eV for the NaFe-, HFe-, NaAl- and HAl-form MTW-type zeolites compared to DFTD2 method. These results indicate that the dispersion correction is essential for zeolite and should be taken into account when calculating adsorption energies and substitution energies. 3.1. Local structures for Fe in MTW 3.1.1. Local structures for NaFe-MTW Figs. 2 and S1 show the stable local structures and relative energies of Fe substituted Na-form MTW zeolite. (Detailed structures were shown in Fig. S1 in Supplementary Materials) It was found

that T(6) site is the most stable site for Fe atom. As shown in Fe(6)-Na-3, the Na+ stays in the cage of T(4,5,6,7). The most stable structures for T(1), T(2), T(3), T(4), (T5) and T(7) sites are Fe(1)-Na3, Fe(2)-Na-1, Fe(3)-Na-3, Fe(4)-Na-4, Fe(5)-Na-2 and Fe(7)-Na-3, respectively, which are 0.21, 0.26, 0.14, 0.10, 0.25 and 0.01 eV less stable than Fe(6)-Na-3. It shows that the energy differences for Fe atom in different T sites are very small, indicating that Fe atoms could be distributed in all kinds of T sites of the Na-form MTWtype zeolite, either in the main channel or in the small cages. In addition, for Fe atom in a certain T site (Fig. S1), the Na+ has several local minima structures. For example, there are three local minima structures, Fe(1)-Na-1, Fe(1)-Na-2 and Fe(1)-Na-3 for Fe in T(1) site. The energy differences are less than 0.30 eV. It should be noted that Na cations stay in the main channel in Fe(1)-Na-3, Fe(2)-Na-1, Fe(3)-Na-3, Fe(4)-Na-4 and Fe(5)-Na-2 for Fe in T(1,2,3,4,5) sites, while the Na cations stay in the small cages in Fe(6)-Na-3 and Fe(7)-Na-3. 3.1.2. Local structures for HFe-MTW The local structures and relative energies of Fe substituted Hform MTW zeolite were show in Figs. 2 and S2. (Detailed structures were shown in Fig. S2 in Supplementary Materials) The corresponding p(1  2  1) cell parameters and coordinates of the most stable structures for Al and Fe incorporated in each T site of MTWtype zeolites are given in the Supplementary Materials. As shown in Fe(3)-H-1 the most stable location for Fe is T(3) site, in which H atom bonds to the nearby O(e) site. The most stable structure for T(1), T(2), T(4), T(5), T(6) and T(7) sites are Fe(1)-H-4, Fe(2)H-4, Fe(4)-H-5, Fe(5)-H-1, Fe(6)-H-1 and Fe(7)-H-4, respectively. The energies for these structures are 0.07, 0.18, 0.04, 0.08, 0.06 and 0.10 eV less stable than Fe(3)-H-1, which indicates that Fe atoms could also distribute in all kinds of T sites of H-form MTW-type zeolite. Similar to the case in NaFe-MTW zeolite, the proton has several local minima structures for each Fe site, e.g. H atom bonds to O(a) and O(c) in Fe(1)-H-1 and Fe(1)-H-2, respectively. The energy difference is 0.22 eV. Similar to the case of AlMTW zeolite [19], the H atoms could stay in both the main channel and the small cages of HFe-MTW zeolite. In Fe(3)-H-1, Fe(5)-H-1 and Fe(6)-H-1, the H atoms are in the main channel, while they stay in the small cages of the zeolite in Fe(1)-H-4, Fe(2)-H-4, Fe(4)-H-5 and Fe(7)-H-4. The reason is probably that the pore sizes of the small cages are suitable to hold the small cations such as Na+ and H+. The results agree well with previous experimental observation. Košová and Cˇejka investigated the acid properties of HAlMTW and HFe-MTW using IR spectra [9]. They also found two kinds of hydroxyl groups (3610 and 3575 cm1 for HAl-MTW, 3630 and 3590 cm1 for HFe-MTW) in the zeolites. According to Chiche et al. [55], the infrared bands at 3610 and 3575 cm1 can be tentatively attributed to hydroxyl groups vibrating in the main channel and the six-membered rings of HAl-MTW structure. The

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Table 1 Relative energies (eV) for Fe substituted MTW-type zeolite calculated with DFT-D2 (DE(DFT-D2)), DFT (DE(DFT)) and dispersion correction (DE(D2)), respectively. The relative energies of the other structures are given with respective to the most stable structure. Structure

DE(DFT-D2)

DE(DFT)

DE(D2)

Structure

DE(DFT-D2)

DE(DFT)

DE(D2)

Fe(1)-Na-1 Fe(1)-Na-2 Fe(1)-Na-3 Fe(2)-Na-1 Fe(2)-Na-2 Fe(2)-Na-3 Fe(3)-Na-1 Fe(3)-Na-2 Fe(3)-Na-3 Fe(3)-Na-4 Fe(4)-Na-1 Fe(4)-Na-2 Fe(4)-Na-3 Fe(4)-Na-4 Fe(5)-Na-1 Fe(5)-Na-2 Fe(5)-Na-3 Fe(5)-Na-4 Fe(5)-Na-5 Fe(6)-Na-1 Fe(6)-Na-2 Fe(6)-Na-3 Fe(6)-Na-4 Fe(7)-Na-1 Fe(7)-Na-2 Fe(7)-Na-3 Fe(7)-Na-4

0.44 0.38 0.21 0.26 0.28 0.30 0.44 0.44 0.14 0.27 0.19 0.94 0.98 0.10 0.74 0.25 0.70 1.11 0.31 1.02 0.46 0 0.14 0.46 0.44 0.01 0.86

0.14 0.17 0.10 0.10 0.03 0.23 0.22 0.13 0.19 0.13 0.02 0.28 0.16 0.15 0.14 0.23 0.42 1.24 0.05 0.23 0.16 0 0.28 0.19 0.03 0.11 0.53

0.57 0.21 0.31 0.36 0.24 0.07 0.66 0.31 0.33 0.14 0.21 1.22 0.81 0.25 0.60 0.48 0.28 0.13 0.26 1.25 0.62 0 0.42 0.27 0.47 0.12 0.33

Fe(1)-H-1 Fe(1)-H-2 Fe(1)-H-3 Fe(1)-H-4 Fe(1)-H-5 Fe(2)-H-1 Fe(2)-H-2 Fe(2)-H-3 Fe(2)-H-4 Fe(2)-H-5 Fe(3)-H-1 Fe(3)-H-2 Fe(3)-H-3 Fe(3)-H-4 Fe(4)-H-1 Fe(4)-H-2 Fe(4)-H-3 Fe(4)-H-4 Fe(4)-H-5 Fe(5)-H-1 Fe(5)-H-2 Fe(5)-H-3 Fe(5)-H-4 Fe(5)-H-5 Fe(6)-H-1 Fe(6)-H-2 Fe(6)-H-3 Fe(6)-H-4 Fe(7)-H-1 Fe(7)-H-2 Fe(7)-H-3 Fe(7)-H-4 Fe(7)-H-5

0.62 0.40 0.18 0.07 0.15 0.18 0.18 0.33 0.18 0.30 0 0.28 0.37 0.05 0.08 0.07 0.26 0.29 0.04 0.08 0.10 0.21 0.40 0.13 0.06 0.10 0.57 0.25 0.24 0.15 0.83 0.10 0.31

0.62 0.51 0.19 0.04 0.27 0.17 0.27 0.47 0.19 0.30 0 0.29 0.38 0.14 0.15 0.22 0.19 0.36 0.09 0.18 0.19 0.33 0.50 0.17 0.22 0.12 0.64 0.38 0.40 0.18 0.58 0.41 0.27

0.00 0.11 0.00 0.11 0.12 0.01 0.09 0.14 0.02 0.00 0 0.01 0.01 0.09 0.07 0.15 0.08 0.08 0.05 0.10 0.09 0.13 0.10 0.04 0.16 0.02 0.07 0.12 0.16 0.03 0.24 0.31 0.03

Bold means the lowest energy for the isomers.

Table 2 Adsorption energies (Eads, eV) for the adsorption of NH3 and pyridine in MTW-type zeolite calculated with DFT-D2 and DFT, respectively. Structures

Eads(DFT-D2)

Eads(DFT)

Structures

Eads(DFT-D2)

Eads(DFT)

Fe(1)-H-4-NH3-1 Fe(1)-H-4-NH3-2 Fe(1)-H-4-NH3-3 Fe(2)-H-4-NH3-1 Fe(2)-H-4-NH3-2 Fe(3)-H-1-NH3-1 Fe(3)-H-1-NH3-2 Fe(4)-H-5-NH3-1 Fe(4)-H-5-NH3-2 Fe(5)-H-1-NH3-1 Fe(5)-H-1-NH3-2 Fe(6)-H-1-NH3-1 Fe(6)-H-1-NH3-2 Fe(7)-H-4-NH3-1 Fe(7)-H-4-NH3-2

1.09 1.07 1.35 1.17 1.52 0.79 1.50 1.19 1.43 1.39 0.71 0.45 1.40 0.34 0.90

0.60 0.36 0.92 0.78 1.16 0.23 1.06 0.78 1.12 1.12 0.30 0.11 1.17 0.09 0.39

Fe(7)-H-4-NH3-3 Fe(1)-H-4-Py-1 Fe(1)-H-4-Py-2 Fe(2)-H-4-Py-1 Fe(2)-H-4-Py-2 Fe(3)-H-1-Py-1 Fe(3)-H-1-Py-2 Fe(4)-H-5-Py-1 Fe(4)-H-5-Py-2 Fe(5)-H-1-Py-1 Fe(5)-H-1-Py-2 Fe(6)-H-1-Py-1 Fe(6)-H-1-Py-2 Fe(7)-H-4-Py-1

1.49 1.41 1.87 1.55 1.97 1.10 1.89 1.43 1.67 1.09 1.95 0.79 1.93 1.72

1.15 0.47 1.03 0.73 1.19 0.05 1.05 0.55 1.12 0.32 1.32 0.01 1.25 0.92

Table 3 Substitution energies (Esub, eV) for the substitution of one Si atom with M (M = Al, Fe) and Y (Y = Na, NH4, H) in MTW-type zeolite calculated with DFT-D2 (and DFT). Substitution sites

NH4-Fe

Na-Fe

T(1) T(2) T(3) T(4) T(5) T(6) T(7)

2.22 2.27 2.43 2.31 2.23 2.26 2.32

4.30 4.25 4.36 4.40 4.26 4.50 4.49

H-Fe (2.90) (2.90) (2.98) (2.94) (3.03) (2.80) (2.90)

1.24 1.13 1.31 1.26 1.23 1.24 1.21

(0.52) (0.31) (0.48) (0.39) (0.30) (0.26) (0.07)

NH4-Al

Na-Al

2.90 2.76 3.01 2.80 2.72 2.96 2.83

4.89 4.85 4.99 4.92 5.08 4.91 5.02

H-Al (3.68) (3.56) (3.61) (3.74) (3.64) (3.46) (3.54)

1.65 1.61 1.72 1.82 1.67 1.66 1.70

(0.96) (0.74) (0.97) (0.96) (0.83) (0.65) (0.71)

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3.2. Structure properties The bond distances and p(1  2  1) cell parameters of MTWtype zeolite before and after the substitution of Si by Fe, which are compared to Al substituted MTW-type zeolite, were shown in Table 4 and Supplementary Materials. It shows that distances of Si–O bonds for pure silica MTW zeolite are in the range of 1.61– 1.63 Å, with the cell volume of 3173.39 Å3. After the substitution of Si by Fe and Al, H–O bond distances are in the range of 0.99– 1.03 Å, which are slightly longer than the calculated H–O bond (0.99 Å) in water molecule, v.s. the Na–O bond distances of 2.20– 2.45 Å. For NaFe-MTW zeolites, the Fe–O bond distances are in the range of 1.82–1.92 Å. In comparison, the Al–O bond of NaAlMTW zeolites are in the range of 1.71–1.78 Å. The results are similar to the Fe–O and Al–O bond distances in bulk a-Fe2O3 and c-Al2O3 [46]. While for HAl-MTW and HFe-MTW, one of the Fe–O and Al– O bonds is elongated to 2.04–2.08 and 1.88–1.92 Å, respectively, since the H atom bonds to the nearby O atom, weakening the corresponding Fe–O and Al–O bonds. It is interesting to note that the cell volume of the zeolite reduces after the substitution of Si by Al and Fe, although the Fe–O and Al–O bonds are longer than the Si–O bonds. A detailed investigation found that not only the bond distances, but also the bond angles change significantly after the substitution of Si by Al and Fe. For example, for T(3) in pure silica MTW zeolite, the \Si(3)–O–Si angles are 180°, 180°, 172.2° and 134.36°, and the \O–Si(3)–O angles are 109.96°, 109.95°, 109.69°, 109.69°, 109.30° and 108.25°. For HAl-MTW, the \Al(3)–O–Si and \O– Al(3)–O angles are 154.51°, 152.53°, 145.73°, 130.58° and 117.68°, 117.38°, 112.31°, 107.46°, 104.03°, 94.74°, respectively. For HFeMTW, the \Fe(3)–O–Si and \O–Fe(3)–O angles are 155.50°, 144.86°, 135.63°, 121.58° and 118.43°, 118.35°, 110.56°, 108.17°, 104.22°, 94.08°, respectively. It indicates that the substitution of Si by Al and Fe results in longer Al–O and Fe–O bonds than the Si–O bonds, while the \Al(3)–O–Si and \Fe(3)–O–Si angles become smaller than the previous \Si(3)–O–Si angles. In order to show the bond properties for the Fe substitution in MTW zeolite, the electron density for different T sites of MTW zeolite before and after Fe substitution were plotted using VESTA and compared with the Al-MTW zeolite [19,56,57]. Fig. 3(a), (b) and (d) show the O–Si–O and O–H bonds are obvious covalent bonds in the T sites. The Al–O and Na–O bonds show obvious ionic property, as shown in Fig. 3(b), (c) and (e). It is interesting that the Fe–O bond in Fig. 3(d) and (e) show the property between the covalent and ionic bonds. 3.3. Substitution energy

Fig. 2. Local structures and relative energies (eV) for Fe-MTW zeolites. H, Na, O, Fe, and Si atoms are in white, violet, red, gray and yellow, respectively. For the H-, and Na-forms Fe-MTW zeolites, the relative energies of the most stable structure 0 eV, and the relative energies of the other structures are given with respective to the most stable structure (only related atoms were selectively shown for good viewing). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3630 and 3590 cm1 bands for HFe-MTW of the experiment results could be attributed to hydroxyl groups vibrating in the main channel and the cages the HFe-MTW zeolite, respectively.

The calculated substitution energies are shown in Table 3. It shows that the substitution energies follow the order: HFe-MTW (1.13 to 1.31 eV) < HAl-MTW (1.61 to 1.82 eV) < NH4FeMTW (2.22 to 2.43 eV) < NH4Al-MTW (2.72 to 3.01 eV) < NaFe-MTW (4.25 to 4.50 eV) < NaAl-MTW (4.85 to 5.08 eV). It indicates that NaAl-form zeolite should be the most favored prescription for the synthesis of MTW-type zeolite from the thermodynamical point of view. In other words, Al atom could be more easily incorporated into zeolite framework than Fe, and Na is a better choice than NH4 and H for MTW-type zeolite synthesis. The result agrees well with the previous experimental results from Košová and Cˇejka [9], in which the incorporation of Al into zeolite framework was more easily than Fe. Direct synthesis of H-form zeolite from ammonium hydroxide without using alkali metal cations can be advantageous over conventional methods by eliminating the ion-exchange step [58]. The substitution energies of H-form MTW-type are less than those of Na-form MTW-type zeolite. It indicates H-form MTW-type zeolite should be more difficult to be synthesized compared to Na-form MTW-type zeolite. Some previous experiments reported the synthesis of NH4-form zeolite,

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Table 4 Bond distances and p(1  2  1) cell volumes of the Al and Fe substituted MTW-type zeolite. T sites

T–O (Å) Si

T1 T2 T3 T4 T5 T6 T7 a

1.62,1.62 1.62,1.63 1.62,1.62 1.63,1.63 1.61,1.62 1.62,1.63 1.61,1.62 1.62,1.63 1.62,1.62 1.62,1.63 1.62,1.62 1.62,1.63 1.62,1.62 1.62,1.62

a

H/Na–O (Å) Al

Fe

Al

H

Na

H

Na

1.71,1.72 1.72,1.89 1.71,1.72 1.72,1.89 1.71,1.72 1.72,1.91 1.71,1.72 1.74,1.89 1.72,1.72 1.73,1.92 1.71,1.72 1.72,1.90 1.71,1.72 1.72,1.88

1.72,1.73 1.75,1.77 1.73,1.73 1.77,1.78 1.72,1.73 1.77,1.78 1.73,1.73 1.77,1.78 1.73,1.75 1.77,1.78 1.73,1.75 1.75,1.78 1.71,1.74 1.76,1.77

1.82,1.82 1.83,2.07 1.82,1.82 1.84,2.07 1.82,1.82 1.83,2.08 1.81,1.82 1.85,2.03 1.82,1.83 1.83,2.07 1.82,1.83 1.83,2.04 1.81,1.81 1.83,2.05

1.83,1.84 1.88,1.88 1.82,1.85 1.89,1.91 1.83,1.85 1.88,1.91 1.83,1.84 1.89,1.91 1.83,1.85 1.88,1.92 1.82,1.86 1.90,1.90 1.81,1.85 1.89,1.90

Fe

Cell volume (Å3)

Al

Fe

H/Na

H/Na

0.99/2.24

1.00/2.26

3066.28/3046.95

3085.51/3030.68

0.99/2.26

1.02/2.20

3022.63/3040.66

3052.80/3041.00

0.99/2.35

0.99/2.25

3059.37/3026.92

3058.31/3033.52

1.03/2.31

1.02/2.28

3046.41/3051.48

3054.52/3018.67

1.02/2.45

0.99/2.19

3047.32/3046.91

3037.21/3056.85

0.99/2.27

0.99/2.28

3020.42/3044.96

3034.51/3020.40

1.01/2.41

0.99/2.37

3018.32/3047.19

3009.67/3036.27

The p(1  2  1) cell volume of the silica-MTW type zeolite is 3173.39 Å3.

Fig. 3. Contour plots of the electron density for Si, Al and Fe in the T sites of MTW-type zeolite. The plots (a), (b), (c), (d) and (e) are obtained from the plane containing the O(g)-Si(4)-O(i), Al(4)-O(h)-H, Na-O(j)-Al(5), Fe(3)-O(e)-H and Na-O(j)-Fe(6), respectively.

which could be converted into H-form zeolite by simple calcinations [58–60]. Our calculated substitution energies for NH4FeMTW and NH4Al-MTW (Table 3) are in the range of 2.22 to 2.43 eV and 2.72 to 3.01 eV, respectively. It indicates that NH4-MTW zeolite was more difficult to be synthesized than Naform MTW zeolite, while more easily than H-MTW zeolite. Similar to Na-MTW and H-MTW zeolite, Fe atoms were found more difficult to be incorporated into NH4-form zeolite than Al atoms. Our calculated results agree well with the reported experiments. The synthesis of NH4-form zeolite is attainable, while it needs a large amount of tetrapropylammonium hydroxide (TPAOH) and crystal seed as structure directing agents [58,59]. For the case of Fe-MTW type zeolite, previous experiments reported the synthesis of Na-form Fe-MTW zeolite [9], while we have not found any reported experiments on the direct synthesis of H-form and NH4-form Fe-MTW zeolites.

Furthermore, it was found that the most favored substitution sites are different for the NaAl-MTW, NaFe-MTW, HAl-MTW and HFe-MTW zeolites, while the most unfavorable site is T(2) for these zeolites. It indicates that T(2) could be most unfavorable site for the substitution of Si by Al and Fe for Na-form and H-form MTW zeolite. For NH4Fe-MTW and NH4Al-MTW zeolites, T(3) sites are the most favored substitution sites for both Fe and Al, and the most unfavored sites are T(1) and T(5), respectively. 3.4. Acid properties of HFe-MTW 3.4.1. NH3 adsorption in HFe-MTW Fig. 4 shows the local structures and adsorption energies for the adsorption of NH3 in HFe-MTW zeolite. Fe(2)-H-4-NH3-2, in which the NH3 molecules bonds to the Brønsted acid H, has the largest adsorption energy of 1.52 eV. It was found that NH3 could be

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Fig. 4. Local structures and adsorption energies (eV) for the adsorption of NH3 in HFeMTW zeolite. H, N, O, Fe, and Si atoms are shown in white, blue, red, gray and yellow, respectively (only related atoms were selectively shown for good viewing). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

adsorbed on both the Lewis acid Fe site and the Brønsted acid H. In Fe(1)-H-4-NH3-1, Fe(2)-H-4-NH3-1, Fe(3)-H-1-NH3-1, Fe(4)-H-5NH3-1, Fe(5)-H-1-NH3-2, Fe(6)-H-1-NH3-1 and Fe(7)-H-4-NH3-1, the NH3 molecule adsorbs on the Lewis acid Fe sites of the zeolite, with the adsorption energies of 1.09, 1.16, 0.79, 1.19, 0.71, 0.45 and 0.34 eV, respectively. In comparison, NH3 molecule adsorbs on the Brønsted acid H sites in Fe(1)-H-4-NH3-3, Fe(2)-H-4-NH3-2, Fe(3)-H-1-NH3-2, Fe(4)-H-5-NH3-2, Fe(5)-H-1NH3-1, Fe(6)-H-1-NH3-2 and Fe(7)-H-4-NH3-3 show the adsorption energies of 1.35, 1.52, 1.50, 1.43, 1.39, 1.40 and 1.49 eV, respectively. The NH3 chemisorbed on the Brønsted acid site reacts with the Brønsted acid site and forms NH+4 species. It indicates that the NH3 molecule adsorbs on the Brønsted acid sites much stronger than on the Lewis Fe sites, as found in the previous experiments of Smirniotis et al. [13]. Furthermore, we considered to put the NH3 molecule in the cages of the zeolite in Fe(1)-H-4-NH3-2 and Fe(7)-H-4-NH3-2. Although the NH3 interacts with the Brønsted acid H and forms NH+4, the adsorption energies are less than in the main channel of the zeolite. It indicates the NH3

molecules are more favored in the main channel than in the small cages of the zeolites. Comparing to the adsorption of NH3 in the HAl-MTW zeolite [19], the adsorption energies for NH3 molecule adsorption on the Lewis acid Al sites of HAl-MTW zeolite (0.34 to 1.10 eV) are slightly less than on the Lewis acid Fe sites of HFe-MTW (0.34 to 1.19 eV), which indicates that the Lewis acidity of Fe site is stronger than that of Al site in MTW-type zeolite. In addition, the adsorption energies for NH3 molecule adsorption on the Brønsted acid sites of HAlMTW zeolite (1.36 to 1.69 eV) are larger than on the Brønsted acid sites of HFe-MTW (1.35 to 1.52 eV), which indicates the Brønsted acidity of HFe-MTW zeolite is weaker than that of HAlMTW. Our results agree well with the previous experimental and computational finding that the Brønsted acidity of the HAl-form zeolite is stronger than that of the HFe-form zeolite [9,26,28]. 3.4.2. Pyridine adsorption in HFe-MTW Fig. 5 shows the local structures and adsorption energies for the adsorption of pyridine in the HFe-MTW. The adsorption of pyridine

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Fig. 5. Local structures and adsorption energies (eV) for the adsorption of pyridine in H-form MTW-type zeolite. H, C, O, Al, and Si atoms are shown in white, black, red, gray and yellow, respectively (only related atoms were selectively shown for good viewing). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

on both the Lewis acid Fe sites and Brønsted acid H sites were calculated. In Fe(1)-H-4-Py-2, Fe(2)-H-4-Py-2, Fe(3)-H-1-Py-2, Fe(4)H-5-Py-2, Fe(5)-H-1-Py-2, Fe(6)-H-1-Py-2 and Fe(7)-H-4-Py-1, the pyridine molecule also interacts with the Brønsted acid H strongly and forms the NC5H6 species similar to the case of the adsorption of NH3. The adsorption energies for the pyridine molecule on the Brønsted acid Si–HO–Fe(1–7) sites of the HFe-MTW are 1.87, 1.97, 1.89, 1.67, 1.95, 1.93 and 1.72 eV, respectively, which are larger than the adsorption energies of NH3 on these sites (1.35, 1.52, 1.50, 1.43, 1.39, 1.40 and 1.49 eV). The adsorption of pyridine on the Lewis acid sites is weaker than on the Brønsted acid site. In Fe(1)-H-4-Py-1, Fe(2)H-4-Py-1, Fe(3)-H-1-Py-1, Fe(4)-H-5-Py-1, Fe(5)-H-1-Py-1 and

Fe(6)-H-1-Py-1 the N atom of pyridine interacts with the Fe atom of the zeolite, and the adsorption energies of these structures are 1.41, 1.55, 1.10, 1.43, 1.09 and 0.79 eV, respectively, which are also larger than the adsorption energies of NH3 on these sites (1.09, 1.16, 0.79, 1.19, 0.71 and 0.45 eV), while smaller than the pyridine adsorbs on the Brønsted acid sites. Comparing to our previous work on the adsorption of pyridine in HAl-MTW zeolite, the results agree well with the previous experimental work that the adsorption of pyridine are stronger in the Brønsted acid sites of HAl-MTW than in the HFe-MTW [9,19]. In order to discover the binding of the NH3 and pyridine to the Lewis and Brønsted acid sites, the adsorption of NH3 and pyridine molecules into the main channel of the zeolite was investigated. It

G. Feng et al. / Microporous and Mesoporous Materials 199 (2014) 83–92

was found that NH3 and pyridine molecules move into the main channel of the MTW zeolite (without binding to the Lewis or Brønsted acid sites) from gas phase are exothermic by 0.18 and 0.69 eV, respectively [19]. To our knowledge, the adsorption of molecules to the active sites of the catalyst always have small energy barrier of around 0.1 eV [61]. The adsorption energies for NH3 on the Lewis and Brønsted acid sites are 0.34 to 1.19 eV and 1.35 to 1.52 eV, respectively. Thus, the binding of the NH3 molecule to the Lewis and Brønsted acid sites could be estimated by Eads(NH3)  0.18 + 0.10, which indicates that the binding of NH3 to the Lewis and Brønsted acid sites of the HFe-MTW zeolite are in the ranges of 0.26–1.11 eV and 1.27–1.44 eV, respectively. The binding of pyridine to the Lewis and Brønsted acid sites of the HFe-MTW zeolite are in the ranges of 0.20–0.96 eV and 1.13– 1.38 eV, respectively. It indicates that the binding of NH3 and pyridine to the HFe-MTW zeolite are similar. In our previous work [19], the binding of pyridine to the Lewis and Brønsted sites of HAl-MTW zeolite are in the range of 0.11–0.74 eV and 1.01– 1.57 eV, respectively, v.s. 0.16–0.92 and 1.18–1.51 eV for the binding of NH3 to the Lewis and Brønsted acid sites. It could be inferred that the binding of NH3 and pyridine to the Lewis Al sites are weaker than to Fe sites of MTW zeolites, while their binding to the Brønsted acid sites of HAl-MTW are stronger than to that of HFe-MTW. It indicates that the Lewis acidities of HFe-MTW zeolite are stronger than those of HAl-MTW zeolite, while the Brønsted acidities of HFe-MTW zeolite are weaker than those of HAl-MTW zeolite.

These results are in well agreement with the previous experiments and provide new insight for the MTW zeolite. Acknowledgements This work is supported by the Major State Basic Research Development Program of China (Grant No. 2009CB623501), SINOPEC project on 2,6-dimethylnaphthalene synthesis, Shenzhen Strategic Emerging Industries Special Fund Program of China (Grant No. GGJS20120619101655715 and JCY20120619101655719), Jiangxi Provincial Department of Science and Technology (No. 20111BDH80023 and 20133BCB23011), ‘‘Gan-po talent 555’’ Project of Jiangxi Province and the Sponsored Program for Cultivating Youths of Outstanding Ability in Jiangxi Normal University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2014. 08.009. References [1] [2] [3] [4] [5] [6] [7]

4. Conclusions [8]

The distribution schemes and acid properties of Fe incorporated MTW-type zeolites were systematically studied using dispersioncorrected density functional theory. It was found that dispersion correction is very important as DFT underestimates the adsorption energies for NH3 and pyridine in HFe-MTW zeolite by 0.23–0.71 and 0.34–1.05 eV, respectively. Similar to the cases of Al-MTW zeolites we studied before, the energy differences for Fe incorporated H-form and Na-form MTW zeolites were less than 0.3 eV, which indicates Fe atoms might distribute in all kinds of T sites. For Fe in each T sites, the H and Na atom could bond to different O atoms, as a result a number of local minima structures could be produced. Substitution energy is firstly introduced to evaluate relative synthesis difficulty for metal incorporated zeolite. For MTW-type zeolites, the calculated substitution energies give the order of NaAl-MTW > NaFe-MTW > NH4Al-MTW > NH4Fe-MTW > HAlMTW > HFe-MTW. Among Na-form, NH4-form and H-form MTWtype zeolite, H-form is the most difficult to be synthesized, Naform is the easiest and NH4-form is in the between. In addition, for a specific form zeolite, the substitution energy shows that Al could be incorporated into MTW-type zeolite more easily than Fe. The results are in line with experimental observations, illustrating a real case that substitution energies can be used to pre-select the prescriptions for zeolite synthesis. In addition, acidities and adsorption of NH3 and pyridine of FeMTW were studied. As indicated by the adsorption and binding energies of NH3 and pyridine to the zeolite, the Lewis acidities of HFe-MTW zeolite are stronger than those of HAl-MTW zeolite, while the Brønsted acidities of HFe-MTW zeolite are weaker than those of HAl-MTW zeolite. Adsorption studies shows both NH3 and pyridine could adsorb on the Lewis sites, while the adsorptions are less stable than on the Brønsted acid sites of MTW zeolite as NH3 and pyridine could be chemisorbed on the Brønsted acid sites to form NH+4 and NC5H+6 species which stabilize the structures.

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