Disorder in Extra-Large Pore Zeolite ITQ-33 Revealed by Single

Communication pubs.acs.org/crystal

Disorder in Extra-Large Pore Zeolite ITQ-33 Revealed by Single Crystal XRD Leifeng Liu,† Zheng-Bao Yu,‡ Hong Chen,†,§ Youqian Deng,∥ Bao-Lin Lee,∥ and Junliang Sun*,†,‡ †

Department of Materials and Environmental Chemistry, Bezelii Center EXSELENT on Porous Materials, Stockholm University, 106 91 Stockholm, Sweden ‡ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China § Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China ∥ Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, 10691 Stockholm, Sweden S Supporting Information *

ABSTRACT: The single crystal of the extra-large pore zeolite, ITQ-33, was obtained and used to explore its crystal structure details. The ITQ-33 structure was found to be disordered with the columnar periodic building unit, explaining the morphology changes upon the different Si/Ge ratio, and the formation of the hierarchical structure from assembling of ITQ-33 nanofibers.

Z

Herein, 1-butyl-3-methylimidazolium hydroxide ([bmIm]OH) was used as an organic structure directing agent (OSDA) in the synthesis. Figure S1 shows the products obtained from different synthesis gel compositions. Three different types of zeolites (ITQ-33, ULT, and MFI) were obtained, while ITQ-33 was strongly preferred when the reagent has a high concentration (H2O/T = 3, where T = Si, Ge, or B). Other factors, such as the amount of Ge, B, and NH4F, have less effect on the product selectivity. In general, ITQ-33 can be synthesized in a broad range of synthesis conditions by using [bmIm]OH as an OSDA. Single crystal was obtained with gel composition of SiO 2 :GeO 2 :[bmIm]OH:NH 4 F:H 2 O = 1:1:0.5:0.1:3 (mole ratio). We also modified the [bmIm]OH molecule by grafting a butyl group to form 1,2-dibutyl-3methylimidazolium hydroxide ([bbmIm]OH), and it could direct the formation of ITQ-33 but with the zeolite UTL as an impurity. The synchrotron SXRD data were collected in Diamond Light Source (U.K.). The crystal structure of ITQ-33 was solved by direct method using SHELXS97 and refined by the full-matrix least-squares calculation on F2, using SHELXL97.10 ITQ-33 crystallizes in a hexagonal space group, P6/mmm, with lattice parameters of a = 19.3095(4) Å and c = 11.5130(4) Å.11 The initial ordered structure is consistent with the one solved from PXRD data. Its asymmetric unit contains four crystallographic independent tetrahedrally connected T atoms (T = Si or Ge) and eight oxygen atoms. The structure has an interconnecting 3D channel system, which includes 18-ring

eolites, as one of the most important catalysts and adsorbents, attract lots of attention from both industry and academia. As one of the most important features, the pore structure of this kind of material determines the sorption characteristics, catalytic behavior, shape selectivity, and many other key factors in the applications.1 Recently many researchers have been focusing on searching zeolites with extra-large pores as they enable the zeolites to process bulkier molecules, improve the diffusion rate, prolong the lifetime, change product selectivity, and so on.2 ITQ-33 synthesized by Prof. Corma’s group is particularly interesting since it is an extra-large pore zeolite with threedimensional (3D) 18 × 10 × 10 channels.3 So far, there are only six zeolites with 3D channels and extra-large pores, among which ITQ-33 shows the second best stability (the best one is ECR-34 with 18 × 8 × 8 ring channels; however, the 8-ring channel is too small for many catalytic reactions).4−8 Furthermore, ITQ-33 shows outstanding catalytic properties on reacting bulk molecules and giving high selectivity in cumene synthesis.9 Determining the crystal structure of zeolites is essential for understanding and exploring its catalytic properties, as it provides detailed and accurate information of the channel system. Until now, the structure of ITQ-33 has only been determined from powder X-ray diffraction (PXRD).3In this work, single crystals of ITQ-33 were obtained and used for single crystal X-ray diffraction studies (SXRD), which provides precise structural information as well as unveils the disorder in the structure with columnar periodic building unit (PBU). On the basis of the refinement results, a structure model of the real ITQ-33 material was proposed. This structure model also offers a good explanation to the morphology changes depending on the Si/Ge ratio in the synthesis gel. © 2013 American Chemical Society

Received: June 10, 2013 Revised: August 1, 2013 Published: August 19, 2013 4168

dx.doi.org/10.1021/cg400880a | Cryst. Growth Des. 2013, 13, 4168−4171

Crystal Growth & Design

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electrons per Å3 form another D4R, which is half of the unit cell away from the original D4R along the c axis. Therefore, in the following refinement, the D4R as well as the oxygen atoms bridging D4R and the column were split into two disordered parts with different heights along the c axis. Consequently, the other two residual peaks at the position shifting from the T3 and T4 atoms by half a unit cell along the c axis appeared. This indicated that the whole structure instead of only the D4Rs should be split into two disordered parts with different heights along the c axis, except the T2 site, which is shared by the two disordered parts with occupancies of 13% and 87%, respectively (Figure 3a). All T sites from both parts could be refined without positional restraints. As shown in Table 1, after adding the disordered part, the R1 value dropped to 7.41% before using squeeze and 7.05% after using squeeze to remove the effect from the guest molecules in the channel. It is worth mentioning that each column should have at least two connecting D4Rs at the same height, otherwise the bonding condition of the T2 atom becomes severely distorted. Following this rule, a possible model of the real ITQ-33 material can be built and shown in Figure 3b. The columns in different colors alternate in heights making the domain roughly tripled along the a and b axes. The single crystal data is from the sample synthesized with a Si/Ge ratio of 1. Although we do not have direct evidence of the occurrence of disorder in the samples with the other samples, it is supposed that in the sample with a high Si/Ge ratio, the disordered domains could not grow together but form individual nanofiber crystals, which are explicitly explained in the later part of this paper. Interestingly, such kind of disorder has not been observed in the other two similar structures mentioned above (ITQ-44 and MWW). There must be some structure features exclusively presented in ITQ-33 that facilitates the disordering. By analyzing the T2 sites which connect the columns and D4Rs, it was found that the distance between T2a-T2b and T2b-T2c is exactly the same (5.75 Å), which is also half of the length of the c axis (11.5 Å). This means the D4R can connect to either T2a-T2b or T2b-T2c with no bond length change and acceptable distortion on bond angles, as illustrated in Figure 3a. However, in ITQ-44, the 3R was replaced by D3R, and the distance between the T2 sites becomes either 5.75 Å or 8.33 Å, which precludes the shift of D4R. For the MWW type zeolite, this kind of disorder is impossible because the structure is asymmetric on the top and bottom sides of the tertiary building block. Twinning and disorder are two concepts people usually used to describe the imperfectness of crystals. For twinning, different components relate to each other by rotation, planar, or point reflection. Therefore, we should refer the phenomenon in ITQ33 as disorder because it only involves translation operation between two domains. Since the two parts are energetically equal, it is supposed that they have the same proportion in the crystal. However, refining the proportion of the two disordered parts results in 13% occupancy for one part and 87% for the other. In fact, the occupancy of 13% reflects the probability of changing heights for the columns but not the real occupancy of one domain. This phenomenon also happens in other disordered structures we have studied previously.14,15 We propose the size of the disordered domain to be the reason. On one hand, when the domain is as small as a few unit cells, the proportion of two parts should be equal after refinement. On the other hand, when the domain is big enough to cover a few hundred unit cells, because the disorder involves only

channels along the c axis and 10-ring channels along the other two crystallographic axes, making ITQ-33 an extra-large pore zeolite with a low framework density of 12.37 T atoms per 1000 Å3. In the ITQ-33 structure, composite building units (CBU) mel, related by a 3-fold rotational symmetry, are merged into tertiary clusters which are further linked along the c axis by 3rings (3R) to create columnar periodic building units (purple part in Figure 1). These columns are then connected by double

Figure 1. The ordered structure of ITQ-33 viewing (a) along the c axis and (b) along the b axis.

4-rings (D4R) in a honeycomb way along the ⟨100⟩ direction to form the 3D framework. The 18-ring channel is along the same direction as the columns with six columns surrounding, as shown in Figure 1a, while the 10-ring is perpendicular to columns and formed between two columns, as shown in Figure 1b. There are also other zeolite structures observed containing mel composite build units as well as being similar to ITQ-33, for instance, ITQ-44 and MWW type zeolites.12 ITQ-44 referred to an IRR-type zeolite (Figure 2a) shows a very similar

Figure 2. Columnar periodic building blocks in (a) ITQ-44, (b) ITQ33, and (c) MWW type zeolites; (d) the mel CBU shared in ITQ-44 and ITQ-33; and (e) the distorted mel CBU in MWW.

structure to ITQ-33 but with a replacement of the 3R with a double 3-ring (D3R). The MWW-type zeolite, an important catalyst in benzene alkylation,13 comprises a similar tertiary cluster as that in ITQ-33, except that it is distorted as the downside shrinks to connect to one T atom while the up side stretches to connect to three separated T atoms shown in Figure 2c. Although the present structure seems to be acceptable, the structure refinement is far from satisfactory as the R1 value of 16.46% is too high considering the data quality (Rint value of 9.95%). This indicates some serious problems in the present structure. By analyzing residual peaks in the Fourier difference map, some hints of a missing part in the structure came out. As found, the first four strong residual peaks with intensity of 9 4169



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Figure 3. Disordered model of ITQ-33. (a) Conjunction region of two disordered parts. (b) A possible model of the real ITQ-33 material, in which the regions in yellow and purple indicate different heights.

Si−O bond to tolerate the distortion between neighboring domains, the nanodomains become connected and form disordered microsized crystals. The hierarchical structure with both micropores and mesopores is greatly desired in order to improve the diffusion rate in zeolites. In the Si-rich condition, assembling of nanofibers creates mesopores (Figure 4, panels a and b), indicating the structural disorder could be utilized to generate the hierarchical structure without assistance of surfactants. In the resultant structure, the composition was refined to be Si19.98Ge26.02O92.00 in one unit cell. Sixty-three percent of the T sites in D4R are occupied by Ge, while the T sites of 3R are 86.6% occupied by silicon. In comparison with the Si−O−Si angle, the Ge−O−Ge angle is smaller, and Ge is strongly preferred on the D4R as well as D3R to relax the geometric strain. However, it is not necessarily to be preferred on 4R or 3R because the geometric strain at these positions is not significant when the external bonding T sites are located at suitable positions. For instance, the aluminosilicate ZSM-18 (MEI type zeolite) also contains 3R as a building unit. To build up a framework with a lower Ge ratio, we can replace D4Rs by 4Rs, which gives rise to a hypothetical structure, as shown in Figure 5b. This hypothetical framework has 3D 12 × 10 × 10

Table 1. Figures of Merit Obtained from Refinements of the Ordered and the Disordered Structurea Rint R1 wR2 GOF

ordered structure

disordered structure

9.95% 16.46% 43.25% 1.987

9.95% 7.05% 22.19% 1.233

a During the refinement of ordered structure, restraint on atom occupancy was required.

translation, in reciprocal space, the diffractions of different parts simply fell on the top of each other. The refinement will end up with the structure with no disorder. While, in between these two extreme conditions, the occupancies of the two disordered parts from the refinement will be between 1 and 0, depending on the size of disordered domains. Figure 4 shows the morphology of crystal changes upon the variation of the Si/Ge ratio in the synthesis gel from the

Figure 4. SEM images of samples from the Si/Ge ratio of (a) 5, (b) 4, and (c) 1. (d−f) The scheme of crystal morphology changes upon the increase of Ge content. Assembling of nanofibers creates mesopores in (a) and (b).

Figure 5. Views along the c axis of the (a) ITQ-33 and the (b) hypothetic structures. The D4R and 4R sites are colored green.

nanofiber crystals in a Si-rich condition (Si/Ge = 5) to tabletlike crystals in a distinct hexagonal shape in a Ge-rich condition (Si/Ge = 1). On the basis of the nature of its microstructure, the mechanism of this morphology development can be deduced. The nanodomains of ITQ-33 can hardly be connected in a Si-rich condition because of the positional mismatch of neighboring domains. While with the increase of the Ge ratio, because the Ge−O has more flexibility than the

ring channel systems, in which the combination of 12 and 10 ring channels is very interesting for catalysis application.16 Besides the direct synthesis, the post-synthesis treatment by inverse sigma transformation also offers the possibility to obtain the hypothetical structure.17 In the case of ITQ-44 with very similar structures, both D4R and D3R would need heterosubstitutions like Ge to stabilize the structure. To replace them 4170



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β = 90°, γ = 120°, V = 3717.59(20) Å3, Z = 92, F000 = 1740, T = 77 K, 31030 reflections collected, 1353 independent reflections (Rint = 0.0995), R1 = 0.0705, wR2 = 0.2219 for [I > 2σ(I)]. Crystallographic Information File can be found in the Supporting Information. (12) Baerlocher, C.; Weber, T.; McCusker, L. B.; Palatinus, L.; Zones, S. I. Science 2011, 333, 1134−1137. (13) Van Miltenburg, A.; Pawlesa, J.; Bouzga, A. M.; Ž ilková, N.; Č ejka, J.; Stöcker, M. Top. Catal. 2009, 52, 1190−1202. (14) Liu, L.; Li, M.; Gao, W.; Akermark, B.; Sun, J. Z. Kristallogr. 2012, 227, 221−226. (15) Huang, S.; Inge, A. K.; Yang, S.; Christensen, K. E.; Zou, X.; Sun, J. Dalton Trans. 2012, 41, 12358−12364. (16) Willhammar, T.; Sun, J.; Wan, W.; Oleynikov, P.; Zhang, D.; Zou, X.; Moliner, M.; Gonzalez, J.; Martínez, C.; Rey, F.; Corma, A. Nat. Chem. 2012, 4, 188−194. (17) Verheyen, E.; Joos, L.; Van Havenbergh, K.; Breynaert, E.; Kasian, N.; Gobechiya, E.; Houthoofd, K.; Martineau, C.; Hinterstein, M.; Taulelle, F.; Van Speybroeck, V.; Waroquier, M.; Bals, S.; Van Tendeloo, G.; Kirschhock, C. E. A.; Martens, J. A. Nat. Mater. 2012, 11, 1059−1064.

by 3R and 4R, respectively, results in the same hypothetical structure. The ITQ-33 single crystal was synthesized and used for SXRD studies. The structure of the extra-large pore zeolite ITQ-33 can be described as the assembling of building columns through sharing D4Rs. An interesting disorder was found in the structure, which can be described as columnar PBUs shift from each other by half of the unit cell along the c axis. A model of the real ITQ-33 material was proposed based on this disordered structure. The disorder also explains different morphologies of obtained crystals at different Si/Ge ratios in the synthesis gel. Benefits from the disorder, the hierarchical structure was obtained from assembling nanofiber crystals into big particles. At the end, a hypothetical zeolite structure with a 12 × 10 × 10 channel system was proposed by replacing the D4R by 4R.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, product diagram, NMR spectrum, powder XRD pattern, TG data, EDS data, table of crystallographic data, and crystallographic information files (CIF) of ITQ-33. In additional, a structure model with two disordered parts regularly alternating is also included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86- 8-6747481. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the Swedish Research Council (VR), National Natural Science Foundation of China (Grant 91222107), and the Swedish Governmental Agency for Innovation Systems (VINNOVA) through the Berzelii Center EXSELENT. We thank the Knut and Alice Wallenberg Foundation for an equipment grant. We thank Dr Chiu Tang for performing PXRD data collection. We also thank Beamline I19 and I11 in Diamond Light Source, Ltd. for data collection.

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REFERENCES

(1) McCusker, L. B.; Baerlocher, C.; Jiří Č ejka, H. v. B. A. C.; Ferdi, S. In Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 2007; 168, 13−37. (2) Jiang, J.; Yu, J.; Corma, A. Angew. Chem., Int. Ed. 2010, 49, 3120− 3145. (3) Corma, A.; Díaz-Cabañas, M. J.; Jordá, J. L.; Martínez, C.; Moliner, M. Nature 2006, 443, 842−845. (4) Sun, J.; Bonneau, C.; Cantín, Á .; Corma, A.; Díaz-Cabanãs, M. J.; Moliner, M.; Zhang, D.; Li, M.; Zou, X. Nature 2009, 458, 1154−1157. (5) Strohmaier, K. G.; Vaughant, D. E. W. J. Am. Chem. Soc. 2003, 125, 16035−16039. (6) Jiang, J.; Jorda, J. L.; Yu, J.; Baumes, L. A.; Mugnaioli, E.; DiazCabanas, M. J.; Kolb, U.; Corma, A. Science 2011, 333, 1131−1134. (7) Jiang, J.; Jorda, J. L.; Diaz-Cabanas, M. J.; Yu, J.; Corma, A. Angew. Chem., Int. Ed. 2010, 49, 4986−4988. (8) Estermann, M.; McCusker, L. B.; Baerlocher, C.; Merrouche, A.; Kessler, H. Nature 1991, 352, 320−323. (9) Moliner, M.; Díaz-Cabañas, M. J.; Fornés, V.; Martínez, C.; Corma, A. J. Catal. 2008, 254, 101−109. (10) Sheldrick, G. M. Acta Crystallogr. 2007, 64, 112−122. (11) Crystal data for ITQ-33: Ge0.22Si0.28O, Mr = 39.72 g/mol, orthorhombic, P6/mmm, a = b = 19.3095(4) Å, c = 11.5130(4) Å, α = 4171


Disorder in Extra-Large Pore Zeolite ITQ-33 Revealed by Single

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