Structure-Directing Agent Governs the Location of Silanol Defects in

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The structure-directing agent governs the location of silanol defects in zeolites. Eddy Dib, Julien Grand, Svetlana Mintova, and Christian Fernandez Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03668 • Publication Date (Web): 30 Oct 2015 Downloaded from http://pubs.acs.org on October 30, 2015

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Chemistry of Materials

The structure-directing agent governs the location of silanol defects in zeolites. Eddy Dib,∗ Julien Grand, Svetlana Mintova, and Christian Fernandez∗ Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6 Bd. du Maréchal Juin, 14050 Caen, France E-mail: [email protected]; [email protected]

Getting precise information about the location of silanols in zeolites is of paramount importance since these defects can strongly affect the materials properties such as their hydrophobicity. 1–3 The defects are also the most probable sites for the coke formation and thus, they are responsible for the catalysts deactivation. 4 Therefore their localization is of crucial importance and is still a very challenging task. Indeed, the conventional diffraction techniques are not sufficient to locate light atoms such as hydrogens due to their low atomic number and their high thermal vibrations. 5 Solid-state Nuclear Magnetic Resonance (SSNMR) can be a powerful tool to solve such a problem. Several types of defects have been identified in the zeolite frameworks including silanols (i) coming from hydrolyzed Si–O–Si bridges; (ii) generated by missing tetrahedral framework atoms and (iii) those present on the external surface of zeolite crystals. 6 In addition, siloxy defects (SiO – ) were identified. They exist in the as-synthesized materials to balance the positive charge of the organic structure directing agents (OSDA). In these defects, the valence of the oxygen atom is approximately equal to one instead of two for Si–O–Si bridges and thus they are stabilized by a SiO – . . . HOSi hydrogen bond. 7 Furthermore, the electronic density of the oxygen involved in a hydrogen bond depends on the O – . . . H donor-acceptor distance. 8,9 Its length was estimated to be 1.68 Å using the empirical correlation between the ∗

OH bond length and the proton (1 H) NMR chemical shift (10.2 ppm). 10 This increases the electronic density of the oxygen atom but is still not sufficient to neutralize the system. Therefore, additional positive charges are needed. It should be noticed that such a 1 H NMR chemical shift for the proton involved in this bond occurs in several as-synthesized zeolites with different topologies (MFI, AFI, MTW, NON, and DDR). This shows the specific geometrical and chemical nature of these defects. However, their location in the zeolite framework is still an open question. 10,11 In this communication, we show that the location and distribution of these defects with respect to tetrapropylammonium (TPA+ ) in the as-synthesized silicalite-1 (pure silica MFItype zeolite synthesized without using any other cation) can be obtained using advanced solidstate 1 H NMR methods. Furthermore, the crucial role of structure directing agent on the defects distribution is emphasized. The conventional 1 H MAS NMR spectrum of as-synthesized silicalite-1 is shown in figure 1A. It displays a group of resonances at 1.0, 1.6 and 3.2 ppm, which are assigned to Hγ , Hβ , Hα in TPA+ . The isolated peak at 10.2 ppm is attributed to the SiO – . . . HOSi defects. Relative intensities of these resonances indicate the presence of one siloxy group per TPA+ molecule. 11 In order to get deeper insight on the location of the defects, advanced two-dimensional (2D) NMR techniques can be used which for instances allows getting precious information

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(8) Brunner, E.; Sternberg, U. Solid-State NMR Investigations on the Nature of Hydrogen Bonds. Prog. Nucl. Magn. Reson. Spectrosc. 1998, 32, 21–57.

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(9) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database. Acta Cryst. B 1985, 41, 244–247. (10) Koller, H.; Weiss, M. Solid State NMR of Porous Materials. Top. Curr. Chem. 2011, 306, 189–227.

(2) Zhou, H.; Mouzon, J.; Farzaneh, A.; Antzutkin, O. N.; Grahn, M.; Hedlund, J. Colloidal Defect-Free Silicalite-1 Single Crystals: Preparation, Structure Characterization, Adsorption, and Separation Properties for Alcohol/Water Mixtures. Langmuir 2015, 31, 8488–8494.

(11) Dib, E.; Mineva, T.; Gaveau, P.; Alonso, B. 14N solid-State NMR: a Sensitive Probe of the Local Order in Zeolites. Phys. Chem. Chem. Phys. 2013, 15, 18349–18352.

(3) Wang, C.-H.; Bai, P.; Siepmann, J. I.; Clark, A. E. Deconstructing Hydrogen-Bond Networks in Confined Nanoporous Materials: Implications for Alcohol-Water Separation. J. Phys. Chem. C 2014, 118, 19723–19732.

(12) Schnell, I.; Brown, S. P.; Low, H. Y.; Ishida, H.; Spiess, H. W. An Investigation of Hydrogen Bonding in Benzoxazine Dimers by Fast Magic-Angle Spinning and DoubleQuantum 1H NMR Spectroscopy. J. Am. Chem. Soc. 1998, 120, 11784–11795.

(4) Lakiss, L.; Ngoye, F.; Canaff, C.; Laforge, S.; Pouilloux, Y.; Qin, Z.; Tarighi, M.; Thomas, K.; Valtchev, V.; Vicente, A.; Pinard, L.; Gilson, J.-P.; Fernandez, C. On the Remarkable Resistance to Coke Formation of Nanometer-Sized and Hierarchical {MFI} Zeolites during Ethanol to Hydrocarbons Transformation. J. Catal. 2015, 328, 165–172.

(13) Schnell, I. Dipolar Recoupling in Fast-MAS Solid-State NMR Spectroscopy. Prog. Nucl. Magn. Reson. Spectrosc. 2004, 45, 145–207. (14) van Koningsveld, H.; van Bekkum, H.; Jansen, J. C. On the Location and Disorder of the Tetrapropylammonium (TPA) Ion in Zeolite ZSM-5 with Improved Framework Accuracy. Acta Cryst. B 1987, 43, 127–132.

(5) Inui, M.; Ikeda, T.; Suzuki, T.; Sugita, K.; Mizukami, F. Quantitative Analysis of Structural Defect in Silicalite by Rietveld Refinements Using X-ray Powder Diffraction and 29Si MAS NMR. Bull. Chem. Soc. J. 2009, 82, 1160–1169.

(15) Brown, S. P. Applications of High-Resolution 1H Solid-State NMR. Solid State Nucl. Magn. Reson. 2012, 41, 1–27. (16) Brown, S. P.; Schnell, I.; Brand, J. D.; Mullen, K.; Spiess, H. W. An Investigation of π − π Packing in a Columnar Hexabenzocoronene by Fast Magic-Angle Spinning and Double-Quantum 1H Solid-State NMR Spectroscopy. J. Am. Chem. Soc. 1999, 121, 6712– 6718.

(6) Chezeau, J.; Delmotte, L.; Guth, J.; Gabelica, Z. Influence of Synthesis Conditions and Post-Synthesis Treatments on the Nature and Quantity of Structural Defects in Highly Siliceous MFI Zeolites: A High-Resolution Solid-State 29Si NMR Study. Zeolites 1991, 11, 598–606.

(17) Shantz, D. F.; Schmedt auf der Gunne, J.; Koller, H.; Lobo, R. F. Multiple-Quantum 1H MAS NMR Studies of Defect Sites in As-Made All-Silica ZSM-12 Zeolite. J. Am. Chem. Soc. 2000, 122, 6659–6663.

(7) Koller, H.; Lobo, R. F.; Burkett, S. L.; Davis, M. E. SiO-... HOSi Hydrogen Bonds in As-Synthesized High-Silica Zeolites. J. Phys. Chem. 1995, 99, 12588–12596.

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Structure-Directing Agent Governs the Location of Silanol Defects in

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