Recrystallization on Alkaline Treated Zeolites in the Presence of Pore

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Recrystallization on Alkaline Treated Zeolites in the Presence of Pore-Directing Agents Eddy Dib, Hussein El Siblani, Shrikant M Kunjir, Aurelie Vicente, Nicolas Nuttens, Danny Verboekend, Bert F Sels, and Christian Fernandez Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01416 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Crystal Growth & Design

Recrystallization on Alkaline Treated Zeolites in the Presence of Pore-Directing Agents Eddy Dib,† Hussein El Siblani,† Shrikant M.Kunjir,† Aurelie Vicente,† Nicolas Nuttens,‡ Danny Verboekend,‡ Bert F. Sels,‡ and Christian Fernandez∗,† †Normandie Univ, ENSICAEN, UNICAEN, CNRS, Laboratoire Catalyse et Spectrochimie, 14000 Caen, France ‡Department M2S, K.U. Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium E-mail: [email protected]

Abstract In previous works aiming at understanding the mesoporous network after alkaline treatment in presence of organic additives, conventional bulk characterization techniques led to the conclusion that the dissolved zeolite does not undergo any kind of recrystallization [Verboekend et al. Cryst. Growth. Des., 2103, 5025-5035]. Here for the first time, we demonstrate using the data obtained from 1 H and 129 Xe NMR spectroscopy that such recrystallization does occur, which lead to the formation of a very thin coating of the mesopore walls. This demonstration is done on a beta (BEA) zeolite treated in presence of TPA+ in an alkaline solution. The formation of a small amount of nanosized crystals or embryonic phases of silicalite1 (MFI) zeolite is evidenced, as well as their homogeneous dispersion on the mesoporous surface of the beta zeolite. We think that these results may explain why a homogeneous mesopore size distribution is obtained, when organic pore-directing agents are used in the zeolite hierarchization process performed in an alkaline medium. 1

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Introduction Zeolites are microporous materials used as catalysts in a large number of industrial applications in petrochemistry. 1 The narrow size of their micropores is a feature that made the success of this material, because it results in the molecular shape selectivity, which helps to control the reactant, transition state or product selectivity. 2 However, the restricted molecular diffusion inside the micropores can often be a limiting factor, because slow diffusion and difficult access to the active sites affect the reactant conversion and the catalyst deactivation. 3–5 This has conducted to many researches dedicated to the creation of mesopores through the zeolite particles, while maintaining as much as possible the original micropores where the catalytic sites reside. 6 Such kind of hierarchical zeolites with a multimodal porosity have proved their efficiency in the last decades. 4,5,7,8 Many strategies exist for obtaining these hierarchical zeolites, e.g., by using dual templating for the micro- and mesoporous domains, or by etching already synthesized zeolites. 9–12 The post-synthesis alkaline treatment of zeolites has demonstrated to be one of the most commercially viable techniques. 9,13,14 This is due to the straightforward nature of the procedure, to a low organic footprint and to excellent performances in various reactions. 15,16 The post-synthetic alkaline treatment aimed at synthesizing hierarchical zeolites is often accompanied by tetra-alkyl ammonium ions. Indeed, the use of pore-directing agents (PDA) were found mandatory to prevent extensive amorphization, during the creation of mesopores, particularly during the hierarchization of sensitive frameworks of beta and USY zeolites. 17 Their ability to direct the mesopores is likely due to their interaction with the external surface, associated to some recrystallization of the dissolved materials, but their true role is still not fully understood. Interestingly, the tetra-propyl ammonium (TPA+ ), known as the best organic structure-directing agent (OSDA) for the MFI topology, is one of the molecules used, but never any recrystallization phenomenon has been evidenced in the earlier studies using conventional bulk characterization techniques. 18 Characterization of such systems at the atomic scale using diffraction techniques is lim2


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ited. Definitely, locating precisely light atoms such as protons is still a very great challenge, although some recent progress has been done using electron diffraction methods. 19 Conversely, nuclear magnetic resonance (NMR) spectroscopy has already proved its tremendous potential in solving such problems. For instances, we have recently shown that the interaction of tetra-alkyl ammonium ions such as TPA+ in zeolite micropores can be realistically described using double quantum 1 H MAS NMR spectroscopy. 20 Thereby, in the present study, we have been using 1 H and

129

Xe NMR spectroscopy, to

bring new insight on the recrystallization occurring in the mesopore network in the case of zeolite beta.

Experimental Section Alkaline Treatment. 100 ml of a mixed aqueous solution of sodium hydroxide (NaOH 0.2 M) and tetra-propyl ammonium bromide (TPABr 0.2 M) was stirred and then heated at a temperature of 338 K. 3.3 g of zeolite beta (BEA framework type, 21 Si/Al = 255) provided by Tosoh (product name: HSZ-980HOA) was added. The mixture was left to react for 30 min at 338 K, after which the solid material was separated via a Büchner filtration and washed with distilled water. The treated zeolite was eventually dried overnight at 373 K. The resulting sample will be referenced as AT-BEA in the following.

X-ray Diffraction. Powder XRD measurements were performed on a STOE Stadi P instrument in transmission mode using CuKα radiation. The crystallinity ratio was determined from the intensity of the diffraction peak at approximately 22.8◦ 2θ.

Thermal Analysis. The carbon content of the zeolite was determined by thermal gravimetric analysis (TGA). Combustion was performed by bringing the samples into a temperatureprogramed oven under an oxygen flow, using a TGA Q500 (TA Instruments) equipped with an automatic sampler. The sample was first heated to 333 K (3 K.min-1 ) and was kept at 3



that temperature for 30 min. Afterwards, the sample was heated to 1073 K (3 K.min-1 ) and was maintained at that temperature for 30 min. The amount of carbon was calculated from the mass loss taking into account weight loss due to water desorption below 423 K.

N2 Sorption. Nitrogen-sorption measurements were executed at 77 K with a Micromeritics TriStar 3000 instrument, controlled by the corresponding software (version 6.03). Sorption was done on a sample previously calcined in static air during 5.5 h at 883 K, using a ramp rate 5 K.min-1 . Prior to the sorption experiment, the samples were degassed overnight under a flow of N2 with heating to 573 K (5 K.min-1 ). The t-plot was used to distinguish between micro- and mesopores (thickness range=0.35–0.50 nm). The mesopore size distribution was calculated using the Barret-Joyner-Halenda (BJH) method applied to the adsorption branch of the N2 isotherms. Nuclear Magnetic Resonance. Conventional proton 1 H magic-angle spinning (MAS) and two-dimensional 1 H double quantum (DQ) NMR spectra were acquired at 500 MHz on a Bruker Avance III-HD 500 (11.7 T), using 1.9-mm outer diameter zirconia rotors spun at 40 kHz, a radio frequency power of 100 kHz and a recycle delay of 2 s. For the 1 H DQ NMR spectra, the recoupling of the double quantum coherence was performed following the well-known BaBa pulse sequence. 22 The time increment for the indirect t1 dimension was equal to 50 µs ( e.g., twice the rotor spinning period). The recoupling time was set equal to four rotor cycles (100 µs). Hyper-polarized

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Xe NMR (HP

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Xe) experiments were performed at 110.6 MHz

Bruker Avance III-HD 400 (9.4 T) spectrometer, 256 scans were summed to record each spectrum, using single 90◦ excitation pulse with a pulse length of 12.5 µs and a recycle delay of 1 s. The hyper-polarized xenon gas was obtained using a home-built xenon polarizer based on the spin-exchange optical pumping (SEOP) technique, 23,24 capable to deliver a continuous flow of HP

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Xe directly to the sample inside the NMR spectrometer. 25

The sample was first compressed into pellets at a pressure of 100 MPa, then crashed and 4


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Table 1: Properties of the parent and alkaline treated beta zeolites

Vmicro (cm3 .g−1 )a Smeso (m2 .g−1 )a Crystallinity (%)b

Parent BEA 0.16 128 100

AT-BEA 0.19 350 81

t-plot method. b Crystallinity determined from the X-ray diffraction peak around 2θ=22.8◦ (see Figure SI-1 in Supplementary Information). a

sieved into small particles of 200-500 µm diameters. Prior to the experiment, the sample was introduced into a home-designed 10 mm O.D. NMR tube and dehydrated under high-vacuum at 673 K overnight.

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Xe NMR spectra were acquired under a continuous recirculating flow

at 50 mL.min-1 of a gas mixture at a pressure of 0.15 MPa and containing 90% He, 6% N2 , and 4% of natural abundance xenon. For all variable temperature (VT) measurements and after each temperature change performed at a speed of 5 K.min-1 , a waiting delay of about 20 min was generally necessary to ensure a homogeneous temperature all over the sample, and this stability was checked by directly monitoring the xenon chemical shifts. Exchange spectroscopy (EXSY) HP-129 Xe NMR spectra were recorded using a basic three pulses (π/2 − t1 − π/2 − τm − π/2 − observe) sequence with a minimum of sixteen different phase combinations in order to select a 0 7→ 1 7→ 0 7→ −1 coherence transfer pathway. The time increment in the indirect t1 dimension was 50 µs. To observe exchange cross peaks with a significant intensity, without losing the signal of xenon due to relaxation, the mixing period (τm ) was set to 500 ms.

Results The hierarchical beta was prepared using established alkaline treatment. Thermal analysis of the treated (though uncalcined) sample reveals that about 17.5 wt.% of TPA+ is present on the sample. After calcination, an enhanced secondary porosity is attained, while the microporosity is preserved (Table 1), proving that the treatment has been successfully per-

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Figure 1: Pore distribution obtained using the BJH method applied to the nitrogen isotherms displayed in Figure SI-2 in Supplementary Information, before (circle) and after (square) the alkaline treatments in presence of tetra-propyl ammonium ions. Dp is the pore width (assuming cylindrical pores) and V is the adsorbed volume.

formed. The BJH analysis of the pore distribution shows that it is rather homogeneous with an average pore width of approximately 5 nm (Figure 1). The conventional 1 H MAS NMR spectrum of the treated zeolite AT-BEA is displayed in Figure 2. It shows a group of resonances between 0 and 4 ppm, which are assigned to the protons of the organic TPA+ molecule. Residual water is observed between 5 and 6 ppm. Finally, a broad isolated resonance is present at 10 ppm. The latter peak is attributed to the hydrogen bond in a defect composed by a siloxy (SiO – ) and silanol (SiOH) in close vicinity. 26 This resonance has been observed in different zeolite frameworks such as as-synthesized MFI, containing a tetra-propyl ammonium as a organic structure-directing agent (OSDA), where the negatively charged siloxy defects counterbalance the positive charge of the OSDA. 20,26,27 Consequently, the existence of a 10 ppm resonance in the alkaline treated BEA zeolite is a strong indication that it is due to the presence of TPA+ ions in the sample. In order to better understand the origin of this resonance at 10 ppm in AT-BEA zeolite, a two-dimensional (2D) double quantum (DQ) 1 H NMR experiment has been performed. This NMR experiment allows getting information on the internuclear distances from the measurements of the through-space dipole-dipole couplings. The recoupling of the DQ coherence under magic-angle spinning conditions can be achieved following the back-to-back (BABA) 6


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Figure 2: 1 H NMR spectra of the AT-BEA zeolite, a BEA zeolite treated by the alkaline solution containing TPA+ . The top trace is the same spectrum with an intensity multiplication factor of 20. This proton spectrum exhibits typical resonances from the TPA molecules (≈0-4 ppm), residual water (≈5.5 ppm) and from the silanols hydrogen-bounded to siloxy defects (≈10.2 ppm denoted with a red star). For comparison, the 1 H NMR spectrum of the original BEA is shown. It exibits only a small resonance around 1.8 - 2 ppm corresponding to external silanols and a broader peak around 5 which may correspond to residual water.

Figure 3: Two-dimensional 1 H double quantum (DQ) NMR spectra of the AT-BEA zeolite recorded using a BABA pulse sequence. The light blue dashed lines indicate the correlation of the siloxy/silanols groups (denoted with a red star) with the TPA+ methyl protons. As expected the water peak at 5.5 ppm is almost not seen on the diagonal owing to the high mobility which prevents double-quantum recoupling between the water protons in the current experimental conditions. However, it is observed a feature that could not be seen on the 1D spectrum, e.g., a significant correlation peak corresponding to a resonance around 7 ppm which may be due to silanols created during the alkaline treatment on and in interaction with some residual water molecules.

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pulse sequence. As displayed on Figure 3, diagonal single-quantum/double-quantum correlation peaks appear for all resonances, which show that all protons have similar neighbors. This is obviously expected for the internal protons of the TPA molecule; however, the observation that this also occurs for the resonance at 10 ppm, reveals that the silanols hydrogen-bounded to siloxy defects are in close vicinity. In addition, the 10 ppm resonance is clearly correlated to the methyl protons of the TPA+ molecule. This is very similar to what we have recently observed for silicalite-1 (a pure silicious MFI zeolite 21 ), and confirms that the existence of the negatively charged siloxy defects is likely due to the presence of TPA+ organic ions. 20 Comparing the intensity of the 10 ppm resonance observed in fig.2 with the resonances of the TPA protons, one can roughly estimate that only 8 to 10% of the TPA detected in this spectrum is related to TPA+ occluded in MFI-type micropores. One reason to explain the observation of the presence of a MFI phase could be that the silica dissolved from the parent BEA zeolite during the alkaline treatment is subjected to recrystallization in presence of TPA+ molecules. All the basic elements for nanozeolite crystallization are present at a convenient temperature, i.e., a silica source from the dissolved BEA zeolite (SiO2 ), a mineralizing agent (NaOH) and a structure-directing agent (TPA+ ). As the tetra-propyl ammonium is generally employed for the crystallization of silicalite-1, we can suggest that some MFI zeolite crystals are produced during such recrystallization. However, the latter is not observed by X-ray diffraction, which may indicate that only a very small amount of such phase is present and/or that the size of the zeolite particles is very small (embryonic phases 28 or nanosized crystals). A valuable technique to prove such a hypothesis is

129

Xe NMR spectroscopy. Indeed, it

has been shown that the xenon molecule has a chemical shift very sensitive to the local environment and in particular to the size of the pore in which it is adsorbed. 29,30 Hence, referring to existing literature, one can easily identify the zeolite phases based on the chemical shifts of the xenon molecular probe. 31,32 However, as the amount and size of the zeolitic phases to

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NMR spectrum of HP 129 Xe adsorbed on the treated AT-BEA zeolite recorded at 235 K. An expansion of the spectrum is made in the 180-100 ppm region to show the small resonance (denoted with the red star) for the xenon adsorbed in the MFI phase.

Figure 4:

Figure 5: Xenon chemical shift vs. the temperature of adsorption for the parent BEA zeolites (circles), the main peak (squares) and the smaller peak chemical shift (stars) for alkaline treated sample and finally the chemical shift of a pure silicalite-1 (triangles).

determine are likely very small, it is important to detect them using sensitivity enhancement techniques, as the

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Xe hyper-polarization by laser pumping (HP

The NMR spectra of HP

129

129

Xe NMR). 33

Xe adsorbed on the alkaline treated BEA zeolite recorded

at different temperatures are shown in the Supplementary Information (Figure SI-3) and in Figure 4 for the lowest temperature of 235 K. A sharp resonance is always observed upfield which corresponds to the free xenon gas. This line is taken as the chemical shift reference and is set to 0 ppm. At all temperatures, the xenon spectra exhibit a resonance in the 70-90 ppm

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Figure 6: Exchange NMR (EXSY) spectrum of HP 129 Xe adsorbed on AT-BEA zeolite at T=235 K and a mixing period of τm =500 ms. The existence of cross peaks reflects the traveling of the xenon molecules between the free gas phase and the micropores of both the MFI and BEA phases. The red circle indicates the expected position of the diagonal peak that should be present if the xenon was exchanging inside the microporous channels of MFI.

range corresponding to the molecules adsorbed in the BEA zeolite channels. 34 Its chemical shift increases as temperature is decreasing (Figure SI-3). Below 270 K, a low-intensity shoulder starts to grow on its left side, and finally, at the lower temperature (235 K) two well-resolved peaks can be observed (Figure 4). The chemical shift of this resonance seems characteristic of the silicalite-1 (MFI) zeolite structure. 32,35,36 As reported on Figure 5, the evolution of the chemical shift of this resonance is very close to that of a pure silicalite-1 recorded in similar conditions, in line with the aforementioned attribution based on the 1 H NMR spectra. The very low intensity of this resonance (about 1% of the resonance for xenon in BEA micropores) reflects a very low concentration of the MFI phase with respect to the BEA phase. Clearly, both 1 H and

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Xe NMR results suggest the formation of MFI phase during the

alkaline treatment of the BEA zeolite in the presence of TPA+ . However, these results do not provide information about the location of both phases. For that purpose, a 2D exchange spectroscopy experiment (EXSY) can be applied to probe the connection between both MFI 10


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and BEA phases. In such 2D experiment, any non-diagonal peak (cross-peak) is a proof that 129

Xe molecules are exhibiting a slow chemical exchange between two different positions.

Only close enough positions are concerned by these cross peaks due to the short mixing times used in the experiment (500 ms) and T1 relaxation. Figure 6 shows cross-peaks at the top of the 2D spectrum, indicating that free xenon has a direct access into both BEA and MFI channels. As well as, a direct connection between MFI and BEA is proved. Moreover, the absence of a diagonal peak corresponding to xenon adsorbed in MFI channels, shows that likely xenon molecules enter and exit MFI phase very rapidly, which suggest that the MFI phase is in the beginning phase of crystallization and distributed in a such way that no direct connection exists between them.

Discussions and Conclusions Using 1 H and

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Xe NMR spectroscopy, the presence of MFI has been evidenced. 1 H NMR

shows that MFI is containing TPA+ ions in interaction with charge-compensating framework defects, i.e., siloxy groups hydrogen-bonded to silanol groups (SiO – · · · HOSi). 20 As abovementioned, the media, in which the BEA undergoes alkaline treatment, is very favorable for MFI crystallization. Indeed, the highly silica parent BEA zeolite is partly dissolved giving a source of silica, the media is alkaline and the presence of TPA+ plays the role of the structure-directing agents. In this condition, it is thus not surprising that nanocrystals or embryonic phases of MFI can be formed. 129

Xe NMR proves unambiguously the presence of tiny MFI particles, which are likely

very well dispersed in the solid.

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Xe EXSY experiments additionally shows that the homo-

geneous dispersion of the MFI phase is close to the opening of the microporous system of the BEA crystal. Indeed, the significant exchange of xenon is observed between BEA and MFI, while no exchange can be detected between MFI phases itself. Explanation is that the MFI particles are very small and homogeneous dispersed on the whole surface of the mesopores

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created during the alkaline treatment. It is worth noting that the amount of TPA+ determined by TGA (17.5 wt.%) after washing and cleaning the sample is relatively high (11 wt.% for pure MFI zeolite).This is another indication of the retention of the TPA molecules by the siliceous network and the occurrence of a strong interaction between them. The high amount is mainly explained by the filling of the formed mesopores by the organic molecules. Fig.2 shows that only 8% of the TPA is likely occluded in silicalite-1 phase. Taking this estimation into consideration, the number of TPA occluded in the MFI (1.4 wt.%) can thus be converted to a number of MFI unit cell per grams of AT-BEA zeolite (4 TPA / MFI uc). This calculation gives around 1019 MFI uc/g. If we assume that MFI at the surface of the mesopores form a mono-layer with a thickness of one unit cell, we obtain a surface of about 50 m2 .g−1 , which is lower than the mesopores surface determined by nitrogen sorption (350 m2 .g−1 ). As this MFI phase was not detected by any other technique (porosity measurement, XRD, TEM...) this could suggest that the coverage of the mesoporous surface by MFI cages is very dispersed. Finally, the formation of MFI when using TPA+ as pore-directing agent (PDA), may explain nicely why it was found mandatory to use such PDA to prevent an extensive amorphization of highly silicic zeolites during desilication in alkaline media and to obtain homogeneous size of mesopores. 17 Indeed, when alkyl-ammonium cations such as TPA+ are present during the treatment, they likely prevent a random amorphization of the parent zeolite by forming a ‘hydrophobic’ assembly of molecules. They interact with the surface producing MFI small cages and prevent hydroxyl anions coming from NaOH to dissolve the material. In conclusion, the MFI appears as a kind of coating of the mesoporous surface. We anticipate that changing the PDA molecules used during the alkaline treatment could be a way to control the nature of the mesopore coating. This could be profitable for applications, as the interfacial coexistence of both phases can generate interesting surface properties and thus could be beneficial in some catalytic and/or adsorption process.

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Acknowledgement C.F., H. E. S. and A.V. thank the Labex EMC3 , FEDER and Region Normandie for funding the NMR equipment in the Laboratoire Catalyse et Spectrochimie, and Sébastien Aiello and Baptiste Rigaud who designed and built the xenon hyperpolariser. E. D. and D. V. thanks respectively the Region Normandie and the Research Foundation Flanders (FWO) for funding their postdoctoral fellowship. N. N. thanks the KU Leuven for financial support (FLOF).

Note The authors declare no competing financial interest.

Supporting Information Available Additional data, i.e., XRD, N2 isotherms) and VT NMR experiments on xenon are reported in the Supporting Information. The PDF file is available free of charge on the ACS Publications website at DOI: xxxxxx.

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For Table of Contents Use Only Recrystallization on Alkaline Treated Zeolites in the Presence of Pore-Directing Agents by Eddy Dib, Hussein El Siblani, Shrikant M.Kunjir, Aurelie Vicente, Nicolas Nuttens, Danny Verboekend, Bert F. Sels, and Christian Fernandez

TOC graphic

Synopsis

Hyperpolarized

129

Xe NMR shows the formation of nanosized crystals or

embryonic phases of silicalite-1 (MFI) at the mesopore surface of alkaline treated beta (BEA) zeolite.

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Recrystallization on Alkaline Treated Zeolites in the Presence of Pore

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