Designing template-free nanosized faujasite- type zeolites

ARTICLES PUBLISHED ONLINE: XX MONTH XXXX | DOI: 10.1038/NMAT4173

Designing template-free nanosized faujasitetype zeolites

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Hussein Awala1, Jean-Pierre Gilson1, Richard Retoux2, Philippe Boullay2, Jean-Michel Goupil1, Valentin Valtchev1 and Svetlana Mintova1* Nanosized faujasite (FAU) crystals have great potential as catalysts or adsorbents to more efficiently process present and forthcoming synthetic and renewable feedstocks in oil refining, petrochemistry and fine chemistry. Here we report on the rational design of template-free synthesis of nanosized FAU zeolites with exceptional properties, including extremely small crystallites (10–15 nm) with a narrow particle size distribution, high crystalline yields (above 80%), micropore volumes (0.30 cm3 g−1 ) comparable to their conventional counterparts (micrometre-sized crystals), Si/Al ratios adjustable between 1.1 and 2.1 (zeolites X or Y) and excellent thermal stability leading to superior catalytic performance in the dealkylation of a bulky molecule, 1,3,5-triisopropylbenzene, probing sites mostly located on the external surface of the nanosized crystals. Another important feature is their excellent colloidal stability, which facilitates a uniform dispersion on supports for applications in catalysis, sorption and thin-to-thick coatings. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

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istorically, synthetic zeolites have played an important role in addressing societal issues such as the more efficient use of fossil or renewable resources and meeting more stringent environmental standards1–7 . In particular, they have revolutionized the fields of heterogeneous catalysis and separation processes because their tunable active sites, located inside a network of micropores, exhibit high activities and exceptional selectivity. In the early 1960s their introduction led to breakthrough innovations in oil refining (for example, fluid catalytic cracking and hydrocracking); others quickly followed in separation and petrochemistry (for example, aromatics processing). A subsequent sustained research effort in academia and industry produced a stream of incremental improvements in zeolite materials and associated processes. Although the chemical composition and pore architecture of zeolites are important, the precise control of the size and shape of their crystallites also has a great influence on their properties in, for instance, catalysis and adsorption. In these fields, one of the drawbacks of zeolites is the occurrence of severe mass transfer limitations, preventing the full use of their potential8 . Highquality X and Y zeolite nanocrystals are expected to greatly lower these transport resistances. The efficiency of separation processes, based mainly on zeolite X, and catalytic conversions, based mainly on zeolite Y, will be significantly improved by switching to these nanosized materials. More efficient zeolites are also needed to convert renewable resources in fuels and chemicals; these emerging feedstocks and their derived platform molecules have oxygenbearing features—that is, they are intrinsically more polar and therefore more prone to diffusion limitations9 . In other applications, not related to catalysis and separation, nanosized zeolites are also likely to bring superior performance10,11 . For instance, ultra-small porous particles, in particular of the FAU type, are highly desirable as drug and contrast agent carriers, and the active components in optical layers and chemical sensors. Zeolites X and Y belong to the FAU family and exhibit a three-dimensional pore structure. Their basic structural units,

the sodalite cages, are assembled to form a spherical supercage, developing a diameter of 1.3 nm with an aperture of 0.71–0.73 nm. Previously, nanocrystalline FAU crystals with a narrow particle size distribution have always been synthesized with the assistance of an organic template, a tetramethylammonium (TMA) cation. By optimizing the type and amount of organic template as well as the composition of the starting precursor suspensions, successful preparations of zeolite Y nanocrystals have been reported12–16 . However, the organic-template approach suffers from several drawbacks. First, they are not environmentally friendly, and they are non-recyclable and expensive. Second, the removal of the template from nanosized zeolites by calcination leads to an irreversible aggregation of nanocrystals in larger solid particles, thus diminishing their expected advantages. Third, the highest crystalline yield of such FAU nanocrystals reported so far is 6–10 wt%—that is, well below those observed (80%) for commercially available micrometre-sized crystals17–22 . Finally, they exhibit a microporosity and specific surface area lower than conventional micrometre-sized zeolites: the highest micropore volume and BET surface area reported for a nanosized FAU zeolite synthesized in the presence of tetramethylammonium cations are 0.12 cm3 g−1 and 448 m2 g−1 , respectively17,20 . Thus, an alternative synthesis providing high-quality nanosized FAU-type zeolites without the above disadvantages is highly desirable, both for fundamental studies and commercial applications. Here we report the engineering of nanosized crystals of FAUtype zeolite without any organic structure-directing agent, under very mild synthesis conditions, leading to a rapid transformation of the initial suspension into a pure and highly crystalline zeolite. This new approach is based in part on our previous studies12,21,23,24 , where the crystallization of a gel network takes place without changing its original particle size. In the present investigation, the reagents are those traditionally used in the large-scale production of zeolites (Supplementary Information). The key factor controlling a uniform nucleation in the system is a homogeneous distribution of reactive

1 Laboratoire

Catalyse et Spectrochimie (LCS), CNRS, ENSICAEN, Université de Caen, 6 boulevard du Maréchal Juin, 14050 Caen, France. 2 CRISMAT, CNRS, ENSICAEN, Université de Caen, 6 boulevard du Maréchal Juin, 14050 Caen, France. *e-mail: [email protected]

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species during the gel preparation. To meet this requirement, the following general procedure is applied: freshly prepared sodium aluminate is used as it supplies mainly monomeric aluminium species25 ; colloidal silica is dissolved in a NaOH solution to provide species of low molecular weight26 ; the two solutions are mixed at 4 ◦ C to decrease the kinetics of silicate and aluminate polymerization to obtain highly uniform precursor particles. The ultimate goal of such a procedure is to reach a uniformity of composition and size of discrete gel precursor particles. Thus, during the ageing process each particle provides a single nucleus, which on heating forms a single crystal. Hence, this breeding of precursor particles leads to uniform single crystals with a size controllable by the synthesis conditions. Moreover, the formation of precursor particles of similar size with identical growth kinetics drastically limits Ostwald ripening of the resulting nanocrystals, ensuring a much narrower particle size distribution. A particularly efficient synthesis of ultra-small FAU zeolite from a water-clear precursor suspension (9 Na2 O: 0.7 Al2 O3 : 10 SiO2 : 160 H2 O) at 50 ◦ C for 45 h is presented (Supplementary Information). As well as the ultra-small zeolite Y crystals (10 nm, sample Y-10), two other Y zeolites (70 nm, sample Y-70 and 400 nm, sample Y-400) are harvested by varying the gel precursor composition and the synthesis conditions (Supplementary Information). The above-described synthesis procedure was also performed with silica and alumina sources commonly used in the commercial manufacturing of zeolites (Supplementary Table 1). The synthesis products are always nanosized crystals with narrow particle size distributions, highlighting the importance of this methodology. However, care should be taken to avoid harvesting mixtures of FAU-type and EMT-type zeolites; the conditions to obtain pure FAU are highlighted in Supplementary Table 1. The intergrowth of FAU and EMT zeolites was reported earlier27,28 . The XRD patterns of these Y zeolites (Y-10, Y-70 and Y-400) are shown in Fig. 1. Crystallites with sizes of 10 nm and 70 nm exhibit broadened Bragg peaks with intensities similar to micrometresized crystals (Fig. 1). Sharper Bragg peaks are present for sample Y-400 (Fig. 1c), illustrating the increased size of its individual particles. The XRD patterns are fitted (Le Bail method) with an Fd 3¯ space group (ICSD card 6315) allowing determination of the respective unit cell parameters (Y-10: a = 25.031(3) Å, Y-70 a = 25.048(2) Å, Y-400: a = 24.829(5) Å) and confirming the purity of the FAU zeolites obtained. A Rietveld refinement using the combined analysis methodology29,30 indicates that Y-10 and Y-70 crystals have an octahedral shape (Supplementary Fig. 1). The size

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Figure 3 | TEM pictures of Y-10 (a) and Y-70 (b) nanosized zeolites; the corresponding high-magnification images of single nanocrystals are presented as insets. 1 2 3

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of the crystals, calculated using the Scherrer equation (9 nm and 38 nm for samples Y-10 and Y-70 respectively), agrees with the dynamic light scattering (DLS) and high-resolution transmission electron microscopy (HRTEM) results. The presence of smaller crystalline domains in ‘single’ zeolite crystals is a commonly observed phenomenon; in our case, it explains the apparent discrepancy between the sizes of Y-70 measured by XRD and TEM. Before the synthesis, the precursor solution of Y-10 is water clear and evolves to a milky suspension after a 28 h hydrothermal treatment at 50 ◦ C (Fig. 2a, inset). The average hydrodynamic diameter of the amorphous and crystalline Y-10 particles changes from 12 nm to 20 nm (polydispersity index of 0.01; Fig. 2a). The DLS analysis on Y-70 shows a substantial difference between the size of the initial precursor (∼20 nm) and the final zeolite crystals (∼70 nm; Fig. 2b). The growth of Y-10 crystals is limited by the low synthesis temperature, with the main crystallization route being through propagation in the gel. In the case of Y-70, the higher synthesis temperature (120 ◦ C) favours Ostwald ripening; thus, larger particles grow at the expense of smaller ones. Y-400 synthesis is performed under standard conditions (100 ◦ C, 12 h); however, the water content is increased and the amount of sodium decreased. These conditions lead to the formation of larger zeolite crystals (400 nm). Crystalline suspensions of Y-10 and Y-70 are stable for over six months, and no sedimentation is observed even for samples with high solid content (Supplementary Fig. 2). The zeta potential of both suspensions, −55 mV, indicates their high colloidal stability (Supplementary Fig. 3). Crystalline particles in final suspensions NATURE MATERIALS | VOL 14 | JANUARY 2015 | www.nature.com/naturematerials

with sizes of roughly 10 nm (Y-10) and 70 nm (Y-70) are shown in the HRTEM images (Fig. 3). They exhibit the lattice fringe spacing (1.2 nm) expected for FAU-type zeolites. The crystal growth process is complete, as even the smallest (Y-10) crystals are well shaped and most of them exhibit a typical octahedral morphology. Octahedral crystals of different orientations are shown in Fig. 3a (inset). Depending on particle orientation, the two-dimensional projection of the crystal on the TEM micrograph can be observed as bi-pyramidal, isometric or even plate-like. The corner of the crystal and four octahedral faces are clearly seen in the top right micrograph of Fig. 3a (inset). Sample Y-70 also exhibits the typical zeolite Y octahedral morphology with well-developed faces (Fig. 3b). The HRTEM, XRD and DLS results are therefore self-consistent. For comparative purposes, scanning electron microscopy (SEM) images of the Y-400 and reference (LZY-62) samples are presented in Supplementary Fig. 4. Again, the typical FAU morphology and a mono-dispersed particle size are observed for Y-400. The porosity and specific surface area of the zeolite samples were characterized by nitrogen adsorption measurements. Zeolites Y-400 (Fig. 4c) and LZY-62 (Supplementary Fig. 5) exhibit a classic type I isotherm, with a sharp uptake at low relative pressures followed by horizontal adsorption and desorption branches. In contrast, samples Y-10 and Y-70 (Fig. 4a,c) exhibit a mix of Type I and IV isotherms with a large H1-type hysteresis. Such a feature is associated with textural pores formed by the close packing of monodispersed and well-shaped nanosized crystallites. The unusually high mesoporosity is attributed to the packing of Y-10 and Y-70 3

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crystals delineating regular mesopores with diameters of 30 nm and 80 nm, respectively. The total pore volumes for samples Y-10 and Y-70 are 1.27 and 0.63 cm3 g−1 , respectively, whereas sample Y-400 has a total pore volume of 0.35 cm3 g−1 (Supplementary Table 2). The large difference in total pore volume is due to the substantial decrease in particle size from 400 nm to 10 nm. The extremely high micropore volume, 0.30 cm3 g−1 , of the smallest (10 nm) nanocrystals is particularly noteworthy as it corresponds to that reported for micrometre-sized highly crystalline FAU zeolites. These results underline the high crystallinity and high quality of the ultra-small crystals; this is not observed when nanosized FAU crystals are synthesized in the presence of organic agents12–16 . Another important feature of Y-10 is its high external surface area, 180 m2 g−1 , opening opportunities for processes taking place specifically on this part of the zeolite. The external surface areas of Y-70 and Y-400 are 60 and 30 m2 g−1 , respectively. The precise control achieved in the synthesis of nanosized FAU is also reflected in their Si/Al ratio. For Y-10, Y-70 and Y-400, the Si/Al ratios are 1.60, 1.65 and 2.10, respectively26 . Inductively coupled plasma (ICP) results are in good agreement with TEMEDS and solid-state 29 Si NMR results (spectra not shown). The versatility of the procedure is further demonstrated by the synthesis of nanosized zeolite X with a particle size of 10 nm (sample X-10) and a Si/Al ratio of 1.10 (Supplementary Fig. 6). The purity of the crystalline phase is confirmed from the fitting of the whole experimental X-ray diffractogram with a cubic FAU ¯ structure (SG: Fd 3). The thermal stability of the nanosized FAU-type crystals, in their sodium form, is assessed by XRD analysis of samples calcined at 550 ◦ C in air. The XRD patterns of Y-10, Y-70 and Y-400 do not change after this treatment; in particular, both the positions and intensities of all Bragg peaks are preserved (Supplementary Fig. 7). Moreover, the 27 Al NMR spectra of the samples in their Hform show no sign of extra framework Al (Supplementary Fig. 8). In addition, thermogravimetric analysis indicates that all three zeolites (Y-10, Y-70 and Y-400) retain their water capacity, related to the quantity of framework Al, after such a thermal treatment (Supplementary Fig. 9). These results highlight that the Y-10, Y-70 and Y-400 materials exhibit the qualities required (see above), to have a strong impact in potential industrial applications. In particular, as well as their 4

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Figure 5 | Conversion of 1,3,5-triisopropylbenzene over Y-10, Y-70, Y-400 and LZY-62 zeolite catalysts. Reaction conditions: PTot = 1.01 × 105 Pa, PTiPBz = 180 Pa, W/F◦TiPBz = 82 kg s mol−1 .

physicochemical properties being highly desirable, their preparation route suggests that their large-scale production could meet strict health, safety and environmental requirements at an affordable cost31 . As far as catalytic applications are concerned, the potentially weakest point of nanosized zeolites, in general, is their relative instability—especially of their external surface under, for instance, thermal or hydrothermal treatments32 . A catalytic test is generally the most sensitive way to probe such a feature33,34 , which tends to escape most spectroscopic techniques. Here, the catalytic properties of the external surface of the nanosized FAU are illustrated by the dealkylation of a bulky molecule, 1,3,5-triisopropylbenzene (TiPBz). Owing to a kinetic diameter of 0.95 nm—that is, well above the pore opening of the FAU-type zeolite (0.73 nm)— TiPBz is commonly used to study the external surface properties of large-pore zeolites20 . Our preliminary tests are performed under identical conditions (PTot = 1.01 × 105 Pa, PTiPBz = 180 Pa, W/F◦ = 82 kg s mol−1 ) in a downflow fixed bed gas phase reactor at two temperatures (200 ◦ C and 225 ◦ C). The TiPBz conversions are reported in Fig. 5, and the associated selectivities in Supplementary Fig. 10. Figure 5 shows that all nanosized zeolite catalysts derived from Y-400, Y-70 and Y-10 are more active than a high-quality commercial sample (LZY-62). Furthermore, there is a clear trend between particle size and activity at both temperatures. However, the Y-10 and Y-70 catalysts exhibit similar performances although their external surface areas are very different and their bulk Si/Al ratios are very close (see above). Further work, outside the scope of this contribution, is needed to ascertain the cause of this observation and relate it, for instance, to Al gradients in the zeolites35,36 or other surface features. The product selectivity (Supplementary Fig. 10) provides further insight into their superior performance, especially at 200 ◦ C. Indeed, although conversion increases from the commercial LZY-62 to the Y-70/Y-10 catalysts, the latter remain very selective towards the (primary) di-alkylated products, further indicating that the external surface is the main locus of the reaction. A more extensive catalytic study, beyond the scope of this contribution, could better quantify these exceptional properties NATURE MATERIALS | VOL 14 | JANUARY 2015 | www.nature.com/naturematerials

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by comparing detailed kinetic data with an in-depth spectroscopic characterization of their external surface acidity37 . Finally, under the test conditions, the catalysts are very stable (no deactivation after 12 h on stream), implying most probably that few turnovers occur inside the micropores, as these would lead to deactivation by pore plugging. In conclusion, the synthesis strategy outlined in this contribution opens the door to the preparation of template-free nanosized FAUtype zeolites under the conditions and with the starting reagents used at present for the mass production of micrometre-sized materials. Extremely small FAU-type nanocrystals are synthesized with exceptional crystalline yields (80%), hitherto reached only for highly crystalline micrometre-sized crystals. The nanocrystals exhibit the porosity expected for highly crystalline FAU-type zeolites and remain stable as colloidal suspension and powders. Their properties and ease of manufacturing are indicative of many potential applications in areas as diverse as catalysis, separation, environmental remediation, decontamination and drug delivery. Moreover, the design of nanoscale devices, including optical layers, de-humidifiers, thin films and membranes, will be possible with these new ultra-small nanocrystals.

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Received 31 July 2014; accepted 13 November 2014; published online XX Month XXXX

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Acknowledgements

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The financial support from the Region of Lower Normandy and the MEET INTEREG EC and MicroGreen (ANR-12-IS08-01) projects is acknowledged.

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Additional information

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Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to S.M.

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Queries for NPG paper nmat4173 Page 1 Query 1: Title has been amended according to style, it was edited so that FAU-type could be defined and to avoid beginning with engineering of. Please pick one among the two options: ‘Designing Template-Free Nanosized faujasite-type Zeolites’ (current title) or ‘Template-free nanosized faujasite-type zeolites’. Query 2: Please expand BET, used just once. Page 2 Query 3: Please define EMT. Query 4: Please define XRD. Page 3 Query 5: Please provide title (to be in bold) that does not refer to the parts themselves for figures 1–4. Query 6: ‘transmission added to the definition of HRTEM, please check. Query 7: Please note that the two supplied versions differ in ‘Scale bar and value’ in Figs 3a, b. Please confirm whether we have followed correct version of figures. Page 4 Query 8: Please check the label on the y-axis of Figure 4 inset. If this is a derivative is the forward slash missing after dV also should the units have nm−1 at the end? Query 9: Please expand BJH, used just once in the caption of Figure 4. Query 10: Please define NMR. Query 11: Please check the term ‘PTot = 1.01105 Pa’ changed to ‘PTot = 1.01 × 105 Pa’ in Figure 5 caption and text. OK? Query 12: Please define W/F◦ . Page 5 Query 13: The title of ref 29 is listed as simply ‘Combined Analysis’ on the Wiley website. We have modified it accordingly. Query 14: Please provide Author Contribution section. This should be short and refer to the authors by their initials.