Mechanism of Zr Incorporation in the Course of Hydrothermal

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Mechanism of Zr Incorporation in the Course of Hydrothermal Synthesis of Zeolite BEA Pavel A. Kots,† Anna V. Zabilska,† Evgeny V. Khramov,‡ Yuriy V. Grigoriev,‡,§ Yan V. Zubavichus,‡ and Irina I. Ivanova*,†,∥ †

Department of Chemistry, Lomonosov Moscow State University, Leninskye Gory 1, bld. 3, 119991 Moscow, Russia National Research Center “Kurchatov Institute”, Kurchatov Square, 1, 123098 Moscow, Russia § Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics” RAS, Lenenskiy prosp., bld. 59, 119333 Moscow, Russia ∥ A.V. Topchiev Institute of Petrochemical Synthesis RAS, Lenenskiy prosp., bld. 29, 119991 Moscow, Russia

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‡

S Supporting Information *

ABSTRACT: The mechanism of Zr-BEA hydrothermal synthesis in fluoride media has been investigated through the detailed characterization of samples obtained at different synthesis times by XRD, XRF, TGA, multinuclear solid-state NMR, FTIR, SEM, TEM with EDS, XAS, and nitrogen sorption. The synthetic procedure involved hydrothermal crystallization of the gel with the following composition: 1SiO2:0.54TEAOH:0.54HF:0.005ZrO2:5.6H2O. The formation of open and closed Lewis acid sites was monitored by FTIR spectroscopy of adsorbed CO, while coordination of Zr was studied by XAS. The results show that the formation of Zr-BEA proceeds by two steps. In the first step, pure silica BEA is crystallized via a solid−solid hydrogel rearrangement mechanism. Zirconium species are occluded in Si-BEA crystals in the form of Zr-rich silicate particles. These particles do not provide for any appreciable Lewis acidity. In the second step, Zr incorporation into T positions of the zeolite structure takes place, leading to the formation of closed Zr sites, which are partially converted into open sites at longer synthesis times. It is demonstrated that the content of open and closed sites can be tuned by variation of the synthesis time.

1. INTRODUCTION Zr-BEA zeolitic material is a promising catalyst for a number of processes related to biomass conversion and synthesis of fine chemicals.1,2 Some recent studies have demonstrated that it is an excellent catalyst for Meerwein−Ponndorf−Verley reductions,1−4 Diels−Alder reactions,5 cascade transformation of citral to mentol,2 ethanol conversion into butadiene,6−8 ringopening aminolysis of epoxides,9 1-butene isomerization,10 methyl levulinate and furfural conversion to γ-valerolactone, and others.11,12 The unique catalytic activity of Zr-BEA is related to isolated Zr atoms acting as Lewis acid sites in tetrahedral framework positions of the siliceous BEA lattice.1−12 Besides that, the hydrophobic nature of the porous system of defect-free zeolite crystals, prepared by a fluoride route, enables catalysis by Lewis acid sites in aqueous medium, which provides an important advantage in biomass conversion. It is generally assumed that the incorporation of zirconium results in the formation of two types of sites: open (OH)−Zr− (OSi)3 with one hydrolyzed Si−O−Zr bond and fully condensed closed Zr−(OSi)4. These two types of sites can be distinguished by FTIR spectroscopy of adsorbed CO.13 An analysis of the literature data suggests that open Zr sites are more active in catalytic transformations requiring Lewis acidity. © XXXX American Chemical Society

Thus, theoretical calculations have demonstrated that open sites are more receptive for glucose isomerization to fructose.14 Catalytic data pointed out that open sites are more active in MPV reduction of crotonaldehyde with ethanol, butadiene synthesis from ethanol,6−8 and glucose isomerization.14,15 However, at present no experimental approach is available for tuning the content of open and closed sites in Zr-BEA zeolite. To determine the optimal experimental strategy for the formation of the required type of active sites, knowledge of the mechanism of Zr incorporation during synthesis is highly desirable. However, although the mechanism of synthesis of all-silica BEA has been extensively studied,16,17 only a few studies are available on the crystallization of heteroatomsubstituted BEA-based materials. Tolborg et al.18 studied Sn incorporation in zeolite BEA. Using SEM-WDS the authors observed a gradient distribution of tin along the crystal of SnBEA. The density of the outer parts was roughly twice as high as in the depleted core of the crystals. However, no explanation Received: June 5, 2018

A

DOI: 10.1021/acs.inorgchem.8b01548 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry for this effect was found. In the case of Zr-BEA the mechanism of heteroatom incorporation remains unexplored. This contribution aims to clarify the mechanism of Zr incorporation during hydrothermal synthesis of Zr-BEA via a fluoride route.

2. EXPERIMENTAL SECTION 2.1. Synthesis Procedure. The synthesis of Zr-BEA zeolite was performed as reported previously.1,2,5 The gel was prepared by mixing of tetraethyl orthosilicate (TEOS), tetraethylammonium hydroxide (TEAOH), zirconyl chloride hydrate (ZrOCl2·8H2O), and hydrofluoric acid in aqueous solution. A detailed description of the gel preparation is given in the Supporting Information. The final gel composition was 1SiO2:0.54TEAOH:0.54HF:0.005ZrO2:5.6H2O. The gel was transferred into Teflon-lined autoclaves, which were placed in a preheated oven and kept at 140 °C for 0.7−25 days. The solid products were recovered by filtration, washed, dried at 100 °C overnight, and calcined in a flow of dry air at 550 °C for 8 h. Samples are denoted as Zr-BEA-x, where x corresponds to the duration of hydrothermal treatment in days. 2.2. Characterization. X-ray diffraction (XRD) analysis was carried out on a D2 Phaser (Bruker) diffractometer. SEM analysis was performed on a Hitachi TM3030 scanning electron microscope. N2 sorption was carried out on outgassed samples using an ASAP-2000 (Micromeritics) instrument. The elemental analysis of the samples was performed using X-ray fluorescence (XRF) techniques on an Axios MAX Advanced spectrometer (PANalytical). MAS NMR spectra were recorded on an AVANCE-II 400 (Bruker) spectrometer with a magnetic field of 9.4 T. XAS measurements were carried out at the “Structural Materials Science” beamline of the Kurchatov synchrotron radiation source.19 TEM and STEM-EDS measurements were performed with a FEI Tecnai Osiris field emission microscope with energy-dispersive EDS detector (Bruker SuperX). Specimen preparation for TEM was carried out using the focused ion beam milling (FIB) technique over an FEI Scios dual beam field emission electron microscope. Lewis acidic sites were characterized by FTIR spectroscopy of adsorbed CO at 77 K. FTIR measurements were carried out on a Nicolet Protégé 460 spectrometer equipped with a liquid nitrogen cooled MCT detector. A detailed description of the experimental techniques and specimen preparation is reported in the Supporting Information.

Figure 1. XRD patterns (a) and crystallization curves and variation of solid yield (b) observed during Zr-BEA synthesis at 140 °C.

indicates that all transformations occur mostly in the solid phase. An analysis of the chemical composition of the solids recovered was carried out by XRF; the content of the template was determined by TGA. The results presented in Table 1 point to the similar compositions of all samples, even those which are poorly crystalline. The Si/Zr molar ratio of the samples remains almost constant within XRF error of ca. 10%. Apparently, the Si/Zr ratio is close to those in the reaction mixture. The content of the organic template also remains constant during the whole synthesis, pointing to the incorporation of the structure directing agent (SDA) in the solid phase in the very early steps of synthesis. However, an analysis of DTG curves indicates significant changes in the state of the SDA during the first 3 days of synthesis (Figure S1). Significant changes were also observed in the local structure of siliceous and fluorine species during the first days of synthesis, as confirmed by solid-state NMR (Figures S2 and S3) and FTIR spectroscopy in the framework vibration region (Figure S4). The 29Si CP MAS NMR spectrum of the Zr-BEA0.7 sample (Figure S3) is dominated by the intense signal of Q3 species at −99.3 ppm with a small contribution of a broad Q4 peak at ca. −108.2 ppm. With crystallization time, the intensity of the line at ca. −99.3 ppm decreases, whereas the signal corresponding to Q4 species increases in intensity, becomes narrower, and finally splits into several lines corresponding to Q4 species in different environments of the zeolite BEA structure. Crystalline samples show a typical spectrum of zeolite BEA with a set of Q4 signals at ca. −106.2, −108.2, and −113.8 ppm. The 19F NMR spectrum for ZrBEA0.7 (Figure S2) shows minor signals at ca. −117.4 and −126.7 ppm corresponding to free fluoride anion and SiF62− anion, respectively.23 After 2 days of synthesis, four NMR signals at ca. −35.0, −37.2, −57.0, and −68.7 ppm appear. According to ref 16, these signals could be assigned to fluoride anions encapsulated in [4354] and [46] cages of zeolite BEA. The intensity of these lines increases with synthesis time, in line with crystallinity growth. The FTIR spectra in the region of 1000−500 cm−1 (Figure S4) show the vibrations typical for zeolite BEA at 621, 575, and 525 cm−1. Their intensities and maxima do not change after the crystallinity reaches 100%. An analysis of the yield of intermediate products of crystallization and their composition suggests that the solid

3. RESULTS AND DISCUSSION 3.1. Mechanism of Zeolite BEA Crystallization. The study of the mechanism of zeolite BEA crystallization was carried out on a series of samples obtained from the same gel batch after different times of hydrothermal treatment. According to XRD data presented in Figure 1a, the reflections typical for zeolite BEA start to appear already after 0.7 day of hydrothermal treatment. Almost no induction period is visible; the development of the crystalline BEA phase starts from the very beginning of heating (Figure 1b). The crystallinity reaches 100% within 5 days, which is faster than the times reported previously for SiBEA (9 days)17 and Zr-BEA (10 days using seeding).1,2 The faster crystallization kinetics observed in this study is most probably due to lower water content in the gel (H2O/SiO2 = 5.6 versus H2O/SiO2 = 9−102,17). This result is in line with the findings of Yakimov et al.20 reported for SnBEA synthesis in fluoride media. A comparison of the crystallization kinetics with the variation of solid yield during the synthesis is shown in Figure 1b. It is interesting to note that the solid yield calculated as the weight of calcined solid per 100 g of SiO2 in the starting gel gives values of ca. 100% and remains almost constant during the whole synthesis. This observation suggests that most of the gel remains in the solid phase during the whole synthesis and B

DOI: 10.1021/acs.inorgchem.8b01548 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Physicochemical Properties of Intermediate and Final Products Obtained during Zr-BEA Synthesis sample

crystallinity, %a

Si/Zr ratio

TEA+,b wt %

Vtotal, cm3/g

Vmicro, cm3/g

cryst size,c μm

0.7 2 2.5 3 5 10 15 20

15 56 78 84 100 100 100 100

220 230 195 180 210

8.3 18.2 18.7 19.5 18.6 18.4 17.9 18.0

0.45 0.25 0.25 0.25 0.26 0.26 0.27 0.26

0.08 0.15 0.18 0.20 0.20 0.20 0.20 0.20

3.9 7.8

176 180

7.3 7.5

7.5

a

The sample obtained after 15 days of synthesis time was used as a standard for the determination of relative zeolite crystallinity. bDetermined by TGA. cDetermined by SEM.

product with the same overall composition as the final zeolite is formed at the very early steps of synthesis. However, the phase composition and the local structures of siliceous, fluorine, and organic species gradually changes during the first 3−5 days of synthesis, pointing to solid−solid type rearrangement of this intermediate product into zeolite ZrBEA.17,21,22,24 The textural characteristics of the intermediate and final products were studied by nitrogen adsorption/desorption over calcined samples. The isotherms are depicted in Figure S5, and the volumes of micropores and mesopores are shown in Table 1. The initial sample (Zr-BEA-0.7) shows the isotherm typical for mesoporous−macroporous materials with nonuniform pore distribution. High nitrogen uptake at a p/p0 value close to 1 and H2 hysteresis type indicate the small size of primary gel particles with interparticle mesoporosity and no well-defined pore size distribution.25 An analysis of the isotherm points to some contribution of micropores (0.08 cm3 g−1). These micropores are most probably occupied by TEA+ clusters in TEA+/silicate composite precursors of zeolite BEA,21,22 suggesting that zeolytic micropores are preformed in the amorphous precursor in the very early steps of synthesis. As the crystallization time increases to 3 days, the contribution of micropores increases to ca. 0.2 cm3 g−1 (Table 1) and the form of the isotherm changes to Langmuirian form (Figure S5), typical for defect-free zeolite materials. Isotherms and hysteresis shapes for samples recovered after 5 and 15 days of synthesis are identical with those of Zr-BEA-3. SEM images (Figure 2) show the products of Zr-BEA crystallization after 0.7−5 days of synthesis. Already after 0.7 day of synthesis (Figure 2a) BEA crystals can be distinguished as octahedra embedded in a spongelike amorphous phase. The amount of amorphous phase decreases after 2 days of synthesis (Figure 2b). For the Zr-BEA-3 sample (Figure 2c), the amorphous phase is absent according to SEM and crystals are quite perfect with smooth facets of intergrown octahedra. The first crystals have a comparatively small size of ca. 3−3.5 μm, while prolongation of the synthesis causes their growth to ca. 7−8 μm due to consecutive consumption of the amorphous phase. To summarize, the above observations point to a solid−solid hydrogel rearrangement mechanism of Zr-BEA crystallization, in accordance with the conclusions made by Serrano et al.17 for crystallization of pure siliceous BEA zeolite. This mechanism accounts for the nucleation stage and the formation of primary crystals. At longer synthesis times (3−5 days) the crystal morphology slightly changes with synthesis time and the solution-mediated transformation cannot be completely excluded.

Figure 2. SEM images of Zr-BEA samples obtained after crystallization for 0.7 (a), 2 (b), 3 (c), and 5 (d) days.

3.2. State and Local Environment of Zr in the Course of Hydrothermal Synthesis Studied by XAS. For the investigation of the state and environment of Zr during synthesis, the XAS technique was selected as the most informative for d-metal dopants in siliceous zeolites.26,27 Figure S6 shows XANES spectra recorded at the Zr K edge for the samples in hydrated and dehydrated forms. All spectra contain two main components at ca. 18018 and 18030 eV and a pre-edge feature at ca. 18006 eV, corresponding to a 1s−4d transition. The former components are usually observed in the spectra of zirconium oxides or silicates.28,29 The intensity ratio of these two components depends on the symmetry of the environment and on coordination number (CN). The preedge feature is usually attributed to Zr atoms in Td symmetry. However, the assignment of the prepeak is not so straightforward. On the basis of XANES spectra it is difficult to come to a conclusion on the local environment of Zr atoms in the samples studied. However, it is clear from Figure S5 that this environment is changing in the course of synthesis. The Fourier-transformed EXAFS data for hydrated and dehydrated Zr-BEA samples are shown in Figure 3. The major maximum due to the Zr−O first coordination sphere is clearly visible. Some minor contributions from the second coordination sphere are also observed in the region of 2.5−5 Å within the noise level. A comparison with the reference data obtained in this work for monoclinic zirconium oxide and mixed zirconium silicate (Figure S7) and with the literature data28,29 suggests that the next neighboring atom to Zr in our samples is silicon. The presence of Zr−O−Zr species C

DOI: 10.1021/acs.inorgchem.8b01548 Inorg. Chem. XXXX, XXX, XXX−XXX

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recently for NbBEA27 and SnBEA.31 According to fitting results (Table 2), Zr resides in a set of environments and the proposed EXAFS fit represents averaged values. Fitting results for samples Zr-BEA-0.7, Zr-BEA-3, and Zr-BEA-5 show octahedral (4 + 2) Zr coordination. This coordination is retained after evacuation, suggesting that Zr is connected to six rigid O−Si fragments,28 which is typical for Zr silicates.28 Due to elongation of two Zr−O distances in the Zr−O shell (ca. 2.0 vs 2.2 Å), coordination of zirconium is better described as square bipyramidal. For the Zr-BEA-5 sample a slight decrease in bond distances is observed after dehydration. This can be caused by restructuring of Zr-containing species after 5 days of hydrothermal treatment. Zr-BEA-10 and Zr-BEA-15 samples show different behavior in comparison to the other samples. For these samples, dehydration leads to a strong decrease in R(Zr−O) signal intensity owing to evolution of water ligands weakly attached to Zr atoms (Figure 3). Fitting results confirm variation of Zr coordination from octahedral (4 + 2) coordination in the hydrated state to distorted-tetrahedral coordination (2 + 2) in the dehydrated state (Table 2). This observation suggests that in these samples Zr atoms are connected with four framework oxygens and two water molecules, which is typical for heteroatom-substituted zeolites.26,27,31 To summarize, XAS results suggest that in all the samples studied Zr is found in the form of silicates; however, its environment is changing during the synthesis. At the early steps of synthesis, zirconium is hexacoordinated and is most probably contained in the form of dense zirconium silicate species occluded during synthesis. The restructuring starts after 5 days of synthesis, evidenced by changing of the bond distances. After 10 h of synthesis the zirconium atoms change their coordination into tetrahedral due to incorporation into the zeolite framework. Interestingly, the main changes in the state and environment of Zr occur within the period of 5−15 days, when zeolite BEA is already crystallized (Figure 1). This result suggests that Zr incorporation occurs mostly via a solid− solid rearrangement mechanism. 3.3. Location of Zr Species in the Zeolite Crystals. In order to visualize the zirconium distribution in the Zr-BEA crystals, STEM coupled with EDS elemental analysis was used. BEA crystals were cut with FIB into 200−400 nm foil, and the cross sections were investigated by EDS. Figure 4 shows highangle annular dark-field (HAADF) images and Zr-EDS maps for Zr-BEA-3, Zr-BEA-5, and Zr-BEA-10 samples. EDS analysis of the Zr-BEA-3 sample shows inhomogeneous Zr distribution. Zirconium forms 100−400 nm aggregates located mainly on the external surface and subsurface layers of the crystal. Part of the Zr-rich aggregates is distributed along the diagonal axes of the crystal. The appearance of these lines in SEM images of crystal cross sections were detected by Tolborg et al.,18 who studied Sn-BEA crystallization in fluoride media. This process, called the “hourglass ef fect”, arises from continuous incorporation of the defects during zeolite crystal growth.18 These regions can form intercrystalline voids where sufficiently small zirconium-containing particles are occluded. These zones should be easily accessible from the outer surface, where large portions of Zr-enriched particles are also localized. The absence of zirconium in other areas of the crystal indicates the relatively weak interaction of zirconium and zeolite at this stage. For Zr-BEA-5 and Zr-BEA-10 samples the distribution is more homogeneous; no zirconium-enriched particles or

Figure 3. Zr K-edge phase-uncorrected k3-weighted Fourier transform EXAFS for Zr-BEA samples hydrated under ambient conditions (solid lines) and dehydrated at 673 K under vacuum (dotted lines).

should lead to a strong signal at ca. 3.0 Å (similar to the ZrO2 reference), comparable in intensity with Zr−O peak,29 which is not observed in our case (Figure 3, Sb). The absence of ZrO2 in the samples was also confirmed by UV−vis (Figure S8). Samples Zr-BEA-10 and Zr-BEA-15 show a minor peak at ca. 3.2 Å. This peak was assigned previously to the presence of isolated ZrO4 tetrahedra, corner or edge shared with SiO4 in the zeolite framework.4,30 The results obtained for the best fit of the first coordination sphere extracted from the EXAFS signal are shown in Table 2. It has to be stressed that the use of two different Zr−O distances in the first shell was mandatory, as has been shown Table 2. Fitting Results of the EXAFS Data for Zr-BEA Samples

Zr-BEA-0.7 degassed

Zr-BEA-3 degassed

Zr-BEA-5 degassed

Zr-BEA-10 degassed

Zr-BEA-15 degassed

R(Zr−O), Å

CN

Debye−Waller factor, Å2

Rf , %

2.03 2.25 2.03 2.26

4.0 2.0 4.0 2.0

0.0023 0.0023 0.0450 0.0026

2.5

2.09 2.30 2.11 2.31

4.0 2.0 4.0 2.0

0.0057 0.0005 0.0049 0.0049

7.0

2.05 2.22 1.98 2.20

4.0 2.0 4.0 2.0

0.0035 0.0006 0.0040 0.0005

2.7

2.08 2.24 2.07 2.23

4.0 2.0 2.0 2.0

0.0032 0.0001 0.0016 0.0032

4.0

2.07 2.24 2.08 2.24

4.0 2.0 2.0 2.0

0.0038 0.0020 0.0015 0.0053

2.4

2.7

4.0

0.7

2.9

2.9

D

DOI: 10.1021/acs.inorgchem.8b01548 Inorg. Chem. XXXX, XXX, XXX−XXX

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are occluded in Si-BEA crystals in the form of zirconium silicate particles. At longer synthesis times these particles undergo solid−solid rearrangement and redistribution in the zeolite crystal, leading to Zr incorporation in the zeolite framework and formation of Zr-BEA. 3.4. Formation of Zr-Containing Lewis Acid Sites. The formation of Lewis acidic sites was studied by FTIR of adsorbed CO (Figure 6). This technique proved to be very

Figure 4. HAADF-TEM-FIB images of the cross sections of ZrBEA-3 (A), ZrBEA-5 (B), and Zr-BEA(10) (C) crystals and corresponding EDS maps showing Zr element distribution: ZrBEA-3 (D), ZrBEA-5 (E), ZrBEA-10 (F). The brightness of the gradient images is not quantitative.

aggregates are detected. Line scan data (Figure S9) confirm a homogeneous Zr distribution for samples Zr-BEA-5 and ZrBEA-10 and point to surface enrichment with Zr in the case of sample Zr-BEA-3. The average Si/Zr ratio is found to be close to 200 in accordance with those expected from gel composition and XRF data. A more detailed study of the Zr-BEA-3 sample by STEMEDS is depicted in Figure 5. The results show that Zr-rich

Figure 6. (a) Difference FTIR spectra in carbonyl stretching region of CO adsorbed at −196 °C over Zr-BEA samples. (b) Variation of crystallinity (●) and relative contents of open (▲) and closed (■) sites with synthesis time.

efficient in differentiation between open and closed Zr sites, which was shown by experiments13 and by DFT calculations.6 The bands of CO adsorbed on open Zr sites are usually located at 2185 and 2165 cm−1, while CO bonded to closed sites gives rise to a 2177 cm−1 band. The spectra of CO adsorbed over ZrBEA-3 and Zr-BEA-5 samples show only two bands at 2157 and 2139 cm−1, which in accordance with previous studies6,13 correspond to CO attached to a silanol group and physically adsorbed CO, respectively. No bands of CO adsorbed on Lewis acid sites are detected over these samples. These bands appear only in the spectra of the samples crystallized for longer periods of time. The Zr-BEA-10 sample shows only a band corresponding to closed sites (2177 cm−1), whereas samples Zr-BEA-15 and Zr-BEA-20 reveal bands corresponding to both closed (2177 cm−1) and open sites (2185 and 2165 cm−1). Variation of the relative contents of open and closed sites with synthesis time is compared with the crystallization curve of zeolite BEA in Figure 6b. The results indicate that Lewis sites appear only after full crystallization of the BEA phase. Pyridine adsorption data (Figure S11) also confirm that strong Lewis acid sites appear in Zr-BEA samples only after 10−15 days of synthesis. According to XRD, NMR, and other data, crystallization is almost complete after ca. 5 days, while no Lewis sites are detected at this stage of synthesis. This observation is in line with XAS and TEM-FIB data and suggests that Lewis sites are generated only by Zr atoms incorporated in the framework tetrahedral positions of zeolite BEA structure. The Zr-rich silicate species occluded in the zeolite at the early steps of synthesis do not produce any Lewis acidity. An increase in open site content in the latter steps of synthesis is accompanied by a decrease in the number of closed sites (Figure 6b). This could be due to the transformation of

Figure 5. STEM-EDS analysis in selected regions of the Zr map of crystal cross-section for the Zr-BEA-3 sample (corresponding Si/Zr ratios are reported on the right).

fragments (points 1−5) have Si/Zr ratios within 5−52. Since particles in points 3−5 (Figure 5) are located inside the crystal, the EDS signal in these regions comes from the whole depth of the foil, leading to the apparent increase of silicon content. The particles localized on the external surface (points 1 and 2) have Si/Zr ratios of 5−9, which corresponds to highly zirconium enriched silicates. To probe the local structure of Zr-containing areas in the ZrBEA-3 sample HRTEM techniques were used. Figure S10 shows a HRTEM image with the corresponding fast Fourier transform (FFT). FFT is used to obtain an analogue of electron diffraction patterns for phase identification. On the basis of FFT the following d spacings are estimated: 3.83, 2.98, 2.63, 2.32, and 1.92 Å. This set of diffraction maxima matches with a known zirconium silicate structure (PDF 85-0659 and 33-1485) or tetragonal zirconium oxide (PDF 42-1164). Thus, putting together HRTEM and XAS data, we can conclude that at the initial steps of synthesis zirconium species E

DOI: 10.1021/acs.inorgchem.8b01548 Inorg. Chem. XXXX, XXX, XXX−XXX

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HRTEM measurements. These species are located predominantly at the external surface and subsurface layers of the crystal, as well as along the diagonal axes of the crystal. At that stage, zirconium is mainly in a square-bipyramidal coordination and has no detectable Lewis acidity. At longer synthesis times, zirconium-containing particles dissolve under the action of reaction media and start to react with Si-BEA. This leads to Zr redistribution in the crystal. Transport of Zr-containing species can be caused by the presence of defects or cavities in the Si-BEA material. After 5 days of synthesis Zr distribution in the crystal becomes homogeneous, as visualized by STEM-EDS. The Zr−O bond distances slightly decrease; however, the coordination number does not change. EXAFS measurements point that Zr occupies mainly square-bipyramidal positions. FTIR of adsorbed CO does not reveal the formation of Lewis sites. Most likely, zirconium forms highly dispersed extraframework clusters of zirconium silicate in BEA cavities or in defect nests of the zeolite structure. A further increase in synthesis time up to 10 days finally leads to the incorporation of Zr into the framework T positions. This is revealed by XAS measurements, confirming tetrahedral coordination of Zr after sample dehydration. FTIR of adsorbed CO indicates that incorporation of Zr atoms into the zeolite framework is accompanied by the formation of closed zirconium Lewis sites. Closed sites are partially converted into open sites at longer synthesis times (during 15−20 days of synthesis). The unraveling of the mechanism of Zr-BEA crystallization provides important information on the formation of Lewis sites in heteroatom-substituted zeolites and shows the way toward their tuning and mastering. It is demonstrated that the rate of Zr incorporation and Lewis site formation is much slower in comparison to the rate of zeolite crystallization. As a consequence, fully crystalline Zr-containing BEA zeolite may be catalytically inactive, if the time of hydrothermal treatment is not sufficient. The overall content of Lewis sites can be tuned by variation of the synthesis time. Furthermore, the results show that the time of hydrothermal treatment also affects the ratio between open and closed Lewis sites. Longer hydrothermal treatment leads to materials with a higher contribution of open sites. Therefore, to obtain catalysts with optimal Lewis acidic properties required for specific catalytic applications, the duration of the hydrothermal treatment should be optimized.

closed sites into open sites via Si−O−Zr bond cleavage under the action of hydrogen fluoride, as a mineralizing agent:

This process is favorable, since the formation of open sites should decrease the structure tension originating from the rather large difference in Zr−O and Si−O bond lengths. 3.5. Mechanism Proposal for Zr Incorporation. On the basis of a detailed multiple-technique characterization of the samples obtained at different synthesis times we propose the mechanism of crystallization depicted in Figure 7. At the initial

4. CONCLUSIONS It is demonstrated that the formation of Zr-BEA in the course of fluoride-mediated hydrothermal synthesis proceeds in two steps. In the first step, pure silica BEA is rapidly crystallized via a solid−solid hydrogel rearrangement mechanism. Zirconium species are incorporated in the form of zirconium silicate particles occluded in Si-BEA crystals, as visualized by STEMEDS and HRTEM of crystal cross-sections. XAS measurements point out that zirconium is hexacoordinated at this step of synthesis and occupies square-bipyramidal positions in zirconium silicate fragments. Such zirconium species do not yield Lewis acid sites, as confirmed by FTIR spectroscopy of adsorbed CO. In the second step of synthesis, Zr-rich fragments start to interact with Si-BEA via a solid−solid rearrangement mechanism, which leads to formation of Zr-BEA with isolated, tetrahedrally coordinated Zr atoms located in T positions of

Figure 7. Proposed mechanism of Zr-BEA crystallization.

stages of synthesis, the crystallization of the pure-silica BEA phase occurs via a solid−solid hydrogel rearrangement mechanism.17,24 After 3 days of synthesis, the crystallization of Si-BEA is nearly complete, and the degree of crystallinity reaches ca. 85%. In the course of relatively fast Si-BEA formation, Zr-containing species are occluded in the form of zirconium silicate fragments, as confirmed by XAS and F

DOI: 10.1021/acs.inorgchem.8b01548 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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the zeolite framework. FTIR spectroscopy of adsorbed CO points that zirconium incorporation into T positions of the zeolite structure is accompanied by the formation of closed Lewis Zr sites. At longer synthesis times, closed Lewis Zr sites are transformed into open sites. These results suggest that the overall content of Lewis sites and the ratio between open and closed sites can be tuned by variation of the synthesis time. To obtain catalysts with optimal Lewis acidic properties required for specific catalytic applications, the duration of the hydrothermal treatment should be optimized.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01548. Extended experimental section, TG-DTA data, MAS NMR spectra, N2 sorption data, and XANES spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for I.I.I.: [email protected]. ORCID

Yan V. Zubavichus: 0000-0003-2266-8944 Irina I. Ivanova: 0000-0002-8742-2892 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the the Russian Science Foundation for financial support (Grant 14-23-00094). P.A.K. gratefully acknowledges Haldor Topsøe A/S for a Ph.D. fellowship. TEM measurements were performed using the equipment of the Shared Research Center FSRC “Crystallography and Photonics” RAS. XAS measurements were performed at the unique scientific facility Kurchatov Synchrotron Radiation Source.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01548 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.8b01548 Inorg. Chem. XXXX, XXX, XXX−XXX