
==== Front
J Phys Chem C Nanomater Interfaces
J Phys Chem C Nanomater Interfaces
jy
jpccck
The Journal of Physical Chemistry. C, Nanomaterials and Interfaces
1932-7447
1932-7455
American Chemical Society

10.1021/acs.jpcc.3c04751
Article
Tracking Lattice Distortion Induced by Defects and Framework Tin in Beta Zeotypes
Bai Yunfei †‡
Taarning Esben †
https://orcid.org/0000-0002-5774-3564
Luthra Mahika §
Lundegaard Lars F. †
Katerinopoulou Anna †
Falsig Hanne †
Nova Ainara §
https://orcid.org/0000-0002-6052-6608
Martinez-Espin Juan S. *†
† Topsoe A/S, Haldor Topso̷es Allé 1, 2800 Kongens Lyngby, Denmark
‡ Aarhus University, Nordre Ringgade 1, 8000 Aarhus C, Denmark
§ Hylleraas Centre for Quantum Molecular Sciences, Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, Blindern, 0315 Oslo, Norway
* Email: JSME@topsoe.com.
18 09 2023
28 09 2023
18 09 2024
127 38 1927819289
14 07 2023
08 09 2023
© 2023 American Chemical Society
2023
American Chemical Society
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

The use of powder X-ray diffraction (PXRD) coupled with lattice parameter refinement is used to investigate the crystal structure of Sn-Beta materials. A newly developed semiempirical PXRD model with a reduced tetragonal unit cell is applied to obtain the characteristic crystallographic features. There is a robust correlation between lattice parameters and the concentration of tin and defects for materials prepared via hydrothermal (HT) and postsynthetic (PT) methods. With tin incorporation, PT Sn-Beta samples, which possess a more defective structure, exhibit an extended interlayer distance in the stacking sequence and expansion of the translation symmetry within the layers, leading to larger unit cell dimensions. In contrast, HT Sn-Beta samples, having fewer defects, show a minimal effect of tin site density on the unit cell volume, whereas lattice distortion is directly correlated to the framework tin density. Furthermore, density functional theory (DFT) studies support an identical trend of lattice distortion following the monoisomorphous substitution of T sites from silicon to tin. These findings highlight that PXRD can serve as a rapid and straightforward characterization method to evaluate both framework defects and heteroatom density, offering a novel approach to monitor structural changes and the possibility to evaluate the catalytic properties of heteroatom-incorporated zeotypes.

H2020 Marie Sklodowska-Curie Actions 10.13039/100010665 859910 document-id-old-9jp3c04751
document-id-new-14jp3c04751
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pmc1 Introduction

Beta zeolitic materials (*BEA topology) have been drawing attention from both academia and industry due to their unique catalytic properties, being used for hydrocarbon conversion, biomass valorization, or even removal of hazardous pollutants from the air.1−4 In this family of materials, we could include purely silicious Beta (Si-Beta), Brønsted acidic Beta (Al-Beta), and Betas with transition metals (e.g., Fe, Co, Cu, Sn, Zr, Ti, and Hf). Typically, Lewis acidic zeolites result from the incorporation of a tetravalent heteroatom, such as tin, zirconium, titanium, and hafnium, in the zeolite framework.5−8 These Lewis acidic materials have been employed in numerous reactions involving biomass due to their affinity to coordinate with hydroxyl and carbonyl groups.9,10 Among all of the heteroatom incorporated zeotypes, Sn-Beta is likely the most studied system because of its versatile applicability in reactions such as Baeyer–Villiger oxidation, Meerwein–Ponndorf–Verley–Oppenauer redox reaction, epoxide ring-opening, carbohydrate isomerization, and aldol and retro-aldol chemistries.11−17

Two widely used protocols for synthesizing Sn-Beta zeotypes are hydrothermal (HT) and postsynthetic (PT) methods. The hydrothermal approach, also known as the bottom-up method, involves simultaneous crystallization and tin incorporation during hydrothermal treatment.11 In contrast, the postsynthesis approach, or top-down method, entails the removal of original aluminum atoms from a parent Al-Beta zeolite, followed by the introduction of tin into the resulting vacancies.18 Although both methods lead to successful tin incorporation, they produce Sn-Beta zeotypes with distinct structural and catalytic properties. Fluoride HT synthesis typically yields defect-free and highly hydrophobic materials with large crystals and high crystallinity, but long crystallization time and use of toxic reagents limit its large-scale industrial production.19 In contrast, PT Sn-Beta typically has a more defective and hydrophilic structure, it requires a shorter synthesis time, and it can be prepared with high tin loading, making it more attractive for large-scale production.20−22 However, tin incorporation might not be completely effective, and as a result, tin oxide can be formed as extraframework species, which have been reported in both HT and PT Sn-Betas. Typically, the fraction of tin in the framework decreases with tin loading.19,22−24

Previous structural studies have provided comprehensive information on the zeotype structure and chemical environment of the active sites.25 Two types of tetrahedral tin species have been reported in Sn-Beta zeotypes: closed tin sites, which are tetrahedrally coordinated to four zeolitic framework oxygen atoms, and open tin sites, which are coordinated to three zeolitic oxygen atoms and one extraframework hydroxyl group.26 Josephson et al. showed that the most stable geometry for these open sites is the ones in which the two −OH groups are opposite to each other (open-out configuration), rather than adjacent (open-in configuration).27 The 3D structures are displayed in Scheme 1 to show the different configurations of those tin sites. Experimental works have also shown that the different environments around the active sites result in a distinct reactivity.28,29 For instance, the open tin sites show stronger Lewis acidity and higher activity for reactions such as glucose isomerization.30,31 The synthesis method has a large influence on the generated tin sites, as the more hydrophilic PT Sn-Betas have commonly a higher open-to-closed tin sites ratio compared to more hydrophobic HT Sn-Betas. It is important to note that a dynamic view of the active sites has been pointed out more recently by different research groups, where interconversion between open and closed sites can occur upon hydration/dehydration.32−35

Scheme 1 3D Structures of Closed Tin Sites (a) and Open Tin Sites with Open-Out (b) and Open-In (c) Configurations at the T2 Position within the Sn-Beta Framework

The active framework sites possess diverse coordination environments and can be distributed among nine distinct crystallographically unique T-site positions in the Beta framework, which significantly complicates structural analysis.2,8,10,26 Numerous efforts have been devoted to studying structure–activity correlations of Sn-Beta. The main studies in this field primarily rely on techniques such as Fourier transform infrared (FTIR) spectroscopy, solid-state nuclear magnetic resonance (SS-NMR), and temperature-programmed desorption (TPD), often using probe molecules for greater sensitivity and specificity.2 TPD is widely used to quantify the acid sites as well as measure their strength using probe molecules. With certain adsorbates, TPD provides information for stoichiometric complexes; however, this method is normally not able to clearly distinguish different types of acid sites with fairly similar strength, such as those in Sn-Beta.36,37 FTIR studies have mainly focused on the investigation of silanols and acid sites. Pyridine and deuterated acetonitrile are two of the most widely used probe molecules, which could identify the Lewis/Brønsted acid sites and open/closed tin sites, respectively.26,38 Silanols and defects of Sn-Beta could also be observed using FTIR by checking either the −OH vibration region or silanol–probe molecule interaction.39 With solid-state NMR, framework tin atoms could be observed directly at an atomic level, making it possible to identify the coordination status and different T sites.25,32,40 However, as the most powerful tool for Sn-Beta characterization, solid-state NMR is limited by high equipment and operation cost. Additionally, the most used isotope for tin NMR study, 119Sn has a natural abundance of only 8.7%, meaning the NMR scan of Sn-Beta could be time-consuming and normally with low resolution.41

From a crystallographic point of view, zeolite Beta is one of the most complex materials in the zeolite family. This structure was independently determined by the groups of Newsam and Higgins et al.42−44 as a stacking-disordered intergrowth of two ordered polymorphs A and B. In the context of reporting and comparing lattice parameters, it is more appropriate to avoid referencing the unit cells of the ordered Beta zeolite polymorphs as they do not represent the actual disordered material. Instead, it is preferable to focus on the translation symmetry within the layers, where a = b = ∼12.6 Å, and the average distance between the layers in the stacking sequence, c = ∼6.6 Å. In this work, a recently published semiempirical PXRD model with a reduced tetragonal unit cell was used to calculate the lattice parameters.45 This model facilitates the fitting of sharp Bragg reflections in the XRD pattern in a Pawley-type fit, while the broad, diffuse peaks are fitted individually to determine their intensities and positions, allowing precise lattice parameter determination and quick analysis for lattice distortions.

Focusing on tin incorporation, previous theoretical studies have predicted that after tin substitution the typical Sn–O average bond length fluctuates from 1.938 to 1.949 Å, depending on the specific T location. These values are longer than those of the Si–O bond, 1.612 Å. Additionally, the Si–O–Sn bond angles are 2°–10° lower compared with Si–O–Si, and the radius change after heteroatom substitution also influences lattice geometry.7,46 Experimentally, changes in the diffraction pattern were observed after dealumination and tin impregnation for PT Sn-Beta zeolites. It has been proposed that the shift of the diffraction peak (302) is connected to crystallographic features within zeolite framework.13,47 However, these methods do not directly reflect the unit cell parameters. The XRD refinement method has been also applied to evaluate the ion-exchange status of zeolites, and the results show strong correlations between the concentration of exchanged ions in the unit cells and their lattice parameters.48−50 Furthermore, changes in the unit cell were recently found to be directly related to the deactivation of various type of zeolites, such as MFI, FER, and CHA, caused by in the accumulation of carbonaceous deposits during the methanol to hydrocarbon reaction.51−54 XRD has also been used extensively over the other most commonly studied metallosilicate, TS-1 (MFI). The titanium content in TS-1 is reported to influence the transition temperature between orthorhombic and monoclinic phases and also change the unit cell volume.55,56

Herein, we focus on the effects on the crystal structure change of Sn-Beta materials upon its preparation. The lattice parameters are evaluated by XRD-LeBail fit using a semiempirical model, with both PT and HT Sn-Beta included. Moreover, FTIR and deuterated acetonitrile adsorption experiments were conducted to show the defects, Lewis acid sites, and their effects on crystal lattice distortion. With DFT calculations of monosubstituted Sn-Beta, we also provide theoretical evidence showing the trend of lattice distortion. Interestingly, our results demonstrate that Sn-Betas prepared with different synthesis protocols possess different features, and both tin and defects concentrations show clear correlation with structural changes. We believe this work might inspire further studies on other metallosilicate materials to understand better their physical structure.

2 Experimental Methods

2.1 Catalyst Preparation

Two sets of Sn-Beta zeotypes were synthesized using previously reported HT and PT methods with minor adjustments.11,57 In a typical HT procedure, 30.6 g of tetraethyl orthosilicate (TEOS, Aldrich, 98%) was mixed with 33.1 g of tetraethylammonium hydroxide (TEAOH, Sigma-Aldrich, 35% in water) under stirring for 60–120 min, until a homogeneous solution was formed. Afterward, a specific amount of tin(IV) chloride pentahydrate (SnCl4·5H2O, Aldrich, 98%) was then dissolved in 2 mL of demineralized water and added to the solution dropwise to control the nominal tin content within the range from 0.3 to 2.6 wt %. The mixture was stirred until a viscous gel was formed. Afterward, 3.1 g of hydrofluoric acid (HF, Fluka, 47–51%) was diluted with 1.6 g of demineralized water and added to the synthesis gel, forming a brittle solid precursor with the approximate composition of 1.0Si:xSn:4.0xCl–:0.55TEA+:0.55F–:7.5H2O. The obtained precursor was then crushed and transferred into an autoclave with a Teflon liner. The hydrothermal process was conducted at 140 °C until full crystallization was achieved. The as-synthesized samples were named HT xSn-Beta, where x represents the tin content in the weight percentage range from 0.3 to 2.6 wt %. The obtained precursors were filtered, washed with abundant water, dried at 80 °C overnight, and calcined in air at 550 °C for 6 h using a temperature ramp of 5 °C/min. Additionally, two blank samples, Si-Beta and SnO2 Si-Beta, were prepared. Si-Beta was prepared in the absence of any tin source, and SnO2 Si-Beta was prepared by incipient wetness impregnation of obtained Si-Beta with tin(IV) chloride solution, with a tin content of 1.1 wt %.

The synthesis of PT Sn-Beta used a commercial Al-Βeta zeolite (Zeolyst, NH4+-form, Si/Al = 19) as the precursor. In the first step, Al-Βeta was calcined at 550 °C (3 °C/min) for 6 h to obtain the proton form. The sample was subsequently dealuminated using a HNO3 solution (Sigma-Aldrich, ≥65%) at 80 °C for 12 h with the acid to zeolite weight ratio of 10 g/1 g. Afterward, the dealuminated sample (DeAl-Beta) was recovered and washed with deionized water until neutral pH. Subsequently, the sample was dried at 80 °C overnight. The obtained DeAl-Beta was then impregnated using the incipient wetness method with a tin(IV) chloride solution to achieve specific tin loadings of 0.1, 0.5, and 1.5 wt %. The DeAl-Beta and impregnated PT Sn-Beta precursors were then dried at 110 °C overnight. The precursors were calcined at 550 °C (3 °C/min) for 6 h to complete the PT synthesis process and were subsequently used for ex situ characterizations.

2.2 Catalyst Characterization

Transmission FTIR experiments with deuterated acetonitrile (CD3CN) adsorption were performed using a Vertex 70 spectrometer. Before the experiment, samples were prepared by pressing around 25 mg of sample powder into pellets of 1 cm diameter and then put into copper envelopes made for the quartz testing cell with KBr windows. Samples were pretreated under vacuum overnight at 450 °C and then cooled to room temperature. The background and blank spectra were acquired under vacuum at room temperature, and afterward, CD3CN was introduced into the cell. During experiments, the samples were first saturated with high CD3CN pressure (above 3 mbar), and the desorption pressure was controlled stepwisely from saturation pressure to around 1 × 10–2 mbar. Under 0.25 mbar of CD3CN pressure, the characteristic peaks of the gas phase and physically adsorbed CD3CN are not as intense, while Lewis acid sites and silanols remain fully titrated and easier to deconvolute. Spectra in this work were in the range of 4000–500 cm–1, by accumulating 128 scans at 2 cm–1 resolution. The obtained spectra were normalized to the zeolite tetrahedral unit vibration region (around 2200 to 1800 cm–1) to directly compare quantities of adsorbed probe molecules among samples, and the background was subtracted from the blank spectrum before adsorption. FTIR data were analyzed using Bruker OPUS software and OriginPro 2023.

Elemental analysis of HT Sn-Beta was conducted to measure tin and silicon contents by an X-ray Fluorescence (XRF) technique, using a Supermini 200 (Rigaku) instrument. The morphology of the HT zeotypes was observed by scanning electron microscopy (SEM), using an XL30 field emission scanning electron microscope equipped with an energy selective backscattered (ESB) detector. For PT Sn-Beta samples, the elemental analysis was performed by inductively coupled plasma–atomic emission spectroscopy (ICP-OES) on a PerkinElmer model Optima 3000 (Varian Vista), and the resulting spectra were compared against calibrations of the investigated elements.

2.3 X-ray Powder Diffraction and Rietveld Refinement

The materials were analyzed by X-ray diffraction using a Panalytical X’Pert Pro instrument system in Bragg–Brentano geometry working in reflectance mode using Cu Kα radiation (λ = 1.541 Å) equipped with a monochromator and Soller, divergence, and antiscatter slits. A portion of a powder sample was mounted into a standard XRD sample holder. The sample was measured in a horizontal spinning sample stage over the 2θ scan range 5°–70° with a step size of 0.017° in ambient air. The resulting diffraction patterns are analyzed using the TOPAS software from Bruker. A semiempirical model was used for Rietveld analysis, where the sharp diffraction peaks are fitted by a LeBail fit and the tetragonal space group P4/m, whereas diffuse diffraction features are fitted by individual peaks. Details of the semiempirical model and calculation are described by Lundegaard et al.45 Standard deviations on the XRD measurements were calculated over HT Si-Beta with multiple measurements and analyses, as presented in Table S4.

2.4 Computational Methodology

Periodic density functional theory calculations were performed using GPAW software in the ASE framework to investigate the geometry change of the unit cell of Polymorph A upon monosubstitution of tin into the zeolite framework.58−60 A plane-wave basis set with a cutoff energy of 800 eV was used.59 The exchange-correlation functional was set to the BEEF-vdW approximation.61 The Fermi–Dirac distribution was used to model the electronic occupations, with a smearing parameter of 0.1 eV. The k-point grid was set to (1,1,1).62 The optimizations were performed using the Broyden– Fletcher–Goldfarb–Shanno (LBFGS) optimizer.63 The structural relaxation of Beta and Sn-Beta was performed in two steps: First, the cell shape was fully relaxed by optimizing the positions of the atoms, while keeping the volume fixed. This was done until the forces on the atoms were less than 0.03 eV/Å. Second, the optimized structure obtained from the first step was used as the starting geometry to reoptimize the unit cell volume with the use of the stress tensor. Here the forces converged to below 0.005 eV/Å.

3 Results and Discussion

3.1 Catalyst Characterization

Elemental compositions of both HT and PT Beta zeotypes are shown in Table 1. The actual silicon/tin ratios are close to that of the nominal values for all the HT Sn-Beta, indicating there was little loss of tin source during synthesis. As shown in Figure 1, SEM images of HT Beta zeotypes with tin content of 0.3, 1.1, and 2.1 wt % and tin-free Si-Beta show the typical capped square bipyramidal (CSBP) morphology of Sn-Beta zeolite.64 A clear trend of morphology change is also observed with increasing tin loading, in which the growth of pyramidal (h0l) faces is more significant with higher tin loading compared with pinacoidal (001) faces, leading to plate-like shape crystals, which is consistent with previous research.19 Tin oxide particles are also found on the Sn-Beta crystals, indicating the formation of extraframework tin oxide during the hydrothermal synthesis process, especially in samples with higher tin loadings. For PT Sn-Beta, a significantly smaller particle size was observed. Given that they were prepared from the same parent material (Al-Beta), PT zeotypes exhibit nearly identical morphologies and crystal size.

Table 1 Elemental Analysis of Zeolitic Materials Prepared

sample no.	sample	tin loading (wt %)	Si/Sn (molar ratio)	
1	HT Si-Beta	0	∞	
2	SnO2 HT Si-Beta	0.97	200	
3	HT 0.3Sn-Beta	0.29	672	
4	HT 0.5Sn-Beta	0.48	410	
5	HT 1.1Sn-Beta	1.05	185	
6	HT 1.3Sn-Beta	1.30	150	
7	HT 2.1Sn-Beta	2.06	94	
8	HT 2.6Sn-Beta	2.61	73	
9	Al-Beta	0	∞	
10	DeAl-Beta	0	∞	
11	PT 0.1Sn-Beta	0.10	1909	
12	PT 0.5Sn-Beta	0.53	357	
13	PT 1.5Sn-Beta	1.51	122	

Figure 1 SEM images of tin-free Si-Beta (a) and HT Sn-Beta zeotypes with tin content of 0.3 (b), 1.1 (c), and 2.1 wt % (d), parent material Al-Beta (e), DeAl-Beta (f), and PT Sn-Beta zeotypes with tin content of 0.1 (g) and 0.5 wt % (h) are shown.

XRD patterns of the HT Beta zeotypes are shown in Figure 2a. Despite variations in morphology and tin content among the samples, XRD analysis did not reveal any significant structural alterations within this group of samples at first sight. However, focusing on the region with 2θ of 25°–35°, exhibited in Figure 2b, two peaks attributed to extraframework tin oxide were observed in SnO2 HT Si-Beta, in which a tin source was impregnated onto a hydrothermal silicious Beta. For HT Sn-Beta samples with a tin content ranging from 0.5 to 1.3 wt %, SnO2 was undetectable using XRD. The sample with the highest tin content, HT 2.6Sn-Beta, is the only exception, where the signal of extraframework SnO2 started to appear, forming two small bumps at 27° and 34°. Collectively, for HT Sn-Beta, the tin source is effectively incorporated into the framework when the tin content is low. With higher tin content, a traceable amount of extraframework SnO2 could be detected by XRD, suggesting the incorporation of framework tin atoms is limited, also raising the necessity to examine the tin sites using more sensitive techniques (e.g., FTIR or NMR).

Figure 2 (a) XRD patterns of HT Beta zeotypes with different tin contents. (b) Zooming-in and stacking plots with 2θ of 25°–35°.

3.2 Lattice Distortion of HT and PT Sn-Beta Zeotypes

3.2.1 Structural Impact of the Sn-Beta Preparation Method

The analysis of lattice geometry, comprising the unit cell volume and lattice parameters, can provide insights into the study of Sn-Beta zeotypes. The refinement of XRD data allows for the acquisition of critical information on lattice changes resulting from ion exchange or heteroatom substitution, which can be used to establish correlations between active sites and catalytic performance.49,51,52 Herein, we aimed to investigate the correlation between tin incorporation and lattice dimensions by measuring a series of Sn-Beta zeotypes with varying tin loadings and synthesis methods. The resulting lattice parameters are presented in Figure 3 and Table S1. When discussing lattice parameters, we choose not to rely on the unit cells of ordered polymorphs as they do not accurately depict the intrinsic disorder of the material. Instead, we utilize the translation symmetry within the layers (a = b = ∼12.6 Å) and the average layer separation in the stacking sequence (c = ∼6.6 Å). To evaluate the overall changes in the geometry of the crystal lattice, we also use the ratio c/a, which gives an idea of the lattice distortion in the unit cell, as done by Lundegaard et al.45 Interestingly, the crystallographic features of the materials are significantly influenced by the preparation method.

Figure 3 Correlation between tin content and lattice parameters of the reduced unit cell, in which interlayer spacing, translation symmetry, the ratio between those two, and cell volume are shown in (a–d), respectively.

In all cases, we see that the average interlayer distance between the stacking sequence, c, is shorter for all HT Sn-Beta materials, whereas the translation symmetry, a, is larger than that in any of the PT Sn-Beta catalysts. The distortion descriptor, c/a, helps to emphasize these differences in the geometry of the crystallographic unit cell. Focusing on the HT sample series, the introduction of tin in the structure expands the interlayer spacing while reducing the translational symmetry within layers, maintaining similar volumes for the reduced unit cell. On the contrary, the PT series shows increases in both crystallographic parameters. Because of the substantial and robust differences between the absolute values for a and c between HT and PT Sn-Beta materials, even at similar tin loadings, another crystallographic arrangement different in this group of materials must be responsible for the observed differences. As highlighted in the introduction, PT Sn-Beta samples are typically more defective in nature than HT Sn-Beta zeotypes due to the preparation method, and we speculate that the different degree of defects could be responsible for the different degrees of distortion.

3.2.2 Defects and Framework Tin Induced Lattice Distortion

To systematically investigate the influence of framework tin sites and structural defects on the crystal lattice, we compare herein the distortion descriptor, c/a, to the density of framework tin and defects. We conducted FTIR characterization using deuterated acetonitrile (CD3CN) as a probe molecule to enable semiquantitative analysis of different species. First, the −OH vibration spectra prior to CD3CN adsorption are presented in Figure S1. In the case of PT Sn-Beta (sample 12), most of the framework and extraframework aluminum has been removed from the parent material (sample 9). Notably, for both Al-Beta and PT Sn-Beta, characteristic bands for hydrogen-bonded silanols are observed, while this signal is absent in HT Sn-Beta (sample 4). With 0.25 mbar of CD3CN pressure, FTIR difference spectra within the C≡N vibrational region for HT and PT Sn-Beta with similar tin content (1.3 and 1.5 wt %, respectively) are shown in Figure 4. As a comparison, the spectrum of the SnO2 Si-Beta is also shown. Following peak deconvolution, characteristic peaks of CD3CN interacting with incorporated open (2316 cm–1) and closed (2308 cm–1) tin sites were observed for all Sn-Beta zeolites.26 Compared to HT Sn-Beta, PT Sn-Beta displayed a peak corresponding to remaining framework Al atoms (2300 cm–1).65 Moreover, peaks corresponding to CD3CN adsorbed on silanol groups (2276 cm–1) and gas-phase/physically adsorbed CD3CN (2267 cm–1) were observed for all samples. The assignment of the signal at 2289 cm–1 is still controversial in the literature, but likely due to loosely grafted tin or small tin oxide clusters.66,67 For comparison, spectra of other samples presented in the article are shown in Figures S2–S4, in which Lewis acidity and defect densities could be tracked.

Figure 4 Three of the representative FTIR difference spectra within C≡N vibration regions of (a) SnO2 HT Si-Beta, (b) HT, and (c) PT Sn-Beta zeotypes at 0.25 mbar CD3CN pressure. Assignments: 2316 cm–1 open Lewis acid, Sn; 2308 cm–1 closed Lewis acid, Sn; 2300 cm–1 Brønsted acid, Al; 2289 cm–1 loosely grafted tin/tin oxide particles (speculative); 2276 cm–1 silanols, SiOH; 2267 cm–1 physisorbed CD3CN.66

As it was suspected that not only tin but also defects could influence the Sn-Beta crystal structure, Figures 5a,b illustrate the correlations between the lattice distortion and the integrated areas corresponding to total framework tin content (open and closed tin sites) and silanols. Focusing first on the effect of silanols, it is confirmed that PT Sn-Beta materials tend to be more defective in nature compared to HT Sn-Beta. In general, the increase in defects distorts Sn-Beta lattice with an increased c/a ratio. Together with Figure 3, these results suggest that the average distance between stacking layers of the Beta framework tends to increase with defect concentration.

Figure 5 Unit cell c to a ratio plotted vs silanol area (a), tin site peak area integration (b), and the ratio between open and closed tin sites (c).

When looking into the influence of the tin content, a linear lattice distortion is observed over the HT Sn-Beta series. Notably, three HT samples with tin content of 0.5, 1.0, and 1.3 wt % (samples 4, 5, and 6) have roughly a similar content in defects, and therefore the lattice distortion is likely attributed to the various levels of framework tin. In contrast, the PT Sn-Beta series does not reflect a clear lattice distortion as a function of tin content, although a certain change is observed compared with the starting dealuminated Beta material. We hypothesize that the larger framework flexibility provided by the defects in PT samples compared to HT samples could enable the heteroatom incorporation without affecting too much the crystal structure. Also, it seems likely that defects have a larger influence on the crystal lattice compared to tin incorporation on framework positions. An attempt to prepare a PT sample with higher tin loading was done (5 wt %), but it did not show more tin incorporation than the sample containing 1.5 wt % tin (Figure S5).

Another important feature of Sn-Beta zeotypes is the type of active sites. As discussed in the Introduction, the open and closed configurations of the Lewis sites are generally different in HT and PT samples. The samples in this article align with literature as shown in Table 2 and Figure 5c. Even though closed sites are more abundant in both HT and PT Sn-Betas, the fraction of open sites is substantially larger than that in PT Sn-Beta zeotypes. We consider that this difference in active sites can also have an impact on the crystal lattice. It is likely that open sites could have an impact on c/a distortion similar to the effect of defects as this type of site would lead to a more flexible crystal structure. Therefore, the offset between Sn-Beta zeotypes synthesized using different methods could be primarily impacted by defects, type, and quantity of active sites. To further investigate this hypothesis, the following section evaluates the impact on the crystal lattice of the type of active sites from a theoretical perspective.

Table 2 Peak Area Integration and Open to Closed Tin Site Ratio in HT and PT Sn-Beta Zeotypes in CD3CN Adsorption Experiments

sample no.	sample	tin content (wt %)	tin site integration	silanol integration	open/closed	
2	SnO2 HT Si-Beta	1.1 (extra-frame)	0	1.0	 	
4	HT 0.5Sn-Beta	0.5	2.0	2.5	0.05	
5	HT 1.1Sn-Beta	1.1	2.7	3.1	0.05	
6	HT 1.3Sn-Beta	1.3	4.2	2.5	0.06	
8	HT 2.6Sn-Beta	2.6	10.5	5.7	0.10	
9	Al-Beta	0	0	11.5	 	
10	DeAl-Beta	0	0	6.7	 	
11	PT 0.1Sn-Beta	0.1	0.3	5.0	0.83	
12	PT 0.5Sn-Beta	0.5	1.1	7.1	0.82	
13	PT 1.5Sn-Beta	1.5	3.7	6.9	0.38	

3.2.3 DFT Calculations of Monosubstituted Sn-Beta

As shown in Section 3.2.2, the systematic change of the HT Sn-Beta unit cell parameters observed in this study demonstrates a clear trend of lattice distortion resulting from tin incorporation, highlighting the need for theoretical investigation. Herein, a simplified polymorph A model was selected for DFT analysis because of its stability and simplicity. Additionally, it has a three-dimensional network of 12-ring pores, making it a suitable candidate for theoretical modifications. Afterward, the obtained results were converted into a stacking sequence model for comparison. To investigate the influence of tin sites, we performed tin monosubstitutions into the nine distinct T-sites using periodic DFT calculations. By substituting silicon with tin atoms at each T-site, we obtained nine models containing one tin atom per unit cell in a closed configuration. The computational models closely resembled the HT samples, as this method retains the structural integrity of the zeolite. The theoretically calculated lattice distortions were compared to the experimental distortions observed from the HT samples, as outlined in the previous section. The relative stability of the nine tin-monosubstituted structures shows small energy differences of approximately 2.2 kcal mol–1 with a preference for the T2 structure, as shown in Table S2. In addition to the closed configurations, we also studied open structures in which the Sn–O–Si bridge was broken to allow for the formation of Sn–OH and Si–OH in two different configurations: open-out and open-in, as shown in Scheme 1. The open-out configuration has the two OH moieties facing away from each other, while in the open-in configuration, they are facing toward each other. After conducting an energy survey of these two configurations at the T2 site, we found that the open-out configuration is 17.2 kcal mol–1 more stable than the open-in due lower electrostatic repulsion between the hydroxyl groups (Table S3). Therefore, the open-out configuration was used to study the distortion in the unit cell along with the closed configuration.

Based on the results obtained, we investigated the unit cell distortion of the Sn-Beta zeotypes. Figure 6 illustrates the change in unit cell parameters when going from Si-Beta to monosubstituted Sn-Beta with closed and open tin sites. Our findings indicate that the unit cell contracts along dimension a and expands along dimension c, resulting in an overall increase in the c/a ratio for all T sites after tin incorporation. Interestingly, this change is more pronounced for the open tin site compared to the closed one. These results are consistent with the experimental parameters obtained for the HT samples, which show an increasing trend in the c/a ratio upon tin insertion and contain a mixture of closed and open sites.

Figure 6 Lattice distortion of the Si-Beta lattice undergoes isomorphous tin substitution with closed and open configurations. (a, b), (c), and (d) show the variations in individual parameters, the ratio between c and a, and unit cell volume for the nine T sites, respectively.

Collectively, the incorporation of tin in the Beta zeolite framework leads to the distortion of the lattice, regardless of the type and location of the tin species. The DFT study demonstrates a higher trend of unit cell stretching (c/a ratio) with open tin sites, which is also observed experimentally for PT Sn-Beta samples, samples showing larger c/a ratios with a higher fraction of open sites. Moreover, the location of the T site in the framework also affects the degree of distortion, whereas all T sites followed the same trend of the c/a ratio change. These findings provide valuable insights into the structural changes that occur with the incorporation of tin into the Beta zeolite framework and emphasize the importance of considering the type and location of the tin species in understanding lattice distortion.

Even though the incorporation of other heteroatoms (e.g., Zr, Ti, Hf, etc.) is not the focus on this article, we believe that a similar methodology could be applied to understand better the metal insertion in such materials, which have been applied in different catalytic applications in the literature.8,68 Indeed, we observe in our preliminary results a different degree of lattice distortion depending on the nature of the metallosilicates (Figure S6), and further studies are ongoing to assess the wider applicability of the XRD method.

4 Conclusion

This study investigates the impact of tin incorporation and defective frameworks on Sn-Beta zeotypes using X-ray powder diffraction with lattice parameter refinement based on a semiempirical model. HT Sn-Beta samples possess a similar speciation of tin sites and a less defective nature. Therefore, the gradual changes in the crystal lattice upon insertion of Sn are linked to the introduction of the heteroatom in the zeolitic framework. While the subcell unit volume remains unperturbed with tin loading, the heteroatom induces the expansion of the average interlayer distance in the stacking sequence, c, and reduces the translational symmetry distances, a and b, resulting in the distortion of the crystal structure. In contrast, PT Sn-Beta zeotypes present a more defective nature and different tin speciation. Consequently, the crystal lattice is less rigid compared to that of HT samples, and smaller changes are observed upon tin incorporation. However, a clear distinct lattice geometry is found with a substantially higher c/a ratio, which is primarily associated with the higher density of defects and open tin sites. Theoretical evidence also confirms that tin incorporation and the type of tin sites will impact the crystal structure of the Sn-Beta zeotypes.

The present study provides novel insights into the effect of defects and tin incorporation on the crystal structure of Sn-Beta zeotypes through a relatively fast to operate technique such as XRD. Moreover, this method can be used as a complementary technique to understand different zeolitic Beta materials and has the potential to serve as a rapid means of evaluating deactivation caused by structural changes and acid site leaching, which could affect the crystal structure.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.3c04751.FTIR spectra within O–H and C≡N vibration of the representative Beta zeotypes, correlation between tin content and Lewis site area integration of CD3CN adsorption spectra, unit cell parameters calculated using reduced cell and semiempirical model, energy differences between T sites and lattice parameters calculated by DFT (PDF)

Supplementary Material

jp3c04751_si_001.pdf

Author Contributions

Conception and design: E.T., J.S.M.-E., Y.B. Data acquisition: Y.B., J.S.M.-E., M.L., A.K. Data analysis: L.F.L., A.K., J.S.M.-E., Y.B., M.L. Supervision: E.T., A.N., H.F.

The authors declare no competing financial interest.

Acknowledgments

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 859910. We thank Aino Nielsen (Topsoe A/S) for the help on FTIR characterization.

Abbreviations

XRD X-ray powder diffraction

TPD temperature-programmed desorption

TGA thermogravimetric analysis

NMR nuclear magnetic resonance

FTIR Fourier-transform infrared spectroscopy

DFT density functional theory.
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