Proton mobility, intrinsic acid strength and acid site location in zeolites

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Proton mobility, intrinsic acid strength and acid site location in zeolites revealed by VTIR and DFT studies Pit Losch, Hrishikesh R. Joshi, Olena Vozniuk, Anna Grünert, Cristina OchoaHernández, Hicham Jabraoui, Michael Badawi, and Wolfgang Schmidt J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11588 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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Journal of the American Chemical Society

Proton mobility, intrinsic acid strength and acid site location in zeolites revealed by VTIR and DFT studies Pit Losch1, Hrishikesh R. Joshi1, Olena Vozniuk1, Anna Gruenert1, Cristina Ochoa-Hernandez1, Hicham Jabraoui2, Michael Badawi2, Wolfgang Schmidt1* 1

Max-Planck-Institut für Kohlenforschung, Department of Heterogeneous Catalysis, Mülheim an der

Ruhr, 45470, Germany 2

Laboratoire Physique et Chimie Théoriques, UMR 7019 CNRS-Université de Lorraine, Saint Avold

57500, France

Keywords: acidity, acid site location, partial deuterium exchange, high temperature study, solid acids Abstract The intrinsic Brønsted acid strength in solid acids relates to the energy required to separate a proton from a conjugate base, for example a negatively charged zeolite framework. The reliable characterization of zeolites’ intrinsic acidity is fundamental to the understanding of acid catalysis and setting in relation solid Brønsted acids with their activity and selectivity. Here, we report an infrared spectroscopic study with partial isotopic deuterium exchange of a series of 15 different acidic aluminosilicate materials, including ZSM-5 zeolites with very few defects. Varying Temperature Infrared spectroscopy (VTIR) permitted estimating activation energies for proton diffusion. Two different proton transfer mechanisms have been distinguished for two different temperature ranges. Si-rich zeolites appeared to be promising proton-transfer materials (Eact. < 40 kJ.mol-1) at temperatures above 150 °C (423 K). Further, a linear bathochromic shift of the Si-(OD)-Al stretching vibration as a function of temperature was observed. It can be assumed that this red-shift is related to the intrinsic O(H/D) bond strength. This observation allowed to extrapolate and estimate precise ν(O-D)@0 K values, which could be attributed to distinct crystallographic locations through Density Functional Theory (DFT) calculations. The developed method was used to reliably determine the likelihood of position of a proton in ZSM-5 zeolites under catalytically relevant conditions (T > 423 K), which has so far never been achieved by any other technique. 1. Introduction:

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Zeolites are essential to guarantee society´s energy supply, as they allow for acid-catalyzed refining of biomass and/or crude oil.1,2 These solid acid catalysts are produced at Mt scale and enable b$ industrial processes. Reliable experimental and theoretical characterization of their intrinsic acid strength as an isolated parameter is however still challenging since it relies on basic probe molecules.3,4 Yet it is important to reach unbiased characterization of zeolite acid strength independent of surface curvature. The Brønsted acidic character of zeolites results from trivalent atoms (typically Al, but also B, Ga or Fe) substituting silicon atoms in tetrahedral positions in a crystalline silicate framework leading to local negative charges. It has been speculated that these acidic bridging hydroxyl (Si-(OH)-Al) sites, herein referred to as Brønsted acid sites (BAS) exhibit different intrinsic acid strengths depending on their concentration and local chemical environment (i.e. spatial separation of the next nearest Al site) as well as on zeolite topology-related parameters, such as Si-(OH)-Al bond lengths and angles. These hypotheses have evolved with the advent of increasingly sophisticated spectroscopic and theoretical techniques. Information on the intrinsic acid strength or O-H bond weakness of zeolitic BAS, and on their location under relevant conditions is extremely valuable for the description of the zeolite acid character independent of their pore topology, thereby enabling multi-parametric analysis and optimization of zeolite catalysts. For instance, superacidity of zeolite acid sites was long thought to be necessary to explain mechanisms occurring in alkane cracking or isomerization and conversion of methanol to hydrocarbons.4,5 The community later agreed that superacid-like reactivities in zeolite catalysis was possible not due to superacidity, but the lowering of transition state potentials via dispersion forces and electrostatic interactions with pore walls.6 We think that independent acidity and topology descriptors will allow addressing entirely new problems, such as the interesting questions whether the intrinsic acid strength is different in hierarchically porous networks compared to purely microporous ones, or whether acid strength is affected by crystal size. An early attempt to describe zeolites´ intrinsic acidities was reported by Mortier who applied Sanderson´s intermediate electronegativity model to predict the effect of proximity between Al sites in a 3D zeolite framework on the positive charge of protons.7 Later, Senchenya et al. showed in a quantum chemical study that the effect of bond angles and lengths can indeed have an effect on the apparent acidity for a reduced cluster. They reasoned, however, that under real catalytic conditions this effect will be averaged and protons on the whole will migrate to the lowest available potential wells. They further concluded that the chemical 2 ACS Paragon Plus Environment

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composition of a zeolite, i.e.; its Al/(Si+Al) ratio, is more relevant to its acid strength than the effect of bond angles and distances.8 Similar to Mortier´s study, Barthomeuf established correlations between intrinsic acidities and Al/(Si+Al) ratios and suggested the aluminum topological density as a descriptor of the intrinsic zeolite acidity.9 As mentioned, these insights were reached with advanced spectroscopic techniques that encompass, but are not limited to, magic angle spinning nuclear magnetic resonance spectroscopy (1H /

27Al

/

29Si

MAS NMR),10,11,12,13,14 infrared spectroscopy with or without probe molecules, and temperature-programmed desorption of basic probe molecules (B-TPD).15 Impedance spectroscopy was used to directly determine the proton conductivity, which can be directly correlated to the activation energy for translational proton transfer between two BAS within zeolites.16 Based on our understanding, such proton conductivities, especially at high temperatures, should depend on the distances between acid sites as elucidated in the study, but also on the intrinsic acid strength of a material. The great interest in developing further techniques for the fundamental understanding of zeolite acidity is underlined by inelastic neutron scattering (INS) at very low temperature (T < 30 K)17,18 and microcalorimetric adsorption-desorption techniques that rely on probe molecules19. While the latter technique is affected by Van der Waals (VdW) interactions, the former suffers from a different bias. INS in combination with computation is a powerful tool for localizing aluminum sites and even protons in a zeolite, albeit only at very low temperatures. Another recent technique to locate protons in materials developed by Palatinus et al. relies on electron diffraction and also requires extensive modeling.20 Derouane et al. comprehensively reviewed advantages and disadvantages of various acidity characterization techniques.21 It can be summarized that techniques which rely on probe molecules are affected by dispersion and electrostatic interactions.22 Hence commonly used acidity descriptors, such as adsorption enthalpies of probe molecules, are only relevant for catalysis if the probe molecules have similar dimensions, electronic and chemical properties as the substrate, transition state or product. Currently the most advanced method for describing solid state acidity without dependence on dispersion and polar interactions is based on theoretical calculations. Jones et al. correlated deprotonation energies (DPE) predicted by DFT to acid catalyzed reactions, e.g. methanol dehydration over concavely porous zeolites and convexly curved polyoxometallates (POMs) both exhibited similar DPEs, but different confinement effects. The difference between deprotonation products (H+), (Z-) and the neutral starting structure (HZ) equals to DPE: EDPE = E(Z-) + E(H+) - E(HZ). Careful interpretation of both theoretical and experimental results 3 ACS Paragon Plus Environment


permitted relevant insights into the combined effects of acid strength and confinement on the reactivity of solid Brønsted acids.23 More recently, ensemble averaging enabled calculation of averaged values that represent more realistic values and it could be confirmed that theoretical intrinsic acid strength is not depending on parameters such as bond angles and lengths or aluminum concentration.24 In contrast to what is described above, these landmark studies conclude that the mobility of protons in the presence of catalytic amounts of water and taking into account the isolating nature of the zeolite framework leads to the dependence between acid strength and the identity of the heteroatom species alone (Al vs. Fe vs. Ga etc.). Acid strength remains unaffected by the location, distribution, or density of these heteroatoms. Unfortunately, these DPEs remain a concept difficult to validate by experimental means. The main drawback of the theoretical approach regarding DPE is that removing a proton from the zeolite without any compensation (adsorption of other molecule or displacement of another proton) leads to a drastic destabilization of the framework. One could thus wonder whether the computed DPEs are then realistic. In microporous zeolites, distance between the positively polarized proton and the negatively charged channel wall can never exceed 2 - 8 Å. Weakly interacting protons will always interact with the delocalized negative charge of the pore walls, especially if one considers the nature of hydrogen bonds (bond distance of up to 2 - 4 Å and angles of 180 - 120°).25 Some attempts have nonetheless been made to address intrinsic acidity, namely by observing the Si-(OH)-Al bond by means of IR spectroscopy. Kazansky et al. suggested that extinction coefficients of acidic O-H bonds determined with a combined 1H-MAS-NMR and Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) approach are proportional to the intrinsic acid strengths. It was reasoned that extinction coefficients of vibrational bands in IR spectra are proportional to the first derivatives of the dipole moments of the corresponding chemical bonds relative to the normal coordinates of their stretching vibrations.26 This report is now presenting an isotopic exchange study with temperature variation. The partial H/D exchange allowed an application of an adapted varying temperature infrared spectroscopy (VTIR) model, permitting the estimation of apparent activation enthalpies for proton diffusion at different temperature ranges (348 – 423 K and 423 - 523 K). It is important to mention that Kreuer et al. reported in their theoretical work on perovskites, amongst others as proton conductors, an independence of activation energy for H or D transfer on isotopic effects was established.27 Therefore, H/D exchange is considered as a valid procedure to monitor H transfer. Silicon-rich zeolites seem to be promising proton transfer materials for high temperature (> 423 K) fuel cell applications. An unprecedented 4 ACS Paragon Plus Environment

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observation of a linear red-shift of the Si-(OD)-Al stretching signal as a function of temperature ΔνOD(T) was made.

Scheme 1. Zeolite acidity characterized without probe molecule. Isotope exchange allows the observation of the more temperature sensitive and localized O-D bond As illustrated in Scheme1, the O-D bond is likely to be more temperature sensitive and more localized compared to the O-H bond due to kinetic isotope effects (KIE). This red shift can be related to the intrinsic acidity of zeolites, as a more polarized bond will be more affected by temperature and lead to a stronger red shift. With the aid of DFT calculations the observation was explored to determine the likelihood of location of protons on specific sites in a zeolite structure at catalytically relevant conditions. 2. Results and Discussion A set of 13 commercial and self-synthesized zeolites was studied. Different Mordenite (Si/Al = 7 and 20), ZSM-5 (Si/Al = 13, 30, 45 and 100), Beta (Si/Al = 13, 18 and 75) and Faujasite (Si/Al = 2.5, 6, 65 and 170) zeolites were compared to standard solid acid materials such as alumina, silica, and Nafion. The textural characterization as well as NH3-TPD data of these materials can be found in Table S1 and Figures S1 in the supplementary material. Our experimental setup relied on a DRIFT cell connected to a mass spectrometer allowing to follow the H/D-exchange during the analysis. (Experimental details are described in the ESI and details are shown in Figures S2 – S4). The sample was first activated in the cell under dry N2 flow at 523 K before the H2O:D2O (1:1) mixture was fed for 10 minutes, followed by closing the system. Spectra at different temperatures were acquired by gradually lowering the temperature, while keeping the atmosphere unchanged. The cell was connected to a mass spectrometer for recording the isotopic distribution in the gas phase, first during the exchange, then at the different temperatures. Figure S3 shows a constant isotopic population in the gas phase during the measurement at high temperatures (regime III) while at lower temperatures water most probably starts condensing. The full range IR spectra of partially D-exchanged ZSM-5(100) zeolite synthesized as reported in literature.28 with only negligible amounts of EFAl species and almost no silanols 5 ACS Paragon Plus Environment


are reported in Figure 1A. These indicate that, in contrast to the O-H stretching region, the O-D stretching region between 2400 - 2900 cm-1 (Figure 1B) is more interesting for performing sensitive studies. Due to a kinetic isotope effect the observed signals are narrower. In Figure 1B two striking observations can be made; first a decrease in signal intensity with lower temperatures will be covered in the first part of this study, meanwhile a red-shift of the BAS signal with higher temperatures will be covered in the second part of this study. The above mentioned intensity variation can be linked to population change. In this context it is important to check whether we were merely observing altered populations of rovibrational states.29 In Figure S5A the first overtone signal for BAS(OD) is showing a decrease in signal intensity from 348 – 423 K, but the difference in signal intensity is significantly lower than the change in absorbance as a function of temperature (ΔAOD(T)) observed at 2400 - 2900 cm-1. The observed BAS(OD) intensity loss with lower temperature is general for the whole series of analyzed samples. As an illustration, in Figure S6, spectra of commercial zeolites ZSM-5(45) (Figure S6A) Mordenite(7) Figure S6B, Faujasite(65) Figure S6C and Beta(12.5) zeolite Figure S6D are shown and all exhibit the above described behavior.

Figure 1. DRIFT spectra of zeolite ZSM-5 with Si/Al = 100: A) Full range spectra with 1:1 exchange with H/D equilibrium at various temperatures (I) O-H stretching region (3300 – 3900 cm-1) and (II) O-D stretching region (2400 – 2900 cm-1); B) Enlarged O-D stretching region. From 423 to 523 K bathochromic shift of Si-(OD)-Al signal changes with increasing temperatures, while the Si-OD peak at 2750 cm-1 remains essentially unchanged. A very elegant spectroscopic method, VTIR developed by Garrone et al., allows determination of adsorption enthalpies using the Van't Hoff equation.30,31,32 It can be assumed 6 ACS Paragon Plus Environment

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that at sufficiently high temperature (typically the herein studied ranges), protons of all acidic OH groups must be in rapid exchange.26 A bond breaking-forming (H/D-exchange) event is observed in this study, therefore the VTIR model has been adapted (see section S.2.3) permitting the evaluation of the energy required for (H/D)+ diffusion. 𝐴

ln(𝐴𝑚𝑎𝑥@523 𝐾 ― 𝐴) =

𝛥𝑆° 𝑅

―

𝛥𝐻°

(1)

𝑅𝑇

The adapted VTIR equation plots the natural logarithm of the relative population of D-BAS sites at a certain temperature, given as the ratio between absorbance (A) on one side, and the difference between the maximum population (Amax@523 K) observed at 523 K and the considered absorbance (A) on the other hand, as a function of temperature (T). The data were extracted from the above presented data as illustrated in Figure S8. The observed chemical process illustrated in Figure S9 can be described as the D-BAS bond breaking. In our case the described ΔH° can be considered as an apparent activation enthalpy for the H/D exchange process. The slope of such a linear plot represents the apparent activation enthalpy (ΔH°.R-1). Different enthalpies are consistently observed in different temperature ranges (Low Temperature (LT): 348 - 423 K and High Temperature (HT): 423 - 523 K) (Figure 2), which is indicative for two distinct proton transfer mechanisms. The intercept (ΔS°.R-1) is not particularly considered herein, yet in principle it relates to the apparent activation entropy of the considered process. Without further details, ΔS° in the range of 423 – 523 K consistently exceed 100 J.(mol.K)-1 while ΔS° in the range of 348 – 423 K remain all below 50 J.(mol.K)-1. The observed transformation is likely to happen in a gaseous phase (high ΔS°) at high temperature, whereas it seems to occur in a more condensed state (low ΔS°) at lower temperatures.



Figure 2. VTIR plots for H/D-exchange on different zeolites as a function of temperature. A) Beta; B) Mordenite; C) Faujasite and D) MFI. (Note: Some zeolites could not be evaluated at low temperatures, (R2(HT) > 0.98, R2(LT) > 0.95) As mentioned above, apparent activation enthalpies for H/D exchange that were determined (Figure 2) indicate two different H/D exchange mechanisms depending on the temperature ranges. It is in agreement with literature to assume that at high temperatures higher apparent activation enthalpies are needed for proton diffusion due to low levels of hydration, opposing to low activation energies at low temperatures with higher hydration levels (see condensation of H2O and D2O at low T by MS Figure S4). In the well-studied case of Nafion, proton transfer activation energies have been linked to different mechanisms. For high hydration levels Eact. between 10 and 20 kJ.mol-1 are reported and up to 30 - 40 kJ.mol-1 for lower hydration levels.33 Our method applied to Nafion, leads to values in very good agreement with the data from literature. Furthermore, it could be concluded from our study that silicon8 ACS Paragon Plus Environment

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rich samples exhibit proton mobility close to that of Nafion, which is a standard material for proton exchange membranes for low temperature fuel cells. However, Nafion fails as a proton conductor in high temperature applications. From the data shown in Figure S9, it can be inferred that non-acidic amorphous silica is apparently also not a good proton conductor at elevated temperatures. Two mechanisms of proton transfer can be considered, i.e., water-chain (Grotthuß-like) mediated (quasi-)transport and a vehicular transport mechanism (diffusing hydronium ions, H3O+) or even direct intra-crystalline proton hopping. Considering MOR(7) in Figure 2B a third transfer mechanism seems to occur (423 – 453 K), however at the moment we cannot clearly see whether it is a third transfer mechanism or simply the simultaneous presence of both the above-mentioned mechanisms. These mechanistic hypotheses are supported by numerous works suggesting that zeolites at 348 – 423 K retain one to three water molecules at their acid site and thus may still contain H7O3+ trimers and H5O2+ dimers and H3O+ cations up to 423 K.34,35 Alberti et al. report a neutron diffraction study indicating that residual H2O participates in the H/D-exchange at the level of the acid sites.36 A previously mentioned Kreuer et al. conducted a theoretical study on H and D transfer and they concluded that ideal proton conductors are characterized by ´soft´ oxygen species, or weak O-H bonds.27 It is therefore reasonable to assume that proton conductivity can be improved with increasing intrinsic acid strength. Numerous, often costly, materials have been studied as potential proton conductor materials for the construction of fuel cell membranes, including perovskites,37 layered oxides,38 mesoporous silica with grafted sulfonic acid groups,39 graphene oxides,40 and metal organic frameworks (MOFs).41

Figure 3. Apparent activation enthalpies for proton transfer as a function of the Al content in the materials. A) LT range (348 - 423 K), ΔH° with error for Nafion as measured with our 9 ACS Paragon Plus Environment


technique is plotted as reference in blue; B) HT range (423 – 523 K), ΔH° with error for Nafion is plotted as reference in red. Several silicon-rich zeolites (Al.(Al+Si)-1 < 0.05 corresponding to Si/Al > 20) scatter at 30 – 60 kJ.mol-1. Lines are only added as guides to the eye. With an increasing interest in these materials, different standard procedures for the measurement of proton diffusion have been established. Impedance spectroscopy and 1HMAS-NMR at different temperatures are commonly used methods.16,38 Proton conductivity on zeolites investigated by means of complex impedance spectroscopy support our findings. Franke et al. determined activation barriers for proton transport in zeolites between 39 - 49 kJ.mol-1 in the temperature range of 373 – 473 K and 74 - 77 kJ.mol-1 for the temperature range of 573 – 773 K.16 Ryder et al. calculated activation energies for proton hopping in ZSM-5 zeolites in a temperature range from 200 - 1000 K. Taking into account the assistance of water in the proton exchange mechanism, they report activation energies as low as 8 - 30 kJ.mol-1 depending on different levels of theory of their models. On a purely theoretical basis they analyzed completely dry hopping for which a 90 - 150 kJ.mol-1 activation energy has been found.42 Sauer et al. meanwhile reported values of around 50 kJ.mol-1 in a Faujasite model.43 Finally, zeolites have already been reported as potent membrane material for methanol fuel cells.44 Given the interest in designing high temperature proton conducting membranes, this work adds a new easy and cheap method to characterize proton conductivity,45,46 but more importantly, with the selected series of materials we were able to show that Si-rich zeolites are most promising for high temperature proton membrane applications (Figure 3). As outlined above in Figure 1, a second observation was made in our experiments, namely a linear bathochromic shift with increasing temperatures of the OD and, for some samples, OH stretching vibration. In the case of ZSM-5(100), the BAS(O-D) stretch band shifts from 2659 cm-1 at 373 K to 2653 cm-1 at 523 K. Wavenumber shifts between different zeolite structures (3550 – 3650 cm-1) as a measure for acid strengths have been excluded over the last five decades, as the discrete IR signatures of different zeolite structures are not affected merely by bond strengths but also strongly by topological phenomena. In contrast to this notion, we are observing indeed a linear bond weakening of acid sites as a function of temperature. QM and MD simulations often do not accurately consider the effect of temperature on bond elongation and in general on bond strength. Even if the development of perturbation theories is an emerging branch of theoretical chemistry, reasonable estimates can also be made from 10 ACS Paragon Plus Environment

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first principles. It is well established for example that Lennard Jones potentials are influenced by temperature.47 Using this temperature dependence for the characterization of inherently weak bonds thus seems very attractive. The latter is well known for the characterization of hydrogen bonds in water. However, to the best of our knowledge, this has never been explored for solid acids. As mentioned, temperature sensitivity is due to weak bonds in acid-base (H-B) bonds for strong acids, in the same way as for hydrogen bonds in water. In these cases the internal energy of a bond is important to the overall bond strength, and the equilibrium bond distance is controlled by both, thermodynamic and quantum mechanical parameters. In general, covalent bond energies can be sufficiently well described by quantum mechanics. The above mentioned relative importance of the internal bond energy leads to a breakdown in the BornOppenheimer approximation, since the energy of the first vibrational excitation of a weak bond can be of the order of kT. According to Dougherty it can be assumed that a linear dependence of apparent hydrogen bond strengths and seemingly also acid bond strength on temperature can be expected.48 Based on these first principles, we assume relation (2) to be valid and interpreted our spectroscopic results accordingly. 𝐸(𝑂𝐷)(T) ∝ ν𝑂𝐷(T)

(2)



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Figure 4. ΔνOD(T) plot for bridging D-BAS of A) D-ZSM-5 samples; B) D-MOR samples; C) D-beta-samples; D) D-(US)Y-samples with different Si/Al-ratios for each.

(For all

regression lines, except D-USY(6) and D-Y(2.5): R2 > 0.93) Accordingly it is worth mentioning that over the last three decades it has become common knowledge in a different field of research on weak bonds in water, that strength of H bonds exhibits a linear relationship with respect to temperature.49,50 Hence, it is reasonable to assume that the weak bond energy of solid mineral acids is also responsible for a red-shift with higher temperatures, indicating a weakening bond due to variations in the internal energy of the bond. Figure 4 presents the temperature-dependent data. In Figure S10A the linear trend of νOD as a function of temperature is plotted with data available in literature. It can be seen that silanol signals exhibit only a marginal red-shift, whereas aluminols or Nafion sulfonic acid sites are not red-shifting at higher temperatures (Figure S10B). The data available in literature, which extend over a wider temperature range, seem to confirm this linear trend for acid aluminosilicates. The variation with temperature is not the same for each zeolite, yet it is consistently linear. One may argue that different slopes may indicate different 12 ACS Paragon Plus Environment

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variations in the internal bond energy and may thus be a direct hint for differences in intrinsic acidities. However, in our opinion the theoretical base for these assumptions is still relatively weak, and these hypotheses will constitute a part of our future investigations. In contrast we used this behavior to extrapolate values to zero Kelvin, i.e., the zero-point energy (ZPE) for the ZSM-5 zeolite samples. Only values for temperatures above 423 K were considered, in order to avoid any interference by residually adsorbed water molecules. At the point of zero energy, vibrations can be accurately calculated with DFT models. ZSM-5 is a zeolite with a rather complex unit cell exhibiting 12 distinct crystallographic tetrahedral Tsites and 26 different oxygen sites. So far no reliable technique is available to locate aluminum, let alone hydronium ions at elevated temperatures, which would be relevant to catalysis. Combining the extrapolated experimental data with DFT-calculated data now allowed an accurate determination of the location of Brønsted acid sites in zeolites under catalytic conditions. In order to avoid excessive recalculation of whole libraries of values, we performed an initial refinement. We used the two self-synthesized ZSM-5 samples having only very few defects and a very high crystal quality, ZSM-5(30) and ZSM-5(100) (Figure S11) because both exhibited a linear temperature dependency for the O-H and O-D stretching bands (Figure 5A and 5B). The extrapolated ν(OH)@0 K were used to determine an approximate isotopic shift at the ZPE of 950 ± 20 cm-1. This shift could now be added to the ν(OD)@0 K of all ZSM-5 samples, including commercial ones, since ν(OD)@0 K could be determined for all of them. This step allowed us to exclude from further consideration roughly 60% (see Table S2) of the possible sites in the MFI structure, thanks to the extensive work of Jones et al.24 The remaining acid sites were independently calculated and vibrations directly extracted for the O-D bonds (details in section S2.1, S2.2 and Table S3).



Figure 5. Linear extrapolation of experimental data for ΔνOH(T) in red and ΔνOD(T) in black to 0 K for A) Model MFI(30); B) Model MFI(100); C) Commercial MFI(45); D) Commercial MFI(13). The results of our models are summed up in Table S3. These values were essential for the determination of discrete acid site locations. While the topic of Al siting in zeolites remains of current interest,51 INS on the other hand, albeit enabling the determination of the location of H cations in zeolites, is only applicable if protons are literally frozen in the zeolite, at 5 – 30 K, close to the ZPE. More relevant to catalysis and sorption processes are conditions where proton mobility needs to be assumed. Owing to our experimental setup and the control over parameters during the measurement, we are confident that our technique permits this. Comparison of the vibrational energies from the DFT calculations with the extrapolated ν(OD)@0 K from our measurements reveals that only a few O-(H/D) sites fit to the experimental data. The most likely positions for Brønsted acid sites in the herein evaluated ZSM-5 samples with Si/Al ratios of 13, 30, 45 and 100 are summarized in Table 1.


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Table 1. Extrapolated ν(O-(H/D))@0 K corresponding to various crystallographic locations of H+/D+. 24 Zeolite(Si/Al) Extrapolated

Likely position of (H/D)+

ν(OD)@0 K / cm-1 ZSM-5(13)

2682 ± 4

Al3-O20; Al6-O5; Al10-O26; Al11O11; Al11-O22

ZSM-5(30)

2670 ± 4

Al1-O1; Al1-O15; Al1-O21; Al6-O6; Al9-O18

ZSM-5(45)

2671 ± 4


ZSM-5(100)

2671 ± 4


Figure 6A and 6B show the most likely positions for O-(H/D) sites in the Al-rich MFI zeolite (ZSM-5(13)) investigated herein. For Si-rich MFI zeolites (ZSM-5(30; 45 and 100)), the most likely locations for O-(H/D) sites are represented in Figure 6C and 6D. Besides the novel method to locate active sites in zeolites at catalytically relevant conditions relying on a combined systematic VTIR / DFT approach, the key finding of our work is also the evidence of populated exposed Brønsted sites (see Figure 6). Pure DFT studies suggest Brønsted sites hidden in small cavities, because of the presence of multiple stabilizing hydrogen bonds.22,52 However, at finite temperature, entropy effects and motion leads protons to be favorably located in more exposed locations. Clearly this is one of the reasons why protons are prone to react with molecules under catalytic conditions. The fact that we found the same likely positions of H+/D+ species for three different ZSM-5 samples strongly supports our approach (see Figure 6C and 6D and Table 1). Indeed, for Si/Al ratios of 30 and higher, all BA sites are isolated and it seems likely that similar sites are occupied preferentially. Regarding the MFI zeolite with the highest Al content (ZSM-5(13)), one should not neglect that protons could also be bound to two Al-O groups (extra-framework aluminum) may affect the nature of Brønsted sites which could as well affect the OH stretching vibrations. Chen et al recently evidenced the presence of Al-O-H-O-Al or Al-H3O+-Al species in low Si/Al ZSM-5 zeolites using isotopic H/D exchange and NMR experiments.12 However, regarding the extensive work on aluminum siting in MFI zeolites, preferential site occupation is likely but could as well be affected by synthesis parameters.53,54,13 The commercial ZSM-5(13) material is synthesized via a template-free route and different preferred Al-sites in that zeolite can be the result of the absence of TPA+ during the synthesis. This is in agreement with what Yokoi et al. report on MFI synthesized in the presence of TPA+ as an SDA opposed to the simultaneous presence of both, TPA+ and Na+.55 The presence of Na+ in the synthesis gel combined with the high Al-concentration is therefore the reason why we observe a different 15 ACS Paragon Plus Environment


acid site distribution for ZSM-5(13) than for the Si-rich samples. It will be interesting to study whether or not this strategy will allow to detect between non Löwenstein type acid sites, i.e. Al-O-Al connectivity which can be stabilized by a proton as it has been calculated.56,57 We plan to extend our method in the future to the location of Brønsted sites to different zeolite frameworks.

Figure 6. Framework scheme of A) and B) Al-rich ZSM-5(13) and C) and D) Si-rich ZSM5(30; 45 and 100) with highlighted positions of the highest likelihood of location of H/Dcations under catalytically relevant conditions (T > 423 K). 3. Conclusion The reliable characterization of Brønsted acidity in microporous aluminosilicates is fundamental to the understanding of acid catalysis and setting in relation solid Brønsted acids with their activity or selectivity. Intrinsic acid strengths under relevant conditions as well as location of Brønsted acid sites are particularly difficult to characterize. In our study, infrared spectroscopy on partially H/D exchanged zeolites allowed addressing a number of critical aspects of zeolite acidity. An analysis at varied temperatures (VTIR) permitted the estimation of apparent activation enthalpies for proton diffusion at different temperature ranges. It could 16 ACS Paragon Plus Environment

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be established that explicitly Si-rich zeolites could be promising H+-transfer materials for high temperature (> 150 °C / 423 K) fuel cell applications. A linear bathochromic shift of the Si-(OD)-Al stretching vibration as a function of temperature was also observed for all zeolites investigated. The observed ΔνOD(T) can be related to the intrinsic O-D bond strength and thus to intrinsic Brønsted acid strength. These ΔνOD(T) depend on the respective zeolite structures. For ZSM-5, extrapolation of experimental ν(OD) data to zero Kelvin and correlating those to data from DFT calculations allowed the determination of distinct locations of acid sites. In summary, IR spectroscopy of partially deuterium exchanged zeolites at different temperatures allowed for direct determination of a number of highly relevant aspects of zeolite acidity. The data were obtained without using any probe molecules and were thus not biased by dispersion forces. Acknowledgement We thank Marcel Blaj and Moritz Krebs for their valuable help in performing and repeating experiments. Bodo Zibrowius is acknowledged for sharing his experience during informative discussions. P.L. is thankful to the Alexander von Humboldt foundation for his fellowship. A.G. thanks the Fonds der Chemischen Industrie (FCI) for her fellowship and finally, the authors are grateful to the Max Planck Society for financial support. H. Joshi would like to thank the IMPRS-Recharge for financial support. H. Jabraoui and M.B. thank the PMMS (Pôle Messin de Modélisation et de Simulation) and GENCI-CCRT/CINES (Grant No. x2018-A0040910433) for providing computer time. Conflict of interest The authors declare no conflict of interest. Corresponding authors *Wolfgang Schmidt. E-mail: [email protected] Supporting Information Experimental procedures; details DFT modeling; table with textural and acidic properties of the zeolites used; tables with results of modeling; figures with NH3-TPD data, DRIFT cell, MS, DRIFT, VTIR, and 27Al MAS NMR data



Graphical Abstract for TOC

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Proton mobility, intrinsic acid strength and acid site location in zeolites

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