Tracking the Chemical Transformations at the BrÃ¸nsted Acid Site upon

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Cite This: Chem. Mater. 2017, 29, 9030-9042

Tracking the Chemical Transformations at the Brønsted Acid Site upon Water-Induced Deprotonation in a Zeolite Pore Aleksei Vjunov,†,¶ Meng Wang,†,¶ Niranjan Govind,‡ Thomas Huthwelker,§ Hui Shi,† Donghai Mei,*,† John L. Fulton,*,† and Johannes A. Lercher*,†,∥ †

Institute for Integrated Catalysis and Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ‡ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States § Swiss Light Source, Paul Scherrer Institut (PSI), 5232 Villigen, Switzerland ∥ Department of Chemistry and Catalysis Research Institute, TU München, Lichtenbergstrasse 4, 85748 Garching, Germany S Supporting Information *

ABSTRACT: The structural changes induced by reversible formation of Brønsted acidic sites and hydronium ions with water in a zeolite with MFI structure are reported as a function of temperature using a combination of physicochemical methods and theory. In the presence of an ample concentration of water, the protons are present as hydrated hydronium ions (H3O+(H2O)n) that are ion-paired to the zeolite. Loss of water molecules hydrating the hydronium ions leads to an unstable free hydronium ion that dissociates to form the hydroxylated T-site. The formation of this SiOHAl species leads to the elongation of one of the four Al−O bonds and causes significant distortion of the tetrahedral symmetry about the Al atom. This distortion leads to the appearance of new pre-edge features in the Al K-edge Xray absorption near edge structure (XANES) spectra. The pre-edge peak assignment is confirmed by time-dependent density functional theory calculation of the XANES spectrum. The XANES spectra are also sensitive to solutes or solvents that are in proximity to the T-site. As temperature increases, the minor fraction of extra-framework Al present in the sample at ambient conditions in octahedral coordination is converted to tetrahedral coordination through the decoordination of H2O ligands.

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INTRODUCTION Zeolites, crystalline microporous tectosilicates, have outstanding shape and size selectivity for adsorbing molecules as well as transforming them in Brønsted and Lewis acid catalyzed reactions.1,2 The Brønsted acid sites of zeolites result from the substitution of Si4+ by Al3+ atoms at tetrahedral positions (Al Tsites) in the framework, leading to a negatively charged tetrahedron,3 which is charge-balanced by H+. The resulting O−H group has a heterolytic bond dissociation energy of approximately 1200 kJ/mol.4 Alternatively, the negative charge may also be balanced by other organic or inorganic cations, including NH4+, metal cations, or hydrated hydronium ion (abbreviated as [H3O(H2O)n]+). Zeolite protons, stabilized either as bridging SiOHAl groups in a dry state or as the confined hydronium ions in the hydrated state, act as Brønsted acids by transferring the proton to reacting substrate molecules.5 The intrinsic strength of these Brønsted acid sites has been vividly debated over the last decades. One of the key points in this debate has been the apparent discrepancy between the strongly covalent character of the zeolite SiOHAl group and its ability to protonate alkanes.6 In more subtle differentiations, the impact of the local chemical composition in the zeolite © 2017 American Chemical Society

framework on the acid strength of a particular SiOHAl group and the impact of the specific location of the SiOHAl group in the lattice on its ability to act as Brønsted acid have also been debated.7−9 Hence, a comprehensive description of the strength and the impact of the location of Brønsted acid sites is a long-standing objective. Summarizing the current state of understanding, the strength of Brønsted acid sites in zeolites does not rival those of super acids10−12 despite the ability to protonate alkanes.13−15 The surprisingly high propensity to form positively charged transition states and intermediates has, in contrast to super acids, been associated with the unique stabilization effect of the ion pairs16−18 that consist of the negatively charged lattice site and the positively charged organic moieties. In addition to the electrostatic interactions with the zeolite lattice, dispersion forces further stabilize the molecular species. Hence, the location of the site and the corresponding steric environment contribute markedly to the ability to form positively charged intermediates. Received: May 25, 2017 Revised: October 6, 2017 Published: October 6, 2017 9030

DOI: 10.1021/acs.chemmater.7b02133 Chem. Mater. 2017, 29, 9030−9042

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Chemistry of Materials

X-ray absorption near edge structure (XANES), as a means to differentiate the state of the SiOHAl group and the Al T-site with increasing concentrations of water41,32 and as a function of temperature. Time-dependent density functional theory (TDDFT)-based calculations are used to assign the spectral features observed in the experimental Al-XANES. IR spectroscopy is used to link this information to the molecular state of adsorbed water.

The intrinsic acid strength has been associated with the strain induced by bond angles of the tetrahedra involved in the formation of the SiOHAl group.8 However, theory as well as experimental results have shown convincingly that the location exerts a measurable, but minor, contribution to the strength of a Brønsted acid site.9 In contrast, the concentration of aluminum in the lattice and, therefore, the overall and local charge distribution dominate the measurable strengths of Brønsted acid sites.19 Recent kinetic studies of methanol dehydration provided a unique insight into the average strength of Brønsted acid sites in zeolites.20,21 In summary, the difference in acid strength between sites within a zeolite is minor, once the SiOHAl groups are sufficiently diluted to statistically ensure that Al−O tetrahedra only have Si−O tetrahedra in the next two neighboring positions (absence of next-nearest Al−O neighbors).22−25 The deviation from the statistical distribution of Al−O tetrahedra (paired next nearest neighboring Al−O tetrahedra) and their impact on acid−base and catalytic properties of zeolites have been extensively studied over the past decades.19,26−29 Obviously, the properties of bridging hydroxyl groups in zeolites and of hydrated hydronium ions in the presence of water differ markedly in their reactivity toward polar and apolar molecules. The specific properties will depend on the concentration and organization of water in the surroundings of SiOHAl groups, which in turn will influence the catalytic properties. While the adsorption of water has been studied before,30 the impact on aluminum has been understood to a far lesser degree. We address in this study, therefore, the interaction of the SiOHAl group with water at varying temperatures in order to understand the mechanism for the reversible formation of zeolite Brønsted acid sites and hydrated hydronium ions. The details of this chemistry are of high importance for the understanding of the catalytic activity of zeolitic materials in the conversion of organic reactants, not only because water is present as an impurity or solvent in many feedstocks but also because water is produced (such as in alcohol elimination reactions), and may, as a consequence, accumulate and form stable species in an active zeolite subtly depending on the reaction conditions.31−33 The protonation of water and the generation of hydronium ions in zeolites has been explored in great detail both experimentally and theoretically.34 For example, IR spectroscopy has been used to study the zeolite acid site−water interactions by monitoring the formation of spectral bands attributable to the OH-stretching and deformation vibrations.35,36,30 Current consensus is that a single water molecule is not protonated, while proton transfer is observed in all zeolites studied so far once at least two of the water molecules are present at an acid site.33 At higher chemical potentials of water, larger clusters form.33,37 What is unclear is the impact of hydronium ion formation and its cluster size on the local zeolite lattice as the number of H2O molecules at a particular SiOHAl site increases. This local rearrangement is important, because it sheds light on the potential hydrolysis of the Al−O tetrahedra (Al T-sites) at high temperatures, as well as the dissolution of Si−O tetrahedra (Si T-sites) at lower temperatures and higher water concentrations.38−40 Therefore, we use a combination of spectroscopic methods to probe the water-induced changes of the Al T-site and the SiOHAl structure in zeolite MFI. We use in situ X-ray absorption spectroscopy (XAS) and specifically the Al K-edge

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EXPERIMENTAL AND THEORETICAL METHODS

Zeolite H-ZSM-5. The zeolite H-ZSM-5 (Si/Al = 45) was obtained from Clariant in the H-form and treated with ammonium hexafluorosilicate (AHFS) to remove extra-framework Al following the procedure reported elsewhere.42 The resulting material had a minimal amount of extra-framework Al according to NMR characterization. This material contains 300 μmol g−1 Brønsted acid sites and 20 μmol g−1 Lewis acid sites, based on the quantification of IR spectra of adsorbed pyridine. Al K-Edge X-ray Absorption Fine Structure. The Al K-edge Xray absorption fine structure (XAFS) experiments were carried out at the PHOENIX II (Photons for the Exploration of Nature by Imaging and XAFS), elliptical undulator beamline at the Swiss Light Source (SLS) at the Paul Scherrer Institute (PSI), Switzerland. The PHOENIX II end station is located at the exit port of the X-Treme beamline.43 Briefly, an elliptical undulator serves as photon source, and a planar grating monochromator is used to generate monochromatic light. Sufficient rejection of high-order harmonics is achieved by an appropriate choice of the cutoff values of the monochromator. Energy calibration was achieved by setting an inflection point of an Al-foil spectrum to 1559.6 eV. The zeolite sample was pressed into a pellet ∼0.5 mm thick and placed in the EXAFS cell (see the Supporting Information). The cell described in the Supporting Information is mounted in a vacuum chamber with a pressure of ∼1 × 10−4 mbar. Dry He (99.995%) was flown through the XAFS cell continuously at a rate of 5 mL/min. The cell was heated using a temperature increment of 5 °C/min. Measurements were performed in fluorescence mode. I0 was measured as total electron yield signal taken from a 0.5 μm thin polyester foil, which was coated with 50 nm of Ni. This I0 detector was held about 1 m upstream of the sample in the beamline vacuum (∼10−6 mbar). The X-ray fluorescence was detected using a single-element Ketec Si-drift diode detector. ATHENA44,45 software was used during the background processing. XANES normalization follows standard protocol used within the Athena software. Briefly the pre-edge baseline is established as a linear line extending between −100 and −10 eV before the main edge. The postedge baseline (second order polynomial) was chosen between +25 and +150 eV above the edge. It is important that the starting point (+25) is at a higher energy than the XANES evaluation region in order to avoid distorting the XANES spectra. Optimization of Al T-Site Structures Using DFT. The calculations were performed following the concept outlined elsewhere.27,46−48 The CP2K program package49,50 was used to perform periodic DFT structure optimizations with a mixed Gaussian and plane wave basis set. Core electrons were represented with norm-conserving Goedecker-Teter-Hutter (GTH) pseudopotentials,51 and the valence electron wave function was expanded in a double-ζ basis set with polarization functions52 along with an auxiliary plane wave basis set with an energy cutoff of 360 Ry. The generalized gradient approximation exchange-correlation functional of Perdew, Burke, and Enzerhof (PBE)53 was used for all calculations. The Broyden-FletcherGoldfarb-Shanno (BFGS) algorithm with SCF convergence criteria of 1.0 × 10−8 a.u. was applied in the structural optimization. To compensate the long-range van der Waals dispersion interactions between water molecules and the MFI zeolite, the DFT-D3 scheme54 with an empirical damped potential term was used in all calculations. All calculated vibrational frequencies presented in this work are unscaled. The Bader charge analysis55 was used to characterize the charge distributions for the ion pairs of the hydronium cluster and MFI framework. 9031


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Figure 1. Sample weight loss (%) (blue) and the corresponding number of water molecules per Al atom in the framework of H-ZSM5 (Si/Al = 45) (magenta) as well as the resulting heat flow (green) are reported for the thermogravimetric analysis as a function of temperature. The fractional areas of the IR bands attributed to hydrogen-bonded OH groups (Fermi diad) forming in the ∼50−150 °C temperature region are also shown in gold. ThermoScientific Nicolet Fourier transform infrared (FTIR) spectrometer using a MCTA detector with a resolution of 4 cm−1. 128 scans were accumulated for each spectrum. For experiments in He flow, the IR cell was purged for 30 min with dry He at 5 mL/min and the sample was then heated at 5 °C/min in a 5 mL/min He flow in 10 °C increments. At each temperature, the sample was equilibrated for 5 min prior to the acquisition of the IR spectrum. For H2O-dosing experiments, the sample was evacuated to 1.0 × 10−7 mbar at 430 °C for 1 h. Then, the sample was cooled to 130 °C; the background spectrum was collected, and water was dosed in increments, allowing 10 min of equilibration at each chosen partial pressure prior to the acquisition of the IR spectra. Thermogravimetric Analysis. The thermogravimetric analysis (TGA) was performed using a Netzsch STA 449C thermal analyzer coupled with a Netzsch QMS 403C quadrupole mass spectrometer (MS). 32.7 mg of sample was loaded in the crucible, and the sample compartment was purged with 15 mL/min dry Ar that was also used as a carrier gas for the MS. Note that the same heating temperature increment as for the XANES and IR studies was used for TG experiments.

To obtain structures of the different Al T-sites in MFI, a single unit cell of the MFI crystal was modified by substituting an Al for a single Si atom to form a negatively charged [Si95O192Al]− cell. For the case where the Brønsted acidic proton is localized at the Al T-site, a proton was placed on one of the O atoms neighboring the Al T-site to compensate the negative charge. For the case where the impact of different water concentrations on the nature of the Brønsted acidic proton was explored, water was added to the zeolite framework randomly in the vicinity of the Al T-site and the whole structure was allowed to optimize while fixing the unit cell parameters. The DFToptimized structures of MFI with varying pore water concentrations were used to calculate the respective IR-band frequencies using the methods reported in the Supporting Information. XANES Calculations with Time-Dependent DFT. XANES calculations were performed at the Al K-edge for two cluster models representing the Al T-site geometries resulting in the localized and delocalized proton conformations (see the Supporting Information) using restricted excitation window TDDFT (or REW-TDDFT) as implemented in the NWChem quantum chemistry program.56,57 The use of TDDFT calculations to accurately predict the location and nature of the XANES spectral features has been demonstrated previously.27 Specifically, this approach quantitatively relates the observed spectral changes to symmetry and structural changes in the first- and second-coordination shells about the absorber Al atom.58 The TDDFT approach involves defining a restricted subspace of single excitations from the relevant core orbitals and no restrictions on the target unoccupied states. This ansatz is possible because core excitation energies are well separated from pure valence−valence excitations. For each Al T-site conformation, the Sapporo-QZP-201259 all electron basis set was used for the single absorbing Al T-site and the nearest O atoms. The Si and O atoms further away were represented with the Stuttgart RLC ECPs.60 The exchange-correlation was treated with the BHLYP functional.61 All boundary O atoms with unsatisfied bonds were passivated with H atoms, and the direction of the OH bond (0.96 Å) was preserved as in bulk MFI. Core hole lifetime broadening for Al is about 0.4 eV while the instrument resolution is about 0.6 eV. The measured bandwidth is substantially larger which is most likely a result of slightly different structural ensembles due to thermal fluctuations at 25 °C and above. Thus, all calculated spectra were Lorentzian broadened by 1 eV and shifted by +15.6 eV to match the experimental spectrum. Infrared (IR) Spectroscopy. The MFI sample for IR measurements was prepared as a self-supporting wafer with a density of approximately 10 mg/cm2. Infrared spectra were recorded on a

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RESULTS AND DISCUSSION Thermogravimetric Analysis (TGA). Thermogravimetry shows the loss of water molecules normalized to the Al T-site (Figure 1). The majority of the weight loss due to water desorption occurred from 40 to 120 °C, accompanied by a pronounced endothermic signal. The total weight loss (0.042 g per gram of sample) suggests the studied MFI contains on average 6 H2O molecules per Al T-site at 25 °C and a pressure of 10−4 mbar H2O. This number is reduced to an average of 2.5 water molecules per Brønsted acid site at 80 °C. The relatively large heat flow measured in the 60−70 °C region is caused by desorption of a total of 45 μmol of water. It includes the breaking of hydrogen bonds and the decoordination of water molecules from the H3O+(H2O)n clusters. The heat flow associated with this process amounts to 55 kJ/mol (H2O) on average. At 130 °C, only one H2O molecule per Al T-site, on average, remained. Further increase in temperature led to a gradual decrease of pore water concentration until water was nearly completely removed from the zeolite pores at ∼250 °C. Beyond 200 °C, the plateau in the heat flow is likely caused by 9032


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Figure 2. IR spectra acquired during water desorption from H-ZSM5 in 5 mL/min He flow (a) and the IR spectra after subtraction of the H-ZSM5 430 °C spectrum (b) are shown. The measurements in (a) and (b) were performed as a temperature series in 10 °C increments. The IR spectra acquired for H-ZSM5 during isothermal (130 °C) adsorption of water are shown in (c), and the water signal obtained by subtraction of the 1.0 × 10−7 mbar H2O H-ZSM5 130 °C spectrum are shown in (d). For H2O adsorption experiments, the sample was pretreated at 430 °C under 1.0 × 10−7 mbar for 1 h. The peaks marked as “A”, “B”, and “C” are assigned to structure “II” in Figure 3.

cm−1) at 430 °C with respect to those typically reported at ambient temperature. In Figure 2b, the spectra of the dry zeolite measured at 430 °C have been subtracted to better illustrate the impact of water on the activated zeolite. At 30−50 °C, a substantial concentration of H-bonded water was present, causing the broad band from 3500 to 2600 cm−1 region (O−H stretching vibration) as well as a broader band around 1600 cm−1 (H−O− H bending vibration).71 The significant decrease of the intensity at higher temperatures shows that a large percentage of water was already removed at ∼70 °C. From 70 to 130 °C, we observed the formation of distinct peaks at 2913, 2466, and 1600 cm−1, corresponding to the A,B,C-triplet of the Fermi resonance.33 This suggests that single water molecules are hydrogen bonded to SiOHAl groups. At temperatures above 150 °C, mainly the bands at 3742 and 3604 cm−1 assigned to the terminal Si−OH and the Si(OH)Al groups were observed. Figure 2c shows a series of IR spectra of water equilibrated with H-ZSM5 at 130 °C at increasing partial pressures. We have chosen this temperature, because at 130 °C (Figures 2a,b) the intensity of the ∼2450 cm−1 peak was approximately half of the intensity observed at 70 °C. The negative intensity of the peak at 3600 cm−1 (Figure 2d) is caused by the red-shift of this

the heat capacity of the MFI and a gradual loss of small amounts of residual water. In Situ IR Spectroscopic Studies. The IR spectra of MFI zeolite during water desorption (by increasing temperature to 430 °C in a He flow) are shown in Figure 2a,b, while the spectra acquired for water adsorption (with increasing partial pressure of water in a range of 1.0 × 10−7 to 1.0 mbar) dosed at 130 °C are compiled in Figure 2c,d. Figure 2a shows the overall IR spectra that include contributions from both the water and the zeolite bands. The bands assigned to hydrogen-bonded OH groups and water are shifted from the zeolite ν(OH) at 3610 cm−1 by approximately 900 cm−1. The resulting ν(OH) mode is split by a Fermi resonance33,62,63 with deformation modes located near 1300 and 850 cm−1. The resulting peaks, typically a triplet of signals, are observed at ∼2900, ∼2400, and ∼1700 cm−1. The spectral assignment of this so-called A,B,C-triplet64,65 has been welldocumented in the literature.66,67 The peaks at 3732, 3682, and 3593 cm−1 are attributed to zeolite terminal Si−OH and Al− OH groups at lattice defects and Si(OH)Al groups, respectively.68−70 The peaks at 1981, 1866, and 1630 cm−1 are attributed to the combinations of MFI lattice vibrations.41 Note that these lattice vibrations are slightly red-shifted (13 9033


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Figure 3. DFT-optimized structures illustrating the process of hydrating the zeolite with increasing numbers of water molecules. For two or more water molecules, the proton dissociates from the T-site forming a hydronium ion with increasing degrees of hydration. The structure assignments of I, II, III, IV, and V are the species discussed in the interpretation of the XANES and IR spectra. The H9O4+ species (not shown) is structurally similar to V.

Brønsted ν(OH) band upon H-bonding with a single water molecule. Zecchina et al.33 noted a strong hysteresis in the IR spectra acquired with decreasing equilibrium pressures compared to spectra taken when the equilibrium pressure increased. The same behavior was observed between the thermal desorption experiment of Figure 2a,b and the isothermal adsorption experiment of Figure 2c,d. This is most prominently seen in the IR bands attributed to the H3O+(H2O)n clusters from 1850 to 1650 cm−1 and from 2800 to 3500 cm−1. One explanation33 for this hysteresis is the presence of a barrier for proton transfer between the Brønsted group and the H3O+(H2O)n cluster. However, DFT calculations show that a H3O+ adjacent to the T-site will instantly transfer the proton with little or no barrier. On the other hand, the stepwise dehydration of the H3O+(H2O)n cluster progresses through a series of increasingly higher barriers, causing a statistical distribution of states. This is in good agreement with the observed states of water. In the desorption experiment at 130 °C (Figure 2a,b), both H3O+(H2O)n clusters and protonated Brønsted sites exist in significant fractions. In the adsorption experiment (Figure 2c,d), the predominant species is the protonated Brønsted site with or without an H-bonding water. Considering that the average concentration of H2O determined by gravimetry is as high as 4 water molecules per SiOHAl group at 70 °C and that the AB doublet begins to appear (Figure 2a), it is concluded that water must be heterogeneously distributed among the SiOAl− sites; i.e., some hydrated hydronium ion clusters exist (giving rise to the broad bands around 3300 cm−1) next to single water molecules, hydrogen bonded to SiOHAl sites (triplet at ∼2900, ∼2400, and ∼1700 cm−1). This heterogeneity is also manifested in the fact that an increasing fraction of the band, characteristic of unperturbed SiOHAl, gradually reappeared with increasing temperature when the coverage was still on average above one H2O per SiOHAl group. T-Site Structures from DFT Optimization. Figure 3 shows several conceptually possible scenarios for water at the Al T-site of a MFI zeolite. In the absence of water, a bridging SiOHAl group is the stable structure (Figure 3 structure “I”). Addition of further water molecules (“II” to “V”) leads to deprotonation and the formation of a hydronium cation (H3O+) that is hydrated and stabilized by additional water molecules. This hydrated cation is localized at the negatively charged Al T-site as an ion-pair species. For structures with more than three water molecules, including structure “V” in Figure 3, the properties increasingly approach those of the bulk hydrated H+.

In order to better understand the interaction of the adsorbed water with the Al T-sites, the MFI Al T4-site, which is located in the 5-member ring that is part of the straight pore channel of the MFI framework, was chosen as the example Al T-site.72 Periodic DFT optimization was then performed to determine the geometry of the BAS (Figure 3, structure “I”). In subsequent steps, one or more water molecules were added to the pore and the systems were reoptimized to determine the nature of the H2O interaction with the BAS site. These structural results are represented in Figure 3 for zero, one, two, three, and five water molecules in the pore, respectively. We find that a single water molecule (Figure 3, structure “II”) cannot sufficiently stabilize the proton and is hence hydrogenbonded to the Al T4-site as a neutral complex. However, even in this neutral complex, we observe a partial redistribution of positive Bader charge between the proton (charge decreases by 0.18 |e|) and the water molecule (charge increases by 0.18 |e|). Two water molecules associated with the SiOHAl group suffice to transfer the proton and form a H5O2+ ion-paired complex with the Al T4-site. Interestingly, we find that the relative stability of this species is quite low and the proton undergoes dynamic rearrangement between the bound (Al−OH-Si) and the mobile (H5O2+) hydronium cluster. The ion-paired protonated water cluster in this case carries a charge of +0.69 |e|. Finally, when three or more (see Figure S4) water molecules are present in the zeolite pore, the proton is transferred from the framework to the water cluster to form a larger hydronium ion cluster that is ion-paired to the Al T4-site. The positive charge associated with the hydronium ion, however, does not increase markedly with the addition of more water molecules and is 0.70 |e|, 0.72 |e|, and 0.72 |e| for H7O3+, H9O4+, and H11O5+, respectively, for the optimized structures. Our DFT results also show that the charges of the four oxygen atoms connected with the T4 site are more negative (0.10−0.15 |e|) than other oxygen atoms on the MFI framework. Even larger charge separation would be expected at higher temperatures where the H3O+(H2O)n clusters are more delocalized from the Al T-site. The presence of various water molecules at the Al T4-site not only creates the ion-paired hydronium cluster but also affects the Al−O bonds at the T-site. Calculations show that the specific Al−O bond length for the proton (H+) connecting with the framework oxygen is ∼0.2 Å longer than the other three Al−O bond lengths. With the increasing water concentration at the BAS, the proton is transferred from the framework oxygen atom to the water cluster. The four Al−O bonds become more equivalent with a maximum bond length difference of ∼0.05 Å (see Figure S2). 9034


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Figure 4. Selected experimental IR spectra taken at various temperatures compared to the IR frequencies calculated using the DFT-optimized structures for different concentrations of water in the zeolite pores. The color coding is reported in the legend. The peaks marked as “A”, “B”, and “C” are assigned to structure “II” in Figure 3. Two of the individual DFT bands have been scaled by 1/2 as shown to improve visualization. DFT frequencies for H7O3+ and H9O4+ are given in Figure S5.

Calculated Wavenumbers of IR Bands of Water Associated with SiOHAl Groups. The DFT-optimized structures of MFI with varying pore water concentrations (Figure 3) are used to calculate the wavenumbers of the associated IR bands (details in the Supporting Information). Figure 4 shows the superposition of the calculated and measured IR bands. For the first configuration, the wavenumber, ν(OH), for the Brønsted site of the SiOHAl group in the absence of water (structure “I”) was determined. The calculated value of 3581 cm−1 is in close agreement with the experimental value of 3604 cm−1 (150 °C). The first added water molecule forms an H-bond to the T-site proton (structure “II”), for which the calculated wavenumbers are 3766, 3657, 2687, and 1599 cm−1. The free O−H stretch bands of this water reside at 3766 and 3657 cm−1, and the positions of these two bands are consistent with features shown more clearly in Figure 2d. For the T-site proton of this structure “II”, the predicted ν(OH) frequency is shifted to 2687 cm−1, which is in excellent agreement with the experimental fundamental that lies between the intense A and B Fermi pair in Figure 4. A higher level of theory67 would be required, however, to predict the resonance splitting of this band. In the presence of 2, 3, 4, or 5 water molecules, the number of calculated bands increases with increasing water concentration. The full set of bands calculated for H3O+(H2O)n with n = 2, 3, 4, or 5 is indicated in Figure 4 (and S5) demonstrating the complex nature of the studied system. It is interesting to note that the calculated frequencies for these various H3O+(H2O)n clusters are similar to experimental measurements of gas phase clusters of comparable sizes.73 At 30 °C, approximately 5 of the predicted bands at 3615, 3750, 3345, 3130, and 2955 cm−1 for structure “V” (H11O5+) are observed in the experimental spectrum at similar

frequencies. The fact that the experimental spectrum shows evidence of these discrete bands suggests that, under ambient conditions, the hydration of the H+ and the confinement of this smaller cluster in the pore near the T-site leads to clusters that significantly differ from those of H+ in bulk water. At temperatures between 70 and 120 °C, we find the Fermi resonance bands at 2900 and 2450 cm−1 for the Brønsted acid site proton hydrogen-bonded with a single water molecule. In this temperature range (Figures 2b and 4), we also observe a broad shoulder near 3400 cm−1 and around 1700 cm−1 that is consistent with smaller H7O3+ or H9O4+ clusters. Al XANES Analysis. In situ Al K-edge XANES was used to experimentally follow structural changes that occur at the T-site within the H-ZSM-5 zeolite upon the removal of pore water with increasing temperature. The acquired spectra shown in Figure 5 demonstrate the impact of pore dehydration on the electronic environment of the Al atoms at the T-sites. Under ambient conditions, we observe a strong peak at 1565.5 eV for the tetrahedral Al.74 Several important spectral changes are observed as the temperature increases: (1) the formation of a distinct pre-edge peak at 1562.5 eV, (2) the formation of a shoulder at 1564 eV, (3) the decrease of the 1565.5 eV peak intensity, (4) a new peak or shoulder at 1566 eV, and (5) a decrease of the spectral intensity in the region between 1568 and 1570 eV. Overall, the changes in the first two features are indicative of the distortion of symmetry of the tetrahedral Al atoms. In a related study, Fulton et al. showed how small distortions of the tetrahedral T-site symmetry altered the Al-environments.58 The changes in the structure of the main peak at 1565.5 eV are directly related to the localization of the proton on the Al T-site as the water desorbs from the pores with increasing temperature. The decrease of intensity at 1570 eV is attributed 9035


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Figure 5. Normalized Al-XANES spectra of HZSM-5 during heating at 5 °C/min in a 5 mL/min He flow are shown. The legend shows the temperatures for the color-coded spectra. Normalization to an edgeheight value of one was applied to the region above 1590 eV. XANES normalization procedure is described in the Experimental and Theoretical Methods.

to the decrease in the concentration of octahedral Al.75 Although the studied zeolite underwent postsynthetic treatment, approximately 6% octahedral Al was still present. This octahedral Al is hypothesized to be either stabilized in the zeolite pore or partially attached to the framework.76,77 Upon heating, the first shell coordination number of such Al species would decrease from 6 to 4 resulting in an Al(OSi)3(OH) species.78 The behavior of this broad peak at 1570 eV is in agreement with findings suggesting that the hydration of acidic zeolites at room temperature leads to the transformation of a fraction of tetrahedral Al to octahedral coordination.79,80 Drake et al. demonstrated that this transformation is reversible in HUSY and H-ZSM5 zeolites.81 Time-Dependent DFT XANES Calculations. In order to determine the XANES peak assignments, we have performed time-dependent density functional theory (TDDFT) calculations of the Al K-edge XANES spectra for the bridging hydroxyl (structure “I” in Figure 3) and for various hydrated species (“II”, “III”, “IV”) in the H-ZSM5 zeolite. The proton localization to form the SiOHAl on the zeolite Al T-site results in a substantial elongation of one of the four Al−O bonds, which in turn causes a significant distortion of the tetrahedral symmetry around the Al atom. The TDDFT XANES calculations are performed using a finite cluster size (∼97 atoms) showing convergence similar to clusters explored previously by Vjunov et al.27 This cluster size was also demonstrated to be representative of the zeolite framework environment investigated by Sauer and co-workers.82 The cluster geometry was based on the T-sites of MFI. The comparison of Al−O distances for all T-sites of H-ZSM5 is shown in Figure S2. Figure 6 compares XANES spectra calculated for two different DFT structural models to spectra taken at three different temperatures, 25, 115, and 205 °C. The TDDFT spectra represent two different structural states, one (“I” in Figure 3) in which the proton is bonded to the bridging oxygen (SiOHAl group) and the second a hydrated site in which the proton is stabilized as hydronium ion hydrated by two water molecules (“IV”). In Figure 6b, structure “IV” shows a band at the “white line” (1565.4 eV) but no distinct bands below this

Figure 6. Normalized experimental Al-XANES spectra (a) at three different temperatures and the corresponding TDDFT calculated spectra (b) for the SiOHAl group (DFT-optimized structure “I” in Figure 3) and for the [SiOAl]−[H(H2O)3]+ ion pair (DFT-optimized structure “IV” in Figure 3). Vertical lines show the approximate location of the observed and predicted transitions. The calculated spectra are shifted by +15.6 eV to overlay with features of the experimental spectrum and were Lorentzian broadened by 1 eV.

energy. On the other hand, for structure “I”, the white line splits into two bands and new bands appear at lower energies near 1563.2 and 1564.2 eV. This same pattern is observed in the experimental spectra of Figure 6a. In structure “I”, the Al− O bond that hosts the bonding H+ is significantly lengthened (∼0.15 Å) and this distortion leads to large changes in the Al atom electronic transitions in the near-edge spectra. Hence, from the series of spectra shown in Figure 5, one can conclude that most of the SiOH groups have reconstituted above 200 °C. There is an additional possibility that thermal effects (115 and 205 °C) could vibrationally distort the symmetry of the Tsite and thereby change the XANES qualitatively and quantitatively. These thermal effects are not included in the DFT-optimized structures used to calculate the spectra in Figure 6b. They were explored by examining short DFT-MD trajectories of the zeolite framework at 25 and 225 °C. As shown in Figure S6, higher temperatures have little or no effect on the pre-edge peak at 1563 eV but do alter some of the features about the “white line” at 1565.5 eV. For this reason, we believe that the fully DFT-optimized structures provide a good representation of the structures that are experimentally measured at higher temperatures. Conceptually, the TDDFT calculations allowed one to assign observed peaks to specific electronic transitions in the region of 1560−1570 eV. For instance, the pre-edge feature at 1563 eV that appears upon pore water removal at elevated temperature 9036


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Chemistry of Materials is assigned primarily to the excitations from the Al 1s−3d level, while the peak underlying the shoulder at 1564 eV is attributed to a mixture of O 3p, Al 3p, and Al 3d states. The formation of the pre-edge feature is a strong indicator of the significant distortion of the tetrahedral symmetry around the Al atom.58 The main peak at 1565.5 eV is due to excitations from Al 1s to a mixture of O 3p and Al 3p states. The TDDFT analysis of spectral features can only be performed up to an energy of 1570 eV. Above this energy, the spectra are dominated by photoelectron scattering processes with nearby atoms that are not included in the linear-response formulation of TDDFT as shown previously.57,83,84 Local T-Site Geometry and XANES. We have also explored how the secondary T-site structure affects the calculated XANES spectra. The XANES calculations (Figure S7) for a T-site proton located on each of the four different O atoms that are bonded to the Al atom at the T4-site show that, while there are significant spectral changes in the amplitudes of the different O sites, the overall band features are approximately identical, regardless of which O bears the H. We have also compared different T-sites (4, 6, 8, 10, and 15) of MFI (see Figure S8) in order to probe how the local environment affects the protonated site. The results are very similar to those for the different T-site O atoms in that the amplitudes of the four XANES bands show some variations, without changing the number of bands. Thus, we conclude that the T4-site represents T-sites in MFI on a general level. Figure 7 shows the calculated changes in the XAS spectrum that occur as the number of water molecules that reside near the T-site for structures (I, II, III, IV) of Figure 3 increases. The pre-edge peak (1563 eV, dashed green line marking the discrete

roots) indicates geometric distortion, i.e., lengthening of one of the Al−O bonds. The “water” peak (1565 eV, blue dashed line) correlates with the presence of water in the pores, and the “empty pore” peaks (at 1564 and 1568 eV) relate to pores that contain no adsorbed water. Figure 7 also reports the four Al−O bond lengths (Å) for each of these optimized structures. When the Al−O bond lengths are equivalent (fully dissociated proton as in Figure 7c), there is no pre-edge peak (see discrete peaks). The pre-edge peak at 1563 eV is most intense, when the H is localized on the T-site (“discrete” green line in Figure 7b). Upon addition of one water (Figure 7a), the intensity of this band decreases as the Al−O bond-length distortion is reduced. Addition of 2 and 3 waters further reduces the amount of Al−O bond length distortion (see red-marked bond lengths in Figure 7a−c). However, the close proximity of this cluster to a specific oxygen of the T-site only partially relaxes the Al site symmetry. The XANES features delineated by the red dashed lines in Figure 7b,c are representative of an unperturbed SiOHAl group. These peaks are absent when water or hydronium clusters reside in the pore. This observation suggests that there is sensitivity to the presence of either a pore solute or solvent molecule near the T-site. This aspect is further explored by the spectra in Figure 8. In this case, two identical T4-site structures

Figure 8. Calculated Al-XANES experimental spectra for (a) a single water H-bonding to the T-site −O−H group (DFT-optimized). In (b), the starting structure in (a) has been frozen but the single pore water has been removed. The four Al−O bond lengths (Å) are given for these structures. The green lines are the series of discrete roots prior to line broadening.

are compared in which the only difference is the presence or absence of a single water molecule localized near the T site. Both structures show the pre-edge peak at 1563 eV, although the band intensity is about 5 times higher in the absence of the water molecule. In addition, the loss of water in the pore leads to an increase in the shoulder at 1564 and the peak at 1566 eV. In these instances, a single, second-shell solute (water) strongly alters the magnitude of the dipole-allowed transition of the distorted T-site thereby significantly altering the intensity of these XANES peaks. This sensitivity to the pore solute provides a spectral capability not readily available with any other

Figure 7. Calculated Al-XANES experimental spectra for a series of DFT-optimized Al T4-site structures. In (a), a single water is Hbonded to the T-site −O−H group; in (b), the T-site is protonated but no pore water; in (c) the T-site is deprotonated in the absence of pore water. The (d) spectrum is the same as (a) but is added for ease of vertical comparison to (e) and (f). In (e) and (f), there is an increasing number of water which deprotonates the T-site. The four Al−O bond lengths (Å) are given for each of these structures, where the Al−O bond that hosts SiOHAl is listed in red. The calculated spectra are shifted by +15.6 eV to match the white line of the experimental spectrum. The green lines are the series of discrete roots prior to line broadening. 9037


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the large majority (>98% according to TG measurement; see Figure 1) of pore water is removed near 300 °C. With increasing temperature, two processes occur in parallel: (1) water is removed from the coordination sphere of the hydronium ion, and (2) when finally only the single H2O is associated forming the hydronium ion, it decomposes, resulting in the localization of the H+ on the oxygen atoms adjacent to the Al T-site. In good agreement with these observations, Nusterer et al. showed that, at very low water coverages, e.g., one water molecule per zeolitic H+, the water molecule tends to adsorb on the acid site forming a neutral complex; however, at higher water concentrations, the zeolitic H+ is present in the form of a hydrated complex that is ion-paired to the negative lattice charge.87 Similar observations were made by Sazama et al.88 Sauer et al. further examined the behavior of water at low concentrations, specifically calculating the energetics for the first water molecule interaction with the zeolitic H+. The results show that, although the neutral complex is slightly preferred over the hydronium ion, the energy difference between the two cases is only ∼10 kJ/mol.89,31 Conversion of Octahedral Al to Tetrahedral Coordination. There is a small Al component (6%) existing as extraframework octahedral Al that also undergoes structural transition at elevated temperature. As reported in the Al XANES analysis section above, we observe a progressive decrease in octahedral Al concentration as the temperature is increased during sample treatment in He flow. The fractions of octahedral and tetrahedral Al are determined from XANES linear combination fitting using the two references, aqueous [Al(H2O)6]3+ and [Al(OH)4]−, reported by Fulton et al.58 The amount (%) of octahedral Al in the studied MFI is graphically shown as a function of temperature in Figure 10. We find that

techniques (e.g., IR). Just this single effect of decreasing the pore water at higher temperature will be in part responsible for the observed changes in the experimental spectra showing increased pre-edge peak intensity at 1563 eV and increases in the XANES peaks at 1564 and 1566 eV. Summary of the Measured and Calculated XANES Features. Combining the insight from experimental spectra and theory allows us to reach several conclusions: (i) At 25 °C, when the sample was exposed to ambient humidity (>5 H2O/ Al T-site), the proton is fully dissociated forming a hydrated hydronium ion. Under these circumstances, all four Al−O bonds are equivalent and only a very weak 1s−3d pre-edge peak at 1563 eV is observed (Figure 5). The presence of pore water leads to a distinct “water” peak at 1565.5 eV; (ii) from 25 to 250 °C, multiple changes occur, as the T-site undergoes dehydration. The +0.4 eV shift in peak at 1565.5 eV between 25 and 115 °C is consistent with a decrease in pore water concentration, while the rise in the pre-edge peak at 1563 eV above 115 °C is consistent with hydroxylation of the T-site; (iii) above 250 °C, all T-sites have reformed the SiOHAl groups again and the Al−O bonds are strongly distorted. These factors, in addition to the lack of any pore water, lead to the strongest pre-edge intensity. Location of the Brønsted Acidic Protons. A linear combination fit of the acquired XANES spectra was performed as a simple approximation in order to determine the fraction of protons located on the Al T-sites (structure “I”, Figure 3) as a function of temperature. By fitting the XANES spectra in Figure 5 over the region from 1560 to 1564.5 eV, we probe the fraction of H+ on the Al T-site as a function of the fraction of H+ delocalized from the T-site in the form of H3O+(H2O)n. We assume that (1) at 25 °C (first data point) ≥99% of H+ is delocalized85 and (2) at 430 °C (last data point) ≥99% of H+ is localized on the oxygen atom in the first shell of Al. The second assumption is justified on the basis of previous reports for zeolite pore water removal at elevated temperatures, e.g., zeolite activation for 1H NMR measurements.86 The results of the linear combination fitting of these spectra in Figure 5 are shown in Figure 9. We observe an S-curve demonstrating that ∼50% of the protons are localized on the Al T-sites at 160 °C while

Figure 10. Decreasing amounts (%) of octahedral Al with increasing temperature are shown. The sample is heated at a rate of 5 °C/min in 5 mL/min He flow. The octahedral Al % is determined from XANES linear combination fitting. The individual data points are color coded and reported in the legend.

approximately 50% of the octahedral Al present at ambient conditions is converted at 115 °C, a trend similar to that observed for the ultrastable HY zeolite.90 The broad temperature range (Figure 10) suggests that there may be several different Al species that are converted to tetrahedral symmetry at different temperatures. Reversibly, at lower temperatures, water molecules can coordinate to these Al sites, forming an octahedral Al symmetry.91,92

Figure 9. Fractions of H+ delocalized as H3O+(H2O)n (blue) are shown as a function of temperature. The sample was heated at a rate of 5 °C/min in 5 mL/min He flow. The fractions are determined from XANES linear combination fitting. The error bars are derived from the linear combination fit analysis of the experimental data. 9038


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CONCLUSIONS Overall, the state of the acid sites in the zeolite depends most stringently on the concentration of water at the Al T-sites. The most significant structural alteration occurs upon dehydration of the localized hydronium ion (H3O+(H2O)n), leading eventually to proton transfer to form the SiOHAl group. The transition, from the zeolite structure having symmetric aluminum oxygen tetrahedra and hydrated hydronium ions, not only is gradual but also occurs in a nonideal or nonequilibrated way, i.e., several structures coexist. As an example, after purging the zeolite with increasing temperatures in He, at 160 °C, approximately half of the protons reside upon the T-site hydroxyl. However, the average concentration of water molecules per Al T atom is significantly below one, a situation in which all protons should reside at the zeolite. TDDFT calculations show that increased intensity of the XANES pre-edge peak at 1563 eV can be associated with the existence of the SiOHAl bridging hydroxyl group. TDDFT calculations show that three other XANES bands between 1564 and 1568 eV are associated with the presence of water, hydronium, or the hydrated cation cluster, H3O+(H2O)n, near the Al T-site. Thus, Al K-edge XANES provides a sensitive method to detect structural and chemical changes about the Tsite during a reaction. The IR spectra and the DFT cluster calculations indicate that the discrete nature of hydrated hydronium ion clusters more resembles clusters in the gas phase than hydrated hydronium ions in aqueous solution. Insight into the specific nature might allow a more detailed comparison between constrained environment and gas phase clusters and might help, therefore, to better understand the impact the restricted space offers.

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manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Abhijeet Karkamkar (PNNL) for assistance with the TGA measurement. The authors thank Dr. Maricruz Sanchez−Sanchez (TU München, Germany) for providing the HZSM-5 sample. We thank Dr. Gary L. Haller for comments on improving the manuscript. This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Award Numbers KC0301020 (JLF, MG, GSK, CJM) and KC030105066418 (NG). Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle under contract DE-AC05-76RL01830. The Al XAFS measurements were performed at the PHOENIX beamline of the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland. We like to thank Dr. Cinthia Piamonteze and Dr. Jan Dreiser for technical support to run the X-Treme beamline, which is required to operate the PHOENIX II branchline. This research also benefitted from computer resources provided by PNNL Institutional Computing (PIC) and EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at PNNL.

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ABBREVIATIONS XAS, X-ray absorption spectroscopy; XAFS, X-ray absorption fine structure; XANES, X-ray absorption near edge structure; DFT, density functional theory; FTIR, Fourier transform infrared; TGA, thermogravimetric analysis

ASSOCIATED CONTENT

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02133. XANES calculations at different temperatures and with phenol and cyclohexanol as pore solutes, details of the Al XANES TDDFT calculations, IR frequency calculations, Al XANES experimental data linear combination fit analysis, and XAFS cell design (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

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

Meng Wang: 0000-0002-3380-3534 Hui Shi: 0000-0003-1180-7443 Donghai Mei: 0000-0002-0286-4182 John L. Fulton: 0000-0001-9361-9803 Johannes A. Lercher: 0000-0002-2495-1404 Author Contributions ¶

A.V. and M.W. contributed equally. A.V., M.W., J.L.F., and J.A.L. developed the concept. A.V., J.L.F., and T.H. performed the XAFS spectroscopic measurements, data analysis, and fitting. N.G. performed XANES calculations. D.M. performed the DFT optimizations and IR band calculations. A.V., M.W., and H.S. performed the IR data acquisition and analysis. The 9039


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