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Delicate Distinction between OH Groups on Proton-Exchanged H‑Chabazite and H‑SAPO-34 Molecular Sieves Istvan Halasz,*,† Bjorn Moden,‡ Anton Petushkov,‡ Jian-Jie Liang,§ and Mukesh Agarwal† †

Research and Development Center, PQ Corporation, 280 Cedar Grove Road, Conshohocken, Pennsylvania 19428, United States Research and Development Center, Zeolyst International, 280 Cedar Grove Road, Conshohocken, Pennsylvania 19428, United States § BIOVIA, Dassault Systèmes, 5005 Wateridge Vista Dr., San Diego, California 92121, United States Downloaded via TEMPLE UNIV on August 1, 2018 at 15:02:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: We observed surprising differences in the FTIR (Fourier transform infrared) hydroxyl spectra of the structurally isomorphous, proton-exchanged H-CHA and H-SAPO-34 molecular sieves when measured by transmission (TR) or diffuse reflectance (DRIFT) techniques. Experimental and density functional theory (DFT) based model evidence is presented in this paper to prove that the essential reason for this spectral difference is that DRIFT emphasizes the vibrations of surface hydroxyl sites. Vibrations of the bulk Brønsted acidic hydroxyls shift to higher frequencies when they become surface species, and the IR beam is reflected from approximately the top ∼15 to 20 Å thick layer of the particles; hence, the proportion of surface related IR bands becomes significant compared to the bulk related ones in the DRIFT spectra while the opposite is valid for the TR spectra. We demonstrate that the surface hydroxyls are Brønsted acidic on both the H-CHA and the H-SAPO-34 particles, and the upshifted vibrations noticed primarily in the DRIFT spectra are Al−OH vibrations on the surface even of H-SAPO-34, not P−OH groups as most researchers believe. We also show that the bulk Brønsted sites might involve HO1, HO2, and HO4 type hydroxyls associated with the known geometrically different oxygen positions on both molecular sieves, but only HO1 surface hydroxyls are associated with the red-shifted vibration intensified in the DRIFT spectra. Moreover, a single surface model cannot account for every vibration observed in DRIFT spectra. From the combination of IR vibrations of three adequate surface models one can as properly match the experimental DRIFT spectra as the TR spectra from the combination of the calculated bulk HO1···HO4 vibrations of these molecular sieve crystals.

1. INTRODUCTION

transform) results are modified by Kubelka−Munk or other functions to compensate for nonlinearity.9−17 Therefore, it came as a total surprise for us to see that the hydroxyl FTIR spectra of proton-exchanged H-CHA and HSAPO-34 samples gave quite different spectra when we incidentally measured their OH contents by both DRIFT and TR technique.18 As an example, illustrated in Figure 1a, a band at 3677 cm−1 is much more intense in the DRIFT spectrum of H-SAPO-34 than in its TR spectrum. In a very similar manner, Figure 1b shows that the 3662 cm−1 band is much more intense in the DRIFT spectrum of H-CHA than in its TR spectrum. Since such comparative FTIR measurements with these two techniques have not been reported on these samples, we surveyed the literature for a potential explanation of this unusual phenomenon. The 3600/3627 cm−1 bands of H-SAPO-34 and the 3594/ 3612 cm−1 bands of H-CHA have been univocally associated in the literature with their Brønsted acidic Si−OH and Al−OH

Chabazite (CHA), an aluminosilicate zeolite, also frequently referred as SSZ-13,1 and its crystallographically isomorphous silicoaluminophosphate, SAPO-34, are well studied molecular sieves with approximately d ∼ 3.8 Å and ∼4.3 Å diameter microchannels,2−4 respectively. These materials are important ingredients of catalysts for the recently developed advanced methanol-to-hydrocarbon (MTH) technology5,6 and the ureaSCR (selective catalytic reduction) based NOx reduction of diesel engine exhausts.7,8 The Brønsted acidic hydroxyl groups (BA-OH) of both zeolites have a pivotal catalytic role in these processes; hence, their accurate characterization is a regular task. Along with all other hydroxyls, this characterization is most frequently made by Fourier transform infrared spectroscopy (FTIR). Depending on the analysts’ preference, two major FTIR sampling techniques are in use: (i) transmission (TR), mostly with thin, self-supported pellets; (ii) diffuse reflectance (DR), carried out with powdered samples.9 Results are generally considered to be identical with each other, especially when the DRIFT (diffuse reflectance infrared Fourier © 2015 American Chemical Society

Received: September 22, 2015 Published: September 27, 2015 24046

DOI: 10.1021/acs.jpcc.5b09247 J. Phys. Chem. C 2015, 119, 24046−24055

Article

The Journal of Physical Chemistry C

Figure 1. FTIR spectra of (a) a H-SAPO-34 sample with nominal composition of Al5.9P5.0Si1.1H1.1O24 and (b) a H-CHA sample with nominal composition of Si5.6Al0.4H0.4O12 show substantial difference when measured by DRIFT vs the transmission (TR) technique.

8T ring and 7T cluster calculations. Smith et al.,21,22 on the other hand, computed with high accuracy that HO2 and HO4 Brønsted acidic sites fit the experimental spectra in both zeolites but also observed shifting the HO4 proton to HO1 position. Lo and Trout23 essentially also support this result. Using the QM-Pot method, Sierka and Sauer24 even found that the protons of the [AlO4] units in various zeolites jump from position to position, but on the H-CHA they found that the HO1 and HO2 positions (see Figure 2) are preferred. Jeanvoine et al.25,26 also suggest that the H-CHA acidic bands are associated with HO1 and HO2 proton positions and in H-SAPO-34 that HO4 protons might contribute to them as well. Hemelsoet et al.27 also identified the HO4 proton being responsible for the HF vibration of H-SAPO-34. Miki Niwa’s group28 computed that the HO3 site of H-CHA cannot be part of the experimental FTIR spectrum of Brønsted acidic sites since it vibrates at lower energies where the experimental spectra are largely empty. They have found that the computed vibrations of the other three hydroxyls fit the experimental spectra. This group28,29 also proved that in general the phosphorus containing zeolites are weaker Brønsted acids than their aluminosilicate structure isomers. The plane-wavebased calculations of Sauer et al.30 also resulted in this conclusion for the H-SAPO-34 and H-CHA structures. The interpretation of the respective 3677 and 3662 cm−1 bands of H-SAPO-34 and H-CHA is more controversial than that of the named Brønsted acidic sites. They do not show up in the computer models of these molecular sieves; i.e., they seem not to be associated with the structural Brønsted acidic sites. In the course of our literature survey we have found that those researchers who use DRIFT technique31−40 generally find these bands more intense than those who use TR technique.41−54 In case of the H-SAPO-34 most researchers assign the band near ∼3680 cm−1 to surface P−OH vibration, following Peri’s55 early empirical assignment from the TR spectra of an amorphous AlPO4. Yet, his spectra also showed the parallel appearance surface Al−OH vibrations near 3800 cm−1, which are absent from our and many other authors’ HSAPO-34 spectra (Figure 1a). The P−OH assignment certainly cannot be valid for the ∼3662 cm−1 band of the H-CHA. To our best knowledge, only Bordiga et al.56 assigned this band to “extra lattice or partially extra lattice Al−OH vibrations”. Many researchers associate the appearance of both the ∼3680 cm−1 H-SAPO-34 and ∼3660 cm−1 H-CHA FTIR bands with sample

sites, respectively. It is clear from the crystal geometry that these low- and high-frequency (LF and HF) vibrational band pairs might actually arise from four different Brønsted protons connected to four, geometrically differently positioned oxygen atoms when they surround an Al3+ ion, isomorphously substituting a tetrahedral Si4+ ion in the aluminosilicate CHA. These oxygens are indicated in Figure 2, which shows the unit

Figure 2. Unit cell of purely siliceous H-SSZ-13 structure shows two interconnected D6R units; its four geometrically distinct oxygen positions are marked (every [SiO4] tetrahedral unit is equivalent in this crystal lattice). Color codes: Si = yellow; O = red.

cell of a SSZ-13 crystal and allows visualizing the CHA structure as interconnected double-six-member siloxane rings (D6R). The geometry of H-SAPO-34 is identical but its tetrahedral building blocks are alternating [PO4]+ and [AlO4]− units, from which some of the P5+ ions are isomorphously substituted by Si4+ ions which generates the Brønsted acidity of this crystal. There is no total agreement between theorists, which of the four possible hydroxyls is responsible for the experimentally observed FTIR vibrations in these two molecular sieves. For example, Shah et al.19 have found that the HO1 and HO3 groups are energetically most stable on both SSZ-13 and H-SAPO-34, but their computed anharmonic νOH vibrations, which are actually seen in the experimental FTIR spectra, do not fit accurately the experimental values. Similar results were reported by Mihaleva et al.20 for H-CHA, based on 24047


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temperature. Next, a Swagelok valve, which separated the PY line from the evacuated sample, was opened and the system was let to equilibrate. After equilibration the Swagelok valve was closed, the sample chamber was evacuated and the sample was cooled from 150 °C to room temperature under vacuum for FTIR measurement. For desorption at higher temperatures, the sample was heated up still under vacuum and kept at the elevated temperature for 2 h. For measurement, it was cooled down again to room temperature. 2.2.2. Refractory Index Measurements. The refractory index of molecular sieve powders was measured by the Becke line method.60 We checked the halo around the particles in a drop of calibrated refractive index liquid by using a Leitz Wetzlar transmission phase contrast microscope. The correct index was gained at the borderline when, upon raising the focus, the higher and lower refractive index liquids caused to move the halo inward or outward, respectively. We used liquid sets differing by n = 0.004 refractivity index. Hence, this determined the accuracy of measurements. 2.3. Molecular Simulation. Density functional theory61 implemented in the program CASTEP62 was used to obtain equilibrium crystal structures. Each structure was geometry optimized using the Broyden, Fletcher, Goldfarb, and Shannon (BFGS) minimizer. Norm-conserving potentials were used. For H, O, Si, Al, and P the valence electrons included were 1s1; 2s2 2p4; 3s2 3p2; 3s2 3p1; and 3s2 3p3, respectively. The plane wave basis set cutoff was 750 eV. The k-point grid was kept to maintain a spacing of ca. 0.08 Å−1. For generalized gradient approximation (GGA) the functional of Perdew, Burke, and Ernzerhof (PBE)63 was employed. The convergence criteria for total energy, maximum force, maximum stress, maximum displacement, and SCF (self-consistent field) iterations were 2e−5 eV/atom, 0.05 eV/Å, 0.1 GPa, 2e−3 Å, and 2e−6 eV/atom, respectively. To simulate the FTIR spectra of periodic crystals, we started with these geometry optimized structures and computed the phonon density of states/vibrational spectra with CASTEP’s implementation of density functional perturbation theory (DFPT).64 A k-spacing of 0.0286 Å−1 was employed in sampling the vibrational density of states with sufficient resolution. The convergence criteria for SCF iterations of the ground state electronic structure was set to 5e−11 eV/atom or lower, to ensure convergence of the DFPT wave functions. To see the deviation of computed bulk spectra from the spectral vibrations of surface species, we calculated the IR spectra of periodic vacuum slabs created from the minimized mol sieve structures. For charge equilibration of the dangling bonds at the discontinued crystal surface, we created various bond “healings” (interconnecting [TO4] tetrahedral units via oxygen bridges; T could be Al, Si, or P atom depending on the material at hand), probed the effect of increased coordination from tetrahedral to higher values, and capped dangling oxygen atoms with protons. These surface modifications were carried out so that the overall vacuum slab became electrically neutral. Because of these changes, each vacuum slab was individually minimized again at the same way as the bulk crystal was using the CASTEP program, and thereafter the DFPT feature of this program was deployed to compute the vibrational spectra from the phonon density of states. The computed FTIR vibrations are illustrated by using Lorentzian line shapes. To approximate the line width of the experimental spectra, we adjusted them to their width at halfheight, usually 20−50 cm−1 depending on the spectrum at

hydrolysis, showing that the intensity of these bands substantially increases in the presence of H2O.41−45,56,57 It has been also reported that the acidic OH groups of both molecular sieves tend to connect at least 1 molecule, but often 3 or 4 H2O molecules, via hydrogen bonds.34,58,59 However, our measurements were carried out on samples calcined at 500 °C or higher temperatures in air, and in addition they were also in situ evacuated at ∼10−5 Torr at 500 °C before both the TR and the DRIFT measurements, which were carried out at room temperature afterward. This pretreatment removes all molecular water from the zeolites as one could confirm from the lack of any measurable 1640 cm−1 FTIR band, associated with the well-known bending vibration of molecular water. It follows that hydrolysis could not take place during our FTIR measurements. What is more, none of these band assignments could answer the question: why can the hydroxyl spectra be substantially different when measured by the TR or DRIFT technique? To clarify this issue, we carried out a number of experiments and DFT (density functional theory) based model calculations. We show in this paper that the distinct sensitivity of the two FTIR sampling techniques toward the surface and bulk hydroxyl groups is the key factor, which is not uniquely restricted to the H-CHA and H-SAPO-34 structures. In the course of this work we also identified and corrected some misconceptions about published vibrational peak assignments and associated every hydroxyl band visible in the TR and the DRIFT spectra with surface and bulk OH species.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Sample Sources and Preparation. Every molecular sieve tested was either a commercial or developmental product of Zeolyst International or its slightly modified version to compare the effect of crystal size and/or elemental composition. Spectroscopic quality pyridine (PY) was purchased from Aldrich. FTIR quality KBr was obtained from the International Crystal Laboratories. Accurate refractive index liquids were purchased from Cargille Laboratories. 2.2. Analytical Techniques. 2.2.1. FTIR (Fourier Transform Infrared) Measurements. Transmisson (TR) FTIR analyses were carried out on a Nicolet 6700 spectrometer from Thermo Scientific by placing self-supported material disks into a sample holder from CIC Photonics, Inc., evacuating it at 500 °C and 5 × 10−6 mbar for at least 2 h and obtaining the hydroxyl spectra after cooling them under vacuum to room temperature (estimated 23 ± 2 °C). DRIFT measurements were made on a Bruker IFS/66 spectrometer. 10% sample powder was thoroughly mixed and ground with KBr, and the mixture was placed either into the sample holder of a Praying Mantis unit from Harrick Scientific, Inc., or into the sample holder of a diffuse IR unit from PIKE Technologies. The sample pretreatment was identical with that of the TR technique. All DRIFT spectra were also measured at room temperature and were converted into Kubelka−Munk (KM) units. For pyridine (PY) adsorption measurements, samples were pretreated the same way as for the OH measurements, but cooled only to 150 °C. Liquid PY was placed into a quartz container and kept under vacuum to avoid moisture inlet. Before adsorption measurements, the liquid was frozen with liquid N2 while the space above the frozen liquid and the overall inlet line was thoroughly evacuated. After this the liquid N2 was removed and the PY was allowed to warm up to room 24048


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Figure 3. Comparison of the FTIR spectra of (a) the commercial, proton-exchanged, H-Y zeolite, CBV 500 and (b) the smaller and larger crystallite versions of the commercial, proton-exchanged ferrierite (H-FER), CP 914C, zeolites measured by the DRIFT and transmission (TR) technique.

Figure 4. Effect of Na exchange on the hydroxyl FTIR bands of (a) a H-SAPO-34 sample with nominal composition of Al3P1.8Si1.2H1.2O12 and (b) a H-CHA sample with nominal composition of Si2.2Al0.8H0.8O12.

DRIFT and the TR techniques. Another factor which can affect the reflection of material particles is their refractory index (n). For various zeolites and amorphous silica samples we found only small variation in this parameter, within the range from n = 1.440 ± 0.004 to n = 1.452 ± 0.004. The differences did not correlate with the appearance or lack of peak shifts between the TR and DRIFT spectra. To see if the most intense bands in the DRIFT spectra of HSAPO-34 and H-CHA are Brønsted acidic or not, we carried out ion exchange with alkaline and other cations. As an example with sodium exchange illustrates in Figure 4, every hydroxyl band got substantially reduced in both materials, no matter which sampling technique was used. Consequently, every hydroxyl has exchangeable, Brønsted acidic character, although those OH groups which are represented by the 3677 and 3670 cm−1 bands in the DRIFT spectra appear to be less prone for the exchange, i.e., probably less acidic than the 3600/3627 cm−1 and 3617/3643 cm−1 band pair represented Brønsted acidic sites in these materials, as discussed in the Introduction. Note that the composition of H-CHA in Figure 4 is somewhat different from that of the high Si/Al ratio H-CHA in Figure 1, which caused some peak shifts, but the difference of its DRIFT and TR spectra remained valid. The bands near 3740 cm−1 are well-known as usually nonacidic or very weakly acidic, isolated terminal silanol groups; hence, the ion exchange might or might not affect them.

hand. We also used GRAMS32 program from Thermo Scientific to combine the adequately increased or decreased computed surface spectra to match the DRIFT spectra.

3. RESULTS AND DISCUSSION After seeing repeated results of different DRIFT vs TR spectra with variously composed H-CHA and H-SAPO-34 samples similar to shown in Figure 1, we investigated the hydroxyl FTIR spectra on a number of other zeolites and amorphous silicas using both DRIFT and TR techniques. In Figure 3a the spectra of a commercial, ultrastabilized (US-Y) zeolite, CBV 500, exemplifies that the differently measured spectra are indeed roughly identical with each other on numerous samples as most researchers expect. Slight differences in the intensity ratios of the various bands are not unusual, but the peak positions are the same. On the other hand, the example of ferrierite (FER) in Figure 3b illustrates that the H-CHA and H-SAPO-34 are not alone with the position differences in peak maxima, which makes the differently measured spectra quite different. It is noteworthy to mention that always some higher wavenumber bands get more intense in the DRIFT measurement. Since DRIFT is a reflection technique, with which the vibrational spectra are known to be affected by the particle size, we tested zeolites of various crystallite sizes. Figure 3b illustrates that this parameter has not been found to have as significant effect on the overall appearance of spectra as the switch between the 24049


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Figure 5. FTIR spectrum of hydroxyl groups on a H-CHA (a) remains largely unchanged when pyridine (PY) is adsorbed onto the sample in a transmission (TR) sample holder at 150 °C but (b) substantially drops when this adsorption is done in a DRIFT cell and recovers upon desorbing the PY at higher temperatures.

Figure 6. Primitive cell of (a) a H-SAPO-34 structure with nominal composition of Al6P5SiHO24; its extralattice proton is connected to an O1 atom; and (b) a H−CHA sample with nominal composition of Si11AlHO24; its extra lattice proton is connected to an O2 atom (see the numbered O atom positions in Figure 2). Color codes: white = H; red = O; pink = Al; yellow = Si; green = P.

surface, to where the PY molecules could not penetrate. Notwithstanding, the intensity drop in this hydroxyl vibration range indicates that these bands mainly represent OH groups so close to the surface that the PY could interact with them, i.e., presumably not further than one or two atomic layers (vide inf ra at surface models). At elevated temperatures up to about 400 °C, the adsorbed PY molecules can be desorbed and the original spectrum recovered. We got exactly the same results on an H-SAPO-34 sample as well. This strong experimental proof for the preferred surface characterization of DRIFT spectra tempted us to learn more about the specificities of these surface hydroxyls, since the facilitated accessibility of external Brønsted acidic sites compared to those, which are in the micropores of these molecular sieves, might even bear of catalytic importance.65 For this end we carried out DFT based model calculations for the IR spectra of bulk and surface hydroxyls both on H-CHA and H-SAPO-34. First, we tested the accuracy of our modeling parameters by computing the bulk Brønsted acidic OH vibrations associated with the isomorphously substituted Al atoms in H-CHA and Si atoms in H-SAPO-34. We considered all four geometrically possible Brønsted sites, indicated in Figure 2, for both structures. To accelerate the calculations, they were made on the primitive cells of these crystals instead of their rhombohedral unit cell structures22,66 shown in Figure

Thus, we had to conclude that neither the chemical composition nor the particle size or the refractory index could cause the appearance of intense bands of acidic hydroxyls which appear in the DRIFT spectra at higher frequencies than the structural Brønsted acidic hydroxyl bands. Therefore, we speculated that these bands might arise from hydroxyls on the particle surfaces magnified by the reflection technique. To check the viability of this hypothesis, we carried out adsorption measurements with pyridine (PY) on these molecular sieves. This molecule is frequently used to measure selectively the Brønsted acidic sites by FTIR spectroscopy on various oxides since it gives a characteristic, distinctly measurable peak at 1540 cm−1. However, its kinetic diameter is about 5.85 Å, which does not allow it to enter the narrow channels of H-CHA and HSAPO-34. Thus, any measurable PY adsorption must occur on the surface of these molecular sieves. As the example of H-CHA illustrates in Figure 5, pyridine virtually did not affect the Brønsted acidic hydroxyl bands in the transmission cell, which shows mainly the bulk acidic sites. However, a dramatic drop in the hydroxyl intensity occurred when the same adsorption was carried out in the DRIFT cell. Consequently the DRIFT spectrum mainly shows surface Brønsted acidic sites. Note that the strongest drop occurred with the 3668 cm−1 band while the lower wavelength bands remained partly unaffected, presumably because they represent “bulk” acidic sites so deep from the 24050


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Figure 7. Experimental TR (copied from Figure 1) and computed OH spectra of H-SAPO-34 and H-CHA with different HOn positions where n = the oxygen position numbers in Figure 2. The model spectra for H-SAPO-34 are as computed and for H-CHA they are shifted up by 0.33%.

confined O3 related hydroxyls vibrate at much lower frequencies, near 3200 cm−1, where experimental FTIR bands were not observed. Consequently, HO3 hydroxyls are only theoretical possibilities, but are not present in the real HSAPO-34 and H-CHA structures. This result fits for example the result reported by Suzuki et al.,28 although their computed HO3 vibration was significantly higher, 3538 cm−1, than ours. For better visibility the computed HO3 bands were omitted from Figure 7, but we show them in Figure S1 in the Supporting Information, along with the detail model structures. As the selected computational parameters gave very reliable vibrational spectra for the bulk hydroxyls, we also used them to compute the surface specific hydroxyls. For this we created two to five primitive cell deep surface slabs. This allowed us to see how far from the surface vibrate the bulk Brønsted acidic sites unbiased and also to control if the calculation went in order; i.e., the bulk Brønsted sites gave the same spectra as seen from the periodic bulk calculation in Figure 7. We created slabs with various hkl index faces; at the top, at two-thirds and at one-half of the top primitive cell; and with putting D6R to the slab surface. Moreover, we tested all four possible hydroxyl positions for each slab variation and closed the inevitable dangling bonds at the surface discontinuation of the periodic lattice in ways which are widely considered to be realistic in the chemistry of metal oxides. These included bridging of oxygens between different tetrahedral units, capping the singular oxygens with hydrogens, making surface oxygens double bounded to the surface cation, or capping the incompletely coordinated top Al, Si, or P atoms with hydroxyl groups. We accepted the surface rearrangement only when the whole periodic slab became electronically neutral. Because of these inevitable distortions on the surface, every surface model had to be geometry optimized and energy minimized before IR model calculations could be carried out. In the course of these virtual experiments we learned several things: (i) The Brønsted acidic sites vibrated totally undisturbed and gave the same spectra as indicated in Figure 7, when they were only in the second or third primitive cell, i.e., quite close to the surface. Considering that the thickness of one cell is about 9.4 Å, their estimated distance from the surface was around 15−25 Å depending on how deep the first primitive cell was cut. In accordance with our deductions from the PY adsorption experiments, this implies that the surface sites become dominating in the DRIFT spectra only if the IR beam in the DRIFT cell does not penetrate deeper than one to three

2. With several parallel calculations we confirmed that the computed vibrational results with the primitive cells are totally identical with those obtained by computing the larger unit cell. Actually, these primitive cells correspond to the trigonal unit cells identified for mineral chabazites long time ago.2,3 One Al and one Si atom were inserted into the model primitive cells of H-CHA and H-SAPO-34, respectively. Figure 6 illustrates these structures showing HO1 and HO2 positioned Brønsted sites. Their nominal compositions are close to the composition of the experimental samples in Figure 1; thus, our computed models were compared to these spectra. As Figure 7 indicates, the computed spectra gave an excellent fit to the experimental TR FTIR bands: in both structures the O1 positioned hydroxyl is responsible for the higher frequency (wavenumber) bands while the O2 and O4 positioned hydroxyls overlap each other at the lower frequency band position. The ΔE (eV) energy differences for the HO1, HO2, HO3, and HO4 bonds, which characterize their stabilities, were found to be 0, 0.03, 0.37, and 0.08 for H-CHA and 0, 0.05, 0.46, and 0.08 for H-SAPO-34, respectively. These values are close to published data except for HO3, which we found much less stable than previously reported.19,26 The structures we used for calculation can be seen in the Supporting Information as .cif files, which allow to see every detail of the lattice parameters and atom positions. As mentioned in the caption of Figure 7, the computed vibrations of H-CHA had to be shifted up by a factor of 1.0033. This shift corresponds to some calculations which also used plane wave approach.19,26 Note however that we are not using the customary notation of scaling factor for computed frequencies, as a scaling factor is typically based on a full range (or a very limited number of subranges) of vibrational frequencies of a target system.67 In the present work, we focus on just the O−H stretching frequencies. The somewhat differing amounts of shifts, zero for H-SAPO-34 and 0.33% for H-CHA, can originate from difference in electron correlation and/or anharmonicity of the vibrations in the target systems,67 given identical basis set treatments in both calculations. Moreover, the difference is small enough to be comparable to an root mean square (rms) of difference between computed and experimental frequencies in the same target system,67 not to mention across different systems. None of the computed vibrations were related to the 3677 and 3662 cm−1 vibrations observed in the DRIFT spectra of HSAPO-34 and H-CHA, respectively (Figure 1). The most 24051


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Figure 8. Experimental DRIFT (copied from Figure 1) and computed OH spectra of surface slabs of H-SAPO-34 and H-CHA. The HO1 and HO2 positions are illustrated in Figure 2. Below the peak position and O assignment it is indicated if the vibration belongs to surface or bulk OH groups.

Figure 9. Surface models for (a) H-SAPO-34 and (b) H-CHA which gave most band for their combined spectra in Figure 8. Color codes are the same as in Figure 6.

constructed from periodic repetition of a truncated unit cell one can only test one type of Brønsted acidic hydroxyls at a time; let us say HO2 connected to either its Al−O or Si−O neighbor in a CHA unit cell. It is also limited how many types of other surface hydroxyls can be modeled within a cell, let us say by capping dangling P−O or Al−O oxygens by H away from the Brønsted acidic sites in a SAPO-34 structure. Same is valid for the interconnecting oxygens between different [TO4] tetrahedra on the surface, where T represents Si, Al, or P atoms. We have found however three surface slab results for both the HCHA and the H-SAPO-34, which could be combined to mimic quite nicely the experimental DRIFT spectra when the peak intensities were adequately adjusted, as Figure 8 illustrates. Figure 9 shows two surfaces for both molecular sieves which accounted for most of their model vibrations. The top figure in Figure 9a shows a H-SAPO-34 surface with HO1 sites connected to Al and Si atoms, resulting in model IR vibrations at 3679 and 3721 cm−1, respectively. On the bottom H-SAPO34 surface in Figure 9a the Al−HO1 bond resulted in 3674 cm−1 computed vibration and the calculated P−OH related vibration was 3559 cm−1. Both calculations gave back the expected vibration near 3627 cm−1 associated with the bulk HO1 Brønsted acidic site. A third surface slab with HO2 Brønsted sites was needed to generate the band at 3588 cm−1, which combined with the other two spectra gave exactly the well-known 3600 cm−1 peak illustrated in Figure 8. This third surface also resulted in a 3745 cm−1 vibration of Al connected

primitive cells. (ii) Very few surface models resulted in vibrations near the 3677 and 3662 cm−1 positions, for which we conducted these calculations. Exclusively HO1 vibrations on surface cuts from 100 and −100 directions gave intense bands near these positions without also generating other bands at positions where the experimental spectra showed nothing. (iii) In the case of H-CHA, the HO1 vibration was close to the 3662 cm−1 target whether it appeared on the surface as Si-connected (3663 cm−1) or Al-connected (3651 cm−1) group. In case of HSAPO-34 the computed 3678 cm−1 band only appeared when the surface HO1 was connected to Al atoms. (iv) Since the experimental DRIFT spectra did not indicate any bands near or above 3800 cm−1, it is unlikely that any significant AlxOy(OH)z type surface accumulation could occur.68−70 In line with this, the cross-section test of our H-CHA and H-SAPO-34 crystallites by EDAX showed totally homogeneous Al dispersion without any surface accumulation. (v) In contrast to the common assignments, these higher frequency bands could never be associated with surface P−OH vibrations, which always gave vibrations below 3570 cm−1. (vi) We tested the option of “healing” the surfaces by using higher than tetrahedral coordinations for Al and P which is known stable state for them in many nonzeolytic oxides and hydroxides. These models resulted in IR spectra quite different from the experimental ones. (vii) None of the computed surfaces could account alone for every IR vibration observed in the experimental DRIFT spectra. The main reason for this is that on a surface, 24052


The Journal of Physical Chemistry C HO2 on the surface, also indicated in Figure 8. In a very similar manner the reader can make the comparison between the HCHA related surface and bulk species shown in Figure 9b and the computed model spectrum in Figure 8. Details of the surface slab models and their computed IR spectra are shown in the Supporting Information. Finally, we wish to address a paradox phenomenon: namely, that the intensity of external isolated silanol groups vibrating near 3740 cm−1 is usually much lower in the DRIFT spectra than the intensity of the 3662 and 3678 cm−1 terminal Brønsted acidic sites. Note that compared to the internal Brønsted site intensities (near 3615 and 3630 cm−1) the intensity of the 3740 cm−1 band is about the same both in the DRIFT and the TR spectra. This suggests that the much higher intensity of the surface Brønsted sites in the DRIFT might be much more abundant on the crystallite surfaces than the nonacidic, isolated, terminal Si−OH. Notwithstanding, in lack of any relevant study, one cannot exclude either that DRIFT has an intrinsic quantitation problem. For example, using reflection Raman measurements for measuring the intensity of Si−O vibrations, it was found that even a 100-fold intensity difference can occur when the vibration belongs to a [SiO4] unit connected to three other neighbors versus the Si−O vibration of a terminal [SiO4] connected to only one other neighbor.71

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09247. Figures S1 and S2 (PDF) Structures of H-SAPO-34 (CIF) Structures of H-CHA (CIF)

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The authors appreciate support and permission for publication from Zeolyst International and PQ Corporation. We thank Ms. Larissa Ding for the electron microscopic and EDAX analysis of our samples.

4. CONCLUSIONS In this study we showed that the TR and DRIFT sampling techniques can result in quite different FTIR spectra. We show experimental and DFT based model evidence for H-CHA and H-SAPO-34 molecular sieves that the reason for the differences is that DRIFT emphasizes the vibrations of surface OH groups when such groups are present on the surface of crystallites. We demonstrated that the surface hydroxyls of these molecular sieves are Brønsted acidic and in contrast to the common belief they represent surface Al−OH and not P−OH groups. The computer models also showed that almost exclusively HO1 hydroxyls appear on the surface and a single surface model cannot account for every vibration observed in DRIFT spectra. We managed to get an excellent fit, however, when combined tree adequate surface slab results for both H-SAPO-34 and HCHA. The presented experimental and computer modeling studies allow a deeper understanding of fine details in the internal and external molecular structure of these practically important molecular sieves. We believe that the presented data adequately prove that the DRIFT technique represents the constitution of a thin surface layer of solid particles in general, which might or might not differ from the bulk composition.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (I.H.). Notes

The authors declare no competing financial interest. 24053


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