Insights into BrÃ¸nsted Acid Sites in the Zeolite ... - ACS Publications

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Insights into Brønsted Acid Sites in the Zeolite Mordenite Dmitry B. Lukyanov,*,† Tanya Vazhnova,† Nikolay Cherkasov,‡ John L. Casci,§ and John J. Birtill∥ †

Catalysis and Reaction Engineering Group, Department of Chemical Engineering, University of Bath, Bath BA2 7AY, United Kingdom ‡ Department of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia § Johnson Matthey Technology Centre Chilton, PO Box 1, Belasis Avenue, Billingham TS23 1LB, United Kingdom ∥ Department of Chemistry, Glasgow University, Glasgow, United Kingdom S Supporting Information *

ABSTRACT: The unique feature of the zeolite catalysts is the presence of catalytically active acidic hydroxyls, also known as Brønsted acid sites (BAS), in the zeolite micropores of molecular dimensions. The accessibility and catalytic properties of BAS depend on their local environment, and it is therefore important to know the exact locations of BAS and the number of BAS in these locations. This paper reports a detailed FT-IR investigation into BAS present in the acidic and partially Na-exchanged samples of industrially important mordenite (MOR) zeolite. Our results demonstrate the existence of (at least) six distinct BAS that can be visualized by six single bands in Fourier self-deconvolution traces of the IR spectra. The quantitative estimates for the amounts of these distinct BAS were obtained using the six-band deconvolution method developed in this work. These estimates show that in the purely acidic H-MOR sample about 25% of BAS are located in eight-membered ring (8-MR) channels (O1−H and O9−H hydroxyls), ∼13% of BAS are at the intersections between the side pockets and 12-MR channels (O5−H hydroxyls), and ∼62% of BAS are located in 12-MR channels (∼39% correspond to O2−H and/or O10−H hydroxyls and the remaining 23% to O3−H and O7−H hydroxyls). These quantitative data demonstrate that the acid sites are distributed quite evenly between oxygen atoms in different crystallographic positions, thus revealing the complexity of the experimental identification of distinct BAS in mordenites and explaining the variety of the earlier suggestions regarding their positions in these zeolites. Zeolite mordenite (MOR),7 one of the most industrially important zeolites,1,2,8,9 represents an excellent example of a zeolite where catalytic properties of BAS are significantly influenced by their local environment.10−13 This zeolite contains two essentially different channels (Figure 1): large 12-membered-ring (12-MR) channels (7.0 × 6.7 Å) and small 8-MR channels (5.7 × 2.6 Å) that run in parallel and are connected by 8-MR openings (4.8 × 3.4 Å).7,14 The latter are also known as side pockets. There are four crystallographically different T atoms (T1−T4) in the MOR structure that are linked to ten crystallographically distinct oxygen atoms (O1− O10). Hence, in principle, there could be ten different acidic hydroxyls (BAS) associated with ten oxygen atoms, e.g., O1−H, O2−H, etc. Note that the existence of a variety of distinct hydroxyls would agree with the established locations of Na+ cations (positions I, IV, and VI in Figure 1) within the channels of Na-containing mordenites.15 Obviously, the accessibility and catalytic properties of the BAS in different locations should be very different due to the different local environments of O1−O10 atoms (Figure 1).

1. INTRODUCTION Zeolites are crystalline microporous aluminosilicates that constitute a very important class of industrial catalysts.1,2 Their frameworks are built from TO4 tetrahedra (T = Si or Al), where every oxygen atom is shared between two T atoms. The chemical composition of a zeolite framework can be therefore expressed as a combination of the SiO2 and AlO2 units. SiO2 units are neutral, but every AlO2 unit has one negative charge that is normally compensated by a cation, e.g., Na+ or K+. Metal cations do not form a part of the framework and can be exchanged by other cations, including protons. In this latter case, zeolites contain catalytically active Brønsted acid sites (BAS), i.e., hydrogen atoms attached to the oxygen atoms of the zeolite framework.3−5 The presence of BAS (acidic hydroxyls) in the confined environment of zeolite micropores (channels) is a characteristic feature of many zeolite catalysts that is responsible for their catalytic properties including shapeselective effects.1−5 The local environment of BAS affects not only the availability of these sites for reactants but is also important for stabilization of reaction intermediates and/or transition states.4−6 Therefore, in order to grasp the mechanisms of zeolite-catalyzed reactions on a molecular level, it is critically important to know the exact locations of BAS within zeolite micropores. © 2014 American Chemical Society

Received: August 26, 2014 Revised: September 19, 2014 Published: September 22, 2014 23918

dx.doi.org/10.1021/jp5086334 | J. Phys. Chem. C 2014, 118, 23918−23929

The Journal of Physical Chemistry C

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bands, which we tentatively assigned to acidic hydroxyls located in the large and small channels of the MOR structure.31 Our work has provided a rationale for the apparent discrepancies between different locations of BAS in mordenites proposed in the literature, but it has not identified the exact locations of BAS. Also, the contradiction between the finding of six distinct locations of BAS in mordenites 31 and the theoretical prediction14 of the existence of maximum four possible locations has not been explained. This current study was undertaken to verify our initial data on the existence of six distinct BAS in the MOR structure31 and to generate insights into exact locations of these sites. To achieve this goal, we conducted a thorough FT-IR investigation of the purely acidic and partially Na-exchanged mordenite samples, including adsorption of NH3 and n-hexane molecules as well as FSD analysis of the IR spectra. We then contrasted our finding of the six distinct BAS to the theoretical analysis14 predicting the existence of only four different BAS in mordenites and arrived at a conclusion relevant to the distribution of BAS (and Al atoms) in different zeolite structures. During the final stage, we analyzed the exact locations of the six distinct BAS and determined their amounts in the MOR samples used in this study.

Figure 1. Structure of the MOR zeolite shown alongside large 12-MR and small 8-MR channels with three positions of Na+ cations (I, IV, and VI) that are marked according to their classification in the original report.15

Thus, to understand the catalytic properties of the distinct BAS in mordenites, we have first to answer the following two questions. How many distinct BAS are in the MOR zeolites? What are the exact positions of these distinct BAS? The answers to these questions are still unknown, although they have been sought in the experimental and theoretical studies for a long time. The early crystallographic studies of the single crystals of the natural mordenite (ptilolite) in the H-form have shown that there is no preferential attachment of protons to any particular framework oxygen atoms,16 thus indicating the possibility of a variety of distinct BAS. This result is in agreement with theoretical studies of the exact location of BAS in MOR zeolites that (i) show that there is no significant difference in terms of energy between different locations17−20 and (ii) designate different locations for the most energetically favorable sites when carried out by different authors that use different methods, e.g., sites O2−H and O6−H,17 site O3−H,20 and site O7−H.21 On the other hand, many research groups that studied BAS in mordenites by IR spectroscopy considered BAS in mordenites as two populations only, one being located in the large 12-MR channels and another one in the small 8MR channels or/and side pockets.10,11,22−27 This consideration was based on the deconvolution of the observed asymmetric IR band at ∼3610 cm−1 into two components that were treated as single bands. Further differentiation between BAS in mordenites has been proposed in a number of experimental and theoretical studies,8,14,17,19,20,29,30 but the suggested numbers of distinct BAS varied (from 3 to 5) and the assignments of these BAS to particular oxygen atoms were contradicting. For example, the preferred location of BAS in the small channels was identified on O1,20,30 O617,28,29 and O98,14,19,30 atoms, while all authors considered the existence of only one distinct type of BAS in the small channels. The agreement on the exact locations of BAS in the large channels was also quite poor, since, in fact, all possible locations available in these channels (see Figure 1) were suggested by different authors: O2,17 O3,20 O2 and O7,8,14,19 O5 and O10,29 O2, O5, and O10.30 Recently, our group reported the first application of the Fourier self-deconvolution (FSD) method for the analysis of IR spectra of functional groups in porous materials.31 Using the 3610 cm−1 band in the IR spectra of acidic MOR zeolites as an example of the FSD-IR analysis, we have concluded that this asymmetric band is, in fact, a superposition of six individual

2. EXPERIMENTAL SECTION 2.1. Zeolite Samples. The initial Na-MOR zeolite (Si/Al = 7.3) was purchased from Zeolyst. This zeolite was ionexchanged at 60 °C (3 h) three times with aqueous solution of NH4NO3 (solution-to-solid ratio was 20 to 1). The obtained NH4MOR sample was dried at room temperature (RT) for about 48 h and was then kept in the sealed container. The sodium content in this sample was below 0.01 wt %, as determined by chemical analysis. Three partially Na-exchanged mordenites were prepared by single ion exchange of the NH4MOR sample with the aqueous NaNO3 solutions of different concentrations (60 °C, 3 h, solution-to-solid ratio was 20). To avoid possible dehydroxylation and dealumination, which can easily occur with MOR zeolites,30,32 all NH4 samples were calcined carefully in the purpose-built IR cell connected to the vacuum system:33,34 the samples were kept under vacuum (10−6 mbar) at RT for 0.5 h and then heated (1 °C/min) to 450 °C and kept at this temperature for 6 h. The IR spectra showed that the NH4 forms were completely converted to Hforms under these conditions. Therefore, the four samples, used in this study, are designated as H-MOR, H82Na18-MOR, H63Na37-MOR, and H45Na55-MOR zeolites in accordance with their Na-exchange levels (0, 18, 37, and 55%, respectively). 2.2. FT-IR Studies. IR spectra of self-supported zeolite disks (10−15 mg/cm2) were recorded at RT using either Nicolet Magna 550 or Bruker Equinox 55 FT-IR spectrometers (both equipped with MCT detectors) and following established procedures.22,33,34 All spectra were collected using 100 scans and a resolution of 2 cm−1. The adsorption of NH3 was carried out at 150 °C to avoid physical adsorption of ammonia, while n-hexane was adsorbed at RT. During stepwise adsorption or desorption of ammonia, the zeolite disk was moved to the heated zone and after treatment was returned to the initial position for recording of a spectrum (in fact, in every case two spectra were recorded). FSD of the IR spectra was carried out using Nicolet OMNIC software, and quantitative deconvolution was performed employing in-house codes written in Matlab R2012a. The methodology and details of the deconvolution approach have been described recently.32 23919



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3. RESULTS AND DISCUSSION This section is organized as follows. Section 3.1 reports the study of the nature and number of acid sites in the purely acidic H-MOR zeolite. In section 3.2 we analyze the asymmetric 3610 cm−1 band in the spectra of the H-MOR zeolite with adsorbed ammonia and n-hexane and conclude that this band is a combination of at least four single bands. In section 3.3 we discuss the IR spectra of the partially Na-exchanged MOR samples and explain the observed shift of the 3610 cm−1 band to higher wavenumbers with increasing Na content. Section 3.4 outlines the main features of Fourier self-deconvolution method and describes its application for the analysis of four MOR samples used in this work. In section 3.5 we discuss the apparent contradiction of our finding of six distinct acidic hydroxyls with the theoretical prediction of existence of maximum four distinct BAS in mordenites and comment on the distribution of BAS and Al atoms in mordenites and other zeolites. We then consider the exact locations of six distinct BAS in mordenites. In section 3.6 we discuss the accessibility of these BAS to the molecules of n-hexane (at 25 °C). Finally, in section 3.7, we describe quantitative deconvolution of the experimentally observed 3610 cm−1 band using six single peaks and consider the amounts of the different BAS in the four MOR samples studied in this work. 3.1. Acid Sites in the Parent H-MOR Zeolite. As it follows from the Introduction, a “perfect” purely acidic zeolite, i.e. the zeolite that has no defects, should have all Al atoms in the framework positions. Therefore, such a zeolite should contain only Brønsted acid sites with their number equal to the number of the framework Al atoms. In practice, however, it is difficult to obtain a perfect zeolite sample because such processes as dehydroxylation and dealumination could easily take place during preparation stages of zeolite samples,8,29,30,32,34−37 including samples of the zeolite mordenite.8,29,30,32,35,37 Both processes lead to the disappearance of BAS and the creation of Lewis acid sites (LAS) and are likely to alter the distribution of BAS in the zeolite structure, since the closely positioned BAS36 and the BAS in certain positions37 are most likely to be affected. Another important feature of the partial dealumination of a zeolite framework that is well established for such zeolites as MOR,35,38 FAU,39−41 and MFI35,42−44 is the creation of the acid sites possessing enhanced activity. This feature becomes critically important in the IR studies combined with quantitative catalytic studies of BAS, as the presence of enhanced activity sites, which are difficult to quantify, may alter the zeolite catalytic activity essentially. Importantly, as has been discussed in the literature,39−41 the presence of enhanced activity sites in acidic zeolites can be unmasked, although not quantified, by detecting Lewis acid sites or/and hydroxyl groups attached to extraframework Al species (these hydroxyls can be normally observed at ∼3665− 75 cm−1). In this work we have attempted to prepare a “perfect” purely acidic H-MOR sample that contains BAS only. The IR spectrum (OH region) of this sample is shown in Figure 2 and features two peaks: the weak peak at 3745 cm−1 corresponds to nonacidic terminal silanol groups and the intensive peak at 3610 cm−1 is associated with acidic hydroxyls that bridge Si and Al framework atoms.22,23 The spectrum gives no indication of the presence of the hydroxyls associated with extraframework Al species and is similar to the IR spectra of acidic mordenites reported in the literature.8,22−27 The

Figure 2. Hydroxyl region of the IR spectrum of the H-MOR zeolite (top curve) and its second derivative (bottom curve, multiplied by 50).

asymmetric shape of the 3610 cm−1 band as well as its second derivative (Figure 2) reveals the heterogeneity of this peak. This feature was first explained in 1993 by Zholobenko et al.22 and Wakabayashi et al.23 by the existence of two overlapping IR bands with the maxima around 3585 and 3610 cm−1. These two bands, termed low-frequency (LF) and high-frequency (HF) bands, were assigned to the BAS located in the small 8-MR and large 12-MR channels, respectively,22,23 and this assignment was later confirmed by many authors.8,24−27 Note that the second derivative of the 3610 cm−1 peak, shown in Figure 2, indicates possible existence of two closely positioned single bands at ∼3581 and ∼3588 cm−1. As stated above, the dealumination of zeolites results in a loss of BAS and is normally associated with the creation of LAS.34 To determine the quantity of the BAS and the presence (or absence) of the LAS in the H-MOR zeolite, we performed stepwise adsorption of ammonia on this zeolite following wellestablished procedures.22,32,33 The ammonia desorption was used for qualitative assessment of the strength of acid sites in this zeolites. The results of adsorption experiments (Figure 3a,b) demonstrate (i) that all acidic hydroxyls are accessible and interact with small NH3 molecules, (ii) that silanol OH groups are not affected by ammonia adsorption, and (iii) that there are no LAS in the H-MOR zeolite, as evidenced from the absence of the ∼1620 cm−1 peak corresponding to NH3 adsorption on LAS.24 The amount of NH3 adsorbed on the H-MOR sample was used to determine the total number of BAS in this sample, as shown in Figure S1 of the Supporting Information. The obtained estimate, that is 1740 μmol/g, is in excellent agreement with the number of BAS calculated on the basis of the Al content (1755 μmol/g) with the assumption of all Al atoms being located in the framework positions. This finding allows us to conclude that the H-MOR zeolite analyzed in this work contains all aluminum atoms in the framework. 3.2. Analysis of the 3610 cm−1 Band in the Purely Acidic MOR Zeolite. Let us now have a closer look at the asymmetric 3610 cm−1 band that is associated with the acidic hydroxyls (BAS) in MOR zeolites. From Figure 3a it is clear that ammonia adsorption on the H-MOR sample at 150 °C results in a progressive decrease of the intensity of this band but has no effect on its position and shape (the bandwidth remains 23920



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Figure 4. Hydroxyl infrared region during NH3 desorption from the H-MOR zeolite at different temperatures (°C).

ref 24 and Figure 3 in ref 27), has not been recognized as an additional single band in the original reports.24,27 Having established the existence of at least two distinct BAS (HF and TF bands) in the large MOR channels, we decided to probe the heterogeneity of the asymmetric 3610 cm−1 peak in the H-MOR zeolite by n-hexane adsorption experiments, similar to the experiments that were used previously in the analysis of the BAS in FER32 and SUZ-433 zeolites. The results are presented in Figure 5, which shows the effect of n-hexane adsorption on the different components of the IR spectrum. The following three observations are worth noting. First, as the amount of adsorbed n-hexane increases, the position of the observed IR peak corresponding to acidic hydroxyls shifts from 3610 to 3591 cm−1 (Figure 5a). Notably, this final frequency is different from the well-known frequency22,23 of the LF band (3585 cm−1). Second, the IR signal, which is removed from the spectrum by adsorbed n-hexane due to its interaction with accessible acidic hydroxyls, is centered at 3616 cm−1 (Figure 5b), which is different from the well-known frequency22,23 of the HF band (3610 cm−1). Both observations reveal that the initially observed 3610 cm−1 band should be a superposition of more than just two single bands (3585 and 3610 cm−1). Moreover, it is clear that the observed 3591 cm−1 band (Figure 5a) cannot be explained by the existence of the three single bands at 3585, 3610 and 3620−22 cm−1. Finally, the comparison of the area of the initial 3610 cm−1 peak with the areas of the remaining 3591 cm−1 (Figure 5a) and disappeared 3616 cm−1 (Figure 5b) peaks allows us to conclude that ∼47% of the initial peak area is not affected by n-hexane adsorption. These data, together with the IR spectra shown in Figure 5, will be considered in more detail in section 3.6 in relation with the discussion of the number and accessibility of the distinct BAS in the H-MOR zeolite. 3.3. IR Spectra of the Partially Na-Exchanged MOR Zeolites. Three Na-containing MOR zeolites were prepared using NH4-MOR as the parent zeolite and were designated according to their Na-exchange levels as H82Na18-MOR, H63Na37-MOR and H45Na55-MOR samples. The IR spectra of these zeolites in the hydroxyl region are shown in Figure 6 together with the spectrum of the parent H-MOR zeolite. As expected, the replacement of acidic protons with the Na+

Figure 3. Hydroxyl (a) and ammonia (b) infrared regions during ammonia adsorption on H-MOR zeolite at 150 °C: initial sample and after addition of different amounts of ammonia (mmol/g).

∼43 cm−1 in all spectra). Such a behavior is in agreement with previous studies22,23 and points out to the ammonia adsorption on all BAS without any preference to the sites located in the small or large channels. A very different situation is observed during ammonia desorption at 200 °C (Figure 4) when NH3 desorbs selectively from the BAS located in the large channels, as evidenced by the high frequency (3620 cm−1) of the observed IR peak. As desorption temperature increases from 200 to 400 °C, the position of the observed band moves from 3620 to 3610 cm−1 (Figure 4). Simultaneously, the bandwidth increases (from 33 to 43 cm−1) and the band shape becomes less symmetric. Importantly, the frequency of the band observed at the desorption temperature of 200 °C (3620 cm−1) is essentially different from the position of the wellknown22,23 HF band (3610−12 cm−1). Therefore, the 3620 cm−1 band is likely to be associated with the BAS that are distinct from the BAS that give rise to the 3610 cm−1 band (notwithstanding that both types should be located in the large MOR channels). Interestingly, this “top-frequency” (TF) band, although it has been previously observed at ∼3622 cm−1 in the partially ion-exchanged Na85H15-MOR zeolites (Figure 3A in 23921



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Figure 6. IR spectra (OH region) of the H-MOR, H82Na18-MOR, H63Na37-MOR, and H45Na55-MOR zeolites.

band at 3600−5 cm−1 was observed.8 To get an insight into the exact number of the individual components in the nonresolved 3610 cm−1 band in the IR spectra of H,Na-MOR zeolites, we have performed Fourier self-deconvolution of these spectra following our initial study of the BAS in purely acidic mordenites with the Si/Al ratios of 6 and 10.31 3.4. Fourier Self-Deconvolution of the IR Spectra of the H,Na-MOR Zeolites. The FSD method was originally proposed for enhancement of the resolution of poorly resolved IR spectra of proteins in the region of vibrations of amide groups.45−47 As in the case with acidic hydroxyls, the IR bands of different amide groups overlap in such a way that they cannot be resolved by the improvement of the instrument resolution, since the distance between the maxima of the single IR bands is too small in comparison with the width of these bands. To overcome this inherent problem in the IR spectra of proteins, Kauppinen et al.45−47 developed a theory of FSD that in practice employs computer processing of spectral data using Fourier transforms. As the result of this processing, the width of the spectral lines can be significantly reduced (by a factor of ∼3), thus increasing the degree to which individual IR bands can be visualized.45 It should be noted however that this enhancement comes at a price of increasing the noise and losing information about band shapes and intensities of the convoluted bands. Significantly, FSD cannot resolve an IR band into single peaks if the individual components are not inherent in the original spectrum. The successful application of FSD depends critically on three parameters. First is the line bandwidth, which is usually defined as the full width of the spectral line at the half-maximum (fwhm). Second is the enhancement factor (EF) that is the ratio of the fwhm before deconvolution to that after deconvolution. The third parameter is the signal-to-noise (STN) ratio in the original IR spectrum, since it has to be sufficiently high to allow successful resolution of the spectrum into individual peaks. For further details of the FSD-IR method and additional (general) guidelines for its use, we refer readers to the original FSD publications and excellent reviews of this subject.45−50 In spite of successful use of the FSD-IR technique in biochemical and biophysical research since 1981,48,49 the first

Figure 5. Adsorption of n-hexane on the H-MOR zeolite at 25 °C. (a) IR spectra of (1) initial sample in the absence of n-hexane and after injection of the different amounts of n-hexane: (2) 0.2, (3) 0.5, (4) 0.8, (5) 1.3, and (6) 1.8 μL. (b) Difference spectra.

cations manifests itself in decreasing intensity of the 3610 cm−1 band. In addition, the shift in the frequency and the change in the bandwidth are evident. Similar changes were observed in previous studies, and the change in the bandwidth was related to the selective removal of the LF band, corresponding to BAS in the small channels, thus leading to a narrower peak in the recorded spectrum.8,11,26,27 Importantly, the change in the position of the maximum of the observed peak (3610 to 3616 cm−1), which is apparent from Figure 6, was not explained in the previous reports.8,11,26,27 This shift, however, can be easily explained now by the presence of (at least) two distinct BAS in the large channels that give rise to the single bands at 3610 and 3620−22 cm−1. The results discussed above reveal the existence of three components (3585, 3610, and 3620−22 cm−1) contributing to the observed asymmetric 3610 cm−1 peak. However, these results cannot explain the IR spectra recorded during n-hexane adsorption experiment (see Figure 5), indicating the presence of at least one more component. This indication agrees well with the IR study of a MOR zeolite (Si/Al = 10) where a weak 23922



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application of this method in the analysis of functional groups in porous materials was reported only recently.31 This first communication highlighted the critical importance of the choice of the bandwidth of the single spectral line in the FSDIR analysis of acidic hydroxyls and demonstrated the validity of all relevant comments and observations made in the original work on FSD of the IR spectra of proteins.45−47 It has also pointed out to relatively narrow single bands of acidic hydroxyls in mordenites that appear to be in the range between 20 and 24 cm−1,31 as opposed to 30−45 cm−1 used previously.11,22−26 As stated in the Introduction, one of the objectives of this work was to verify the FSD results reported in the first communication.31 Therefore, during the first phase of this study we investigated the effects of the bandwidth and EF on the FSD traces of the IR spectra of four MOR zeolites. The results of this work are illustrated by the FSD traces (Figures 7 and 8) obtained for the H-MOR and H82Na18-MOR samples only, since the FSD traces of the H63Na37-MOR and H45Na55-MOR samples, as will be shown below, were poorly resolved. Analysis

Figure 8. FSD traces of the IR spectrum of the H82Na18-MOR zeolite. (a) FSD was carried out with the EF of 3.2 and the bandwidth of 18 (bottom), 20, 22, 24, and 26 cm−1. (b) FSD was carried out with the bandwidth of 22 cm−1 and the EF of 2.8 (bottom), 3.0, 3.2, 3.4, and 3.6. The original IR spectrum is shown in Figure 6.

of Figures 7 and 8 confirms the main conclusion of the earlier FSD-IR study of three purely acidic mordenites31 that is the existence of the six single bands in the IR spectrum of acidic hydroxyls in mordenites. Examination of the FSD traces in terms of the quality of their resolution and the degree of overdeconvolution (troughs and wavelike features) points out that the optimum FSD traces are obtained with the bandwidth of 20−22 cm−1 and the EF of 3.1−3.2. These optimum values agree very well with the values reported earlier (fwhm = 22 cm−1, EF = 3.0).31 Figure 9 compares the FSD traces of four MOR samples that were generated using the fwhm of 22 cm−1 and the EF of 3.1. These traces demonstrate that, as Na-exchange progresses, the intensities of the six peaks change in different ways, thus yielding a basis for meaningful explanation of the shift in the position and the change in the bandwidth of the observed peak. Moreover, these FSD traces provide a compelling opportunity to visualize the exchange of acidic protons in different locations, which we consider in section 3.5.

Figure 7. FSD traces of the IR spectrum of acidic hydroxyls in the HMOR zeolite. (a) FSD was carried out with the EF of 3.2 and the bandwidth of 18 (bottom), 20, 22, 24, and 26 cm−1. (b) FSD was carried out with the bandwidth of 22 cm−1 and the EF of 2.8 (bottom), 3.0, 3.2, 3.4, and 3.6. The original IR spectrum is shown in Figure 6. 23923



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(Figure 1) two atoms (O4 and O8) cannot be involved in the formation of BAS. This conclusion follows from the fact that all acidic hydroxyls are accessible and interact with NH3 molecules (Figure 3a), while, as concluded by Alberti,14 the hydroxyls O4−H and O8−H, if they do exist, would not be accessible even to these small molecules. Two single bands, observed at 3581 and 3590 cm−1 (Figure 9), can be easily assigned to two of the three oxygen atoms (O1, O6, and O9) located in the small channels (Figure 1). This assignment is based on the well-known fact22−26 that the acidic hydroxyls in the small channels are responsible for the LF band around 3585 cm−1, which was considered as a single band in these previous studies. Our work demonstrates that at least two oxygen atoms (among O1, O6, and O9) are involved in the formation of BAS in the small channels of mordenites. This new insight links together the earlier (contradicting) suggestions on the preferable location of BAS on the O1,20,30 O6,17,28,29 and O98,14,19,30 atoms. The oxygen atoms O1 and O9 are preferred over O6 atom in our assignment because they are linked to T3 positions (Figure 1) that are much more likely to be occupied by Al atoms14,37,52 in comparison with T1 positions that are associated with the O6 atoms. Note that the existence of the O6−H hydroxyls cannot be ruled out on the basis of the available data, as their stretching frequency can coincide with the vibration of other hydroxyls (at 3581 or 3590 cm−1). The 3599 cm−1 band may belong to either the LF or HF parts of the IR spectrum, and its assignment is therefore not straightforward. According to Marie et al.,8 pyridine adsorption on an acidic mordenite (Si/Al = 10) has led to the disappearance of the 3610 and 3600−5 cm−1 bands, thus indicating the association of the 3599 cm−1 band with the BAS in the large channels (oxygen atoms O2, O3, O7, and O10) or at the entrance of the side pockets close to the large channels (oxygen atom O5). Before proceeding with further discussion of the exact locations of the BAS, let us note that the same five oxygen atoms (O2, O3, O5, O7, and O10) should be considered for the acidic hydroxyls vibrating at 3609, 3617, and 3625 cm−1, as this high-frequency part of the IR spectra disappear upon pyridine adsorption.8,9,22−26 The locations of the distinct BAS in the MOR channels should be directly related to the locations of metal cations in the metal-exchanged mordenites. The latter have been established in a series of neat papers by Schlenker et al. for different cations (see, for example, refs 15 and 53), and the locations of the Na+ cations are shown as positions I, IV, and VI in Figure 1 (the ion-exchange positions are marked according to their classification in the original report15). Clearly, position I corresponds to the acidic hydroxyls O1−H and O9−H, as it follows from the assignment of the 3581 and 3590 cm−1 peaks. Figure 1 demonstrates that position IV is related to the acidic hydroxyls that may involve oxygen atoms O2, O5, and O10 and that position VI is linked to the oxygen atoms O3 and O7. Now let us consider the FSD traces shown in Figure 9. Introduction of the first Na+ cations into H-MOR zeolite results in a dramatic decrease in the intensities of the two LF bands (3581 and 3590 cm−1) and two HF bands (3599 and 3609 cm−1), leaving the intensities of the two TF bands (3617 and 3625 cm−1) unchanged. Further increase in the Na content (compare the spectra of the H82Na18-MOR and H63Na37-MOR zeolites) leads to suppression of the LF and HF bands, while the intensities of the TF bands appear to remain stable. Only, when the Na content is above 50%, these TF bands appear to

Figure 9. FSD traces of the IR spectra of the H-MOR, H82Na18-MOR, H63Na37-MOR, and H45Na55-MOR zeolites. FSD was performed with the bandwidth of 22 cm−1 and the EF of 3.1. The original IR spectra of these zeolites are shown in Figure 6.

3.5. Location of the Distinct BAS in the MOR Zeolites. In this section we discuss the assignment of the six distinct BAS to specific oxygen atoms in the MOR framework. Let us start this discussion with consideration of the apparent contradiction of our finding of the six distinct BAS in mordenites with the analysis of the possible different locations of BAS in these zeolites.14 This analysis was focused on the identification of different sets of framework oxygen atoms that can host BAS. With a number of clearly identified and justified assumptions, it has led to the conclusion that the maximum number of the BAS in the different crystallographic locations could be no more than four. Notably, this original work was noted but not questioned in the later studies of different BAS in mordenites.8,29,30 Our work, on the other hand, points out that the number of distinct BAS in mordenites is no less than six (see Figures 7−9), thus questioning the validity of the presented FSD-IR results or earlier analysis.14 Thorough consideration of the original study reveals, however, that one assumption, which was used in the analysis, was not explicitly noted in the paper, quite likely because of its “obvious” nature. Indeed, it was “silently” assumed that a unit cell should be considered as a repeating unit for description of the distribution of BAS in the MOR framework. This assumption looks quite natural at the first glance, but we have not found experimental or theoretical evidence that the ideal unit cell is the right unit for the analysis of the distributions of BAS in zeolites. The occurrence of six distinct BAS in mordenites, as evident from Figures 7−9, indicate that distribution of BAS (and, concurrently, of aluminum atoms) is not necessarily repeated from one unit cell to another. This view does not imply that distributions of BAS and Al atoms in mordenites are completely random, and therefore, it is not in contradiction with the results of extensive studies of the aluminum distribution in zeolite frameworks.51 According to our interpretation of the FSD-IR data, six single peaks in the FSD traces shown in Figure 9 correspond to six BAS that are associated with six different oxygen atoms in the MOR framework. Note that among the ten crystallographically different oxygen atoms (O1−O10) in the MOR structure 23924



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that remains in the observed spectrum (Figure 5a) has the maximum at 3591 cm−1 (LF part). Significantly, this latter peak is not positioned between two single bands (3581 and 3590 cm−1), as one would expect from the assignment of these bands to the O1−H and O9−H hydroxyls. Hence, its position at 3591 cm−1 suggests that it is likely to be a superposition of three single peaks with the wavenumbers at 3581, 3590, and 3599 cm−1. The appearance of the 3599 cm−1 band here can be explained by quite possible restricted access to the O5−H hydroxyls that give rise to this band (see section 3.5). Accordingly, we suggest that the O5−H hydroxyls are not accessible to the n-hexane molecules at ∼25 °C but appear to be accessible for interaction with much stronger base, pyridine, at 150 °C, as was noted by Marie et al.8 On the other hand, the O2−H and O10−H hydroxyls, which vibrate in the large channels and give rise to the 3609 cm−1 single band, should be accessible not only for pyridine but also for n-hexane. Given the high accessibility of the O3−H and O7−H hydroxyls (IR bands at 3617 and 3625 cm−1), this conception neatly explains the position of the peak (3616 cm−1) that was removed from the spectrum upon n-hexane adsorption (Figure 5). Hence, if our results and their interpretation are correct, then four acidic hydroxyls, O2−H and O10−H (3609 cm−1 band) and O3−H and O7−H (3617 and 3625 cm−1 bands), which interact with n-hexane molecules, should be responsible for about 53% of the area of the observed 3610 cm−1 band. This prediction is in excellent agreement with the value of 55% that has been obtained, as shown in section 3.7 (Table 2), by the six-band quantitative deconvolution of the experimental IR spectrum of the H-MOR zeolite. Note that here we are using the values of areas which are not affected by possible uncertainties associated with the values of extinction coefficients (the latter are used at the next stage of our analysis that is the determination of the numbers of acidic hydroxyls responsible for the IR bands). 3.7. Amounts of the Distinct BAS in the H,Na-MOR Zeolites. The amounts of the different BAS were determined on the basis of the quantitative deconvolution of the 3610 cm−1 band. This was performed using six Voigt functions, as these functions describe most accurately the combination of the Gaussian and Lorentzian components that arise due to chemical inhomogeneity of the nearest neighbors of BAS and collision-like broadening caused by perturbation with hydrogen bonds.32,58−60 Every band was defined by four parameters: position, width, fraction of the Lorentzian component, and intensity. During the deconvolution, however, only intensities of the single bands were optimized, while the positions, widths, and Lorentzian fractions were predetermined. The positions were selected based on the Fourier self-deconvolution results (Figure 9), and the Lorentzian fractions were changing linearly from 75% to 15% with the band position in accordance with our recent analysis of deconvolution methods.32 Bandwidths and relative extinction coefficients were calculated using the experimental correlations (1) and (2) that were established by Makarova et al.55 for the IR bands of acidic hydroxyls perturbed by various bases.

be slightly affected (Figure 9). The discussed changes reveal that the six single bands can be considered as three pairs, where each pair corresponds to a particular ion-exchange position. The pair of the LF (3581 and 3590 cm−1) bands has been already assigned to the O1−H and O9−H hydroxyls vibrating in the small channels. The TF (3617 and 3625 cm−1) pair of bands can be assigned to the O3−H and O7−H hydroxyls (position VI) because, due to their location, these hydroxyls are likely to vibrate in the largest openings, and therefore, their frequencies should be the highest.4,54−57 Hence, the remaining two HF bands (3599 and 3609 cm−1) should be associated with at least two of the three oxygen atoms O2, O5, and O10, which correspond to the ion-exchange position IV. The assignment of the six single IR bands to the BAS in specific locations, when considered together with the spectra shown in Figure 9, reveals the following steps in the ionexchange process. At the beginning, the Na+ cations occupy positions I and IV in such a way that at the 50% exchange level nearly all positions I appear to be occupied. At this level, a few Na cations are placed in the positions VI, while many positions IV are still available. According to the literature data,24,27 we can conclude that positions IV are fully occupied at ∼85% Naexchange level, as the IR band of acidic hydroxyls in H15Na85MOR samples is centered at ∼3622 cm−1 which is between 3617 and 3625 cm−1, as one would expect on the basis of the FSD-IR results and their interpretation given in this paper. Note that the scheme of the Na-exchange in mordenites drawn above agrees with the analysis based on the crystallographic studies of these zeolites.15 We will proceed now to the refinement of our assignment of the two HF bands (3599 and 3609 cm−1). As noted above, these bands can be associated with three oxygen atoms that are O2, O5, and O10. Consideration of Figure 1 shows that O2 atom is shared between T2 and T4 atoms, O5 links two T2 atoms, and O10 links two T4 atoms. Given the obvious difference in the intensities of the 3599 and 3609 cm−1 peaks (Figure 9) and the preferred location of Al atoms in the T3 and T4 positions as opposed to the T1 and T2 positions,14,37,52 we can conclude that the less intense 3599 cm−1 band is very likely to be associated with the O5−H hydroxyl group. Thus, the 3609 cm−1 band can be associated with either one of the O2−H and O10−H hydroxyls or with both of them (indeed, there is no data or reason known to us that would rule out one of these two groups). Importantly, our assignment of the 3599 and 3609 cm−1 peaks is in line with the fact that the local environment of the O5−H hydroxyls is more constrained than the local environments of the O2−H and O10−H groups (see Figure 1), and, therefore, the frequencies of vibrations of the latter groups could be expected54−57 to be slightly higher in comparison with the O5−H hydroxyls. 3.6. Accessibility of the Distinct BAS in the H-MOR Zeolite. Having completed the detailed assignment of the six distinct IR bands to the different BAS, we are now in the position to explain the results of the n-hexane adsorption experiments (Figure 5) and comment on the relative amounts of the different BAS in the H-MOR zeolite. In section 3.2 we have established that n-hexane adsorption on H-MOR zeolite results in the 53% decrease in the area of the initial 3610 cm−1 peak. This decrease is clearly associated with the interaction of n-hexane molecules with the acidic hydroxyls located in the large channels of the H-MOR zeolite, as the peak that is removed from the initial spectrum (Figure 5b) is centered at 3616 cm−1 (TF and HF parts) and the peak

a = a0(1 + 0.010ΔνOH)

(1)

ε = ε0(1 + 0.018ΔνOH)

(2)

In eqs 1 and 2, a0 and ε0 are the fwhm and the extinction coefficient, respectively, of the least perturbed band (3625 cm−1, in our case), and a and ε are the fwhm and the extinction 23925



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The deconvolution procedure, as described above, leads to a range of solutions rather than a single (unique) solution. The uncertainties of the deconvolution are associated mainly with the use of relatively wide and strongly overlapping single bands. These uncertainties, however, can be decreased significantly (i) by taking into account qualitative results of the FSD-IR (Figure 9) and (ii) by assuming that absolute absorbance of a specific single band cannot increase with the increase in the sodium content in the MOR samples. The final results of the deconvolution are presented in Figure 10, and the areas of the six single bands in the IR spectra of four MOR samples are compared in Table 2.

coefficient of the band perturbed by hydrogen bonds and vibrating at the wavenumber that is lower by ΔνOH (cm−1) than the 3625 cm−1. The values of all parameters of the single bands used in the deconvolution are presented in Table 1. Table 1. Parameters of the Single Bands Used in the Quantitative Deconvolution of the 3610 cm−1 Band band no.

position (cm−1)

fwhm (cm−1)

Lorentzian fraction (%)

relative extinction coeff

1 2 3 4 5 6

3581 3590 3599 3609 3617 3625

25.9 24.3 22.7 20.9 19.4 18.0

75.0 60.7 47.6 34.6 25.4 15.0

1.83 1.63 1.45 1.27 1.14 1.00

Table 2. Band Area Fractions Obtained by the Six-Band Deconvolution

The deconvolution was performed by optimizing the intensities of the bands to minimize squared deviation between the experimental and simulated (sum of the six components) spectra. The uncertainty of the deconvolution was estimated by repeating the procedure 104 times with randomly generated initial parameters of the bands. The positions of the bands and fwhm were allowed to vary within ±0.5 cm−1 from the initial values (because of the uncertainty of their experimental estimation), and the Lorentzian fractions of the bands were fixed.

band (cm−1)

H100MOR

H82Na18-MOR

H63Na37-MOR

H45Na55-MOR

3581 3590 3599 3609 3617 3625

15.2 16.2 13.7 36.7 9.8 8.4

10.1 15.0 14.3 35.7 13.1 11.8

4.1 3.7 23.4 35.1 17.7 16.0

3.8 3.5 19.7 29.4 20.9 22.7

The fractions of different BAS in four MOR samples were calculated using the extinction coefficients and the area fractions of the bands:

Figure 10. Deconvolution of the hydroxyl region of the IR spectra of four MOR samples using six single bands: (a) H-MOR, (b) H82Na18-MOR, (c) H63Na37-MOR, and (d) H45Na55-MOR. The parameters of the single IR bands used in the deconvolution are given in Table 1. 23926


The Journal of Physical Chemistry C Ni = 100% ×

Article

Ai /εi 6 ∑ j = 1 Aj /εj

These two BAS are linked to the Na-exchange position I, as can be seen in Figure 1. The 3599 cm−1 single band is assigned to O5−H hydroxyls that vibrate in the intersection between side pockets and 12-MR channels, and the 3609 cm−1 single band corresponds to O2−H and/or O10−H hydroxyls vibrating in the 12-MR channels. These acidic hydroxyls (O2−H, O5−H, and O10−H) are associated with the Na-exchange position IV (Figure 1). The last pair of the single IR bands (3617 and 3625 cm−1) corresponds to O3−H and O7−H hydroxyls that are located in the 12-MR channels and are linked to the Naexchange position VI. The quantitative estimates of the amounts of the six distinct BAS in the purely acidic and partially Na-exchanged mordenite samples were obtained using the six-band deconvolution method developed in this work. These estimates (Table 3) show that in the purely acidic H-MOR sample, used in this work, about ∼25% of BAS are located in the 8-MR channels (O1−H and O9−H hydroxyls), ∼13% of BAS are associated with O5 oxygen atoms at the intersection between the side pockets and 12-MR channels, and ∼62% of BAS are located in the 12-MR channels (∼39% correspond to O2−H and/or O10−H hydroxyls and the remaining 23% to O3−H and O7− H hydroxyls). Importantly, these data are in excellent quantitative agreement with the independent experimental data on n-hexane adsorption and demonstrates that the acidic hydroxyls O5−H, O6−H, and O9−H are not accessible to nhexane molecules at 25 °C. Finally, the quantitative data on the distribution of different BAS in mordenites, obtained in this work, demonstrate that these acid sites are distributed quite evenly between oxygen atoms in different crystallographic positions. This result reveals the complexity of the identification of distinct BAS in mordenites and explains the variety of the earlier suggested positions of BAS in these zeolites. The FSD-IR method appears to provide a unique opportunity to visualize the distribution of different BAS in zeolites as well as their exchange with metal cations.

(3)

where Ni is the number of BAS corresponding to ith single band (see Table 1), εi is the relative extinction coefficient of the ith band, and Ai is the estimate of the area fraction of the ith band. The distribution of different BAS, as characterized by six single IR bands, is presented in Table 3. These data are in very Table 3. Distribution (%) of Distinct BAS Calculated Using the Six-Band Deconvolution band (cm−1)

H100MOR

H82Na18-MOR

H63Na37-MOR

H45Na55-MOR

3581 3590 3599 3609 3617 3625

11.5 13.4 12.8 39.1 11.7 11.5

7.4 12.1 13.0 36.9 15.1 15.5

2.9 2.8 20.2 34.6 19.5 20.0

2.6 2.6 16.6 28.2 22.4 27.6

good agreement with the data on n-hexane adsorption, as noted in section 3.6, and quantifies the data on the exchange of acidic protons by Na cations in different positions. Significantly, the results in Table 3 reveal that there is no single specific crystallographic location that dominates in formation of BAS in the MOR structure. Instead, the BAS are distributed quite evenly between different locations with some preference of the formation on the O2 and O10 oxygen atoms in the MOR framework (let us recall that the most intensive 3609 cm−1 band has been assigned to the O2−H and O10−H hydroxyls). Hence, the data in Table 3 provide strong independent experimental support to the crystallographic16 and theoretical17−20 studies that demonstrate the absence of preferential attachment of protons to any particular framework oxygen atoms. In addition, these data (Table 3) point out to some preference of the location of aluminum atoms in T4 crystallographic positions (O2 and O10 oxygen atoms are linked to T4 crystallographic positions, as can be seen in Figure 1), thus supporting the earlier studies of aluminum siting in mordenites.37,52

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 shows the effect of adsorbed ammonia on the intensity of the 3610 cm−1 band and was used for calculation of the number of BAS in the H-MOR zeolite. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSION A detailed investigation into Brønsted acid sites in the purely acidic and partially Na-exchanged MOR zeolites was carried out using FT-IR spectroscopy. As a result, the presence of the six distinct BAS in the MOR zeolites, as was recently reported,31 has been confirmed. This finding was contrasted to the earlier prediction of maximum four distinct BAS in mordenites14 and led to the conclusion that a repeating unit larger than a unit cell is required for the description of the distributions of BAS and associated aluminum atoms in mordenites and other zeolites. This conclusion has then been extended into suggestion that siting of BAS and aluminum atoms in zeolite frameworks may not follow any repeating pattern. The assignment of the six single IR bands, clearly visible in the FSD traces of the IR spectra of the four MOR samples, to the BAS located in the different positions of the MOR channels has been completed using the experimental data on adsorption of ammonia and n-hexane in a combination with the structural information available in the literature.7,14,15 Two single bands (3581 and 3590 cm−1) have been assigned to acidic hydroxyls O1−H and O9−H that vibrate in the small 8-MR channels.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.B.L.). Present Address

N.C.: School of Biological, Biomedical and Environmental Sciences, University of Hull, Hull, HU6 7RX, United Kingdom. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS D.B.L. thanks the Department of Chemical Engineering at the University of Bath for the start-up funds that were used by the authors to build the IR system.

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REFERENCES

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