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Lewis Acidity Inherent to the Framework of Zeolite Mordenite Manoj Ravi,† Vitaly L. Sushkevich,‡ and Jeroen A. van Bokhoven*,†,‡ †

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland ‡ Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland

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S Supporting Information *

ABSTRACT: Despite being used extensively as heterogeneous catalysts and supports at the academic and industrial levels alike, the nature of aluminum-based Lewis acidity in zeolites is not completely understood. In this study, we discovered a correlation between Lewis acidity and aluminum structure in the framework of zeolite mordenite. The amount of octahedrally coordinated aluminum in these samples present under wet conditions correlates to the number of Lewis acid sites as detected by Fourier-transform infrared spectroscopy of adsorbed probe molecules in the fully dehydrated state. We illustrate that these aluminum species, which are better considered as framework-associated and not extra-framework aluminum, have Lewis acidic properties when treated under vacuum at high temperature. This observation constitutes important progress in understanding the structure of Lewis acid sites. The Lewis acid sites are framework aluminum species which undergo a change in coordination, from tetrahedral to octahedral, depending on the charge-balancing cation and degree of hydration. These Lewis acid sites exist in the case of charge balancing by protons.

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INTRODUCTION Zeolites continue to be an indispensable class of materials in catalysis research, as well as in chemical industries, to catalyze a wide array of reactions that includes cracking,1 alkylation,2 acylation,3 and other reactions of interest for the production of fine chemicals.4,5 Over the years, a number of synthesis and characterization techniques have been developed for these microporous, crystalline aluminosilicates. The structure of aluminum species in zeolites and that of catalytically active sites, particularly those of Lewis acidic aluminum sites remain a focal point in zeolite research. Lewis acid sites in zeolites, often obtained by introducing heteroatoms such as titanium, tin, and zirconium, have been exploited to catalyze reactions such as epoxidation,6 aldol condensation,7 and Baeyer−Villiger oxidation.8 In general, Lewis acidic zeolites also find extensive applications in the conversion of biomass.9 This includes the conversion of sugars to lactates and dehydration of sugars to furan compounds.10 Particularly for zeolites, 27Al MAS NMR is the most common technique used to characterize different aluminum species in terms of their coordination,11 while the acidic properties of zeolites can be examined by a range of methods,12 including 1H MAS NMR,13,14 Fourier transform infrared (FTIR) spectroscopy of adsorbed probe molecules,15 such as pyridine and carbon monoxide, and NH 3-temperature programmed desorption.16 In a 27Al MAS NMR spectrum of a hydrated zeolite, tetrahedrally coordinated framework aluminum (Td) is characterized by a narrow resonance between about 50 and 60 ppm and octahedrally coordinated aluminum (Oh) by a resonance around 0 ppm. In some zeolites, such as zeolite BEA, the resonance at 0 ppm is © 2019 American Chemical Society

generally a superposition of two contributions: well-ordered octahedral aluminum and distorted octahedral aluminum.17 Numerous studies, with many of them based on zeolite BEA, have tried to discern the role of these octahedrally coordinated aluminum species, distinguishing them as extra-framework or framework-associated, depending on the aluminum’s ability to reversibly change its coordination.18−22 In the case of the latter, octahedrally coordinated aluminum in protonic zeolites quantitatively reverts to tetrahedral coordination on heating the zeolite to higher temperatures or exchanging the zeolite to its sodium or ammonium form.18−25 By primarily studying the effects of steaming and activity in the Meerwein−Ponndorf− Verley (MPV) reaction, a wealth of novel, if at times conflicting, findings have emerged. Octahedral aluminum species connected to the framework structure of zeolite BEA was found to exhibit characteristics of Lewis acid sites.26 However, the absence of a correlation between catalytic activity in the Lewis acid-catalyzed MPV reduction of ketones and the relative amounts of octahedrally coordinated aluminum in different H-BEA samples led to the conclusion that such aluminum species are not mandatory for catalytic activity.27 Likewise, it was reported that framework aluminum atoms in a nontetrahedral coordination may be associated with Lewis acidic properties28,29 and consequently, partially hydrolyzed framework aluminum was proposed as the precursor for the MPV active site.23 Furthermore, quantitative studies hint at cationic extra-framework aluminum species being located at Received: April 17, 2019 Revised: May 21, 2019 Published: May 28, 2019 15139

DOI: 10.1021/acs.jpcc.9b03620 J. Phys. Chem. C 2019, 123, 15139−15144

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The Journal of Physical Chemistry C ion-exchange positions in zeolite BEA.17 Emanating from these studies is a lack of consensus on the structure of Lewis acid sites in zeolites, a vague understanding of the role of framework aluminum in catalytic activity, and an improbability to relate aluminum of a particular structure or coordination definitively to Lewis acidity.

The relative amount of aluminum in the octahedral coordination was found based on the relative area of the peak centered at 0 ppm. FTIR Spectroscopy of CO Adsorbed over Zeolites. IR spectra were recorded on a Thermo Nicolet iS50 FTIR spectrometer equipped with a DTGS detector at a 4 cm−1 optical resolution and by carrying out 128 scans. Prior to the measurements, the sample (about 20 mg) was pressed in selfsupporting discs and activated in the IR transmission cell attached to a vacuum line at 723 K for 4 h. A low temperature vacuum cell cooled with liquid nitrogen was used for carbon monoxide (CO) adsorption measurements. Calibrated aliquots of the gas were introduced into the cell and the spectra were collected immediately. The pressure was measured by a Pfeiffer gauge. Difference spectra were obtained by the subtraction of the spectra of the activated samples from the spectra of the samples with the adsorbate. The presented spectra were normalized to the weight of the samples and compared at similar CO coverage. The subtraction was performed using the OMNIC 9.1 software package. FTIR Spectroscopy of Pyridine Adsorbed over Zeolites. The activation procedure was the same as the one described above using CO as the probe molecule. For pyridine (Py) adsorption, the samples were exposed to 3 Torr of Py at 423 K for 30 min for the complete diffusion of probe molecules and then, evacuated at different temperatures for 30 min. For quantifications, molar integral extinction coefficients of 2.22 cm/μmol and 1.67 cm/μmol were used for Lewis and Brønsted acid sites, respectively.37 The peaks at 1455 and 1555 cm−1 were used for the determination of Lewis and Brønsted acid sites, respectively. Atomic Absorption Spectroscopy. The silicon and aluminum content of various zeolites were determined by AAS. The zeolites (20−50 mg) were dissolved in a mixture of hydrofluoric acid (2−5 mL) and 60% nitric acid (2−5 mL), and diluted with deionized-water (25−40 mL). The mixture was stirred at room temperature and diluted to the range of calibration. The resulting solutions were measured with an Agilent SpectrAA 220FS instrument.

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EXPERIMENTAL SECTION Materials Preparation. Mordenite zeolites of different Si/ Al ratios were obtained from commercial suppliers as follows: CBV10A, Si/Al = 6.5, in sodium form; CBV21A, Si/Al = 10, in ammonium form; CBV90A, Si/Al = 46, in proton form were purchased from Zeolyst International. Zeoflair 800, Si/Al = 8.5, in sodium form; Zeoflair 810, Si/Al = 19, in proton form were obtained from Zeochem. Ammonium Exchange Procedure. As-received MOR zeolites were converted to the ammonium form by the following procedure: 1 g of zeolite was stirred in 80 mL of 0.1−0.5 M solution of ammonium nitrate (>99%, SigmaAldrich) at room temperature or 353 K overnight with pH monitoring. The suspension was then filtered at room temperature and rinsed with 300 mL of deionized water. This procedure was repeated twice to obtain the mordenite zeolite in the ammonium form. The ammonium form was calcined in an oven in static air at 823 K for 6 h with a heating ramp rate of 1 K min−1. The calcined H-forms are labeled HMOR(8), H-MOR(9), H-MOR(11), H-MOR(19), and HMOR(48) with the number in the brackets indicating the Si/Al ratio of the zeolite. Acid Treatment Procedure. Acid treatment of the MOR samples resulting in proton-exchanged MOR was performed as follows: Na-MOR was stirred in suitable concentrations of nitric acid (0.1−1 M) for the desired time (1−24 h) at the desired temperatures (298−323 K). The resulting solids were washed extensively with Milli-Q water and dried at 373 K overnight. Subsequently, the sample was calcined in static air at 823 K for 6 h with a heating ramp rate of 1 K min−1. Sodium Exchange Procedure. The following procedure was used to exchange zeolites to the sodium form: 1 g of zeolite was stirred in 60 mL of 0.1 M solution of sodium nitrate (>99%, Sigma-Aldrich) at room temperature overnight with pH monitoring. The suspension was then filtered and rinsed with 300 mL of deionized water. The procedure was repeated twice. The zeolite was dried overnight at 373 K in a drying oven and subsequently calcined in static air at 823 K for 6 h with a heating ramp rate of 1 K min−1.

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RESULTS AND DISCUSSION While often in the past, extra-framework aluminum has conveniently been cited as the reason for Lewis acidity, curated data suggests that such a correlation does not necessarily hold true at all times (Figure S1). This does not mean that extra-framework debris does not have Lewis acid sites, but that such acidity can also stem from other sites that are inherent to the zeolite framework structure and the endeavor of the present work is to identify such sites. To address the pressing question of the Lewis acidityaluminum structure relationship, we turned to MOR zeolites. Mordenite is widely used as a heterogeneous catalyst and support in extensively researched reactions, such as hydroisomerization of hydrocarbons,30 methane to methanol,31 and selective catalytic reduction.32 Five MOR zeolite samples, with different Si/Al ratios given in brackets, were used in this study. Physical characterization by nitrogen adsorption−desorption indicate the high micropore volume of the MOR samples irrespective of the aluminum content (Table 1). The sample with the highest Si/Al ratio, H-MOR(48), has the highest mesoporous volume (approx. 0.11 cm3/g) and this might be due to the postsynthetic treatment of the zeolite by the supplier to remove aluminum.

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MATERIAL CHARACTERIZATION Nitrogen Physisorption. N2 sorption−desorption isotherms were measured at 77 K using a Micromeritics Tristar automatic surface area and pore size analyzer. Prior to the measurements, the samples were degassed for 4 h under vacuum for accurate mass measurements. Solid State 27Al-MAS-NMR. 27Al-MAS-NMR spectra of the zeolites were recorded on a Bruker 400 MHz Ultra-Shield spectrometer. The spectra were accumulated from 3000 scans using a 4 mm probe at a sample spinning rate of 10 kHz. The 27 Al chemical shift was referenced to AlNH4(SO4)2·12H2O at −0.54 ppm and the spectra were acquired using a pulse sequence at a pulse angle of 90° and a recovery delay of 1 s. The mass of the sample loaded into the ZrO2 rotor was recorded to normalize the corresponding spectra per unit mass. 15140

DOI: 10.1021/acs.jpcc.9b03620 J. Phys. Chem. C 2019, 123, 15139−15144

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The Journal of Physical Chemistry C

species back into the framework of the zeolite is generally not possible. However with the MOR zeolites, we observe an enrichment of the framework with tetrahedrally coordinated aluminum upon the exchange of the protonic zeolite to its ammonium form (Table S1), thereby demonstrating the importance to view the octahedral species as being linked to the framework. The flexibility of the aluminum coordination in zeolite MOR has been observed by XANES at the Al K-edge,21 and such framework-associated aluminum has previously been identified in zeolites such as ZSM-5,24 zeolite Y,25 and BEA.18 Likewise, it has been shown that aluminum eliminated from the framework of zeolites BEA and MOR can easily be reinserted back into the framework.35 Interestingly, the possibility to drive octahedral aluminum in H-MOR back into the tetrahedral coordination has also been hypothesized during copper exchange.36 The acid sites in zeolite mordenite were probed by FTIR spectroscopy using pyridine and carbon monoxide as probe molecules. The two molecules differ in terms of strength and size and may be used in parallel to obtain deeper insight on the nature of acid sites being probed. Figure 2 shows the FTIR spectra of adsorbed pyridine over the calcined H-MOR samples. The peaks at 1455 and 1621 cm−1 are associated to the interaction of pyridine with Lewis acid sites. The bands at 1545 and 1635 cm−1 are assigned to Brønsted bound pyridine and that at 1490 cm−1 is structure insensitive.37,38

Table 1. Characterization of MOR Zeolites Used in This Study sample (bulk Si/Al)

micropore volume [cm3/g]a

total pore volume [cm3/g]b

Al content [μmol/g]c

octahedral Al relative to total Al [%]d

H-MOR(8) H-MOR(9) H-MOR(11) H-MOR(19) H-MOR(48)

0.18 0.20 0.18 0.19 0.17

0.21 0.23 0.22 0.22 0.28

1815 1621 1402 818 302

12 13 11 10 1

a

From the t-plot of nitrogen-physisorption experiments. bBased on single point adsorption at p/po = 0.97. cCalculated based on the Si/Al ratio obtained from AAS. dDetermined from the area under the 0 ppm peak of 27Al MAS NMR spectra.

The 27Al MAS NMR spectra of the H-MOR zeolites after being calcined in static air and stored under ambient conditions show two resonances: a peak at 55 ppm, associated to tetrahedral framework aluminum (Td) and a sharp peak centered at 0 ppm, indicative of well-ordered octahedral aluminum species (Oh) with the absence of distorted octahedral aluminum (Figure 1). The tetrahedral peak has a

Figure 1. 27Al MAS NMR spectra of the H-MOR zeolites of different Si/Al ratios.

broad shoulder attached to it, potentially suggesting the presence of other aluminum sites in a distorted tetrahedral coordination. In addition, this broad extension is more apparent in samples with higher aluminum content. The population of aluminum in the octahedral coordination is not present in the sodium or ammonium forms, which both show only one resonance characteristic of tetrahedrally coordinated framework aluminum (Figure S3). Moreover, this octahedrally coordinated aluminum species is best considered as framework-associated and not as extra-framework aluminum. This aluminum species adopts a typical framework tetrahedral coordination on back-exchanging to its sodium or ammonium form (Figure S3), and hence, the octahedral aluminum can be visualized as at least being partially attached to the zeolite framework or linked to its original location. Aluminum species that are characterized by such a sharp peak at around 0 ppm in other zeolite frameworks are flexible to be reinserted into the framework.33,34 Extra-framework aluminum with an octahedral coordination generally results in a broader feature at around 0 ppm and reinsertion of this

Figure 2. FTIR spectra of adsorbed pyridine over H-MOR zeolites of different Si/Al ratios. (L) indicates Lewis bound pyridine, (B) indicates Brønsted bound pyridine, and (L + B) indicates pyridine associated with both Brønsted and Lewis sites. 15141

DOI: 10.1021/acs.jpcc.9b03620 J. Phys. Chem. C 2019, 123, 15139−15144

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The Journal of Physical Chemistry C Likewise, these acid sites were examined with carbon monoxide as the probe molecule (Figure 3). The absorption

Figure 4. Correlating Lewis acidity determined by FTIR spectroscopy of adsorbed pyridine (squares, left ordinate axis) and adsorbed carbon monoxide (diamonds, right ordinate axis) in H-MOR to the amount of octahedral aluminum determined by 27Al MAS NMR. Quantitative data on Lewis acidity in the experiments with pyridine were extracted based on the peak at 1455 cm−1. Relative amounts of Lewis acidity, based on experiments with carbon monoxide, were calculated by the sum of the areas of the deconvoluted peaks centered at 2223 and 2198 cm−1.

high temperatures under vacuum. Thus, these results, which are obtained by using two vastly varying techniques in terms of sample environment, aid in unequivocally assigning sixcoordinated aluminum in H-MOR to Lewis acidic properties. The generation of octahedral aluminum in zeolites from tetrahedral framework aluminum is driven by the adsorption of water and hydrolysis reactions.25,39,40 Bearing this in mind, the relationship between octahedral aluminum and Lewis acidity (Figure 4) is intuitive to envisage because an aluminum species that is capable of changing its coordination from tetrahedral to octahedral by interacting with lone pairs of water is concurrent with the definition of Lewis acidity. The y-intercept of this correlation, which refers to the Lewis acidity in the absence of any octahedral aluminum, is likely to originate from other Lewis-acidic aluminum species, such as distorted fourcoordinated and/or penta-coordinated aluminum. The broad shoulder of the peak at 58 ppm in the 27Al MAS NMR spectra (Figure 1) suggests the presence of such species in H-MOR. It is vital to understand that the structure probed by FTIR spectroscopy is not the same as that in 27Al MAS NMR as the two techniques differ in their pretreatments of the zeolite sample and these varying treatments bring about a change in the aluminum coordination.18,19,23,25,41,42 While 27Al MAS NMR is measured on samples stored under ambient conditions, FTIR spectroscopy entails the degassing of the zeolite under vacuum. We demonstrate this difference in sample pretreatment by mimicking the wet environs of an NMR experiment in an FTIR spectroscopic measurement using carbon monoxide as the probe molecule. Upon preadsorbing very small quantities of water (20 μmol g−1, approximately 10% of total Lewis acid content) before exposure to carbon monoxide, we observe a decrease in the absorbance band at 2223 and 2187 cm−1, associated with three-coordinated Lewis acidic aluminum and Brønsted acid sites, respectively, (Figure S4). This shows the tendency of the water molecules to adsorb over Lewis and Brønsted acid sites and induce a change in coordination of aluminum that can be regarded as associated to the framework. As has been reported in the past, moisture drives the formation of octahedrally coordinated aluminum in H-mordenite and this aluminum species is unstable at temperatures higher than 395 K, quantitatively reverting back to tetrahedral coordination.21

Figure 3. FTIR spectra of adsorbed carbon monoxide over H-MOR zeolites of different Si/Al ratios. (B) Indicates Brønsted acid sites probed by carbon monoxide. Lewis acidic aluminum in coordination with carbon monoxide are denoted as (Al3c-CO)δ+ and (Al5c-CO)δ+.

feature at 2223 cm−1 is characteristic of the interaction of carbon monoxide with three-coordinated Lewis acidic aluminum while that at 2198 cm−1 is attributed to fivecoordinated Lewis acidic aluminum probed by carbon monoxide. The band at 2173 cm−1 arises from the interaction of carbon monoxide with Brønsted acid sites in the zeolite and the feature at 2138 cm−1 is due to physisorbed liquid-like carbon monoxide. All samples have a greater proportion of three-coordinated Lewis acidic aluminum than five-coordinated Lewis acidic aluminum sites. Furthermore, appreciable amounts of the five-coordinated Lewis acidic species can only be found in samples with high aluminum content (Figure 3). These three- and five-coordinated Lewis acidic aluminum sites, as observed here by FTIR spectroscopy, are due to dehydration during sample pretreatment. The removal of water from the coordination sphere facilitates the interaction of carbon monoxide with such sites. Figure 4 illustrates a linear relation between the amount of octahedrally coordinated aluminum under wet conditions of a solid-state NMR experiment and the Lewis acidity as detected by FTIR spectroscopy in the H-MOR samples. This correlation, which is observed irrespective of the choice of the probe molecule, suggests that aluminum species that are in the octahedral coordination at room temperature in a hydrated environment are associated with Lewis acidity when treated at 15142

DOI: 10.1021/acs.jpcc.9b03620 J. Phys. Chem. C 2019, 123, 15139−15144

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The other takeaway from Figure 4 is the influence of the Si/ Al ratio on Lewis acidity. The higher Lewis acidity in samples with greater aluminum content (Table 1) underscores the importance of having framework aluminum close to each other to generate sites with Lewis acidity. Furthermore, our experiments highlight the necessity to have protons as charge-compensating cations in order to generate Lewis-acidic aluminum sites. When examined by FTIR spectroscopy using carbon monoxide as the probe molecule, the MOR zeolite in its sodium form did not show any signature for Lewis acidic aluminum sites (Figure S5). As expected, this sample also had very little to no octahedral aluminum under the conditions of an NMR experiment (Figure S3). The simultaneous absence of Lewis acidity and octahedral aluminum in the hydrated state reaffirms the relationship of six-coordinated aluminum to Lewis acidity. Subsequently, treating Na-MOR(9) in nitric acid solution results in the generation of Lewis acidity (absorbance at 2223 cm−1, Figure S5) and a population of aluminum species with the octahedral coordination (acid treated MOR(9), Figure S6). Thus, the octahedrally coordinated aluminum species is only present in the case of proton-exchanged zeolites, suggesting that two close Brønsted acid sites are needed to generate the defective site able to change the aluminum coordination into octahedral. Besides, this observation is in good agreement with in situ XAS experiments that ascribe to the formation of octahedral aluminum, without the loss of aluminum from the zeolite lattice, to the interaction of water with Brønsted acid sites.40

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b03620. Figures showing curated data for the correlation of Lewis acidity with extra-framework aluminum, nitrogen adsorption−desorption isotherms, the reversible octahedral−tetrahedral aluminum transformation in zeolite MOR, effect of preadsorbed water on Lewis and Brønsted acidity, FTIR spectra of adsorbed carbon monoxide, and 27Al MAS NMR spectra of Na-MOR(9) and acid treated MOR(9) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vitaly L. Sushkevich: 0000-0002-3788-8969 Jeroen A. van Bokhoven: 0000-0002-4166-2284 Author Contributions

M.R. synthesized the samples, performed and analyzed the NMR experiments, and prepared the manuscript. V.L.S. performed and analyzed the FTIR experiments and provided feedback during the preparation of the manuscript. J.A.v.B. devised the overall idea, supervised the experimental progress and provided feedback during the preparation of the manuscript.

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Notes

The authors declare no competing financial interest.

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CONCLUSIONS To summarize, the experiments in this study constitute a prominent step forward in understanding the structure of aluminum-based Lewis acidity in zeolites. Aluminum species in octahedral coordination under hydrated conditions are associated with Lewis acidity in the dehydrated environment of FTIR spectroscopy. These Lewis acid sites only form in protonic zeolites that have close aluminum sites, and such Lewis acidity is intrinsic to the zeolite framework. The structure of Lewis acid sites in zeolites is a function of the conditions, and the octahedral aluminum describing this Lewis acidity is associated to the framework. In addition, as reflected by the offset in the FTIR-NMR correlation, there do exist other aluminum species in the zeolite with Lewis acidic properties. In light of our findings, octahedral aluminum, that is associated to the framework, can be associated to Lewis acidity without ambivalence, and the existence of such sites is strictly influenced by the Si/Al ratio of the zeolite. We propose that this correlation of octahedral frameworkassociated aluminum to Lewis acidity holds true for zeolite topologies beyond just mordenite. Gil et al. studied the acidic properties of SSZ-35 and SSZ-33 using infrared spectroscopy and MAS NMR.34 NH4-SSZ-35 was found to possess two tetrahedral aluminum sites and no octahedral aluminum. Meanwhile, the proton form of SSZ-33 exhibited a sharp resonance at around 0 ppm in the 27Al MAS NMR and this octahedral aluminum species was shown to adopt a tetrahedral coordination on exchange with ammonium nitrate. The Lewis acidity per unit cell of this sample was far greater than that of SSZ-35 and we postulate that this difference in Lewis acidity could be explained by the vastly different amounts of octahedral aluminum in the two samples.

ACKNOWLEDGMENTS The authors gratefully acknowledge the ESI Platform, Paul Scherrer Institute and ETH Zurich for financial support and the assistance of Dr. René Verel for NMR measurements. Manoj thanks Teng Li and Amy Knorpp for help with AAS measurements.

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DOI: 10.1021/acs.jpcc.9b03620 J. Phys. Chem. C 2019, 123, 15139−15144