Nature of Enhanced Brønsted Acidity Induced by Extraframework Aluminum in an Ultrastabilized Faujasite Zeolite: An In Situ NMR Study.

The enhancing effect of extraframework Al (EFAl) species on the acidity of bridging hydroxyl groups in a steam-calcined faujasite zeolite (ultrastabilized Y, USY) was investigated by in situ monitoring the H/D exchange reaction between benzene and deuterated zeolites by 1H MAS NMR spectroscopy. This exchange reaction involves Brønsted acid sites (BAS) located in sodalite cages and supercages. In a reference faujasite zeolite free from EFAl, both populations of BAS are equally and relatively slowly reactive toward C6H6. In USY, in stark contrast, the H/D exchange of sodalite hydroxyl groups is significantly faster than that of hydroxyl groups located in the faujasite supercages, even though benzene has only access to the supercages. This evidences selective enhancement of BAS near Lewis acidic EFAl species, which according to the NMR findings are located in the faujasite sodalite cages.

Introduction

Faujasite zeolite (framework code FAU, zeolite Y, Figure ) is widely used as an acid catalyst for (hydro)cracking operations in the oil refining industry. In its as-synthesized form, the zeolite has a high density of framework aluminum (FAl) and exhibits only weak intrinsic acidity and limited hydrothermal stability. The Brønsted acidity and stability of such a Y zeolite can be strongly enhanced by removing Al from the framework (dealumination). Dealumination leads to ultrastabilized zeolite Y (USY), which is typically achieved by calcination of as-prepared Y zeolite in the presence of steam at temperatures in the range of 550–800 °C. The higher intrinsic Brønsted acidity of dealuminated zeolite relates to the increased concentration of isolated FAl sites (i.e., no framework Al in the second coordination shell).[1,2] A substantial part of Al atoms extracted from the framework are relocated to positions outside the framework (extraframework Al, EFAl). While typically a part of these EFAl species are removed by further chemical treatment, it is known that residual EFAl species influence the acidity and reactivity of steam-calcined zeolites.[2] This can for instance be appreciated from the higher intrinsic acidity and acid activity of a USY zeolite in comparison with an EFAl-free zeolite Y reference with a comparable framework Al content, obtained by selective dealumination by ammonium hexafluorosilicate, (NH4)2SiF6.[2−7] Although the promoting effect of EFAl species on the acidity of the BAS in USY zeolites has been investigated before,[8−15] important aspects such as the nature of the EFAl species, their location inside the faujasite micropores, and the way they enhance the intrinsic acidity of bridging hydroxyl groups are not understood yet.

Figure 1

Faujasite unit cell and cation exchange sites. Reproduced with permission from ref (16); copyright 2011 American Chemical Society.

Recently, Van Bokhoven et al. demonstrated that EFAl formed during steam calcination of an NH4Y zeolite predominantly occupies the SI′ sites inside the faujasite sodalite cages (Figure a).[17] The structural details of such EFAl species are still unknown. Although most of the previous studies considered cationic mononuclear Al-oxo complexes as EFAl species,[18,19] recent findings indicate that the coordinative unsaturation of such Al centers and the high basicity of the terminal oxygen ligands will cause such complexes to rearrange into multinuclear Al-oxo/hydroxo clusters inside sodalite cages.[20] A recent density functional theory (DFT) study by Liu et al.[21] confirmed that such multinuclear complexes are preferentially located in sodalite cages (Figure S2, Supporting Information). Oxygenated and hydroxylated Al3+ cations condense within sodalite cages to form clustered O- and OH-bridged species, bearing a higher formal positive charge compensated through the direct interaction with lattice oxygen anions. In a subsequent work, it was shown that such EFAl species in faujasite strongly affect the acidity of the BAS, leading to a higher activity toward alkane activation.[22]

Solid-state NMR spectroscopy is a powerful technique for studying acid sites in zeolites.[23−25,27] Al NMR spectroscopy can distinguish the different types of Al in zeolites. Recently, White et al. used one- and two-dimensional 1H NMR spectroscopy to directly detect different types of acid sites in HZSM-5 and to reveal direct proton exchange between them.[26] Another possibility lies in the study of probe molecules that interact with the acid sites, e.g., acetone (13C NMR),[27−29] deuterated pyridine (1H NMR),[25] and trimethylphosphine (31P NMR).[30,31] For example, the adsorption of 2-13C-acetone was used as a molecular probe in an NMR study to demonstrate the influence of EFAl on zeolite acidity.[32] In these works, the strong interplay between BAS and EFAl species was emphasized.

Herein, we employ in situ NMR spectroscopy to determine the acidity of a Y zeolite by following the H/D exchange of BAS with benzene. The suitability of this method for acidity characterization was first demonstrated by the group of Haw using NMR spectroscopy,[33] while our group later extended this approach using IR spectroscopy.[34] The sensitivity of IR spectroscopy allows quantification of strong BAS in zeolites, clays, and even amorphous silica–aluminas, which contain very few of such acid sites.[35,36] For a range of EFAl-free Y zeolites, Almutairi et al. showed a strong correlation between acidity measured by CO IR spectroscopy and the H/D exchange rate with deuterated benzene as determined by IR spectroscopy.[2] Selective substitution of AlF with Si atoms can be achieved by treating a parent NH4Y zeolite with ammonium hexafluorosilicate (NH4)2SiF6 (AHFS).[1] Thus, the H/D exchange rate can directly indicate the acidity of such an EFAl-free Y zeolite. The much higher H/D exchange rate of Y zeolites promoted by EFAl sites confirmed their enhancing effect on the BAS. Nevertheless, there was no clear correlation between the amount of EFAl sites and the Brønsted acidity. These H/D exchange IR spectroscopy studies showed that protons in sodalite cages exchange at similar rates as those in the more accessible supercages. In the present work, we turn back to 1H NMR spectroscopy to follow the H/D exchange under mild conditions for two Y zeolites, one which was dealuminated by treatment with ammonium hexafluorosilicate such that it is free from EFAl and another one which is a commercial steam-calcined USY zeolite containing EFAl. The nature and location of EFAl species in USY were also investigated by NMR spectroscopy. The peculiar result of our current efforts is that, in the presence of EFAl sites, the sodalite hydroxyl groups exchange substantially faster than the hydroxyl groups located in the faujasite supercages even though benzene has only access to the supercages. The role of EFAl-oxo species stabilized in sodalite cages as acidity-enhancing species will be discussed.

Materials and Methods

Sample Preparation

Steam-calcined zeolite USY (Si/Al 4.05) was received from Zeolyst. Dealuminated Y zeolite AHFSY (Si/Al 4.15) was prepared according to a literature procedure involving the isomorphous substitution of Al by Si using ammonium hexafluorosilicate (AHFS).[3]

H/D Exchange

The parent zeolite was dehydrated at 500 °C (heating rate 2 °C/min) for 12 h in a dynamic vacuum (p < 10–5 mbar). The dehydrated sample was deuterated by exposure to 10 mbar of D2O gas at 150 °C for 30 min, followed by evacuation. This procedure was repeated two times. The sample was then evacuated at 450 °C for another 12 h. The two deuterated samples are denoted as D-USY and D-AHFSY. H/D exchange was carried out in a 4 mm zirconium NMR rotor. The dehydrated deuterated zeolite was loaded in the NMR rotor in a glovebox under inert (N2) atmosphere. The rotor was then placed in a glass tube, connected to a manifold setup, and evacuated for 2 h. Thereafter, the tube containing the sample rotor was cooled to −40 °C, and the sample was brought in contact with a benzene reservoir (partial pressure of 4 mbar) for 1 min. The rotor was then tightly closed with a boron nitride cap and transferred to the NMR probe-head kept at −30 °C.

Solid-State NMR Spectroscopy

NMR measurements were performed on a Bruker DMX500 spectrometer operating at 500, 99, and 132 MHz for 1H, 29Si, and 27Al, respectively. The NMR measurements were carried out using a 4 mm MAS probe-head with a sample rotation rate of 12.5 kHz. 1H NMR spectra were obtained by a Hahn-echo pulse sequence of p1–τ1–p2–τ1–aq with a 90° pulse p1 = 5 μs and a 180° p2 = 10 μs. The interscan delay was chosen to be 120 s in order to obtain quantitative spectra. The same pulse sequence was used for T2(1H) filtered spectra by varying the τ1 time. T1rho(1H) filtered spectra were recorded by introducing a lock pulse in the Hahn-echo pulse sequence (p1–tlock pulse–τ1–p2–τ1–aq) with τ1 = 2.5 us and a variable tlock pulse duration. Two-dimensional 1H MAS NMR exchange experiments were performed using the 90°–t1–90°–tmix–90°–t2 pulse sequence with evolution time t1 and signal detection time t2. A mixing time tmix of 0.3 and 1 s was used. 27Al NMR spectra were recorded with a single-pulse sequence with a 18° pulse duration of 1 μs and an interscan delay of 1 s. MQMAS spectra were recorded by a three-pulse sequence p1–t1–p2–τ–p3–t2 for triple-quantum generation and zero-quantum filtering (strong pulses p1 = 3.4 μs and p2 = 1.4 μs at a nutation frequency ν1 = 100 kHz, a soft pulse p3 = 11 μs at ν1 = 8 kHz, a filter time τ = 20 μs, and an interscan delay 0.2 s). 27Al–{1H} TRAnsfer of Population in DOuble Resonance (TRAPDOR)[37] spectra were recorded with the irradiation on and off on the 27Al nuclei of 795 μs, prior to the echo pulse, and an interscan delay of 10 s. Quantitative 29Si NMR spectra were recorded using a high-power proton decoupling direct excitation (DE) pulse sequence with a 90° pulse duration of 3 μs and an interscan delay of 360 s. The 1H and 29Si pulse powers were adjusted to obtain a 90° pulse length equal to 5 μs in tetramethylsilane (TMS). An Al(NO3)3 solution was used to adjust the 27Al pulse power. The temperature of the sample was controlled by a BVT300 variable temperature control unit and nitrogen gas. For low temperature, the nitrogen gas flow was cooled through a heat exchanger inserted in liquid nitrogen. The appropriate temperature was adjusted by using a heating element in the NMR probe-head. The true temperature inside the probe was calibrated using ethylene glycol and methanol for high and low temperatures, respectively.

X-ray Diffraction (XRD)

XRD patterns of zeolites were recorded on a Bruker D4 Endeavor Diffractometer using Cu Kα radiation with a wavelength of 1.54056 Å. 2θ angles from 5 to 60° were measured with a step size of 0.077° and a time per step of 1 s. The catalysts were ground and pressed in sample holders for measurements. Crystallinity of the zeolite samples was calculated from the XRD patterns using the Topas software.

Elemental Analysis

The Al content in zeolite catalysts was determined by elemental analysis, which was carried out by ICP-OES analysis on a Spectro Ciros CCD ICP optical emission spectrometer with axial plasma viewing. For the ICP measurements, the samples were dissolved in a 1.5 mL solution of an HF/HNO3/H2O (1:1:1) mixture.

Results

Basic Characterization

The XRD patterns shown in Figure a indicate that USY and AHFSY are highly crystalline and exhibit the expected cubic crystal structure with nearly similar lattice constants (Table ). The correlation between the FAl content in zeolite and the lattice constant can be used to determine the FAl content in zeolite Y.[1,38] Using this method, the FAl contents of the two zeolites were found to be comparable, i.e., 3.0 mmol/g for USY and 3.3 mmol/g for AHFSY.

Figure 2

(a) XRD patterns and (b) 29Si MAS NMR spectra of steam-calcined USY and selectively dealuminated AHFSY, respectively. Both XRD and 29Si NMR data are used to determine the framework Al content (Tables and 2).

Table 1

Al Content, c(Al), Derived from ICP, and Framework Al Content, c(FAl), from the XRD Lattice Constant a0

	c(Al)a (mmol/g)	a0 (Å)	FAl/u.c.b	c(FAl)c (mmol/g)
USY	4.18	24.477	26	3.0
AHFSY	3.66	24.536	32	3.3

From ICP elemental analysis.

Number of FAl atoms per unit cell (u.c.): 107.1(a0 – 24.238).[1]

From FAl/u.c. assuming u.c. composition Si192–AlO384H.

Table 2

29Si NMR Peak Intensities of Si Atoms with Varying Numbers of Al Neighbors in the Zeolite Lattice and the Framework Al Content, c(FAl), Derived Thereof

	I2/I3/I4a29Si NMR	Si/Alb	I2/I3/I4c	c(FAl)d (mmol/g)
USY	13/35/47	5.4	16/31/37	2.6
AHFSY	17/53/30	4.6	17/49/24	3.4

Ratio of NMR intensities I (%) of Q silicon atoms with Si(OSi)4–(OAl) coordination (I1 = 100 – I2 – I3 – I4).

Calculated from Si/Al = ∑I/∑nI.[41]

For random Al distribution in lattice without Al–O–Al pairs and Al/Si ratio p: .

From Si/Al, assuming u.c. composition Si192–AlO384H.

The 27Al NMR spectrum of the AHFSY zeolite shows one symmetric peak at ∼62 ppm (Figure , right top), which can be assigned to FAl. In comparison, the 27Al NMR spectrum of USY contains two additional peaks, i.e., an octahedral Al signal at ∼0 ppm (Figure , left top) belonging to EFAl and a broad peak around 40 ppm. The nature of these species was investigated by 27Al MQMAS spectroscopy, which removes the second-order quadrupolar line broadening, thereby enhancing resolution and distinguishing between chemical-shift heterogeneity and quadrupolar broadening. The resulting 27Al MQMAS NMR spectra (Figure , left) confirm that USY has both FAl with tetrahedral (T) and octahedral (O) oxygen coordination. The signal at 40 ppm in the 1D spectrum appears as a quadrupolar broadened tetrahedral signal (T′ in Figure ) in the MQMAS spectrum. This distortion of some tetrahedral FAl is probably caused by the presence of EFAl in USY in the vicinity of FAl as will be confirmed below in 1H 2D correlation NMR experiments.

Figure 3

2D MQMAS 27Al MAS NMR spectra of USY and AHFSY along with projected spectra along the axes. For comparison, quantitative 1D 27Al NMR spectra are plotted at the top (red).

The FAl content can also be estimated from the intensity of the 27Al NMR signal at the tetrahedral position in the 1D spectrum. The calculated FAl content was 1.4 mmol/g for USY and 3.2 mmol/g for AHFSY. However, 27Al NMR spectroscopy is not always accurate in determining the Al population due to the possible NMR invisibility of some Al nuclei, especially in steam-calcined zeolites like USY. To verify these FAl values, we used 29Si MAS NMR spectroscopy as a third method.[39−41] The similar FAl contents of the two zeolite samples are confirmed by the relative occurrences of Si atoms Q with different coordinations Si(OSi)4–(OAl) in the zeolite lattice. The respective Si/FAl ratios calculated from the 29Si MAS NMR intensities (Figure b) are 5.4 and 4.6, corresponding to 30.1 and 34.1 FAl atoms per unit cell (T192O384, T = Si or Al). The framework Al (FAl) contents of USY and AHFSY are thus in the same range (Table ).

Before studying the H/D exchange between these samples and benzene, we investigate their proton and deuterium forms by 1H NMR spectroscopy (Figure ). The resonance at 1.9 ppm belongs to silanol moieties, while the signals at 3.9 and 4.6 ppm can be assigned to BAS in the sodalite cages and supercages, respectively. USY shows another signal at 2.8 ppm attributable to EFAl hydroxyl groups. These assignments are supported by the 1H–27Al TRAPDOR effect on the 1H NMR intensities. TRAPDOR is an NMR technique based on the dipolar coupling between a quadrupolar nucleus, such as 27Al, and a nucleus with spin 1/2 such as 1H. The technique relies on continuous irradiation of the 27Al spins during a rotor-synchronized spin–echo pulse sequence of the 1H channel.[37] By comparison of the proton echo intensities in experiments with and without irradiation on 27Al, the extent of the TRAPDOR effect can be monitored. The higher the intensity difference between the two echoes, the greater the dipolar coupling between the two nuclei is, and thus, the closer the two nuclei are. As can be seen in Figure , the signals at 2.8 and 4.0–4.6 ppm are affected by the irradiation on 27Al and can be assigned to EFAl hydroxyl groups (AlOH species) and BAS groups in supercages and sodalite cages, respectively. The signals at 0.8, 1.3, and 1.9 ppm are respectively attributed to nonacidic silanol groups, arising from framework defects, and to silanol groups.

Figure 4

1H MAS NMR spectra of dehydrated (top) USY and (bottom) AHFSY in (red) proton and (green) deuterium form as well as (black) loaded with benzene at the start of the H/D exchange at 55 °C. The figure also shows (gray) spectra recorded at 25 and 100 °C.

Figure 5

1H–{27Al} TRAPDOR effect in partialy H/D-exchanged USY zeolite. The spectra were recorded at the end of in situ H/D exchange at 55 °C (blue and green lines without and with 27Al irradiation, respectively).

The 1H NMR peak intensities for USY and AHFSY (red lines in Figure ) can be quantitatively interpreted in terms of concentrations (Table ). As AHFSY contains no EFAl, the BAS density can be taken equal to the FAl density of 34 u.c.–1 as determined by 29Si NMR spectroscopy. The 3.4× lower combined peak area of BAS in USY indicates a BAS density of ∼10 u.c.–1. The remaining 20 FAl u.c.–1 in USY should then be compensated in charge by cationic EFAl species. The overall Al content of USY from ICP equals 48 u.c.–1, suggesting that ∼18 Al atoms u.c.–1 are present as EFAl species. The peak area ratios between the overlapping 1H NMR signals of the two types of BAS are practically the same for USY and AHSFY within the spectral deconvolution error (Table ). From the benzene peak area, the total benzene loading before the start of the H/D exchange was 0.31 and 0.50 mmol/g for USY and AHFSY, respectively. This corresponds to, respectively, four and six benzene molecules per unit cell for USY and AHFSY.

Table 3

Concentration of Proton Species in USY and AHFSY Derived from 1H NMR Intensities

	δH	USY	AHFSY
	(ppm)	mmol/g	mmol/g
BASsodalite	4.6	0.57 ± 0.01	1.85 ± 0.01
BASsuper	3.9	0.39 ± 0.01	1.40 ± 0.01
Si–OH	1.8	0.13 ± 0.03	0.07 ± 0.01
Al–OH	2.6	0.40 ± 0.01	0.06 ± 0.01

H/D exchange between the deuterated zeolites and benzene was initially investigated on D-USY at different temperatures. Figure shows a stack plot of the 1H MAS NMR spectra recorded at different temperatures between −30 and 100 °C. The reaction starts at 25 °C with the H/D exchange at the sodalite position followed by H/D exchange of the supercage position. At a temperature of 100 °C, the proton signals of the BAS show an advanced exchange and a peak shape comparable to the fully protonated sample before deuteration and benzene exposure. At this stage, the silanol and EFAl hydroxyl groups are only slightly affected by exposure to benzene, which is expected due to the low acidity of these moieties.[42] The chemical shift of the supercage BAS protons is slightly larger than that of the supercage protons in the zeolites without benzene. In fact, the selective change of this signal confirms its assignment to the supercage BAS protons, because benzene cannot access the sodalite cages and, therefore, mainly interacts with the supercage BAS protons.

Figure 6

In situ1H NMR of H/D exchange between D-USY and benzene, recorded at different temperatures starting from −30 °C. The spectra were obtained after a specific time at the indicated temperature. Between −30 and 35 °C, the spectra were obtained after 2 h. The 55 °C spectrum was obtained after 14 h of H/D exchange, and the 100 °C spectrum was obtained after 1 h. For comparison, the spectrum of the dehydrated sample is added (red line).

The H/D exchange behavior in USY and AHFSY zeolites was then monitored at a constant temperature of 55 °C during 14 h, by recording quantitative 1H NMR spectra each 0.5 h. A series of spectra for USY and AHFSY at different times is given in Figure . As can be seen, the total signal intensity of the BAS signal at 4.0 and 4.5 ppm in both zeolites significantly increased during these experiments. A key difference noted is that whereas the sodalite sites in the USY zeolite exchange faster than the supercage sites, these two populations exchange at the same rate in the AHFSY zeolite.

Figure 7

In situ1H NMR of H/D exchange between D-USY and benzene as a function time at 55 °C. Spectra were recorded every 0.5 h up to 14 h in total.

As the sodalite and supercage proton signals overlap, line shape deconvolution was required to obtain quantitative information on the distinct exchange behavior (Figure S3). Deconvolution was done by use of the dmfit2008 program.[43] A Gaussian line shape was used where the positions and the widths of the sodalite and supercage resonances were kept fixed within the series of spectra. Figure shows the relative increase in the proton signals of BAS of USY and of AHFSY as a function of the exchange time. Confirming the qualitative difference above, the deconvolution confirms that the sodalite and supercage deuteroxyl (OD) sites in AHFSY exchange in the same manner, while for USY, the sodalite OD sites exchange much faster than the supercage OD sites.

Figure 8

Relative protonation of OD sites in USY and AHFSY based on deconvolution of the in situ1H NMR spectra recorded at 55 °C during H/D exchange of D-USY with C6H6 (cf., Figure ).

The faster H/D exchange of sodalite sites is unexpected, because it is well-known that benzene cannot enter the sodalite cages. The exclusive location of benzene in the supercage is confirmed by a selective chemical-shift change of the supercage BAS signal from 3.9 to 4.1 ppm due to benzene, whereas the shift of the sodalite-cage BAS stays unaltered (Figure ). Furthermore, after benzene absorption, the transversal 1H NMR relaxation is only accelerated for the supercage BAS (Figure ). The selective relaxation enhancement by a factor of 2 is caused by the dipole interactions between the supercage BAS protons and the relatively high concentration of benzene protons in the supercage. The absence of any benzene-induced relaxation effect on the BAS signal at 4.6 ppm also contradicts any tentative explanation that benzene has accidentally shifted part of the supercage BAS signals underneath the sodalite BAS signal in the 1H NMR spectrum.

Figure 9

Relative 1H NMR Hahn-echo spectra at varied echo times of fully protonated (a) USY, (b) AHFSY, (c) USY with benzene, and (d) AHFSY with benzene. The used echo times 2τ are 0.005, 2, 20, 40, and 80 ms in the direction of decreasing signal intensity. The numbers above the signals indicate the transversal relaxation time T2(1H) determined from the signal decay vs. 2τ.

2D spin-exchange spectroscopy (2D EXSY) provides information about the proximity of chemically different protons.[26,44] For a sufficiently long mixing time, the protons that are close in space will show cross-peaks in the 2D NMR spectrum. As can be seen in the 2D EXSY spectrum of USY measured at a mixing time of 1 s (Figure ), the sodalite, supercage, and EF proton spins show such cross-peaks. The relative intensities of these cross-peaks follow the total relative intensities in the sample as can be appreciated from the horizontal section spectra. This means that the proton magnetization at the three sites is mixed within an exchange time of 1 s. USY with adsorbed benzene shows, in addition to the previous exchange picture, cross-peaks with benzene protons. In contrast, the signal due to the silanol does not show cross-peaks with other hydroxyl groups in the zeolite. These findings point to the spatial separation of BAS and EF protons from the silanol groups at the external surface of the zeolite crystallites. Interestingly, in the presence of benzene molecules, the spin-exchange behavior of the EFAl proton spins with the supercage spins differs from the exchange without benzene. The amount of the exchanged supercage proton intensity with the EF hydroxyl groups is lower in the presence of benzene, while the intensity of sodalite BAS signal remains the same (i.e., by comparison of the horizontal section in Figure ). This proves that benzene interacts selectively with BAS in the supercages.

Figure 10

(left) 2D EXSY spectra measured at room temperature for USY with benzene still remaining in the USY sample (bottom) and after the complete removal of benzene (top). The spectra were acquired at a MAS rate of 10 kHz and a mixing time of 1 s. (right) Horizontal sections are shown at F1 1H NMR shifts corresponding to proton spins of hydroxyl groups associated with (a) EFAl, (b) supercage BAS, (c) sodalite BAS, and (d) C6H6.

Discussion

For the investigation of the influence of EFAl on the acidity situation in a Y zeolite, an EFAl-containing zeolite USY prepared by steaming was compared to an EFAl-free and chemically dealuminated AHFSY zeolite with comparable Si/FAl ratios. The two samples have similar XRD patterns, reflecting their high crystallinity. The FAl content in AHFSY (Table ), determined from the XRD lattice constant and 27Al MAS NMR (Table S1) and 29Si MAS NMR (Table ) spectra are similar and comparable to the total Al content in the sample determined by elemental analysis. This proves that the AHFSY zeolite is free from EFAl. As follows from the 27Al MQMAS NMR spectrum in Figure , the steamed USY sample contains octahedrally coordinated EFAl at ∼0 ppm, a horizontally broadened peak at ∼40 ppm, and a peak at ∼56 ppm. The peak at ∼40 ppm can be assigned to distorted FAl species.[45] The signal at ∼56 ppm lies on the diagonal, which therefore can be attributed to tetrahedrally coordinated FAlIV[45] or FAl species compensated in charge by cationic EFAl instead of protons.[2] The FAl content in USY zeolite determined by 27Al NMR spectra is lower than the value derived from the unit cell volume, which can be attributed to the NMR invisibility of part of Al caused by strong quadrupolar interactions. The number of BAS in USY is 3 times lower than in AHFSY, as can be concluded from a comparison of the peak areas in 1H NMR spectra (Table ). The distribution of the BAS over the sodalite cages and the supercages is nearly the same in both zeolites. The observed low BAS concentration in USY can be explained by their replacement by cationic EFAl. The high AlOH content as follows from the 1H NMR spectrum of USY is an indication that the cationic EFAl partially appears as Al(OH)(3– where x equals 1 or 2. The silanol density in USY is twice that in AHFSY but 3 times lower than the number of AlOH groups. This low silanol density in USY is probably because, parallel to hydrolysis reactions occurring during steaming, a healing reaction between the SiOH groups takes place, leading to siliceous regions.[41] The FAl content in USY and AHFSY, as extracted from XRD lattice constant values, is approximately similar. This forms a good basis for the investigation of the effect of EFAL species on the reactivity of the steamed zeolite. Differences in acidity and reactivity between the two zeolites, which could arise from the effect of framework Si/Al ratio, can therefore be neglected and instead correlated to the effect of EFAl species.

Before discussing the H/D exchange results in detail, some aspects concerning the diffusion and adsorption of benzene molecules in zeolite Y should be clarified. The benzene molecule has a kinetic diameter of ∼6 Å,[46] which is larger than the 6-membered ring (2.3 Å) of the sodalite cages and smaller than the 12-membered ring (7.4 Å) windows of the supercages. Therefore, benzene can freely diffuse in zeolite Y and will exclusively sit in the supercages and the pore windows interconnecting them. This is in line with the observed selective shift change and the decrease in the transversal relaxation T2(1H) of the supercage protons due to the presence of benzene. In contrast, the sodalite proton shift and T2(1H) relaxation do not change (Figures and 9), implying that benzene cannot enter the sodalite cages. The downfield shift of the supercage signal originates from the effect of the induced magnetic field by the circulating electrons in the benzene aromatic ring.[47] The effective field felt by the supercage protons depends on the orientation of the benzene molecule. When the benzene is facially coordinated to the supercage BAS, the proton will be more shielded, and hence, the peak position will shift upfield. The opposite will happen when the benzene ring and the proton are nearly in the same plane (Figure ).

Figure 11

1H NMR shift dependence on the relative benzene molecule orientation.

This could mean that the peripheral edge of benzene is more oriented in the direction of the supercage BAS such that the protons spend some time in the deshielding region. This configuration could be stabilized by weak or transient complexation of the benzene hydrogen atoms with zeolite oxygen atoms. An alternative explanation could be that the observed small downfield shift is the sum of two effects, namely (i) the upfield shift due to the planar coordination of benzene to the supercage BAS and (ii) a weak hydrogen bond along the molecular C6 axis.

1H NMR spectroscopy during H/D exchange is a frequently used method to measure the acidity and reactivity of the hydroxyl groups in a faujasite zeolite by monitoring the proton transfer between the BAS and reactants or model molecules, such as benzene.[27,42,48] In all studies we found, however, the researchers focused on the total exchange of the BAS with adsorbed molecules. In the current study, we addressed the site-selective H/D exchange of sodalite and supercage hydroxyl groups. We started from almost completely deuterated zeolites and low benzene loading. Following the H/D exchange at low temperature in this manner provides a good contrast in the H/D exchange reactivity and selectivity of the different hydroxyl sites in the two investigated zeolites.

The USY zeolite showed a higher H/D exchange reactivity with benzene than AHFSY. This behavior agrees with our earlier H/D exchange IR study,[2] which was attributed to the presence of EFAl. Interesting is the exchange of the sodalite BAS with benzene. Although benzene cannot enter the small cages, the sodalite BAS can exchange with molecules that are exclusively sitting in the supercage cavities. This can be explained by perturbation of the local zeolitic structure. These will lead to a flip of the SiO(3)Al and SiO(4)Al bonds in the direction of the supercage, when a guest molecule approaches the inner zeolite wall. The similarity in reactivity between sodalite and supercage BAS in AHFSY (Figure bottom) indicates that all sites possess the same acidity. The faster H/D exchange of the sodalite BAS compared to supercage BAS in USY (Figure , top) implies that the reaction of the sodalite BAS is influenced by EFAl species in a different manner than the reaction with the supercage BAS. A possible cause may be the localization of EFAl species in USY zeolite. Van Bokhoven and co-workers[17] showed in their in situ XRPD and XAS investigation of the dealumination of NH4Y by steaming that the formed EFAl species occupy the SI′ site, which is at the external base of the hexagonal prism just inside the sodalite cage. Mota et al.[18,19] mentioned in their work that occupation of the SI′ site in the sodalite cage in the steamed Y zeolite by Al3+ close to O(3) oxygen atoms and at the center of the cage (U site) by Al(OH)2+ leads to an increase in the acid strength of sodalite protons (O(3)H) and a decrease in the supercage (O(1)H) acid strength. They also mentioned that the role of the EFAl is to stabilize the conjugate base, formed upon deprotonation. Florian et al.[16] demonstrated in their work on La in a Y zeolite that La cations are predominantly stabilized within sodalite cages.

To gain more insight in this, more detailed experimental information about the location of EFAl in the zeolite and the interaction between hydroxyl groups and benzene molecules is needed. For this purpose, we applied 1H NMR relaxometry and 2D 1H NMR in this study. Spin–spin or transversal relaxation originates from the dipolar interaction of a spin with local magnetic fields generated by other neighboring 1H spins. This dipolar interaction falls off rapidly as ∼1/r3 with the distance r between two spins and, hence, is only effective over short distances (<0.5 nm).[49] The similarity in T2 relaxation behavior between supercage and sodalite-cage protons in AHFSY (Figure ) indicates that the environments of the protons at the two types of sites are similar. This is in line with the picture outlined above about the similar acidity of the two types of BAS in AHFSY. The high proton density in benzene causes neighboring 1H spins in the supercage to relax faster than without benzene. This reveals valuable information about the relative location of the different hydroxyl groups in the zeolite. With benzene present in the zeolites, the transversal relaxation of the protons in the sodalite cages remains unchanged, whereas the relaxation of the supercage protons becomes roughly twice as fast in both zeolites. This is because benzene can freely pass through the 12-membered ring window, which affects the T2(1H) time of the supercage BAS. As a result of the inaccessibility of the sodalite cage for benzene, the T2(1H) time of the sodalite BAS remains unaffected. The slight decrease in T2(1H) relaxation time of the EFAl OH in USY suggests that the EFAl is positioned further away from the high proton density of benzene in comparison to the supercage BAS. In USY, benzene shows, in addition to the bulk chemical shift around 7.3 ppm, a downfield shift at ∼8.0 ppm. An explanation for this shift could be that some of benzene molecules are highly deshielded as result of a stronger H–benzene bond with negative framework oxygen atoms. The appearance of cross-peaks in 2D EXSY 1H NMR spectra between bridged and EFAl hydroxyl groups suggests that the EFAl species are in close proximity to the BAS. The silanol protons show no cross-peaks with the other hydroxyl groups and are therefore exclusively at the external surface of the zeolite crystals. This is in good agreement with the unaltered T2 of the silanol protons upon benzene adsorption and with the results reported by Mildner and Freude.[42]

In the presence of benzene, the situation outlined above is changed. The cross-peak signal arising from the interaction of the supercage and the EF protons is slightly attenuated (Figure ). The attenuation of the cross-peak by benzene indicates that a fraction of the EF proton intensity is transferred to benzene. This could be explained by considering EF proton spins exchanging indirectly, via supercage protons, with the benzene. This is again an indication for the selective exchange of benzene with the supercage BAS protons. T1ρ(1H) relaxometry, which is sensitive to short-range proximity (Figure S1), suggests also that cationic EFAl species are located in close proximity of the sodalite BAS. The H/D exchange behavior between the BAS in USY and benzene as well as the obtained experimental information from T2(1H), T1ρ(1H) relaxometry, and 2D EXSY NMR spectroscopy indicate that the cationic EFAl is located in the sodalite cage.

In Figure , the H/D exchange reaction between the BAS and benzene in the presence of EFAl is proposed. Upon deprotonation, the negatively charged zeolite surface is stabilized by a cationic EFAl complex. Consequently, the sodalite BAS reactivity is increased by the presence of EFAL species.

Figure 12

Stabilization of the negatively charged zeolite surface by EFAl cations.

Conclusion

We carried out an in situ1H MAS NMR study of the H/D exchange reaction between benzene and BAS in EFAl-free dealuminated AHFSY and steam-calcined USY zeolites with a main focus on unraveling the nature of the promoting effect of EFAl on zeolitic BAS. The NMR approach here differs from our earlier in situ IR spectroscopy approach by starting from the deuterated zeolite instead of the proton form and involving continuous exposure to low benzene concentrations instead of transient pulses at high benzene densities. Benzene selectively changes the 1H NMR chemical shift of the supercage BAS yet does not influence the shift of the sodalite-cage BAS. This is consistent with the accepted picture that C6H6 exclusively adsorbs in the supercages, because the sodalite cages are inaccessible for benzene. Nevertheless, in the case of the EFAl-free AHFSY zeolite, our in situ NMR study shows that the sodalite and the supercage BAS undergo equally fast H/D exchange. On the contrary, in USY, the H/D exchange of the sodalite-cage BAS is strongly and, more importantly, selectively enhanced compared to that in AHFSY. Already at 25 °C, the sodalite-cage BAS become significantly protonated within a day, and at 55 °C, the H/D exchange under the specific loading conditions (initial Hbenzene/DBAS ratio ≈ 2) approaches the equilibrium protonation within several hours.

Given the predominant interactions of benzene with the supercage BAS, the most stable supposedly axial binding configuration of C6H6 to this BAS type, through weak hydrogen bonding along the benzene C6 axis, is not of direct importance for the H/D exchange reaction. H/D exchange is most likely controlled by a less stable peripheral binding state with the hydrogen bonding in the benzene plane. In such a configuration, the approachability of sodalite and supercage BAS for benzene located in the supercage may be more comparable. In addition, the local zeolite framework seems sufficiently flexible to allow for temporary flips of the Al–OH–Si bridges, such that the sodalite BAS may transiently point into the supercages. Such lattice flips alone, however, would at most explain roughly equal H/D exchange rates of the two BAS types, as indeed observed for AHFSY. The strong selective enhancement of reactivity of the sodalite-cage BAS in USY thus appears to be caused by the EFAl species in the sodalite cages of this zeolite.

As indicated by the spatial proximity derived from the equal proton spin-diffusion driven T1ρ(1H) relaxation behavior of protons in the EFAl species and the sodalite-cage BAS, the sodalite-cage BAS closely interacts with the EFAl. This is in line with earlier DFT computations,[21] which showed the preference of EFAl to be stabilized in the faujasite sodalite cages. The promoting role of EFAl cationic species is therefore to stabilize the negative charge on the lattice upon deprotonation of the sodalite BAS. This leads to a strong and selective enhancement of the acidity of the sodalite-cage BAS observed in the in situ1H NMR of H/D exchange in USY.