Exposing active sites in zeolites with fluoropyridines as NMR probe molecules

Abstract

Active sites in zeolites are crucial for catalysis, but their identification remains challenging due to structural complexity. Solid-state NMR, a key tool for studying zeolites, can struggle with directly detecting acidic properties due to the quadrupolar nature of ubiquitous metal dopants. Here, we use fluorinated pyridine as a probe to identify active sites in HY and HZSM-5, two important solid acid catalysts. ¹⁹F NMR effectively detects binding environments, while ¹⁹F–²⁷Al polarization transfer reveals elusive penta-coordinated Al^V Brønsted acid sites and shows that the dominant active sites are distorted framework-associated Al^IV and Al^V sites with large quadrupolar coupling constants, shedding light on catalytic properties. Low-temperature (100 K) NMR proves essential for capturing these interactions. This work enhances the understanding of zeolite active sites and highlights the broad applicability of fluorinated probe molecules for surface characterization, offering a cost-effective, highly sensitive approach for catalytic studies.

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Introduction

Zeolites are the most widely used catalyst class for the cracking of hydrocarbons.^1–4 They also show promise for methanol-to-olefin (MTO) fuel production and for the processing of other types of renewable biomass into usable fuels and products,^5–8 as well as for the upcycling of polyolefin plastics.^9–11 Starting from a porous silica (SiO₂) structure, doping of metals such as Al into the zeolite framework introduces acid sites that can be active for catalysis.^6,12–15 Much work over many decades has been undertaken to try to optimise the framework structures and pore sizes,^16–20 the amount of acid sites, the ratio of types of acid sites (Brønsted : Lewis),^21,22 and also the synergy between different acid sites and dopants (such as single atoms sites^23–25 or nanoparticles^26,27) for particular applications. Therefore, knowledge of how the atomic-level structure relates to catalytic mechanisms is essential.

Solid-state NMR (ssNMR) spectroscopy can provide local information about ¹H,^{28–30 29}Si,^{31–33 17}O,^34–36 and ²⁷Al^28–32,37 sites. Although Al (and other metals) can be inserted into the framework in tetrahedral (Al^IV) positions, it is well known that there are a number of different extra-framework aluminium (EFAl) sites present in such materials: Al^IV, penta-coordinated Al^V, and hexa-coordinated Al^VI.^14,32,38 Different synthesis/doping methods and post synthetic treatments have been applied to change the silicon : aluminium ratio (SAR) of zeolites as well as their pore structures, thus varying the Brønsted acid site (BAS) : Lewis acid site (LAS) ratio and their accessibilities.³⁰ This leads to different functional properties.

Computational chemistry techniques have been applied to model how reagents bind, elucidating reaction pathways,^39–41 and to investigate the types of active site present,^42–45 so that structure can be related to function. Nevertheless, the nature of some of these sites remains ambiguous even after frequent investigations. One such site is the penta-coordinated EFAl^V moiety;^32,46 it has generally been treated as a LAS and its distorted geometry leads to it being a highly active species.^47–49 However, recent work from Deng and coworkers suggests that there is also the potential for BAS-type activity for Al^V sites.³² Structural ambiguities such as these hinder a comprehensive understanding of the catalytic activity of these vital porous materials.

Analysis using adsorption of probe molecules can give the types and relative strengths of the acid sites present. Probe molecules are commonly used with IR spectroscopy, temperature programmed desorption (TPD), as well as NMR spectroscopy. For NMR, the main probe molecules can be split into two categories derived from their NMR-active isotopes used to characterise surfaces: ³¹P-based, e.g. trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO),^50,51 and ¹⁵N-based, e.g. pyridine (Py).⁵⁰ These are used because they interact strongly with active sites and their spy nuclei have high sensitivities to environmental changes owing to large chemical shift ranges. However, TMP and TMPO are hazardous (TMP is highly toxic and flammable).^21,51,52 Py is commonly used with FT-IR to give rapid assignment of acid sites and their strengths. Ammonia and Py have proven to be particularly useful for the characterization of acid sites through the combination of FT-IR and TPD for quasi-quantitative assignment of site coverage.^53–59 Ammonia can be preferred for TPD owing to its lower cost, smaller size, lower desorption temperatures, and reduced hazards compared to Py. The utility of Py as a probe molecule with NMR spectroscopy has been recently highlighted for example by Copéret and coworkers who used adsorption of ¹⁵N-enriched Py combined with variable temperature ¹⁵N NMR and DFT studies to elucidate the presence of a pseudo tricoordinated framework-associated LAS in the zeolite mordenite.⁴⁶ Its distorted nature suggests that it is likely very active in catalysis, but this needs to be further investigated.⁴⁶ The use of isotopically-enriched Py was required as ¹⁵N has low NMR receptivity (0.04% compared to ¹H) but ¹⁵N-Py is costly (∼2 k€ per g).

Both categories of probe molecule exhibit multiple binding modes. TMPO adsorbs to both BAS and LAS and shows a total of 4 LAS and BAS coordination modes/strengths for zeolite Y (H form, HY)^52,60 and binding to 5 different BASs for HZSM-5.⁵¹ The relative spatial proximities between the active sites can be determined using 2D correlation NMR spectroscopy involving ³¹P from TMPO.⁶⁰ Similarly, monomethylamine (MMA) studies for HZSM-5 and HY have been used to examine spatial correlations whilst showing multiple binding sites for both zeolites.^53,61 Combining NMR spectroscopy of probe molecules with synchrotron resonant soft X-ray diffraction and neutron powder diffraction has very recently enabled the identification of the specific locations of tetrahedral framework aluminium in HZSM-5 ⁶² but not framework-associated or extra-framework sites owing to their lack of long-range order.

Similar to ³¹P, ¹⁹F has favourable NMR properties, including high receptivity (>12 times that of ³¹P) and a wide chemical shift range. Fluorine is also predominantly found naturally only in minerals, so it rarely contributes to signals from most synthetic zeolitic analytes and thus resonance overlap is minimal, and resolution is high. This means that it is in principle an ideal spy nucleus; indeed, 4-fluoronitrobenzene and 4-fluoroaniline have been used to measure zeolite acidity,⁶³ 4-fluoroacetophenone has been used to do so quantitatively,^64,65 and CF₄ has been used to probe pore dimensions and accessibility in a similar manner to ¹²⁹Xe NMR,⁶⁶ while ammonium hexafluorosilicate has been used to investigate dealumination processes.⁶⁷ Functionalised pyridines containing ¹⁹F such as 2- and 3-fluoropyridine (2FP and 3FP, respectively) are widely commercially available at low cost (∼10 € per g). TPD experiments have shown that they bind to active sites in zeolites⁶⁸ and therefore they have the potential to be useful probe molecules for NMR.

Herein, the utility of fluoropyridines as NMR probe molecules is demonstrated. Zeolites HY (SAR = 2.6) and HZSM-5 (SAR = 40) were selected as the focus of this investigation due to their wide industrial relevance^69–73 as well as both having similar active sites but different pore sizes, structures, and active site distributions. Pyridine has a kinetic diameter of ∼5.3 Å, so it can fit into the medium-sized pores of the 10-membered rings (10-MR) (∼5.5 Å) of HZSM-5 (ref. 74) but will be tightly constrained by the small size (see Fig. 1). Pyridine fits more easily into the large-pore 12-MR supercages of HY (pore opening ∼7.4 Å). However, pyridine is too large to fit into the smaller sodalite cages of HY. Here, high-field NMR and low temperature NMR are used for increased resolution and sensitivity, respectively, enabling multiple binding modes/strengths to be detected for fluoropyridines adsorbed on HY and HZSM-5 via ¹⁹F NMR. Notably, polarisation transfer from the probe molecule (¹⁹F) to the zeolites (²⁷Al) has shown the unexpected presence of pentacoordinated framework-associated BAS and that these are the preferred adsorption sites for both zeolites. This highlights the importance of using probe molecules to gain detailed insights into the active sites so that structure can be related to function and thus so that structures can be effectively rationally optimised.

Fig. 1 Structures of zeolite Y (a) and ZSM-5 (b); only oxygen (red) and silicon/aluminium (blue) sites are represented. FT-IR spectra (c and d) of zeolite HY (c) and zeolite HZSM-5 (d), dosed with 3FP (top), 2FP (middle) and undosed (bottom) with relevant acid site bindings shown on the spectra. Wavenumbers and assignment are tabulated in Table S7.†

Results and discussion

Pyridine is commonly used as a probe molecule for IR studies of active sites in zeolites since the resulting spectra can be used to discriminate between protonated pyridine (Py-H⁺), pyridine adsorbed at a weak BAS through H-bonding (Py-B), and pyridine adsorbed at a LAS (Py-L).^75,76 Fig. 1 shows the absorbance-mode FT-IR spectral region of 1800–1350 cm⁻¹ for HY and HZSM-5 dosed with 2FP and 3FP as well as the pure zeolites and fluoropyridines, for comparison. The peaks observed for the 2FP-dosed zeolites are very similar to those expected for equivalent Py adsorption. These show clear protonated species (2FP-H⁺@HY and 2FP-H⁺@HZSM-5) and adsorption at LAS (2FP-L@HY and 2FP-L@HZSM-5) for both zeolites. 3FP does not have the same spectral separation of (ν_8a) vibrational modes as Py or 2FP but the 3FP-dosed zeolites exhibit bands corresponding to 3FP-H⁺ with higher wavenumbers (1565 cm⁻¹ for HY and 1571 cm⁻¹ for HZSM-5; cf. 1547 and 1543 cm⁻¹ for 2FP-dosed zeolites, respectively) and bands corresponding to adsorption at weak LAS with lower wavenumbers (1434 cm⁻¹ for HY and 1433 cm⁻¹ for HZSM-5; cf. 1440 and 1439 cm⁻¹ for 2FP-dosed zeolites, respectively). This suggests that 2FP and 3FP have different adsorption geometries and/or binding strengths and these are dependent on the type of active site. These variations highlight the sensitivity of fluoropyridines as probe molecules.

As the FT-IR spectra demonstrated that both 3FP and 2FP bind to active sites in zeolites HY and HZSM-5, they were both investigated using conventional ¹⁹F solid-state NMR spectroscopy at 9.4 T field strength. A TPD study shows different preferential bindings for 2FP and 3FP to zeolite acid sites in HZSM-5 and H-Mordenite with 3FP having a higher adsorption enthalpy.⁶⁸ Disappointingly, the {¹H–}¹⁹F cross-polarisation magic angle spinning (CPMAS) NMR spectra of 2FP@HY and 2FP@HZSM-5 are almost identical (see Fig. S1†). They both show a large component (δ{¹⁹F} ≈ −70 ppm) and two smaller components (δ{¹⁹F} ≈ −70 and −80 ppm), with the smaller components exhibiting sizeable chemical shift anisotropies (CSA). Cross-polarisation was used to be selective to species that are strongly adsorbed; CPMAS uses the dipolar interaction (here between ¹H and ¹⁹F) to transfer polarisation between spins and this can be averaged-out under fast isotropic motion, leading to poor transfer efficiencies. Nevertheless, the larger peak displays negligible CSA, indicating dynamical averaging (see Fig. S1 and Table S8†). Thus, this larger peak can be assigned to physisorbed species and the smaller peaks to species bound to active sites. Although CPMAS is not inherently quantitative, ¹⁹F direct excitation with careful sample preparation can enable a quantitative analysis of acid sites, as has been shown previously.⁶⁴

The {¹H–}¹⁹F CPMAS NMR spectra of 3FP@HY and 3FP@HZSM-5 are substantially different, even at moderate field strengths (9.4 T, see Fig. 2, S1, and S2†). This is a first indication that 3FP can be used to differentiate active sites between similar zeolite solid acid catalysts. For 3FP, however, the smaller, broader, higher anisotropy peaks are deshielded (i.e. higher ppm) compared to the narrower peaks from physisorbed species (δ{¹⁹F} ≈ −130 ppm). This infers, as expected, that the position of the F on the pyridine ring (e.g. 2FP vs. 3FP) plays a role in the electronic interaction of the probe molecule with the zeolite surface and this is reflected in its chemical shielding. The direct excitation ¹⁹F MAS NMR spectra of 3FP@HY and 3FP@HZSM-5 (Fig. 2) are similar to the respective CPMAS NMR spectra but it is clear from the reduced relative presence of spinning side bands that there is a large proportion of dynamic species that are not detected via CP. However, comparing these spectra to those of 3FP@silicalite-1 (a pure silica MFI-type zeolite) and 3FP@γ-alumina (a nanoparticulate Lewis acid catalyst) shows that 3FP interacts strongly with acid sites in the porous acid catalysts since the ¹⁹F resonances for 3FP@silicalite-1 and 3FP@γ-alumina correspond to mainly physisorbed species (Fig. 2a and b). Since 3FP shows a greater difference for ¹⁹F NMR spectra between HY and HZSM-5 compared to 2FP, 3FP will be the focus of the rest of the study.

Fig. 2 9.4 T ¹⁹F direct excitation (a–d) and {¹H–}¹⁹F CP (e and f) MAS NMR spectra recorded with ¹H decoupling at ambient temperature. Spectra shown are of 3FP-doped samples of silicalite-1 (blue) (a), γ-alumina (purple) (b), HY (black) (c and e), and HZSM-5 (red) (d and f). † Denotes artifacts from decoupling. (a and b) were recorded with 12 kHz MAS frequency and (c–f) were recorded with 8 kHz MAS frequency.

To reduce the dynamics of adsorbed 3FP, the doped zeolites were cooled to 260 K. Very similar {¹H–}¹⁹F CPMAS NMR spectra are observed compared to those recorded at ambient temperatures (Fig. S2†). However, there are some notable differences. The ¹⁹F resonances assigned to physisorbed 3FP have become broader and now present a larger CSA, as would be expected with reduced dynamics. The highest shifted (most down-field) ¹⁹F isotropic resonance observed for 3FP@HZSM-5 appears at a lower chemical shift compared to 3FP@HY (cf. δ{¹⁹F} ≈ −118 ppm for HZSM-5 and −112 ppm for HY) but it has very similar CSA parameters: δ_CSA and η (see Fig. 2, S1, and S2, and Table S8†). This indicates that 3FP is adsorbed in a similar mode, but with a different strength. Notably, there are ¹⁹F resonances observed with intermediate chemical shifts (δ{¹⁹F} ≈ −120 to −128 ppm), and the associated CSA tensors have asymmetry parameters, η, close to 1, whereas the highest shifted resonances have asymmetry parameters closer to 0; this indicates that the binding mode that results in the corresponding ¹⁹F CSA tensor is very different. Previous NH₃-TPD measurements on HY (with the silicon-to-aluminium ratio, SAR, = 2.6) show a balanced weak : strong acidity (0.8 : 1.0) where the total active acidity is from a 1.0 : 1.1 BAS : LAS ratio and that the majority of LAS show weak acidity whereas the BAS show strong acidity.⁷⁷ For HZSM-5 with lower Al content (SAR = 38), NH₃-TPD studies show a greater proportion of BAS compared to LAS and weaker LAS but stronger BAS than HY.⁷⁸ Therefore, it is likely that the downfield ¹⁹F resonances stem from 3FP-L and the intermediate ¹⁹F resonances stem from 3FP-B owing to their relative amounts in each zeolite (Table S8†) and that downfield shifts (higher ppm) indicate greater acid strengths. The ¹⁹F spectrum and resonances are generally broader for 3FP@HY compared to 3FP@HZSM-5. This is consistent with the NH₃-TPD analysis that highlights the complexity and variation of acidic strength of HY,^78,79 leading to a distribution of ¹⁹F chemical shifts that are not dynamically averaged. Therefore, NMR chemical shifts and their anisotropies from 3FP ¹⁹F resonances can be used to distinguish LAS and BAS in zeolites, as well as their strengths, with higher deshielding (more positive ppm) stemming from both stronger binding and from 3FP-L.

The impact of the 3FP doping can also be observed via ²⁷Al MAS NMR spectra (Fig. S3†). Framework tetrahedral aluminium-27 resonances (δ{²⁷Al} ≈ 60 ppm) are perturbed upon adsorption of 3FP. For both zeolites there is a slight broadening of these signals, particularly at the high-field (lower ppm) side, and this effect is larger for HY than HZSM-5. Furthermore, the resonances corresponding to hexa-coordinated extra-framework aluminium (EFAl^VI) species (δ{²⁷Al} ≈ 0 ppm) for both zeolites, which are much more prevalent in HY, are reduced upon adsorption of 3FP. This is likely due to direct binding of 3FP to these LAS (vide supra) or 3FP may lead to an annealing effect on the zeolites as pyridines can help relax distorted Al centres.⁴⁶

To gain higher spectral resolution from 3FP adsorbed on the zeolites, high-field (20.0 T) NMR analysis was employed and combined with fast MAS (33 kHz). It is clear from high-resolution ¹H MAS NMR spectra recorded at ∼260 K (Fig. S4†) that a highly acidic proton is observed (δ{¹H} ≈ 15 ppm) upon adsorption of 3FP to zeolite HY. A similar chemical shift is observed (δ{¹H} ≈ 15 ppm) for d₅-pyridine adsorbed to HY and is assigned to Py-H⁺.³² The assignment to 3FP-H⁺ is confirmed here through a ¹H–¹H dipolar correlation spectrum (Fig. S5†), which shows a correlation for this resonance with 3FP protons. The absence of a similar peak for 3FP@HZSM-5 is attributed to the lower amount of BAS compared to HY. It is also clear that the ¹H from 3FP are more dynamic for 3FP@HZSM-5 compared to 3FP@HY, as demonstrated by the narrower associated resonances (δ{¹H} ≈ 6–10 ppm). This is consistent with the analysis of the 9.4 T 260 K {¹H–}¹⁹F CPMAS NMR spectra (vide supra). This could be considered counterintuitive since the pores of HZSM-5 are smaller than those of HY and thus 3FP should be more tightly constrained in the former. This infers that 3FP interacts more strongly with the pores in HY, either due to the nature of the active sites or to the larger proportion of these. Ambient temperature high-field acquisition of {¹H–}¹⁹F CPMAS NMR spectra of 3FP@HY and 3FP@HZSM-5 was challenging. This again highlights the mobility of 3FP in the pores. With cooling (∼260 K), to counteract frictional sample heating under MAS, high-resolution {¹H–}¹⁹F CPMAS NMR spectra could be obtained (Fig. 3). Deconvolution of the 3FP@HY ¹⁹F NMR spectrum shows two clusters of peaks (δ{¹⁹F} ≈ −104 to −116 ppm and −118 to −134 ppm). Even at high-field with fast MAS, the ¹⁹F resolution is not sufficient to resolve individual peaks; the deconvolution is non-definitive but has been fit with the minimum number of peaks required for a suitable reproduction of the spectrum (full fitting parameters can be found in Table S10†). This deconvolution demonstrates the benefits of high-field as it is evident that there are multiple ¹⁹F resonances and thus adsorption modes/strengths. As noted above, the downfield (higher ppm) cluster is assigned to 3FP-L and the upfield cluster is from 3FP-B and physisorbed 3FP. For 3FP@HZSM-5 the observed ¹⁹F chemical shift range is more limited but there are still two discrete clusters (δ{¹⁹F} ≈ −114 to −120 ppm and −120 to −134 ppm), with the downfield and upfield clusters assigned to 3FP-L and 3FP-B/physisorbed, respectively (vide supra). The sample cooling has provided further evidence for 3FP adsorption to strong BAS through the formation of 3FP-H⁺ and, combined with high-fields, the ¹⁹F NMR spectra show that there are a variety of binding modes and strengths at both LAS and BAS and that the variety is greater for HY compared to HZSM-5, as is the strength of LAS.

Fig. 3 20 T {¹H–}¹⁹F CPMAS NMR spectra recorded with cooling (∼260 K) and using a MAS frequency of ∼33 kHz. Spectra shown are of (a) 3FP-dosed zeolite HY (black) and (b) 3FP-dosed HZSM-5 (red), with deconvolutions (purple) and the envelope (green). Fitted parameters are given in Table S10.†

Pyridine has previously been shown to adsorb to common zeolites with multiple binding modes.^58,76 This is consistent with the number of major adsorption sites found for 3FP for the two zeolites studied herein. To further reduce the dynamics of adsorbed 3FP and also to increase the NMR sensitivity, MAS NMR spectra were collected at 100 K and 14.1 T. This enabled {¹⁹F–}²⁷Al CPMAS NMR spectra to be recorded (Fig. 4), which show where 3FP is adsorbed in the zeolites; this was not possible previously at higher temperatures owing to reduced experimental sensitivity (Fig. S6†). As expected, ²⁷Al resonances are detected near ¹⁹F spins at δ{²⁷Al} ≈ 60 ppm for both HY and HZSM-5, which correspond to 3FP-B since these BAS arise from framework tetrahedral Al^IV. These ²⁷Al resonances can be deconvoluted into two contributions: (i) framework Al^IV-1 (C_Q ≈ 2.9 MHz, δ_iso{²⁷Al} ≈ 64 ppm) and (ii) framework-associated Al^IV-2 (C_Q ≈ 6.4 MHz, δ_iso{²⁷Al} ≈ 65 ppm).^38,47,80 Their relative contributions to the {¹⁹F–}²⁷Al CPMAS NMR spectra (1.0 : 1.2 for HY and 1 : 2 for HZSM-5, see Table S11†) are different compared to the quantitative direct excitation ²⁷Al MAS NMR spectra (1.0 : 0.5 for undosed HY and 1.0 : 0.4 for undosed HZSM-5, Fig. S9 and Table S12†). This demonstrates that 3FP adsorbs near Al^IV-2 sites more preferentially than near Al^IV-1 sites. This could be expected since these sites protrude into the pores and have been shown to be responsible for enhanced catalytic activity.⁸⁰ It is notable that the relative intensity of 3FP adsorption to Al^IV-2 compared to Al^IV-1 is greater for HZSM-5 compared to HY; this may explain the results of calorimetry studies of the two zeolites which show greater pyridine adsorption enthalpy for HZSM-5 than HY.⁸¹

Fig. 4 14.1 T, 100 K {¹⁹F–}²⁷Al CPMAS NMR spectra (a and b) and DNP-enhanced {¹H–}¹⁵N CPMAS NMR spectrum (c) of 3FP-dosed zeolites HY (black) (a and c) and HZSM-5 (red) (b). The corresponding ²⁷Al direct excitation MAS NMR spectra are shown with dashed lines in (a and b) and spectral deconvolutions are given in Fig. S8.† The MAS frequency was 8 kHz for all spectra and ²⁷Al background correction is shown in Fig. S7.†

Surprisingly, and importantly, a different ²⁷Al resonance is detected (δ{²⁷Al} ≈ 20 ppm) as the primary adsorption site for 3FP in both zeolites (∼8 : 1 compared to Al^IV-1). This resonance is not detected in direct excitation ²⁷Al MAS NMR spectra nor in a ¹H–²⁷Al dipolar correlation MAS NMR spectrum, even at high field (see Fig. S10 and S11†); only an Al^V site stemming from EFAl^V is detected for HY (δ{²⁷Al} ≈ 30 ppm). This demonstrates the utility of {¹⁹F–}²⁷Al CP with adsorbed 3FP to reveal previously undetectable Al sites; they are likely scarce yet have a high propensity for adsorption. To assign this resonance to 3FP-B or 3FP-L, a complementary ¹⁵N NMR spectrum was recorded for 3FP@HY (Fig. 4c). Owing to the low NMR receptivity of ¹⁵N (e.g. natural isotopic abundance of 0.4%), the {¹H–}¹⁵N CPMAS NMR spectrum was enhanced using dynamic nuclear polarization.^82–84 The resulting spectrum shows a single ¹⁵N resonance (δ{¹⁵N} ≈ 205 ppm) that corresponds to Py-B.^58,59 There is no observed signal for Py-L (expected δ{¹⁵N} ≈ 240 ppm^46,85). Therefore, the primary adsorption site for 3FP is a BAS with low ²⁷Al chemical shift.

Recent studies have presented active sites in zeolites that could be responsible for the primary adsorption site for 3FP. Yakimov et al. used adsorption of pyridine on mordenite zeolite combined with low temperature MAS NMR spectroscopy and DFT calculations and showed that Py-B (C_Q ≈ 1.8 MHz, δ_iso{²⁷Al} ≈ 54 ppm) were prevalent and that framework-associated pseudo tri-coordinated Al LAS existed and could also adsorb Py (C_Q ≈ 9.4 MHz, δ_iso{²⁷Al} ≈ 58 ppm).⁴⁶ Zheng et al. also used Py-adsorption but for HY and with 2D correlation MAS NMR spectroscopy demonstrated the existence of unprecedented penta-coordinated Al^V-BAS (C_Q ≈ 5.6 MHz, δ_ιso{²⁷Al} ≈ 35 ppm).³² The latter study was performed at room temperature, and it is known that quadrupolar parameters can be dynamically averaged.⁴⁶ The values extracted from our low temperature measurements (Table S11†) for the primary adsorption site for 3FP on HY and HZSM-5 (C_Q ≈ 7.5 MHz, δ_iso{²⁷Al} ≈ 30 ppm) match well to those reported for Al^V-BAS, accounting for reduced dynamics. Moreover, a very recent theoretical and experimental study by Wang et al.⁸⁶ showed that “NMR-invisible” tri-coordinated Al sites in ultrastable Y (USY) zeolite become detectable upon hydration and that this adsorption of water leads to framework-associated Al^IV-2 and, importantly, Al^V-BAS (C_Q ≈ 9.3 MHz, δ_iso{²⁷Al} ≈ 32 ppm). USY is prepared from HY via calcination that dealuminates the framework, creating tri-coordinated Al defects. Therefore, the evidence here suggests that these sites can also be found in HY and HZSM-5 in low concentrations and that adsorption of 3FP is preferentially to these difficult-to-detect Al^V-BAS structures and {¹⁹F–}²⁷Al CPMAS NMR facilitates this observation. This highlights the challenge of correlating structure with activity in heterogeneous catalysts. However, probe molecules can effectively target active sites and, when combined with solid-state NMR analysis, provide detailed insights into the accessibility and acidity of these sites.

Conclusion

This study demonstrates the efficacy of 3-fluoropyridine (3FP) as a powerful probe molecule for elucidating the nature of active sites in zeolites (exemplified by HY and HZSM-5). 2-fluoropyridine (2FP) was also investigated but shows inferior properties compared to 3FP in terms of adsorption strength and resulting ¹⁹F chemical shift perturbations. Nevertheless, it may find value for certain applications. The advantages of 3FP are that: (i) it is readily applicable as a probe molecule for a variety of analytical tools including TPD, IR, and NMR, facilitating complementary characterisations; (ii) it is relatively inexpensive; (iii) it is relatively small so can fit in micropores; (iv) it can bind (as Py does) to both BAS and LAS; and (v) it can differentiate acid strengths through chemical shifts of both ¹⁵N and ¹⁹F. The limitations are that it does not fit inside the smallest zeolite pores, it could block moderately-sized pores preventing a full quantitative analysis, and that it displays substantial dynamics at ambient temperature. Through conventional, high-field, and low-temperature ¹⁹F MAS NMR techniques various active sites have been identified, including previously hard-to-detect Al^V Brønsted acid sites that are in low concentrations but apparently crucial for adsorption and thus catalytic activity. The local aluminium environments, which are generally deemed as the active sites or adjacent to active sites (BAS), are very similar for zeolites HY and HZSM-5 in terms of where 3-fluoropyridine is bound; their ²⁷Al NMR signatures after polarisation transfer from ¹⁹F are almost identical. However, the ¹⁹F NMR response from the fluoropyridine is very different. This method provides valuable insight into different activity for seemingly similar sites. This concept is well known for zeolites, but not so well understood. Such findings provide further evidence for the nuanced role of zeolite active site environments in determining molecular interactions, reinforcing the need for detailed spectroscopic characterisation to fully understand structure–function relationships in zeolite catalysis.

Experimental procedures

Details regarding the experimental procedures can be found in the ESI.†

Data availability

Data for this article, including NMR and FT-IR data are available at Figshare at https://doi.org/10.48420/28378607.v1.

Author contributions

D. L. conceptualized the original idea. J. H., Y. M., and R. Z. designed the adsorption conditions and performed the standard characterization of all samples. J. H. collected, interpreted, and finalized the solid-state NMR data with input and supervision from P. C. V. for the low temperature MAS NMR and from D. I. for the high-field MAS NMR. S. Y. and X. F. supervised the synthetic and adsorption work and D. L. supervised the solid-state NMR spectroscopy. J. H. wrote the first draft of the manuscript and the ESI.† S. Y., X. F., and D. L. revised the article for submission, with input from all coauthors.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

The UK High-Field Solid-State NMR Facility used in this research was funded by EPSRC and BBSRC (EP/T015063/1), as well as the University of Warwick including via part funding through Birmingham Science City Advanced Materials Projects 1 and 2 supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF). The low temperature (100 K) MAS NMR experiments were performed at the Nottingham DNP MAS NMR Facility which is funded by the University of Nottingham and EPSRC (EP/L022524/1 and EP/R042853/1). We also thank the EPSRC and The University of Manchester for funding the 16.4 T NMR spectrometer (EP/V007580/1) used in this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ay00507h

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