27Al (3Q) MAS NMR spectroscopies

Complex analysis of the state and location of framework Al atoms regarding channels and their distribution between Al pairs and single Al atoms were obtained by employing ²⁷Al (3Q) and Co(II) ion exchange to determine Al pairs. Information on Al siting of the Al_1Al atoms in ZSM-5_(s) was obtained from the 2D plot analysis of the ²⁷Al 3Q MAS NMR spectra of the hydrated Na samples combined for quantitative analysis with the simulation of the ²⁷Al single-pulse experiment. The resonances with an isotropic chemical shift of 56.7 and 56.2  ppm predominate in ZSM-5_(s) (95% of Al). According to our previous study, these values can be attributed to the calculated ²⁷Al chemical shifts of 56.7, 57.0, 57.1, 57.1 and 57.2 ppm, which were assigned to Al atoms substituted in the T11, T21, T2, T16 and T23 sites, respectively, of monoclinic ZSM-5^6,12. Only the T16 site of those assignable to the ²⁷Al NMR resonance at 56.4  ppm is not located at the channel intersection. This together with the fact that both ZSM-5 samples were prepared using TPA⁺ ions, located at the channel intersections, allows us to suggest that the Al atoms of the ²⁷Al resonance at 56.4 and 56.0 ppm are most likely present at channel intersections. Thus, the framework Al atom in the ZSM-5_(s) sample is mostly (85%) located at the channel intersections_.

The ²⁷Al (3Q) NMR spectra analysis of Al location cannot be employed with ZSM-5_(p) containing predominantly Al pairs as close Al atoms significantly affect ²⁷Al resonances^7,12,27,39. On the other hand, the zeolite containing prevailingly Al pairs provides advantageously quantitative exchange of Co(II) ions (Co_max) at the individual Al pair sites, which, in dehydrated Co-zeolites, can be used for the determination of positions of these Co(II) ions in the individual rings and channels. Quantitative analysis of d–d transitions of individual bare Co(II) ions (as probes of charge-balancing Al pairs), obtained from Vis spectra of dehydrated Co(II)-ZSM-5, provided infomation on the location of Al pairs in the framework rings and channel system^26,27,39. UV–Vis diffuse-reflectance spectra of fully exchanged Co(II)-ZSM-5 samples dehydrated at 480 °C were recorded on a Perkin-Elmer Lambda 950 UV–Vis–NIR spectrometer equipped with integrating sphere for diffuse-reflectance measurements covered by Spectralon®, which also served as a reference. The reflectance was recalculated using the Schuster–Kubelka–Munk function F(R_∞) = (1 − R_∞)²/2R_∞, where R_∞ is the diffuse reflectance from a semi-infinite layer and F(R_∞) is proportional to the absorption coefficient. Refer to details of the d–d vibrations of the individual Co(II) ions and distribution of Al in the MFI framework.

The concentration of Al atoms in Al pairs in individual (α, β, γ) 6MRs was calculated using Eq. (1). Co-oxo species were not detected in Co-ZSM-5_(p)  and Co-ZSM-5_(s)³.

The concentrations of the individual bare Co(II)-α, Co(II)-β and Co(II)-γ ions were determined from the quantitative analysis of the characteristic d–d transitions of the Vis spectra of dehydrated Co-ZSM-5 zeolites. Deconvolution of the UV–Vis spectrum Co-ZSM-5_(p) is shown in Fig. 8 (upper panel).

Diffuse-reflectance UV–Vis spectrum of dehydrated ( 480 °C) Co(II)-ZSM-5 with a high portion of Al pairs (a), and spectrum deconvolution to components describing Co(II) in α-, β- and γ-rings (b). Measurement sequence: IR-RS, rapid scan IR measurement (one spectrum collected within 1 s); GC, gas chromatography measurement; IR gas cell, one RS-IR spectrum followed by one spectrum of gaseous products collected in the PIKE IR gas cell (c).

The concentration of single Al_1Al atoms was determined by Eq. (2)

where Al_total was derived from chemical analysis.