Solid-State NMR Spectroscopy for the Study of Local Environments

2.4.1 General Principles12,13

Nuclear magnetic resonance (NMR) spectroscopy is applicable to the study of nuclei with spin quantum number I ≠ 0. A nucleus has (2I + 1) spin energy levels, which are degenerate and distinguished by the quantum number mI (mI = -I, -I + 1… +I). These levels split in the presence of a strong external magnetic field (B0). This splitting is called the Zeeman effect (Fig. 2.11).14The resonance phenomenon occurs when a radiation of energyhν matches the energy gap (ΔE) between the permitted transition levels (selection

rule: ΔmI = ± 1). The ‘resonance frequency’ ν depends on the chemical environment of

the atom (i.e. a chemical shift, δiso). As a consequence this technique is applied here for the structural study of local environments.

Fig. 2.11The Zeeman effect for a nucleus (I = 1/2) in presence of a magnetic field B0.

The nuclear spin transitions are characterised by extremely small energy differences. The simple application of radiofrequency pulses (rf) enables the observation of the resonance phenomenon. The resonance frequencies depend also on the magnetic field strength. In order to make comparable experiments performed with different instruments, these frequencies are reported as chemical shifts (δiso) in ppm relative to an external standard compound. ΔE α  E B0 No Field Applied Field

The simplest NMR experiment can be visualised by a vector model and can be divided into three parts, once the nuclear spin is placed into an external magnetic field B0: i) the perturbation by applying a pulse of rf radiation, ii) the detection of the resonance during its evolution until the signal decay to zero and iii) recovery time referred to as the relaxation delay to achieve the re-establishment of the initial state prior to the perturbation (Fig. 2.12).

Fig. 2.12Schematic representation of the three steps in a typical NMR experiment.

In the presence of an external magnetic field B0, a bulk magnetisation (M) is generated along the z axis due to the slight excess of population in the α-state over that in the - state. This M can be seen as the sum of all the magnetic vectors (Fig. 2.13 a) precessing on the surface of a cone at the Larmor frequency (the equivalent to the resonance frequency of that nucleus). In a frame rotating at rf(the frequency of an applied pulse) the effect of the pulse is to rotate M into thexyplane (transverse magnetisation Mxy, Fig. 2.13 b). In the detection stage after the application of the rf pulse, the system tends to the equilibrium condition (relaxation). In this way, the transfer magnetisation dephases along thexy plane with a characteristic relaxation time T2. This dephasing induces a voltage in the coil which is recorded as a function of time and is called a Free Induction Decay (FID). The FID, a time-domain signal, is converted by the mathematical procedure Fourier transformation (FT) into a frequency-domain spectrum. In the recovery step the equilibrium populations are allowed to fully re-establish according to a characteristic relaxation time T1. C 13 H 1 Decoupling_Detection FT t FID ν NMR spectrum Recovering Perturbation

Fig. 2.13Schematic representation of the bulk magnetization (M) in presence of B0(a) and the torque of M

when the rf pulse is applied (b).

2.4.2 NMR in Solids15

The interactions detected by NMR are anisotropic, they depend upon orientation with respect to the applied magnetic field B0. In solution state, these interactions are averaged, due to the fast tumbling motion becoming isotropic, but in the solid state the molecules can be considered effectively static. This produces broadened signals as a consequence of collecting the signal of all the spins in their different orientations. The main interactions responsible for the broadening are i) the dipolar interaction, ii) the chemical shift anisotropy (CSA) and iii) the quadrupolar coupling.

Dipolar Coupling

The dipolar coupling or dipole-dipole coupling is the through space magnetic interaction of pairs of nuclei (the local magnetic field Blocat a nucleus I generated by a nucleus S) and is independent of the applied field. This interaction depends on the positions of the nuclei and it is angular dependent via a factor equal to (3cos2θIS – 1). This interaction is often the most influential for nuclei with spin I = 1/2.

Chemical Shift Anisotropy (CSA)

The chemical shift is produced by the interaction of the electrons which shield the nucleus and the applied magnetic field (B0). The distribution of those electrons is not always spherically symmetric; therefore the chemical shift is anisotropic depending on the orientation of the nuclei of the sample with respect to B0 and its influence in

M y y x M z a) B0 x z b)

broadening increases with the strength of the external field applied.16Nevertheless, like the dipolar interactions the chemical shift anisotropy (CSA) depends on (3cos2θIS– 1).17

Quadrupolar Coupling

The nuclei with spin I > 1/2 have a non-spherically symmetrical distribution of the nuclear charge and possess an electric quadrupole moment (eQ) which interacts strongly with any electric field gradient (EFG) generated at the nucleus by the surrounding electron density. The magnitude of this quadrupolar interaction is described by the quadrupolar coupling constant CQ. This interaction is most significant for nuclei with spin I > 1/2 and is sometimes so important that the signals disappear beyond detection. The quadrupolar interaction is often so large (many MHz) that the effect on the NMR spectrum needs to be considered to both first and second order.

Magic Angle Spinning (MAS) and Multiple Quantum (MQ) MAS NMR

Solid-state NMR requires Magic Angle Spinning (MAS) to obtain high resolution spectra. In MAS NMR, the sample is rotated at 10-30 kHz around an axis at an angle of 54.736º to the applied magnetic field B0.18At this angle the function (3cos2θ – 1) becomes zero. Therefore the broadening due to the dipolar coupling and the CSA are removed. The resultant spectra for nuclei with I = 1/2 have narrow resonances related to specific local environments of the nuclei in the framework. In contrast, for nuclei with I > 1/2 the spectra show additional broadening due to second order quadrupolar interactions. In this case the Multiple Quantum (MQ) MAS NMR19 experiment enables the separation of isotropic and anisotropic information in two-dimensional (2D) spectra, thereby improving the resolution between different sites.

2D spectra20 can be generally generated in four steps: preparation (P), evolution (E), mixing (M) and detection (D) as described in figure 2.14.

P E M D

1

Fig. 2.14Schematic representation of a typical 2D NMR experiment.

In the preparation period one or more pulses perturb the system to evolve in a period of time t1. The information is converted into observed signals by an analogous scheme of pulse(s) in the mixing period which are detected in the period of time t2. The t1 period is varied providing the indirect time dimension for the two-dimensional FT spectrum (Fig. 2.15).

Fig. 2.15Generation of a 2D spectrum by variation of t1and detection of the signals in t2, from reference

21.

For the specific MQ MAS NMR experiment after a shearing transformation, the projection along F1gives the isotropic spectrum and F2the quadrupolar frequencies.

Cross Polarisation Experiment

Spectra can be detected using the Cross Polarisation (CP) technique to increase the signal of the nuclei under study by transferring the magnetization from abundant nuclei such as 1

H. CP is mainly applied to enhance ‘weak’ signals of nuclei that have low natural abundance, such as13C and29Si. This technique involves the application of simultaneous pulses on both nuclei for a period of time referred to as the contact time. In this interval the transverse magnetisation (Mxy) previously created on the high abundant nuclei is transferred, via the dipolar interaction, to the less sensitive nuclei (Fig. 2.16).

H 1 C 13 CP CP

Fig. 2.16Schematic representation of a typical CP NMR experiment.

CP spectra are not quantitative because nuclei closer to 1H are preferentially enhanced. For quantitative data the spectra need to be collected using Direct Polarisation (DP). Table 2.3 summarises the natural abundance and spin of the nuclei studied in this thesis:

Table 2.3Relevant NMR information of the nuclei studied in this work.22

Nucleus Natural abundance % Spin

99.99 1/2 1.07 1/2 99.63 1 1 H 13 C 14 N 15 N 27 Al 0.37 100 1/2 5/2 29 Si 4.70 1/2 31 P 100 1/2 2.4.3 NMR Experiments 13

C, 15N MAS NMR gave information about the organic molecule used as template. Commonly13C NMR is used to show that the molecule remains intact within the pores.

The nuclei 31P,29Si and 27Al are part of the framework and MAS NMR of these gives information about local environments (Table 2.4).

Table 2.4Typical chemical shifts for the nuclei of the framework in their local environments, using as reference phosphoric acid, tetramethylsilane (TMS) and aluminium (III) chloride respectively.

Nucleus Environment Typical chemical shift (ppm) Ref

31 P Tetrahedral P(4Mg) –19 23 P(1Al, 3Mg) P(2Al, 2Mg) P(3Al, 1Mg) P(4Al) –30 29 Si Tetrahedral Si(4Al) –83 to –87 24 Si(3Al, 1Si) –88 to –94 Si(2Al, 2Si) –93 to –99 Si(1Al, 3Si) –97 to –107 Si(4Si) –103 to –114 27 Al Tetrahedral 30 - 50 25 6-coordinate _{0 - 22}

In this thesis 31P MAS NMR spectra were applied for the study of MgAPO materials at different magnesium loadings. Following the procedure of Barrie and Klinowski23 the assignment of the resonances in the spectra assumes that the substitution of aluminium for magnesium in the AlPO lattice increases the chemical shift of the adjacent phosphorus, and this effect increases with the number of adjacent aluminium atoms

substituted. The most negative chemical shift is attributed to P(4Al), the next to P(3Al, 1Mg) and so on (Fig. 2.17). This study is described in detail in Chapter 4.

Fig. 2.1731P MAS NMR and the deconvoluted curves for MgAPO STA-14, where magnesium substitutes 20% of the aluminium sites.

In the case of SAPO materials, 29Si and27Al MAS NMR as well as27Al MQ MAS NMR were applied in this thesis to study the incorporation of silicon into the framework by replacing phosphorus sites at different loadings. 29Si MAS NMR spectra show characteristic chemical shifts for specific local environments denoted as Si(nAl, (4-n)Si), where n is the number of aluminium atoms and 4-n the number of silicon atoms (see Table 2.4 and Fig. 2.18). The incorporation of silicon into the framework influences the neighbouring aluminium atoms and therefore the 27Al MAS NMR spectra. Because 27Al MAS NMR is very complex due to the quadrupolar effect, 27Al MQ MAS NMR was applied to observe the influence of the silicon content on the aluminium environments. As a result, it has been demonstrated by the analysis of the samples at different silicon content (from 8% up to 30% silicon content) that the weak signal at 9 ppm in the 27Al MAS NMR spectra can be attributed to Al(1Si,3P) environments (Fig. 2.19). The full discussion is given in Chapter 3.

P (AlO)4 P(AlO)3 (MgO)1 P(AlO)2 (MgO)2 P(AlO)1 (MgO)3 ppm P(AlO)4

Fig. 2.18 29Si MAS NMR spectra of as-made SAPO STA-7 with Si/Al ratio ca. 0.3 in both cases but the sample on the right was synthesised by S. Warrender using fluoride anions, favouring the formation of silica islands.

Fig. 2.1927Al MAS NMR (top) and27Al MQ MAS NMR (bottom) spectra for SAPO STA-7 at Si/Al 0.08 and 0.30, respectively. Samples provided by S. J. Warrender.

-400 -200 0 200 ppm -300 -200 -100 0 100 200 300 ppm -150 -130 -110 -90 -70 -50 ppm -130 -110 -90 -70 ppm

The experimental parameters for the spectra collected in this work are summarised in each experimental section of the chapters. In general the following parameters are included:

 Freq (MHz), the resonance frequency of the specific nucleus at the magnetic field of the instrument.

 Acq time (ms), the acquisition time (t2) for the FID collection.

 Relaxation delay (s), the recovery time required between each acquisition to allow nuclei to recover their equilibrium populations prior to the application of a new pulse cycle.

 Polarisation (abbreviated as polaris.), to denote cross polarisation (CP) or direct polarisation (DP) experiment.

 Contact time (ms), the time of transferring the magnetisation in the CP experiment.

 Pulse time (μs), the duration of the pulse applied.

 Spin rate (kHz), the velocity at which the sample is spun at MAS conditions.