Verified Syntheses of Zeolitic Materials

2nd Revised Edition

Product characterization by NMR

Michael Stöcker
SINTEF Applied Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway

The impact of solid-state NMR as a powerful tool for studies of micro- and mesoporous molecular sieves has been dramatic during recent years. Tremendous progress has been made, aiming towards enhanced resolution, sensitivity and improved multinuclear capabilities. Solid-state NMR is nowadays a well established technique for characterization of zeolites and related materials with respect to structure elucidation, catalytic behavior and mobility properties.

Solid-state NMR is a complementary technique to XRD, since both crystalline and amorphous material as well as powders can be investigated. While XRD (preferably on single crystals) provides information about the long range ordering and periodicities, NMR allows investigations on the short range ordering (local environment) and structure.

The potential of high-resolution solid-state NMR has been known for a long time. However, the challenge has always been to overcome the problems in connection with recording solid-state NMR spectra with sufficient resolution. Distinct nuclear spin interactions like chemical shift anisotropy (CSA), dipolar and quadrupolar interactions which lead to excessive line broadening, are averaged in liquids due to fast thermal/molecular motions of molecules, but are operative in the rigid lattice of solids (the molecules are less mobile). As a consequence, the fine structure is lost since broad lines are obtained, hiding the essential information of analytical character. In addition, long spin-lattice relaxation times, due to the lack of translation- and rotation-motions, are controlling the repetition of a NMR experiment and, consequently, the entire recording time. During recent years, several techniques have been developed for averaging those interactions/phenomena to zero, or reduce them to the isotropic values, allowing the registration of high-resolution NMR spectra of solids as well. Those techniques and their relations to the mentioned interactions / phenomena are:

Techniques

Handling of the following interactions / phenomena

Dipolar decoupling (DD)

Heteronuclear dipolar interactions

Multiple pulse sequences (MPS)

Homonuclear dipolar interactions

Magic angle spinning (MAS)

Chemical shift anisotropy (CSA), dipolar and first-order quadrupolar interactions

Dynamic angle spinning (DAS)

Second-order quadrupolar interactions

Double orientation rotation (DOR) or Multiple-quantum NMR Cross polarization (CP)

Long spin-lattice relaxation times

During MAS, the sample is rotated quickly about an angle of θ = 54║44' in relation to the axis of the external magnetic field, and the three interactions mentioned have a dependence on the second-order Legendre polynomial 3 cos2θ -1. That means, if θ is chosen to be 54°44' (the so-called magic angle), the expression 3 cos2θ -1 becomes equal to zero and the interactions are averaged to zero or reduced to the isotropic values.

Quadrupolar nuclei interact not only with the magnetic field in which the sample is placed but also with the electric field gradient. The combination of both effects results in an anisotropy that can no longer be removed by MAS alone. A detailed analysis of the averaging process of quadrupolar nuclei shows that second-order quadrupolar interactions depend on a fourth-order Legendre polynomial 35 cos4θ -3 cos2θ +3. Therefore, the introduction of two independent angles averages the effect of both tensors and high resolution solid-state NMR spectra of nuclei possessing quadrupole moments can be recorded, in principle, two different techniques can be used: Applying DAS, the sample is rotated sequentially about two different angles, whereas during DOR, the sample is spun simultaneously about the two axes. However, using multiple-quantum NMR spectroscopy the same information can be obtained.

The spinning speed of a rotor during MAS or DAS/DOR experiments should be at least in the range of the line width of the signal recorded under static conditions, otherwise the main resonance line is accompanied by a series of spinning side bands occurring at integral multiples of the spinning speed. Cross polarization (CP) allows transfer of magnetization (or polarization) from an abundant species (usually 1H) to a dilute species which is under observation. The benefits are primarily an intensity enhancement of the dilute spin signal and a reduction of the recycle time between experiments, since the rate-determining relaxation time is now that of the abundant species. Usually the relaxation of the abundant spins are much faster than the dilute spin relaxation. In order to obtain optimum line narrowing and improved sensitivity in a solid-state NMR spectrum, the experimental techniques mentioned are often applied in combination, as, for example, GP/MAS or CRAMPS (combined rotation and multiple pulse spectroscopy).

All of the relevant basic nuclei contributing to the framework of zeolites and AIPO4 molecular sieves are detectable to NMR investigations by their natural isotopes (natural abundance in parentheses): 29Si (4.7%), 27Al (10096), 31P (100%) and 17O (0.037%). Both 27Al and 31P spectra are easily detected within reasonable time, however, 27Al has a quadrupole moment which can cause line broadening due to interaction with the electric field gradient. Investigations of 17O NMR can be done by using enriched material, since the natural abundance is quite low.

The obtained resonance lines for 31P and 29Si are usually narrow, and, due to their important role as framework elements (besides 27Al), these nuclei have been widely used in solid-state NMR studies of microporous and mesoporous molecular sieves for structural investigations. The most important application of 29Si NMR is due to the relationship between the 29Si chemical shift sensitivity and the degree of condensation of the Si-O tetrahedra, that is, the number and type of tetrahedrally coordinated atoms connected to a given SiO4 unit Si (n Al), with n = 0, 1, 2, 3 or 4 [chemical shift range: -80 to -115 ppm, with the high-field signal for Si (O Al)]. Here n indicates the number of Al atoms sharing oxygens with the SiO4 tetrahedron under consideration.Differences in chemical shifts between Si (n Al) and Si (n+1 Al) are about 5-6 ppm. Furthermore, 29Si MAS NMR spectra can be used to calculate the framework Si / Al ratio from the NMR signal intensities (I) according to eq. (1):

 
4
 
Σ
ISi(nAl)
Si
 
n=0
––––
=
––––––––––––––––––––
Al
 
4
 
Σ
0.25 nISi(nAl)
 
n=0

 

27Al NMR spectra reveal the existence of extra-framework Al (about 0 ppm) besides the lattice aluminum (tetrahedrally coordinated Al at about 40-65 ppm). However, in special cases aluminum atoms can be ╬invisible" and are not observable by NMR The introduction of DAS and DOR as well as nutation NMR (where in a two-dimensional way the effect of the quadrupole interaction is separated from other line broadening interactions) allow a much more detailed insight with respect to the structural information available by 27Al NMR Usually, the only Al transition recorded in microporous and mesoporous materials is the central + 1/2 <=> -1/2 transition, which is dependent only on the second order quadrupolar interaction. This interaction decreases with increasing magnetic field strength, and better resolution can be obtained by applying higher magnetic fields and/or DAS/DOR Both 29Si and 27Al MAS NMR spectra are widely used to follow dealumination processes, as well as direct synthesis procedures concerning 29Si NMR.

Solid-state 1H NMR of protons, OH groups, adsorbed water, organic sorbates and probe molecules containing hydrogen in microporous and mesoporous molecular sieves has been developed as a usable method for getting information about different kinds of hydrogens in terminal or bridging OH groups, varying environments for hydrogen containing probe molecules and, finally, acidity investigations. In this way, four distinct types of protons have been identified and quantified by their chemical shifts: (1) non-acidic, terminal SiOH groups (1.5-2 ppm), (2) AlOH groups at non-framework Al (2.6-3.6 ppm), (3) acidic, bridging hydroxyl groups SiO(H)Al (3.6-5.6 ppm) and (4) ammonium ions (6.5-7.6 ppm). Cross polarization measurements are used to emphasize signals of nuclei connected to hydrogen containing environments.

Other nuclei which can substitute isomorphously for the usual framework elements in microporous and mesoporous materials are observable by solid-state NMR, for example, 11B, 73Ge and 69,7lGa. Charge compensating cations, like 7Li, 23Na, 39K, 133Cs or 195Pt, are suitable for NMR measurements. However, most of those elements possess a quadrupole moment which usually limits the application. Furthermore, organic compounds used as templates during hydrothermal synthesis or as sorbates in the zeolite framework (adsorbed guest molecules can cause frame transitions) as well as catalytic organic reactions on microporous and mesoporous molecular sieves (even in situ) can be detected by applying 13C CP/MAS NMR.

Finally, l29Xe (natural abundance of 26.4%) is a very suitable and sensitive isotope for probing the pore architecture of zeolites and AlPO4Ôs. The extended Xe electron cloud is easily deformable due to interactions, for example, the Xe atoms and the channel wall of a zeolite framework. The deformation results in a large low-field shift of the Xe resonance. Probe molecules like water and hydrocarbons have been used to study the pore architecture of mesoporous materials by monitoring the 1H NMR intensity of the liquid water signal when decreasing the temperature. The intensity of the liquid water 1H NMR signal drops drastically when the water is frozen, however, the temperature for this transition depends strongly on the pore diameter of the porous material. A surprising consistency between the nitrogen adsorptiondesorption isotherms and the proton NMR signal intensity versus temperature was observed (see Figure 1).

Click here for Figure 1

The introduction of two-dimensional (2D) solid-state NMR spectroscopy enables us to follow the three dimensional connectivities within a zeolite lattice (Si-O-Si), by applying 2D homonuclear 29Si COSY (Correlated Spectroscopy) or 2D 29Si INADEQUATE (Incredible Natural Abundance Double Quantum Transfer Experiment) sequences.The mobility/dynamics of small molecules and the intracrystalline mass transfer in porous materials can be followed by diffusion NMR measurements. Most known is the pulsed field gradient (PFG) technique, where the spins have a state which is spatially dependent upon their location along B, (with pulses of linear gradients of B0).