Transfer Of Population In Double Resonance Research Articles

Identification of phosphorus sites in amorphous LiPON thin film by observing internuclear proximities

Amorphous lithium phosphorus oxynitrides (LiPON), prepared by reactive magnetron sputtering, have become the electrolytes of choice for all-solid-state thin film microbatteries since its discovery in early 1990s. Nevertheless, there is still a lack of understanding of their atomic-level structure and its influence on ionic conductivity. Solid-state NMR spectroscopy represents a promising technique to determine the atomic-level structure of LiPON glasses but is challenging owing to its low sensitivity in the case of thin film materials. Recently, 31P solid-state NMR spectra of LiPON thin films were acquired under magic-angle spinning (MAS) conditions and assigned with the help of density functional theory (DFT) calculations of NMR parameters. However, the identification of the different P local environments in these materials is still a challenge owing to their amorphous structure and the lack of resolution of the 31P MAS NMR spectra. We show herein how the NMR observation of internuclear proximities helps to establish the nature of P sites in LiPON thin films. The 31P-14N proximities are probed by a transfer of population in double resonance (TRAPDOR) experiment, whereas 31P-31P proximities are observed using one-dimensional (1D) 31P double-quantum (DQ)-filtered and two-dimensional (2D) 31P homonuclear correlation spectra as well as dipolar dephasing experiments using DQ-DRENAR (DQ-based dipolar-recoupling effects nuclear alignment reduction) technique. The obtained NMR data further support the recently proposed assignment of 31P NMR signals of LiPON thin films. With the help of this assignment, the simulation of the quantitative 1D 31P NMR spectrum indicates that PO43- orthophosphate anions prevail in LiPON thin films and N atoms are mainly incorporated in [O3PNPO3]5− dimeric anions. PO3N4− isolated tetrahedra and [O3POPO3]4− anions are also present but in smaller amounts.

Improved resolution for spin-3/2 isotopes in solids via the indirect NMR detection of triple-quantum coherences using the T-HMQC sequence

The indirect NMR detection of quadrupolar nuclei in solids under magic-angle spinning (MAS) is possible with the through-space HMQC (heteronuclear multiple-quantum coherence) scheme incorporating the TRAPDOR (transfer of population in double-resonance) dipolar recoupling. This sequence, called T-HMQC, exhibits limited t1-noise. In this contribution, with the help of numerical simulations of spin dynamics, we show that most of the time, the fastest coherence transfer in the T-HMQC scheme is achieved when TRAPDOR recoupling employs the highest radiofrequency (rf) field compatible with the probe specifications. We also demonstrate how the indirect detection of the triple-quantum (3Q) coherences of spin-3/2 quadrupolar nuclei in solids improves the spectral resolution for these isotopes. The sequence is then called T-HMQC3. We demonstrate the gain in resolution provided by this sequence for the indirect proton detection of 35Cl nuclei in l-histidine∙HCl and l-cysteine∙HCl, as well as that of 23Na isotope in NaH2PO4. These experiments indicate that the gain in resolution depends on the relative values of the chemical and quadrupolar-induced shifts (QIS) for the different spin-3/2 species. In the case of NaH2PO4, we show that the transfer efficiency of the T-HMQC3 sequence employing an rf-field of 80 kHz with a MAS frequency of 62.5 kHz reaches 75% of that of the t1-noise eliminated (TONE) dipolar-mediated HMQC (D-HMQC) scheme.

Indirect NMR detection via proton of nuclei subject to large anisotropic interactions, such as 14N, 195Pt, and 35Cl, using the T-HMQC sequence.

Recently, the T-hetero-nuclear multiple quantum coherence (T-HMQC) sequence using the TRAPDOR (transfer of population in double resonance) recoupling has been introduced for the indirect detection via protons of quadrupolar nuclei with spin I = 1 (14N) or 3/2 (35Cl) in solids at fast magic-angle spinning (MAS). The sequence is simple as it only uses four rectangular pulses and exhibits low t1-noise because the recoupling pulses are applied to the indirectly detected isotope, I. We demonstrate that this sequence is applicable for the detection via protons of spin-1/2 nuclei subject to large chemical shift anisotropy, such as 195Pt. We also report the proton detection of double-quantum (2Q) coherences of 14N nuclei using this sequence. This 2Q version is more robust to the adjustment of the magic angle and the instabilities of the MAS frequencies than its parent single-quantum (1Q) version since the 2Q coherences are not broadened by the first-order quadrupole interaction. In practice, than its 1Q counterpart for the indirect detection of 14N nuclei, the 2Q variant benefits from a slightly higher resolution and comparable sensitivity. In this article, we derive for the first time the Hamiltonian that describes the spin dynamics during the TRAPDOR recoupling. This Hamiltonian demonstrates the importance of the adiabaticity parameter as well as the role of third-order terms in the effective Hamiltonian. The effects of offsets, radio-frequency field, and recoupling time on the efficiency of the T-HMQC sequence are analyzed numerically as well as with experimental detection via protons of 195Pt nuclei in a mixture of cis- and trans-platin and that of 14N and 35Cl isotopes in l-histidine HCl.

Enhanced Fluoride Uptake by Layered Double Hydroxides under Alkaline Conditions: Solid-State NMR Evidence of the Role of Surface >MgOH Sites.

Layered double hydroxides (LDHs) are potential low-cost filter materials for use in fluoride removal from drinking water, but molecular-scale defluoridation mechanisms are lacking. In this research, we employed 19F solid-state NMR spectroscopy to identify fluoride sorption products on 2:1 MgAl LDH and to reveal the relationship between fluoride sorption and the LDH structure. A set of six 19F NMR peaks centered at -140, -148, -156, -163, -176, and -183 ppm was resolved. Combining quantum chemical calculations based on density function theory (DFT) and 19F{27Al} transfer of populations in double resonance (TRAPDOR) analysis, we could assign the peaks at -140, -148, -156, and -163 ppm to Al-F (F coordinated to surface Al) and those at -176 and -183 ppm to Mg-F (F coordinated to surface Mg only). Interestingly, the spectroscopic data reveal that the formation of Al-F is the predominant mode of F- sorption at low pH, whereas the formation of Mg-F is predominant at high pH (or a higher Mg/Al ratio). This finding supports the fact that the F- uptake of 2:1 MgAl LDH was nearly six times that of activated alumina at pH 9. Overall, we explicitly revealed the different roles of the surface >MgOH and >AlOH sites of LDHs in defluoridation, which explained why the use of classic activated alumina for defluoridation is limited at high pH. The findings from this research may also provide new insights into material screening for potential filters for F- removal under alkaline conditions.

High-Resolution NMR of S = 3/2 Quadrupole Nuclei by Detection of Double-Quantum Satellite Transitions via Protons.

Indirect NMR detection via protons under fast magic-angle spinning can help overcome the low sensitivity and resolution of low-γ quadrupole nuclei such as 35Cl. A robust and efficient method is presented for indirectly acquiring the double-quantum satellite-transition (DQ-ST) spectra of quadrupole nuclei. For a spin S = 3/2, the DQ-STs have a much smaller second-order quadrupolar broadening, one-ninth compared to that of the central transition. Thus, they can provide a factor of up to 18 in resolution enhancement. The indirect detection of DQ-STs via protons is carried out using the heteronuclear multiple-quantum coherence (HMQC) experiment with the transfer of populations in double-resonance (TRAPDOR) recoupling mechanism. The resolution enhancement by detecting DQ-STs and the high efficiency of the TRAPDOR-HMQC experiment are demonstrated by 35Cl NMR of several active pharmaceutical ingredients (APIs).

Inspecting the Structure and Formation of Molecular Sieve SAPO-34 via 17O Solid-State NMR Spectroscopy

Silicoaluminophosphates (SAPOs) are microporous frameworks with Brønsted acid sites that can be used as acidic catalysts. A firm understanding of SAPO structure, formation, and crystallinity is necessary for understanding and expanding SAPO applications in heterogeneous catalysis. Solid-state 17O NMR (SSNMR) spectroscopy is an ideal tool to probe structure and formation of SAPO-based materials; the 17O quadrupolar and chemical shift interactions are exquisitely sensitive to local electronic and magnetic environments. In this work, a pure trigonal SAPO-34 molecular sieve synthesized via the dry-gel conversion (DGC) method was investigated using a combination of 17O magic-angle spinning, 17O triple-quantum magic-angle spinning, 17O[27Al] transfer of population in double-resonance, and 17O[31P] rotational-echo double-resonance SSNMR spectroscopy, complemented by powder X-ray diffraction along with 27Al, 29Si, and 31P multinuclear SSNMR experiments. The four observed 17O resonances were simulated to extract NMR parameters, with each resonance assigned to individual oxygen local environments and connectivities in SAPO-34. The incorporation of oxygen from 17O-labeled water (i.e., H217O) during the DGC formation of pure trigonal SAPO-34 has also been investigated at various time intervals and stages of crystallization using 17O SSNMR. There is direct involvement of 17O-enriched water vapor during the DGC crystallization process of trigonal SAPO-34. The initial dry gel is amorphous, transforming to a crystalline layered AlPO4 phase during the first hour of heating and then progressing to a semicrystalline phase after 4 h of heating; formation of the crystalline trigonal SAPO-34 product was found to be complete after 2 days.

Network oxygen sites in calcium aluminoborosilicate glasses: Results from 17O{27Al} and 17O{11B} double resonance NMR

11B, 17O, and 27Al NMR has widely been used to quantify numerous aspects of oxide glass structure, but many issues remain to be resolved, including basic issues of assignment of overlapping 17O resonances. Double resonance NMR, involving cation spin pairs such as 27Al and 11B, has provided further details of network connectivities in some multicomponent glasses; very recent work on 27Al-17O has allowed unique identification of non-bridging oxygen (NBO) on aluminum cations in lanthanum aluminoborosilicate glasses [1]. Here we have explored 17O{27Al} and 17O{11B} TRAnsfer of Population in DOuble-Resonance (TRAPDOR) NMR experiments in several calcium aluminosilicate and aluminoborosilicate glasses. We have demonstrated clear distinction among resonances for 17O associated with boron and/or aluminum, including quantification of NBO associated with boron, aluminum and silicon in these multicomponent glasses. Additionally, spectral components for bridging oxygens and non-bridging oxygens with aluminum, boron, and silicon can also be at least in part separated, allowing for comparison to proportions expected if cations were connected via random mixing.

Oxidation Products of NaAlH4 Studied by Solid-State NMR and X-ray Diffraction

The oxidation products of NaAlH4 were studied using solid-state nuclear magnetic resonance experiments combined with X-ray diffraction (XRD) experiments. We studied the coordination of aluminum with oxygen using single-pulse 27Al experiments. 27Al−{1H} rotational echo double-resonance, 23Na−{27Al}/27Al−{23Na} transfer of population in double-resonance (TRAPDOR), and combined 27Al−23Na TRAPDOR−multiple-quantum magic-angle spinning (MQMAS) experiments were used to get qualitative information about Al−H and Al−Na proximities. These NMR experiments show that the intermediate oxidation product is an amorphous sodium aluminum hydroxide with Al in a tetrahedral coordinated site. A 27Al MQMAS experiment shows distributions of chemical shift and quadrupolar coupling parameters. Therefore, the material is probably highly disordered, and multiple compounds may be present. XRD measurements show that this phase indeed lacks long-range order. The end product contains mainly octahedral coordinated aluminum hydroxide. Crystalline aluminum hydroxides, consisting of gibbsite and bayerite, and sodium carbonate hydrates, Na2CO3·H2O and Na3(CO3)2·2H2O, are observed by XRD measurements. Our results show that relatively harmless and nontoxic compounds are formed. However, the materials should handled carefully because of the strong basicity of compounds like sodium carbonate.

Characterization of the gel phases formed in the synthesis of microporous gallophosphate, cloverite

This work examines the formation of a microporous gallophosphate with an extra large pore system, namely cloverite (structure type: CLO) in fluoride medium. The amorphous gallophosphate gel containing the key precursor to cloverite was characterized and its evolution followed by 31P, 71Ga, 19F and 1H MAS NMR and several dipolar-coupling based double-resonance methods including 31P{71Ga} and 19F{71Ga} rotational-echo adiabatic passage double-resonance (REAPDOR), 31P{19F} and 19F{31P} rotational-echo double-resonance (REDOR), and 1H{31P} REDOR and 1H{71Ga} transfer of populations in double-resonance (TRAPDOR). The results indicate that upon mixing the Ga and P sources in the presence of structure directing agent (SDA) and HF, an amorphous oxyfluorinated gallophosphate precursor containing single 4-rings (4Rs) with alternating Ga–O–P linkage forms immediately. The Ga in the 4R is 6-coordinated and the two Ga in the 4R are bridged by a fluorine. Heating quickly converts the 4Rs to double four-membered rings (D4Rs), from which the entire CLO structure is built. The amount of the D4R rapidly increases with increasing heating time. The presence of fluoride ions greatly facilitates the formation of the 4Rs via bridging two Ga sites and the hydrothermal treatment promotes D4Rs via condensation. Interaction between SDA molecule and the phosphorus site in the gel is also probed. Further, the local structure of Ga sites in cloverite is characterized by 71Ga MAS NMR at ultrahigh magnetic field of 21.1 T by obtaining their electric field gradient tensor parameters.

Insights into Oxygen Exchange Between Gaseous O2and Supported Vanadium Oxide Catalysts via17O NMR

Vanadium oxide reference compounds, KVO3 and V2O5, and supported vanadium oxide catalysts (Al2O3, TiO2, and SiO2) were investigated using magic angle sample spinning 17O NMR. All samples were 17O-enriched using gas−solid exchange. Extraction of chemical shift and quadrupolar coupling information for the model compounds KVO3 and V2O5 was performed via the simulation of MAS spectra obtained in one-pulse experiments and the observations were consistent with their known bulk structures. For the supported vanadia catalysts, it was found that the oxygen exchange process is dominated by 17O signal from the catalyst oxide supports. Spectra obtained via rotor-synchronized spin echoes revealed additional wide lines for Al2O3 and TiO2 supported vanadia catalysts that arise from 17O in the surface vanadia species of the catalysts. Additional 17O−51V TRAPDOR (TRAnsfer of Populations in DOuble Resonance) experiments support this assignment. The wide lines suggest that the local environments of the 17O nuclei associated...

TRAPDOR double-resonance and high-resolution MAS NMR for structural and template studies in zeolite ZSM-5

A combination of 27Al magic-angle spinning (MAS)/multiple-quantum (MQ) MAS, and 27Al–{ 14N} TRAnsfer of Population in DOuble-Resonance (TRAPDOR) nuclear magnetic resonance (NMR) was used to study aluminium environments in zeolite ZSM-5. 27Al–{ 14N} TRAPDOR experiments, in combination with 14N NMR were employed to show that the two tetrahedral peaks observed in the 27Al MAS/3Q-MAS spectra of as-synthesized ZSM-5 are due to aluminium atoms occupying crystallographically inequivalent T-sites. A 13C–{ 27Al} TRAPDOR experiment was used to study the template, tetrapropyl ammonium bromide (TPABr), in the three-dimensional pore system of ZSM-5. The inequivalency of the methyl groups of TPA was observed in the 13C–{ 27Al} TRAPDOR spectra of as-synthesized ZSM-5 and the motion of the methyl end of the propyl chain appeared to be more restricted in the sinusoidal channel than in the straight channel.

A solid-state NMR study of the formation of molecular sieve SAPO-34

This work examined the formation of a catalytically important microporous material, SAPO-34, in the presence of HF under hydrothermal synthesis conditions. The local environments of P, Al, F and Si atoms in several solid phases obtained at different stages of crystallization were characterized by several solid-state NMR techniques including 31P, 27Al, 19F and 29Si MAS, 27Al triple-quantum MAS, 31P{ 27Al} transfer of populations in double-resonance, 27Al{ 31P} rotational-echo double-resonance (REDOR), 27Al→ 31P heteronuclear correlation spectroscopy, 31P{ 19F} and 27Al{ 19F} REDOR as well as 1H→ 31P cross polarization. The NMR results provide the new insights into the formation of SAPO-34.

NMR spectroscopy of naturally occurring surface-adsorbed fluoride on Georgia kaolinite

AbstractUsing 19F magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, we show that most of the fluoride present in the KGa-lb reference kaolinite from Washington County, Georgia, occurs as a surface-adsorbed species bonded to Al. This surface fluoride can be removed from the <2 µm fraction by acid wash, but is largely retained in the coarse fraction. Correlation of integrated 19F NMR peak intensities with fluoride sorption experiments indicates a bulk F content of ∼144 ppm for KGa-1b, of which ∼30% substitutes for hydroxyl sites in the mineral structure and the remaining 70% occurs adsorbed on particle surfaces, corresponding to an edge surface fluoride density of ∼0.7 F− nm−2. 19F{27Al} TRAPDOR (TRAnsfer of Populations in DOuble Resonance) NMR data for the original kaolinite and for products of F− sorption experiments at pH 4 show that all of the observed 19F signals arise from fluoride bonded to Al atoms. Furthermore, bridging Al-F-Al sites and terminal Al-F give distinctly different TRAPDOR fractions allowing assignment of resolved peaks based on the number of Al in the first coordination sphere. This result was confirmed for fluoride adsorbed to the surface of gibbsite from aqueous suspension. No evidence was found for Si-F-type environments on the kaolinite surfaces.

Physical properties and in vitro bioactivity of hierarchical porous silica–HAP composites

In two recent papers (J. Andersson, S. Areva, B. Spliethoff and M. Lindén, Biomaterials, 2005, 26, 6827–6835 and J. Andersson, E. Johannessen, S. Areva, M. Järn and M. Lindén, J. Nanosci. Nanotechnol., 2006, 6, 2438–2444) we have presented new means of synthesizing silica–calcium phosphate (hydroxyapatite or tricalcium phosphate) composite materials, where the calcium phosphate is covered by a mesoporous layer of silica. These materials are bifunctional biomaterials, as they can be used both as drug carrier matrices and osteoconductive materials. Some of these materials, especially if synthesized according to a one-pot method, exhibit a very high in vitro bioactivity, and nucleate and grow calcium phosphate on their surfaces in less than 24 h if exposed to a simulated body fluid. In the present study, we have carried out a thorough characterization of the one-pot sol–gel derived composite materials by the means of solid state 29Si magic angle spinning (MAS) NMR, 31P MAS NMR, 23Na MAS NMR, and transfer of population in double resonance (TRAPDOR) NMR spectroscopy, scanning electron microscopy, and transmission electron microscopy. The aim of the study is to relate the material properties to the in vitro bioactivity. The reason for high bioactivity of the composites cannot be ascribed to the silica content, but primarily to the presence of a highly soluble second calcium phosphate phase, NaCaPO4, co-existing with the hydroxyapatite in the hybrid material. Furthermore, the hydroxyapatite becomes increasingly calcium deficient with increasing silica content, which adds to increase the bioactivity. Also the overall crystallinity of the apatitic calcium phosphate phase could contribute to the bioactivity of the composites in vitro.

Multinuclear Solid-State High-Resolution and13C -{27Al} Double-Resonance Magic-Angle Spinning NMR Studies on Aluminum Alkoxides

A combination of 27Al magic-angle spinning (MAS)/multiple quantum (MQ)-MAS, 13C-1H CPMAS, and 13C-{27Al} transfer of population in double-resonance (TRAPDOR) nuclear magnetic resonance (NMR) were used for the structural elucidation of the aluminum alkoxides aluminum ethoxide, aluminum isopropoxide, and aluminum tertiarybutoxide. Aluminum alkoxides exist as oligomers with aluminum in different coordinations. High-resolution 27Al MAS NMR experiments with high-spinning speed distinguished the aluminum atoms in different environments. The 27Al MAS NMR spectrum gave well-resolved powder patterns with different coordinations. Z-filter MQ-MAS was performed to obtain the number and types of aluminum environments in the oligomeric structure. 13C-1H CPMAS chemical shifts resolved the different carbon species (-CH3, =CH2, =CH-, and =C=) in the structures. 13C-{27Al} TRAPDOR experiments were employed to obtain relative Al-C dipolar interactions and to distinguish between terminal and bridging alkoxides in the crystallographic structures. The complete characterization of selected aluminum alkoxides using advanced NMR methods has evidenced the tetrameric structure for aluminum isopropoxide and the dimeric structure for aluminum tertiary-butoxide, as reported in the literature, and proposed a polymeric structure for aluminum ethoxide.

Acid sites in mesoporous Al-SBA-15 material as revealed by solid-state NMR spectroscopy

The nature of acid sites in Al-SBA-15 materials with various Si/Al ratios was studied by various NMR techniques including 27Al, 29Si, 1H, 31P MAS (magic-angle spinning), and some double-resonance methods such as 1H/ 27Al transfer of population in double resonance (TRAPDOR). XRD and 27Al NMR results indicate that Al has been successfully incorporated into the framework of SBA-15 materials by post-synthesis method. The bridging hydroxyl groups (SiOHAl) associated with Brönsted acid sites are invisible either in 1H MAS or in 1H/ 27Al TRAPDOR spectra of the dehydrated Al-SBA-15 materials. However, after the adsorption of basic probe molecules, such as trimethylphosphine oxide (TMPO), trimethylphosphine (TMP) and acetone, 31P MAS and 13C CP MAS NMR experiments strongly support the presence of Brönsted acid sites. NMR results suggest that Brönsted acid sites in dehydrated Al-SBA-15 materials are in the form of terminal silanol groups in the vicinity of aluminum atoms that can be induced to generate the bridging hydroxyl groups by the adsorbed basic probe molecules. Although the acid strength of Al-SBA-15 materials detected by different probe molecules is different, it is comparable to that of microporous zeolites. It is noteworthy that no Lewis acid site can be found in the Al-SBA-15 materials.

Brönsted and Lewis Acidity of the BF3/γ-Al2O3 Alkylation Catalyst as Revealed by Solid-State NMR Spectroscopy and DFT Quantum Chemical Calculations

Multinuclear solid-state NMR techniques and DFT quantum chemical calculations were employed to investigate the detailed structure of acid sites on the BF3/gamma-Al2O3 alkylation catalyst. The NMR experiment results indicate that gaseous BF3 is able to react with the hydroxyl groups present on the surface of gamma-Al2O3, leading to the formation of new Brönsted and Lewis acid sites. The 1H/11B and 1H/27Al TRAPDOR (TRAnsfer of Population in DOuble Resonance) experiments suggest that the 3.7 ppm signal in 1H NMR spectra of the BF3/gamma-Al2O3 catalyst is due to a bridging B-OH-Al group that acts as a Brönsted acid site of the catalyst. On the other hand, a Lewis acid site on the surface of the catalysts, as revealed by 31P MAS and 31P/27Al TRAPDOR NMR of adsorbed trimethylphosphine, is associated with three-coordinate -OBF2 species. 13C NMR of adsorbed 2-13C-acetone indicates that the Brönsted acid strength of the catalyst is slightly stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which is in good agreement with theoretical prediction. In addition, DFT calculations also reveal the detailed structure of various acid sites formed on the BF3/gamma-Al2O3 catalyst and the interaction of probe molecules with these sites.

Post-synthesis alumination of mesoporous silica SBA-15 with high framework aluminum content using ammonium hexafluoroaluminate

Incorporation of high aluminum contents into the silica framework of SBA-15 was achieved by treating SBA-15 with an aqueous (NH 4) 3AlF 6 solution at room temperature under alkaline conditions. Characterization by powder X-ray diffraction (XRD), N 2 adsorption–desorption, transmission electron microscopy (TEM), FTIR, and multinuclear solid-state NMR measurements was carried out to evaluate the efficiency of this alumination method. 27Al magic angle spinning (MAS), 1H/ 27Al TRAnsfer of Population in DOuble Resonance (TRAPDOR) NMR, and cumene cracking measurements demonstrated that the aluminated SBA-15 materials contained exclusively tetrahedrally coordinated aluminum and exhibited well-developed Brønsted acidity. A high level of framework aluminum substitution, up to an Si/Al ratio near 5, without any significant loss in the textural properties of SBA-15, was achieved via the present post-synthesis route. 19F MAS NMR was helpful in gaining more insights into the mechanism of the present alumination process.

Probing the Local Environments of Phosphorus in Aluminophosphate-Based Mesostructured Lamellar Materials by Solid-State NMR Spectroscopy

We have characterized a series of mesostructured aluminophosphate (AlPO)-based lamellar materials by several solid-state NMR techniques. In particular, we were able to estimate the average number of Al atoms and identify the nature of other ancillary groups in the second coordination sphere for each P site. Our work has shown that a combination of several dipolar coupling-based (31)P/(27)Al double-resonance techniques such as transfer of population in double-resonance (TRAPDOR), rotational echo double-resonance (REDOR), and heteronuclear correlation spectroscopy (HETCOR) as well as (1)H --> (31)P cross polarization (CP) can provide more detailed structural information regarding the local environments of P and Al atoms in AlPO-based mesostructured materials, which is not readily available from straightforward (31)P and (27)Al magic-angle spinning (MAS) experiments.

Using Trimethylphosphine as a Probe Molecule to Study the Acid Sites in Al−MCM-41 Materials by Solid-State NMR Spectroscopy

Using trimethylphosphine (TMP) as a probe molecule, acid sites in a series of Al-MCM-41 materials with Si/Al ratios ranging from 16 to 80 were investigated by various solid-state NMR techniques including Si-29, P-31, Al-27, H-1 magic-angle spinning (MAS), and some double- or triple-resonance methods such as P-31 --> H-1 cross polarization (CP), H-1/Al-27, and H-1/P-31/Al-27 TRAPDOR (transfer of population in double resonance). By means of the H-1/Al-27 TRAPDOR technique, which has the ability to establish a correlation between 1H and Al-27, the hydroxyl groups associated with Al could be discriminated from the silanol groups. Two signals at 3.5 and 1.9 ppm were observed in the H-1/Al-27 TRAPDOR spectra; they are likely due to the bridging hydroxyl (SiOHAl) and aluminum hydroxyl groups, respectively. P-31 MAS NMR strongly supports the formation of zeolitelike Bronsted acid sites in the mesoporous material after the incorporation of aluminum. The concentration of the Bronsted sites determined from P-31 MAS NMR is about 2 orders of magnitude lower than that of total SiOH groups, which renders their direct observation by H-1 MAS NMR spectroscopy difficult. However, these sites can be well studied by P-31 --> H-1 CP/MAS and various triple-resonance TRAPDOR NMR techniques, which provide a more detailed description of the interactions between TMP and the different hydroxyl groups in the Al-MCM-41 materials. Although the acid strength of the Bronsted sites in the Al-MCM-41 materials is lower than that of microporous zeolites such as HY, these sites can protonate TMP molecules. Some of the framework T-site aluminum atoms are not associated with the bridging hydroxyl groups and are probably present as Al-OH. No Lewis acid sites were found in our samples, though the Al-27 MAS spectra show the existence of five-coordinate extra framework aluminum.