Site connectivities in silver borophosphate glasses: new results from 11B{31P} and 31P{11B} rotational echo double resonance NMR spectroscopy

Local and medium range order in the glass system 50Ag2O-50[(B2O3)x-(P2O5)(1-x)] (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) have been investigated by high-resolution solid state nuclear magnetic resonance (NMR) techniques. The detailed local site distribution has been derived from deconvolution analysis of the 11B and 31P magic-angle spinning (MAS) NMR signals. Quantitative information regarding the extent of boron-oxygen-phosphorus connectivity has been obtained on the basis of 11B[31P} and 31P{11B} rotational echo double resonance experiments. Incorporation of borate into silver metaphosphate glasses produces four-coordinate BO4/2- sites, which crosslink the metaphosphate chains, resulting in a significant increase in the glass transition temperature. Furthermore, the presence of borate favors the disproportionation of P(2) chain-like units into P(1) and P(3) sites, an effect not observed in binary alkali phosphate glasses. Finally, borate incorporation beyond x=0.3 results in the formation of neutral BO3/2 units, indicating some net charge transfer from the borate to the phosphate network former species. This latter result corresponds to the general metal ion scavenging effect observed for phosphate species in other mixed network former glass systems. In the present system, the effect is relatively moderate, however, suggesting that anionic BO4/2- groups are stabilized by the interaction with the phosphate groups.     

Determination of J coupling constants between spin-1/2 and quadrupolar nuclei in inorganic solids from spin echo and refocused INEPT experiments: a case study on AlPO₄ berlinite

A systematic study utilizing rotor-synchronized homonuclear ((31)P, (27)Al) and heteronuclear ({(31)P}(27)Al and {(27)Al}(31)P) spin echo, and {(27)Al}(31)P refocused INEPT experiments (employing soft pulses for selective excitation of the central transition for the quadrupolar (27)Al (I=5/2)) have been performed on AlPO(4) berlinite at 30 kHz MAS to better understand the J modulation behavior involving half-integer quadrupolar nuclei in solid materials with framework structure. Analyses of the J modulation on either the (27)Al or (31)P coherence in both the {(31)P}(27)Al and {(27)Al}(31)P spin echo experiments, and both periods of the refocused INEPT experiment yield consistent results for the (2)J(AlP) (Al-O-P) coupling constant (ca. 25 Hz). It is noted that the coupling of each (27)Al to four (31)P spins during the first ((27)Al) evolution period of the refocused INEPT, and the populations of (31)P coupled to different numbers (0-4) of (27)Al in the ± 1/2 Zeeman states during (31)P coherence evolution, which have been neglected in previous studies, must be taken into account for proper treatment. Analysis of J modulation on the spin ((27)Al) coupled to spin-1/2 nuclei in general gives more accurate results. Weak long-range homonuclear (4)J(PP) (P-O-Al-O-P) coupling was also observed from the (31)P spin echo and INADEQUATE experiments.     

Combined 17O NMR and 11B-31P double resonance NMR studies of sodium borophosphate glasses

17O enriched sodium borophosphate glasses were prepared from isotopically enriched NaPO3 and H3BO3. These glasses have been studied by 17O, 11B and 31P NMR including 17O and 11B multiple quantum magic angle sample spinning (MQMAS), 11B-31P heteronuclear correlation (HETCOR) NMR and 11B{31P} rotational echo double resonance (REDOR). For comparison, the crystalline borophosphates BPO4 and Na5B2P3O13 were included in the investigations. The latter compound shows three sharp 31P resonances at -0.2, -2 and -8 ppm and two BO4 sites that can only be resolved by MQMAS. The 17O NMR spectra were recorded using both the static echo method at medium magnetic field (9.4 T) as well as MAS and MQMAS methods at high field (17.6 T). In total, five oxygen sites were identified in these borophosphate glasses: P-O-P, Na...O-P, P-O-B, B-O-B, Na...O-B. However, these five sites are not present simultaneously in any of the glasses. The 17O MQMAS spectra prove that P-O-B links play a major role in borophosphate glasses. These results are confirmed by the complementary 11B MAS spectra that show the presence of asymmetric and symmetric trigonal groups BO3a and BO3s and two tetrahedral BO4 units. 11B{31P} REDOR NMR is used to give independent information to assign the 11B lines to structural units present in the glasses. These REDOR measurements reveal that B-O-P bonds are present for each borate unit, including the BO3 groups. Particularly, a structural proposal for the two different BO4 resonances is given in terms of a different number of bonded phosphate tetrahedra. The 31P MAS spectra are usually broad and not well resolved. It is shown by 11B-31P HETCOR NMR that a possible structural assignment of a 31P signal at about -20 ppm to Q2 units as in binary sodium phosphate glasses is wrong and that the phosphate tetrahedron belonging to this resonance must be connected to borate groups.     

Multinuclear NMR studies on the sol-gel preparation of sodium aluminophosphate glasses

Sodium aluminophosphate gels and glasses in the system NaPO₃–Al₂O₃ with P/Al ratios ranging from 9:1 to 1:1 have been synthesized by a novel sol–gel route based on the reaction of aluminum lactate with sodium polyphosphate in aqueous solution. The route from the solution to the gel and the final glass was monitored in situ by liquid and solid state NMR techniques, characterizing the influence of composition and pH on the hydrolysis, polymerization, and vitrification processes. The site distribution in the xerogels is strongly influenced by the gel-processing temperature. At temperatures near 150°C ligation with lactate groups can be nearly suppressed, resulting in maximum Al/P connectivity in the gel. Annealing the gels at temperatures near 400°C produces significant structural rearrangements, resulting in a glassy network that has close structural similarity to the glasses derived from the usual melt-cooling procedure at 1100–1450°C. This has been confirmed by extensive ²⁷Al, ³¹P and ²³Na MAS NMR as well as ²⁷Al{³¹P} and ²⁷Al {¹H} double resonance experiments. Compared to melt-cooling, the sol–gel process permits a significant extension of the glass-forming region towards higher alumina contents.     

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.     

Multinuclear (27Al, 29Si, 47,49Ti) solid-state NMR of titanium substituted zeolite USY

Multinuclear solid-state NMR spectroscopy, employing 29Si MAS,27Al MAS/3Q-MAS and (47,49)Ti wide-line experiments, has been used for the structural characterization of titanium substituted ultra-stable zeolite Y (Ti-USY). 27Al MAS experiments show the presence of aluminum in four (Al(IV)), five (Al(V)), and six (Al(VI)) coordination, whereas the multiplicity within Al(IV) and Al(VI) is revealed by 27Al 3Q-MAS experiments. Two different tetrahedral and octahedral Al environments are resolved and their isotropic chemical shifts (delta(CS)) and second-order quadrupole interaction parameters (P(Q)) have been determined by a graphical analysis of the 3Q-MAS spectra. The emergence of signal with higher intensity at -101 ppm in the 29Si MAS spectrum of Ti-USY samples indicates the possible occurrence of Q4(3Si,1Ti) type silicon environments due to titanium substitution in the faujasite framework. High-field (11.74T) operation, using a probehead specially designed to handle a large sample volume, has enabled the acquisition of 47,49Ti static spectra and identification of the titanium environment in the zeolite. The chemical shielding and electric field gradient tensors for the titanium environment in the zeolite have been determined by a computer simulation of the quadrupolar broadened static 47,49Ti NMR spectra.     

Al coordination and water speciation in hydrous aluminosilicate glasses: direct evidence from high-resolution heteronuclear 1H-27Al correlation NMR

In order to shed light on the dissolution mechanisms of water in depolymerized aluminosilicate melts/glasses, a comprehensive one- (1D) and two-dimensional (2D) NMR study has been carried out on hydrous Ca- and Mg-aluminosilicate glasses of a haplobasaltic composition. The applied techniques include 1D 1H MAS NMR and 27Al-->1H cross-polarization (CP) MAS NMR, and 2D 1H NOESY and double-quantum (DQ) MAS NMR, 27Al triple-quantum (3Q) MAS NMR and 27Al-->1H heteronuclear correlation (HETCOR) and 3QMAS/HETCOR NMR. Ab initio calculations were also performed to place additional constraints on the 1H NMR characteristics of AlOH and Si(OH)Al groups. This study has revealed, for the first time, the presence of free OH (i.e. (Ca, Mg)OH), SiOH and AlOH species, in addition to molecular H2O, in hydrous glasses of a depolymerized aluminosilicate composition. The AlOH groups are mostly associated with four-coordinate Al, but some are associated with five- and six-coordinate Al.     

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.     

High-field 19.6T 27Al solid-state MAS NMR of in vitro aluminated brain tissue

The combination of (27)Al high-field solid-state NMR (19.6T) with rapid spinning speeds (17.8 kHz) is used to acquire (27)Al NMR spectra of total RNA human brain temporal lobe tissues exposed to 0.10 mM Al(3+) (as AlCl(3)) and of human retinal pigment epithelial cells (ARPE-19), grown in 0.10 mM AlCl(3). The spectra of these model systems show multiple Al(3+) binding sites, good signal/noise ratios and apparent chemical shift dispersions. A single broad peak (-3 to 11 ppm) is seen for the aluminated ARPE-19 cells, consistent with reported solution-state NMR chemical shifts of Al-transferrin. The aluminated brain tissue has a considerably different (27)Al MAS NMR spectrum. In addition to the transferrin-type resonance, additional peaks are seen. Tentative assignments include: -9 to -3 ppm, octahedral AlO(6) (phosphate and water); 9 ppm, condensed AlO(6) units (Al-O-Al bridges); 24 ppm, tetrahedral AlO(3)N and/or octahedral Al-carbonate; and 35 ppm, more N-substituted aluminum and /or tetrahedral AlO(4). Thus, brain tissue is susceptible to a broad range of coordination by aluminum. Furthermore, the moderate (27)Al C(Q) values (all less than 10 MHz) suggest future NMR studies may be performed at 9.4T and a spin rate of 20 kHz.     

Characterization of local environments in crystalline borophosphates using single and double resonance NMR

(11)B and (31)P magic-angle spinning as well as (11)B{(31)P} and (31)P{(11)B} rotational echo double resonance (REDOR) NMR have been applied to characterize the local environments in the crystalline borophosphates K(3)[BP(3)O(9)(OH)(3)], NH(4)[ZnBP(2)O(8)] and Rb(3)[B(2)P(3)O(11)(OH)(2)]. Dipolar second moment values extracted from the REDOR curves at short evolution times (DeltaS/S(0) < or = 0.2) are in reasonable agreement with those calculated from the internuclear distances in the corresponding crystal structures. In particular, the method is found to be useful for distinguishing between boron and phosphorus local environments with different numbers of B-O-P connectivities, making REDOR a well-suited tool for medium-range order investigations in glasses.     

A Al MAS, MQMAS and off-resonance nutation NMR study of aluminium containing silica-based sol-gel materials

Aluminium containing hybrid materials were prepared via the sol-gel method using aluminium sec-butoxide complexed with ethylacetoacetate (Al(OBu^s)₂EAA or Al(OBu^s)₃/EAA mixtures). As silanes, phenyltrimethoxysilane (PhTMS) or phenyltriethoxysilane (PhTES), 3-glycidoxypropyl trimethoxysilane (Glymo) and tetraethylorthosilicate (TEOS) were used. After room temperature drying of the samples the ²⁷Al single pulse excitation (SPE) magic angle spinning (MAS) NMR shows that octahedral (5 ppm) and tetrahedral (55 ppm) coordinated aluminium species are present in the materials. The relative amount of these two species depends on the preparation method. However, the Al(IV)/Al(VI) ratio is lower than 3 (typically 2.3) in all materials, indicating the presence of a small amount of an aluminate phase. Annealing of the samples at 100, 150 and 200 °C results in the formation of an extra signal at 30 ppm (peak maximum measured at 11.7 T). Based on the resonance frequency this signal is generally assigned to a pentahedrally coordinated aluminium species. Hydration/dehydration processes of annealed samples were studied with ²⁷Al SPE MAS NMR, multiple-quantum MAS NMR (MQMAS) and off-resonance nutation NMR. Upon hydration of the annealed sample the signal intensity around 30 ppm decreases in intensity and at the same time the intensity of the signal around 55 ppm increases by the same amount (tetrahedrally coordinated aluminium). The MQMAS spectra reveal that the signal around 30 ppm is not caused by a fivefold-coordinated aluminium species but mainly by tetrahedrally coordinated aluminium species in a distorted environment, experiencing large quadrupole induced shifts and small chemical shifts due to conformational changes in the polymeric network. From the MQMAS NMR spectra it can be concluded that the linebroadening observed in the ²⁷A1 MAS NMR spectra is due to both a distribution in isotropic chemical shifts and a distribution in quadrupole coupling constants (C_qcc = e²qQ/h). Hydration of the sample results in a decrease of the average C_qcc for the tetrahedrally coordinated aluminium from 6 to 4 MHz, whereas the average C_qcc of the octahedrally coordinated aluminium is hardly influenced (4 MHz). These MQMAS results are confirmed by off-resonance nutation experiments.     

稀土掺杂P2O5-Al2O3-BaO玻璃的制备及其结构研究

稀土掺杂磷酸盐玻璃具有优异的光学和光谱特性,在激光介质、有色滤光材料等领域中有着重要的应用。在P2O5_Al2O3_BaO_Sm2O3(PABS)玻璃形成能力研究的基础上,借助MAS NMR、红外光谱等分析手段,研究了玻璃的结构、玻璃组成与热处理等对玻璃结构的影响。结果表明:不同组成PAB(S)玻璃的31P MAS NMR谱在-20ppm~-25ppm范围内均存在单一的特征信号峰,对应于磷氧多面体[PO4]的Q2型结构;在18ppm、-12ppm和-36ppm处27Al的MAS NMR谱的三个特征信号峰分别对应于27Al的[AlO5][、AlO6]、[AlO6]配位结构,稀土离子的掺入以及热处理均使得[AlO6]向[AlO5]转变;玻璃的网络结构主要由Q2型[PO4]、[AlO5]和[AlO6]构成,并P_O_P、P_O_Al的形式相连接。玻璃在1383 cm-1处出现的吸收峰可能是玻璃结构中形成了P_O_Sm键所致。     

27Al multiple-quantum MAS NMR of mechanically treated bayerite (alpha-Al(OH)3) and silica mixtures

Two-dimensional 27Al multiple-quantum magic angle spinning (MQMAS) NMR experiments are used to study mixtures of bayerite (alpha-Al(OH)3) with either silicic acid (SiO2.nH2O) or silica gel (SiO2) that have been ground together for varying lengths of time. This mechanical treatment produces changes in the 27Al MAS and MQMAS NMR spectra that correspond to the formation of new Al species. Mean values of the quadrupolar interaction (PQ) and isotropic chemical shift (deltacs) are extracted from the two-dimensional 27Al NMR spectra for each of these species. The presence of significant distributions of both 27Al quadrupolar and chemical shift parameters is demonstrated and the effect of grinding duration on the magnitudes of these distributions is discussed.     

含硼分子筛的11B VAS NMR和1H MAS NMR研究

以变角旋转(VAS) NMR方法考察了三配位硼的存在状态,得到其四极作用常数C_q为2.4MHz,电场梯度不对称系数η为0.0.硼引入骨架形成的酸性羟基质子的信号位于2.3ppm,¹H{¹¹B}双共振实验证实其与硼原子的关联很弱.与6.5ppm分子筛吸水峰同时出现的2.7ppm的信号也被认为是吸水的信号,与非骨架铝羟基质子的信号在同一位置,但可通过¹H{²⁷Al}双共振实验加以判别.     

Effect of alkali-earth ions on the local structure of LaAlO3-La0.67A0.33MnO3 (A = Ca, Sr, Ba) diluted solid solutions: 27Al NMR studies

²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) was studied for diluted alkali earth metal-doped lanthanum manganite solid solutions in the lanthanum aluminate (1 − y)LaAlO₃-yLa_0.67 A _0.33MnO₃ (A = Ca, Sr, Ba) with y =0, 2, 3, and 5 mol %. The spectra depended on the dopant species and showed higher substitutional ordering for the Ba-containing mixed crystals. Magnetically shifted lines were observed in all solid solutions and were attributed to Al in the octahedral oxygen environment near manganese trivalent ions. Nonlinear dependences of their intensity were referred to the manganese-rich cluster formation. An additional MAS NMR line corresponding to aluminum at sites different from the octahedral site in pure LaAlO₃ was observed only in solutions doped with Ba. 3Q MAS NMR revealed that the broadening of this line is governed mainly by quadrupole coupling and made it possible to calculate the isotropic chemical shift. The article was submitted by the authors in English.     

Y型沸石脱铝机制和铝状态的NMR研究

对Y型沸石原粉及几种不同脱铝样品进行了几种不同NMR方法的测量,确定了不同处理过程的脱铝机制和骨架内外铝状态的变化,²⁹Si MAS NMR谱给出了骨架Si、Al分布的信息,反映了不同方法处理样品其脱铝机制的差异,据此可以控制Y沸石脱铝过程获得更高的硅铝比,²⁷Al MAS NMR谱、二维章动NMR及¹H-²⁷Al CP/MAS NMR测量表明:水热处理的Y沸石中存在4种不同的铝状态。综合²⁹Si和²⁷Al MAS NMR观测,可相当程度地推断Y型沸石分子筛脱铝改性所发生的内部过程。     

High-resolution heteronuclear correlation spectra between 31P and 27Al in microporous aluminophosphates

A solid-state nuclear magnetic resonance (NMR) experiment, which provides high-resolution two-dimensional heteronuclear correlation (HETCOR) spectra between 27Al and 31P, is described. The first part of the experiment uses triple-quantum or quintuple-quantum magic-angle spinning (MQMAS) NMR of spin-5/2 nuclei (27Al) to produce an isotropic echo that is unaffected by the second-order quadrupolar broadening. The magnetization is then transferred to the spin-1/2 (31P) nuclei via cross-polarization (CP), resulting in isotropic resolution in both spectral dimensions. To illustrate its usefulness, this method (referred to as MQHETCOR) is applied to two important microporous framework aluminophosphates, hydrated VPI-5 and AIPO4-40.     

丝光沸石催化剂的固体高分辨核磁共振研究

利用²⁹Si MAS NMR及²⁷Al MAS NMR技术研究了丝光沸石催化剂制备过程中的结构变化,并利用Al的四极作用大小来区分重叠在一起的不同的Al物种.     

Multinuclear MAS NMR study of the microporous aluminophosphate AlPO4-17 and the related silicoaluminophosphate SAPO-17.

AlPO4-17 and SAPO-17 in their as-synthesized, calcined, and calcined and subsequently rehydrated forms have been studied by 27Al and 31P MAS NMR. Pronounced structural changes caused by template removal and rehydration can unambiguously be attributed to a change in the coordination number (on calcination: 5-->4, on rehydration/dehydration: 4<-->6) of part of the framework aluminium atoms. The different resonance lines can be assigned to crystallographically inequivalent sites present in the modified ERI framework. 27Al quadrupolar coupling parameters of the two aluminium sites in the calcined AlPO4-17 (CQ = 4.4 MHz and 2.1 MHz) were determined by recording spectra at different field strengths. The isotropic chemical shifts were obtained from rotation sidebands of the (+/- 3/2, +/- 1/2) satellite transitions. 2D 27Al nutation MAS NMR was used to corroborate the line assignment for the as-synthesized and the rehydrated AlPO4-17. By using 13C MAS NMR it was shown that the occluded template (cyclohexylamine) is present as ions. For the first time, a splitting of the 29Si NMR line caused by crystallographically inequivalent sites in a SAPO-type material is reported. The line splitting of 4.3 ppm is comparable with that observed for isostructural aluminosilicate erionite.     

Identification of fluorine sites at the surface of fluorinated gamma-alumina by two-dimensional MAS NMR

By means of 27Al triple quantum Magic-Angle Spinning Nuclear Magnetic Resonance (3QMAS NMR) and 27Al[19F] WISE MAS NMR, we were able to detect three different Al-F sites on the surface of fluorinated gamma-alumina. Three 19F resonances at 9, 20, and 33 ppm (from C6F6) correlated to 27Al resonances in the octahedral range. While the positions of the maxima in the 27Al dimension were ill-defined due to the inherently low efficiency of the 27Al[19F] CPMAS process, the center of gravity of the lines shifted significantly upfield in that dimension with increasing wt.% F. Tentatively, these three resonances were assigned to (VI)Al(O(6-n)Fn) (n = 1, 2, 3) environments on the F/gamma-Al2O3 surface. At F contents above levels corresponding to the full fluorination of the gamma-Al2O3 surface, neoformation of an AlF3 x 3H2O phase was also evidenced with an 19F resonance at -8 ppm and with an 27Al resonance at -17 ppm.