MQMAS NMR Research Articles

Structure and phase changes of alumina produced by flame hydrolysis.

Fumed alumina from the combustion of AlCl3 produced nano particles with specific areas from 30 to 220m2g-1 (BET) which were characterized by powder X-ray diffraction, 27Al solid-state NMR and transmission electron microscopy. During the short-lived synthesis, highly disordered γ-alumina progressively transforms into a mixture of δ and θ-alumina. For the γ-alumina particles, only for particles with the highest specific area a significant amount of five-coordinated Al can be found which is only partially located in the particle surface. Water can be bound reversibly and increases the coordination number of aluminium atoms in the particle surface. For the well-crystallized mixture of δ and θ-alumina, high resolution and high S/N powder XRD pattern features a large number of superstructure reflections along with the commonly observed diffuse reflections. 27Al MQMAS NMR provides a total of 8 crystallographic sites with an unusually high resolution in the tetrahedral region, with 4 distinct AlO4 sites pertaining to the δ phase alone. The results suggest that the δ-alumina phase produced in this process can be described as an ordered structure.

Modulating hierarchical ZSM-5 crystals from etching-assisted growth with substituted phenols

The rational modulation of zeolite microstructures tailoring crystal morphology and hierarchy, is an effective way to promote their mass transport and catalytic performance. However, practical realization remains challenging. We recently demonstrated a novel etching-assisted crystallization process that allows fine control over zeolite architecture by eliminating intermediate defective nanocrystals with active proton-bearing organic molecules such as phenol. Here, we investigated in detail the phenolic etching effect on the crystallization of ZSM-5 zeolites using a combination of techniques including diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and 2D 27Al multiple quantum magic angle spinning nuclear magnetic resonance (MQ MAS NMR) and systematically elucidated the structure modulation with a homologous series of phenolic compounds. Time-series characterizations confirmed the initial seed formation, intermediate dissolution, and final perfected crystallization stages during ZSM-5 synthesis in the presence of phenolic compounds. Since deprotonation to phenolate anion in-situ is required during alkaline zeolite synthesis, the electron-withdrawing (EW) groups on the phenol para-position lead to bigger ZSM-5 crystals with more mesopores, while the phenolic molecules with para- electron-donating (ED) groups exhibit less significant effect. Under the same initial pH, the phenolic compounds show unparalleled compatibility with zeolite crystallization by stabilizing the system pH values, whereas inorganic acids invariably fail the zeolite crystallization. Given the remarkable kinetic control over zeolite growth through the simultaneous but opposite nucleophilic etching process, our observation provides a versatile way to manipulate zeolite crystallization with high tunability.

Solid-state NMR study on dealumination mechanism of H-MOR zeolite by high-temperature hydrothermal treatment

The acidity of zeolites which plays an important role in various catalytic reactions is closely related to framework aluminum. After dealumination, not only the Si/Al ratio of zeolites increases, but also new aluminum species are generated. Furthermore, the pore structure and catalytic performance of zeolites are affected. High-temperature hydrothermal treatment, as a frequently-used method of dealumination, is widely concerned. However, the dealumination mechanism of zeolite during the hydrothermal treatment is not clear. In this work, the existence of different Al species in H-MOR during the process of high-temperature hydrothermal treatment was confirmed by 27Al MQ MAS NMR. It was found that the second tetrahedrally coordinated framework aluminum (Al(IV)-2) species is the precursor of amorphous non-framework aluminum transformed from framework aluminum of H-MOR during high-temperature hydrothermal treatment. Besides, the 1H-1H DQ-SQ NMR was used to study spatial proximity between different hydroxyl species. Finally, the dealumination process of H-MOR zeolite is revealed: Al(IV)-1 species firstly transformed into Al(IV)-2 species, and then evolved into pentahedrally coordinated aluminum (Al(V)) species and hexahedrally coordinated aluminum (Al(VI)) species.

Quantitatively Mapping the Distribution of Intrinsic Acid Sites in Mordenite Zeolite by High-Field 23Na Solid-State Nuclear Magnetic Resonance.

It is of great significance to accurately quantify the Brønsted acid sites (BASs) at different positions of mordenite (MOR) zeolite. However, H-MOR obtained from Na-MOR can hardly avoid dealumination under hydrothermal conditions, which causes difficulty in the acid characterization. Herein, 23Na-27Al D-HMQC was performed combined with high-field 23Na MQ MAS NMR and DFT calculation, which provided an unambiguous attribution of the 23Na chemical shifts and further helped to improve the resolution of 27Al MAS NMR. By fitting the 23Na and 1H MAS NMR spectra of Na/H-MOR, the intrinsic BAS contents in different T-sites were measured by characterizing the location and content of sodium ions. These Na/H-MOR zeolites with various acid distributions were used for DME carbonylation and showed that the amount of BASs in the T3 site was proportional to the activity of carbonylation. This study provides a new method for investigating the intrinsic acid properties of zeolites.

Characterization of Monochromate and Hemichromate AFm Phases and Chromate-Containing Ettringite by 1H, 27Al, and 53Cr MAS NMR Spectroscopy

The calcium aluminate hydrate AFm and AFt phases formed upon hydration of Portland cement have an important role in the stabilization and solidification of hazardous chromate ions in hardened cement. AFm monochromate (Ca4[Al(OH)6]2(CrO4)·12H2O), AFm hemichromate (Ca4[Al(OH)6]2(CrO4)0.5(OH)·12H2O) and the chromate-containing AFt phase, Ca6[Al(OH)6]2-(CrO4)3·24H2O, were synthesized and investigated by 1H, 27Al, and 53Cr MAS NMR spectroscopy. 27Al quadrupolar coupling parameters (CQ, ηQ) and isotropic chemical shifts (δiso) were determined for the three phases, including two distinct Al sites in chromate-AFt, as observed by 27Al MAS and MQMAS NMR. Two dominant peaks are apparent in the 1H MAS NMR spectra of each of the phases. For the AFm phases, these resonances are assigned to framework hydroxyl groups (1.7–2.0 ppm) and water molecules/hydroxyls (5.0–5.5 ppm) in the interlayer. For chromate-AFt, the peaks are ascribed to framework hydroxyl groups in the [Ca6Al2(OH)12]6+ columns (~1.4 ppm) and water molecules (~4.8 ppm) associated with the Ca ions. 53Cr MAS NMR spectra acquired at 22.3 T for the samples show a narrow resonance for both chromate AFm phases, whereas indications of three distinct Cr resonances are apparent for the chromate AFt. The absence of any second-order quadrupolar effects in the 53Cr NMR spectra strongly suggests that the chromate ions are highly mobile in the anionic sites of the AFm and AFt structures. The NMR data reported in this work are in agreement with the reported crystal structures for the chromate AFm and AFt phases and may be useful for identification and characterization of chromate fixation in cementitious systems, complementing information gained from conventional powder X-ray diffraction studies.

NMR Spectroscopic Characterization of Flame‐Made Amorphous Silica‐Alumina for Cyclohexanol and Glyceraldehyde Conversion

AbstractAmorphous silica‐aluminas (ASAs), possessing both Brønsted acid sites (BAS) and Lewis acid sites (LAS), are important bifunctional catalysts in various industrial applications. Solid‐state NMR spectroscopy has been widely used for characterizing the local structure and the surface sites of ASAs with probe molecules. In this work, four‐, five‐ and six‐coordinated Al species have been observed on the flame‐made ASAs by 27Al MQ MAS NMR experiment. 1H/27Al TRAPDOR MAS NMR experiments confirmed that surface Al species contribute to the formation of BAS and protonate ammonia probe molecules. The adsorption of ammonia on Lewis acidic Al sites (δ1H=3.0 ppm) was evidenced by various 1H MAS NMR experiments on samples dehydrated at different temperatures, allowing the distinction from ammonium ions (δ1H=6.7 ppm) formed at BAS. The signal of ammonia adsorbed on LAS increased with increasing Al content in the ASAs. These properties together with the absence of pore diffusional constraints render the flame‐made ASAs excellent catalysts for cyclohexanol dehydration and the conversion of glyceraldehyde in ethanol to ethyl lactate, outperforming the performance of other ASAs or zeolites.

A Comparison on the Hydrothermal Stability of Nano-sized H-ZSM-5 Zeolite Modified by Ammonium Dihydrogen Phosphate and Trimethylphosphate

In this study, solid NMR characterizations including 29Si MAS NMR, 31P MAS NMR, 27Al MAS NMR, and 27 Al MQ MAS NMR were carried out to get insights on the difference of ammonium dihydrogen phosphate (NH4H2PO4) and trimethylphosphate ((CH3)3PO4) modified nano-sized H-ZSM-5 zeolites. Results show that (CH3)3PO4 is more suitable for the phosphorus modification of the H-ZSM-5 zeolite, because it can effectively interact with aluminum species and facilitate the formation of distorted tetra-coordinated framework Al bonded with phosphorous species. This distorted framework Al species can better maintain the acidity of original framework Al during steaming treatment at high temperatures, even up to 800 °C. Relating the NH3-TPD characterization results with the catalytic cracking performance of full range FCC gasoline over the as-prepared P/H-ZSM-5 samples, we found out that the phosphorus-stabilized distorted framework Al sites are highly acidic and active. The (CH3)3PO4-modified H-ZSM-5 zeolite exhibits bright future in the application of important commercial processes operated under severe hydrothermal conditions.

Alumina: discriminative analysis using 3D correlation of solid-state NMR parameters.

Synthetic transition aluminas (χ, κ, θ, γ, δ, η, ρ) exhibit unique adsorptive and catalytic properties leading to numerous practical applications. Generated by thermal transformation of aluminium (oxy)hydroxides, structural differences between them arise from the variability of aluminium coordination numbers and degree of dehydroxylation. Unequivocal identification of these phases using X-ray diffraction has proven to be very difficult. Quadrupolar interactions of 27Al nuclei, highly sensitive to each site symmetry, render advanced 27Al solid-state NMR a unique spectroscopic tool to fingerprint and identify the different phases. In this paper, 27Al NMR spectroscopic data on alumina reported in literature are collected in a comprehensive library. Based on this dataset, a new 3D correlative method of NMR parameters is presented, enabling fingerprinting and identification of such phases. Providing a gold standard from crystalline samples, this approach demonstrates that any sort of crystalline, ill crystallized or amorphous, mixed periodic or aperiodically ordered transition alumina can now be assessed beyond the current limitations of characterisation. Adopting the presented approach as a standard characterisation of alumina samples will readily reveal NMR parameter-structure-property relations suitable to develop new or improved applications of alumina. Methodological guidance is provided to assist consistent implementation of this characterisation throughout the fields involved.

Unraveling the Redox Couples of VIII/VIV Mixed-Valent Na3V2(PO4)2O1.6F1.4 Cathode by Parallel-Mode EPR and In Situ/Ex Situ NMR

We herein present a study on the sodiation/desodiation behavior of Na3V2(PO4)2O1.6F1.4 cathode with I4/mmm tetragonal space group by the combination of dual-mode electron paramagnetic resonance (EPR) and in situ/ex situ solid-state nuclear magnetic resonance (SSNMR) techniques. The complementary parallel/perpendicular mode EPR, in situ 23Na NMR, 23Na/51V MAS NMR, and two-dimensional 23Na MQMAS NMR experiments substantiate that (i) both VIV and VIII are electrochemically active in this material, with the former transforming to VV, whereas the latter transforming to VIV; (ii) VIV continually converts to VV during the whole charge process, whereas VIII mostly reacts at the higher-potential charge plateau region; (iii) the Na+ extraction/intercalation process of Na3V2(PO4)2O1.6F1.4 is a widespread solid-solution mechanism; (iv) electronic spin hopping might occur locally between VIII and VV ions generated during the electrochemical reaction, resulting in the inconsecutive shift in 23Na resonance. This study s...

Identification of Distinct Framework Aluminum Sites in Zeolite ZSM-23: A Combined Computational and Experimental 27Al NMR Study

ZSM-23 (MTT) is a silicon-rich zeolite with one-dimensional, 10-membered ring channels, which has recently attracted interest as a promising catalyst in aromatic-free methanol-to-hydrocarbons conversion. To obtain a better understanding of the catalytic activity and ultimately to design a better catalyst, it is crucial to locate the active sites in the zeolite framework. This work investigates the tetrahedral aluminum framework sites in two zeolite H-ZSM-23 samples by experimental and computational 27Al NMR spectroscopy. 27Al MQMAS NMR experiments at six different magnetic fields (4.7–22.3 T) were utilized to resolve distinct Al sites. The detected tetrahedral framework Al sites were assigned to the specific tetrahedral sites in the crystal structure by DFT calculations of the 27Al chemical shieldings. A comprehensive investigation of the structural model, basis set, and exchange–correlation potential used in the DFT calculations was performed. Two avenues were pursued for calculating the 27Al isotropic c...

Distribution of Aluminum over the Tetrahedral Sites in ZSM-5 Zeolites and Their Evolution after Steam Treatment

Zeolite ZSM-5 is one of the most widely used zeolites, in particular in heterogeneous catalysis. This work investigates the incorporation of Al in the silica framework of monoclinic ZSM-5 and the Al speciation during steam treatment for four ZSM-5 samples with different Si/Al ratios, using ultrahigh field 27Al NMR (22.3 T), 29Si NMR, X-ray diffraction, and IR spectroscopy. 27Al MQMAS NMR at 22.3 T allows identification and quantification of 10 distinct tetrahedral framework resonances which are assigned to the crystallographic sites by their average T–O–T angles, determined from powder XRD patterns for the same samples. A clear Al site preference is observed and found to be dependent on the Si/Al ratio of the parent zeolite. The framework sites facing the intersections in ZSM-5 are found to be most prone to dealumination, whereas Al in the straight and sinusoidal-shaped channels are more stable toward steam treatment at high temperatures. Extra-framework Al species with 5- and 6-fold coordination and two ...

RE2O3-alkaline earth-aluminosilicate fiber glasses: Melt properties, crystallization, and the network structures

Glasses and/or glass fibers of a new glass system, RE2O3 (RE = Sc,Y, La)-MgO-CaO-Al2O3-SiO2, were studied in detail, covering rare earth (RE) effects on high temperature viscosity, liquidus temperature and crystalline phases, softening and glass transition temperature, and speciation reactions of aluminum (AlOx, x = 4, 5) and silicate network (Qn, n = 4, 3, 2) using 27Al and 29Si MAS NMR and Raman spectroscopic methods. Rare earth oxides (RE2O3) were shown to have a dual functionality, i.e., decreasing melt viscosity like a network modifier depolymerizing glass network, but increasing viscosity at low temperature, i.e., glass softening temperature (Tsoft) and transition temperature (Tg), strengthening the glass network functioning like Al2O3. Liquidus temperature (TLiq) was found to be sensitive to the type of RE2O3 modifications in terms of their ionic field strength (IFS); Sc3+ of the highest IFS increased, but La3+ of the lowest IFS decreased TLiq. Effect of Y3+ (intermediate IFS) on TLiq resembles Sc3+ only at higher concentration. The overall results suggest that RE ions of different IFS have different “true” solubility limits in the host glass matrix, above which RE2Si2O7 crystallizes. All of the glasses with and without RE2O3 contained predominately four-fold coordinated aluminum, AlO4; yet replacement of CaO by RE2O3 increased AlO5 according to 27Al MQMAS NMR spectroscopic measurements. Raman spectroscopic results suggested that depolymerization of the silicate network.

Mapping Al Distributions in SSZ-13 Zeolites from 23Na Solid-State NMR Spectroscopy and DFT Calculations

Al distributions and locations of corresponding nonframework cations in SSZ-13 zeolites have significant impact on their catalytic and adsorption properties. Herein, we demonstrate the feasibility of elucidating Na+ locations and correlated Al distributions in a series of Na-SSZ-13 zeolites with Si/Al ratios ranging from 4 to 48 by combining 23Na MQ MAS NMR spectroscopy and DFT calculations. As only one crystallographically inequivalent T site exists in CHA cages, the isolated Al and the Al pair (Al–O–Si–O–Si–O–Al) and (Al–O–Si–O–Al) in 6-membered ring (MR) distributions and the corresponding Na+ locations are clearly discriminated. In the high-silica Na-SSZ-13 zeolite, Na+ ions are mainly located at the 6-MR SIIa0 site corresponding to one Al substitution in the CHA cage. However, a small amount of the Al pair in 6-MR is still observable even with an Si/Al ratio of 48, corresponding to two Na+ ions located at 6-MR SIIa1 and 8-MR SIII’a1 sites, respectively. With more Al substitutions in the CHA cage, a p...

Determination of Acid Site Location in Dealuminated MCM-68 by 27Al MQMAS NMR and FT-IR Spectroscopy with Probe Molecules

A series of MCM-68 zeolites with different Si/Al ratios were prepared by treatment with nitric acid and compared with beta zeolites. Speciation of aluminum and location of acid sites changed depending on the Si/Al ratio. The location of Bronsted acid sites in MCM-68 samples was able to be classified by FT-IR measurements with pyridine and 2,6-di-tert-butylpyridine as probe molecules, and the number of Bronsted acid sites was quantified according to the locations. For high-aluminum MCM-68, Bronsted acid sites were broadly distributed in both the 12-ring channel and 10-ring windows as well as inside the supercage. The Bronsted acid sites in the 12-ring channel were easily removed by the acid treatment, and consequently, high-silica MCM-68 had Bronsted acid sites predominantly in the 10-ring windows and inside the supercage. 27Al MQMAS NMR spectra of high-silica MCM-68 showed two specific peaks assignable to T6 and T7 sites, which did not face the 12-ring channel, forming the Bronsted acid sites highly toler...

27Al MAS NMR Studies of HBEA Zeolite at Low to High Magnetic Fields

27Al single pulse (SP) MAS NMR spectra of HBEA zeolites with high Si/Al ratios of 71 and 75 were obtained at three magnetic field strengths of 7.05, 11.75, and 19.97 T. High field 27Al MAS NMR spectra acquired at 19.97 T show significantly improved spectral resolution, resulting in at least two well-resolved tetrahedral-Al NMR peaks. Based on the results obtained from 27Al MAS and MQMAS NMR acquired at 19.97 T, four different quadrupole peaks are used to deconvolute the 27Al SP MAS spectra acquired at various fields by using the same set of quadrupole coupling constants, asymmetric parameters and relative integrated peak intensities for the tetrahedral Al peaks. The line shapes of individual peaks change from typical quadrupole line shape at low field to essentially symmetrical line shapes at high field. We demonstrate that, for fully hydrated HBEA zeolites, the effect of second-order quadrupole interaction can be ignored, and quantitative spectral analysis can be performed by directly fitting the high fi...

Understanding the structural origin of crystalline phase transformations in nepheline (NaAlSiO4)‐based glass‐ceramics

AbstractNepheline (Na6K2Al8Si8O32) is a rock‐forming tectosilicate mineral which is by far the most abundant of the feldspathoids. The crystallization in nepheline‐based glass‐ceramics proceeds through several polymorphic transformations — mainly orthorhombic, hexagonal, cubic — depending on their thermochemistry. However, the fundamental science governing these transformations is poorly understood. In this article, an attempt has been made to elucidate the structural drivers controlling these polymorphic transformations in nepheline‐based glass‐ceramics. Accordingly, two different sets of glasses (meta‐aluminous and per‐alkaline) have been designed in the system Na2O–CaO–Al2O3–SiO2 in the crystallization field of nepheline and synthesized by the melt‐quench technique. The detailed structural analysis of glasses has been performed by 29Si, 27Al, and 23Na magic‐angle spinning — nuclear magnetic resonance (MAS NMR), and multiple‐quantum MAS NMR spectroscopy, while the crystalline phase transformations in these glasses have been studied under isothermal and non‐isothermal conditions using differential scanning calorimetry (DSC), X‐ray diffraction (XRD), and MQMAS NMR. Results indicate that the sequence of polymorphic phase transformations in these glass‐ceramics is dictated by the compositional chemistry of the parent glasses and the local environments of different species in the glass structure; for example, the sodium environment in glasses became highly ordered with decreasing Na2O/CaO ratio, thus favoring the formation of hexagonal nepheline, while the cubic polymorph was the stable phase in SiO2–poor glass‐ceramics with (Na2O+CaO)/Al2O3 > 1. The structural origins of these crystalline phase transformations have been discussed in the paper.

Release of full catalytic capacity of desilicated ZSM-5 in MTH reaction: Al migration along mesopore introduction and post engineering

Alkali-treatment is an effective method for improving ZSM-5 lifetime in methanol to hydrocarbon (MTH) reaction. In this paper, the effect of alkali-treatment on framework Al migration in ZSM-5 was firstly studied by 27Al 2D MQMAS NMR experiment. The result clearly demonstrates that part of framework Al in ZSM-5 zeolite is migrated into extra-framework position along with the mesopore introduction. Such Al migration is the reason of Lewis acidity formation in zeolite during alkali-treatment. Lewis acidity, together with internal hydroxyl group, partly neutralizes the benefit of mesopore introduction in MTH reaction. Post-treatment of mesoporous ZSM-5 sample in FeF3/NH4HF2 can selectively diminish the Lewis acidity and internal hydroxyl group. The improvement of catalyst lifetime by such post-treatment clearly proves the negative effects of Lewis acidity and internal hydroxyl group in MTH reaction, and the benefits of mesopore in MTH reaction is potentially overlooked.

Resolution of the Two Aluminum Sites in Ettringite by 27Al MAS and MQMAS NMR at Very High Magnetic Field (22.3 T)

Ettringite (Ca6[Al(OH)6]2(SO4)3·26H2O) is the first hydration product formed during Portland cement hydration. 27Al MAS NMR has been used in a wide number of studies to detect and quantify ettringite in hydrated cement blends by the observation of a single, narrow resonance at 13–14 ppm. This work reports the first observation of resonances from two distinct Al sites in octahedral coordination for ettringite, employing 27Al MAS and MQMAS NMR at an ultrahigh magnetic field (22.3 T). Thereby, the 27Al NMR spectra are in agreement with the most accepted trigonal model for the ettringite structure. 27Al quadrupole coupling parameters and isotropic chemical shifts for the two Al sites are determined from simulations and least-squares optimization of slow-speed 27Al MAS NMR spectra of the satellite transitions. These data reveal that the local environments for the two octahedral Al sites are very similar, in accord with the most recent XRD refinements of the ettringite structure. Finally, the significant improv...

Tail gas catalyzed N2O decomposition over Fe-beta zeolite. On the promoting role of framework connected AlO6 sites in the vicinity of Fe by controlled dealumination during exchange

A novel route to prepare highly active and stable N2O decomposition catalysts is presented, based on Fe-exchanged beta zeolite. The procedure consists of liquid phase Fe(III) exchange at low pH. By varying the pH systematically from 3.5 to 0, using nitric acid during each Fe(III)-exchange procedure, the degree of dealumination was controlled, verified by ICP and NMR. Dealumination changes the presence of neighbouring octahedral Al sites of the Fe sites, improving the performance for this reaction. The so-obtained catalysts exhibit a remarkable enhancement in activity, for an optimal pH of 1. Further optimization by increasing the Fe content is possible. The optimal formulation showed good conversion levels, comparable to a benchmark Fe-ferrierite catalyst. The catalyst stability under tail gas conditions containing NO, O2 and H2O was excellent, without any appreciable activity decay during 70h time on stream. Based on characterisation and data analysis from ICP, single pulse excitation NMR, MQ MAS NMR, N2 physisorption, TPR(H2) analysis and apparent activation energies, the improved catalytic performance is attributed to an increased concentration of active sites. Temperature programmed reduction experiments reveal significant changes in the Fe(III) reducibility pattern with the presence of two reduction peaks; tentatively attributed to the interaction of the Fe-oxo species with electron withdrawing extraframework AlO6 species, causing a delayed reduction. A low-temperature peak is attributed to Fe-species exchanged on zeolitic AlO4 sites, which are partially charged by the presence of the neighbouring extraframework AlO6 sites. Improved mass transport phenomena due to acid leaching is ruled out. The increased activity is rationalized by an active site model, whose concentration increases by selectively washing out the distorted extraframework AlO6 species under acidic (optimal) conditions, liberating active Fe species.

Effects of Composition and Structure of Mg/Al Oxides on Their Activity and Selectivity for the Condensation of Methyl Ketones

The effects of chemical composition and pretreatment on Mg–Al hydrotalcites and alumina-supported MgO were evaluated for the gas-phase, self-condensation reaction of C3–C5 biomass-derived methyl ketones. We show that the selectivity toward the acyclic dimer enone and the cyclic enone trimer can be tuned by controlling the temperature of hydrotalcite calcination. Methyl ketone cyclization is promoted by Lewis acidic sites present on the hydrotalcite catalysts. XRD and thermal decomposition analysis reveal that the formation of periclase MgO starts above 623 K accompanied by complete disappearance of the hydrotalcite structure and is accompanied by an increase in hydroxyl condensation as the formation of well-crystallized periclase. 27Al MQMAS and 25Mg MAS NMR show that at progressively higher temperatures, Al3+ cations diffuses out of the octahedral brucite layers and incorporate into the tetrahedral and octahedral sites of the MgO matrix thereby creating defects to compensate the excess positive charge ge...