Dehydroxylate Phase Research Articles

Preparation and characterization of lanthanum nitrate nanoparticles by a polyol method

We present the synthesis of lanthanum nitrate nanoparticles via the polyol method. A crystallographic study of La(NO3)3·4H2O nanoparticles was carried out using the Rietveld method and XRD to detect the phases of the yield: α (monoclinic) and β (orthorhombic) phases were obtained. Thermogravimetric analysis showed a unique dehydration process in the 65 °C–245 °C temperature range and a dehydroxylation phase in the 333 °C–558 °C temperature range. Using TEM–EDX, and DLS, the purity of the material was evaluated; a spherical shape morphology and monodispersity were observed. The absorbance at 1340–1440 cm−1 and 1500–1600 cm−1 (IR regions) of the lanthanum nanoparticles was deconvolved to increase the resolution of the chemical imprint in these regions.

The impact of kaolin dehydroxylation on the porosity and mechanical integrity of kaolin based ceramics using different pore formers

Thermally induced lattice defects in kaolin play a key role in the mechanical integrity of kaolin based ceramics during heat treatment due to their phase transformations. The effects of three types of pore formers (sawdust, styrofoam and powdery high density polyethylene) on the porosity and mechanical integrity of kaolin based ceramics have been studied based on their batch formulations. The porosity of the ceramic bodies was experimentally determined, while the structure and chemistry of the materials were elucidated via X-ray diffraction (XRD), scanning electron microscopy (SEM), FTIR, DSC/TG/DTA, and zeta potential. The kaolin-based porous samples sintered at 1150°C exhibited mullite phase transformation attributed to recrystallization effects. Morphologically, open pores were distributed on the sample surfaces due to larger pathways and available channels of eviction of the pore formers. Compressive strength decreased linearly as apparent porosity increased signifying its correlation. The compressive strength showed to be much more sensitive to defects created by dehydroxylation as micro cracks were observed on the sample with HDPE as pore former which could be as a result of large volume change accompanying phase transformations and sensitivity to the dehydroxylation phase. It is noted that the new pore former (HDPE) used in this study produced ceramic bodies with porosity as high as 67% and hence the least compressive strength.

Adsorption of fluoride by waste iron oxide: The effects of solution pH, major coexisting anions, and adsorbent calcination temperature

In this study, a waste iron oxide material (BT3), which is a by-product of the fluidized-bed Fenton reaction (FBR-Fenton), was thermally treated between 200 and 900°C and was used as an efficient adsorbent for the removal of fluoride ions in an aqueous system. The highest fluoride adsorption capacity occurred at the termination of the BT3 goethite dehydroxylation phase at about 300°C calcination where both the volume of nanopores formed by dehydroxylation and the specific surface area reached their maximum values. Above 300°C, BT3 transformed to the hematite phase in which fluoride adsorption capacity decreased as calcination temperature increased. On the other hand, the effect of pH on the fluoride adsorption capacity of BT3 for various initial fluoride concentrations was examined. The optimum pH value was found to be about 4. After that efficiency decreased as pH became more alkaline. Finally, coexisting anions affected the fluoride adsorption capacity of BT3 at pH 3.9±0.2 in this order: PO43−>SO42−>Cl−>NO3−.

Thermal expansion and dehydroxylation of phengite micas

Phengite samples (2M 1 and 3T politypes) and a synthetic end-member muscovite specimen were studied by in situ high-temperature synchrotron radiation X-ray diffraction. The measured volume thermal expansion of 2M 1 phengite (<α V> ≈ 36.6 × 10−6 K−1) was systematically greater than <α V> of the 3T polytype (≈33.3 × 10−6 K−1). A positive linear correlation between the average thermal expansion on (001) plane and the mean tetrahedral rotation angle at ambient condition is proposed on the ground of new measurements and literature data. Dehydroxylation processes were observed in 2M 1, starting at 1,000 K in 3T at 800 and 945 K in synthetic muscovite. Rietveld refinements allowed a determination of structural variations upon heating of phengite samples and their dehydroxylate phases. The phengite structure expands by regularizing the tetrahedral sheet and by reducing the bond length differences between the outer and inner coordination shell of the interlayer site. The dehydroxylate phase derived from 2M 1 is characterized by fivefold polyhedra in the low temperature form as a consequence of two OH groups reacting to form H2O + O (residual). The dehydroxylate exhibits an increase of the cation–cation distances along the M–Or–M bonds with respect to low-temperature phengite structures. For the 3T phase, we were unable to achieve completion of dehydroxylation. The refined structural model of the dehydroxylate phase shows two hydroxyl sites, but at a short distance from one another. This result suggests that the dehydroxylation reaction did not proceed to completion.

Imaging of dioctahedral 2:1 layers by high-resolution transmission electron microscopy (HRTEM): Possibility of recording the dehydroxylate

High-resolution transmission electron microscopy (HRTEM) images of dioctahedral 2:1 phyllosilicates (muscovite, paragonite, pyrophyllite, etc.) acquired by intense electron radiation may not record the natural state but rather the dehydroxylate phase, e.g., NaAl2Si3AlO11 in the case of paragonite, NaAl2Si3AlO10(OH)2. Intense electron radiation on paragonite changes its electron diffraction pattern by a small increase in cell edges and a considerable decrease of the β angle, which are consistent with dehydroxylation. Comparison between experimental and simulated HRTEM images also indicates that the experimental image contrast is in better agreement with that for the dehydroxylate structure than the natural state. Thus, special care is necessary when analyzing the structures of dioctahedral 2:1 phyllosilicates from their HRTEM images, e.g., positions of octahedral cations that were proposed to change by migration during the dehydroxylation process

The Effect of Heating on the Surface Area, Porosity and Surface Acidity of a Bentonite

AbstractThe Hançılı bentonite from Turkey shows significant changes in surface area, micro- and mesoporosity, surface acidity and acid strength with heating from 100 to 900°C for 2 h. The specific surface area (S) and specific micro-mesopore volume (V) of the original and heated samples were evaluated from N2 adsorption and desorption data, respectively, by standard methods. The adsorption of n-butylamine from the solution in cyclohexane on the samples was used to determine the total surface acidity (nm) and the adsorption equilibrium constant (K) was taken as a measure of the acid strength. S, V and nm having initial values of 98 m2g−1, 0.080 cm3g−1 and 4.8 × 10−4 mol g−1, respectively, stayed approximately constant as the temperature increased to 450°C and then decreased almost in parallel with each other, reaching their minimum or zero at 900°C. The total surface acidity, in general, declines with increasing temperature. The most acidic sites, however, increase with heating, and especially at dehydration and dehydroxylation. Acid strength reaches its maximum during the dehydroxylation phase at ∼600°C. It was concluded that the total surface acidity does not necessarily parallel the strength of the most acid sites.

Characterization of 2M pyrophyllite associated with argillic alteration in steam-heated environment, Solo, Mabini, Philippines.

Localized deposit of Fe-rich pyrophyllite was encountered in the kaolinite-dominated alteration halo in Solo, Mabini, Philippines. The environment of formation of this dioctahedral mineral is related to low-temperature advanced argillic alteration found in shallow volcanic region. The pyrophyllite crystallizes as a two-layer monoclinic (2M) structure with the following unit cells, a=5.16A, b=8.95A, c=18.70A, β=99.87° while its dehydroxylate form yields a slightly different unit cell parameters with a=5.17A, b=9.02A, c=18.76A, β=99.80°. The expansion of b on dehydroxylation arises largely from the rearrangement of the Al ions in the octahedral layer and the insertions of oxygen ions in the Al planes. The obtained tetrahedral rotation (twisting of SiO4 groups) of 11°52' in pyrophyllite is closely comparable to the 9°40' angle of twist in the dehydroxylate. This strongly suggests that the tetrahedral networks of the original pyrophyllite are almost similarly twisted in the dehydroxylate phase. X-ray powder data demonstrate that the dehydroxylation process is accompanied by a significant enlargement of basal spacing with an observed expansion of about 1.99%. Thermal decomposition of pyrophyllite shows a single dehydration event, which takes place over a temperature range of 500-700°C with its peak at 604°C. The occurrence of this low-temperature dehydroxylation can be regarded as a consequence of significant replacement of Al by Fe in the octahedral layer and the presence of some impurities. Chemical data of pyrophyllite obtained from EDX analysis give an average structural formula of Ca0.01K0.10(Al1.81Fe3+0.16Ti0.01)(Si3.88Al0.12)O10(OH)2 with an average Si/Al3++Fe3+ ratio of 1.86. Analyses of representative samples indicate that the octahedral site is filled with 1.76 to 1.84 Al cations, which is relatively lower than the 1.90 to 1.95 Al cations exhibited by ordinary pyrophyllite. This significant deficiency of Al is believed to be compensated by considerably high Fe3+ occupancy in the octahedral site. The source of Fe can be correlated to the previous decomposition of minor amount of pyrite associated with argillic alteration. SEM observations show common disoriented stacks of pyrophyllite plates, but the most visible component of the samples are the loosely compacted stacks in which thin elongated blade-like particles are randomly dispersed in the form of relatively porous aggregates. The coexistence of pyrophyllite and kaolinite in this environment is a consequence of sluggish crystallization of quartz caused by local variation of silica activity, which explains the crystallization of pyrophyllite instead of stable kaolinite at low temperature (<300°C).

Dehydration and decomposition of pyrophyllite at high pressures: electrical conductivity and X-ray diffraction studies to 5 GPa

The dehydration and decomposition of South African pyrophyllite were studied in the pressure range 2.5–5.0 GPa and in the temperature (T) range 295–1473 K using both in situ electrical conductivity measurements and X-ray diffraction studies on the recovered samples. Activation energies for conduction (Qc) vary in the range 0.02–0.07 eV for T ≤ 500 K where the dominant conduction mode is electronic, and Qc is in the range 1.10–1.28 eV for T ≥ 500 K where ionic conduction dominates. Abrupt changes in the isobaric temperature dependence of conductivity mark the onset of dehydration and subsequent decomposition into kyanite plus quartz–coesite. At 2.5 GPa, South African pyrophyllite forms the dehydroxylate phase at 760 K with a pressure dependence of ~30 K/GPa and complete decomposition follows at 1080 K with a pressure dependence of ~41 K/GPa. The resulting pressure–temperature phase diagram is in very good agreement with many previous studies at 1 atm (101.325 kPa).

Dehydroxylation induced structural transformations in montmorillonite: an isothermal FTIR study

Fourier transform infrared (FTIR) measurements on thermally shocked (290<T<1100 K, t=1 and 24 h) and thermally soaked (T=803 K, 0<t<25 h) Ca-montmorillonite from Texas show that the dehydroxylation temperature for this mineral is much lower than previously reported in the literature and depends upon the thermal soak time used. The effect of thermal shock temperature on vibrational frequencies, originating from the silicate sheet, and the intensity of Al-OH-Al stretching mode suggest that Camontmorillonite dehydroxylates at 903 <T<923 K and 773<T<823 K for 1 h and 24 h thermally shocked samples, respectively. Our similar measurements on the Mn and Fe exchanged-montmorillomte from Texas demonstrate that the dehydroxylation temperature range is also affected by the type of cations present in the interlayer. The thermal soak (T=803 K, 0<t<25 h) measurement on Ca-montmorillonite indicates that the dehydroxylation of this montmorillonite proceeds via the development of intermediate structural phases. As noted by previous authors, the dehydroxylation of Mg2+ and Fe3+ octahedral sites does not affect the overall structure. However, if ∼75 percent of the hydroxyls attached to Al3+ are lost, then the lattice manifests an intermediate structural phase. In this intermediate phase, the structure of both octahedral and silicate layers is affected. On a further loss of hydroxyls (∼90% of Al3+ hydroxyls), the final montmorillonite dehydroxylate phase develops. The vibrational analysis of an isothermally treated sample suggests that the final phase is induced due to the rearrangement of the silicate oxygens, which leave the coordination around Al to be 5. The dehydroxylation results of montmorillonite (Texas) have been discussed in terms of known mechanisms, and it appears that the dehydroxylation starts at the surface and proceeds via proton delocalization at trans hydroxyl positions, followed by the protons' migration across the vacant cation sites with the formation of H2O molecules below the hexagonal holes.

The thermal reactions of montmorillonite studied by high-resolution solid-state29Si and27Al NMR

Only very slight changes are observed in the29Si and27Al solid-sate magic-angle spinning NMR spectra of a montmorillonite containing almost equal numbers of octahedral aluminium and magnesium ions when its interlayer water is driven off by heating. The29Si NMR spectra are unaffected by dehydroxylation which begins at ∼ 450° C, but the27Al spectra show a decrease in total intensity, possibly due to the formation of 5-coordinated aluminium, with a slight increase in the intensity of the tetrahedral aluminium resonance. On the basis of these results, a structural model is proposed for the dehydroxylate phase and its formation mechanism is discussed. The destruction of the dehydroxylate X-ray pattern at ∼ 850° C and the subsequent recrystallization of the high-temperature products (β-quartz, enstatite and high-cordierite at ∼ 1100° C;β-cristobalite, enstatite and sapphirine at ∼ 1200° C) is accompanied by changes in the silicon and aluminium NMR spectra and in the57Fe Mbssbauer spectra which are fully consistent with the known structural features of these phases.